|HAI LAMA AT TALLINNA ONTHE OTH

US009957582B2

(12) United States Patent ( 10) Patent No.: US 9, 957,582 B2

Puerta Velasquez et al. (45) Date of Patent: May 1 , 2018
(54 ) PRECOATED SHEETS FOR (2013.01); C21D 8 /0236 (2013.01); C210
MANUFACTURING PRESS -HARDENED 8 / 0257 (2013.01); C22C 38 /002 ( 2013 .01);
COATED STEEL PARTS C22C 38 /02 ( 2013 .01) ; C22C 38 /04 ( 2013 .01 ) ;
C22C 38 /06 (2013.01 ); C22C 38 /28 (2013 .01 );
(71 ) Applicant: ArcelorMittal, Luxembourg (LU ) C22C 38 /32 (2013.01 ); C23C 2 /06 (2013.01 );
(72 ) Inventors : Juan David Puerta Velasquez, Metz C23C 2 / 12 (2013 .01); C23C 2 / 28 (2013 .01 )
(FR ); Jonas Staudte, (58 ) Field of Classification Search
Montigny-les -Metz (FR ); Pascal ??? ............... C22C 38/02; C22C 38 /0432
Drillet, Rozerieulles (FR ) USPC .................. 148 /333 ; 428 / 457
See application file for complete search history .
(73 ) Assignee: ArcelorMittal, Luxembourg (LU )
(56 ) References Cited
( * ) Notice: Subject to any disclaimer, the term of this U . S . PATENT DOCUMENTS
patent is extended or adjusted under 35
U .S .C . 154 (b ) by 0 days. days . 6 ,296 , 805 B1 10 / 2001 Laurent et al.
6 ,564 ,604 B25 /2003 Kefferstein et al.
(21) Appl. No.: 15/609,841 (Continued )
(22 ) Filed : May 31, 2017 FOREIGN PATENT DOCUMENTS
(65 ) Prior Publication Data DE 1294402 5 /2008
US 2017/0260599 A1 Sep . 14, 2017 FR 2780984 1/2000
(Continued )
Related U .S . Application Data Primary Examiner — Brian Walck
(62 ) Division of application No. 14 /426 ,523 , filed as (74 ) Attorney, Agent, or Firm — Davidson , Davidson &
application No . PCT/ IB 2013/001914 on Sep . 6 . 2013 . Kappel, LLC ; Jennifer L . O 'Connell ; William C . Gehris
( 30 ) Foreign Application Priority Data (57) ABSTRACT
A cold -rolled sheet is provided . The cold - rolled sheet
Sep. 6 , 2012 (WO ).................. PCT/FR2012/000350 includes a steel substrate with a carbon content C , between
0 .07 % and 0 . 5 % , expressed by weight, and a metal pre
(51 ) Int. Cl. coating on at least the two principal faces of the steel
C22C 38 /02 ( 2006 .01) substrate . The substrate has a decarburized area on the
C22C 38 /04 ( 2006 .01) surface of each of the two principal faces. The depth p50 % of
(Continued ) the decarburized area is between 6 and 30 micrometers , and
(52) U . S . CI. P500 is the depth at which the carbon content is equal to 50 %
CPC . C211 8 /0278 (2013 . 01 ); B32B 15 /012 of the content Co . The sheet does not contain a layer of iron
( 2013 . 01); B32B 15 /013 (2013.01) ; C21D oxide between the substrate and the metal pre -coating .
6 / 002 ( 2013.01) ; C211 6 / 005 ( 2013 .01 );
C21D 6 / 008 (2013 .01 ); C21D 8 /0205 6 Claims, 8 Drawing Sheets

N?
 US 9,957 ,582 B2
Page 2

(51) Int. Ci.

C22C 38 /32 ( 2006 .01)
C210 8 /02 ( 2006 .01)
B32B 15 /01 ( 2006 .01)
C23C 2 /28 ( 2006 .01 )
C22C 38 /28 (2006 .01)
C22C 38 /06 ( 2006 .01 )
C22C 38 /00 (2006 .01)
C21D 6 /00 ( 2006 .01)
C23C 2 / 12 ( 2006 . 01)
C23C 2 /06 ( 2006 . 01)
(56 ) References Cited
U . S . PATENT DOCUMENTS
7, 687,152 B2 3 /2010 Ikematsu et al .
7 , 867,344 B2 1/ 2011 Kusumi et al.
2001/ 0042393 A1 11/ 2001 Kefferestein et al.
2010 /0050730 A1 3 / 2010 Buschsieweke et al.
2010 /0193081 A1 8 / 2010 Mataigne et al.
2011/0076477 A1 3 /2011 Brandstaetter et al .
2016 /0222486 Al 8/ 2016 Moulin et al.
FOREIGN PATENT DOCUMENTS
2807447
FP
JP
H04276057
2001234290
10 / 2001
10 / 1992
8 /2001
JP 2006104546 4 / 2006
JP 2010043323 2 / 2010
KR 100760152 6 /2006
wo WO2009080292 7 /2009
U . S . Patent May 1, 2018 Sheet 1 of 8 US 9 ,957,582 B2

20 m

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U . S . Patent May 1, 2018 Sheet 2 of 8 US 9,957,582 B2

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U . S . Patent May 1, 2018 Sheet 3 of 8 US 9,957,582 B2

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U . S . Patent May 1, 2018 Sheet 4 of 8 US 9,957,582 B2

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U . S . Patent May 1, 2018 Sheet 5 of 8 US 9,957,582 B2

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U . S . Patent May 1, 2018 Sheet 6 of 8 US 9, 957,582 B2

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U . S . Patent May 1, 2018 Sheet 7 of 8 US 9,957,582 B2

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U . S . Patent May 1, 2018 Sheet 8 of 8 US 9, 957,582 B2

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 US 9, 957,582 B2
PRECOATED SHEETS FOR Document WO2009080292 discloses a process that
MANUFACTURING PRESS -HARDENED makes it possible to increase the bending angle of a hardened
COATED STEEL PARTS part . According to this process , a steel sheet is heated in an
annealing furnace to a temperature between 650 and 800° C .
This is a divisional of U . S . patent application Ser. No . 5 to obtain a layer of oxide that is significantly thicker than 0 .3
14 /426 ,523 , filed Mar. 6 , 2015 which is a National Stage of micrometers . Certain alloy elements of the steel are oxidized
International Application PCT/ IB2013 /001914 , filed Sep . 6 , underneath this oxide layer. This oxide layer is then partly
2013 which claims priority to International Application reduced so that it has a thickness greater than 0 . 3 microm
PCT /FR2012/000350 , filed Sep . 6 , 2012 . All of the above eters . The extreme surface of the reduced oxide layer
applications are hereby incorporated by reference herein . 10 consists of pure iron . The sheet is then coated using a hot - dip
This invention relates to a process for the fabrication of process . After this step , the sheet has the following different
parts from a steel sheet cold - rolled and annealed , pre - coated , layers in succession : the steel substrate comprising the
heated , stamped and then hardened during cooling by hold - oxidized elements in the vicinity of the surface ( internal
ing it inside a press tool; these parts are intended in par - oxidation ), this substrate being topped by a partly reduced
ticular to be used as structural elements in automotive 15 oxide layer, which is itself topped by the coating applied
vehicles to perform anti-intrusion or energy absorption using a hot- dip process . During the subsequent step of the
functions. Parts of this type can also be used , for example , austenitization of the blank and /or during the shaping and
for the fabrication of tools or parts of agriculturalmachines . cooling, a thin ductile layer is formed under the coating such
that the cracks formed during the coating propagate less
BACKGROUND 20 easily into this underlying layer during the forming process.
However, the layer of oxides that is present when the
In this type of application , the goal is to manufacture steel sheet is immersed in the metal coating bath can have an
parts that combine high mechanical strength , high impact undesirable effect in terms of the adherence of the hot- dip
strength , good corrosion resistance and good dimensional coating to this layer.
accuracy . This combination is particularly desirable in the 25 It would therefore be desirable to have a fabrication
automobile industry , where attempts are being made to process that does not have this disadvantage and that would
significantly reduce the weight of the vehicles . Anti-intru - make it possible to obtain simultaneously , after press hard
sion and structural parts , as well as other parts that contrib - ening, high levels of tensile strength and bendability .
ute to the safety of automotive vehicles such as bumpers, It is also known that industrial fabrication conditions
door or center pillar reinforcements, for example , require the 30 inevitably include a certain variability such as , for example ,
above mentioned characteristics, for example . This weight of the temperature cycle during the annealing of the sheet
reduction can be achieved in particular thanks to the use of before it is coated , and the composition and / or the dew point
steel parts with a martensitic or bainitic -martensitic micro - of the atmosphere of the continuous annealing furnaces ,
structure . which can vary slightly during a given fabrication sequence
The fabrication of parts of this type is described in prior 35 or can vary from one fabrication run to another. Even if the
art publications FR2780984 and FR2807447 , according to maximum precautions are taken to minimize these varia
which a blank cut in a steel sheet for heat treatment and tions, it would be desirable to have a fabrication process
pre -coated with a metal or metal alloy is heated in a furnace such that themechanical characteristics , and in particular the
and then hot formed . The pre -coating can be aluminum or an bendability , obtained after press hardening are as insensitive
aluminum -based alloy , zinc or a zinc alloy. During the 40 as possible to this potential variation of the fabrication
heating in the furnace , the pre -coating provides protection conditions. An additional objective is a fabrication process
for the surface of the steel against decarburization and the that results in good isotropy of the parts after hot stamping ,
formation of calamine. During the heating in the furnace , i.e ., in which the bendability is not highly dependent on the
this pre - coating becomes alloyed with the steel substrate to direction of stress in relation to the direction in which the
form a compound suitable for hot forming and that does not 45 sheet is rolled .
cause any deterioration of the tooling .Holding the part in the It is also known that the hold time of the blanks in the
tooling after forming has been performed makes possible a furnace during the austenitization step during hot stamping
rapid cooling that leads to the formation of hardened micro - can influence the mechanical characteristics of the parts . It
structures that have very high mechanical characteristics . A would therefore be desirable to have a fabrication process
process of this type is known as press hardening . 50 that is less sensitive to the hold time in the furnace to achieve
As a rule , the mechanical characteristics of the parts a high level of reproducibility of the mechanical character
obtained in this manner are evaluated by means of tensile istics of the parts .
strength and hardness tests . The above referenced docu - In the case of parts fabricated from sheets pre -coated with
ments also describe fabrication processes that make it pos - zinc or zinc alloy , the objective is to have a process that
sible to obtain a mechanical strength (or maximum tensile 55 makes it possible to weld these parts without the risk of
strength ) Rm of 1500 MPa starting with a steel blank that embrittlement of the grain boundaries caused by penetration
has an initial strength Rm of 500 MPa before heating and of liquid zinc.
rapid cooling.
However, the service conditions of certain hardened and SUMMARY OF THE INVENTION
coated parts require not only a high level of strength Rm but 60
also good bendability. This parameter does in fact appear to An object of this invention is to solve the problems cited
be more pertinent than the measured elongation at failure in above by means of an economical fabrication process.
traction to guarantee that the part has sufficient ductility to Surprisingly, the inventors have shown that a high bend
absorb deformations or impacts without risk of rupture , in ability of the parts is achieved when a decarburized area of
particular in the areas corresponding to local stress concen - 65 a specific thickness is present underneath the metal pre
trations due to the geometry of the part or to the potential coating , before the press hardening of the part. Surprisingly ,
presence of micro -defects on the surface of the parts . this specific decarburization before hardening produces
 US 9,957,582 B2
bending results that are not dependent to a great extent on between 6 and 30 micrometers , whereby psov, is the depth at
the continuous annealing conditions before coating and that which the carbon content is equal to 50 % of said content Co,
reflect good isotropy in relation to the rolling direction ; high and to obtain a sheet that has no iron oxide layer on its
bending values are achieved in spite of the presence of surface , followed by a pre -coating with metal or a metal
oxides in this decarburized area , which corresponds to an 5 alloy on the annealed steel that serves as the substrate . The
oxygen enrichment in this area . pre - coated steel is then cut to obtain a blank , the blank is
For this purpose the present invention provides a cold then optionally cold stamped , and then the blank is heated to
rolled sheet that is annealed and pre - coated for the fabrica - a temperature T , in a furnace to give the steel an at least
tion of press hardened parts , composed of a steel substrate partly austenitic structure . The heated blank is removed from
for heat treatment with a carbon content Co between 0 .07 % 10 the furnace and is transferred into a press or a forming
and 0 .5 % , whereby this content is expressed by weight, and device , and the blank is then hot formed or hot sized to
a metal pre - coating on at least one of the two principal faces obtain a part that is then cooled in the press or the forming
of the steel substrate , characterized in that the substrate device to give it a martensitic or bainitic -martensitic micro
comprises a decarburized area on the surface of each of the structure by hardening.
two principal faces , whereby the depth p50 % of the decar- 15 An additional object of the invention is a fabrication
burized area is between 6 and 30 micrometers , whereby process characterized in that the pre - coating is performed
P50 % is the depth at which the carbon content is equal to 50 % continuously using a hot-dip process by passage through a
of the content Co , and in that the sheet does not contain a bath .
layer of iron oxide between the substrate and the metal In one preferred embodiment, the metal pre -coating is
pre - coating. The depth 50 % of the decarburized area is 20 aluminum or an aluminum alloy.
advantageously between 9 and 30 micrometers , and very The pre -coating is preferably zinc or a zinc alloy .
advantageously between 12 and 30 micrometers. In one particular embodiment , the metal pre - coating is
In one preferential embodiment, the metal pre-coating of composed of a layer of aluminum or an aluminum -based
the sheet is aluminum or an aluminum alloy. alloy topped by a layer of zinc or a zinc alloy.
In another preferential embodiment, the metal pre - coating 25 The depth p50o is advantageously between 9 and 30
is zinc or a zinc alloy. micrometers, and very advantageously between 12 and 30
The metal pre -coating can preferably be composed of a micrometers .
layer of aluminum or an aluminum -based alloy topped by a An additional object of the invention is a process char
layer of zinc or a zinc alloy . acterized in that the composition of the steel substrate
The composition of the steel substrate advantageously 30 comprises , whereby the contents are expressed by weight :
includes , whereby the contents are expressed by weight: 0 .07 % sCs0.5 % , 0 .5 % sMn< 3 % , 0 .02 % sSis0 .5 % ,
0 .07 % sCs0 .5 % , 0 .5 % sMns3 % , 0 .02 % sSis0 .5 % , 0 .01 % sCrs1 % , Tis0 .2 % . Als0 .25 % , Ss0 . 05 % , P < 0 . 1 %
0 .01 % sCrs1 % , Tis0 .2 % , Als0 .25 % , Ss0 .05 % , Ps0 .1 % , 0 .0005 % < B < 0 .010 % , optionally 0 .0005 % sCas0 .005 % ,
0 .0005 % sBs0 .010 % , optionally 0 .0005 % sCas0 .005 % , whereby the remainder of the composition consists of iron
whereby the remainder of the composition consists of iron 35 and the unavoidable impurities resulting from processing .
and the unavoidable impurities resulting from processing. In one particular embodiment, the composition of the
Very advantageously , the composition of the steel sub - steel substrate includes, whereby the contents are expressed
strate includes , whereby the contents are expressed by by weight: 0 .09 % Cs0 .38 % , 0 .8 % sMns1.5 % ,
weight: 0 .09 % sCs0 . 38 % , 0 . 8 % sMns1. 5 % , 0 . 1 % sSis0 .35 % 0 .01 % sCrs0 . 3 % , 0 . 02 % sTis0 . 1 % ,
0 . 1 % sSis0 .35 % 0 .01 % sCrs0 . 3 % , 0 .02 % sTis0 . 1 % , 40 0 .001 % sAls0 .25 % , S 0 . 05 % , Ps0 . 1 % ,
0 .001% sAls0 .25 % , S50 .05 % , P < 0 .1 % , 0 .002 % sBs0 .005 % , optionally 0.0005 % sCas0 .005 % ,
0 .002 % sBs0 . 005 % , optionally 0 .0005 % Cas0 .005 % , whereby the remainder of the composition consists of iron
whereby the remainder of the composition consists of iron and the unavoidable impurities resulting from processing .
and the unavoidable impurities resulting from processing. In one particular embodiment of the process, the compo
In one preferred embodiment, the composition of the steel 45 sition of the steel substrate comprises : 0 . 15 % < C < 0 .25 % .
substrate includes, whereby the contents are expressed by The temperature TR is preferably greater than or equal to
weight: 0 . 15 % sCs0 .25 % . the temperature Ac3 of the steel.
The steel substrate advantageously has an oxygen content The cold -rolled steel sheet preferably has an oxygen
O , and underneath the interface between the pre - coating and content Oo; it is annealed to obtain in the substrate , at the
the substrate , at a depth of between 0 and 5 micrometers 50 conclusion of the annealing, underneath the interface
measured from the interface , an average oxygen content Ome between the pre - coating and the substrate , at a depth of
whereby Om0 , is greater than 15. between 0 and 5 micrometers measured from the interface ,
Advantageously, the substrate of the pre -coated steel an average oxygen content Om , whereby 0m / 0 , is greater
contains , in an area located between zero and 5 micrometers
eters than 15 .
below the interface between the pre -coating and the sub - 55 Preferably the cold -rolled sheet is annealed to obtain , in
strate , oxides that include at least one element selected from the substrate at the conclusion of the annealing , underneath
a group consisting of titanium , silicon , manganese, alumi- the interface between the pre -coating and the substrate, at a
num and chromium , whereby the density of the oxides with depth located between 0 and 5 micrometers from the inter
a diameter greater than 1 micrometer in this area is greater face , oxides that include at least one element selected from
than 50 /mm . 60 a group consisting of titanium , silicon , manganese, alumi
The present invention additionally provides a process for num , chromium , whereby the density of the oxides with a
the fabrication of a coated and hardened steel part compris- diameter greater than 1 micrometer in this area is greater
ing the successive steps, according to which a cold - rolled than 50 /mm².
steel sheet is procured for heat treatment having a carbon The present invention further provides a fabrication pro
content Co between 0 .07 % and 0 . 5 % . The rolled steel is 65 cess as described in any one of the embodiments described
annealed to obtain , upon completion of the annealing, a above , wherein the annealing conditions include the follow
decarburization of the surface of the sheet over a depth P50 % ing steps in succession : after having procured the cold -rolled
 US 9, 957,582 B2
sheet, it is heated as it travels through a radiant tube furnace The present invention even further provides a fabrication
or a resistance furnace or an induction furnace or a furnace process, the annealing conditions of which include the
combining at least any two of these means, to a temperature following steps in succession : after having procured a cold
Tla between 600° C . and Ac1 + 40° C ., whereby Acl des rolled steel sheet, it is pre-heated as it travels through a
ignates the temperature of the start of the austenitic trans - 5 furnace, the pre -heating being carried out in a zone of a
formation as the steel is heated , in a zone of the furnace furnace heated by direct flame, whereby the sheet is pre
where the atmosphere A1 contains from 2 to 15 % by volume heated to a temperature T1b between 550 and 750° C . in an
of hydrogen , preferably 3 -5 % by volume of hydrogen , the atmosphere resulting from the combustion of a mixture of
balance being nitrogen and unavoidable impurities , with a air and natural gas in which the air/gas ratio is between 1 and
dew point between - 60 and - 15° C . The sheet is then heated 10 1 .2 . The sheet is heated from the temperature T1b to a
from the temperature Tia to a temperature T2a between 720 temperature T2b between 760 and 830° C . in a second zone
and 860° C ., whereby at least one element selected from of the furnace heated by radiant tubes or by resistances or by
among liquid water, steam or oxygen is injected into the induction or by any combination of at least two of these
furnace starting at the temperature Tla to obtain , in the 16 means, in which the atmosphere contains from 3 to 40 % by
section of the furnace between the temperature Tla and the volume of hydrogen , the balance being nitrogen and
temperature T2a , an atmosphere A2a with a dew point PR unavoidable impurities, whereby the dew point is less than
between - 15° C . and the temperature Te of the iron /iron - 30° C ., and whereby the interval of time between the
oxide equilibrium dew point, whereby the interval of time instant when the sheet is at the temperature T1b and the
between the instant when the sheet is at the temperature Tla 20 instant when it reaches the temperature T2b is at least 30
and the instant when the sheet reaches the temperature T2a seconds. The sheet is held at a temperature Tm between T2b
is greater than or equal to 30 seconds . The sheet is then held and T2b + 40° C ., under an atmosphere A3 which is reductive
at a temperature Tm between T2a and T2a + 40° C ., under an for iron , and is then cooled in an atmosphere , such that no
atmosphere A3 which is reductive for iron , and is then surface re - oxidation of the iron occurs , to a temperature T3.
cooled to a temperature T3, in an atmosphere A4 such that 25 The sheet is then pre -coated by hot- dip coating in a metal
no surface reoxidation of the iron occurs. The sheet is then bath at the temperature Tbm , it being understood that the
pre - coated by hot - dip coating in a metal bath at the tem - temperature T3 is between Tbm - 10° C . and Tbm + 50° C .
perature Tbm , it being understood that the temperature T3 is In one preferred embodiment, the temperature T2b is
between Tbm - 10° C . and Tbm + 50° C . greater than Acl.
Preferably, the dew point PR of the atmosphere A2a is 30
between - 15 and + 17° C ., very preferably between - 15 and BRIEF DESCRIPTION OF THE DRAWINGS
- 10° C .
The present invention also provides a fabrication process Additional characteristics and advantages of the invention
in which the annealing conditions include the following
steps in succession : after having procured the cold - rolled 35 will be made clear in the following description , which is
provided by way of example , and refers to the accompany
sheet, it is heated as it travels through a radianttube furnace ing figures , in which :
or a resistance furnace or an induction furnace or a furnace
combining at least any two of these means, to a temperature FIG . 1 shows the microstructure of a pre -coated steel
Tla between 600° C . and Ac1+ 40° C ., whereby Acl des sheet according to the present invention and intended for the
jgnates the temperature of the start of the austenitic trans - 40 fabrication of press hardened parts .
formation as the steel is heated , in a zone of the furnace FIG . 2 schematically illustrates the definition of the depth
where the atmosphere A1 contains from 2 to 15 % by volume of the soft area d measured by microhardness underneath the
of hydrogen , preferably 3 -5 % by volume of hydrogen , the coating of a press hardened part.
balance being nitrogen and unavoidable impurities, with a FIG . 3 schematically illustrates the definition , for a pre
dew point between -60 and - 15° C . The sheet is then heated 45 coated sheet or blank, of the depth of decarburization of the
from the temperature Tla to a temperature T2a between 720 surface p500% , measured by glow discharge optical emission
and 860° C ., whereby at least one element selected from spectroscopy, underneath the pre - coating of the sheet or
among liquid water, steam or oxygen is injected into the blank before press hardening .
furnace starting at the temperature Tla to obtain , in the FIG . 4 illustrates the variation of the critical bending
section of the furnace between the temperature Tia and the 50 angle a , of a press hardened part, as a function of the depth
temperature T2a , an atmosphere A2b which is oxidizing for of the soft superficial area, whereby the latter is measured by
iron , whereby the interval of time between the instant when micro -hardness under the coating.
the sheet is at the temperature Tla and the instant when the FIG . 5 illustrates the variation of the critical bending
sheet reaches the temperature T2a is greater than or equal to angle a of a press hardened part, as a function of the depth
30 seconds . The sheet is then held at a temperature Tm 55 of decarburization P500 , the latter being measured on the
between T2a and T2a + 40° C . under an atmosphere A3 pre -coated blank before hot stamping and hardening.
which is reductive for iron , whereby the complete reduction FIG . 6 shows the influence of the dew point in a particular
of the layer of iron formed in this atmosphere A2b occurs no zone of the furnace during the annealing before pre -coating
later than the end of the hold at the temperature Tm . The on the critical bending angle of the part after hot stamping.
sheet is then cooled , in an atmosphere A4 such that no 60 FIG . 7 shows the influence of the dew point in a particular
surface re - oxidation of the iron occurs , up to a temperature zone of the furnace during the annealing before pre - coating
T3 , then the sheet is pre - coated by hot-dip coating in a metal on the depth of decarburization P50 % , whereby this latter
bath at the temperature Tbm , it being understood that the parameter is measured on the pre - coated blank before hot
temperature T3 is between Tbm - 10° C . and Tbm + 50° C . stamping and hardening.
In one advantageous embodiment, the temperature Tla is 65 FIG . 8 shows the microstructure of the steel underneath
greater than Acl , i.e ., the austenitic transformation tempera -
the zinc coating, after press hardening, for a dew point of
ture during the heating of the steel substrate. - 27° C .
 US 9, 957,582 B2
FIG . I also shows the microstructure of the steel under - after the hot forming operation . Above a concentration equal
neath the zinc coating , after press hardening, for a dew point to 1 % (preferably 0 . 3 % ), the effect of the chromium on the
of - 7° C . uniformity of the mechanical properties in the part is satu
FIG . 10 illustrates the variation , before hot stamping, of rated .
the carbon content of the steel substrate of two pre -coated 5 Aluminum is an element that promotes deoxidation and
steel sheets in the vicinity of their interface with the pre - the precipitation of nitrogen . In excessive quantity , coarse
coating, whereby the annealing of the sheets was carried out aluminates are formed during processing that tend to reduce
in an atmosphere A2a with a dew point of - 27° C . or - 7° C . the ductility , which is reason to limit the aluminum content
FIG . 11 illustrates the variation of the carbon content of to 0 . 25 % by weight. A minimum content of 0 .001 % makes
two hot stamped steel parts in the vicinity of the interface 10 it possible to deoxidize the steel in the liquid state during
with the coating of these parts , whereby the annealing of the processing .
sheets used for the fabrication of these parts was carried out in excessive quantities, sulfur and phosphorus lead to
in an atmosphere A2a with a dew point of - 27° C . or - 7° C . increased brittleness . That is why it is preferable to limit
FIGS. 12 and 13 illustrate the internal oxides formed respective concentrations of these elements to 0 . 05 and 0 . 1 %
during the annealing in the steel substrate in the vicinity of 15 by weight .
the surface. Boron , the concentration of which must be between
FIGS . 14 and 15 show two spectra of the energy disper- 0 .0005 and 0 .010 % by weight, and preferably between 0 .002
sive X - ray spectroscopy of these oxides . and 0 . 005 % by weight, is an element that plays a major role
FIG . 16 shows the variation of the relative oxygen content in hardenability . Below a concentration of 0 .0005 % , a
(0 /0 .) under the pre - coating in sheet fabricated according to 20 sufficient effect on hardenability is not achieved . The full
the present invention . effect is obtained for a concentration of 0 .002 % . The maxi
mum boron contentmust be less than 0 .010 % and preferably
DETAILED DESCRIPTION 0 .005 % to avoid reducing toughness.
Titanium has a high affinity for nitrogen . It protects the
The thickness of the cold - rolled sheet used in the process 25 boron so that this element is in free form so that it can exert
according to a preferred embodiment of the present inven - its full effect on the hardenability. Above 0 .2 % , however,
tion is preferably between approximately 0 . 5 and 2 .6 mm , there is a risk of the formation of coarse titanium nitrides in
the thickness range used in , among other applications, the the liquid steel that have an undesirable effect on its tough
fabrication of structural or reinforcement parts for the auto - ness . It is preferably between 0 . 02 and 0 . 1 % .
mobile industry . 30 Optionally , the steel can also contain calcium in a quantity
The steel is a steel for heat treatment, i.e ., it is a steel between 0 . 0005 and 0 . 005 % : by combining with oxygen and
capable of hardening after austenitization and rapid cooling sulfur, the calcium makes it possible to prevent the forma
by quenching . tion of large inclusions, which have an undesirable effect on
The steel advantageously contains the following ele - the ductility of the sheets or the parts fabricated from them .
ments , whereby the composition is expressed by weight: 35 The remainder of the composition of the steel consists of
a carbon content between 0 .07 and 0 .5 % , preferably iron and the inevitable impurities resulting from processing,
between 0 .09 and 0 . 38 % by weight, and very preferably in particular oxygen , which is present in the form of oxides .
between 0 . 15 and 0 . 25 % by weight. This element plays a A preferred steel is 22MnB5 containing 0 .20 -0 . 25 % C ,
major role in the hardenability and mechanical strength 1 . 1 - 1. 35 % Mn, 0 . 15 -0 .35 % Si, 0 .02 - 0 . 06 % A1, 0 . 02 -0 .05 %
obtained after the cooling that follows the austenitization 40 Ti, 0 .02 -0 .25 % Cr, 0 .002 -0 .004 % B , the balance consisting
treatment. Below a content of 0 . 07 wt. % , the suitability for of iron and unavoidable impurities.
hardening is reduced and the mechanical strength is insuf The inventors have sought primarily the conditions that
ficient after press hardening . A content of 0 . 15 % C makes it would make it possible to achieve good bendability after
possible to guarantee sufficient hardenability in the most press hardening. This characteristic is measured by subject
highly hot formed areas . Beyond a content of 0 .5 % by 45 ing the part to a three -point bend . The part is bent progres
weight, the risk of the formation of defects is increased sively on rollers with three -point flexing, whereby the
during hardening , in particular for the thickest parts . It also applied load is measured simultaneously. The critical bend
becomes difficult to guarantee ductility during the bending i ng angle a . when cracks appear in the part is measured ,
of parts after press hardening. A carbon content between whereby this phenomenon is accompanied by an instanta
0 .09 and 0 .38 % makes it possible to obtain a strength Rm50 neous decrease in the applied load . Test conditions of this
between approximately 1000 and 2050 MPa when the type are described in DIN VDA 238 - 100 . For a breaking
microstructure of the part is completely martensitic . load Rm on the order of 1300 - 1600 MPa, a critical bending
in addition to its role as a deoxidizing agent, manganese angle greater than 55° is required to satisfy the specifica
also has a major effect on hardenability , in particular when tions. Preferably , even a critical bending angle greater than
its content by weight is greater than 0 . 5 % and preferably 55 60° is required to satisfy the most severe conditions of use .
greater than 0 . 8 % . Nevertheless , it is preferable to limit its B y means of a fabrication process that will be described
addition to 3 % by weight, and very preferable to limit it to in detail below , the inventors have fabricated parts , starting
1 .5 % to avoid excessive segregation . with blanks of 22MnB5 steel 1. 2 mm thick , galvannealed ,
The silicon content of the steel must be between 0 .02 and hot stamped after heating to 880° C . and held at that
0 .5 % by weight, and preferably between 0 . 1 % and 0 .35 % . In 60 temperature for 5 minutes, that differ only by the presence
addition to its role in the deoxidation of the liquid steel, this of a more or less large soft layer located underneath the
element contributes to the hardening of the steel, although its coating . The process for the determination of the depth of
content must nevertheless be limited to prevent the exces - this soft area is illustrated schematically in FIG . 2. After
sive formation of oxides and to avoid undesirable effects on press hardening, the part consists of a steel substrate for heat
the dip - coatability . 65 treatment 6 and a coating 4 that is separated from the
Above a level greater than 0 .01 % , chromium increases the substrate by the interface 5 . It should be noted that this
hardenability and contributes to achieving high strength diagram makes no attempt to reproduce the respective
 US 9,957,582 B2
10
dimensions of the different areas . Hardness measurements relation to the rolling direction , the depth of the decarbur
are taken under a very low load ( e . g ., Vickers hardness under ization p50 must not be less than 12 micrometers . Surpris
a load of 50 grams, HV0 .05 ) in the substrate starting at the ingly, it has nevertheless been observed that beyond a depth
interface 5 , to obtain the curve 7 illustrating the micro P50 % of 30 micrometers, the bendability is not improved and
hardness profile . From that, the value d that characterizes the 5 even becomes slightly worse when the flexing is applied in
depth of the soft area is obtained . FIG . 4 shows the critical the direction perpendicular to rolling . In addition , the dif
bending angle a measured for values of d varying approxi- ference in bendability between the directions parallel to and
mately between 30 and 40 micrometers . For a small depth of perpendicular to rolling has a tendency to increase . There
the soft area , the hot stamped parts will not satisfy the fore , to satisfy the mechanical requirements , the value of
requirement a 255º. However, for deeper soft areas , it has 10 P500 must be between 6 and 30 micrometers, preferably
been observed that the ratio is affected by wide scattering between 9 and 30 , and very preferably between 12 and 30
For a given value of d , e . g ., 35 micrometers , it is not possible micrometers .
to determine with certainty whether the hot stamped part The process according to the present invention is
will or will not satisfy the required criterion . It has also been described in greater detail below . First a steel for heat
observed that the microstructures corresponding to these 15 treatment is procured as described above . This steel is in the
variable -width soft areas are very similar after press hard form of flat, cold rolled sheet. The annealing heat treatment
ening. In addition , the microstructure of these soft areas can explained below has the particular purpose of effecting a
be completely martensitic , i. e ., it is not possible to easily recrystallization of the microstructure work hardened by the
distinguish them using conventional optic microscopy . In cold rolling. After an optional degreasing and electrolytic
other words, the inventors have shown that neither the depth 20 cleaning to obtain a surface free of contamination , a depth
of the soft areas measured on the press hardened parts nor of decarburization P50 % between 6 and 30 micrometers can
the optical microstructure observations of the soft areas of be obtained by means of the following processes .
these parts are parameters that make it possible to reliably In a first embodiment, the sheet is subjected to a heat
guarantee a minimum value for the bending angle . treatment as it travels through a radiant tube furnace, or a
Surprisingly, the inventors have shown that to obtain the 25 furnace heated by resistance , or by induction , or by any
desired result it is necessary to determine the depth of combination of these differentmeans . These means offer the
decarburization not on the press hardened part but on the characteristic to adjust, independently of the heating means,
pre - coated sheet or blank before hardening . The determina - the atmosphere prevailing in the different parts of the
tion process is illustrated in FIG . 3 , the diagram of which furnace . The furnace comprises a plurality of zones (pre
does not attempt to reproduce the respective dimensions of 30 heating, heating , hold , cooling ) where different temperature
the different areas to scale : the sheet or the blank is com - and /or atmosphere conditions prevail. The sheet is pre
posed of a steel substrate 10 and a pre - coating 8 separated heated to a temperature Tla in a zone where the atmosphere
from the substrate by the interface 9 . Starting from this (designated A1) contains from 2 to 15 % by volume hydro
interface, glow discharge optical emission spectrometry gen , preferably 3 -5 % by volume hydrogen , the balance
(GDOES ) , a technique that is in itself known , is used to 35 being nitrogen and unavoidable impurities in the gas , with a
measure the depth p50 at which the carbon content is equal dew pointbetween - 60 and - 15° C . It is known that the dew
to 50 % of the nominal carbon content Co of the substrate 10 . point characterizes the oxidation potential of the atmosphere
The concentration profile can exhibit a regular decrease of in question . The traveling sheet then passes into another
carbon from the substrate to the interface (profile 11 ) or even zone of the furnace where , starting at a temperature Tla ,
a minimum located not far from the interface (profile 12 ). 40 water is injected in liquid or vapor form , or oxygen , or a
This latter case reflects a localized carbon enrichment near combination of these different elements , to increase the dew
the extreme surface that in practice has no influence on the point of the atmosphere . The injection must not be carried
mechanical properties after hot stamping . In the case of the out at a temperature Tla less than 600° C ., which would
profile 12 , the depth P50 % to be taken into consideration is result in a low -temperature oxidation of the iron . The
located beyond this very superficial enrichment, as illus- 45 injection is preferably carried out at a temperature Tla
trated in FIG . 3 . Thanks to a fabrication process that will be greater than Ac1, the starting temperature of the austenitic
described in detail below , the inventors have fabricated transformation of the steel as it is heated . That is because ,
parts , starting with galvannealed blanks of 22MnB5 steel 1 . 2 above this temperature , carbon is in solid solution in aus
mm thick that differ only by the presence of a more or less tenite , i.e ., in a form more suitable for the decarburization
large decarburized layer located underneath the coating . 50 phenomenon that is to take place . The injection is preferably
These sheets were cut to obtain blanks that were heated in carried out at a temperature Tla less than or equal to
the furnace to 880° C . for 5 minutes, then hot stamped to Ac1 + 40° C . This temperature range above Acl will be
obtain the parts. These parts were subjected to bending tests , preferred to obtain a greater depth of decarburization P50 %
the results of which are illustrated in FIG . 5 , whereby the e .g ., greater than 9 or 12 micrometers . Beyond Ac1 + 40° C .
flexing during the bending is exerted either in a direction 55 there is a risk of increasing the austenitic grain size and of
parallel to the rolling direction ( curve 13 ) or in a perpen - causing the formation of bainitic and /or martensitic com
dicular direction (curve 14 ). In contrast to the results pre - pounds in the steel substrate during the cooling that follows
sented in FIG . 4 , it should be noted that the depth of the the annealing.
decarburized area before press hardening makes it possible The injection is carried out so that the dew point PR of the
to satisfactorily predict the properties of the part after press 60 atmosphere A2a of this section of the furnace is between
hardening . To obtain a critical bending angle a 255° (bend - 15° C . and the temperature Te of the dew point of the
ing in the direction parallel to rolling ), the depth of the iron /iron oxide thermodynamic equilibrium . In the tempera
decarburized area P50 % must not be less than 6 micrometers. ture range in question , the iron oxide formed can be FeO or
For this condition to be satisfied , regardless of the orienta - Fe2O2. The lowest equilibrium temperature Te correspond
tion in relation to the rolling direction , the depth of the 65 ing to the formation of one oxide or the other will be
decarburization p50 %, must not be less than 9 micrometers . selected . This temperature Te can be determined , for
To obtain a value a _255°, regardless of the orientation in example , by reference to the publication : JANAF Thermo
 US 9,957,582 B2
12
mechanical Tables, 3rd Edition , Part II, Journal of Physical certain additive elements (Mn, Si, Al, Cr, Ti). Underneath
and Chemical Reference Data , Volume 14 , 1985 , Supple this oxide layer, internal oxides of Mn, Si, Al, Cr or Ti are
ment No. 1, published by the American Chemical Society formed and an oxygen enrichment occurs . A superficial
and the American Institute of Physics for the National decarburization occurs at the same time as the oxidation of
Bureau of Standards . Under these injection conditions, a 5 the iron . The sheet then exits the injection section at a
selective internal oxidation of certain additive elements temperature T2a which is between 720 and 860° C . and
present in the steel (Mn, Si, Al, Cr, Ti) is effected without enters a holding zone at a hold temperature Tm between T2a
any surface oxidation of the iron . The internal oxidation can and T2a + 40° C . The interval of time between the instant
have a depth extending to approximately 5 micrometers when the sheet is at the temperature Tla and the instant
below the surface for Mn , Si, Al and Cr. There is an oxygen 10 when it reaches the temperature T2a must be at least 30
enrichment in this surface area , the average oxygen content seconds to obtain a depth of decarburization P50 % between
of which is designated by Om . If the nominal oxygen content 6 and 30 micrometers. Beyond that, in the holding zone, the
of the steel substrate is designated by Oo, the ratio 0 0 . sheet is held at the temperature Tm in an atmosphere A3
that characterizes the surface enrichment in oxygen is which is reductive for iron , whereby the conditions are
greater than 15 . 15 selected so that the complete reduction of the iron oxide
The oxides are located between 0 and 5 micrometers layer occurs not later than the end of the hold at the
below the interface between the pre -coating and the sub temperature Tm . For this purpose, for example, an atmo
strate . For a dew point PR higher than - 15° C ., the density sphere containing from 2 to 15 % by volume ofhydrogen can
of oxides with a diameter greater than 1 micrometer in this be selected , preferably 3 -5 % by volume hydrogen , the
area is greater than 50 /mm². The diameter is defined as 20 balance consisting of nitrogen and unavoidable impurities in
follows: starting with a metallographic section , the diameter the gas, with a dew point between - 60 and - 15° C . for a
of the circle that would have the same surface area is sufficient length of time for the complete reduction of the
determined . It is known that oxides generally have an superficial iron oxide layer to occur in this zone . After the
unfavorable influence on ductility on account of their role as reduction of this iron oxide layer, the sheet contains oxides
sites of incipient failure . As explained below , the specific 25 of Mn , Si, Al or Cr located between 0 and 5 micrometers
nature of the internal oxides formed under the conditions of beneath the surface , whereby the density of oxides with a
the invention does not have any undesirable effect on the diameter greater than 1 micrometer in this area is greater
suitability for bending after press hardening. than 50 /mm2. The local oxygen enrichment is such that the
Under these conditions, a superficial decarburization ratio 0 / 0 , is greater than 15 .
occurs . When the dew point is higher than the temperature 30 The cooling step that follows is described below .
Te of the dew point corresponding to the iron / iron oxide In a third embodiment, the thermal cycle for the annealing
equilibrium , the atmosphere becomes oxidizing for iron . of the sheet combines different means of heating; the pre
During the subsequent annealing steps, there is a potential heating step is carried out in a zone of a direct- flame furnace
risk of not completely reducing the iron oxide and causing (“ DFF” ). As it travels through the furnace , the sheet is
the local appearance of coating defects corresponding to the 35 preheated to a temperature T1b between 550 and 750° C . in
local presence of unreduced superficial oxides . The tem a zone where the atmosphere results from the combustion of
perature Te is a function of the temperature and concentra - a mixture of air and natural gas. According to an embodi
tion of hydrogen in the atmosphere . By way of illustration , ment of the invention , the air/ gas ratio is between 1 and 1 . 2 ,
for an atmosphere containing 97 . 5 % nitrogen and 2 .5 % it being understood that the air- gas combustion in a stoi
hydrogen , Te = + 9° C . at 800° C . For an atmosphere contain - 40 chiometric ratio , is of 1. These pre -heating conditions result
ing 95 % nitrogen and 5 % hydrogen , Te = + 18° C . at 800° C . in the formation of a superficial layer of iron oxide , the
The sheet then exits the section in which the injection was thickness of which is between 0 . 10 and 0 . 25 micrometers .
performed at a temperature T2a which is between 720 and Underneath this oxide layer, internal oxides ofMn, Si, A1, Cr
860° C . and enters into a holding zone at a temperature Tm or Ti are formed and an oxygen enrichment occurs . At the
between T2a and T2a + 40° C . The interval of time between 45 exit from this pre -heating zone in the DFF furnace , the sheet
the instant when the sheet is at the temperature Tla and the enters a second furnace zone heated by radiant tubes (RTF )
instant when it reaches the temperature T2a must be at least or by resistances, or by induction , or by any combination of
30 seconds to obtain a depth of decarburization P50 % these different means. The atmosphere contains from 3 to
between 6 and 30 micrometers . 40 % by volumehydrogen , the balance consisting of nitrogen
Optionally, the atmosphere in the beginning of the holding 50 and unavoidable impurities , with a dew point of less than
zone can be identical to thatof the preceding zone , i.e ., it can
- 30° C . In this second zone , the sheet is heated to a
have a dew point between temperature T2b between 760 and 830° C . Preferably, T2b
- 15 and Te. The sheet can then be cooled or it can be held is greater than Act, which makes possible a faster decar
at the temperature Tm under an atmosphere A3 containing burization on account of the presence of carbon in solid
from 2 to 15 % by volume hydrogen , preferably 3 - 5 % by 55 solution in the austenite . The interval of time between the
volume hydrogen , the balance consisting of nitrogen and instant when the sheet is at the temperature T1b and the
unavoidable impurities in the gas, with a dew point between instant when it reaches the temperature T2b must be at least
- 60 and - 15° C ., these conditions being reductive for iron . 30 seconds to obtain a depth of decarburization P50 % of
The cooling step that follows is described below . between 6 and 30 micrometers . These conditions lead to a
In a second embodiment, the fabrication process starts in 60 complete reduction of the superficial layer of iron oxide
a manner identical to the process described above, up to the created in the preceding step , and to the intended superficial
step of the injection at the temperature Tla between 600° C . decarburization . After the reduction of this iron oxide layer,
and Ac1 + 40° C ., preferably higher than Acl . At this tem the sheet contains oxides ofMn , Si, Al or Cr located between
perature a quantity of water, steam or oxygen is injected to O and 5 micrometers below the surface , whereby the density
obtain , in this zone of the furnace , an atmosphere designated 65 of oxides with a diameter greater than 1 micrometer in this
A2b which is oxidizing for iron . These conditions cause a area is greater than 50 /mm2. The local oxygen enrichment is
complete oxidation of the surface , i.e., of the iron and of such that the ratio Om /0 , is greater than 15 .
 US 9 ,957,582 B2
14
The sheet then travels into a holding zone at a hold austenitic structure to the steel substrate . Tr can be between
temperature Tm between T2b and T2b + 40° C . A (temperature of the start of the austenitic transformation
The rest of the process is identical in the three embodi of the steel when heated ) and Ac3 ( temperature of the end of
ments described above . The sheet is cooled to a temperature the austenitic transformation ), in particular when the objec
T3 in an atmosphere A4 so that no superficial re -oxidation 5 tive is to achieve bainitic -martensitic microstructures after
of the iron occurs. For example, an atmosphere containing cooling in the press . On the other hand , the temperature TR
from 2 to 70 % by volume hydrogen can be used , the balance will be higher than Az if the objective is a mostly marten
consisting of nitrogen and unavoidable impurities in the gas, sitic microstructure in the final part. The blanks are prefer
with a dew point between - 60 and - 30° C . The sheet that ably heated in a furnace under an ordinary atmosphere ;
subsequently enters into the pre - coating bath is therefore 10 substratethisandstepthe, alloying
during takes place between the steel of the
pre - coating. The term “ pre - coating" is used
completely free of superficial iron oxide . The temperature
T3 is close to the temperature Tbm , the temperature of the tolayer designate the alloy before heating, and “ coating " the alloy
formed during the heating that immediately precedes
pre - coating bath , to prevent a thermal disruption of the bath . the hot stamping. The heat treatment in the furnace therefore
For this reason , the temperature T3 will be between Tbm modifies the nature of the pre-coating and its geometry
10° C . and Tbm + 50° C . Therefore , for a pre - coating with 15 because the thickness of the final coating is greater than that
zinc, the temperature T3 will be between 450 and 510° C . of the pre -coating. The coating formed by alloying protects
For a pre -coating in an aluminum -silicon bath , the tempera -the underlying steel from oxidation and additional decar
ture T3 will be between 660 and 720° C . burization and is appropriate for subsequent hot -shaping, in
The pre - coating can be aluminum or an aluminum - based particular in a stamping press . The alloying occurs over the
alloy . In the latter case, the sheet can advantageously be 20 entire thickness of the coating. Depending on the composi
obtained by continuous dipping in an aluminum -silicon tion of the pre -coating, one or more intermetallic phases are
alloy bath that contains, in percent by weight, 7 -15 % silicon , formed in this alloy layer and/or an alloy in the form of a
3 to 20 % iron , optionally between 15 and 30 ppm calcium , solid solution . The iron enrichment of the coating results in
the remainder consisting of aluminum and the unavoidable a rapid elevation of its melting point. The coatings formed
impurities resulting from processing . 25 also have the advantage that they are adherent and suitable
The pre - coating can also be zinc or a zinc alloy . In for the potential hot forming operations and rapid cooling
particular, it can be continuous hot dip galvanized (“GI” ), that are to follow .
containing 0 .25 -0 .70 % Al, 0 .01-0 . 1 % Fe , the balance being The blank is held at a temperature TR to ensure the
zinc and unavoidable impurities resulting from processing. uniformity of its internal temperature . Depending on the
The pre -coating can also be galvannealed (“ GA ” ), contain - 30 thickness of the blank, which can be in the range between
ing 0 . 15 -0 .4 % A1, 6 - 15 % Fe , the balance being zinc and 0 .5 and 2 .6 mm , for example , the hold time at the tempera
unavoidable impurities resulting from processing . The pre - ture T [sic ; Tr] can vary from 30 seconds to 15 minutes.
coating can also be a zinc - aluminum -magnesium alloy con The heated blank is then extracted from the furnace and
taining 1 - 15 % Al, 0 .5 - 5 % Mg, 0 .01 -0 . 1 % Fe, the balance transferred to the tooling, whereby this transfer is performed
being zinc and unavoidable impurities resulting from pro - 35 rapidly so as not to cause the transformation of the austenite
cessing. The pre -coating can also be an alloy containing during cooling . In one variant embodiment, the blank is
4 -6 % A1, 0 .01-0 . 1 % Fe , the balance being zinc and unavoid heated in the vicinity of the tooling , then hot formed without
able impurities resulting from processing . transfer. The blank is then hot stamped to obtain the final
The pre -coating can also be an aluminum -zinc alloy geometry of the part. Other modes of hot deformation are
containing 40 - 45 % Zn , 3 - 10 % Fe and 1 - 3 % Si, the balance 40 also possible , for example shaping between rollers, a process
being zinc and unavoidable impurities resulting from pro generally known as “ roll shaping .” If the blank has already
cessing. been previously cold stamped , the step that follows the
The pre - coating can also be composed of a superposition extraction of the blank from the furnace can be simply a
of layers , e.g., after the deposition by dip coating of a layer conformation in a press tool. In this case , the conformation
of aluminum or aluminum alloy , one or more subsequent 45 is characterized by a smaller force applied by the tooling on
deposits of zinc or zinc alloy can be laid down , e . g ., by the part, and the purpose is to finish the final geometry of the
electrodeposition or by vacuum deposition : PVD (Physical part and to avoid potential deformations during cooling .
Vapor Deposition ) and /or CVD (Chemical Vapor Deposi- Optionally , it is also possible to heat only a portion of the
tion ), whereby these deposition processes are known in blank , or to cool the stamped part differently in its different
themselves. 50 areas , in which case these variants result in parts that are
At this stage, by means of the processes described above , hardened non - uniformly , whereby certain areas are signifi
a sheet is obtained that is composed of a steel substrate , the cantly harder, while other areas have a lower mechanical
depth ofdecarburization P50 % of which is between 6 and 30 strength but greater ductility .
micrometers, topped by a pre -coating, without a layer of iron After the stamping or conformation step , the part is held
oxide present between the substrate and the pre - coating . 55 in the tooling , which may optionally be cooled , to ensure its
FIG . 1 presents an example of a sheet of this type, whereby effective cooling by thermal conduction .
the steel substrate 1 comprises a specific superficial decar - Depending on the cooling rate and the hardenability of the
burized area 2 topped by a galvannealed pre - coating 1. steel of the substrate, the final microstructure is martensitic
This sheet is then cut to obtain a blank , the geometry of or bainitic -martensitic .
which is in a defined relation to the final geometry of the 60 The following results , which are presented by way of a
intended part. Optionally, it is possible to cold stamp the part non - restrictive example , demonstrate the advantageous
to bring it more or less close to the final geometry of the characteristics achieved by an embodiment of the invention .
intended part. In the case of a small cold deformation , the
process can be supplemented by a hot deformation , as will EXAMPLE 1
be explained below . 65
This flat or pre -stamped blank is then heated to a tem A cold rolled sheet 1.2 mm thick is procured , the com
perature TR capable of conferring a partly or completely position of which , expressed in percentby weight ( % ), is the
 US 9 ,957,582 B2
15 16
following, with the remainder consisting of iron and area is such that the ratio Om0 , is greater than 15 . It is equal
unavoidable impurities resulting from processing: to 15 . 1 for PR = - 3° C . and to 17 . 4 for PR = + 2° C .
The pre -coated sheets are then cut to obtain blanks
suitable for stamping . These blanks were heated to a tem
C Mn Si Cr Ti A1 S P B 0 5 perature of 880° C . in a furnace under an ordinary atmo
0.22 1.17 0.24 0. 19 0 .040 0.032 0.003 0.013 0.003 0.0014 sphere . After a hold time of five minutes in the furnace
(including a period of 4 minutes for the heating phase ), the
blanks were extracted and immediately stamped . After hot
The temperature Acl of this steel composition is 724° C . stamping , the parts were cooled in the press at a rate greater
The sheet is preheated as it travels through a radiant tube 10 than 30° C . per second to obtain a totally martensitic
furnace under an atmosphere Al of nitrogen containing structure in the steel substrate . The ultimate tensile strength
4 .7 % by volume hydrogen with a dew point of -31° C . to a Rm obtained on the hardened parts is typically on the order
temperature Tla of600° C ., after which water is injected to of 1500 MPa.
obtain an atmosphere A2a with a dew point PR . Different The critical bending angle a of these parts wasmeasured
tests have been performed by modifying the flow ofwater 15 by a three -point bending test performed with two outside
injected into the furnace , to vary the dew point PR between rollers with a diameter of 30 mm and a central blade with a
- 27° C . ( obtained by means of the injection of a relatively very small radius.
large quantity ofwater ) and + 2° C . In all the tests , the sheet a FIG . 6 illustrates the variation of the critical angle a , as
function of the dew point PR after injection of water
was then heated from the temperature Tla to the temperature de 20 starting at the temperature Tla . When PR is less than - 15°
T2a equal to 780° C . in the atmosphere A2a for 110 seconds, 2 C ., the bending angle obtained has an unsatisfactory value of
which achieves a decarburization and a selective internal less than 55º . When PR exceeds the temperature Te of + 17°
oxidation of Mn, Si, Al, Cr and Ti, whereby these oxides are C ., there is a potential risk of not completely reducing the
formed in the immediate vicinity of the surface of the sheet . iron oxide during the subsequent hold , and causing the local
At the temperature T2a , the dew point of the iron /iron oxide 25 appearance of coating defects corresponding to the local
equilibrium is + 17° C . The sheet then enters a zone of the presence of unreduced superficial oxides . In the range of an
furnace where it is held at the temperature Tm of 780° C . embodiment of the invention , the bending angle varies little
under an atmosphere A3 containing nitrogen and 7 % hydro as a function of the dew point. Between - 15 and - 7° C ., the
gen , which is reductive for iron . The sheet is then cooled as average increase is 0 .79° per ° C ., while the variation is
it travels into another zone of the furnace under an atmo - 30 greater below - 15° C . (1. 050 per ° C .). When PR is between
sphere A4 containing 10 % hydrogen , to a temperature T3 of - 15 and - 10° C ., a particularly interesting range is detected ,
470° C ., and pre - coated by dipping in a bath at the tem - because the bending angle is practically independent of the
perature Tm of 462° C . containing zinc and 0 . 125 % alumi- dew point . In other words, in this particular range , any
num as well as unavoidable impurities . No superficial re potential undesirable fluctuation of the quantity of water
oxidation of the iron occurs in the steps of holding and 35 injected during the annealing into the furnace has no con
cooling in the atmosphere A4 . Immediately after pre - coat - sequence on the suitability for bending after hot stamping ,
ing, the sheet is reheated to a temperature of 540° C . to which makes it possible to guarantee a high degree of
obtain a galvannealed pre - coating (“ GA ” ) , i.e ., one contain - stability of the characteristics of the stamped and press
ing 9 % iron . The result is a sheet that does not contain a layer hardened parts. It can also be observed that this good
of iron oxide between the steel substrate and the gal- 40 bendability is obtained in spite of the presence of oxides
vannealed pre - coating . FIG . 12 illustrates the observations formed below the pre -coating. Without being bound to a
made under the conditions of an embodiment of the inven - theory , it is thought that the incipient damage starting from
tion, oxides formed during selective internal oxidation , these oxides has a tendency to be delayed by the intrinsic
which are visible in the steel substrate immediately under- toughness of the area with reduced carbon content in which
neath the pre - coating . These oxides can be isolated or 45 they are located .
aligned along the grain boundaries. On the basis of an Tests have also been performed by simultaneously vary
analysis of the ray energy dispersive spectroscopy of these ing PR and the temperature Tla, whereby the latter is 720°
oxides (“ EDS” ), it has been shown that they are oxides of C . (i.e ., Ac1 - 4° C .) or 760° C . (Ac1+ 36° C .). FIG . 7
manganese, silicon and aluminum as illustrated in FIGS. 14 illustrates the influence of the temperature Tla and of the
and 15 , which exhibit the characteristic peaks corresponding 50 dew point PR on the depth of decarburization P50 % before
to these elements. The peak for iron is due to the matrix hot stamping, measured by glow discharge optical emission
surrounding the oxides . spectroscopy. When the dew point is too low , the decarbur
FIG . 13 illustrates the presence of oxides with a diameter ized depth does not reach the value required by an embodi
greater than 1 micrometers , the density of which is greater ment of the invention ( result marked “ A ” in FIG . 7 ). A
than 50 /mm? in the area located between 0 and 5 microm - 55 sufficiently high dew point, with a temperature Tla slightly
eters below the surface of the steel sheet. below Ac1, makes it possible to achieve the required depth
FIG . 16 illustrates the variation of the relative oxygen (result “ B ” ) . Heating to a higher temperature Tla (Ac1 + 36°
content (0 / 0 . ) under the pre -coating in the substrate , mea . C .) makes it possible to significantly increase the depth of
sured by glow discharge optical emission spectrometry , decarburization P50 % (result “ C ” ) .
whereby Oo designates the nominal oxygen content of the 60 After the hot stamped parts obtained were polished and
substrate . This variation has been measured for values of the etched using the reagent Nital, the microstructure under the
dew point PR of - 3° C . and + 2° C . The depth p = 0 corre - coating that results from the alloying by diffusion between
sponds to the interface between the substrate and the pre - the zinc of the initial pre -coating and the steel of the
coating. In an area located between 0 and 5 micrometers substrate was observed using optical microscopy. FIG . 8
below the surface of the substrate , there is an increase in the 65 also illustrates the coating 15 and the underlying steel 16 for
oxygen content corresponding to the presence of the oxides an annealing with a dew point PR = - 27° C . FIG . 9 illustrates
mentioned above . The local enrichment in oxygen in this the coating 17 and the underlying steel 18 for an annealing
 US 9, 957,582 B2
17 18
with a dew point PR = - 7° C . In spite of the significant is then constituted by a ferrite microstructure or by a mixture
difference in bendability between the two specimens ) (209), of ferrite and austenite . In the presence of liquid zinc , this
no significant micro - structural differences were detected microstructure displays a reduced sensitivity to cracking
between the two specimensafter hot stamping, in spite of the compared to the austenitic structure.
difference of decarburization existing between them before 5
hot stamping . EXAMPLE 2
FIG . 10 illustrates the variation , before hot stamping, of
the carbon content of the two sheets annealed in an atmo Sheets pre - coated with zinc using the process described
sphere A2a with the dew point PR of - 27° C . or - 7° C . This above were fabricated , with the exception that the sheets
variation , measured by glow discharge optical emission 10 have a thickness of 1 .8 mm and were not reheated to 540°
spectrometry in the steel substrate , is expressed in FIG . 10 C . after hot - dip coating , as a result of which their coating is
as a function of the depth below the interface between the galvanized and not galvannealed .
steel and the pre - coating . The measured local content (C ) The fabrication conditions were selected to obtain a sheet
was determined in relation to the nominal carbon content Co with a decarburized depth p50 % of 6 micrometers. The sheets
to obtain the variation of the relative carbon content C / C . . 15 were cut to obtain blanks that were austenitized at a tem
It should be noted that the decarburized areas are very perature of 880° C . in a furnace under an ordinary atmo
different under the two annealing conditions , whereby the sphere. After a total hold time of up to 10 minutes in the
depth ofdecarburization P500 is 15 micrometers for PR = - 7° furnace, the blanks were extracted , immediately hot stamped
C . and 3 micrometers for PR = - 27° C . If we consider the and press hardened . The following table indicates the varia
totality of the decarburized area , the depth of decarburiza - 20 tion of the criticalbending angle & as a function of the total
tion measured after annealing with PR = -7° C . is greater by hold time of the part in the furnace .
approximately 35 micrometers than that measured after
annealing at PR = - 27° C .
After hot stamping of the sheets , the same process was Hold time (minutes) Bending angle ac o
used to determine the variation of the carbon content under 25 5 .5 57 .5
the coating of the parts thus obtained . FIG . 11 illustrates the 55
variation of the relative carbon content C /Co of these parts . 10 54
It can thereby be shown that the decarburized area is
essentially identical under the two annealing conditions. It therefore appears that the blanks can remain in the
That indicates that the heating in the furnace prior to the 30 furnace up to 7 minutes before being hot stamped , while still
press hardening treatment leads to a diffusion of carbon meeting the requirements . This makes it possible to solve the
toward the decarburized surface of the steel . The determi problems encountered on hot stamping lines, when an inci
nation of the decarburization after hot stamping does not dent on the line makes it necessary to keep the blanks in the
make it possible to determine that the annealing with PRO furnace longer than planned . The present invention makes
7° C . will lead to satisfactory bending results , while the 35 this flexibility possible , thereby eliminating the unnecessary
annealing at PR = - 27° C . will not achieve the required level. rejection of the blanks . It can also be observed that beyond
Although incomplete , this homogenization of the carbon 7 minutes, the increase in hold time leads only to a very
makes it possible , however, to obtain , in the steel located small decrease in the bending angle , which indicates that the
immediately beneath the coating , a sufficient carbon content process -according to an embodiment of the invention guar
to cause a martensitic hardening under the cooling condi - 40 antees a high level of safety , in the event of a drift in relation
tions linked to the hot stamping, as illustrated in FIGS . 8 and to the nominal heat treatment parameters during the hot
9 . However , the intrinsic toughness characteristics of the stamping , and makes it possible to achieve a high degree of
martensite created under these conditions depends on the reproducibility of themechanical characteristics of the parts .
decarburization
ation conditions
conditions which
which result
result inin particular
particular from
from Therefore, the invention makes possible the fabrication of
the choice of the temperature PR . Therefore , the effective 45 pre -coated sheets and coated parts with very high strength
testing of the suitability for bending of the hot stamped parts characteristics and bendability and with good isotropy .
must be performed on the sheets or the blanks before the hot under very satisfactory economic conditions. These parts
stamping operation and not after , contrary to what would be can advantageously be used as structural or reinforcement
expected by a person skilled in the art . parts in the field of automobile construction .
In addition , the hot stamped parts fabricated from the 50
sheets pre -coated with zinc or zinc alloy decarburized What is claimed is:
according to an embodiment of the invention display a 1 . A precoated , cold -rolled and annealed sheet with a
particular aptitude for welding by spot welding. It appears thickness between 0 .5 and 2 .6 mm for the fabrication of
that, after heating and hot stamping, there is a decarburized press hardened parts , comprising :
layer beneath the coating. It is known that resistance welding 55 a steel substrate for heat treatment with a carbon content
leads to a very large local temperature elevation because the Co between 0 .07 % and 0 .5 % , expressed by weight;
fusion is achieved in the molten core that constitutes the a metal pre -coating on at least the two principal faces of
joint between the welded components . In welded joints the steel substrate, the metal pre -coating being alumi
executed on conventional hot stamped parts , there is an num , an aluminum alloy, zinc or a zinc alloy, or is
embrittlement of the austenitic grain boundaries by penetra - 60 composed of a layer of aluminum or an aluminum
tion of the zinc of the coating , which is then liquid on based alloy topped by a layer of zinc or a zinc alloy;
account of the temperature elevation during the welding . a composition of the steel substrate contains, expressed in
According to an embodiment of the invention , the presence percent by weight:
of an area in which the carbon is very depleted under the
coating leads to a local increase in the transformation 65 0 .07 % C50 .5 % ;
temperature Ac3 into austenite during the heating . Depend
ing on the carbon content, the structure at high temperature 0 .5 % sMn< 3 % ;
 US 9 ,957,582 B2
0 .02 % sSis0 .5 % ;
19
0 .1 % sSis0 .35 % ;
20
0 .01% sCrs1 % ; 0 .01% sCrs0 . 3 % ;
Tis0. 2 % ; 0 .02 % s Tis0 .1 % ;
Als0 .25 % ; 0 .001% Als0 .25 % ;
S50 .05 % ; Ss0.05 % ;
Ps0. 1 % ; 10 Ps0 . 1 % ;
0 .0005 % sB 0 .010 % ; and 0 .002 % sBs0 .005 % ; and
a remainder of the composition being iron and the the remainder of the composition being iron and the
unavoidable impurities resulting from processing ; unavoidable impurities resulting from processing.
the steel substrate including a decarburized area on the 15 3 . The pre - coated sheet as recited in claim 1 , wherein the
surface of each of the two principal faces , the depth composition of the steel substrate contains, expressed in
P50 % of the decarburized area being between 6 and 30 percent by weight, 0 . 15 % sCs0 . 25 % .
micrometers , P50 % being the depth at which the carbon 4 . The pre - coated sheet as recited in claim 1 , wherein the
content is equal to 50 % of the content Co, substrate contains , in an area located between 0 and 5
the sheet not including a layer of iron oxide between the 20 micrometers below the interface between the pre - coating
substrate and the metal pre - coating , and and the substrate, oxides that include at least one element
the steel substrate having an oxygen content O , and selected from a group consisting of titanium , silicon , man
having, underneath an interface between the pre - coat- ganese , aluminum , chromium , a density of the oxides with
ing and the substrate , at a depth located between 0 and a diameter greater than one micrometer in this area is greater
5 micrometers starting from the interface , an average 25 than 50 /mm - .
oxygen content Om , whereby Om/ 0 , is greater than 15 . 5 . The pre-coated sheet as recited in claim 1, wherein the
2 . The pre -coated sheet as recited in claim 1 , wherein the composition of the steel substrate includes
composition of the steel substrate contains, expressed in 0 . 0005 % sCas0 . 005 % .
percent by weight: 6 . The pre -coated sheet as recited in claim 2 , wherein the
0 .09 % sCs0 . 38 % ;
30 composition of the steel substrate includes
0 .0005 % sCas0.005 % .
0 . 8 % sMns1. 5 % ; o * * * *