22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais

06 a 10 de Novembro de 2016, Natal, RN, Brasil

Microstructure and Texture Evolution of Different High Manganese Cast

Steels During Hot Deformation and Subsequent Treatment

M.N.S. Lima1, W.M. Ferreira2, C.D. Andrade1, H.F.G. de Abreu1, J. Klug1, M. Masoumi1,*.

1
Universidade Federal do Ceará, Campus Universitário do Pici – Centro de Tecnologia –
Depto. de Engenharia Metalúrgica e de Materiais - CEP: 60440-554 - Fortaleza – CE.
2
Universidade Federal do Piauí, Campus Universitário Ministro Petrônio Portella –Centro
de Tecnologia – Curso de Engenharia Mecânica - CEP:64049-550 - Teresina – PI.
* mohammad@alu.ufc.br

Abstract

Microstructure and texture evolution were studied in two different austenitic high
manganese cast steels in each processing condition. Special attention was paid to
the effects of hot deformation and subsequent treatment on grain orientation
behavior. The roles of Mn and C elements as well as heat treatment processes
were investigated by Thermo-Calc. The texture evolutions in the as-cast, solution
heat treatment, as-rolled and subsequent treatment were explored via orientation
distribution function. The results showed that face-centred cube austenite was
developed in steels. Strong {110}<115> texture component was characterized in
as-cast in both alloys. Then, the inhomogeneity microstructure and the pronounced
microsegregations were removed by annealing and Brass {110}<112>, {110}<111>
and {221}<102> components were formed. Finally, cube {001}<100> component
was developed during hot rolling in samples.

Keywords: very high carbon steel, high manganese steel, crystallographic texture.

1. Introduction

Carbon is considered as the most important enhance the hardness and

strength of steels, even in absence of other alloying elements. This current study
focused on the microstructure and texture evolution of ultrahigh carbon steel (more
than 1.0 wt.%) steels. The ultrahigh carbon content causes an excessive strength
and work-hardening with poor formability, leading to very cleavage and brittle
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06 a 10 de Novembro de 2016, Natal, RN, Brasil

fracture during deformation even at high temperature [1,2]. Although these

materials have an excessive strength and work-hardening, very poor formability is
a main determinal factor to use in industries. Continuous demanding of new
materials in high pressure-temperature applications also requires the industry to
develop new materials with high specific strength and high work-hardening
capacity and good formability. To fulfill these requirements, ultrahigh carbon-
manganese steels were casted in this work by aiming the Transformation-Induced
Plasticity (TRIP) theory [3]. The research effort to characterize the microstructure
and texture evolutions under different processing conditions (i.e. as-cast, annealing
and hot rolling).

From the crystallographic aspect, shear deformation facilitates dislocation

motion across preferred slip systems. It is leading to change in inter-atomic
spacing and crystal orientation. The lattice reorientation through heat treatment
and deformation is considered as a unique characteristic in crystalline materials.
Oh et al. [4] reported that in the high Mn steels with face-centered cubic (FCC)
structure the grain orientation takes place on the <111> direction parallel to rolling
direction (RD). Rolling of high Mn steels with low stacking-fault-energy (SFE)
develops brass {110}<112>, Copper {112}<111> texture components [5,6,7]. It is
well-known that the strain energy which is generated during deformation can
release by optimizing the grain orientation. Therefore, the main focus of the current
work was put on the impact of the microstructure and crystallographic texture
obtained in different processing conditions/ routes on the resulting mechanical
properties.

2. Experimental

Two type of high-carbo-manganese plates of this work were casted in in the

Foundry Laboratory of the Federal University of Ceará (LAF-UFC). The proper
elements were melted in vacuum induction furnace protected with Ar gas. Then,
the result molten steel cased to plate ingot. The chemical compositions of both
alloys are listed in Table 1. For the investigations in this work, as-cast plate with a
thickness of 40 mm and a width of 100 mm was cast. The cooling rate was
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

approximately 150°C/s. To remove the inhomogeneity microstructure and the

pronounced microsegregations, the subsequent annealing was applied. In this
work, the annealing was conducted at 1000°C for two hours according the Thermo-
Calc results based on its chemical composition. Finally, hot rolling was performed
at the initial rolling temperature of 1100°C, and at the finishing temperature of
around 1000°C. The final hot rolling plate was about 20 mm thick. The plates are
then gone through air cooling.

Table1: Chemical composition of both cast steels (wt.%).

C Mn Si P S Ni Cr Mo Cu Fe
1st alloy 1.17 28.08 0.82 0.030 0.010 0.037 0.088 0.20 0.25 bal.
2nd alloy 1.12 18.92 0.75 0.032 0.009 0.035 0.085 0.18 0.23 bal.

Using the Thermo-Calc software and the TCFE6 database, the phase
diagrams of the proposed compositions were constructed as a function of the
variation of the C and Mn percentages. Also, the weight fraction diagram of each
phase was constructed as a function of temperature. The results of the
experimental systems were compared with the commercial steel diagrams.

Microstructural studies carried out using the optical microscopes (Zeiss

model, Axio Imager 2) and scanning electron. The samples were ground and
polished with diamond paste with a particle size 6, 3 and 1μm. For better
preparation of the samples that were subjected to buffing of alumina 0.05 m and
further automated final polishing for a period of 5-8 hours per sample in Buehler
model 69-1100 1000 minimet device with abrasive silica diluted in distilled water.
Then, the sample were etched by 2% Nital solution was applied (2 ml HNO3 + 98ml
ethyl alcohol), for a time of (5/10) seconds, to be followed the grain boundaries of
the microstructure. Shortly after another etching was conducted with sodium
metabisulphite solution (Na2S2O5) dissolved in distilled water in composition (5g
Na2S2O5 + 50 ml distilled water) for a time (5-10) seconds to be observed dendritic
formations of gross microstructure of fusion.

Phase analysis and quantitative texture measurements by means of X-ray

diffraction (XRD) were performed using a fully automated texture goniometer that
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

was operated in back reflection mode at 40kV and 45mA with Co Kα radiation.
Four incomplete pole figures {111}, {200}, {220}, and {311} were measured and
used for calculation of the orientation distribution functions (ODFs) by using the
series expansion method with positivity criterion by Mtex free Matlab toolbox for
analyzing and modeling crystallographic textures.

3. Results and discussion

The weight fraction of phase diagram for the both alloys was calculated in
accordance with the chemical composition shown in Fig. 1. Computational program
Thermo-Calc was aimed to determine the formation of possible precipitated phases
during casting and post heat treatments. It is observed that at temperature around
1000°C the cementite and austenite are developed, resulting in elimination the as-
cast grain structure with inhomogeneous grain distribution.

Fig. 1: Weight fraction of phase diagram for the a) 1st alloy and b) 2nd alloy.

It is also shown that in 1st alloy a large amount of carbides also developed
such as Mo. However, with a reduction in carbon content and maintaining the high
manganese content during austenite stabilizing (i.e. increasing the austenitic
phase), the cementite (Fe3C) microconstituintes reduced significantly. In second
alloy, by Fe dissolving, the continues development of FCC structure is observed. It
is concluded that the austenite is the final microstructure of both alloys regardless
the cooling rate.
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

Fig. 2 shows the micrographs of as-cast alloys obtained by an optical

microscope. A fine dendritic solidification structure with short-wave length
microsegregations [8] of alloying elements is characterized by optimal micrographs
as shown in Fig. 2. The microstructure shows an inhomogeneous grain structure
with comparably large, elongated grains, resulted in a significant anisotropy and
poor mechanical properties are indicated from the microstructure. By the
comparing of two alloys with different Mn content. It could be concluded that the
dendrites and their arm spacing varied with increasing manganese content. These
structures which improved by the nucleation and growth of the austenite grain size
are affected by the manganese content.

Fig. 2: Dendrites of as-cast alloys a) 1st alloy and b) 2nd alloy.

Fig. 3 shows the microstructure of as-cast sample obtained by optical

microscopy. The austenite grains with a large amount of porosities with precipitates
are found in these samples. It is observed that air-cooling after casting developed a
large number of porosity and voids due to high cooling rate. Thus, removing the
undesirable porosities and dissolving the segregations and achieving the desired
microstructure with homogenous grain size distribution annealing treatment are
required. The second alloy with lower Mn content had coarse austenite grains. The
austenite grain sizes are calculated 86±6 and 158±12 μm by Axio Imager 2
installed in Zeiss model optical microscope in both samples, respectively.
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

Fig. 3: Micrographs of as-cast alloys a) 1st alloy and b) 2nd alloy.

In order to better and more precious of microstructural studies of as-cast

sample, the samples were analyzed by scanning electron microscopy SEM and
energy dispersive spectrometer (EDS). The SEM micrographs of as-cast alloys are
presented in Fig. 4. The first sample had austenite grains with fine distribution of
pearlite microconstituintes were characterized. Moreover, a significant amount of
microvoids and precipitates were identified in this sample. The EDS result in this
region is presented in Fig. 5a. Carbides and oxides were identified which indicate a
poor mechanical properties. The SEM and EDS studies also conducted in second
alloy, no pearlite microconstituintes was found, whereas aluminium carbide
precipitates developed, Fig. 4b and 5b.

Fig. 4: SEM micrographs of as-cast alloys a) 1st alloy and b) 2nd alloy.
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

Fig. 5: EDS analyzes of as-cast alloys a) 1st alloy and b) 2nd alloy.

As it was reported, many precipitates such as carbide and aluminium oxide

precipitations were identified in as-cast samples of both materials. Nitrogen
adsorption isotherms on aluminum oxide samples, is resulting in a very low
mechanical properties in compression behaviour. Therefore annealing treatment
performed according to results from the computational program Thermo-Calc on
1000°C to eliminate undesirable precipitates and microsegregations. A
homogeneous distribution of pearlite microconstituintes in austenite phase is
observed in microstructures of both alloys after annealing treatment. Moreover,
there no microsegregations were observed in this condition, Fig. 6.

Fig. 6: Fig. 4: SEM micrographs of annealed samples a) 1st alloy and b) 2nd alloy.
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

The influence of the heat treatment parameters on the microstructural properties,

such as grain structure, texture, and element distribution, was analyzed and
correlated with the mechanical properties. It is well-known that in the cube
structure (i.e. body and face-centered cube) according to the dislocation motion
and Taylor factor factory [9]; the grains were laid on compact planes such as {110},
{111} can provide enough slip systems to inaugurate and facilitate deformation
[10]. However, the {001} grains (which developed at very high temperatures by
phase transformation and recrystallization) due to large atomic distances, poor
mechanical properties [11] are achieved.

Fig. 7: Calculated ODF in first alloy at a) and b)

In order to investigate texture evolution in alloys, macrotexture analysis was

conducted in both samples. Fig. 7 shows the related ODF at in first
alloy. The {110}<115> texture components in as-cast sample, while a large number
of porosities and microvoids are leading to early fracture. Brass {110}<112>,
{110}<111> and {221}<102> components are characterized in this sample after
annealing. This means that the annealing can remove successfully the
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

inhomogeneous as-cast structure and developed newly organized structure. Then,

rotated copper {114}<110> and cube {001}<100> components were formed during
rolling at high temperature. High carbon-Manganese steels are considered as low
SFE materials [12], thus dynamic recrystallization occurred in these steels during
hot deformation. Our findings are fully consistent with mentioned proceed.

Fig. 8: Calculated ODF in second alloy at a) and b)

Macrotexture investigation was carried out in second alloy of all conditions and
their ODFs were calculated and presented in Fig. 8. Besides {110}<115>
component that developed in first sample, Brass {110}<112> and {110}<225>
texture components also were identified in second sample with lower Mn content.
Furthermore, {110}<115>, Brass and {110}<111> components were developed as
expected. However, cube {001}<100> component with low mechanical properties
were formed significantly in this alloy after hot deformation, because of low SFE
and dynamic recrystallization occurrence.
 22º CBECiMat - Congresso Brasileiro de Engenharia e Ciência dos Materiais
06 a 10 de Novembro de 2016, Natal, RN, Brasil

Conclusion

Microstructure and texture evolution were studied in two different austenitic high
manganese cast steels in each processing condition. The main focus of the current
work was put on the impact of the microstructure and crystallographic texture
obtained in different processing conditions/ routes on the resulting mechanical
properties. The results revealed that face-centred cube austenite was developed in
both alloy steels. Strong {110}<115> texture component was characterized in as-
cast in both alloys. Then, the inhomogeneity microstructure and the pronounced
microsegregations were removed by annealing and Brass {110}<112>, {110}<111>
and {221}<102> components were formed. Finally, cube {001}<100> component
was developed during hot rolling in samples.

Acknowledgments

The authors acknowledge the Coordenação de Aperfeiçoamento de Pessoal de

Nível Superior (CAPES) for financial support and the research board of the
Universidade Federal do Ceará and Laboratório de Caracterização de Materiais
(LACAM) for providing the research facilities of this work.

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