HIGH STRENGTH HOT ROLLED MEDIUM MANGANESE STEEL WITH AND METHOD OF PRODUCING THE SAME
TECHNICAL FIELD
[0001] This invention relates to medium manganese steel with high
strength and moderate ductility, and more particularly to high strength hot rolled medium manganese steel.
[0002] This invention further relates to a process for the production of
medium manganese steel with high strength and moderate ductility by a thermo-mechanical processing route, for large scale production of such steel. This steel will be mostly used in the advanced automobile application, where high strength and ductility is main concern. In this process as melted materials are subjected to hot rolling, followed by short time annealing treatment. The processed steel shows ultimate tensile strength in the range of 800 MPa - 1580 MPa and elongation 7-18%. Specifically, the invention relates to a process for the production of ultra high strength austenite -martensite/ferrite two phase steel, which would be suitable for making automobile parts. However, in austenitic phase deformation takes place by both twinning and strain induced martensitic transformation.
BACKGROUND
[0003] Medium Mn steels are important class of materials which are
known for their excellent combination of ductility and strength. The material contains low to high amount of Mn (4-9 wt. %), low to high amount of carbon (0.2-0.6 wt. %) and silicon (<1wt. %) for desirable strength and elongation. Compared to high Mn steels, the medium Mn steels have very high strength and moderate ductility. High Mn steels suffer from various drawbacks such as delayed cracking and melting of Mn. So it is very important to reduce Mn content for real application of this grade of steels. But reduction of Mn introduces two phase austenite plus martensite phases in microstructure. Therefore, for enhancement in ductility an additional deformation mechanism is highly desirable with changing the chemistry. The stabilization of certain amount of ferrite phase could be a possibility. Elongation due to ferrite phase could add to overall ductility. Ferrite phase also enhances impact and shock absorption.

[0004] The processes based on thermo-mechanical route, which are
known to result in partitioning of the austenite phase in a subsequent amount to increase the strength and ductility in medium Mn steels. In recent years, various thermo-mechanical processing routes have been developed for the generation of dual phase microstructures in many Mn containing steels [ Lee, S.J., Lee, S. and De Cooman B.C., Mn partitioning during the intercritical annealing of ultrafine-grained 6% Mn transformation-induced plasticity steel, Scripta Materialia, 2011. 64(7), pp. 649-652 ; Lee, S., et al., Localized deformation in multiphase, ultra-fine-grained 6 Pct Mn transformation-induced plasticity steel, Metallurgical and Materials Transactions A, 2011. 42(12), pp. 3638-3651 ; Suh, D. W. et al., Influence of Al on the microstructural evolution and mechanical behavior of low-carbon, manganese transformation-induced-plasticity steel, Metallurgical and Materials Transactions A, 2010. 41(2), pp. 397-408; HajyAkbary, F., et al., A quantitative investigation of the effect of Mn segregation on microstructural properties of quenching and partitioning steels, Scripta Materialia, 2017. 137, pp. 27-30 ]. This thermo-mechanical processing route leads to low to very high austenite phase fraction in microstructure [Lee, S., Lee, S. J. and De Cooman B.C., Austenite stability of ultrafine-grained transformation-induced plasticity steel with Mn partitioning, Scripta materialia, 2011. 65(3): pp. 225-228.]. Other than conventional processing, special annealing treatment has been included in processing schedule, known as inter-critical annealing treatment (ICT). In ICT treatment, the materials undergo annealing treatment at temperature close to austenitic phase region to enable partitioning of Mn and carbon. The Mn and carbon further stabilize austenite phase in microstructure. A distinct advantage of this process is that it can be scaled up for large-scale production in industry, in relatively simple and cheaper ways. In the paper entitled ‘Austenite stability of ultrafine-grained transformation-induced plasticity steel with Mn partitioning’ published in the Scripta Materialia, 65(2011), pp.-225-228, the authors have shown that Mn partitioning takes place during short time annealing treatment in medium Mn steels. The partitioned Mn causes stability of austenite phase in final microstructure.
[0005] Mechanical properties of the present invention are much better
than properties reported in the similar level of Mn containing steels [ Lee, S., K. Lee and De Cooman B.C., Observation of the TWIP+TRIP plasticity-enhanced mechanism in Al-added 6 wt pct medium Mn steel, Metallurgical and Materials

Transactions A, 2015. 46(6): pp.2356-2363 ; Zhang, Y., et al., Influence of temperature and grain size on austenite stability in medium manganese steels, Metallurgical and Materials Transactions A, 2017. 48(5): pp. 2140-2149; Cao, W., et al., Microstructure and mechanical properties of Fe-5Mn-0.2C steel processed by ART annealing. Materials Science and Engineering: A, 2011. 528(22): pp.6661-6666].
OBJECTS OF THE INVENTION
[0006] It is therefore an object of this invention to propose medium
manganese steel with high strength and moderate ductility and a process for the production thereof.
[0007] It is a further object of this invention to propose medium
manganese TWIP/TRIP steel comprising of ultra high strength (1.0 - 1.6 GPa) and moderate ductility (15-20%) suitable for making long steel strip for automobile applications.
[0008] Another object of this invention is to propose medium
manganese steel with high strength and moderate ductility, and having a microstructure with high fraction of austenite through short annealing period that enhances the strength and ductility in medium Mn steels.
[0009] Yet another object of this invention is to propose medium
manganese TWIP/TRIP steels with nearly fully austenitic microstructure.
[00010] It is a still further object of this invention to medium manganese
dual phase medium Mn TWIP/TRIP steels for large scale production in industry of high strength and ductility for making automobile parts.
[00011] These and other objects and advantages of the invention will be
apparent to a person skilled in the art after a consideration of the ensuing description taken in conjunction with the accompanying drawings which illustrate the preferred embodiments of the invention.
SUMMARY OF THE INVENTION
[00012] According to this invention is provided a medium manganese
steel with high strength and moderate ductility, comprising 3.5-9.5 wt.%

Manganese (Mn), 0.2-0.70 wt.% Carbon (C), 0.1-0.5wt.% Silicon (Si), 0.01-0.03wt.% Nitrogen (N), 0.001- 0.002 wt.% Phosphorus(P), 0.001-0.003 wt.% Sulphur(S) and the residue being Iron (Fe), said steel having ultra high strength of 1.0-1.6 GPa and moderate ductility of 15-20%.
[00013] According to this invention is further provided a process for
the production of medium manganese steel with high strength and moderate ductility, comprising the steps of melting steel and subsequent heating at 1000-1100C for 3-6 hrs to provide a homogenous composition, hot rolling the melted steel by multi-step rolling process, at a temperature in the range of 500-1000˚C, whereby a thickness reduction of approximately 80-90% takes place, followed by soaking and cooling the hot-rolled steel and subjecting the cooled hot-rolled steel to short time annealing for a period in the range of 5s to 3600s, to obtain the dual phase steel of high strength and moderate ductility.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[00014] Figure 1: Schematic representation of material processing schedule according to the invention.
[00015] Figure 2: Microstructure of processed material showing austenite plus ferrite phase. (a) Microstructure of steel-1 consisting mostly ferrite phase with Cementite precipitate, (b) Microstructure of steel-2 alloy consisting austenite plus martensite phase, (c) Microstructure of steel-3 alloy, fully austenite.
[00016] Figure 3: X-ray diffraction patterns of processed samples. The patterns show presence of austenite phase in microstructures. Steel-3 sample shows 100% austenite in microstructure, whereas steel-2 and steel-1 samples show approximately 40% and 10% austenite phase, respectively.
DETAILED DESCRIPTION OF THE INVENTION
[00017] According to this invention is provided medium manganese steel with high strength and moderate ductility, comprising 3.5-9.5 wt.% Manganese (Mn), 0.2-0.70 wt.% Carbon (C), 0.1-0.5wt.% Silicon (Si), 0.01-

0.03wt.% Nitrogen (N), 0.001-0.002 wt.% Phosphorus(P), 0.001-0.003 wt.% Sulphur(S) and residue being Iron (Fe).
[00018] According to this invention is further provided a process for the
production of medium manganese steel with high strength and moderate ductility, comprising the steps of melting steel to provide a homogenous composition, hot rolling the melted steel by multi-step rolling process, at a temperature in the range of 500-1000˚C, whereby a thickness reduction of approximately 80-90% takes place, followed by soaking and cooling the hot-rolled steel and subjecting the cooled hot-rolled steel to short time annealing for a period in the range of 5s to 3600s, to obtain the dual phase steel of high strength and moderate ductility.
[00019] The final microstructure of said steel consists of austenite phase varying from 10 to 100% depending upon the composition and the yield strength, ultimate tensile strength and elongation are 500-1000 MPa, 800 -1580 MPa and 7-18 % elongation, respectively.
[00020] The medium manganese steel has a single phase as well as dual phase microstructure as layer type, one layer of austenite phase and one layer of martensite/ferrite phase arranged together. The two phases will follow orientation relationship, the closed packed plane of austenite phase parallel to the closed packed plane of ferrite phase.
[00021] In accordance with this invention the as melted materials undergo hot rolling and followed by short time annealing treatment. The processed steel showed ultimate tensile strength in the range of 800 MPa - 1580 MPa and elongation 7-18%. Specifically, the invention relates to developing a process of producing ultra high strength austenite - martensite/ferrite two phase steel, which would be suitable for making automobile parts. However, in austenitic phase deformation takes place by both twinning and strain induced martensitic transformation. The process comprises the steps of providing a melting procedure in a suitable furnace for making the suitable alloy with homogenous composition throughout the microstructure. A hot rolling unit is needed for rolling the as-melted material. Soaking is performed in the temperature range 900-1200⁰C. Following soaking cooling is performed from soaking temperature at a rate in the range of 5-100C/s using air or other

cooling medium such as water. Hot rolling is performed by multi-step rolling process, at a temperature in the range of 500-1000°C, preferably at about 650°C and involves thickness reduction of approximately 80-90%. The hot rolled sheet is subjected to short time annealing preferably 5s-3600s annealing treatment to increase substantial amount of austenitic phase fraction in the microstructure, after cooling to room temperature using either water or air.
[00022] The invention provides a method of producing fine grained dual phase microstructure for making long products. Figure 1 shows the schematic view of thermo-mechanical processing schedule used for this experiment.
[00023] In accordance with a preferred embodiment of this invention, the process for producing single and dual phase steel of high strength and moderate ductility of medium Mn steel has the following steps.
[00024] The samples were melted in an arc melting furnace in argon
atmosphere. The as melted materials were hot rolled to 80-90% thickness reduction at 650ᵒC. Hot rolling has been carried out using multiple passes. The hot rolling temperature has been kept very close to the austenite finishing temperature. Following the hot rolling, a short time annealing treatment has been carried out at the same temperature. Annealing time was 3 min to retain the significant amount of austenite plus martensite/ferrite phases in microstructure. Water quenching was done after short time annealing treatment. Mechanical properties of the hot rolled and short time annealed sample have been assessed.
[00025] In one aspect of the invention, a process has been provided for
producing single as well as dual phase TWIP/TRIP steel in medium Mn steels leading to very high strength and moderate ductility, suitable for making long steel strip in a continuous process. Dual phase microstructure (austenite + martensite/ferrite) of medium Mn steels has been produced through a newly proposed special thermo-mechanical processing route. The resulting fine grained dual phase microstructure has very high strength and moderate ductility and can be used to manufacture long strip in steel industry, requiring for high strength and reasonable ductility.

[00026] In the second aspect of the invention, a thermo-mechanical
processing route has been provided to produce fine grained medium Mn steels with high strength and moderate ductility.
[00027] The present invention relates to a method of development of
advanced high strength steel sheet comprising following compositions. Table 1 shows the chemical compositions (wt.%) of the steels according to the invention.
Table 1: Chemical compositions (wt.%) of the developed steel

[00028] The Mn and C are important elements for stabilizing significant
amount of austenite phase fraction in the microstructure. High percentage of Mn is causes several problems, such as delayed cracking and melting, so it is important to reduce Mn content without much compromising in mechanical properties. Very high C content also creates problem in welding.
[00029] The microstructure contains two phases, austenite and ferrite.
Both phases are ductile and both contribute in strength and ductility. In austenitic phase, deformation takes place by twinning and strain induced martensitic transformation. The continuous formation of twining and martensitic transformation during straining causes ultra high strength and moderate ductility in this steel. In this new improved thermo-mechanical route provides simple way to produce fine grained dual phase, austenite plus martensite/ferrite microstructure compared to any other conventional thermo-mechanical processing route.

[00030] The invention will now be explained in greater details with the
help of the following non-limiting examples. However, the examples are not to be construed as limiting the scope of the invention.
EXAMPLES
[00031] The following examples depict steel compositions along with
corresponding properties.
[00032] Examples (1-3) with exact parameters and corresponding
mechanical properties is depicted in the example table. All three steels were melted using arc melting and cast. These steels were heated to 800-1100C for homogenization 2-6 hrs. Subsequently roughened and cooled in the temperature range 500-750°C and deformed by hot rolling with multiple passes. During hot rolling uniform temperature was maintained after each rolling pass by through intermediate annealing using furnace set at predetermined temperature 500-750ºC. Total 70-90% deformation was applied to achieve thickness in the range 0.8-2 mm. The steel was air cooled after hot rolling.

[00033] The steels produced according to the invention have been subjected to tests to evaluate their structure and physical properties.
[00034] Figure 2 shows the microstructure of processed material showing austenite plus ferrite phase:
(a) Microstructure of steel-1 alloy consisting mostly ferrite phase with Cementite precipitate,
(b) Microstructure of steel-2 alloy consisting austenite plus martensite phase,

(c) Microstructure of steel-3 alloy, fully austenite. Figure 2 depicts the resulting microstructure of the processed sample. After thermo-mechanical processing, the average grain size becomes considerably finer, and the morphology of phase distribution is lamellar type, one layer of austenite and one layer of ferrite phase, are arranged together.
[00035] Figure 3: X-ray diffraction patterns of processed samples. The
patterns show presence of austenite phase in microstructures. Steel-3 sample shows 100% austenite in microstructure, whereas steel-2 and steel-1 samples show approximately 40% and 10% austenite phase, respectively. The X-ray patterns shows presence of 100% austenite phase in microstructure for Steel-3 sample. Approximately 40% and 10% austenite phase present in microstructure for Steel 1 & 2 samples, respectively.
[00036] Table 2 shows the tensile properties of the tested samples and
data available in literature.
Table 2: Tensile properties of the developed steel product

[00037] Table 2 depicts the mechanical properties of thermo-mechanical
processed steels according to the invention. In tensile testing 6 mm gauge length, 2 mm width and 0.8 mm thickness sample dimension was used. The tensile test was carried out at constant strain rate of 10-3 sec-1 at room temperature.
[00038] The steels have shown the yield strength, ultimate tensile
strength and elongation as 525 - 935 MPa, 800 - 1580 MPa and 7-18 % elongation, respectively, depending upon the composition.
[00039] The advantages of the steel produced according to the invention
are that the new steel has excellent combination of tensile strength and ductility, therefore, attractive for automotive structural application and

several other areas where good combination of tensile strength and elongation required.
[00040] The steel has kind of microstructure, therefore, expected to be
useful for application where toughness and load bearing capability is required.
[00041] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purpose of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

WE CLAIM:

1. A medium manganese steel with high strength and moderate ductility, comprising
3.5-9.5 wt.% Manganese (Mn)
0.2-0.70 wt.% Carbon (C)
0.1 - 0.5 wt.% Silicon (Si)
- 0.03 wt.% Nitrogen (N)
- 0.002 wt.% Phosphorus (P)
- 0.003 wt.% Sulphur (S) and residue being Iron (Fe),
said steel having ultra high strength of 1.0-1.6 GPa and moderate ductility of 15-20%.
2. The medium manganese steel as claimed in Claim 1, wherein the final
microstructure of said steel consists of austenite phase varying from 10 to
100% depending upon the composition and the yield strength, ultimate
tensile strength and elongation are 500 - 1000 MPa, 800 - 1580 MPa and 7-
18 % elongation, respectively.
3. The medium manganese steel as claimed in Claim 1, having a single
phase as well
as dual phase microstructure as layer type, one layer of austenite phase and one layer of martensite/ferrite phase arranged together.
4. The medium manganese steel as claimed in Claims 1 and 3, wherein the two phases will follow orientation relationship, the closed packed plane of austenite phase parallel to the closed packed plane of ferrite phase.
5. A process for the production of a medium manganese steel with high strength and moderate ductility, comprising the steps of melting steel to provide a homogenous composition,
hot rolling the melted steed by multi-step rolling process, at a temperature in the range of 500-1000˚C, whereby a thickness reduction of approximately 80-90% takes place, followed by soaking and cooling the hot-rolled steel and subjecting the cooled hot-rolled steel to short time annealing for a period in the range of

5s to 3600s, to obtain the dual phase steel of high strength and moderate ductility.
6. The process as claimed in Claim 5, wherein soaking is performed at a
temperature in the range of 900-1200˚C.
7. The process as claimed in Claim 5, wherein cooling is effected at a rate of
5-100˚C/s.
8. The process as claimed in Claim 5, wherein cooling is effected using air or other cooling medium such as water.
9. The process as claimed in Claim 5, wherein short time annealing is effected for a period of 5s to 3600s.