Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
MEDIUM MANGANESE STEEL WITH THIRD GENERATION AHSS PROPERTIES AND METHOD OF PRODUCING THE SAME INVOLVING ANNEALING, AIR COOLING AND HARDEN & TEMPERING HEAT TREATMENT.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to a low carbon (0. 2%C) with medium manganese (6.67% Mn) steel and a carbide suppressing element Al (<1%Al) and method of producing the same through selective heat treatment route. More particularly, the present invention is directed to a low carbon (0. 2%C) with medium manganese (6.67% Mn) steel and a carbide suppressing element Al (<1%Al) which is melted and hot worked as a wrought product. The product in the form of strip was subjected to a variety of heat treatments including annealing, air cooling and harden and tempering that can give third generation advanced high strength steel(AHSS) properties. The heat treatments involved annealing, fast air cooling, hardening and tempering. Under certain heat treatment condition, the steel exhibits a tensile strength greater than 1000MPa and total elongation greater than 30% with the product of tensile strength and elongation exceeding 30 GPa % conforming to the third generation advanced high strength steel. The steel would be beneficial for light weighting automotive components that enhances fuel efficiency with excellent crash resistance properties.

BACKGROUND OF THE INVENTION
Automotive engineers are continuously working for the weight reduction of the automobile for the fuel efficiency, reduction in CO2 emissions and greater safety of the passengers. Hence, many new steels and materials are being developed to fulfil the requirement. Third generation advanced high strength steels are growing in demand for such purposes for their excellent combination of strength and ductility. In the present invention a medium manganese steel was developed and subjected to a range of heat treatment such as annealing, air cooling and harden & tempering after inter critical and full austenitization condition to achieve third generation advanced high strength steel properties.
Prior art
A multistep annealing treatment to improve the mechanical properties in a medium manganese steel is reported earlier (CN113025790A) whereas the present invention is a single step heat treatment. The composition claimed comprising mass fraction 0.02-0.1%C, 0.1-0.3%Si, 4-8%Mn, 0.008-0.025%Ti whereas in the present invention the carbon content is higher 0.17%C and there is the introduction of 0.78%Al which is a carbide suppressing element that differs from the prior art invention. The prior art invention relates to multistep annealing, whereas in the present invention, single step annealing, air cooling and harden and tempering are examined.

A dual phase high strength steel of composition (wt. %) 0.2-0.5%C-4-7%Mn with Fe balanced reported (CN112251679B) with pearlite transformation treatment, austenite reverse transformation treatment and tempering with microstructure of pearlite-like lamellar structure alternately composed of 15-35% of austenite and 65-85% of martensite lamella, unlike in the present invention where there is 0.78% Al associated with carbide suppression and retained austenite is promoted. The structure is predominantly bainitic in nature in the present invention with retained austenite that promotes TRIP effect.

Cu-Al alloyed high-strength medium manganese steel with 0.15 to 0.18% C, 10 to 10.32 % Mn, 1.9 to 2.1 %Al and 1.8 to 2.2% Cu containing steel was reported in patent CN114525443A. The hot rolled plate can reach 866 to 1061MPa, the elongation is 35.2% to 47.1%, and the product of strength and elongation is 35-40 GPa%. Unlike the prior art, the present invention has lower Mn content of 6.67% and Al content is also lower at 0.78% Al and the Cu content has a residual value of 0.029% and is not deliberately alloyed. The present steel is designed to be much leaner yet meet the properties meeting third generation AHSS. The heat treatment process deployed in the prior art is much time consuming compared to the present invention.

A low carbon ( 0.15% to 0.2% of C) medium Mn ( 2.5% to 3.0% of Mn) steel (CN107916359A) with carbide suppressing elements of 0.5% to 0.55% of Si, and1.45% to 1.5% of Al, containing steel comprises of smelting, continuous casting, hot rolling, annealing, acid pickling, cold rolling and annealing followed by inter critical annealing which increases retained austenite content and ensures the mechanical stability of retained austenite and finer grain size and a residual stress free microstructure that impart TRIP effect enhancing the strength and plasticity of the steel. Unlike the steel in the prior art, the present invention has lower Si content (0.3%), and the Mn content is almost double (6.67% Mn) and the carbide suppressing element Al is much lower at 0.78% and responds to third generation AHSS heat treatment to realize the properties.

A hot-rolled low carbon (0.18 to 0.22 %C) medium manganese (6 to 9%Mn) steel plate with high hole expansion rate, high strength and high elongation (CN111961982B)with Al content varying between 0 to 4% Al. and the steel is subjected to multiple steps heat treatment with longer soaking durations. Unlike the prior art, the steel in the present invention is on less than 0.2% C and Al content is less than 0.78% that stabilizes the austenite. The leaner composition of the steel was converted to a wrought product and subjected to simple heat treatments annealing, air cooling and hardening and tempering where excellent mechanical properties are realized.

Method for improving deformation capability of medium manganese steel with TRIP (Transformation-Induced Plasticity) effect (CN106636905A) was reported in a low carbon steel (0.02~ 0.50% C) Mn content much lower than the present invention at 3.5 to 6.0% and the residual elements N=0.006%;O=30ppm;P=0.015%;S=0.020%. In the present invention the Mn content is 6.67% and the steel has 0.78% Al for suppressing the carbide formation. The processing and microstructure in the invented steel is different from the prior art data.

Low carbon (0.05 to 0.4 % C) medium manganese (3 to 10 % Mn) steel plate with carbide suppressing elements less than 2% each of Si and Al content. The steel is further micro-alloyed with less than 0.2% each of V and T (CN114262778A). Unlike in the prior art, the invented steel is leaner as it does not have micro-alloying elements. The present invented steel does not have micro-alloying elements V andTi. The microstructure reported in the prior art is completely different compared to the microstructures generated in the present invention.

A low carbon (0.1 to 0.3% of C), medium manganese (4 to 12% Mn) steel with a high product of strength and elongation and preparation method thereof (CN112410681A)has been reported. unlike the prior art the present invention has deliberate addition of 0.78% Al that suppresses carbide formation. The prior art steel is subjected to a multi- step long duration heat treatment while the invented steel involves single stage heat treatments with shorted duration to develop a different range of microstructure and mechanical properties.

Low-cost heat treatment method of low carbon (0.1-0.2% C) and medium manganese steel (4.5 to 5.5 % Mn) steel (CN108866296A) with (less than 0.05% Al) and less than 0.1 %Si. The carbide suppressing elements Si and Al are very low. The present invention has 6.67% Mn and 0.78% Al. The prior art steel has multiple steps of heat treatment adopted whereas the heat treatment adopted in the present invention in a single step heat treatment.

A medium Mn steel is reported in quenched and tempered condition to achieve third generation properties (XU Juan-ping et al., Chinese Journal of Engineering, 2019, 41(5): 557-572) where a 10Mn and 4% Al is used to realize third generation AHSS properties whereas in the present invention a lower Mn (6.67) and lower Al (0.78%) used for the realization of third generation AHSS properties.

Thus, the composition of the medium Mn steel is a leaner one in its category and the heat treatment is simple such as annealing, fast air cooling and hardening and tempering heat treatment that gives a unique range of properties that gives third Generation AHSS properties at selected processing conditions.

OBJECTS OF THE INVENTION
The basic object of the invention is to develop a lean composition of a medium carbon steel with low carbon content (<0.2%C) medium Mn content (6 to 7%Mn) and lean carbide suppressing element % Al (< 1 %) steel that meets advanced high strength steel properties that is required to generate unique range of properties.
It is another objective of present invention that wherein the prior art Medium Mn steels, used the Si or Al content between 1.5 and 3% to generate retained austenite, the medium Mn steel of present invention has <1% Al content and this lean chemistry shows good retention of the retained austenite and Third Gen Advanced high strength steel properties.
Another object of the invention is to convert the cast steel into a wrought product by hot deformation using either hot forging or hot rolling.
Yet another object of the invention is to subject the wrought steel to various heat treatment schedules that develop martensiticmicrostructures with high retained austenite content that enhances the mechanical properties by TRIP effect.
A still furtherobject of the invention is to subject thin strips of the steel to thermal processing schedules such as furnace annealing, fast air cooling and hardening and tempering heat treatment to achieve a wide range of advanced high strength steel properties, some of which meet the third generation AHSS properties (UTS>1000MPa and TE>30% with tensile toughness > 30GPa %)

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to medium manganese steel comprising of a steel composition having low carbon content between 0.10 to 0.25 % and preferably 0.17%C; medium manganese (content between 6.0 to 7.0 %Mn and preferably 6.67% Mn) alloyed with lowest autenite promoting element including carbide suppressing element between 0.7 to 0.8% and preferably 0.78%Al,residual elements Si less than 0.35% and typically 0.3%; S less than 0.005% and typically 0.001%; P content less than 0.02% and typically 0.013%; residual Cr < 0.1% and typically 0.085%; Ni < 0.02% and typically 0.011%; Cu Less than 0.05% and typically 0.029% favouring wrought steel based third generation AHSS properties selectively under annealing, air cooling and harden and tempering treatment.

A further aspect of the present invention is directed to said medium manganese steel wherein the said steel has Ac1 temperature in the range of 643 to 675oC with typical value of 659oC; Ac3 temperature in the range of 709 to 751oC with typical value of 730 oC; Bs temperature in the range of 231 to 287oC with typical value of 259 0C and Ms temperature of 173 to 271 with typical value of 222 oC .

A still further aspect of the present invention is directed to said medium manganese steel which is wrought steel of 2 to 5 mm thickness preferably 2mm thickness enabling said third generation AHSS properties selectively under annealing, air cooling and harden and tempering heat treatment.

A still further aspect of the present invention is directed to said medium manganese steel adapted to generate a wide range of mechanical properties and microstructures including hot worked and heat treated in inter-critical temperature at 7000C selectively (i) yield strength in the range of 300-340MPa with typical value of 320MPa, ultimate tensile strength in the range of 1223 to 1243 MPa with typical value of 1233 MPa, elongation in the range of 24.36-24.66 with typical value of 24.5 % and the product of both (tensile toughness) in the range of 29.79 to 30.65 GPa. % with typical vale of 30 GPa %; (ii) furnace annealed third Gen AHSS properties; (iii) microstructure as furnace annealed product having coarse intercritical ferrite, martensite and 20% retained austenite in film form inter woven with martensite phase.

The steel composition as given above that was melted could be forged by heating the billet at a temperature range between 1100 and 1200 oC with typical 1150oCand held for 2 to 4 hour typically 3 hours subjected to a wrought product with reduction of 60-80% with typical 70% reduction in cross section.

A further aspect of the present invention is directed to said steel as per composition given above and hot forge processing to 2mm strip was subjected to austenitization above Ac3 and in the intercritical austenitization is followed by annealing in furnace, air cooling and hardened and tempered heat treatment.

A still further aspect of the present invention is directed to said medium manganese steel adapted to generate a wide range of mechanical properties and microstructures including hot worked austenitized in the intercritical temperature range of 720 and 700 oC followed by air cooling having tensile strengths in the third Gen AHSS range; (ii) steel austenitized at 720 oC and air cooled having yield strength in the range of 568 to 604MPa with typical value of 586MPa, ultimate tensile strength in the range of 1090-1138MPa with typical value of 1114MPa, elongation of 27.24-27.76% with typical value of 27.5%, tensile toughness in the range of 29.69 to 31.59GPa. % with typical value of 30.6 GPa %. and having microstructure of lath martensite with intercritical ferrite and retained austenite in film form, interwoven with martensite phase imparting the above properties; (iii) steel inter-critically treated at 700 oC followed by air cooling having microstructure with 20% retained austenite, yield strength in the range of 536 to 566 MPa with typical value of 551MPa, ultimate tensile strength in the range of 1139 to 1179 MPa with typical vale of 1159 MPa with elongation in the range of 31.51-31.85% with typical value of 31.7 % and tensile toughness in the range of 35.89 to 37.55% with typical value of 36.7 %.; (iv) with the austenitization temperature above Ac3, at 770 oC followed by furnace annealing, with yield strength in the range of 680-726MPa with typical value of 703MPa, ultimate tensile strength in the range of 1764 to 1804MPa with typical value of 1114 MPa with elongation in the range of 14.7 to 15% with typical value of 14.8%. and the tensile toughness in the range of 25.93 to 27.03GPa. % with typical value of 26.4 GPa% where the microstructure showed 9.5% retained austenite in the annealed condition. A still further aspect of the present invention is directed to said medium manganese steel adapted to generate a wide range of mechanical properties and microstructures including hot worked austenitized for 2 min in the intercritical temperature range of 680, 700 and 720 oC temperatures and also above Ac3 temperature at 780 oC followed by hardening in water;(ii) steel as-hardened especially from 720 and 780 C having yield strength in the range of 506-628MPa, ultimate tensile strength in the range of 1054 to 1084MPa with excellent elongation of 27.83-32.01%, which corresponds to a tensile toughness of 29.67 to 33.90 GPa % and with microstructure shows martensite laths with intricate association of retained austenite. Lower the intercritical austenitization temperature, lower strength and ductility were realized in the as-hardened condition.

A still further aspect of the present invention is directed to said medium manganese steel adapted to generate a wide range of mechanical properties and microstructures including (i) hot worked and austenitized for 1 to 5min with typical 2 min in the intercritical temperature range of 680, 700 and 720 oC temperatures and also above Ac3 temperature at 780 oC followed by hardening in water and followed by tempering for 10- 20min with typical 15 min at 200, 400 and 600 oC, wherein the steel austenitized above Ac3 at 780 oC followed by tempering at 400 and 600 oC having third Gen AHSS properties ; (iii) The steel tempered at 400 oC gave yield strength in the range of 550-598MPa with typical value of 574MPa, ultimate tensile strength in the range of 1237-1281MPa with typical 1259 MPa and elongations in the range of 27.63-28.23 with typical value of 27.9% with tensile toughness in the range of 34.18 to 36.16GPa. % with typical 35 GPa % (iv) steel tempered at 600 oC gave yield strength in the range of 530-564MPa, ultimate tensile strength in the range of 1143 to 1169MPa with typical value of 1156 MPa, elongation in the range of 31.29 to 31.83% with typical value of 31.6% and tensile toughness in the range of 35.76 to 37.21GPa% with typical value of 36.4 GPa % wherein the microstructure comprised of predominantly tempered martensite along with the presence of significant retained austenite.
Yet another aspect of the present invention is directed to a process for the manufacture of medium manganese steel as described above including:
(i)providing a selective steel composition having low carbon content between 0.10 to 0.25 % and preferably 0.17%C; medium manganese (content between 6.0 to 7.0 % Mn and preferably 6.67% Mn) alloyed with lowest austenite promoting element including carbide suppressing element between 0.7 to 0.8% and preferably 0.78%Al ,residual elements Si less than 0.35% and typically 0.3%; S less than 0.005% and typically 0.001%; P content less than 0.02% and typically 0.013%; residual Cr < 0.1% and typically 0.085%; Ni < 0.02% and typically 0.011%; Cu Less than 0.05% and typically 0.029%;
(ii) producing wrought steel thereof;
(iii) generating AHSS properties in the steel following selectively the steps of annealing, air cooling and harden and tempering treatment.

A further aspect of the present invention is directed to said method wherein the steel is made involving Ac1 temperature in the range of 643 to 675oC with typical value of 659 oC; Ac3 temperature in the range of 709 to 751oC with typical value of 730 oC; Bs temperature in the range of 231 to 287oC with typical value of 259 0C and Ms temperature of 173 to 271oC with typical value of 222 oC .

A still further aspect of the present invention is directed to said method wherein the steel composition is melted, forged by heating the billet at a temperature range between 1100 and 1200 oC and held for 2to 4preferably 3 hours, at the typical forging temperature of 1150 oC, the steel made to a wrought product with 60to 80 % reductionpreferably about 70% reduction in cross section, the wrought steel either in the form of forging or in the form of rolling, preferably the later, was made to 2to 5mm preferably 2mm thick strips and the strips were subjected to a variety of heat treatments annealing, forced air cooling and hardening and tempering to realize AHSS properties.

A still further aspect of the present invention is directed to saidmethod wherein the strip is subjected to austenitization above Ac3 and in the intercritical austenitization and is followed by annealing in furnace, air cooling and hardened and tempered heat treatment.

A still further aspect of the present invention is directed to said method comprising said hot working and heat treated selectively (i) for annealing in the inter critical temperature (700oC) generates a wide range of mechanical properties and microstructures, the steel exhibits yield strength in the range of 300-340MPa with typical value of 320MPa, ultimate tensile strength in the range of 1223 to 1243 MPa with typical value of 1233 MPa, elongation in the range of 24.36-24.66 with typical value of 24.5 % and the product of both (tensile toughness) in the range of 29.79 to 30.65 GPa. % with typical vale of 30 GPa %; (ii) The steel in the furnace annealed condition provided third Gen AHSS properties when the steel is in the inter critical austenitization at 690 to 710oCpreferably 700 oC held for 2to 10 min preferably 5 min duration followed by furnace cooling. In medium Mn steels such properties in furnace annealed condition do not give high properties, the microstructure in the furnace annealed condition having coarse intercritical ferrite, martensite and 20% retained austenite in film form inter woven with martensite phase.

A still further aspect of the present invention is directed to said method comprising said hot working and heat treated selectively which is austenitized in the intercritical temperature range of 725to 690oC preferably 720 and 700 oC followed by air cooling to obtain tensile strengths in the third Gen AHSS range, the steel austenitized at 715 to 725oC with typically 720 oC and air cooling showed yield strength in the range of 568 to 604MPa with typical value of 586MPa, ultimate tensile strength in the range of 1090-1138MPa with typical value of 1114MPa, elongation of 27.24-27.76% with typical value of 27.5%, tensile toughness in the range of 29.69 to 31.59GPa. % with typical value of 30.6 GPa %, the microstructure having lath martensite with intercritical ferrite and retained austenite in film form, interwoven with martensite phase imparting the above properties, wherein when the steel was inter-critically treated at 690to 710oC preferably 700 oC followed by air cooling gave similar microstructure with 20% retained austenite, the yield strength obtained in the range of 536 to 566 MPa with typical value of 551MPa, ultimate tensile strength in the range of 1139 to 1179 MPa with typical vale of 1159 MPa with elongation in the range of 31.51-31.85% with typical value of 31.7 % and tensile toughness in the range of 35.89 to 37.55% with typical value of 36.7 %. and wherein when the austenitization temperature is above Ac3, in the range of 760-780oC with typically at 770 oC followed by furnace annealing, the yield strength in the range of 680-726MPa with typical value of 703MPa, ultimate tensile strength in the range of 1764 to 1804MPa with typical value of 1114 MPa with elongation in the range of 14.7 to 15% with typical value of 14.8%. and the tensile toughness in the range of 25.93 to 27.03GPa. % with typical value of 26.4 GPa% where the microstructure showed 9.5% retained austenite in the annealed condition.

Another aspect of the present invention is directed to said method comprising said hot working and heat treated selectively austenitized for 1 to 5 preferably 2 min in the intercritical temperature range of 680, 700 and 720 oC temperatures and also above Ac3 temperature at 780 oC followed by hardening in water, the steel in the as-hardened condition especially from 720 and 780 C showed yield strength in the range of 506-628MPa, ultimate tensile strength in the range of 1054 to 1084MPa with excellent elongation of 27.83-32.01%, which corresponds to a tensile toughness of 29.67 to 33.90 GPa %. the microstructure shows martensite laths with intricate association of retained austenite,lower the intercritical austenitization temperature, lower strength and ductility were realized in the as-hardened condition.

Yet another aspect of the present invention is directed to said method comprising said hot working and heat treated selectively was austenitized for 1to 5min preferably 2 min in the intercritical temperature range of 680, 700 and 720 oC temperatures and also above Ac3 temperature at 780 oC followed by hardening in water and followed by tempering for 10-20min typically 15 min at 200, 400 and 600 oC, the steel austenitized above Ac3 at 780 oC followed by tempering at 400 and 600 oC gave third Gen AHSS properties, the steel tempered at 400 oC gave yield strength in the range of 550-598MPa with typical value of 574MPa, ultimate tensile strength in the range of 1237-1281MPa with typical 1259 MPa and elongations in the range of 27.63-28.23 with typical value of 27.9% with tensile toughness in the range of 34.18 to 36.16GPa. % with typical 35 GPa%, the steel when tempered at 600 oC gave yield strength in the range of 530-564MPa, ultimate tensile strength in the range of 1143 to 1169MPa with typical value of 1156 MPa, elongation in the range of 31.29 to 31.83% with typical value of 31.6% and tensile toughness in the range of 35.76 to 37.21GPa% with typical value of 36.4 GPa %, the microstructure comprised of predominantly tempered martensite along with the presence of significant retained austenite.

The above and other aspects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1: Annealing heat treatment cycle of medium manganese steel.
Fig.2: Optical microstructure of (a)700oC/5min/FC and (b)780oC/5min/FC, with (c) and (d) their corresponding scanning electron micrographs respectively.
Fig. 3: Stress strain diagram of the annealed medium manganese steels.
Fig. 4: XRD of the annealed medium manganese steels.
Fig. 5: Heat Treatment cycle for air cooled medium manganese steel.
Fig. 6:Microstructure of the medium manganese steel (a) 700oC/5min/AC (b) 720oC/5min/AC (c) 770oC/5min/AC (d) 900oC /5min/AC.
Fig. 7:Microstructure of the medium manganese steel (a) 900oC/5min/AC (b) 770oC/5min/AC (c) 700oC/5min/AC, (d)-(f) their corresponding scanning electron micrographs.
Fig. 8: Stress-strain diagram of the medium manganese steel at various air cooled condition.
Fig. 9: X-Ray Diffraction of the steels under various air cooled conditions.
Fig.10:Hardening and tempering Heat Treatment cycle of the medium manganese steel.
Fig. 11: Microstructure of the medium Mn steel austenized at (a) 680oC (b) 700 oC(c) 720oC (d) 780 oC for 2min followed by water quenching. Showing complete lath martensitic structure.
Fig.12:SEM micrograph of medium Mn steel austenitized at (a) 680oC(b) 700 oC (c) 720 oC (d) 780 oC, held for 2 min followed by water quenching
Fig.13: SEM micrograph of the medium Mn steel tempered at 200, 400 and 600 oC for 15 min after austenitizing at 700 oC and 780 oC for 2 min followed by water quenching.
Fig. 14:. XRD of inter critical austenization (a) 700oC/WQ and austenization above A3 (b) 780oC/ WQ followed by tempering at 200 400 & 600 oC.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to a lean composition of a medium carbon steel with low carbon content (<0.2%C) medium Mn content (6 to 7%Mn) and lean carbide suppressing element % Al (< 1 %) steel that meets advanced high strength steel(AHSS) properties that is required to generate unique range of properties.
Chemical composition of the low carbon, medium manganese steel is shown in Table-1. The steel has low carbon content that ensures good weldability and formability. The steel is a low carbon steel with C between 0.10 and 0.25% and more specifically 0.17% suitable for welding and formability. It has Mn content between 6.0 to 7.0 % range and 6.67% more specifically, which on heat treatment promotes significant retained austenite in the steel. In addition, the steel is alloyed with 0.7 to 0.8%Al range and a more specific Al content of 0.78% Al. Thus, keeping the alloy design principle above, a new medium Mn steel with lean carbide suppressing element has been designed with Al. All other elements are residual level. The designed composition responded to a wide variety of thermal processing that gave a unique range of mechanical properties.

Table 1:Chemical composition in wt.% of the medium manganese steel.
C Mn S P Si Al Cr Ni Cu
Range 0.10 – 0.25 6.0-
7.0 <0.01 <0.02 <0.35 0.7-0.8 <0.1 <0.02 <0.05
Actual 0.17 6.67 0.001 0.013 0.30 0.78 0.085 0.011 0.029
Ac1=659 oC; Ac3=730 oC; Bs =259 oC; Ms= 222 oC

Thin strips of the steel are subjected to thermal processing conditions that promote a unique range of microstructure and mechanical properties. The present steel is subjected to three types of heat treatment.
The steel designed was air-induction melted in a laboratory induction furnace. The as-cast steel bar was heated to temperatures between 1200 and 1100 oC, typically 1150 oC and soaked@ rate of 1 h per inch typically for 3 hours in a 160 mm dia cast steel ingot. The steel was converted to a wrought product by hot forging at 1150 oC in a 1 ton hammer press with 70% reduction. The steel was reheated, when the temperature fell below 900 oC. The steel was forged to a bar of 55x 55 mm cross section. The steel so made was converted to 2 mm thick strips for further heat treatment. The heat treatment processing and properties are elucidated below. The critical transformation point of the steel was obtained from JMat Pro software (Bs and Ms) and DSC experiment (Ac1 and Ac3)is shown in Table 1.

1. ANNEALED MEDIUM Mn AHSS
One set of samples of the 2mm thick steel strip of the invented medium manganese steel were used for the annealing heat treatment as per schedule shown in Fig. 1. Sample was soaked above AC3 temperature at 780 oC with 5 min soaking and another set of samples was soaked in the intercritical austenitization temperature (between Ac1 and Ac3)typically at 700oC soaked for 5min, followed by furnace cooled to room temperature. The annealed samples were tensile tested for mechanical property evaluation and microstructural analysis using optical and scanning electron microscopy in addition to X-ray diffraction for quantification of retained austenite. The optical microstructure of the steel at the heat treated condition is shown in Fig.2. In both cases, the steel transforms to martensite and retained austenite. The martensite lath is intimately interwoven with retained austenite, white phase as seen in the SEM microstructure in Fig.2. The mechanical properties of the steel in the annealed condition, is given in Table 2. It is seen that the steel austenitized from above Ac3 temperature and quenched has ultimate tensile strength in the range of 1391 to 1441MPa with typical value of 1416 MPa, yield strength in the range of 633 to 703MPa with typical value of 668 MPa and elongation in the range of 7.65 to 8.05% with typical value of 7.8% and the tensile toughness in the range of 10.64 to 11.60 GPa. % with typical value of 11.26 GPa.%. The main strengthening is derived from lath martensite and it is reasonably good properties. The steel annealed from intercritical temperature at 700 oC, leads to fine martensite, some amount of soft intercritical ferrite along with stabilized retained austenite rich in carbon content. The ultimate tensile strength is in the range of 1223 to 1243MPa with typical value of 1233 MPa, yield strength in the range of 300 to 340MPa with typical value of 320MPa and elongation in the range of 24.36 to 24.66 % with typical value of 24.5%. The tensile toughness is in the range of 29.79 to 30.65 GPa. % with typical value of 30 GPa.%. The better plasticity of the steel is due to intercritical austenitization, where a softer ferrite along with martensite and lath type retained austenite is observed. The tensile behaviour of the steel is shown in Fig. 3. The presence of retained austenite in intercritically austenitized sample against sample fully austenitized as shown in XRD examination in Fig. 4. The invention records annealing treatment in a unique medium Mn steels where ultra high strength levels with martensite and retained austenite are observed when the steel austenitized above Ac3 temperature and some amount of ferrite in addition to the above phases found when austenitized in the inter critical zone.

Table 2:Mechanical properties of annealed medium manganese steels.
Condition YS, MPa UTS, MPa TE, % UTS*TE, GPa %
780/5min/FC 668±35 1416±25 7.85±0.20 11.26±0.48
700/5min/FC 320±20 1233±10 24.51±0.15 30.23±0.43

2. AIR COOLED MEDIUM Mn AHSS
Four sets of the steel strip samples were subject to four different austenitization temperature followed by cooling in air, a treatment close to normalizing in low alloy low carbon steel. Two austenitization temperatures, 770 and 900 oC were above Ac3 temperature followed by air cooling as in Fig. 5.The carbide dissolution will enhance at higher austenitization temperature of 900 oCwhere the grains may coarsen as well and hence 770 oC austenitization temperature was also studied. In addition, two temperatures in the inter critical temperatures 700 and 720 oC is chosen to generate some volume of intercritical ferrite content in the steel. The austenite from fully austenitized condition transforms to martensite and lowersthe contents of retained austenite with increasing austenitization temperature. The intercritically austenitized steel at 700 and 720 oC followed by fast air cooling shows a carbon partition during inter critical treatment. The carbon enriched retained austenite transforms to martensite and retained austenite along with inter critical ferrite. The intercritical ferrites are blocky as shown in Fig.6 and Fig.7. The mechanical properties of the steel in the fast air cooled condition, is shown in Table-3.
Table 3:Heat Treatment Condition and Mechanical Properties of the steel
Heat Treatment Condition YS, MPa UTS, MPa TE, % UTS*TE,
GPa %
900oC/5min/AC 706±20 1693±35 9.51±0.20 16.09±0.67
770oC/5min/AC 703±23 1784±20 14.85±0.15 26.48±0.56
720oC/5min/AC 586±18 1114±24 27.50±0.26 30.63±0.95
700oC/5min/AC 551±15 1159±20 31.68±0.17 36.74±0.83

The steel austenitized above Ac3 and close to it at 770 oC shows much higher ultimate tensile strength in the range of 1764 to 1804 MPa with typical value of 1784 MPa, yield strength in the range of 620 to 726MPawith typical value of 703MPa and elongations in the range of 14.7 to 15%with typical value of 14.85% with the tensile toughness in the range of 25.93 to 27.06 GPa. % with typical value of 26.48 GPa.%. The relative plasticity of the steels can be assessed from the tensile chart as in Fig.8. The plasticity is improved because the steel in this condition shows higher retained austenite than at still higher temperature as evidenced from XRD charts as shown in Fig. 9. The microstructure is predominantly martensite with higher amount of retained austenite when the steel is austenitized at 770 oC followed by air cooling. At the highest austenitization temperature of 900 oC, the steel shows ultra high ultimate tensile strength in the range of 1658-1728MPa with typical value of 1693 MPa but shows lowest elongation in the range of 9.31-9.71% with typical value of 9.5%, with the tensile toughness in the range of 15.44 to 16.78GPa% with typical value of 16 GPa.% as in Table 3. This may be due to coarser structure and the yield strength is in the range of 686 to 726 MPa with typical value of 706 MPa. The steel has less than 1.5% retained austenite as shown in Fig.9.

The steel austenitized at the intercritical temperature range at 700 and 720 oC induces softer ferrite and carbon rich austenite. This steel when subject to fast air cooling, tends to retain higher content of retained austenite due to the carbon partitioning, which tends to form retained austenite which is aided by 0.78% Al content that suppress the carbide content. The retained austenite content was 20% at 700 oC and 27% retained austenite at 720 oC austenitizing followed by air cooling. Accordingly, the steel austenitized at 700 oC followed by air cooling which showed ultimate tensile strength in the range of 1139 to 1179MPa% with typical value of 1159 MPa, yield strength in the range of 536 to 566MPa with typical vale of 551 MPa and elongations in the range of 31.51 to 31.85% with typical vale of 31.6% with tensile toughness in the range of 35.89 to 37.55GPa% with typical value of 36.74 GPa % and20.3%retained austenite. The actual microstructure has blocky ferrite, lath martensite and retained austenite in film form. The steel austenitized at 720 oC shows more retained austenite and lower ferrite content along with martensite. The ultimate tensile strength is in the range of 1090 to1138 MPa with typical value of 1114 MPa, yield strength in the range of 568 to 604MPa with typical value of 586MPa, elongations in the range of 27.24 to 27.76% with typical value of 27.5% with tensile toughness in the range of 29.69 to 31.59 GPa. % with typical value of 30.63GPa %.The tensile chart of the intercritically treated steel, is also shown in Fig. 8. Thus, the intercritical treatment followed by fast air cooling gives an ultra high strength steel of 1100 MPa class with excellent ductility.

3. HARDENED AND TEMPERED MEDIUM Mn AHSS
The invented medium Mn steel is subjected to hardening and tempering, which is not a common heat treatment for such steels. At certain condition, excellent properties were obtained in this condition as well.
The base steel as in Table 1 was subject to full austenitization at 780 oC and intercritical austenitization at 680, 700 and 720 oC soaked for 2 min followed by water quenching and the samples austenitized at every temperature was subject to tempering at 200, 400 and 600 oC for 15 min holding as per cycle shown in Fig.10. The steel on quenching from fully austenitic condition tends to form martensite and significant quantities of retained austenite(40%) at 780 oC austenitization (Fig.11,12& 13). The steel when tempered post quenching tend to form tempered martensite and a significant amount of retained austenite, is transformed to carbides and ferrite. When the steel is intercritically austenitizedat 680, 700 and 720 oC and soaked for 2 min, it tends to partition carbon between the blocky intercritical ferrite and austenite, and on quenching, the austenite portion tends to form partly martensite and retained austenite. The XRD charts in Fig.14 and Table 4 shows that the as-quenched steel shows blocky intercritical ferrite, lath martensite and significant fraction of retained austenite. The retained austenite is filmy along the lath martensite. The steel on tempering shows that there is ferrite and tempered carbide. The austenite content decreases on tempering as shown in Table 4. The presence of high Mn enables Mn partition to austenite causing retained austenite to exist.
Table 4: Retained Austenite fraction obtained through XRD.
Austenitization
Temperature, oC Tempering Temperature, oC Amount of retained austenite by XRD (%)
700 WQ 11.17
700 200 9.05
700 400 6.52
700 600 0
780 WQ 40.15
780 200 24.03
780 400 42.85
780 600 34.02

Table 5: Mechanical properties of the hardened and tempered medium manganese steels.
Austenitization Temperature, oC Tempering Temperature, oC YS, MPa UTS, MPa TE, % UTS*TE,
GPa %
680 Not tempered 702±25 1041±33 18.21±0.22 18.96±0.83
680 200 830±15 1108±24 11.23±0.15 12.44±44
680 400 789±17 1123±16 15.74±0.18 17.68±45
680 600 621±22 1034±19 16.90±0.25 17.48±58
700 Not tempered 694±12 1057±22 18.75±0.19 19.82±61
700 200 839±30 1104±13 18.48±0.22 20.41±67
700 400 891±18 1140±09 19.16±0.28 21.85±49
700 600 639±09 1038±15 19.14±0.26 19.87±56
720 Not tempered 526±20 1084±18 28.11±0.28 30.46±0.81
720 200 825±18 1273±22 12.09±0.24 15.39±0.57
720 400 874±12 1293±18 14.10±0.22 18.23±0.54
720 600 632±19 1058±13 15.27±0.29 16.15±0.51
780 Not tempered 605±23 1054±05 31.78±0.23 33.50±0.40
780 200 486±25 1269±16 21.35±0.28 26.74±0.70
780 400 574±24 1259±22 27.93±0.30 35.16±0.99
780 600 547±17 1156±13 31.56±0.27 36.48±0.72
Interestingly the invented steel in the as-quenched condition itself shows good ductility and tensile strength in the range 1041 to 1084 MPa with elongation between 18 to 32%. The yield strength is in the range of 526 to 702 MPa and tensile toughness in the range of 19-33GPa.%. The steel as quenched from 780 and 720 oC, shows excellent ductility with 1050 MPa ultimate tensile strength. When the inter critical temperature decreases there is increased formation of inter critical ferrite and less martensite. On tempering, less amount of tempered martensite is formed. Hence, the total elongation decreases. Especially, the steel tempered at lower tempering temperature shows lower ductility. Thus, the steel austenitized at the intercritical temperature followed by tempering, in general, shows inferior ductility. However, the steel quenched from a fully austenitized condition followed by tempering shows better ductility. Tempering at higher tempering temperature in steel quenched from 780 oC shows excellent ductility and strength. The steel tempered at 400 and 600 oC shows 1259 MPa and 1156 MPa respectively with corresponding elongation 28% and 31% which is in the third Generation AHSS level. The intercriticallyaustenitized and quenched and tempered gives lower ductility and lower strength. The plastic property of the steel appears to be better when the steel is quenched from above Ac3 and tempered at temperature above 400 oC.

, Claims:WE CLAIM:
1. Medium manganese steel comprising of a steel composition having low carbon content between 0.10 to 0.25 % and preferably 0.17%C; medium manganese (content between 6.0 to 7.0 %Mn and preferably 6.67% Mn) alloyed with lowest austenite promoting element including carbide suppressing element between 0.7 to 0.8% and preferably 0.78%Al,residual elements Si less than 0.35% and typically 0.3%; S less than 0.005% and typically 0.001%; P content less than 0.02% and typically 0.013%; residual Cr < 0.1% and typically 0.085%; Ni < 0.02% and typically 0.011%; Cu Less than 0.05% and typically 0.029% favouring wrought steel based third generation AHSS properties selectively under annealing, air cooling and harden and tempering treatment.
2. The medium manganese steel as claimed in claim 1 wherein the said steel has Ac1 temperature in the range of 643 to 675oC with typical value of 659 oC; Ac3 temperature in the range of 709 to 751oC with typical value of 730 oC; Bs temperature in the range of 231 to 287oC with typical value of 259 0C and Ms temperature of 173 to 271oC with typical value of 222 oC.
3. The medium manganese steel as claimed in anyone of claims 1 or 2 which is wrought steel of 2 to 5 mm thickness preferably 2mm thickness enabling said third generation AHSS properties selectively under annealing, air cooling and harden and tempering treatment.
4. The medium manganese steel as claimed in anyone of claims 1 to 3 adapted to generate a wide range of mechanical properties and microstructures including hot worked and heat treated in inter-critical temperature 690-710oC preferably at 7000C selectively (i) yield strength in the range of 300-340MPa with typical value of 320MPa, ultimate tensile strength in the range of 1223 to 1243 MPa with typical value of 1233 MPa, elongation in the range of 24.36-24.66 with typical value of 24.5 % and the product of both (tensile toughness) in the range of 29.79 to 30.65 GPa. % with typical vale of 30 GPa %; (ii) furnace annealed third Gen AHSS properties ; (iii) microstructure as furnace annealed product having coarse intercritical ferrite, martensite and 20% retained austenite in film form inter woven with martensite phase.
5. The steel composition as per claim (1) that was melted could be forged by heating the billet at a temperature range between 1100 and 1200 oC and held for 3 hours. At the typical forging temperature of 1150 oC, the steel was made to a wrought product with 70% reduction in cross section.
6. The steel as per composition in claim (1) and hot forge processing as in claim (2) is subjected to processing as in claim (2) with 2mm strip was subjected to austenitization above Ac3 and in the intercritical austenitization is followed by annealing in furnace, air cooling and hardened and tempered heat treatment.
7. The medium manganese steel as claimed in anyone of claims 1 to 3 adapted to generate a wide range of mechanical properties and microstructures including hot worked (i) austenitized in the intercritical temperature range of 725 to 690oC with typically at 720 and 700 oC followed by air cooling having tensile strengths in the third Gen AHSS range; (ii) steel austenitized in the temperature range of 715-725oC with typically at 720 oC for 2-10min and typically 5min and air cooled having yield strength in the range of 568 to 604MPa with typical value of 586MPa, ultimate tensile strength in the range of 1090-1138MPa with typical value of 1114MPa, elongation of 27.24-27.76% with typical value of 27.5%, tensile toughness in the range of 29.69 to 31.59GPa. % with typical value of 30.6 GPa %. and having microstructure of lath martensite with intercritical ferrite and retained austenite in film form, interwoven with martensite phase imparting the above properties; (iii) steel inter-critically treated in the temperature range of 690-710oC with typically at 700 oC for 2-10min and typically 5 min followed by air cooling having microstructure with 20% retained austenite, yield strength in the range of 536 to 566 MPa with typical value of 551MPa, ultimate tensile strength in the range of 1139 to 1179 MPa with typical vale of 1159 MPa with elongation in the range of 31.51-31.85% with typical value of 31.7 % and tensile toughness in the range of 35.89 to 37.55% with typical value of 36.7 %. ; (iv) with the austenitization temperature above Ac3, in the range of 760-780oC and typically at 770 oC followed by furnace annealing, with yield strength in the range of 680-726MPa with typical value of 703MPa, ultimate tensile strength in the range of 1764 to 1804MPa with typical value of 1114 MPa with elongation in the range of 14.7 to 15% with typical value of 14.8%. and the tensile toughness in the range of 25.93 to 27.03GPa. % with typical value of 26.4 GPa% where the microstructure showed 9.5% retained austenite in the annealed condition.
8. The medium manganese steel as claimed in anyone of claims 1 to 3 adapted to generate a wide range of mechanical properties and microstructures including hot worked (i) austenitized for 1-5min and typically for 2 min in the intercritical temperature range of 680, 700 and 720 oC and also above Ac3 temperature at 780 oC followed by hardening in water;(ii) steel as-hardened especially from 720 and 780 C having yield strength in the range of 506-628MPa, ultimate tensile strength in the range of 1054 to 1084MPa with excellent elongation of 27.83-32.01%, which corresponds to a tensile toughness of 29.67 to 33.90 GPa % and with microstructure shows martensite laths with intricate association of retained austenite. Lower the intercritical austenitization temperature, lower strength and ductility were realized in the as-hardened condition.
9. The medium manganese steel as claimed in anyone of claims 1 to 3 adapted to generate a wide range of mechanical properties and microstructures including (i) hot worked and austenitized for 1-5 min and typically 2 min in the intercritical temperature range of 680, 700 and 720 oC temperatures and also above Ac3 temperature in the range of 770-790oC and typically at 780 oC followed by hardening in water and followed by tempering for 10-20min and typically 15 min at 200, 400 and 600 oC. wherein the steel austenitized above Ac3 at 780 oC followed by tempering at 400 and 600 oC having third Gen AHSS properties; (iii) The steel tempered at 400 oC gave yield strength in the range of 550-598MPa with typical value of 574MPa, ultimate tensile strength in the range of 1237-1281MPa with typical 1259 MPa and elongations in the range of 27.63-28.23 with typical value of 27.9% with tensile toughness in the range of 34.18 to 36.16GPa. % with typical 35 GPa % (iv) steel tempered at 600 oC gave yield strength in the range of 530-564MPa, ultimate tensile strength in the range of 1143 to 1169MPa with typical value of 1156 MPa, elongation in the range of 31.29 to 31.83% with typical value of 31.6% and tensile toughness in the range of 35.76 to 37.21GPa% with typical value of 36.4 GPa %wherein the microstructure comprised of predominantly tempered martensite along with the presence of significant retained austenite.
10. A method for the manufacture of medium manganese steel as claimed in anyone of claims 1 to 9 including :(i)providing a selective steel composition having low carbon content between 0.10 to 0.25 % and preferably 0.17%C; medium manganese (content between 6.0 to 7.0 %Mn and preferably 6.67% Mn) alloyed with lowest autenite promoting element including carbide suppressing element between 0.7 to 0.8% and preferably 0.78%Al ,residual elements Si less than 0.35% and typically 0.3%; S less than 0.005% and typically 0.001%; P content less than 0.02% and typically 0.013%; residual Cr < 0.1% and typically 0.085%; Ni < 0.02% and typically 0.011%; Cu Less than 0.05% and typically 0.029% ;
(ii) producing wrought steel thereof;
(iii) generating AHSS properties in the steel following selectively the steps of annealing, air cooling and harden and tempering treatment.
11. The method as claimed in claim 10 wherein the steel is made involving Ac1 temperature in the range of 643 to 675oC with typical value of 659 oC; Ac3 temperature in the range of 709 to 751oC with typical value of 730 oC; Bs temperature in the range of 231 to 287oC with typical value of 259 0C and Ms temperature of 173 to 271 with typical value of 222 oC .
12. The method as claimed in anyone of claims 10 or 11 wherein the steel composition is melted, forged by heating the billet at a temperature range between 1100 and 1200 oC and held for 2 to 4. preferably 3 hours, at the typical forging temperature of 1150 oC, the steel made to a wrought product with 60 to 80% preferably about 70% reduction in cross section, the wrought steel either in the form of forging or in the form of rolling, preferably the later, was made to 1 to 5mm preferably 2mm thick strips and the strips were subjected to a variety of heat treatments annealing, forced air cooling and hardening and tempering to realize AHSS properties.
13. The method as claimed in anyone of claims 10 to12 wherein the strip is subjected to austenitization above Ac3 and in the intercritical austenitization and is followed by annealing in furnace, air cooling and hardened and tempered heat treatment.
14. The method as claimed in anyone of claims 10 to 13 comprising said hot working and heat treated selectively (i) for annealing in the inter critical temperature (700oC) generates a wide range of mechanical properties and microstructures, the steel exhibits yield strength in the range of 300-340MPa with typical value of 320MPa, ultimate tensile strength in the range of 1223 to 1243 MPa with typical value of 1233 MPa, elongation in the range of 24.36-24.66 with typical value of 24.5 % and the product of both (tensile toughness) in the range of 29.79 to 30.65 GPa. % with typical vale of 30 GPa %; (ii) The steel in the furnace annealed condition provided third Gen AHSS properties when the steel is in the inter critical austenitization at 690to 710oC preferably 700 oC held for 2to 10min preferably 5 min duration followed by furnace cooling. In medium Mn steels such properties in furnace annealed condition do not give high properties, the microstructure in the furnace annealed condition having coarse intercritical ferrite, martensite and 20% retained austenite in film form inter woven with martensite phase.
15. The method as claimed in anyone of claims 10 to 14 comprising said hot working and heat treated selectively which is austenitized in the intercritical temperature range of 725 to 690oC preferably 720 and 700 oC followed by air cooling to obtain tensile strengths in the third Gen AHSS range, the steel austenitized in the range of 715-725oC with typically at 720 oC and air cooling showed yield strength in the range of 568 to 604MPa with typical value of 586MPa, ultimate tensile strength in the range of 1090-1138MPa with typical value of 1114MPa, elongation of 27.24-27.76% with typical value of 27.5%, tensile toughness in the range of 29.69 to 31.59GPa. % with typical value of 30.6 GPa %, the microstructure having lath martensite with intercritical ferrite and retained austenite in film form, interwoven with martensite phase imparting the above properties, wherein when the steel was inter-critically treated at 690 to 710oCpreferably 700 oC followed by air cooling gave similar microstructure with 20% retained austenite, the yield strength obtained in the range of 536 to 566 MPa with typical value of 551MPa, ultimate tensile strength in the range of 1139 to 1179 MPa with typical vale of 1159 MPa with elongation in the range of 31.51-31.85% with typical value of 31.7 % and tensile toughness in the range of 35.89 to 37.55% with typical value of 36.7 %. and wherein when the austenitization temperature is just above Ac3, in the range of 760-780oC at 770 oC for 2-10 min preferably 5min followed by furnace annealing, the yield strength in the range of 680-726MPa with typical value of 703MPa, ultimate tensile strength in the range of 1764 to 1804MPa with typical value of 1114 MPa with elongation in the range of 14.7 to 15% with typical value of 14.8%. and the tensile toughness in the range of 25.93 to 27.03GPa. % with typical value of 26.4 GPa% where the microstructure showed 9.5% retained austenite in the annealed condition.
16. The method as claimed in anyone of claims 10 to 15 comprising said hot working and heat treated selectively austenitized for 1 to 5min preferably 2 min in the intercritical temperature range of 680, 700 and 720 oC temperatures and also above Ac3 temperature at 780 oC followed by hardening in water, the steel in the as-hardened condition especially from 720 and 780 C showed yield strength in the range of 506-628MPa, ultimate tensile strength in the range of 1054 to 1084MPa with excellent elongation of 27.83-32.01%, which corresponds to a tensile toughness of 29.67 to 33.90 GPa %. the microstructure shows martensite laths with intricate association of retained austenite, lower the intercritical austenitization temperature, lower strength and ductility were realized in the as-hardened condition.
17. The method as claimed in anyone of claims 10 to 16 comprising said hot working and heat treated selectively was austenitized for 1 to 5 min preferably 2 min in the intercritical temperature range of 680, 700 and 720 oC temperatures and also above Ac3 temperature at 780 oC followed by hardening in water and followed by tempering for 15 min at 200, 400 and 600 oC., the steel austenitized above Ac3 at 780 oC followed by tempering at 400 and 600 oC gave third Gen AHSS properties, the steel tempered at 400 oC gave yield strength in the range of 550-598MPa with typical value of 574MPa, ultimate tensile strength in the range of 1237-1281MPa with typical 1259 MPa and elongations in the range of 27.63-28.23 with typical value of 27.9% with tensile toughness in the range of 34.18 to 36.16GPa. % with typical 35 GPa%, the steel when tempered at 600 oC gave yield strength in the range of 530-564MPa, ultimate tensile strength in the range of 1143 to 1169MPa with typical value of 1156 MPa, elongation in the range of 31.29 to 31.83% with typical value of 31.6% and tensile toughness in the range of 35.76 to 37.21GPa% with typical value of 36.4 GPa%, the microstructure comprised of predominantly tempered martensite along with the presence of significant retained austenite.

Dated This The 26th Day Of September, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199