Description:FIELD OF INVENTION
The present invention relates to the development of a unique composition of medium manganese steel followed by its process of development involving subjecting to austenite reverted transformation giving rise to third gen AHSS properties with the product having strength and elongation exceeding 30 GPa%. The process involved hot working of the steel by hot forging or hot rolling followed by air cooling which develops a microstructure consisting of martensite and retained austenite. The steel is then reheated to a temperature in the inter critical temperature range where there is formation of the reverted austenite at various holding conditions. The microstructure after reheating consists of ferrite, bainite, martensite and reverted austenite.
BACKGROUND OF THE INVENTION
Automotive industry is looking for new materials for the weight reduction of automotive components for the improved fuel efficiency, reduced carbon emission and greater safety of the passengers. Hence innovative compositions, processing conditions are being employed to maximize the strength and elongations to cater the need of automotive components. Medium Mn steels with 3 to 12 % Mn are promising materials for third generation advanced high strength steels for automotive applications.

EP2746409A1 reported a steel of composition 0.09 to 0.14 wt. %C, 4.0 to 4.5 wt. % Mn, 0.005 to 1% Al, 0.4% Cr, 0.3 to 0.6% Si and 40 to 60 ppm N. In addition, the (Si+Cr+Al) in the range of 0.3 to 1.3 wt.% as a ferrite stabilizing formulation. The steel was subjected to primary annealing at 810 oC held for 5 min and cooled at 25 to 200 K/s cooling rate subjected to secondary annealing in the temperature range of 650oC held for 4 h and cooled. As against the above the investigated steel has higher Mn content of 6.7% and Al content 0.78%. There is no addition of Cr in the investigated steel. The processing condition in the prior art involves cold rolled material with first stage annealing above Ac3 for 5 min and second stage reheating between Ac3 and Ac1 for 4 hours. The present patent involves deformation at 1150oC followed by air cooling followed by reheating in the temperature range between 600 and 650oC for a brief holding time of 5 min. The properties in the prior art has tensile strength between 773 to 944 MPa and elongation between 32 to 28% that results in a strength and elongation between 25 to 26 GPa. As against the prior art the investigated steel gave tensile properties in the range Tensile strength between 1010 to 1410 and elongation range from 14 to 31%. At specific conditions of reheating the product of strength and elongation in the present patent range from 32 to 37 GPa%.
CN114807524A reported use of a medium Mn steel with the composition of 0.25 to 0.35%C; 7 to 9%Mn; 1.5 to 2.5%Al; =0.005 %P, =0.007 %S, and balance Fe. Against this the present patent has lower Mn content of 6.67% and Al content of 0.78%. The austenite reverted transformation involved forging and hot rolling at 1150oC (typically 1100-1300oC for 1-3h for multi-pass rolling) with a finishing rolling at 900oC. The steel so made is subject to partial austenitization to generate 43.5 to 46 % retained austenite and rest ferrite in the first stage. This is followed by second stage annealing, below Ac3 between 740-780oC with a hold time of 10-15min followed by water quenching. The present patent however involves deformation at 1150oC which is well above Ac3 in fully austenitic range, followed by air cooling followed by reheating in the temperature range between 600 and 650oC with a brief holding time of 5 min followed by air cooling. The prior art claims a product of strength and elongation to be 45 to 60 GPa.%, while the steel invented with lower Mn content and cooling in first stage from above Ac3, shows the product of strength and elongation between 20 and 37 GPa%.
CN111961982A reported use of a medium Mn steel with a high hole expansion rate of 35-73 %. The steel with 0.18 to 0.22% C, 6 to 9 % Mn and less than 4% Al composition cast, hot forged and subject to multi pass rolling and heat treatment gave 553 to 593 MPa yield strength, of 900 to 1307 MPa tensile strength, and 32 to 40 % elongation. The processing as per the prior art involves after hot rolling the temperature of the asynchronous hot rolled plate is preserved at 650 to 800oC with hold time 10 to 60 min, followed by rapid quenching. As against the prior-art single stage processing, the invented steel was subjected to two stage annealing. The initial annealing above Ac3 followed by air cooling and it was further annealed at a temperature within inter critical temperature for a brief holding of 5 min. The peak strength when the second austenitization was between 660 to 720oC held for 5 min. The property range of invented steel with modified processing shows 337 to 647 MPa yield strength, 1010 to 1410 MPa tensile strength and 14 to 31% elongation.
Bleck et al., [Wolfgang Bleck, Tarek Allam, and Alexander Gramlich, Berg HuettenmaennMonatsh, Alloying and Processing of Medium Manganese Steels for Forging Applications (2022) Vol. 167 (11): 534–537], reported a medium manganese steel with ART heat treatment, with respect to annealing of air cooled medium manganese steel, which showed martensite about 3 % retained austenite. Tempered martensite and decomposition of retained austenite above 250oC. Higher tempering temperatures at 350oC gives temper embrittlement and 450oC results is carbide coagulation with strength decrease and ductility increase. Heating above 650oC nucleated austenite net work where reverted austenite transformation is observed.

Traversing the prior arts, it was observed that very few works have been done in the field of development of a unique composition of medium manganese steel followed by its process of development involving subjecting to austenite reverted transformation giving rise to third gen AHSS properties with the product having strength and elongation exceeding 30 GPa%.

Hence, in the present work, third generation advanced high strength steels for automotive applications were processed involving hot working the steel by hot forging or hot rolling followed by air cooling which develops a microstructure consisting of martensite and retained austenite followed by reheating it to a temperature in the inter critical temperature range wherein there is formation of the reverted austenite at various holding conditions.
OBJECTIVE OF THE INVENTION
The main objective of this invention is to develop Low carbon Medium manganese steel having 0.17% C, 6.67% Mn and 0.78% Al for automotive applications with enhanced mechanical properties in a steel, on austenite reverted transformation showing high levels of retained austenite in a tempered martensite structure that improves mechanical properties in terms of tensile strength and ductility.
Another objective of the present advancement is to process low carbon Medium manganese steel by developing processing condition by reverted austenite transformation such that the mechanical properties show greater than 550 MPa yield strength, 1010 to 1100 MPa tensile strength, and 17 to 23% elongation, that gives the product strength and elongation exceeding 30 GPa.%.
SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a Low carbon Medium manganese steel having composition including major alloying elements comprising of carbon content between 0.10 and 0.2%; Mn content between 6 to 7% and Al content between 0.5 and 1.0 and optionally having residual elements S less than 0.03%, P less than 0.03%, Si content between 0.2 and 0.5 %, Cr and Ni content each less than 0.1%, and Cu less than 0.07%.

A further aspect of the present invention is directed to the low carbon medium manganese steel having austenite content as a function of reverted austenite holding temperature variable between 9.5 to 39.5% and secondary martensite.

A still further aspect of the present invention is directed to the low carbon medium manganese steel having said temperature dependent variable reverted transformation austenite % and related variable strength characteristics selected from:
(i) Which is a reheated steel between 660 to 720oC having high retained austenite content in the range of 26.8 to 39.5% show yield strength between 337 and 647 MPa and tensile strength between 1010and 1410 MPa and elongation between 14 and 31%.

(ii) austenite % at a temperature of 660 and 680oC showing yield strength between 563 to 647 MPa and tensile strength in the range 1010 to 1036 MPa and total elongation 22 to 31%, the product of strength and elongation is between 22 to 32 GPa % conforming to Third Gen AHSS and having high yield ratio in the range of 0.54 to 0.64.

(iii) reverted austenitization temperature of 700 to 720oC where the high austenite content along with tempered martensite gives lowest yield strength between 337 and 350 MPa while the tensile strength show 1258 to 1410 MPa with elongation 29 to 14% with the product of tensile strength and ductility values are 37 to 20 GPa % conforming to Third Gen AHSS and having low yield ratio in the range of 0.25-0.27.

A still further aspect of the present invention is directed to a process for manufacture of Low carbon Medium manganese steel comprising:

(a) Providing said select steel composition;
(b) Melting in any primary and secondary steel making processes including Basic oxygen furnace, Electric arc furnace followed by secondary steel making lade furnace and also through induction furnace with graded alloying;
(c) Casting as ingot or in continuous casting as slabs;
(d) the slabs thus obtained being processed by hot deformation by forging or hot rolling or a combination there of and converted into hot rolled sheets.

Another aspect of the present invention is directed to the process comprising subjecting to hot deformation at a temperature in the range of 1100 to a finish deformation temperature of 900oC followed by air cooling;
reheating the steel air cooled to room temperature to a reverted austenite formation temperature within the range of intercritical temperature of the steel and held for a brief duration of 1 to 10 minutes preferably about 5 minutes;
with steel heated to a specific temperature above Ac1 for promoting the reformation of retained austenite, the retained austenite so formed is stabilized by the partitioning of carbon and manganese in the austenite.

Another aspect of the present invention is directed to the process comprising selective casting and hot deformation and subjected to reverted austenite heat treatment to form reverted austenite content varying between 9.5% and 39.5%, the secondary phase being primary martensite wherein the retained austenite content varied as a function of reverted austenite holding temperature, the austenite content varied between 9.5 to 39.5% by varying the temperature with the formation of secondary martensite wherein the residual reverted austenite content peaked at temperature between 680 and 720oC with value of retained austenite from 28% to as high as 39%.

Yet another aspect of the present invention is directed to the process wherein selective steel making, casting and hot deformation processing and heat treatment and formation of reverted austenite provide a versatile range of mechanical properties, with the steel reheated between 660 to 720oC, with high retained austenite content in the range of 26.8 to 39.5% providing yield strength between 337 and 647 MPa and tensile strength between 1010 and 1410 MPa and elongation between 14 and 31%.

A still further aspect of the present invention is directed to the process including the selective steel making, casting and hot deformation processing heat treatment and formation of reverted austenite at a reverted transformation austenite % at a temperature of 660 and 680oC shows yield strength between 563 to 647 MPa and tensile strength in the range 1010 to 1036 MPa and total elongation 22 to 31% with generation of the product of strength and elongation is between 22 to 32 GPa % thus conforming to Third Gen AHSS.

Another aspect of the present invention is directed to the process including selective steel making, casting and hot deformation processing formation of reverted austenite, maximum austenite formation at a reverted austenitization temperature of 700 to 720oC where the high austenite content along with tempered martensite gives lowest yield strength between 337 and 350 MPa while the tensile strength show 1258 to 1410 MPa with elongation 29 to 14% and wherein the product of tensile strength and ductility values are 37 to 20 GPa % conforming to Third Gen AHSS.

Yet another aspect of the present invention is directed to the process comprising selective steel making, casting and hot deformation processing, heat treatment adopted and formation of reverted austenite and mechanical properties have high yield ratio in the range of 0.54 to 0.64.

Another aspect of the present invention is directed to the process comprising the selective steel making, casting and hot deformation processing, heat treatment and formation of reverted austenite and mechanical properties having low yield ratio in the range of 0.25-0.27.

The advancement is described here under in greater detail in relation to the following non-limiting exemplary illustrations as per the accompanying figures wherein:
Figure 1: Heat Treatment Cycle of medium manganese steel for ART
Figure 2: Phase fraction calculation through Thermocalc software (2020b).
Figure 3: SEM micrograph of (a) as-received, (b) 600oC/5min/AC, (c) 680oC/5min/AC, (d) 700oC/5min/AC, (e)760oC/5min/AC, (f) 800oC/5min/AC.
Figure 4: XRD on selected samples for the retained austenite evaluation.
Figure 5 (a) Engineering stress-strain diagram of the medium manganese steel at selected annealing condition (b) Change in the YS, UTS and total elongation with annealing temperature.

DETAILED DESCRIPTION OF THE INVENTION
In the present study, a medium Mn steel with 0.17 % C, 6.67% Mn and 0.78 % Al was hot forged from a cast ingot, was air cooled. Due to the high alloying content, the C-curve in the CCT diagram is shifted to the right and even on air cooling martensite is promoted. The martensite is lath type as carbon content is 0.17%. Thus, initially the steel after hot forging was air cooled to form martensite. This is followed by reheating the steel to a temperature in the inter-critical range between 600 and 800oC at a step interval of of 20oC. Reheating nucleates austenite along the martensite laths and is stabilized by the high Mn content. Thus, the austenite formed along the martensite lath results in high retained austenite in the matrix after the steel is air cooled to room temperature. The residual austenite at certain condition of processing leads to high mechanical properties that satisfy the third Gen AHSS.

EXAMPLES
The chemical composition of the steel in the present invention is summarized in Table-1. The steel has carbon content in the range 0.1 and 0.2%, which ensures that the steel developed has good formability and weldability. The low carbon content promotes lath martensite in the matrix. The steel is alloyed with 6 to 7% Mn content which results significant enlargement of austenite field that results in austenite formation at lower austenitization temperature as the Ac1 decreases with increase in Mn content. In addition, the high Mn content shifts the CCT curve to the right and even on air cooling the steel forms martensite phase. The steel is further alloyed with Al in a range between 0.5 and 1% which increases the Ms temperature and suppresses the cementite precipitation that retains the austenite phase.

The austenite reverted transformation heat treatment cycle of the steel is shown in Fig.1. It involves initial hot forging of the steel at about 1150oC with a forge finish temperature of the order of 900oC. Air cooling the steel in this condition to room temperature promotes the formation of lath martensite. The 2 to 5 mm thick sheets air cooled after hot deformation, forms martensite. The martensite fraction of close to 94% and about 6% retained austenite remains in the steel theoretically.
Reheating the steel above Ac1 temperature increases the austenite content. To assess the austenite content theoretically, the phase constitution can be assessed from Thermocalc software. The equilibrium phases formed for the steel is given in Fig.2. The Ac1 temperature is below 400oC and the Ac3 temperature is 752oC. Hence, at equilibrium conditions, some austenite is retained at low temperatures. Reheating the steel to temperatures 600 to 740 oC progressively increases the retained austenite content from 4% to 92%. Above 752oC, 100 % austenite would be formed. The Ac1temperature was found to be less than 400oC and the Ac3 temperature is752oC. The steel was reheated to temperatures between 600 and 740oC at 20oC interval to generate increasing austenite content from 18.7% to 92% in the inter-critical temperature as per Thermocalc data in Fig.2. The final austenite that remains on further air cooling is given in Table 2. Air cooling results in transformation of some of austenite back to martensite.
The SEM micrographs of the steel in the initial as-forged and air cooled condition shows fully lath martensitic microstructure Fig.3(a). The retained austenite content of the forged steel does not show detectable austenite as seen in XRD data in Fig. 4, which implies that the retained austenite is less than 5%. The steel in this condition is reheated to temperature in the inter-critical range, when the steel is reheated in the range 600oC to 800oC and held for 5 min followed by air cooling the microstructure showing reverted austenite as shown in Figs. 3(b)-(f). The XRD data in Fig.4 for the corresponding conditions show that the reverted austenite content has a peak value at 700oC in the temperature range between 600 and 800oC as in Table 2. The microstructure shows martensite laths and the austenite is nucleated along the lath boundaries.

The engineering stress-strain diagram of the steel heat treated at varying second annealing temperature of 6.67% Mn Medium Mn steel is shown in Fig.5 (a). The steel gives a versatile range of mechanical properties, as in Table 3. The mechanical properties as a function of second annealing temperature where reverted austenite formed is given in Fig. 5(b). It is seen that the there is a gradual fall in yield strength from 681 MPa at 600oC to 663 MPa at 680oC temperature. There is a sudden fall in strength between 660 and 740oC. The yield strength has lowest value at 337 to 350 MPa at temperature between 700 to 720oC. Beyond 740oC, the yield strength is restored to 560 to 630 MPa. The tensile strength does not follow the trend. The tensile strength gradually decreases from 1100 MPa at 600oC to 1036 MPa at 700oC. Beyond 720oC annealing temperature, the tensile strength increases from 1258 MPa at 720oC to a peak value of 1568 MPa at 760oC. Beyond 760oC, the tensile strength decreases to 1436 MPa at 800oC. Depending on the second annealing temperature, the elongation has a significantly high value between 640 and 740oC. The elongation value between 600 to 640 is around 18% and between 640 and 720oC, it peaks at 32 to 37% elongation. The elongation is at a value lower than 10% between 740 to 800oC. The peak value in strength and lowering of yield strength and enhancement of elongation is attributed to the peak value of retained austenite seen as shown in Table 2.
Very good combination of strength and elongations were found when the steel in annealed at a temperature range of 660oC to 620oC, where the product of strength and elongation exceeds 23 GPa % to peak values 32 and 37 GPa % at annealing temperature of 680 and 700oC. However, the yield strength is 563 MPa at 680oC, which qualifies for third Gen AHSS.

Table-1: Chemical composition of the medium manganese steel in wt.%.
C Mn Al Si S P Cr Ni Cu
Range 0.10-0.20 6.0-7.0 0.5-1.0 0.2-0.5 0.03 max 0.03
max 0.1
max 0.1
max 0.07
max
Actual 0.17 6.67 0.78 0.30 0.001 0.013 0.085 0.011 0.029

Table-2: Retained austenite evaluated through XRD.
Heat Treatment Condition of the Steel Austenite at the second austenitization temperature, oC Holding time
min

Retained Austenite after cooling the steel to room temperature (%)
As forged 4
600 18.71 5 13.29
680 40.0 5 28.5
700 46 5 39.5
720 53 5 26.8
780 71 5 9.5
800 92 5 7.4

Table-3: Mechanical Properties of the steel at various ART annealing temperatures.
Temperature
of second annealing Thermocalc based
Austenite Final Austenite reversion, % Yield strength
(MPa) Tensile strength
(MPa) Total Elongation,
(%) Yield Ratio
UTS.%E
(GPa.%)

As Forged 4 573 1377 14.07 0.42 19.37
600 32 13.29 681 1100 17.70 0.62 19.47
620 40 - 673 1100 17.30 0.61 19.03
640 46 - 645 1050 17.90 0.61 18.80
660 53 - 647 1010 22.60 0.64 22.83
680 61 28.5 563 1036 30.97 0.54 32.07
700 71 39.5 337 1258 29.60 0.27 37.24
720 81 26.8 350 1410 14.44 0.25 20.35
740 92 - 562 1492 7.47 0.38 11.15
760 100 - 595 1568 9.17 0.38 14.38
780 100 9.5 630 1462 6.74 0.43 9.85
800 100 7.4 577 1436 10.37 0.40 14.89

ADVANTAGE OF THE INVENTION:
The Low carbon Medium manganese steel having 0.17% C, 6.67% Mn and 0.78% Al for automotive applications with enhanced mechanical properties in steel, on austenite reverted transformation showing high levels of retained austenite in a tempered martensite structure that improves mechanical properties in terms of tensile strength and ductility are best suited for automotive applications , Claims:We Claim:
1. A Low carbon Medium manganese steel having composition including major alloying elements comprising of carbon content between 0.10 and 0.2%; Mn content between 6 to 7% and Al content between 0.5 and 1.0 and optionally having residual elements S less than 0.03%, P less than 0.03%, Si content between 0.2 and 0.5 %, Cr and Ni content each less than 0.1%, and Cu less than 0.07%.

2. The low carbon medium manganese steel as claimed in claim 1 having austenite content as a function of reverted austenite holding temperature variable between 9.5 to 39.5% and secondary martensite.

3. The low carbon medium manganese steel as claimed in anyone of claims 1 or 2 having said temperature dependent variable reverted transformation austenite % and related variable strength characteristics selected from :

(i) Which is a reheated steel between 660 to 720oC having high retained austenite content in the range of 26.8 to 39.5% show yield strength between 337 and 647 MPa and tensile strength between 1010and 1410 MPa and elongation between 14 and 31%.

(ii) austenite % at a temperature of 660 and 680oC showing yield strength between 563 to 647 MPa and tensile strength in the range 1010 to 1036 MPa and total elongation 22 to 31%, the product of strength and elongation is between 22 to 32 GPa % conforming to Third Gen AHSS and having high yield ratio in the range of 0.54 to 0.64.

(iii) reverted austenitization temperature of 700 to 720oC where the high austenite content along with tempered martensite gives lowest yield strength between 337 and 350 MPa while the tensile strength show 1258 to 1410 MPa with elongation 29 to 14% with the product of tensile strength and ductility values are 37 to 20 GPa % conforming to Third Gen AHSS and having low yield ratio in the range of 0.25-0.27.

4. A process for manufacture of Low carbon Medium manganese steel as claimed in anyone of claims 1 to 3 comprising:

(a) Providing said select steel composition;
(b) Melting in any primary and secondary steel making processes including Basic oxygen furnace, Electric arc furnace followed by secondary steel making lade furnace and also through induction furnace with graded alloying;
(c) Casting as ingot or in continuous casting as slabs;
(d) the slabs thus obtained being processed by hot deformation by forging or hot rolling or a combination there of and converted into hot rolled sheets.

5. The process as claimed in claim 4 comprising subjecting to hot deformation at a temperature in the range of 1100 to a finish deformation temperature of 900oC followed by air cooling;

reheating the steel air cooled to room temperature to a reverted austenite formation temperature within the range of intercritical temperature of the steel and held for a brief duration of 1 to 10 minutes preferably about 5 minutes;
with steel heated to a specific temperature above Ac1 for promoting the reformation of retained austenite, the retained austenite so formed is stabilized by the partitioning of carbon and manganese in the austenite.

6. The process as claimed in anyone of claims 4 or 5 comprising selective casting and hot deformation and subjected to reverted austenite heat treatment to form reverted austenite content varying between 9.5% and 39.5%, the secondary phase being primary martensite wherein the retained austenite content varied as a function of reverted austenite holding temperature, the austenite content varied between 9.5 to 39.5% by varying the temperature with the formation of secondary martensite wherein the residual reverted austenite content peaked at temperature between 680 and 720oC with value of retained austenite from 28% to as high as 39%.

7. The process as claimed in anyone of claims 4 or 5 wherein selective steel making, casting and hot deformation processing and heat treatment and formation of reverted austenite provide a versatile range of mechanical properties, with the steel reheated between 660 to 720oC, with high retained austenite content in the range of 26.8 to 39.5% providing yield strength between 337 and 647 MPa and tensile strength between 1010 and 1410 MPa and elongation between 14 and 31%.

8. The process as claimed in anyone of claims 4 or 5 including the selective steel making, casting and hot deformation processing heat treatment and formation of reverted austenite at a reverted transformation austenite % at a temperature of 660 and 680oC shows yield strength between 563 to 647 MPa and tensile strength in the range 1010 to 1036 MPa and total elongation 22 to 31% with generation of the product of strength and elongation is between 22 to 32 GPa % thus conforming to Third Gen AHSS.

9. The process as claimed in anyone of claims 4 or 5 including selective steel making, casting and hot deformation processing formation of reverted austenite, maximum austenite formation at a reverted austenitization temperature of 700 to 720oC where the high austenite content along with tempered martensite gives lowest yield strength between 337 and 350 MPa while the tensile strength show 1258 to 1410 MPa with elongation 29 to 14% and wherein the product of tensile strength and ductility values are 37 to 20 GPa % conforming to Third Gen AHSS.

10. The process as claimed in anyone of claims 4 or 5 comprising selective steel making, casting and hot deformation processing, heat treatment adopted and formation of reverted austenite and mechanical properties have high yield ratio in the range of 0.54 to 0.64.

11. The process as claimed in anyone of claims 4 or 5 comprising the selective steel making, casting and hot deformation processing, heat treatment and formation of reverted austenite and mechanical properties having low yield ratio in the range of 0.25-0.27.

Dated this the 21st day of July, 2023
Anjan Sen

Of Anjan Sen & Associates

(Applicants Agent)

IN/PA-199