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 :
A LOW CARBON LEAN ALLOYED ULTRA HIGH STRENGTH QUENCH AND PARTITIONED STEEL FOR CRASH RESISTANCE APPLICATION AND METHOD OF PRODUCING THE SAME.



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
Present invention relates to a low carbon lean alloyed high yield strength (>660MPa), ultra high ultimate tensile strength steel (> 1200 MPa) of hot rolled product with very good ductility (>17% elongation)conforming to third generation advanced high strength steel (AHSS)and a method of producing the same. The steel with (wt. %) 0.17% carbon, 1.35% Si and 1.73% Mn, 0.04%Ti and 0.02% Nb produced by conventional steel making process and rolled to hot rolled product of thickness in the range of 2 to 5mm for the automotive crash resistance applications. The steel so produced subjected to single stage quench and partitioning (Q&P) heat treatment after austenization above AC3 temperature followed by salt bath quenching below the martensitic start temperature. The steel having microstructure predominantly of tempered martensite with some amount of retained austenite.
The steel subjected to Q&P achieved yield strength between 660-710MPa, ultimate tensile strength between 1200 to 1320MPa to achieve tensile toughness greater than 20GPa.%. The steel properties are in the third generation regime for the automotive applications suitable for the weight reduction to improve fuel efficiency, reduce CO2 emission and greater safety by the crash resistance property.

BACKGROUND OF THE INVENTION
Third generation advanced high strength steels (AHSS) are growing demand in the automotive industry for their excellent combination of strength and ductility to improve fuel efficiency, reduce CO2 emissions and provide greater safety to the passenger by excellent crash resistance properties. According to the third generation AHSS steel definition the steel should have tensile toughness (product of ultimate tensile strength and total elongation) in the range of 20GPa. % to 40GPa.%. TRIP aided bainitic ferrite (TBF), Medium manganese TRIP, Delta TRIP and Quench and partitioned (Q&P) steels fall in the third generation AHSS categories. In the present invention a low carbon steel alloyed with Si and Mn with micro alloying of Nb and Ti produced through conventional steel making and hot rolled in the range of 2 to 5mm has been used to achieve third generation AHSS properties through single stage Q&P heat treatment. Single stage Q&P is much simpler to implement in industry compared to two stage Q&P cycle.

Prior art
CN113061698A prior art reports invention of Q&P steelfrom a chemical composition as mentioned in Table-1, which clearly distinguish and compares the differences of the prior art with the proposed inventions. The present invention differs in chemical composition from said prior art w.r.t several alloying elements such as Mn, Cr, Ni, Mo and V. The reference prior art has used alloying elements (like Cr, Ni and Mo) and processed through Q&P treatment to achieve the required mechanical properties. However, in the proposed invention these alloying elements are not present for developing Q&Psteel.Moreover the processing conditions in both the processes are entirely different. The prior art proposed 2 stage heat treatment process as mentioned in detail: rapidly transferring the sample from the salt bath at 770 ? to an oil bath at 130 ?, rapidly cooling the sample to 130 ?, heatingthe sample to 400 ?, preserving the heat for 500s, and then cooling to room temperature. However in the proposed invention our methodology adoption is different as shown in figure 2. So, the prior art CN113061698Adiffers in both chemical composition of the steel material as well as the processing conditions to achieve Q&P steel. Moreover, the mechanicalproperties (UTS) in the prior art is 1600-2000MPa where as in the present invention 1200-1320MPa.
CN111440987A prior art reports invention of Q&P steelfrom a chemical composition as mentioned in the Table-1, which clearly distinguish and compares the differences of the prior art with the proposed inventions. The composition of the steel is close to the present composition, however, the prior art invention contains V whereas in the present invention Nb is the micro-alloying which differs from the prior art invention.In this prior art the inventors mentions the use of 2 stage heat treatment where they anneal the steel materials at 820-930 ? followed by slowly cooling to 660-700 ? at a cooling speed of 5-10 ?/s which further cools rapidly to 200-300 ? at a cooling speed of not lessthan 60 ?/s which further heated to 360-460 ? for 150-550 s to distribute the temperature of 360-460 ? for 150-550 s and finally cooled to room temperature. However in the proposed invention our methodology adoption for Q&P heat treatment is different as shown in figure-2. So, the prior art CN111440987Adiffers in both chemical composition of the steel material as well as the processing conditions to achieve Q&P steel. Moreover, the mechanicalproperties reported in the prior art is close to 980MPa tensile strength whereas in the present invention it is quite superior in the range of 1200-1320MPa.
CN103820613A prior art reports invention of Q&P steel from C-Mn-Al series TRIP 590 steel whereas in the present invention Si containing steel having Ti and Nb as micro alloying elementis used. The tensile toughness reported in the prior art is 21-22 GPa% whereas in the present invention it is >20GPa%.In the prior art they used 2 stage heat treatment and their heat treatment processes includepre-processed TRIP590 cold-roll steel sheets were annealed at 1,000-1,150 °C for 2-5 min followed by rapidly cooling the steel sheets to 200-220 °C for 10-30 s followed by quenching. The quenched steel was then rapidly heatedto 350-370 °C for 30-60 s and finally, water quenched. The austenization temperature used in the prior art invention is 1000 to 1150oC which is much higher compared to the present invention (900oC).However in the proposed invention our methodology adoption for Q&P heat treatment is different as shown in figure -2. So, the prior art CN103820613A differs in both chemical composition of the steel material as well as the processing conditions to achieve Q&P steel.
KR101694875B1 prior art reports invention of Q&P steel from a chemical composition as mentioned in the Table-1, which clearly distinguish and compares the differences of the prior art with the proposed inventions. The composition of the steel is close to the present composition except the additional Nb micro alloy addition as compared to the prior art invention. The heat treatment process used in prior art is heating the rolled pieces at a temperature of 800 to 900 ° C are rapidly cooled to 500 to 600 ° C at a cooling rateof 50 ° C / s, air-cooled for 5 to 10 seconds followed by cooling to Ms-Mf temperature range at a cooling rate of more than 50 ° C to obtain a structureof pro-eutectoid ferrite, martensite and retained austenite. Finally, after reeling, the steel is slowly cooled to room temperature toobtain a high strength hot rolled Q & P steel. However, in the proposed invention the steel is heated in at 900 °C followed by cooling below 400 ° C which is different than the temperature of 500-600 °C cited in the reference prior art. The detailed heat treatment process of the proposed invention is shown as a graphical form in Figure 2. The properties reported in the prior art invention with yield strength 700MPa, tensile strength 1300MPa and total elongation 10% or more with 20% retained austenite whereas in the proposed invention the properties such as yield strength 650-700MPa, ultimate tensile strength 1200-1320MPa and total elongation in the range of 16 to 17% with retained austenite content in the range of 6-12%.The prior art production process is completely different compared to the present invention.
CN102766818B prior art reports invention of Q&P steel from a chemical composition as mentioned in the Table-1, which clearly distinguish and compares the differences of the prior art with the proposed inventions. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Mn, Al, Cr, Mo and Nb content. Mn and Al content in the prior art is significantly different than the proposed invention. The Cr and Mo is intentionally added to the cited prior art whereas Nb is an intentionally added alloying element in the proposed invention. The cited reference patent have boron content in the range of 0.002-0.004%. The Si & Al content is added in such a way that its total content should be greater than 1% (Si + Al > 1%). In the heat treatment of the cited prior art, steel sample is heated to 950 ? in salt bath furnace for 30 minutes duration for complete austenitizing followed by oil quench to room temperature. The quenched steel is heated again to a temperature of 450 ? for 8-10 seconds for carbon partitioning followed by quenching at a cooling rates of 38 ?/s. The chemical composition and Q&P process cited in reference prior art is significantly different than the proposed invention with different level of mechanical properties.
CN103805851A prior art reports invention of Q&P steel from a chemical composition as mentioned in the Table-1, which clearly distinguish and compares the differences of the prior art with the proposed inventions. The chemical composition of steel cited in the reference prior art is almost similar except the Ti and Nb content of the proposed invention. Ti is added intentionally to a higher value than the cited prior art to restrict the grain growth of austenite phase and thus achieve finer martensite with enhanced properties. Nb is also added intentionally to the proposed invention.
In the claimed prior art, the Q&P steel is manufactured through hot rolling process where heating Strand or ingot heating, 1100 ~ 1200 ? of Heating temperatures, soaking time 1 ~ 2 hour followed by hot rolling 1000 ~ 1070? of start rolling temperatures, more than 950 ? and accumulative total deflection >=50%, subsequentlyintermediate blank treat temperature to 800 ~ 850 ?, then carry out last 3 ~ 6 passage rollings followed by press quenching Finish to gaugefinishes rear steel plate and is cooled to 150 ~ 250 ? with the cooling rate of >50 ?/s, obtain martensite+residual austenite bodytissue, after finally batching, slowly cool to room temperature. The UTS reported >1400MPa and elongation >10% reported in the prior art whereas in the present invention UTS in the range of 1200-1320MPa and elongation in the range of 16-17%. Based on the above mentioned details the proposed patent is different than the prior art cited document.
JP2019504202A prior art reports invention of Q&P steel from a chemical composition as mentioned in the Table-1, which clearly distinguish and compares the differences of the prior art with the proposed inventions. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Cr, Mo and V content. The Cr, Mo and V is intentionally added to the cited prior art which is different than proposed invention.In the claimed prior art, the Q&P steel is manufactured through hot rolling process where as in the proposed invention the Q&P process adopted is a heat treatment through salt bath single stage heat treatment process, as mention in figure-2.Based on the above mentioned details the proposed patent is different than the prior art cited document.
In one of the publications (Quenching and Partitioning Steel Heat Treatment, Li Wang and John G. Speer, Metallogr. Microstruct. Anal. (2013) 2:268–281, DOI 10.1007/s13632-013-0082-8) mentioned the Q&P treatment with a different chemical composition than the proposed invention as mentioned in the Table-1. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Ti and Nb content. The Ti and Nb is intentionally added in the proposed invention to achieve Q&P steel which differs from the cited prior art. In the claimed prior art, the Q&P steel is manufactured throughheat treatment where steel is heated to a temperature above Ac3 (annealing temperature) followed by slow cooling to a temperature below Ar3 (~ 740 ?) followed by quenching to a temperature between Ms and Mf with a cooling rate higher than 50 ?/s.Based on the above mentioned details the proposed patent is different than the prior art cited document.
In one of the publications (Thermal Stability of Austenite and Properties of Quenching &Partitioning (Q&P) Treated AHSS, R. M. Wu, L. Wang and X. J. Jin, Physics Procedia 50 (2013) 8 – 12) mentioned the Q&P treatment with a different chemical composition than the proposed invention as mentioned in the Table-1. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Ti and Nb content. The Ti and Nb is intentionally added in the proposed invention to achieve Q&P steel which differs from the cited prior art. In the claimed prior art, the Q&P steel is manufactured throughheat treatment where steel was austenitized at 900? for 5 minutes, followed by quenching into salt bath at 320? for 60 seconds, then furtherquenched into water at room temperature. Though the heat treatment process is near about same w.r.t the heating and quenching temperature, the holding time for carbon participation is different than the proposed invention. Based on the above mentioned details the proposed patent is different than the prior art cited document.
In one of the publications (The effect of pre-quenching process on microstructure and mechanical properties in a Nb-microalloyed low carbon Q-P steel, Jun Zhang, Hua Ding, Jingwei Zhao, University of Wollongong, Research Online) mentioned the Q&P treatment with a different chemical composition than the proposed invention as mentioned in the Table-1. The chemical composition of steel cited in the reference prior art differs with the proposed invention w.r.t the Ti content. The Timicro alloying element is intentionally added in the proposed invention to achieve Q&P steel which differs from the cited prior art.
In the claimed prior art, the Q&P steel is manufactured through heat treatment process where two different routes are adopted to achieve Q&P steel and thus named as two groups based on heat treatment process (group 1 & 2). The first group was austenitized at 910 ? for 3 min, and then quenched to 220 ?. Subsequently, these quenched specimens wereheld at 400 ? in a time range from 5s to 500s. Finally, a second quenching process was carriedout to room temperature. The second group was treated by a pre-quenchingprocess, from 910 ? to room temperature. Then the samples were austenitized at 850 ? for 3 min followed by quenched to 220 ? and partitioned at400 ? from 5s to 500s. Finally, the partitioned samples were quenched to room temperature. The heat treatment process adopted in the proposed invention is quite different than the cited prior art. Based on the above mentioned details the proposed patent is different than the prior art cited document.

OBJECTS OF THE INVENTION
The object of the invention is to develop a low carbon (wt. %) (<0.17%C) lean alloyed steel preferably with1.35% Siand 1.73% Mn, micro alloyed with 0.04% Ti and 0.02% Nb processed to have hot rolled product of thickness in the range of 2-5mm which when subjected to single stage quench and partitioning heat treatment after austenization above Ac3 temperature provide third generation advanced high strength steel properties with high yield strength ultra high ultimate tensile strength (greater than 1200MPa) and tensile toughness greater than 20GPa, suitable for crash resistance applications.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a low carbon lean alloyed steel having composition comprising by wt. % 0.03 to 0.3% and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01and 0.06% preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04% ; Al content between 0.04 and 0.08% preferably about 0.06%; with < 0.005 preferably 0.002 to 0.004 wt.% S and , 0.03% preferably 0.015 to 0.017 wt.% P, and 0.003-0.006% N and balance iron, based quench and partitioned heat treated third generation advanced high strength steel(AHSS) properties steel.

A further aspect of the present invention is directed to said low carbon lean alloyed steel having yield strength in the range of 650 to 700MPa, ultimate tensile strength of 1200 to 1300MPa with total elongation of 15-17% to achieve tensile toughness greater than 20GPa%.

A still further aspect of the present invention is directed to said low carbon lean alloyed steel having thickness in the range 2 to 5mm with a microstructure comprising pre-dominently martensite microstructure with 75 to 85% martensite and retained austenite in the range of 6-12% by volume fraction.

A still further aspect of the present invention is directed to said low carbon lean alloyed steel wherein the microstructure and mechanical properties achieved conform to third generation Advanced High Strength Steels with higher yield ratio (0.59-0.65) adapted for automotive components including crash resistance applications.

Another aspect of the present invention is directed to a process to produce low carbon lean alloyed hot rolled steel sheet comprising
(i) providing seelctive lean alloy steel of composition preferably comprisingby wt. % 0.03 to 0.3% and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01and 0.06% preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04% ; Al content between 0.04 and 0.08% preferably about 0.06%; with < 0.005 preferably 0.002 to 0.004 wt.% S and , 0.03% preferably 0.015 to 0.017 wt.% P, and 0.003-0.006% N and balance iron,
(ii) carrying out conventional steel making through basic oxygen furnace and ladle furnace, continuous casting route followed by hot rolling to have product of thickness in the range 2 to 5mm;
(iii) subjecting said hot rolled steel to quench and partitioning heat treatment cycle involving austenitization above Ac3 temperature at 900oC for 2-5 min followed by quenching in salt bath at 380-390oC and preferrably 387oC for a time period of 1-5min and water quenched to 20 oC to achieve desired microstructure and AHSS properties.

A further aspect of the present invention is directed to said process wherein said steel making comprising primary steel making process using hot metal from blast furnace in a EAF or BOF furnace, followed by secondary steel making process using a Ladle steel making route, where the desired composition was achieved with ferro alloy addition, followed by casting the same through continuous casting in slab caster, hot rolled the cast slab to produce steel strip with thockness between 2 to 5 mm withyield strength 670-690 MPa, ultimate tensile strength in the range of 840 to 890MPa, total elongations in the range of 11.5 to 12.5 %, yield ratio in the range of 0.78 to 0.80 and tensile toughness (product of ultimate tensile strength to total elongation) between 10.25- to 10.51GPa%.

A still further aspect of the present invention is directed to said process wherein (i) austenitizzed at above Ac3 temperature at 900oC for 2-5min followd by Q&P at 262, 330 and 387oC the steel produced retained austenite in thin film form in the range of 3-15% with tempered martensitic microstructure and (ii) when austenitized above Ac3at 900oC for 2-5 min followed by quenching in salt bath at 380-390oC and preferrably 387oC for a time period of 1-5min and water quenched to get pre-dominently martensite microstructurein the range of 75 to 85% with retained austenite in the range of 6-12% to give yieldstrength in the range of 650 to 700MPa, ultimate tensile strength of 1200 to 1300MPa with total elongation of 15-17% to achieve tensile toughness greater than 20GPa.%.

A further aspect of the present invention is directed to saidprocess wherein said hot rolling was carried out comprising
Slab Reheating temperature 1250oC,
Roughing Mill temperature: 1210oC,
Finishing temperature: 890oC,
Coiling Temperature: 570oC,
No of passes: 6.

The above 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: Pseudo-binary phase diagram of the steel with the phase field as a function of temperature and carbon content.
Fig. 2: Single stage quench and partitioning(Q&P) heat treatment cycle for the steel.
Fig. 3: Optical Microstructure of the steel subjected to different Q&P heat treatment.
Fig. 4: SEM micrograph of the steel subjected to different Q&P heat treatment cycle.
Fig. 5: XRD of the steel subjected to Q&P heat treatment.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to the development of a low carbon lean alloyed steel composition range and typical composition as per Table 1, adapted for a quench and partitioning heat treatment cycle, to achieve the Third Gen Advanced High Strength Steel with tensile toughness greater 20 GPa.% and 25GPa% when subjected to austenitization above Ac3 or in the intercritical range respectively. The steel used is comprised of of 0.03 to 0.20 wt. % carbon preferably 0.17%C; 1.5 to 1.8 wt. % Mn, preferably 1.73% Mn; 1.0 to 1.5% Si with Si content of 1.35% ; microalloyed with 0.01 to 0.04% Nb and 0.03 to 0.05 wt.% Ti, and 0.003-0.006% N. The residual Al varied between 0.04 and 0.08%Al. The residual element Al is maintained in the range 0.04 to 0.08 wt. % Al, and 0.002 to 0.004 wt.% S and 0.015 to 0.017 wt.% P.
Steel composition, processing details and properties achieved are given in following tables:
Table 1: Chemical composition (wt. %) and critical temperatures (oC) of the steel.
Table-2: Steel Making and Hot Rolling Parameters
Table 3: Mechanical properties of the steel and retained austenite content subjected to Q&P heat treatment cycle.
The base steel produced in the present invention through primary steel making process using hot metal from blast furnace in a EAF or BOF furnace, followed by secondary steel making process using a Ladle steel making route, where the desired composition was achieved with ferro alloy addition, followed by casting the same through continuous casting in slab caster, hot rolled the cast slab to produce steel strip between 2 to 5 mm withyield strength 670-690 MPa, ultimate tensile strength in the range of 840 to 890MPa, total elongations in the range of 11.5 to 12.5 %, yield ratio in the range of 0.78 to 0.80 and tensile toughness (product of ultimate tensile strength to total elongation) between 10.25- to 10.51GPa%.
Another aspect of the present invention is directed to a process for manufacture of the lean alloyed steel including processing. The said lean alloyed steel composition adapted for variable thermal processing comprising of 0.03 to 0.20 wt. % carbon preferably 0.17% C, Mn 1.5 to 1.8 wt. % preferably 1.73% Mn, Si 1 to 1.5 wt.% preferably 1.35%Si, Nb 0.01 to 0.04 wt. %, 0.03 to 0.05 wt.% Ti preferably 0.04%Ti, and less than 0.003-0.006% N, 0.04 to 0.08 wt. % Al, 0.002 to 0.004 wt.% S and 0.015 to 0.017 wt%P under primary steel making process, followed by secondary steel making process including ferroalloy addition, followed by casting the same through continuous casting in slab caster to produce hot rolled strip between 2-5mm thickness.
Another aspect of the present invention is the design of heat treatment cycle where the single stage quench and partitioning (Q&P)is carriedoutwith austenization above Ac3 temperature at 900oC for 2-5 min followed by quenching in salt bath between260-390oC preferrably at 262, 330 and 387oC for a time period of 1-10min and water quenched with the patenting temperature range of partitioning between 380-390oC preferrably 387oC followed by water quenching.
The lean steel austenitized at 900oC for 2-5min followd by Q&P at 262, 330 and 362oC the steel contains retained austenite in the range of 1-16% with 70-80% martensite and 10-15% bainite. When austenitizzed at above Ac3 temperature at 900oC for 2-5min followd by Q&P at 262, 330 and 387oC the steel contains retained austenite in thin film form in the range of 3-15% with tempered martensitic microstructure.
The steel when austenitized above Ac3 at 900oC for 2-5 min followed by quenching in salt bath at 380-390oC and preferrably 387oC for a time period of 1-5min and water quenched to get pre-dominently martensite microstructure with retained austenite in the range of 6-12% to give yield strength in the range of 650 to 700MPa, ultimate tensile strength of 1200 to 1300MPa with total elongation of 15-17% to achieve tensile toughness greater than 20GPa.%.

The chemical composition of the steel is shown in Table-1. The composition of the steel can be mapped from the phase diagram shown in Fig. 1. The pseudo binary phase diagram gives the composition helps to estimate the equilibrium phases in the inter critical temperature range by Lever Rule. The steel making, continuous casting and hot rolling parameters are summarized in Table-2. The Q&P heat treatment cycle applied to the steel is shown in Fig. 2. The optical microstructure of the Q&P steel is shown in Fig. 3 and the SEM micrograph in Fig. 4. The XRD of the steel at different Q&P cycles is shown in Fig.5. The mechanical properties of the steel along with the retained austenite content obtained through XRD are summarized in Table-3.

Table 1 Composition of the steel ( wt.%) of present invention
S. No Steel composition Details C Mn Si Al Ti Cr Ni Mo Nb V S P N
Wt %
1 Selected
range of elements 0.03-0.2 1.0-1.8 1-1.5 0.040-0.08 0.03-0.05 0.01-0.04 0.002-0.004 0.015-0.017 0.003-0.006
2 Typical sample 0.17 1.73 1.35 0.06 0.04 0.02 0.003 0.016 0.005

Table 2: Steel Making and Hot Rolling Parameters
Steel making Making Blasite furnace Hot metal ? 180 ton Basic oxygen furnace ? Ladle steel making
Continuous casting Slab size (Length *Width*thick=7250*1270*220 mm3)
Hot rolling Slab Reheating temperature 1250oC
Roughing Mill temperature: 1210oC
Finishing temperature: 890oC
Coiling Temperature: 570oC
No of passes: = 6

Table-3: Mechanical Properties of the steel
Aust. Temp. oC Q&P Temp. oC Aust. Time, min YS, MPa UTS, MPa TE, % YR UTS*TE, GPa.% Retained Austenite, %
As-Rolled 670±20 865±25 12.00±0.50 0.79±0.01 10.38±0.13 4
900
5min 262 1 669±17 1434±28 16.54±0.32 0.50±0.01 23.72±0.01 3.2
5 706±11 1411±13 16.05±0.28 0.53±0.01 22.65±0.19 3.85
10 746±18 1401±23 16.10±0.29 0.55±0.01 22.56±0.04 3.81
15 754±15 1380±32 15.73±0.33 0.50±0.01 21.70±0.05 5.22
330 1 603±17 1198±21 12.90±0.37 0.55±0.01 15.46±0.17 6.97
5 649±21 1171±18 14.82±0.39 0.61±0.02 17.35±0.19 8.44
10 734±22 1212±27 15.25±0.55 0.65±0.01 18.48±0.25 6.78
15 742±22 1133±12 13.13±0.40 0.50±0.02 14.88±0.30 6.54
387 1 661±12 1321±19 15.67±0.52 0.59±0.01 20.69±0.39 12.27
5 711±16 1198±31 16.91±0.22 0.65±0.01 20.27±0.26 5.84
10 739±14 1130±19 11.91±0.29 0.70±0.01 13.46±0.10 15.22
15 888±30 1262±25 12.04±0.36 0.47±0.02 15.19±0.15 12.86
, Claims:We Claim:

1. A low carbon lean alloyed steel having composition comprising by wt. % 0.03 to 0.3% and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01and 0.06% preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04% ; Al content between 0.04 and 0.08% preferably about 0.06%; with < 0.005 preferably 0.002 to 0.004 wt.% S and , 0.03% preferably 0.015 to 0.017 wt.% P, and 0.003-0.006% N and balance iron, based quench and partitioned heat treated third generation advanced high strength steel(AHSS).
2. The low carbon lean alloyed steel as claimed in claim 1, having yield strength in the range of 650 to 700MPa, ultimate tensile strength of 1200 to 1300MPa with total elongation of 15-17% to achieve tensile toughness greater than 20GPa%.
3. The low carbon lean alloyed steel as claimed in anyone of claims 1 or 2 having thickness in the range 2 to 5mm with a microstructure comprising pre-dominently martensite microstructure with 75.to 85.% martensite and retained austenite in the range of 6-12% by volume fraction.
4. The low carbon lean alloyed steel as claimed in anyone of claims 1 to 3 wherein the microstructure and mechanical properties achieved conform to third generation Advanced High Strength Steels with higher yield ratio (0.59-0.65) adapted for automotive components including crash resistance applications.
5. A process to produce low carbon hot rolled steel sheet as claimed in claims 1 to 4 comprising
(i) providing selective lean alloy steel of composition preferably comprisingby wt. % 0.03 to 0.3% and preferably around 0.17% C; Mn content varying between 1.5 and 2.5% preferably about 1.7%; Si content varying between 1.0 to 2.5 % preferably about 1.35%; Nb content between 0.01and 0.06% preferably about 0.02%; Ti content between 0.03 and 0.05% preferably 0.04% ; Al content between 0.04 and 0.08% preferably about 0.06%; with < 0.005 preferably 0.002 to 0.004 wt.% S and , 0.03% preferably 0.015 to 0.017 wt.% P, and 0.003-0.006% N and balance iron, (ii) carrying out conventional steel making through basic oxygen furnace and ladle furnace, continuous casting route followed by hot rolling to have product of thickness in the range 2 to 5mm;
(ii) subjecting said hot rolled steel to quench and partitioning heat treatment cycle involving austenitization above Ac3 temperature at 900oC for 2-5 min followed by quenching in salt bath at 380-390oC and preferrably 387oC for a time period of 1-5min and water quenched to 20oC to achieve desired microstructure and AHSS properties.
6. The process as claimed in claim 5 wherein said steel making comprising primary steel making process using hot metal from blast furnace in a EAF or BOF furnace, followed by secondary steel making process using a Ladle steel making route, where the desired composition was achieved with ferro alloy addition, followed by casting the same through continuous casting in slab caster, hot rolled the cast slab to produce steel strip with thockness between 2 to 5 mm withyield strength 670-690 MPa, ultimate tensile strength in the range of 840 to 890Mpa, total elongations in the range of 11.5 to 12.5 %, yield ratio in the range of 0.78 to 0.80 and tensile toughness (product of ultimate tensile strength to total elongation) between 10.25- to 10.51Gpa%.
.
7. The process as claimed in claims 5 or 6 wherein (i) austenitizzed at above Ac3 temperature at 900oC for 2-5min followd by Q&P at 262, 330 and 387oC the steel produced retained austenite in thin film form in the range of 3-15% with tempered martensitic microstructure and (ii) when austenitized above Ac3at 900oC for 2-5 min followed by quenching in salt bath at 380-390oC and preferrably 387oC for a time period of 1-5min and water quenched to get pre-dominently martensite microstructure with retained austenite in the range of 6-12% to give yieldstrength in the range of 650 to 700Mpa, ultimate tensile strength of 1200 to 1300Mpa with total elongation of 15-17% to achieve tensile toughness greater than 20Gpa.%.
8. The process as claimed in anyone of claims 5 to 7 wherein said hot rolling was carried out comprising
Slab Reheating temperature 1250oC,
Roughing Mill temperature: 1210oC,
Finishing temperature: 890oC,
Coiling Temperature: 570oC,
No of passes: 6.

Dated this the 21st day of August, 2023
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199