Description:FIELD OF THE INVENTION
The present invention relates to a method of development of low carbon ultra-high strength steel (> 1000 MPa) of hot rolled product with very good ductility (> 25% elongation) which will be in the third generation advanced high strength steel regime. The steel with (wt.%) 0.17% carbon, 1.35% Si and 1.73% Mn, micro alloyed with 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 applications. The steel so produced is subjected to single stage quench and partitioning (Q&P) heat treatment after austenization in the inter critical temperature followed by salt bath quenching below the martensitic start temperature. The steel so produced have microstructure of inter critical ferrite in the range of 26-32%, martensite in the range of 40-50% and retained austenite in the range of 12 to 16% with 10-15% bainite.
The steel achieved yield strength between 460-530MPa, ultimate tensile strength between 1060 to 1260MPa to achieve tensile toughness (product of ultimate tensile strength and total elongation) greater than 25GPa. %. The steels properties are in the third generation regime for the automotive applications. The steel has very good elongation of 20 to 25% and yield ratio in the range of 0.42-0.43% indicating its potential for formability applications in automotive industry.

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.
CN113061698A reports invention of Q&P steel from a chemical composition: C: 0.20 to 0.45wt. %, Mn: 2.0 to 8.0wt. %, Si: 1.0to 3.0wt. %, Al: 0 to 1.5 wt.%, Cr: 0 to 1.5 wt.%, Ni: 0 to 3 wt.%, V: 0 to 0.5 wt.%, Mo: 0 to 1.0 wt.%, and Nb: 0 to 0.5 wt.%. 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. The prior art proposes two stage heat treatment process as mentioned in detail: rapidly transferring the sample from the salt bath at 770°C to an oil bath at 130°C, rapidly cooling the sample to 130°C heating the sample to 400°C preserving the heat for 500s, and then cooling to room temperature. The mechanical properties of the Q&P is 1600-2000MPa.

CN111440987A reports invention of Q&P steel from a chemical composition: C: 0.13 to 0.20 wt. %, Si: 1.0 to 1.8 wt. %, Mn: 1.3 to 2.0 wt. %, Ti: 0.02 to 0.05 wt. %, V: 0.01 to 0.03 wt. %, Al: 0.02 to 0.05 wt. %, P: =0.015 wt. %, S: =0.005 wt. %, N: =0.004 wt. %, O: =30ppm, the rest is Fe and unavoidable impurities. The prior art contains V as the micro alloy. It mentions the use of two stage heat treatment which involves annealing the steel materials at 820-930°C followed by slowly cooling to 660-700°C at a cooling speed of 5-10°C/s which further cools rapidly to 200-300°C at a cooling speed of not less than 60°C/s which is further heated to 360-460°C for 150-550s to distribute the temperature of 360-460°C for 150-550s and finally cooled to room temperature. The mechanical properties of the Q&P steel is close to 980MPa tensile strength.

CN103820613A relates to development of Q&P steel from C-Mn-Al series TRIP 590 steel. The tensile toughness of the steel reported in the prior art is 21-22 GPa %. The prior art employs two stage heat treatment and the heat treatment processes include annealing of pre-processed TRIP590 cold-roll steel sheets 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 heated to 350-370°C for 30-60s and finally, water quenched. The austenization temperature used in the prior art invention is 1000 to 1150°C which is much higher.
KR101694875B1 relates to the development of Q&P steel from a chemical composition: C: 0.15 to 0.40 wt. %; Si: 1.0 to 2.0 wt. %; Mn: 1.5 to 3.0 wt. %; P: 0.015 wt. % or less; S: 0.005 wt. % or less; Al: 0.3 to 1.0 wt. %; N: 0.006 wt. % or less; Ti: 0.005 to 0.015 wt. % and the balance being Fe and inevitable impurities The heat treatment process involves heating the rolled pieces at a temperature of 800 to 900°C followed by rapidly cooling to 500 to 600°C at a cooling rate of 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 structure of pro-eutectoid ferrite, martensite and retained austenite. Finally, after reeling, the steel is slowly cooled to room temperature to obtain a high strength hot rolled Q & P steel. The Q&P steel reported yield strength of 700MPa, tensile strength of 1300MPa and total elongation of 10% or more with 20% retained austenite.

CN102766818B discloses development of Q&P steel from a chemical composition: 0.15-0.3 wt. % of C; 0.3-0.5 wt. % of Mn; 0.5-1 wt. % of Cr; 0.2-0.6 wt. % of Mo; 0.2-1.5 wt. % of Si; 0.02-1.0 wt.% of Al; 0.002-0.004 wt. % of B; 0.02-0.05 wt. % of Ti; less than 0.01wt. % of S and less than 0.015 wt. % of P, wherein (Si+Al)>1.0 wt. %.The Cr and Mo are intentionally added to the cited prior and have boron content in the range of 0.002-0.004 wt. %. The Si & Al are added in such a way that its total content should be greater than 1 wt. % (Si + Al > 1 wt. %). In the heat treatment, steel sample is heated to 950°C 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°C for 8-10 seconds for carbon partitioning followed by quenching at a cooling rates of 38°C/s.

CN103805851A relates to the development of Q&P steel from a chemical composition: C: 0.20 to 0.40 wt. %; Si: 0.8 to 2.0 wt. %; Mn 1.5 to 3.0 wt. %; and P=0.015 wt. %. S=0.005 wt. %, Al 0.02 to 0.08 wt. %, N=0.006 wt. %, Ti 0.005 to 0.015 wt. %, O=30ppm, the rest is Fe and inevitable impurities. Q&P steel is manufactured through hot rolling process involving heating Strand or ingot heating at temperature of 1100 to 1200°C for 1-2 hour followed by hot rolling where the rolling temperature is 1000~1070°C, the multi-pass pressure is above 950°C and the cumulative deformation is =50% , subsequently intermediate billet is heated to 800~850°C then carried out last 3 ~ 6 passage rollings followed by press quenching which involves cooling of the steel plate after hot rolling to 150~250°C with the cooling rate of >50°C/s to obtain martensite + residual austenite structure and finally coiled and slowly cooled to room temperature. The ultimate tensile strength (UTS) reported >1400MPa and elongation >10% reported in the prior art.

JP2019504202A relates to development of Q&P steel from a chemical composition: 0.15 to 0.25 wt.% C; 0.6 to 2.0 wt. % Si; 1.5 to 3.0 wt.% Mn; 0.01 to 0.1 wt.% sol. Al; 0.01 to 0.5 wt.% Cr; 0.005 to 0.2 wt.% Mo; 0.001 to 0.05 wt.% P; 0.001 to 0.05 wt.% S; 0.001 to 0.01 wt. % N; 0.003 to 0.1 wt.% Nb; 0.003 to 0.1 wt.% titanium Ti; 0.003 to 0.01 wt. % V; 0.0005 to 0.005 wt.% B; and a balance of Fe and unavoidable impurities. The Cr, Mo and V is intentionally added to the cited prior art. The Q&P steel is manufactured through hot rolling process where the heated slab is hot-rolled at a temperature between 850° C and 1150° C to obtain a hot-rolled steel sheet. The hot-rolled steel sheet is cooled to a cooling end temperature in a range between 200° C to 400° C at an average cooling rate in a range between 50 to 100°C/sec and coiled. The hot-rolled steel sheet demonstrated tensile strength thereof is 1200 MPa or more and the ductility thereof is 10% or more.

Li Wang et al., Metallogr. Microstruct. Anal. (2013) 2:268–281, DOI 10.1007/s13632-013-0082-8; reported Q&P treatment with chemical composition: C: 0.15–0.30 wt.%; Mn: 1.5–3.0 wt.%; Si: 1.0–2.0 wt.%; Al: 0.02–0.06 wt.%; P: <0.015 wt.% and Si: <0.01 wt.%. The Q&P steel is manufactured through heat 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°C/s. After quenching, the steel is usually reheated to a higher temperature (partitioning temperature) and held for a couple of minutes. Finally, the stable carbon-enriched austenite is retained when the steel is cooled to room temperature.
R. M. Wu et al., Physics Procedia 50 (2013) 8 – 12; discloses Q&P treatment with a different chemical composition: C: 0.20 wt.%; Si: 1.42 wt.% ; Mn: 1.87 wt.% ; Al: 0.0405 wt.% ; P: 0.012 wt.% ; S: 0.0006 wt.%; the rest is Fe. The Q&P steel is manufactured through heat treatment where steel was austenitized at 900°C for 5 minutes, followed by quenching into salt bath at 320°C for 60 seconds and then further quenched into water at room temperature. The steel subjected to Q&P treatment exhibited the combination of high tensile strength (1311MPa) and relatively high elongation (13.6%).

Jun Zhang et al., University of Wollongong, Research Online mentions the Q&P treatment with a different chemical composition: C: 0.18 wt. %; Mn:1.44 wt.%; Si: 1.48 wt.%; Al: 0.15 wt.% and Nb: 0.025 wt.%. 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°C for 3 min, and then quenched to 220°C. Subsequently, these quenched specimens were held at 400°C in a time range from 5s to 500s. Finally, a second quenching process was carried out to room temperature. The second group was treated by a pre-quenching process, from 910°C to room temperature. Then the samples were austenitized at 850°C for 3 min followed by quenched to 220°C and partitioned at 400°C from 5s to 500s. Finally, the partitioned samples were quenched to room temperature. The steel subjected to Q&P treatment demonstrated a combination of ultimate tensile strength (1000MPa) and total elongation (above 30%).

In the present invention a low carbon steel alloyed with Si and Mn having additional 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. As compared to the two stage Q&P process, single stage Q&P is simple and easy to implement in industry.

OBJECTS OF THE INVENTION

The basic object of the present invention is development of a low carbon ultra-high strength steel of hot rolled product with very good ductility and can be in the third generation advanced high strength steel regime.
Another object of the present invention is to provide a low carbon ultra-high strength steel having yield strength in the range of 460-530MPa.
Yet another object of the present invention is to provide a low carbon ultra-high strength steel having ultimate tensile strength in the range of 1060-1260MPa with total elongation in the range of 20-25% to achieve tensile toughness greater than 25GPa. %
Further object of the present invention is to provide a low carbon ultra-high strength steel having yield ratio in the range of 0.42-0.43%.
Yet another object of the present invention is to provide process of development of low carbon ultra-high strength steel of hot rolled product with very good ductility which will be in the third generation advanced high strength steel regime.
Further object of the present invention is to provide a process of development of low carbon ultra-high strength steel which involves single stage quench and partitioning (Q&P) heat treatment.
Another object of the present invention is to provide a low carbon ultra-high strength steel having microstructure of inter critical ferrite in the range of 26-32%, martensite in the range of 40-50% and retained austenite in the range of 12 to 16% with 10-15% bainite.
Yet another object of the present invention is to provide a third generation advanced high strength steel having great potential for formability applications of automotive components.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a third generation AHSS steel with low carbon based steel composition of (0.03 to 0.3 wt. %) 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 0.01 and 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 S content less than 0.005 and P content < 0.03% and attaining ultra-high strength (greater than 1000MPa) preferably in the range of 1050MPa .. to 1280MPa.. and tensile toughness greater than 25GPa. % preferably in the range of 25.25 to 27GPa.% as single stage quench and partition heat treated steel.
A further aspect of the present invention is directed to a third generation AHSS steel including 0.02% Cr, 0.008% each of Ni and Cu, Mo content of 0.001 and V content of 0.002%.
Yet another aspect of the present invention is directed to a third generation AHSS steel having yield strength in the range of 460-530 MPa, Ultimate tensile strength in the range of 1060-1260 MPa with total elongation in the range of 20-25% and desired toughness greater than 25 GPa% and yield ratio of 0.42 to 0.43.
A still further aspect of the present invention is directed to a third generation AHSS steel comprising of microstructure of inter critical ferrite between 26-32%, tempered martensite between 40-50%, bainite between 10-15% and retained austenite between 12 to 16% providing mechanical properties including yield strength between 460-530MPa, ultimate tensile strength between 1060 to 1260MPa to achieve tensile toughness greater than 25GPa.% with elongation between 20-25% and yield ratio of 0.42 to 0.43 for formability application in automotive components.
Another aspect of the present invention is directed to a process for the manufacture of the third generation AHSS steel comprising the steps of:
(i) providing lean steel based on the steel composition comprising low carbon based steel composition of (0.03 to 0.3 wt. %) 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 0.01 and 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 S content less than 0.005 and P content < 0.03%;
(ii) subjecting the lean steel of said composition to quench and partitioning heat treatment cycle as heating in the inter critical temperature between 820-840°C, preferably at 830°C for 2-5 min followed by quenching in salt bath at 355-365°C preferably at 362°C for a time period of 1-10min and water quenched to achieve microstructure of inter critical ferrite, tempered martensite and retained austenite in the range of 12-16% such as to attain mechanical properties including yield strength in the range of 460-530MPa, ultimate tensile strength in the range of 1060 to 1260MPa with total elongation in the range of 20-25% to achieve tensile toughness greater than 25GPa.%.
Yet another aspect of the present invention is directed to a process comprising subjecting said lean alloy steel of composition (wt.%) 0.17%C, 1.35% Si, 1.73% Mn, 0.04% Ti, 0.02% Nb obtained following conventional steel making followed by hot rolled product of thickness in the range 2 to 5mm which is subjected to quench and partitioning (Q&P) after intercritical austenization at 830°C with inter critical austenite of 71%, austempering between 355 to 365°C and finally water quenching for desired controlled microstructure based end characteristics.

BRIEF DESCRIPTION OF ACCOMPANYING FIGURES

Figure 1: Pseudo-binary phase diagram of the steel with the phase field as a function of temperature and carbon content.
Figure 2: Single stage quench and partitioning heat treatment cycle for the steel.
Figure 3: Optical Microstructure of the steel subjected to Q&P heat treatment.
Figure 4: SEM micrograph of the steel subjected to Q&P heat treatment cycle.
Figure 5: XRD of the steel subjected to Q&P heat treatment cycle.
DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the development of a 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 Generation Advanced High Strength Steel with tensile toughness greater than 25GPa% when subjected to intercritical austenitization. The steel used is comprised 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.

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 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 carried out at inter critical austenization between 820 to 840°C, preferably 830°C for 2 to 5 min followed by quenching in salt bath between 230 to 365°C preferably at 236, 310 and 362°C for a time period of 1-10min and water quenched to achieve microstructure of inter critical ferrite, tempered martensite and retained austenite in the range of 12-16% to give mechanical properties such as yield strength in the range of 460-530MPa, ultimate tensile strength in the range of 1060 to 1260MPa with total elongation in the range of 20-25% to achieve tensile toughness greater than 25GPa.%. The yield ratio for the invented steel is in the range of 0.42 to 0.43.
The aspects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non-limiting illustrative drawings.

Examples

Example 1: Chemical Composition of the Lean Alloyed Steel
The present example is related to the development of a lean alloyed steel composition range and typical composition (Table 1) adapted for a quench and partitioning heat treatment cycle, to achieve the Third Generation Advanced High Strength Steel with tensile toughness greater than 25GPa % when subjected to intercritical austenitization.
The steel used is comprised 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.
The steel is a low carbon steel with 1.72% Mn, 1.35% Si as major alloying elements along with microalloying elements Ti and Nb that forms finer grain size by the formation of carbonitrides. Ti is added at a higher value to restrict the grain growth of austenite phase and thus achieve finer martensite with enhanced properties. The steel is aluminium killed and has 0.06% Al that refines the grain size by the formation of AlN phase. The rest of the residual elements in the steel include 0.02% Cr, 0.008% each of Ni and Cu, Mo content of 0.001 and V content of 0.002%. The Mn content enlarges the austenite field, lowers the A1 temperature. The Si content suppresses the formation of cementite and has a tendency to enable the formation of austenite.
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.
Table 1: Chemical composition of lean alloyed steel in wt. %:
C Mn Si Al Ti Cr Ni Mo Nb V S P N
Range 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
Actual 0.17 1.73 1.35 0.06 0.04 0.02 0.003 0.016 0.005

Example 2: Process of Manufacture of Lean Alloyed Steel
The present example is directed to a process for manufacture of the lean alloyed steel including processing. The lean alloyed steel composition 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 is adapted for variable thermal processing 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.
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 with yield 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 10.51 GPa %.
The steel making, continuous casting and hot rolling parameters are summarized in Table 2.

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

Example 3: Design of Heat Treatment Cycle
The present example relates to the design of heat treatment cycle where single stage Quench & Partitioning step is carried out at intercritical austenization between 820 to 840°C, preferably 830°C for 2 to 5 min followed by quenching in salt bath between 230 to 365°C preferably at 236, 310 and 362°C for a time period of 1-10min and water quenched.
The Q&P heat treatment cycle applied to the steel is shown in Fig. 2. It consists of austenitizating the hot rolled steel at 830°C for 2-5 min which is having approximately 79% retained austenite and then quenching in a salt bath at 236, 310 and 362°C for 1-15min time for the partitioning and then water quenched.
The temperatures were selected with calculation of initial retained austenite content at temperatures of 830°C as 71% at the austenization stage calculated from the lever rule from the phase diagram obtained through Thermocalc (2020b) software. The rest of the phase being ferrite.
The fraction of martensite formed (fQTm) as a function of the quench temperature (QT) is given by Koinsteinen-Maurberger equation as follows;
fQTm = 1–exp (-1.1 x 10-2 (Ms-QT)) ----- Eq. (1)
The quench and partitioning temperatures are selected by calculating from the equation to have 20, 40 and 80% residual austenite content at the three quench temperatures selected for each austenitization.
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-3: Mechanical Properties of the steel and retained austenite content subjected to Q&P heat treatment cycle.

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
830
5min 236 1 547±16 1325±17 18.10±0.35 0.41±0.01 23.98±16 5.5
5 540±9 1273±23 16.67±0.29 0.42±0.01 21.23±0.1 8.85
10 520±13 1266±18 18.05±0.22 0.41±0.01 22.84±0.5 5.65
15 467±14 1227±28 19.67±0.19 0.38±0.01 24.14±0.32 5.86
310 1 487±8 1121±17 20.26±0.17 0.43±0.01 22.48±0.15 0.8
5 449±6 1088±13 21.51±0.24 0.41±0.01 23.41±0.02 1.1
10 403±11 1071±21 20.84±0.26 0.38±0.01 22.31±0.16 1.7
15 430±15 1077±15 20.88±0.27 0.40±0.02 22.49±0.02 2.2
362 1 528±17 1259±24 20.16±0.29 0.42±0.01 25.39±0.12
13.36
5 463±14 1067±13 25.28±0.33 0.43±0.02 26.98±0.02
11.72
10 494±9 1145±18 24.47±0.39 0.43±0.01 28.01±0.01
16.24
15 554±13 1060±13 21.86±0.23 0.47±0.01 23.17±0.04 8.69

The lean steel inter critical austenitized at 830°C for 2-5min followed by Q&P at 236, 310 and 362°C the steel contains retained austenite in the range of 1-16% with 40-50% martensite and 10-15% bainite with around 30% inter critical ferrite.
The steel when austenitized in the inter critical zone at 830°C for 2-5 min followed by quenching in salt bath at 355-365°C preferably at 362°C for a time period of 1-10min and water quenched to achieve microstructure of inter critical ferrite, tempered martensite and retained austenite in the range of 12-16% to give mechanical properties such as yield strength in the range of 460-530MPa, ultimate tensile strength in the range of 1060 to 1260MPa with total elongation in the range of 20-25% to achieve tensile toughness greater than 25GPa.%. The yield ratio for the steel of present invention is in the range of 0.42 to 0.43. , Claims:We claim:
1. Third generation AHSS steel with low carbon based steel composition of (0.03 to 0.3 wt. %) 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 0.01 and 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 S content less than 0.005 and P content < 0.03% and attaining ultra-high strength (greater than 1000MPa) preferably in the range of 1050MPa to 1280MPa and tensile toughness greater than 25GPa. % preferably in the range of 25.25 to 27GPa.% as single stage quench and partition heat treated steel.

2. The third generation AHSS steel as claimed in claim 1 including 0.02% Cr, 0.008% each of Ni and Cu, Mo content of 0.001 and V content of 0.002%.

3. The third generation AHSS steel as claimed in anyone of claims 1 or 2 having yield strength in the range of 460-530 MPa, Ultimate tensile strength in the range of 1060-1260 MPa with total elongation in the range of 20-25% and desired toughness greater than 25 GPa% and yield ratio of 0.42 to 0.43.

4. The third generation AHSS steel as claimed in anyone of claims 1 to 3 comprising of microstructure of inter critical ferrite between 26-32%, tempered martensite between 40-50%, bainite between 10-15% and retained austenite between 12 to 16% providing mechanical properties including yield strength between 460-530MPa, ultimate tensile strength between 1060 to 1260MPa to achieve tensile toughness greater than 25GPa.% with elongation between 20-25% and yield ratio of 0.42 to 0.43 for formability application in automotive components.

5. A process for the manufacture of the third generation AHSS steel as claimed in anyone of claims 1 to 4 comprising the steps of :
(i) providing lean steel based on the steel composition comprising low carbon based steel composition of (0.03 to 0.3 wt. %) 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 0.01 and 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 S content less than 0.005 and P content < 0.03%;
(ii) subjecting the lean steel of said composition to quench and partitioning heat treatment cycle as heating in the inter critical temperature between 820-840°C, preferably at 830°C for 2-5 min followed by quenching in salt bath at 355-365°C preferably at 362°C for a time period of 1-10min and water quenched to achieve microstructure of inter critical ferrite, tempered martensite and retained austenite in the range of 12-16% such as to attain mechanical properties including yield strength in the range of 460-530MPa, ultimate tensile strength in the range of 1060 to 1260MPa with total elongation in the range of 20-25% to achieve tensile toughness greater than 25GPa.%.

6. The process as claimed in claim 5 comprising subjecting said lean alloy steel of composition (wt.%) 0.17%C, 1.35% Si, 1.73% Mn, 0.04% Ti, 0.02% Nb obtained following conventional steel making followed by hot rolled product of thickness in the range 2 to 5mm which is subjected to quench and partitioning (Q&P) after intercritical austenization at 830°C with inter critical austenite of 71%, austempering between 355 to 365°C and finally water quenching for desired controlled microstructure based end characteristics.

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