Cold rolled and heat-treated steel sheet and method of manufacturing the same

Patent metadata

Field	Value
Publication number	US-12486551-B2
Application number	US-202017784848-A
Country	US
Kind code	B2
Filing date	Dec 16, 2020
Priority date	Dec 19, 2019
Publication date	Dec 2, 2025
Grant date	Dec 2, 2025

Abstract

A cold rolled and heat-treated steel sheet having a composition including, by weight percent C: 0.12-0.25% Mn: 3.0-8.0%, Si: 0.70-1.50%, Al: 0.3-1.2%, B: 0.0002-0.004%, S≤0.010%, P≤0.020%, N≤0.008%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, and having a microstructure consisting of, in surface fraction: between 5% and 45% of ferrite, between 25% and 85% of partitioned martensite, the partitioned martensite having a carbides density strictly less than 2×10 6 /mm 2 , between 10% and 30% of retained austenite, less than 8% of fresh martensite, a part of the fresh martensite being combined with retained austenite in the shape of martensite-austenite islands in total surface fraction less than 10%.

First claim

What is claimed is:

1 . A cold rolled and heat-treated steel sheet, made of a steel having a composition comprising, by weight percent:
C: 0.12-0.25% Mn: 3.0-8.0% Si: 0.70-1.50% Al: 0.3-1.2% B: 0.0002-0.004% S≤0.010% P≤0.020% N≤0.008% and optionally one or more of the following elements, in weight percentage: Mo≤0.5% V≤0.2% Nb≤0.06% Ti≤0.05% a remainder of the composition being iron and unavoidable impurities resulting from processing; the steel sheet having a microstructure consisting of, in surface fraction: between 5% and 45% of ferrite; between 25% and 85% of partitioned martensite, the partitioned martensite having a carbides density strictly less than 2×10 6 /mm 2 ; between 10% and 30% of retained austenite; and less than 8% of fresh martensite, a part of the fresh martensite being combined with retained austenite in the shape of martensite-austenite (M-A) islands in total surface fraction of 1% to less than 10%, and wherein fresh martensite and martensite-austenite islands size are less than 0.7 μm, the steel sheet having a yield strength YS expressed in MPa, a tensile strength TS expressed in MPa, a uniform elongation UE expressed in %, a total elongation TE expressed in %, a hole expansion ratio HER expressed in %, and a silicon content expressed in weight percent satisfying the following equation:
( YS*UE+TS*TE+TS*HER )/% Si>65000.

2 . The cold rolled and heat-treated steel sheet as recited in claim 1 wherein the manganese content is comprised between 3.0% and 5.0%.

3 . The cold rolled and heat-treated steel sheet as recited in claim 1 wherein the silicon content is comprised between 0.80% and 1.30%.

4 . The cold rolled and heat-treated steel sheet as recited in claim 1 wherein the microstructure includes at most 6% of fresh martensite.

5 . The cold rolled and heat-treated steel sheet as recited in claim 1 wherein the yield strength YS is higher than 950 MPa.

6 . The cold rolled and heat-treated steel sheet as recited in claim 1 wherein the tensile strength TS is higher than 1180 MPa.

7 . The cold rolled and heat-treated steel sheet as recited in claim 1 wherein the uniform elongation is higher than 10%.

8 . The cold rolled and heat-treated steel sheet as recited in claim 1 wherein the hole expansion ratio is higher than 25%.

9 . The cold rolled and heat-treated steel sheet of claim 1 , wherein the steel has a composition consisting of, by weight percent:
C: 0.12-0.25% Mn: 3.0-8.0% Si: 0.70-1.50% Al: 0.3-1.2% B: 0.0002-0.004% S≤0.010% P≤0.020% N≤0.008% and optionally one or more of the following elements, in weight percentage: Mo≤0.5% V≤0.2% Nb≤0.06% Ti≤0.05% a remainder of the composition being iron and unavoidable impurities resulting from processing.

10 . A method for manufacturing a cold rolled and heat-treated steel sheet, comprising the following successive steps:
casting a steel to obtain a semi-finished product having a composition comprising, by weight percent:
C: 0.12-0.25%
Mn: 3.0-8.0%
Si: 0.70-1.50%
Al: 0.3-1.2%
B: 0.0002-0.004%
S≤0.010%
P≤0.020%
N≤0.008%
and optionally one or more of the following elements, in weight percentage:
Mo≤0.5%
V≤0.2%
Nb≤0.06%
Ti≤0.05%
a remainder of the composition being iron and unavoidable impurities resulting from processing; reheating the semi-finished product at a temperature T reheat between 1150° C. and 1300° C.; hot rolling the reheated semi-finished product with a finish rolling temperature FRT between 800° C. and 950° C. to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a coiling temperature T coil between 200° C. and 700° C.; annealing the hot rolled steel sheet at a first annealing temperature TA1 between 550° C. and 700° C., and maintaining the steel sheet at the TA1 temperature for a holding time tA1 between 30 s and 50 h; cold rolling the hot-rolled steel sheet to obtain a cold rolled steel sheet; reheating the cold-rolled steel sheet to a second annealing temperature TA2 above Ae3-10° C., and maintaining the steel sheet at the TA2 temperature for a holding time tA2 between 1 s and 1000 s, so to obtain, upon annealing, a microstructure comprising martensite and bainite, the sum of the martensite and bainite being greater than 80%, strictly less than 20% of ferrite, and strictly less than 20% of a sum of martensite-austenite (M-A) islands and carbides, Ae3 being calculated from the formula:
Ae 3=890−20*√% C+20*% Si−30*% Mn+130*% Al

reheating the cold rolled steel sheet to a temperature TA3 strictly lower than Ae3 and higher than (Ae1+Ae3)/2, and maintaining the steel sheet at said annealing temperature TA3 for a holding time tA3 comprised between 3 s and 1000 s, Ae1 being calculated from the formula:
Ae 1=670+15*% Si−13*% Mn+18*% Al

quenching the cold rolled steel sheet to a quenching temperature TQ lower than (Ms-50° C.), to obtain a quenched steel sheet, Ms being calculated from the formula:
Ms= 560−(30*% Mn+13*% Si−15*% Al+12*% Mo)−600*(1−exp(−0.96*% C))

reheating the quenched steel sheet to a partitioning temperature TP comprised between 350° C. and 550° C., and maintaining the quenched steel sheet at said partitioning temperature for a partitioning time comprised between 1 s and 1000 s; and cooling the steel sheet to the room temperature, to obtain the cold rolled and heat-treated steel sheet as recited in claim 1 .

Assignees

• Arcelormittal

Inventors

• Perlade Astrid
• Zhu Kangying
• Jung Coralie
• Kegel Frederic

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/14

  containing titanium or zirconium · CPC title

• C22C38/12

  containing tungsten, tantalum, molybdenum, vanadium, or niobium · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C38/04 Primary

  containing manganese · CPC title