	US 12,264,375 B2
	Method of heat treating a cold rolled steel strip
Shangping Chen, Beverwijk (NL); Richard Mostert, Heemskerk (NL); Maxim Peter Aarnts, Castricum (NL); and Stefanus Matheus Cornelis Van Bohemen, Leiden (NL)
Assigned to TATA STEEL IJMUIDEN B.V., Velsen-Noord (NL)
Appl. No. 17/596,676
Filed by TATA STEEL IJMUIDEN B.V., Velsen-Noord (NL)
PCT Filed Jun. 11, 2020, PCT No. PCT/EP2020/066208 § 371(c)(1), (2) Date Dec. 16, 2021, PCT Pub. No. WO2020/254187, PCT Pub. Date Dec. 24, 2020.
Claims priority of application No. 19180700 (EP), filed on Jun. 17, 2019.
Prior Publication US 2022/0316021 A1, Oct. 6, 2022
Int. Cl. C21D 8/02 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/04 (2006.01); C22C 38/06 (2006.01); C23C 2/06 (2006.01); C23C 2/40 (2006.01)

CPC C21D 8/0236 (2013.01) [C21D 8/0247 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C23C 2/06 (2013.01); C23C 2/40 (2013.01); C21D 2211/001 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)]

21 Claims

1. A method of heat treating a cold rolled steel strip, which method comprises the steps of:

a) soaking an uncoated cold rolled steel strip within a temperature range of (Ac3−20) to (Ac3+20) for a soaking time t2 of 1-200 seconds, thereby obtaining a cold rolled steel strip having an austenitic microstructure;

b) cooling of the uncoated soaked steel strip resulting from step a) to a temperature T4 in the range of Ms-(Ms−200);

c) heating the uncoated cooled steel strip resulting from step b) to a temperature range of Bs-Ms;

d) heat treating the uncoated heated steel strip in the temperature range of Bs-Ms for a period of time t5 of 30-120 seconds;

d1) optionally a coating step of coating the heat treated steel strip with a protective coating following the heat treating:

e) cooling the heat treated, optionally coated, steel strip to ambient temperature;

such that the heat treated, optionally coated, steel strip has a microstructure (in vol. %) comprising:

polygonal ferrite (PF): 0-10;

polygonal ferrite (PF)+acicular ferrite (AF)+higher bainitic ferrite (HBF): 5-30;

lower bainitic ferrite (LBF)+partitioned martensite (PM): 50-85,

retained austenite (RA): 5-20;

martensite (M): 0-15;

wherein the steel strip has a composition (in mass percent) comprising

C: 0.15-0.28;

Mn: 1.70-3.00;

Si: 0.50-2.00;

Al: 0.01-0.60;

P: less than 0.050;

S: less than 0.020;

N: less than 0.0080;

wherein the sum (Si+Al) is ≥0.60; and

wherein 10C+Mn+Cr≥3.85 and 8.5≤(Mn+Cr)/C≤16;

optionally one or more elements selected from

0<Cr≤1.00;

0<Cu≤0.20;

0<Ni≤0.50;

0<Mo≤0.50;

0<Nb≤0.10;

0<V≤0.10;

0<Ti≤0.10;

0<B≤0.0030;

0<Ca≤0.0050;

0<REM≤0.0100, wherein REM is one or more rare earth metals;

and the remainder being iron and inevitable impurities,

wherein the steel strip resulting from step e) has Tensile strength (TS) of at least 980 Mpa and Total elongation (TE) of at least 13%;

wherein the steel strip resulting from step e) has at least one property selected from the group consisting of:

Yield strength (YS) of at least 550 Mpa,

Hole expansion capacity (HEC) of at least 20%, and

Bending angle (BA) of at least 80°.