Heat treatment of cold rolled steel strip

Patent metadata

Field	Value
Publication number	US-12522889-B2
Application number	US-202017596673-A
Country	US
Kind code	B2
Filing date	Jun 11, 2020
Priority date	Jun 17, 2019
Publication date	Jan 13, 2026
Grant date	Jan 13, 2026

Abstract

A method of heat treating a high strength cold rolled steel strip including
a) soaking a cold rolled steel strip, b) cooling the soaked steel strip c) heat treating the cooled strip; d) cooling the heat treated steel strip to ambient temperature range; such that the steel strip has a microstructure including various ferrites, retained austenite and martensite. The main components in the steel composition includes carbon, manganese, silicon and aluminium in addition to iron.

First claim

The invention claimed is:

1 . A method of heat treating a cold rolled steel strip, which method comprises the steps of:
a) heating an uncoated cold rolled steel strip from a first temperature, wherein the first temperature is room temperature, to a temperature T 1 in the range of 680-740° C. at a single constant heating rate V 1 of 10.0-30.0° C./s; and further heating the uncoated cold rolled steel strip from the temperature T 1 to a soaking temperature T 2 within a soaking temperature range of (Ac3−20)-(Ac3+20) at a heating rate V 2 of 0.5-4.0° C./s, a1) then soaking the uncoated cold rolled steel strip at the soaking temperature T 2 within the temperature range of (Ac3−20) to (Ac3+20) for a soaking time t 2 of 30-150 seconds, thereby obtaining a cold rolled steel strip having an austenitic microstructure; b) cooling the uncoated soaked steel strip resulting from step a1) to a temperature T 4 in the range of Bn-Ms, wherein acicular ferrite is formed during the cooling in a temperature between Bs and Ms; wherein step b) comprises a substep of cooling the soaked steel strip from step a) to a temperature T 3 in the range of 750-600° C., at a cooling rate V 3 of 2.0-15.0° C./s; and further comprising a substep of cooling the soaked steel strip from a temperature T 3 to T 4 at a cooling rate V 4 of 20.0-60.0° C./s; c) increasing temperature of the uncoated cooled strip obtained in step b) to a temperature T 5 in a temperature range between Bs and T 4 , to transform austenite to carbide-free LBF and c1) heat treating the uncoated cooled strip obtained in step c) at the temperature T 5 in a range between the temperature Bs and T 4 for a period of time t 5 of 30-300 seconds; c2) following heat treatment of step c1) optionally providing the heated treated steel strip with a coating, d) cooling the heat treated, optionally coated, steel strip to a temperature T 7 in the range of ((Ms−50) to Mf) at a cooling rate of V 7 of 5.0-10.0° C./s and further cooling the heat treated, optionally coated, steel strip to ambient temperature at a cooling rate of V 8 of 5.0-20.0° C./s; 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)+carbide-free higher bainitic ferrite (HBF): 5-30; carbide-free lower bainitic ferrite (LBF): 45-80; retained austenite (RA): 5-20; fresh martensite (M): 0-20; wherein carbide-free higher bainitic ferrite (HBF) is present; 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 of (Si+Al) is ≥0.60; and wherein 10C+Mn+Cr≥3.85 and 8.5≤(Mn+Cr)/C≤16; and optionally one or more elements selected from 0<Cr≤0.35; 0<Cu≤0.20; 0<Ni≤0.50; 0<Mo≤0.30; 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.

2 . The method according to claim 1 , wherein the microstructure comprises 52-80 vol. % carbide-free lower bainitic ferrite (LBF).

3 . The method according to claim 1 wherein step b) comprises cooling the soaked steel strip from step a) to the temperature T 4 at a cooling rate sufficient to avoid pearlite formation.

4 . The method according to claim 1 , wherein step b) comprises a substep of cooling the soaked steel strip from a temperature T 3 in the range of 800-550° C., to T 4 at a cooling rate V 4 of at least 15° C./s.

5 . The method according to claim 1 , wherein the microstructure of the steel is carbide-free.

6 . The method according to claim 1 , wherein step c) is performed at least partially by latent heat produced by the bainite transformation.

7 . The method according to claim 1 , wherein the heat treating step c) is performed in the range of Bn-(Ms+50).

8 . The method according to claim 1 , comprising a further heat treatment step between steps c1) and d) of heating the uncoated steel strip resulting from step c1) from the temperature T 5 to a temperature T 6 in the range of Bs-Bn.

9 . The method according to claim 1 , comprising a further heat treatment step between steps c1) and d) of heating the steel strip resulting from step c1) from the temperature T 5 to a temperature T 6 in the range of Bs-Bn, wherein the further heat treatment step comprises a hot dip galvanizing treatment.

10 . The method according to claim 1 , following heat treatment of step c1) further comprising a coating step of coating the heated treated steel strip with a protective coating.

11 . The method according to claim 1 , wherein the microstructure comprises in vol. %:
polygonal ferrite (PF) 0-5; polygonal ferrite (PF)+acicular ferrite (AF)+carbide-free higher bainitic ferrite (HBF): 10-25; lower bainitic ferrite (LBF): 50-75; retained austenite (RA): 7-15; fresh martensite (M): 0-15; and/or wherein the C content in retained austenite (RA) is 0.90 wt. % or more.

12 . The method according to claim 1 , wherein the resulting steel strip has at least one of the properties:
Yield strength (YS) is at least 550 MPa; and/or Tensile strength (TS) is at least 980 MPa; and/or Total elongation (TE) is at least 13%; and/or Hole expansion capacity (HEC) is at least 20%; and/or Bending angle (BA) is at least 80°.

13 . The method according to claim 1 , wherein polygonal ferrite (PF): 4-5, and
polygonal ferrite (PF)+acicular ferrite (AF)+carbide-free higher bainitic ferrite (HBF): 19-23.

14 . The method according to claim 13 , wherein tensile strength is 1127-1153 MPa.

15 . The method according to claim 14 , wherein HEC=>25%.

16 . The method according to claim 15 , wherein UE=>10.5% and TE=>14.9%.

17 . The method according to claim 1 , wherein step b) comprises a substep of cooling the soaked steel strip from step a) to a temperature T 3 in the range of 750-600° C., at a cooling rate V 3 of 3.0-10.0° C./s.

18 . A heat treated cold rolled steel strip made by the method of claim 1 and having a composition (in mass %) 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 of (Si+Al) is ≥0.60; and
wherein 10C+Mn+Cr≥3.85 and 8.5≤(Mn+Cr)/C≤16; and
optionally one or more elements selected from
0<Cr≤0.35;
0<Cu≤0.20;
0<Ni≤0.50;
0<Mo≤0.30;
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;
and 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): 45-80;
retained austenite (RA): 5-20;
fresh martensite (M): 0-20.

19 . The heat treated cold rolled steel strip according to claim 18 having at least one of the properties:
Yield strength (YS) is at least 550 MPa; and/or
Tensile strength (TS) is at least 980 MPa; and/or
Total elongation (TE) is at least 13%; and/or
Hole expansion capacity (HEC) is at least 20%; and/or
Bending angle (BA) is at least 80°.

20 . A method of heat treating a cold rolled steel strip, which method comprises the steps of:
a) heating an uncoated cold rolled steel strip from room temperature to a temperature T 1

Assignees

• Tata Steel Ijmuiden Bv

Inventors

• Chen Shangping
• Mostert Richard
• Aarnts Maxim Peter
• Van Bohemen Stefanus Matheus Cornelis

Classifications

• C21D8/02

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

• C23C2/40

  Plates; Strips · CPC title

• C22C38/38

  with more than 1.5% by weight of manganese · CPC title

• C22C38/28

  with titanium or zirconium · CPC title

• C22C38/26

  with niobium or tantalum · CPC title