Method for producing a high strength steel sheet having improved ductility and formability, and obtained steel sheet

Patent metadata

Field	Value
Publication number	US-2026098316-A1
Application number	US-202519417035-A
Country	US
Kind code	A1
Filing date	Dec 11, 2025
Priority date	Dec 21, 2015
Publication date	Apr 9, 2026
Grant date	—

Abstract

A method for producing a steel sheet, the method containing the following successive steps:
providing a cold-rolled steel sheet, the chemical composition of the steel containing in weight %: 0.15%≤C≤0.23%, 2.0%≤Mn≤2.8%, 1.0%≤Si≤2.1%, 0.02%≤Al≤1.0%, with 1.7%≤Si+Al≤2.1%, 0≤Nb≤0.035%, 0≤Mo≤0.3%, 0≤Cr≤0.4%, the remainder being Fe and unavoidable impurities, annealing the steel sheet at an annealing temperature T A so as to obtain a structure comprising at least 65% of austenite and up to 35% of intercritical ferrite, quenching the sheet at a cooling rate of at least 20° C./s from a temperature of at least 600° C. down to a quenching temperature QT comprised between Ms−170° C. and Ms−80° C., heating the sheet up to a partitioning temperature PT between 350° C. and 450° C. and maintaining the sheet at this temperature for a partitioning time Pt comprised between 80 s and 440 s, immediately cooling the sheet down to the room temperature,

the steel sheet having a final microstructure consisting of, in surface fraction:
between 40% and 70% of tempered martensite, between 7% and 15% of retained austenite, between 15% and 35% of ferrite, at most 5% of fresh martensite, at most 15% of bainite.

First claim

What is claimed is:

1 : A steel sheet, wherein the steel sheet is made of a steel having a chemical composition containing by weight:

0.15
%

≤
C
≤

0.23
%

,

2.
%

≤
Mn
≤

2.8
%

,

1.
%

≤
Si
≤

2.1
%

,

0.02
%

≤
Al
≤

1.
%

,

with

1.7
%

≤

Si
+
Al

≤

2.1
%

,

0
≤
Nb
≤

0.035
%

,

0
≤
Mo
≤

0.3
%

,

0
≤

Cr
≤

0.4
%

,

and
a remainder, the remainder being Fe and unavoidable impurities,
said steel sheet having a microstructure consisting of, in surface fraction:
between 40% and 70% of tempered martensite, the tempered martensite having a C content lower than 0.03%,
between 7% and 15% of retained austenite,
between 15% and 35% of ferrite,
at most 5% of fresh martensite, and
at most 15% of bainite.

2 : The steel sheet according to claim 1 , wherein the ferrite comprises, with respect to a whole structure of the steel sheet, between 0% and 15% of intercritical ferrite and between 0% and 35% of transformation ferrite.

3 : The steel sheet according to claim 1 , wherein the retained austenite has a C content comprised between 0.9% and 1.2%.

4 : The steel sheet according to claim 1 , wherein the steel sheet has a yield strength of at least 900 MPa, a tensile strength of at least 1180 MPa, a total elongation of at least 12%, measured according to ISO standard 6892-1, and a hole expansion ratio HER of at least 25%, measured according to the ISO standard 16630:2009.

5 : The steel sheet according to claim 1 , wherein the chemical composition of the steel satisfies at least one of the following conditions:

C
≥

0.16
%

,

C
≤

0.21
%

,

Mn
≥

2.2
%

,

Mn
≤

2.7
%

,

0.01
%

≤
Nb

,

Mo
≤

0.05
%

,

or

Mo
≥

0.1
%

,

Cr
≤

0.05
%

,

or

Cr
≥

0.1

%
.

6 : The steel sheet according to claim 1 , wherein the chemical composition of the steel is such that C+Si/10≤0.30% and Al≥6(C+Mn/10)−2.5%.

7 : The steel sheet according to claim 6 , wherein the chemical composition of the steel is such that 1.0%≤Si<1.3% and 0.5%<Al≤1.0%.

8 : The steel sheet according to claim 7 , wherein 1.0%≤Si<1.2% and 0.6%≤Al≤1.0%.

9 : The steel sheet according to claim 1 , wherein the chemical composition of the steel is such that 1.3%≤Si≤2.1% and 0.02%≤Al≤0.5%.

10 : The steel sheet according to claim 7 , wherein the steel sheet is coated with Zn or a Zn alloy coating, the coating resulting from an electrochemical method or a vacuum coating process.

11 : The steel sheet according to claim 1 , wherein the steel sheet has a thickness comprised between 0.7 and 3 mm.

12 : The steel sheet according to claim 1 , wherein the surface fraction of fresh martensite is between 2% and 5% and/or wherein the surface fraction of retained austenite is between 11% and 15%.

Assignees

• Arcelormittal

Inventors

• Gospodinova Maya
• Venkatasurya Pavan

Classifications

• C22C38/38

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

• C22C38/26

  with niobium or tantalum · CPC title

• C22C38/22

  with molybdenum or tungsten · CPC title

• C22C38/06

  containing aluminium · CPC title

• C22C38/02

  containing silicon · CPC title