Method for producing an ultra high strength coated or not coated steel sheet and obtained sheet

Patent metadata

Field	Value
Publication number	US-10378077-B2
Application number	US-201415323232-A
Country	US
Kind code	B2
Filing date	Jul 3, 2014
Priority date	Jul 3, 2014
Publication date	Aug 13, 2019
Grant date	Aug 13, 2019

Abstract

A method for producing a cold rolled steel sheet having a tensile strength ≥1470 MPa and a total elongation TE≥19%, the method comprising the steps of annealing at an annealing temperature AT≥Ac3 a non-treated steel sheet whose chemical composition contains in weight %: 0.34%≤C≤0.40%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0%<Cr≤0.5%, 0%<Mo≤0.3%, 0.01%≤A1≤0.07%, the remainder being Fe and unavoidable impurities, quenching the annealed steel sheet by cooling it to a quenching temperature QT<Ms transformation point and between 150° C. and 250° C., and making a partitioning treatment by reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 420° C. and maintaining the steel sheet at this temperature during a partitioning time Pt between 15 seconds and 120 seconds.

First claim

The invention claimed is:

1. A non-coated steel sheet made of a steel comprising:
a chemical composition including in weight %:
0.34%≤C≤0.40%;

1.50%≤Mn≤2.30%;

1.50≤Si≤2.40%;

0.35%≤Cr≤0.45%;

0.07%≤Mo≤0.20%; and

0.01%≤Al≤0.08%;

a remainder being Fe and unavoidable impurities; and

a structure including, by volume, at least 60% of martensite and between 12% and 15% of residual austenite;
the steel sheet having a yield strength higher than 880 MPa, a tensile strength higher than 1520 MPa and a total elongation of at least 20%.

2. A method for producing the non-coated steel sheet according to claim 1 , the method comprising the steps of:
annealing at an annealing temperature AT a cold rolled steel sheet made of a steel having a chemical composition including in weight %:
0.34%≤C≤0.40%;

1.50%≤Mn≤2.30%;

1.50≤Si≤2.40%;

0.35%≤Cr≤0.45%;

0.07%≤Mo≤0.20%; and

0.01%≤Al≤0.08%;

a remainder being Fe and unavoidable impurities, the annealing temperature AT being higher than an Ac3 transformation point of the steel, to obtain an annealed steel sheet; quenching the annealed steel sheet by cooling the annealed steel sheet down to a quenching temperature QT lower than an Ms transformation point of the steel and between 200° C. and 230° C., to obtain a quenched steel sheet;
performing a partitioning treatment by reheating the quenched steel sheet at a partitioning temperature PT between 350° C. and 450° C. and maintaining the quenched steel sheet at the partitioning temperature PT during a partitioning time Pt between 15 seconds and 120 seconds;
and after partitioning, cooling the steel sheet to room temperature in order to obtain the non-coated steel sheet having the structure including, by volume, at least 60% of martensite and between 12% and 15% of residual austenite;
the non-coated steel sheet having a yield strength higher than 880 MPa, a tensile strength higher than 1520 MPa and a total elongation of at least 20%.

3. The method according to claim 2 , wherein the steps are performed successively.

4. The method according to claim 2 , wherein the annealing temperature AT is between 870° C. and 930° C.

5. A coated steel sheet made of a steel comprising:
a chemical composition comprising in weight %:
0.34%≤C≤0.40%;

1.50%≤Mn≤2.30%;

1.50≤Si≤2.40%;

0.35%≤Cr≤0.45%;

0.07%≤Mo≤0.20%; and

0.01%≤Al≤0.08%;

a remainder being Fe and unavoidable impurities; and

a structure including, by volume, at least 60% of martensite and between 12% and 15% of residual austenite;
the steel sheet being galvanized,
the steel sheet having a tensile strength higher than 1510 MPa and a total elongation of at least 20%.

6. A method for producing the coated steel sheet according to claim 5 , the method comprising the steps of:
annealing at an annealing temperature AT a cold rolled steel sheet made of a steel having a chemical composition including in weight %:
0.34%≤C≤0.40%;

1.50%≤Mn≤2.30%;

1.50≤Si≤2.40%;

0.35%≤Cr≤0.45%;

0.07%≤Mo≤0.20%; and

0.01%≤Al≤0.08%;

a remainder being Fe and unavoidable impurities, the annealing temperature AT being higher than an Ac3 transformation point of the steel, to obtain an annealed steel sheet; quenching the annealed steel sheet by cooling the annealed steel sheet down to a quenching temperature QT lower than an Ms transformation point of the steel and between 200° C. and 230° C., to obtain a quenched steel sheet;
performing a partitioning treatment by reheating the quenched steel sheet at a partitioning temperature PT between 350° C. and 450° C. and maintaining the quenched steel sheet at the partitioning temperature PT during a partitioning time Pt between 25 seconds and 55 seconds; and
galvanizing the steel sheet then cooling the steel sheet to room temperature in order to obtain the coated steel sheet having the structure including, by volume, at least 60% of martensite and between 12% and 15% of residual austenite;
the steel sheet having a tensile strength higher than 1510 MPa and a total elongation of at least 20%.

7. The method according to claim 6 , wherein the steps are performed successively.

Assignees

• Arcelormittal

Inventors

• Girina Olga A
• Panahi Damon

Classifications

• C21D8/02 Primary

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

• C21D2211/001

  Austenite · CPC title

• Y10T428/12799

  Next to Fe-base component [e.g., galvanized] · CPC title

• C21D2211/008

  Martensite · CPC title

• C21D1/25

  Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten") · CPC title