Method for producing an ultra high strength galvannealed steel sheet and obtained galvannealed steel sheet

Patent metadata

Field	Value
Publication number	US-11512362-B2
Application number	US-202017087019-A
Country	US
Kind code	B2
Filing date	Nov 2, 2020
Priority date	Dec 29, 2015
Publication date	Nov 29, 2022
Grant date	Nov 29, 2022

Abstract

A method for producing a coated steel sheet having a tensile strength TS of at least 1450 MPa and a total elongation TE of at least 17% includes the successive steps of providing a cold rolled steel sheet made of a steel having a chemical composition comprising, in weight %: 0.34%≤C≤0.45%, 1.50%≤Mn≤2.30%, 1.50≤Si≤2.40%, 0%<Cr≤0.7%, 0%≤Mo≤0.3%, 0.10%≤Al≤0.7%, and optionally 0%≤Nb≤0.05%, the remainder being Fe and unavoidable impurities, annealing the cold-rolled steel sheet at an annealing temperature AT higher than the Ac3 transformation point of the steel, quenching the annealed steel sheet by cooling it down to a quenching temperature QT lower than the Ms transformation point of the steel and comprised between 150° C. and 250° C., and reheating the quenched steel sheet to a partitioning temperature PT between 350° C. and 450° C. and maintaining the steel sheet at the partitioning temperature PT for a partitioning time Pt of at least 80 s, and coating the steel sheet by galvannealing, with an alloying temperature GAT comprised between 470° C. and 520° C.

First claim

What is claimed is:

1. A method for producing a galvannealed steel sheet, the method comprising successive steps of:
providing a cold rolled steel sheet made of a steel having a chemical composition including, by weight:
0.34%≤C≤0.45%
1.50%≤Mn≤2.30%
1.50≤Si≤2.40%
0%<Cr≤0.7%
0%≤Mo≤0.3%
0.10%≤Al≤0.7%,
optionally 0%≤Nb≤0.05%, and

a remainder, the remainder including Fe and unavoidable impurities;
annealing the cold rolled steel sheet at an annealing temperature AT higher than an Ac3 transformation point of the steel;
quenching the steel sheet by cooling the steel sheet down to a quenching temperature QT, lower than a Ms transformation point of the steel, between 150° C. and 250° C.;
reheating the steel sheet to a partitioning temperature PT between 350° C. and 450° C.;
maintaining the steel sheet at the partitioning temperature PT for a partitioning time Pt of at least 80 s; and
coating the steel sheet by hot dip coating in a zinc bath followed by galvannealing, with an alloying temperature GAT between 470° C. and 520° C., the steel sheet being heated to the alloying temperature with a heating rate of at least 20° C./s.

2. The method according to claim 1 , wherein during the quenching step, the steel sheet is cooled down to the quenching temperature QT at a cooling rate to avoid ferrite formation upon cooling, and to obtain a quenched steel sheet having a structure consisting of martensite and austenite.

3. The method according to claim 2 , wherein the cooling rate is greater than or equal to 20° C./s.

4. The method according to claim 1 , wherein the quenching temperature is between 200° C. and 230° C.

5. The method according to claim 1 , wherein the partitioning time Pt is between 100 s and 300 s.

6. The method according to claim 1 , wherein the annealing temperature AT is between 870° C. and 930° C.

7. The method according to claim 1 , wherein the alloying temperature GAT is between 480° C. and 500° C.

8. The method according to claim 1 , wherein the steel sheet is maintained at the alloying temperature GAT for a time GAt between 5 s and 15 s.

9. The method according to claim 1 , wherein the chemical composition of the steel includes, by weight, 0.10%≤Al≤0.30%.

10. The method according to claim 1 , wherein the chemical composition of the steel includes, by weight, 0.15%≤Al≤0.7%.

11. The method according to claim 1 , wherein the chemical composition of the steel includes, by weight, 0.03%≤Nb≤0.05%.

12. The method according to claim 1 , wherein the galvannealed steel sheet has a tensile strength TS of at least 1450M/Pa and a total elongation TE of at least 17%.

13. The method according to claim 1 , wherein providing the cold rolled steel sheet includes hot-rolling a steel sheet with a finish rolling temperature less than 850° C. then cold rolling.

14. The method according to claim 1 , wherein to be heated to the annealing temperature, the cold rolled steel sheet is first heated to a temperature of 600° C. at a heating rate below 20° C./s, then heated again to a temperature of 800° C. at a heating below 10° C./s and eventually heated to the annealing temperature at a heating rate below 5° C./s.

Assignees

• Arcelormittal

Inventors

• Girina Olga A
• Panahi Damon

Classifications

• C21D8/02

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

• B32B2311/30

  Iron, e.g. steel · CPC title

• C22C38/22

  with molybdenum or tungsten · CPC title

• Y10T428/12972

  Containing 0.01-1.7% carbon [i.e., steel] · CPC title

• C21D9/46 Primary

  for sheet metals · CPC title