Cold rolled and annealed steel sheet, method of production thereof and use of such steel to produce vehicle parts

Patent metadata

Field	Value
Publication number	US-12509741-B2
Application number	US-202318124997-A
Country	US
Kind code	B2
Filing date	Mar 22, 2023
Priority date	May 24, 2016
Publication date	Dec 30, 2025
Grant date	Dec 30, 2025

Abstract

A cold rolled and annealed steel sheet includes by weight: 0.6≤C≤1.3%, 15.0≤Mn≤35%, 5≤Al≤15%, Si≤2.40%, S≤0.03%, P≤0.1%, N≤0.1%, possibly one or more optional elements chosen among Ni, Cr and Cu in an respective amount of up to 4.0%, up to 3.0% and up to 3.0% and possibly one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%, the remainder of the composition making up of iron and inevitable impurities resulting from the elaboration, the microstructure of the sheet including optionally up to 3% of kappa carbides, optionally up to 10.0% of granular ferrite, the remainder being made of austenite, the average grain size and average aspect ratio of the austenite being respectively below 6 μm and comprised between 1.5 and 6 and the average grain size and average aspect ratio of the ferrite, when present, being respectively below 5 μm and below 3.0.

First claim

What is claimed is:

1 . A cold rolled and annealed steel sheet comprising by weight:
0.6≤C≤1.3%, 28.88≤Mn≤35%, 5≤Al≤12%, Si≤2.40%, S≤0.03%, P≤0.1%, N≤0.1%, the remainder of the composition making up of iron and inevitable impurities resulting from elaboration, the microstructure of said sheet comprising optionally up to 3% of kappa carbides, optionally up to 10% of granular ferrite, the remainder being made of austenite, an average grain size of the austenite being below 6 μm, an average aspect ratio of the austenite being between 1.5 and 6, an average grain size of the ferrite, when present, being below 5 μm, and an average aspect ratio of the ferrite, when present, being below 3.0; wherein the steel sheet has an ultimate tensile strength of at least 900 MPa, a yield strength of at least 700 MPa and a uniform elongation of at least 30%.

2 . The steel sheet according to claim 1 , wherein the steel sheet comprises the kappa carbides, and the kappa carbides are intragranular kappa carbides.

3 . The steel sheet according to claim 1 , comprising 5≤Al≤10%.

4 . The steel sheet according to claim 3 , wherein the steel sheet has an ultimate tensile strength of at least 1000 MPa, a yield strength of at least 800 MPa and a uniform elongation of at least 30%.

5 . The steel sheet according to claim 3 , wherein the steel sheet comprises the kappa carbides, the kappa carbides are intragranular kappa carbides, and the microstructure of the steel sheet is free from intergranular kappa carbides.

6 . The steel sheet of claim 5 , wherein the steel sheet is free from granular ferrite.

7 . The steel sheet according to claim 5 , wherein the microstructure consists of the intragranular kappa carbides, up to 10% granular ferrite and austenite.

8 . The steel sheet according to claim 7 , wherein the microstructure consists of the intragranular kappa carbides and austenite.

9 . The steel sheet according to claim 3 , wherein the uniform elongation is at least 31.3%.

10 . The steel sheet according to claim 3 , wherein the average aspect ratio of the austenite is between 1.5 and 2.6.

11 . The steel sheet according to claim 10 , wherein the microstructure comprises 1.3 to 6% granular ferrite.

12 . The steel sheet according to claim 1 , wherein the composition consists of C, Mn, Al, Si, S, P, N, iron and the inevitable impurities resulting from elaboration.

13 . The steel sheet according to claim 1 , wherein the microstructure of said steel sheet includes no more than 6% granular ferrite.

14 . The steel sheet according to claim 13 , wherein the microstructure of said steel sheet is free from intergranular kappa carbides, and the steel sheet has a density below 7.2.

15 . The steel sheet according to claim 1 , wherein the microstructure of said steel sheet includes no more than 5% granular ferrite.

16 . The steel sheet according to claim 1 , wherein the microstructure of said steel sheet includes no more than 3% granular ferrite.

17 . The steel sheet according to claim 1 , wherein the steel sheet further comprises one or more elements chosen among Ni, Cr and Cu in a respective amount of up to 4.0%, up to 3.0% and up to 3.0%.

18 . The steel sheet according to claim 17 , wherein the steel sheet further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%.

19 . The steel sheet according to claim 17 , wherein the respective amount of Cu is below 0.1%.

20 . The steel sheet according to claim 1 , wherein the steel sheet further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%.

21 . The steel sheet according to claim 1 , wherein the carbon content is between 0.8 and 1.0%.

22 . The steel sheet according to claim 1 , wherein the aluminum content is between 8.5 and 10%.

23 . The steel sheet according to claim 1 , wherein the steel sheet is covered by a metallic coating.

24 . The steel sheet according to claim 1 , wherein the steel sheet is covered by an aluminum-based coating or a zinc-based coating.

25 . A method for producing the steel sheet according to claim 1 , comprising:
feeding a slab having a composition including, by weight: 0.6 ≤C≤1.3%, 28.88 ≤Mn≤35%, 5 ≤Al≤12, Si≤2.40%, S≤0.03%, P≤0.1%, N≤0.1%, the remainder of the composition making up of iron and inevitable impurities resulting from elaboration; reheating the slab at a temperature above 1000° C. and hot rolling the slab with a final rolling temperature of at least 800° C. to obtain a hot rolled steel sheet; coiling the hot rolled steel sheet at a temperature of 400° C. or less; cold-rolling such hot rolled steel sheet at a reduction comprised between 30 and 80%; performing, in a single annealing step, annealing such cold rolled sheet by heating it up to an annealing temperature between 700 and 1000° C., holding it at such temperature during less than 5 minutes and cooling it at a rate of at least 30° C./s, and thereby producing the steel sheet according to claim 1 .

26 . The method according to claim 25 , wherein the composition further comprises one or more elements chosen among Ni, Cr and Cu in an respective amount of up to 4.0%, up to 3.0% and up to 3.0%.

27 . The method according to claim 25 , wherein the composition further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%.

28 . The method according to claim 27 , wherein the steel sheet further comprises one or more elements chosen among B, Ta, Zr, Nb, V, Ti, Mo, and W in a cumulated amount of up to 2.0%.

29 . The method according to claim 25 , wherein the annealing temperature is between 800 and 950° C.

30 . The method according to claim 25 , wherein the coiling temperature is between 350 and 400° C.

31 . The method according to claim 25 , further comprising applying a metallic coating to said sheet after said cooling.

32 . The method according to claim 25 , wherein in the single annealing step, the cold rolled sheet is held at the annealing temperature for 1 to 3 minutes.

33 . A structural or safety part of a vehicle comprising the steel sheet of claim 1 , flexibly rolled into said structural or safety part.

34 . A vehicle comprising the part according to claim 33 .

35 . The steel sheet according to claim 1 , comprising 0.8 ≤C≤1.3%.

Assignees

• Arcelormittal

Inventors

• Soler Michel
• Garat Xavier

Classifications

• C21D8/02 Primary

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

• C22C38/002

  containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60 · CPC title

• C21D8/0263

  following hot rolling · CPC title

• C21D6/008

  containing Si · CPC title

• B32B15/013

  one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium · CPC title