Cold rolled and annealed steel sheet and method of manufacturing the same

Patent metadata

Field	Value
Publication number	US-11530461-B2
Application number	US-201816766328-A
Country	US
Kind code	B2
Filing date	Nov 21, 2018
Priority date	Dec 5, 2017
Publication date	Dec 20, 2022
Grant date	Dec 20, 2022

Abstract

A cold-rolled and heat-treated steel sheet having a microstructure consisting of, in surface fraction: between 10% and 30% of retained austenite, the retained austenite being present as films having an aspect ratio of at least 3 and as Martensite Austenite islands, less than 8% of the Martensite Austenite islands having a size above 0.5 μm, at most 1% of fresh martensite, at most 50% of tempered martensite, and recovered martensite containing precipitates of at least one element chosen among niobium, titanium and vanadium. A method for manufacturing the cold-rolled and heat-treated steel sheet is also described.

First claim

What is claimed is:

1. A cold-rolled and heat-treated steel sheet, made of a steel having a composition comprising by weight percent:
C: 0.03-0.25%,
Mn: 3.5-8%,
Si: 0.5-2.0%,
Al: 0.03-2.0%,
Ti≤0.080%,
Nb≤0.080%,
V≤0.2%,
V+Ti+Nb>0.01%,
S≤0.010%,
P≤0.020%
N≤0.008%,

and optionally including at least one of the following elements, in weight percentage:
Mo: 0.1-0.5%,
Cr: 0.01-1%,
B: 0.0005-0.004%,

a remainder of the composition being iron and unavoidable impurities resulting from processing,
the cold-rolled and heat-treated steel sheet having a microstructure consisting of, in surface area fraction:
between 10% and 30% of retained austenite, the retained austenite being present as austenite films having an aspect ratio of at least 3 and as Martensite Austenite islands, less than 8% of the Martensite Austenite islands having a size above 0.5 μm,
at most 1% of fresh martensite,
at most 50% of tempered martensite, and
recovered martensite containing precipitates of at least one element chosen from the group consisting of: niobium, titanium and vanadium.

2. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein aluminium content is at most 1.2%.

3. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein niobium content is at least 0.010%.

4. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein carbon content is at least 0.10%.

5. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein a surface area fraction ratio between the Martensite Austenite islands having the size above 0.5 μm and the austenite films is below 1.0.

6. The cold-rolled and heat-treated steel sheet as recited in claim 5 wherein the surface area fraction ratio is below 0.5.

7. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the surface area fraction of the fresh martensite is below 1%.

8. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein less than 5% of the Martensite Austenite islands have the size above 0.5 μm.

9. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the surface area fraction of the austenite films having an aspect ratio above 3 is at least 5%.

10. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the cold-rolled and heat-treated steel sheet is coated with Zn or a Zn alloy.

11. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the cold-rolled and heat-treated steel sheet is coated with Al or a Al alloy.

12. The cold-rolled and heat-treated steel sheet as recited in claim 1 wherein the cold-rolled and heat-treated steel sheet has a yield strength YS of at least 750 MPa, a tensile strength TS of at least 1000 MPa, a uniform elongation UE of at least 12%, and a hole expansion ratio HER of at least 20%.

13. A method for manufacturing a cold-rolled and heat-treated steel sheet, comprising the following successive steps:
casting a steel so as to obtain a slab, the steel having a composition comprising by weight percent:
C: 0.03-0.25%,
Mn: 3.5-8%,
Si: 0.5-2.0%,
Al: 0.03-2.0%,
Ti≤0.080%,
Nb≤0.080%,
V≤0.2%,
V+Ti+Nb>0.01%,
S≤0.010%,
P≤0.020%,
N≤0.008%,

and optionally including at least one of the following elements, in weight percentage:
Mo: 0.1-0.5%,
Cr: 0.01-1%,
B: 0.0005-0.004%,

a remainder of the composition being iron and unavoidable impurities resulting from processing;
reheating the slab at a temperature T reheat between 1150° C. and 1300° C.;
hot rolling the reheated slab at a temperature higher than Ar3 to obtain a hot rolled steel sheet;
coiling the hot rolled steel sheet at a coiling temperature T coil between 20° C. and 600° C.;
annealing the hot rolled steel sheet, to obtain a hot-rolled and annealed steel sheet;
cold rolling the hot-rolled and annealed steel sheet so as to obtain a cold rolled steel sheet;
reheating the cold-rolled steel sheet to a first annealing temperature between Ae3 and Ae3+100° C. and maintaining the cold-rolled steel sheet at the first annealing temperature for a holding time between 30 s and 600 s, so as to obtain, upon annealing, a fully austenitic structure;
quenching the cold-rolled steel sheet at a cooling rate between 0.5° C./s and 200° C./s, to a quenching temperature between 20° C. and Ms-50° C. and maintaining the cold-rolled steel sheet at the quenching temperature for a holding time between 1 and 200 s;
reheating the cold-rolled steel sheet to a second annealing temperature comprised between TΘ and Ae3-30° C., TΘ designating the temperature above which the cementite is dissolved upon heating, and maintaining the cold-rolled steel sheet at the second annealing temperature for a time between 100 s and 2000 s;
cooling the cold-rolled steel sheet to the room temperature; and
tempering the cold-rolled steel sheet, at a temperature between 170° C. and 500° C., for a holding time between 3 and 1200 s to obtain a cold-rolled and heat treated steel sheet as recited in claim 1 .

14. The method as recited in claim 13 wherein the annealing of the hot-rolled steel sheet is a batch annealing, performed at a temperature 500° C. and 680° C. for 1000 s to 50000 s.

15. The method as recited in claim 13 wherein the reheating rate of the cold rolled steel sheet to the first annealing temperature is between 1 and 200° C./s.

16. A method for producing a spot welded joint of at least two steel sheets, comprising the steps of:
providing the cold-rolled and heat-treated steel sheet manufactured according to the method as recited in claim 13 ;
providing a second steel sheet; and
spot welding the cold-rolled and heat-treated steel sheet to the second steel sheet.

17. The method as recited in claim 16 wherein the second steel sheet is a cold-rolled and heat-treated steel sheet manufactured according to the method as recited in claim 13 .

18. A method for producing a spot welded joint of at least two steel sheets, comprising the steps of:
providing the cold-rolled and heat-treated steel sheet as recited in claim 1 ;
providing a second steel sheet; and
spot welding the cold-rolled and heat-treated steel sheet to the second steel sheet.

19. The method as recited in claim 18 wherein the second steel sheet is a cold-rolled and heat-treated steel sheet as recited in claim 1 .

Assignees

• Arcelormittal

Inventors

• Zhu Kangying
• Perlade Astrid
• Jung Coralie
• Kegel Frederic

Classifications

• C21D8/02

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

• C22C38/001

  containing N · CPC title

• B23K11/163

  Welding of coated materials · CPC title

• C21D6/008

  containing Si · CPC title

• B32B15/04

  comprising metal as the main or only constituent of a layer, {which is} next to another layer of {the same or of} a {different material (next to a bituminous or tarry layer B32B11/08; next to a water-setting substance layer B32B13/06; next to a glass layer B32B17/061; next to a cellulosic plastic layer B32B23/042)} · CPC title