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

Patent metadata

Field	Value
Publication number	US-2020347471-A1
Application number	US-201816765207-A
Country	US
Kind code	A1
Filing date	Nov 21, 2018
Priority date	Dec 5, 2017
Publication date	Nov 5, 2020
Grant date	—

Abstract

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

First claim

1 - 17 . (canceled)

18 . 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: 0.1-2.0% 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 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 such Martensite Austenite islands having a size above 0.5 μm, at most 10% of fresh martensite, and recovered martensite containing precipitates of at least one element chosen from the group consisting of: niobium, titanium and vanadium.

19 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein aluminium content is at most 1.2%.

20 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein niobium content is at least 0.010%.

21 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein carbon content is at least 0.10%.

22 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein a fraction ratio between the Martensite Austenite islands having the size above 0.5 μm and the austenite films is below 1.0.

23 . The cold-rolled and heat-treated steel sheet as recited in claim 22 wherein the fraction ratio is below 0.5.

24 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the surface fraction of the fresh martensite is below 5%.

25 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein less than 5% of the Martensite Austenite islands have the size above 0.5 μm.

26 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the surface fraction of the austenite films having an aspect ratio above 3 is at least 8%.

27 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the cold-rolled and heat-treated steel sheet is coated with Zn or a Zn alloy.

28 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the cold-rolled and heat-treated steel sheet is coated with Al or a Al alloy.

29 . The cold-rolled and heat-treated steel sheet as recited in claim 18 wherein the cold-rolled and heat-treated steel sheet has a yield strength YS of at least 700 MPa, a tensile strength TS of at least 900 MPa, a uniform elongation UE of at least 12%, and a hole expansion ratio HER of at least 20%.

30 . 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: 0.1-2.0%
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 above 860° 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 between TΘ and 720° C., TΘ designating the temperature above which 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, to obtain a cold-rolled and heat treated steel sheet.

31 . The method as recited in claim 30 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.

32 . The method as recited in claim 30 wherein the reheating rate of the cold rolled steel sheet to the first annealing temperature is between 1 and 200° C./s.

33 . 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 30 ; providing a second steel sheet; spot welding the cold-rolled and heat-treated steel sheet to the second steel sheet.

34 . The method as recited in claim 33 wherein the second steel sheet is a cold-rolled and heat-treated steel sheet manufactured according to the method as recited in claim 30 .

35 . 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 18 ; providing a second steel sheet; spot welding the cold-rolled and heat-treated steel sheet to the second steel sheet.

36 . The method as recited in claim 35 wherein the second steel sheet is a cold-rolled and heat-treated steel sheet as recited in claim 18 .

Assignees

• Arcelormittal

Inventors

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

Classifications

• C21D1/26 Primary

  Methods of annealing · CPC title

• C21D6/005

  containing Mn · CPC title

• C21D1/78

  Combined heat-treatments not provided for above · CPC title

• C21D8/0447

  characterised by the heat treatment · CPC title

• C21D8/0473

  Final recrystallisation annealing · CPC title