High strength steel and manufacturing method

Patent metadata

Field	Value
Publication number	US-10597745-B2
Application number	US-201415102386-A
Country	US
Kind code	B2
Filing date	Dec 5, 2014
Priority date	Dec 11, 2013
Publication date	Mar 24, 2020
Grant date	Mar 24, 2020

Abstract

A cold-rolled and annealed steel sheet is provided, the chemical composition of which comprises, the contents being expressed by weight percent:
0.10≤C≤0.13% 2.4≤Mn≤2.8% 0.30≤Si≤0.55% 0.30≤Cr≤0.56% 0.020≤Ti≤0.050% 0.0020≤B≤0.0040% 0.005≤Al≤0.050% Mo≤0.010% Nb≤0.040% 0.002≤N≤0.008% S≤0.005% P≤0.020%,

the remainder consisting of iron and unavoidable impurities resulting from the smelting, the steel sheet having a microstructure consisting of, in surface proportion, martensite and/or lower bainite, said martensite comprising fresh martensite and/or self-tempered martensite, the sum of the surface proportions of martensite and lower bainite being comprised between 60 to 95%, 4 to 35% of low carbide containing bainite, 0 to 5% of ferrite, and less than 5% of retained austenite in island form.

First claim

What is claimed is:

1. A cold-rolled and annealed steel sheet, having a chemical composition consisting of with contents expressed by weight percent:
0.10≤C≤0.13%
2.4≤Mn≤2.8%
0.30≤Si≤0.55%
0.30≤Cr≤0.56%
0.020≤Ti≤0.050%
0.0020≤B≤0.0040%
0.005≤Al≤0.050%
Mo≤0.010%
Nb≤0.040%
0.002≤N≤0.008%
S≤0.005%
P≤0.020%,
and a remainder consisting of iron and unavoidable impurities resulting from smelting, wherein the steel sheet has a microstructure consisting of, in surface proportions:
martensite and/or lower bainite, a sum of the surface proportions of martensite and lower bainite being comprised between 60 and 95%,
4 to 35% of low carbide containing bainite, containing less than 100 carbides per surface unit of 100 square micrometers,
0 to 5% of ferrite, and
less than 5% of retained austenite in island form, and

wherein the martensite consists of fresh martensite and/or self-tempered martensite, a sum of the surface proportions of self-tempered martensite and lower bainite being comprised between 40 and 95%.

2. The steel sheet according to claim 1 , wherein a surface proportion of fresh martensite is comprised between 4% and 20%.

3. The steel sheet according to claim 1 , wherein a surface proportion of fresh martensite is comprised between 4% and 15%.

4. The steel sheet according to claim 1 , wherein said self-tempered martensite and said lower bainite contain rod-shaped carbides oriented in directions <111> of martensitic and bainite laths.

5. The steel sheet according to claim 1 , wherein the surface proportion of ferrite is comprised between 4 and 5%.

6. The steel sheet according to claim 1 , wherein the retained austenite islands have a smallest dimension smaller than 50 nanometers.

7. The steel sheet according to claim 1 , wherein a fraction of former austenite grains created by the annealing whose size is less than one micrometer represents less than 10% of a total population of said former austenite grains.

8. The steel sheet according to claim 1 , wherein said steel sheet has a tensile strength comprised between 1180 MPa and 1320 MPa, and a hole expansion ratio Ac % greater than or equal to 40%.

9. The steel sheet according to claim 1 , wherein said steel sheet has a thickness comprised between 0.7 mm and 1.5 mm, and said steel sheet has a bending angle greater than or equal to 55°.

10. The steel sheet according to claim 1 , wherein Mn is present in the chemical composition in an amount, by weight percent, comprised between 2.5% and 2.8%.

11. The steel sheet according to claim 1 , wherein Si is present in the chemical composition in an amount, by weight percent, comprised between 0.30 and 0.5%.

12. The steel sheet according to claim 1 , wherein Al is present in the chemical composition in an amount, by weight percent, comprised between 0.005% and 0.030%.

13. The steel sheet according to claim 1 , wherein said steel sheet comprises a zinc or zinc alloy coating, obtained through continuous dip coating.

14. The steel sheet according to claim 13 , wherein said zinc or zinc alloy coating is a galvannealed coating, said zinc or zinc alloy coating comprising 7 to 12% of iron.

15. The steel sheet according to claim 1 , wherein said steel sheet comprises a zinc or zinc alloy coating, obtained through vacuum deposition.

16. A method for manufacturing a cold-rolled and annealed steel sheet according to claim 1 , comprising the following successive steps:
providing a semi-finished steel having a chemical composition consisting of, with contents expressed by weight percent:
0.10≤C≤0.13%
2.4≤Mn≤2.8%
0.30≤Si≤0.55%
0.30≤Cr≤0.56%
0.020≤Ti≤0.050%
0.0020≤B≤0.0040%
0.005≤Al≤0.050%
Mo≤0.010%
Nb≤0.040%
0.002≤N≤0.008%
S≤0.005%
P≤0.020%

and a remainder consisting of iron and unavoidable impurities resulting from smelting, then
heating said semi-finished steel to a temperature T reheat greater than or equal to 1250° C., then
hot-rolling said semi-finished steel, with an end of rolling temperature greater than a temperature Ar3 of the beginning of transformation of austenite upon cooling, to obtain a hot-rolled steel sheet, then
cooling said hot-rolled steel sheet at a rate sufficient to avoid a formation of ferrite and perlite, then
coiling said hot-rolled steel sheet at a temperature below 580° C., then
cold-rolling said hot-rolled steel sheet to obtain a cold-rolled steel sheet, then
reheating said cold-rolled steel sheet between 600° C. and Acl, Acl designating a beginning of austenitic transformation temperature upon heating, at a heating rate V R comprised between 1 and 20° C./s, then
reheating said cold-rolled steel sheet to a temperature Tm comprised between Ac3′−10° C. and Ac3′+30° C., and holding said cold-rolled steel sheet at said temperature Tm for a time Dm comprised between 50 and 150 seconds, with Ac3′=Min{Ac3+1200/Dm; 1000° C.}, where Ac3 and Ac3′ are expressed in degrees Celsius and Dm in seconds, and where Ac3 designates an end of austenitic transformation temperature upon heating as determined independently from a holding time at that temperature Ac3, then
cooling the steel sheet at a rate comprised between 10 and 150° C./s to a temperature Te comprised between 460° C. and 490° C., then
holding said steel sheet at the temperature Te for a time comprised between 5 and 150 seconds, then
coating the steel sheet by continuous dipping in a zinc or zinc alloy bath at a temperature TZn comprised between 450° C. and 480° C., said temperatures Te and TZn being such that 0≤(Te-TZn)<10° C.

17. The method according to claim 16 , further comprising a step of heating the coated steel sheet to a temperature comprised between 490° C. and 550° C. for a time t G comprised between 10 s and 40 s.

18. A method for manufacturing a cold-rolled and annealed steel sheet according to claim 1 , comprising the following successive steps:
providing a semi-finished steel having a chemical composition consisting of, with contents expressed by weight percent:
0.10≤C≤0.13%
2.4≤Mn≤2.8%
0.30≤Si≤0.55%
0.30≤Cr≤0.56%
0.020≤Ti≤0.050%
0.0020≤B≤0.0040%
0.005≤Al≤0.050%
Mo≤0.010%
Nb≤0.040%
0.002≤N≤0.008%
S≤0.005%
P≤0.020%

and a remainder consisting of iron and unavoidable impurities resulting from smelting, then
heating said semi-finished steel to a temperature T reheat greater than or equal to 1250° C., then
hot-rolling said semi-finished steel, with an end of rolling temperature greater than Ar3, to obtain a hot-rolled steel sheet, then
cooling said hot-rolled steel sheet at a rate sufficient to avoid a formation of ferrite and perlite, then
coiling said hot-rolled steel sheet at a temperature below 580° C., then
cold-rolling said hot-rolled steel sheet to obtain a cold-rolled steel sheet, then
reheating said cold-rolled steel sheet between 600° C. and Acl, Acl designating a beginning of austenitic transformation temperature upon heating, at a heating rate V R comprised between 1 and 20° C./s, then
reheating said cold-rolled steel sheet to a temperature Tm comprised between Ac3-10° C. and Ac3+30° C., and holding said cold-rolled steel sheet at the temperature Tm for a time Dm comprised between 50 and 150 seconds, with Ac3′=Min{Ac3+1200/Dm; 1000° C.}, where Ac3 and Ac3′ are expressed in degrees Celsius and Dm in seconds, and where Ac3 designates an end of austenitic transformation temperature upon heating as determined independently from a holding time at that temperature Ac3, then
cooling the steel sheet at a rate comprised between 10 and 100° C./s to a temperature Te comprised between

Assignees

• Arcelormittal

Inventors

• Drillet Josée
• Hebert Véronique

Classifications

• C22C38/38 Primary

  with more than 1.5% by weight of manganese · CPC title

• C22C38/32

  with boron · CPC title

• C23C2/40

  Plates; Strips · CPC title

• C23C2/06

  Zinc or cadmium or alloys based thereon · CPC title

• C23C14/16

  on metallic substrates or on substrates of boron or silicon · CPC title