Manufacturing method for very high-strength, cold-rolled, dual-phase steel sheets

Patent metadata

Field	Value
Publication number	US-10190187-B2
Application number	US-201615097039-A
Country	US
Kind code	B2
Filing date	Apr 12, 2016
Priority date	May 21, 2008
Publication date	Jan 29, 2019
Grant date	Jan 29, 2019

Abstract

The present invention provides a cold-rolled and annealed Dual-Phase steel sheet having strength from 980 to 1100 MPa and a breaking elongation greater than 9%. The composition includes the contents being expressed by weight: 0.055%≤C≤0.095%, 2%≤Mn≤2.6%, 0.005%≤Si≤0.35%, S≤0.005%, P≤0.050%, 0.1≤Al≤0.3%, 0.05%≤Mo≤0.25%, 0.2%≤Cr≤0.5%, Cr+2Mo≤0.6%, Ni≤0.1%, 0.010≤Nb≤0.040%, 0.010≤Ti≤0.050%, 0.0005≤B≤0.0025%, and 0.002%≤N≤0.007%. The remainder of the composition includes iron and inevitable impurities resulting from the smelting. A manufacturing method is also provided.

First claim

What is claimed is:

1. A manufacturing method for a cold-rolled and annealed Dual-Phase steel sheet, comprising the steps of:
supplying a steel sheet having a composition comprising:
0.055%≤C≤0.095%;
2%≤Mn≤2.6%;
0.005%≤Si≤0.35%;
S≤0.005%;
P≤0.050
0.1≤Al≤0.3%;
0.05%≤Mo≤0.25%;
0.2%≤Cr≤0.5%;
Cr+2Mo≤0.6%;
Ni≤0.1%;
0.010≤Nb≤0.040%;
0.010≤Ti≤0.050%;
0.0005≤B≤0.0025% and
0.002%≤N≤0.007%,
a remainder of the composition comprising iron and the inevitable impurities resulting from smelting;

casting the steel sheet as a semi-finished product;
bringing the semi-finished product to a temperature T R in which 1150° C. ≤T R ≤1250° C.;
hot-rolling the semi-finished product with an end-of-rolling temperature TFL ≥A r3 to obtain a hot-rolled product;
coiling the hot-rolled product at a temperature T bob in which 500° C. ≤T bob ≤570° C.;
descaling the hot-rolled product;
cold-rolling with a reduction from 30 to 80% to obtain a cold-rolled product;
heating the cold-rolled product at a rate 1° C./s≤Vc ≤5° C/s to an annealing temperature T M in which Ac1+40° C. ≤T M ≤Ac3-30° C. at which the product is held for a time 30s≤t M ≤300s so as to obtain a heated and annealed product with a structure comprising austenite and a non-recrystallized ferrite fraction less than or equal to 15%;
cooling the product to a temperature less than a temperature Ms at a rate V high enough for all of the austenite to transform to martensite;
the steel sheet having a tensile strength between 980 and 1100 MPA and a microstructure consisting of 40 to 65% ferrite, 35 to 50% martensite and 0 to 10% bainite.

2. The manufacturing method as recited in claim 1 , further comprising the steps of:
cooling the heated and annealed product at a rate V R high enough to prevent transformation of the austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T zn is reached;
continuously galvanizing the product by immersion in a bath of zinc or Zn alloy at a temperature 450° C. ≤T zn ≤480° C. to obtain a galvanized product;
cooling the galvanized product to ambient temperature at a rate V′ R greater than 4° C./s to obtain a cold-rolled, annealed and galvanized steel sheet.

3. The manufacturing method as recited in claim 1 , further comprising the steps of:
cooling the heated and annealed product at a rate V R high enough to prevent transformation of the austenite to ferrite, until a temperature close to the hot-dip galvanizing temperature T zn is reached;
continuously galvanized the product by immersion in a bath of zinc or Zn alloy at a temperature 450° C. ≤T zn ≤480° C. to obtain a galvanized product;
heating the galvanized product to a temperature TG from 490 to 550° C. for a time t G from 10 to 40s to obtain a galvannealed product;
cooling the galvannealed product to ambient temperature at a rate V″ R greater than 4° C./s, to obtain a cold-rolled and galvannealed steel sheet.

4. The manufacturing method as recited in claim 1 wherein the temperature T M is from 760 to 830° C.

5. The manufacturing method as recited in claim 2 , wherein the rate of cooling V R is greater than or equal to 15° C./s.

6. The manufacturing method as recited in claim 3 , wherein the rate of cooling V R is greater than or equal to 15° C./s.

7. A method for manufacturing a structural or safety part for a motor vehicle comprising:
the manufacturing method recited in claim 1 .

8. The manufacturing method as recited in claim 1 , wherein the microstructure of the cold-rolled and annealed dual-phase steel sheet consists of ferrite and martensite.

9. The manufacturing method as recited in claim 1 , wherein the composition of the steel sheet consists essentially of:
0.055%≤C≤0.095%;
2%≤Mn≤2.6%;
0.005%≤Si≤0.35%;
S≤0.005%;
P≤0.050%;
0.1≤Al≤0.3%;
0.05%≤Mo≤0.25%;
0.2%≤Cr≤0.5%;
Cr+2Mo≤0.6%;
Ni≤0.1%;
0.010≤Nb≤0.040%;
0.010≤Ti≤0.050%;
0.0005≤B≤0.0025% and
0.002%≤N≤0.007%,
a remainder of the composition consisting of iron and the inevitable impurities resulting from smelting.

Assignees

• Arcelormittal

Inventors

• Moulin Antoine
• Sardoy Veronique
• Vinci Catherine
• Restrepo Gacess Gloria
• Watershoot Tom
• Goune Mohamed

Classifications

• C21D8/02

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

• C21D6/008

  containing Si · CPC title

• C21D9/46 Primary

  for sheet metals · CPC title

• C21D6/004

  containing Cr and Ni · CPC title

• C21D8/021

  involving particular fabrication steps or treatments of ingots or slabs · CPC title