Method for Manufacturing a High Strength Steel Sheet and Sheet Obtained by the Method

Patent metadata

Field	Value
Publication number	US-2017137909-A1
Application number	US-201515323200-A
Country	US
Kind code	A1
Filing date	Jul 3, 2015
Priority date	Jul 3, 2014
Publication date	May 18, 2017
Grant date	—

Abstract

A method for manufacturing a high-strength steel sheet having a tensile strength of more than 1100 MPa and a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation of at least 10%, made of a steel containing in percent by weight: 0.1%≦C≦0.25%, 4.5%≦Mn≦10%, 1%≦Si≦3%, 0.03%≦Al≦2.5%, the remainder being Fe and impurities resulting from the smelting, the composition being such that CMn Index=Cx(1+Mn/3.5)≦0.6, the method comprising the steps of annealing a rolled sheet made of said steel by soaking it at an annealing temperature TA higher than the Ac transformation point of the steel but less than 1000° C., cooling the annealed sheet to a quenching temperature QT between 190° C. and 80° C. at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, maintaining the steel sheet at an overaging temperature PT between 350° C. and 500° C. for an overaging time Pt of more than 5 s cooling the sheet down to the ambient temperature. Sheet obtained.

First claim

1 - 25 . (canceled)

26 . A method for manufacturing a steel sheet having a tensile strength of more than 1100 MPa, a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation TE of at least 10.0%, comprising the steps of:
providing a steel sheet, the steel having a chemical composition including in percent by weight:
0.1%≦C≦0.25%;

4.5%≦Mn≦10%;

1%≦Si≦3%; and

0.03%≦Al≦2.5%;

a remainder being Fe and impurities resulting from the smelting, and
the chemical composition including a CMnIndex=Cx(1+Mn/3.5)≦0.6;
annealing a rolled sheet made of the steel by soaking the rolled sheet at an annealing temperature TA higher than the Ac3 transformation point of the steel but less than 1000° C., and maintaining the rolled sheet at the annealing temperature for 60 s to 200 s, to obtain an annealed sheet; cooling the annealed sheet to a quenching temperature QT between 190° and 80° C., at a cooling speed sufficient to obtain a quenched sheet having a structure just after cooling including martensite and retained austenite, the quenching temperature QT being such that the steel sheet produced by the method has a final structure including at least 20% of retained austenite and at least 65% of martensite, a sum of ferrite and bainite contents being less than 10%; maintaining the quenched sheet at an overaging temperature PT between 350° C. and 500° C. for an overaging time Pt of more than 5 s; cooling the steel sheet down to ambient temperature.

27 . The method according to claim 26 , wherein the chemical composition of the steel includes:
0.15%≦C≦0.25%;
4.5%≦Mn≦5.5%;
1.4%≦Si≦1.8%;
0.03%≦Al≦2.5%; and
C x (1+Mn/3.5)≦0.6, and
wherein the annealing temperature AT is higher than 760° C., the quenching temperature QT is less than 170° C. and the final structure of the steel sheet includes between 20% and 30% of retained austenite.

28 . The method according to claim 27 , wherein the quenching temperature QT is less than 150° C.

29 . The method according to claim 27 , wherein the overaging temperature PT is between 380° C. and 470° C. and the overaging time Pt is between 90 s and 600 s.

30 . The method according to claim 26 , wherein the chemical composition of the steel includes:
0.15%≦C≦0.25%;
6.5%≦Mn≦7.5%;
1.4%≦Si≦1.8%;
0.03%≦Al≦2.5%; and
C x (1+Mn/3.5)≦0.6;
wherein the annealing temperature AT is higher than 710° C., the quenching temperature QT is between 120° C. and 180° C., the overaging temperature PT is between 350° C. and 470° C. and the overaging time Pt is between 5 s and 600 s.

31 . A method for manufacturing a steel sheet having a tensile strength of more than 1100 MPa, a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation of at least 10.0%, comprising the steps of:
providing a steel having a chemical composition containing in percent by weight:
0.15%≦C≦0.25%;

4.5%≦Mn≦5.5%;

1.4%≦Si≦1.8%; and

0.03%≦Al≦2.5%;

a remainder being Fe and impurities resulting from the smelting, and
the chemical composition including a CMnIndex=Cx(1+Mn/3.5)≦0.6;
annealing a rolled sheet made of the steel by soaking the rolled sheet at an annealing temperature TA higher than the Ac 1 transformation point of the steel but less than 1000° C. and less than the Ac 3 transformation point of the steel, to obtain an annealed sheet; cooling the annealed sheet to a quenching temperature QT between 110° and 170° C., at a cooling speed sufficient to obtain a quenched sheet having a structure just after cooling including martensite and retained austenite; maintaining the quenched sheet at an overaging temperature PT between 350° C. and 500° C. for an overaging time Pt between 5 s and 600 s; cooling the steel sheet down to ambient temperature; the steel sheet having a final structure including at least 15% of ferrite, at least 50% of martensite and at least 15% of retained austenite.

32 . The method according to claim 26 , wherein the chemical composition of the steel includes 0.03%≦Al≦0.5%.

33 . The method according to claim 32 , wherein the chemical composition of the steel includes Si+Al≧1.4%.

34 . The method according to claim 26 , wherein the chemical composition of the steel is includes 1.4%≦Al≦2.5%.

35 . The method according to claim 26 , wherein the overaging temperature PT is between 440° C. and 470° C. and the overaging time Pt is between 5 s and 60 s.

36 . The method according to claim 25 , wherein at least part of the step of maintaining the quenched sheet at the overaging temperature PT includes passing the quenched sheet in a hot dip coating bath.

37 . The method according to claim 36 , further comprising, after passing the sheet in the hot dip coating bath and before cooling to ambient temperature, a step of maintaining the steel sheet at a temperature between 480° C. and 570° C.

38 . The method according to claim 26 , wherein the annealing, the quenching and the overaging are performed on a continuous heat treatment line.

39 . The method according to claim 26 , wherein the rolled sheet is prepared by rolling and wherein rolling includes hot rolling and cold rolling.

40 . A steel sheet having a tensile strength of more than 1100 MPa, a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation TE of at least 10.0%, comprising:
a steel having a chemical composition including, in weight percent:
0.1%≦C≦0.25%;

4.5%≦Mn≦10%;

1%≦Si≦3%; and

0.03%≦Al≦2.5%;

a remainder being Fe and impurities resulting from the smelting, and
the chemical composition including a CMnIndex=Cx(1+Mn/3.5)≦0.6; and
a structure including at least 20% of retained austenite and at least 65% of martensite and a sum of ferrite and bainite contents being less than 10%.

41 . The steel sheet according to claim 15 , wherein the chemical composition of the steel includes:
0.15%≦C≦0.25%;
4.5%≦Mn≦5.5%;
1.4%≦Si≦1.8%;
0.03≦Al≦2.5%; and
CMnIndex=C x (1+Mn/3.5)≦0.6.

42 . The steel sheet according to claim 41 , wherein the yield strength YS is higher than 1100 MPa, the tensile strength TS is higher than 1350 MPa, the uniform elongation UE is of more than 10% and the total elongation TE is of more than 12%.

43 . The steel sheet according to claim 40 , wherein the chemical composition of the steel includes:
0.15%≦C≦0.25%;
6.5%≦Mn≦7.5%;
1.4%≦Si≦1.8%;
0.03≦Al≦2.5%; and
C x (1+Mn/3.5)≦0.6, and
wherein the yield strength YS is higher than 1000 MPa and the tensile strength TS is higher than 1100 MPa.

44 . A steel sheet having a tensile strength of more than 1100 MPa, a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation TE of at least 10.0%, comprising:
a steel having a chemical composition including, in weight percent:
0.15%≦C≦0.25%;

4.5%≦Mn≦5.5%;

1.4≦Si≦1.8%; and

0.03≦Al≦2.5%;

a remainder being Fe and impurities resulting from the smelting, and
the chemical composition including a CMnIndex=Cx(1+Mn/3.5)≦0.6;
a structure including at least 15% of ferrite, at least 50% of martensite and at least 15% of retained austenite.

45 . The steel sheet according to claim 40 , wherein the chemical composition of the steel includes 0.03%≦Al≦0.5%.

46 . The steel sheet according to claim 45 , wherein the chemical composition of the steel includes Si+Al≧1.4%.

Assignees

• Arcelormittal

Inventors

• Xu Wei
• Arlazarov Artem

Classifications

• C21D8/02

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

• C21D1/25

  Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten") · CPC title

• C21D2211/008

  Martensite · CPC title

• C21D2211/002

  Bainite · CPC title

• C22C38/02

  containing silicon · CPC title