Thick-walled high-strength hot rolled steel sheet with excellent low-temperature toughness and method of producing same

Patent metadata

Field	Value
Publication number	US-9493865-B2
Application number	US-201514635053-A
Country	US
Kind code	B2
Filing date	Mar 2, 2015
Priority date	Jul 31, 2008
Publication date	Nov 15, 2016
Grant date	Nov 15, 2016

Abstract

A thick-walled high-strength hot rolled steel sheet has a high tensile strength TS of 521 MPa or more and excellent low-temperature toughness. The steel material forming the sheet contains 0.02%-0.08% C, 0.01%-0.10% Nb, and 0.001%-0.05% Ti and is heated; C, Ti, and Nb satisfies (Ti+(Nb/2))/C<4.

First claim

The invention claimed is:

1. A method of producing a thick-walled high-strength hot rolled steel sheet comprising:
heating a steel material containing, on a mass percent basis,
0.02%-0.08% C, 0.01%-0.50% Si,
0.5%-1.8% Mn, 0.025% or less P,
0.005% or less S, 0.005%-0.10% Al,
0.01%-0.10% Nb, 0.001%-0.05% Ti,
the balance being Fe, and incidental impurities, C, Ti, and Nb being contained to satisfy expression (1):
(Ti+(Nb/2))/C<4  (1);

performing hot rolling including rough rolling and finish rolling;
performing accelerated cooling at an average cooling rate of 100° C./s or more at a position 1 mm from a surface of the steel sheet in the thickness direction and at an average cooling rate of 10° C./s or more at a middle position of the steel sheet in the thickness direction to a cooling stop temperature of BFS or lower at a middle position of the steel sheet in the thickness direction, wherein BFS is defined by expression (2):
BFS (° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni−1.5CR  (2);

and
performing coiling at a coiling temperature of BFS0 or lower at the middle position of the steel sheet in the thickness direction, the BFS0 being defined by expression (3):
BFS 0 (° C.)=770−300C−70Mn−70Cr−170Mo−40Cu−40Ni  (3)

where in expressions (1), (2), and (3), C, Ti, Nb, Mn, Cr, Mo, Cu, and Ni each represent a proportion (percent by mass) thereof, and CR represents a cooling rate (° C./s) at the middle position of the steel sheet in the thickness direction.

2. The method according to claim 1 , further comprising:
performing scale removal treatment with a scale breaker before the rough rolling and before the finish rolling, wherein in the hot rolling, the finish entry temperature (FET) is 800° C. to 1050° C., and finish delivery temperature (FDT) is 750° C. to 950° C.

3. The method according to claim 1 ,
wherein, in the accelerated cooling, when a carbon equivalent Ceq is 0.37% or less, an average cooling rate at a position 1 mm from a surface of the steel sheet in the thickness direction is 10° C./s or more, and when the carbon equivalent Ceq exceeds 0.37%, the average cooling rate is 10 to 200° C./s, the carbon equivalent Ceq is defined by expression (4):
Ceq (%)=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15  (4)

where C, Ti, Mn, Cr, Mo, V, Cu, and Ni each represent the proportion thereof (percent by mass).

4. The method according to claim 1 , wherein the coiling is performed at a coiling temperature of 300° C. or higher at a middle position of the steel sheet in the thickness direction.

5. The method according to claim 1 , further comprising, on a mass percent basis, one or more selected from the group consisting of 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.

6. The method according to claim 1 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

7. The method according to claim 2 , further comprising, on a mass percent basis, one or more selected from the group consisting of 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.

8. The method according to claim 3 , further comprising, on a mass percent basis, one or more selected from the group consisting of 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.

9. The method according to claim 4 , further comprising, on a mass percent basis, one or more selected from the group consisting of 0.01%-0.10% V, 0.01%-0.50% Mo, 0.01%-1.0% Cr, 0.01%-0.50% Cu, and 0.01%-0.50% Ni.

10. The method according to claim 2 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

11. The method according to claim 3 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

12. The method according to claim 4 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

13. The method according to claim 5 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

14. The method according to claim 7 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

15. The method according to claim 8 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

16. The method according to claim 9 , further comprising, on a mass percent basis, 0.0005%-0.005% Ca.

Assignees

• Jfe Steel Corp

Inventors

• Kami Chikara
• Nakata Hiroshi
• Nakagawa Kinya

Classifications

• C22C38/12

  containing tungsten, tantalum, molybdenum, vanadium, or niobium · CPC title

• C21D1/25

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

• C22C38/08

  containing nickel {(C22C38/105 takes precedence)} · CPC title

• C22C38/38

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

• C22C38/26

  with niobium or tantalum · CPC title