High-strength, high-toughness steel plate, and method for producing the same

Patent metadata

Field	Value
Publication number	US-10544478-B2
Application number	US-201615562291-A
Country	US
Kind code	B2
Filing date	Mar 25, 2016
Priority date	Mar 31, 2015
Publication date	Jan 28, 2020
Grant date	Jan 28, 2020

Abstract

A high strength/highstrength, high toughness steel sheet having tensile strength, Charpy impact absorption energy, and ductile fracture rate are equal to or greater than specified values comprises, in mass %, 0.03-0.08% of C, 0.01-0.50% of Si, 1.5-2.5% of Mn, 0.001-0.010% of P, 0.0030% or less of S, 0.01-0.08% of Al, 0.010-0.080% of Nb, 0.005-0.025% of Ti, 0.001-0.006% of N, at least one substance selected from among 0.01-1.00% of Cu, 0.01-1.00% of Ni, 0.01-1.00% of Cr, 0.01-1.00% of Mo, 0.01-0.10% of V, and 0.0005-0.0030% of B, and a remainder of Fe and unavoidable impurities. At a position at ½ the sheet thickness, the area ratio of island-like martensite is less than 3%, the area ratio of bainite is 90% or more, and the average particle size of cementite within the bainite is 0.5 μm or less.

First claim

The invention claimed is:

1. A high-strength, high-toughness steel plate having a composition containing, by mass %,
C: 0.03% or more and 0.08% or less,
Si: 0.01% or more and 0.50% or less,
Mn: 1.5% or more and 2.5% or less,
P: 0.001% or more and 0.010% or less,
S: 0.0030 or less,
Al: 0.01% or more and 0.08% or less,
Nb: 0.010% or more and 0.080% or less,
Ti: 0.005% or more and 0.025% or less,
N: 0.001% or more and 0.006% or less, and further containing at least one selected from
Cu: 0.01% or more and 1.00% or less,
Ni: 0.01% or more and 1.00% or less,
Cr: 0.01% or more and 1.00% or less,
Mo: 0.01% or more and 1.00% or less,
V: 0.01% or more and 0.10% or less, and
B: 0.0005% or more and 0.0030% or less, with the balance being Fe and unavoidable impurities,
wherein the steel plate has a microstructure in which an area fraction of Martensite-Austenite constituent at a ½ position in a thickness direction is less than 3%,
an area fraction of bainite at the ½ position in the thickness direction is 90% or more, and
an average particle size of cementite present in the bainite at the ½ position in the thickness direction is 0.5 μm or less.

2. The high-strength, high-toughness steel plate according to claim 1 , wherein the composition further contains, by mass %, at least one selected from
Ca: 0.0005% or more and 0.0100% or less,
REM: 0.0005% or more and 0.0200% or less,
Zr: 0.0005% or more and 0.0300% or less, and
Mg: 0.0005% or more and 0.0100% or less.

3. A method for producing the high-strength, high-toughness steel plate according to claim 1 , the method comprising:
heating a steel slab to 1000° C. or higher and 1250° C. or lower;
performing rolling in an austenite recrystallization temperature range;
performing rolling at an accumulated rolling reduction ratio of 60% or more in an austenite non-recrystallization temperature range;
finishing the rolling at a temperature of (Ar 3 temperature +50° C.) or higher and (Ar 3 temperature +150° C.) or lower;
performing accelerated cooling from a cooling start temperature of Ar 3 temperature or higher and (Ar 3 temperature +100° C.) or lower to a cooling stop temperature of Ms temperature or higher and (Ms temperature +100° C.) or lower at a cooling rate of 10° C./s or more and 80° C./s or less;
holding the temperature of the steel in a range of the cooling stop temperature ±50° C. for 50 s or longer and shorter than 300 s; and then
performing natural cooling to a temperature range of 100° C. or lower.

4. A method for producing the high-strength, high-toughness steel plate according to claim 2 , the method comprising:
heating a steel slab to 1000° C. or higher and 1250° C. or lower;
performing rolling in an austenite recrystallization temperature range;
performing rolling at an accumulated rolling reduction ratio of 60% or more in an austenite non-recrystallization temperature range;
finishing the rolling at a temperature of (Ar 3 temperature +50° C.) or higher and (Ar 3 temperature +150° C.) or lower;
performing accelerated cooling from a cooling start temperature of Ar 3 temperature or higher and (Ar 3 temperature +100° C.) or lower to a cooling stop temperature of Ms temperature or higher and (Ms temperature +100° C.) or lower at a cooling rate of 10° C./s or more and 80° C./s or less;
holding the temperature of the steel in a range of the cooling stop temperature ±50° C. for 50 s or longer and shorter than 300 s; and then
performing natural cooling to a temperature range of 100° C. or lower.

5. The high-strength, high-toughness steel plate according to claim 1 , wherein the steel plate includes a base metal having:
i) a tensile strength of 625 MPa or more, measured in accordance with API-5L,
ii) a Charpy impact absorbed energy at −40° C. of 375 J or more, measured in accordance with ASTM A370, and
iii) a percent ductile fracture (SA value) as determined by a DWTT at −40° C. of 85% or more.

6. The high-strength, high-toughness steel plate according to claim 2 , wherein the steel plate includes a base metal having:
i) a tensile strength of 625 MPa or more, measured in accordance with API-5L,
ii) a Charpy impact absorbed energy at −40° C. of 375 J or more, measured in accordance with ASTM A370, and
iii) a percent ductile fracture (SA value) as determined by a DWTT at −40° C. of 85% or more.

7. The method of claim 3 , wherein after the step of accelerated cooling, a reheating step is not performed.

8. The method of claim 4 , wherein after the step of accelerated cooling, a reheating step is not performed.

Assignees

• Jfe Steel Corp

Inventors

• Kimura Hideyuki
• Ota Shusaku
• Ishikawa Nobuyuki
• Kakihara Shinichi
• Nagao Ryo

Classifications

• C21D8/02

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

• C22C38/20

  with copper · CPC title

• C21D2211/002

  Bainite · CPC title

• C22C38/002

  containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60 · CPC title

• C22C38/005

  containing rare earths, i.e. Sc, Y, Lanthanides · CPC title