Steel plate for high-strength and high-toughness steel pipes and method for producing steel plate

Patent metadata

Field	Value
Publication number	US-11236405-B2
Application number	US-201716072717-A
Country	US
Kind code	B2
Filing date	Jan 23, 2017
Priority date	Jan 29, 2016
Publication date	Feb 1, 2022
Grant date	Feb 1, 2022

Abstract

A steel plate for high-strength and high-toughness steel pipes has a chemical composition containing, by mass %, C: 0.03% or more and 0.08% or less, Si: more than 0.05% 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, and N: 0.001% or more and 0.006% or less, and further containing, by mass %, 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 inevitable impurities. The steel plate has a microstructure in which an area fraction of ferrite at a ½ position of a thickness of the steel plate is 20% or more and 80% or less and deformed ferrite constitutes 50% or more and 100% or less of the ferrite.

First claim

The invention claimed is:

1. A steel plate for high-strength and high-toughness steel pipes, the steel plate having a chemical composition containing, by mass %,
C: 0.03% or more and 0.08% or less,
Si: more than 0.05% 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, and
N: 0.001% or more and 0.006% or less, and further containing, by mass %, 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 inevitable impurities,
wherein the steel plate has a microstructure in which an area fraction of ferrite at a ½ position of a thickness of the steel plate is 20% or more and 80% or less and deformed ferrite constitutes 50% or more and 100% or less of the ferrite,
wherein separations that occur in a fractured surface of a test piece of the steel plate have a separation index (SI −55° C. ) of 0.10 mm −1 or more provided that the test piece is subjected to a DWTT test (Drop Weight Tear Test) at a test temperature of −55° C., the separation index being defined by formula (1):
SI −55° C. (mm −1 )=ΣLi/ A   (1)

where ΣLi: a total of lengths (mm) of separations having a length of 1 mm or more existing in an evaluation region (A) of the test piece for the DWTT test,
A: an area (mm 2 ) of the evaluation region of the test piece for the DWTT test, the evaluation region being a region excluding a first portion and a second portion in the test piece, the first portion having a dimension extending from a press notch side to the evaluation region, the second portion having a dimension extending from a drop weight impact side to the evaluation region, the dimension of the first portion and the dimension of the second portion each being equal to a thickness, t, of the test piece (in a case that the thickness t<19 mm) or each being 19 mm (in a case that the thickness t≥19 mm),

wherein the steel plate has a tensile strength of 625 MPa or more, and a Charpy impact absorbed energy at −55° C. (vE −55° C. ) of 160 J or more,
wherein the steel plate has a percent ductile fracture of 85% or more, as determined by a DWTT test at −55° C.

2. The steel plate according to claim 1 for high-strength and high-toughness steel pipes, wherein the chemical 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 a steel plate for high-strength and high-toughness steel pipes, the method being formulated to produce the steel plate according to claim 1 for high-strength and high-toughness steel pipes, the method comprising:
hot rolling, the hot rolling being carried out by heating a steel slab to a range of 1000° C. or higher and 1250° C. or lower, rolling the steel slab in an austenite recrystallization temperature range, thereafter rolling is performed in a range of an Ar 3 temperature or higher and (Ar 3 temperature+150° C.) or lower, at an accumulated rolling reduction ratio of 50% or more, and thereafter rolling is performed in a range of (the Ar 3 temperature−50° C.) or higher and lower than the Ar 3 temperature, at an accumulated rolling reduction ratio of more than 50%; and
cooling, the cooling being carried out, immediately after the hot rolling, by cooling the steel plate by accelerated cooling at a cooling rate of 10° C./s or higher and 80° C./s or lower to a cooling stop temperature of 250° C. or higher and 450° C. or lower, and thereafter naturally cooling the steel plate to a temperature range of 100° C. or lower.

4. A method for producing a steel plate for high-strength and high-toughness steel pipes, the method being formulated to produce the steel plate according to claim 2 for high-strength and high-toughness steel pipes, the method comprising:
hot rolling, the hot rolling being carried out by heating a steel slab to a range of 1000° C. or higher and 1250° C. or lower, rolling the steel slab in an austenite recrystallization temperature range, thereafter rolling is performed in a range of an Ar 3 temperature or higher and (Ar 3 temperature+150° C.) or lower, at an accumulated rolling reduction ratio of 50% or more, and thereafter rolling is performed in a range of (the Ar 3 temperature−50° C.) or higher and lower than the Ar 3 temperature, at an accumulated rolling reduction ratio of more than 50%; and
cooling, the cooling being carried out, immediately after the hot rolling, by cooling the steel plate by accelerated cooling at a cooling rate of 10° C./s or higher and 80° C./s or lower to a cooling stop temperature of 250° C. or higher and 450° C. or lower, and thereafter naturally cooling the steel plate to a temperature range of 100° C. or lower.

5. The steel plate according to claim 1 for high-strength and high-toughness steel pipes, wherein the deformed ferrite is a ferrite having an aspect ratio of 3 or more, the aspect ratio being a ratio of the ferrite grain length in the rolling direction to the ferrite grain length in the thickness direction.

6. The steel plate according to claim 2 for high-strength and high-toughness steel pipes, wherein the deformed ferrite is a ferrite having an aspect ratio of 3 or more, the aspect ratio being a ratio of the ferrite grain length in the rolling direction to the ferrite grain length in the thickness direction.

7. The method according to claim 3 for high-strength and high-toughness steel pipes, wherein the deformed ferrite is a ferrite having an aspect ratio of 3 or more, the aspect ratio being a ratio of the ferrite grain length in the rolling direction to the ferrite grain length in the thickness direction.

8. The method according to claim 4 for high-strength and high-toughness steel pipes, wherein the deformed ferrite is a ferrite having an aspect ratio of 3 or more, the aspect ratio being a ratio of the ferrite grain length in the rolling direction to the ferrite grain length in the thickness direction.

9. The steel plate according to claim 1 for high-strength and high-toughness steel pipes, wherein the steel plate has the microstructure in which an area fraction of bainite at the ½ position of the thickness of the steel plate is 20% or more and 80% or less and a total area fraction of the microstructure, other than ferrite, deformed ferrite and bainite, is 10% or less.

10. The steel plate according to claim 2 for high-strength and high-toughness steel pipes, wherein the steel plate has the microstructure in which an area fraction of bainite at the ½ position of the thickness of the steel plate is 20% or more and 80% or less and a total area fraction of the microstructure, other than ferrite, deformed ferrite and bainite, is 10% or less.

11. The method according to claim 3 for high-strength and high-toughness steel pipes, wherein the steel plate has the microstructure in which an area fraction of bainite at the ½ position of the thickness of the steel plate is 20% or more and 80% or less and a total area fraction of the microstructure, other than ferrite, deformed ferrite and bainite, is 10% or less.

12. The method according to claim 4 for high-strength and high-toughness steel pipes, wherein the steel plate has the microstructure in which an area fraction of bainite at the ½ position of the thickness

Assignees

• Jfe Steel Corp

Inventors

• Kimura Hideyuki
• Nagao Ryo
• Ishikawa Nobuyuki
• Hase Kazukuni

Classifications

• C21D8/10

  during manufacturing of tubular bodies · CPC title

• C21D8/02 Primary

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

• C22C38/48

  with niobium or tantalum · CPC title

• C22C38/46

  with vanadium · CPC title

• C22C38/42

  with copper · CPC title