Steel material for line pipes, method for producing the same, and method for producing line pipe

Patent metadata

Field	Value
Publication number	US-11401568-B2
Application number	US-201916964661-A
Country	US
Kind code	B2
Filing date	Jan 22, 2019
Priority date	Jan 30, 2018
Publication date	Aug 2, 2022
Grant date	Aug 2, 2022

Abstract

A method for producing a steel material for line pipes which has a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method including heating a steel having a specific composition to a temperature of 1000° C. to 1200° C.; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature +20° C.) or less is 50% or more, and a rolling finish temperature is the Ar3 transformation point or more and 790° C. or less; and subsequently performing accelerated cooling from a cooling start temperature of the Ar3 transformation point or more, at a cooling rate of 10° C./s or more, until the temperature of a surface of a steel plate reaches 300° C. to 500° C.

First claim

The invention claimed is:

1. A method for producing a steel material for line pipes, the steel material having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising heating a steel having a composition containing, by mass,
C: 0.030% to 0.10%,
Si: 0.01% to 0.20%,
Mn: 1.0% to 2.0%,
Nb: 0.005% to 0.050%,
Ti: 0.005% to 0.025%, and
Al: 0.08% or less,
the composition further containing one or more elements selected from, by mass,
Cu: 0.5% or less,
Ni: 1.0% or less,
Cr: 1.0% or less,
Mo: 0.5% or less, and
V: 0.1% or less,
wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 or less, and an Ar 3 transformation point represented by Formula (3) is 750° C. or less, with the balance being Fe and inevitable impurities, to a temperature of 1000° C. to 1200° C.; performing hot rolling such that a cumulative rolling reduction ratio in a non-recrystallization temperature range is 60% or more, such that a cumulative rolling reduction ratio in a temperature range of (a rolling finish temperature+20° C.) or less is 50% or more, and such that a rolling finish temperature is the Ar 3 transformation point or more and 790° C. or less; and subsequently performing accelerated cooling from a cooling start temperature of the Ar 3 transformation point or more, at a cooling rate of 10° C./s or more, until the temperature of a surface of a steel plate reaches 300° C. to 500° C., such that a metal microstructure is formed in the steel material that is composed primarily of bainite, wherein an area fraction of polygonal ferrite at a position of ¼ plate thickness is 10% or less, an area fraction of martensite-austenite constituent at the position of ¼ plate thickness is 5% or less, and an average grain size of bainite at a position of ½ plate thickness is 10 μm or less,
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1)

Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10  (2)

Ar 3 (° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo  (3)

wherein, in Formulae (1) to (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

2. A method for producing a line pipe having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising cold forming a steel material for line pipes produced by the method according to claim 1 into a steel pipe shape by butting opposed edges of the steel material to one another; joining the butting edges to each other by seam welding; and subsequently performing pipe expansion at an expansion ratio of 1.2% or less to produce a steel pipe.

3. A steel material for line pipes, the steel material having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the steel material comprising a composition containing, by mass,
C: 0.030% to 0.10%,
Si: 0.01% to 0.20%,
Mn: 1.0% to 2.0%,
Nb: 0.005% to 0.050%,
Ti: 0.005% to 0.025%, and
Al: 0.08% or less,
the composition further containing one or more elements selected from, by mass,
Cu: 0.5% or less,
Ni: 1.0% or less,
Cr: 1.0% or less,
Mo: 0.5% or less, and
V: 0.1% or less,
wherein a Ceq value represented by Formula (1) is 0.350 or more, a Pcm value represented by Formula (2) is 0.20 or less, and an Ar 3 transformation point represented by Formula (3) is 750° C. or less, with the balance being Fe and inevitable impurities,
the steel material further comprising a metal microstructure composed primarily of bainite, wherein an area fraction of polygonal ferrite at a position of ¼ plate thickness is 10% or less, an area fraction of martensite-austenite constituent at the position of ¼ plate thickness is 5% or less, and an average grain size of bainite at a position of ½ plate thickness is 10 μm or less,
Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5  (1)

Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10  (2)

Ar 3 (° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo  (3)

wherein, in Formulae (1) to (3), the symbol of each element represents the content (mass %) of the element and is zero when the composition does not contain the element.

4. A method for producing a line pipe having a tensile strength of 570 MPa or more, a compressive strength of 440 MPa or more, and a thickness of 30 mm or more, the method comprising cold forming a steel material for line pipes according to claim 3 into a steel pipe shape by butting opposed edges of the steel material to one another; joining the butting edges to each other by seam welding; and subsequently performing pipe expansion at an expansion ratio of 1.2% or less to produce a steel pipe.

Assignees

• Jfe Steel Corp

Inventors

• Yasuda Kyono
• Yokota Tomoyuki
• Tanizawa Akihiko
• Muraoka Ryuji
• Hase Kazukuni

Classifications

• C21D8/10

  during manufacturing of tubular bodies · CPC title

• C21D7/13 Primary

  by hot working · CPC title

• C22C38/001

  containing N · CPC title

• C22C38/16

  containing copper · CPC title

• C22C38/50

  with titanium or zirconium · CPC title