WELDED STEEL PIPE FOR LINEPIPE HAVING HIGH COMPRESSIVE STRENGTH AND HIGH FRACTURE TOUGHNESS AND MANUFACTURING METHOD THEREOF

• US 20120305122A1
• Filed: 11/25/2010
• Published: 12/06/2012
• Est. Priority Date: 11/25/2009
• Status: Abandoned Application

• Current Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Original Assignee: JFE Steel Corporation (JFE Holdings Incorporated)
• Inventors: Ishikawa, Nobuyuki, Tanizawa, Akihiko, Sueyoshi, Hitoshi, Horie, Masayuki, Kiyoto, Yasumitsu
• Application Number:
  US13/511,822
• Publication Number:
  US 20120305122A1
• Time in Patent Office: Days
• US Class Current: 138/177
• CPC Class Codes: B23K 9/0282 for welding tube sections
• CPC Class Codes: B23K 9/18 Submerged-arc welding
• CPC Class Codes: C21D 2211/002 Bainite
• CPC Class Codes: C21D 8/10 during manufacturing of tubular bodies
• CPC Class Codes: C21D 8/105 of ferrous alloys
• CPC Class Codes: C21D 9/08 for tubular bodies or pipes
• CPC Class Codes: C21D 9/14 wear-resistant or pressure-resistant pipes
• CPC Class Codes: C22C 38/001 containing N
• CPC Class Codes: C22C 38/002 containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• CPC Class Codes: C22C 38/02 containing silicon
• CPC Class Codes: C22C 38/04 containing manganese
• CPC Class Codes: C22C 38/06 containing aluminium
• CPC Class Codes: C22C 38/08 containing nickel C22C38/105 takes precedence
• CPC Class Codes: C22C 38/12 containing tungsten, tantalum, molybdenum, vanadium, or niobium
• CPC Class Codes: C22C 38/14 containing titanium or zirconium

Abstract

Provided are a steel pipe for a linepipe having a heavy wall thickness and excellent fracture toughness in a base material, and methods of producing the same. A welded heat affected zone is achieved by suppressing lowering of yield stress caused by a Bauschinger effect by optimizing the metal microstructure of a steel plate. A steel pipe has a composition which contains by mass % 0.03 to 0.08% C, 0.10% or less Si, 1.00 to 2.00% Mn, 0.010% or less P, 0.0030% or less S, 0.06% or less Al, 0.005 to 0.020% Nb, 0.005 to 0.025% Ti, 0.0010 to 0.0060% N, and Fe and unavoidable impurities as a balance. Ti(%)/N(%) is a value which falls within a range of 2 to 4, and a Ceq value is 0.30 or more.

Claims

1. A welded steel pipe for a linepipe having the composition which contains by mass % 0.03 to 0.08% C, 0.10% or less Si, 1.00 to 2.00% Mn, 0.010% or less P, 0.0030% or less S, 0.06% or less Al, 0.005 to 0.020% Nb, 0.005 to 0.025% Ti, 0.0010 to 0.0060% N, and Fe and unavoidable impurities as a balance, wherein

C(%)−

0.065Nb(%) is 0.025 or more,

Ti(%)/N(%) is a value which falls within a range of 2 to 4, and

a Ceq value is 0.30 or more,

a base material having metal microstructure where a fraction of bainite is 80% or more, a fraction of M-A constituent (MA) is 3% or less, a fraction of cementite is 5% or less, and an average grain size of bainite is 5 μ

m or less, and

a welded heat affected zone having metal microstructure where a fraction of bainite is 90% or more and a fraction of M-A constituent (MA) is 3% or less,

wherein the Ceq value is expressed by the following formula;

Ceq=C(%)+Mn(%)/6+{Cr(%)+Mo(%)+V(%)}/5+{Cu(%)+Ni(%)}/15

2. The welded steel pipe for a linepipe according to claim 1, wherein the composition further contains by mass % one or two kinds or more selected from a group consisting of 0.50% or less Cu, 1.0% or less Ni, 0.50% or less Cr, 0.50% or less Mo, 0.10% or less V, and 0.0005 to 0.0035% Ca, and C(%)−

0.065Nb(%)−

0.025Mo(%)−

0.057V(%) is 0.025 or more.

3. A method of manufacturing a welded steel pipe for a linepipe, wherein steel having the composition described in claim 1 is heated to a temperature which falls within a range of 950 to 1200°

C., is subjected to hot rolling where a rolling reduction rate in a no-recrystallization temperature range is set to 60% or more and a rolling completion temperature falls within a range of Ar₃ to (Ar₃+70°

C.), and subsequently, is subjected to accelerated cooling at a cooling rate of 10°

C./sec or more from a temperature of (Ar₃−

30°

C.) or above to a temperature which falls within a range of more than 300°

C. to 420°

C. thus a steel plate being manufactured, the steel plate is formed into a steel pipe shape by cold forming, seam welding is applied to a butt portion of the steel pipe shape to form a steel pipe, and the steel pipe is subjected to pipe expansion with an expansion rate of 0.4% to 1.2%.

4. The method of manufacturing a welded steel pipe for a linepipe according to claim 3, wherein the steel plate is subjected to reheating succeeding the accelerated cooling such that a steel plate surface temperature falls within a range of 500 to 700°

C. and a steel plate center temperature becomes below 550°

C.