High strength steel plate for sour-resistant line pipe and method for producing same, and high strength steel pipe or tube using high strength steel plate for sour-resistant line pipe

Abstract

Provided is a high strength steel plate for a sour-resistant line pipe with excellent low-temperature toughness as well as HIC and SSCC resistance. The high strength steel plate for a sour-resistant line pipe has a chemical composition containing, by mass %, C: 0.030-0.060%, Si: 0.01-0.50%, Mn: 0.80-1.80%, P: 0.015% or less, S: 0.0015% or less, Al: 0.010-0.080%, Cr: 0.05-0.50%, Nb: 0.005-0.080%, N: 0.0010-0.0080%, and Ca: 0.0005-0.0050%, with the balance being Fe and inevitable impurities, in which a microstructure at 0.25 mm below a surface of the steel plate consists of granular bainite and tempered martensite austenite constituent, a maximum grain size is 80 μm or less and an average grain size is 20 μm or less at a mid-thickness position of the steel plate, a brittle-ductility transition temperature in a Charpy impact test is −100° C. or lower, and a tensile strength is 535 MPa or more.

1 . A high strength steel plate for a sour-resistant line pipe comprising a chemical composition containing, by mass %, C: 0.030% or more and 0.060% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.80% or more and 1.80% or less, P: 0.015% or less, S: 0.0015% or less, Al: 0.010% or more and 0.080% or less, Cr: 0.05% or more and 0.50% or less, Nb: 0.005% or more and 0.080% or less, N: 0.0010% or more and 0.0080% or less, and Ca: 0.0005% or more and 0.0050% or less, with the balance being Fe and inevitable impurities, wherein
 a microstructure at 0.25 mm below a surface of the steel plate consists of granular bainite and tempered martensite austenite constituent,   a maximum grain size is 80 μm or less and an average grain size is 20 μm or less at a mid-thickness position of the steel plate,   a brittle-ductility transition temperature in a Charpy impact test is −100° C. or lower, and a tensile strength is 535 MPa or more.   
     
     
         2 . The high strength steel plate for a sour-resistant line pipe according to  claim 1 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less. 
     
     
         3 . The high strength steel plate for a sour-resistant line pipe according to  claim 1 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of V: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, Zr: 0.0005% or more and 0.02% or less, Mg: 0.0005% or more and 0.02% or less, and REM: 0.0005% or more and 0.02% or less. 
     
     
         4 . A method for producing a high strength steel plate for a sour-resistant line pipe, the method comprising:
 heating a slab having a chemical composition containing, by mass %, C: 0.030% or more and 0.060% or less, Si: 0.01% or more and 0.50% or less, Mn: 0.80% or more and 1.80% or less, P: 0.015% or less, S: 0.0015% or less, Al: 0.010% or more and 0.080% or less, Cr: 0.05% or more and 0.50% or less, Nb: 0.005% or more and 0.080% or less, N: 0.0010% or more and 0.0080% or less, and Ca: 0.0005% or more and 0.0050% or less, with the balance being Fe and inevitable impurities to a temperature of 1000° C. or higher and 1250° C. or lower;   then subjecting the slab to hot rolling, which satisfies a total rolling reduction in a recrystallization temperature range being 35% or more and 55% or less and a rolling reduction in a final rolling pass in the recrystallization temperature range being 10% or more to obtain a steel plate; and   then subjecting the steel plate to controlled cooling under a set of conditions including:
 a temperature of a surface of the steel plate at the start of cooling being equal to or higher than Ar 3  point; 
 an average cooling rate in a temperature range from 750° C. to 550° C. in terms of a temperature at 0.25 mm below the surface of the steel plate being 15° C./s or higher and 35° C./s or lower; 
 an average cooling rate in a temperature range from 750° C. to 550° C. in terms of a temperature at a mid-thickness position of the steel plate being 15° C./s or higher; and 
 a cooling stop temperature in terms of temperatures at 0.25 mm below the surface of the steel plate and at the mid-thickness position of the steel plate being 350° C. or higher and 550° C. or lower. 
   
     
     
         5 . The method for producing a high strength steel plate for a sour-resistant line pipe according to  claim 4 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Cu: 0.30% or less, Ni: 0.10% or less, and Mo: 0.50% or less. 
     
     
         6 . The method for producing a high strength steel plate for a sour-resistant line pipe according to  claim 4 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of V: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, Zr: 0.0005% or more and 0.02% or less, Mg: 0.0005% or more and 0.02% or less, and REM: 0.0005% or more and 0.02% or less. 
     
     
         7 . A high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe according to  claim 1 . 
     
     
         8 . A high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe according to  claim 2 . 
     
     
         9 . The high strength steel plate for a sour-resistant line pipe according to  claim 2 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of V: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, Zr: 0.0005% or more and 0.02% or less, Mg: 0.0005% or more and 0.02% or less, and REM: 0.0005% or more and 0.02% or less. 
     
     
         10 . The method for producing a high strength steel plate for a sour-resistant line pipe according to  claim 5 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of V: 0.005% or more and 0.1% or less, Ti: 0.005% or more and 0.1% or less, Zr: 0.0005% or more and 0.02% or less, Mg: 0.0005% or more and 0.02% or less, and REM: 0.0005% or more and 0.02% or less. 
     
     
         11 . A high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe according to  claim 3 . 
     
     
         12 . A high strength steel pipe or tube using the high strength steel plate for a sour-resistant line pipe according to  claim 9 .