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

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 0.30%,
• 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 Ar3 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 to a rolling finish temperature +20° C. or less is 50% or more, and such that a rolling finish temperature is the Ar3 transformation point or more and 790° C. or less, the rolling finish temperature being an average temperature of a steel plate;
• subsequently performing accelerated cooling from a temperature of the Ar3 transformation point or more, at a cooling rate of 10° C./s or more, to a cooling stop temperature of 200° ° C. to 450° C., the cooling stop temperature being an average temperature of the steel plate;
• and then performing reheating such that the temperature of a surface of the steel plate is 350° C. to 550° C. and such that the temperature of the center of the steel plate is less than 550° C.,
• 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)
• Ar3(° 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.