A high-strength martensitic stainless steel pipe excellent in toughness and hot workability, which comprises by mass percent, C: 0.18 to 0.22%, Si: 0.1 to 0.5%, Mn: 0.40 to 1.00%, P: 0.011 to 0.018%, S: 0.003% or less, Cr: 11.50 to 13.50%, Ni: 0.5% or less, Al: 0.0005 to 0.003%, N: 0.012 to 0.040%, Cu: 0.25% or less, Ti: 0.05% or less, V: 0.02 to 0.18%, Mo: 0 to 0.05%, Nb: 0 to 0.009%, B: 0.0010% or less, and Ca: 0.0050% or less, with the balance being Fe and impurities, and having a yield strength of 650 MPa or more and a toughness exceeding 70 J/cm2 by impact value in the Charpy impact test at 0 degrees C. using V-notch test pieces.
A high-strength martensitic stainless steel pipe excellent in toughness and hot workability, which comprises by mass percent, C: 0.18 to 0.21%, Si: 0.1 to 0.5%, Mn: 0.40 to 0.70%, P: 0.011 to 0.018%, S: 0.003% or less, Cr: 11.50 to 13.50%, Ni: 0.5% or less, Al: 0.0005 to 0.003%, N: 0.012 to 0.032%, Cu: 0.25% or less, Ti: 0.05% or less, V: 0.04 to 0.18%, Mo: 0 to 0.05%, Nb: 0.002 to 0.009%, B: 0.0010% or less, and Ca: 0.0050% or less, with the balance being Fe and impurities, and having a value of fn represented by the following formula (A) satisfying 0 to 80, a yield strength of 750 MPa or more, and a toughness exceeding 50 J/cm2 by impact value in the Charpy impact test at 0 degrees C. using V-notch test pieces:
fn=50Mo 500 (V-0.04) 5000Nb (A),
wherein an element symbol appearing in the formula (A) represents the content by mass percent of the corresponding element included in the steel.
A method for producing the high-strength martensitic stainless steel pipe as described in above (1) , which comprises:
heating the steel pipe, which is made by use of the martensitic steel, having the chemical composition according to above (1) as the material and cooled to room temperature by atmospheric cooling or air cooling, at a temperature T1, which exists in the temperature range of 930 to 980 degrees C., for 5 to 30 minutes;
then cooling the steel pipe from the temperature T1 to a temperature T2, which exists in the temperature range of 600 to 350 degrees C., at a cooling rate of 1 to 40 degrees C./sec;
successively cooling the steel pipe from the temperature T2 to a temperature T3, which exists in the temperature range of 300 to 150 degrees C., and from the temperature range lower than the temperature T3 to room temperature at cooling rates of less than 1 degrees C./sec and of 5 to 40 degrees C./sec, respectively; and,
performing a bend straightening treatment at a straightener outlet temperature of 510 degrees C. or higher successively to tempering treatment at 610 to 750 degrees C.
A method for producing the high-strength martensitic stainless steel pipe as described in above (2), which comprises:
heating the steel pipe, which is made by use of the martensitic steel, having the chemical composition according to above (2) as the material and cooled to room temperature by atmospheric cooling or air cooling, at a temperature T1, which exists in the temperature range of 930 to 980 degrees C., for 5 to 30 minutes;
then cooling the steel pipe from the temperature T1 to a temperature T2, which exists in the temperature range of 600 to 350 degrees C., at a cooling rate of 1 to 40 degrees C./sec;
successively cooling the steel pipe from the temperature T2 to a temperature T3, which exists in the temperature range of 300 to 150 degrees C., and from the temperature range lower than the temperature T3 to room temperature at cooling rates of less than 1 degrees C./sec and of 5 to 40 degrees C./sec, respectively; and,
performing a bend straightening treatment at a straightener outlet temperature of 510 degrees C. or higher successively to tempering treatment at 610 to 750 degrees C.
The inventions for the high-strength martensitic stainless steel pipe described in above (1) and (2), and the inventions for the methods for producing high-strength martensitic stainless steel pipes described in (3) and (4) are called "Invention (1)" to "Invention (4)", respectively, or often collectively called "the present invention".
|