Method for making high-strength high-toughness martensitic stainless steel seamless pipe

A method for making a high-strength high-toughness martensitic stainless steel seamless pipe comprising:

• heating a martensitic stainless steel raw material to an austenitic range;
• piercing and elongating the raw material to form an original pipe;
• cooling the original pipe to form a structure substantially composed of martensite in the original pipe;
• reheating the original pipe to a temperature in a dual-phase range between the Ac1 transformation point and the Ac3 transformation point directly after cooling the original pipe to form the structure substantially composed of martensite;
• finish-rolling the original pipe at an initial rolling temperature T (° C.) between the Ac1 transformation point and the Ac3 transformation point;
• cooling the original pipe to form a processed pipe having a predetermined size;
• and
• tempering the processed pipe at a temperature below the Ac1 transformation point such that the steel has an absorbed energy (E−40)L per unit area of a longitudinal direction (L direction) and an absorbed energy (E−40)C per unit area of a circumferential direction (C direction) of about 180 J/cm2 or more, and
• a ratio (E−40)C/(E−40)L of about 8.0 or more.

A method for making a high-strength high-toughness martensitic stainless steel seamless pipe consisting essentially of:

• heating a martensitic stainless steel raw material to an austenitic range;
• piercing and elongating the raw material to form an original pipe:
• cooling the original pipe to form a structure substantially composed of martensite in the original pipe;
• reheating the original pipe to a temperature in a dual-phase range between the Ac1 transformation point and the Ac3 transformation point;
• finish-rolling the original pipe at an initial rolling temperature T (° C.) between the Ac1 transformation point and the Ac3 transformation point;
• cooling the original pipe to form a processed pipe having a predetermined size;
• and
• tempering the processed pipe at a temperature below the Ac1 transformation point.