High-strength martensitic stainless steel and method for making the same

A high strength martensitic stainless steel consisting essentially of:

• 0.06 wt. % or less C, 12 to 16 wt. % Cr, 1 wt. % or less Si, 2 wt. % or less Mn, 0.5 to 8 wt. % Ni, 0.1 to 2.5 wt. % Mo, 0.3 to 4 wt. % Cu, 0.05 wt. % or less N, at least one element selected from the group consisting of 0.01 to 0.1 wt. % V, 0.01 to 0.1 wt. % Nb, and the balance being Fe and inevitable impurities;
• said steel having an area ratio of a .delta.-ferrite phase of 10% or less expressed as a percent;
• said steel including at least 30 fine copper precipitates per 1 square micron meter (.mu.m.sup.2);
• and
• said steel having a 0.2% yield stress of 75 kg/mm.sup.2 or more and a charpy impact energy of 10 kg-m or more.

A method for manufacturing a high strength martensitic stainless steel comprising the steps of:

• preparing a martensitic stainless steel steel consisting essentially of 0.06 wt. % or less C, 12 to 16 wt. % Cr, 1 wt. % or less Si, 2 wt. % or less Mn, 0.5 to 8 wt. % Ni, 0.1 to 2.5 wt. % Mo, 0.3 to 4 wt. % Cu, 0.05 wt. % or less N, and the balance being Fe and inevitable impurities;
• austenitizing said martensitic stainless steel at a temperature of Ac.sub.3 transformation point to 980.degree. C. to produce a austenitized martensitic steel;
• cooling the austenitized martensitic stainless steel;
• tempering the cooled stainless steel to disperse fine Cu precipitate grains in a matrix at a tempering temperature (T.degree.C.) of 500.degree. C. to the lower one of either 630.degree. C. or Ac.sub.1 transformation point and at a tempering time (t hour), said tempering temperature and said tempering time satisfying the following equation:
• 15200.ltoreq.(20+log t)(273+T).ltoreq.17800.

A method for manufacturing a high strength martensitic stainless steel comprising the steps of:

• preparing a martensitic stainless steel steel consisting essentially of 0.06 wt. % or less C, 12 to 16 wt. % Cr, 1 wt. % or less Si, 2 wt. % or less Mn, 0.5 to 8 wt. % Ni, 0.1 to 2.5 wt. % Mo, 0.3 to 4 wt. % Cu, 0.05 wt. % or less N, at least one element selected from the group consisting of 0.01 to 0.1 wt. % V and 0.01 to 0.1 wt. % Nb and the balance being Fe and inevitable impurities;
• and the balance being Fe and inevitable impurities;
• austenitizing said martensitic stainless steel at a temperature of Ac.sub.3 transformation point to 980.degree. C. to produce a austenitized martensitic steel;
• cooling the austenitized martensitic stainless steel;
• tempering the cooled stainless steel to disperse fine Cu precipitate grains in a matrix at a tempering temperature (T.degree.C.) of 500.degree. C. to the lower one of either 630.degree. C. or Ac.sub.1 transformation point and at a tempering time (t hour), said tempering temperature and said tempering time satisfying the following equation:
• 15200.ltoreq.(20+log t)(273+T).ltoreq.17800.

A high strength martensitic stainless steel having a composition consisting essentially of:

• 0.06 wt. % or less C, 12 to 16 wt. % Cr, 1 wt. % or less Si, 2 wt. % or less Mn, 0.5 to 8 wt. % Ni, 0.1 to 2.5 wt. % Mo, 0.3 to 4 wt. % Cu, 0.05 wt. % or less N, at least one element selected from the group consisting of 0.01 wt. % to 0.1 wt. % V, 0.01 to 0.1 wt. % Nb, and optionally at least one additional element selected from the group consisting of 0.01 to 0.10 wt. % Al, 4 wt. % or less W, 0.2 wt. % or less Ti, 0.2 wt. % or less Zr, 0.2 wt. % or less Ta, 0.2 wt. % or less Hf, 0.01 wt. % or less of Ca and 0.02 wt. % or less of a rare earth metal;
• and the balance being Fe and inevitable impurities including no more than 0.04 wt. % P and no more than 0.01 wt. % S;
• said steel having an area ratio of a .delta.-ferrite phase of 10% or less expressed as a percent;
• said steel including at least 30 fine copper precipitates per 1 .mu.m.sup.2 ;
• and
• said steel having a 0.2% yield stress of 75 kg/mm.sup.2 or more and a charpy impact energy of 10 kg-m or more.