High strength and high toughness stainless steel sheet and method for producing thereof

A high strength and high toughness stainless steel sheet essentially consisting of;

• 0.01 to 0.2 wt. % C, 0.1 to 2 wt. % Si, 0.1 to 2 wt. % Mn, 4 to 11 wt. % Ni, 0.08 to 0.9 wt. % Cu, 13 to 20 wt. % Cr, 0.01 to 0.2 wt. % N, 0.0005 to 0.0025 wt. % sol.Al, 0.002 to 0.01 wt. % O, 0.009 wt. % or less S, and the balance being Fe and inevitable impurities;
• said inevitable impurities existing as nonmetallic inclusions, a composition of said nonmetallic inclusions being in a range enclosed with lines connecting nine points in a phase diagram in a 3-component system of "Al.sub.2 O.sub.3 -MnO-SiO.sub.2 " given below;
• Point 1 (Al.sub.2 O.sub.3 :
• 21%, MnO:
• 12%, SiO.sub.2 :
• 67%),
• Point 2 (Al.sub.2 O.sub.3 :
• 19%, MnO:
• 21%, SiO.sub.2 :
• 60%),
• Point 3 (Al.sub.2 O.sub.3 :
• 15%, MnO:
• 30%, SiO.sub.2 :
• 55%),
• Point 4 (Al.sub.2 O.sub.3 :
• 5%, MnO:
• 46%, SiO.sub.2 :
• 49%),
• Point 5 (Al.sub.2 O.sub.3 :
• 5%, MnO:
• 68%, SiO.sub.2 :
• 27%),
• Point 6 (Al.sub.2 O.sub.3 :
• 20%, MnO:
• 61%, SiO.sub.2 :
• 19%),
• Point 7 (Al.sub.2 O.sub.3 :
• 27.5%, MnO:
• 505, SiO.sub.2 :
• 22.5%),
• Point 8 (Al.sub.2 O.sub.3 :
• 30%, MnO:
• 38%, SiO.sub.2 :
• 32%),
• Point 9 (Al.sub.2 O.sub.3 :
• 33%, MnO:
• 27%, SiO.sub.2 :
• 40%);
• the steel sheet containing 40 to 90% martensite;
• and
• the steel sheet having a 1400 N/mm.sup.2 or more tensile stress when a tensile strain is 1.0%.

A method for producing a stainless steel sheet for the comprising the steps of:

• preparing a stainless steel strip consisting essentially of 0.01 to 0.2 wt. % C, 0.1 to 2 wt. % Si, 0.1 to 2 wt. % Mn, 4 to 11 wt. % Ni, 13 to 20 wt. % Cr, 0.01 to 0.2 wt. % N, 0.0005 to 0.0025 wt. % sol.Al, 0.002 to 0.01 wt. % O, 0.009 wt. % or less S, and the balance being Fe and inevitable impurities;
• said inevitable impurities existing as nonmetallic inclusions, said nonmetallic inclusions being in a range enclosed with the lines connecting nine points in a phase diagram in 3-component system of "Al.sub.2 O.sub.3 -MnO-SiO.sub.2 " given below;
• Point 1 (Al.sub.2 O.sub.3 :
• 21%, MnO:
• 12%, SiO.sub.2 :
• 67%),
• Point 2 (Al.sub.2 O.sub.3 :
• 19%, MnO:
• 21%, SiO.sub.2 :
• 60%),
• Point 3 (Al.sub.2 O.sub.3 :
• 15%, MnO:
• 30%, SiO.sub.2 :
• 55%),
• Point 4 (Al.sub.2 O.sub.3 :
• 5%, MnO:
• 46%, SiO.sub.2 :
• 49%),
• Point 5 (Al.sub.2 O.sub.3 :
• 5%, MnO:
• 68%, SiO.sub.2 :
• 27%),
• Point 6 (Al.sub.2 O.sub.3 :
• 20%, MnO:
• 61%, SiO.sub.2 :
• 19%),
• Point 7 (Al.sub.2 O.sub.3 :
• 27.5%, MnO:
• 50%, SiO.sub.2 22.5%),
• Point 8 (Al.sub.2 O.sub.3 :
• 30%, MnO:
• 38%, SiO.sub.2 :
• 32%),
• Point 9 (Al.sub.2 O.sub.3 :
• 33%, MnO:
• 27%, SiO.sub.2 :
• 40%);
• applying to the steel strip a process of first cold rolling (CR.sub.1)--first intermediate annealing--second cold rolling (CR.sub.2)--second intermediate annealing--third cold rolling (CR.sub.3)--the final annealing--fourth cold rolling (CR.sub.4)--low temperature heat treatment;
• cold reduction ratios of the first-, second- and third cold rolling, each being 30% to 60%;
• annealing temperatures in the first-, second- and final annealing being in a range of 950.degree. C. to 1100.degree. C.;
• a reduction ratio of the fourth cold rolling being 66% to 76%;
• the low temperature heat treatment ranging from 300.degree. C. to 600.degree. C. afor a period 0.1 sec to 300 sec.

A high strength and high toughness stainless steel sheet produced by a method comrising the steps of:

• preparing a stainless steel strip consisting essentially of 0.01 to 0.2 wt. % C, 0.1 to 2 wt. % Si, 0.1 to 2 wt. % Mn, 4 to 11 wt. % Ni, 13 to 20 wt. % Cr, 0.01 to 0.2 wt. % N, 0.0005 to 0.0025 wt. % sol.Al, 0.002 to 0.01 wt. % O, 0.009 wt. % or less S, and the balance being Fe and inevitable impurities;
• said inevitable impurities existing as nonmetallic inclusions, said nonmetallic inclusions being in a range enclosed with the lines connecting nine points in the phase diagram in 3-component system of "Al.sub.2 O.sub.3 -MnO-SiO.sub.2 " given below;
• Point 1 (Al.sub.2 O.sub.3 :
• 21%, MnO:
• 12%, SiO.sub.2 :
• 67%),
• Point 2 (Al.sub.2 O.sub.3 :
• 19%, MnO:
• 21%, SiO.sub.2 :
• 60%),
• Point 3 (Al.sub.2 O.sub.3 :
• 15%, MnO:
• 30%, SiO.sub.2 :
• 55%),
• Point 4 (Al.sub.2 O.sub.3 :
• 5%, MnO:
• 46%, SiO.sub.2 :
• 49%),
• Point 5 (Al.sub.2 O.sub.3 :
• 5%, MnO:
• 68%, SiO.sub.2 :
• 27%),
• Point 6 (Al.sub.2 O.sub.3 :
• 20%, MnO:
• 61%, SiO.sub.2 :
• 19%),
• Point 3 (Al.sub.2 O.sub.3 :
• 27.5%, MnO:
• 50%, SiO.sub.2 :
• 22.5%),
• Point 8 (Al.sub.2 O.sub.3 :
• 30%, MnO:
• 38%, SiO.sub.2 :
• 32%),
• Point 9 (Al.sub.2 O.sub.3 :
• 33%, MnO:
• 27%, SiO.sub.2 :
• 40%);
• applying to the stainless strip a process of first cold rolling (CR.sub.1)--first intermediate annealing--second cold rolling (CR.sub.2)--second intermediate annealing--third cold rolling (CR.sub.3)--final annealing--fourth cold rolling (CR.sub.4)--low temperature heat treatment;
• cold reduction ratios of the first-, second- and third cold rolling, each being 30% to 60%;
• annealing temperatures in the first-, second- and last annealing being in a range of 950.degree. C. to 1100.degree. C.;
• a cold reduction ratio of the fourth cold rolling being 66% to 76%;
• the low temperature heat treatment ranging from 300.degree. C. to 600.degree. C. for a period 0.1 sec to 300 sec.