Austenitic stainless steel sheet and a method for its manufacture

Abstract

An austenitic stainless steel sheet for springs having both a high strength and excellent formability has a chemical composition comprising C: 0.01-0.15%, Si: at most 3.0%, Mn: at most 3.0%, Cr: 10.0-30.0%, Ni: 4.0-20.0%, N: at most 0.40%, and a remainder of Fe and impurities, and it has a metallurgical structure such that the austenite content γs (%) in the surface region of the steel sheet and the austenite content γc (%) in the center region of the sheet thickness satisfy (γs+γc)/2≦55 and γs/γc≧0.10, with the remaining structure being primarily strain-induced martensite.

1 . An austenitic stainless steel sheet characterized by having a chemical composition comprising, in mass %, C: 0.01-0.15%, Si: at most 3.0%, Mn: at most 3.0%, Cr: 10.0-30.0%, Ni: 4.0-20.0%, N: at most 0.40%, and a remainder of Fe and impurities, and by having a metallurgical structure in which the austenite content γs (%) in a surface region of the steel sheet and the austenite content γc (%) in a center region of the sheet thickness satisfy (γs+γc)/2≦55 and γs/γc≧0.10, with the remaining structure being primarily strain-induced martensite. 
     
     
         2 . An austenitic stainless steel sheet as set forth in  claim 1  wherein the chemical composition contains, in mass %, at least one of Mo: at most 3.0% and Cu: at most 3.0% in place of a portion of Fe. 
     
     
         3 . An austenitic stainless steel sheet as set forth in  claim 1  wherein the chemical composition contains, in mass %, at least one of Ti: at most 0.50%, Nb: at most 0.50%, and V: at most 1.0% in place of a portion of Fe. 
     
     
         4 . A method for manufacturing an austenitic stainless steel sheet as set forth in  claim 1  characterized by performing hot rolling of a steel having the above-described chemical composition, then performing cold rolling and annealing of the resulting hot rolled steel sheet to obtain an annealed cold rolled steel sheet, and subjecting the annealed cold rolled steel sheet to temper rolling with the number of passes being at least the reduction (%)/10. 
     
     
         5 . A method as set forth in  claim 4  wherein the average grain diameter of the austenite grains of the annealed cold rolled steel sheet before temper rolling is at most 5 μm. 
     
     
         6 . An austenitic stainless steel sheet as set forth in  claim 2  wherein the chemical composition contains, in mass %, at least one of Ti: at most 0.50%, Nb: at most 0.50%, and V: at most 1.0% in place of a portion of Fe. 
     
     
         7 . A method for manufacturing an austenitic stainless steel sheet as set forth in  claim 2  characterized by performing hot rolling of a steel having the above-described chemical composition, then performing cold rolling and annealing of the resulting hot rolled steel sheet to obtain an annealed cold rolled steel sheet, and subjecting the annealed cold rolled steel sheet to temper rolling with the number of passes being at least the reduction (%)/10. 
     
     
         8 . A method for manufacturing an austenitic stainless steel sheet as set forth in  claim 3  characterized by performing hot rolling of a steel having the above-described chemical composition, then performing cold rolling and annealing of the resulting hot rolled steel sheet to obtain an annealed cold rolled steel sheet, and subjecting the annealed cold rolled steel sheet to temper rolling with the number of passes being at least the reduction (%)/10. 
     
     
         9 . A method for manufacturing an austenitic stainless steel sheet as set forth in  claim 6  characterized by performing hot rolling of a steel having the above-described chemical composition, then performing cold rolling and annealing of the resulting hot rolled steel sheet to obtain an annealed cold rolled steel sheet, and subjecting the annealed cold rolled steel sheet to temper rolling with the number of passes being at least the reduction (%)/10.