Metastable austenite stainless steel strip or steel sheet and manufacturing method thereof

Abstract

A metastable austenite stainless steel strip of high strength and ductility. The steel strip including C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe, a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ T phase and a γ R phase, a sum of the γ T phase and the γ R phase is 15 to 50 volume %, and a γ T phase area ratio defined by formula (2) (=100×(total area ratio of γ T phase of entire observation area)) is between 1% and 20% inclusive, and 0.2% yield strength (YS) of 1400 N/mm 2 to 1900 N/mm 2 where a value of YS-EL balance satisfies at least 21,000 to 48,000.

What is claimed is: 
     
         1 . A metastable austenite stainless steel strip or steel sheet comprising:
 a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe and unavoidable impurities;   a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ T  phase and a γ R  phase, a total of the γ T  phase and the γ R  phase is 15 to 50 volume %, and a γ T  phase area ratio defined by the following formula (2) is between 1% and 20% inclusive; and   0.2% yield strength (YS) of 1400 N/mm 2  to 1900 N/mm 2  where a value of YS-EL balance derived from the formula (1) satisfies at least 21,000 to 48,000, wherein
   YS-EL balance=YS×EL  (1) and
 
   γ T  phase area ratio (%)=100×(total area ratio of γ T  phase of entire observation area)  (2), where
 
   the α′ phase is a strain induced martensite phase, the Y phase is a combination of the γ T  phase and the γ R  phase, the γ R  phase is a reverse transformation austenite phase in which an area per crystal grain is between 5 μm 2  and 20 μm 2  inclusive, the γ R  phase is an austenite phase other than the γ T  phase, and YS is 0.2% yield strength and EL is elongation.   
     
     
         2 . The stainless steel strip or steel sheet according to  claim 1 , wherein the 0.2% yield strength (YS) is 1550 N/mm 2  to 1900 N/mm 2  and the value of YS-EL balance derived from the formula (1) satisfies at least 35,000 to 48,000. 
     
     
         3 . The stainless steel strip or steel sheet according to  claim 1 , wherein, in the mass percent composition, Fe is partly replaced with one or two elements selected from a group of Al: 0.1 to 3.5% and Ti: 0.1 to 3.5%. 
     
     
         4 . The stainless steel strip or steel sheet according to  claim 1 , wherein HV is 450 or more. 
     
     
         5 . A manufacturing method of a metastable austenite stainless steel strip or steel sheet, the method comprising:
 preparing a stainless steel strip or steel sheet including a mass percent composition of C: 0.05 to 0.15%, Si: 0.05 to 1%, Mn: 2% or less, Cr: 16 to 18%, Ni: 4 to 11%, Mo: 2.5 to 3.5%, Cu: 0.4 to 1.0% with a remaining part of Fe and unavoidable impurities;   performing cold rolling to the stainless steel strip or steel sheet in order to form 50 volume % or more of a strain induced martensite phase (α′ phase) from the austenite phase (γ phase); and   performing a low temperature heat treatment to the stainless steel strip or steel sheet with the strain induced martensite phase (α′ phase) within a temperature range of 250 to 480° C. in order to grow a austenite phase (γ T  phase) from the martensite phase (α′ phase) formed in the strain induced martensite phase formation, wherein the metastable austenite stainless steel strip or steel sheet has the following metal structure and mechanical property,   a metal structure is a dual phase structure of an α′ phase and a γ phase where the γ phase is composed of a γ T  phase and a γ R  phase, a total of the γ T  phase and the γ R  phase is 15 to 50 volume %, and a γ T  phase area ratio defined by the following formula (2) is between 1% and 20% inclusive,   a mechanical property satisfies 0.2% yield strength (YS) of 1400 N/mm 2  to 1900 N/mm 2  where a value of YS-EL balance derived from the formula (1) satisfies at least 21,000 to 48,000,
   YS-EL balance=YS×EL  (1) and
 
   γ T  phase area ratio (%)=100×(total area ratio of γ T  phase of entire observation area)  (2), where
 
   the α′ phase is a strain induced martensite phase, the Y phase is a combination of the γ T  phase and the γ R  phase, the γ R  phase is a reverse transformation austenite phase in which an area per crystal grain is between 5 μm 2  and 20 μm 2  inclusive, the γ R  phase is an austenite phase other than the γ T  phase, and YS is 0.2% yield strength and EL is elongation.   
     
     
         6 . The manufacturing method according to  claim 5 , wherein the 0.2% yield strength (YS) is 1550 N/mm 2  to 1900 N/mm 2  and the value of YS-EL balance derived from the formula (1) satisfies at least 35,000 to 48,000. 
     
     
         7 . The manufacturing method according to  claim 7 , wherein the stainless steel strip or steel sheet contains, in the mass percent composition, Fe is partly replaced with one or two elements selected from a group of Al: 0.1 to 3.5% and Ti: 0.1 to 3.5%. 
     
     
         8 . The manufacturing method according to  claim 5 , wherein the stainless steel strip or steel sheet has HV of 450 or more.