High-strength steel sheets and processes for production of the same

Abstract

A high strength steel sheet with both excellent elongation and stretch-flanging performance is provided. The high strength steel sheet of the present invention comprises, in percent by mass, C: 0.05 to 0.3%, Si: 0.01 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 0.1%, and Fe and inevitable impurities as the remainder, and has a structure mainly composed of tempered martensite and annealed bainite. The space factor of the tempered martensite is 50 to 95%, the space factor of the annealed bainite is 5 to 30%, and the mean grain size of the tempered martensite is 10 μm or smaller in terms of the equivalent of a circle diameter. The steel sheet has a tensile strength of 590 MPa or higher. The high strength steel sheet of the present invention has a space factor of the martensite phase which is a main component of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter; in the martensite phase, the space factor of the martensite phase having a grain size of 10 μm or larger in terms of the equivalent of a circle diameter is 15% or lower; and the space factor of the retained austenite phase in the metal structure is 3% or lower. The high strength steel sheet of the present invention is a dual phase steel sheet mainly composed of a ferrite phase and martensite, and the space factor of the ferrite phase is 5 to 30%, and the space factor of the martensite phase is 50 to 95%. Moreover, the ferrite phase is annealed martensite.

1 . A high strength steel sheet which comprises, in percent by mass, C: 0.05 to 0.3%; Si: 3% or less (not including 0%.); Mn: 0.5 to 3.0%; Al: 0.01 to 0.1%; and the remainder comprising iron and inevitable impurities, the high strength steel sheet having a space factor of a martensite phase which is a main component of a metal structure of 50% or higher, and a tensile strength of 590 MPa or higher. 
     
     
         2 . The high strength steel sheet according to  claim 1 , wherein the structure which is a main part of the metal structure is the martensite phase, which is tempered martensite, and finely dispersed annealed bainite; the space factor of the tempered martensite is 50 to 95%; the space factor of the annealed bainite is 5 to 30%; and a mean grain size of the tempered martensite is 10 μm or lower in terms of the equivalent of a circle diameter. 
     
     
         3 . The high strength steel sheet according to  claim 1 , wherein the space factor of the martensite phase which is a main part of the metal structure is 80% or higher; the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter; the space factor of the martensite phase having a grain size of 10 μm or larger in terms of the equivalent of a circle diameter in the martensite phase is 15% or lower; and the space factor of the retained austenite phase in the metal structure is 3% or lower. 
     
     
         4 . The high strength steel sheet according to  claim 3 , wherein the martensite phase is a tempered martensite phase; an annealed martensite phase is contained as the metal structure other than the martensite phase and the retained austenite phase; and the space factor of the annealed martensite phase is 3 to 20%. 
     
     
         5 . The high strength steel sheet according to  claim 1 , wherein the structure which is a main part of the metal structure is the martensite phase and a ferrite phase; the space factor of the martensite phase is 50 to 95%; the space factor of the ferrite phase is 5 to 30%; and the mean grain size of the martensite phase is 10 μm or smaller in terms of the equivalent of a circle diameter. 
     
     
         6 . The high strength steel sheet according to  claim 5 , wherein the ferrite phase is annealed martensite. 
     
     
         7 . The high strength steel sheet according to  claim 1 , wherein at least an element selected from Ti, Nb, V and Zr is further contained in an amount of 0.01 to 1% by mass in total. 
     
     
         8 . The high strength steel sheet according to  claim 1 , wherein Ni and/or Cu are/is further contained in an amount of 1% by mass or lower in total. 
     
     
         9 . The high strength steel sheet according to  claim 1 , wherein Cr: 2% by mass or less and/or Mo: 1% by mass or less are/is further contained. 
     
     
         10 . The high strength steel sheet according to  claim 1 , wherein 0.0001 to 0.005% by mass of B is further contained. 
     
     
         11 . The high strength steel sheet according to  claim 1 , wherein Ca and/or REM are/is further contained in an amount of 0.003% by mass or lower in total. 
     
     
         12 . A method for manufacturing a high strength steel sheet according to  claim 2 , the method comprising using a steel sheet having a space factor of bainite in the entire metal structure of 90% or higher as a material steel sheet; heating and retaining the material steel sheet at a temperature of (Ac 3  point −100° C.) or higher but not higher than Ac 3  point for 0 to 2400 seconds (including 0 seconds); then cooling the material steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher; and subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550° C. for 60 to 1200 seconds. 
     
     
         13 . A method for manufacturing a high strength steel sheet according to  claim 3 , the method comprising using a steel sheet in which the total space factor of the martensite phase and/or of the retained austenite phase in the entire metal structure is 90% or higher as a material steel sheet; heating and retaining the steel sheet at a temperature of (Ac 3  point −100° C.) or higher but not higher than Ac 3  point for 30 to 1200 seconds; cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher; and further conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 500° C. for 60 to 1200 seconds. 
     
     
         14 . A method for manufacturing a high strength steel sheet according to  claim 5 , the method comprising providing a total space factor of the martensite phase and/or bainite phase in the entire metal structure is 90% or higher; using a steel sheet having a grain size of the former austenite of 20 μm or smaller in terms of the equivalent of a circle diameter as a material steel sheet; heating and retaining the steel sheet at a temperature of (Ac 3  point −100° C.) or higher but not higher than Ac 3  point for 1 to 2400 seconds; then cooling the steel sheet to a transformation start temperature of martensite, Ms point, or lower at an average cooling rate of 10° C./sec. or higher; and subsequently conducting a heat treatment in which the steel sheet is heated and retained at a temperature of 300 to 550° C. for 60 to 1200 seconds.