Multi-phase steel sheet excellent in hole expandability and method of producing the same

Abstract

The present invention provides a steel sheet excellent in both a balance between strength and elongation and a balance between strength and hole expandability, in other words, a multi-phase steel sheet having an excellent balance between strength and hole expandability. The present invention is a multi-phase steel sheet excellent in hole expandability characterized in that: the steel sheet contains, as chemical components in mass, C: 0.03 to 0.15%, P: not more than 0.010%, S: not more than 0.003%, and either one or both of Si and Al in a total amount of 0.5 to 4%, and one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with the balance consisting of Fe and unavoidable impurities; the microstructure at a section of the steel sheet is composed of either one or both of retained austenite and martensite which account(s) for 3 to 30% in total in area percentage and the balance consisting of either one or both of ferrite and bainite; the maximum length of the crystal grains in the microstructure is not more than 10 microns; and the number of inclusions 20 microns or larger in size at a section of the steel sheet is not more than 0.3 piece per square millimeter.

1 - 6 . (canceled) 
     
     
         7 : A method of producing a multi-phase steel sheet excellent in hole expandability, which steel sheet contains, as chemical components in mass,
 C: 0.03 to 0.15%,   P: not more than 0.010%,   S: not more than 0.003%, and   
       either one or both of Si and Al in a total amount of 0.5 to 4%, and one or more of Mn, Ni, Cr, Mo and Cu in a total amount of 0.5 to 4%, with the balance consisting of Fe and unavoidable impurities, characterized by:
 when molten steel with said components is refined, circulating the molten steel not less than 1.5 times after flux for desulfurization is added at the time of the desulfurization of the molten steel; 
 further, when a steel sheet is produced by hot-rolling a slab obtained by casting said molten steel, conducting the finish rolling by controlling the finish-rolling entry temperature to 950° C. or higher and the finish-rolling exit temperature within the range from 780 to 920° C.; and 
 coiling the steel sheet thus obtained at a temperature of 500° C. or lower. 
 
     
     
         8 : A method of producing a multi-phase steel sheet excellent in hole expandability according to  claim 7 , characterized in that the steel sheet further contains, as chemical components in mass, one or more of Nb, V and Ti in a total amount of 0.3% or less. 
     
     
         9 . A method of producing a multi-phase steel sheet excellent in hole expandability according to  claim 7 , characterized in that the steel sheet further contains, as a chemical component in mass, B of 0.01% or less. 
     
     
         10 : A method of producing a multi-phase steel sheet excellent in hole expandability according to  claim 7 , characterized in that the steel sheet further contains, as chemical components in mass, either one or both of Ca of 0.01% or less and REM of 0.05% or less.