Steel sheet with excellent bendability

Abstract

A steel sheet with excellent bendability in which the main structure is composed of retained austenite (5-30 area %) and ferrite (no less than 50 area %) such that there exist no more than 40 carbide grains per 2000 μm 2 between the retained austenite and the ferrite. The steel sheet is suitable for automobiles which exhibits bending properties (bendability and tight bendability) even though its strength is as high as 600 to 1400 MPa.

What is claimed is:  
     
         1 . A steel sheet with excellent bendability comprising: 
 C: from 0.06 mass % to 0.25 mass %;    at least one of Si and Al: total 0.5-3 mass %;    Mn: from 0.5 mass % to 3 mass %;    P: no more than 0.15 mass % (excluding 0 mass %); and    S: no more than 0.02 mass % (excluding 0 mass %),    wherein the main structure of said steel sheet comprises retained austenite of 5-30 area % and ferrite of no less than 50 area %,    and wherein there exist no more than 40 carbide grains per 2000 μm 2  in said steel sheet.    
     
     
         2 . The steel sheet as defined in  claim 1 , further comprising at least one member selected from the group consisting of: 
 Mo: no more than 1 mass % (excluding 0 mass %);    Ni: no more than 0.5 mass % (excluding 0 mass %); and    Cu: no more than 0.5 mass % (excluding 0 mass %).    
     
     
         3 . The steel sheet as defined in  claim 1 , further comprising at least one of Ca of no more than 0.003 mass % (excluding 0 mass %) and rare earth element of no more than 0.003 mass % (excluding 0 mass %).  
     
     
         4 . The steel sheet as defined in  claim 1 , wherein the main structure of said steel sheet comprises retained austenite of 5-20 area %.  
     
     
         5 . The steel sheet as defined in  claim 1 , wherein there exist no more than 30 carbide grains per 2000 μm 2  in said steel sheet.