Ultrahigh strength steel sheet and strength part for automobile utilizing the same

Abstract

There is provided an ultrahigh strength steel sheet containing 0.10 to 0.40 mass % of C, 0.01 to 3.5 mass % of Cr, at least one selected from the group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5 mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of P and 0.01 mass % or less of S as impurities and the balance being Fe and unavoidable impurities based on the total mass of the steel sheet and having a base structure of lower bainite, a prior austenite grain size of 30 μm or smaller and a tensile strength of 980 MPa or higher. There is also provided an automotive strength part using the ultrahigh strength steel sheet.

1 . An ultrahigh strength steel sheet comprising 0.10 to 0.40 mass % of C, 0.01 to 3.5 mass % of Cr. at least one selected from the group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5 mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of P and 0.01 mass % or less of S as impurities and the balance being Fe and unavoidable impurities based on the total mass of the steel sheet and having a base structure of either lower bainite, tempered lower bainite or tempered martensite, a prior austenite grain size of 30 μm or smaller and a tensile strength of 980 MPa or higher. 
   
   
       2 - 3 . (canceled) 
   
   
       4 . The ultrahigh strength steel sheet according to  claim 1 , wherein the steel sheet further comprises at least one of 0.1 to 3.0 mass % of Cu and 0.1 to 3.0 mass % of Ni based on the total mass of the steel sheet. 
   
   
       5 . The ultrahigh strength steel sheet according to  claim 1 , wherein the steel sheet further comprises at least one of 0.01 to 2.5 mass % of Si and 0.1 to 1.0 mass % of Mn based on the total mass of the steel sheet. 
   
   
       6 . The ultrahigh strength steel sheet according to  claim 1 , wherein the steel sheet further comprises 0.001 to 0.1 mass % of Al based on the total mass of the steel sheet. 
   
   
       7 . The ultrahigh strength steel sheet according to  claim 1 , wherein the steel sheet has an average prior austenite grain size of 3 to 10 μm. 
   
   
       8 . The ultrahigh strength steel sheet according to  claim 1 , wherein the steel sheet is either a hot rolled steel sheet or a cold rolled steel sheet. 
   
   
       9 . The ultrahigh strength steel sheet according to  claim 1 , wherein the steel sheet is surface treated by zinc plating. 
   
   
       10 . The ultrahigh strength steel sheet according to  claim 1 , wherein the steel sheet is treated by film lamination. 
   
   
       11 . An automotive strength part using the ultrahigh strength steel sheet according to  claim 1 . 
   
   
       12 . The automotive strength part according to  claim 11 , wherein the automotive strength part is formed by subjecting the ultrahigh strength steel sheet to any of press molding process, hydroform process and blow molding process 
   
   
       13 . The automotive strength part according to  claim 11 , wherein the automotive strength part has a cut-processed portion.