High strength thin steel sheet excelling in weldability and process for producing the same

Abstract

Provided are a high strength thin steel sheet having tensile strength of about 800 MPa or more, and a manufacturing method thereof. The thin steel sheet is mainly used for construction materials, home appliances, and automobiles. The thin steel sheet has excellent plating characteristic, welding characteristic, bending workability, and hole expansion ratio. The thin steel sheet includes, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, SoLAl: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and includes Fe and other inevitable impurities as a remainder. Here, Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27. Also, the manufacturing method can secure workability of the thin steel sheet.

1 . A high strength thin steel sheet having an excellent welding characteristic, the thin steel sheet comprising: in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder,
 wherein Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27.   
     
     
         2 . The thin steel sheet of  claim 1 , wherein the steel sheet has a structure comprising at least one selected from the group consisting of bainite and bainitic ferrite occupying about 40% or more, and ferrite and martensite phases occupying the remainder. 
     
     
         3 . The thin steel sheet of  claim 1 , wherein the steel sheet comprises a hot-dip galvanized (GI) layer or a galvannealed (GA) layer on a surface of the steel sheet. 
     
     
         4 . A method for manufacturing a high strength thin steel sheet having an excellent welding characteristic, the method comprising:
 reheating a slab of the steel sheet comprising, in weight %, C: 0.02-0.20%, Si: 1.5% or less, Mn: 1.5-3.0%, P: 0.001-0.10%, S: 0.010% or less, Sol.Al: 0.01-0.40%, N: 0.020% or less, Cr: 0.3-1.5%, B: 0.0010-0.0060%, Sb: 0.001-0.10%, and including at least one material selected from the group consisting of Ti: 0.003-0.08%, Nb: 0.003-0.08%, and Mo: 0.003-0.08%, and including Fe and other inevitable impurities as a remainder, wherein Si, Mn, B, Sb, P, and S meet conditions of 5<(Si/Mn+150B)/Sb<20 and C+Mn/20+Si/30+2P+4S<0.27, and rolling and winding the slab at a temperature of a finish rolling exit side between the Ar 3  transformation point and 950° C.;   pickling a wound hot rolled steel sheet and performing cold rolling on the same at a reduction ratio of 40-80%; and   performing continuous annealing on an obtained cold rolled steel sheet at a temperature range of 740-860° C., cooling the cold rolled steel sheet down to 250-600° C. at a cooling rate satisfying a condition of −5 Log CR+25C−17Si+40Cr+13,000B>30 in a cooling rate range of 3-150° C./s, and cooling the same at a cooling rate of 5° C./minute or more.   
     
     
         5 . The method of  claim 4 , wherein the steel sheet has a structure comprising at least one selected from the group consisting of bainite and bainitic ferrite occupying 40% or more, and ferrite and martensite phases occupying the remainder. 
     
     
         6 . The method of  claim 4 , further comprising performing hot-dip galvanizing (GI) or galvannealing (GA).