High strength and high formability steel sheet having excellent spot weldability, and method for manufacturing same

Abstract

The present invention relates to a steel sheet for use in automobiles, etc., and to a steel sheet that has high strength and high formability and is superb in terms of spot weldability, and a manufacturing method therefor.

1 . A high-strength and high-formability steel sheet having excellent spot weldability, comprising by weight:
 0.05 to 0.10% of carbon (C), 0.3% or less (excluding 0%) of silicon (Si), 2.0 to 2.5% of manganese (Mn), 0.05% or less (excluding 0%) of titanium (Ti), 0.1% or less (excluding 0%) of niobium (Nb), 1.5% or less (excluding 0%) of chromium (Cr), 0.1% or less of phosphorus (P), and 0.01% or less of sulfur (S), with a balance of Fe and inevitable impurities,   wherein a microstructure of the steel sheet at a point of ¼ of a thickness (t) of the steel sheet includes, by volume fraction, 65 to 85% of a soft phase and a remainder of a hard phase,   wherein a surface layer portion of the steel sheet includes a sound layer, and a thickness of the sound layer is 5 to 50 μm.   
     
     
         2 . The high-strength and high-formability steel sheet having excellent spot weldability of  claim 1 , wherein the sound layer is a ferrite main phase with a grain size of 6 to 20 μm. 
     
     
         3 . The high-strength and high-formability steel sheet having excellent spot weldability of  claim 1 , wherein the soft phase comprises, by area fraction, 60% or more of recrystallized ferrite and 5% or less of non-recrystallized ferrite. 
     
     
         4 . The high-strength and high-formability steel sheet having excellent spot weldability of  claim 1 , wherein the hard phase comprises martensite or a mixed structure of martensite and a trace amount of bainite. 
     
     
         5 . The high-strength and high-formability steel sheet having excellent spot weldability of  claim 1 , wherein the hard phase has an aspect ratio of 1.2 or less. 
     
     
         6 . The high-strength and high-formability steel sheet having excellent spot weldability of  claim 1 , further comprising:
 a plating layer on a surface of the steel sheet.   
     
     
         7 . The high-strength and high-formability steel sheet having excellent spot weldability of  claim 1 , wherein the steel sheet has a tensile strength (TS) of 780 MPa or more and an elongation (El) of 18% or more. 
     
     
         8 . A method for manufacturing a high-strength and high-formability steel sheet having excellent spot weldability, the method comprising:
 heating a steel slab comprising, by weight: 0.05 to 0.10% of carbon (C), 0.3% or less (excluding 0%) of silicon (Si), 2.0 to 2.5% of manganese (Mn), 0.05% or less (excluding 0%) of titanium (Ti), 0.1% or less (excluding 0%) of niobium (Nb), 1.5% or less (excluding 0%) of chromium (Cr), 0.1% or less of phosphorus (P), and 0.01% or less of sulfur (S), with a balance of Fe and inevitable impurities, to a temperature within a range of 1100 to 1300° C.;   hot rolling the heated steel slab, and cooling and hot rolling the steel slab so that a surface temperature of a material thereof is Ar3 or lower for a predetermined period of time during finish hot rolling;   coiling and cooling the hot-rolled steel sheet after the hot rolling;   cold rolling the cooled hot-rolled steel sheet at a reduction ratio of 70 to 90%;   heating and maintaining the cold-rolled steel sheet to a temperature range of Ac1 to Ac1+30° C. after the cold rolling; and   slowly cooling the cold-rolled steel sheet at an average cooling rate of 1 to 10° C./s to a temperature within a range of 650 to 700° C., and then rapidly cooling the cold-rolled steel sheet at an average cooling rate of 5 to 50° C./s to a temperature within a range of 300 to 580° C.   
     
     
         9 . The method for manufacturing a high-strength and high-formability steel sheet having excellent spot weldability of  claim 8 , wherein the temperature of the material during the finish hot rolling is Ar3 to 1000° C. 
     
     
         10 . The method for manufacturing a high-strength and high-formability steel sheet having excellent spot weldability of  claim 8 , wherein the coiling is performed at a temperature within a range of 400 to 700° C. and the cooling is performed at a cooling rate 0.1° C./s or less. 
     
     
         11 . The method for manufacturing a high-strength and high-formability steel sheet having excellent spot weldability of  claim 8 , further comprising:
 performing an over-aging treatment for 200 to 800 seconds after the rapid cooling.   
     
     
         12 . The method for manufacturing a high-strength and high-formability steel sheet having excellent spot weldability of  claim 8 , wherein, during the finish hot rolling, the surface is cooled by spraying water at least once between rolling passes. 
     
     
         13 . The method for manufacturing a high-strength and high-formability steel sheet having excellent spot weldability of  claim 8 , further comprising:
 forming a plating layer.