High-strength steel sheet and method for producing the same

Abstract

Provided are a high-strength steel sheet and a method for producing the same. A high-strength steel sheet has a composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities and also has a microstructure containing ferrite and pearlite. The volume fraction of the ferrite is 70% to 97%. The volume fraction of the pearlite is 3% or more. The volume fraction of cementite present at grain boundaries of the ferrite is 2% or less. The sum of the volume fractions of phases other than the ferrite, the pearlite, and the cementite is less than 3%. The average grain size of the ferrite is 7 μm or less.

1 . A high-strength steel sheet having a composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities, the high-strength steel sheet having a microstructure containing ferrite and pearlite, wherein the volume fraction of the ferrite is 70% to 97%, the volume fraction of the pearlite is 3% or more, the volume fraction of cementite present at grain boundaries of the ferrite is 2% or less, the sum of the volume fractions of phases other than the ferrite, the pearlite, and the cementite is less than 3%, and the average grain size of the ferrite is 7 μm or less. 
     
     
         2 . The high-strength steel sheet according to  claim 1 , further containing at least one selected from the group consisting of 0.01% to 1.0% Cr, 0.01% to 0.1% Ti, and 0.01% to 0.1% V on a mass basis. 
     
     
         3 . The high-strength steel sheet according to  claim 1 , further having a tensile strength TS of 600 MPa to 700 MPa. 
     
     
         4 . The high-strength steel sheet according to  claim 1 , further having a hole expansion ratio λ of 80% or more. 
     
     
         5 . The high-strength steel sheet according to  claim 1 , wherein the volume fraction of the ferrite is 80% to 95%. 
     
     
         6 . The high-strength steel sheet according to  claim 1 , wherein the volume fraction of the pearlite is 3% to 30%. 
     
     
         7 . The high-strength steel sheet according to  claim 1 , wherein the volume fraction of the pearlite is 5% to 28%. 
     
     
         8 . A method for producing a high-strength steel sheet, comprising:
 a step of preparing a steel slab having a chemical composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities;   a step of hot-rolling the steel slab into a hot-rolled sheet; and   a step of annealing the hot-rolled sheet in such a way that the hot-rolled sheet is heated to a two-phase temperature range between the Ac 1  transformation temperature and the Ac 3  transformation temperature, is cooled to a temperature range of 450° C. to 600° C. at an average cooling rate of 5° C./s to 30° C./s, and is then held at this temperature range for 100 s or more.   
     
     
         9 . The method for producing the high-strength steel sheet according to  claim 8 , wherein the steel slab further contains at least one selected from the group consisting of 0.01% to 1.0% Cr, 0.01% to 0.1% Ti, and 0.01% to 0.1% V on a mass basis. 
     
     
         10 . The method for producing the high-strength steel sheet according to  claim 8 , wherein the annealing step includes heating to the two-phase temperature range between the Ac 1  transformation temperature and the Ac 3  transformation temperature, cooling to a temperature range of 450° C. to 600° C. at an average cooling rate of 10° C./s to 20° C./s, and then holding at this temperature range for 100 s to 300 s.