High tensile strength hot-rolled steel sheet having superior strain aging hardenability and method for producing the same

Abstract

A high tensile strength hot-rolled steel sheet having superior strain aging hardenability, which has high formability and stable quality characteristics, and in which satisfactory strength is obtained when the steel sheet is formed into automotive components, thus enabling the reduction in weight of automobile bodies. Specifically, a method for producing a high tensile strength hot-rolled steel sheet having superior strain aging hardenability with a BH of 80 MPa or more, a ΔTS of 40 MPa or more, and a tensile strength of 440 MPa or more includes the steps of heating a steel slab to 1,000° C. or more, the steel slab containing, in percent by mass, 0.15% or less of C, 0.45% or less of Si, 3.0% or less of Mn, 0.08% or less of P, 0.02% or less of S, 0.02% or less of Al, 0.0050% to 0.0250% of N, and optionally 0.1% or less in total of at least one of more than 0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V, the ratio N (mass %)/Al (mass %) being 0.86 or more; rough-rolling the steel slab to form a sheet bar; finish-rolling the sheet bar at a finishing temperature of 800° C. or more; cooling at a cooling rate of 20° C. to 40° C./s or more within 0.5 second after the finish-rolling; and coiling at a temperature of 650° C. to 450° C. or less.

1 . A high tensile strength hot-rolled steel sheet having superior strain aging hardenability comprising: in percent by mass,
 0.15% or less of C;   0.45% or less of Si;   3.0% or less of Mn;   0.08% or less of P;   0.02% or less of S;   less than 0.02% of Al;   0.0050% to 0.0250% of N; and   the balance being Fe and incidental impurities,   the ratio N (mass %)/Al (mass %) being 0.86 or more,   N in the dissolved state being 0.0030% or more, wherein the hot-rolled steel sheet has a ferrite phase with an average grain size of 10 μm or less.   
   
   
       2 . The steel sheet according to  claim 1 , having a BH of 92 MPa or more. 
   
   
       3 . The steel sheet according to  claim 1 , having a ΔTS of 52 MPa or more. 
   
   
       4 . The steel sheet according to  claim 1 , having a BH of 92 MPa or more and a ΔTS of 52 MPa or more. 
   
   
       5 . A high tensile strength hot-rolled steel sheet having superior strain aging hardenability with a tensile strength of 440 MPa or more comprising: in percent by mass,
 0.15% or less of C;   0.45% or less of Si;   3.0% or less of Mn;   0.08% or less of P;   0.02% or less of S;   less than 0.02% of Al;   0.0050% to 0.0250% of N; and   the balance being Fe and incidental impurities,   the ratio N (mass %)/Al (mass %) being 0.86 or more, N in the dissolved state being 0.0030% or more,   wherein the hot-rolled steel sheet has a structure in which the areal rate of the ferrite phase having an average grain size of 10 μm or less is 50% or more.   
   
   
       6 . A steel sheet according to  claim 5 , further comprising at least one selected from the group consisting of the following Group a to Group d:
 Group a: 1.0% or less in total of at least one of Cu, Ni, Cr, and Mo   Group b: 0.1% or less in total of at least one of Nb, Ti, and V   Group c: 0.0030% or less of B   Group d: 0.0010% to 0.010% in total of at least one of Ca and REM.   
   
   
       7 . A high tensile strength hot-rolled plated steel sheet produced by electroplating or hot-dip plating a steel sheet according to  claim 6 . 
   
   
       8 . A steel sheet according to  claim 5 , wherein the high tensile strength hot-rolled sheet has a thickness of 4.0 mm or less. 
   
   
       9 . A high tensile strength hot-rolled plated steel sheet produced by electroplating or hot-dip plating a steel sheet according to  claim 8 . 
   
   
       10 . A high tensile strength hot-rolled plated steel sheet produced by electroplating or hot-dip plating a steel sheet according to  claim 5 . 
   
   
       11 . The steel sheet according to  claim 5 , having a BH of 92 MPa or more. 
   
   
       12 . The steel sheet according to  claim 5 , having a ΔTS of 52 MPa or more. 
   
   
       13 . The steel sheet according to  claim 5 , having a BH of 92 MPa or more and a ΔTS of 52 MPa or more. 
   
   
       14 . A high tensile strength hot-rolled steel sheet having superior strain aging hardenability with a BH of 80 MPa or more, a ΔTS of 40 MPa or more, and a tensile strength of 440 MPa or more comprising, in percent by mass,
 0.15% or less of C;   0.45% or less of Si;   3.0% or less of Mn;   0.08% or less of P;   0.02% or less of S;   less than 0.02% of Al;   0.0050% to 0.0250% of N; and   the balance being Fe and incidental impurities,   the ratio N (mass %)/AI (mass %) being 0.86 or more, N in the dissolved state being 0.0030% or more,   wherein the hot-rolled steel sheet has a structure in which the areal rate of the ferrite phase having an average grain size of 10 μm or less is 70% or more, and the areal rate of the martensite phase is 5% or more.   
   
   
       15 . A high tensile strength hot-rolled steel sheet having superior strain aging hardenability comprising: in percent by mass,
 0.03% to 0.1% of C;   0.45% or less of Si;   1.0% to 3.0% of Mn;   0.08% or less of P;   0.02% or less of S;   
     less than 0.02% of Al;
 0.0050% to 0.0250% of N; 
 0.1% or less in total of at least one of more than 0.02% to 0.1% of Nb and more than 0.02% to 0.1% of V; and 
 the balance being Fe and incidental impurities, 
 the ratio N (mass %)/Al (mass %) being 0.86 or more, 
 N in the dissolved state being 0.0030% or more, 
 the total of precipitated Nb and precipitated V being 0.015% or more, 
 wherein the hot-rolled steel sheet has a structure in which the areal rate of the ferrite phase having an average grain size of 10 μm or less is 80% or more, and the average grain size of a precipitate comprising a Nb carbonitride or a V carbonitride is 0.05 μm or less. 
 
   
   
       16 . The steel sheet according to  claim 15 , having a BH of 92 MPa or more. 
   
   
       17 . The steel sheet according to  claim 15 , having a ΔTS of 52 MPa or more. 
   
   
       18 . The steel sheet according to  claim 15 , having a BH of 92 MPa or more and a ΔTS of 52 MPa or more.