High tensile hot-rolled steel sheet having excellent strain aging hardening properties and method for producing the same

Abstract

The present invention provides 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, 2.0% 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 0.3 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;    2.0% 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    the balance being Fe and incidental impurities, the ratio N (mass %)/Al (mass %) being 0.3 or more, N in the dissolved state being 0.0010% or more.    
     
     
         2 . 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;    2.0% 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    the balance being Fe and incidental impurities,    the ratio N (mass %)/Al (mass %) being 0.3 or more, N in the dissolved state being 0.0010% 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.    
     
     
         3 . A steel sheet according to  claim 2  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.  
 
     
     
         4 . A steel sheet according to either  claim 2  or  3 , wherein the high tensile strength hot-rolled sheet has a thickness of 4.0 mm or less.  
     
     
         5 . A high tensile strength hot-rolled plated steel sheet produced by electroplating or hot-dip plating a steel sheet according to any one of  claims 2  to  4 .  
     
     
         6 . A method for producing a high tensile strength hot-rolled steel sheet having superior strain aging hardenability with a tensile strength of 440 MPa or more comprising the steps of: 
 heating a steel slab to 1,000° C. or more, the steel slab comprising: in percent by mass, 
 0.15% or less of C;  
 2.0% 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 further comprising at least one selected from the group consisting of the following Group a to Group d, the ratio N (mass %)/Al (mass %) being 0.3 or more: 
 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;  
 
   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./s or more within 0.5 second after the finish-rolling; and    coiling at a temperature of 650° C. or less.    
     
     
         7 . A method according to according to  claim 6 , further comprising the step of performing at least one of skin pass rolling and leveling with an elongation of 1.5% to 10% after the coiling step is performed.  
     
     
         8 . A method according to either  claim 6  or  7 , further comprising the step of joining consecutive sheet bars to each other between the steps of rough-rolling and finish-rolling.  
     
     
         9 . A method according to any one of  claims 6  to  8 , further comprising the step of using at least one of a sheet bar edge heater for heating a widthwise end of the sheet bar and a sheet bar heater for heating a lengthwise end of the sheet bar between the steps of rough-rolling and finish-rolling.  
     
     
         10 . 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;  
 2.0% 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  
 the balance being Fe and incidental impurities,  
 the ratio N (mass %)/Al (mass %) being 0.3 or more, N in the dissolved state being 0.0010% 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.  
 
     
     
         11 . 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 comprising the steps of: 
 heating a steel slab to 1,000° C. or more, the steel slab comprising: in percent by mass, 
 0.15% or less of C;  
 2.0% 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 further comprising at least one selected from the group consisting of the following Group a to Group d, the ratio N (mass %)/Al (mass %) being 0.3 or more: 
 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;  
 
 
 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./s or more within 0.5 second after the finish-rolling; and  
 coiling at a temperature of 450° C. or less.  
 
     
     
         12 . 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;    2.0% or less of Si;    1.0% to 3.0% 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;    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 0.3 or more,    N in the dissolved state being 0.0010% 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.    
     
     
         13 . A method for producing a high tensile strength hot-rolled steel sheet having superior strain aging hardenability comprising the steps of: 
 heating a steel slab to 1,100° C. or more, the steel slab comprising: in percent by mass, 
 0.03% to 0.1% of C;  
 2.0% or less of Si;  
 1.0% to 3.0% 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:  
 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;  
   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 40° C./s or more within 0.5 second after the finish-rolling; and    coiling in the temperature range of 550 to 650° C.