High-strength hot-rolled steel sheet and method for producing the same (as amended)

Abstract

A steel slab having a composition containing C: more than 0.07% and 0.2% or less, Si: 2.0% or less, Mn: 1.0% to 3.0%, Al: 0.1% or less, Ti: 0.05% to 0.3%, and V: 0.05% to 0.3% on a mass percent basis is heated to 1100° C. or more and is subjected to rough rolling and finish rolling. In the finish rolling, the total rolling reduction of two final passes is 30% or more, and the finish rolling temperature ranges from (A r3 transformation temperature) to (A r3 transformation temperature+120° C.). Cooling is started within 2 seconds after the finish rolling. Coiling is performed at an average cooling rate of 40° C./s or more at a coiling temperature in the range of 300° C. to 500° C. The resulting high-strength hot-rolled steel sheet has a microstructure in which a bainite phase constitutes more than 90% by volume, the average lath interval of bainite is 0.45 μm or less, and the ratio of Fe-based carbide precipitated in bainite lath to all Fe-based carbide is 10% or more, has a high tensile strength of 900 MPa or more, and has significantly improved punchability in mass production.

1 . A high-strength hot-rolled steel sheet having a composition and a microstructure, the composition containing, on a mass percent basis,
 C: more than 0.07% and 0.2% or less, Si: 2.0% or less,   Mn: 1.0% to 3.0%, P: 0.05% or less,   S: 0.005% or less, Al: 0.1% or less,   N: 0.01% or less, Ti: 0.05% to 0.3%,   V: 0.05% to 0.3%, and   
       the balance being Fe and incidental impurities,
 the microstructure comprising a bainite of more than 90% by volume, the bainite having an average lath interval of 0.45 μm or less, and Fe-based carbide precipitated in bainite lath grains having a ratio of 10% or more by number to all Fe-based carbide. 
 
     
     
         2 . The high-strength hot-rolled steel sheet according to  claim 1 , wherein the composition further contains at least one selected from Nb: 0.005% to 0.2% and B: 0.0002% to 0.0030% on a mass percent basis. 
     
     
         3 . The high-strength hot-rolled steel sheet according to  claim 1 , wherein the composition further contains at least one selected from Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, and Sn: 0.005% to 0.3% on a mass percent basis. 
     
     
         4 . The high-strength hot-rolled steel sheet according to  claim 1 , wherein the composition further contains at least one selected from Mo: 0.002% to 0.3% and Cr: 0.002% to 0.3% on a mass percent basis. 
     
     
         5 . The high-strength hot-rolled steel sheet according to  claim 1 , wherein the composition further contains at least one selected from Ca: 0.0002% to 0.004% and REM: 0.0002% to 0.004% on a mass percent basis. 
     
     
         6 . A hot-dip galvanized steel sheet, having a hot-dip galvanizing layer or alloyed hot-dip galvanizing layer on a surface of the high-strength hot-rolled steel sheet according to  claim 1 . 
     
     
         7 . A method for producing a high-strength hot-rolled steel sheet, comprising:
 heating a steel slab and subjecting the steel slab to hot rolling having rough rolling and finish rolling to form a hot-rolled steel sheet,   wherein the steel slab has a composition containing, on a mass percent basis,   C: more than 0.07% and 0.2% or less, Si: 2.0% or less,   Mn: 1.0% to 3.0%, P: 0.05% or less,   S: 0.005% or less, Al: 0.1% or less,   N: 0.01% or less, Ti: 0.05% to 0.3%,   V: 0.05% to 0.3%, and   the balance being Fe and incidental impurities,   wherein the hot rolling includes heating the steel slab to 1100° C. or more, a total rolling reduction of two final passes in the finish rolling being 30% or more, a finish-rolling finishing temperature being in a range of (A r3  transformation temperature) to (A r3  transformation temperature+120° C.), starting cooling within 2 seconds after the finish rolling, cooling the hot-rolled steel sheet to a coiling temperature at an average cooling rate of 40° C./s or more, and coiling the hot-rolled steel sheet at a coiling temperature: 300° C. to 500° C.   
     
     
         8 . The method for producing a high-strength hot-rolled steel sheet according to  claim 7 , wherein the composition further contains at least one selected from Nb: 0.005% to 0.2% and B: 0.0002% to 0.0030% on a mass percent basis. 
     
     
         9 . The method for producing a high-strength hot-rolled steel sheet according to  claim 7 , wherein the composition further contains at least one selected from Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, and Sn: 0.005% to 0.3% on a mass percent basis. 
     
     
         10 . The method for producing a high-strength hot-rolled steel sheet according to  claim 7 , wherein the composition further contains at least one selected from Mo: 0.002% to 0.3% and Cr: 0.002% to 0.3% on a mass percent basis. 
     
     
         11 . The method for producing a high-strength hot-rolled steel sheet according to  claim 7 , wherein the composition further contains at least one selected from Ca: 0.0002% to 0.004% and REM: 0.0002% to 0.004% on a mass percent basis. 
     
     
         12 . A method for producing a hot-dip galvanized steel sheet, comprising:
 pickling a high-strength hot-rolled steel sheet produced by the method for producing a high-strength hot-rolled steel sheet according to  claim 7 , and subjecting the high-strength hot-rolled steel sheet to annealing and plating treatment to form a coated steel sheet,   wherein the annealing is performed at a soaking temperature of 730° C. or less, and the plating treatment after the annealing includes passing the high-strength hot-rolled steel sheet through a hot-dip galvanizing bath to form a hot-dip galvanizing layer on a surface of the high-strength hot-rolled steel sheet and optionally subjecting the high-strength hot-rolled steel sheet to alloying treatment for alloying the hot-dip galvanizing layer.   
     
     
         13 . A high-strength hot-rolled steel sheet having a composition containing, on a mass percent basis,
 C: 0.05% to 0.15%, Si: 1.5% or less,   Mn: 1.0% to 2.0%, P: 0.05% or less,   S: 0.005% or less, Al: 0.1% or less,   N: 0.01% or less, Ti: 0.05% to 0.2%, and   the balance being Fe and incidental impurities,   wherein the high-strength hot-rolled steel sheet has a microstructure comprising a bainite of more than 92% by volume, the bainite having an average lath interval of 0.60 μM or less, and Fe-based carbide precipitated in bainite lath grains having a ratio of 10% or more by number to all Fe-based carbide.   
     
     
         14 . The high-strength hot-rolled steel sheet according to  claim 13 , wherein the composition further contains at least one selected from Nb: 0.005% to 0.2% and B: 0.0002% to 0.0030% on a mass percent basis. 
     
     
         15 . The high-strength hot-rolled steel sheet according to  claim 13 , wherein the composition further contains at least one selected from Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, and Sn: 0.005% to 0.3% on a mass percent basis. 
     
     
         16 . The high-strength hot-rolled steel sheet according to  claim 13 , wherein the composition further contains at least one selected from Mo: 0.002% to 0.3% and Cr: 0.002% to 0.3% on a mass percent basis. 
     
     
         17 . The high-strength hot-rolled steel sheet according to  claim 13 , wherein the composition further contains at least one selected from Ca: 0.0002% to 0.004% and REM: 0.0002% to 0.004% on a mass percent basis. 
     
     
         18 . A hot-dip galvanized steel sheet, having a hot-dip galvanizing layer or alloyed hot-dip galvanizing layer on a surface of the high-strength hot-rolled steel sheet according to  claim 13 . 
     
     
         19 . A method for producing a high-strength hot-rolled steel sheet, comprising:
 heating a steel slab and subjecting the steel slab to hot rolling having rough rolling and finish rolling to form a hot-rolled steel sheet,   wherein the steel slab contains, on a mass percent basis,   C: 0.05% to 0.15%, Si: 1.5% or less,   Mn: 1.0% to 2.0%, P: 0.05% or less,   S: 0.005% or less, Al: 0.1% or less,   N: 0.01% or less, Ti: 0.05% to 0.2%, and   the balance being Fe and incidental impurities,   wherein the hot rolling includes heating the steel slab to 1100° C. or more, a total rolling reduction of two final passes in the finish rolling being 30% or more, a finish-rolling finishing temperature being in a range of (Ar3 transformation temperature) to (Ar3 transformation temperature+120° C.), starting cooling within 2 seconds after the finish rolling, cooling the hot-rolled steel sheet to a coiling temperature at an average cooling rate of 50° C./s or more, and coiling the hot-rolled steel sheet at a coiling temperature: 300° C. to 500° C.   
     
     
         20 . The method for producing a high-strength hot-rolled steel sheet according to  claim 19 , wherein the composition further contains at least one from Nb: 0.005% to 0.2% and B: 0.0002% to 0.0030% on a mass percent basis. 
     
     
         21 . The method for producing a high-strength hot-rolled steel sheet according to  claim 19 , wherein the composition further contains at least one selected from Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, and Sn: 0.005% to 0.3% on a mass percent basis. 
     
     
         22 . The method for producing a high-strength hot-rolled steel sheet according to  claim 19 , wherein the composition further contains at least one selected from Mo: 0.002% to 0.3% and Cr: 0.002% to 0.3% on a mass percent basis. 
     
     
         23 . The method for producing a high-strength hot-rolled steel sheet according to  claim 19 , wherein the composition further contains at least one selected from Ca: 0.0002% to 0.004% and REM: 0.0002% to 0.004% on a mass percent basis. 
     
     
         24 . A method for producing a hot-dip galvanized steel sheet, comprising:
 pickling a high-strength hot-rolled steel sheet produced by the method for producing a high-strength hot-rolled steel sheet according to  claim 19 , and subjecting the high-strength hot-rolled steel sheet to annealing and plating treatment to form a coated steel sheet having a coated layer on a surface thereof,   wherein the annealing is performed at a soaking temperature of 730° C. or less, and the plating treatment after the annealing includes passing the high-strength hot-rolled steel sheet through a hot-dip galvanizing bath to form a hot-dip galvanizing layer on a surface of the hot-rolled steel sheet and optionally subjecting the high-strength hot-rolled steel sheet to alloying treatment for alloying the hot-dip galvanizing layer.