High strength hot-rolled steel sheet and method of producing the same

Abstract

A high-strength hot-rolled steel sheet has a tensile strength TS of 980 MPa or more and is manufactured by controlling the steel sheet to have a chemical composition containing C: more than 0.1% and 0.2% or less, Si: 0.5% or more and 3.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.05% or less, S: 0.004% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.15% or less, V: more than 0.10% and 0.30% or less, and the balance being Fe and inevitable impurities. Surface regions have a microstructure including mainly a ferrite phase, and an inner region has a microstructure including mainly a bainite phase. The proportions of the surface regions in the thickness direction of the steel sheet are 1.0% or more and 5.0% or less of the whole thickness respectively from the upper and lower surfaces of the steel sheet.

1 .- 4 . (canceled) 
     
     
         5 . A high-strength hot-rolled steel sheet, comprising:
 a chemical composition containing, by mass %,   C: more than 0.1% and 0.2% or less, Si: 0.5% or more and 3.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.05% or less, S: 0.004% or less, Al: 0.10% or less,   N: 0.008% or less, Ti: 0.05% or more and 0.15% or less,   V: more than 0.10% and 0.30% or less, and the balance being Fe and inevitable impurities,   surface regions including mainly a ferrite phase, and   an inner region including mainly a bainite phase,   the surface regions having a microstructure including mainly a ferrite phase in an amount of 80% or more in terms of area fraction and the balance being at least one selected from a bainite phase, a martensite phase, and a retained austenite phase in an amount of 0% or more and 20% or less in terms of area fraction,   the inner region having a microstructure including mainly a bainite phase in an amount of more than 90% in terms of area fraction and the balance being at least one selected from a ferrite phase, a martensite phase, and a retained austenite phase in an amount of 0% or more and less than 10% in terms of area fraction, and   proportions of the surface regions in a thickness direction of the steel sheet being 1.0% or more and 5.0% or less of a whole thickness, respectively, from upper and lower surfaces of the steel sheet.   
     
     
         6 . The high-strength hot-rolled steel sheet according to  claim 5 , wherein the chemical composition further contains, by mass %, at least one, selected from the group consisting of Nb: 0.003% or more and 0.2% or less, B: 0.0002% or more and 0.0015% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.2% or less, and Mo: 0.005% or more and 0.2% or less. 
     
     
         7 . The high-strength hot-rolled steel sheet according to  claim 5 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less. 
     
     
         8 . The high-strength hot-rolled steel sheet according to  claim 6 , wherein the chemical composition further contains, by mass %, at least one selected from the group consisting of Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less. 
     
     
         9 . A method of manufacturing a high-strength hot-rolled steel sheet comprising:
 heating a steel slab having the chemical composition according to  claim 5  in a temperature range of 1250° C. or higher,   holding the heated steel slab in the temperature range for 3600 seconds or more,   performing hot rolling including rough rolling and finish rolling under conditions that a finish delivery temperature is 840° C. or higher and 940° C. or lower,   starting cooling immediately after hot rolling has been performed,   performing cooling at an average cooling rate of 25° C./s or more, and   performing coiling at a coiling temperature of 350° C. or higher and 500° C. or lower.   
     
     
         10 . A method of manufacturing a high-strength hot-rolled steel sheet comprising:
 heating a steel slab having the chemical composition according to  claim 6  in a temperature range of 1250° C. or higher,   holding the heated steel slab in the temperature range for 3600 seconds or more,   performing hot rolling including rough rolling and finish rolling under conditions that a finish delivery temperature is 840° C. or higher and 940° C. or lower,   starting cooling immediately after hot rolling has been performed,   performing cooling at an average cooling rate of 25° C./s or more, and   performing coiling at a coiling temperature of 350° C. or higher and 500° C. or lower.   
     
     
         11 . A method of manufacturing a high-strength hot-rolled steel sheet comprising:
 heating a steel slab having the chemical composition according to  claim 7  in a temperature range of 1250° C. or higher,   holding the heated steel slab in the temperature range for 3600 seconds or more,   performing hot rolling including rough rolling and finish rolling under conditions that a finish delivery temperature is 840° C. or higher and 940° C. or lower,   starting cooling immediately after hot rolling has been performed,   performing cooling at an average cooling rate of 25° C./s or more, and   performing coiling at a coiling temperature of 350° C. or higher and 500° C. or lower.   
     
     
         12 . The method according to  claim 9 , wherein the chemical composition further contains: 1) at least one, selected from the group consisting of Nb: 0.003% or more and 0.2% or less, B: 0.0002% or more and 0.0015% or less, Cu: 0.005% or more and 0.2% or less, Ni: 0.005% or more and 0.2% or less, Cr: 0.005% or more and 0.2% or less, and Mo: 0.005% or more and 0.2% or less, and 2) at least one selected from the group consisting of Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less.