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

Abstract

A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more, has a composition containing, on a percent by mass basis, C: 0.05% or more and 0.18% or less, Si: 1.0% or less, Mn: 1.0% or more and 3.5% or less, P: 0.04% or less, S: 0.006% or less, Al: 0.10% or less, N: 0.008% or less, Ti: 0.05% or more and 0.20% or less, V: more than 0.1% and 0.3% or less, and the balance being Fe and incidental impurities, and has a microstructure including a primary phase and a secondary phase, the primary phase being a bainite phase having an area fraction of more than 85%, the secondary phase being at least one of ferrite phase, martensite phase, and retained austenite phase, the secondary phase having an area fraction of 0% or more and less than 15% in total, the bainite phase having an average lath interval of laths of 400 nm or less, and the laths having an average long axis length of 5.0 μm or less.

1 . A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more, comprising a composition and a microstructure,
 the composition containing, on a percent by mass basis,   C: 0.05% or more and 0.18% or less, Si: 1.0% or less,   Mn: 1.0% or more and 3.5% or less, P: 0.04% or less,   S: 0.006% or less, Al: 0.10% or less,   N: 0.008% or less, Ti: 0.05% or more and 0.20% or less,   V: more than 0.1% and 0.3% or less, and   the balance being Fe and incidental impurities, and   the microstructure comprising a primary phase and a secondary phase,   the primary phase being a bainite phase having an area fraction of more than 85%,   the secondary phase being at least one of ferrite phase, martensite phase, and retained austenite phase, the secondary phase having an area fraction of 0% or more and less than 15% in total,   the bainite phase having an average lath interval of laths of 400 nm or less, and the laths having an average long axis length of 5.0 μm or less.   
     
     
         2 . The high strength hot rolled steel sheet according to  claim 1 , wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% or more and 0.4% or less, B: 0.0002% or more and 0.0020% 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 0.4% or less, and Mo: 0.005% or more and 0.4% or less. 
     
     
         3 . The high strength hot rolled steel sheet according to  claim 1 , wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0002% or more and 0.01% or less and REM: 0.0002% or more and 0.01% or less. 
     
     
         4 . A method for manufacturing a high strength hot rolled steel sheet, comprising:
 heating a steel material having the composition according  claim 1  to 1,200° C. or higher,   applying hot rolling having rough rolling and finish rolling, the finish rolling having an accumulated rolling reduction of 50% or more in a temperature range of 1,000° C. or lower and a finishing temperature of 820° C. or higher and 930° C. or lower,   starting cooling within 4.0 s after the hot rolling,   performing cooling at an average cooling rate of 20° C./s or more, and   performing coiling at a coiling temperature of 300° C. or higher and 450° C. or lower.   
     
     
         5 . A high strength hot rolled steel sheet having a tensile strength TS of 980 MPa or more, comprising a composition and a microstructure,
 the composition containing, on a percent by mass basis,   C: more than 0.1% and 0.2% or less, Si: 1.0% or less,   Mn: 1.5% to 2.5%, P: 0.05% or less,   S: 0.005% or less, Al: 0.10% or less,   N: 0.007% or less, Ti: 0.07% to 0.2%,   V: more than 0.1% and 0.3% or less, and   the balance being Fe and incidental impurities,
 the microstructure comprising a primary phase and the remainder other than the primary phase, 
   the primary phase being a bainite phase having an area fraction of 90% or more,   the remainder being at least one selected from martensite phase, austenite phase and ferrite phase and having an area fraction of 10% or less, and   cementite dispersed in the microstructure having a mass percent of 0.8% or less and an average grain size of 150 nm or less.   
     
     
         6 . The high strength hot rolled steel sheet according to  claim 5 , wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3%. 
     
     
         7 . The high strength hot rolled steel sheet according to  claim 5 , wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01%. 
     
     
         8 . A method for manufacturing a high strength hot rolled steel sheet comprising: heating a steel material, applying hot rolling having rough rolling and finish rolling, applying cooling having two stages of first stage cooling and second stage cooling, and performing coiling to produce a hot rolled steel sheet,
 wherein   the steel material is specified to be a steel material having a composition containing, on a percent by mass basis,   C: more than 0.1% and 0.2% or less, Si: 1.0% or less,   Mn: 1.5% to 2.5%, P: 0.05% or less,   S: 0.005% or less, Al: 0.10% or less,   N: 0.007% or less, Ti: 0.07% to 0.2%,   V: more than 0.1% and 0.3% or less, and   the balance being Fe and incidental impurities,   the heating is a treatment to heat the steel material to 1,200° C. or higher,   the finish rolling is rolling with finishing temperature: 850° C. to 950° C.,   the first stage cooling is cooling in which cooling is started within 1.5 s of completion of the finish rolling and cooling to a first stage cooling stop temperature of 500° C. to 600° C. is performed at an average cooling rate of 20° C./s to 80° C./s,   the second stage cooling is cooling in which cooling to a second stage cooling stop temperature of 330° C. to 470° C. is performed at an average cooling rate of 90° C./s or more within 3 s of completion of the first stage cooling, and   after completion of the second stage cooling, coiling is performed, where the coiling temperature is the second stage cooling stop temperature.   
     
     
         9 . The method for manufacturing a high strength hot rolled steel sheet, according to  claim 8 , wherein the composition further contains, on a percent by mass basis, at least one selected from Nb: 0.005% to 0.1%, B: 0.0002% to 0.002%, Cu: 0.005% to 0.3%, Ni: 0.005% to 0.3%, Cr: 0.005% to 0.3%, and Mo: 0.005% to 0.3%. 
     
     
         10 . The method for manufacturing a high strength hot rolled steel sheet, according to  claim 8 , wherein the composition further contains, on a percent by mass basis, at least one selected from Ca: 0.0003% to 0.01% and REM: 0.0003% to 0.01%.