High yield ratio hot rolled steel sheet which has excellent low temperature impact energy absorption and haz softening resistance and method of production of same

Abstract

Hot rolled steel sheet which has a maximum tensile strength of 600 MPa or more and has an excellent low temperature impact energy absorption and HAZ softening resistance and a method of production of the same are provided, that is, sheet which contains, by mass %, C: 0.04 to 0.09%, Si: 0.4% or less, Mn: 1.2 to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02 to 0.09%, Ti: 0.02 to 0.07%, and N: 0.005% or less, where 2.0≦Mn+8[% Ti]+12[% Nb]≦2.6, has a balance of Fe and unavoidable impurities, has an area percentage of pearlite of 5% or less, has a total area percentage of martensite and retained austenite of 0.5% or less, has a balance of a metal structure of ferrite and/or bainite, has an average grain size of ferrite and bainite of 10 μm or less, has an average particle size of alloy carbonitrides with incoherent interfaces which contain Ti and Nb of 20 nm or less, and has a yield ratio of 0.85 or more.

1 . High yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising, by mass %,
 C: 0.04 to 0.09%,   Si: 0.4% or less,   Mn: 1.2 to 2.0%,   P: 0.1% or less,   S: 0.02% or less,   Al: 1.0% or less,   Nb: 0.02 to 0.09%,   Ti: 0.02 to 0.07%, and   N: 0.005% or less,   a balance of Fe and unavoidable impurities,   where 2.0≦Mn+8[% Ti]+12[% Nb]≦2.6, and   having a metal structure which comprises an area percentage of pearlite of 5% or less, a total area percentage of martensite and retained austenite of 0.5% or less, and a balance of one or both of ferrite and bainite,   having an average grain size of ferrite and bainite of 10 μm or less,   having an average grain size of alloy carbonitrides with incoherent interfaces which contain Ti and Nb of 20 nm or less,   having a yield ratio of 0.85 or more, and   having a maximum tensile strength of 600 MPa or more.   
     
     
         2 . The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to  claim 1 , characterized by further comprising, by mass %, V: 0.01 to 0.12%. 
     
     
         3 . The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to  claim 1  or  2 , characterized by further comprising, by mass%, one or more of Cr, Cu, Ni, and Mo in a total of 0.02 to 2.0%. 
     
     
         4 . The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to any one of  claims 1  to  3 , characterized by further comprising, by mass%, B: 0.0003 to 0.005%. 
     
     
         5 . The high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to any one of  claims 1  to  4 , characterized by further comprising, by mass%, one or more of Ca, Mg, La, and Ce in a total of 0.0003 to 0.01%. 
     
     
         6 . High yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized that the high yield ratio hot rolled steel sheet according to any one of  claims 1  to  5  is plated or alloy plated on a surface. 
     
     
         7 . A method of production of high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising,
 heating a steel slab having a composition according to any one of  claims 1  to  5  to 1150° C. or more,   rough rolling the heated steel slab, finishing the rough rolling at temperature between 1000° C. and 1080° C., wherein a maximum rolling interval in the rough rolling which is performed at 1150° C. or less is 45 sec or less,   after the rough rolling, holding the steel slab for a holding time t 1  (sec) which satisfies the following formula (1), then starting finish rolling,   performing finish rolling with a final rolling temperature Tf which satisfies the following formula (2) so as to obtain a steel sheet,   starting water cooling of the steel sheet within 3 seconds after the finish rolling, then cooling the steel sheet to temperature of 700° C. or less at a lowest cooling rate of 8° C./sec or more, and   coiling the steel sheet at temperature between 530° C. and 650° C.
   1000×([% Ti]+[% Nb])>t1   formula (1)
 
     Tf> 830+400([% Ti]+[% Nb])   formula (2)
 
   
     
     
         8 . The method of production of high yield ratio hot rolled steel sheet according to  claim 7  characterized in that a final rolling temperature Tf satisfies the following formula (3).
     Tf> 830+800([% Ti]+[% Nb])   formula (3)
 
 
     
     
         9 . A method of production of high yield ratio hot rolled plated steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance characterized by comprising,
 pickling the hot rolled steel sheet which was obtained by the method of production according to  claim 7  or  8 , heating the steel sheet at an Ac3 temperature or less, then dipping the steel sheet in a plating bath to plate the surface of the steel sheet.   
     
     
         10 . The method of production of high yield ratio hot rolled plated steel sheet which has an excellent low temperature impact energy absorption and HAZ softening resistance according to  claim 9  characterized by further comprising alloying the plated steel sheet after said plating.