Bake-hardening hot-rolled steel sheet with excellent workability and method for manufacturing the same

Abstract

According to an exemplary embodiment of the present invention, a bake-hardening hot-rolled steel sheet with excellent workability can be provided. The exemplary steel sheet can contain, in terms of mass %, C of about 0.01 to 0.2%, Si of about 0.01 to 2%, Mn of about 0.1 to 2%, P of about 0.1% or less, S of about 0.03% or less, Al of about 0.001 to 0.1%, N of about 0.01% or less, Nb of about 0.005 to 0.05%, and as the remainder, Fe and unavoidable impurities. A microstructure of the steel sheet can be a polygonal ferrite and/or a continuous-cooled microstructure having an average particle diameter of about 2 μm to 8 μm, and the grain boundary abundance ratio of solute C and/or solute N may be about 0.28 or lower. An exemplary embodiment of the present invention can also be directed to a method for manufacturing a hot-rolled steel sheet. In this exemplary method, a slab containing aforementioned components can be heated to at least a temperature satisfying the equation of SRT (° C.)=6670/{2.26−log(% Nb)(% C)}−273. The heated slab can be subjected to a rough rolling, a finish rolling under a condition where the final temperature can be in a range of the Ar 3 transformation point temperature or higher and (Ar 3 transformation point temperature+100° C.) or lower to obtain a rolled steel. The rolled steel can be cooled at a cooling rate of about 80° C./sec or higher from cooling initiation down to a temperature range of about 500° C. or lower to obtain a hot rolled steel sheet; and such hot rolled steel sheet can be coiled.

1 - 10 . (canceled) 
   
   
       11 . A bake-hardening hot-rolled steel sheet having a particular workability, comprising:
 at least one portion which comprises, in terms of mass %:
 C of about 0.01 to 0.2%, 
 Si of about 0.01 to 2%, 
 Mn of about 0.1 to 2%, 
 P of about 0.1% or less, 
 S of about 0.03% or less, 
 Al of about 0.001 to 0.1%, 
 N of about 0.01% or less, 
 Nb of about 0.005 to 0.05%, and 
 a remainder thereof with Fe and unavoidable impurities, 
   wherein the at least one portion has at least one of a polygonal ferrite microstructure or a continuous-cooled microstructure having an average particle diameter of about 2 μm to 8 μm, and   wherein the at least one portion has a grain boundary abundance ratio of at least one of solute C or solute N that is about 0.28 or lower.   
   
   
       12 . The bake-hardening hot-rolled steel sheet according to  claim 11 , wherein the at least one portion comprises, in terms of mass %, at least one of:
 Ti of about 0.001 to 0.02%,   B of about 0.0002 to 0.002%,   Cu of about 0.2 to 1.2%,   Ni of about 0.1 to 0.6%,   Mo of about 0.05 to 1%,   V of about 0.02 to 0.2%, or   Cr of about 0.01 to 1%.   
   
   
       13 . The bake-hardening hot-rolled steel sheet according to  claim 11 , wherein the at least one portion comprises, in terms of mass %, at least one of:
 Ca of about 0.0005 to 0.005%, and   REM of about 0.0005 to 0.02%.   
   
   
       14 . The bake-hardening hot-rolled steel sheet according to  claim 11 , wherein the at least one portion is galvanized. 
   
   
       15 . A method for manufacturing a bake-hardening hot-rolled steel sheet with a particular workability, comprising:
 heating at least one portion of a slab containing, in terms of mass %,
 C of about 0.01 to 0.2%, 
 Si of about 0.01 to 2%, 
 Mn of about 0.1 to 2%, 
 P of about 0.1% or less, 
 S of about 0.03% or less, 
 Al of about 0.001 to 0.1%, 
 N of about 0.01% or less, 
 Nb of about 0.005 to 0.05%, and 
 a remainder thereof with Fe and unavoidable impurities, wherein the at least one portion is heated at least to a temperature satisfying the following equation:
     SRT (° C.)=6670/{2.26−log(% Nb)(% C)}−273; 
 
   hot rolling the at least one heated portion; wherein during the hot rolling, the heated slab is rough rolled so as to obtain a rough rolled bar;   finish rolling the rough rolled bar under a condition where a final temperature is in a range between approximately Ar 3  transformation point temperature or higher and approximately Ar 3  transformation point temperature +100° C. or lower so as to obtain a rolled steel;   cooling the rolled steel at a cooling rate of about 80° C./sec or higher from a cooling initiation temperature in the finish rolling to a temperature range of about 500° C. or lower, so as to obtain a hot rolled steel sheet; and   coiling the hot rolled steel sheet.   
   
   
       16 . The method according to  claim 15 , wherein the initiation temperature of the finish rolling is set to about 1000° C. or higher. 
   
   
       17 . The method according to  claim 15 , further comprising heating at least one of the rough rolled bar or the rolled steel at least one of until an initiation of the finish rolling or during the finish rolling. 
   
   
       18 . The method according to  claim 15 , further comprising descaling the at least one portion from a completion of the rough rolling until an initiation of the finish rolling. 
   
   
       19 . The method according to  claim 15 , further comprising immersing the hot-rolled steel sheet obtained by the hot rolling in a galvanizing bath so as to galvanize a surface of the hot-rolled steel sheet. 
   
   
       20 . The method according to  claim 19 , further comprising performing an alloying treatment after a galvanization of the at least one portion.