Hot rolled steel sheet having an ultrafine grain structure and process for producing steel sheet

Abstract

A hot rolled steel sheet comprises ultrafine ferrite grains as a main phase and fine second phase particles. The ferrite grains have an average grain size of not less than 2 μm but less than 4 μm. The second phase has an average particle size of not more than 8 μm and in not less than 80% of the second phase, the spacing of the second phase particle with the closest second phase particle is not less than the second phase particle size. The steel sheet has an ultrafine grain structure, superior mechanical characteristics, reduced anisotropy in its mechanical characteristics and high formability. A process for producing the steel sheet is also disclosed.

What is claimed is:  
     
         1 . A hot rolled steel sheet having an ultrafine grain structure, comprising ferrite as a main phase and a second phase, said ferrite having an average grain size of not less than 2 μm but less than 4 μm, said second phase having an average particle size of not more than 8 μm and in not less than 80% of the second phase, the spacing of the second phase particle with the closest second phase particle being not less than the second phase particle size.  
     
     
         2 . The hot rolled steel sheet according to    claim 1   , wherein said second phase particles have an aspect ratio of not more than 2.0.  
     
     
         3 . The hot rolled steel sheet according to    claim 1   , wherein said second phase is at least one phase selected from the group consisting of pearlite, bainite, martensite and retained austenite.  
     
     
         4 . The hot rolled steel sheet according to    claim 1   , comprising: by weight percent, 
 C: more than 0.01 to 0.3%;    Si: not more than 2.0%;    Mn: not more than 3.0%;    P: not more than 0.5%; and    Ti: 0.03 to 0.3%; and    the balance Fe and incidental impurities.    
     
     
         5 . The hot rolled steel sheet according to    claim 1   , comprising: by weight percent, 
 C: more than 0.01 to 0.3%;    Si: not more than 2.0%;    Mn: not more than 3.0%;    P: not more than 0.5%;    Ti: 0.03 to 0.3%;    and at least one component selected from the group consisting of the components of at least one of the following Groups A to C;    Group A: Nb: not more than 0.3%, and V: not more than 0.3%;    Group B: Cu: not more than 1.0%, Mo: not more than 1.0%, Ni: not more than 1.0%, and Cr:    not more than 1.0%; and    Group C: Ca, REM and B in a total amount of not more than 0.005%; and    the balance Fe and incidental impurities.    
     
     
         6 . The hot rolled steel sheet according to    claim 1   , further comprising Al in an amount of not more than 0.2% by weight.  
     
     
         7 . A process for producing a hot rolled steel sheet having an ultrafine grain structure, comprising: 
 casting steel comprising, by weight percent, C: more than 0.01 to 0.3% and Ti: 0.03 to 0.3%;    cooling the cast steel to a temperature not higher than 1150° C., or re-heating the cast steel at a temperature not greater than 1150° C.;    hot-rolling the cast steel performing light reduction for at least three passes in a low-temperature region of a dynamic recrystallization temperature of austenite;    cooling the rolled steel starting within about 2 seconds after completion of the hot rolling and at a cooling rate not less than 30° C./sec; and    coiling the cooled steel sheet at a temperature of from 350 to 550° C.    
     
     
         8 . The process according to    claim 7   , wherein the light reduction in a low-temperature region of the dynamic recrystallization temperature comprises a final pass at a reduction of 13 to 30%, and at least two passes at a reduction of not more than 20%.  
     
     
         9 . The process according to    claim 7   , wherein the light reduction in a low-temperature region of the dynamic recrystallization temperature comprises 3 passes or 4 passes.  
     
     
         10 . The process according to    claim 7   , wherein the low-temperature region of the dynamic recrystallization temperature is between a lower-limit temperature of dynamic recrystallization determined prior to the hot rolling and said lower-limit temperature plus 80° C.  
     
     
         11 . The process according to    claim 7   , wherein the low-temperature region of the dynamic recrystallization temperature is between a lower-limit temperature of dynamic recrystallization determined prior to the hot rolling and said lower-limit temperature plus 60°C.  
     
     
         12 . The process according to    claim 7   , wherein the steel sheet comprises: by weight percent, 
 C: more than 0.01 to 0.3%;    Si: not more than 2.0%;    Mn: not more than 3.0%;    P: not more than 0.5%; and    Ti: 0.03 to 0.3%; and    the balance Fe and incidental impurities.    
     
     
         13 . The process according to    claim 7   , wherein the steel sheet comprises: by weight percent, 
 C: more than 0.01 to 0.3%;    Si: not more than 2.0%;    Mn: not more than 3.0%;    P: not more than 0.5%;    Ti: 0.03 to 0.3%; and    at least one component selected from the group consisting of the components of at least one of the following Groups A to C;    Group A: Nb: not more than 0.3% and V: not more than 0.3%;    Group B: Cu: not more than 1.0%, Mo: not more than 1.0%, Ni: not more than 1.0% and Cr:    not more than 1.0%; and    Group C: Ca, REM and B in a total amount of not more than 0.005%; and    the balance Fe and incidental impurities.    
     
     
         14 . The process according to    claim 7   , wherein the light reduction in a low-temperature region of the dynamic recrystallization temperature is performed while a steel material to be rolled is being heated between rolling stands in a rolling mill.  
     
     
         15 . The process according to    claim 7   , wherein the light reduction in a low-temperature region of the dynamic recrystallization temperature is performed while rolling rolls are being heated.  
     
     
         16 . The process according to    claim 7   , wherein lubrication rolling is conducted during the hot rolling.