Warm rolling method

Abstract

A multi-directional warm-rolling method for manufacturing an ultrafine grain steel material with an ultrafine grain structure of 3 μm or smaller in average grain size. When roiling of two passes or more is performed for a steel material in the rolling temperature range of 350 to 800° C., a rolling by an oval shape caliber and a rolling by the other shape caliber an performed at leas one time so that a large amount of strain can be introduced into the material by a simple means with less section reduction ratio and less number of passes. Steel materials having the ultrafine grain structure and excellent strength and ductility can be manufactured by this method.

1 . A warm rolling method for manufacturing an ultrafine grain steel material having an ultrafine grain structure of average grain size of 3 μm or less, which comprises rolling by a caliber of oval shape and rolling by a caliber of other shape at least once or more each when rolling two passes or more to a steel material in a rolling temperature range of 350 to 800° C.  
   
   
       2 . The warm rolling method of  claim 1 , wherein rolling by a caliber of oval shape is followed by rolling by a caliber of other shape.  
   
   
       3 . The warm rolling method of  claim 1 , wherein the caliber of other shape is square or round shape.  
   
   
       4 . The warm rolling method of  claim 1 , wherein the rolling by oval shape caliber is executed by 2 times or more to maximum N/2 times or less in the case of N>2, wherein N is the total number of passes.  
   
   
       5 . The warm rolling method of  claim 1 , wherein continuous two passes rolling are executed.  
   
   
       6 . The warm rolling method of  claim 5 , wherein the section reduction ratio after square shape caliber rolling from material is 20% or more in two passes rolling by calibers of oval shape and square shape.  
   
   
       7 . The warm rolling method of  claim 1 , wherein the section reduction ratio is, in combination rolling of two passes rolling by calibers of oval shape and square shape, 40% or more in combination rolling by two times, and the section reduction ratio is 60% or more in combination rolling by three times.  
   
   
       8 . The warm rolling method of  claim 1 , further including a rolling step in which the maximum shorter axis length of material after rolling by oval shape caliber is 75% or less of the material diagonal length before oval rolling.  
   
   
       9 . The warm rolling method of  claim 1 , wherein a plastic strain of 1.5 or more is introduced at least in a region of 50 vol. % inside of the material.  
   
   
       10 . The warm rolling method of  claim 9 , wherein a plastic strain of 2 or more is introduced in a region of 90 vol. % or more inside of the material.  
   
   
       11 . The warm rolling method of  claim 1 , wherein the rolling condition parameter Z expressed in the following formula (1) is 11 or more in the case that crystal structure of Fe just before rolling is bcc such as the structure of ferrite, bainite, martensite, pearlite or other, or 20 or more in the case that structure just before rolling is austenite and Fe crystal structure is fcc;  
     
       
         
           
             
               
                 
                   Z 
                   = 
                   
                     log 
                     ⁡ 
                     
                       [ 
                       
                         
                           ɛ 
                           t 
                         
                         ⁢ 
                         
                           exp 
                           ⁡ 
                           
                             ( 
                             
                               Q 
                               
                                 8.31 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     T 
                                     + 
                                     273 
                                   
                                   ) 
                                 
                               
                             
                             ) 
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
       ε: strain  
       t: time from start of rolling till end (s)  
       T: rolling temperature (° C., average of rolling temperature of each pass in the case of multi-pass rolling)  
       Q: 254,000 if structure just before rolling is a primary phase of ferrite, bainite, martensite or pearlite; or 300,00 if mother phase is austenite.  
     
   
   
       12 . The warm rolling method of  claim 1 , wherein the section reduction ratio of initial material to final rolling is 90% or less.  
   
   
       13 . The warm rolling method of  claim 1 , wherein an ultrafine grain steel having an average grain size of C section or L section of 3 microns or less is manufactured.  
   
   
       14 . The warm rolling method of  claim 1 , wherein an ultrafine grain steel having an average grain size of C section or L section of 1 micron or less is manufactured.  
   
   
       15 . The warm rolling method of  claim 2 , wherein the caliber of other shape is square or round shape.  
   
   
       16 . The warm rolling method of  claim 2 , wherein the rolling by oval shape caliber is executed by 2 times or more to maximum N/2 times or less in the case of N>2, wherein N is the total number of passes.  
   
   
       17 . The warm rolling method of  claim 3 , wherein the rolling by oval shape caliber is executed by 2 times or more to maximum N/2 times or less in the case of N>2, wherein N is the total number of passes.  
   
   
       18 . The warm rolling method of  claim 15 , wherein the rolling by oval shape caliber is executed by 2 times or more to maximum N/2 times or less in the case of N>2, wherein N is the total number of passes.  
   
   
       19 . The warm rolling method of  claim 2 , wherein continuous two passes rolling are executed.  
   
   
       20 . The warm rolling method of  claim 3 , wherein continuous two passes rolling are executed.