Process for producing steel bar wire rod for cold working
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a process for producing a steel bar wire rod for cold working. More particularly, the present invention is concerned with a process for producing a soft steel bar wire rod for cold working that can improve the softening level after spheroidization annealing to facilitate the subsequent cold working such as cutting, cold forging and machining in the production of various bolt parts, automobile parts, construction machine parts, bearing parts, etc. 
     2. Description of the Related Art 
     The majority of various bolt parts, automobile parts, construction machine parts, bearing parts, etc. has hitherto been produced by subjecting a steel bar wire rod to cold forming such as cutting, cold forging or machining. In cold forming, the rolled material is usually so hard that it is difficult to perform cold working. For this reason, spheroidization annealing is conducted prior to cold forming for the purpose of improving the cold workability. At the present time, however, the softening level is not satisfactory, so that, for example, in the case of cold forging, a further softening of the steel material is desired in the art for the purpose of further improving the tool life. 
     On the other hand, Japanese Examined Patent Publication (Kokoku) No. 41-19283 discloses a method of preliminarily treating a steel for spheroidization annealing characterized in that a steel bar wire rod is subjected to working of 30% or more at a temperature of from 200° C. to the recrystallization temperature (this temperature is 400° C. in the Example). Although according to this method, the spheroidizing of carbide is accelerated by the spheroidization annealing, and the intervals of carbide particles become so small that it is difficult to attain satisfactory softening. For this reason, at the present time, this technique is not always used in the art. 
     Accordingly, an object of the present invention is to provide a process for producing a soft steel bar wire rod for cold working that can realize an excellent softening level through conventional spheroidization annealing. 
     SUMMARY OF THE INVENTION 
     The present inventors have made extensive and intensive studies with a view to realizing an excellent softening level through conventional spheroidization annealing and, as a result, have found the following facts. 
     In order to realize an excellent softening level through conventional spheroidization annealing, it is important to satisfy the following two requirements. 
     (1) Undissolved cementite particles (spheroidal carbide) produced from a plate cementite constituting a pearlite structure are allowed to remain at large intervals in a suitable amount, preferably in an amount of from about 2×10 5  to 6×10 5  particles/mm 2  at the holding for spheroidization annealing. 
     (2) Austenite particles are coarsened to 20 μm or less at the holding for spheroidization annealing. 
     The following means are useful for satisfying the above-described requirement (1). 
     1 Hot rolling is conducted at a temperature just above the Ar 3  point or just above Arcm with a total reduction of area of 30% or more to form a pearlite having large lamellar intervals at the time of completion of the transformation. 
     2 Thereafter, hot rolling is conducted at a temperature of from (Ac 1  -400) to Ac 1  ° C. with a total reduction of area of 10 to 70% to divide the plate cementite into sections and agglomerate the section. 
     3 Further, when the material after finish hot rolling is cooled to 300° C. at an average cooling rate of 1° C./sec or less, it becomes easy for undissolved cementite particles to remain in a suitable amount at large intervals at the holding for spheroidization annealing. 
     In order to satisfy the above-described requirement (2), it is necessary to conduct hot rolling at a temperature just above the Ar 3  point or just above the Arcm with a total reduction of area of 30% or more. 
     The present invention has been made based on the above-described novel finding, and the subject matter of the present invention resides in a process for producing a steel bar wire rod for cold working, comprising heating a steel comprising, in terms of % by weight (% is hereinafter by weight), 0.1 to 1.5% of C and 0.25 to 2.0% of Mn with the balance consisting of Fe and unavoidable impurities to 900° to 1250° C., hot-rolling the heated steel at a temperature of from Ar 3  to (Ar 3  +200)° C. or Arcm to (Arcm+200)° C. with a total reduction of area of 30% or more, cooling the hot-rolled material to complete a ferrite/pearlite transformation or a pro-eutectoid cementite/pearlite transformation and subjecting the transformed material to finish hot rolling at a temperature of from (Ac 1  -400) to Ac 1  ° C. with a total reduction of area of 10 to 70%. If necessary, this process may further comprise the step of subjecting the material after the finish hot rolling to 300° C. at an average cooling rate of 1° C./sec or less. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The material contemplated in the present invention is a steel composed mainly of 0.1 to 1.5% of C and 0.25 to 2.0% of Mn. The reason for the limitation of the contents of C and Mn will now be described. 
     In the steel bar wire rod contemplated in the present invention, a steel bar wire rod is produced according to the process of the present invention and subjected to cold working and then hardening and tempering to ensure a predetermined strength and toughness. In this case, when the C content is less than 0.1%, sufficient strength is not obtained while when it exceeds 1.5%, the toughness deteriorates. For this reason, the C content was limited to 0.1 to 1.5%. 
     Mn is important for ensuring the hardenability and dissolving Mn in the cementite to stabilize the cementite ((Fe, Mn) 3  C) in austenite for the purpose of allowing undissolved cementite particles to remain in a suitable amount at large intervals at the holding time for spheroidization annealing. When the content is less than 0.25%, the effect is small while when the content exceeds 2%, the effect is saturated. For this reason, the Mn content was limited to 0.25 to 2.0%. 
     The incorporation of C and Mn is indispensable to the present invention. Besides these elements, if necessary, Si and Al may be incorporated as an deoxidizing element in an amount of from 0.03 to 1.00% and an amount of from 0.015 to 0.05%, respectively, Cr, Mo and Ni may be incorporated in an amount of from 0.01 to 2.0%, an amount of from 0.01 to 1.0% and an amount of from 0.1 to 3.5%, respectively, for the purpose of increasing the hardenability, Nb, V, Ti and N may be incorporated in an amount of from 0.005 to 0.1%, an amount of from 0.03 to 0.3%, an amount of from 0.005 to 0.04% and an amount of from 0.003 to 0.020%, respectively, for the purpose of regulating the particle size, and S may be incorporated in an amount of from 0.01 to 0.15% for the purpose of improving the machinability. 
     In the present invention, the above-described steel is heated to 900° to 1250° C., hot rolling is conducted at a temperature of from Ar 3  to (Ar 3  +200)° C. or Arcm to (Arcm+200)° C. with a total reduction of area of 30% or more, and a ferrite/pearlite transformation or a proeutectoid cementite/pearlite transformation is then completed. The reason for this will now be described. The reason for this will now be described. The reason for the limitation of the heating temperature to 900° to 1250° C. is that when the heating temperature is below 900° C., the rolling temperature in an austenite region becomes so low that the refinement of the austenite grain by rolling in a recrystallization region is unsatisfactory while when the heating temperature exceeds 1250° C., the austenite crystal grain is significantly coarsened. Then, rolling is conducted at a temperature of from Ar 3  to (Ar 3  +200)° C. or Arcm to (Arcm+200)° C. with a total reduction ratio of 30% or more for the purpose of refining the austenite grain through recrystallization and, at the same time, forming a pearlite having large lamellar intervals and reducing the austenite grain diameter at the holding for spheroidization annealing. When the rolling temperature exceeds (Ar 3  +200)° C. or (Arcm+200)° C. and the total reduction of area is less than 30%, the intended effect is small while when the rolling temperature is below Ar 3  or Arcm, the refinement of the austenite grain through recrystallization is unsatisfactory. For this reason, the rolling should be conducted under conditions of a temperature in the range of from Ar 3  to (Ar 3  +200)° C. or Arcm to (Arcm+200)° C. and a total reduction of area of 30% or more. 
     In the present invention, the finish hot rolling is conducted at a temperature of from (Ac 1  -400) to Ac 1  ° C. with a total reduction of area of 10 to 70%. This is because the plate cementite is divided into sections and agglomerated for the purpose of allowing undissolved cementite particles to remain in a suitable amount at large intervals at the holding for spheroidization annealing. When the rolling temperature exceeds Ac 1  and the total reduction ratio is less than 10%, this effect is small while when the rolling temperature is below (Ac 1  -400)° C. and the total reduction of area exceeds 70%, the work hardening of the ferrite matrix becomes so large that the ferrite matrix cannot be sufficiently softened in the subsequent spheroidization annealing. For this reason, the rolling should be conducted under conditions of a temperature of from (Ac 1  -400) to Ac 1  ° C. and a total reduction of area of 10 to 70%. 
     The spheroidization annealing is conducted under conventional conditions, that is, by holding the material at a temperature of from 700° to 820° C. for 2 to 7 hr and then gradually cooling the heated material to a temperature of from 600° to 720° C. at a cooling rate of 0.1 to 1.0° C./min. 
     As described in claim 2, in the present invention, if necessary, the material after finish hot rolling may be cooled to 300° C. at an average cooling rate of 1° C./sec or less. This is because Mn is dissolved in the cementite to stabilize the cementite in austenite for the purpose of allowing undissolved cementite particles to remain in a suitable amount at large intervals at the holding for spheroidization annealing and this effect is significant when the average cooling rate is 1° C./sec or less. 
    
    
     The effect of the present invention will now be described in more detail with reference to the following Examples. 
     EXAMPLE 
     Chemical ingredients of materials under testing are listed in Table 1. 
     These materials were produced by a melt process in a converter and continuously cast. They were subjected to blooming to form blooms having a size of 162 mm square and rolled into a round bar steel material having a size of 20 to 50 mm under conditions specified in Table 2. The spheroidization annealing was carried out on these rolled materials under conditions specified in Table 3. The properties of the spheroidized materials are also given in Table 3. 
     The evaluation of the spheroidized materials was conducted on the basic of two properties, that is, the tensile strength and the degree of spheroidizing specified in JIS G3539, and the target of the quality of the annealed material was set to spheroidizing degree of No. 2 or less corresponding to the standard of the conventional cold forged steel. 
     As is apparent from Table 3, all the examples of the present invention exhibited a spheroidizing degree of No. 2 or less, and the tensile strength as well is on a lower level. Among the examples of the present invention, level Nos. 9, 10 and 13 are examples wherein the average cooling rate to 300° C. was 1° C./sec. These examples exhibited a further improvement in both the spheroidizing degree and the tensile strength. 
     On the other hand, level Nos. 2 and 3 are respectively a comparative example wherein the soaking temperature was lower than the temperature range specified in the present invention and a comparative example wherein the soaking temperature was higher than the temperature range specified in the present invention. Level No. 5 is a comparative example wherein the total reduction of area by rolling at a temperature of from Ar 3  to (Ar 3  +200)° C. was lower than the lower limit of the total reduction of area specified in the present invention. Level Nos. 6 and 11 are each a comparative example wherein the total reduction of area by rolling at a temperature in the range of from (Ac 1  400) to Ac 1  ° C. was lower than the lower limit of the total reduction of area specified in the present invention. Level Nos. 16, 17 and 18 were each a comparative example wherein the Mn content was lower than the lower limit of the Mn content specified in the present invention. In all of these comparative examples, the spheroidizing degree was No. 3 or more, and the softening degree as well was not satisfactory. Further, level No. 12 is a comparative example wherein the total reduction of area by rolling at a temperature of from (Ac 1  -400) to Ac 1  ° C. was higher than the upper limit of the total reduction of area specified in the present invention. In this case, although a good degree of spheroidization can be attained, the softening degree is not satisfactory compared with level No. 10, i.e., an example of the present invention having the same steel material as No. 12. 
     
                                           TABLE 1                                 
__________________________________________________________________________
Steel                                                                     
   Classif-                                                               
No.                                                                       
   ication                                                                
         C  Mn Si C r                                                     
                     AL Mo Ni Nb V  Ti N  S                               
__________________________________________________________________________
A  Present                                                                
         0.20                                                             
            1.10                                                          
               0.25                                                       
                  0.83                                                    
                     0.035                                                
                        -- -- -- -- -- 0.006                              
                                          0.018                           
   invention                                                              
B  Present                                                                
         0.47                                                             
            0.79                                                          
               0.29                                                       
                  -- 0.027                                                
                        -- -- -- -- -- 0.012                              
                                          0.035                           
   invention                                                              
C  Present                                                                
         0.98                                                             
            0.54                                                          
               0.27                                                       
                  1.08                                                    
                     0.034                                                
                        -- -- -- -- -- 0.009                              
                                          0.021                           
   invention                                                              
D  Present                                                                
         1.39                                                             
            0.85                                                          
               0.31                                                       
                  0.57                                                    
                     0.043                                                
                        -- -- -- -- -- 0.007                              
                                          0.019                           
   invention                                                              
E  Present                                                                
         0.39                                                             
            0.74                                                          
               0.26                                                       
                  1.72                                                    
                     0.038                                                
                        0.18                                              
                           -- -- -- -- 0.008                              
                                          0.027                           
   invention                                                              
F  Present                                                                
         0.33                                                             
            1.65                                                          
               0.21                                                       
                  -- 0.027                                                
                        0.13                                              
                           2.9                                            
                              -- -- -- 0.013                              
                                          0.016                           
   invention                                                              
G  Present                                                                
         0.55                                                             
            0.81                                                          
               0.27                                                       
                  0.91                                                    
                     0.032                                                
                        -- -- -- 0.17                                     
                                    0.010                                 
                                       0.010                              
                                          0.017                           
   invention                                                              
H  Present                                                                
         0.65                                                             
            0.83                                                          
               0.69                                                       
                  1.12                                                    
                     0.034                                                
                        0.05                                              
                           -- 0.04                                        
                                 -- -- 0.008                              
                                          0.025                           
   invention                                                              
I  Comp. Ex.                                                              
         0.53                                                             
            0.20                                                          
               0.27                                                       
                  -- 0.045                                                
                        -- -- -- -- -- 0.009                              
                                          0.023                           
J  &#34;     0.18                                                             
            0.22                                                          
               0.29                                                       
                  0.12                                                    
                     0.028                                                
                        0.01                                              
                           -- -- -- -- 0.012                              
                                          0.015                           
K  &#34;     1.27                                                             
            0.19                                                          
               0.32                                                       
                  0.09                                                    
                     0.034                                                
                        -- -- -- -- -- 0.008                              
                                          0.017                           
__________________________________________________________________________
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
                                Total reduction Total reduction           
                                of area in      of area                   
                                                        Average           
                                temp. range     temp. range               
                                                        rate of           
Rolling        Soaking                                                    
                    Ar.sub.3 to (AR.sub.3 + 200) or                       
                                described on                              
                                        (AC.sub.1 - 400)                  
                                                described                 
                                                        cooling           
material                                                                  
     Classif-                                                             
           Steel                                                          
               temp.                                                      
                    Arcm to (Arcm + 200)                                  
                                the left                                  
                                        to Ac.sub.1                       
                                                the left                  
                                                        to 300°    
                                                        C.                
No.  ication                                                              
           No. °C.                                                 
                    °C.  %       °C.                        
                                                %       °C./sec    
__________________________________________________________________________
 a*  Present                                                              
           A   1050  800 to 1000                                          
                                60      330 to 730                        
                                                60      1.5               
     invention                                                            
b    Comp. Ex.                                                            
           A    850 &#34;           &#34;       &#34;       40      &#34;                 
c    &#34;     A   1300 &#34;           &#34;       &#34;       &#34;       &#34;                 
 d*  Present                                                              
           B   1150 750 to 950  40      &#34;       &#34;       1.2               
     invention                                                            
e    Comp. Ex.                                                            
           B   1150 &#34;           20      &#34;       &#34;       &#34;                 
f    &#34;     B   1150 &#34;           40      &#34;        5      &#34;                 
 g*  Present                                                              
           C   1100 780 to 980  50      340 to 740                        
                                                20      &#34;                 
     invention                                                            
 h*  Present                                                              
           D   1100  800 to 1000                                          
                                60      320 to 720                        
                                                &#34;       &#34;                 
     invention                                                            
 i*  Present                                                              
           E   1050 760 to 960  40      360 to 760                        
                                                40      0.6               
     invention                                                            
 j*  Present                                                              
           F   1050 720 to 920  &#34;       290 to 690                        
                                                &#34;       &#34;                 
     invention                                                            
k    Comp. Ex.                                                            
           F   1050 &#34;           &#34;       &#34;        5      &#34;                 
l    &#34;     F   1050 &#34;           &#34;       &#34;       80      &#34;                 
 m*  Present                                                              
           F   1050 &#34;           &#34;       &#34;       60      0.4               
     invention                                                            
 n*  Present                                                              
           G   1050 740 to 940  &#34;       370 to 770                        
                                                40      1.2               
     invention                                                            
 o*  Present                                                              
           H   1100 &#34;           &#34;       &#34;       &#34;       &#34;                 
     invention                                                            
p    Comp. Ex.                                                            
           I   1050 760 to 960  &#34;       390 to 790                        
                                                &#34;       &#34;                 
q    &#34;     J   1100  820 to 1020                                          
                                &#34;       340 to 740                        
                                                &#34;       &#34;                 
r    &#34;     K   1050 790 to 990  &#34;       330 to 730                        
                                                &#34;       &#34;                 
__________________________________________________________________________
 Note) *Example of the present invention                                  
 
    
     
                                           TABLE 3                                 
__________________________________________________________________________
               Spheroidizing conditions Quality of annealing material     
          Rolling     Gradual cooling                                     
                               Gradual cooling                            
                                        Tensile                           
                                              Spheroidizing               
Level                                                                     
    Classif-                                                              
          material                                                        
               Heating temp.                                              
                      termination temp.                                   
                               rate     strength                          
                                              degree                      
No. ication                                                               
          No.  °C.                                                 
                      °C.                                          
                               °C./min                             
                                        kgf/mm.sup.2                      
                                              No.                         
__________________________________________________________________________
 1* Present                                                               
          a    770    650      0.25     45.2  2                           
    invention                                                             
2   Comp. Ex.                                                             
          b    &#34;      &#34;        &#34;        48.5  3                           
3   &#34;     c    &#34;      &#34;        &#34;        49.3  4                           
 4* Present                                                               
          d    750    670      &#34;        53.1  2                           
    invention                                                             
5   Comp. Ex.                                                             
          e    &#34;      &#34;        &#34;        58.2  3                           
6   &#34;     f    &#34;      &#34;        &#34;        60.1  3                           
 7* Present                                                               
          g    780    690      &#34;        66.4  2                           
    invention                                                             
 8* Present                                                               
          h    790    680      &#34;        83.7  2                           
    invention                                                             
 9* Present                                                               
          i    780    &#34;        &#34;        55.9  1                           
    invention                                                             
10* Present                                                               
          j    730    640      &#34;        63.2  1                           
    invention                                                             
11  Comp. Ex.                                                             
          k    &#34;      &#34;        &#34;        67.8  3                           
12  &#34;     l    &#34;      &#34;        &#34;        65.4  1                           
13* Present                                                               
          m    &#34;      &#34;        &#34;        62.8  1                           
    invention                                                             
14* Present                                                               
          n    750    670      &#34;        60.1  2                           
    invention                                                             
15* Present                                                               
          o    &#34;      &#34;        &#34;        67.9  2                           
    invention                                                             
16  Comp. Ex.                                                             
          p    760    &#34;        &#34;        58.4  4                           
17  &#34;     q    &#34;      &#34;        &#34;        49.6  4                           
18  &#34;     r    780    &#34;        &#34;        87.4  4                           
__________________________________________________________________________
 Note) *Example of the present invention