Steel wire for machine structural parts and method for manufacturing the same

Abstract

A steel wire for machine structural parts, including respective predetermined contents of C, Si, Mn, P, S, Al, Cr, N, and iron, wherein when a total content of Cr and Mn (% by mass) in cementite in the metallurgical microstructure is expressed as {Cr+Mn}, a total content of Cr and Mn (% by mass) in steel is expressed as [Cr+Mn], and a C content (% by mass) of the steel is expressed as [C], a concentration ratio {Cr+Mn}/[Cr+Mn] is (0.5[C] +0.040) or more, and an average circular-equivalent diameter of all the cementite is (1.668-2.13[C]) μm or more and (1.863-2.13[C]) μm or less.

1 . A steel wire for machine structural parts, which satisfies a chemical composition comprising:
 C: 0.05% by mass to 0.60% by mass;   Si: 0.005% by mass to 0.50% by mass;   Mn: 0.30% by mass to 1.20% by mass;   P: more than 0% by mass and 0.050% by mass or less;   S: more than 0% by mass and 0.050% by mass or less;   Al: 0.001% by mass to 0.10% by mass;   Cr: more than 0% by mass and 1.5% by mass or less; and   N: more than 0% by mass and 0.02% by mass or less, with the balance being iron and inevitable impurities, wherein, when a total content of Cr and Mn (% by mass) in cementite in the metallurgical microstructure is expressed as {Cr+Mn}, a total content of Cr and Mn (% by mass) in steel is expressed as [Cr+Mn], and a C content (% by mass) of the steel is expressed as [C], a concentration ratio {Cr+Mn}/[Cr+Mn] is (0.5[C]+0.040) or more, and wherein, when a C content (% by mass) of a steel is expressed as [C], an average circular-equivalent diameter of all the cementite is (1.668-2.13[C]) μm or more and (1.863-2.13[C]) μm or less.   
     
     
         2 . The steel wire for machine structural parts according to  claim 1 , which satisfies one or more of the following (a) to (c):
 (a) further comprising one or more selected from the group consisting of:
 Cu: more than 0% by mass and 0.25% by mass or less, 
 Ni: more than 0% by mass and 0.25% by mass or less, 
 Mo: more than 0% by mass and 0.50% by mass or less, and 
 B: more than 0% by mass and 0.01% by mass or less, 
   (b) further comprising one or more selected from the group consisting of:
 Ti: more than 0% by mass and 0.2% by mass or less, 
 Nb: more than 0% by mass and 0.2% by mass or less, and 
 V: more than 0% by mass and 0.5% by mass or less, and 
   (c) further comprising one or more selected from the group consisting of:
 Mg: more than 0% by mass and 0.02% by mass or less, 
 Ca: more than 0% by mass and 0.05% by mass or less, 
 Li: more than 0% by mass and 0.02% by mass or less, and 
 REM: more than 0% by mass and 0.05% by mass or less. 
   
     
     
         3 . The steel wire for machine structural parts according to  claim 1 , wherein an average ferrite grain size is 30 μm or less. 
     
     
         4 . A method for manufacturing the steel wire for machine structural parts according to  claim 1 , the method comprising:
 subjecting a bar steel satisfying the chemical composition to spheroidizing annealing, the spheroidizing annealing including the following processes (1) to (3):   (1) heating the steel to a temperature T1 of (A1+8° C.) to (A1+31° C.), and then heating and holding the steel at the temperature T1 for more than 1 hour and 6 hours or less;   (2) performing a cooling-heating process two to six times in total, wherein the cooling-heating process includes cooling the steel to a temperature T2 of higher than 650° C. and (A1−17° C) or lower, and then heating the steel to a temperature T3 of (A1+8° C.) to (A1+31° C.) at an average temperature increase rate of 75° C./hour to 160° C./hour; and   (3) cooling the steel from the temperature T3 of the final cooling-heating process, where A1 is calculated by the following equation (1):
   A1(° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni]  (1)
 
   
       where an expression [element] represents the content of each element (% by mass), and the content of an element not contained is zero. 
     
     
         5 . The method for manufacturing the steel wire for machine structural parts according to  claim 4 , wherein the bar steel is a steel wire obtained by subjecting a wire rod to wire drawing at an area reduction ratio of more than 5%. 
     
     
         6 . The steel wire for machine structural parts according to  claim 2 , wherein an average ferrite grain size is 30 μm or less. 
     
     
         7 . A method for manufacturing the steel wire for machine structural parts according to  claim 2 , the method comprising:
 subjecting a bar steel satisfying the chemical composition to spheroidizing annealing, the spheroidizing annealing including the following processes (1) to (3):   (1) heating the steel to a temperature T1 of (A1+8° C.) to (A1+31° C.), and then heating and holding the steel at the temperature T1 for more than 1 hour and 6 hours or less;   (2) performing a cooling-heating process two to six times in total, wherein the cooling-heating process includes cooling the steel to a temperature T2 of higher than 650° C. and (A1−17° C.) or lower, and then heating the steel to a temperature T3 of (A1+8° C.) to (A1+31° C.) at an average temperature increase rate of 75° C./hour to 160° C./hour; and   (3) cooling the steel from the temperature T3 of the final cooling-heating process, where A1 is calculated by the following equation (1):
   A1(° C.)=723+29.1×[Si]−10.7×[Mn]+16.9×[Cr]−16.9×[Ni]  (1)
 
   
       where an expression [element] represents the content of each element (% by mass), and the content of an element not contained is zero. 
     
     
         8 . The method for manufacturing the steel wire for machine structural parts according to  claim 7 , wherein the bar steel is a steel wire obtained by subjecting a wire rod to wire drawing at an area reduction ratio of more than 5%. 
     
     
         9 . The steel wire for machine structural parts according to  claim 1 , having a C content of 0.10% by mass to 0.55% by mass. 
     
     
         10 . The steel wire for machine structural parts according to  claim 9 , having a C content of 0.15% by mass to 0.50% by mass. 
     
     
         11 . The steel wire for machine structural parts according to  claim 1 , having a Si content of 0.010% by mass to 0.40% by mass. 
     
     
         12 . The steel wire for machine structural parts according to  claim 11 , having a Si content of 0.050% by mass to 0.35% by mass. 
     
     
         13 . The steel wire for machine structural parts according to  claim 1 , having a Mn content of 0.35% by mass to 1.10% by mass. 
     
     
         14 . The steel wire for machine structural parts according to  claim 13 , having a Mn content of 0.40% by mass to 1.00% by mass. 
     
     
         15 . The steel wire for machine structural parts according to  claim 1 , having an Al content 0.010% by mass to 0.08% by mass. 
     
     
         16 . The steel wire for machine structural parts according to  claim 1 , having a Cr content of 0.01% by mass to 1.40% by mass. 
     
     
         17 . The steel wire for machine structural parts according to  claim 16 , having a Cr content of 0.10% by mass to 1.25% by mass. 
     
     
         18 . The steel wire for machine structural parts according to  claim 2 , having a Mo content of 0.05% by mass to 0.40% by mass. 
     
     
         19 . The steel wire for machine structural parts according to  claim 3 , wherein an average ferrite grain size is 25 μm or less. 
     
     
         20 . The steel wire for machine structural parts according to  claim 19 , wherein an average ferrite grain size is 20 μm or less.