Steel wire rod and manufacturing method of steel wire rod

Abstract

A steel wire rod according to an aspect of the present invention has a chemical composition in a predetermined range, in which a structure in a central part includes 80 area % to 100 area % of pearlite and a total of 0 area % or more and less than 20 area % of proeutectoid ferrite, proeutectoid cementite, martensite, and bainite; an average lamellar spacing of the pearlite in the central part is 50 nm to 100 nm; an average length of lamellar cementite in the central part is 1.9 μm or less; an average pearlite block size in the central part is 15.0 μm to 30.0 μm; a structure in a surface part includes 70 area % to 100 area % of the pearlite; and an average pearlite block size in the surface part is 0.40 times or more and 0.87 times or less the average pearlite block size in the central part.

1 . A steel wire rod comprising, as a chemical composition, by unit mass %:
 C: 0.60% to 1.10%;   Si: 0.005% to 0.350%;   Mn: 0.10% to 0.90%;   Cr: 0.010% to 0.300%;   N: 0.0100% or less;   P: 0.030% or less;   S: 0.030% or less;   Al: 0% to 0.070%;   Ti: 0% to 0.030%;   V: 0% to 0.100%;   Nb: 0% to 0.050%;   Mo: 0% to 0.20%;   B: 0% to 0.0030%; and   a remainder including Fe and impurities,   wherein, when a distance from a circumferential surface to a central axis of the steel wire rod is defined as r in units of mm, a structure in a central part which is a region from the central axis to r×0.5 includes 80 area % to 100 area % of pearlite and a total of 0 area % or more and less than 20 area % of proeutectoid ferrite, proeutectoid cementite, martensite, and bainite,   an average lamellar spacing of the pearlite in the central part is 50 nm to 100 nm,   an average length of lamellar cementite in the central part is 1.9 μm or less,   an average pearlite block size in the central part is 15.0 μm to 30.0 μm,   a structure in a surface part which is a region from the circumferential surface to r×0.1 includes 70 area % to 100 area % of the pearlite, and   an average pearlite block size in the surface part is 0.40 times or more and 0.87 times or less the average pearlite block size in the central part.   
     
     
         2 . The steel wire rod according to  claim 1  comprising, as the chemical composition, by unit mass %:
 Si: 0.100% to 0.350%, wherein 
 the average pearlite block size in the surface part is 17.0 μm or less, and a relationship between a tensile strength TS [MPa] and an electric resistivity ρ [μΩ·cm] of the steel wire rod satisfies Formula (1),
   ρ≤0.0155× TS+ 1.25  Formula (1),
 
 
 
     
     
         3 . The steel wire rod according to  claim 1  comprising, as the chemical composition, by unit mass %:
 Si: 0.005% or more and less than 0.100%, wherein 
 the average pearlite block size in the surface part is 0.80 times or less the average pearlite block size in the central part, and 
 a relationship between a tensile strength TS [MPa] and an electric resistivity ρ [μΩ·cm] of the steel wire rod satisfies Formula (2),
   ρ≤0.0155× TS− 0.95  Formula (2),
 
 
 
     
     
         4 . The steel wire rod according to  claim 1  comprising, as the chemical composition, by unit mass %:
 at least one or more selected from the group consisting of
 Al: 0.001% to 0.070%, 
 Ti: 0.002% to 0.030%, 
 V: more than 0% and 0.100% or less, 
 Nb: more than 0% and 0.050% or less, 
 Mo: more than 0% and 0.20% or less, and 
 B: 0.0003% to 0.0030%. 
 
 
     
     
         5 . A manufacturing method of a steel wire rod, comprising:
 casting a bloom including, as a chemical composition, by unit mass %,
 C: 0.60% to 1.10%, 
 Si: 0.005% to 0.350%, 
 Mn: 0.10% to 0.90%, 
 Cr: 0.010% to 0.300%, 
 N: 0.0100% or less, 
 P: 0.030% or less, 
 S: 0.030% or less, 
 Al: 0% to 0.070%, 
 Ti: 0% to 0.030%, 
 V: 0% to 0.100%, 
 Nb: 0% to 0.050%, 
 Mo: 0% to 0.20%, 
 B: 0% to 0.0030%, and 
 a remainder including Fe and impurities; 
   heating the bloom to a heating temperature in a temperature range of 1150° C. or higher and 1250° C. or lower;   retaining a temperature of the bloom at the heating temperature for 600 seconds to 7200 seconds;   hot rolling the bloom after the retaining so that a finishing temperature is 950° C. or higher and 1050° C. or lower to obtain a hot rolled steel;   water cooling the hot rolled steel to a temperature range of 780° C. or higher and 840° C. or lower;   winding the hot rolled steel after the water cooling in the temperature range of 780° C. or higher and 840° C. or lower;   patenting the hot rolled steel after the winding by immersing the hot rolled steel in a molten salt at a temperature of 450° C. or higher and T1° C. or lower defined by Formula (3) for 20 seconds to 200 seconds, within 9 seconds to 25 seconds after the winding; and   tempering the hot rolled steel after the patenting to obtain the steel wire rod by heating the hot rolled steel to a tempering temperature in a temperature range of 540° C. to 600° C., retaining the hot rolled steel at the tempering temperature for 30 seconds to 600 seconds, and cooling the hot rolled steel to room temperature,
     T 1[° C.]=− r ′ [mm]×16+580  Formula (3)
 
   where r′ is a distance from a circumferential surface to a central axis of the hot rolled steel in units of mm.   
     
     
         6 . The manufacturing method of a steel wire rod according to  claim 5 ,
 wherein the steel wire rod includes, as the chemical composition, by unit mass %, at least one or more selected from the group consisting of
 Al: 0.001% to 0.070%, 
 Ti: 0.002% to 0.030%, 
 V: more than 0% and 0.100% or less, 
 Nb: more than 0% and 0.050% or less, 
 Mo: more than 0% and 0.20% or less, and 
 B: 0.0003% to 0.0030%. 
   
     
     
         7 . The steel wire rod according to  claim 2  comprising, as the chemical composition, by unit mass %:
 at least one or more selected from the group consisting of
 Al: 0.001% to 0.070%, 
 Ti: 0.002% to 0.030%, 
 V: more than 0% and 0.100% or less, 
 Nb: more than 0% and 0.050% or less, 
 Mo: more than 0% and 0.20% or less, and 
 B: 0.0003% to 0.0030%. 
 
 
     
     
         8 . The steel wire rod according to  claim 3  comprising, as the chemical composition, by unit mass %:
 at least one or more selected from the group consisting of
 Al: 0.001% to 0.070%, 
 Ti: 0.002% to 0.030%, 
 V: more than 0% and 0.100% or less, 
 Nb: more than 0% and 0.050% or less, 
 Mo: more than 0% and 0.20% or less, and 
 B: 0.0003% to 0.0030%.