Steel Material, Process of Fabricating Steel Material, and Apparatus of Fabricating Steel Material

Abstract

According to one aspect of the present invention, there is provided a process of fabricating a steel material by performing a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, wherein the heat treatment comprises a heating step in which a portion having a certain depth from a surface of the steel material is rapidly heated by induction heating or direct energization heating, and a cooling step in which the steel material, which has been subject to the heating step, is rapidly cooled a predetermined time after the heating step, and a heating temperature in the heating step is Ac1 transforming point or more.

1 . A process of fabricating a steel material by performing a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material,
 wherein the heat treatment comprises a heating step for rapidly heating a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating, and a cooling step for rapidly cooling the steel material subjected to the heating step after a predetermined time from the heating step, and   a heating temperature in the heating step is Ac1 transforming point or more.   
     
     
         2 . The process of fabricating a steel material according to  claim 1 , wherein a time from the heating step to the cooling step is not more than a predetermined time determined according to a steel grade, a wire diameter, a heating temperature, and a heating time. 
     
     
         3 . The process of fabricating a steel material according to  claim 1 , wherein a process condition in the heat treatment is determined based upon a heat conductivity characteristic of the steel material after the rapid heating to the surface. 
     
     
         4 . The process of fabricating a steel material according to  claim 1 , wherein the process condition in the heat treatment is a time integration value of the temperature of the steel material, and is determined based upon a tempering progression value that indicates a progression state of tempering of the steel material. 
     
     
         5 . The process of fabricating a steel material according to  claim 4 , further comprising a step of calculating the heat conductivity characteristic of the steel material or the tempering progression value,
 wherein the process condition in the heat treatment is determined based upon the calculated heat conductivity characteristic or the tempering progression value.   
     
     
         6 . The process of fabricating a steel material according to  claim 4 , wherein the process condition in the heat treatment is set such that the tempering progression value at a surface layer becomes 1.5 times or more the tempering progression value at a central part. 
     
     
         7 . The process of fabricating a steel material according to  claim 4 , wherein the time from the heating step to the cooling step is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at the central part. 
     
     
         8 . The process of fabricating a steel material according to  claim 1 , wherein the process condition in the heat treatment is a combination including at least two of a frequency, an input electric energy, a heating temperature, a heating time, and a natural cooling time. 
     
     
         9 . The process of fabricating a steel material according to  claim 8 , further comprising a step of calculating the heat conductivity characteristic of the steel material or the tempering progression value,
 wherein the process condition in the heat treatment is determined based upon the calculated heat conductivity characteristic or the tempering progression value.   
     
     
         10 . The process of fabricating a steel material according to  claim 8 , wherein the time from the heating step to the cooling step is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at the central part. 
     
     
         11 . The process of fabricating a steel material according to  claim 1 , wherein the steel material is a steel wire or a steel rod. 
     
     
         12 . The process of fabricating a steel material according to  claim 1 , wherein, after a quenching process including a heating process and a cooling process is performed to the steel material, the heating step and the cooling step are respectively performed once as the tempering process. 
     
     
         13 . A steel material that is subject to the heating step and the cooling step according to  claim 1 ,
 wherein a difference between hardness in the vicinity of a surface layer and hardness at a position toward a central part from a position of 10% from the surface layer in a radial direction is HV50 or more, and a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm 2  or more.   
     
     
         14 . A steel material that is subject to the heating step and the cooling step according to  claim 1 ,
 wherein all cross-sections have a tempered martensite structure, hardness of a surface layer is HV380 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1420 N/mm 2  or more, and   hardness at a portion near a central part from the surface layer is uniform.   
     
     
         15 . A steel material that is subject to the heating step and the cooling step according to  claim 1 ,
 wherein all cross-sections have a tempered martensite structure, hardness of a surface layer is HV420 or less, a tensile strength obtained when a tensile test is conducted with a No. 2 test piece in JISZ2201 is 1600 N/mm 2  or more, and   hardness at a portion near a central part from the surface layer is uniform.   
     
     
         16 . An apparatus of fabricating a steel material that performs a heat treatment to a steel material having high strength, in order to reduce hardness at one part of the steel material to less than hardness at other parts of the steel material, the apparatus comprising:
 a heating unit configured to rapidly heat a portion from a surface of the steel material to a certain depth by induction heating or direct energization heating; and   a cooling unit configured to rapidly cool the steel material subjected to the heating after a predetermined time from the heating,   wherein a heating temperature of the steel material by the heating unit is Ac1 transforming point or more.   
     
     
         17 . The apparatus of fabricating a steel material according to  claim 16 ,
 wherein a time from an end of the process by the heating unit to a start of the process by the cooling unit is not more than a predetermined time determined according to a steel grade, a wire diameter, a heating temperature, and a heating time,   a process condition in the heat treatment is determined based upon a combination including at least two of a heat conductivity characteristic of the steel material after the rapid heating to the surface, a tempering progression value indicating a progression state of tempering of the steel material, a frequency, an input electric energy, a heating temperature, a heating time, and a natural cooling time,   the process condition in the heat treatment is set such that the tempering progression value at the surface layer becomes 1.5 times or more the tempering progression value at a central part,   a time from the heating step to the cooling step is not more than a predetermined time determined according to a steel grade, a wire diameter, a heating temperature, and a heating time,   the steel material is a steel wire or a steel rod,   the apparatus further comprises:   a conveying unit configured to continuously convey the steel material along a predetermined path passing the heating unit and the cooling unit, wherein the process condition includes a distance and a conveying speed in the conveying path; and   a quenching unit configured to perform a heating process and a cooling process to the steel material,   one heating unit and one cooling unit are provided at a downstream side of the quenching unit in the conveying path, and   the apparatus further comprises a control unit configured to control the process condition in the heat treatment based upon a calculation result of the heat conductivity characteristic of the steel material or the tempering progression value.