Steel plate member and method of producing the steel plate member

Abstract

A method of producing a steel plate member including: quenching a steel plate by heating a steel plate to a temperature which is higher than a first transformation temperature at which austenitization is completed, and then cooling the steel plate at a cooling rate higher than an upper critical cooling rate; reheating a second area of the steel plate, without reheating a first area of the steel plate, to a temperature which is higher than a second transformation temperature, at which austenitization starts, and is lower than the first transformation temperature; and cooling the steel plate at a cooling rate lower than a lower critical cooling rate.

What is claimed is: 
     
         1 . A method of producing a steel plate member comprising:
 quenching a steel plate by heating the steel plate to a temperature which is higher than a first transformation temperature at which austenitization is completed and then cooling the steel plate at a cooling rate higher than an upper critical cooling rate;   reheating a second area of the steel plate, without reheating a first area of the steel plate, to a temperature which is higher than a second transformation temperature, at which austenitization starts, and is lower than the first transformation temperature; and   cooling the reheated steel plate at a cooling rate lower than a lower critical cooling rate to obtain the steel plate member in which a hard area composed of martensite is formed in the first area, a soft area containing tempered martensite in addition to ferrite and pearlite is formed in the second area, and an area composed of only tempered martensite is formed in a third area between the first area and the second area.   
     
     
         2 . The method of producing the steel plate member according to  claim 1 , wherein the second area of the steel plate is reheated by induction heating. 
     
     
         3 . The method of producing the steel plate member according to  claim 1 , wherein, after the steel plate is heated, a press processing is performed on the steel plate before the steel plate is cooled during quenching. 
     
     
         4 . The method of producing the steel plate member according to  claim 1 , wherein:
 the third area is heated to a temperature which is lower than the second transformation temperature by heat conduction from the second area being reheated; and   a structure of the third area changes from martensite to tempered martensite.   
     
     
         5 . A steel plate member comprising:
 a hard area composed of martensite;   a soft area containing ferrite and pearlite; and   a third area which is formed between the hard area and the soft area, wherein: the soft area further contains tempered martensite; and   the third area includes an area composed of only tempered martensite.