Steel plate member and method of producing the same

Abstract

A method of producing a steel plate member includes: heating a steel plate member to a temperature higher than an austenite transformation finish temperature and subsequently cooling the steel plate member at a cooling rate higher than an upper critical cooling rate; and softening the steel plate member by reheating the steel plate member after the cooling of the steel plate member.

What is claimed is: 
     
         1 . A method of producing a steel plate member, comprising:
 heating a steel plate member to a temperature higher than an austenite transformation finish temperature and subsequently cooling the steel plate member at a cooling rate higher than an upper critical cooling rate; and   softening the steel plate member by reheating the steel plate member after the cooling of the steel plate member,   wherein the softening of the steel plate member includes, without reheating a first region of the steel plate member, reheating a second region of the steel plate member to a temperature between an austenite transformation start temperature and the austenite transformation finish temperature and subsequently cooling the steel plate member at a cooling rate higher than the upper critical cooling rate to form a hard region containing martensite in the first region, a soft region containing the martensite and tempered martensite in the second region, and a region consisting of the tempered martensite in a boundary region between the first region and the second region.   
     
     
         2 . The method of producing a steel plate member according to  claim 1 , wherein in the softening of the steel plate member, the second region is reheated by induction heating. 
     
     
         3 . The method of producing a steel plate member according to  claim 1 , wherein the steel plate member is press-formed after the heating of the steel plate member and before the cooling of the steel plate member. 
     
     
         4 . The method of producing a steel plate member according to  claim 2 , wherein the second region is reheated by a high-frequency induction heating apparatus including a coil and a high-frequency power source, the coil includes a plate member having a U-shaped cross-section, and the second region is inserted inside the coil and inductively heated. 
     
     
         5 . A steel plate member comprising:
 a hard region containing martensite;   a soft region softer than the hard region, the soft region containing martensite and tempered martensite; and   a boundary region located between the hard region and the soft region, the boundary region including a region consisting of tempered martensite.   
     
     
         6 . The steel plate member according to  claim 5 , wherein the steel plate member is a steel plate member for a pillar as a component of an automobile body and is joined at the soft region to another metal member. 
     
     
         7 . The steel plate member according to  claim 6 , wherein a flange portion provided at an end of the steel plate member includes the soft region.