Method of forming steel sheet, equipment for forming the same, and formed product

Abstract

A formed product of a steel sheet is provided. The formed product includes a surface portion adjacent to an exterior surface and a central portion surrounded by the surface portion. A first phase has a maximum volume fraction in the surface portion, and a second phase has a minimum volume fraction in the surface portion. The second phase has a maximum volume fraction in the central portion, and the first phase has a minimum volume fraction in the central portion.

What is claimed is: 
     
         1 . A method of forming a steel sheet, the method comprising:
 preparing a steel sheet including a surface portion and a central portion;   thermally treating the steel sheet;   performing a first phase transformation process cooling a surface of the thermally treated steel sheet to phase-transform a structure of the surface portion of the thermally treated steel sheet;   forming the steel sheet after the performing of the first phase transformation process; and   performing a second phase transformation process cooling the formed steel sheet to phase-transform a structure of the central portion of the formed steel sheet.   
     
     
         2 . The method of  claim 1 , wherein a phase of the structure of the central portion of the steel sheet is maintained during the performing of the first phase transformation process. 
     
     
         3 . The method of  claim 2 , wherein a phase of the structure of the surface portion of the steel sheet is maintained during the forming of the steel sheet and the performing of the second phase transformation process. 
     
     
         4 . The method of  claim 1 , wherein the thermally treated steel sheet has an austenite structure,
 wherein the structure of the surface portion of the thermally treated steel sheet is phase-transformed to a ferrite structure by the first phase transformation process,   wherein the austenite structure of the central portion of the thermally treated steel sheet is maintained during the performing of the first phase transformation process,   wherein the central portion of the formed steel sheet has a structure having a higher strength than the ferrite structure by the second phase transformation process, and   wherein the ferrite structure of the surface portion of the formed steel sheet is maintained during the performing of the second phase transformation process.   
     
     
         5 . The method of  claim 1 , wherein an elongation of the surface portion phase-transformed by the first phase transformation process is higher than an elongation of the central portion phase-transformed by the second phase transformation process, and
 wherein a strength of the central portion phase-transformed by the second phase transformation process is higher than a strength of the surface portion phase-transformed by the first phase transformation process.   
     
     
         6 . The method of  claim 1 , wherein the performing of the first phase transformation process comprises:
 supplying compressed air to the surface portion of the steel sheet.   
     
     
         7 . The method of  claim 1 , wherein the performing of the first phase transformation process comprises:
 jetting a liquid temperature-reducing agent to the surface portion of the steel sheet.   
     
     
         8 . The method of  claim 1 , wherein a step of forming the steel sheet and the second phase transformation process are performed in the same apparatus. 
     
     
         9 . A formed product comprising:
 a surface portion adjacent to an exterior surface; and   a central portion surrounded by the surface portion,   wherein a first phase has a maximum volume fraction in the surface portion,   wherein a second phase has a minimum volume fraction in the surface portion,   wherein the second phase has a maximum volume fraction in the central portion, and   wherein the first phase has a minimum volume fraction in the central portion.   
     
     
         10 . The formed product of  claim 9 , wherein a structure of the first phase is a ferrite structure, and
 wherein a structure of the second phase has a higher strength than the ferrite structure.   
     
     
         11 . The formed product of  claim 9 , wherein the surface portion has only the first phase, and
 wherein the central portion has only the second phase.   
     
     
         12 . The formed product of  claim 9 , wherein the volume fraction of the first phase gradually decreases from the surface portion to the central portion,
 wherein the volume fraction of the second phase gradually increases from the surface portion to the central portion,   wherein the volume fraction of the second phase gradually decreases from the central portion to the surface portion, and   wherein the volume fraction of the first phase gradually increases from the central portion to the surface portion.   
     
     
         13 . The formed product of  claim 9 , wherein a structure of the first phase has a higher elongation than a structure of the second phase, and
 wherein the structure of the second phase has a higher strength than the structure of the first phase.   
     
     
         14 . The formed product of  claim 9 , further comprising a mixed portion between the surface portion and the central portion including the first phase and the second phase. 
     
     
         15 . Equipment for forming a steel sheet, the equipment comprising:
 a cutting part cutting a steel sheet;   a heat supply part thermally treating the cut steel sheet;   a surface cooling part selectively cooling a surface portion of the thermally treated steel sheet; and   a forming/cooling part forming the steel sheet having the cooled surface portion and cooling a central portion of the formed steel sheet.   
     
     
         16 . The equipment of  claim 15 , wherein the surface cooling part comprises:
 a first surface cooling part cooling a first surface of the steel sheet; and   a second surface cooling part cooling a second surface of the steel sheet opposite to the first surface.   
     
     
         17 . The equipment of  claim 15 , wherein the first surface cooling part and the second surface cooling part supply a temperature-reducing agent to the first surface and the second surface in a direction opposite to a direction in which the steel sheet is transferred. 
     
     
         18 . The equipment of  claim 17 , wherein temperature-reducing agent supply nozzles supplying the temperature-reducing agent are disposed obliquely to the first and second surfaces of the steel sheet. 
     
     
         19 . The equipment of  claim 15 , wherein the surface cooling part supplies compressed air to selectively cool a surface portion of the steel sheet. 
     
     
         20 . The equipment of  claim 15 , wherein the surface cooling part selectively cools the surface portion of the thermally treated steel sheet except the central portion to phase-transform a structure of the surface portion, and
 wherein the forming/cooling part cools the formed steel sheet to phase-transform a structure of the central portion except the surface portion.