Hot-rolled steel sheet, method of manufacturing the same, and equipment for manufacturing the same

Abstract

A method of manufacturing a hot-rolled steel sheet is explained. The method includes hot-rolling a slab, performing a first phase transformation process cooling a surface of the hot-rolled slab to phase-transform a structure of a surface portion of the hot-rolled slab, performing a second phase transformation process to phase-transform a structure of a central portion of the hot-rolled slab after the first phase transformation process, and coiling the slab including the phase-transformed surface portion and the phase-transformed central portion.

What is claimed is: 
     
         1 . A method of manufacturing a hot-rolled steel sheet, the method comprising:
 hot-rolling a slab;   performing a first phase transformation process cooling a surface of the hot-rolled slab to phase-transform a structure of a surface portion of the hot-rolled slab;   performing a second phase transformation process to phase-transform a structure of a central portion of the hot-rolled slab after the first phase transformation process; and   coiling the slab including the phase-transformed surface portion and the phase-transformed central portion.   
     
     
         2 . The method of  claim 1 , wherein a phase of the structure of the central portion of the hot-rolled slab is maintained during the first phase transformation process. 
     
     
         3 . The method of  claim 2 , wherein a phase of the structure of the surface portion of the hot-rolled slab is maintained, during the second phase transformation process. 
     
     
         4 . The method of  claim 1 , wherein performing the first phase transformation process comprises:
 providing compressed air to the surface portion of the hot-rolled slab.   
     
     
         5 . The method of  claim 1 , wherein performing the first phase transformation process comprises:
 jetting a liquid temperature-reducing material to the surface portion of the hot-rolled slab.   
     
     
         6 . The method of  claim 1 , wherein the hot-rolled slab has an austenite structure,
 wherein the surface portion of the hot-rolled slab has a ferrite structure by the first phase transformation process, and the central portion of the hot-rolled slab has the austenite structure after the first phase transformation process and before the second phase transformation process, and   wherein the central portion of the hot-rolled slab has a structure having a higher strength than the ferrite structure by the second phase transformation process, and the surface portion of the hot-rolled slab has the ferrite structure after the second phase transformation process.   
     
     
         7 . 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.   
     
     
         8 . The method of  claim 1 , wherein performing the second phase transformation process comprises: water-cooling the hot-roiled slab. 
     
     
         9 . The method of  claim 1 , further comprising:
 tempering the coded slab.   
     
     
         10 . The method of  claim 1 , wherein the first phase transformation process and the second phase transformation process are performed using different apparatuses from each other. 
     
     
         11 . A hot-rolled steel sheet comprising:
 a first surface;   a second surface opposite to the first surface; and   a central portion between the first and second surfaces,   wherein a first phase has a maximum volume fraction and a second phase has a minimum volume fraction at the first surface and the second surface, and   wherein the first phase has a minimum volume fraction and the second phase has a maximum volume fraction in the central portion.   
     
     
         12 . The hot-rolled steel sheet of  claim 11 , 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.   
     
     
         13 . The hot-rolled steel sheet of  claim 11 , 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 hot-rolled steel sheet of  claim 11 , wherein first and second portions respectively adjacent to the first and second surfaces have only the first phase, and
 wherein the central portion has only the second phase.   
     
     
         15 . The hot-rolled steel sheet of  claim 11 , wherein the volume fraction of the first phase progressively decreases from the first surface to the central portion, and the volume fraction of the second phase progressively increases from the first surface to the central portion, and
 wherein the volume fraction of the first phase progressively increases from the central portion to the second surface, and the volume fraction of the second phase progressively decreases from the central portion to the second surface.   
     
     
         16 . Equipment for manufacturing a hot-rolled steel sheet, the equipment comprising:
 a roller hot-rolling a slab;   a transport table transporting the hot-rolled slab;   a first surface cooling part disposed over the transport table, the first surface cooling part cooling a first surface of the hot-rolled slab;   a second surface cooling part spaced apart from the first surface cooling part with the transport table therebetween, the second surface cooling part cooling a second surface of the hot-rolled slab opposite to the first surface; and   a third cooling part water-cooling the slab cooled by the first and second surface cooling parts.   
     
     
         17 . The equipment of  claim 16 , wherein the transport table transports the slab in a first direction, and
 wherein each of the first and second surface cooling parts comprises:   a temperature-reducing material supply line extending in a second direction perpendicular to the first direction; and   a temperature-reducing material supply nozzle supplying a temperature-reducing material supplied from the temperature-reducing material supply line to the first surface or the second surface of the hot-rolled slab.   
     
     
         18 . The equipment of  claim 17 , wherein the temperature-reducing material supply nozzle supplies the temperature-reducing material to the first surface or the second surface of the slab in an opposite direction to the first direction. 
     
     
         19 . The equipment of  claim 17 , wherein the temperature-reducing material supply nozzle is disposed to be oblique with respect to the first surface or the second surface of the slab. 
     
     
         20 . The equipment of  claim 16 , wherein a first portion of the slab adjacent to the first surface and a second portion of the slab adjacent to the second surface are phase-transformed by the first and second surface cooling, parts, and
 wherein a phased of a central portion of the slab between the first and second portions is maintained when the first and second portions are phase-transformed.