Preparation method of steel product having different strengths using laser heat treatment, and heat hardened steel used therein

Abstract

Disclosed are a preparation method of a steel product capable of local reinforcement using laser heat treatment, and a heat hardened steel used in the method. According to the present invention, the preparation method of a steel part comprises the following steps: (a) preparing a material comprising 0.1-0.5 wt % of C, 0.1-0.5 wt % of Si, 0.5-3.0 wt % of Mn, 0.1 wt % or less of P, 0.05 wt % or less of S, 0.01-1.0 wt % of Cr, 0.1 wt % or less of Al, 0.2 wt % or less of Ti, 0.0005-0.08 wt % of B, and the balance of Fe and inevitable impurities; (b) preparing a formed product by forming the material into a predetermined shape; and (c) locally reinforcing the high strength portion by carrying out laser heat treatment on a portion requiring high strength at the formed product.

1 . A method for manufacturing a steel product, comprising:
 (a) preparing a material comprising: by weight %, 0.1˜0.5% of carbon (C) ; 0.1˜0.5% of silicon (Si); 0.5˜3.0% of manganese (Mn); 0.1% or less of phosphorus (P); 0.05% or less of sulfur (S); 0.01˜1.0% of chromium (Cr); 0.1% or less of aluminum (Al); 0.2% or less of titanium (Ti); 0.0005˜0.08% of boron (B); and the balance of Fe and unavoidable impurities;   (b) producing a formed body by forming the material into a predetermined shape; and   (c) performing laser heat treatment to a part requiring high strength (high strength part) in the formed body, and locally strengthening the high strength part.   
     
     
         2 . The method according to  claim 1 , wherein, in step (a), a layer selected from Al plating layer, Al—Si plating layer, Zn—Ni plating layer, Zn plating layer, Zn—Al plating layer and high-temperature oxidation-resistant resin coating layer is formed on the surface of the material. 
     
     
         3 . The method according to  claim 1 , wherein, in step (c), the laser heat treatment irradiating a laser on the high strength part and heating the high strength part to a temperature higher than Ac3, and then cooling to a temperature lower than Ms at a cooling rate of 5˜300° C./sec. 
     
     
         4 . The method according to  claim 1 , wherein, in step (a), the material has a tensile strength of 400˜990 MPa and an elongation of 10˜40%. 
     
     
         5 . The method according to  claim 4 , wherein, in step (c), laser irradiating time and laser strength is adjusted for the tensile strength of the high strength portion is 1200˜1900 MPa in the laser heat treatment. 
     
     
         6 . A heat treated hardened steel comprising:
 by weight %, 0.1˜0.5% of carbon (C) ; 0.1˜0.5% of silicon (Si); 0.5˜3.0% of manganese (Mn); 0.1% or less of phosphorus (P); 0.05% or less of sulfur (5); 0.01˜1.0% of chromium (Cr); 0.1% or less of aluminum (Al); 0.2% or less of titanium (Ti); 0.0005˜0.08% of boron (B); and the balance of Fe and unavoidable impurities;   having a tensile strength of 400˜990 MPa and an elongation of 10˜40% before heat treatment, and   having a tensile strength of 1200˜1900 MPa and an elongation of 1˜13% after heat treatment.   
     
     
         7 . The heat treated hardened steel according to  claim 6 , wherein the steel has a layer selected from Al plating layer, Al—Si plating layer, Zn—Ni plating layer, Zn plating layer, Zn—Al plating layer and high-temperature oxidation-resistant resin coating layer is formed on the surface of the steel.