Forming method, heat treatment system, and formed product

Abstract

In heating step S 101 , a steel sheet is heated and made in an austenite state. In heating step S 101 , the whole region of the steel sheet is evenly heated, and the whole region of the steel sheet is made in the austenite state. In cooling step S 102 , only a first region set on the steel sheet in the austenite state is forcibly cooled (rapidly cooled) within a temperature range of a range where martensitic transformation does not occur. In cooling step S 102 , a second region other than the first region is cooled by natural cooling to maintain a state in which a temperature is higher than in the first region.

1 . A forming method comprising:
 a heating step of heating a steel sheet having a plating layer of aluminum added with silicon formed on a surface of the steel sheet, to make the steel sheet in an austenite state, and to alloy a whole of the plating layer to have a composition ratio of iron which is not less than a composition ratio of iron of an γ-phase alloy of iron, aluminum and silicon, and set a thickness of a diffusion layer formed on the side of the steel sheet on which the plating layer is formed not more than 10 μm;   a cooling step of forcibly cooling only a first region set on the steel sheet in the austenite state within a temperature range where martensitic transformation does not occur;   a reheating step of, after the cooling step, heating the steel sheet under conditions that do not allow the diffusion layer to grow such that the second region is made in the austenite state or maintain the austenite state of the second region, and   a forming step of hot-press-forming the steel sheet including the first region that is not in the austenite state, and a second region other than the first region, which is in the austenite state,   wherein   
       the forming step is performed after the reheating step. 
     
     
         2 . The forming method according to  claim 1 , wherein
 in the cooling step, the first region is cooled to a temperature at which a ferrite/pearlite phase grows.   
     
     
         3 . The forming method according to  claim 1 , wherein
 the heating step includes heating a whole region of the steel sheet evenly.   
     
     
         4 . The forming method according to  claim 1 , wherein
 the cooling step includes forcibly cooling only the first region, outside a heating furnace in which the heating step is performed, to a temperature where a ferrite/pearlite phase is formed within a temperature range where martensitic transformation does not occur, and naturally cooling the second region to a temperature lower than a temperature at which transformation to austenite starts,   the forming step includes hot-press-forming the steel sheet including the first region that is not in the austenite state and the second region in the austenite state, and   reheating step includes heating the steel sheet, immediately after the cooling step, to make the second region in the austenite state, and maintaining a state in which the first region is rapidly cooled for a predetermined time to grow a ferrite/pearlite phase.   
     
     
         5 . The forming method according to  claim 4 , wherein
 the cooling step including maintaining a state in which the temperature of the second region is higher than the first region,   the reheating step includes heating the steel sheet under a condition of such a range that the first region is not made in the austenite state, and   the forming step includes transforming only the second region to martensite.   
     
     
         6 . The forming method according to  claim 1 , wherein
 the forming step includes hot-press-forming the steel sheet including the first region that is not in the austenite state and the second region in the austenite state, setting a strength of the first region to not more than 780 MPa without a quenching process, and setting a strength of the second region to not less than 1,300 MPa after having undergone a quenching process, and   the diffusion layer is evenly formed with a thickness of not more than 10 μm in a whole region including the first region and the second region.   
     
     
         7 . A heat treatment system configured to form, on a steel sheet, a first region that is not in an austenite state and a second region other than the first region to form a region to be partially plastically deformed by hot-press-forming the steel sheet for which a plating layer of aluminum added with silicon is formed on a surface, comprising:
 a heat treatment apparatus configured to heat the steel sheet and make the steel sheet in the austenite state, alloy a whole of the plating layer to have a composition ratio of iron which is not less than a composition ratio of iron of an γ-phase alloy of iron, aluminum and silicon, and set a thickness of a diffusion layer formed on the side of the steel sheet on which the plating layer is formed not more than 10 μm;   a cooling processing apparatus configured to forcibly cool only the first region set on the steel sheet in the austenite state within a temperature range where martensitic transformation does not occur; and   a reheat treatment apparatus configured to heat the steel sheet processed by the cooling processing apparatus under such a condition that the diffusion layer does not grow to make the second region in the austenite state or heat the steel sheet processed by the cooling processing apparatus under such a condition that the diffusion layer does not grow to maintain the austenite state of the second region.   
     
     
         8 . A formed product obtained by forming a steel sheet for which a plating layer of aluminum added with silicon is formed on a surface, comprising:
 a first region having a strength of not more than 780 MPa without a quenching process and a hardness of not more than 220 HV, the first region not being in a martensite state; and   a second region having a strength of not less than 1,300 MPa after having undergone a quenching process and a hardness of not less than 400 HV, the second region being in the martensite state,   wherein a whole of the plating layer is alloyed,   the formed product comprises a diffusion layer in which a composition ratio of iron is not less than a composition ratio of γ-phase iron in an alloy of iron, aluminum and silicon and which is formed on a steel sheet side of the plating layer,   the diffusion layer is evenly formed with a thickness of not more than 10 μm in a whole region including the first region and the second region, and   the formed product further comprises a boundary region formed between the first region and the second region and having a width up to 50 mm, in which a hardness gradually transitions.