Method for heating steel plate and method for manufacturing hot-pressed product

Abstract

A steel plate to be heated is a blank having a first region and a second region adjacent to the first region. The blank is heated by direct resistance heating. A jet of cooling medium is applied to the first region during the direct resistance heating such that a temperature of the first region is kept lower than a quenching region while heating the second region to be equal to or higher than the quenching temperature. To provide a clear boundary between the first and second regions, the jet of cooling medium is applied along a slant direction such that the cooling medium expands along the boundary between the first and second regions. Alternatively, a partition member is provided along the boundary between the first and second regions. The heated blank is then press-formed and cooled in a press die to obtain a hot-pressed product.

1 . A method for heating a steel plate, the steel plate being a blank having a first region and a second region adjacent to the first region, the method comprising:
 heating the blank by direct resistance heating; and   applying a jet of cooling medium to the first region on at least one of a front surface and a back surface of the blank during the direct resistance heating such that a temperature of the first region is kept lower than a quenching region while heating the second region to be equal to or higher than the quenching temperature,   wherein the jet of cooling medium is applied along a slant direction that is inclined toward the second region from a direction perpendicular to the at least one of the front surface and the back surface in the first region such that the jet of cooling medium expands along a boundary between the first region and the second region; or   wherein a partition member is provided to extend along the boundary between the first region and the second region on the at least one of the front surface and the back surface of the blank.   
     
     
         2 . The method according to  claim 1 , wherein the jet of cooling medium is applied along the slant direction such that the jet of cooling medium expands in a form of a curtain along the boundary between the first region and the second region. 
     
     
         3 . The heating method according to  claim 2 , wherein the first region includes an edge of the blank, and
 wherein the jet of cooling medium applied to the first region is caused to flow off the edge of the blank.   
     
     
         4 . The heating method according to  claim 2 , wherein the first region is a closed region surrounded by the second region, and
 wherein the jet of cooling medium applied to the first region is caused to flow from a circumference of the first region toward a center of the first region.   
     
     
         5 . The heating method according to  claim 1 , wherein the partition member is provided to extend along the boundary between the first region and the second region on the at least one of the front surface and the back surface of the blank. 
     
     
         6 . The heating method according to  claim 5 , wherein the partition member is provided to contact the at least one of the front surface and the back surface. 
     
     
         7 . The heating method according to  claim 5 , wherein the first region includes an edge of the blank, and
 wherein the partition member extends alongside the edge of the blank to cause the jet of cooling medium applied to the first region to flow off the edge of the blank.   
     
     
         8 . The heating method according to  claim 5 , wherein the first region is a closed region surrounded by the second region, and
 wherein the partition member has a cylindrical shape to cause the jet of cooling medium applied to the first region to flow from a center of the first region toward a circumference of the first region.   
     
     
         9 . The heating method according to  claim 8 , wherein an inner cylinder is inserted inside the partition member to apply the jet of cooling medium toward the center of the first region through the inner cylinder. 
     
     
         10 . The heating method according to  claim 1 , wherein the jet of cooling medium is applied in a state in which the first region is supported on at least one of the front surface and the back surface to suppress bending of the blank. 
     
     
         11 . The heating method according to  claim 10 , wherein the first region is point-supported at one or more locations on at least one of the front surface and the back surface. 
     
     
         12 . The heating method according to  claim 1 ,
 wherein the blank is rectangular and has a constant sectional area along a longitudinal direction of the blank, and   wherein the heating of the blank by the direct resistance heating comprises applying electric current to pass through the blank via a pair of electrodes fixed at longitudinal ends of the blank.   
     
     
         13 . The heating method according to  claim 1 , wherein the blank is non-rectangular and has a sectional area monotonously decreasing along a longitudinal direction of the blank from a first end of the blank to a second end of the blank, and wherein the heating of the blank by the direct resistance heating comprises:
 placing a pair of electrodes on the first end of the blank; and   moving one of the electrodes in the longitudinal direction toward the second end of the blank while applying electric current to pass through a portion of the blank between the pair of electrodes.   
     
     
         14 . The heating method according to  claim 13 , wherein the application of the jet of cooling medium to the first region is started after the one of the electrodes has passed the first region. 
     
     
         15 . The heating method according to  claim 1 , wherein the temperature of the first region is kept lower than an Ac1 transformation point of the blank while heating the second region to be equal to or higher than an Ac3 transformation point of the blank. 
     
     
         16 . A method for manufacturing a hot-pressed product, the method comprising:
 heating the blank by the method according to  claim 1 ;   press-forming the heated blank by a press die; and   cooling the blank inside the press die to quench the second region.