Patent Publication Number: US-2021164069-A1

Title: Steel plate member and method of producing the same

Description:
INCORPORATION BY REFERENCE 
     This is a divisional of U.S. application Ser. No. 16/185,078 filed Nov. 9, 2018, which claims priority to Japanese Patent Application No. 2017-243904 filed on Dec. 20, 2017. the disclosure of which, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a steel plate member and a method of producing the same and particularly relates to a steel plate member including a hard region containing martensite and a soft region softer than the hard region and a method of producing the steel plate member. 
     2. Description of Related Art 
     In recent years, steel plate members including a hard region resistant to impact and a soft region for joining to another metal member have been developed, for example, as structural members for automobiles. Japanese Patent Application Publication No. 2012-144773 discloses a technique of forming a hard region and a soft region in one steel plate member by locally heating only a region of the steel plate member to a temperature higher than an austenite transformation finish temperature A 3  and quenching the heated region. 
     SUMMARY 
     The inventors have found the following problems concerning a steel plate member including a hard region and a soft region and a method of producing the steel plate member. As disclosed in JP 2012-144773 A, local heating of only a region of a steel plate member to a temperature higher than the austenite transformation finish temperature A 3  converts the microstructure of the region to an austenite single phase. Thus, the region becomes a hard region containing martensite after quenching. On the other hand, in a region heated only to a temperature lower than an austenite transformation start temperature A 1 , austenite does not form. After quenching, therefore, this region remains a soft region containing ferrite and pearlite as it is before quenching. 
     A boundary region between the hard region and the soft region is naturally heated to a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3 , and part of ferrite and pearlite in the boundary region is converted to austenite. Thus, after quenching, the boundary region has an unstable microstructure containing martensite which is hard and further containing ferrite and pearlite which are soft. This unfortunately results in high susceptibility to fracture in the boundary region between the hard region and soft region and hence low local ductility or bendability. 
     The present disclosure provides a steel plate member with reduced susceptibility to fracture in a boundary region between a hard region and a soft region and a method of producing the steel plate member. 
     A method of producing a steel plate member according to a first aspect of the present disclosure includes: heating a steel plate member to a temperature higher than an austenite transformation finish temperature and subsequently cooling the steel plate member at a cooling rate higher than an upper critical cooling rate (quenching step); and softening the steel plate member by reheating the steel plate member after the cooling of the steel plate member (tempering step), wherein the tempering step includes, without reheating a first region of the steel plate member, reheating a second region of the steel plate member to a temperature between an austenite transformation start temperature and the austenite transformation finish temperature and subsequently cooling the steel plate member at a cooling rate higher than the upper critical cooling rate to form a hard region containing martensite in the first region, a soft region containing martensite and tempered martensite in the second region, and a region consisting of tempered martensite in a boundary region between the first region and the second region. 
     In the method of producing a steel plate member according to the first aspect of the present disclosure, the tempering step includes, without reheating a first region of a steel plate member, reheating a second region of the steel plate member to a temperature between an austenite transformation start temperature and an austenite transformation finish temperature and subsequently cooling the steel plate member at a cooling rate higher than an upper critical cooling rate. This results in formation of a hard region containing martensite in the first region, formation of a soft region including martensite and tempered martensite in the second region, and formation of a region consisting of tempered martensite in a boundary region between the first region and the second region. Thus, an unstable microstructure containing martensite which is hard and further containing ferrite and pearlite which are soft is not formed in the boundary region, in consequence of which fracture in the boundary region can be prevented. 
     According to the first aspect, in the tempering step, the second region may be reheated by induction heating. With this feature, the second region of the steel plate member can be rapidly heated and, in addition, the temperature to which the second region is heated can be accurately controlled to a temperature between the austenite transformation start temperature and the austenite transformation finish temperature. 
     According to the first aspect, in the quenching step, the steel plate member may be press-formed after the heating of the steel plate member and before the cooling of the steel plate member. With this feature, spring back occurring in cold press can be avoided and, at the same time, a high-strength steel plate member can be obtained as a result of the cooling subsequent to the press forming. 
     According to the first aspect, the second region may be reheated by a high-frequency induction heating apparatus including a coil and a high-frequency power source, and the coil may include a plate member having a U-shaped cross-section, and the second region may be inserted inside the coil and inductively heated. 
     A steel plate member according to a second aspect of the present disclosure is a steel plate member including a hard region containing martensite, a soft region softer than the hard region, and a boundary region located between the hard region and the soft region, the soft region including martensite and tempered martensite, the boundary region including a region consisting of tempered martensite. 
     In the boundary region of the steel plate member according to the second aspect, a region consisting of tempered martensite is formed, while no unstable microstructure containing martensite which is hard and further containing ferrite and pearlite which are soft is formed. Thus, fracture in the boundary region can be prevented. 
     The steel plate member according to the second aspect may be a steel plate member for a pillar as a component of an automobile body and may be joined at the soft region to another metal member. With this feature, post-joining fracture can be prevented. 
     The soft region may be included in a flange portion provided at an end of the steel plate member according to the second aspect. With this feature, the soft region can easily be formed by induction heating. 
     The present disclosure can provide a steel plate member with reduced susceptibility to fracture in a boundary region between a hard region and a soft region and a method of producing the steel plate member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a temperature chart illustrating a method of producing a steel plate member according to a first embodiment; 
         FIG. 2A  is a schematic plan view illustrating a change in microstructure of a steel plate member during a local tempering step; 
         FIG. 2B  is a schematic plan view illustrating a change in microstructure of the steel plate member during the local tempering step; 
         FIG. 2C  is a schematic plan view illustrating a change in microstructure of the steel plate member during the local tempering step; 
         FIG. 3  is a schematic plan view of the steel plate member according to the first embodiment; 
         FIG. 4  is a temperature chart illustrating a method of producing a steel plate member according to a comparative example for the first embodiment; 
         FIG. 5  is a schematic plan view of a steel plate member according to the comparative example for the first embodiment; 
         FIG. 6  is a schematic perspective view of an induction heating apparatus used in a method of producing a steel plate member according to a second embodiment; 
         FIG. 7  is a temperature chart for induction heating of steel plates having different thicknesses; 
         FIG. 8A  is a perspective view of an example of the steel plate member according to the second embodiment; 
         FIG. 8B  is a perspective view of an example of the steel plate member according to the second embodiment; 
         FIG. 9  is a temperature chart illustrating the conditions of local tempering according to an Example of the second embodiment; 
         FIG. 10  is a graph showing the hardness distribution of a steel plate member produced by the method of producing a steel plate member according to an Example of the second embodiment; 
         FIG. 11  shows photographs of the microstructure of a steel plate member produced by the method of producing a steel plate member according to the second embodiment; and 
         FIG. 12  is a graph showing the hardness and critical bending angle of a boundary region  13  of a steel plate member according to an Example in comparison with those of a boundary region  23  of a steel plate member according to a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, specific embodiments employing the present disclosure will be described in detail with reference to the drawings. It should be noted that the present disclosure is not limited to the embodiments described below. For clarity of explanation, the following description and the drawings are simplified as appropriate. 
     First Embodiment 
     Method of Producing Steel Plate Member 
     First, a method of producing a steel plate member according to a first embodiment will be described with reference to  FIG. 1 . The method of producing a steel plate member according to the first embodiment is suitable as a method of producing a steel plate member including a hard region resistant to impact and a soft region for joining to another metal member, the steel plate member being for use, for example, in automobiles. An example of the steel plate member for use in automobiles is a steel plate member for a pillar as a component of an automobile body. 
       FIG. 1  is a temperature chart illustrating the method of producing a steel plate member according to the first embodiment. In  FIG. 1 , the abscissa represents the time (s), and the ordinate represents the temperature (° C.). As shown in  FIG. 1 , the method of producing a steel plate member according to the first embodiment includes a quenching step and a local tempering step. In the method of producing a steel plate member according to the first embodiment, the quenching step is performed first, followed by the local tempering step. 
     In the quenching step, first, the entire steel plate member is heated to a temperature higher than an austenite transformation finish temperature A 3 . As a result of this heating, the microstructure of the entire steel plate member converts from ferrite and pearlite to an austenite single phase. Subsequently, the steel plate member is cooled at a cooling rate higher than an upper critical cooling rate. This causes martensite transformation of the steel plate member, thus converting the microstructure of the entire steel plate member to martensite which is hard. 
     The steel plate member is preferably press-formed after the heating of the steel plate member and before the cooling of the steel plate member. This process is hot press, which enables obtaining a high-strength steel plate member through tempering subsequent to the press forming while avoiding spring back which may occur in the case of cold press. Such hot press is generally called hot stamping. The steel plate used for hot stamping is, for example, but not limited to, a steel plate made of manganese-boron steel and having a thickness of about 1 to 4 mm. 
     Next, in the local tempering step, only a region of the steel plate member is locally reheated and thus softened. Specifically, as shown in  FIG. 1 , only a second region  12  of the steel plate member is reheated to a temperature between an austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3  without reheating of a first region  11  of the steel plate member. In this process, a boundary region  13  between the first region  11  and the second region  12  is heated to a temperature lower than the austenite transformation start temperature A 1  by heat conduction from the second region  12  and thermally affected. The tempering temperature in common cases is lower than the austenite transformation start temperature A 1 ; thus, the heating temperature in the local tempering step according to the first embodiment is higher than the tempering temperature in common cases. 
     Subsequently, the steel plate member is cooled at a cooling rate higher than the upper critical cooling rate to cause martensite transformation of the second region  12 .  FIG. 1  schematically shows a martensite transformation start temperature Ms, a martensite transformation finish temperature Mf, and a ferrite/pearlite nose in a continuous cooling transformation (CCT) phase diagram. That is,  FIG. 1  schematically illustrates the process in which the second region  12  having been reheated to a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3  is cooled at a cooling rate higher than the upper critical cooling rate. 
       FIGS. 2A to 2C  are schematic plan views illustrating the change in microstructure of the steel plate member during the local tempering step. First, the microstructure of a steel plate member  10  shown in  FIG. 2A  which has been subjected to the quenching step but has yet to be subjected to the local tempering step will be described. As shown in  FIG. 2A , the microstructure of the entire steel plate member  10  contains martensite M before the local tempering step. 
     Next, the microstructure of the steel plate member  10  shown in  FIG. 2B  which is being heated in the local tempering step will be described. As shown in the temperature chart of  FIG. 1 , only the second region  12  of the steel plate member  10  is reheated to a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3  during the heating in the local tempering step. 
     Thus, as shown in  FIG. 2B , the heating in the local tempering step causes conversion of the martensite M to tempered martensite TM and further conversion of part of the tempered martensite TM to austenite A in the second region  12 . As a result, the microstructure of the second region  12  becomes a mixed structure of the tempered martensite TM and the austenite A. In the vicinity of the boundary region  13 , the temperature of the heated second region  12  is close to the austenite transformation start temperature A 1 , so that the amount of the austenite A decreases while the amount of the tempered martensite TM increases. The term “tempered martensite TM” is used herein to collectively refer to all structures resulting from the reheating-induced softening of the martensite M and encompasses troostite and sorbite. 
     Meanwhile, the first region  11  of the steel plate member  10  is not reheated and thus is not thermally affected. The microstructure of the first region  11  therefore remains unchanged from the martensite M. The boundary region  13  between the first region  11  and the second region  12  is heated to a temperature lower than the austenite transformation start temperature A 1  by heat conduction from the second region  12  and thermally affected. This converts the microstructure of the boundary region  13  from the martensite M to the tempered martensite TM. 
     More specifically, the temperature of the heated boundary region  13  becomes higher and closer to the austenite transformation start temperature A 1  with decreasing distance from the second region  12 . For this reason, the second region  12 -side of the boundary region  13 , which is a major portion of the boundary region  13 , has a microstructure consisting of the tempered martensite TM. In the vicinity of the first region  11  in the boundary region  13 , both the tempered martensite TM and the martensite M are present. With decreasing distance from the first region  11  which contains the martensite M, the amount of the tempered martensite TM decreases and the amount of the martensite M increases. The boundary region  13  becomes softer gradually in the direction from the first region  11  which is hard to the second region  12  which is soft, because, as described above, the amount of the tempered martensite TM heated to a high temperature increases with decreasing distance from the second region  12 . 
     Next, the microstructure of the steel plate member  10  shown in  FIG. 2C  which has been subjected to the local tempering step will be described. As shown in  FIG. 1 , the cooling process in the local tempering step cools the steel plate member  10  at a cooling rate higher than the upper critical cooling rate. Thus, as shown in  FIG. 2C , the austenite A formed in the second region  12  during heating converts to the martensite M. As a result, the microstructure of the second region  12  becomes a mixed structure of the tempered martensite TM and the martensite M. The microstructure of the first region  11  remains unchanged from the martensite M. The microstructure of the boundary region  13  remains unchanged from the tempered martensite TM. 
     Configuration of Steel Plate Member 
     Next, a steel plate member according to the first embodiment will be described with reference to  FIG. 3 .  FIG. 3  is a schematic plan view of a steel plate member according to the first embodiment. The steel plate member according to the first embodiment is a steel plate member produced by the method of producing the steel plate member according to the first embodiment which is illustrated in  FIG. 1 . 
     As shown in  FIG. 3 , the steel plate member  10  according to the first embodiment includes a first region  11 , a second region  12 , and a boundary region  13 .  FIG. 3  schematically shows the respective microstructures of the first region  11 , the second region  12 , and the boundary region  13 . The microstructures of the steel plate member  10  shown in  FIG. 3  are identical to the microstructures of the steel plate member  10  shown in  FIG. 2C  which has been subjected to the local tempering step. 
     As shown in  FIG. 3 , the first region  11  is a hard region containing the martensite M which is hard. The second region  12  is a soft region containing the martensite M and the tempered martensite TM. In the vicinity of the boundary region  13  in the second region  12 , the amount of the martensite M gradually decreases with decreasing distance from the boundary region  13 . The second region  12 , which is soft, is provided mainly for the purpose of joining the steel plate member  10  to another metal member by spot welding or laser welding. Joining at the soft region rather than at the hard region can prevent post-joining fracture. 
     The boundary region  13  is formed between the first region  11  and the second region  12 . The second region  12 -side of the boundary region  13 , which is a major portion of the boundary region  13 , has a microstructure consisting of the tempered martensite TM. In the vicinity of the first region  11  in the boundary region  13 , both the tempered martensite TM and the martensite M are present. With decreasing distance from the first region  11 , the amount of the tempered martensite TM decreases and the amount of the martensite M increases. 
     Steel Plate Member According to Comparative Example and Method of Producing the Same 
     Hereinafter, a steel plate member according to a comparative example for the first embodiment and a method of producing the steel plate member will be described with reference to  FIG. 4  and  FIG. 5 .  FIG. 4  is a temperature chart illustrating the method of producing the steel plate member according to the comparative example for the first embodiment.  FIG. 5  is a schematic plan view of the steel plate member according to the comparative example for the first embodiment. First, the method of producing the steel plate member according to the comparative example will be described with reference to  FIG. 4 . In  FIG. 4 , the abscissa represents the time (s) and the ordinate represents the temperature (° C.). As shown in  FIG. 4 , the steel plate member according to the comparative example includes only a local quenching step. 
     In the local quenching step, only a region of the steel plate member is locally heated to a temperature higher than the austenite transformation finish temperature A 3 . Specifically, as shown in  FIG. 4 , only a first region  21  of the steel plate member is heated to a temperature higher than the austenite transformation finish temperature A 3  without heating of a second region  22  of the steel plate member. As a result of this heating, the microstructure of the first region  21  converts from ferrite and pearlite to an austenite single phase. The second region  22  includes a region heated to a temperature lower than the austenite transformation start temperature A 1 . The microstructure of the second region  22  remains unchanged from ferrite and pearlite. 
     A boundary region  23  between the first region  21  and the second region  22  is heated to a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3  by heat conduction from the second region  22 . This converts part of ferrite and pearlite to austenite in the microstructure of the boundary region  23 . Thus, the microstructure of the boundary region  23  becomes a mixed structure of ferrite, pearlite, and austenite. 
     Subsequently, the steel plate member is cooled at a cooling rate higher than an upper critical cooling rate. This causes all of the austenite to undergo martensite transformation, leading to a change of the microstructure of the first region  21  into martensite which is hard. The microstructure of the boundary region  23  becomes a mixed structure of ferrite which is soft, pearlite which is soft, and martensite which is hard. The microstructure of the second region  22  remains unchanged from ferrite and pearlite. 
     Next, the steel plate member according to the comparative example for the first embodiment will be described with reference to  FIG. 5 . The steel plate member according to the comparative example is a steel plate member produced by the method of producing the steel plate member according to the comparative example which is illustrated in  FIG. 4 . As shown in  FIG. 5 , a steel plate member  20  according to the comparative example includes a first region  21 , a second region  22 , and a boundary region  23 .  FIG. 5  schematically shows the respective microstructures of the first region  21 , the second region  22 , and the boundary region  23 . 
     As shown in  FIG. 5 , the first region  21  is a hard region containing martensite which is hard. The second region  22  is a soft region containing ferrite F and pearlite P (ferrite/pearlite FP) which are soft. The boundary region  23  is formed between the first region  21  and the second region  22  and is a mixed structure of the ferrite/pearlite FP which is soft and the martensite M which is hard. 
     In the steel plate member  20  according to the comparative example, as described above, the boundary region  23  has an unstable microstructure containing both the martensite M which is hard and the ferrite/pearlite FP which is soft. Thus, fracture is likely to occur in the boundary region between the hard region and the soft region. 
     Advantage Effects of Steel Plate Member According to First Embodiment and Method of Producing the Same 
     Next, the advantage effects of the steel plate member according to the first embodiment and the method of producing the steel plate member will be described. As described above, the steel plate member  20  according to the comparative example, which is shown in  FIG. 5 , has the boundary region  23  having an unstable microstructure containing both the martensite M which is hard and the ferrite/pearlite FP which is soft. 
     By contrast, as shown in  FIG. 3 , the boundary region  13  of the steel plate member  10  according to the first embodiment has a microstructure consisting of the tempered martensite TM. This can prevent fracture in the boundary region  13  between the first region  11  which is hard and the second region  12  which is soft. In the vicinity of the first region  11  in the boundary region  13 , both the tempered martensite TM and the martensite M are present; however, fracture can be prevented because of a small difference in hardness between the tempered martensite TM and the martensite M adjacent to each other. 
     Furthermore, the boundary region  13  of the steel plate member  10  according to the first embodiment is heated to a higher temperature and thus becomes softer with decreasing distance from the second region  12  heated in the local tempering step. That is, the boundary region  13  of the steel plate member  10  according to the first embodiment becomes softer gradually in the direction from the first region  11 -side which is hard to the second region  12 -side which is soft. Thus, fracture in the boundary region  13  between the first region  11  which is hard and the second region  12  which is soft can be more effectively prevented. 
     In the steel plate member  20  according to the comparative example, the second region  22  which is a soft region contains the ferrite/pearlite FP, while in the steel plate member  10  according to the present embodiment, the second region  12  which is a soft region contains the martensite M and the tempered martensite TM. The second region  12  of the steel plate member  10  according to the present embodiment is therefore not as soft as the second region  22  of the steel plate member  20  according to the comparative example. However, the second region  12  is soft enough to prevent fracture which may occur after joining to another metal member. 
     The second region  12  of the steel plate member  10  according to the present embodiment has a mixed structure, just as does the boundary region  23  of the steel plate member  20  according to the comparative example. However, the difference in hardness between the martensite M and the tempered martensite TM in the second region  12  of the steel plate member  10  according to the present embodiment is smaller than the difference in hardness between the martensite M and the ferrite/pearlite FP in the boundary region  23  of the steel plate member  20  according to the comparative example. Fracture is therefore not likely to occur in the second region  12  either. 
     Second Embodiment 
     Method of Producing Steel Plate Member 
     Next, a method of producing a steel plate member according to a second embodiment will be described with reference to  FIG. 6 .  FIG. 6  is a schematic perspective view of an induction heating apparatus used in the method of producing a steel plate member according to the second embodiment. In the method of producing a steel plate member according to the first embodiment, the way of heating the steel plate member  10  in the local tempering step is not particularly limited. By contrast, in the method of producing a steel plate member according to the second embodiment, an induction heating apparatus  30  shown in  FIG. 6  is used to heat the steel plate member  10  in the local tempering step. The other features of the second embodiment are the same as those of the first embodiment and will therefore not be described in detail. 
     As shown in  FIG. 6 , the induction heating apparatus  30  is a high-frequency induction heating apparatus including a coil  31  and a high-frequency power source  32 . As shown in  FIG. 6 , the coil  31  is a horizontally-positioned plate member having a U-shaped cross-section. The high-frequency power source  32  is connected to the two open ends of the coil  31 . Only the second region  12  of the steel plate member  10  is inserted inside the coil  31  and inductively heated to a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3 . Being placed outside the coil  31 , the first region  11  of the steel plate member  10  is not inductively heated and is not thermally affected by heat conduction from the second region  12 . The boundary region  13  between the first region  11  and the second region  12  is heated to a temperature lower than the austenite transformation start temperature A 1  by heat conduction from the second region  12  and thermally affected. 
     In the high-frequency induction heating, the heating efficiency abruptly drops at the Curie point where the steel plate member  10  loses the magnetic properties, and thus the increase in the temperature of the heated region becomes difficult around the Curie point. Since austenite is non-magnetic while martensite, ferrite, and pearlite are ferromagnetic, the Curie point lies between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3 . 
     Thus, with the use of high-frequency induction heating, only the second region  12  of the steel plate member  10  can be rapidly heated, and the temperature to which the second region  12  is heated can be easily and accurately kept at a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3 . Since rapid heating of only the second region  12  of the steel plate member  10  is possible, the boundary region  13  thermally affected by heat conduction from the second region  12  can be narrowed. Thanks to the fact that the temperature to which the second region  12  is heated can be easily and accurately kept at a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3 , variation in the structure of the tempered second region  12  can be reduced. 
       FIG. 7  is a temperature chart for induction heating of steel plates having different thicknesses. The abscissa represents the time, and the ordinate represents the temperature. Both of the steel plates PL 1  and PL 2  are steel plates for hot stamping which are made of manganese-boron steel (22MnB5 steel). The steel plate PL 2  is thicker by about 1 mm than the steel plate PL 1 . It thus takes a longer time to increase the temperature of the steel plate PL 2  than to increase the temperature of the steel plate PL 1 . However, the use of high-frequency induction heating led to successful rapid heating of both of the steel plates PL 1  and PL 2 . Additionally, for the both of the steel plates PL 1  and PL 2  having different thicknesses, the temperature to which the second region was heated was easily and accurately kept at a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3  by virtue of the use of high-frequency induction heating. 
     Configuration of Steel Plate Member 
     Next, the configuration of an example of the steel plate member according to the second embodiment will be described with reference to  FIGS. 8A and 8B .  FIGS. 8A and 8B  are perspective views of an example of the steel plate member according to the second embodiment. The steel plate member according to the second embodiment is a steel plate member produced by the method of producing the steel plate member according to the second embodiment. A steel plate member  50  shown in  FIGS. 8A and 8B  is a steel plate member for a pillar as a component of an automobile body and is specifically a center pillar reinforcement. In  FIGS. 8A and 8B  are shown the steel plate member  50  that has yet to be subjected to the local tempering step and the steel plate member  50  that has been subjected to the local tempering step, and the shape of the steel plate member  50  remains the same before and after the local tempering step. 
     It should be understood that the right-handed xyz Cartesian coordinate system shown in  FIGS. 8A and 8B  is one defined for convenience of explanation of the positional relationship among the constituting elements of the steel plate member. The intended use and shape of the steel plate member  50  shown in  FIGS. 8A and 8B  are only illustrative, and the intended use and shape of the steel plate member according to the present embodiment are not limited in any respect. 
     As shown in  FIGS. 8A and 8B , the steel plate member  50  according to the second embodiment includes a body portion  51 , an upper flange portion  52 , and a lower flange portion  53 . As shown in  FIGS. 8A and 8B , the body portion  51  is a portion having a hat-shaped cross-section and includes a top plate  51   a  extending lengthwise in the z-axis direction, side walls  51   b , and flange portions  51   c . More specifically, a pair of side walls  51   b  extend perpendicularly from the ends in the width direction (direction of the x-axis) of the top plate  51   a  extending lengthwise in the z-axis direction. Further, the flange portions  51   c  project outwardly from the ends in the height direction (direction of the y-axis) of each side wall  51   b.    
     The body portion  51  is slightly curved such that the body portion  51  bulges as a whole in the negative direction of the y-axis. Additionally, the upper end portion (end portion in the positive direction of the z-axis) and the lower end portion (end portion in the negative direction of the z-axis) of the body portion  51  are widened in the width direction (direction of the x-axis) and are T-shaped in plan view. The degree of widening in the width direction is greater in the lower end portion than in the upper end portion. 
     The upper flange portion  52  includes a plate surface extending perpendicularly from the upper end portion of the body portion  51  in the negative direction of the y-axis and a plate surface projecting from the end in the height direction (direction of the y-axis) of the above plate surface outwardly in the length direction of the body portion  51  (in the positive direction of the z-axis). Thus, the upper flange portion  52  is a portion extending lengthwise in the width direction (direction of the x-axis) to present an L-shaped cross-section. The lower flange portion  53  is a flat plate-shaped portion projecting from the lower end of the top plate  51   a  outwardly in the length direction (in the negative direction of the z-axis) and extending lengthwise in the width direction (direction of the x-axis). 
     The steel plate member  50  according to the second embodiment, which is shown in  FIGS. 8A and 8B , is produced by press-forming a flat plate in the quenching step described above. Thus, the microstructure of the steel plate member  50  shown in  FIG. 8A  which has yet to be subjected to the local tempering step contains the martensite M over the entire steel plate member  50 . 
     In the steel plate member  50  shown in  FIG. 8B  which has been subjected to the local tempering step, a second region  12   a  containing the martensite M and the tempered martensite TM is formed in a band shape at the outer edge of each of the pair of flange portions  51   c  of the body portion  51 . That is, in the local tempering step, the second region  12   a  is locally heated by induction heating. Inwardly of the second region  12   a  is formed a boundary region  13   a  consisting of the tempered martensite TM. 
     Likewise, a second region  12   b  containing the martensite M and the tempered martensite TM is formed in a band shape at the outer edge of the upper flange portion  52 . That is, in the local tempering step, the second region  12   b  is locally heated by induction heating. Inwardly of the second region  12   b  is formed a boundary region  13   b  consisting of the tempered martensite TM. 
     Likewise, a second region  12   c  containing the martensite M and the tempered martensite TM is formed in a band shape at the outer edge of the lower flange portion  53 . That is, in the local tempering step, the second region  12   c  is locally heated by induction heating. Inwardly of the second region  12   c  is formed a boundary region  13   c  consisting of the tempered martensite TM. 
     In the steel plate member  50  shown in  FIG. 8B  which has been subjected to the local tempering step, the region other than the second regions  12   a ,  12   b , and  12   c  and the boundary regions  13   a ,  13   b , and  13   c  is the first region  11  containing the martensite M. 
     The steel plate member  50  thus produced is joined to another metal member by spot welding or other means at the soft second regions  12   a ,  12   b , and  12   c  provided in the flange portion  51   c , upper flange portion  52 , and lower flange portion  53 , respectively. Joining at the soft regions rather than at hard regions can prevent post-joining fracture. Since the flange portion  51   c , upper flange portion  52 , and lower flange portion  53  are provided at the ends of the steel plate member  50 , the soft regions can be easily formed by induction heating. 
     Additionally, in the steel plate member  50 , the second region  12   a  is formed in a band shape which, as shown in  FIG. 8A , extends over the entire length of the body portion  51 . Thus, there may be a risk that the cooling process in the local tempering step causes a change in the dimensions of the steel plate member  50  and hence a deterioration of the dimensional accuracy. In the method of producing a steel plate member according to the present embodiment, the cooling rate in the cooling process in the local tempering step is high, which permits the cooling to be performed with the steel plate member  50  being held by a cooling mold or other means. This makes it possible to reduce the dimensional change of the steel plate member  50  during the cooling process in the local tempering step. 
     Examples 
     The following describes an Example of the second embodiment in which the steel plate member is inductively heated in the local tempering step. The steel plate member used was a quenched steel plate for hot stamping which was made of manganese-boron steel (22MnB5 steel) and which had a thickness of 2.0 mm, a width of 100 mm, and a length of 300 mm. 
       FIG. 9  is a temperature chart illustrating the conditions of the local tempering according to the Example of the second embodiment. The abscissa represents the time and the ordinate represents the temperature. In  FIG. 9  are shown temperature profiles in the four regions, namely the first region  11 , the second region  12 , the first region-side of the boundary region  13 , and the second region-side of the boundary region  13 . As shown in  FIG. 9 , only the second region  12  of the steel plate member was heated to a temperature between the austenite transformation start temperature A 1  and the austenite transformation finish temperature A 3 , and then the steel plate member was cooled at a cooling rate higher than the upper critical cooling rate to cause martensite transformation of the second region  12 . 
     As described in detail below, the local tempering according to the Example resulted in formation of a hard region containing the martensite M in the first region  11  shown in  FIG. 9 . In the second region  12  shown in  FIG. 9  was formed a soft region containing the martensite M and the tempered martensite TM. Both the first region-side of the boundary region  13  and the second region-side of the boundary region  13  which are shown in  FIG. 9  had a microstructure consisting of the tempered martensite TM. The second region-side of the boundary region  13 , which was heated to a higher temperature than the first region-side of the boundary region  13 , had lower hardness than the first region-side of the boundary region  13 . 
       FIG. 10  and  FIG. 11  show the hardness distribution and photographs of the microstructure of the steel plate member produced by the method of producing the steel plate member according to the Example of the second embodiment, respectively.  FIG. 10  is a graph showing the hardness distribution of the steel plate member produced by the method of producing the steel plate member according to the second embodiment.  FIG. 11  shows photographs of the microstructure of the steel plate member produced by the method of producing the steel plate member according to the second embodiment. 
     As shown in  FIG. 10 , a boundary region  13  is formed between a hard first region  11  having a high Vickers hardness (HV) and a soft second region  12  having a low Vickers hardness (HV). As shown in  FIG. 10 , the boundary region  13  thermally affected by heat conduction from the second region  12  was able to be narrowed to about 40 mm by the use of high-frequency induction heating. It was also found that the boundary region  13  became softer gradually in the direction from the hard first region  11  having a hardness of about 480 HV to the soft second region  12  having a hardness of about 300 HV. In the vicinity of the second region  12  in the boundary region  13  consisting of tempered martensite, the hardness decreased to about 250 HV, while in the second region  12  containing martensite and tempered martensite, the hardness increased to about 300 HV. 
     As shown in  FIG. 11 , the microstructure of the first region  11  was a structure containing martensite. The microstructure of the second region  12  was a mixed structure of martensite and tempered martensite. The microstructure of the boundary region  13  was a structure consisting of tempered martensite. Thus, microstructures similar to the microstructures shown in  FIG. 3  were actually obtained. 
       FIG. 12  is a graph showing the hardness and critical bending angle of the boundary region  13  of the steel plate member according to the Example in comparison with those of a boundary region  23  of a steel plate member according to a comparative example. The bending test for determining the critical bending angle (deg) was conducted in accordance with VDA 238-100, a standard specified by German Association of the Automotive Industry. As shown in  FIG. 12 , the Vickers hardness (HV) of the boundary region  13  in the Example was comparable to the Vickers hardness (HV) of the boundary region  23  in the comparative example. 
     As shown in  FIG. 12 , the critical bending angle (deg) of the boundary region  13  in the Example was twice or more the critical bending angle (deg) of the boundary region  23  in the comparative example, which demonstrated a significant improvement in bendability. The reason for the low bendability of the boundary region  23  in the comparative example is that, as described with reference to  FIG. 5 , the boundary region  23  has an unstable microstructure containing both the martensite M which is hard and the ferrite/pearlite FP which is soft. The high bendability of the boundary region  13  in the Example can be attributed to the fact that the boundary region  13  has a structure consisting of tempered martensite. 
     The present disclosure is not limited to the embodiments described above, and modifications can be made as appropriate without departing from the gist of the present disclosure.