JP6173873B2 - Steel plate heating method - Google Patents

Description

  The present invention relates to a method for heating a steel sheet in which a steel sheet having a non-uniform cross-sectional area perpendicular to the energizing direction is heated by direct current heating.

  Hot pressing or the like that transforms the structure of a heated steel sheet by rapid cooling and forms it into a specific shape requires heating the steel sheet to austenite completion temperature (Ac3) or higher. Direct current heating, which is a heating means for the steel plate in such a case, has the advantage of easily improving the productivity because the heating time is short, but the steel plate with a non-uniform cross-sectional area perpendicular to the current supply direction is uniform. Since it cannot heat, it has the fault that a utilization object is restrict | limited. From this, various steel plate heating methods have been proposed in order to uniformly heat a steel plate having a non-uniform cross-sectional area perpendicular to the energization direction by direct current heating (Patent Document 1 and Patent Document 2).

  Patent Document 1 discloses a method for heating a steel sheet in which the temperature distribution of the steel sheet is adjusted by adjusting the power supplied to the electrode attached to the steel sheet (blank) having a non-uniform cross-sectional area perpendicular to the energization direction. Is disclosed (Patent Document 1 [Claim 1]). The electric power supplied to the electrode controls the strength of the electric current supplied to the electrode by adding a resistance between the power source and the electrode (Patent Document 1 [Claim 2]), or the energization time of the electric current supplied to the electrode (Patent Literature 1 [Claim 3]) to adjust. In order to locally heat the periphery of the part where the electrode is in contact, the tip of the electrode has a spherical shape with a radius of 20 mm or less (Patent Document 1 [Claim 4]).

  Patent Document 2 discloses that the steel plates sandwiched between the pair of bar electrodes and the widths of the pair of bar electrodes arranged at opposite ends of a steel plate (blank) having a non-uniform cross-sectional area perpendicular to the energizing direction are made the same. Disclosed is a heating method (electric heating method) of a steel sheet in which the region is formed into a rectangle (Patent Document 1 [Claim 1]). A steel plate having a non-uniform cross-sectional area perpendicular to the energizing direction is provided with an auxiliary portion that protrudes outward from an end portion, and the steel plate is divided into a plurality of rectangles by electrodes sandwiching the auxiliary portion (Patent Literature 1 and [Claims]. 2]). In the pair of bar electrodes assigned to each rectangle, one side of the pair of bar electrodes having the maximum inter-electrode distance is used as a ground electrode, and the potential applied to each electrode is linear with respect to the distance from the ground electrode. (Patent Document 1 [Claim 3]).

Japanese Patent Laid-Open No. 2002-248525 JP 2011-183418

  The heating method of the steel sheet disclosed in Patent Document 1 and Patent Document 2 is to assign a plurality of electrodes to a steel sheet having a non-uniform cross-sectional area orthogonal to the energizing direction, and to control the magnitude of current flowing between the electrodes and the energizing time. Similar in that the entire steel sheet is heated. However, since the Joule heat that heats the steel plate is proportional to the square of the current, the change in the Joule heat that occurs even if the current intensity or energization time is slightly increased or decreased increases, and the cross-sectional area perpendicular to the energization direction is uniform. It is difficult to uniformly heat the entire steel sheet, which is considered to cause uneven heating. This is a problem that cannot be ignored as the number of electrodes increases.

  In addition, the current intensity and energization time must be readjusted only by slightly changing the arrangement position of each electrode with respect to the steel sheet having a non-uniform cross-sectional area perpendicular to the energization direction. As described above, since the Joule heat for heating the steel plate is proportional to the square of the current, readjustment is difficult, and there is a possibility that the waiting time until heating the steel plate having a non-uniform cross-sectional area perpendicular to the energizing direction may be increased. is there. This means that there is a possibility of losing the advantage of direct current heating that can be heated in a short time in actual production site use, and thus causes a problem of lowering productivity.

  In addition, a heating device with a large number of electrodes has a complicated structure, which increases the manufacturing cost including the control system, shortens the mean time between failures due to the increased number of failure points, and increases the operating cost. There are also problems that follow. Thus, the heating method of the steel sheet disclosed in Patent Document 1 or Patent Document 2 using a plurality of electrodes is a problem in which unevenness in heating occurs, a problem that impairs the advantage of increasing productivity, and the manufacturing cost and operation of the heating device. There was a problem of high costs. Therefore, in order to solve such problems, a study was made to develop a method for heating a steel sheet in which a steel sheet having a non-uniform cross-sectional area perpendicular to the direction of current conduction is heated directly by current heating.

  What has been developed as a result of the study is a method of heating a steel sheet that directly heats a steel sheet having a non-uniform cross-sectional area perpendicular to the energizing direction by dividing the steel sheet in the energizing direction connecting a pair of electrodes sandwiching the steel sheet. In the first step, the entire steel sheet is heated below the austenite transformation start temperature (Ac1), and in the second step, the temperature drop is inversely proportional to the cross-sectional area perpendicular to the energizing direction of each cooling region. Cooling with a width forms a temperature gradient that descends from the largest to the smallest of the cross-sectional area. In the third step, the entire steel sheet is energized in the direction of energization to bring the steel sheet above the austenite transformation completion temperature (Ac3). It is a heating method of a steel plate that uniformly heats the whole.

  The cooling region set by “dividing the steel plate in the energizing direction connecting a pair of electrodes sandwiching the steel plate” is a cooling unit divided by a virtual boundary line orthogonal to the energizing direction, and each cooling unit or A cooling means corresponding to a plurality or all of them is assigned, and it means a certain range that is cooled with different temperature drop widths. The length of each cooling region in the energization direction is determined by the assigned cooling means so that a temperature gradient continuous with the adjacent cooling region is formed. “Heating the entire steel sheet below the austenite transformation start temperature (Ac1)” may be a target temperature that does not exceed the austenite transformation start temperature (Ac1), but preferably the limit not exceeding the austenite transformation start temperature (Ac1). Heat to the preheating temperature (PT) set as high as possible.

  “Cross-sectional area orthogonal to the energizing direction of each cooling region” is the average value of either one of both end surfaces orthogonal to the energizing direction of the cooling region or the average value of the both end surfaces when the set cooling region is cut out from the steel sheet. . When the size of the both end surfaces of the cooling region is hardly changed, the end surfaces on the same side in all the cooling regions are compared as cross-sectional areas representing the respective cooling regions. However, when the difference in the size of both end faces of the cooling region cannot be ignored, each cooling region compares the average value of the both end surfaces as a cross-sectional area representing each cooling region. Each cooling region can be freely set in the length in the energization direction, so that the length is shortened to make the end faces almost the same size, or the length is increased to make a difference in the end face sizes. It is good also considering the average value of both as a cross-sectional area of each cooling area | region.

  “Cooling with the temperature drop width inversely proportional to the cross-sectional area (perpendicular to the energization direction)” means that if the cross-sectional area orthogonal to the energization direction is large, the temperature drop width from the temperature heated in the first step is small, and If the cross-sectional area orthogonal is small, it means that the temperature drop from the temperature is large. There are as many temperature drop widths as there are cooling areas. For example, if there are four cooling areas, four different temperature drop widths are set. In this case, the difference in the temperature drop width set in each adjacent cooling region may be the same or different. `` Temperature gradient descending from a large cross-sectional area to a small area '' means that the temperature at the large cross-sectional area is high and the temperature is lowered toward the small cross-sectional area and the temperature is relatively low To do.

  The method for heating a steel sheet of the present invention is a place where the heated steel sheet is easily heated by direct current heating before starting the austenite transformation = cooling a cooling region having a small cross-sectional area perpendicular to the current-carrying direction with a large temperature drop width, In addition, it is difficult to be heated by direct current heating = a cooling region having a large cross-sectional area orthogonal to the current supply direction is cooled or not cooled with a small temperature drop width (in this case, the temperature drop width is “0”), thereby orthogonal to the current supply direction. A temperature gradient descending from a large cross-sectional area toward a small area is formed on the entire steel sheet. As a result, when energized again and heated, the entire steel sheet is uniformly heated to the austenite transformation completion temperature (Ac3) or higher, for example, the quenching temperature (QT) set to the austenite transformation completion temperature or higher.

  In the first step, the entire steel sheet is heated to a temperature lower than the austenite transformation start temperature (Ac1). When the heating temperature is not uniform throughout the steel sheet, the maximum temperature may be the austenite transformation start temperature (Ac1). From this, the 1st process can heat the whole steel plate below austenite transformation start temperature (Ac1) by direct current heating, furnace heating, infrared heating, or induction heating. In direct current heating, a steel sheet is heated by Joule heat generated by energizing a pair of electrodes sandwiching the steel sheet. In the furnace heating, the steel sheet is heated in an electric furnace or the like. Infrared heating heats a steel plate by irradiating high-energy infrared rays. In the induction heating, the steel sheet is heated by Joule heat generated by electromagnetic induction applied to the steel sheet. From the viewpoint of reducing the manufacturing cost, it is preferable to use direct current heating used in the third step also in the first step.

  In the second step, the cooling means is not limited as long as the cooling range can be varied for each cooling region. For example, in the second step, the temperature of the cooling gas blown to each cooling region is made different, or the amount of the cooling gas blown to each cooling region is made different so that the cross-sectional area perpendicular to the energizing direction is changed from a large part to a small part. Create a temperature gradient down. The cooling gas may be a gas having a temperature relatively lower than that of the steel plate heated to a temperature lower than the austenite transformation start temperature (Ac1). Since such cooling gas diffuses from the sprayed point to the surroundings, the temperature drop width continuously varies from a large cross-sectional area perpendicular to the energizing direction to a small one, and a continuous temperature gradient can be formed. When the temperature or the air volume of the cooling gas is different, the heat capacity of the cooling gas that affects the temperature drop width is increased or decreased.

  In the second step, the temperature of the cooling block pressed against each cooling region is varied, or the contact time of the cooling block pressed against each cooling region is varied so that the cross-sectional area perpendicular to the energization direction is large. A temperature gradient that descends from a small point toward a small point may be formed. The cooling block may be made of a metal or a ceramic having a high thermal conductivity, and may be a mass having a temperature relatively lower than that of a steel plate heated to a temperature lower than the austenite transformation start temperature (Ac1). Since such a cooling block has a larger specific heat than the cooling gas, the temperature drop time can be shortened. If the temperature of the cooling block is different, a difference in the temperature gradient of the temperature drop occurs, and the temperature drop width at the same time can be different. In addition, when the contact time of the cooling block is different, a difference occurs in the amount of heat exchanged, and a difference in temperature drop during the same time can be made.

  In the third step, the cooling means is not limited as long as the entire steel sheet can be rapidly cooled. Thus, in the third step, the present invention can be used for hot pressing if the entire steel sheet is formed into a product shape by a forming die pressed against the entire steel sheet and the entire steel sheet is rapidly cooled by heat conduction to the forming mold. If the forming die is provided with a cooling gas outlet and a recovery port, the forming die can also be used in the second step using cooling gas as long as a steel plate is not formed. Further, if the forming die is divided and pressed against the steel plate, the forming die can be used for the second step using the cooling block.

  The present invention provides a method for heating a steel sheet in which a steel sheet having a non-uniform cross-sectional area perpendicular to the energizing direction is heated by direct current heating. This is because conventional automotive parts that have been difficult to heat using direct current heating, such as pillars that gradually increase in cross-sectional area, also use direct current heating to produce austenite transformation in a short time without uneven heating. It means that it can be heated to the completion temperature (Ac3). Thereby, the utilization range of the hot press processing using direct electric heating can be expanded, and the productivity of the automotive member made of steel plate to be quenched can be improved.

  In the heating method of the steel sheet of the present invention, in the second step, the steel sheet is cooled with a temperature drop width that is inversely proportional to the cross-sectional area orthogonal to the energizing direction of each cooling region, and a temperature gradient that decreases from a large cross-sectional area to a small area is formed. It is characterized in that At this time, even if the cooling region is intermittently cooled in the energization direction and the temperature gradient is given, the temperature gradient is not stable due to local thermal equilibrium in the steel plate heated to a temperature lower than the austenite transformation start temperature (Ac1) in advance. In addition to equalization, the entire steel sheet is immediately heated to the austenite transformation completion temperature (Ac3) or higher, so heating unevenness does not become a problem.

  Further, in the method for heating a steel plate of the present invention, in the second step, a time for once cooling the steel plate is added, but the heating of the steel plate prior to the cooling is less than the austenite transformation start temperature (Ac1). Therefore, even if the heating and cooling are combined, it does not take much time, so the processing time is almost the same as that of conventional direct current heating that simply heats to the austenite transformation completion temperature (Ac3) or higher. . For this reason, the productivity of direct current heating is not impaired by using the present invention, but rather the effect of expanding the application target for improving the productivity by using direct current heating is obtained.

  And the heating method of the steel plate of this invention is the same as the number of the electrodes used for direct current heating only by adding the cooling means for cooling a part in a 2nd process. Such control is also simple, for example, it is only necessary to switch between energization and cutoff, so that the manufacturing cost of the heating device is not increased so much. In addition, since the number of electrodes is the same as the conventional one and the control of the cooling means and the electrodes is simple, the operating cost of the heating device can be sufficiently suppressed. As described above, the present invention has an effect of expanding the application range of the direct current heating, that is, the hot press processing using the direct current heating, while suppressing the manufacturing cost and the operation cost.

  The feature of the present invention lies in the second step of forming a temperature gradient in the energizing direction after heating the entire steel sheet to less than the austenite transformation start temperature (Ac1), and thus heating below the austenite transformation start temperature (Ac1) is a heating means. It doesn't matter. For this reason, in addition to using direct current heating in accordance with the third step, it is possible to use furnace heating capable of batch processing at low operating costs, or use infrared heating or induction heating in combination with other production facilities. . Such extensive use of the heating means in the first step provides the advantage of making the construction of production equipment flexible when applying the present invention.

  Since the cooling gas that is the cooling means in the second step has a small specific heat, it is difficult to provide a temperature difference compared to the cooling block. However, since it diffuses to the surroundings from the sprayed point, one cooling gas injection nozzle is provided at a time. Even if the cooling regions to be allocated are arranged in large divided units that are long in the energizing direction, it is easy to form a smoothly continuous temperature gradient in the energizing direction. On the other hand, the cooling block which is the cooling means of the second step has a large specific heat and uniformly cools the contact range, so it is difficult to make the temperature gradient smoothly continuous. Since the fluctuation of the temperature gradient is absorbed, there is no problem. Rather, the temperature gradient can be formed in a short time by increasing the temperature drop difference between adjacent cooling regions. The cooling gas and the cooling block may be used alone, or may be used in combination with the short and long of both.

  The method for heating a steel sheet of the present invention can be used for hot pressing by using a forming die as a cooling means after the third step. Hot pressing using the present invention can uniformly heat the entire steel sheet to the austenite transformation completion temperature (Ac3) even if the steel sheet has a non-uniform cross-sectional area perpendicular to the energizing direction. Heat treatment (perlite transformation, bainite transformation or martensitic transformation) can be performed. Thus, the method for heating a steel sheet according to the present invention realizes heat treatment of the steel sheet without causing uneven heating, without impairing productivity, and suppressing the manufacturing cost and operating cost of the heating device.

It is a graph showing an example of the heat processing pattern based on this invention. It is a graph showing another example of the heat processing pattern based on this invention. It is a top view showing the preparatory stage of the heat processing by a direct current heating apparatus. It is a top view showing the heating step of the 1st process by a direct energization heating device. It is a top view showing the heating step of the 1st process by an induction heating device. It is a top view showing the heating step of the 1st process by a furnace heating device. It is a top view showing the heating step of the 1st process by an infrared thermal device. It is a top view showing the temperature fall stage of the 2nd process by the air cooling of a cooling device. It is a side view showing the temperature-falling stage of the 2nd process by an air-cooling cooling device. It is a side view showing the temperature-falling stage of the 2nd process by the cooling device of heat conduction. It is a side view showing the heating step of the 3rd process by a direct energization heating device. It is a side view showing the temperature-falling stage of the hot press process by a shaping | molding die. It is a perspective view showing the steel plate shape of an Example. It is a graph showing distribution of the Vickers hardness of the length direction of the steel plate of an Example.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The “energization direction” in the following description means that the direct current heating used in the first step or the third step ignores the fact that the direction of current flow is reversed because the power source 22 is alternating current, and the axis of the current flowing in the steel plate 1 In each figure showing direct current heating, it is defined as the direction toward the left edge of the right electrode 21 with reference to the right edge of the left electrode 21 as viewed in the drawing, and heating other than direct current heating used in the first step It is also used to explain the means.

  As shown in FIG. 1 or FIG. 2, the heating method according to the present invention is a heat treatment in the first half in which a steel sheet 1 (see FIG. 3 and the following) whose cross-sectional area perpendicular to the energizing direction is not uniform is quenched by direct energization heating. Applied. This example is an example in which the steel sheet 1 having a non-uniform cross-sectional area perpendicular to the energizing direction of the direct current heating in the third step is uniformly quenched and hot pressed by the forming die 5 (see FIG. 12 below). is there. In order to simplify the description, the steel plate 1 of the present example has a constant thickness and a plane-view trapezoidal shape that is symmetrical with respect to an axis extending in the energizing direction. For this reason, the cross-sectional area is proportional to the plate width.

  In the case where the first step is direct current heating, the steel plate 1 having a non-uniform cross-sectional area perpendicular to the energizing direction has a small cross-sectional area because the electric resistance is high when the cross-sectional area is small (where the plate width is narrow). The temperature rises so as to form a temperature gradient TG1 (see FIG. 4 below) that rises from a large cross-sectional area toward a small one. For example, the point A upstream in the energizing direction, the point B in the middle, and the point C downstream (see FIG. 3 and subsequent figures) are heated (the temperature rising curve is separated in FIG. 1), and the cross-sectional area When the smallest point A reaches the preheating temperature (PT) set below the austenite transformation start temperature (Ac1), the energization is stopped and the process proceeds to the second step.

  Further, when the first step is induction heating and magnetic lines of force are formed in the energizing direction of the steel plate 1, a temperature gradient TG1 rising from a large cross-sectional area to a small one as in the case of the direct energization (see FIG. 5 described later). ). However, when the first step is induction heating and magnetic lines of force perpendicular to the surface of the steel plate 1 are formed, a partial temperature increase difference of the steel plate 1 does not occur, so that the temperature gradient TG1 in the energization direction can be made constant. In this case, although depending on the configuration of the induction heating device, the entire steel plate 1 cannot often be heated at one time. For example, the steel plate 1 is sequentially heated in several sections while moving the induction heating device.

  On the other hand, when the first step is furnace heating or infrared heating, even if the steel plate 1 has a non-uniform cross-sectional area perpendicular to the energizing direction, a partial temperature increase difference of the steel plate 1 does not occur. A similar temperature gradient TG1 is formed (see FIG. 6 or FIG. 7 below). For example, the points A, B, and C (see FIG. 3 and subsequent figures) that are separated in the energization direction rise uniformly in temperature (FIG. 1, the temperature curves match), and the points A and B When all the points C reach the preheating temperature (PT), the heating is stopped, and the process proceeds to the second step.

  When the first step is direct current heating, both ends of the long steel plate 1 are sandwiched between the electrodes 21 and 21 of the direct current heating device 2 as shown in FIG. The electrodes 21 and 21 are connected by a power source (AC power source) 22, and a current flows directly to the steel plate 1 through the electrodes 21 and 21. Thereby, as shown in FIG. 4, the steel sheet 1 generates Joule heat having itself as a resistor, and the range sandwiched between the electrodes 21 and 21 is heated. The portion sandwiched between the electrodes 21 and 21 remains as a non-heat treated region (non-quenched portion). In the steel sheet 1 targeted in this example, the direction in which the current flows = the cross-sectional area orthogonal to the energizing direction is not uniform, so the generated Joule heat increases in inverse proportion to the cross-sectional area.

  For this reason, when the first step is direct current heating, the point A of the small cross-sectional area is higher than the point C of the small cross-sectional area, and the point B in between is heated to an intermediate temperature. As a result, a temperature gradient TG1 that rises from a large cross-sectional area to a small one is formed (see the temperature distribution graph in FIG. 4). In this example, since the trapezoidal steel plate 1 having a different cross-sectional area in proportion to the plate width is heated, the temperature gradient TG1 is a straight line, but the cross-sectional area is irregularly increased or decreased in the energizing direction. In the case of a steel plate, the temperature gradient TG1 is also a curve that increases and decreases.

  When the first process is induction heating, as shown in FIG. 5, the steel sheet 1 is carried into the induction heating device 6 incorporating the induction coil 61 by the conveyance line 63, and the induction coil 61 is supplied by the current supplied from the power source 62. The magnetic field formed by the magnetic field generates an induced current in the direction of energization of the steel plate 1. Thereby, the steel plate 1 generates Joule heat using itself as a resistor. The steel plate 1 targeted in this example has a non-uniform cross-sectional area perpendicular to the direction in which the current flows = the direction of energization. Therefore, as with direct energization heating, the generated Joule heat increases in inverse proportion to the cross-sectional area.

  For this reason, even when the first step is induction heating, the point A having a small cross-sectional area is higher than the point C having a small cross-sectional area, and the point B, which is between the two, is heated to an intermediate temperature. Thus, a temperature gradient TG1 that rises from a large cross-sectional area toward a small area is formed (see the temperature distribution graph in FIG. 5). In this example, since the trapezoidal steel plate 1 having a different cross-sectional area in proportion to the plate width is heated, the temperature gradient TG1 is a straight line, but the cross-sectional area is irregularly increased or decreased in the energizing direction. In the case of a steel plate, the temperature gradient TG1 is also a curve that increases and decreases.

  When the first step is furnace heating, as shown in FIG. 6, for example, the steel plate 1 housed in the furnace heating device 3 incorporating an electric heater (not shown) heated by a current supplied from the power source 31 is heated. . The heating source may be a gas heater. Although furnace heating takes time compared with direct current heating or induction heating, the furnace heating is excellent in that a plurality of steel plates 1 can be heated at one time (in FIG. 6, two steel plates 1 are used). heating). Moreover, since all the steel plates 1 are all heated uniformly, a constant temperature at which the points A, B and C are heated to the same preheating temperature (PT) regardless of the cross-sectional area. A gradient TG1 is formed (see the temperature distribution graph in FIG. 6). The same applies to irregular steel sheets whose cross-sectional area increases or decreases in the energizing direction.

  When the first process is infrared heating, as shown in FIG. 7, the steel plate 1 is carried by the transfer line 73 to the infrared heating device 7 including the infrared irradiation unit 71 connected to the power source 72, and the infrared irradiation field The steel plate 1 is heated by irradiating the steel plate 1 with infrared rays. Infrared heating, although the time for heating the steel sheet 1 takes longer than that for direct current heating or induction heating, does not take the time required for furnace heating, and the entire steel sheet 1 can be heated uniformly. Specifically, regardless of the cross-sectional area, a constant temperature gradient TG1 is formed in which the points A, B and C are heated to the same preheating temperature (PT) (see the graph of temperature distribution in FIG. 7). ). The same applies to irregular steel sheets whose cross-sectional area increases or decreases in the energizing direction.

  As shown in FIG. 8, the steel plate 1 heated to the preheating temperature (PT) in the first step by direct current heating has the set first cooling region 11, second cooling region 12, and third temperature. Cooling is performed by changing the temperature drop width of each cooling region 13 to form a temperature gradient TG2 in which the inclined temperature gradient TG1 formed in the first step is reversed. If the temperature gradient TG1 is tilted by direct current heating (Fig. 4) or induction heating (Fig. 5), the temperature gradient TG2 having the opposite gradient with the above temperature difference is changed so that the temperature difference caused by heating changes in the direction of current flow. Form. On the other hand, if the temperature gradient TG1 is constant by furnace heating (FIG. 6) or infrared heating (FIG. 7), the temperature difference caused by the heating is measured in advance, and the temperature gradient TG2 having a reverse slope with the temperature difference is measured. Form.

  In this example, as shown in FIG. 9, the cooling device 4 including the first air blowing blocks 41, 41, the second air blowing blocks 42, 42, and the third air blowing blocks 43, 43 facing each other with the steel plate 1 interposed therebetween. The first cooling region 11, the second cooling region 12, and the third cooling region 13 are each air-cooled. The first air blowing block 41, the second air blowing block 42, and the third air blowing block 43 are different in the number of air blowing nozzles 44 for blowing the same amount of air (cooling gas) 45, and the air 45 is blown onto the steel plate 1. The temperature drop width is different. The temperature of the air 45 blown out from each blower nozzle 44 of the first blower block 41, the second blower block 42, and the third blower block 43 may be varied to provide a difference in the temperature drop width.

  When the steel plate 1 is air-cooled by the air 45, the air 45 diffuses to the surroundings, and the steel plate 1 is cooled beyond the first cooling region 11, the second cooling region 12, and the third cooling region 13. Further, since the specific heat of the air 45 is small, only the portion where the air 45 collides is not rapidly cooled, but rather the cooling unevenness is mitigated by the high thermal conductivity of the steel plate 1. For this reason, the cooling device 4 of the present example has the first cooling region 11, the second cooling region 12 and the left cooling region (left half in FIGS. 8 and 6) in which the steel plate 1 having a particularly large temperature drop width is thinned. While only the third cooling region 13 is concentrated and air-cooled, a temperature gradient TG2 that smoothly inclines in the energization direction is formed using the diffusion of the air 45 and the heat conduction of the steel plate 1.

  In the second step, when it is desired to shorten the cooling time, for example, as shown in FIG. 10, the first cooling blocks 46 and 46 made of metal or ceramics facing each other with the steel plate 1 interposed therebetween, the second cooling blocks 47 and 47, and A cooling device 4 composed of third cooling blocks 48 and 48 is used. The first cooling block 46, the second cooling block 47 and the third cooling block 48 are pressed against the first cooling region 11, the second cooling region 12 and the third cooling region 13, respectively, and absorb the heat to rapidly cool the steel plate 1. To do. In this case, the first cooling block 46, the second cooling block 47, and the third cooling block 48 absorb heat from the pressed portion, and the only means for relaxing the temperature difference from the surroundings is the heat conduction of the steel plate 1. Although the temperature gradient TG2 inclined in the direction fluctuates, the rattling is absorbed by the subsequent heating in the third step and does not affect the heat treatment.

  In the third step, the steel plate 1 on which the temperature gradient TG2 (see FIG. 8) is formed is directly sandwiched between both ends of the steel plate 1 as shown in FIG. 11, as in the case where the first step is direct current heating. A current is passed directly from the power source 22 to the steel plate 1 via the electrodes 21 and 21 of the electric heating device 2, and the range sandwiched between the electrodes 21 and 21 reaches a quenching temperature (QT) higher than the austenite transformation completion temperature (Ac3). Heat. At this time, the point A having a small cross-sectional area has a higher temperature rise than the points B and C having a large cross-sectional area, but the temperature at the start of heating is lower than the points B and C. The points reach the quenching temperature (QT) equally at almost the same elapsed time.

  The steel plate 1 that has reached a uniform quenching temperature (QT) as a whole, after holding the quenching temperature (QT) for a predetermined quenching time (tq), as shown in FIG. When the formed mold 5 is pressed, a press-formed product 14 is obtained (hot press processing). The forming die 5 is formed by plastically deforming the steel plate 1 in accordance with the shape of the die, and at the same time absorbs abruptly heat from the steel plate 1 by heat conduction, and at least martensite transformation start temperature (Ms), preferably martensite transformation completion temperature ( The steel sheet 1 is quenched by lowering the temperature to MF).

  The steel sheet 1 to which the present invention is directed has a non-uniform cross-sectional area perpendicular to the energization direction, so that it is difficult to perform uniform quenching by direct energization heating. In the present invention, by forming a temperature gradient TG2 that takes into account the difference in temperature rise in the second step, the steel sheet 1 having a non-uniform cross-sectional area perpendicular to the energizing direction is uniformly set to the quenching temperature (QT), and the hardness To achieve quenching without unevenness. Thus, if the present invention is used, the steel plate 1 that has been difficult to use can be heated using direct current heating, so that the advantages of direct current heating (for example, improvement in productivity) can be enjoyed. .

  It was confirmed by the heat treatment method of the present invention whether or not a steel sheet having a non-uniform cross-sectional area perpendicular to the energizing direction could be uniformly quenched. As shown in FIG. 13, the steel plate used in the examples is a manganese boron steel (22MnB5) plate material suitable for quenching (austenite completion temperature Ac3 = 900 ° C.), length 280 m × width 148 mm (short). Side) to width 198 mm (long side) × thickness 1.4 mm. In the examples, the whole was uniformly heated to less than the austenite transformation start temperature (Ac1) by furnace heating, then the short side half was cooled to around 200 ° C. by air cooling, and then the alternating current was supplied to the length of the steel sheet by an energizing device. Energized in the vertical direction and heated above the austenite transformation completion temperature (Ac3), then sandwiched between metal blocks using a hot press machine, and the whole was uniformly cooled to the martensite transformation completion temperature (MF) .

  Specifically, the steel plate is divided into two in the energizing direction connecting a pair of electrodes sandwiching the steel plate, and a left half (lower left half in FIG. 13) cooling region is set. Then, as a first step, a muffle furnace (indirect The entire steel plate is uniformly heated to 600 ° C using a flame furnace, and then, as a second step, air at room temperature (around 23 ° C) is blown from the factory-installed blow gun (air gun) to the cooling area of the steel plate (left Half) Spray the surface evenly to cool it to 200 ° C, and in the third step, an alternating current with a frequency of 60 Hz and 950 A was directly applied to the steel sheet by means of a current-carrying device that sandwiched 40 mm from both ends in the longitudinal direction of the steel sheet. The whole is heated to 930 ° C over a period of time, and finally is sandwiched between metal blocks using a press machine. The entire steel plate is uniformly cooled to 200 ° C or less while applying a pressing pressure of 600 tons, and the whole is quenched. Site transformation).

  The results of measuring Vickers hardness at eight points (points a to h) on the axis connecting the intermediate points in the length direction and width direction (points a and h in FIG. 13) of the range in which the electrodes are in contact with each other. Shown in 14. The points a and h in the range where the electrodes are in contact with each other were not heat-treated and the hardness was 175 HV (Vickers hardness), whereas the quenched points b to g had a minimum hardness of 426 HV ( b point), the maximum hardness is within 451 HV (c point and d point) and the hardness difference is within 30 HV, and it can be seen that the hardness is almost uniform. From this, it has been confirmed that by utilizing the present invention for the heat treatment of a steel sheet, even the steel sheet having a non-uniform cross-sectional area perpendicular to the energizing direction can be uniformly quenched.

1 Steel plate
11 First cooling zone
12 Second cooling zone
13 Third cooling region 2 Direct current heating device 3 Furnace heating device 4 Cooling device
41 First blower block
42 2nd ventilation block
43 3rd ventilation block
44 Blower nozzle
45 Air
46 1st cooling block
47 Second cooling block
48 3rd cooling block 5 Mold 6 Induction heating device 7 Infrared heating device
Ac1 Austenite transformation start temperature
PT preheating temperature
Ac3 Austenite transformation completion temperature
QT quenching temperature
tq Quenching time
Ms Martensite transformation start temperature
Mf Martensite transformation completion temperature
TG1 First step temperature gradient
TG2 Temperature gradient of the second process

Claims (10)

A heating method for a steel sheet in which a steel sheet having a non-uniform cross-sectional area perpendicular to the energization direction is directly heated by energization heating,
Dividing the steel plate in the energizing direction connecting a pair of electrodes sandwiching the steel plate, and setting a plurality of cooling regions,
In the first step, the entire steel sheet is heated below the austenite transformation start temperature (Ac1)
In the second step, cooling is performed with a temperature drop width that is inversely proportional to the cross-sectional area orthogonal to the energization direction of each cooling region to form a temperature gradient that decreases from a large area of the cross-sectional area toward a small area,
The third step is a method for heating a steel sheet in which the entire steel sheet is energized in the direction of energization to uniformly heat the entire steel sheet to an austenite transformation completion temperature (Ac3) or higher. The method for heating a steel sheet according to claim 1, wherein the first step heats the entire steel sheet to less than the austenite transformation start temperature (Ac1) by direct current heating. The method for heating a steel sheet according to claim 1, wherein the first step heats the entire steel sheet to less than the austenite transformation start temperature (Ac1) by furnace heating. The method of heating a steel sheet according to claim 1, wherein the first step heats the entire steel sheet to less than the austenite transformation start temperature (Ac1) by infrared heating. The method for heating a steel sheet according to claim 1, wherein the first step heats the entire steel sheet to less than the austenite transformation start temperature (Ac1) by induction heating. The second step forms a temperature gradient that decreases from a large cross-sectional area perpendicular to the energizing direction to a small one by varying the temperature of the cooling gas blown to each cooling region. Heating method for steel sheet. 6. The second step according to any one of claims 1 to 5, wherein the second step forms a temperature gradient that decreases from a large cross-sectional area perpendicular to the energizing direction to a small one by varying the amount of cooling gas blown to each cooling region. Heating method for steel sheet. 6. The second step forms a temperature gradient that decreases from a large cross-sectional area perpendicular to the energizing direction to a small one by varying the temperature of the cooling block pressed against each cooling region. Method of heating steel sheet. The second step forms a temperature gradient that decreases from a large cross-sectional area perpendicular to the energizing direction to a small one by changing the contact time of the cooling block pressed against each cooling region. The heating method of the steel plate as described. The third step is a method for heating a steel plate according to any one of claims 1 to 9, wherein the whole steel plate is formed into a product shape by a forming die pressed against the whole steel plate, and the whole steel plate is rapidly cooled by heat conduction to the forming die.

Images