A temperature-raising bainite forming process

Abstract: The present invention has been made with a view to providing a temperature-raising bainite forming process for a steel material capable of reducing the time required for the thermal treatment and the cycle time of a thermal treatment device without necessitating any special means for handling the steel material. According to the present invention, the steel material is heated to a temperature higher than the austenitic transformation point, and quenched temporarily to an intermediate point temperature higher than the martensitic transformation point. Then, the temperature of the steel material is again raised towards the range corresponding to bainitic transformation to form a bainitic structure. Then, the step of raising the temperature is discontinued before the temperature corresponding to the austenitic transformation point is reached, and the steel material is quenched subsequently. In the above-described steps of heating and raising the temperature towards the range corresponding to bainitic transformation, only the portion to be treated by the bainite forming process is irradiated locally with a high-density energy beam.

Description: The present invention relates to a temperature-raising bainite forming process for treating a steel material to form a bainitic structure. It is conventionally known to transform the structure of the steel material into the bainitic structure in order to improve its extensibility, drawability, robustness, and the like. Conventionally, to obtain the bainitic structure, it has been proposed to execute bainitic hardening subsequently followed by bainitic tempering, an isothermal treatment such as an austempering process, and the like. According to the former method, that is, "bainitic hardening subsequently followed by bainitic tempering", the steel material is first heated to a temperature higher than an austenitic transformation point temperature and then quenched to a temperature lower than a martensitic transformation point temperature, thereby accomplishing the bainitic hardening temporarily. Then, the hardened steel material is again heated to a temperature range corresponding to bainitic transformation to generate bainitic structure. According to the latter method, that is, the austempering process, as shown with a dotted line 39 in Fig.1, the steel material is first heated to a temperature higher than the austenitic transformation point temperature and then quenched to a temperature higher than the martensitic transformation point temperature. Then, the temperature reached at this time is kept as it is over a long period of time such as one to five hours, until the S curve is crossed and the bainitic transformation area is entered. However, the above conventional methods have the following drawbacks. In the case of the former, that is, "bainitic hardening subsequently followed by bainitic tempering", two separate heating steps, that is, bainitic hardening and bainitic tempering are required. Thus, there is a considerably long time span required for the entire thermal treatment and a relatively great thermal energy loss caused. Besides, it is necessary to handle the hardened material and the tempered material separately in order to prevent quality deterioration resulting from erroneous omission of the tempering process. Thus, the handling of the steel materials becomes complicated. On the other hand, in the case of the latter method, that is, the austempering process, the thermal treatment is executed continuously. Therefore, in comparison with the former case, the thermal energy loss is smaller and the handling of the steel material is easier. However, the austempering process, as described above, requires a relatively long isothermal treatment to obtain the bainitic structure. Therefore, such time-consuming thermal treatment results in a long cycle time of the thermal treatment device, thereby deteriorating productivity. In consideration of such conventional problems, it is an object of the present invention to provide a temperature-raising bainite forming process for a steel material capable of reducing the thermal treatment time and the cycle time of a thermal treatment device without any special means for handling the steel material. GB 601 588 A discloses a steel heat treatment comprising the steps of heating the steel to a temperature higher than the austenitic transformation point temperature, quenching it to an intermediate point temperature higher than the martensitic transformation point temperature, reheating it so that its structure is transformed to other than coarse pearlite or martensite, and cooling it. The invention is defined by the appended claims. The most remarkable aspect of the present invention lies in that the steel material heated to a temperature higher than the austenitic transformation point temperature is quenched temporarily by self cooling to said intermediate point temperature and subsequently reheated towards a temperature range corresponding to bainitic transformation to generate bainitic structure for improvement in quality. The steel material to be treated according to the present invention, the quality of which is improved by generating bainitic structure, may be a carbon steel such as S50C, S23C, or S10C, an alloy steel such as SNCM, SCR, or SCM, and a tool steel such as SK, SKD, SKH, or SKS. The aforementioned intermediate point temperature is a temperature at which the quenching is discontinued prior to raising the temperature of the steel material again towards the temperature range corresponding to bainitic transformation after going through the steps of heating the material to a temperature higher than the austenitic temperature and subsequently quenching the material. The intermediate point temperature is higher than the martensitic transformation point temperature. If the intermediate point temperature is lower than the martensitic transformation point temperature, martensitic transformation will be started, thereby hindering the progress of bainitic transformation. The intermediate point temperature is rendered higher than the martensitic transformation point temperature. Furthermore, the bainitic transformation range is represented by what is called an S curve (TTT curve) as will be described later with reference to Fig. 1. The aforementioned raised temperature is lower than the austenitic transformation temperature. If vice versa, a problem such as resumption of austenitic transformation will be brought about. The cooling following the aforementioned raising of the temperature may be self cooling, air cooling, or oil quenching. The bainitic structure, as defined in claim 11, is at least one selected from a group comprising upper bainite, lower bainite, and sorbite. In the present invention, all these structures are collectively referred to as bainitic structure. In the following, operation of the present invention will be described. According to the temperature-raising bainite forming process of the present invention, the material is quenched by self cooling to the intermediate point temperature after being heated to a temperature higher than the austenitic transformation point temperature, and then reheated towards a temperature range corresponding to bainitic transformation. Thus, as time passes, the S bainitic transformation area is crossed by a characteristic line representing temperature variation substantially perpendicularly. Accordingly, formation of the bainitic structure can be completed within a short period of time. Therefore, the entire thermal treatment time is reduced and thereby the cycle time of the thermal treatment device is also reduced. Preferably, as defined in claim 2 of the present invention, the aforementioned step of raising the temperature towards a temperature range corresponding to bainitic transformation is executed from a bainitic transformation starting range to a bainitic transformation ending range. Thus, complete bainitic transformation becomes possible, and a steel material mostly comprising the bainitic structure can be obtained. Furthermore, it is preferable, as defined in claim 3 of the present invention, that the step of heating the material to the temperature higher than the austenitic transformation point temperature and the step of raising the temperature from the intermediate point temperature towards the range corresponding to bainitic transformation are executed by locally irradiating a portion of the steel material to be improved with a high-density energy beam. Thus, both the steps of heating the material to the temperature higher than the austenitic transformation point temperature and raising the temperature towards the range corresponding to bainitic transformation can be executed with good responsiveness. In particular, it is possible to provide any local portion of the steel material to be improved with the bainitic structure efficiently. Thus, the high-density energy beam irradiation is especially advantageous when a local portion of the steel material needs to be improved. The high-density energy beam may be, for example, electron beam or laser beam. High-density energy for high frequency heating may also be utilized, although this is not a beam. In the present invention, these are collectively referred to as high-density energy beam. The electron beam is generated by applying a high voltage to an electron beam gun. The laser beam is generated by applying a high voltage to a laser oscillator. The high-density energy beam is emitted separately and locally, in the steps of heating the material to a temperature higher than the austenitic transformation temperature point and raising the temperature towards the range corresponding to bainitic transformation respectively. More preferably, as defined in claim 3 of the present invention, the step of raising the temperature from the intermediate point temperature to the temperature range corresponding to bainitic transformation is executed gradually or repeated a plurality of times. According to the gradual raising of the temperature as described above, the intensity level in irradiating with the high-density energy beam is controlled or pulse-controlled so that the heat pattern from the intermediate point temperature towards the temperature range corresponding to bainitic transformation is changed gradually or like pulses. For example, the temperature is first kept at a constant value and then raised, or the temperature is first raised gradually and then quickly (see Fig.5). Furthermore, the optimum heat pattern may be set depending on the material to be used so that a desired bainitic structure can surely be obtained. In this case, too, as described above, the heat pattern of quenching can be changed. For example, the step of quenching is first executed quickly, and then gradually (see Fig.6). In this way, the temperature variation curve during the step of quenching can be controlled such that the curve lies above the martensitic transformation point temperature (intermediate point temperature) without crossing the nose of the S curve. Besides, smooth transition from the step of quenching to that of raising the temperature can be achieved. Furthermore, as defined in claim 4 of the present invention, the aforementioned high-density energy beam includes a heating beam for heating the portion of the steel material to be improved to a temperature higher than the austenitic transformation point temperature and a temperature-raising beam for raising the temperature towards the range corresponding to bainitic transformation. The heating beam is used to heat the portion to be improved, and the temperature-raising beam is used to continuously irradiate the portion to be improved after the portion has been quenched by self cooling to the intermediate point temperature. In this case, since the portion of the steel material to be improved is irradiated with the heating beam and the temperature-raising beam successively, the aforementioned two steps of thermal treatment (heating the material to a temperature higher than the austenitic transformation point temperature and raising the temperature towards a range corresponding to bainitic transformation) can be carried out successively. Thus, the steps of heating, quenching, and raising the temperature can be executed with better responsiveness. The aforementioned quenching can be accomplished by providing a certain time interval between the irradiation with the heating beam and the irradiation with the temperature-raising beam. More specifically, during the time interval, the heat given to the portion to be improved by the heating beam is rapidly transmitted to the inside of the steel material and to the outside, thereby quenching the steel material rapidly. The aforementioned time interval is necessary for the temperature of the portion of the steel material to be improved to reach the aforementioned intermediate point temperature. As described in claim 5 of the present invention, the high-density energy beam is emitted from a single beam generating source and divided to irradiate a plurality of portions. In this case, the single high-density beam is divided into a plurality of beams using a deflection control device or the like. In this way, a plurality of portions of the steel material to be improved can be irradiated simultaneously with the divided high-density beam, thereby achieving compactness of an irradiation equipment. According to claim 6 of the present invention, a surface layer of the portion to be improved is melted when heated to the temperature higher than the austenitic transformation point temperature. In this case, if it is desirable to increase a hardening depth or to harden a low-carbon steel to a greater depth, the melted portion is austenitized in an extremely short period of time. Thus, the time required for thermal treatment is further reduced. In addition, since only the temperature of the surface layer is raised, self cooling can be employed in the step of quenching. Preferably, as defined in claim 7 of the present invention, the step of quenching by self cooling is executed at the rate of 10 3 °C/min. or more. The rate less than 10 3 °C/min is problematic, because ferrite + pearlite transformation may be started. However, it is preferable to set the upper limit of the quenching rate to 10 7 °C/min. Furthermore, according to claim 8 of the present invention, the intermediate point temperature is lower than the temperature corresponding to the nose of the S curve representing the bainitic transformation range. In this case, the intermediate point temperature is set below the nose of the S curve (see Fig. 1). Thus, the bainitic structure can be obtained with certainty. Fig. 1 is a solid-line diagram illustrating the S curve-heat pattern relationship according to a first embodiment. Fig. 2 shows a condition under which a high-density energy beam is radiated according to the first embodiment. (A) is a side view and (B) is a plan view. Fig. 3 is an explanatory diagram of a thermal treatment device according to a second embodiment. Fig. 4 is an explanatory diagram of the condition under which the high-density energy beam is radiated according to the second embodiment. Fig. 5 is a solid-line diagram illustrating the S curve-heat pattern relationship according to a third embodiment. Fig. 6 is a solid-line diagram illustrating the S curve-heat pattern relationship according to a fourth embodiment. Fig. 7 is an explanatory diagram of the lock-up clutch piston according to a fifth embodiment. Fig. 8 is an explanatory diagram showing an example of the locus of the electron beam on an irradiated portion according to a sixth embodiment. Fig. 9 is an explanatory diagram showing an example of a deflection waveform of the electron beam according to the sixth embodiment. Fig. 10 is an explanatory diagram showing another example of the locus of the electron beam on an irradiated portion according to a seventh embodiment. Fig. 11 is an explanatory diagram showing an example of the deflection waveform of the electron beam for irradiation according to the seventh embodiment. Fig. 12 is a side view of a detent spring according to an eighth embodiment. Fig. 13 is a plan view of the detent spring according to the eighth embodiment. Fig. 14 is a plan view of a diaphragm spring according to a ninth embodiment. Fig. 15 is a side view of the diaphragm spring according to the ninth embodiment. The temperature-raising bainite forming process according to a first embodiment of the present invention will be described with reference to Figs. 1 and 2. As shown in Fig. 1, according to the bainite forming process of the first embodiment, a steel material 2 to be treated (Fig.2) is first heated to a temperature 31 higher than an austenitic transformation point Ae1 (a straight line 310), and subsequently quenched by self cooling to an intermediate point temperature 32 higher than a martensitic transformation point Ms (a straight line 340) temporarily. Then, the temperature is raised again from the intermediate point temperature 32 towards a range 37 (straight line 330) corresponding to bainitic transformation to form bainitic structure. Then, the step of raising the temperature is discontinued at a temperature (33) before reaching the austenitic transformation point. Thereafter, the temperature is lowered (a straight line 340). Fig.1 shows S curves 36, 360 (TTT curves), plotted with y-axis representing time (logarithmic scale) and x-axis representing temperature (°C). Shown herein are the bainite forming process 3 (with a solid line) according to the present invention and a conventional austempering process 39 (with a dotted line). A time difference T (as shown in the lower right-hand region in the graph) between the temperature-raising bainite forming process 3 and the austempering process 39 represents the time reduced by the present invention. In this embodiment, the step of raising the temperature towards the aforementioned bainitic transformation range is executed from a bainitic transformation starting range to a bainitic transformation ending range (see a line 330 extending diagonally upward across the area defined between the two S curves 36, 360). In this embodiment, as shown in Figs. 1 and 2, during the thermal treatment, a portion 20 of the steel material 2 to be improved is locally irradiated with high-density energy beams 11 and 12. More specifically, as illustrated in (A) and (B) of Fig.2, a high-density energy beam 10 emitted from a high-density energy beam generating source 1 is divided by a deflection lens into a heating beam 11 and a temperature-raising beam 12. While the steel material 2 is moved in the direction of arrow as shown in Fig. 2, the aforementioned portion 20 to be improved is first irradiated with the heating beam 11. Subsequently, the irradiated portion 21 is heated to a temperature higher than the austenitic transformation point temperature. Then, the material 2 is irradiated with the temperature-raising beam 12, which follows the heating beam 11, thereby raising the temperature to the range corresponding to bainitic transformation to form a bainitic structure in the irradiated portion 22. After being irradiated with the heating beam 11, the portion 20 to be improved of the steel material is quenched quickly to by self cooling the aforementioned intermediate point temperature, before it is irradiated again with the temperature-raising beam 12. As described in the foregoing, according to this embodiment, the temperature 31 higher than the austenitic transformation point temperature is lowered to the intermediate point temperature 32 quickly and is again raised towards the range 37 corresponding to bainitic transformation. Thus, it is possible to raise the temperature in a short period to the range corresponding to bainitic transformation defined by the aforementioned S curves. Accordingly, generation of the bainitic structure can be completed within a short period of time. Furthermore, since the time required for the entire thermal treatment can be reduced, the cycle time of the thermal treatment device can be reduced, too. In addition, since the thermal treatment can be executed in a single operation, the steel material necessitates no special handling system. Furthermore, according to this embodiment, the step of raising the temperature to the range corresponding to bainitic transformation is executed from the bainitic transformation starting range to the bainitic transformation ending range. Thus, the bainitic structure is obtained substantially over the entire portion 20 to be treated of the steel material 2. In addition, according to this embodiment, the aforementioned steps of heating and raising the temperature are executed by irradiation with the high-density energy beam, so that the bainitic structure is obtained only on the portion 20 to be improved, not over the entire steel material 2. In other words, the steel material 2 can be improved partially, thereby giving a desired extensibility and robustness only to the treated portion. In this embodiment, the high-density energy beam is emitted from the single generating source 1 and is divided into the heating beam 11 and the temperature-raising beam 12 to continuously irradiate the steel material 2 moving thereunder. Thus, the thermal treatment device can be designed more compactly, and the time required for the bainite formation process can be reduced. Furthermore, the intermediate point temperature 32 is set to a temperature below the nose 361 of the S curve 360. According to a second embodiment, as shown in Figs. 3 and 4, in addition to the features of the temperature-raising bainite forming process of the first embodiment, an annular portion 20 to be treated (Fig. 4) of the steel material 2 is irradiated with the heating beam 11 and the temperature-raising beam 12 successively while the steel material 2 is rotating. The steel material 2 to be treated in this embodiment is a lock-up clutch piston used for a torque converter. The piston has a shape of a plate (see Figs. 3 and 7). The bainitic structure has to be obtained over the annular portion of the lock-up clutch piston (Fig. 4). The thermal treatment device for the above purpose, as shown in Fig. 3, comprises a working chamber 19 for storing the steel material 2 therein, the beam generating source 1 for radiating the heating beam 11 and temperature-raising beam 12 into the working chamber 19, and deflection coils 111 and 112 for dividing the high-density energy beam 10 emitted from the beam generating source 1 into the heating beam 11 and temperature-raising beam 12. Moreover, a vacuumizing and air exhausting device 16 for reducing the internal pressure of the working chamber 19 and a high-speed deflection control device 110 for the high-density energy beam deflected by the deflection coils 111 and 112 are provided. The outputs of both the beams can be controlled freely by varying the frequency and waveform of the current flowing through the deflection coils 111 and 112. These devices are controlled by a general control device 17. Furthermore, a motor 150 for rotating a mounting base 15 on which the portion 20 to be improved of the steel material 2 is disposed under the working chamber 19. In implementing the temperature-raising bainite forming process using the above-described thermal treatment device, the motor 150 for rotation is first actuated to cause the steel material 2 to rotate in the direction of arrow as shown in Fig. 4. Then, the working chamber 19 is vacuumized by the vaccumizing and air exhausting device 16. Then, as shown in Figs. 3 and 4, the steel material 2 is first irradiated with the heating beam 11 and subsequently, after a certain time interval, irradiated with the temperature-raising beam 12. Thus, as shown in Fig. 4, the bainitic structure can be formed in the annular portion of the steel material 2. The effect presented by this embodiment is similar to that of the first embodiment. According to a third embodiment, as shown in Fig. 5, the step of raising the temperature from the intermediate point temperature towards the range corresponding to bainitic transformation is executed gradually or repeated a plurality of times. The heat pattern H as shown in Fig.5 illustrates an example in which the temperature is lowered to the intermediate point temperature quickly, kept constant for a short period of time, raised gradually, and raised rapidly to go through the range corresponding to bainitic transformation. Furthermore, the heating pattern K in Fig.5 shows an example in which the step of raising the temperature is executed a plurality of times. In this manner, a relatively fine bainitic structure can be obtained within a short period of time. The effect presented by this embodiment is also similar to that of the first embodiment. A fourth embodiment is, as shown in Fig.6, an example in which the step of quenching the material from the temperature higher than the austenitic transformation point to the intermediate point temperature is executed gradually. The heat pattern C in Fig.6 represents an example in which the temperature is lowered to the intermediate point temperature quickly and gradually, and then raised quickly to go through the range corresponding to bainitic transformation. Thus, a relatively fine bainitic structure can be obtained within a short period of time. The effect presented by this embodiment is also similar to that of the first embodiment. In this embodiment, the bainite forming process and device according to the embodiments 1 and 2 are employed. The steel material to be treated in this embodiment is, as show in Fig. 7, a lock-up clutch piston 41 for a torque converter. The lock-up clutch piston 41 is partially fixed by caulking to a damper device for absorbing the fluctuation of the torque transmitted in a torque converter. Reference numeral 43 in Fig. 7 denotes a hole for fixing the lock-up clutch piston. The damper device, as shown in Fig. 7, comprises a driven plate 51 integrally rotated with a turbine liner and springs 52 and 53. In this embodiment, as shown in Fig. 7, the springs 52 are designed for the first stage and disposed at 8 portions along the circumference of the lock-up clutch piston 41, while the springs 53 are designed for the second stage and disposed at 4 portions along the circumference of the lock-up clutch piston 41. The springs 53 are alternately provided in the springs 52. Furthermore, the diameter and longitudinal dimension of the spring 53 are smaller than those of the spring 52. Accordingly, the spring 53 starts to yield when the spiral angle of the spring 52 has reached a set value and the transmitted torque has reached a bending point. Thus, the rotation transmitted from a front cover through a friction member is further transmitted to a turbine hub through the damper device. In this case, the springs 52 and 53 are compressed to absorb the fluctuation of the transmitted torque during the transmission of the rotation. These springs also play a role in absorbing vibration or noise produced when an abrupt change in the output torque of an engine is transmitted to a transmission apparatus (not shown). When the lock-up clutch piston 41 is driven in the normal direction (when the lock-up clutch is in an engaged state and the lock-up clutch piston 41 is caused to rotate counterclockwise in Fig. 7) and when it is driven in the reverse direction (when the lock-up clutch piston 41 is caused to rotate clockwise in Fig. 7 to apply engine braking or the like), the spring 52 is compressed. Therefore, at this time, the spring 52 is likely to slide over a flat portion 411 of the lock-up clutch piston 41. This gives rise to a problem that friction is caused between the flat portion 411 of the lock-up clutch piston 41 and the spring 52 as a result of the sliding movement therebetween. The lock-up clutch piston 41 is provided with a doughnut-shaped spring receiving portion 40 (as shown by hatching in Fig. 7) for contact with the spring 52. Since the spring receiving portion 40 of the lock-up clutch piston is required to exhibit sufficient abrasion resistance and robustness, the spring receiving portion (about 3mm thick) needs to include a partially formed bainitic structure (0.1-0.2mm thick). The material used for the above member is S23C. In implementing the temperature-raising bainite forming process, the electron beam as a high-density energy beam as described in the first and second embodiments is employed for the steps of heating and raising the temperature. The above-described electron beam generating device is capable of producing an output of 5KW. With this device the electron beam is radiated at the feeding rate of 10m/min. The above-described member is rotated at 25 rpm, and a portion thereof corresponding to a radius of 127mm is irradiated successively with the heating beam 11 as an electron beam of 3.5KW and the temperature-raising beam 12 as an electron beam of 1.5KW (Figs. 2 through 4). The distance between the beams 11 and 12 on the irradiated portion is 20mm, and the deflection loci of both the beams 11 and 12 are 5mm in x-axis direction and 10mm in y-axis direction respectively. After the steel material 2 has been irradiated with the beam 11, it is cooled quickly by self cooling to the intermediate point temperature before being irradiated with the beam 12 subsequently. In this case, the Vickers hardness of the surface of the steel material 2 is 450. According to the conventional method, this value is only attained by repeating the tempering process at 250°C twice after the hardening process. The bainitic structure is observed in the spring receiving portion of the above-described member, while the ferrite-pearlite structure remains in the other portions. An example of the irradiation locus of an electron beam according to a sixth embodiment is shown in Fig. 8. In this embodiment, the electron beam is radiated according to two circular deflection loci C 1 and C 2 . In this case, the areas 25 and 26 to be thermally treated, corresponding to the portions to be irradiated with the heating beam and the temperature-raising beam respectively, are irradiated with the electron beam according to the circular deflection loci C 1 and C 2 respectively. During the irradiation, the material to be treated is caused to rotate about a central axis thereof. Thus, the locus of the electron beam in each of the areas 25 and 26 to be thermally treated is moved in the direction of arrow H. Furthermore, each of the circular deflection loci C 1 and C 2 generates a sinusodial deflection waveform in the directions of x-axis and y-axis and is formed by the combination of deflections . Moreover, by changing each of the circular deflection loci C 1 and C 2 to alternately irradiate the areas 25 and 26 to be thermally treated with the electron beam, a deflection waveform w1 as shown in Fig. 9 is generated and superposed on the deflection waveform in the direction of y-axis. Thus, the area 25 to be thermally treated is irradiated with the electron beam during the period t1 through which the voltage V E is positive, while the area 26 to be thermally treated is irradiated with the electron beams during the period t2 through which the voltage V E is negative. Furthermore, with respect to the deflection waveform w 1 , by setting the period t1 shorter and the period t2 longer, it is possible to adjust the energy for irradiating the areas 25 and 26 to be thermally treated. A seventh embodiment, as shown in Fig. 10, is another example in which areas 27 and 28 to be thermally treated are irradiated with the electron beam. In this case, the electron beam is emitted according to two plane deflection loci C 3 and C 4 . That is, the areas 27 and 28 to be thermally treated are irradiated with the electron beam according to the plane deflection loci C 3 and C 4 respectively. During the irradiation, the material to be treated is caused to rotate about the central axis thereof. Thus, also in this case, the locus of the electron beam in the areas 27 and 28 to be thermally treated is moved in the direction of arrow H. Furthermore, each of the plane deflection loci C 3 and C 4 is formed by generating a deflection voltage of a triangular wave in the directions of x-axis and y-axis. By changing the plane deflection loci C 3 and C 4 to irradiate the areas 27 and 28 to be thermally treated with the electron beam, the deflection waveform w 1 as shown in Fig. 11 is superposed on the triangular wave in the directions of x-axis and y-axis. It is also possible to combine the circular deflection with the plane deflection or to deflect the electron beam to provide it with a linear or elliptical locus. In respect of all the other features, this embodiment is similar to the sixth embodiment. Although the material to be treated in the above-described embodiment is a lock-up clutch piston for a torque converter, the present invention is applicable to any steel material having a surface layer portion that needs hardening either entirely or partially, such as a sliding plate portion of a multi-plate frictional engagement device, a portion at which two members are connected to each other or to which one member is connected by means of a snap ring, etc, an oil pump plate, a seal ring, and the like. An eighth embodiment, as shown in Figs. 12 and 13, is an example in which a detent spring 6 is partially improved. The detent spring 6 is employed in a shift apparatus of an automatic transmission and comprises a front end portion 61 for mounting a roller thereon, a concave portion 62 for accommodating a detent lever therein, and a fixed portion 63. The fixed portion 63 is provided with a mounting hole 64. In the case of the detent spring 6, the portion 60 ( indicated by an alternate long and short dash line) to be improved, which requires robustness, is treated by the bainite forming process according to the present invention. The aforementioned detent spring 6 is made of SK5. As described in the embodiment, the improvement is made by irradiating the portion to be improved with two kinds of electron beams. In respect of all the other features, this embodiment is similar to the second embodiment. The effect of this embodiment is similar to that of the second embodiment. Conventionally, however, the above-described improvement is made by hardening and tempering the detent spring entirely. A ninth embodiment, as shown in Figs. 14 and 15, is an example in which a diaphragm spring 7 is partially improved. The diaphragm spring 7 is employed in a clutch disk of an automobile, and comprises a conic base portion 71 and a radial spring portion 75 which is radially divided by holes 73 radially extending from the center and provided with front end portions 72. The portion 70 (indicated by alternate long and short dash lines) to be improved, including the aforementioned spring portion 75, is treated by the bainite forming process according to the present invention. The diaphragm spring 7 is made of S50C. In respect of all the other features, this embodiment is similar to the eighth embodiment. Conventionally, however, the diaphragm spring 7 is treated entirely by the austempering process. The temperature-raising bainite treating process according to the present invention is capable of reducing the time required for the entire thermal treatment and the cycle time of the thermal treatment device without requiring any special means for handling the steel material.

Claims: A bainite forming process, comprising the steps of: a) heating a steel material to a temperature higher than an austenitic transformation point temperature; b) quenching the steel material to an intermediate point temperature higher than a martensitic transformation point temperature; c) reheating the steel material towards a temperature range corresponding to bainitic transformation from the intermediate point temperature to form a bainitic structure; d) discontinuing the temperature-raising process before the austenitic transformation point temperature is reached; and e) cooling the steel material, characterized in that said step a) and said step c) are executed by locally irradiating a portion of the steel material to be improved with a high-density energy beam or by high frequency heating and said step b) is executed by self cooling. Process according to claim 1, wherein said step c) is executed from a bainitic transformation starting range to a bainitic transformation ending range. Process according to claim 1 or 2, wherein said step c) is repeated a plurality of times. Process according to one of claims 1 through 3, wherein said high-density energy beam includes a heating beam for heating said portion of the steel material to be improved to a temperature higher than said austenitic transformation point temperature and a temperature-raising beam for raising the temperature towards said range corresponding to bainitic transformation, said heating beam is used to heat the portion to be improved, and said temperature-raising beam is used to continuously irradiate the portion to be improved after said portion has been quenched to the intermediate point temperature. Process according to one of claims 1 through 4, wherein said high-density energy beam is emitted from a single beam generating source and divided to irradiate a plurality of portions. Process according to one of claims 1 through 5, wherein a surface layer of the portion to be improved is melted when heated to said temperature higher than the austenitic transformation point temperature. Process according to any one of claims 1 through 6, wherein said quenching is executed at the rate of 10 3 °C /min. or more. Process according to any one of claims 1 through 7, wherein said intermediate point temperature is lower than a temperature corresponding to the nose of an S curve representing the bainitic transformation range. Process according to any one of claims 1 through 8, wherein said bainitic structure is at least one selected from a group comprising upper bainite, lower bainite, and sorbite.

Inventors: MARUKI MICHIO, OHBAYASHI KOUJI, SUZUKI TAKATOSHI, MARUKI, MICHIO, OHBAYASHI, KOUJI, SUZUKI, TAKATOSHI, Suzuki, Takatoshi, K. K. Toyota Chuo Kenkyusho
Applicants: AISIN AW CO, AISIN AW CO., LTD.
Application year: 1997
Main classification: C21D1
IPC codes: C21D5, C21D1, C21D9, C21D6