JP2005133215A - Heat treatment system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To uniformly micronize crystal grains in the whole body of a part by strictly controlling the heating temperature in secondary heat treatment in a heat treatment system in which a nitrogen-enriched layer is formed in primary heat treatment and a re-quenching is applied in the secondary heat treatment. <P>SOLUTION: A bearing part is cooled to the temperature below an A<SB>1</SB>transformation point after heating to the temperature exceeding the A<SB>1</SB>transformation point with a primary heat treatment apparatus 1, to form the nitrogen-enriched layer on the surface thereof. The bearing part after applying the primary heat treatment, is applied to the heat treatment with a secondary heat treatment apparatus 2 which cools to the temperature below the A<SB>1</SB>transformation point after heating to the temperature exceeding the A<SB>1</SB>transformation point. In a heater 21 in the secondary heat treatment apparatus 2, an induction-heating is applied and the temperature of the induction-heated bearing part, is detected and a feedback control is applied to the heater 21 according to the detected value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a heat treatment system for performing two-stage heat treatment (primary heat treatment and secondary heat treatment) on a steel part.

Japanese Patent Application Laid-Open No. 2003-226918 discloses a heat treatment method suitable for steel mechanical parts that require a high rolling fatigue life, such as rolling bearing parts. This, after the steel for the bearing parts were carbonitrided in the carbonitriding temperature exceeding the A ₁ transformation point, cooled to a temperature below the A ₁ transformation point, then the temperature of the carbonitriding process by A ₁ transformation point or more Quenching is performed by reheating to a lower quenching temperature range (790 ° C. to 830 ° C.).

According to this method, the presence of the carbonitriding layer on the surface layer increases the hardness of the bearing component, and the quenching temperature during reheating is suppressed to a temperature at which austenite crystal grains do not easily grow. The diameter can be reduced to 8 μm or less. As a result, the grain boundary strength is increased, so that effects such as improvement of rolling fatigue life and improvement of crack resistance can be obtained.
JP 2003-226918 A

  In the invention disclosed in the above publication, it is necessary to strictly control the heating temperature of the secondary heat treatment in order to uniformly refine the crystal grains in the entire part. At this time, as in the conventional general quenching process, secondary heating was performed in an atmospheric furnace and the atmospheric temperature in the furnace was measured. Deviation may occur, and strict temperature control is difficult.

  Therefore, an object of the present invention is to strictly control the heating temperature in the secondary heat treatment in a heat treatment system in which a nitrogen-enriched layer is formed by the primary heat treatment and re-quenched by the secondary heat treatment.

In order to achieve this object, the heat treatment system according to the present invention is a _primary system in which a steel part is heated to a temperature exceeding the A ₁ transformation point and then cooled to below the A ₁ transformation point to form a nitrogen-enriched layer on the surface. performing a heat treatment apparatus, the steel part after the primary heat treatment, after heating to a temperature above the a ₁ transformation point, and a second heat treatment apparatus for cooling to below the a ₁ transformation point, the induction heating in the secondary heat treatment apparatus At the same time, the temperature of the steel part to be induction-heated is detected, and the induction heater is feedback-controlled according to the detected value.

According to this heat treatment system, since the nitrogen-enriched layer in which nitrogen is diffused is formed on the surface by the heat treatment in the primary heat treatment apparatus, the surface hardness of the steel part is increased. On the other hand, the austenite grains in the steel structure are coarsened after the primary heat treatment, but after that, induction heating is performed to a secondary heating temperature exceeding the A ₁ transformation point, so that the control of the heating temperature and the heating time is performed. Through the heat treatment, the austenite grains in the microstructure of the steel part after heat treatment can be refined. For example, fine grains having a grain size number exceeding 10 by the austenite grain size test method defined in JIS G0551 can be obtained. Is possible. From the above characteristics, it is possible to improve wear resistance and crack resistance as compared with normal products, and to further greatly improve the rolling fatigue life.

  In the present invention, induction heating such as high-frequency heating is performed with a secondary heat treatment apparatus, the temperature of the steel part to be induction-heated is detected, and the heating condition of the induction heater is feedback-controlled according to the detected value. Therefore, based on the temperature of the actual steel part, the secondary heating temperature can be accurately and accurately maintained in a narrow temperature range, and a high quality steel part with a fine grain size can be obtained throughout the part. Can be obtained.

  In this case, in order to minimize the temperature error, it is desirable to detect the temperature of the steel part to be induction-heated with a non-contact type temperature sensor.

  As a means for forming the nitrogen-enriched layer by the primary heat treatment, carbonitriding is desirable, and gas carbonitriding is particularly preferred in view of cost and quality. The gas carbonitriding can be performed in an atmosphere furnace using, for example, an atmosphere gas obtained by adding ammonia to a carburizing gas.

  As described above, according to the present invention, when the nitrogen-enriched layer is formed by the primary heat treatment and the re-quenching is performed by the secondary heat treatment, the heating temperature in the secondary heat treatment apparatus can be accurately managed. Therefore, it is possible to prevent uneven heating in the secondary heat treatment and to uniformly refine the crystal grain size in the entire part, and to stabilize the quality of the steel part.

  Hereinafter, an embodiment of the present invention applied to a bearing component as an example of a steel component will be described.

  FIG. 1 conceptually shows the configuration of a heat treatment system according to the present invention. As shown in the figure, this heat treatment system includes a primary heat treatment apparatus 1, a secondary heat treatment apparatus 2, cleaning apparatuses 3 and 5, and a tempering apparatus 6. Bearing parts formed in a forming process (not shown) such as forging → turning are sequentially transferred to the primary heat treatment apparatus 1 and the secondary heat treatment apparatus 2 and heated and cooled by the respective apparatuses to be subjected to the primary heat treatment and further to the secondary heat treatment. Heat treatment is applied.

  The bearing component here means a bearing component of a rolling bearing such as a ball bearing, a tapered roller bearing, a roller bearing, or a needle roller bearing. FIG. 2 shows, as an example, a deep groove ball bearing 4 having an outer ring 41, an inner ring 42, and rolling elements (balls) 43 as main components, and among these components, an outer ring 41 that is in rolling contact with a mating member, The inner ring 42 and the rolling element 43 correspond to the bearing parts referred to herein. As materials for these bearing parts, in addition to bearing steel such as SUJ2, C: 0.6 to 1.3 wt%, Si: 0.3 to 3.0 wt%, Mn: 0.2 to 1.5 wt%, Cr : 0.3 to 5.0 wt%, Ni: 0.1 to 3 wt% (desirably Mo: 0.05 to less than 0.25 wt%, V: 0.05 to 1.0 wt% further included) Bearing steel and medium carbon containing C: 0.4-0.8 wt%, Si: 0.2-0.9 wt%, Mn: 0.7-1.3 wt%, Cr: 0.7 wt% or less Steel or the like can also be used.

The primary heat treatment apparatus 1 includes a heater 11 and a cooler 12. In FIG. 1, a continuous type is illustrated as the heater 11, but a batch type furnace can also be used. The heater 11 is constituted by, for example, an atmospheric furnace that uses an atmospheric gas obtained by adding ammonia to a carburizing gas. In the furnace of the heater 11, the bearing parts are heated at a temperature T1 (800 ° C. to 900 ° C., for example, 850 ° C.) exceeding the A ₁ transformation point as shown in FIG. heating). As a result, activated nitrogen diffuses into the surface layer and the surface layer of the bearing component is hardened (gas carbonitriding). Since the heater 11 is basically intended to form a nitrogen-enriched layer on the surface, it may be at least nitrided and carburization is not necessarily required. However, depending on the conditions, for example, when decarburization is a concern or when the amount of carbon in the steel used is small and sufficient hardness cannot be ensured, carburizing is indispensable in addition to nitriding. As the heater 11, a vacuum furnace, a salt bath furnace, an induction heater, or the like can be used. The heated bearing parts are cooled (for example, oil-cooled) to the Ms point or less by the cooler 12 and further transferred to the washing machine 3 to remove the cooling liquid.

As shown in FIG. 1, the bearing parts carbonitrided by the primary heat treatment apparatus 1 are supplied to the secondary heat treatment apparatus 2 via a conveying means such as a conveyor. The secondary heat treatment apparatus 2 performs induction hardening, and includes a heater 21 and a cooler 22. In the heater 21, as shown in FIG. 3, the bearing component is induction-heated at a secondary heating temperature T2 (for example, 880 ° C. to 900 ° C.) equal to or higher than the A ₁ transformation point for a predetermined time (for example, 1.5 to 2 seconds). . Although FIG. 3 illustrates the case where the secondary heating temperature T2 is lower than the primary heating temperature T1, the secondary heating temperature T2 may be equal to or higher than T1. In the induction heating, the heating temperature and the heating time can be precisely controlled and the treatment is performed in a short time, so that the austenite crystal grains in the microstructure of the bearing part can be refined. At this time, whether or not the austenite crystal grains are refined can be evaluated by the product of the heating temperature and the heating time. For example, when the maximum heating temperature in the induction heater 21 is low, the heating time is correspondingly increased. By increasing the length, the austenite crystal grains can be made finer.

  After the heating is completed, the bearing component is transferred to the cooler 22 where it is cooled (for example, oil-cooled) below the Ms point and quenched. The cooling can be performed by the cooler 22 independent from the heater 21 as shown, or can be performed while being held in the induction heating position in the heater 21. In order to ensure the dimensional accuracy after quenching, the secondary heat treatment apparatus 2 can also perform mold quenching after induction heating, so that thin parts such as an outer ring and an inner ring of a rolling bearing, an outer ring of a tapered roller bearing, It is possible to improve the accuracy of components having a non-uniform thickness such as an inner ring and to stably obtain bearing performance. The mold quenching means a process of quenching in a state where the article to be heated is constrained by a mold, and includes press quenching that constrains the mold by applying pressure.

  The bearing parts that have been subjected to the secondary heat treatment are taken out from the secondary heat treatment apparatus 2, washed and removed of the cooling liquid by the cleaning apparatus 5, and then transferred to the tempering apparatus 6, as shown in FIG. For example, tempering is performed at 180 ° C. This tempering is desirably performed by induction heating such as high-frequency heating in order to improve the processing efficiency by shortening the heating time.

  The heater 21 of the secondary heat treatment apparatus 2 is provided with a sensor 9 that detects the temperature (surface temperature) of the bearing component that is induction-heated in a non-contact manner. For example, an infrared temperature sensor can be used as the sensor 9. In the heater 21, the bearing component is held with a predetermined clearance with respect to an inductor (not shown), and the sensor 9 measures the temperature of the held bearing component in a non-contact manner, and the detected value is a control device. 8 is transmitted. Based on the detected temperature data, the control device 8 determines whether or not the bearing component to be heated has reached the prescribed secondary heating temperature T2, and further whether or not it is within a predetermined temperature range. Accordingly, feedback control of the induction heater 21 is performed. The induction heater 21 is controlled mainly by changing the input power to the inductor and the heating time.

  In the above description, oil cooling is exemplified as the cooling method in the primary heat treatment apparatus 1 and the secondary heat treatment apparatus 2, but other cooling methods such as water cooling, air cooling, gas cooling, etc. can also be adopted. Different cooling methods may be employed for the heat treatment apparatus 1 and the secondary heat treatment apparatus 2. In this embodiment, the cleaning devices 3 and 5 are installed because the oil cooling is performed in both the primary heat treatment and the secondary heat treatment. However, this type of cleaning device is not necessary for water cooling, air cooling, or gas cooling. It becomes.

  The primary heating temperature T1, the secondary heating temperature T2, and the tempering temperature T3 described above exemplify the case where the bearing steel SUJ2 is used as the steel material. Depending on the type of steel used, these temperatures T1, T2, and T3 may be different from the above examples.

  In the bearing parts heat-treated in the above process, since a nitrogen-enriched layer (nitrogen content 0.1 to 0.7 wt%) is formed on the surface layer, high hardness exceeding Hv700 is obtained, and in the microstructure As the austenite grains are refined, the austenite grain size exceeds # 10. Further, good physical property values far exceeding conventional products such as a fracture stress value of bearing parts of 2650 MPa or more, a hydrogen concentration in steel of 0.5 ppm or less, and a retained austenite amount of 13 to 25% in steel can be obtained. Therefore, crack resistance strength, wear resistance, and the like can be improved from the above, and a remarkable effect can be obtained in improving the rolling fatigue life.

  The heater 21 of the secondary heating device 2 is an induction heater that uses an electromagnetic induction phenomenon to generate heat by directly converting electrical energy into thermal energy in a steel structure. By adjusting the heating conditions, the amount of heating can be controlled easily and accurately. Therefore, the secondary heating temperature T2 can be accurately maintained in a predetermined temperature range by feedback control of the heating condition of the heater 21 by the control device 8 according to the detection value of the sensor 9. Moreover, since the induction heating is a piece-by-piece heating, there is no uneven heating depending on the charging position in the furnace as in the case of using an atmospheric furnace. From the above, it is possible to obtain steel parts with a uniform crystal grain size throughout the entire part, and have the effects specific to the above two-stage heat treatment, such as ensuring wear resistance and crack resistance, or improving rolling fatigue life. It becomes possible to obtain stably.

  In addition, induction heating has the advantages that local heating is possible and the hardened layer depth can be selected freely, rapid heating and quenching are possible, and fatigue strength can be increased by surface compressive residual stress. This is also beneficial for further cost reduction, higher quality, and improved fatigue life of bearing parts.

  In the above description, bearing parts are exemplified as heat treatment targets. However, the present invention is not limited to this, and mechanical parts (for example, components of constant velocity universal joints) that require a high rolling fatigue life, and steel are also included. Can be widely applied to manufactured parts in general.

It is sectional drawing which shows schematic structure of the heat processing system concerning this invention. It is sectional drawing of a deep groove ball bearing. It is a cycle diagram of the heat processing in the said heat processing system.

Explanation of symbols

1 Primary heat treatment device 2 Secondary heat treatment device 3 Cleaning device 4 Deep groove ball bearing (rolling bearing)
5 Cleaning Device 6 Tempering Device 8 Control Device 9 Sensor 11 Heating Machine 12 Cooling Machine 21 Heating Machine (Induction Heating Machine)
22 Cooling machine 41 Outer ring 42 Inner ring 43 Rolling element

Claims (3)

After the steel parts were heated to a temperature above the A ₁ transformation point, and primary heat treatment apparatus for forming a nitriding layer on the surface by cooling to below the A ₁ transformation point, the steel parts after the primary heat treatment, A ₁ after heating to a temperature exceeding the transformation point, and a second heat treatment apparatus for cooling to below the a ₁ transformation point, performs induction heating in the secondary heat treatment device detects the temperature of the steel component to be inductively heated, A heat treatment system that performs feedback control of an induction heater according to the detected value.   The heat treatment system according to claim 1, wherein the temperature of the steel part to be induction-heated is detected by a non-contact type temperature sensor.   The heat treatment system according to claim 1 or 2, wherein gas carbonitriding is performed by a primary heat treatment apparatus.

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