JP2005133214A - Heat treatment system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the efficiency of heat treatment in the whole system when a nitrogen-enriched layer is formed in primary heat treatment and a re-quenching is applied in secondary heat treatment. <P>SOLUTION: A bearing part 41 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 41 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. The respective secondary heat treatment apparatus 2 is parallel disposed at a plurality of positions, and an induction-heating is applied in each secondary heat treatment apparatus. <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

  As described above, in the invention disclosed in the above publication, the heat treatment is performed twice in combination with the primary and the secondary. However, since the required heating times are different between the primary heat treatment and the secondary heat treatment, the primary heat treatment and the secondary heat treatment as they are. The consistency of the heating time of heat treatment may not be achieved, and the heat treated product may stagnate in the line, or the waiting time of each heat treatment device may be increased, resulting in a decrease in heat treatment efficiency.

  Therefore, an object of the present invention is to improve the heat treatment efficiency of the entire system when a nitrogen-enriched layer is formed by a primary heat treatment and re-quenched by a 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. and 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, in parallel a plurality of secondary heat treatment apparatus It is arranged.

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 system of the present invention, since the plurality of secondary heat treatment apparatuses are arranged in parallel as described above, the secondary heat treatment can be performed simultaneously at a plurality of locations, and the heat treatment efficiency of the secondary heat treatment can be increased.

  In this case, it is preferable to perform induction heating in each secondary heat treatment apparatus. Induction heating has better work efficiency than an atmosphere furnace such as a combustion furnace, and the heating time is short. Therefore, by performing this induction heating in parallel at multiple locations, the heating efficiency in the secondary heat treatment jumps. Increase. Accordingly, the heat treatment efficiency of the primary heat treatment and the secondary heat treatment can be matched (balanced), whereby the heat treatment efficiency can be improved in the entire system.

  In this case, if mold quenching is performed with a secondary heat treatment device, a heat-treated product with high deformation accuracy with little deformation can be obtained, and particularly good dimensional accuracy can be ensured even for thin-walled components and components with non-uniform thickness. Is possible. 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.

  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 re-quenched by the secondary heat treatment, the heat treatment efficiency in the secondary heat treatment is dramatically increased. Therefore, it becomes possible to match the heat treatment efficiency of the primary heat treatment and the secondary heat treatment, and thereby the heat treatment efficiency can be improved in the entire system.

  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 illustrated, this heat treatment system includes a primary heat treatment apparatus 1, a cleaning apparatus 3, a plurality of secondary heat treatment apparatuses 2 arranged in parallel to the primary heat treatment apparatus 1 and the cleaning apparatus 3, a cleaning apparatus 5, and a tempering apparatus 6. Composed. 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 perform the primary heat treatment and the secondary heat treatment. Applied. A cleaning device 5 and a tempering device 6 are arranged at the subsequent stage of the secondary heat treatment apparatus 2 in each row.

  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 tapered roller bearing 4 having an outer ring 41, an inner ring 42, and rolling elements (conical rollers) 43 as main components, and the outer ring 41 that is in rolling contact with a mating member among these components. The inner ring 42 and the rolling element 43 correspond to the bearing parts here. 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 heater 11, the bearing component is heated at a temperature T1 (800 ° C. to 900 ° C., for example, 850 ° C.) exceeding the A ₁ transformation point for a predetermined time, for example, 40 minutes (primary heating) as shown in FIG. . 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, bearing parts carbonitrided by the primary heat treatment apparatus 1 (the outer ring 41 of the tapered roller bearing is illustrated in the drawing) are subjected to high-frequency heating or the like via a conveying means such as a conveyor (not shown). It is distributed and supplied to any secondary heat treatment apparatus 2 that performs induction heating. In each secondary heat treatment apparatus 2, the bearing component 41 is disposed on the inner periphery of the inductor 21 and, as shown in FIG. 3, is predetermined at a secondary heating temperature T 2 (for example, 880 ° C. to 900 ° C.) that is equal to or higher than the A ₁ transformation point. Induction heating is performed for a 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 completion of the heating, as shown in FIG. 1, the bearing component 41 is fitted into, for example, a mold 22, and is cooled to below the Ms point with a coolant such as oil while being held therein (mold). Quenching). A quenching may be performed by ejecting a coolant such as oil from the pores provided in various places of the mold 22. Cooling can be performed at an induction heating position as shown in the figure, or can be performed by transferring to a position different from the induction heating position.

  The bearing parts for which the secondary heat treatment has been completed are taken out from the respective secondary heat treatment apparatuses 2, and after the cooling liquid is washed and removed by the respective washing machines 5, the bearing parts are transferred to the respective tempering machines 6, and as shown in FIG. Tempering is performed at T3 (for example, 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.

  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.

  In the present invention, as described above, the bearing parts 41 that have passed through the common primary heat treatment apparatus 1 are distributed to the plurality of secondary heat treatment apparatuses 2 to simultaneously advance the secondary heating, and each secondary heat treatment apparatus 2 heats in a short time. The high frequency heating is completed. Thereby, the heating efficiency in the secondary heat treatment is remarkably increased, so that the heat treatment efficiency of the primary heat treatment and the secondary heat treatment can be matched (balanced), and the heat treatment efficiency can be improved in the entire system. The number of secondary heat treatment apparatuses 2 can be arbitrarily selected within a range in which both can be balanced in consideration of the difference in heat treatment efficiency between the primary treatment side and the secondary treatment side. In the said embodiment, although the number of the primary heat processing apparatuses 1 is made into one, the heat processing efficiency between several secondary heat processing apparatuses 2 can also be balanced by arranging this in parallel in several places (this In this case, the number of secondary heat treatment apparatuses 2 is larger than that of the primary heat treatment apparatus 1).

  In addition, the high-frequency heating performed in the secondary heat treatment apparatus 2 is less likely to be thermally deformed in principle, and die quenching is performed after the heating, so that a bearing component having high dimensional accuracy with little heat deformation can be obtained at low cost and rolling. Good dimensional accuracy can be ensured even for thin parts such as the outer ring and inner ring of the bearing, particularly parts having a non-uniform thickness such as the outer ring 41 and inner ring 42 of the tapered roller bearing. Therefore, the quality of the bearing parts is improved, and it is possible to ensure stable bearing performance.

  Induction heating can be used to heat individual components even piece-by-piece, local heating is possible, and the depth of the hardened layer can be freely selected. This has the advantage that the fatigue strength can be increased by this, so that it is effective for further lowering the cost, improving the quality and improving the fatigue life of the 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 tapered roller bearing. It is a cycle diagram of the heat processing in the said heat processing system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Primary heat processing apparatus 2 Secondary heat processing apparatus 3 Cleaning apparatus 4 Tapered roller bearing (rolling bearing)
5 Cleaning device 6 Tempering device 11 Heating machine 12 Cooling machine 21 Inductor 22 Mold 41 Outer ring 42 Inner ring 43 Rolling element

Claims (4)

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, the heat treatment system, characterized in that a second heat treatment apparatus for cooling to below the a ₁ transformation point, to place a plurality of secondary heat treatment apparatus in parallel.   The heat processing system of Claim 1 which performs induction heating with each secondary heat processing apparatus.   The heat treatment system according to claim 1 or 2, wherein mold hardening is performed in each secondary heat treatment apparatus.   The heat treatment system according to any one of claims 1 to 3, wherein gas carbonitriding is performed with a primary heat treatment apparatus.

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