JP2012097317A - High frequency quenching method and manufacturing method of product made from steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high frequency quenching method in which the hardness of a surface is high and the toughness of the inside is excellent using a material easily shaped.SOLUTION: In the gearwheel 1, a steel made part material is subjected to spherodizing processing considering cutting performance. First, the gearwheel 1 is subjected to the first high frequency quenching step. In the first high frequency quenching step, the oscillating frequency of the high frequency oscillator 10 is set high. Next the induction heating coil 7 is energized with high frequency alternate current with frequency lower than the previous time and the gearwheel 1 is subjected to the second high frequency heating step. As a result, the surface area heated in the first high frequency heating step previously becomes an austenite structure where remaining spherical carbide disappears and the spherical carbide is substantially perfectly disappears. On the other hand, the area deeper than that becomes the austenite structure including the spherical carbide. The reheated the area in the surface is converted to a martensite structure where the spherical provide does not remain when the gearwheel 1 is quenched. The area first heated in the second high frequency heating step is converted to the martensite structure including the spherical carbide.

Description

  The present invention relates to a method of induction quenching, and relates to a method capable of improving surface toughness and internal toughness by induction quenching. Moreover, this invention relates to the manufacturing method of the product which uses steel as a raw material, and includes induction hardening as one of the processes.

One of the surface hardening methods for steel parts is induction hardening. Here, induction hardening is a hardening method characterized in that an induction coil is brought close to a workpiece formed of a steel part material, and the workpiece is heated by supplying a high-frequency current to the induction coil.
That is, in the induction hardening, a high frequency current is applied to the induction coil to generate an alternating magnetic field in the induction coil, and a workpiece is placed in the alternating magnetic field to generate a secondary current in the workpiece itself. In this method, the workpiece itself is heated and quenched.

JP 2003-213327 A

Induction hardening has the advantage that the time required for heating is shorter than, for example, carburizing and hardening.
On the other hand, it is said that a work subjected to induction hardening is inferior in toughness as compared with a work subjected to hardening by carburizing.

That is, the secondary current generated in the workpiece by the high frequency alternating magnetic field flows mainly on the surface of the workpiece. Therefore, only the surface part of the workpiece is locally heated. For example, assuming that the gears are induction hardened, the workpiece after quenching has a cross-sectional quenching pattern as shown in FIG. That is, when a gear is formed using a carbon steel material for machine structure (SC material) as a raw material, this tooth portion is induction-hardened, and this cross section is etched and observed, a quenching pattern as shown in FIG. 7 is obtained. This tendency is remarkable when the module is a small gear (module 8 or less).
Therefore, when a gear having a small tooth whose whole tooth portion is heated to the inside is heated at high frequency and rapidly cooled from that state, the entire tooth portion A is substantially completely martensified as shown in FIG. The hub part B is not martensitic at all.
Therefore, the tooth surface is hard and excellent in wear resistance, but the toughness of the root portion is low, and the overall impact resistance of the tooth is low. As a result, when receiving a shocking load, the tooth may break from the root.

On the other hand, when only the surface side of the gear is heated at a high frequency for a short time with an appropriate frequency, thinly quenched, and further tempered, a quenching pattern as shown in FIG. 8 is obtained.
In this case, even if the surface of the tooth portion A has a Vickers hardness of about HV720, the inside of the tooth is the hardness of the material before quenching, and the Vickers hardness is about HV230. However, the rigidity is low. Therefore, also in this case, the gear has a weak tooth root strength. Further, a region having a low hardness is generated between the center portion (inside) of the tooth and the surface portion.
Even if only the surface of the gear is heated for a short time with an appropriate frequency using a tempered material and thinly quenched, the hardness of the entire gear becomes low due to the decrease in hardness of the transition layer.

  Further, even if the whole tooth is induction-hardened as shown in FIG. 7 or only the tooth surface is induction-hardened as shown in FIG. The difference in hardness from this part (inside) was so great that the strength transition at the boundary between the two was steep and sometimes the boundary between the two was destroyed when subjected to an impact.

  That is, as described above, the hardened part is about HV720 in terms of Vickers hardness, while the part adjacent to this is the hardness of the material before quenching, and is about HV230 in terms of Vickers hardness. However, the difference in hardness between the two is significant. For this reason, when subjected to an impact, the boundary between the two is destroyed, and the tooth surface may be chipped.

  In addition, when steel products are manufactured using induction hardening, there is a problem that the carbon content of the material is large and machinability and malleability are poor as compared with carburizing and quenching.

  That is, in the case of manufacturing steel products using steel part materials as raw materials, most of them are machined in the middle. For example, a steel part material is subjected to a cutting process such as a lathe process, a milling process, and a hobbing process, and then a quenching process is performed to complete a product. Alternatively, a steel part material is plastically deformed (forged) and formed into a predetermined shape, and then a quenching process is performed to complete the product.

On the other hand, when induction hardening is adopted as a quenching measure, the steel parts material to be used is limited and carbon steel material for machine structure (SC material) or alloy steel for machine structure is used. It is necessary to use steel parts material containing medium or higher. More specifically, when adopting induction hardening, it is necessary to perform molding using medium carbon steel or high carbon steel.
On the other hand, carburizing and quenching can use steel part materials (low carbon steel) having a low carbon content.
In general, steel parts containing a large amount of carbon, such as carbon steel materials for machine structures (SC materials), are harder than steel part materials having a low carbon content. That is, the induction hardening material is harder than the carburizing material. Therefore, there was a complaint that cutting and plastic deformation are difficult and molding is difficult.

  Further, when the steel material is spheroidized (spheroidizing annealing), spheroidized carbides appear in the structure and the machinability is improved. That is, the spheroidized material is softer than the normalizing material and the tempered material. However, on the other hand, the material that has been spheroidized does not diffuse carbides when heated for a short time. Therefore, in the induction hardening characterized by short-time heating, it is difficult to diffuse the carbides, and the normal induction hardening cannot exhibit sufficient hardness.

  Accordingly, the present invention focuses on the above-described problems of the prior art, and an object of the present invention is to develop a high-frequency quenching method having high surface hardness and excellent internal toughness. At the same time, it is an object of the present invention to develop a method for manufacturing a quenched product that is easy to cut and forge, has high surface hardness, and can be hardened with excellent toughness inside. To do.

  In order to solve the above-mentioned problems, the present inventor earnestly researched and induction-hardened steel parts having a structure containing spherical carbide twice, and in the first first induction heating process, If the oscillation frequency of the high-frequency oscillator is set high and the oscillation frequency of the high-frequency oscillator is set low in the second second high-frequency heating process, the inside of the steel part material will be quenched in an ideal state. I found

  The invention according to claim 1 completed on the basis of this finding includes a first high-frequency heating step of high-frequency heating a steel part having a structure containing a spheroidized carbide, and the steel subjected to the first high-frequency heating step. A high-frequency quenching method characterized by sequentially performing a second high-frequency heating step of high-frequency induction heating a manufactured part at a frequency lower than the frequency in the first high-frequency heating step, and a rapid cooling step of rapidly cooling the steel part. It is.

  According to the second aspect of the present invention, the surface of the steel part is heated to a temperature much higher than the Ac3 point by the first high-frequency heating step, and then slowly cooled or allowed to cool, and then the second high-frequency heating step. The surface of the steel part is heated to a temperature deeper than the heating depth in the previous first high-frequency heating step and near the Ac3 point over a longer time than the previous first high-frequency heating step, and then the rapid cooling step is performed. The induction hardening method according to claim 1, wherein the induction hardening method is performed.

  The invention according to claim 3 is the induction hardening according to claim 1 or 2, wherein the frequency in the first high-frequency heating step is 50 kHz or more, and the frequency in the second high-frequency heating step is 30 kHz or less. Is the method.

  Further, the frequency in the first high-frequency heating step is desirably 80 kHz or more.

  The invention according to claim 4 includes a spheronization step of subjecting the steel material to spheroidizing annealing, and a forming step of cutting and / or forging the steel material into a predetermined workpiece shape. It is a manufacturing method of a product made of steel, which is hardened by the induction hardening method of any one of 1 to 3.

  According to the present invention, the steel part is excellent in that the hardness of the surface is high and the inside has a medium hardness and a deep toughness region. In addition, since the material before quenching has a structure in which carbides are spheroidized, the hardness before quenching is low, and molding performed before quenching is easy.

It is a flowchart which shows the process of the manufacturing method of the product which uses the steel of embodiment of this invention as a raw material. It is a conceptual diagram of the induction hardening apparatus used by embodiment of this invention. It is a partial cross section figure of the steel parts (gear) before performing induction hardening. It is a partial cross section figure of the steel parts (gear) in the case of the 1st high frequency heating process. It is a partial cross section figure of the steel parts (gear) in the case of the 2nd high frequency heating process. 1 is a partial cross-sectional view of a steel part (gear) subjected to induction hardening according to an embodiment of the present invention. It is a partial cross section figure of the steel parts (gear) which hardened the whole tooth | gear by the induction hardening method of the prior art. It is a partial cross section figure of the steel parts (gear) which hardened only the surface part of the tooth | gear by the induction hardening method of the prior art.

Embodiments of the present invention will be further described below. In the present embodiment, a case where the gear is quenched will be described as an example.
The workpiece employed in this embodiment is a gear (steel part) 1. The gear 1 is made of carbon steel such as S45C and S50C, or alloy steel such as chrome molybdenum steel (SCM). In this embodiment, AISI 4150 or SCM440 is used.

A gear (steel part) 1 as a workpiece is formed by cutting a material using a known hob for gear cutting, but in this embodiment, a steel part as a material prior to forming by cutting. Is spheroidized (spheroidizing annealing). That is, carbides are precipitated in a spherical shape in the structure. The method of the spheroidizing treatment is known, and there are the following four types as typical methods.
That is, in the first method, a steel part is heated to a temperature between A1 temperature and A3 temperature or Acm temperature and gradually cooled.
In the second method, the steel part is heated to a temperature between the A1 temperature and the A3 temperature or the Acm temperature, and then kept at a temperature equal to or lower than the A1 temperature and then cooled.
In the third method, the steel part is heated to a temperature between A1 temperature and A3 temperature or Acm temperature, and then maintained at a temperature equal to or lower than the A1 temperature. To do.
In the fourth method, the steel part is heated to the A1 temperature and held at the A1 temperature.
The spheroidized material includes spheroidized carbides. The spheroidized material is softer than normalizing materials and tempered materials. However, as described above, the material subjected to the spheroidization treatment does not progress in the diffusion of carbides when heated for a short period of time. Therefore, the diffusion of carbides is difficult in the induction hardening characterized by short time heating. Quenching cannot exhibit sufficient hardness.

And in this embodiment, the gearwheel 1 is shape | molded using the steel parts material which performed the spheroidization process.
A steel material having a structure having such a spheroidized carbide has high machinability, and a tooth profile can be easily formed using a known hob.
As shown in FIG. 3, the formed gear 1 has a structure in which carbides are spheroidized at any part.

Subsequently, the formed gear 1 is induction-quenched.
FIG. 2 is a conceptual diagram of the induction hardening apparatus 2. The induction hardening device 2 includes a high frequency power source 5, a current transformer 6 and an induction heating coil 7.
The high frequency power source 5 includes a high frequency oscillator 10 and can output a high frequency alternating current having a desired frequency.
The induction heating coil 7 is, for example, a known circle type coil, and has a size that covers the circumference of the gear 1. The shape and structure of the induction heating coil 7 are arbitrary, and may be a simple circular shape or a concavo-convex shape corresponding to the tooth profile. A semi-open type coil may also be used. Furthermore, the coil of the structure which raises a tooth | gear one by one may be used.

In the present embodiment, the gear 1 is first subjected to the first high-frequency heating process. That is, the oscillation frequency of the high frequency oscillator 10 is set high, and a high frequency high frequency alternating current is applied to the induction heating coil 7.
As a result, a secondary current flows through the gear 1, but since the current supplied to the induction heating coil 7 has a high frequency, the secondary current flows only on the surface of the gear 1. Therefore, as shown in FIG. 4, only the surface of the gear 1 generates heat, and the inside does not generate heat. However, since heat conduction is received from the heat generating region, the temperature rises inside, but the internal temperature rise is small, so that the carbide in the structure does not melt and diffuse.

In the first high-frequency heating step, the surface temperature of the gear 1 is set to a temperature that is much higher than the Ac3 point, and carbide that is a difficult-to-diffuse structure is preliminarily diffused to some extent.
The surface temperature of the gear 1 is set to a temperature that exceeds the Ac3 point by 200 degrees Celsius or more. When the heating time is extremely short, specifically, the surface temperature is desirably 1000 degrees Celsius or more.
When alloy steel such as AISI 4150 is used, it is desirable that the temperature be higher because carbide diffusion is worse than carbon steel. It is recommended to raise the temperature to about 1200 degrees Celsius instantaneously.
Further, it is recommended that the thickness of the heat generation diffusion region is about 1 mm to 2 mm.
The time during which the heat generation region is maintained at a high temperature is very short, specifically, about 0.1 to 0.5 seconds. The time during which the heat generating region is maintained at a high temperature is a time within a range in which internal diffusion of carbide due to overheating can be suppressed.

As a result, the vicinity of the surface has an austenite structure, and the spheroidized carbide in the structure dissolves and diffuses. On the other hand, the internal non-heated region is substantially unchanged in structure and maintains a state in which spherical carbides are precipitated.
By allowing the gear 1 to cool after the first high-frequency heating step, a structure state is obtained in which spherical carbides can be completely diffused during the subsequent second high-frequency heating step.
In the present embodiment, the induction heating coil 7 is energized for a certain time, and then the energization is stopped and left. As a result, the temperature of the gear 1 is lowered due to the effect of heat sinking on the inside. Further, when the temperature of the gear 1 is lowered, the surface portion becomes an ordinary structure or a fine pearlite structure by self-cooling, and a structure in which a part of undissolved spherical carbide remains therein. On the other hand, the deeper part maintains a structure containing many spherical carbides.

Next, the gear 1 is subjected to a second high-frequency heating process. That is, the oscillation frequency of the high-frequency oscillator 10 is lowered, or the gear 1 is moved to another station, a high-frequency alternating current having a lower frequency than that of the previous induction is supplied to another induction heating coil 7, and the gear 1 is brought close to the other coil. .
As a result, a secondary current flows through the gear 1, but since the current supplied to the induction heating coil 7 has a low frequency, the secondary current flows not only on the surface of the gear 1 but also in the deep portion. Therefore, as shown in FIG. 5, heat is generated from the tooth root of the gear 1 to a deeper region.
Moreover, in a 2nd high frequency heating process, it heats to temperature lower than an above described 1st high frequency heating process. More specifically, the heating region is heated so that the surface temperature is about the Ac3 point. More desirably, heating is performed at around 800 degrees Celsius.

  During the second high-frequency heating step, the time during which the heat generation region is maintained at a high temperature is much longer than that during the first high-frequency heating step. The reference heating time is about 2 to 3 times that of the gear 1 module.

  In addition, it is recommended that the thickness (depth) of the heat generating region be several mm or more from the tooth base.

As a result, the surface region previously heated to a high temperature by the first high-frequency heating process and pre-diffused is reheated during the second high-frequency heating process, and the remaining spherical carbide diffuses and disappears, and the inherent carbon It becomes a uniform austenite structure according to the amount. That is, the reheat region in FIG. 5 has a structure in which the spheroidized carbides have almost completely disappeared.
On the other hand, the region deeper than that is a region heated for the first time in the second high-frequency heating step, and becomes an austenite structure containing undissolved spherical carbides.

When the gear 1 is rapidly cooled, the surface reheat region is transformed into only a martensite structure. In addition, the region heated for the first time by the second high-frequency heating step is transformed into a mixed structure of undissolved spherical carbide and low carbon martensite.
After quenching, reheat and temper as necessary.

  The hardness distribution of each part is as shown in FIG. 6. The surface layer has a very hard structure with a Vickers hardness of about HV740, the intermediate layer has a slightly hard structure with a Vickers hardness of about HV450, and the deepest layer has a Vickers hardness. It becomes a soft structure with a hardness of about HV200 or less.

Therefore, the tooth surface is excellent in wear resistance, and the inside and root portion of the tooth show high strength and toughness.
Furthermore, since the difference in hardness at the interface of each layer is small, no delamination occurs. Therefore, the gear 1 after induction hardening has an ideal hardness distribution and is strong.

  In the embodiment described above, the present invention has been described by taking a gear as an example, but steel parts are not limited to those formed in a gear. The product adopting the present invention is not limited, but it is recommended to adopt the present invention for a product that always receives both impact and friction. Further, when the present invention is applied to a gear, when the module is applied to a gear of 10 or less, a more remarkable effect is exhibited.

  The present invention has high hardness on the surface, excellent wear resistance, internal toughness higher than that of the material, the intermediate transition layer can also have a gentle strength (hardness) gradient, and is suitable for high-strength members A quenching method is provided.

1 Gears (steel parts)
2 Induction hardening 5 High frequency power supply 10 High frequency oscillator

Claims (4)

  A first high-frequency heating step for high-frequency heating a steel part having a structure containing spheroidized carbides, and a frequency lower than the frequency during the first high-frequency heating step for the steel part that has undergone the first high-frequency heating step A high-frequency quenching method comprising sequentially performing a second high-frequency heating step of high-frequency induction heating in step 1 and a rapid cooling step of rapidly cooling the steel part.   In the first high-frequency heating process, the surface of the steel part is heated to a temperature much higher than the Ac3 point, and then slowly cooled or allowed to cool. 2. The method according to claim 1, wherein the temperature is raised to a temperature deeper than a heating depth in one high-frequency heating step and in the vicinity of the Ac3 point over a longer time than the first high-frequency heating step, and then a rapid cooling step is performed. The induction hardening method as described.   The induction hardening method according to claim 1 or 2, wherein the frequency in the first high-frequency heating step is 50 kHz or more, and the frequency in the second high-frequency heating step is 30 kHz or less.   A spheroidizing step of subjecting the steel material to spheroidizing annealing, and a forming step of cutting and / or forging the steel material into a predetermined workpiece shape, and thereafter induction heating according to any one of claims 1 to 3. A method for producing a product made of steel, which is quenched by a method of placing.

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