JP5577573B2 - Vacuum carburizing method and vacuum carburizing apparatus - Google Patents

Description

  The present invention relates to a vacuum carburizing treatment method and a vacuum carburizing treatment apparatus, and more particularly to a vacuum carburizing treatment method and a vacuum carburizing treatment apparatus suitable for use in high alloy steel.

  In the surface hardening method of high alloy steel, tempering such as quenching and tempering is generally used. In addition, when higher hardness is required, surface treatment such as nitriding treatment or PVD coating treatment may be performed. However, since these methods are methods for attaching foreign substances to the structure of the base material, there are problems such as peeling. On the other hand, the vacuum carburizing process is a process that reinforces the base material itself, and is therefore a method with less fear of peeling.

The vacuum carburizing process is one of the carburizing processes for increasing the hardness of the surface layer part by carburizing and quenching the surface layer part of the metal workpiece. Examples of the vacuum carburizing treatment include those described in Patent Document 1 and Patent Document 2 below.
In the vacuum carburizing process shown in Patent Document 1, the workpiece is heated to a predetermined temperature in an extremely low pressure state in the heating chamber, and after the carburizing gas such as acetylene is charged in the heating chamber and the workpiece is carburized, The supply of the carburizing gas is stopped, and the heating chamber is again brought into an extremely low pressure state to diffuse carbon near the surface of the object to be processed into the interior, cool down to the quenching temperature, and then cool with oil.
The vacuum carburizing process shown in Patent Document 2 is a furnace (Patent Document 1) at the initial stage of diffusion in the vacuum carburizing process as in Patent Document 1 in order to improve excessive carburization of the surface (especially corners) of the workpiece. The decarburizing gas is introduced into the heating chamber) to reduce or remove cementite on the surface of the workpiece.
Patent Document 3 discloses one of plasma carburizing methods.
JP-A-8-325701 JP 2004-115893 A JP-A-10-158780

  However, in the methods described in Patent Documents 1 to 3, the content of carbon in the base material is not sufficient, and the desired hardness may not be obtained. In addition, there is a problem that the hardened surface layer is shallow and the improvement of toughness is insufficient.

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a vacuum carburizing method and a vacuum carburizing apparatus that can sufficiently improve the hardness and toughness of an object to be processed.

  In order to achieve the above object, a vacuum carburizing method according to the present invention includes a temperature raising step of raising the temperature of a workpiece in a heating chamber to a predetermined temperature, and the temperature of the workpiece to the predetermined temperature. A carburizing step of carburizing the workpiece by supplying a carburizing gas into the heating chamber from a state where the heating chamber containing the workpiece is decompressed to an extremely low pressure atmosphere in a state of reaching the workpiece; and the carburizing step Thereafter, the supply of the carburizing gas is stopped and a diffusion step of diffusing carbon, which is a constituent element of the carburizing gas, from the surface to the inside of the object to be processed, and after the diffusion step, the object to be processed is A cooling step for cooling, wherein the carburizing step and the diffusion step are alternately repeated a plurality of times.

  In the vacuum carburizing method of the present invention, in mass%, C: 0.31% to 0.6%, Si: 0.1% to 0.6%, Mn: 0.3% to 1.0%, Ni : 0.05% to 0.6%, Cr: 3.0% to less than 5.0%, one or two of Mo or W is equivalent to Mo (Mo + 1 / 2W): 0.8% to 4 0.0%, V or Nb, one or two of them is equivalent to V (V + 1 / 2Nb): 0.5% to 1.5%, and the high-alloy steel consisting of Fe and inevitable impurities in the balance is treated. It is desirable to make it.

  Alternatively, in mass%, C: 0.8% to 1.6%, Si: 0.4% or less, Mn: 0.6% or less, P: 0.03% or less, S: 0.03% or less, Cr: 8.0% to 13.0%, Mo: 0.8% to 2.0%, V: 0.2% to 0.5%, Cu: less than 0.25%, Ni: 0.5% It is desirable to use the following high carbon steel as the workpiece.

  In the vacuum carburizing method according to the present invention, the treatment may be performed in a nitrogen gas atmosphere in all or a part of the last diffusion step among a plurality of repetitions.

  The carburizing step and the diffusion step may be alternately repeated a plurality of times, and after a cooling step, a reheating step of reheating the object to be processed may be further provided.

  Furthermore, the reheating step includes a temperature raising step for raising the temperature of the object to be processed to a predetermined temperature, and a holding step for holding the object to be processed at the predetermined temperature. In some cases, the treatment may be performed in a nitrogen gas atmosphere.

  In a first vacuum carburizing apparatus of the present invention, a workpiece having a heating chamber having a heater, a cooling chamber having a cooler, and a workpiece to be processed in the heating chamber having reached a predetermined temperature. The carburizing gas is supplied to the heating chamber from a state where the accommodated heating chamber is depressurized to an extremely low pressure atmosphere, so that the workpiece is carburized, and the supply of the carburizing gas is stopped to configure the carburizing gas. After the element carbon is diffused from the surface of the object to be processed to the inside, the carburization and the diffusion are alternately repeated a plurality of times, and then the object to be processed is cooled in the cooling chamber. And a control means for controlling the cooling chamber.

  The second vacuum carburizing apparatus of the present invention includes a heating chamber having a heater and a cooler, and the heating chamber in which the workpiece is accommodated in a state where the workpiece in the heating chamber has reached a predetermined temperature. The carburized gas is supplied into the heating chamber from a state where the pressure is reduced to an extremely low pressure atmosphere, the workpiece is carburized, the supply of the carburizing gas is stopped, and carbon which is a constituent element of the carburizing gas is added. Control means for controlling the heating chamber so as to cool the workpiece in the heating chamber after diffusing from the surface of the workpiece to the inside and repeating the carburization and the diffusion alternately a plurality of times; , Provided.

  In general, when only a single carburizing process is performed, a strong affinity element such as carbon (C) is included in addition to iron (Fe), which is the main constituent element, in the base material of the workpiece. Solubilization tends to occur and carbon tends to accumulate only on the surface. On the other hand, according to the vacuum carburizing treatment method of the present invention, by repeating the carburizing step and the diffusion step a plurality of times alternately, the carbon that has entered the surface of the workpiece in the first carburizing step is It diffuses inside the object to be processed in the diffusion process, carbon is replenished from the surface again in the second carburizing process, and after the carbon concentration on the surface rises, the carbon diffuses inside the object to be processed in the next diffusion process. . At this time, in the second diffusion step, the carbon that has entered the workpiece in the first carburizing step undergoes two diffusion treatments, and thus further diffuses into the workpiece. Such behavior is repeated a plurality of times.

  That is, according to the vacuum carburizing treatment method of the present invention, by repeating the set of the carburizing step and the diffusion step a plurality of times, the carbon is replenished from the surface of the workpiece in each carburizing step, and the carbon in each diffusion step. Moves from the surface of the workpiece to the inside. Thereby, the gradient of the carbon concentration profile in the object to be processed becomes gentle while moving in the direction in which the concentration increases as a whole. As a result, the surface hardened layer can be formed deeply while maintaining the level of carbon content in the base material of the object to be processed, so that the object to be processed having excellent hardness and toughness can be obtained without causing surface peeling. Can do.

  In addition, when a high alloy steel having the above composition is used as an object to be treated, the additive elements other than carbon do not inhibit the diffusion of carbon, improve hardenability and toughness, and precipitate fine carbides. Since it is obtained, it is preferable as an object to be processed of the present invention.

  Further, in all or part of the last diffusion step and the holding step in the reheating step, when processing is performed in a nitrogen gas atmosphere, nitride such as chromium nitride is formed on the surface of the object to be processed. Surface hardness and wear resistance can be improved.

  In addition, when the workpiece is reheated after repeating the carburizing step and the diffusion step a plurality of times alternately and passing through the cooling step, the carbon concentration gradient in the workpiece can be made gentler. The toughness of the workpiece can be improved.

  According to the first and second vacuum carburizing apparatuses of the present invention, the heating chamber and the cooling chamber are controlled so that the carburizing process and the diffusion process are alternately repeated a plurality of times, as in the vacuum carburizing process of the present invention. Therefore, the carbon content in the base material can be sufficiently increased, the surface hardened layer can be formed deeply, and a workpiece having excellent hardness and toughness can be obtained.

Hereinafter, an embodiment of a vacuum carburizing apparatus and a vacuum carburizing method according to the present invention will be described with reference to the drawings.
In each of the following drawings, the scale of each member is appropriately changed in order to make each member a recognizable size.

FIG. 1 is a cross-sectional view showing the configuration of the vacuum carburizing apparatus of the present embodiment.
As shown in FIG. 1, the vacuum carburizing apparatus of the present embodiment is a two-chamber type apparatus that includes a case 1, a heating chamber 2, and a cooling chamber 3, and performs heating and cooling in separate chambers. The case 1 has a substantially cylindrical shape, is installed with the axis line horizontal, the heating chamber 2 is accommodated at one side separated by the approximate center in the axial direction, and the other is a cooling chamber 3. An opening / closing mechanism 12 that opens or closes the cooling chamber 3 by raising and lowering the door 11 that opens and closes the inlet 3 a of the cooling chamber 3 is provided at a substantially central portion in the axial direction of the case 1.

  The heating chamber 2 includes a heat insulating partition wall 21, a heater 22, a power supply unit 23 (shown in FIG. 3), a cooler 24, and a mounting table 25. Here, FIG. 2 is a perspective view showing the shape of the heater 22. FIG. 3 is a schematic diagram showing an attachment structure of the heater 22 to the heat insulating partition wall 21 and an electrical connection between the heater 22 and the power supply unit 23.

  As shown in FIG. 3, the heat insulating partition 21 is formed by interposing a heat insulating material 21c between a metal outer wall 21a and a graphite inner wall 21b. Moreover, as shown in FIG. 1, doors 21d and 21e are provided on the upper and lower surfaces of the heat insulating partition wall 21, respectively.

As shown in FIG. 2, the heater 22 includes three heaters H1 to H3 of the same type. Each of the heaters H1 to H3 includes a hollow thin shaft portion g1, a solid thin shaft portion g2, a solid thick shaft portion g3, connectors c1 to c3, and a power feeding shaft portion m. The hollow thin shaft portion g1, the solid thin shaft portion g2 and the solid thin shaft portion g3 are made of graphite. The feeding shaft portion m is made of metal.
The connector c1 has a rectangular parallelepiped shape, and is provided with connecting portions a1 and b1 that are opposite to each other in each of the regions divided in half in the longitudinal direction, and has a hollow thin shaft portion g1 and a solid thin portion. The shaft part g2 is connected to be energized. The connector c2 has an L shape so that the two connection portions a2 and b2 face each other in the orthogonal direction, and connects the hollow thin shaft portions g1 to each other so as to be energized. The connector c3 is formed by connecting two connection portions a3 and b3 facing in the same direction so as to be separated from each other, and connects the hollow thin shaft portions g1 so as to be energized.

  The hollow thin shaft portion g1 is arranged so as to form a rectangle with four pieces, and three corners of the rectangle are connected by a connector c2. The solid thin shaft portion g2 is connected to one end of each of the two hollow thin shaft portions g1 forming one remaining corner of the rectangle by the connector c1, and the other is the connection portion a3 of the connector c3. , B3. The end opposite to the end attached to the connector c1 of the solid thin shaft portion g2 is connected to one end portion of the solid thick shaft portion g3, and the other end portion of the solid thick shaft portion g3 is connected to the other end portion of the solid thick shaft portion g3. A feeding shaft portion m is attached.

The four hollow thin shaft portions g1, the solid thin shaft portion g2, the solid thick shaft portion g3, the connector c1, the three connectors c2, and the power feeding shaft portion m as described above form a pair. Each heater H1-H3 is comprised by connecting by c3.
The hollow thin shaft portion g1, the solid thin shaft portion g2, and the solid thin shaft portion g3 have different easiness to generate heat due to the difference in their cross-sectional areas. Heat is likely to be generated in the order of the thin shaft portion g2 and the solid thick shaft portion g3, and the solid thick shaft portion g3 is difficult to generate heat.

As shown in FIG. 3, the power supply shaft portion m is hollow, and a cooling pipe t is accommodated therein. Cooling water that suppresses a temperature rise due to energization is circulated through the cooling pipe t.
The heaters H <b> 1 to H <b> 3 are supported by a heater support portion 26 provided in a part of the heat insulating partition wall 21. The heater support portion 26 is made of ceramics and is formed in a substantially cylindrical shape whose inner diameter is larger than the diameter of the solid thick shaft portion g3, and the axial direction of the cylinder is parallel to the thickness direction of the heat insulating partition wall 21. It fixes so that each edge part may be located in the inner side and the outer side of the heat insulation partition 21, respectively. An opening 26a having the same diameter as that of the solid thick shaft portion g3, which is smaller than the inner diameter of the cylinder, is provided at an end located outside the heat insulating partition wall 21. The solid thick shaft portion is provided in the opening 26a. Each heater H1-H3 is supported by g3 being fitted.

Further, the feeding shaft portion m is led out of the case 1 through an opening 1 a provided in the case 1. A gap between the opening 1a and the power feeding shaft portion m is sealed by being closed with a sealing material 1b. A power supply unit 23 is connected to the power supply shaft unit m.
The power supply unit 23 includes a power supply 23a, a breaker 23b, a thyristor 23c, a temperature controller 23d, a transformer 23e, a resistor 23f, and an ammeter 23g.

The power source 23a is connected to the power supply shaft portion m through the breaker 23b, the thyristor 23c, and the transformer 23e, and supplies power to the power supply shaft portion m. The breaker 23b cuts off power when the load on the circuit exceeds the allowable range, and prevents the circuit from being overloaded.
The thyristor 23c cooperates with the temperature controller 23d to turn on the circuit until the temperature of the heaters H1 to H3 reaches a predetermined temperature, and releases the conduction when the temperature of the heaters H1 to H3 reaches the predetermined temperature. The transformer 23e converts the voltage of power supplied from the power source 23a into a predetermined value.
The resistor 23f and the ammeter 23g are disposed in the middle of a circuit that is branched from the circuit between the transformer 23e and the feeding shaft portion m and grounded. The ammeter 23g measures the ground fault current.

  As shown in FIG. 1, the cooler 24 is provided in the upper part of the heat insulation partition 21, and has the heat exchanger 24a and the fan 24b. The heat exchanger 24 a is for removing heat from the gas heated in the heating chamber 2. The fan 24b circulates the gas in the heating chamber 2 and the case 1. When cooling the inside of the heating chamber 2, the doors 21d and 21e of the heat insulating partition wall 21 are opened, and the gas in the heating chamber 2 and the case 1 is circulated by the fan 24b and cooled by the heat exchanger 24a. The temperature in the heating chamber 2 and the temperature of the workpiece W in the heating chamber 2 are decreased.

The mounting table 25 is configured to have a rectangular frame and a plurality of rollers, and each roller is arranged in parallel with two opposite sides of the frame, and the other two sides of the frame. The two ends are supported rotatably. Such a mounting table 25 is installed so that the rotation axis of each roller is orthogonal to the transport direction, and facilitates the transfer of the workpiece W. The workpiece W is evenly heated from the lower surface side by being placed on the placement table 25.
In the vacuum state, the higher the temperature, the lower the vapor pressure is evaporated in order, so that each part exposed to a high temperature in the heating chamber 2 can increase the temperature of the heating chamber 2 to about 1300 ° C. Use a material that does not evaporate.

The cooling chamber 3 is a room for cooling the workpiece W, and includes a cooler 31, a current plate 32, and a mounting table 33.
The cooler 31 includes a heat exchanger 31a and a fan 31b. The heat exchanger 31 a is for removing heat from the gas in the cooling chamber 3. The fan 31 b circulates gas at a high pressure in the cooling chamber 3.
The rectifying plate 32 is a combination of a lattice box partitioned in a lattice shape and punching metal, and is disposed above and below the position in the cooling chamber 3 where the workpiece W is placed, The flow direction of the gas in 3 is adjusted. The mounting table 33 has substantially the same structure as the mounting table 25 installed in the heating chamber 2 and is disposed at the same height as the mounting table 25.

  The vacuum carburizing apparatus of the present embodiment is omitted in the drawings, but the temperature, pressure, heating rate, cooling rate, processing time of each process, various gas flow rates, etc. in the heating chamber 2 and the cooling chamber 3 are omitted. A control unit (control means) for controlling various processing conditions is provided.

Next, a vacuum carburizing method performed by the vacuum carburizing apparatus having the above configuration will be described with reference to FIG.
FIG. 4 is a diagram illustrating a processing time, a processing temperature, and a pressure profile for each process.
Here, an example is shown in which a hot die steel (SKD61) or high strength hot die steel used for an aluminum die casting die is used as a workpiece.

  In particular, as an object to be processed, C: 0.31% to 0.6%, Si: 0.1% to 0.6%, Mn: 0.3% to 1.0%, Ni: 0% by mass. 0.05% to 0.6%, Cr: 3.0% to less than 5.0%, one or two of Mo or W being Mo equivalent (Mo + 1 / 2W): 0.8% to 4.0 It is desirable to use a high alloy steel in which any one or two of%, V or Nb is V equivalent (V + 1 / 2Nb): 0.5% to 1.5%, and the balance is Fe and inevitable impurities.

  C is an element necessary for improving hardness and strength by imparting a sufficient matrix hardness by quenching and tempering and forming a carbide by combining with Cr, Mo, V, Nb and the like. However, if it is less than 0.31%, the effect cannot be sufficiently obtained, and if it exceeds 0.6%, toughness and hot workability are deteriorated, which is not preferable.

  Si is an element necessary as a deoxidizer during steelmaking. However, if it is less than 0.1%, the effect cannot be obtained sufficiently, and if it exceeds 0.6%, the toughness is lowered and the diffusion of C during carburization is inhibited, which is not preferable.

  Ni is an element that contributes to improving hardenability and toughness. However, if it is less than 0.05%, the effect cannot be sufficiently obtained, and if it exceeds 0.6%, the machinability is lowered, which is not preferable.

  Cr is an element that contributes to improvement in hardness and hardenability. However, if it is less than 3.0%, the effect cannot be sufficiently obtained, and if it is 5.0% or more, coarse carbides are likely to be generated during solidification, and grain boundary carbides are likely to precipitate and grow during carburization, resulting in toughness. Is not preferable.

  Mo and W can be added singly or in combination, and are elements that precipitate fine carbides during tempering and improve softening resistance, hardness, and high-temperature strength. In addition, fine carbides are precipitated during carburizing to improve wear resistance and erosion resistance, and the fine carbides suppress the coarsening of crystal grains during carburizing and contribute to securing toughness. However, if it is less than 0.8%, the effect cannot be sufficiently obtained, and if it exceeds 4.0%, the amount of fine carbides precipitated during tempering increases and the toughness and hot workability are deteriorated.

  V and Nb can be added singly or in combination, and dissolve in the matrix during quenching heating, precipitate fine carbides that do not easily agglomerate during tempering, increase softening resistance in the high temperature range, and provide large high temperature strength. In addition, it has the effect of suppressing the coarsening of crystal grains and ensuring toughness. However, if it is less than 0.5%, the effect cannot be sufficiently obtained, and if it exceeds 1.5%, the toughness and hot workability are deteriorated.

  As shown in FIG. 4, the vacuum carburizing process of the present embodiment includes: A: first temperature raising step, B: temperature rising holding step, C: second temperature raising step, D: pre-carburizing holding step, E: first 1 carburizing step, F: first diffusion step, G: second carburizing step, H: second diffusion step, I: cooling step.

  In the temperature raising, carburizing, and diffusion steps, first, the workpiece W is placed at a position surrounded by the heaters H1 to H3 in the heating chamber 2. Subsequently, the heating chamber 2 is evacuated and the inside of the heating chamber 2 is depressurized to a vacuum state (very low pressure atmosphere). Here, in a general vacuum carburizing process, “vacuum” refers to about 10 kPa or less, which is about 1/10 of the atmospheric pressure, but in this embodiment, 1 Pa or less is defined as “vacuum”.

  Next, the heater 22 is energized to raise the temperature in the heating chamber 2 (first temperature raising step). Although the temperature may be raised to a target temperature (1030 ° C. in the present embodiment) at a time in one temperature raising step, the temperature raising step is divided into two in this embodiment. Raise the temperature inside. The heating rate is 10 ° C./min.

  Next, the heating chamber 2 is held at 850 ° C. for 40 minutes (temperature rising holding step). Even if the workpiece W has a certain heat capacity, the temperature of the workpiece W can easily follow the temperature in the heating chamber 2 by providing this holding step. As a result, the temperature at the time of shifting to the next pre-carburizing holding process can be accurately controlled.

  Next, the heater 22 is energized to raise the temperature in the heating chamber 2 from 850 ° C. to the target 1030 ° C. (second temperature raising step). In the second temperature raising step, similarly to the first temperature raising step, the temperature raising rate is 10 ° C./min.

When the temperature in the heating chamber 2 reaches 1030 ° C., the process proceeds to the pre-carburizing holding process.
In the pre-carburizing holding step, the temperature in the heating chamber 2 is held at 1030 ° C., which is the temperature at the end of the second temperature raising step, for 40 minutes. By passing through the pre-carburizing holding step, the temperature of the workpiece W is made uniform at 1030 ° C. from the surface to the inside.

  Next, in the first carburizing step, a carburizing gas such as acetylene is supplied into the heating chamber 2. The gas flow rate when supplying the carburizing gas is, for example, 5 L (liter) / min, and the carburizing gas is blown from both side surfaces and the lower surface of the heating chamber 2. At this time, the pressure in the heating chamber rises from the pressure in the pre-carburizing holding process to a predetermined pressure. In the first carburizing step, the workpiece W is carburized by being exposed to a high temperature carburizing gas atmosphere of 1030 ° C. in the heating chamber 2 for 20 minutes.

  Next, in the first diffusion step, the carburizing gas is exhausted from the inside of the heating chamber 2 and is controlled to be in a vacuum state equivalent to the pressure up to the pre-carburizing holding step. And the temperature in the heating chamber 2 shall be 1030 degreeC same as a 1st carburizing process, and it hold | maintains for 200 minutes. Through this first diffusion step, carbon that has entered the vicinity of the surface of the workpiece W is diffused from the surface of the workpiece W to the inside.

  Next, as in the previous two steps, the carburizing gas is supplied into the heating chamber 2 to perform the second carburizing step, and the carburizing gas is exhausted from the heating chamber 2 to perform the second diffusion step. During this time, the temperature in the heating chamber 2 is kept at 1030 ° C. The pressure in the heating chamber 2 is set so that the second carburizing step is the same as the first carburizing step, and the second diffusion step is the same as the first diffusion step. The gas flow rate for carburizing in the second carburizing step is the same as that in the first carburizing step. The processing time is 7 minutes for the second carburizing step and 85 minutes for the second diffusion step, both of which are shorter than the first carburizing step and the first diffusion step. If other processing conditions are the same, the carbon at the surface carbon concentration, the effective carburization depth, and the effective carburization depth depends on the processing time of the first and second carburizing steps and the processing time of the first and second diffusion steps. Since the concentration is determined, the treatment time for each carburizing step and the treatment time for each diffusion step may be set as appropriate according to the desired carbon concentration profile.

When the carburizing step and the diffusion step are repeated a plurality of times, nitrogen gas is introduced into the heating chamber 2 in all or part of the last diffusion step (second diffusion step in this example), Alternatively, the diffusion process may be performed. In this case, the workpiece W is nitrided in the diffusion step. By forming the chromium nitride layer on the surface of the workpiece W, the effect of improving the surface hardness and wear resistance is obtained.
If the above nitriding process is performed when the carburizing process and the diffusion process are performed only once, the chromium nitride layer formed on the surface of the workpiece W will be placed under a high temperature process / vacuum process for a long time. The chromium nitride layer may be altered. On the other hand, when the carburizing step and the diffusion step are repeated a plurality of times, if the nitriding treatment is performed in the last step, the chromium nitride layer on the surface of the workpiece W is left under a high temperature treatment / vacuum treatment for a long time. This can be avoided, and alteration of the chromium nitride layer can be prevented.

  Finally, the workpiece W is transferred to the cooling chamber 3 and the workpiece W is cooled from a high temperature of 1030 ° C. to room temperature (cooling step). In the present embodiment, nitrogen gas, which is an inert gas, is supplied into the cooling chamber 3 from above, and the nitrogen gas is circulated at a high pressure of, for example, about 3 bar by the fan 31b of the cooler 31 to forcibly cool the workpiece W. To do.

  The vacuum carburizing process of this embodiment is completed by the above process. However, after the cooling step, a reheating step of heating the workpiece W again may be performed as shown in FIG. In this example, in the reheating step, the workpiece W is heated from room temperature to 850 ° C. at a temperature rising rate of 10 ° C./min (first temperature rising step) and then held at 850 ° C. for 40 minutes (during the temperature rising). Holding step), and then the temperature is raised from 850 ° C. to 1030 ° C. at a temperature raising rate of 10 ° C./min (second temperature raising step), and then held at 1030 ° C. for 40 minutes (holding step). Thus, by adding the reheating step, the carbon concentration gradient in the workpiece W can be made gentle, and the toughness of the workpiece W can be further improved.

(add to)
Further, when the above reheating step is added, the treatment may be performed in a nitrogen gas atmosphere in the same manner as described above in all or part of the last holding step. In this case, since the workpiece W is nitrided in the holding step, the same effects as described above, such as improvement in surface hardness and wear resistance, can be obtained.

  According to the vacuum carburizing process using the vacuum carburizing apparatus of the present embodiment, carbon is replenished from the surface of the workpiece W in each carburizing process by alternately repeating the carburizing process and the diffusion process twice. In each diffusion step, carbon moves from the surface of the workpiece W to the inside. Thereby, the gradient of the carbon concentration profile in the workpiece W becomes gentle while moving in the direction in which the concentration increases as a whole. As a result, the surface hardened layer can be formed deeply while maintaining the level of the carbon content in the base material of the object to be processed W, so that the object is excellent in hardness and toughness without causing surface peeling. You can get things.

  The inventor has determined by simulation how much the carbon concentration profile in the workpiece is different between the conventional method in which the carburizing step and the diffusion step are performed only once, and the method of the present embodiment. FIG. 6 is a graph showing the simulation results. The horizontal axis in FIG. 6 is the depth (mm) from the surface of the workpiece, and the vertical axis is the carbon concentration (mass%). Curves indicated by reference signs A1 and A2 indicate carbon concentration profiles of a conventional method in which carburization and diffusion are performed once, respectively, reference sign A1 indicates a carburization: 40 minutes, diffusion: 45 minutes sample, reference sign A2 indicates carburization: A sample of 60 minutes and diffusion: 300 minutes is shown. Curves indicated by reference numerals B1 and B2 indicate the carbon concentration profile of the method of the present embodiment in which carburization and diffusion are set twice, and reference numeral B1 indicates the first carburization: 20 minutes, the first diffusion: 200 minutes, 2 Carburization: Sample for 7 minutes, Second diffusion: Sample for 85 minutes, B2 indicates a sample obtained by adding a reheating step to the heat treatment of the sample of B1. Conditions other than those described above are as described in the above embodiment.

  As shown in FIG. 6, in the conventional sample A1 in which carburization and diffusion are performed once each, the surface concentration is as high as 0.85% or more, but the concentration rapidly decreases as the depth from the surface increases. It can be seen that the depth is reduced to 0.4% or less at the position of 2 mm in depth. For this reason, in the to-be-processed object processed on these conditions, improvement of hardness or toughness is not enough, and there exists a possibility that peeling may arise in the location where carbon concentration is changing rapidly. On the other hand, in the sample B1 of this embodiment in which carburization and diffusion were alternately performed twice, although the surface concentration is lower than the sample A1, a level of about 0.7% can be secured, and the concentration gradient It can be seen that is sufficiently smooth compared to sample A1. Thereby, hardness and toughness can be improved and occurrence of peeling can be suppressed. Further, in the sample B2 to which reheating is added, the concentration gradient is further gentler than that of the sample B1, and the same effect can be obtained. On the other hand, with the conventional sample A2, a gentle concentration gradient similar to that of the samples B1 and B2 can be obtained. However, a long processing time of carburization: 60 minutes and diffusion: 300 minutes is necessary, and the productivity is inferior. is there.

  In the said embodiment, although the example of steel for hot metal mold | die (SKD61) and steel for high intensity | strength hot metal mold | die was given as a material of the to-be-processed object W, SKD11 which is steel for high carbon hot metal mold | die other than that. Can also be used. SKD11 is mass%, C: 0.8% to 1.6%, Si: 0.4% or less, Mn: 0.6% or less, P: 0.03% or less, S: 0.03% or less Cr: 8.0% to 13.0%, Mo: 0.8% to 2.0%, V: 0.2% to 0.5%, Cu: less than 0.25%, Ni: 0.5 % Of various elements.

  When using this SKD11, in order to increase the effect, it is desirable to perform the treatment at a temperature higher by about + 50 ° C. than the normal quenching temperature (generally 1025 ° C. to 1050 ° C.). However, when treated at such a temperature, the Acm point (carbon solid solution limit value) increases, and the carbon concentration on the surface increases, while the crystal grain size increases. In this case, a reheating step is essential. That is, by cooling once after quenching, holding at a general quenching temperature after reheating, and quenching, crystal grain enlargement due to excessive temperature rise is eliminated, and a quenched structure is obtained.

When carburizing a high carbon material such as SKD11, the carbon concentration near the surface immediately becomes saturated, and a certain amount or more of carbon cannot be carburized. Therefore, it is possible to increase the amount of carbon to be carburized by repeating the following procedure.
(1) Carburizing the high carbon material (the carbon concentration near the surface is saturated).
(2) Diffusion treatment is performed to diffuse carbon that has been carburized near the surface to the inside of the material to be treated. Thereby, the carbon concentration on the surface of the material to be processed is not saturated.
(3) Since the carbon concentration on the surface of the material to be processed is no longer saturated, carburizing to the vicinity of the surface becomes possible by performing the carburizing process again.
(4) A desired amount of carbon can be carburized by repeating the above steps (1) to (3).

  In the case of SKD61, it was stated that “if the carbon content exceeds 0.6%, the toughness and hot workability are lowered, which is not preferable” (see paragraph [0032]). On the other hand, in SKD11, carburized carbon combines with Cr, Mo, V, etc. to form a carbide, and the hardness and strength are improved. In particular, since the Cr content is higher than that of SKD61, there is no problem even if the carbon content exceeds 0.6%. That is, the description “if the carbon content exceeds 0.6% is not preferable because the toughness and hot workability are deteriorated” describes the circumstances specific to SKD61.

In the above embodiment, the two-chamber type vacuum carburizing apparatus shown in FIG. 1 has been described. However, the carburizing process and the diffusion process are alternately performed using the vacuum carburizing apparatus of another embodiment as in the above embodiment. It is also possible to perform repeated vacuum carburization.
FIG. 7 is a schematic view showing an example of a form of a vacuum carburizing apparatus. As shown in FIG. 7, the vacuum carburizing apparatus includes a single chamber type, a continuous type, and a separate type of transfer device in addition to the two-chamber type of the above embodiment.
The single chamber type is a configuration in which only a heating chamber is provided without a cooling dedicated chamber, and a cooler is provided in the heating chamber. Since the cooler is in the heating chamber, the single chamber type can be used when a steel material with good hardenability is the material to be treated because the temperature decreasing rate is slow.

  The continuous type is a form used when a large number of workpieces W are continuously vacuum carburized, and includes a preheating chamber, a first heating chamber, a second heating chamber, and a cooling chamber. The second heating chamber is provided with a cooler. In such a continuous type, for example, a preheating (temperature raising) process is performed in the preheating chamber, a pre-carburizing holding process, a carburizing process and a diffusion process are performed in the first heating chamber, and a normalizing process, A vacuum carburizing process is performed by a procedure of performing a heating process and a pre-quenching holding process and performing a quenching process in a cooling chamber. Since the workpieces W sequentially move in the processing chamber as the process proceeds, vacuum carburization of a large number of workpieces W can be performed one after another.

In the separate type of transfer device, the heating chamber 2 and the cooling chamber 3 of the above-described embodiment are not provided in the same case 1, but are provided separately, and a transfer device for transferring the workpiece W moving between the two processing chambers is provided. It is a thing. Each process of the vacuum carburizing process is performed in the heating chamber from the preheating process to the pre-quenching holding process in the same manner as in the above embodiment, and the quenching process is performed in the cooling chamber.
Here, the number of heating chambers is not limited to one, and a plurality of heating chambers may be installed. In the vacuum carburizing process, the time required for the heating chamber is longer than the time required for the cooling chamber. Therefore, if the number of heating chambers and cooling chambers is 1: 1, the free time of the cooling chamber becomes longer. However, the number of heating chambers is increased according to the number of objects to be processed, and the objects to be processed are sequentially transferred from the multiple heating chambers to the cooling chamber, thereby reducing the free time of the cooling chamber and effectively using the cooling chamber. it can. In that case, the vacuum carburizing process can be performed efficiently. When a plurality of heating chambers are provided, at least one of them may be provided with a cooler, and the other heaters may be provided without a cooler.

  As an example of the separate type of the transport device, in addition to the illustrated one, one further including a main container and a preparation chamber can be considered. The main container is, for example, a cylindrical sealed container, and one or more heating chambers, cooling chambers, and a preparation chamber are radially connected to the outer peripheral surface of the cylindrical main container, and a transfer device is provided in the main container. Stored. The transfer device rotates in the main container between positions connected to any of the heating chamber, the cooling chamber, and the preparation chamber.

In such a vacuum carburizing apparatus, when a workpiece is put into the preparation chamber, the transfer device transfers the workpiece from the preparation chamber to the heating chamber, and also transfers the workpiece from the heating chamber to the cooling chamber. Then, the workpiece is transferred from the cooling chamber to the preparation chamber. And what is necessary is just to take out a to-be-processed object from a preparation room.
According to the vacuum carburizing apparatus, since the object to be processed always passes through the main container when being transported between the chambers, the object to be processed is subjected to the vacuum carburizing process after being put into the preparation chamber, and the preparation chamber It is possible to ensure that the outside air is not touched until it is taken out. In addition, while another object to be processed can be taken in and out of the preparation chamber while the object to be processed is charged in the heating chamber or the cooling chamber, a vacuum carburizing process is performed when vacuum carburizing a plurality of objects to be processed. Each room of the device can be used effectively.
The shape of the main container is an example, and the main container may be any container that houses the transfer device and is connected to the heating chamber, the cooling chamber, and the preparation chamber.

  Furthermore, by using a transfer device with a heater and / or a cooler, it is possible to transfer between the heating chamber and the cooling chamber while controlling the temperature of the object to be processed. Further, when the heating chamber or the cooling chamber and the transfer device are communicated with each other when the workpiece is transferred, the heater (or cooler) of the transfer device and the temperature in the heating chamber (or the temperature in the cooling chamber) The temperature can be adjusted to the same level. And the to-be-processed object after a vacuum carburizing process can be cooled to normal temperature with the cooler of a conveying apparatus.

As shown in FIG. 8, a convection heating fan F and a motor M that rotationally drives the convection heating fan F may be further provided as components of the heater 22. The convection heating fan F and the motor M constitute a gas convection device.
In such a configuration, for example, when the temperature is raised from a low temperature as in the temperature raising step, an inert gas is charged into the heating chamber 2 to place the workpiece W in an inert atmosphere and convection is performed by the motor M. By energizing the heaters H1 to H3 and generating heat while rotating the heating fan F, the workpiece W can be quickly heated uniformly.
Moreover, in the said embodiment, although it was set as the cooler 31 which circulates high pressure gas and cooled the to-be-processed object W, in implementation, a cooler cools the to-be-processed object W by oil cooling, Also good.

  The technical scope of the present invention is not limited to the above embodiment, and various modifications can be made without departing from the spirit of the present invention. For example, in the above embodiment, the carburizing step and the diffusion step are alternately repeated twice, but may be repeated three times or more. At this time, the processing time of each carburizing step and each diffusion step may be the same or different, and can be appropriately set so as to obtain a target concentration profile.

It is sectional drawing which showed the structure of the vacuum carburizing apparatus of one Embodiment of this invention. It is a perspective view which shows the shape of the heater in the vacuum carburizing apparatus. It is a schematic diagram which shows the attachment structure of the heater with respect to the heat insulation partition in the vacuum carburizing apparatus, and the electrical connection of a heater and a power supply part. It is a figure which shows the profile of the process time for every process, process temperature, and pressure. It is a figure which shows the profile of the process time of a reheating process, process temperature, and a pressure. It is a figure which shows the simulation result of the carbon concentration profile in a to-be-processed object. It is a schematic diagram which shows the example of the other form of the vacuum carburizing processing apparatus in one Embodiment of this invention. It is sectional drawing which showed the structure of the vacuum carburizing processing apparatus in other embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Case, 11 ... Door, 12 ... Opening / closing mechanism, 1a ... Opening, 1b ... Sealing material, 2 ... Heating chamber, 21 ... Heat insulation partition, 21a ... Outer shell, 21b ... Inner shell, 21c ... Heat insulation material, 21d, 21e ... Door, 22 ... Heater, H1-H3 ... Heater, g1 ... Hollow thin shaft portion, g2 ... Solid thin shaft portion, g3 ... Solid thick shaft portion, m ... Feeding shaft portion, t ... Cooling pipe, c1-c3 Connector, a1, b1, a2, b2, a3, b3 ... Connection part, 23 ... Power supply part, 23a ... Power supply, 23b ... Breaker, 23c ... Thyristor, 23d ... Temperature controller, 23e ... Transformer, 23f ... Resistor , 23g ... ammeter, 24 ... cooler, 24a ... heat exchanger, 24b ... fan, 25 ... mounting table, 26 ... heater support, 26a ... opening, 3 ... cooling chamber, 3a ... inlet, 31 ... cooler, 31a ... heat exchanger, 31b ... fan, 32 ... rectifying plate, 33 ... mounting , W ... object to be processed, F ... convection heating fan, M ... motor.

Claims (7)

A temperature raising step for raising the temperature of the workpiece in the heating chamber to a predetermined temperature;
In a state where the temperature of the object to be processed has reached the predetermined temperature, the carburizing gas is supplied into the heating chamber from a state where the pressure in the heating chamber containing the object to be processed is reduced to 1 Pa or less. Carburizing process for carburizing things,
After the carburizing step, the diffusion step of stopping the supply of the carburizing gas and diffusing carbon, which is a constituent element of the carburizing gas, from the surface of the workpiece to the inside;
A cooling step for cooling the workpiece after the diffusion step,
Just repeat a plurality of times and the diffusion step and the carburization step alternately,
A vacuum carburizing method characterized in that processing is performed in a nitrogen gas atmosphere in all or part of the last diffusion step among a plurality of repetitions .   In mass%, C: 0.31% to 0.6%, Si: 0.1% to 0.6%, Mn: 0.3% to 1.0%, Ni: 0.05% to 0.6 %, Cr: 3.0% to less than 5.0%, one or two of Mo or W is equivalent to Mo (Mo + 1 / 2W): 0.8% to 4.0%, either V or Nb One or two of them is a V equivalent (V + 1 / 2Nb): 0.5% to 1.5%, and a high alloy steel consisting of Fe and inevitable impurities as the balance is used as the workpiece. Item 2. The vacuum carburizing method according to Item 1.   In mass%, C: 0.8% to 1.6%, Si: 0.4% or less, Mn: 0.6% or less, P: 0.03% or less, S: 0.03% or less, Cr: 8.0% to 13.0%, Mo: 0.8% to 2.0%, V: 0.2% to 0.5%, Cu: less than 0.25%, Ni: 0.5% or less 2. The vacuum carburizing method according to claim 1, wherein high-carbon steel is used as the workpiece. 4. The method according to claim 1 , further comprising a reheating step in which the carburizing step and the diffusion step are alternately repeated a plurality of times, and after the cooling step, the workpiece is heated again. The vacuum carburizing method according to item. The reheating step includes a temperature raising step for raising the temperature of the workpiece to a predetermined temperature, and a holding step for holding the workpiece at the predetermined temperature, and all or part of the holding step. 5. The vacuum carburizing method according to claim 4, wherein the treatment is performed in a nitrogen gas atmosphere . A heating chamber having a heater;
A cooling chamber having a cooler;
In a state where the object to be processed in the heating chamber reaches a predetermined temperature, the carburizing gas is supplied into the heating chamber from a state where the pressure in the heating chamber containing the object to be processed is reduced to 1 Pa or less. Carburize the object, stop the supply of the carburizing gas to diffuse carbon, which is a constituent element of the carburizing gas, from the surface of the workpiece to the inside, and repeat the carburizing and the diffusion a plurality of times alternately. And a control means for controlling the heating chamber and the cooling chamber so that the object to be processed is cooled in the cooling chamber after all or part of the last diffusion is performed in a nitrogen gas atmosphere. A vacuum carburizing apparatus characterized by that. A heating chamber having a heater and a cooler;
In a state where the object to be processed in the heating chamber reaches a predetermined temperature, the carburizing gas is supplied into the heating chamber from a state where the pressure in the heating chamber containing the object to be processed is reduced to 1 Pa or less. Carburize the object, stop the supply of the carburizing gas to diffuse carbon, which is a constituent element of the carburizing gas, from the surface of the workpiece to the inside, and repeat the carburizing and the diffusion a plurality of times alternately. And a control means for controlling the heating chamber so that the object to be treated is cooled in the heating chamber after all or part of the last diffusion is performed in a nitrogen gas atmosphere. Vacuum carburizing treatment equipment.

Images