JP4821810B2 - Carburizing heat treatment method and carburizing source material - Google Patents

Description

  The present invention relates to a carburizing heat treatment method for a steel material and a carburizing source material used therefor.

  Steel materials are frequently used for various machine parts from the viewpoints of strength, rigidity, cost, and the like. When a steel material is used, it is easy to obtain a member having desired strength, ductility, and the like by appropriately selecting the type of alloy element, its content, heat treatment conditions, and the like.

  However, since strength (hardness) and ductility are in a trade-off relationship, it is easy to obtain a steel member with various characteristics balanced at a high level only by heat treatment such as general quenching and tempering (tempering treatment). is not. Therefore, in terms of fatigue resistance, wear resistance, etc., carburizing heat treatment (surface hardening) that only hardens the surface where high hardness is required and the inside is relatively soft, and strength and ductility can be balanced at a high level. One of the methods is often used.

Carburizing heat treatment is a treatment in which carbon (C) is infiltrated from the surface of a steel material having a relatively small C content, such as case-hardened steel, and the C concentration in the surface portion is increased before quenching. is there. As a result, a so-called “hard and soft” metal structure is obtained in which only the surface portion that tends to be the starting point of breakage or wear is reinforced while maintaining ductility as a whole. Thereby, the steel member excellent in both fatigue resistance, wear resistance, etc. and impact resistance can be obtained.
Such a carburizing heat treatment method includes a solid carburizing method, a gas carburizing method, and a liquid carburizing method depending on differences in carburizing sources (carburizing agents) that are generally used.

  Of these, the gas carburizing method that is currently used frequently is relatively easy to control the formed carburized layer. This gas carburizing method is generally performed by using a gas obtained by modifying a hydrocarbon gas (RX gas) as a carburizing source and adjusting the CO concentration and temperature by controlling the atmosphere. However, the gas carburizing method relies on an equilibrium (gas-solid equilibrium) between the CO gas and the steel material surface. For this reason, in order to form a carburized layer having a thickness (carburizing depth) of about 0.6 to 1 mm, the material to be treated (steel member) is heated in a carburizing gas of about 800 to 900 ° C. for 2 to 4 hours. It will be necessary.

  Recently, however, demands for reducing CO2 emissions, saving energy, and speeding up processes have become stronger. Therefore, a vacuum carburizing method capable of reducing the environmental load, saving energy, shortening the processing time and the like is being used with respect to the conventional gas carburizing method. The vacuum carburizing method is a method in which carbon (Gr) generated by decomposition of a small amount of a hydrocarbon-based gas (carburizing gas) introduced into a vacuum furnace is adhered to the surface of a steel material and carburized. According to the vacuum carburizing method, carburization occurs due to an equilibrium (solid-solid equilibrium) between Gr and iron (Fe), so that the carburizing ability (C penetrating power) is high, and a short time carburizing process is possible. . In addition, the indication regarding a vacuum carburizing method exists in the following patent document etc., for example.

JP 2006-161119 A JP 2007-322036 A JP 2007-224357 A

  However, when the vacuum carburizing method is performed, a portion (excess carburized portion) in which carbon permeates excessively is formed in the surface portion of the steel member in a network shape along the grain boundary of the surface portion of the steel member. The excessive carburized portion is made of carbide, and the network-like carbide causes the surface portion of the steel member to become brittle. Therefore, in the vacuum carburizing method, once the carburizing treatment is performed, a diffusion process for further diffusing the infiltrated C further into the interior is provided to eliminate such a network-like carbide (excess carburizing portion).

  However, for this reason, even if the carburizing process itself is completed in about ten minutes at the most, the diffusion process requires several tens of minutes separately. As a result, from the overall viewpoint, even if the vacuum carburizing method is used, it cannot be said that the processing time is necessarily shortened sufficiently.

  This invention is made in view of such a situation, and it aims at providing the carburizing heat treatment method which can form the carburized layer in which a network-like carbide etc. are not formed in the surface part of a steel member in a short time. To do.

  As a result of extensive research and trial and error, the present inventor has produced an excessive carburized portion (such as a network-like carbide) by using iron carbide, which is a compound of Fe and C, as a carburizing source. And succeeded in forming a preferable carburized layer in a short time. And by developing this result, the present inventor has completed various inventions described below.

<Carburizing heat treatment method>
(1) The carburizing heat treatment method of the present invention is a method for carburizing heat treatment of a steel material in which C is permeated from the surface portion of the steel material to the inside to form a carburized layer having a higher C concentration in the surface portion than in the steel material. Because
A contact step in which a carburizing source material made of iron carbide, which is a compound of Fe and C, is brought into contact with the surface of the steel material; and the steel material up to a carburizing temperature equal to or higher than a temperature at which at least C in the carburizing source material diffuses into the steel material. It comprises a carburizing heating step for heating the steel and a cooling step for cooling the steel material after the carburizing heating step.

(2) The carburizing heat treatment method of the present invention is epoch-making in that iron carbide is used as a carburizing source, and is completely different from the conventional one. According to the carburizing heat treatment method of the present invention, a hard and homogeneous carburized layer having no excessive carburized portion can be obtained in a short time. Moreover, not only is the C penetration time into the surface of the steel material short, but also the diffusion process required when vacuum carburizing, etc. is unnecessary, and the entire carburizing heat treatment can be completed in a very short time. It is.

Although the mechanism by which a preferable carburized layer can be formed in a short time by the carburizing heat treatment method of the present invention is not necessarily clear, the present state is considered as follows.
First, in the case of the conventional vacuum carburizing method, Gr adhering to the surface of the steel material becomes a carburizing source. However, it is considered that Gr cannot directly equilibrate with the austenite phase (γ), which is a high-temperature phase of a normal material (steel material), and maintains a three-phase equilibrium between the γ phase and cementite (θ) on the surface. Therefore, when vacuum carburizing is performed, cementite is inevitably generated in the austenite phase, and the equilibrium C concentration of the surface portion of the steel material becomes a high concentration of 4% or more. As a result, an excessive carburized portion as described above is formed.

  On the other hand, according to the carburizing heat treatment method of the present invention, since iron carbide that directly equilibrates with the γ phase is used as the carburizing source, the steel material under high temperature is in contact with the iron carbide in contact with the surface portion. The γ phase alone is in an equilibrium state. For this reason, the C concentration in the surface portion of the steel material is maintained at about 1 to 2%, which is the equilibrium solid solubility limit (solid solution limit concentration) of iron carbide, and cementite is not generated inside the austenite phase.

  Thus, according to the carburizing heat treatment method of the present invention, C is rapidly infiltrated from the iron carbide into the surface portion of the steel material, but no excessive carburized portion or the like is formed on the surface portion of the steel material. As a result, the entire carburizing heat treatment can be completed in a very short time without requiring a diffusion step.

(3) By the way, a representative example of iron carbide which is a carburizing source used in the present invention is cementite represented by M ₃ C (M: metal element). However, Fe-C binary cementite is a metastable phase and can be decomposed into Fe (γ phase) and Gr at high temperatures. For this reason, if Fe3C is used as it is, for example, the carburizing temperature cannot be increased, and rapid processing becomes difficult. Therefore, in order to facilitate quick carburizing heat treatment, it is preferable to use iron carbide that is more stable even in a high temperature region as a carburizing source. In this regard, the present inventor has already stated that iron carbide can be stabilized even at high temperatures by containing a specific element such as Cr or Mo, that is, it is not easily decomposed into a γ phase and Gr. Is heading.

  Therefore, it is preferable that the iron carbide as the carburizing source material according to the carburizing heat treatment method of the present invention contains a stabilizing element that suppresses and stabilizes at least the carburizing temperature of the steel material.

<Carburizing source>
As described above, the present invention is epoch-making in that iron carbide is used as a carburizing source, and is completely different from conventional carburizing heat treatment methods. Therefore, the present invention can be grasped as the following carburizing source material.

  That is, the present invention provides the supply of C used for carburizing heat treatment of a steel material in which C penetrates from the surface portion of the steel material to the inside to form a carburized layer having a higher C concentration in the surface portion than the inside of the steel material. It may be a carburizing source material which is a source, and the carburizing source material is made of iron carbide which is a compound of Fe and C.

<Others>
(1) The “steel material” in this specification may be a steel material itself (raw material) or a member close to the final product. Usually, the carburizing heat treatment is often performed at the final stage of production, but after the carburizing heat treatment, iron carbide contacting the surface, scale removal, shot processing, and machining may be performed. Moreover, the form of the steel material does not matter.

  In general, steel refers to steel having a C content of about 0.02 to 2.1% by mass (hereinafter simply referred to as “%”), but the steel material of the present invention is a material to be subjected to carburizing heat treatment ( Any substrate can be used. That is, the steel material only needs to have Fe as a main component, and the amount of C therein is not limited. For example, the C amount in the steel material before the carburizing heat treatment may be 0%, and conversely, cast iron or the like whose apparent C amount exceeds 2.1% may be used.

(2) Unless otherwise specified, “x to y” in this specification includes the lower limit x and the upper limit y. Further, the lower limit and the upper limit described in the present specification can be arbitrarily combined to constitute a range such as “ab”.

  The present invention will be described in more detail with reference to embodiments of the invention. In addition, the content demonstrated by this specification including the following embodiment is applicable not only to the carburizing heat treatment method of the steel materials which concerns on this invention but the carburizing source material used for it. Also, it should be noted that which embodiment is the best depends on the target, required performance, and the like.

  By the way, in addition to the configuration of the present invention described above, one or two or more arbitrarily selected from the configurations listed below may be further added. At this time, the category of the invention is not a problem. For example, if it is the structure regarding a carburizing source material, it cannot be overemphasized that it is related also to the carburizing heat treatment method. Further, the invention of the carburizing heat treatment method related to “method” can be an invention related to “thing” (steel member subjected to carburizing heat treatment) if understood as a product-by-process.

(1) Iron carbide Iron carbide is basically a compound of Fe and C. A typical example is M ₃ C type cementite as described above. This iron carbide does not have to be composed only of Fe and C. It is preferable that the iron carbide maintains its M ₃ C type cementite structure even under a high temperature environment. In particular, it is preferable that the steel material is stable in a range below the temperature at which the steel material starts to liquefy (eutectic temperature). By using such stable iron carbide, it is possible to perform carburizing heat treatment at a temperature higher than the conventional carburizing temperature (800 to 900 ° C.), and it is possible to quickly form a preferable carburized layer having no excess carburized portion or the like. It becomes.

  Specifically, iron carbide becomes stable even in a high temperature region by containing a specific element in iron carbide. As such a stabilizing element, Cr, Mo, W, V, and Nb can be used. The iron carbide may contain only one of these stabilizing elements, or may contain two or more. If there are too few stabilizing elements, there will be no effect, and if there are too many stabilizing elements, it will change to the carbide | carbonized_material which has the structure peculiar to the element, and will also become high cost.

  From such a viewpoint, the content of the stabilizing element in the iron carbide is determined. However, the preferable range varies depending on the type of element. For example, when the total amount of iron carbide is 100 mass% (simply referred to as “%” as appropriate), Cr is preferably 0.6 to 12%. The upper and lower limits can be arbitrarily selected within the numerical range, and in particular, numerical values arbitrarily selected from 0.8%, 1%, 3%, 5%, 8%, 10%, and 11% It is preferable.

  Similarly, Mo is preferably 0.6 to 9%. The upper and lower limits can be arbitrarily selected within the numerical range, but it is particularly preferable that the upper and lower limits are numerical values arbitrarily selected from 0.8%, 1%, 3%, 5% and 8%.

(2) Steel material The composition of the steel material is not limited as long as it is composed mainly of Fe. However, since the steel material is a material to be carburized and heat treated, it is usually a low-carbon steel and contains a relatively large amount of alloying elements that enhance the hardenability. Specifically, what is generally referred to as case-hardened steel, such as SCr material, SCM material, SNCM material, etc., which are standardized by JIS as alloy steel for machine structure, is typical. By the way, the composition of such steel materials is as follows: C: 0.1 to 0.4%, Cr: 0.9 to 1.2%, Mo: 0.15 to 0.3, when the entire steel material is 100% by mass. %, Si: 0.15-1.5%, Mn: 0.5-2.7%.

When the carburizing heat treatment of the present invention is performed, it is natural that the concentration of C in the surface portion increases, and the concentration of elements other than C in the surface portion can also be higher than in the inside thereof. This is because elements (for example, stabilizing elements) other than C contained in the carburizing source material can diffuse to the steel material side to some extent. As a result, if the chemical composition of the surface portion of the steel material after the carburizing heat treatment is investigated, it may be determined whether or not the carburizing heat treatment method of the present invention has been implemented. For example, when Cr, which is a stabilizing element, is contained in iron carbide and Cr, which enhances hardenability, is also contained in the steel material, which is a material to be treated, Can be different. However, the diffusion coefficient of Cr or the like in the carburizing temperature (about one to about 10 ^quarters) considerably smaller than the diffusion coefficient and C, also in the case of carburizing heat treatment method of the present invention, considerable short carburizing time. For this reason, there is no extreme concentration difference between elements inside the steel material and the surface portion with respect to elements other than C, and there is no extreme concentration difference that destroys the original characteristics of the steel material.

(3) Contacting step and carburizing source material The contacting step is a step of bringing the carburizing source material into contact with the surface of the steel material. The carburized layer formed by the carburizing heat treatment is only about 1 mm. For this reason, the carburizing source material itself may be thin.

  Based on this, an appropriate contact process according to the form of the carburizing source material may be performed. For example, if the carburizing source material is an iron carbide powder (iron carbide powder), the contacting step may be an adhesion step for attaching the iron carbide powder to the surface of the steel material.

Moreover, if the slurry which melt | dissolved this iron carbide powder in the solvent is used, a contact process can be easily performed as an application | coating process which apply | coats the slurry to the surface of steel materials. This application process may be any of a brush application process, a spray process, a dipping process, and the like. At this time, the solvent for preparing the slurry may be water or a volatile solvent such as alcohol. The number of times of application may be one or more times as long as a uniform iron carbide powder layer is formed on the surface of the steel material.
When performing the adhesion process or the coating process, the particle size of the iron carbide powder is preferably 150 μm or less, more preferably 45 μm or less, in order to speed up the carburizing heat treatment.

  Of course, if the iron carbide powder is used as it is, it may be peeled off or dropped off before completion of the carburizing heat treatment. Therefore, when a carburizing source foil formed of iron carbide in a tape shape is used as a carburizing source material, an efficient and reliable contact process can be performed. In this case, a contact process turns into a sticking process which sticks a carburizing source foil on the surface of steel materials, for example. Since the carburizing source foil itself is usually not tacky, it is preferable to impart tackiness or mechanically press the carburizing source foil against the surface of the steel material.

  The carburizing source material may be melted or fired (sintered) material. However, when a mixed powder obtained by mixing iron powder, Gr powder, and iron alloy powder is heated to produce iron carbide to produce a carburized source material, it is easy to obtain a carburized source material having a desired shape.

(4) Carburizing heating step The carburizing heating step is a step of heating the steel material having the carburizing source material in contact with the surface to a carburizing temperature equal to or higher than the temperature at which at least C in the carburizing source material diffuses into the steel material.
In the case of the present invention, the atmosphere in which the carburizing heating step is performed is not limited. However, in order to suppress oxidation of the steel material, decomposition of the iron carbide, and the like, it is preferable to perform in an inert gas atmosphere.

  The carburizing temperature may be equal to or higher than the temperature at which the steel material transforms into the austenite phase (A1 transformation point) and equal to or lower than the eutectic temperature of the steel material. However, from the viewpoint of speeding up the carburizing heat treatment, the carburizing temperature is preferably 900 ° C. or higher, 950 ° C. or higher, 1000 ° C. or higher, 1050 ° C. or higher, and 1100 ° C. or higher. On the other hand, from the viewpoint of energy saving and the like, the carburizing temperature is preferably 1150 ° C. or lower, 1100 ° C. or lower, and 1050 ° C. or lower, although it depends on the stability of the steel material or iron carbide. The upper and lower limits of these carburizing temperatures can be arbitrarily combined.

  Incidentally, when carburizing heat treatment was performed at a carburizing temperature of 1100 ° C. using the method of the present invention, C could be diffused to a depth of about 0.4 mm in 4 minutes and about 0.9 mm in 12 minutes.

(5) Cooling process A cooling process is a process of cooling the steel materials heated at the carburizing heating process. This cooling may be slow cooling such as furnace cooling, but is usually rapid cooling for quenching. The degree of rapid cooling may be water cooling, hot water cooling, or oil cooling as long as the metal structure of the surface portion of the steel material is at least partially transformed into martensite. In addition, after the cooling step, heat treatment such as tempering may be separately performed to adjust the mechanical properties of the steel material.

(6) Examples of steel materials (steel members) subjected to the carburizing heat treatment method of the present invention include various pulleys, transmission synchro hubs, engine connecting rods, hub sleeves, sprockets, various gears (ring gears, parking gears). , Pinion gear, etc.).

The present invention will be described more specifically with reference to examples.
<Manufacture of carburizing source>
(1) As raw material powder for producing a carburizing source material, carbonyl iron powder (Fukuda Metals, average particle size 5 μm), graphite powder (Gr powder), SUS430 (JIS) fine powder (Kobe Steel,- 45 μm) was prepared. The SUS430 fine powder is for addition of Cr. These various powders were blended so that the total composition (atomic%) was as follows.
Fe: 70.98%, C: 25.00%, Cr: 4.01%
This is converted into mass% as follows.
Fe: 88.62%, C: 6.714%, Cr: 4.66%
This composition is obtained by substituting about 5% by mass of Fe into Cr when the total amount of Fe in Fe—C binary system cementite (Fe 3 C) is 100% by mass.

  FIG. 1 shows the result of calculating the equilibrium diagram between (Fe5% Cr) 3C instead of Fe3C and pure Fe using the TCFE3 (Thermocalc Co., Ltd.) database. From this calculated phase diagram, it can be seen that iron carbide (Cr-added cementite) composed of (Fe5% Cr) 3C can stably coexist with the austenite phase from the A1 transformation temperature to the eutectic temperature of Fe.

(2) The powder blended with the above various powders was placed in a ball mill and mixed in an Ar gas atmosphere at room temperature for about 10 hours. The mass ratio of the φ8 mm steel balls used at this time and the blended powder was 10: 1.

An appropriate amount of an aqueous binder (manufactured by Yuken Kogyo Co., Ltd., DB20) was added to the mixed powder thus obtained, and the mixed powder was formed into a tape shape having a thickness of about 0.75 mm by the doctor blade method. This molded body was heated in an Ar gas atmosphere furnace at 1100 ° C. for 30 minutes to obtain a carburized source foil made of Cr-added cementite (hereinafter referred to as “Cr-θ foil”).
A 0.2 mm thick Gr sheet manufactured by Toyo Carbon Co., Ltd. was prepared as a carburizing source for comparison.

<Carburizing heat treatment>
(1) A φ12 mm cylindrical test piece (steel material) made of case-hardened steel (JIS: SCM420H) was prepared as a material to be subjected to carburizing heat treatment. A Cr-θ foil was sandwiched while applying a constant load (49 MPa) to the carburizing source foil placed on the outer surface of the test piece using an in-atmosphere hot working test apparatus (contact process).

The test piece in this state was heated by high frequency induction (carburization heating process). The heating temperature (carburizing temperature) at this time was 950 ° C. or 1100 ° C., and the heating time was 4 minutes or 12 minutes.
The test piece immediately after the high frequency induction heating was quenched with Ar gas (cooling step). This cooling time was 3 minutes.
Since the initial temperature increase rate was 20 ° C./s, the time required for this carburizing heat treatment was about 8 minutes or about 16 minutes in total.
(2) The Cr-θ foil used in the carburizing heat treatment was replaced with a Gr sheet for comparison, and a test specimen for comparison was manufactured in the same process.

<Measurement>
(1) First, the above Cr-θ foil itself was subjected to X-ray analysis. The X-ray diffraction result (XRD) is shown in FIG. 2A. Moreover, the microscope picture which observed the carburizing source foil with the scanning electron microscope (SEM) is shown to FIG. 2B.

(2) A test of carburizing heat treatment (1100 ° C. × 4 minutes) using a Gr sheet in FIG. 3A and FIG. 3B for the metallographic structure of the surface layer of the test piece subjected to carburizing heat treatment (1100 ° C. × 4 minutes) using Cr-θ foil The metal structures of the surface layer cross-sections of the pieces are shown in FIGS. 4A to 4C, respectively. In FIGS. 3B and 4C, the internal structure that has not been carburized is bainite, and both confirmed that the same cooling was performed.

  Incidentally, each metal structure was observed using an optical microscope. The micrographs of FIGS. 3A, 3B, 4A, and 4B are obtained by observing a cross section of a test piece that was corroded with a repeller reagent. The micrograph of FIG. 4C is a picric acid-sodium hydroxide-based reagent and a carbide. The cross-section of the test piece that preferentially corroded was observed.

(3) For various test pieces that were carburized and heat treated using Cr-θ foil and various test pieces that were carburized and heat treated using a Gr sheet, the Vickers was sequentially applied in the direction of the depth of the test piece from the carburized surface (carburized depth). Hardness (load 50 g) was measured. The hardness distribution (correlation between Vickers hardness and carburization depth) of the various test pieces thus obtained is shown in FIGS. 5 and 6, respectively.

<Evaluation>
(1) First, it can be seen from the state diagram of FIG. 1 that the Cr-θ foil is stable up to about 1160 ° C. and thermodynamically balanced with the γ phase of the material to be treated. Further, as can be seen from the X-ray diffraction diagram of FIG. 2A, only the α-Fe and Gr of the raw material powder were detected in the green body before heating, whereas the green body after heating was almost cementite (( Fe5% Cr) 3C) single phase was confirmed. However, even in this case, α-Fe and Gr remain slightly, and the ratio is estimated to be about 95% of the whole.

  Moreover, it turned out that the carburizing source foil is porous from the micrograph shown in FIG. 2B. It is considered that the pores were generated by an exothermic reaction accompanied with the synthesis of cementite from the raw material powder.

(2) As can be seen from FIGS. 3A and 3B, extremely fine lath martensite is formed in the surface layer portion of the test piece subjected to the carburizing heat treatment using the Cr-θ foil from directly under the remaining Cr-θ foil layer. Is formed. And as it becomes deeper than that, a mixed structure containing a black phase that seems to be bainite is formed. Although it is not clear as a metal structure other than these, the presence of a retained austenite phase is also conceivable.

  On the other hand, as can be seen from FIGS. 4A and 4B, in the surface layer portion of the test piece subjected to the carburizing heat treatment using the Gr sheet, first, a metal structure of a high carbon region is present immediately below the remaining Gr sheet layer. . This high carbon region metallographic structure consists of specific acicular martensite and retained austenite phases. Moreover, as can be seen from the photomicrograph shown in FIG. 4C, it was also found that network-like carbides were precipitated along the grain boundaries (γ grain boundaries) of the austenite phase. The metal structure of a deeper part than this was similar to the metal structure of the test piece carburized and heat treated using the Cr-θ foil.

(3) As can be seen from FIG. 5, in the case of the carburized heat treatment using Cr-θ foil, the higher the carburizing temperature and the longer the carburizing time, the greater the internal hardness. . In all cases where the carburizing temperature and carburizing time were different, the overall hardness distribution tended to monotonously decrease toward the inside. Moreover, the Vickers hardness of the outermost layer portion was stable at about 850 to 1000 Hv in all cases.

  Thus, it was confirmed that a carburized layer having stable hardness was formed even when the carburizing temperature or carburizing time was appropriately changed. Conversely, a carburized layer having a desired thickness can be easily obtained by changing the carburizing temperature or carburizing time, and the carburizing heat treatment method according to the present invention is also excellent in controllability. In addition, in this case, almost no carbide is seen in the vicinity of the surface of the test piece, and the diffusion process required when vacuum carburizing with hydrocarbon gas is unnecessary, and the carburizing heat treatment is completed within a short time. Can be made.

  On the other hand, as can be seen from FIG. 6, in the case of the carburized heat-treated test piece using the Gr sheet, the hardness distribution of the surface layer portion was unstable regardless of the length of the carburizing time. In either case, the hardness tended to decrease rapidly after increasing once toward the inside.

  In carburizing with a Gr sheet, the hardness of the surface layer fluctuates greatly from Hv 700 to 900, reaches a maximum hardness somewhat inside, and then rapidly decreases. According to the analysis by an X-ray microanalyzer (EPMA), the positions of the highest hardness in 4 minutes and 12 minutes were consistent at C concentration of 0.8 to 0.9%. Therefore, the carburized region having a higher C concentration on the surface does not lead to an increase in hardness (excess carburization). This is presumably because the range in which the hardness varies corresponds to the acicular martensite structure of FIG. 4 (a), and there are many residual γ phases. Incidentally, in the case of a test piece subjected to a carburizing heat treatment using a Cr-θ foil, a decrease in hardness was slight even in a surface layer portion where the C concentration exceeded 1%.

(4) Based on the above results, when carburizing heat treatment is performed using the Cr-θ foil according to one embodiment of the present invention, when carburizing heat treatment is performed using a Gr sheet, hydrocarbon-based gas (such as methane) is used. Table 1 summarizes the results of examining the carbon diffusion depth and the presence or absence of grain boundary carbides when vacuum carburized and heat-treated using a general RX gas. In addition, about the carburizing depth, the simulation value was shown suitably.

  As can be seen from Table 1, when carburizing heat treatment was performed using Cr-θ foil, grain boundary carbides (network-like carbides) were not observed unlike carburizing by other methods. Moreover, the carburization depth was equal to or greater than that of the conventional carburizing method.

It is a Fe- (Fe5% Cr) 3C type | system | group phase diagram. It is an X-ray diffraction pattern (XRD) of Cr-theta foil which is a carburizing source material used in this example. It is the SEM photograph which observed the Cr-theta foil. It is a metal structure photograph of the surface layer cross section of the test piece heat-carburized using Cr-theta foil. It is a metal structure photograph inside the test piece which carburized and heat-treated using Cr-theta foil. It is a metal structure photograph of the surface layer cross section of the test piece heat-carburized using a Gr sheet. It is a metal structure photograph inside the test piece which carburized and heat-treated using the Gr sheet. It is a metal structure photograph of the surface layer cross section of the test piece heat-carburized using a Gr sheet. It is a hardness distribution figure of the surface layer cross section of the test piece heat-carburized using Cr-theta foil. It is a hardness distribution figure of the surface layer cross section of the test piece heat-carburized using a Gr sheet.

Claims (9)

A carburizing heat treatment method for a steel material in which carbon (C) is infiltrated from the surface portion of the steel material to the inside to form a carburized layer having a higher C concentration in the surface portion than the inside of the steel material,
A contacting step in which a carburizing source material made of iron carbide which is a compound of iron (Fe) and C is brought into contact with the surface of the steel material;
A carburizing heating step of heating the steel material to a carburizing temperature equal to or higher than a temperature at which C in the carburizing source material diffuses into the steel material;
A cooling step for cooling the steel material after the carburizing heating step;
A carburizing heat treatment method for a steel material, comprising:   The carburizing heat treatment method for a steel material according to claim 1, wherein the carburizing source material includes a stabilizing element that suppresses and stabilizes the decomposition of the iron carbide at least up to the carburizing temperature.   3. The stabilization element according to claim 2, wherein the stabilizing element is chromium (Cr), and the Cr is 0.6 to 12% when the entire iron carbide is 100 mass% (hereinafter simply referred to as “%”). Carburizing heat treatment method for steel. The method for carburizing heat treatment of a steel material according to any one of claims 1 to ₃ , wherein the iron carbide is M ₃ C type (M: metal element). The carburizing source material is an iron carbide powder made of the iron carbide powder,
The said contact process is an adhesion process which makes this iron carbide powder adhere to the surface of the said steel material, The carburizing heat treatment method of the steel material in any one of Claims 1-4. The carburizing source material is a carburizing source foil obtained by forming the iron carbide into a tape shape,
The said contact process is a sticking process which affixes this carburizing source foil on the surface of the said steel material, The carburizing heat treatment method of the steel material in any one of Claims 1-4. A carburizing source serving as a supply source of C used for carburizing heat treatment of a steel material in which C is permeated from the surface portion of the steel material to the inside to form a carburized layer having a C concentration higher than that inside the steel material. Material,
The carburizing source material is composed of iron carbide which is a compound of Fe and C.   The carburizing source material according to claim 7, wherein the iron carbide is obtained by heating a mixed powder obtained by mixing iron powder, Gr powder, and iron alloy powder.   The carburizing source material according to claim 7 or 8, which is in the form of powder or tape.