JP4188307B2 - Carburized parts and manufacturing method thereof - Google Patents

Description

  The present invention relates to a carburized component and a manufacturing method thereof.

  A gear as a power transmission component of an automobile or the like is a component in which a root fracture that occurs at the root where a bending stress acts and a fracture (pitting phenomenon) that occurs near the pitch point due to sliding are problems. In order to satisfy the characteristics that can withstand these, a method of carburizing the surface of the component has been widely used, and further improvements have been made by combining various materials and heat treatments.

  In particular, in recent years, materials have been developed to suppress grain boundary oxide layers and carburized abnormal layers during carburizing, which are harmful to tooth root destruction, and high strength has been achieved by shot peening and the like.

  On the other hand, research on the pitting phenomenon has been actively conducted, and it has been found that prevention of softening of the material is effective in improving the strength. Slip occurs on the tooth surface of the gear, and heat is generated immediately below the tooth surface due to repeated contact. The temperature at this time is considered to be in a temperature range of about 200 ° C. to 300 ° C., and it is considered that the material is softened by this heat generation, and thus pitting destruction occurs. Therefore, prevention of softening of the material in the temperature range of about 200 to 300 ° C. is effective in improving the pitting failure, and material added with Si, Cr, Mo or the like as an alloy element having excellent softening resistance in this temperature range Has been developed.

JP-A-6-158266

  However, with the recent increase in output of automobiles and the like, the gears are required to have higher strength, but the above-described materials cannot cope with them.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a carburized component and a method of manufacturing the same that achieve high strength of a power transmission component such as a gear.

Means for solving the problems / effects of the invention

In order to solve the above problems, the carburized parts of the present invention are:
In mass%, C: 0.01 to 0.30%, Si: 0.80 to 1.50%, Mn: 0.30 to 1.20%, Cr: 2.12 to 5.5% , The balance is made of steel consisting of Fe and inevitable impurities ,
When the primary carburizing process is performed at a temperature not lower than the Acm point, the steel is rapidly cooled to the A1 point or lower, and then subjected to the secondary carburizing process at a temperature not lower than the A1 point and not higher than the Acm point. A carbide having an average C concentration from the surface of 0.2 mm to a depth of 0.2 mm to 1.61 % to 3.0%, a carbide area ratio from the surface to a depth of 50 μm of 15% to 60%, and a size of 10 μm or less. Carbides are finely dispersed and deposited so as to occupy 90% or more of the total, and the grain boundary oxide layer depth is 1 μm or less.

  Further, one or two of Mo: 0.2 to 1.0% and V: 0.2 to 1.0% can be added as a steel component.

  The present invention has the following basic features. That is, by performing high-concentration vacuum carburization, carbides are precipitated in a fine and large amount on the surface of the part, and the surface grain boundary oxide layer is substantially eliminated to increase the surface hardness and strength. Moreover, the temper softening resistance in the 200 ° C. to 300 ° C. region is enhanced by high Si, which is made possible by vacuum carburization, and good surface fatigue strength is obtained. In order to obtain these characteristics, components and conditions in an appropriate range described in detail below are required.

C: 0.10 to 0.30%
C is an essential element for securing the strength of the component, and needs to be contained by 0.10% or more. On the other hand, excessive content increases the material hardness, so that the machinability deteriorates and it becomes difficult to process parts. Therefore, the upper limit is made 0.30%.

・ Si: 0.80 to 1.52 %
Si is an element to be contained as a deoxidizer during melting and an element that plays an important role in the present invention. That is, since the temper softening resistance in the 200 ° C. to 300 ° C. region is increased by dissolving in the matrix, high surface fatigue strength can be obtained. Moreover, since the solid solubility in the carbide is low and the Si concentration of the base material is increased, the coarse growth of the carbide is suppressed. Furthermore, when a large amount of carbide is precipitated, Si having a low solid solubility in the carbide is concentrated in the matrix, and the temper softening resistance of the matrix is further improved. In order to acquire these effects, it is necessary to contain 0.80% or more. On the other hand, the excessive content hinders the precipitation of carbides and the carburized surface reaction and significantly lowers the carburizability, and also lowers the ductility and easily causes cracking during plastic working, so the upper limit is 1.52 %. To do.
Si is an element that promotes grain boundary oxidation in the case of normal gas carburization, and this grain boundary oxidation layer causes the impact strength and fatigue strength at the tooth base to decrease. Therefore, in the case of gas carburizing, a large amount of Si cannot be added. However, by using vacuum carburizing as described above, the problem of grain boundary oxidation is eliminated, so that high Si can be achieved.

Mn: 0.30 to 1.20%
Mn is contained as a deoxidizer at the time of melting, and has an effect of improving hardenability, so it is necessary to contain 0.30% or more. In the present invention, an element having an effect of improving the hardenability such as Cr is also contained at the same time. However, since the element such as Cr forms a carbide, it is sufficient to increase the Cr content depending on the amount of the carbide. Hardenability may not be ensured. Therefore, adjustment of the Mn content is effective for obtaining the required hardenability. On the other hand, excessive content decreases the machinability by increasing the material hardness, so the upper limit is made 1.20%.

・ Cr: 2.0-5.5%
Cr is an element that plays an important role in the present invention. It is necessary to contain 2.0% or more as a carbide forming element or as an element for improving hardenability. On the other hand, excessive inclusion reduces the machinability by increasing the material hardness and facilitates the formation of network carbides at the grain boundaries, so the upper limit is made 5.5%.

Mo: 0.2 to 1.0%
Mo, like Cr, combines with C to form carbides, and has an effect of improving the softening resistance in the temperature range of 200 ° C. to 300 ° C. and improving the pitting strength. In order to acquire these effects, it is preferable to make it contain 0.2% or more. On the other hand, excessive content reduces the machinability by increasing the material hardness and increases the material cost, so the upper limit is preferably made 1.0%.

・ V: 0.2-1.0%
V, like Cr · Mo, combines with C to generate carbides, and has the effect of improving softening resistance and improving pitting characteristics by generating MC-based carbides. In order to acquire these effects, it is preferable to make it contain 0.2% or more. On the other hand, excessive content reduces the machinability by increasing the material hardness, so the upper limit is preferably made 1.0%.

・ Carburizing treatment: Vacuum carburizing treatment (1000 Pa or less)
The carburized component of the present invention is subjected to vacuum carburizing treatment. Since the grain boundary oxide layer can be reduced by the vacuum carburizing treatment, the strength of the carburized component can be increased.
As described above, Si is added as an essential component in the present invention. Si is an element that promotes grain boundary oxidation during normal gas carburization, and this grain boundary oxidation contributes to a decrease in impact strength and fatigue strength at the tooth base. Therefore, in the case of normal gas carburization, it is very difficult to increase the Si content. However, according to the vacuum carburizing treatment, the formation of the grain boundary oxide layer is suppressed, and therefore, high Si can be easily realized.

-Grain boundary oxide layer depth: 1 μm or less The grain boundary oxide layer causes a decrease in fatigue strength and pitting resistance, and the degree of decrease increases as the depth increases. Therefore, in the carburized part of the present invention, the depth of the grain boundary oxide layer from the surface of the steel after the vacuum carburizing process is set to 1 μm or less.

・ Average C concentration from surface to 0.2mm depth: 1.2% or more and 3.0% or less In normal carburizing treatment, eutectoid carburizing treatment is aimed at 0.8% which is the amount of eutectoid C. Is common. However, since the present invention improves the pitting resistance by precipitating carbides on the surface layer of the steel material and increasing the softening resistance, it is necessary to contain C in the amount of eutectoid C (0.8%) or more. Furthermore, even if the carbide is precipitated, the surface fatigue strength cannot be improved unless the amount of carbide necessary for enhancing the softening resistance is obtained. Therefore, a sufficient amount of C is included to achieve such improvement. There is also a need.
From these viewpoints, the average C concentration (hereinafter also referred to as surface C concentration) from the steel surface to a depth of 0.2 mm is set to 1.2% or more. The reason why the depth is 0.2 mm from the surface of the steel is that the hardness in the region is important from the viewpoint of pitting resistance. On the other hand, excessive inclusion will produce large carbides, and will cause a lack of hardenability of the substrate, leading to a decrease in strength. Therefore, the upper limit of the surface C concentration is set to 3.0%.

Carbide area ratio from the surface to a depth of 50 μm: 15% or more and 60% or less Precipitation of carbide increases the surface hardness, improves softening resistance in the 200 to 300 ° C. region, and improves pitting resistance. However, when the carbide area ratio from the surface to a depth of 50 μm is less than 15%, the softening resistance is not sufficiently improved, and a sufficient strength improvement effect cannot be obtained. On the other hand, if the area ratio of carbide exceeds 60%, the softening resistance is improved, but the carbide tends to precipitate in a network form along the grain boundary as the size increases, so bending in addition to the surface fatigue strength Reduces fatigue strength. An observation example of the obtained carbide is shown in FIG.

・ Carbide is finely dispersed and precipitated so that carbides with dimensions of 10 μm or less occupy 90% or more of the whole. Carbides are hard particles, and are the starting point of fatigue fracture like non-metallic inclusions such as Al oxide and Ti nitride. Sometimes. Therefore, it is desirable that the carbide is small, and it is necessary to control the size to 10 μm or less in order not to exist as a starting point of fatigue fracture. Therefore, the carbides are controlled to be finely dispersed and precipitated so that carbides having a size of 10 μm or less occupy 90% or more of the whole. An observation example of the obtained carbide is shown in FIG.

  In order to manufacture the carburized parts described above, the carburized part manufacturing method of the present invention is such that after the primary carburizing treatment is performed on the steel containing the steel components at a temperature of Acm point or higher, it is rapidly cooled to A1 point or lower. Then, a secondary carburizing process is performed at a temperature not lower than the A1 point and not higher than the Acm point. That is, as shown in FIGS. 1 (a) and 1 (b), first, the primary carburizing process is performed so as not to precipitate carbide at a temperature higher than the Acm point where the solid solubility limit of C is large and carbide does not precipitate (between ab and ab). ). Next, it is rapidly cooled below the A1 point to bring C into a supersaturated state (between bc). After that, it is heated again to a temperature higher than the A1 point to uniformly precipitate fine carbide nuclei from the supersaturated substrate of C (between de: see the upper part of FIG. 2), followed by further secondary carburizing treatment. Precipitation nuclei are grown (between ef: see the lower part of FIG. 2). By performing such a multi-stage carburizing process, carburization at a high C concentration in which carbides are finely dispersed can be performed without causing precipitation of network carbides. On the other hand, as shown in FIG. 3, when carburizing to a high C concentration region that is less than the Acm point, a net-like coarse carbide is very easily generated. Note that the carburizing process is performed by the vacuum carburizing process (1000 Pa or less) as described above.

  Moreover, after the said secondary carburizing process, a peening process can be performed as needed, and thereby further strengthening can be achieved. For example, shot peening (S / P) or water jet peening (W / J) can be applied to the peening process.

Hereinafter, tests conducted for confirming the effects of the present invention will be described.
First, steel having the chemical composition shown in Table 1 was melted in a 150 kg high frequency vacuum induction furnace. The obtained steel ingot was rolled or hot-forged into a round bar having a diameter of 90 mm, and further hot-forged into a steel bar having a diameter of 22 to 32 as necessary to obtain a test material.
In addition, in the composition of the comparative example in Table 1, those that deviate from the composition range specified in the present invention include a downward arrow (↓) when lower than the lower limit, and an upward arrow (↑) when higher than the upper limit. ) Is attached.

The following evaluation was performed on the obtained test materials.
(1) Manufacturability evaluation Manufacturability was evaluated by evaluating the hardness after annealing.
A φ32 × 100 L round bar test piece was subjected to an annealing treatment of 920 ° C. × 1 hour, and further subjected to an annealing treatment of 760 ° C. × 5 hours, and the hardness of the cross section to the R / 2 position was measured. The hardness measurement is JIS
It was based on Z 2245 (B scale), and the index was HRB 90 or less.

(2) Carburizing Basic Characteristic Evaluation (2-1) Carburizing Treatment Method A round bar test piece of φ10 × 100 L was produced from a forged steel bar of φ22 and used as a carburizing test piece.
The carburization process was performed using a vacuum carburizing furnace, using propane as the carburizing gas, and controlling the propane gas flow rate, diffusion time, and carburizing temperature to control the surface C concentration. The carburizing treatment was performed under two levels of conditions such that the surface C concentration was 1.5% and 2.5%.
Further, in Example 3, carburizing treatment was performed in the range of 0.8 to 3.2% in order to investigate the influence of the surface C concentration.
The carburizing conditions are as follows.
-Primary carburization treatment: Carburizing treatment is performed at 1100 ° C for 70 minutes so that the C concentration on the outermost surface is about 1.2%, and then quenched by gas cooling to a temperature range of 500 ° C or less, so that the carbide C was infiltrated into the steel to a high concentration range where it did not precipitate.
-Secondary carburizing treatment: After carrying out the precipitation treatment in a temperature range of 850 to 900 ° C. depending on the target carburizing concentration, and further in the temperature range of 850 to 1000 ° C. depending on the target C concentration. Carburizing treatment was performed for 90 minutes, and quenching treatment was performed in an oil bath at 130 ° C. Moreover, the tempering process of 180 degreeC * 120min was implemented after the quenching process.

(2-2) Evaluation items Hereinafter, the evaluated items will be described. The evaluation results are shown in Table 2. Table 3 shows the evaluation results of Example 3 in which the surface C concentration was changed.
-Surface C density | concentration After carburizing process, C density | concentration was measured from the Dalai powder to the position of 0.2 mm from the surface of a process test piece.
・ Carbide area ratio After polishing the cross section of the round bar test piece that had been carburized and quenched and tempered, and corroded with picral, photographed the position of 50 μm from the outermost surface with SEM (observation magnification 3000 times), The area ratio was measured by image analysis.
-Carbide size It observed on the same conditions as the above, and measured the area ratio which the carbide | carbonized_material of 10 micrometers or less occupies.
-Presence / absence of reticulated carbide Observed under the same conditions as above, the presence / absence of reticulated carbide was investigated.
-Existence of incompletely hardened structure After polishing the cross section of the round bar test piece that had been carburized and quenched and tempered and corroded with nital, the position of 50 μm from the outermost surface was observed with an optical microscope, and incomplete The presence or absence of a hardened structure was investigated.
・ Depth of grain boundary oxide layer The cross section of a round bar test piece that has been carburized and quenched and tempered is polished, the uncorroded state is observed with an optical microscope, and the layer that appears black along the outermost grain boundary The depth was measured.
-Temper softening resistance The round bar test piece which performed the carburizing quenching and tempering process was further tempered at 300 degreeC x 180 minutes, the cross section was grind | polished, and the hardness of the position of 50 micrometers from the outermost surface was measured. The hardness measurement is JIS
In accordance with Z 2244 (HV 0.3), Hv 750 or more was set as an index that can be expected to have a sufficient strength improvement effect (≧ 30%: SCR420—gas eutectoid carburization contrast).

  According to Table 2, all of Examples 1 to 10 have no problem in manufacturability (annealing hardness ≦ HRB90), and incompletely hardened structure and carbide on the network, grain boundary oxidation, etc. causing strength deterioration are not seen, A sufficient tempering hardness (≧ 750 Hv) at 300 ° C. is obtained. On the other hand, Comparative Examples 2, 4, 6, 8 to 10 have high hardness after annealing, and have a problem in manufacturability. In Comparative Examples 3 and 8, since Si is low or Cr is high, fine dispersion control of carbides is not sufficient, and network carbides and other coarse carbides are generated, and there is a concern about strength reduction. In Comparative Example 4, since Si is too high, there is a problem in manufacturability, and the carburizing property is hindered and sufficient carburizing cannot be performed. In Comparative Examples 5 and 7, the amount of Cr · Mn is low, the hardenability is insufficient, an incompletely hardened structure is seen, and there is a concern about strength reduction.

  According to Table 3, carburization with a surface C concentration of less than 1.2% improves the surface fatigue strength but does not provide a sufficient strength improvement effect (≧ 30%). On the other hand, when carburization with a surface C concentration exceeding 3.0%, a sufficient tempering hardness of 300 ° C. is obtained, but network carbides and coarse carbides are seen, and a sufficient strength improvement effect is not obtained.

(3) Surface fatigue strength evaluation The surface fatigue strength was evaluated by a roller pitting tester, and the load surface pressure at which no pitting was generated in 107 cycles was defined as the surface fatigue strength. Specifically, first, a φ32 mm round bar was heated and held at 950 ° C. and then gradually cooled and softened, and then a roller pitting test piece having a test part diameter of 26 mm was produced by machining. Moreover, SUJ2 was used for the counter roller of the test piece, and a quenching and tempering treatment was performed so as to have a hardness of HRC61. In addition, the curvature radius of a large roller is 150R and 700R.
The carburizing treatment is performed at the same time as the carburizing treatment performed for conducting the basic evaluation test of the invention steel. In addition, a part of the roller pitting test piece after the carburizing treatment was tempered by holding at 300 ° C. for 3 hours, and the carbon concentration, the carbide area ratio, the maximum carbide size, the tempering hardness, and the like were also evaluated.
As for the surface fatigue strength, the surface fatigue strength of the gas eutectoid carburized material of JIS-SCR420 is defined as 1.0, and the strength of each material is described by an index. In addition, it was set as the parameter | index that there exists a sufficient strength improvement effect of 30% or more compared with JIS-SCR420H gas eutectoid carburized steel.
The above evaluation results are shown in Table 4.

  According to Table 4, Examples 1 to 10 all have a sufficient strength improvement effect (≧ 30%). On the other hand, in Comparative Example 1, the strength of the core is insufficient and the strength is low. In Comparative Examples 2, 4, 6, 9, and 10, the strength is sufficiently improved, but there is a problem in manufacturability. In Comparative Examples 3 and 8, mesh-like carbides and other coarse carbides are generated, and a sufficient strength improvement effect cannot be obtained. In Comparative Examples 5 and 7, the Cr / Mn amount is low, the hardenability is insufficient, an incompletely hardened structure is observed, and a sufficient strength improvement effect is not obtained.

  From the above tests, it was confirmed that the present invention had a fine and large amount of carbides precipitated on the surface, substantially no grain boundary oxide layer on the surface, and high surface hardness and strength.

Explanatory drawing of the carburizing process which concerns on the manufacturing method of the carburized component of this invention Cross-sectional schematic view and cross-sectional observation view of steel during the carburizing process of FIG. The figure explaining the example of the carburizing process different from this invention, and a cross-sectional observation figure Cross-sectional observation view of carburized parts of the present invention

Claims (3)

In mass%, C: 0.01 to 0.30%, Si: 0.80 to 1.50%, Mn: 0.30 to 1.20%, Cr: 2.12 to 5.5% , The balance is made of steel consisting of Fe and inevitable impurities ,
After performing the primary carburizing treatment at a temperature not lower than the Acm point, it is rapidly cooled below the A1 point, and then at a temperature not lower than the A1 point and not higher than the Acm point, the surface C concentration is higher than the surface C concentration after the primary carburizing. Thus, when the carburizing treatment is performed, the average C concentration from the steel surface to the depth of 0.2 mm is 1.61 % to 3.0%, and the carbide area from the surface to the depth of 50 μm. The carbide is finely dispersed and precipitated so that the carbide is not less than 15% and not more than 60%, and the carbide having a size of 10 μm or less accounts for 90% or more of the whole, and the grain boundary oxide layer depth is 1 μm or less. Features carburized parts.   The carburized component according to claim 1, further comprising one or two of Mo: 0.2 to 1.0% and V: 0.2 to 1.0% as a steel component. In mass%, C: 0.01 to 0.30%, Si: 0.80 to 1.50%, Mn: 0.30 to 1.20%, Cr: 2.12 to 5.5% , Against the steel whose balance is Fe and inevitable impurities,
  After performing the primary carburizing treatment at a temperature not lower than the Acm point, it is rapidly cooled below the A1 point, and then at a temperature not lower than the A1 point and not higher than the Acm point, the surface C concentration is higher than the surface C concentration after the primary carburizing. Thus, by performing the vacuum carburizing process in which the secondary carburizing process is performed, the average C concentration from the surface of the steel to the depth of 0.2 mm is 1.61% to 3.0%, and the carbide area from the surface to the depth of 50 μm The carbide is finely dispersed and precipitated so that the carbide having a rate of 15% to 60% and occupying 90% or more of the size is 10 μm or less, and further the grain boundary oxide layer depth is 1 μm or less. A method for manufacturing a carburized part.

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