JP2019014931A - Heat treatment method for steel material component - Google Patents

Abstract

To provide a heat treatment method for a steel material component capable of suppressing the generation of the creep deformation of a steel material component while suppressing the increasing carburization time.SOLUTION: A steel material component 1 is subjected to carburization treatment at a carburization temperature of 900 to 980°C, and the steel material component is subjected to nitriding treatment at a nitriding temperature of 820 to 900°C and is successively subjected to hardening treatment.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a heat treatment method for steel parts.

Carburizing and quenching heat treatment of steel parts is a technology that improves the wear resistance and impact strength of the surface of steel parts by infiltrating carbon into the surface of steel parts and forming a hard and dense carbon martensite structure on the surface by rapid cooling. It is. Carbon diffusion rate of carburizing steel parts, depending on the diffusion equation ^{∂c / ∂t = D (∂ 2} c / ∂x 2), the diffusion coefficient D in the case of a metal _material, D = D 0 exp (-Ea / KT). D ₀ is the material frequency factor, Ea is the diffusion activation energy, k is the gas constant, T is the temperature, and the diffusion coefficient D increases as the temperature T increases, so the carburizing temperature is increased to shorten the carburizing time. It is effective to do. However, when the metal is held at a temperature higher than a certain temperature, creep deformation occurs, and deformation due to gravity occurs according to the holding time. The scale of creep deformation depends on the Larson Miller parameter P = T (C + logtr) when the processing temperature is T, the material constant C, and the processing time tr. The amount of deformation increases. For this reason, in order to suppress the creep deformation at the time of carburizing heat treatment, a method of carburizing the surface of the steel material at a carburizing temperature of 800 to 1200 ° C. is known (Patent Document 1).

JP 2008-121064 A

  However, since the carburizing process is performed at a temperature of 1200 ° C. or lower in the conventional method described above, the relational expression D = D0exp (−Ea / kT) of the diffusion coefficient is compared with the maximum carburizing temperature of 1400 ° C. below the melting point of the steel material part. Accordingly, there is a problem that the carburizing time is about three times longer.

  The problem to be solved by the present invention is to provide a heat treatment method for steel parts that can suppress the occurrence of creep deformation of the steel parts while suppressing an increase in carburizing time.

  The present invention solves the above-mentioned problems by subjecting a steel part to a carburizing treatment at a temperature lower than a normal carburizing temperature and then nitriding the steel part at a nitriding temperature equal to or lower than the carburizing temperature.

  According to the present invention, the occurrence of creep deformation can be suppressed by carburizing at a temperature lower than the normal carburizing temperature. In addition, the surface hardness of steel parts can be increased by nitriding after carburizing. This also suppresses the occurrence of creep deformation. Therefore, even if the carburizing time at low temperature is shortened, the surface is hardened by nitriding, so that the total heat treatment process can suppress the occurrence of creep deformation of steel parts while suppressing the increase in carburizing time. it can.

It is a perspective view which shows an example of the steel material component applied to the heat processing method of the steel material component which concerns on this invention. It is a graph which shows the relationship between temperature and time of one Embodiment of the heat processing of the steel material parts which concern on this invention. It is a graph which shows the hardness by the carburizing process of FIG. 2, and the hardness by a nitriding process. It is a graph which shows the relationship between temperature and time of other embodiment of the heat processing of the steel material parts based on this invention. It is a graph which shows the result of having measured the amount of creep deformation of Example 1 and comparative example 1 of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing an example of a steel part applied to a heat treatment method for a steel part according to the present invention. The steel material part 1 to show in figure is a part which comprises the multilink of a variable compression ratio engine. For example, as described in FIGS. 6 and 7 of Japanese Patent Application Laid-Open No. 2017-088922, this multi-link is a configuration in which a pair of steel parts 1 are symmetrically combined with each other and coupled with screws, and the crankshaft is rotated. The shaft rotates as much as the compression ratio is changed.

  The steel part 1 of this embodiment includes a semicircular bearing portion 11, a pair of pin press-fit portions 12, and a screw portion 13. A bearing portion 11 is provided between the pair of pin press-fit portions 12 and the screw portion 13. The bearing 11 constitutes a crankshaft bearing. The pair of pin press-fitting portions 12 have holes for press-fitting pins for connecting the upper link or the control link. The screw portion 13 has a screw hole 13A for screwing the screw. Although illustration is omitted, a screw hole through which a screw is inserted is formed between the pair of pin press-fit portions 12, and the screw inserted into the screw hole is screwed into the screw hole 13 </ b> A of the other steel material part 1. Match.

  The steel part 1 of this embodiment first forms a steel part 1 having a link-shaped outer shape by forging a raw material. In this forging process, a hole 13 </ b> B having no screw groove is formed in the screw portion 13 of the steel material part 1. Next, chamfering is performed to remove the black skin on the surface of the steel part 1. Thereafter, a surface hardening treatment such as quenching is performed, and the entire surface of the steel material part 1 is hardened by heat treatment. Examples of the surface hardening treatment include heat treatment such as quenching, carburizing and quenching, nitriding and quenching, or carbonitriding and quenching. Hereinafter, the heat treatment method according to the present invention will be described on the assumption that the material of the steel part 1 is made of the chromium steel steel SCr420H and the entire surface of the steel part 1 is subjected to carbonitriding and quenching. The chromium steel steel SCr420H is composed of 0.17 to 0.23% by weight of C, 0.85 to 1.20% by weight of Cr, and 0.15 to 0.35% by weight of Si as components other than iron. , Mn 0.60-1.00% by weight, P 0.03% by weight or less, S 0.03% by weight or less, Ni 0.25% by weight or less, and Cu 0.3% by weight or less It is. However, the steel part according to the present invention is not limited to the chrome steel material, and other steel materials can be used. Moreover, even if it is a case where it consists of chromium steel materials, it is not limited at all to the illustrated temperature.

  As described above, in view of the trade-off relationship that the carburizing time becomes long if the carburizing temperature is lowered in order to suppress the creep deformation that occurs at the high carburizing temperature, in this embodiment, the carburizing temperature is made lower than the normal temperature. In addition, the surface hardness of the steel material part 1 is increased by nitriding after the carburizing treatment, thereby shortening the carburizing time. That is, the problem of carburizing time is compensated by nitriding treatment.

  FIG. 2 is a graph showing the relationship between temperature and time in one embodiment of the heat treatment of steel parts according to the present invention. In the heat treatment method for the steel material part 1 according to the present embodiment, first, the steel material part 1 is put into a heat treatment furnace at time t0 to t1, and the temperature is raised to the carburizing temperature. The carburizing temperature is, for example, 900 to 980 ° C., and the carburizing time is appropriately determined depending on the hardness required for the steel material part 1. By performing the carburizing process at a low temperature of 900 to 980 ° C. at time t1 to t2, structurally low strength and dimensional accuracy (particularly, the pair of pin press-in portions 12) of the steel part 1, such as the pair of pin press-in portions 12. Creep deformation is prevented from occurring in a portion where the distance L) is required.

  When the carburizing process is completed at time t2, the ambient temperature in the heat treatment furnace is subsequently lowered to 820 to 900 ° C. at time t2 to t3, and nitrogen gas is introduced into the heat treatment furnace to perform nitriding at time t3 to t4. Process. As a result, a nitrided layer is formed in the surface area of the carburized layer formed on the surface of the steel part 1 as shown in the lower cross-sectional view of FIG. Note that the chromium oxide layer shown in the cross-sectional view is a thin and dense Cr—C—O film formed on the surface of the steel part 1 by carburizing treatment, and in order to inhibit the diffusion of nitrogen, the nitriding treatment times t3 to t4 are: The time is set so that the surface of the steel part 1 has sufficient hardness.

  The dotted line in FIG. 3 shows the hardness with respect to the depth from the surface of the steel part 1 by the carburizing process at time t1 to t2, and the solid line in FIG. 3 is by the carburizing process at time t1 to t2 and the nitriding process at time t2 to t3. It is a graph which shows the hardness with respect to the depth from the surface of the steel material component 1. FIG. As shown in the figure, the surface hardness is increased by the carburizing process, but the surface hardness is further increased by the nitriding process. As described above, the nitriding treatment in which the surface of the steel part 1 is impregnated with nitrogen has an advantage that the strength can be further improved in addition to the carburizing treatment, but the nitriding treatment can be performed only at a low temperature and the diffusion rate is slow. For this reason, there is a drawback that the nitriding depth for ensuring the strength cannot be increased. However, in the present embodiment, after the carburizing treatment, the temperature is lowered to the quenching holding temperature, and a nitriding hard layer can be formed near the surface by injecting a nitriding gas into the atmosphere of the heat treatment furnace. Thereby, by ensuring the hardness in the vicinity of the surface of the steel material part 1 by nitriding, the carburizing time at a lower temperature than usual can be shortened, and the total carburizing time can be reduced. The amount of creep deformation can be reduced.

  When the nitriding process is finished at time t4, the steel material part 1 is quenched at time t4 to t5. By the above heat treatment process, it is possible to improve the wear resistance and impact strength of the surface of the steel part 1 while suppressing the occurrence of creep deformation.

  FIG. 4 is a graph showing the relationship between temperature and time in another embodiment of the heat treatment of steel parts according to the present invention. In the heat treatment method for the steel material part 1 according to the present embodiment, first, the steel material part 1 is put into a heat treatment furnace at time t0 to t1, and the temperature is raised to the carburizing temperature. The carburizing temperature is, for example, 1050 ° C., and the carburizing time is appropriately determined depending on the hardness required for the steel material part 1. By performing the carburizing process at a low temperature of 1050 ° C. at the time t1 to t2, the dimensional accuracy of the steel part 1, particularly the pair of pin press-fit portions 12, such as the pair of pin press-fit portions 12 (between the pair of pin press-fit portions 12). Creep deformation is suppressed from occurring at the site where the distance L) is required. Moreover, since the carburizing temperature is higher than that in the embodiment shown in FIG. 3 described above, the carburizing time can be shortened.

  As shown in the sectional view at time t2 in FIG. 4, a carburizing process produces a thin and dense Cr—C—O film (chromium oxide layer) on the surface of the steel part 1, and this chromium oxide layer is formed during the subsequent nitriding process. Inhibits the diffusion of nitrogen. For this reason, in the heat treatment of the embodiment shown in FIG. 3 described above, it is necessary to ensure a sufficient nitriding time.

  On the other hand, in this embodiment, the steel material part 1 which finished the carburizing process at time t2 is cooled to 650 ° C. or lower, preferably 600 ° C. or lower at time t2 to t3, and the steel material part 1 and the chromium oxide layer are The chromium oxide layer is destroyed and removed by applying thermal stress to the chromium oxide layer using the difference in thermal expansion coefficient. This cooling treatment time may be set to an appropriate time according to the state of destruction of the chromium oxide layer. After that, the temperature of the heat treatment furnace is raised again to a nitriding temperature of 820 to 900 ° C. The cooling treatment at this time t2 to t4 is an additional treatment compared to the embodiment of FIG. The removal of the chromium layer offsets the shortening of the next nitriding time. Also, the total heat treatment time can be shortened by setting the carburizing temperature to a relatively high 1050 ° C. even at a low temperature.

  At time t3 to t4, the atmospheric temperature in the heat treatment furnace is lowered to 820 to 900 ° C., and nitrogen gas is introduced into the heat treatment furnace to perform nitriding treatment at time t4 to t5. As a result, as shown in the cross-sectional view in the lower diagram of FIG. 4, a nitride layer is formed in the surface area of the carburized layer formed on the surface of the steel part 1 from which the chromium oxide layer has been removed. When the nitriding process is completed at time t5, the steel material part 1 is quenched at times t5 to t6. By the above heat treatment process, it is possible to improve the wear resistance and impact strength of the surface of the steel part 1 while suppressing the occurrence of creep deformation.

  FIG. 2 shows the heat treatment (Example 1) shown in FIG. 2 so that the effective hardened layer depth (ECD) is 1.1 ± 0.3 mm for the steel part 1 made of the chromium steel steel SCr420H shown in FIG. The heat treatment shown in 4 (Example 2) was performed. Further, as Comparative Example 1, only the carburizing process (the nitriding process after the carburizing process was not performed) was performed on the same steel material part 1. Table 1 shows the Rockwell hardness (HRC) of the surface of each steel part 1 after these heat treatments, the creep deformation amount of the distance L between the pair of pin press-fit portions 12 shown in FIG. 1, and the total heat treatment time.

  Comparing Examples 1 and 2 with Comparative Example 1, both Examples 1 and 2 have a high surface hardness, a small amount of creep deformation, and a short processing time. In particular, the effect of reducing the amount of creep deformation and shortening the processing time of Example 2 is remarkable.

  For the steel part 1 made of the chromium steel steel SCr420H shown in FIG. 1, the number of samples is increased so that the effective hardened layer depth (ECD) is 1.3, and the heat treatment (Example 1) shown in FIG. 2 is performed. Then, the creep deformation amount of the distance L between the pair of pin press-fitting portions 12 shown in FIG. 1 was measured. Further, as Comparative Example 1, only the carburizing process (the nitriding process after the carburizing process is not performed) is performed on the same steel material part 1, and the creep deformation amount of the distance L between the pair of pin press-fit portions 12 illustrated in FIG. Was measured. The result is shown in FIG. The creep deformation amount of Example 1 is smaller than that of Comparative Example 1.

  As described above, according to the heat treatment method for steel parts of the present embodiment, the occurrence of creep deformation can be suppressed by performing the carburizing process at 900 to 1200 ° C., which is lower than the normal carburizing temperature. In addition to this, the surface hardness of the steel part 1 can be increased by performing nitriding after the carburizing treatment, and the occurrence of creep deformation can also be suppressed. Therefore, even if the carburizing time at low temperature is shortened, the surface is hardened by nitriding, so that the total heat treatment process can suppress the occurrence of creep deformation of steel parts while suppressing the increase in carburizing time. it can.

  Further, according to the heat treatment method for steel parts of this embodiment, the steel part 1 that has been carburized is cooled to 650 ° C. or lower, preferably 600 ° C. or lower, and thus formed on the surface of the steel part 1 by carburizing. The chromium oxide layer can be destroyed and removed. As a result, in the subsequent nitriding treatment, the chromium oxide layer that inhibits the diffusion of nitrogen is removed, so that the nitriding treatment can be completed in a short time.

DESCRIPTION OF SYMBOLS 1 ... Steel material part 11 ... Bearing part 12 pin press-fit part 13 ... Screw part 13A ... Screw hole

Claims (4)

After carburizing steel parts at a carburizing temperature of 900-1200 ° C,
A heat treatment method for a steel part, wherein the steel part is nitrided at a nitriding temperature lower than the carburizing temperature. After carburizing steel parts at a carburizing temperature of 900-980 ° C,
Then, the heat processing method of the steel material components which carries out the nitriding process of the said steel material components at the nitriding temperature of 820-900 degreeC. After carburizing steel parts at a carburizing temperature of 1020 to 1200 ° C,
The steel part is cooled to 650 ° C. or lower,
Then, the heat processing method of the steel material components which nitrides the said steel material components at the nitriding temperature of 820-900 degreeC.   The heat treatment method for steel parts according to any one of claims 1 to 3, wherein a quenching process is performed following the nitriding process.

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