JP6313928B2 - Heat treatment method for steel - Google Patents

Description

This invention also relates to the heat treatment method of steel.

  Conventionally, various heat treatments such as quenching and tempering of steel materials have been performed for the purpose of increasing the strength and toughness of metal members, improving the wear resistance performance of the surface, improving the fatigue strength characteristics, and the like. Various methods have been used for the heat treatment method at this time depending on the purpose of use of the steel material.

  Among the heat treatment of the steel material, since quality improvement that cannot be achieved by a single quenching can be achieved, a method of performing a quenching process twice has been studied. For example, Patent Document 1 discloses an invention for the purpose of “providing a manufacturing method and use of a high-strength, high-corrosion-resistant martensitic stainless steel having the same strength as cast stainless steel and having excellent underwater fatigue strength and toughness”. Is disclosed, and a heat treatment method for a steel material including two quenching steps is disclosed.

And in patent document 1, "by weight, C; 0.01-0.10%, Si; 0.1-1.5%, Mn; 0.1-2.0%, Cr; 12-17% , Ni; 3 to 7%, Co; 0.1 to 3%, Mo; 0.1 to 2.0%, V; 0.05 to 0.5%, mainly composed of martensite and retained austenite In a high strength and high corrosion resistance martensitic stainless steel having a mixed structure and a residual austenite content of 35% or less by volume and a δ ferrite content of 15% or less by area, the balance is made of Fe and inevitable impurities. A step of quenching the composition at a temperature of A _C3 or higher, a step of intermediate quenching at a temperature between A _C1 and A _C3 at least once after that step, and a step of quenching at a temperature of A _C1 or lower after that step. Including a step of returning Time employs a highly corrosion-resistant method for manufacturing a martensitic stainless steel. ". That is, in this patent document 1, “a step of quenching a steel material having a constant composition at a temperature of AC ₃ point or higher”, “a step of intermediate quenching at a temperature between A _C1 and A _C3 at least once after that step”. "it can be seen that has a" Next step of tempering at a _C1 temperature below ".

  By adopting the manufacturing method disclosed in Patent Document 1, it can be seen that a high-strength, high-corrosion-resistant martensitic stainless steel of the level disclosed in Patent Document 1 can be obtained.

Japanese Patent No. 2678263

  However, in the case of a high-strength, high-corrosion-resistant martensitic stainless steel obtained by the manufacturing method disclosed in Patent Document 1, the hardness is not sufficient, and the field of use as a product is limited.

  Therefore, in the market, there has been a demand for a steel quenching method that can impart higher hardness to steel materials.

  Accordingly, the inventors of the present invention have come up with the invention described below as a result of intensive studies. Hereinafter, it will be described separately for each invention.

  In the heat treatment method for steel according to the present invention, the steel type has a high frequency twice for a steel material that is a stainless steel having a carbon content of 0.1% by mass to 1.5% by mass and a Cr content of 10% by mass or more. A heat treatment method including a quenching step using a quenching method includes the following steps.

First quenching step: The steel material is heated at a first heating temperature T1 set in a range of 1000 ° C to 1100 ° C, then cooled to room temperature, quenched, and left in the atmosphere.
2nd hardening process: The steel material which the 1st hardening process was complete | finished is heated by 2nd heating temperature T2 set to the range of 850 degreeC-1200 degreeC, and it cools and quenches to room temperature.

  In the steel material heat treatment method according to the present invention, it is also preferable to repeat the second quenching step twice or more.

  In the heat treatment method for steel according to the present invention, the tempering heating temperature T3 is preferably a temperature set in a range of 170 ° C to 650 ° C.

In the heat treatment method for steel according to the present invention, a steel after quenching, iron - that heating was heated to A _c1 line below the tempering heating temperature T3 in the carbon phase diagram, adding a tempering step of cooling to room temperature Is preferred.

  In the steel material heat treatment method according to the present invention, it is preferable that a sub-zero treatment is performed after the end of the second quenching step, and then a chilling treatment step is performed in which the steel material is left in the atmosphere until the temperature returns to room temperature.

  It is preferable that the amount of retained austenite of the steel material is 35% or less by using the steel material heat treatment method described above.

  By adopting the above-described heat treatment method for steel, the hardness of the steel can be stably improved. Therefore, it becomes possible to make the steel material which could not be used depending on the application conventionally by using the steel material heat treatment method according to the present invention to be usable steel material. The steel material heat treatment method according to the present invention particularly comprises stainless steel having a carbon content of 0.1% by mass or more and a Cr content of 10% by mass or more, or a carbon content of 0.6% by mass or more. This is a strengthening method suitable for carbon steel.

It is a conceptual diagram for showing the heating history of the heat processing method of the steel materials which concerns on this invention. It is a schematic diagram for demonstrating the hardness measurement position in a sample. It is a photograph of the crystal structure of Example 1-1 to Example 1-6. It is a photograph of the crystal structure of Comparative Example 1 (Comparative Sample 1-A to Comparative Sample 1-C). It is a photograph of the crystal structure of Comparative Example 2 (Comparative Sample 2-A to Comparative Sample 2-F). It is a photograph of the crystal structure of Comparative Example 3 (Comparative Sample 3-A to Comparative Sample 3-C).

  Hereinafter, the steel material heat treatment method according to the present invention and the high-hardness steel material obtained by this heat treatment method will be described in detail.

Form of heat treatment method for steel material: The heat treatment method for steel material according to the present invention includes two or more quenching steps for the steel material. Hereinafter, although it demonstrates for every process, the heating system used in this invention is demonstrated first. In FIG. 1, the conceptual diagram of the heat processing method of this steel material is shown. In FIG. 1, the heating region is denoted by reference symbol R, the holding region is denoted by symbol H, the cooling region is denoted by symbol C, and the deep cooling treatment region is denoted by reference symbol S ₀ .

Heating method: In the heat treatment method for steel according to the present invention, it is preferable to employ a high-frequency heating method for heating when performing the quenching treatment. This is because the quenching process by the high-frequency heating method has a higher heating rate than the general quenching process and can quickly heat the surface of the steel material to be heated. That is, the heating method in each quenching step described below is a condition when the high-frequency heating method is adopted. Note that the heating times R ₁ , R ₂ , and R ₃ shown in FIG. 1 are times converted by {Tn (n = 1, 2, 3) −RT (room temperature)} / [heating rate], respectively. .

First quenching step: In this first quenching step, the steel material is heated at a first heating temperature T1 set in a range of 1000 ° C. to 1200 ° C., held for H ₁ hour, cooled to room temperature, quenched, and into the atmosphere. put. Here, there are no particular limitations on the shape, size, etc. of the steel material, but it is necessary to be able to achieve the heating rate during heating described below.

  The first heating temperature T1 set in the range of 850 ° C. to 1200 ° C. is employed as the quenching temperature used in the first quenching step. When the temperature of the 1st heating temperature T1 is less than 850 degreeC, it becomes difficult to become a perfect hardened structure | tissue and it becomes difficult to obtain desired hardness stably. On the other hand, depending on the composition of the steel material, the temperature of the first heating temperature T1 may be over 1200 ° C. and heated to about 1400 ° C. However, when the temperature of the first heating temperature T1 exceeds 1200 ° C., the amount of retained austenite becomes excessive and the tendency to cause a decrease in hardness becomes remarkable. This increase in the amount of retained austenite becomes remarkable particularly when high carbon chromium steel (for example, SUJ material or the like) or stainless steel (for example, SUS material or the like) is used.

  And the 1st heating temperature T1 in the case of using "stainless steel provided with the carbon content of 0.1 mass% or more and the Cr content of 10 mass% or more" is set to the temperature of the range of 1000 to 1200 degreeC. Is more preferable from the viewpoint of obtaining a stable quenched structure. On the other hand, it is more preferable that the first heating temperature T1 in the case of using “carbon steel having a carbon content of 0.6 mass% or more” is set to a temperature in the range of 850 ° C. to 1100 ° C. In the case of carbon steel, when heated at a temperature exceeding 1100 ° C., the amount of retained austenite becomes excessive and tends to cause a decrease in hardness.

In addition, here, a high-frequency heating method, which will be described later, is adopted as a heating method, and the temperature is raised in a short time because heating for a long time using other heating methods reduces productivity and cures. This is because it is not preferable because it is cured to an unnecessary portion. When this steel material is heated to the first heating temperature T1 set in the range of 850 ° C. to 1200 ° C. (R _{1 in} FIG. ₁ ), the first heating temperature T1 holds the steel material for 3 seconds to 120 seconds. (H _{1 in} FIG. ₁ ). If this holding time (H ₁ ) is less than 3 seconds, even if the heating temperature is brought close to 1200 ° C., it cannot be uniformly austenitic, and a sufficient quenching effect cannot be obtained. It becomes difficult to increase the hardness of the. On the other hand, when this holding time (H ₁ ) exceeds 120 seconds, a decarburization phenomenon occurs on the surface of the product, and the surface hardness decreases, which is not preferable.

  When the heating in the first quenching process is completed, it is cooled to room temperature, quenched, and left in the air. There is no particular limitation on the cooling method at this time.

Second quenching step: In this second quenching step, the first quenching step is completed, and the steel material that has been once cooled to room temperature and martensite is austenitized and quenched. If the heat treatment method of the steel material according to the present invention does not include the second quenching step, the amount of retained austenite increases when the heating temperature is increased to obtain high hardness, and the desired hardness is obtained. You can't get it. In addition, it is preferable to set the 2nd heating temperature T2 at this time in the range of 850 degreeC-1200 degreeC. When the second heating temperature T2 is less than 850 ° C., it is not preferable because it is impossible to obtain the hardness that is expected to be obtained in the second quenching step for the steel material. On the other hand, when the 2nd heating temperature T2 exceeds 1200 degreeC, since the decarburization phenomenon occurs in the surface of steel materials and surface hardness falls, it is unpreferable.

  At this time, it is also preferable to set the first heating temperature T1 in the range of “first heating temperature T1 ± 200 ° C.” as shown in FIG. By satisfying this condition, the hardness of the steel material can be increased more stably. If the 2nd heating temperature T2 employ | adopted at the 2nd hardening process in such a case is said concretely, it is preferable to employ | adopt the temperature of the range of 950 degreeC-1200 degreeC. That is, for example, “When 1000 ° C. is adopted as the first heating temperature T1, the second heating temperature T2 is a temperature in the range of 950 ° C. to 1200 ° C.” and “1200 ° C. is adopted as the first heating temperature T1. In this case, the second heating temperature T2 is a temperature in the range of 1000 ° C. to 1200 ° C. ”. By maintaining the relationship between the first heating temperature T1 and the second heating temperature T2, the effect of improving the hardness of the steel material is further stabilized.

As described above, when the steel material is heated to the second heating temperature T2 set in the range of 850 ° C. to 1200 ° C. by the high frequency heating method, the second heating temperature T2 is maintained for 3 seconds to 7 seconds (see FIG. 1 H ₂ ). If this holding time (H ₂ ) is less than 3 seconds, even if the heating temperature is brought close to 1200 ° C., it cannot be uniformly austenitized, and a sufficient quenching effect cannot be obtained. It becomes difficult to increase the hardness of the. On the other hand, when the holding time (H ₂ ) exceeds 7 seconds, the effect of increasing the hardness of the product is saturated, and thus a further increase in hardness cannot be expected.

The second heating temperature T2 when the carbon content of the steel material to be heat-treated is 0.77% by mass or less is “A _C3 + 50 ° C.”, and the second heating when the carbon content exceeds 0.77% by mass. As the temperature T2, it is preferable to adopt a temperature of “ _AC1 + 50 ° C.” because a more stable hardness increasing effect can be obtained.

  In addition, it is preferable to repeat the 2nd hardening process employ | adopted by this invention described above twice or more. By repeating the second quenching process twice or more, carbon is diffused stepwise, and the heat treatment conditions specified in the present invention can be selected to further improve the hardness of the steel material.

Cooling method: In the cooling in the first quenching step and the second quenching step described above, it is preferable to employ a cooling rate in a range not depending on the nose of the continuous cooling transformation curve (CCT curve). Accordingly, the required cooling times C ₁ , C ₂ , and C ₃ shown in FIG. 1 are times converted by (Tn (n = 1, 2, 3) −RT (room temperature)) / [cooling rate], respectively. .

Cryogenic treatment process: This cryogenic treatment process is an arbitrary process performed according to the properties and required quality of the steel material. When the quenching process described above is completed and a deep cooling process is performed, it is preferable to move to the deep cooling process as quickly as possible. The deep cooling treatment here refers to an operation of immersing in liquid air or dry ice (−40 ° C. to −196 ° C.) immediately after quenching, and is also referred to as sub-zero treatment. Since the amount of retained austenite in the steel structure is reduced and the amount of martensite is increased by the deep cooling treatment, the hardness can be improved.

Hereinafter, the deep cooling treatment in the heat treatment method for steel according to the present application starts contact between the steel material and liquid nitrogen within 60 seconds after the end of the second quenching step. Then, using liquid nitrogen, at a cryogenic treatment temperatures (-196 ° C.), maintained for 30 minutes or more _(S 0 in Fig. 1). When the time (S ₀ ) of this deep cooling treatment is less than 30 minutes, the amount of retained austenite becomes excessive, and it becomes difficult to sufficiently improve the hardness of the steel material.

  The deep-cooled steel material is then left in the atmosphere until it returns to room temperature (Rn in FIG. 1). In this way, the temperature is returned to room temperature without heating.

Then, the steel was returned to room temperature through a cryogenic process, iron - heated was heated to A _C1-wire following tempering heating temperature T3 in the carbon phase diagram, it is possible to add tempering step of cooling to room temperature preferable. The tempering process of this invention is demonstrated below.

Tempering step: This tempering step is also an optional step performed according to the properties and required quality of the steel material in the same manner as the deep cooling treatment step. That is, after returning to room temperature on standing steel (including the case where after the cryogenic treatment process) after tempering step, the iron - and heated to the tempering heating temperature T3 below _C1 line A in the carbon phase diagram Heat (R _{3 in} FIG. 1). Here, it is not preferable to perform tempering heating at a temperature exceeding the _AC1 line because sufficient hardness cannot be obtained as a product. Specifically, the tempering heating temperature T3 at this time is preferably set in a range of 170 ° C to 650 ° C. When this tempering heating temperature T3 is less than 170 ° C., an appropriate crystal structure to be obtained by tempering cannot be obtained, and the hardness of the steel material is not stable. On the other hand, when the tempering heating temperature T3 exceeds 650 ° C., the crystal structure suitable for increasing the hardness tends to disappear, which is not preferable.

When the steel is heated to heated to _{A C1-wire} following tempering heating temperature T3, for 60 minutes to 150 minutes at the tempering heating temperature T3, it holds _(H 3 in Figure 1). When the heating in this tempering process is completed, it is cooled to room temperature (C _{3 in} FIG. 1).

  Moreover, the heat processing method of the steel materials which concerns on this invention is equipped with stainless steel provided with carbon content of 0.1 mass% or more and Cr content of 10 mass% or more, or carbon content of 0.6 mass% or more. Carbon steel can be suitably used as a steel material. According to the heat treatment method for steel materials according to the present invention, the high carbon chromium steel (for example, SUJ material) and the stainless steel (for example, SUS material) that are likely to increase the amount of retained austenite during quenching are quenched. Even so, it is possible to suppress an increase in the amount of retained austenite.

Form of high-hardness steel: The high-hardness steel obtained by the heat treatment method for steel according to the present invention is a high-hardness steel material having a martensite structure strengthened by the heat treatment method for steel described above. And the amount of retained austenite in the crystal structure of this steel material is 35% or less, “stainless steel having a carbon content of 0.1% by mass or more and a Cr content of 10% by mass or more”, or “0.6 It is “carbon steel having a carbon content of at least mass%”. Here, by clarifying the amount of retained austenite as 35% or less, the characteristics of the crystal structure of the high-hardness steel material whose hardness is sufficiently improved become clear. In addition, by adopting the above-described heat treatment method for steel materials according to the present invention, for high carbon chromium steel (for example, SUJ material) and stainless steel (for example, SUS material) in which the amount of retained austenite tends to increase during quenching. Even if quenching is performed, the amount of retained austenite can be reduced to 35% or less.

  A steel material having a retained austenite content of 35% or less in the crystal structure has a Vickers hardness (HV 0.3) of 80% or more of the maximum hardness in the C content hardness curve. Specifically, it is assumed that the Vickers hardness (HV 0.3) is 630 or more.

  Hereinafter, the heat treatment method and effect of the steel material according to the present invention will be described more specifically through examples and comparative examples. In these, the sample used for heat processing used the rod of length 10mm x diameter 3mm.

  The rod-shaped sample used in Example 1 has a composition containing the components shown in Table 1 below as components other than iron.

  In accordance with the procedure described below, six types of samples corresponding to the examples (Examples 1-1 to 1-6) were obtained. In addition, the following table | surface, it shows so that the heating conditions etc. of an Example and a comparative example can be compared.

Heating method: In the heat treatment method for steel according to the present invention, it is preferable to employ a high-frequency heating method for heating when performing the quenching treatment. This is because the heating rate is fast and the steel material to be heated can be quickly heated. That is, the heating method when performing quenching in each step described below employs a high-frequency heating method, and a heating rate of 190 ° C./second or more is employed. And the furnace heating was employ | adopted as the heating system when performing tempering. Therefore, the heating times R ₁ , R ₂ , and R ₃ shown in FIG. 1 are times converted by (Tn (n = 1, 2, 3) −RT (room temperature)) / [heating rate], respectively. .

First quenching step: In the first quenching step, the sample was heated to a first heating temperature T1 set at three levels of 1000 ° C, 1100 ° C, and 1200 ° C. Atsushi Nobori time R ₁ in this case, to be 5 seconds to set the condition of the high frequency heating apparatus. Then, the sample has reached a predetermined heating temperature, the first heating temperature T1 5 seconds (H _1), and held. Thereafter, using helium gas, the sample in which the crystal structure was austenitized by heating was rapidly cooled to room temperature, thereby converting the crystal structure into martensite. The quenching described above was completed, and the mixture was once cooled to room temperature and left at room temperature for 3 minutes (H _{RT1 in} FIG. 1).

Second quenching step: In this second quenching step, the first quenching step was completed, and the steel material that had been once cooled to room temperature and turned into martensite was again heated to the second heating temperature T2 and quenched. As shown in Table 1, the second heating temperature T2 at this time was set at two levels of 1000 ° C. and 1100 ° C. within the range of “first heating temperature T1 ± 200 ° C.” with reference to the first heating temperature T1. . Heating time R ₂ at this time is such that the 5 seconds to set the condition of the high frequency heating apparatus. The sample that reached the predetermined heating temperature was held at the second heating temperature T2 for 5 seconds (H ₂ ). Thereafter, using helium gas, the sample in which the crystal structure was austenitized by heating was rapidly cooled to room temperature, thereby converting the crystal structure into martensite. When the quenching described above was completed and the cooling to room temperature was completed, the deep cooling process step was performed within 60 seconds ( _{HRT2 in} FIG. 1).

Cryogenic treatment step: In the cryogenic treatment step, the steel material that has undergone the second quenching step is brought into contact with liquid nitrogen and maintained at a cryogenic treatment temperature (-196 ° C.) for 180 minutes (S _{0 in} FIG. 1). . The deep-cooled sample was left in the atmosphere until it returned to room temperature (Rn in FIG. 1), and gradually returned to room temperature.

Tempering process: deep cooling after the processing step, the sample was returned to room temperature, the iron - and heated was heated to A _C1-wire following tempering heating temperature T3 (170 ° C.) in the carbon phase diagram (R ₃ in FIG. 1 ). Then, for 120 minutes at the tempering heating temperature T3, and held _(H 3 in Figure 1). Then, it stood to cool and the implementation sample 1-1-the implementation sample 1-6 of the variation shown in Table 2 were obtained.

  And various evaluation was performed using these samples. The evaluation method for this sample was applied to all the samples of Examples and Comparative Examples. The various evaluation items will be described below.

Hardness measurement: As shown in FIG. 2, the hardness of the sample was measured by cutting the rod-shaped sample after quenching in a longitudinal section, and the hardness at the measurement points indicated as M1 to M5 in FIG. Measurement was performed and the average value was defined as hardness. The hardness measured at this time was Vickers hardness (HV 0.3).

Measurement of amount of retained austenite: The amount of retained austenite of the sample was measured by applying an X-ray diffraction analysis method to the crystal structure after quenching. For this measurement, a residual stress measuring device using a one-dimensional counter (PSPC: Position Sensitive Proportional Counter) manufactured by Rigaku Corporation was used. The measurement conditions at this time are shown in Table 3 below.

Crystal structure observation: The surface of Example 1-1 to Example 1-6 was etched using a Villela reagent and observed with a metal microscope (magnification: 400 times). The observation results are shown in FIG.

  The rod-shaped sample used in Example 2 has a composition containing the components shown in Table 4 below as components other than iron. In Example 2, the deep cooling process and the tempering process performed in Example 1 are omitted.

Heating method: The same high-frequency heating method as in Example 1 is adopted. Therefore, detailed description here is omitted to avoid redundant description.

First quenching step: In the first quenching step, the sample was heated to a first heating temperature T1 set at three levels of 850 ° C, 950 ° C, and 1050 ° C. Atsushi Nobori time R ₁ in this case, to be 5 seconds to set the condition of the high frequency heating apparatus. Then, the sample has reached a predetermined heating temperature, the first heating temperature T1 5 seconds (H _1), and held. Thereafter, using helium gas, the sample in which the crystal structure was austenitized by heating was rapidly cooled to room temperature, thereby converting the crystal structure into martensite. The quenching described above was completed, and the mixture was once cooled to room temperature and left at room temperature for 3 minutes (H _{RT1 in} FIG. 1).

Second quenching step: In this second quenching step, the first quenching step was completed, and the steel material that had been once cooled to room temperature and turned into martensite was again heated to the second heating temperature T2 and quenched. As shown in Table 5, the second heating temperature T2 at this time was set at two levels of 850 ° C. and 950 ° C. Heating time R ₂ at this time is such that the 5 seconds to set the condition of the high frequency heating apparatus. The sample that reached the predetermined heating temperature was held at the second heating temperature T2 for 5 seconds (H ₂ ). Thereafter, using helium gas, the sample in which the crystal structure was austenitized by heating was rapidly cooled to room temperature, thereby converting the crystal structure into martensite. The quenching described above was completed, and cooling to room temperature was performed. And the implementation sample 2-1 to the implementation sample 2-6 of the variation shown in Table 2 were obtained.

  In addition, since the crystal structure observation of Example 2-1 to Example 2-6 is the same as that of Example 1, the description is omitted.

Comparative example

[Comparative Example 1: Comparison with Example 1]
In this comparative example 1, the second quenching process in the example is omitted. Therefore, after the completion of the first quenching step, a deep cooling treatment step was immediately performed, and a tempering step was performed to obtain Comparative Sample 1-A, Comparative Sample 1-B, and Comparative Sample 1-C having variations shown in Table 6.

  And various evaluation was performed like these Examples using these samples. The evaluation method for this sample is as described above. The evaluation results are shown in Table 10 together with the evaluation results of the examples.

  The crystal structures of Comparative Sample 1-A to Comparative Sample 1-C of Comparative Example 1 were also observed in the same manner as in Example 1, and the observation results are shown in FIG.

[Comparative Example 2: Comparison with Example 1]
In this comparative example 2, the deep cooling process in an Example is abbreviate | omitted. Therefore, after completion of the first quenching process and the second quenching process, the samples returned to room temperature were subjected to a tempering process, and variations of Comparative Sample 2-A to Comparative Sample 2-F shown in Table 7 were obtained.

  And various evaluation was performed like these Examples using these samples. The evaluation method for this sample is as described above. The evaluation results are shown in Table 11 together with the evaluation results of the examples.

  The crystal structures of Comparative Sample 2-A to Comparative Sample 1-F of Comparative Example 2 are also observed in the same manner as in Example 1. The observation results are shown in FIG.

[Comparative Example 3: Comparison with Example 1]
In the comparative example 1, the second quenching process and the deep cooling process in the example are omitted. Therefore, after the completion of the first quenching process, the sample returned to room temperature was subjected to a tempering process, and Comparative Samples 3-A to 3-C of variations shown in Table 8 were obtained.

  And various evaluation was performed like these Examples using these samples. The evaluation method for this sample is as described above. The evaluation results are shown in Table 12 together with the evaluation results of the examples.

  The crystal structures of Comparative Sample 3-A to Comparative Sample 3-C of Comparative Example 3 were also observed in the same manner as in Example 1, and the observation results are shown in FIG.

[Comparative Example 4: Comparison with Example 2]
In Comparative Example 4, the second quenching process in Example 2 is omitted. Therefore, the 1st hardening process was performed and the comparative sample 4-A, the comparative sample 4-B, and the comparative sample 4-C of the variation shown in Table 9 were obtained.

  And various evaluation was performed like these Examples using these samples. The evaluation method for this sample is as described above. The evaluation results are shown in Table 13 together with the evaluation results of the examples.

  Observation of the crystal structures of Comparative Sample 4-A to Comparative Sample 4-C of Comparative Example 4 is omitted because the crystal structure of Example 2 as a comparison target is omitted.

[Contrast between Example 1 and Comparative Example 1]
The comparison between Example 1 and Comparative Example 1 is for verifying the effect of providing the second quenching step. Therefore, in the comparative example 1, the 2nd hardening process in Example 1 is abbreviate | omitted. Table 10 below shows the evaluation results in comparison.

  As can be seen from Table 10, the minimum value of the Vickers hardness of the working sample is 583 (HV 0.3). The comparative sample 1-A and the comparative sample 1-C show only values below the minimum value of this working sample. However, the value of the Vickers hardness of the comparative sample 1-B is 606 (HV 0.3), which is the same value as the working sample. At this time, the amount of retained austenite of Comparative Sample 1-B is 15.5%.

  However, when a sample equivalent to the amount of retained austenite of Comparative Sample 1-B is searched for among the samples, Example 3 has the same amount of retained austenite. And the value of the Vickers hardness of the implementation sample 3 at this time is 637 (HV0.3), and the value of the Vickers hardness of the comparative sample 1-B is considerably higher than 606 (HV0.3).

  Considering these facts, if the amount of retained austenite in the crystal structure is the same, it is understood that it is more useful to improve the hardness of the steel material by adopting the heat treatment method of the steel material according to the present invention. .

[Contrast between Example 1 and Comparative Example 2]
The comparison between Example 1 and Comparative Example 2 is for verifying the effect of providing the deep cooling process. Therefore, in Comparative Example 2, the deep cooling process in Example 1 is omitted. In Table 11 below, the evaluation results are shown in comparison.

  As can be seen from Table 11, the minimum value of the Vickers hardness of the working sample is 583 (HV 0.3). On the other hand, any sample of Comparative Sample 2-A to Comparative Sample 2-F shows only a value equal to or lower than the lowest value of this working sample. The samples of Comparative Sample 2-A to Comparative Sample 2-F have a high amount of retained austenite because they are not subjected to deep cooling treatment if the conditions of the first quenching step and the second quenching step are the same as those of the working sample. I understand that.

  When attention is paid to the amount of retained austenite, the following can be understood. In Comparative Sample 2-A to Comparative Sample 2-F, it can be understood that the hardness of the sample decreases as the amount of retained austenite increases. However, when Example Sample 1 to Example Sample 6 were each compared with a comparative sample having a low amount of retained austenite, all of the example samples were harder than the comparative sample.

  Considering these, the deep cooling process is considered to be an essential process in the heat treatment method for steel according to the present invention. That is, the amount of retained austenite in the crystal structure is reduced by combining the “first quenching process” and the “second quenching process” and the “deep cooling process” provided in the heat treatment method for steel according to the present invention. It can be understood that the hardness of the steel material can be improved.

[Contrast between Example 1 and Comparative Example 3]
This comparison between Example 1 and Comparative Example 3 is for verifying what effect is produced when the second quenching process and the deep cooling process do not exist. In other words, this is a comparison between the effect of improving the hardness of the steel material by one simple quenching and the effect of improving the hardness of the steel material by the heat treatment method of the steel material according to the present invention. Therefore, in Comparative Example 3, the second quenching process and the deep cooling process in Example 1 are omitted. Table 12 below shows the comparison of the evaluation results.

  As can be seen from Table 12, the minimum value of the Vickers hardness of the working sample is 583 (HV 0.3). On the other hand, any sample of Comparative Sample 3-A to Comparative Sample 3-C shows only a value equal to or lower than the lowest value of this working sample. That is, it can be said that the samples of Comparative Sample 3-A to Comparative Sample 3-C have a high amount of retained austenite because there is no deep cooling treatment if the conditions of the first quenching step are the same as those of the working sample. Therefore, for example, when a product requiring a hardness of about 600 (HV 0.3) is subjected to the same heat treatment as the samples of Comparative Sample 3-A to Comparative Sample 3-C, the desired hardness is obtained. Is not preferable because it is difficult to guarantee product quality.

  In consideration of these matters, it is considered that the second quenching process and the deep cooling process are indispensable processes in the steel material heat treatment method according to the present invention. That is, the amount of retained austenite in the crystal structure is reduced by combining the three steps of the “first quenching step”, “second quenching step”, and “deep cooling treatment step” provided in the heat treatment method for steel according to the present invention. It can be understood that the hardness of the steel material can be reduced and stably improved.

[Contrast between Example 2 and Comparative Example 4]
The comparison between Example 2 and Comparative Example 4 is to verify the effect of providing the second quenching step in the heat treatment of “carbon steel having a carbon content of 0.6 mass% to 1.5 mass%”. belongs to. Therefore, in the comparative example 4, the 2nd hardening process in Example 2 is abbreviate | omitted. Table 13 below shows the evaluation results in comparison.

  As can be seen from Table 13, the minimum value of the Vickers hardness of the working sample is 808 (HV 0.3). On the other hand, the comparative sample 4-A and the comparative sample 4-C show only values below the lowest value of this working sample. However, the value of Vickers hardness of Comparative Sample 4-B is 841 (HV 0.3), which is the same value as that of the working sample.

  At this time, the amount of retained austenite of Comparative Sample 4-B is 16.8%. Therefore, when a sample equivalent to the amount of retained austenite of Comparative Sample 4-B is searched for among Samples 2-1 to 2-6, there is no perfect match, but Sample 2-2 is close. Has retained austenite content. The value of Vickers hardness of the implementation sample 2-2 at this time is 862 (HV0.3), and even if the amount of retained austenite is large, the value of Vickers hardness 841 (HV0.3) of the comparative sample 4-B Even higher.

  Considering these facts, if the amount of retained austenite in the crystal structure is the same, it is understood that it is more useful to improve the hardness of the steel material by adopting the heat treatment method of the steel material according to the present invention. .

  According to the heat treatment method for steel according to the present invention, higher hardness can be imparted to the steel. And since the amount of retained austenite can be reduced by employ | adopting the heat processing method of the steel material which concerns on this invention compared with one hardening, the improvement effect of hardness is fully acquired. Therefore, the steel material heat treatment method according to the present invention can be suitably used for parts requiring high hardness, such as rocket parts and aircraft parts. In addition, the steel material heat treatment method according to the present invention can be used with an existing high-frequency heating type heat treatment apparatus and does not require a new facility investment, and thus is excellent in economic efficiency.

R ₁ , R ₂ , R ₃ heating region H ₁ , H ₂ , H ₃ holding region H _RT1 , H _RT2 room temperature holding region C ₁ , C ₂ , C ₃ cooling region S ₀ deep cooling treatment region 1 rod-shaped sample 2 longitudinal section Surface M1, M2, M3, M4, M5 Hardness measurement points

Claims (6)

A heat treatment including a quenching process using two induction quenching methods for a steel material in which the steel type is stainless steel having a carbon content of 0.1% by mass to 1.5% by mass and a Cr content of 10% by mass or more. In the method, the heat processing method of the steel materials characterized by including the process shown below.
First quenching step: The steel material is heated at a first heating temperature T1 set in a range of 1000 ° C to 1100 ° C, then cooled to room temperature, quenched, and left in the atmosphere.
2nd hardening process: The steel material which the 1st hardening process was complete | finished is heated by 2nd heating temperature T2 set to the range of 850 degreeC-1200 degreeC, and it cools and quenches to room temperature. The steel material heat treatment method according to claim 1 , wherein the second quenching step is repeated two or more times. The finished steel quenching process, iron - heated was heated to A _C1-wire following tempering heating temperature T3 in the carbon phase diagram, according to claim 1 or claim 2 by adding the tempering step of cooling to room temperature Heat treatment method for steel. The said tempering heating temperature T3 is the temperature set to the range of 170 to 650 degreeC, The heat processing method of the steel materials of Claim 3 . The steel material heat treatment method according to any one of claims 1 to 4 , further comprising a chilling treatment step of performing a sub-zero treatment after the second quenching step and then leaving the steel material in the atmosphere until the temperature returns to room temperature. . The method for heat treating a steel material according to any one of claims 1 to 5 , wherein the amount of retained austenite of the steel material is 35% or less.