JP2016017212A - Carburizing and quenching method for steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carburizing and quenching method where, even if the surface layer of a steel is subjected to carburizing treatment, and the steel is subjected to primary cooling till its refining, the amount of the strain applied to the steel can be reduced.SOLUTION: The structure of a steel made of a ferrite structure and a pearlite structure is heated so as to be an austenite, the surface layer of the steel in the state of the austenite structure is subjected to carburizing treatment with a carburizing gas, after the carburizing treatment, the surface layer of the steel is cooled to a temperature below the recrystallization temperature of the steel so that the austenite structure of the steel can be retained, thus dislocation is introduced into the austenite structure of the surface layer, the surface layer of the steel made of the austenite structure into which dislocation is introduced is temperature-increased to a temperature more than the recrystallization temperature of the steel or more, thus the austenite structure of the surface layer of the steel is refined with the dislocation as the nucleus, and subsequently, the structure of the steel is transformed into a martensite structure.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a carburizing and quenching method for a steel material in which the surface layer of the steel material is carburized using a carburizing gas and then refined.

  Conventionally, carburizing treatment has been applied to steel materials in order to improve the durability of the surface of the steel materials. One of the carburizing processes is a gas carburizing process. In the gas carburizing process, a carburizing gas is brought into contact with the surface of a heated steel material, so that carbon is dissolved and diffused from the surface of the steel material to its surface layer. Here, in order to increase the solid solution amount and solid solution diffusion rate of carbon in the steel material, it is preferable that the heating temperature of the steel material during the carburizing process is high. However, if the heating temperature of the steel material during the carburizing process is increased, the austenite crystal grains of the steel material become coarse, and the toughness of the steel material is reduced.

In view of such points, for example, a carburizing and quenching method as shown in FIG. 6 has been proposed (see, for example, Patent Document 1). Here, first, as steel consisting of ferrite structure and pearlite structure is austenitic structure, after heating the steel to a carburizing temperature Ta in excess of A ₃ transformation point, carburizing carburizing gas steel austenitic structure. Next, the austenite structure is transformed into a martensite structure by primarily cooling the carburized steel material to a cooling temperature Tb below the martensite transformation point Ms. The primary cooled steel material is reheated to a temperature rise temperature Tc equal to or higher than the recrystallization temperature Tr to transform the martensite structure into an austenite structure, and the surface layer austenite structure is refined. Finally, the refined austenite structure is transformed into a martensite structure by secondary cooling (quenching is performed). In this way, a refined martensite structure is formed on the carburized surface layer of the steel material.

Further, as another carburizing and quenching method, a carburizing and quenching method as shown in FIG. 7 has been proposed (see, for example, Patent Document 2). Here, first, similarly to the carburizing method described above, the steel material transformed into austenitic structure at carburization temperature TA greater than the A ₃ transformation point carburizing carburizing gas. Next, the carburized steel material is primarily cooled to a cooling temperature TB exceeding the martensitic transformation point Ms, and the cooling temperature TB is maintained for a certain time. Thereby, the austenite structure of at least the surface layer of the steel material is transformed into a ferrite structure and a pearlite structure (or bainite structure). Thereafter, in the same manner as the carburizing and quenching method described above, the steel material is reheated to a temperature rise temperature TC equal to or higher than the recrystallization temperature Tr to transform it into an austenite structure, and then the austenite structure is converted into a martensite structure by secondary cooling. Transform (harden). Thus, also in this case, a refined martensite structure is formed on the surface layer of the steel material that has been carburized.

Japanese Patent Laid-Open No. 11-217626 JP 2009-52119 A

  However, when the carburizing and quenching method shown in FIG. 6 is performed, distortion occurs in the steel material when the steel material is transformed from the austenite structure to the martensite structure during the primary cooling, and during the secondary cooling after reheating (during quenching). Is accompanied by the same transformation, further strain is imparted to the steel material.

  Similarly, even when the carburizing and quenching method shown in FIG. 7 is performed, the steel material is maintained at a constant cooling temperature TB after the primary cooling, so that the steel material is transformed from the austenite structure to the ferrite structure and the pearlite structure (or bainite structure). The steel material will be distorted. Similarly to FIG. 6, even during secondary cooling after reheating (quenching), since the austenite structure is transformed to the martensite structure, strain is further imparted to the steel material.

  As described above, in the carburizing and quenching method shown in FIGS. 6 and 7, the steel material is primarily cooled during the carburizing treatment of the steel layer and refined, and the austenite structure of the surface layer is transformed once. The amount of strain of the steel material becomes larger than that of only quenching without quenching (without cooling twice).

  The present invention has been made in view of the above points, and the object of the present invention is to provide a steel material even if the steel material is primarily cooled during the carburizing treatment and refinement of the steel surface layer. An object of the present invention is to provide a carburizing and quenching method for steel that can reduce the amount of strain applied.

  As a result of intensive studies, the inventors transformed the steel structure into an austenite structure by heating at the time of carburizing treatment, and finally quenching to transform the austenite structure to the martensite structure by cooling the steel material. In the meantime, new knowledge was obtained that the strain generated in the steel material can be reduced by maintaining the steel material structure in the austenite structure regardless of the cooling of the steel material on the way.

  The present invention is based on the inventors' new knowledge, and the steel carburizing and quenching method according to the present invention heats a steel material composed of a ferrite structure and a pearlite structure, and transforms the structure of the steel material into an austenite structure. A step of carburizing the surface layer of the steel material of the austenite structure with a carburizing gas, and after the carburizing treatment, the surface layer of the steel material is maintained at a temperature lower than the recrystallization temperature of the steel material so as to maintain the austenite structure of the steel material. The step of introducing dislocations into the austenite structure of the surface layer of the steel material by cooling to a temperature, and by raising the temperature of the surface layer of the steel material composed of the austenite structure into which the dislocation has been introduced to a temperature equal to or higher than the recrystallization temperature of the steel material , A step of refining the austenite structure of the surface layer of the steel material with the dislocation as a nucleus, and a steel material having the austenite structure refined On cooling, characterized in that it comprises a hardening step of transforming the austenitic structure of the steel in martensitic structure, at least.

First, in the present invention, a steel material having a ferrite structure and a pearlite structure is prepared and heated until the ferrite structure and the pearlite structure are transformed into an austenite structure. Specifically, steel of A ₁ transformation point or more, preferably heated to A ₃ transformation point or more temperature (described later carburization temperature). Next, by bringing the carburizing gas into contact with the surface of the steel material having an austenitic structure heated and held at the carburizing temperature, the carbon of the carburizing gas is dissolved from the surface of the steel material, and the carbon is diffused into the surface layer (inside the steel material). As a result, the surface layer of the steel material is carburized with the carburizing gas.

Next, after carburizing, the surface layer of the steel material is primarily cooled to a temperature lower than the recrystallization temperature of the steel material so as to maintain the austenite structure of the steel material, thereby introducing dislocations serving as recrystallization nuclei into the austenitic structure. Here, a temperature below the recrystallization temperature (i.e., a temperature lower than the A ₁ transformation point) because it is cooled, the austenite structure which not transformed structure is maintained in the state of supercooled austenite. Thus, since the dislocation is introduced into the austenite structure of the surface layer due to thermal contraction of the steel material by cooling in a state where the austenite structure is maintained, the structure of the steel material is not transformed, and strain due to the transformation is not imparted to the steel material.

  In addition, if the dislocation described above is introduced in a state where the surface layer structure of the steel material is maintained in the austenite structure, and the austenite structure of the surface layer can be refined in the process described later, the cooling conditions are particularly limited. is not.

  Next, the surface layer of the steel material having an austenite structure into which dislocations are introduced is heated to a temperature equal to or higher than the recrystallization temperature of the steel material without transformation, thereby refining the surface layer structure of the steel material with dislocations as the core.

  In the present invention, “increasing the temperature of the surface layer of the steel material to a temperature equal to or higher than the recrystallization temperature of the steel material” means that the surface layer of the steel material is heated by an external heating device to increase the temperature (reheating). This includes both cases where the temperature of the surface layer of the steel material is raised by self-recuperation using the heat inside the steel material.

  In this way, the surface layer of the steel material is refined with the dislocations of the surface layer as the core, so that the toughness of the carburized layer that is the surface layer of the steel material is improved. In addition, since the austenite structure is refined before the austenite structure into which the dislocation is introduced is transformed into a ferrite structure, a pearlite structure, or a bainite structure, strain due to the transformation is not imparted to the steel. Finally, in the quenching process, the steel material whose surface austenite structure has been refined is cooled to transform the austenite structure of the steel material into a martensite structure.

  As described above, according to the present invention, the steel structure is transformed into an austenite structure, and the austenite structure is maintained without transformation, and after carburizing and refining the surface layer of the steel material, the steel material is transformed into a martensite structure. I am letting. Thereby, while being able to refine | miniaturize the structure | tissue of the carburized surface layer, the distortion amount of steel materials can be reduced rather than the conventional one.

  As long as the above-described change in structure can be controlled, the method for raising the temperature of the surface layer of the steel material in the step of refining is not particularly limited. However, as a more preferable aspect, in the step of miniaturization, the temperature of the surface layer of the steel material is increased by self-reheating of the steel material.

  Here, “self-recuperation of steel” in the present invention refers to heat that is input to the steel in the carburizing process and remains in the steel after the step of introducing dislocation by primary cooling. It is the heat which can reheat the surface layer of steel materials with the heat of.

  According to this aspect, since the austenite structure of the surface layer of the steel material is refined using dislocation as a nucleus by utilizing self-recuperation of the steel material, it is not necessary to reheat the steel material from the outside by a heater or the like. Thereby, an austenite structure | tissue can be refined | miniaturized cheaply and rapidly. In addition, since the steel material can be rapidly cooled from the cooling in the process of introducing dislocation to the temperature rise of the surface layer of the steel in the refinement process, the austenite structure (supercooled austenite structure) is transformed into another structure. The surface layer of the steel material can be refined quickly before.

  As a more preferable aspect, in the carburizing process, the steel material is heated to 1000 ° C. or more, and in the step of introducing the dislocation, the surface layer of the steel material is cooled at a cooling rate of 100 ° C./second or more.

  According to the present invention, by heating the steel material to 1000 ° C. or more in the carburizing process, a sufficient amount of heat can be input into the steel material, and in the step of introducing the dislocation, the surface layer of the steel material Is cooled at a cooling rate of 100 ° C./second or more, so that dislocations serving as nuclei for refining (recrystallization nuclei) can be sufficiently introduced into the surface layer of the steel material. Thereby, the austenite structure of the surface layer can be refined more reliably.

  Here, when the steel material is heated below 1000 ° C., the heat input to the steel material is insufficient or the temperature before cooling is low, so the temperature of the surface layer of the steel material does not rise to the recrystallization temperature during self-recuperation. In some cases, dislocations that become the core of refinement during cooling cannot be sufficiently imparted to the surface layer of the steel material. Furthermore, even when the steel material is heated at 1000 ° C. or higher, when the cooling rate is less than 100 ° C./second, it is not possible to sufficiently impart dislocations that become the core of refinement when cooling, The surface layer of steel may not be refined.

  According to the present invention, the steel material is transformed into an austenite structure, and while maintaining this structure, the surface layer of the steel material is carburized and refined, so that strain generated in the steel material can be reduced.

The figure which showed the temperature profile of the steel materials for demonstrating the carburizing quenching method of the steel materials which concern on embodiment of this invention. The typical conceptual diagram of the carburizing processing apparatus which performs suitably the carburizing quenching method of the steel materials shown in FIG. The typical conceptual diagram of the carburizing processing apparatus which concerns on a modification. The figure which showed the result of the distortion amount (ratio) of the steel materials of Example 1 and Comparative Examples 1-3. The structure photograph after secondary cooling after the carburizing process of the steel materials according to Example 7 and Comparative Example 7. The figure which showed the temperature profile of the steel materials for demonstrating the conventional carburizing quenching method of steel materials. The figure which showed the temperature profile of the steel materials for demonstrating the carburizing hardening method of the other conventional steel materials.

  Below, with reference to drawings of FIGS. 1-3, the carburizing-and-quenching method of the steel materials which concern on embodiment of this invention is demonstrated.

  FIG. 1 is a view showing a temperature profile of a steel material for explaining a steel carburizing and quenching method according to an embodiment of the present invention. FIG. 2 is a schematic conceptual diagram of a carburizing apparatus 1A that suitably performs the carburizing and quenching method for steel shown in FIG. FIG. 3 is a schematic conceptual diagram of a carburizing apparatus 1B according to a modification. 3 differs from the carburizing apparatus 1A shown in FIG. 2 in that a reheating furnace 41 is further provided.

1. Steel material (material to be treated) A steel material that performs the carburizing and quenching method according to the present embodiment is a steel material having a ferrite structure and a pearlite structure. As a steel material suitable for such a carburizing and quenching method, for example, C: 0.1 to 0.4 mass%, Si: 0.15 to 0.35 mass%, Mn: 0.55 to 0.95 mass% , P: 0.03 mass% or less, S: 0.03 mass% or less, and the balance is a steel material composed of unavoidable impurities and iron.

  Here, C is an element for ensuring the strength and hardness of the steel material (base material) to be carburized. When C is less than 0.1% by mass, the hardness of the base material itself is low. Insufficient time may be required for the carburizing process described later. On the other hand, when C exceeds 0.4% by mass, the machinability and cold forgeability of the steel material before processing are lowered, and the hardness of the base material itself is improved, so that the toughness of the steel material itself is improved. May fall.

  Si is an element for improving the hardenability. When Si is less than 0.15% by mass, the hardenability of the steel material may be lowered. On the other hand, when Si exceeds 0.35 mass%, the machinability and toughness of the steel material may be deteriorated.

  Mn is an element for improving the hardenability like Si, and when the Mn is less than 0.55% by mass, the hardenability of the steel may be lowered. On the other hand, when Mn exceeds 0.95 mass%, the machinability of the steel material may be lowered.

  A smaller amount of P and S is desirable, and when P exceeds 0.03% by mass, segregation is likely to occur, and the impact resistance of the steel material may be reduced. When S exceeds 0.03 mass%, hot workability may be reduced.

  Further, if necessary, the above-described components may include Ni: 0.25% by mass or less, Cr: 0.8-1.3% by mass, Mo: 0.1-0.4% by mass, Ni, At least one of Cr and Mo may be added to the steel material.

  When Ni exceeds 0.25 mass%, the hardness of the steel material may increase and the toughness of the steel material may decrease. Cr is an element for improving hardenability and improving wear resistance and corrosion resistance. When Cr is less than 0.8% by mass, the hardenability of the steel may be lowered. On the other hand, when Cr exceeds 1.3% by mass, the hardness of the steel material may increase and the toughness of the steel material may decrease.

  Mo is an element for improving the hardenability. When Mo is less than 0.1% by mass, the hardenability may be lowered. On the other hand, when Mo exceeds 0.4 mass%, the hardness of the steel material may increase and the toughness of the steel material may decrease.

  Specific steel materials suitable for the carburization treatment include chromium molybdenum steel (JIS standard: SCr415-435), chromium molybdenum steel (JIS standard: SCM415-435), and the like.

2. Carburizing treatment process In this embodiment, the steel materials mentioned above are prepared and a carburizing treatment process as shown in FIG. 1 is performed. Specifically, ferrite structure and pearlite structure of the steel material described above is a steel to transform to austenite structure A ₁ transformation point or more, more preferably heated to A ₃ transformation point or above the temperature (carburization temperature) T1 ( (See S1 in FIG. 1).

  In the present embodiment, as shown in FIG. 2, the steel material W is placed on a receiving jig 61 arranged in the transfer area 6, the steel material W is moved to the heating furnace 2 together with the receiving jig 61, and the heating furnace is heated in a reduced pressure atmosphere. The steel material W is heated using the high frequency induction heating coil 21 in 2 under the above conditions.

For example, steel chromium steel (e.g. JIS standard: SCr420) or chromium molybdenum steel (e.g. JIS standard: SCM420) when you select the, _{A 1} transformation point around 730 ° C., _{A 3} transformation point is around 900 ° C. Therefore, it is heated to at least 730 ° C. or more, preferably 900 ° C. to 1200 ° C. to transform the structure into an austenite structure.

  Next, as shown in FIG. 1, at least the surface layer of the steel material in the austenite structure heated to the carburizing temperature T1 is carburized with a carburizing gas (see S2 in FIG. 1). Specifically, as shown in FIG. 2, the steel material W heated in the heating furnace 2 is transported to the carburizing furnace 3 together with the receiving jig 61. In the carburizing treatment furnace 3, the carburizing gas G is brought into contact with the surface of the steel material W in the austenite structure state while the temperature of the steel material W is maintained at the above-described carburizing temperature T1 by the heater 31, and carbon is fixed to the surface layer of the steel material W. Dissolve and diffuse. Here, the carburizing gas G can include hydrocarbon-based gas, and is a general gas used for carburizing treatment such as acetylene gas and propane gas.

3. Primary cooling process (dislocation introduction process)
In the primary cooling step, the surface layer of the steel material is primarily treated to a cooling temperature T2 lower than the recrystallization temperature Tr of the steel material so that the austenite structure of the carburized steel material is maintained (so that the temperature is not lower than the martensite transformation point Ms). By cooling, dislocations are introduced into the austenite structure of the surface layer (see S3 in FIG. 1). In the present embodiment, even if the primary cooling to a cooling temperature T2 lower than lower recrystallization temperature Tr than the A ₁ transformation point, austenite organization immediately without transformation to other tissues, is maintained as supercooled austenite.

  Thus, in this embodiment, as shown in FIG. 6, the austenite structure is maintained because the austenite structure is not cooled until it becomes a martensite structure (cooling temperature Tb) at the time of primary cooling. Thereby, in this embodiment, the distortion | strain to the steel materials resulting from transformation does not generate | occur | produce in a primary cooling process.

  The dislocations introduced in the primary cooling step are dislocations that become recrystallization nuclei, and become the nuclei for refining the austenite structure when the steel material is heated to a recrystallization temperature Tr or higher as described later. In the carburizing process, assuming that the steel is heated to 1000 ° C. or higher up to the inside, if the steel layer is cooled at a cooling rate of 100 ° C./sec to 300 ° C./sec in the primary cooling step, the austenite structure is obtained. The dislocation described above can be introduced more reliably. According to the inventor's experiment, when the cooling rate is less than 100 ° C./second, the structure of the surface layer may not be refined, and it is difficult to perform cooling at a cooling rate exceeding 300 ° C./second.

  In the present embodiment, as shown in FIG. 2, the steel material W whose surface layer is carburized in the carburizing furnace 3 is transported to the cooling chamber 5 together with the receiving jig 61. In the cooling chamber 5, the surface layer of the steel material W is so formed that the water Q discharged from the cooling nozzle 51 causes the surface temperature of the steel material W to exceed the martensite transformation point Ms of the surface layer and lower than the recrystallization temperature Tr of the steel material. Cool down.

  For example, when chromium steel (for example, JIS standard: SCr420) is selected as the steel material, the martensite transformation point Ms is 430 ° C., and when chromium molybdenum steel (for example, JIS standard: SCM420) is selected, the martensitic transformation point Ms is 400 ° C. Further, the recrystallization temperature of these steel materials varies depending on the influence of the residual stress of the steel materials, but is generally in the range of more than 450 ° C. and 600 ° C. or less.

  Therefore, in the case of chromium steel (for example, JIS standard: SCr420), the surface layer is maintained in the state in which the austenite structure is maintained if the first cooling step is performed at a cooling temperature T2 that exceeds at least the martensitic transformation point Ms430 ° C and not higher than 450 ° C. Dislocations can be introduced into the austenite structure. Similarly, when chromium molybdenum steel (for example, JIS standard: SCM420) is selected, the austenite structure is maintained if the first cooling step is performed at a cooling temperature T2 that exceeds at least the martensitic transformation point Ms400 ° C and not higher than 450 ° C. In the state, dislocations can be introduced into the austenite structure of the surface layer.

4). Reheating process (miniaturization process)
In the reheating step, the surface layer of the steel material having the austenite structure into which the dislocation has been introduced is raised to a temperature rise temperature T3 equal to or higher than the recrystallization temperature of the steel material (reheated), whereby the austenite of the steel material surface layer with the dislocation as a nucleus. A structure | tissue is refined | miniaturized (refer S4 of FIG. 1).

  In the present embodiment, as shown in FIG. 2, the steel material W whose surface layer is primarily cooled in the cooling chamber 5 is moved together with the receiving jig 61 to the transfer area 6, and the temperature of the surface layer of the steel material is increased by self-reheating of the steel material W. The temperature T3 is increased.

  Specifically, the heat that is input into the steel material W in the heating furnace 2 and the carburizing furnace 3 (that is, the carburizing process), and the heat remaining in the steel material W when the steel material W is cooled in the cooling chamber 5, It is transmitted to the surface layer of the steel material W, the surface layer of the steel material W is heated, and the temperature of the surface layer is raised to the temperature increase temperature T3.

  If the surface layer of the steel material can be reheated to the temperature increase temperature T3 so that the temperature profile shown in FIG. 1 is obtained, for example, as shown in FIG. May be further provided, and the steel material W may be reheated to the temperature rise temperature T3 using the high frequency induction heating coil 41.

  However, in the present embodiment, as shown in FIG. 2, the austenite structure of the surface layer of the steel material W is refined by using dislocations as the core material while making use of the self-recuperation of the steel material W. Therefore, as shown in FIG. It is not necessary to provide a new reheating furnace 4. Thereby, an austenite structure | tissue can be refined | miniaturized cheaply and rapidly.

  Moreover, from the cooling of the steel material W in the dislocation introduction process to the temperature increase of the surface layer of the steel material in the miniaturization process can be performed quickly. For this reason, before the austenite structure (supercooled austenite) is transformed into another structure (ferrite structure and pearlite structure (or bainite structure) as shown in FIG. 7, the austenite structure on the surface layer of the steel material is rapidly refined. can do. In addition, although the refinement process is expressed as a “reheating process” for the sake of convenience, this “reheating” means that the surface layer of the steel material is heated while remaining due to the above-described self-reheating in this specification. This includes cases where

Here, when chromium steel (for example, JIS standard: SCr420) or chromium molybdenum steel (for example, JIS standard: SCM420) is selected as the steel material, the rate of temperature rise of the steel material is preferably 50 ° C./second or more. When it is slower than this rate of temperature rise, the residual stress accompanying dislocation inside the steel material is released, and the austenite structure of the surface layer of the steel material may not be refined with the dislocation as a nucleus. Further, when the heating temperature T3 is 950 ° C. or more beyond the A ₃ transformation point is sometimes austenite structure will be coarse.

5). Secondary cooling process (quenching process)
In the secondary cooling step, the steel material with a refined surface layer structure is cooled to transform the steel material structure into a martensite structure (see S5 in FIG. 1). This process is a process aimed at quenching the surface layer of the steel material, and the austenite structure of the surface layer can be transformed into a martensite structure by cooling the steel material to a martensite transformation point Ms or less at a predetermined cooling rate. .

  In the present embodiment, as shown in FIG. 2, the steel material W whose surface layer is reheated and refined is transported to the cooling chamber 5 together with the receiving jig 61. In the cooling chamber 5, the water Q discharged from the cooling nozzle 51 cools the surface layer of the steel material W so that the surface temperature of the steel material W is equal to or lower than the martensite transformation point Ms.

  Thus, in this embodiment, after transforming the structure of the steel material to an austenite structure in the carburizing treatment step, maintaining the austenite structure without transformation, carburizing and refining the surface layer of the steel material, Transformed into a martensite organization. That is, unlike the past, even when the steel material is cooled to introduce dislocations that become the core of the refinement of the structure during the primary cooling after the carburizing treatment, the austenite structure is maintained without being transformed, Generation of strain to the steel material can be suppressed. According to the steel material that has undergone such a series of steps, the structure of the carburized surface layer can be refined, and the strain amount of the entire steel material can be reduced as compared with the conventional steel material.

  Hereinafter, the present invention will be described by way of examples. In the example shown below, the carburizing apparatus shown in FIG. 2 is used.

[Example 1]
A spur gear (steel material) made of chromium steel (JIS standard: SCr420) having a diameter of 170.7 mm, a pitch of 7.5 mm, and an overball diameter (OBD) of 171.7 mm was prepared. The structure of chromium steel is a structure composed of a ferrite structure and a pearlite structure.

Next, carburizing treatment was performed under the conditions shown in Table 1. The inside of the heating furnace was evacuated to a reduced pressure atmosphere, the steel material was put into the heating furnace, and the steel material was heated to 1000 ° C. in 80 seconds using a high frequency induction heating coil. Incidentally, since the A ₃ transformation point of the steel material is around 900 ° C., the steel material of the tissue by heating, it is transformed into martensite structure from ferrite structure and pearlite structure.

  Next, acetylene gas 20 L / min is introduced into the carburizing treatment furnace as the carburizing gas, the temperature of the steel material is maintained at 1000 ° C. with a heater, and the carburizing time 115 at a carburizing temperature (T1) of 1000 ° C. and a pressure of 1100 Pa. Carburizing gas is brought into contact with the surface of the steel material for 2 seconds so that carbon is dissolved in the surface layer, and then the steel material is heated at the carburizing temperature T1 for a diffusion time of 280 seconds without using the carburizing gas (in a reduced pressure atmosphere in which the carburizing gas is discharged). Then, the solid solution carbon was diffused in the surface layer of the steel material (carburization treatment time for a total of 395 seconds).

  Next, the steel material after the carburizing treatment was conveyed to a cooling chamber and sprayed with water, whereby the surface layer (specifically, the surface) of the steel material was primarily cooled to a cooling rate of 200 ° C./second and a cooling temperature (T2) of 450 ° C. Note that the recrystallization temperature Tr is estimated to be around 490 ° C., and in the primary cooling, it is cooled to the recrystallization temperature or lower. Dislocation was introduced into the surface layer of the steel material by this primary cooling.

  Next, the steel material was taken out from the cooling chamber, and the surface layer of the steel material was reheated by self-reheating of the steel material (in the course of renewal) to refine the austenite structure of the surface layer of the steel material. Thereby, the surface layer of the steel material was reheated to a temperature increase rate of 100 ° C./second and a temperature increase temperature (T3) of 900 ° C. (a temperature equal to or higher than the recrystallization temperature Tr) by the heat inside the steel material.

  The steel material after reheating was conveyed again to the cooling chamber, and the steel material was secondarily cooled to room temperature at a cooling rate of 100 ° C./second by spraying water on the steel material, and the steel material was quenched. Thereby, the structure of the steel material was transformed into a martensite group.

[Comparative Example 1]
In the same manner as in Example 1, a steel material was prepared, and carburizing and quenching was performed on the steel material under the conditions shown in Table 1. The difference from Example 1 is that carburizing treatment is performed at a carburizing temperature (Ta) of 1000 ° C. so as to obtain a temperature profile similar to the temperature profile shown in FIG. The primary cooling to Tb) is to transform the austenite structure into a martensite structure. Then, it reheated by the high frequency to the temperature increase rate of 100 degree-C / sec and the temperature increase temperature (Tc) of 900 degreeC by the high frequency heating, and was secondary-cooled to room temperature with water at the cooling rate of 100 degree C / second.

[Comparative Example 2]
The same steel material as in Example 1 was prepared, and carburizing and quenching was performed on the steel material under the conditions shown in Table 1. The difference from Example 1 is that the carburizing process is performed at a carburizing temperature (TA) of 1000 ° C. so as to obtain a temperature profile similar to the temperature profile shown in FIG. TB) The primary cooling to 500 ° C. was maintained at this temperature (500 ° C.) for 7200 seconds to transform the austenite structure into a bainite structure. Thereafter, reheating was not performed (different from FIG. 7 (no TC)), and secondary cooling was performed to room temperature with water at a cooling rate of 100 ° C./sec.

[Comparative Example 3]
The same steel material as in Example 1 was prepared, and carburizing and quenching was performed on the steel material under the conditions shown in Table 1. The difference from Example 1 is that the carburizing process is performed at a carburizing temperature (TA) of 1000 ° C. so that a temperature profile similar to the temperature profile shown in FIG. TB) The primary cooling to 650 ° C. was maintained at this temperature (650 ° C.) for 7200 seconds to transform the austenite structure into a ferrite structure and a pearlite structure. Thereafter, reheating was not performed (different from FIG. 7 (no TC)), and secondary cooling was performed to room temperature with water at a cooling rate of 100 ° C./sec.

<Measurement of strain>
For the steel materials (gears) of Example 1 and Comparative Examples 1 to 3, the overball diameter (OBD) before and after carburizing and quenching was measured, and the strain after the carburizing and quenching was measured from this value. The result is shown in FIG. In FIG. 4, when the strain amount of the steel material of Comparative Example 1 is 1, the strain amounts of the steel materials of Example 1 and Comparative Examples 2 and 3 are shown.

(Results and Discussion)
As shown in FIG. 4, the strain amount of the steel material of Example 1 was the smallest value as compared with those of Comparative Examples 1 to 3. Moreover, the thing of Example 1 was reduced 74% with respect to the distortion amount of the steel material of the comparative example 1. FIG.

  From this result, in the carburizing and quenching method of Example 1, the steel material is transformed into an austenite structure, and while maintaining the austenite structure, the surface layer of the steel material is carburized and refined, thereby reducing strain generated in the steel material. It is thought that was made. On the other hand, in the case of Comparative Example 1, strain is generated in the steel material when transformed from the austenite structure to the martensite structure during the primary cooling, and the strain amount generated in the steel material than in Example 1 due to the strain at the time of transformation. Seems to have grown. Similarly, in the case of Comparative Example 2, the austenite structure transformed into a bainite structure when maintained at a constant temperature TB after primary cooling, and in the case of Comparative Example 3, when the austenite structure transformed into a ferrite structure and a pearlite structure, It is considered that a strain occurred in the steel material, and the strain amount generated in the steel material was larger than that in Example 1 due to the strain at the time of transformation.

  In the following Examples 2 to 8 and Comparative Examples 4 to 8, when the surface layer of the steel material was heated (self-recovery) after the primary cooling, the austenite structure of the surface layer of the steel material was fine. It is an example for confirming the conditions suitable for being converted into.

[Example 2]
As in Example 1, carburizing and quenching of steel was performed. The difference from Example 1 is that a steel material made of columnar chrome steel (JIS standard: SCr420) having a diameter of 18 mm and a length of 20 mm was prepared, and carburizing and quenching was performed under the conditions shown in Table 2. The conditions of the carburizing process differ from Example 1.

  Specifically, the carburizing process of Example 2 differs from that of Example 1 in that the carburizing temperature (T1) is 950 ° C. and the pressure is 1100 Pa, the carburizing time is 198 seconds, and the subsequent carbon diffusion time. The carburizing process was performed for 502 seconds (total carburizing time of 700 seconds).

  Therefore, from these differences, the rate of temperature increase and the temperature increase rate (T3) when the surface layer of the steel material is reheated by self-recuperation of the steel material (T3) are also those of Example 1 as shown in Table 2. Is different.

[Examples 3 to 8]
The carburizing and quenching method for steel was performed in the same manner as in Example 2. The points shown in Table 2 differ from Examples 3 to 8 in Example 2. Due to this difference, the rate of temperature rise and the temperature rise (T3) when the surface layer of the steel material is reheated by self-recuperation (steering) of the steel material are also different from those of Example 2 as shown in Table 2. Is different.

  Example 3 is different from Example 2 in that carburization is performed at a carburization temperature (T1) of 1000 ° C. for a carburization time of 115 seconds and a subsequent carbon diffusion time of 280 seconds (total carburization time of 395 seconds). It is the point that went.

  The difference between Example 4 and Example 2 is that the carburizing process is performed at a carburizing temperature (T1) of 1000 ° C. for a carburizing time of 115 seconds and a subsequent carbon diffusion time of 280 seconds (total carburizing time of 395 seconds). It is the point which performed and the point which made the primary cooling rate with water 100 degree-C / sec.

  Example 5 differs from Example 2 in that the carburization temperature (T1) is 1100 ° C., and the carburization process is performed for 115 seconds, followed by a carbon diffusion time of 280 seconds (the total carburization time is 395 seconds). It is the point that went.

  Example 6 differs from Example 2 in that the carburizing process is performed at a carburizing temperature (T1) of 1100 ° C. with a carburizing time of 70 seconds and a subsequent carbon diffusion time of 142 seconds (a total carburizing time of 212 seconds). It is the point which performed and the point which made the primary cooling rate with water 100 degree-C / sec.

  Example 7 differs from Example 2 in that the carburizing temperature (T1) is 1200 ° C., and the carburizing time is 137 seconds, and then the carbon diffusion time is 43 seconds (the carburizing time is 180 seconds in total). It is the point that went.

  Example 8 differs from Example 2 in that the carburizing temperature (T1) is 1200 ° C., and the carburizing time is 137 seconds, and then the carbon diffusion time is 43 seconds (the carburizing time is 180 seconds in total). It is the point which performed and the point which made the primary cooling rate with water 100 degree-C / sec.

[Comparative Examples 4 to 8]
Similarly to Example 2, carburizing and quenching of steel materials according to Comparative Examples 4 to 8 was performed. The points shown in Table 2 are that Comparative Examples 4 to 8 differ from Example 2.

  Comparative Example 4 is different from Example 2 in that the primary cooling rate with water is set to 100 ° C./second.

  Comparative Example 5 is different from Example 2 in that the primary cooling rate with water is 50 ° C./second.

  Comparative Example 6 is different from Example 2 in that the carburizing temperature (T1) is 1000 ° C., and the carburizing time is 115 seconds, and then the carbon diffusion time is 280 seconds (the carburizing time is 395 seconds in total). It is the point which performed and the point which made primary cooling rate with water 50 degrees C / second.

  The comparative example 7 is different from the second embodiment in that the carburizing temperature (T1) is 1100 ° C., the carburizing time is 70 seconds, and the carbon diffusion time is 142 seconds (the carburizing time is 212 seconds in total). It is the point which performed and the point which made primary cooling rate with water 50 degrees C / second.

  Comparative Example 8 differs from Example 2 in that the carburizing temperature (T1) is 1200 ° C., and the carburizing time is 137 seconds, and the carbon diffusion time is 43 seconds (the carburizing time is 180 seconds in total). It is the point which performed and the point which made primary cooling rate with water 50 degrees C / second.

  Since Examples 2-8 and Comparative Examples 4-8 differ in at least one of the carburizing temperature, carburizing time, and primary cooling rate, the surface layer of the steel material was reheated by self-recuperation (steering) of the steel material. As shown in Table 2, the rate of temperature rise and the temperature rise (T3) of the surface layer (specifically, the surface) of the steel material are also different.

<Measurement of crystal grain size>
The crystal grain sizes of the steel materials after secondary cooling according to Examples 2 to 8 and Comparative Examples 4 to 8 were measured. Moreover, in order to confirm the refinement | miniaturization by reheating of Examples 2-8 and Comparative Examples 4-8, the steel materials rapidly cooled to room temperature by primary cooling are prepared after the carburizing process of Examples 2-8 and Comparative Examples 4-8. And according to JIS G 0551, the crystal grain size of this steel material and the steel material after the above-mentioned secondary cooling was measured. Table 2 shows the crystal grain size numbers of these steel materials. Furthermore, the cross section including the surface layer of the steel materials of Example 7 and Comparative Example 7 was observed with a microscope. The result is shown in FIG.

(Results and Discussion)
In the steel materials according to Examples 2 to 8, unlike those of Comparative Examples 4 to 8, the austenite structure of the steel material was refined by self-recuperation, and the martensite structure after secondary cooling was also refined.

  The steel material according to Comparative Example 4 had a carburizing temperature as low as 950 ° C. and was not higher than the recrystallization temperature Tr (500 ° C.) due to self-recuperation, and the surface layer structure of the steel material was not refined by self-recuperation. Conceivable.

  In the steel materials according to Comparative Examples 5 to 7, since the primary cooling rate was not sufficient, the dislocation causing the refinement could not be sufficiently introduced into the austenite structure of the steel material, and the recrystallization temperature Tr ( It is considered that the structure of the surface layer of the steel material was not refined by self-recuperation for two reasons that the temperature was not higher than 490 ° C.).

  In addition, the steel material according to Comparative Example 8 had a primary cooling rate that was not sufficient, so that dislocations that caused refinement could not be sufficiently introduced into the austenite structure of the steel material. Even if the temperature rises (495 ° C.) above the crystal temperature (490 ° C.), it is considered that the material has not been refined.

  Further, Table 3 below summarizes whether or not the steel materials were refined with respect to the conditions of the carburizing temperature and the primary cooling rate of the steel materials according to Examples 2 to 8 and Comparative Examples 4 to 8 described above. From this result, it is considered that when the carburization temperature (T1) is set to 1000 ° C. or higher and the primary cooling rate is set to 100 ° C./second or higher, the austenite structure of the steel material is surely refined by self-recuperation. However, even in the case of Example 2 that does not satisfy this condition, since the austenite structure of the steel material is refined by self-recuperation, the carburizing temperature (T1) is 1000 ° C. or higher and the primary cooling rate is 100 ° C./second or higher. Even if not satisfied, the austenite structure of the steel may be refined by self-recuperation.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various designs can be made without departing from the spirit of the present invention described in the claims. It can be changed.

1A, 1B: Carburizing apparatus, 2: Heating furnace, 3: Carburizing furnace, 4: Reheating furnace, 5: Cooling chamber, 6: Transfer area, 21: High frequency induction heating coil, 31: Heater, 41: High frequency induction Heating coil, 51: cooling nozzle, 61 receiving jig, Q: water, W: steel

Claims (3)

Heating a steel material composed of a ferrite structure and a pearlite structure, transforming the structure of the steel material into an austenite structure, and then carburizing the surface layer of the steel material of the austenite structure with a carburizing gas; and
After the carburizing treatment, the step of introducing dislocations into the austenite structure of the surface of the steel material by cooling the surface layer of the steel material to a temperature lower than the recrystallization temperature of the steel material so as to maintain the austenite structure of the steel material;
A step of refining the austenite structure of the surface layer of the steel material with the dislocation as a core by raising the surface layer of the steel material composed of the austenite structure into which the dislocation has been introduced to a temperature equal to or higher than the recrystallization temperature of the steel material;
A steel carburizing and quenching method, comprising: a quenching step of cooling the steel material having a refined austenite structure to transform the austenite structure of the steel material into a martensite structure.   The method for carburizing and quenching a steel material according to claim 1, wherein in the step of refining, the temperature of the surface layer of the steel material is increased by self-recuperation of the steel material. In the carburizing step, the steel material is heated to 1000 ° C. or higher,
The method for carburizing and quenching a steel material according to claim 2, wherein in the step of introducing the dislocation, a surface layer of the steel material is cooled at a cooling rate of 100 ° C / second or more.

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