JP2011219846A - Method for manufacturing machine structural member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a machine structural member with which workability can be made to be satisfactory even if a steel raw material for improving the temper softening resistance is used.SOLUTION: This method includes; a step, in which the steel raw material composed by mass% of 0.3-1.5% C, 0.2-2.0% Mn and one or more kinds intentionally selected from the group of 0.5-2.0% Si, 0.1-1.5% Cr, 0.1-1.5% Mo, 0.05-0.5% V and 0.005-0.2% Nb, and the balance Fe with inevitable impurities, is softening-treated to make the metallographic structure composed of the ferrite having ≥30% area ratio and the pearite, or the metallographic structure composed of the ferrite and the spheroidal carbide or the metallographic structure composed of the ferrite and the granular carbide; a step for work-treating the softening-treated steel raw material into a prescribed shape; and a step, in which at least two times of high frequency-induction heat-treatment are performed to the work-treated steel raw material and the metallographic structure on the surface layer is made to the martensite and the metallographic structure in the intermediate layer adjoined to the surface layer is made to a tempered-martensite or a tempered-martensite, ferrite and pearite.

Description

  The present invention relates to an improvement in a method of manufacturing a mechanical structural component such as a gear by forming a hardened layer on the surface of a steel material by induction hardening.

  When performing induction hardening twice on carbon steel, the inventors conducted induction heating and quenching in which the hardening depth of the second induction hardening is shallower than that of the first induction hardening. A martensitic structure hardened to a depth of 3.0 mm or less from the surface, an incompletely quenched area adjacent to the quenched area and a mixed structure of martensite and ferrite, and adjacent to the incompletely quenched area. A temper softening region having a depth of 2.5 to 7.0 times the depth of the region, and tempering for a minimum hardness value at the boundary between the incomplete quenching region and the temper softening region A method of manufacturing a surface-quenched steel whose difference from the maximum hardness value in the softened region is within HV150 has been proposed (Patent Document 1).

  According to this method, the entire portion that has been quenched in the first heat treatment but not in the second heat treatment is tempered and softened, so that high compressive stress remains uniformly over the entire surface hardened region, resulting in high fatigue. Not only can the strength be obtained, but also the internal tensile stress can be relieved to improve the reliability of the member against breakage.

  However, since the material applied to this method is carbon steel, the temper softening resistance is low. For this reason, for example, a gear manufactured by this method is easily tempered and softened when heated with a frictional resistance during use. For this reason, there remains room for improving the pitching characteristics of the gear manufactured by this method.

  By the way, it is known to increase the temper softening resistance of carbon steel by adding Si, V or the like to the carbon steel (Non-patent Document 1).

  Further, a method is known in which surface fatigue characteristics are improved by performing induction hardening twice on carbon steel added with Si, V, etc. (Patent Document 2).

JP 2007-111985 A Japanese Patent Laid-Open No. 7-118791

"Effects of effective hardened layer depth and silicon content on pitching life of induction-hardened materials", Electric Steel, 71-1, (2000), pp. 19-28, Koichiro Inoue, Sadayuki Nakamura, 2000

  However, when Si, V, or the like is added to carbon steel, the hardness of the steel material increases, and when a high hardness steel material is processed into a gear or the like, workability such as machinability deteriorates.

Moreover, since the AC ₃ transformation point of steel rises when Si is added, the steel material must be at a higher temperature when the steel material is austenitized by high-frequency heating. As a result, problems such as deformation of the steel material tend to occur.

The present invention provides a method for producing a machine structural component that does not deteriorate workability even when a steel material to which a component that enhances temper softening resistance is added is used. Further, the present invention provides an AC ₃ transformation of steel. The present invention provides a method for manufacturing a machine structural component capable of suppressing deformation of a steel material even if a steel material to which a component for raising the point is added is used, and includes the following steps.

(1) By mass%, C: 0.3-1.5%, Mn: 0.2-2.0%, Si: 0.5-2.0%, Cr: 0.1-1.5%, Mo: 0.1-1.5%, V: 0.05-0.5% , And Nb: one or more selected from the group consisting of 0.005 to 0.2%, and a steel material composed of the balance Fe and unavoidable impurities, and a ferrite and pearlite having a ferrite area ratio of 30% or more A metal structure selected from the group consisting of a metal structure consisting of, a metal structure consisting of ferrite and spherical carbide, and a metal structure consisting of ferrite and granular cementite,
A process of processing the steel material softened in the process into a predetermined shape;
A process of performing at least two induction heat treatments on the processed steel material, and making the metal structure of the surface layer martensite, and subsequent intermediate layer tempered martensite, or tempered martensite and ferrite and pearlite,
A method for manufacturing a machine structural part comprising:

  The present invention adds a component that increases the temper softening resistance of steel, such as Si, V, Nb, etc. to the steel material, and when producing steel products such as gears with improved pitching fatigue characteristics, a predetermined softening treatment is performed. Then, workability can be ensured by processing the steel material.

For example, Si is an element that raises AC ₁ and AC ₃ . For this reason, if the steel material is subjected to one ultra-rapid short-time induction hardening, the AC ₃ point increases and the ferrite area of the previous structure is large, so it is necessary to heat it to a high temperature. As a result, the deformation of the steel material increases. Furthermore, sufficient austenite cannot be obtained even when heated at a high temperature, and undissolved ferrite or the like remains in the quenched structure, so that the required strength of the part cannot be obtained.

  In the present invention, by induction hardening for a long time at a low heating temperature, the amount of change in deformation is constant and the value is small (constant deformation). Furthermore, since the structure becomes martensite at that time, it becomes possible to austenite even if it is heated at a temperature lower than one high temperature short time in the second induction hardening. For this reason, a constant low deformation can be obtained and a more uniform martensite structure can be obtained by performing ultra-rapid short-time heating and low-temperature induction hardening in the next second heating. As a result, a high strength constant low deformation part can be made.

  Moreover, hardness becomes higher than a base part by the structure | tissue of a 1st hardening part becoming a tempered martensite. As a result, high fatigue strength and high static bending strength can be obtained.

It is an external view of the product (gear: W) concerning the example and comparative example of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail.

(Steel material)
This steel material includes a predetermined amount of components necessary for manufacturing steel products such as gears that require high bending strength, and also includes a predetermined amount of components that increase the temper softening resistance of the steel. Furthermore, the steel material further contains an element that improves the hardenability of the steel, if necessary. Moreover, each component is suitably adjusted within the scope of the present invention depending on the characteristics required depending on the application of the steel product.

  Steel materials according to the present invention are: C: 0.3-1.5%, Mn: 0.2-2.0%, Si: 0.5-2.0%, Cr: 0.1-1.5%, Mo: 0.1-1.5%, V: 0.05-0.5 %, Nb: one or two or more selected from the group consisting of 0.005 to 0.2%, and the balance Fe and unavoidable impurities. Furthermore, 1 type or 2 types selected from the group which consists of B: 0.0005-0.05%, Ti: 0.02-0.05%, and Ni: 0.01-1.5% are included as needed.

Hereinafter, the reason for addition of each additive component and the reason for limiting the addition range will be described.

C: 0.3-1.5%
C is an element necessary for ensuring strength, and determines the hardness after quenching. When the amount is less than 0.3%, the hardness is as low as 550 HV or less, and thus it is difficult to apply to sliding parts such as gears. For this reason, the lower limit was made 0.3%. On the other hand, if it exceeds 1.5%, the toughness decreases, so the upper limit was made 1.5%.

Mn: 0.2-1.5%
Mn is an element that improves hardenability. In order to ensure hardenability, 0.2% or more is necessary. For this reason, the lower limit was made 0.2%. On the other hand, even if added over 1.5%, the hardenability becomes excessive and the toughness deteriorates. In addition, the workability also decreases, so the upper limit was made 1.5%.

[Selective elements: Si, Cr, Mo, V, Nb]
These elements are all elements that increase the temper softening resistance.

Si: 0.5-2.0%
Si is an element that increases the temper softening resistance. This improves the tooth surface strength, but 0.5% or more is necessary to ensure the same tooth surface strength as that of conventional carburizing and quenching. For this reason, the lower limit was made 0.5%. On the other hand, if the content exceeds 2.0%, the hardness increases due to the solid solution strengthening of ferrite, leading to a decrease in machinability, so the upper limit was made 2.0%.

Cr: 0.1-1.5%
Cr increases temper softening resistance and also improves hardenability. If it is less than 0.1%, the effect of increasing the temper softening resistance is hardly exhibited, so the lower limit was made 0.1%. On the other hand, when it exceeds 1.5%, the effect of increasing the softening resistance is saturated and the workability is also lowered, so the upper limit was made 1.5%.

Mo: 0.1-1.5%
Mo has the effects of increasing the resistance to temper softening and strengthening the hardened layer to improve the bending fatigue strength. If it is less than 0.1%, this effect is hardly exhibited, so the lower limit was made 0.1%. On the other hand, if it exceeds 1.5%, the effect is saturated and the workability deteriorates, so the upper limit was made 1.5%.

V: 0.05-0.5%
V particularly has the effect of synergistically increasing the temper softening resistance when coexisting with Si. In particular, it is an effective element for preventing the phenomenon of breaking in a short life due to rolling fatigue. V has the effect of refining the grain boundaries of the steel. In order to exhibit these effects, 0.05% or more is required in the Si-containing range. However, the effect is saturated even if it is added excessively, so the upper limit is made 0.5%.

Nb: 0.005-0.2%
Nb has the effect of synergistically increasing the temper softening resistance, particularly when coexisting with Si. In particular, it is an effective element for preventing the phenomenon of breaking in a short life due to rolling fatigue. Nb also has the effect of refining the grain boundaries of steel. In order to exert these effects, 0.005% or more is required in the Si-containing range. However, the effect is saturated even if it is added excessively, so the upper limit is made 0.2%.

[Selective elements: B, Ti, Ni]
These elements are elements that improve hardenability.

B: 0.0005-0.05%
B is an element that improves hardenability and not only improves fatigue properties by strengthening grain boundaries but also improves strength. For that effect, 0.0005% or more is necessary, and even if added over 0.05%, the effect is saturated.

Ti: 0.02 to 0.05%
Ti is an element that improves hardenability, refines crystal grains by forming carbonitride, and improves tooth root bending fatigue strength. 0.02% or more is required for grain refinement, and the effect is saturated even if added over 0.05%.

Other ingredients Ni: 0.01-1.5%
Ni is an element that improves hardenability. If it is less than 0.01%, this effect is hardly exhibited, so the lower limit was made 0.01%. On the other hand, Ni is an expensive element, and if it is too much, it causes burning cracks, so 1.5% was made the upper limit.

  In addition, if the steel raw material applied to this invention method is a range which does not inhibit the characteristic of this invention, another component (for example, Te, Ca, Mg, Zr etc.) according to the use (various machine structural components). It is allowed to contain.

(Softening treatment)
The steel material having the above composition is high in hardness as it is, and therefore, processing takes time and effort. For this reason, in the present invention, the steel material is preliminarily softened before the processing, so that the steel material has a soft metal structure that is easy to process. Examples of the soft metal structure include a metal structure composed of ferrite and pearlite having a ferrite area ratio of 30% or more, a metal structure composed of ferrite and spherical carbide, or a metal structure composed of ferrite and granulated pearlite. The reason why the lower limit of the ferrite area ratio is specified in the metal structure composed of ferrite and pearlite is that the workability deteriorates when the ferrite area ratio is less than 30%. The metal structure subjected to the softening treatment of the present invention usually has a hardness of 280 HV or less, preferably about 190 to 270 HV. This is about the same as or less than the hardness of a carbon steel material with good workability. The ferrite area ratio is measured by an image analyzer.

  Specific heat treatment methods (softening treatments) for obtaining the metal structure and hardness include, for example, the following methods i) to d).

B) Complete annealing of the steel material: After maintaining at a temperature of AC ₃ or AC ₁ or higher, the steel material is gradually cooled to a temperature of Ar ₁ or lower.

B) Spherical annealing treatment: (a) Hold steel material at a temperature just below AC ₁ point for a long time, (b) Repeat heating and cooling at a temperature just below AC ₁ point, or (c) Directly below AC ₁ point or After heating to a temperature between AC ₁ and ACm, the furnace is cooled very slowly or maintained at a temperature just below the AC ₁ point.

C) Normalizing treatment: After heating to a temperature of AC ₃ or ACm to austenite, cooling in a quiet atmosphere.

D) Normalizing and annealing are performed by performing cooling control during hot and warm forging and cooling after rolling.

  The above processing itself is widely known to those skilled in the art. Therefore, those skilled in the art can perform the softening process based on the above description to make the metal structure and hardness of the steel material easily processed (for example, about 190 to 270 HV).

(Processing)
Various types of processing (forging, cold forging, rolling, etc., arbitrary processing such as rolling, pressing, cutting, turning, drilling, etc., and combinations of these processing) when manufacturing the machine structural component according to the present invention However, the processing described here means machining such as cutting, turning, and drilling performed before the heat treatment.

  According to the present invention, since the steel material is softened in advance, the processing (machining) can be easily performed.

(Induction hardening)
In the present invention, after the processing, induction hardening is performed at least twice, so that the metal structure of the steel material is martensite from the surface, followed by tempered martensite, tempered martensite, ferrite and pearlite. Suppose it is a base organization

  In order to obtain such a structure, a known method of performing induction hardening at least twice described in Patent Document 1, Patent Document 2, and documents cited in these documents is used as it is or using its principle. The processing conditions can be appropriately modified and applied.

  When these known two-stage induction hardening methods are described generally and schematically, first, by the first induction hardening, the desired mechanical structural parts “martensite, tempered martensite, tempered martensite and ferrite and Targeting a region to be an “interlayer composed of pearlite”, this region is made an austenite structure and then rapidly cooled (quenched) to obtain a martensite structure.

  Next, the second induction hardening is performed by using a region shallower than the target region of the first induction hardening, that is, a region (surface layer) to be a martensite layer as a final product as a target. By this quenching, the surface layer is a martensite layer, and the intermediate layer following this layer is a tempered martensite, a tempered martensite, and a metal structure of ferrite and pearlite.

(First induction hardening)
The steel material before the first induction hardening has a softened metal structure. For this reason, the heating temperature of the first induction hardening is increased, and the heating time is increased to obtain a region of a predetermined depth from the surface (“martensite, tempered martensite, tempered martensite, ferrite, pearlite, The region to be an intermediate layer consisting of “) is made to have a sufficient austenite structure. Sufficient austenitic structure means substantially 100% austenite in hypoeutectoid steel and substantially austenite structure and carbide in hypereutectoid steel.

  In order to obtain such a metal structure, the first heat treatment temperature and heat treatment time vary depending on the depth of the region of the austenite structure to be obtained. For example, in a small mechanical structure component such as a gear, the heating temperature is preferably 950 ° C. or higher, but if it exceeds 1150 ° C., crystal grains become coarse, which is not preferable. In addition, the heating time is preferably 10 seconds or more in order to obtain sufficient austenite for the cured layer having a desired depth, but if it exceeds 60 seconds, the cured layer becomes deeper, the deformation becomes larger, This is not preferable because the crystal grains become coarse.

  The frequency used for the first high frequency heat treatment is not particularly limited, but is preferably 3 kHz to 30 kHz from the viewpoint of increasing the depth of the hardened layer. And it quenches after such a heating.

(Second induction hardening)
The second induction hardening is shallower than the first induction hardening, that is, in the martensite structure obtained by the first induction hardening, induction hardening is performed on the surface layer region to be the martensite layer. . As a result, the surface layer becomes a martensite layer, and the intermediate layer following the surface layer becomes tempered martensite, tempered martensite, ferrite and pearlite.

Heating conditions such as the second heat treatment temperature and heat treatment time for obtaining such a metal structure vary depending on the depth of the region of the austenite structure to be obtained. For small mechanical structural parts such as gears, an example is a method of heating to 600 ° C. in 5 seconds, then allowing to cool for 3 seconds, heating at 950 ° C. for 0.5 seconds, and then quenching.

The heating temperature is preferably 900 ° C. or higher, but if it exceeds 1100 ° C., the crystal grains become coarse, so 900 ° C. or higher and 1100 ° C. or lower is preferable. In addition, the heating time is preferably 0.3 seconds or more in order to obtain sufficient austenite for the cured layer having a desired depth, but if it exceeds 1 second, the cured layer becomes deeper and deformation becomes larger. This is not preferable because the crystal grains become coarse.

  The frequency used for the second high-frequency heat treatment is not particularly limited, but is preferably 25 kHz to 200 kHz from the viewpoint of making the hardened layer shallower than the first time.

  Then, tempering is performed after the second induction heat treatment to obtain the product according to the present invention.

  In addition, as long as the object of the present invention is not hindered, the present invention may be subjected to another heat treatment between the first quenching and the second quenching or after the second quenching, Is possible.

(Product characteristics)
The product made by the method of the present invention has a roller pitching characteristic of, for example, about 2800 to 3000 MPa because the steel contains a component that increases the temper softening resistance.

(Use)
The method of the present invention is effective in the manufacture of mechanical structural parts that require fatigue characteristics and wear characteristics, particularly small mechanical structural parts such as gears.

  Steel materials (Examples 1 to 8 and Comparative Examples 1 to 3) containing the components described in Table 1 were prepared. Example steel types 1 to 8 are steel materials according to the present invention, which are excellent in pitching characteristics, but are hard-to-process steel types having a high material hardness. Comparative steel types 1 to 3 are steel types having low material hardness and excellent workability but inferior pitching characteristics.

  In the embodiment of the present invention, after forging the steel grade of the embodiment, softening treatment (pre-heat treatment) is performed to obtain a predetermined metal structure, and then processing to a predetermined shape (post-hobbing and grinding) is performed to obtain a desired tooth. The shape was a mold. Thereafter, predetermined induction hardening and tempering were performed to obtain a desired product (gear). Table 2 shows the softening conditions and the metal structure of the steel material before processing (after softening), and Table 3 shows the hardness (HV) and workability (drill resistance N · cm) of the steel material before processing. . Table 4 shows the heat treatment conditions for the first induction hardening, and Tables 5 and 6 show the heat treatment conditions for the second induction hardening. The specifications of the gears used for the test are as follows.

Module 3
40 teeth
Pressure angle 20 °
Tooth width 20
Outer diameter 126
For comparison with these examples, the steel material was forged, subjected to a predetermined pre-heat treatment, and then subjected to a predetermined processing (machining) to obtain a desired tooth shape. Thereafter, predetermined induction hardening and tempering were performed to obtain a desired product (gear). These are referred to as Comparative Examples 1 to 8, and the heat treatment conditions and the metal structure of the steel material before processing (after pre-heat treatment) are shown in Table 2, the hardness (HV) and workability (drill resistance) of the steel material before processing. N · cm) is also shown in Table 3. The heat treatment conditions for induction hardening are also shown in Tables 5 and 6.

Moreover, also about the comparison steel grade, after forging the steel raw material, after carrying out predetermined | prescribed heat processing, it processed into a predetermined shape (machine processing), and was set as the desired tooth-type shape. Thereafter, predetermined induction hardening and tempering were performed to obtain a desired product (gear). These are referred to as Comparative Examples 9 to 11, and the pre-heat treatment conditions and the metal structure of the steel material before processing (after pre-heat treatment) are shown in Table 2, the hardness (HV) and workability (drill resistance) of the steel material before processing. N · cm) is shown in Table 3. The heat treatment conditions for induction hardening are also shown in Tables 5 and 6.

In Table 2, F + P (Fn): Ferrite + Pearlite (Numerical value n indicates ferrite area percentage)
Tempering M: Tempered martensite F + Spherical C: Ferrite + Spherical cementite

Claims (4)

In mass%, C: 0.3-1.5%, Mn: 0.2-2.0%, Si: 0.5-2.0%, Cr: 0.1-1.5%, Mo: 0.1-1.5%, V: 0.05-0.5% and Nb: A metal consisting of ferrite and pearlite with a ferrite area ratio of 30% or more by softening a steel material consisting of one or more selected from the group consisting of 0.005 to 0.2% and the balance Fe and inevitable impurities A structure, a metal structure consisting of ferrite and spherical carbide, and a metal structure selected from the group consisting of a metal structure consisting of ferrite and granular cementite;
A process of processing the steel material softened in the process into a predetermined shape;
A process of performing at least two induction heat treatments on the processed steel material, and making the metal structure of the surface layer martensite, and subsequent intermediate layer tempered martensite, or tempered martensite and ferrite and pearlite,
A method for manufacturing a machine structural part comprising:   The steel material contains one or two selected from the group consisting of B: 0.0005 to 0.05%, Ti: 0.02 to 0.05% and Ni: 0.01 to 1.5% in mass%. Manufacturing method for machine structural parts.   The manufacturing method of the machine structural component of Claim 1 or 2 which softens a steel raw material and makes the hardness of a steel raw material 280HV or less.   The first heat treatment performed on the processed steel material is a martensite layer in a predetermined region of the steel material by high-frequency heat treatment, and the second heat treatment is martensite obtained by the first heat treatment. Among the layers, the surface layer side is martensite, and the intermediate layer subsequent to the surface layer is tempered martensite, or tempered martensite and ferrite and pearlite. Production method.

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