JP5969204B2 - Induction hardened gear having excellent wear resistance and surface fatigue characteristics and method for producing the same - Google Patents

Description

  The present invention relates to a gear having wear resistance and surface fatigue characteristics suitable for use in automobiles and various industrial machines, and a method for manufacturing the same.

  In recent years, gears used in automobiles and the like have been required to be reduced in size in accordance with the reduction in weight of the vehicle body due to energy saving, but the load has increased due to higher output of the engine. The gear breakage is largely classified, and is caused by tooth impact fracture, tooth root bending fatigue fracture and tooth surface pressure fatigue (also referred to as surface fatigue) fracture.

  Conventionally, gears are formed using case-hardened steel such as JISSCr420, and surface treatment such as carburization is performed. However, in carburizing, there is a large drop in strength due to the effects of grain boundary oxidation and carburizing abnormal layers, and in order to avoid this, it is necessary to use a material with a high alloy addition system or to perform heat treatment in a vacuum atmosphere, resulting in considerable cost increase Is required.

  Therefore, in recent years, gears obtained by subjecting a material with a small amount of alloy addition to surface hardening by induction hardening have been proposed.

  For example, Patent Document 1 describes that for induction-hardened parts, the bending fatigue strength and rolling fatigue strength are improved by defining the size and number of inclusions in the material as well as the component definition. .

  However, although the bending fatigue strength and rolling fatigue strength are improved by defining the size and number of inclusions, when controlling the size and number of inclusions, the temperature conditions during casting and rolling are controlled according to the composition of the inclusions. It is difficult to manufacture a product because it is necessary to do so.

  In Patent Document 2, the amount of C and Si in the component composition is increased to improve wear resistance and impact resistance characteristics. However, since the amount of C is high, internal toughness is low and sufficient impact characteristics can be improved. I can't.

  Patent Document 3 describes an induction-hardened component that improves hardenability without increasing the amount of C by adding B and at the same time improves internal toughness. However, even if the addition of B alone improves the impact characteristics, the surface fatigue strength required for the gear does not improve.

  Patent Document 4 describes a gear excellent in impact characteristics, bending fatigue characteristics, and surface fatigue characteristics, which is a result of research conducted by the present inventors on gears created by induction hardening. However, these characteristics of the developed gear were good, but the amount of wear was large, the frequency of replacement of the lubricating oil increased, and the loss of power performance of the gear unit increased due to wear, resulting in transmission loss. increased.

Japanese Patent Laid-Open No. 11-1749 Japanese Patent Publication No. 04-8497 Japanese Patent No. 3402562 JP 2007-92107 A

  The present invention solves the problems in Patent Documents 1 to 4 above. Specifically, the impact strength and bending fatigue strength using the materials that can be manufactured by the current general equipment are maintained at the same level as conventional gears. However, it is an object of the present invention to provide a gear having excellent wear resistance and better surface fatigue characteristics than a conventional gear and a method for manufacturing the same.

In order to achieve the above-mentioned problems, the inventors have improved the wear resistance, surface fatigue characteristics, that is, surface fatigue strength, based on the following findings (1) to (5) obtained through repeated studies. As a component composition of this material, it is effective to secure a bending fatigue property by managing hardenability, to improve surface fatigue strength by increasing temper softening resistance, and to further increase the Ac ₃ transformation point. I found out.
(1) In order to obtain excellent performance equal to or better than that of the current induction-hardened gear after induction hardening, it is necessary to make the hardness distribution equivalent to that of the conventional steel. Hardenability index: Adjustment by D value Is effective.
(2) If the temper softening resistance is increased by containing appropriate amounts of Si and Cr, softening due to heat generation on the gear contact surface is suppressed, and cracking of the tooth surface can be prevented when the gear is driven.
(3) The temper softening resistance is arranged by the value of 10Si + Cr (where Si and Cr are the contents (mass%)).
(4) The precipitation of carbides is controlled in the quenched and tempered structure before induction hardening, and fine carbides are dispersed in the hardened layer after induction hardening. Thereby, the hardness of the induction-hardened portion is further improved, the wear resistance is improved due to the presence of the carbide, and softening hardly occurs during tempering.
(5) The bending fatigue strength is improved by the crystal grain refinement effect peculiar to induction hardening. However, when the Ac3 transformation point is further increased and the crystal grain size after induction heating is refined, the bending fatigue strength is remarkably improved.
(5) The refinement of crystal grains after induction hardening by increasing the Ac ₃ transformation point greatly increases impact characteristics and surface fatigue strength.

The present invention has been made by further investigation based on the obtained knowledge. In mass%, C: 0.25 to 0.65%, Si: 0.70 to 2.00% or more, Mn: 0.30 to 2.00%, Cr: 1.50% or less, 1) The value of Z calculated by the formula is 15 ≦ Z ≦ 30, the remainder has a composition composed of Fe and inevitable impurities, the structure of the induction-hardened portion is mainly tempered martensite, and the particle size is less than 300 nm inside. An induction-hardened gear excellent in wear resistance and surface fatigue characteristics, characterized in that 90 or more carbides are finely dispersed per 100 μm ² .
Z = 10Si + Cr + 50 (D × Ceq) / A (1)
Here, Si and Cr show the amount (mass%) which each element contains. D is a hardenability index, Ceq is a carbon equivalent, A is an Ac ₃ transformation point, and is a value calculated by the equations (2), (3), and (4).
D = 8.76 * √ (C) * (1 + 0.64 * Si) * (1 + 4.1 * Mn) * (1 + 2.33 * Cr) (2)
However, in the formula, each alloy element is the content (% by mass), and when B is added, the D value is twice the value by this formula.
Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + 0.023 (3)
However, the content of each alloy element in the formula (mass%).
A = 921-203√C + 44.7 * Si-30 * Mn-11 * Cr (4)
However, the content of each alloy element in the formula (mass%).
2. The component composition further contains one or more of Nb: 0.010 to 0.060%, Ti: 0.005 to 0.050%, and B: 0.0005 to 0.0100% by mass%. 2. An induction-hardened gear having excellent wear resistance and surface fatigue characteristics according to 1.
The steel having the composition described in 3.1 or 2 is quenched and tempered after hot forging, then processed into a gear shape, induction hardened and tempered as a surface hardening heat treatment, and the structure of the induction hardening portion A method for producing a gear excellent in wear resistance and surface fatigue characteristics, characterized in that 90 or more carbides having a particle size of less than 300 nm are finely dispersed in 100 μm ² inside, mainly comprising tempered martensite .
4). 4. The method for producing a gear excellent in wear resistance and surface fatigue characteristics according to 3, wherein the tooth surface is further shot peened or polished after the induction hardening and tempering.

  According to the present invention, a gear having excellent wear resistance and surface fatigue characteristics that can be manufactured using equipment and a component composition at low cost and its manufacturing conditions can be obtained, which is extremely useful industrially.

It is a figure which shows the pattern of the induction heat processing used in the Example, (a) shows induction hardening conditions, (b) shows tempering conditions. The current steel No. used in the examples. The figure which shows the pattern of the carburizing quenching and tempering process of 38 steel. The figure explaining a roller pitching test piece.

In the present invention, 1. 1. Component composition of steel used as material Defines the microstructure of the induction hardening part of the gear. Each limitation reason is described below.
[Component Composition] In the following description,% is mass%.
C: 0.25 to 0.65%
C is necessary for ensuring the strength, and determines the surface hardness after induction hardening. If the content is less than 0.25%, the surface hardness decreases to 500 HV or less, so that the strength as a gear cannot be ensured. On the other hand, if it exceeds 0.65%, the toughness inside the gear decreases and the fatigue crack progresses quickly, so that the bending fatigue characteristics deteriorate, so 0.25 to 0.65%.

Si: 0.70 to 2.00%
Si is an element effective in increasing the temper softening resistance and thereby improving the surface fatigue characteristics. In order to obtain the effect, the content is made 0.70% or more. Further, since the improvement in resistance to temper softening is due to the effect of delaying the precipitation of carbides, the carbides can be finely precipitated after quenching and tempering before induction hardening. As a result, even in rapid heating by induction hardening, carbides are easily dissolved, and the surface hardness after induction hardening can be increased. Furthermore, there is an effect that the Ac ₃ transformation point is raised and the crystal grains are refined by induction hardening. Since the effect mentioned above will be saturated if it exceeds 2.00%, content is made into 0.70 to 2.00%.

Mn: 0.30 to 2.00%
Mn is an element that enhances hardenability, and is 0.30% or more in order to obtain the effect. On the other hand, if it exceeds 2.00%, the hardenability is excessively increased, the toughness is deteriorated, and the bending fatigue characteristics are lowered. Moreover, workability also deteriorates, so 0.30 to 2.00%.

Cr: 1.50% or less Cr is contained because it is an element that enhances hardenability and resistance to softening by tempering and greatly affects carbide precipitation during tempering. If the content exceeds 1.50%, the hardenability becomes too high, the toughness inside the gear is deteriorated, and the bending fatigue strength is lowered, so the content is made 1.50% or less.

15 ≦ Z ≦ 30
Z is a parameter in consideration of temper softening resistance, hardenability, and Ac ₃ transformation point, and Z = 10Si + Cr + 50 (DxCeq) / A. Here, Si and Cr show the amount (mass%) which each element contains. D is a hardenability index, Ceq is a carbon equivalent, A is an Ac ₃ transformation point, and is a value calculated by the following equations (2) to (4).
D = 8.76 * √ (C) * (1 + 0.64 * Si) * (1 + 4.1 * Mn) * (1 + 2.33 * Cr) (2)
However, in the formula, each alloy element is the content (% by mass), and when B is added, the D value is twice the value by this formula.
Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + 0.023 (3)
However, the content of each alloy element in the formula (mass%).
A = 921-203√C + 44.7 * Si-30 * Mn-11 * Cr (4)
However, the content of each alloy element in the formula (mass%).

  When the value of Z is 15 or more and 30 or less, the surface fatigue strength can be improved, and the impact strength and bending fatigue strength can be maintained at the same level as conventional parts. If the value of Z is less than 15, there is no effect of improving the surface fatigue strength, and if it exceeds 30, the hardenability becomes too high, causing cracking, or the hardness becomes too high and the machinability is lowered.

  The above is the basic component composition of the present invention, and the remainder is Fe and inevitable impurities. The contents of P and oxygen as unavoidable impurities are preferably as low as possible. Furthermore, when improving a characteristic, 1 or more types of Nb, Ti, and B can be contained.

Nb: 0.010 to 0.060%
Nb refines crystal grains by forming carbonitride and improves bending fatigue strength. Although 0.010% or more is necessary for making the crystal grains finer, the effect is saturated even if the content exceeds 0.060%. Therefore, when it contains Nb, it is made into 0.010 to 0.060%.

Ti: 0.005 to 0.050%
Ti refines crystal grains by forming carbonitride and improves bending fatigue strength. Although 0.005% or more is necessary to make the crystal grains fine, the effect is saturated even if the content exceeds 0.050%. Therefore, when it contains Ti, it is 0.005 to 0.050%.

B: 0.0005 to 0.0100%
B is effective in increasing the hardenability. The effect is obtained at 0.0005% or more, but the effect is saturated even if the content exceeds 0.0100%. Therefore, when it contains B, it is made into 0.0005 to 0.0100%.

  In order to improve machinability in the steel of the present invention, free cutting elements such as S, Pb, Se, and Ca are added.

[Microstructure of induction hardening part of gear]
The microstructure of the induction hardening portion is defined as a microstructure mainly composed of tempered martensite in which 90 or more carbides having a particle size of less than 300 nm are finely dispersed per 100 μm ² . The microstructure of the induction-hardened part is mainly tempered martensite so that the strength, toughness and wear resistance required for gears can be obtained. It is assumed that the subject includes at least 95%. The carbides present in the tempered martensite are effective for wear resistance, but when the particle size exceeds 300 nm, the fatigue strength is reduced and the fatigue strength is reduced as in the case of inclusions.

On the other hand, even when the particle diameter is as small as less than 300 nm, the wear resistance cannot be improved when the density of the carbide is as small as less than 90 per 100 μm ² . Further, the fine dispersion is necessary to improve the wear resistance. Therefore, it was limited as described above. The microstructure observation method will be described in detail in Examples.

  The gear according to the present invention is manufactured by subjecting steel having the above component composition to hot forging, followed by quenching and tempering, then processing into a gear shape, and induction hardening and tempering as surface hardening heat treatment. After induction hardening and tempering, shot peening or polishing may be further performed on the tooth surface.

  Hot forging is performed at 1000 to 1300 ° C., then cooled to room temperature, reheated to 850 to 950 ° C., then poured into oil at 60 to 130 ° C. and quenched, or air cooled to 780 to 880 ° C. after forging. After that, similarly, it is put into oil at 60 to 130 ° C. and quenched. Then, it is desirable to reheat to 450-700 degreeC, temper, and process into a gear shape. The induction hardening and tempering conditions are appropriately selected so that the microstructure of the induction hardening portion after induction hardening and tempering satisfies the above-mentioned definition of the microstructure. In the present invention, the induction hardening portion is a region where the structure and hardness have been changed by induction hardening and tempering.

  When shot peening is performed on the tooth surface of the gear, (1) shot peening using particles having a particle size of 0.05 to 0.1 mmΦ and a hardness of 700 HV or more, and (2) particle size of 0.4 to 1. Shot peening performed using particles having a hardness of 700 HV or more having a particle size of 0.05 to 0.1 mmΦ after performing using particles having a hardness of 700 HV or more of 2 mmΦ, and (3) a particle size of 0.4 to 0.4 Either shot peening is performed by mixing grains having a hardness of 700 HV or more with a diameter of 1.2 mmΦ and grains having a hardness of 700 HV or more with a thickness of 0.05 to 0.1 mmΦ. As the shot peening, the fatigue characteristics are improved in the order of (1) to (3), but the proper selection is made in consideration of the manufacturing cost. Hereinafter, the present invention will be described in more detail in comparison with comparative examples by way of examples.

Steels having chemical components shown in Table 1 were melted and used as test materials. No. shown in the table. Nos. 1 , 3 to 20 are steels having a composition within the range of the present invention. 21 to 37 are steels having a component composition outside the scope of the present invention. No. 22 is JIS SCr420 steel which is a conventional steel.

  The ingot made of the above-mentioned steel was prepared into a round bar steel having a diameter of 32 to 70 mm by hot rolling. Steels 1 to 37 were quenched and tempered after normalizing treatment, and normal steel was subjected to normalizing treatment.

  No. Steels 1-37 are sampled from hardened and tempered steel bars, and conventional steels are taken from steel bars after normalizing treatment, 20 mmφ round bars, JIS No. 3 impact test pieces, Ono-type rotary bending fatigue test pieces, and roller pitching test pieces. did.

  No. for each round bar and each fatigue test piece. The steels 1 to 37 were subjected to induction hardening and tempering treatment shown in FIG. 1 (FIG. 1 (a) was induction hardening conditions and FIG. 1 (b) was tempering conditions). No. The 38 steel was subjected to carburizing quenching and tempering treatment shown in FIG. 2, and then the surface hardness, internal hardness and effective hardened layer depth were investigated, and the impact test, the rotating bending fatigue test and the roller pitching test were performed.

  No. 18 steel materials were finely adjusted for quenching and tempering conditions and induction heating conditions, and the number of carbide particles having a particle size of less than 300 nm deposited at the induction hardening site was changed, and the same test was performed. The contents of each survey are described below.

[Effective hardened layer depth, surface hardness, internal hardness investigation, carbide precipitation amount]
A 20φ (mm) round bar is cut and For steel Nos. 1 to 37, induction quenching and tempering are performed after quenching and tempering, and for conventional steel, after carburizing quenching and tempering, the hardness distribution of the cross section is measured, and the depth at which 550 HV is obtained in terms of Vickers hardness is investigated. The effective hardened layer depth. Furthermore, the hardness at a depth of 50 μm from the surface was defined as the surface hardness, and was measured using a Vickers hardness meter together with the internal (non-hardened portion) hardness. No. For steels 1 to 37, the induction-quenched sites were observed by FE-SEM, and the number per 100 μm ² of carbides having a particle size of less than 300 nm was counted. The particle size was the average value of the minor axis and the major axis.

[Impact characteristics]
A JIS No. 3 impact test piece was prepared using each steel material. The steels 1 to 37 were subjected to induction quenching and tempering, and the conventional materials were subjected to carburizing quenching and tempering, and then the impact value at a test temperature of 20 ° C. was examined by a Charpy tester.

[Rotating bending fatigue characteristics]
A test piece having a parallel part diameter of 10 mm was taken from a round steel bar having a diameter of 32 mm, and a notch (notch coefficient: 1.4) having a depth of 1.5 mm was attached to the parallel part in the direction perpendicular to the parallel part. A rotating bending fatigue test piece was prepared. No. The steels 1 to 37 were induction-quenched and tempered, and the conventional steels were carburized and tempered. It performs rotation bending fatigue test as fatigue limit of 10 ⁷ times using fatigue tester Ono-type rotating bending was measured rotating bending fatigue strength.

[Surface fatigue characteristics]
The surface pressure fatigue characteristics were investigated by a roller pitching test. A test piece having a cylindrical portion with a diameter of 26 mm and a width of 28 mm as shown in FIG. 3 was prepared from a round steel bar having a diameter of 32 mm. Further, using a round steel bar having a diameter of 70 mm, the diameter was set to 135 mm by forging, and then a normalizing process was performed to produce a large roller having a diameter of 130 mm and a width of 18 mm.

Subsequently, about a roller-shaped test piece and a large roller, it is No. The steels 1 to 37 were subjected to induction quenching and tempering, and the conventional steels were subjected to carburizing and tempering. Using a roller pitching tester, the test was conducted 10 ⁷ times with a fatigue limit. The test condition is the number of revolutions: 1500 r. p. m, slip ratio 40%, lubricant: mission oil, oil temperature: 120 ° C.

  Further, the test was conducted 10 million times at a surface pressure of 3000 MPa, and the amount of wear was calculated from the difference in the test piece weight before and after the test.

Table 2 shows the survey results .

  Comparative Example No. No. 22 has a C content lower than the range of the present invention, so the hardenability is too low and the surface hardness is low. Moreover, the hardened layer depth is too shallow. Therefore, rotational bending fatigue strength and surface fatigue strength were reduced.

  Comparative Example No. No. 23 has a Si content lower than the range of the present invention. Therefore, the softening resistance was too low and the surface fatigue strength was lowered. Comparative Example No. In No. 24, the Si content was higher than the range of the present invention, so that the toughness was low, and the impact value and the rotational bending fatigue strength were reduced.

  Comparative Example No. In No. 25, since the Mn content was lower than the range of the present invention, the hardenability was too low and the effective hardened layer depth was too shallow, so the overall strength was insufficient and the rotary bending fatigue strength was lowered. Comparative Example No. No. 26 has too high hardenability because the Mn content is higher than the range of the present invention. Therefore, the impact value and the rotational bending fatigue strength decreased.

  Comparative Example No. 27 has a Cr content higher than the range of the present invention. As a result, the toughness decreased and the bending fatigue strength decreased. Comparative Example No. In Nos. 28 and 29, since the Nb content was lower than the range of the present invention, the crystal grains became larger and the impact value and the bending fatigue strength were lowered.

Comparative Example No. In 30, 31, and 35, since the Ti content was lower than the range of the present invention, the crystal grains became large, and the impact value and the bending fatigue strength were lowered.
Comparative Example No. Nos. 32, 33, and 34 have a B content lower than the range of the present invention, so that the hardenability is insufficient, the hardened layer depth is shallow, and the internal hardness is low, so that the overall strength is insufficient and the rotational bending Fatigue strength was insufficient.

  Comparative Example No. In 36, the value of Z was lower than the range of the present invention, and the surface fatigue strength decreased. Comparative Example No. No. 37 has a Z value higher than the range of the present invention, and has high hardenability, the hardening depth and internal hardness are too high, the impact value decreases, and the rotary bending fatigue strength also decreases.

On the other hand, No. which is steel of the present invention. Nos. 1 , 3 to 20 are comparative example Nos. 21-37 and conventional steel No. Compared to 38, favorable results were obtained in impact characteristics, rotational bending fatigue characteristics, and surface fatigue characteristics.

Table 3 shows the results of investigating the rotational bending fatigue characteristics and the surface fatigue characteristics by changing the number of carbide particles having a particle size of less than 300 nm to be precipitated at the induction-quenched site. 18-5, 18-6, 18-7 in which the number of carbides having a diameter of 300 nm or less is smaller than the range of the present invention has a large wear amount. 18-1,18-2,18-3,18-4 is within range of the present invention is good less resistance to friction耗性wear amount relative thereto.

Claims (4)

C: 0.25 to 0.49%, Si: 0.70 to 2.00 %, Mn: 0.30 to 2.00%, Cr: 1.50% or less, ) The value of Z calculated by the formula is 15 ≦ Z ≦ 30, the remainder has a component composition consisting of Fe and inevitable impurities, and the structure of the induction-hardened part is mainly tempered martensite, inside the induction-hardened part. carbides having a particle size of less than 300nm and an excellent wear resistance and surface fatigue properties, characterized in that it is dispersed in 100 [mu] m ² per 90 or more fine, induction hardening gears grinding耗量is less than 24 mg.
Z = 10Si + Cr + 50 (D × Ceq) / A (1)
Here, Si and Cr show the amount (mass%) which each element contains. D is a hardenability index, Ceq is a carbon equivalent, A is an Ac ₃ transformation point, and is a value calculated by the equations (2), (3), and (4).
D = 8.76 * √ (C) * (1 + 0.64 * Si) * (1 + 4.1 * Mn) * (1 + 2.33 * Cr) (2)
However, in the formula, each alloy element is the content (% by mass), and when B is added, the D value is twice the value by this formula.
Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + 0.023 (3)
However, the content of each alloy element in the formula (mass%).
A = 921-203√C + 44.7 * Si-30 * Mn-11 * Cr (4)
However, the content of each alloy element in the formula (mass%).   The component composition further contains one or more of Nb: 0.010 to 0.060%, Ti: 0.005 to 0.050%, and B: 0.0005 to 0.0100% by mass%. The induction-hardened gear excellent in wear resistance and surface fatigue characteristics according to claim 1. The steel having the composition of claim 1 or 2 is hot forged, quenched and tempered, then processed into a gear shape, induction hardened and tempered as a surface hardening heat treatment, and the structure of the induction hardened portion is tempered. characterized in that it is assumed that the carbide particle size of less than 300nm therein was ² per 90 or more 100μm finely dispersed martensite mainly, friction耗量is below 24mg having excellent abrasion resistance and surface fatigue properties Induction hardening gear manufacturing method.   4. The method of manufacturing an induction-hardened gear excellent in wear resistance and surface fatigue characteristics according to claim 3, further comprising performing shot peening or polishing on the tooth surface after the induction hardening and tempering.

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