JP4723338B2 - Steel for induction-hardened gears excellent in impact characteristics, bending fatigue characteristics, and surface fatigue characteristics, and a manufacturing method of gears - Google Patents

Description

  The present invention relates to a steel for induction-hardened gears and a method for producing a gear using the same, and is suitable for gears that are required for excellent impact characteristics, bending fatigue strength and surface fatigue strength, used in automobiles and various industrial machines. About things.

  In recent years, gears used in vehicles such as automobiles have been required to be downsized as the weight of the vehicle body has been reduced due to energy savings, while the load on the gears due to higher engine output has increased. There is a strong demand for further improvement.

  Gear breakage is broadly divided into impact resistance fracture of teeth, bending fatigue fracture of tooth roots, and surface pressure fatigue fracture of tooth surfaces. Conventionally, gears are molded using case-hardened steel such as JIS SCr420, Durability has been improved by surface treatment such as carburizing.

  However, carburization is greatly reduced 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 high alloy material or heat treatment in a vacuum atmosphere, which inevitably increases production costs. It was.

  Therefore, in recent years, a method for manufacturing a gear using surface hardening by induction hardening, which is inexpensive in terms of equipment and can use a low alloy material composition, has been studied.

  Patent Document 1 describes that the induction fatigue-hardened parts are made to improve the bending fatigue strength and rolling fatigue strength by prescribing the size and shape of inclusions in the material as well as the component prescription.

  Patent Document 2 describes that a component composition with an increased C and Si content is used to improve wear resistance and impact fatigue resistance.

Patent Document 3 describes that by adding B, the hardenability is improved without increasing the C content, and at the same time, the internal toughness is improved.
Japanese Patent Laid-Open No. 11-1749 Japanese Patent Laid-Open No. 04-8497 Japanese Patent No. 3402562

  However, in actual production, it is difficult to produce a steel material that satisfies the definition of the size and number of inclusions described in Patent Document 1, and the production load such as adjustment of operating conditions increases. Further, since the component composition described in Patent Document 2 has a high C content, there is a concern that the internal toughness decreases and sufficient impact characteristics cannot be improved, and is a feature of the component composition described in Patent Document 3. Even if the impact characteristics are improved by adding B alone, the surface fatigue characteristics necessary for gears are not improved.

  Therefore, the present invention is a material that can be manufactured even with current general equipment, and is capable of mass production and has superior impact characteristics, root bending fatigue strength and tooth surface fatigue strength than conventional gears. Is to provide.

In order to achieve the above-mentioned problems, the present inventors have made extensive studies and obtained the following new findings.
(1) In order to obtain excellent characteristics equivalent to or better than the current induction-hardened gears after induction hardening, it is necessary to make the hardness distribution equivalent to that of conventional steel, and adjustment by the hardenability index: D value is effective It is.

  (2) Increasing the temper softening resistance by containing appropriate amounts of Si and Cr suppresses softening due to heat generation on the gear contact surface, and can prevent cracking of the tooth surface when the gear is driven. Is organized by the value of 10Si + Cr (where Si and Cr are the contents).

  (3) Increasing the temper softening resistance with Si and Cr has the effect of making the carbide finer in the quenched and tempered structure before induction hardening, facilitating solid solution of the carbide by induction hardening, and improving the hardness after induction hardening. can get.

  (4) The bending fatigue strength is improved by the effect of crystal grain refinement unique to induction hardening, but is further improved when the Ac3 transformation point is further increased and the crystal grain size after induction heating is refined.

  (5) The refinement of crystal grains after induction hardening by increasing the Ac3 transformation point greatly increases impact characteristics and surface fatigue strength.

The present invention has been made based on further studies based on the knowledge obtained, that is, the present invention,
1. C: 0.25 to 0.50 mass%, Si: 1.5 to 2.00 mass%, Mn: 0.3 to 0.78 mass%, Cr: 0.51 mass% or less, formula (1) A steel for induction-quenched gears, which is excellent in impact characteristics, bending fatigue characteristics, and surface fatigue characteristics, in which Z is 18 ≦ Z ≦ 38, balance Fe and inevitable impurities.
Z = 10Si + Cr + 50 (D × Ceq) / A (1)
However, Si and Cr are contents (mass%), D is a hardenability index (mm), Ceq is a carbon equivalent (mass%), and A is an Ac3 transformation point (° C.). The hardenability index D (mm) is determined by D = 8.76 * √C * (1 + 0.64 * Si) * (1 + 4.1 * Mn) * (1 + 2.33 * Cr), and the carbon equivalent Ceq (mass%) ) Is obtained by Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + 0.023, and Ac3 transformation point A (° C.) is obtained by A = 921-203√C + 44.7 * Si-30 * Mn-11 * Cr. In each formula, C, Si, Mn, and Cr are contained in the steel (mass%).

  In addition to the component composition described in 2.1, one or more of Nb: 0.010 to 0.060 mass%, Ti: 0.005 to 0.05 mass%, and B: 0.0005 to 0.01 mass% Induction hardened gear steel excellent in impact characteristics, bending fatigue characteristics, and surface pressure fatigue characteristics.

  3. Impact characteristics characterized by subjecting the steel having the component composition described in 3.1 or 2 to forging, quenching and tempering, and then forming a gear shape by machining and then subjecting the teeth to induction hardening. , A method for manufacturing a gear excellent in bending fatigue characteristics and surface pressure fatigue characteristics.

4). 4. The method for producing a gear excellent in impact characteristics, bending fatigue characteristics, and surface pressure fatigue characteristics according to 3 , wherein the tooth surfaces are further subjected to shot peening or polishing after induction hardening.

  A gear manufactured by the manufacturing method according to 5.3 or 4 and having excellent impact characteristics, bending fatigue characteristics, and surface pressure fatigue characteristics.

  ADVANTAGE OF THE INVENTION According to this invention, the steel material used for the gear and the manufacturing method of a gear using the said steel material in which the outstanding impact fatigue strength, bending fatigue strength, and surface fatigue strength are requested | required are obtained, and it is very useful industrially.

  The reasons for limiting the component composition will be described below.

C: 0.25 to 0.50 mass%
C is necessary for securing the strength, and the amount thereof determines the surface hardness after induction hardening. If the amount is less than 0.25 mass%, the surface hardness is reduced to 500 HV or less, so that the strength as a gear cannot be secured.

  On the other hand, if it is more than 0.50 mass%, not only the toughness inside the gear is lowered and the impact value is lowered, but also the fatigue crack progresses quickly, so that the bending fatigue characteristics are lowered. Moreover, since the material hardness increases too much, the workability deteriorates. Therefore, the C content is limited to 0.25 to 0.50 mass%.

Si: 1.50 mass% or more Si is an element effective for increasing the temper softening resistance, thereby improving the surface fatigue characteristics. Moreover, the precipitation of carbides is delayed, the carbides are refined in the quenching / tempering structure before induction hardening, the carbides are easily dissolved even by rapid heating by induction hardening, and the surface hardness after induction hardening is increased.

Also, the Ac ₃ transformation point is raised to refine the crystal grains during induction hardening. Although an increase in temper softening resistance can be obtained even at 0.8% or more, 1.50 mass% or more is necessary to obtain an Ac ₃ transformation point effective for crystal grain refinement. Therefore, the Si content is set to 1.50 mass% or more.

Mn: 0.3 to 2.0 mass%
Mn is an element that enhances hardenability. In order to ensure hardenability, the amount of Mn added must be 0.3% or more. Even if added over 2.0 mass%, the hardenability becomes excessive, the toughness is deteriorated, and the bending fatigue properties are lowered. Moreover, workability also deteriorates. Therefore, the Mn content is set to 0.3 to 2.0 mass% or less.

Cr: 0.70 mass% or less Cr is an element improving the temper softening resistance as well as a hardenability improving element. However, when the content exceeds 0.70 mass%, the effect of increasing the softening resistance is saturated and the hardenability becomes too high, so that the toughness inside the gear is deteriorated and the bending fatigue strength is lowered. Therefore, the Cr content is set to 0.70 mass% or less.

18 ≦ Z (= 10 Si + Cr + 50 (D × Ceq) / A) ≦ 38,
However, Si and Cr are the contents (mass%), D is the hardenability index (mm), Ceq is the carbon equivalent (mass%), and A is the Ac ₃ transformation point (° C.).

  Z is a value obtained by a parameter formula (= 10 Si + Cr + 50 (D × Ceq) / A), and makes steels of the above compositional range a gear steel excellent in all of impact characteristics, bending fatigue characteristics and surface pressure fatigue characteristics, If the value is less than 18, any of the characteristics deteriorates. On the other hand, if it exceeds 38, the hardenability becomes excessive, and cracking occurs, or the hardness becomes too high and the machinability deteriorates, so Z is set to 18 or more and 38 or less.

  The hardenability index D (mm) is determined by D = 8.76 * √C * (1 + 0.64 * Si) * (1 + 4.1 * Mn) * (1 + 2.33 * Cr), and the carbon equivalent Ceq ( mass%) is obtained by Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + 0.023, and Ac3 transformation point A (° C.) is obtained by A = 921-203√C + 44.7 * Si-30 * Mn-11 * Cr. . In each formula, C, Si, Mn, and Cr are contained in the steel (mass%).

  Although sufficient characteristics can be obtained with the basic component composition of the present invention as described above, one or more of Nb, Ti and B can be contained when further improving the characteristics.

Nb: 0.010 to 0.060 mass%
Nb refines crystal grains by forming carbonitride and improves the root bending fatigue strength. In order to refine the crystal grains, 0.010 mass% or more is required. On the other hand, the effect is saturated even if added exceeding 0.060 mass%. Therefore, when adding, Nb content shall be 0.010-0.060 mass%.

Ti: 0.005 to 0.050 mass%
Ti refines crystal grains by forming carbonitrides and improves tooth root bending fatigue strength. In order to refine crystal grains, 0.005 mass% or more is necessary. On the other hand, the effect is saturated even if added in excess of 0.050 mass%. Therefore, when adding, Ti content shall be 0.005-0.050 mass%.

B: 0.0005 to 0.0100 mass%
B is effective in increasing the hardenability. The effect is obtained at 0.0005 mass% or more, and is saturated when it exceeds 0.0100 mass%. Therefore, when adding, B addition amount shall be 0.0005-0.0100 mass%. When adding B, it calculated | required by the said calculation formula (D = 8.76 * (root) C * (1 + 0.64 * Si) * (1 + 4.1 * Mn) * (1 + 2.33 * Cr)) as D value. Use a value doubled.

  In the steel of the present invention, it is desirable to reduce the P and oxygen contents as inevitable impurities as much as possible. In order to improve machinability, free cutting elements such as S, Pb, Se, and Ca may be included.

  The steel of the present invention can be produced by a conventional method, and the production conditions are not particularly specified. When manufacturing gears using the steel of the present invention, after forging, quenching and tempering, and then machining into a gear shape and then subjecting the teeth to induction hardening, depending on the desired performance After the induction hardening process, it is also possible to perform shot peening or polishing on the tooth surface.

  The gear manufactured by the manufacturing method described above has excellent impact characteristics, bending fatigue characteristics, and surface fatigue characteristics. Hereinafter, the effects of the present invention will be specifically described with reference to examples.

Steels having chemical components shown in Table 1 were melted, and ingots were prepared into round bar steels having a diameter of 32 to 70 mm by hot rolling. In Table 1, no. Nos. 1 to 13 (No. 2 is a missing number) are developed steels having a component composition within the scope of the present invention. 14 to 21 are comparative steels having a component composition outside the scope of the present invention. No. 22 is JIS SCr420 steel which is a conventional case-hardened steel.

  Among the obtained round bars, those made of developed steel and comparative steel are subjected to quenching and tempering after normalizing treatment, and those made of conventional steel are subjected only to normalizing treatment, then 20 mmφ round bar, JIS3 No. impact test piece, Ono type rotating bending fatigue test piece, and roller pitching test piece were collected.

The contents of each survey are described below.
1. Investigation of effective hardened layer depth, surface hardness, internal hardness Cut 20mmφ round bar and subject to induction quenching / tempering treatment for samples taken from developed steel and comparative steel, and carburizing / tempering treatment for those taken from conventional steel After the application, the hardness distribution of the cross section was measured, and the effective depth of the hardened layer was determined by obtaining the Vickers hardness of 550 HV. 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 (internal hardness).

  FIG. 1 shows induction hardening / tempering treatment, (a) shows induction hardening, and (b) shows a heat treatment pattern of tempering treatment. FIG. 2 shows carburizing and quenching treatment, (a) shows carburizing and quenching, and (b) shows a heat treatment pattern of tempering treatment. All the heat treatment patterns were in accordance with those at the time of manufacturing the gears.

2. Impact characteristics investigation Of the JIS No. 3 impact test specimens collected from the developed steel, comparative steel and conventional steel, those extracted from the developed steel and comparative steel are subjected to induction hardening and tempering treatment (see Fig. 1) and then from the conventional steel. The collected sample was subjected to carburizing, quenching and tempering treatment (according to FIG. 2), and the impact value at a test temperature of 20 ° C. was obtained by a Charpy tester. The test was performed on three samples and the average value was obtained.

3. Rotating bending fatigue characteristics A specimen with a diameter of 10mm was taken from a round bar with a diameter of 32mm, and a notch with a depth of 3mm in the circumferential direction (perpendicular to the central axis) of the parallel part (notch coefficient: 1.4). A rotating bending fatigue test piece was prepared by machining the entire circumference. For samples taken from the developed steel and comparative steel, after induction hardening and tempering (according to Fig. 1), for those taken from conventional steel, after carburizing and tempering treatment (according to Fig. 2), Ono type performs rotation bending fatigue test ¹⁰⁷ times using rotating bending fatigue tester as fatigue limit was determined rotary bending fatigue strength.

4). Roller pitching test (contact pressure fatigue characteristics)
A roller pitching test piece having a cylindrical portion having a diameter of 26 mm and a width of 28 mm as shown in FIG. 3 was prepared from a round bar having a diameter of 32 mm.

  In addition, a round bar having a diameter of 70 mm was made to have a diameter of 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. Next, among the roller pitching specimens and large rollers, those extracted from the developed steel and comparative steel are subjected to induction quenching and tempering (according to FIG. 1), and conventional steel is subjected to carburizing quenching and tempering (according to FIG. 2). did.

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 of 40%, lubricant: mission oil, oil temperature: 120 ° C.

The obtained results are shown in Table 2. From the table, the developed steel No. Nos. 1 to 13 (where No. 2 is a missing number) are impact strength (impact value), rotational bending fatigue properties (rotational bending fatigue strength), and surface fatigue properties (surface pressure fatigue strength). It was confirmed that the characteristics superior to 22 were obtained.

  On the other hand, Comparative Steel No. No. 14 has a C content higher than the range of the present invention, the internal hardness becomes excessive, and the impact value and rotational bending fatigue strength are inferior to the developed steel. Comparative steel No. No. 15 has a C content lower than the range of the present invention, the hardenability is lowered, and the surface hardness is low. In addition, the hardened layer depth is shallow, and the rotational bending fatigue strength and surface fatigue strength are inferior to the developed steel.

  Comparative steel No. No. 16 has a Si content lower than the range of the present invention, and is inferior to the developed steel in impact characteristics (impact value), rotational bending fatigue strength, and surface fatigue strength. Comparative steel No. No. 17 has a Mn content larger than the range of the present invention, the hardenability becomes excessive, the internal hardness is high, and the impact characteristics (impact value) and rotational bending fatigue strength are inferior to the developed steel.

  Comparative steel No. No. 18 has a Mn content lower than the range of the present invention, the hardenability is lowered, the effective hardened layer depth is too shallow, the strength of the matrix is insufficient, and the rotational bending fatigue strength is inferior to the developed steel.

  Comparative steel No. No. 19 has a Cr content higher than the range of the present invention, has an excessive hardenability, and is inferior to the developed steel in impact characteristics (impact value) and rotational bending fatigue strength.

  Comparative steel No. 20 and 21 have a Z value outside the range of the present invention. As a result, no. No. 20 has a rotational bending fatigue characteristic of No. 20. No. 21 is inferior to the developed steel in impact characteristics (impact value) and surface fatigue characteristics.

The induction hardening and the tempering process are shown, (a) is an induction hardening, (b) is a figure which shows the heat processing pattern of a tempering process. The figure which shows the heat processing pattern of carburizing quenching and tempering, (a) is carburizing and quenching, and (b) is tempering. The figure which shows an example of a roller pitching test piece.

Claims (5)

C: 0.25 to 0.50 mass%, Si: 1.5 to 2.00 mass%, Mn: 0.3 to 0.78 mass%, Cr: 0.51 mass% or less, formula (1) A steel for induction-quenched gears, which is excellent in impact characteristics, bending fatigue characteristics, and surface fatigue characteristics, in which Z is 18 ≦ Z ≦ 38, balance Fe and inevitable impurities.
Z = 10Si + Cr + 50 (D × Ceq) / A (1)
However, Si and Cr are contents (mass%), D is a hardenability index (mm), Ceq is a carbon equivalent (mass%), and A is an Ac3 transformation point (° C.). The hardenability index D (mm) is determined by D = 8.76 * √C * (1 + 0.64 * Si) * (1 + 4.1 * Mn) * (1 + 2.33 * Cr), and the carbon equivalent Ceq (mass%) ) Is obtained by Ceq = C + Si / 7 + Mn / 5 + Cr / 9 + 0.023, and Ac3 transformation point A (° C.) is obtained by A = 921-203√C + 44.7 * Si-30 * Mn-11 * Cr. In each formula, C, Si, Mn, and Cr are contained in the steel (mass%).   The component composition according to claim 1, further comprising one or more of Nb: 0.010 to 0.060 mass%, Ti: 0.005 to 0.05 mass%, and B: 0.0005 to 0.01 mass%. Induction hardened gear steel excellent in impact characteristics, bending fatigue characteristics, and surface pressure fatigue characteristics. Impact characteristics characterized by subjecting the steel having the component composition according to claim 1 or 2 to forging, quenching and tempering, and thereafter forming a gear shape by machining, and then subjecting teeth to induction hardening treatment. , A method for manufacturing a gear excellent in bending fatigue characteristics and surface pressure fatigue characteristics. The method for producing a gear excellent in impact characteristics, bending fatigue characteristics, and surface pressure fatigue characteristics according to claim 3 , wherein the tooth surfaces are further subjected to shot peening or polishing after induction hardening. A gear manufactured by the manufacturing method according to claim 3 or 4 and having excellent impact characteristics, bending fatigue characteristics, and surface pressure fatigue characteristics.

Images