JP4938475B2 - Gear steel excellent in impact fatigue resistance and gears using the same - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a carburized gear steel and a carburized gear using the same, and has a rotational bending fatigue strength equivalent to that of JIS SCM822 case-hardened steel, and is suitable for automobiles and various industrial machines that require particularly high impact fatigue strength. About things.

  In recent years, gears used in automobiles and the like have been required to be downsized with the reduction of the weight of the vehicle body due to energy saving, but on the other hand, the load has increased due to the high output of the engine. Improvement is an issue.

  In general, the durability of a gear is determined by impact resistance fracture of a tooth, bending fatigue fracture of a tooth root, and surface pressure fatigue fracture of a tooth surface.

  In gears used in impact stresses such as automobile differentials, destruction may occur early due to high impact loads, and various improvements in impact characteristics have been studied (Patent Documents 1 to 5). ).

  That is, in Patent Document 1, Mo is added in order to improve the toughness of the carburized layer, Mn, Cr, and P are decreased to reduce the grain boundary strength of the carburized layer, and Mo / (10Si + 100P + Mn + Cr) is obtained. It has been proposed to improve impact properties by defining a lower limit and by defining a range of carburized hardened layer depth.

  Patent Document 2 proposes that the toughness is improved by controlling the quenching cooling rate range to an appropriate range according to the component composition so that the inside of the gear is a mixed structure of martensite and bainite.

  In Patent Document 3, the microstructure is defined in the same manner as Patent Document 2, and the microstructure is a mixed structure of martensite and troostite that improves internal toughness, and the range of addition amounts of Mn and Cr is defined. And the method of suppressing the fall of internal hardness by restrict | limiting Mo addition amount and restrict | limiting the amount of troostite is proposed.

Further, Patent Document 4 proposes a steel in which Mo is added to the component composition described in Patent Document 3. Patent Document 5 proposes a steel material for bevel gears in which the composite additive amount of Mn, Cr and Mo is limited in the component composition to suppress the hardness of the steel material and the impact characteristics are improved without impairing the cold forgeability. Yes.
Japanese Examined Patent Publication No. 7-100840 Japanese Patent No. 3094856 Japanese Patent No. 3329177 Japanese Patent No. 3733504 Japanese Patent No. 3319684

  However, in the method described in Patent Document 1, even if the impact characteristics can be improved, it is necessary to add a large amount of Mo, which is an expensive alloy, or to significantly extend the carburizing time if a large amount of Mo is not added. As a result, the product cost or the manufacturing cost is greatly increased.

  In the method described in Patent Document 2, since the bainite structure is included in the microstructure, it is possible to improve the toughness and increase the impact value.

  However, if a bainite structure is contained inside, the internal hardness is reduced, so that the gears are easily deformed by impact, and there is a concern that the gear may be damaged when the impact force is repeated.

  In the method described in Patent Document 3, since the composite addition amount of Mn and Cr is specified and the addition amount of Mo is regulated, the grain boundary oxidation generated near the surface layer increases, and oxides of Mn and Cr are formed. Therefore, the hardenability is lowered and an incompletely hardened layer is formed on the surface layer.

  Therefore, even if the internal hardness can be ensured, the surface layer is likely to be broken due to a decrease in the surface layer hardness, and as a result, all fatigue strengths including impact fatigue are reduced.

  In the case of the method described in Patent Document 4, even if Mo is added, the hardness inside the gear is reduced due to the true tooth. Therefore, even if the impact characteristics are improved, the fatigue strength such as internal bending fatigue is reduced. In the case of the method described in Patent Document 5, when the gear is shaped by hot forging, the hardness is low and the fatigue strength other than impact is reduced.

  Therefore, an object of the present invention is to provide a low-cost steel for gears that can provide excellent impact fatigue strength without deteriorating the characteristics as a gear and a gear using the steel.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive studies on a gear steel having a low-cost component composition that provides an excellent impact fatigue strength by a conventionally used carburizing method, which is inexpensive as a production cost. went.

As a result, in the case of gears, it is necessary not to cause deformation due to impact force with respect to repeated impact stress. To that end, it is possible to obtain an appropriate hardness distribution, and an intergranular oxide layer depth and internal hardness that affect the hardness distribution. We found out that it was important and obtained the following component design guidelines.
1. Strengthening of prior austenite grain boundaries is most important for improving impact fatigue properties, and P is reduced to suppress embrittlement of prior austenite grain boundaries.
2. Further, B is dissolved in the steel to preferentially segregate at the prior austenite grain boundaries, thereby suppressing P grain boundary segregation.
3. In order to make solute B exist in steel, N having a strong bonding force with B is bonded with Ti or Al. However, when Ti is used, TiN precipitated during melting is relatively large, sharp and hard. Due to the inclusion, it tends to be a starting point of fatigue, and the surface fatigue strength and bending fatigue strength are reduced.
4). When Al is added and N is fixed using the equilibrium relationship with B and N, BN and AlN precipitate in the steel. Since AlN is fine, crystal grains are refined and impact fatigue strength is improved. Further, since AlN is fine, it does not become a starting point of fatigue, and the fatigue strength can be improved as compared with the addition of Ti.
5. For the impact fatigue strength, the balance between the grain boundary oxide layer depth, the internal hardness, and the carburized hardness distribution (hardened layer depth) is important, and there is an optimum range.

The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. In mass%, C: 0.16 to 0.30%, Si: less than 0.40%, Mn: 0.50% or more, P: 0.015% or less, Cu: 0.30% or less, Cr: 2 Less than 0.000%, Mo: 0.16-0.50%, Al: 0.060-0.110%, B: 0.0004% or more, N: 0.0065% or less, (1) formula The effective content of the hardened layer depth E, internal hardness D, and grain boundary oxide layer depth P obtained after carburizing and tempering is as follows. Steel for gears with excellent impact fatigue resistance that satisfies the requirements.
I (= 14/27 × Al + 14 / 10.8 × B−N) ≧ 0.03 (1)
However, Al, B, and N in the formula (1) indicate the content (mass%).
170 ≦ S (= E / 2 × DP / 2) ≦ 250 (2)
However, E: Effective hardened layer depth (mm), D: Internal hardness (HV), P: Grain boundary oxide layer depth (micrometer) is shown.
2. Furthermore, it is excellent in impact fatigue resistance according to 1, wherein at least one of Ti: less than 0.020%, Nb: 0.050% or less, and V: 0.030-0.200% is added. Steel for gears.
3. A gear excellent in impact fatigue resistance obtained by machining or carving after forging using the steel material described in 3.1 or 2 and then carburizing and quenching.
A gear excellent in impact fatigue resistance, which is formed into a gear shape by the process of 4.3 and further subjected to shot peening or polishing on the tooth surface.

  According to the present invention, a gear having a bending fatigue strength equal to or higher than that of conventional steel and having an excellent impact fatigue strength is obtained, which is extremely useful industrially.

The reasons for limiting the constituent elements of the present invention will be described below. In the explanation,% is mass%.
C: 0.16-0.30%
C is necessary for securing the strength, and its amount determines the internal hardness. If the amount is less than 0.16%, the surface hardness decreases to 300 HV or less, and the strength as a gear cannot be secured. On the other hand, if it exceeds 0.30%, the hardness of the material is excessively increased, so that the workability is reduced and the production cost is increased.

Si: Less than 0.40% Si is an element that is easily oxidized, promotes grain boundary oxidation on the carburized surface, and generates an incompletely hardened layer. Since the content becomes significant when the content is 0.40% or more, the content is made less than 0.40%.

Mn: 0.50% or more Mn is an element that enhances hardenability, but if the added amount is less than 0.50%, hardenability cannot be ensured. Therefore, the Mn addition amount is 0.50% or more.

P: 0.015% or less P segregates at the grain boundary, embrittles the grain boundary, and lowers the grain boundary strength. As the added amount increases, the generation and propagation of fatigue cracks are more likely to occur, so 0.015% or less.

Cu: 0.30% or less Cu decreases the hot workability such as rolling and forging, and when processed into a gear or the like, the processing accuracy deteriorates and the fatigue characteristics deteriorate, so the content is made 0.30% or less.

Cr: less than 2.00% Cr is an element that enhances hardenability and increases temper softening resistance. However, when the added amount is 2.00% or more, 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 not only the impact value is lowered but also the bending fatigue strength is lowered. To do. Therefore, the Cr addition amount is limited to less than 2.00%.

Mo: 0.16-0.50%
Mo is an element effective for improving the hardenability, but if it is less than 0.16%, the hardenability cannot be secured, so the content is made 0.16% or more. Since Mo is also an expensive element, the upper limit is 0.50% and the content is 0.16 to 0.50%.

Al: 0.060 to 0.110%
Al is an element necessary for ensuring solid solution B in the equilibrium between N and B. If the added amount is less than 0.060%, the effect cannot be obtained, and if added over 0.110%, there is a risk of casting abnormalities during melting, so 0.060 to 0.110%.

B: 0.0004% or more B dissolves in the steel and segregates at the grain boundaries to improve the hardenability and compensate for the decrease in hardenability due to low Si content. Moreover, in order to prevent grain boundary segregation of P, improve grain boundary strength, and improve fatigue characteristics, the content is made 0.0004% or more.

N: 0.0065% or less Since N combines with B to form BN, the lower N is better for securing solid solution B. However, if N is 0.0065% or less in the equilibrium relationship of Al, N, and B, solid solution B can be secured, so the content is made 0.0065% or less.

I (= 14/27 × Al + 14 / 10.8 × B−N): 0.03 or more This parameter I determines the balance of Al, B, and N in order to secure a solid solution B that improves impact fatigue strength. As an index, when the value of this parameter I is less than 0.03, B cannot be secured in the steel, so 0.03 or more. Al, B, and N in the formula indicate the content (mass%).

  The above is the basic component composition of the present invention. When further improving the characteristics, one or more of Ti, Nb, and V are added.

Ti: Less than 0.020% Ti is an element that is most easily bonded to N and is effective in securing solid solution B. However, if it is added excessively, a large amount of coarse TiN having a hard and sharp shape is formed, which becomes a starting point of fracture of bending fatigue and impact fatigue, and lowers the strength. The effect is significant when 0.020% or more is added. Therefore, when adding Ti, it is less than 0.020%.

Nb: 0.050% or less Nb refines crystal grains and strengthens grain boundaries to improve fatigue strength, but the effect is saturated at 0.050%. Therefore, when adding Nb, it is made into 0.050% or less.

V: 0.030-0.200%
V increases the internal fatigue strength after carburizing to improve the overall fatigue strength. The effect appears at 0.030% or more, but when it exceeds 0.200%, it becomes saturated. Therefore, when V is added, the content is made 0.030 to 0.200%.

  The S and oxygen contents as unavoidable impurities are desirably as low as possible within the range allowed by the cost. Moreover, you may contain free-cutting elements, such as S, Pb, Se, and Ca, as needed in order to improve machinability.

170 ≦ S (= E / 2 × DP / 2) ≦ 250
(However, E: Effective hardened layer depth (mm), D: Internal hardness (HV), P: Grain boundary oxide layer depth (μm))
This parameter S is for imparting excellent impact fatigue resistance, and is set to 170 or more and 250 or less. No. in Table 1 Create impact fatigue test specimens using 1 to 3 round steel bars, carburize and temper under various conditions, and create impact fatigue test specimens with different grain boundary oxide depth, internal hardness, and effective hardened layer depth. Prepared and conducted an impact fatigue test. The results are shown in Table 2 and FIG.

  The impact fatigue test results are as follows: E: effective hardened layer depth (mm), D: internal hardness (HV), P: grain boundary oxide layer depth (μm) S (= E / 2 × DP / 2) ), An excellent impact fatigue test result with a parameter S value of 170 or more and 250 or less and an impact fatigue strength of 2.5 N · m or more is obtained.

  When the effective hardened layer depth is 0.98 mm or more, the grain boundary oxide layer is 12 μm or more, and the internal hardness is 417 Hv or more, excellent rotational bending fatigue characteristics with a rotational bending fatigue strength of 650 MPa or more can be obtained. For automobiles and various industrial machines As such, it is more preferable.

  In addition, the numerical value itself in each unit system is used for E, D, and P when calculating the parameter S.

  When producing a gear from the steel for gear according to the present invention, it is melt cast by a conventional method to form a billet, and after hot rolling, preforming as a gear is performed.

  Next, it is machined or machined after forging to form a gear shape, and then carburized and quenched, and if necessary, the tooth surface is further subjected to shot peening or polishing to obtain a final product. The carburizing and quenching treatment is performed at a carburizing temperature of 900 to 1050 ° C, a quenching temperature of 800 to 900 ° C, and tempering within a range of 120 to 250 ° C.

  Steels having chemical components shown in Table 3 were melted and used as test steels. In the table, No. Nos. 1 to 15 are steels of the present invention having compositions within the scope of the present invention. 16 to 28 are comparative steels outside the scope of the present invention. No. 29 is a conventional steel of JIS SCM 822 case hardening steel.

  The ingots of the present invention steel, the comparative steel, and the conventional steel prepared as above were prepared by hot rolling into a round bar steel having a diameter of 32 mm, and the obtained round bar steel was subjected to a normalization treatment.

  A 20 mmφ round bar, an Ono-type rotary bending fatigue test piece, and an impact fatigue test piece were collected from the round bar after the normalizing treatment. After carburizing and tempering the round bar and each fatigue test piece under the conditions shown in Fig. 1, investigation of surface hardness, internal hardness, effective hardened layer depth, impact fatigue test and rotating bending fatigue test Carried out. The details of each survey are described below.

Intergranular oxidation layer depth, effective hardened layer depth, surface hardness, internal hardness investigation Invented steel, comparative steel and conventional steel 20φ round bar, carburizing quenching and tempering, cutting, maximum grain boundary oxidation The layer depth was measured with an optical microscope. Further, the hardness distribution of the cross section was measured, and the depth at which 550 HV was obtained in terms of Vickers hardness was investigated and set as the “effective hardened layer depth”. Further, the hardness at a depth of 5 mm from the surface layer was defined as “internal hardness” and measured using a Vickers hardness tester.

Impact Fatigue Properties Investigation Test specimens shown in FIG. 2 were prepared from round steel bars with a diameter of 32 mm, carburized and tempered under the conditions shown in FIG. 1, and then fractured by a falling weight type impact fatigue tester at 200 repetitions. The impact energy to investigate was investigated.

Rotating Bending Fatigue Properties From a round steel bar having a diameter of 32 mm, a test piece having a diameter and a parallel part diameter of 8 mm as shown in FIG. 3 was taken, and a notch with a depth of 2 mm perpendicular to the parallel part was obtained (notch coefficient: A rotating bending fatigue test piece having 1.34) applied to the entire circumference was prepared.

Carburizing quenching and tempering treatment under the conditions shown in FIG. 1 is performed on the total number of the test pieces obtained (invention steel, comparative steel, and conventional steel), and then 10 ⁷ times using an Ono rotary bending fatigue tester. A rotating bending fatigue test was performed as the fatigue limit, and the rotating bending fatigue strength was measured.

  Table 4 shows the results of the above investigation. The steel according to the present invention has an effective hardened layer depth of 0.98 mm or more, a grain boundary oxide layer of 12 μm or more, an internal hardness of 417 Hv or more, a rotational bending fatigue strength of 654 MPa or more, and an impact fatigue strength of 2.7 N · m or more. Comparative steel No. 16-No. No. 28 and the conventional steel (JISSCM822 case-hardened steel) (No. 29 conventional steel (JISSCM822 case-hardened steel) had a rotational bending fatigue strength of 650 MPa and an impact fatigue strength of 2.0 N · m).

  That is, comparative steel No. No. 16 had a C content lower than the range of the present invention, so the internal hardness was too low, and the impact fatigue strength was good, but the rotational bending fatigue strength was reduced.

  Comparative steel No. In No. 17, since the Si content is higher than the range of the present invention, the grain boundary oxide layer is deep and the surface hardness is low. Therefore, impact fatigue strength, rotational bending fatigue strength, and surface fatigue strength decreased.

  Comparative steel No. No. 18 has too low hardenability because the Mn content is lower than the range of the present invention. Therefore, since the effective hardened layer depth was shallow and the internal hardness was low, impact fatigue strength and rotational bending fatigue strength were reduced.

  Comparative steel No. In No. 19, since the P content was higher than the range of the present invention, the grain boundary strength was insufficient and the impact fatigue strength was lowered.

  Comparative steel No. In No. 20, since the Cu content was higher than the range of the present invention, the processing accuracy of the test piece was poor, and the rotary bending fatigue strength and the impact fatigue strength were reduced.

  Comparative steel No. No. 21 has too high hardenability because the Cr content is higher than the range of the present invention. As a result, the impact fatigue strength decreased.

  Comparative steel No. Since No. 22 had Mo content lower than the range of the present invention, hardenability became insufficient, and the effective hardened layer depth and internal hardness fell. As a result, the rotational bending fatigue strength and impact fatigue strength decreased.

  Comparative steel No. In No. 23, since the V content was lower than the range of the present invention, the internal hardness decreased. As a result, the rotating bending fatigue strength decreased.

  Comparative steel No. In No. 24, since the B content was lower than the range of the present invention, the grain boundary strengthening effect could not be obtained, the effective hardened layer depth became shallow, and the rotary bending fatigue strength and the impact fatigue strength decreased.

  In Comparative Steel No. 25, the amount of Al added was lower than the range of the present invention. As a result, the grain boundary strengthening effect due to solute B was not obtained, the effective hardened layer depth was shallow, and the internal hardness was low. Therefore, the rotational bending fatigue strength and the impact fatigue strength were reduced.

  Comparative steel No. No. 26 has an N addition amount higher than the range of the present invention. As a result, the solid solution B could not be secured, the effective hardened layer depth was shallow, and the internal hardness was low. Therefore, the rotational bending fatigue strength and the impact fatigue strength were reduced.

  Comparative steel No. No. 27 has a Ti addition amount higher than the range of the present invention. Therefore, fatigue failure due to the TiN starting point is likely to occur, and the rotational bending fatigue characteristics and the impact fatigue characteristics are deteriorated.

  Comparative steel No. 28 has an Al addition amount and an I value lower than the scope of the present invention. As a result, the grain boundary strengthening effect due to the solid solution B was not obtained and the hardenability was lowered, so that the effective hardened layer depth became shallow. Therefore, the rotational bending fatigue strength and the impact fatigue strength were reduced.

The figure which shows the carburizing quenching and tempering process conditions. The figure which shows the shape of an impact fatigue test piece. The figure which shows the shape of an Ono type | formula rotation bending fatigue test piece. The figure which shows the influence of S (= E / 2xDP / 2) value which gives to an impact fatigue test result.

Claims (4)

In mass%, C: 0.16 to 0.30%, Si: less than 0.40%, Mn: 0.50% or more, P: 0.015% or less, Cu: 0.30% or less, Cr: 2 Less than 0.000%, Mo: 0.16-0.50%, Al: 0.060-0.110%, B: 0.0004% or more, N: 0.0065% or less, (1) formula The effective content of the hardened layer depth E, internal hardness D, and grain boundary oxide layer depth P obtained after carburizing and tempering is as follows. Steel for gears with excellent impact fatigue resistance that satisfies the requirements.
I (= 14/27 × Al + 14 / 10.8 × B−N) ≧ 0.03 (1)
However, Al, B, and N in the formula (1) indicate the content (mass%).
170 ≦ S (= E / 2 × DP / 2) ≦ 250 (2)
However, E: Effective hardened layer depth (mm), D: Internal hardness (HV), P: Grain boundary oxide layer depth (micrometer) is shown.   The impact fatigue resistance according to claim 1, further comprising adding at least one of mass%, Ti: less than 0.020%, Nb: 0.050% or less, and V: 0.030 to 0.200%. Excellent gear steel.   A gear excellent in impact fatigue resistance, obtained by subjecting the steel material according to claim 1 or 2 to a gear shape by machining or forging after machining or carburizing and quenching.   A gear excellent in impact fatigue resistance, which is formed into a gear shape by the process of claim 3 and further subjected to shot peening or polishing on the tooth surface.

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