JP2008179848A - Steel for gear having superior impact fatigue resistance and contact fatigue strength, and gear using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide steel for a carburized gear suitable for use in automobiles and various types of industrial machines, which is required to have excellent impact fatigue characteristics and superior contact fatigue strength, and to provide a gear using the same. <P>SOLUTION: The steel has a composition which comprises, by mass%, 0.15 to 0.30% C, more than 0.50 but less than 1.50% Si, 0.50% or more Mn, 0.015% or less P, 0.30% or less Cu, less than 2.00% Cr, less than 0.50% Mo, 0.006 to 0.110% Al, less than 0.02% Ti, 0.0004% or more B, 0.0065% or less N, at least one element of Nb and V, as needed, while satisfying the expression (1): I(=14/27×Al+14/10.8×B-N)≥0.03, wherein Al, B and N represent contents (mass%), and the balance Fe with unavoidable impurities. Furthermore, the steel has such an S value after having been carburized, quenched and tempered as to satisfy the expression (2); 170≤S=E/2xD-P/2≤250, wherein (E) represents the effective depth (mm) of a hardened layer, (D) represents the hardness (HV) of the inner part and (P) represents the depth (μm) of an oxidized layer in a grain boundary. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel for gears and a gear using the same, and particularly relates to a steel suitable for automobiles and various industrial machines that require excellent impact fatigue resistance and surface fatigue strength.

  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, since the gear is shaped by hot forging, the hardness is low, and 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 obtain excellent impact fatigue strength and surface fatigue strength without deteriorating the characteristics as a gear, and a gear using the same.

  In order to solve the above-mentioned problems, the present inventors have earnestly studied a steel for gears having a low-cost component composition that can provide excellent impact fatigue strength and surface fatigue strength by carburizing by a conventional method, which is inexpensive as a production cost. Study was carried out.

As a result, in the case of gears, in addition to improving toughness against repeated impact stresses, it is necessary to have a proof strength that does not cause deformation due to impact force. When the proof strength is low, even if toughness is improved, actual durability Was found to be inferior, and the following component design guidelines were obtained.
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. When solid solution B is present in the steel, N, which has a strong binding force with B, is bonded with Ti or Al. However, TiN that precipitates during melting is relatively large, sharp and hard inclusions, It tends to be a starting point, and 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. Therefore, the solid solution B is secured mainly by adjusting the Al—B—N balance, and subordinated by adding Ti.
5. There is an optimum range for the amount of Si that suppresses grain boundary oxidation during carburizing.
6). In order to improve both surface fatigue strength and impact fatigue strength, the balance between grain boundary oxidation and internal hardness and carburized hardness distribution is important, and the most optimal range exists.

The present invention has been made by further studying the obtained knowledge, that is, the present invention,
1. In mass%, C: 0.15 to 0.30%, Si: more than 0.50% to less than 1.50%, Mn: 0.50% or more, P: 0.015% or less, Cu: 0.30 %: Cr: 2.00% or less, Mo: 0.50% or less, Al: 0.006 to 0.110% or less, Ti: less than 0.02%, B: 0.0004% or more, N: 0 .0065% or less, the balance satisfying the formula (1) has a composition composed of Fe and inevitable impurities, and the effective hardened layer depth, internal hardness, and grain boundary oxidation obtained after carburizing and tempering Gear steel with excellent impact fatigue resistance and surface fatigue strength that satisfies the S value of equation (2) for layer depth.
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. Further, the gear steel excellent in impact fatigue resistance and surface fatigue strength according to 1, wherein any one or more of Nb: 0.050% or less and V: 0.030-0.200% is added.
3. A gear excellent in impact fatigue resistance and surface fatigue strength, obtained by machining the steel material described in 1 or 2 after machining or forging to obtain a gear shape and then carburizing and quenching.
A gear having excellent impact fatigue resistance and surface fatigue strength, 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 excellent impact fatigue strength and surface fatigue strength and a bending fatigue strength equivalent to that of conventional steel 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: 0.15-0.30%
C is necessary for securing the strength, and its amount determines the internal hardness. If the amount is less than 0.15%, the surface hardness decreases to 300 HV or less, and the strength as a gear cannot be secured. On the other hand, if it is more than 0.30%, the material becomes excessively hard, the workability is lowered, and the production cost is increased, so the content is made 0.15% or more and 0.30% or less.

Si: more than 0.50% to 1.50%
Si is an easily oxidizable element that promotes oxidation at the grain boundaries on the carburized surface and tends to form an incompletely hardened layer. However, if it exceeds 0.50%, surface oxidation occurs not only at the grain boundaries. Oxidation proceeds even inside the crystal grains and is uniformly oxidized as a whole.

  For this reason, the generation and propagation of cracks due to the notch effect of the brittle grain boundary oxide layer is reduced, and the uniform oxide layer improves the conformability of the gear. Moreover, since the temper softening resistance is improved, the surface fatigue strength is improved.

  However, when the amount is 1.50% or more, the effect is saturated, and further addition only increases the alloy cost. Therefore, the content is set to exceed 0.50 to 1.50%.

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 amount of Mn added is limited to 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: 2.00% or less Cr is an element improving the temper softening resistance as well as a hardenability improving element. However, when the added amount exceeds 2.00%, the effect of increasing the softening resistance is saturated, and the hardenability becomes too high, so the toughness inside the gear is deteriorated, not only the impact value is lowered but also the bending fatigue strength is lowered. To do. Therefore, the Cr addition amount is set to 2.00% or less.

Mo: 0.50% or less Mo is an element effective for improving the hardenability. The effect becomes higher as the addition amount increases, but is 0.50% or less because of an expensive element.

Al: 0.006 to 0.110%
Al is added for securing solid solution B and deoxidation. The effect of deoxidation is not obtained if the addition amount is less than 0.006%, and if too much is added, there is a risk of casting abnormalities at the time of melting, so 0.006 to 0.110%.

Ti: It is necessary to combine N with other elements in order to secure a solid solution B of less than 0.02%. Ti is an effective element because it has a stronger bonding force with N than B, and is added so as to complement the effect of the Al-B-N balance described later. However, when 0.02% or more is added, coarse TiN is generated and tends to become a starting point of fatigue cracks, so that the fatigue strength decreases. Therefore, Ti is made less than 0.02%.

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 is Al, B for segregating at the grain boundaries to ensure solid solution B that improves impact fatigue strength. , N balance index, and if this parameter value is less than 0.03, B cannot be secured in the steel, so it is set to 0.03 or more.

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

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 limits to 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 is obtained at 0.030% or more, but when it exceeds 0.200%, it is saturated, so when V is added, the content is made 0.030 to 0.200%.

  The contents of S and oxygen as unavoidable impurities are preferably 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 roller pitting (surface fatigue) characteristics, and is set to 170 or more and 250 or less. No. in Table 1 Create an impact fatigue test piece and a roller pitching test piece using 1 to 3 round steel bars, carburize and temper under various conditions, and have different grain boundary oxide layer depth, internal hardness, and effective hardened layer depth. An impact fatigue test piece was prepared, and an impact fatigue test and a roller pitching test were performed. The results are shown in Table 2 and FIG.

  Impact fatigue test results and roller pitting (surface fatigue) characteristics are as follows: E: effective hardened layer depth (mm), D: internal hardness (HV), P: grain boundary oxide layer depth (μm) parameter S (= E / 2 × DP / 2), and when the parameter S value is 170 or more and 250 or less, an excellent impact fatigue strength of 2.5 N · m or more is obtained, and 150 or more and 300 or less. It has been found that excellent surface fatigue strength of 3000 MPa or more can be obtained.

  That is, if the value of the parameter S is 170 or more and 250 or less, both the impact fatigue strength and the surface fatigue strength are improved, but if out of the range, either or both of the impact fatigue strength and the surface fatigue strength are reduced. .

  When the effective hardened layer depth is 0.80 mm or more, the grain boundary oxide layer is 13 μm or more, and the internal hardness is 424 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. No. in the table. Nos. 1 to 16 are steels of the present invention, which have a component composition within the scope of the present invention. Reference numerals 17 to 27 are comparative steels having a component composition outside the scope of the present invention. No. 28 is JISSCM822 case hardening steel which is a conventional 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, a roller pitching 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. 2, investigation of surface hardness, internal hardness, effective hardened layer depth, impact fatigue test and rotational bending fatigue test Carried out. The details of each survey are described below.

Intergranular oxide layer depth, effective hardened layer depth, internal hardness investigation invention steel, comparative steel and conventional steel 20φ round bar, carburizing quenching and tempering, then cutting, maximum grain boundary oxide layer depth The thickness 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 was set as “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 pieces shown in FIG. 3 were prepared from each round steel bar having a diameter of 32 mm, carburized and tempered under the conditions shown in FIG. The impact energy destroyed at 200 times was investigated.

Test specimens having a parallel part diameter of 26 mm shown in FIG. 4 were collected from a round steel bar having a surface fatigue characteristic of 32 mm in diameter and subjected to carburizing and tempering under the conditions shown in FIG. A roller pitching fatigue test was performed using these test pieces, and the surface fatigue strength was evaluated with 10 ⁷ times as the fatigue limit.

  Table 4 shows the survey results. The steel according to the present invention has an effective hardened layer depth of 0.90 mm or more, a grain boundary oxide layer of 10 μm or more, an internal hardness of 417 Hv or more, a surface fatigue strength of 3000 MPa or more, and an impact fatigue strength of 2.7 N · m or more. Comparative steel No. 17-No. 27 and the conventional steel (JISSCM822 case-hardened steel) (No. 28 conventional steel (JISSCM822 case-hardened steel) had surface fatigue strength of 2700 MPa and impact fatigue strength of 2.0 N · m).

  That is, comparative steel No. No. 17 had an internal hardness that was too low because the C content was lower than the range of the present invention. As a result, the surface fatigue strength decreased.

  Comparative steel No. No. 18 has a deep grain boundary oxide layer because the Si content is lower than the range of the present invention. Therefore, impact fatigue strength and surface fatigue strength were reduced.

  Comparative steel No. No. 19 has a Mn content lower than the range of the present invention, so that the hardenability is too low. Therefore, since the effective hardened layer depth was shallow and the internal hardness was low, impact fatigue strength and surface fatigue strength were reduced.

  Comparative steel No. No. 20 had a P content higher than the range of the present invention, so the grain boundary strength was insufficient, and the impact fatigue strength was lowered.

  Comparative steel No. In No. 21, the Cu content was higher than the range of the present invention, so that the processing accuracy of the test piece was deteriorated. Therefore, both impact fatigue strength and surface fatigue strength decreased.

  Comparative steel No. Since the Cr content is higher than the range of the present invention, the hardenability is too high. As a result, the 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 surface 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 action was not obtained, the effective hardened layer depth became shallow, and the surface fatigue strength and the impact fatigue strength decreased.

  Comparative steel No. In No. 25, since the Ti content was higher than the range of the present invention, a large amount of coarse fatigue failure due to TiN occurred in the roller pitting fatigue test and the impact fatigue test. Therefore, surface fatigue strength and 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 became shallow, and the impact fatigue strength decreased.

  Comparative steel No. 27 has an I value lower than the range of the present invention. As a result, the solid solution B could not be secured, the effective hardened layer depth became shallow, and the internal hardness was too low, so that the impact fatigue strength and the surface fatigue strength were lowered.

The figure which shows the influence of S (= E / 2xDP / 2) value which exerts on an impact fatigue test result and roller pitting (surface fatigue) characteristic. 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 a roller pitching fatigue test piece.

Claims (4)

In mass%, C: 0.15 to 0.30%, Si: more than 0.50% to less than 1.50%, Mn: 0.50% or more, P: 0.015% or less, Cu: 0.30 %: Cr: 2.00% or less, Mo: 0.50% or less, Al: 0.006 to 0.110% or less, Ti: less than 0.02%, B: 0.0004% or more, N: 0 .0065% or less, the balance satisfying the formula (1) has a composition composed of Fe and inevitable impurities, and the effective hardened layer depth, internal hardness, and grain boundary oxidation obtained after carburizing and tempering Gear steel with excellent impact fatigue resistance and surface fatigue strength that satisfies the S value of equation (2) for layer depth.
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.   Furthermore, the steel for gears excellent in the impact fatigue resistance property and surface fatigue strength of Claim 1 which adds any 1 or more of Nb: 0.050% or less and V: 0.030-0.200%.   A gear excellent in impact fatigue resistance and surface fatigue strength obtained by machining the steel material according to claim 1 or 2 after machining or forging to obtain a gear shape and then carburizing and quenching.   A gear excellent in impact fatigue resistance and surface fatigue strength obtained by forming a gear shape by the process of claim 3 and further subjecting the tooth surface to shot peening or polishing.

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