JP4116787B2 - Steel member - Google Patents

Description

【００４１】
【表２】

【００４２】
【表３】

【００４３】
【表４】

【００４４】
【表５】

【００４５】
表１，２において、Ｎｏ．１〜１８は本発明の規定要件を満たす実施例であり、回転曲げ疲労強度が８５０ＭＰａ以上と高く、ピッチング寿命も長寿命を示している。更に、適量の快削性元素を添加したＮｏ．２，４，６，８，９，１０，１４，１５，１８は優れた快削性を示している。
【００４６】
これらに対し、表３，４におけるＮｏ．１９〜２７は、本発明で定める何れかの要件を欠く比較例であり、Ｎｏ．１９，２１，２２，２５〜２７は、直径０．５μｍ以下の炭化物の面密度が６．０個／１０μｍ²未満で、全炭化物数に占める直径０．５μｍ以下の炭化物数の割合が８０％未満であるため、粗大な炭化物の存在によって回転曲げ疲労強度が劣り、また直径０．５μｍ以下の炭化物の面密度が低いためピッチング寿命が短い。
【００４７】
またＮｏ．２３では、Ｍｏ量が少ないため炭化物数不足となって表層が硬度不足となり、ピッチング寿命が短くなる他、回転曲げ疲労強度も乏しい。Ｎｏ．２１は、表層に不完全焼入れ組織が生成しており、微細炭化物の数も割合も少ないためピッチング寿命が短く、回転曲げ疲労強度が乏しい。
【００４８】
Ｎｏ．２０，２２，２４では、Ｓｉ量、Ｃｒ量またはＭｏ量が多過ぎるためか、焼ならし後の硬さが高く、切削性および冷鍛加工性が悪くて鋼部材としての適性を欠く。
【００４９】
【発明の効果】
本発明は以上の様に構成されており、心部の化学成分を規定すると共に、浸炭焼入・焼戻し処理後におけるオーステナイト結晶粒度番号、表層部の微細炭化物の面密度と微細炭化物数の割合を特定することによって、高い曲げ疲労強度を維持しつつ優れたピッチング寿命を示す機械構造用鋼部品や、優れた耐摩耗性を発揮する工具として使用できる鋼部材を提供し得ることになった。
【００５０】
特に、上記鋼部材を、高度の耐ピッチング性と疲労強度の要求される歯車や軸類に使用することで、該歯車や軸類の耐久性を著しく高めることができた。
【図面の簡単な説明】
【図１】性能評価試験に用いたローラピッチング試験片の寸法・形状を示す図である。
【図２】実験で採用した高濃度浸炭処理パターンを示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel member, and more specifically, a steel part for mechanical structure having excellent pitting resistance and fatigue strength such as gears, sliding parts, shafts, etc. used at high surface pressure, The present invention relates to a steel member used as a tool or the like having excellent wear characteristics.
[0002]
Below, the case where it uses as a steel part for machine structures among steel members is demonstrated mainly.
[0003]
[Prior art]
Machine structural steel parts are widely used as power transmission parts in, for example, automobiles, construction machines, and industrial machines. As steel materials used for such mechanical structural steel parts, Cr case-hardened steel, Cr-Mo case-hardened steel, Ni-Cr-Mo case-hardened steel, which have been defined in JLS G4104, G4105, G4103, etc. There have been used steel parts for machine structural use that have been subjected to surface hardening treatment such as carburizing treatment or carbonitriding treatment after molding these steels.
[0004]
However, in recent years, as the demand for higher stress and smaller and lighter parts of automobiles, construction machinery, industrial machinery, etc. has increased, load stresses such as gears for power transmission have been increasing. Conventional steels for machine structural use and surface-hardened steels are difficult to adapt to such harsh usage environments.
[0005]
In view of such a situation, in order to suppress peeling damage on the contact surface accompanying increase in contact surface pressure, that is, pitting damage, the hardness of the surface layer portion is increased by increasing the C concentration of the surface layer portion and finely depositing carbides. For example, a high concentration carburizing method, a CD (Carbide Dispersion) carburizing method, a hypereutectoid carburizing method and the like are employed. JP-A-6-158266 discloses C: 0.10 to 0.3%, Si: less than 1.0%, Mn: 0.3 to 1.5%, P: less than 0.020%, Cr : Over 1.50%, and the mass ratio satisfies “4.5 <(8 · Si + 3Cr) <13.5”, Al: 0.010 to 0.050%, N: 0.005 to 0.00. A method of manufacturing a high surface pressure component is disclosed in which a steel material composed of 025% and the balance Fe is carburized so that the surface carbon concentration is 0.7 to 1.2%, and then quenched and tempered.
[0006]
In JP-A-6-25823, C: 0.05 to 0.3%, Si: 0.05 to 2%, Mn: 0.3 to 2%, Cr: 2 to 8%, S: 0 0.03% or less, A1: 0.015 to 0.06%, N: 0.005 to 0.02%, the balance being Fe and inevitable impurities, P (phosphorus) in the inevitable impurities being 0.02 %, And O (oxygen) is controlled to 0.002% or less, and parts made of the steel material are subjected to carburizing or carbonitriding treatment and quenching / tempering treatment. A carburized steel part on which carbide or carbonitride having an average particle size of 5 μm or less is deposited is disclosed.
[0007]
However, the steel parts for mechanical structures manufactured by the methods disclosed in JP-A-6-158266 and JP-A-6-25823 have a high C content and a large number of coarse carbides having a diameter of about several μm are precipitated. Therefore, although the pitting resistance can be satisfied, there is a problem that the bending fatigue strength is inferior.
[0008]
[Problems to be solved by the invention]
The present invention has been made by paying attention to the above-mentioned circumstances, and its purpose is to provide wear resistance in addition to steel parts for mechanical structures that can enhance the pitting resistance without impairing the bending fatigue characteristics. It is an object of the present invention to provide a steel member that can be used as an excellent tool.
[0009]
[Means for Solving the Problems]
The steel member according to the present invention, which has been able to solve the above problems, has C: 0.10 to 0.30%, Si: 0.35 to 1.0%, Mn: 0.20 to 1. 0%, Cr: 1.0 to 1.31 %, Mo: 0.05 to 0.6%, and A1: 0.005 to 0.05%, Nb: 0.005 to 0.05%, Ti: one or two or more elements selected from the group consisting of 0.005 to 0.1%, N: 0.008 to 0.05%, the balance being iron and inevitable impurities, Carbide having an austenite grain size number of 8.5 or more after charcoal quenching and tempering and a diameter of 0.5 μm or less in the surface layer within 50 μm from the surface when observed with a scanning electron microscope at a magnification of 8000 times The surface density is 6.0 pieces / 10 μm ² or more, and the diameter occupies the total number of carbides. It has a gist in that the ratio of the number of carbides of 0.5 μm or less is 80% or more, and the amount of retained austenite at a depth of 50 μm from the steel sheet surface is 20% or less (excluding 0%). This steel member has a surface hardness of HV800 or higher.
[0010]
When this steel member contains one or more elements selected from the group consisting of Zr: 0.01 to 0.15% and Pb: 0.09% or less , excellent machinability is achieved. Is also preferable because it can be added together.
[0011]
Note that the above “center” refers to the inside of steel in which the carbon concentration distribution that is not reached by the carburized carbon from the surface is substantially constant.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Under the above-mentioned problems, the inventors have repeatedly studied for the development of steel parts for machine structures with improved pitting resistance without harming the steel members, particularly bending fatigue characteristics. In addition to prescribing the chemical composition of the material, if a large amount of fine carbides are deposited on the surface layer part from the surface to 50 μm and the surface layer hardness is increased to HV800 or more, the pitting resistance as a steel part for machine structures is greatly improved. I found out. However, in order to ensure the high level of fatigue strength required for machine parts, the size and number of carbides in the surface layer portion within 50 μm from the surface must be strictly controlled, and the carbide has a diameter of 0.5 μm or less. With a surface density of 6.0 pieces / 10 μm ² or more and a ratio of the number of carbides with a diameter of 0.5 μm or less in the total number of carbides of 80% or more is the required characteristics described above as steel parts for machine structures Thus, the present invention has been conceived.
[0013]
Hereinafter, the reason for limiting each numerical value defined in the present invention will be described in detail. First, the surface density of the carbide in the surface layer portion from the surface to 50 μm, the surface density of 0.5 μm or less must be 6.0 pieces / 10 μm ² or more, and the surface density is 6.0 pieces / If it is less than 10 μm ² , the level of pitting resistance intended by the present invention cannot be obtained. In addition, the proportion of carbides having a diameter of 0.5 μm or less in the total number of carbides must be 80% or more, and if it is less than 80%, carbides having a diameter of more than 20 μm and exceeding 20% are the origin of destruction. This adversely affects the bending fatigue strength of mechanical structural steel parts. However, if the number of carbides having a diameter of more than 0.5 μm is less than 20% in the total number of carbides, the effect on fatigue strength is small, and there is no practical problem.
[0014]
The surface density is more preferably 8.0 pieces / 10 μm ² or more, more preferably 10.0 pieces / 10 μm ² or more, and a diameter of 0.5 μm, in order to ensure excellent pitting resistance without deteriorating bending fatigue characteristics. A more preferable ratio of the following carbides in the total number of carbides is 90% or more, and more preferably 95% or more.
[0015]
Furthermore, in the present invention, in addition to the above-mentioned requirements, it is necessary to use a steel material having an appropriate chemical component in the core, and by combining them, the hardness of the surface layer is ensured to be HV800 or more, and excellent pitting resistance It is possible to satisfy both the properties and the bending fatigue strength. Hereinafter, the reason for limitation of each component is demonstrated about the chemical component of the steel materials defined by this invention.
[0016]
C: 0.10 to 0.30%
C is an element indispensable for securing the core hardness required as a steel member. If it is less than 0.10%, the core hardness is insufficient, and the strength is insufficient for steel parts and tools for machine structures. . However, if the amount of C becomes too large, the core hardness becomes excessively high and cold forging workability and machinability deteriorate, so it must be suppressed to 0.30% or less. From such a viewpoint, the more preferable lower limit of C is 0.15%, and the more preferable upper limit is 0.25%.
[0017]
Si: 0.15-1.0%
Si is one of the most important elements in the present invention, and has an effect of refining carbides precipitated in the surface layer portion of the steel material. When the steel material of the present invention is carburized, it is fine in the surface layer portion. This is an important requirement for precipitating a large number of carbides and causing carbides having a diameter of 0.5 μm or less to be present in an area density of 6.0 / 10 μm ² or more. In addition, an appropriate amount of Si strengthens the steel matrix and also exhibits the effect of further increasing the hardness of the surface layer. If the Si content in the steel is less than 0.15%, such an effect is not sufficiently exerted, the surface carbide is coarsened and the surface density is decreased, and a satisfactory surface hardness cannot be obtained. On the other hand, if the amount of Si exceeds 1.0%, the hardness after spheroidizing annealing becomes too high when producing a steel member, and the machinability and cold forgeability become poor. From such a viewpoint, the more preferable lower limit of Si is 0.35%, and the more preferable upper limit is 0.6%.
[0018]
Mn: 0.20 to 1.0%
Mn acts as a deoxidizer, reduces the amount of oxide inclusions and improves the internal quality of the member, and also works effectively to ensure the effective hardened layer depth and core hardness. In order to exert such an action effectively, Mn should be contained 0.20% or more, more preferably 0.25% or more. If it is less than 0.20%, the effective hardened layer depth and the core hardness are insufficient. Therefore, sufficient fatigue strength cannot be obtained. On the other hand, when Mn exceeds 1.0%, the amount of retained austenite in the surface layer portion becomes excessively large, and the surface layer portion becomes insufficient in hardness. From such a viewpoint, the more preferable content rate of Mn is 0.25% or more and 0.60% or less.
[0019]
Cr: 1.0-2.0%
Cr is also an important element in the present invention, and by adding an appropriate amount of Cr, it becomes possible to promote precipitation of carbides during carburizing. By the way, if the Cr content is less than 1.0%, the above effect is not exhibited effectively, and the hardenability of the steel member is also reduced, and the required effective hardened layer depth and core hardness are insufficient. Fatigue strength deteriorates. However, if over 2.0% and Cr is excessively contained, the core hardness becomes too high, resulting in poor machinability. A more preferable lower limit of Cr is 1.25%, and a more preferable upper limit is 1.50%.
[0020]
Mo: 0.05-0.6%
Mo is also an important element and plays an important role in increasing the precipitation of carbides in the carburized layer in the surface layer portion, increasing the hardenability of the steel matrix, and hardening the surface layer portion. Incidentally, if Mo is less than 0.05%, not only the carbide is insufficiently generated, but also an incompletely hardened structure is generated in the surface layer portion, which causes a deterioration in fatigue strength. On the other hand, if the Mo content exceeds 0.6%, the steel member becomes excessively hard, and the machinability and cold workability become poor. A more preferable content of Mo is 0.15% or more and 0.5% or less.
[0021]
By the way, in the present invention, as described above, the surface density of the carbide having a diameter of 0.5 μm or less in the surface layer portion within 50 μm from the surface is 6.0 / 10 μm ² or more, and the diameter accounting for the total number of carbides is 0.5 μm. In order to precipitate carbide so that the ratio of the following number of carbides is 80% or more, it is essential to secure 8.5 or more in the austenite grain size number of steel, which is a machine part, after carburizing and tempering. It becomes a requirement. This is because one of the precipitation sites where carbon that excessively penetrates during carburization precipitates as carbide is an austenite grain boundary, and by increasing the austenite crystal grain boundary by refining the crystal grain, This is because a large number of carbides can be precipitated.
[0022]
By the way, when the austenite grain size number is less than 8.5, since there are few carbide precipitation sites, a large number of coarse carbides with a diameter of more than 0.5 μm are precipitated, and the surface density of carbides with a diameter of 0.5 μm or less is reduced, resulting in pitting resistance. And bending fatigue properties are inferior.
[0023]
A more preferable austenite grain size number is 9.0 or more, and even more preferably 9.5 or more, in order to more effectively exhibit the effect of such an increase in precipitation sites.
[0024]
Thus, in order to obtain a steel with a large austenite grain size number in the core structure, it is necessary to suppress the growth of austenite grains in the heating process for carburizing. As a specific method for that purpose, One or two or more elements selected from the group consisting of: Al: 0.005-0.05%, Nb: 0.005-0.05%, Ti: 0.005-0.1%; N: The method of making 0.008-0.05% exist is mentioned. That is, by containing these elements, precipitation of AlN, NbN, TiN, etc. generated in the steel can be used to suppress the growth of austenite crystal grains during carburizing heating.
[0025]
If the amount of A1 in the steel is less than 0.005%, the amount of Nb is less than 0.005% or the amount of Ti is less than 0.005%, the effect of suppressing the coarsening of crystal grains is not effectively exhibited, and the core structure The austenite grain size number cannot be increased to 8.5 or more. On the other hand, when the Al content exceeds 0.05%, the Nb content exceeds 0.05%, or the Ti content exceeds 0.1%, the above-mentioned effects are not only saturated, but also alumina inclusions and TiN. The formation of this causes problems such as a decrease in bending fatigue strength and a decrease in cold workability.
[0026]
Further, in the present invention, in addition to the above elements, Ca: 0.0005 to 0.05%, Zr: 0.01 to 0.15%, Bi: 0.05% or less, S: 0.12% or less, If one or more elements selected from the group consisting of Pb: 0.09% or less and Mg: 0.02% or less are included, the machinability when producing a steel member can be greatly improved. preferable. Incidentally, if the content of these selected elements is less than the lower limit, the above-mentioned effects are not fully exhibited, while the above-mentioned effects of each element are saturated near the upper limit, and rather a large amount of coarse composite inclusions are generated and bent. It may cause troubles such as deterioration of fatigue strength and pitting resistance.
[0027]
Furthermore, in the present invention, the amount of retained austenite in the surface layer portion is preferably 5% or more and 35% or less, more preferably 10% or more and 20% or less. The reason for this is that if the amount of retained austenite is less than 5%, the toughness will be lowered and the compatibility with other parts when used as a machine structural part will deteriorate, causing impact fatigue failure and noise, and conversely 35%. If it exceeds, the surface hardness is lowered and the pitting resistance becomes insufficient.
[0028]
Although it is a surface hardening treatment method adopted when obtaining the steel member of the present invention, it is desirable to adopt high-concentration carburization or CD carburization in order to finely precipitate carbides.
[0029]
As a specific method of the surface curing treatment method, for example,
(1) Carburize by holding for about 8 hours at about 930 ° C in an atmosphere with a carbon potential of about 1.20% in the first step, and then hold at about 830 ° C for about 45 minutes in the second step after that. There is a method of reheating and quenching by oil quenching. In order to obtain a carbide precipitation form defined in the present invention, the surface layer is maintained at 930 ° C. for 2 hours in an atmosphere having a carbon potential of 1.15%. To 50 μm position until the average carbon concentration becomes 0.9%, then gas cooling and cooling to near room temperature, then heating at a rate of 25 ° C./min or more and again at 850 ° C. As a more specific example, there is a method in which after holding in a 0.85% atmosphere for 1 hour, quenching with cold oil and finally tempering. Other,
(2) A surface hardening treatment method that can be used in such a manner that a process of heating to about 1000 ° C. in an atmosphere having a carbon potential of about 1.30%, then cooling to about 700 ° C. and then heating to about 880 ° C. is repeated. One of them.
[0030]
As described above, the steel member of the present invention has high hardness by precipitating carbide on the surface of the member, and is excellent in wear resistance. Therefore, tools such as a hexagon wrench holder, a concrete drill holder, a forging die, etc. It can also be used as
[0031]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, but may be implemented with appropriate modifications within a range that can meet the purpose described above and below. Any of these may be included in the technical scope of the present invention.
[0032]
Example 1
Steel materials having the composition shown in Tables 1 and 3 were melted in a small furnace, subjected to solution treatment and normalizing treatment after hot forging, and then machined to have a diameter of 10 mm × 130 mm (two pieces) and a diameter of 26 mm. It processed into the shape of a * 130mm round bar test piece, a rotation bending fatigue test piece, and a roller pitching test piece. FIG. 1 shows the shape of a roller pitching test piece. As the rotating bending fatigue test piece, a notched fatigue test piece having a shape factor of 2.0 was used. These test pieces were subjected to high-concentration carburizing treatment in the pattern shown in FIG. In FIG. 2, in the first stage, high-concentration carburization is performed by leaving it in an atmosphere having a carbon potential (Cp) of 1.2%. Further, in the second stage, reheating and quenching is performed, and fine carbides are formed in the carburized portion of the surface layer. And was tempered in the third stage.
[0033]
After completion of tempering, the presence or absence of an incompletely quenched structure, the number and size of carbides, and the measurement of the crystal grain size number were measured by using one round bar test piece having a diameter of 10 mm.
[0034]
(1) Presence / absence of incompletely hardened structure: After cutting and polishing a round bar test piece, SEM observation at a magnification of 8,000 times was performed at a 50 μm position from the surface to determine the presence or absence of incompletely hardened structure.
(2) Number and size of carbides: Image processing is performed on the above 8,000 times photograph to determine the number and size of carbides. In addition, the method of calculating | requiring the diameter of a carbide | carbonized_material calculates | requires as a diameter of the circle | round | yen which becomes the same area, after measuring the area | region of a carbide | carbonized_material.
[0035]
(3) Crystal grain size number: Measured according to J1S G0551.
[0036]
Furthermore, after subjecting the test piece after tempering to shot peening, the remainder of the round bar test piece having a diameter of 10 mm is taken out and the residual austenite and hardness are measured. The roller pitching test was subjected to finish polishing, the rotary bending fatigue test was used as it was, and the hardness after normalization was measured under the following conditions.
[0037]
The retained austenite was measured at a position of 50 μm depth from the surface by X-ray diffraction, and the hardness was measured at a position of 50 μm from the surface and a central position in the diameter direction in the cut section. The rotational bending fatigue test was performed at a rotational speed of 3,600 rpm, and the stress that did not break even after reaching 10 ⁷ times was determined as the bending fatigue strength. The following conditions were adopted for the roller pitching test, and the pitching life was determined when the pitching damage occurred.
[0038]
[Roller pitching test conditions]
Surface pressure: 3.7 GPa, rotation speed: 1,500 rpm, slip rate: −40%, oil temperature: 80 ° C., mating roller: JlS G4805 high carbon chrome bearing steel SUJ2
[Hardness test after normalization]
Conducted in accordance with JIS Z2244, test load is 98N.
[0039]
The test results are shown in Tables 2, 4 and 5.
[0040]
[Table 1]

[0041]
[Table 2]

[0042]
[Table 3]

[0043]
[Table 4]

[0044]
[Table 5]

[0045]
In Tables 1 and 2, no. Examples 1 to 18 are examples that satisfy the prescribed requirements of the present invention. The rotational bending fatigue strength is as high as 850 MPa or more, and the pitching life is also long. Furthermore, no. 2, 4, 6, 8, 9, 10, 14, 15, 18 show excellent free-cutting properties.
[0046]
On the other hand, No. Nos. 19 to 27 are comparative examples lacking any requirement defined in the present invention. 19, 21, 22, 25 to 27 have a surface density of carbides of 0.5 μm or less in diameter of less than 6.0 / 10 μm ² , and the ratio of the number of carbides of 0.5 μm or less in diameter to the total number of carbides is 80%. Therefore, the rotational bending fatigue strength is inferior due to the presence of coarse carbides, and the surface density of carbides having a diameter of 0.5 μm or less is low, so that the pitching life is short.
[0047]
No. In No. 23, since the amount of Mo is small, the number of carbides is insufficient, the surface layer becomes insufficient in hardness, the pitching life is shortened, and the rotational bending fatigue strength is also poor. No. No. 21 has an incompletely quenched structure on the surface layer, and since the number and proportion of fine carbides are small, the pitching life is short and the rotational bending fatigue strength is poor.
[0048]
No. In 20, 22, and 24, the amount of Si, Cr, or Mo is excessive, or the hardness after normalization is high, and the machinability and cold forging workability are poor, so that the steel member is not suitable.
[0049]
【The invention's effect】
The present invention is configured as described above, defines the chemical components of the core, and determines the ratio of the austenite grain size number after carburizing and tempering, the surface density of fine carbides and the number of fine carbides in the surface layer part. By specifying, it has become possible to provide a steel member for mechanical structure that exhibits excellent pitching life while maintaining high bending fatigue strength, and a steel member that can be used as a tool that exhibits excellent wear resistance.
[0050]
In particular, the durability of the gears and shafts can be remarkably enhanced by using the steel member for gears and shafts that require high pitting resistance and fatigue strength.
[Brief description of the drawings]
FIG. 1 is a diagram showing dimensions and shapes of roller pitching test pieces used in a performance evaluation test.
FIG. 2 is a diagram showing a high-concentration carburizing treatment pattern employed in the experiment.

Claims (3)

The core is C: 0.10 to 0.30% (meaning mass% in the case of chemical components, the same shall apply hereinafter), Si: 0.35 to 1.0%, Mn: 0.20 to 1.0%, Cr: 1.0 to 1.31 %, Mo: 0.05 to 0.6%, and A1: 0.005 to 0.05%, Nb: 0.005 to 0.05%, Ti: One or two or more elements selected from the group consisting of 0.005 to 0.1% and N: 0.008 to 0.05%, the balance being iron and inevitable impurities, and carburizing The austenite grain size number after quenching and tempering is 8.5 or more, and the surface density of carbides having a diameter of 0.5 μm or less in the surface layer within 50 μm from the surface when observed with a scanning electron microscope at a magnification of 8000 times in There 6.0 pieces / 10 [mu] m ² or more, carbonization of the following diameters 0.5μm to total all the carbides And the ratio of the number of 80% or more, the steel member characterized by the amount of residual austenite at a depth of 50μm from the further surface of the steel sheet is 20% or less (not including 0%). The steel according to claim 1, wherein the core contains one or more elements selected from the group consisting of Zr: 0.01 to 0.15% and Pb: 0.09% or less as other elements. Element.   The steel member according to claim 1, wherein the steel member is a machine structural component.

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