JP2012052189A - Manufacturing method of machine part formed of steel material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a machine part formed of a steel material which can impart high rigidity and high toughness only to a prescribed region by using inexpensive low-carbon steel as the steel material, in the manufacturing method of the machine part which is manufactured by using the steel material (steel plate) through plastic processing and quenching processing.SOLUTION: The manufacturing method of the machine part manufactures a steel material processed product (workpiece) formed of the low-carbon steel material into a prescribed part shape by plastic processing through a quenching process. The method includes (1) a nitriding process which is applied to the workpiece, and obtains a nitriding layer at not lower than 1.5 mass% of a nitrogen concentration whose depth is 10 μm by using a nitrogen gas, and at not lower than 0.10 mass% of a nitrogen concentration whose depth is 50 μm, and (2) a high-frequency quenching process which makes the workpiece after applied with the nitriding process martensitic by rapidly cooling it in a short time after making it autstenitic by heating only a necessary region by using a high frequency.

Description

  The present invention relates to a method for manufacturing a steel machine part. In particular, machine parts that require high hardness and toughness (toughness) such as gears, pulleys, sprockets, etc. are manufactured through plastic processing and / or machining such as press blanking of steel materials. It relates to a preferred invention.

  Here, the case where gears in transportation equipment and the like are manufactured from steel plates (steel materials) through plastic processing (blanking, etc.) and cutting processing will be described. However, when pulleys and sprockets are manufactured by plastic processing and cutting processing. In addition, the present invention is applicable.

  When manufacturing the above gears by blanking from a steel plate, conventionally, in order to give the required hardness to the meshing part of the gears after blanking, it is usually handled by conventional carburizing or carbonitriding Was.

  However, in carburizing and carbonitriding, grain boundary oxides are deposited on the surface of the workpiece (steel material) during carburizing, so the hardenability is reduced and the fatigue strength (fatigue strenngth) is improved by quenching. The effect is inhibited.

  In addition, although it does not affect the patentability of this invention, patent documents 1-3 etc. can be mentioned as a prior art document.

  Patent Document 1 states that “the surface layer does not have a nitrogen compound layer, is provided with a nitrogen diffusion layer in which nitrogen is dissolved at a concentration of 0.05 to 1.50% from the surface to a predetermined depth, and is subjected to quenching treatment. Nitrogen-hardened product (Claim 1 etc.) is disclosed in Patent Document 2 as “a steel rolling member that diffuses and penetrates one or more alloy elements from a surface layer and carburizes the surface layer. , Rolling members that have been subjected to quenching or quenching and tempering treatment after carburizing and / or nitriding treatment (Claim 1 and the like) are disclosed in Patent Document 3 as "cutting gear teeth into their final shapes. Then, the gear teeth are carburized, and the gears are gradually cooled or annealed, and only the surfaces of the gear teeth are heated and induction hardened "(summary, etc.), respectively.

  Further, Non-Patent Document 1 describes a paper titled “Gas Carbonitriding”, and Non-Patent Document 2 describes a paper titled “Basic Research on Induction Hardening of Soft Nitrided Steel”.

JP 2007-46088 A JP 2004-107709 A JP-A-11-343520

Takishima, “Gas carbonitriding”, published by Japan Heat Treatment Technology Association, Heat Treatment No.10, No.1, February 1969, p568-572. Takase et al., “Basic Research on Induction Hardening of Soft Nitrided Steel” published by Japan Heat Treatment Technology Association, Heat Treatment No.17, No.3, June 1977, p377-381.

  In view of the above, the present invention is a low cost, low cost, steel material in a manufacturing method of a machine part in which a steel material product (work) prepared from a steel material (steel plate) by plastic working and / or machining is subjected to quenching. Carbon steel can be used to provide high hardness and toughness only at specified parts, and when attaching machine parts, when it is caulked or welded as post-processing, steel machine parts that do not cause caulking cracks or fire cracks An object is to provide a manufacturing method.

  Therefore, as a result of diligent efforts to solve the above-mentioned problems, the present inventors have come up with a method for manufacturing a steel machine part having the following configuration.

A method of manufacturing a steel machine part that manufactures a steel product (work) prepared from low-carbon steel (carbon content less than 0.3% by mass) into a predetermined part shape by plastic working and / or machining. Because
A nitrogen concentration of 10 μm in depth: 1.5% by mass or less and a depth of 50 μm using a nitriding gas consisting essentially of a neutral gas and a nitriding agent in a sealed chamber with respect to the workpiece. Nitrogen concentration: a nitriding treatment step for obtaining a nitriding layer of 0.10% by mass or more, and
The work after the nitriding treatment step is induction-heated by induction heating only to a required portion to austenite, and then rapidly cooling to martensite without time,
It is characterized by including each process of these.

It is a flowchart for demonstrating the manufacturing method of the steel-made machine components of this invention. It is a phase diagram of the Fe-N system (revised in “Metal Data Book” Maruzen, 2004.2 edited by the Japan Institute of Metals). It is a plane dimension figure of the spur gear which is a test piece used for each example. It is a heating time chart of the nitriding process in Examples A and D of the present invention. It is a graph which shows the surface hardness (HV) and nitriding depth (micrometer) after the nitriding process in Examples A-F of the present invention. It is a graph which shows the surface hardness (HV) after induction hardening of two different conditions in Example AF of this invention, and the effective depth (micrometer) after one induction hardening.

  Hereinafter, modes for carrying out the present invention will be described. In the following description, “%” indicating a concentration or the like means “% by mass” unless otherwise specified.

  Highly conceptually, the present invention relates to a method of manufacturing a machine part in which a processed steel material (work) processed from a low carbon steel material (carbon content less than 0.3% by mass) into a predetermined part shape is subjected to quenching.

  Here, a case where a plate-like mechanical part (for example, a gear) is manufactured through blanking (plastic working) by using “SPCC” for cold rolled steel sheet as a low carbon steel material will be described as an example. The present invention can also be applied to machine parts other than gears using other low carbon steel sheets, and other steel materials such as strip steel, die steel, and steel bars. Further, the present invention can be applied not only to plastic working but also to machine parts (workpieces) formed by cutting / grinding machining.

  The outline of the method of the present invention is as shown in FIG. Hereinafter, each process will be described in detail.

(1) Workpiece preparation process:
A plate work is prepared by blanking a low carbon steel plate into a gear shape. After blanking, chamfering is usually performed by machining (cutting or grinding). Note that the workpiece may be prepared by machining from the beginning instead of blanking.

(2) Nitrogen treatment process:
Nitrogen treatment is performed on the entire surface of the plate-like workpiece.

  Nitrogen treatment conditions at this time are as follows: a nitrogen concentration of 10 μm in depth of 1.5% or less using a nitriding gas consisting essentially of a neutral gas and a nitriding agent in a sealed chamber with respect to the workpiece. Although it varies depending on the nitriding heating temperature, it is preferably 0.2 to 0.9%, and the nitrogen concentration at a depth of 50 μm is 0.10% or more (depending on the nitriding heating temperature, desirably 0.15 to 0.7%) nitriding layer can be obtained. It is more desirable that the nitrogen concentration at a depth of 100 μm be 0.5% (or 0.2%) or less.

If the nitrogen concentration at a depth of 10 μm is excessive, it is difficult to ensure surface hardness and toughness. During high-frequency heating, nitrogen atoms (N) dissolved in austenite become molecular nitrogen (N ₂ ), denitrifying (departing) through grain boundary diffusion or connected vacancies, and many voids are formed. It is estimated that it is easy to be done.

Nitrogen treatment conditions for ensuring the nitrogen concentration are substantially composed only of N ₂ (neutral gas) and NH ₃ (nitriding agent), and a nitriding agent mixing ratio: 20 to 50 vol% (desirably 30 to 45 vol). %) Composition. The heating conditions are usually selected from the range of 650 to 850 ° C. × 35 to 120 min, but the austenite region having a high N solid solubility limit, A ₁ transformation point: 727 ° C. or higher is desirable in the Fe—C phase diagram, Is preferably 740-840 ° C. × 60-100 min.

If the mixing ratio of the nitriding agent is too high, the temperature of the heating conditions is too low, or the time is too long, the nitrogen concentration tends to be excessive, and further, N atoms dissolved (invaded) in the austenite are associated. As a result, N molecules (N ₂ ) are formed, and voids are formed at the austenite grain boundaries, so that retained austenite (γ) is easily generated (resulting in a decrease in hardness).

  On the contrary, when the mixing ratio of the nitriding agent is too low, the temperature of the heating condition is too high, or the time is too short, the nitrogen concentration becomes too low, and it is difficult to ensure the required surface hardness.

  It has been confirmed that a lower nitriding agent mixing ratio is advantageous in terms of cost and that it is easy to stably obtain high hardness and high toughness on the surface side.

  In addition, after the nitriding treatment, prior to the high-frequency heating, when performing using a carburizing nitriding apparatus, a quenching treatment is usually performed, but it is not inevitable in view of distortion.

(3) Induction hardening process:
In this step, the workpiece after the nitriding treatment is heated at high frequency only at a required portion (tooth portion) to austenite to partially generate austenite, and then instantaneously cooled to martensite.

That is, the A ₁ line in the Fe—C phase diagram, which is the temperature at which partial austenitization is possible, is set appropriately at a temperature of 727 ° C. or higher, preferably in the range of 800 to 950 ° C.

  Specifically, high-frequency heating conditions: output 20 to 35 kW, heating and holding time 3.5 to 1.0 s, and rapid cooling conditions: 20 to 40 ° C. × 1 to 10 s.

When the high-frequency heating time is short, the N diffusion to the deep part is small, the decrease in the N concentration on the surface side is suppressed, and the amount of retained austenite (hereinafter “γ _R ”) tends to be relatively large. On the other hand, when the induction hardening (heating) time is long, N diffusion to the deep part is large, the surface side N concentration is lowered, and the surface side γ _R amount tends to be relatively small.

Upon rapid cooling in the induction hardening process, transformation from austenite (γ) to martensite occurs, and martensite transformation is accompanied by volume expansion. Therefore, gamma ratio martensitic transformation rate When it is large in _R is smaller compressive residual stress due to the small next martensite expansion, difficult to obtain a high hardness and high toughness steel mechanical parts. Conversely, gamma ratio of _R can be expected to improve the high hardness and high toughness in a certain the steel mechanical parts due to compressive residual stress becomes larger martensite transformation rate is due to the large next martensite expansion small.

  Thus, a machine part having a surface hardness (Vickers hardness: JIS Z 2244 (load 100 g), the same shall apply hereinafter) of 650 HV or higher, desirably 750 HV or higher, and even 800 HV or higher can be easily manufactured.

  The chemical components of “SPCC” are C: 0.12% or less, Mn: 0.50% or less, P: 0.040% or less, S: 0.045% or less (quoted from [JIS G 3141]).

  Examples carried out to confirm the effects of the present invention will be described.

  The test specimen used was a low-carbon steel plate (SPCC) (thickness: 2.8 mm) blanked and then rounded by cutting to form a plate-like gear with dimensional specifications as shown in FIG.

  In addition, the test method of each characteristic was performed as follows.

1) Surface hardness (HV):
The measurement was performed according to JISZ2244 under the condition of a load of 100 g using a micro Vickers hardness tester.

2) Nitriding depth after nitriding treatment (μm)
The metal cross-sectional structure photograph (400 times) taken with an optical microscope was used to determine the depth at which the ferrite phase / martensite phase appeared to be substantially equivalent.

3) Effective depth of high-frequency treatment (μm)
Judgment was made with a Vickers hardness meter by a depth up to a hardness of HV550.

(1) Nitrogen treatment process:
Nitrogen treatment is performed in order from the front side: a cooling chamber that also serves as a purge chamber connected to the loading / unloading device, an intermediate chamber, and finally a heating chamber (nitrogenation chamber). An in-out direct carburizing furnace (manufactured by Nippon Techno Co., Ltd.) (not shown) was used.

Due to the vacuum-tight structure, the atmosphere of the heating chamber in which the nitriding treatment is performed is easily made substantially only of N ₂ (neutral gas) and NH ₃ (nitriding agent). Oxide formation at the grain boundaries (which contributes to a decrease in hardness and toughness) is suppressed.

Furthermore, oxygen is exhausted by evacuating the oxygen atmosphere in the purge chamber reliably in a short time by vacuum purge. In other words, with the work loaded in the purge chamber, the purge chamber is closed and closed by closing the inlet / outlet vacuum doors (first and second vacuum doors) and the heating chamber heat insulating doors of the purge chamber provided on the partition wall, respectively. After evacuating the chamber, the intermediate chamber, and the nitriding chamber, each chamber is decompressed (atmospheric pressure) with N ₂ gas. The heating chamber (nitrogenation chamber) is heated to a set temperature. After opening the inlet / outlet vacuum door and the heating chamber insulation door of the purge chamber and carrying the workpiece into the heating chamber (nitrification chamber), the heating chamber insulation door and the second vacuum door of the purge chamber are closed, and the heating chamber ( Nitrating chamber) is used as a sealed chamber, N ₂ and NH ₃ are introduced, and the nitriding agent mixing ratio is set to a predetermined value.

  Nitrogen treatment was performed under the conditions shown in Table 1. A heating time chart of the nitriding chamber in Examples A and D is shown in FIG.

That is, after evacuating to 50 to 80 Pa in a state where the temperature in the nitriding chamber filled with N ₂ is maintained at 700 ° C., N ₂ is introduced and returned to atmospheric pressure (101.3 kPa). . At that time, the temperature slightly decreases as shown in FIG. Thereafter, the test piece (workpiece) is inserted into the nitriding chamber, heated up to the set temperature (800 ° C. in the example), and then soaked for a uniform temperature distribution in the nitriding chamber (40 After that, N ₂ (neutral gas) and NH ₃ (nitriding agent) were introduced into the nitriding chamber to obtain a predetermined nitriding agent mixing ratio, and a nitriding treatment was performed.

  Then, after the nitriding treatment, the workpiece (test piece) is moved to the quenching chamber through the intermediate chamber by tensile drive, and is rapidly cooled (oil quenching: 85 ° C. × 10 min), and the test piece is removed from the carburizing and nitriding furnace. Was taken out.

  About each workpiece | work prepared in this way, the distance (depth)-nitrogen concentration from the surface was performed using the commercially available EMPA (Electron Probe Micro Analyzer) apparatus.

  Table 1 shows the nitrogen concentrations at depths of 10 μm, 50 μm, and 100 μm in the workpieces of the examples.

  It can be seen that the nitriding within the scope of the present invention is performed in any of the examples.

  Moreover, while measuring the surface hardness (HV) after a nitriding process, the hardening depth (micrometer) was measured and the summarized result is shown in FIG. FIG. 5 shows that the nitriding depth tends to increase as the nitriding temperature increases, and the surface hardness after the nitriding treatment tends to decrease. That is, it can be seen that when the temperature of the nitriding treatment is high and the time is long, the nitrogen concentration on the surface side tends to be low.

(2) Induction hardening process:
About each test piece after the said nitriding process, induction hardening was performed only on the tooth | gear part on the following two conditions using the vacuum oscillator (350 kHz, 50 kW).
・ 34kW (8.5KV × 4.0A) × 1.6s
・ 21kW (7.0kV × 3.0A) × 3.0s

  Table 1 and FIG. 6 summarize the results of surface hardness (HV) and effective depth (550 HV) (μm) measured by the above method.

  In any of the examples, a surface hardness of 750 HV or higher was obtained, and an effective depth of 50 μm or higher was confirmed.

  In particular, a surface hardness (HV) of 800 HV or higher was obtained, and a target value of 100 μm or higher was obtained at an effective depth (550 HV) (μm). This is a surface hardness characteristic that was difficult to obtain by conventional induction hardening.

Claims (5)

A machine part manufacturing method for manufacturing a steel product (work) prepared from low carbon steel (carbon content less than 0.3% by mass) into a predetermined part shape by plastic working and / or machining. ,
Using a nitriding gas consisting essentially of a neutral gas and a nitriding agent in a sealed chamber, the nitrogen concentration at a depth of 10 μm is 1.5 mass% or less and the depth is 50 μm with respect to the workpiece. Nitrogen concentration: a nitriding treatment step for obtaining a nitriding layer of 0.10% by mass or more, and
The work after the nitriding treatment step is induction-heated by induction heating only to a required portion to austenite, and then rapidly cooling to martensite without time,
The manufacturing method of the steel-made machine parts characterized by including each process of these. 2. The method for manufacturing a steel machine part according to claim 1, wherein the heating temperature in the nitriding treatment step is equal to or higher than the A ₁ transformation point (eutectoid reaction line: 727 ° C.) of the Fe—C phase diagram. As the nitriding gas in the nitriding treatment step, a composition having substantially a composition of N ₂ (neutral gas) and NH ₃ (nitriding agent) and a nitriding agent mixing ratio of 20 to 50 vol% is used. 740-850 ° C. × 35-120 min,
In the induction hardening step, high-frequency conditions: output 20-35 kW, heating and holding time 3.5-1.0 s, rapid cooling conditions: 20-40 ° C. × 1-10 s, surface hardness (Vickers hardness: JIS Z 2244 (load 100 g), the same shall apply hereinafter.) Obtain machine parts of 650 HV or higher.
The method for manufacturing a steel machine part according to claim 1 or 2.   The method for manufacturing a steel machine part according to claim 1, 2 or 3, wherein the plastic working is blanking.   A plate-shaped gear manufacturing method, wherein the plate-shaped gear is manufactured by induction-hardening only the tooth portion using the method for manufacturing a steel machine part according to claim 4.

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