JP4900251B2 - Manufacturing method of nitrided parts - Google Patents

Description

  The present invention relates to a method of manufacturing a nitrided part used for industrial parts and automobile parts such as industrial machines and automobiles after performing a graphite precipitation treatment, followed by a nitriding treatment, and in particular, advantageously improving the nitriding characteristics thereof. It is intended to be illustrated.

  Steel materials used for machine parts such as industrial machines and automobiles are machined into a predetermined shape by cutting, cold forging, or so-called cold working using them together, and then by quenching and tempering or nitriding. It is manufactured by the method of ensuring the required characteristics.

  As a means for improving the machinability of such steel for machine structural use, a method of adding a free-cutting element such as Pb, S, Bi, P or the like alone or in combination to the steel is generally used. In particular, Pb is frequently used because it has an extremely strong effect of improving machinability. However, on the other hand, Pb is an element harmful to the human body, and a large exhaust facility is required in the manufacturing process of steel materials and the machining process of machine parts, and at the same time, there is a great problem from the viewpoint of recycling of steel materials.

  On the other hand, in order to improve the cold forgeability of the steel material, it is desirable to reduce the elements such as Pb, S, Te, Bi, and P as described above.

As a method for enabling these contradictory alloy designs, a method of graphitizing C in steel has been proposed (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 51-57621

By the way, the nitriding treatment for steel is to form a harder nitride layer on the surface of the steel to improve wear resistance, fatigue strength, etc., and is widely used in the field of machine parts and the like. .
However, if a nitriding treatment is applied to a steel material using graphite precipitation as a means for improving cold forgeability and machinability, the graphite phase will not be nitrided, so the nitride layer thickness and nitride layer A problem arises in the adhesion.
That is, in the conventional technology as described above, it has been difficult to apply the graphite precipitation method for the purpose of improving cold workability in a process based on nitriding.

  The present invention advantageously solves the problems of the prior art as described above. Even if Pb is not used, the coverage is equal to or better than that of conventional Pb-added free-cutting steel without harming the cold forgeability. The purpose is to propose an advantageous manufacturing method of nitrided parts that can be cold-cut by machinability and has excellent adhesion of the nitrided layer after nitriding.

Now, in order to achieve the above object, the inventors have earnestly devised a method for industrially and stably producing a steel material having excellent cold workability such as machinability and cold forgeability and nitriding property. As a result of repeated research, the following findings were obtained.
Since the microstructure of the steel surface layer affects the nitriding properties, it is necessary to utilize the lubricating effect of graphite during cold working such as cold forging and cutting. What is necessary is just to make it the state which graphite does not exist abundantly on the surface to perform.
In other words, it has been found that it is extremely effective to achieve the intended purpose to reduce the abundance of graphite in the surface layer portion during nitriding while utilizing graphite during cold working.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.1-0.8%, Si: 0.5-2.0%,
Mn: 0.1 to 2.0%, B: 0.0003 to 0.0150%,
Al: 0.005-0.1%, N: 0.0015-0.0150%,
O: 0.0030% or less, P: 0.020% or less, and S: 0.035% or less, with the balance being Fe and unavoidable impurities, after hot working, and after heat treatment for the purpose of graphite precipitation Then, after cold working, the steel was heated and held at a temperature of Ac ₁ point or higher, and the graphite deposited on the surface was dissolved in the base material to reduce the graphite phase to an area ratio of 0.5% or less . A method for producing a nitrided part, comprising performing nitriding in a temperature range of 450 to 650 ° C.

2. In the above 1, the steel material is further in mass%,
Ni: 0.1-3.0%, Cu: 0.1-3.0%,
Co: 0.1-3.0%, Mo: 0.05-1.0%,
V: 0.05-0.5%, Nb: 0.005-0.05%,
Ti: 0.005-0.05%, Zr: 0.005-0.2% and
REM: 0.0005-0.2%
A method for producing a nitrided part, comprising a composition containing one or more selected from among the above.

Thus, according to the present invention, a nitrided part that is not only excellent in cold workability such as machinability and cold forgeability but also excellent in nitriding characteristics can be obtained industrially stably.
This makes it possible to produce a steel material that balances cold workability and nitriding characteristics without using elements such as Pb that adversely affect the human body.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
C: 0.1-0.8%
C is an essential element for forming the graphite phase and securing the strength as a machine structural component. If the C content is less than 0.1%, it is difficult to ensure the graphite phase necessary for ensuring machinability, so addition of 0.1% or more is required. Not only the deformation resistance at the time of rolling increases, but also the deformability decreases and the generation of cracks and flaws in the hot rolled material increases, so the content is limited to 0.8%.

Si: 0.5-2.0%
Since Si does not dissolve in cementite but has the effect of promoting graphitization by destabilizing cementite, it is actively added. However, if the content is less than 0.5%, the effect of addition is poor. On the other hand, if the content exceeds 2.0%, the deformability during hot working is reduced and the hardness after precipitation of graphite is increased. Si was limited to the range of 0.5 to 2.0%.

Mn: 0.1-2.0%
Mn is not only effective for deoxidizing steel, but is also an element that is useful for hardenability, so it is actively added. On the other hand, it dissolves in cementite and has the negative effect of inhibiting graphitization. . Here, if the amount of Mn is less than 0.1%, there is no effect on deoxidation, so addition of at least 0.1% is required. However, if it exceeds 2.0%, graphitization is inhibited, so Mn is 0.1 to 2.0. % Range.

B: 0.0003-0.0150%
B combines with N in the steel to form BN, which acts as a nucleus for crystallization of graphite and promotes graphitization and has the effect of refining graphite grains. Further, B is an important component in the present invention because B is an element useful for enhancing the hardenability of steel and ensuring the strength after quenching. Here, if the amount of B is less than 0.0003%, the effect of improving graphitization and hardenability is small, so addition of 0.0003% or more is required. However, if added over 0.0150%, B dissolves in the cementite. By stabilizing cementite, however, graphitization is adversely inhibited, so B was limited to a range of 0.0003 to 0.0150%.

Al: 0.005 to 0.1%
Al reacts with N in the steel to form AlN, which acts effectively as a nucleation site of graphite, thereby promoting graphitization, so it is actively added. Here, if the content is less than 0.005%, the effect is small, so at least 0.005% addition is required. On the other hand, when it exceeds 0.1%, a large number of Al-based oxides are formed in the casting process, and this oxide alone not only becomes a starting point for fatigue fracture, but also extremely coarse graphite grains with this oxide as a nucleus. It is formed. In addition, since the Al-based oxide is hard, the machinability is lowered by wearing the tool during cutting. For these reasons, the upper limit of Al content was set to 0.1%.

N: 0.0015-0.0150%
N combines with B to form BN, and this BN serves as a nucleus for crystallization of graphite, thereby remarkably reducing the size of the graphite grains and promoting graphitization. Therefore, the essential element in the present invention It is. If the N content is less than 0.0015%, BN is not sufficiently formed. On the other hand, if added over 0.0150%, cracking of the slab is promoted during continuous casting, so N is in the range of 0.0015 to 0.0150%. Limited.

O: 0.0030% or less O forms oxide-based non-metallic inclusions and decreases cold forgeability, machinability and fatigue strength. Therefore, it is desirable to reduce as much as possible, but up to 0.0030% is acceptable. Is done.

P: 0.020% or less P is an element that not only inhibits graphitization but also deteriorates cold forgeability by embrittlement of the ferrite layer. Moreover, by segregating at the grain boundaries during quenching and tempering and reducing the grain boundary strength, the resistance to the propagation of fatigue cracks is reduced and the fatigue strength is degraded. Therefore, it is desirable to reduce P as much as possible, but it is acceptable up to 0.020%.

S: 0.035% or less S forms MnS in steel, which becomes a starting point of crack generation during cold forging and deteriorates cold forgeability. In addition, MnS not only becomes a starting point for fatigue fracture, but also forms coarse graphite by acting as a nucleus for crystallization of graphite, which causes a decrease in fatigue strength. However, it is acceptable up to 0.035%.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Ni, Cu, Co: 0.1-3.0% each
Ni, Cu and Co are all elements that promote graphitization. Moreover, since it has the effect | action which improves hardenability, it becomes possible to improve hardenability, without inhibiting graphitization. However, if the content is less than 0.1%, the effect is small. On the other hand, if the content exceeds 3.0%, the effect is saturated. Shall be allowed to.

Mo: 0.05-1.0%
Mo not only enhances hardenability but also has a feature that its distribution to cementite is small compared to alloy elements such as Mn and Cr. For this reason, the hardenability of the steel material can be enhanced with almost no inhibition of graphitization. Moreover, since the steel material to which Mo is added has high tempering and softening resistance, it is possible to improve the hardness at the same tempering temperature, and as a result, it is possible to improve the fatigue strength. In addition, because of its high hardenability, it is easy to form a bainite structure that forms fine graphite in the state of hot rolling, and as a result, the melting of the graphite during quenching is completed in a short time. be able to. For this reason, it is used when it is necessary to further improve the fatigue strength. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, if the content exceeds 1.0%, graphitization is inhibited, and cold forgeability and Since the machinability is lowered, Mo is limited to the range of 0.05 to 1.0%.

V: 0.05-0.5%, Nb: 0.005-0.05%
V and Nb are both carbide-forming elements, but hardly inhibit solidification in cementite and thus do not significantly inhibit graphitization. Further, it is an element that forms carbonitride and raises not only the strength by its precipitation strengthening action but also improves the hardenability, and therefore contributes effectively when the fatigue strength needs to be improved. However, if the V content is less than 0.05%, the effect of addition is poor. On the other hand, if the content exceeds 0.5%, the effect is saturated, so V is added in the range of 0.05 to 0.5%. Also, if the Nb content is less than 0.005%, the effect of addition is still poor. On the other hand, if the content exceeds 0.05%, the effect is saturated, so Nb is added in the range of 0.005 to 0.05%.

Ti: 0.005-0.05%, Zr: 0.005-0.2%
Ti and Zr both form carbonitrides, which act as nuclei for crystallization of graphite, thereby refining the graphite grains, and thus contribute effectively when it is necessary to further refine the graphite grains. . Further, by forming carbonitride, it becomes possible to make B act effectively on the hardenability during quenching. In order to exert such effects, it is necessary to add 0.005% or more of both Ti and Zr. On the other hand, if Ti and Zr are contained in amounts exceeding 0.05% and 0.2%, N for forming BN is insufficient, and as a result, the graphite grains become coarse and the graphitization time becomes extremely long. Ti and Zr should be added in the range of 0.005 to 0.05% and 0.005 to 0.2%, respectively.

REM: 0.0005-0.2%
REMs such as La and Ce combine with S to form (La, Ce) S. This becomes the core of graphitization, promotes graphitization and refines the graphite grains, and contributes effectively when it is necessary to refine the graphite grains and promote graphitization. However, if the amount of REM is less than 0.0005%, the effect of addition is poor. On the other hand, if the amount exceeds 0.2%, the effect is saturated, so REM should be added in the range of 0.0005 to 0.2%.

Next, a method for producing the steel of the present invention will be described.
The steel adjusted to the above suitable component composition is melted in a conventionally known converter or electric furnace, and then made into a steel material by a continuous casting method or an ingot-bundling method.
Next, the steel material is subjected to hot working such as hot rolling and hot forging, and then heat treatment for precipitating graphite. The heat treatment for precipitation of graphite is preferably performed at a temperature of 600 to 760 ° C. under the condition of holding for 0.3 hours or more. This is because if the processing temperature is less than 600 ° C, graphite that can provide sufficient lubrication effect during cold working such as cold forging and cutting cannot be precipitated, while if it exceeds 760 ° C, This is because the steel structure being held becomes the austenite center and sufficient graphite precipitation cannot be obtained. If the treatment time is less than 0.3 hours, a sufficient amount of graphite cannot be deposited.
Here, in order to fully utilize the lubricating effect of graphite during cold working such as cold forging and cutting, the precipitation amount of graphite is preferably 5% or more of the C amount in steel.

After the heat treatment for precipitation of the graphite, cold working such as cold forging and cutting is performed.
This cold working includes not only ordinary cold forging and cutting, but also peripheral turning and drilling.

  Next, nitriding treatment is performed. Prior to this nitriding treatment, it is necessary to remove the graphite on the steel surface layer portion. This is because if graphite exists in the surface layer of the steel material, the graphite phase will not be nitrided even if nitriding is performed, so the thickness of the nitrided layer varies or the adhesion of the nitrided layer is low. This is because a problem of deterioration occurs.

Here, as a method of removing graphite from the steel surface layer before nitriding treatment, the steel material is heated and held at a temperature of Ac ₁ point or higher so that graphite precipitated on the surface is dissolved in the base material (surface The graphite solid solution method) is preferred.

In order to dissolve the graphite precipitated on the surface of the steel material in the base material according to the above method, it is important to heat and hold the steel material at a temperature of Ac ₁ point or higher. This is because if the heating and holding temperature is lower than the Ac ₁ point, the graphite on the surface of the steel cannot be sufficiently dissolved in the base material, and the area ratio of the graphite phase in the surface layer is reduced to 0.5% or less. Because you can't.

As described above, after removing graphite in the surface layer portion of the steel material, nitriding treatment is performed.
In the nitriding treatment, it is necessary to perform the nitriding treatment in a temperature range of 450 to 650 ° C. in a nitrogen atmosphere such as a salt bath containing nitrogen or a gas. The nitriding time is preferably 1 to 7 hours.

  As described above, during the cold working, it is possible to perform a smooth cold working by allowing the graphite phase to exist in the surface layer portion as well as the inside of the steel material. By eliminating the graphite phase as much as possible, it is possible to effectively perform nitriding, and to obtain a nitrided part having excellent nitriding characteristics without peeling of the nitrided layer.

Hereinafter, the present invention will be described based on examples.
Steels having the composition shown in Table 1 were melted in a converter, then bloomed by continuous casting, then billet-rolled, and further bar-rolled to give a 35 mmφ steel bar. Then, after heat-processing for graphite precipitation on the conditions shown in Table 2, cutting and cold forging were performed. Next, prior to the nitriding treatment, after removing the graphite on the steel surface layer portion using the surface graphite solid solution method under the conditions shown in Table 2, in a gas atmosphere where NH ₃ : RX = 1: 1 is mixed. Nitriding treatment was performed by air cooling after holding at 3 ° C. for 3 hours to obtain a nitrided part. In addition, for the sake of comparison, the graphite removal treatment before the nitriding treatment was omitted for a part.
The graphitization rate, machinability, cold forgeability, surface area graphite area ratio after graphite removal treatment and nitriding characteristics of the bar steel after graphitization treatment were investigated.
The obtained results are also shown in Table 2.

In addition, the graphitization rate and the evaluation method of each characteristic are as follows.
-Graphitization rate Specimens for optical microscope observation are collected from 1 / 4d part of steel bar and do not corrode after polishing. Using an image analyzer, the cross section is 5 points, and the optical microscope image is 10 times magnified at 400 magnifications for each location. The area ratio of graphite was measured. Next, the graphitization rate was defined as follows by the ratio of the graphite area ratio of the 1/4 d part to the value when all of the additive C was graphitized.
(Measured graphite area ratio) / (graphite area ratio when all of added C is graphitized) × 100 (%)

・ Machinability A drilling test was conducted on steel bars. The tool used was a straight drill with a coating of 4 mm in diameter, dry cutting was performed at a feed rate of 0.15 mm / rev, and the machinability was evaluated based on the drilling depth until the cutting became impossible.

-Cold forgeability A cylindrical test piece of 15mmφ x 22.5mmL was made from a bar steel, subjected to a compression test using a 300t press, and the deformation resistance was calculated from the load during the test. Here, the deformation resistance when the compression rate (height reduction rate) is 60% is shown. Further, the number of repetitions was 10, and the presence or absence of cracking on the side surface of the test piece was confirmed. The compression rate at which cracking occurred in half of the test piece after the test was used as a deformability index as a limit compression rate.

・ Surface area graphite area ratio Specimens for optical microscope observation were collected from the surface layer part of the steel bar and did not corrode after polishing. Using an image analyzer, the cross-section was measured over 10 fields of view of the optical microscope image at 400 times magnification. The area ratio of graphite was measured.

-Nitriding characteristics The nitriding characteristics were evaluated by the adhesion of the nitride layer. That is, after the gas soft nitriding treatment, shot projection was performed on the surface of the steel bar, and the presence or absence of delamination was investigated by subsequent surface observation, and the adhesion of the nitrided layer was evaluated.

As apparent from Table 2, all the inventive examples have excellent machinability equivalent to or better than steel P (No. 14) corresponding to Pb-added S45C, and also have good cold forgeability. It was excellent. Furthermore, it can be seen that all of the inventive examples have no detachment after shot projection and are excellent in nitriding characteristics.
On the other hand, no precipitation of graphite was observed after annealing at 740 ° C. for 7 hours in any of Nos. 11 to 14 in which the composition of steel deviated from the appropriate range of the present invention. Both the properties were significantly inferior to the inventive examples.
In addition, even if the steel component composition satisfies the appropriate range of the present invention, No. 2, 4 and 5 in which the manufacturing conditions do not satisfy the range of the present invention, many graphite particles remain in the surface layer during nitriding. Therefore, the adhesion of the nitride layer was inferior to that of the inventive examples.

Claims (1)

% By mass
C: 0.1-0.8%, Si: 0.5-2.0%,
Mn: 0.1 to 2.0%, B: 0.0003 to 0.0150%,
Al: 0.005-0.1%, N: 0.0015-0.0150%,
O: 0.0030% or less, P: 0.020% or less, and S: 0.035% or less, with the balance being Fe and unavoidable impurities, after hot working, and after heat treatment for the purpose of graphite precipitation Then, after cold working, the steel was heated and held at a temperature of Ac ₁ point or higher, and the graphite deposited on the surface was dissolved in the base material to reduce the graphite phase to an area ratio of 0.5% or less . A method for producing a nitrided part, comprising performing nitriding in a temperature range of 450 to 650 ° C.