JP2623124B2 - Steel material for nitriding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a steel material for nitriding, and more particularly, to a material having a recrystallization softening temperature higher than a nitriding temperature and having a high center hardness and a high thickness after nitriding. The present invention relates to a nitriding steel material improved so as to obtain a compound layer.

[Background of the Invention]

In the field of manufacturing automotive parts and office machine parts,
A method of performing carburizing and quenching treatment on a low carbon steel sheet in order to impart wear resistance and fatigue resistance characteristics is often used. In these methods, it carburizing temperature is higher than the A ₃ transformation point of the steel,
Due to rapid cooling during quenching, heat treatment distortion is unavoidable, especially in products with low plate pressure.

Therefore, for products, especially heat treatment distortion is a problem, the process temperature is applied nitriding processing such as low gas nitrocarburizing or Tufftride than the A ₁ transformation point of the steel, thereby imparting abrasion resistance and fatigue resistance That is being done. In general, the material used for the nitriding treatment is ordinary steel or low alloy steel.

In order to obtain high surface hardness and high core hardness, C
Medium-carbon low-alloy steels containing nitride-forming elements such as r, Al, and Ti are used, and such low-alloy steels generally have very good nitriding properties. Therefore, especially when the product thickness is thinner than 0.8 mm, when the nitriding treatment is applied, the thinner the thickness, the higher the hardness as the surface layer up to the center of the thickness, and it becomes very brittle and low stress. There is a problem that it is easily broken. In order to avoid this, if the processing time is shortened to reduce the center hardness and secure toughness, the compound layer of iron nitride (Fe ₃ N or Fe ₄ N) formed on the surface
Is very thin, and the intended wear resistance cannot be exhibited.
In addition, the conventional low alloying steel for nitriding generally has a large alloy addition amount and a high material cost due to alloy element addition.

On the other hand, ordinary steel (such as hot-rolled mild steel sheet and cold-rolled steel sheet) is inexpensive and has a feature that a thick compound layer is easily formed on the surface. On the other hand, there is a fundamental problem that it is difficult to generate fine iron nitride and high hardness cannot be obtained directly below the compound layer. In addition, since the material is soft, the hardness of the central portion is low, and there is a problem that high fatigue strength cannot be obtained.

[Object of the invention]

The present invention is intended to solve the above-mentioned problems of the conventional nitriding treatment. Even when the nitriding treatment is applied to a thin plate having a thickness of 0.8 mm or less, the wear resistance is maintained without deteriorating the toughness. In order to develop a steel material for nitriding that can exhibit fatigue resistance characteristics, a new material that can overcome the problems of insufficient surface hardness and central part hardness after nitriding, which are the drawbacks of using ordinary steel, is considered. It is something to offer.

[Summary of the Invention]

According to the present invention, there is provided a steel material for nitriding having a recrystallization softening temperature of 580 ° C. or higher, which is higher than the nitriding temperature, and having good nitriding characteristics. 0.08%, Mn: 0.10 to 0.60%, N: 50 ppm or less, and Ti: 4 (% C +% N) to [4 (% C +% N) +0.2]%, Nb: 7.8 (% C +% N)-[7.8 (% C +% N) +0.2]
%, Zr: 7.6 (% C +% N) to [7.6 (% C +% N) +0.2]
%, V: 5.7 (% C +% N) to [5.7 (% C +% N) +0.2]%, and the balance consists of iron and unavoidable impurities. In the steel material for nitriding subjected to the nitriding treatment, which has been subjected to cold working, furthermore, Cr:
Nitriding steel material added with 0.10 to 0.80% and / or Al: 0.05 to 0.50%, and further, with respect to the Cr and Al added steel, Ti,
An object of the present invention is to provide a steel material for nitriding in which P is added in an amount of 0.03 to 0.20% instead of Nb, Zr, and V.

[Detailed Description of the Invention]

The effects of the steel material for nitriding according to the present invention will be specifically described in Examples described later. The reasons for defining the content ranges of the respective component elements contained in the steel as described above and the effects of the respective elements will be described individually. The outline is as follows.

C: A fundamental feature of the present invention is that the C content is reduced and a small amount of Ti, Nb, Zr, V or the like is added to increase the recrystallization softening temperature. If the C content exceeds 0.08%, large amounts of Ti, Nb, Zr, V, etc., which increase the recrystallization softening temperature, are required. On the other hand, in order to decarbonize the C content to less than 0.002%, the degassing time during melting takes a long time, and the production cost increases. The present invention is intended to achieve the above-mentioned object by reducing the cost with low carbon and low alloy components, and therefore, the C content is 0.002 to 0.002.
0.08%.

Mn: Mn is an essential element in the steel of the present invention in order to secure material strength. If the Mn content is less than 0.1%, the material strength of the steel for nitriding is insufficient, and even if added over 0.6%, the hardness of the surface layer just below the surface compound layer such as iron nitride and the hardness at the center increases. Does not contribute. For this reason, the Mn content is in the range of 0.1 to 0.6%.

N: N content is Ti, added to increase the recrystallization softening temperature,
It affects the amount of Nb, Zr, V added. When the amount of N is high, the amount of these elements added increases, and the production cost increases. And the amount of N
If it exceeds 50 ppm, the precipitation of carbonitrides of Ti, Nb, Zr, and V enlarges, and the toughness of the material decreases. For this reason, it is necessary to limit the amount of N to 50 ppm or less.

Ti, Nb, Zr, V: These elements are carbonitride forming elements,
Further, by forming a solid solution in the matrix, the recrystallization softening temperature after cold working can be increased to 550 ° C or higher.

Ti is less than 4 times (% C +% N), Nb is (% C +% N)
Less than 7.8 times, Zr is less than 7.6 times (% C +% N), V is (%
If it is less than 5.7 times (C +% N), all contribute only to the formation of carbonitride of each element, and the recrystallization softening temperature cannot be increased.

On the other hand, when Ti exceeds [4 (% C +% N) +0.2]% and Nb
Exceeds [7.8 (% C +% N) +0.2]% and Zr becomes [7.6
(% C +% N) +0.2]%, V is [5.7 (% C +%
N) +0.2]%, the effect of increasing the recrystallization softening temperature is saturated even if added.

Therefore, the addition ranges of these elements are Ti: 4 (% C +% N) to [4 (% C +% N) +0.2]%, Nb: 7.8 (% C +% N) to [7.8 (% C +% N). ) +0.2]
%, Zr: 7.6 (% C +% N) to [7.6 (% C +% N) +0.2]
%, V: 5.7 (% C +% N) to [5.7 (% C +% N) +0.2]%. As a result, the recrystallization softening temperature after cold working becomes higher than the normal nitriding treatment temperature (580 ° C or less), and the high hardness in the non-recrystallized state is maintained even after the nitriding treatment. Is obtained.

P: P also increases the recrystallization softening temperature by forming a solid solution in the matrix, but its effect is less than 0.03%, and its effect is saturated even if it exceeds 0.20%.
The amount is added in the range of 0.03 to 0.20%.

Cr: Cr is added to increase the surface hardness immediately below the surface compound layer such as iron nitride. However, if it is less than 0.10%, it has no effect, and if it exceeds 0.8%, it is formed on the surface during nitriding. The thickness of the compound layer is reduced (the formation of iron nitride on the surface is suppressed), and as a result, the wear resistance is reduced.
The Cr content is in the range of 0.10 to 0.08%.

Al: Al is also added to increase the surface hardness immediately below the compound layer. However, if it is less than 0.05%, it has no effect. If it exceeds 0.50%, the thickness of the compound layer formed on the surface during nitriding treatment is reduced. Is reduced (the formation of iron nitride on the surface is suppressed) and the wear resistance is reduced. Therefore, the Al content is in the range of 0.05 to 0.50%.

As described above, according to the present invention, an appropriate amount of Ti, Nb, Zr, V, and P, which have the effect of increasing the recrystallization temperature when dissolved, are dissolved in a matrix of low carbon steel, and even at the nitriding temperature. In addition to suppressing recrystallization, the balance of components is assured to ensure sufficient material strength even with low carbon. Such an effect is obtained when the material before nitriding is a cold-worked material. Demonstrates its true value for the first time. In other words, the hardness of the material itself is secured by cold working,
This hardness (hardness reaching the center) even by nitriding
Is not softened by recrystallization. For this purpose, it is necessary that the product to be subjected to the nitriding treatment is sufficiently cold-worked. If the material is a sheet material, this cold working can generally be cold rolling, in which case the final step of manufacturing the steel strip or steel sheet only needs to be the cold rolling step (final finish annealing). Without steel strip or steel sheet). In the case of the steels targeted in the present invention, if the cold working ratio (cold rolling ratio) is less than 20%, the hardness after cold working is low. It cannot be maintained sufficiently. For this reason, it is preferable that the steel sheet is subjected to the nitriding treatment in a state where the cold working rate (cold rolling rate) is 20% or more. In that case, when forming to the required product shape and then nitriding, it is generally necessary that the material before the forming process has been subjected to cold working of at least 20%.

As described above, according to the present invention, after making C of 0.08% or less and N of 50 ppm or less, an appropriate amount of solid solution is formed in Ti, Nb, Zr, V and a matrix. (1) The iron nitride compound layer is formed on the outermost surface after nitriding by using a trace amount so as not to inhibit the formation of the iron nitride compound layer (Fe ₃ N or Fe ₄ N). (2) The hardness at the center is maintained at the time of cold working by suppressing recrystallization at the time of nitriding due to solid solution of the alloy element, and (3) ) The hardness just below the outermost compound layer is achieved by the combined effect of a large amount of means that the hardness is secured by the formation of compounds during the nitriding treatment by adding an appropriate amount of Cr or Al. It can be provided to the market at a lower cost than nitriding steel made of conventional alloy steel.

Hereinafter, the effects of the present invention will be specifically described with reference to typical examples.

[Example] Steel having the chemical composition shown in Table 1 was melted using a converter or a converter and degassing equipment, and was continuously cast into an approximately 11-ton slab, which was then hot-rolled under normal rolling conditions. Rolling was performed to produce a hot-rolled sheet with a thickness of 2.0 mm, followed by acid washing and then cold-rolling to a thickness of 0.5 mm.

Hereinafter, steel having a chemical composition range defined by the present invention is referred to as “the present invention steel”, and other steels are referred to as “comparative steel”.

In Table 1, A is a comparative steel that is a normal cold-rolled steel sheet, B is a comparative steel in which the amount of Ti added is greater than the range of the present invention, and C is a comparative steel in which the amount of Cr is greater than the range of the present invention. This is a comparative steel. On the other hand, D is the steel of the present invention in which Ti is added in the range of the present invention, E is the steel of the present invention in which Cr and Ti are added in the range of the present invention, and F is the steel of the present invention in which Cr, Ti, and Nb are added in the range of the present invention. , G is the steel of the present invention in which Nb is added in the range of the present invention, H is the steel of the present invention in which Cr and Nb are added in the range of the present invention, I is the steel of the present invention in which Nb and Al are added in the range of the present invention, and J is P And the steel of the present invention wherein Al and Al were added within the scope of the present invention.

Of these cold-rolled materials, comparative steel A, inventive steel E and H
Each sample was heated from 400 ° C to 800 ° C for 60 minutes, and the change in hardness was examined. The results are shown in FIG. As can be seen from the annealing softening curve in FIG. 1, Comparative Steel A completely softens at 550 ° C., while Steels E and H of the present invention soften at 650 ° C., which is 100 ° C. higher. Therefore, the steel of the present invention is in an unrecrystallized state at the ordinary nitriding temperature of 580 ° C., and does not soften at the nitriding temperature unlike the comparative steel.

Next, for each steel sample, RX gas: NH ₃
Gas nitrocarburizing was performed in an atmosphere in which the gas ratio was adjusted to 1: 1. The cross-sectional hardness distribution from the surface of the obtained nitrided material was measured. The obtained results are shown in FIG. 2 and FIG. The thickness of the compound layer on the surface was measured, and the results are shown in Table 2. The composition of any steel compound layer is ε
-Fe ₃ N is the main component. Table 2 also shows the hardness at the center.

The following is clear from the results in Table 2 and FIGS.

A compound layer thickness of the comparative steel hardness of 8.0μm and thick but the center is low and H _V 272.

Comparative Steel B and C of Ti or Cr is added more than the present invention range, H _V 600 nitriding characteristic is very good center hardness
As shown above, it is very hard. However, the thickness of the compound layer involved in abrasion resistance is extremely thin, 3.3 to 3.5 μm.
Therefore, this alloy steel becomes brittle due to nitriding at a thin plate thickness, and cannot enjoy the wear resistance effect due to the formation of iron nitride on the surface unlike ordinary steel.

In contrast, the D, E, F, G, H, I, and J networks according to the present invention are:
Compound layer thickness center hardness just below thick and compound layer and 7.5~9.0μm also shows the H _V 400 to 500, sufficient center hardness due to the production of sufficient surface compound layer by nitridation treatment and recrystallization softening suppression Characteristic of the present invention, namely, the maintenance of

Also, as can be seen in FIGS.
The hardness at the position of μm is higher in the E, F, H, I, and J steels to which Cr and Al are added than in the D and G steels to which Cr and Al are not added. Therefore, an appropriate amount of Cr and Al clearly has an effect of increasing the hardness immediately below the compound layer. However, D and G steels containing only Ti and Nb, which increase the recrystallization softening temperature, are comparative steels A
The hardness of the central part is remarkably increased as compared with that of the first embodiment, and the effect of the present invention is sufficiently exhibited.

In order to examine the influence of strength and toughness due to the improvement of the center hardness according to the present invention, a V-shaped block shown in FIG. 4 was used.
0.4mm ^t × 15mm ^w × 40mm ^L was pressed 4.0mm after the tip of the punch came in contact with the sample, and after unloading, the deformation H (m
m) was measured. The results are shown in Table 3.

As can be seen from Table 3, comparative steels B and C, which contain more Ti and Cr than the scope of the present invention, have very good nitriding properties and very high hardness up to the center as described above, It can be seen that the material breaks and is very brittle in material.

Comparative steel A, which has a low center hardness, has a large deformation of 6.2 mm and is easily deformed because of its low strength, and is not suitable for applications requiring strength and fatigue characteristics.

On the other hand, the steels of the present invention all show a smaller amount of deformation than the comparative steels and have excellent properties in terms of strength. In particular, the amount of deformation of each of the E, F, H, I, and J networks added with Cr and Al, which increase the hardness just below the compound layer, is extremely small, and exhibits excellent strength characteristics.

As shown in the above examples, according to the present invention, a nitriding steel is provided, in which a thick directional compound layer is formed by nitriding and the recrystallization softening temperature is high. As a result, the thickness is especially low.
Nitriding of thin sheets of 8 mm or less can simultaneously exhibit wear resistance, fatigue resistance and toughness not found in conventional materials.

[Brief description of the drawings]

FIG. 1 is a diagram showing recrystallization softening curves of comparative steel A and inventive steels E and H, and FIGS. 2 and 3 show cross-sectional hardness distributions after gas nitrocarburizing treatment of each steel in Examples. Fig. 4 is a schematic cross-sectional view showing a test jig for measuring the amount of deformation due to processing of the gas nitrocarburized material. Fig. 5 is a side view showing the shape of a test piece for measuring the amount of deformation in the test of Fig. 4. FIG.

Claims (6)

(57) [Claims] C: 0.002 to 0.08%, Mn: 0.10 to 0.60%, N: 50 ppm or less, and Ti: 4 (% C +% N) to [4 (% C +% N) +0 .2]%, Nb: 7.8 (% C +% N) to [7.8 (% C +% N) +0.2]
%, Zr: 7.6 (% C +% N) to [7.6 (% C +% N) +0.2]
%, V: 5.7 (% C +% N) to [5.7 (% C +% N) +0.2]%, and the balance consists of iron and unavoidable impurities. Steel material for nitriding that has been subjected to cold working and is subjected to nitriding. 2. C: 0.002 to 0.08%, Mn: 0.10 to 0.60%, Cr: 0.10 to 0.80%, N: 50 ppm or less, and Ti: 4 (% C +% N) to [4 (% C +% N) +0.2]%, Nb: 7.8 (% C +% N) to [7.8 (% C +% N) +0.2]
%, Zr: 7.6 (% C +% N) to [7.6 (% C +% N) +0.2]
%, V: 5.7 (% C +% N) to [5.7 (% C +% N) +0.2]%, and the balance consists of iron and unavoidable impurities. Steel material for nitriding that has been subjected to cold working and is subjected to nitriding. C: 0.002 to 0.08%, Mn: 0.10 to 0.60%, Al: 0.05 to 0.50%, N: 50 ppm or less, and Ti: 4 (% C +% N) to [4 (% C +% N) +0.2]%, Nb: 7.8 (% C +% N) to [7.8 (% C +% N) +0.2]
%, Zr: 7.6 (% C +% N) to [7.6 (% C +% N) +0.2]
%, V: 5.7 (% C +% N) to [5.7 (% C +% N) +0.2]%, and the balance consists of iron and unavoidable impurities. Steel material for nitriding that has been subjected to cold working and is subjected to nitriding. 4. C: 0.002 to 0.08%, Mn: 0.10 to 0.60%, Cr: 0.10 to 0.80%, Al: 0.05 to 0.50%, N: 50 ppm or less, and Ti: 4 (% C +% N)-[4 (% C +% N) +0.2]%, Nb: 7.8 (% C +% N)-[7.8 (% C +% N) +0.2]
%, Zr: 7.6 (% C +% N) to [7.6 (% C +% N) +0.2]
%, V: 5.7 (% C +% N) to [5.7 (% C +% N) +0.2]%, and the balance consists of iron and unavoidable impurities. Steel material for nitriding that has been subjected to cold working and is subjected to nitriding. 5. C: 0.002 to 0.08%, Mn: 0.10 to 0.60%, N: 50 ppm or less, P: 0.03 to 0.20%, Cr: 0.10 to 0.80%, Al: Nitriding steel material for nitriding, containing one or two or more of 0.05 to 0.50%, with the balance being iron and unavoidable impurities and cold worked to 20% or more. 6. The method according to claim 1, wherein the cold working is cold rolling.
Or the steel material for nitriding according to 5.