JPH0881734A - Steel for nitriding treatment and production therof - Google Patents

Abstract

PURPOSE: To produce a steel for nitriding treatment in which a hardened layer after the nitriding treatment is thick, furthermore, the hardness of the core part is high, and moreover, as for machinability, the level equal to that of the conventional steel for nitriding treatment is secured. CONSTITUTION: This steel for nitriding treatment has a compsn. contg., as essential components, by weight, 0.10 to 0.40% C, <=1.50% Mn, <=2.00% Cr, 0.05 to 0.60% V and 0.05 to 1.00% Al, and the balance Fe with inevitable impurities, and in the matrix of iron, fine-grained VC is dispersed. The parts free from the softening of the core part caused by high temp. and long time holding in the process of nitriding, high in strength and having a long service life compared to the conventional steel for nitriding treatment can be obtd.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to nitriding steel,
More specifically, the present invention relates to a nitriding steel which is useful as a material for gears because the hardness of the core of the material is high and the hardened layer on the surface becomes thicker when nitriding.

[0002]

2. Description of the Related Art Gears, sleeves, liners, etc. are required not only to have excellent mechanical strength as a whole, but also to have a high surface hardness and excellent wear resistance.
It is manufactured using a nitriding steel such as S standard SACM645 as a starting material. When manufacturing the above-mentioned component, first, a predetermined nitriding steel is machined to manufacture an intermediate material having a final shape of the target component or a shape similar to the final shape.

Then, the intermediate material is, for example, 570 ° C.
The nitriding treatment is performed by leaving it in the surrounding N ₂ atmosphere or NH ₃ atmosphere for a desired time. Due to the diffusion of nitrogen that progresses in this process, a hardened layer containing a nitride as a main component is formed on the surface of the intermediate material, the surface of the intermediate material has a higher hardness than the core portion, and the surface of the intermediate material is selectively removed. Wear resistance is improved.

From the above-mentioned usage form of the nitriding steel, it is natural that the nitriding steel is excellent in mechanical strength as a whole, and it is obvious that the nitriding steel has excellent machinability during the production of the intermediate material. From the point of view, it is required that the material is soft and easy to work and that a hardened layer having high hardness and excellent wear resistance is formed on the surface during nitriding treatment.

[0005]

By the way, when a high-strength component is to be obtained, it is preferable that the nitriding steel to be used also has a high-strength and hard core portion. However, when the nitriding steel itself used has a high hardness, the workability during the above-mentioned machining is deteriorated. That is, from the viewpoint of machining, it is desirable that the nitriding steel used is soft.

Therefore, it is preferable that the nitriding steel has a hard core portion from the viewpoint of providing a high-strength component and a soft core portion from the viewpoint of machining. Characteristics are required. Therefore, in order to satisfy the contradictory characteristics described above, a steel with excellent hardenability is adopted as the nitriding steel, and the steel is machined while it is relatively soft before quenching, and then the quenching is performed. After improvement, it has been attempted to harden the surface by nitriding.

However, since the nitriding treatment is carried out by holding it at a temperature of around 570 ° C. for a long time as described above, the tempered nitriding steel undergoes temper softening during the nitriding treatment to increase its strength. The effect of quenching performed for the purpose may be wasted. Further, in the above-mentioned JIS standard SACM645, the hardness of the hardened layer formed by the nitriding treatment is high, but the hardened layer is thin and brittle, so that the hardened layer may be peeled off by an impact load. is there.

The present invention solves the above-mentioned problems in the conventional nitriding steel and provides a component having higher strength. Therefore, the hardened layer after nitriding is thick and the hardness of the core is high. Moreover, it is an object of the present invention to provide a nitriding steel in which machinability is secured at the same level as the conventional nitriding steel.

[0009]

In order to achieve the above object, in the present invention, C: 0.10 to 0.40% by weight, Mn: 1.
50% by weight or less, Cr: 2.00% by weight or less, V: 0.05
.About.0.60% by weight and Al: 0.05 to 1.00% by weight as essential components, the balance consisting of Fe and unavoidable impurities, characterized in that fine particles VC are dispersed in the iron base. A nitriding steel (hereinafter, referred to as a nitriding steel A) is provided.

Here, C is a component that contributes to the improvement of the hardness of the core by forming a fine VC by combining with V by controlling the cooling rate after hot working, which will be described later. It is set to 10 to 0.40% by weight. Content is 0.1
If the amount is less than 0% by weight, the above effects cannot be sufficiently exhibited and the ferrite phase is increased, so that the material is softened and it becomes inconvenient for high strength. On the contrary, 0.4
If it is more than 0% by weight, machinability and cold workability are deteriorated, and the thickness of the hardened layer formed during the nitriding treatment is reduced, resulting in deterioration of toughness. The preferred content is 0.15-0.25
% By weight.

[0011] Mn functions to increase the hardenability of steel and wear resistance, but if it is contained too much, it produces retained austenite and causes a decrease in toughness, so its content is 1. Limit to 50% by weight or less. The preferred content is 0.50 to 1.20% by weight. Cr is a component that promotes the nitriding reaction to increase the thickness of the surface-hardened layer and thus contributes to the improvement of the wear resistance of steel. However, the above effect is saturated even if the content is too large, so the content is set to 2.00% by weight or less. A more preferable content is 0.5 to 1.0% by weight.

V is a fine carbide VC associated with carbon
This serves to uniformly disperse in the iron matrix, improve the hardness of the core, and increase the softening resistance of the base material during the nitriding treatment. Further, the toughness of the core is improved by refining the crystal grains of the iron base itself. Furthermore, V that was not associated with carbon
Is combined with nitrogen during nitriding to form a hard nitride,
It contributes to the improvement of the surface hardness and also serves to increase the thickness of the hardened layer.

The content of V is set to 0.05 to 0.60% by weight. When the content of V is less than 0.05% by weight, it is difficult to obtain the above effects because the amount of C is insufficient with respect to the amount of C described above, and the content of more than 0.60% by weight is contained. Even if it does, the above-mentioned effect will be saturated. The preferred content is 0.10 to 0.30% by weight. Al is
It is a component that promotes the nitriding reaction to increase the hardness of the surface hardened layer and thus contributes to the improvement of the wear resistance of steel. If this element is not contained, the hardness of the hardened layer after the nitriding treatment becomes low. However, if it is contained in a too large amount, it will combine with nitrogen only at the surface to form a nitride, and nitrogen will not diffuse inside. Therefore, the thickness of the hardened layer due to the nitriding treatment becomes thin. Therefore, the content of Al is 0.
It is set to 05 to 1.00% by weight. The preferred content is 0.1
~ 0.2% by weight.

The nitrided steel A of the present invention comprises the above-mentioned components as essential components. In addition to the essential components, Si: 2.0
0% by weight or less, Ti: 1.50% by weight or less, Nb: 0.02
~ 1.50% by weight, Ta: 0.02 to 1.50% by weight, Mo:
The iron matrix is further strengthened by blending one or more selected from the group consisting of 3.00% by weight or less, B: 0.05% by weight or less, and Ni: 2% by weight or less.

In this nitriding steel (hereinafter referred to as nitriding steel B), Si functions to improve resistance to temper softening, but if it is contained in too much amount, the iron matrix itself softens, Since it causes a decrease in malleability, its content is preferably limited to 2.00% by weight or less. Ti combines with nitrogen at the time of nitriding treatment to form a hard nitride to improve the surface hardness, but even if it is contained in a large amount, the above effect is saturated,
The content is preferably limited to 1.50% by weight or less.

Nb and Ta combine with nitrogen during the nitriding treatment to form a hard nitride to improve the hardness of the surface, but the effect is saturated even if contained in a large amount. On the contrary, when the content is too small, the thickness of the hardened layer becomes thin and the hardness of the surface also decreases.
The contents of Nb and Ta are 0.02 to 1.50, respectively.
It is preferable to set it to the weight percent.

Mo acts as an iron-base strengthening element to improve the hardenability of steel and the resistance to temper softening, but if it is contained in a too large amount, the thickness of the hardened layer is increased. Since it works to decrease, M
The content of o is preferably limited to 3.00% by weight or less. Similar to Mo, B also acts as an iron-base strengthening element and acts to improve the hardenability of steel, but if it is contained in a too large amount, it acts to reduce the thickness of the hardened layer. Therefore, the content is preferably limited to 0.05% by weight or less.

Ni acts as a toughness improving element,
If it is contained in an excessively large amount, the toughness improving effect is saturated, and the cost is simply increased. Therefore, N
It is preferable that i is 2% by weight or less. In addition to the above-mentioned nitrided steel A or nitrided steel B, S: 0.40% by weight
Below, Pb: 0.40% by weight or less, Te: 0.40% by weight or less, Se: 0.30% by weight or less, Ca: 0.30% by weight or less, Bi: 0.50% by weight or less, Sb: 0 By blending one or more selected from the group consisting of 0.30% by weight or less, it is possible to obtain a nitriding steel (hereinafter referred to as nitrided steel C) having further improved machinability.

S, Pb, Te, Se, Ca, Bi, Sb
Both work to improve the machinability of steel,
If each of these elements is contained in a too large amount, the hot workability and toughness will deteriorate, so S is 0.40% by weight or less,
Pb is 0.40 wt% or less, Te is 0.40 wt% or less, S
e is 0.30% by weight or less, Ca is 0.30% by weight or less, Bi
Is preferably limited to 0.50% by weight or less and Sb is limited to 0.30% by weight or less.

The structure of each of the above-mentioned nitrided steels is such that fine VC is uniformly dispersed in the matrix structure composed of ferrite and pearlite. The fine particles VC are precipitated by controlling the cooling rate after the hot working described later, and are finely dispersed in the matrix structure to increase the hardness of the core of each nitrided steel, and are obtained even after the nitriding treatment. Contributes to higher strength of target parts.

In order to obtain the steel for nitriding treatment of the present invention, first, the steel materials A, B and C having the above-mentioned composition are subjected to the following heat treatment, whereby V is once changed to the iron base. Make a solid solution inside. In this heat treatment, when the heating temperature is lower than 1000 ° C., V does not completely form a solid solution in the iron matrix, and when it is higher than 1300 ° C., the crystal grains become coarse and the strength is lowered. Therefore, the heating temperature is 1000 ° C to 130 ° C.
Set to 0 ° C. The preferred heating temperature is 1000-12
It is 50 ° C.

Then, hot working is performed. Hot working is
It may be hot rolling or hot forging. The hot working finish temperature is not particularly limited, but is usually preferably about 800 to 1100 ° C. Then, the material after the hot working is immediately forcibly cooled at a cooling rate to be described later to promote the precipitation of the intended VC.

The cooling rate at this time is 5 ° C./min to 20 within a temperature range from the hot working end temperature to a temperature of 500 ° C.
Set to 0 ° C / min. When the cooling rate is 5 ° C./minute or less, the iron matrix itself becomes soft and the strength improving effect becomes poor. Further, if it is 200 ° C./min or more, a machinability is deteriorated because a hard bainite structure is generated in the structure, although it depends on the composition of the steel.

In the cooling process after the hot working, the nitriding steel has a cooling end temperature of 727 ° C.
When the temperature is lower than the above, the structures of ferrite and pearlite do not change, and the precipitation of VC also stops at around 550 ° C. Therefore, if the temperature is cooled to below that temperature at the cooling rate, no structural change occurs. The nitriding steel obtained in this way is processed into the shape of the final part or a shape close to it, and nitriding is performed. Served as.

[0025]

The steel for nitriding treatment according to the present invention is heated at a predetermined temperature before hot working to form a solid solution of V component in the matrix structure, and by controlling the cooling rate after hot working, the iron matrix is produced. It is possible to precipitate fine VC in a state of being uniformly dispersed therein. In addition, by controlling the contents of Cr, Al, and V that combine with nitrogen to form a hard nitride, the thickness of the hardened layer on the surface formed in the nitriding process is increased,
In addition, it can have high hardness.

[0026]

【Example】

Examples 1 to 7 and Comparative Examples 1 to 4 Various steel materials having the compositions shown in Table 1 were melted to have a diameter of 80 mm,
A cylindrical test piece having a length of 400 mm was produced. First,
The test piece was heated to 1100 ° C. and hot forged until the temperature shown in Table 2 was reached, so that the diameter was 40 mm and the length was 16 mm.
Processing was performed up to 00 mm, and subsequently, cooling was performed from the indicated hot forging end temperature to 500 ° C. at the cooling rates shown in Table 2.

The surface hardness of each test piece after cooling was measured with a Vickers hardness meter. Next, gas soft nitriding treatment was performed on these test pieces. The treatment conditions of the gas soft nitriding treatment were as follows: the test piece was held at 570 ° C. for 5 hours in an atmosphere of NH ₃ : N ₂ : CO ₂ = 5.5: 2.0: 2.5, and then the test piece was kept at 100%. It was put in oil at ℃ and cooled.

The surface hardness and the hardness of the core of each test piece subjected to the gas nitrocarburizing treatment were measured with a Vickers hardness meter. Further, the test piece is cut into a surface perpendicular to the longitudinal direction, the hardness is measured from the side face vicinity portion of the cross section toward the center with a Vickers hardness meter to obtain a hardness distribution, and the Vickers hardness is 500 Hv or more. The thickness from the side surface of the existing portion was measured, and the thickness of the hardened layer by the nitriding treatment was measured.

Subsequently, a roller pitting test was performed on the test pieces whose hardness had been measured, and the life of each test piece was measured. In the roller pitting test, a load roller 2 having a diameter D of 130 mm was brought into contact with a test portion 1a of a cylindrical test piece 1 having a diameter d of 26 mm as shown in FIG. Under the conditions, the number of repetitions until the test part 1a was damaged was evaluated.

Surface pressure: 200 MPa. Slip rate: -40%. Lubricant: Gear oil (temperature 80 ° C). The above results are collectively shown in Table 2.

[0031]

[Table 1]

[0032]

[Table 2]

Examples 8 to 11 and Comparative Examples 5 to 8 For the steel materials used in Example 6, except that the cooling rate and / or the cooling end temperature from the hot forging end temperature were changed. The same process was performed. After reaching the cooling end temperature, cooling was performed in oil. Table 3 shows the characteristics of the obtained treated materials.

[0034]

[Table 3]

As is clear from the results shown in Table 2, the nitriding steel of the present invention has no softening of the core due to the high temperature during the nitriding treatment and long-term holding, as compared with the steels of the comparative examples. The hardness of the core shows a high value, and the thickness of the hardened layer by the nitriding treatment also shows a value about twice. Further, it can be seen that the nitriding steel of the present invention is superior to the steels of Comparative Examples also in terms of life.

Further, from the results shown in Table 3, when the cooling end temperature and the cooling rate were out of the ranges specified in the method of the present invention, the desired microstructure could not be obtained, which was superior to Example 6 as the steel for nitriding treatment. You can see that you can't get anything.

[0037]

As is clear from the above description, in the nitriding steel of the present invention, fine VC is finely dispersed in the iron matrix by controlling the cooling rate after hot working. Therefore, the hardness of the core is improved, and a high-strength component can be obtained.

Further, since the fine VC dispersed in the iron matrix which has improved the hardness of the core is stable even when heated up to 800 ° C., it is heated up to around 570 ° C. by the nitriding treatment and held for a long time. However, the nitriding steel does not soften. Therefore, it is possible to provide steel for nitriding treatment that is resistant to temper softening, and omit conventional heat treatment such as quenching for adjusting core hardness that is performed before nitriding treatment. You can

Further, by controlling the composition of Cr, Al, and V which forms a hard nitride by combining with nitrogen, the thickness of the hardened layer on the surface generated in the nitriding process can be made thick and high in hardness. As a result, the strength of the hardened layer itself is increased, which contributes to a longer service life of the component.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing configurations of a load roller and a test piece in a roller pitting test.

[Explanation of symbols]

 1 Cylindrical test piece 1a Test part 2 Load roller d Diameter (of test part) D Diameter (of load roller)

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location C22C 38/60 C23C 8/26

Claims (6)

[Claims] 1. C: 0.10 to 0.40% by weight, Mn: 1.5
0% by weight or less, Cr: 2.00% by weight or less, V: 0.05-
0.60% by weight, Al: 0.05 to 1.00% by weight is contained as an essential component, and the balance consists of Fe and inevitable impurities.
A steel for nitriding treatment, characterized in that fine particles VC are dispersed in an iron base. 2. In addition to the above essential components, Si: 2.0
0% by weight or less, Ti: 1.50% by weight or less, Nb: 0.02
~ 1.50% by weight, Ta: 0.02 to 1.50% by weight, Mo:
The nitriding steel according to claim 1, wherein one or two or more selected from the group consisting of 3.00% by weight or less, B: 0.05% by weight or less, and Ni: 2% by weight or less is mixed. 3. In addition to the essential components, S: 0.40
% By weight, Pb: 0.40% by weight or less, Te: 0.40% by weight or less, Se: 0.30% by weight or less, Ca: 0.30% by weight or less, Bi: 0.50% by weight or less, Sb : 0.30% by weight
The nitriding steel according to claim 1 or 2, wherein one or more selected from the group consisting of the following is compounded. 4. C: 0.10 to 0.40% by weight, Mn: 1.5
0% by weight or less, Cr: 2.00% by weight or less, V: 0.05-
After the steel material containing 0.60% by weight and Al: 0.05 to 1.00% by weight as the essential components and the balance consisting of Fe and inevitable impurities is heated to a temperature of 1000 ° C to 1300 ° C, hot working is performed. After completion of the hot working, the temperature of the steel material is 5 ° C./min to 2 between the hot working finish temperature and the temperature of 500 ° C.
A method for producing nitriding steel, which comprises cooling at a cooling rate of 00 ° C./min. 5. The steel material further comprises Si: 2.00% by weight or less, Ti: 1.50% by weight or less, and Nb: 0.02 to 1.5.
0% by weight, Ta: 0.02 to 1.50% by weight, Mo: 3.00
The method for producing nitriding steel according to claim 4, wherein one or more kinds selected from the group consisting of wt% or less, B: 0.05 wt% or less, and Ni: 2 wt% or less are blended. 6. The steel material further comprises S: 0.40% by weight.
Below, Pb: 0.40% by weight or less, Te: 0.40% by weight or less, Se: 0.30% by weight or less, Ca: 0.30% by weight or less, Bi: 0.50% by weight or less, Sb: 0 The method for producing a nitriding steel according to claim 4 or 5, wherein one or more selected from the group consisting of 0.30% by weight or less is mixed.