JPS58171558A - Tough nitriding steel - Google Patents

Abstract

PURPOSE:To obtain a tough nitriding steel with increased resistance to temper brittleness during nitriding by adding proper amounts of C, Ni, Cr, Mn, Mo, V, etc. and a very small amount of sol. Al to Fe. CONSTITUTION:This tough nitriding steel consists of, by weight, 0.25-0.50% C, <=0.50% Si, <=1.00 Mn, <=0.020% P, <=0.020% S, 1.50-2.50% Ni, 1.50-2.50% Cr, 0.20-0.70% Mo, 0.05-0.20% V, 0.010-0.10% sol. Al, <=0.0020% O, and the balance Fe with inevitable impurities. said proper amounts of C, Ni, Cr, Mo, V, etc. are added to provided high toughness to the steel, and Al deteriorating the toughness is restructed to said very small amount. The amounts of P and S are reduced, the amounts of Si, Mn, Ni and Cr are made proper, and Mo and V are effectively added to enhance the tempering resistance. Cr, Mo and V are added by said proper amounts to enhance the nitriding characteristics.

Description

[Detailed Description of the Invention] The steel of the present invention has high toughness due to the presence of specific components, and is highly resistant to deterioration in toughness (so-called tempering brittleness) that occurs during nitriding treatment or during tempering prior to the nitriding treatment. The present invention relates to a tough nitriding steel having the following characteristics.

It has been known for a long time that when a hardened steel material is tempered to a temperature range of about 450 to 600 C, the toughness decreases to zero during heating or cooling, and this is called phosphorus reversion embrittlement.

With the development of analytical instruments such as mochi chika and AES (Auger electron spectrometer), it has become clear that the precipitation of impurity elements such as P and S at grain boundaries is a cause of temper brittleness.

The electrification treatment is generally carried out for several hours at a temperature range of around 500 to 580C, where tempering brittleness is most likely to occur.
In some cases, the treatment is carried out for a long time, 100 hours or more. Therefore, in order to prevent the decrease in toughness that occurs during nitriding, MO is generally added. For example, J.I.
In 8ACM645 of S'jji steel, Mo-0,15
~0.30% is added. However, it has been confirmed that the addition of Mo also causes a decrease in the toughness of the core portion during the nitriding process. For example, it is clearly stated on page 333 of the Japan Institute of Metals (New Metal Course, New Edition Materials Edition, Tekko So).In general, snow treatment is performed for the purpose of improving wear resistance and fatigue strength. Until now, the decrease in toughness that often occurs during nitriding has not been considered a practical problem.

However, when applying nitriding treatment to machine parts that require repeated impact resistance in addition to wear resistance and seizure resistance, the special nitriding steel has a high surface hardness that reduces wear resistance and seizure resistance. However, due to its low toughness, repeated impacts can lead to breakage.

Snow treatment is often applied to ordinary strong steels such as chromium steel and chrome-molybdenum steel, in addition to specialized nitriding copper.

However, in this case, since the surface hardness after nitriding is low, wear resistance and seizure resistance are insufficient. Additionally, due to temper brittleness that occurs during nitriding, the expected toughness may not be obtained in some cases.

The steel of the present invention has the above-mentioned wear resistance and seizure resistance.
This book was developed as a material suitable for applications requiring high impact resistance, and its gist is that C: 0.25 to 0.50% St: 0.50S or less Mn: 1.00 or less P: 0.020 or less S: 0.0201 or less Ni: 1.50 to 2.50 or less Cr: 1
．． 50 to 2.50% or less Mo: 0.20 to 0.7 (
1% V: 0.05~0.20S so4At: 0.010~0.10%0:
This is a nitriding steel that has strong toughness and high resistance to tempering embrittlement that occurs during nitriding, with Sa being 0.0020 or less with the remainder being Fe and unavoidable impurities.

Next, the reason for limiting the chemical composition of the steel of the present invention will be described.

CFi is the most basic element that plays an important role in improving hardenability and maintaining hardness and strength after quenching and tempering. As C increases, the intensity increases, but
When it exceeds 50 inches, it becomes saturated, and on the other hand, the toughness decreases. Moreover, when C increases, nitriding properties are also inhibited. Therefore, the upper limit is set to 0.50%.

On the other hand, the lower the C content, the deeper the penetration depth of nitrogen during nitriding, but since the required hardenability and strength cannot be obtained, the lower limit is set at 0.25%.

St is used as a deoxidizing agent, and also improves hardenability.
It is effective in strengthening the ferritic ground by solid solution, but the effect increases by 9%, and if it is in too much, the toughness decreases.
In terms of tempering brittleness, it is also preferable to have a smaller amount. Furthermore, since it also inhibits nitriding properties, the upper limit is set to o and s o s.

Like SS, Mn is also used as a descaling agent, but it has a great effect on improving hardenability, improves hardness and strength after quenching and tempering, and also has an effective effect on nitriding properties. However, if the amount is too large, the machinability and hot workability are reduced. Also, for temper brittleness, less is better, so the upper limit is set to 1. Let it be OO*.

Both P and S significantly reduce the toughness, and by segregating to the grain boundaries, cause brittleness due to burning, so it is desirable to reduce the amount as much as possible.

Reproductive potential as well? 1 and 0.020% or less.

Ni is added to improve hardenability and toughness, but
It is particularly effective in improving toughness. However, if there are too many
High annealing hardness significantly reduces workability.

It also inhibits nitriding properties and increases susceptibility to temper brittleness.

In the steel of the present invention, Ni is 1650-2. Within the range of s o %, the best balanced value of these properties is obtained.

Or has a great effect of improving hardenability and strength,
It also has a great effect on nitriding properties, increasing both the surface hardness and the nitriding depth of the numerous Nemuro-kai layers. However, if the amount is too high, K forms a strong nitrided layer, which prevents nitrogen from penetrating and, conversely, reduces the hardening depth. To obtain the hardness and deep hardening depth necessary for wear resistance and seizure resistance, Ni and the following (DMo, V
The connection with the luck 1c weight should be in the 1.50-2.50% C1 range.

M improves hardenability, strength, and toughness, and also has an effective effect on nitriding properties. In particular, it has a significant effect on temper brittleness, but if it exceeds 0.70%, the workability decreases, and if it exceeds 0.70%, the resistance to temper brittleness decreases. become. Therefore, the upper limit is set to 0.701. Further, if the addition amount is less than 0.201, resistance to temper brittleness is insufficient. Therefore, the lower limit is set to 0.20%.

(2) has high resistance to temper softening, improves hardenability, strength and toughness, and improves nitriding properties. In addition, it also has the effect of making the crystal grains finer6,6, which also improves toughness.

However, in order to exhibit these effects, it is necessary to add o, oss or more, and when it exceeds 0.20%, the @ shock transition temperature begins to rise.

Therefore, the upper limit is set to 0.20% and the lower limit is set to 0.05%.

By precipitating as 5ojAtFi, kLN, polycrystalline grains are refined and toughness is improved. It is also one of the basic elements of nitrided steel, and although it significantly improves the surface hardness of the nitrided layer, it also reduces the toughness of the core. Therefore, in the present invention, which emphasizes toughness, the surface hardness of the nitrided layer is improved with other elements such as Or%MO1, and At is added to refine the crystal grains. If it is less than 0.010n, crystal grains tend to become coarse due to heating during quenching, so the lower limit is set to 0.010n. Above 0+10-, the crystal grains tend to become coarse again, and as the 5oLkl in the steel base increases, the toughness decreases, so the upper limits are set as o and ios.

O generally exists as oxide-based inclusions, and since these oxide-based inclusions act as stress concentration sources, they reduce toughness and fatigue strength. Therefore, the lower the value of 0, the more desirable it is, but currently the upper limit that can be realized at any time on an industrial level is 0.
．． 00201, and in the steel of the present invention, it is 0.0
If it is 020- or less, the purpose can be sufficiently achieved. Therefore, the upper limit of 00 is set to 0.0020.

The steel of the present invention has superior toughness compared to conventional nitriding steels, and also has excellent needle-recovery brittleness compared to general tough steels. Furthermore, the nitriding treatment provides high surface hardness. It is characterized by the ability to obtain a hardening depth. As described above, there is no conventional steel for nitriding that has high toughness and high resistance to deterioration in toughness during nitriding or during tempering prior to nitriding.

The characteristics of the composition of the steel of the present invention are as follows: (1) In order to have high toughness, C, Ni,
Or.

Appropriate amounts of alloying elements such as M and V were added, and only a trace amount of At, which impairs toughness, was added.

(2) In order to improve needle return brittleness, impurity elements such as %P%S are reduced, and St, Mn%Ni
, the amount of Cr was appropriately limited, and MO1■ was added effectively.

(3) Appropriate amounts of Cr, Mo, and V were added to improve the nitriding properties.

By doing so, we were able to successfully combine these percentage characteristics.

Next, the characteristics of the present invention will be explained together with examples.

Table 1 shows the chemical composition and 500% of the invention steel and comparative steel.
CX20 hr-+530UX 40 hr Surface hardness of nitrided layer when gas nitriding treatment is applied (hardness at a position 25μ from the surface) and 1 (data on effective hardening depth, steel S properties and embrittlement properties in V-550) Figure 1 shows the hardness distribution of the nitrided layer under the above nitriding conditions.As seen in the figure, SACM 645 has high surface hardness because it contains At. However, the gradient of the hardness distribution is very steep.Thus, it shows good wear resistance, but the nitride layer tends to peel off under repeated impact loads, and this peeling becomes the starting point. It tends to easily lead to fatigue failure.In the case of general strong steel, the surface hardness is at most about HV-650, and the t* hardening depth is not very deep.On the other hand, the steel of the present invention has a hardness of HV-650. It can be seen that a surface hardness of about 100% was obtained, and that the hardening depth was deep and good nitriding properties were obtained.

Next, in order to investigate the case where the hardness was quenched and tempered to 1 (RC35) and the embrittlement characteristics, it was then hardened to 500 (
Charpy impact test after 1'X100hr isothermal embrittlement treatment (Tl53 test piece: 2-U notch)
The results are shown in Figure 2.

This test steel exhibits high impact values both in normal quenching and tempering conditions as well as in isothermal embrittlement treatment, indicating that it has excellent toughness and needle reversibility embrittlement. .

In order to investigate the adaptability to mechanical parts that are subjected to particularly shocking repeated loads, the nitriding conditions were
Table 2 shows the results of the pine wood type cyclic impact fatigue test using the same material as shown in the figure. Table 2 also shows the results when nitriding was not performed up to 1 of quenching and tempering in order to investigate the impact fatigue properties of the core. This result also shows that the steel of the present invention is superior to the comparative steel in terms of repeated impact resistance both for the core and for the entire core including the nitrided layer.

Table 2 Pine wood impact test results (Impact energy: 30/-cs) As stated above, the steel of the present invention has both high toughness and excellent needle reversibility brittleness, and there is no special manufacturing process. It can be easily manufactured by ordinary manufacturing methods without using other means, and is therefore suitable for use in various mechanical parts that require wear resistance, and will greatly contribute to the industry.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the hardness distribution of the nitrided layer of the inventive steel and comparative steel, and Figure 2 is a diagram comparing the degree of embrittlement when the inventive steel and comparative steel are subjected to embrittlement treatment. .

Claims (1)

[Claims] Weight ratio C: O40~0.50'1l st: o, so - or less Mn: 1.0 G - or less p: o, ozo - or less 8: 0.02096 or less Ni: 1. 50-2.50chi Cr: 1.50-2.501 Mo: 0.20-0.70SV: 0.05-0.20chi 5otAt: 0.010-0.10chi〇2 0.0
A strong nitriding steel having strong toughness and high resistance to temper brittleness that occurs during nitriding, characterized by comprising 02056 or less with the remainder re and unavoidable impurities.