JPS59129756A - High strength carburizing steel for hardening for cross pin of driving shaft - Google Patents

Abstract

PURPOSE:To obtain a high strength carburizing steel for hardening for the cross pins of a driving shaft with improved fatigue strength by specifying the ratio of Ni/Cr, the amount of Cr+Mn+Ni and the ratio of sol. Al/N in a low C-Mn-Ni- Cr-Mo steel, and reducing the amount of O. CONSTITUTION:This steel is an alloy steel consisting of, by weight, 0.15-0.25% C, 0.10-0.35% Si, 0.60-1.50% Mn, 1.00-1.50% Cr, 3.50-4.50% Ni, 0.05-0.30% Mo, 0.015-0.030% sol. Al, 0.010-0.018% N, <=0.0020% O and the balance Fe with inevitable impurities while satisfying relations represented by equations 3.0<=Ni/Cr<=4.0, 5.5<=(Mn+Cr+Ni)<=7.0 and 1.1<=sol.Al/N<=2.0. The steel has high toughness and fatigue strength in spite of its high hardness, and the steel provides the most suitable performance to the cross pins of a driving shaft used under severe conditions.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-strength carburized and quenched steel for drive shaft cross bins having extremely high fatigue strength.

During use, the drive shaft cross bin receives repeated bending loads on the trunnion and multiple contact pressures on the rollers.

In order to prevent destructive accidents such as breakage and damage accidents such as raking of rolling parts, the core has high hardness and high toughness, and the carburized surface layer has high hardness and is resistant to braking and wear. In addition to having excellent properties, it is also required to have high resistance to cracking caused by repeated loads and impact loads.

For conventional drive shaft cross bins, Cr-Mo series, N1-C
Carburizing and hardening steels whose basic components are r-Mo yarn or Nl-Cr-Mo-V system have been used, but when the usage conditions are severe and high loads are applied, it is necessary to meet the above-mentioned required characteristics. The reality is that there is no carburized and hardened steel that satisfies both requirements.

JIS SN0M8 when relatively high load is applied.
15.

Ni-Cr-Mo carburized hardened steel such as SNCM616 is used for drive shaft cross bins, but SN0M81
5, the core strength is insufficient to obtain long life, p, SN0
In M616, network-like carbides tend to precipitate during carburizing, and braking resistance and fatigue strength are not sufficient.

The object of the present invention is to provide a high-strength steel for carburizing and quenching that has characteristics that meet the above requirements under severe conditions. That is, the steel of the present invention has a low C-Mn-Ni-Cr-M
o-based as a basic component.Furthermore, in order to stabilize the carburizing and quenching process, improve the toughness and flaking resistance of the carburized hardened layer, and make the core part highly hard and tough to obtain a long life. N
i/Cr ratio, (Cr, +Mn+Ni) amount, s Q t
The AllN ratio is optimized and the amount of oxygen in the steel is reduced.

The diameter of the trunnion part of the drive shaft cross bin is 100mm~
When the core has a relatively large cross section such as 400 mmφ, high hardenability is required to make the core hard. For example, JIS SN0M8, which has been used conventionally.
15 steel or SNCM616 steel has insufficient hardenability in the case of large cross sections, and the core hardness after carburizing and quenching is less than about HRC40.Also, as mentioned above, in SN0M616, reticular carbides occur in the carburized layer. It was difficult to obtain high fatigue strength. Conventionally, in many cases of broken drive shaft cross-bin trunnions, the hardness is HRC40 or less. In order to obtain high hardness, it is sufficient to increase the amount of C and alloying elements, but high toughness cannot be obtained at the same time by simply enriching the alloying elements and increasing the hardenability. Furthermore, in conventional carburized and quenched parts, when the core hardness is increased to a high hardness of about HRC 40 or higher, mechanical properties such as tensile strength are improved, but there is a fear that fatigue strength may be decreased. The present invention provides a high-strength carburized and quenched steel suitable for use in drive shaft cross bins, which eliminates the drawbacks of conventional steels.

In the present invention, the Nl/Cr ratio, (Cr + Mn + N1) amount 5
By optimizing the otAt/N ratio, the core has a high hardness of HRC43 or higher, 140 kgf7111111
High tensile strength of 11 klil fm/cm21
High toughness and 65 kgf/Wn with impact fatigue life of 1.2 x 10' cycles or more on Charpy value on d
This was done based on the fact that it was found in the course of research that two or more high fatigue strengths could be obtained at the same time.

First, in the steel of the present invention, hardenability is improved by increasing the amount of (Cr + Mn + Ni), and a stable and dense microstructure can be obtained by making the Ni/Cr ratio appropriate.

Furthermore, drive shaft cross bins require long carburizing treatment to thicken the carburized layer, and are required to have stable grain size characteristics, but with the steel of the present invention, the grain size remains fine even after long carburizing. It is a grain.

Conventionally, in order to stabilize the grain size and obtain fine grain size, v, N
It is well known to add elements such as B, Ti, etc., but in the steel of the present invention, the above-mentioned grain refinement is achieved by limiting 5otAt and Ni to a relatively narrow range and keeping the moLAllN ratio within a certain range. Since grains do not become coarse even during long carburization without adding any elements, a core with high hardness and high toughness is obtained with a good balance of the basic components.

On the other hand, the properties of the carburized surface layer and subsurface, which are subjected to the contact pressure of the rolling elements, do not have a large effect on flaking resistance. In the steel of the present invention, the limitation of Nl/Cr ratio and Mn, CyNl f
The balance of tt prevents the generation of network carbides, which are unfavorable for flaking resistance and fatigue strength, and also controls the content of oxygen, which forms harmful oxides in the steel, to a low value. Since the oxide-based inclusions that are a source of internal stress concentration are significantly reduced, it has extremely high resistance to crack initiation against surface contact fatigue, bending fatigue, and impact loads.

Due to the above-mentioned factors, the steel of the present invention has high toughness and high fatigue strength despite its high hardness, making it possible to provide optimal performance to drive shaft cross bins used under severe conditions.

Next, the components of the steel of the present invention will be explained in detail.

(Carbon (C)) The steel of the present invention is subjected to surface treatment by carburizing, and then subjected to appropriate heat treatment such as quenching and tempering before being put into practical use.

This is to obtain a hardened surface layer by carburizing and to obtain low carbon martensite with high impact resistance in the center part (or non-carburized part) of the part. However, if the amount of soft carbon is low, pressure ferrite remains during quenching, making it impossible to obtain high hardness, and instead impairing impact resistance and reducing toughness. On the other hand, if the soft carbon content is high, the hardness is high and the toughness is low.

In the steel of the present invention, in consideration of practical financial resources, it is necessary to take other alloying elements into account and set the amount to 0.15 to 0.25% in order to make the hardness of the central part of the part HnC40 to HRC48.

Therefore, the lower limit of C is set to 0.15%, and the upper limit is set to 0.25%.

(2) Silicon (St) Silicon in steel improves temper softening resistance, hardenability, and financial strength, but when its content increases, it inhibits carburizability and hardens ferrite, thereby inhibiting rigidity. .

Therefore, the upper limit was set at 0.35%, and the lower limit was set at 0.101 in consideration of mass productivity and economic efficiency in steelmaking operations, and in order to obtain the necessary financial strength.

(3) Manganese (Mn) Mn in steel plays a major role in adjusting hardenability, and is preferably used to improve hardenability partly because it is inexpensive. The steel of the present invention also contains other hardenability adjusting elements (C, 81, N
It is desirable that the content be at least 0.60%, taking into account the amounts of i, Cr, MO). Therefore, the lower limit of Mn is 0.60%
shall be.

The more Mn improves hardenability, the better, but if it is contained in a large amount, hot workability and rigidity deteriorate.
It is necessary to keep the upper limit of Mn to 1.5 inches. Therefore, M
The upper limit of n is set to 1.5.

(4) Chromium (Cr) The purpose of adding Cr in the present invention is to provide necessary hardenability, improve carburizability, and facilitate its adjustment.

Further, in order to provide toughness and wear resistance to the carburized surface layer, the steel of the present invention requires at least 1 to 00 Cr. Therefore, the lower limit of Cr is set as i, ooeI). The above-mentioned effects increase as the amount of Cr contained increases; however, if it exceeds 1.5 inches, reticular carbides are likely to occur in the carburized surface layer, which is undesirable, but considering economic efficiency, the upper limit of Cr is set at 1.5 inches. shall be.

(5) Nickel (Ni) The purpose of adding nickel to steel is to provide necessary hardenability and improve toughness after quenching and tempering.

It is also effective in controlling the surface carbon concentration during carburizing.

In the steel of the present invention, the above effects cannot be sufficiently obtained at a content of 3.5% or less. Therefore, the lower limit of Nl was set to 3.50%. When the nickel content increases, the transformation point becomes more pronounced due to p-carburization, and the residual austenite on the high-carbon surface becomes excessive.
Surface hardness decreases and wear resistance and flaking resistance deteriorate. Therefore, the upper limit of N1 was set to 4.50 inches.

(6) Molybdenum (Mo) The purpose of adding Mo to steel is to provide necessary hardenability, improve mechanical properties, and particularly improve toughness after quenching and tempering.

In the steel of the present invention, these effects cannot be sufficiently obtained with molybdenum content below 0.05%, so 0.05% molybdenum is set as the lower limit. M
The hardenability of o increases as the amount added increases, but as the amount of addition increases, it forms side carbides and becomes a solid solution in the steel, conversely worsening the hardenability. Therefore, taking economic efficiency into account, the upper limit was set at 0.30%.

(7) Aluminum (soth) The addition of aluminum effectively acts to adjust the oxygen level and grain size. For such an effect, N
Considering the amount, the lower limit was set at 0.015%. Also At
When a large amount of
The upper limit was set at 0.030% because the development of dendrites in the cast structure becomes remarkable and the crystal grains become coarse.

(8) Nitrogen (f) N in steel mainly exists in the form of AtN and has the effect of refining crystal grains. In the steel of the present invention, the lower limit is 0.0104 and the upper limit is 0.018 as the range in which AtN acts most effectively, taking into account the 5otAtA ratio as well as the 801 At content.
%. If it is less than 0.010%, the above effect will not be obtained regardless of the amount of 5otAl(D), and if it exceeds 0.018%, the crystal grains will become coarser and the impact value will decrease.

(9) Oxygen (0) Oxygen forms oxides with At, 81 or seeds in steel and is particularly harmful to flaking properties. The effect of reducing large harmful oxide inclusions is when the amount of oxygen is reduced to 0.00.
The purpose cannot be achieved unless it is kept below 20%. Therefore, the upper limit was set to 0.0020%.

αONi/Cr It is necessary to add Cr to obtain the necessary hardenability, but C
In order to suppress the tendency of generation of reticulated coal substances during carburizing due to the increase in r, it is necessary to simultaneously increase the amount of Ni, and there is an optimum ratio between Ni and Cr.

When carburizing is carried out for a long time and the Ni/Cr ratio is low, the C concentration in the carburized surface layer tends to be excessive, resulting in formation of network-like carbides and a decrease in strength. Furthermore, when the N i /Cr ratio is high, carburizability is reduced, residual austenite becomes excessive, and surface hardness is reduced. The upper limit of Ni/Cr was set to 4.0 and the lower limit was set to 3.0 in order to stabilize the carburizability, make the structure dense, and obtain high strength and high toughness.

α1) Mn+Cr+Nj Mn * Cr r Ni are all effective elements for improving hardenability, but if the amount of these elements is too large, the core hardness becomes too high and the toughness deteriorates. Moreover, if the amount of elements is insufficient, the required strength cannot be obtained. The core hardness of the large drive shaft cross bin is HRC40 to HRC48 depending on the size.
In order to train the material to the appropriate hardness, Mu + Cr −)−
The upper limit of the Ni amount was set to 7.0 tI), and the lower limit was set to 5.5 eI).

02 5oLALAJ sotAt and N have a grain refining effect in the form of AtN in steel. In order to make the crystal grains fine and prevent them from becoming coarse during carburizing and quenching, it is necessary that a certain amount or more of AtN be finely precipitated. 5otAt/N force exceeds 2.0,
Alternatively, even if it is less than 1.1, it is possible to precipitate ktN if it contains 5otAt and N75 in a certain amount, but 8Δ does not precipitate finely and does not work effectively for grain refinement. In order to effectively and stably precipitate AtN even in the case of long-time carburization, it is necessary to limit noLAL/N to a certain range, so the upper limit of 5otAt/N is set to 2.0. The lower limit was set to 1.1.

Next, examples of the steel of the present invention will be described. Table 1 shows the chemical compositions of the example of the present invention, the conventional case hardening steel for cross bins, and comparative steel, respectively. These coppers were prepared to a size of 200wnφ and carburized at 920-940°C, then heated to 830-830°C.
It was oil quenched from 860°C, further heated to 780 to 810°C, oil quenched, and then tempered at a temperature of 150 to 180°C.

Steels A, B, C, D, E and F are examples of steels according to the invention, and G is J.
IS SNCM616 H and work are JIS SNCM8
15 J, K.

L, M, and N indicate the chemical composition of the steels tested for comparison.

Table 2 shows the mechanical properties of the steels in Table 1 tested on specimens prepared by extraction from the core (non-carburized part) of the heat-treated material.

As is clear from Table 2, the steel of the present invention has significantly higher fatigue strength and superior toughness than conventional steel.

FIG. 1 shows an example of the hardness distribution curve of the carburized surface layer of the steel of the present invention when subjected to the heat treatment described above, in comparison with an example of the conventional steel. The steel of the present invention has a high carburized surface hardness and a deep carburized depth. Therefore, it has excellent surface resistance, IK+4 resistance, and flaking resistance.

FIG. 2 is an example of hardness distribution curves obtained by measuring hardness from the surfaces of 200φ1 test pieces of the present invention steel and conventional steel, JIS SNCM815 steel and JIS SNCM616 $1, which were subjected to the above heat treatment. It is clear that even with the steel of the present invention having a relatively large cross section, sufficiently high hardness can be obtained up to the core.

FIG. 3 is an example of the Ono rotary bending fatigue characteristics of the steel of the present invention. The test pieces were machined and finished to a parallel part of 8mm diameter and 30mm diameter, respectively, and then carburized, quenched, and tempered.

FIG. 4 is a diagram showing the relationship between the tensile strength and rotating bending fatigue limit of the steel of the present invention, conventional steel, and comparative steel.

In contrast to the correlation curves (indicated by dotted lines) drawn for the conventional steel and comparative steel, the steel of the present invention has a reed on the upper side, indicating that it has a high fatigue limit with respect to tensile strength.

FIG. 5 is a diagram showing the relationship between impact fatigue life and tensile strength. In this case as well, the life of the inventive steel is 6 to 10XIO'cycles for the conventional steel and comparative steel, whereas
The inventive steel shows a much higher life of 12-13×10' cycles.

The steel of the present invention has significantly higher fatigue strength than, for example, JIS SNCM815 steel.

As described above, the inventive iron has improved hardenability due to the basic components and a limited Ni/Cr ratio, Mn + Cr + Ni.
It is a steel that is ideal for drive shaft cross bins because it has high strength, high toughness, and a long fatigue life due to the component balance and limited 5otAt/'N ratio, as well as a reduction in the amount of oxygen in the steel. .

[Brief explanation of the drawing]

Figure 1 shows the relationship between the distance from the surface of the carburized layer and the hardness of the inventive steel and conventional steel. Figure 2 shows the distance from the surface to the center of the inventive steel and conventional steel after carburizing, quenching and tempering. Figure 3 is an S-N curve diagram showing the Ono rotary bending fatigue characteristics of the inventive steel and conventional steel, and Figure 4 is the tensile strength and rotary bending fatigue of the inventive steel and conventional steel. FIG. 5 is a diagram showing the relationship between the tensile strength and impact fatigue life of the steel of the present invention and the conventional steel. Honda Kodaira 1 Fig. 1 Distance in A plane 5 (77 + scraps) Nail 52 Fig. 2183 Fig. φ8φ30 Wood φ Do B month fg 1 mouth 1 Stress repeatability (n) 4th L: <1 +, 'i '3 1-do 1 Draw 3 more strong Nisushi deception ka 2

Claims (1)

[Claims] Heavy Jet: %TeCo, 15-0.25%, si
O, 10-0,3596, Mn 0.60-1.50
q6, Cr 1.00-1.50%, Nl 3.5
0% ~ 4.5 (1, Mo 0.05 ~ '0.30'
16.5oIAI. 0.015-0.030%, NO,010-0.0
Contains 18%, OO, 0020 or less, and the balance is free from Fe and unavoidably contained impurities.
, Ni/Cr≦4.0,5.5'Z (Mn 10Cr
+N1) Sl near, 0 and 1.1 < 5ot
A high-strength carburizing and quenching steel for a drive shaft cross bin, characterized by having an At/N ratio of 2.0.