JPH0499846A - High strength steel for gear - Google Patents

Abstract

PURPOSE:To obtain a high strength steel for gear capable of improving the fatigue strength of gear by the ordinary carburizing and quenching alone without causing remarkable increase in cost by specifying a composition consisting of C, Si, Mn, Ni, Cr, Mo, Al, P, S, and Fe. CONSTITUTION:This steel is a high strength steel for gear characterized by having a composition consisting of, by weight ratio, 0.10-0.35% C, <=0.15% Si, 0.5-2.0% Mn, <=2.5% Ni, <=0.35% Cr, <=1% Mo, <=0.1% Ag, <=0.020% P, <=0.030% S, and the balance Fe with inevitable impurities and further containing, if necessary, one or more kinds among <=0.2% Nb, <=0.2% V, and <=0.2% Ti. In this steel, Si and Cr are reduced, and Mn is increased, instead, by which the reduction of inferior hardened layer and the improvement of fatigue strength are made possible and also machinability can be improved.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention provides a high-strength case-hardened steel suitable for use in mechanical parts that are subjected to high repeated loads and require fatigue strength, transfer fatigue strength, and wear resistance. , particularly regarding high-strength gear steels that require high fatigue strength.

[Conventional technology and its technical issues]

Since fatigue strength is important for gears, which are power transmission parts for automobiles, it has been customary to machine case hardened steel and then carburize and harden it. However, in recent automobiles, the engine output has increased, the gear size has been reduced, and the weight has been reduced, and as a result, it has become necessary to increase the strength of the car. In some cases, the fatigue strength of the gear may be insufficient and it may easily break. Therefore.

There is a demand for high-strength gear steels that have higher fatigue strength than these.

In a carburized and quenched gear, a grain boundary oxidation layer with a poorly quenched layer is formed directly under the surface layer of the gear during carburization.

This grain boundary oxidation layer is formed by preferentially oxidizing Si, Mn, Cr, etc. in the steel during carburizing, and is surrounded by an abnormal layer of poor quenching due to decreased hardenability. These become the starting points for fatigue failure of gears and cause insufficient fatigue strength. Therefore, as a means to improve the fatigue strength of gears, it is essential to reduce this grain boundary oxidation layer.

Even when using conventional SCM steel or SNCM steel,
As a method of preventing the formation of a grain boundary oxidation layer, there are a method of vacuum carburizing treatment and a method of removing the grain boundary oxidation layer by grinding after carburizing and quenching.

However, all of these methods have the disadvantage that processing costs are higher than normal carburizing and quenching, and productivity is significantly reduced because mass production is not possible, making them difficult to use practically.

On the other hand, recently, in terms of steel materials for gears, high-strength steel for gears in which grain boundary oxidation layers are reduced only by ordinary carburizing and quenching has been developed. For example, JP-A No. 60-243252 and JP-A No. 63-121640 disclose that Si, Mn, and Cr, which tend to form grain boundary oxidation layers, are reduced, and instead grain boundary oxidation layers are not formed, and A gear steel with increased Ni and Mo content has been disclosed, which can also prevent a poorly hardened layer that promotes fatigue fracture. Further, Japanese Patent Application Laid-Open No. 64-47838 discloses a gear steel in which Mn and Cr are increased in order to prevent a poorly hardened layer without adding expensive Ni and Mo.

The former is S as disclosed in JP-A-60-243252 and JP-A-63-121640.
A gear steel with reduced i, Mn, and Cr and increased Ni and Mo can reduce grain boundary oxidation layers and prevent hardened layers, and improve the fatigue strength of gears. However, Ni
, when the amount of Mo added is small, the effect is small;
If a large amount of Ni, M=4-0 is added, the cost will increase and the material hardness will also increase, resulting in poor machinability.

In addition, the latter, that is, as disclosed in JP-A No. 64-47838, without adding Ni and Mo, Mn, C
Gear steels with increased r have a large cost advantage, but are not sufficiently effective in reducing grain boundary oxidation layers and preventing poorly quenched layers, and the fatigue strength of gears is lower than that of gears with increased Ni and Mo content. It is not as good as industrial steel.

[Means to solve the problem]

The present invention was devised to solve the above-mentioned problems, and its purpose is to create a high-strength gear steel that can improve the fatigue strength of gears only by ordinary carburizing and quenching. The objective is to provide this without significantly increasing costs.

In order to achieve this objective, the present inventors discovered that the main components forming the grain boundary oxidation layer are Si, Mn,
Since Cr is produced by combining with oxygen, the effects of Si, Mn, and Cr on the grain boundary oxidation layer, the poorly hardened layer, and the fatigue strength after carburizing and quenching were investigated in detail.

As a result, it was found that among these components, Si is the most harmful to fatigue strength, followed by Cr, while Mn has a small effect on fatigue strength. Therefore, the present invention has been completed by discovering that the fatigue strength of gears can be improved by skillfully adjusting the components of Si, Mn, and Cr. That is, the first invention steel in the present invention has a weight ratio of C: 0.10-0.35% and Si: 0°15.
% or less, Mn: 0.5-2.0%, Ni: 2°5%
Below, Cr: 0.35% or less, Mo: 1% or less, Al:
0.1% or less, P: 0.020% or less, s: 0. oao
% or less, with the remainder consisting of Fe and unavoidable impurities, and the second invention steel is: C: 0.10-0.35%, Si: 0° 15
% or less, Mn 70.5-2.0%, Ni: 2° or less, Cr: 0.35% or less, Mo: 1% or less, Afl
: 0.1% or less, P: 0.020% or less, S: 0.0
In addition to high-strength gear steel containing 30% or less Nb:
0.2% or less, V: 0.2% or less, and the remainder consists of Fe and unavoidable impurities, and the third invention steel is C: 0.10-0.35%,
Si: 0.15% or less, Mn: 0.5 to 2.0%,
Ni: 2.5% or less, Cr: 0.35% or less, Mo
: 1% or less, AIl: 0.1% or less, P: 0.
020% or less, S: 0.030% or less, and further Nb: 0.2% or less, ■=0.
2% or less, Ti: 0.2% or less, and the balance is Fe and inevitable impurities.

The present invention will be explained in detail below.

As mentioned earlier, Si and M forming the grain boundary oxidation layer
It is desirable to reduce n and Cr as much as possible, but reducing them all will reduce hardenability, make it easier to form a poorly hardened layer, and make it difficult to ensure the strength of the base material. A large amount of expensive N1 and Mo must be added.

The first invention steel contains Si and Cr, which are the most harmful to fatigue strength.
By decreasing Mn and increasing Mn, which has a small effect, it is possible to avoid adding excessive amounts of expensive N1 and Mo.
This made it possible to reduce the number of poorly hardened layers and improve the fatigue strength of gears. Furthermore, since there is no need to add large amounts of Nj and Mo, machinability can be improved.

According to the second invention steel, in addition to the components of the first invention steel, Nb,
By adding a small amount of V, the fatigue strength of the - layer can be improved.

According to the third invention steel, in addition to the components of the first invention steel, Nb,
By selectively adding small amounts of V and Ti, the fatigue strength of the - layer can be improved.

In addition, high-strength gears are often subjected to shot peening treatment to improve fatigue strength.
It was confirmed that each of the steels of the present invention is effective in improving fatigue strength after shot peening.

The reasons for limiting each component in the steel of the present invention are as follows. Note that all X's are weight %.

Regarding C: C is a basic element necessary to guarantee the strength of the base material by carburizing and quenching, and it is necessary to add at least 0.10%. However, when added in large amounts, it leads to deterioration of toughness and machinability, so the upper limit was set at 0°35%.

Regarding Si: Si is an element with the strongest affinity for oxygen, and the deepest part of the grain boundary oxidation layer is mostly Si-based oxide. When the content exceeds 0.15%, the grain boundary oxidation layer becomes significantly deep,
Since the fatigue strength decreases, the upper limit was limited to 0.15% to improve the fatigue strength.

Regarding Mn: Mn is an effective and inexpensive element for improving hardenability, reducing the number of defective hardening layers, and ensuring the strength of the base material, and it is necessary to add at least 0.50%. be. However, M
n is an element that has a strong oxygen affinity and forms a grain boundary oxidation layer.
Also, if added in large amounts, the material becomes hard and machinability is impaired, so the upper limit was set at 2.0%.

Regarding Ni: Ni is an element effective not only in improving hardenability but also in improving toughness. Further, in order to reduce the abnormal structure caused by poor quenching that occurs near the grain boundary oxidation layer and obtain a uniform quenched structure up to the outermost layer, it is desirable to actively add it as necessary. This effect increases as the amount added increases, but if added in a large amount, the material hardens and machinability is impaired, and Ni is an expensive element, so the upper limit should be set to 2.
゜5%.

Regarding Cr: Cr is an element that improves hardenability like Mn, but
Since the oxygen affinity is strong and the grain boundary oxidation layer becomes deep, which significantly reduces fatigue strength, the second limit was set at 0°35%.

Regarding Mo: Like Ni, Mo is an effective element for improving hardenability, toughness, and fatigue strength by 10%. However, even if added in a large amount, this effect would be saturated and it would be economically disadvantageous, so the upper limit was set at 1%.

Regarding Al: Al is an element that is effective in combining with nitrogen to generate AlN, refining crystal grains, and improving toughness and fatigue strength, but when added in large amounts, alumina-based inclusions increase. Therefore, the upper limit was set to 0.1%.

Regarding P: P segregates at grain boundaries and deteriorates toughness, and if contained in a large amount, fatigue strength decreases, so P is set to 00020% or less.

Regarding S: S is an effective element for improving machinability, but if contained in a large amount, cleanliness will be impaired, so the content was set to 0.030% or less.

Regarding Nb, ■, and Ti: Nb, V, and Ti are effective elements for generating carbon and nitrides, refining crystal grains, and improving toughness and fatigue strength. In addition, the yield strength of the carburized part and base metal part is increased,
Contributes to improving fatigue strength.

However, even if a large amount is added, this effect becomes saturated,
Also, since it is economically disadvantageous, the upper limit is set at 0.2
%.

〔Example〕

Table 1 shows the chemical components of the invention steel and comparative steel. Nα
No. 1 to N.alpha.10 are the steels of the present invention, No.11 to N.alpha.14 are the conventional JIS steel types, and Nn15 to N.alpha.20 are the comparison steels.

A round bar with a diameter of 20 to 90 mm was produced from each drawing by hot rolling and forging, and after standard treatment at 925°C, this round bar was processed into a fatigue test piece. Subsequently, each fatigue test piece was carburized and quenched, and its carburizing and quenching properties, rotary bending fatigue properties, gear fatigue properties, and transfer fatigue properties were investigated.

(a) Carburizing and quenching characteristics Carburizing and quenching characteristics were investigated using rotary bending fatigue test pieces. Carburizing and quenching is carried out at 930℃ for 5 hours, then at 840℃ for 3 hours.
After holding 0mln, oil quenching is performed at 180°C for 2hrs.
Tempering was performed. Table 2 shows the carburizing and quenching characteristics. In the steel of the present invention, all grain boundary oxidation layers are 10 μm or less, which is significantly reduced compared to 20 μm or more in conventional steel, and no poorly quenched structure is observed. Carburizing properties such as carburized layer depth and hardness distribution are equivalent to conventional steel and comparative steel.

(b) Rotating bending fatigue characteristics An hourglass-shaped specimen (applied concentration factor α = 1.1) with a center diameter of 8 mm was prepared from a round bar with a diameter of 20 mm, and the 10' fatigue strength was determined by the Ono type rotating bending fatigue test. , the bending fatigue strength was evaluated. Further, after carburizing and quenching, test pieces were prepared that were subjected to shot peening treatment (arc height 0, 6 nwIIA, coverage 200%), and fatigue strength was determined in the same manner.

Table 3 shows the fatigue strength of each stroke. The steel of the present invention is the conventional steel N
011 to Nα14 and comparative steels Nα15, 16, and 20, and comparable to comparative steels Nα17 to Nα19 containing large amounts of Ni and Mo.

Invention #ANa 2~Na with addition of Nb, V, and Ti
In No. 10, further improvement in fatigue strength was obtained.

FIG. 1 shows the relationship between the amounts of Ni and Mo added and the fatigue strength. The steel of the present invention has higher fatigue strength than conventional steel and comparative steel at the same addition amount. In other words, expensive Ni,
This makes it possible to ensure fatigue strength without adding a large amount of Mo.

The above-mentioned effects of the steel of the present invention were the same in test specimens subjected to shot peening treatment, and it was confirmed that the present invention is also effective in improving fatigue strength after shot peening.

(c) Gear fatigue characteristics: From a round bar with a diameter of 90 mm, an outer diameter of 75 fields was cut by cutting.
A test gear with module 2.5 and 28 teeth was created.

Here, only the conventional llNα11 was subjected to grinding after carburizing and quenching to completely remove the grain boundary oxidation layer and poorly quenched structure. The gear fatigue test was conducted using a power circulation gear fatigue tester at a rotational speed of 3000 rpm, and the torque value that did not cause damage after 10 cycles was determined as the dedendum strength of the gear.

Table 3 shows the gear fatigue strength of each stroke. The tooth root strength of the steel of the present invention, as well as the rotational bending fatigue strength, is improved compared to the conventional steel and comparative steel, and the comparative steel N with large amounts of Ni and Mo added.
0. 19, and is comparable to, and even exceeds, conventional steel No. 11, in which the grain boundary oxidation layer and poorly hardened structure were completely removed by tooth surface grinding.

In addition, while the conventional steel Nα'4 suffers from pitting in the low torque region, the steel of the present invention does not cause pitting due to the prevention of poorly quenched structures, and the surface pressure strength is also strengthened. .

(iv) Transfer fatigue characteristics A test piece of φ12×22nn+ was prepared from the surface layer of a round bar with a diameter of 65 mm. In the transfer fatigue test, a cylindrical transfer fatigue tester was used to determine the 10% failure life at a surface pressure of 600 kgf/nn+'' and a rotational speed of 4600 Orpm, and evaluate the surface pressure strength. invented steel Nα
3 to Nα9 also have improved rolling fatigue life.

Table 2 Table 3 Note 2 The gear durability torque of Nα11 is the value after grinding.

〔Effect of the invention〕

According to the present invention as described above, a high-strength gear steel with superior tooth root strength and surface pressure strength compared to conventional steel can be provided at a low price without adding excessively expensive Ni and Mo. This has the excellent effect of providing a practical gear steel that can be used in all mechanical parts that are subjected to high cyclic loads and require fatigue strength, transfer fatigue strength, and wear resistance. It will be done.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the amounts of Ni and Mo added and the bending fatigue strength in the steel of the present invention, conventional steel, and comparative steel.

Claims (3)

[Claims] (1) Weight ratio: C: 0.10-0.35%, Si: 0.
15% or less, Mn: 0.5-2.0%, Ni: 2.5%
Below, Cr: 0.35% or less, Mo: 1% or less, Al:
0.1% or less, P: 0.020% or less, S: 0.030
% or less, with the remainder consisting of Fe and unavoidable impurities. (2) C: 0.10-0.35%, Si: 0.
15% or less, Mn: 0.5-2.0%, Ni: 2.5%
Below, Cr: 0.35% or less, Mo: 1% or less, Al:
0.1% or less, P: 0.020% or less, S: 0.030
% or less, and further contains at least one of Nb: 0.2% or less and V: 0.2% or less, with the balance consisting of Fe and inevitable impurities. (3) Weight ratio: C: 0.10-0.35%, Si: 0.
15% or less, Mn: 0.5-2.0%, Ni: 2.5%
Below, Cr: 0.35% or less, Mo: 1% or less, Al:
0.1% or less, P: 0.020% or less, S: 0.030
% or less, further containing one or more of Nb: 0.2% or less, V: 0.2% or less, Ti: 0.2% or less, and the remainder F
A high-strength gear steel characterized by comprising e and inevitable impurities.