JP2934485B2 - High-strength gear steel and high-strength gear that can be rapidly carburized - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to gears and gear steels used in automobiles and industrial machines and required to have high fatigue strength, and in particular, high strength capable of carburizing in a short time. The present invention relates to gear steel and high-strength gears.

[Prior Art] Low gear case hardening steels such as SCr420 and SCM420 are used for gears requiring high fatigue strength, and are generally manufactured by carburizing and quenching at around 900 ° C. However, in the carburizing and quenching treatment described above, an incompletely quenched layer is formed in the outermost layer of the carburizing, and since the layer is a soft layer, it causes a decrease in the fatigue strength at the root and the tooth surface.

The present inventors have developed a high-strength technology by optimizing components to be contained in steel and optimizing shot peening processing conditions for the purpose of improving the root fatigue strength.
This is proposed in JP-A-1-306521 and JP-A-1-306545. However, according to these methods, the fatigue strength at the root is improved, but the fatigue strength at the tooth surface is still low, leaving room for improvement.

Further, conventionally, a long time of 5 hours or more is required to obtain high fatigue strength by carburizing and quenching at about 900 ° C. In order to shorten this carburizing treatment time, it is not inconceivable to raise the carburizing temperature to, for example, about 950 ° C. or more, but in the chemical composition of the working steel described above, the crystal grains are coarsened and the fatigue strength is reduced. As a result, the carburizing temperature cannot be raised above 950 ° C. JP-A-62-1843 and JP-A-62-4819 show that by adding a large amount of Si, a two-phase structure of (α + γ) is formed and the crystal grains are coarsened even when the carburizing temperature is around 1000 ° C. Is disclosed that can suppress the occurrence of stagnation. However, in this method, the effect of improving the physical properties by the high temperature treatment is not as expected,
Inadequate in terms of fatigue strength at root and tooth surface.

[Problem to be Solved by the Invention] The present invention has been made by focusing on the above circumstances,
An object of the present invention is to provide a gear steel and a gear that can be rapidly carburized and have excellent fatigue strength at the root and the tooth surface.

[Means for Solving the Problems] The present invention that has achieved the above object is as follows: C: 0.1 to 0.2% Si: 0.05 to 0.10% Mn: 0.3 to 1.0% V: more than 0.5% and 1.5% or less Nb: 0.005 to 0.07 % Cr: 0.5 to 1.5%, and preferably further contains 0.5% of Mo.
High-strength gear steel capable of rapidly carburizing characterized by containing 2.0% and the balance being Fe and unavoidable impurities. The larger the Nb content is 0.040% or more, the larger the steel. This is desirable because fatigue strength can be obtained.

In addition, by using the high-strength gear steel, an average of 10 or more carbides of 3 μm or less is formed on the martensite ground over a range of 0.3 mm or more from the surface of the outermost layer.
A high-strength gear having a dispersity of 00 μm ^{2 and} a core portion having a ferrite area ratio of more than 5% and less than 60% and a balance of an austenite phase having a two-phase structure is obtained.

[Operation] The present inventors presuppose that rapid carburization is performed at a high temperature of 950 to 1,050 ° C, and during the intensive research on the relationship between the steel composition and the fatigue strength on the tooth surface and the tooth root, 950 to 1,050 ° C.
Under the conditions of high-temperature carburizing described above, it has been found that the composite addition of V and Nb is extremely effective in improving the fatigue strength of the tooth surface, and has reached the present invention.

In other words, V forms a two-phase structure state of ferrite and austenite by high-temperature carburization at 950 to 1,050 ° C to prevent crystal grains from being coarsened, and generates fine carbides on the outermost surface layer of the carburized steel to provide high fatigue strength on the tooth surface. It is thought to give.

In addition, when carburizing treatment is performed using this steel, as the carburization progresses, the outermost layer changes from a two-phase structure state of ferrite phase and austenite phase to a single-phase structure state of austenite phase, and the crystal by the appearance of mixed phase The effect of suppressing grain coarsening cannot be maintained. However, in this steel, by adding Nb and precipitating fine carbonitrides such as NbC, coarsening of crystal grains can be suppressed by a so-called pinning effect, and the fatigue strength of the tooth surface is kept high. is there.

Hereinafter, the reasons for limiting each component of the high-strength gear steel according to the present invention will be described.

C: 0.1 to 0.2% When the carbon content is less than 0.1%, the ferrite area ratio in the core of the gear increases, so that a predetermined strength cannot be obtained even after carburizing and quenching, and the fatigue strength at the tooth root decreases. On the other hand, if it exceeds 0.2%, it becomes difficult to obtain a two-phase structure by high-temperature carburizing treatment.
Therefore, the C content is set to 0.1% or more and 0.2% or less.

Si: 0.05 to 0.10% Si is added for the purpose of deoxidizing steel materials and is contained in an amount of 0.05% or more. However, if it is too much, grain boundary oxidation occurs remarkably during carburizing treatment, and the fatigue strength of the tooth base and tooth surface decreases. 0.
The upper limit was 10%.

Mn: 0.3 to 1.0% Mn is also added for the purpose of deoxidizing steel materials like Si, and is contained in an amount of 0.3% or more. However, too much Mn results in deterioration of machinability and generation of grain boundary oxides, and causes an increase in tooth root and tooth surface. Since the fatigue strength is reduced, the upper limit is set to 1.0%.

V: more than 0.5% and less than 1.5% V forms a two-phase microstructure in high-temperature carburizing at 950 ° C or higher to prevent crystal grains from coarsening, and forms fine carbide on the outermost surface of the carburized surface to form tooth surfaces. It is an element that gives high fatigue strength. However, if it is less than 0.5%, the carburization treatment cannot be accelerated because the two-phase structure state cannot be obtained unless it is lower than 950 ° C. On the other hand, if it exceeds 1.5%, the ferrite area ratio increases, resulting in a decrease in fatigue strength and cost. Not a good idea. Therefore, the V content is set to more than 0.5% and 1.5% or less.

Nb: 0.005 to 0.07% Nb is used to suppress the coarsening of crystal grains in the outermost surface layer of carburized steel.
It is necessary to contain 005% or more, and desirably 0.040% or more. On the other hand, if the content is too large, Nb carbon / nitride is generated and the workability such as machinability is impaired, so the upper limit was made 0.07%.

Cr: 0.5 to 1.5% Cr improves the hardenability and is effective in improving the strength of the core structure by high-temperature carburizing and quenching. However, if it is less than 0.5%, sufficient hardenability cannot be obtained, and if it exceeds 1.5%, the grain size becomes large. The content of Cr is 0.5-1.5% because it forms a network carbide in the interface and lowers the strength.
And

Mo: 0.5 to 2% Mo is an element effective for hardenability like Cr, and is particularly added in an amount of 0.5% or more to secure the hardenability of the carburized surface layer.
%, The combined effect with V causes the ferrite area ratio to increase too much, resulting in a decrease in fatigue strength. Therefore, the upper limit was set to 2.0%.

FIG. 1 is a graph showing the relationship between core hardness and ferrite area ratio after carburizing, quenching and tempering. If the ferrite area ratio is 60% or more, the core hardness becomes 200 Hv or less, and it is impossible to achieve high strength of the gear. On the other hand, if the ferrite area ratio is 5% or less, the effect of suppressing austenite crystal grain coarsening is lost and fatigue strength is reduced. Therefore, the area ratio of ferrite is more than 5% and 60%
Set to less than.

In order to achieve the high strength of the gear, it is effective to disperse carbides of 3 μm or less at an average ratio of 10 or more / 100 μm ² . The size and the degree of dispersion of the carbide vary greatly depending on the carburizing conditions. When the carburizing temperature exceeds 1050 ° C., the degree of dispersion of the carbide becomes 10 or more / 100 μm ^{2 on} average, but the size exceeds 3 μm. In addition, the martensitic structure of the matrix cannot be precipitated and hardened. On the other hand, at a carburizing temperature of less than 950 ° C., the size of the carbide is 3 μm or less, but the average degree of dispersion of the carbide is less than 10/100 μm ^2, and high strength is not achieved because austenite remains.

[Examples] Table 1 shows the chemical components of Invention Steels 1 to 6 and Comparative Steels 1 to 5. After forging these test steels, normalizing treatment was performed to process test specimens. Inventive steels 1 to 6 and comparative steels 1 to 3 were carburized for 2.5 hours in methane gas at the carburizing temperatures shown in Tables 2 and 3, and comparative steels 4 and 5 were carburized for 5 hours in methane gas at 925 ° C. Was given. Next, the furnace was cooled to 850 ° C., held for 30 minutes, quenched by oil cooling, and then cooled to 180 ° C.
For 2 hours and air-cooled.

Tables 2 and 3 show the carburizing characteristics and fatigue test results of the test pieces subjected to the above treatment. The rotating bending fatigue test was performed at a rotation speed of 3600 rpm using a smooth test piece having a parallel portion of 8 mm, and the rolling fatigue strength was measured under the conditions of a surface pressure of 600 kgf / mm ² .

Nos. 1 to 9 are high-strength gears according to the present invention, which satisfy the conditions of the present invention in all of the chemical composition, the number and size of carbides in the carburized layer, and the area ratio of ferrite in the core, enabling rapid carburization. It is also found that the carburizing characteristics and the fatigue strength are excellent.

Nos. 10 to 24 are comparative examples in which at least one of the above conditions of the present invention is not satisfied, and are inferior in carburizing characteristics and / or fatigue strength.

[Effects of the Invention] Since the present invention is configured as described above, it is possible to provide a gear steel and a high-strength gear that can perform a rapid carburizing treatment and have excellent fatigue strength at the root and the tooth surface. It became.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between core hardness and ferrite area ratio when carburizing, quenching and tempering are performed using the steel of the present invention.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-62-199765 (JP, A) JP-A-63-62859 (JP, A) JP-A-4-45244 (JP, A) (58) Field (Int.Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] C: 0.1 to 0.2% (% by weight, hereinafter the same) Si: 0.05 to 0.10% Mn: 0.3 to 1.0% V: more than 0.5% and 1.5% or less Nb: 0.005 to 0.07% Cr: 0.5 to 1.5% A high-strength gear steel that can be rapidly carburized, the balance comprising Fe and unavoidable impurities. 2. C: 0.1-0.2%, Si: 0.05-0.10%, Mn: 0.3-
1.0%, V: more than 0.5% and 1.5% or less, Nb: 0.005 to 0.07%, Cr: 0.5 to 1.5
%, Mo: 0.5 to 2.0%, with the balance being Fe and unavoidable impurities, a high-strength carburizing steel that can be rapidly carburized. 3. The high-strength gear steel according to claim 1, which contains 0.040% or more of Nb. 4. A high-strength gear comprising the steel for high-strength gears capable of rapid carburization according to any one of claims 1 to 3, wherein martensite extends over a range of 0.3 mm or more from the surface of the outermost layer. In the ground, carbides of 3μm or less exist with an average dispersity of 10 or more / 100μm ² , while in the core, the ferrite area ratio is more than 5% and less than 60%, and the rest consists of a two-phase structure of austenite phase. And high strength gears.