JPH073391A - Surface hardened high strength parts small in heat treating strain - Google Patents

Abstract

PURPOSE:To dvelop a steel for high strength machine structural use requiring surface hardness and small in heat treating strains by icorporating specified amounts of V, Mo, Al, N or the like into steel. CONSTITUTION:Machine structural parts are produced by case hardening steel obtd. by incorprating steel having a fundamental compsn. constituted of, by weight, 0.03 to 0.3% C, 0.03 to 0.35% Si, 0.3 to 3% Mn, <0.03% P and < 0.035% S, and the balance Fe with >0.1 to 0.5% V, 0.015 to 0.06% Al and 0.002 to 0.02% N ro furthermore incorporated with 0.03 to 1.5% Cr, 0.02 to 2% Ni, 0.02 to 2% Cu, 0.005 to 0.1% Ti and 0.005 to 0.1% Nb independently or compositely. The structure of thee surface layer face of the parts is formed of a mixed one of martensite and retained austenite, the structure of the core part is formed of a mixed one of ferrite and martensite, the volume ratio of the ferrite is regulated to $40%, V compounds having <=0.1mum size are precepitated in the ferrite and the amt. of the precipitated V compounds/ the amt. of V to be added >0.5 is regulated, by which the high strength machine structural parts whose surface has high hardness and free from heat treating strains can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to parts for various machine structures, particularly parts requiring surface hardness, such as gears used in various parts of automobiles, and particularly to those having small heat treatment distortion and high strength. It is related to various parts. In the following description, an application example to a gear for an automobile will be described as a representative example, but the parts of the present invention are not limited to the gears for an automobile or the parts for an automobile, and the strain due to heat treatment is small and the high strength is high. It is applicable to all required mechanical structural parts.

[0002]

2. Description of the Related Art With the spread of automobiles worldwide,
The impact on environmental problems from a global scale has come to be taken up, and it is an urgent task to improve the fuel efficiency and output of automobile engines for the purpose of reducing CO ₂ gas emissions. In order to address these issues, it is considered necessary to promote higher strength and lighter weight of automobile parts, and carburizing and quenching treatment has been conventionally performed as one of the means for achieving higher strength. ing.

On the other hand, there has been a strong desire from the automobile users to reduce the gear noise during the driving of the automobile, and the demand is becoming stronger as the performance of the automobile becomes higher. . For these reasons, in recent years, there has been a demand for realization of high-strength parts having less heat treatment distortion after carburizing and quenching from the viewpoint of improving quietness in addition to higher strength.

As a technique for reducing heat treatment distortion, a two-step heat treatment method (for example, Japanese Patent Laid-Open No. 58-113316) in which reheating and quenching treatment is performed after carburizing and cooling, and 200 ° C. after carburizing diffusion
Marquenching method of cooling and holding in moderate nitrate or nitrite, and further air cooling, method of adjusting chemical components and reducing heat treatment distortion by performing optimum carburizing treatment (for example, JP-A-2-298250) Is proposed. Further, a low strain carburizing steel (for example, Japanese Patent Laid-Open No. 60-50795) has been proposed which achieves low strain by adjusting the content of C and Mn to define the critical cooling rate. However, the technology proposed so far is still inadequate from the viewpoint of satisfying both characteristics of ensuring strength and reducing heat treatment strain.

The present invention has been made in order to solve these technical problems, and its purpose is to achieve high precision by reducing heat treatment strain, and also to have a high level of both properties of high strength. It is to provide a surface-hardened high-strength component that can be achieved by.

[0006]

The present invention capable of achieving the above object means C: 0.03 to 0.3%, Si: 0.03 to
0.35%, Mn: 0.3 to 3%, P: 0.03% or less, S: 0.035% or less, Mo: 0.1 to 1%, V:
More than 0.1 to 0.5%, Al: 0.015 to 0.06%,
N: 0.002-0.05% each, the balance being a part molded using a case-hardening net consisting of Fe and unavoidable impurities, and the surface layer is a mixed structure consisting mainly of martensite and retained austenite. In addition, the core has a mixed structure mainly composed of ferrite and martensite, the ferrite volume ratio of the core is 40% or more, and the V compound having a size of 0.1 μm or less is contained in the ferrite. The surface-hardened high-strength component is characterized in that the ratio of the amount of precipitated V compound and the amount of added V (amount of precipitated V compound / amount of added V) is 0.5 or more.

[0007]

The present inventors have made various studies from the viewpoint of satisfying both characteristics of high precision and high strength. As a result, they have found that the above objects can be achieved satisfactorily by specifying the chemical composition of the case-hardening steel to be used and appropriately adjusting the metallographic structure of the component after being made into a product, and completed the present invention. First, the reasons for limiting the chemical components in the invention are as follows.

C: 0.03 to 0.3% C is an element useful for guaranteeing the strength of the core of the component, and for this purpose, addition of 0.03% or more is required. However, if added excessively, the toughness deteriorates, machinability and cold forgeability deteriorate, and the workability deteriorates, and the two-phase temperature range narrows, making it difficult to secure stable core hardness. Therefore, it should be 0.3% or less.

Si: 0.03 to 0.35% Si is an element useful as a deoxidizing agent at the time of melting, and for this purpose, 0.03% or more must be added. On the other hand, excessive addition promotes intergranular oxidation, leads to a decrease in bending fatigue strength, and cold forgeability decreases, so the upper limit was set to 0.35%.

Mn: 0.3-3% Mn is an important element as a deoxidizing agent and for securing strength and hardenability, and it is necessary to add 0.3% or more, but 3% If it exceeds 1.0, the cold workability is deteriorated and segregation to the grain boundaries is increased, so that the grain boundary strength is lowered and, as a result, the bending fatigue strength is lowered. Therefore, the upper limit is set to 3%.

P: 0.03% or less P segregates at the grain boundaries to lower the toughness.
The upper limit was set to 0.03%. S: 0.035% or less S is a machinability-improving element and exerts a machinability improving action commensurate with the amount added, but excessive addition adversely affects cold forgeability, so 0.035% The following was set.

Mo: 0.1 to 1% Mo is an element useful for suppressing the grain boundary oxide layer, ensuring hardenability, forming retained austenite, and the like. The above addition is required, but 1%
Since the above-mentioned effect is saturated when the value exceeds, the upper limit was set to 1%.

V: more than 0.1 to 0.5% V is one of the important elements in the present invention, and the precipitation of the V compound in ferrite increases the strength after quenching. In order to exert such an effect, it is necessary to add more than 0.1%, but excessive addition causes coarsening of the V compound and the above effect reaches saturation. Therefore, the upper limit is set to 0. It was set at 5%.

Al: 0.015 to 0.06% Al is an element contained in steel as a deoxidizing agent, and also combines with N in steel to form AlN and prevents coarsening of crystal grains. In order to exert this effect, it is necessary to add 0.015% or more, but since the above effect saturates from around 0.06%, the upper limit was set to 0.06%.

N: 0.002-0.05% N is Al, V, (and Ti, N described later) in the steel.
b) and the like are combined with each other to form a nitride, which exhibits an effect of suppressing coarsening of crystal grains. In order to exert this effect, it is necessary to add 0.002% or more, but since this effect reaches saturation soon, 0.05% is set as the upper limit.

The case-hardening net used in the present invention contains the above-mentioned elements as basic components and the balance Fe and unavoidable impurities. If necessary, a predetermined amount of Cr, Ni, Cu, Ti,
Nb or the like may be added. However, among the above-mentioned optional additional elements, Cu is at least N for the reason described later.
i must be added simultaneously with i. The reasons for limiting the range of addition when adding these elements are as follows.

Cr: 0.03 to 1.5% Cr is an element useful for improving hardenability, and it is necessary to add 0.03% or more to exert this effect, but 1.5%.
Exceeding 1.0, grain boundary oxidation of the carburized layer will increase, so 1.5
% Was set as the upper limit. Ni: 0.02 to 2% Ni is an element useful for refining the structure after carburizing and ensuring stable core hardness, and 0.02% or more is added to exert this effect. However, if 2% is exceeded, this effect saturates, so 2% was defined as the upper limit.

Cu: 0.02 to 2% Cu is an element useful for preventing grain boundary oxidation, and it is necessary to add 0.02% or more to exert this effect.
If the content exceeds%, this effect saturates, so 2.0% was set as the upper limit. However, since hot workability deteriorates when Cu is added alone, when Cu is added, it is necessary to simultaneously add Ni having the effect of improving hot workability within the above range.

Ti: 0.005 to 0.1% Ti is an element which is combined with N to form a nitride and is useful for refining crystal grains. To exert this effect, 0.005
%, It is necessary to add 0.1% or more, but this effect reaches saturation soon, so 0.1% was set as the upper limit. Nb: 0.005 to 0.1% Nb combines with N in steel to form a nitride and suppresses coarsening of crystal grains. In order to exert this effect, 0.
It is necessary to add 005% or more, but excessive addition causes a reduction in cold workability, so 0.1% was set as the upper limit.

The object of the present invention is not achieved only by specifying the chemical composition of case-hardening steel, but it is necessary to satisfy other requirements as described above. Next, the reason why the ferrite volume ratio of the core portion, the particle size and the proportion of the precipitated V compound in the ferrite are specified will be described. Core ferrite volume ratio: 40% or more There is a correlation between the ferrite volume ratio and heat treatment strain, and in order to reduce heat treatment strain, the core ferrite volume ratio must be 40% or more. Size of V compound deposited in ferrite: 0.1 μm
Hereinafter, the V compound that effectively acts to increase the ferrite hardness is 0.
It has a size of 1 μm or less, and if it exceeds 0.1 μm, the precipitation of the V compound does not contribute to the increase in ferrite hardness. Precipitated V compound amount / added V amount: 0.5 or more When the value of deposited V compound amount / added V amount is 0.5 or more,
Since an increase in ferrite hardness was observed, the value of the amount of precipitated V compound / the amount of added V was set to 0.5 or more. The amount of the precipitated V compound in the present invention is a value obtained by measuring the amount of the precipitated V compound remaining in a mesh obtained by laminating several 0.2 μm mesh layers by the ICP-extraction residual analysis method.

In the high strength component of the present invention, "the surface layer has a mixed structure mainly composed of martensite and retained austenite, and the core has a mixed structure mainly composed of ferrite and martensite". The requirement is that the surface layer is mainly composed of martensite and retained austenite that can obtain a hard structure for the purpose of improving fatigue strength, and the core is mainly composed of ferrite and martensite necessary to achieve heat treatment strain reduction. It is an organization. Also in order to each such mixing organization, carburizing, carburizing cooled after the surface hardening treatment such as nitriding, then A _C3 transformation point or above when the C content in the composition of the used steel is 0.4% 850 It suffices to perform a treatment such as heating at a temperature of ℃ or less and holding and quenching.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any design changes made to the gist of the preceding and the following will be applied to the present invention. It is included in the technical scope.

[0023]

EXAMPLES Steel materials having the chemical compositions shown in Table 1 (Steel No. 1
~ 26) and machined into a module: 2.0,
Pressure angle: 20 °, Number of teeth: 28 (Mating gear number of teeth: 31
Gears), tooth width: 15 mm, carburized at 900 ° C. for 3 hours, cooled in a furnace, carbonitrided at 650 to 850 ° C. for 30 minutes, and then quenched in oil.
A tempering treatment was performed at 0 ° C. for 2 hours.

[0024]

[Table 1]

After the tempering treatment, the volume fraction of the core ferrite was measured by the image analysis processing, and the amount of precipitated V compound was measured by the IPC-extraction residual analysis method. After the tempering treatment, shot particles of HRC50 were used, and the projection speed was 80.
Shot peening was performed at m / s and a gear fatigue test was performed. Furthermore, in order to investigate the behavior of end face runout,
The axial test piece shown in FIG. 2 was produced by machining and subjected to the above carburizing treatment and tempering treatment. After that, the bending of the shaft (end face runout) at the position shown in FIG. 2 was measured. Table 2 shows core volume fraction of ferrite, size of precipitated V compound, ratio (amount of precipitated V compound / amount of added V), gear fatigue strength, end face runout, etc.
Fig. 1 shows the gear fatigue test results.

[0026]

[Table 2]

As is apparent from Table 2, the high-strength component of the present invention can stably secure a ferrite volume ratio of 40% or more, and as a result, end face runout, that is, heat treatment distortion can be reduced. Further, by satisfying (precipitated V compound amount / added V amount) of 0.5 or more, the ferrite hardness is kept high, and the gear fatigue strength is also excellent. It demonstrates its outstanding features without regret. On the other hand, the comparative examples lacking any of the requirements specified in the present invention are inferior in at least one of the characteristics.

That is, in Steel No. 19, since the amount of C is less than the lower limit specified in the present invention, the hardness of the core portion is lowered and the gear fatigue strength is lowered. In Steel No. 20, precipitation hardening was not observed because V was not added, and the gear fatigue strength was low. Steel No. 21 satisfies the chemical composition, but since the amount of V compound deposited is small, the gear fatigue strength is reduced. In Steel Nos. 22 and 23, the C content was larger than the upper limit specified in the present invention, so the core ferrite volume ratio was small and the end face runout was large. Steel No. 24 had a martensite single-phase structure in the core due to tempering treatment after carburizing and quenching, and no precipitation of V compound was observed, and in addition, end face runout occurred greatly. Steel No. 25 has a coarse V compound due to excessive addition of Mo. In Steel No. 26, the V content was larger than the upper limit specified in the present invention, the V compound became coarse, and the gear fatigue strength decreased.

[0029]

According to the present invention, which has the above-described structure, a surface-hardened high-strength component having less strain after heat treatment and having excellent fatigue strength can be realized. Then, by using such a component, reduction of gear noise of the automobile and reduction in size and weight of the component can be achieved.

[Brief description of drawings]

FIG. 1 is a graph showing gear fatigue test results.

FIG. 2 is a schematic explanatory view showing a shape of an end face runout measurement test piece and an end face runout measurement position.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Morifumi Nakamura 2 Nadahamahigashi-cho, Nada-ku, Kobe-shi, Hyogo Stock Company Kobe Steel Works Kobe Steel Works (72) Inventor Mikishi Hirahara Shinichi Fuchu-cho, Aki-gun, Hiroshima Prefecture No. Mazda Co., Ltd. (72) Inventor Yukio Arimi No. 3 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Co., Ltd.

Claims (6)

[Claims] 1. C: 0.03-0.3% (meaning weight%; the same applies hereinafter), Si: 0.03-0.35%, Mn:
0.3-3%, P: 0.03% or less, S: 0.035%
Hereinafter, Mo: 0.1 to 1%, V: more than 0.1 to 0.5%,
Al: 0.015 to 0.06%, N: 0.002 to 0.
It is a part formed by using a case wire containing 0.05% each and the balance Fe and unavoidable impurities. The surface layer has a mixed structure mainly composed of martensite and retained austenite, and the core part is ferrite. It has a mixed structure mainly composed of martensite, has a ferrite volume ratio of 40% or more in the core portion, and has a ferrite content of 0.
A V compound having a size of 1 μm or less is precipitated, and the ratio of the amount of the precipitated V compound and the amount of added V (amount of precipitated V compound / added V
Amount) is 0.5 or more, a surface-hardened high-strength component with little heat treatment distortion. 2. The surface-hardened high-strength component according to claim 1, wherein a case-hardening net further containing Cr: 0.03 to 1.5% is used. 3. A surface-hardened high-strength component according to claim 1, which further comprises a case-hardening net containing Ni: 0.02 to 2%. 4. The surface-hardened high-strength component according to claim 3, wherein a case-hardening net further containing Cu: 0.02 to 2% is used. 5. The surface-hardened high-strength component according to claim 1, which further comprises a case-hardening net containing Ti: 0.005 to 0.1%. 6. The surface-hardened high-strength component according to claim 1, which further comprises a case-hardening net containing Nb: 0.005-0.1%.