JP3436867B2 - Induction hardened part excellent in strength and fatigue resistance and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction hardened part having excellent static strength and fatigue resistance and a method for manufacturing the same, and more particularly to a gear for a differential gear of an automobile in which breakage of a tooth root due to fatigue is a problem. Induction-hardened parts and the manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, an automobile differential device is often used after being subjected to a surface hardening treatment, and carburizing, nitriding and induction hardening are adopted as the surface hardening method. Among these, "carburizing" is aimed at hardening by the matrix having high toughness and high carbonization of the surface layer of the material.
It is mainly applied to materials such as gears for the purpose of improving fatigue strength. However, the carburizing process is mainly performed in a batch process in a gas atmosphere, and a large amount of energy and cost are consumed, for example, having a heating and holding temperature of around 930 ° C. for several hours or more. In addition, in the actual operation, there is a problem that the environment tends to be deteriorated due to the treatment of carburized materials and the like, and there is a problem that it is difficult to inline.

Therefore, in order to solve these problems, studies have been conducted to obtain desired strength characteristics only by induction hardening. This is because induction hardening is very advantageous for shortening the time for surface hardening treatment, reducing energy, and further for making the environment cleaner.

Several reports have been made on the induction hardening treatment. For example, JP-A-5-33101
The publication discloses a proposal regarding non-heat treated steel for induction hardening. This is to ensure the required hardness of the matrix (core portion) itself by adjusting the amounts of C, Mn and Cr. However, as can be seen from the description of the example, the amount of C is set to 0.52% or less from the viewpoint of toughness. In other words, in order to substitute for carburized steel, it is necessary to make the surface layer hardness as high as that of carburized steel (Vickers hardness of about 700 or more), whereas the matrix (core) when the C content is 0.55% or more. However, it cannot be said that it is a sufficient alternative steel for carburizing because the measures to reduce the toughness of the part) and the low cycle fatigue are insufficient.

Further, it is known that grain refinement is effective as a means for improving the toughness of high carbon steel. For example, Japanese Patent Laid-Open No. 61-147849 discloses that fine ferrite and fine lath martensite are obtained by quenching after forging in a non-recrystallized region. Further, JP-A-5-9576 discloses that a fine ferrite pearlite structure is obtained by rolling a non-heat treated steel bar in a non-recrystallized region and accelerating cooling after rolling.
However, both steels are completely different from induction hardened steels, and since the steel types are different, the strength level has not reached the hardness equivalent to carburized steel (Vickers hardness of about 700 or more). In addition, both of the characteristics of mixing fine ferrite in the structure of the hardened layer are characteristic, but the mixing of ferrite promotes a yield phenomenon that is harmful to fatigue of gears. It was not preferable.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides an induction-hardened component which is economical and has strength and fatigue resistance equivalent to a carburized material only by subjecting the component to induction-hardening treatment, and a method for producing the same. In particular, the present invention intends to convert from the carburizing process to the induction hardening process for parts used for gears for automobile differentials.

[0007]

The gist of the present invention is as follows. (1) C: 0.40% or more and 0.70% or less by mass %,
Si: 0.05% or more and 0.80% or less, Mn: 0.50
% To 2.00%, S: 0.01% to 0.03%
Hereinafter, V: 0.30% or more and 1.00% or less, Al: 0.
It contains 010% or more and 0.050% or less, N: 0.0050% or more and 0.0200% or less, P is 0.030% or less, and the balance is Fe and unavoidable impurities. Induction-hardened parts with excellent static strength and fatigue resistance, characterized in that the grain size of the site is JIS No. 14 or above. (2) In addition to the components described in (1), further in mass% , N
b: The induction-hardened component according to (1), which contains 0.05% or more and 0.50% or less. (3) In addition to the components described in (1) or (2), further mass
% , Cr: 0.1% or more and 1.50% or less, The induction hardened component according to (1) or (2), characterized in that

(4) C: 0.40% or more by mass% ,
70% or less, Si: 0.05% or more and 0.80% or less, M
n: 0.50% to 2.00%, S: 0.01% to 0.03%, V: 0.30% to 1.00%, Al: 0.010% to 0.050% Below, N:
Containing 0.0050% to 0.0200% or less, P is at 0.030% or less, the steel and the balance Fe and unavoidable impurities as a material, the said workpiece 900 ° C. or higher 1150
After heating at a temperature below ℃, 600 ℃ below 850 ℃
2 times warm processing with a processing rate of 30% or more
The method for producing an induction-hardened component having excellent static strength and fatigue resistance is characterized in that the above-mentioned and then induction hardening treatment is performed so that the grain size of martensite in the hardened layer is 14 or more in JIS grain size number. (5) In addition to the components described in (4), further in mass% , N
b: The method for producing an induction-hardened component according to (4), characterized in that steel containing 0.05% or more and 0.50% or less is used as a raw material. (6) In addition to the components described in (4) or (5), further mass
% , Cr: 0.1% or more and 1.50% or less of steel is used as a raw material, The manufacturing method of the induction hardening component of (4) or (5) characterized by the above-mentioned.

According to the present inventions (1) to (3), the grain size of the martensite structure of the hardened layer after induction hardening is JIS No. 10 (particle size 12.
4 μm), whereas in the present invention, the average number of particles is 14 or more (grain size 3.1 μm or less), the static strength of the hardened layer, impact characteristics and fatigue resistance are higher than those of conventional induction hardened materials. The characteristics are significantly improved, and the performance targeted by the present invention can be obtained.

According to the present inventions (4) to (6) , the steel for machine structural use having the required components is processed in the unrecrystallized region at least two times of large strain processing of 30% or more. By performing the warm working of the above, large strain processing of 60% or more with the cumulative working amount, then allowing to cool to room temperature, and then induction hardening, a structure with extremely fine former austenite grain size was obtained. To be That is, by large strain processing in the non-recrystallized region,
Transformation of austenite to fine ferrite / ferrite structure due to increase of γ grain boundary and introduction of deformation zone,
Then, when induction hardening is performed, the grain refinement effect of the preceding structure and the rapid heating effect by high frequency are combined, the austenite grains at the time of reheating are refined, and the quenching structure is also refined.

Here, as a result of adding a predetermined amount of V or further adding Nb, V carbonitrides and Nb carbonitrides are produced during cooling in warm forging, and the microstructure after cooling is refined. After that, when induction hardening is applied, the grain refinement effect of the preceding structure and the rapid heating effect by high frequency are combined, and the austenite grains at the time of reheating are further refined.

In order to make the grain-refining effect of V carbonitride and Nb carbonitride work effectively during warm forging, heating is performed once to a temperature between 900 ° C. and 1150 ° C. , and V carbonitride and Nb The carbonitride is once made into a solid solution and then precipitated in the subsequent cooling step, thereby exerting a large refining effect in cooperation with large strain processing.

Due to the combination of the above-mentioned warm working and induction hardening, the grain size of the martensite structure of the hardened layer after induction hardening is 10 in JIS in the case of ordinary induction hardened steel.
No. 14 (particle size 12.4 μm), on the other hand, according to the present invention, an average number of 14 or more (particle size 3.1 μm or less) can be obtained. Strength,
The impact characteristics and fatigue resistance characteristics are significantly improved, and the performance targeted by the present invention can be obtained.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. The chemical composition of the material steel of the present invention is as follows. C has a function of ensuring a desired strength in steel (core portion) and further a function of ensuring a surface hardness after induction hardening, but when the content thereof is less than 0.40%, the above-mentioned effect is desired. However, if the content exceeds 0.70%, the toughness deteriorates. Therefore, C
The content was set to 0.40% to 0.70%, but 0.53% to 0.70% to secure the above effect more stably.
It is preferably adjusted to 0%.

Si is contained as a deoxidizing material during steel making and is an element for improving the strength of steel, and its content is adjusted according to the required strength. However, the Si content needs to be 0.05% or more in order to make the deoxidizing effect effective. On the other hand, if it exceeds 0.80%, the toughness and ductility of the steel become low, and at the same time, the Workability is reduced. Therefore, the Si content is determined to be 0.05% or more and 0.80% or less.

Like Si, Mn is an element for improving the strength of steel, and its content is adjusted according to the required strength. Therefore, in order to make this effect effective, it is necessary to secure the content of 0.50% or more. However, the Mn content is 2.
If it exceeds 00%, the hardenability is excessively improved, and the production of the bainite structure or the island martensite structure is promoted during the production of the raw material and the warm forging, so that the workability is deteriorated. Therefore, the Mn content is set to 0.50% to 2.00%, but in order to secure the above effect more stably, it is set to 0.
It is preferably adjusted to 70% to 1.50%.

Like Mn, Cr is an element for improving the strength of steel materials, and the characteristics of the developed steel are not impaired even if a predetermined amount is added according to the required strength and the size of parts. However,
If the Cr content is less than 0.1%, the desired effect due to the above action cannot be obtained, while if it exceeds 1.50%, the hardenability is excessively improved and the bainite structure or The formation of the island martensite structure is promoted and the workability is lowered. Therefore, when adding, it is preferable to adjust to 0.1% or more and 1.50% or less.

Mo and Ni have the function of reducing grain boundary segregation and strengthening austenite, and the addition of a predetermined amount does not impair the characteristics of the developed steel.

P deteriorates the toughness of the hardened layer. Especially 0.
If it exceeds 030%, the toughness will deteriorate, so
Although the P content is set to 0.030% or less, it is preferably adjusted to 0.020% or less.

S is an element effective in improving machinability. If the content of S is less than 0.01%, the desired effect due to the action cannot be obtained. On the other hand, if it exceeds 0.03%, S of the steel material cannot be obtained. Ductility is significantly reduced. Therefore, the S content is set to 0.01% or more and 0.03% or less.

V is an element effective for enhancing hardenability and fatigue strength, and is an element for forming carbonitrides and refining crystal grains, in order to secure a desired effect. It is necessary to contain 0.30% or more. But,
If it exceeds 1.00%, the effect is saturated. Therefore, the V content is set to 0.30% to 1.00%.

Nb is an element that produces a nitride and refines the crystal grains, and 0.050 is necessary to secure a desired effect.
% Or more is necessary. But 0.50
If it exceeds%, the effect is saturated. Therefore, the Nb content is set to 0.050% to 0.50%.

Al is an element which has a deoxidizing action and has a grain refining effect as AlN, and it is necessary to contain 0.010% or more in order to secure a desired effect. However, if it exceeds 0.050%, not only the effect is saturated, but also the workability and fatigue characteristics of the steel material are deteriorated. Therefore, the Al content is 0.010% to 0.050.
%, But preferably 0.015% to 0.030
It is good to adjust to%.

N is an element having a strong affinity with Al, V and Nb, and is an element for precipitating crystal grains by precipitating Al, V and Nb as nitrides in steel. N content is 0.
If it is less than 0050%, the desired effect due to the above-mentioned action cannot be obtained, while if it exceeds 0.0200%, the toughness decreases. Therefore, the N content is 0.0050
% To 0.0200%.

In addition, in the steel of the present invention, the above-mentioned effects are impaired even if free-cutting elements such as Pb, Bi, Te, Ca, etc., which improve the machinability, are added in addition to the respective components according to the present invention. There is no. Therefore, if more machinability is desired for finishing the part after warm forging, one or more of the above free-cutting elements may be added.

The crystal grain condition of the hardened layer of the present invention will be described below. The toughness and strength of the induction hardened steel are strongly influenced by the grain size of the hardened layer when the hardness of the hardened layer and the hardness of the core are substantially constant. As shown in FIG. 1, the impact value and the three-point bending strength are improved with the refinement of crystal grains,
In particular, it can be remarkably improved by making the particle size to be JIS No. 14 (3.1 μm) or more. Therefore, the crystal grain size of the cured layer was determined to be JIS No. 14 or above.

The conditions of the warm working of the present invention will be described below.
The warm working is a step of forming the shape of the component at the same time as miniaturizing the structure before induction heat treatment by warm forging or warm rolling. Generally, carburized parts or induction hardened parts are machined after hot forging. However, warm forging is an indispensable step in order to achieve an extremely fine structure after induction hardening while omitting heat treatment and annealing before the induction surface treatment. That is, the material is subjected to large strain processing in the non-recrystallized region and then allowed to cool to room temperature to sufficiently refine the structure before induction hardening.

In this case, if the heating temperature of the warm forging before forging is less than 900 ° C., the solid solution of V and Nb is insufficient, and the desired effect described above cannot be obtained, and if it exceeds 1150 ° C., the unrecrystallized region is not obtained. The heating temperature is 900
It is better to adjust the temperature to 1150 ° C. or higher. The processing start temperature is 600 ° C or higher because it goes through processing in the non-recrystallization temperature range.
It is preferable to adjust the temperature to 850 ° C. or lower, preferably 600 ° C. or higher and 700 ° C. or lower.

Further, the large strain working in the non-recrystallized region is an effect of transforming austenite into a fine ferrite / ferrite structure by increasing the γ grain boundaries and introducing a deformation zone, and refining the structure before induction hardening. There is. in this case,
If the single processing rate is less than 30%, the grain refining effect is insufficient, and if the cumulative processing rate is less than 60%, the grain refining effect is insufficient. Therefore, it was determined that the large strain processing in the non-recrystallized region should be performed twice or more at a processing rate of 30% or more.

Although cooling after the large strain processing is basically allowed to cool, the above-mentioned effects are not impaired even if air cooling or water cooling is performed as a measure for handling.

The induction hardening conditions of the present invention are described below.
After the warm working, a finishing machining process is performed, and then an induction surface quenching process is performed. In this case, if the temperature is lower than 850 ° C., it cannot be sufficiently dissolved by a short time treatment such as high frequency heating, and if the temperature is higher than 1100 ° C., the growth of crystal grains occurs and the structure refining effect cannot be sufficiently exerted. Therefore, the heating temperature is 850 ° C to 1100 ° C, preferably 850 ° C.
It is better to adjust the temperature to 900 ° C. After induction heating, water quenching is performed, and tempering is performed if necessary.

[0032]

Example Steels having the chemical composition shown in Table 1 were melted in a vacuum melting furnace and cast into ingots. Then, the obtained ingot was formed into a round steel bar having a diameter of 50 mm by hot forging. Subsequently, the heating temperature, the processing start temperature and the processing rate were variously changed, and warm forging was performed. The heating temperature is 850, 900, 9
50, 1100, 1150, 1200 ° C., and the working start temperature of warm forging is 500, 600, 650, 750, 8
The temperature was set to 00, 850, and 950 ° C. A thermocouple was embedded at a position of 1/2 the radius of the round steel bar, and forging was started when the temperature reached a predetermined temperature by cooling. Further, the processing rate was set to 20, 30, 40, 60% once, and the same processing rate was repeated twice or more, and the cumulative processing rate was variously changed from 20% to 91%. After the warm working, the round steel bar was cooled to room temperature by cooling. Then, by machining from round bar steel, 10 mm square, 70 mm long,
Three-point bending test piece with a semicircular cutout of 2 mmR in the center, grip portion 15 mmφ, parallel portion 9 mmφ notch (ρ = 1,
Ono-type rotary bending fatigue test piece with α = 1.66) and 2
A 0 mmφ and 200 mmL length test piece for material investigation was prepared. For the various processing conditions, see Tables 2 and 3.
Are summarized in. The chemical components are shown in comparison with Table 1.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

Next, each test material was subjected to induction hardening. The surface hardness, the grain size of the hardened layer, the three-point bending strength, and the rotational bending fatigue strength achieved by this were investigated. The survey results are shown in Tables 2 and 3. The induction hardening conditions at this time were: frequency: 3 kHz output: 100 kW moving speed: 5
It was mm / s. For comparison, the results of induction hardening and carburizing treatment of conventional steel without warm forging were also investigated.

The survey results shown in Tables 2 and 3 show the following. Here, in the method of the present invention (using A1 to A13 and carried out under the processing conditions within the scope of the present invention), the induction hardened hardened layer has a crystal grain size of 14 according to the JIS grain size number.
It achieves a very fine grain size as high as or better than No. 2, has a high surface hardness, a high three-point bending strength and a high rotational bending fatigue strength, and has characteristics equal to or higher than those of ordinary carburized steel (C2). In other words, it can be seen that although the developed method is an induction hardening treatment, it has excellent characteristics equivalent to or better than the carburizing treatment.

On the other hand, when steels (B1 to B13) having components outside the scope of the present invention are treated under the processing conditions within the scope of the present invention, B1 and B2 do not produce fine grain steel of No. 14 or above. Therefore, strength and fatigue strength are low, and B4 to B
In No. 13, although fine particles of No. 14 and above can be obtained, it can be seen that excellent characteristics comparable to those of the method of the present invention cannot be obtained due to deterioration of toughness and lack of hardness. Furthermore, when steel (A1) having a component within the scope of the present invention is processed under processing conditions outside the scope of the present invention, fine grains of No. 14 and above cannot be obtained, so the strength and fatigue strength are the same as those of conventional induction hardened steels. Although it is slightly better than (C1), it is clear that it does not reach the level of ordinary carburized steel (C2).

[0039]

As described above, the induction hardened parts of the present invention can obtain static strength and bending fatigue strength equal to or higher than those of carburized steel only by induction hardening, and conventionally, carburized. Instead of the conventional steel grade, it will contribute to the improvement of the performance of various machine structural parts and the reduction of the manufacturing cost, and will have useful effects in industry.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of grain size of a hardened layer on impact value and three-point bending strength.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI C22C 38/38 C22C 38/38 F16H 55/06 F16H 55/06 (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 C21D 1/00-11/00 F16H 55/06

Claims (6)

(57) [Claims] 1. In mass% , C: 0.40% or more and 0.70% or less, Si: 0.05% or more and 0.80% or less, Mn: 0.50% or more and 2.00% or less, S: 0.01% or more and 0.03% or less, V: 0.30% or more and 1.00% or less, Al: 0.010% or more and 0.050% or less, N: 0.0050% or more and 0.0200% or less Included, P is 0.030% or less, the balance is Fe and unavoidable impurities, and the grain size of martensite in the induction hardening layer is JI.
Induction-hardened parts with excellent static strength and fatigue resistance, characterized by having an S grain size number of 14 or more. 2. In yet mass% in addition to the ingredients mentioned in claim 1, Nb: induction hardening component according to claim 1, characterized in that it contains 0.05% to 0.50% or less. In 3. Further mass% in addition to the ingredients mentioned in claim 1 or 2, Cr: according to claim 1 or 2, characterized in that it contains 0.1% to 1.50% or less Induction hardened parts. 4. In mass% , C: 0.40% or more and 0.70% or less, Si: 0.05% or more and 0.80% or less, Mn: 0.50% or more and 2.00% or less, S: 0.01% or more and 0.03% or less, V: 0.30% or more and 1.00% or less, Al: 0.010% or more and 0.050% or less, N: 0.0050% or more and 0.0200% or less Contained, P is 0.030% or less, and a steel composed of the balance Fe and unavoidable impurities is used as a material, and the material is heated at a temperature of 900 ° C. or more and 1150 ° C. or less.
After that, processing that starts processing at a temperature of 600 ° C or below 850 ° C
Static strength and fatigue resistance, characterized in that the grain size of martensite in the hardened layer is 14 or more in JIS grain size number after performing the warm working of the rate of 30% or more twice or more and then the induction hardening treatment. Manufacturing method of induction hardened parts with excellent characteristics. 5. The steel according to claim 4, characterized in that, in addition to the components described in claim 4, a steel containing Nb: 0.05% or more and 0.50% or less by mass % is used as a raw material. Manufacturing method of induction hardened parts. 6. A steel material containing, in addition to the components described in claim 4 or 5, a mass % of Cr: 0.1% or more and 1.50% or less, as a raw material. Alternatively, the method for producing an induction-hardened component according to item 5.