JPH1171633A - Induction hardened parts excellent in strength and fatigue resistance and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain parts having static strength and fatigue resistance equal to those of a carburizing material only by executing induction hardening treatment after formation into parts, by specifying the componental compsn. of a steel and regulating the grain size of martensite in an induction hardened layer to a specified value or above. SOLUTION: A steel contg., by weight, 0.40 to 0.70% C, 0.05 to 0.80% Si, 0.50 to 2.00% Mn, 0.01 to 0.03% S, 0.30 to 1.00% V, 0.010 to 0.050% Al, 0.0050 to 0.0200% N, <=0.030% P, and the balance Fe is used as a stock and is subjected to large strain warm working of >=30% working ratio in an unrecrystallized temp. range for >= two times. Then, induction hardening is executed to regulate the grain size of martensite in the hardened layer to No. >=14 by the JIS size number. Furthermore, the above components may be incorporated with 0.05 to 0.50% Nb or 0.1 to 1.50% Cr as well. Moreover, the temp. of the warm working is preferably regulated to 900 to 1100 deg.C and the working starting temp. to 600 to 800 deg.C.

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 of manufacturing the same, and more particularly, to a gear for a differential gear of an automobile in which tooth root breakage due to fatigue is a problem. And a method of manufacturing the same.

[0002]

2. Description of the Related Art Conventionally, automobile differentials are often used after being subjected to a surface hardening treatment, and carburizing, nitriding and induction hardening are employed as surface hardening methods. Among these, "carburizing" aims to harden the matrix by increasing the carbon surface layer of the material with high toughness,
It is mainly applied to gears and other materials for the purpose of improving fatigue strength. However, in the carburizing process, a batch process in a gas atmosphere is predominant, and a lot of energy and cost are consumed, for example, a heating and holding at around 930 ° C. for several hours or more. In addition, in actual operation, there is a problem that the environment is liable to be deteriorated due to the treatment of carburizing material and the like, and there is a problem that it is difficult to in-line.

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

Several reports have been made on the induction hardening process. For example, JP-A-5-33101
In the publication, a proposal regarding a non-heat treated steel for induction hardening is disclosed. This is to ensure the required hardness of the matrix (core) itself by adjusting the amounts of C, Mn and Cr. However, as can be seen from the description of the example, the C content is set to 0.52% or less from the viewpoint of toughness. In other words, in order to substitute carburized steel, it is necessary to make the surface layer hardness comparable to that of carburized steel (about 700 or more in Vickers hardness), while the matrix (core) when the C content is 0.55% or more is required. However, because the measures to reduce the toughness and low cycle fatigue of the part (1) were insufficient, they could not be said to be sufficient as a carburizing alternative steel.

It is known that refinement of crystal grains is effective as a means for improving the toughness of high carbon steel. For example, Japanese Patent Application Laid-Open No. 61-147849 discloses that fine ferrite and fine lath martensite are obtained by forging in a non-recrystallized region and then quenching. Further, JP-A-5-9576 discloses that a fine ferrite pearlite structure is obtained by rolling a non-heat-treated steel bar in an unrecrystallized region and accelerated cooling after the rolling.
However, both are steel materials completely different from induction hardened steels, and since the steel types are different, the strength level has not reached hardness as high as carburized steel (about 700 or more in Vickers hardness). In addition, the mixture of fine ferrite in the structure of the hardened layer is a feature of both, but the mixture of ferrite promotes a yield phenomenon that is detrimental to gear fatigue. It was not good.

[0006]

SUMMARY OF THE INVENTION The present invention provides an induction hardened part which is economical only by performing induction hardening after forming the part and has strength and fatigue resistance comparable to those of carburized materials, and a method of manufacturing the same. In particular, an attempt is made to switch from carburizing to induction hardening for parts used in gears for differential gears of automobiles and the like.

[0007]

The gist of the present invention is as follows. (1) By weight%, C: 0.40% to 0.70%,
Si: 0.05% or more and 0.80% or less, Mn: 0.50
% To 2.00%, S: 0.01% to 0.03%
V: 0.30% or more and 1.00% or less;
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 consists of Fe and unavoidable impurities. An induction hardened part having excellent static strength and fatigue resistance, wherein the crystal grain size of the site is 14 or more in JIS grain size number. (2) In addition to the components described in (1), N
b: The induction hardened part according to (1), which contains 0.05% or more and 0.50% or less. (3) The composition as described in (1) or (2), further comprising Cr: 0.1% or more and 1.50% or less by weight in addition to the components described in (1) or (2). Induction hardened parts.

(4) By weight%, C: 0.40% or more.
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% Hereinafter, N:
It contains 0.0050% or more and 0.0200% or less, P is 0.030% or less, and is made of steel consisting of the balance of Fe and unavoidable impurities, and has a working rate of 30% in a non-recrystallization temperature range.
The above-mentioned large strain warm working is performed twice or more, and then induction hardening is performed, and the grain size of martensite in the hardened layer is 14 or more in JIS grain size number. Method of manufacturing induction hardened parts excellent in quality. (5) In addition to the components described in (4), N
b: The method for producing an induction hardened component according to (4), wherein the steel is a steel containing 0.05% or more and 0.50% or less. (6) In addition to the components described in (4) or (5), a steel further containing Cr: 0.1% or more and 1.50% or less by weight is used as a material (4). Or the method of manufacturing the induction hardened part according to (5). (7) The manufacturing of the induction hardened parts according to (4) to (6), wherein the heating temperature of the warm working is 900 ° C. or more and 1100 ° C. or less, and the working start temperature is 600 ° C. or more and 800 ° C. or less. Method.

According to the present inventions (1) to (3), the grain size of the martensite structure of the hardened layer after induction hardening is 10th in the JIS grain size number for a normal induction hardened steel.
4 μm), whereas in the present invention, the average number is 14 or more (particle size of 3.1 μm or less). Therefore, compared with the conventional induction hardened material, the static strength, impact characteristics, and fatigue resistance of the hardened layer are higher. The characteristics are greatly improved, and the performance targeted by the present invention can be obtained.

According to the present inventions (4) to (7), a steel for machine structural use having a required component is processed twice or more in a large strain of 30% or more in an unrecrystallized region. A large strain of 60% or more in cumulative working amount is carried out, then cooled to room temperature, and then subjected to induction hardening to obtain a structure with an extremely fine austenite grain size. Can be In other words, by large strain processing in the non-recrystallized area,
Transformation from austenite to fine ferrite / ferrite structure due to increase of γ grain boundary and introduction of deformation zone,
Thereafter, when induction hardening is performed, the effect of refining the prestructure and the effect of rapid heating by high frequency are combined, so that austenite grains during reheating are refined, and the quenched structure is also refined.

Here, as a result of adding a predetermined amount of V or further adding Nb, V carbonitride and Nb carbonitride are generated during cooling in warm forging, and the structure after cooling is refined. Then, when induction hardening is performed, the effect of refining the prestructure and the effect of rapid heating by high frequency are combined, and the austenite grains during reheating are further refined.

In order to make the grain refinement effect of V carbonitride and Nb carbonitride work effectively at the time of warm forging, it is heated once to a temperature between 900 ° C. and 1100 ° C. By temporarily dissolving the carbonitride and precipitating it in the subsequent cooling step, a large refining effect is exhibited in cooperation with large strain processing.

[0013] By 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 grain size number for ordinary induction hardened steel.
According to the present invention, on the other hand, the average number is 14 or more (the particle size is 3.1 μm or less), whereas the number of the hardened layer is smaller than that of the conventional induction hardened material. Strength,
The impact characteristics and fatigue resistance characteristics are significantly improved, and the performance targeted by the present invention can be obtained.

[0014]

Embodiments of the present invention will be described below in detail. The chemical composition of the material steel of the present invention is as follows. C has the effect of ensuring the desired strength of the steel (core) and the effect of ensuring the surface hardness after induction hardening. If the content is less than 0.40%, the desired effect due to the above-mentioned effect is obtained. Cannot be obtained. On the other hand, if the content exceeds 0.70%, the toughness deteriorates. Therefore, C
Although the content was determined to be 0.40% to 0.70%, in order to secure the above effect more stably, the content was 0.53% to 0.70%.
It is preferable to adjust to 0%.

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

Mn is an element for improving the strength of a steel material like Si, and its content is adjusted according to the required strength. Therefore, it is necessary to secure a content of 0.50% or more to make this effect effective. However, when the Mn content is 2.
If it exceeds 00%, the hardenability is excessively improved, and the formation of a bainite structure or an island-like martensite structure during material production and warm forging is promoted, and workability is reduced. Therefore, the Mn content is determined to be 0.50% to 2.00%.
It is preferable to adjust from 70% to 1.50%.

Cr, like Mn, is an element for improving the strength of a steel material. Even if a predetermined amount is added in accordance with the required strength and the size of parts, the characteristics of the developed steel are not impaired. However,
If the Cr content is less than 0.1%, the desired effect cannot be obtained by the above-mentioned action, while if it exceeds 1.50%, the hardenability is excessively improved, so that the bainite structure or The formation of an island-like martensite structure is promoted, and workability is reduced. Therefore, when adding, it is preferable to adjust to 0.1% or more and 1.50% or less.

Mo and Ni have the effect of reducing grain boundary segregation and strengthening austenite, and even if they are added in predetermined amounts, the properties of the developed steel are not impaired.

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

S is an element effective for improving the machinability. If its content is less than 0.01%, the desired effect cannot be obtained by the effect, while if it exceeds 0.03%, the content of the steel material cannot be improved. The ductility is greatly reduced. Therefore, the S content is determined to be 0.01% or more and 0.03% or less.

V is an element effective for improving hardenability and improving fatigue strength, and is an element that forms carbonitrides and refines crystal grains. It is necessary to contain 0.30% or more. But,
If it exceeds 1.00%, the effect is saturated. Therefore, the V content was determined to be 0.30% to 1.00%.

Nb is an element that forms nitrides and refines crystal grains.
% Or more. However, 0.50
%, The effect saturates. Therefore, the Nb content was determined to be 0.050% to 0.50%.

Al is an element which has a deoxidizing effect 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 the content exceeds 0.050%, not only the effect is saturated, but also the workability and fatigue characteristics of the steel material deteriorate. Therefore, the Al content ranges from 0.010% to 0.050.
%, But preferably 0.015% to 0.030%.
It is better to adjust to%.

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

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

The crystal grain size conditions of the cured layer of the present invention will be described below. The toughness and strength of induction hardened steel are strongly affected by the crystal grain size of the hardened layer when the hardness and the core hardness of the hardened layer are substantially constant. As shown in FIG. 1, the impact value and the three-point bending strength are improved with the refinement of the crystal grains.
In particular, it is remarkably improved by reducing the size to 14 or more (3.1 μm) in JIS particle size number. Therefore, the crystal grain size of the hardened layer was determined to be 14 or more in JIS grain size number.

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

In this case, if the heating temperature before the forging in the warm forging is lower than 900 ° C., the solid solution of V and Nb is insufficient, so that the above-mentioned desired effects cannot be obtained. Heating temperature is 900
It is preferable to adjust the temperature to 1150 ° C. or higher. The processing start temperature is adjusted to be 600 ° C. or more and 800 ° C. or less, preferably 600 ° C. or more and 700 ° C. or less in order to perform processing in the non-recrystallization temperature range.

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

The cooling after the large strain processing is basically performed by cooling, but the above-mentioned effect is not impaired even if air cooling or water cooling is performed due to handling measures.

The induction hardening conditions of the present invention will be described below.
After the warm working, a finishing machining is performed, and then a high frequency surface quenching is performed. In this case, if the temperature is lower than 850 ° C., the solution cannot be sufficiently dissolved by a short-time treatment of high-frequency heating, and if the temperature is higher than 1100 ° C., crystal grains grow, and the effect of refining the structure cannot be sufficiently exhibited. Therefore, the heating temperature is 850 ° C. to 1100 ° C., preferably 850 ° C.
The temperature should be adjusted to 900 ° C. After the high frequency heating, water quenching is performed, and tempering is performed as necessary.

[0032]

EXAMPLES Steel having the chemical components shown in Table 1 was melted in a vacuum melting furnace and cast into ingots. Next, 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. Heating temperature is 850, 900, 9
50, 1100, 1150, 1200 ° C., and the warming forging process starting temperatures are 500, 600, 650, 750, 8
00, 850 and 950 ° C. The working temperature was such that a thermocouple was embedded at a position 位置 of the radius of the round bar, and forging was started when the temperature reached a predetermined temperature by cooling. The processing rate was set to 20, 30, 40, and 60% at one time, and the same processing rate was repeated twice or more, and the cumulative processing rate was variously changed from 20% to 91%. After warm working, the round bar was allowed to cool to room temperature by cooling. Then, 10mm square, 70mm long by machining from round bar steel,
A three-point bending test piece with a semicircular notch of 2 mmR at the center, a gripping part of 15 mmφ, and a parallel part of 9 mmφ with a notch (ρ = 1,
α = 1.66) Ono-type rotating bending fatigue test piece and 2
A test piece for material investigation of 0 mmφ and length of 200 mmL was prepared. Tables 2 and 3 show various processing conditions.
Are shown together. 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 crystal grain size of the hardened layer, the three-point bending strength, and the rotating bending fatigue strength thus achieved 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 mm / s. For comparison, the results of induction hardening and carburizing of conventional steel without warm forging were also investigated.

The following can be seen from the survey results shown in Tables 2 and 3. Here, in the method of the present invention (implemented under the processing conditions within the range of the present invention using A1 to A13), the hardened layer subjected to induction quenching has a crystal grain size of all JIS grain sizes of 14
It achieves an extremely fine grain size of at least one, has a high surface hardness, a high three-point bending strength and a high rotational bending fatigue strength, and has properties equivalent 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, excellent characteristics equal to or better than the carburizing treatment can be obtained.

On the other hand, when steels (B1 to B13) having components outside the range of the present invention are processed under the processing conditions within the range of the present invention, fine grained steel of No. 14 or more cannot be obtained with B1 and B2. Therefore, strength and fatigue strength are low, and B4 to B
In the case of No. 13, fine particles of No. 14 or more can be obtained, but it is understood that excellent properties comparable to the method of the present invention cannot be obtained due to a decrease in toughness and insufficient hardness. Further, when a steel (A1) having a component within the range of the present invention is processed under processing conditions outside the range of the present invention, fine grains of No. 14 or more cannot be obtained, and the strength and fatigue strength are the same as those of the conventional induction hardened steel. Although slightly better than (C1), it is clear that it is not as good as ordinary carburized steel (C2).

[0039]

As described above, the induction hardened part of the present invention can obtain a static strength and a bending fatigue strength equal to or higher than those of the carburized steel only by the induction hardening treatment. In addition to the steel type, there is an industrially useful effect such as greatly contributing to the improvement of the performance of various mechanical structural parts and the reduction of manufacturing costs.

[Brief description of the drawings]

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

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/38 C22C 38/38 F16H 55/06 F16H 55/06

Claims (7)

[Claims] 1. C: 0.40% or more and 0.70% by weight
%, Si: 0.05% to 0.80%, Mn:
0.50% to 2.00%, S: 0.01% to 0.03%, V: 0.30% to 1.00%,
Al: 0.010% to 0.050%, N: 0.0
050% or more and 0.0200% or less;
30% or less, the balance being Fe and unavoidable impurities, and a high-frequency quenching hardened layer having a martensite crystal grain size of 14 or more in JIS grain size number, characterized by excellent static strength and fatigue resistance. Hardened parts. 2. The induction hardened part according to claim 1, further comprising Nb: 0.05% to 0.50% by weight in addition to the components described in claim 1. 3. The high-frequency wave according to claim 1, further comprising Cr in an amount of 0.1% to 1.50% by weight in addition to the components described in claim 1 or 2. Hardened parts. 4. C: 0.40% to 0.70% by weight
%, Si: 0.05% to 0.80%, Mn:
0.50% to 2.00%, S: 0.01% to 0.03%, V: 0.30% to 1.00%,
Al: 0.010% to 0.050%, N: 0.0
050% or more and 0.0200% or less;
30% or less, the steel consisting of the balance of Fe and unavoidable impurities is used as a material, and a large strain warm working at a working rate of 30% or more is performed twice or more in a non-recrystallization temperature range, and then induction hardening is performed to obtain a hardened layer. Grain size of martensite is JIS
A method for producing an induction hardened part having excellent static strength and fatigue resistance, characterized in that the particle size number is 14 or more. 5. The steel according to claim 4, wherein the steel further contains Nb: 0.05% to 0.50% by weight in addition to the component according to claim 4. Manufacturing method of induction hardened parts. 6. A steel material containing Cr: 0.1% or more and 1.50% or less by weight in addition to the components according to claim 4 or 5. Or the method for producing an induction hardened part according to 5. 7. The heating temperature for warm working is 900 ° C. or higher.
00 ° C or less and processing start temperature is 600 ° C or more and 800
7. The method for producing an induction hardened part according to claim 4, wherein the temperature is not higher than 0.degree.