JPH0641630A - Softening heat treatment method of steel for power transmitting part - Google Patents

Abstract

PURPOSE:To provide a softening heat treatment method capable of increasing the cutting property in the cutting without adverse effects on the strength and toughness after the carburization hardening when the steel to be used as the power transmitting part of a gear or the like to be used for automobiles, construction machines, industrial machines or the like is manufactured in the procedure of the warm or hot machining, the normalizing, the cutting, the carburizing or the carbo-nitriding hardening. CONSTITUTION:Steel where the requirements C: 0.1-0.4%, Si: 0.15% or lower Mn: 0.3-2%, Cr: 0.2-2% are met, and Mo: 0.3-1% and/or Ni: 0.4-4.5% is satisfied is used. The steel is held at the temperature of 840-950 deg.C in the normalizing process to be executed after the hot machining and before the cutting, and then the temperature is decreased, and held for 20-150 minutes at the temperature of 650-750 deg.C, and cooled, obtaining the structure consisting essentially of ferrite perlite.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a softening heat treatment method for steel materials used as various power transmission parts for automobiles, construction machines, industrial machines and the like, and particularly to the softening heat treatment performed before cutting and carburizing or carbonitriding. The present invention relates to a softening heat treatment method devised so that a hardness equivalent to that of a conventional material can be secured without impairing toughness and machinability by appropriately controlling heating temperature, cooling conditions, and the like in the curing step.

[0002]

2. Description of the Related Art In recent years, the trend toward higher output and higher performance in automobiles has tended to accelerate, and power transmission parts such as gears used in these vehicles are also required to have higher strength. In particular, gears are required to have excellent fatigue resistance and wear resistance because high bending stress and contact stress are applied to the tooth root due to high speed rotation. Materials used for such gears include Cr-type case-hardened steel, Cr-Mo-type case-hardened steel and Ni-Cr-Mo-type case-hardened steel, which have been specified in JIS G4104, G4105 and 4103, respectively. Has been used.

However, when these JIS standard steels are used, alloying elements such as Mn and Cr are selectively oxidized during the carburizing or carbonitriding treatment for hardening, and the hardness of the surface layer portion after quenching is reduced. There is a problem of forming a poor incompletely hardened layer, which makes it difficult to achieve high strength.

Therefore, in order to achieve the high strength required for gears and the like, a high-strength steel for gears has been proposed in which an increasing amount of alloying elements such as Mo and Ni that hardly form oxides is added. (JP-A-1-306545: Steel for carburizing gears with excellent fatigue strength).
However, when this steel material is subjected to the normalizing treatment similar to that of the conventional steel, there is a problem that the hardness is not sufficiently lowered and the machinability is lowered.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and its purpose is to have excellent toughness and hardness equivalent to those of conventional steels, and to have machinability as well. An object of the present invention is to provide a softening heat treatment method by which an excellent steel material for a power transmission member can be obtained.

[0006]

The constitution of the softening heat treatment method according to the present invention which was able to solve the above problems is as follows: C: 0.1 to 0.4% by weight, Si: 0.15% or less, Mn: 0.3 to 2
%, Cr: 0.2 to 2%, and Mo: 0.3
~ 1% and / or Ni: 0.4-4.5% steel material is used, warm or hot working, normalizing, cutting,
When carburizing or carbonitriding quenching is performed to manufacture a power transmission / transmission component, in the normalizing process step 840 to 950 ℃
After heating and holding, the temperature is lowered, and when it reaches 650 to 750 ℃, it is held for 20 to 150 minutes and then cooled, or in the normalizing step, the self heat of warm or hot working is used, and then It has a gist that the structure is made mainly of ferrite / pearlite by cooling for 30 to 200 minutes after cooling and reaching 650 to 750 ° C.

[0007]

In the present invention, the steel material having the specified chemical composition as described above is used, and the steel material is subjected to warm or hot working → normalizing →
Cutting → Carburizing or carbo-nitriding quenching, when manufacturing power transmission parts, especially by controlling the temperature condition of normalizing well
The structure is mainly composed of pearlite, so that the hardness equivalent to that of the conventional material can be ensured without impairing the toughness and machinability of the final carburized or carbonitrided quenched product. First, the reason for defining the chemical composition of the steel used in the present invention will be explained.

C is an indispensable element for ensuring a sufficient core hardness and ensuring an effective hardened layer depth by carburizing, and must be contained in an amount of 0.1% or more. However, if it exceeds 0.4%, not only the toughness and machinability deteriorate, but also the heat treatment strain increases and the dimensional accuracy deteriorates.
It was 0.4%.

Although Si is added for deoxidation, it is an element that is more easily oxidized than Fe, and if it is too much, the grain boundary oxide layer becomes deep and the bending fatigue strength decreases, so it was made 0.15% or less. Mn is not only an effective element as a deoxidizing / desulfurizing element at the time of melting, but also an important element for ensuring the core hardness and the effective hardened layer depth of the carburized product. It must be contained in an amount of 0.3% or more to be effective. However, if it exceeds 2%, toughness and machinability deteriorate.

[0010] Cr is also an element useful for imparting the hardness of the carburizing core and the effective hardened layer depth, and it is necessary to contain Cr in an amount of 0.3% or more. However, if added in excess of 2%, coarse carbides precipitate at the grain boundaries and deteriorate the pitting strength.

Mo is an element that has a lower affinity for oxygen than Fe and is less likely to form an oxide, and has an effect of significantly improving hardenability in a high C region such as a carburized surface layer. Therefore, by adding an appropriate amount of Mo, M
It exerts the effect of compensating for the decrease in hardenability caused by the grain boundary oxidation of n, Cr, etc., suppressing the formation of an incompletely hardened layer, and strengthening the surface layer portion. Moreover, it has the effect of increasing the fatigue strength after shot peening by lowering the Ms point of the carburized surface layer and increasing the amount of retained austenite after quenching.
Must be contained by 0.3% or more. However, these effects saturate at about 1.0%, so any further addition is useless.

Ni, like Mo, does not easily form an oxide,
It suppresses the formation of an incompletely hardened layer and strengthens the surface layer. Moreover, it has the effect of increasing the amount of retained austenite in the surface layer portion and increasing the toughness of the carburized layer, and it is necessary to contain 0.4 or more in order to exert such effects effectively. But since their effects saturate at about 4.5%,
Any further addition is useless. Mo and Ni are the same substances in that they have the effects of improving the hardenability and increasing the retained austenite as described above, and therefore Mo and Ni are
And Ni may be contained individually, or two kinds may be contained at the same time.

In the present invention, a steel material satisfying the requirements of the above-mentioned composition is used, and after the hot or hot working of the steel material, the holding temperature and the cooling condition at the time of normalizing the steel material are well controlled to make the structure of ferrite・ It is extremely important to make the organization mainly perlite. However, if the heating temperature during normalizing treatment is less than 840 ° C., the structure does not become a perfect γ structure, so that the desired normalizing treatment effect cannot be obtained. On the other hand, if the heating temperature exceeds 950 ° C, the γ grains are significantly coarsened and the toughness becomes poor. Therefore, the primary heating temperature was set to 840 to 950 ° C. The holding time at this time varies depending on the mass of the manufactured parts, but is preferably about 20 to 120 minutes.

Next, after lowering the temperature to 650 to 750 ° C., 20
Hold for ~ 150 minutes, then cool. This secondary heating temperature
At temperatures below 650 ° C, bainite is formed, while at temperatures above 750 ° C, ferrite / pearlite transformation does not complete within a prescribed time, and untransformed γ phase increases and bainite and martensite are formed in the subsequent cooling process. It tends to occur. Also, if the holding time is less than 20 minutes, the ferrite-pearlite transformation does not complete, and this transformation does not occur in the above temperature range.
It takes almost 150 minutes to complete, so holding it any further is meaningless.

In the above description, the temperature conditions in the case where the material is cooled once after the warm or hot working and then normalized are explained. However, the primary heating can be performed by using the self-heat during the warm or hot working as it is. It is possible to obtain a similar ferrite-pearlite structure by omitting the above, cooling to 650 to 750 ° C, holding for 30 to 200 minutes, and then cooling.

The steel material that has been subjected to the normalizing treatment under the above-mentioned conditions has a structure mainly composed of ferrite / pearlite and exhibits excellent machinability, and the carburizing or carbonitriding quenching after that makes the core portion excellent in toughness. The surface layer portion is remarkably hardened while keeping it, and a power transmission member having high toughness and excellent wear resistance can be obtained.

[0017]

EFFECTS OF THE INVENTION The present invention is constituted as described above, and normalizes by making the structure after the normalization a structure mainly composed of ferrite / pearlite even though it is a steel material containing a relatively large amount of alloying elements. The machinability afterward can be remarkably improved, and the hardness after carburizing or carbonitriding and quenching can be increased to the level equal to or higher than that of the conventional steel.

[0018]

EXAMPLES The constitution and effects of the present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. Example 1 A steel material having the composition shown in Table 1 was hot forged to a diameter of 40 mm, and then the temperature condition of the present invention shown in FIG. 1 was satisfied, that is, 900 ° C.
After 30 minutes of primary heating, increase the secondary heating temperature to 600-80.
Normalizing treatment was carried out at 0 ° C. and holding time varied in the range of 15 to 160 minutes, and the hardness and texture of the obtained sample were measured. The hardness measurement results are shown in Table 2, and a representative microstructure photograph is shown in FIG.

As is clear from Table 2, the hardness of the normalizing treatment under the conditions satisfying the specified requirements of the present invention is higher than those obtained by the comparative methods 1a to 1c and the conventional method. Remarkably low. However, if the secondary heating temperature is less than 650 ° C (Comparative method 1a) or exceeds 750 ° C (Comparative method 1b),
Alternatively, when the secondary heating time is less than 20 minutes (Comparative Method 1c), the decrease in hardness is insufficient, and satisfactory machinability cannot be expected. Further, even if the normalizing condition is proper, sufficient machinability cannot be obtained by the comparative method 1e in which the steel material composition deviates from the specified requirement.

In comparison method 1d, the holding time during secondary heating is
It was set for 150 minutes or longer, and the structure is mainly ferrite / pearlite, and the hardness is sufficiently low, but the holding time is too long, and the productivity is poor. 2 (A) to 2 (C) show the crystal structures of the normalized steel materials obtained by the present invention method 1, the comparative method 1a and the conventional method 1b in Table 2, which were obtained by the method of the present invention. Has a ferrite-pearlite structure similar to that of JIS standard "SCr420" which is subjected to normalizing treatment, whereas the one obtained by the comparative method has a ferrite-pearlite-bainite three-phase structure. .

Example 2 The same steel material as in Example 1 was used, the primary heating was omitted by utilizing the self-heating during hot forging as shown in FIG. 3, and then the secondary heating temperature was 600-800. ℃, holding time 25 ~ 21
Normalizing treatment was performed by changing the time to 0 minutes, and the hardness of the obtained sample was measured and the structure was observed. Table 3 shows the hardness measurement results.
4 and a representative microstructure photograph are shown in FIG.

As is clear from Table 3, the hardness of the method of the present invention is remarkably low, and the hardness is almost the same as that of the conventional method. However, if the heating temperature is below 650 ° C (comparative method 2a) or exceeds 750 ° C (comparative method 2b),
When the heating time is less than 30 minutes (comparative method 2c), the hardness is higher and the machinability is lowered as compared with the conventional method when the temperature is out of the specified range of the present invention. Further, even if the normalizing condition is proper, in the comparative method 2e in which the steel material composition deviates from the specified requirements, the hardness is not sufficiently lowered and satisfactory machinability cannot be obtained. Further, as is clear from the structure of FIG. 4, in the method according to the present invention [FIG. 4 (A)], the conventional method J
Normalized version of IS standard SCr420 [Fig. 4
(C)], a ferrite-pearlite structure similar to that of (C)] is obtained, whereas bainite is formed in the one obtained by the comparative method 2a which deviates from the prescribed requirements of the present invention [FIG. 4 (B)].

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

[Brief description of drawings]

FIG. 1 is a heat pattern showing normalizing conditions adopted in Example 1.

2 is a steel material obtained in Example 1, that is, (A) invention steel, (B)
It is a figure which shows each metal structure of comparative steel and (C) conventional steel.

3 is a heat pattern showing normalizing conditions adopted in Example 2. FIG.

4] Steel materials obtained in Example 2, namely (A) steel of the present invention, (B)
It is a figure which shows each metal structure of comparative steel and (C) conventional steel.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shugoro Adachi 2 Nadahamahigashi-cho, Nada-ku, Kobe-shi, Hyogo Stock Company Kobe Steel Works Kobe Steel Works

Claims (2)

[Claims] 1. C: 0.1-0.4% by weight, Si: 0.15
%, Mn: 0.3 to 2%, Cr: 0.2 to 2%, and Mo: 0.3 to 1% and / or Ni:
When using steel containing 0.4 to 4.5% and performing hot or hot working, normalizing, cutting, carburizing or carbonitriding and quenching to manufacture power transmission components, the normalizing treatment process is performed at 840 to After heating and holding at 950 ° C, the temperature is lowered to 650
A softening heat treatment method for steel materials for power transmission parts, characterized in that the structure is made mainly of ferrite / pearlite by holding it for 20 to 150 minutes when it reaches ~ 750 ° C and then cooling it. 2. C: 0.1-0.4% by weight%, Si: 0.15
%, Mn: 0.3 to 2%, Cr: 0.2 to 2%, and Mo: 0.3 to 1% and / or Ni: 0.
Using steel material containing 4-4.5%, warm or hot working,
When manufacturing power transmission components by normalizing, cutting, carburizing or carbonitriding quenching, in the normalizing treatment step, the self heat of warm or hot working is used, and then the temperature is lowered to 650 ~ A softening heat treatment method for steel for power transmission parts, characterized in that the structure is made mainly of ferrite / pearlite by holding it for 30 to 200 minutes when it reaches 750 ° C and then cooling it.