JPS5934788B2 - Manufacturing method of surface nitrided steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing steel that is surface hardened by nitrocarburizing treatment and that also has extremely high core strength.

For example, mechanical parts used under high surface pressure, such as automobile parts, are required to have wear resistance, pitting resistance, seizure resistance, etc., so surface treatments such as carburizing or nitriding are often applied. .

However, in order to satisfy the above-mentioned rough beach properties, it is necessary that the hardness of the surface layer is 700 or more in terms of Vickers hardness (Hv), and that the hardening depth is sufficiently deep. This hardening depth is required to be 0.3 or more at a depth with a Vickers hardness of 500 or more, considering the maximum shear stress below the surface. Conventionally, in order to form the above-mentioned surface hardening layer,
Case-hardened steel is used as a material and undergoes carburizing and quenching.

Although sufficient surface hardness and hardening depth can be obtained with this method, distortion occurs during quenching, leading to poor precision as a mechanical part, requiring work to remove the distortion, and a certain amount of running stock is required. However, there are major drawbacks such as decreased productivity and increased costs. In contrast to the above carburizing and quenching method, as a surface hardening method that causes less distortion,
A nitriding method is known in which nitrogen is infiltrated into steel at low temperatures (approximately 500°C) to form nitrides with alloying elements that have a high affinity for nitrogen, and the surface is hardened by creating large strains in the iron lattice. SACM steel (medium C-Cr-Mo-Al steel) is specified by JIS as a typical steel type for this purpose.

However, this nitriding method is not suitable for mass production because it requires an extremely long treatment time of 50 to 100 hours in order to obtain the desired hardness and hardening depth.

In addition to these surface hardening methods, there is a method conventionally called soft nitriding, one of which is gas soft nitriding.

In the gas nitrocarburizing method, parts are treated in a mixed gas of ammonia gas, an endothermic gas such as propane gas, or butane (hereinafter referred to as RX gas), or an exothermic gas (hereinafter referred to as NX gas), and a part of the gas is treated with nitrogen. Carbon is infiltrated into steel to harden the surface layer. This method is suitable for the mass production of automobile parts, etc., because it does not cause distortion in the treated object like the carburizing-quenching method, and it does not require a long time like the nitriding method. However, the development of steel types suitable for this has not yet been sufficient. Furthermore, when not only surface hardness but also core strength of the part is required, the desired properties cannot be obtained by conventional processing methods using conventional carburizing and nitriding steels. An object of the present invention is to provide a method for manufacturing a steel product that has sufficient performance not only in surface hardness and hardening depth but also in core strength.

The above purpose is to use a material steel with a special composition containing Ti, to perform solution treatment by heating this Q steel in a specific temperature range and rapidly cooling it, and to perform nitrocarburizing after this solution treatment. This is achieved by a combination of:

That is, the present invention provides C: 0.03 to 0.25(f) and Ti: 4×C(:f
)+0.01% to 4×C%+1.00I) is heated at 900 to 1300°C, rapidly cooled from this temperature range, and then subjected to soft nitriding treatment in a temperature range of 450 to 700°C. The subject matter is a method for producing surface nitrided steel characterized by the following.

The steel material used in the method of the present invention must contain C and Ti as essential components, but it is also desirable to contain the following components within their respective specified ranges.

Si: 0.01-1.0 (: fl) Mn: 0.1-1.5% Cr: 0.1-2.5% Al: 0.08 (f) or less In addition to the above components, strength, 201) to increase toughness
The following Ni and/or MO may be included, and in order to improve machinability, 0.35% or less Pb,
O. It is also possible to contain S or the like in an amount of 3% or less.

As the nitriding method, the Tufftrade method, ion nitriding method, gas soft nitriding method, etc. can be adopted, but the gas soft nitriding method is the most practical for efficient production of mass-produced parts.

According to the method of the present invention, the surface hardness is Hv7OO or more, the hardening depth is sufficient, and the core hardness is Hl7OO.
~350 steel products can be manufactured.

Furthermore, as will be described later, by selecting the combination of the solution treatment to increase the core hardness and the nitrocarburizing treatment, the strength of the core can be adjusted according to the performance required of the product. Further, the solution treatment may be performed not only by reheating the cold material, but also by performing processing such as hot forging in a temperature range of 900° C. or higher, and immediately cooling with water or oil.
Since nitrocarburizing does not cause distortion to the product, if cutting, surface finishing, etc. are performed prior to this treatment,
Highly accurate surface-hardened mechanical parts can be manufactured efficiently. Next, reasons for limiting each requirement in the present invention will be described.

First, regarding the components contained in the steel material, if C is less than 0.03%, the hardness of the core cannot be made sufficiently high;
25 (if present above F6, 900℃~1300℃
If the amount of TiC that is not dissolved during heating increases, it will only cause harmful effects such as deterioration of toughness and increase in the amount of Ti required, and will not be useful for increasing the strength of the core.

Ti is important for ensuring surface hardness and increasing hardening depth after soft nitriding, and is also important for precipitating TiC during soft nitriding to increase the hardness of the core. The surface hardening effect of Ti is due to the formation of nitrides (TiN) by combining with nitrogen that enters the steel during soft nitriding treatment, so the effective amount of Ti for the formation of nitrides is , is extremely important for improving surface hardness and expanding hardening depth. Effective Ti is obtained by subtracting the Ti amount forming TiC from the total Ti amount, that is, total Ti-C%×4. As the amount of effective Ti increases, the hardening depth increases, but if it exists in excess of 1.0%, it causes embrittlement of the fabric and increases cost.

Furthermore, if the effective Ti content is less than 0.01%, the surface hardness and hardening depth after soft nitriding treatment will be insufficient. Therefore,
The Ti content varies from 4×C(L+0.01% to 4×C(f
l) should be in the range +1.0%. Si and Mn
is necessary for adjusting the cleanliness ratio of molten steel during steel melting and the strength of the steel relative to the base material, Si is 1.0% and Mn is 1.0%.
Addition of more than 5% is unfavorable for workability, machinability, etc. Cr is necessary to ensure surface hardness during nitriding, but the presence of 2.5% or more reduces the hardening depth during short-time soft nitriding.

Al is necessary for particle size adjustment and for improving and stabilizing the Ti yield as well as having a deoxidizing effect.

In the steelmaking process, deoxidation is performed by adding Al prior to the addition of Ti, but in order to stably keep Ti at the target value, 0.
Al below 08(!l) is sufficient, and A exceeding it is sufficient.
l is not preferable because it increases Al2O3 in the steel. In addition, excessive M forms AIN in the surface layer during soft-nitriding treatment, inhibits the penetration of N into the interior, and reduces the hardening depth. In addition to these components, up to 2% of Ni, MO or 0
．． Up to 35% Pb,O. It may contain up to 3% S.

The upper limit values of these components are values that do not cause any adverse effects other than the purpose of addition. Next, regarding the conditions for solution treatment and soft nitriding treatment, solution treatment is to dissolve TiC, which is precipitated in the subsequent soft nitriding and acts on core hardening, as shown in Fig. 5, which will be described later. The heating temperature is 900℃
If the temperature is lower than 130 °C, there is almost no solid solution.
If the temperature exceeds 0°C, the toughness of the product will be extremely deteriorated, which is not preferable.

Soft nitriding is surface hardening by nitriding, and it is necessary to perform the treatment at a temperature that satisfies both of these properties in order to achieve the required soft nitriding properties and to harden the core by precipitating TiC. .

If this temperature is lower than 450°C, a hardened surface layer with sufficient hardness will not be formed. On the other hand, when treated at a temperature exceeding 700° C., the hardness of the core decreases, as can be seen from FIG. 4 below. If it is necessary to particularly increase the hardness of the core, a temperature range of 500 to 650° C. may be selected as shown in FIG. Next, the present invention will be specifically explained with reference to Examples.

Eight types of steel shown in Table 1 were melted and heated to 1100℃ and 1
After heating to 250℃, 1000 and 1100℃ respectively
When finished by forging and immediately cooled with water, oil, or forced air, or reheated after forging to 1100℃ and 1250℃
45f when water cooled, oil cooled, or forced air cooled immediately after heating.
A disk-shaped test piece of 40 f x 10 t was made from the material and subjected to gas nitrocarburizing at 570°C x 4 h in ammonia gas + RX gas. The hardness distribution of the oil-cooled material after nitrocarburizing at 1100°C for 1 hour is shown in Figures 1 and 2, and the hardness of Ya 1, Black 2, and Black 6 steels after solution treatment is shown in Figure 3.

(The hardness after normalizing is shown in Table 1.) Table 1 Nakaya 1 to Kuro 6
are the steels that are the subject of the present invention; Ya 1 and Black 2 are steels with the basic composition; Black 3 is a steel with an increased amount of S to improve machinability; Ya 4 is a steel that further contains Pb. , Black 5 is a steel in which MO is added to improve strength for large members, and Black 6 is a steel in which the amount of C is lowered to reduce the amount of Ti.

In addition, Black 7, which was listed as a comparative steel, does not contain Ti and is mainly composed of A.
The nitriding property improved by l and Cr, 0.8, contains Ti as well as Al and Cr, but has no effective Ti. Figures 1 and 2 are at 1100℃ after forging.
The target steel of the present invention, which was reheated for 1 hour and immediately cooled in oil to increase the core hardness, was heated to
A comparison is made between a case in which the core hardness was not increased by performing gas nitrocarburizing for 4 hours, and a case in which comparative steel was subjected to gas nitrocarburizing under the same conditions.

The core hardness of the steel subject to the present invention is increased by solution treatment (forging and quenching), and further increased by subsequent nitrocarburizing.

It can be seen that it is possible to increase the hardness by precipitating TiC simultaneously with nitrocarburizing. The hardness of the core becomes higher than that of the normalized material by water cooling, oil cooling, or forced air cooling after solution treatment or forging, as shown in sample Jf6 in Figure 1.
．． 1 and black 2 core hardness, sample f) in Figure 3. 1
and comparison with the hardness of Black 2 at 1100℃ x 1h oil cooling,
This can also be seen from the relationship between the aging temperature and hardness after solution treatment shown in FIG. On the other hand, the core hardness after soft nitriding is equal to the hardness after solution treatment and precipitation hardening of TiC due to soft nitriding.

FIG. 5 shows the relationship between solution temperature and hardness, and the hardness after solution treatment depends on the temperature. Regarding the soft-nitriding property of the steel subject to the present invention, the hardness decreases from the surface toward the inside, but the gradient of the decrease is gradual.

As seen in FIG. 1, this tendency is similar to that of the steel in which hardness is not increased by solution treatment, but as shown in FIG. 2, the tendency is markedly different from that of the comparative steel. Thus, in addition to high surface hardness, deep hardening depth and high core hardness greatly contribute to improving the wear resistance and pitting resistance, especially in products used under high loads. As explained above, according to the present invention, surface hardness,
Machine parts with sufficient hardening depth, high core strength, and significantly improved wear resistance, pitting resistance, and other properties can now be manufactured efficiently, especially for automotive parts. When manufacturing mass-produced products, it exhibits excellent effects that cannot be matched by conventional carburizing and quenching methods and nitriding methods.

[Brief explanation of the drawing]

Figure 1 shows the soft nitriding properties of the solution treated material at 1100°C x 1 hour oil cooling, and the soft nitriding properties of the normalized material.
Fig. 3 shows hardness when solid solution of TiC obtained by solution treatment or rapid cooling after forging, Fig. 4 shows precipitation hardening. FIG. 5 is a graph showing the relationship between the hardness after solution treatment and the solution treatment temperature, that is, the temperature at which solution treatment is possible.

Claims (1)

[Claims] 1 C0.03-0.25% and Ti4×C%+0.01
%~4×C%+1.0% steel containing 900~13
After heating at 00℃ and quenching from this temperature range, 450~
A method for producing surface nitrided steel, characterized by performing soft nitriding in a temperature range of 700°C. 2. A method for producing surface nitrided steel according to claim 1, characterized in that steel having the following composition is used as the material steel. C0.03~0.25% Si0.01~1.0% Mn0.1~1.5% Cr0.1~2.5% Ti4×C%+0.01~4×C%+1.0% Al0 .08% or less Fe balance