JPH0368753A - Production of nitrided steel member - Google Patents

Abstract

PURPOSE:To form a hardened layer over the whole region of the surface part by applying machining to a steel member, carburizing the member into the part at a specific depth from the surface so that carburization concentration reaches a specific percentage, and then carrying out ion nitriding treatment. CONSTITUTION:A steel member is subjected to machining. Subsequently, slight- degree carburizing treatment is applied to the above steel member so that carburization concentration in the part from the surface to the depth of 0.05-0.5mm from the surface is regulated to 0.3-0.7%. Then, ion nitriding treatment is performed. By this method, the strength of the steel member can be sufficiently improved.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a method of manufacturing a nitrided steel member.

[Prior art] Steel nitriding involves infiltrating nitrogen into a steel member to form a hardened layer containing iron nitride in a portion from the surface to a depth of several hundred μm (hereinafter referred to as the surface portion). This treatment method is conventionally known as one of the surface hardening treatment methods for steel, and this treatment method is often used particularly for mechanical structural parts such as gears because heat treatment distortion is relatively small. Specifically, the AI transformation point of the iron-carbon phase diagram (approximately 730°C
) under suitable temperature conditions (e.g. 500-570°C
) In this case, the M member is exposed to a nitrogen-based atmospheric gas containing ammonia gas to generate iron nitride on the surface, or under an appropriate temperature condition below the A1 transformation point. Conventionally, a salt bath nitriding method has been used in which a steel member is subjected to a salt bath using molten cyanate or the like to generate iron nitrides on the surface thereof.

However, the above-mentioned conventional nitriding method for sea bream has problems such as the processing time required (several tens of hours).
A soft nitriding treatment method has been proposed that uses a cyanide compound, RX gas (endothermic modified gas), etc. to generate iron nitride on the surface of a steel member in a few hours (for example,
(See Japanese Patent Publication No. 61-31184).

All of the conventional nitriding methods described above have the advantage of being relatively inexpensive and capable of performing nitriding in large quantities. However, in these treatment methods, the rate of supply of nitrogen to the steel member is regulated by the thermochemical equilibrium relationship on the surface of the steel member, so a sufficient amount of nitrogen does not enter the steel member, resulting in insufficient hardness. No hardened layer is formed. Therefore, there is a problem that the strength of the steel member cannot be sufficiently increased. On the other hand, as mechanical structural parts have become smaller and lighter in recent years, higher strength is required of steel members. It has been demanded.

Therefore, a steel member is placed in a container filled with an atmospheric gas consisting of a mixed gas of nitrogen and hydrogen at low pressure (several Torr), while a positive voltage is applied to the container side and a negative voltage is applied to the steel member side. An ion nitriding method has been proposed in which a glow discharge is caused between these materials to ionize nitrogen gas, and the ionized nitrogen is caused to penetrate into the steel member using electric force. According to this ion nitriding treatment method, it is possible to forcibly infiltrate a large amount of nitrogen into the steel member by electric force, regardless of the thermochemical equilibrium relationship. can be generated.

[Problems to be Solved by the Invention] Therefore, it can be said that according to the above-mentioned ion nitriding method, the hardness of the surface portion of a steel member can basically be increased compared to a nitriding method that does not ionize nitrogen. .

However, when the inventors of the present application carefully measured the hardness of various depths on the surface of the steel member subjected to the conventional ion nitriding treatment, it was found that the hardness from the surface was 50 to 1.
Although sufficient hardness was obtained in the 100 tt portion, sufficient hardness was not obtained in the extremely shallow portion (hereinafter referred to as the outermost surface portion) with a depth of 25 μm or less, and for this reason, the steel It was found that the strength of the member as a whole was lacking.

The present invention has been made in view of the above-mentioned conventional problems, and it is possible to sufficiently increase the hardness over the entire surface area without reducing the hardness of the outermost surface part of the steel member, and to increase the strength of the steel member mechanically. It is an object of the present invention to provide a method for manufacturing a nitrided steel member that can sufficiently increase the size and weight of structural parts.

[Means for Solving the Problems] The inventors of the present application have conducted detailed research into the cause of the decrease in hardness of the outermost surface portion of steel members subjected to the conventional ion nitriding treatment, and have found the following facts. discovered.

That is, in the ion nitriding treatment of sea bream, a low-pressure mixed gas containing nitrogen gas to create nitrogen ions and hydrogen gas to activate the steel surface is generally used as the atmospheric gas. A so-called decarburization phenomenon occurs in which a considerable portion of the carbon at the outermost surface of the steel member combines with hydrogen in the atmospheric gas and is lost from the steel member. For this reason, the carbon concentration decreases in the outermost surface portion of the steel member, resulting in a decrease in hardness.

The inventors of the present application focused on this fact, and if the carbon concentration at the outermost surface of the steel member is appropriately increased in advance, the carbon at the outermost surface will combine with hydrogen in the atmospheric gas during ion nitriding. We thought that even if carbon was lost, the carbon concentration could still be maintained at an appropriate value. Based on such consideration, and in order to achieve the above object, the present invention machine-processes a steel member, and then reduces the carburization concentration at a depth of 0.05 to 0.5 am from the surface. Provided is a method for producing a nitrided steel member, characterized in that a weak carburizing treatment is performed to give a carbon content of 0.3 to 0.7%, and then an ion nitriding treatment is performed.

[Operations and Effects of the Invention] According to the present invention, charcoal is removed from the surface of the steel member to a depth of 0.05 to 0.5111 m by mild carburizing treatment.
Since the a degree is increased to 0.3 to 0.7%, even if the carbon at the outermost surface is lost by combining with hydrogen in the atmospheric gas when ion nitriding is performed afterwards, the carbon at the outermost surface will be removed. The concentration is maintained at an appropriate value and no decrease in hardness occurs. Therefore, a hardened layer having sufficient hardness can be formed over the entire surface portion by the ion nitriding treatment, and the strength of the steel member can be sufficiently increased.

[Example] Hereinafter, an example of the method for manufacturing a nitrided steel member according to the present invention will be specifically described according to the flowchart shown in FIG.

In step S2, a steel member having an appropriate composition depending on the intended use is prepared (see Table 2). The present invention can be applied to general structural steel, but it can also be applied to chromium (Cr), molybdenum (MO), vanadium (V), aluminum (A
It is particularly effective for alloy steels containing I), titanium (T i), and the like.

In step S2, the steel member having the above-mentioned composition is formed by hot forging into an appropriate shape corresponding to the final part shape of the mechanical structural part to be manufactured from the steel member.

In step S3, a normalizing treatment (normalizing) is performed on the hot-forged steel member in order to refine and make the grain size uniform. This normalizing treatment is carried out in a conventional manner by heating the steel member to an appropriate temperature (for example, 900° C.) and then allowing it to cool in the atmosphere.

In step S4, a low-temperature annealing treatment (low-temperature annealing) is performed on the steel member in order to remove internal distortion of the steel member. This low-temperature annealing treatment heats steel members to 300-600°C.
This is carried out by the usual method of holding at a suitable temperature within the range of 0.5 to 2 hours, followed by gradual cooling, and in particular, internal strains in steel members where bainite transformation has occurred are effectively removed.

It should be noted that the processes from step 82 to step S4 described above are not essential, and may be performed only when necessary depending on the material of the steel member, the shape of the final product, the required quality, etc.

In step S5, the steel member is machined into the same shape as the final product (mechanical structural part). For example, in gears, teeth are formed by machining.

In step S6, a weak carburizing process is performed on the machined steel member. In the ion nitriding process in step S8, which will be described later, decarburization occurs at the outermost surface of the steel member due to hydrogen in the atmospheric gas, so to compensate for this, the carbon concentration at the surface is increased.

In this weak carburizing treatment, the carburizing concentration is 0.3 to 0.7
It is preferable to set it to an appropriate value within the range of %. The reason why the carburizing concentration is limited in this way is that if the carburizing concentration is less than 0.3%, it will not be possible to sufficiently compensate for the decrease in carbon due to the decarburization phenomenon during ion nitriding treatment, and the hardness of the outermost surface of the steel member will be reduced. On the other hand, if the carburizing concentration exceeds 0.7%, the carbon concentration in the surface area becomes too high, and this carbon blocks the progress of nitriding and sufficiently increases the hardness of the surface area. This is because it is not possible. Note that the carbon concentration of nitriding steel used as mechanical structural parts is usually 0.1 to 0.5%.

The carburization concentration is controlled by adjusting the composition of the atmospheric gas, as will be explained later.

Further, the carburizing depth is preferably set to an appropriate value within the range of 0.05 to 0.5 rrlm. The reason why the carburizing depth is limited in this way is that if the carburizing depth is less than 0.05 mm, it is not possible to secure the necessary carbon concentration in the outermost part of the steel member. This is because if it exceeds mm, the carbon concentration in the internal region where decarburization does not occur during ion nitriding becomes higher than necessary. The carburization depth is controlled by adjusting the treatment time, as will be explained later.

As the atmospheric gas for performing the above-mentioned weak carburizing treatment, a mixture of RX gas whose composition is shown in Table 1 and air added in a range of 0 to 5% is used.

Table 1 Composition of RX gas [Unit: vol. Here, there is a certain correspondence based on a thermochemical equilibrium relationship between the amount of air added to the RX gas and the carbon concentration (carburized concentration) in the surface area after weak carburizing treatment. The carburization concentration can be freely adjusted by changing the amount of air added. For example, when the amount of air added is 0%, the carburization concentration is 0.7%, and when the amount of air added is 5%, the carburization concentration is 0.3%. Utilizing this, the carburization concentration is adjusted to a desired value within the range of 0.3 to 0.7% by adjusting the amount of air added to the RX gas within the range of 0 to 5%. In addition, if it is necessary to perform ordinary carburizing treatment with a carburizing concentration of 0.8 to 0.9%, an enriched gas such as propane (Cs Hs ) may be added to the RX gas.

Under an atmosphere gas adjusted to a predetermined composition in this way, the steel member is heated to an appropriate temperature within the range of 800 to 900°C, and this temperature is maintained for an appropriate time within the range of IO to 60 minutes (hereinafter referred to as , this time is called carburizing time). Here, according to the basic theory, the carburizing depth is approximately proportional to the 0.5th power of the carburizing time, so the carburizing depth will be a desired value within the range of 0.05 to 0.5 mm. Set the carburizing time as follows.

After this, the steel member is cooled, and since the purpose of the mild carburizing treatment is to increase the carbon concentration in the surface portion of the steel member, a rapid cooling method (quenching) can be used as a cooling method after the mild carburizing treatment. Note that, of course, a slow cooling method can be used.

With this control method, the carbon concentration in the surface area of the rope member to a desired depth within the range of 0.05 to 0.5 mm is reduced to within the range of 0.3 to 0.7%. Adjusted to desired value.

In step S7, in order to restore the toughness of the steel member, 1
Tempering is carried out at a suitable temperature within the range of 50-600°C for 0.5-5 hours. Note that this tempering need only be performed when rapid cooling (quenching) is performed in step S6, and can be omitted when slow cooling is performed.

In step S8, the rope member is subjected to ion nitriding treatment, and iron nitride is generated on the surface portion thereof to form a hardened layer. This ion nitriding method is a commonly used method, so a detailed explanation will be omitted.
For example, when N and H2 are mixed at a volume ratio of 6:4,
Using an atmospheric gas of Torr, there is a
This is done by applying a voltage of 50V and heating at 570°C for 3 hours. As mentioned above, since the carbon concentration at the surface portion of the steel member is increased in step S6, even if decarburization occurs at the outermost surface portion due to hydrogen in the atmospheric gas, the carbon concentration is maintained at an appropriate value and the hardness is reduced. does not occur. Therefore, the strength of the steel member can be sufficiently increased.

In step S6, shot peening is applied to the surface of the steel member to increase the residual compressive stress in the surface portion, thereby further increasing the strength of the steel member.

In order to confirm the effects of the present invention, the surface hardness Hv of the steel member manufactured using the manufacturing method according to the present invention and the surface hardness Hv of the steel member manufactured using the conventional ion nitriding treatment method were compared.
A comparative experiment was conducted, and the experimental method and experimental results will be explained below.

First, a round bar with a diameter of 30 mm was made by hot rolling using steel materials with three compositions, A, B, and C as shown in Table 2. After heating this round bar to 900°C, Three types of urine test pieces were prepared by normalizing treatment, and three original test pieces made from three types of steel, A, B, and C, were tested under the following conditions. Only ion nitriding treatment was performed, and these were designated as Samples 1, 2.3, respectively (hereinafter, these are collectively referred to as conventional examples).

Atmospheric gas N1:Ht= 6 +4 gas pressure
7 Torr Applied voltage: 450 ■ Heating conditions: 570°C for 3 hours In addition, several original test specimens made from three types of steel, A, B, and C, were heated at various temperatures as shown in Table 3. After performing the carburizing treatment, the same ion nitriding treatment as in samples 1 to 3 was performed, and these were designated as samples 4 to 9. In addition, when cooling rapidly after weak carburizing treatment, flI4al
When the K material was immersed in oil and slowly cooled, the steel member was cooled in a furnace at a rate of about 0.5° C./sec.

Then, the hardness Hv of Samples 1 to 9 was measured at positions at depths of 25.50 and 100 μm from the surface, respectively. The results are shown in Table 3.

As is clear from Table 3, when comparing samples made from the same steel material, the hardness at a depth of 25 μm, which corresponds to the outermost surface area where decarburization occurs due to ion nitriding treatment, is lower than that of weak carburizing treatment. Regardless of the conditions, the sample according to the present invention is significantly higher than the conventional example. In addition, the hardness at a depth of 50 and 100 μm is also
With a few exceptions, the hardness of the samples according to the present invention is higher than that of the conventional example.

From this experimental result, according to the manufacturing method according to the present invention,
It can be seen that the hardness of the outermost surface portion can be significantly increased, and the strength of the nitrided steel member can be significantly increased by one factor.

[Brief explanation of drawings]

FIG. 1 is a flowchart showing a method for manufacturing a nitrided steel member according to the present invention.

Claims (1)

[Claims] (1) Machine processing is performed on the steel member, and then mild carburization is performed so that the carburization concentration at a depth of 0.05 to 0.5 mm from the surface is 0.3 to 0.7%. 1. A method for manufacturing a nitrided steel member, which comprises applying ion nitriding treatment.