JPH0772328B2 - High nitrogen content austenitic sintered stainless steel and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high nitrogen content austenitic sintered alloy and a method for producing the same.

More specifically, the present invention relates to a high nitrogen content austenitic sintered alloy that uses nitride fine powder as an N supply source by using a powder metallurgy method, and a method for producing the same.

(Prior Art) N is a typical austenite-forming element along with C, Ni, Mn, and Co. As an element that improves corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance, and the tensile strength of steel at high temperatures. It has been added as an enhancing element.

However, in the conventional melting material, there is a limit to the amount added due to the problems of solubility in molten steel, bubble generation during solidification, bubbling, and in commercial steel there is no more than 0.45% in high heat resistant high strength steel with high Mn content. To some extent, only examples of inclusion are seen.

For example, the following are known literatures regarding high N content steel.

Japanese Patent Publication No. 50-8967, Japanese Patent Publication No. 56-20151, Japanese Patent Publication No. 41-14805, No. 51-29966, No. 51-31086, No. 51-31087, No. 51-31203, No. 50-4172. No. 50-5650 and No. 50-24886, all of which are based on the dissolution method, but even in the past, the advantage of high N content has been fully recognized, and many efforts have been made for that purpose. You can see that

In addition, it is possible to increase the N by a method such as melting in a high pressure atmosphere or casting, but anyway in a small scale experiment,
Practical application on an industrial scale is extremely difficult, and the restrictions on the device are extremely large.

For example, "Journal of the Japan Institute of Metals" Vol.34, No.2, 1970, pp.1
88-194 describes the experimental results of equilibrating N under high temperature and high pressure, and it was reported that about 1.4% N ₂ was absorbed in Fe-Mn (27% Mn) steel. .

As described above, much effort has been made to increase the N content from the past, but it is still not practical to put it into practical use on an industrial scale, and it is not possible to add it beyond the solid solubility limit of nitrogen. It was the current situation that was considered possible.

Incidentally, it is known to produce a sintered product from an iron-based powder and a metal nitride powder by a powder metallurgy method for wear resistance of cutting tools and bearings (for example, JP-A-56-77360 and Japanese Patent Publication No. 57-54539). However, all of these are intended to utilize the stability and heat resistance of the nitride itself at high temperatures as they are, and it has been attempted to increase the amount of solute N and thereby form an austenite structure. There wasn't. Further, the nitride in that case is considered to be a carbide and an average.

(Problems to be Solved by the Invention) From the viewpoint of improving pitting corrosion resistance, crevice corrosion resistance, or increasing strength, the higher the N concentration in the steel, the more desirable it is, but since N is a gas component, There is a limit to the solid solubility, and the upper limit of the amount added is determined from the viewpoint of preventing bubbles from forming during solidification. In the past, methods such as component adjustment such as high Mn content, melting under pressure, casting, etc. were adopted for the purpose of increasing the amount of solid solution in molten stainless steel.
The amount of N added was at most about 0.45% (Example: Japanese Patent Publication No. 50-
8967).

An object of the present invention is to provide a high nitrogen content austenitic alloy, which has been difficult to manufacture by the conventional melting method, ingot making method, or casting method, and a manufacturing method thereof.

Another object of the present invention is to provide a highly nitrogen-containing highly corrosion-resistant austenitic alloy having excellent pitting corrosion resistance and crevice corrosion resistance, and a method for producing the same.

Another object of the present invention is to provide a high-nitrogen-containing, high-corrosion austenitic alloy having remarkably excellent pitting corrosion resistance and crevice corrosion resistance and high strength, and a method for producing the same.

Another object of the present invention is to provide a high-strength and high-nitrogen-containing austenitic alloy that is remarkably excellent in oxidation resistance at high temperatures and a method for producing the same.

(Means for Solving Problems) Here, the gist of the present invention is that a matrix phase derived from a martensitic, ferritic, two-phase, or austenitic stainless steel powder and an N-diffused phase are used. A high-nitrogen austenitic sintering with excellent corrosion resistance, consisting of a dispersed phase based on decomposition / diffusion of the metal nitride fine powder it has, and having a density of 95% or more of the theoretical density and an average N content of 0.5% or more. It is stainless steel. According to still another feature, the invention provides for filling an unburned mixture of metal nitride fine powder and stainless steel powder into a metal container, degassing and sealing, followed by densification and sintering, Subsequently, a method for producing a high nitrogen-containing austenitic sintered stainless steel, which comprises a step of holding the obtained sintered body at a temperature of 900 ° C. or higher and a melting point or lower to diffuse N in the metal nitride to the stainless steel powder side Is.

That is, according to the present invention, the problem as described above, which has been caused by conventionally adopting the steps of material dissolution and nitrogen solid solution, is solved by using the technique of the powder metallurgy method. It is an attempt to solve all at once. Since nitrogen is added in the solid state from the beginning, that is, it is added in the form of a compound, there is no problem of solid solubility limit, bubbling, etc. Moreover, the nitrogen compound added in the form of fine powder is decomposed by passing through the diffusion treatment step, Along with this, the diffusion of nitrogen is also accelerated. Therefore, in the present invention, the metal nitride is added in the form of fine powder. Preferably particle size
It is a fine powder of 80 μm or less, and more preferably 10 μm or less. The presence of the nitrogen diffusion phase thus produced characterizes the metallurgical structure of the sintered alloy according to the invention.

(Operation) Next, the manufacturing process in the present invention will be described in detail.

First, according to the present invention, after the stainless steel powder and the metal nitride powder are uniformly mixed, the mixture is filled in a steel capsule, and vacuum is drawn at room temperature or while heating to deaerate the inside, or after deaerating. Fill a steel capsule with N ₂ gas and seal it.

Stainless steel powder is produced by an atomizing method, a pulverizing method, an intergranular corrosion method, etc., but since the oxygen content is low, a non-oxidizing atomizing medium such as N ₂ gas, Ar gas, or He gas is used. Those produced are preferred.
Among such atomized steel powders, atomized steel powders having an average particle size of 200 μm or less, preferably 100 μm or less are desirable, and the lower the oxygen concentration in the steel powder, the better, and preferably 0.03% or less. The stainless steel powder may be any of martensitic, ferritic, austenitic, two-phase ferritic, or austenitic.

On the other hand, it is preferable that the metal nitride has a high N concentration and is stable at room temperature, but easily decomposes at a temperature of 1000 ° C. or higher. Examples of such a metal nitride used in the present invention include Cr ₂ N and / or Cr ₂ N which are the main components.
In addition to Cr-based nitrides composed of CrN, Fe composed mainly of Fe ₂ N and / or Fe ₄ N containing Fe and N as main components.
There are system nitrides, V system nitrides, Si system nitrides, Al system nitrides, Mg system nitrides, and the like, and compound nitrides thereof may be used. Fe-Cr system, Fe-V system, Fe-Mn system, Fe-Cr-
V type, Fe-Cr-V-Mn type and the like. The metal nitride system may be selected depending on the intended alloy system component, as well as manufacturability and economy.

When Cr ₂ N is used, the problem of deterioration of corrosion resistance due to Cr ₂ N precipitation, which is a problem with conventional high N steels, is small.
As the metal nitride fine powder, Cr ₂ N fine powder is preferable.

It is sufficient that the metal nitride fine powder is a well-known commercially available product, or the metal powder may be treated by heating with N ₂ gas at the time of compacting to generate a nitride. The average particle size of the metal nitride used in the present invention is preferably small, generally 80 μm or less, and more preferably 10 μm or less. The metal nitride is easily oxidized in the manufacturing process, but the lower the oxygen concentration in the metal nitride, the better. Desirably, the oxygen concentration in the metal nitride is 0.2% or less.

The steel capsule may be made of carbon steel or stainless steel, but it is more preferable that the carbon concentration in the steel is low. This is to prevent carburization from the container during the heat treatment. The C content is 0.03% or less for carbon steel and 0 for stainless steel.
02% or less is most desirable. The holding temperature at the time of evacuation may be room temperature, but heating is more effective than the purpose of removing water inside. It is desirable to evacuate at a higher temperature within a temperature range where the metal nitride does not decompose. Filling the capsule with N ₂ gas after degassing has the effect of further improving heat transfer during heating.

Then, it is densified and sintered. Densification and sintering may be carried out simultaneously by hot isostatic pressing (HIP), or cold isostatic pressing and hot forging, hot extrusion, hot drawing, heat It may be performed in a combination of hot rolling. By maintaining the temperature at 900 ° C. or higher, decomposition of the metal nitride, solid solution of N on the stainless steel side, and diffusion proceed, and a nitrogen diffusion phase is formed.

FIG. 1 is an explanatory view schematically showing a metallurgical structure of a high nitrogen content austenitic sintered stainless steel according to the present invention, in which a matrix phase 10 is derived from stainless steel powder, As described above, an austenitic structure is formed as a whole by nitrogen diffusion. The dispersed phase 20 is derived from fine powder of metal nitride, and is composed of a nitrogen diffusion phase obtained by decomposing and diffusing metal nitride. As will be described later, although a new nitride may be precipitated depending on the subsequent heat treatment, the metal nitride compounded with the powder is once decomposed and diffused to form a dispersed phase. Is. In the sintered stainless steel according to the present invention, such N-diffused phase and dispersed phase are not observed microscopically and only appear as a concentration gradient, but the drawings are schematic for convenience of explanation. It was done.

Here, the “nitrogen diffusion phase” is a solid solution region having a nitrogen concentration gradient formed due to the decomposition / diffusion of the metal nitride compounded as the metal nitride fine powder. It should be noted that this solid-dissolved nitrogen may also be Cr ₂ N or depending on the final thermal history.
It may be precipitated as a metal nitride of CrN.

In the case of the austenitic sintered alloy according to the present invention, the C content is not particularly limited as long as it is derived from the alloy powder and is not particularly limited, but it is for the purpose of corrosion resistance and the formation of carbides is limited as much as possible. 0.02 to ensure the required corrosion resistance
% Or less is preferable.

Thus, according to the method of the present invention, it is possible to easily produce a high nitrogen content austenitic alloy steel having any N concentration in the steel. Generally, the density of the austenitic sintered alloy thus obtained is 95% or more, preferably 98% or more of the theoretical density, and the N content is 0.5% or more in average concentration. Moreover, the sintered alloy steel according to the present invention has a high N content, and the pitting corrosion resistance and the crevice corrosion resistance are remarkably improved, and the strength can be increased. Further, according to the present invention, production of a high nitrogen content austenitic stainless steel, which has been conventionally considered as one of difficult-to-machine materials, is facilitated because steel powder is filled in the metal production container. .

Next, the present invention will be described in more detail by way of examples.

Example After mixing each stainless steel powder having the composition and particle size shown in Table 1 and Table 2 with Cr-based nitride fine powder, a carbon steel capsule having a C concentration in the steel of 0.018% was formed. While filling and heating, the inside of the container was degassed by vacuum drawing and hermetically sealed.

The stainless steel powders shown in Table 1 are stainless steel powders produced by the gas atomizing method using the comparative steels (Experiment Nos. 11, 12, 13) shown in Table 3 as raw materials. A commercially available reagent was used as the Cr-based nitride fine powder shown in Table 2. For some of the invention steels in Table 3 (Experiment No. 5), the Cr-based nitride fine powders shown in Table 2 were crushed by a ball mill to have an average particle size of 2 μm.

The evacuation condition is 1 × 10 ⁻³ mmHg, and the heating temperature at that time is 550 ° C. × 1 hr. Next, this was subjected to densification and sintering by the steps shown in Table 3 and subjected to a test.

When the densification and the sintering were simultaneously performed by the hot isostatic pressing (HIP), the sintering was performed at 1350 ° C. for 1 hour while applying a pressure of 2000 atm in a nitrogen gas atmosphere. The heating temperature is 1000 ° C or higher for the purpose of promoting decomposition of metal nitride and diffusion of N,
It is more desirable that the temperature is higher than the melting point. The weight of one capsule is 8.5 kg.

In the hot forging, the sintered body was heated to 1250 ° C. and held for 1 hour, and then hot forged to a finished dimension of thickness 30 mm × width 80 mm × length L. After forging, it was air cooled.

After hot rolling at 1250 ° C and holding for 1 hour, the thickness is 7m
It was hot rolled to m × width 80 mm × length L.

For hot extrusion, after preheating the sintered body to 900 ° C, 125 in a high frequency furnace.
It was carried out by heating to 0 ° C.

The refining was carried out by holding at 1120 ° C or 1230 ° C and then cooling with water.

In the invention steels 1, 2, 4, and 10 in Table 3, precipitation of Cr-based nitride was observed in the matrix phase derived from the alloy powder.
On the other hand, in invention steels 3, 5, 6, 7, 8 and 9, no precipitation of Cr-based nitrides was observed in the matrix phase derived from the alloy powder. This depends on the refining temperature.

Test pieces were cut out from the HIP material and plate material of each high N content austenitic stainless steel obtained in this way and subjected to a tensile test at room temperature and 600 ° C, a Charpy impact test, a pitting potential measurement in artificial seawater, an acid resistance Each chemical test was carried out. The results of each test are summarized in Table 3.

The tensile test was carried out using a round bar tensile test piece having a diameter of 5 mm and a length of 30 mm in the parallel portion. The Charpy impact test was carried out at −20 ° C. using JIS No. 4 half size (thickness 5 mm) with 2 mm V notch. The pitting potential was measured in Ar degassed artificial seawater at 80 ° C. and evaluated by the potential Vc at which the current density was 1000 μA / cm ² . In the oxidation resistance test, heating was repeated 6 times under the conditions of heating at 1300 ° C. for 6 hours in the air and air cooling, and then the oxidation weight loss after removing the oxide on the sample surface was evaluated. The density of the obtained sintered body was substantially 100% of the theoretical density in each case.

The proof stress of the invention steel is remarkably higher than that of the comparative steel, and the pitting potential is remarkably improved. This is due to the high N. Charpy impact absorption energy is also 5.6kgf-m / cm
^It has a sufficiently high value of ² or more. Invention Steel 10 is Comparative Steel 13
Compared with the above, the weight loss due to oxidation is greatly reduced, and the strength is markedly increased at high temperatures.

(Effects of the Invention) As is clear from the above description, the austenitic sintered alloy obtained by the present invention has high strength not only in corrosion resistance but also in oxidation resistance, and therefore only for corrosion resistance purposes. However, it can be advantageously used for oxidation resistance purposes and also for high strength purposes.

The properties of the steel of the present invention do not depend on the shape, and can be obtained regardless of whether it is a sintered product, a forged product, a plate or pipe, or a seamless steel pipe.

Therefore, the application range of the steel according to the present invention in the industrial field is extremely wide.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating the structure of a sintered stainless steel according to the present invention.

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Claims (3)

[Claims] 1. A matrix phase derived from stainless steel powder and a disperse phase based on decomposition / diffusion of fine metal nitride powder having an N diffusion phase, which has a density of 95% or more of the theoretical density and an average N content. A high-nitrogen-containing austenitic sintered stainless steel with excellent corrosion resistance with an amount of 0.5% or more. 2. An unburned mixture of fine metal nitride powder and stainless steel powder is filled in a metal container, degassed and sealed, then densified and sintered, and then the obtained sintering is performed. Body 900
A method for producing a high nitrogen-containing austenitic sintered stainless steel, which comprises the step of diffusing N in a metal nitride to the stainless steel powder side while maintaining the temperature at not lower than 0 ° C and not higher than the melting point. 3. The method according to claim 2, wherein the average particle size of the metal nitride fine powder is 80 μm or less.