JPH02240235A - Manufacture of high nitrogen-containing austenitic alloy - Google Patents

Abstract

PURPOSE:To manufacture a high nitrogen austenitic alloy having excellent strength and corrosion resistance with high efficiency by packing metallic nitride fine powder and alloy powder into a metallic vessel and subjecting the mixed body to hot working under specified conditions. CONSTITUTION:The unsintered mixed body of fine powder of Cr series metallic nitride or the like and alloy powder of stainless steel or the like is packed into a metallic vessel made of steel or the like, which is deaerated and sealed. The mixed body is subjected to hot working a cross-sectional reduction ratio of >=5 at the temp. of 900 deg.C to the m.p. or below to refine the metallic nitride and to form an alloy phase. The hot working is executed by forging, rolling, extruding or the like. In this way, a sintering stage under high temp. and high pressure is unnecessitated, by which the high nitrogen-contg. austenitic alloy can be manufactured with high efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing a high nitrogen content austenitic alloy.

More specifically, the present invention relates to a method of manufacturing a high nitrogen content austenitic sintered alloy using powder metallurgy and using fine nitride powder as a nitrogen source.

(Prior art) N is a typical austenite-forming element along with C, Nis, Mn, Co, etc., and is used as an element that improves corrosion resistance, especially pitting corrosion resistance and crevice corrosion resistance, and improves tensile strength of steel at high temperatures. It has been added to steel as an element to increase strength.

However, there are limits to the amount of conventional molten metals that can be added due to problems such as solubility in molten steel, bubble generation during solidification, and bubbling. There are only cases where the content is about 0.45% at most.

All of these methods are based on the melting method, but it can be seen that the advantages of high nitrogen content in steel have been well recognized in the past, and many efforts have been made for this purpose.

Other methods of achieving high nitrogen content, such as melting or casting in a high-pressure atmosphere, can be considered, but apart from small-scale experiments, practical application on an industrial scale is extremely difficult, and there are also limitations on equipment. It's big.

For example, [Journal of the Japan Institute of Metals J Vol, 34, Hiroshi 2.1
970, pp. 188-194 describes the experimental results of nitrogen equilibration under high temperature and high pressure, and according to this, F.
6-Mn (27%Mn)! About lti about 1.
It has been reported that 4% of nitrogen was absorbed.

As described above, much effort has been made to increase the nitrogen content, but it is still difficult to put it into practical use on an industrial scale, and it is still difficult to add more than the solid solubility limit of nitrogen. At present, this was thought to be possible.

In other words, from the viewpoint of improving pitting corrosion resistance, crevice corrosion resistance, or increasing strength, the higher the nitrogen concentration in steel, the more desirable it is, but since nitrogen is a gas component, there is a limit to its solid solubility in molten steel. Furthermore, the upper limit of the amount added is determined from the viewpoint of preventing the generation of bubbles during solidification. In other words, in the past, methods such as component adjustment such as increasing Mn content, melting under pressure, and casting were adopted for the purpose of increasing the amount of solid solution in molten stainless steel. As disclosed, the amount of nitrogen added is at most 0.4
It was about 5%.

By the way, manufacturing sintered crystals from iron-based powder and metal nitride powder by powder metallurgy is widely known as a method for manufacturing products such as cutting tools and bearings that require wear resistance.

However, these methods all attempt to utilize the stability and heat resistance at high temperatures that nitrides themselves have, and do not attempt to form an austenite structure by increasing the amount of solid solution nitrogen. It wasn't. In this case, the nitride is considered to be equivalent to the carbide.

Therefore, the present inventors, in Japanese Patent Application Laid-Open No. 61-227153, mixed metal nitride fine powder and alloy powder and sintered the mixture in a metal container to decompose the metal nitride and diffuse it to the alloy side. We proposed a means to increase the strength and corrosion resistance of steel by

(Problems to be Solved by the Invention) According to the means disclosed in JP-A No. 61-227153, a high nitrogen-containing austenitic alloy having an arbitrary nitrogen concentration in steel and having high strength and high corrosion resistance can be produced. It can be manufactured reliably.

However, as a result of further investigation by the present inventors, it was found that in order to obtain a high nitrogen content austenitic alloy by this method, a sintering process under high temperature and high pressure conditions is required, and this sintering process usually takes several hours. The degree of productivity that needs to be done,
It was found that the manufacturing cost was low and practical application on an industrial scale was relatively difficult.

That is, conventionally, it has not been possible to provide a method for reliably producing a high nitrogen content austenitic alloy with high efficiency, that is, in a short period of time.

It is therefore an object of the present invention to provide a method for producing a highly nitrogen-containing austenitic alloy having excellent strength and corrosion resistance with high efficiency.

(Means for Solving the Problems) In order to solve the above problems, the present inventors first proposed the method disclosed in JP-A No. 61-227153, in which a metal nitride fine powder and an alloy powder are combined. The reason why it was considered necessary to heat the mixture and hold it under high temperature and pressure to decompose the metal nitrides was examined in detail again. In other words, it was previously believed that unless the alloy was maintained at high temperature and high pressure, the nitrogen gas generated as the nitride decomposed would form bubbles and a healthy alloy structure could not be obtained.

Here, the inventors of the present invention filled a mixed powder of metal nitride and alloy powder into a metal container instead of holding it under high temperature and high pressure conditions as described above, that is, without going through the sintering process. However, it has been found that a sound alloy structure can be obtained without generating nitrogen gas by hot working the alloy after it has been degassed and sealed, and by appropriately setting the hot working conditions.

In other words, by hot working the above mixed powder at a temperature of 900°C or higher and lower than the melting point with a cross-sectional reduction ratio of 5 or more, the metal nitrides are dispersed within and between the alloy powder in a more finely pulverized form. The present invention was completed based on the finding that the alloy powders adhere to each other and form an alloy.

The gist of the present invention is to fill a metal container with an unsintered mixture of metal nitride fine powder and alloy powder, deaerate it,
This is a method for producing a high nitrogen-containing austenitic alloy, which includes a step of hot working at a cross-sectional reduction ratio of 5 or more at a temperature of 900° C. or higher and lower than the melting point after sealing.

In the present invention, the "cross-sectional reduction ratio" refers to (cross-sectional area before processing)/(cross-sectional area after processing), and "filling" means to fill the metal nitride fine powder by using vibration etc. as necessary. This refers to the state in which an unsintered mixture of powder and alloy powder is charged into a metal container and filled to capacity.

Further, in the present invention, "hot working" refers to a processing method used when manufacturing a product by a powder metallurgy method, and specifically includes forging, rolling, extrusion, or combination thereof.

Note that the "melting point" is the melting point of the alloy powder.

(Function) Next, the manufacturing process in the present invention will be explained in detail. In the following description of the present invention, stainless steel powder is used as the alloy powder, and a steel capsule is used as the metal container.

First, according to the present invention, after uniformly mixing stainless steel powder and metal nitride powder, jil! ! Fill the capsule and degas the inside by vacuuming at room temperature or while heating.
Alternatively, after degassing, the steel capsule is filled with N8 gas and sealed.

Stainless steel copper powder is manufactured by atomization, pulverization, intergranular corrosion, etc., but from the viewpoint of low oxygen content, non-oxidizing spray media such as N8 gas, ^r gas, H
It is preferable to use one manufactured by a gas atomization method using e-gas or the like. Among such atomized copper powders, atomized copper powders with an average particle size of 200 mm or less, preferably 100 mm or less are desirable, and the lower the oxygen concentration in the steel powder, the better.

It is preferably 0.03% or less.

On the other hand, it is preferable that the metal nitride has a high N concentration and is stable at room temperature, but easily decomposed at temperatures of 1000°C or higher.As such metal nitrides used in the present invention, Cr and N Mainly Cr with main component
Cr-based nitride consisting of J and/or CrN, Pa-based nitride consisting mainly of Fe1N and/or Fe4N whose main components are Fe and N, ■-based nitride, Si-based nitride, M-based nitride, M, Fe-based nitrides may be used, or composite nitrides of these may be used.
-Cr system, Fe-V system, Fe-Mn system, Fe-Cr
-V system, Fe-Cr -V -Mn system, etc. In this case, the metal nitride system may be appropriately selected depending on the intended alloy system components, manufacturability, economic efficiency, etc.

In addition, when Cr□N is used, the problem of corrosion resistance deterioration due to Cr1N precipitation, which is a problem with high nitrogen steel made by conventional melting methods, does not occur in the present invention, so CrJ is used as the metal nitride fine powder. Fine powder is preferred.

In this way, it is sufficient to use the metal nitride fine powder as long as it is a well-known commercially available powder, or it can be treated by heating with N2 gas in advance when producing the metal powder or filling it into a metal container to form the nitride. It may be generated.

The average particle size of the metal nitride used in the present invention is desirably small, generally preferably 80 us or less, and more desirably an average particle size of 10 μm or less.

Metal nitrides are easily oxidized during 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 blending ratio of each powder is limited by the deterioration of toughness due to undissolved nitrides in the resulting alloy. For example, when using Cr-N powder, it should be 20% or less, more preferably 10% or less, based on the alloy powder. It's good to do that.

The steel capsule serving as a metal container filled with the powder mixture may be made of either carbon steel or stainless steel, but it is more preferable that the steel constituting this container has a low carbon content.
This is to prevent carburization from the container during heat treatment. The carbon content is most preferably 0.03% or less for carbon steel, and 0.02% or less for stainless steel.The holding temperature during vacuuming may be room temperature, but for the purpose of removing internal moisture, Heating is more effective.

The degree of vacuum degassing is preferably about 10-S to 10" Torr. This is to remove gas adhering to the surface, and there is no particular restriction as far as that. After the degassing is completed, seam welding etc. is performed to bond the metal. Seal the container.

Further, it is desirable to perform evacuation at a higher temperature within a temperature range in which the metal nitride does not decompose.

Filling the capsule with N□ gas after degassing has the effect of further improving heat transfer during heating, which is preferable.

Next, the unsintered mixture filled in this metal container was
This hot working method involves hot working at a temperature above 0°C and below the melting point to refine the metal nitride and form an alloy phase. Below 900°C, alloying is not sufficient because the deformation of the alloy powder is small. For example, forging, rolling, extrusion, and combinations thereof may be used, and according to these methods, plate material,
Can be formed into pipes and rods. The degree of hot working requires a cross-sectional reduction ratio of 5 or more. As mentioned above, the cross-sectional reduction ratio here refers to (cross-sectional area before processing)/(cross-sectional area after processing).If this cross-sectional reduction ratio is less than 5, metal nitrides become finer and alloy powders adhere to each other. But it becomes an imperfect, healthy alloy! This is because a k11 weave cannot be obtained.

Thus, according to the method of the present invention, a high nitrogen-containing austenitic alloy can be produced easily and efficiently without the need for a sintering process lasting several hours at high temperature and high pressure. Of course, the specific structure of the resulting austenitic alloy, such as austenitic stainless steel, can be freely changed by changing the composition of the initial starting powder. In particular, according to the invention, the N content is between 0.01 and 2.
．． It can be set to 0%.

Further, the present invention will be explained in detail along with examples thereof.

Note that this is merely an example of the present invention, and the present invention is not unduly limited thereby.

Examples Each stainless steel powder (melting point: 1390-1468°C) having the structure and particle size shown in Tables 1 and 2, respectively.
After mixing various Cr-based nitride powders at a ratio of 3.4 to 6.3%, steel with a diameter of 80 mm and a carbon concentration of 0.018% was prepared.
The mixture was filled into a carbon steel capsule with a diameter of 180 m and a height of 180 m, and the inside was evacuated and sealed while heating.

The conditions for evacuation were lXl0-'--〇, and the heating conditions at that time were 550°C x 1h''.

Next, after heating this at 1200°C for 1 hour, the diameter was 301I.
It was hot extruded into a 1 liter bar.

Also, for Ag3, the extrusion diameter was changed from 30mm to 75mm.
1, and some of them were subjected to old P under conditions of 1300° C., 2000 atm, and 4 hours instead of hot working.

Samples 1 to 9 were cut out from the high nitrogen-containing austenitic stainless steel thus obtained and subjected to a tensile test, a Charpy impact test, and a pitting potential measurement in artificial seawater.

The results of each test are summarized in Table 3.

(The following is a margin) The tensile test was conducted using a round bar tensile test piece whose parallel portion had a diameter of 5 mm and a length of 30+ ui. Charpy impact test is 2 Tsuru-
■JIS No. 4 half size with notch (thickness 5 an)
The test was carried out at -20°C. The pitting potential was measured in artificial seawater at 80°C degassed with Ar at a current density of 100μA/
The evaluation was made based on the potential Vc at which the temperature was 100 cm.

As is clear from Table 3, the Charpy impact absorption energy and pitting corrosion potential of Samples 1 to 4 obtained by the method of the present invention are comparable to those of conventionally obtained high nitrogen content stainless steels. It can be seen that it is excellent.

On the other hand, Sample-5 to Sample-9 are comparative samples.

Samples No. 5 to No. 8 had a cross-sectional reduction ratio of 4.5 to 1.8, which was smaller than the range of the present invention, so a healthy alloy structure could not be obtained, and the Charpy impact absorption energy and pitting corrosion potential were significantly reduced. It's stored away.

In addition, sample Hidden 9 is a sample that has been subjected to the old P in which hot working is performed with a cross-sectional reduction ratio of 5 or more, and although it has excellent mechanical properties, it is necessary to perform further processing after this. It can be seen that the productivity is significantly reduced compared to this method.

(Effects of the Invention) As is clear from the above explanation, the present invention allows hot working to be performed at a high temperature of 900°C or higher and with a cross-sectional reduction ratio of 5 or more, without going through a sintering process at high temperature and high pressure. This makes it possible to produce a high-nitrogen austenitic alloy with high efficiency, that is, in a short period of time.

The practical significance of the present invention having such effects is remarkable.

Claims (1)

[Claims] A process of filling an unsintered mixture of fine metal nitride powder and alloy powder into a metal container, degassing it, sealing it, and then hot working it at a temperature of 900°C or more and less than the melting point and with a cross-sectional reduction ratio of 5 or more. A method for producing a high nitrogen content austenitic alloy comprising: