KR101323041B1 - Austenitic Stainless Steel for Gas Nitriding and gas nitriding method of the same - Google Patents

Abstract

The present invention finds that the activity of Cr and the high temperature nitrogen utilization increase as a result of adding a small amount of Mo to AISI 304 stainless steel through phase equilibrium calculation, and excellent corrosion resistance through the alloy design based on this The low temperature and high temperature of the AISI 304 series austenitic stainless steel for gas-nitriding and the nitriding layer with excellent surface hardness and excellent corrosion resistance are excellent in nitriding ability of low temperature ammonia gas and high temperature nitrogen gas. Provide a gas nitriding method.

The composition of the invention steel for nitriding is% by weight C: greater than 0 and less than 0.08, Cr: 18 and 20, Ni: 8 and 10, Si: greater than 0 and less than 1.0, Mn: 1-2 and P: greater than 0 and less than 0.045, S : More than 0 and less than 0.030, N: more than 0 and less than 0.10, Mo: 0.1 to 3.0 and the balance are composed of Fe and other unavoidable impurities.

Low Temperature Gas Nitriding, High Temperature Solid Nitriding, Austenitic, Stainless Steel, Mo

Description

Austenitic Stainless Steel for Gas Nitriding and gas nitriding method of the same}

The present invention relates to a gas nitriding method of austenitic stainless steel for gas nitriding and its austenitic stainless steel, and more particularly, the effect of the addition of Mo on the phase equilibrium and nitriding behavior of the conventional AISI 304 stainless steel As the addition increases the activity of Cr and increases the nitriding ability of the steel, it is a new austenitic stainless steel for AIDS 304 and its stainless steel, which has excellent nitriding ability of low temperature ammonia gas and nitriding of high temperature nitrogen gas. Provide a nitriding method.

In general, the gas nitriding technique is a surface hardening technique first proposed by A. Fry in 1923, and heats Al or Cr-containing nitride at a temperature of about 500 ^o C and keeps it heated for 10-100 hours in an ammonia gas atmosphere. This is a technology for forming a surface nitride layer consisting of a very hard nitride image on the surface of the steel. Unlike carburizing and high-frequency curing, this gas nitriding technology is heated to around 500 ^o C in an ammonia gas atmosphere to penetrate the activated N atoms obtained from pyrolysis of ammonia gas into the steel surface to form a surface nitriding layer, thereby surface hardening. To achieve.

In addition, since the gas nitriding technology is not hardened by the change of the microstructure of the steel, subsequent heat treatment such as quenching after nitriding is not required. In addition, since the nitriding treatment is carried out in the nitriding temperature range of 500-600 ^o C, unlike other surface hardening methods, it is made on ferrite, so there is no dimensional change after nitriding treatment and residual stress acts as compressive stress on the outermost layer. Therefore, it is excellent in wear resistance and at the same time excellent in fatigue resistance and corrosion resistance is relatively widely used in the industry.

On the other hand, the gas nitriding method forms a surface nitriding layer by infiltrating activated N generated by the decomposition of ammonia occurring on the steel surface as follows by diffusion. In the structure of the (NH ₃ = N + 3H) surface nitride layer, the outermost surface layer is formed with a surface layer of Fe ₂ N having a hexagonal crystal structure, which is one of the nitride images, followed by Fe ₄ N having a face-centered cubic structure. This is followed by the formation of the diffusion layer, which is the main cause of surface hardening. The diffusion layer shows a structure in which precipitates of nitrides such as AlN and CrN are finely dispersed in the ferrite matrix. In this diffusion layer, fine precipitates are uniformly dispersed in the ferrite matrix, thereby making it difficult to transfer dislocations, thereby exhibiting high wear resistance and hardness.

On the other hand, studies on the gas nitriding of stainless steels such as AISI 304 using ammonia gas are well known to accelerate the nitriding rate between 500 and 600 ^o C. The reason is known to form Cr nitrides such as Cr ₂ N and CrN during nitriding. However, when these Cr nitrided images are formed, Cr deficiency occurs in the austenite matrix, and as a result, the density of the passivation film is reduced, thereby reducing the corrosion resistance.

Accordingly, an object of the present invention is to provide a new nitriding austenitic stainless steel gas nitridation method of AISI 304 series, which has very good nitriding ability of ammonia gas, especially S phase, using ammonia gas nitriding through phase equilibrium calculation. It is.

In addition, another object of the present invention is a new nitriding for the AISI 304 stainless steel series having excellent high temperature solid solution nitriding ability using N ₂ gas (or N ₂ and H ₂ mixed gas) through phase equilibrium calculation It is to provide an austenitic stainless steel and a gas nitriding method thereof.

In order to achieve the above object, the present invention firstly contains C: more than 0 and less than 0.08, Si: more than 0 and less than 1.0, Mn: 1 and 2, P: more than 0 and less than 0.01, S: more than 0, less than 0.03, and N: 0. Less than 0.10 or less, Cr: 18 to 20, Ni: 8 to 10, Mo: 0.1 to 3.0 and the balance provide austenitic stainless steel for gas nitriding composed of Fe and other unavoidable impurities.

In the present invention, Mo is preferably 0.5 to 2.5 by weight.

In addition, in the present invention, the austenitic stainless steel having the composition described above forms a low temperature S phase (expanded austenite) on the surface through nitriding with nitriding gas in a low temperature region of 300 to 450 ^° C. or less to cure the surface. Provided is a method of gas nitriding austenitic stainless steel.

In the present invention, the nitriding gas is ammonia or a mixed gas of ammonia and hydrogen.

In addition, the present invention provides a method of gas nitriding austenitic stainless steel to harden the surface of the austenitic stainless steel made of the above composition through the nitriding treatment with nitriding gas at a high temperature of 900 ~ 1200 ^° C or more. do.

In the present invention, the nitriding gas in the high temperature region is nitrogen or a mixed gas of nitrogen and hydrogen.

In the present invention, by adding a small amount of Mo to the conventional AISI 304 stainless steel, due to the increase in the activity of Cr, the corrosion resistance is excellent at room temperature compared to the conventional AISI 304 stainless steel, and specially gas nitriding (low temperature ammonia gas nitriding and high temperature nitrogen gas nitriding) The effect gets an excellent effect. In other words, when the steel of the present invention was subjected to ammonia gas nitridation, the formation of S phase, which is a low temperature phase, was promoted due to the increase in Cr activity and the grain refining effect.

In addition, even in the case of high temperature solid solution nitriding treatment using nitrogen gas (or nitrogen and hydrogen mixed gas), due to the effect of increasing the activity of Cr and stabilization of high temperature austenite, the steel of the present invention is superior to AISI 304 stainless steel. Showed excellent high temperature solid solution nitriding ability. As a result, the surface hardness was significantly increased and the corrosion resistance was also significantly increased.

Hereinafter, the present invention will be described in more detail with reference to the drawings.

First, austenitic AISI 304 stainless steel is a representative austenitic stainless steel widely used as a corrosion resistant material or a high temperature material for pressure vessels, heat exchangers, and piping in the oil refining industry because of its excellent ductility, toughness, high temperature strength, and especially corrosion resistance. to be. However, AISI304 stainless steels have been limited in their application due to their high coefficient of friction, low wear and corrosion resistance. Efforts to improve the utilization of AISI 304 stainless steels by solving such problems have been continually sought. As one of the methods, gas nitriding technology using ammonia gas has traditionally been steadily studied.

Gas nitriding technology using ammonia gas decomposes ammonia gas by surface reaction of AISI 304 stainless steel between 450 ~ 600 ℃ and penetrates activated nitrogen gas from steel surface to form surface nitride layer to achieve surface hardening. In addition, to improve the corrosion resistance. In particular, such a temperature range is known to be a very favorable temperature condition for the nitriding process because it has the advantage that the nitriding rate is very fast. However, the problem at this time is the fact that Cr nitrides such as Cr ₂ N and CrN are formed during nitriding. When these Cr nitrides are formed, the wear resistance is excellent, but Cr deficiency occurs in the austenite matrix, and as a result, the density of the passivation film is reduced, thereby reducing the corrosion resistance.

In order to improve such a problem, low temperature ammonia gas nitriding technology conducted at 450 ° C. or less has been studied. The low temperature ammonia gas nitriding technology uses the fact that the formation rate of Cr nitride is very slow at a low temperature below 450 ° C, so that ammonia gas nitriding is carried out at low temperature, thereby eliminating the formation of Cr nitride and forming a low temperature phase called S phase. will be. The S phase, which is a low temperature phase, is a phase formed by the supersaturation of nitrogen atoms in the austenite phase, also called the expanded austenite phase, which strengthens the austenite matrix by expanding the lattice without affecting the distribution of Cr. To play a role. Therefore, low temperature ammonia gas nitriding is a method of obtaining a surface nitride layer showing excellent fatigue strength and wear resistance while maintaining good corrosion resistance.

However, since the low temperature gas nitriding method performs gas nitriding at a low temperature of less than 450 ^° C., in the conventional AISI 304 steel, the nitriding proceeds extremely slowly, and thus it is difficult to sufficiently obtain a nitride layer made of S phase. Therefore, this technique has been mainly developed in nitriding techniques having high nitriding potential, such as ion nitriding, and its application has been difficult in conventional gas nitriding techniques using ammonia gas.

Therefore, one of the objectives of the present invention was to develop a new nitriding steel of AISI304 series having excellent ability to form S phase at low temperature, and to provide a low temperature nitriding technology using ammonia gas. Another more important object of the present invention is the solid solution nitriding that can be utilized for in-line nitriding by remarkably improving the high temperature solid solution nitriding rate using nitrogen gas (or nitrogen + hydrogen mixed gas) through the development of the same molten steel. An attempt was made to provide a technology.

On the other hand, in the present invention, in addition to the above-mentioned low temperature gas nitriding, the equilibrium of N ₂ gas (or a mixed gas of N ₂ and H ₂ ) and austenite at a high temperature of 1,000 ^o C or more, preferably 1,000 ^o C to 1200 ^o C It is a method to solidify the surface by enhancing the nitrogen atom on the austenite by using the reaction to improve the corrosion resistance. With the recent sharp rise in raw material prices and global capacity expansion, the profitability of general-purpose stainless steels has been falling and sales competition has intensified. Such high temperature employment nitriding technology is more suitable for the creation of new markets and applications through the development of high value-added stainless steel products. The need is gradually increasing.

In particular, the high temperature solid solution nitriding technology has the advantage of combining the nitriding process with the in-line manufacturing process of stainless steel plate as it is performed at high temperature, thereby creating high added value and reducing manufacturing cost. The advantage is that you can pursue it at the same time. Since the in-line nitriding process has to be carried out within a few minutes at high temperature, solute nitriding has become an important problem to be solved in order to increase the rate of solid solution nitriding. Therefore, in the present invention, a new nitriding austenitic stainless steel based on AISI 304 stainless steel having excellent high temperature solid solution nitriding ability using N ₂ gas (or N ₂ and H ₂ mixed gas) through phase equilibrium calculation. Can be provided.

Next will be described in detail with respect to the base material component of the present invention. In the present invention, as a result of studying the phase equilibrium through the state diagram calculation for the AISI 304 series stainless steel with Mo added, the following facts were found. First, the addition of trace amounts of Mo to AISI 304 series stainless steel resulted in a significant increase in Cr activity. Secondly, the addition of trace amounts of Mo increased the solubility of nitrogen atoms in high temperature solid solution nitriding. Third, the grain size of the austenite phase can be refined by adjusting the Mo content to be added.

On the basis of the results of such phase equilibrium calculations, a new austenitic stainless steel alloy for gas nitridation has been developed, characterized in that it is prepared by adding a predetermined Mo to a conventional AISI 304 stainless steel. The reason for limiting the ingredients of the base material component of the present invention will be described in more detail.

The austenitic stainless steel according to the present invention is first in weight% C: greater than 0 and less than 0.08, Si: greater than 0 and less than 1.0, Mn: 1-2, P: greater than 0 and less than 0.045, S: greater than 0 and less than 0.03, N: Above 0 and below 0.1, Cr: 18 ~ 20, Ni: 8 ~ 10, Mo: 0.1 ~ 3.0 and the balance consists of Fe and other unavoidable impurities.

In the present invention, C acts as an austenite stabilizing element and contributes to increasing the hardenability and strength of the steel. It also plays a role in increasing nitrogen solubility in austenite to a limited extent. If the content is more than 0.08wt.%, The chromium carbide is formed during annealing to promote chromium sensitization, thereby reducing the corrosion resistance. Therefore, the C content is preferably more than 0 but limited to 0.08% or less.

Si is advantageous for high temperature oxidation resistance, but when the addition amount is too high, the hot workability is lowered, and in particular, since the inclusion of linear inclusion defects due to the increase of Si inclusions is concerned, the content of Si is more than 0 but limited to 1.0% or less. It is desirable to.

Mn is an austenite stabilizing element that promotes hardenability of steel. In particular, it is an element that increases the nitrogen solute in austenite, and its content is small at 1.0% or less, and when the amount is large, the high temperature oxidation resistance is lowered. Therefore, the addition amount is preferably limited to 2% or less.

P segregates at grain boundaries during solidification, inhibits hot workability, and causes high temperature brittleness. Therefore, the content of P is preferably greater than 0 but limited to 0.045% or less.

S is an element that inhibits hot workability, and segregation at the austenite grain boundary during solidification causes linear defects during hot rolling. Therefore, the content thereof is more than 0 but preferably limited to 0.030% or less.

N is an element that increases the yield strength of the material to twice the carbon when it is solid solution, and thus inhibits the cold workability, that is, the formability of the material, and promotes the age crack generated after molding. In addition, if excessively included, crystallization such as CrN may be formed during solidification, which may inhibit corrosion resistance. Therefore, the content thereof is more than 0 but preferably limited to 0.1% or less.

Cr is an essential element for showing excellent corrosion resistance and has excellent hardenability. If the content is less than 18%, it may not exhibit sufficient corrosion resistance. If the content is more than 20%, formation of the delta ferrite phase is encouraged, making it difficult to obtain a stable austenite phase.

Ni is an essential element for stabilizing the austenite phase at room temperature and has excellent hardenability. It is difficult to stabilize austenite at room temperature below 8 wt., And to promote chromium sensitization by reducing the solubility of carbon at 10 wt. Nickel, on the other hand, is an element that reduces the austenite solubility of nitrogen. Therefore, Ni content of more than 10 wt.% Adversely affects the solute nitride effect.

Mo is an important element that distinguishes it from the conventional AISI 304 steel in the present invention steel, which has fine grains through teltaferrite formation at annealing temperature and increases Cr activity, so that the nitriding rate is not only low temperature ammonia gas nitriding but also high temperature nitrogen gas solid solution nitriding. Serves to significantly increase. At the same time, it significantly improves the corrosion resistance of AISI 304 stainless steel by improving the fitting potential. If the content is less than 0.1%, the effect is insignificant, and if the content is greater than 3.0%, the fraction of teltaferrite increases, adversely affecting the corrosion resistance and mechanical properties. Therefore, the range should be 0.1% or more but 3.0% or less.

On the other hand, more preferable optimum Mo content conditions are as follows. That is, when the Mo content is about 0.5 wt%, more preferably 0.65 wt% or less, it is difficult to expect a grain refining effect because it is located in the austenite single phase region at about 1150 ^° C., the homogenization temperature. On the other hand, if the Mo content is about 2.5 wt%, more preferably 2.2 wt% or more, there is no austenite single phase region (ignoring the presence of M ₂₃ C ₆ ) at high temperature, so that the high temperature teltaferrite phase may exist up to low temperature. The possibility of forming a sigma image increases. Therefore, the optimum Mo composition of the present invention is 0.5 wt% ≦ Mo ≦ 2.5 wt%. However, in consideration of the inaccuracy of the equilibrium calculation database and the kinetic uncertainties such as diffusion, the composition range of the Mo addition effect found in the present invention will be extended to 0.1 wt% ≦ Mo ≦ 3.0 wt%.

1 shows a state diagram calculation result of the present invention improved composition by adding a general AISI 304 stainless steel and 1.5 wt% Mo. The left state diagram is a state diagram of Fe18Cr8Ni, which is a general AISI 304 stainless steel, and the right side is a state diagram of Fe18Cr8Ni1.5Mn, according to the present invention. As can be seen from both state diagrams, it can be seen that the state diagrams of the two steel grades are similar in form. However, in the case of the present invention steel added 1.5wt% Mo, compared to the conventional steel without Mo, the state diagram shows a form in which the parallel content is moved to the smaller Cr content (1.5 to 2.0 wt%). In other words, the phase equilibrium of the Fe-Ni-Cr-Mo quaternary alloy containing a small amount of Mo is treated (approximately) as if the Fe-Ni-Cr ternary alloy increased by the amount of Mo added to the parent Cr composition. You can do it. From this result, the role of Mo in the case of adding a small amount of Mo can be interpreted as if Mo plays a role of increasing the activity of Cr by that much.

Figure 2 shows the results of calculating the Mo addition effect on the Fe18Cr8Ni1Mn-xN system. When a small amount of Mo was added, the first two phase regions of the hot ferrite phase (telta phase) and the austenite phase were relatively enlarged. Second, the boundary of austenite / (austenite + Cr ₂ N) moved to higher N. The second result shows that the addition of Mo increases the solubility of nitrogen in austenite. As described above, the reason why the solubility of nitrogen in the austenite phase in the Mo-containing steel is increased is because Mo plays a role of increasing Cr activity. Cr is known to have the strongest effect of increasing nitrogen solubility in austenite. However, when the amount of Mo is less than 0.1wt%, the effects occurring in FIGS. 1 and 2 are insignificant, and when the Mo content is more than 3.0wt%, the sigma phase is easily formed from the delta-ferrite phase, which is a high temperature phase, and the corrosion properties and mechanical properties of the stainless steel. Will adversely affect the nature.

Fe18Cr8Ni1Mn-xMo state diagram calculation results in Figure 3 shows this well. Conventional AISI 304 stainless steel containing no Mo is present in the austenite single phase region at the homogenization temperature, based on the commonly performed homogenization temperature of 1150 ^° C. In comparison, the Mo-modified AISI 304 stainless steel containing a small amount of Mo (1.5 wt.%) Is present in the delta-ferrite and austenite two-phase regions at high temperatures at the homogenization temperature.

This can be seen through the optical microscope tissue photograph of FIG. 4. FIG. 4 is a texture photograph showing the delta-ferrite distribution of the homogenized specimen, showing both the conventional AISI 304 stainless steel and the inventive steel. As can be seen in the figure, the conventional AISI 304 steel is present as an austenitic single phase, while in the stainless steel of the present invention it can be observed that a significant amount of the telta-ferrite phase is dispersed at the austenite matrix. When calculating this through the state diagram shown in FIG. 3, in the case of the inventive steel, a telta ferrite phase of 10% may be present in the austenite matrix in molar fraction at the homogenization temperature. As the temperature decreases, the delta-ferrite phase slowly decreases and disappears below 1060 ^o C, and at lower temperatures it has an austenite single-phase structure. In addition, the reduction in temperature is expected in the calculation of the equilibrium to the presence of carbide phase, sigma phase and ferrite phase, but it is known that the formation of these equilibrium phases are extremely limited under normal cooling conditions.

On the other hand, the present invention finds a new fact that a small amount of telta-ferrite present at such a homogenization temperature plays a role of remarkably minimizing austenite grain structure by preventing grain coarsening occurring during homogenization and during hot working. It was based together.

This fact can be confirmed from the optical microscope histogram shown in FIG. 5. In the case of the stainless steel of the present invention under the same homogenization conditions, it can be observed that the grain size was drastically reduced to less than 1/2 of the conventional AISI 304 stainless steel. If the grain size is refined, the mechanical properties of the steel will not only be excellent, but the grain boundary area will increase rapidly, and the nitriding rate will be greatly improved. This effect is expected to be more prominent in low temperature nitriding than in high temperature nitriding, since the role of diffusion through grain boundaries is generally more important at low temperatures.

In general, stainless steel is a criterion more important than the mechanical properties. In order to investigate the corrosion resistance of the present invention, the anode polarization curve in the 3.5% NaCl corrosion solution was measured and compared with the polarization curve of the conventional AISI 304 stainless steel. 6 shows measurement results of the polarization polarization curves of both steel grades. As can be seen from the figure, the corrosion potential of the present invention tended to be higher, the passive current density was decreased, and in particular, the fitting potential was significantly increased, indicating that the corrosion resistance of the present invention was superior to AISI304. . From this fact, it can be seen that the small amount of telta-ferrite phase, which may be present at room temperature in Mo-added steel, does not significantly affect the corrosion resistance of stainless steel.

The reason why the corrosion resistance of the alloy of the present invention is increased compared to that of AISI 304 stainless steel is that Mo, as shown in the phase equilibrium calculation result of FIG. 1, increases the activity of Cr, thereby increasing the density of the Cr dynamic coating. This is because the degree increased. The fact that Mo addition enhances the activity of Cr was confirmed in the ammonia gas nitriding test. As a result of addition of Mo, it was confirmed that the formation of Cr ₂ N or CrN tended to be promoted at low temperature.

Next, a gas nitriding test was conducted to find the nitriding potential of the austenitic stainless steel for nitriding developed in the present invention. The results will be described below. The gas nitriding test consists of the following three tests: first, a general nitriding test using ammonia gas (near nitriding temperature: near 500 ^o C), second, a low temperature nitriding test using ammonia gas (nitriding temperature: below 450 ^o C), and Third, high temperature solid solution nitriding tests (nitriding temperature: around 1100 ^° C.) using N ₂ (N ₂ + H ₂ ) gas were carried out, respectively. In each case, conventional AISI 304 stainless steel was used as a comparison steel to compare the nitriding effects. Representative compositions of the inventive steel and AISI 304 stainless steel used in this example are shown in the following table.

division C Si Mn P S Cr Ni Mo N AISI304 0.058 0.437 1.06 0.004 <0.002 18.10 8.10 - 0.040 The present invention 0.062 0.394 1.08 0.004 <0.002 18.21 8.14 1.50 0.031

Example  One:

7 shows the results of gas nitriding the AISI 304 stainless steel and the inventive steel for 20 hours at 530 ^° C. in a flowing ammonia atmosphere (1,000 ml / min). It can be observed that in both steels a nitride layer of 50 μm or more was formed. When comparing the nitride layers of the two steels, it can be seen that the thickness of the nitride layer is significantly increased in the steel of the present invention. According to the measurement results of the hardness change according to the change in the cross-sectional depth, it can be seen that both steels formed a nitride layer having a high and constant hardness composed of a nitride phase from the surface to a specific depth. Comparing the thickness of the nitride layer, it can be seen that the AISI 304 stainless steel is about 70 μm while the steel of the present invention is about 100 μm, and the thickness of the nitride layer is increased by about 1/2 times in the steel of the present invention. As a result of the X-ray diffraction test, it can be seen that the nitride layer is mainly composed of a nitride phase of Fe ₄ N (Fe ₃ N), Cr ₂ N, and CrN. In addition, strong oxide peaks (Fe ₂ O ₃ ) were observed, indicating that a considerable amount of oxide was formed on the outermost surface.

Example  2 :

FIG. 8 shows the results of gas nitriding with an ammonia gas atmosphere (100 ml / min) flowing through AISI 304 stainless steel and the inventive steel for 6 hours at low temperature nitriding temperature (450C). Although the nitriding temperature is low, it can be seen that gas nitriding has progressed to some extent. In particular, in the steel of the present invention, the nitride layer is formed to a considerable depth despite the low temperature. In the hardness change measurement results according to the change in the cross-sectional depth, unlike in Example 1, a region with a constant hardness is not observed on the surface. In addition, in the case of AISI 304 stainless steel, it can be seen that the surface hardness value is significantly small and the depth is shallow. On the other hand, in the case of the alloy of the present invention, no surface nitride layer having a constant hardness is found. However, it can be seen that the surface hardness is much higher than AISI 304 and its depth is more than doubled. As a result of the x-ray diffraction test, it was found that the nitride layer formed at this time was S phase, which is a low temperature phase and not a bridge and a nitride phase. From this, it can be seen that the alloy steel of the present invention has the effect of significantly promoting not only the nitriding phase but also the formation of the S phase, which is a low temperature phase. This is also because Mo included in the alloy of the present invention increases the activity of Cr and thereby activates the penetration of activated nitrogen atoms formed by pyrolysis of ammonia gas. At this time, since the temperature is low, Cr is difficult to move, and thus formation of Cr nitride phase is suppressed and only expanded austenite phase in which nitrogen is supersaturated due to the penetration of nitrogen atoms is formed. However, the gas nitriding process according to the present invention at a temperature below 300 ^o C is rather to retain more than 300 ^o C may be a problem in thus forming the nitride layer is difficult to form the low-temperature phase S.

Example  3:

FIG. 9 shows the result of high temperature solid solution nitriding treatment for 1075 ^° C., for 3 hours under a nitrogen and hydrogen mixed gas atmosphere (N ₂ 100ml / min + H ₂ 300ml / min). Observation of the cross-sectional texture photograph after nitriding treatment is not easy to determine whether a nitride layer is formed in both steels. Micro Vickers hardness measurements, however, showed that both steels (tested on two specimens each) had a high hardness on the surface of 200-220 HV0.2 and decreased exponentially toward the center of the specimen, as if the diffusion of nitrogen. It shows a form that shows a concentration gradient. Compared with the hardness value (150 HV0.2) of the base material, in the case of AISI 304 stainless steel, the solid solution nitriding has not yet proceeded to the center of the specimen, but it can be seen that the solid solution nitriding has already been performed to the center of the specimen in the present steel. . In addition, if the standard of solid solution nitride layer formation is set to 200 HV0.2, it can be seen that in the case of AISI 304 stainless steel, the thickness of the solid solution nitride layer is 50 μm or less, whereas 100 μm or more is formed in the steel of the present invention. X-ray diffraction results show no formation of nitride on the surface. It can be seen that this change in cross-sectional hardness is due to pure solid solution nitriding. Comparing the x-ray diffraction results, in both cases the peak on the austenite phase is the main peak, but in the case of AISI 304 stainless steel, some alpha martensite peaks are observed. However, in the steel of the present invention, such an alpha martensite peak is not observed and only a peak of the austenite phase tends to appear strong. This fact is in good agreement with the fact that in the steel containing a small amount of Mo, the austenite single phase region expands as the concentration of nitrogen increases, as described in the state diagram calculation result of the present invention shown in FIG. 2. For this reason, the solubility of nitrogen increases and the grain size is fine, so the diffusion rate of nitrogen is relatively increased. Along with this fact, it is believed that in the present alloy, especially the activity of Cr increases, the high temperature solid solution nitriding rate is significantly increased compared to AISI 304 stainless steel. On the other hand, when the solid solution nitriding temperature is lowered, the austenite (A) / (A + Cr 2 N) boundary composition of nitrogen decreases in FIG. 2, so that the solubility of nitrogen in austenite decreases, and when the temperature increases (from FIG. 2). The austenite equilibrium solubility of nitrogen gas is reduced. From this, in order to employ a sufficient amount of nitrogen atoms (without precipitation of Cr2N) in austenite, the solid solution nitriding temperature is preferably 1000 1200C.

Example  4:

FIG. 10 shows the results of measuring the polarization polarization curves of AISI 304 steel and the inventive steel before and after solid solution nitriding treatment in 3.5% NaCl corrosion solution in order to investigate the effect of high temperature solid solution nitriding treatment on the corrosion resistance. As can be seen in the figure, the AISI 304 steel showed similar corrosion behavior before and after solid solution nitriding. On the other hand, in the inventive steel, the corrosion potential increased and the passivation current density decreased after the solid solution nitridation treatment. In particular, the pitting potential increased significantly. The result was a very high pitting potential of more than 500 mV (SCE). As a result of the high temperature solid solution nitriding treatment of the present invention, not only the surface hardness but also the corrosion resistance was remarkably improved, and the solid solution nitriding effect was found to be superior to that of AISI304 steel.

1 is a graph showing a comparison of the state diagram of Fe 18 Cr 8 Ni without Mo and Fe 18 Cr 8 Ni 1.5 Mo with Mo according to the present invention.

FIG. 2 is a graph illustrating a state diagram of a Fe18Cr8Ni1Mn-xN system in which a state diagram of the AISI 304 steel without Mo and the present invention steel with Mo is compared.

Figure 3 is a graph showing the state diagram of the Fe-added Fe18Cr8Ni1Mn-xMo according to the present invention and the results of the phase fraction change according to the temperature change when the Mo content is 1.5%.

FIG. 4 is an optical micrograph (Electrolytic etched-10N KOH) showing the distribution of telta-ferrite in a homogenized specimen, and is a comparative drawing of the conventional AISI 304 steel and the inventive steel.

5 is a chemical etched-aqua regia showing the grain structure: a comparison of the existing AISI 304 steel with the inventive steel.

6 is a graph showing the measurement results of the polarization polarization curve in 3.5% NaCl corrosion solution, a comparison of the conventional AISI 304 steel and the present invention steel.

FIG. 7A is a cross-sectional SEM photograph of each of the conventional AISI 304 steel and the inventive steel, showing the results of ammonia gas nitriding treatment (20 hours, 100% NH ₃ , 1 L / min) at 530 ° C., which is a normal temperature.

FIG. 7B is a graph showing the cross-sectional hardness (Vickers hardness) distribution of the conventional AISI 304 steel and the inventive steel in connection with FIG. 7A.

FIG. 7C is a diagram of X-ray diffraction test results of the conventional AISI 304 steel and the inventive steel in connection with FIG. 7A.

FIG. 8A shows the results of ammonia gas nitriding treatment (6 hours, 100% NH ₃ , 100 mL / min) at 450 ° C., which is lower than the normal temperature. .

FIG. 8B is a graph showing the cross-sectional hardness (Vickers hardness) distribution of AISI 304 steel and the inventive steel in connection with FIG. 8A.

FIG. 8C is an X-ray diffraction test result diagram of AISI 304 steel and the inventive steel in connection with FIG. 8A.

Figure 9a is a cross-sectional structure photograph of the conventional AISI 304 steel and the present invention steel showing the results of the nitrogen gas solid solution nitriding treatment (3 hours, 100mL / min N ₂ + 300mL / minH ₂ ) at a high temperature zone 1075 ℃.

FIG. 9B is a graph showing the cross-sectional hardness (microbicus hardness) distribution of the conventional AISI 304 steel and the inventive steel in connection with FIG. 9A.

FIG. 9C is an X-ray diffraction test result diagram of the conventional AISI 304 steel and the inventive steel in conjunction with FIG. 9A.

Figure 10a, 10b is conventional AISI304 steel (a) in the present invention steel (b) each of the high temperature employed nitride _{(500mlN 2 + 300mlH 2, 1090C} , 1h) anodic polarization song in a 3.5% NaCl corrosion solution measured before and after the line in Is a graph showing comparison.

Claims (7)

By weight% C: more than 0 and less than 0.08, Si: more than 0 and less than 1.0, Mn: 1 and 2, P: more than 0 and less than 0.045, S: more than 0 and less than 0.03, N: more than 0 and less than 0.1, Cr: 18 and 20, Ni: 8-10, Mo: 0.5-2.5, and the remainder is austenitic stainless steel made of Fe and other unavoidable impurities, The austenitic stainless steel has a low-temperature S phase (expanded austenite) as a nitride layer on the surface, the low-phase S phase is expanded to a phase formed by supersaturation of nitrogen atoms in the austenite phase (expanded austenite) Austenitic stainless steel for gas nitride. delete delete By weight% C: more than 0 and less than 0.08, Si: more than 0 and less than 1.0, Mn: 1 and 2, P: more than 0 and less than 0.045, S: more than 0 and less than 0.03, N: more than 0 and less than 0.1, Cr: 18 and 20, Ni: 8 ~ 10, Mo: 0.5 ~ 2.5 and remainder of low temperature S phase (expanded austenite) through nitriding of austenitic stainless steel made of Fe and other unavoidable impurities with nitriding gas at low temperature below 300 ~ 450 ℃ ) To form a surface to cure the surface, wherein the low temperature S phase is an expanded austenite phase formed by the supersaturation of nitrogen atoms in the austenite phase. 5. The method of claim 4, The nitriding gas is ammonia or a gas nitriding method of austenitic stainless steel is a mixed gas of ammonia and hydrogen. By weight% C: more than 0 and less than 0.08, Si: more than 0 and less than 1.0, Mn: 1 and 2, P: more than 0 and less than 0.045, S: more than 0 and less than 0.03, N: more than 0 and less than 0.1, Cr: 18 and 20, Ni: 8 ~ 10, Mo: 0.5 ~ 2.5, and the balance of austenite stainless steel made of Fe and other unavoidable impurities at high temperature of 1000 ~ 1200 ℃ through nitriding by nitriding gas to harden the surface Gas Nitriding Method of Austenitic Stainless Steel. The method according to claim 6, The nitriding gas is nitrogen or a mixture of nitrogen and hydrogen gas nitriding method of austenitic stainless steel.

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