KR20000051876A - A method for producing austenite stainless steel dissolved high content of nitrogen - Google Patents

Abstract

PURPOSE: A method producing austenite stainless steel containing large portion of nitrogen is provided to increase nitrogen solubility with a minimum equipment investment cost. CONSTITUTION: Nitride alloyed steel is charged into a melting furnace as a basic material. In order to increase nitrogen solubility, chemical elements are added in the molten nitride alloy. Atmosphere pressure is variably increased to suppress nitrogen gas evaporation from the molten metal. The added chemical elements are chosen among manganese, chrome, nickel and carbon. Atmosphere pressure is pressurized by nitrogen gas. Nitrogen contained in the steel has a stable structure even thought concentration is several times as high as the present material, so that the austenite stainless steel has high tenacity, high corrosion resistance and non-magnetism.

Description

A method for producing austenite stainless steel dissolved high content of nitrogen}

The present invention relates to a method for manufacturing austenitic stainless steel used as various structural materials, frames, designs, parts, and cores, and more specifically, using a conventional pressurized induction furnace, using a new alloy component and composition design method. By using iron nitride alloy as a basic raw material and manufacturing under pressurized nitrogen atmosphere, austenite containing a large amount of nitrogen that can have a stable structure while allowing nitrogen of a concentration several times higher than that of the conventional one can be dissolved in the mother alloy. It relates to a method of manufacturing stainless steel.

Stainless steel is a steel with high corrosion resistance and is used for chemical containers and pipes. It is also used for special purposes by varying the added elemental elements. In particular, austenite stainless steel has better corrosion resistance than ferrite or martensite stainless steel, good mechanical properties at high and low temperatures, good impact resistance and cold drawing. As well as easy to form and weld, it has high creep strength at high temperatures and good toughness at low temperatures, and is widely used to account for about 70% of the total production of stainless steel. + Ni ≧ 22% (Cr> 16%, Ni> 6%) and the representative standard component is 18 (Cr) -8 (Ni), known as 18-8 type stainless steel.

Among the various properties of the austenitic stainless steel, the most important one in practical use is nonmagnetic properties. This is a unique property not available in other stainless steels. In addition to components, materials in the machinery industry, such as cylinders, pipes, valves and freezers, and military equipment such as missiles and tanks, as well as food, paper, and liquor containers and materials for automobiles, construction, aircraft, and medical equipment. It is widely used for distribution electric materials. Moreover, it is excellent in workability and can be used not only for various wire rods, but also for the manufacture of a micro wire of several tens of micrometers in diameter, and is also used for various other filters.

However, despite having such excellent properties, austenitic stainless steels have lower mechanical strengths than ferritic or martensitic systems and thus have limited application.

In order to overcome this problem, the proposed method was to add nitrogen. Since nitrogen stabilizes the austenite structure and also increases the strength property proportionally, it is useful to contain as much nitrogen as possible, but since it is a gas element, It was not easy to contain more than some amount in the alloy.

As one of the conventional methods for manufacturing nitrogen-containing stainless steel, a method of preparing nitrogen-containing steel by injecting a flux containing a high concentration of nitrogen into a molten metal and dissolving the nitrogen in the flux in the molten metal has been used. However, when the stainless steel is manufactured by the conventional method, unwanted components are present in the steel due to the other components included in the flux, which may cause deterioration of the quality of the stainless steel. There was a problem that a process is required. In addition, this method has a problem that it is difficult to control the expensive equipment cost, productivity, and composition control because the pressurization to a very high air pressure.

In addition, among the manufacturing methods for containing a larger amount of nitrogen in the alloy, currently industrialized manufacturing methods worldwide include the Pressurized Electroslag Remelting (PESR) method and the Plasma Arc Remelting method.

PESR method is produced by using Si ₃ N ₄ slag as the main raw material and applying ultra-high pressure of 42 atm in the melting furnace. There is a problem in that the productivity is reduced because there is a limit to the amount of dissolution per batch.

In addition, since nitrogen solubility in molten steel generally increases proportionally as the partial pressure of nitrogen increases (Sivert's law), it is necessary to apply ultra high pressure of several tens of atmospheres or more to obtain thousands of ppm solubility. There is a problem in that it is necessary to manufacture a dedicated equipment equipped with-, equipment for cooling, cooling equipment, etc., and a huge facility investment cost is required to separately manufacture dedicated equipment for high nitrogen steel.

On the other hand, the Plasma Arc Remelting method is a method of dissolving nitrogen in an alloy by using an ultra-high temperature arc plasma in a nitrogen atmosphere. Since nitrogen is a gas element, in principle, it must be dissociated in nitrogen monoatomic state to be incorporated into the molten metal to penetrate into the metal. It is a method using the easy.

Using this method, the nitrogen atom dissociated by the ultra-high temperature arc plasma can penetrate into the molten metal to increase the concentration of nitrogen, but the concentration of the dissociated nitrogen ions is only a few% of the total nitrogen atmosphere pressure. In addition, the surface of the molten metal reacts with nitrogen gas to form a thick nitride layer on the surface of the molten metal to block the penetration of nitrogen molecules into the molten metal. There was a problem in that the concentration of nitrogen in the alloy could not be increased more than a certain amount such that the actual amount of nitrogen dissolved by dissociation did not exceed 1000 to 2000 ppm. As a result, in order to increase the nitrogen solubility of the molten metal by a certain concentration, it was found that it is impossible to be metallurgically dependent solely on dissociated nitrogen ions.

Accordingly, the present invention has been made to solve the above problems, and an object of the present invention is to provide a method for producing austenitic stainless steel containing a large amount of nitrogen to minimize the equipment investment cost and increase nitrogen solubility as much as possible.

Figure 1 shows the relationship with the lattice constant for the concentration of nitrogen in the alloy,

2 shows the results of X-ray diffraction analysis on the specimen in the cast state.

Method for producing austenitic stainless steel containing a large amount of nitrogen according to the present invention for achieving the above object, the first step of charging by melting the alloying iron nitride as a basic raw material in the melting furnace; A second step of injecting and dissolving additional elements for increasing nitrogen solubility in the molten alloy; And a third step of variably raising the atmospheric pressure in the melting furnace to suppress nitrogen gas evaporation from the molten metal, wherein the melting furnace uses a pressurized induction melting furnace, and the additional element is manganese, chromium, nickel. And carbon, and the atmospheric pressure is characterized by being pressurized by nitrogen gas.

Hereinafter, a preferred embodiment of the method for producing austenitic stainless steel containing a large amount of nitrogen according to the present invention will be described in detail with reference to the accompanying drawings.

First, the appropriate melting equipment, alloy composition and selection of the basic raw materials will be described as follows.

Selection of Appropriate Melting Facilities

In the present invention, in order to avoid the problems of the conventional method that required a huge equipment investment cost, an appropriate melting equipment was selected to use the existing equipment as it is or to use only a slight reinforcement.

In order to contain a large amount of nitrogen in the molten metal, two dissolution conditions must be met. The first is that when charging the raw materials to start dissolution, it is not necessary to facilitate the selective evaporation of the raw materials by suddenly adding too high a heat source from the beginning. Because each additional element has a different melting point and evaporation temperature, when a high-temperature heat source is suddenly put in, the low melting point metal melts first and evaporates, making it difficult to control the alloy composition. Especially in the case of nitride, since the evaporation of nitrogen gas occurs rapidly when dissolving it should be more careful. Therefore, it is judged that an induction melting furnace using eddy current loss heat of the material itself is most suitable as a melting facility.

Secondly, when nitrogen alloy is charged and dissolved as a basic raw material, since nitrogen is basically a gas element, nitrogen gas exceeding the thermodynamic solubility must be discharged to the outside when dissolved, and thus nitrogen generated after the nitride alloy is dissolved. It is necessary to pressurize to some extent in order to suppress evaporation of a gas as much as possible, and in this invention, when the pressing force was about 1 atmosphere (atmospheric pressure), it turned out that sufficient effect can be acquired.

In conclusion, it is considered that an induction furnace capable of pressurization of 1 atmosphere or more is most suitable as a melting facility. Of course, it is preferable to use nitrogen as a pressurized gas.

Design of Proper Alloy Composition

Since the solubility of nitrogen largely depends on the type and amount of alloy components, it is possible to maximize the solubility by appropriately selecting the alloy components and composition ratios.

In general, the solubility of nitrogen in molten Fe-based alloys is determined by the interaction between nitrogen and the additive elements, and the interaction between all atoms is a chemical reaction involving positive or negative heat of reaction. The degree of reaction can be explained by the sign and magnitude of the reaction sequence, and the reaction between atoms, ie the degree of interaction, can be predicted from the interaction coefficient e _n ^(j) (interaction parameter).

Numerous studies have shown that the heat of nitrogen dissolution (heat of reaction) in molten Fe-based alloys is proportional to the interaction coefficient. Finally, the nitrogen solubility and interaction coefficient have the following relationship.

Therefore, the solubility of nitrogen is determined by the interaction between the additional elements used in the present invention and nitrogen, and furthermore, when high-pressure nitrogen is applied during dissolution, the evaporation of nitrogen gas can be suppressed as much as possible. We believe that employment can be maximized.

After all, in order to contain a large amount of nitrogen in the prepared austenitic stainless steel, it is preferable to add a large amount of additional elements that can increase the solubility of nitrogen, in the present invention, the solubility of nitrogen using an additive element having a composition range as follows: Increased.

Mn: 5 (wt.%) ≤ X ≤ 30 (wt.%)

Cr: 5 (wt.%) ≤ X ≤ 25 (wt.%)

Ni: 1 (wt.%) ≤ X ≤ 12 (wt.%)

C: 0.01 (wt.%) ≤ X ≤ 1.0 (wt.%)

In addition, in order to increase the solubility of nitrogen above a certain concentration, in the initial alloy design, nitrogen alloyed with Ni was alloyed as a basic raw material. Therefore, a large amount of nitrogen mixed in the molten metal can be secured from the nitride alloy used as the basic raw material. The nitride alloys that can be used as the basic raw materials include chromium iron alloys, manganese nitride alloys, molybdenum nitride iron alloys, chromium molybdenum iron alloys, silicon nitride alloys, and iron nitride alloys. Nitride alloys can be used.

By the production method according to the present invention using the melting facility and the alloy design method, up to about 9,000 ppm was successfully dissolved in the alloy.

Example

Basic components of the alloys, but to Step Fe-Mn-Cr-Ni- N, F 2 as one example in the present embodiment, - 25% Mn- 12% Cr - 1% of Ni-based alloys were prepared according to the basic composition, A large amount of Mn was added to contain as much nitrogen as possible, and the amount of Ni was kept to a minimum to produce lower cost alloy materials.

In order to obtain an alloy containing higher concentration of nitrogen, as a charging raw material in the molten metal, pure metal of each component was used as a raw material together with various iron nitride alloys such as chromium iron alloy (6 wt.% N), and the melting furnace was pressurized. A vacuum induction melting furnace (VIM) was used. The initial vacuum was 6 × 10 ⁻⁵ torr, nitrogen (N ₂ ) gas was injected to atmospheric pressure to form a clean dissolution atmosphere, and then the secondary vacuum was 6 × 10. ^{It was set to -5} torr and dissolved by injecting nitrogen gas at a constant pressure.

In order to suppress the nitrogen gas evaporated upon melting, the nitrogen partial pressure was changed to 0 atm, 1 atm, 1.5 atm, and 2 atm, and high nitrogen steel containing various concentrations of nitrogen was produced. The change in solubility of nitrogen was investigated.

For the alloy thus prepared, component analysis was performed using ICP, and the analysis of nitrogen was performed using a gas analyzer. Table 1 below shows the results of the analysis.

Fe Mn Cr Ni N C Si P _N2 (atm) A Bal. 26.1 12.5 0.99 - 0.05 0.98 0 B Bal. 26.9 12.3 1.03 0.51 0.03 0.79 One C Bal. 27.8 12.1 1.06 0.58 0.03 0.79 1.5 D Bal. 25.0 12.4 1.05 0.67 0.03 0.92 2

According to the table, in the case of high nitrogen steel that can be manufactured by domestic special steel makers, the concentration of nitrogen is only about 2,300 ppm (0.23 wt.%), Whereas nitrogen in the alloy prepared according to the embodiment of the present invention is used. It can be seen that the concentration gradually increases as the partial pressure of nitrogen increases, and when the nitrogen partial pressure is 2 atm, it exhibits a very high nitrogen content of 6,700 ppm.

The alloy of this embodiment exhibited a higher concentration of nitrogen than that of the conventional alloy, in which a solid solution of nitrogen was used in addition to pressurizing nitrogen above atmospheric pressure and using chromium alloy ferroalloy as a raw material. It is judged that the height is because Mn is contained in a large amount.

Increasing the nitrogen concentration by increasing the partial pressure of nitrogen changes the lattice parameter. FIG. 1 shows the relationship with the lattice constant for the concentration of nitrogen in the alloy. As the nitrogen concentration increases, the lattice constant also increases. It can be seen that the increase is because nitrogen is invaded into the alloy as a solid solution.

On the other hand, by employing nitrogen in the alloy can be stabilized austenite structure, and further it is possible to achieve a non-magnetic and high-strength at the same time, the most important point in the present invention is that the fine structure of the alloy produced 100% To have austenitic structure, the optical microscope observation was performed to investigate the solidified structure in the casting state, using the X-ray diffractometer (XRD) for each of the A, B, C, D in Table 1 Phase analysis was performed to determine whether the austenite structure required by the present invention was obtained.

Figure 2 shows the results of X-ray diffraction analysis of the specimen in the cast state, an alloy containing a high concentration of nitrogen in addition to the austenite stabilizing elements Mn, Ni, that is, single phase austenite structure in alloys B, C, D Only appeared, and it can be seen that in addition to the austenite structure, other phases exist only in the alloy A containing no nitrogen. The black circle in alloy A was identified as (chromium, iron) carbide ((Cr, Fe) ₇ C ₃ ).

Therefore, it can be seen that the melt production process developed in the present invention is a very useful process for producing high nitrogen austenitic stainless steel of thousands of ppm or more.

Table 2 below shows all the alloying components and their composition and the amount of nitrogen analyzed using the manufacturing method according to the present invention. The ppm unit for nitrogen and the wt% for other compositions were used. As a result of designing an alloy system of a certain composition through the simulation of alloy design and dissolving it at high frequency, the concentration of nitrogen changed according to the nitrogen partial pressure, and the change of nitrogen was linearly proportional to the square root of the nitrogen partial pressure. Could know.

Fe Mn Cr Ni N C Si P _N2 One Bal. 25.46 12.17 1.05 3445 0.671 0.50 One 2 Bal. 25.64 11.63 0.98 4824 0.765 1.06 One 3 Bal. 23.82 11.98 1.12 - 0.701 0.45 One 4 Bal. 23.86 11.93 1.13 - 0.804 0.35 One 5 Bal. 24.42 16.07 1.11 5978 0.889 0.50 One 6 Bal. 23.59 13.92 1.26 5241 0.281 1.16 One 7 Bal. 24.74 14.02 1.17 5305 0.278 0.59 One 8 Bal. 21.56 16.44 1.08 5292 0.746 0.94 1.5 9 Bal. 18.91 15.26 1.14 3887 0.804 1.31 1.5 10 Bal. 25.75 15.52 1.31 7143 0.684 0.60 1.5 11 Bal. 24.37 16.64 1.09 8122 0.706 0.51 1.5

Fe Mn Cr Ni N C Si P _N2 12 Bal. 26.09 15.33 1.09 8011 0.552 0.53 1.5 13 Bal. 25.20 14.13 1.05 8930 0.565 0.39 1.5 14 Bal. 26.65 8.64 1.06 0 0.504 0.43 0 15 Bal. 26.28 15.52 1.08 7236 0.694 0.49 2 16 Bal. 25.25 15.86 1.12 6401 0.525 0.58 One 17 Bal. 22.10 12.38 1.10 - 0.017 0.67 One 18 Bal. 21.01 13.01 1.10 - 0.021 0.66 One 19 Bal. 21.38 12.75 1.06 - 0.022 0.67 One 20 Bal. 19.93 12.35 1.10 - 0.016 0.94 One 21 Bal. 24.10 12.03 0.98 0 0.013 1.13 0 22 Bal. 24.35 10.81 0.98 0 0.008 1.14 0 23 Bal. 25.83 11.86 1.02 7783 0.029 0.77 One 24 Bal. 26.71 10.35 1.08 6265 0.026 0.81 One 25 Bal. 25.68 11.35 1.01 6490 0.023 0.83 1.5 26 Bal. 25.18 12.04 1.01 6928 0.026 0.91 1.5 27 Bal. 23.27 11.16 1.06 4689 0.029 1.17 One 28 Bal. 21.97 11.91 1.05 4219 0.029 1.58 One 29 Bal. 24.36 11.68 1.05 4969 0.029 1.15 1.5 30 Bal. 23.99 11.89 1.06 4589 0.036 1.65 1.5 31 Bal. 23.27 11.92 1.06 5273 0.039 1.30 2 32 Bal. 22.64 11.82 1.05 5514 0.035 1.46 2 33 Bal. 19.67 12.54 1.01 22 0.017 1.26 0 34 Bal. 21.16 12.34 1.04 5287 0.021 0.85 One 35 Bal. 18.68 12.14 1.09 4739 0.036 1.52 1.5 36 Bal. 27.86 12.14 1.06 5789 0.033 0.791 1.5 37 Bal. 24.98 12.39 1.05 6730 0.031 0.918 2 38 Bal. 27.30 12.05 1.06 6927 0.032 0.723 2 39 Bal. 26.14 12.51 0.985 42 0.052 0.978 0 40 Bal. 28.90 10.62 0.997 1105 0.052 0.427 0 41 Bal. 26.70 12.27 1.09 4368 0.027 1.13 One 42 Bal. 26.05 12.44 1.04 6284 0.032 0.664 1.5 43 Bal. 26.92 12.31 1.03 5121 0.029 0.794 One

The method for producing austenitic stainless steel containing a large amount of nitrogen according to the present invention configured as described above does not require an additional capital investment by using a pressurized induction melting furnace, which is a basic facility of a special steel company, Since the concentration is several times higher than that of conventional materials and has a stable structure, it has high strength material properties, high toughness, high corrosion resistance and non-magnetic properties, and is applicable to structural materials, various kinds of parts, and steel core materials. Of course, instead of adding a large amount of Mn in order to increase the nitrogen concentration, expensive Ni is a very useful invention that can produce more economical steel by suppressing as much as possible.

Claims (8)

A first step of charging molten alloy iron as a basic raw material in a melting furnace to dissolve it; A second step of injecting and dissolving additional elements for increasing nitrogen solubility in the molten alloy; A method for producing austenite stainless steel containing a large amount of nitrogen comprising a third step of increasing the atmospheric pressure in the melting furnace to suppress the evaporation of nitrogen gas from the molten metal. The method of claim 1, The method for producing austenitic stainless steel containing a large amount of nitrogen, characterized in that the melting of the iron nitride and the additional element is carried out by gradual heating. The method of claim 1, The melting furnace is a method for producing austenitic stainless steel containing a large amount of nitrogen, characterized in that the pressure induction melting furnace. The method of claim 1, The method for producing austenitic stainless steel containing a large amount of nitrogen, characterized in that the iron nitride is selected from the group consisting of chromium nitride iron, manganese nitride iron, molybdenum nitride alloy, chromium molybdenum alloy iron and silicon nitride alloy iron . The method of claim 1, The addition element is a method for producing austenitic stainless steel containing a large amount of nitrogen, characterized in that manganese, chromium, nickel and carbon. The method of claim 5, The composition range of each additional element is manganese 5 (wt.%) Or more and 30 (wt.%) Or less, chromium 5 (wt.%) Or more and 25 (wt.%) Or less, nickel 1 (wt.%) Or more 12 (wt.%) or less, carbon (0.01 (wt.%)) or more, 1.0 (wt.%) or less characterized in that a large amount of nitrogen containing austenitic stainless steel production method. The method of claim 1, The atmospheric pressure is a method for producing austenitic stainless steel containing a large amount of nitrogen, characterized in that the pressurized by nitrogen gas. The method of claim 7, wherein The atmospheric pressure is a method for producing austenitic stainless steel containing a large amount of nitrogen, characterized in that more than 1 atm.