KR100445246B1 - High Pitting Resistant and High Ni bearing duplex stainless steel - Google Patents

Abstract

In the present invention, Cr: 19.0 to 35.0, Ni: 10.1 to 15.0%, Mo: 0.1 to 3.0%, Mn: 3.5% or less, Si: 3.5% or less, C: 0.5% or less, N: 0.12 to 0.50% And Cu: 3.5% or less, W: 0.1% to 5.0%, Co: 0.1% to 6.0%, and provide a high-resistance two-phase stainless steel of high nickel and low molybdenum, wherein the remainder is composed of inevitable impurities and iron.

Description

High Pitting Resistant and High Ni bearing duplex stainless steel}

The present invention relates to high nickel duplex stainless steel composed of ferritic and austenitic two-phase structures with excellent resistance to formulation.

The most influential elements in the formal resistance of stainless steels are chromium, nickel, molybdenum, tungsten, nitrogen, etc. The quantitative formulating of the effect of these elements on corrosion resistance is the official resistance index. In other words, the formula for the representative Pitting Resistance Equivalent (PRE) is equal to% Cr + 3.3 (% Mo + 0.5% W) + 30% N. In particular, two-phase stainless steel has a very low corrosion resistance by controlling the nickel content of the austenitic stainless steel very low and adding molybdenum relatively high so that the microstructure is composed of two phases of austenite and ferrite. To obtain stainless steel.

According to Korean Patent Registration No. 10-0215727 obtained by the inventor, the main alloy composition of the austenitic-ferrite two-phase structure is molybdenum 3-5% or less, tungsten 3-6.5%, chromium 19-30%, nickel 5-8.5%, nitrogen 0.2-0.45%. As described above, the conventional corrosion resistant two-phase stainless steel is added with molybdenum or tungsten in a large amount as shown in the formula resistance index formula above to maintain high corrosion resistance.

Meanwhile, according to Korean Patent Registration No. 0143481, the main alloy composition of austenitic + ferritic two-phase stainless steel having excellent corrosion resistance, high temperature oxidation resistance, hot workability and impact toughness is 5-8% nickel, 22-27% chromium, and mol It consists of 1-2% of ribdenium, 3-5% of tungsten and 0.13-0.27% of nitrogen. In addition, according to the Republic of Korea Patent Publication No. 2000-0043778, the main alloy composition of two-phase stainless steel capable of high heat input welding is 5-8% nickel, 22-24% chromium, 1.6-2% molybdenum, 2-2.7% tungsten , 0.13-0.27% nitrogen. In addition, according to Korean Patent Publication No. 1999-0072736, the main alloy composition of two-phase stainless steel resistant to corrosive soda is 3-10% nickel, 28-35% chromium, 1-4% molybdenum, 2% or less tungsten, It consists of 0.2-0.6% nitrogen. In addition, the main alloy composition of the two-phase stainless steel of the US patent US4391635 is 3-6% nickel, 22-30% chromium, 0.5-1.5% molybdenum, 0.1% or less nitrogen. In addition, the main alloy composition of the two-phase stainless steel of the US patent US5284530 is 3-9% nickel, 26-30% chromium, 3-4.5% molybdenum, 0.25-0.35% nitrogen. In addition, the main alloy composition of the two-phase stainless steel of the US patent US6048413 is 3-9% nickel, 20-30% chromium, 3-8% molybdenum, 0.2-0.5% nitrogen. In addition, the main alloy composition of the two-phase stainless steel of the US patent US5733387 is 5-8% nickel, 22-27% chromium, 1-2% molybdenum, 2-5% tungsten, 0.13-0.3% nitrogen. As such, the conventional two-phase stainless steel is made of ferrite + austenite duplex stainless steel by adding low nickel to obtain a two-phase structure.

The present invention is a stainless steel consisting of two phases of ferrite phase and austenite phase, which is not shown in the prior arts as described above, and is a two-phase stainless steel having excellent formal resistance to a corrosive environment containing a large amount of chloride ions. The purpose is to provide.

Figure 1. Official resistance index and critical formula temperature for alloys 2001, 2301, 2701, 2901, 3301

Figure 2. Official resistance index and critical formula temperature of the comparative alloy

3. Proportional relationship between the formula resistance of the alloy of Figure 1 and the nickel content of the alloy component

Fig. 4. Critical formula temperature and formula resistance index of alloys 2002, 2302, 2702, 2902, 3302

5. Proportional relationship between the formula resistance and nickel of the alloy of FIG.

In order to achieve the above object, the present invention provides a weight% of Cr: 19.0 to 35.0, Ni: 10.1 to 15.0%, Mo: 0.1 to 3.0%, Mn: 3.5% or less, Si: 3.5% or less, and C: 0.5% In the following, N: 0.12 to 0.50%, Cu: 3.5% or less, W: 0.1 to 5.0%, Co: 0.1 to 6.0%, and the pneumatic resistance of high nickel and low molybdenum is characterized by consisting of inevitable impurities and iron. Provides two-phase stainless steel.

Hereinafter, the reason for numerical limitation of this invention is demonstrated.

(1) Carbon 0.50% or less:

As a representative element to stabilize the austenite phase, it is very important for maintaining mechanical strength. In order to prevent the deterioration of mechanical properties due to the addition of high nickel, it is preferable to increase the carbon content. However, if a large amount is added, carbides are precipitated, which may weaken the corrosion resistance, so it is limited to 0.50% or less.

(2) Nickel 10.1 ~ 15.0%:

It is a strong austenite stabilizing element that is useful in terms of corrosion resistance and contains at least 10.1% or more to adjust the ferrite / austenite fraction in two-phase stainless steel in order to evenly distribute the alloying elements useful in the ferritic and austenite phases. There is. It should be added in consideration of chromium equivalent and nickel equivalent relationship, and limited to maximum 15.0% in consideration of relationship with ordinary ratio and cost reduction.

(3) Chromium 19.0-35.0%:

The most important and basic element for maintaining the corrosion resistance of stainless steel, but at least 12% is required for the minimum corrosion resistance, 19% in consideration of chromium equivalent and nickel equivalent because the austenitic-ferrite two-phase structure must be obtained in the alloy of the present invention It should contain at least chromium. However, when a large amount is added, an intermetallic compound is generated, which may significantly weaken the corrosion characteristics and mechanical properties, so it is limited to a maximum of 35.0%.

(4) Molybdenum 0.1-3.0%:

Together with chromium, it acts to stabilize the ferrite phase as an important element for maintaining corrosion resistance. In the annealing state, it is very useful in terms of mechanical properties and corrosion resistance, so it should be added at least 0.1%. Chromium equivalent and nickel equivalent as it is a representative element to generate intermetallic compounds adversely affected by aging heat treatment or hot rolling and welding. And it is limited to 3.0% or less in terms of corrosion resistance and phase stability.

(5) Tungsten 0.1-5.0%:

The effect on corrosion resistance as ferrite stabilizing element is about 50% smaller than that of molybdenum but has a similar tendency and should be added at least 0.1%. The amount should be adjusted according to the content of chromium and molybdenum, and it is limited to 5.0% or less in terms of phase stability, mechanical properties and corrosion resistance since it may promote the formation of harmful intermetallic compounds when a large amount is added. 3.5% or less:

As an element to stabilize the ferrite structure, it has a deoxidation effect during dissolution and refining, and an element that increases oxidation resistance, but when added more than 3.5%, the toughness and ductility of the steel decreases and the corrosion resistance also decreases rapidly, so it is limited to a maximum of 3.5% or less.

(7) Manganese 3.5% or less:

It is an element that stabilizes austenite structure and is an element that increases the high capacity of nitrogen. In order to improve the corrosion resistance by increasing the nitrogen content, an appropriate amount of manganese is an essential element. In addition, it forms a manganese sulfide by combining with sulfur in the steel to prevent the occurrence of hot brittleness and deoxidation effect during refining, but the corrosion resistance is reduced when added a large amount is limited to less than 3.5%.

(8) Nitrogen 0.12-0.5%:

It is a useful element that improves the resistance to formulas, and its effect is about 30 times more than chromium. As a strong austenite stabilizing element, it is one of the most important elements in terms of corrosion resistance. When present simultaneously with molybdenum, the synergistic effect greatly improves the corrosion resistance. Compensation of mechanical properties can be obtained by adding nitrogen when carbon is lowered for the purpose of improving the intergranular corrosion resistance. It should be added in consideration of balance with carbon, chromium, nickel, molybdenum, tungsten, and austenite-ferrite phase ratio. More than 0.12% is preferable in terms of corrosion resistance. If possible, limit the maximum to 0.5% or less.

(9) Copper 3.5% or less:

It has the effect of greatly improving acid resistance, and enhances the strength by strengthening the matrix structure as an austenite stabilizing element. If the ratio is not appropriate, the ratio of element to chromium and molybdenum may be reduced, so the resistance to the formula may be weakened.

(10) Cobalt 0.1-6.0%:

Cobalt is an austenite stabilizing element and should be added at least 0.1% since it is an element that improves mechanical properties and corrosion resistance.

Hereinafter, an Example is given and this invention is demonstrated in detail.

First, the alloys used in the embodiments of the present invention are electrolytic iron, chromium, molybdenum, nickel, tungsten, copper, Fe-Si, Fe-Mn, cobalt, Fe-Cr-N, etc., which have pure commercial grades. And dissolved in a magnesia crucible under an atmospheric atmosphere using a high frequency induction furnace.

After injecting molten metal into a fully preheated quadrilateral mold or sand mold, an ingot of 30 kg was manufactured, and the hot rolled ingot was subjected to grinding or machining to process the ingot to an appropriate size, followed by annealing heat treatment under an appropriate annealing heat treatment condition. And evaluated.

The following test was done with the specimen produced as mentioned above.

* PREW =% Cr + 3.3 (% Mo + 0.5% W) + 30% N

* Creq = Cr + 2Si + 1.5Mo + 0.75W + 5V + 5.5Al + 1.75Nb + 1.5Ti

* Nieq = Ni + 0.5Mn + 30C + 0.3Cu + 25N + Co

Table 1 shows the chemical composition of the alloy and the comparative alloy of the present invention, Figure 1 shows the alloys 2001, 2301, 2701, 2901, 3301 annealing heat treatment for 30 minutes at 1150 ^o C after the water-cooled specimens ASTM G It shows the critical formula temperature obtained in 6% FeCl ₃ + 1% HCl aqueous solution by 48-00 C method. The official test vessel used a 1 liter glass reactor and a condenser was installed on top to prevent evaporation of the solution. The specimens were tested at an angle of 45 ^o using wires insulated with rubber tubing and replaced with fresh solution at each test. The surface of the specimen was wet ground with SiC abrasive up to # 600 and then stored in a desiccator until testing.

The alloy resistance index (PREW) of alloy 2001 was 42.6 and the critical formula temperature was estimated to be 70 ^° C. The PREW of alloy 2301 was 44.4 and the critical formula temperature was 65 ^o C. The PREW of alloy 2701 is 47.1 and the critical formula temperature is 80 ^o C. The PREW of alloy 2901 was 46.2 and the critical formula temperature was 85 ^o C. Alloy 3301 had a PREW of 45.8 and a critical formula temperature of 90 ^o C.

In general, the relationship between the formal resistance of stainless steel and the index of resistance is known to be linearly proportional (MB Rockel, W. Herda, and U. Brill, Proceedings of International Conference on Stainless Steels, p. 78 (1991)). ). However, as shown in FIG. 1, the present invention alloy may have a low proportionality between the official resistance index and the official resistance, that is, the critical formula temperature.

2 is a diagram showing the formula resistance index and the critical formula temperature of the comparative alloy. The official resistance index of the comparative alloy 1000 was 51.1, and the critical formula temperature was estimated at 75 ^° C. The official resistance index of the comparative alloy 1001 was 42.7, and the critical formula temperature was evaluated at 60 ^° C. The official resistance index of the comparative alloy 1002 was 51, and the critical formula temperature was estimated at 75 ^° C. The official resistance index of comparative alloy 1003 was 48.4, and the critical formula temperature was estimated to be 70 ^° C. Comparing FIG. 1 and FIG. 2, it can be seen that the critical formula temperature, that is, the formula resistance of the alloy of the present invention was better evaluated even when the formula resistance index of the alloy of the present invention was smaller than that of the comparison alloy.

The cause was shown to be related to nickel in the alloying components as shown in FIG. That is, Figure 3 is a diagram showing the critical formula temperature, that is, the official resistance of the alloy according to the nickel content of the alloy composition. As can be seen from the figure, as the nickel content of the alloying components increases, the formal resistance is improved. The official resistivity shown in FIG. 1 does not show a proportional relationship with the official resistance index of the alloy, but in FIG. 3, it maintains a proportional relationship. It is a two-phase stainless steel in which the alloy of the present invention has a two-phase structure of (austenite + ferrite), and the ratio of these two phases can be controlled by the chromium equivalent and the nickel equivalent. Up to now, the approach to improve the corrosion resistance has been to improve the official resistance by increasing the chromium equivalent of chromium, molybdenum, tungsten, etc. to increase the official resistance index, or to increase the official resistance of nickel equivalent nitrogen by increasing the official resistance Increasing the index has improved formula resistance. In particular, since nitrogen is an austenite stabilizing element which plays an important role in controlling the ferrite content in two-phase stainless steel, and an element capable of increasing the formal resistance by 30 times or more than chromium, many inventions have been concentrated on nitrogen. Nickel, on the other hand, is an element that stabilizes austenite structure, such as nitrogen, but has little effect on formal resistance. Therefore, many inventions have been focused on the advantages of reducing the nickel content in austenitic stainless steel to introduce a ferrite structure to create a two-phase structure and reduce the expensive nickel content. However, as shown in Figure 3 (austenite + ferrite) when the amount of nickel is added in the alloy composition to obtain a two-phase structure than the nickel content of the conventional two-phase stainless steel, it is found that the formal resistance is greatly improved. Can be.

<Example 1>

The alloy having the chemical composition of Table 1 was cut to a size of 1 × 2 in and subjected to annealing heat treatment. Annealing heat treatment conditions were maintained for 30 minutes at a temperature of 1,150 ^° C and cooled to water to prepare a specimen.

The official test vessel used a 1 liter glass reactor and a condenser was installed on top to prevent evaporation of the solution. The specimens were tested at an angle of 45 ^o using wires insulated with rubber tubing and replaced with fresh solution at each test. The surface of the specimen was wet ground with SiC abrasive up to # 600 and then stored in a desiccator until testing.

Critical formula temperature was determined in 6% FeCl ₃ + 1% HCl solution according to ASTM G48-00 C method. No weight loss was found at 50 ^° C. for both the comparative alloy and the inventive alloy at all annealing conditions. Thus, the critical formula temperature of each condition was obtained. 4 is a graph showing the formula resistance index and critical formula temperature for alloys 2002, 2302, 2702, 2902, and 3302 to which cobalt or copper is added in small amounts. As can be seen from the figure, the formal resistivity shows a low linear proportional relationship with the formula resistance index of the alloy, and it can be seen that it is much better than the formula resistivity of the comparative alloy of FIG. In order to have excellent formal resistance in stainless steel, the formula resistance index (PREW), which is basically composed of alloying elements, must be high. However, two-phase stainless steel is composed of two phases of austenite and ferritic phase. Moreover, austenite phase contains mostly nitrogen to increase corrosion resistance, whereas ferrite phase is concentrated with chromium, tungsten and molybdenum. . Nitrogen is hardly dissolved in the ferritic phase (up to 0.05% solid solution), and the remaining content is concentrated in the austenite phase, which shows very high segregation of alloying elements between phases. However, nickel is an element that stabilizes an austenite phase like nitrogen, but is different from nitrogen in that it is distributed with a large amount of solid solution in the ferrite phase.

In addition, as shown in Figure 5, the official resistance of the alloys 2002, 2302, 2702, 2902, 3302 as a result of showing the nickel content, showing a similar tendency to Figure 3 to support the characteristics of high nickel two-phase stainless steel.

According to the present invention as described above, despite the higher nickel content and lower molybdenum content than the conventional two-phase stainless steel, the ferrite phase and the austenite phase are properly balanced, and the alloy components are uniform in the austenitic phase and the ferrite phase. This results in a two-phase stainless steel that maximizes resistance to the formulation.

Claims (2)

It is composed of Cr: 19.0 to 35.0, Mo: 0.1 to 3.0, Mn: 3.5 or less, Ni: 10.1 to 15.0, Si: 3.5 or less, C: 0.5 or less, N: 0.12 to 0.5, and the rest is inevitable High nickel two-phase stainless steel with excellent formula resistance, characterized in that made. The method of claim 1, High nickel two-phase stainless steel having excellent formula resistance, characterized in that one or two or more of W: 0.1 to 5.0, Cu: 3.5 or less, and Co: 0.1 to 6.0 are added to the composition.