KR100325708B1 - Cr-RICH FERRITIC STAINLESS STEEL WITH EXCELLENT CORROSION RESISTANCE AGAINST SEA WATER - Google Patents

Abstract

PURPOSE: Provided is a Cr-rich ferritic stainless steel with excellent corrosion resistance against sea water with the addition of Cr and Al to conventional ferritic stainless steel. CONSTITUTION: The Cr-rich ferritic stainless steel with excellent corrosion resistance against sea water comprises 0.01 wt.% or less of C, 0.02 wt.% or less of N, 0.5 wt.% or less of Si, 0.5 wt.% or less of Mn, 0.03 wt.% or less of P, 0.004 wt.% or less of S, 0.01 wt.% or less of O, 0.18 wt.% or less of Ti, 0.18 wt.% or less of Nb, 1.0 wt.% or less of Cu, Al 0.03 to 0.13 wt.%, 4 wt.% or less of Mo, Cr 21 to 35 wt.%, a balance of Fe and incidental impurities, wherein (Ti+Nb) wt.% is less than 9x(C+N) wt.% and C+N wt.% is less than 0.03 wt.%.

Description

High chromium ferritic stainless steel with excellent seawater corrosion resistance.

The present invention relates to a ferritic stainless steel used as a material for seawater heat exchange engines and building exterior walls, and more specifically, by adding Al and Cu to the existing ferritic steel to improve air durability and seawater corrosion resistance (seawater resistance). An increased high chromium ferritic stainless steel.

Usually, ferritic stainless steels with Cr and Mo contents of 26% and 3% or more, respectively, have seawater resistance and are therefore used as seawater heat exchangers or exterior wall materials in coastal areas. However, due to its weak toughness, its use is limited, and many studies have been conducted to develop steel sheets having high corrosion resistance and toughness. In general, to improve toughness, the Cr, Mo, C, N content should be reduced. However, the reduction of Cr and Mo content lowers the corrosion resistance, and the reduction of the content of C and N is not only difficult in process but also requires a large cost. Therefore, there is a need for a method of increasing corrosion resistance and improving toughness by adding microalloys at a constant Cr, Mo, C, and N concentration.

In order to increase the toughness of ferritic stainless steel, Alleghey Ludlum Co., Ltd. added 29% of Ni to improve the toughness without reducing corrosion resistance by adding about 2% of Ni (29% Cr-4% Mo-). 2% Ni) steel grade is produced and marketed. In addition, according to Yoshihiro et al., It is reported that the addition of 0.06 to 0.2% P improves the corrosion resistance (EP 0 603 402 Al). In addition, according to Korean Application No. 1996-61973, the toughness is improved by the complex addition of Nb and AL. Is being reported.

Accordingly, an object of the present invention is to provide a ferritic stainless steel which is improved in corrosion resistance and toughness by adding microalloy elements without further refining facilities and reducing Cr and Mo contents compared to the above ferritic steel.

1 is a graph showing a change in corrosion resistance according to the alloy component,

2 is a graph showing the change in toughness according to the alloy component.

In order to achieve the above-described object, the present invention provides a weight percentage of a high chromium ferritic stainless steel in terms of weight%, C: 0.01% or less, N: 0.02% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.03% S: 0.004% or less, O: 0.01% or less, Ti: 0.18% or less, Nb: 0.18% or less, Cu: 1.0% or less, Al: 0.03 to 0.13%, Mo: 4% or less, Cr: 21 to 35 %, The balance consists of Fe and inevitable impurities,

It is characterized by providing a high chromium ferritic stainless steel having excellent seawater corrosion resistance satisfying the conditions of% (Ti + Nb) <9 × (C + N) and C + N <0.03%.

Hereinafter, the limited reason for the composition of the inventive steel will be described in detail.

C and N: The maximum of each of these element contents is limited to 0.01% and 0.02% to minimize the decrease in toughness. However, the less these contents, the more the properties of the material, e.g. elongation, impact toughness, corrosion resistance, etc. improves, so the minimum is not limited.

Si: It is an element which increases deoxidation and oxidation resistance. The Si content is limited to within 0.5% to suppress the reduction of toughness.

Mn: Element that increases deoxidation. However, the inclusion MnS reduces the corrosion resistance, limiting the Mn content to within 0.5%.

P: Limits the P content to 0.03% or less as it reduces toughness as well as corrosion resistance.

S: Since MnS is formed to reduce corrosion resistance, the S content is limited to 0.004% or less.

O: Increase the inclusion content to reduce toughness and corrosion resistance. Therefore, it is good to suppress the O content as much as possible and limit it to 0.01% or less.

Cu: Not only increases corrosion resistance in a reducing atmosphere but also increases pitting resistance. However, excessive addition reduces stress corrosion resistance and hot workability, thus limiting the addition amount to 1.0% or less.

Al: Element added for deoxidation. The addition of Al increases the corrosion resistance. However, if the oxygen content is less than 0.003%, attention is required because it increases the amount of oxidative inclusions in the steel. When the Al content is less than 0.03%, no improvement in corrosion resistance is observed. When the Al content is more than 0.13%, only the content of inclusions is increased without improving the corrosion resistance. Therefore, Al should be controlled in the range of 0.03% to 0.13%.

Nb, Ti: Since it brings about the deterioration of element or toughness added to prevent sensitization, it is required to minimize the addition amount in consideration of corrosion resistance. That is, it is preferable to limit Nb to 0.18% or less and Ti to 0.18% or less. In the case of Cr ferritic steel, the content of (Ti + Nb) is preferably not more than 9 times the content of (C + N), and more preferably about 5 times the content of (C + N). In particular, in the case of Ti, the addition of harmfulness is required to increase the weld bendability and the corrosion resistance, but it is preferable to add a complex with Nb because the over addition causes surface defects.

Cr, Mo: To increase the corrosion resistance but decrease the toughness, it should be added in the appropriate range of Cr and Mo. If the Cr content is too low, it does not affect the improvement of corrosion resistance, so it should be more than 21%, but the Cr content is 35% If exceeds, the strength is too high and the elongation is low, resulting in poor moldability, and when cooled, the toughness decreases rapidly due to sigma precipitation. In addition, Mo increases the corrosion resistance, but if the content exceeds 4%, the elongation is lowered, resulting in poor moldability, and when cooled, the toughness decreases rapidly due to sigma precipitation, so it is limited to 4% or less.

Hereinafter, the present invention will be described in detail through examples.

(Example)

The stainless steel having the chemical composition as shown in Table 1 was dissolved in a vacuum induction furnace to prepare 30Kg ingots, and these ingots were subjected to homogenization by heat treatment at 1200 ° C. for 2 hours in an Ar atmosphere. After the heat treatment, hot-rolled and water cooled to a thickness of 6mm. These plates were annealed in a heat treatment furnace at 1000 ° C. for 1 minute and then cold rolled to 3 mm thick. Finally, the specimens were made by cold annealing at 950 ° C. for 30 seconds, and each specimen was ground to SIC # 200 before the experiment. In addition, experiments were also conducted on commercially available SUS304 and SUS316 to compare the corrosion resistance. Nos. 1 to 11, 13, 19, and 20 in Tables 1 to 4 represent comparative materials as test materials, Nos. 16 to 18 represent invention materials as test materials, and Nos. 21 and 22 are commercial materials. Indicates. In addition, each component content of Table 1 represents weight%.

In Table 2, two experiments were used to evaluate the grain boundary corrosion resistance according to the addition of stabilizing elements. The former is an electropotentiokinetic reactivation (EPR) experiment (Corrosion vol 40 1984, 584-593) and the latter is a modified strauss test (ASTMA262-91A-F). 2M H 2 SO ₄ + 0.5M NaCl + 0.05M KSCN solution was used for the former experiment. The former is an experiment to quantify grain boundary corrosion resistance, which means that the lower the EPR value, the better the resistance. On the other hand, the latter is a commonly used test method used to check the occurrence of intergranular corrosion. All specimens used in the intergranular corrosion resistance test were subjected to sensitization heat treatment at 620 ° C. for 10 min before the intergranular corrosion evaluation. In the case of the test material, the EPR value decreased as the stabilization ratio was increased. According to the Strauss test results, no sensitization occurred even when the stabilization ratio ((Ti + Nb) / (C + N)) was about 9 (Comparative Material 1). A stabilization ratio of 9 would be sufficient to prevent sensitization. do. In addition, even when stabilizing elements (Ti, Nb) were not added (Comparative Materials 19 and 20), the EPR value was lower than that of the commercially available material, and the value was similar to that of the steel having a stabilization ratio of 9, which is excellent in resistance to grain boundary corrosion. It was. In general, even if the stabilization element is over-added to the minimum amount to prevent intergranular corrosion, there is no significant change in the intergranular corrosion resistance, but in the case of the steel of the present invention, it is required to add as few stabilizing elements as possible because toughness is lowered. Therefore, the stabilization ratio was set to 9 or less in consideration of toughness and intergranular corrosion.

Table 3 was performed in 30, 32.5 ℃ FeCl ₃ solution (ASTMG48A) artificially made a gap in order to measure the crack corrosion resistance according to the change of alloying elements. The gap corrosion resistance increased with decreasing inclusion content. In particular, the crack corrosion resistance of Al alone was increased compared to the comparative materials (1,20), but the degree of Al was high (10,11), and relatively poor (8,9). It was also inadequate at the time of addition of Cu alone (13). However, the inventive inventive material 16, in which Al and Cu were added in combination, exhibited excellent corrosion resistance.

Figure 1 was measured the current in the formula potential and passivation film through the polarization test in 40 ℃ 2.5M NaC1 solution in order to measure the corrosion resistance in brine atmosphere. The formula potential of the composite material added with Cu and Al (Heat No. 16, 18) was the highest and the current was smallest in the passivation film. That is, the seawater corrosion resistance was the best when the two elements were combined.

Figure 2 shows the change in toughness according to the total content (C + N + O + Nb + Ti) of stabilizing elements and impurities. The lower the soft brittle transition temperature (DBTT) means the higher the toughness. Toughness was measured for all of the sensitized heat treated specimens. When the content of these concentrations is 0.15% or more, since DBTT has reached room temperature, it is required to reduce the sum of these total concentrations to 0.15% or less.

Table 4 shows the comparative materials (1,3,6,6,10,13,20), the inventors (16) and the commercials (21,22) from the shoreline to measure the atmospheric corrosion resistance as experimental materials in a coastal atmosphere. An experiment was conducted for three months at a beach 30 meters away from Pohang and Wolpo. The degree of corrosion was determined by measuring the degree of decrease in surface whiteness. The relative amount of change after converting the whiteness of the material before exposure to the relative value of 100 was measured. That is, when the whiteness is 100%, it means that no corrosion occurs and that as the whiteness decreases, the corrosion accelerates. Corrosion resistance improved with decreasing inclusion content and increasing stabilizing element content. As shown in Table 3 and FIG. 1, the fact that Al addition improves the gap corrosion resistance and corrosion resistance shows that the comparative material 10, which is an Al-addition steel as shown in Table 4, improves the air durability (air corrosion resistance). It explains well. However, the comparative material 13, which is a single additive steel of Cu, further improved the atmospheric durability unlike the results of reducing the gap corrosion resistance and the corrosion resistance (Table 3, Fig. 1). The different results above are due to the following causes. In other words, since Cu does not participate in the formation of the passivation film, resistance in gap corrosion and passivation regions is lowered, but Cu is considered to have strong resistance to atmospheric corrosiveness because Cu exhibits strong corrosion resistance in the activating region and sulfur dioxide atmosphere. Therefore, the inventive material 16 having a complex addition of Cu and Al exhibited the best atmospheric durability because the influence of each of these additional elements was compounded.

In the present invention, the additional refinery and the addition of fine alloying elements to the existing ferritic steel without reducing the Cr and Mo content to improve the corrosion resistance and toughness, and at the same time to add Al and Cu composite effect to increase the air durability and sea water resistance have.

Claims (1)

In ferritic stainless steel, the chemical composition is in weight%, C: 0.01% or less, N: 0.02% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.03% or less, S: 0.004% or less, O: 0.01% or less, Ti: 0.18% or less, Nb: 0.18% or less, Cu: 1.0% or less, Al: 0.03 to 0.13%, Mo: 4% or less, Cr: 21 to 35%, and the balance is Fe And inevitable impurities, A high chromium ferritic stainless steel having excellent seawater corrosion resistance, characterized by satisfying the conditions of% (Ti + Nb) <9 × (C + N) and C + N <0.03%.

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