JPH0121863B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an inexpensive austenitic stainless steel that exhibits excellent corrosion resistance in neutral salt environments such as coastal areas and places that come into contact with living water. Traditionally, devices and equipment used in neutral salt environments, such as various types of chemical equipment, heat exchange tubes, desalination plants, water heaters, and food processing equipment, have been made of SUS304, SUS316, etc. in consideration of corrosion resistance. Austenitic stainless steel is widely used, but
Still, many accidents have been reported due to pitting corrosion, crevice corrosion, stress corrosion cracking, etc. Furthermore, in the case of SUS316 steel, more than 2% Mo (hereinafter % is expressed as a weight percentage) is added. Therefore, there was also the disadvantage that the price was relatively high. On the other hand, in recent years, materials that exhibit relatively good stress corrosion cracking resistance even in neutral salt environments have been developed.
Attempts have also begun to use Mo-containing high-purity ferrite stainless steel, such as SUS444, but such Mo-containing high-purity ferrite stainless steel
The properties in terms of formability, weldability, and toughness were not good, and there were many problems that still needed to be solved before it could be applied to the above-mentioned equipment. From the above-mentioned viewpoint, the present inventors have conventionally
It compensates for the shortcomings of SUS304 and SUS316 steel, or Mo-containing high-purity ferrite stainless steel, which often cause problems in coastal areas and living water environments, and has excellent pitting corrosion resistance, crevice corrosion resistance, and stress corrosion cracking resistance in neutral salt environments. As a result of our research to provide a steel that exhibits sufficient effects on corrosion resistance and other corrosion resistance, and is also excellent in forming and assembly processing, we developed a steel based on the composition of 18-8 type austenitic stainless steel, and its Mo By reducing the amount added, Si,
In addition to appropriately controlling the amounts of Cu and N added, if the B content that unavoidably mixes into the steel is limited to a predetermined value or less, the synergistic effect is to improve corrosion resistance in a neutral salt environment. They discovered that it is possible to create an austenitic stainless steel that not only has improved oxidation resistance but also exhibits excellent high-temperature oxidation resistance and is less expensive. This invention was made based on the above findings, and austenitic stainless steel is made of austenitic stainless steel, C: 0.08% or less, Si: more than 2.0% and 4.0% or less, Mn: 2.00% or less, Cr: 16.00 to 20.00%, Ni. : 8.00 to 13.00%, Mo: 0.30 to 1.50%, Cu: 0.30 to 2.00%, N: 0.05 to 0.30%, and if necessary, further contains Nb: less than 0.10%, and Formula, Ni-bal. = Ni (%) + 30 [C (%) + N (%)] + 0.5 Mn (%) - 1.1 [Cr (%) + 1.5 Si (%) + Mo (%) + 0.5 Nb (%) )] Ni-bal. expressed as +8.2 is in the range of -2.5 to +1.0, Fe and unavoidable impurities: remaining, however, among the impurities, B: 0.0020% or less (20ppm or less) The composition is characterized in that it further improves corrosion resistance in a neutral salt environment and also provides excellent high-temperature oxidation resistance. Incidentally, the austenitic stainless steel of this invention can be produced by any method of production, and can also be produced by casting,
Regardless of the product form, such as forged products or rolled materials, it exhibits excellent corrosion resistance in a neutral salt environment, and
It exhibits excellent oxidation resistance even in environments exposed to high temperatures, with no change in its properties. Next, the reason why the composition of the stainless steel of the present invention is limited as described above will be explained. (a) Excessive addition of C causes carbides to precipitate in the heat-affected zone during welding, deteriorating corrosion resistance.
The content is preferably as low as possible. In particular, if the C content exceeds 0.08%, the corrosion resistance deteriorates significantly, so the content was set at 0.08% or less. (b) Si The Si component is a particularly important element in the steel of the present invention, and has the effect of increasing the pitting corrosion resistance, hydrochloric acid resistance, stress corrosion cracking resistance, and high temperature oxidation resistance of the steel as described later. When the amount exceeds 2.0%, the effect becomes noticeable, and when the amount is less than 2.0%, the desired effect cannot be obtained, and the desired pitting corrosion resistance, hydrochloric acid resistance, stress corrosion cracking resistance,
Furthermore, it is not possible to create steel that is resistant to high-temperature oxidation. On the other hand, if Si is contained in excess of 4.0%,
In addition to forming ferrite, it also deteriorates formability and hot workability, so the content was set at more than 2.0% and less than 4.0%. (c) Mn In addition to acting as a deoxidizing agent in the production of stainless steel, the Mn component also combines with S to form manganese sulfide and prevents hot embrittlement. If Mn exceeds 2.00%, the internal properties of the material will deteriorate, so the Mn content was set at 2.00% or less. (d) Cr The chromium component is an element that has strong resistance to the corrosion of stainless steel, and in order to ensure the corrosion resistance of austenitic stainless steel, the content must be at least 16.00% or more. However, if the content exceeds 20.00% in coexistence with the Ni component, the ferrite phase will increase and formability and hot workability will deteriorate, so the Cr content is set at 16.00 to 20.00%. Ta. (e) Ni The Ni component is a strong austenite stabilizing element, which is extremely important for ensuring a perfect austenite structure, and also works to improve stress corrosion cracking resistance, which is a drawback of austenitic stainless steel. However, if the content is less than 8.00%, the desired effect cannot be obtained in the above action, while if the content exceeds 13.00%, not only will hot workability be impaired, but Since this is not preferable from an economic point of view, the Ni content was set at 8.00 to 13.00%. (f) Mo The Mo component has the effect of improving the pitting corrosion resistance, acid resistance, and crevice corrosion resistance of steel, and if the content is 0.30% or more, it may coexist with other elements such as N.
Corrosion resistance equivalent to or higher than SUS316 can be ensured. However, increasing the Mo content will lead to disadvantages in terms of economic efficiency, and this tendency becomes especially noticeable when the content exceeds 1.50%.
It was set at 0.30-1.50%. (g) Cu The Cu component has the effect of further improving sulfuric acid resistance, crevice corrosion resistance, and stress corrosion cracking resistance, but if its content is less than 0.30%, the desired effects cannot be obtained. On the other hand, if the content exceeds 2.00%, the hot workability and weldability of the steel will deteriorate, so the content should be reduced from 0.30 to 2.00%.
%. Note that it is preferable to set the Cu content to 0.50 to 2.00% in order to obtain more reliable effects. (h) N The N component has the effect of further improving the pitting corrosion resistance and crevice corrosion resistance of steel, as well as ensuring its strength, but if the N content is less than 0.05%, the desired effect may not be achieved. can't get the other
If the content exceeds 0.30%, not only the stress corrosion cracking resistance will deteriorate, but also the hot workability and formability will deteriorate.
It was set at 0.05-0.30%. (i) Nb Since the Nb component has the effect of suppressing susceptibility to intergranular corrosion due to carbide precipitation, etc., it can be added when it is necessary to further improve intergranular corrosion resistance. amount is 0.10
If the Nb content exceeds 0.10%, hot workability, weldability, and cleanliness deteriorate, so the Nb content was set at less than 0.10%. (j) Ni−bal. Formula, Ni−bal.=Ni (%) + 30 [C (%) + N
(%)]+0.5Mn(%)-1.1[Cr(%)+1.5Si(%)
+
Ni− expressed as Mo (%) + 0.5Nb (%)] + 8.2
Even if bal. is less than -0.25 or more than +1.0, cracks will occur during hot forging or hot rolling of steel, so in order to stabilize hot workability, Ni−bal. was set at −0.25 to +1.0. (k) B B is an element that is mixed in as an impurity from raw materials, such as boron-containing stainless steel scrap, refractories, and melting furnace residue, and promotes grain boundary precipitation of carbides and deteriorates corrosion resistance. The lower the content, the better. When the B content exceeds 0.0020% (20ppm), the corrosion resistance tends to deteriorate significantly, especially
Since the effect of suppressing carbide grain boundary precipitation when Nb and N are added in combination is inhibited, B
The upper limit of content was set at 0.0020%. Next, the present invention will be specifically explained using examples and comparing with comparative examples. Examples First, steel samples (invention steels 1 to 19 and comparative steels 20 to 27) having the chemical compositions shown in Table 1 were melted in the atmosphere and manufactured by casting. Next, for each of these samples, the second
We conducted various corrosion tests as shown in the table, and

[Table] (Note) * indicates that the amount of the component is outside the range of the present invention.

[Table] Eating habits were investigated. The results thus obtained are shown in Table 2 and Figures 3 to 5. The results shown in Table 2 show that materials to which Cu is actively added have excellent sulfuric acid resistance and improved crevice corrosion resistance. The crevice corrosion potential for evaluating the crevice corrosion resistance of steel was measured using the following procedure. Figure 1 is a schematic diagram of the test equipment used to examine crevice corrosion resistance, and Figure 2 is a perspective view of its main parts. Sample 1 was obtained by polishing the surface of the sample (30 mm x 30 mm) with #600 abrasive paper, spot welding conductor wire 2 was applied, and the four end faces were coated with vinyl chloride paint 3. Next, this was inserted into the test tank 4, and a gap forming plate 5 made of silicone rubber was placed thereon. The gap forming plate 5 is provided with a support rod 7 to which a weight 6 is attached, so that a load of 700 kg can be applied thereto. Thereafter, a 0.5 mol NaCl aqueous solution 8 was injected into the test tank 4, the temperature was maintained at 40°C, a predetermined potential was applied to the sample 1, and the sample was left for 24 hours, after which the presence or absence of crevice corrosion was examined. The crevice corrosion potential is
The potential applied to sample 1 was varied at 25 mV intervals, and the highest potential at which crevice corrosion did not occur was set.
According to this method, the poorer the crevice corrosion resistance of the steel, the more likely it is that crevice corrosion will occur at a lower potential, making it possible to accurately evaluate the crevice corrosion resistance. In addition, in FIG. 1, what is indicated by the reference numeral 9 is a platinum counter electrode. The results shown in Table 2 also clearly show that pitting corrosion resistance and stress corrosion cracking resistance are improved by increasing the Si content. Figure 3 shows Si content and pitting potential in a 0.5M NaCl aqueous solution (40℃) (according to JISG0577)
Figure 4 is a diagram showing the relationship between Si content and stress, which was derived by investigating the presence or absence of cracking after 300 hours in a stress corrosion cracking test (according to JISG0576) in a boiling magnesium chloride aqueous solution. This is a plot diagram showing the relationship with corrosion cracking. From these figures 3 and 4, it can be seen that when the Si content exceeds 2%, pitting corrosion resistance and stress corrosion cracking resistance are significantly improved. it is obvious. Furthermore, the results shown in Table 2 also clearly show that the effect of improving pitting corrosion resistance by the addition of N is extremely significant. In addition, 20% NaCl + 1%
As a result of measuring the crack initiation time in an aqueous solution of Na ₂ Cr ₂ O ₇ 2H ₂ O, it was observed that cracks occurred in the steel of the present invention for a longer period of time than in the comparison steel. Even in the gas-liquid interface immersion test of spot welded specimens (1 ton x 10 x 30 plates spot welded to 1 ton x 15 x 40 plates), no stress corrosion cracking was observed in the steel of the present invention. In contrast, conventional
The occurrence of cracks was clearly confirmed in Comparative Steels 26 and 27, which are SUS304 and SUS316 steels. Moreover, Si
It is clear that materials with increased content exhibit excellent oxidation resistance even in repeated oxidation test results at 1000°C in the atmosphere. Figure 5 shows the repeated oxidation test at 1000℃ in the atmosphere, after 400 cycles (30 minutes heating - 10 minutes air cooling).
It is a diagram showing the relationship between Si content and oxidation loss.
This figure also shows that the high-temperature oxidation resistance is extremely good when the Si content exceeds 2%. Overall, all of the above-mentioned test results clearly demonstrate the excellent corrosion resistance of the steel of the present invention in a neutral salt aqueous solution. As mentioned above, according to the present invention, even when used in domestic water environments such as water supply and sewage systems, or seawater environments, it does not cause corrosion problems like conventional SUS304 steel or SUS316 steel, and is cost effective. It is possible to obtain low-cost, highly corrosion-resistant austenitic stainless steel, and it brings about industrially useful effects, such as making it possible to further extend the life of equipment used in the above-mentioned neutral salt environment. .

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram of the test equipment used to investigate crevice corrosion resistance, Figure 2 is a schematic perspective view of the main parts of the test equipment shown in Figure 1, and Figure 3 is the Si content and pitting corrosion of steel. A diagram showing the relationship with electric potential, Figure 4 shows the Si of steel.
FIG. 5 is a plot diagram showing the relationship between Si content and stress corrosion cracking resistance, and FIG. 5 is a diagram showing the relationship between Si content and high temperature oxidation loss of steel.

Claims (1)

[Claims] 1 C: 0.08% or less, Si: more than 2.0% and 4.0% or less, Mn: 2.00% or less, Cr: 16.00 to 20.00%, Ni: 8.00 to 13.00%, Mo: 0.30 to 1.50%, Contains Cu: 0.30 to 2.00%, N: 0.05 to 0.30%, and the formula: Ni-bal.=Ni (%) + 30 [C (%) + N (%)] + 0.5 Mn (%) - 1.1 [ Cr(%) +1.5Si(%)+Mo(%)]+8.2 Ni-bal. expressed as -2.5 to +1.0, Fe and unavoidable impurities: remaining, however, among these impurities , B: An austenitic stainless steel with excellent corrosion resistance in a neutral salt environment, characterized by having a component composition (weight %) of 0.0020 or less. 2 C: 0.08% or less, Si: over 2.0% and 4.0% or less, Mn: 2.00% or less, Cr: 16.00-20.00%, Ni: 8.00-13.00%, Mo: 0.30-1.50%, Cu: 0.30-2.00% , N: 0.05 to 0.30%, and further contains Nb: less than 0.10%, and the formula, Ni−bal.=Ni (%) + 30 [C (%) + N (%)] +0 .5Mn (%) −1.1 [Cr (%) +1.5Si (%) + Mo (%) + 0.5Nb (%)] +8.2 Ni−bal. is in the range of −2.5 to +1.0. , Fe and unavoidable impurities: Remaining, however, among the impurities, B: 0.0020% or less. steel.