JPH02232344A - Stainless steel excellent in resistance to sticking of marine organism - Google Patents

Abstract

PURPOSE:To improve resistance to the sticking of marine organism by combinedly adding specific amounts of Cu and one or more elements among As, Bi, Sb, and Sn to a high-Cr and high-Mo ferritic stainless steel having a specific composition. CONSTITUTION:This ferritic stainless steel has a composition consisting of, by weight, <=0.02% C, <=1.5% Si, <=3.0% Mn, 25.0-30.0% Cr, 0.5-4.0% Ni, 3.5-6.0% Mo, 0.05-0.5% Cu, <=0.02% N, <=0.03% P, <=0.002% S, 0.03-0.5% of one or more elements among As, Bi, Sb, and Sn, and the balance Fe. This steel has superior resistance to the sticking of marine organism and also has excellent seawater corrosion resistance, such as crevice corrosion resistance and seawater erosion resistance. In the above steel, Cu has functions of improving acid resistance and reducing the amount of marine organism sticking, but the effects are insufficient when Cu content is below the lower limit and an adverse effect is produced on corrosion resistance when Cu content is over the upper limit. As, Bi, etc., are noxious to marine organism and produce a remarkable effect of inhibiting the sticking of marine organism by coexistence with Cu. Moreover, intergranular corrosion resistance can be improved by further adding prescribed amounts of one or more elements among Ti, Nb, and Zr.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to ferritic stainless steel that has excellent biofouling resistance and seawater resistance and is popular as a material for marine structures and equipment that uses seawater. It is something.

Conventional technology and its issues In recent years, various offshore structures have been constructed in various places, and the number of equipment that uses seawater is also increasing significantly. Research into steel materials exhibiting corrosion resistance has also made remarkable progress, and for example, high Cr/high Mo stainless steel with excellent seawater resistance called "super stainless steel" has been put into practical use.

However, it has been pointed out that such super stainless steel has a significant amount of biofouling such as shellfish and algae, and for example, when the super stainless steel is applied to heat exchangers such as condenser tubes, chemical injection ( There was no choice but to carry out mechanical cleaning (processing by injecting chlorine, etc. into the environmental seawater) and mechanical cleaning (carried out by inserting sponge balls, etc.). However, the above-mentioned "environmental and operational measures" taken to prevent the adhesion of marine organisms bring about great disadvantages in terms of equipment management and costs, and there is a need for "equipment measures" that can alleviate such inconveniences. Countermeasures from the material side were strongly desired.

However, there are reports that it is effective to use copper alloys as materials for equipment that uses seawater to prevent marine organisms from adhering to them; however, copper alloys do not have sufficient erosion resistance, and In the case of exchangers, the type of process fluid is restricted due to corrosion resistance, so this method has not been a fundamental solution to the problem.

Therefore, the purpose of the present invention is to improve the biofouling resistance of the seawater-resistant high Cr/high Mo stainless steel (super stainless steel a), which is excellent in terms of corrosion resistance and erosion resistance, from the viewpoint of steel material composition, For example, our goal is to provide a seawater-resistant material that not only has excellent corrosion resistance against heat exchange media, but also exhibits sufficiently excellent performance in terms of corrosion resistance, erosion resistance, and biofouling resistance in a seawater environment. It was written.

As a result of repeated research and repeated numerous experiments in order to achieve the above object, the inventors of the present invention discovered (al) a high Cr/high Regarding Mo stainless steel (super stainless steel), ferritic type is generally better in crevice corrosion resistance in seawater than austenitic type, and therefore ferritic type high Cr/high galvanic type is recommended for seawater environment equipment. It is advantageous to apply 0 stainless steel, Yama) If a specific amount of Cu is added to this high Cr/high Mo ferritic stainless steel, and one or more of As, Bi, sb or Sn is added in combination. (C) Furthermore, the above-mentioned high Cr/high Mo ferritic stainless steel has a significantly reduced adhesion of living things in seawater without any particular adverse effect on the useful properties of the high Cr/high Mo ferritic stainless steel. A predetermined amount of Ti. We were able to obtain new findings such as the fact that when one or more of Nb and Zr is contained, intergranular corrosion resistance is further improved, resulting in a material with better seawater resistance.

The present invention has been made based on the above findings, etc. rc: 0.02% or less (hereinafter, % representing component ratio is expressed as weight percentage) Si: 1.5% or less, Mn: 3.0%
Below, Cr: 25.0-30.0%, Ni
: 0.5~4.0%, Mo: 3.5~6.0%,
Cu: 0.05-0.5%N7 0.0
2% or less, P: 0.03% or less, S: 0.
002% or less, As. Bi + one or more of Sb and Sn: 0.0
Contains 3-0.5% {However, Cr(X) + 3Mo
(χ)≧38.01, or one or more of Ti, Nb, and Zr: 0.03 to 0.8%
It also contains ferritic stainless steel with the balance consisting of Fe and unavoidable impurities, giving the stainless steel excellent biofouling resistance and seawater resistance. It is something to do.

Hereinafter, the reason why the composition of the steel is limited as described above in the present invention will be explained in detail together with the effects of each component.

(Function) a) C Since C precipitates at grain boundaries as chromium carbide and promotes sensitization to corrosion, it is desirable that the amount of C be low. In particular, when the C content exceeds 0.02%, the toughness and intergranular corrosion resistance of the steel decrease below the desired values, so the C content was limited to 0.02% or less.

b) St Although the St component is an effective element for improving acid resistance and pitting corrosion resistance (from this point of view, it is desirable to ensure a content of at least 0.04%), it should not be contained in an amount exceeding 1.5%. S
The i content was set at 1.5% or less.

c) Mn The Mn component is effective as a deoxidizing agent for steel, but if it is contained in an amount exceeding 3.0%, the amount of sulfides produced will increase, leading to deterioration of pitting corrosion resistance. The amount was set at 3.0% or less.

d) Cr Cr is an effective element for improving corrosion resistance such as acid resistance, pitting corrosion resistance, and crevice corrosion resistance, but its content is 25.0%.
If it is less than 100%, it is particularly resistant to pitting corrosion in seawater. The crevice corrosion resistance will not be sufficient, and on the other hand, if the content exceeds 30.0%, precipitation of sigma phase etc. will become significant, leading to deterioration of hot workability and significant embrittlement. is 25.0
It was set at ~30.0%.

e) Ni Ni component has the effect of increasing the toughness of ferritic stainless steel, but if its content is less than 0.5%, the desired effect due to the above effect cannot be obtained; If the amount exceeds that amount, the steel will have two phases: ferrite and austenite! It becomes a JI weave or austenite single-phase structure, and its crevice corrosion resistance in seawater deteriorates. Therefore, in order to realize a ferritic single phase steel with desired toughness and excellent crevice corrosion resistance, the Ni content was determined to be 0.5 to 4.0%.

f) Mo Mo, like Cr, is an essential component for improving the pitting and crevice corrosion resistance of steel, but if its content is less than 3.5%, it will cause crevice corrosion in seawater. increased receptivity,
On the other hand, if hO is contained in an amount exceeding 6.0%, precipitation of the sigma phase is promoted and embrittlement is likely to occur. Therefore, the Mo content is set at 3.5 to 6.0%.

In addition, Cr and Mo, which are essential for improving pitting corrosion resistance and crevice corrosion resistance, are Cr(χ)+3Mo(
When the following condition is satisfied: χ)≧38.0, pitting corrosion resistance and crevice corrosion resistance are extremely effectively improved.

g) Cu Cu is an essential component for increasing the acid resistance of steel and reducing the amount of biofouling. However, the Cu content is 0.0
If the Cu content is less than 5%, the above effects will not be sufficient. On the other hand, if the Cu content exceeds 0.5%, the precipitates will increase and the corrosion resistance will be adversely affected. Therefore, the Cu content should be 0.05%.
It was set at ~0.5%.

h) N N, like C, is a harmful element that reduces the toughness of ferritic stainless steel and promotes sensitization due to chromium nitride precipitation, so it is better to reduce it as much as possible. In particular, when the N content exceeds 0.02%, the above-mentioned harmful effects are significantly increased, so the N content was limited to 0.02% or less.

i) PP Although P is an impurity element that is unavoidably contained in steel, it is desirable to keep its content as low as possible because it causes a decrease in weldability and hot workability. However, if the P content exceeds 0.03%, the weldability deteriorates significantly, so the P content was limited to 0.03% or less.

j) S S, like P, is an impurity element that is unavoidably contained in steel, but it is preferably as low as possible because MnS, which is a starting point for pitting corrosion, is particularly likely to form. Furthermore, S is a harmful element that also has a negative effect on hot workability. In particular, in the case of the stainless steel of the present invention with a high Cr content, the S content exceeds 0.002%.
The S content was limited to 0.002% or less because the deterioration of hot workability becomes extremely significant when S is contained.

k) As, Bi. sb and Sn Both of these elements act as poisonous substances to marine organisms, and in particular, when they coexist with Cu, they exhibit a remarkable effect in suppressing the attachment of organisms to the steel surface.

Therefore, one or more of these components can be included under the condition of combined addition with Cu, but if the total content is less than 0.03%, the biofouling inhibiting effect will not be sufficient; The addition of large amounts of these components is not desirable from the perspective of protecting the quality of environmental seawater, and in order to avoid water pollution, the total content of these components was limited to 0.5% or less.

1) Ti. Nb and Zr Each of these elements has the effect of fixing and stabilizing C and N in the steel, suppressing the sensitization of the steel, and further improving the intergranular corrosion resistance. A species or two or more species may be contained, but the total content of the above components is 0.03%.
If it is less than (C(χ)10N(
χ)} If the amount is not 10 times or more, the fixation and stabilization of C and N will not be sufficient. On the other hand, the above ingredients total 0.8%
Even if it is added in excess of 10%, the improvement in intergranular corrosion resistance will be saturated. Therefore, when adding Ti, Nb and Zr, the total content of one or more of these is 0.03 to 0.8%, and C(χ) + N
(X) was determined as the value that satisfies.

Next, the effects of the present invention will be explained in more detail with reference to Examples.

<Example> First, steels 1 to 15 having the chemical composition shown in Table 1 were melted by vacuum melting method, and then forged, hot rolled, and then processed in the usual manner.
After heat treatment and cooling, solution heat treatment was performed to obtain a test plate.

Next, a biofouling test piece (50
fl width x 100n length x 3mm thickness), crevice corrosion test piece (2
0 fin width x 30 fin length x 3 fin thickness and 12l1 width x 30 crane length x
311 thickness) and erosion test piece (5011 width x 1
00 No. 2 surface length x 31 m thickness) was cut out and its biofouling resistance, crevice corrosion resistance, and erosion resistance were investigated.

The results of these tests are also compared with the results of similar tests performed on aluminum brass, and are shown in Table 2.

In addition, in the crevice corrosion test, as shown in the side view in Fig. 1, the two large and small test pieces (1) and (21) are superimposed,
A titanium bolt (3) and nut (4) were passed through a hole drilled in the center and tightened, and the sample was immersed in raw seawater for half a year.

In addition, for the bioadhesion test, similar to the crevice corrosion test, the test piece was immersed in raw seawater for half a year, and after the test, the attached shellfish was scraped off to examine the amount of adhesion.

Furthermore, the erosion test was conducted at a flow rate of 5 m perpendicular to the surface of the test piece.
The test was carried out by injecting artificial seawater (equivalent to ASTM-D1141) at a speed of 1.5 m/sec and investigating the erosion situation.

As is clear from the results shown in Table 2, the ferritic stainless steel according to the present invention has no occurrence of crevice corrosion or erosion, and there is no adhesion of organisms at all, or even if there is adhesion, it is only 20 mg. The amount of adhesion is only below,
It can be seen that it has excellent seawater resistance and biofouling resistance.

On the other hand, in the case of comparative stainless steels, the amount of shellfish adhesion was large. And crevice corrosion is Cr(
Crevice corrosion does not occur when the amount of χ)+3Mo(χ) is 38.0 or more (Comparative Examples 12 and 13), but when it is less than 38.0, crevice corrosion is observed.

In addition, in the case of aluminum brass, which is a copper alloy, there is no biological attachment such as shellfish, but it has been confirmed that the front surface corrodes in seawater.
It is also clear that the erosion resistance is significantly inferior.

Summary of Effects> As explained above, the present invention provides a ferritic stainless steel that not only has excellent biofouling resistance but also has sufficient seawater resistance such as crevice corrosion resistance and seawater erosion resistance. It can provide extremely useful effects industrially, such as greatly contributing to improving the performance of marine structures and equipment that uses seawater (for example, seawater heat exchangers, condenser pipes, etc.).

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the assembly of a crevice corrosion test piece. In the drawings: 1...Large-sized test piece. 2...Small test piece. 3
...Titanium bolt, 4...Titanium Nantes.

Claims (2)

[Claims] (1) In weight%, C: 0.02% or less, Si: 1.5% or less, Mn: 3.
0% or less, Cr: 25.0-30.0%, Ni: 0.5
~4.0%, Mo: 3.5~6.0%, Cu: 0.05
~0.5%, N: 0.02% or less, P: 0.03% or less, S: 0.002% or less, one or more of As, Bi, Sb and Sn: 0.03-0.5% and the remainder is F
A ferritic stainless steel with excellent biofouling resistance and seawater resistance, characterized in that it consists of E and unavoidable impurities and has a composition satisfying Cr (%) + 3Mo (%) ≧ 38.0. . (2) In weight%, C: 0.02% or less, Si: 1.5% or less, Mn: 3.
0% or less, Cr: 25.0-30.0%, Ni: 0.5
~4.0%, Mo: 3.5~6.0%, Cu: 0.05
~0.5%, N: 0.02% or less, P: 0.03% or less, S: 0.002% or less, one or more of As, Bi, Sb and Sn: 0.03-0.5% , Ti, Nb, and Zr: 0.03 to 0.8%, and the remainder is F.
and unavoidable impurities, and Cr (%) + 3Mo (%) ≧ 38.0 and [Ti (%) + Nb (%) + Zr (%)] / [C (%)
+N (%)] ≧10 A ferritic stainless steel having excellent biofouling resistance and seawater resistance.