JP3691283B2 - Inexpensive stainless steel for natural seawater - Google Patents

Description

【００２３】
腐食試験片は固溶化熱処理を終えた板材から２５幅×５０長さ×４mm厚さの寸法の鋼片を切り出し、全面を４００番のエメリー紙で湿式研磨をおこなったのち、アセトン中で超音波による脱脂を約１０分おこない、熱風乾燥し、試験前重量を測定した。腐食試験は実自然海水中に３ヶ月間浸漬することでおこなった。所定の時間経過後、腐食試験片に付着している藻類、貝類、海綿類などの生物を丹念に歯ブラシなどを用いて水洗・除去し、アセトン中脱脂後、熱風乾燥し、試験前後の重量差から腐食速度を算出した。また、倍率５倍のルーペを用いて腐食発生の有無を観察した。これより、腐食速度が０．１mm／ｙ以下若しくは腐食の発生が観られない材料を耐食性良好として○印で、また、そうでない材料を×印として耐食性の判定（評価）をおこなった。表１の腐食速度は本発明鋼が比較鋼に比べて腐食速度が低く０．１mm／ｙ以下であり、極めて優れた耐食性を有する材料であることがわかる。
【００２４】
【発明の効果】
以上に述べたように本発明鋼により、自然海水中での耐すきま腐食性を大幅に改善することが可能となり、実自然海水中で使用できる廉価な構造用材料を開発することができた。
【図面の簡単な説明】
【図１】図１は、ＳＵＳ３０４ステンレス鋼の自然電位の経時変化を自然海水と煮沸海水とで比較したものである。自然海水（微生物生息）中での自然電位は最大約＋２００ｍＶに達するのに対し、煮沸海水（微生物死滅）中では＋５０ｍＶにしか達しない。
【図２】自然海水中で３ヶ月間暴露試験をおこなったステンレス鋼の腐食速度を示している。ＶもしくはＣｕを添加したステンレス鋼は、これらを含まないステンレス鋼に比べて腐食速度が極めて低い。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an inexpensive stainless steel for natural seawater having excellent corrosion resistance in an environment where natural seawater is stored and transported, such as tanks for seawater storage and line pipes for seawater transportation, and in an environment where it is in direct contact with natural seawater such as estuaries and sluices. It is about steel.
[0002]
[Prior art]
Conventionally, equipment for direct contact with natural seawater, such as seawater storage tanks, seawater transport line pipes, estuary weirs, sluices, etc., depends on the seawater concentration and temperature conditions, carbon steel, low alloy steel, stainless steel, copper alloy, titanium Etc. are used properly. In addition to this, organic coated steel, FRP, and the like are often used.
In stainless steel, crevice corrosion in the seawater environment and stress corrosion cracking may occur under operating conditions of about 60 ° C or higher. Therefore, 20% Cr-25% is generally used as seawater resistant stainless steel. Austenitic stainless steels such as Ni-4.5% Mo-1.5% Cu steel and 20% Cr-24% Ni-6% Mo steel, and 28% Cr-1.2% Ni-3. Ferritic stainless steels such as 5% Mo steel and 29% Cr-2% Ni-4% Mo steel are used.
However, these stainless steels are expensive stainless steels that are used in relatively high temperature regions such as seawater desalination plants and seawater heat exchangers. In contrast, in general natural seawater, relatively inexpensive stainless steel such as SUS304 steel and SUS316L steel is used. However, in order to avoid occurrence of pitting corrosion and crevice corrosion, it is necessary to perform anticorrosion.
[0003]
[Problems to be solved by the invention]
Unlike seawater and groundwater, natural seawater contains many salts and microorganisms. For this reason, a biofilm is formed on the surface of stainless steel, and microorganisms accumulate after a certain amount of time.If organisms such as sponges, barnacles, and mussels adhere to the stainless steel surface, a gap is formed in the gap between the metal and the organism. It is necessary to develop stainless steel with excellent crevice corrosion resistance because corrosion tends to occur.
The present invention is an inexpensive stainless steel that is excellent in corrosion resistance applicable to such a natural seawater environment and can be used as a facility material for natural seawater storage tanks, natural seawater transport line pipes, estuaries weirs, sluices, etc. It is something to be offered.
[0004]
[Means for Solving the Problems]
As a result of intensive studies on stainless steels having excellent corrosion resistance in the above environment, the inventors have completed the present invention by identifying the optimum steel components. The gist of the present invention is as follows. is there.
[0005]
(1) In mass %,
C: 0.005 to 0.05%, Si: 0.01 to 1%,
Mn: 0.1 to 2%, P: 0.04% or less,
S: 0.005% or less, Cr: 17-25%,
Ni: 15-20%, N: 0.01-0.3%,
O: 0.005% or less, V: 1.02-4 %
Containing, inexpensive type natural seawater stainless steel balance being composed of iron and unavoidable impurities.
(2) Furthermore, in mass%,
Cu: 0.5 to 4%
The low-cost stainless steel for natural seawater according to (1) above, characterized by comprising
( 3 ) Furthermore, in mass %,
Ca: 0.001 to 0.03%, Ce: 0.001 to 0.03%
One or contain two, the following (1) above, wherein the satisfying formula (1) or inexpensive type natural seawater stainless steel according to (2) of the.
S + O− (0.8 × Ca) − (0.3 × Ce) ≦ 40 (1)
However, S, O, Ca, and Ce in the formula indicate the content (ppm) of each element in steel.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
From the above viewpoints, the present inventors firstly made the natural potential of stainless steel in natural seawater, which is the environment to which the material is exposed, and in natural seawater that has been boiled (boiling, 2 hours). Measurements were made. The result is shown in FIG. The natural potential in boiling seawater initially shows a base potential of about -200 mV, but then gradually transitions to a noble direction, and finally the potential becomes noble to about -50 mV.
[0007]
On the other hand, natural seawater is initially at the same base potential as boiling seawater, but abrupt potential nomination (about 200 mV) has been observed from about the third day after immersion. Thereafter, the plateau state of the electric potential continues during the 6th to 9th days, and the potential gradually transitions to the base direction from the end of the 9th day, and tends to approach the potential of the boiling seawater. Observing the natural seawater, the seawater has become cloudy since the 9th day. Here, if the microorganisms inhabit the natural seawater and the microorganisms die in the boiling seawater, the rapid noble phenomenon of the potential in the natural seawater is due to the action of the microorganism (the action of the metabolite of the microorganism). It is considered a thing.
[0008]
In addition, the natural seawater became cloudy after the 9th day passed, and it was found that the dead bodies were dead due to the loss of nutrients from the microorganisms, and that the potential was close to boiling seawater because the microorganisms were dead. Therefore, the inventors cannot evaluate the corrosion resistance test using the conventional ASTM artificial seawater in order to develop stainless steel that is resistant to natural seawater corrosion, but based on the corrosion resistance evaluation in natural seawater where microorganisms live. The conclusion was reached that it was necessary.
[0009]
Therefore, the present inventors conducted a long-term exposure test of various stainless steels in natural seawater based on the above findings. As a result, as shown in FIG. 2 as an example of the test results, it was found that the stainless steel added with V or Cu in the steel exhibits much superior corrosion resistance compared to the stainless steel not containing these. Based on this knowledge, the inventors limited the elements that should be contained in the steel material that can be used in natural seawater and the amount thereof, and completed the steel of the present invention.
[0010]
It describes the reasons for limiting the configuration requirements of the present invention will be described below.
C is harmful to the corrosion resistance of stainless steel, but a certain content is necessary from the viewpoint of strength. With an extremely low C content of less than 0.005%, the production cost increases. Moreover, it was 0.05% or less than 0.005% in order to significantly degrade the ultra-Ell and corrosion resistance of 0.05%.
[0011]
Si has a Si content of 1.0% or less as a component range of normal stainless steel that can be hot-rolled within a range that does not affect the corrosion resistance. Further, if the Si amount is less than 0.01%, the manufacturing cost becomes high.
[0012]
Mn is an austenite stabilizing element and can be added as an alternative to expensive Ni. However, since the corrosion resistance in natural seawater targeted by the present invention exceeds 2%, the effect is saturated. The upper limit of the amount was 2.0% or less. Further, if the amount of Mn is less than 0.1%, the manufacturing cost for the reduction increases, so the content was made 0.1% or more.
[0013]
P is desirably small in terms of corrosion resistance and hot workability. If it exceeds 0.04%, the hot workability is extremely deteriorated. Therefore, the P content is set to 0.04% or less. More preferably, the P content is 0.02% or less.
[0014]
S is an element that significantly affects the hot workability rather than the corrosion resistance, and the lower the amount, the better. Therefore, the S content is set to 0.005% (50 ppm) or less.
[0015]
O, like S, is an element that significantly affects hot workability, and the lower the better. O was limited to 0.005% (50 ppm) or less obtained by a normal stainless steel manufacturing method.
[0016]
Ca and Ce are selectively added as a deoxidizer and desulfurizer for molten steel, usually in the range of 0.001% to 0.03%. Even if added over 0.03%, the deoxidation effect and the desulfurization effect are saturated. In addition, for S and O, by combined addition with Ca and Ce,
S + O− (0.8 × Ca (ppm)) − (0.3 × Ce (ppm))
By adding a Ca amount and a Ce amount satisfying a value of 40 or less, O in the low S steel is fixed to prevent the formation of MnS, and the hot workability is greatly improved.
[0017]
Cr is a basic component of the present invention. In order to obtain good corrosion resistance in natural seawater, addition of 17% or more is necessary even if it coexists with Ni, N, V, and Cu. The corrosion resistance improves as the Cr content increases, but if it exceeds 25%, the manufacturability becomes extremely difficult and becomes economically expensive. Therefore, the range of Cr amount is limited to 17% or more and 25% or less.
[0018]
Ni is a basic component of the stainless steel of the present invention together with Cr. In order to facilitate the production of a stainless steel plate, the metal structure must be in an austenite phase, and Ni addition is essential. The minimum amount of Ni for making the steel of the present invention an austenitic phase is 15%. Moreover, when there is too much Ni amount, a price will become high. The upper limit of the amount of Ni that is economically inexpensive and maintains the austenite phase is 20%.
[0019]
As described above, the addition of V and / or Cu is essential to maintain the corrosion resistance in natural seawater.
When V is less than 1.02 %, sufficient corrosion resistance in natural seawater cannot be obtained. The corrosion resistance improves as the amount of V increases, but if added over 4%, the hot workability of the stainless steel is remarkably deteriorated, making the steel production difficult and economically expensive. Therefore, even when Cu has an upper limit of V content limited to 4%, sufficient corrosion resistance in natural seawater cannot be obtained with addition of less than 0.5%. Moreover, when it adds exceeding 4%, the hot workability of stainless steel will deteriorate remarkably and steel manufacture will become difficult. Therefore, the upper limit of the amount of Cu is limited to 4%.
When V and Cu are added in combination, the total amount is preferably 4% or less.
[0020]
N is a strong austenite-forming element and has a function of stabilizing the passive film of stainless steel. To obtain stable corrosion resistance, N must be added in an amount of 0.01% or more. If the upper limit is 0.3% or less, other properties such as toughness are not affected.
[0021]
【Example】
The present invention will be described below based on examples.
Table 1 shows the chemical composition and corrosion resistance evaluation results of the steels of the present invention and comparative steels, which were each melted by an electric furnace vacuum melting method and cast into a mold to prepare ingots. Thereafter, a soaking treatment is performed at 1150 to 1250 ° C. for 0.5 to 1 hour, and after surface preparation, it is heated again to 1250 ° C., hot-rolled to a thickness of 6 mm, heated at 1100 ° C. for 30 minutes, Quenching solution heat treatment was performed, and corrosion test pieces were collected and subjected to actual natural seawater exposure tests.
[0022]
[Table 1]

[0023]
For the corrosion test piece, a steel piece having a size of 25 width × 50 length × 4 mm thickness was cut out from the plate material that had been subjected to the solution heat treatment, the entire surface was wet-polished with No. 400 emery paper, and then ultrasonicated in acetone. Was degreased for about 10 minutes, dried with hot air, and the weight before the test was measured. The corrosion test was conducted by immersing in real natural seawater for 3 months. After a predetermined period of time, organisms such as algae, shellfish, and sponges adhering to the corrosion test piece are carefully washed and removed with a toothbrush, etc., degreased in acetone, dried with hot air, and the weight difference between before and after the test. From this, the corrosion rate was calculated. Moreover, the presence or absence of corrosion generation was observed using a magnifying glass having a magnification of 5 times. From this, the corrosion resistance was judged (evaluated) with a circle mark indicating that the corrosion rate was 0.1 mm / y or less or the occurrence of corrosion was not good, and the material having no corrosion was marked with X. The corrosion rate in Table 1 shows that the steel of the present invention has a corrosion rate lower than that of the comparative steel and is 0.1 mm / y or less, and is a material having extremely excellent corrosion resistance.
[0024]
【The invention's effect】
As described above, the steel according to the present invention can greatly improve the crevice corrosion resistance in natural seawater, and has been able to develop an inexpensive structural material that can be used in actual natural seawater.
[Brief description of the drawings]
FIG. 1 is a comparison of the natural potential of SUS304 stainless steel over time between natural seawater and boiling seawater. The natural potential in natural seawater (microbe inhabitants) reaches a maximum of about +200 mV, whereas it reaches only +50 mV in boiling seawater (microbe killed).
FIG. 2 shows the corrosion rate of stainless steel subjected to an exposure test in natural seawater for 3 months. Stainless steel to which V or Cu is added has a very low corrosion rate compared to stainless steel not containing these.

Claims (3)

% By mass
C: 0.005 to 0.05%,
Si: 0.01 to 1%,
Mn: 0.1 to 2%,
P: 0.04% or less,
S: 0.005% or less,
Cr: 17 to 25%,
Ni: 15-20%,
N: 0.01-0.3%
O: 0.005% or less,
V : 1.02 to 4%
Containing, inexpensive type natural seawater stainless steel balance being composed of iron and unavoidable impurities. In addition,
Cu: 0.5 to 4%
The low-cost stainless steel for natural seawater according to claim 1, comprising: Furthermore in mass%,
Ca: 0.001 to 0.03%,
Ce: 0.001 to 0.03%
One or contain two, the following (1) inexpensive die according to claim 1 or 2, characterized by satisfying the equation natural seawater for stainless steel.
S + O− (0.8 × Ca) − (0.3 × Ce) ≦ 40 (1)
However, S, O, Ca, and Ce in the formula indicate the content (ppm) of each element in steel.

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