WO2012005327A1

WO2012005327A1 - Corrosion-resistant steel material for cargo oil tank

Info

Publication number: WO2012005327A1
Application number: PCT/JP2011/065595
Authority: WO
Inventors: 鹿島　和幸; 幸　英昭
Original assignee: 住友金属工業株式会社
Priority date: 2010-07-09
Filing date: 2011-07-07
Publication date: 2012-01-12
Also published as: JPWO2012005327A1; JP4941620B2; KR20120035935A; CN102686760A

Abstract

Disclosed is a corrosion-resistant steel material for a cargo oil tank, which has excellent resistance to general corrosion and local corrosion and is characterized by containing, in mass%, 0.01-0.2% of C, 0.01-1.0% of Si, 0.05-2.0% of Mn, 0.002-0.1% of P, 0.01% or less of S, 0.01-2.0% of Cu, 0.01-1.0% of Ni, more than 0% but less than 0.01% of W and 0.1% or less of Al, with the balance made up of Fe and impurities. The corrosion-resistant steel material for a cargo oil tank may additionally contain one or more elements selected from among Cr, Mo, Ti, Zr, Sb, Sn, Nb, V, B, Ca, Mg and REM. The surface of the corrosion-resistant steel material for a cargo oil tank may be coated with a sulfide layer or an anti-corrosion coating film.

Description

Corrosion resistant steel for cargo oil tanks

The present invention relates to a steel material for a cargo oil tank used for a crude oil tank in a tanker.

There are two main types of corrosion forms in tanker cargo oil tanks. One is general corrosion that occurs in the gas phase portion of the top plate portion, and the other is local corrosion that occurs in the bottom plate portion. In particular, when loading crude oil containing hydrogen sulfide (H ₂ S), part of H ₂ S contained in the crude oil moves into the gas phase, which makes the corrosive environment extremely severe.

In such a corrosive environment as described above, it is easy to cause full corrosion on the back of the deck, which is the ceiling of the cargo oil tank, and there have been reports of cases where the corrosion rate is very large at 0.3 mm / year or more. Further, pitting corrosion is likely to occur on the bottom plate of the cargo oil tank, and there are cases where the pitting corrosion growth rate is as high as several mm / year.

For this reason, painting on the inner surface of the steel material of the cargo oil tank is partly performed, but the initial painting cost and the cost of repainting every 10 years are large. Moreover, even when the tank bottom plate is painted, pitting corrosion may occur from a defective portion of the coating film. Therefore, the actual situation is that the plate thickness design considering the corrosion allowance is carried out, and it is considered as a countermeasure against the general corrosion and the local corrosion. For example, a plate thickness design that takes into account the corrosion allowance, such as 2 mm corrosion allowance for 20 years of use, has been performed. In addition, the bottom plate is regularly inspected, and those with a large pitting corrosion depth are repaired by overlay welding, but this creates a problem with enormous maintenance costs. .

However, when designing the plate thickness considering the corrosion allowance, not only will the tank manufacturing cost increase because the thickness of the steel will increase by that amount, but the crude oil load will decrease by the plate thickness considering the corrosion allowance. There are also disadvantages. Accordingly, there is a strong demand for the development of a steel material for cargo oil tanks that can reduce the corrosion allowance and that can prevent an increase in cost and has excellent corrosion resistance.

In addition, welding is performed to build an oil tank at the shipbuilding stage, and since there is a welded joint, not only corrosion resistance is good, but also the strength, toughness, weldability, etc. of the welded joint are excellent. A material is desired.

As steel for cargo oil tanks, for example, Patent Document 1 proposes steel containing Cu and Mg as essential components, and Patent Literature 2 proposes steel containing Cr and Al as essential components. However, when the crude oil containing H _{2 _S,} H _{2 S} has not been considered at all for effects against corrosion, Therefore, sufficient corrosion resistance is obtained at the cargo oil tank to be mounted on an actual ship There was no case. In particular, since the influence of H ₂ S is extremely large in the environment of the crude oil tank bottom plate, it is essential to ensure corrosion resistance in the H ₂ S existence environment.

Further, the steel material disclosed in Patent Document 3 containing Cu and Ni as essential components is said to have improved overall corrosion resistance and pitting corrosion resistance in the cargo oil tank.

However, although this steel material certainly improves the corrosion resistance, since it contains expensive alloy components such as Cu and Ni, there is a problem that the melting cost of the steel material becomes high. In particular, in recent years, the prices of these elements have soared, and even the low content of alloy components increases the cost of the alloy components, which is a significant cost increase compared to the coating specifications of ordinary steel materials.

Further, in Patent Document 4, Cu: 0.05-2%, Ni: 0.01-1%, W: 0.01-1%, N: 0.001-0.01%, and O (oxygen) : Steel material containing 0.0001 to 0.005% as an essential component is disclosed, and it is said that both the general corrosion resistance and the local corrosion resistance in the cargo oil tank are improved.

However, since this steel material contains expensive alloy components such as Cu and Ni, there remains a problem that the melting cost of the steel material becomes high.

Japanese Patent Laid-Open No. 2000-17381 JP 2001-107180 A Japanese Patent Laid-Open No. 2003-82435 JP 2005-325439 A

The present invention has been made in view of the above situation, and an object of the present invention is to provide a corrosion-resistant steel material for a cargo oil tank that has excellent resistance to general corrosion and local corrosion and has high cost performance.

In order to achieve the above-mentioned problems, the inventors conducted an experiment on the overall corrosion occurring in the gas phase portion of the top plate portion and the local corrosion occurring in the bottom plate portion, simulating the corrosive environment caused by crude oil on an actual ship. It was. That is, for the gas phase, in the wet and dry repeatedly environment containing inert gas and H _{2 S,} were reproduced experiment corrosion product layer found in the deck behind the actual ship loaded with crude oil containing H 2 _S. And about the baseplate part, the experiment which simulated the pitting corrosion generation | occurrence | production from the oil-coat defect part in a high concentration chloride solution was conducted.

This experiment was performed on the steels having various chemical compositions used in the examples described later using the test apparatus shown in FIGS. 1 is a reproduction test apparatus for a gas phase part, and FIG. 2 is a reproduction test apparatus for a bottom plate part.

As a result, the following findings (a) to (c) were obtained regarding the corrosion resistance of the gas phase part and the bottom plate part.

(A) In the reproducibility test of the gas phase part, that is, the test on the overall corrosion occurring in the tank top plate part, it was found that the corrosion rate hardly depends on the time regardless of the presence or absence of alloy elements. Therefore, in the gas phase environment, the corrosion protection effect by the corrosion products is small, and it is necessary to improve the corrosion resistance of the base material itself by containing the alloy element.

In the entire corrosive environment, it is effective to contain elements such as Cu, Ni and W, and the effect is further increased by compounding these elements. In particular, by containing Cu and W in combination, the corrosion resistance of the base material itself is improved by suppressing the anodic dissolution reaction of the steel material.

(b) In the reproducibility test of the bottom plate part, that is, the test on the local corrosion that occurs in the bottom plate part, the pitting corrosion rate at the initial stage of corrosion is almost the same depending on the steel type, but it turns out that the pitting corrosion rate decreases with time depending on the steel type. did. Therefore, the anticorrosion effect by the corrosion products is dominant in the bottom plate environment. That is, if a corrosion product that delays the progress of corrosion at the initial stage of corrosion can be formed on the surface of the steel material, the corrosion resistance can be improved.

When a steel material is placed in a locally corrosive environment, an iron rust (β-FeOOH) layer is usually formed on the surface of the steel material. However, when Cu, Ni, and W are contained, a sulfide layer is first formed on the steel material surface, and then an iron rust layer is formed. Since this sulfide layer remarkably suppresses the anodic dissolution reaction, it contributes to the improvement of corrosion resistance. In particular, a layer containing W sulfide or further Mo sulfide exhibits cation selectivity and has an effect of suppressing permeation of Cl 2 ⁻ ions through the sulfide layer. When this layer is formed, the contribution to the improvement of the corrosion resistance becomes particularly large.

(c) As described above, it is important to contain Cu, Ni, and W in both a general corrosion environment and a local corrosion environment, but in order to obtain high corrosion resistance in both environments, Cu , Ni and W are required to have appropriate contents.

The present invention has been completed based on these findings, and the gist of the invention resides in the following corrosion-resistant steel materials for cargo oil tanks (1) to (7).

(1)% by weight, C: 0.01 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.05 to 2.0%, P: 0.002 to 0.1% , S: 0.01% or less, Cu: 0.01-2.0%, Ni: 0.01-1.0%, W: more than 0% and less than 0.01%, Al: 0.1% or less A corrosion-resistant steel material for a cargo oil tank, characterized by comprising a balance Fe and impurities.

(2) In mass%, instead of part of Fe, Cr: 5.0% or less, Mo: 1.0% or less, Ti: 0.2% or less, Zr: 0.2% or less, Sb: 0 3. Corrosion resistant steel material for cargo oil tank according to (1) above, containing one or more of 3% or less and Sn: 0.3% or less.

(3) In mass%, in place of part of Fe, Nb: not more than 0.1%, V: not more than 0.2% and B: not more than 0.01%, containing one or more The corrosion-resistant steel material for cargo oil tanks according to (1) or (2) above.

(4) In mass%, instead of a part of Fe, it contains one or more of Ca: 0.01% or less, Mg: 0.01 or less and REM: 0.01% or less The corrosion-resistant steel material for a cargo oil tank according to any one of (1) to (3) above.

(5) The corrosion-resistant steel material for a cargo oil tank according to any one of (1) to (4) above, which has a Cu, Ni and W sulfide or Mo sulfide layer on the surface.

(6) The corrosion-resistant steel material for cargo oil tanks according to any one of (1) to (4) above, wherein the surface of the ridge is covered with an anticorrosion film.

(7) The surface is covered with an anticorrosion film through an intermediate layer made of sulfide of Cu, Ni and W or further sulfide of Mo. Corrosion-resistant steel material for cargo oil tanks as described.

According to the present invention, it is possible to provide a corrosion-resistant steel material for cargo oil tanks having excellent resistance to general corrosion and local corrosion.

1 shows a reproduction test apparatus for a gas phase part. The reproduction | regeneration test apparatus of a baseplate part is shown. An acid immersion test apparatus is shown.

Hereinafter, the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition C: 0.01 to 0.2%
C is an element necessary for securing strength as a material, and a content of 0.01% or more is necessary. However, if the content exceeds 0.2%, weldability decreases. Further, as the C content increases, the amount of cementite that becomes a cathode in an acidic environment and promotes corrosion increases, and the weldability deteriorates. For this reason, the upper limit was made 0.2%. A preferable upper limit is 0.15%, and a preferable lower limit is 0.04%.

Si: 0.01 to 1.0%
Si is an element necessary for deoxidation, and in order to obtain a sufficient deoxidation effect, it is necessary to contain 0.01% or more. However, if the content exceeds 1%, the toughness of the base material and the welded joint is impaired. Therefore, the Si content is set to 0.01 to 1.0%. A preferable upper limit is 0.8%, and a more preferable upper limit is 0.5%. A preferred lower limit is 0.04%, and a more preferred lower limit is 0.10%.

Mn: 0.05 to 2.0%
Mn is an element having an effect of increasing the strength of steel at a low cost, and a content of 0.05% or more is necessary to obtain this effect. However, if the content exceeds 2.0%, weldability deteriorates and joint toughness also deteriorates. Therefore, the Mn content is set to 0.05 to 2.0%. A preferable upper limit is 1.8%, and a more preferable upper limit is 1.6%. A preferred lower limit is 0.3%, and a more preferred lower limit is 0.5%.

P: 0.002 to 0.1%
P has the effect of improving the general corrosion resistance and pitting resistance. Moreover, although acid resistance deteriorates, so that there is usually much content of P, acid resistance improves by containing P in Cu containing steel. The effect of improving acid resistance in such general corrosion resistance and pitting corrosion resistance and the effect of improving acid resistance in Cu-containing steel are exhibited by containing 0.002% or more of P. However, if the content exceeds 0.1%, the weldability is significantly reduced. Therefore, the P content is set to 0.002 to 0.1%. A preferable upper limit is 0.08%, and a more preferable upper limit is 0.06%. A preferred lower limit is 0.003%, and a more preferred lower limit is 0.004%.

S: 0.01% or less S is unavoidably present as an impurity in steel. However, if the content exceeds 0.01%, a large amount of MnS is produced in the steel, and MnS becomes a starting point of corrosion, resulting in overall corrosion and pitting corrosion. Therefore, the S content is set to 0.01% or less. A preferable upper limit is 0.008%, and a more preferable upper limit is 0.005%. In addition, the lower the S content, the better.

Cu: 0.01 to 2.0%
Cu is an element that not only improves the overall corrosion resistance, but also forms a sulfide layer together with S to improve the pitting corrosion resistance under the bottom plate environment (local corrosion environment) in the cargo oil tank. This effect is exhibited by containing 0.01% or more of Cu, but even if Cu is contained exceeding 2.0%, the effect is not only saturated, but also for preventing cracking during hot rolling. Since the amount of Ni contained in the steel also increases, the cost increases. Therefore, the Cu content is set to 0.01 to 2.0%. A preferable upper limit is 1.8%, and a more preferable upper limit is 1.5%. A preferred lower limit is 0.05%, and a more preferred lower limit is 0.10%. Details of the sulfide layer will be described later.

Ni: 0.01 to 1.0%
Ni, as well as Cu, is an element that not only improves the overall corrosion resistance, but also forms a sulfide layer together with S to improve the pitting corrosion resistance under the bottom plate environment (local corrosion environment) in the cargo oil tank. . This effect is exhibited when the content is 0.01% or more. However, if the content exceeds 1.0%, the effect is not only saturated but also the cost is increased. Therefore, the Ni content is set to 0.01 to 1.0%. A preferable upper limit is 0.9%, and a more preferable upper limit is 0.8%. A preferred lower limit is 0.05%, and a more preferred lower limit is 0.1%. Details of the sulfide layer will be described later.

W: more than 0% and less than 0.01% W is an element that improves acid resistance and improves overall corrosion resistance. W also has the effect of increasing the overall corrosion resistance by combining with other elements and the effect of improving the pitting corrosion resistance by forming a corrosion-resistant sulfide layer together with S in a wet hydrogen sulfide environment. These effects can be obtained by containing a trace amount of W. However, if W is contained in an amount of 0.01% or more, an effect commensurate with the cost cannot be obtained, and deterioration of weldability is also a concern. Therefore, the W content is more than 0% and less than 0.01%. Details of the sulfide layer will be described later.

Al: 0.1% or less Al is an element effective for deoxidation of steel, but since Si is contained in the present invention, deoxidation is performed with Si. Therefore, since it is not always necessary to deoxidize with Al, it is not necessary to contain Al. However, in addition to Si, the composite deoxidation can be performed by further adding Al. In this case, when Al is contained 0.005% or more, it can be effectively deoxidized. On the other hand, if the Al content exceeds 0.1%, not only the overall corrosiveness is remarkably deteriorated but also the nitride is coarsened, resulting in a decrease in toughness. Therefore, the upper limit of the Al content when Al is contained is set to 0.1% or less. A preferred upper limit is 0.05%.

The corrosion-resistant steel material for cargo oil tank according to the present invention has the above-described elements, and the balance is made of Fe and impurities. The impurities are components that are mixed due to various factors in the manufacturing process including raw materials such as ores and scraps when industrially manufacturing steel materials, and do not adversely affect the present invention. Means what is allowed.

Corrosion resistant steel material for cargo oil tank according to the present invention may be replaced with a part of Fe, if necessary, among Cr, Mo, Ti, Zr, Sb, Sn, Nb, V, B, Ca, Mg, REM. 1 type, or 2 or more types of elements can be contained.

These elements can be classified into the following three groups.

(i) As the first group, Cr: 5.0% or less, Mo: 1.0% or less, Ti: 0.2% or less, Zr: 0.2% or less, Sb: 0.3% or less and Sn: One or more of 0.3% or less.

(Ii) The second group is one or more of Nb: 0.1% or less, V: 0.2% or less, and B: 0.01% or less.

(Iii) The third group is one or more of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less.

Hereinafter, each of these elements will be described for each group.

(i) First group: Cr, Mo, Ti, Zr, Sb and Sn
Cr: 5.0% or less Cr can be contained as necessary. When Cr is contained alone, the corrosion resistance in an acid environment is lowered. However, when it is contained in combination with Cu, a highly protective rust layer is formed in a repeated wet and dry environment, and the overall corrosion resistance is improved. However, if the Cr content exceeds 5.0%, the effect is not only saturated, but also the weldability is reduced and the cost is increased. Therefore, the upper limit of the Cr content is 5.0%. A preferable upper limit is 4.5%, and a more preferable upper limit is 4.0%. In addition, in order to acquire the effect by containing Cr stably, it is preferable that Cr content shall be 0.5% or more. More preferably, it is 1.0% or more.

Mo: 1.0% or less Mo can be contained as necessary. Mo is an element that improves acid resistance, and has the effect of improving overall corrosion resistance in a dry and wet repeated environment with acidic water. In addition, there is an effect of improving the pitting resistance by forming a corrosion-resistant sulfide layer together with S in a wet hydrogen sulfide environment. However, even if Mo is contained in excess of 1.0%, the effect is saturated, weldability is impaired, and cost is increased. Therefore, the upper limit of the Mo content when Mo is contained is 1.0%. A preferable upper limit is 0.5%, and a more preferable upper limit is 0.4%. In addition, in order to acquire the effect by containing Mo stably, it is preferable to contain Mo 0.01% or more. More preferably, it is 0.1% or more, More preferably, it is 0.2% or more.

Ti: 0.2% or less Ti can be contained as necessary. Ti has the effect | action which raises the intensity | strength of steel. Ti also has an effect of improving the toughness of steel and an effect of suppressing the generation of MnS as a starting point of corrosion and increasing the resistance to general corrosion and pitting corrosion by forming TiS. Furthermore, since the coarsening of the crystal grains is suppressed by the dispersion of TiN, the toughness of the high heat input weld is improved. However, even if Ti is contained in excess of 0.2%, the above effect is saturated and the cost is increased. Therefore, the upper limit of the Ti content when Ti is contained is 0.2%. A preferable upper limit is 0.15%, and a more preferable upper limit is 0.1%. In addition, in order to acquire the effect by containing Ti stably, it is preferable to contain Ti 0.005% or more. More preferably, it is 0.01% or more, More preferably, it is 0.015% or more.

Zr: 0.2% or less Zr can be contained if necessary. Zr, like Ti, preferentially forms sulfides and has the effect of suppressing the generation of MnS. In addition, Zr is an element that hardly forms nitrides as compared with Ti, and has a feature that sulfides are formed more efficiently. However, when Zr is contained exceeding 0.2%, the toughness is reduced. Therefore, the upper limit of the Zr content when Zr is contained is 0.2%. A preferable upper limit is 0.15%, and a more preferable upper limit is 0.1%. In addition, in order to obtain stably the effect by containing Zr, it is preferable to contain Zr 0.005% or more. More preferably, it is 0.01% or more, More preferably, it is 0.02% or more.

Sb: 0.3% or less Sb can be contained as required. Sb has the effect of improving the overall corrosion resistance in a wet and dry repeated environment and increasing the acid resistance. Furthermore, it has the effect | action which improves pitting corrosion resistance by improving the corrosion resistance in the environment where the pH of a pitting part is low. However, even if Sb is contained in excess of 0.3%, the above effect is saturated. Therefore, the upper limit of the Sb content when Sb is contained is 0.3%. A preferable upper limit is 0.25%, and a more preferable upper limit is 0.2%. In addition, in order to obtain stably the effect by containing Sb, it is preferable to contain Sb 0.03% or more. More preferably, it is 0.05% or more.

Sn: 0.3% or less Sn can be contained as necessary. Sn is an element that improves the corrosion resistance in an acid environment, and has the effect of improving the overall corrosion resistance in a dry and wet repeated environment with acidic water. Moreover, it has the effect | action which improves pitting corrosion resistance by improving the corrosion resistance in the environment where the pH of a pitting corrosion part is low. However, even if Sn is contained in excess of 0.3%, the above effect is not only saturated, but the toughness of the base metal and the high heat input welded joint is significantly deteriorated. Therefore, the upper limit of the Sn content when Sn is contained is 0.3%. A preferable upper limit is 0.25%, and a more preferable upper limit is 0.2%. In addition, in order to acquire the effect by containing Sn stably, it is preferable to contain 0.01% or more of Sn. More preferably, it is 0.02% or more, More preferably, it is 0.03% or more.

(ii) Second group: Nb, V and B
Nb: 0.1% or less Nb can be contained as necessary. Nb is an element having an effect of increasing the strength of steel. However, when Nb exceeds 0.1%, toughness deteriorates. Therefore, the upper limit of the Nb content when Nb is contained is 0.1%. A preferable upper limit is 0.08%, and a more preferable upper limit is 0.05%. In addition, in order to obtain the effect by containing Nb stably, it is preferable to contain Nb 0.001% or more. More preferably, it is 0.005% or more, More preferably, it is 0.01% or more.

V: 0.2% or less V can be contained as necessary. V is an element having an effect of increasing the strength of steel. However, when V is contained exceeding 0.2%, toughness and weldability deteriorate. Therefore, the upper limit of the V content when V is contained is 0.2%. A preferable upper limit is 0.15%. In addition, in order to acquire the effect by containing V stably, it is preferable to contain V 0.005% or more. More preferably, it is 0.01% or more.

B: 0.01% or less B can be contained if necessary. B is an element having an effect of increasing the strength of steel. However, when B exceeds 0.01%, toughness deteriorates. Therefore, the upper limit of the B content when B is contained is 0.01%. A preferable upper limit is 0.008%, and a more preferable upper limit is 0.005%. In addition, in order to acquire the effect by containing B stably, it is preferable to contain B 0.0002% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.0008% or more.

(iii) Third group: Ca, Mg and REM
Ca: 0.01% or less Ca can be contained as necessary. Ca dissolves in water at the time of a corrosion reaction and becomes alkaline, and has an action of suppressing pH reduction at the steel material interface. For this reason, the corrosion resistance of bare steel and a coating part improves. However, this effect is saturated even if Ca is contained exceeding 0.01%. Therefore, the upper limit of the Ca content when Ca is contained is 0.01%. A preferable upper limit is 0.008%, and a more preferable upper limit is 0.005%. In addition, in order to acquire the effect by containing Ca stably, it is preferable to contain 0.0002% or more of Ca. More preferably, it is 0.0005% or more, More preferably, it is 0.001% or more.

Mg: 0.01% or less Mg can be contained as required. Mg, like Ca, has the effect of improving the corrosion resistance by suppressing the pH drop at the steel material interface during the corrosion reaction. However, the effect is saturated even if Mg is contained exceeding 0.01%. Therefore, the upper limit of the Mg content when Mg is contained is 0.01%. A preferable upper limit is 0.008%, and a more preferable upper limit is 0.005%. In addition, in order to acquire the effect by containing Mg stably, it is preferable to contain Mg 0.0002% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.001% or more.

REM: 0.01% or less REM can be contained as necessary. REM has the effect of improving the weldability of steel. However, even if REM exceeds 0.01%, not only this effect is saturated but also the cost of the steel material increases. Therefore, the upper limit of the REM content when REM is contained is 0.01%. A preferable upper limit is 0.008%, and a more preferable upper limit is 0.005%. In addition, in order to acquire the effect by containing REM stably, it is preferable to contain REM 0.0001% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.001% or more.

Here, REM is a general term for 17 elements in which Y and Sc are combined with 15 elements of lanthanoid, and one or more of these elements can be contained. Note that the content of REM means the total content of these elements.

(B) Sulfide layer A sulfide layer contains Cu, W, Ni, or Mo further in steel materials, and is formed by using it in the bottom plate environment of a cargo oil tank. Therefore, it is not necessary to form a sulfide layer when shipping steel. Cargo to use the initial by using as a bottom plate steel oil tank H _{2 S} and Cl ^- proceeds constant pitting attack of, after a certain period of time, the sulfide layer is formed. Sulfide layer reduces the concentration of H _{2 S} in the steel material surface, suppresses the anodic dissolution of steel, W sulfide or even Mo sulfide particular a cation selective is Cl ^- inhibiting the transmission of. This slows down the progress of pitting corrosion and improves corrosion resistance.

Analysis by EPMA (Electron Probe Microanalyzer) revealed that the sulfide layer is formed in the order of Mo sulfide, Cu sulfide, W sulfide, and Ni sulfide from the inside (steel material side). The sulfide layer was formed in this order because of the solubility of each sulfide and the critical S ²⁻ concentration for sulfide formation from the Cu, W, Ni, and Mo ion concentrations estimated from the pitting corrosion growth rate. When calculated, the concentration is considered to be in the order of Mo, Cu, W, and Ni.

Once the sulfide layer is formed, an iron rust (β-FeOOH) layer that is normally formed on the surface of the steel material may be formed thereon. The iron rust layer does not have the effect of reducing the H ₂ S concentration or the effect of suppressing the permeation of Cl ⁻ , but H ₂ S and Cl ⁻ that have permeated the iron rust layer are blocked by the sulfide layer, so that excellent corrosion resistance is achieved Demonstrate.

The sulfide layer may be partially damaged by cleaning the cargo oil tank. Even in such a case, since the sulfide layer is formed again by use, the corrosion resistance is not lowered.

Although the sulfide layer has been described above, the steel material of the present invention can also be used as a tank top plate portion where a sulfide layer is not formed, and when used in a full corrosive environment, the corrosion resistance of the base material itself causes corrosion. Progress can be suppressed.

(C) About anticorrosion film The steel material of the present invention described above exhibits good corrosion resistance even when used as it is, and can reduce the corrosion allowance. However, when the surface is covered with an anticorrosion coating made of an organic resin or metal, the durability of the anticorrosion coating is improved and the corrosion resistance is further improved, which is more suitable for use as a corrosion resistant steel material for cargo oil tanks.

Here, examples of the anticorrosion coating made of an organic resin include vinyl butyral, epoxy, urethane, and phthalic acid resin coatings, and examples of the anticorrosion coating made of metal include a plating coating and a thermal spray coating of Zn or Al. be able to.

In addition, the durability of the anticorrosion film is improved because the corrosion of the steel material of the present invention, which is the base, is remarkably suppressed. This is considered to be because of this.

The treatment with the above anticorrosion coating may be performed by a usual method. Moreover, it is not always necessary to apply the anticorrosion coating to the entire surface of the steel material, and only one surface of the steel material as the surface exposed to the corrosive environment may be subjected to the anticorrosion treatment. Alternatively, only a part of the steel material that is exposed to the corrosive environment may be subjected to anticorrosion treatment.

(D) Manufacturing Method The steel material of the present invention can be manufactured as follows. However, the manufacturing method of the steel material of the present invention is not limited to this manufacturing method.

Slab having a composition defined in the present invention, in which the content of S is kept low and RH, DH, electromagnetic stirring, etc. are performed in the steelmaking stage.

This slab is hot-rolled under the conditions that the heating temperature is about 1100 ° C. to 1200 ° C., the rolling reduction per rolling is 3% or more, and the rolling finish temperature is about 700 to 900 ° C. After the rolling is finished, it is allowed to cool in the air, or a temperature range from a temperature of Ar ₃ or higher to at least about 570 ° C. is cooled at a cooling rate of 5 ° C./s or higher, and then cooled in the air. Through the above steps, the steel material of the present invention can be manufactured. In addition, all the above-mentioned temperature is the temperature in the surface part of steel materials.

23 types of steel having the chemical composition shown in Table 1 were melted using a vacuum melting furnace to form a 150 kg steel ingot, and then hot forged by an ordinary method to produce a block having a thickness of 60 mm.

Next, the block was heated at 1120 ° C. for 1 hour, hot-rolled, finished to a thickness of 20 mm at 850 ° C., and then allowed to cool to room temperature.

A test piece having a width of 25 mm, a length of 50 mm, and a thickness of 4 mm was taken from each steel plate having a thickness of 20 mm and subjected to a corrosion test simulating the environment behind the deck of an actual ship. This corrosion test assumes a cargo tank gas phase. Here, in particular, for the test steel according to the invention example of steel type 1, a modified epoxy paint is spray-coated to form an anticorrosion film of about 200 μm, and a cross bark is attached to the anticorrosion film to partially bullion. Exposed and subjected to a similar corrosion test.

That is, as shown in the top plate test of FIG. 1, an acrylic lid having a gas supply port with a sample specimen attached to the lower surface while preparing a glass container in which ion exchange water is placed in the lower third portion. Was used to seal the upper end of the glass container.

Next, the sealed glass container was placed in a thermostatic bath, and a temperature cycle of 50 ° C. × 20 hours → 25 ° C. × 4 hours was applied for 56 days. At that time, the corrosive gas in the cargo tank was simulated in the gas phase portion in the glass container, and the gas A having the following composition was blown from the gas supply port.
[Gas A] 5% by volume, 5% O ₂ -13% CO ₂ -0.01% SO ₂ -0.05% H ₂ S-residual N ₂
After the 56-day corrosion test, the corrosion rate (total corrosion rate) in units of “mm / year” was determined from the reduced mass of each specimen. Table 2 shows the test result as “Test 1”. In Table 2, for the test steel (steel type 1 (with coating)) on which the anticorrosion coating was formed, the corrosion rate of the bare metal exposed portion was determined.

This test was conducted using the same test piece as in Example 1 and assuming the bottom plate of an actual ship.

That is, as seen in the bottom plate test of FIG. 2, a glass container containing a 10% NaCl solution at 40 ° C. is prepared, and a corrosion test piece is immersed in the solution.

Next, the sealed glass container was placed in a thermostatic bath, and an immersion test was performed for 28 days. Gas B having the following composition was blown from the gas supply port.
[Gas B] 5% by volume, 5% O ₂ -13% CO ₂ -0.01% SO ₂ -0.2% H ₂ S-residual N ₂
The corrosion test piece was prepared by applying a simulated oil coat (mixture of crude oil and rust) on a test piece taken from a steel plate except for a 5 mm diameter circular portion.

The measurement of pitting corrosion depth was carried out using a micrometer based on the portion where pitting corrosion did not occur, that is, the simulated oil coat application portion, in the corrosion test piece after the test. Here, the maximum value of the depth at the pitting corrosion occurrence portion was adopted as the pitting corrosion depth.

After the 28-day corrosion test, the pitting corrosion rate in units of “mm / year” was determined from the pitting depth of each test piece. Table 2 shows the test results as “Test 2”. As in Example 1, in Table 2, the corrosion rate of the bare metal exposed portion was determined for the test steel (steel type 1 (with coating)) on which the anticorrosion coating was formed.

A test was taken from each steel plate having a thickness of 20 mm produced in the same manner as in Example 1, and a test piece having a width of 40 mm, a length of 50 mm, and a thickness of 4 mm was taken to simulate the environment in the pitting corrosion of the cargo tank bottom plate. Carried out.

That is, as seen in the acid immersion test of FIG. 3, HCl was added to a 10% NaCl solution at 30 ° C., and the test piece was immersed in a solution adjusted to pH 0.85. The test period was 72 hours, and the solution was changed every 24 hours in order to minimize the impact on corrosion due to solution degradation.

After the 72-hour corrosion test, the corrosion rate in “mm / year” was determined from the reduced mass of each test piece. Table 3 shows the test result as “Test 3”. As in Example 1, in Table 2, the corrosion rate of the bare metal exposed portion was determined for the test steel (steel type 1 (with coating)) on which the anticorrosion coating was formed.

As can be seen from the corrosion test results shown in Table 2, in Comparative Example 21, the corrosion resistance is not sufficient in Tests 1, 2, and 3 because the alloy elements are not appropriately added. In Comparative Example 22, the corrosion resistance in Test 3 is low because the amount of P is not appropriate.

On the other hand, it can be seen that the inventive examples (1 to 20) show good corrosion resistance in Tests 1, 2 and 3.

Claims

In mass%, C: 0.01 to 0.2%, Si: 0.01 to 1.0%, Mn: 0.05 to 2.0%, P: 0.002 to 0.1%, S: 0.01% or less, Cu: 0.01 to 2.0%, Ni: 0.01 to 1.0%, W: more than 0% and less than 0.01%, Al: containing 0.1% or less Further, a corrosion-resistant steel material for cargo oil tanks, comprising the balance Fe and impurities.
In mass%, instead of part of Fe, Cr: 5.0% or less, Mo: 1.0% or less, Ti: 0.2% or less, Zr: 0.2% or less, Sb: 0.3% The corrosion resistant steel material for a cargo oil tank according to claim 1, characterized in that it contains one or more of the following and Sn: 0.3% or less.
It is characterized by containing one or more of Nb: 0.1% or less, V: 0.2% or less, and B: 0.01% or less in place of part of Fe in mass%. The corrosion-resistant steel material for cargo oil tanks according to claim 1 or 2.
It is characterized by containing one or more of Ca: 0.01% or less, Mg: 0.01% or less, and REM: 0.01% or less in place of part of Fe in mass%. The corrosion-resistant steel material for cargo oil tanks according to any one of claims 1 to 3.
The corrosion-resistant steel material for cargo oil tanks according to any one of claims 1 to 4, wherein the surface has a sulfide layer of Cu, Ni and W or further a sulfide of Mo.
The corrosion-resistant steel material for cargo oil tanks according to any one of claims 1 to 4, wherein the surface is coated with an anticorrosion film.
The cargo according to any one of claims 1 to 4, wherein the surface is coated with an anticorrosion film through an intermediate layer made of a sulfide of Cu, Ni and W or further a sulfide of Mo. Corrosion resistant steel for oil tanks.