TWI410503B - Corrosion resistant steel for crude oil sump and its manufacturing method, and crude oil sump - Google Patents

Abstract

Provided is a steel product for a crude oil tank which possesses excellent general corrosion resistance and excellent local corrosion resistance and also exhibits such excellent corrosion resistances even when the steel product is used in a state where Zn is present in a surface of the steel product. To be more specific, provided is a corrosion resistance steel product for a crude oil tank having a composition which contains by mass% 0.001 to 0.16% C, 1.5% or less Si, 0.1 to 2.5% Mn, 0.025 or less P, 0.01% or less S, 0.005 to 0.1% Al, 0.001 to 0.008% N, 0.008 to 0.35% Cu, more than 0.1% and 0.5% or less Cr, 0.005 to 0.3% Sn, and 0.01% or less Mo, and a value of A1 defined by the following formula is set to 0 or less. Note A �¢ 1 = 28 × C + 2000 × P 2 + 27000 × S 2 + 0.0083 × 1 / Cu + 0.027 × 1 / Cr + 95 × Mo + 0.00098 × 1 / Sn - 6.

Description

Corrosion-resistant steel for crude oil tanks and its manufacturing method, as well as crude oil tank

The present invention relates to an oil tank suitable for use in a crude oil tanker or an oil tank for transporting or storing crude oil (hereinafter collectively referred to as "crude oil tank") (steel) Products), in particular, a general corrosion and a crude oil sump that can be mitigated on the surface of a steel part of a top part or a side wall part, a bottom part of a crude oil sump A steel that is locally corroded on a bottom plate. Further, the steel material for a crude oil oil tank of the present invention contains a thick steel plate, a thin steel sheet, and a shaped steel.

It is known that the inner surface of a crude oil sump currently used for a tanker, particularly the steel of the back side of the upper deck and the upper part of the upper side, undergoes overall corrosion. The reasons for the general corrosion are as follows.

(1) Dew drop and alternate wetting and drying on the steel sheet surface caused by the temperature difference between day and night,

(2) In the oil tank of the crude oil, the enclosed inert gas is used for explosion protection (about 5 vol% of O ₂ , about 13 vol% of CO _{2 ,} about 0.01 vol% of SO ₂ , and N ₂ as the residual part). The dissolution of dew condensation water by O ₂ , CO ₂ , SO ₂ in the boiler or the exhaust gas of the engine

(3) the incorporation of corrosive gas such as H ₂ S volatilized from crude oil into dew condensation water,

(4) Residue of salt water used in cleaning of the crude oil tank. These were obtained from the investigation of the actual dock inspection by the detection of strong acidity of dew condensation water, sulfate ion and chloride ion.

Furthermore, an iron rust generated by corrosion can oxidize H ₂ S as a catalyst, and a solid S (elemental sulfur) is formed in a layer form in rust, and such a corrosion product. It will fall off due to easy peeling and will accumulate at the bottom of the crude oil tank. Therefore, the current situation is that in every 2.5 years of dock inspection, a huge cost will be spent to recover the maintenance and repair of the upper tank or the deposited material at the bottom of the tank.

In addition, in the bottom plate of the oil tank of the oil tanker, a corrosion inhibition function of the crude oil itself or a protective coating from the crude oil formed on the inner surface of the crude oil tank may be used (hereinafter referred to as "oil film" ") Corrosion inhibition, and it is considered that corrosion does not occur on the steel used. However, recent research has found that bowl-shaped local corrosion (pitting corrosion) occurs on the steel of the oil sump bottom. As for the causes of local corrosion, the following items can be cited, but they are only inferences, and the reasons for this are not clear.

(1) The presence of sodium chloride as a representative of a mixture of high-concentration salts of condensed water (brine),

(2) Detachment of the grease film due to excessive washing,

(3) The high concentration of sulfide in crude oil,

(4) The high concentration of O ₂ , CO ₂ and SO ₂ in the inert gas enclosed by the explosion-proof in the crude oil tank

(5) participation of microorganisms, etc.,

In addition, in the analysis of the retained water in the crude oil tank at the time of the dock inspection, high concentrations of chloride ions and sulfate ions were detected.

Therefore, the most effective method for suppressing the above-mentioned general corrosion or local corrosion is to apply a heavy coating on the surface of the steel to break the steel from the corrosion environment. However, it has been pointed out that in the coating operation of the crude oil tank, the coating area is enlarged, and due to the deterioration of the coating film, it is necessary to perform the coating every 10 years, which causes a huge expense in inspection and painting. It has also been pointed out that the damage of the recoated film in the crude oil tank environment will promote corrosion.

In response to the above-mentioned problems of corrosion, it is proposed to have several corrosion-resistant steels which have improved corrosion resistance of the steel itself and corrosion resistance in the crude oil tank environment.

For example, Patent Document 1 discloses a corrosion resistant steel for a cargo oil tank which is added to a steel containing C: 0.01 to 0.3% by mass, and an appropriate amount of Si, Mn, and P, S and Ni: 0.05 to 3% and then selectively added Mo, Cu, Cr, W, Ca, Ti, Nb, V, B to improve the resistance to general corrosion or local corrosion.

In addition, in the case of a dry and wet environment containing H ₂ S, if the content of Cr exceeds 0.05% by mass, the general corrosion resistance and the pitting resistance are remarkably lowered, and the content of Cr is 0.05% by mass or less.

Further, Patent Document 2 discloses a corrosion-resistant steel for a crude oil oil tank which is added with an appropriate amount of Si, Mn, P, S, and Cu on a steel containing C: 0.001% to 0.2% by mass: 0.01 to 1.5%, Al: 0.001 to 0.3%, N: 0.001 to 0.01%, and addition of at least one of Mo: 0.01 to 0.2% or W: 0.01 to 0.5% to obtain general corrosion resistance and local corrosion resistance It is excellent and can suppress the generation of corrosion products containing solid S.

Further, Patent Document 3 discloses an anti-corrosion steel for a cargo oil tank which is added with an appropriate amount of Si and Mn on a steel containing C: 0.01 to 0.2% by mass%. , P and Ni: 0.01 to 2%, Cu: 0.05 to 2%, W: 0.01 to 1%, and then Cr, Al, N, and O are selectively added, and the addition amount of Cu, Ni, and W is further specified by a parameter formula. , and make the overall corrosion or local corrosion upwards.

Further, Patent Document 4 discloses a corrosion-resistant steel for a cargo oil tank which is added with an appropriate amount of Si and Mn on a steel containing C: 0.01 to 0.2% by mass%. , P, Cr, Al, and Ni: 0.01 to 1%, Cu: 0.05 to 2%, Sn: 0.01 to 0.2%, and then selectively add Mo, W, Ti, Zr, Sb, Ca, Mg, Nb, V , B, and make it resistant to general corrosion or local corrosion upwards.

[Previous Technical Literature]

[Patent Literature]

[Patent Document 1] JP-A-2003-082435

[Patent Document 2] JP-A-2004-204344

[Patent Document 3] JP-A-2005-325439

[Patent Document 4] JP-A-2007-270196

However, when the corrosion resistant steel disclosed in the above Patent Documents 1 to 4 is used in the crude oil oil tank, the resistance to the overall corrosion when it is used for the upper portion of the crude oil oil tank (hereinafter referred to as "total corrosion resistance") Or, resistance to local corrosion when used in a crude oil tank bottom plate (hereinafter referred to as "local corrosion resistance") is hardly said to be sufficiently sufficient.

This shows that in the development of various corrosion resistant steels for general corrosion in the upper deck of the crude oil tank or local corrosion of the bottom plate, it is not sufficient to carry out the corrosion resistance test only by simulating the respective corrosion environments. This means that the corrosion test in the laboratory does not correctly correct the actual environment because it contains many elements of the accelerating test, or some of the corrosion factor is omitted. In the case of reproduction, especially in the development of corrosion resistant steel for crude oil tanks, chloride ions and sulfate ions must be added to the test environment.

Further, in the inventions described in Patent Documents 3 and 4, it is considered that when the crude oil is not stowed, the ballast tank located outside the cargo oil tank is stowed with seawater to have both crude oil. The corrosion resistance in corrosive environments and seawater corrosive environments is the target technology. However, in the seawater corrosion environment, the corrosion resistance of the corrosion-resistant coating film on the outside of the cargo oil tank is focused on the corrosion resistance of the steel itself, but it is caused by the steel. There is no consideration in the improvement of the corrosion resistance of the corrosion-resistant element contained in the zinc-based primer and the improvement of the corrosion resistance in the state where the coating film exists on the surface of the steel material, that is, the corrosion resistance after coating.

However, the corrosion resistance after coating which has not been considered in Patent Documents 3 and 4 is increased upwards. Although it is extremely important and effective in attempting to increase the life of corrosion-resistant steel for crude oil tankers, at present, there is no energy in reality. Implement these technologies.

Accordingly, the present invention has been made in order to solve the above problems, and an object thereof is to provide an inner surface of a crude oil oil tank, particularly for use in an upper deck and a side panel, which has excellent overall corrosion resistance and is used in a crude oil sump bottom plate. When it is used in a state where Zn is present on the surface of the steel material, the steel material for crude oil oil sump which exhibits remarkable and excellent overall corrosion resistance and local corrosion resistance, a method for producing the same, and the use of the steel material Crude oil tank.

In order to achieve the above problems, the inventors first extracted a corrosion test in which the factors relating to general corrosion in the crude oil tank were combined and the factors were combined. As a result, the overall corrosion generated in the crude oil tank was successfully reproduced, and the following findings were obtained regarding the dominant corrosion factor and the corrosion mechanism of the overall corrosion.

The inert gas enclosed in the crude oil tank for the explosion-proof contains water vapor. Therefore, condensation can occur on the surface of the steel wall of the oil sump during the temperature difference between day and night in navigation. In the dew condensation water, CO ₂ (diacidified carbon) or O ₂ (acid), SO ₂ (diacidified sulfur), and H ₂ S (sulfurized water) which are volatilized from crude oil, which are dissolved in an inert gas component, form a sulfate-containing ion. Corrosive acidic solution. It is also necessary to consider the chloride ions that the crude oil tanks bring in due to seawater washing. A corrosive acidic solution in which these components are dissolved is concentrated during the rise of the temperature of the steel sheet, and overall corrosion occurs on the surface of the steel sheet. Even if rust formed on the surface of the steel sheet acts as a catalyst and S (sulfur) is precipitated from H ₂ S, a layered rust layer of rust and sulfur is formed, and the rust layer on the surface of the steel sheet becomes brittle and unprotected. Corrosion will continue.

Therefore, the inventors investigated the influence of various alloying elements which may cause overall corrosion of the surface of the steel sheet in an environment in which dew condensation water containing a sulfate ion and a chloride ion exists. As a result, it was confirmed that the addition of Cu, Cr, and Sn densifies the rust layer on the surface of the steel sheet formed in the environment for the steel material for the crude oil sump, and the corrosion resistance is improved upward. The addition of W and Sb promotes the formation of a dense rust layer, which increases its overall corrosion resistance. In other words, the inventors have found that a steel material for a crude oil sump excellent in general corrosion resistance can be obtained by mainly adding Cu, Cr, and Sn, and further adding W and Sb in an appropriate amount.

Next, the inventors extracted the factors related to the local corrosion of the crude oil tank bottom plate to perform the corrosion test by combining the factors. As a result, the local corrosion generated by the bottom plate of the crude oil tank was successfully reproduced in the same manner as the overall corrosion, and the following findings were obtained regarding the governing factors and corrosion mechanisms of the local corrosion.

The local corrosion of the bowl type actually produced in the bottom of the crude oil tank is due to the fact that O ₂ and H ₂ S contained in the solution retained on the bottom plate are the main dominating factors, specifically, O ₂ and H ₂ S coexist, and Both the O ₂ concentration and the H ₂ S concentration are locally corroded in a certain range of environments (in an aqueous solution in which the O ₂ concentration: 2 to 8 vol%, and the H ₂ S concentration: 0.1 to 5 vol% of the gas is saturated). In other words, in an environment of low O ₂ concentration and low H ₂ S concentration, H ₂ S is oxidized to precipitate solid S. This precipitated solid S will form a local battery between the bottom of the crude oil sump and cause localized corrosion on the surface of the steel. This localized corrosion is further promoted and grown if it is in an acidic environment in which chloride ions and sulfate ions are present.

Therefore, the inventors investigated the influence of various alloying elements which may cause local corrosion in the environment of the above low O ₂ concentration and low H ₂ S concentration. As a result, it was confirmed that the addition of W densifies the rust layer on the surface of the steel sheet formed in the environment for the steel material for the crude oil oil tank, and the local corrosion resistance is improved upward, and it is confirmed that Sn and Sb are The addition contributes to the formation of a dense rust layer containing W, which is enhanced in local corrosion resistance. Further, it was confirmed that in the acidic corrosive environment in which both the chloride ion and the sulfate ion exist simultaneously, the addition of Mo deteriorates the corrosion resistance. In other words, in addition to the addition of W, by adding Sn and Sb in an appropriate amount and limiting the Mo content, a steel material for a crude oil sump excellent in local corrosion resistance can be obtained.

From the results of the above findings, it is found that by adapting the contents of Cu, Cr, Sn, W and Sb, it is possible to obtain excellent resistance to general corrosion when used in the inner surface of a crude oil tank, and at the same time, when used in a crude oil tank bottom plate. Excellent local corrosion resistance means that steel for crude oil sump excellent in corrosion resistance can be obtained regardless of any part used in the crude oil tank.

Further, the inventors have found that the steel materials having the above-mentioned Cu, Cr, Sn, W, and Sb contents have excellent corrosion resistance even in the uncoated state, and the metal-containing Zn or Zn is applied to the surface. In addition to greatly extending the coating life of the compound, the coating is also significantly improved in overall corrosion resistance and local corrosion resistance. Moreover, the influence of the microstructure of the steel in the steel of the present invention on the corrosion resistance was investigated, and the perlite having an area ratio of 2% or more was generated, and the corrosion resistance was improved upward.

The present invention is based on the above findings and is further reviewed.

That is, the present invention is a corrosion-resistant steel material for a crude oil oil tank, which is characterized by containing C: 0.001 to 0.16 mass%, Si: 1.5 mass% or less, Mn: 0.1 to 2.5 mass%, P: 0.025 mass% or less, and S: 0.01. Mass% or less, Al: 0.005 to 0.1% by mass, N: 0.001 to 0.008% by mass, Cu: 0.008 to 0.35% by mass, Cr: 0.1% by mass to 0.5% by mass or less, Sn: 0.005 to 0.3% by mass, and Mo : 0.01% by mass or less, the remainder being Fe and unavoidable impurities, wherein the value of A1 defined by the following formula (1) is 0 or less;

A1=28×[C]+2000×[P] ² +27000×[S] ² +0.0083×(1/[Cu])+0.027×(1/[Cr])+95×[Mo]+0.00098× (1/[Sn])-6 ‧‧‧(1)

Here, the content (% by mass) of each of the elements [C], [P], [S], [Cu], [Cr], [Mo], and [Sn] in the above formula.

The corrosion resistant steel material for a crude oil oil tank of the present invention further contains Ni: 0.005 to 0.4% by mass in addition to the above-described component composition, and the value of A2 defined by the following formula (2) is 0 or less;

A2=28×[C]+2000×[P] ² +27000×[S] ² +0.0083×(1/[Cu])+2×[Ni]+0.027×(1/[Cr])+95× [Mo]+0.00098×(1/[Sn])-6 ‧‧‧(2)

Here, the content (% by mass) of each of the elements [C], [P], [S], [Cu], [Ni], [Cr], [Mo], and [Sn] in the above formula.

In addition to the above-described component composition, the corrosion-resistant steel material for a crude oil oil tank of the present invention further contains one or two selected from the group consisting of W: 0.001 to 0.5% by mass and Sb: 0.005 to 0.3% by mass, and is as follows ( 3) The value of A3 defined by the formula is below 0;

A3=28×[C]+2000×[P] ² +27000×[S] ² +0.0083×(1/[Cu])+2×[Ni]+0.027×(1/[Cr])+95× [Mo]+0.00098×(1/[Sn])+0.0019×(1/([Sb]+[W]))-6.5 ‧‧‧(3)

Here, the elements of the [C], [P], [S], [Cu], [Ni], [Cr], [Mo], [Sn], [Sb], and [W] systems in the above formula Content (% by mass).

Further, the corrosion resistant steel material for a crude oil oil tank of the present invention further contains Nb: 0.002 to 0.1% by mass, V: 0.002 to 0.1% by mass, Ti: 0.001 to 0.1% by mass, and B: 0.01% by mass, in addition to the above-described component composition. One or two or more selected from the following.

In addition to the above-described component composition, the corrosion-resistant steel material for a crude oil oil tank of the present invention further contains one or two selected from the group consisting of Ca: 0.0002 to 0.005 mass% and REM: 0.0005 to 0.015 mass%.

Further, in the corrosion-resistant steel material for a crude oil oil tank of the present invention, the micro-structure of the steel sheet having a thickness of 1/4 is contained in an amount of 2 to 20% of the iron.

Further, the corrosion resistant steel material for a crude oil oil tank of the present invention is formed by forming a coating film containing a metal Zn or Zn compound on the surface of the steel material.

Further, in the corrosion resistant steel material for a crude oil oil tank of the present invention, the Zn content in the coating film is 1.0 g/m ² or more.

Moreover, the present invention proposes a method for producing a corrosion-resistant steel material for a crude oil oil tank, which is obtained by heating a steel material having the above-described composition to 1000 to 1350 ° C, and then performing a hot rolling between a rolling temperature of 750 ° C or higher. The cooling rate at 2 ° C/sec or more is cooled to 650 ° C or lower and 450 ° C or higher.

Still another aspect of the invention is a crude oil sump using the above steel.

According to the present invention, it is possible to inexpensively provide a steel which does not cause overall corrosion or local corrosion in any part of a crude oil tank for oil tanks of crude oil tankers or oil tanks for transporting or storing crude oil, and has industrially extreme Good results.

[Best Mode for Carrying Out the Invention]

Hereinafter, the reason why the component composition of the steel material for crude oil oil grooves of the present invention is limited to the above range will be described.

C: 0.001 to 0.16 mass%

In the present invention, in order to obtain a desired strength, it is necessary to contain 0.001% by mass or more. On the other hand, C not only deteriorates the corrosion resistance as the content increases, but when it is added in excess of 0.16 mass%, the weldability and the toughness of the welded heat affected zone are deteriorated. Therefore, C is set in the range of 0.001 to 0.16 mass%. In addition, from the viewpoint of improving strength and toughness, it is preferably in the range of 0.01 to 0.15 mass%, more preferably in the range of 0.05 to 0.15 mass%.

Si: 1.5% by mass or less

The Si function acts as a deoxidizing agent and is also an element which increases the strength. When it is added in an amount of more than 1.5% by mass, the toughness of the steel is lowered. Therefore, in the present invention, Si is limited to a range of 1.5% by mass or less. In addition, in the acidic environment, since the corrosion resistance is increased by the formation of the anti-corrosion film, it is preferable to increase the corrosion resistance in an acidic environment by adding 0.2 to 1.5% by mass. A range of 0.3 to 1.5% by mass is more preferable.

Mn: 0.1 to 2.5% by mass

In the present invention, Mn is an element which increases the strength of the steel material, and in order to obtain a desired strength, it is necessary to contain 0.1% by mass or more. On the other hand, when it is added in an amount of more than 2.5% by mass, the toughness and weldability of the steel are lowered, and segregation is promoted to cause unevenness in the composition of the steel sheet. Therefore, Mn is set in the range of 0.1 to 2.5% by mass. In addition, from the viewpoint of maintaining high strength and suppressing formation of inclusions which deteriorate corrosion resistance, it is preferably in the range of 0.5 to 1.6% by mass, more preferably in the range of 0.8 to 1.4% by mass.

P: 0.025 mass% or less

P is a harmful element which causes segregation at the grain boundary to lower the toughness of the steel and also lowers the corrosion resistance, and the content thereof is preferably as low as possible. In particular, when the content is more than 0.025% by mass, the center segregation is promoted to cause unevenness in the composition of the steel sheet, and the toughness is remarkably lowered. Therefore, P is 0.025% by mass or less. In addition, if the P is reduced to less than 0.003 mass%, the manufacturing cost is greatly increased, and the lower limit of P is preferably about 0.003 mass%, and the general corrosion resistance in the acid environment is improved. From the viewpoint of upward improvement, it is preferable to set it to 0.010% by mass or less. Further, it is more preferably 0.009% by mass or less.

S: 0.01% by mass or less

The S system forms a non-metal inclusion MnS to become a starting point of corrosion and a harmful element which is resistant to local corrosion and general corrosion resistance, and its content is preferably as low as possible. In particular, when the content is more than 0.01% by mass, the local corrosion resistance and the general corrosion resistance are remarkably lowered. In the present invention, the upper limit of S is 0.01% by mass. In addition, from the viewpoint of further improving the corrosion resistance, it is preferably 0.0020% by mass or less. However, since the production cost is greatly increased by extremely reducing S, it is actually 0.0002 to 0.0020% by mass. Further, it is preferably 0.0009 mass% or less.

Al: 0.005 to 0.1% by mass

Al acts as an element of the deacidifying agent, and in the present invention, it must be contained in an amount of 0.005% by mass or more. On the other hand, when it adds more than 0.1 mass%, the toughness of steel will fall. Therefore, Al is preferably in the range of 0.005 to 0.1% by mass, preferably in the range of 0.01 to 0.05% by mass, more preferably in the range of 0.02 to 0.04% by mass.

N: 0.001 to 0.008% by mass

N is necessary to increase the toughness of the steel and to increase the mechanical properties of the weld joint part, and it is necessary to add 0.001% by mass or more. However, when the addition exceeds 0.008% by mass, the solid solution N increases, and the toughness of the joint portion is remarkably lowered depending on the welding conditions. Therefore, N is in the range of 0.001 to 0.008% by mass, preferably 0.002 to 0.005% by mass, and more preferably 0.002 to 0.004% by mass.

Cu: 0.008 to 0.35 mass%

Cu has an effect of forming an anticorrosion coat to suppress overall corrosion, and is an element which must be added in the present invention. However, if it is less than 0.008% by mass, the above effects are not obtained. On the other hand, Cu is remarkably improved by the addition of Sn, and the overall corrosion resistance is remarkably improved. However, when it is added in an amount of more than 0.35% by mass, the inter-heat processability is lowered, which may impair the manufacturability. Therefore, Cu is set in the range of 0.008 to 0.35 mass%. In addition, the addition effect of Cu is more saturated as the amount of addition increases, and it is preferably in the range of 0.008 to 0.15 mass%, and more preferably in the range of 0.01 to 0.14 mass% from the viewpoint of cost and effect. .

Cr: 0.1% by mass or more and 0.5% by mass or less

In addition to forming a protective coating on the surface of the steel material together with Cu, Cr enhances the overall corrosion resistance in an acidic environment, and also has an effect of improving the strength of the steel material, and is an element which must be added in the present invention. In particular, in an acidic environment containing sulfate ions and chloride ions, Cr forms an oxide layer to cover the surface of the steel, which has the effect of reducing the overall corrosion rate. Further, since Cr is densely densified together with Cu, even in the state of being coated with a zinc-rich primer, the Zn compound can remain in the rust layer for a long time, and even if it contains corrosion resistance after coating, It still contributes greatly to the upward improvement of corrosion resistance. In addition, since the addition amount of Cu can be suppressed by the effect of increasing the corrosion resistance by the addition of Cr, the effect of reducing the inter-heat workability caused by the coexistence of Cu and Sn is reduced. However, when Cr is added in an amount of 0.1% by mass or less, the above-described effect of addition is not obtained. On the other hand, when the amount is more than 0.5% by mass, the above effects are saturated, and the cost is increased and the weldability is deteriorated. Therefore, the Cr is added in an amount of more than 0.1% by mass and not more than 0.5% by mass, preferably in the range of 0.11% to 0.3% by mass, more preferably in the range of 0.12% to 0.2% by mass.

Sn: 0.005 to 0.3% by mass

Sn is a composite effect with Cu, or a composite effect of Cu and W when W is added as described later, in addition to forming a dense rust layer to suppress overall corrosion in an acidic environment, and also suppressing local corrosion. The role is an element that must be added in the present invention. However, when it is less than 0.005 mass%, the above-mentioned addition effect is not obtained. On the other hand, when it is more than 0.3 mass%, deterioration of hot workability and toughness may occur. Therefore, Sn is in the range of 0.005 to 0.3% by mass, more preferably in the range of 0.02 to 0.1% by mass, and still more preferably in the range of 0.03 to 0.09% by mass.

Mo: 0.01% by mass or less

Mo generally has the same effect as W and is considered to be an element that enhances corrosion resistance upward. However, the inventors have newly discovered that, in contrast to W, an insoluble salt is formed in an acidic saline environment, and Mo forms a soluble salt in an acidic saline environment, which does not exert a barrier effect, particularly in the case where the Mo content exceeds 0.01 mass. When a large amount is present, the corrosion resistance in an acidic saline environment may be deteriorated. Therefore, in the present invention, the content of Mo is limited to 0.01% by mass or less, preferably 0.008% by mass or less, more preferably 0.005% by mass or less.

The above elements are essential components of the steel of the present invention. However, in order to achieve excellent corrosion resistance and local corrosion resistance, the steel material of the present invention is required to contain not only the above components in the above composition range but also the value of A1 defined by the following formula (1) of 0 or less. Even better, the value of A1 is -1 or less.

Remember

A1=28×[C]+2000×[P] ² +27000×[S] ² +0.0083×(1/[Cu])+0.027×(1/[Cr])+95×[Mo]+0.00098× (1/[Sn])-6 ‧‧‧(1)

Here, the content (% by mass) of each of the [C], [P], [S], [Cu], [Cr], [Mo], and [Sn] systems in the above formula

The above formula (1) is an empirical formula showing an empirical formula including the corrosion resistance index of each element related to general corrosion resistance and local corrosion resistance obtained by the corrosion test carried out in the present invention, and it is known that If the value of A1 exceeds 0, it is no longer possible to ensure either or both of general corrosion resistance and local corrosion resistance. Further, in the above formula (1), regarding the influence of each element on the corrosion resistance, the element of the primary and secondary term means that the more the element is added, the more the general corrosion resistance and the local corrosion resistance are lowered, and on the other hand, The elements in the inverse term indicate that the more added, the more general corrosion resistance and local corrosion resistance. In other words, C and Mo are corrosion-reducing elements, P and S are corrosion-reducing elements, Cu, Cr, and Sn-based corrosion-improving elements that are affected by the second-order content.

In addition to the above basic components, the steel material of the present invention may further contain Ni in the following range.

Ni: 0.005 to 0.4% by mass

Ni is added by being compounded with Cu, and has an effect of suppressing deterioration of hot workability. However, when the addition is less than 0.005% by mass, the above effects cannot be obtained, and on the other hand, when the addition exceeds 0.4% by mass, the cost increases. Therefore, Ni is preferably added in the range of 0.005 to 0.4% by mass. Further, from the viewpoint of cost and effect, it is more preferably in the range of 0.005 to 0.15% by mass, and more preferably in the range of 0.005 to 0.1% by mass. Further, it is more preferably in the range of 0.03 to 0.1% by mass.

Further, when Ni is added, it is necessary to include each component in place of the value of A1 by setting the value of A2 defined by the following formula (2) to 0 or less. Even better, the value of A2 is -1 or less.

Here, it is understood from the formula (2) that Ni is an element which lowers corrosion resistance.

Remember

A2=28×[C]+2000×[P] ² +27000×[S] ² +0.0083×(1/[Cu])+2×[Ni]+0.027×(1/[Cr])+95× [Mo]+0.00098×(1/[Sn])-6 ‧‧‧(2)

Here, [C], [P], [S], [Cu], [Ni], [Cr], [Mo], and [Sn] in the above formula represent the content (% by mass) of each element.

Further, in addition to the above components, the steel material of the present invention may be added to one or two selected from the group consisting of Sb and W in the following range.

Sb: 0.005 to 0.3% by mass

Similarly to Sn, the Sb system has a composite effect with Cu or a composite effect of Cu and W when W is added as described later to form a dense rust layer and has the effect of suppressing corrosion in an acidic environment. Features can be added when added. However, if it is added in an amount of less than 0.005% by mass, the effect is not obtained. On the other hand, when the addition exceeds 0.3% by mass, the effect is saturated and the workability is also lowered. Therefore, when Sb is added, it is preferably in the range of 0.005 to 0.3% by mass, more preferably in the range of 0.02 to 0.15% by mass, and still more preferably in the range of 0.03 to 0.09% by mass.

W: 0.001 to 0.5% by mass

The WO ₄ ^2- ion formed in a corrosive environment exhibits a barrier effect on anion such as chloride ions and forms an insolubility of FeWO ₄ to suppress the progress of corrosion. Further, it also has an effect of densifying the rust layer formed on the surface of the steel sheet. Further, W has a chemical and physical effect, and has an effect of suppressing local corrosion and overall corrosion in a corrosive environment in which H ₂ S and Cl ^- are present. However, if it is less than 0.001% by mass, a sufficient effect of addition cannot be obtained. On the other hand, when it is added in an amount of more than 0.5% by mass, not only the effect is saturated, but also the cost is increased. Therefore, when W is added, it is preferably in the range of 0.001 to 0.5% by mass, more preferably in the range of 0.02 to 0.1% by mass, and still more preferably in the range of 0.03 to 0.09% by mass.

Further, when Sb and/or W are added in addition to the above Ni, it is necessary to replace the value of A1 or A2 with the value of A3 defined by the following formula (3), and to contain each element. Even better, the value of A3 is -1 or less.

Here, it is understood from the formula (3) that Sb and W are elements which enhance the corrosion resistance upward.

Remember

A3=28×[C]+2000×[P] ² +27000×[S] ² +0.0083×(1/[Cu])+2×[Ni]+0.027×(1/[Cr])+95× [Mo]+0.00098×(1/[Sn])+0.0019(1/([Sb]+[W]))-6.5 ‧‧‧(3)

Here, [C], [P], [S], [Cu], [Ni], [Cr], [Mo], [Sn], [Sb], and [W] in the above formula represent the respective elements. Content (% by mass).

In addition, in the steel material of the present invention, in addition to the above-mentioned components, one or two or more selected from the group consisting of Nb, V, Ti and B may be added in the following range.

Nb: 0.002 to 0.1% by mass

Nb is an element added for the purpose of increasing the strength and toughness of steel. However, if it is less than 0.002 mass%, it will have no effect. On the other hand, if it exceeds 0.1 mass%, the effect will be saturated. Therefore, when Nb is added, it is preferably in the range of 0.002 to 0.1% by mass, more preferably in the range of 0.004 to 0.05% by mass, even more preferably in the range of 0.005 to 0.01% by mass.

V: 0.002 to 0.1% by mass

The V system is an element added for the purpose of increasing the strength of the steel. However, if it is less than 0.002% by mass, there is no effect of increasing the strength. On the other hand, when it is added in excess of 0.1% by mass, the toughness is lowered. Therefore, in the case of addition, it is preferably in the range of 0.002 to 0.1% by mass, more preferably in the range of 0.003 to 0.05% by mass, even more preferably in the range of 0.004 to 0.01% by mass.

Ti: 0.001 to 0.1% by mass

Ti is an element added for the purpose of increasing the strength and toughness of steel. However, if it is less than 0.001% by mass, the effect is not obtained. On the other hand, if it exceeds 0.1% by mass, the effect is saturated. Therefore, in the case of addition, it is preferably in the range of 0.001 to 0.1% by mass, more preferably in the range of 0.005 to 0.03% by mass, and still more preferably in the range of 0.006 to 0.02% by mass.

B: 0.01% by mass or less

B is an element added for the purpose of increasing the strength of the steel, and the effect can be obtained by adding 0.0003 mass% or more. However, when the addition is more than 0.01% by mass, since the toughness is lowered, the addition is preferably 0.01% by mass or less, more preferably 0.0003 to 0.002% by mass, and still more preferably 0.0003 to 0.0015% by mass.

In addition, the steel of the present invention is improved in ductility and toughness, and one or two selected from the group consisting of Ca and REM may be further added in the following range in addition to the above components.

Ca: 0.0002 to 0.005 mass%

Ca has an effect of improving ductility and toughness by morphological control of inclusions, and has an effect of improving corrosion resistance in a coating state. These features are added upwards for the purpose of adding. However, when the addition is less than 0.0002% by mass, there is no effect. On the other hand, when it is added in an amount exceeding 0.005% by mass, the toughness is lowered. Therefore, in the case of addition, it is preferably in the range of 0.0002 to 0.005 mass%, and more preferably in the range of 0.001 to 0.005 mass%, and more preferably in the range of 0.001 to 0.003 mass% from the viewpoint of improvement in corrosion resistance. .

REM: 0.0005 to 0.015 mass%

REM (Rare Earth Metal) means a rare earth element having an atomic order of 57 to 71. In general, it may be added using a mixture containing a mixture of La, Ce, Pr, Nd or the like. This REM controls the morphology of the inclusions and has the effect of increasing the ductility and toughness. However, if it is less than 0.0005 mass%, it does not have an effect. On the other hand, when it adds more than 0.015 mass%, toughness will fall. Therefore, in the case of the addition, it is preferably in the range of 0.0005 to 0.015 mass%, and further preferably in the range of 0.005 to 0.015 mass%, and in the range of 0.005 to 0.01 mass%, from the viewpoint of improving the corrosion resistance upward. Better.

Further, the remainder of the steel material of the present invention other than the above components is formed of Fe and unavoidable impurities. However, the steel material of the present invention does not exclude other elements if it does not impair the effects of the present invention, and for example, if it is O, it may contain 0.008% or less.

Next, the microstructure of the steel material for a crude oil tank of the present invention will be described.

In the steel material of the present invention, the fine structure at a position 1/4 of the thickness t is formed by a composite structure composed of ferrite, pearlite and bainite transformation. It is preferable to contain a wave of iron in an area ratio of 2 to 20%.

In general, in the method of controlling the strength of steel having the same composition, although various kinds of tissue control methods are used, among them, water cooling after hot rolling is one of the most commonly used methods. When the steel material having the chemical composition of the present invention is subjected to slow cooling after hot rolling, a fine structure composed of ferrite iron and ferrite may be formed, but water cooling is performed. In the case of rapid cooling treatment, the above-mentioned wave iron will change into a tougher iron structure with higher strength. In particular, the higher the cooling rate, the lower the cooling stop temperature, and the higher the ratio of the toughened iron structure, eventually becoming the two-phase structure of the ferrite iron and the toughened iron.

However, the toughened iron structure has the property of accelerating corrosion in an acid environment due to the small dispersed structure of the cementite. Therefore, it is possible to improve the corrosion resistance by allowing a certain amount of the Borne iron structure to remain and suppressing the fine dispersion of the ferritic carbon. However, it is clear that the area ratio of the wave iron is 2% or more because the corrosion resistance caused by the residual iron is increased upward. On the other hand, if the area ratio of the Borne iron structure exceeds 20%, the toughness may be lowered. Therefore, in the steel material of the present invention, in order to obtain more excellent corrosion resistance, the area ratio of the ferrite in the fine structure is preferably controlled in the range of 2 to 20%. Here, the reason why the measurement position of the fine structure is a position of 1/4 of the thickness of the steel material is that, in a thick steel material such as a shipbuilding thickness, the position of 1/4 of the thickness of the steel plate can represent the full thickness. Moreover, even if the machined surface of the steel is exposed to a corrosive environment, it can satisfy the comprehensive corrosion resistance from the surface layer of the steel to the center of the plate thickness. Further, the corrosion-resistant steel material for a crude oil oil tank of the present invention having a fine structure has a tensile strength of 315 MPa or more and a tensile strength of 440 MPa or more. Further, if a predetermined strength is obtained, the toughened iron structure does not matter if it does not exist.

Next, a method for producing a steel material for a crude oil sump according to the present invention will be described.

The steel material of the present invention can be produced by the same method as the conventional steel material by controlling the component composition to the above-described range of the present invention. For example, a secondary refining furnace such as a steel converter, an electric furnace, or a vacuum degassing equipment, etc., except for adjusting the main five elements of C, Si, Mn, P, and S The contents of Cu, Cr, Sn, and Mo are also adjusted to the extent of the present invention, and other alloying elements are added as needed to melt the steel suitable for the present invention. Then, the molten steel is made into a steel slab (steel sheet) by a continuous casting method or a block-blocking calendering method, and the steel sheet is directly reheated after being cooled or cooled. Perform hot intercalation.

From the viewpoint of ensuring corrosion resistance and mechanical properties, it is necessary to select a suitable rolling temperature and a reduction ratio and control the fine structure. Specifically, it must be adjusted to After the steel material having the composition of the above-mentioned suitable range is heated to 1000 to 1350 ° C, the finishing temperature is 750 ° C or higher, and the inter-heat rolling is performed, and the cooling is performed at 2 ° C /sec or more to 650 ° C or less and 450 ° C or more. The cooling stops the temperature.

Slab heating temperature: 1000～1350°C

When the heating temperature is lower than 1000 ° C, the deformation resistance becomes large, and the inter-heat rolling is difficult. On the other hand, if the heating exceeds 1350 ° C, it will cause the occurrence of surface marks, and the scale loss or the fuel basic unit will increase. It is preferably in the range of 1100 to 1300 °C.

Hot roll finishing temperature: 750 ° C or higher

The temperature after calendering is necessary to be 750 ° C or higher. If it is lower than 750 ° C, the waiting time until the steel reaches a predetermined rolling temperature occurs, the rolling efficiency is lowered, or the rolling force is increased due to an increase in deformation resistance. It is difficult to carry out calendering.

Cooling rate after hot rolling: 2 ° C / sec or more, cooling stop temperature: 650 ° C or less, 450 ° C or more

It is necessary to cool at a cooling rate of 2 ° C /sec or more after cooling between heat. If it is less than 2 ° C / sec, the ferrite iron will be coarsened and the stress will be lowered. On the other hand, the upper limit of the cooling rate is not particularly limited, and may be 80 ° C / sec or less which is generally obtained by water cooling.

Further, the cooling stop temperature is required to be 650 ° C or lower and 450 ° C or higher. If it exceeds 650 ° C, the ferrite iron will coarsen and the stress will decrease. On the other hand, if it is lower than 450 ° C, the fraction of the Borne iron will be less than 2%.

In general, a steel material used in a crude oil tank of a tanker or the like is a coating material by applying a primer containing a metal Zn or a Zn compound (hereinafter collectively referred to as "zinc primer". ")) to improve the local corrosion resistance and general corrosion resistance. These steels are coated with a zinc-rich primer after being subjected to shot blasting on the surface. Depending on the surface state of the steel plate, etc., the substrate may not be completely covered, in order to completely To cover the entire surface, it is necessary to have a coating film thickness of a certain amount or more (for example, 15 μm or more).

In this case, the steel material for a crude oil sump of the present invention produced by the above method using the steel material having the above-described composition is characterized in that it has excellent corrosion resistance (corrosion resistance) not only in the uncoated state. It is resistant to local corrosiveness and is excellent in corrosion resistance after coating. In particular, the steel material for a crude oil sump of the present invention is obtained by converting a coating amount of a primer containing a metal Zn or a Zn compound into a Zn content of 1.0 g/m ² or more, thereby achieving local corrosion resistance and comprehensive resistance. Corrosiveness is increased upwards. In addition, when it is 2.5 g/m ² or more, more excellent local corrosion resistance and general corrosion resistance can be obtained. Further, from the viewpoint of local corrosion resistance and general corrosion resistance, although the upper limit of the coating amount of the zinc-rich primer is not set, if the coating film of the zinc-rich primer becomes thick, the cutting property or the weldability is not obtained. The thickness of the upper limit should preferably be 100 μm.

The relationship between the coating thickness of the zinc-rich primer and the Zn content of the steel surface depends on the Zn content in the zinc-rich primer, but in general, if the average coating thickness is 15 μm or more, it is covered. The entire surface of the steel material can be ensured to be converted into a coating amount of Zn content of 1.0 g/m ² or more without considering the type of zinc-rich primer.

In addition, the Zn content of the surface of the steel sheet can be obtained by, for example, cutting a plurality of small pieces (for example, 10 pieces) of 30 mm square from the steel material, completely dissolving and recovering the coating film or the rust layer of the surface, and analyzing the amount of Zn contained therein. Seek.

[Example 1]

The steel having the composition shown in Tables 1-1 to 1-4 is melted using a converter or the like, and a steel preform having a thickness of 200 mm is formed by a continuous casting method, and after heating the steel embryos to 1200 ° C, The post-rolling was performed by calendering at a temperature of 800 ° C to be rolled to a thickness of 25 mm, and then cooled to 580 ° C at a cooling rate of 30 ° C / sec to produce steel sheets No. 1 to 35.

In addition, the area ratio of the pulverized iron was measured for the fine structure in the 1/4 position of the steel plate observation plate thickness, and all of the steel sheets were confirmed, and the area ratio of the pulverized iron in the fine structure was 2% or more.

Further, in the steels of Nos. 1 and 8 of Table 1, the steel sheets having different area ratios of the ferro-conducting iron in the fine structure were produced by changing the cooling rate and the cooling stop temperature after the hot rolling.

Then, a test piece having a length of 50 mm, a width of 50 mm, and a thickness of 5 mm was taken out as a test surface at a position of 1/4 of the thickness of each of the obtained steel sheets as described above, and subjected to bead blasting on the surface thereof. The test piece in the uncoated state after direct bead blasting, and the three types of test pieces each having a zinc-rich primer having a thickness of 5 to 10 μm, 15 to 25 μm, and 50 to 70 μm, respectively, have four surface states. Corrosion test piece. Then, on the test surface of the test piece of 50 mm × 50 mm, a portion of the center of the local corrosion origin of 5 mm Φ was left, and the sludge containing the crude oil component actually taken out by the oil tanker was uniformly applied. Further, in terms of the content per unit area (coating amount) of Zn, if the coating state is uniform, it is proportional to the thickness of the zinc-rich primer, and when the thickness of the zinc-rich primer is 15 μm, generally, regardless of the rich The type of the zinc primer can be ensured to be converted into a Zn coating amount of 1.0 g/m ² or more.

Next, the test piece was supplied to the test liquid of the test apparatus of the structure shown in Fig. 1 and subjected to a partial corrosion test for 1 month. The test device is composed of a double structure of a corrosion test bath 2 and a constant-temperature bath 3. In the corrosion test tank 2, the injection can be generated and generated in the actual crude oil tank bottom plate. The test solution 6 which partially corrodes the same local corrosion. This test liquid 6 is a mother liquid containing 10 mass% NaCl aqueous solution containing 5000 mass ppm of sulfate ions, and is introduced into the mother liquid to adjust to CO ₂ : 13 vol% + O ₂ : 5 vol% O ₂ + SO ₂ : 0.01 vol %+H ₂ S: a solution of a mixed gas of a concentration ratio of 0.3 vol% and dissolved therein. Further, the mixed gas control gas (adjustable gas) portion of the residue 4 is based inert N ₂ gas (inert nitrogen gas). In the above test apparatus, in order to continuously supply the mixed gas 4, the test liquid 6 is constantly stirred. Further, the temperature of the test liquid 6 was adjusted by the temperature of the water 7 placed in the constant temperature bath 3, and was maintained at 40 °C.

After the end of the above corrosion test, the rust generated on the surface of the test piece was removed, and the depth of the localized corrosion occurred was measured by a depth meter while visually observing the corrosion configuration, and evaluated on the basis of the following criteria. Resistant to local corrosiveness.

<Evaluation of local corrosion resistance>

AA ◎: no local corrosion occurred

A○: The depth of local corrosion is less than 0.5mm

B△: the depth of local corrosion is above 0.5mm and less than 1mm

C×: the depth of local corrosion is 1mm or more

The results of the above partial corrosion test are shown in Tables 2 and 3. As can be seen from Table 2, the steel sheets of the invention examples of Nos. 1 to 21 which are suitable for the present invention are coated with a zinc-rich primer, and the evaluation of local corrosion resistance shows AA ◎ or A ○, even in the absence of coating. When local corrosion occurs in the state, the maximum depth is also controlled to be less than 0.5 mm, showing good local corrosion resistance. In particular, those coated with a zinc-rich primer having a thickness of 15 μm or more means that the zinc-rich primer has a uniform coating state, and the Zn content is 1.0 g/m ² or more, and those of No. 3 to 21 are AA ◎. From this, it was confirmed that the local corrosion resistance was further increased upward by the coating of the zinc-rich primer.

On the other hand, in the steel sheets of Nos. 22 to 35 which do not satisfy the conditions of the present invention, it is intended that the content of P, S, and Mo exceeds at least one of the contents of Cu, Cr, and Sn which is lower than the range of the present invention. In the scope of the present invention, or any steel sheet having an index of corrosion resistance A1 to A3 exceeding 0, not only the case where the zinc-rich primer is not applied, but also the evaluation of the local corrosion resistance even in the case of coating It is C × or B △. That is, the steel sheet of the comparative example is not only poor in local corrosion resistance in the uncoated state, but even in the case where the zinc-rich primer is applied, the local corrosion resistance is only slightly improved.

In addition, Table 3 shows the results of the local corrosion resistance in the uncoated state, which was carried out in the same manner as described above, using a steel sheet in which the area ratio of the iron in the fine structure was changed. As can be seen from Table 3, it is confirmed that the steel sheet having a fine structure of 2% or more in area ratio is resistant to the steel sheet having a fine structure composed of the toughened iron containing no iron. The tendency to increase local corrosivity.

[Embodiment 2]

A rectangular test piece having a length of 50 mm, a width of 25 mm, and a thickness of 4 mm was taken out at a position of 1/4 of the thickness of the steel sheets of Nos. 1 to 35 obtained in Example 1, and the surface was subjected to bead blasting, and Examples 1 In the same manner, a test material in which the direct-bead rust-removing and non-coating state is produced, and the thickness of each of the zinc-rich primers (the ratio of the area per unit area of Zn) is 5 to 10 μm and 15 to 25 μm. And a test piece having three kinds of surface states, such as a test piece of three types, such as 50 to 70 μm, in total. Further, in the test piece coated with the zinc-rich primer, in order to accelerate the corrosion, an X-shaped cut of the surface of the steel material was performed on the surface to be tested, thereby simulating the damaged portion. Further, the film damage at this time was 1.0% in terms of area ratio.

Next, the test piece was supplied to a comprehensive corrosion test using the test apparatus shown in Fig. 2 which simulates the corrosive environment in the crude oil tank. This corrosion test apparatus is composed of a corrosion test tank 12 and a temperature control plate 13, and in the corrosion test tank 12, in order to maintain a saturated vapor pressure, water 16 can be injected and the temperature is maintained at 30 °C. In addition, in the interior of the corrosion test tank, in order to simulate the corrosion environment in the crude oil tank, the saturated steam pressure (dew point: 30 ° C) can be filled with CO ₂ : 13 vol%, O ₂ : 5 vol%, SO ₂ : 0.01 Vol%, H ₂ S: 0.01 vol%, and the residual portion is a mixed gas of N ₂ . The test piece is placed under the temperature control plate disposed above the corrosion test tank, and is heated by a heater and a cooling device at 25 ° C × 1 hour / 50 ° C × 5 hours, and the temperature is raised and lowered. One cycle (8 hours), continuous for 28 days, simulates the overall corrosion caused by condensation of dew. In addition, in order to impart sulfate ions and chloride ions to the surface (test surface) of the test piece, it is possible to apply a mixture of sodium sulfate and sodium chloride equivalent to 1000 ppm by mass of sulfate ions and 10,000 ppm by mass of chloride ions. After 500 μL of the aqueous solution was dried, it was supplied to the test. Further, after the start of the test, sulfate ions and chloride ions were supplied for a whole week.

After the end of the above corrosion test, the test piece without the coating state is obtained by removing the rust generated on the surface of the test piece and determining the thickness reduction due to corrosion from the change before and after the test. The corrosion resistance was 1 year and the corrosion resistance was evaluated on the basis of the following criteria.

<Evaluation of general corrosion resistance of uncoated materials>

A○: Corrosion speed is less than 0.2mm/year

B△: Corrosion rate is 0.2mm/year or more and less than 0.8mm/year

C×: corrosion rate is 0.8mm/year or more

Moreover, the area ratio of the rust generated on the surface of each test piece and the coating film was measured about the primer coating material, and the general corrosion resistance was evaluated based on the following criteria.

<Evaluation of general corrosion resistance of primer coating materials>

A○: rust area ratio is less than 25%

B△: the rust area ratio is 25% or more and less than 50%

C×: rust area ratio is 50% or more

The results of the above comprehensive corrosion test are shown in Tables 4 and 5. As can be seen from Table 4, the steel sheets according to the invention examples of Nos. 1 to 21 of the present invention are evaluated as A○ in the total corrosion resistance of the non-coated materials, and coated with a zinc-rich primer at the same time. The general corrosion resistance is also all A○, that is, it can be confirmed that the steel sheet of the invention has good corrosion resistance not only in the uncoated state, but also coated by the zinc-rich primer, and further Has good resistance to general corrosion.

On the other hand, in the steel sheets No. 22 to 35 of the comparative example, not only when the zinc-rich primer was not applied, but also when the coating was applied, the evaluation of the general corrosion resistance was only C× or B Δ. In one case, its resistance to general corrosion is poor.

Further, Table 5 shows that the steel sheet having the change in the area ratio of the ferrite in the fine structure obtained in Example 1 was used, and the overall corrosion test in the uncoated state was carried out, and the comprehensive evaluation was performed on the same basis as above. The result of corrosiveness. As can be seen from Table 5, the steel sheet having an area ratio of 2% or more of the ferrite has a tendency to improve the overall corrosion resistance similarly to the local corrosion resistance.

[Industrial availability]

The technology of the present invention is not limited to oil tanks for crude oil tankers or steel oil tanks for oil tanks for transporting or storing crude oil, and for other fields of steel used in similar corrosive environments, including primer coating or The general coating situation is also quite applicable.

[Table 1-1]

[Table 1-2]

[Table 1-3]

[Table 1-4]

[Table 2]

<Evaluation of local corrosion resistance>

AA ◎: no local corrosion occurred

A○: The depth of local corrosion is less than 0.5mm

B△: the depth of local corrosion is above 0.5mm and less than 1mm

C×: the depth of local corrosion is 1mm or more

[table 3]

<Evaluation of local corrosion resistance>

AA ◎: no local corrosion occurred

A○: The depth of local corrosion is less than 0.5mm

B△: The depth of local corrosion is above 0.5mm and less than 1.0mm

C×: the depth of local corrosion is 1.0mm or more

[Table 4]

<Evaluation of general corrosion resistance of uncoated materials>

A○: Corrosion speed is less than 0.2mm/year

B△: Corrosion rate is 0.2mm/year or more and less than 0.8mm/year

C×: corrosion rate is 0.8mm/year or more

<Evaluation of general corrosion resistance of primer coating materials>

A○: rust area ratio is less than 25%

B△: the rust area ratio is 25% or more and less than 50%

C×: rust area ratio is 50% or more

[table 5]

<Evaluation of general corrosion resistance of uncoated materials>

A○: Corrosion speed is less than 0.2mm/year

B△: Corrosion rate is 0.2mm/year or more and less than 0.8mm/year

C×: corrosion rate is 0.8mm/year or more

1,11. . . Test piece

2, 12. . . Corrosion test tank

3. . . Constant-temperature bath

4, 14. . . Introduced gas

5, 15. . . Exhaust gas

6,16. . . Test liquid

7. . . water

13: Temperature control board

Fig. 1 is a view showing a local corrosion test apparatus.

Fig. 2 is a view showing a general corrosion test apparatus.

Claims (10)

A corrosion-resistant steel material for a crude oil oil tank, characterized by containing C: 0.001 to 0.16 mass%, Si: 1.5 mass% or less, Mn: 0.1 to 2.5 mass%, P: 0.025 mass% or less, S: 0.01 mass% or less, and Al : 0.005 to 0.1% by mass, N: 0.001 to 0.008% by mass, Cu: 0.008 to 0.35 mass%, Cr: more than 0.1% by mass and 0.5% by mass or less, Sn: 0.005 to 0.3% by mass, and Mo: 0.01 mass % or less, and a remainder of Fe and inevitable impurities into, wherein the following formula (1) as defined hereinafter of A1 is 0; referred A1 = 28 × [C] + 2000 × [P] 2 +27000 ×[S] ² +0.0083×(1/[Cu])+0.027×(1/[Cr])+95×[Mo]+0.00098×(1/[Sn])-6‧‧‧(1) Here, the content (% by mass) of each element of [C], [P], [S], [Cu], [Cr], [Mo], and [Sn] in the above formula. The requested item of a crude oil tank with a corrosion-resistant steel, wherein, in addition to the component composition further contains Ni: 0.005 ~ 0.4% by mass, and is represented by the following formula (2) as defined hereinafter of A2 is 0; referred A2 = 28×[C]+2000×[P] ² +27000×[S] ² +0.0083×(1/[Cu])+2×[Ni]+0.027×(1/[Cr])+95×[Mo ]+0.00098×(1/[Sn])-6‧‧‧(2) Here, [C], [P], [S], [Cu], [Ni], [Cr], The content (% by mass) of each element of [Mo] and [Sn]. The corrosion-resistant steel material for a crude oil tank of claim 1 or 2, which further comprises one or two selected from the group consisting of W: 0.001 to 0.5% by mass and Sb: 0.005 to 0.3% by mass, in addition to the above-mentioned component composition, and defined the following equation (3) A3 is 0 or less; referred A3 = 28 × [C] + 2000 × [P] 2 + 27000 × [S] 2 + 0.0083 × (1 / [Cu]) + 2×[Ni]+0.027×(1/[Cr])+95×[Mo]+0.00098×(1/[Sn])+0.0019×(1/([Sb]+[W]))-6.5‧ ‧ (3) Here, [C], [P], [S], [Cu], [Ni], [Cr], [Mo], [Sn], [Sb], and [W] in the above formula 〕 is the content of each element (% by mass). The corrosion-resistant steel material for a crude oil tank of claim 1 or 2, further comprising Nb: 0.002 to 0.1% by mass, V: 0.002 to 0.1% by mass, Ti: 0.001 to 0.1% by mass, in addition to the above component composition. And B: one or more selected from the group consisting of 0.01% by mass or less. The corrosion-resistant steel material for a crude oil oil tank according to claim 1 or 2, which further comprises one or two selected from the group consisting of Ca: 0.0002 to 0.005 mass% and REM: 0.0005 to 0.015 mass%, in addition to the above-described component composition. The corrosion-resistant steel material for a crude oil tank of claim 1 or 2, wherein the micro-structure of the steel sheet having a thickness of 1/4 is a pearlite having an area ratio of 2 to 20%. The corrosion-resistant steel material for a crude oil tank of claim 1 or 2 is formed by forming a coating film containing a metal Zn or Zn compound on the surface of the steel material. The corrosion-resistant steel material for a crude oil tank of claim 7, wherein the Zn content in the coating film is 1.0 g/m ² or more. A method for producing a corrosion-resistant steel material for a crude oil oil tank, characterized in that the steel material having the composition of any one of claims 1 to 5 is heated to 1000 to 1350 ° C, and the post-rolling temperature is 750 ° C or higher. The inter-heat rolling is performed, and is cooled to a cooling stop temperature of 650 ° C or lower and 450 ° C or higher at a cooling rate of 2 ° C /sec or more. A crude oil sump characterized by the use of the steel of any one of claims 1-8.