KR20160049023A - Corrosion resistant steel for crude oil tank, manufacturing method therefor, and crude oil tank - Google Patents

Abstract

Provided is a steel material for a crude oil tank which is excellent in corrosion resistance on the inner surface and on corrosion resistance and has excellent corrosion resistance even when it is used in the presence of Zn on the surface of the steel material. Specifically, it is preferable that the steel material contains, as mass%, 0.001 to 0.16% of C, 1.5% or less of Si, 0.1 to 2.5% of Mn, 0.025% or less of P, 0.01% or less of S, 0.005 to 0.1% Wherein the content of Mo is not more than 0.01% and the value of A1 defined by the following formula is not more than 0: 0.001 to 0.008%, Cu: 0.008 to 0.35%, Cr: more than 0.1% to 0.5% Corrosion resistant steel.
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A1 = 28 × [C] + 2000 × [P] 2 + 27000 × [S] 2 + 0.0083 × (1 / [Cu]) + 0.027 × (1 / [Cr]) + 95 × [Mo] + 0.00098 × (1 / [Sn]) - 6

Description

TECHNICAL FIELD [0001] The present invention relates to a corrosion resistant steel material for a crude oil tank, a method of manufacturing the same, and a crude oil tank,

The present invention relates to a tank for transporting or storing an oil tank or crude oil of a crude oil tanker (hereinafter collectively referred to as a crude oil tank) (Hereinafter referred to as " general ") steel products which are preferred for use in a crude oil tank, such as a top part, a sidewall part and a bottom part of a crude oil tank corrosion and a local corrosion occurring on a bottom plate of a crude oil tank.

Further, the steel material for a crude oil tank of the present invention includes a thick steel plate, a thin steel sheet and a shaped steel.

It is known that overall surface corrosion occurs in the inner surface of the crude oil tank of the tanker, particularly in the steel used for the back side of the upper deck and the upper part of the side wall. The cause of the whole surface corrosion is,

(1) dew drop and alternate wetting and drying on the steel sheet surface due to day and night temperature difference,

(2) An inert gas (about 5 vol% of O ₂ , about 13 vol% of CO ₂ , about 0.01 vol% of SO ₂ , and N ₂ of the remainder) enclosed in the crude oil tank for explosion protection Dissolving the number of dew condensation of O ₂ , CO ₂ , and SO _{2 in} the exhaust gas of the boiler or the engine which is the exhaust gas of the engine,

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

(4) Residual of salt water used for cleaning of crude oil tank

And the like. These can be guessed from the fact that in the actual dock inspection, the number of strong acidity dewatering, sulfate ion and chloride ion are detected .

In addition, H ₂ S is oxidized using iron rust produced by corrosion as a catalyst, so that elemental sulfur is generated in layer form in the ruthenium, and the corrosion products thereof are, So that it is deposited on the bottom of the crude oil tank. For this reason, in the inspection of the dock every 2.5 years, maintenance and repair of the upper part of the tank and recovery of the deposited material of the tank bottom are carried out at a high cost.

On the other hand, in the bottom plate of the crude oil tank of the tanker, a corrosion inhibition function of the crude oil itself or a protective coating derived from crude oil (hereinafter referred to as "oil coat") formed on the inner surface of the crude oil tank, It has been considered that corrosion does not occur in the steel material to be used. However, in a recent study, it has been found that bowl-shaped local corrosion (pitting corrosion) occurs in the steel of the tank bottom. As a cause of local corrosion,

(1) the presence of brine dissolved at a high concentration of salts such as sodium chloride,

(2) detachment of the oil coat by excess cleaning,

(3) high concentration of sulfide in crude oil,

(4) High concentration of O ₂ , CO ₂ and SO ₂ in the inert gas sealed for explosion-proof in the crude oil tank,

(5) involvement of microorganisms

Are all examples, all of which are not yet clear. In the analysis of the water remaining in the crude oil tank at the time of the actual poisoning inspection, chloride ions and sulfate ions at a high concentration are detected.

However, the most effective method for suppressing the whole surface corrosion or local corrosion is a method of severing the steel material from the corrosion environment by performing heavy coating on the surface of the steel material. However, the coating operation of the crude oil tank is enormous in its application area. Further, due to the deterioration of the coating film, it is necessary to coat again once in about 10 years, so that a lot of cost is incurred in inspection and painting. It has been pointed out that, in the damaged portion of the intermediate coat film, corrosion is rather promoted in the crude oil tank environment.

With respect to such corrosion problems, there have been proposed several corrosion resistant steels which have improved corrosion resistance of the steel itself and have corrosion resistance even in a crude oil environment.

For example, Patent Document 1 discloses a method of adding a proper amount of Si, Mn, P, and S and 0.05 to 3% Ni to a steel containing 0.01 to 0.3% of C by mass% There is disclosed a corrosion resistant steel for a cargo oil tank which is improved in resistance to whole surface corrosion and local corrosion by adding Mo, Cu, Cr, W, Ca, Ti, Nb, V and B.

Further, in a dry and wet repetitive environment including H ₂ S, when the content of Cr exceeds 0.05 mass%, deterioration of the ingot corrosion resistance and pitting resistance becomes significant, so that the content of Cr should be 0.05 mass% or less Lt; / RTI >

Patent Document 2 discloses that a steel containing 0.001 to 0.2% of C by mass contains Si, Mn, P, and S in an appropriate amount, 0.01 to 1.5% of Cu, 0.001 to 0.3% of Al, 0.001 By adding at least one of Mo: 0.01 to 0.2% or W: 0.01 to 0.5% in addition to 0.01 to 1.0% of Mo and further to produce corrosion products having solid corrosion resistance and local corrosion resistance, Resistant steel for use in crude oil tanks.

Patent Document 3 discloses that a steel containing 0.01 to 0.2% of C by mass contains Si, Mn and P in an appropriate amount, 0.01 to 2% of Ni, 0.05 to 2% of Cu, 1% of Cr, Al, N, and O are added, and the amount of addition of Cu, Ni, and W is further defined by a parameter formula to increase corrosion resistance and local corrosion of the cargo oil tank .

Patent Document 4 discloses that a steel containing 0.01 to 0.2% of C by mass contains Si, Mn, P, Cr and Al in an appropriate amount, 0.01 to 1% of Ni, 0.05 to 2% of Cu, The present invention relates to a method for producing a steel sheet having improved corrosion resistance and local corrosion resistance by adding 0.01 to 0.2% of Sn and further optionally adding Mo, W, Ti, Zr, Sb, Ca, Mg, Nb, A corrosion resistant steel for a tank is disclosed.

(Patent Document 1) Japanese Laid-Open Patent Publication No. 2003-082435

(Patent Document 2) Japanese Laid-Open Patent Publication No. 2004-204344

(Patent Document 3) Japanese Laid-Open Patent Publication No. 2005-325439

(Patent Document 4) Japanese Laid-Open Patent Publication No. 2007-270196

However, when the corrosion resistant steel disclosed in the above Patent Documents 1 to 4 is applied to a crude oil tank, resistance to the overall surface corrosion (hereinafter referred to as " internal surface corrosion resistance ") when used in the upper portion of the crude oil tank, Resistance to local corrosion (hereinafter referred to as " local corrosion resistance ") when used in a tank bottom plate is not necessarily sufficient.

This is insufficient for simply corrosion resistance test simulating the respective corrosion environments in order to develop a corrosion resistant steel against the whole surface corrosion of the upper deck of the crude oil tank and the local corrosion of the bottom plate respectively have. That being said, corrosion tests in laboratories include elements of accelerating test, so that some corrosion factors are omitted or the actual environment is reproduced accurately In particular, in the development of corrosion resistant steels for crude oil tanks, it is necessary to add chloride ions and sulfate ions in the test environment.

The inventions described in Patent Documents 3 and 4 are designed so that when crude oil is not loaded, balance of corrosion resistance in a crude oil corrosion environment and a sea water corrosion environment is considered in consideration of loading of seawater in a ballast tank outside the cargo oil tank It is a target technology. However, with respect to seawater corrosion environments, these techniques pay attention to the corrosion resistance of the steel itself as the corrosion resistance after deterioration of the anticorrosive coating film on the outer surface of the cargo oil tank. However, due to the synergistic effect of Zn in the zinc- No consideration is given to the improvement of the corrosion resistance in a state in which a coated film is present on the surface of the steel material, that is, the so-called corrosion resistance after painting.

However, in Patent Literatures 3 and 4, it is very important and effective to improve the corrosion resistance after painting, which is not considered in order to improve the longevity of the corrosion resistant steel material for a crude oil tanker. It is a fact not to do.

The present invention has been developed in order to solve the above problems, and an object of the present invention is to provide an excellent inner surface corrosion resistance when used for an inner surface of a crude oil tank, particularly an upper deck and a side plate, A steel material for a crude oil tank exhibiting remarkably excellent internal surface corrosion resistance and local corrosion resistance when used in a state where Zn is present on the surface of a steel material and a method for producing the steel material, .

In order to achieve the above object, the inventors of the present invention first conducted a corrosion test in which factors related to the entire surface corrosion in the crude oil tank were extracted and the factors were combined. As a result, we succeeded in reproducing the whole surface corrosion occurring in the crude oil tank, and obtained the following knowledge about the dominant factor of the whole surface corrosion and the corrosion mechanism.

The inert gas enclosed in the crude oil tank for explosion-proof includes water vapor. Therefore, the temperature difference between day and night during the voyage causes condensation on the surface of the steel material on the inner wall of the tank. The condensation water contains CO ₂ (carbon dioxide), O ₂ (oxygen), SO ₂ (sulfur dioxide), and H ₂ S (hydrogen sulfide), which are volatile components from crude oil, To produce an acidic solution. It is also necessary to consider chloride ions introduced by the sea water washing of the crude oil tank. The corrosive acid solution in which these components are dissolved is concentrated in the course of the rise of the steel sheet temperature, and causes the whole surface corrosion on the surface of the steel sheet. Further, since the green oxide formed on the surface of the steel sheet serves as a catalyst and S (sulfur) is precipitated from H ₂ S and the green oxide layer in which the sulfur oxides and sulfur are layered is formed, the green oxide layer on the surface of the steel sheet becomes soft and has no protection, Corrosion continues.

Thus, the inventors investigated the influence of various alloying elements on the whole surface corrosion of the steel sheet surface in the presence of dewatered water containing sulfate ions and chloride ions. As a result, it is found that the addition of Cu, Cr, and Sn improves the corrosion resistance of the ingot surface by densifying the green layer on the surface of the steel sheet formed in the environment used as the steel material for the crude oil tank, And it is confirmed that the corrosion resistance of the inner surface is improved. That is, it has been found that a steel for a crude oil tank excellent in corrosion resistance on the ingot surface is obtained by additionally adding W and Sb in addition to Cu, Cr and Sn in an appropriate amount.

Next, the inventors extracted factors involved in local corrosion of the bottom of the crude oil tank, and conducted a corrosion test in which these factors were combined. As a result, we succeeded in reproducing the local corrosion caused by the bottom of the crude oil tank as well as the whole surface corrosion, and obtained the following knowledge about the dominant factor of the local corrosion and the corrosion mechanism.

O ₂ and H ₂ S contained in the solution staying on the bottom plate act as a main dominant factor in the local corrosion of the bowl type which occurs in the actual crude oil tank bottom plate. Specifically, O ₂ and H ₂ S coexist (In an aqueous solution saturated with a gas having an O ₂ concentration of 2 to 8 vol% and an H ₂ S concentration of 0.1 to 5 vol%) in an environment in which both the O ₂ concentration and the H ₂ S concentration exist Local corrosion occurs. In short, under an environment of low O ₂ concentration and low H ₂ S concentration, H ₂ S is oxidized to precipitate solid S. The precipitated solid S forms a local cell between itself and the crude tank bottom plate, causing local corrosion on the surface of the steel material. This local corrosion is further promoted and grown under acidic conditions in which chloride and sulfate ions are present.

Thus, the inventors investigated the influence of various alloying elements on occurrence of local corrosion under the environment of the low O ₂ concentration and the low H ₂ S concentration. As a result, it is found that the addition of W improves the local corrosion resistance by densifying the green layer on the surface of the steel sheet formed in the environment used as the steel for the crude oil tank, and the addition of Sn and Sb, To improve the local corrosion resistance. Further, in an acidic corrosion environment in which both chloride ion and sulfate ion exist at the same time, it was confirmed that the addition of Mo deteriorates the corrosion resistance. That is, in addition to the addition of W, by adding an appropriate amount of Sn and Sb, and by restricting the Mo content, a steel material for a crude oil tank excellent in local corrosion resistance is obtained.

From the above findings, it has been found that by appropriately adjusting the contents of Cu, Cr, Sn, W, and Sb, it is possible to obtain an excellent corrosion resistance on the inner surface of the crude oil tank, That is, it was found that a steel material for a crude oil tank excellent in corrosion resistance can be obtained even if it is used in any part of the crude oil tank.

Further, the inventors of the present invention have found out that the steel materials obtained by appropriately adjusting the contents of Cu, Cr, Sn, W and Sb have excellent corrosion resistance even in the unpatterned state. In the case of using a coating containing metal Zn or Zn compounds on the surface, It has been found that the life span can be largely extended and the corrosion resistance on the inner surface is improved and the local corrosion resistance is remarkably improved. In the steel material of the present invention, the influence of the microstructure of the steel on the corrosion resistance was investigated. As a result, it was found that the corrosion resistance can be improved by producing 2% or more of perlite as the area ratio.

The present invention has been further made based on the above findings.

That is, the present invention relates to a steel sheet comprising: 0.001 to 0.16 mass% of C, 1.5 mass% or less of Si, 0.1 to 2.5 mass% of Mn, 0.025 mass% of P or less, 0.01 mass% or less of S, 0.001 to 0.008 mass% of N, 0.008 to 0.35 mass% of Cu, more than 0.1 mass% and 0.5 mass% or more of Cr and 0.005 to 0.3 mass% of Mo, 0.01 mass% or less of Mo, And inevitable impurities, and is represented by the following formula (1);

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

The content (mass%) of each of the elements [C], [P], [S], [Cu], [Cr], [Mo]

Is a value of 0 or less.

The corrosion resistant steel material for a crude oil tank of the present invention further contains 0.005 to 0.4 mass% of Ni in addition to the above-mentioned composition,

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

The content (mass%) of each of the elements [C], [P], [S], [Cu], [Ni], [Cr], [Mo]

Is 0 or less.

The corrosion resistant steel material for a crude oil tank of the present invention may further contain one or more kinds selected from 0.001 to 0.5 mass% of W and 0.005 to 0.3 mass% of Sb,

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

[C], [P], [S], [Cu], [Ni], [Cr], [Mo], [Sn], [Sb] and [W] of the respective elements Content (mass%)

The value of A3 defined in " 0 "

The corrosion resistant steel material for a crude oil tank of the present invention may further contain, in addition to the above-mentioned composition, at least one selected from the group consisting of 0.002 to 0.1 mass% of Nb, 0.002 to 0.1 mass% of V, 0.001 to 0.1 mass% of Ti and 0.01 mass% Or a mixture of two or more species.

Further, the corrosion resistant steel material for a crude oil tank of the present invention is characterized by further containing one or two selected from the group consisting of 0.0002 to 0.005 mass% of Ca and 0.0005 to 0.015 mass% of REM in addition to the above composition.

The corrosion resistant steel material for a crude oil tank of the present invention is characterized in that the microstructure at a position of 1/4 of the plate thickness of the steel contains 2 to 20% pearlite in area ratio.

The corrosion resistant steel material for a crude oil tank of the present invention is characterized in that a coating film containing metal Zn or Zn compound is formed on the surface of a steel material.

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

Further, the present invention is characterized in that the steel material having the above-mentioned composition is heated to 1000 to 1350 캜, hot-rolled at a rolling finish temperature of 750 캜 or higher, cooled at a cooling rate of 2 캜 / sec or more, And cooling the steel to a stopping temperature.

Further, the present invention is a crude oil tank characterized by using the above steel material.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a steel material which does not cause corrosion of the whole surface or local corrosion even at a portion of a crude oil tank such as a tank for transporting or storing oil tanks or crude oil, It has a remarkable effect.

1 is a view for explaining a local corrosion test apparatus.
2 is a view for explaining an entire surface corrosion test apparatus.

The reason why the composition of the steel material for a crude oil tank of the present invention is limited to the above range will be described.

C: 0.001 to 0.16 mass%

C is an element for increasing the strength of the steel, and in the present invention, it is necessary to contain 0.001 mass% or more in order to obtain a desired strength. On the other hand, C not only deteriorates the corrosion resistance with an increase in the content, but additions exceeding 0.16 mass% deteriorate the weldability and the toughness of the welded heat affected zone. Therefore, C is in the range of 0.001 to 0.16 mass%. From the viewpoint of further improving the strength and toughness, a range of 0.01 to 0.15 mass% is preferable. And more preferably 0.05 to 0.15 mass%.

Si: 1.5 mass% or less

Si acts as a deoxidizing agent and increases the strength, and addition of more than 1.5 mass% decreases the toughness of the steel. Therefore, in the present invention, Si is limited to a range of 1.5 mass% or less. From the viewpoint of improving corrosion resistance in an acidic environment, Si is preferably added in a range of 0.2 to 1.5 mass%, more preferably in a range of 0.3 to 1.5 mass% mass% is more preferable.

Mn: 0.1 to 2.5 mass%

Mn is an element for increasing the strength of the steel material, and in the present invention, it is necessary to contain 0.1 mass% or more of Mn in order to obtain a desired strength. On the other hand, the addition of more than 2.5 mass% deteriorates the toughness and weldability of steel, and promotes segregation, resulting in nonuniformity of steel sheet composition. Therefore, the Mn content is in the range of 0.1 to 2.5 mass%. From the viewpoint of suppressing formation of inclusions that maintain high strength and deteriorate corrosion resistance, the range is preferably 0.5 to 1.6 mass%, and more preferably 0.8 to 1.4 mass%.

P: not more than 0.025 mass%

P is a harmful element which is segregated in grain boundaries to deteriorate the toughness of steel and deteriorate corrosion resistance, and is preferably reduced as much as possible. In particular, when the content is more than 0.025 mass%, the central segregation is promoted to cause irregularity in the composition of the steel sheet, and the toughness remarkably decreases, so that the content of P is 0.025 mass% or less. In addition, the reduction of P to less than 0.003 mass% leads to an increase in the production cost, so that the lower limit of P is preferably about 0.003 mass%, and the corrosion resistance of the inner surface in the acid environment is improved , It is preferable to set it to 0.010 mass% or less. Further, it is more preferably 0.009 mass% or less.

S: 0.01 mass% or less

S is a harmful element that forms a non-metal inclusion MnS, which is a starting point of corrosion and lowers local corrosion resistance and corrosion resistance on the inner surface, and is preferably reduced as much as possible. In particular, the content exceeding 0.01 mass% leads to a significant reduction in local corrosion resistance and corrosion resistance on the ingot surface, and therefore, in the present invention, the upper limit of S is set at 0.01 mass%. From the viewpoint of further improving the corrosion resistance, 0.0020 mass% or less is preferable. However, extreme reduction of S leads to an increase in the manufacturing cost, so that it is practically 0.0002 to 0.0020 mass%. Further, it is more preferably 0.0009 mass% or less.

Al: 0.005-0.1 mass%

Al is an element that acts as a deoxidizing agent, and in the present invention, it is necessary to contain Al in an amount of 0.005 mass% or more. On the other hand, when it is added in an amount exceeding 0.1 mass%, the toughness of the steel decreases. Therefore, the content of Al is in the range of 0.005 to 0.1 mass%. And preferably 0.01 to 0.05 mass%. And more preferably 0.02 to 0.04 mass%.

N: 0.001 to 0.008 mass%

N is required to be added in an amount of 0.001 mass% or more in order to improve the toughness of the steel and to improve the mechanical properties of the weld joint part. However, the addition of more than 0.008 mass% leads to an increase in solid solution N, and significantly decreases the toughness of the joint portion depending on the welding conditions. Therefore, N is in the range of 0.001 to 0.008 mass%. , Preferably 0.002 to 0.005 mass%, and more preferably 0.002 to 0.004 mass%.

Cu: 0.008 to 0.35 mass%

Cu forms an anticorrosion coat to inhibit corrosion of the entire surface, and in the present invention, it is an element which is required to be added. However, if it is less than 0.008 mass%, the above effect can not be obtained. On the other hand, when Cu is added in combination with Sn, the corrosion resistance on the inner surface is remarkably improved. When it is added in an amount exceeding 0.35 mass%, the hot workability is deteriorated and the composition is deteriorated. Therefore, the Cu content is in the range of 0.008 to 0.35 mass%. In addition, since the effect of Cu addition saturates as the addition amount increases, it is preferably in the range of 0.008 to 0.15 mass% in terms of cost-effectiveness. And more preferably in the range of 0.01 to 0.14 mass%.

Cr: more than 0.1 mass% and less than 0.5 mass%

Cr is an element which, together with Cu, forms a protective coating on the surface of a steel material and improves the corrosion resistance of the ingot surface in an acidic environment, in addition to enhancing the strength of the steel material. Particularly, in an acidic environment containing sulfate ions and chloride ions, Cr forms an oxide layer to cover the surface of the steel and has an effect of lowering the entire surface corrosion rate. In addition, since the green layer is densified together with Cu, Cr makes the Zn compound remain in the green layer for a long time even in the state in which zinc primer is coated, and contributes greatly to improvement in corrosion resistance, including corrosion resistance after painting. Further, since the addition amount of Cu can be suppressed by the effect of improving the corrosion resistance by the addition of Cr, there is an effect of alleviating the deterioration of hot workability occurring under coexistence of Cu and Sn. However, when the Cr content is less than 0.1 mass%, the above-mentioned effect can not be obtained. On the other hand, when the Cr content exceeds 0.5 mass%, the effect is saturated and the cost is increased and the weldability is deteriorated. Therefore, Cr is added in a range of more than 0.1 mass% to 0.5 mass% or less. And more preferably in the range of 0.11 to 0.3 mass%. And more preferably in the range of 0.12 to 0.2 mass%.

Sn: 0.005 to 0.3 mass%

Sn is formed by a complex effect with Cu or by a combined effect of Cu and W when W is added as will be described later to form a dense green layer to suppress the whole surface corrosion under an acidic environment, It has an action of suppressing corrosion, and in the present invention, it is an element which is required to be added. However, when the amount is less than 0.005 mass%, the above-mentioned addition effect is not obtained, while additions exceeding 0.3 mass% cause deterioration of hot workability and toughness. Therefore, Sn is set in a range of 0.005 to 0.3 mass%. And more preferably in the range of 0.02 to 0.1 mass%. And more preferably 0.03 to 0.09 mass%.

Mo: 0.01 mass% or less

Mo generally has the same action as W and is considered to be an element for improving corrosion resistance. However, the inventors have found that W forms insoluble salts in an acidic salt water environment, Mo forms soluble salts in an acidic salt water environment and does not exhibit a barrier effect. In particular, when the Mo content exceeds 0.01 mass% , The corrosion resistance in the acidic brine environment is deteriorated. Therefore, in the present invention, the content of Mo is limited to 0.01 mass% or less. Preferably 0.008 mass% or less, and more preferably 0.005 mass% or less.

The above elements are basic components of the steel material of the present invention. However, in order for the steel material of the present invention to have excellent corrosion resistance on the inner surface side and resistance to local corrosion, it is necessary that not only the above-mentioned components fall within the above composition range, but also the value of A1 defined by the following formula It needs to be done. Also preferably, the value of A1 is -1 or less.

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

The content (mass%) of each of the elements [C], [P], [S], [Cu], [Cr], [Mo]

The formula (1) is an empirical formula showing an index of corrosion resistance summarizing the influence of each element on the internal surface corrosion resistance and the internal corrosion resistance obtained in the corrosion test conducted in the present invention. When the value of A1 exceeds 0, It is impossible to secure either or both of the inner surface corrosion resistance and the local corrosion resistance. In the above formula (1), it is understood that, as to the effect on the corrosion resistance of each element, the elements in the primary and secondary terms decrease in the inner surface corrosion resistance and the inner corrosion resistance as the element is added, And the corrosion resistance of the inner surface is improved and the local corrosion resistance is improved as the number of the elements of the protons increases. In short, C and Mo are elements for reducing corrosion resistance, P and S are elements for decreasing corrosion resistance, which affects the square of the content, Cu, Cr and Sn are elements for improving corrosion resistance.

 The steel of the present invention may further contain Ni in addition to the above-mentioned basic components in the following range.

Ni: 0.005 to 0.4 mass%

Ni has a function of suppressing deterioration of hot workability by being added in combination with Cu. However, when the amount is less than 0.005 mass%, the above effect can not be obtained, while when it exceeds 0.4 mass%, the cost increases. Therefore, Ni is preferably added in an amount of 0.005 to 0.4 mass%. From the viewpoint of cost-effectiveness, the range of 0.005 to 0.15 mass% is more preferable. And more preferably in the range of 0.005 to 0.1 mass%. Further, it is more preferably in the range of 0.03 to 0.1 mass%.

When Ni is added, it is necessary to contain each component so that the value of A2 defined by the following formula (2) is 0 or less instead of the value of A1. Further, the value of A2 is preferably -1 or less.

Here, as can be seen from the formula (2), Ni is an element that degrades corrosion resistance.

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

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

In addition to the above-mentioned components, one or two of Sb and W may be added to the steel material of the present invention in the following ranges.

Sb: 0.005 to 0.3 mass%

Sb, like Sn, acts to suppress corrosion in an acidic environment by forming a dense green layer by the combined effect with Cu or by the combined effect of Cu and W when W is added as described later And can be added when it is desired to further improve this property. However, the addition of less than 0.005 mass% is not effective, while the addition of more than 0.3 mass% saturates the effect and reduces the workability. Therefore, in the case of adding Sb, it is preferable that the content is in the range of 0.005 to 0.3 mass%. And more preferably 0.02 to 0.15 mass%. And more preferably 0.03 to 0.09 mass%.

W: 0.001 to 0.5 mass%

W shows that the WO ₄ ^2- ions formed in a corrosive environment exhibit a barrier effect on anions such as chloride ions and form insolubility FeWO ₄ , . Further, there is an effect of densifying the green layer formed on the surface of the steel sheet. W has an effect of inhibiting the progress of local corrosion and whole surface corrosion in a corrosive environment in which H ₂ S and Cl ^- are present due to their chemical and physical effects. However, when the content is less than 0.001 mass%, sufficient addition effect can not be obtained. On the other hand, when the content exceeds 0.5 mass%, the effect is saturated and the cost is increased. Therefore, in the case of adding W, it is preferable that the content is in the range of 0.001 to 0.5 mass%. And more preferably in the range of 0.02 to 0.1 mass%. And more preferably 0.03 to 0.09 mass%.

When Sb and / or W is added in addition to Ni, it is necessary to contain each element so that the value of A3 defined by the following formula (3) is 0 or less instead of the value of A1 or A2 have. Further, the value of A3 is preferably -1 or less.

Here, as can be seen from the formula (3), Sb and W are elements for improving the corrosion resistance.

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

[C], [P], [S], [Cu], [Ni], [Cr], [Mo], [Sn], [Sb] and [W] of the respective elements (Mass%).

Further, in order to improve the strength and toughness of the steel material of the present invention, one or more selected from Nb, V, Ti and B in addition to the above components may be added in the range described below.

Nb: 0.002 to 0.1 mass%

Nb is an element added for the purpose of improving the strength and toughness of steel. However, when the content is less than 0.002 mass%, the effect is not effective. On the other hand, when the content exceeds 0.1 mass%, the effect is saturated. Therefore, in the case of adding Nb, it is preferable that the content is in the range of 0.002 to 0.1 mass%. And more preferably in the range of 0.004 to 0.05 mass%. And more preferably in the range of 0.005 to 0.01 mass%.

V: 0.002 to 0.1 mass%

V is an element added for the purpose of improving the strength of steel. However, when the content is less than 0.002 mass%, there is no effect of improving the strength. On the other hand, when the content exceeds 0.1 mass%, the toughness is lowered. Therefore, when added, it is preferably in the range of 0.002 to 0.1 mass%. And more preferably in the range of 0.003 to 0.05 mass%. And more preferably in the range of 0.004 to 0.01 mass%.

Ti: 0.001 to 0.1 mass%

Ti is an element added for the purpose of improving the strength and toughness of steel. However, when the content is less than 0.001 mass%, the effect is not exhibited, while when it exceeds 0.1 mass%, the effect is saturated. Therefore, when added, it is preferably in the range of 0.001 to 0.1 mass%. And more preferably 0.005 to 0.03 mass%. And more preferably in the range of 0.006 to 0.02 mass%.

B: not more than 0.01 mass%

B is an element added for the purpose of improving the strength of steel, and its effect is obtained by adding 0.0003 mass% or more. However, the addition of more than 0.01 mass% reduces the toughness, and when it is added, it is preferably 0.01 mass% or less. And more preferably in the range of 0.0003 to 0.002 mass%. And more preferably in the range of 0.0003 to 0.0015 mass%.

Further, since the steel material of the present invention improves softness and toughness, one or two of Ca and REM may be added in addition to the above components in the following ranges.

Ca: 0.0002 to 0.005 mass%

Ca has an effect of improving ductility and toughness by morphological control of inclusions and also has an effect of improving corrosion resistance in a coating state. . However, when the content is less than 0.0002 mass%, the effect is not obtained, whereas when the content exceeds 0.005 mass%, the toughness is lowered. Therefore, in the case of the addition, it is preferably in the range of 0.0002 to 0.005 mass%. From the viewpoint of improving the corrosion resistance, the range of 0.001 to 0.005 mass% is more preferable. And more preferably 0.001 to 0.003 mass%.

REM: 0.0005 to 0.015 mass%

REM (Rare Earth Metal) means a rare earth element having an atomic number of 57 to 71, and may be added by using mischmetal which is a mixture containing La, Ce, Pr, Nd and the like in general. This REM has an effect of controlling the shape of inclusions and improving ductility and toughness. However, when the amount is less than 0.0005 mass%, the effect is not obtained, whereas when it exceeds 0.015 mass%, the toughness is lowered. Therefore, when added, it is preferably in the range of 0.0005 to 0.015 mass%. From the viewpoint of improving the corrosion resistance, the range of 0.005 to 0.015 mass% is more preferable. And more preferably in the range of 0.005 to 0.01 mass%.

Further, in the steel material of the present invention, the balance other than the above-mentioned components is composed of Fe and inevitable impurities. However, the steel material of the present invention does not inhibit the content of other elements as long as the effect of the present invention is not impaired. For example, if the content is 0%, the steel material 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.

The steel material of the present invention has a composite structure in which the microstructure at a position 1/4 of the plate thickness t is composed of ferrite, pearlite and bainite transformation, It is preferable to include 2 to 20% of pearlite.

In general, various kinds of texture control methods are used as a method of controlling the strength of a steel having the same component composition, among which water cooling after hot rolling is one of the most used methods. The steel material having the composition of the present invention forms a microstructure composed of ferrite and pearlite when subjected to slow cooling after hot rolling. When a quenching treatment typified by water cooling is performed, the pearlite has a higher strength Is changed to a high bainite structure. In particular, the higher the cooling rate and the lower the cooling stop temperature, the higher the ratio of the bainite structure, and finally the bainite structure of ferrite and bainite.

However, since the bainite structure is a micro dispersion structure of cementite, it has a property of accelerating corrosion in an acid environment. Therefore, a certain amount of pearlite structure is allowed to remain, and the fine dispersion of the cementite is suppressed, whereby the corrosion resistance can be improved. The area ratio of the pearlite is 2% or more when the effect of improving the corrosion resistance by allowing the pearlite to remain clearly appears. On the other hand, if the area ratio of the pearlite structure exceeds 20%, the toughness is lowered, which is not preferable. Therefore, in the steel material of the present invention, in order to obtain better corrosion resistance, it is preferable to control the area ratio of pearlite in the microstructure to a range of 2 to 20%. The reason why the measurement position of the microstructure is located at 1/4 of the plate thickness of the steel is that in the case of a steel plate having a large plate thickness such as for shipbuilding, This is because, even if the machined surface of the steel material is exposed to the corrosive environment, it is possible to satisfy the corrosion resistance on the whole surface from the surface layer of the steel material to the center of the plate thickness. The corrosion resistant steel material for a crude oil tank of the present invention having a microstructure generally has a yield strength of 315 MPa or more and a tensile strength of 440 MPa or more. Further, if a predetermined strength is obtained, the bainite structure may not be present.

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

The steel material of the present invention can be produced in the same manner as the conventional steel material by using a steel material whose composition is controlled within the range of the present invention. For example, in a secondary refining furnace such as a steel converter, an electric furnace, and a vacuum degassing equipment, the main five elements C, Si, Mn, P and S, Cu , The content of Cr, Sn and Mo is adjusted to the range of the present invention, and other alloying elements are added, if necessary, and the steel suitable for the present invention is dissolved. Thereafter, the molten steel is made into a steel slab (steel slab) by a continuous casting method, a coarse ingot-breaking rolling method, etc., and the steel strip is reheated and left as it is or after being cooled and subjected to hot rolling.

From the viewpoint of ensuring corrosion resistance and mechanical properties, the hot rolling condition needs to control the microstructure by selecting an appropriate rolling temperature and pressing ratio, and more specifically, The steel material having the composition should be heated to 1000 to 1350 캜 and then hot rolled at a rolling finishing temperature of 750 캜 or more to be cooled to a cooling stop temperature of not less than 650 캜 and not less than 450 캜 at a rate of not less than 2 캜 / .

Slab heating temperature: 1000 to 1350 DEG C

If the heating temperature is less than 1000 ° C, the deformation resistance is large and hot rolling becomes difficult. On the other hand, heating exceeding 1350 DEG C causes surface marks, or increases scale loss or fuel basic unit. And preferably in the range of 1100 to 1300 占 폚.

Hot roll finishing temperature: 750 ℃ or higher

The finish temperature of hot rolling should be 750 ° C or higher. When the temperature is lower than 750 ° C, a waiting time until the steel material reaches a predetermined rolling temperature occurs, so that the rolling efficiency is lowered or the rolling force is increased due to an increase in deformation resistance. And it is difficult to roll it.

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

The cooling rate after hot rolling needs to be cooled to 2 DEG C / sec or more. Below 2 캜 / sec, the ferrite is coarse and the yield stress is lowered. On the other hand, the upper limit of the cooling rate is not particularly limited, but may be about 80 캜 / sec or less obtained by ordinary water cooling.

The cooling stop temperature should be 650 ° C or lower and 450 ° C or higher. If the temperature is higher than 650 ° C, the ferrite becomes coarse and the yield stress lowers. On the other hand, when the temperature is lower than 450 ° C, the pearlite fraction becomes less than 2%.

Generally, the steel material used for the crude oil tank of the tanker is coated with a paint such as a primer containing metal Zn or Zn compound (hereinafter, collectively referred to as " zinc primer "), And the corrosion resistance of the inner surface is improved. Since these steel materials are subjected to shot blasting on the surface and then coated with zinc primer, depending on the surface condition such as the roughness of the steel sheet, the base material may not be completely covered, It is required that a coating film thickness of a predetermined amount or more (for example, 15 占 퐉 or more) is required.

In this respect, the steel material for a crude oil tank of the present invention produced by the above method using a steel material having the above-mentioned composition is not only excellent in corrosion resistance (corrosion resistance on the inner surface of the steel body and locally corrosive) even in the unpainted state, It is also characterized by excellent. Particularly, in the steel material for a crude oil tank of the present invention, the coating amount of the primer containing a metal Zn or Zn compound is 1.0 g / m 2 or more in terms of the Zn content, thereby remarkably improving the local corrosion resistance and the corrosion- have. If it is 2.5 g / m < 2 > or more, superior local corrosion resistance and corrosion resistance on the ingot surface can be obtained. In addition, from the viewpoints of local corrosion resistance and corrosion resistance on the inner surface, the upper limit of the zinc primer application amount is not set, but when the coating film of the zinc primer becomes thick, the cuttability and weldability are lowered. Therefore, the upper limit thickness is preferably 100 占 퐉.

The relationship between the coating thickness of the zinc primer and the Zn content on the steel surface depends on the Zn content in the zinc primer. Generally, if the average coating thickness is 15 m or more, the entire surface of the steel material can be covered, A coating amount of 1.0 g / m < 2 > or more can be ensured in terms of Zn content irrespective of the type of primer.

The Zn content on the surface of the steel sheet can be determined by cutting out a plurality of small pieces (for example, 10 pieces) of 30 mm in width and 30 mm from the steel material, dissolving and recovering all of the coating film or green layer on the surface thereof, Can be obtained by analyzing Zn content.

[Example 1]

A steel having the composition shown in Tables 1 to 4 was melted by using a converter or the like and formed into a slab having a thickness of 200 mm by a continuous casting method and these slabs were heated to 1200 占 폚 and then finished at a finish rolling temperature of 800 占 폚 And then rolled at a plate thickness of 25 mm and then cooled to 580 캜 at a cooling rate of 30 캜 / sec to produce steel plates No. 1 to No. 35.

For these steel sheets, the microstructure at a plate thickness of 1/4 was observed to measure the area ratio of pearlite, and it was confirmed that the area ratio of pearlite in the microstructure of all these steel sheets was 2% or more Respectively.

For the steels of Nos. 1 and 8 in Tables 1 to 4, steel sheets having different pore size areas in the microstructure were produced by changing the cooling rate and the cooling stop temperature after hot rolling.

Subsequently, a test piece having a length of 50 mm, a width of 50 mm and a thickness of 5 mm and having a plate thickness of 1/4 of the steel sheet obtained as described above as a surface to be tested was sampled and shot blasted on the surface, Three types of test specimens painted in a shot blast state and a zinc primer in a thickness of 5 to 10 μm, 15 to 25 μm and 50 to 70 μm, and four kinds of corrosion test pieces having a total surface state . Thereafter, the sludge containing the crude oil component collected from the actual tanker was uniformly coated on the test surface of 50 mm x 50 mm of the test piece, leaving a portion of 5 mm in the center serving as a starting point of local corrosion. When the uniform coating state is proportional to the thickness of the zinc primer and the thickness of the zinc primer is 15 占 퐉, the content (coating amount) of Zn per unit area (coating amount) It is possible to secure 1.0 g / m < 2 > or more.

Subsequently, the test piece was subjected to a local corrosion test in which it was immersed in the test liquid of the test apparatus of the structure shown in Fig. 1 for one month. This test apparatus 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 local portion generated from the bottom of the actual crude oil tank A test liquid (6) is injected which can cause the same local corrosion as corrosion. This test liquid 6 was prepared by adding 10 mass% NaCl aqueous solution containing 5000 mass ppm of sulfate ion as a mother liquor to a solution of CO ₂ : 13 vol% + O ₂ : 5% volO ₂ + SO ₂ : 0.01 vol% And a mixed gas (4) adjusted to a concentration ratio of H ₂ S: 0.3 vol% was introduced and dissolved. In addition, an adjustable gas, which is the remaining portion of the mixed gas 4, was an inert N ₂ gas (inert nitrogen gas). In this test apparatus, since the mixed gas 4 is continuously supplied, the test liquid 6 is always stirred. The temperature of the test liquid 6 was maintained at 40 占 폚 by adjusting the temperature of the water 7 placed in the thermostatic chamber 3. Then,

After completion of the corrosion test, the rust formed on the surface of the test piece was removed, and the corrosion configuration was visually observed. The depth of generated local corrosion was measured by a depth meter, and the following criteria To evaluate the local corrosion resistance.

≪ Evaluation of local corrosion resistance &

AA ◎: No occurrence of local corrosion

A ○: depth of local corrosion less than 0.5 ㎜

B △: depth of local corrosion 0.5 ㎜ or more and less than 1 ㎜

C x: depth of local corrosion 1 mm or more

The results of the above local corrosion test are shown in Tables 5 and 6. < tb > < TABLE > It can be seen from Table 5 that the steel sheets of Inventive Nos. 1 to 21 according to the present invention exhibit all AA corrosion or A ○, regardless of whether zinc primer is applied or not, and that local corrosion The maximum depth thereof is suppressed to less than 0.5 mm, and it has good local corrosion resistance. Particularly, in the case where the zinc primer is coated by 15 탆 or more, that is, the application state of the zinc primer is uniform and the Zn content is 1.0 g / m 2 or more, all of Nos. 3 to 21 are AA ◎ and the application of the zinc primer , It was confirmed that the local corrosion resistance was remarkably improved.

On the other hand, at least one of the steel sheets No. 22 to No. 35 of Comparative Examples which do not satisfy the conditions of the present invention, that is, the content of Cu, Cr and Sn is less than the range of the present invention and the content of P, S, The steel plates having a value exceeding the range or the values of the indexes A1 to A3 of the corrosion resistance exceeding 0 are not only applied to the case where the zinc primer is not applied but also when the zinc primer is applied, Or B ?. That is, the steel sheet of the comparative example not only has inferior local corrosion resistance in unpatterned state, but also improves even when the zinc primer is applied.

Table 6 shows the results of evaluating the corrosion resistance in the unpatterned state in the same manner as above using a steel plate in which the area ratio of pearlite in the microstructure was changed. It was confirmed from Table 6 that the microstructural steel sheet containing pearlite in an area ratio of 2% or more as compared with the microstructured steel sheet composed only of bainite containing no pearlite tended to have improved local corrosion resistance.

Example 2

A rectangular test piece having a length of 50 mm, a width of 25 mm and a thickness of 4 mm was sampled from the plate having a thickness of 1/4 of the steel plates No. 1 to No. 35 obtained in Example 1 and the surface was subjected to shot blasting, As in the case of Example 1, the test materials in a shot blast state and the zinc primer (proportional to the content of Zn per unit area) were adjusted to three levels of 5 to 10 mu m, 15 to 25 mu m and 50 to 70 mu m To prepare corrosion test pieces having four types of surface states in total. Further, in order to accelerate corrosion, a test piece coated with a zinc primer was subjected to X-shaped cutting to the surface of the steel to be tested, and this was used as a simulated damage point. The damage of the coating film at this time was 1.0% in area ratio.

The test specimens were then subjected to a full surface corrosion test using the test apparatus shown in Fig. 2 to simulate the corrosion environment in the crude oil tank. The corrosion test apparatus is composed of a corrosion test tank 12 and a temperature control plate 13. Water 16 is injected into the corrosion test tank 12 to maintain the saturated vapor pressure, Lt; 0 > C. In order to simulate the corrosive environment in the crude oil tank, CO ₂ : 13 vol%, O ₂ : 5 vol%, SO ₂ : 0.01 vol%, H ₂ S: 0.01 vol% ₂ is filled under a saturated water vapor pressure (dew point: 30 占 폚). The test piece was placed under the temperature control plate provided on the upper part of the corrosion test tank and heated by a heater and a cooling device at 25 ° C for 1 hour / 50 ° C for 5 hours, Hour), and this was given for 28 days, so that the entire surface corrosion due to the number of condensation could be simulated. 500 占 퐇 of an aqueous solution prepared by mixing sodium sulfate and sodium chloride corresponding to 1000 mass ppm of sulfate ion and 10000 mass ppm of chloride ion was applied to the surface of the test piece (surface to be tested) to give sulfate ion and chloride ion, Lt; / RTI > After the start of the test, sulfate ions and chloride ions were supplied every one week.

After completion of the corrosion test, the unprinted test pieces were subjected to corrosion test to remove the rust formed on the surface of the test piece, and then, from the mass change before and after the test, the plate thickness loss by corrosion was obtained, The corrosion resistance of the inner surface was evaluated.

≪ Evaluation of corrosion resistance of the inner surface of unpainted material &

A ○: Corrosion rate less than 0.2 mm / year

B △: corrosion rate 0.2 mm / year or more 0.8 mm / year or less

C x: corrosion rate 0.8 mm / year or more

As to the primer coating material, the rust area ratio under the surface of each test piece and under the coating film was measured, and the corrosion resistance of the inner surface was evaluated by the following criteria.

≪ Evaluation of corrosion resistance of the inner surface of the primer coating material &

A ○: Less than 25% of rust area

B △: Rust area ratio 25% or more and less than 50%

C ×: Rust area ratio 50% or more

The results of the whole surface corrosion test are shown in Tables 7 and 8. From Table 7, it can be seen that the inventive steel sheets Nos. 1 to 21 according to the present invention have good evaluation of corrosion resistance on the inner surface of the unpainted material, and that the corrosion resistance of the inner surface of the steel sheet coated with the zinc primer is all A , That is, the steel sheet of the present invention was found to have better internal surface corrosion resistance in unpatterned state and better internal surface corrosion resistance by zinc primer coating.

On the other hand, in the steel sheets Nos. 22 to 35 of the comparative examples, not only the zinc primer was applied but also the corrosion resistance of the inner surface was evaluated by C x or B △ even when the zinc primer was applied, .

In Table 8, a steel sheet in which the area ratio of pearlite in the microstructure was changed, obtained in Example 1, was subjected to a whole surface corrosion test in a non-coated state, and the corrosion resistance . From Table 5, it was found that a steel sheet having an area ratio of pearlite of 2% or more tends to have improved corrosion resistance on the ingot surface as well as local corrosion resistance.

Industrial availability

The technique of the present invention is not limited to a steel material for a crude oil tank such as a tank for transporting or storing crude oil or crude oil of a crude oil tanker and a primer coating or a normal coating for a steel material of other fields used in a similar corrosive environment The present invention can be suitably applied to the case of using in combination.

1, 11: Specimen
2, 12: corrosion test tank
3: constant-temperature bath
4, 14: Introduction gas
5, 15: Exhaust gas
6, 16: Test liquid
7: water
13: Temperature control plate

Claims (7)

C: 0.001 to 0.16 mass%, Si: more than 0 mass% to 1.5 mass%, Mn: 0.1 to 2.5 mass%, P: more than 0 mass% : 0.005 to 0.1 mass%, N: 0.001 to 0.008 mass%, Cu: 0.052 to 0.35 mass%, more than 0.1 mass% and 0.5 mass% or more of Cr, 0.005 to 0.3 mass% And the balance of Fe and inevitable impurities, 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]) + 0.00098 × (1 / [Sn]) - 6 ... (2)
The content (mass%) of each of the elements [C], [P], [S], [Cu], [Cr], [Ni] The method according to claim 1,
Resistant steel material for a crude oil tank further containing at least one of the following optional element groups (A) to (C) in addition to the above-mentioned component composition.
(A) W: 0.001 to 0.5 mass% and Sb: 0.005 to 0.3 mass%, and the value of A3 defined in the following formula (3) is 0 or less
A3 = 28 占 [C] + 2000 占 P ² + 27000 占 [S] ² + 0.0083 占 1 / [Cu] + 2 占 Ni + 0.027 占 1 / (1 / [Sn]) + 0.0019 x (1 / ([Sb] + [W])) - 6.5 (3)
(B) 0.002 to 0.1 mass% of Nb, 0.002 to 0.1 mass% of V, 0.001 to 0.1 mass% of Ti, and 0.01 mass% or less of B
(C) 0.0002 to 0.005 mass% of Ca and 0.0005 to 0.015 mass% of REM
[C], [P], [S], [Cu], [Ni], [Cr], [Sn], [Sb] and [W] in the formula (3) (mass%) 3. The method according to claim 1 or 2,
A corrosion resistant steel material for a crude oil tank, wherein the microstructure at a position of 1/4 of the plate thickness of the steel contains 2 to 20% of pearlite in an area ratio. 3. The method according to claim 1 or 2,
A corrosion resistant steel material for a crude oil tank, wherein a coating film containing a metal Zn or Zn compound is formed on the surface of the steel material. 5. The method of claim 4,
A corrosion resistant steel material for a crude oil tank having a Zn content of 1.0 g / m 2 or more in a coating film. A steel material having the composition described in claim 1 or 2 is heated to 1000 to 1350 占 폚 and hot rolled at a rolling finish temperature of 750 占 폚 or more to obtain a hot rolled steel sheet at a cooling rate of 2 占 폚 / Or more to a cooling stop temperature equal to or higher than the cooling stop temperature. A crude oil tank characterized by using the steel material according to claim 1 or 2.

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