KR20160075717A - Steel for crude oil tank and crude oil tank - Google Patents

Abstract

The steel according to claim 1, wherein the composition of the steel is 0.03 to 0.18% of C, 0.03 to 1.50% of Si, 0.1 to 2.0% of Mn, 0.025% or less of P, 0.010% or less of S, 0.005 to 0.10% 0.008% or less, Cu: 0.05 to 0.4%, and the balance of Fe and inevitable impurities, and the dislocation density of the steel is expressed by the following equation (1):?? 4 × 10 ¹⁶ × [% Cu] ^2.8 (1) where [% Cu] is a range that satisfies the Cu content (mass%) in the steel material, The corrosion resistance of the inner surface of the bottom plate of the crude oil tank and the corrosion resistance of the interior of the bottom plate of the crude oil tank are excellent.

Description

STEEL FOR CRUDE OIL TANK AND CRUDE OIL TANK

The present invention relates to a tank for transporting or storing crude oil or crude oil of a crude oil tanker formed by welding a steel material (hereinafter collectively referred to as " crude oil tank "). More specifically, the present invention relates to a steel material for a crude oil tank in which front corrosion occurring in the ceiling portion or side wall portion of the crude oil tank and the local corrosion occurring in the bottom portion of the crude oil tank are reduced, and a crude oil tank composed of the steel material.

Further, the steel material for the crude oil tank of the present invention includes a post-steel plate, a thin steel plate and a section steel.

It is known that the steel used for the inner surface of the crude oil tank of the tanker, especially on the upper deck backside and above the side walls, is subject to frontal corrosion. As a cause of this frontal corrosion,

[1] Repeated condensation on the surface of the steel sheet due to temperature difference between day and night and drying (dry)

[2] In an inert gas (about 4 vol% of O _2, about 13 vol% of CO _2, about 0.01 vol% of SO ₂ , exhaust gas of an engine or the like having a typical composition of N ₂ as the remainder) enclosed in the crude oil tank for explosion- O _2, the penetration of condensation on the number of CO _2, SO _2,

[3] Penetration of corrosive gas such as H ₂ S volatilized from crude oil into condensation water,

[4] Residual of seawater used for washing crude oil tank.

These are generally known from the fact that sulfate ions and chloride ions are detected in the strongly acidic dewaxed water in a dock inspection of an actual ship, which is carried out every 2.5 years.

Further, when H ₂ S is oxidized as a catalyst of iron rust generated by corrosion, the solid S is generated in a layer form in the iron green, but these corrosion products are easily peeled off and deposited on the bottom of the crude oil tank. Therefore, in the inspection of the dock, it is a phenomenon that maintenance is performed on the upper part of the tank or collection of sediments on the tank bottom is carried out at a high cost.

On the other hand, in the case of steel used as a bottom plate of a crude oil tank of a tanker, corrosion has been conventionally caused by the corrosion inhibiting action of the crude oil itself and the corrosion inhibiting action of a protective coat (oil coat) derived from crude oil formed on the inner surface of the crude oil tank I thought it was not. However, recent studies have made it clear that local corrosion (formula) of the rice air type occurs in the steel of the tank bottom plate.

As a cause of such local corrosion,

[1] The presence of flocculated water, which is highly soluble in salts, such as sodium chloride,

[2] Leakage of oil coat by excess cleaning,

[3] High concentration of sulfide contained in crude oil,

[4] O ₂ , CO ₂ , and SO ₂ in an inert gas for explosion-proof penetration into condensation water And the like.

Actually, at the time of inspection of a dock of an actual ship, a high concentration of chloride ion and sulfate ion are detected as a result of analyzing the water staying in the crude oil tank.

However, the most effective method for preventing the above-mentioned front corrosion and local corrosion is to apply heavy coating on the surface of the steel material and to prevent the steel material from the corrosive environment. However, the coating operation of the crude oil tank is not only vast in its area of application, but also requires coating replacement at least once every 10 years due to deterioration of the coating film, resulting in enormous cost for inspection and painting. In addition, it is pointed out that the portion of the coating film damaged by the coating is promoted rather than corrosion under the corrosive environment of the crude oil tank.

For such corrosion problems, several techniques have been proposed to improve the corrosion resistance of the steel itself and to improve the corrosion resistance of the crude oil tank in a corrosive environment.

For example, Japanese Patent Application Laid-Open No. 2004-328706 discloses a steel sheet comprising 0.001 to 0.2% of C, 0.01 to 2.5% of Si, 0.1 to 2% of Mn, 0.03% or less of P, 0.02% or less of S, 0.001 to 0.3% of Al, 0.001 to 0.01% of N, 0.01 to 0.5% of Mo and 0.01 to 1% of W and the balance of Fe and unavoidable impurities Discloses a technique of forming a welded joint so that the content of Cu, Mo, and W in the weld metal satisfies the following three formulas when the welded joints are welded to each other.

3? Cu content (mass%) of the weld metal / Cu content (mass%) of the steel? 0.15

3? (Mo content of weld metal + W content (mass%)) / (Mo content of steel + W content (mass%))? 0.15

-0.3≤ Cu content (mass%) of the weld metal - Cu content (mass%) of the steel ≤0.5

Patent Document 2 discloses a steel sheet comprising 0.001 to 0.2% of C, 0.01 to 2.5% of Si, 0.1 to 2% of Mn, 0.03% or less of P, 0.02% or less of S, 0.01 to 1.5% of Cu, 0.001 to 0.3% of Al, 0.001 to 0.01% of N, 0.01 to 0.5% of Mo and 0.01 to 1% of W and the balance of Fe and unavoidable impurities Discloses a technique of forming a welded joint so that the content of Cu, Mo, and W in the weld metal satisfies the following two equations when welding each other to form a crude oil tank.

3? Cu content (mass%) of the weld metal / Cu content (mass%) of the steel? 0.15

3? (Mo content of weld metal + W content (mass%)) / (Mo content of steel + W content (mass%))? 0.15

Patent Document 1: Japanese Patent Application Laid-Open No. 2005-21981 Patent Document 2: Japanese Patent Application Laid-Open No. 2005-23421

In order to conserve the marine environment and safely operate the crude oil tanker, it is important to prevent the leakage of crude oil from the crude oil tank and to prevent the occurrence of through holes due to corrosion in the crude oil tank. For this reason, the corrosion conditions of the bottom plate of the crude oil tank are investigated temporarily for docks every 2.5 years, and maintenance is carried out for the formula exceeding 4 mm deep. In order to reduce the maintenance cost of the crude oil tanker, Application of corrosion resistant steel to tankers has been proposed as one of the means to inhibit the occurrence of excess formaldehyde.

However, in the technologies described in Patent Documents 1 and 2, it is difficult to suppress the local corrosion (formula) occurring in the tank bottom plate and the welded joint to 4 mm or less in 2.5 years. It has been found that the pH of the solution in the formula generated in the tank bottom plate and welded part in the actual corrosion investigation of the ship in recent years is 1.0 or less. Generally, it is well known that corrosion of steel in an acidic liquid is controlled by the hydrogen reduction reaction, and the corrosion rate is drastically increased with a decrease in pH. Therefore, in the immersion test at pH 2.0 as described in the examples of Patent Documents 1 and 2, the corrosion environment in the actual vessel can not be sufficiently reflected.

On the other hand, it is about the suppression of the front corrosion occurring in the tank top plate, but it is about 0.11 mm / y even when the corrosion rate is the lowest among the inventions described in Patent Documents 1 and 2. On the other hand, the corrosion rate of the corrosion resistant steel applied to the top plate is required to be 0.08 mm / y or less because the actual service life of the crude oil tanker is 25 years and the design corrosion ratio of the tank top plate is about 2 mm. Particularly, in the case of a lounge welded to a tanker top plate, both sides are exposed to the corrosive environment inside the tanker. Therefore, when corrosion resistant steel having a corrosion rate exceeding 0.1 mm / y is applied, In the technique described in Japanese Patent Application Laid-Open No. 2000-342708, the coating is not desired to be omitted.

SUMMARY OF THE INVENTION The present invention has been developed in view of the above phenomenon, and it is an object of the present invention to provide a steel material for a crude oil tank which is excellent both in corrosion resistance on the inner surface in the upper plate of the crude oil tank, The present invention is directed to providing a crude oil tank which is composed of a crude oil tank.

However, the inventors have repeated intensive studies for solving the above problems.

As a result, it was found that the above-mentioned front corrosion and local corrosion can be remarkably reduced by appropriately controlling the dislocation density in the relation between the steel composition and the dislocation density of the steel, especially the Cu amount and Sn amount.

The present invention is based on the above knowledge.

That is, the structure of the present invention is as follows.

1. A steel sheet comprising, by mass%, 0.03 to 0.18% of C, 0.03 to 1.50% of Si, 0.1 to 2.0% of Mn, 0.025% or less of P, 0.010% or less of S, 0.005 to 0.10% And the balance of Fe and inevitable impurities, wherein the dislocation density? Of the steel satisfies the following formula (1) in relation to the Cu content:

?? 4 × 10 ¹⁶ × [% Cu] ^2.8 --- (1)

[% Cu] is the Cu content (% by mass) in the steel material,

2. The steel material for a crude oil tank according to 1 above, wherein the steel further contains 0.005 to 0.4% of Sn by mass% and the dislocation density? Of the steel satisfies the following formula (2) :

?? 4 × 10 ¹⁶ × ([% Cu] + [% Sn]) ^2.8 --- (2)

[% Cu] and [% Sn] are respectively the contents of Cu and Sn (mass%) in the steel,

3. The steel according to claim 1, wherein the steel contains 0.005 to 0.4% of Ni, 0.01 to 0.2% of Cr, 0.005 to 0.5% of Mo, 0.005 to 0.5% of W, 0.005 to 0.4% of Sb, 0.001 to 0.1% of Nb, Of at least one member selected from the group consisting of Ti: 0.001-0.1%, V: 0.002-0.2%, Ca: 0.0002-0.01%, Mg: 0.0002-0.01%, and REM: 0.0002-0.015% The steel material for a crude oil tank according to 1 or 2.

4. A crude oil tank manufactured using the steel material for a crude oil tank according to any one of 1 to 3 above.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to effectively suppress frontal corrosion and local corrosion occurring in a tank for transporting or storing oil tanks or crude oil in a crude oil tanker, and is extremely useful in industry.

1 is a view for explaining a test apparatus used for a front corrosion test as an embodiment of the present invention.
Fig. 2 is a view for explaining a test apparatus used in the official test as an embodiment of the present invention. Fig.

Hereinafter, the present invention will be described in detail.

First, 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. The "% " marking on the component means% by mass unless otherwise specified.

C: 0.03 to 0.18%

C is an element for increasing the strength of the steel. In the present invention, 0.03% or more is added to secure the desired strength (490 to 620 MPa). However, addition of C exceeding 0.18% lowers the weldability and toughness of the weld heat affected zone. Therefore, the C content is in the range of 0.03 to 0.18%. And preferably in the range of 0.06 to 0.16%.

Si: 0.03 to 1.50%

Si is an element added as a deoxidizing agent, but it is also an element effective for increasing the strength of steel. Therefore, in the present invention, Si is added in an amount of 0.03% or more to secure a desired strength. However, Si addition of more than 1.50% lowers the toughness of the steel. Therefore, the amount of Si is set in the range of 0.03 to 1.50%. And preferably 0.05 to 0.40%.

Mn: 0.1 to 2.0%

Mn is an element for increasing the strength of steel. In the present invention, Mn is added in an amount of 0.1% or more to obtain a desired strength. However, addition of Mn in excess of 2.0% lowers the toughness and weldability of the steel. Therefore, the Mn content is in the range of 0.1 to 2.0%. And preferably in the range of 0.80 to 1.60%.

P: not more than 0.025%

P is a harmful element which is segregated at the grain boundary and lowers the toughness of the steel. Therefore, it is desirable to reduce the P as much as possible. Particularly, when P is contained in excess of 0.025%, the toughness is greatly reduced. If P is contained in an amount exceeding 0.025%, the corrosion resistance in the tank oil tank is adversely affected. Therefore, the amount of P should be 0.025% or less. It is preferably 0.015% or less.

S: not more than 0.010%

S forms a nonmetallic inclusion MnS to become a starting point of local corrosion and is a harmful element that lowers resistance to local corrosion. Particularly, when S is contained in excess of 0.010%, the local corrosion resistance is remarkably lowered. Therefore, the allowable upper limit of the amount of S is 0.010%. It is preferably 0.005% or less.

Al: 0.005 to 0.10%

Al is an element to be added as a deoxidizing agent, and is added in an amount of 0.005% or more in the present invention. However, if Al is added in excess of 0.10%, the toughness of the steel decreases, so the upper limit of the amount of Al is 0.10%.

N: not more than 0.008%

Since N is a harmful element that deteriorates toughness, it is desirable to reduce the amount of N as much as possible. Particularly, when N is contained in an amount exceeding 0.008%, the lowering of the toughness becomes large, so the upper limit of the amount of N is 0.008%.

Cu: 0.05 to 0.4%

Cu not only enhances the strength of steel but also is present in rust generated by corrosion of steel and inhibits the diffusion of Cl- ions promoting corrosion, and thus is an essential additive element that has an effect of enhancing corrosion resistance. These effects are not sufficiently obtained when Cu is added in an amount of less than 0.05%. On the other hand, when Cu is added in an amount exceeding 0.4%, the effect of improving the corrosion resistance is saturated, and there is a possibility that problems such as surface cracking occur during hot working. Therefore, the amount of Cu is set in the range of 0.05 to 0.4%. And preferably in the range of 0.06 to 0.35%.

Sn: 0.005 to 0.4%

Sn is a useful element contributing to the suppression of local corrosion and frontal corrosion of steel by forming a dense green layer upon being blown into the green layer at the time of corrosion. This effect is exhibited when Sn is added in an amount of 0.005% or more. However, when Sn is added in an amount exceeding 0.4%, not only the low-temperature toughness is lowered but also defects are generated at the time of welding. Therefore, the amount of Sn is in the range of 0.005 to 0.4%. , Preferably in the range of 0.01 to 0.2%, more preferably in the range of 0.01 to 0.1%.

Although the basic components have been described above, in the present invention, besides the components described above, the following elements can be suitably contained.

Cr: 0.01 to 0.2%

Cr migrates into the green layer with the progress of corrosion and blocks the intrusion of Cl- into the green layer, thereby suppressing the concentration of Cl- at the interface between the green layer and the substrate, thereby contributing to improvement of corrosion resistance. When a Zn-containing primer is applied to the surface of a steel material, a complex oxide of Cr or Zn centering on Fe can be formed to sustain Zn on the surface of the steel sheet over a long period of time, thereby remarkably improving the corrosion resistance have. The above effect is particularly remarkable in a portion contacting with a liquid containing a high concentration of salt separated from crude oil fractions such as a bottom plate portion of a tanker oil tank, and a steel containing the Cr is treated with a Zn-containing primer treatment The corrosion resistance can be remarkably improved as compared with a steel material not containing Cr. The effect of Cr is not sufficient when the amount of Cr is less than 0.01%, while if it exceeds 0.2%, the toughness of the welded portion is deteriorated. Therefore, the amount of Cr is set in the range of 0.01 to 0.2%. And preferably in the range of 0.05 to 0.20%.

Mg: 0.0002 to 0.01%

Mg not only contributes to improving the toughness of the weld heat affected zone but also exists in the rust generated by the corrosion of the steel, thereby enhancing the corrosion resistance. These effects are not sufficiently obtained when the amount of Mg is less than 0.0002%, and when the amount exceeds 0.01%, the toughness is lowered. Therefore, the amount of Mg is in the range of 0.0002 to 0.01%.

Ni: 0.005 to 0.4%

Ni has the effect of improving the corrosion resistance in the exposed state and the corrosion resistance in the state where the epoxy coating is applied to the zinc primer by making the generated green particles fine. Therefore, Ni is added when it is desired to further improve the corrosion resistance. The above effect is expressed by addition of Ni of 0.005% or more. On the other hand, when Ni is added in excess of 0.4%, the effect is saturated. Therefore, Ni is preferably added in the range of 0.005 to 0.4%. And preferably in the range of 0.08 to 0.35%.

Sb: 0.005-0.4%

Sb not only suppresses the formula in the tank bottom tank bottom plate but also has the effect of suppressing the front surface corrosion on the tanker upper deck. The above effect is expressed by addition of Sb of 0.005% or more, but the effect is saturated even when Sb is added in excess of 0.4%. Therefore, Sb is preferably added in a range of 0.005 to 0.4%.

Nb: 0.001 to 0.1%, Ti: 0.001 to 0.1%, V: 0.002 to 0.2%

Nb, Ti, and V are all elements that increase the strength of steel, and can be appropriately selected depending on the required strength. In order to obtain the above effect, Nb and Ti are preferably added in an amount of 0.001% or more, and V is added in an amount of 0.002% or more. However, Nb and Ti exceed 0.1% and V exceeds 0.2%, respectively, and toughness decreases. Therefore, Nb, Ti and V are preferably added in the above ranges respectively.

Ca: 0.0002 to 0.01%, REM: 0.0002 to 0.015%

Both Ca and REM are effective in improving the toughness of the weld heat affected zone and can be added as needed. The above effect can be obtained by adding 0.0002% or more of Ca and 0.0002% or more of REM. When Ca is added in excess of 0.01% and REM is added in excess of 0.015%, the toughness is lowered. Therefore, Ca and REM are preferably added in the above ranges respectively.

Mo: 0.005 to 0.5%, W: 0.005 to 0.5%

Mo and W not only inhibit the formula in the tank bottom tank bottom, but also have the effect of suppressing the front surface corrosion of the upper tank deck. The effects of Mo and W are expressed by the addition of at least 0.005%, respectively, but when it exceeds 0.5%, the effect reaches saturation. Therefore, it is preferable that the amounts of Mo and W are 0.005 to 0.5%, respectively. , More preferably 0.01 to 0.3%, and most preferably 0.02 to 0.2%.

The reason why Mo and W have the above-mentioned corrosion resistance improving effect is that MoO ₄ ^{2 -} and WO ₄ ^{2 -} are produced in the rust which is generated as the steel sheet is corroded, and the MoO ₄ ^{2 -} and WO ₄ ^{2 -} This is because the presence of chloride ions suppresses invasion of the surface of the steel sheet. It is also believed that the corrosion of the steel is suppressed by the inhibitor action by the adsorption of MoO ₄ ^2- and WO ₄ ^{2 - on} the surface of the steel.

Next, the transition density of the steel material specified in the present invention will be described.

The corrosion-resistant steel of the present invention is obtained by adding various kinds of corrosion-resistant elements to a predetermined amount of steel as described above, whereby various kinds of corrosion-resistant elements are concentrated on the rust layer on the surface of the steel material formed in the corrosive environment of the tank- It suppresses diffusion of corrosion factor and reduces corrosion rate of steel.

On the other hand, the formation of the transition attributable to the production process can not be avoided in the steel material, but since this transition is thermodynamically unstable, it functions as an anode site in which iron dissolves in the corrosive environment. The green layer formed on the surface of the corrosion resistant steel has a protective effect and has the effect of reducing the corrosion rate of the steel but its function is not perfect and when the density of transition on the surface of the steel layer below the green layer is large, And further satisfactory corrosion resistance can not be obtained.

The protective property of the green layer is mainly determined by the Cu concentration in the steel, or the concentration of Cu and Sn when Sn is contained. The higher the Cu and Sn concentrations, the better the protection. For this reason, the allowable dislocation density also varies depending on the amount of Cu and the amount of Sn.

Therefore, the inventors of the present invention have investigated the relationship between the protection of the green layer and the amount of Cu or Sn, and as a result, it has been found that the dislocation density α is controlled in the range given by the following expressions (1) and (2) , It is possible to obtain good protection of the green layer.

?? 4 × 10 ¹⁶ × [% Cu] ^2.8 --- (1)

?? 4 × 10 ¹⁶ × ([% Cu] + [% Sn]) ^2.8 --- (2)

[% Cu] and [% Sn] are respectively the contents of Cu and Sn (mass%) in the steel,

The steel material for a crude oil tank of the present invention is preferably produced by the following method.

That is, the steel material of the present invention can be obtained by using a steel material (slab) obtained by continuously casting a steel prepared by the composition described above by using a known refining process such as electric furnace, electric furnace or vacuum degassing, , And then the material is reheated and then hot-rolled to form a post-steel plate, a thin steel plate, a section steel, or the like.

If the heating temperature is less than 900 占 폚, the deformation resistance is large and it becomes difficult to carry out the hot rolling. If the heating temperature exceeds 1200 占 폚, the austenite The lip is coarsened to cause deterioration of toughness, and the scale loss due to oxidation becomes remarkable, and the yield is lowered. More preferably, the heating temperature is in the range of 1000 to 1150 占 폚.

When the steel sheet is rolled into a steel sheet having a desired shape and dimensions by hot rolling, the finishing rolling finishing temperature is preferably 700 ° C or higher. If the finish rolling finish temperature is less than 700 캜, the deformation resistance of the steel becomes large, the rolling load becomes large and the rolling becomes difficult or the waiting time till the rolling material reaches the predetermined rolling temperature occurs. Because.

The cooling of the steel material after hot rolling may be performed by air cooling or accelerated cooling. However, in order to obtain a higher strength, it is preferable to perform accelerated cooling. When accelerated cooling is carried out, it is preferable that the cooling rate is 2 to 80 캜 / s and the cooling stop temperature is 650 to 400 캜. When the cooling rate is less than 2 DEG C / s and the cooling stop temperature is more than 650 DEG C, the effect of accelerated cooling is small and sufficient high strength is not achieved. On the other hand, when the cooling rate is more than 80 DEG C / This is because the toughness of the steel material is lowered or the shape of the steel material is distorted.

Example

Table 1 shows the results. 1 to 37 were made into a steel ingot by a vacuum melting furnace to be a steel ingot or by a solvent in a converter to form a steel slab by continuous casting. After reheating them to 1150 占 폚, the finish rolling finish temperature To form a post-steel plate having a plate thickness of 25 mm, and then cooled to a cooling stop temperature shown in Table 2 at a water-cooling rate of 10 DEG C / s.

The thus obtained No. The steel sheets 1 to 37 were subjected to a dew condensation test and an acid resistance test, and their corrosion resistance was evaluated. The dislocation density of the steel was also measured.

That is, a front corrosion test (dew condensation test) simulating the upper surface of the upper deck and a local corrosion resistance test (acid resistance test) simulating the tank bottom plate environment were carried out in the following manner.

[1] Front corrosion test simulating the upper tank deck environment (condensation test)

In order to evaluate the corrosion resistance against the front corrosion on the back surface of the tanker upper deck, for each of the steel plates of Nos. 1 to 37 above, a square having a width of 25 mm, a length of 60 mm, and a thickness of 5 mm And the surface thereof was polished with an emery paper of number 600. Next, the back surface and the end surface were sealed with tape so as not to corrode, and a front corrosion test was conducted using the corrosion test apparatus shown in Fig.

This corrosion test apparatus is composed of a corrosion test tank 2 and a temperature control plate 3. Water 6 held at a temperature of 30 ° C is injected into the corrosion test tank 2, A mixed gas composed of 13 vol% CO ₂ , 4 vol% O ₂ , 0.01 vol% SO ₂ , 0.05 vol% H ₂ S and the remainder N ₂ was introduced into the corrosion test tank ₂ through the introduction gas pipe 4, It is filled with water vapor of supersaturation, and the corrosion environment on the back of the upper deck of the crude tank is being reproduced. The corrosion test piece 1 was set on the upper surface of the test tank and the corrosion test piece 1 was passed through a temperature control plate 3 containing a heater and a cooling device at 25 DEG C for 1.5 hours + 22.5 hours as one cycle was repeatedly applied for 21, 49, 77 and 98 days to generate condensation water on the surface of the test piece 1 to cause frontal corrosion. 1, '5' represents an exhaust gas pipe from the test tank.

After the above corrosion test, the rust on the surface of each test piece was removed, and the decrease in mass due to corrosion was determined from the mass change before and after the test. From this value, the plate thickness reduction amount per one year (corrosion rate on one side) was converted. When the corrosion amount is less than 2 mm, the front corrosion resistance is good (O). When the corrosion amount is more than 2 mm, the front surface corrosion resistance is poor (X) ).

[2] Local corrosion test simulating the environment of the tank bottom tank (acid test)

In order to evaluate the corrosion resistance of the bottom plate of the tanker lubricant bottom, the steel plates of Nos. 1 to 37 were placed in a rectangular shape having a width of 25 mm, a length of 60 mm, and a thickness of 5 mm The piece was cut out and its surface was polished with an emery paper of number 600.

Next, a 10% aqueous solution of NaCl was prepared with concentrated hydrochloric acid to prepare a test solution having a Cl ion concentration of 10% and pH of 0.85. The test solution was suspended in a hole of 3 mm in diameter through the upper part of the test piece, Were immersed in 2 L of test solution for 168 hours. In addition, the test solution was previously kept at 30 캜 and exchanged with a new test solution every 24 hours.

The apparatus used for the corrosion test is shown in Fig. In this corrosion test apparatus, the test solution 10 is placed in the corrosion test tank 8 and the test piece 7 is placed in the corrosion test tank 8 in the dual structure of the corrosion test tank 8 and the constant temperature tank 9, ). ≪ / RTI > The temperature of the test solution (10) is maintained by adjusting the temperature of the water (12) put in the thermostat (9).

After the above corrosion test, the generated rust on the surface of the test piece was removed, and the difference in mass before and after the test was obtained. The difference in the plate thickness was repre- sented by the total surface area, and the amount of decrease in plate thickness per year (corrosion rate on both sides) was obtained. As a result, the local corrosion resistance was evaluated as good (.largecircle.) When the corrosion rate was 1.0 mm / y or less, and the local corrosion resistance was evaluated as poor (x) when the corrosion rate was 1.0 mm / y or more.

[3] Measurement of dislocation density of steel

No. after the acid test was carried out. Test specimens of 20 x 20 x 5 mm were cut out from the test specimens 1 to 37, and the surface of 1 mm on the surface of the original steel was taken as the measurement surface. The diffraction peaks of the (110), (211) and (220) planes of the steel material were measured using an X-ray diffraction measuring apparatus, and the respective diffraction angles 2? And half width? M were obtained for each test piece.

The horizontal axis represents sin? /? And the vertical axis represents? Cos? /?, And the measurement results of the above crystal planes are plotted.

Here,? Represents an X-ray wavelength of 1.789 ANGSTROM, and? Represents a half-width of true diffraction peak, respectively, and was obtained from the actual half-width? M and the non-distortion half width? S by the formula (3).

In addition, a standard sample of Si powder was used as the non-distortion standard sample (? S at the peak position was obtained from the interpolation calculation by the parabolic approximation).

^{β = (βm 2 -βs 2)} 0.5 --- (3)

An approximate curve was drawn by the least-squares method for the above-mentioned float 3 points, and the distortion? Was calculated from the slope as shown in the equation (4), and the dislocation density? Was obtained from the equation (5).

?? cos? /? = 0.9 / D + 2?? sin? /?

α = 14.4 ε ² / b ² (5)

B is a Burgers vector of 0.25 nm,

D represents the crystallite size.

The obtained results are shown in Table 2.

[Table 1]

[Table 2]

As shown in Table 2, the steel plates Nos. 1-4, 7-10, and 13-36 satisfying the conditions of the present invention were subjected to a front corrosion test simulating the upper deck back surface and a local corrosion test simulating the tank bottom plate environment All of them exhibited good corrosion resistance.

On the other hand, in the case of the steel sheet No. 2 which does not satisfy the condition of the present invention. 5, 6, 11, 12 and 37 could not obtain good results in any corrosion resistance test.

1, 7; Corrosion test pieces 2, 8; Corrosion test tank
3; Temperature control plate 4; Introduced gas pipe
5; Exhaust gas pipes 6 and 12; water
9; Constant temperature bath 10; Test solution
11; gut

Claims (4)

In terms of% by mass,
C: 0.03 to 0.18%
Si: 0.03 to 1.50%
Mn: 0.1 to 2.0%
P: 0.025% or less,
S: 0.010% or less,
Al: 0.005 to 0.10%
N: 0.008% or less,
Cu: 0.05 to 0.4%
And the remainder being Fe and unavoidable impurities, wherein the steel material for a crude oil tank satisfying the following formula (1) in relation to the dislocation density? Of the steel material and the Cu content:
?? 4 × 10 ¹⁶ × [% Cu] ^2.8 --- (1)
However, [% Cu] is the Cu content (mass%) in the steel material. The method according to claim 1,
The steel material is in mass%
Sn: 0.005 to 0.4%
And the dislocation density? Of the steel satisfies the following formula (2) in relation to the content of Cu and Sn:
?? 4 × 10 ¹⁶ × ([% Cu] + [% Sn]) ^2.8 --- (2)
[% Cu] and [% Sn] are Cu and Sn contents (% by mass) in the steel, respectively. 3. The method according to claim 1 or 2,
The steel material, in mass%
Ni: 0.005 to 0.4%
Cr: 0.01 to 0.2%
Mo: 0.005 to 0.5%
W: 0.005 to 0.5%
Sb: 0.005 to 0.4%
Nb: 0.001 to 0.1%
Ti: 0.001 to 0.1%
V: 0.002 to 0.2%
Ca: 0.0002 to 0.01%
Mg: 0.0002 to 0.01%
REM: 0.0002-0.015%
And at least one kind selected from the group consisting of iron and iron. A crude oil tank manufactured using the steel material for a crude oil tank according to any one of claims 1 to 3.

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