KR20080097479A - Corroson-resistant steel material for crude oil storage tank, and crude oil storage tank - Google Patents

Abstract

Disclosed is a steel material which, when used in an oil tank for an oil tanker, a tank for transporting a crude oil, a tank for storage of a crude oil or the like, can reduce a local corrosion on a bottom plate or a general corrosion on a top board or a side board. The steel material for a crude oil storage tank has the following chemical composition: C: 0.001-0.16 mass%, Si: 0.01-1.5 mass%; Mn: 0.1-2.5 mass%; P: 0.025 mass% or less; S: 0.01 mass% or less; Al: 0.005-0.1 mass%; N: 0.001-0.008 mass%; W: 0.001-0.5 mass%; and Cr: not less than 0.06 mass% and less than 0.20 mass%; with the remainder being Fe and unavoidable impurities.

Description

Corrosion-Resistant Steel and Crude Oil Storage Tanks for Crude Oil Storage Tanks {CORROSON-RESISTANT STEEL MATERIAL FOR CRUDE OIL STORAGE TANK, AND CRUDE OIL STORAGE TANK}

The present invention can reduce local corrosion occurring at the bottom plate and entire surface corrosion occurring at the top plate or the side plate when the steel material used in the crude oil tank is not applied or when the primer is applied. The present invention relates to a corrosion resistant steel material and a crude oil tank made of the steel material. In addition, the crude oil tank referred to in the present invention is a generic term for an oil depot of an oil tanker, a tank for transporting crude oil, a tank for storing crude oil, etc., and a steel material for a crude oil tank, a later steel sheet, foil (Iii) Steel sheet and shape steel are also included.

On the upper inner surface of the tanker's crude oil tank (behind the upper deck), the inert gas (O ₂ : 5 vol%, CO ₂ : 13 vol%, SO ₂ : 0.01 vol%) and the balance N ₂ contained in the crude oil tank for explosion protection It is known to cause full corrosion by corrosive gases such as O ₂ , CO ₂ , SO ₂ , H ₂ S volatilized from crude oil, and the like contained in a representative composition (exhaust gas such as a boiler or engine).

In addition, the H ₂ S is oxidized by the catalytic action of iron rust generated by corrosion to become a solid S, and it is present in the layered form in the iron rust. Then, these corrosion products easily peel off and are deposited on the bottom of the crude oil tank. Therefore, in the dock inspection of the tanker performed every 2.5 years, the maintenance of the tank upper part and the removal of a deposit are carried out with enormous cost.

On the other hand, the steel used for the bottom plate of the crude oil tank of a tanker has the corrosion inhibitory effect of the protective film derived from the crude oil produced | generated on the inner surface of a crude oil tank, and the crude oil protection tank (henceforth "crude oil protective film"). As a result, corrosion was not considered to occur. However, in recent years, it has become clear that a bowl-type local corrosion occurs in steel materials used for tank bottom plates.

As a cause of such bowl type local corrosion,

(1) the presence of agglomerated water in which salts representative of sodium chloride are dissolved at high concentrations,

(2) separation of the crude oil protective film by excessive washing,

(3) high concentration of sulfides contained in crude oil;

(4) high concentration of O ₂ , CO ₂ , SO ₂ contained in the inert gas for explosion protection,

(5) involvement of microorganisms,

Although such items may be mentioned, they are all within the estimation region, and no clear cause is yet known.

The most effective method of suppressing the corrosion as described above is a method of applying a layered coating on the surface of the steel to shield the steel from the corrosive environment. However, since the coating area of the crude oil tank is enormous and needs to be repainted about once every 10 years, a huge cost is required for the construction and inspection, and under the crude oil tank environment, the double coat is applied. In one case, it is pointed out that the corrosion of the coating-film damaged part is rather encouraged.

Therefore, corrosion resistant steels having corrosion resistance have been proposed even under an environment such as a crude oil tank. For example, Patent Document 1 adds an appropriate amount of Si, Mn, P, and S to C: 0.01 to 0.3 mass% of steel, further Ni: 0.05 to 3 mass%, optionally Mo, Cu, Cr. There is disclosed a corrosion resistant steel having excellent front surface corrosion resistance and local corrosion resistance having a composition containing W, Ca, Ti, Nb, V, and B.

In addition, Patent Document 2 discloses an appropriate amount of Si, Mn, P, and S in a steel of C: 0.001 to 0.2 mass%, 0.01 to 1.5 mass%, Al: 0.001 to 0.3 mass%, and N: 0.001 to 0.0l. It contains mass% and further contains at least one of Mo: 0.01-0.2 mass% or W: 0.01-0.5 mass%, and the formation of a corrosion product containing solid S having excellent front surface corrosion resistance and local corrosion resistance. A corrosion resistant steel that can be suppressed is disclosed.

Patent Document 1: Japanese Unexamined Patent Publication No. 2003-082435

Patent Document 2: Japanese Unexamined Patent Publication No. 2004-204344

However, when the corrosion resistant steels of Patent Documents 1 and 2 are used as steel materials for crude oil tanks, they exhibit excellent inhibitory effects on the entire surface corrosion occurring at the top of the crude oil tank, but are resistant to local corrosion occurring at the bottom of the crude oil tank. It is difficult to say that "internal corrosion" is sufficient.

Accordingly, an object of the present invention is to solve the above problems, and has excellent front surface corrosion resistance when used in the upper part of the oil tank (upper deck and side plate), and excellent local corrosion resistance even when used in a crude oil tank bottom plate. In addition, when using in the primer application | coating state, it is providing the steel material for crude oil tanks excellent in local corrosion resistance excellent in the effect of extending | stretching paint life.

The inventors first extracted the factors involved in the local corrosion of the crude oil tank bottom plate, combined these factors, performed the corrosion test, and succeeded in reproducing the local corrosion occurring in the crude oil tank bottom plate. . As a result, the following findings were obtained about the governing factors and the mechanism of local corrosion occurring in the crude oil tank bottom plate.

That is, in the bowl type local corrosion occurring in the actual oil tank bottom plate, O ₂ (oxygen) and H ₂ S (hydrogen sulfide) contained in the liquid play an important function as a governing factor for local corrosion, and in particular, O ₂ And H ₂ S coexist, and in an environment where both the O ₂ partial pressure and the H ₂ S partial pressure are low, specifically, the O ₂ partial pressure: 2 to 8 vol% and the H ₂ S partial pressure: 5 to 20 vol% Local corrosion occurs in an aqueous solution of saturated gas. That is, under low O ₂ and partial pressure and low H ₂ S partial pressure environments, H ₂ S is oxidized to precipitate solid S, and a local battery is formed between the crude oil tank bottom plate and the solid S, so that local corrosion occurs on the steel surface. do. In particular, in an acidic environment in which chloride ions (Cl ⁻ ) are present, local corrosion is promoted and grows.

Therefore, the inventors earnestly examined the influence of various alloying elements that reach local corrosion occurring under the environment of the low O ₂ partial pressure and low H ₂ S partial pressure. As a result, by the addition of W and Cr, the rust layer formed on the surface of the steel sheet is densified under the use environment of the crude oil tank steel, thereby improving the local corrosion resistance and the corrosion resistance of the entire surface, and the addition of SD, Sb or Mo. Helps the formation of a dense rust layer containing W, and can improve local corrosion resistance and front surface corrosion resistance more. In other words, it was found that by optimizing mainly the content of W and Cr, and further, Sn, Sb, and Mo, steel materials for crude oil tanks excellent in both of local corrosion resistance and corrosion resistance of the front surface were obtained.

Moreover, when the said steel material was used in the state which apply | coated the primer containing Zn on the surface, it discovered that while the coating life is prolonged remarkably, local corrosion resistance and whole surface corrosion resistance also improved.

This invention is completed based on the said knowledge, adding further examination.

That is, in the present invention, C: 0.001-0.16 mass%, Si: 0.01-1.5 mass%, Mn: 0.1-2.5 mass%, P: 0.025 mass% or less, S: 0.01 mass% or less, Al: 0.005-0.1 mass %, N: 0.001-0.008 mass%, W: 0.001-0.5 mass%, Cr: 0.06 mass% or more and less than 0.20 mass%, remainder is a steel material for crude oil tanks which consists of Fe and an unavoidable impurity.

The steel material of this invention contains the component of at least 1 group of the following A-D group in addition to the said component composition further, It is characterized by the above-mentioned.

Group A; Sn: 0.005 to 0.3 mass% and Sb: 0.005 to 0.3 mass%

Group B; Mo: 0.001-0.5 mass%

C group; Nb: 0.001 to 0.1 mass%, V: 0.002 to 0.1 mass%, Ti: 0.001 to 0.1 mass% and B: 0.01 mass% or less

Group D; One or two selected from Ca: 0.0002 to 0.005 mass% and REM: 0.0005 to 0.015 mass%

In addition, the steel material of the present invention may be coated with a primer containing Zn on its surface in order to improve corrosion resistance.

Moreover, this invention is a crude oil tank comprised with the steel materials described above.

Advantageous Effects of Invention According to the present invention, steel materials having excellent front surface corrosion resistance and local corrosion resistance can be provided at low cost not only in the uncoated state but also in the primer coating state. Therefore, the steel material of this invention can be used suitably as structural materials, such as a tank for transport or storage of crude oil.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the test apparatus used for the local corrosion test used in Example 1. FIG.

FIG. 2 is a view for explaining a test apparatus used for the front surface corrosion test used in Example 2. FIG.

Explanation of symbols on the main parts of the drawings

1, 8: Test piece 2, 9: Corrosion test tank

3: thermostat 4, 11: gas introduction

5, 12: gas discharge 6: test liquid

7, 13: water 10: temperature control plate

Implement the invention  Best form for

The reason for limiting the component composition of the steel material for crude oil tank of this invention as mentioned above is demonstrated.

C: 0.001 to 0.16 mass%

C is an element which increases the strength of steel, and in the present invention, it is required to contain 0.001 mass% or more in order to obtain a desired strength. On the other hand, containing exceeding 0.16 mass% reduces the weldability and the toughness of a weld heat influence part. For this reason, C shall be in the range of 0.001-0.16 mass%. In addition, in order to have both characteristics of strength and toughness, the range of 0.01-0.15 mass% is preferable.

Si: 0.01 to 1.5 mass%

Si is usually added as a deoxidizer, but as an element to increase the strength of the steel, the present invention requires 0.01 mass% or more. However, additions exceeding 1.5 mass% lower the toughness of the steel. For this reason, Si is taken as the range of 0.01-1.5 mass%. In addition, Si forms an anticorrosive coating under an acidic environment and contributes to improvement of corrosion resistance. In order to acquire this effect, the range of 0.2-1.5 mass% is preferable.

Mn: 0.1 to 2.5 mass%

Mn is an element that improves the strength of the steel, and in the present invention, 0.1 mass% or more is added in order to obtain a desired strength. On the other hand, addition exceeding 2.5 mass% reduces the toughness and weldability of steel. For this reason, Mn is made into the range of 0.1-2.5 mass%. In addition, the range of 0.5-1.6 mass% is preferable from a viewpoint of suppressing formation of the inclusion which ensures strength and deteriorates corrosion resistance, More preferably, it is 0.8-1.4 mass%.

P: 0.025 mass% or less

P is a harmful element which segregates at grain boundaries and reduces the toughness of steel, and it is preferable to reduce P as much as possible. In particular, when the content exceeds 0.025 mass%, the toughness is greatly reduced, so that P is made 0.025 mass% or less. In addition, since the reduction to less than 0.005 mass% causes an increase in manufacturing cost, the lower limit is preferably about 0.005 mass%.

S: 0.01 mass% or less

S is a harmful element which forms MnS which is a nonmetallic inclusion, becomes a starting point of local corrosion, and reduces local corrosion resistance, and it is preferable to reduce it as much as possible. In particular, the content exceeding 0.01 mass% causes a significant decrease in local corrosion resistance, so the upper limit of S is 0.01 mass%. In addition, since the reduction to less than 0.002 mass% causes an increase in manufacturing cost, the lower limit of S is preferably about 0.002 mass%.

Al: 0.005 to 0.1 mass%

Al is an element added as a deoxidizer, and in the present invention, Al is added at least 0.005 mass%. However, when it exceeds 0.1 mass%, since toughness of steel will fall, the upper limit of Al shall be 0.1 mass%. Preferably, it is in the range of 0.01 to 0.05 mass%.

N: 0.001 to 0.008 mass%

N is a component that lowers toughness and is preferably reduced as much as possible. In particular, when it contains 0.008 mass% or more, since toughness will fall largely, an upper limit shall be 0.008 mass%. However, industrially, it is difficult to reduce it to less than 0.001 mass%. Therefore, N is in the range of 0.001 to 0.008 mass%.

W: 0.001-0.5 mass%

W is an essential additional element important for improving corrosion resistance in the present invention. By adding W, WO ₄ ^{2 -} ions formed in a corrosive environment exhibit a barrier effect against anions such as chloride ions, and form insoluble FeWO ₄ to suppress the progress of corrosion. Further, the rust layer formed on the surface of the steel sheet is very dense by containing W. The addition of W suppresses the progress of full surface corrosion and the growth of local corrosion in the corrosive environment in which H ₂ S and Cl ⁻ are present by such chemical and physical actions. Therefore, the steel material for crude oil tank which improves local corrosion resistance and also excellent corrosion resistance on all surfaces is obtained.

When the Zn-containing primer is applied to the surface of the steel, Zn in the primer is introduced into the densified rust layer containing W to form a composite oxide of W or Zn centered on Fe, and the steel sheet over a long period of time. Zn can remain on the surface. Therefore, compared with the steel material which does not contain W, generation | occurrence | production of local corrosion can be suppressed over a long period of time.

The above-mentioned effect of improving the corrosion resistance of W is not sufficiently expressed when less than 0.001 mass%. On the other hand, when it exceeds 0.5 mass%, the effect is saturated and the cost is increased. Therefore, W is in the range of 0.001 to 0.5 mass%.

Cr: 0.06 mass% or more and less than 0.20 mass%

As the corrosion progresses, Cr moves to the rust layer, blocks the penetration of Cl ⁻ into the rust layer, and reduces the concentration of Cl ⁻ at the interface between the rust layer and the ground iron. In the case where the Zn-containing primer is applied, Zn can remain on the surface of the steel sheet for a long time by forming a complex oxide of Cr or Zn around Fe. As a result, generation of local corrosion can be suppressed over a long time compared with the steel material which does not contain Cr. However, if the effect is less than 0.06 mass%, this effect is not sufficiently obtained. On the other hand, the weld toughness deteriorates at 0.20 mass% or more. Therefore, Cr is made into the range of 0.06 mass% or more and less than 0.20 mass%.

Although the steel material of this invention makes the said component a basic component, in order to further improve corrosion resistance, it can contain 1 type (s) or 2 or more types chosen from Sn, Sb, and Mo in the following range.

Sn: 0.005 to 0.3 mass%

Sn has the effect of improving the acid resistance of the formed dense rust layer by the composite effect of W and Cr, and suppressing corrosion. However, at 0.005 mass% or less, the above effect cannot be obtained, while addition of more than 0.3 mass% results in deterioration of hot workability and toughness. Therefore, it is preferable to contain Sn in the range of 0.005-0.3 mass%.

Sb: 0.005 to 0.3 mass%

Sb has the effect of improving the acid resistance of the formed dense rust layer by the composite effect of W and Cr and suppressing corrosion, similar to Sn. However, when the content is 0.005 mass% or less, the above effect is not obtained. On the other hand, when the addition exceeds 0.3 mass%, the effect is saturated and the workability is also lowered. Therefore, it is preferable to contain Sb in 0.005 to 0.3 mass%.

Mo: 0.001-0.5 mass%

When Mo is contained together with W or Cr, Mo improves the front corrosion resistance and local corrosion resistance, and promotes the formation of a dense rust layer by the combined effect of W, Cr and Sn or Sb, and further improves the corrosion resistance. There is a function to let. The above effect is obtained at a content of 0.001 mass% or more. If the content exceeds 0.5 mass%, the effect may be saturated and the cost may increase. Therefore, when it contains, the range of 0.001-0.5 mass% is preferable.

In addition to the above components, the steel of the present invention may further contain one or two or more selected from Nb, V, Ti, and B in the following ranges for the purpose of improving the steel strength.

Nb: 0.001 to 0.1 mass%

Nb is an element added for the purpose of improving the strength of steel. When the amount is less than 0.001 mass%, the effect is small. On the other hand, since the toughness decreases when the amount is more than 0.1 mass%, the range of 0.001 to 0.1 mass% is preferable.

V: 0.002 to 0.1 mass%

V is an element added for the purpose of improving the strength of steel. When the amount is less than 0.002 mass%, the effect of improving strength is small. On the other hand, since the toughness decreases when the amount exceeds 0.1 mass%, the range of 0.002 to 0.1 mass% is preferable.

Ti: 0.001-0.1 mass%

Ti is an element added for the purpose of improving the strength and toughness of the steel. If the amount is less than 0.001 mass%, the above effect is small. On the other hand, if the amount exceeds 0.1 mass%, the effect is saturated. When added, the range of 0.001 to 0.1 mass% is preferable.

B: 0.01 mass% or less

B is an element added for the purpose of improving the strength of steel. However, when it exceeds 0.01 mass%, since toughness falls, when adding, 0.01 mass% or less is preferable.

In addition to the above components, the steel of the present invention may further contain one or two kinds selected from Ca and REM in the following ranges for the purpose of improving the ductility and toughness.

Ca: 0.0002 to 0.005 mass%

Ca has the effect of improving the ductility and toughness of the steel through the shape control of inclusions. If it is less than 0.0002 mass%, there is no effect, On the other hand, if it exceeds 0.005 mass%, since toughness will fall, it is preferable to contain in 0.0002-0.005 mass%.

REM: 0.0005 ~ 0.015 mass%

REM has the effect of improving the ductility and toughness through the shape control of inclusions. If it is less than 0.0005 mass%, the effect is small, whereas when it exceeds 0.015 mass%, toughness falls. Therefore, in the case of addition, the range of 0.0005 to 0.015 mass% is preferable.

The steel material of this invention WHEREIN: The remainder other than the said component consists of Fe and an unavoidable impurity. However, if it is in the range which does not impair the effect of this invention, it does not prevent containing a component other than the above, For example, O is 0.008 mass% or less, Cu is 0.05 mass% or less, Ni is 0.05 mass% If it is below, it can accept.

Cu is supposed to contribute to the improvement of overall corrosion in an environment containing hydrogen sulfide. However, when Cu is added to the steel of the present invention, Cu is not only limited in improving the local corrosion resistance but also causes a significant decrease in hot workability. Although it is not possible, it may contain as 0.05 mass% or less as unavoidable impurity. Moreover, even if it adds to the steel of this invention, Ni does not confirm the improvement effect of corrosion resistance and local corrosion resistance, and since it becomes a cost raising factor, it is not added but may contain it if it is 0.05 mass% or less as an unavoidable impurity.

Next, the preferable manufacturing method of the steel material of this invention is demonstrated.

The steel material of this invention finishes the steel adjusted by the said component composition to various shapes, such as a thick steel plate, a thin steel plate, and a shaped steel, in the same way as a conventional steel. For example, the steel of the present invention is a converter or a snow furnace, and the main five elements (C, Si, Mn, P, S) are adjusted in the scope of the present invention by a commonly known method such as a vacuum degassing apparatus. At the same time, other alloying elements are added to the solvent according to the required properties, and the steel is then made into a steel slab by a continuous casting method or the like, and the steel slab is immediately reheated and hot rolled immediately or after cooling. It is preferable to set it as a product.

The hot rolling conditions are not particularly subject to corrosion resistance steel, but from the viewpoint of securing the mechanical properties required as steel materials used for crude oil tanks and the like, it is preferable to control them at an appropriate hot rolling temperature, rolling reduction ratio and the like. After hot rolling, it is preferable to control a cooling rate according to desired mechanical characteristics. For example, in the case of using a high-strength steel having a tensile strength of 490 N / kPa or higher, the finishing temperature of hot rolling is preferably 750 ° C or higher, and then cooled to 700 ° C or lower at a cooling rate of 2 ° C / sec or higher. . When finishing temperature is less than 750 degreeC, deformation resistance is large and shape control becomes difficult. Moreover, when cooling rate is less than 2 degree-C / sec, or cooling stop temperature exceeds 700 degreeC, tensile strength of 490 N / Pa class or more is hard to be obtained.

When using as a steel material for crude oil tanks, the steel material of this invention obtained by the above can apply | coat a primer containing Zn, and can greatly improve local corrosion resistance and a front surface corrosion resistance. Generally, the steel sheet is primer-coated after the shot blasting is applied to the surface thereof, but in order to uniformly cover the entire surface of the steel sheet, a certain coating thickness or more is required, and in order to improve the local corrosion resistance and the entire surface corrosion resistance, Zn It is preferable that the thickness to apply | coat the primer containing is made into 5 micrometers or more. In addition, the upper limit of the coating amount is not formed from the viewpoint of local corrosion resistance and corrosion resistance of the entire surface. However, when the primer is thick, the cutting property, weldability and economical efficiency deteriorate, so the upper limit is preferably 100 µm.

Example 1

Steels having various component compositions of N0.1 to 33 shown in Table 1 were dissolved in a vacuum melting furnace or converter, and the steel slabs were reheated to 1200 ° C, followed by hot rolling to bring the finishing finish temperature to 800 ° C. A thick steel sheet having a thickness of 16 mm was used.

From the steel plates N0.1-33 obtained in this way, the square small piece of width 50mm x length 50mmx thickness 15mm is cut out, and shot blasting is carried out to the surface, and the coating film thickness of an inorganic zinc primer is 0 micrometer (no coating ), 5-10 micrometers, 15-25 micrometers, and the test piece apply | coated divided into four types, 50-70 micrometers were produced. Subsequently, the test piece was masked with anticorrosive paint on the end face and the back face, and the sludge containing the crude oil component collected from the actual tanker was uniformly applied only to the upper face, which was the test face, to obtain a corrosion test piece. Here, two kinds of test pieces having different surface states of the corrosion test pieces were produced. That is, the test piece (test 1) which apply | coated and tested the sludge uniformly to the test piece, and the thing which mixed sulfur by 50% by weight ratio on a sludge (it is called a sulfur mixing sludge) on the part of the center part 2mmφ of a test piece hereafter, And other parts are test pieces (test 2) which apply | coated only sludge uniformly. In the test 2, a sulfur mixed sludge part becomes a starting point of local corrosion, and promotes local corrosion. This makes it possible to grasp more clearly the influence of the steel component, the influence of the primer, and the combination of the steel component and the primer on the local corrosion inhibition. Moreover, it turns out that this test method has a higher correlation with the exposure test in solid line than Test 1.

These test pieces were then used for the corrosion test immersed in test liquid 6 of the corrosion test apparatus shown in FIG. 1 for 1 month. This corrosion test apparatus is a dual type apparatus of the corrosion test tank (2) and the thermostat (3), and the test solution (6) in the corrosion test tank (2) that can generate local corrosion similar to that generated in the actual oil tank bottom plate. This is included. The test liquid 6, the artificial seawater prescribed in ASTMD1141 test with the mother liquor, and while this solution, 5% vol O ₂ + 10 vol% H by adjusting the partial pressure ratio of ₂ S, was used to introduce a gas mixture (4) comprising the balance being N ₂ gas. The temperature of the test liquid 6 was kept at 50 degreeC by adjusting the temperature of the water 7 put into the thermostat 3. In addition, since the mixed gas 4 is continuously supplied, the test liquid 6 is always stirred.

After the said corrosion test, the rust produced on the surface of the test piece was removed, the corrosion form was visually observed, the corrosion depth of the local corrosion generating part was measured by the dip meter, and it evaluated according to the following rank classification.

1: no local corrosion

2: local corrosion depth less than 0.1 mm

3: local corrosion depth 0.1 mm or more and less than 0.2 mm

4: local corrosion depth 0.2 mm or more and less than 0.6 mm

5: local corrosion depth 0.6 mm or more and less than 1.0 mm

6: local corrosion depth 1.0 mm or more and less than 1.5 mm

7: Local corrosion depth 1.5 mm or more and less than 2.0 mm

8: local corrosion depth 2.0 mm or more

The course of the local corrosion test is shown in Table 2. In the test 1, the evaluation of local corrosion test is Rank 1-3, and the local corrosion depth is suppressed to less than 0.2 mm in the steel plate of N0.1-25 suitable for the component composition of this invention. In particular, all of which applied the zinc primer 5 micrometers or more are rank 1, and local corrosion does not generate | occur | produce. On the other hand, the steel plates of N0.26-31 except the steel plates of N0.32-33 were inferior to the invention example among the comparative steel plates which deviate from the component composition of this invention.

Next, in the test 2, although the local corrosion was promoted more than the test 1, the difference between the types of steel, especially the difference between the types of steel in the zinc primer application | coating state can be seen clearly. That is, the local corrosion of the invention steel sheets 1 to 25 is smaller than the comparative steel sheets 26 to 31 in any of the zinc primer uncoated state and the coated state, similarly to the test 1, but in the test 1, the same level as the invention steel plate It can be seen that comparative steel sheets N0.32 to 33, which were locally corrosive, were inferior to the inventive steel sheet in this test.

As mentioned above, it turns out that the local corrosion resistance of the steel of this invention is excellent.

Example 2

From the steel sheets of N0.1-33 same as those used in Example 1, a rectangular small piece having a width of 25 mm x length 48 mm x thickness 4 mm is cut out, and the surface is subjected to shot blasting, and then the coating film thickness of the inorganic zinc primer. To a test piece coated with 4 types of 0 µm (no coating), 5 to 10 µm, 15 to 25 µm, and 50 to 70 µm, and then the steel df surface on the coating film surface in order to accelerate the corrosion resistance test. The X-shaped cutter scratch which reached | attained was made into the corrosion test piece so that a damage area ratio might be 1.0%.

These test pieces were subjected to a full-face corrosion test that was likely to occur in an environment inside a crude oil tank upper deck using the corrosion test apparatus shown in FIG. 2. This corrosion test apparatus is comprised from the corrosion test tank 9 and the temperature control plate 10, water 13 is injected into the corrosion test tank 9, and the temperature is maintained at 40 degreeC. Then, the front corrosion test, to a corrosive environment simulating a a crude oil tank, the said water (13), 12 vol% CO 2 _{and, 5 vol% O 2, 0.01} vol% SO 2, 0.1 vol% H 2 A mixed gas composed of S and the balance N ₂ was introduced to fill the corrosion test tank 9 with the supersaturated water vapor pressure, and the temperature of the test piece was further changed by the heater and the cooling device through the temperature control panel 10. The cycle was repeatedly changed for 20 days at 0 ° C. × 4 hours + 50 ° C. × 4 hours, so that corrosion due to condensation water could be simulated.

After the above-mentioned whole corrosion test, each test piece was evaluated as follows.

<Zink Primer Unpainted Material>

From the mass change before and after the test, the plate | board thickness loss by corrosion was calculated | required, this was converted into the corrosion plate thickness per year, and it evaluated according to the following rank classifications.

1: corrosion rate less than 0.10 mm / year

2: corrosion rate 0.10 mm / year or more and less than 0.25 mm / year

3: corrosion rate 0.25 mm / year or more and less than 0.50 mm / year

4: corrosion rate 0.50 mm / year or more and less than 1.00 mm / year

5: corrosion rate 1.00 mm / year or more

<Zink primer coating material>

The area ratio of the rust which generate | occur | produced on the surface of each test piece and under coating film was measured, and it evaluated according to the following rank classifications.

1: less than 5% rust area rate

2: rust area rate more than 5% less than 15%

3: rust area rate more than 15% less than 25%

4: rust area rate more than 25% less than 50%

5: rust area rate more than 50%

The result of the said front corrosion test was shown together in Table 2. From the result of Table 2, the steel sheets of N0.1-25 suitable for this invention are all favorable in the rank 1-2 for all corrosion-resistant front surfaces. On the other hand, the comparative steels 26 to 33 were inferior to the inventive steels not only when the inorganic zinc primer was applied but also when applied. As mentioned above, it turns out that the corrosion resistance of the whole surface of invention steel is excellent.

TABLE 1

TABLE 2

[Table 2] Continue

Claims (7)

C: 0.001 to 0.16 mass%, Si: 0.01 to 1.5 mass%, Mn: 0.1 to 2.5 mass%, P: 0.025 mass% or less, S: 0.01 mass% or less, Al: 0.005 to 0.1 mass%, N: 0.001 to Steel for crude oil tanks containing 0.008 mass%, W: 0.001-0.5 mass% and Cr: 0.06 mass% or more and less than 0.20 mass%, with the balance being Fe and unavoidable impurities. The steel material for a crude oil tank according to claim 1, further comprising one or two kinds selected from Sn: 0.005 to 0.3 mass% and Sb: 0.005 to 0.3 mass% in addition to the component composition. The steel material for a crude oil tank according to claim 1 or 2, further comprising Mo: 0.001-0.5 mass% in addition to the component composition. The method according to any one of claims 1 to 3, wherein in addition to the component composition, Nb: 0.001 to 0.1 mass%, V: 0.002 to 0.1 mass%, Ti: 0.001 to 0.1 mass%, and B: 0.01 mass Steel material for crude oil tanks containing 1 type (s) or 2 or more types chosen from% or less. The method according to any one of claims 1 to 4, further comprising one or two selected from Ca: 0.0002 to 0.005 mass% and REM: 0.0005 to 0.015 mass% in addition to the ingredient composition. Steel for crude oil tanks The steel material for a crude oil tank according to any one of claims 1 to 5, wherein a primer coating containing Zn is applied to the surface of the steel material. Crude oil tank comprised from the steel materials in any one of Claims 1-6.

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