CN1329545C - Crude oil tank with welding seam having excellent anti-corrosivity - Google Patents

Abstract

The present invention provides a crude oil tank for corrosion of crude oil that occurs in a steel oil tank, in which the entire crude oil tank including welded joints can exhibit approximately the same excellent corrosion resistance and can suppress corrosion of solid sulfur components The formation of products (residues). Welding to form the crude oil tank, the steel used contains (in mass %) as basic components: C: 0.001-0.2%, Si: 0.01-2.5%, Mn: 0.1-2%, Cu: 0.01-1.5%, Al: 0.001-0.3%, N: 0.001-0.01%; and, it also contains: one or both of Mo: 0.01-0.5% and W: 0.01-1%, the Cu content of the weld metal in the welded joint part /Cu content of steel and (Mo content of weld metal + W content)/(Mo content of steel + W content are 0.15 to 3, so localized corrosion of joints can be suppressed and the corrosion resistance of the crude oil tank as a whole can be improved.

Description

Crude Oil Tank with Welded Seams with Excellent Corrosion Resistance

technical field

The present invention relates to a crude oil tank including welded joints (joints) in a crude oil corrosion environment of a crude oil tanker formed by a welded structure, a steel oil tank for transporting or storing crude oil such as above ground or underground crude oil containers The entire crude oil tank can exhibit approximately the same excellent corrosion resistance, and can also suppress the generation of corrosion products (sludge) containing solid sulfur components.

Background technique

Welded structural steel with excellent strength and weldability should be used in the oil tanks of crude oil tankers that transport crude oil or the steel tanks that transport or store crude oil in above-ground or underground crude oil containers that store crude oil.

Moisture, salt or corrosive gas components contained in crude oil will put steel in a corrosive environment. In particular, on the inner surface of the oil tank of a crude oil tanker, volatile components in crude oil or mixed seawater, salt in oil field salt water, inert gas transported in the oil tank for explosion prevention, engine exhaust gas of so-called ships, and temperature changes between day and night The resulting condensation will form a unique corrosion environment, which will corrode and reduce the wall thickness of the steel plate. Due to the thinning of the steel plate corrosion, it is difficult to maintain the required hull strength, so the steel plate must be switched (cutting off the corroded parts and welding new parts), which will increase the cost. In addition to the aforementioned corrosion damage, a large amount of solid sulfur components (hereinafter, referred to as solid S) are generated and precipitated on the steel surface of the inner surface of the steel oil tank. It should be taken into account that the rust on the inner surface of the corroded deck acts as a catalyst, and _SO2 and _H2S in the gas phase react to form solid S. Generation of new rust due to corrosion of steel and precipitation of solid S alternately occur, whereby layered corrosion products of rust and solid S are deposited. Since the solid S is brittle, the product composed of rust and solid S tends to peel off, fall off, and accumulate as residue at the bottom of the oil tank. It is said that the amount of residue recovered through regular inspections is more than 300 tons in VLCCs. Therefore, in terms of maintenance and management, it is strongly required to reduce residues mainly composed of solid S.

That is, as a steel plate for a crude oil tank, it is required to use a corrosion-resistant steel plate that has excellent corrosion resistance and generates less residues containing solid S.

As a technique for simultaneously realizing the anticorrosion of steel and the reduction of residues mainly composed of solid S, coating and lining anticorrosion are generally used, and anticorrosion by zinc or aluminum spraying has been proposed (for example, Japan Shipbuilding Association No. 242 Research Department, issued in March 2013, "Research on New Corrosion Conditions of Crude Oil Tankers" 2012 Report). However, in addition to the so-called economic problems related to construction costs, microscopic defects and aging corrosion will inevitably occur during the construction of the anti-corrosion layer. Therefore, even if coating and lining are used, periodic inspections and inspections are unavoidable. repair. On the other hand, no technology has been proposed to achieve both corrosion resistance and residue reduction by virtue of the characteristics of steel materials.

Although there are not very few countermeasures and technical solutions for steel materials, they are all limited to the improvement of corrosion resistance. For example, steel for shipbuilding has been proposed that has excellent corrosion resistance in use environments such as ship shell plates, ballast tanks, cargo oil tanks, and cargo holds of mining ships (see, for example, JP-A-2002-17381). This corrosion-resistant steel for shipbuilding contains appropriate amounts of C, Si, Mn, P, S, and Al, and contains Cu: 0.01 to 2.00%, and Mg: 0.0002 to 0.0150%, so it can improve the corrosion resistance of the entire surface and Local corrosion resistance. In addition, there have been proposed corrosion-resistant steels for oil tanks that have excellent corrosion resistance for use in fuel tanks and excellent weldability as shipbuilding structural steels. The corrosion-resistant steel is a S-Cu-Ni-Cr-Al steel containing a very small amount of P-. In order to ensure weldability, the upper limit of the total amount of alloy addition is stipulated by the formula value of this steel. The explosion-proof prime mover exhaust gas has excellent corrosion resistance in the oil tank (for example, see Japanese Patent Application Laid-Open No. 2002-107179). In addition, the above-mentioned other corrosion-resistant steel is a steel containing a small amount of P-very small amount of S-Cu-Ni-Cr-Al. In order to ensure weldability, the upper limit of the total amount of alloy addition is stipulated by the formula value of this steel, and the imported The internal corrosion of the oil tank caused by the exhaust gas of the explosion-proof prime mover in the oil tank has excellent corrosion resistance (see, for example, JP-A-2002-107180). In addition, there have been proposed steel materials having excellent resistance to crude oil containers capable of exhibiting excellent corrosion resistance against corrosion occurring in containers for transporting or storing crude oil, and methods for producing the same (see, for example, JP-A-2002-173736). The corrosion-resistant steel proposed here adds Cu: 0.5-1.5%, Ni: 0.5-3.0%, Cr: 0.5-2.0%, and is limited in the range of 1.0≤0.3 Cu+2.0-Cr-0.5 Cu≤3.8, so that Suppresses localized corrosion that accompanies an increase in the amount of alloy added, and this steel has excellent corrosion resistance in the gaseous and liquid phases of crude oil tanks.

However, although any one of the above-mentioned corrosion-resistant steels is given in the environment of the crude oil tank, the solution of the corrosion resistance of the steel itself is not disclosed, that is, by suppressing the corrosion resistance of the steel itself, especially in the Precipitation of solid S that is largely generated and exfoliated in the gas phase of the oil tank. Therefore, in the application of welded structures such as containers, from the viewpoint of improving the reliability of the structure and prolonging the service life, it is desired to develop a material that is excellent in corrosion resistance, can suppress the generation of residues mainly composed of solid S, and has excellent welding workability. Structural steel.

On the other hand, the crude oil tank has a generally welded structure, and since the coating or lining is not fully implemented, the welded joints are inevitably in the crude oil tank environment. In conventional arc welding or gas electric welding, welding wire or flux is melted to form weld metal. Therefore, in general, the composition and structure of weld metal are different from those of steel. In a corrosive environment, when metals with large differences in chemical composition or structure are adjacent to each other, metals with relatively low electrochemical properties are selectively corroded, and different metals are prone to corrosion. Therefore, there is a concern that if selective corrosion occurs, large localized corrosion will occur.

In the case of making a crude oil tank with ordinary steel that cannot particularly improve corrosion resistance, since the steel with a very large surface area is electrochemically inferior regardless of the welding method or welding material, there is no selectivity. Corrosion of welded joints. However, if the crude oil tank is formed of steel with excellent corrosion resistance, the electrochemical properties of the weld metal will be poor due to the welding method or welding material, and the weld metal will be selectively corroded, so that the crude oil tank as a whole will be damaged. There is a possibility of impairing corrosion resistance. Therefore, in order to achieve good corrosion resistance of the whole crude oil tank formed by welding structure in the crude oil tank environment, not only the steel material but also the welded joint must be considered. However, no technology that can meet this requirement has been found so far. .

Contents of the invention

In order to solve the above-mentioned problems, the object of the present invention is to provide a crude oil oil tank, which can be used in the crude oil corrosion environment of the oil tank of the crude oil tanker formed by the welded structure, the steel oil tank for transporting or storing crude oil on the ground or underground crude oil container, etc. The entire crude oil tank including welded joints can exhibit approximately the same excellent corrosion resistance, and can also suppress the generation of corrosion products (sludge) containing solid sulfur components.

In order to solve the above-mentioned problems, the present inventors found that the chemical composition of the steel is related to the corrosion resistance of the steel on the inside of the crude oil tank plate surface constituting the normal gas phase part, and investigated the influence of the chemical composition of the steel. The chemical composition of the steel is used as the basis, and Cr is not added substantially. By adding a specific amount of any one or two of Mo, W, and Cu, the amount of P and S added as impurities is limited, thereby improving the Corrosion resistance in the environment, while greatly reducing the formation of residues. By carefully studying the relationship between the existence state of Mo and W and the corrosion resistance, it can be found that when Mo and W exist in a solid solution state, it is more ideal for corrosion resistance. In addition, after careful study of the welding metal, the chemical composition of the steel, and the necessary conditions for the metal structure required to achieve the same corrosion resistance as the steel at the welded joint when the steel materials are welded to each other, it was found that by making the weld metal and the steel The content ratio of Cu, Mo and W among them is in a specific range, which can make the steel and the welded joint containing weld metal exhibit the same good corrosion resistance, and it is also newly discovered that the proper formation of the structure of the steel can also be effectively Improve the corrosion resistance of joints.

Since the present invention is mainly made based on the above findings, the main contents thereof are as follows.

A crude oil tank with welded joints with excellent corrosion resistance, characterized in that the steel used to form the crude oil tank contains (in mass %): C: 0.001-0.2%, Si: 0.01-2.5%, Mn: 0.1- 2%, P: 0.03% or less, S: 0.02% or less, Cu: 0.01-1.5%, Al: 0.001-0.3%, N: 0.001-0.01%; and, the steel also contains: Mo: 0.01-0.5% and W: one or two of 0.01% to 1%, the rest is composed of Fe and unavoidable impurities; when steel materials are welded to each other to form a crude oil tank, the weld metals Cu, Mo, W in the welded joint part The content of should satisfy the following formulas (1) and (2):

3≥Cu content of weld metal (mass%)/Cu content of steel (mass%)≥0.15 (1)

3≥(Mo content of weld metal + W content (mass%))

/(Mo content of steel + W content (mass%) ≥ 0.15 (2)

(2) According to the crude oil tank with welded joints with excellent corrosion resistance according to the above-mentioned (1), it is characterized in that: the content of weld metal Cu, Mo and W in the welded joint part should satisfy the following formula respectively (3) and (4):

1.5≥Cu content of weld metal (mass%)/Cu content of steel (mass%)≥0.3 (3)

1.5≥(Mo content of weld metal + W content (mass%))

/(Mo content of steel + W content (mass%) ≥ 0.3 (4)

(3) The crude oil tank having a welded joint excellent in corrosion resistance according to any one of the above (1) or (2), characterized in that: the amount of solid solution Mo and the amount of solid solution W of the steel should satisfy the following Formula (5):

Solid solution Mo+solid solution W≥0.005% (5)

(4) The crude oil tank having a welded joint excellent in corrosion resistance according to any one of (1) to (3), wherein the Cr content in the steel material is less than 0.1%.

(5) The crude oil tank having a welded joint excellent in corrosion resistance according to any one of the above (1) to (4), wherein the steel material further contains Ni: 0.1 to 3%, Co: 0.1 to 1 or 2 of 3%.

(6) The crude oil tank having a welded joint excellent in corrosion resistance according to any one of the above (1) to (5), characterized in that the steel material further contains Sb: 0.01 to 0.3%, Sn: 0.01 to 0.3%, Pb: 0.01 to 0.3%, As: 0.01 to 0.3%, Bi: 0.01 to 0.3%, Se: 0.01 to 0.3%, one or more.

(7) The crude oil tank having a welded joint excellent in corrosion resistance according to any one of the above (1) to (6), wherein the steel material further contains Nb: 0.002 to 0.2%, and V: 0.005 to 0.5%, Ti: 0.002 to 0.2%, Ta: 0.005 to 0.5%, Zr: 0.005 to 0.5%, and B: 0.0002 to 0.005%, one or more.

(8) The crude oil tank having a welded joint excellent in corrosion resistance according to any one of the above (1) to (7), wherein the steel material further contains Mg: 0.0001% to 0.01%, Ca: 0.0005% to 0.01%, Y: 0.0001 to 0.1%, La: 0.005 to 0.1%, Ce: 0.005 to 0.1%, or 1 or more.

(9) The crude oil tank having a welded joint excellent in corrosion resistance according to any one of the above (1) to (8), characterized in that the microstructure of the steel material is composed of bainite and martensite Composition of at least one or two, and the total area ratio of bainite and martensite is 30% or more.

Description of drawings

FIG. 1 is a schematic diagram showing the procedure for obtaining a test piece in a corrosion resistance test of a joint.

Figure 2 is a structural diagram of the corrosion test device.

Fig. 3 is a view illustrating a temperature cycle attached to a test piece.

Detailed ways

In order to overcome the aforementioned problems and achieve its purpose, the present invention provides the following specific means.

First, the reasons for limiting the constituent elements and the contents of the steel materials forming the crude oil tank will be described. The unit of % of the component content in a specification is mass %.

Since removing C to a content below 0.001% will significantly hinder industrial economy, the content of C should be greater than 0.001%, and when used as a strengthening element, its content is preferably greater than 0.002%. On the other hand, if the content exceeds 0.2%, the excessive content will cause deterioration of weldability or joint toughness, etc., which is unfavorable as steel for welded structures. Therefore, in the present invention, 0.001 to 0.2% is used as a limit. scope.

Si is necessary as a deoxidizing element, and its content must be greater than 0.01% in order to exhibit the deoxidizing effect. Si is an element that has not only the effect of improving the corrosion resistance of the entire surface but also the effect of improving the local corrosion resistance. In order to achieve this effect, its content is preferably greater than 0.1%. On the other hand, if the content of Si is too large, adhesion of hot-rolled scale occurs (reduced scale peeling property), and defects caused by scale increase, so in the present invention, the upper limit is limited to 2.5%. In particular, in the case of steel that requires strict corrosion resistance, weldability, base material, and joint toughness, the upper limit is preferably 0.5%.

In order to ensure the strength of the steel, the Mn content must be greater than 0.1%. On the other hand, if it exceeds 2%, the weldability will be deteriorated, or the susceptibility to intergranular embrittlement will be increased. Therefore, in the present invention, the range of Mn should be limited to 0.1-2%.

P is an impurity element, and it should be limited to 0.03% or less because the weldability will be deteriorated if it exceeds 0.03%. In particular, when it is less than 0.015%, it is preferable because it exerts a favorable influence on corrosion resistance and weldability.

S is also an impurity element, and when it exceeds 0.02%, it tends to increase the amount of residue generation. In addition, the upper limit is made 0.02% because the mechanical properties, especially the ductility, are remarkably deteriorated. In terms of corrosion resistance and mechanical properties, the S content is preferably as small as possible, and therefore, less than 0.01% is particularly preferred.

If the content of Cu, Mo, and W is greater than 0.01% at the same time, the corrosion resistance can be effectively improved, and the effect of suppressing the formation of solid S can also be produced. Because content exceeds 1.5%, their effect just reaches saturation almost, on the contrary, can promote the surface cracking of steel sheet, the deterioration of joint toughness etc., and significantly increase adverse effect, therefore, its upper limit is 1.5% in the present invention . In view of the balance between corrosion resistance, the effect of suppressing generation of residue, and manufacturability, it is more preferably 0.01 to 0.5%.

Al is an element useful for deoxidation, and is an element effective for refining the grain size of the heated austenite of the base material by forming AlN. In addition, it has the effect of suppressing the generation of corrosive products including solid S, and thus is beneficial. However, in order to exert its effect, its content must be greater than 0.001%. On the other hand, if the content exceeds 0.3% and becomes excessive, coarse oxides will be formed to deteriorate the ductility, so it must be limited within the range of 0.001% to 0.3%.

Although N in the solid solution state will adversely affect the ductility and toughness, so it is not ideal, but because the combination of V, Al or Ti can effectively affect the refinement of austenite grain size and precipitation strengthening, so , if a small amount is used, the mechanical properties can be effectively improved. In addition, it is industrially impossible to completely remove N in steel, and it is not preferable because a reduction beyond the necessary level would impose an excessive load on the manufacturing process. Therefore, the lower limit should be 0.001% as a range within which adverse effects on ductility and toughness are allowable, industrial control is possible, and load on the manufacturing process is allowable. If the content is too large, solid solution N may increase, which may adversely affect ductility or toughness, so the upper limit of the allowable range should be limited to 0.01%.

Mo and W, like Cu, are important elements for corrosion resistance and precipitation suppression of solid S, and their content must be greater than 0.01% at the same time as Cu. Mo and W have substantially the same effect, and the range of Mo is 0.01 to 0.5%, and the range of W is 0.01 to 1%, and Mo or W must be contained, or both elements must be contained. If the contents of Mo and W are greater than 0.01% at the same time, there will be a significant effect on the corrosion resistance and the suppression of the precipitation of solid S. If the Mo content exceeds 0.5%, and the W content exceeds 1%, on the one hand, the effect of improving the corrosion resistance and the precipitation suppression of solid S will be saturated, and on the other hand, the weldability or toughness will be deteriorated. Therefore, it should be added Mo is limited to 0.01 to 0.5%, and W is limited to 0.01 to 1%. In addition, in order to suppress the formation of precipitates and secure solid solutions of Mo and W, it is more preferable to set the upper limits of Mo and W to 0.1% and 0.2%, respectively.

The above-mentioned ranges of Mo and W are essential conditions. In order to exert the effect on the corrosion resistance more effectively, it is more preferable to ensure that the solid solution amounts of Mo and W are more than a certain amount after the contents are set within the above-mentioned ranges. That is, when Mo and W form coarse precipitates, a depletion layer of the element is formed around them, and the corrosion resistance effect is impaired. Therefore, Mo and W are preferably present as uniformly as possible. Since Mo and W in a solid solution state have the same effect on corrosion resistance, as shown in the formula (5), if the total solid solution amount of the two elements is 0.005% or more, the corrosion resistance can be greatly improved.

In addition, in the present invention, the solid-solution Mo and W that can effectively improve corrosion resistance refer to the amounts obtained by removing the precipitated amounts obtained by sampling residue analysis from the total content. That is, in the sample residue analysis, in the case of extremely fine precipitates that are regarded as solid solution, they are considered to exist uniformly in the steel in a solid solution state, and therefore, can have an effective effect on corrosion resistance. effect.

Although the basic requirements and the reasons for the limitation of the chemical composition of the steel of the present invention are given above, in the present invention, for the purpose of improving various properties of the steel, the limitation of the chemical composition is also selectively given.

Cr is a strengthening element and can be added as needed to adjust the strength. However, since Cr is the element that most accelerates the progress of localized corrosion, if the content exceeds 0.1%, it will deteriorate the corrosion resistance in the crude oil environment. , and will slightly promote the formation of solid S. Therefore, in the present invention, its content of more than 0.1% is not desirable. Therefore, this element should not be intentionally contained, and even if it is contained, it is preferably 0.1% or less.

Ni and Co are elements effective in improving the toughness of the base material or HAZ, and also have the effect of improving corrosion resistance and suppressing residue in steel materials containing Cu and Mo. After the contents of both elements are greater than 0.1%, the effect of improving toughness or improving corrosion resistance can clearly appear. On the other hand, since these two elements are expensive elements, an excess content of both elements exceeding 3% is economically inappropriate and leads to deterioration of weldability. , Co, its content should be limited to 0.1% to 3%.

Since the contents of Sb, Sn, Pb, As, Bi, and Se are all more than 0.01%, in order to have corrosion resistance, especially the effect of inhibiting the development of localized corrosion at the liquid phase part, it is necessary to contain The lower limit of these elements is 0.01%. Since the content of these elements exceeds 0.3%, the content will be excessive and the effect will be saturated. Therefore, considering the adverse effects on other characteristics and economic efficiency, the upper limit should be 0.3%.

Nb, V, Ti, Ta, Zr, and B are effective elements that can increase the strength of steel in small amounts, and their contents are mainly determined according to the needs for strength adjustment. In order to express the respective effects, the Nb content should be 0.002% or more, the V content should be 0.005% or more, the Ti content should be 0.002% or more, the Ta content should be 0.005% or more, and the Zr content should be 0.005% or more. The content of B should be 0.0002% or more. On the other hand, since the content of Nb exceeds 0.2%, the content of V exceeds 0.5%, the content of Ti exceeds 0.2%, the content of Ta exceeds 0.5%, the content of Zr exceeds 0.5%, and the content of B exceeds 0.005%, resulting in significant toughness. deterioration and is therefore not ideal. Therefore, as required, in the case of containing Nb, V, Ti, Ta, Zr, B, the content of Nb should be limited to 0.002-0.2%, the content of V should be limited to 0.005-0.5%, and the content of Ti should be limited to 0.002-0.2%, the content of Ta should be limited to 0.005-0.5%, the content of Zr should be limited to 0.005-0.5%, and the content of B should be limited to 0.0002-0.005%.

Mg, Ca, Y, La, and Ce are effective in controlling the shape of inclusions, and are effective in improving ductility. In addition, they can also effectively improve the HAZ toughness of welded joints with large heat input, but they The inhibitory effect on residue generation by immobilizing S is weak, therefore, their content should be determined as needed. The content of various elements in the present invention should determine the lower limit value according to the lower limit that can show the effect, wherein, the lower limit value of Mg is 0.0001%, the lower limit value of Ca is 0.0005%, the lower limit value of Y is 0.0001%, La The lower limit of Ce is 0.005%, and the lower limit of Ce is 0.005%. On the other hand, the upper limit should be determined according to whether the coarsening of inclusions has an adverse effect on mechanical properties, especially ductility and toughness. In the present invention, from this point of view, the upper limit of Mg and Ca is 0.01 %, the upper limit of Y, La, and Ce is 0.1%.

The reasons for the limitation of the chemical composition in the present invention are given above. In addition, by specifying the microstructure of the steel material, it is also possible to secure an improvement in the localized corrosion resistance of the welded joint portion. That is, when steel materials having the above-mentioned composition ranges are welded to each other, and the composition ratio of Cu, Mo, and W of the weld metal and steel materials in the weld joint is specified in an appropriate range as described later, the weld metal and the steel materials The substructure of the heat-affected part of the welding is composed of at least a low-temperature transformation structure containing acicular ferrite or bainite. In this case, the microstructure of the steel is at least one of bainite and martensite or In the two configurations, the total area ratio of the bainite and martensite is preferably 30% or more. When the total area ratio of bainite and martensite is less than 30%, if the main structure of ferrite or ferrite-pearlite is formed, the corrosion on the steel side will be selectively aggravated, so it will inevitably lead to the corrosion of the steel. Corrosion resistance deteriorates. When the total area ratio of bainite and martensite is 30% or more, the structure, weld metal, weld heat-affected part, and steel are approximately the same in terms of corrosion resistance, localized corrosion is difficult to occur, and crude oil tank The overall corrosion resistance will steadily increase.

Based on the above reasons, when steel materials with specified composition and structure are welded to each other to form a crude oil tank, in order to improve the uniform corrosion resistance of the welded joint and the base metal as a whole, and effectively show the corrosion resistance of the weld metal and steel, improve the crude oil tank Overall corrosion resistance, the balance of the chemical composition of the weld metal and steel is important, especially the ratio of Cu, Mo, W weld metal and steel required to develop corrosion resistance must satisfy the following (1) and (2).

3≥Cu content of weld metal (mass%)/Cu content of steel (mass%)≥0.15 (1)

3≥(Mo content of weld metal + W content (mass%))

/(Mo content of steel + W content (mass%) ≥ 0.15 (2)

Regarding Cu, if the mass % in the weld metal/mass % in the steel material exceeds 3 as in the formula (1), since the steel material from the welding heat-affected part near the weld metal to the base material will be selectively corroded, it is not good. ideal. On the other hand, if the mass % of Cu in the weld metal/mass % in steel is less than 0.15, the electrochemical performance of the weld metal is poor, so it should be avoided as much as possible in order to avoid localized corrosion of the weld metal. Therefore, its mass % is 3-0.15, Preferably it is 3-0.3. In addition, Mo and W must be specified in the same way. Since Mo and W have approximately the same effect on the corrosion state, it is only necessary to specify the total amount of Mo and W, which is the same as Cu. As shown in (2), the difference between Mo and W In the total amount, mass % in weld metal/mass % in steel must be 3 to 0.15, preferably 3 to 0.3. When the total amount of Cu, Mo, and W is close to 1 in mass % in the weld metal/mass % in the steel material, one of the steel materials will be selectively corroded. As shown in (3) and (4), the total amount of Cu, Mo, and W is preferably in the range of 1.5 to 0.3 in mass % in weld metal/mass % in steel material.

Below, the effects of the present invention will be described in detail through examples. In addition, the present invention should not be limited to the Examples described below.

Example

The effects of the present invention will be described based on examples. The test steel is made into steel plate by vacuum melting or converter melting. The steel plate is made into a steel plate with a thickness of 15 to 50 mm adjusted to predetermined strength and toughness by water-cooling heat treatment (TMCP) or reheating quenching and tempering (QT) after hot rolling. In the comparative examples, steel sheets produced by ordinary hot rolling (AR) and controlled rolling (CR) are also included. The chemical composition of steel is shown in Table 1, 2.

Table 1

Partition steel sheet number Chemical composition (mass%) C Si Mn P S Al N Cu Ni Co. Cr Mo W Example of the invention 1 0.17 0.41 1.20 0.011 0.006 0.033 0.0040 0.31 - - - 0.053 - 2 0.13 0.26 1.52 0.010 0.004 0.041 0.0029 0.36 - - 0.008 0.061 - 3 0.11 0.17 1.43 0.006 0.002 0.015 0.0038 0.40 0.29 - - 0.070 - 4 0.07 0.18 1.53 0.007 0.003 0.015 0.0028 0.31 0.38 - 0.005 0.066 - 5 0.10 0.21 1.44 0.008 0.003 0.012 0.0027 0.25 0.26 - - - 0.066 6 0.12 0.31 1.27 0.006 0.004 0.028 0.0039 0.36 0.28 - 0.005 0.028 0.122 7 0.07 0.09 1.36 0.008 0.002 0.048 0.0035 0.28 0.16 0.12 0.006 0.072 0.089 8 0.11 0.15 0.89 0.007 0.001 0.065 0.0051 0.30 1.52 - 0.012 0.096 0.113 9 0.08 0.11 1.51 0.006 0.003 0.018 0.0027 0.35 0.22 0.16 - 0.058 - 10 0.08 0.15 1.55 0.005 0.002 0.017 0.0034 0.27 - 0.21 - 0.039 0.075 comparative example 11 0.13 0.48 1.26 0.015 0.003 0.042 0.0035 - - - - - - 12 0.11 0.26 1.45 0.011 0.003 0.018 0.0046 0.35 0.26 - 0.006 - - 13 0.12 0.32 1.40 0.010 0.003 0.030 0.0030 0.31 - - 0.012 0.542 - 14 0.26 0.27 1.39 0.013 0.004 0.038 0.0025 0.21 0.11 0.11 0.008 0.053 - 15 0.14 0.33 1.06 0.011 0.003 0.009 0.0052 0.14 0.51 - 0.315 0.050 -

Table 2

Distinguish steel sheet number Chemical composition (mass %) Nb Ta V Ti Zr B Sb Sn Pb As Bi Se Mg Ca Y La Ce Example of the invention 1 - - - - - - - - - - - - - - - - - 2 0.013 - - - - - - - - - - - - - - - - 3 0.009 - - 0.015 - - - - 0.012 - - - - 0.0015 - - - 4 0.005 - - 0.009 - 0.0011 0.015 - - - - 0.010 - - - - 0.006 5 0.015 0.051 - 0.010 - - - 0.049 - - 0.042 - 0.0011 - - - - 6 - 0.078 0075 - 0.013 - - 0.038 - 0.044 - 0.025 - - 0.0011 - - 7 0.011 0.022 0.044 0.008 0.010 0.0011 - - - - 0.050 - - - - 0.005 - 8 - - 0.073 - - 0.0016 0.020 - 0.015 0.010 - 0.021 - 0.0018 0.0021 - - 9 - - - 0.012 - 0.0011 0.015 0.030 0.010 - - 0.018 - 0.0009 - - 0.008 10 - 0.051 - 0.013 - 0.0015 - - - - - - 0.0011 0.0020 0.0026 0.011 0.008 comparative example 11 0.015 - - 0.012 - - - - - - - - - - - - - 12 0.009 - - 0.009 - - - - - - - - - 0.0016 - - - 13 0.015 - 0.022 - - - - - - - - 0.0015 - - - - 14 0.010 - - 0.008 - - - - - - - - - - 0.0025 - - 15 - - 0.051 - - - - - - - 0.010 - 0.0013 - - -

table 3

Partition steel plate number Steel plate number Thickness of steel sheet (mm) Manufacturing method of steel plate (Note 1) Steel plate thickness (mm)       Microstructure (Note 2) Solid solution Mo, W   Mechanical properties (Note 3) Organization B(%) M(%) B+M(%) Solid solution Mo(%) Solid solution W(%)  Solid solution Mo+W(%) Yield stress (MPa) Tensile strength (MPa) Charpy vTrs(℃) Example of the invention A1 1 150 TMCP 15 F+B+M 43 11 54 0.023 - 0.023 432 567 -45 A2 2 200 TMCP 20 F+B+M 59 6 65 0.021 - 0.021 458 579 -38 A3 3 200 TMCP 25 F+B+M 35 9 44 0.033 - 0.033 435 550 -54 A4 4 150 TMCP 15 F+B+M 40 3 43 0.026 - 0.026 442 556 -69 A5 5 200 TMCP 20 F+B+M 38 4 42 - 0.028 0.028 442 547 -53 A6 6 250 QT 30 B+M 34 66 100 0.011 0.031 0.042 619 703 -48 A7 7 200 TMCP 25 F+B+M 63 twenty one 84 0.030 0.028 0.058 553 629 -50 A8 8 250 QT 50 B+M 31 69 100 0.026 0.025 0.051 657 728 -71 A9 9 150 TMCP 20 F+B+M 36 7 43 0.024 - 0.024 440 552 -65 A10 10 200 TMCP 25 F+B+M 33 6 39 0.019 0.024 0.043 426 531 -62 A11 3 200 QT 15 B+M 54 46 100 0.026 - 0.026 442 573 -39 A12 6 250 TMCP 20 F+B+M 49 37 86 0.013 0.028 0.041 584 689 -45 A13 8 250 TMCP 50 F+B+M 57 16 73 0.030 0.031 0.061 469 563 -66 comparative example B1 11 150 CR 15 F+P 0 0 0 - - 0 368 497 -40 B2 12 200 AR 20 F+P 0 0 0 - - 0 426 537 -26 B3 13 150 QT 20 m 0 0 100 0.003 - 0.003 755 794 -5 B4 14 200 TMCP 20 F+B+M 13 81 94 0.008 - 0.008 704 762 18 B5 15 200 N 25 F+P 0 0 0 0.018 - 0.018 397 543 -twenty one

Note 1) AR: Ordinary rolling: CR: Controlled rolling: TMCP: Water-cooled processing heat processor: QT: Reheating quenching and tempering (AR for rolling): N: Normalizing

Note 2) F: Ferrite: B: Bainite: M: Martensite: P: Pearlite

Note 3) The foot of the test piece is obtained from the central part of the thickness of the plate along the rolling direction and at a right angle

Table 3 shows the manufacturing method of the steel sheet, the structure, the amount of solid solution Mo and W, and the mechanical properties. In the measurement of bainite and martensite phases in the structure, the magnification of 1000 to 5000 is taken by a scanning electron microscope with more than 10 fields of view at each position 2mm below the surface, 1/4 of the plate thickness, and the center of the plate thickness magnification of the tissue photograph, and the average area ratio (area ratio in the observed cross-section, %) was obtained by an image analysis device. The amounts of solid solution Mo and W were determined by residue analysis of samples taken from the whole thickness of the descaled steel sheet. The tensile properties of the steel sheet (base material) are measured at room temperature using a round bar tensile test piece from the central part of the sheet thickness in a direction perpendicular to the rolling direction. The toughness of the steel plate is also tested at various temperatures from the center of the plate thickness in a direction at right angles to the rolling direction, using a standard 2mm V-notch Charpy impact test piece, in order to obtain the fracture transition critical temperature (vTrs).

For the steel plates in Table 3 manufactured using steel sheets with the chemical compositions shown in Tables 1 and 2, coated arc welded (SMAW) or submerged arc welded (SAW) joints are manufactured using welding materials with the chemical composition shown in Table 4. seam. The composition of Table 4 is the chemical composition of the welding rod (hand rod) in SMAW welding, and shows the composition of the welding wire in SAW welding. In addition, in SAW welding, materials equivalent to JIS Z3352 are used for the flux.

Table 4

Welding material (No. Ma) Chemical composition (mass%) C Si Mn P S Cu Mo W W1 0.11 0.32 1.78 0.012 0.006 0.424 0.090 - W2 0.10 0.32 1.82 0.010 0.006 0.631 0.112 - W3 0.12 0.32 1.69 0.009 0.005 0.471 0.063 0.031 W4 0.11 0.68 1.78 0.011 0.005 0.286 0.052 - W5 0.11 0.32 1.80 0.010 0.005 0.407 0.016 0.055 W6 0.10 0.32 1.78 0.010 0.005 0.417 0.019 0.068 W7 0.11 0.32 1.78 0.009 0.005 0.423 0.232 0.019 W8 0.11 0.32 1.78 0.010 0.005 0.371 0.016 0.222 W9 0.12 0.31 1.77 0.010 0.005 0.364 0.098 - W10 0.11 0.32 1.78 0.010 0.003 0.327 0.059 0.084 W11 0.11 0.68 1.78 0.015 0.005 0.424 0.090 - W12 0.11 0.33 1.78 0.010 0.006 0.560 0.118 0.019 W13 0.09 0.32 1.78 0.010 0.005 0.400 0.120 0.093 W14 0.11 0.68 1.75 0.010 0.005 0.275 0.051 - W15 0.11 0.30 1.78 0.013 0.003 0.503 0.041 0.061 W16 0.11 0.32 1.78 0.010 0.005 0.429 0.091 - W17 0.11 0.32 1.78 0.010 0.004 0.364 0.089 - W18 0.10 0.34 1.77 0.010 0.005 0.210 - - W19 0.11 0.32 1.78 0.014 0.005 0.381 0.070 - W20 0.12 0.32 1.78 0.010 0.007 0.497 0.091 - W21 0.11 0.32 1.78 0.011 0.005 0.757 0.334 - W22 0.10 0.31 1.79 0.011 0.005 - - - W23 0.11 0.30 1.78 0.010 0.004 1.021 0.366 0.043

The grooves are all V-shaped grooves. In Table 5, the welding conditions, the amounts of Cu, Mo, and W in the weld metal (WM), and the composition ratio of the weld metal and the steel sheet in the total of Cu, Mo, and W (Mo+W) are shown. By changing the welding material, the composition ratio of these materials can be changed to include a range other than that of the present invention.

table 5

Partition seam number Steel plate number     Welding Conditions    Mass of Cu, M0, W in weld metal (%)  Welding metal mass%/mass% in steel plate Welding method (Note 1) Welding materials Heat input (KJ/mm) Cu Mo W Cu Mo+W Example of the invention WA1 A1 SAW W1 3 0.39 0.079 0 1.26 1.49 WA2 A2 SAW W2 3 0.55 0.097 0 1.53 1.59 WA3 A3 SAW W3 4.5 0.45 0.065 0.022 1.13 1.24 WA4 A4 SMAW W4 1.5 0.29 0.054 0 0.94 0.62 WA5 A5 SAW W5 3 0.36 0.011 0.058 1.44 1.05 WA6 A6 SAW W6 4.5 0.40 0.022 0.084 1.11 0.71 WA7 A7 SAW W7 3 0.38 0.184 0.040 1.36 1.39 WA8 A8 SAW W8 4.5 0.35 0.040 0.189 1.17 1.10 WA9 A9 SAW W9 3 0.36 0.086 0 1.03 1.48 WA10 A10 SAW W10 4.5 0.31 0.053 0.081 1.15 1.18 WA11 A11 SMAW W11 1.5 0.42 0.087 0 1.05 1.24 WA12 A12 SAW W12 3 0.50 0.091 0.050 1.39 0.94 WA13 A13 SAW W13 4.5 0.37 0.113 0.099 1.23 1.01 WA14 A1 SMAW W14 1.5 0.28 0.051 0 0.90 0.96 WA15 A2 SAW W15 3 0.46 0.047 0.043 1.28 1.48 comparative example WB1 B1 SAW W16 3 0.30 0.064 0 - - WB2 B2 SAW W17 3 0.36 0.062 0 1.03 - WB3 B3 SMAW W18 1.5 0.24 0.078 0 0.77 0.14 WB4 B4 SAW W19 3 0.33 0.065 0 1.57 1.23 WB5 B5 SAW W20 3 0.39 0.079 0 2.79 1.58 WB6 A3 SAW W21 4.5 0.65 0.255 0 1.63 3.64 WB7 A5 SAW W22 3 0.07 0.000 0.008 0.28 0.12 WB8 A5 SAW W23 3 0.79 0.256 0.050 3.16 4.64

Note 1) SMAW: coated arc welding; SAW: buried arc welding

Table 6 is a test for evaluating the corrosion resistance and selective corrosion tendency of joints, and Table 7 is a test for mainly evaluating the corrosion resistance of the entire surface of steel materials and the generation of residues.

The test conditions for evaluating the corrosion resistance of joints in Table 6 are as follows.

Test pieces were obtained from the welded joints in Table 5, and a corrosion test of the welded joints in an environment simulating the crude oil tank environment was performed. As shown in the schematic diagram in Figure 1, from the position of 1mm on the surface of the steel plate in the welded joint, in the way of including weld metal (WM), welding heat-affected zone (HAZ), and base metal (BM), obtain a length of 80mm, For a test piece with a width of 40mm and a thickness of 4mm, the entire surface of the test piece is mechanically ground. After 600 times of wet grinding, only one surface of the 80mm×40mm surface layer remains, and the end surface and the inner surface are covered with paint. This test piece was immersed in two kinds of corrosion solutions of pH 2.0 and 1 volume % HCl aqueous solution in which 20 mass % of NaCl was dissolved. The immersion conditions are carried out under the conditions of liquid temperature 30°C and immersion time 336 hours, and the maximum corrosion depth at each position of the weld metal (WM), welding heat-affected zone (HAZ), and base metal (BM) is measured, and converted Evaluation was performed as corrosion rate (mm/year).

Table 6

Partition seam number Steel plate number    Maximum corrosion rate (mm/year)(Note 1) WM HAZ BM Example of the invention WA1 A1 0.19 0.20 0.21 WA2 A2 0.18 0.18 0.20 WA3 A3 0.18 0.17 0.16 WA4 A4 0.22 0.19 0.20 WA5 A5 0.17 0.22 0.19 WA6 A6 0.19 0.19 0.21 WA7 A7 0.16 0.17 0.17 WA8 A8 0.19 0.19 0.20 WA9 A9 0.16 0.17 0.17 WA10 A10 0.17 0.17 0.19 WA11 A11 0.18 0.18 0.17 WA12 A12 0.19 0.20 0.20 WA13 A13 0.18 0.20 0.21 WA14 A1 0.22 0.21 0.20 WA15 A2 0.18 0.17 0.20 Comparative example WB1 B1 0.23 1.24 1.45 WB2 B2 0.22 0.95 0.88 WB3 B3 1.54 0.31 0.29 WB4 B4 0.25 0.24 0.21 WB5 B5 0.28 1.61 1.72 WB6 A3 0.32 0.98 0.97 WB7 A5 1.38 0.25 0.19 WB8 A5 0.35 1.44 1.46

Note 1) Corrosion conditions: Adjust the pH to 2.0 with 1N HCl and contain 20% NaCl

Solution -30℃×336h

Composed of weld metal (WM), weld heat-affected zone (HAZ), base metal (BM)

The maximum corrosion depth of the position (mm) conversion (mm/year)

Table 7

Partition Steel plate number steel sheet number Relative Corrosion Rate (Note 1) Relative residue generation rate (Note 2) Example of the invention A1 1 25.5 28.3 A2 2 24.9 25.1 A3 3 23.4 20.8 A4 4 23.6 22.0 A5 5 24.1 22.2 A6 6 22.6 23.6 A7 7 21.4 19.8 A8 8 22.7 16.6 A9 9 24.3 24.1 A10 10 22.1 18.6 A11 3 22.9 20.1 A12 6 22.3 23.4 A13 8 22.5 16.3 Comparative example B1 11 100 100 B2 12 95.3 96.1 B3 13 41.3 26.7 B4 14 29.6 25.1 B5 15 63.1 111.2

Note 1) Take comparative example B1 plus corrosion rate (0.52mm/y) as a relative value of 100

Note 2) The mass of the corrosion product containing precipitated solid S in Comparative Example B1 (1180 mg/test piece)

as a relative value of 100

Next, the corrosion test conditions for investigating the corrosivity of the entire surface of steel materials and the generation of residues are as follows.

A test piece having a length of 40 mm, a width of 40 mm, and a thickness of 4 mm was obtained from the steel plate shown in Table 3 so that the 1/4 position of the plate thickness of the steel plate was the center of the test piece. The entire surface of the test piece was mechanically ground, and after performing wet grinding 600 times, a surface of 40 mm×40 mm remained, and the inner surface and the end surface were covered with paint. The corrosion rate of the test steel and the generation rate of the residue mainly composed of solid S were evaluated using the test apparatus shown in FIG. 2 . In Table 8, the compositions of the gases used in the corrosion tests are shown.

Table 8

gas composition CO ₂ H ₂ S O ₂ N ₂ Concentration 12% by volume 500ppm 5% by volume margin

The gas passes through the dew point adjustment water tank 2, and after being adjusted to a certain dew point (30°C), it is transported into the test chamber 3. Before the corrosion test, the NaCl aqueous solution was applied on the surface of the test piece 4 and dried so that the NaCl adhesion amount was 1000 mg/m ² , and it was placed horizontally on the constant temperature heating plate 5 in the test chamber. By controlling the heating controller 6, as shown in FIG. 3 , a temperature cycle of 20° C.×1 hour and 40° C.×1 hour totaling 2 hours/cycle is given to produce repeated drying and wetting on the surface of the test piece. After 720 cycles, the corrosion rate was evaluated by the corrosion loss, and the residue generation rate was evaluated by the generated substances on the surface of the test piece. In addition, chemical analysis and X-ray analysis were performed on the product, and iron hydroxide (rust) and solid S were confirmed by preliminary tests.

In the examples, first, with regard to the mechanical properties, as shown in FIG. 3, it can be seen from Table 3 that the steel plates with steel plate numbers A1 to A13 satisfying the requirements of the present invention all have sufficient base material properties as steel for welded structures. . In the comparative example, since the steel sheet numbers B3 and B4 were excessive in Mo and Cu as component contents, the toughness was remarkably deteriorated compared with the steel sheet having the chemical composition of the present invention.

For corrosion resistance, first look at the corrosion resistance of the joints in Table 6. Among the welded joints with joint numbers WA1-WA15 that satisfy the chemical composition of the steel and the chemical composition ratio of the weld metal and the steel according to the present invention, Regardless of the welding method and heat input, WM, HAZ, and BM all produce roughly uniform corrosion, and their corrosion rates are also very low.

On the other hand, in the case of the welded joints of the joint numbers WB1 to WB8 in the comparative example, as shown below, it can be seen that the selective corrosion of specific parts does not satisfy the requirements of the present invention. The speed is significantly increased, and as a crude oil tank formed by welding structure, the corrosion resistance is significantly deteriorated compared with the present invention.

That is, the chemical composition of the steel materials with joint numbers WB1 to WB2 does not contain all or part of the elements necessary to ensure corrosion resistance. Since the chemical composition does not satisfy the present invention, the corrosion resistance of the steel materials themselves is poor, so , Compared with WM, the corrosion rate of HAZ and BM increases significantly.

In the joint number WB3, since the Mo content is too large, the toughness of the steel plate is insufficient as shown in Fig. The composition ratio, that is, (Mo content of weld metal + W content (mass %))/(Mo content of steel + W content (mass %)) is too small, therefore, in the corrosion test of the joint, due to selective corrosion WM, therefore, has corrosion resistance problems with crude oil tanks.

In the joint number WB4, since the amount of C is too large, the toughness of the steel material is insufficient as structural steel.

In the joint number WB5, Cr is intentionally added, and because the content is too large, the corrosion rate of BM is significantly increased, so it is not ideal.

In the joint number WB6, the composition ratio of WM and steel is too large for Cu and (Mo+W), so although the corrosion of WM can be suppressed, the corrosion rate of HAZ and BM near WM is different from that of It is not preferable because the ratio increases when only steel materials are corroded.

In the joint number WB7, for Cu and (Mo+W), since the composition ratio of WM and steel is too small, WM is selectively corroded, because the corrosion rate of WM will be significantly increased, therefore, The corrosion resistance of the crude oil tank is insufficient.

The joint number WB8 is the same as the joint number WB6. For Cu and (Mo+W), since the composition ratio of WM and steel is too large, although the corrosion of WM can be suppressed, the HAZ near WM and Since the corrosion rate of BM increases compared with the case where only steel materials are corroded, it is not preferable.

Next, for steel sheets, the results of Table 6, which mainly studied the overall surface corrosion resistance and residue resistance, show that the corrosion rate and residue generation rate of steel sheets with the chemical composition of the present invention are A1-A13. Compared with the steel plate number B1 of the comparative example of any one of Cu, Mo, and W, it has excellent corrosion resistance and residue resistance on the entire surface while ensuring that it is reduced to 30% or less. In the crude oil tank made of steel satisfying the requirements of the present invention, the base material portion other than the joint exhibits good corrosion resistance and residue resistance.

On the other hand, it can be seen that since the steel sheets of the steel sheet numbers B1, B2, and B5 of the comparative example do not satisfy the requirements regarding the chemical composition in the present invention, the entire surface of the steel sheet itself is poor in corrosion resistance and/or residue resistance. , in a crude oil tank formed of steel that does not satisfy the requirements of the present invention, it cannot be expected that the crude oil tank as a whole has sufficient corrosion resistance.

That is, since the steel plate number B1 does not contain any of Cu, Mo, and W necessary for the development of corrosion resistance, the corrosion resistance and residue resistance of the steel plate itself are lower than those of the examples of the present invention. very bad.

Since the steel plate number B2 does not contain Mo and W, the requirements necessary for the development of corrosion resistance cannot be achieved. Therefore, compared with the examples of the present invention, the corrosion resistance and residue resistance of the steel plate itself are inferior.

Since the steel plate number is B5, since the amount of Cr is high, it is not preferable because the formation of residues is significantly promoted. The corrosion resistance of the entire surface is also relatively poor.

In addition, in the steel sheet numbers B3 and B4 in the comparative example, since both Mo and C are excessive as the component content, the corrosion resistance can be improved, but the toughness is lower than that of the steel sheet having the chemical composition of the present invention. Significantly worse, they are examples of insufficient mechanical properties as structural steels.

From the above examples, it can be seen that according to the present invention, the entire crude oil tank including welded joints exhibits approximately the same excellent corrosion resistance, and can also suppress the generation of corrosion products (residue) containing solid sulfur components.

The present invention provides a crude oil tank including a crude oil tank with welded joints under the crude oil corrosion environment of a crude oil tanker formed by a welded structure, a steel tank for transporting or storing crude oil such as an aboveground or underground crude oil container Overall, it can show approximately the same excellent corrosion resistance, and can also suppress the formation of corrosion products (residue) containing solid sulfur components, thereby contributing to the improvement of crude oil tanks, steel structures with crude oil tanks, and ships. Long-term reliability, safety and economy.

Claims (8)

1. A crude oil tank with a welded joint excellent in corrosion resistance, characterized in that: the steel used to form the crude oil tank contains, in mass %: C: 0.001-0.2%, Si: 0.01-2.5%, Mn: 0.1 to 2%, P: 0.03% or less, S: 0.02% or less, Cu: 0.01 to 1.5%, Al: 0.001 to 0.3%, and N: 0.001 to 0.01% as basic components; Contains: one or both of Mo: 0.01-0.1% and W: 0.01-1%, the rest is composed of Fe and unavoidable impurities, and the amount of solid solution Mo and solid solution W of the steel satisfies the relationship: Solid solution Mo + solid solution W ≥ 0.005%; wherein when the steel is welded to each other to form a crude oil tank, the content of the weld metal Cu, Mo, and W in the welded joint part should satisfy the following formulas (1) and (2) respectively : 3≥Cu content of weld metal (mass%)/Cu content of steel (mass%)≥0.15 (1) 3≥(Mo content of weld metal + W content (mass%)) /(Mo content of steel + W content (mass%)) ≥ 0.15 (2). 2. The crude oil tank with welded joints excellent in corrosion resistance according to claim 1, characterized in that: the content of weld metal Cu, Mo, W in the welded joint part should satisfy the following formula (3) respectively and (4): 1.5≥Cu content of weld metal (mass%)/Cu content of steel (mass%)≥0.3 (3) 1.5≥(Mo content of weld metal + W content (mass%)) /(Mo content of steel + W content (mass%)) ≥ 0.3 (4). 3. The crude oil tank having welded joints with excellent corrosion resistance according to claim 1 or 2, characterized in that the Cr content in the steel is less than 0.1%. 4. The crude oil tank having welded joints with excellent corrosion resistance according to any one of claims 1 to 3, characterized in that the steel material also contains Ni: 0.1 to 3%, Co: 0.1 to 1 or 2 of 3%. 5. The crude oil tank with welded joints with excellent corrosion resistance according to any one of claims 1 to 4, characterized in that the steel material also contains Sb: 0.01 to 0.3%, Sn: 0.01 to 0.01% by mass % 0.3%, Pb: 0.01 to 0.3%, As: 0.01 to 0.3%, Bi: 0.01 to 0.3%, Se: 0.01 to 0.3%, one or more. 6. The crude oil tank with welded joints excellent in corrosion resistance according to any one of claims 1 to 5, characterized in that the steel material also contains Nb: 0.002 to 0.2%, V: 0.005 to 0.5%, Ti: 0.002 to 0.2%, Ta: 0.005 to 0.5%, Zr: 0.005 to 0.5%, and B: 0.0002 to 0.005%, one or more. 7. The crude oil tank having welded joints with excellent corrosion resistance according to any one of claims 1-6, characterized in that the steel material also contains Mg: 0.0001-0.01%, Ca: 0.0005-0.01% by mass % %, Y: 0.0001 to 0.1%, La: 0.005 to 0.1%, and Ce: 0.005 to 0.1%, one or more. 8. The crude oil tank with welded joints with excellent corrosion resistance according to any one of claims 1 to 7, characterized in that: the microstructure of the steel material is at least one of bainite and martensite or Two kinds of constitutions, the total area ratio of the above-mentioned bainite and martensite is 30% or more.