JP2012117137A - Steel for crude oil tank having excellent corrosion resistance, welded joint, and crude oil tank - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel for a crude oil tank capable of reducing greatly overall corrosion or local corrosion generated in the crude oil tank, and to provide a welded joint and the crude oil tank.SOLUTION: When manufacturing a crude oil tank by welding a steel containing in mass%, 0.03-0.16% C, 0.05-1.50% Si, 0.1-2.0% Mn, ≤0.025% P, ≤0.010% S, 0.005-0.10% Al, ≤0.008% N, 0.0001-0.01% Hf and 0.03-0.4% Cu, and also containing one or two or more kinds selected from 0.01-1.0% W, 0.01-0.5% Mo, 0.005-0.2% Sn and 0.005-0.4% Sb, a welded joint is formed, in which a welded metal part satisfies following relations: 1<(Cu content in a welded metal/Cu content in a base metal)≤50, and 0.25≤(Cu content in the welded metal/total content of Mo and W in the welded metal)≤3.

Description

  The present invention relates to an oil tank of a crude oil tanker constructed by welding steel materials and a tank for transporting or storing crude oil (hereinafter collectively referred to as “crude oil tank”). Steel materials for crude oil tanks with excellent corrosion resistance that can reduce overall corrosion that occurs on the ceiling, side walls, and bottom and local corrosion that occurs on the bottom plate of crude oil tanks, welded joints welded with these steel materials, and those steel materials The present invention relates to a crude oil tank composed of welded joints. The steel materials used in the crude oil tank of the present invention include thick steel plates, thin steel plates and shaped steels.

It is known that the steel used on the inner surface of a tanker's crude oil tank, particularly on the back of the upper deck and the upper part of the side wall, is totally corroded. The cause of this general corrosion is that sulfate ions and chloride ions are detected in highly acidic condensed water in dock inspections of actual ships every 2.5 years.
(1) Repeated condensation and drying (wet and dry) on the steel sheet surface due to temperature difference between day and night,
(2) Inert gas sealed in the crude oil tank for explosion protection (O ₂ about 4 vol%, CO ₂ about 13 vol%, SO ₂ about 0.01 vol%, boiler N ₂ or the exhaust gas of the engine, etc.) Dissolution of O ₂ , CO ₂ , SO ₂ in condensed water,
(3) Dissolution of corrosive gas such as H ₂ S volatilized from crude oil into condensed water,
(4) Residual seawater used for cleaning crude oil tanks,
Etc. are considered.

Furthermore, when H ₂ S is oxidized using iron rust generated by corrosion as a catalyst, solid S is formed in layers in the iron rust, but these corrosion products are easily peeled off and dropped off, resulting in crude oil tanks. Deposit at the bottom of the. For this reason, in the dock inspection, the current situation is that repair of the upper part of the tank and collection of deposits at the bottom of the tank are performed with great expense.

On the other hand, the steel used for the bottom plate of a tanker's crude oil tank, etc. does not corrode due to the corrosion inhibition effect of the crude oil itself or the corrosion inhibition effect of the protective coat (oil coat) derived from the crude oil formed on the inner surface of the crude oil tank. It was considered a thing. However, recent research has revealed that bowl-shaped local corrosion (pitting corrosion) occurs in the steel plate of the tank bottom plate. As a cause of this local corrosion,
(1) presence of condensed water in which salts represented by sodium chloride are dissolved at a high concentration;
(2) Oil coat detachment due to excessive cleaning,
(3) High concentration of sulfides contained in crude oil,
(4) Increasing the concentration of O ₂ , CO ₂ , SO _2, etc. in the inert gas for explosion protection dissolved in dew condensation water,
Etc. are considered. In fact, at the time of a dock inspection of an actual ship, when water accumulated in the crude oil tank is analyzed, high concentrations of chloride ions and sulfate ions are detected.

  By the way, the simplest and most effective method for preventing the above-mentioned general corrosion and local corrosion is to apply heavy coating on the surface of the steel material to shield the steel material from the corrosive environment. However, the painting operation of crude oil tanks requires an enormous area to be applied, and the paint film must be repainted once every 10 years due to the deterioration of the coating film. To do. Furthermore, it has been pointed out that corrosion is promoted on the damaged part of the heavy-painted coating film in the corrosive environment of the crude oil tank.

For the corrosion problem as described above, several techniques for improving the corrosion resistance of the steel material itself in the corrosive environment of the crude oil tank have been proposed. For example, in Patent Document 1, in mass%, C: 0.001 to 0.2%, Si: 0.01 to 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%, N: 0.001 to 0.01%, and Mo: 0.0. When forming a welded joint by welding steel materials containing 01 to 0.5% and W: 0.01 to 1% or two types, the balance being Fe and inevitable impurities, in the weld metal A technique for forming a welded joint so that the contents of Cu, Mo, and W satisfy the following three formulas is disclosed.
0.15 ≦ Cu content of weld metal (mass%) / Cu content of steel (mass%) ≦ 3
0.15 ≦ (Mo content of weld metal + W content (mass%)) / (Mo content of steel + W content (mass%)) ≦ 3
−0.3 ≦ (Cu content of weld metal (mass%) − Cu content of steel (mass%)) ≦ 0.5

Moreover, in patent document 2, 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 to 1.5%, Al: 0.001 to 0.3%, N: 0.001 to 0.01%, and Mo: 0 When forming a crude oil tank by welding steel materials containing 0.01 to 0.5% and W: 0.01 to 1% or two of which are composed of Fe and inevitable impurities. A technique for forming a welded joint so that the content of Cu, Mo, W in the inside satisfies the following two formulas is disclosed.
0.15 ≦ Cu content of weld metal (mass%) / Cu content of steel (mass%) ≦ 3
0.15 ≦ (Mo content of weld metal + W content (mass%)) / (Mo content of steel + W content (mass%)) ≦ 3

Japanese Patent Laid-Open No. 2005-21981 Japanese Patent Laid-Open No. 2005-23421

  However, with the techniques described in Patent Document 1 and Patent Document 2, it is difficult to suppress local corrosion (pitting corrosion) generated in the tanker bottom plate and the welded joint to 4 mm or less in 2.5 years. This is because, in recent years, corrosion surveys of actual ships have revealed that the pH of the solution inside the pitting corrosion generated on the tanker bottom plate and the welded portion is 1.0 or less. In general, it is known that the corrosion rate of a steel material in an acidic solution is rate-limited by a hydrogen reduction reaction, and increases dramatically as the pH decreases. Therefore, the immersion test at pH 2.0 as described in the examples of Patent Documents 1 and 2 does not sufficiently reflect the corrosive environment in the actual ship.

  Accordingly, an object of the present invention is to provide a steel material for a crude oil tank having excellent overall corrosion resistance on the top plate of a crude oil tank such as a tanker oil tank and a local corrosion resistance on the bottom plate of the crude oil tank, and a welded joint obtained by welding the steel material. And to provide a crude oil tank composed of those steel materials and welded joints.

  The inventors have intensively studied to solve the above problems. As a result, the corrosion resistance of the steel material can be improved by controlling the component composition of the steel within an appropriate range, and is included in the weld metal of the welded joint when the steel material having the component composition is welded to form a crude oil tank. By controlling the content of Cu, Mo and W within an appropriate range, not only the steel materials constituting the crude oil tank, but also the overall corrosion and local corrosion that occur in the welded joints can be reduced, and the overall corrosion resistance of the crude oil tank is significantly improved. The inventors have found that it can be improved and completed the present invention.

  That is, the present invention includes C: 0.03-0.16 mass%, Si: 0.05-1.50 mass%, Mn: 0.1-2.0 mass%, P: 0.025 mass% or less, S: 0 0.010 mass% or less, Al: 0.005 to 0.10 mass%, N: 0.008 mass% or less, Hf: 0.0001 to 0.01 mass%, Cu: 0.03 to 0.4 mass%, and , W: 0.01 to 1.0 mass%, Mo: 0.01 to 0.5 mass%, Sn: 0.005 to 0.2 mass%, and Sb: 0.005 to 0.4 mass% It is a steel material for a crude oil tank that contains seeds or two or more kinds, and the balance consists of Fe and inevitable impurities.

In addition to the above component composition, the steel material for crude oil tank according to the present invention further includes at least one component selected from the following groups A to E.
Group A; Ni: 0.005 to 0.4 mass%, Co: 0.01 to 0.4 mass%, and Cr: 0.01 to 0.3 mass%, or one or more B groups; Ta: 0.001 to 0.1 mass%, Nb: 0.001 to 0.1 mass%, Ti: 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 One or two or more C groups selected from 0.2 mass%; Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass%, Y: 0.0001 to 0.1 mass% 1 type or 2 types or more chosen from among D groups; B: 0.0002-0.003 mass%
Group E; Bi: 0.001 to 0.1 mass%

Moreover, this invention is a welded joint which uses any one of the above steel materials as a base material, and the composition of the weld metal part is the following formulas (1) and (2);
1 <(Cu content in weld metal / Cu content in base material) ≦ 5 (1)
0.25 ≦ (Cu content in weld metal / total content of Mo and W in weld metal) ≦ 3
... (2)
It is the welded joint characterized by satisfy | filling.

  Moreover, this invention is a crude oil tank characterized by having said weld joint.

  According to the present invention, the overall corrosion and local corrosion generated in a crude oil tank formed by welding, such as an oil tank of a crude oil tanker or a tank for transporting or storing crude oil, can be observed in all parts including not only steel plates but also welded joints. Can be suppressed. Therefore, according to the present invention, it is possible to reduce the repair load of the ship and to extend the repair period, and as a result, the service life of the ship can be greatly extended, so that there is a remarkable industrial effect.

In the Example of this invention, it is a figure explaining the test apparatus used for the general corrosion test. In the Example of this invention, it is a figure explaining the test apparatus used for the pitting corrosion test.

First, the component composition of the steel material used for the crude oil tank of the present invention will be described.
C: 0.03-0.16 mass%
C is an element that enhances the strength of steel. In the present invention, 0.03 mass% or more is added in order to secure a desired strength. On the other hand, addition exceeding 0.16 mass% reduces weldability and toughness of the heat affected zone. Therefore, C is set to a range of 0.03 to 0.16 mass%.

Si: 0.05-1.50 mass%
Si is an element added as a deoxidizer, but is also an effective element for increasing the strength of steel. Therefore, in the present invention, 0.05 mass% or more is added in order to ensure a desired strength. However, addition exceeding 1.50 mass% reduces the toughness of steel. Therefore, Si is set to a range of 0.05 to 1.50 mass%.

Mn: 0.1 to 2.0 mass%
Mn is an element that increases the strength of 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 2.0 mass% reduces the toughness and weldability of steel. Therefore, Mn is in the range of 0.1 to 2.0 mass%.

P: 0.025 mass% or less P is a harmful element that segregates at the grain boundaries and lowers the toughness of the steel, and is desirably reduced as much as possible. In particular, if added over 0.025 mass%, the toughness is greatly reduced. Moreover, when P is added exceeding 0.025 mass%, it will also have a bad influence on the corrosion resistance in a tank oil tank. Therefore, P is set to 0.025 mass% or less. Preferably it is 0.015 mass% or less.

S: 0.010 mass% or less S is a harmful element that forms MnS, which is a non-metallic inclusion, and serves as a starting point for local corrosion and reduces local corrosion resistance. It is desirable to reduce it as much as possible. In particular, addition exceeding 0.010 mass% leads to a significant decrease in local corrosion resistance. Therefore, the upper limit of S is 0.010 mass%. Preferably, it is 0.005 mass% or less.

Al: 0.005-0.10 mass%
Al is an element added as a deoxidizer, and 0.005 mass% or more is added in the present invention. However, if added in excess of 0.10 mass%, the toughness of the steel decreases, so the upper limit of Al is 0.10 mass%.

N: 0.008 mass% or less N is a harmful element that lowers toughness, and is desirably reduced as much as possible. In particular, if added over 0.008 mass%, the toughness is greatly lowered, so the upper limit is made 0.008 mass%.

Hf: 0.0001 to 0.01 mass%
Hf is an element that is taken into the rust layer generated by corrosion and has an effect of suppressing the overall corrosion of the steel material by forming a dense rust layer. Such an effect can be obtained by adding 0.0001 mass% or more. On the other hand, addition exceeding 0.01 mass% is not preferable because it causes a decrease in low-temperature toughness. Therefore, Hf is added in the range of 0.0001 to 0.01 mass%.

Cu: 0.03-0.4 mass%
Cu is an element that increases the strength of the steel, and also exists in the rust generated by the corrosion of the steel and has the effect of increasing the corrosion resistance. These effects cannot be sufficiently obtained with addition of less than 0.03 mass%. On the other hand, addition over 0.4 mass% saturates the effect of improving corrosion resistance and causes problems such as surface cracking during hot working. . Therefore, Cu needs to be added in the range of 0.03 to 0.4 mass% from the viewpoint of stably producing the steel material of the present invention.

The steel material of this invention needs to contain 1 type, or 2 or more types chosen from W, Mo, Sn, and Sb other than the said component in the following range.
W: 0.01-1.0 mass%
W has the effect of suppressing pitting corrosion at the tanker tank bottom plate and the effect of suppressing overall corrosion of the tanker upper deck. The above effect is manifested by addition of 0.01 mass% or more. However, if it exceeds 1.0 mass%, the effect is saturated. Therefore, W is added in the range of 0.01 to 1.0 mass%. Preferably it is 0.01-0.5 mass%, More preferably, it is the range of 0.02-0.3 mass%.

The reason why W has the above-described effect of improving corrosion resistance is that WO ₄ ²⁻ is generated in the rust generated as the steel sheet corrodes, and the presence of this WO ₄ ²⁻ causes chloride ions to be generated. Intrusion into the steel sheet surface is suppressed, and furthermore, FeWO ₄ is generated at the site where the pH is lowered, such as the anode part on the steel sheet surface, and the presence of this FeWO ₄ also suppresses the entry of chloride ions into the steel sheet surface. Because it is done. Further, even by the inhibitor effect of adsorption to WO ₄ ^2-steel surfaces, believed to corrosion of the steel material is prevented.

Mo: 0.01-0.5 mass%
Mo not only suppresses pitting corrosion on the bottom plate of the tanker tank, but also improves overall corrosion resistance on the back of the tanker's upper deck and corrosion resistance after painting in a corrosive environment where salt water is repeatedly immersed and highly humid like a ballast tank. There is an effect to make. The effect of Mo is manifested by addition of 0.01 mass% or more, but the effect is saturated when it exceeds 0.5 mass%. Therefore, Mo is set to a range of 0.01 to 0.5 mass%. Preferably it is 0.02-0.5 mass%, More preferably, it is the range of 0.03-0.4 mass%.
The reason why Mo has the above-described effect of improving corrosion resistance is that, like W, MoO ₄ ²⁻ is generated in the rust generated along with corrosion of the steel sheet, and the presence of MoO ₄ ²⁻ causes chloride ions. This is thought to be because the penetration of steel into the steel sheet surface is suppressed.

Sn: 0.005-0.2 mass%, Sb: 0.005-0.4 mass%
Sn and Sb have the effect of suppressing pitting corrosion on the tanker tank bottom plate and the effect of suppressing the overall corrosion of the tanker upper deck. The above effects are manifested by the addition of Sn: 0.005 mass% or more and Sb: 0.005 mass% or more. On the other hand, even if Sn: more than 0.2 mass% and Sb: more than 0.4 mass% are added, the effect is saturated. Furthermore, the addition of a large amount of Sn promotes surface cracking during hot working with Cu. Therefore, Sn and Sb are preferably added in the above ranges.

Moreover, it is preferable that the steel material of this invention contains the 1 type (s) or 2 or more types chosen from Ni, Co, and Cr other than the said essential component in the following range.
Ni: 0.005-0.4 mass%, Co: 0.01-0.4 mass%
Ni and Co have the effect of refining the generated rust particles to improve the corrosion resistance in the bare state and the corrosion resistance in the state where the epoxy primer is applied to the zinc primer. Therefore, these elements are preferably added in an auxiliary manner when it is desired to further improve the corrosion resistance. The above effects are manifested by adding Ni: 0.005 mass% or more and Co: 0.01 mass% or more. On the other hand, even if Ni is added in excess of 0.4 mass% and Co is added in excess of 0.4 mass%, the effect is saturated. Further, Ni has an effect of suppressing surface cracks during hot working that occur in steel containing Cu or Sn. Therefore, Ni and Co are preferably added within the above ranges.

Cr: 0.01-0.3 mass%
Cr moves into the rust layer with the progress of corrosion and blocks the penetration of Cl ^{− into} the rust layer, thereby suppressing the concentration of Cl ^{− at} the interface between the rust layer and the ground iron. In addition, when a Zn-containing primer is applied, a complex oxide of Cr or Zn centering on Fe can be formed, and Zn can be kept on the surface of the steel sheet for a long period of time, thus dramatically improving the corrosion resistance. Can do. The above-mentioned effect is particularly remarkable in a portion that comes into contact with a liquid containing a high concentration of salt separated from a crude oil component, such as a bottom plate portion of a tanker oil tank, and a Zn-containing primer treatment is applied to the steel material in the above-mentioned portion containing Cr. As a result, the corrosion resistance can be remarkably improved as compared with a steel material not containing Cr. For the effect of Cr, addition of less than 0.01 mass% is not sufficient. On the other hand, addition of more than 0.3 mass% deteriorates the toughness of the weld. Therefore, Cr is preferably added in a range of 0.01 to 0.3 mass%.

Further, in the steel material of the present invention, Ta, Nb, Ti, V and Zr can be further added in the following range in addition to the above components for the purpose of increasing the strength of the steel.
Ta: 0.001 to 0.1 mass%, Nb: 0.001 to 0.1 mass%, Ti: 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 One or more selected from 0.2 mass% Ta, Nb, Ti, Zr and V are elements that increase the strength of the steel material, and are appropriately selected and added according to the required strength. Can do. In order to acquire the said effect, it is preferable to add Ta, Nb, Ti, and Zr 0.001 mass% or more, respectively, and V to add 0.002 mass% or more. However, when Ta, Nb, Ti, Zr exceeds 0.1 mass%, and V exceeds 0.2 mass%, toughness decreases, so Ta, Nb, Ti, Zr, V are within the above ranges. Is preferably added.

Furthermore, in order to increase the strength or improve the toughness, the steel material of the present invention can further contain Ca, REM and Y in the following ranges in addition to the above components.
Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass%, and Y: 0.0001 to 0.1 mass%, or one or more selected from Ca, REM, and Y, Both are effective in improving the toughness of the weld heat affected zone and can be added as necessary. The above effect can be obtained by adding Ca: 0.0002 mass% or more, REM: 0.0002 mass% or more, Y: 0.0001 mass% or more, but Ca: 0.01 mass%, REM: 0.015 mass%, Y: If added in excess of 0.1 mass%, the toughness is reduced, so Ca, REM, and Y are preferably added in the above ranges.

Furthermore, the steel material of this invention can contain B in the following range in addition to the said component.
B: 0.0002 to 0.003 mass%
B is an element that increases the strength of the steel material, and can be added as necessary. In order to acquire the said effect, adding 0.0002 mass% or more is preferable. However, when it exceeds 0.003 mass%, toughness will fall. Therefore, it is preferable to add B in the range of 0.0002 to 0.003 mass%.

Furthermore, in addition to the said component, the steel material of this invention can contain Bi in the following range further.
Bi: 0.001 to 0.1 mass%
Bi is an element that enhances the local corrosion resistance of steel, and can be added as necessary. In order to acquire the said effect, adding 0.001 mass% or more is preferable. However, if added over 0.1 mass%, the toughness decreases. Therefore, Bi is preferably added in the range of 0.001 to 0.1 mass%.

Next, the steel material used for the crude oil tank of the present invention is preferably produced by the following method.
The steel material of the present invention is prepared by melting a steel adjusted to a component composition suitable for the present invention using a known refining process such as a converter, an electric furnace, vacuum degassing, etc. It is preferable that a steel material (steel slab) is formed by a rolling method, and then the material is reheated and hot-rolled to obtain a steel material such as a thick steel plate, a thin steel plate, and a shaped steel.

  The reheating temperature before the hot rolling is preferably 900 to 1200 ° C. If heating temperature is less than 900 degreeC, a deformation resistance is large and it will become difficult to hot-roll. On the other hand, when the heating temperature exceeds 1200 ° C., the austenite grains become coarse, leading to a decrease in toughness, and a scale loss due to oxidation becomes remarkable, resulting in a decrease in yield. A more preferable heating temperature is 1000 to 1150 ° C.

  Further, when rolling into a steel material having a desired shape and size by hot rolling, the finish rolling finishing temperature is preferably 750 ° C. or higher. If the temperature is lower than 750 ° C., the deformation resistance of the steel increases, the rolling load increases, it becomes difficult to roll, and a waiting time occurs until the rolled material reaches a predetermined rolling temperature, so the rolling efficiency decreases. It is because.

  The steel material after hot rolling may be cooled by either air cooling or accelerated cooling. However, when higher strength is desired, accelerated cooling is preferable. In addition, when performing accelerated cooling, it is preferable to make a cooling rate into the range of 2-80 degree-C / sec and the cooling stop temperature in the range of 650-300 degreeC. When the cooling rate is less than 2 ° C./sec and the cooling stop temperature exceeds 650 ° C., the effect of accelerated cooling is small and sufficient strength cannot be achieved. On the other hand, if the cooling rate exceeds 80 ° C./sec and the cooling stop temperature is less than 300 ° C., the toughness of the obtained steel material may be reduced, or the shape of the steel material may be distorted.

Next, a weld joint of a crude oil tank formed by welding the steel material of the present invention will be described.
In the welded joint of the crude oil tank formed by welding steel plates manufactured by adjusting to the above-mentioned proper components, Cu in the weld metal and Cu in the base material (steel material) are represented by the following formula (1):
1 <(Cu content in weld metal / Cu content in base material) ≦ 5 (1)
It is preferable to contain and satisfy.
When the corrosion resistance of the weld metal is inferior to the corrosion resistance of the base material (steel material), in the acid immersion test that simulates the inside of the pitting corrosion of the tanker bottom plate described later, the dissolution of the metal in the welded portion is promoted. Here, if Sn or Sb is added to the weld metal in order to improve the corrosion resistance of the weld metal to the same level as the base material (steel material), the low temperature toughness of the welded joint cannot be secured. Therefore, it is necessary to improve the corrosion resistance of the weld metal to the same level as the base material (steel material) without containing Sn or Sb in the weld metal. Therefore, in the present invention, as means for improving the corrosion resistance of the weld metal, it is preferable to limit the value of (Cu content in the weld metal / Cu content in the base metal) to the range defined by the above formula (1).

  When (Cu content in the weld metal / Cu content in the base metal) is 1 or less, the corrosion resistance of the weld metal is inferior to that of the base material (steel material). In the simulated acid immersion test, the dissolution of the metal in the welded portion is promoted. On the other hand, when (Cu content in the weld metal / Cu content in the base metal) exceeds 5, not only the cost of the welding material (welding wire) is increased by adding more Cu than necessary, but also welding. Since the corrosion resistance of the metal greatly exceeds that of the base material, selective corrosion of the base material occurs in an actual corrosive environment. Therefore, it is preferable that Cu in the weld metal and Cu in the base material (steel material) satisfy the above formula (1).

In the weld joint of the crude oil tank of the present invention, Cu, Mo and W in the weld metal are represented by the following formula (2):
0.25 ≦ (Cu content in weld metal / total content of Mo and W in weld metal) ≦ 3
... (2)
It is preferable to contain and satisfy.
The inventors have found that when Cu and Mo or W are added in combination, the corrosion resistance of the welded joint is greatly improved by the synergistic effect of these elements. However, when (Cu content in weld metal / total content of Mo and W in weld metal) in the formula (2) is less than 0.25, the content of Mo or W in the weld metal On the other hand, since the content of Cu is too low and the above synergistic effect cannot be expected, the corrosion resistance of the welded joint is lowered. On the other hand, when (Cu content in the weld metal / total content of Mo and W in the weld metal) in the formula (2) exceeds 3, Mo or Mo compared to the Cu content in the weld metal Since the W content is too low and the above synergistic effect cannot be expected, the corrosion resistance of the welded joint is lowered. Therefore, it is preferable that Cu, Mo, and W in the weld metal satisfy the above formula (2). In addition, as long as the content of Mo and W in a weld metal is in the range with which the said content satisfy | fills the said Formula, it does not need to contain any one of Mo and W.

  In order to control the contents of Cu, Mo and W in the weld metal within the above range, a welding material (welding wire) used for welding is appropriately selected according to the component composition of the steel (base material) and welding conditions. It is preferable to select. For example, a welding wire having a composition obtained by dividing the target composition of Cu, Mo and W in the weld metal by the base material dilution rate is produced and welded using this.

The welding method used for welding the crude oil tank according to the present invention is a single-sided one-pass submerged arc welding method such as FAB welding, FCB welding, RF welding, carbon dioxide arc welding (CO ₂ welding), or the like. However, when the chemical component composition of the weld metal is controlled within an appropriate range, a welding method using a welding wire is preferable.

  No. shown in Table 1-1 and Table 1-2. Steel having different component compositions of 1-41 is melted in a vacuum melting furnace to form a steel ingot, or melted in a converter, continuously cast into a steel slab, these are reheated to 1150 ° C., and then finish-rolled Hot rolling with an end temperature of 800 ° C. was performed to obtain a thick steel plate having a thickness of 25 mm.

No. obtained in this way. Thick steel plates 1 to 41 were subjected to a general corrosion test (condensation test) simulating the back of the upper deck below and a local corrosion test (acid resistance test) simulating the tanker bottom plate environment.
(1) Full corrosion test (condensation test) simulating the tanker upper deck environment
In order to evaluate the corrosion resistance against general corrosion on the back of the upper tanker, Cut a rectangular piece of width 25mm x length 60mm x thickness 5mm from thick steel plates 1 to 41, polish the surface with 600th emery paper and seal it with tape so that the back and end faces do not corrode. A corrosion resistance test piece was prepared, and a full surface corrosion test was performed 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 having a temperature of 36 ° C. is injected into the corrosion test tank 2, and the water 6 Into the corrosion test tank 2, a mixed gas (introduction gas 4) consisting of 12 vol% CO ₂ , 4 vol% O ₂ , 0.01 vol% SO ₂ , 0.05 vol% H ₂ S and the balance N ₂ is introduced. It is filled with supersaturated water vapor and reproduces the corrosive environment behind the upper deck of a crude oil tank. Then, a temperature change with a cycle of 25 ° C. × 3 hours + 50 ° C. × 21 hours is applied to the corrosion test piece 1 set on the upper and rear surfaces of the test tank via a temperature control plate 3 incorporating a heater and a cooling device. It is repeatedly applied for one day to cause dew condensation water on the surface of the test piece 1 to cause overall corrosion. In FIG. 1, 5 indicates the exhaust gas from the test tank.
After the above test, the rust on the surface of each test piece was removed, the amount of mass decrease due to corrosion was determined from the change in mass before and after the test, and this value was converted into a reduction in sheet thickness (corrosion rate on one side) per year. As a result, when the corrosion rate is 0.08 mm / y or less and no local corrosion is observed in either the base metal part or the welded part, the overall corrosion resistance is good (O), more than 0.08 mm / y, or When the local corrosion was visually recognized in either the base metal part or the welded part, the overall corrosion resistance was evaluated as poor (x).

(2) Local corrosion test (acid resistance test) simulating the tanker tank bottom plate environment
In order to evaluate pitting corrosion resistance against pitting corrosion in the bottom plate of the tanker oil tank part, Cut out a rectangular piece of width 25 mm x length 60 mm x thickness 5 mm from the thick steel plates 1 to 41 so that the weld metal is parallel to the center of the test piece in the width direction and the entire surface is 600 emery paper. A corrosion resistance test piece was prepared by polishing.
Next, a 10 mass% NaCl aqueous solution was prepared using concentrated hydrochloric acid to prepare a test solution having a Cl ion concentration of 10 mass% and a pH of 0.85. The test solution was suspended in a 3 mmφ hole opened in the upper part of the test piece, and suspended per test piece. A corrosion test was conducted by immersing in 2 L of test solution for 168 hours. The test solution was preheated and maintained at 30 ° C. and replaced with a new test solution every 24 hours.
The apparatus used for the corrosion test is shown in FIG. This corrosion test apparatus is a double-type apparatus of a corrosion test tank 8 and a thermostatic tank 9, and the test solution 10 is put in the corrosion test tank 8, and the test piece 7 is suspended and immersed in the tegs 11 therein. ing. The temperature of the test solution 10 is maintained by adjusting the temperature of the water 12 placed in the thermostatic chamber 9.
After removing the rust generated on the surface of the test piece after the corrosion test, the mass difference before and after the test is calculated, the difference is divided by the total surface area, and the reduction in thickness (corrosion rate on both sides) per year is calculated. It was. As a result, when the corrosion rate is 1.0 mm / y or less and local corrosion is not visually recognized in the base metal part and the welded part, the local corrosion resistance is good (◯), and the corrosion rate exceeds 1.0 mm / y. Alternatively, the case where local corrosion was visually observed in either the base metal part or the welded part was evaluated as poor (×) local corrosion resistance.

  The results of the magnetic particle flaw detection test and the corrosion resistance test are also shown in Tables 1-1 and 1-2. From these tables, No. satisfying the conditions of the present invention is obtained. The thick steel plates 1 to 33 exhibit good corrosion resistance in both the dew condensation test simulating the upper deck back and the acid resistance test simulating the tanker bottom plate environment, while the No. 1 does not satisfy the conditions of the present invention. It can be seen that the 34 to 41 thick steel plates have not obtained good results in any of the corrosion resistance tests.

No. described in Table 1-1. No. 2 described in Table 2-2. No. 22 steel plates were welded together by various welding methods described in Table 2 1 to 10 welded joints were produced. In addition, the heat input of each welding method was 146 kJ / cm for FCB welding, 180 kJ / cm for FAB welding, 1.5 kJ / cm for CO ₂ welding, and all the grooves were V grooves. Further, the composition control of Cu, Mo and W in the weld metal of each welded joint is performed by setting the target composition of Cu, Mo and W to the base material dilution ratio (CO ₂ welding is about 11%, FAB welding is about 47%, FCB welding 67 is A welding wire having a composition obtained by rebating by about%) was prepared, and welding was performed using this. Also, for FCB welding, flux (PF-I55E / manufactured by Kobe Steel) and reverse flux (PF-I50R / manufactured by Kobe Steel), and for FAB welding, flux (PF-I52E / ( Kobe Steel, Ltd.), filler (RR-2 / Kobe Steel) and backing material (FA-B1 / Kobe) were used.

After measuring the contents of Cu, Mo and W in the weld metal using the atomic absorption analysis method for the welded joint produced as described above, the following full corrosion test (condensation test) simulating the back of the upper deck ) And a local corrosion resistance test (acid resistance test) simulating the tanker bottom plate environment.
(1) Full corrosion test (condensation test) simulating the tanker upper deck environment
No. A rectangular piece of width 25 mm × length 60 mm × thickness 5 mm so that the weld metal is located in the center in parallel with the width direction of the test piece from the position of the plate thickness ¼ of the thick steel plate welded joint of 1-10. After cutting out and polishing the surface with 600th emery paper, a test piece was prepared by sealing with a tape so that the back surface and the end surface were not corroded. Using the corrosion test apparatus shown in FIG. A general corrosion test (condensation test) was performed under the same conditions, and the corrosion resistance was evaluated according to the same criteria.
(2) Local corrosion test (acid resistance test) simulating the tanker tank bottom plate environment
No. A rectangular piece of width 25 mm × length 60 mm × thickness 5 mm so that the weld metal is located in parallel with the width direction of the test piece from the position of the thickness 1/4 of the thick steel plate welded joint of 1-10. Cut out and polished the entire surface with 600th emery paper to produce a test piece. Using the corrosion test apparatus shown in FIG. 2, a local corrosion test (acid resistance test) was performed under the same conditions as in Example 1. Corrosion resistance was evaluated on the basis.

  The results of the corrosion resistance test are also shown in Table 2. From Table 2, No. satisfying the conditions of the present invention is obtained. The welded joints 1 to 4, 7 and 8 satisfy the conditions of the present invention while exhibiting good corrosion resistance in both the dew condensation test simulating the upper deck back and the acid resistance test simulating the tanker bottom plate environment. No. It can be seen that the thick steel plates of 5, 6, 9 and 10 do not give good results in any of the corrosion resistance tests.

DESCRIPTION OF SYMBOLS 1, 7: Test piece 2, 8: Corrosion test tank 3: Temperature control plate 4: Introducing gas 5: Exhaust gas 6, 12: Water 9: Thermostatic bath 10: Test liquid 11: Tegus

Claims (8)

C: 0.03-0.16 mass%,
Si: 0.05-1.50 mass%,
Mn: 0.1 to 2.0 mass%,
P: 0.025 mass% or less,
S: 0.010 mass% or less,
Al: 0.005 to 0.10 mass%,
N: 0.008 mass% or less,
Hf: 0.0001 to 0.01 mass%,
Cu: 0.03-0.4mass% is contained, and
W: 0.01 to 1.0 mass%,
Mo: 0.01-0.5 mass%,
A steel material for a crude oil tank containing one or more selected from Sn: 0.005-0.2 mass% and Sb: 0.005-0.4 mass%, the balance being Fe and inevitable impurities. In addition to the above component composition, the steel material further includes:
Ni: 0.005 to 0.4 mass%,
The steel material for a crude oil tank according to claim 1, comprising one or more selected from Co: 0.01 to 0.4 mass% and Cr: 0.01 to 0.3 mass%. . In addition to the above component composition, the steel material further includes:
Ta: 0.001 to 0.1 mass%,
Nb: 0.001 to 0.1 mass%,
Ti: 0.001 to 0.1 mass%,
The crude oil tank according to claim 1 or 2, comprising one or more selected from Zr: 0.001 to 0.1 mass% and V: 0.002 to 0.2 mass%. Steel material. In addition to the above component composition, the steel material further includes:
Ca: 0.0002 to 0.01 mass%,
REM: 0.0002 to 0.015 mass%,
Y: 0.0001 to 0.1 mass%,
The steel material for a crude oil tank according to any one of claims 1 to 3, comprising one or more selected from among the above. The steel material according to any one of claims 1 to 4, wherein the steel material further contains B: 0.0002 to 0.003 mass% in addition to the component composition. The said steel materials contain Bi: 0.001-0.1mass% further in addition to the said component composition, The steel materials for crude oil tanks of any one of Claims 1-5 characterized by the above-mentioned. It is a welded joint which uses the steel material of any one of Claims 1-6 as a base material, Comprising: The component composition of a weld metal part satisfy | fills following (1) Formula and (2) Formula, Fittings.
1 <(Cu content in weld metal / Cu content in base metal) ≦ 5 (1)
0.25 ≦ (Cu content in weld metal / total content of Mo and W in weld metal) ≦ 3
... (2) A crude oil tank comprising the welded joint according to claim 7.

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