JP2003105487A - Corrosion resistant steel sheet for tank having excellent corrosion resistance in weld zone, and welding method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corrosion resistant steel sheet for a tank which has excellent corrosion resistance in a base metal used in a primer-coated state or in a weld zone subjected to no primer coating, which enables the use of a conventional welding wire for carbon steel, and to provide a welding method therefor. SOLUTION: The corrosion resistant steel sheet for the tank having excellent corrosion resistance in a weld zone has chemical components containing, by mass, <=0.16% C, <=1.5% Si, <=2.0% Mn, <=0.05% P, <=0.01% S, and 0.15 to 1.4% Cu, and the balance substantially Fe, and in which the value of Pcm expressed by the following inequality (1) is <=0.24. In welding, a welding wire for carbon steel is used: Pcm=C+Si/30+Mn/20+Cr/20+Cu/20+Ni/60+Mo/15+V /10+5B<=0.24 (1); wherein the elemental symbols denote each the mass% of an element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tanker steel, and more particularly to a corrosion resistant steel plate for a coin oil tank which is excellent in corrosion resistance of a welded portion for an oil renting tank and used in combination with a primer coating, and a welding method thereof.

[0002]

2. Description of the Related Art Since the regulation of the International Maritime Organization obliges a double hull structure of a new tanker, a new problem has arisen with respect to corrosion of ship structures such as oil tanks, ballast tanks and deck plates. For ballast tank materials that are repeatedly subjected to seawater environments and wet high temperature environments, steel materials that are resistant to seawater corrosion, corrosion in the splash zone environment, and condensation environment corrosion are required. Even in the deck plate corresponding to the ballast tank part, the corrosive environment on the back surface is assumed to be the same as that in the upper part in the ballast tank.

As steel materials suitable for these applications, Cu-P, Cu-Cr and Cu-P-Cr steel materials having excellent corrosion resistance are disclosed in Japanese Patent Laid-Open No. 7-
It is disclosed in Japanese Patent No. 267182. Steel materials produced by this technology are C: 0.15% or less, Si: 0.02 to 1.5%, Mn: 0.2 to 5.0
%, S: 0.005% or less, Cu: 0.1 to 1.0%, P: 0.01 to 0.15%
In addition, one or more of Ni: 0 to 1.5%, Nb: 0 to 0.03%, and Mo: 0 to 1.0%, V: 0 to 1.0%, and W: 0 to 1.0%, It is a low alloy steel for ballast tanks containing one or two of Al: 0 to 1.0% and Ti: 0 to 0.5%.

Further, JP-A-7-310141 and JP-A-8-246048 disclose a corrosion resistant steel containing about 1 to 3% of Cr, which exhibits corrosion resistance usable in a ballast tank environment. There is. The corrosion-resistant steel described in JP-A-7-310141 has Cr: 0.5 to 3.5% as a main component and Ni: 1.
Steel containing 5% or less, one or more of Mo: 0.8% or less, or one or more of Nb: 0.005 to 0.05% and Ti: 0.005 to 0.05%. The corrosion resistant steel described in JP-A-8-246048 has C: 0.1% or less, Si: 0.10 to 0.80%, Mn:
1.50% or less, Al: 0.005-0.050%, Cr: 1.0-3.0%, T
Steel containing i: 0.005 to 0.03% and N: 0.0020 to 0.0120%.

Further, in Japanese Unexamined Patent Publication No. 2001-214236, C: 0.003% to 0.30%, S is defined as the addition of Cu, Ni, Cr, Mo, Sb and Sn is effective for general corrosion resistance and local corrosion.
i: 2.0% or less, Mn: 2.0% or less, P: 0.050% or less, S: 0.050%
In addition to this, Cu: 0.01 to 2.0%, Ni: 0.01 to 7.0%, Cr:
0.01-10.0%, Mo: 0.01-4.0%, Sb: 0.01-0.3%, Sn:
A corrosion resistant steel containing 0.01 to 0.3% of one or more kinds is disclosed.

[0006]

On the other hand, in recent tankers, especially large-sized oil tankers called VLCCs, there are cases where significant corrosion is found on the inner surface of the oil tank. Although this phenomenon has not been a problem in the past, it has recently attracted attention as a new corrosion problem. At present, the classification and estimation of the corrosion mechanism used for ballast tanks are not performed, but it is an urgent task to prevent the corrosion of the oil tank or the deck plate of the relevant oil tank.

As a means for solving the above-mentioned corrosion problem of the material for the oil tank, it is effective to apply the primer coating and the heavy coating to the steel plate before assembling. However, in many cases, the coating is damaged as it is used, and so-called under-coating corrosion occurs in which rust is generated from the surface-damaged portion and the coating film is destroyed. Therefore, in normal use environment, at most 5
It takes about 10 years to see the same level of corrosion as naked use without painting, and maintenance costs will be enormous.

Further, in the steel material for shipbuilding, the weldability and the material management are emphasized from the viewpoint of construction cost. High-efficiency submerged arc welding, especially single sided single layer FCB (flux / copper / backing) welding, is commonly used for welding upper steel sheets such as VLCC in order to shorten the construction period. At present, mild steel or tensile strength 49
When carbon steel such as 0 N / mm ² class high-strength steel sheet is used as a steel material for shipbuilding, commercially available mild steel or tensile strength TS490 N / mm ² class whose composition is almost the same as the base material composition considering weldability. A high-strength steel welding wire (hereinafter referred to as a carbon steel welding wire) is used as an ordinary welding material. However, when using a steel sheet with a composition different from the carbon steel currently used for the base material of the steel for shipbuilding, it has the same composition as the steel sheet in order to make the corrosion resistance in the base material and the welded part equal. Although it is necessary to use eutectic wire, when eutectic wire is used, the cost of the welding material is higher than that of the welding wire for carbon steel, and hot cracking of the welded part occurs. There is a problem that weldability deteriorates. Furthermore, when changing the welding material only for a part of the hull, such as when using a common metal wire only for welding the upper steel plate such as VLCC, the same carbon steel wire is used for all the welding of the hull. However, it is more difficult to control the welding material, and there is a risk of increasing costs in terms of material control.

The steel for ballast tanks described in the above-mentioned JP-A-7-310141 or JP-A-8-246048 has a large amount of additive elements according to the examples, and not only the weldability is remarkably poor but also the alloy. The effect of improving corrosion resistance is small when used in combination with painting to meet the cost increase. That is, it cannot be said that it is always easy to use as a steel material for a currency oil tank.
This is also the case with the technique described in Japanese Patent Application Laid-Open No. 7-267182, and in the embodiment, when Cu is added, 0.5 to 0.14
%, P is 0.045 to 0.14%, and Cr is 1 to 5%. JP 2001-21423 A
Also in the steel described in Japanese Patent No. 6, according to the example, 0.30% or more is required when Cu is added alone, and 0.50% or more is required when Ni is added in combination with Cu and Ni. Furthermore, when using a welding wire having the same chemical composition as those of these steels as a metallurgical wire, it is easy to cause high temperature cracking of the welded portion as described above, and further increase the cost of the welding material and material management for each hull part. It is feared that this will lead to an increase in construction costs due to the difficulty of construction.

The present invention solves the above problems and has excellent corrosion resistance in the base material used in the primer coating state and the welded portion not subjected to the primer coating, and the conventional welding wire for carbon steel can be used. It is an object of the present invention to provide a corrosion-resistant steel plate for a coin oil tank having excellent corrosion resistance of a welded portion and a welding method thereof.

[0011]

DISCLOSURE OF THE INVENTION The present inventors have conducted a detailed investigation of the corrosive environment acting on steel in a freight oil tank. As a result, it has been clarified that the influence of the engine exhaust gas introduced into the freight oil tank and the influence of hydrogen sulfide in the volatile component of the crude oil are large as the corrosion factors in the freight oil tank of recent large tankers. Exhaust gas is introduced to prevent explosion due to volatile components from crude oil.However, in addition to oxygen and nitrogen, exhaust gas contains corrosive substances such as a considerable amount of carbon dioxide gas, SOx, and in some cases H ₂ S. Gas is also included.

Therefore, when a temperature cycle is present in the presence of these corrosive gases and acid dew point corrosion acts, rust having a less dense structure develops from the microscopically damaged portions of the coating film. . This rust develops and progresses between the coating film and the steel material, and finally the coating film is continuously peeled off. Further, hydrogen sulfide contained in the volatile component acts on the coating film peeling portion of the steel material to promote corrosion. This corrosion due to hydrogen sulfide is widely seen in the environment of contact with crude oil such as oil well pipes. Such corrosion is a problem mainly in the upper part of the oil-rental tank where gas accumulates, and the corrosive atmosphere in the upper part of the oil-rental tank as described above is hereinafter referred to as "tank environment".

It has been found that the corrosion mechanism in the freight oil tank is as described above, but with respect to acid dew point corrosion,
Even if a steel material having an improved corrosion resistance by using an alloy is used, practical corrosion resistance cannot be obtained by bare use without coating, so the steel sheet needs to be coated and used. However, since there is a problem that the rust of the coating film damaged portion and the coating-free welded portion as described above spreads only by coating, the present inventors have
From the viewpoint of minimizing the progress of rust in the steel plate base metal under the coating film and in the welded part when used barely and prolonging the coating film life, the steel plate base metal of the corrosion resistant steel and its components are based on the chemical composition of the base metal. The present invention has been completed by diligently studying the component design for both of the welds having the above dilution. In the component design of the steel material of the present invention, not only the corrosion resistance of the base material, but also the corrosion resistance of the welded portion when using a carbon steel welding material, and further when the application of large heat input welding exceeding 100 kJ / cm Weldability and mechanical properties were also considered as important factors.

The present invention has been made based on the above studies, and its features are as follows.

(A) In a corrosion-resistant steel plate for a currency oil tank used in a primer-painted state, the chemical component mass% is
, C: 0.16% or less, Si: 1.5% or less, Mn: 2.0% or less, P: 0.
05% or less, S: 0.01% or less, Cu: 0.15% to 1.4%,
The balance consists essentially of Fe and is expressed by the following formula (1).
Corrosion-resistant steel plate for currency oil tanks with excellent corrosion resistance of welds, which has a Pcm value of 0.24 or less.

[0016]     Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ≦ 0.24 (1) However, the element symbols indicate the mass% of each element.

(B) The corrosion-resistant steel plate for a currency oil tank having excellent corrosion resistance of the welded portion according to (a), which further contains Ni: 0.1 to 0.7 mass%.

(C) A coin having excellent corrosion resistance of the welded part according to (a) or (b), which further contains one or two of Cr: 0.5% or less and Mo: 0.5% or less. Corrosion resistant steel plate for oil tanks.

(D) Nb: 0.05 mass% or less, V: 0.10 m
Ass% or less, Ti: 0.05 mass% or less, and one or more of them are included. (a) to (c) Corrosion resistance for a cargo oil tank excellent in corrosion resistance of a welded part steel sheet.

(E) The corrosion-resistant steel sheet for a coin oil tank having excellent corrosion resistance of the welded portion according to any one of (a) to (d), which further contains Al: 0.5 mass% or less.

(F) The corrosion-resistant steel plate for a currency oil tank, which further comprises B: 0.01% or less and which has excellent corrosion resistance of the welded portion according to any one of (a) to (e).

(G) A method for welding a corrosion resistant steel plate for a fuel oil tank according to any one of (a) to (f), characterized in that the welding is performed using a welding wire for carbon steel. Welding method for corrosion resistant steel plates for currency oil tanks with excellent corrosion resistance.

(H) The method for welding a corrosion-resistant steel sheet for a currency oil tank having excellent corrosion resistance of the welded part according to (g), characterized in that the welding is performed under the condition that the base material dilution ratio is 65% or more.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The corrosion-resistant steel plate for a coin oil tank and the welding method thereof according to the present invention will be described below.

First, the reasons for limiting the chemical components will be described.
All units are mass%.

C: 0.16% or less C is an element useful for strengthening steel, but excessive addition adversely affects weldability and corrosion resistance, so the upper limit of addition is 0.16%.
%. More preferably, the upper limit of the added amount is 0.15%.

Si: 1.5% or less Si is an element useful for deoxidizing steel, but excessive addition adversely affects welding workability, so the upper limit of the addition amount is made 1.5%.

Mn: 2.0% or less Mn is an element effective for strengthening and improving toughness of steel, but excessive addition impairs weldability, so the upper limit of addition is 2.0%. More preferably, the upper limit of the amount added is 1.5%.
On the other hand, addition of 0.2% or more is preferable in order to secure the strength of steel.

P: 0.035% or less P is an element that reduces weldability, and the lower the content, the more desirable. From the above viewpoint, up to 0.035% is an allowable range, and the content is 0.035% or less.

S: 0.01% or less S is an element that deteriorates the hot workability and weld crack resistance of steel, and the lower the content, the more desirable. From the above viewpoint, 0.
Up to 01% is within the allowable range, and the upper limit of the content is 0.01
%. More preferably, the upper limit of the amount added is 0.005%.

Cu: 0.15-1.4% Cu significantly improves the undercoat corrosion resistance of steel in a tank environment. Addition of 0.1% or more is effective, but considering not only the base metal but also the corrosion resistance of the weld that becomes a component diluted from the base metal component by welding (there is base metal dilution), % Or more must be added. However, if the addition exceeds 1.4%, the tendency of hot cracking in the weld becomes remarkable. Therefore,
The addition amount is 0.15 to 1.4%.

In the present invention, Ni may be contained for the purpose of further improving the corrosion resistance of the steel sheet.

Ni: 0.1-0.7% Ni is an expensive additive element, but it is effective in improving corrosion resistance.
In addition, it has the effect of suppressing damage to the weldability by Cu. Considering not only the base metal, but also the corrosion resistance of the weld where the base metal is diluted, the effect is not clear unless 0.1% or more is added to the base metal, but even if it exceeds 0.7%, the effect saturates. Then
On the contrary, the economical efficiency of steel is impaired and the weld cracking property is also reduced. Therefore, the addition amount is set to 0.1 to 0.7%.

In the present invention, one or two of the following Cr and Mo may be contained for the purpose of further improving the strength of the steel sheet.

Cr: 0.5% or less Cr has a small anticorrosion effect in a tank environment, but since it improves the strength characteristics of steel, it can be used in a limited amount. If the amount added exceeds 0.5%, the cold crack resistance and welding workability also deteriorate, so the amount added should be 0.5% or less.

Mo: 0.5% or less Mo is an element harmful to the corrosion resistance in a corresponding use environment, but it can be used in a limited amount in order to improve the strength characteristics of steel. Addition of more than 0.5% remarkably reduces the corrosion resistance under the coating film, so the addition amount is 0.5% or less.

In the present invention, for the purpose of further improving the weldability of the steel sheet, one or more of Nb, V and Ti shown below may be contained.

Nb: 0.05% or less, V: 0.10% or less, Ti: 0.05%
Hereinafter, Nb, V, and Ti can combine with carbon in steel to form a carbide and reduce the effect of carbon on weldability, so a certain amount of addition can be selected. However, Nb is 0.05%, V is 0.1
When 0% and Ti are added in excess of 0.05% respectively, a large amount of carbide is precipitated and cracks easily occur during welding. Therefore, Nb is 0.05% or less, V is 0.10% or less, T
i should be 0.05% or less.

In the present invention, Al may be contained for the purpose of further improving the corrosion resistance of the steel sheet.

Al: 0.5% or less Al is necessary for deoxidation and improves the corrosion resistance under the coating film in a tank environment, so that it can be appropriately added. 0.5%
If it is added, the amount of slag frequently occurs during welding, and the workability is significantly reduced.

In the present invention, B may be contained for the purpose of further improving the workability of the steel sheet.

B: 0.01% or less B is an additive element effective in improving hot workability and can be selected and added. However, if it exceeds 0.01%, the tendency of welding hot cracking becomes remarkable, so that The addition amount is 0.01% or less.

Pcm value: 0.24 or less (Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B (1)) In the present invention, the above In addition to the restrictions on chemical components as described above, the Pcm value corresponding to the above formula (1) is specified. This is a characteristic value indicating the susceptibility to welding cracks, and if this value exceeds 0.24, the cold crack occurrence rate during welding will increase significantly, so Pcm
It is necessary to keep the value below 0.24.

Among the chemical components of the steel of the present invention, the balance other than the above chemical components is substantially Fe. "The balance is essentially F
The term “e” means that the effect of the present invention is not lost.
It means that those containing other trace elements including inevitable impurities can be included in the scope of the present invention.

The steel having the above chemical composition can be produced by the same method as that for ordinary steel. For example, in the melting of steel, the main five elements C, Si, Mn, P, and S are adjusted within the scope of the invention in a converter or the like, and other alloying elements are added as necessary.

Thereafter, the slab obtained by continuous casting or the like is rolled as it is or after being cooled. There is no particular limitation on the rolling conditions for the corrosion resistant steel, but it is necessary to secure an appropriate reduction ratio from the viewpoint of mechanical properties.

During rolling, a high-strength steel material having a tensile strength of 490 N / mm ² grade or more can be obtained by controlling the cooling rate after hot rolling. For example, the finishing temperature of hot rolling is set to 750 ° C. or higher, and then it is cooled to 600 ° C. or lower at a cooling rate of 2 ° C./s or higher. When the finishing temperature is less than 750 ° C, the deformation resistance becomes large and defective shapes are likely to occur, which is not preferable. If the cooling rate is less than 2 ° C / s or the cooling stop temperature exceeds 680 ° C, tensile strength of 490 N / mm ² or higher cannot be obtained.

The steel sheet of the present invention is usually used in a primer-coated state. As the primer coating, organic or inorganic coating or a coating film with a rust preventive primer is applied. The type of coating and the primer are not limited, but it is most effective to use a coating film made of an inorganic zinc primer.

Next, the method of welding the corrosion-resistant steel plate of the present invention will be described.

When welding the corrosion-resistant steel sheet of the present invention, a welding method generally employed as a welding method for steel plates for shipbuilding may be used, and there is no particular limitation. The feature is that a welding wire can be used. By welding using a commercially available welding wire for carbon steel, the welded portion exhibits good corrosion resistance without deterioration of welding efficiency and weldability, and the cost of the welding material does not increase. The welding wire for carbon steel is also suitable for high heat input welding. Any commercially available welding wire for carbon steel can be used for welding the corrosion-resistant steel plate of the present invention without particular limitation. As the composition of the welding wire for carbon steel (tensile strength TS490N / mm ² grade), C: 0.18% or less, Si: 0.15% or less, Mn: 0.2 to 2.8%,
P: 0.03% or less, S: preferably 0.03% or less, 0.09% C-1.9% Mn-
0.01% P-0.01% S (mass%) is typical. In addition to the commercially available welding wire for carbon steel, a welding wire having a composition similar to that of the welding wire for carbon steel can be used, but if a common metal wire having substantially the same composition as the corrosion-resistant steel plate of the present invention is used. ,
The weldability and workability may deteriorate and the cost of the welding material may increase.

When welding the corrosion-resistant steel sheet of the present invention, it is preferable to weld under the condition that the base metal dilution ratio is 65% or more. The base metal dilution ratio is the ratio of the component composition of the welded portion to the component composition of the base material, and the base material dilution ratio of Cu can be used in the present invention. If the base metal dilution rate is less than 65%, Cu in the welded part
The amount may be diluted and may not provide the required corrosion resistance in the tank environment. If FCB welding or the like is used, it is possible to make the base metal dilution rate 65% or more without reducing the welding efficiency.

By using the welding method according to the present invention, the damaged portion of the coating film exhibits excellent corrosion resistance in a tank environment, and it is not necessary to use the common metal alloy wire that has been conventionally used. By the welding method using the welding material, it is possible to obtain a corrosion-resistant steel sheet for tankers that exhibits excellent corrosion resistance even in a welded portion having no coating film.

By using the corrosion-resistant steel plate of the present invention, it is possible to reduce the corrosion rate at the welded portion by about 10% as compared with the conventional case. Corrosion weight loss is 0.1m in tank environment
It is estimated to be m / year or more, but the excess is 2 at the welded part.
Since it is about 5 mm, reducing the corrosion rate of the welded portion by about 10% makes it unnecessary to replace the deck during the 20 years that is the typical life of the ship.

The corrosion-resistant steel plate for a currency oil tank of the present invention is coated with a primer and used as a structure or a deck plate above the currency oil tank of a VLCC tanker. When welding the corrosion resistant steel plate for a coin oil tank of the present invention, welding is performed using a welding material for carbon steel, and the welded portion can be used as it is without being coated with a primer. Further, even when it is used as a structure such as a beam or a column in the tank or in the ceiling portion, suitable corrosion resistance and mechanical properties can be exhibited.

[0055]

EXAMPLES Steel sheets having the chemical compositions shown in Tables 1 and 2 (No. 1 to
42) was coated with a primer and welded using a welding wire to prepare samples (No. 1 to 54). No primer coating was applied to some of the samples.

When melting steel, all melting is 150 k
Casting was also carried out in vacuum with a vacuum induction melting furnace. 50k
After forming into an ingot, it was heated to 1200 ° C and hot-rolled to obtain a plate material having a thickness of 25 mm. This 25 mm plate material was used for evaluation of weld crackability. Further, it was reheated to 1180 ° C. and hot-rolled to obtain a plate material having a thickness of 6 mm, which was subjected to a corrosion resistance evaluation test of the base material.

To evaluate the hot cracking resistance, a V-shaped groove was cut on a plate material having a thickness of 25 mm, and tab members were welded to both ends of the joint.
A butt-welded joint was produced, and a method for comparatively evaluating the presence or absence of cracks in the weld metal after cooling was used. The welding method is FCB welding, and the welding material is a commercially available tensile strength of 50 kg class (490 N / mm ²
Grade) carbon steel welding wire, and a common metal wire containing Cu and Ni equivalent to the base metal melted in the laboratory. Welding conditions are voltage 38-45V, current 1000-1250A, welding speed 40cm /
The heat input was 139 kJ / cm. The base metal dilution rate under these welding conditions was approximately 70%. An X-ray transmission method was used to detect cracks. At the same time, the bead disturbance due to the generation of slag and the deterioration of workability were also evaluated.

On the other hand, for the evaluation of cold cracking susceptibility during welding, a y-type weld cracking test defined by Japanese Industrial Standard JIS Z3158 was carried out, and the presence or absence of cracks after cooling the steel sheet was evaluated. Welding uses a commercially available 490 N / mm ² grade coated arc welding wire for carbon steel, welding conditions are voltage 24 V, current 170 A, welding speed 1
The heat input was 5 cm / min and the heat input was 16 kJ / cm. The test was conducted at 25 ° C.
A cross-section cutting method was used to detect cracks.

Further, regarding the corrosion resistance, both the base material and the welded portion were investigated. Regarding the corrosion resistance of the base material, a corrosion test piece having a size of 6 mm x 55 mm x 45 mm was cut out from a plate material having a thickness of 6 mm, and the whole surface was subjected to a primer treatment (primer coating) and subjected to a corrosion test. For primer treatment, a test piece that had been shot rust-removed in advance was spray-coated with a zinc-based primer using a solvent containing an alkyl silicate resin varnish containing a certain proportion of zinc dust-containing pigment, and dried at room temperature for 24 hours. Let The weight ratio of the zinc dust-containing pigment was compared using 43% and 48%. In order to accelerate the evaluation of corrosion resistance, an X-shaped cutting that reaches the surface of the steel material was applied to the test surface, and this was used as a simulated damage point to evaluate the surface rust after the corrosion test and the progress of rust under the coating film by the surface area ratio. . The damage area ratio before the test is 1.
It was 0%.

The corrosion resistance of the welded part was measured by using a plate material having a thickness of 25 mm coated with a primer under the above-mentioned FCB welding conditions to prepare an FCB welded joint, and dimensioning the welded part at the center.
A corrosion test piece of 25 mm thickness × 45 mm width × 150 mm length was cut out and subjected to a corrosion test. In addition, the welded part was left bare without applying a primer, and a 2 to 5 mm extra area was added.

In the corrosion test, the test piece was exposed in an atmosphere and a temperature cycle simulating the environmental conditions in the freight oil tank to evaluate the expansion rate of the corroded portion. The simulated environment in the freight oil tank was filled with a mixed gas of gas composition 10% CO ₂ , 8% O ₂ , 0.02% SOx, 0.1% H ₂ S and the balance N ₂ under supersaturated water vapor pressure, The atmosphere. The test piece inserted in this atmosphere was subjected to a repeated temperature cycle of 30 ° C / 50 ° C for 30 days with one heater and a cooling device, one cycle of one day, so that corrosion due to dew condensation water could be simulated.

Tables 1 and 2 show the results of component analysis of the steels of the present invention and comparative steels, and Pcm according to the above formula (1), and Table 3 shows the results of evaluations of weldability and corrosion resistance using the above-described evaluation method. Indicated. In Table 3, the type of primer applied to the surface (43: weight ratio of zinc dust pigment 43%, 48: 48%, 48% of the same), some of which were not coated with the primer, is also shown. Regarding weldability, ◎ indicates that there is no hot crack in the butt welding test, workability is good, and there is no cold crack, and any of hot crack, workability deterioration, and cold crack is recognized. Is indicated by x. Regarding the corrosion resistance of the base metal, ⊚ indicates that the rust (corrosion) area ratio (unit%) under the simulated environment is 5% or less, and x indicates that it exceeds 5%. Regarding the corrosion resistance of the welded portion, Cu, Ni are outside the scope of the present invention.
Is relative evaluation with the corrosion weight loss of the steel sheet 35 with no addition as 1, and the relative corrosion weight loss is 0.9 or less is shown as ◎ with a good, 0.9
Those that exceeded were indicated by x.

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

Coating the corrosion resistant steel plate of the present invention with a primer,
Sample welded using carbon steel welding wire (No. 1,
2, 7-13, 17-22, 25-36) have both weld crack resistance and workability during welding, and have good corrosiveness in the base metal and welds, making them suitable for use as steel for oil tanks. It turned out to be suitable. On the other hand, samples that were not coated with a primer or that used a wire for co-metal welding (No. 3 to 6, 14
No. 16, 23, 24, 37 to 54) had insufficient weld crack resistance, welding workability, or corrosion resistance of the base metal or weld.

FIG. 1 shows the relationship between the Pcm value and the y-type weld cracking test result. According to FIG. 1, when the value of Pcm is 0.24 or less, weld cracking does not occur (no crack in the figure), and when it is larger than that, weld cracking occurs (crack in the figure). Therefore, if the Pcm value exceeds 0.24, the incidence of cold cracking during welding will increase significantly, so it is necessary to maintain the Pcm value at 0.24 or less.

FIG. 2 is a diagram showing the relationship between the relative corrosion weight loss in the dew condensation corrosion test of the welded portion and the Cu addition amount of the steel sheet for the samples with the Ni addition amount of 0.7% or less of the steel sheet. According to FIG. 2, when the Cu addition amount of the steel sheet is less than 0.15%, the effect of Cu addition on the corrosion resistance of the welded portion in the tank environment is not sufficiently exhibited. Addition of 0.1% or more Cu is effective for improving the corrosion resistance of steel sheets, but the components of the steel sheet are diluted in the welded portion,
0.15 Cu was added to the base steel plate to reduce the Cu content.
% Or more must be added.

From the above, by applying a primer coating to the corrosion-resistant steel plate of the present invention and performing welding using an appropriate welding wire, corrosion resistance and weldability satisfying the performance as a steel for a fuel oil tank can be obtained. I found out that

[0070]

EFFECT OF THE INVENTION The corrosion-resistant steel sheet for a coin oil tank of the present invention described above has excellent corrosion resistance compared to conventional corrosion-resistant steel sheets in both the base material and the welded portion when applied to the structural material of a currency oil tank, In addition, a carbon steel welding wire that has been generally used in the past can be used, and it is possible to improve weldability and reduce alloy cost and welding material cost. In addition, material management is simple, and cost can be reduced in this respect as well.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a Pcm value and a y-type weld cracking test result.

FIG. 2 is a diagram showing a relationship between a Cu addition amount of a steel sheet and a relative corrosion amount at a welded portion.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hideki Kimura             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Minoru Matsuda             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. F-term (reference) 4E001 AA03 CA07 EA10                 4E081 YL04

Claims (8)

[Claims] 1. A corrosion-resistant steel plate for a currency oil tank used in a primer-coated state, wherein the chemical composition, in mass%, is C:
0.16% or less, Si: 1.5% or less, Mn: 2.0% or less, P: 0.05% or less, S: 0.01% or less, Cu: 0.15% to 1.4%, with the balance consisting essentially of Fe. A corrosion-resistant steel plate for a coin oil tank having excellent corrosion resistance of a weld, which has a Pcm value represented by the formula (1) of 0.24 or less. Pcm = C + Si / 30 + Mn / 20 + Cr / 20 + Cu / 20 + Ni / 60 + Mo / 15 + V / 10 + 5B ≦ 0.24 (1) However, the element symbols are mass% of each element. Show. 2. The corrosion-resistant steel plate for a currency oil tank having excellent corrosion resistance of the welded portion according to claim 1, further comprising Ni: 0.1 to 0.7 mass%. 3. A coin oil having excellent corrosion resistance of the welded portion according to claim 1 or 2, further comprising one or two of Cr: 0.5% or less and Mo: 0.5% or less. Corrosion resistant steel plate for tanks. 4. Nb: 0.05 mass% or less, V: 0.10 mass
% Or less and Ti: 0.05 mass% or less, and one or more of them are contained. Corrosion resistant steel plate for a coin oil tank having excellent corrosion resistance of a welded part according to any one of claims 1 to 3. . 5. The corrosion-resistant steel plate for a coin oil tank having excellent corrosion resistance of a welded portion according to claim 1, further comprising Al: 0.5 mass% or less. 6. The corrosion-resistant steel plate for a currency oil tank, which further comprises B: 0.01% or less, which is excellent in corrosion resistance of the welded portion according to any one of claims 1 to 5. 7. The method for welding a corrosion resistant steel plate for a currency oil tank according to claim 1, wherein the welding is performed using a welding wire for carbon steel, and the corrosion resistance of a welded portion is characterized. Excellent welding method for corrosion resistant steel plates for oil tanks. 8. The method for welding a corrosion-resistant steel sheet for a currency oil tank having excellent corrosion resistance of a welded portion according to claim 7, wherein the welding is performed under the condition that the base material dilution ratio is 65% or more.