KR20120130283A - Welded joint having excellent corrosion resistance and crude oil tank - Google Patents

Abstract

Provided are a welded joint capable of significantly reducing global corrosion and local corrosion occurring in a crude oil tank, and a crude oil tank having the welded joint. Specifically, in mass%, C: 0.03 to 0.16%, Si: 0.05 to 1.50%, Mn: 0.1 to 2.0%, P: 0.025% or less, S: 0.010% or less, Al: 0.005 to 0.10%, N: 0.008% or less, Cr: more than 0.1%, 0.5% or less, Cu: 0.03-0.4%, W: 0.01-1.0%, Mo: 0.01-0.5%, Sn: 0.001-0.2%, and Sb: 0.001-- Steel materials containing one or two or more selected from 0.4% are welded to each other, and Cu, Mo, and W in the weld metal are 3 <{(Mo + W) in the weld metal} / {(Mo + W) in the base metal} A crude oil tank having a welded joint satisfying ≦ 8 and 0.2 ≦ (Cu in weld metal) / {(Mo + W) in weld metal} ≦ 3.

Description

WELDED JOINT HAVING EXCELLENT CORROSION RESISTANCE AND CRUDE OIL TANK}

The present invention is collectively referred to as a "crude oil tank" (hereinafter referred to as a "crude oil tank") for transporting or storing an oil tank or a crude oil of a crude oil tanker formed by welding steel materials. Specifically, a crude oil having a welded joint which reduces local corrosion (pitting corrosion) in the crude oil tank and a welded joint thereof It's about tanks. In addition, steel materials used in the crude oil tank of the present invention include a thick steel plate, a thin steel sheet and a shaped steel.

It is known that full surface corrosion occurs on the inner surface of the tanker's crude oil tank, especially on the back side of the sun deck and on the side walls. As a cause of this whole surface corrosion,

(1) repetition of dew drop and drying (wetting and drying) on the surface of steel sheet by day and night temperature differences;

(2) Inert gases encapsulated in the crude oil tank for explosion proof (O ₂ about 5 vol%, CO ₂ about 13 vol%, SO ₂ about 0.01 vol%, balance N ₂ as a representative composition ( dissolution of dew condensation water of O ₂ , CO ₂ , SO ₂ in boilers or engine exhaust gases,

(3) dissolution of corrosive gas such as H ₂ S volatilized from crude oil into condensed water;

(4) Residues of seawater used to clean crude oil tanks

And the like. These are also estimated from the fact that sulfuric acid ions and chloride ions (Cl ⁻ ) are detected in the strongly acidic condensed water in the investigation of the solid inspection conducted every 2.5 years. You can find out.

Further, when H ₂ S is oxidized using iron rust produced by corrosion as a catalyst, solid S is formed in a layer in the iron rust, and these corrosion products are easily peeled off and fall off, and are deposited at the bottom of the crude oil tank. Therefore, in the poison test, it is a present situation that the maintenance of the tank top and the collection of the deposit of the tank bottom are performed at high cost.

On the other hand, the steel used for the bottom plate of the tanker's crude oil tank or the like inhibits the corrosion of the crude oil itself and the corrosion of the protective coating derived from the crude oil (oil coating) formed on the inner surface of the crude oil tank. Corrosion suppression action was thought to prevent corrosion. However, recent studies have revealed that bowl-shaped local corrosion (pitting corrosion) occurs in the steel of the tank bottom plate. As a cause of this local corrosion,

(1) the presence of high concentrations of brine, in which salts representative of sodium chloride are dissolved;

(2) detachment of the oil coat due to excess washing,

(3) high concentration of sulfides in crude oil;

(4) Items such as high concentration of O ₂ , CO ₂ , SO _2, etc. in the explosion-proof inert gas dissolved in condensation water are exemplified. In fact, high concentrations of chloride ions and sulfate ions have been detected in the results of analyzing the water retained in the crude oil tank at the time of solid line poison test.

By the way, the most effective method of preventing the above-mentioned whole surface corrosion or local corrosion is to coat the steel surface with heavy coating, and to block the steel material from the corrosion environment. However, the painting operation of the crude oil tank has a large area to be coated, and also needs to be repainted about once every ten years due to deterioration of the coating film, which incurs a high cost for inspection and painting. In addition, it is pointed out that the part where the middle coating film was damaged is encouraged rather than corrosion under the corrosion environment of a crude oil tank.

As for the above-mentioned corrosion problem, several techniques have been proposed to improve the corrosion resistance of the steel material itself and to improve the corrosion resistance under the corrosion environment of the crude oil tank. For example, in Patent Literature 1, 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%, Mo: 0.01-0.5% and W: 0.01-1%, 1 or 2 types, remainder Fe and an unavoidable impurity The technique which forms a welded joint so that content of Cu, Mo, and W in a weld metal may satisfy | fill the following three formulas when welding the steel materials which consist of these to form a welded joint is disclosed.

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

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

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

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: 0.01-1.5% , Al: 0.001-0.3%, N: 0.001-0.01%, Mo: 0.01-0.5% and W: 0.01-1%, 1 or 2 types, and remainder as Fe and an unavoidable impurity The technique which forms a welded joint so that content of Cu, Mo, and W in a weld metal may satisfy | fill the following two formulas when welding the steel materials formed and forming a crude oil tank is disclosed.

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

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

Japanese Laid-Open Patent Publication 2005-21981 Japanese Laid-Open Patent Publication 2005-23421

However, in the technique of patent document 1 and patent document 2, it is difficult to suppress local corrosion (formula) which arises in a tanker bottom plate and a welded joint to 4 mm or less in 2.5 years. However, recent corrosion investigations have shown that the pH of the formula internal solution occurring in the tanker bottom plate and the weld is less than 1.0 (Ref. 242 Study Group Study on the New Corrosion Behavior of Crude Oil Tankers 2001 Report / See the Japan Shipbuilding Research Division). In general, it is well known that steel corrosion in an acidic solution is controlled by a hydrogen reduction reaction, and the corrosion rate is greatly increased with the drop of pH. Therefore, it is because the immersion test in pH 2.0 described in the Example of the said patent document 1 and patent document 2 does not fully reflect the corrosion environment in a solid line.

Moreover, although the steel materials of the prior arts, such as said patent document 1 and patent document 2, add Cu as an essential corrosion resistance improvement element, since addition of Cu causes surface cracks at the time of hot rolling, it inhibits manufacturing stability. I have a problem.

Therefore, the object of the present invention is that the whole surface corrosion resistance and local corrosion resistance in a crude oil tank such as a tanker oil tank part formed by welding steel having excellent manufacturability that does not cause problems such as cracking during hot rolling. It is an object of the present invention to provide a crude oil tank having an excellent welded joint and the welded joint.

The inventors earnestly studied for the solution of the said subject. As a result, the crude oil tank is controlled by controlling the content of Cu, Mo, and W contained in the weld metal of the weld joint in an appropriate range when forming a crude oil tank by welding steel materials having improved corrosion resistance by controlling the composition of steel in an appropriate range. It was found that the entire surface corrosion and the local corrosion generated in the welded seam can be remarkably reduced, thereby completing the present invention.

That is, in the present invention, C: 0.03 to 0.16 mass%, Si: 0.05 to 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, Cr: more than 0.1 mass%, 0.5 mass% or less, Cu: 0.03-0.4 mass%, W: 0.01-1.0 mass%, Mo: 0.01-0.5 mass%, It contains 1 type (s) or 2 or more types chosen from Sn: 0.001-0.2 mass% and Sb: 0.001-0.4 mass%, and the said component further contains following formula (1);

X value ^{= (1-0.8 × Cu 0 .5)} × {1- (0.8 × W + 0.4 × Mo) 0.3} × {1- (Sn + 0.4 × Sb) 0.3} × {1- (0.05 × Cr + 0. 03 × Ni + 0.03 × Co) ^0.3 } × {1 + 2 × (S / 0.01 + P / 0.025)}... (One)

X value defined by 0.5 or less, following (2) Formula;

Z value = (1 + 10 x Sn) x (Cu-0.7 x Ni)... (2)

In the welded joint of a crude oil tank containing Z so that the Z value defined by (0.1) may be 0.15 or less and remainder is formed by welding steel materials which consist of Fe and an unavoidable impurity, Cu in a base metal and the weld metal of a welded joint, Mo and W are following (3) Formula;

3 <{(Mo + W) in weld metal} / {(Mo + W) in base metal} ≤8. (3)

And (4) formula;

0.2 ≦ (Cu in weld metal) / {(Mo + W) in weld metal} ≦ 3. (4)

It is a welded joint excellent in corrosion resistance, characterized by containing and satisfying. Here, the element symbol in each said formula represents content (mass%) of the element.

Moreover, the steel material used for the welded joint of this invention is characterized by containing the component of at least 1 group further from the following A-D group in addition to the said component composition.

             doing

Group A; 1 type or 2 types selected from 0.005 to 0.4 mass% of Ni and 0.01 to 0.4 mass% of Co

Group B; Nb: 0.001-0.1 mass%, Ti: 0.001-0.1 mass%, Zr: 0.001-0.1 mass%, and V: 0.002-0.2 mass%

C group; 1 type or 2 or more types selected from Ca: 0.0002-0.01 mass%, REM: 0.0002-0.015 mass%, and Y: 0.0001-0.1 mass%

Group D; B: 0.0002 to 0.003 mass%

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

Industrial Applicability According to the present invention, it is possible to suppress the entire surface corrosion and local corrosion occurring 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, in all parts including not only steel sheets but also welded joints. Because of this, it has a special effect in the industry.

BRIEF DESCRIPTION OF THE DRAWINGS In the Example of this invention, it is a figure explaining the test apparatus used for the whole surface corrosion test.
FIG. 2 is a diagram illustrating a test apparatus used for a formal test in an embodiment of the present invention. FIG.
3 is a diagram showing the effect (bottom plate and top plate) of the weld metal composition on the corrosion resistance of the corrosion resistant welded steel joint for tankers.

First, the component composition of the steel material used for the crude oil tank of this invention is demonstrated.

C: 0.03 to 0.16 mass%

C is an element that increases the strength of the steel, and in the present invention, 0.03 mass% or more is added to secure desired strength. On the other hand, addition of more than 0.16 mass% lowers the weldability and toughness of the weld heat affected zone. Therefore, C is in the range of 0.03 to 0.16 mass%. The range of 0.06-0.16 mass% is more preferable.

Si: 0.05-1.50 mass%

Although Si is an element added as a deoxidizer, it is also an element effective in raising the strength of steel. Therefore, in this invention, in order to ensure desired intensity | strength, it adds 0.05 mass% or more. However, addition exceeding 1.50 mass% reduces the toughness of steel. Therefore, Si is made into the range of 0.05-1.50 mass%. The range of 0.15-0.50 mass% is more preferable.

Mn: 0.1 to 2.0 mass%

Mn is an element which raises the strength of steel, and in the present invention, 0.1 mass% or more is added to obtain desired strength. On the other hand, addition exceeding 2.0 mass% reduces the toughness and weldability of steel. Therefore, Mn is made into the range of 0.1-2.0 mass%. In addition, from the viewpoint of maintaining high strength and suppressing formation of inclusions that degrade corrosion resistance, the range of 0.5 to 1.6 mass% is preferable, and the range of 0.7 to 1.4 mass% is more preferable.

P: 0.025 mass% or less

P is a harmful element which segregates at grain boundaries and reduces the toughness of steel, and it is preferable to reduce it as much as possible. In particular, when it exceeds 0.025 mass%, toughness will fall largely. Moreover, when P exceeds 0.025 mass%, it will also adversely affect the corrosion resistance in a tank oil tank. Therefore, P is made into 0.025 mass% or less. Preferably it is 0.015 mass% or less.

S: 0.010 mass% or less

S is a harmful element which forms MnS which is a non-metal inclusion and becomes a starting point of local corrosion, and reduces local corrosion resistance, and it is preferable to reduce it as much as possible. In particular, additions exceeding 0.010 mass% lead to a significant decrease in local corrosion resistance. Therefore, the upper limit of S is made into 0.010 mass%. Preferably it is 0.005 mass% or less.

Al: 0.005 to 0.10 mass%

Al is an element added as a deoxidizer, and is added in the present invention at 0.005 mass% or more. However, when the content exceeds 0.10 mass%, the toughness of the steel decreases, so the upper limit of Al is made 0.10 mass%. Preferably, it is in the range of 0.01 to 0.05 mass%. The range of 0.02-0.04 mass% is more preferable.

N: 0.008 mass% or less

N is a harmful element which reduces toughness, and it is preferable to reduce N as much as possible. In particular, when it exceeds 0.008 mass%, since toughness will fall largely, an upper limit shall be 0.008 mass%. Preferably it is 0.006 mass% or less, More preferably, it is 0.004 mass% or less.

Cr: 0.1 mass% or more and 0.5 mass% or less

Cr transfers 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, it is possible to form a complex oxide of Cr or Zn centered on Fe and to keep Zn on the surface of the steel sheet for a long time, so that the corrosion resistance can be remarkably improved. The above-mentioned effect is particularly noticeable in a portion in contact with a liquid containing a high concentration of salt separated from the crude oil fraction, such as the bottom plate portion of the tanker oil tank, and by subjecting the steel material of the portion containing Cr to Zn-containing primer treatment Compared with the steel which does not contain Cr, corrosion resistance can be improved significantly. The effect of Cr is not enough to add 0.1 mass% or less, while addition of more than 0.5 mass% deteriorates the toughness of the welded portion. Therefore, Cr is made into the range of more than 0.1 mass% and 0.5 mass% or less. The range of 0.11-0.3 mass% is more preferable. The range of 0.12-0.2 mass% is more preferable.

Cu: 0.03 to 0.4 mass%

Cu is an element that increases the strength of the steel, and exists in rust generated by corrosion of the steel and has an effect of improving corrosion resistance. These effects are not sufficiently obtained by addition of less than 0.03 mass%. On the other hand, addition of more than 0.4 mass% causes problems such as surface cracking during hot working, in addition to saturation of the corrosion resistance improving effect. Therefore, it is necessary to add Cu in the range of 0.03-0.4 mass% from a viewpoint of stably manufacturing the steel material of this invention. Moreover, since the effect of Cu addition becomes saturated with the increase of addition amount, the range of 0.008-0.15 mass% is preferable from a cost vs. effect. The range of 0.01-0.14 mass% is more preferable.

The steel material of this invention needs to contain 1 type (s) or 2 or more types chosen from W, Mo, Sn, and Sb other than the said component in the following range.

W: 0.01 to 1.0 mass%

W has the effect of suppressing the formula in the tanker tank bottom plate, and also has the effect of suppressing the entire surface corrosion of the tanker upper deck portion. The said effect is expressed by addition of 0.01 mass% or more. However, if it exceeds 1.0 mass%, the effect is saturated. Therefore, W is added in 0.01-1.0 mass%. Preferably it is 0.01-0.5 mass%, More preferably, it is the range of 0.02-0.3 mass%.

In addition, the reason why W has the above corrosion resistance improving effect is that WO ₄ ^{2 −} is formed in the rust generated due to corrosion of the steel sheet, and the presence of WO ₄ ^{2 −} causes chloride ions. inhibited from breaking on the surface of the steel sheet and, also in a pH of lower portion of the anode portion (anode) of the steel sheet surface, the FeWO ₄ is generated, the attack on the surface of the steel sheet of the chloride ion by the presence of the FeWO ₄ inhibition Because it becomes. In addition, WO ₄ ^{2 -} by the inhibitor action (inhibitory action) by adsorption on the surface of the steel, it is considered that the corrosion inhibition of steel.

Mo: 0.01-0.5 mass%

Mo not only suppresses the formula in the tanker tank bottom plate, but also the corrosion resistance of the entire surface of the rear surface of the tanker upper deck, and the corrosion resistance after coating in a corrosive environment in which salt water immersion and high wetness are repeated like a ballast tank. Has the effect of improving. The effect of Mo is expressed by addition of 0.01 mass% or more, but when it exceeds 0.5 mass%, the effect is saturated. Therefore, Mo is made into the range of 0.01-0.5 mass%. Preferably it is 0.02-0.5 mass%, More preferably, it is the range of 0.03-0.4 mass%.

In addition, the reason why Mo has a corrosion resistance improving effect as described above, as in W, MoO ₄ ² in the melt is generated accompanying the corrosion of the steel sheet ^- is generated, the MoO ₄ ^{2 -} by the presence of chloride ions It is considered that intrusion into the surface of the steel sheet is suppressed.

Sn: 0.001-0.2 mass%, Sb: 0.001-0.4 mass%

Sn and Sb have the effect of suppressing the formula in the tanker tank bottom plate, and also have the effect of suppressing the entire surface corrosion of the tanker upper deck portion. The above effects are expressed by addition of at least 0.001 mass% of Sn and at least 0.001 mass% of Sb. On the other hand, even if it adds in excess of Sn: 0.2 mass% and Sb: 0.4 mass%, the effect is saturated. In addition, addition of a large amount of Sn encourages surface cracking during hot working with Cu. Therefore, it is preferable to add Sn and Sb in the said range, respectively.

Moreover, it is preferable that the steel material of this invention contains 1 type or 2 types selected from Ni and Co in the following range other than the said essential component.

Ni: 0.005 to 0.4 mass%, Co: 0.01 to 0.4 mass%

Ni and Co have an effect which refine | miniaturizes the produced | generated rust particle, and improves corrosion resistance in the bare state and corrosion resistance in the state in which the epoxy coating was given to the zinc primer. Therefore, when these elements want to improve corrosion resistance more, it is preferable to add them auxiliaryly. The said effect is expressed by addition of 0.005 mass% or more of Ni and 0.01 mass% or more of Co. On the other hand, even if it adds exceeding 0.4 mass% of Ni and exceeding 0.4 mass% of Co, the effect is saturated. Moreover, Ni has the effect of suppressing the surface crack at the time of hot working which arises in steel containing Cu and Sn. Therefore, it is preferable to add Ni and Co in the said range, respectively.

Moreover, in addition to the said essential component and the optional addition component (Ni, Co) containing in the said appropriate range, the steel materials of this invention are further represented by following formula (1);

X value ^{= (1-0.8 × Cu 0 .5)} × {1- (0.8 × W + 0.4 × Mo) 0.3} × {1- (Sn + 0.4 × Sb) 0.3} × {1- (0.05 × Cr + 0. 03 × Ni + 0.03 × Co) ^0.3 } × {1 + 2 × (S / 0.01 + P / 0.025)}... (One)

Here, the element symbol in the said formula represents content (mass%) of the element, and the element which is not contained is calculated as 0 (zero).

It is necessary to contain so that X value defined by may satisfy 0.5 or less.

Equation (1) is a formula for evaluating the influence of each component on the corrosion in the tanker oil tank. The coefficient of the component for improving the corrosion resistance is negative, and the coefficient of the component for deteriorating the corrosion resistance is indicated as plus. Therefore, the steel with a small value of X is excellent in corrosion resistance. The inventors examined the relationship between the value of X and the corrosion resistance of the steel material in the corrosive environment in the tanker oil tank. As a result, when X is 0.5 or less, the corrosion resistance in the corrosive environment in the tanker tank is excellent. It was found that the corrosion resistance deteriorated. Therefore, in the steel material of this invention, when determining content of P, S, Cr, Cu, W, Mo, Sn, Sb, Ni, and Co, it is necessary to design components so that the said X value may be 0.5 or less. .

Moreover, in addition to containing the said component in said appropriate range, the steel materials of this invention are Cu, Sn, and Ni are following formula (2);

Z value = (1 + 10 x Sn) x (Cu-0.7 x Ni)... (2)

Here, the element symbol in the said formula represents content (mass%) of the element, and the element which is not contained is calculated as 0 (zero).

It is necessary to contain so that Z value defined by may become 0.15 or less. The reason is Cu is an element that causes surface cracks during hot working, and Sn is an element that promotes cracking by Cu. On the other hand, Ni is an effective element for preventing the harmful effects of the elements, but in order to express the above effects of Ni, it is necessary to satisfy the above formula (2) and add Ni.

Moreover, the steel material of this invention can add 1 type (s) or 2 or more types selected from Nb, Ti, V, and Zr in addition to the said component in order to raise the intensity | strength of steel in the following range.

Nb: 0.001-0.1 mass%, Ti: 0.001-0.1 mass%, Zr: 0.001-0.1 mass%, and V: 0.002-0.2 mass%

Nb, Ti, Zr, and V are all elements which raise steel strength, and can be suitably selected and added according to the intensity | strength required. In order to acquire the said effect, it is preferable to add Nb, Ti, Zr 0.001 mass% or more, and V 0.002 mass% or more, respectively. However, when Nb, Ti, Zr exceeds 0.1 mass% and V exceeds 0.2 mass%, respectively, toughness falls, It is preferable to add Nb, Ti, Zr, V in the said range, respectively. Do.

In addition, the steel materials of the present invention may be added, in addition to the above components, one or two or more selected from Ca, REM, and Y in the following ranges in order to increase the strength or improve the toughness. .

Ca: 0.0002 to 0.01 mass%, REM: 0.0002 to 0.015 mass% and Y: 0.0001 to 0.1 mass%

Ca, REM, and Y are all effective in the toughness improvement of a welding heat influence part, and can be added as needed. The effect is obtained by addition of 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: 0.1 mass% When lowering, the toughness is rather deteriorated, and Ca, REM and Y are preferably added in the above ranges, respectively.

Moreover, the steel material of this invention can contain B further in the following range in addition to the said component.

B: 0.0002 to 0.003 mass%

B is an element which raises the strength of steel materials, and can be added as needed. In order to acquire the said effect, it is preferable to add 0.0002 mass% or more. However, when it exceeds 0.003 mass%, toughness will fall. Therefore, it is preferable to add B in 0.0002 to 0.003 mass%.

Moreover, it is preferable to manufacture the steel materials used for the crude oil tank of this invention by the following method.

That is, the steel material of the present invention is a known refinery process such as a steel converter, an electric furnace, vacuum degassing equipment, etc. ) To a steel material (slab) by a continuous casting process or a steel-clad rolling method, and then the material is reheated and then hot rolled. It is preferable to set it as steel materials, such as a steel plate, a thin steel plate, and a shaped steel.

It is preferable to make reheating temperature before the said hot rolling into the temperature of 900-1200 degreeC. If heating temperature is less than 900 degreeC, deformation resistance is large and it becomes difficult to hot roll. On the other hand, when the heating temperature exceeds 1200 ° C, austenite grains are coarsened, and in addition to causing toughness, scale loss due to oxidation is remarkable, and yield ratio is increased. This is because it is degraded. More preferable heating temperature is 1000-1150 degreeC.

Moreover, when rolling by steel materials of a desired shape and a dimension by hot rolling, it is preferable to make finish rolling finish temperature into 750 degreeC or more. This is because at less than 750 ° C., the deformation resistance of the steel is increased to increase the rolling load, which makes it difficult to roll or to cause a waiting time until the rolled material reaches a predetermined rolling temperature, thereby reducing the rolling efficiency.

The cooling of the steel material after hot rolling may be any method of air cooling or accelerated cooling, but if it is desired to obtain higher strength, it is preferable to accelerate cooling. Moreover, when performing acceleration cooling, it is preferable to make cooling rate into 2 to 80 degreeC / sec, and cooling stop temperature in the range of 650 to 300 degreeC. If the cooling rate is less than 2 ° C / sec and the cooling stop temperature is more than 650 ° C, the effect of accelerated cooling is small and sufficient high strength is not achieved. On the other hand, when cooling rate exceeds 80 degreeC / sec and cooling stop temperature is less than 300 degreeC, toughness of the steel material obtained may fall or deformation may arise in the shape of steel material.

Next, the welded joint of the crude oil tank formed by welding the steel material of this invention is demonstrated.

As for the welding joint of the crude oil tank formed by welding the steel plates manufactured by adjusting to the said appropriate component, Mo and W in a weld metal are following formula (3);

3 <{(Mo + W) in weld metal} / {(Mo + W) in base metal} ≤8. (3)

It needs to satisfy and contain.

Generally, when corrosion resistance of a weld metal is inferior to the corrosion resistance of a base material (steel material), dissolution of a weld metal is accelerated | stimulated by the acid immersion test which simulated the formula inside of the tanker bottom plate mentioned later. In addition, in order to increase the corrosion resistance of the weld metal to the same level as the base material (steel material), when Sn or Sb is added to the weld metal, low-temperature toughness of the welded joint cannot be secured. Therefore, it is necessary to improve the corrosion resistance of a weld metal to the same grade as a base material (steel material), without containing Sn and Sb in a weld metal. Then, this invention is characterized by restricting the value of {(Mo + W)} / {(Mo + W) in a base metal} to the range prescribed | regulated by said Formula (3) as a corrosion resistance improvement means of a weld metal.

When {Mo + W in weld metal} / (Mo + W in base metal) is 3 or less, since the corrosion resistance of a weld metal is inferior to that of a base material (steel material), acid immersion which simulated the formula inside of the tanker bottom plate mentioned later In the test, dissolution of the weld metal is accelerated. On the other hand, when {(Mo + W)} / {(Mo + W) in a base metal} exceeds 8, the cost of a welding material (welding wire) will not only rise by addition of Mo or W more than required, Since the corrosion resistance of the weld metal greatly exceeds that of the base material, selective corrosion of the base material occurs in a real corrosion environment. Therefore, Mo and W in a weld metal need to satisfy said Formula (3). The range of 3-7 is more preferable for {Mo + W in weld metal} / (Mo + W in base metal). The range of 3-6 is more preferable.

Moreover, as for the welding joint of the crude oil tank of this invention, Cu, Mo, and W in a weld metal are following (4) Formula;

0.2 ≦ (Cu in weld metal) / {(Mo + W) in weld metal} ≦ 3. (4)

It needs to satisfy and contain.

The inventors found out that the corrosion resistance of a welded joint improves significantly by the synergistic effect of those elements, when Cu and Mo or W are used together. However, when (Cu in a weld metal) / ((Mo + W) in a weld metal) in (4) Formula is less than 0.2, content of Cu is remarkably low compared with content of Mo or W in a weld metal, and the said synergistic effect Since it cannot be expected, the corrosion resistance of a welded joint falls. On the other hand, when (Cu in a weld metal) / ((Mo + W) in a weld metal) in (4) Formula exceeds 3, content of Mo or W is too low compared with Cu content in a weld metal, and also the said rise Since the effect can not be expected, the corrosion resistance of a welded joint falls. Therefore, Cu, Mo, and W in a weld metal need to satisfy said formula (4). In addition, content of Mo and W in a weld metal may not contain either Mo and W as long as the sum total content exists in the range which satisfy | fills the said formula. As for (Cu in a welding metal) / ((Mo + W) in a welding metal), the range of 0.5-3 is more preferable. The range of 0.5-2.5 is more preferable.

In addition, the inventors of the present invention, in addition to the above conditions, the welded joint of the present invention, Cu in the weld metal and the base metal (steel material) is represented by the following formula (5);

1? (Cu in weld metal) / (Cu in base material)? (5)

When it satisfy | filled, it discovered that the corrosion resistance in all the parts containing a base material (steel material) and a welded joint improves further.

As described above, the present invention does not contain Sn or Sb in the weld metal in order to improve the corrosion resistance of the weld metal to the same level as the base material (steel material) without lowering the low temperature toughness of the weld joint. The point is to control the value of Mo + W)} / {(Mo + W)} in a base material to the appropriate range prescribed | regulated by Formula (3). However, in order to improve the corrosion resistance of a weld metal more stably, it is more preferable that the value of (Cu in a weld metal) / (Cu in a base material) is 1 or more in addition to said (3) Formula. On the other hand, when (Cu in a welding metal) / (Cu in a base material) exceeds 8, the cost of a welding material (welding wire) will not only rise by addition of Cu more than necessary, but the corrosion resistance of a welding metal will Since it greatly exceeds, there exists a possibility that the selective corrosion of a base material may generate | occur | produce. Therefore, it is preferable that Cu in a weld metal and a base material (steel material) satisfy | fills (5) Formula.

In addition, in order to control content of Cu, Mo, and W in the said welding metal in the said range, the welding material (weld wire) used for welding suitably according to the component composition of a steel material (base material), and welding conditions. It is desirable to choose. For example, a welding wire having a composition obtained by converting target compositions of Cu, Mo, and W in a weld metal into dilution ratios of a base material is produced, and is welded using the same.

The welding method used for welding the crude oil tank of the present invention is high-heat welding such as FAB welding, FCB welding, RF welding, which is a one-side submerged arc welding method in one pass. input welding) or carbon dioxide gas arc welding (CO ₂ welding (CO ₂ arc welding)) and may be used and the like, such as small heat input welding (small-heat input welding), view of controlling the chemical component composition of the appropriate range of the weld metal Needs to be a welding method using a welding wire.

In addition, FAB welding is a registered trademark of the welding method of Kobe Steel Works Co., Ltd. Here, a method of forming a back side bead by applying a backing material composed of glass tape, solid flux, etc. directly to the back of the steel sheet, and by one pass welding. . In addition, FCB welding is a registered trademark of the welding method of Kobe Steel Works Co., Ltd., A method of distributing a back dam flux on a copper plate, and a method of forming a back side bead by one pass welding on the back of a steel plate. In addition, the RF method is a registered trademark of the welding method of Kobe Steel Works Co., Ltd., and puts the jig frame in which the rug flux is overlapped under the back dam flux including the welding thermosetting resin on the back of the steel plate, and presses the flux in the frame to perform one pass welding. Say how to form the back bead.

Example

No. 1 shown in Table 1-1 and Table 1-2. Steels having different component compositions of 1 to 36 are melted in a vacuum melting furnace to form ingots, or they are melted in converters and continuously cast to steel slabs, and these are reheated to 1150 ° C, and the finish rolling end temperature is 800 ° C. Rolling was performed to obtain a thick steel sheet having a plate thickness of 25 mm.

No. thus obtained. With respect to the thick steel plate of 1-36, the presence or absence of the crack of the steel plate surface was investigated by the magnetic particle test, and it was determined that (circle) and crack were detected as x that the crack was not detected.

Next, the No. Each steel plate of 1-36 was welded by the welding method of Table 2-1 and Table 2-2, and the welding joint was produced. Further, the heat input for each welding method, welding FCB is 146 kJ / ㎝, FAB welding is 180 kJ / ㎝, CO ₂ welding was set to 1.5 kJ / ㎝. All improvements were made to V improvements. Here, the composition control of Cu, Mo, and W in the weld metal of each welded joint is based on the target composition of Cu, Mo, and W as the base material dilution rate (about 11% CO ₂ welding, about 47% FAB welding, about 67% FCB welding). The welding wire which has a composition calculated | required in conversion to was produced, and it welded using this. For FCB welding, flux (PF-I55E / Kobe Steel Mill) and flux (PF-I50R / Kobe Steel Mill, Ltd.) and FAB welding flux (PF-I52E / (Note) ) Kobe Steel Mill), filler (RR-2 / Kobe Steel Mill) and backing material (FA-B1 / Kobe Steel Mill) were used, respectively.

In addition, as mentioned above, FCB welding refers to the method of forming a back side bead by 1-pass welding by spreading a flux on the copper plate and spreading | dispersing a flux. In addition, FAB welding means the method of forming a back side bead by applying a backing material comprised from glass tape, solid flux, etc. directly to the back surface of a steel plate, by 1 pass welding.

About the welded joint produced as mentioned above, content of Cu, Mo, and W in a weld metal was measured using atomic absorption spectrometry.

In addition, with the following method, the whole surface corrosion test which simulated the upper deck and the local corrosion test which simulated the tanker bottom plate environment were implemented.

(1) Full surface corrosion test simulating tanker upper deck environment

In order to evaluate the corrosion resistance to corrosion of the entire surface on the back of the tanker upper deck, the above-mentioned No. From the position of the plate thickness 1/4 of the thick steel plate welded joint of 1-36, the rectangular small piece of width 25mm x length 60mm * thickness 5mm is cut out so that a weld metal may be located in the center parallel to the width direction of a test piece. The surface was polished with 600th emery paper. In order that a back surface and a cross section may not be corroded, it sealed with the tape and the whole surface corrosion test was done using the corrosion test apparatus shown in FIG.

This corrosion test apparatus is composed of a corrosion test bath (2) and a temperature control plate (3), and the corrosion test tank (2) is filled with water (6) at a temperature of 36 deg. In addition, in the water 6, a mixed gas consisting of 12 vol% CO ₂ , 5 vol% O ₂ , 0.01 vol% SO ₂ , 0.3 vol% H ₂ S, and the balance N ₂ (introduction gas 4) The inside of the corrosion test tank (2) was filled with supersaturated water vapor to reproduce the corrosion environment behind the crude oil tank upper deck. And the corrosion test piece (1) set on the back of this test tank is 25 degreeC x via the temperature-controlled plate (3) which integrated the electric heater and the cooling system. The temperature change which makes 3 cycles +50 degreeCx21 hours into 1 cycle is repeated for 180 days, the dew condensation water is made to surface of the test piece 1, and the whole surface corrosion is caused. In FIG. 1, 5 shows the discharge gas from a test tank.

After the test, rust on the surface of each test piece was removed, and from the mass change before and after the test, a decrease in mass due to corrosion was obtained, and from this value, it was converted into a plate thickness loss per year (corrosion rate on one side). As a result, the corrosion resistance is good when the corrosion rate is 0.08 mm / year or less and no local corrosion is observed in the base material part or the weld part (○), the corrosion resistance is greater than 0.08 mm / year, or the base part part or the weld part. In either case, when local corrosion was visually confirmed, the whole surface corrosion resistance was evaluated as poor (x).

(2) Local corrosion (official) test simulating tanker tank bottom plate environment

In order to evaluate corrosion resistance to the formula in a tanker oil tank bottom plate, the said No. From the position of the plate thickness 1/4 of the thick steel plate welded joint of 1-36, the rectangular small piece of width 25mm x length 60mm * thickness 5mm is cut out so that a weld metal may be located in the center parallel to the width direction of a test piece. The whole surface was polished with the 600th emery paper.

Subsequently, a 10 mass% NaCl aqueous solution was prepared using a concentrated hydrochloric acid to prepare a test solution having a concentration of 10 mass% of Cl ions at a pH of 0.85, and a string was passed through a 3 mmφ hole formed in the upper portion of the test piece. Suspension was performed and the corrosion test which immersed in 2 L test solution for 168 hours was performed with respect to 1 test piece. In addition, the test solution was previously heated and maintained at 30 ° C. and replaced with a new test solution every 24 hours.

The apparatus used for the said corrosion test is shown in FIG. This corrosion test apparatus is a dual-type apparatus of the corrosion test tank 8 and the thermostat 9, the test solution 10 is contained in the corrosion test tank 8, and the test piece 7 is connected to the string 11 therein. It is hung and dipped. The temperature of the test solution 10 is maintained by adjusting the temperature of the water 12 put in the constant-temperature bath 9.

After the said corrosion test, after removing the rust produced on the test piece surface, the mass difference before and behind a test was calculated | required, this difference was converted into the total surface area, and the plate thickness reduction amount (corrosion rate of both surfaces) per year was calculated | required. As a result, when the corrosion rate is 0.8 mm / year or less and local corrosion is not visually confirmed in the base material part and the weld part, the local corrosion resistance is good (○), and the corrosion rate is more than 0.8 mm / year and 1.0 mm / year or less. In addition, when the local corrosion is not visually confirmed in the base material and the welded part, the local corrosion resistance is generally good (△), the corrosion rate is higher than 1.0 mm / year, or the local corrosion is caused in any of the base material and the welded part. The case where it confirmed visually was evaluated that internal corrosion resistance was bad (x).

The result of the said magnetic particle test and the result of a corrosion resistance test were shown to Table 2-1 and Table 2-2 with the X value and Z value calculated | required from the component composition of each steel plate. From these tables, the base metal and the weld metal satisfy the component composition of the present invention, and satisfy the conditions of the X value and the Z value. The thick steel plates of 1 to 4, 6, 7 and 10 to 29 exhibited no corrosion at the time of rolling, and exhibited good corrosion resistance in all of the corrosion resistance tests that simulated the upper deck and the tanker base plate environment. On the other hand, No. which does not satisfy the conditions of this invention. The thick steel plates of 5, 8, 9, and 30-36 do not obtain a favorable result also in any corrosion resistance test.

3 is No. A diagram showing the effect of the welded metal composition (bottom plate and top plate) on the corrosion resistance of the tanker corrosion resistant welded seam for the thick steel plates of 1 to 36, wherein the abscissa is (Cu in the weld metal) / {(Mo + W in the weld metal)} The vertical axis is plotted as ((Mo + W) in the weld metal} / {(Mo + W) in the base metal}.

From FIG. 3, (Cu in a welding metal) / {(Mo + W) in a welding metal} is the range of 0.2-3, and {Mo + W in a welding metal} / (Mo + W) in a base material has a range of 3-8. When satisfied, good corrosion resistance (circle mark and △ mark in Table 2-1 and Table 2-2) was shown in both of the corrosion resistance test simulated behind the upper deck and the corrosion resistance test simulated in the tanker bottom plate environment (in FIG. 3). ◆ display). In addition, in FIG. 3, x showed either the corrosion resistance test which simulated the upper deck and the corrosion resistance test which simulated the tanker bottom plate environment.

[표 1-2]

[표 2-1]

[표 2-2]

1, 7: test piece
2, 8: corrosion test tank
3: temperature control plate
4: introduction gas
5: exhaust gas
6, 12: water
9: thermostat
10: test solution
11: string
TABLE 1-1

TABLE 1-2

[Table 2-1]

[Table 2-2]

Claims (6)

C: 0.03 to 0.16 mass%,
Si: 0.05-1.50 mass%,
Mn: 0.1-2.0 mass%,
P: 0.025 mass% or less,
S: 0.010 mass% or less,
Al: 0.005-0.10 mass%,
N: 0.008 mass% or less,
Cr: 0.1 mass% or more and 0.5 mass% or less,
Cu: 0.03-0.4 mass%, and further
1 or 2 or more types selected from W: 0.01-1.0 mass%, Mo: 0.01-0.5 mass%, Sn: 0.001-0.2 mass%, and Sb: 0.001-0.4 mass%,
Moreover, the said component contains the steel materials which the X value defined by following formula (1) becomes 0.5 or less, and the Z value defined by following formula (2) becomes 0.15 or less, and remainder consists of Fe and an unavoidable impurity. The welding joint of the crude oil tank formed by welding WHEREIN: The welding joint which Cu, Mo, and W in the base metal and the weld metal in a weld joint satisfy | fill the following formula (3) and (4) formula.
doing
X value ^{= (1-0.8 × Cu 0 .5)} × {1- (0.8 × W + 0.4 × Mo) 0.3} × {1- (Sn + 0.4 × Sb) 0.3} × {1- (0.05 × Cr + 0. 03 × Ni + 0.03 × Co) ^0.3 } × {1 + 2 × (S / 0.01 + P / 0.025)}... (One)
Z value = (1 + 10 x Sn) x (Cu-0.7 x Ni)... (2)
3 <{(Mo + W) in weld metal} / {(Mo + W) in base metal} ≤8. (3)
0.2 ≦ (Cu in weld metal) / {(Mo + W) in weld metal} ≦ 3. (4)
Here, the element symbol in the said formula represents content (mass%) of the element. The method of claim 1,
The said steel material is a welded joint containing 1 type or 2 types further selected from 0.005 to 0.4 mass% of Ni and 0.01 to 0.4 mass% of Co in addition to the said component composition. The method according to claim 1 or 2,
In addition to the above-mentioned component composition, the steel is further one or two selected from Nb: 0.001-0.1 mass%, Ti: 0.001-0.1 mass%, Zr: 0.001-0.1 mass%, and V: 0.002-0.2 mass%. Welded joints containing more than one species. The method according to any one of claims 1 to 3,
The said steel material further contains 1 type (s) or 2 or more types chosen from Ca: 0.0002-0.01 mass%, REM: 0.0002-0.015 mass%, and Y: 0.0001-0.1 mass% in addition to the said component composition. The method according to any one of claims 1 to 4,
The said steel material further contains B: 0.0002-0.003 mass% in addition to the said component composition. Crude oil tank which has a welded joint in any one of Claims 1-5.

Images