KR20190062479A - Crude oil tankers and crude oil tankers - Google Patents

Abstract

Provided is a steel material for a crude oil tanker that has both a predetermined composition of components and a Sn segregation degree of less than 18, thereby achieving excellent internal corrosion resistance and local corrosion resistance and excellent anti-lamellar tear properties.

Description

Crude oil tankers and crude oil tankers

The present invention relates to a crude oil tank (crude oil tank) of a crude oil tanker formed by welding a steel material, particularly a bottom plate (ceiling portion) or a side wall upper portion where corrosion of the entire surface occurs, Which is excellent in corrosion resistance and anti-lamellar tear property.

The present invention also relates to a crude oil tanker using the above-mentioned steel material.

It is known that the steel used for the inner surface of the crude oil tank of the crude oil tanker, particularly the upper back plate and the upper portion of the side wall, is subjected to frontal corrosion.

As a cause of this frontal corrosion,

(1) repetition of condensation and drying (dry) on the surface of steel sheet due to day and night temperature difference,

(2) Inner be introduced for the explosion-proof into the oil tank agent gas (O _2: from about 4 vol%, CO _2: about 13 vol%, SO _2: boiler or engine to approximately 0.01 vol%, the balance N ₂ with a representative composition of O _2, the number of condensation of CO _2, SO ₂ contained in the exhaust gas and the like), penetration,

(3) penetration of corrosive gas such as H ₂ S volatilized in crude oil into the dew water channel,

(4) Residual seawater used to clean crude oil tanks

And the like. These can be found from the fact that sulfate ions or chloride ions are detected in the strongly acidic dew condensation water in the solid-line toxicity test, which is usually conducted every 2.5 years.

Further, when H ₂ S is oxidized by the iron oxide generated by the corrosion, the solid S is generated in the layer of the dense wood. The iron oxide generated as a layer of the solid S easily peels off and is deposited on the bottom of the crude oil tank. For this reason, in the toxic inspection of the crude oil tanker, it is in the present situation that repair at the upper part of the crude oil tank or collection of the deposit at the bottom of the tank is carried out at a high cost.

On the other hand, in a steel material such as a crude oil tank bottom plate, corrosion is not caused by the corrosion inhibiting action of the crude oil itself or the corrosion inhibiting action of a protective coat (oil coat) derived from crude oil formed on the inner surface of the crude oil tank Thought.

However, recent studies have revealed that in the steel of the crude tank bottom plate, local corrosion (formula) of the air-borne type occurs.

As a cause of such local corrosion,

(1) the presence of flocculated water having a high concentration of salts represented by sodium chloride,

(2) detachment of the oil coat by excess cleaning,

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

(4) high concentration of O ₂ , CO ₂ , SO ₂ and the like in the explosion inert gas injected into the condensation water,

And the like. Actually, at the time of inspecting the solid line, the water contained in the crude oil tank was analyzed, and chloride ion and sulfate ion at a high concentration were detected.

In order to prevent the above-mentioned front surface corrosion or local corrosion of the inner surface of the crude oil tank, it is effective to coat the surface of the steel material to shield the steel material from the corrosive environment.

However, the coating operation of the crude oil tank is vast in its area of application, and the paint film needs to be repainted once every ten years by deterioration of the coating film. For this reason, a large cost is incurred in the inspection and painting of the crude oil tank. In addition, it is pointed out that, if the coating film is damaged, corrosion in such a damaged part is rather promoted under the corrosive environment of the crude oil tank.

Therefore, development of a steel material capable of preventing frontal corrosion and local corrosion of the inner surface of a crude oil tank without coating is desired.

As such a steel material, for example, in Patent Document 1,

P: 0.05% or less, S: 0.01% or less, Ni: 0.05 to 3%, Mo: 1% or less, C: 0.01 to 0.3%, C: 0.02 to 1% , Cu: not more than 1%, Cr: not more than 2%, W: not more than 1%, Ca: not more than 0.01%, Ti: not more than 0.1%, Nb: not more than 0.1%, V: not more than 0.1%, B: And a steel material for a cargo oil tank consisting of the remainder Fe and impurities.

.

In Patent Document 2,

0.01 to 1% of Si, 0.01 to 1% of Si, 0.05 to 2% of Mn, 0.05 to 2% of P, 0.01 to 1% of S, 0.01 to 1% of Ni, %, Sn: 0.01 to 0.2%, Cr: not more than 0.1%, Al: not more than 0.1%, and the balance Fe and impurities.

.

Japanese Patent Application Laid-Open No. 2003-82435 Japanese Patent Application Laid-Open No. 2007-270196

The crude oil tank of the crude oil tanker typically welds the bottom plate, the hopper plate, the upper deck plate and the longitudinal member, and the welded joint receives tensile stress in the plate thickness direction. It has become clear recently that there is a risk that a lamellar tear will occur in such a welded joint. Here, the term "lamellar tear" refers to a welded joint subjected to tensile stress in the thickness direction of a cruciform joint, a T joint, a corner joint, etc., and cracks develop in the steel material in a direction parallel to the surface of the steel sheet by tensile stress, .

Therefore, the steel material for the crude oil tanker is required to have excellent resistance to lamellar tear in addition to the above-mentioned corrosion resistance against the front surface corrosion and local corrosion of the inner surface of the crude oil tank.

In this respect, in the steel material of Reference 1, mechanical properties such as the inner lamellar tear property are not considered at all. Also, in the steel material of the cited document 2, no consideration is given to the inner lamellar tear property.

Thus, in Cited Documents 1 and 2, the risk of generating a lamellar tear in a welded joint is not taken into consideration at all. Therefore, when the steel material of Cited Documents 1 and 2 is used in a crude oil tank of an actual crude oil tanker , It is feared that a lamellar tear will occur in the weld joint.

SUMMARY OF THE INVENTION The present invention has been developed in view of the above-described circumstances, and has an excellent corrosion resistance against the front surface corrosion on the inner surface of the upper deck of the crude oil tank of the crude oil tanker and on the upper surface of the side wall, It is an object of the present invention to provide a steel material excellent in crude steel tanker having a lamellar tear property.

It is another object of the present invention to provide a crude oil tanker using the steel material for a crude oil tanker.

By the way, the inventors of the present invention have conducted extensive research for solving the above-mentioned problems and obtained the following findings.

(1) Addition of Sn and reduction of S are effective for improvement of the local corrosion environment in the bottom plate of the crude oil tank of the crude oil tanker, that is, the corrosion resistance in the official environment (hereinafter also referred to as local corrosion resistance).

(2) In order to improve the corrosion resistance in the front corrosion environment (hereinafter also referred to as "front surface corrosion resistance") on the back surface of the upper tank of the crude oil tanker or on the upper surface of the side wall, Cu, Ni, Sb, W, Mo, Si, or a combination of two or more thereof.

(3) On the other hand, from the viewpoint of the inner lamellar tear property, it is effective to reduce the amount of S in the steel and to reduce Sn.

In this way, from the viewpoint of improving the corrosion resistance (inner surface corrosion resistance and local corrosion resistance) in the corrosion environment on the inner surface of the crude oil tank of the crude oil tanker, addition of Sn is effective, but from the viewpoint of anti- It is valid. Therefore, the inventors have repeatedly studied on the basis of the above-described knowledge so that corrosion resistance and lamellar tear resistance can be achieved at the same time.

As a result,

(4) When the center segregation of Sn is suppressed and Sn is diffused to the entire steel material as much as possible, it is possible to obtain excellent lamellar tear properties even when Sn contains a predetermined amount, that is, It was found that the corrosion resistance in the corrosive environment of the inner surface of the crude oil tank of the crude oil tanker and the resistance to lamellar tear can be both satisfied.

In addition,

(5) It was found that the lamellar tear resistance was further improved by strictly controlling the amount of Sn according to the amount of S.

The present invention has been further studied and completed based on the above findings.

That is, the structure of the present invention is as follows.

1.% by mass,

C: 0.03 to 0.18%

Mn: 0.10 to 2.00%

P: 0.030% or less,

S: 0.0070% or less,

Al: 0.001 to 0.100%,

Sn: 0.01 to 0.20% and

N: 0.0080% or less,

Cu: 0.01 to 0.50%

Ni: 0.01 to 0.50%

0.01 to 0.30% Sb,

W: 0.01 to 0.50%

Mo: 0.01 to 0.50% and

Si: 0.01 to 1.50%

, And the balance of Fe and inevitable impurities,

Steels for crude oil tankers with Sn segregation less than 18.

Here, the Sn segregation degree is defined by the following equation (1).

[Sn Snittness] = [Sn concentration in center segregation] / [Sn concentration in average] --- (1)

2. The steel material for a crude oil tanker according to 1 above, wherein the S content and the Sn content in the above-mentioned composition satisfy the following relation (2).

10000 x [% S] x [% Sn] ² ? 1.40 - (2)

Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the composition of the components.

3. The composition according to claim 1,

Cr: 0.01 to 0.50% and

Co: 0.01 to 0.50%

, The steel material for a crude oil tanker according to (1) or (2) above.

4. The composition according to claim 1,

Ti: 0.001 to 0.100%,

Zr: 0.001 to 0.100%,

Nb: 0.001 to 0.100% and

V: 0.001 to 0.100%

The steel material for a crude oil tanker according to any one of 1 to 3 above, which contains one or two or more species selected from the group consisting of:

5. The composition according to claim 1,

Ca: 0.0001 to 0.0100%,

Mg: 0.0001 to 0.0200% and

REM: 0.0002 to 0.2000%

The steel material for a crude oil tanker according to any one of 1 to 4,

6. The composition of claim 1,

B: 0.0001 to 0.0300%

The steel material for a crude oil tanker according to any one of 1 to 5 above,

7. A crude oil tanker using the steel material for a crude oil tank according to any one of 1 to 6 above.

According to the present invention, it is possible to obtain a steel material for a crude oil tanker which is excellent in corrosion resistance in the corrosive environment of the inner surface of the crude oil tank of the crude oil tanker, that is, both of the inner surface corrosion resistance and the local corrosion resistance, and also has excellent lamellar tear resistance.

By using the steel material for the crude oil tanker of the present invention in the crude oil tank of the crude oil tanker, it becomes possible to reduce the cost of inspection and coating of the crude oil tank while ensuring high safety.

1 is a schematic view of a test apparatus used for a front corrosion test (dew condensation test).
2 is a schematic view of a test apparatus used for a local corrosion test (acid resistance test).

Hereinafter, the present invention will be described in detail. First, the reason why the composition of steel components is limited to the above range in the present invention will be described. In addition, the units of the content of elements in the composition of the steel are all "% by mass ", but they are simply expressed as "% "

C: 0.03 to 0.18%

C is an element necessary for securing strength of the steel. In order to obtain such an effect, the amount of C is 0.03% or more. However, when the C content exceeds 0.18%, the weldability and the toughness of the weld heat affected zone deteriorate. Therefore, the C content is in the range of 0.03 to 0.18%. , Preferably not less than 0.04% and not more than 0.16%.

Mn: 0.10 to 2.00%

Mn is an element for increasing the strength of the steel. In order to obtain such an effect, the amount of Mn should be 0.10% or more. However, if the amount of Mn exceeds 2.00%, the toughness and weldability of the steel decrease. Further, due to the center segregation of Mn, the lamellar tear resistance also deteriorates. Therefore, the Mn content is in the range of 0.10 to 2.00%. , Preferably not less than 0.60% and not more than 1.80%. More preferably, it is 0.80% or more and 1.60% or less.

P: not more than 0.030%

P deteriorates toughness and weldability. Therefore, the amount of P is 0.030% or less. Preferably 0.025% or less. More preferably, it is 0.015% or less. The lower limit is not particularly limited, but is preferably 0.003%.

S: not more than 0.0070%

S is an important element involved in local corrosion and intramolecular tearing. That is, S is a harmful element that forms a non-metallic inclusion MnS and becomes a starting point of local corrosion and lowers local corrosion resistance. Therefore, it is preferable that S is reduced as much as possible. Particularly, when the S content exceeds 0.0080%, the local corrosion resistance is remarkably lowered. Further, coarse MnS is a starting point of the lamellar tear. In particular, if the amount of S exceeds 0.0070%, the lamellar tear property is significantly deteriorated. Therefore, from the viewpoint of compatibility between the local corrosion resistance and the inner lamellar tear properties, the S content is 0.0070% or less. And preferably 0.0060% or less. More preferably, it is 0.0050% or less. The lower limit is not particularly limited, but is preferably 0.0003%.

Al: 0.001 to 0.10%

Al is an element added as a deoxidizer, and the amount of Al is 0.001% or more. However, if the amount of Al exceeds 0.10%, the toughness of the steel decreases. Therefore, the amount of Al is set in the range of 0.001 to 0.10%.

Sn: 0.01 to 0.20%

Sn is an element which is necessary for improving the local corrosion resistance and the corrosion resistance of the inside and is an element which decreases the lamellar tearing property while enhancing the important element related to the inner lamellar tear property, in other words, the corrosion resistance.

In other words, Sn prevents the diffusion of Cl ^- (chloride ion) that promotes corrosion by forming an insoluble film on the surface of a steel in a strong acidic local corrosion environment such as a bottom plate of a crude oil tank, Height is effective. In addition, Sn suppresses the diffusion of an anionic species such as SO ₄ ^2- which promotes corrosion by entering the rust on the surface of the steel in a weakly acidic, frontal corrosion environment such as the upper deck of a crude oil tank, It is effective. These effects are expressed by setting the amount of Sn to 0.01% or more. Of the Cu, Ni, Sb, W, Mo, and Si to be described later, Cu, Ni, and Sb , W, and Mo are not added, it is possible to exhibit good corrosion resistance.

On the other hand, Sn tends to segregate in the center portion of the steel, and in such a segregation portion, the hardness is remarkably increased, so that the lamellar tear property is deteriorated. In particular, when the Sn content exceeds 0.20%, the lamellar tear resistance deteriorates greatly. Therefore, from the viewpoint of securing the resistance to lamellar tearing, the amount of Sn is set to 0.20% or less. It is preferably 0.15% or less. More preferably, it is 0.10% or less.

N: 0.0080% or less

Since N is a harmful element that deteriorates toughness, it is desirable to reduce N as much as possible. In particular, when the N content exceeds 0.0080%, the decrease in toughness becomes large. Therefore, the N content is 0.0080% or less. Preferably 0.0070%. The lower limit is not particularly limited, but is preferably 0.0005%.

At least one member selected from the group consisting of 0.01 to 0.50% of Cu, 0.01 to 0.50% of Ni, 0.01 to 0.30% of Sb, 0.01 to 0.50% of W, 0.01 to 0.50% of Mo and 0.01 to 0.50% of Si

Cu, Ni, Sb, W, Mo and Si are elements for improving the corrosion resistance in the front corrosion environment such as the upper deck of the crude tank of the crude oil tanker.

As described above, although Sn is an effective element for improving the corrosion resistance, it can not be contained in a large amount from the viewpoint of resistance to lamellar tear. Therefore, in order to obtain excellent corrosion resistance in a front corrosion environment such as an upper deck of a crude oil tank of a crude oil tanker, 0.01 to 0.50% of Cu, 0.01 to 0.50% of Ni, 0.01 to 0.30% of Sb and 0.01 to 0.50% , Mo: 0.01 to 0.50%, and Si: 0.01 to 1.50%.

Here, Cu, Ni, and Sb, respectively, along with the progress of corrosion, and glass as Cu ^2+, Ni ²⁺ and Sb ³⁺ from the steel surface, in conjunction with corrosion factor, S ^2-, CuS, NiS, Sb ₂ S ₃ . As a result, permeation of S ^{2- from} the steel interface is suppressed. In addition, W, Mo and Si are liberated as WO ₄ ^2- , MoO ₄ ^2- and SiO ₄ ^4- , respectively, and enter the rust to impart cateonic permselectivity to rust, and SO ₄ ^2- S ^{2- and} so on.

These effects become present when the above-mentioned Sn (anticorrosive) action coexists, and the amounts of Cu, Ni, Sb, W, Mo, and Si are each 0.01% or more. However, if a large amount of elements is added, the weldability and toughness are deteriorated, which is disadvantageous from the viewpoint of cost.

Therefore, the Cu content is in the range of 0.01 to 0.50%, the Ni content is in the range of 0.01 to 0.50%, the Sb content is in the range of 0.01 to 0.30%, the W content is in the range of 0.01 to 0.50%, the Mo content is in the range of 0.01 to 0.50% The range and the amount of Si are set in the range of 0.01 to 1.50%.

Preferably, the amount of Cu is not less than 0.02% and not more than 0.40%, the amount of Ni is not less than 0.02% and not more than 0.40%, the amount of Sb is not less than 0.02% and not more than 0.25%, the amount of W is not less than 0.02% Is 0.02% or more and 0.40% or less, and the amount of Si is 0.01% or more and 1.00% or less.

As described above, the lowering mechanism of the internal lamellar tear property by Sn is different from the lowering mechanism of internal lamellar tear by S. However, the degradation of the inner lamellar tear by S and Sn acts synergistically with each other. Therefore, from the viewpoint of further improving the lamellar tear resistance, it is preferable that the content of S and Sn satisfy the relation of the following formula (2).

10000 x [% S] x [% Sn] ² ? 1.40 - (2)

Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the composition of the components.

(2) above means that the influence of the Sn amount on the lamellar tear resistance is very large compared to the influence of the S amount. That is, strict control of Sn means that it is particularly important in securing the resistance to lamellar tear.

Here, 10000 x [% S] x [% Sn] ² is more preferably 1.20 or less. The lower limit of 10000 x [% S] x [% Sn] ² is not particularly limited, but is preferably 0.001.

Further, in suppressing the lamellar tear, it is needless to say that both the amount of S and the amount of Sn are limited to the above-mentioned range.

The basic components have been described above. In the steel material for a crude oil tanker of the present invention, the following elements can be appropriately contained.

0.01 to 0.50% of Cr, and 0.01 to 0.50% of Co.

Cr and Co move to the inside of the green layer with the progress of corrosion and block intrusion of Cl ^{- into} the green layer, thereby suppressing the concentration of Cl ^- in the interface between the green layer and the substrate, thereby contributing to improvement of corrosion resistance . When the Zn-containing primer is applied to the surface of the steel material, Cr and Co can form a composite oxide with Zn or the like around Fe, thereby enabling Zn to remain on the surface of the steel sheet for a long period of time. . Such an effect becomes particularly conspicuous in a portion in contact with a liquid containing a high concentration of salt separated from crude oil fractions, such as a bottom plate of a crude oil tank of a crude oil tanker. That is, when a steel material is treated with a Zn-containing primer and the steel material is used in a portion contacting with a liquid containing a high concentration of salt separated from crude oil fractions, in a steel containing Cr or Co, The corrosion resistance is significantly improved as compared with a steel material which does not have a high corrosion resistance.

Such an effect is not sufficiently obtained when the amount of Cr or Co is less than 0.01%. On the other hand, if the amount of Cr or Co exceeds 0.50%, the toughness of the welded portion is deteriorated. As for Cr, the pH at the corrosion part is lowered as an element causing the hydrolysis reaction. In other words, if Cr is excessively added, the corrosion resistance as a total may be deteriorated.

Therefore, when Cr and Co are contained, the content thereof is in the range of 0.01 to 0.50%. , Preferably not less than 0.02% and not more than 0.30%. More preferably, it is not less than 0.03% and not more than 0.20%.

0.001 to 0.100% of Ti, 0.001 to 0.100% of Zr, 0.001 to 0.100% of Nb, and 0.001 to 0.100% of V,

Ti, Zr, Nb and V can be added singly or in combination from the viewpoint of ensuring desired strength. However, if any element is contained excessively, the toughness and weldability are deteriorated. Therefore, when Ti, Zr, Nb and V are contained, the amount thereof is in the range of 0.001 to 0.100%. , Preferably not less than 0.005% and not more than 0.050%.

At least one selected from 0.0001 to 0.0100% of Ca, 0.0001 to 0.0200% of Mg, and 0.0002 to 0.2000% of REM

Ca, Mg and REM can be added singly or in combination from the viewpoint of improving the toughness of the welded portion. However, if any element is contained excessively, the toughness of the welded portion is deteriorated. In addition, the cost also increases. Therefore, when Ca, Mg and REM are contained, the Ca amount is 0.0001 to 0.0100%, the Mg amount is 0.0001 to 0.0200%, and the REM amount is 0.0002 to 0.2000%.

B: 0.0001 to 0.0300%

B is an element improving the quenching of the steel. From the viewpoint of securing a desired strength, B can be contained. From this point of view, it is effective to set the amount of B to 0.0001% or more. However, if B is excessively contained, in particular, when the B content exceeds 0.0300%, a significant deterioration in toughness is caused. Therefore, when B is contained, the content thereof is in the range of 0.0001 to 0.0300%.

The other components are Fe and inevitable impurities.

As described above, the composition of the steel material for the crude oil tanker of the present invention has been described. However, in the steel material for crude oil tanker of the present invention, it is very important to control the Sn segregation degree as follows.

Sn segregation: less than 18

By the center segregation of Sn, the hardness of the segregation part increases greatly. Such a segregation portion becomes a starting point of the occurrence of a lamellar tear. In other words, in order to secure an excellent lamellar tear property in the composition of the component containing Sn, it is important to suppress the center segregation of Sn and suppress the hardness increase of the segregation part. From this point of view, the Sn segregation degree should be less than 18. Preferably less than 16. More preferably 15 or less. The lower limit is not particularly limited, but is preferably 2.

The term "Sn" as used herein means an average Sn concentration obtained by line analysis of an electron beam microanalyzer (hereinafter referred to as "EPMA") on a section cut perpendicular to the rolling direction of the steel material Of Sn at the center segregation.

Specifically, assuming that the thickness of the steel is t (mm) and the width (direction perpendicular to the rolling direction and thickness direction of the steel) is W (mm), the cross section cut parallel to the rolling direction of the steel (0.5 ± 0.1) × t in the direction of the thickness of the steel material in the direction perpendicular to the surface of the steel sheet (the surface area including the center position in the thickness direction of the steel material), the beam diameter: 20 占 퐉 and pitch: 20 占 퐉, the EPMA surface analysis of Sn is carried out. Further, the EPMA surface analysis of Sn is performed in three cross-sectional fields of view at positions of 1/4 × W, ½ × W, and 3/4 × W.

Subsequently, from the EPMA surface analysis, a position having the highest Sn concentration in each cross-sectional field of view was selected, and at each of the positions, a beam diameter of 5 占 퐉 and a pitch of 5 占 퐉 in the thickness direction of the steel, Perform EPMA line analysis. In addition, in the EPMA line analysis, regions up to 25 μm from the front and back surfaces of the steel are excluded.

Then, the maximum value of the Sn concentration (mass concentration) is obtained for each measurement line, and these average values are set as the Sn concentration (mass concentration) of the center segregation portion. The Sn concentration of the center segregation portion is calculated as an arithmetic average The value obtained by dividing the average Sn concentration (mass concentration) is regarded as Sn segregation.

In other words,

[Sn Snittness] = [Sn concentration in the center segregation] / [Sn concentration in average]

to be.

As described above, the steel material for a crude oil tanker of the present invention has a Sn segregation degree which suppresses the center segregation of Sn, that is, the degree of center segregation of Sn, from a viewpoint of ensuring excellent lamellar tear characteristics, Or less. Here, the Sn segregation degree changes largely depending on the production conditions even if the composition of the components is the same. For this reason, in order to suppress center segregation of Sn, it is very important to appropriately control the manufacturing method of the steel.

Hereinafter, a method for producing a steel material for a crude oil tanker according to the present invention will be described in detail.

That is, the steel material of the present invention can be obtained by a method in which a steel material adjusted to the above-described composition is subjected to a solvent treatment using a known refining process such as electric furnace, electric furnace, vacuum degassing, or the like and is subjected to continuous casting or agglomeration- (Slab), and then the steel material is reheated as required, followed by hot rolling to obtain a steel plate or a section steel. The thickness of the steel material is not particularly limited, but is preferably 2 to 100 mm. And more preferably 3 to 80 mm. More preferably 4 to 60 mm.

Here, in the case of continuous casting, the casting speed (drawing speed) is preferably 0.3 to 2.8 m / min. When the casting speed is less than 0.3 m / min, the operating efficiency is deteriorated. On the other hand, if the casting speed exceeds 2.8 m / min, surface temperature unevenness occurs, and molten steel supply to the inside of the cast is insufficient, and center segregation of Sn is promoted. More preferably 0.4 m / min or more and 2.6 m / min or less from the viewpoint of suppressing center segregation of Sn. More preferably 1.5 m / min or less.

It is also preferable to carry out a soft reduction method in which the cast steel having an unstored layer at the end of solidification is gradually cast by a rolling roll group at a reduced total amount and a descending speed to the extent of the sum of the amount of coagulation shrinkage and the amount of heat shrinkage.

Next, when the steel material is hot-rolled into a desired dimensional shape, it is preferable to heat the steel material to a temperature of 900 to 1350 占 폚. When the heating temperature is less than 900 ° C, the deformation resistance is large and the hot rolling becomes difficult. On the other hand, if the heating temperature exceeds 1350 ° C, surface shake may occur or scale loss or fuel unit may increase.

Particularly, the higher the heating temperature is, the more the diffusion of Sn in the center segregation portion is promoted, which is advantageous from the viewpoint of ensuring the resistance to lamellar tear. From this point of view, the heating temperature is more preferably 1030 DEG C or higher.

The holding time at the heating temperature is preferably 60 min or more. As a result, diffusion of Sn in the center segregation portion is sufficiently promoted. More preferably not less than 150 min. The upper limit is not particularly limited, but is preferably 1000 min.

In the case where the temperature of the steel material is originally in the range of 1030 to 1350 占 폚 and the steel material has been held in the temperature range for 60 minutes or more, it may be supplied to the hot rolling without heating. The hot-rolled sheet obtained after hot-rolling may be subjected to reheating treatment, acidic rolling or cold-rolling to obtain a cold-rolled sheet having a predetermined thickness.

In the hot rolling, it is preferable to set the finishing rolling finishing temperature to 650 DEG C or higher. When the finish rolling finish temperature is less than 650 캜, the rolling load increases due to the increase in deformation resistance, and it is difficult to perform rolling.

The cooling after the hot rolling may be either air cooling or accelerated cooling. However, in order to obtain a higher strength, it is preferable to perform accelerated cooling.

Here, when accelerated cooling is performed, it is preferable that the cooling rate is 2 to 100 캜 / s and the cooling stop temperature is 700 to 400 캜. That is, when the cooling rate is less than 2 占 폚 / s and / or the cooling stop temperature is more than 700 占 폚, the effect of accelerated cooling is small and sufficient high strength may not be achieved. On the other hand, when the cooling rate is higher than 100 deg. C / s and / or the cooling stop temperature is lower than 400 deg. C, the toughness of the steel may decrease or the shape of the steel may be deformed. However, this is not necessarily the case when heat treatment is carried out in a post-process.

Example

The steel having the composition shown in Table 1 (the balance being Fe and inevitable impurities) was dissolved in a converter and was continuously cast by the conditions shown in Table 2 to form a steel slab. These steel slabs were reheated to 1150 占 폚, held under the conditions shown in Table 2, and subjected to hot rolling at a final finish rolling temperature of 800 占 폚 to obtain a steel sheet having a thickness of 40 mm. The cooling after the hot rolling was performed by cooling (accelerated cooling) at a cooling rate of 10 ° C / s and a cooling stop temperature of 550 ° C.

Then, the Sn segregation degree in the obtained steel sheet was obtained by the above method. The results are shown in Table 2.

In addition, with respect to the steel sheet obtained as described above, a corrosion test (condensation test) simulating the environment on the upper deck of the crude tank tank of the crude oil tanker and a local corrosion test (acid test) simulating the bottom plate environment were carried out Respectively.

(1) Front corrosion test (condensation test)

In order to evaluate the corrosion resistance (front surface corrosion resistance) against the front corrosion on the back surface of the upper tank of the crude oil tank of the crude oil tank, from the position of 1 mm of the surface to the steel plates of Nos. 1 to 58 above, A piece of square having a size of 60 mm and a thickness of 5 mm was cut out, and the surface was polished with an emery paper having a number of 600. Then, the back surface and the end surface were sealed with a tape so as not to corrode to prepare a test piece 1, and a front corrosion test was conducted using the corrosion testing apparatus shown in Fig. This corrosion testing apparatus is composed of a corrosion test tank 2 and a temperature control plate 3. Water 6 maintained at a temperature of 30 캜 was introduced into the corrosion test tank 2 and 13 vol% CO ₂ and 4 vol% O ₂ , 0.01 vol% SO ₂ , 0.05 vol% H ₂ S and the remainder N ₂ was introduced to fill the corrosion test tank 2 with supersaturated water vapor so that the upper deck of the crude oil tank was exposed to the corrosive environment . The test piece 1 is set on the back surface of the corrosion test tank 2 and the test piece 1 is heated at 25 ° C for 1.5 hours and 50 ° C through a temperature control plate 3 containing a heater and a cooling device. Deg.] C for 22.5 hours as one cycle was repeatedly applied for 21, 49, 77 and 98 days to generate condensation water on the surface of the test piece 1 to cause frontal corrosion. 1, reference numeral 5 denotes an exhaust gas pipe extending from the test tank.

After the above corrosion test, the rust on the surface of each test piece was removed, and a sense of mass due to corrosion was obtained from the change in mass before and after the test. From this value, the plate thickness reduction amount per one year (corrosion rate on one side) was converted. Then, the predicted hand amount after 25 years from the value of the 4 test periods was determined. When the corrosion amount was 2.0 mm or less, the front corrosion resistance was good (O). When the corrosion amount was 2.0 mm or more, the front corrosion resistance was evaluated as poor Respectively.

(2) Local corrosion test (acid test)

In order to evaluate the corrosion resistance (local corrosion resistance) in the local corrosion environment (formula) in the bottom plate of the crude oil tank of the crude oil tanker, from the position of 1 mm from the surface to the above steel plates No. 1 to 58, A piece of square having a width of 25 mm, a length of 60 mm, and a thickness of 5 mm was cut out, and the surface was polished with an emery paper of 600 number to prepare a test piece.

Then, a 10% by mass NaCl aqueous solution was prepared by using concentrated hydrochloric acid to prepare a test solution having a Cl ion concentration of 10% by mass and a pH of 0.85. The test solution was suspended in a hole having a diameter of 3 mm in the upper part of the test piece, Each test piece was subjected to a corrosion test in which it was immersed in 2 L of test solution for 168 hours. In addition, the test solution was heated and maintained at 30 캜 in advance, 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 device having a double structure of a corrosion test tank 8 and a constant temperature bath 9. The test solution 10 is contained in the corrosion test tank 8 and the test piece 7 is placed in the corrosion test tank 8 ). The temperature of the test solution (10) is maintained by adjusting the temperature of the water (12) put in the thermostatic chamber (9).

After the corrosion test described above, the rust formed on the surface of the test piece was removed, and the mass difference before and after the test was obtained. The difference in the plate thickness per year (corrosion rate on both sides) was obtained by dividing the difference by the total surface area. As a result, the local corrosion resistance was evaluated as good (.largecircle.) When the corrosion rate was 1.0 mm / y or less and the poor corrosion resistance (x) when the corrosion rate was 1.0 mm / y sec.

In addition, the following criteria were used to evaluate my lamellar toughness.

(3) Evaluation of the property of the lamellar tiar

Rules for ClassNK Steel Wire In accordance with the same inspection procedure (Part 2, Part K), the steel sheets Nos. 1 to 58 obtained as described above were subjected to a tensile test in the thickness direction (Z direction) The reduction of area (RA) was calculated. Then, on the basis of the calculated sectional shrinkage ratio (RA), the lamellar tear resistance was evaluated based on the following criteria.

◎ (Pass, especially excellent): 70 or more

○ (Pass): 35 or more and less than 70

△ (Failed): 25 to less than 35

× (failure): less than 25

The evaluation results of (1) to (3) are shown in Table 2. The overall evaluation in Table 2 shows that the evaluations of (1) to (3) above are all "O" or "⊚" Quot; or " x " was " failed ".

[Table 1]

[Table 1 (continued)]

As shown in Table 2, all of the inventive examples have excellent inner surface corrosion resistance, local corrosion resistance, and further excellent anti-lamellar tear properties.

On the other hand, in the comparative example, sufficient characteristics are not obtained for at least one of the frontal corrosion resistance, the local corrosion resistance and the anti-lamellar tear property.

That is, in Comparative Examples Nos. 42, 48, and 52, the S amount exceeds the upper limit, so that sufficient characteristics are not obtained for the resistance to local corrosion and the resistance to lamellar tear.

Further, in Comparative Examples Nos. 43, 47, and 50, the Sn amount exceeded the upper limit, so that sufficient characteristics were not obtained for the lamellar tear resistance.

In Comparative Example No. 44, since the amount of S exceeded the upper limit, and Cu, Ni, Sb, W, Mo, and Si were not contained in a predetermined amount, sufficient corrosion resistance, corrosion resistance and anti- Characteristics can not be obtained.

In Comparative Example No. 45, since the amount of Sn is lower than the lower limit, sufficient characteristics are not obtained for the inner surface corrosion resistance and the local corrosion resistance.

In Comparative Example No. 46, since the S amount and the Sn amount exceed the upper limit, sufficient characteristics can not be obtained with respect to the local corrosion resistance and the anti-lamellar tear property.

In Comparative Example No. 49, since Cu, Ni, Sb, W, Mo and Si were not contained in a predetermined amount, sufficient characteristics were not obtained for the internal corrosion resistance.

In Comparative Example No. 51, since the amount of S exceeds the upper limit and the amount of Sn falls below the lower limit, sufficient characteristics are not obtained for the inner surface corrosion resistance, the inner corrosion resistance and the anti-lamellar tear property.

In Comparative Examples Nos. 53 to 56, since the Sn segregation degree exceeds the upper limit, sufficient characteristics can not be obtained with respect to the resistance to lamellar tear.

1, 7: Specimen
2, 8: corrosion test tank
3: Temperature control plate
4: Introduced gas pipe
5: Exhaust gas pipe
6, 12: water
9: thermostat
10: Test solution
11:

Claims (7)

In terms of% by mass,
C: 0.03 to 0.18%
Mn: 0.10 to 2.00%
P: 0.030% or less,
S: 0.0070% or less,
Al: 0.001 to 0.100%,
Sn: 0.01 to 0.20% and
N: 0.0080% or less,
Cu: 0.01 to 0.50%
Ni: 0.01 to 0.50%
0.01 to 0.30% Sb,
W: 0.01 to 0.50%
Mo: 0.01 to 0.50% and
Si: 0.01 to 1.50%
, And the balance of Fe and inevitable impurities,
Steels for crude oil tankers with Sn segregation less than 18.
Here, the Sn segregation degree is defined by the following equation (1).
[Sn Snittness] = [Sn concentration in center segregation] / [Sn concentration in average] --- (1) The method according to claim 1,
Wherein the S content and the Sn content in the composition satisfy the relation of the following formula (2).
10000 x [% S] x [% Sn] ² ? 1.40 - (2)
Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the composition of the components. 3. The method according to claim 1 or 2,
Wherein the composition of the component further comprises, by mass%
Cr: 0.01 to 0.50% and
Co: 0.01 to 0.50%
, And the steel material for a crude oil tanker containing one or two selected from among the above. 4. The method according to any one of claims 1 to 3,
Wherein the composition of the component further comprises, by mass%
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100%,
Nb: 0.001 to 0.100% and
V: 0.001 to 0.100%
A steel material for a crude oil tanker. 5. The method according to any one of claims 1 to 4,
Wherein the composition of the component further comprises, by mass%
Ca: 0.0001 to 0.0100%,
Mg: 0.0001 to 0.0200% and
REM: 0.0002 to 0.2000%
A steel material for a crude oil tanker. 6. The method according to any one of claims 1 to 5,
Wherein the composition of the component further comprises, by mass%
B: 0.0001 to 0.0300%
By weight based on the total weight of the crude steel tanker. A crude oil tanker using the steel material for a crude oil tank according to any one of claims 1 to 6.

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