TW201814063A - Steel for crude oil tanker and crude oil tanker - Google Patents

Abstract

Provided is steel for a crude oil tanker for which excellent uniform corrosion resistance and excellent local corrosion resistance together with excellent lamellar tear resistance have been established as a result of the steel having a specified component composition and an Sn segregation of less than 18.

Description

Crude oil tanker steel and crude oil tanker

[0001] The present invention relates to a crude oil tank (oil tank portion) of a crude oil tanker formed by butt welding of steel materials, particularly an upper deck back surface (top) or an upper portion of a side wall that may cause full corrosion, and local corrosion (pitting corrosion) may occur. Any one of the bottom plates of the oil tank portion can be applied to a steel material for a crude oil tanker having excellent corrosion resistance and laminar tear resistance. Furthermore, the present invention relates to a crude oil tanker using the above-mentioned steel materials.

[0002] It is known that the internal surface of the crude oil tank of the crude oil tanker, especially the steel used on the back of the upper deck and the upper part of the side wall, will cause comprehensive corrosion. [0003] The reasons for this comprehensive corrosion include: (1) condensation and drying (wet and dry) on the surface of the steel plate repeatedly due to the temperature difference between day and night; (2) an inert gas enclosed in a crude oil tank for explosion protection (O ₂ : about 4 vol%, CO ₂ : about 13 vol%, SO ₂ : about 0.01 vol%, and the rest are exhaust gas from a boiler or engine composed of N ₂ as a representative) O ₂ , CO ₂ , SO ₂ contained Dissolved in condensed water; (3) Corrosive gases such as H ₂ S volatilized from crude oil are dissolved in condensed water; (4) Seawater residues used to clean crude oil tanks. In these cases, it is usually possible to detect sulfuric acid ions or chloride ions in the strong acidic condensate during routine dock inspections of physical ships every two and a half years. [0004] In addition, when H ₂ S is oxidized by using rust generated by corrosion as a catalyst, solid S is generated in layers in the rust. The rust formed in the layered solid S is easily peeled off and accumulated on the bottom of the crude oil tank. Therefore, the routine dock inspection of crude oil tankers requires huge expenses. The current situation lies in the repair of the upper part of the crude oil tank or the recovery of deposits at the bottom of the tank each time. [0005] On the other hand, for steel materials such as a bottom plate of a crude oil tank, it has been conventionally considered that the corrosion suppression of a protective coating (oil coating) derived from the crude oil itself due to the corrosion inhibitory action of crude oil or the crude oil formed on the inner surface of the crude oil tank Function without corrosion. However, recent studies have clarified that the steel of the bottom plate of crude oil tanks will cause localized corrosion (pitting corrosion) of the bowl type. [0006] The causes of such local corrosion include: (1) the presence of high-concentration condensed water containing salts such as sodium chloride; (2) oil coating caused by excessive washing detachment layer; sulfide (3) contained in the crude oil to a high concentration; (4) was dissolved in water proof condensed with an inert gas O _2, CO _2, to a high concentration of SO ₂ and the like. In fact, during a routine dock inspection of a physical ship, the water remaining in the crude oil tank was analyzed, and high concentrations of chloride and sulfate ions were detected. [0007] To prevent the overall corrosion or local corrosion of the inner surface of the crude oil tank, it is effective to coat the surface of the steel to isolate the steel from the corrosive environment. However, the coating area of the crude oil tank is very large, and it needs to be re-coated every 10 years due to the deterioration of the coating film. Therefore, the inspection or coating of the crude oil tank requires a large amount of expense. Furthermore, it has been pointed out that once the coating film is damaged, such damaged parts will promote corrosion under the corrosive environment of the crude oil tank. [0008] Therefore, it is desired to develop a steel that can prevent full corrosion or local corrosion of the inner surface of the crude oil tank without coating. [0009] As such a steel material, for example, Patent Document 1 discloses: "A steel material for cargo oil tanks, which contains C: 0.01 to 0.3% by mass, Si: 0.02 to 1%, and Mn: 0.05 to 2 %, P: 0.05% or less, S: 0.01% or less, Ni: 0.05 ~ 3%, Mo: 1% or less, Cu: 1% or less, Cr: 2% or less, W: 1% or less, Ca: 0.01% or less , Ti: 0.1% or less, Nb: 0.1% or less, V: 0.1% or less, B: 0.05% or less, and the rest are composed of Fe and impurities. " [0010] Moreover, Patent Document 2 discloses: "A steel for cargo oil tanks, which contains C: 0.01 to 0.2%, Si: 0.01 to 1%, Mn: 0.05 to 2%, and P: 0.05% or less, S: 0.01% or less, Ni: 0.01 ~ 1%, Cu: 0.05 ~ 2%, Sn: 0.01 ~ 0.2%, Cr: 0.1% or less, Al: 0.1% or less, and the rest are composed of Fe and impurities Constituted. ". [Prior Art Document] [Patent Document] [0011] [Patent Document 1] Japanese Patent Laid-Open No. 2003-82435 [Patent Document 2] Japanese Patent Laid-Open No. 2007-270196

[Problems to be Solved by the Invention] [0012] In addition, the crude oil tank of a crude oil tanker is usually welded with a bottom plate and a hopper feed plate, an upper deck back plate and a longitudinal beam, etc., and the welded connection portion will be subjected to tensile stress in the thickness direction of the plate. . The risk of lamellar tearing in such welded joints has recently been clarified. Here, the so-called lamellar tear refers to a welded connection that undergoes tensile stress in the thickness direction of the plate, such as a cross-shaped connection, a T-shaped connection, and a square connection. Due to the tensile stress, it is parallel to the surface of the steel plate. Direction, deepening cracks inside the steel and cracking. Therefore, in addition to the corrosion resistance of the entire surface or local corrosion of the inside surface of the above-mentioned antigen tank, the steel for crude oil tankers also requires excellent lamellar tear resistance. [0013] In this regard, the steel material of Document 1 is cited without considering any mechanical properties such as lamellar tear resistance. Moreover, the steel materials cited in Reference 2 have not taken into consideration any resistance to lamellar tearing. [0014] In this way, references 1 and 2 are cited, because the risk of lamellar tearing at the welded joint is not considered at all. Therefore, when the steels cited in references 1 and 2 are used in the crude oil tanks of actual crude oil tankers, There is a possibility that lamellar tearing may occur at the welded joint. [0015] The present invention has been developed in view of the above-mentioned circumstances, and therefore provides an antigen tanker with a full corrosion on the back surface of the deck or the upper part of the side wall on the inner surface of the crude oil tank of the oil tanker. The purpose is to use steel for crude oil tankers with excellent tearability. Moreover, this invention aims at providing the crude oil tanker which used the said steel material for crude oil tankers. [Means for Solving the Problem] [0016] Therefore, the inventors of this case have made repeated researches to solve the above-mentioned problems, and obtained the following insights: (1) The bottom plate of the crude oil tank of the crude oil tanker should be improved in a locally corrosive environment, that is, in a pore corrosion environment The corrosion resistance (hereinafter also referred to as local corrosion resistance) is effective when Sn is added and S is reduced. (2) To improve the corrosion resistance of the upper deck back surface or the upper side wall of the crude oil tanker of the crude oil tanker in a comprehensive corrosive environment (hereinafter also referred to as comprehensive corrosion resistance), a compound selected from Cu, Ni, Sb, W, and One or more of Mo and Si are effective. (3) On the other hand, from the viewpoint of laminar tear resistance, it is effective to reduce the amount of S in steel and at the same time reduce Sn. [0017] In this way, from the viewpoint of improving the corrosion resistance (corrosion resistance and local corrosion resistance) of the inner surface of the crude oil tank of a crude oil tanker under a corrosive environment, the addition of Sn is effective, but it is based on lamellar tear resistance. In the opinion, reducing Sn is effective. Therefore, based on the above findings, the inventors of the present case have conducted further studies in order to have both corrosion resistance and lamellar tear resistance. [0018] As a result, the following insights were obtained: (4) As long as the central segregation of Sn is suppressed and Sn is diffused as much as possible to the entire steel, even if it contains a predetermined amount of Sn, excellent lamellar tear resistance can be obtained; that is, as long as Properly adjusting the amount of Sn, while suppressing the central segregation of Sn, and making Sn diffuse to the entire steel, can have both the corrosion resistance and lamellar tear resistance of the inner surface of the crude oil tank of a crude oil tanker in a corrosive environment. In addition, the following findings were obtained: (5) Strictly controlling the amount of Sn based on the amount of S can further improve the resistance to lamellar tearing. The present invention was completed based on the above findings and further research. [0019] That is, the gist of the present invention is structured as follows. 1. A steel material for a crude oil tanker, comprising: C: 0.03 to 0.18%, Mn: 0.10 to 2.00%, P: 0.030% or less, S: 0.0070% or less, and Al: 0.001 to 0.100% in mass% , Sn: 0.01 ~ 0.20% and N: 0.0080% or less, at the same time containing Cu: 0.01 ~ 0.50%, Ni: 0.01 ~ 0.50%, Sb: 0.01 ~ 0.30%, W: 0.01 ~ 0.50%, Mo: 0.01 ~ 0.50% and Si: One or more of 0.01 ~ 1.50%, and the rest is composed of Fe and unavoidable impurities, and the degree of Sn segregation is less than 18; Here, the degree of Sn segregation is It is defined by the following formula (1): [Sn segregation degree] = [Sn concentration in the center segregation part] / [average Sn concentration] --- (1). [0020] 2. The steel for crude oil tankers according to the above 1, wherein the S content and the Sn content in the aforementioned component composition satisfy the relationship of the following formula (2): Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the component composition, respectively. [0021] 3. The steel for crude oil tankers according to the above 1 or 2, wherein the aforementioned composition further contains one or two selected from the group consisting of Cr: 0.01 to 0.50% and Co: 0.01 to 0.50% by mass. [0022] 4. The steel material for crude oil tankers according to any one of the foregoing 1 to 3, wherein the aforementioned component composition further contains a material selected from the group consisting of Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, and Nb: 0.001 in mass%. ~ 0.100% and V: One or more of 0.001 ~ 0.100%. [0023] 5. The steel for crude oil tankers according to any one of the foregoing 1 to 4, wherein the aforementioned component composition further contains a member selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.0002 in terms of mass%. ~ 0.2000% of one or more of them. [0024] 6. The steel material for a crude oil tanker according to any one of 1 to 5 above, wherein the aforementioned component composition further contains B: 0.0001 to 0.0300% by mass%. [0025] 7. A crude oil tanker, which is formed by using the steel material for a crude oil tanker according to any one of 1 to 6 above. [Effects of the Invention] According to the present invention, it is possible to obtain the corrosion resistance of the inner surface of a crude oil tank of a crude oil tanker under a corrosive environment, that is, the overall corrosion resistance and the local corrosion resistance are excellent, and the lamellar tear resistance is excellent. It is also an excellent steel for crude oil tankers. In addition, by using the steel material for a crude oil tanker of the present invention in a crude oil tank of a crude oil tanker, high safety can be ensured, and at the same time, the cost required for inspection or painting of the crude oil tank can be reduced.

[Mode for Carrying Out the Invention] [0028] Hereinafter, the present invention will be specifically described. First, the reason why the component composition of steel is limited to the aforementioned range in the present invention will be described. In addition, the unit of the content of the elements in the composition of the steel is "mass%". Hereinafter, unless otherwise specified, it is only expressed in "%". [0029] C: 0.03 to 0.18% C is an element required to ensure the strength of the steel. In order to obtain this effect, the amount of C is taken as 0.03% or more. However, if the amount of C exceeds 0.18%, the weldability and the toughness of the heat-affected zone are deteriorated. Therefore, the amount of C is in the range of 0.03 to 0.18%. It is preferably 0.04% or more and 0.16% or less. [0030] Mn: 0.10 to 2.00% Mn is an element used to increase the strength of steel. In order to obtain this effect, the amount of Mn is taken to be 0.10% or more. However, when the Mn content exceeds 2.00%, the toughness and weldability of the steel are deteriorated. In addition, due to the segregation of the center of Mn, laminar tear resistance is also deteriorated. Therefore, the Mn content is in the range of 0.10 to 2.00%. It is preferably 0.60% or more and 1.80% or less. More preferably, it is 0.80% or more and 1.60% or less. [0031] P: 0.030% or less P deteriorates toughness and weldability. Therefore, the amount of P is taken as 0.030% or less. It is preferably 0.025% or less. It is more preferably 0.015% or less. In addition, the lower limit is not particularly limited, but 0.003% is preferable. [0032] S: 0.0070% or less S is an important element involved in local corrosion resistance and lamellar tear resistance. That is, S is a harmful element that forms MnS, which is a non-metallic inclusion, and becomes a starting point of local corrosion, thereby deteriorating local corrosion resistance. Therefore, the S system is preferably reduced as much as possible. In particular, if the amount of S exceeds 0.0080%, the local corrosion resistance is significantly deteriorated. In addition, a large volume of MnS becomes the starting point of lamellar tearing. In particular, when the amount of S exceeds 0.0070%, the lamellar tear resistance is significantly deteriorated. Therefore, from the viewpoint of having both local corrosion resistance and lamellar tear resistance, the S content is 0.0070% or less. It is preferably 0.0060% or less. It is more preferably 0.0050% or less. In addition, the lower limit is not particularly limited, but 0.0003% is preferable. [0033] Al: 0.001 to 0.10% Al is an element added as a deoxidizing agent, and the amount of Al is 0.001% or more. However, if the Al content exceeds 0.10%, the toughness of the steel is deteriorated. Therefore, the Al content is in the range of 0.001 to 0.10%. [0034] Sn: 0.01 ~ 0.20% Sn is an element required to improve local corrosion resistance and overall corrosion resistance, and is an important element participating in lamellar tear resistance; in other words, it can improve corrosion resistance, but On the other hand, an element that deteriorates lamellar tear resistance. I.e., Sn and the like in the bottom of the oil tank localized strongly acidic corrosive environment, the steel surface to form insoluble film, and promotes corrosion inhibiting Cl ^- (chloride ion) of diffusion, thereby, improve the corrosion that is Sexual effect. In addition, in a weakly acidic and fully corrosive environment such as the back of the deck above the crude oil tank, Sn will be incorporated into the rust on the surface of the steel, and inhibit the diffusion of anion species such as SO ₄ ^2- , which promotes corrosion. It has the effect of improving the corrosion resistance. These effects can be exhibited by taking the amount of Sn to 0.01% or more. In addition, especially in a comprehensive corrosive environment such as the back of the upper deck, the effect of adding Sn is great. By adding Sn to 0.05% or more, even if Cu, Ni, Sb, W, Mo, and Si are not added as described below Cu, Ni, Sb, W, and Mo can also exhibit good corrosion resistance. On the other hand, since Sn tends to segregate in the center portion of the steel, and in such a segregated portion, the hardness is remarkably increased, so that the lamellar tear resistance is deteriorated. In particular, when the amount of Sn exceeds 0.20%, the lamellar tear resistance is significantly deteriorated. Therefore, from the viewpoint of ensuring laminar tear resistance, the amount of Sn is 0.20% or less. It is preferably 0.15% or less. It is more preferably 0.10% or less. [0035] N: 0.0080% or less Since N is a harmful element that deteriorates toughness, it is better to reduce it as much as possible. In particular, when the amount of N exceeds 0.0080%, toughness is significantly deteriorated. Therefore, the amount of N is taken to be 0.0080% or less. It is preferably 0.0070%. In addition, the lower limit is not particularly limited, but it is preferably 0.0005%. [0036] One selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Sb: 0.01 to 0.30%, W: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and Si: 0.01 to 1.50% Or two or more kinds of Cu, Ni, Sb, W, Mo, and Si are elements that improve the corrosion resistance of the crude oil tanker's crude oil tank, such as the upper deck of the crude oil tanker, in a comprehensive corrosive environment. As described above, although Sn is an element that can effectively improve the corrosion resistance, it cannot be contained in large amounts from the viewpoint of laminar tear resistance. Therefore, in order to obtain the excellent corrosion resistance of the crude oil tanker's crude oil tanks and other decks in a comprehensive corrosive environment, they need to be selected from the group consisting of Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, and Sb: 0.01 to 0.30. %, W: 0.01 to 0.50%, Mo: 0.01 to 0.50%, and Si: 0.01 to 1.50%. Here, Cu, Ni, and Sb will each be released from the surface of the steel with Cu ²⁺ , Ni ²⁺ and Sb ³⁺ as the corrosion progresses, and combined with the corrosion factor S ^2- to form CuS, NiS, Sb ₂ S ₃ . As a result, penetration of S ^{2 -to} the steel interface can be suppressed. In addition, W, Mo, and Si are freed with WO ₄ ^2- , MoO ₄ ^2-, and SiO ₄ ^4- , respectively, and incorporated into the rust to impart cation selective permeability to the rust, and to electrically suppress SO ₄ ^2- or S ^2- Penetration of corrosive anions into the steel interface. These effects are more significant when the above-mentioned anticorrosive effect of Sn coexists, and the amounts of Cu, Ni, Sb, W, Mo, and Si are 0.01% or more, respectively. However, if any of these elements is contained in a large amount, weldability and toughness are deteriorated, which is disadvantageous from the viewpoint of cost. Therefore, the amount of Cu is in the range of 0.01 to 0.50%, the amount of Ni is in the range of 0.01 to 0.50%, the amount of Sb is in the range of 0.01 to 0.30%, the amount of W is in the range of 0.01 to 0.50%, and the amount of Mo is taken In the range of 0.01 to 0.50%, the amount of Si is in the range of 0.01 to 1.50%. Preferably, the Cu content is 0.02% or more and 0.40% or less, the Ni content is 0.02% or more and 0.40% or less, the Sb content is 0.02% or more and 0.25% or less, the W content is 0.02% or more and 0.40% or less, Mo The amount is 0.02% or more and 0.40% or less, and the Si amount is 0.01% or more and 1.00% or less. [0037] As described above, the lamellar tear resistance deterioration mechanism caused by Sn is different from the lamellar tear resistance deterioration mechanism caused by S. However, the lamellar tear resistance deterioration caused by S and Sn acts synergistically with each other. Therefore, from the viewpoint of further improving the laminar tear resistance, it is desirable to satisfy the relationship of the following formula (2) with respect to the contents of S and Sn: Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the component composition, respectively. [0038] The above formula (2) means that the effect of the amount of Sn on lamellar tear resistance is much greater than the effect of the amount of S on it. That is, it means that it is particularly important to strictly control Sn in ensuring laminar tear resistance. Here, 10000 × [% S] × [% Sn] ^{2 is more} preferably 1.20 or less. The lower limit of 10000 × [% S] × [% Sn] ² is not particularly limited, but is preferably 0.001. In addition, when suppressing lamellar tearing, it should be assumed that both the S amount and the Sn amount are limited to the above range. [0039] Although the basic components have been described above, the steel for a crude oil tanker of the present invention may suitably contain the following elements. One or two kinds of Cr and Co selected from Cr: 0.01 to 0.50% and Co: 0.01 to 0.50% will be transferred to the rust layer as the corrosion progresses, and by blocking Cl ^- intrusion into the rust layer, It can suppress the concentration of Cl ^- at the interface between the rust layer and the base iron, thereby contributing to the improvement of corrosion resistance. In addition, when a Zn-containing primer is applied to the surface of a steel material, Cr and Co form a composite oxide with Zn and the like around Fe, and Zn can persist on the surface of the steel plate over a long period of time, thereby further improving corrosion resistance. Such an effect is particularly remarkable in a portion in contact with a liquid separated from a crude oil oil and containing a high concentration of salt, such as a bottom plate of a crude oil tank of a crude oil tanker. That is, when a Zn-containing primer is applied to a steel material and the steel material is used in a portion in contact with a liquid containing a high concentration of salts separated from crude oil, a steel material containing Cr or Co and a steel material containing no such elements are used. Compared with steel, the corrosion resistance has been greatly improved. Such an effect cannot be sufficiently obtained when the amount of Cr or Co is less than 0.01%. On the other hand, when the amount of Cr or Co exceeds 0.50%, the toughness of the welded portion is deteriorated. In addition, Cr is an element that undergoes a hydrolysis reaction, and lowers the pH of the corroded portion. That is, if Cr is excessively added, the overall corrosion resistance may be deteriorated. Therefore, when it is made to contain Cr and Co, the amounts are each in the range of 0.01 to 0.50%. It is preferably at least 0.02% and at most 0.30%. More preferably, it is 0.03% or more and 0.20% or less. [0040] One or more Ti, Zr, Nb, and V selected from 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%, based on the guarantee The viewpoint of desired strength can be added individually or in combination. However, when any of these elements is contained excessively, toughness and weldability are deteriorated. Therefore, when Ti, Zr, Nb, and V are contained, the amounts are all in the range of 0.001 to 0.100%. It is preferably 0.005% or more and 0.050% or less. [0041] One or more Ca, Mg, and REM selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.0002 to 0.2000%. From the viewpoint of improving the toughness of the welded part, they can be separated. Or added in combination. However, if any of these elements is contained in an excessive amount, the toughness of the welded portion is deteriorated instead. It also increases costs. Therefore, when Ca, Mg, and REM are contained, the amount of Ca is 0.0001 to 0.0100%, the amount of Mg is 0.0001 to 0.0200%, and the amount of REM is 0.0002 to 0.2000%. [0042] B: 0.0001 to 0.0300% B is an element that improves the hardenability of the steel. From the viewpoint of ensuring the required strength, B may be contained. From this viewpoint, it is effective to set the amount of B to 0.0001% or more. However, if B is contained excessively, especially when the amount of B exceeds 0.0300%, toughness will be significantly deteriorated. Therefore, when B is contained, the amount is in the range of 0.0001 to 0.0300%. [0043] Components other than the above are Fe and unavoidable impurities. [0044] The composition of the steel for crude oil tankers of the present invention has been described above, but for the steel for crude oil tankers of the present invention, it is extremely important to control the degree of Sn segregation as follows. Sn segregation degree: less than 18 Due to the central segregation of Sn, the hardness of the segregation portion is greatly increased. In addition, such a segregation portion becomes a starting point for the occurrence of lamellar tearing. That is, in order to ensure excellent lamellar tear resistance in a composition containing Sn, it is important to suppress the center segregation of Sn and increase the hardness of the segregated portion. From this viewpoint, the degree of Sn segregation is determined to be less than 18. It is preferably less than 16. It is more preferably 15 or less. The lower limit is not particularly limited, but is preferably set to 2. [0045] The term “Sn segregation degree” used herein refers to an electron beam microanalyzer (hereinafter referred to as EPMA) on a cross section (a cross section perpendicular to the surface of the steel) cut parallel to the rolling direction of the steel. The ratio of the Sn concentration in the central segregation to the average Sn concentration obtained from the line analysis. Specifically, when the thickness of the steel material is t (mm) and the width (the direction perpendicular to the rolling direction and thickness direction of the steel material) is W (mm), first, cut out parallel to the rolling direction of the steel material. The thickness direction of the steel section (section perpendicular to the steel surface): (0.5 ± 0.1) × t, rolling direction: 15mm surface area (that is, the surface area including the center position of the thickness direction of the steel), with the beam diameter: The EPMA surface analysis of Sn was performed under the conditions of 20 μm and a pitch of 20 μm. In addition, Sn's EPMA surface analysis was performed at three cross-sectional fields of view: 1/4 × W, 1/2 × W, and 3/4 × W. Next, the position of the highest Sn concentration in each of the cross-sectional views was selected from the EPMA surface analysis, and the EPMA line analysis of Sn was performed at the positions along the thickness direction of the steel material under the conditions of beam diameter: 5 μm and pitch: 5 μm. In addition, when the EPMA line analysis was performed, the areas before 25 μm were excluded from the front and back surfaces of the steel. Then, the maximum value of the Sn concentration (mass concentration) is obtained for each measurement line, and the average value is used as the Sn concentration (mass concentration) of the central segregation portion, and the Sn concentration of the central segregation portion is divided by the measurement line. The arithmetic mean of all the measured values, that is, the value obtained by averaging the Sn concentration (mass concentration) was taken as the degree of Sn segregation. That is, [Sn segregation degree] = [Sn concentration in the center segregation part] / [average Sn concentration]. [0046] As described above, from the viewpoint of ensuring excellent lamellar tear resistance, the steel for crude oil tankers of the present invention is extremely important to suppress the segregation of Sn, that is, the segregation of Sn, which indicates the degree of Sn segregation. The degree is controlled below a predetermined value. Here, even if the composition of the components is the same, the degree of Sn segregation varies greatly depending on the manufacturing conditions. Therefore, in order to suppress the center segregation of Sn, it is extremely important to appropriately control the manufacturing method of the steel. Hereinafter, a preferred method for manufacturing a steel material for a crude oil tanker according to the present invention will be described. [0047] That is, the steel material of the present invention can be melted by a known refining process such as a converter or an electric furnace, vacuum degassing, and the like, and the steel can be melted by a continuous casting method or agglomerate-separation method. The block rolling method is used to prepare a steel material (steel blank, slab), and then the steel material is reheated as required, and then hot rolled to produce a steel plate or a shaped steel. The thickness of the steel material is not particularly limited, but is preferably 2 to 100 mm. More preferably, it is 3 to 80 mm. More preferably, it is 4 to 60 mm. Here, when continuous casting is adopted, the casting speed (pulling speed) is preferably 0.3 to 2.8 m / min. If the casting speed is less than 0.3m / min, the work efficiency will be deteriorated. On the other hand, if the casting speed exceeds 2.8 m / min, uneven surface temperature occurs, and molten steel cannot be sufficiently supplied into the flat billet to promote the center segregation of Sn. From the viewpoint of suppressing center segregation of Sn, it is more preferably 0.4 m / min or more and 2.6 m / min or less. More preferably, it is 1.5 m / min or less. In addition, a light-rolling method is preferably performed, in which a flat embryo with an unsolidified layer at the end of solidification is used, while the total amount of rolling and the rolling speed are equivalent to the sum of the solidification shrinkage and the heat shrinkage. Casting was performed while rolling by the rolling roller group slowly. [0048] Next, when the above-mentioned steel material is hot-rolled into a desired size and shape, it is preferably heated to a temperature of 900 ° C to 1350 ° C. The heating temperature is less than 900 ° C, the deformation resistance is large, and hot rolling is not easy. On the other hand, if the heating temperature exceeds 1350 ° C, surface marks may be generated, scale loss, or fuel unit increase. In addition, in particular, the higher the heating temperature, the more the diffusion of Sn in the central segregation portion can be promoted, so it is advantageous from the viewpoint of ensuring laminar tear resistance. From this viewpoint, the heating temperature is more preferably 1030 ° C or higher. The holding time at the heating temperature is preferably 60 minutes or more. This can sufficiently promote the diffusion of Sn in the central segregation portion. More preferably, it is 150 min or more. In addition, the upper limit thereof is not particularly limited, but it is preferably 1000 min. [0049] In addition, when the temperature of the steel material is originally in the range of 1030 to 1350 ° C, and it is maintained at this temperature range for more than 60 minutes, it can be directly supplied to hot rolling without heating. In addition, the hot-rolled sheet obtained after hot rolling may be subjected to reheating treatment, acid, and cold-rolled to form a cold-rolled sheet having a predetermined thickness. In hot rolling, the finishing rolling temperature is preferably 650 ° C or higher. The finishing rolling temperature is less than 650 ° C, which increases the rolling load due to the increase in deformation resistance, and it is difficult to implement rolling. [0050] The cooling after hot rolling may be either gas cooling or accelerated cooling. If higher strength is desired, accelerated cooling is preferred. Here, when performing accelerated cooling, it is preferable to set the cooling rate to 2 to 100 ° C / s and the cooling stop temperature to 700 to 400 ° C. That is, if the cooling rate does not reach 2 ° C / s and / or the cooling stop temperature exceeds 700 ° C, the effect of accelerated cooling is small, and sufficient strength cannot be achieved. On the other hand, when the cooling rate exceeds 100 ° C / s and / or the cooling stop temperature does not reach 400 ° C, the toughness of the steel material is deteriorated or the shape of the steel material is deformed. However, this is not the case when heat treatment is performed in subsequent steps. [Examples] [0051] Steel with the composition shown in Table 1 (the rest being Fe and unavoidable impurities) was melted in a converter, and a steel blank was produced by continuous casting under the conditions shown in Table 2. After heating these steel slabs to 1150 ° C. again, the steel slabs were maintained under the conditions shown in Table 2 and hot-rolled at a finish rolling temperature of 800 ° C. to obtain a steel sheet having a thickness of 40 mm. The cooling system after hot rolling was water-cooled (accelerated cooling) at a cooling rate of 10 ° C / s and a cooling stop temperature: 550 ° C. Then, the degree of Sn segregation in the obtained steel sheet was determined according to the method described above. The results are shown in Table 2. [0052] Furthermore, for the steel plate obtained in the above manner, the comprehensive corrosion test (condensation test) and the local corrosion test (acid resistance test) to simulate the environment of the bottom plate are performed separately for the environment on the back of the upper deck of the crude oil tank of the crude oil tanker. ). (1) Comprehensive corrosion test (condensation test) In order to evaluate the comprehensive corrosion resistance (total corrosion resistance) on the back of the upper deck of the crude oil tank of the crude oil tanker (all-round corrosion resistance), each of the above-mentioned steel plates No. 1 to 58 was measured from the surface. At a position of 1 mm, a rectangular small piece with a width of 25 mm × a length of 60 mm × a thickness of 5 mm was cut out, and the surface was polished with emery paper with an abrasive grain size of 600. Next, a test piece 1 was prepared by sealing with tape to prevent corrosion on the back surface and the end surface, and a full corrosion test was performed using the corrosion test apparatus shown in FIG. 1. This corrosion test device is composed of a corrosion test tank 2 and a temperature control plate 3. Water 6 maintained at a temperature of 30 ° C. was injected into the corrosion test tank 2, and 13 vol% CO ₂ , 4 vol% O ₂ , 0.01 vol% SO ₂ , and 0.05 vol% were introduced into the water 6 through the gas introduction pipe 4. H ₂ S and the rest is a mixed gas composed of N ₂ ; thereby, supersaturated water vapor is filled in the corrosion test tank 2, and the corrosion environment on the back of the deck above the crude oil tank is reproduced. Then, a test piece 1 is mounted on the upper and back surfaces of the corrosion test tank 2 and the test piece 1 is repeatedly given a temperature of 25 ° C × 1.5 hours + 50 ° C × via a temperature control plate 3 with a built-in heater and cooling device. A temperature change of 22.5 hours for one cycle reached 21, 49, 77, and 98 days, which caused condensation water to form on the surface of the test piece 1 and caused comprehensive corrosion. In the first figure, reference numeral 5 denotes an exhaust pipe from a test tank. After the above-mentioned corrosion test, the rust on the surface of each test piece was removed, and the mass reduction due to corrosion was obtained from the mass change before and after the test, and this value was converted into a reduction in plate thickness per year (corrosion rate on one side). Then, the predicted loss amount after 25 years was obtained from the values of the four test periods. If the corrosion amount is 2.0 mm or less, the overall corrosion resistance is rated as good (○); when it exceeds 2.0 mm, the total corrosion resistance is rated. Bad (×). [0053] (2) Local corrosion test (acid resistance test) In order to evaluate the corrosion resistance (local corrosion resistance) of the bottom plate of the crude oil tank of a crude oil tanker in a local corrosion environment (pitting corrosion), A rectangular piece with a width of 25 mm × length of 60 mm × thickness of 5 mm was cut out from a position of 1 mm on the surface of the steel plate, and the surface was polished with emery paper having an abrasive grain size of 600 to prepare a test piece. Next, a 10% by mass NaCl aqueous solution was prepared using a concentrated hydrochloric acid to prepare a test solution having a Cl ion concentration of 10% by mass and a pH of 0.85, and the fishing line was suspended through a 3 mmf hole formed in the upper part of the test piece. The test piece was subjected to a corrosion test immersed in 2 L of a test solution for 168 hours. In addition, the test solution was heated in advance and kept at 30 ° C, and the test solution was replaced with a new one every 24 hours. FIG. 2 shows an apparatus used in the corrosion test. This corrosion test device is a dual structure device of a corrosion test tank 8 and a constant temperature tank 9. The corrosion test tank 8 is filled with the above-mentioned test solution 10, and the test piece 7 is suspended by a fishing line 11 and immersed therein. The temperature of the test solution 10 is maintained by adjusting the temperature of the water 12 charged in the thermostatic bath 9. After the above-mentioned corrosion test, the rust generated on the surface of the test piece was removed, and the difference in mass before and after the test was obtained, and the difference was divided by the total surface area to determine the reduction in plate thickness per year (corrosion rate on both sides). As a result, the local corrosion resistance was rated as good (○) when the corrosion rate was 1.0 mm / y or less, and the local corrosion resistance was rated as poor (×) when the corrosion rate was over 1.0 mm / y. [0054] Further, the evaluation of laminar tear resistance was performed in the following manner. (3) The evaluation of laminar tear resistance is based on the ClassNK steel ship rules and the inspection methods (Chapter 2 of Chapter K). The steel plate thickness direction (Z direction) is applied to the steel plates No. 1 to 58 obtained in the above manner. ), And calculate the reduction of area (RA). Then, based on the calculated shrinkage ratio (RA), laminar tear resistance was evaluated according to the following criteria. ◎ (Passed, Excellent): 70 or more ○ (Passed): 35 or more and less than 70 △ (Failed): 25 or more and less than 35 × (Failed): Less than 25 [0055] Will (1) ~ (3) The evaluation results are recorded in Table 2. In addition, the comprehensive evaluation in Table 2 is rated as "Pass" when all the evaluations (1) to (3) above are "○" or "◎", and any one of the evaluations (1) to (3) has "△" or "×" is rated as "Failed". [0056] [0057] [0058] As shown in Table 2, the examples of the invention both have excellent general corrosion resistance and local corrosion resistance, and also have excellent lamellar tear resistance. On the other hand, in the comparative example, sufficient characteristics were not obtained with respect to at least one of general corrosion resistance, local corrosion resistance, and lamellar tear resistance. [0059] That is, in Comparative Examples Nos. 42, 48, and 52, since the amount of S exceeded the upper limit, local corrosion resistance and lamellar tear resistance were not obtained, and sufficient characteristics were not obtained. In Comparative Examples Nos. 43, 47, and 50, since the amount of Sn exceeded the upper limit, lamellar tear resistance was not obtained, and sufficient characteristics were not obtained. Comparative Example No. 44 failed to obtain general corrosion resistance, local corrosion resistance, and lamellar tear resistance because the amount of S exceeded the upper limit and did not contain a predetermined amount of Cu, Ni, Sb, W, Mo, and Si. Full characteristics. In Comparative Example No. 45, since the amount of Sn was lower than the lower limit, it had insufficient general corrosion resistance and local corrosion resistance, and sufficient characteristics could not be obtained. In Comparative Example No. 46, since the amount of S and the amount of Sn exceeded the upper limits, local corrosion resistance and lamellar tear resistance were not obtained, and sufficient characteristics were not obtained. Comparative Example No. 49 did not contain a predetermined amount of Cu, Ni, Sb, W, Mo, and Si, so it had full corrosion resistance and failed to obtain sufficient characteristics. In Comparative Example No. 51, since the amount of S exceeds the upper limit and the amount of Sn is lower than the lower limit, it has insufficient general corrosion resistance, local corrosion resistance, and lamellar tear resistance, and sufficient characteristics cannot be obtained. In Comparative Examples Nos. 53 to 56, since the degree of Sn segregation exceeded the upper limit, lamellar tear resistance was not obtained, and sufficient characteristics were not obtained.

[0060]

1,7‧‧‧test strips

2,8‧‧‧Corrosion test tank

3‧‧‧ temperature control board

4‧‧‧Gas introduction pipe

5‧‧‧ exhaust pipe

6, 12‧‧‧ water

9‧‧‧ constant temperature bath

10‧‧‧Test solution

11‧‧‧ fishing line

[0027] FIG. 1 is a schematic diagram of a test device used for a comprehensive corrosion test (condensation test). Figure 2 is a schematic diagram of a test apparatus used for a local corrosion test (acid resistance test).

Claims (7)

A steel material for a crude oil tanker, comprising: C: 0.03 to 0.18% in mass%, 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 ~ 0.20% and N: 0.0080% or less, and simultaneously selected from Cu: 0.01 ~ 0.50%, Ni: 0.01 ~ 0.50%, Sb: 0.01 ~ 0.30%, W: 0.01 ~ 0.50%, Mo: 0.01 ~ 0.50 % And Si: 0.01 ~ 1.50% 1 or more of, and the rest are composed of Fe and unavoidable impurities, and the degree of Sn segregation is less than 18; 此 Here, the degree of Sn segregation is as follows Definition of formula (1): [Sn segregation degree] = [Sn concentration in central segregation part] / [average Sn concentration] --- (1). For example, the steel for a crude oil tanker according to claim 1, wherein the S content and the Sn content in the aforementioned component composition satisfy the relationship of the following formula (2): Here, [% S] and [% Sn] are the contents (mass%) of S and Sn in the component composition, respectively. For example, the steel for a crude oil tanker according to claim 1 or 2, wherein the aforementioned composition further contains one or two selected from the group consisting of Cr: 0.01 to 0.50% and Co: 0.01 to 0.50% in mass%. The steel for crude oil tankers according to any one of claims 1 to 3, wherein the aforementioned composition further contains a mass selected from Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, Nb: 0.001 to 0.100%, and V: 0.001 ~ 0.100% One or more of them. The steel for crude oil tankers according to any one of claims 1 to 4, wherein the aforementioned composition further contains a member selected from the group consisting of Ca: 0.0001 to 0.0100% by mass%, Mg: 0.0001 to 0.0200%, and REM: 0.0002 to 0.2000%. 1 or more of them. The steel for crude oil tankers according to any one of claims 1 to 5, wherein the aforementioned component composition further contains B: 0.0001 to 0.0300% by mass%. A crude oil tanker made of the steel material for a crude oil tanker according to any one of claims 1 to 6.