TWI654316B - Steel and crude oil tankers for crude oil tankers - Google Patents

Abstract

The present invention provides a crude oil tanker which can be combined with a predetermined composition and a degree of Sn segregation of less than 18, and which is excellent in overall corrosion resistance, local corrosion resistance, and excellent laminar tear resistance. Use steel.

Description

Steel and crude oil tankers for crude oil tankers

[0001] The present invention relates to a crude oil tank (oil groove portion) of a crude oil tanker formed by welding steel materials, particularly an upper deck back (top) or upper side wall of the upper deck which is generally corroded, and local corrosion (pore erosion) occurs. Any of the base plates of the oil groove portion can be applied to steel materials for crude oil tankers excellent in corrosion resistance and layer tear resistance. Further, the present invention relates to a crude oil tanker using the above steel material.

[0002] It is known that the inner surface of a crude oil tank of a crude oil tanker, especially the steel used on the back side of the upper deck and the upper part of the side wall, will cause overall corrosion. [0003] The causes of this general corrosion include: (1) condensation and drying (dry and wet) on the surface of the steel sheet due to temperature difference between day and night; (2) inert gas sealed in the crude oil tank for explosion-proof (O ₂ : about 4 vol%, CO ₂ : about 13 vol%, SO ₂ : about 0.01 vol%, and the rest is O ₂ , CO ₂ , SO ₂ contained in a boiler or an engine exhaust gas represented by N ₂ ) (3) a corrosive gas such as H ₂ S volatilized from crude oil is dissolved in condensed water; (4) used for washing seawater in a crude oil tank. In such cases, it is common to detect sulfate or chloride ions in strongly acidic condensate water during routine shipyard inspections of physical ships conducted every two and a half years. Further, when H ₂ S is oxidized by the rust generated by the corrosion as a catalyst, the solid S is formed in a layer form in the rust. The rust which is formed in such a layer by the solid S is easily peeled off and deposited on the bottom of the crude oil tank. Therefore, in the case of a routine dock inspection of a crude oil tanker, it is costly. The current situation is that each time the repair of the upper part of the crude oil tank or the recovery of the deposit at the bottom of the tank is carried out. [0005] On the other hand, in the case of a steel material such as a crude oil tank bottom plate, corrosion suppression of a protective coating (oil coating) derived from crude oil itself or a crude oil formed on the inner surface of a crude oil tank is generally considered. Function without causing corrosion. However, according to recent research, the steel of the bottom plate of the crude oil tank produces localized corrosion (pore erosion) of the bowl type. [0006] The reason for causing such local corrosion is as follows: (1) there is a condensed water in which a salt represented by sodium chloride is dissolved at a high concentration; (2) an oil coating is caused by excessive washing. The layer is detached; (3) the sulfide contained in the crude oil is at a high concentration; (4) the high concentration of O ₂ , CO ₂ , SO _{2 ,} etc. in the inert gas for explosion prevention dissolved in the condensed water. In fact, at the time of the routine dock inspection of the physical ship, the analysis of the water retained in the crude oil tank detected a high concentration of chloride ions and sulfate ions. [0007] In order to prevent overall corrosion or local corrosion of the inner surface of the above crude oil tank, it is effective to apply the coating on the surface of the steel material to isolate the steel material from the corrosive environment. However, the coating operation of the crude oil tank is very large, and it needs to be recoated once every 10 years due to deterioration of the coating film. Therefore, the inspection and painting of the crude oil tank requires a large cost for inspection or painting. Furthermore, it has been pointed out that once the coating film is damaged, the damaged portion may contribute to corrosion in the corrosive environment of the crude oil tank. [0008] Therefore, it has been desired to develop a steel material that can prevent overall corrosion or local corrosion of the inner surface of the crude oil tank without performing coating. [0009] As such a steel material, for example, Patent Document 1 discloses: "A steel material for cargo oil tanks containing 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 to 3%, Mo: 1% or less, Cu: 1% or less, Cr: 2% or less, W: 1% or less, and 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 balance is composed of Fe and impurities. [0010] Further, Patent Document 2 discloses: "A steel material for cargo oil tanks containing C: 0.01 to 0.2% by mass, Si: 0.01 to 1%, Mn: 0.05 to 2%, P: 0.05% or less, S: 0.01% or less, Ni: 0.01 to 1%, Cu: 0.05 to 2%, Sn: 0.01 to 0.2%, Cr: 0.1% or less, Al: 0.1% or less, and the balance of Fe and impurities Composition.". [Prior Art Document] [Patent Document 1] [Patent Document 1] JP-A-2003-82435 (Patent Document 2) JP-A-2007-270196

[Problems to be Solved by the Invention] [0012] In addition, the crude oil tank of the crude oil tanker usually welds the bottom plate and the funnel feed plate, the upper deck back plate and the longitudinal beam, etc., and at the welded joint portion, it is subjected to tensile stress along the thickness direction. . At such welded joints, the risk of lamellar tearing has recently been elucidated. Here, the term "layered tear" refers to a welded joint that is subjected to tensile stress in a thickness direction in a cross-shaped joint portion, a T-shaped joint portion, a square joint portion, etc., and is parallel to the steel sheet surface due to tensile stress. In the direction of the steel, the crack is deepened inside the steel to cause cracking. Therefore, in addition to the corrosion resistance of the overall corrosion or local corrosion of the inner surface of the above-mentioned antigen oil tank, the steel material for the crude oil tanker is also required to have excellent laminar tear resistance. [0013] In this regard, the steel of the literature 1 is not considered to have any mechanical properties such as resistance to lamellar tearing. Moreover, the steel of the literature 2 has not been considered to be any one which is resistant to lamellar tearing. [0014] Thus, citing documents 1 and 2, the risk of laminar tearing at the welded joint portion is not considered at all, and therefore, when the steel materials cited in Documents 1 and 2 are used in the crude oil tank of the actual crude oil tanker, There is a delamination of the layered tear at the welded joint. [0015] The present invention has been made in view of the above-mentioned state of the art, and is intended to provide an antigenic oil tanker in the inner surface of the crude oil tank, the upper surface of the deck or the upper part of the side wall, and the corrosion resistance of the local corrosion of the bottom plate is excellent, and the layer is resistant. For the purpose of steel for crude oil tankers with excellent tearing properties. Moreover, the present invention has an object of providing a crude oil tanker using the above-described steel material for a crude oil tanker. [Means for Solving the Problem] [0016] Accordingly, the inventors of the present invention have made efforts to study the above problems in order to solve the above problems, and have obtained the following findings: (1) To raise the bottom plate of the crude oil tanker in a local corrosive environment, that is, in a pitting environment The corrosion resistance (hereinafter also referred to as local corrosion resistance) is effective in adding Sn and reducing S. (2) To improve the corrosion resistance of the back of the deck or the upper part of the upper side of the crude oil tanker in the overall corrosive environment (hereinafter also referred to as general corrosion resistance), and to add Cu, Ni, Sb, W together with Sn. One or more of Mo and Si are effective. (3) On the other hand, based on the viewpoint of resistance to lamellar tearing, it is effective to reduce the amount of S in the steel while reducing the Sn system. [0017] Thus, based on the corrosion resistance (corrosion resistance and local corrosion resistance) of the inner surface of the crude oil tank of the crude oil tanker in the corrosive environment, the addition of the Sn system is effective, but based on the layer tear resistance. From the point of view, reducing Sn is effective. Therefore, the inventors of the present invention have further studied in order to have both corrosion resistance and lamellar tear resistance based on the above findings. [0018] As a result, the following findings are obtained: (4) As long as the center segregation of Sn is suppressed and Sn is strongly diffused toward the entire steel material, excellent lamellar tear resistance can be obtained even if a predetermined amount of Sn is contained; that is, as long as When the amount of Sn is appropriately adjusted and the center segregation of Sn is suppressed, and Sn is diffused toward the entire steel material, the corrosion resistance and the lamellar tear resistance of the inner surface of the crude oil tank of the crude oil tanker in a corrosive environment can be achieved. Further, the following findings were obtained: (5) The amount of Sn is strictly controlled in accordance with the amount of S, and the layer tear resistance can be further improved. The present invention has been completed based on the above findings and further research. [0019] That is, the gist of the present invention is as follows. A steel material for a crude oil tanker comprising C: 0.03 to 0.18% by mass, Mn: 0.10 to 2.00%, P: 0.030% or less, S: 0.0070% or less, and Al: 0.001 to 0.100%. Sn: 0.01 to 0.20% and N: 0.0080% or less, and contain 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. ~0.50% and Si: 0.01~1.50% of one or more of them, and the rest consists of Fe and inevitable impurities, and the Sn segregation degree is less than 18; here, the Sn segregation system It is defined by the following formula (1): [Sn segregation degree] = [Sn concentration of the center segregation part] / [Average Sn concentration] - (1). [0020] 2. The steel material for a crude oil tanker according to the above 1, wherein the S content and the Sn content in the composition of the composition satisfy the relationship of the following formula (2): Here, [%S] and [%Sn] are the contents (% by mass) of S and Sn in the component composition, respectively. 3. The steel material for a crude oil tanker according to the above 1 or 2, wherein the component composition further contains one or two selected from the group consisting of Cr: 0.01% to 0.50% by mass and 0.01% to 0.50% by weight. 4. The steel material for a crude oil tanker according to any one of the above 1 to 3, wherein the component composition further contains, by mass%, Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, and Nb: 0.001. ~0.100% and V: 0.001~0.100% One or more of them. [0023] The steel material for a crude oil tanker according to any one of the above 1 to 4, wherein the component composition further contains, by mass%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0200%, and REM: 0.0002 ~0.2000% of one or more. [0024] The steel material for a crude oil tanker according to any one of the above 1 to 5, wherein the component composition further contains B: 0.0001 to 0.0300% by mass%. [0025] A crude oil tanker obtained by using the steel material for a crude oil tanker according to any one of the above 1 to 6. [Effects of the Invention] According to the present invention, it is possible to obtain corrosion resistance of a crude oil tank inner surface of a crude oil tanker in a corrosive environment, that is, excellent in overall corrosion resistance and local corrosion resistance, and resistance to lamellar tearing Also excellent in steel for crude oil tankers. Moreover, by using the steel material for crude oil tanker of the present invention for the crude oil tank of the crude oil tanker, high safety can be ensured, and 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 above range in the present invention will be described. In addition, the unit of the content of the element in the composition of the steel is "% by mass", and the following is only expressed by "%" unless otherwise specified. [0029] C: 0.03 to 0.18% C is an element required to secure the strength of the steel. In order to obtain such an effect, the amount of C is 0.03% or more. However, if the amount of C exceeds 0.18%, the weldability and the toughness of the welded heat affected zone may deteriorate. 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 for increasing the strength of steel. In order to obtain such an effect, the amount of Mn is 0.10% or more. However, if the amount of Mn exceeds 2.00%, the toughness and weldability of the steel may deteriorate. Further, the layer-like tear property is also deteriorated due to segregation at the center of Mn. Therefore, the amount of Mn 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. P: 0.030% or less P deteriorates toughness and weldability. Therefore, the amount of P is taken to be 0.030% or less. It is preferably 0.025% or less. More preferably, it is 0.015% or less. Further, the lower limit thereof is not particularly limited, and is preferably 0.003%. [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 which forms MnS, which is a non-metallic inclusion and becomes a starting point of local corrosion and deteriorates local corrosion resistance. Therefore, the S system is preferably reduced as much as possible. In particular, when the amount of S exceeds 0.0080%, the local corrosion resistance is remarkably deteriorated. Also, large volumes of MnS can be the starting point for lamellar tears. In particular, when the amount of S exceeds 0.0070%, the layer tear resistance is largely deteriorated. Therefore, the S amount is 0.0070% or less from the viewpoint of both local corrosion resistance and lamellar tear resistance. It is preferably 0.0060% or less. More preferably, it is 0.0050% or less. Further, the lower limit thereof is not particularly limited, and is preferably 0.0003%. [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 amount of Al exceeds 0.10%, the toughness of the steel deteriorates. Therefore, the amount of Al is in the range of 0.001 to 0.10%. [0034] Sn: 0.01 to 0.20% Sn is an element required to enhance local corrosion resistance and general corrosion resistance, and is an important element involved in the resistance to lamellar tearing; in other words, it can improve corrosion resistance, but On the other hand, an element which deteriorates the lamellar tearing property. That is, Sn forms a poorly soluble film on the surface of the steel in a highly acidic localized corrosive environment such as the bottom of the crude oil tank, and suppresses the diffusion of Cl ^- (chloride ions) which promotes corrosion, thereby improving corrosion resistance. Sexual effect. In addition, Sn is incorporated into the rust on the surface of the steel in a weakly acidic general corrosive environment such as the back of the deck above the crude oil tank, thereby suppressing the diffusion of anionic species such as SO ₄ ^{2- which} promotes corrosion, thereby It has the effect of improving corrosion resistance. These effects can be exhibited by taking the amount of Sn by 0.01% or more. Further, in particular, in the general corrosion environment such as the back of the upper deck, the effect of adding Sn is large, and the amount of Sn is 0.05% or more, even if Cu, Ni, Sb, W, Mo, and Si described later are not added. Cu, Ni, Sb, W, and Mo also exhibit good corrosion resistance. On the other hand, since Sn tends to be segregated in the center portion of the steel material, the hardness is remarkably increased in such a segregation portion, so that the lamellar tear resistance is deteriorated. In particular, when the amount of Sn exceeds 0.20%, the layer tear resistance is greatly deteriorated. Therefore, the amount of Sn is 0.20% or less from the viewpoint of ensuring the resistance to lamellar tearing. It is preferably 0.15% or less. More preferably, it is 0.10% or less. N: 0.0080% or less N is a harmful element which deteriorates toughness, and therefore it is preferable to reduce it as much as possible. In particular, when the amount of N exceeds 0.0080%, the toughness is largely deteriorated. Therefore, the amount of N is taken to be 0.0080% or less. It is preferably 0.0070%. Further, the lower limit thereof is not particularly limited, and is preferably 0.0005%. [0036] one selected from the group consisting of 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 types of Cu, Ni, Sb, W, Mo, and Si are elements which improve the corrosion resistance of the upper deck of the crude oil tanker, such as the upper deck of the crude oil tank, in a general corrosive environment. For example, although Sn is an element which can effectively improve corrosion resistance, it cannot be contained in a large amount based on the viewpoint of resistance to lamellar tearing. Therefore, in order to obtain excellent corrosion resistance of the upper deck of the crude oil tanker in a general corrosive environment, it is required to contain a selected from 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%, one or more. Here, each of Cu, Ni, and Sb is freed from Cu ²⁺ , Ni ^{2+ ,} and Sb ³⁺ from the surface of the steel as the corrosion proceeds, and combines with the S ^{2 of the} corrosion factor to form CuS, NiS, and Sb ₂ S. ₃ . As a result, penetration of the S ^2- to steel interface can be suppressed. In addition, W, Mo and Si are each freed from WO ₄ ^2- , MoO ₄ ^2- and SiO ₄ ^4- , incorporated into rust, imparting cation selective permeability to rust, and electrically inhibiting SO ₄ ^2- or Penetration of S2 ^- corrosive anions to the steel interface. These effects are more remarkable when the anti-etching action of Sn described above coexists, and the amounts of Cu, Ni, Sb, W, Mo, and Si are respectively 0.01% or more. However, when any of the elements is contained in a large amount, the weldability or toughness is 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%, and 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 amount of Cu is 0.02% or more and 0.40% or less, the amount of Ni is 0.02% or more and 0.40% or less, the amount of Sb is 0.02% or more and 0.25% or less, and the amount of W is 0.02% or more and 0.40% or less. The amount 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. Further, as described above, the deterioration mechanism for the layer tear resistance caused by Sn is different from the deterioration mechanism for the layer tear resistance caused by S. However, the deterioration of the layer tear resistance caused by S and Sn acts synergistically with each other. Therefore, based on the viewpoint of further improving the resistance to lamellar tearing, the content of S and Sn should preferably satisfy the relationship of the following formula (2): Here, [%S] and [%Sn] are the contents (% by mass) of S and Sn in the component composition, respectively. [0038] The above formula (2) means that the influence of the amount of Sn on the lamellar tear resistance is much larger than the influence of the S amount. That is, it means that it is particularly important to strictly control Sn in ensuring the resistance to lamellar tearing. Here, 10000 × [% S] × [% Sn] ^{2 is more} preferably set to 1.20 or less. The lower limit of 10000 × [% S] × [% Sn ² ] is not particularly limited, and is preferably 0.001. Further, in order to suppress the lamellar tear, it is reasonable to assume that both the amount of S and the amount of Sn are limited to the above range. In the above, the basic component has been described. However, the steel material 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 the group consisting of Cr: 0.01 to 0.50% and Co: 0.01 to 0.50% are transferred to the rust layer as corrosion proceeds, thereby blocking the intrusion of Cl ^- into the rust layer. The concentration of Cl ^- at the interface between the rust layer and the base iron can be suppressed, thereby contributing to an improvement in corrosion resistance. Further, when the Zn-containing primer is applied to the surface of the steel material, Cr and Co form a composite oxide with Zn or the like around Fe, and Zn can be continuously deposited on the surface of the steel sheet for a long period of time, whereby the corrosion resistance can be further improved. Such an effect, particularly such as a bottom plate of a crude oil tank of a crude oil tanker, is particularly remarkable in a portion in contact with a liquid containing a high concentration of salt separated from the crude oil. That is, when the steel is subjected to a Zn-containing primer treatment and the steel is used in a portion that is in contact with a liquid containing a high concentration of salt separated from the crude oil, a steel containing Cr or Co, and a material containing no such element Corrosion resistance has been greatly improved compared to steel. Such an effect cannot be sufficiently obtained when the amount of Cr or the amount of Co is less than 0.01%. On the other hand, when the amount of Cr or the amount of Co exceeds 0.50%, the toughness of the welded portion is deteriorated. Further, in the case of Cr, an element which undergoes a hydrolysis reaction lowers the pH of the corrosion portion. That is, when Cr is excessively added, there is a possibility that the overall corrosion resistance is deteriorated. Therefore, when Cr and Co are contained, the amount thereof is in the range of 0.01 to 0.50%. It is preferably 0.02% or more and 0.30% or less. More preferably, it is 0.03% or more and 0.20% or less. [0040] One or more of Ti, Zr, Nb, and V selected from the group consisting of 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 ensuring The viewpoint of the desired strength can be added individually or in combination. However, when any of the elements is excessively contained, 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%. It is preferably 0.005% or more and 0.050% or less. One or more of 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%, and may be separately based on the viewpoint of toughness of the welded portion. Or added in combination. However, when any one of the elements is excessively contained, the toughness of the welded portion is deteriorated instead. Also, it will increase 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 which improves the hardenability of the steel material. Further, it is possible to contain B based on the viewpoint of ensuring the required strength. From such a viewpoint, it is effective to take B amount of 0.0001% or more. However, if B is excessively contained, especially if the amount of B exceeds 0.0300%, the toughness is largely deteriorated. Therefore, when B is contained, the amount thereof is in the range of 0.0001 to 0.0300%. [0043] The components other than the above are Fe and inevitable impurities. In the above, the component composition of the steel material for a crude oil tanker of the present invention has been described. However, it is extremely important to control the Sn segregation degree system as the steel material for a crude oil tanker of the present invention as follows. Sn segregation degree: less than 18 Due to the center segregation of Sn, the hardness of the segregation portion is greatly increased. Moreover, such a segregation portion becomes a starting point for the occurrence of a layered tear. In other words, in order to secure excellent lamellar tear resistance in the composition of Sn, it is important to suppress center segregation of Sn and suppress an increase in hardness of the segregation portion. From this point of view, the degree of Sn segregation is determined to be less than 18. It is preferably less than 16. More preferably 15 or less. The lower limit is not particularly limited, and is preferably set to 2. [0045] Further, the term "Sn segregation degree" as used herein refers to a cross section (a cross section perpendicular to the surface of the steel material) which is cut in parallel with the rolling direction of the steel material, and is represented by an electron beam microanalyzer (hereinafter referred to as EPMA). The ratio of the Sn concentration to the average Sn concentration in the center segregation portion obtained by 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 the thickness direction of the steel material) is W (mm), first, it is cut in parallel with the rolling direction of the steel material. Thickness direction of the steel material of the cross section (section perpendicular to the steel surface): (0.5 ± 0.1) × t, rolling direction: 15 mm surface area (ie, the surface area including the center position of the steel in the thickness direction), the beam diameter: The EPMA surface analysis of Sn was carried out under conditions of 20 μm and pitch: 20 μm. Further, the EPMA surface analysis of Sn was carried out in the three cross-sectional fields at positions of 1/4×W, 1/2×W, and 3/4×W. Next, the position where the Sn concentration was the highest in the cross-sectional field of view was selected by the EPMA surface analysis, and EPMA line analysis of Sn was performed at the position in the thickness direction of the steel material at a beam diameter of 5 μm and a pitch of 5 μm. In addition, in the EPMA line analysis, the area before the 25 μm was excluded from the front and back sides 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 center segregation portion, and the Sn concentration of the center segregation portion is divided by the measurement line. The arithmetic mean value of all the measured values, that is, the value obtained by the average Sn concentration (mass concentration) is taken as the Sn segregation degree. That is, [Sn segregation degree] = [Sn concentration of the center segregation portion] / [Average Sn concentration]. [0046] As described above, the steel material for a crude oil tanker of the present invention is extremely important in suppressing the center segregation of Sn, that is, the Sn segregation indicating the degree of center segregation of Sn, from the viewpoint of ensuring excellent lamellar tear resistance. The degree is controlled below the established value. Here, even if the composition of the components is the same, the degree of Sn segregation greatly changes depending on the manufacturing conditions. Therefore, in order to suppress the center segregation of Sn, it is extremely important to appropriately control the method of manufacturing the steel. Hereinafter, a preferred method for producing a steel material for a crude oil tanker of 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, or by continuous casting or agglomeration. The block rolling method is used to form a steel material (steel blank, slab), and then the steel material is further heated as required, and then hot rolled to obtain a steel sheet or a steel or the like. Further, the thickness of the steel material is not particularly limited, but is preferably 2 to 100 mm. More preferably 3~80mm. More preferably, it is 4~60mm. Here, in the case of continuous casting, the casting speed (pull speed) is preferably from 0.3 to 2.8 m/min. The casting speed is less than 0.3 m/min, and the work efficiency is deteriorated. On the other hand, when the casting speed exceeds 2.8 m/min, surface temperature unevenness occurs, and molten steel cannot be sufficiently supplied to the inside of the flat embryo to promote 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. Further, it is preferable to carry out a light rolling method in which a flat embryo having an unsolidified layer at the end of solidification is used, and the total amount of rolling and the rolling speed corresponding to the sum of the amount of solidification shrinkage and the amount of heat shrinkage are borrowed. Casting is carried out while the rolling roll group is slowly rolling. [0048] Next, when the steel material described above is hot rolled into a desired size, it is preferably heated to a temperature of from 900 ° C to 1350 ° C. When the heating temperature is less than 900 ° C, the deformation resistance is large and it is difficult to perform hot rolling. On the other hand, if the heating temperature exceeds 1350 ° C, surface marks may be generated or the scale loss or the fuel original unit may be increased. Further, in particular, the higher the heating temperature, the more the diffusion of Sn in the center segregation portion is promoted, and therefore it is advantageous from the viewpoint of ensuring the resistance to lamellar tearing. From such a viewpoint, the heating temperature is more preferably 1030 ° C or more. Further, the holding time at the above heating temperature is preferably 60 minutes or more. Thereby, the diffusion of Sn in the center segregation portion can be sufficiently promoted. More preferably 150min or more. Further, the upper limit thereof is not particularly limited, and it is preferably 1000 minutes. [0049] Further, when the temperature of the steel material is originally in the range of 1030 to 1350 ° C, and is maintained at this temperature range of 60 min or more, hot rolling can be directly supplied without heating. Further, the hot-rolled sheet obtained after the hot rolling may be subjected to reheating treatment, acidification, and cold rolling to obtain a cold-rolled sheet having a predetermined thickness. In hot rolling, the finish rolling temperature is preferably 650 ° C or higher. When the finishing rolling temperature is less than 650 ° C, the rolling load increases due to an increase in deformation resistance, and rolling is not easy. [0050] The cooling after hot rolling may be any method of gas cooling and accelerated cooling, and in order to obtain higher strength, it is preferred to perform accelerated cooling. Here, in the case of performing accelerated cooling, it is preferred to set the cooling rate to 2 to 100 ° C/s and the cooling stop temperature to 700 to 400 ° C. That is, when the cooling rate is less than 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 may deteriorate or the shape of the steel material may be deformed. However, this is not the case when heat treatment is carried out in the subsequent steps. [Examples] Steel having the composition shown in Table 1 (the balance being Fe and inevitable impurities) was melted in a converter, and a steel preform was produced by continuous casting under the conditions shown in Table 2. These steel blanks were again heated to 1,150 ° C, and then held under the conditions shown in Table 2, and further subjected to hot rolling at a finish rolling temperature of 800 ° C to obtain a steel sheet having a thickness of 40 mm. Further, the cooling after the hot rolling was performed by cooling with a cooling rate of 10 ° C/s and a cooling stop temperature of 550 ° C (accelerated cooling). Then, according to the above method, the degree of segregation of Sn in the obtained steel sheet was obtained. The results are also recorded in Table 2. [0052] Furthermore, for the steel sheets obtained as described above, the overall corrosion test (condensation test) of the back surface environment of the crude oil tank on the crude oil tanker and the local corrosion test of the simulated bottom plate environment (acid resistance test) are respectively carried out according to the following essentials. ). (1) General Corrosion Test (Condensation Test) To evaluate the corrosion resistance (total corrosion resistance) of the back of the deck above the crude oil tank of the crude oil tanker, and to the surface of the above No. 1~58 steel sheet, from the surface At a position of 1 mm, a rectangular piece having a width of 25 mm, a length of 60 mm, and a thickness of 5 mm was cut out, and the surface was ground with a diamond grain of abrasive grain size of 600. Next, the test piece 1 was prepared by sealing with a tape to prevent corrosion of the back surface and the end surface, and the overall corrosion test was performed using the corrosion test apparatus shown in Fig. 1. This corrosion test apparatus is composed of a corrosion test tank 2 and a temperature control plate 3. In the corrosion test tank 2, water 6 having a temperature maintained at 30 ° C is injected, and in the water 6 , 13 vol% CO ₂ , 4 vol% O ₂ , 0.01 vol% SO ₂ , 0.05 vol% are introduced through the gas introduction pipe 4 . H ₂ S and the remainder are a mixed gas of N ₂ ; thereby, supersaturated water vapor is filled in the corrosion test tank 2 to reproduce the corrosive environment on the back of the deck above the crude oil tank. Then, the test piece 1 is attached to the upper back surface of the corrosion test tank 2, and the test piece 1 is repeatedly applied with a temperature control plate 3 having a heater and a cooling device therein at 25 ° C × 1.5 hours + 50 ° C × The temperature change of 1 cycle for 1 cycle was 21, 49, 77, and 98 days, and condensed water was formed on the surface of the test piece 1, resulting in overall corrosion. In addition, in Fig. 1, reference numeral 5 denotes an exhaust pipe from the test tank. After the above-mentioned corrosion test, the rust on the surface of each test piece was removed, and the mass loss caused by the corrosion was determined from the change in mass before and after the test, and the value was converted into the amount of reduction in thickness per one year (corrosion rate on one side). Then, the predicted loss amount after 25 years was obtained from the values of the four test periods, and the general corrosion resistance was evaluated as good (○) when the corrosion amount was 2.0 mm or less, and the general corrosion resistance was evaluated when the corrosion amount was more than 2.0 mm. Bad (×). (2) Local Corrosion Test (Acid Resistance Test) To evaluate the corrosion resistance (corrosion resistance) of the bottom plate of the crude oil tank of the crude oil tanker under the local corrosive environment (pitting corrosion), and to the above No. 1 to 58 For the steel sheets, rectangular pieces having a width of 25 mm, a length of 60 mm, and a thickness of 5 mm were cut out from the surface at a position of 1 mm, and the surface was polished with a diamond grain of abrasive grain size of 600 to prepare a test piece. Then, a test solution having a Cl ion concentration of 10% by mass and a pH of 0.85 prepared by using a 10% by mass aqueous NaCl solution was prepared by using concentrated hydrochloric acid, and the fishing line was suspended by a hole of 3 mmf which was passed through the upper portion of the test piece. The test piece was subjected to a corrosion test immersed in a 2 L test solution for 168 hours. Further, the test solution was previously heated and kept at 30 ° C, and a new test solution was replaced every 24 hours. Fig. 2 shows the apparatus used in the above corrosion test. The corrosion test apparatus is a device having a double structure of the corrosion test tank 8 and the constant temperature bath 9, and the test solution 10 is placed in the corrosion test tank 8, and the test piece 7 is suspended by the 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 constant temperature bath 9. After the above 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 in the total surface area was divided by the difference to obtain the amount of reduction in thickness per one year (corrosion rate on both sides). As a result, when the corrosion rate was 1.0 mm/y or less, the local corrosion resistance was evaluated as good (○); when the corrosion rate exceeded 1.0 mm/y, the local corrosion resistance was evaluated as poor (×). Further, the evaluation of the layer tear resistance is performed in the following manner. (3) Evaluation of the resistance to lamellar tearing According to the rules of the ClassNK steel ship and the inspection method (Chapter 2 of K), the plate thickness direction of the steel plate of No. 1 to 58 obtained as described above (Z direction) The tensile test was performed, and the reduction ratio (RA, Reduction of Area) was calculated. Then, based on the calculated shrinkage ratio (RA), the layer tear resistance was evaluated on the basis of the following criteria. ◎ (qualified, excellent): 70 or more ○ (qualified): 35 or more and less than 70 △ (failed): 25 or more and less than 35 × (failed): less than 25 [0055] (1) ~ The evaluation results of (3) are also shown in Table 2. In addition, the comprehensive assessment in Table 2 is rated as "qualified" when all the evaluations of (1) to (3) above are "○" or "◎", and any one of the evaluations of (1) to (3) has "△" or "×" was rated as "failed". [0056] [0057] As shown in Table 2, the invention examples both have excellent general corrosion resistance and local corrosion resistance, and have excellent laminar tear resistance. On the other hand, in the comparative example, at least one of general corrosion resistance, local corrosion resistance, and lamellar tear resistance was not obtained, and sufficient characteristics were not obtained. 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 obtained, and sufficient characteristics were not obtained. Further, in Comparative Examples Nos. 43, 47, and 50, since the amount of Sn exceeded the upper limit, the layer tear property was prevented, and sufficient characteristics were not obtained. 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, the total corrosion resistance, the local corrosion resistance, and the lamellar tear resistance were not obtained. Fully characterized. In Comparative Example No. 45, since the amount of Sn was less than the lower limit, overall corrosion resistance and local corrosion resistance were obtained, and sufficient characteristics were not obtained. In Comparative Example No. 46, since the amount of S and the amount of Sn exceeded the upper limit, local corrosion resistance and lamellar tear resistance were obtained, and sufficient characteristics were not obtained. In Comparative Example No. 49, since Cu, Ni, Sb, W, Mo, and Si were not contained, the total corrosion resistance was obtained, and sufficient characteristics were not obtained. In Comparative Example No. 51, since the amount of S exceeded the upper limit and the amount of Sn was less than the lower limit, the overall corrosion resistance, the local corrosion resistance, and the layer tear resistance were not obtained, and sufficient characteristics were not obtained. In Comparative Examples Nos. 53 to 56, since the Sn segregation degree exceeded the upper limit, the layer tear property was prevented, and sufficient characteristics were not obtained.

[0060]

1,7‧‧‧ test strips

2,8‧‧‧corrosion test tank

3‧‧‧temperature control board

4‧‧‧ gas introduction tube

5‧‧‧Exhaust pipe

6,12‧‧‧ water

9‧‧‧ thermostatic bath

10‧‧‧ test solution

11‧‧‧ Fishing line

[0027] FIG. 1 is a schematic view of a test apparatus for a general corrosion test (coagulation test). Figure 2 is a schematic view of a test apparatus for a local corrosion test (acid resistance test).

Claims (4)

A steel material for a crude oil tanker, comprising: C: 0.03 to 0.18% by 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, including 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 to 1.50%, and the remainder consists of Fe and inevitable impurities, and the Sn segregation degree is less than 18; here, the Sn segregation system is under Formula (1) is defined as: [Sn segregation degree] = [Sn concentration of center segregation portion] / [Average Sn concentration] - (1). The steel material for a crude oil tanker according to claim 1, wherein the S content and the Sn content in the composition of the composition satisfy the relationship of the following formula (2): 10000 × [% S] × [% Sn] ² ≦ 1.40 - (2 ) thereto, [% S] and [% Sn] are the content of the S component composition and Sn (mass%). The steel material for a crude oil tanker according to claim 1 or 2, wherein the component composition further comprises one or more of the following groups (A) to (D); (A) in mass% It is one or two selected from the group consisting of Cr: 0.01 to 0.50% and Co: 0.01 to 0.50%, and (B) is selected from the group consisting of Ti: 0.001 to 0.100%, Zr: 0.001 to 0.100%, and Nb: One or more of 0.001 to 0.100% and V: 0.001 to 0.100%, and (C) is 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% by mass%. One or two or more of them, (D) is B: 0.0001 to 0.0300% by mass%. A crude oil tanker which is obtained by using a steel material for a crude oil tanker according to any one of claims 1 to 3.