KR20090098775A - Double hull fuel tanker - Google Patents

Abstract

A double hull fuel tanker is provided to be applied to a ship having a ballast tank with superior durability and to improve the stability of the ship. A double hull fuel tanker uses a steel material as tank bottom plate. The steel material comprises C 0.01~0.3%, Si 0.01~2%, Al 0.005~0.1%, Cu 0.01~1%, Cr 0.01~1%, Ca 0.0001~0.005%, Ti 0.005~0.2%, Ni 0.01~1%, and the rest Fe and inevitable impurities. The steel material includes pearlite 5~25%, bainite less than 20%, martensite less than 10%, and the rest ferrite as to area rate. In the steel material the number of inclusion having a circle equivalent diameter greater than 20mum per unit area is 0.20/mm^2 or less. The steel material includes one or more selected from the group consisting of Mg 0.005% or less, Sr 0.005% or less, and Ba 0.005% or less.

Description

Double Tank Oil Tanker {DOUBLE HULL FUEL TANKER}

TECHNICAL FIELD The present invention relates to a steel for ships used as a main structural material in ships such as crude oil tankers, cargo ships, ships, passenger ships, warships, and the like. The present invention relates to a tank bottom plate of a crude oil tanker for storing marine steels, crude oil or crude oil sludge which exhibits excellent corrosion resistance even under high corrosive environments exposed to salts derived from seawater or high temperature and high humidity. Bottom plate ”may be omitted.) And steel materials (especially thick steel plates) which are excellent in corrosion resistance used for ballast tanks added to ships for improving the stability of a ship. The ship's steel material of this invention is useful especially in the structure of a ballast tank and a crude oil tank, such as a crude oil tank bottom plate etc. according to the embodiment. Moreover, this invention also provides the ship which has the durable ballast tank comprised by the steel material of this invention.

Literature information of the prior art

[Document 1] Japanese Unexamined Patent Application Publication No. 06-322477

[Document 2] Japanese Unexamined Patent Publication No. 2004-190123

[Document 3] Japanese Unexamined Patent Publication No. 2003-82435

[Document 4] Japanese Unexamined Patent Publication No. 2005-97709

[Document 5] Japanese Unexamined Patent Publication No. 2006-9128

[Document 6] Japanese Unexamined Patent Publication No. 2000-17381

[Document 7] Japanese Unexamined Patent Publication No. 2005-171332

[Document 8] Japanese Unexamined Patent Publication No. 7-34196

[Document 9] Japanese Unexamined Patent Publication No. 2004-169048

Steel materials used as major structural materials (for example, outer plates, ballast tanks, crude oil tanks, etc.) in the above-mentioned various vessels are often exposed to salts caused by seawater and high temperature and high humidity, and thus are often subjected to corrosion damage.

Specifically, ship's ballast tanks are used to inject and discharge seawater in response to changes in dripping conditions, etc., so that the steel used is exposed to a very harsh corrosive environment, such as a immersion of seawater and a repetition of a wet atmosphere containing salt. do. In addition, although the inside of a ballast tank is high temperature by irradiation to the upper deck of sunlight, since a hull is cooled by seawater, the temperature difference gradient is provided to the anticorrosive coating film in a tank. When a temperature difference gradient is applied to the anticorrosive coating film, moisture is more likely to invade the steel through the coating film due to the pressure difference due to the temperature difference, and corrosion under the coating film is promoted. It is in a strict environment.

In a crude oil tank bottom plate, the problem that a steel material receives severe local corrosion by the water (retention water) which stayed in the tank bottom containing a high concentration of chloride, sulfur content derived from crude oil, etc., and the perforation is presently present.

Such corrosion may cause an accident such as a ship sinking or an oil spill from a crude oil tanker. Therefore, it is necessary to provide some anticorrosive means to steel materials. As anticorrosive means performed so far, (a) coating, (b) electric system, and the like are well known in the past.

Among the coatings represented by heavy coating, there is a high possibility that a coating film defect may exist, and the coating film may be flawed due to a collision in the manufacturing process, and thus, raw material steel is often exposed. . In such steel exposed portions, the steel is corroded locally and intensively, leading to premature leakage of the received crude oil. Although the protection of steel by rust-proof and anticorrosive sheet is comparatively recognized, the effect is recognized, but there is a problem that corrosion in the exposed part of steel on the upper part of the sheet is inevitable, as in the coating.

The electric system is very effective for a part completely immersed in an electrolyte solution having high conductivity such as seawater. However, in the gas phase part in the crude oil tank or the ballast tank, since there is no electrolyte aqueous solution, an electric circuit is not formed and the anticorrosive effect is not exhibited. In addition, in the crude oil tank bottom plate, since there is not enough retained water to act as an aqueous electrolyte solution (thin water thickness), the distance between the sacrificial anode is small and the electrical method is not suitable. In addition, when the dielectric anode for corrosion prevention is lost due to abnormal consumption or dropping, severe corrosion may proceed immediately.

In addition to the above means, Japanese Patent Laid-Open No. 06-322477 discloses that by adding one or more of Cu, Cr, Ni, and Mo, the corrosion resistance can be improved in crude oil including hydrogen sulfide and water, and also Nb, Ti, and V. By adding one or more of these, it is disclosed that the ferrite particles can be made finer and strengthened, thereby lowering the crack growth rate of advancing the ferrite particles. In Japanese Unexamined Patent Publication No. 2004-190123, on the basis of the chemical composition of general welded structural steel, Cr is substantially free of additives, and either a certain amount of Mo, W, or both and Cu are appropriately added in combination to provide an impurity. By limiting the addition amounts of phosphorus P and S, it is disclosed that the corrosion resistance in the environment behind the crude oil tank deck which always becomes a gas phase part can be improved. Japanese Laid-Open Patent Publication No. 2003-82435 discloses that corrosion resistance of steel materials can be improved by appropriately adjusting chemical components, inclusions, and structures.

Japanese Unexamined Patent Application Publication No. 2005-97709 discloses improving the local corrosion resistance of steel for crude oil tank bottom plate by adjusting the chemical component, in particular, by adjusting the Ni content to an appropriate amount and then increasing the N content. In Japanese Patent Laid-Open No. 2004-204344, on the basis of the chemical composition of general welded structural steels, Cr is substantially free of additives, and either a certain amount of Mo, W or both and Cu are added, and P is an impurity. By limiting the amount of S added, Al is added to reduce the local corrosion damage of the crude oil tank steel. In Japanese Patent Laid-Open No. 2006-9128, it is disclosed to improve the corrosion resistance of ship steels by using a predetermined amount of Co and Mg in combination, and by adjusting the chemical composition appropriately.

Japanese Unexamined Patent Publication No. 2000-17381 also proposes a corrosion resistant steel having improved corrosion resistance of the steel itself by adjusting a chemical component or the like. Moreover, Japanese Unexamined Patent Application Publication No. 2005-171332 also proposes a steel material having improved corrosion resistance by adjusting a chemical component and using a zinc rich primer. In addition, Japanese Patent Laid-Open No. 7-34196 also proposes a durability improving technique combining a corrosion resistant steel material and a resin coating.

In Japanese Patent Laid-Open No. 2004-169048, the chemical composition of steel materials, the composition, size and number of sulfide-based inclusions and oxide-based inclusions in steel are optimized to improve the corrosion resistance in the oil tank environment. A crude oil bath steel in which HAZ (weld heat affected zone) toughness is improved by substitution heat welding is disclosed.

By such an improvement technique, it has become possible to secure its own corrosion resistance compared with the conventional member. However, the improvement of corrosion resistance is calculated | required continuously for ship steel materials. Then, the objective of this invention is providing the ship steel material which shows the outstanding corrosion resistance.

In the first basic aspect of the present invention, a ship steel material capable of achieving the above object is

C: 0.01 to 0.3% (meaning of mass%, the same applies hereinafter),

Si: 0.01-2%,

Mn: 0.01 to 2%,

Al: 0.005% to 0.1%,

Cu: 0.01 to 1%,

Ni: 0.01 to 1%, and

Cr: O.O1-1%,

Containing, the remainder being composed of Fe and inevitable impurities,

Ferritic is the largest structure, the bainite and / or martensite is less than 30% in total (including 0) is a marine steel material excellent in corrosion resistance. Moreover, the 1st basic aspect of this invention contains the 1st Embodiment-3rd Embodiment mentioned later.

In the 2nd basic aspect (4th embodiment) of this invention, the ship steel material which could achieve the said objective is,

C: 0.01 to 0.30%,

Si: 0.01 to 2.0%,

Mn: 0.01 to 2.0%,

Al: 0.005% to 0.10%,

S: 0.010% or less

Each containing, furthermore,

a group element (one or more selected from Mg, Ca, Sr, and Ba): 0.0005% to 0.020%

b group element (1 or more types chosen from Ti, Zr, and Hf): It contains 0.005 to 0.20%, and the ratio of the total content S (a) of the said group a element to the total content S (b) of the b group element In the range of S (a) / S (b) in the range of 0.01 to 1, the average particle diameter of the sulfide inclusions and the oxide inclusions each having a balance of Fe and an unavoidable impurity and also satisfying the following requirements is 1; It is -100 micrometers, and these are ship steel materials excellent in corrosion resistance characterized by the presence of 20-2000 pieces per 1 mm <2> of a rolling direction cross section, respectively.

Sulfide inclusions: sulfide inclusions having a circle equivalent diameter of at least 0.5 µm containing 1 to 30% of the total amount of the group a elements and / or 1 to 30% of the total amount of the group b elements;

Oxide inclusions: An oxide inclusion having a circular equivalent diameter of 0.5 µm or more containing 1 to 30% of the total amount of the group a elements and / or 1 to 30% of the total amount of the group b elements.

Marine steel of the present invention is characterized in that the chemical composition is appropriately controlled in order to improve the corrosion resistance. Hereinafter, first, the chemical component composition will be described.

The steel for ships of this invention contains C, Si, Mn, and Al as a basic component. C is an element necessary for securing the strength of the material. However, when it contains too much, toughness will deteriorate. Si and Al are elements necessary for deoxidation and ensuring strength. However, when it contains too much, weldability will deteriorate. Mn, like Si, is required for deoxidation and strength. However, when it contains too much, toughness will deteriorate. Therefore, in the steel of the present invention, the amount of C is 0.01 to 0.3%, the amount of Si is 0.01 to 2%, the amount of Mn is 0.01 to 2%, and the amount of Al is in the range of 0.005 to 0.1%.

The steel material of this invention contains inevitable impurities, such as P and S. However, P and S are an element which degrades toughness, weldability, etc., It is preferable that the amount of P and S in the steel of this invention is as low as possible, and it is recommended that they are 0.01% or less, respectively.

According to the level of corrosion resistance required for the steel of the present invention, an element having an action of improving corrosion resistance, for example, Ca, Mg, Sr, Ba, Ti, Zr, Hf, Ni, Co, Cu, Cr, La, Ce , Nd, Sm, Se, Sb, Bi and / or Te may be contained. Moreover, what is necessary is just to adjust content of these corrosion resistance improvement elements suitably according to the characteristic requested | required of steel materials.

Ca, Mg, Sr, and Ba have the effect of suppressing the pH drop due to hydrolysis of Fe dissolved by corrosion. By this action, the pH drop in a portion where hydrogen ion diffusion such as a coating film / steel material interface or a structural gap portion is hard to occur can be suppressed, and the corrosion resistance (particularly, internal gap corrosion resistance) of the portion can be improved. Moreover, when Mg coexists with Co, it has a remarkable effect on the improvement of corrosion resistance. However, excessively containing these deteriorates workability and weldability. Therefore, when Ca, Mg, Sr, and Ba are contained in the steel of the present invention, each amount must be controlled to 0.020% or less. Moreover, in each embodiment of this invention, content of Ca, Mg, Sr, and Ba and the following elements is determined in more detail according to the performance calculated | required.

Ti, Zr, and Hf have an effect of stabilizing the surface rust film formed in a corrosive environment, and are effective elements for expressing the corrosion inhibitory effect by the surface rust film for a long time. However, when these amounts are excessive, weldability and workability (especially hot workability) will deteriorate. Therefore, in the steel materials of the present invention, when Ti, Zr and Hf are contained, the respective amounts should be controlled to 0.2% or less.

Ni and Co have an effect of forming a dense surface rust coating in a corrosive environment, and Ni and Co are elements that exhibit a corrosion inhibiting effect by the surface rust coating. However, when these amounts are excessive, weldability and hot workability will deteriorate. Therefore, when Ni and Co are contained in the steel of this invention, each amount should be controlled to 5.0% or less.

Cu and Cr have an action of promoting the formation of a dense surface rust coating which greatly contributes to the improvement of corrosion resistance, and are elements that improve corrosion resistance by increasing environmental barrier properties. In addition, an appropriate amount of Cr is effective for improving toughness. However, when it contains too much, weldability and hot workability will deteriorate. Therefore, when Cu and Cr are contained in the steel of this invention, each amount should be in 0.01 to 5.0% of range.

La, Ce, Nd, Sm, and Se have an effect of further improving corrosion resistance, and in particular, promote the effect of suppressing pH lowering by Ca or the like and stabilizing effect of the surface rust coating by Ti or the like. However, when it contains too much, workability and weldability will deteriorate. Therefore, in the steel materials of the present invention, the amounts of La, Ce, Nd and Sm should be controlled to 0.01% or less and Se amount to 0.1% or less, respectively.

Sb, Bi, and Te are elements which promote the rust densification effect | action by Cu etc. and improve corrosion resistance. However, when it contains too much, workability and weldability will deteriorate. Therefore, in the steel materials of the present invention, the amounts of Sb and Bi should be controlled to 0.2% or less, and the amount of Te to 0.1% or less, respectively.

The steel material of this invention may contain the element which improves the characteristic of steel materials besides the said element, for example, B, V, Nb, Ta, etc. B and V are effective elements for improving strength (especially B for improving hardenability). Ta is an element that contributes to the reduction of the ferrite grain size by the grain refining action. Nb has the action of both of these (strength improvement and grain refinement). However, when these amounts are excessive, toughness will deteriorate. Therefore, in the steel materials of the present invention, the amount of B should be controlled to 0.01% or less, and the amount of V, Nb and Ta should be controlled to 0.5% or less, respectively.

Moreover, the corrosion resistance of steel materials can be improved further by controlling the composition ratio of a chemical component suitably in the steel materials of this invention. In more detail, Ca, Mg, Sr, and Ba, which have an effect of suppressing the pH drop, are group a, Ti, Zr, and Hf, which are effective in stabilizing the surface rust coating, are group b, and Co and Ni, group c. When classified into, the a to c group elements having different effects act synergistically. Therefore, corrosion resistance can be improved further by controlling suitably the ratio of the sum total content of a group element, b group element, and c group element.

Specifically, the total content of the group a elements is S (a)% (meaning the mass%, which is the same below), the total content of the group b elements is S (b)%, and the total content of the group c elements is S (c). In the case of)%, from the viewpoint of the corrosion resistance of the entire surface, preferably 20 ≦ S (c) / S (a) ≦ 350, more preferably 50 ≦ S (c) / S (a) ≦ 170 It is recommended to adjust the content of these elements as much as possible. Moreover, it is recommended to adjust so that it may become 1.00 <S (c) / S (b) <60, more preferably 10 <S (c) / S (b) <30 from the viewpoint of gap corrosion resistance. In addition, in order to increase the overall corrosion resistance, it is preferable to adjust such that O1 ≤ S (a) / S (b) ≤ 1, more preferably 0.02 ≤ S (a) / S (b) ≤ 0.8 do.

The inventors have found that ship steel materials excellent in corrosion resistance can be obtained by appropriately adjusting chemical composition and controlling metal structures and / or non-metallic inclusions appropriately.

The marine steel material in the 1st basic aspect of this invention implement | achieves the outstanding corrosion resistance by adjusting chemical composition in the appropriate range. First, the chemical composition of the steel of the first basic aspect of the present invention will be described.

<C: 0.01 to 0.3%>

The amount of C of the first basic aspect of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.04% or more, and preferably 0.3% or less, from the viewpoint of securing the strength and the deterioration of the toughness described above. It is 0.28% or less, More preferably, it is 0.26% or less.

<Si: 0.01 to 2%>

Si amount of the first basic aspect of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more, and 2% or less, from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

<Mn: 0.01 to 2%>

The amount of Mn of the first basic aspect of the present invention is 0.01% or more, preferably 0.05% or more, more preferably 0.10% or more, 2% or less, from the viewpoints of deoxidation and strength securing and toughness deterioration described above. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

<Al: 0.005 to 0.1%>

Al amount of the 1st basic aspect of this invention is 0.005% or more, Preferably it is 0.010% or more, More preferably, it is 0.015% or more from a viewpoint of the above-mentioned deoxidation, ensuring strength, and weldability deterioration, 0.1% or less, Preferably it is 0.09% or less, More preferably, it is 0.08% or less,

<Cu: 0.01 to 1%>

Cu amount of the 1st basic aspect of this invention is 0.01% or more, Preferably it is 0.05% or more from a viewpoint of corrosion resistance improvement mentioned above (especially the formation of dense surface rust film formation), and weldability and hot workability deterioration, 1 % Or less, Preferably it is 0.9% or less.

<Ni: 0.01 to 1%>

The amount of Ni of the first basic aspect of the present invention is 0.01% or more, preferably 0.02% or more, 1% from the viewpoints of the above-described corrosion resistance improvement (particularly, formation of a dense surface rust coating) and deterioration of weldability and hot workability. Hereinafter, Preferably it is 0.9% or less.

<Cr: O.O1-1%>

The Cr amount of the first basic aspect of the present invention is 0.01% or more, preferably 0.05% or more, from the viewpoints of the above-described corrosion resistance improvement (especially for the formation of a dense surface rust coating) and toughness improvement, and deterioration of weldability and hot workability. It is 1% or less, Preferably it is 0.8% or less.

The basic component of the ship steel of the 1st basic aspect of this invention is as above-mentioned, and a remainder part consists of iron and an unavoidable impurity (for example, P, S, O, N, H, Mo, W, etc.). However, the unavoidable impurity needs to be in an amount so as not to impair the characteristics of the steel. In addition to the above components, the steel materials of the first basic aspect of the present invention are also effective to contain, as necessary, elements that are used in the first to third embodiments described later as optional elements.

As for the steel materials for ships in the 1st basic aspect of this invention, in addition to adjustment of a chemical composition, metal structure is appropriately controlled and corrosion resistance is improved. Specifically, by making ferrite the maximum structure and making bainite and / or martensite less than 30% (including 0) in total, the corrosion resistance can be further improved.

In the ship steel materials in the 1st basic aspect of this invention, ferrite has the largest area ratio. Since ferrite is excellent in weldability and workability and small in susceptibility to stress corrosion cracking by chlorides, ferrite is preferable as a structure of a structural member used in a chloride environment called seawater.

On the other hand, bainite and martensite are effective structures for improving the strength and toughness of steel materials. However, when the bainite and / or martensite is excessively present in the ferrite, the corrosion of the bainite and / or martensite portion is promoted and the corrosion resistance is lowered. In order to prevent this corrosion resistance fall, it is recommended to set the area ratio of bainite + martensite to less than 30% in total (including the case where either bainite, martensite, or both are 0).

In the present invention, when the thickness of the steel is 6 mm or more, in the present invention, the area ratio is 3 mm deeper than the surface, and when the thickness of the steel is less than 6 mm, the area ratio of the steel is 1/2 of the thickness of the steel. The observation magnification of 400 times and the observation visual field of 150 micrometers x 200 micrometers or more are observed with the optical microscope, and the average value of the area ratio obtained by arbitrary 30 visual fields is employ | adopted. In addition, the surface as a reference of the depth means the surface of the steel subjected to the rolling force.

Next, the first to third embodiments included in the first basic aspect of the present invention will be described in detail. First, the first embodiment of the present invention,

C: 0.01 to 0.3%,

Si: 0.01-2%,

Mn: 0.01 to 2%,

Al: 0.005% to 0.1%,

P: 0.01% or less (does not contain 0%),

S: 0.005% or less (not including 0%),

Cu: 0.01 to 1%,

Ni: 0.01 to 1%, and

Cr: O.O1-1%

Containing, the remainder being composed of Fe and inevitable impurities,

The area ratio is pearlite: 5 to 25%, bainite: less than 20%, martensite: less than 10%, and the remainder has a structure composed of ferrite,

The ferrite grain size is 15 µm or less, and the aspect ratio of pearlite is 3.0 to 20, characterized in that the steel.

In the ship steel of the 1st aspect of this invention, as needed,

(1) Ca: 0.005% or less (does not contain 0%), Mg: 0.005% or less (does not contain 0%), Sr: 0.005% or less (does not contain 0%), Ba: 0.005% or less (Does not contain 0%), La: 0.005% or less (does not contain 0%), Ce: 0.005% or less (does not contain 0%), Nd: 0.005% or less (does not contain 0%) At least one element selected from the group consisting of Sm: 0.005% or less (without O%), and Se: 0.1% or less (without O%),

(2) Ti is selected from the group consisting of 0.2% or less (does not contain 0%), Zr: 0.2% or less (does not contain 0%), and Hf: 0.2% or less (does not contain 0%) At least one kind of element,

(3) Sb: 0.2% or less (does not contain 0%), Bi: 0.2% or less (does not contain 0%), and Te: 0.1% or less (does not contain 0%) At least one kind of element,

(4) Co: 1% or less (does not contain 0%),

(5) Nb: 0.5% or less (does not contain 0%) and / or Ta: 0.5% or less (does not contain 0%), or

(6) It is also effective to contain B: 0.01% or less (does not contain 0%) and / or V: 0.5% or less (does not contain 0%), and the like. Properties are further improved. The ship steel materials of 1st Embodiment of this invention may satisfy | fill two or more of these requirements (1)-(6) simultaneously.

The marine steel material according to the first embodiment of the present invention exhibits excellent corrosion resistance even under severe corrosive environments by appropriately adjusting the (I) chemical composition and (II) the metal structure. Durability can be secured. Below, the chemical component composition of the steel material by 1st aspect of this invention is demonstrated first.

<C: 0.01 to 0.3%>

The amount of C of the first embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.04% or more, and preferably 0.3% or less, from the viewpoint of securing the strength and the deterioration of the toughness described above. It is 0.28% or less, More preferably, it is 0.26% or less.

<Si: 0.01 to 2%>

The Si amount of the first embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more, 2% or less, from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

<Mn: 0.01 to 2%>

The amount of Mn in the first embodiment of the present invention is 0.01% or more, preferably 0.05% or more, more preferably 0.10% or more, 2% or less, from the viewpoints of deoxidation and strength securing and toughness deterioration described above. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

<Al: 0.005 to 0.1%>

The amount of Al in the first embodiment of the present invention is 0.005% or more, preferably 0.010% or more, more preferably 0.015% or more, 0.1% or less, from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Preferably it is 0.09% or less, More preferably, it is 0.08% or less.

<P: 0.01% or less (does not include 0%)>

In the first embodiment of the present invention, as described above, it is recommended that the amount of P harmful to toughness, weldability, and the like be 0.01% or less, preferably 0.005% or less.

<S: 0.005% or less (not including 0%)>

In the first embodiment of the present invention, as described above, it is recommended that the amount of S harmful to toughness, weldability, and the like be 0.005% or less, preferably 0.004% or less.

<Cu: 0.01 to 1%>

Cu amount of the 1st aspect of this invention is 0.01% or more, Preferably it is 0.05% or more from a viewpoint of the above-mentioned corrosion resistance improvement (especially the formation of dense surface rust film formation), and weldability and hot workability deterioration, 1 % Or less, Preferably it is 0.9% or less.

<Ni: 0.01 to 1%>

The amount of Ni in the first embodiment of the present invention is 0.01% or more, preferably 0.02% or more, 1% from the viewpoint of the above-described corrosion resistance improvement (particularly, formation of a dense surface rust coating) and deterioration of weldability and hot workability. Hereinafter, Preferably it is 0.9% or less.

<Cr: O.O1-1%>

The Cr amount of the first embodiment of the present invention is 0.01% or more, preferably 0.05% or more, from the viewpoints of the above-described corrosion resistance improvement (especially for the formation of dense surface rust coating) and toughness improvement, and deterioration of weldability and hot workability. It is 1% or less, Preferably it is 0.8% or less.

The basic component of the ship steel of the 1st aspect of this invention is as above-mentioned, and a remainder consists of iron and an unavoidable impurity (for example, O, N, H, Mo, W, etc.). However, the unavoidable impurity needs to be in an amount so as not to impair the characteristics of the steel. In addition, the steel material of the first embodiment of the present invention is effective to contain the following optional components and the like, in addition to the components described above, and the steel properties are further improved depending on the kind of the components to be contained.

<Ca: 0.005% or less, Mg: 0.005% or less, Sr: 0.005% or less, Ba: 0.005% or less, La: 0.005% or less, Ce: 0.005% or less, Nd: 0.005% or less, Sm: 0.005% or less, and Se: at least one element selected from the group consisting of 0.1% or less>

Ca, Mg, Sr, Ba, La, Ce, Nd, Sm, and Se (preferably Ca, Mg, La, Ce, and Se) are selection elements effective for improving corrosion resistance. As described above, Ca, Mg, Sr and Ba are each preferably 0.0001% or more, more preferably, to the steel materials of the first embodiment of the present invention in order to improve corrosion resistance, particularly to suppress the pH drop. Is in an amount of at least 0.0005%, La, Ce, Nd and Sm, respectively, preferably in an amount of at least 0.0001%, more preferably at least 0.0003%, and Se, preferably at least 0.001%, more preferably 0.002 It is recommended to contain in an amount of% or more. On the other hand, in the case of containing them, the amount of Ca, Mg, Sr, Ba, La, Ce, Nd and Sm is 0.005% or less, preferably 0.004% or less, respectively, from the viewpoint of workability and weldability deterioration described above. Is adjusted to 0.1% or less, preferably 0.05% or less.

<Ti: 0.2% or less, Zr: 0.2% or less, and Hf: at least one element selected from the group consisting of 0.2% or less>

Ti, Zr and Hf are selection elements effective for improving corrosion resistance. As described above, in order to improve the corrosion resistance, particularly to stabilize the surface rust coating, the steels of the first embodiment of the present invention are preferably Ti, Zr and Hf, preferably 0.005% or more, more preferably, respectively. It is recommended to contain 0.008% or more. On the other hand, it is recommended to adjust Ti, Zr, and Hf amounts to 0.2% or less, preferably 0.15% or less, respectively, from the viewpoint of the above-described weldability and workability deterioration.

<At least one element selected from the group consisting of Sb: 0.2% or less, Bi: 0.2% or less, and Te: 0.1% or less>

Sb, Bi and Te are selection elements effective for improving corrosion resistance. As described above, in order to improve the corrosion resistance, in particular, to promote the action of Cu and the like, Sb and Bi, respectively, preferably 0.005% or more, and more preferably 0.008, in the steel materials of the first embodiment of the present invention. It is recommended to contain Te in an amount of at least%, preferably in an amount of at least 0.001%, more preferably at least 0.002%. On the other hand, from the viewpoint of workability and weldability deterioration described above, when these are included, the amounts of Sb and Bi are respectively 0.2% or less, preferably 0.15% or less, and the Te amount is 0.1% or less, preferably 0.05% or less. It is recommended to adjust with.

<Co: 1% or less>

Co is a selection element effective for improving corrosion resistance. As described above, in order to improve the corrosion resistance and to form a particularly dense surface rust coating, the steel of the first embodiment of the present invention preferably contains Co, preferably 0.01% or more, and more preferably 0.02% or more. It is recommended to let. On the other hand, when it contains from a viewpoint of weldability and hot workability deterioration, it is recommended to adjust Co amount to 1% or less, Preferably it is 0.8% or less.

<Nb: 0.5% or less and / or Ta: 0.5% or less>

Nb and Ta, as described above, are selective elements having a grain refining action and contributing to the reduction of the ferrite grain size. As a result of this, the ferrite grain size is further reduced, and the corrosion resistance can be further improved. It is recommended to contain Nb and Ta, respectively, Preferably it is 0.003% or more, More preferably, 0.005% or more in the steel materials of 1st Embodiment of this invention from a viewpoint of a grain refinement action. On the other hand, from the viewpoint of the above-mentioned toughness deterioration, when including these, it is recommended to adjust Nb and Ta amount to 0.5% or less, respectively, Preferably it is 0.4% or less.

<B: 0.01% or less and / or V: 0.5% or less>

As described above, B and V are both selective elements effective for improving the strength. B is preferably in an amount of 0.0001% or more, more preferably 0.0003% or more, and V in the steel materials of the first embodiment of the present invention from the viewpoint of strength improvement, and preferably 0.01% or more, and more preferably It is recommended to contain in an amount of 0.02% or more. On the other hand, from the viewpoint of the above-mentioned toughness deterioration, when including these, it is recommended to adjust B amount to 0.01% or less, Preferably 0.009% or less, and V amount to 0.5% or less.

In order to improve corrosion resistance, the marine steel material of the 1st aspect of this invention is that metal structure is suitably controlled, specifically, an area ratio is 5-25% of pearlite, less than 20% of bainite, and martensite : It is less than 10%, and remainder has one structure which has a structure which consists of ferrite. In addition, the meaning and measurement method of a tissue area ratio are the same as the above-mentioned.

In the steel materials of the first embodiment of the present invention, ferrite has the largest area ratio. It is because ferrite is preferable as a structure of the structural member used in the chloride environment called sea water as mentioned above.

Pearlite is necessary in order to acquire the strength requested | required of the steel materials of 1st Embodiment of this invention. In addition, when the pearlite area ratio is too small, the pearlite tends to be scattered locally, and the tendency to generate local corrosion such as formula and gap corrosion increases. From the viewpoint of strength and local corrosion resistance, in the first embodiment of the present invention, the pearlite area ratio is set to 5% or more. However, the increase in pearlite causes deterioration of toughness and weldability, and also acts as a cathode site of the corrosion reaction, resulting in deterioration of corrosion resistance (corrosion resistance of the entire surface). Therefore, the pearlite area ratio was set to 25% or less. The more preferable range of pearlite area ratio is 8 to 20%.

As described above, bainite adversely affects corrosion resistance, and in particular, lowers local corrosion resistance. Therefore, it is recommended that the bainite area ratio in the first embodiment of the present invention be less than 20%, preferably less than 18%.

As mentioned above, martensite adversely affects corrosion resistance, and also deteriorates toughness and weldability. Therefore, it is recommended that the martensite area ratio in the first embodiment of the present invention be less than 10%, preferably less than 8%.

The ship steel of the first embodiment of the present invention is characterized in that the ferrite grain size is 15 µm or less and the aspect ratio of pearlite is controlled to 3.0 to 20 in order to improve the corrosion resistance.

The value of the ferrite particle diameter in the 1st Embodiment of this invention is the thickness of steel materials, when the thickness of steel materials is less than 6 mm in the site | part which is 3 mm deeper than the surface, when the thickness of steel materials is 6 mm or more. In 1/2 part, the average value of the circle equivalent diameter of the ferrite obtained by observing with an optical microscope in the observation magnification of 400 times the principle, and 150 micrometers x 200 micrometers or more, and the visual field obtained by arbitrary 30 visual fields is employ | adopted. The equivalent circle diameter can be obtained by an appropriate image analysis device or software. Moreover, it is preferable to perform appropriate etching, such as nital etching, to the sample to observe.

The aspect ratio of the pearlite in the first embodiment of the present invention refers to JIS H7402 (Method of short fiber aspect ratio test in fiber-reinforced metal), and indicates the distance between the maximum two points on the outer circumference of the pearlite contour. Is a value calculated as Lp / Dp when the distance divided by the pearlite contour in the line passing perpendicular to the long diameter direction of the pearlite and passing through the midpoint of the pearlite long diameter is a pearlite short diameter: Dp. define. The value of the aspect ratio of pearlite in this invention is the site | part of 1/2 the thickness of steel materials, when the thickness of steel materials is less than 6 mm in the site | part of 3 mm deeper than the surface, when the thickness of steel materials is 6 mm or more. WHEREIN: The observation magnification of 400 times the principle, and 150 micrometers x 200 micrometers or more of observation fields are observed with an optical microscope, and the average value of Lp / Dp obtained by arbitrary 10 views is employ | adopted.

When the ferrite particle size is reduced, the homogeneity of the corrosion reaction increases, and therefore, corrosion resistance, in particular, local corrosion resistance can be improved. In addition, P and S, which adversely affect the corrosion resistance, tend to segregate in the grain boundary, but by reducing the ferrite grain size, the grain boundary can be increased and the grain boundary concentration of the impurities can be lowered, thereby achieving higher corrosion resistance. In order to fully exhibit such an effect, it is recommended that the ferrite particle size be 15 µm or less, more preferably 10 µm or less.

Moreover, corrosion resistance can be improved by increasing the aspect ratio of pearlite. Although this mechanism is unclear, when the pearlite / ferrite interface is increased, it is estimated that the impurity concentration segregating at the interface is reduced, thereby improving the corrosion resistance. However, the present invention is not limited to this estimation. In order to improve corrosion resistance, it is recommended to make the aspect ratio of pearlite 3.0 or more, Preferably it is 6 or more, More preferably, it is 12 or more. On the other hand, the upper limit of the aspect ratio of pearlite was set to 20 from the viewpoint of material anisotropy.

The ship steel material of 1st Embodiment of this invention can be manufactured, for example by the following method. First, by general solvent methods such as a converter and an electric furnace, the solvent is performed while adjusting the chemical component composition so as to satisfy the above range, and the ingot is formed by a general casting method such as a continuous casting method or an ingot method. In order to reduce gas components, such as O, N, and H, it is preferable to perform out-of-furnace refining, such as vacuum degassing methods, such as a DH method and an RH method, as needed. In addition, for deoxidation, it is preferable to use a kicked steel from the viewpoint of mechanical properties or weldability, and from the viewpoint of corrosion resistance, Al-kilted steel with less segregation of harmful impurities such as P and S is more preferable.

After heating the obtained steel ingot in the temperature range of 1000-1200 degreeC, it is preferable to hot-roll and set it as desired dimension shape. If the heating temperature before rolling is less than 1000 degreeC, as a solid solution degree, such as Si, Mn, Ni, P, and S, will become low, a structure will become nonuniform and local corrosion will arise easily. On the other hand, when heating temperature exceeds 1200 degreeC, austenite particle will coarsen and it will become difficult to obtain the ferrite particle diameter of 15 micrometers or less.

It is preferable to control hot rolling finish temperature to 750-850 degreeC in order to suppress austenite coarsening by recrystallization in rolling, and to ensure toughness. By adjusting the total rolling reduction of a hot rolling rate to 80 to 97% of range, the aspect ratio of pearlite can be 3.0-20.

By controlling the cooling rate from the end of hot rolling to 500 degreeC in the range of 0.5-15 degreeC / sec, pearlite: 5 to 25%, bainite: less than 20%, martensite: less than 10%, and remainder is ferrite It can form a tissue. When the cooling rate is less than 0.5 deg. C / sec, the generation of carbides becomes remarkable, the cathode site in the corrosion reaction increases, and the corrosion resistance may deteriorate. On the other hand, when the cooling rate exceeds 15 ° C / sec, the toughness deterioration due to martensite transformation may increase.

Since the ship steel of the 1st aspect of this invention is excellent in corrosion resistance, and can be used by specification (no treatment), you may use it by giving surface treatment, such as coating, plating, or chemical conversion treatment. When using the steel material of this invention as a structural material of the cargo oil tank of a crude oil tanker, it is preferable to use the inner crude oil tank side by naked specification (no treatment), and the outer ballast tank side by a coating specification. However, the crude oil tank side may be treated with zinc rich paint or shop primer for the purpose of initial rust prevention, if necessary. As a paint on the ballast tank side, a tar epoxy resin paint, a modified epoxy resin paint, or other typical heavy anticorrosive paint etc. are mentioned. In addition, the steel materials of the present invention may be used in combination with other anticorrosive methods such as an electric system (dielectric anode method, external power supply method).

The marine steel materials of the first embodiment of the present invention exhibit excellent corrosion resistance even under severe corrosive environments and can ensure durability over a long period of time. Therefore, the marine steel materials in ships such as crude oil tankers, cargo ships, ships, passenger ships, warships, etc. It is useful as structural materials, such as a board | plate, ballast tank, and a crude oil tank.

Next, a second embodiment of the present invention will be described in detail. 2nd aspect of this invention relates to the steel materials used especially for a crude oil tank bottom plate among ship steel materials. The corrosive environment to which this crude oil tank bottom plate is exposed is severe, and as a result, the improvement of corrosion resistance (especially local corrosion resistance) is constantly required as steel materials used. Moreover, in the crude oil tanker of the double-half (double angle) structure, the outer surface of a tank bottom plate becomes a ballast tank side, and it is normally used in the state which applied the coating by epoxy paint. Since the ballast tank side receives corrosion action by the salt in order to fill the seawater at the time of emptying, and receives corrosion action by high temperature and high humidity during the transportation of crude oil, excellent paint corrosion resistance is required. Therefore, in the crude oil tank bottom plate of the structure to be doubled, it is required to be excellent in both local corrosion resistance (inner surface side) and coating corrosion resistance (ballast tank side). Accordingly, an object of the second embodiment of the present invention is to provide a steel material for a crude oil tank bottom plate having excellent local corrosion resistance (inner surface side in contact with crude oil) and paint corrosion resistance (ballast tank side in contact with seawater).

According to a second aspect of the present invention, the above object can be achieved,

C: 0.01 to 0.3%,

Si: 0.01-2%,

Mn: 0.01 to 2%,

Al: 0.005% to 0.1%,

Cu: 0.01 to 1%,

Cr: O.O1-1%,

Ca: 0.0001 to 0.005%,

Ti: 0.005 to 0.2%, and

Ni: 0.01 to 1%

Containing, the remainder being composed of Fe and inevitable impurities,

The area ratio is pearlite: 5 to 25%, bainite: less than 20%, martensite: less than 10%, and the remainder has a structure composed of ferrite,

The number per unit area of nonmetallic inclusions whose round equivalent diameter exceeds 20 micrometers is 0.20 piece / mm <2> or less steel materials for crude oil tank bottom plates.

In the steel material for crude oil tank bottom plate of the second embodiment of the present invention, if necessary, (1) Mg: 0.005% or less (not including 0%), Sr: 0.005% or less (not containing 0%), And Ba: at least one element selected from the group consisting of 0.005% or less (not including 0%), (2) Zr: 0.2% or less (not including 0%) and / or Hf: 0.2% or less (0% not included), (3) Co: 1% or less (does not contain 0%), (4) La: 0.01% or less (does not contain 0%), Ce: 0.01% or less (0 %), Nd: 0.01% or less (does not contain 0%), Sm: 0.01% or less (does not contain 0%), and Se: 0.1% or less (does not contain O%) At least one element selected from the group consisting of: (5) Sb: 0.2% or less (does not contain 0%), Bi: 0.2% or less (does not contain 0%), and Te: 0.1% or less (0% At least one element selected from the group consisting of: (6) B: 0.01% or less (does not contain 0%), V: 0.5% or less (does not contain 0%), and Nb: It is also effective to contain at least one element selected from the group consisting of 0.5% or less (not containing 0%), or (7) Ta: 0.5% or less (not containing 0%), and the like. According to the kind of component to make, the characteristic of the crude oil tank bottom plate steels will further improve. The steel for crude oil tank bottom plate of 2nd Embodiment of this invention may satisfy | fill two or more of these requirements (1)-(7) simultaneously.

In the steel for crude oil tank bottom plate according to the second embodiment of the present invention, Ca, Mg and Sr are group a, Ti, Zr and Hf are group b, Co and Ni are group c, When the sum total content is S (a)% (mean of mass%, the same applies hereinafter), the sum content of the b group element is S (b)% and the sum content of the c group element is S (c)%, It is preferable to adjust the total content of the c-group elements so as to satisfy the following formulas (1) and (2).

20 ≤ S (c) / S (a) ≤ 350

1.00 ≤ S (c) / S (b) ≤ 60 (2)

Since the steel material for crude oil tank bottom plates of 2nd aspect of this invention is excellent in both local corrosion resistance and paint corrosion resistance, it is especially preferable to use it as a tank bottom plate of a crude oil tanker of a double split structure.

According to the second embodiment of the present invention, (I) appropriately adjusting the chemical component composition, in particular Ca, which is effective in suppressing the pH drop, Ti, which is effective in stabilizing the surface rust coating, and dense surface rust, etc. By containing Ni etc. which have an effect of forming a film in appropriate quantity, (II) appropriately controlling a metal structure, and (III) suppressing the nonmetallic inclusion which exists in steel materials, both local corrosion resistance and paint corrosion resistance This excellent crude oil tank bottom steel can be realized. Below, the chemical component composition of the steel material by 1st aspect of this invention is demonstrated first.

<C: 0.01 to 0.3%>

The amount of C in the second embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.04% or more, and preferably 0.3% or less, from the viewpoint of securing the strength and the deterioration of the toughness described above. It is 0.28% or less, More preferably, it is 0.26% or less.

<Si: 0.01 to 2%>

Si amount of the second embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more, 2% or less, from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

<Mn: 0.01 to 2%>

The amount of Mn in the second embodiment of the present invention is 0.01% or more, preferably 0.05% or more, more preferably 0.10% or more, 2% or less, from the viewpoints of deoxidation and strength securing and toughness deterioration described above. Preferably it is 1.8% or less, More preferably, it is 1.6% or less.

<Al: 0.005 to 0.1%>

The amount of Al in the second embodiment of the present invention is 0.005% or more, preferably 0.010% or more, more preferably 0.015% or more, and 0.1% from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Hereinafter, Preferably it is 0.09% or less, More preferably, it is 0.08% or less.

<Cu: 0.01 to 1%>

Cu amount of the 2nd aspect of this invention is 0.01% or more, Preferably it is 0.05% or more from a viewpoint of the above-mentioned corrosion resistance improvement (especially the formation of dense surface rust film formation), and weldability and hot workability deterioration, 1 % Or less, Preferably it is 0.9% or less.

<Cr: O.O1-1%>

The Cr amount of the second embodiment of the present invention is 0.01% or more, preferably 0.05% or more, from the viewpoints of the above-described corrosion resistance improvement (especially for the formation of dense surface rust coating) and toughness improvement, and deterioration of weldability and hot workability. It is 1% or less, Preferably it is 0.8% or less.

<Ca: 0.0001 to 0.005%>

The Ca amount of the second embodiment of the present invention is 0.0001% or more, preferably 0.0005% or more, and 0.005% from the viewpoints of the above-described corrosion resistance improvement (especially suppression of pH drop) and toughness improvement, and workability and weldability deterioration. Hereinafter, Preferably it is 0.004% or less.

<Ti: 0.005 to 0.2%>

Ti amount of the 2nd aspect of this invention is 0.005% or more, Preferably it is 0.008% or more from a viewpoint of corrosion resistance improvement (stabilization of surface rust film), toughness improvement, and weldability and workability deterioration mentioned above, 0.2% Hereinafter, Preferably it is 0.15% or less.

<Ni: 0.01 to 1%>

The amount of Ni in the second embodiment of the present invention is 0.01% or more, preferably 0.02% or more, 1% from the viewpoints of the above-described corrosion resistance improvement (particularly, formation of a dense surface rust coating) and deterioration of weldability and hot workability. Hereinafter, Preferably it is 0.9% or less.

The basic components of the steel for crude oil tank bottom plate of the second embodiment of the present invention are as described above, and the balance is substantially iron and inevitable impurities (for example, P, S, O, N, H, Mo, W, etc.). It is done. However, the unavoidable impurity needs to be in an amount so as not to impair the characteristics of the steel. The steel material of the second embodiment of the present invention is also effective to contain the following optional components and the like, in addition to the above components, if necessary, and the characteristics of the steel can be further improved depending on the type of the components to be contained.

<Mg: at least one element selected from the group consisting of 0.005% or less, Sr: 0.005% or less, and Ba: 0.005% or less>

Mg, Sr and Ba are selectable elements effective for improving corrosion resistance. As described above, in order to improve the corrosion resistance, in particular to suppress the pH decrease, Mg, Sr, and Ba, respectively, preferably 0.0001% or more, more preferably, in the steel of the second embodiment of the present invention. It is recommended to contain in an amount of 0.0005% or more. On the other hand, it is recommended to adjust Mg, Sr, and Ba amount to 0.005% or less, preferably 0.004% or less, respectively, from the viewpoint of workability and weldability deterioration mentioned above.

<Zr: 0.2% or less and / or Hf: 0.2% or less>

Zr and Hf are selection elements effective for improving corrosion resistance. As described above, Zr and Hf are each preferably 0.005% or more, more preferably 0.008, in the steel materials of the second embodiment of the present invention in order to improve corrosion resistance, particularly to stabilize the surface rust coating. It is recommended to contain in an amount of% or more. On the other hand, it is recommended to adjust Zr and Hf amount to 0.2% or less, Preferably it is 0.15% or less when it contains these from a viewpoint of weldability and workability deterioration mentioned above.

<Co: 1% or less>

Co is a selection element effective for improving corrosion resistance. As described above, in order to improve the corrosion resistance, particularly to form a dense surface rust coating, the amount of Co in the steel of the second embodiment of the present invention is preferably 0.01% or more, more preferably 0.02% or more. It is recommended to contain as. On the other hand, when it contains from a viewpoint of weldability and hot workability deterioration, it is recommended to adjust Co amount to 1% or less, Preferably it is 0.8% or less.

At least one selected from the group consisting of <La: 0.01% or less, Ce: 0.01% or less, Nd: 0.01% or less, Sm: 0.01% or less (not including O%), and Se: 0.1% or less Element of>

La, Ce, Nd, Sm, and Se are all selective elements effective for improving corrosion resistance. As described above, La, Ce, Nd, and Sm, respectively, are preferably 0.0001% in the steel materials of the second embodiment of the present invention in order to improve the corrosion resistance, particularly to promote the action of Ca or Ti. As mentioned above, More preferably, it is recommended to contain Se in the quantity of 0.0005% or more, Preferably it is 0.001% or more, More preferably, it is 0.002% or more. On the other hand, from the viewpoint of workability and weldability deterioration mentioned above, when these are contained, the amount of La, Ce, Nd and Sm is respectively 0.01% or less, preferably 0.008% or less, and the Se amount is 0.1% or less, preferably. Preferably, it is recommended to adjust to 0.05% or less.

<At least one element selected from the group consisting of Sb: 0.2% or less, Bi: 0.2% or less, and Te: 0.1% or less>

Sb, Bi and Te are elements effective for improving corrosion resistance. The description of these amounts is the same as that of the first embodiment of the present invention described above.

<B: at least one element selected from the group consisting of 0.01% or less, V: 0.5% or less, and Nb: 0.5% or less>

B, V, and NB are all selective elements effective for improving the strength. In order to improve the strength, B is preferably used in the steel of the second embodiment of the present invention in an amount of 0.0001% or more, more preferably 0.0003% or more, preferably 0.01% or more, and more preferably It is recommended to contain NB in an amount of 0.02% or more, and NB in an amount of preferably 0.003% or more, more preferably 0.005% or more. On the other hand, from the viewpoint of the above-mentioned toughness deterioration, when it contains, B amount is 0.01% or less, More preferably, it is 0.009% or less, V amount is 0.5% or less, More preferably, 0.45% or less, Nb amount It is recommended to adjust to 0.5% or less, preferably 0.45% or less.

<Ta: 0.5% or less>

Ta is a selection element effective for grain refinement. The description of this amount is the same as that of the first embodiment of the present invention described above.

As a preferable selection element, At least 1 sort (s) chosen from the group which consists of Mg and Sr, At least 1 sort (s) chosen from the group which consists of Zr and Hf; Co and; At least 1 sort (s) chosen from the group which consists of La, Ce, Nd, and Sm, and at least 1 sort (s) chosen from the group which consists of B, V, and Nb.

Moreover, in the steel for crude oil tank bottom plate of 2nd aspect of this invention, the composition ratio of a chemical component, specifically, as mentioned above, S (c) / S (a) ratio and S (c) / S (a) ratio By controlling appropriately, the corrosion resistance of steel materials can be improved further. The description of the S (c) / S (a) ratio and the S (c) / S (a) ratio in the second embodiment of the present invention is the same as described above.

The steel for crude oil tank bottom plate of 2nd aspect of this invention is one of the characteristics that the area ratio of a metal structure is appropriately controlled in order to improve corrosion resistance. The description regarding the area ratio of the metal structure in the 2nd Embodiment of this invention is the same as that of the 1st Embodiment mentioned above.

The steel material for crude oil tank bottom plates of 2nd aspect of this invention is one of the characteristics that the nonmetallic inclusion which exists in steel materials is suppressed in order to improve corrosion resistance. This nonmetallic inclusion may be a starting point of local corrosion. Here, the "non-metallic inclusion" in the second embodiment of the present invention means sulfides such as MnS, oxides such as Al ₂ O ₃ or SiO ₂ and nitrides such as TiN.

In the environment of a crude oil tank bottom plate, large nonmetallic inclusions having a circle equivalent diameter exceeding 20 µm are particularly harmful, and the number of the inclusions is 0.22 / mm2 or less, more preferably 0.11 / mm2 or less. It is recommended.

In the second embodiment of the present invention, the number per unit area of the nonmetallic inclusions exceeding 20 μm is greater than 6 mm when the thickness of the steel is 6 mm or more. Is a number per unit area of the inclusions obtained by observing with an optical microscope at an observation magnification of 400 times the principle and an observation field of 150 μm × 200 μm or more at a half of the steel thickness, and obtained at an arbitrary 30 field of view. The average value of is adopted. In addition, the surface used as a reference | standard of depth means the surface of the steel material to which the force was applied in rolling. Moreover, the observation method of an optical microscope conforms to "the microscope test method of the nonmetallic inclusion of steel" described in JIS G0555.

The steel for crude oil tank bottom plate of 2nd Embodiment of this invention can be manufactured by the following method, for example. First, by using general solvent methods such as converters and electric furnaces, the solvent is formed while adjusting the chemical composition so as to satisfy the above range, and the steel is cast into a general casting method such as a continuous casting method or an ingot method. Moreover, as a deoxidation type, it is preferable to use a kicked steel from a mechanical characteristic or a weldability viewpoint, More preferably, Al kicked steel is recommended. The reduction of the number per unit area of the nonmetallic inclusion having a circular equivalent diameter of more than 20 µm to 0.22 / mm2 or less may be achieved by, for example, an external furnace refining such as a vacuum degassing method such as the DH method or the RH method. Can be achieved.

After heating the obtained steel ingot in the temperature range of 1100-1200 degreeC, it is preferable to hot-roll and set it as desired dimension shape. At this time, by controlling the hot rolling end temperature to 750 to 850 ° C, and controlling the cooling rate from after the end of hot rolling to 500 ° C in the range of 0.5 to 15 ° C / sec, pearlite: 5 to 25%, bainite: 20 It is less than%, martensite: less than 10%, and the remainder can obtain the steel material of the structure which consists of ferrite.

Since the steel for crude oil tank bottom plate of the 2nd aspect of this invention is excellent in both local corrosion resistance and corrosion resistance of a coating, it is a tank bottom plate of the crude oil tanker of a double split structure, especially the crude oil tank side is naked (no treatment), It is preferable to coat and use a ballast tank side. However, the crude oil tank side may also process zinc rich paint and shop primer for the purpose of initial rust prevention as needed. As a paint on the ballast tank side, a tar epoxy resin paint, a modified epoxy resin paint, or other typical heavy anticorrosive paint etc. are mentioned. Moreover, you may use the steel material for crude oil tank bottom plates of this invention in combination with other anticorrosive methods, such as an electric system (dielectric anode method and an external power supply method).

Next, a third embodiment of the present invention will be described in detail. The third embodiment of the present invention relates to steel materials used in ballast tanks, particularly among ship steels. It is an object of the third embodiment of the present invention to provide a ballast tank steel material and a ballast tank which contribute to reduction of economic losses such as maintenance cost of a ship and extension of a dog period (time loss) and further improvement in ship safety. In addition, in the third embodiment of the present invention, a ballast effective for improving durability at a strict site as a corrosion environment at a high temperature such as the upper side of the upper deck where the electric system does not work (not submerged in seawater even in ballast), a heating part in the dropping, or the vicinity of the engine, etc. To provide steel and ballast tanks for tanks.

The third embodiment of the present invention, which has achieved the above object,

C: 0.01 to 0.30%,

Si: 0.01 to 2.0%,

Mn: 0.01 to 2.0%,

P: 0.01% or less,

S: 0.0005% to 0.005%,

Al: 0.005% to 0.10%,

Cu: 0.1% to 1.0%,

Ni: 0.01 to 1.0%,

Cr: 0.01 to 0.5%

Containing a residual portion consisting of Fe and an unavoidable impurity, mainly composed of ferrite, having an area ratio of bainite and / or martensite of less than 30% (including 0) in total, and an average of sulfide-based inclusions It is a steel material for ballast tanks excellent in corrosion resistance, characterized by having a particle diameter of 0.5 to 10 µm.

In the steel material for ballast tank of the 3rd aspect of this invention, in addition to the said component, (1) Mg: 0.0001-0.005%, Ca: 0.0001-0.005%, Sr: 0.0001-0.005%, Ba: 0.0001 as needed To 0.005%, La: 0.0001 to 0.005%, Ce: 0.0001 to 0.005%, Nd: 0.0001 to 0.005%, Sm: 0.0001 to 0.005%, and Se: 0.001% to 0.1% One or more or two or more selected from the group consisting of (2) Co: 0.005 to 0.20%, Ti: 0.005 to 0.20%, Zr: 0.005 to 0.20%, and Hf: 0.005 to 0.20%, (3) One or two or more selected from the group consisting of Sb: 0.005 to 0.2%, Bi: 0.005 to 0.2%, and Te: 0.001 to 0.1%, (4) B: 0.0001 to 0.010%, V: 0.0l to 0.50 % And Nb: one or two selected from the group consisting of 0.003 to 0.50% It is also effective to contain more than a kind or (5) Ta: 0.003 to 0.5%, and the like, and the properties of the steel are further improved depending on the kind of the component to be contained. The steel material for ballast tanks of 3rd Embodiment of this invention may satisfy 2 or more of these requirements (1)-(5) simultaneously.

In the steel for ballast tanks of 3rd aspect of this invention, it is preferable to form the anticorrosive coating film directly on the steel material surface. Moreover, in the 3rd aspect of this invention, the ship which has a ballast tank comprised by the steel material for ballast tanks mentioned above is also provided.

As a result of intensive studies, the present inventors have found that, in addition to the components of C, Si, Mn, and Al, the present inventors appropriately adjust the additive elements such as Cu and Ni, optimize the state of the structure and inclusions, and also directly control the steel materials. By coating the paint, it was found that the above object of the third embodiment of the present invention can be achieved.

In the anticorrosive coating film, chemical substances causing corrosion such as moisture, salt, oxygen and the like penetrate the coating film, and steel corrosion starts to occur under the coating film. By using the steel of the third embodiment of the present invention in a ballast tank environment, the steel corrosion generated under the anticorrosive coating film can be effectively suppressed, so that the expansion pressure of the corrosion product is smaller than that of conventional steel, and the expansion of the anticorrosive coating film This becomes less likely to occur, resulting in the life of the anticorrosive coating film. In addition, even when defects or scratches such as pinholes are present in the anticorrosive coating film and the steel materials are exposed, the steel materials of the third embodiment of the present invention have a small corrosion propagation rate at the exposed portion, and therefore, the steel sheet is subjected to enormous corrosion damage such as perforation of steel sheets. It's hard to reach. Below, limitation of the component range of the steel materials of 3rd Embodiment of this invention is demonstrated.

<C: 0.01 to 0.30%>

C is an element necessary for securing the strength of the material. It is necessary to contain 0.01% or more in order to ensure the lowest strength (although the thickness of steel, but usually about 400 MPa) as a structural member of the petroleum tank. From the viewpoint of securing such strength and deterioration of toughness described above, the amount of C in the third embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.04% or more, and 0.30% or less, preferably It is 0.28% or less, More preferably, it is 0.26% or less.

<Si: 0.01% to 2.0%>

Si amount of the third embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more, and 2.0% or less, from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Preferably it is 1.80% or less, More preferably, it is 1.60% or less.

<Mn: 0.01 to 2.0%>

The amount of Mn in the third embodiment of the present invention is 0.01% or more, preferably 0.05% or more, more preferably 0.10% or more, and 2.0% or less, from the viewpoints of deoxidation and strength securing and toughness deterioration described above. Preferably it is 1.80% or less, More preferably, it is 1.60% or less.

<P: 0.01% or less>

P is an element which improves seawater resistance by addition of 0.02% or more. However, P is an element which degrades toughness and weldability, and it is preferable to suppress content as much as possible. In the third embodiment of the present invention, the seawater resistance improvement effect of P is not used with emphasis on toughness and weldability, and the allowable upper limit of P is set to 0.01%.

<S: 0.0005 to 0.005%>

S is also an element that degrades toughness and weldability, and it is preferable to suppress the content as much as possible. The upper limit to which S is permissible is up to 0.005%, and when it exceeds this, weldability as a steel material for ballast tanks cannot be ensured. Therefore, in the 3rd Embodiment of this invention, S amount was made into 0.005% or less.

<Al: 0.005 to 0.10%>

Al amount of the 3rd aspect of this invention is 0.005% or more, Preferably it is 0.010% or more, More preferably, it is 0.015% or more from a viewpoint of deoxidation, the strength ensured, and weldability deterioration mentioned above, 0.10% or less, Preferably it is 0.090% or less, More preferably, it is 0.080% or less.

<Cu: 0.01 to 1.0%>

Cu is an element effective for improving corrosion resistance. Cu has the effect of suppressing the corrosion reaction occurring under the anticorrosive coating film, and is an element having the effect of suppressing the corrosion under the coating film due to the corrosion under the coating film that is likely to occur in the thin film portion of the coating or the like. Moreover, in a coating film defect part, it also has the effect | action which densifies product rust when a steel material receives corrosion, and is an element effective also in expressing the effect which suppresses corrosion progression of a coating film scratch part. In order to exert these effects, the amount of Cu in the third embodiment of the present invention is 0.01% or more, preferably 0.05% or more. On the other hand, in order to prevent the above-mentioned deterioration of weldability and hot workability, Cu amount is 1.0% or less, Preferably it is 0.90% or less.

<Ni: 0.01 to 1.0%>

Ni is effective for improving corrosion resistance. Ni, like Cu, is an element having the effect of suppressing the coating film expansion due to corrosion under the coating film, and the effect of suppressing the corrosion progress of the coating film scratch. In addition, Ni is an element necessary also for preventing the red heat brittleness by addition of Cu. In order to exert such an effect, the Ni amount of the third embodiment of the present invention is 0.01% or more, preferably 0.05% or more. On the other hand, in order to prevent the above-mentioned deterioration of weldability and hot workability, Ni amount is 1.0% or less, Preferably it is 0.90% or more.

<Cr: 0.01 to 0.5%>

Cr is an element effective for improving corrosion resistance. Cr has an effect of suppressing primer consumption under an anticorrosive coating film, and is also an effective element for expressing an effect of suppressing corrosion progression of a scratch on the coating film. In addition, an appropriate amount of Cr is effective for improving toughness, and is also an element necessary for obtaining mechanical properties required as a ballast tank material. In order to exert these effects, the amount of Cr in the third embodiment of the present invention is 0.01% or more, preferably 0.05% or more. On the other hand, in order to prevent deterioration of weldability and hot workability mentioned above, Cr amount is 0.5% or less, Preferably it is 0.45% or less.

The ballast tank steel of the third embodiment of the present invention is effective to contain the following optional components in addition to the above components, if necessary, and the characteristics of the steel can be further improved depending on the type of components to be contained.

<Mg: 0.0001 to 0.005%, Ca: 0.0001 to 0.005%, Sr: 0.0001 to 0.005%, Ba: 0.0001 to 0.005%, La: 0.0001 to 0.005%, Ce: 0.0001 to 0.005%, Nd: 0.0001 to 0.005%, 1 or 2 or more types selected from the group consisting of Sm: 0.0001 to 0.005%, and Se: 0.001% to 0.1%>

Mg, Ca, Sr, Ba, La, Ce, Nd, Sm, and Se are selection elements effective for improving corrosion resistance. These exhibit the effect of suppressing the promotion of corrosion due to the pH decrease and are effective in suppressing the coating film expansion. Therefore, Mg, Ca, Sr, Ba, La, Ce, Nd and Sm are preferably in the amount of 0.0001% or more, more preferably 0.0005% or more, in the steel of the third embodiment of the present invention. Preferably it is contained in an amount of 0.001% or more, more preferably 0.002% or more. On the other hand, from the viewpoint of workability and weldability deterioration mentioned above, when it contains, Mg, Ca, Sr, Ba, La, Ce, Nd and Sm amount is 0.005% or less, Preferably it is 0.004% or less, Se amount , 0.1% or less, preferably adjusted to 0.05% or less.

<1 or 2 or more types selected from the group consisting of Co: 0.005 to 0.20%, Ti: 0.005 to 0.20%, Zr: 0.005 to 0.20%, and Hf: 0.005 to 0.20%>

Co, Ti, Zr and Hf are selection elements effective for improving corrosion resistance. Specifically, in the chloride corrosion environment, Co has a function of densifying rust generated, Ti, Zr, and Hf have a function of stabilizing rust, and are all elements that suppress corrosion progress in coating film scratches. Therefore, Co, preferably 0.005% or more, more preferably 0.02% or more of Ti, Zr and Hf, preferably 0.005% or more, and more preferably 0.008, in the steel of the third embodiment of the present invention. It is recommended to contain in an amount of% or more. On the other hand, from the viewpoint of the above-described deterioration of weldability and workability, when it is contained, the amount of Co is 020% or less, preferably 0.18% or less, and the amount of Ti, Zr and Hf is 0.20% or less, preferably 0.15% It is recommended to adjust to the following.

<1 or 2 or more types selected from the group consisting of Sb: 0.005 to 0.2%, Bi: 0.005 to 0.2%, and Te: 0.001 to 0.1%>

Sb, Bi and Te are selection elements effective for improving corrosion resistance. The description of these amounts is the same as that of the first embodiment of the present invention described above.

<B: 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50% of at least one kind or at least two kinds selected from the group>

B, V and NB are selection elements effective for improving mechanical properties (strength). In order to improve the strength, in the steel of the third embodiment of the present invention, B is preferably 0.0001% or more, more preferably 0.0003% or more, and V is preferably 0.01% or more, and more preferably It is recommended to contain NB in an amount of 0.02% or more, preferably 0.003% or more, more preferably 0.005% or more. On the other hand, in view of the above-mentioned toughness degradation, when it contains, B amount is adjusted to 0.010% or less, preferably 0.0090% or less, and V and Nb amounts are respectively adjusted to 0.50% or less, preferably 0.45% or less. It is recommended.

<Ta: 0.003 to 0.5%>

Ta is a selection element effective for grain refinement. The description regarding this quantity is the same as that of the 1st Embodiment of this invention mentioned above.

As a preferable selection element, 1 type, or 2 or more types chosen from the group which consists of Mg, Ca, and Sr; One or two or more selected from the group consisting of Co, Ti, and Zr; 1 type, or 2 or more types selected from the group which consists of B, V, and Nb is mentioned.

The components of the steel for ballast tanks of the 3rd aspect of this invention are as above, and remainder consists of iron and an unavoidable impurity. As an unavoidable impurity element, O, N, H, Mo, W, etc. are mentioned, for example, It is preferable not to exceed 0.1%, and what does not exceed 0.01% is also recommended.

<Organization>

As for the structure of the steel material by 3rd aspect of this invention, it is preferable to make ferrite excellent in weldability and workability as a main body (area 50% or more in total). Further, ferrite is advantageous as a structural member in a chloride environment called seawater because of its low susceptibility to stress corrosion cracking by chlorides. The steel of the present invention is required to control the area ratio of the other, bainite and / or martensite mainly based on ferrite. The steel material of the present invention is also allowed to include a structure other than these ferrites, bainite and martensite, such as pearlite, as part of its configuration.

As described above, bainite and / or martensite adversely affect corrosion resistance, and according to the third embodiment of the present invention, the area ratio of bainite + martensite is less than 30% (either bainite or martensite, or It is recommended to include the case where both are zero). In addition, the meaning and the measuring method of a structure area ratio are the same as the above-mentioned.

<Sulfide inclusions>

As the non-metallic inclusions, inclusions such as sulfides, oxides, nitrides or carbides are present in the steel. Among these, sulfide-based inclusions such as MnS are the starting point of corrosion, and thus are the most harmful inclusions from the viewpoint of corrosion resistance as steel materials for ballast tanks. In the case where the size and number of sulfide-based inclusions are controlled, when the size is increased and the number is reduced, significant local corrosion starting from it occurs, leading to early drilling and the like. On the contrary, when the size is made small and the number is increased, the corrosion starting point under the anticorrosive coating film increases, which promotes the expansion of the anticorrosive coating film. In the steelmaking process, it was found that the above-mentioned adverse effects can be minimized by reducing the sulfide inclusions by using an appropriate desulfurization apparatus and adjusting the size. Specifically, the harmfulness can be minimized by setting the average particle diameter (circle equivalent diameter) of the sulfide inclusions to 0.5 to 10 m.

The sulfide inclusions defined in the third embodiment of the present invention are adjusted to the above-described component range at the time of secondary refining, and the ratio of Mn / (S + Mg + Ca + Sr) is 10 As mentioned above, it can obtain by bubbling by inert gas, such as Ar, and making it fully stir, making component adjustment so that it may become less than 700. FIG. Mn has a function of inhibiting nucleation of sulfide inclusions, and S, Mg, Ca, and Sr, on the contrary, have a function of promoting formation of the inclusion. For this reason, the ratio of Mn / (S + Mg + Ca + Sr) is related to the size of the sulfide inclusions, and if the ratio is large, nucleation of the sulfide inclusions is suppressed and the inclusions tend to coarsen. In this way, the average particle diameter of the sulfide inclusions can be controlled by adjusting the ratio.

In the third embodiment of the present invention, when the thickness of the sulfide inclusions is 6 mm or more, the average particle diameter of the steel is 1 mm of the thickness of the steel when the thickness of the steel is less than 6 mm in a 3 mm deeper portion than the surface. At the site | part of / 2, the round equivalent diameter of arbitrary 30 sulfide type interference | inclusions is measured and the average value obtained is employ | adopted. The measurement of the equivalent circular diameter of the sulfide-based inclusions can be obtained by observing a mirror-polished sample with a scanning electron microscope (SEM) at an appropriate magnification of about 1000 to 5000 times, and performing image analysis or the like.

The sulfide inclusions in the third embodiment of the present invention, as illustrated in FIG. 1, compare the intensities of peaks corresponding to S, O, N and C in the EDX spectrum, and thus the peak intensity of S. Is the sulfide-based inclusion. The EDX analysis can be performed at the time of SEM observation for measuring the average particle diameter, and the sulfide-based inclusions can be confirmed.

<Manufacturing method>

The steel material of 3rd Embodiment of this invention can be manufactured, for example by the following method. Secondary refining including a component adjustment and a temperature adjustment is performed with respect to the molten steel which went out to the ladle from the converter or the electric furnace using the RH vacuum degassing apparatus. In order to control the sulfide-based inclusions, it is necessary to carry out bubbling with an inert gas such as Ar at the time of secondary refining to sufficiently stir the molten steel. After that, the steel is formed by a general casting method such as a continuous casting method or an ingot method. Moreover, as a deoxidation type, it is preferable to use a kicked steel from a mechanical characteristic or a weldability viewpoint, More preferably, Al kicked steel is recommended.

After heating the obtained steel ingot in the temperature range of 1100-1200 degreeC, it is preferable to hot-roll and set it as desired dimension shape. At this time, predetermined structure can be obtained by controlling hot rolling finish temperature to 700-850 degreeC, and controlling the cooling rate from after completion | finish of hot rolling to 500 degreeC in the range of 0.1-15 degreeC / sec.

<Anticorrosive paint>

In the period from the time of shipment from the steel mill to the application of the anticorrosive coating, primers such as zinc rich primers are usually applied for the purpose of preventing rust generation of steel materials. Prior to the application of the anticorrosive paint, it is common to purify the surface to be coated with a wire brush, sand pepper, or the like for the purpose of removing rust, oil, and the like, which have occurred in primer defects that adversely affect coating. The steel material for ballast tanks of 3rd aspect of this invention removes the said primer and remaining black skin (mill scale) before the coating of an epoxy resin anticorrosive coating, and is effective for improving corrosion resistance by apply | coating a paint directly to steel materials. Since an alloying element directly acts on a corrosion reaction, the effect of improving paint corrosion resistance can be obtained further. As a method of removing a primer and the like, methods such as sand blast, shot blast and grinding can be appropriately selected in consideration of workability and the like.

As the anticorrosive paint to be applied to the steel for ballast tank of the third embodiment of the present invention, epoxy resin paints such as tar epoxy resin paints and modified epoxy resin paints are preferable, but urethane resin paints, acrylic resin paints, and the like, as apparent from the effect thereof. Even when the paint is used, the film-life effect of the coating film appears. Moreover, it does not impair the effect of the other system methods, such as an electrical system (dielectric anode method, an external power supply method), and can also use together.

Next, the 2nd basic aspect (4th embodiment) of this invention is demonstrated in detail. In the fourth embodiment of the present invention, the so-called gap corrosion proceeds rapidly, particularly in the "gap" part caused by contact between foreign matter and steel, structural causes, or damage to the anticorrosive coating film, thereby greatly shortening the lifespan. The conventional anticorrosive technique is made by paying attention to the situation which cannot be said to satisfy the corrosion resistance to such a corrosion part. Accordingly, an object of the fourth embodiment of the present invention is to show a level of corrosion resistance that can be used in a high corrosion environment even in the absence of anticorrosive coating or electrical coating. In particular, it has excellent durability against gap corrosion and at the same time, salt adhesion and wetting due to seawater. It is to provide a marine steel material which exhibits excellent anticorrosion durability against corrosion caused by repetition of the environment. Especially, as a raw material for tankers which transport petroleum fuels, such as crude oil, even when used as structural materials, such as a tank which directly contacts with petroleum fuels, such as crude oil, the steel material for ships which show the outstanding corrosion resistance is provided.

The steel material for ships according to the fourth embodiment of the present invention, which was able to achieve the above object,

C: 0.01 to 0.30%,

Si: 0.01 to 2.0%,

Mn: 0.01 to 2.0%,

Al: 0.005% to 0.10%,

S: 0.010% or less

Each containing, furthermore,

a group element (one or more selected from Mg, Ca, Sr, and Ba): 0.0005% to 0.020%

b group element (1 or more types chosen from Ti, Zr, and Hf): It contains 0.005 to 0.20%, and the ratio of the total content S (a) of the said group a element to the total content S (b) of the b group element In the range of S (a) / S (b) in the range of 0.01 to 1, the average particle diameter of the sulfide inclusions and the oxide inclusions is 1 to 1, with the remainder being made of steel of Fe and unavoidable impurities, and also satisfying the following requirements. The gist exists in the part which is 10 micrometers, and there exist 20-200 each of these in 1 mm <2> of a cross section of a rolling direction.

Sulfide inclusions: sulfide inclusions having a circle equivalent diameter of at least 0.5 µm containing 1 to 30% of the total amount of the group a elements and / or 1 to 30% of the total amount of the group b elements;

Oxide inclusions: An oxide inclusion having a circular equivalent diameter of 0.5 µm or more containing 1 to 30% of the total amount of the group a elements and / or 1 to 30% of the total amount of the group b elements.

The ship steel according to the fourth embodiment of the present invention is, as another element, (1) Cr: 0.01 to 5.0%, Cu: 0.01 to 5.0%, Ni: 0.01 to 5.0%, and Co: 0.01 to 5.0% At least one selected from the group consisting of (2) La: 0.0001 to 0.005%, Ce: 0.0001 to 0.005%, Nd: 0.0001 to 0.005%, Sm: 0.0001 to 0.005%, and Se: 0.001% to 0.1% At least one selected from the group consisting of: (3) at least one selected from the group consisting of Sb: 0.005 to 0.2%, Bi: 0.005 to 0.2%, and Te: 0.001 to 0.1%, and (4) B: 0.0001 to It may contain at least one selected from the group consisting of 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%, or (5) Ta: 0.003 to 0.5%. The steel material for ships which concerns on 4th aspect of this invention may satisfy | fill two or more of these requirements (1)-(5) simultaneously.

And the steel materials for ships of 4th Embodiment of this invention make the most effective use as a tank material of a crude oil tanker, taking advantage of the outstanding corrosion resistance.

According to the marine steel of the fourth embodiment of the present invention, the composition of steel is appropriately adjusted, and in particular, a predetermined group a element (one or more of Mg, Ca, Sr, and Ba) and group b element (Ti, Zr, Anticorrosive coating by containing an appropriate amount of one or more of Hf), adjusting the content ratio, and controlling the size and number of sulfide inclusions and oxide inclusions containing these a-group elements and b-group elements within a predetermined range. In addition, it is possible to provide a ship steel material that exhibits sufficient corrosion resistance even without performing electrical methods, and in particular, improves durability against interstitial corrosion, and also under severe corrosion environments caused by salt adhesion caused by seawater and repeated wet environments. It is possible to provide a ship steel material exhibiting excellent anticorrosion durability. The steel for ships of the 4th aspect of this invention provided with such a characteristic is useful as materials, such as a shell plate and a ballast tank, in ships, such as a crude oil tanker, a cargo ship, a ship, a passenger ship, and a warship. Very useful as a material.

The present inventors have earnestly studied on the basis of the desired achievement as described above. As a result, an appropriate amount of a-group element (at least one of Mg, Ca, Sr, and Ba) and b-group element (at least one of Ti, Zr, and Hf) having a pH lowering inhibitory action or a stabilizing action of produced rust is contained. By controlling their content ratio appropriately, it is possible to adjust the size and number of sulfide inclusions and oxide inclusions containing these elements well, thereby obtaining a high-performance marine steel material that can solve the above problems. The fourth embodiment of the present invention has been completed.

Moreover, in the 4th Embodiment of this invention, in order to satisfy the basic characteristics as steel materials, it is necessary to adjust basic components, such as C, Si, Mn, and Al suitably, First, the reason for limitation of these basic components is revealed.

<C: 0.01 to 0.30%>

C is an important element in order to secure the structural strength of the steel, and in order to secure the minimum strength (although it is about 400 MPa depending on the thickness of the steel) as a structural member of the ship, it is necessary to contain C 0.01% or more. From the viewpoint of securing such strength and deterioration of the toughness described above, the amount of C in the fourth embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.04% or more, and preferably 0.30% or less. It is 0.28% or less, More preferably, it is 0.26% or less.

<Si: 0.01% to 2.0%>

Si amount of the fourth embodiment of the present invention is 0.01% or more, preferably 0.02% or more, more preferably 0.05% or more, and 2.0% or less, from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Preferably it is 1.5% or less, More preferably, it is 1.0% or less.

<Mn: 0.01 to 2.0%>

The amount of Mn of the fourth embodiment of the present invention is 0.01% or more, preferably 0.05% or more, more preferably 0.10% or more, and 2.0% or less, from the viewpoints of deoxidation and strength securing and toughness deterioration described above. Preferably it is 1.80% or less, More preferably, it is 1.60% or less.

<Al: 0.005 to 0.10%>

The amount of Al in the fourth embodiment of the present invention is 0.005% or more, preferably 0.010% or more, more preferably 0.015% or more, and 0.10% or less, from the viewpoints of deoxidation and strength securing and weldability deterioration described above. Preferably it is 0.050% or less, More preferably, it is 0.040% or less.

<S: 0.010% or less>

In the fourth embodiment of the present invention, as described above, it is recommended to adjust the amount of S harmful to toughness, weldability and the like to 0.1% or less, preferably 0.008% or less.

<a group element (at least one selected from Mg, Ca, Sr, and Ba): 0.0005% to 0.020%>

In the 4th Embodiment of this invention, content of these elements contained in group a is 0.0005% or more in total from a viewpoint of the above-mentioned corrosion resistance improvement (especially suppression of pH fall), and moldability and weldability deterioration. Preferably it is 0.0008% or more, 0.020% or less, Preferably it is 0.015% or less.

<b group element (at least 1 type chosen from Ti, Zr, Hf): 0.005 to 0.20%>

In the 4th Embodiment of this invention, content of these elements contained in group b is 0.005% or more in total from a viewpoint of the above-mentioned corrosion resistance improvement (especially stabilization of surface rust film), and workability and weldability deterioration. Preferably it is 0.008% or more, 0.20% or less, Preferably it is 0.15% or less.

In the fourth embodiment of the present invention, in order to synergistically exhibit the pH lowering inhibitory action by the a-group element and the stabilization effect of the surface rust coating by the b-group element, the total content S of the a-group element (a ) And the ratio of the total content S (b) of the group b elements are appropriately controlled. As mentioned above, in order to improve comprehensive corrosion resistance, the S (a) / S (b) ratio in 4th aspect of this invention is 0.01 or more, Preferably it is 0.02 or more, Preferably it is 1 or less, Preferably it is 0.8 It is as follows.

The essential constituent elements of the ship steel according to the fourth embodiment of the present invention are as described above, and the remaining portion is inevitable to be mixed from steelmaking raw materials (iron ore, secondary raw materials, scraps, etc.) or manufacturing processes (for example, P, O, N, W, Mo, etc.), but these also contain a trace amount as long as they do not impair the characteristics of steel materials. However, since too many of these allowable components will adversely affect toughness etc., even if it is a large amount, it is for suppressing to 0.5% or less, More preferably, about 0.1% or less.

In the fourth embodiment of the present invention, the physical properties can be further improved by actively containing the selected element as shown next as another element. Hereinafter, they supplement with respect to their selection element and the addition effect.

<At least one selected from the group consisting of Cr: 0.01 to 5.0%, Cu: 0.01 to 5.0%, Ni: 0.01 to 5.0%, and Co: 0.01 to 5.0%>

Cr, Cu, Ni, and Co are all selective elements which contribute to the corrosion resistance improvement of steel materials. As described above, in order to improve the corrosion resistance, particularly to form a dense surface rust coating, or to promote the formation thereof, Cr, Cu, Ni, and Co, respectively, are used in the steel materials of the fourth embodiment of the present invention. Preferably, it is recommended to contain in 0.01% or more, More preferably, in the amount of 0.05% or more. On the other hand, it is recommended to adjust Cr, Cu, Ni, and Co amounts to 5.0% or less, preferably 4.50% or less, respectively, from the viewpoint of the above-described weldability and hot workability deterioration.

<At least one selected from the group consisting of La: 0.0001 to 0.005%, Ce: 0.0001 to 0.005%, Nd: 0.0001 to 0.005%, Sm: 0.0001 to 0.005%, and Se: 0.001% to 0.1%>

La, Ce, Nd, Sm, and Se are selection elements effective for improving corrosion resistance. The description of these amounts is the same as that of the first embodiment of the present invention described above.

<Sb: 0.005 to 0.2%, Bi: 0.005 to 0.2%, and Te: at least one selected from the group consisting of 0.001 to 0.1%>

Sb, Bi and Te are selection elements effective for improving corrosion resistance. The description of these amounts is the same as that of the first embodiment of the present invention described above.

<B: at least one kind selected from the group consisting of 0.0001 to 0.010%, V: 0.01 to 0.50%, and Nb: 0.003 to 0.50%>

B, V, and NB are all selective elements which contribute to further improvement of strength. In order to improve such strength, B is preferably used in the steel of the fourth embodiment of the present invention in an amount of 0.0001% or more, more preferably 0.0003% or more, preferably 0.01% or more, and more preferably It is recommended to contain NB in an amount of 0.02% or more, preferably 0.003% or more, more preferably 0.005% or more. On the other hand, from the viewpoint of the above-mentioned toughness deterioration, when these are contained, the amount of B is 0.010% or less, preferably 0.0090% or less, and the V and Nb amounts are 0.50% or less, preferably 0.45% or less, respectively. It is recommended to adjust.

<Ta: 0.003 to 0.5%>

Ta is a selection element effective for grain refinement. The description regarding this quantity is the same as that of the 1st Embodiment of this invention mentioned above.

As a preferable selection element, At least 1 sort (s) chosen from the group which consists of Cr, Cu, Ni, and Co; At least 1 sort (s) chosen from the group which consists of B, V, and Nb is mentioned.

Although the above is the reason for limitation regarding the chemical component, in the fourth embodiment of the present invention, the sulfide-based inclusions and the oxide-based inclusions in the steel are contained in the predetermined group a and group b elements, and the size and number thereof are appropriately adjusted. Control is important for obtaining excellent corrosion resistance. Hereinafter, these inclusions are demonstrated.

In an aqueous environment containing chloride ions, sulfide-based inclusions are eluted in water one after another to become the starting point of corrosion. By the way, when the sulfide inclusions contain a predetermined amount of the a-group element and the b-group element, it was found that dissolution of the sulfide-based inclusions in an aqueous environment containing chloride ions can be suppressed. In a corrosive environment containing sulfur content (elemental sulfur, hydrogen sulfide gas, etc.) derived from petroleum, when a predetermined amount of the a-group element and the b-group element is contained in the oxide inclusion, the dissolution of the oxide inclusion is promoted. This promotes their elution in water and raises the pH of the aqueous environment. As a result, it was confirmed that the pH reduction caused by the hydrolysis reaction in the local anode in which iron was dissolved is suppressed, the corrosion reaction is suppressed, and it contributes to the improvement of corrosion resistance.

Therefore, in the fourth embodiment of the present invention, the sulfide inclusions and the oxide inclusions contain an appropriate amount of the elements included in the a and b groups to suppress the dissolution of the sulfide inclusions and to increase the pH by dissolving the oxide inclusions. Corrosion suppression effect accompanying them is exhibited relatively, and corrosion resistance is greatly improved.

In order to exert such an effect effectively, the sulfide inclusions and the oxide inclusions each contain 1 to 30% of the total amount of the group a elements and / or 1 to 30% of the total amount of the b group elements, and those sulfide inclusions. Although the equivalent circle diameter is 0.5 micrometer or more among peroxide inclusions, it is necessary to control average particle diameter and number to an appropriate range. Namely, in the fourth embodiment of the present invention, sulfide inclusions and oxide inclusions containing an appropriate amount of the a-group element and / or the b-group element, each having a circle equivalent diameter of 0.5 µm or more but an average particle diameter of 1 to 10 µm, respectively. In addition, it is very important that they are present in the range of 20 to 200 in each mm 2 of the steel cross section, in order to effectively exhibit the above characteristics and to exhibit high levels of corrosion resistance.

Moreover, about the said sulfide type and oxide type inclusions, the sample surface which mirror-polished the cross section of the test-provided steel to about 1 micrometer with diamond paste etc. was observed with the scanning electron microscope (SEM), and the energy dispersing type X Elemental analysis by preliminary analysis (EDX) confirmed the inclusion components of those inclusions, and the sulfide-based inclusions (for example, see FIG. 2) having the highest S (sulfur) content among the S, O, and N contained in the inclusions. ), And the one with the highest O (oxygen) content was classified as an oxide inclusion (see, for example, FIG. 3).

When the content of the a-group element or the b-group element contained in these sulfide-based inclusions and oxide-based inclusions is less than 1%, respectively, the effect of improving the corrosion resistance at the level intended in the fourth embodiment of the present invention is not exhibited. When the excess amount exceeds 30%, the mechanical properties of the steel are inferior. Therefore, the content of the a-group element and the b-group element included in these inclusions is set in the range of 1 to 30%, respectively. More preferable content is 2% or more, 28% or less, More preferably, they are 3% or more and 25% or less.

However, if the particle size of the sulfide inclusions is too small, the fine sulfide inclusions are dispersed as a whole, so that corrosion of the entire surface tends to occur and the corrosion resistance is lowered. On the contrary, if the particle size of the sulfide inclusions is too large, the mechanical properties of the steel will deteriorate. Moreover, when the particle size of an oxide type interference | inclusion is too small, the pH raise effect by melt | dissolution to an aqueous atmosphere will become inadequate, and pH fall due to the hydrolysis reaction in the local anode part which dissolves iron is not able to be suppressed, and it is satisfied. It is difficult to achieve the effect of improving fine corrosion resistance. On the contrary, when the particle size of the oxide inclusions is too large, the mechanical properties of the steel material deteriorate as in the case of the sulfide inclusions. From this, the sulfide inclusions and the oxide inclusions should be adjusted in the range of 1 to 10 mu m in the average particle diameter (average value of the circle equivalent diameter), and more preferably in the range of 2.0 to 9.0 mu m.

In addition, the particle size of these sulfide inclusions and oxide inclusions is mirror-polished (for example, about 1 micrometer with diamond paste) of the test-provided steel material, and is photographed by the scanning electron microscope (SEM). Photographs were taken by image analysis, and 100 arbitrary arbitrary diameters whose circle equivalent diameters were considered to be approximately 0.5 µm or more were selected from the respective inclusions, and their equivalent circle diameters were obtained, and the average value was taken as the average particle diameter.

Next, with respect to the number of these inclusions, if the number of sulfide inclusions is too small, the individual sulfide inclusions will become too large to deteriorate the mechanical properties of the steel, and the mechanical properties of the steel will deteriorate similarly even if the number itself is too large. . In addition, when the number of oxide-based inclusions is insufficient, the pH synergism by dissolution in the aqueous environment described above is lowered, and the pH reduction inhibitory effect due to hydrolysis reaction with the local anode where iron dissolution occurs is insufficient, resulting in insufficient corrosion resistance. If the number is too large, the mechanical properties of the steel deteriorate. Accordingly, in the fourth embodiment of the present invention, the number of sulfide inclusions and oxide inclusions having the size (circle equivalent diameter of 0.5 μm or more) observed per 1 mm 2 of the steel cross section is set to 200 to 2000, respectively. . More preferable number is 500-1500.

The number of sulfide inclusions and oxide inclusions described herein is obtained by performing a mirror polishing (for example, about 1 μm with diamond paste) on the cross section of the test-provided steel, and then applying the magnification surface by scanning electron microscope (SEM) to 400 magnification. It observed twice and calculated | required as an average value from the number of sulfide type interference | inclusions and oxide type interference | inclusion of the said size (circle equivalent diameter is 0.5 micrometer or more) observed in arbitrary 10 visual fields, respectively.

The ship steel of the 4th aspect of this invention adjusts chemical composition suitably as mentioned above, and (I) content ratio of group a element and group b element in steel S (a) / S (b) Excellent corrosion resistance is achieved by controlling the size and number of sulfide-based or oxide-based inclusions containing (II) a-group elements and b-group elements in a predetermined amount. Therefore, unlike the steel materials of the 1st basic aspect of this invention mentioned above, the ship steel materials of the 4th aspect of this invention, ie, the 2nd basic aspect of this invention, are not limited to metal structure.

Next, the manufacturing method of the said ship steel material which concerns on 4th aspect of this invention is demonstrated. The steel materials of the fourth embodiment of the present invention can be produced by the following method. That is, secondary refining including component adjustment and temperature adjustment is performed on the molten steel melted in the converter or the electric furnace by the conventional method and outgoing to the ladle using the RH vacuum degassing apparatus. In the 4th aspect of this invention, the a-group element and b-group element which comprise the said sulfide type interference | inclusion and oxide type interference | inclusion are added at the time of this secondary refining, and are adjusted so that it may become a predetermined component composition. In the secondary refining process, it is also possible to process using apparatuses other than RH, such as de-S process by LF (ladle refining) as needed. As a deoxidation type in secondary refining, it is preferable to use a kilted steel from a viewpoint of obtaining a steel material excellent in mechanical characteristics or weldability, More preferably, it is preferable to use Al-kilted steel. After secondary refining, it is ingot by general casting methods, such as a continuous casting method and an ingot method.

The steel ingot thus obtained is heated to a temperature range of 1100 to 1200 ° C, followed by hot rolling to obtain a desired dimensional shape. At this time, it is preferable that the end temperature of hot rolling shall be 750-850 degreeC, and to control the cooling rate of the temperature range to 500 degreeC after completion | finish of hot rolling in the range of 0.1-15 degreeC / sec.

In order to adjust the size and number of the sulfide inclusions and the oxide inclusions, which are important components in the fourth embodiment of the present invention, especially in carrying out the production method, the a group element and the b group It is important to add the element during secondary refining. When these elements are added at the time of secondary refining and the addition amount is adjusted so that the ratio of the group a element to the group b element is within a predetermined range, the group a element and the group b element can be contained in the sulfide inclusion and the oxide inclusion. The size and number of inclusions can be controlled to a suitable range. If component adjustment is performed by adding a-group element or b-group element before the secondary refining, too many inclusion components or coarse and coarse inclusions are easily generated, and the size and number of inclusions can be suppressed to the above-mentioned suitable range. There will be no.

In addition, it is also important to adjust the slab heating temperature before hot rolling to 1100-1200 degreeC. When heating temperature exceeds 1200 degreeC, even if a chemical component is appropriate, the number of inclusions will increase and it will become impossible to achieve the objective of 4th embodiment of this invention.

The marine steel material of the fourth embodiment of the present invention exhibits excellent corrosion resistance even if the steel material itself is not coated by the above characteristics. However, as shown in the later examples, the modified epoxy resin paint or the like may be used. It is also possible to use together with other anticorrosive methods, such as the various heavy anticorrosive coating, zinc rich paint, shop primer, and electric system which are represented. In the case where coating is applied in such a manner, as shown in the later examples, the corrosion resistance of the coating film itself (coating film scratch resistance) is also excellent.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not restrict | limited by the following example, It changes and implements suitably in the range which may be suitable for the meaning of the previous and the following. Of course, it is possible, and they are all included in the technical scope of this invention. In addition, in a later example, "%" means the "mass%" unless there is particular notice.

First Example (Examples of the Invention and Comparative Example of the First Aspect of the Present Invention)

<Production of Test Offering Material>

After performing converter converter, the slab which has a chemical component composition shown to following Table 1 and 2 was created by the continuous casting method. These slabs were hot rolled under the conditions shown in Tables 3 and 4 to produce steel sheets. In Tables 3 and 4, the heating temperature before hot rolling is "heating temperature", the hot rolling end temperature is "rolling end temperature", and the total reduction rate in hot rolling is 500 deg. C from the end of hot rolling. The cooling rate up to was described as "cooling rate".

The test member A of the size of 300x300x10 (mm) was produced by cutting and surface-grinding the steel plate obtained as mentioned above. The external shape of the test member A is shown in FIG. In addition, as shown in Fig. 5, four small test members of 60 x 60 x 5 (mm) are brought into contact with a large test member (the same as the test member A) of 300 x 300 x 10 (mm), The test member B in which the clearance gap was formed was produced. The small test member and the large test member for forming the gap were made to have the same surface treatment as the test member A using a steel material having the same chemical composition. And the hole of 10 mm (phi) was opened in the center of a small test member to the base material side (large test member side), and it fixed with the screw made of M8 plastics.

Moreover, as a test member used in the simulation environment of a ballast tank, the test member C (FIG. 6) which performed the modified epoxy resin coating (average film thickness: 50 micrometers) on the whole surface was also produced. In addition, in order to investigate the degree of corrosion progress when the anticorrosive coating film is damaged and the steel of the raw material is exposed, one cut scratch (length: 300 mm, width: about 0.5 mm) reaching the test member C to the raw material is applied. Test member D (Fig. 7) formed with a cutter knife was also produced. On the test surfaces of these test members C and D, a pure zinc piece having a size of 20 mm φ × 10 mm was attached so that the distance between the center of the zinc piece and the end of the test member was 20 mm, and the electric system was applied. In any of the test members, only one surface (test surface) for evaluating the corrosion situation was coated on one surface, and the surfaces other than the test surface were coated with silicone sealant to prevent corrosion.

Corrosion Test

(I) Test in simulated environment of crude oil tank

The test members A and B were immersed horizontally in a crude oil simulated solvent in which crude oil sludge collected from a crude oil tanker and natural seawater collected from Kagogawa City, Hyogo Prefecture were mixed at a volume ratio of 1: 1, and a partial pressure ratio 5% O _2- a mixed gas of _{0.5% H 2 S-l0%} CO `( balance of N ₂₎ was introduced into the test tank. The number of test members is 10 pieces in each of test members A and B, and the test period is 1 year. After the end of the test, the test member A was removed from the corrosion product such as rust by a cathodic electrolytic method (JIS K8284) in an aqueous solution of ammonium hydrogen citrate. Moreover, also about test member B, the small test member for gap formation was removed, and the corrosion product was removed in the same way.

In test members A and B tested in the simulation environment, the overall surface corrosion resistance, corrosion uniformity, and gap corrosion resistance were evaluated according to the criteria shown in Table 5. The results of the corrosion test are shown in Tables 6 and 7.

(II) Test in simulated environment of ballast tank

"High temperature which assumed the period in which the test members C and D were installed perpendicularly to the sealed corrosion test tank, and the time in which the seawater was introduce | transduced in the ballast tank, and the empty period of the ballast tank by loading crude oil was assumed. "High humidity state" was repeatedly applied every two weeks. The seawater used at this time was natural seawater collected from Hyogo Kakogawa City, and the seawater temperature was maintained at 30 ° C. In the high temperature, high humidity state, temperature control and humidification were performed so that it might be 40 degreeC of atmospheric temperature, and 90% of RH or more. The number of test members is 10 pieces, respectively, for test members C and D, and the total test period is one year.

In test members C and D tested in the simulation environment, the coating film expandability and the coating film scratch resistance were evaluated based on the criteria shown in Table 5. The results of the corrosion test are shown in Tables 6 and 7.

(1) Whole surface corrosion resistance and corrosion uniformity

In the test member A, the mass change before and after the test was converted into the average sheet thickness reduction Dave (mm), the average value of ten test members was computed, and the whole surface corrosion resistance of each test provision member was evaluated. Moreover, the maximum erosion depth Dmax (mm) of the test member A was calculated | required using the needle-contacting three-dimensional shape measuring apparatus, Dmax / Dave was calculated, and corrosion uniformity was evaluated.

(2) gap corrosion resistance

In the test member B, using the needle contact type three-dimensional shape measuring device, the gap corrosion depth on the large test member side was measured, and the maximum value of the ten test members was evaluated as the maximum gap corrosion depth Dcrev (mm) to evaluate the gap corrosion resistance. did.

(3) coating film expandability

In the test member C, observation was performed suitably, time (days) until the expansion which can be seen visually to a coating film was measured, and the coating film expandability was evaluated.

(4) coating film scratch resistance

In the test member D, the coating film expansion width (mm) perpendicular | vertical to cut damage was measured by vernier, the maximum value of 10 test members was computed as the maximum expansion width, and the coating film scratch part corrosion resistance was evaluated.

Steel Nos. 1 to 6, which did not satisfy the requirements of the first embodiment of the present invention, had insufficient corrosion resistance. In detail, steel number 1 is the conventional steel which does not contain Cu, Ni, and Cr, and is a bad result in all the items of a corrosion test. Steel No. 2 containing Cu, Ni, and Cr improved the corrosion uniformity and the coating film expandability, but other items resulted in poor results. This is probably because the pearlite area ratio is excessive. Steel material 3 is a result of poor corrosion uniformity and coating film expandability. This is probably because the bainite area ratio is excessive. Steel No. 4 results in poor corrosion uniformity and coating film expandability. This is presumably because the ferrite grain size is too large. Steel material No. 5 is the result of poor overall surface corrosion resistance, gap corrosion resistance, and coating-film flaw corrosion resistance. This is presumably because the aspect ratio of pearlite is too small. Steel No. 6 results in poor coating film expandability and poor coating film scratch resistance. This is presumably because the amount of Cr is too small.

On the other hand, steel numbers 7-34 which satisfy all the requirements of the 1st aspect of this invention show the outstanding corrosion resistance in any simulation environment of a crude oil tank and a ballast tank, and it turns out that it is preferable as a ship steel. 8 shows a graph showing the relationship between the corrosion resistance of the entire surface and the aspect ratio of pearlite. As shown in Tables 3, 4, 6, 7, and 8, when the aspect ratio of pearlite is 3 or more, a good result (△) can be obtained in overall surface corrosion resistance, and when it is 6 or more, excellent results (○) It can be obtained, and if it is 12 or more, a very good result (?) Can be obtained.

Second Example (Examples of the Invention and Comparative Example of the Second Embodiment of the Present Invention)

<Production of test provision member>

The steel materials of the chemical component shown in Table 8 and Table 9 were melted in the converter, the slab produced by the continuous casting method was heated to 1150 degreeC, and then hot rolling was performed, and the steel plate was produced. Steel structure was adjusted by changing the hot rolling finish temperature at the time of manufacture, and the cooling rate from the end of hot rolling to 500 degreeC. The number of nonmetallic inclusions whose round equivalent diameter exceeds 20 micrometers was adjusted by increasing and decreasing the degassing-processing time by RH method.

In Table 10, the hot rolling end temperature is referred to as "rolling end temperature", the cooling rate from the end of hot rolling to 500 ° C is referred to as "cooling rate", the area ratio of the steel structure is defined as "structure area ratio", and the circle equivalent diameter. The number of nonmetallic inclusions exceeding 20 micrometers is described as a "nonmetallic inclusion." In addition, the values of S (c) / S (a) and S (c) / S (b) are also described in Table 10.

Except having a thickness of 25 mm, the test member A (FIG. 4) of the magnitude | size of 300 * 300 * 25 (mm) was created by the method similar to a 1st Example. Using the test member A having a thickness of 25 mm, a test member B (FIG. 5) having a gap portion was produced in the same manner as in the first embodiment. In addition, except that the thickness was 25 mm, the test member C (Fig. 6) in which the modified epoxy resin coating (average film thickness: 50 µm) was applied to the entire surface in the same manner as in the first embodiment, and one cut scratch ( The test member D (FIG. 7) which formed length: 300 mm and width: about 0.5 mm) was also produced. In addition, the test surface of the test members C and D was attached to a pure zinc member having a size of 20 mm 占 25 mm in accordance with their thickness, and subjected to the electric system.

Corrosion Test

(I) Test in simulated environment of crude oil tank

The test in the simulated environment of the crude oil tank was conducted on the test members A and B in the same manner as in the first example, and the above-described overall surface corrosion resistance, corrosion uniformity, and gap corrosion resistance were evaluated. Table 5). The results of the corrosion test are shown in Table 11.

(II) Test in simulated environment of ballast tank

The test in the simulated environment of a ballast tank evaluated the above-mentioned coating-film expandability and coating film scratch part corrosion resistance with respect to test member C and D (decision criterion: said table 5). The results of the corrosion test are shown in Table 11.

Steel number 35 is a conventional steel that does not contain Cu, Cr, Ca, Ni, and Ti, and has poor results in all items of the corrosion test. Steel No. 36 is slightly improved in overall surface corrosion resistance and coated film expandability compared with No. 35, but the Cr content does not reach the predetermined amount specified in the second embodiment of the present invention, which is a poor result in other items. . The steel materials 37 and 38 satisfy the requirements of the chemical component composition prescribed | regulated in the 2nd Embodiment of this invention, but since the pearlite area ratio and bainite area ratio exceed the prescribed value, respectively, the improvement of corrosion resistance is inadequate. The steel number 39 also satisfies the requirements of the chemical component composition according to the second embodiment of the present invention, but does not satisfy the requirements of the number of nonmetallic inclusions, and thus the gap corrosion resistance and the coating film expandability are poor.

On the other hand, steel numbers 40-61 which satisfy | fill all the requirements of the 2nd aspect of this invention show the outstanding corrosion resistance in any simulation environment of a crude oil tank and a ballast tank, and it turns out that it is preferable as steel materials for crude oil tank bottom plates. . Graphs showing the relationship between total surface corrosion resistance and S (c) / S (a) values and the gap corrosion resistance and S (c) / S (b) values for these steel materials Nos. 9 and 10 are shown.

As shown in Table 10 and Table 11, and Fig. 9, in the total surface corrosion resistance, in the range of 20 ≦ S (c) / S (a) ≦ 350, it is a good result (○), and 50 ≦ S ( c) / S (a) ≤ 170, a very good result (?). In addition, as shown in Tables 10 and 11 and FIG. 10, in the range of 1.00 ≦ S (c) / S (b) ≦ 60 in the gap corrosion resistance, it is a good result (○), and 10 ≦ S (c). In the range of / S (b) ≤ 30, very good results (?) Are obtained. Thus, by controlling S (c) / S (a) and S (c) / S (b) appropriately, corrosion resistance (especially whole surface corrosion resistance and gap corrosion resistance) can be improved.

Third Example (Examples of the Invention and Comparative Example of the Third Embodiment of the Present Invention)

<Production of test provision member>

The molten steel outgoing from the converter was subjected to bubbling with Ar gas using an RH vacuum degassing apparatus, and the composition was adjusted to the composition shown in Table 12 while stirring the molten steel to form a steel ingot by the continuous casting method. The deoxidation type is Al Kilde steel. The steel ingot thus obtained was heated to 1150 ° C., and then hot rolled to prepare a steel sheet having a thickness of 19 mm. At this time, the hot rolling end temperature and the cooling rate from after completion | finish of hot rolling to 500 degreeC are as showing in Table 13. The area ratio of bainite + martensite (B + M) and the average particle size of sulfide inclusions of the steel thus obtained are shown in Table 13. In addition, the structure of the remainder part of the same steel material has ferrite F as a main body (area 50% or more), and some contain other pearlite (P).

The test member of the magnitude | size of 100 * 10 * 10 * (10) was cut out from the steel plate obtained in this way. The entire surface of the test member was polished with a wet rotary polishing machine (polishing paper # 600), and after washing with water and washing with acetone, zinc-rich primer was applied so that the average film thickness was 15 µm (± 3 µm), and at least 24 hours. It was dried in a desiccator. Next, as shown in Table 13, pretreatment was performed and various anticorrosive coatings were applied by airless spraying. In addition, the brush mounting performed as a pretreatment of painting is a process for removing garbage, dust, etc. on the to-be-painted surface. In addition, sand blasting was performed until all zinc rich primers disappeared.

The outer shape of the test member E thus obtained is as shown in FIG. In the test, in order to examine the degree of corrosion progress when the anticorrosive coating film was scratched and the steel of the raw material was exposed, one cut scratch reaching the raw material having a length of 10 mm and a width of about 0.5 mm was placed on the test surface side ( The test member F (FIG. 12) formed with the cutter knife at the high temperature side at the time of a corrosion test was also used.

Corrosion test method

The laboratory evaluation test method which simulated the inside of a ballast tank is as follows. The test members E and F were installed vertically in the test tank which satisfied the artificial seawater of a test liquid, the temperature of the test surface side of a test member was adjusted to 40 degreeC, and the back surface was adjusted to 10 degreeC, and the temperature difference gradient was applied to the anticorrosive coating film. Granted. In addition, the test member F with cut scratches provided the test member so that a cut scratch might become a high temperature side. As shown in FIG. 13, the test member was held for two weeks in a state where the whole test member was submerged (simulating the ballast state of the cavity), and then the artificial seawater was drained and the test member was exposed to the surface as shown in FIG. (Simulated dripping state) was maintained for one week. In the state of Fig. 14, artificial seawater remained below the test member in the test tank, and the temperature difference gradient of the test member was maintained by the temperature difference of the gas phase part. In the case where a temperature difference gradient is provided, moisture permeation of the coating film is promoted from the high temperature side to the low side. Therefore, the high temperature side (40 degreeC) in which corrosion under a coating film becomes remarkable was made into the test surface (evaluation surface). In the evaluation test, the states of Figs. 13 and 14 were repeated, and continued for a total of 24 weeks (168 days). The number of test members provided for the test was 5 pieces of E and F, respectively.

In the test member E, the time until coating film expansion by the expansion pressure of the corrosion product in a coating film / steel interface was measured, and the coating expandability was evaluated. The time until the coating film expansion was observed was visually confirmed once a day, and the time until the coating film expansion was recognized by one of the five test members provided for the test, respectively.

In the test member F, after completion of the test (after 24 weeks have elapsed), the expansion width (the width expanded in the direction perpendicular to the cut scratch) of the anticorrosive coating film was measured with a noggles, and the maximum value of the five test members provided for the test, respectively. The coating film scratch part corrosion resistance was evaluated by (maximum expansion width).

Corrosion test results

Corrosion test results are shown in Table 14. As a general anticorrosive coating film, No. 62, in which a standardized 250 μm modified epoxy resin coating film was formed, was inferior to both of the coating film expandability (△) and the coating film scratch resistance (×), and assumed a portion where the anticorrosive coating film became thin. The number 63 drops very much to x. Nos. 64 and 65 are Cu content and Cr content, respectively, which do not satisfy the prescribed values of the third embodiment of the present invention, so that the improvement in coating corrosion resistance is insufficient, and they are not suitable as steel materials for ballast tanks. Nos. 66 and 67 indicate that the component range of each additional element of the third embodiment of the present invention satisfies the prescribed values, but the area ratio of bainite + martensite and the average particle diameter of the sulfide-based inclusions do not meet the requirements, respectively. The improvement of coating corrosion resistance is slightly inadequate, and it is not satisfactory as steel materials for ballast tanks.

On the other hand, it is clear that all the things controlled by the component range of 3rd Embodiment of this invention (No. 68-88) have improved corrosion resistance to the level more than (circle). In particular, the removal of the shop primer and applying the anticorrosive coating directly to the steel material (No. 69, No. 73, No. 80, No. 87) are not limited to those remaining in the shop primer (No. 68, No. 72, No. 79, No. 86). It can be seen that the effect of improving the corrosion resistance is high. In addition, the effect of improving the coating corrosion resistance is recognized in both of the modified epoxy resin and the tar epoxy resin, and the film thickness is also recognized in any film thickness of 50 and 250 µm.

As described above, the steel of the third embodiment of the present invention is excellent in corrosion resistance of paint and delays coating film deterioration starting from coating film expansion, thereby suppressing the development of corrosion from steel exposed parts such as coating film defects and scratches. It can know that it is suitable as steel materials for ballast tanks. Therefore, the ballast tank comprised by the steel material of 3rd aspect of this invention also turns out to have the outstanding durability easily.

Fourth Example (Examples of the Invention and Comparative Example of the Fourth Aspect of the Present Invention)

<No test provided>

The steel materials of the chemical component compositions shown in Tables 15 and 16 below were converted into solvents in accordance with the addition time of the group a and b elements shown in Table 17, dephosphorized and desulfurized by secondary refining, followed by continuous casting. Made by slab. Subsequently, the obtained slab was reheated to the heating temperature shown in Table 17, and then hot rolled to obtain a steel sheet including the average particle diameter and the number of sulfide inclusions and oxide inclusions shown in Table 18.

After mirror-polished the rolling direction cross section of each test provision steel plate using the 1 micrometer diamond paste, the size and number of sulfide type inclusions and oxide type inclusions contained in a test provision member were SEM (scanning electron microscope: 5000 times magnification). And EDX (energy dispersive fluorescence X-ray analyzer).

That is, the chemical composition of each inclusion in five arbitrarily selected places was measured by EDX, and the average particle diameter (circle) was determined by SEM observation for sulfide inclusions and oxide inclusions satisfying the contents of a predetermined group a and b elements. The average value of equivalent diameter: Lave) and the average value N of the number of existences per 1 mm <2> were calculated | required. 2, an example of the secondary electron image by the SEM of a sulfide type interference | inclusion, the EDX analysis chart of the arrow part of the said electron phase is shown in FIG. 3, The secondary electron image by the SEM of an oxide type interference | inclusion, and this electronic image An example of the EDX analysis chart of an arrow part is shown.

In addition, each steel sheet thus obtained was cut and subjected to surface grinding to finally produce a test member G having a size of 100 × 100 × 25 (mm) (FIG. 15), and as shown in FIG. Four small test members of * 5 (mm) were contacted with a large test member (the same as the said test member G) of 100 * 100 * 25 (mm), and the test member H which provided the clearance part was produced. The small test member and the large test member for forming gaps were made of steel having the same chemical composition, and the surface treatment was the same as that of the test member G. Then, a 5 mm diameter hole was drilled in the center of the small test member, and a screw hole was drilled in the substrate (large test member) side and fixed with a screw made of plastic.

In addition, test member I (FIG. 17) in which modified epoxy resin coating (bottom coating: zinc-rich primer) with an average thickness of 250 µm was applied to the entire surface of a test member having the same size as test member G was also used. On one side of the test member I, the cut damage that reaches the raw material with a cutter knife (length: 100 mm, width) in order to examine the degree of corrosion progress when the coating film for the anticorrosion occurs and the steel of the raw material is exposed. : About 0.5 mm).

The corrosion test was done by the following method using five test members G, H, and I, respectively.

Corrosion Test Method X: Marine Simulation Environment

First, the marine environment to which a ship is exposed was simulated, and the complex cycle corrosion test by repeating a seawater spray test and a constant temperature and humidity test was done. In the seawater spray test, the test provision members (each test member G to I) are installed in the test tank at an angle of 60 ° from the horizontal, and 35 ° C artificial seawater (salt water with a Cl concentration of 2%) is continuously sprayed in the mist. did. The spraying quantity at this time was previously adjusted so that 1.5 +/- 0.3 mL of artificial seawater per hour may be collected in arbitrary positions in the circular dish of the area cm <2> installed horizontally in the test tank. In addition, the constant temperature / humidity test was performed in the test tank adjusted to temperature: 60 degreeC, and humidity: 95%, inclining and installing each test provision member at the angle of 60 degrees from horizontal. Sea water spray test: 4 hours, constant temperature and humidity test: 4 hours were made into 1 cycle, these were performed in turn, and the test provision member was corroded. The total test time was six months.

(1) Whole surface corrosion resistance and corrosion uniformity

The whole surface corrosion resistance and corrosion uniformity were evaluated by the method similar to Example 1 except having used five test member G instead of ten test member A. In addition, the weight measurement and the plate | board thickness measurement after a test were performed after removing corrosion products, such as rust, by the negative electrode electrolytic method (JIS K8284) in 10% of ammonium hydrogen citrate aqueous solution.

(2) gap corrosion resistance

The gap corrosion resistance was evaluated in the same manner as in the first example except that five test members H were used instead of ten test members B. FIG. In addition, the maximum gap corrosion depth Dcrev (mm) removed the corrosion product by the said cathodic electrolytic method, and measured it using the needle contact-type three-dimensional shape measuring apparatus "SE3500" made by Kosaka Research Institute.

(3) coating film scratch resistance

The coating film scratch part corrosion resistance was evaluated by the method similar to Example 1 except having used five test member I instead of ten test member D. FIG.

The evaluation criteria of the entire surface corrosion resistance (Dave), corrosion uniformity (Dmax / Dave), gap corrosion resistance (Dcrev), coating film scratch resistance (maximum expansion width) are as shown in Table 19 below. The results are shown in Table 20 and Table 21 together with the results of the later fifth example.

From the result of the said experiment, it can think as follows. Contents and content ratios of predetermined group a elements (one or more of Mg, Ca, Sr, and Ba) and group b elements (one or more of Ti, Zr, and Hf) satisfy the requirements of the fourth embodiment of the present invention. Not satisfying (numbers 90 to 93), or satisfying the content and content ratio of the predetermined group a and b group elements, the group a and b elements are added before secondary refining, or the slab heating temperature is appropriate. It is known that the average particle diameter (Lave) and the number N of the sulfide inclusions and the oxide inclusions do not meet the prescribed values of the fourth embodiment of the present invention (numbers 94 to 96) by exceeding the temperature. Compared to (No. 89), the corrosion resistance of the entire surface is slightly improved in any corrosion test, but sufficient improvement in corrosion uniformity and gap corrosion resistance is not recognized, and it is insufficient as corrosion resistance of ship steels.

On the other hand, in contrast to the provisions of the fourth embodiment of the present invention, the content and ratio of the predetermined group a and group b elements are appropriate, and the average particle diameter (Lave) and number of sulfide inclusions and oxide inclusions ( It is understood that the test providing members (numbers 97 to 117) in which N) is appropriately controlled exhibit all excellent corrosion resistance compared to conventional steels (number 89), and are excellent as corrosion resistance steel for ships.

In addition, it can be seen that the corrosion resistance of the steel material is further improved when an appropriate amount of corrosion resistance improving elements such as Cu, Cr, Ni, and Co are contained in the steel. Among these, in the steel type to which Cu and Cr were added (for example, number 98, 99, etc.), it can be seen that the corrosion resistance of the whole surface greatly improved. In addition, in the steel type (for example, number 105, 106, etc.) to which the appropriate amount of Ni was added, the corrosion resistance of a coating film flaw part is improved significantly. Moreover, in the type of steel to which Co was added (for example, numbers 101, 104, etc.), corrosion uniformity is improved significantly.

As described above, in addition to the adjustment of the chemical components, the sulfide-based inclusions and the oxide-based inclusions containing a predetermined amount of the a-group element and the b-group element are contained so as to have a predetermined amount of the a-group element and the b-group element. By appropriately controlling the size and number, it is possible to obtain a marine steel material which is remarkably excellent in corrosion resistance.

Fifth Example (Examples of the Invention and Comparative Example of the Fourth Aspect of the Present Invention)

<No test provided>

Steel materials containing the chemical component compositions shown in Tables 15 and 16 above and the inclusions shown in Table 18 were produced in the same manner as in the fourth embodiment described above. Subsequently, cutting and surface grinding were carried out, and test members G, H, and I of FIGS. 15 to 17 were produced in the same manner as in the fourth example, and each of five test members was used for corrosion testing. The corrosion test method at this time is as follows.

Corrosion test method Y: crude oil tank simulation environment

In a simulated environment of a crude oil tank in the same manner as in Example 1, except that test member G was used instead of test member A, test member H was used instead of test member B, and test member I was used instead of test member C. The corrosion test of was done.

Using the test members G to I in the same manner as in the fourth example, the overall surface corrosion resistance, corrosion uniformity, gap corrosion resistance, and coating film scratch resistance were evaluated. Evaluation criteria for the overall surface corrosion resistance (Dave), corrosion uniformity (Dmax / Dave), gap corrosion resistance (Dcrev), coating film scratch resistance (maximum expansion width) are as shown in Table 22 below.

Corrosion test results

Corrosion test results are as summarized in Table 20 and Table 21 above. In contrast to the definition of the fourth embodiment of the present invention, the content ratio of the group a elements (one or more of Mg, Ca, Sr, and Ba) and the group b elements (one or more of Ti, Zr, and Hf) is appropriately adjusted. Not included, the ratio is appropriate but the content does not meet the specified requirements (numbers 90 to 93), or the content ratio and content of the group a and b elements satisfy the specified requirements, No. 94 to 96, where the average particle diameter (Lave) and number (N) of the sulfide inclusions and the oxide inclusions do not meet the specified requirements by adding the group b element before the secondary refining or by the slab heating temperature exceeding an appropriate range. Although the overall surface corrosion resistance improves slightly compared with the conventional steel (No. 89), the improvement effect is not recognized in corrosion uniformity, gap corrosion resistance, etc., and it is insufficient as corrosion resistance of ship steel materials.

On the other hand, in contrast with the provisions of the fourth embodiment of the present invention, the content and content ratio of the a-group element and the b-group element are appropriate, and the average particle diameter (Lave) and the number (N) of the sulfide inclusions and the oxide inclusions. () (No. 97-117) in the appropriate range show the corrosion resistance excellent compared with the conventional steel (No. 89), and it turns out that it is preferable as corrosion-resistant steel for ships.

Moreover, what contained the appropriate amount of corrosion resistance improvement elements, such as Cu, Cr, Ni, and Co, is further improving the corrosion resistance of steel materials. Among these, in the steel materials (for example, numbers 98 and 99 etc.) which added the appropriate amount of Cu and Cr, the corrosion resistance of the whole surface is greatly improved. In addition, in the steel material to which Ni was added (for example, number 105, 106 etc.), the significant improvement of coating-film flaw corrosion resistance is recognized. In addition, in the steel material to which Co was added (for example, numbers 101, 104, etc.), corrosion uniformity is greatly improved.

As described above, the sulfide-based inclusions and oxides containing a predetermined amount of a-group and b-group elements are contained by appropriately adjusting the chemical components, and containing a predetermined a-group element and a b-group element. By controlling the size and number of the system inclusions, it can be seen that ship steel materials having higher durability can be obtained.

Fig. 18 shows the ratio S (a) / of the total content S (a) of the group a elements of the steel materials and the total content S (b) of the b group elements on the basis of the results of the fourth and fifth embodiments. It is a graph summarizing the relationship between S (b)) and integrated corrosion resistance. That is, the S (a) / S (b) ratio is taken on the horizontal axis, and the vertical axis is the integrated determination of corrosion resistance (if there are two judgments, a plot is made between them), and the S (a) / S (b) ratio is The `` ◇ '' and S (a) / S (b) ratios that do not satisfy O.O1 to 1.O are in the range of O.O1 to 1.0, but any one of the size and number of both inclusions The above-mentioned "@" and an embodiment that satisfies all the prescribed requirements of the fourth embodiment of the present invention are indicated by "o".

As is also apparent from this graph, all of the examples of the present invention that satisfy the prescribed requirements of the fourth embodiment of the present invention can achieve good results of? Or more in integrated evaluation of corrosion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an example of a sulfide-based inclusion in the third embodiment of the present invention, in which the upper figure is an SEM photograph thereof, and the lower figure is an EDX spectrum of an arrow part of the upper figure.

Fig. 2 is a chart showing secondary electron images by SEM of sulfide inclusions in the steel according to the fourth embodiment of the present invention, and EDX analysis results of the inclusions.

Fig. 3 is a chart showing secondary electron images by SEM of oxide inclusions in the steel of the fourth embodiment of the present invention and EDX analysis results of the inclusions.

4 is an explanatory view showing the external shape of the test member A used in the first and second embodiments.

Fig. 5 is an explanatory diagram showing the external shape of the test member B used in the first and second embodiments.

Fig. 6 is an explanatory diagram showing the external shape of the test member C used in the first and second embodiments.

Fig. 7 is an explanatory diagram showing the external shape of the test member D used in the first and second embodiments.

Fig. 8 is a graph showing the relationship between the total surface corrosion resistance and the aspect ratio of pearlite in steels Nos. 7 to 34 produced in the first embodiment.

Fig. 9 is a graph showing the relationship between total surface corrosion resistance and S (c) / S (a) values in steel materials 40 to 61 produced in Example 2;

Fig. 10 is a graph showing the relationship between the gap corrosion resistance and the S (c) / S (b) values in the steel materials 40 to 61 produced in the second embodiment.

Fig. 11 is an explanatory diagram showing the appearance of the test member E used in the third embodiment.

Fig. 12 is an explanatory diagram showing the external shape of the test member F (formed cut scratch) used in the third embodiment.

Fig. 13 illustrates the corrosion test method of the third embodiment, showing a state in which artificial seawater as a test liquid is held at a high level, and a test member is provided in a state where the whole of the test member is submerged.

Fig. 14 illustrates the corrosion test method of the third embodiment, showing a state in which artificial seawater as a test liquid is kept at a low level and installed in a state where the test member is exposed on the surface of the test.

Fig. 15 is an explanatory diagram showing the external shape of the test member G used in the fourth and fifth embodiments.

Fig. 16 is an explanatory diagram showing the external shape of the test member H used in the fourth and fifth embodiments.

Fig. 17 is an explanatory diagram showing the external shape of the test member I used in the fourth and fifth embodiments.

Fig. 18 shows the ratio <S (a) / S (b) of the total content S (a) of the group a elements and the total content S (b) of the group b elements of the test-provided steels obtained in Examples 4 and 5; A graph showing the relationship between >> and integrated corrosion resistance.

Claims (12)

C: 0.01 to 0.3%, Si: 0.01-2%, Mn: 0.0l to 2%, Al: 0.005% to 0.1%, Cu: 0.01 to 1%, Cr: O.O1-1%, Ca: 0.0001 to 0.005%, Ti: 0.005 to 0.2%, and Ni: 0.01 to 1% Containing, the remainder being composed of Fe and inevitable impurities, The area ratio is pearlite: 5 to 25%, bainite: less than 20%, martensite: less than 10%, and the remainder has a structure composed of ferrite, A crude oil tanker having a double split structure, characterized in that a tank bottom plate is provided with steel for crude oil tank bottom plate having a number of non-metallic inclusions having a circle equivalent diameter exceeding 20 µm, which is 0.20 pieces / mm 2 or less. The steel material for crude oil tank bottom plate according to claim 1, wherein the steel material for crude oil tank bottom plate is Mg: 0.005% or less (0%), Sr: 0.005% or less (0%), and Ba: 0.005% or less (0). Crude oil tanker having a double partition structure containing at least one element selected from the group consisting of%). The structure of claim 1, wherein the steel for crude oil tank bottom plate further comprises at least one of Mg: 0.005% or less (not containing 0%), and Sr: 0.005% or less (not containing 0%). Crude oil tanker. The structure of claim 1, wherein the steel for crude oil tank bottom plate further comprises at least one of Zr: 0.2% or less (not containing 0%) and Hf: 0.2% or less (not containing 0%). Crude oil tanker. The crude oil tanker according to claim 1, wherein the steel for crude oil tank bottom plate further contains Co: 1% or less (not containing 0%). The crude oil tank bottom plate steel according to claim 1, wherein the steel for crude oil tank bottom plate is La: 0.01% or less (does not contain 0%), Ce: 0.01% or less (does not contain 0%), Nd: 0.01% or less (0% Containing at least one element selected from the group consisting of S): Sm: 0.01% or less (does not contain 0%), and Se: 0.1% or less (does not contain O%) Crude oil tanker structure to double. The crude oil tank bottom plate steel according to claim 1, wherein the steel for crude oil tank bottom plate is La: 0.01% or less (does not contain 0%), Ce: 0.01% or less (does not contain 0%), Nd: 0.01% or less (0% And a Sm: crude oil tanker having a double partition structure containing at least one element selected from the group consisting of 0.01% or less (not containing 0%). The steel material for crude oil tank bottom plate according to claim 1, wherein the steel for crude oil tank bottom plate is made of Sb: 0.2% or less (does not contain 0%), Bi: 0.2% or less (does not contain 0%), and Te: 0.1% or less (0). Crude oil tanker having a double partition structure containing at least one element selected from the group consisting of%). The steel material for crude oil tank bottom plate according to claim 1, wherein the steel material for the crude oil tank bottom plate is B: 0.01% or less (does not contain 0%), V: 0.5% or less (does not contain 0%), and Nb: 0.5% or less (0 Crude oil tanker having a double partition structure containing at least one element selected from the group consisting of%). The crude oil tanker according to claim 1, wherein the steel for crude oil tank bottom plate further contains Ta: 0.5% or less (not containing 0%). The crude oil tanker according to claim 1, wherein the steel for crude oil tank bottom plate further comprises at least one of the following (a) to (f). (a) Mg: 0.005% or less (does not contain 0%), Sr: 0.005% or less (does not contain 0%), and Ba: 0.005% or less (does not contain 0%) At least one, (b) at least one of Zr: 0.2% or less (not including 0%), Hf: 0.2% or less (not including 0%), (c) Co: 1% or less (not including 0%), (d) La: 0.01% or less (does not contain 0%), Ce: 0.01% or less (does not contain 0%), Nd: 0.01% or less (does not contain 0%), Sm: 0.01% or less (Not containing 0%), and Se: at least one element selected from the group consisting of 0.1% or less (not containing 0%), (e) Sb: 0.2% or less (does not contain 0%), Bi: 0.2% or less (does not contain 0%), and Te: 0.1% or less (does not contain 0%) At least one kind of element, (f) from the group consisting of B: 0.01% or less (does not contain 0%), V: 0.5% or less (does not contain 0%), and Nb: 0.5% or less (does not contain 0%) At least one element selected. 12. The crude oil tank bottom plate steel according to any one of claims 1 to 11, wherein Ca, Mg and Sr are classified into a group, Ti, Zr and Hf are classified into b group, and Co and Ni are classified into c group. The total content of the group a elements is S (a)% (the meaning of mass%, hereinafter the same), the total content of the group b elements is S (b)%, and the total content of the group c elements is S (c)% In such a case, a crude oil tanker having a structure to be doubled that satisfies the following formulas (1) and (2). 20 ≤ S (c) / S (a) ≤ 350 1.00 ≤ S (c) / S (b) ≤ 60 (2)

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