JP2007277616A - Steel material for bottom plate of crude oil tank having excellent corrosion resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for the bottom plate of a crude oil tank in which both of local corrosion resistance and coating corrosion resistance are excellent. <P>SOLUTION: The steel material for the bottom plate of a crude oil tank has a composition comprising 0.01 to 0.3% C, 0.01 to 2% Si, 0.01 to 2% Mn, 0.005 to 0.1% Al, 0.01 to 1% Cu, 0.01 to 1% Cr, 0.0001 to 0.005% Ca, 0.005 to 0.2% Ti and 0.01 to 1% Ni, and the balance Fe with inevitable impurities, and has a structure composed of, by area ratio, 5 to 25% pearlite, <20% bainite and <10% martensite, and the balance ferrite, and in which the number per unit area of nonmetallic inclusions whose diameter of the equivalent circle exceeds 20 μm is ≤0.20 pieces/mm<SP>2</SP>. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel material having excellent corrosion resistance used as a tank bottom plate of a crude oil tanker (hereinafter sometimes abbreviated as “crude oil tank bottom plate”) for storing crude oil or crude oil sludge.

  In recent years, a problem has arisen in the bottom plate of crude oil tanks that the steel material is subjected to severe local corrosion due to moisture (residual water) accumulated in the tank bottom containing chloride at a high concentration and sulfur derived from crude oil. It has become. Such corrosion of the tank steel material may cause a serious accident such as the sinking of a crude oil tanker. For this reason, it is necessary to apply some anticorrosion means to the steel material.

  As the conventional anticorrosion means, (i) painting, (ii) rust / corrosion prevention sheet, (iii) electrocorrosion prevention, etc. are well known and put into practical use. Of these, in coatings represented by heavy coating, there is a high possibility that coating film defects exist, and the coating film may be damaged due to a collision or the like in the manufacturing process, so that the base steel material is often exposed. In such a steel exposed portion, the steel material corrodes locally and intensively, leading to early leakage of stored petroleum. Although protection of the steel material by the anticorrosion / corrosion protection sheet is relatively effective, there is a problem that corrosion of the scratched part of the steel material is unavoidable as in the case of coating.

  Electrocorrosion is very effective for a part completely immersed in an aqueous electrolyte solution having a high conductivity such as seawater. However, in the crude oil tank bottom plate, there is not enough retained water acting as an electrolyte aqueous solution (thin residence water thickness is thin), so that the distance on which the sacrificial anode acts is small and unsuitable for anticorrosion.

In addition to the above means, in Patent Document 1, by adjusting the chemical components, particularly by making the Ni content appropriate, and increasing the N content, the local corrosion resistance of the steel for the bottom plate of the crude oil tank can be improved. It is disclosed to improve. In Patent Document 2, on the basis of the chemical composition of general welded structural steel, Cr is substantially non-added, and a specific amount of either Mo or W or both and Cu are added in combination to form impurities. It is disclosed that the amount of P and S added is limited, and Al is added to reduce local corrosion damage of crude oil tank steel. Patent Document 3 discloses that a predetermined amount of Co and Mg are contained in combination, and the corrosion resistance of the marine steel is improved by appropriately adjusting the chemical composition.
Japanese Patent Laying-Open No. 2005-97709 (in particular, claims, paragraph 0012) Japanese Patent Application Laid-Open No. 2004-204344 (in particular, claims, paragraph 0060) Japanese Patent Laying-Open No. 2006-9128 (in particular, claims, paragraph 0010)

  However, the corrosive environment to which the bottom plate of the crude oil tank is exposed is severe, and the steel materials used for this purpose are constantly required to be further improved in corrosion resistance (particularly local corrosion resistance). Further, in a crude tanker having a double hull (double shell) structure, the outer surface of the tank bottom plate is the ballast tank side and is usually used in a state of being coated with an epoxy paint or the like. The ballast tank side is filled with seawater when it is empty, and therefore receives a corrosive action due to its salt content. On the other hand, it is subject to a corrosive action due to high temperature and humidity when transporting crude oil, and therefore requires excellent coating corrosion resistance. Accordingly, the bottom plate of the crude tank having a double hull structure is required to have both excellent local corrosion resistance (inner surface side) and coating corrosion resistance (ballast tank side).

  The present invention has been made paying attention to the circumstances as described above, and its purpose is excellent in both local corrosion resistance (inner side in contact with crude oil) and coating corrosion resistance (in ballast tank side in contact with seawater). It is to provide steel materials for crude oil tank bottom plates.

The steel material for the bottom plate of the crude oil tank of the present invention capable of achieving the above object is as follows.
C: 0.01 to 0.3% (meaning mass%, the same shall apply hereinafter)
Si: 0.01-2%
Mn: 0.01-2%
Al: 0.005 to 0.1%,
Cu: 0.01 to 1%,
Cr: 0.01-1%,
Ca: 0.0001 to 0.005%,
Ti: 0.005 to 0.2%, and Ni: 0.01 to 1%
And the balance has a composition consisting of Fe and inevitable impurities,
In area ratio, pearlite: 5 to 25%, bainite: less than 20%, martensite: less than 10%, the balance has a structure made of ferrite,
The number per unit area of a circle nonmetallic inclusions equivalent diameter exceeds 20μm, characterized in that 0.20 pieces / mm ² or less.

  Further, in the steel plate for crude oil tank bottom plate according to the present invention, (a) Mg: 0.005% or less (not including 0%) and / or Sr: 0.005% or less (including 0%) as necessary. (B) Zr: 0.2% or less (excluding 0%) and / or Hf: 0.2% or less (excluding 0%), (c) Co: 1% or less (0% Not included), (d) La: 0.01% or less (not including 0%), Ce: 0.01% or less (not including 0%), Nd: 0.01% or less (not including 0%) ), And Sm: 0.01% or less (excluding 0%), and / or (e) B: 0.01% or less (excluding 0%), V: selected from the group consisting of 0.5% or less (not including 0%) and Nb: 0.5% or less (not including 0%) Is at least one element, such as is also effective to contain, depending on the type of component to be contained, so that the characteristics of the steel material for a crude oil tank bottom plate is further improved.

In this crude steel tank bottom plate steel, Ca, Mg and Sr are classified into group a, Ti, Zr and Hf are classified into group b, Co and Ni are classified into group c, and the total content of group a elements is S (a). % (Meaning of mass%, the same applies hereinafter), the total content of group b elements being S (b)%, and the total content of group c elements being S (c)%, the total of elements a to c It is preferable to adjust the content so as to satisfy the following formulas (1) and (2).
20 ≦ S (c) / S (a) ≦ 350 (1)
1.00 ≦ S (c) / S (b) ≦ 60 (2)

  The steel material for a crude oil tank bottom plate of the present invention is excellent in both local corrosion resistance and paint corrosion resistance, and therefore is preferably used as a tank bottom plate of a crude tanker having a double hull structure.

  In the steel plate for crude oil tank bottom plate of the present invention, (I) appropriately adjusting the chemical component composition, in particular, Ca having an effect of suppressing pH decrease, Ti having an effect of stabilizing the surface rust film, and the like, and Containing an appropriate amount of Ni or the like that has the effect of forming a dense surface rust film, (II) appropriately controlling the steel material structure, and (III) suppressing non-metallic inclusions present in the steel material. As a result, it was possible to realize a steel plate for a crude oil tank bottom plate that was excellent in both local corrosion resistance and paint corrosion resistance.

  The steel material for crude oil tank bottom plate of the present invention is characterized in that the chemical composition is appropriately adjusted in order to improve the corrosion resistance. Therefore, first, the chemical composition of the steel material will be described.

<C: 0.01 to 0.3%>
C is an element necessary for ensuring the strength of the material. In order to obtain the strength required as a structural member of a crude oil tank, it is necessary to contain 0.01% or more. However, if the content exceeds 0.3%, the toughness deteriorates. Therefore, the C amount is set to 0.01 to 0.3%. The minimum with the preferable amount of C is 0.02%, More preferably, it is 0.04% or more. The upper limit is preferably 0.28%, more preferably 0.26% or less.

<Si: 0.01-2%>
Si is a necessary element for deoxidation and securing strength, and the strength required as a structural member cannot be secured unless it is less than 0.01%. However, if the content exceeds 2%, weldability deteriorates. Therefore, the Si amount is determined to be 0.01-2%. The minimum with the preferable amount of Si is 0.02%, More preferably, it is 0.05% or more. The preferable upper limit is 1.8%, more preferably 1.6% or less.

<Mn: 0.01-2%>
Similar to Si, Mn is also necessary for deoxidation and securing strength, and if it is less than 0.01%, the strength required as a structural member cannot be secured. However, if the content exceeds 2%, the toughness deteriorates. Therefore, the amount of Mn is set to 0.01 to 2%. The minimum with the preferable amount of Mn is 0.05%, More preferably, it is 0.10% or more. The preferable upper limit is 1.8%, more preferably 1.6% or less.

<Al: 0.005-0.1%>
Al is also necessary for deoxidation and securing of strength, similarly to Si and Mn, and 0.005% or more is necessary for sufficient deoxidation. However, if the content exceeds 0.1%, weldability is impaired. Therefore, the Al content is determined to be 0.005 to 0.1%. The minimum with the preferable amount of Al is 0.010%, More preferably, it is 0.015% or more. The preferable upper limit is 0.09%, more preferably 0.08% or less.

<Cu: 0.01 to 1%>
Cu is an element effective for improving corrosion resistance. Specifically, Cu has an effect of increasing the denseness of the surface rust film, and is an element effective for improving the environmental barrier properties and improving the corrosion resistance. In order to fully exhibit these effects, it is necessary to contain 0.01% or more, but when it contains exceeding 1% excessively, weldability and hot workability will deteriorate. Therefore, the amount of Cu is set to 0.01 to 1%. A preferable lower limit of the amount of Cu is 0.05%, and a preferable upper limit is 0.9%.

<Cr: 0.01 to 1%>
Cr is an element effective for improving corrosion resistance. Specifically, Cr, like Cu, has an effect of increasing the denseness of the surface rust film, and is an element that improves the environmental barrier properties and improves the corrosion resistance. Moreover, an appropriate amount of Cr is effective for improving toughness, and is an element necessary for obtaining mechanical properties necessary as a steel material for a crude oil tank bottom plate. In order to fully exhibit these effects, it is necessary to contain 0.01% or more, but when it contains exceeding 1% excessively, weldability and hot workability will deteriorate. Therefore, the Cr amount is set to 0.01 to 1%. The preferable lower limit of the Cr amount is 0.05%, and the preferable upper limit is 0.8%.

<Ca: 0.0001 to 0.005%>
Ca is an element effective for improving corrosion resistance. In detail, Ca has the effect | action which suppresses the pH fall by the hydrolysis of Fe melt | dissolved by corrosion. Due to this action, the crevice corrosion resistance can be particularly improved by suppressing the decrease in pH and the promotion of corrosion in the structural crevice where the diffusion of hydrogen ions to the outside hardly occurs. This action is effectively exerted by containing Ca in an amount of 0.0001% or more. However, if over 0.005% is contained, workability and weldability are deteriorated. Therefore, the Ca content is determined to be 0.0001 to 0.005%. A preferable lower limit of the Ca content is 0.0005%, and a preferable upper limit is 0.004%.

<Ti: 0.005-0.2%>
Ti is an element effective for improving corrosion resistance. Specifically, Ti has an action of stabilizing a surface rust film formed in a corrosive environment, and is an element effective for developing a corrosion inhibiting effect by the surface rust film over a long period of time. In order to exhibit this effect, it is preferable to contain 0.005% or more. However, if the content is excessive, weldability and hot workability deteriorate, so 0.2% or less is preferable. Therefore, the Ti amount is determined to be 0.005 to 0.2%. The preferable lower limit of the Ti amount is 0.008%, and the preferable upper limit is 0.15%.

<Ni: 0.01 to 1%>
Ni is effective in improving corrosion resistance. Specifically, Ni has an action of forming a dense surface rust film in a corrosive environment, and is an element that exhibits a corrosion inhibition effect by the surface rust film. Further, Ni has an effect of preventing red heat brittleness due to containing Cu. In order to exhibit such an effect, it is preferable to make it contain 0.01% or more. However, if the content exceeds 1%, weldability and hot workability deteriorate. Therefore, the amount of Ni is set to 0.01 to 1%. The preferable lower limit of the amount of Ni is 0.02%, and the preferable upper limit is 0.9%.

  The basic components of the steel plate for crude oil tank bottom plate of the present invention are as described above, and the balance is made of iron and inevitable impurities (for example, P, S, O, N, H, Mo, W, etc.). However, the inevitable impurities need to be an amount that does not hinder the properties of the steel material.

  In addition to the above components, the steel material for a crude oil tank bottom plate of the present invention includes (a) Mg: 0.005% or less (excluding 0%) and / or Sr: 0.005% or less (as required) (B) Zr: 0.2% or less (not including 0%) and / or Hf: 0.2% or less (not including 0%), (c) Co: 1% or less (Not including 0%), (D) La: 0.01% or less (not including 0%), Ce: 0.01% or less (not including 0%), Nd: 0.01% or less (0 %), And Sm: at least one element selected from the group consisting of 0.01% or less (excluding 0%), and / or (e) B: 0.01% or less (0% Not including), V: 0.5% or less (not including 0%), and Nb: 0.5% or less (not including 0%) At least one element selected, etc. is also effective to contain, depending on the type of component to be contained, so that the characteristics of the steel material for a crude oil tank bottom plate is further improved.

<Mg: 0.005% or less and / or Sr: 0.005% or less>
Mg and Sr are effective elements for improving the corrosion resistance, and can be contained as necessary. In detail, like Ca, these have the effect | action which suppresses the pH fall by the hydrolysis of Fe melt | dissolved by corrosion, and suppress the corrosion promotion by pH fall. This action is particularly effective for improving crevice corrosion resistance. Mg is particularly effective in improving corrosion resistance when Co is present. In order to sufficiently exert such an action, it is recommended that Mg and Sr are each preferably contained in an amount of 0.0001% or more, more preferably 0.0005% or more. However, when these are contained excessively, workability and weldability are deteriorated. Therefore, when contained, the upper limit of the amount of Mg and Sr is 0.005%, and more preferably 0.004% or less.

<Zr: 0.2% or less and / or Hf: 0.2% or less>
Zr and Hf are effective elements for improving corrosion resistance, and can be contained as necessary. Specifically, like Ti, these elements have the effect of stabilizing the surface rust film formed in the corrosive environment, and are effective elements for expressing the corrosion inhibition effect by the surface rust film over a long period of time. . In order to fully exhibit this effect, it is recommended that Zr and Hf are each preferably contained in an amount of 0.005% or more, more preferably 0.008% or more. However, when these amounts are excessive, weldability and hot workability deteriorate. Therefore, when contained, the upper limit of the amount of Zr and Hf is 0.2%, more preferably 0.15% or less.

<Co: 1% or less>
Co is an element effective for improving the corrosion resistance, and can be contained as necessary. Specifically, Co, like Ni, has an action of forming a dense surface rust film in a corrosive environment, and is an element that exhibits a corrosion inhibition effect by the surface rust film. In particular, due to the coexistence with Mg, a remarkable corrosion resistance effect is obtained. In order to sufficiently exhibit such effects, it is recommended that Co be contained in an amount of preferably 0.01% or more, more preferably 0.02% or more. However, when the amount of Co becomes excessive, weldability and hot workability deteriorate. Therefore, when it is contained, the upper limit of the amount of Co is 1%, more preferably 0.8% or less.

<At least one element selected from the group consisting of La: 0.01% or less, Ce: 0.01% or less, Nd: 0.01% or less, and Sm: 0.01% or less>
La, Ce, Nd, and Sm are all effective elements for improving corrosion resistance, and can be contained as necessary. Specifically, these have the effect of further enhancing the corrosion resistance by promoting the effect of suppressing pH decrease by Ca and the like and the effect of stabilizing the surface rust film by Ti and the like. In order to sufficiently exhibit such an action, it is recommended that La, Ce, Nd and Sm are each preferably contained in an amount of 0.0001% or more, more preferably 0.0005% or more. However, when these are contained excessively, workability and weldability deteriorate. Therefore, when contained, the upper limits of the amounts of La, Ce, Nd and Sm are each 0.01%, and more preferably 0.008%.

<At least one element selected from the group consisting of B: 0.01% or less, V: 0.5% or less, and Nb: 0.5% or less>
B, V and Nb are all effective elements for improving the strength, and can be contained as required. In order to fully exhibit these effects (especially B from the viewpoint of improving hardenability), B is preferably 0.0001% or more, more preferably 0.0003% or more, and V is preferably Is recommended to be contained at 0.01% or more, more preferably 0.02% or more, and Nb is preferably contained at 0.003% or more, more preferably 0.005% or more. However, when these are contained excessively, the toughness of the steel material deteriorates. Therefore, when contained, the upper limit of the B amount is 0.01%, more preferably 0.009% or less, and the upper limit of the V amount is 0.5%, more preferably 0.45% or less. The upper limit of the amount of Nb is 0.5%, more preferably 0.45% or less.

  As described above, Ca, Mg and Sr, which have the effect of suppressing pH reduction, are in the a group, Ti, Zr and Hf, which have the effect of stabilizing the surface rust film, are in the b group, and Co and Ni are in the c group. When classified, the a to c group elements having different effects act synergistically. Therefore, the corrosion resistance can be further improved by appropriately controlling the ratio of the total content of the a group element, the b group element, and the c group element.

  Specifically, the total content of group a elements is S (a)% (meaning mass%, the same applies hereinafter), the total content of group b elements is S (b)%, and the total content of group c elements is In the case of S (c)%, from the viewpoint of general corrosion resistance, it is preferably 20 ≦ S (c) / S (a) ≦ 350, more preferably 50 ≦ S (c) / S (a) ≦ 170. It is recommended to adjust the total content of the a group element and the c group element so that From the viewpoint of crevice corrosion resistance, the group b is preferably 1.00 ≦ S (c) / S (b) ≦ 60, more preferably 10 ≦ S (c) / S (b) ≦ 30. It is recommended to adjust the total content of elements and group c elements.

  In order to improve corrosion resistance, the steel material for the crude oil tank bottom plate of the present invention has a steel material structure appropriately controlled, specifically, in terms of area ratio, pearlite: 5 to 25%, bainite: less than 20%, Martensite: less than 10%, and one of the characteristics is that the balance has a structure made of ferrite.

  In the present invention, the area ratio of the structure is, in principle, in a portion having a depth of 3 mm from the surface when the thickness of the steel material is 6 mm or more, and in a half of the thickness of the steel material when the thickness of the steel material is less than 6 mm. Observation with an optical microscope at an observation magnification of 400 times and an observation visual field of 150 μm × 200 μm or more, and an average value of area ratios obtained in arbitrary 30 visual fields is adopted. In addition, the surface used as the reference | standard of a depth means the surface of steel materials with which force was applied by rolling.

  As described above, the steel structure of the present invention is mainly composed of ferrite. Ferrite is preferable as a structure of a structural member used in a chloride environment called seawater because it is excellent in weldability and workability and has low sensitivity to stress corrosion cracking due to chloride.

  Perlite is necessary to obtain the strength of the steel material. If the pearlite area ratio is too small, the pearlite tends to be locally scattered, and the tendency to cause local corrosion such as pitting corrosion and crevice corrosion increases. From the viewpoint of strength and local corrosion resistance, the pearlite area ratio was set to 5% or more. However, in addition to causing deterioration of toughness and weldability, the increase in pearlite acts as a cathodic site for corrosion reaction and deteriorates corrosion resistance (total corrosion resistance). Therefore, the pearlite area ratio was set to 25% or less. A more preferable range of the pearlite area ratio is 8 to 20%.

  Bainite is an effective structure for improving the toughness of steel materials. However, if bainite is present in the ferrite, corrosion of the bainite portion is promoted and local corrosion resistance is lowered. Therefore, it is recommended that the bainite area ratio is less than 20%, preferably less than 18%.

  Martensite is an effective structure for improving strength, but it impairs toughness and weldability. Therefore, from the viewpoint of securing toughness and weldability, it is recommended that the martensite area ratio is less than 10%, preferably less than 8%.

The steel material for crude oil tank bottom plate of the present invention is characterized in that non-metallic inclusions present in the steel material are suppressed in order to improve corrosion resistance. This non-metallic inclusion may be a starting point for local corrosion. Here, the “non-metallic inclusion” in the present invention means a sulfide type inclusion such as MnS, an oxide type such as Al ₂ O ₃ or SiO ₂ and a nitride type inclusion such as TiN.

In the environment of the crude oil tank bottom plate, large non-metallic inclusions having an equivalent circle diameter exceeding 20 μm are particularly harmful, and the number of inclusions is 0.20 pieces / mm ² or less, more preferably 0.1 pieces / mm ² or less. It is recommended to do.

  In the present invention, the number per unit area of non-metallic inclusions exceeding 20 μm is the thickness of the steel material when the thickness of the steel material is 6 mm or more and the thickness of the steel material is less than 6 mm at a portion 3 mm deep from the surface. The average of the number per unit area of the inclusions obtained in an arbitrary 30 visual fields when observed with an optical microscope at an observation magnification of 400 times and an observation visual field of 150 μm × 200 μm or more in principle Adopt value. In addition, the surface used as the reference | standard of a depth means the surface of steel materials with which force was applied by rolling. The observation method of the optical microscope is in accordance with “Microscopic test method for non-metallic inclusions in steel” described in JIS G0555.

The steel material for a crude oil tank bottom plate of the present invention can be manufactured, for example, by the following method. First, the ingot is prepared by a normal casting method such as a converter or an electric furnace while adjusting the chemical composition so that the chemical composition satisfies the above range. And As a deoxidation type, it is preferable to use killed steel from the viewpoint of mechanical properties and weldability, and Al killed steel is more preferable. Here, reducing the number per unit area of non-metallic inclusions having an equivalent circle diameter of more than 20 μm to 0.20 pieces / mm ² or less is, for example, outside refining such as a vacuum degassing method such as DH method or RH method. Can be achieved by performing

  Subsequently, after heating the obtained steel ingot to a temperature range of 1100 to 1200 ° C., it is preferable to perform hot rolling to obtain a desired size and shape. At this time, the end temperature of hot rolling is controlled to 750 to 850 ° C., and the cooling rate from the end of hot rolling to 500 ° C. is controlled in the range of 0.5 to 15 ° C./second, whereby pearlite: 5 Steel material having a structure of ˜25%, bainite: less than 20%, martensite: less than 10%, and the balance being composed of ferrite can be obtained.

  The steel plate for crude oil tank bottom plate according to the present invention is excellent in both local corrosion resistance and paint corrosion resistance. Therefore, as a tank bottom plate of a double hull structure crude oil tanker, the crude oil tank side is bare (untreated), and the ballast tank The side is preferably painted and used. However, the crude oil tank may also be subjected to a treatment such as zinc rich paint or shop primer for the purpose of preventing initial rust as needed. Examples of the paint on the ballast tank side include tar epoxy resin-based paints, modified epoxy resin paints, and other representative heavy anticorrosion paints. Further, the steel material for the bottom plate of the crude oil tank of the present invention may be used in combination with other anticorrosion methods such as cathodic protection (galvanic anode method, external power supply method).

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

<Production of test material>
Steel materials having chemical components shown in Table 1 were melted in a converter, and a slab produced by a continuous casting method was heated to 1150 ° C. and then hot-rolled to produce a steel plate. The steel structure was adjusted by changing the hot rolling end temperature during production and the cooling rate from the end of hot rolling to 500 ° C. The number of non-metallic inclusions having an equivalent circle diameter exceeding 20 μm was adjusted by increasing or decreasing the degassing treatment time by the RH method.

  Table 2 shows the hot rolling end temperature as “rolling end temperature”, the cooling rate from the end of hot rolling to 500 ° C. as “cooling rate”, the area ratio of steel structure as “structure area ratio”, and the circle equivalent The number of nonmetallic inclusions having a diameter exceeding 20 μm is described as “nonmetallic inclusions”. Table 2 also describes the values of S (c) / S (a) and S (c) / S (b).

  By cutting and surface grinding the steel plate obtained as described above, a test piece A having a size of 300 × 300 × 25 (mm) was produced as a test piece used in a simulated environment on the crude oil tank side. The external shape of the test piece A is shown in FIG. Further, as shown in FIG. 2, four small test pieces of 60 × 60 × 5 (mm) are brought into contact with a large test piece of 300 × 300 × 25 (mm) (the same as the test piece A), A test piece B having a clearance was formed. The small test piece and the large test piece for forming the gap were steel materials having the same chemical composition, and the surface finish was the same as that of the test piece A. Then, a 10 mmφ hole was opened in the center of the small test piece on the base material side (large test piece side), and fixed with an M8 plastic screw.

  Furthermore, as a test piece used in the simulated environment on the ballast tank side, a test piece C (FIG. 3) in which a modified epoxy resin coating (average film thickness: 50 μm) was applied to the entire surface was also produced. In addition, in order to investigate the degree of corrosion progression when the base steel material is exposed due to scratches on the anticorrosive coating film, one cut ridge reaching the base to the test piece C (length: 100 mm, width: about 0.5 mm) A test piece D (FIG. 4) formed with a cutter knife was also produced. On the test surfaces of these test pieces C and D, a pure zinc piece having a size of 20 mmφ × 25 mm was attached so that the distance between the center of the zinc piece and the end of the test piece was 20 mm, and was subjected to cathodic protection. In each test piece, only one surface (test surface) was evaluated for corrosion status, and the surfaces other than the test surface were coated with a silicone sealant to prevent corrosion.

<Corrosion test>
(I) Test in a simulated environment on the crude oil tank side Specimens A and B were added to a crude oil simulation solvent in which crude oil sludge collected from a crude oil tanker and natural seawater collected in Kakogawa City, Hyogo Prefecture were mixed at a volume ratio of 1: 1. Were immersed horizontally, and a mixed gas of 5% O ₂ -0.5% H ₂ S-10% CO ₂ (remainder N ₂ ) in a partial pressure ratio was introduced into the test tank. The number of test pieces is 10 for each of the test pieces A and B, and the test period is one year. After completion of the test, the test piece A was subjected to removal of corrosion products such as iron rust by a cathodic electrolysis method (JIS K8284) in a diammonium hydrogen citrate aqueous solution. For test piece B, the small test piece for forming the gap was removed, and the corrosion products were removed in the same manner.
Test specimens A and B tested in the simulated environment were evaluated for overall corrosion resistance, corrosion uniformity, and crevice corrosion resistance according to the criteria shown in Table 3. The results of the corrosion test are shown in Table 4.

(II) Test in simulated environment on the ballast tank side Test pieces C and D were installed vertically in a sealed corrosion test tank, and “seawater immersed state” assuming the period of seawater introduced into the ballast tank, and crude oil The “high temperature and high humidity state” assuming a period when the ballast tank is empty was repeatedly applied every 2 weeks. The seawater used at this time was natural seawater collected in Kakogawa City, Hyogo Prefecture, and the seawater temperature was kept at 30 ° C. In a high temperature and high humidity state, the temperature was adjusted and humidified so that the atmospheric temperature was 40 ° C. and the humidity was 90% RH or higher. The number of test pieces is 10 for each of the test pieces C and D, and the total test period is one year.
With respect to the test pieces C and D tested in the simulated environment, the coating film swelling resistance and coating film butt corrosion resistance were evaluated according to the criteria shown in Table 3. The results of the corrosion test are shown in Table 4.

(1) Overall corrosion resistance and corrosion uniformity In test piece A, the change in mass before and after the test was converted to an average thickness reduction amount D-ave (mm), and the average value of 10 test pieces was calculated. The overall corrosion resistance of the test material was evaluated. Moreover, the maximum erosion depth D-max (mm) of the test piece A was calculated | required using the stylus type three-dimensional shape measuring apparatus, D-max / D-ave was calculated, and corrosion uniformity was evaluated.

(2) Crevice corrosion resistance Measure the crevice corrosion depth on the large test piece side using the stylus type three-dimensional shape measuring device on test piece B, and set the maximum value of 10 test pieces to the maximum crevice corrosion depth D. As crev (mm), crevice corrosion resistance was evaluated.

(3) Coating film swell resistance Test piece C was appropriately observed, and the time (day) until swelling that can be visually confirmed on the coating film was measured to evaluate the swell resistance of the coating film.

(4) Corrosion resistance of coating film ridge part Measure the coating film swelling width (mm) in the direction perpendicular to the cut scratches with test piece D with calipers, and calculate the maximum value of 10 test pieces as the maximum swelling width. The corrosion resistance was evaluated.

  Steel No. Reference numeral 1 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. 2 is No.2. Compared to 1, the overall corrosion resistance and the film swellability are slightly improved, but the Cr amount is less than the predetermined amount, resulting in poor results in other items. Steel No. 3 and no. No. 4 satisfies the requirements of the chemical component composition defined in the present invention, but the pearlite area ratio and the bainite area ratio exceed the specified values, respectively, so that the corrosion resistance is insufficiently improved. Steel No. No. 5 also satisfies the requirements for the chemical composition, but does not meet the requirements for the number of non-metallic inclusions, so the crevice corrosion resistance and the coating film swelling resistance are poor.

  On the other hand, steel No. satisfying all the requirements of the present invention. Nos. 6 to 27 show excellent corrosion resistance in both simulated environments on the crude oil tank side and the ballast tank side, and it is understood that the steel materials for the crude oil tank bottom plate are preferable. These steel materials No. 6 to 27 are graphs showing the relationship between the general corrosion resistance and the S (c) / S (a) value, and the relationship between the crevice corrosion resistance and the S (c) / S (b) value. And shown in FIG.

  As shown in Tables 2 and 4 and FIG. 5, in the general corrosion resistance, in the range of 20 ≦ S (c) / S (a) ≦ 350, good results (◯) are obtained, and 50 ≦ S ( In the range of c) / S (a) ≦ 170, extremely good results (◎) are obtained. Further, as shown in Tables 2 and 4 and FIG. 6, in the crevice corrosion resistance, good results (◯) were obtained in the range of 1.00 ≦ S (c) / S (b) ≦ 60. In the range of ≦ S (c) / S (b) ≦ 30, extremely good results (◎) are obtained. Thus, by appropriately controlling S (c) / S (a) and S (c) / S (b), the corrosion resistance (especially, general corrosion resistance and crevice corrosion resistance) can be further improved. Can do.

It is explanatory drawing which shows the external appearance shape of the test piece A used for the corrosion test. It is explanatory drawing which shows the external appearance shape of the test piece B used for the corrosion test. It is explanatory drawing which shows the external appearance shape of the test piece C used for the corrosion test. It is explanatory drawing which shows the external appearance shape of the test piece D used for the corrosion test. The steel material No. produced in the Example. It is a graph which shows the relationship between the general corrosion resistance and S (c) / S (a) value in 6-27. The steel material No. produced in the Example. It is a graph which shows the relationship between crevice corrosion resistance and S (c) / S (b) value in 6-27.

Claims (8)

C: 0.01 to 0.3% (meaning mass%, the same shall apply hereinafter)
Si: 0.01-2%
Mn: 0.01-2%
Al: 0.005 to 0.1%,
Cu: 0.01 to 1%,
Cr: 0.01-1%,
Ca: 0.0001 to 0.005%,
Ti: 0.005 to 0.2%, and Ni: 0.01 to 1%
And the balance has a composition consisting of Fe and inevitable impurities,
In area ratio, pearlite: 5 to 25%, bainite: less than 20%, martensite: less than 10%, the balance has a structure made of ferrite,
A steel material for a bottom plate of a crude oil tank excellent in corrosion resistance, characterized in that the number per unit area of non-metallic inclusions having an equivalent circle diameter exceeding 20 μm is 0.20 pieces / mm ² or less.   The steel material for a crude oil tank bottom plate according to claim 1, further comprising Mg: 0.005% or less (not including 0%) and / or Sr: 0.005% or less (not including 0%).   The steel material for a crude oil tank bottom plate according to claim 1 or 2, further comprising Zr: 0.2% or less (not including 0%) and / or Hf: 0.2% or less (not including 0%).   Furthermore, Co: Steel material for crude oil tank bottom plates in any one of Claims 1-3 containing 1% or less (excluding 0%). Ca, Mg and Sr are classified into a group, Ti, Zr and Hf are classified into b group, Co and Ni are classified into c group, and the total content of group a elements is S (a)% (meaning mass%, hereinafter The same formula (1) and (2) are satisfied when the total content of the b group elements is S (b)% and the total content of the c group elements is S (c)%. Steel material for crude oil tank bottom plate in any one of -4.
20 ≦ S (c) / S (a) ≦ 350 (1)
1.00 ≦ S (c) / S (b) ≦ 60 (2)   Furthermore, La: 0.01% or less (not including 0%), Ce: 0.01% or less (not including 0%), Nd: 0.01% or less (not including 0%), and Sm: 0 The steel material for a crude oil tank bottom plate according to any one of claims 1 to 5, comprising at least one element selected from the group consisting of 0.01% or less (not including 0%).   Further, from the group consisting of B: 0.01% or less (not including 0%), V: 0.5% or less (not including 0%), and Nb: 0.5% or less (not including 0%) The steel material for a crude oil tank bottom plate according to any one of claims 1 to 6, comprising at least one element selected.   The steel material for a crude oil tank bottom plate according to any one of claims 1 to 7, which is used for a tank bottom plate of a crude tanker having a double hull structure.