JP4668141B2 - Steel material for ballast tank with excellent corrosion resistance and ballast tank with excellent durability - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a steel material (particularly a thick steel plate) having excellent corrosion resistance used for a ballast tank added to a ship for improving the stability of the ship, and a ship having a ballast tank having excellent durability constituted thereby. is there.

  Since the ship's ballast tank injects and discharges seawater in response to changes in the state of cargo, the steel used is exposed to extremely harsh corrosive environments of seawater immersion and repeated humid atmosphere containing salt. Corrosion damage to the ballast tank leads to serious accidents such as sinking due to perforations, and ocean spillage of cargo such as crude oil and chemicals, so it is necessary to provide some anticorrosion measures to the steel. Ballast tanks are generally coated with heavy anti-corrosion coating, and from the viewpoint of improving safety and reliability, an anti-corrosion method such as a galvanic anode method is often used in combination.

  By using an anticorrosion method such as tar epoxy resin coating, corrosion of the steel material can be suppressed to some extent. However, the environmental barrier properties are not perfect even with the anticorrosion coating, and chemical substances that cause corrosion such as moisture, salt and oxygen penetrate the coating and eventually cause corrosion of the steel material. When steel material corrosion occurs under the anticorrosion coating, the anticorrosion coating swells due to the expansion pressure of the corrosion product, destroys the coating and leads to exposure of the steel, and the anticorrosion action is lost. Further, in reality, there is a high possibility that the coating film has a defect, and the coating film may be damaged by a collision or the like at the time of ship construction, so that the base steel material may be exposed. In addition, there are not a few cases where extremely thin portions of the anticorrosion coating film are formed, such as edge portions of steel materials and poorly constructed portions. In such an exposed steel part, the steel material corrodes locally and intensively, and seawater permeates at an early stage where the coating film is thin, causing corrosion under the coating film. In addition, the anti-corrosion method is a very effective anti-corrosion method during the period when seawater is injected into the ballast tank (when empty), but it is necessary for the electrochemical reaction when there is no seawater (when loading). Since there is no electrolyte aqueous solution, the anticorrosive effect does not work. Furthermore, even if seawater is injected into the ballast tank, the anticorrosion effect does not naturally act in a space portion where seawater is not in contact, such as the upper deck.

  Furthermore, although the inside of the ballast tank is hot due to irradiation of the upper deck of sunlight, since the hull is cooled by seawater, a temperature difference gradient is given to the anticorrosion coating film in the tank. When a temperature difference gradient is applied to the anti-corrosion coating, moisture easily penetrates into the steel through the coating due to the osmotic pressure due to the temperature difference, and corrosion under the coating is promoted. The ballast tank is a harsh environment for anticorrosion coatings.

  As described above, the currently commonly used anticorrosion methods require repainting and repainting at the time of regular inspection and repair with dogs relatively soon after the ship enters service. Maintenance costs and extended dog periods (time loss) ) And other economic losses.

In addition to the above technique, a corrosion-resistant steel material in which the corrosion resistance of the steel material itself is improved by adjusting chemical components has been proposed (for example, Patent Document 1). In addition, a steel material having improved corrosion resistance by using a chemical component adjustment and a zinc rich primer has been proposed (for example, Patent Document 2). Furthermore, the durability improvement technique which combined corrosion-resistant steel materials and resin coating is also proposed (for example, patent document 3). However, the improvement in corrosion resistance by these techniques cannot be said to be sufficient, and the contribution to the reduction of the economic loss is small, and more effective anticorrosion methods are required.
Japanese Patent Laid-Open No. 2000-17371 JP 2005-171332 A Japanese Patent Laid-Open No. 7-34196

  The present invention has been made paying attention to the above-mentioned circumstances, and its purpose is to contribute to the reduction of economic loss such as ship maintenance cost and dog period extension (time loss) and further improvement of ship safety. An object of the present invention is to provide a steel material for a ballast tank and a ballast tank.

  In addition, the present invention provides a ballast tank that is effective in improving durability in a severe part as a corrosive environment at high temperatures such as the back side of the upper deck where the anticorrosion does not act (does not soak in seawater during ballasting), the heating part of the load and the vicinity of the engine. Steel and ballast tanks are provided.

The present invention according to claim 1
C: 0.01 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 2.0%,
Mn: 0.01 to 2.0%,
P: 0.01% or less,
S: 0.0005 to 0.005% or less,
Al: 0.005 to 0.10%,
Cu: 0.1 to 1.0%
Ni: 0.01 to 1.0%,
Cr: 0.01 to 0.5%
The balance is composed of Fe and inevitable impurities, has a structure mainly composed of ferrite, and has an area ratio of bainite and / or martensite of less than 30% (including 0). A steel material for ballast tanks having excellent corrosion resistance, characterized in that the average particle size is 0.5 to 10 μm.

The present invention according to claim 2
further,
Mg: 0.0001 to 0.005%,
Ca: 0.0001 to 0.005%,
Sr: 0.0001 to 0.005%,
2. The steel material for ballast tanks having excellent corrosion resistance according to claim 1, comprising one or more selected from the group consisting of:

Further, the present invention according to claim 3 provides
further,
Co: 0.005 to 0.20%,
Ti: 0.005 to 0.20%,
Zr: 0.005 to 0.20%,
The steel material for ballast tanks having excellent corrosion resistance according to claim 1 or 2, which contains one or more selected from the group consisting of:

The present invention according to claim 4 provides
further,
B: 0.0001 to 0.010%,
V: 0.01 to 0.50%,
Nb: 0.003 to 0.50%,
It is a steel material for ballast tanks excellent in corrosion resistance in any one of Claims 1-3 containing 1 type, or 2 or more types chosen from the group which consists of.

The present invention according to claim 5
The steel material for a ballast tank excellent in corrosion resistance according to any one of claims 1 to 4, wherein the anticorrosion coating film is directly formed on the surface of the steel material.

  The present invention according to claim 6 is a ship having a ballast tank constituted by the steel material for ballast tank according to any one of claims 1 to 5.

  As a result of intensive research, the present inventors have appropriately adjusted additive elements such as Cu and Ni in addition to basic components such as C, Si, Mn, and Al to optimize the state of the structure and inclusions. In addition to the above, it has been found that the above-described problems can be solved by directly applying an anticorrosion paint to the steel material.

As described above, the deterioration of the anticorrosion coating film starts when chemical substances that cause corrosion, such as moisture, salt, and oxygen, penetrate into the coating film and cause corrosion of steel under the coating film. By using the steel material of the present invention in a ballast tank environment, corrosion of the steel material that occurs under the anticorrosion coating can be effectively suppressed, so that the expansion pressure of the corrosion product is reduced compared to conventional steel, and the anticorrosion coating swells. It becomes difficult and prolongs the life of the anticorrosion coating. In addition, even when defects and scratches such as pinholes are present in the anticorrosion coating film and the steel material is exposed, the steel material of the present invention has a small corrosion progress rate at the exposed portion, so that the steel plate is perforated. Less likely to cause corrosion damage. The reasons for limiting the component range of the steel of the present invention will be described below.
C: 0.01 to 0.30%
C is an element necessary for ensuring the strength of the material. In order to obtain the minimum strength as a structural member of the petroleum tank, that is, about 400 MPa (depending on the thickness of the steel material used), it is necessary to contain 0.01% or more. However, if the content exceeds 0.30%, the toughness deteriorates. For these reasons, the C content range was set to 0.01 to 0.30%. In addition, the minimum with preferable C content is 0.02%, More preferably, it is good to set it as 0.04% or more. Moreover, the upper limit with preferable C content is 0.28%, More preferably, it is good to set it as 0.26% or less.
Si: 0.01 to 2.0%
Si is a necessary element for deoxidation and securing strength, and the minimum strength as a structural member cannot be secured unless it is less than 0.01%. However, if the content exceeds 2.0%, the weldability deteriorates. In addition, the minimum with preferable Si content is 0.02%, More preferably, it is good to set it as 0.05% or more. Moreover, the upper limit with preferable Si content is 1.80%, It is good to set it as 1.60% or less more preferably.
Mn: 0.01 to 2.0%
Mn is also necessary for deoxidation and securing strength in the same manner as Si, and if it is less than 0.01%, the minimum strength as a structural member cannot be secured. However, if the content exceeds 2.0%, the toughness deteriorates. In addition, the minimum with preferable Mn content is 0.05%, It is good to set it as 0.10% or more more preferably. Moreover, the upper limit with preferable Mn content is 1.80%, More preferably, it is good to set it as 1.60% or less.
P: 0.01% or less P is an element that improves seawater resistance by addition of 0.02% or more. However, P is an element that deteriorates toughness and weldability, and the content is preferably suppressed as much as possible. In the present invention, emphasis is placed on toughness and weldability, and the effect of improving the seawater resistance of P is not used, and the allowable upper limit of P is set to 0.01%.
S: 0.0005 to 0.005%
S is an element that deteriorates toughness and weldability, and the content is preferably suppressed as much as possible. The allowable upper limit of S is up to 0.01%, and if it exceeds this, weldability as a steel material for ballast tanks cannot be ensured. Therefore, S is set to 0.01% or less.
Al: 0.005-0.10%
Al is also necessary for deoxidation and securing of strength in the same manner as Si and Mn, and if less than 0.005%, there is no effect on deoxidation. However, if added over 0.10%, the weldability is impaired, so the range of the amount of Al added is set to 0.005 to 0.10%. In addition, the minimum with preferable Al content is 0.010%, It is good to set it as 0.015% or more more preferably. Moreover, the upper limit with preferable Al content is 0.080%, It is good to set it as 0.090% or less more preferably.
Cu: 0.01 to 1.0%
Cu is an element effective for improving corrosion resistance. Cu has an action of suppressing the corrosion reaction occurring under the anticorrosion coating film, and is an element having the effect of suppressing the swelling of the coating film due to the undercoat corrosion that easily occurs in the thin film portion of the coating. In addition, it has an effect of densifying the generated rust when the steel material is corroded in the coating film defect part, and is an effective element for expressing the effect of suppressing the corrosion progress of the coating film scratched part. . In order to exert these effects, it is necessary to contain 0.01% or more in any case, but if contained excessively, weldability and hot workability deteriorate, so 1.0% or less. There is a need to. The more preferable lower limit when Cu is contained is 0.05%, and the more preferable upper limit is 0.90%.
Ni: 0.01 to 1.0%
Ni is effective in improving corrosion resistance. Ni, like Cu, has the effect of suppressing the corrosion reaction that occurs under the anticorrosion coating, and is an element that has the effect of suppressing the swelling of the coating due to the undercoat corrosion that tends to occur in the thin film portion of the coating. is there. In addition, it has an effect of densifying the generated rust when the steel material is corroded in the coating film defect part, and is an effective element for expressing the effect of suppressing the corrosion progress of the coating film scratched part. . Ni is an element necessary to prevent red heat brittleness due to Cu addition. In order to exert such effects, it is preferable to contain 0.01% or more. However, if the added amount is excessive, weldability and hot workability deteriorate, so 1.0% or less is preferable. The more preferable lower limit when these elements are contained is 0.05%, and the more preferable upper limit is 0.90%.
Cr: 0.01 to 0.5%
Cr is an element effective for improving corrosion resistance. Cr has an action of suppressing primer consumption under the anticorrosion coating film, and is an element effective for exhibiting an effect of suppressing corrosion progress of the coating film scratch. Further, an appropriate amount of Cr is effective for improving toughness, and is an element necessary for obtaining mechanical properties necessary as a ballast tank material. In order to exhibit these effects, it is necessary to contain 0.01% or more, but if it is contained excessively, weldability and hot workability deteriorate, so 0.5% or less is necessary. There is. The more preferable lower limit when Cr is contained is 0.05%, and the more preferable upper limit is 0.45%.

In addition to the above components, the steel material for a crude oil tank bottom plate of the present invention may include (1) Mg: 0.0001 to 0.005%%, Ca: 0.0001 to 0.005%, Sr: 0 if necessary. One or more selected from the group consisting of 0.0001 to 0.005%, (2) Co: 0.005 to 0.20%, Ti: 0.005 to 0.20%, Zr: 0.005 to 0.005. 20% or more selected from the group consisting of: (3) B: 0.0001 to 0.010%, V: 0.01 to 0.50% and Nb: 0.003 to 0.50% It is also effective to contain one or more selected from the group, etc., and the characteristics of the steel material will be further improved according to the type of component to be contained.
Ca: 0.0001 to 0.005%
Ca is an element effective for improving corrosion resistance. Since diffusion of hydrogen ions to the outside hardly occurs at the coating film / steel material interface, the pH decreases due to the hydrolysis of Fe ions dissolved by the steel material corrosion, and the corrosion under the coating film is further accelerated. Ca has an action of mitigating such a decrease in pH, exhibits an effect of suppressing corrosion promotion due to a decrease in pH, and is effective in suppressing swelling of the coating film. Such an effect is effectively exhibited by containing 0.0001% or more of Ca. However, if over 0.005% is contained, workability and weldability are deteriorated. A more preferred lower limit is 0.0005%, and a more preferred upper limit is 0.004%.
Mg: 0.0001 to 0.005%
Mg is an effective element for improving corrosion resistance. Mg, like Ca, has an action of mitigating pH reduction, exhibits an effect of suppressing corrosion promotion due to pH reduction, and is effective in suppressing swelling of the coating film. Such an effect is effectively exhibited by containing 0.0001% or more, and is particularly effective when Co is present. However, if over 0.005% is contained, workability and weldability are deteriorated. A more preferable lower limit of Mg is 0.0005%, and a more preferable upper limit is 0.004%.
Sr: 0.001 to 0.005%
Sr is an element effective for improving corrosion resistance. Sr, like Ca and Mg, has an action of mitigating pH reduction, exhibits an effect of suppressing corrosion promotion due to pH reduction, and is effective in suppressing film swelling. Such an effect is effectively exhibited by containing 0.0001% or more. However, if over 0.005% is contained, workability and weldability are deteriorated. A more preferable lower limit of Sr is 0.0005%, and a more preferable upper limit is 0.004%.
Co: 0.005 to 0.20%
Co is an element effective for improving corrosion resistance. Co has an action of densifying rust generated in a chloride corrosive environment, and is an element that suppresses the progress of corrosion at the scratches on the coating film. In order to exhibit such an effect, it is preferable to contain 0.005% or more. However, if the addition amount is excessive, weldability and hot workability deteriorate, so 0.2% or less is preferable. The more preferable lower limit when these elements are contained is 0.02%, and the more preferable upper limit is 0.8%.
Ti: 0.005 to 0.20%
Ti is an element effective for improving corrosion resistance. Ti has an action of densifying rust generated in a chloride corrosive environment, and is an element that suppresses the progress of corrosion at the scratches on the coating film. In order to exhibit such an effect, it is preferable to contain 0.005% or more. However, if the addition amount is excessive, weldability and hot workability deteriorate, so 0.20% or less is preferable. The more preferable lower limit when Ti is contained is 0.008%, and the more preferable upper limit is 0.15%.
Zr: 0.005 to 0.20%
Zr is an element effective for improving corrosion resistance. Zr, like Ti, has an action of densifying rust generated in a chloride corrosive environment, and is an element that suppresses corrosion progress in the scratches on the coating film. In order to exhibit such an effect, it is preferable to contain 0.005% or more. However, if the addition amount is excessive, weldability and hot workability deteriorate, so 0.20% or less is preferable. A more preferable lower limit when containing Zr is 0.008%, and a more preferable upper limit is 0.15%.
One or more selected from the group consisting of B: 0.0001 to 0.010%, V: 0.01 to 0.50% and Nb: 0.003 to 0.50% B, V and Nb are all It is an effective element for improving mechanical properties. Of these, B is contained in an amount of 0.0001% or more, which improves the hardenability and is effective in improving the strength. . V is effective for improving the strength by containing 0.01% or more, but if it exceeds 0.50%, the toughness of the steel material is deteriorated, which is not preferable. Nb is effective for improving the strength by containing 0.003% or more, but if it exceeds 0.50% and it is contained excessively, the toughness of the steel will be deteriorated. The more preferable lower limit of these elements is 0.0003% for B, 0.02% for V, and 0.005% for Nb. Further, more preferable upper limit is 0.0090% for B, 0.45% for V, and 0.45% for Nb.

The components of the steel material for ballast tanks of the present invention are as described above, and the balance consists of iron and inevitable impurities. Inevitable impurity elements include, for example, O, N, H, Mo, W, etc., preferably not exceeding 0.1%, and more preferably not exceeding 0.01%. .
Structure It is preferable that the structure of the steel material of the present invention is mainly composed of ferrite excellent in weldability and workability (total area ratio is 50% or more). Moreover, since ferrite is less sensitive to stress corrosion cracking due to chloride, it is advantageous as a structural member in a chloride environment called seawater. The steel of the present invention is mainly composed of ferrite, and in addition, it is necessary to control the area ratio of bainite and / or martensite. The steel material of the present invention is allowed even when it contains a structure other than ferrite, bainite and martensite, such as pearlite, as a part of its structure.

  Bainite and martensite are effective structures for improving the strength and toughness of steel materials. However, if bainite and / or martensite is present more than necessary in the ferrite, corrosion of the bainite and / or martensite portion is promoted under the anticorrosive coating, and coating corrosion resistance is lowered. In order to prevent such a decrease in corrosion resistance, it is recommended that the area ratio of bainite + martensite be less than 30% (including the case where either bainite, martensite, or both are 0).

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.
Sulfide inclusions As non-metallic inclusions, inclusions such as sulfides, oxides, nitrides or carbides are present in the steel. Of these, sulfide inclusions such as MnS are the starting point of corrosion, and are the most harmful inclusions from the viewpoint of corrosion resistance as steel materials for ballast tanks. When the size and number of sulfide inclusions are controlled, if the size is increased and the number is decreased, significant local corrosion occurs starting from that and leads to early drilling. On the other hand, when the size is reduced and the number is increased, the number of corrosion starting points under the anticorrosive coating film increases and the occurrence of swelling of the anticorrosive coating film is promoted. In addition to reducing sulfide inclusions using a suitable desulfurization apparatus in the steelmaking process, it has been found that the adverse effects described above can be minimized by adjusting the size of the inclusions. Specifically, the harmfulness can be minimized by setting the average particle diameter (equivalent circle diameter) of the sulfide inclusions to 0.5 to 10 μm.

  In the sulfide inclusions defined in the present invention, for example, in the secondary refining, in addition to adjusting to the above component range, the ratio of Mn / (S + Mg + Ca + Sr) is more than 10 and less than 700. It is possible to obtain the molten steel by sufficiently stirring the molten steel by bubbling with an inert gas such as Ar while adjusting the components. Mn has an action of suppressing nucleation of sulfide inclusions, and S, Mg, Ca, and Sr have an action of promoting the formation of the inclusions. For this reason, the ratio of Mn / (S + Mg + Ca + Sr) is related to the size of sulfide inclusions. If the ratio is large, the nucleation of sulfide inclusions is suppressed and the generated inclusions tend to be coarse. It is in. Thus, the average particle diameter of sulfide inclusions can be controlled by adjusting the ratio.

  In the present invention, the average particle size of the sulfide inclusions is ½ of the thickness of the steel material when the steel material thickness is 6 mm or more and the steel material thickness is less than 6 mm at a portion 3 mm deep from the surface. In this part, the equivalent circle diameter of 30 arbitrary sulfide inclusions is measured, and the obtained average value is adopted. The equivalent circle diameter of sulfide inclusions can be measured 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. it can.

As exemplified in FIG. 1, the sulfide inclusion in the present invention has the highest peak intensity of S in the EDX spectrum by comparing the intensity of peaks corresponding to S, O, N and C. Is referred to as sulfide inclusions. EDX analysis can be performed during SEM observation for average particle size measurement to confirm sulfide inclusions.
Manufacturing method The steel material of this invention can be manufactured, for example with the following method. Secondary refining, including component adjustment and temperature adjustment, is performed on molten steel that has been discharged from a converter or electric furnace to a ladle using an RH vacuum degasser. In order to control sulfide inclusions, it is necessary to sufficiently stir the molten steel by bubbling with an inert gas such as Ar during secondary refining. Then, it is made into a steel ingot by a normal casting method such as a continuous casting method or an ingot-making method. 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.

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 hot rolling end temperature is controlled to 700 to 850 ° C., and the cooling rate from the end of hot rolling to 500 ° C. is controlled in the range of 0.1 to 15 ° C./second, whereby a predetermined structure is obtained. can get.
Anticorrosion paint During the period from shipping from steelworks to application of anticorrosion paint, a primer such as zinc rich primer is usually applied for the purpose of preventing rusting of the steel material. Before applying the anticorrosive paint, the surface to be coated is generally cleaned with a wire brush or sand pepper for the purpose of removing rust or oil generated in the defective primer portion which adversely affects the coating. The steel material for ballast tank of the present invention improves the corrosion resistance by removing the primer and the remaining black skin (mill scale) before applying the epoxy resin-based anticorrosion paint and applying the paint directly to the steel material. Since the alloy element effective for the above acts directly on the corrosion reaction, a further effect of improving the coating corrosion resistance can be obtained. As a method for removing the primer and the like, a method such as sand blasting, shot blasting, and grinding can be appropriately selected in consideration of workability and the like.

  As the anticorrosion paint to be applied to the steel material for ballast tank of the present invention, an epoxy resin paint such as a tar epoxy resin paint or a modified epoxy resin paint is preferable, but a urethane resin system or an acrylic resin system is apparent from its effect. Even when paints such as these are used, the effect of extending the life of the coating film is exhibited.

  In addition, the effects of other anticorrosion methods such as cathodic protection (galvanic anode method, external power supply method) are not harmed, and it is possible to use them together.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified 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.
(Example)
[Production of test materials]
The molten steel produced from the converter is subjected to bubbling with Ar gas using an RH vacuum degassing device, and the components are adjusted to the composition shown in Table 1 while stirring the molten steel. It was. The deoxidizing form is Al killed steel. The obtained steel ingot was heated to 1150 ° C., and then hot rolled to produce a steel plate having a thickness of 19 mm. At this time, the hot rolling end temperature and the cooling rate from the end of hot rolling to 500 ° C. are as shown in Table 2. Table 2 also shows the area ratio of bainite + martensite (B + M) of the obtained steel material and the average particle size of sulfide inclusions. In addition, all the structures of the remainder of the steel material are mainly composed of ferrite (F) (area ratio of 50% or more), and part thereof includes other pearlite (P).

  A test piece having a size of 100 × 100 × 10 (mm) was cut out from the obtained steel plate. The entire surface of the test piece is polished and finished with a wet rotary polishing machine (abrasive paper; # 600), washed with water and acetone, then applied with a zinc rich primer so that the average film thickness is 15 μm (± 3 μm), and over 24 hours. Dry in a desiccator. Next, as shown in Table 2, pretreatment was performed and various anticorrosive paints were applied by airless spray. Note that brushing applied as a pretreatment for painting is a treatment for removing dust, dust, and the like on the surface to be painted. Moreover, sandblasting was performed until all zinc rich primers disappeared.

The appearance of the test piece A thus obtained is as shown in FIG. In the test, in order to investigate the degree of corrosion progress when the base steel material is exposed due to scratches on the anticorrosive coating film, a cut surface that reaches the base of length: 100 mm and width: about 0.5 mm is tested. Test piece B (FIG. 3) formed with a cutter knife on the side (high temperature side during the corrosion test) was also used.
[Corrosion test method]
The laboratory evaluation test method simulating the inside of the ballast tank is as follows. Test pieces A and B are installed vertically in a test tank filled with artificial seawater of the test solution, the temperature on the test side of the test piece is adjusted to 40 ° C., and the back side is adjusted to 10 ° C. A temperature difference gradient was applied. In addition, the test piece B with the cut flaw was placed so that the cut flaw was on the high temperature side. As shown in FIG. 4, the whole test piece is submerged in water (simulating an empty ballast state) for 2 weeks, and then the artificial seawater is drained to expose the test piece on the water surface as shown in FIG. The held state (simulated load state) was held for one week. In the state of FIG. 5, artificial seawater was left below the test piece in the test tank, and the temperature difference gradient of the test piece was maintained by the temperature difference in the gas phase. When a temperature difference gradient is applied, moisture penetration of the coating film is promoted from the higher temperature side to the lower temperature side. Accordingly, the high temperature side (40 ° C.) at which corrosion under the coating film becomes significant was taken as the test surface (evaluation surface). In the evaluation test, the states of FIGS. 4 and 5 were repeated and continued for a total of 24 weeks (168 days). The number of test pieces subjected to the test is 5 for each of A and B. In test piece A, the time until the coating film swells due to the expansion pressure of the corrosion product at the coating film / steel material interface is measured. The blister resistance was evaluated. The time until the occurrence of the swelling of the coating film was observed until the appearance of the swelling of the coating film was observed in any one of the five test pieces tested by visual observation once a day.

For test piece B, the swelling width of the anticorrosion coating film (width swollen in the direction perpendicular to the cut wound) was measured with a caliper after completion of the test (after 24 weeks), and each of the five test pieces tested The coating film scratch resistance was evaluated by the maximum value (maximum swollen width).
[Corrosion test results]
The results of the corrosion test are shown in Table 3. As a normal anticorrosive coating film, a standard 250 μm modified epoxy resin coating film was formed. No. 1 was inferior in both the swelling resistance of coating film (Δ) and the scratch resistance of the coating film (×), and No. 1 which assumed the part where the anticorrosion coating film became thin. 1 is very inferior in both cases. No. 3 and no. No. 4 has a Cu content and a Cr content that are less than the specified values, respectively, so that the coating corrosion resistance is insufficiently improved and is not suitable as a steel material for ballast tanks. No. 5 and no. Although the component range of each additive element satisfies the specified value, the area ratio of bainite + martensite and the average particle size of sulfide inclusions do not satisfy the specifications, so the coating corrosion resistance improvement is slightly insufficient. As a steel material for ballast tanks, it is not satisfactory.

  On the other hand, it is clear that the corrosion resistance of all those controlled to the component range of the present invention (Nos. 7 to 27) is improved to a level of ◯ or higher. In particular, the products (No. 8, No. 12, No. 19, No. 26) in which the shop primer was removed and the anticorrosion paint was directly applied to the steel material were those with the remaining shop primer (No. 7, No. 11, It can be seen that the effect of improving the corrosion resistance is higher than those of No. 18 and No. 25). Further, the effect of improving the coating corrosion resistance is observed in both the modified epoxy resin and the tar epoxy resin, and the film thickness is also observed at any film thickness of 50 and 250 μm.

  As described above, the steel material of the present invention has excellent coating corrosion resistance, delays coating film deterioration starting from coating film swelling, and can suppress the corrosion progress from exposed steel parts such as coating film defects and scratches, It turns out that it is preferable as a steel material for ballast tanks. Therefore, it is easily found that the ballast tank made of the steel material of the present invention has excellent durability.

An example of the sulfide inclusions referred to in the present invention is shown, the upper figure is the SEM photograph, and the figure is the EDX spectrum of the arrow part in the upper figure. It is a top view which shows the general-view shape of the test piece A used for the Example. It is a top view which shows the general-view shape of the test piece B (what formed the cut collar) used for the Example. It is a figure which demonstrates the corrosion test method used for the Example, and shows a mode that the artificial seawater as a test liquid is maintained high, and the test piece was installed in the state which the whole was submerged to this. It is a figure which demonstrates the corrosion test method used for the Example, and shows a mode that the artificial seawater as a test liquid is maintained low, and it has installed in the state which exposed the test piece on the water surface to this.

Claims (6)

C: 0.01 to 0.30% (meaning mass%, the same shall apply hereinafter)
Si: 0.01 to 2.0%,
Mn: 0.01 to 2.0%,
P: 0.01% or less,
S: 0.0005 to 0.005% or less,
Al: 0.005 to 0.10%,
Cu: 0.1 to 1.0%
Ni: 0.01 to 1.0%,
Cr: 0.01 to 0.5%
The balance is composed of Fe and inevitable impurities, has a structure mainly composed of ferrite, and has an area ratio of bainite and / or martensite of less than 30% (including 0). A steel material for ballast tanks excellent in corrosion resistance, characterized by having an average particle size of 0.5 to 10 μm. further,
Mg: 0.0001 to 0.005%,
Ca: 0.0001 to 0.005%,
Sr: 0.0001 to 0.005%,
The steel material for ballast tanks having excellent corrosion resistance according to claim 1, comprising one or more selected from the group consisting of: further,
Co: 0.005 to 0.20%,
Ti: 0.005 to 0.20%,
Zr: 0.005 to 0.20%,
The steel material for ballast tanks having excellent corrosion resistance according to claim 1 or 2, comprising one or more selected from the group consisting of: further,
B: 0.0001 to 0.010%,
V: 0.01 to 0.50%,
Nb: 0.003 to 0.50%,
The steel material for ballast tanks excellent in corrosion resistance according to any one of claims 1 to 3, comprising one or more selected from the group consisting of: The steel material for ballast tanks excellent in corrosion resistance in any one of Claims 1-4 which formed the anti-corrosion coating film directly on the steel material surface. The ship which has a ballast tank comprised with the steel material for ballast tanks in any one of Claims 1-5.

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