WO2013111355A1

WO2013111355A1 - Corrosion-resistant steel for hold of coal carrying vessel or coal/ore carrying vessel

Info

Publication number: WO2013111355A1
Application number: PCT/JP2012/061624
Authority: WO
Inventors: 真孝面田; 釣　之郎; 務小森; 星野　俊幸
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2012-01-25
Filing date: 2012-04-25
Publication date: 2013-08-01
Also published as: JP5862323B2; EP2808411B1; JP2012177190A; EP2808411A4; KR20140105862A; EP2808411A1; CN104105806A

Abstract

The purpose of the present invention is to provide a corrosion-resistant steel for a hold of a coal carrying vessel and a coal/ore carrying vessel, which does not undergo corrosion after removal of a coating film therefrom under an environment in which drying and humidification are repeated and the pH value is low. Specifically provided is a corrosion-resistant steel for a hold of a coal carrying vessel or a coal/ore carrying vessel, which is characterized in that a material for the steel has a component composition containing 0.010 to 0.200 mass% of C, 0.05 to 0.50 mass% of Si, 0.10 to 2.0 mass% of Mn, 0.0250 mass% or less of P, 0.010 mass% or less of S, 0.0050 to 0.10 mass% of Al, 0.010 to 0.50 mass% of Sb and 0.0010 to 0.0080 mass% of N, with the remainder being Fe and unavoidable impurities.

Description

Corrosion resistant steel for holding coal ships or coal / ore combined ships

The present invention relates to a steel material having excellent corrosion resistance used for a hold (also called a hold) of a coal carrier or a coal / ore carrier (ore / coal carrier).

In the bulk carrier, maritime accidents became an international problem in the early 1990s. In particular, many accidents were reported on coal ships and coal / iron ore combined ships, most of which were damage in the hold. Bulk cargo ships are loaded directly on the hold, so they are easily affected by corrosive loads. Corrosion in the hold (hereinafter also referred to as “hold”), especially the hold of coal ships and coal / iron ore combined ships. It is considered a problem that the strength locally decreases due to pitting corrosion at the inner side wall (side shell). Cases where this pitting corrosion has progressed remarkably and cases where the plate thickness of the side frame that secures the strength of the ship has been extremely reduced have been reported. When the change standard of the hold side wall steel is 70% or less of the drawing plate thickness, and the change standard of the hold rib steel is 75% or less of the drawing plate thickness (however, the drawing plate thickness-corrosion allowance-larger than the corrosion allowance thickness) It is not necessary to have a value).

The side wall portion of the bulk cargo ship where pitting corrosion occurs is a single-hull, and the cargo and seawater are separated from each other only by one steel material. The temperature in the hold rises due to self-heating of coal. Therefore, dew condensation water is likely to occur on the side wall of the hold due to the temperature difference between the seawater and the hold. Since coal SO ₄ ²⁻ dissolves in a place where dew condensation water is generated on the side wall of the cargo hold and reacts with the dew condensation water to produce sulfuric acid, the inside of the hold has a low pH environment in which sulfuric acid corrosion is likely to occur. Therefore, for low pH environment, “suppress hydrogen generation reaction”, SO ₄ ²⁻ base iron-rust which becomes counter anion of iron dissolution transmission to the interface for the _{^{(SO 4 2- permeation to rust /}} steel interface), _"SO ^{4 2-rust} permeation inhibiting _{^{(inhibition of SO 4 2- permeation in}} rust layers) " two corrosion mechanisms that (Corrosion protection mechanism) is required.

As a countermeasure against corrosion in such a hold (corrosion control method), a modified epoxy-based coating is applied in the hold to a coating thickness of about 150 to 200 μm. However, since the paint is often peeled off due to mechanical damage due to coal or iron ore, heavy scratches and heavy machinery during loading and unloading, sufficient anticorrosion effect (effect of corrosion) (protection) is not obtained.

Therefore, as a countermeasure against corrosion, a method of periodically repainting or partially repairing is taken, but since such a method is very expensive, it is necessary to maintain the ship. The challenge is to reduce the life cycle cost, including the cost of maintenance.

By the way, as corrosion-resistant steel for ships, steels developed for cargo oil tanks and ballast tanks are known.

The upper deck side of the cargo oil tank is an O ₂ , CO ₂ , SO ₂ or the like contained in an inert gas that is blown into the tank in order to prevent explosion protection. It is exposed to an environment of corrosive gas such as H ₂ S that volatilizes from crude oil. Although the bottom plate has a protective film derived from crude oil ( also referred to as “oil coat”) (protective film), local corrosion of the bowl-shaped is caused at the point where the film is peeled off. Exposed to the resulting environment. For example, Patent Document 1 proposes a corrosion-resistant steel using the anti-corrosion mechanism of “improvement of corrosion resistance by suppressing pH decrease” and “improvement of local corrosion resistance by fine dispersion of sulfide”.

In addition, the ballast tank plays a role of enabling stable navigation of the ship by injecting seawater when there is no cargo, and is placed in a very severe corrosive environment (severe corrosive environment). The back side of the upper deck of the ballast tank is not immersed in seawater and is not in a state where it is splashed with seawater, so cathodic protection does not function, and this part is caused by sunlight. Since the temperature of the steel material rises, it becomes a severe corrosive environment and receives severe corrosion. Moreover, although the side wall surface and the bottom surface of the ballast tank are completely immersed in seawater and are in a corrosive environment, the anticorrosive action functions.

However, when operating without cargo, seawater is not injected into the ballast tank, and since the entire ballast tank does not have anti-corrosion protection, it is subject to severe corrosion due to repeated wet and dry environments and residual adhered salt. . For example, in Patent Document 2, it is possible to suppress the passage of Cl ⁻ by densifying rust, and in Patent Document 3, a corrosion prevention mechanism that electrochemically suppresses the transmission of Cl ⁻ by WO ₄ ^2−. Corrosion-resistant steel has been proposed.

As described above, in a coal ship and a coal / ore combined ship, in a low pH environment where sulfuric acid is concentrated by repeated drying and wetting, suppression of hydrogen generation reaction and permeation of SO ₄ ^{2− to} the rust-steel interface are prevented. Must be suppressed. As described above, in the hold of the coal ship and the coal / ore combined ship and the ballast tank and the oil tank, the corrosion environment and the anticorrosion mechanism are different, so that the corrosion resistant steel for the ballast tank and the oil tank cannot be diverted as it is. For this reason, as a steel for a coal ship or a coal / ore combined ship holding, an original material design and characteristic evaluation are required.

Further,

Patent Documents

1, 4 and 5 are known as conventional techniques referring to a coal ship or a coal / ore combined ship holding application. As chemical composition compositions of corrosion-resistant steel for shipbuilding under the use environment of coal ships and coal / ore combined ships, Patent Document 1 discloses steel materials containing Cu and Mg as essential component compositions, and Patent Document 4 discloses Cu and Ni. Steel materials having Sn and Sn as essential components, and Patent Document 5 disclose steel materials having Cu and Sn as essential components for the purpose of further improving cost.

Japanese Patent Laid-Open No. 2000-17371 JP 2008-144204 A JP 2007-46148 A JP 2007-262555 A JP 2008-174768 A

However, since the steel materials disclosed in Patent Document 1 are intended for excellent steel materials in common use environments such as ship outer plates, ballast tanks, cargo oil tanks, ore ship cargo holds, etc., the corrosion resistance of steel materials is evaluated. As a method, the results of corrosion tests of cargo oil tanks and ballast tanks are cited as good, but no test results have been shown that take into account the hold use environment of coal ships and coal / ore combined ships.

In Patent Documents 4 and 5, although corrosion resistance under a coating film simulating the environment of a coal ship or coal / ore combined ship is evaluated, mechanical damage caused by coal or iron ore is inevitable in a hold use environment. The evaluation test assuming the situation where peeling easily occurs and the evaluation of the maximum pitting corrosion depth which is the standard for switching the steel sheet are not performed.

As described above, in the development of steel materials with excellent corrosion resistance for use in coal ships or coal / ore combined ships, the corrosion environment unique to coal ships or coal / ore combined ships is taken into account, and at the same time, the coated film is peeled off. Despite the importance of evaluating the corrosion of steel in the absence of this, this viewpoint has not been considered in the prior art.

Therefore, an object of the present invention is to provide a corrosion-resistant steel for holding a coal ship or a coal / ore combined ship that can suppress corrosion after peeling of a coating film in a dry and wet repeated and low pH environment.

Generally, a ship is constructed by welding steel materials such as steel plate, steel plate, shaped steel, steel bar, etc., and the surface of the steel material is anticorrosive. Used with a coating applied. However, in a coal ship or coal / ore combined ship hold environment, the coating is easily peeled off due to mechanical damage of the coal / ore, and the steel material is exposed to cyclic wet and dry environment and a low pH environment. Here, the steel material which developed corrosion resistance even after peeling of the anticorrosion coating film on the surface of the steel material was developed.

Therefore, the present inventors have developed a test method simulating the environment in a coal ship or a coal / ore combined-use ship hold and examined the influence of each alloy element using the test method. As a result, Further, the addition of Cu and Ni has been found to improve the corrosion resistance of the steel material after peeling of the coating film of the coal ship or coal / ore combined-use ship hold, and the present invention has been completed. A test method simulating the environment in the coal / ore combined ship hold will be described later in Examples.
1. The component composition of the steel material is C: 0.010 to 0.200 mass%, Si: 0.05 to 0.50 mass%, Mn: 0.10 to 2.0 mass%, P: 0.0250 mass% or less, S: 0 0.010 mass% or less, Al: 0.0050 to 0.10 mass%, Sb: 0.010 to 0.50 mass%, N: 0.0010 to 0.0080 mass%, and the balance from Fe and inevitable impurities Corrosion resistant steel for holding a coal ship or a combined coal / ore ship.
2. 2. Coal according to 1, further comprising at least one selected from Cu: 0.010 to 1.0 mass% and Ni: 0.010 to 1.0 mass% in addition to the steel material Corrosion resistant steel for holding ships or coal / ore combined ships.
3. Corrosion-resistant steel for holding a coal ship or a coal / ore combined ship according to 1 or 2, wherein the steel material is Cr: 0.050 mass% or less.
4). In addition to the steel material, any one of 1-3, further comprising at least one selected from W: 0.005-0.5 mass% and Mo: 0.005-0.5 mass% Corrosion-resistant steel for holding a coal ship or coal / ore combined ship according to any one of the above.
5. In addition to the steel materials, Ti: 0.0010 to 0.030 mass%, Nb: 0.0010 to 0.030 mass%, Zr: 0.0010 to 0.030 mass%, and V: 0.0020 to 0.20 mass%. The corrosion-resistant steel for holding a coal ship or a coal / ore combined ship according to any one of 1 to 4, characterized by containing at least one selected from among them.
6). Corrosion-resistant steel for holding a coal ship or a coal / ore combined ship according to any one of 1 to 5, further comprising Ca: 0.0005 to 0.0040 mass% in addition to the steel material .
7). In addition to the steel material, one or more selected from REM: 0.0001 to 0.0150 mass% and Y: 0.0001 to 0.10 mass% are contained. Corrosion resistant steel for holding coal ships or coal / ore combined ships.
8). In addition to the steel material, it contains at least one selected from Se: 0.0005 to 0.50 mass%, Te: 0.0005 to 0.50 mass%, and Co: 0.010 to 0.50 mass%. The corrosion-resistant steel for holding a coal ship and a coal / ore combined ship according to any one of 1 to 7,

ADVANTAGE OF THE INVENTION According to this invention, the corrosion resistance for the coal ship which can suppress the corrosion after peeling of a coating film in a coal ship, a coal and ore combined ship hold | maintenance, and a low pH environment, and a coal and ore combined ship hold Steel can be obtained.

The figure which shows an example of the temperature-humidity cycle of a coal corrosion test. The figure which estimates the maximum sheet thickness reduction of coal ship or coal and ore combined use hold steel material 25 years later. The figure which shows the mapping result of S after the coal corrosion test of the example of this invention and a comparative example by an electron beam microanalyzer.

Hereinafter, modes for carrying out the present invention will be described. First, the reason why the component composition of the steel material is limited to the above range in the present invention will be described.

C: 0.010-0.200 mass%
C is an element effective for increasing the strength of steel, and in the present invention, it is necessary to contain 0.010 mass% or more in order to ensure the strength. On the other hand, the content exceeding 0.200 mass% decreases the weldability and the toughness of the weld heat affected zone. Therefore, C is set to a range of 0.010 to 0.200 mass%. Furthermore, it is preferably in the range of 0.050 to 0.150 mass%.

Si: 0.05 to 0.50 mass%
Si is added as a deoxidizer and is an element that enhances the strength of steel. In the present invention, it contains 0.05 mass% or more. However, since the content exceeding 0.50 mass% deteriorates the toughness of steel, the upper limit of Si is set to 0.50 mass%. In addition, Si improves the corrosion resistance by forming an anticorrosion film in an acidic environment. In order to obtain this effect, the range is preferably 0.20 to 0.40 mass%.

Mn: 0.10 to 2.0 mass%
Mn is an element that can increase the strength of the steel at a low cost and can prevent hot brittleness, so it is contained in an amount of 0.10 mass% or more. However, if the content exceeds 2.0 mass%, the toughness and weldability of the steel are reduced, so Mn is set to 2.0 mass% or less. From the viewpoint of securing strength and suppressing inclusions, the range is preferably 0.80 to 1.4 mass%.

P: 0.0250 mass% or less P is a harmful element that deteriorates not only the base metal toughness of steel but also the weldability and weld toughness by segregating at the grain boundaries, so it is desirable to reduce it as much as possible. In particular, when the P content exceeds 0.0250 mass%, the deterioration of the base metal toughness and weld zone toughness increases. Therefore, P is set to 0.0250 mass% or less. Preferably, it is 0.0150 mass% or less.

S: 0.010 mass% or less S forms MnS which is a starting point of local corrosion, and reduces local corrosion resistance. Furthermore, since it is a harmful element that deteriorates the toughness and weldability of steel, it is desirable to reduce it as much as possible. In the present invention, it is limited to 0.010 mass% or less. Preferably it is 0.007 mass% or less, More preferably, it is 0.005 mass% or less.

Al: 0.0050 to 0.10 mass%
Al is added as a deoxidizer. For this purpose, the content of 0.0050 mass% or more is required, but the content exceeding 0.10 mass% lowers the toughness of the weld metal part when welding. Therefore, Al is limited to the range of 0.0050 to 0.10 mass%. Preferably, it is 0.010 to 0.050 mass%.

Sb: 0.010 to 0.50 mass%
When Sb contains 0.010 mass% or more as an alloy element in the steel material, it is concentrated near the ground iron in a low pH environment. Since Sb has a large hydrogen overvoltage, the hydrogen generation reaction is suppressed at the portion where Sb is deposited, and the corrosion resistance is improved. Furthermore, the corrosion product is made dense, and diffusion of H ₂ O, O ₂ , SO ₄ ²⁻ , and Cl ⁻ into the ground iron is suppressed.

On the other hand, when Sb is added exceeding 0.50 mass%, the toughness is lowered. Therefore, Sb was limited to a range of 0.010 to 0.50 mass%. Preferably, it is in the range of 0.010 to 0.30 mass%, and more preferably in the range of 0.010 to 0.20 mass%.

N: 0.0010 to 0.0080 mass%
N is an element that lowers toughness, and is desirably reduced as much as possible. However, it is difficult to reduce it to less than 0.0010 mass% industrially. On the other hand, when it contains exceeding 0.0080 mass%, the remarkable toughness deterioration will be caused. Therefore, in the present invention, N is limited to the range of 0.0010 to 0.0080 mass%. Preferably, the content is 0.0010 to 0.0050 mass%.

Furthermore, in addition to the above essential components, the steel material of the present invention can contain one or more selected from Cu and Ni in the following range.

Cu: 0.010 to 1.0 mass%
Cu densifies corrosion products and suppresses the diffusion of H ₂ O, O ₂ , SO ₄ ²⁻ , and Cl ⁻ into the ground iron. Thereby, the corrosion resistance of steel improves. This effect is manifested with a content of 0.010 mass% or more, but as the amount added increases, the weldability and the toughness of the base material decrease. Therefore, when Cu is contained, it is preferably in the range of 0.010 to 1.0 mass%. More preferably, it is in the range of 0.010 to 0.50 mass%. More preferably, it is in the range of 0.010 to 0.35 mass%. Cu also has the effect of improving the corrosion resistance by forming Cu ₂ Sb, which is an intermetallic compound in the presence of Sb.

Ni: 0.010 to 1.0 mass%
Ni, like Cu, densifies the corrosion products and suppresses diffusion of H ₂ O, O ₂ , SO ₄ ²⁻ , and Cl ⁻ into the ground iron. Thereby, the corrosion resistance of steel improves. This effect is manifested when the content is 0.010 mass% or more. However, if the content exceeds 1.0 mass%, the effect is saturated and the cost is increased, so when Ni is contained, 0.010 to 1.0 mass%. A range is preferable. More preferably, it is in the range of 0.010 to 0.50 mass%.

The steel material of the present invention can further contain Cr in the following range in addition to the above components.

Cr: 0.050 mass% or less Cr is an element that lowers the corrosion resistance because it causes hydrolysis in a low pH environment, so it may be added without addition. Although it can be added for strength adjustment, particularly when its content exceeds 0.050 mass%, the corrosion resistance is significantly reduced. Therefore, when Cr is contained, its content should be 0.050 mass% or less. preferable. More preferably, it is 0.030 mass% or less.

W: 0.005 to 0.5 mass% and Mo: 0.005 to 0.5 mass%
When W and Mo are eluted from the base material, they form oxygen acid, which electrically repels the anion, prevents the anion from penetrating to the surface of the ground iron, and improves the corrosion resistance. Furthermore, Mo and W improve corrosion resistance by forming a hardly soluble corrosive substance such as FeMoO ₄ or FeWO ₄ . In order to acquire these effects, it is preferable to contain 0.005 mass% or more of all. However, even if added over 0.5 mass%, the effect is not only saturated, but also the cost increases. Therefore, when it is contained, the content is preferably set to 0.5 mass% or less. More preferably, it is 0.010 to 0.3 mass%.

In addition to the above components, the steel material of the present invention may further contain one or more selected from Ti, Nb, Zr and V in the following range for the purpose of improving the strength.

Ti: 0.0010 to 0.030 mass%, Nb: 0.0010 to 0.030 mass%, Zr: 0.0010 to 0.030 mass%, V: 0.0020 to 0.20 mass%, one or more types Ti , Nb, Zr and V are all elements that increase the strength of the steel, and can be selected and contained according to the required strength. In order to obtain such an effect, it is preferable to contain Ti, Nb, and Zr in an amount of 0.0010 mass% or more, and V in an amount of 0.0020 mass% or more. However, Ti, Nb, and Zr are all 0.030 mass%, and if V is contained in excess of 0.20 mass%, the toughness decreases, so when Ti, Nb, Zr, and V are contained, Each of them is preferably contained in the above range. More preferably, Ti: 0.0050 to 0.020 mass%, Nb: 0.0050 to 0.020 mass%, Zr: 0.0050 to 0.020 mass%, and V: 0.0050 to 0.10 mass%.

In addition to the above components, the steel material of the present invention can further contain Ca in the following range.
Ca: 0.0005 to 0.0040 mass%
Ca is an element that increases the ductility and toughness of steel by controlling the form of inclusions. In order to exhibit such an effect, it is preferable to contain at least 0.0005 mass%. However, if it is contained excessively, coarse inclusions are formed and the toughness of the base material is deteriorated. Therefore, when it is contained, the upper limit is preferably made 0.0040 mass%. More preferably, it is 0.0010 to 0.0030 mass%.

In addition to the above components, one or more selected from REM and Y can be added to the steel material of the present invention in the following range for the purpose of improving toughness.

REM: 0.0001 to 0.0150 mass%, Y: 0.0001 to 0.10 mass%
REM (rare earth metal) and Y are both elements that increase the toughness of the weld heat affected zone, and can be contained as necessary. This effect is obtained when both REM and Y are contained in an amount of 0.0001 mass% or more. However, when REM contains 0.0150 mass% and Y exceeds 0.10 mass%, the toughness is deteriorated. Therefore, when REM and Y are contained, it is preferable to set the above ranges.

In addition to the above components, the steel material of the present invention may further contain one or more selected from Se, Te, and Co in the following range for the purpose of improving the strength.

One or more of Se: 0.0005 to 0.50 mass%, Te: 0.0005 to 0.50 mass%, and Co: 0.010 to 0.50 mass% increase the strength of the steel. It is an element and can be contained as required. In order to obtain this effect, Se and Te are preferably contained in an amount of 0.0005 mass% or more, and Co is preferably contained in an amount of 0.010 mass% or more. However, any of Se, Te, and Co is contained in an amount exceeding 0.50 mass%. When it contains, it is preferable to set it as said range.

Of the chemical components in the present invention, components other than those described above are Fe and inevitable impurities. However, as long as the effects of the present invention are not lost, the inclusion of components other than those described above is not rejected. For example, Mg: 0.0001 to 0.010 mass% can be contained for the purpose of improving toughness.

On the other hand, as will be shown later in Examples, even if Sn is contained instead of Sb, there is no effect of suppressing the corrosion weight loss and the maximum pitting corrosion depth. Furthermore, Sn coexists with Cu lowers the melting point of Cu and further lowers the solid solubility in iron, so that Cu precipitates at grain boundaries on the surface of the steel material and causes hot cracking. Therefore, Sn is not added, but if its content is less than 0.005 mass%, it does not cause hot cracking and is acceptable as an impurity.

Next, although the suitable manufacturing method of the corrosion-resistant steel material which concerns on this invention is demonstrated, the manufacturing method which can apply this invention is not restricted to this.

The steel material obtained by continuous casting or the like is hot-rolled as it is or after re-cooling after cooling. The heat treatment conditions for exerting the corrosion resistance are not limited, but it is preferable to ensure an appropriate reduction ratio from the viewpoint of mechanical properties. When the finishing temperature of hot rolling is less than 750 ° C., deformation resistance increases and defective shape occurs. Therefore, the finishing temperature is preferably 750 ° C. or higher.

For example, a steel material having a tensile strength of 490 MPa or higher can be manufactured by controlling the cooling rate to 600 ° C. or lower at a finishing temperature of 750 ° C. or higher and then a cooling rate of 150 ° C./min or higher.

The steel shown in Table 1 was made into a slab by continuous casting after melting in a vacuum melting furnace or melting in a converter. Next, the slab was charged into a heating furnace and heated to 1200 ° C., and a steel plate having a thickness of 25 mm was formed by hot rolling at a finish rolling finishing temperature of 800 ° C.

As a result of investigating the mechanism of pitting corrosion that most affects the destruction of the ship due to the corrosion in the hold of the coal ship and the coal / ore combined ship, the present inventors have found the following. The side wall of the bulk carrier is a single hull, and the cargo and seawater are separated from each other only by one piece of steel. Therefore, due to the temperature difference between the seawater and the hold, dew condensation water is generated on the side wall of the hold, the steel material and the surface of the coal are wet, and the H ₂ SO ₄ -derived substance adsorbed on the surface of the coal oozes into the water film. Pitting corrosion progresses under the coal forming the meniscus, and H ⁺ is consumed in the meniscus portion due to the corrosion of the steel material, so the H ⁺ concentration decreases. On the other hand, since a large amount of H ⁺ exists on the coal surface, a difference in H ⁺ concentration is produced between the coal surface and the meniscus portion. It is considered that the difference in chemical potential is the driving force (driving force), and H ⁺ is supplied to the meniscus portion from the coal surface. Unreacted H ⁺ adheres to the coal surface again during the drying process, and is used for the corrosion reaction in the next dew condensation process. This process takes place in a long-term cycle, causing more corrosion in the meniscus area and pitting corrosion. Will be formed. Based on this mechanism, the following conditions were used to simulate pitting corrosion in the hold of coal ships and coal / ore combined ships.

(Example 1)
First, in order to measure the maximum pitting corrosion depth using the steel sheet shown in Table 1, an example was obtained by the following procedure (this test method is referred to as corrosion test a).
A test piece of 5 mm ^t × 50 mm ^W × 75 mm ^L was collected from the steel plate having the components shown in Table 1, and the surface of the test piece was shot blasted to provide a scale or oil content. ) Was removed. By using this surface as a test surface, the corrosion resistance of the steel material after coating film peeling was evaluated. After the back and end surfaces are coated with a silicon base adhesive tape, they are fitted into an acrylic cell, and 5 g of coal is laid on the acrylic cell, and a temperature and humidity chamber is used. 1 and atmosphere A (temperature 60 ° C., humidity 95%, 20 hours), and atmosphere B (temperature 30 ° C., humidity 95%, 3 hours). A temperature and humidity cycle with a transition time of 0.5 hours was given for 28 days. Here, the symbol “⇔” is used in the sense of repetition (the same applies hereinafter). In addition, 5 g of coal was weighed, immersed in 100 ml of distilled water at room temperature for 2 hours, filtered, and the coal leachate diluted to 200 ml having a pH of 3.0 was used. In this example, the test is conducted under such conditions, thereby simulating a temperature / humidity environment and a dew condensation state that greatly affect the corrosion in the hold of the coal ship and the coal / ore combined ship. After the test, using a rust remover, the rust of each test piece was peeled off, and the weight loss of the steel material was measured to obtain the amount of corrosion. Moreover, it measured using the generated maximum pitting depth depth meter. The results are shown in Table 2.

From Table 2, the test number No. 1-a to 27-a (the numerical part of the test number is the same as the steel plate number; the same applies hereinafter), In any of 33-a to 40-a, the weight loss and the maximum pitting depth are good compared to the comparative material, the weight loss is 2.5 g or less, and the maximum pitting depth is suppressed to 0.30 mm or less. You can see that On the other hand, test number No. which is a comparative material. 28-a and no. 29-a contains Cr in excess of 0.050 mass%. 30-a and no. Since 32-a does not contain Sb but contains Sn, the weight loss was 2.7 g or more, and the maximum pitting depth was 0.35 mm or more. Test No. Since 31-a does not contain Sb, the weight loss is 2.71 g and the maximum pitting corrosion depth is 0.34 mm, although the amount of other elements is within the scope of the present invention. Corrosion resistance was inferior compared.
(Example 2)
Next, an example for estimating the maximum thickness reduction after 25 years is shown. In the same manner as in Example 1, test pieces of 5 mm ^t × 50 mm ^W × 75 mm ^L were collected from the steel plates shown in Table 1. The surface of the test piece was shot blasted to remove scale and oil on the surface, and this surface was used as a test surface to evaluate the corrosion resistance of the steel material after peeling the coating film. After coating the back and end faces with a silicone seal, it is fitted into an acrylic jig, and 5 g of coal is laid on it, and the atmosphere A (temperature 60 ° C., humidity 95%, 20 hours) ⇔ Atmosphere B (temperature 30 ° C., humidity 95%, 3 hours) A temperature and humidity cycle with a transition time of 0.5 hours was applied for 28, 56, 84, 168, and 336 days. And).

In addition, 5 g of coal was weighed and immersed in 100 ml of distilled water at room temperature for 2 hours, and then filtered, and the coal leachate diluted to 200 ml had a pH of 3.0. In this example, the test is conducted under such conditions, thereby simulating a temperature / humidity environment and a dew condensation state that greatly affect the corrosion in the hold of the coal ship and the coal / ore combined ship. After the test, the rust of each test piece was peeled off using a rust remover and measured using a maximum pitting depth depth meter for each period. However, the value of the maximum pitting depth increases as the target area increases. Therefore, in order to predict the maximum pitting depth for each period on an actual ship, the maximum pitting depth of the area corresponding to the actual ship hold from the measured value in this test piece area using extreme value statistics (extreme value statistics). Was calculated. Here, because the hold rib, which is the application site of the developed steel, is corroded from both sides, the maximum pitting depth for each period is doubled, and the life of the ship is estimated by extrapolating those values. The maximum thickness reduction after 25 years was estimated. The results are shown in Table 3. It is assumed that the plate thickness of the application site is 15 to 20 mm, the corrosion allowance is 3.5 to 4.0 mm, and the corrosion thickness is 0.5 mm. Based on the standard for steel sheet switching in the CSR-B edition (IACS common structure rule for bulk carriers), the maximum thickness reduction criteria after 25 years was set to 4.0 mm.

In addition, using an electron probe micro-analysis, the present invention example No. 37-b and Comparative Example No. S mapping of the rust cross section of 44-b was performed. The electron microanalyzer uses EPMA1600 manufactured by Shimadzu Corporation, and has an acceleration voltage of 20 kV, a beam diameter of 1 μm, and a region of 100 × 100 μm at a pitch of 0.4 μm in the X and Y directions. It was measured.

Fig. 2 shows a graph that estimates the maximum thickness reduction after 25 years. Here, the maximum thickness reduction is the thickness of the steel plate where the thickness is reduced most due to local corrosion from the drawing thickness in the ship. Invention Example No. 37-b and Comparative Example No. 44-b is described. The maximum plate thickness reduction in each period used for creating FIG. 2 was as follows in Invention Example 37-b. 28 days: 0.85 mm, 56 days: 1.11 mm, 84 days: 1.28 mm, 168 days: 1.36 mm, 336 days: 1.47 mm. Furthermore, in Comparative Example 44-b, the following was true. 28 days: 0.96 mm, 56 days: 1.39 mm, 84 days: 1.62 mm, 168 days: 1.91 mm, 336 days: 2.11 mm. In addition, test numbers No. 1-b to 27-b, test no. In any of 33-b to 40-b, the estimated maximum thickness reduction after 25 years was 4.0 mm or less, which is the criterion. Further, only the addition of Sb was removed from the claims. Since 31-b did not satisfy the criteria, it can be seen that Sb greatly affects the corrosion protection in this environment.

FIG. 3 shows a mapping result of S by an electron microanalyzer of the cross section of the rust after 84 days in the corrosion test b. No. which is a comparative example. In No. 44-b, there is an interfacial layer with a lot of S between the rust layer and the ground iron, whereas in the example of the present invention, In 37-b, the interface layer with many S is hardly seen. From this, in the present invention example, the penetration of SO ₄ ²⁻ to the rust-steel interface is suppressed by the densification of rust by Sb and the electrical repulsion of SO ₄ ²⁻ by the oxygen acid of W. It is estimated to be. From this, it can be seen that the present invention is a steel material that forms a rust layer that suppresses permeation of SO ₄ ^{2− in} a coal ship or a coal / ore combined ship hold environment.

As described above, the effect of the present invention was confirmed. In this example, the method shown in FIG. 1 was used as a test method simulating the environment in the coal ship or coal / ore combined-use ship hold, but it was extremely consistent with the case where it was actually installed in the hold and evaluated. Some results have been obtained. Conditions such as atmosphere A and B, transition time, cycle, coal adjustment method, pH value of coal leachate, etc. are not limited to the above-mentioned examples, and the usage environment within the steel hold Depending on the situation, it can be changed appropriately.

The steel material according to the present invention is used as a constituent member of a coal ship or a coal / ore combined ship hold that is easily peeled off due to mechanical damage of coal or ore and is exposed to repeated dry and wet conditions and a low pH environment. Can do.

Claims

The component composition of the steel material is C: 0.010 to 0.200 mass%, Si: 0.05 to 0.50 mass%, Mn: 0.10 to 2.0 mass%, P: 0.0250 mass% or less, S: 0 0.010 mass% or less, Al: 0.0050 to 0.10 mass%, Sb: 0.010 to 0.50 mass%, N: 0.0010 to 0.0080 mass%, and the balance from Fe and inevitable impurities Corrosion-resistant steel for holding coal ships or coal / ore combined ships.
The coal ship or coal according to claim 1, further comprising at least one selected from Cu: 0.010 to 1.0 mass% and Ni: 0.010 to 1.0 mass% in addition to the steel material.・ Corrosion-resistant steel for holding ore combined ships.
The corrosion resistant steel for holding a coal ship or a coal / ore combined ship according to claim 1 or 2, wherein the steel material has Cr: 0.050 mass% or less.
4. The method according to claim 1, further comprising at least one selected from W: 0.005-0.5 mass% and Mo: 0.005-0.5 mass% in addition to the steel material. Corrosion-resistant steel for holding coal ships or coal / ore combined ships as described in 1.
In addition to the steel materials, Ti: 0.0010 to 0.030 mass%, Nb: 0.0010 to 0.030 mass%, Zr: 0.0010 to 0.030 mass%, and V: 0.0020 to 0.20 mass%. The corrosion-resistant steel for holding a coal ship or a coal / ore combined ship according to any one of claims 1 to 4, comprising at least one selected from among them.
6. The corrosion-resistant steel for holding a coal ship or a combined coal / ore ship according to any one of claims 1 to 5, further containing Ca: 0.0005 to 0.0040 mass% in addition to the steel material.
7. In addition to the steel material, one or more selected from REM: 0.0001 to 0.0150 mass% and Y: 0.0001 to 0.10 mass% are contained. Corrosion-resistant steel for holding coal ships or coal / ore combined ships.
In addition to the steel material, it contains at least one selected from Se: 0.0005 to 0.50 mass%, Te: 0.0005 to 0.50 mass%, and Co: 0.010 to 0.50 mass%. Item 8. A corrosion-resistant steel for holding a coal ship or a coal / ore combined ship according to any one of items 1 to 7.