JP5702683B2 - Corrosion-resistant steel for bulk carriers and hold of bulk carriers - Google Patents

Description

  The present invention relates to a corrosion-resistant steel material used as a structural member of a bulk carrier and a cargo hold of a bulk carrier constructed using the corrosion-resistant steel material, and more particularly, sulfur content such as coal and iron ore. The present invention relates to a corrosion-resistant steel material for a bulk cargo ship that can be suitably used in a corrosive environment caused by a cargo including a cargo, and a hold of a bulk cargo ship.

  Bulk carriers, or bulk carriers and bulkers are cargo vessels designed to store and transport bulk cargo such as unpacked grain, ore and cement in the hold. In the present specification, a bulk carrier including a bulk carrier and a bulker will be described. In the case of this bulk carrier, the corrosion of the steel material occurs due to the corrosive action of seawater like other ships, but when bulk cargo such as coal or iron ore contains sulfur (S), Since the corrosion of the steel material is further accelerated under the influence of the S component, the corrosion damage tends to become more prominent than other ships. In particular, steel material corrosion often appears more prominently in a hold where bulk cargoes containing these S components are in direct contact.

  In this way, when the corrosion of steel materials such as the hold is significant, the strength of the structure is reduced due to the plate thickness wear of the steel materials, and in some cases formed by the corrosion of the steel materials. Inundation from the hole may cause marine accidents such as sinking, and it has been essential to adopt an effective bulk carrier.

  As anticorrosion means, conventionally, bulk cargo ships are subjected to anticorrosion coating such as painting with epoxy resin paint, and cathodic protection to install a sacrificial anode such as zinc so as to short-circuit with steel. It was general.

  Of these anti-corrosion methods, anti-corrosion coating is a particularly commonly used method. However, due to various external factors and aging deterioration, the coating may become wrinkled or the coating may peel off. There were many, and it was difficult to maintain anticorrosion performance for many years, and maintenance for inspection and repair of anticorrosion coating was indispensable. Furthermore, there are many cases where it is necessary to assemble a scaffold for the inspection and repair of the coating, and there is a problem that the time and cost required for the maintenance are increased.

  Particularly in the bulk of a bulk carrier, paint peeling may occur due to contact with the bucket during cargo handling, and the paint may be scraped by cargo in contact with vibration during voyage. In this way, the bulk of a bulk cargo ship, particularly its inner wall surface, ceiling surface, floor surface, etc., was significantly damaged by coating compared with the outer plate and ballast tank of the ship, and its coating life was particularly short.

  In addition, in the case of ballast tanks, if anticorrosion is applied to the interior, seawater is injected and corrosion tends to proceed, and the anticorrosion action by the sacrificial anode appears effectively. Since it is not filled with an aqueous electrolyte solution such as seawater that is indispensable for anticorrosion and is merely a corrosive environment in a humid atmosphere, it is difficult to effectively obtain an anticorrosive action.

  Against this background, bulk carriers are required to take more effective anti-corrosion measures so far in order to improve their safety and extend their life. Against this background, for example, Patent Document 1 and Patent Document 2 have also proposed techniques relating to a corrosion-resistant steel material in which the corrosion resistance of the steel material itself is improved by adjusting chemical components and hardness. By applying these steel materials, it has become possible to ensure corrosion resistance superior to conventional anticorrosion means.

  However, even if these technologies are adopted, it is impossible to obtain a sufficient effect of improving corrosion resistance. In the severe environment of bulk carriers, it is still impossible to demonstrate sufficient corrosion resistance, and it is more effective. The development of technology related to anticorrosion has been awaited.

  Under such circumstances, the present inventors also control the content of the additive element appropriately, and as a container for storing or transporting (transporting) minerals or ores, the structure having a long life with excellent corrosion resistance Patent Document 3 proposes a steel material that can be used as a member. However, even in the technology described in Patent Document 3, the corrosion of steel materials due to the S component has not been sufficiently studied, and it is further necessary to use it for a bulk cargo ship that transports coal or iron ore containing the S component. There was room for improvement.

JP 2007-262555 A JP 2008-174768 A JP 2010-100902 A

  The present invention has been made as a solution to the above-mentioned conventional problems, and exhibits excellent long-term corrosion resistance even in severe corrosive environments caused by bulk cargo containing S, and is suitable as a structural member for bulk cargo ships. It is an object of the present invention to provide a bulk cargo ship corrosion-resistant steel material that can be used for a bulk cargo ship and a bulk cargo ship built using the corrosion-resistant steel material.

Invention of Claim 1 is the mass%, C: 0.04-0.30%, Si: 0.28-0.60%, Mn: 0.1-2.0%, P: 0.010 -0.040%, S: 0.003-0.025%, Al: 0.010-0.10%, Cu: 0.10-1.0%, Ni: 0.10% or less (0% N: 0.0030 to 0.010% is contained, the balance is made of Fe and inevitable impurities, CP1 defined by the following formula (1) is 0.10 or more, and the following (2 CP2 defined by the formula is 0.80 to 7.00, and is a corrosion-resistant steel material for bulk cargo ships characterized by excellent corrosion resistance in a corrosive environment caused by cargo containing sulfur. .
CP1 = [Si] + 2 × [Cu] −5 × [Ni] (1) Formula CP2 = ([P] / 4 + [S]) / [N] (2) where the above formula [] Is the content (% by mass) of each element.

  The invention according to claim 2 is the corrosion-resistant steel material for bulk cargo ship according to claim 1, which further contains Cr: 0.01 to 0.3% by mass%.

According to a third aspect of the invention, in mass%, further, Ca: a Bulk ship corrosion resistant steel according to claim 1 or 2, characterized in that it contains from 0.0003 to 0.005%.

The invention according to claim 4 is in mass%, further Ti: 0.001 to 0.05%, V: 0.001 to 0.05%, Nb: 0.001 to 0.05%, B: 0. The corrosion-resistant steel material for bulk carriers according to any one of claims 1 to 3 , characterized by containing .0003-0.005% of one or more.

The invention according to claim 5 is the corrosion-resistant steel material for a bulk cargo ship according to any one of claims 1 to 4 , wherein a coating layer containing Zn and having a thickness of 5 μm or more is formed on the surface.

A sixth aspect of the present invention is a bulk carrier of a bulk cargo ship which is configured by using the steel material according to any one of the first to fifth aspects.

  The corrosion-resistant steel material for bulk carriers according to the present invention exhibits excellent long-term corrosion resistance even under severe corrosive environment caused by bulk cargo containing S, and can be suitably used as a structural member for bulk carriers. .

  Further, according to the bulk of the bulk carrier of the present invention, even bulk cargo including S can be accommodated without problems of long-term corrosion resistance.

It is a top view which shows the bare test piece used for the corrosion test in the Example. It is a top view which shows the surface treatment test piece which formed the coating layer containing Zn used for the corrosion test in the Example in the half of the surface.

  Hereinafter, the present invention will be described in more detail based on embodiments.

  The inventors of the present invention are eager to find a steel material that exhibits excellent long-term corrosion resistance even in a severe corrosive environment caused by a bulk cargo containing S, and can be suitably used as a structural member of a bulk cargo ship. Repeated research. As a result, CP1, which is obtained from the content of C, Si, Mn, P, S, Al, Cu, Ni and N in the steel material and the formula [Si] + 2 × [Cu] -5 × [Ni], Corrosion resistance even under severe corrosive environment caused by bulk cargo including S content of bulk cargo ship by strictly adjusting CP2 calculated from the formula [P] / 4 + [S]) / [N] It was found that an excellent steel material could be obtained, and the present invention was completed.

The inside of a bulk carrier or the like is a wet corrosive environment in which corrosion proceeds due to a thin water film formed by condensation on the surface of a steel material constituting the hold. In the case where bulk cargo such as coal or iron ore contains S content, the present inventors consider that the corrosion of the steel material is promoted by the S content of the bulk cargo especially in view of the fact that corrosion of the steel material is likely to proceed. Studied. As a result, the acceleration of corrosion is that when condensation occurs on bulk cargo such as coal and iron ore, S and moisture contained in bulk cargo react with each other to produce sulfuric acid, which promotes corrosion of steel materials. In addition to that, it was found that S itself is also involved in the electrochemical reaction of corrosion and further promotes corrosion of steel materials. That is, it was discovered that S contained in the bulk cargo reacts with hydrogen ions in the dew condensation water to cause a cathode reaction of S + H ⁺ + 2e → HS ⁻ , further accelerating corrosion by sulfuric acid.

  As a result of intensive studies on the development of corrosion resistance of steel against corrosion promotion due to S content derived from bulk cargo, Si oxide and Cu are formed on the surface of the steel by appropriately adjusting the content of chemical components in the steel. It has been found that a composite corrosion product film mainly composed of sulfides can be formed, and that the corrosion rate of the steel material can be greatly suppressed by the composite corrosion product film.

  The composite corrosion product film composed of an oxide of Si and a sulfide of Cu is stable in a corrosive environment due to S, and is a steel material against an electrochemical reaction between sulfuric acid produced by reaction of S and moisture and S itself. It is considered that the corrosion resistance is expressed. Such expression of corrosion resistance is considered to be particularly effective in a wet corrosive environment in which corrosion proceeds due to a thin water film such as the inside of a bulk carrier.

  As explained above, the corrosion resistant steel material for bulk cargo ship and the bulk carrier of the bulk cargo ship according to the present invention has a corrosion resistance by forming a composite corrosion product film of Si oxide and Cu sulfide on the surface of the steel material. In addition to adding appropriate amounts of Si and Cu to the steel material, the P, S, Ni, and N contents are set to appropriate contents, and CP1 and CP2 are adjusted. There is a need to. Moreover, in order to ensure the basic characteristics (mechanical characteristics and weldability) as a steel material, it is necessary to appropriately control the contents of C, Mn, and Al. The reason for limiting the component ranges of these essential additive elements will be described below. All units are described as%, but indicate mass%. In the following explanations other than the essential additive elements,% indicates mass% in the same manner.

・ C: 0.04 to 0.30%
C is a basic additive element necessary for securing the strength of the steel material. In order to obtain the strength required as a steel material for bulk carriers, it is necessary to contain at least 0.04% or more. However, when C is excessively contained, toughness is deteriorated in addition to deterioration of corrosion resistance. In order not to cause such adverse effects due to the addition of C, the C content needs to be suppressed to 0.30% at most. In addition, the minimum with preferable content of C is 0.045%, It is good to set it as 0.05% or more more preferably. Moreover, the upper limit with preferable content of C is 0.29%, It is good to set it as 0.28% or less more preferably.

・ Si: 0.28-0.60%
Si is an essential element for forming a composite corrosion product film on the surface of a steel material, and is necessary for ensuring corrosion resistance in the corrosive environment of a bulk carrier. It is also an element necessary for deoxidation and securing strength. In order to express such an action, it is necessary to contain at least 0.28% or more. However, if Si is excessively contained exceeding 0.60%, the weldability is deteriorated. In addition, the minimum with preferable content of Si is 0.29%, More preferably, it is good to set it as 0.30% or more. Moreover, the upper limit with preferable content of Si is 0.58%, More preferably, it is good to set it as 0.55% or less.

Mn: 0.1 to 2.0%
Mn is an element necessary for deoxidation and securing strength, and the minimum strength as a structural member cannot be secured unless it is less than 0.1%. However, if the content exceeds 2.0%, the toughness deteriorates. In addition, the minimum with preferable content of Mn is 0.15%, More preferably, it is good to set it as 0.20% or more. Moreover, the upper limit with preferable Mn content is 1.9%, More preferably, it is good to set it as 1.8% or less.

・ P: 0.010 to 0.040%
P is an element that forms a composite corrosion product film on the surface of a steel material in the corrosive environment of a bulk cargo ship, and an element that, when dissolved, produces a phosphate that acts as an inhibitor to enhance corrosion resistance. It is an indispensable additive element in the steel material of the present invention. Such an inhibitory effect due to phosphate is particularly pronounced in a humid environment where corrosion due to the formation of a thin water film proceeds. In order to express such an effect, it is necessary to contain 0.010% or more of P. However, if P is contained excessively, toughness and weldability deteriorate, so the content is at most 0.040%. In addition, the minimum with preferable content of P is 0.011%, More preferably, it is good to set it as 0.012% or more. Moreover, the upper limit with preferable P content is 0.038%, It is good to set it as 0.035% or less more preferably.

S: 0.003 to 0.025%
S is an element that forms Cu sulfide on the steel surface together with the dissolved Cu after dissolving in the corrosive environment of a bulk carrier, and forms a composite corrosion product film on the surface of the steel with Si oxide. By doing so, the effect of reducing the corrosion reaction of the steel material is exhibited. Therefore, it is an element necessary for improving the corrosion resistance of steel. In order to express such an action, it is necessary to contain 0.003% or more of S. However, as in the case of P, when S is excessively contained, toughness and weldability deteriorate, so the allowable content is 0.025% at most. In addition, the minimum with preferable content of S is 0.004%, It is good to set it as 0.005% or more more preferably. Moreover, the upper limit with preferable S is 0.024%, It is good to set it as 0.023% or less more preferably.

・ Al: 0.010-0.10%
Al is also an element necessary for deoxidation and securing strength, like Si and Mn described above. In order to exhibit such an action effectively, it is necessary to contain 0.010% or more. However, if the content exceeds 0.10%, weldability is impaired, so the addition of Al is limited to 0.10%. In addition, the minimum with preferable content of Al is 0.011%, More preferably, it is good to set it as 0.012% or more. Moreover, the upper limit with preferable content of Al is 0.095%, More preferably, it is good to set it as 0.090% or less.

Cu: 0.10 to 1.0%
Cu is an element that forms a sulfide on the steel surface together with the dissolved S after dissolving in the corrosive environment of a bulk carrier, and forms a complex corrosion product film on the surface of the steel together with an oxide of Si. This exerts the effect of reducing the corrosion reaction of the steel material. Therefore, it is an element necessary for improving the corrosion resistance of steel. In order to express such an action, it is necessary to contain Cu by 0.10% or more. However, if it is contained excessively, the weldability and hot workability of the steel material deteriorate, so it is necessary to make it 1.0% or less. The minimum with preferable content of Cu is 0.11%, and a more preferable minimum is 0.12%. Moreover, the upper limit with preferable Cu content is 0.95%, and a more preferable upper limit is 0.90%.

Ni: 0.10% or less (excluding 0%)
Ni exerts the effect of improving the toughness of the steel material, but also exhibits the effect of hindering the formation of a composite corrosion product film by the Si oxide and Cu sulfide necessary for ensuring the corrosion resistance, thereby deteriorating the corrosion resistance of the steel material. It also has the effect of letting it go. Therefore, the Ni content should be as low as possible. However, such an adverse effect becomes significant when the content exceeds 0.10%, so the upper limit was made 0.10%. The upper limit with the preferable amount of Ni is 0.09%, and 0.08% or less is still more preferable.

・ N: 0.0030-0.010%
N has an action of promoting the formation of a composite corrosion product film made of an oxide of Si and a sulfide of Cu necessary for ensuring corrosion resistance, and is an element necessary for ensuring the corrosion resistance of a steel material. In order to develop such an effect, N needs to be contained in an amount of 0.0030% or more. However, if N is excessively contained, not only the formation of the composite corrosion product film is inhibited, but also the solid solution N increases, which adversely affects the ductility and toughness of the steel material, so the upper limit is made 0.010%. . In addition, the minimum with preferable content of N is 0.0033%, More preferably, it is 0.0035% or more. Moreover, the upper limit with preferable addition amount of N is 0.09%, and a more preferable upper limit is 0.08%.

CP1 = [Si] + 2 × [Cu] -5 × [Ni]: 0.10 or more The CP1 value obtained from [Si] + 2 × [Cu] -5 × [Ni] is necessary for the expression of corrosion resistance of the present invention. It is a parameter that represents the ability to form a composite corrosion product film with a simple Si oxide and Cu sulfide. In addition to adjusting the Si, Cu, and Ni contents to the above ranges, the steel material of the present invention needs to have a CP1 value of 0.10 or more. When the CP1 value is less than 0.10, formation of the composite corrosion product film becomes insufficient, and the corrosion resistance is not improved. If the CP1 value is 0.10 or more, a composite corrosion product film is formed. Therefore, the upper limit of the CP1 value is not particularly defined in the present invention. Weldability and hot workability deteriorated. Therefore, the CP1 value is preferably 2.60 or less determined from the upper limits of the addition amount of Si and the addition amount of Cu.

CP2 = ([P] / 4 + [S]) / [N] : 0.80 to 7.00
The ratio (mass ratio) of the addition amount of P / 4 + S and N has an influence on the formation of the composite corrosion product film by the oxide of Si and the sulfide of Cu necessary for the development of the corrosion resistance of the present invention. The CP2 value obtained from P] / 4 + [S]) / [N] is also a parameter representing the ability to form a composite corrosion product film. When the CP2 value is less than 0.80, the amount of N relative to the amount of (P / 4 + S) becomes excessive, and a composite corrosion product film is not formed. On the other hand, if the CP2 value exceeds 7.00, the amount of N relative to the amount of (P / 4 + S) is insufficient, and in this case also, the composite corrosion product film is not formed. Therefore, it is necessary to adjust CP2 value to 0.80-7.00 from a viewpoint of corrosion resistance expression.

  The above is the reason for limiting the component range of the essential additive elements of the steel material of the present invention, with the balance being iron and inevitable impurities. Inevitable impurities include O, H and the like, and these elements may be contained to the extent that they do not impair various properties of the steel material. However, by suppressing the total content of these inevitable impurities to 0.1% or less, preferably 0.09% or less, the corrosion resistance effect according to the present invention can be maximized.

  The steel material of the present invention is more effective if it contains the following elements. The reason for limiting the component range when these elements are contained will be described below.

・ Cr: 0.01-0.3%
Cr forms an oxide film on the surface of the steel material, and further improves the protection by the composite corrosion product film and reduces the corrosion reaction. Therefore, Cr is an effective element for improving the corrosion resistance. In order to exert such an effect, it is preferable to contain 0.01% or more. However, if Cr is excessively contained, the corrosion resistance is deteriorated in addition to lowering the pH of the corrosion tip, and also weldability and hot workability are deteriorated. preferable. The more preferable lower limit when Cr is contained is 0.02%, and more preferably 0.03% or more. Moreover, a more preferable upper limit when Cr is contained is 0.28%, and more preferably 0.26% or less.

Sn: 0.005-0.3%, Sb: 0.005-0.3%
Sn and Sb are effective elements for improving corrosion resistance by forming a sulfide film on the surface of the steel material, further improving the protection by the composite corrosion product film and reducing the corrosion reaction. In order to exert such an effect, it is preferable to contain 0.005% or more of each. However, when Sn and Sb are contained excessively, toughness and weldability are deteriorated. In addition, the more preferable lower limit when Sn and Sb are contained is 0.008%, respectively, and more preferably 0.01% or more. Moreover, the more preferable upper limit when Sn and Sb are contained is 0.28% respectively, and it is still more preferable to set it as 0.26% or less respectively.

・ Ca: 0.0003 to 0.005%
Ca is an element effective for improving corrosion resistance. Ca has an action of mitigating the pH drop at the tip of corrosion and is effective in exhibiting the effect of suppressing the promotion of corrosion due to the pH drop and exhibiting corrosion resistance. Such an effect is effectively exhibited by containing Ca in a amount of 0.0003% or more. However, if over 0.005% is contained, workability and weldability are deteriorated. The more preferable lower limit when Ca is contained is 0.0005%, and more preferably 0.0008% or more. A more preferable upper limit when Ca is contained is 0.0045%, and 0.004% or less is still more preferable.

Ti: 0.001-0.05%, V: 0.001-0.05%, Nb: 0.001-0.05%, B: 0.0003-0.005%
Ti, V, Nb and B are effective elements for improving the strength of the steel material. When Ti, V, and Nb are contained, the content is preferably 0.001% or more, respectively. However, if excessively contained, the toughness of the steel material deteriorates, so each content should be 0.05% or less. Is preferred. More preferable lower limits when Ti, V, and Nb are contained are each 0.002%, and more preferably 0.003% or more. Further, the more preferable upper limit when Ti, V, and Nb are contained is 0.045%, and more preferably 0.04% or less. On the other hand, the content when B is contained is preferably 0.0003% or more, but if it is contained excessively, the workability and weldability of the steel material deteriorate, so 0.005% or less is preferable. . The more preferable lower limit when B is contained is 0.0004%, and more preferably 0.0005% or more. Moreover, the more preferable upper limit when B is contained is 0.0045%, and further preferably 0.004% or less.

  The corrosion-resistant steel material for bulk carriers according to the present invention exhibits excellent corrosion resistance without forming a coating layer on its surface by coating or the like, but further has a coating layer having a thickness of 5 μm or more containing Zn on its surface. By forming, the corrosion resistance is further improved. Zn reacts with S derived from bulk cargo to generate a zinc sulfide precipitate film on the surface of the steel material, and has the effect of suppressing the corrosion progress of the steel material. In the case where the thickness of the coating layer containing Zn is not sufficient, the thickness of the zinc sulfide precipitate film becomes thin, and sufficient protective properties cannot be obtained. From such a viewpoint, the thickness of the coating layer containing Zn is preferably 5 μm or more, more preferably 10 μm or more. However, when the thickness of the coating layer containing Zn is too thick, the weldability of the steel material is deteriorated. Therefore, the thickness of the coating layer containing Zn is preferably 50 μm or less. As the coating layer containing Zn, for example, a zinc primer applied for the purpose of primary rust prevention of a steel material can be applied.

  In addition, the surface of the corrosion-resistant steel material for bulk carriers according to the present invention can be coated as necessary. By coating the surface, it is possible to suppress the corrosion progress of the steel material when the steel material is exposed due to deterioration of the coating film such as wrinkles and peeling, and the safety of the bulk carrier can be improved. The paint used for painting is not particularly limited, and includes epoxy resin paint, chlorinated rubber paint, acrylic resin paint, urethane resin paint, phthalic acid resin paint, phenol resin paint, silicon resin paint, fluororesin paint, etc. Can be mentioned. Among these paints, it is recommended to use an epoxy resin-based paint from the viewpoint of adhesion to a steel material or a coating layer containing Zn formed as necessary. Any epoxy resin-based paint may be used as long as it contains an epoxy resin as a vehicle (color developing agent), and examples thereof include a modified epoxy resin paint and a tar epoxy resin paint.

  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.

[Production of test materials]
Steel materials having various component compositions shown in Table 1 were melted in an electric furnace to obtain a 40 kg steel ingot. The obtained steel ingot was heated to 1150 ° C. and then hot-rolled to obtain a steel material having a plate thickness of 10 mm. At this time, the hot rolling end temperature was appropriately adjusted in the range of 650 to 850 ° C., and the cooling rate from the end of hot rolling to 500 ° C. within the range of 0.1 to 15 ° C./second or less.

  From the steel material, a bare test piece a shown in FIG. 1 for evaluating the corrosion resistance of the steel material itself and a surface treatment test piece b shown in FIG. 2 in which the coating layer 1 containing Zn was formed on the surface were prepared.

  First, a specimen having a size of 50 × 30 × 5 (mm) was cut out from a steel material, the entire surface was polished to SiC # 600 with a wet rotary polishing machine, and further subjected to water washing and acetone washing was bare. It was set as the test piece a. Note that a suspension hole 2 of 3 mmφ was formed at the end of the test piece for suspension during the corrosion test.

  Moreover, the surface treatment test piece b which formed the inorganic zinc rich primer layer (the coating layer 1 containing Zn) in the half of the surface was produced. First, a test piece having a size of 50 × 30 × 5 (mm) was cut out from the steel material, and the entire surface was subjected to sandblasting so that the ten-point average roughness (Rz) of the surface was 30 to 40 μm. Washed with water and acetone. Thereafter, an inorganic zinc-rich primer was applied to approximately half the surface of the test piece and dried to obtain a surface-treated test piece b. The thickness of the coating layer containing Zn (described as a Zn layer in Table 3) 1 determined from the change in mass before and after applying the inorganic zinc rich primer is as shown in Table 3. The surface-treated test piece b was also provided with a 3 mmφ suspension hole 2 at the end of the test piece for suspension during the corrosion test. This surface treatment test piece b was used for corrosion resistance evaluation in a state where the coating layer 1 containing Zn was partially peeled off.

[Corrosion test method]
An immersion corrosion test using a mixed solution (test solution) of sulfuric acid and sulfur (S) was carried out as a corrosion test simulating a bulk cargo ship carrying bulk cargo such as coal containing S or iron ore. Specifically, a test solution was prepared by suspending 10 g of powdered S reagent in 1 L of sulfuric acid aqueous solution whose pH was adjusted to 2.0. The temperature of the test solution at this time was 30 ° C., and the immersion period was 72 hours. For the corrosion test, No. 1 shown in Table 1 was used. Three to one specimens 1 to 33 were prepared and used. In the test, the mass change of the test piece before and after the test was measured, and the corrosion rate of each test piece was obtained. The mass of the test piece after the test was measured after removing the corrosion products on the surface by cathodic electrolysis in a 10% by mass ammonium hydrogen citrate aqueous solution at 30 ° C., further washing with water and acetone, and drying. did.

[Test results]
Table 2 shows the results of the corrosion test using the bare specimen a. The corrosion rate is No. It is shown as a relative value when the corrosion rate of No. 1 steel (usually used steel) is 100. In addition, the overall evaluation is indicated by “◯” when the corrosion rate is 60 or less, “OO” when the corrosion rate is 50 or less, and “XX” when the corrosion rate is 40 or less. The bare specimens “a”, “XX”, and “XXX” were evaluated as being capable of being used as a corrosion-resistant steel material for bulk cargo ships exhibiting excellent long-term corrosion resistance even in a corrosive environment.

  No. 2-No. In the steel material No. 7, in order, the contents of Si, P, S, Cu, Ni, and N do not satisfy the ranges of the chemical composition defined in the present invention, respectively. .1) and no improvement in corrosion resistance was observed in corrosive environments. No. 8-No. Since the steel material No. 10 does not satisfy the ranges defined in the present invention, CP1 or CP2 is No. 10, respectively. 2-No. Similarly to the steel material of No. 7, the corrosion rate is the same level as that of the steel material (No. 1) that is normally used, and no improvement in corrosion resistance was observed in a corrosive environment.

  On the other hand, No. 1 satisfying the requirements defined in the present invention. 11-No. All of the steel materials No. 37 have a corrosion rate of 60 or less, resulting in excellent corrosion resistance. These corrosion resistances can be obtained by appropriately controlling the contents of C, Si, Mn, P, S, Al, Cu, Ni, and N, and the values of CP1 and CP2. It is expressed by the anticorrosive action of the composite corrosion product film mainly composed of sulfide.

  Next, Table 3 shows the test results of the corrosion test using the surface-treated test piece b in which the coating layer 1 containing Zn is formed on approximately half of the surface. This is a corrosion test simulating a state where the coating layer 1 containing Zn is partially peeled off. The corrosion rate indicates the corrosion rate of the almost half surface on which the inorganic zinc rich primer layer (Zn-containing coating layer 1) was formed. The relative value when the corrosion rate of steel No. 1 is 100 is shown.

  No. A is an inorganic zinc rich primer layer (Zn-containing coating layer 1) formed on substantially half of the surface of a steel material (No. 1) that is usually used, and its corrosion rate is inorganic containing Zn. It was almost the same as that in which the zinc rich primer layer was not formed, the inorganic zinc rich primer layer eluted early, and the anticorrosive effect by the Z inorganic zinc rich primer layer was not recognized. No. B, No. E, No. G is the steel material No. 13, no. 16, no. An inorganic zinc rich primer layer (Zn-containing coating layer 1) is formed on almost half of the surface of 17 but the thickness of the coating layer 1 is less than 5 μm. It was almost the same level as that in which the primer layer was not formed.

  On the other hand, when the inorganic zinc rich primer layer (Zn-containing coating layer 1) having a thickness of 5 μm or more is formed, the corrosion rate is 10% or more compared to the case where the inorganic zinc rich primer layer is not formed. The corrosion resistance was improved by forming the inorganic zinc rich primer layer.

  As described above, the surface treatment with the steel material and Zn-containing coating layer 1 of the present invention has excellent corrosion resistance in a corrosive environment with both sulfuric acid and sulfur, and bulk loading for loading bulk cargo containing S. It can be used very effectively as a structural member of a cargo ship.

DESCRIPTION OF SYMBOLS 1 ... Cover layer 2 containing Zn ... Hanging hole a ... Bare specimen b ... Surface treatment specimen

Claims (6)

In mass%, C: 0.04 to 0.30%, Si: 0.28 to 0.60%, Mn: 0.1 to 2.0%, P: 0.010 to 0.040%, S: 0.003 to 0.025%, Al: 0.010 to 0.10%, Cu: 0.10 to 1.0%, Ni: 0.10% or less (excluding 0%), N: 0.0. Containing 0030-0.010%, the balance consisting of Fe and inevitable impurities,
CP1 defined by the following formula (1) is 0.10 or more,
CP2 defined by the following formula (2) is 0.80 to 7.00,
Corrosion-resistant steel for bulk carriers, which has excellent corrosion resistance in corrosive environments caused by cargo containing sulfur.
CP1 = [Si] + 2 × [Cu] −5 × [Ni] (1) Formula CP2 = ([P] / 4 + [S]) / [N] (2) where the above formula [] Is the content (% by mass) of each element.   The corrosion-resistant steel material for bulk carriers according to claim 1, further comprising Cr: 0.01 to 0.3% by mass%. The corrosion-resistant steel material for bulk carriers according to claim 1 or 2 , further comprising Ca: 0.0003 to 0.005% by mass%. Further, Ti: 0.001 to 0.05%, V: 0.001 to 0.05%, Nb: 0.001 to 0.05%, B: 0.0003 to 0.005%. The corrosion-resistant steel material for a bulk carrier according to any one of claims 1 to 3 , comprising one or more kinds. The corrosion-resistant steel material for bulk carriers according to any one of claims 1 to 4 , wherein a coating layer containing Zn and having a thickness of 5 µm or more is formed on the surface. A bulkhead of a bulk carrier, which is configured using the steel material according to any one of claims 1 to 5 .

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