JP5839151B1 - Steel, a ballast tank and a hold of a ship using this steel, and a ship provided with this ballast tank or a hold - Google Patents

Abstract

This steel material has a predetermined chemical component, has a soft structure and a hard structure, and an Sn concentration ratio, which is a ratio of the Sn concentration in the hard tissue to the Sn concentration in the soft tissue, is 1.2 or more and 6 Less than 0.0.

Description

  The present invention relates to a steel material having excellent corrosion resistance, which is used in a corrosive environment containing chloride, such as a seawater environment. Moreover, this invention relates to the ballast tank and ship hold of a ship formed using this steel material excellent in corrosion resistance. Moreover, this invention relates to the ship provided with these ballast tanks or a hold.

It is well known that the influence of chloride is extremely large as a factor that accelerates corrosion of steel materials. In particular, steel materials used in ships such as ballast tanks, outer panels and superstructures, bridges and other structures in coastal areas, port sheet piles and pipe piles, offshore structures, offshore wind power generation facilities, etc. It is extremely susceptible to corrosion because it is exposed to repeated wet and dry environments. Steels installed in seawater are also less susceptible to corrosion, although not as much as steels installed in dry and wet environments. Steel materials used in beach areas do not receive seawater splashes, but corrosion is accelerated by the arrival of sea salt particles. Also in the inland area, in order to prevent road surface freezing in winter, spraying of an antifreezing agent containing chloride is sometimes performed, and corrosion of steel due to chloride is a problem.
Furthermore, the inside of the hold, which is a compartment for loading cargo for ships that carry coal, iron ore, etc., is not directly exposed to the seawater environment, but because it is washed with seawater, corrosion of steel due to chloride is a problem. Become. In addition, corrosion of steel materials due to chlorides also becomes a problem in tanks of salt carriers. In addition, since the tanker's crude oil tank has a severe corrosive environment in which drain water, which is a high-concentration chloride solution, is present, corrosion of the steel material becomes a problem. In addition, corrosion of steel due to chloride is a problem in oil sand drilling and transportation facilities. Thus, corrosion of steel materials by chloride is a big problem.

  In an environment where corrosion due to chloride is a problem as described above, steel materials are usually used by coating. However, since the coating film deteriorates and corrosion occurs and progresses from thin parts such as steel edges, maintenance such as repainting is indispensable when using steel materials for a long time in a structure. is there. For example, when repainting, depending on the structure, it may be necessary to install a scaffolding, resulting in a huge maintenance cost, and a large amount of VOC (volatile organic compounds) that are harmful to the human body due to painting. It is a problem to occur. Under these circumstances, there has been a strong demand for the development of a steel material that is excellent in corrosion resistance even if it is not painted, or a steel material that can extend the period until recoating is required.

As steel materials with improved corrosion resistance under such chloride environments, for example, Patent Documents 1 to 3 describe that Sn is 0.005 to 0.3 mass%, 0.02 to 0.40 mass%, and 0, respectively. by containing .01～0.50 wt%, chloride ion ^- steel obtained by improving the corrosion resistance in an environment containing chlorine (Cl ion) are disclosed.
In Patent Document 4, W: 0.01 to 0.5 mass%, Mo: one or more of 0.02 to 0.5 mass%, and Sn: 0.001 to 0.2 mass% , Sb: Steel materials including one or more of 0.01 to 0.2% by mass and capable of extending the period until repair coating in a seawater corrosive environment are disclosed.

Japanese Unexamined Patent Publication No. 2010-064110 Japanese Unexamined Patent Publication No. 2012-057236 Japanese Unexamined Patent Publication No. 2012-255184 Japanese Unexamined Patent Publication No. 2009-046750

  As described above, it is disclosed that a steel material containing Sn or a steel material containing one or more of Sn and Sb and one or more of W and Mo has excellent corrosion resistance in a corrosive environment containing chloride. Yes. However, as a result of performing a corrosion test on a steel material containing Sn and / or W, the inventors of the present invention simply contained steel material containing Sn, or one or more of Sn and Sb and one or more of W and Mo. It was confirmed that sufficient corrosion resistance could not be secured depending on the corrosive environment even when using the steel material.

Specifically, the following corrosion tests were conducted.
Steel plates A to C having chemical components shown in Table 1 were prepared, and these steel plates were used to simulate two types of corrosion tests simulating the corrosive environment of the ballast tank, that is, SAEJ 2334 test and wave tank test (hereinafter referred to as “WT test”). ”). In both the SAEJ2334 test and the WT test, a test piece having a corrosion-resistant film formed on the surface of the steel sheet was used.

The SAEJ2334 test will be described.
The SAEJ2334 test is an accelerated deterioration test in which the conditions of repeated wet and dry (wet → salt deposition → dry) are performed in one cycle (24 hours in total), and simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd. is there. The SAEJ2334 test was performed under the following conditions as one cycle. The corrosion forms under the following conditions are similar to those in the atmospheric exposure test.
(Test conditions)
-Wet: 50 ° C, 100% RH, 6 hours,
Adherence of salt: 0.5 mass% NaCl, 0.1 mass% CaCl ₂ , 0.075 mass% NaHCO ₃ aqueous solution immersion, 0.25 hours,
Drying: 60 ° C., 50% RH, 17.75 hours

  In the SAEJ2334 test, test pieces of length 60 mm × width 100 mm × thickness 3 mm collected from each steel plate (steel plates A, B, C) having a thickness of 20 mm were used. The surface of each test piece is subjected to shot blast treatment, and after shot blast treatment, a modified epoxy paint (“NOVA 2000” manufactured by China Paint Co., Ltd.) is spray-coated on the surface of the steel plate to obtain 350 μm. An anticorrosive film having a coating thickness was formed. After forming the anticorrosion film, for each test piece, a cross-shaped ridge was formed on the anticorrosion film, thereby exposing a part of the steel plate as the base.

Evaluation in the SAEJ2334 test was performed by the following (a) and (b).
(A) The maximum corrosion depth (maximum value of the corrosion depth from the steel material surface) of the steel material as the base was measured at the position where the ridge portion of the anticorrosion film was formed.
(B) In order to evaluate the area of the portion of the anticorrosive film that peeled off from the heel, the film peeled area ratio (%) was determined. Specifically, the part from which the anticorrosion film peeled (the part that developed from the collar and peeled off) was removed with a cutter or the like, and the removed part was used as the film peeling part. Then, using the binarization processing of the image processing software, a value of (film peeled portion area) / (test piece area) × 100 was obtained and used as the film peeled area ratio (%). A test piece area means the area of the surface in which the collar part was formed among six surfaces of a test piece.

  In the SAEJ2334 test, the case where the maximum corrosion depth was 0.45 mm or less and the film peeling area ratio was 60% or less was regarded as acceptable.

Next, the wave tank test (WT test) will be described.
The wave tank test (WT test) is a test that simulates the environment in a ship's ballast tank. The WT test was performed under the following conditions simulating the back side of the deck of the ship's ballast tank (position (2) in FIG. 1).
(Test conditions)
-Under the temperature cycle (temperature of a test piece) which repeats "50 degreeC, 12 hours" and "20 degreeC, 12 hours", the seawater splashes which scattered from the seawater surface are made to adhere to the test piece surface.

  In the WT test, test pieces of length 140 mm × width 30 mm × thickness 2.5 mm collected from each steel plate (steel plates A to C) having a thickness of 20 mm were used. The surface of each collected test piece was spray-coated with a modified epoxy paint (“Banno 500” manufactured by China Paint Co., Ltd.) to form an anticorrosion film having a thickness of 350 μm. Then, as shown in FIG. 2B, in the central part of the test piece, a 10 mm-long linear ridge extending in the width direction of the test piece was formed on the anticorrosion film, and a part of the steel material as a base was exposed. .

Evaluation in the WT test was performed by the following (c) and (d).
(C) The maximum corrosion depth (maximum value of the corrosion depth from the steel material surface) of the steel material as the base was measured at the position where the ridge portion of the anticorrosion film was formed.
(D) In order to evaluate the area of the part which developed from the buttocks of the anticorrosion film and was peeled off, the film peeled area ratio (%) was determined. Specifically, the part from which the anticorrosion film peeled (the part that developed from the collar and peeled off) was removed with a cutter or the like, and the removed part was used as the film peeling part. Then, using the binarization processing of the image processing software, the value of (film peeling area) / (30 mm × 100 mm area centering on the film collar) × 100 is obtained and the film peeling area ratio (%) did. Here, the reason why the film peeling area ratio was standardized using an area of 30 mm × 100 mm as a denominator is that it is unlikely that the film peeling proceeds with a size larger than this area.
In the WT test, the case where the maximum corrosion depth was 0.3 mm or less and the film peeling area ratio was 35% or less was regarded as acceptable.

The results of the above test are shown in Table 2.
The results of the SAEJ2334 test were good for all of the steel plates A, B, and C. However, the steel plates A and B gave bad results in the film peeling area ratio and the maximum corrosion depth in the WT test. On the other hand, the steel sheet C showed good results in both the SAEJ 2334 test and the WT test.
In steel plates A and B, the SAEJ2334 test showed good results, but the WT test gave bad results. The reason for this is that under the conditions of the WT test, the wetting time is long and the penetration of water under the coating film increases, so that the pH increases due to the dissolution reaction of Fe ²⁺ , and the coating film is compared with the SAEJ2334 test. It is thought that this is because peeling is further promoted.

  In view of the above circumstances, an object of the present invention is to provide a steel material that can ensure excellent corrosion resistance even in a severe environment including chloride, such as the back side of the deck of a ship's ballast tank.

The present inventors received the above test results, and examined the structures of the test pieces of the steel plates A to C by observation with an optical microscope. As a result, in order to ensure excellent corrosion resistance even under harsh conditions such as the WT test, a new possibility has been found that it is important to control not only the chemical composition of the steel sheet but also its structure.
The inventors further conducted a detailed investigation on the above findings. Specifically, steel sheets A1 to A4 were prepared by changing the production conditions as shown in Table 3 for steel having the same chemical composition as steel sheet A in Table 1. Moreover, the WT test was done using the test extract | collected from these steel plates A1-A4. The test conditions were the same as those described above.

  Table 4 shows the results of the WT test performed on the steel plates A1 to A4. In the steel plate A3, good results were obtained for both the film peeling area ratio and the maximum corrosion depth. Further, when the structures of the steel plates A1 to A4 were examined in detail, the ratio of the Sn concentration in the hard tissue to the Sn concentration in the soft tissue (Sn concentration in the hard structure / Sn concentration in the soft tissue, hereinafter referred to as the Sn concentration ratio). It was found that by controlling the value within a predetermined range, excellent corrosion resistance can be ensured even under severe conditions such as the WT test.

The reason why the Sn concentration ratio changes depending on the manufacturing conditions is considered as follows.
(I) In the case of manufacturing conditions for steel plate A1 Cooling after rolling is air cooling, and the cooling rate is slow. Therefore, a soft structure (soft phase) such as ferrite is likely to grow, and a soft structure centering on ferrite or the like and a layered (band-shaped) hard structure (hard phase) are formed. Further, since the cooling rate is slow, Sn diffusion from the soft tissue to the hard tissue is likely to occur, and the Sn concentration ratio becomes high.
(Ii) In the case of manufacturing conditions for the steel sheet A2, Sn has a low melting point compared to other elements, and thus diffuses even between 550 and 400 ° C, but when strongly cooled (water cooled) to 650 ° C to 400 ° C, Sn cannot be sufficiently diffused into the hard tissue, and the Sn concentration ratio becomes small.
(Iii) In the case of manufacturing conditions for steel plate A3 After rolling, the temperature range up to 550 ° C. or higher is strongly cooled, thereby forming a structure in which a soft structure and a hard structure are dispersed. Thereafter, Sn is diffused moderately in the dispersed hard phase by being slowly cooled to 550 ° C to 400 ° C.
(Iv) In the case of manufacturing conditions for the steel sheet A4 Since the steel sheet is strongly cooled to 450 ° C. after rolling, the structure becomes the center of the hard phase, and Sn does not sufficiently diffuse into the hard phase, so the Sn concentration ratio becomes low.

  The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) The steel material according to one embodiment of the present invention has a chemical composition of mass%, C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.05 to 3 0.00%, Sn: 0.01 to 0.50%, O: 0.0001 to 0.0100%, Cu: 0 to less than 0.10%, Cr: 0 to less than 0.10%, Mo: 0 to 0 Less than 0.050%, W: 0 to less than 0.050%, Cu + Cr: 0 to less than 0.10%, Mo + W: 0 to less than 0.050%, Sb: 0 to less than 0.05%, Ni: 0 to 0 0.05%, Nb: 0 to 0.050%, V: 0 to 0.050%, Ti: 0 to 0.020%, Al: 0 to 0.100%, Ca: 0 to less than 0.0100% , Mg: 0~0.0100%, REM: 0~0.0100%, P: 0.05% or less, S: 0.01% or less, the balance be Fe and impurities; And the soft tissue is a ferrite, pearlite, bainite, and has a hard structure is martensite; Sn concentration ratio is the ratio of the Sn concentration of the hard tissue for the Sn concentration of the soft tissue is 1.2 or more It is less than 6.0.
(2) In the steel material according to the above (1), the chemical composition may contain Cu + Cr: 0 to less than 0.05% by mass%.
(3) In the steel material according to the above (1) or (2), the chemical composition may contain Mo + W: 0.0005 to less than 0.050% by mass%.
(4) In the steel material according to any one of (1) to (3), the chemical composition is mass%, Nb: 0.001 to 0.050%, and V: 0.005 to 0.00. 050%, Ti: 0.001 to 0.020%, Al: 0.01 to 0.100%, Ca: 0.0002 to less than 0.0100%, Mg: 0.0002 to 0.0100%, and REM : One or more selected from 0.0002 to 0.0100% may be contained.
(5) In the steel material as described in any one of (1) to (4) above, the surface may be coated with an anticorrosive film having a thickness of 20 μm or more.
(6) A ballast tank or a hold according to another aspect of the present invention is formed using the steel material according to any one of (1) to (5) above.
(7) The ship which concerns on another aspect of this invention is provided with the ballast tank or ship hold as described in said (6).

  According to the above aspect of the present invention, it is possible to provide a steel material having excellent corrosion resistance even in a corrosive environment containing chloride. Moreover, according to this invention, the ship provided with the ballast tank and ship hold | maintenance of a ship formed using this steel material excellent in corrosion resistance, and these ballast tanks and hold ship can be provided.

It is a schematic diagram of the test tank used for a wave tank test (WT test). It is an example of the test piece used for a wave tank test (WT test). It is an example of the test piece used for a wave tank test (WT test). It is a schematic diagram which shows the ballast tank, ship hold, and ship which apply the steel materials which concern on this embodiment.

  A steel material according to an embodiment of the present invention (hereinafter may be referred to as a steel material according to the present embodiment) will be described in detail. “%” Of the content of each element indicates “mass%”.

About chemical composition (chemical component) In the steel material which concerns on this embodiment, the reason which prescribes | regulates the chemical composition is as follows.

C: 0.01 to 0.20%
C is an element that improves the strength of the steel material. In order to obtain this effect, the lower limit of the C content is set to 0.01%. A preferable lower limit of the C content is 0.02%, and a more preferable lower limit of the C content is 0.03%. The lower limit of the C content may be 0.05%, 0.07%, or 0.09%. On the other hand, if the C content exceeds 0.20%, the weldability is significantly reduced. Moreover, with the increase in the C content, the amount of cementite that acts as a cathode in a low pH environment and promotes corrosion increases, and the corrosion resistance of the steel material decreases. For this reason, the upper limit of the C content is set to 0.20%. The upper limit of the preferable C content is 0.18%, and the more preferable upper limit of the C content is 0.16%. The upper limit of the C content may be 0.15% or 0.13%.

Si: 0.01-1.00%
Si is an element necessary for deoxidation. In order to obtain a sufficient deoxidizing effect, it is necessary to contain 0.01% or more. A preferable lower limit of the Si content is 0.03%, and a more preferable lower limit of the Si content is 0.05%. The lower limit of the Si content may be 0.10%, 0.15%, or 0.20%. On the other hand, if the Si content exceeds 1.00%, the toughness of the base material and the welded joint is impaired. For this reason, the upper limit of Si content is made 1.00%. The upper limit of the preferable Si content is 0.80%, and the more preferable upper limit of the Si content is 0.60%. The upper limit of the Si content may be 0.50%, 0.40%, or 0.30%.

Mn: 0.05 to 3.00%
Mn is an element having an effect of increasing the strength of the steel material at a low cost. In order to obtain this effect, the lower limit of the Mn content is set to 0.05%. The lower limit of the preferable Mn content is 0.20%, and the lower limit of the more preferable Mn content is 0.40%. The lower limit of the Mn content may be 0.60%, 0.80%, or 0.90%. On the other hand, if the Mn content exceeds 3.00%, weldability and joint toughness deteriorate. For this reason, the upper limit of the Mn content is set to 3.00%. The upper limit of the preferable Mn content is 2.50%, and the more preferable upper limit of the Mn content is 2.00%. The upper limit of the Mn content may be 1.80%, 1.60%, or 1.50%.

Sn: 0.01 to 0.50%
Sn is an important element in the steel material according to the present embodiment. Sn is dissolved as ^{Sn 2+,} by lowering the concentration of ^{Fe 3+} by ^{2Fe 3+ + Sn 2+ → 2Fe 2+} + Sn 4+ comprising reaction inhibiting corrosion reaction. Moreover, Sn remarkably suppresses the anodic dissolution reaction of the steel material in a low pH chloride environment, thereby greatly improving the corrosion resistance of the steel material in a chloride corrosion environment. In order to obtain these effects, the lower limit of the Sn content needs to be 0.01%. A preferable lower limit of the Sn content is 0.03%, and a more preferable lower limit of the Sn content is 0.05%. The lower limit of the Sn content may be 0.08%, 0.12%, 0.16%, or 0.19%. On the other hand, when the Sn content exceeds 0.50%, not only the above effects are saturated, but also the toughness of the base material and the high heat input welded joint is deteriorated. Therefore, the upper limit of the Sn content is 0.50%. The upper limit of the preferable Sn content is 0.45%, and the upper limit of the more preferable Sn content is 0.40%. The upper limit of the Sn content may be 0.35% or 0.30%.

O: 0.0001 to 0.0100%
O (oxygen) is an important element in the steel material according to the present embodiment, similarly to Sn. O improves the toughness of the welded joint by containing a small amount. In order to obtain this effect, the lower limit of the O content needs to be 0.0001%. A preferable lower limit of the O content is 0.0002% or more, and a more preferable lower limit of the O content is 0.0003%. The lower limit of the O content may be 0.0005%, 0.0010%, 0.0015%, or 0.0019%. On the other hand, O forms oxides such as SnO and SnO ₂ . Therefore, if the O content exceeds 0.0100%, the Sn concentration in the hard tissue cannot be sufficiently ensured. Moreover, since the said oxide becomes a starting point of corrosion, the corrosion resistance of steel materials falls. Therefore, the upper limit of the O content is 0.0100%. A preferable upper limit of the O content is 0.0090%, and a more preferable lower limit of the O content is 0.0080%. The upper limit of the O content may be 0.0060%, 0.0040%, or 0.0030%.

Cr: 0 to less than 0.10% Cr is generally considered to be an element that improves the corrosion resistance of steel. However, the present inventors have found that the corrosion resistance of steel materials deteriorates when Cr is contained in a corrosive environment containing chloride as assumed in the present embodiment. A smaller Cr content is preferable, and the lower limit of the content is 0%. On the other hand, considering the case of mixing as impurities, the upper limit of Cr content is set to less than 0.10%. The Cr content is preferably limited to 0.07% or less or less than 0.05%, more preferably 0.03% or less or 0.02% or less. It is more preferable to limit the Cr content to 0.01% or less.

Cu: 0 to less than 0.10% Cu is generally considered to be an element that improves the corrosion resistance of steel. However, the present inventors have found that the corrosion resistance of the steel material decreases when Cu is contained in a corrosive environment containing chloride as assumed in the present embodiment. A smaller Cu content is preferable, and the lower limit of the Cu content is 0%. On the other hand, the upper limit of the Cu content is set to less than 0.10% in consideration of the case where it is mixed as an impurity. In order to improve the corrosion resistance, the Cu content is preferably limited to 0.07% or less or 0.05% or less, and more preferably 0.03% or less or 0.02% or less. It is even more preferable to limit the Cu content to 0.01% or less.
When the steel material contains Cu, Cu and Sn coexist. In this case, rolling cracks may occur depending on the manufacturing method. In order to suppress rolling cracks, it is important to reduce the ratio of Cu content to Sn content (Cu / Sn). When Cu is contained, Cu / Sn is preferably 1.0 or less. More preferably, Cu / Sn is 0.5 or less or 0.3 or less.

Cu + Cr: 0 to less than 0.10% As described above, Cr and Cu are elements that lower the corrosion resistance of steel in a corrosive environment containing chloride. Therefore, when these elements are contained simultaneously, it is necessary to limit not only the content of each element but also the total content. That is, it is necessary to limit the total content of Cu and Cr to less than 0.10%. Preferably, it is less than 0.07%, more preferably less than 0.05%, still more preferably less than 0.04%, still more preferably less than 0.03%.

Mo: 0 to less than 0.050% When the Mo content is 0.050% or more, the corrosion resistance may be lowered, and the cost of the steel material is significantly increased. Therefore, it is preferable that the Mo content is small, and the Mo content is less than 0.050%. Preferably, the Mo content is 0.040% or less. The upper limit of the Mo content may be 0.030%, 0.020%, 0.010%, or 0.005%. In order to improve the corrosion resistance, it is preferable that the Mo content is small, and the lower limit of the Mo content is 0%. However, the lower limit of the Mo content may be set to 0.010% or 0.020% in order to improve characteristics such as strength or toughness.

W: 0 to less than 0.050% When the W content is 0.050% or more, the corrosion resistance may be lowered, and the cost of the steel material is significantly increased. Therefore, it is preferable that the Mo content is small, and the W content is less than 0.050%. More preferably, the W content is 0.040%. The upper limit of the W content may be 0.030%, 0.020%, 0.010%, or 0.005%. In order to improve the corrosion resistance, it is preferable that the W content is small, and the lower limit of the W content is 0%. However, to improve properties such as strength or toughness, the lower limit of Mo may be 0.010% or 0.020%.

Mo + W: 0 to less than 0.050% In order to improve corrosion resistance, it is necessary to limit not only the content of each element but also the total content of Mo and W. That is, the total content of Mo and W is limited to less than 0.050%. The upper limit of the total content may be 0.030%, 0.020%, 0.010%, or 0.005%. In order to improve the corrosion resistance, it is preferable that the total content is small. However, in order to improve properties such as strength or toughness, the lower limit of the total content may be 0.005%, 0.010%, or 0.020%. There is no problem.

The steel material according to the present embodiment basically has the above components and the balance is Fe and impurities. In addition to the above components, the steel material is selected from the following elements as necessary. One or more components may be included.
Moreover, in this embodiment, an impurity means the component mixed by raw materials, such as an ore and a scrap, and other factors, when manufacturing steel materials industrially.

Sb: 0 to 0.05%
Sb is an element that improves acid resistance. However, even if Sb exceeds 0.05%, not only the effect is saturated, but also the toughness of the steel material is deteriorated. Therefore, the Sb content is 0.05% or less. The upper limit of the Sb content may be 0.04% or 0.03%. The Sb content is not essential, and the lower limit of the Sb content is 0%. However, in order to improve acid resistance, the lower limit of the Sb content may be 0.005%, 0.010%, or 0.015%.

Ni: 0 to 0.05%
Ni is generally considered to improve the corrosion resistance of steel, like Cu. However, the present inventors have found that, in a corrosive environment containing chloride as assumed in the present embodiment, the corrosion resistance of the steel material is reduced when Ni is contained. A lower Ni content is preferable, and the lower limit of the Ni content is 0%. Considering the case of mixing as an impurity, the upper limit of Ni content is set to 0.05%. In order to improve the corrosion resistance, the Ni content is preferably limited to 0.03% or less or 0.02% or less, and more preferably limited to 0.01% or less.

Nb: 0 to 0.050%
Nb is an element that increases the strength of the steel material. However, when the Nb content exceeds 0.050%, the above effect is saturated. Therefore, the Nb content when contained is 0.050% or less. If necessary, the Nb content may be 0.030% or less or 0.020% or less. Since Nb is not necessarily contained, the lower limit of the Nb content is 0%. However, in order to obtain the effect of improving the strength, Nb may be contained by 0.001% or more, 0.003% or more, or 0 You may make it contain 0.005% or more.

V: 0 to 0.050%
V is an element that increases the strength of the steel material in the same manner as Nb. V, like Mo and W, dissolves in a corrosive environment (in an aqueous solution) and exists in the form of oxyacid ions, and suppresses transmission of chloride ions in the rust layer. However, when the V content exceeds 0.050%, not only the above effects are saturated, but also the cost is remarkably increased. Therefore, when V is included, the V content is 0.050% or less. The V content may be 0.040% or less or 0.030% or less. Since V is not necessarily contained, the lower limit of the V content is 0%. However, in order to obtain the above effect, V may be contained by 0.005% or more or 0.010% or more.

Ti: 0 to 0.020%
Ti is effective for deoxidation of steel materials and suppresses the formation of MnS, which is a starting point for corrosion of steel materials. However, when the Ti content exceeds 0.020%, not only the above effects are saturated, but also the cost of the steel material increases. Therefore, when Ti is contained, the Ti content is 0.020% or less. The Ti content is preferably 0.015% or less. Since Ti is not necessarily contained, the lower limit of the Ti content is 0%. However, in order to obtain the above effect, 0.005% or more or 0.008% or more of Ti may be contained.

Al: 0 to 0.100%
Al is an element effective for deoxidation of steel materials. In this embodiment, since Si is contained in the steel material, deoxidation is performed by Si. Therefore, deoxidation treatment with Al is not always necessary, and the lower limit of the Al content is 0%. However, in addition to Si, deoxidation with Al may be performed. In order to obtain the deoxidation effect by Al, the Al content is preferably 0.010% or more, more preferably 0.020% or more, and further preferably 0.030% or more. On the other hand, when the Al content exceeds 0.100%, the corrosion resistance of the steel material in a low pH environment decreases, and the corrosion resistance of the steel material in a chloride corrosion environment decreases. On the other hand, if the Al content exceeds 0.100%, the nitride becomes coarse, which causes a decrease in the toughness of the steel material. Therefore, the Al content when contained is 0.100% or less. The upper limit of the preferable Al content is 0.060%, and the more preferable upper limit of the Al content is 0.045%.

Ca: 0 to less than 0.0100% Ca is present in the form of an oxide in the steel material, and suppresses the decrease in pH at the interface in the corrosion reaction part, thereby suppressing corrosion. When obtaining said effect, it is preferable to contain Ca 0.0002% or more, and it is more preferable to contain 0.0005% or more. On the other hand, when the Ca content is 0.0100% or more, the above effect is saturated. Therefore, when Ca is contained, the Ca content is less than 0.0100%. The Ca content may be 0.0050% or less or 0.0030% or less. Since it is not always necessary to contain Ca, the lower limit of the Ca content is 0%.

Mg: 0 to 0.0100%
Mg, like Ca, suppresses the decrease in pH at the interface in the corrosion reaction part and suppresses corrosion of the steel material. When obtaining the above effect, the Mg content is preferably 0.0002% or more, and more preferably 0.0005% or more. On the other hand, when the Mg content exceeds 0.0100%, the above effect is saturated. Therefore, the Mg content when contained is 0.0100% or less. The Mg content may be 0.0050% or less or 0.0030% or less. Since it is not always necessary to contain Mg, the lower limit of the Mg content is 0%.

REM: 0 to 0.0100%
REM (rare earth element) is an element that improves the weldability of steel. When obtaining this effect, the REM content is preferably 0.0002% or more, and more preferably 0.0005% or more. On the other hand, when the REM content exceeds 0.0100%, the above effect is saturated. Therefore, the REM content when contained is 0.0100% or less. The upper limit of the REM content may be 0.0050% or 0.0030%. It is not always necessary to contain REM, and the lower limit of the REM content is 0%. In the present embodiment, REM is a generic name for 17 elements in which Y and Sc are added to 15 elements of a lanthanoid. The steel material which concerns on this embodiment can contain 1 or more types of these 17 elements in steel materials, and REM content means the total content of these elements.

  Among impurities, the following elements need to be strictly limited in content.

P: 0.050% or less P is an element present as an impurity in the steel material. P is an element that lowers the acid resistance of the steel material, and lowers the corrosion resistance of the steel material in a chloride corrosion environment where the pH of the corrosion interface is lowered. Further, P decreases the weldability of the steel material and the toughness of the weld heat affected zone. Therefore, the P content is limited to 0.050% or less. The P content is preferably limited to 0.040% or less, and more preferably limited to less than 0.030%. In order to improve the toughness of the weld heat affected zone, the upper limit of the P content may be 0.020%, 0.015%, or 0.010%. Although it is not easy to completely remove P, it is not necessary to eliminate it, and the lower limit of the P content is 0%.

S: 0.010% or less S is an element present as an impurity in the steel material. S forms MnS as a starting point of corrosion in the steel material. When the S content exceeds 0.010%, the corrosion resistance of the steel material is significantly reduced. For this reason, the S content is limited to 0.010% or less. The S content is preferably limited to 0.008% or less, more preferably limited to 0.006% or less, and further preferably limited to 0.004% or less. It is not easy to completely remove S, but it is not necessary to eliminate it, and the lower limit of the S content is 0%.

The microstructure of the steel material according to this embodiment will be described.
The steel material according to the present embodiment has a hard structure and a soft structure. In the present embodiment, the hard structure is pearlite, bainite, and martensite, and the soft structure is ferrite. The ratio between the hard structure and the soft structure may be determined in accordance with the strength design of the steel material, and is not particularly limited. However, in order to ensure the strength and toughness necessary for the hull structural steel, the present embodiment The microstructure of the steel material according to the present invention is preferably a composite structure containing pearlite and ferrite, and is preferably area% (area ratio) and the ferrite structure is 80% or less of the entire structure.
Moreover, the steel material according to the present embodiment has a structure in which a hard structure and a soft structure are laminated and / or dispersed. When the Sn concentration ratio described later is controlled within a predetermined range, a structure in which a hard tissue and a soft tissue are dispersed is preferable. In addition, when the average particle diameter described later is reduced, a structure in which a hard structure and a soft structure are dispersed is preferable. The structure in which the hard structure and the soft structure are dispersed refers to a structure in which the hard structure and the soft structure are dispersed in the steel material.

  When the crystal grains of the steel material are coarsened, the mechanical properties of the steel material are deteriorated and intragranular corrosion is likely to occur. There is a risk that the crystal grains that have undergone intragranular corrosion become the starting point of corrosion. Therefore, the average particle diameter of the soft tissue and the average particle diameter of the hard tissue are each preferably 15 μm or less, and more preferably 10 μm or less. Note that the “average grain size” is defined as a grain boundary at a structure boundary having an orientation difference of 15 ° or more when evaluated by EBSP (Electron Backscatter Diffraction Pattern). A portion surrounded by the grain boundary is defined as a grain boundary. It can be calculated as a crystal grain. Specifically, for example, by using the EBSP method, observation of 5 fields or more is performed at a magnification of 2000 times, a structure boundary having an orientation difference of 15 ° or more is regarded as a crystal grain boundary, and an area of each crystal grain is obtained. Then, the equivalent circle diameter is calculated from each obtained area, the calculated value is regarded as the crystal grain size of each crystal grain, and the average value thereof is defined as the average grain size. In the evaluation by the EBSP method, for example, a cross section of a test piece cut out from a steel material is observed. More specifically, for example, a cross section parallel to the rolling direction and the plate thickness direction of the steel material is observed from a direction perpendicular to the cross section. The region to be observed is, for example, a position that is 1/4 of the plate thickness from the surface of the steel material.

  Generally, in a neutral chloride aqueous solution, it is said that the soft tissue has excellent corrosion resistance and the hard tissue has low corrosion resistance. Here, in a severe corrosive environment that contains chloride and is repeatedly dried and wet, the thin water film adhering to the steel material surface changes to an acidic chloride aqueous solution. For this reason, in steel materials used in the corrosive environment as described above, the hard structure corroded in neutral chloride aqueous solution starts as a starting point, and the corrosion progresses to develop full corrosion including the soft structure. Sometimes.

As described above, the hard tissue has lower corrosion resistance than the soft tissue, and the hard tissue is a starting point for corrosion. However, if there is much Sn in the hard structure, the progress of corrosion can be avoided, and the progress of the overall corrosion of the steel including the soft structure can be suppressed.
In the steel material according to the present embodiment, in order to prevent such corrosion, the Sn concentration in each structure is controlled so that the Sn concentration in the hard structure is 1.2 times or more the Sn concentration in the soft structure. As described above, Sn ions dissolved by corrosion improve the corrosion resistance of the steel material. Therefore, if Sn is present at a high concentration in the hard structure that corrodes in advance, the initial corrosion in the hard structure can be avoided, and the progress of corrosion to the entire steel material can be prevented. However, when the Sn concentration in the hard tissue is more than 6 times the Sn concentration in the soft tissue, a potential difference is generated between the hard tissue and the soft tissue, and the soft tissue having excellent corrosion resistance is preferentially corroded over the hard tissue. So rather the corrosion resistance is reduced. Therefore, in order to further exhibit the corrosion resistance effect of Sn, it is necessary that the Sn concentration in the hard tissue is 1.2 times or more and less than 6.0 times the Sn concentration in the soft tissue. Preferably, the Sn concentration in the hard tissue is 1.3 times or more, more preferably 1.5 times or more, still more preferably 1.7 times or more, even more preferably 2.0 times the Sn concentration in the soft tissue. That's it. Preferably, the Sn concentration in the hard tissue is 5.0 times or less, more preferably 4.0 times or less, and even more preferably 3.5 times or less the Sn concentration in the soft tissue.

  The steel material according to the present embodiment has excellent corrosion resistance even if it is used as it is because it has the above-described chemical components and structure. However, the corrosion resistance of the steel material can be further improved by forming an anticorrosion film on the surface of the steel material by painting or the like. Specifically, for example, the surface of the steel material can be covered with an anticorrosion coating made of an organic resin.

  Here, examples of the anticorrosion film made of an organic resin include vinyl butyral, epoxy, urethane, and phthalic acid resin films. One or more of these resins may be laminated to form an anticorrosion film. The film thickness of the anticorrosive film (film thickness when dried) is preferably 20 μm or more, and more preferably 50 μm or more. The lower limit of the film thickness may be 100 μm or 150 μm. When the film thickness of the anticorrosion film (thickness at the time of drying) exceeds 500 μm, peeling of the anticorrosion film may progress due to a thermal cycle due to a difference in thermal expansion coefficient between the resin and the steel material. Therefore, the film thickness of the anticorrosion film is preferably 500 μm or less. The upper limit of the film thickness may be 400 μm or 300 μm.

  Moreover, in the steel material which concerns on this embodiment, durability of an anticorrosion film improves compared with the conventional steel material. As a result, the corrosion resistance of the steel material is further improved. The reason why the durability of the anticorrosive film is improved is as follows. That is, as described above, in the steel material according to the present embodiment, corrosion can be remarkably suppressed regardless of the presence or absence of the anticorrosion film. For this reason, even when the anticorrosion film has a defective part, it can suppress that the steel material as a foundation | substrate is corroded from the said defective part. Thereby, the expansion | swelling and peeling of an anticorrosion film resulting from corrosion of steel materials can be suppressed. As a result, it is considered that the durability of the anticorrosion film is improved.

The steel material according to the present embodiment has excellent corrosion resistance even in a corrosive environment containing chloride. Therefore, the steel material according to the present embodiment is suitably used as a material for a ship's ballast tank, a bulk carrier of a bulk carrier such as a coal carrier or an ore carrier.
In a ballast tank or a hold of a ship formed using the steel material according to the present embodiment, corrosion is suppressed, so that the frequency of maintenance such as repainting can be reduced. In addition, in ships equipped with these ballast tanks and holdhouses, it is possible to reduce operational costs due to lower maintenance frequency, and to prevent the replacement of steel (thinned due to corrosion) to steel with the required thickness (repair). The effects of improving safety and reducing repair costs can be obtained.

  Hereinafter, the preferable manufacturing method of the steel material which concerns on this embodiment is demonstrated. The steel material according to the present embodiment is not limited to the production method as long as the chemical components, the structure, and the like are satisfied, but the following production method is preferable because it can be easily produced.

In manufacturing the steel material according to the present embodiment, in order to increase the Sn concentration in the hard structure, it is preferable to use, for example, a slab in which the contents of S and O (oxygen) that easily form a compound with Sn are suppressed. The reason is that Sn forms a compound with O (oxygen) or S, and forms tin oxide (SnO, SnO ₂ etc.) or tin sulfide (SnS, SnS ₂ ), and these compounds are formed in the steel material. This is because Sn in the steel material decreases. When melting a slab with a low S and O content, it is desirable to manage deoxidation and / or desulfurization as shown below.

  That is, with regard to deoxidation, Si and Mn are added in advance at the initial stage of melting to perform preliminary deoxidation, and after the dissolved oxygen concentration in the molten steel is set to 30 ppm or less, deoxidization is performed again by adding Al. It is desirable. At this time, if necessary, Ti having a deoxidizing effect may be added together with Al. On the other hand, desulfurization is desirably performed by adding quick lime together with a slag modifier to slag formed by melting. The addition of Sn is desirably performed after deoxidation to reduce the dissolved oxygen concentration in the molten steel to 30 ppm or less and / or to desulfurize the dissolved sulfur concentration in the molten steel to 100 ppm or less. In the case of deoxidation, Al may be added for preliminary deoxidation, and the dissolved oxygen concentration in the molten steel may be 30 ppm or less, and then Si and Mn may be added to perform deoxidation again. In addition, what is necessary is just to adjust chemical components other than S and O by the well-known method in the preferable range mentioned above.

Depending on the composition of the steel material, the slab that suppresses the content of S and O, which can easily form a compound with Sn as described above, is heated at about 1000 to 1150 ° C. What is necessary is just to hot-roll on the conditions whose rolling finish temperature is about 700-900 degreeC more than 3.0%.
The steel material after hot rolling is, for example, water-cooled (weakly water-cooled) at an average cooling rate of 1.0 to 3.0 ° C./s in a temperature range of up to 650 ° C. after the end of rolling, and then continuously from 650 ° C. to 550 The temperature range of ° C. is cooled with water at an average cooling rate of 3.0 to 25 ° C./s (strong water cooling), and the temperature range of 550 ° C. to 400 ° C. is cooled at an average cooling rate of 0.01 to 1.0 ° C./s. Cooling) and then air cooling to room temperature. The temperature said here is the temperature of the steel surface.
Sn has the property of being easily concentrated in hard tissues. When the cooling rate in the temperature range of 650 ° C. or higher is slow, such as less than 1.0 ° C./s (for example, 0.8 ° C./s), a macroscopic band structure is formed in soft tissue and hard tissue. In addition, Sn is concentrated in the hard tissue, and the Sn concentration ratio between the hard tissue and the soft tissue (Sn concentration in the hard tissue / Sn concentration in the soft tissue, hereinafter referred to as the Sn concentration ratio) is 6.0 or more. There is a risk. In this case, there is a possibility that the toughness of the steel material starting from the hard structure may be reduced, and when the steel material is corroded, the soft structure is preferentially dissolved, whereby the corrosion progresses locally.
In addition, when strong cooling is performed uniformly in a temperature range of up to 400 ° C. after rolling, the structure becomes the center of the hard phase, and the Sn concentration ratio may be lowered.
Further, when the cooling rate of 550 ° C. to 400 ° C. exceeds 1.0 ° C., Sn is not sufficiently diffused into the hard tissue, and the Sn concentration ratio becomes small.

By producing a steel material by the method as described above, both the soft structure and the hard structure are refined, and the Sn concentration in the hard structure is 1.2 times or more and less than 6.0 times the Sn concentration in the soft structure (that is, The Sn concentration ratio is 1.2 or more and less than 6.0), and the hard structure is finely dispersed in the steel material. Thereby, when a steel material is corroded, it becomes a more uniform corrosion form. When the hard structure is coarse, when the Sn concentration ratio of the hard structure and the soft structure (Sn concentration in the hard structure / Sn concentration in the soft structure) is 6.0 or more, the steel material is corroded. Since soft tissue is preferentially dissolved, it becomes a local corrosion form.
The steel material according to the present embodiment may be a steel plate. Although it is not necessary to specifically limit the plate thickness of the steel plate, the lower limit of the plate thickness may be 6 mm or 10 mm, and the upper limit of the plate thickness may be 50 mm or 40 mm.
The strength (tensile strength) of the steel material according to the present embodiment is not particularly limited. However, considering application to an actual structure, it may be 400 MPa or more and 600 MPa or less.

  When forming an anticorrosion film on the surface of the steel material concerning this embodiment, an anticorrosion film can be formed by a publicly known usual method. In addition, the whole surface of steel materials may be coat | covered with the anti-corrosion film, and only a part of surface of steel materials may be coat | covered with the anti-corrosion film. Specifically, the anticorrosion film may be formed only on a portion exposed to a corrosive environment (for example, one side of a steel material). When using a steel material as a steel pipe, an anticorrosion film may be formed only on the outer or inner surface of the steel pipe. Before forming the anticorrosive film, the surface of the steel material may be subjected to chemical conversion treatment. In the chemical conversion treatment, a treatment agent prepared from zinc, titanium, zirconium, chromium, a silane compound, or the like can be used.

  What is necessary is just to perform by a well-known method, when forming a ballast tank and a hold using the steel materials which concern on this embodiment. Moreover, what is necessary is just to perform by a well-known method also when constructing | assembling the ship provided with the ballast tank and ship hold using the steel materials which concern on these this embodiment. Drawing 3 is a mimetic diagram showing an example of composition of a vessel provided with a ballast tank and a hold, and these ballast tanks and a hold, formed using steel materials concerning this embodiment.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  Twenty-seven types of steel were melted using a vacuum melting furnace to form a 50 kg steel ingot, and then hot forged by a normal method to produce blocks (steel Nos. 1-27) having a thickness of 60 mm. Table 5 shows the chemical composition of the produced block.

  Next, as shown in Table 6, each of the above blocks was heated at 1100 ° C. to 1120 ° C. for 1 hour or more and then hot-rolled to a finish rolling temperature of 750 to 900 ° C. to obtain a steel plate having a thickness of 20 mm. Obtained. Thereafter, the steel sheet was cooled to room temperature under various cooling conditions. It was set as the steel plate of 1-30. The cooling start temperature in Table 6 indicates the temperature at which water cooling is started after rolling, the cooling rate 1 is the average cooling rate at the end of rolling to 650 ° C, the cooling rate 2 is the average cooling rate at 650 ° C to 550 ° C, The cooling rate 3 is an average cooling rate at 550 to 400 ° C.

  A steel material having a length of 20 mm, a width of 15 mm, and a thickness of 20 mm was cut out from the above steel plates (Test Nos. 1 to 30) to prepare a micro sample. Each micro sample was mirror-polished and then corroded using a nital etchant. Then, each micro sample was observed with a microscope and SEM (scanning electron microscope). Simultaneously with the SEM observation, the EDX (energy dispersive X-ray spectrometer: JED2300 manufactured by JEOL Ltd., observation magnification: 3500 times, visual field width: 46 μm × 33 μm), the Sn concentration (A) of the hard tissue and the soft tissue The Sn concentration (B) was quantitatively analyzed. Then, the Sn concentration ratio (= (A) / (B)) between the hard tissue and the soft tissue was determined.

  As a result, as shown in Table 7, in Test Nos. 1 to 24 according to the examples of the present invention, the Sn concentration ratio between the hard tissue and the soft tissue was 1.2 or more and less than 6.0. Although not shown in Table 7, all the steel sheets of the inventive examples included a soft layer and a hard phase, and the area ratio of the ferrite structure as the soft layer was 80% or less of the entire structure.

  On the other hand, in the test Nos. 25 to 27 and 29 according to the comparative example, the Sn concentration ratio was out of the range of the present invention. Test No. 28 according to the comparative example does not contain Sn. In test No. 30 according to the comparative example, the Sn concentration ratio was outside the specified range of the present invention, but the total content of Cu + Cr was out of the range of the present invention.

  Apart from the above observations, the following corrosion tests were carried out. As the corrosion test, a SAEJ2334 test and a wave tank test (WT test) were performed, which simulated the corrosive environment of the ballast tank. As the WT test, two types of tests, a WT test (1) and a WT test (2) described later, were performed. In the SAEJ2334 test, two types of test pieces were used: a test piece having no anticorrosion film and a test piece having an anticorrosion film. In the WT test (1) and the WT test (2), a test piece having an anticorrosion film was used.

First, the SAEJ2334 test will be described. The SAEJ2334 test is an accelerated deterioration test performed under the following conditions of repeated wet and dry (wet → salt deposition → dry) as one cycle (24 hours in total), and simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd. It is a test.
-Wet: 50 ° C, 100% RH, 6 hours,
Adhesion of salt: 0.5% by mass NaCl, 0.1% by mass CaCl2, 0.075% by mass NaHCO ₃ aqueous solution, 0.25 hours,
Drying: 60 ° C., 50% RH, 17.75 hours

  In the SAEJ2334 test, a test piece having a length of 60 mm, a width of 100 mm, and a thickness of 3 mm collected from each steel plate (test Nos. 1 to 30) having a thickness of 20 mm was used. The surface of each test piece is subjected to shot blasting, and the test piece having an anticorrosive film is subjected to the above shot blasting treatment and then two kinds of modified epoxy paints (A: “NOVA 2000” manufactured by China Paint Co., Ltd.). ”, B:“ Banno 500 ”manufactured by China Paint Co., Ltd.) was spray-coated on the surface of the steel material to form an anticorrosion film. For each test piece having the anticorrosion film, a cross-shaped ridge was formed on the anticorrosion film to expose a part of the steel material as a base.

  Next, the wave tank test (WT test) will be described. The WT test is a test performed in a test tank simulating the corrosive environment of a ship's ballast tank as shown in FIG. The test cycle consists of two weeks in a state where artificial seawater (salt water) is stored in the test tank and one week in which the test tank is empty in order to simulate the environment in the ballast tank. The temperature of the artificial seawater was maintained at 35 ° C., and the splash of the artificial seawater was attached to the test piece by swinging the test tank.

  Of the wave tank test (WT test), the WT test (1) is a test that simulates the environment of the side surface of the ballast tank, as shown in FIG. In the WT test (1), seawater splashes scattered from the seawater surface adhere to the surface of the test piece. In the WT test (1), a test piece having a length of 290 mm, a width of 30 mm, and a thickness of 2.5 mm collected from each steel plate (test Nos. 1 to 30) having a thickness of 20 mm was used. On the surface of each collected specimen, either one of the above-mentioned two types of modified epoxy paints (A: “Nova 2000” manufactured by China Paint Co., Ltd., B: “Banno 500” manufactured by China Paint Co., Ltd.) Was spray-coated to form an anticorrosion film. And as shown to FIG. 2A, the lower part (immersion zone) of the test piece was immersed in artificial seawater. A pair of linear ridges (length 10 mm) extending in the width direction of the test piece is formed on the anticorrosion film in the upper part of the test piece, that is, the region not splashed in the artificial seawater (splash zone). A part was exposed. More specifically, the linear ridges were formed at a position 20 mm above the seawater surface and a position 110 mm above the seawater surface, respectively. The numbers in FIGS. 2A and 2B represent dimensions (unit: mm).

  Of the wave tank test (WT test), the WT test (2) is a test simulating the environment on the back side of the deck as shown in FIG. In the WT test (2), seawater droplets scattered from the seawater surface adhere to the test piece surface under a temperature cycle (temperature of the test piece) that repeats “50 ° C., 12 hours” and “20 ° C., 12 hours”. Let In the WT test (2), a test piece of length 140 mm × width 30 mm × thickness 2.5 mm collected from each steel plate (test Nos. 1 to 30) having a thickness of 20 mm was used. On the surface of each collected specimen, either one of the above-mentioned two types of modified epoxy paints (A: “Nova 2000” manufactured by China Paint Co., Ltd., B: “Banno 500” manufactured by China Paint Co., Ltd.) Was spray-coated to form an anticorrosion film. And as shown to FIG. 2B, in the center part of a test piece, the linear wrinkles (length 10mm) extended in the width direction of a test piece are formed in an anticorrosion film, and a part of steel materials as a foundation | substrate are exposed. It was.

  Evaluation in the SAEJ2334 test was performed as follows. A uniform rust layer was formed on the entire surface of each test piece on which the anticorrosion film was not formed after the test. For each of these test pieces, the amount of corrosion was determined. The “corrosion amount” was determined as an average thickness reduction amount of the test piece when the surface rust layer was removed. Specifically, the plate thickness reduction amount was calculated using the weight reduction amount of the test piece before and after the test and the surface area of the test piece, and was defined as the corrosion amount.

  About each test piece which formed the anticorrosion film, the maximum corrosion depth (maximum value of the corrosion depth from the steel material surface) of the steel material as a foundation | substrate was measured in the position in which the collar part of the anticorrosion film was formed. Moreover, in order to evaluate the area of the part which progressed from the collar part and peeled among anticorrosion films, the film peeling area rate (%) was calculated | required. Specifically, the part from which the anticorrosion film peeled (the part that developed from the collar and peeled off) was removed with a cutter or the like, and the removed part was used as the film peeling part. Then, using the binarization processing of the image processing software, a value of (film peeled portion area) / (test piece area) × 100 was obtained and used as the film peeled area ratio (%). In addition, a test piece area means the area of the surface in which the collar part was formed among six surfaces of a test piece.

  In addition, the pass / fail judgment standard in the SAEJ2334 test was set to pass 0.60 mm or less for the corrosion amount. Moreover, about the said maximum corrosion depth in the collar part of an anticorrosion film, 0.45 mm or less was set as the pass. Furthermore, the film peeling area ratio was determined to be 60% or less.

  Evaluation in the WT test was performed as follows. First, in order to evaluate the area of the portion of the anticorrosive coating that peeled off from the heel portion, the peeled film area ratio (%) was determined. Specifically, the part from which the anticorrosion film peeled (the part that developed from the collar and peeled off) was removed with a cutter or the like, and the removed part was used as the film peeling part. Then, using the binarization processing of the image processing software, the value of (film peeling area) / (30 mm × 100 mm area centering on the film collar) × 100 is obtained and the film peeling area ratio (%) did. Here, the reason why the film peeling area ratio was standardized using an area of 30 mm × 100 mm as a denominator is that it is unlikely that the film peeling proceeds with a size larger than this area. In addition, as a pass / fail judgment standard in the WT test, a film peeling area ratio of 35% or less was accepted for both the WT test (1) and the WT test (2). About the maximum corrosion depth, 0.3 mm or less was set as the pass.

  The test results are shown in Table 7 above. In Table 7, “on WT (1)” is the evaluation result at the buttocks (see FIG. 2A) 110 mm above the sea level, and “under WT (1)” is the position 20 mm above the sea level. The evaluation result in a buttocks (refer FIG. 2A) is shown.

  As shown in Table 7, in the test Nos. 1 to 24 according to the examples of the present invention, in the SAEJ2334 test, the corrosion amount of the test piece without the coating is as small as 0.60 mm or less, and the maximum corrosion is also observed for the test piece with the coating. The depth is as small as 0.45 mm or less, and the film peeling area ratio is as low as 60% or less. Further, in both the WT test (1) and the WT test (2), the film peeling area ratio is as low as 35% or less. Furthermore, in steel materials Nos. 1 to 24, sufficient toughness was obtained by suppressing the Sn content to 0.5% or less.

  On the other hand, the test No. 25 according to the comparative example has an Sn concentration ratio of 6.0 or more, and WT (1) upper film peeling area ratio, WT (1) lower film peeling area ratio, and WT (2) film in the WT test. The peel area ratio and WT (2) maximum corrosion depth did not satisfy the target values.

  In Test Nos. 26 and 27 according to the comparative examples, the Sn concentration ratio is lower than 1.2, and the WT (1) upper film peeling area ratio, WT (1) lower film peeling area ratio, and WT (2) in the WT test. ) Delamination area ratio, WT (2) Maximum corrosion depth did not satisfy the target value. In Test No. 29 according to the comparative example, the O content exceeds 0.0100%, and the Sn concentration ratio is less than 1.2, and the non-coating corrosion resistance, the film peeling area ratio, and the maximum corrosion in the SAEJ2334 test. Depth, WT (1) upper film peeling area ratio, WT (1) lower film peeling area ratio, WT (2) film peeling area ratio, and WT (2) maximum corrosion depth satisfy the target values in the WT test I did not.

  Test No. according to the comparative example. No. 28 does not contain Sn, corrosion amount in SAEJ2334 test, film peeling area rate and maximum corrosion depth, WT (1) upper film peeling area rate, WT (1) lower film peeling area in WT test Rate, WT (2) film peeling area rate, and WT (2) maximum corrosion depth did not satisfy the target values.

  Test No. according to the comparative example. No. 30 is Sn content and Sn concentration ratio is in the range of the present invention, but the total content of Cu + Cr is 0.10% or more, the film peeling area ratio and the maximum corrosion depth in SAEJ2334 test, and WT WT (1) upper film peeling area ratio, WT (1) lower film peeling area ratio, WT (2) film peeling area ratio, and WT (2) maximum corrosion depth in the test did not satisfy the target values.

  From the above test results, it can be seen that the steel material according to the present invention can provide excellent corrosion resistance while preventing a decrease in toughness.

  ADVANTAGE OF THE INVENTION According to this invention, the steel materials which have the outstanding corrosion resistance also in the corrosive environment containing a chloride can be provided.

1 Ship 2 Ballast Tank 3 Funakura

Claims (7)

Chemical composition is mass%,
C: 0.01-0.20%
Si: 0.01 to 1.00%,
Mn: 0.05 to 3.00%,
Sn: 0.01 to 0.50%,
O: 0.0001 to 0.0100%,
Cu: 0 to less than 0.10%,
Cr: 0 to less than 0.10%,
Mo: 0 to less than 0.050%,
W: 0 to less than 0.050%,
Cu + Cr: 0 to less than 0.10%,
Mo + W: 0 to less than 0.050%,
Sb: 0 to less than 0.05%,
Ni: 0 to 0.05%,
Nb: 0 to 0.050%,
V: 0 to 0.050%,
Ti: 0 to 0.020%,
Al: 0 to 0.100%,
Ca: 0 to less than 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100%,
P: 0.05% or less,
S: 0.01% or less,
Balance: Fe and impurities;
Having a soft structure that is ferrite and a hard structure that is pearlite, bainite, and martensite ;
The Sn concentration ratio, which is the ratio of the Sn concentration in the hard tissue to the Sn concentration in the soft tissue, is 1.2 or more and less than 6.0;
A steel material characterized by that. The chemical composition is mass%,
The steel material according to claim 1, comprising Cu + Cr: 0 to less than 0.05%. The chemical composition is mass%,
Mo + W: The steel material of Claim 1 containing 0.0005 to less than 0.050%. The chemical composition is mass%,
Nb: 0.001 to 0.050%, V: 0.005 to 0.050%, Ti: 0.001 to 0.020%, Al: 0.01 to 0.100%, Ca: 0.0002 to It contains 1 or more types selected from less than 0.0100%, Mg: 0.0002-0.0100%, and REM: 0.0002-0.0100% of Claim 1 to 3 characterized by the above-mentioned. Steel material as described in any one of Claims.   The steel material according to any one of claims 1 to 4, wherein the surface is coated with an anticorrosive film having a thickness of 20 µm or more.   A ballast tank or a hold formed using the steel material according to any one of claims 1 to 5.   A ship provided with the ballast tank or the hold of Claim 6.

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