TWI531660B - Steel, ballast tank and ship hold using the steel, and ship including the ballast tank and the ship hold - Google Patents

Description

Steel, ballast tanks and cabins of ships using the steel, and the ballast tanks or cabins Ship Technical field

The present invention relates to a steel material which is excellent in corrosion resistance and can be used in a corrosive environment containing chloride in a seawater environment. Further, the present invention relates to a ballast tank and a ship cabin of a ship formed using a steel material excellent in corrosion resistance. Furthermore, the invention relates to a ship having such ballast tanks or cabins.

Background technique

Among the factors that accelerate the corrosion of steel, the influence of chloride is extremely well known. In particular, steels used in ship's ballast tanks, outer and superstructures, bridges in coastal areas, sheet piles and pipe piles in harbor facilities, offshore structures and offshore wind power plants are not only directly It is exposed to the alternating wet and dry environment and is extremely susceptible to corrosion. Although the steel set in seawater is less corroded than steel placed in an environment of alternating wet and dry conditions, it is also susceptible to corrosion. The steel used in the coastal area will not be affected by the sea water spray, but it will Corrosion is promoted by flying sea salt particles. In addition, even in inland areas, there are cases in which anti-freezing agents containing chlorides are sprayed in winter to prevent road surface freezing, so that there is also a problem of corrosion of steel materials caused by chlorides.

Furthermore, the ship carrying coal and iron ore, etc., the block used to load the cargo, that is, the interior of the ship, is not directly exposed to the seawater environment, but is exposed to chlorides caused by the use of seawater for washing and the like. The problem of steel corrosion. In addition, the cabin of the salt transport ship also faces the problem of corrosion of steel caused by chloride. Moreover, since there is a severe corrosive environment in which there is a high concentration of chloride solution in the oil tank of the tanker, there is a problem of corrosion of the steel. In addition, excavation and conveying equipment in oil sands are also facing the problem of corrosion of steel caused by chloride. As mentioned, corrosion of steel caused by chloride is indeed a major problem.

In the environment where the problem of corrosion caused by chloride is caused as described above, the steel is usually used after being coated. However, since the coating film may be cracked or corroded and continuously corroded from a portion where the thickness of the coating film such as the edge of the steel is thin, the steel material must be subjected to maintenance such as recoating under long-term use. For example, when repainting, it is necessary to provide a foot pedal or the like depending on the structure, which results in a large maintenance cost and a large amount of VOC (volatile) which is harmful to the human body due to painting. Organic compounds) and so on. From such a viewpoint, it has been strongly desired to develop a steel material which can have excellent corrosion resistance even if it is not coated, or a steel material which can extend the period of time required for recoating.

The steel material which has improved corrosion resistance in such a chloride environment, for example, contains Sn in an amount of 0.005 to 0.3% by mass, 0.02 to 0.40% by mass, and 0.01 to 0.50 by mass, as disclosed in Patent Documents 1 to 3. %, a steel that improves corrosion resistance in an environment containing chloride ions (Cl ^- ions).

Further, Patent Document 4 discloses a steel material which can extend the repair coating period in a seawater corrosion environment, and contains W: 0.01 to 0.5% by mass and Mo: 0.02 to 0.5% by mass, and Sn: 0.001 to 0.2. One or more of mass % and Sb: 0.01 to 0.2 mass%.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2010-064110

Patent Document 2: Japanese Patent Laid-Open Publication No. 2012-057236

Patent Document 3: Japanese Patent Laid-Open Publication No. 2012-255184

Patent Document 4: Japanese Patent Laid-Open Publication No. 2009-046750

As described above, a Sn-containing steel material or a steel material containing one or more of Sn and Sb and one or more of W and Mo is disclosed, and has excellent corrosion resistance in a corrosive environment containing chloride. However, the inventors of the present invention conducted corrosion tests on steel materials containing Sn and/or W, and confirmed that only a steel containing Sn or a steel containing one or more of Sn and Sb and one or more of W and Mo were used for various corrosive environments. Still not enough to ensure adequate corrosion resistance.

Specifically, the following corrosion test was performed.

Steel plates A to C containing chemical compositions as shown in Table 1 were prepared, and two corrosion tests simulating the corrosive environment of the ballast tanks, namely the SAEJ2334 test and the wave tank test (hereinafter referred to as "WT test"), were carried out using the steel plates. . in In any of the SAE J2334 test and the WT test, a test piece in which an anti-corrosion film was formed on the surface of the steel sheet was used.

Description of the SAEJ2334 test.

The so-called SAEJ2334 test is an accelerated degradation test in which a dry-wet alternating (wetting→adhering salt→drying) condition is used as a cycle (total 24 hours), and it is a harsh corrosion environment simulating a salt content of more than 1 mdd. Test. The SAE J2334 test was carried out under the following conditions. The corrosion pattern under the following conditions is similar to the corrosion pattern of the atmospheric exposure test.

(Test conditions)

‧ Wetting: 50 ° C, 100% RH, 6 hours; ‧ Adhesion salt: impregnation of 0.5% by mass of NaCl, 0.1% by mass of CaCl ₂ , 0.075% by mass of NaHCO ₃ aqueous solution, 0.25 hours; ‧ Drying: 60 ° C, 50% RH, 17.75 hours

In the SAE J2334 test, a test piece having a length of 60 mm, a width of 100 mm, and a thickness of 3 mm was taken out from each of the steel sheets (steel sheets A, B, and C) having a thickness of 20 mm. The surface of each test piece was subjected to bead blasting treatment, and after the bead blasting treatment, a modified epoxy type coating ("NOVA 2000" manufactured by China National Coatings Co., Ltd.) was sprayed on the surface of the steel sheet to form an anti-corrosion coating film having a thickness of 350 μm. Membrane. After the formation of the anti-corrosion film, each test piece was formed into a cross-shaped flaw on the anti-corrosion film to partially expose one of the steel sheets as the base.

The evaluation of the SAE J2334 test was carried out in accordance with the following (a) and (b).

(a) The maximum corrosion depth (the maximum value of the corrosion depth from the steel surface) of the steel material as the base was measured at the position where the scratch of the corrosion-resistant film was formed.

(b) In order to evaluate the area of the portion peeled off from the scar portion in the anti-corrosion film, the peeling area ratio (%) of the film was examined. Specifically, the portion where the anti-corrosion film is peeled off (the portion peeled off from the scar portion) is removed by a cutter or the like, and the removed portion is used as a film peeling portion. Then, using the binarization processing of the image processing software, the value of (film peeling area) / (test piece area) × 100, which is the film peeling area ratio (%), was calculated. The area of the test piece refers to the area of the surface on which the flaw portion is formed among the six faces of the test piece.

In the SAE J2334 test, when the maximum corrosion depth was 0.45 mm or less and the film peeling area ratio was 60% or less, it was judged to be acceptable.

Next, a description of the wave tank test (WT test). The so-called wave tank test (WT test) is a test that simulates the environment in a ship's ballast tank. The WT test was carried out in accordance with the following conditions for simulating the back side of the deck of the ship's ballast tank (the position of (2) of Fig. 1).

(Test conditions)

‧In the temperature cycle of "50 ° C, 12 hours" and "20 ° C, 12 hours" (the temperature of the test piece), the seawater spray splashed from the sea surface was attached to the surface of the test piece.

In the WT test, a test piece having a length of 140 mm, a width of 30 mm, and a thickness of 2.5 mm was taken out from each of the steel sheets (steel plates A to C) having a thickness of 20 mm. A modified epoxy coating ("BANNOH 500" manufactured by China National Coatings Co., Ltd.) was sprayed on the surface of each of the cut test pieces to form an anti-corrosion film having a coating film thickness of 350 μm. Then, as shown in FIG. 2B, in the center portion of the test piece, a linear flaw extending along the width direction of the test piece and having a length of 10 mm was formed on the etching resist to expose a part of the steel sheet as the base.

The evaluation of the WT test was carried out in accordance with the following (c) and (d).

(c) The maximum corrosion depth (the maximum value of the corrosion depth from the steel surface) of the steel material as the base was measured at the position where the corrosion-resistant film formed the flaw portion.

(d) In order to evaluate the area of the portion peeled off from the development of the flaw portion of the anti-corrosion film, the peeling area ratio (%) of the film was examined. Specifically, the portion where the anti-corrosion film is peeled off (the portion peeled off from the scar portion) is removed by a cutter or the like, and the removed portion is used as a film peeling portion. Then, using the binarization processing of the image processing software, the value of (the area of the peeled portion of the film) / (the area of 30 mm × 100 mm centered on the film scar portion) × 100 is calculated, that is, the peeling area ratio (%) of the film. Here, the reason why the area ratio of the film peeling is standardized by using the area of 30 mm × 100 mm as the denominator is that the peeling of the film does not progress to the size of the area or more.

In the WT test, when the maximum corrosion depth was 0.3 mm or less and the film peeling area ratio was 35% or less, it was judged to be acceptable.

The above test results are shown in Table 2.

The results of any of the steel plates A, B, and C in the SAEJ2334 test are good. it is good. However, in the WT test, the peeling area ratio of the film and the maximum corrosion depth of the steel sheets A and B were unfavorable results. On the other hand, the steel sheet C showed good results in both the SAEJ2334 test and the WT test.

In the case of the steel sheets A and B, although it showed good results in the SAEJ2334 test, it was a poor result in the WT test. The reason for this is presumably because in the conditions of the WT test, the penetration of water under the coating film is increased due to the long wetting time, so that the pH value due to the dissolution reaction of Fe ²⁺ is increased, so the test with SAEJ2334 is carried out. The ratio is further to promote the peeling of the film.

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

The inventors of the present invention obtained the above experimental results and investigated the structure of the test pieces of the steel sheets A to C by an optical microscope. A new discovery has been made in the survey results, that is, in order to ensure excellent corrosion resistance under such severe conditions as the WT test, not only the chemical composition of the steel sheet but also the control of its structure is important.

The inventors of the present invention conducted further investigations on the above findings. Specifically, the steel having the same chemical composition as that of the steel sheet A of Table 1 was changed into the steel sheets A1 to A4 as shown in Table 3. Further, the test pieces were taken from the steel sheets A1 to A4 and subjected to a WT test. The test conditions are the same as those described above.

The results of the WT test on the steel sheets A1 to A4 are shown in Table 4. In the steel sheet A3, both the peeling area ratio and the maximum corrosion depth of the film obtained good results. Further, after examining the structure of the steel sheets A1 to A4 in detail, it was found that the ratio of the Sn concentration in the hard structure to the Sn concentration in the soft tissue (the Sn concentration in the hard tissue/the Sn concentration in the soft tissue, When the Sn concentration ratio is controlled within a predetermined range, excellent corrosion resistance can be ensured even under such severe conditions as the WT test.

The reason why the Sn concentration is changed according to different manufacturing conditions is as follows.

(i) Condition of manufacturing conditions of steel plate A1

The cooling after rolling is air cooling, and the cooling rate is slow. Therefore, a soft structure (soft phase) such as fertilized iron is likely to grow, and a soft structure centering on the ferrite iron or the like and a layered (ribbon) hard structure (hard phase) are formed. Further, since the cooling rate is slow, it is easy to cause Sn to diffuse from the soft tissue to the hard structure, and the Sn concentration ratio becomes high.

(ii) Manufacturing conditions of steel plate A2

Since Sn has a lower melting point than other elements, it diffuses between 550 ° C and 400 ° C. However, when it is strongly cooled (water cooled) to 650 ° C to 400 ° C, Sn cannot be sufficiently diffused into the hard structure, resulting in The Sn concentration ratio becomes small.

(iii) Case of manufacturing conditions of steel plate A3

After rolling, by soft cooling to a temperature range of 550 ° C or higher, a structure in which a soft tissue and a hard tissue are dispersed can be obtained. Thereafter, by gentle cooling at 550 ° C to 400 ° C, Sn is moderately diffused to a dispersed hard phase.

(iv) Manufacturing conditions of steel plate A4

Since it is strongly cooled to 450 ° C after rolling, the structure is mainly a hard phase, and since Sn cannot be sufficiently diffused to the hard phase, 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) A steel material according to one aspect of the present invention has a chemical composition in terms of mass%: C: 0.01 to 0.20%, Si: 0.01 to 1.00%, Mn: 0.05 to 3.00%, and Sn: 0.01 to 0.50%, O. : 0.0001~0.0100%, Cu: 0~ less than 0.10%, Cr: 0~ less than 0.10%, Mo: 0~ less than 0.050%, W: 0~ less than 0.050%, Cu+Cr: 0~ less than 0.10%, Mo+ W: 0~ less than 0.050%, Sb: 0~ less than 0.05%, Ni: 0~0.05%, Nb: 0~0.050%, V: 0~0.050%, Ti: 0~0.020%, Al: 0~0.100% , Ca: 0~ less than 0.0100%, Mg: 0~0.0100%, REM: 0~0.0100%, P: 0.05% or less, S: 0.01% or less, and the remainder: Fe and impurities; and having soft tissue and hard tissue The ratio of the Sn concentration in the hard tissue to the Sn concentration in the soft tissue, that is, the Sn concentration ratio is 1.2 or more and less than 6.0.

(2) The steel material according to the above (1), wherein the chemical composition may contain Cu+Cr: 0 to less than 0.05% by mass%.

(3) The steel material according to the above (1) or (2), wherein the chemical composition may contain Mo+W: 0.0005 to less than 0.050% by mass%.

(4) The steel material according to any one of the above-mentioned items, wherein the chemical composition may be one or more selected from the group consisting of Nb: 0.001 to 0.050%, and V. : 0.005 to 0.050%, 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: 0.0002 to 0.0100%.

(5) The steel material according to any one of the above (1) to (4), which may have an anti-corrosion film having a film thickness of 20 μm or more.

(6) A ballast tank or a ship cabin according to another aspect of the invention is formed by using the steel material according to any one of the above (1) to (4).

(7) A ship according to another aspect of the present invention includes the ballast tank or the ship cabin described in the above (6).

According to the above aspect of the invention, it is possible to provide a steel material which has excellent corrosion resistance even in a corrosive environment containing chloride. Moreover, according to the present invention, it is possible to provide a ballast tank and a ship cabin of a ship formed using a steel material excellent in corrosion resistance, and a ship including the ballast tank and the ship cabin.

1‧‧‧ ship

2‧‧‧ ballast tank

3‧‧‧the cabin

Figure 1 is a schematic illustration of a test tank used in a wave tank test (WT test).

Fig. 2A is an example of a test piece used in a wave tank test (WT test).

Fig. 2B is an example of a test piece used in the wave tank test (WT test).

Fig. 3 is a schematic view showing a ballast tank, a ship cabin, and a ship to which the steel material according to the embodiment is applied.

Used to implement the type of invention

The steel material according to an embodiment of the present invention (hereinafter sometimes referred to as the steel material of the present embodiment) will be described in detail. "%" of the content of each element means "% by mass".

About chemical composition (chemical composition)

The reason for the chemical composition of the steel material of the present embodiment is as follows.

C: 0.01~0.20%

C is an element used to increase the strength of steel. In order to obtain this effect, the lower limit of the C content is set to 0.01%. A suitable 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 also be set to 0.05%, 0.07% or 0.09%. On the other hand, once the C content exceeds 0.20%, the weldability will be significantly lowered. Further, while the C content is increased, the amount of stellite carbon which is a cathode and promotes corrosion in a low pH environment is also increased, and the corrosion resistance of the steel material is lowered. Therefore, the upper limit of the C content is set to 0.20%. A suitable upper limit of the C content is 0.18%, and a more suitable upper limit of the C content is 0.16%. The upper limit of the C content can also be set to 0.15% or 0.13%.

Si: 0.01~1.00%

Si is an essential element for deoxidation. In order to obtain a sufficient deoxidation effect, it is necessary to contain 0.01% or more. A suitable lower Si content is 0.03%, and a more suitable Si content lower limit is 0.05%. The lower limit of the Si content may also be set to 0.10%, 0.15% or 0.20%. On the other hand, once the Si content exceeds 1.00%, the toughness of the base material and the welded joint portion is impaired. Therefore, the upper limit of the Si content is set to 1.00%. A suitable Si content upper limit is 0.80%, and a more suitable Si content upper limit is 0.60%. The upper limit of the Si content may also be set to 0.50%, 0.40% or 0.30%.

Mn: 0.05~3.00%

Mn is an element that has a low cost and has an effect of increasing the strength of the steel. In order to obtain this effect, the lower limit of the Mn content is set to 0.05%. A suitable lower limit of the Mn content is 0.20%, and a more preferable lower limit of the Mn content is 0.40%. Lower limit of Mn content It can also be set to 0.60%, 0.80% or 0.90%. On the other hand, once the Mn content exceeds 3.00%, the weldability and joint toughness will deteriorate. Therefore, the upper limit of the Mn content is set to 3.00%. A suitable upper limit of the Mn content is 2.50%, and a more suitable upper limit of the Mn content is 2.00%. The upper limit of the Mn content may also be set to 1.80%, 1.60% or 1.50%.

Sn: 0.01~0.50%

Sn is an important element in the steel material of the present embodiment. Sn dissolves into Sn ²⁺ and the concentration of Fe ³⁺ is lowered by the reaction of 2Fe ³⁺ +Sn ²⁺ →2Fe ²⁺ +Sn ⁴⁺ to suppress the corrosion reaction. Moreover, since Sn can significantly suppress the anodic dissolution reaction of the steel in a low pH chloride environment, the corrosion resistance of the steel in a chloride corrosive environment can be greatly improved. In order to obtain such effects, the lower limit of the Sn content must be set to 0.01%. A suitable lower Sn content is 0.03%, and a more suitable Sn content lower limit is 0.05%. The lower limit of the Sn content may also be set to 0.08%, 0.12%, 0.16% or 0.19%. On the other hand, once the Sn content exceeds 0.50%, not only the above effect is saturated, but the toughness of the base material and the high heat-injection joint is deteriorated. Therefore, the upper limit of the Sn content is set to 0.50%. A suitable upper Sn content is 0.45%, and a more suitable Sn content upper limit is 0.40%. The upper limit of the Sn content may also be set to 0.35% or 0.30%.

O: 0.0001~0.0100%

O (oxygen) and Sn are both important elements in the steel of the present embodiment. The toughness of the welded joint can be improved by containing a trace amount of O. In order to obtain this effect, the lower limit of the O content must be set to 0.0001%. A suitable 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 also be 0.0005%, 0.0010%, 0.0015% or 0.0019%. On the other hand, O forms an oxide such as SnO or SnO ₂ . Therefore, once the O content exceeds 0.0100%, the Sn concentration in the hard tissue cannot be sufficiently ensured. Further, the above oxide becomes a corrosion starting point, thereby lowering the corrosion resistance of the steel material. Therefore, the upper limit of the O content is set to be 0.0100%. A suitable 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 also be set to 0.0060%, 0.0040% or 0.0030%.

Cr: 0~ less than 0.10%

In general, Cr is considered to be an element that enhances the corrosion resistance of steel. However, the inventors of the present invention have found that the corrosion resistance of the steel material deteriorates when Cr is contained in a corrosive environment containing chloride as assumed in the present embodiment. Therefore, the Cr content should be less, and the lower limit of the content is 0%. On the other hand, it is considered that the impurity content is mixed, and the upper limit of the Cr content is less than 0.10%. The Cr content is preferably limited to 0.07% or less or less than 0.05%, and is preferably limited to 0.03% or less or 0.02% or less. The Cr content is more preferably limited to 0.01% or less.

Cu: 0~ less than 0.10%

In general, Cu is considered to be an element that enhances the corrosion resistance of steel. However, the inventors of the present invention have found that the corrosion resistance of the steel material is lowered when Cu is contained in a corrosive environment containing chloride as assumed in the present embodiment. Therefore, the Cu content is preferably small, and the lower limit of the Cu content is 0%. On the other hand, it is considered that the impurity content is mixed, and the upper limit of the Cu content is less than 0.10%. In order to improve the corrosion resistance, the Cu content is preferably limited to 0.07% or less or 0.05% or less, and is preferably limited to 0.03% or less or 0.02% or less. The Cu content is more preferably limited to 0.01% or less.

When the steel contains Cu, it will become a case where Cu and Sn coexist. at this time, Depending on the manufacturing method, rolling may occur. In order to suppress the rolling, it is important to reduce the ratio of Cu content to Sn content (Cu/Sn). When Cu is contained, Cu/Sn is preferably set to 1.0 or less. Cu/Sn is preferably set to 0.5 or less or 0.3 or less.

Cu+Cr: 0~ less than 0.10%

As described above, in a corrosive environment containing chloride, Cu and Cr are elements which can lower the corrosion resistance of the steel. Therefore, in the case where these elements are contained at the same time, not only the content of each element but also the total content needs to be limited. That is, the total content of Cu and Cr needs to be limited to less than 0.10%. It is preferably less than 0.07%, more preferably less than 0.05%, more preferably less than 0.04%, still more preferably less than 0.03%.

Mo: 0~ less than 0.050%

Once the Mo content is 0.050% or more, not only the corrosion resistance will decrease, but also the steel cost will be greatly increased. Therefore, the Mo content is preferably small, and the Mo content is set to be less than 0.050%. A suitable Mo content is 0.040% or less. The upper limit of the Mo content may also be set to 0.030%, 0.020%, 0.010% or 0.005%. In order to improve the corrosion resistance, the Mo content is preferably small, and the lower limit of the Mo content is 0%. However, in order to improve characteristics such as strength or toughness, the lower limit of the Mo content may be set to 0.010% or 0.020%.

W: 0~ less than 0.050%

Once the W content is 0.050% or more, not only the corrosion resistance will decrease, but also the steel cost will be greatly increased. Therefore, the Mo content is preferably small, and the W content is set to be less than 0.050%. A suitable W content is 0.040%. The upper limit of the W content may also be set to 0.030%, 0.020%, 0.010% or 0.005%. In order to improve corrosion resistance, The W content is preferably small, and the lower limit of the W content is 0%. However, in order to improve characteristics such as strength and toughness, the lower limit of the Mo content may be set to 0.010% or 0.020%.

Mo+W: 0~ less than 0.050%

In order to improve the corrosion resistance, it is necessary to limit the total content for Mo and W not only the content of the respective elements. That is, the total content of Mo and W is limited to less than 0.050%. The upper limit of the total content may also be set to 0.030%, 0.020%, 0.010% or 0.005%. In order to improve the corrosion resistance, the total content is preferably small. However, in order to improve characteristics such as strength and toughness, the lower limit of the total content may be 0.005%, 0.010% or 0.020%.

The steel material of the present embodiment has substantially the above-mentioned components and the remainder is Fe and impurities. However, in addition to the above components, one or more components selected from the following elements may be contained as required.

Further, the impurity described in the present embodiment refers to a component in which the steel material is mixed in the industrial production due to raw materials such as ore and chips or other factors.

Sb: 0~0.05%

Sb is an element which improves oxidation resistance. However, when the content of Sb exceeds 0.05%, not only the above effects are saturated, but also the toughness of the steel material or the like is deteriorated. Here, the Sb content is 0.05% or less. The upper limit of the Sb content can also be set to 0.04% or 0.03%. Since it is not necessary to contain Sb, the lower limit of the Sb content is set to 0%. However, in order to improve oxidation resistance, the lower limit of the Sb content may be set to 0.005%, 0.010% or 0.015%.

Ni: 0~0.05%

In general, Ni is considered to be an element that enhances the corrosion resistance of steel as well as Cu. However, the inventors of the present invention found that it is assumed as in the present embodiment. In a corrosive environment containing chloride, the corrosion resistance of the steel is reduced once Ni is contained. Therefore, the Ni content is preferably small, and the lower limit of the Ni content is 0%. Considering the case where impurities are mixed, the upper limit of Ni content is 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 is preferably limited to 0.01% or less.

Nb: 0~0.050%

Nb is an element that enhances the strength of steel. However, when the Nb content exceeds 0.050%, the aforementioned effect is saturated. Therefore, when Nb is contained, the content is made 0.05% or less. The Nb content may be set to 0.030% or less or 0.020% or less depending on the demand. Since it is not necessary to contain Nb, the lower limit of the Nb content is 0%. However, in order to obtain an effect of improving the strength, Nb may be contained in an amount of 0.001% or more, or may be contained in an amount of 0.003% or more or 0.005% or more.

V: 0~0.050%

V and Nb are both elements that increase the strength of the steel. Further, V, like Mo and W, is dissolved in a corrosive environment (in an aqueous solution) and exists as an oxyacid ion to suppress penetration 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 greatly increased. Therefore, when V is contained, the content is set to 0.050% or less. The V content may be set to 0.040% or less or 0.030% or less. Since it is not necessary to contain V, the lower limit of the V content is 0%; however, in order to obtain the above effects, the content V may be 0.005% or more or 0.010% or more.

Ti: 0~0.020%

Ti not only has an effect on the deoxidation of steel, but also suppresses the formation of MnS which will become the starting point of corrosion of steel. However, when the Ti content exceeds 0.020%, not only the foregoing The effect is saturated and the cost of steel is increased. Therefore, when Ti is contained, the content is made 0.020% or less. The Ti content is suitably set to 0.015% or less. Since it is not necessary to contain Ti, the lower limit of the Ti content is 0%; however, in order to obtain the above effects, Ti may be contained in an amount of 0.005% or more or 0.008% or more.

Al: 0~0.100%

Al is an element that has an effect on the deoxidation of steel. In the present embodiment, since Si is contained in the steel material, oxygen can be deoxidized through the Si. Therefore, it is not necessary to carry out the deoxidation treatment by permeating Al, and the lower limit of the Al content is set to 0%. However, it is also possible to further deoxidize with Al in addition to Si. In order to obtain the deoxidation effect by Al, the Al content is preferably set to 0.010% or more, more preferably 0.020% or more, and still more preferably 0.030% or more. On the other hand, once the Al content exceeds 0.100%, the corrosion resistance of the steel material in a low pH environment is lowered, resulting in a decrease in the corrosion resistance of the steel material under the chloride corrosion mirroring. Further, when the Al content exceeds 0.100%, the toughness of the steel material is lowered due to the coarsening of the nitride. Therefore, when Al is contained, the content is made 0.100% or less. A suitable upper limit of Al content is 0.060%, and a more suitable upper limit of Al content is 0.045%.

Ca: 0~ less than 0.0100%

Ca is present in the form of an oxide in the steel material, and the pH drop at the interface of the corrosion reaction portion can be suppressed to suppress corrosion. In order to obtain the above effects, it is preferred to contain Ca in an amount of 0.0002% or more, and more preferably 0.0005% or more. On the other hand, when the Ca content is 0.0100% or more, the aforementioned effects are saturated. Therefore, when Ca is contained, its content is less than 0.0100%. The Ca content may be set to 0.0050% or less or 0.0030% or less. Since it is not necessary to contain Ca, the lower limit of the Ca content is 0%.

Mg: 0~0.0100%

Both Mg and Ca suppress the pH drop at the interface of the corrosion reaction portion to suppress corrosion of the steel material. In order to obtain the above effects, it is preferred to contain the Mg content in an amount of 0.0002% or more, and more preferably 0.0005% or more. On the other hand, once the Mg content exceeds 0.0100%, the aforementioned effects are saturated. Therefore, when Mg is contained, the content is made 0.0100% or less. The Mg content may be set to 0.0050% or less or 0.0030% or less. Since it is not necessary to contain Mg, the lower limit of the Mg content is 0%.

REM: 0~0.0100%

REM (rare earth element) is an element that enhances the weldability of steel. In order to obtain the aforementioned effects, the REM content is preferably set to 0.0002% or more, and is preferably set to 0.0005% or more. On the other hand, once the REM content exceeds 0.0100%, the aforementioned effect is saturated. Therefore, when REM is contained, the content is made 0.0100% or less. The upper limit of the REM content can also be set to 0.0050% or 0.0030%. Since it is not necessary to contain REM, the lower limit of REM content is 0%. The REM described in the present embodiment is a general term for a total of 17 elements of Y and a total of 17 elements of Y and Sc. The steel material of the present embodiment may contain one or more of these 17 elements in the steel material, and the REM content means the total content of these elements.

Among the impurities, the content of the following elements must be strictly limited.

P: 0.050% or less

P is an element that exists as an impurity in steel. P is an element which can reduce the oxidation resistance of the steel material, and can also lower the corrosion resistance of the steel material in a chloride corrosion environment where the pH of the corrosion interface is low. Moreover, P can also make the weldability of steel and The toughness of the welded heat affected zone is lowered. For this reason, the P content is limited to 0.050% or less. The P content is preferably limited to 0.040% or less, and is preferably limited to less than 0.030%. In order to increase the toughness of the heat affected portion of the fusion, the upper limit of the P content may be set to 0.020%, 0.015% or 0.010%. Although it is not easy to completely remove P, it is not necessary to remove P, so the lower limit of P content is 0%.

S: 0.010% or less

S is an element that exists as an impurity in steel. S forms MnS which becomes the starting point of corrosion in the steel. Once the S content exceeds 0.010%, the corrosion resistance of the steel material will be significantly reduced. Therefore, the S content is limited to 0.010% or less. The S content is preferably limited to 0.008% or less, more preferably 0.006% or less, and is more preferably 0.004% or less. Although it is not easy to completely remove S, it is not necessary to remove S, so the lower limit of S content is 0%.

Description of the microstructure of the steel material of the present embodiment.

The steel material of this embodiment has a soft structure and a hard structure. In the present embodiment, the hard structure refers to ferrite, toughened iron, and 麻田散铁, and the soft tissue refers to ferrite. The ratio of the soft structure to the hard structure can be determined in accordance with the steel strength design, and is not particularly limited; however, in order to ensure the strength and toughness necessary for the steel for the hull structure, the microstructure of the steel of the present embodiment should be contained. The composite structure of Borne iron and ferrite iron, in terms of area% (area ratio), fertilized iron structure should account for less than 80% of the whole organization.

Moreover, the steel material of this embodiment has a structure in which a hard structure and a soft structure are laminated and/or dispersed. In the case where the Sn concentration ratio is controlled within a predetermined range to be described later, it is preferable that the hard tissue and the soft tissue are dispersed. Further, in the case of reducing the average particle diameter described later, it is also preferable to be a hard tissue and The soft tissue is a dispersed tissue. The so-called hard tissue and soft tissue are dispersed structures, which refer to the structure in which the hard and soft tissues are dispersed in the steel.

Once the steel grains are coarsened, in addition to the mechanical properties of the steel, the intragranular corrosion is also likely to occur. The grain etched in the crystal may become the starting point of corrosion. Therefore, the average particle diameter of the soft tissue and the average particle diameter of the hard structure are preferably 15 μm or less, and preferably 10 μm or less. In addition, the "average particle diameter" is evaluated by an EBSP (Electron Backscatter Diffraction Pattern), and a grain boundary having a phase difference of 15 or more is defined as a grain boundary and surrounded by the grain boundary. The part is regarded as a grain and thus calculated. Specifically, for example, five or more fields of view are observed at a magnification of 2000 times using an EBSP method, and a grain boundary having a phase difference of 15° or more is regarded as a grain boundary, and the area of each crystal grain is obtained. Next, the equivalent circle diameter was calculated from each of the obtained areas, and the calculated numerical value was regarded as the particle diameter of each crystal grain, and the average value of these particle diameters was regarded as the average particle diameter. In the evaluation by the EBSP method, for example, the cross section of the test piece cut out from the steel material is observed. More specifically, for example, a cross section parallel to the rolling direction and the thickness direction of the steel material is observed from a direction perpendicular to the cross section. The area to be observed, for example, is 1/4 of the thickness from the surface of the steel.

In general, in a neutral chloride aqueous solution, the soft structure is excellent in corrosion resistance and the hard structure has low corrosion resistance. Here, in a severe corrosive environment containing chloride and alternating wet and dry, the thin water film adhered to the surface of the steel material changes to an acidic chloride aqueous solution. Therefore, in the case of steel used in the corrosive environment, it has been in a neutral chloride aqueous solution. The hard-cured hard structure will become the starting point and corrode, and develop into a comprehensive corrosion that is also covered by soft tissues.

As described above, the corrosion resistance of the hard structure is inferior to that of the soft tissue, and it becomes a starting point of corrosion. Therefore, when the amount of Sn in the hard structure is increased, the progress of corrosion can be avoided, and the progress of comprehensive corrosion including the soft tissue can be suppressed.

In order to prevent such corrosion, the steel material of the present embodiment controls the concentration of Sn in each structure so that the Sn concentration in the hard structure is 1.2 times or more the Sn concentration in the soft structure. As mentioned earlier, Sn ions dissolved by corrosion can improve the corrosion resistance of the steel. Therefore, if Sn can be present in a high concentration in a hard structure which is corroded first, initial corrosion in the hard structure can be avoided, and corrosion can be prevented from progressing to the entire steel material. However, when the concentration of Sn in the hard tissue is more than 6 times the concentration of Sn in the soft tissue, a potential difference is generated between the hard tissue and the soft tissue, so that the soft tissue having excellent corrosion resistance is corroded in preference to the hard tissue. , resulting in reduced corrosion resistance. Therefore, in order to more exert the corrosion resistance by Sn, the Sn concentration in the hard structure must be 1.2 times or more and less than 6.0 times the Sn concentration in the soft tissue. The concentration of Sn in the hard tissue is preferably 1.3 times or more of the Sn concentration in the soft tissue, more preferably 1.5 times or more, more preferably 1.7 times or more, and still more preferably 2.0 times or more. The concentration of Sn in the hard structure is preferably 5.0 times or less, more preferably 4.0 times or less, and more preferably 3.5 times or less, of the Sn concentration in the soft tissue.

Since the steel material of the present embodiment has the above chemical composition and structure, it has excellent corrosion resistance even when the steel material is directly used. However, by forming an anti-corrosion film on the surface of the steel by painting or the like, the steel can be further improved. Corrosion resistance. Specifically, for example, the surface of the steel material can be coated by an anti-corrosion film made of an organic resin.

Here, the anti-corrosion film made of the organic resin may, for example, be a resin film such as polyvinyl butyral, epoxy resin, urethane or phthalic acid. One or more of these resins may also be laminated as an anti-corrosion film. The film thickness (film thickness at the time of drying) of the anti-corrosion film is preferably 20 μm or more, and more preferably 50 μm or more. The lower limit of the film thickness can also be set to 100 μm or 150 μm. When the film thickness of the anti-corrosion film (film thickness at the time of drying) exceeds 500 μm, the anti-corrosion film may be peeled off due to thermal cycling due to the difference in thermal expansion coefficient between the resin and the steel material. Therefore, the film thickness of the anti-corrosion film is preferably set to 500 μm or less. The upper limit of the film thickness can also be set to 400 μm or 300 μm.

Further, the steel material of the present embodiment can improve the durability of the anti-corrosion film as compared with the known steel material. Thus, the corrosion resistance of the steel material can be further improved. The reason why the durability of the anti-corrosion film can be improved is considered as follows. That is, as described above, the steel material of the present embodiment can effectively suppress corrosion irrespective of the presence or absence of the anti-corrosion film. Therefore, even in the case where the corrosion-resistant film has a defective portion, corrosion of the steel sheet as the base from the defective portion can be suppressed. Thereby, it is possible to suppress the expansion and peeling of the anti-corrosion film due to corrosion of the steel material. Based on this result, it is considered that the durability of the anti-corrosion film can be improved.

The steel material of the present embodiment can have excellent corrosion resistance even in a corrosive environment containing chloride. Therefore, the steel material of the present embodiment is suitable for use in the cabin material of a bulk carrier such as a ballast tank, a coal carrier, or a mining ship of a ship.

a ballast tank or a ship cabin of a ship formed by the steel material of the present embodiment, The maintenance frequency of repainting and the like can be reduced because corrosion is suppressed. In addition, ships equipped with such ballast tanks or cabins can achieve the following effects: the shipping cost can be reduced due to the reduced maintenance frequency, and the steel material which is prevented from thinning due to corrosion should be replaced (repaired) into steel having the necessary thickness. Increased security and reduced repair costs.

Hereinafter, a suitable method for producing the steel material according to the present embodiment will be described. The steel material of the present embodiment does not need to be limited in the production method if it satisfies the chemical composition and the structure. However, the following production method is suitable because it is easy to manufacture.

In the production of the steel material of the present embodiment, in order to increase the Sn concentration in the hard structure, for example, a flat embryo having a low content of S and O (oxygen) which are easily formed with the Sn-forming compound can be used. The reason is that since Sn forms a compound with O (oxygen) or S, that is, forms tin oxide (SnO, SnO _{2 ,} etc.) or tin sulfide (SnS, SnS _2, etc.), once these compounds are formed in steel, in steel Sn will be reduced. In the case of melting a flat embryo which has a low S and O content, deoxidation and/or desulfurization management should be carried out as follows.

That is, it is preferable to deoxidize before adding Si and Mn in advance in the initial stage of melting to perform pre-deoxidation so that the dissolved oxygen concentration in the molten steel is 30 ppm or less, and then adding Al to perform deoxidation again. At this time, Ti having the same deoxidizing effect as Al may be added as needed. On the other hand, the desulfurization is preferably carried out by introducing a slag modifier and quicklime to the slag formed by the melting. The addition of Sn is preferably carried out after deoxidation so that the dissolved oxygen concentration in the molten steel is 30 ppm or less and/or desulfurization so that the sulfur concentration in the molten steel is 100 ppm or less. In addition, when deoxidizing, Al may be added to perform pre-deoxidation so that the dissolved oxygen concentration in the molten steel is After 30 ppm or less, Si and Mn were added to perform deoxidation again. Further, chemical components other than S and O can be adjusted by a known method within the above-described appropriate range.

As described above, the flat embryo which has a low content of S and O (oxygen) which is easy to form a compound with Sn may be subjected to heating, for example, at a temperature of about 1000 to 1150 ° C for each rolling delay, depending on the composition of the steel material. The rolling reduction ratio of the pass is 3.0% or more, and the rolling end temperature is about 700 to 900 ° C.

The steel after hot rolling is, for example, water-cooled (weak water-cooled) at an average cooling rate of 1.0 to 3.0 ° C/s in the temperature range up to 650 ° C after the end of rolling, and then in the temperature range of 650 to 550 ° C. The average cooling rate of 3.0~25°C/s is water-cooled (strong water cooling), then water-cooled (slow water cooling) at an average cooling rate of 0.01~1.0°C/s in the temperature range of 550°C~400°C, after which air cooling is performed. To room temperature. The temperature referred to herein means the temperature of the surface of the steel material.

Sn has a property of being easily thickened in a hard tissue. In the temperature range above 650 ° C, if the cooling rate is less than 1.0 ° C / s (for example, 0.8 ° C / s), the slow speed, not only in the soft tissue and hard tissue will form a giant band structure, at the same time, The Sn in the hard structure becomes thick, 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. In this case, in addition to the possibility that the toughness of the steel material is lowered from the hard tissue as the starting point, the soft structure is first dissolved and localized corrosion occurs when the steel material is corroded.

Further, if strong cooling is performed in the temperature range of -400 ° C after rolling, the structure will be mainly a hard phase, and the Sn concentration ratio will be lowered.

Further, when the cooling rate of 550 ° C to 400 ° C exceeds 1.0 ° C, Sn does not sufficiently diffuse into the hard structure, resulting in a decrease in the Sn concentration ratio.

By manufacturing the steel by the above method, both the soft structure and the hard structure can refine the crystal grains, and the Sn concentration ratio in the hard tissue can be 1.2 times or more and less than 6.0 times the Sn concentration in the soft tissue (that is, the Sn concentration ratio is 1.2 and less than 6.0), thereby further finely dispersing the hard tissue in the steel. Thereby, when the steel is corroded, a more uniform corrosion form can be obtained. When the coarse structure causes the Sn concentration ratio between the soft tissue and the hard tissue (Sn concentration in the hard tissue/Sn concentration in the soft tissue) to be 6.0 or more, the soft tissue is first dissolved when the steel is corroded, so that a local portion is formed. Sexual corrosion pattern.

The steel material of this embodiment may be a steel sheet. Although the thickness of the steel sheet is not particularly limited, the lower limit of the thickness may be 6 mm or 10 mm, and the upper limit of the thickness may be 50 mm or 40 mm.

The strength (tensile strength) of the steel material of the present embodiment is not particularly limited. However, considering the application to the physical structure, it may be set to 400 MPa or more and 600 MPa or less.

When the anti-corrosion film is formed on the surface of the steel material of the present embodiment, the anti-corrosion film can be formed by a conventionally known method. In addition, the anti-corrosion film may be coated on the entire surface of the steel material, or the anti-corrosion film may be coated on only part of the surface of the steel material. Specifically, the anti-corrosion film may be formed only in a portion that will contact the corrosive environment (for example, one side of the steel material). When steel is applied to a steel pipe, an anti-corrosion film can be formed only on the outer or inner surface of the steel pipe. Before the formation of the anti-corrosion film, the surface of the steel may be subjected to a chemical change film treatment. Chemical As the treatment film, a treatment agent prepared from zinc, titanium, zirconium, chromium, a decane compound or the like can be used.

When the ballast tank or the ship cabin is formed by applying the steel material of the present embodiment, it can be carried out by a known method. Further, when a ship having a ballast tank or a ship formed by applying the steel materials of the present embodiment is constructed, it can be carried out by a known method. Fig. 3 is a schematic view showing an example of a ballast tank or a ship cabin formed by applying the steel material of the present embodiment, and a ship having the ballast tank or the ship cabin.

Hereinafter, the present invention will be specifically described by way of examples, but the invention is not limited to the examples.

Example

After 27 kinds of steels were melted in a vacuum melting furnace and 50 kg of steel blocks were prepared, hot forging was performed by a general method to produce a bulk material (steel No. 1 to 27) having a thickness of 60 mm. The chemical composition of the fabricated block is shown in Table 5.

Next, the block was heated at 1,100 ° C to 1,120 ° C for one hour or more as shown in Table 6, and then hot rolled at a rolling end temperature of 750 to 900 ° C to obtain a steel plate having a thickness of 20 mm. Thereafter, the steel sheet was cooled to room temperature under various cooling conditions to prepare steel sheets of Test Nos. 1 to 30. In addition, in Table 6, the cooling start temperature refers to the temperature at which water cooling starts after rolling; the cooling rate 1 refers to the average cooling rate at the end of rolling to 650 ° C; and the cooling rate 2 refers to 650 ° C to 550 ° C The average cooling rate below; the cooling rate 3 refers to the 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 steel sheet (Test No. 1 to 30) to prepare a micro sample. Each micro sample was mirror-polished and then etched using a Nital etching solution. After that, for each A small sample was observed by microscopy and SEM (scanning electron microscope). At the same time as SEM observation, the concentration of Sn in hard tissues was quantitatively analyzed by EDX (energy scattering X-ray spectrometer: JED2300 manufactured by JEOL Ltd., observation magnification: 3500 times, field width: 46 μm × 33 μm). And the Sn concentration of soft tissue (B). Then, the Sn concentration ratio of the hard tissue to the soft tissue (=(A)/(B)) was obtained.

As a result, as shown in Table 7, in Test Nos. 1 to 24 of the present invention example, the Sn concentration ratio of the hard structure to the soft tissue was 1.2 or more and less than 6.0. Further, although not shown in Table 7, any of the examples of the present invention and the steel sheet contain a soft layer and a hard phase, and the area ratio of the soft layer (i.e., the ferrite structure) is 80% or less of the total structure.

On the other hand, in Test Nos. 25 to 27 and 29 of the comparative example, the Sn concentration ratio was outside the range of the present invention. Test No. 28 of the comparative example did not contain Sn. In Test No. 30 of the comparative example, although the Sn concentration ratio falls within the range defined by the present invention, the total content of Cu + Cr falls outside the range of the present invention.

Unlike the foregoing observations, the corrosion test described below was carried out. The corrosion test is a SAEJ2334 test and a wave tank test (WT test) performed to simulate the corrosive environment of the ballast tank. The WT test was carried out in the following two tests, namely the WT test (1) and the WT test (2). In the SAE J2334 test, the following two test pieces were used, which were a test piece without an anti-corrosion film and a test piece having an anti-corrosion film. In the WT test (1) and the WT test (2), a test piece having an anti-corrosion film was used.

First, the SAEJ2334 test will be explained. The so-called SAEJ2334 test is an accelerated degradation test using a dry-wet alternating (wetting→adhering salt→drying) condition as a cycle (24 hours total), and simulating severe corrosion such as fly salt content exceeding 1 mdd. Environmental testing.

‧ Wetting: 50 ° C, 100% RH, 6 hours; ‧ Adhesion salt: impregnation of 0.5% by mass of NaCl, 0.1% by mass of CaCl ₂ , 0.075% by mass of NaHCO ₃ aqueous solution, 0.25 hours; ‧ Drying: 60 ° C, 50% RH, 17.75 hours

In the SAE J2334 test, a test piece having a length of 60 mm, a width of 100 mm, and a thickness of 3 mm was cut out from each of the steel sheets having a thickness of 20 mm (Test Nos. 1 to 30). The surface of each test piece was subjected to bead blasting treatment, and the test piece having the anti-corrosion film was sprayed with two modified epoxy-based paints on the surface of the steel material after the bead blasting treatment (A: manufactured by China National Paint Co., Ltd.) NOVA 2000", B: "BANNOH 500" manufactured by China National Coatings Co., Ltd.) to form an anti-corrosion film. For each test piece having an anti-corrosion film, a cross-shaped flaw was formed on the anti-corrosion film, and a part of the steel plate as a base was exposed.

Next, the wave tank test (WT test) will be described. The so-called WT test, shown in Figure 1, is a test conducted by a test tank to simulate the corrosive environment of a ship's ballast tank. In order to simulate the environment in the ballast tank, the test period is composed of two weeks for the state in which the test tank stores artificial seawater (saline) and one week for the test tank. The temperature of the artificial seawater was maintained at 35 ° C, and the test tank was shaken to cause the spray of the artificial seawater to adhere to the test piece.

In the wave tank test (WT test), the WT test (1) is an experiment conducted as shown in Fig. 1, which simulates the environment on the side of the ballast tank. In the WT test (1), seawater spray spattered from the sea surface was attached 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 was cut out from each of the steel sheets having a thickness of 20 mm (Test Nos. 1 to 30). For the surface of each test piece after the cutting, the above two modified epoxy coatings (A: "NOVA 2000" manufactured by China National Coatings Co., Ltd., B: "BANNOH 500" manufactured by China Coatings Co., Ltd.) were sprayed. One, whereby an anti-corrosion film is formed. Then, as shown in Fig. 2A, the lower portion (water immersion area) of the test piece was immersed in artificial seawater. On the upper part of the test piece, that is, the area (splash area) which is not immersed in the artificial seawater, a pair of linear flaws (length 10 mm) extending in the width direction of the test piece are formed on the anti-corrosion film to make the steel plate as the base. Part of it is exposed. More specifically, the linear flaws are formed at a position above the seawater surface of 20 mm and at a position above the seawater surface of 110 mm. Further, the size (unit: mm) is as shown by the numbers in FIGS. 2A and 2B.

In the wave tank test (WT test), the WT test (2) is an experiment conducted on the back side of the simulated press deck as shown in Fig. 1. in In the WT test (2), the seawater spray splashed from the sea surface was attached to the test under repeated "50 ° C, 12 hours" and "20 ° C, 12 hours" temperature cycles (temperature of the test piece). Sheet surface. In the WT test (2), a test piece having a length of 140 mm, a width of 30 mm, and a thickness of 2.5 mm was taken out from each of the steel sheets having a thickness of 20 mm (Test Nos. 1 to 30). For the surface of each of the cut test pieces, the above two modified epoxy-based paints (A: "NOVA 2000" manufactured by China National Coatings Co., Ltd., B: "BANNOH 500" manufactured by China Paint Co., Ltd.) were sprayed. One, whereby an anti-corrosion film is formed. Then, as shown in FIG. 2B, a linear flaw (length: 10 mm) extending in the width direction of the test piece was formed on the anti-corrosion film in the center portion of the test piece, and a part of the steel plate as the base was exposed.

The evaluation of the SAE J2334 test was carried out as follows. Each test piece in which the anti-corrosion film was not formed was formed with a uniform rust layer over the entire surface of the test after the test. The amount of corrosion of each of the test pieces was determined. The "corrosion amount" is obtained by reducing the average thickness of the test piece after removing the surface rust layer. Specifically, the amount of thickness reduction was calculated using the amount of weight reduction of the test piece before and after the test and the surface area of the test piece, and this was used as the amount of corrosion.

For each test piece in which the anti-corrosion film was formed, the maximum corrosion depth (the maximum value of the corrosion depth from the steel surface) of the steel material as the base was measured at the position where the anti-corrosion film was formed with the flaw portion. Further, in order to evaluate the area of the portion of the anti-corrosion film which was peeled off from the development of the scar portion, the film peeling area ratio (%) was examined. Specifically, the portion where the anti-corrosion film is peeled off (the portion peeled off from the scar portion) is removed by a cutter or the like, and the aforementioned Part of it is used as a peeling part of the film. Then, using the binarization processing of the image processing software, the value of (film peeling area) / (test piece area) × 100, which is the film peeling area ratio (%), was calculated. Further, the area of the test piece refers to the area of the surface on which the flaw portion is formed among the six faces of the test piece.

In addition, the criteria for judging the SAEJ2334 test are judged to be acceptable for the amount of corrosion of 0.60 mm or less. Further, the maximum corrosion depth of the eroded film scar portion was judged to be acceptable if it was 0.45 mm or less. Further, the film peeling area ratio of 60% or less was judged to be acceptable.

The evaluation of the WT test was carried out as follows. First, in order to evaluate the area of the portion of the anti-corrosion film which was peeled off from the development of the scar portion, the peeling area ratio (%) of the film was examined. Specifically, the portion where the anti-corrosion film is peeled off (the portion peeled off from the scar portion) is removed by a cutter or the like, and the removed portion is used as a film peeling portion. Then, using the binarization processing of the image processing software, the value of (the area of the peeled portion of the film) / (the area of 30 mm × 100 mm centered on the film scar portion) × 100 is calculated, that is, the peeling area ratio (%) of the film. Here, the reason why the area ratio of the film peeling is standardized by using the area of 30 mm × 100 mm as the denominator is that the peeling of the film does not progress to the size of the area or more. In addition, the criteria for the acceptance of the WT test were judged to be acceptable regardless of whether the WT test (1) or the WT test (2), that is, the peeling area ratio of the film was 35% or less. For the maximum corrosion depth, those below 0.3 mm were judged to be qualified.

The experimental results are shown in Table 7 above. In addition, "WT (1)" shown in Table 7 is a scar portion at a position above the sea water surface 110 mm (refer to The evaluation result of FIG. 2A) and "under WT (1)" are the evaluation results of the scar part (refer FIG. 2A) of the position of 20 mm above the sea surface.

As shown in Table 7, in Test Nos. 1 to 24 of the present invention, in the SAE J2334 test, the amount of corrosion of the test piece having no film was 0.60 mm or less and the value was small; even in the case of a test piece having a film, the maximum was The corrosion depth is below 0.45 mm and the value is also small, and the peeling area ratio of the film is below 60% and the value is also low. Further, regardless of the WT test (1) or the WT test (2), the peeling area ratio of the film was 35% or less and the value was low. In addition, in the steel materials No. 1 to 24, since the Sn content is suppressed to 0.5% or less, sufficient toughness can be obtained.

On the other hand, in Test No. 25 of the comparative example, since the Sn concentration ratio was 6.0 or more, the WT (1) epithelial peeling area ratio, WT (1) epidermis peeling area ratio, WT (2) film in the WT test. Both the peeling area ratio and the maximum corrosion depth of WT(2) could not meet the target value.

In Test Nos. 26 and 27 of the comparative example, since the Sn concentration ratio was less than 1.2, the WT (1) epithelial membrane peeling area ratio, WT (1) epidermis peeling area ratio, and WT (2) film in the WT test. Both the peeling area ratio and the maximum corrosion depth of WT(2) could not meet the target value. Further, in Test No. 29 of the comparative example, since the O content exceeded 0.0100% and the Sn concentration ratio was less than 1.2, the coating-free corrosion resistance, the peeling area ratio and the maximum corrosion depth in the SAE J2334 test, and the WT test were performed. The WT(1) epithelial membrane peeling area ratio, WT(1) lower membrane peeling area ratio, WT(2) membrane peeling area ratio, and WT(2) maximum corrosion depth were all unable to satisfy the target value.

In Test No. 28 of the comparative example, since the amount of corrosion was not contained in the SAEJ2334 test, the peeling area ratio and the maximum corrosion depth in the SAEJ2334 test. The degree of WT(1) epithelial peeling area ratio, WT(1) lower film peeling area ratio, WT(2) peeling area ratio, and WT(2) maximum corrosion depth in the WT test could not satisfy the target value.

In Test No. 30 of the comparative example, although the ratio of Sn content to Sn concentration was within the range of the present invention, since the total content of Cu + Cr was 0.10% or more, the peeling area ratio and the maximum corrosion depth of the SAE J2334 test were obtained. And the WT (1) epithelial membrane peeling area ratio, WT (1) lower membrane peeling area ratio, WT (2) peeling area ratio, and WT (2) maximum corrosion depth in the WT test could not satisfy the target value.

As is apparent from the above test results, by using the steel material of the present invention, it is possible to obtain a reduction in toughness and an excellent corrosion resistance.

Industrial availability

According to the present invention, it is possible to provide a steel material which can have excellent corrosion resistance even in a corrosive environment containing chloride.

Claims (8)

A steel material characterized by a chemical composition of: C: 0.01 to 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~ less than 0.10%, Cr: 0~ less than 0.10%, Mo: 0~ less than 0.050%, W: 0~ less than 0.050%, Cu+Cr: 0~ less than 0.10%, Mo+W: 0~ less than 0.050 %, Sb: 0~ less than 0.05%, Ni: 0~0.05%, Nb: 0~0.050%, V: 0~0.050%, Ti: 0~0.020%, Al: 0~0.100%, Ca: 0~ less than 0.0100%, Mg: 0~0.0100%, REM: 0~0.0100%, P: less than 0.05%, S: less than 0.01%, and the remainder: Fe and impurities; and has soft tissue of ferrite iron and hard tissue of Borne, toughened iron and granulated iron; and the concentration of Sn in the hard tissue is relative to the softness The ratio of the concentration of Sn in the tissue, that is, the concentration ratio of Sn is 1.2 or more and less than 6.0. The steel material according to claim 1, wherein the chemical composition comprises Cu+Cr: 0 to less than 0.05% by mass%. The steel material according to claim 1, wherein the chemical composition comprises Mo+W: 0.0005 to less than 0.050% by mass%. The steel material according to any one of claims 1 to 3, wherein the chemical composition is at least one selected from the group consisting of Nb: 0.001 to 0.050%, V: 0.005 to 0.050%, Ti: 0.001~ 0.020%, Al: 0.01 to 0.100%, Ca: 0.0002 to less than 0.0100%, Mg: 0.0002 to 0.0100%, and REM: 0.0002 to 0.0100%. The steel material according to any one of claims 1 to 3, which is coated with an anti-corrosion film having a film thickness of 20 μm or more. The steel material of claim 4 is coated with an anti-corrosion film having a film thickness of 20 μm or more. A ballast tank or cabin using steel as claimed in any one of claims 1 to 6 The material formed by the person. A ship having a ballast tank or a cabin as claimed in claim 7.