WO2016098234A1 - 鋼材、この鋼材を用いた船舶のバラストタンク及び船倉、並びにこのバラストタンクまたは船倉を備える船舶 - Google Patents
鋼材、この鋼材を用いた船舶のバラストタンク及び船倉、並びにこのバラストタンクまたは船倉を備える船舶 Download PDFInfo
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
Definitions
- the present invention relates to a steel material having excellent corrosion resistance that 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.
- Patent Documents 1 to 3 Sn is 0.005 to 0.3 mass%, 0.02 to 0.40 mass%, and 0, respectively.
- a steel material having a corrosion resistance improved in an environment containing chloride ions (Cl 2 ⁇ ions) by containing 0.01 to 0.50 mass% is disclosed.
- Patent Document 4 discloses that W: 0.01 to 0.5% by mass, Mo: one or more of 0.02 to 0.5% by mass, and Sn: 0.001 to 0.2% by mass.
- Sb A steel material containing at least one of 0.01 to 0.2% by mass and capable of extending the period until repair coating in a seawater corrosive environment is 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
- 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.
- 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.
- 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 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 ⁇ m.
- 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 test piece area means the area of the surface in which the collar part was formed among six surfaces of a test piece.
- 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.
- 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 ⁇ m.
- 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. .
- 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.
- 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.
- 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.
- an object of the present invention is to provide a steel material that can ensure excellent corrosion resistance even in a severe environment containing chloride, such as the back side of the deck of a ship's ballast tank.
- the present inventors investigated 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. Further, a WT test was performed using tests taken from these steel plates A1 to 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.
- good results were obtained for both the film peeling area ratio and the maximum corrosion depth.
- the ratio of the Sn concentration in the hard structure to the Sn concentration in the soft structure (Sn concentration in the hard structure / Sn concentration in the soft structure, 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 present invention has been completed based on the above findings, and the gist thereof is as follows.
- 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 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: less than 0 to less than 0.05%, Ni: 0 to less than 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:
- the chemical composition may contain Cu + Cr: 0 to less than 0.05% by mass%.
- the chemical composition may contain Mo + W: 0.0005 to less than 0.050% by mass%.
- 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.
- the surface may be coated with an anticorrosive film having a thickness of 20 ⁇ m or more.
- 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 (4) above.
- the ship which concerns on another aspect of this invention is provided with the ballast tank or ship hold as described in said (6).
- maintenance of a ship formed using this steel material excellent in corrosion resistance, and these ballast tanks and hold ship can be provided.
- 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%”.
- C 0.01 to 0.20%
- 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%.
- 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 to 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%.
- 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.
- 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%.
- 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.
- 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.
- 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%.
- 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
- 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%.
- 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.
- 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.
- a smaller Cr content is preferable, and the lower limit of the content is 0%.
- 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.
- 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%.
- 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.
- 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.
- Cu and Sn coexist.
- rolling cracks may occur depending on the manufacturing method.
- 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%
- 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% If 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%
- 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%.
- the W content is small, and the lower limit of the W content is 0%.
- the lower limit of Mo may be 0.010% or 0.020%.
- Mo + W 0 to less than 0.050%
- the upper limit of the total content may be 0.030%, 0.020%, 0.010%, or 0.005%.
- 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.
- the steel material is selected from the following elements as necessary.
- One or more components may be included.
- 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.
- 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%.
- the upper limit of Ni content is set to 0.05%.
- 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.
- 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.
- 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.
- Si 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%.
- the Al content is preferably 0.010% or more, more preferably 0.020% or more, and further preferably 0.030% or more.
- 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.
- 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 a decrease in pH at the interface in the corrosion reaction part to suppress corrosion.
- 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.
- the Mg content is preferably 0.0002% or more, and more preferably 0.0005% or more.
- 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.
- the REM content is preferably 0.0002% or more, and more preferably 0.0005% or more.
- 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%.
- 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.
- the following elements must 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%.
- 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.
- 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.
- the hard structure is pearlite, bainite, and martensite
- 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.
- 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.
- 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.
- a structure in which a hard tissue and a soft tissue are dispersed is preferable.
- 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.
- the average particle diameter of the soft tissue and the average particle diameter of the hard tissue are each preferably 15 ⁇ m or less, and more preferably 10 ⁇ m or less.
- the “average grain size” is a grain boundary defined by an EBSP (ElectronscBackscatter Diffraction Pattern: electron beam backscattering pattern) with an orientation difference of 15 ° or more, and a portion surrounded by the grain boundary It can be calculated as a crystal grain.
- 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.
- 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.
- the soft tissue has excellent corrosion resistance and the hard tissue has low corrosion resistance.
- the thin water film adhering to the steel material surface changes to an acidic chloride aqueous solution.
- 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.
- the hard tissue has lower corrosion resistance than the soft tissue, and the hard tissue is a starting point for 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.
- 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.
- 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.
- 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.
- 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 the above-described chemical composition and structure, and thus has excellent corrosion resistance even when used as it is.
- 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.
- the surface of the steel material can be covered with an anticorrosion coating made of an organic resin.
- 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 ⁇ m or more, and more preferably 50 ⁇ m or more.
- the lower limit of the film thickness may be 100 ⁇ m or 150 ⁇ m.
- 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 ⁇ m or less.
- the upper limit of the film thickness may be 400 ⁇ m or 300 ⁇ m.
- the durability of the anticorrosion film is improved as compared with the conventional steel material.
- 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
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- the slab can be formed at a temperature of about 1000 to 1150 ° C., and the rolling reduction per rolling pass can be suppressed.
- Hot rolling may be performed under conditions of 3.0% or more and a rolling finishing temperature of about 700 to 900 ° C.
- the steel material after hot rolling is subjected to water cooling (weak water cooling) at an average cooling rate of 1.0 to 3.0 ° C./s in a temperature range from the end of rolling to 650 ° C., and then continuously from 650 ° C. to 550 ° C.
- the temperature range of ° C is water-cooled (strong water cooling) at an average cooling rate of 3.0 to 25 ° C / s, and the temperature range of 550 ° C to 400 ° C is cooled at a mean cooling rate of 0.01 to 1.0 ° C / s (slow Cooling) and then air cooling to room temperature.
- the temperature said here is the temperature of the steel material surface.
- Sn has a property of being easily concentrated in a hard structure.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- the anticorrosion film can be formed by a known ordinary method.
- the whole surface of steel materials may be coat
- the anticorrosion film may be formed only on a portion exposed to a corrosive environment (for example, one side of a steel material).
- an anticorrosion film may be formed only on the outer or inner surface of the steel pipe.
- the surface of the steel material may be subjected to chemical conversion treatment.
- a treatment agent prepared from zinc, titanium, zirconium, chromium, a silane compound, or the like can be used.
- FIG. 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.
- each of the above blocks was heated at 1100 ° C. to 1120 ° C. for 1 hour or more and 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. 1 to 30 steel plates were used.
- the cooling start temperature in Table 6 indicates the temperature at which water cooling starts after rolling, the cooling rate 1 is the average cooling rate from 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 ⁇ width of 15 mm ⁇ thickness of 20 mm was cut out from the above steel plates (test Nos. 1 to 30) to prepare a micro sample.
- the Sn concentration ratio between the hard tissue and the soft tissue was 1.2 or more and less than 6.0.
- 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.
- 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.
- 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.
- 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
- 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.
- a cross-shaped ridge was formed on the anticorrosion film to expose a part of the steel material as a base.
- 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.
- the WT test (1) is a test that simulates the environment of the side surface of the ballast tank, as shown in FIG.
- seawater splashes scattered from the seawater surface adhere to the surface of the test piece.
- test pieces each having a length of 290 mm ⁇ width of 30 mm ⁇ thickness of 2.5 mm collected from each steel plate having a thickness of 20 mm were used.
- the WT test (2) is a test that simulates the environment on the back side of the deck as shown in FIG.
- 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”.
- test pieces each having a length of 140 mm ⁇ width of 30 mm ⁇ thickness of 2.5 mm collected from each steel plate (test Nos. 1 to 30) having a thickness of 20 mm were used.
- the 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.
- the maximum corrosion depth of the steel material as the base (the maximum value of the corrosion depth from the steel material surface) was measured at the position where the collar portion of the anticorrosion coating was formed.
- the film peeling area rate (%) was calculated
- 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.
- the pass / fail judgment standard in the SAEJ2334 test was set to pass 0.60 mm or less for the amount of corrosion. 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.
- the 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.
- 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.
- 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 corrosion amount of the test piece without the coating is as small as 0.60 mm or less, and the test piece with the coating also has the maximum corrosion.
- the depth is as small as 0.45 mm or less, and the film peeling area ratio is as low as 60% or less.
- the film peeling area ratio is as low as 35% or less.
- sufficient toughness was obtained by suppressing the Sn content to 0.5% or less.
- 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.
- 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.
- 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. related to 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. related to 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.
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Abstract
Description
さらには、石炭および鉄鉱石等を運搬する船舶の貨物を積載する区画である船倉内は、直接海水環境にはさらされないが、海水による洗浄等が行われるので塩化物による鋼材の腐食が問題となる。また、塩運搬船のタンクなどにおいても塩化物による鋼材の腐食が問題となる。また、タンカーの原油タンク内は、高濃度塩化物溶液であるドレン水が存在する厳しい腐食環境となっているので、鋼材の腐食が問題となる。その他、オイルサンドの掘削・輸送設備においても塩化物による鋼材の腐食が問題となる。このように、塩化物による鋼材の腐食は大きな問題となっている。
また、特許文献4には、W:0.01~0.5質量%、Mo:0.02~0.5質量%のうち1種以上、及び、Sn:0.001~0.2質量%、Sb:0.01~0.2質量%のうち1種以上を含み、海水腐食環境下において補修塗装までの期間の延長が可能な鋼材が開示されている。
表1に示す化学成分を有する鋼板A~Cを準備し、これらの鋼板を用いて、バラストタンクの腐食環境を模擬した2種類の腐食試験、すなわち、SAEJ2334試験及びウエーブタンク試験(以下「WT試験」とする)を行った。SAEJ2334試験、WT試験のいずれにおいても鋼板の表面に防食皮膜を形成させた試験片を用いた。
SAEJ2334試験とは、乾湿繰り返し(湿潤→塩分付着→乾燥)の条件を1サイクル(合計24時間)として行う加速劣化試験であり、飛来塩分量が1mddを超えるような厳しい腐食環境を模擬する試験である。SAEJ2334試験は以下の条件を1サイクルとして行った。下記の条件下における腐食形態は、大気暴露試験の腐食形態と類似している。
(試験条件)
・湿潤:50℃、100%RH、6時間、
・塩分付着:0.5質量%NaCl、0.1質量%CaCl2、0.075質量%NaHCO3水溶液浸漬、0.25時間、
・乾燥:60℃、50%RH、17.75時間
(a)防食皮膜の疵部が形成された位置において、下地としての鋼材の最大腐食深さ(鋼材表面からの腐食深さの最大値)を測定した。
(b)防食皮膜のうち疵部から進展して剥離した部分の面積を評価するために、皮膜剥離面積率(%)を求めた。具体的には、防食皮膜が剥離した部分(疵部から進展して剥離した部分)をカッター等によって取り除き、上記取り除いた部分を皮膜剥離部とした。そして、画像処理ソフトの2値化処理を用いて、(皮膜剥離部面積)/(試験片面積)×100の値を求めて皮膜剥離面積率(%)とした。試験片面積とは、試験片の6つの面うち疵部が形成された面の面積を意味する。
ウエーブタンク試験(WT試験)は、船舶のバラストタンク内の環境を模擬した試験である。WT試験は、船舶のバラストタンクの甲板の裏面側(図1の(2)の位置)を模擬した以下の条件で行った。
(試験条件)
・「50℃、12時間」と「20℃、12時間」を繰り返す温度サイクル(試験片の温度)のもとで、海水面から飛散した海水飛沫を試験片表面に付着させる。
(c)防食皮膜の疵部が形成された位置において、下地としての鋼材の最大腐食深さ(鋼材表面からの腐食深さの最大値)を測定した。
(d)防食皮膜の疵部から進展して剥離した部分の面積を評価するために、皮膜剥離面積率(%)を求めた。具体的には、防食皮膜が剥離した部分(疵部から進展して剥離した部分)をカッター等によって取り除き、上記取り除いた部分を皮膜剥離部とした。そして、画像処理ソフトの2値化処理を用いて、(皮膜剥離部面積)/(皮膜疵部を中心とした30mm×100mmの面積)×100の値を求めて皮膜剥離面積率(%)とした。ここで、30mm×100mmの面積を分母として皮膜剥離面積率を標準化したのは、この面積以上の大きさで皮膜剥離が進むことは考えられないためである。
WT試験では、最大腐食深さが0.3mm以下で、かつ、皮膜剥離面積率35%以下の場合を合格とした。
鋼板A、B、Cのいずれについても、SAEJ2334試験の結果は良好であった。しかしながら、鋼板A及びBは、WT試験において皮膜剥離面積率及び最大腐食深さにおいて、悪い結果となった。一方、鋼板CはSAEJ2334試験とWT試験の両方の試験において、良好な結果を示した。
鋼板A及びBにおいて、SAEJ2334試験では良好な結果を示したものの、WT試験では、悪い結果となった。その理由として、WT試験の条件では、濡れ時間が長く、塗膜下への水の浸透が大きくなるので、Fe2+の溶解反応によるpHの上昇が起こり、SAEJ2334試験の場合に比べて、塗膜剥離がより促進されるためであると考えられる。
本発明者らは、更に、上記知見に関し、詳細な調査を行った。具体的には、表1の鋼板Aと同じ化学成分を有する鋼に対し、製造条件を表3に示すように変化させて、鋼板A1~A4を作成した。また、これらの鋼板A1~A4から採取した試験を用いてWT試験を行った。試験条件は、上述した条件と同じ条件とした。
(i)鋼板A1の製造条件の場合
圧延後の冷却は空冷であり、冷却速度が遅い。そのため、フェライトなどの軟質組織(軟質相)が成長しやすく、フェライトなどを中心とする軟質組織と層状(バンド状)の硬質組織(硬質相)とが形成される。また、冷却速度が遅いので、軟質組織から硬質組織へのSnの拡散が起こりやすく、Sn濃度比が高くなる。
(ii)鋼板A2の製造条件の場合
Snは融点が他の元素に比べて低いため、550~400℃間でも拡散するものの、650℃~400℃まで強冷却(水冷)された場合には、Snが十分硬質組織に拡散できず、Sn濃度比が小さくなる。
(iii)鋼板A3の製造条件の場合
圧延後、550℃以上までの温度域を強冷却されることにより、軟質組織と硬質組織が分散した組織となる。その後、550℃~400℃を緩冷却されることで、分散した硬質相に適度にSnが拡散する。
(iv)鋼板A4の製造条件の場合
圧延後に450℃まで強冷却されるため、組織が硬質相中心となり、また、Snが硬質相に十分拡散しないため、Sn濃度比が低くなる。
(2)上記(1)に記載の鋼材では、前記化学組成が、質量%で、Cu+Cr:0~0.05%未満を含有してもよい。
(3)上記(1)または(2)に記載の鋼材では、前記化学組成が、質量%で、Mo+W:0.0005~0.050%未満を含有してもよい。
(4)上記(1)~(3)のいずれか一項に記載の鋼材では、前記化学組成が、質量%で、Nb:0.001~0.050%、V:0.005~0.050%、Ti:0.001~0.020%、Al:0.01~0.100%、Ca:0.0002~0.0100%未満、Mg:0.0002~0.0100%、およびREM:0.0002~0.0100%、から選択される1種以上を含有してもよい。
(5)上記(1)~(4)のいずれか一項に記載の鋼材では、表面が、膜厚が20μm以上の防食皮膜により被覆されていてもよい。
(6)本発明の別の態様に係るバラストタンクまたは船倉は、上記(1)~(4)のいずれか一項一項に記載の鋼材を用いて形成される。
(7)本発明の別の態様に係る船舶は、上記(6)に記載のバラストタンクまたは船倉を備える。
本実施形態に係る鋼材において、その化学組成を規定する理由は次のとおりである。
Cは、鋼材の強度を向上させる元素である。この効果を得るため、C含有量の下限を0.01%とする。好ましいC含有量の下限は、0.02%であり、より好ましいC含有量の下限は0.03%である。C含有量の下限を、0.05%、0.07%又は0.09%としてもよい。一方、C含有量が0.20%を超えると溶接性が著しく低下する。また、C含有量の増大とともに、pHが低い環境でカソードとなって腐食を促進するセメンタイトの生成量が増大し、鋼材の耐食性が低下する。このため、C含有量の上限を0.20%とする。好ましいC含有量の上限は、0.18%であり、より好ましいC含有量の上限は0.16%である。C含有量の上限を、0.15%又は0.13%としてもよい。
Siは脱酸に必要な元素である。十分な脱酸効果を得るため、0.01%以上含有させる必要がある。好ましいSi含有量の下限は、0.03%であり、より好ましいSi含有量の下限は0.05%である。Si含有量の下限を、0.10%、0.15%又は0.20%としてもよい。一方、Si含有量が1.00%を超えると、母材および溶接継手部の靱性が損なわれる。このため、Si含有量の上限を1.00%とする。好ましいSi含有量の上限は、0.80%であり、より好ましいSi含有量の上限は0.60%である。Si含有量の上限を、0.50%、0.40%又は0.30%としてもよい。
Mnは低コストで鋼材の強度を高める作用を有する元素である。この効果を得るため、Mn含有量の下限を0.05%とする。好ましいMn含有量の下限は、0.20%であり、より好ましいMn含有量の下限は0.40%である。Mn含有量の下限を、0.60%、0.80%又は0.90%としてもよい。一方、Mn含有量が3.00%を超えると、溶接性及び継手靭性が劣化する。このため、Mn含有量の上限を3.00%とする。好ましいMn含有量の上限は、2.50%であり、より好ましいMn含有量の上限は2.00%である。Mn含有量の上限を、1.80%、1.60%又は1.50%としてもよい。
Snは本実施形態に係る鋼材において重要な元素である。SnはSn2+として溶解し、2Fe3++Sn2+→2Fe2++Sn4+なる反応によりFe3+の濃度を低下させることで、腐食反応を抑制する。また、Snは、低pH塩化物環境において鋼材のアノード溶解反応を著しく抑制するので、塩化物腐食環境における鋼材の耐食性を大幅に向上させる。これらの効果を得るため、Sn含有量の下限を0.01%とする必要がある。好ましいSn含有量の下限は、0.03%であり、より好ましいSn含有量の下限は0.05%である。Sn含有量の下限を、0.08%、0.12%、0.16%又は0.19%としてもよい。一方、Sn含有量が0.50%を超えると、上記の効果が飽和するだけでなく、母材および大入熱溶接継手の靭性が劣化する。したがって、Sn含有量の上限は0.50%とする。好ましいSn含有量の上限は、0.45%であり、より好ましいSn含有量の上限は0.40%である。Sn含有量の上限を、0.35%又は0.30%としてもよい。
O(酸素)はSnと同様に本実施形態に係る鋼材において重要な元素である。Oは微量の含有により溶接継手の靭性を向上させる。この効果を得るには、O含有量の下限を0.0001%とする必要がある。好ましいO含有量の下限は、0.0002%以上であり、より好ましいO含有量の下限は、0.0003%である。O含有量の下限を、0.0005%、0.0010%、0.0015%又は0.0019%としてもよい。一方、OはSnOおよびSnO2等の酸化物を形成する。そのため、O含有量が0.0100%を超えると、硬質組織中におけるSn濃度を十分に確保できない。また、上記酸化物は腐食の起点となるので、鋼材の耐食性が低下する。したがって、O含有量の上限は0.0100%とする。好ましいO含有量の上限は、0.0090%であり、より好ましいO含有量の下限は、0.0080%である。O含有量の上限を、0.0060%、0.0040%又は0.0030%としてもよい。
Crは一般に、鋼材の耐食性を向上させる元素であると考えられている。しかしながら、本発明者らは、本実施形態で想定されるような塩化物を含む腐食環境下では、Crを含有すると鋼材の耐食性が悪化することを見出した。Cr含有量は、少ない方が好ましく、含有量の下限を0%とする。一方、不純物として混入する場合を考慮し、Cr含有量の上限を0.10%未満とする。Cr含有量は、0.07%以下又は0.05%未満に制限することが好ましく、0.03%以下又は0.02%以下に制限することがより好ましい。Cr含有量を0.01%以下に制限することがより一層好ましい。
Cuは一般に、鋼材の耐食性を向上させる元素であると考えられている。しかしながら、本発明者らは、本実施形態で想定されるような塩化物を含む腐食環境下では、Cuを含有すると鋼材の耐食性が低下することを見出した。Cu含有量は、少ない方が好ましく、Cu含有量の下限を0%とする。一方、不純物として混入する場合を考慮し、Cu含有量の上限を0.10%未満とする。耐食性の向上のためには、Cu含有量を0.07%以下又は0.05%以下に制限することが好ましく、0.03%以下又は0.02%以下に制限することがより好ましい。Cu含有量を0.01%以下に制限することがより一層好ましい。
鋼材がCuを含有する場合にはCuとSnが共存することになる。この場合、製造方法によっては圧延割れが生じることがある。圧延割れを抑制するためには、Sn含有量に対するCu含有量の比(Cu/Sn)を小さくすることが重要となる。Cuを含有させる場合、Cu/Snを1.0以下とすることが好ましい。Cu/Snを0.5以下又は0.3以下とすることがより好ましい。
上述したように、Cr及びCuは塩化物を含む腐食環境下において、鋼材の耐食性を低下させる元素である。そのため、これらの元素を同時に含有する場合、個々の元素の含有量だけでなく、合計含有量を制限する必要がある。すなわち、Cu及びCrの合計含有量を0.10%未満に制限する必要がある。好ましくは、0.07%未満、より好ましくは0.05%未満、さらに好ましくは0.04%未満、より一層好ましくは0.03%未満である。
Mo含有量が0.050%以上であると、耐食性が低下する場合があることに加え、鋼材のコストが大幅に上昇する。したがって、Mo含有量は少ない方が好ましく、Mo含有量は0.050%未満とする。好ましくは、Mo含有量は、0.040%以下である。Mo含有量の上限を0.030%、0.020%、0.010%又は0.005%としてもよい。耐食性の改善のためにはMo含有量は少ない方が好ましく、Mo含有量の下限は0%である。しかし、強度又は靭性などの特性向上のため、Mo含有量の下限を0.010%又は0.020%としてもよい。
W含有量が0.050%以上であると、耐食性が低下する場合もあることに加え、鋼材のコストが大幅に上昇する。したがって、Mo含有量は少ない方が好ましく、W含有量は0.050%未満とする。より好ましくは、W含有量は、0.040%である。W含有量の上限を0.030%、0.020%、0.010%又は0.005%としてもよい。耐食性の改善のためにはW含有量は少ない方が好ましく、W含有量の下限は0%である。しかし、強度又は靭性などの特性向上のため、Mo下限を0.010%又は0.020%としても差し支えない。
耐食性の向上のため、Mo及びWについては個々の元素の含有量だけでなく、合計含有量を制限する必要がある。すなわち、Mo及びWの合計含有量を0.050%未満に制限する。この合計含有量の上限を0.030%、0.020%、0.010%又は0.005%としてもよい。耐食性の改善のためにはこの合計含有量は少ない方が好ましいが、強度又は靭性などの特性向上のため、合計含有量の下限を0.005%、0.010%又は0.020%としても差し支えない。
また、本実施形態において、不純物とは、鋼材を工業的に製造する際に、鉱石、スクラップ等の原料、その他の要因により混入する成分を意味する。
Sbは、耐酸性を向上させる元素である。しかし、Sbを0.05%超含有させても、その効果が飽和するだけでなく、鋼材の靭性等の劣化を招く。そこで、Sbの含有量は0.05%以下とする。Sb含有量の上限を、0.04%又は0.03%としてもよい。Sbの含有は必須でなく、Sb含有量の下限は0%である。しかし、耐酸性を向上させるために、Sb含有量の下限を0.005%、0.010%又は0.015%としてもよい。
Niは一般に、Cuと同様に、鋼材の耐食性を向上させると考えられている。しかしながら、本発明者らは、本実施形態で想定されるような塩化物を含む腐食環境下では、Niを含有すると鋼材の耐食性が低下することを見出した。Ni含有量は、少ない方が好ましく、Ni含有量の下限は0%である。不純物として混入する場合を考慮し、Ni含有量の上限を0.05%とする。耐食性の向上のため、Ni含有量を0.03%以下又は0.02%以下に制限することが好ましく、0.01%以下に制限することがより好ましい。
Nbは鋼材の強度を上昇させる元素である。しかし、Nb含有量が0.050%を超えると上記の効果が飽和する。したがって、含有させる場合のNb含有量は0.050%以下とする。必要に応じて、Nb含有量を、0.030%以下又0.020%以下としてもよい。Nbは必ずしも含有させる必要がないのでNb含有量の下限は0%であるが、強度向上の効果を得るために、Nbを0.001%以上含有させてもよく、0.003%以上又は0.005%以上含有させてもよい。
Vは、Nbと同様に鋼材の強度を上昇させる元素である。また、Vは、MoおよびWと同様に、腐食環境中(水溶液中)に溶解して酸素酸イオンの形で存在し、さび層中の塩化物イオンの透過を抑制する。しかし、V含有量が0.050%を超えると、上記の効果が飽和するだけでなく、コストが著しく上昇する。したがって、含有させる場合のV含有量は0.050%以下とする。V含有量を、0.040%以下又は0.030%以下としてもよい。Vは必ずしも含有させる必要がないのでV含有量の下限は0%であるが、上記の効果を得るために、Vを0.005%以上又は0.010%以上含有させてもよい。
Tiは、鋼材の脱酸に有効であり、かつ鋼材の腐食の起点となるMnSの形成を抑える。しかし、Ti含有量が0.020%を超えると、上記の効果が飽和するだけでなく、鋼材のコストが上昇する。したがって、含有させる場合のTi含有量は0.020%以下とする。Ti含有量は、0.015%以下とすることが好ましい。Tiは必ずしも含有させる必要がないのでTi含有量の下限は0%であるが、上記の効果を得るために、Tiを0.005%以上又は0.008%以上含有させてもよい。
Alは、鋼材の脱酸に有効な元素である。本実施形態では鋼材中にSiを含有させるので、Siによって脱酸が行われる。よって、Alで脱酸処理することは必ずしも必要でなく、Al含有量の下限は0%とする。しかし、Siに加えて、さらにAlによる脱酸を行ってもよい。Alによる脱酸効果を得るためには、Al含有量を0.010%以上とすることが好ましく、0.020%以上とすることがより好ましく、0.030%以上とすることがさらに好ましい。一方、Al含有量が0.100%を超えると、低pH環境における鋼材の耐食性が低下することによって塩化物腐食環境における鋼材の耐食性が低下する。また、Al含有量が0.100%を超えると窒化物が粗大化することによって鋼材の靱性の低下を引き起こす。したがって、含有させる場合のAl含有量は、0.100%以下とする。好ましいAl含有量の上限は、0.060%であり、より好ましいAl含有量の上限は0.045%である。
Caは鋼材中に酸化物の形で存在し、腐食反応部における界面のpHの低下を抑制して、腐食を抑える。上記の効果を得る場合、Caを0.0002%以上含有させることが好ましく、0.0005%以上含有させることがより好ましい。一方、Ca含有量が0.0100%以上であると、上記の効果が飽和する。したがって、含有させる場合のCa含有量は、0.0100%未満とする。Ca含有量を0.0050%以下又は0.0030%以下としてもよい。必ずしもCaを含有させる必要はないので、Caの含有量の下限は0%である。
Mgは、Caと同様に、腐食反応部における界面のpHの低下を抑制して、鋼材の腐食を抑える。上記の効果を得る場合、Mg含有量を0.0002%以上含有させることが好ましく、0.0005%以上含有させることがより好ましい。一方、Mg含有量が0.0100%を超えると、上記の効果が飽和する。したがって、含有させる場合のMg含有量は0.0100%以下とする。Mg含有量は、0.0050%以下又は0.0030%以下としてもよい。必ずしもMgを含有させる必要はないので、Mgの含有量の下限は0%である。
REM(希土類元素)は鋼材の溶接性を向上させる元素である。この効果を得る場合、REM含有量を0.0002%以上とすることが好ましく、0.0005%以上とすることがより好ましい。一方、REM含有量が0.0100%を超えると上記の効果が飽和する。したがって、含有させる場合のREM含有量は0.0100%以下とする。REM含有量の上限を、0.0050%又は0.0030%としてもよい。必ずしもREMを含有させる必要はなく、REMの含有量の下限は0%である。本実施形態において、REMとは、ランタノイドの15元素にYおよびScを合わせた17元素の総称である。本実施形態に係る鋼材は、これらの17元素のうちの1種以上を鋼材に含有することができ、REM含有量は、これらの元素の合計含有量を意味する。
Pは鋼材中に不純物として存在する元素である。Pは鋼材の耐酸性を低下させる元素であり、腐食界面のpHが低下する塩化物腐食環境においては鋼材の耐食性を低下させる。また、Pは鋼材の溶接性および溶接熱影響部の靭性を低下させる。そのため、P含有量を0.050%以下に制限する。P含有量は、0.040%以下に制限することが好ましく、0.030%未満に制限することがより好ましい。溶接熱影響部の靭性の向上のため、P含有量の上限を0.020%、0.015%又は0.010%としてもよい。Pを完全に取り除くことは容易ではないが、これを排除する必要はなく、P含有量の下限は0%である。
Sは鋼材中に不純物として存在する元素である。Sは鋼材中に腐食の起点となるMnSを形成する。S含有量が0.010%を超えると、鋼材の耐食性の低下が顕著になる。このため、S含有量は0.010%以下に制限する。S含有量は、0.008%以下に制限することが好ましく、0.006%以下に制限することがより好ましく、0.004%以下に制限することがさらに好ましい。Sを完全に取り除くことは容易ではないが、これを排除する必要はなく、S含有量の下限は0%である。
本実施形態に係る鋼材は、硬質組織と軟質組織とを有する。本実施形態において、硬質組織とはパーライト、ベーナイト、およびマルテンサイトであり、軟質組織とはフェライトである。硬質組織と軟質組織との割合は、鋼材の強度設計に合わせて決定すればよく、特に限定する必要はないが、船体構造用鋼として必要な強度および靭性を確保するためには、本実施形態に係る鋼材のミクロ組織は、パーライトおよびフェライトを含む複合組織であることが好ましく、面積%(面積率)で、フェライト組織が全組織の80%以下であることが好ましい。
また、本実施形態に係る鋼材は、硬質組織と軟質組織とが積層および/または分散した組織構造を有する。後述するSn濃度比を所定の範囲内に制御する場合、硬質組織と軟質組織とが分散した組織が好ましい。また、後述する平均粒径を小さくする場合も、硬質組織と軟質組織とが分散した組織が好ましい。硬質組織と軟質組織とが分散した組織とは、鋼材中において、硬質組織および軟質組織がそれぞれ分散して存在している組織構造のことをいう。
本実施形態に係る鋼材では、このような腐食を防止するために、各組織中のSn濃度を制御し、硬質組織中のSn濃度を軟質組織中のSn濃度の1.2倍以上とする。先に述べたように、腐食により溶解したSnイオンは、鋼材の耐食性を向上させる。したがって、先行して腐食する硬質組織中にSnが高濃度で存在させれば、硬質組織での初期腐食を回避でき、鋼材全体への腐食進行を防止することができる。しかしながら、硬質組織中のSn濃度が軟質組織中のSn濃度の6倍以上になると、硬質組織と軟質組織との間で電位差が生じ、耐食性に優れる軟質組織が硬質組織に優先して腐食されるので、むしろ耐食性が低下する。したがって、Snによる耐食効果をより発揮するためには、硬質組織中のSn濃度が軟質組織中のSn濃度の1.2倍以上6.0倍未満であることが必要である。好ましくは、硬質組織中のSn濃度は、軟質組織中のSn濃度の1.3倍以上、より好ましくは1.5倍以上、さらに好ましくは1.7倍以上、より一層好ましくは2.0倍以上である。好ましくは、硬質組織中のSn濃度は、軟質組織中のSn濃度の5.0倍以下、より好ましくは4.0倍以下、さらに好ましくは3.5倍以下である。
本実施形態に係る鋼材を用いて形成された、船舶のバラストタンクまたは船倉では、腐食が抑制されるため、再塗装などのメンテナンスの頻度を低下させることができる。また、これらのバラストタンクや船倉を備えた船舶では、メンテナンス頻度の低下による運航コストの削減や、腐食により板厚が薄くなった鋼材の所要の板厚の鋼材への取り換え(補修)を防止できることによる安全性の向上や補修コスト低減などの効果が得られる。
熱間圧延後の鋼材は、例えば、圧延終了後~650℃までの温度域を1.0~3.0℃/sの平均冷却速度で水冷(弱水冷)した後、続けて650℃~550℃の温度域を平均冷却速度3.0~25℃/sで水冷(強水冷)し、550℃~400℃の温度域を平均冷却速度0.01~1.0℃/sで冷却(緩冷却)し、その後、室温まで空冷すればよい。ここで言う温度とは、鋼材表面の温度である。
Snは硬質組織において濃化しやすい性質を有する。650℃以上の温度域での冷却速度が、1.0℃/s未満(例えば0.8℃/s)のように遅い場合には、軟質組織および硬質組織に、巨視的なバンド組織が形成されるとともに、Snが硬質組織に濃化し、硬質組織と軟質組織とのSn濃度比(硬質組織中のSn濃度/軟質組織中のSn濃度、以下Sn濃度比とする)が6.0以上になるおそれがある。この場合、硬質組織を起点とする鋼材の靭性低下の可能性があるとともに、鋼材が腐食される際に軟質組織が優先的に溶解されることによって、局部的に腐食が進む。
また、圧延後~400℃の温度域において一様に強冷却を行うと、組織が硬質相中心となり、Sn濃度比が低くなる可能性がある。
また、550℃~400℃の冷却速度が1.0℃超となった場合、Snが十分硬質組織に拡散されず、Sn濃度比が小さくなる。
本実施形態に係る鋼材は、鋼板であってもよい。鋼板の板厚を特に限定する必要はないが、板厚の下限を6mm又は10mmとしてよく、板厚の上限を50mm又は40mmとしてもよい。
本実施形態に係る鋼材の強度(引張強さ)は、特に限定する必要はない。しかしながら、実構造体への適用を考慮して、400MPa以上600MPa以下としてもよい。
・湿潤:50℃、100%RH、6時間、
・塩分付着:0.5質量%NaCl、0.1質量%CaCl2、0.075質量%NaHCO3水溶液浸漬、0.25時間、
・乾燥:60℃、50%RH、17.75時間
2 バラストタンク
3 船倉
Claims (7)
- 化学組成が、質量%で、
C:0.01~0.20%、
Si:0.01~1.00%、
Mn:0.05~3.00%、
Sn:0.01~0.50%、
O:0.0001~0.0100%、
Cu:0~0.10%未満、
Cr:0~0.10%未満、
Mo:0~0.050%未満、
W:0~0.050%未満、
Cu+Cr:0~0.10%未満、
Mo+W:0~0.050%未満、
Sb:0~0.05%未満、
Ni:0~0.05%、
Nb:0~0.050%、
V:0~0.050%、
Ti:0~0.020%、
Al:0~0.100%、
Ca:0~0.0100%未満、
Mg:0~0.0100%、
REM:0~0.0100%、
P:0.05%以下、
S:0.01%以下、
残部:Feおよび不純物
であり;
軟質組織と硬質組織とを有し;
前記軟質組織中のSn濃度に対する前記硬質組織中のSn濃度の比であるSn濃度比が1.2以上6.0未満である;
ことを特徴とする鋼材。 - 前記化学組成が、質量%で、
Cu+Cr:0~0.05%未満
を含有することを特徴とする請求項1に記載の鋼材。 - 前記化学組成が、質量%で、
Mo+W:0.0005~0.050%未満
を含有することを特徴とする請求項1に記載の鋼材。 - 前記化学組成が、質量%で、
Nb:0.001~0.050%、V:0.005~0.050%、Ti:0.001~0.020%、Al:0.01~0.100%、Ca:0.0002~0.0100%未満、Mg:0.0002~0.0100%、およびREM:0.0002~0.0100%、から選択される1種以上を含有する
ことを特徴とする請求項1から3のいずれか一項に記載の鋼材。 - 表面が、膜厚が20μm以上の防食皮膜により被覆されていることを特徴とする請求項1から4のいずれか一項に記載の鋼材。
- 請求項1から5のいずれか一項に記載の鋼材を用いて形成されたバラストタンクまたは船倉。
- 請求項6に記載のバラストタンクまたは船倉を備える船舶。
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CN201480077212.8A CN106103769B (zh) | 2014-12-18 | 2014-12-18 | 钢材、使用该钢材的船舶的压载舱和船舱、以及具备该压载舱或船舱的船舶 |
JP2015520755A JP5839151B1 (ja) | 2014-12-18 | 2014-12-18 | 鋼材、この鋼材を用いた船舶のバラストタンク及び船倉、並びにこのバラストタンクまたは船倉を備える船舶 |
KR1020167025558A KR101715581B1 (ko) | 2014-12-18 | 2014-12-18 | 강재, 이 강재를 사용한 선박의 밸러스트 탱크 및 선창, 및 이 밸러스트 탱크 또는 선창을 구비하는 선박 |
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EP3744871A4 (en) * | 2018-01-26 | 2021-05-19 | Nippon Steel Corporation | NET FOR ANCHORING CHAIN AND ANCHORING CHAIN |
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WO2019003448A1 (ja) * | 2017-06-30 | 2019-01-03 | Jfeスチール株式会社 | 熱間プレス部材およびその製造方法ならびに熱間プレス用冷延鋼板 |
CN107904512A (zh) * | 2017-12-15 | 2018-04-13 | 苏州赛斯德工程设备有限公司 | 一种防腐蚀钢材及其加工工艺 |
CN110536973B (zh) * | 2018-03-16 | 2020-08-18 | 日本制铁株式会社 | 煤/矿石运输船货舱用钢板 |
CN112272712A (zh) * | 2018-10-31 | 2021-01-26 | 日本制铁株式会社 | 煤专用船或矿/煤兼用船的船舱用耐蚀钢及船舱 |
JP6813127B1 (ja) * | 2019-11-13 | 2021-01-13 | 日本製鉄株式会社 | 鋼材 |
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