DK1990437T3

DK1990437T3 - CORROSION-RESISTANT STEEL MATERIAL FOR SHIP AND VESSELS

Info

Publication number: DK1990437T3
Application number: DK07707040.7T
Authority: DK
Inventors: Kazuhiko Shiotani; Tsutomu Komori; Toshiyuki Hoshino
Original assignee: Jfe Steel Corp
Priority date: 2006-02-27
Filing date: 2007-01-12
Publication date: 2016-09-19
Also published as: KR101115275B1; WO2007097142A1; CN101389782A; EP1990437A1; EP1990437B1; KR20110084462A; EP1990437A4; CN101389782B; KR20080090541A

Description

DESCRIPTION

Technical Field [0001] The present invention relates to anti-corrosion Steel used for ships such as a coal ship, an ore carrier, an ore coal carrier, a crude oil tanker, an LPG carrier, an LNG carrier, a Chemical tanker, a container ship, a bulk carrier, a log carrier, a chip carrier, a refrigerated cargo ship, a puré car carrier, a heavy load carrier, a roll-on/roll-off ship, a limestone carrier, and a cement carrier, and particularly relates to anti-corrosion Steel for ship used for a ballast tank and the like under a severe corrosión environment with seawater. The anti-corrosion Steel for ship described in the invention ineludes a thick Steel píate, sheet Steel, shape sheet, and bar Steel.

Background Art [0002] Since a ballast tank of a ship serves to enable stable navigation of a ship by being poured with seawater while no cargo is loaded, the tank is placed in an extremely severe corrosión environment. Therefore, an anti-corrosion paint film using epoxy paint is formed, in addition, cathodic protection is applied for preventing corrosión of Steel used for the ballast tank.

[0003] However, even if such anti-corrosion measures are taken, corrosión environment of the ballast tank is still in a severe condition. That is, when the ballast tank is poured with seawater, in the case that cathodic protection works, corrosión can be inhibited from being developed in a región that is perfectly dipped in the seawater. However, a región near a top of the ballast tank, a back of an upper deck is particularly not dipped in seawater, and situated while being sprayed with seawater. Therefore, cathodic protection does not work in such a región. Furthermore, since the región is increased in temperature of a Steel píate by sunlight, the región is in a more severe corrosión environment, and consequently drastically corroded. Moreover, when the ballast tank is not poured with seawater, cathodic protection does not work at all, and consequently the tank is drastically corroded due to an effect of remaining adhesión salinity.

[0004] Life of an anti-corrosion paint film of a ballast tank is said to be typically about 10 years under such a drastic corrosión resistance, that is, half the life (20 years) of.a ship. It is actual situation that corrosión resistance is kept by performing repair painting in the remaining ten years. However, since the ballast tank is in the severe corrosión environment as above, even if the repair painting is carried out, an effect of the painting is hard to be kept for a long time. Moreover, since the repair painting is performed as operation in a narrowspace, operation environment is not preferable.. Thus, it is desirable to develop a Steel material having an excellent corrosión resistance, which can lengthen a period before the repair painting to the utmost, and can reduce an operation load of the repair painting to the utmost.

[0005] Thus, several technologies have been proposed to improve corrosión resistance of Steel itself used for a región placed in a severe corrosión environment such as the ballast tank. For example, patent document 1 discloses anti-corrosion low alloy Steel including Steel containing C of 0.20 mass% or less, the Steel being added with Cu of 0.05 to 0.50 mass% and W of 0.01 to less than 0.05 mass% as elemente that improves corrosión resistance, and furthermore added with one or at least two of Ge, Sn, Pb, As, Sb, Bi, Te and Be in a range of 0.01 to 0.2 mass% as the elements. Patent document 2 discloses anti-corrosion low alloy Steel including Steel containing C of 0.20 mass% or less, the Steel being added with Cu of 0.05 to 0. 50 mass% and W of 0. 05 to 0. 5 mass% as elements that improves corrosión resistance, and furthermore added with one or at least two of Ge, Sn, Pb, As, Sb, Bi, Te and Be in a range of 0.01 to 0.2 mass% as the elements. Patent document 3 discloses anti-corrosion low alloy Steel including Steel containing C of 0.15 mass% or less, which is added with Cu of 0.05 to less than 0.15 mass% and W of 0.05 to 0.5 mass%.

[0006] Patent document 4 discloses a ballast tank in which anti-corrosion low alloy Steel, which includes Steel containing C of 0.15 mass% or less, the Steel being added with P of 0.03 to 0.10 mass%, Cu of 0.1 to 1.0 mass%, and Ni of 0.1 to 1.0 mass% as elements that improve corrosión resistance, is coated with anti-corrosion paint such as tar epoxy paint, puré epoxy paint, epoxy paint without solvent, and urethane paint, and then covered with resin. This technology is intended to lengthen the life of anti-corrosion painting by improving corrosión resistance of Steel itself, and thereby achieve a ship being free from maintenance over 20 to 30 years corresponding to the useful life of a ship.

[0007] Patent document 5 makes a proposal that Steel containing C of 0.15 mass% or less is added with Cr of 0.2 to 5 mass% as an element that improves corrosión resistance in order to achieve a ship being free from maintenance. Furthermore, patent document 6 proposes an anti-corrosion method of a ballast tank characterized in that Steel containing C of 0.15 mass% or less, which is added with Cr of 0.2 to 5 mass% as an element that improves corrosión resistance, is used as a componential material, and oxygen gas concentration within a ballast tank has a valué in a ratio of 0.5 or less with respect to a valué of oxygen gas concentration in air.

[0008] Patent document 7 makes a proposal that Steel containing C of 0.1 mass% or less is added with Cr of 0.5 to 3.5 mass% to improve corrosión resistance, so that a ship being free from maintenance is achieved. Furthermore, patent document 8 discloses Steel for ship in which Steel containing C of 0.001 to 0.025 mass% is added with Ni of 0.1 to 4.0 mass% to improve paint-film damage resistance, so that maintenance cost for repair painting and the like is reduced.

[0009] Patent document 9 discloses Steel for ship in which Steel containing C of 0.01 to 0.25 mass% is added with Cu of 0.01 to 2.00 mass% and Mg of 0.0002 to 0.0150 mass% so as to have corrosión resistance in use environment such as a Shell of a ship, a ballast tank, a cargo oil tank, and an iron-ore cargo hold. Furthermore, patent document 10 discloses Steel ¡n whlch Steel contalnlng C of 0.001 to 0.2 mass% is compositely added with Mo, W and Cu, and limited in the added amount of P and S as impurities, thereby general corrosión or local corrosión that may occur in a crude oil tank ¡s inhibited.

[0010] However, each of the patent documente 1 to 3 does not make adequate investigaron on corrosión resistance under presence of a paint film of Zinc-primer or epoxy palnt belng typically coated on Steel configuring a ballast tank or the like. Therefore, further investigation is necessary for improving corrosión resistance under presence of the paint film.

[0011] The Steel described in the patent document 4 is added with a comparatively large amount of P, 0.03 to 0.10 mass%, to improve corrosión resistance of base metal, which is problematic in a point of weldability and toughness of a weld. The Steel described in each of the patent documents 5 and 6 is added with a comparatively large amount of Cr, 0. 2 to 5 mass%. The Steel described in the patent document 7 is added with a comparatively large amount of Cr, 0.5 to 3.5 mass%. Either of them is problematic in the point of weldability and toughness of a weld, in addition, problematic in that manufacturing cost is increased. The steel described in the patent document 8 is comparatively low in C content, and comparatively high in Ni content, which is problematic in that manufacturing cost is increased.

[0012] The Steel described in the patent document 9 is problematic in that Mg is essentially added, which causes unstableness in yield of steel manufacture, leading to unstableness in mechanical properties of the Steel. Furthermore, the steel described in the patent document 10 is anti-corrosion steel used in the crude oil tank, namely, used under an environment having H2S, and therefore the steel is unclear in corrosión resistance in the ballast tank having no H2S. Furthermore, since corrosión resistance has not been investigated in a condition that zinc-primer being typically used for steel for a ballast tank is coated, further investigation on corrosión resistance is necessary to be used for a ballast tank.

Patent document 1: JP-A-48-050921 Patent document 2: JP-A-48-050922 Patent document 3: JP-A-48-050924 Patent document 4: JP-A-07-034197 Patent document 5: JP-A-07-034196 Patent document 6: JP-A-07-034270 Patent document 7: JP-A-07-310141 Patent document 8: JP-A-2002-266052 Patent document 9: JP-A-2000-017381 Patent document 10: JP-A-2004-204344

Disclosure of the Invention [0013] Generally, a ship is built by welding steel materials such as a thick steel píate, sheet steel, shape Steel, and bar steel, and surfaces of the steel materials are applied with anti-corrosion painting before use. The anti-corrosion painting is typically applied in a manner that zinc-primer is coated for primary rust prevention, and epoxy paint ¡s coated as secondary painting (maln painting) after subassembly or maln assembly is perfbrmed. Therefore, the major part of steel surface of a ship has a double-layer structure thereon, which ineludes a zinc primer paint film and an epoxy paint film. However, since the zinc-primer is burned out by welding heat on a weld, zinc primer is repainted on the weld as touch up paint for rust prevention in a period after welding and before main painting. However, when the period before the main painting is short, repainting of zinc primer is not performed in some case. After a ship is built, the ship enters Service, and in a ship that has been used for many years, there is a portion where the paint film is degraded and thus does not adequately act as a paint film, or there is a portion where the paint film is separated, so that a steel píate is bared.

[0014] As a result, three conditions exist on a surface of steel of a ship in Service, including a portion of the double-layer structure in which a zinc primer paint film and an epoxy paint film are formed, a portion where only an epoxy paint film is formed, and a portion where the steel is bared. In any condition, steel having exeellent corrosión resistance is required to improve corrosión resistance of the ship.

[0015] Thus, an object of the invention is to provide anti-corrosion steel for ship at low cost, which exhibits exeellent corrosión resistance without depending on a surface condition of steel even under a severe corrosión environment such as a ballast tank of a ship, whereby a period before repair painting can be lengthened, consequently operation load of the repair painting can be reduced.

[0016] The inventors made earnest study for developing the Steel that exhibits exeellent corrosión resistance without depending on the surface condition of the Steel even under the severe corrosión environment caused by seawater. As a result, the inventors found that W and Cr are contained as essential elements, in addition, elemente that improve corrosión resistance such as Sb and Sn are contained in an appropriate range, thereby the Steel that exhibits excellent corrosión resistance can be obtained in any of the conditions of the double-layer structure of a zinc primer paint film and an epoxy paint film, only an epoxy paint film, and bare Steel, and consequently they completed the invention.

[0017] That is, the invention ineludes anti-corrosion Steel for ship containing C of 0.03 to 0.25 mass%, Si of 0.05 to 0.50 mass%, Mn of 0.1 to 2.0 mass%, P of 0.025 mass% or less, S of 0.01 mass% or less, Al of 0.005 to 0.10 mass%, W of 0.01 to 1.0 mass%, Cr of 0.01 mass% or more and less than 0.20 mass%, N of 0.001 to 0.008 mass%, and the remainder including Fe and inevitable impurities.

[0018] The Steel of the invention is characterized by containing a component in at least one group between the following groups A and B, in addition to the above composition.

[0019] Group A; one or two selected from Sb of 0. 001 to 0. 3 mass% and Sn of 0.001 to 0.3 mass%.

[0020] Group B; one or at least two selected from Ni of 0.005 to 0.25 mass%, Mo of 0.01 to 0.5 mass%, and Co of 0.01 to 1.0 mass.

[0021] Moreover, the Steel of the invention is characterized by containing a component in at least one group among the following groups C to E, in addition to the above composition.

[0022] Group C; one or at least two selected from M) of 0.001 to 0.1 mass%, Ti of 0.001 to 0. 1 mass%, Zr of 0. 001 to 0.1 mass%, and V of 0.002 to 0.2 mass%.

[0023] Group D; B of 0.0002 to 0.003 mass%.

[0024] Group E; one or at least two selected from Ca of 0.0002 to 0.01 mass%, REM of 0.0002 to 0.015 mass%, and Y of 0.0001 to 0.1 mass%.

[0025] Moreover, Steel of the invention is characterized in that an epoxy paint film or a zinc primer paint film is formed on a surface of the Steel, or both the zinc primer paint film and the epoxy paint film are formed thereon.

[0026] According to the invention, Steel can be provided, the Steel having excellent corrosión resistance even under a severe corrosión environment caused by seawater, which significantly contributes to lengthening of a period before repair painting, and reduction in operation load of repair painting.

Best Mode for Carrying Out the Invention [0027] The inventors conducted the following experiment to develop Steel having excellent corrosión resistance in any of the three portions that may exist on a surface of Steel of a ship in Service, that is, a portion of the double-layer structure of a zinc primer paint film and an epoxy paint film, a portion of only an epoxy paint film, and a bare Steel portion.

[0028] Steel being added with various alloy elements were ingoted, then the ingots were hot-rolled into hot-rolled plates 5 mm in thickness, and then test pieces 5 mm thick, 100 mm wide, and 200 mm long, and test pieces 5 mm thick, 50 mm wide, and 150 mm long were taken from the hot-rolled plates. Then, a surface of each of the test pieces was subjected to shot blasting to remove scales or oil from the surface, and then the test pieces were subjected to the following three types of surface treatment, so that exposure test pieces were prepared.

[0029] Condition A: A double-layer film of a zinc primer film (about 15 pm) and a tar epoxy resin paint film (about 100 pm) is formed on a surface of a test piece.

[0030] Condition B: Asingle-layer film of a tar epoxy resin paint film (about 100 pm) is formed on a surface of a test piece.

[0031] Condition C: A surface of a test piece is subjected to shot blasting and remains bare (with no anti-corrosion film).

[0032] Then, the test pieces were subjected to a salt-spray alternate-drying-and-wetting corrosión test, which simulates a corrosión environment corresponding to a back of an upper deck of a ballast tank of an actual ship, based on a condition that an exposure test was performed by 132 eyeles, each eyele including spraying of 5% NaCI solution at 35°C for 2 hr, leaving to stand at 60°C and RH25% for 4 hr, and leaving to stand at 50°C and RH95% for 2 hr, thereby the test pieces were evaluated in corrosión resistance. For the test pieces of the conditions A and B, each test piece having a paint film, corrosión resistance was evaluated in a way that a scratch in 80 mm in length, which reached a surface of base Steel, was formed in a straight fine by a box cutter through the paint film before the test, and area of a swollen paint film generated around the scratch was measured for evaluation after the test. For the test pieces having no paint film of the condition C, corrosión resistance was evaluated in a way that the test pieces were derusted after the test, and the average amount of decrease in thickness was calculated for evaluation from the amount of change in weight (amount of decrease) between the derusted test piece and a test piece before the test.

[0033] By summarizing results of the corrosión test, effects of respective alloy elemente on corrosión resistance are collected in Table 1. Table 1

(Effect on corrosión resistance) [0034] 0; no effect 1 to 2; some effect 4; large effect 5 to 6; significantly large effect To briefly describe the results, 1. 1) In the case of the condition A (double-layer paint film of the zinc primer paint film and the tar epoxy resin paint film), the most effective element for improving corrosión resistance is Cr, followed by W, and tbllowed by Sb. 2. 2) In the case of the condition B (only the tar epoxy resin paint film), the most effective element for improving corrosión resistance is W, followed by Sb, and followed by Sn. 3. 3) In the case of the condition C (bare Steel), the most effective element for improving corrosión resistance is W, followed by Sb, and followed by Sn. 4. 4) When W and Cr are compositely contained, corrosión resistance at the condition A is improved compared with a case that each is singly contained, and furthermore, when Sb and Sn are contained in addition to them, a significant effect is exhibited in any of the conditions A, B and C. 5. 5) Mo slightly improves corrosión resistance in any of the conditions A, B and C, and Ni and Co slightly improve corrosión resistance in the conditions A and C.

[0035] Based on the test results, the invention was designed to use a componential System in which W and Cr were compositely contained as essential elemente for improving corrosión resistance. Furthermore, the invention was designed in a way that when more excellent corrosión resistance was required, a component design was used, in which one or two selected from Sb and Sn were contained in addition to W and Cr. Moreover, the invention was designed in a way that when further more excellent corrosión resistance was required, one or two or more selected from Ni, Mo and Co were additionally contained.

[0036] Next, a composition that the anti-corrosion Steel for ship of the invention must have is specifically described. C: 0.03 to 0.25 mass% [0037] C is an effective element for increasing strength of Steel, and needs to be contained by 0. 03 mass% or more to obtain a desired strength in the invention. On the other hand, when C is contained by more than 0.25 mass%, toughness of HAZ (weld heat-affected zone) is reduced. Accordingly, C is contained in a range of 0.03 to 0. 25 mass%. From a viewpoint of naturally obtaining certain strength together with certain toughness by rolling, C is preferably contained in a range of 0.05 to 0.20 mass%.

Si: 0.05 to 0.50 mass% [0038] Si is an element to be added as a deoxidizer, or added for increasing strength of Steel, and contained by 0.05 mass% or more in the invention. However, when Si is added by more than 0.50 mass%, toughness of Steel is degraded, therefore an upper limit of Si is specified to be 0.50 mass%.

Mn: 0.1 to 2.0 mass% [0039] Mn is an element having an effect of preventing hot shortness, and increasing strength of Steel, and added by 0.1 mass% or more. However, when Mn is added by more than 2.0 mass%, toughness of Steel and weldability are reduced, therefore Mn is contained to be 2.0 mass% or less. Preferably, Mn is contained in a range of 0.5 to 1.6 mass%. P: 0.025 mass%or less [0040] P is a harmful element that may degrade toughness of Steel as mother material, and furthermore degrade weldability and toughness of a weld, and therefore P is preferably reduced to the utmost. In particular, when the content of P exceeds 0. 025 mass%, toughness of mother material and toughness of a weld are more significantly reduced. Accordingly, P is contained to be 0.025 mass% or less. Preferably, P is contained to be 0.014 mass% or less. S: 0.01 mass%or less [0041] Since S is a harmful element that may degrade toughness of Steel and weldability, S is preferably reduced to the utmost, and contained to be 0.01 mass% or less in the invention.

Al: 0.005 to 0.10 mass% [0042] Al is an element to be added as a deoxidizer, and added by 0.005 mass% or more. However, when Al is contained by more than 0.10 mass%, Al3+ eluted due to corrosión of base Steel reduces pH of a surface of the base Steel, leading to degradation in corrosión resistance, therefore an upper limit of Al is specified to be 0.10 mass%. W: 0.01 to 1.0 mass% [0043] W improves corrosión resistance under presence of both the zinc primer paint film and the epoxy paint film, and significantly improves corrosión resistance under presence of only the epoxy paint film. Moreover, W significantly improves corrosión resistance of Steel even if the Steel is bare. Therefore, W is one of the most important elemente for improving corrosión resistance in the Steel of the invention. The effect is exhibited in W content of 0.01 mass% or more. However, when W content is more than 1.0 mass%, the effect is saturated. Accordingly, the content of W is in a range of 0.01 to 1.0 mass%.

[0044] The reason why W has the effect of improving corrosión resistance is because as a Steel píate is corroded, WO42' is produced in produced rust, and presence of the WO42· prevente chloride ions from entering a Steel píate surface, and furthermore sparingly-soluble -FeW04 is produced in a región having decreased pH such as an anode area of the Steel píate surface, and presence of the FeWC>4 also prevente chloride ions from entering the Steel píate surface. The chloride ions are thus prevented from entering the Steel píate surface, thereby corrosión of the Steel píate is effectively inhibited. Moreover, inhibitor operation of WO42· also inhibits corrosión of Steel.

Cr: 0.01 mass%or more and less than 0.20 mass% [0045] Since Cr exhibits excellent corrosión resistance under presence of both the zinc primer paint film and the epoxy paint film, Cr is one of the important elemente in the Steel of the invention. It is presumed that under presence of zinc primer, Zn in the zinc primer is eluted, so that Zn-based corrosión products such as ZnO and ZnCÍ2*4Zn(OH)2 are produced, and Cr acts on the Zn-based corrosión products so as to further improve corrosión prevention of base Steel given by the Zn-based corrosión products. Such a corrosión resistance ¡mprovement effect of Cr under presence of the zinc primer is exhibited at the content of Cr of 0.01 mass% or more. However, when Cr is contained by 0.20 mass% or more, toughness of a weld is degraded. Accordingly, the content of Cr is in a range of 0.01 mass% or more and less than 0.20 mass%.

[0046] N is a harmful component for toughness, and desirably reduced to the utmost in order to achieve improvement in toughness. However, N is industrially hard to be decreased to less than 0.001 mass%. Conversely, when N is contained by 0.008 mass% or more, toughness is significantly degraded. Accordingly, the content of N is in a range of 0.001 to 0.008 mass% in the invention.

[0047] The Steel of the invention may contain the following componente in addition to the above componente for the purpose of further improving corrosión resistance.

One or two of 0.001 to 0.3 mass%Sb and 0.001 to 0.3 mass%Sn [0048] Sb has an effect of improving corrosión resistance under presence of both the zinc primer paint film and the epoxy paint film, under presence of only the epoxy paint film, and in a condition of bare Steel. Sn has an effect of improving corrosión resistance under presence of only the epoxy paint film, and in the condition of bare Steel. The reason for the effects of Sb and Sn is considerad to be that corrosión is inhibited in a región having decreased pH such as an anode area of the Steel píate surface. While the effects are exhibited when either of Sn and Sb is contained by 0.001 mass% or more, in the case that the content is more than 0.3 mass%, toughness of each of mother material and HAZ is degraded, therefore each of Sn and Sb is preferably contained in a range of 0.001 to 0.3 mass%.

One or at least two of 0.005 to 0.25 mass%Ni, 0. 01 to 0.5 mass%Mo, and 0.01 to 1.0 mass%Co [0049] Ni, Mo and Co slightly improve corrosión resistance under presence of both the zinc primer paint film and the epoxy paint film, and in the condition of bare Steel. Furthermore, Mo slightly improves corrosión resistance even under presence of only the epoxy paint film. Therefore, when corrosión resistance is desired to be further improved, the elemente may be supplementarily contained. The reason for the effects of Ni, Mo and Co is considerad to be that they act to refine rust partióles, in addition, Mo produces M0O42· in rust and thus prevente chloride ions from entering a Steel píate surface. The effects are exhibited in the Ni content of 0.005 mass% or more, Mo content of 0.01 mass% or more, and Co content of 0.01 mass% or more respectively.

[0050] However, even if Ni of more than 0.25 mass%, Mo of more than 0.5 mass%, and Co of more than 1.0 mass% are added respectively, the respective effects are saturated, which is economically disadvantageous. Accordingly, Ni, Mo and Co are preferably contained in the above range respectively.

[0051] Furthermore, the Steel of the invention may contain the following components ¡n addition to the above components for ¡ncreaslng strength of Steel and/or improving toughness. One or at least two of 0.001 to 0.1 mass% Nb, 0.001 to 0.1 mass% Ti, 0.001 to 0.1 mass% Zr, and 0.002 to 0.2 mass% V

[0052] Any of Nb, Ti, Zr and V is an element that improves strength of Steel, and can be selectively contained depending on required strength. To obtain such an effect, preferably, each of Nb, Ti and Zr is contained by 0.001 mass% or more, and V is contained by 0.002 mass% or more. However, when Nb, Ti or Zr is added by more than 0.1 mass%, and when V is added by more than 0.2 mass%, toughness is reduced, therefore each of Nb, Ti, Zr and V is preferably added with each of the above valúes being specified as an upper limit valué. B: 0.0002 to 0.003 mass%

[0053] B is an element that improves strength of steel, and can be contained as needed. To obtain the effect, B is preferably contained by 0.0002 mass% or more. However, when B is added by more than 0.003 mass%, toughness is degraded. Accordingly, B is preferably contained in a range of 0.0002 to 0.003 mass%. One or at least two of 0.0002 to 0.01 mass% Ca, 0.0002 to 0. 015 mass% REM, and 0.0001 to0.1 mass%Y

[0054] Any of Ca, REM and Y is an element having an effect of improving toughness of a weld heat-affected zone, and can be selectively contained as specified above. The Steel of the invention preferably contains Fe and inevitable impurities as components other than the above.

[0055] Next, a preferable method of manufacturing the anti-corrosion steel according to the invention is described.

[0056] Preferably, molten Steel having the composition is produced by a typically known method such as a converter or an electric furnace, then formed into a Steel material such as a slab or billet by a typically known method such as a continuous casting method or an ingot making method. It is appreciated that the molten Steel may be additionally subjected to treatment such as ladle metallurgy or vacuum degassing.

[0057] Next, the Steel material is preferably heated to a temperature of 1050 to 1250°C, then hot-rolled into desired size and shape. Alternatively, when temperature of the Steel material is high in a level at which the Steel material can be hot-rolled, the Steel material is preferably directly hot-rolled into Steel having the desired size and shape without being heated, or with being merely soaked.

[0058] In hot rolling, to secure strength, hot-finish-rolling finishing temperature and a cooling rate after hot finish rolling are preferably appropriately adjusted, wherein the hot-finish-rolling finishing temperature is preferably 700°C or more, and cooling after hot finish rolling is finished is preferably performed by air cooling or accelerated cooling at a cooling rate of 100 °C/s or less. Reheating may be performed after cooling.

Example [0059] Steel having a composition shown in Table 2 was produced by a vacuum melting furnace or a converter, then slabs were loaded into a heating furnace and heated to 1150°C, and then hot-rolled into thick Steel plates 25 mm in thickness, and then the Steel plates obtained in such a way were examined in tensile and impact properties of mother material. Moreover, a heat cycle corresponding to submerge welding with input heat quantity of 150 kJ/cm was applied to the Steel plates to simúlate HAZ portions, and the simulated HAZ portions were provided for evaluation of an impact property (simulated-HAZ impact property).

[0060] Next, test pieces 5 mm thick, 100 mm wide, and 200 mm long, and test pieces 5 mm thick, 50 mm wide, and 150 mm long were taken from the respective thick Steel plates, then a surface of each of the test pieces was subjected to shot blasting, and then the test pieces were subjected to surface treatment at the following conditions A to C, so that exposure test pieces were prepared.

[0061] Condition A: Adouble-layer film of a zinc primer film (about 15 pm) and a tar epoxy resin paint film (about 200 pm) was formed on a surface of a test piece. Condition B: A single-layer film of a tar epoxy resin paint film (about 200 pm) was formed on a surface of a test piece. Condition C: A surface of a test piece was subjected to shot blasting and remained bare (with no anti-corrosion film).

[0062] The test pieces of the conditions A and B, each test piece having a paint film, were provided with a scratch in 80 mm in length in a straight line, which reached a surface of base Steel, by a box cutter through the paint film.

[0063] Then, the test pieces were attached to a back of an upper deck of a ballast tank of an actual ship so as to be provided for an exposure test. A period of the exposure test was three years, and corrosión environment of the ballast tank was set as follows: about 20 days as a period in which seawater was filled in the ballast tank, and about 20 days as a period in which seawater was not filled therein were combined as one cycle, and the cycle was repeated. For the test pieces of the conditions A and B, each test piece having the paint film, corrosión resistance in the exposure test was evaluated in a way that area of a swollen paint film generated around the scratch was measured for evaluation. For the test pieces of the condition C having no paint film, corrosión resistance was evaluated in a way that the test pieces were derusted after the test, and the average amount of decrease in thickness was calculated from the amount of change in weight (amount of decrease) between the derusted test piece and a test piece before the test, and then assuming that No. 21 Steel without containing any particular element that improves corrosión resistance was base Steel (100), a ratio of a valué of each test piece to a valué of the base Steel was calculated for evaluation.

[0064] Table 3 shows results of tensile and impact tests, and Table 4 shows results of exposure for two years and exposure for three years. From the result of the exposure for three years in Table 4, it is known that Steel of each of Nos. 1 to 20 as inventive examples, which satisfy the composition of the invention, is 50% or less in swollen-paint-film area and thickness-decrease-amount with respect to the base Steel (No. 21) in any of the test pieces at the conditions A to C, and therefore has excellent corrosión resistance. In the Steel of No. 20, while the ratio to the base Steel is 73% at the condition of both the zinc primer paint film and the tar epoxy paint film in a result of the exposure for two years, the ratio is 42% in a result of the exposure for three years, showing the anti-corrosion effect of W and Cr being exhibited.

[0065] On the contrary, in Steel of each of Nos. 22 to 24 that do not satisfy the composition of the invention, even if corrosión resistance is improved compared with the base Steel (No. 21), the ratio to the base Steel is more than 50% in some condition. In No. 26, since Al content exceeds the upper limit valué, corrosión resistance is degraded in all the conditions. In Steel of each of Nos. 25 and 27, while a ratio of corrosión resistance to the base Steel is 50% or less, an impact property of a weld is significantly degraded.

Industrial Applicability [0066] The anti-corrosion Steel for ship of the invention has excellent corrosión resistance under corrosión environment due to seawater, therefore the Steel can be used for a ballast tank of a ship, in addition, can be applied to other uses where the Steel is used in similar corrosión environment.

Table 2

REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disdaims all liability in this regard.

Patent documente cited in the description • JP4805Q921A [0012] • JP48050922A Γ00121 • JP48050924A Γ00121 • JP7034197Af00121 • JP7034196A Γ0012i • JP7034270A fñt)123 • JF73-!0141Ar0012f • JP20022660S2A f01)121 • JP2000017381A ΓΟΟΙ Sf • JP2004204344A Γ0Ω12Ί

Claims

1. Anti-corrosion figures for ships consisting of: 0.03 to 0.25 mass% C, 0.05 to 0.50 mass% Si, 0.1 to 2.0 mass% Mn, 0.025 mass -% or less P, 0.01 mass% or less S, 0.005 to 0.10 mass% Al, 0.01 to 1.0 mass% W, for 0.01 mass% or more and less than 0.20 mass% Cr, 0.001 to 0.008 mass% N and wherein the residue, including Fe and inevitable impurities, further optionally contains: One or two selected from 0.001 to 0.3 mass% Sb and 0.001 to 0.3 mass% Sn in addition to the composition, and / or one or at least two selected from 0.005 to 0.25 mass% Ni, 0.01 to 0.5 mass% Mo and 0.01 to 1.0 mass -% Co in addition to the composition and / or one or at least two selected from 0.001 to 0.1 mass% Nb, 0.001 to 0.1 mass% Ti, for 0.001 to 0.1 mass% Zr and 0.002 to 0 , 2 mass% V in addition to the composition and / or 0.0002 to 0.003 mass% B in addition to the composition and / or one or at least two selected from 0.0002 to 0.01 mass% Ca, 0.0002 to 0.015 mass% REM and 0.0001 to 0.1 mass% Y in addition to the composition.

2. Anti-corrosion steel for ships according to claim 1, characterized in that: An epoxy paint film is formed on a surface of the steel.

Ship anti-corrosion steel according to claim 1 or 2, characterized in that: A zinc primer film is formed on a surface of the steel.