Classifications

• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B63—SHIPS OR OTHER WATERBORNE VESSELS; RELATED EQUIPMENT
  • B63B—SHIPS OR OTHER WATERBORNE VESSELS; EQUIPMENT FOR SHIPPING 
  • B63B3/00—Hulls characterised by their structure or component parts
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
  • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
  • C21D7/06—Modifying the physical properties of iron or steel by deformation by cold working of the surface by shot-peening or the like
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
• • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • 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
• • 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
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
• • 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
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling

Definitions

• 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 pure 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 corrosion environment with seawater.
• the anti-corrosion steel for ship described in the invention includes a thick steel plate, sheet steel, shape sheet, and bar steel.
• 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 corrosion environment. Therefore, an anti-corrosion paint film using epoxy paint is formed, in addition, cathodic protection is applied for preventing corrosion of steel used for the ballast tank.
• Life of an anti-corrosion paint film of a ballast tank is said to be typically about 10 years under such a drastic corrosion resistance, that is, half the life (20 years) of.a ship. It is actual situation that corrosion resistance is kept by performing repair painting in the remaining ten years.
• the ballast tank is in the severe corrosion 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.
• the repair painting is performed as operation in a narrow space, operation environment is not preferable..
• it is desirable to develop a steel material having an excellent corrosion 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.
• 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 elements that improves corrosion 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.
• 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%.
• 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 corrosion resistance, is coated with anti-corrosion paint such as tar epoxy paint, pure 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 corrosion 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.
• 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 corrosion 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 corrosion resistance, is used as a componential material, and oxygen gas concentration within a ballast tank has a value in a ratio of 0.5 or less with respect to a value of oxygen gas concentration in air.
• 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 corrosion 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.
• 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 corrosion 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 in which steel containing 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 corrosion or local corrosion that may occur in a crude oil tank is inhibited.
• each of the patent documents 1 to 3 does not make adequate investigation on corrosion resistance under presence of a paint film of Zinc-primer or epoxy paint being typically coated on steel configuring a ballast tank or the like. Therefore, further investigation is necessary for improving corrosion resistance under presence of the paint film.
• 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 corrosion 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.
• 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.
• the steel described in the patent document 10 is anti-corrosion steel used in the crude oil tank, namely, used under an environment having H 2 S, and therefore the steel is unclear in corrosion resistance in the ballast tank having no H 2 S.
• corrosion 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 corrosion resistance is necessary to be used for a ballast tank.
• a ship is built by welding steel materials such as a thick steel plate, 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 is coated as secondary painting (main painting) after subassembly or main assembly is performed. Therefore, the major part of steel surface of a ship has a double-layer structure thereon, which includes a zinc primer paint film and an epoxy paint film.
• zinc primer is repainted on the weld as touch up paint for rust prevention in a period after welding and before main painting.
• an object of the invention is to provide anti-corrosion steel for ship at low cost, which exhibits excellent corrosion resistance without depending on a surface condition of steel even under a severe corrosion 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.
• the inventors made earnest study for developing the steel that exhibits excellent corrosion resistance without depending on the surface condition of the steel even under the severe corrosion environment caused by seawater.
• W and Cr are contained as essential elements, in addition, elements that improve corrosion resistance such as Sb and Sn are contained in an appropriate range, thereby the steel that exhibits excellent corrosion 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.
• the invention includes 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.
• 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.
• Group A one or two selected from Sb of 0. 001 to 0. 3 mass% and Sn of 0.001 to 0.3 mass%.
• 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.
• 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.
• Group C one or at least two selected from Nb 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%.
• 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%.
• 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.
• steel can be provided, the steel having excellent corrosion resistance even under a severe corrosion environment caused by seawater, which significantly contributes to lengthening of a period before repair painting, and reduction in operation load of repair painting.
• the inventors conducted the following experiment to develop steel having excellent corrosion 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.
• 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.
• a double-layer film of a zinc primer film (about 15 ⁇ m) and a tar epoxy resin paint film (about 100 ⁇ m) is formed on a surface of a test piece.
• Condition B A single-layer film of a tar epoxy resin paint film (about 100 ⁇ m) is formed on a surface of a test piece.
• Condition C A surface of a test piece is subjected to shot blasting and remains bare (with no anti-corrosion film).
• test pieces were subjected to a salt-spray alternate-drying-and-wetting corrosion test, which simulates a corrosion 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 cycles, each cycle including spraying of 5% NaCl 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 corrosion resistance.
• a salt-spray alternate-drying-and-wetting corrosion test which simulates a corrosion 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 cycles, each cycle including spraying of 5% NaCl 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
• each test piece having a paint film corrosion 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 line 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.
• corrosion 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.
• the invention was designed to use a componential system in which W and Cr were compositely contained as essential elements for improving corrosion resistance. Furthermore, the invention was designed in a way that when more excellent corrosion 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 corrosion resistance was required, one or two or more selected from Ni, Mo and Co were additionally contained.
• 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.
• toughness of HAZ welding heat-affected zone
• C is contained in a range of 0.03 to 0. 25 mass%.
• C is preferably contained in a range of 0.05 to 0.20 mass%.
• 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 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 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.
• P is contained to be 0.025 mass% or less.
• P is contained to be 0.014 mass% or less.
• 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 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%, Al 3+ eluted due to corrosion of base steel reduces pH of a surface of the base steel, leading to degradation in corrosion resistance, therefore an upper limit of Al is specified to be 0.10 mass%.
• W improves corrosion resistance under presence of both the zinc primer paint film and the epoxy paint film, and significantly improves corrosion resistance under presence of only the epoxy paint film. Moreover, W significantly improves corrosion resistance of steel even if the steel is bare. Therefore, W is one of the most important elements for improving corrosion 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%.
• W has the effect of improving corrosion resistance
• WO 4 2- is produced in produced rust
• presence of the WO 4 2- prevents chloride ions from entering a steel plate surface
• furthermore sparingly-soluble -FeWO 4 is produced in a region having decreased pH such as an anode area of the steel plate surface
• presence of the FeWO 4 also prevents chloride ions from entering the steel plate surface.
• the chloride ions are thus prevented from entering the steel plate surface, thereby corrosion of the steel plate is effectively inhibited.
• inhibitor operation of WO 4 2- also inhibits corrosion of steel.
• Cr Since Cr exhibits excellent corrosion resistance under presence of both the zinc primer paint film and the epoxy paint film, Cr is one of the important elements 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 corrosion products such as ZnO and ZnCl 2 •4Zn(OH) 2 are produced, and Cr acts on the Zn-based corrosion products so as to further improve corrosion prevention of base steel given by the Zn-based corrosion products. Such a corrosion resistance improvement 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%.
• N is a harmful component for toughness, and desirably reduced to the utmost in order to achieve improvement in toughness.
• N is industrially hard to be decreased to less than 0.001 mass%.
• toughness is significantly degraded. Accordingly, the content of N is in a range of 0.001 to 0.008 mass% in the invention.
• the steel of the invention may contain the following components in addition to the above components for the purpose of further improving corrosion resistance.
• Sb has an effect of improving corrosion 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 corrosion 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 considered to be that corrosion is inhibited in a region having decreased pH such as an anode area of the steel plate 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%.
• Ni, Mo and Co slightly improve corrosion 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 corrosion resistance even under presence of only the epoxy paint film. Therefore, when corrosion resistance is desired to be further improved, the elements may be supplementarily contained.
• the reason for the effects of Ni, Mo and Co is considered to be that they act to refine rust particles, in addition, Mo produces MoO 4 2- in rust and thus prevents chloride ions from entering a steel plate 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.
• Ni, Mo and Co are preferably contained in the above range respectively.
• the steel of the invention may contain the following components in addition to the above components for increasing strength of steel and/or improving toughness.
• Nb, Ti, Zr and V is an element that improves strength of steel, and can be selectively contained depending on required strength.
• each of Nb, Ti and Zr is contained by 0.001 mass% or more, and V is contained by 0.002 mass% or more.
• 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 values being specified as an upper limit value.
• 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 to 0.1 mass% Y
• 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.
• 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.
• the steel material is preferably heated to a temperature of 1050 to 1250°C, then hot-rolled into desired size and shape.
• the steel material is preferably directly hot-rolled into steel having the desired size and shape without being heated, or with being merely soaked.
• 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.
• 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 simulate HAZ portions, and the simulated HAZ portions were provided for evaluation of an impact property (simulated-HAZ impact property).
• 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.
• Condition A A double-layer film of a zinc primer film (about 15 ⁇ m) and a tar epoxy resin paint film (about 200 ⁇ m) was formed on a surface of a test piece.
• Condition B A single-layer film of a tar epoxy resin paint film (about 200 ⁇ m) 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).
• 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.
• 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 corrosion 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.
• corrosion 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.
• Table 3 shows results of tensile and impact tests
• 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 corrosion resistance.
• 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.
• the anti-corrosion steel for ship of the invention has excellent corrosion resistance under corrosion 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 corrosion environment.

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• 2007
  • 2007-01-12 EP EP07707040.7A patent/EP1990437B1/en not_active Not-in-force
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