WO2011145739A1 - 耐食性に優れる溶接継手および原油タンク - Google Patents
耐食性に優れる溶接継手および原油タンク Download PDFInfo
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- WO2011145739A1 WO2011145739A1 PCT/JP2011/061698 JP2011061698W WO2011145739A1 WO 2011145739 A1 WO2011145739 A1 WO 2011145739A1 JP 2011061698 W JP2011061698 W JP 2011061698W WO 2011145739 A1 WO2011145739 A1 WO 2011145739A1
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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
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
- B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
- B23K35/24—Selection of soldering or welding materials proper
- B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
- B23K35/22—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by the composition or nature of the material
- B23K35/24—Selection of soldering or welding materials proper
- B23K35/30—Selection of soldering or welding materials proper with the principal constituent melting at less than 1550 degrees C
- B23K35/3053—Fe as the principal constituent
- B23K35/3073—Fe as the principal constituent with Mn as next major constituent
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K9/00—Arc welding or cutting
- B23K9/0026—Arc welding or cutting specially adapted for particular articles or work
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K9/00—Arc welding or cutting
- B23K9/02—Seam welding; Backing means; Inserts
- B23K9/025—Seam welding; Backing means; Inserts for rectilinear seams
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K9/00—Arc welding or cutting
- B23K9/18—Submerged-arc welding
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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
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous 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
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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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
-
- 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
-
- 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
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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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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2101/00—Articles made by soldering, welding or cutting
- B23K2101/04—Tubular or hollow articles
- B23K2101/12—Vessels
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
- B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
- B23K2103/00—Materials to be soldered, welded or cut
- B23K2103/02—Iron or ferrous alloys
- B23K2103/04—Steel or steel alloys
Definitions
- the present invention relates to a crude oil tanker oil tank (oil tank) formed by welding steel materials or a tank for transporting or storing crude oil (hereinafter referred to as a crude oil tank).
- a crude oil tank formed by welding steel materials or a tank for transporting or storing crude oil (hereinafter referred to as a crude oil tank).
- the steel materials used in the crude oil tank of the present invention include thick steel plate, thin steel sheet, and shaped steel.
- the steel used for the bottom plate of a tanker's crude oil tank, etc. has the effect of inhibiting the corrosion of the crude oil itself and the corrosion of a protective coating (oil coating) derived from the crude oil formed on the inner surface of the crude oil tank. It was thought that corrosion did not occur due to the suppression action (corrosion suppression action). However, recent studies have revealed that bowl-shaped local corrosion (pitting corrosion) occurs in the steel plate of the tank bottom plate.
- the most effective method for preventing the above-described general corrosion and local corrosion is to apply a heavy coating to the surface of the steel material to shield the steel material from the corrosive environment.
- the painting operation of crude oil tanks requires an enormous area to be applied, and the paint film must be repainted once every 10 years due to the deterioration of the coating film. To do.
- corrosion is promoted on the damaged part of the heavy-painted coating film in the corrosive environment of the crude oil tank.
- Patent Document 1 For the corrosion problem as described above, several techniques for improving the corrosion resistance of the steel material itself in the corrosive environment of the crude oil tank have been proposed. For example, in Patent Document 1, in mass%, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, P: 0.03% or less, S: 0.02% or less, Cu: 0.01 to 1.5%, Al: 0.001 to 0.3%, N: 0.001 to 0.01%, and Mo: 0.001%.
- Patent Document 2 in mass%, C: 0.001 to 0.2%, Si: 0.01 to 2.5%, Mn: 0.1 to 2%, P: 0.03% or less , S: 0.02% or less, Cu: 0.01 to 1.5%, Al: 0.001 to 0.3%, N: 0.001 to 0.01%, and Mo: 0
- C 0.001 to 0.2%
- Si 0.01 to 2.5%
- Mn 0.1 to 2%
- P 0.03% or less
- S 0.02% or less
- Cu 0.01 to 1.5%
- Al 0.001 to 0.3%
- N 0.001 to 0.01%
- Mo 0
- Patent Document 1 and Patent Document 2 described above contain Cu as an essential element for improving corrosion resistance
- the addition of Cu causes surface cracks during hot rolling, so that production stability is improved. Have the problem of harming.
- an object of the present invention is to provide general corrosion resistance in a crude oil tank such as a tanker oil tank formed by welding a steel material having excellent manufacturability without causing problems such as cracking during hot rolling.
- Another object of the present invention is to provide a welded joint having excellent local corrosion resistance and a crude oil tank having the welded joint.
- the inventors have intensively studied to solve the above problems. As a result, when forming a crude oil tank by welding steel materials with improved corrosion resistance by controlling the component composition of steel, the content of Cu, Mo and W contained in the weld metal of the welded joint is within the proper range. It has been found that by controlling to the above, it is possible to remarkably reduce the general corrosion and local corrosion generated in the welded joint of the crude oil tank, and the present invention has been completed.
- the steel material used for the welded joint of the present invention further contains at least one component of the following groups A to D.
- Group A Ni: 0.005 to 0.4 mass% and Co: 0.01 to 0.4 mass% or one selected from Group B; Nb: 0.001 to 0.1 mass%, Ti : 0.001 to 0.1 mass%, Zr: 0.001 to 0.1 mass%, and V: 0.002 to 0.2 mass%, one or more C groups; Ca: 0.0002 ⁇ 0.01 mass%, REM: 0.0002 to 0.015 mass%, and Y: 0.0001 to 0.1 mass%, one or more D groups; B: 0.0002 to 0.003 mass %
- the present invention is a crude oil tank having the above-mentioned weld joint.
- the overall corrosion and local corrosion generated in a crude oil tank formed by welding such as an oil tank of a crude oil tanker or a tank for transporting or storing crude oil, can be observed in all parts including not only steel plates but also welded joints. Since it can be suppressed, it has a remarkable industrial effect.
- Example of this invention it is a figure explaining the test apparatus used for the general corrosion test. In the Example of this invention, it is a figure explaining the test apparatus used for the pitting corrosion test. It is a figure which shows the influence (a baseplate and a top plate) of the weld metal composition on the corrosion resistance of the corrosion-resistant steel welded joint for tankers.
- C 0.03-0.16 mass%
- C is an element that enhances the strength of steel.
- 0.03 mass% or more is added in order to secure a desired strength.
- addition exceeding 0.16 mass% lowers the weldability and the toughness of the welded heat affected zone. Therefore, C is set to a range of 0.03 to 0.16 mass%. A range of 0.06 to 0.16 mass% is more preferable.
- Si 0.05-1.50 mass%
- Si is an element added as a deoxidizer, but is also an effective element for increasing the strength of steel. Therefore, in the present invention, 0.05 mass% or more is added in order to ensure a desired strength. However, addition exceeding 1.50 mass% reduces the toughness of steel. Therefore, Si is set to a range of 0.05 to 1.50 mass%. A range of 0.15 to 0.50 mass% is more preferable.
- Mn 0.1 to 2.0 mass%
- Mn is an element that increases the strength of steel, and in the present invention, 0.1 mass% or more is added in order to obtain a desired strength. On the other hand, addition exceeding 2.0 mass% reduces the toughness and weldability of steel. Therefore, Mn is in the range of 0.1 to 2.0 mass%. From the viewpoint of maintaining high strength and suppressing the formation of inclusions that deteriorate the corrosion resistance, a range of 0.5 to 1.6 mass% is preferable, and a range of 0.7 to 1.4 mass% is more preferable. preferable.
- P 0.025 mass% or less
- P is a harmful element that segregates at the grain boundaries and lowers the toughness of the steel, and is desirably reduced as much as possible. In particular, if added over 0.025 mass%, the toughness is greatly reduced. Moreover, when P is added exceeding 0.025 mass%, it will also have a bad influence on the corrosion resistance in a tank oil tank. Therefore, P is set to 0.025 mass% or less. Preferably it is 0.015 mass% or less.
- S 0.010 mass% or less
- S is a harmful element that forms MnS, which is a non-metal inclusion, and becomes a starting point of local corrosion, and reduces local corrosion resistance. Is desirable. In particular, addition exceeding 0.010 mass% leads to a significant decrease in local corrosion resistance. Therefore, the upper limit of S is 0.010 mass%. Preferably, it is 0.005 mass% or less.
- Al 0.005 to 0.10 mass%
- Al is an element added as a deoxidizer, and 0.005 mass% or more is added in the present invention. However, if added in excess of 0.10 mass%, the toughness of the steel decreases, so the upper limit of Al is 0.10 mass%. Preferably, it is in the range of 0.01 to 0.05 mass%. A range of 0.02 to 0.04 mass% is more preferable.
- N 0.008 mass% or less
- N is a harmful element that lowers toughness, and is desirably reduced as much as possible.
- the toughness is greatly lowered, so the upper limit is made 0.008 mass%.
- it is 0.006 mass% or less, More preferably, it is 0.004 mass% or less.
- the above-mentioned effect is particularly remarkable in a portion that comes into contact with a liquid containing a high concentration of salt separated from a crude oil component, such as a bottom plate portion of a tanker oil tank, and a Zn-containing primer treatment is applied to the steel material in the above-mentioned portion containing Cr.
- a Zn-containing primer treatment is applied to the steel material in the above-mentioned portion containing Cr.
- the corrosion resistance can be remarkably improved as compared with a steel material not containing Cr.
- addition of 0.1 mass% or less is not sufficient, while addition exceeding 0.5 mass% deteriorates the toughness of the weld. Therefore, Cr is in the range of more than 0.1 mass% and less than 0.5 mass%. A range of 0.11 to 0.3 mass% is more preferable. The range of 0.12 to 0.2 mass% is more preferable.
- Cu 0.03-0.4 mass%
- Cu is an element that increases the strength of the steel, and also exists in the rust generated by the corrosion of the steel and has the effect of increasing the corrosion resistance. These effects cannot be sufficiently obtained with addition of less than 0.03 mass%.
- addition over 0.4 mass% saturates the effect of improving corrosion resistance and causes problems such as surface cracking during hot working. . Therefore, from the viewpoint of stably producing the steel material of the present invention, Cu needs to be added in the range of 0.03 to 0.4 mass%.
- the range of 0.008 to 0.15 mass% is preferable from the viewpoint of cost effectiveness. A range of 0.01 to 0.14 mass% is more preferable.
- the steel material of this invention needs to contain 1 type, or 2 or more types chosen from W, Mo, Sn, and Sb other than the said component in the following range.
- W: 0.01 to 1.0 mass% W has the effect of suppressing pitting corrosion at the tanker tank bottom plate and the effect of suppressing overall corrosion of the tanker upper deck. The above effect is manifested by addition of 0.01 mass% or more. However, if it exceeds 1.0 mass%, the effect is saturated. Therefore, W is added in the range of 0.01 to 1.0 mass%. The range is preferably 0.01 to 0.5 mass%, more preferably 0.02 to 0.3 mass%.
- W has the above-described effect of improving corrosion resistance
- WO 4 2 ⁇ is generated in the rust generated as the steel sheet corrodes, and the presence of this WO 4 2 ⁇ causes chloride ions ( chloride ion) can be suppressed from entering the surface of the steel sheet, further, in the site where pH is lowered such anode portion of the steel sheet surface (anode), generated by FeWO 4, the chloride ions by the presence of this FeWO 4 It is because the penetration
- Mo 0.01 to 0.5 mass% Mo not only suppresses pitting corrosion on the bottom plate of the tanker oil tank, but also after coating in a corrosive environment that repeats salt water immersion and high humidity like the ballast tank, as well as the overall corrosion resistance of the tanker backside. There is an effect of improving the corrosion resistance of.
- the effect of Mo is manifested by addition of 0.01 mass% or more, but the effect is saturated when it exceeds 0.5 mass%. Therefore, Mo is in the range of 0.01 to 0.5 mass%. The range is preferably 0.02 to 0.5 mass%, more preferably 0.03 to 0.4 mass%.
- Mo has the above-described effect of improving corrosion resistance is that, like W, MoO 4 2 ⁇ is generated in the rust generated along with corrosion of the steel sheet, and the presence of MoO 4 2 ⁇ causes chloride ions. This is thought to be because the penetration of steel into the steel sheet surface is suppressed.
- Sn: 0.001 to 0.2 mass%, Sb: 0.001 to 0.4 mass% Sn and Sb have the effect of suppressing pitting corrosion on the tanker tank bottom plate and the effect of suppressing the overall corrosion of the tanker upper deck.
- the above effects are manifested by the addition of Sn: 0.001 mass% or more and Sb: 0.001 mass% or more.
- Sn: more than 0.2 mass% and Sb: more than 0.4 mass% are added, the effect is saturated.
- Sn and Sb are preferably added in the above ranges.
- the steel material of this invention contains 1 type or 2 types chosen from Ni and Co other than the said essential component in the following range.
- Ni and Co have the effect of refining the generated rust particles to improve the corrosion resistance in the bare state and the corrosion resistance in the state where the epoxy primer is applied to the zinc primer. . Therefore, these elements are preferably added in an auxiliary manner when it is desired to further improve the corrosion resistance.
- the above effects are manifested by adding Ni: 0.005 mass% or more and Co: 0.01 mass% or more.
- Ni has an effect of suppressing surface cracks during hot working that occur in steel containing Cu or Sn. Therefore, Ni and Co are preferably added within the above ranges.
- the element symbol in the above formula indicates the content (mass%) of the element, and the element not contained is calculated as 0 (zero). It is necessary to contain so that the X value defined by may satisfy 0.5 or less.
- the above equation (1) is an equation for evaluating the influence of each component on the corrosion in the tanker oil tank, and the coefficient of the component that improves the corrosion resistance is expressed as negative, and the coefficient of the component that deteriorates the corrosion resistance is expressed as positive. Yes. Therefore, the steel material having a smaller value of X has better corrosion resistance.
- the inventors have found that if X is 0.5 or less, the corrosion resistance in the corrosive environment in the tanker oil tank is reduced. Although it is excellent, it discovered that the said corrosion resistance deteriorated when X exceeded 0.5. Therefore, in determining the content of P, S, Cr, Cu, W, Mo, Sn, Sb, Ni and Co, the steel material of the present invention is designed so that the X value is 0.5 or less. There is a need.
- the element symbol in the above formula indicates the content (mass%) of the element, and the element not contained is calculated as 0 (zero). It is necessary to contain so that the Z value defined by (1) may be 0.15 or less.
- Cu is an element that causes surface cracking during hot working
- Sn is an element that promotes cracking due to Cu.
- Ni is an effective element for preventing the harmful effects of the above elements, but in order to exhibit the above effects of Ni, it is necessary to satisfy the above formula (2) and to add Ni. .
- the steel material of the present invention may further contain one or more selected from Nb, Ti, V and Zr in the following range in addition to the above components. it can.
- Nb 0.001 to 0.1 mass%
- Ti 0.001 to 0.1 mass%
- Zr 0.001 to 0.1 mass%
- V 0.002 to 0.2 mass%
- Nb, Ti, Zr and V are all elements that increase the strength of the steel material, and can be appropriately selected and added according to the required strength.
- Nb, Ti, and Zr each 0.001 mass% or more, and V to add 0.002 mass% or more.
- the toughness decreases, so Nb, Ti, Zr, V are added within the above ranges, respectively. Is preferred.
- the steel material of the present invention further includes one or more selected from Ca, REM and Y in the following range in addition to the above components.
- Ca 0.0002 to 0.01 mass%
- REM 0.0002 to 0.015 mass%
- Y 0.0001 to 0.1 mass%
- Ca, REM, and Y are all effective in improving the toughness of the weld heat affected zone, and can be added as necessary.
- the above effect can be obtained by adding Ca: 0.0002 mass% or more, REM: 0.0002 mass% or more, Y: 0.0001 mass% or more, but Ca: 0.01 mass%, REM: 0.015 mass%, Y: If added in excess of 0.1 mass%, the toughness is reduced, so Ca, REM, and Y are preferably added in the above ranges.
- the steel material of this invention can contain B in the following range in addition to the said component.
- B 0.0002 to 0.003 mass%
- B is an element that increases the strength of the steel material, and can be added as necessary. In order to acquire the said effect, adding 0.0002 mass% or more is preferable. However, when it exceeds 0.003 mass%, toughness will fall. Therefore, B is preferably added in the range of 0.0002 to 0.003 mass%.
- the steel material of the present invention is obtained by converting a steel adjusted to a component composition suitable for the present invention into a known refining process (steel converter, electric furnace, vacuum degassing equipment) or the like. It is melted using a refinery process and made into a steel material (slab slab) by continuous casting process or ingot-bundling rolling process. It is preferable that the steel material is rolled (hot rolled) to be a steel material such as a thick steel plate, a thin steel plate and a shaped steel.
- the reheating temperature before the hot rolling is preferably 900 to 1200 ° C. If heating temperature is less than 900 degreeC, a deformation resistance is large and it will become difficult to hot-roll. On the other hand, when the heating temperature exceeds 1200 ° C., austenite grains are coarsened, resulting in a decrease in toughness, and a scale loss due to oxidation becomes significant, resulting in a decrease in yield ratio. It is. A more preferable heating temperature is 1000 to 1150 ° C.
- the finish rolling finish temperature is preferably 750 ° C. or higher. If the temperature is lower than 750 ° C., the deformation resistance of the steel increases, the rolling load increases, it becomes difficult to roll, and a waiting time occurs until the rolled material reaches a predetermined rolling temperature, so the rolling efficiency decreases. It is because.
- the steel material after hot rolling may be cooled by either air cooling or accelerated cooling, but accelerated cooling is preferred when higher strength is desired.
- the cooling rate is 2 to 80 ° C./sec and the cooling stop temperature is 650 to 300 ° C.
- the cooling rate is less than 2 ° C./sec and the cooling stop temperature exceeds 650 ° C.
- the effect of accelerated cooling is small and sufficient strength cannot be achieved.
- the cooling rate exceeds 80 ° C./sec and the cooling stop temperature is less than 300 ° C., the toughness of the obtained steel material may be reduced, or the shape of the steel material may be distorted.
- the present invention limits the value of ⁇ (Mo + W) ⁇ / ⁇ (Mo + W) ⁇ in the base metal to the range defined by the above formula (3) as means for improving the corrosion resistance of the weld metal.
- Cu, Mo and W in the weld metal are represented by the following formula (4): 0.2 ⁇ (Cu in weld metal) / ⁇ (Mo + W) in weld metal ⁇ ⁇ 3 (4) It is necessary to satisfy and contain.
- the inventors have found that when Cu and Mo or W are added in combination, the corrosion resistance of the welded joint is greatly improved by the synergistic effect of these elements.
- (Cu in the weld metal) / ⁇ (Mo + W) in the weld metal ⁇ in the formula (4) is less than 0.2, the Cu is compared with the Mo or W content in the weld metal. Therefore, the corrosion resistance of the welded joint is lowered.
- the inventors of the present invention further have a Cu in the weld metal and a base material (steel material) represented by the following formula (5): 1 ⁇ (Cu in weld metal) / (Cu in base material) ⁇ 8 (5)
- the corrosion resistance in all the parts including a base material (steel material) and a welded joint further improved.
- the present invention improves the corrosion resistance of the weld metal to the same level as the base material (steel material) without reducing the low temperature toughness of the welded joint, so that the weld metal does not contain Sn or Sb.
- the value of (Mo + W) ⁇ / ⁇ (Mo + W) ⁇ in the base metal should be 1 or more. More preferred.
- (Cu in the weld metal) / (Cu in the base metal) exceeds 8
- addition of Cu more than necessary increases not only the cost of the welding material (welding wire) but also the corrosion resistance of the weld metal.
- the welding material used for welding is used depending on the component composition of the steel (base material) and the welding conditions. )) Is preferably selected as appropriate.
- a welding wire having a composition obtained by dividing the target composition of Cu, Mo, and W in the weld metal by the dilution ratio of the base material is produced and welded using this.
- the welding method used for welding the crude oil tank of the present invention is a high heat input welding (high welding such as FAB welding, FCB welding, RF welding, which is a one-sided submerged arc welding method (one-side submerged arc welding)).
- high welding such as FAB welding, FCB welding, RF welding
- -heat input welding or carbonate gas arc welding (CO 2 welding (CO 2 arc welding)
- CO 2 welding CO 2 arc welding
- small heat input welding such as) (small-heat input welding) or the like
- the chemical composition of the weld metal From the viewpoint of controlling to an appropriate range, it is necessary that the welding method uses a welding wire.
- FAB welding is a registered trademark related to the welding method at Kobe Steel, Ltd., and a back bead is formed by one-pass welding by directly applying a backing material composed of glass tape, solid flux, etc. to the back surface of the steel plate.
- FCB welding is a registered trademark related to the welding method of Kobe Steel, Ltd., and spraying a backing flux on a copper plate and pressing it against the back of the steel plate to form a back bead by one-pass welding.
- the RF method is a registered trademark related to the welding method of Kobe Steel, Ltd., and a jig frame with an underlay flux piled under a backing flux containing a welding thermosetting resin is applied to the back of the steel plate. A method of forming a back bead by one-pass welding by pressing the flux.
- the steel plates 1 to 36 were welded together by the welding methods described in Table 2-1 and Table 2-2 to prepare welded joints.
- the heat input of each welding method was 146 kJ / cm for FCB welding, 180 kJ / cm for FAB welding, and 1.5 kJ / cm for CO 2 welding. All the grooves were V grooves.
- the composition control of Cu, Mo, and W in the weld metal of each welded joint is performed by setting the target composition of Cu, Mo, and W to the base material dilution ratio (CO 2 welding is about 11%, FAB welding is about 47%, FCB welding A welding wire having a composition obtained by rebating by about 67% was produced, and welding was performed using this.
- FCB welding For FCB welding, a flux (PF-I55E / manufactured by Kobe Steel) and a back flux (PF-I50R / manufactured by Kobe Steel), and for FAB welding, a flux (PF-I52E / ( Kobe Steel, Ltd.), filler (RR-2 / Kobe Steel) and backing material (FA-B1 / Kobe) were used.
- FCB welding refers to a method in which a backing flux is spread on a copper plate and pressed against the back surface of the steel plate to form a back bead by one-pass welding.
- FAB welding refers to a method in which a backing material composed of glass tape, solid flux or the like is directly applied to the back surface of a steel plate to form a back bead by one-pass welding.
- This corrosion test apparatus is composed of a corrosion test bath 2 and a temperature control plate 3, and water 6 having a temperature of 36 ° C. is injected into the corrosion test bath 2, and In the water 6, a mixed gas (introduction gas 4) consisting of 12 vol% CO 2 , 5 vol% O 2 , 0.01 vol% SO 2 , 0.3 vol% H 2 S and the balance N 2 is introduced to corrode.
- the test tank 2 is filled with supersaturated water vapor to reproduce the corrosive environment behind the upper deck of the crude oil tank.
- the corrosion test piece 1 set on the upper and rear surfaces of the test tank is set to 25 ° C. ⁇ 3 hours via a temperature control plate 3 including a heater (electric heater) and a cooling system (cooling system).
- a temperature change with + 50 ° C. ⁇ 21 hours as one cycle was repeatedly applied for 180 days to cause dew condensation on the surface of the test piece 1 to cause overall corrosion.
- 5 indicates the exhaust gas from the test tank.
- the overall corrosion resistance is good ( ⁇ ), or more than 0.08 mm / year, or When the local corrosion was visually recognized in either the base metal part or the welded part, the overall corrosion resistance was evaluated as poor (x).
- a corrosion test was performed in which the test piece was immersed in 2 L of the test solution for 168 hours.
- the test solution was preheated and maintained at 30 ° C. and replaced with a new test solution every 24 hours.
- the apparatus used for the corrosion test is shown in FIG.
- This corrosion test apparatus is a double-type apparatus of a corrosion test tank 8 and a thermostatic tank 9, and the test solution 10 is put in the corrosion test tank 8, and the test piece 7 is suspended and immersed in the tegs 11 therein. ing.
- the temperature of the test solution 10 is maintained by adjusting the temperature of the water 12 placed in a constant-temperature bath 9.
- the mass difference before and after the test is calculated, the difference is divided by the total surface area, and the reduction in thickness (corrosion rate on both sides) per year is calculated. It was. As a result, when the corrosion rate is 0.8 mm / year or less and local corrosion is not visually recognized in the base metal part and the welded part, the local corrosion resistance is good ( ⁇ ), and the corrosion rate exceeds 0.8 mm / year.
- the corrosion rate is 1.0 mm / year or less and no local corrosion is visually recognized in the base metal part and the welded part, the local corrosion resistance is generally good ( ⁇ ), the corrosion rate is over 1.0 mm / year, or the mother The case where local corrosion was visually recognized in either the material part or the welded part was evaluated as poor ( ⁇ ) in the local corrosion resistance.
- Figure 1 shows the influence of the weld metal composition (bottom plate and top plate) on the corrosion resistance of tanker corrosion resistant steel welded joints for thick steel plates 1 to 36, with the horizontal axis (Cu in the weld metal) / ⁇ in the weld metal (Mo + W) ⁇ , and the vertical axis is plotted as ⁇ (Mo + W) ⁇ in the weld metal / ⁇ (Mo + W) ⁇ in the base metal. From FIG.
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Abstract
Description
(1)昼夜の温度差による鋼板表面への結露(dew drop)と乾燥(乾湿(wetting and drying))の繰り返し、
(2)原油タンク内に防爆用に封入されるイナートガス(inert gases)(O2約5vol%、CO2約13vol%、SO2約0.01vol%、残部N2を代表組成とするボイラ(boiler)あるいはエンジン(engine)の排ガス(exhaust gas)等)中のO2,CO2,SO2の結露水(dew condensation water)への溶け込み、
(3)原油から揮発するH2S等腐食性ガス(corrosive gas)の結露水への溶け込み、
(4)原油タンクの洗浄に使用された海水の残留
などが挙げられる。これらは、2.5年毎に行われる実船のドック検査(dock inspection)の調査で、強酸性の結露水中に、硫酸イオンや塩化物イオン(chloride ion)(Cl−)が検出されていることからも窺い知ることができる。
(1)塩化ナトリウム(sodium chloride)を代表とする塩類が高濃度に溶解した凝集水(brine)の存在、
(2)過剰な洗浄によるオイルコートの離脱、
(3)原油中に含まれる硫化物(sulfide)の高濃度化、
(4)結露水に溶け込んだ防爆用イナートガス中のO2、CO2、SO2等の高濃度化、などの項目が挙げられている。実際、実船のドック検査時に、原油タンク内に滞留した水を分析した結果では、高濃度の塩化物イオンと硫酸イオンが検出されている。
3≧溶接金属のCu含有量(質量%)/鋼材のCu含有量(質量%)≧0.15
3≧(溶接金属のMo含有量+W含有量(質量%))/(鋼材のMo含有量+W含有量(質量%))≧0.15
−0.3≦(溶接金属のCu含有量(質量%)−鋼材のCu含有量(質量%))≦0.5
3≧溶接金属のCu含有量(質量%)/鋼材のCu含有量(質量%)≧0.15
3≧(溶接金属のMo含有量+W含有量(質量%))/(鋼材のMo含有量+W含有量(質量%))≧0.15
X値=(1−0.8×Cu0.5)×{1−(0.8×W+0.4×Mo)0.3}×{1−(Sn+0.4×Sb)0.3}×{1−(0.05×Cr+0.03×Ni+0.03×Co)0.3}×{1+2×(S/0.01+P/0.025)} ・・・(1)
で定義されるX値が0.5以下、下記(2)式;
Z値=(1+10×Sn)×(Cu−0.7×Ni) ・・・(2)
で定義されるZ値が0.15以下となるよう含有し、残部がFeおよび不可避的不純物からなる鋼材同士を溶接して形成される原油タンクの溶接継手において、母材中および溶接継手の溶接金属中におけるCu,MoおよびWが、下記(3)式;
3<{溶接金属中の(Mo+W)}/{母材中の(Mo+W)}≦8 ・・・(3)
および(4)式;
0.2≦(溶接金属中のCu)/{溶接金属中の(Mo+W)}≦3 ・・・(4)
を満たして含有することを特徴とする耐食性に優れる溶接継手である。ここで、上記各式中の元素記号は、その元素の含有量(mass%)を示す。
記
A群;Ni:0.005~0.4mass%およびCo:0.01~0.4mass%のうちから選ばれる1種または2種
B群;Nb:0.001~0.1mass%、Ti:0.001~0.1mass%、Zr:0.001~0.1mass%およびV:0.002~0.2mass%のうちから選ばれる1種または2種以上
C群;Ca:0.0002~0.01mass%、REM:0.0002~0.015mass%およびY:0.0001~0.1mass%のうちから選ばれる1種または2種以上
D群;B:0.0002~0.003mass%
C:0.03~0.16mass%
Cは、鋼の強度を高める元素であり、本発明では、所望の強度を確保するため、0.03mass%以上添加する。一方、0.16mass%を超える添加は、溶接性(weldability)および溶接熱影響部(welded heat affected zone)の靭性(toughness)を低下させる。よって、Cは0.03~0.16mass%の範囲とする。0.06~0.16mass%の範囲がより好ましい。
Siは、脱酸剤として添加される元素であるが、鋼の強度を高めるのに有効な元素でもある。そこで、本発明では、所望の強度を確保するため、0.05mass%以上添加する。しかし、1.50mass%を超える添加は、鋼の靭性を低下させる。よって、Siは0.05~1.50mass%の範囲とする。0.15~0.50mass%の範囲がより好ましい。
Mnは、鋼の強度を高める元素であり、本発明では、所望の強度を得るため、0.1mass%以上添加する。一方、2.0mass%を超える添加は、鋼の靭性および溶接性を低下させる。よって、Mnは0.1~2.0mass%の範囲とする。なお、高強度を維持し、かつ、耐食性を劣化させる介在物の形成を抑制する観点からは、0.5~1.6mass%の範囲が好ましく、0.7~1.4mass%の範囲がより好ましい。
Pは、粒界に偏析して鋼の靭性を低下させる有害な元素であり、できる限り低減するのが望ましい。特に、0.025mass%を超えて添加すると、靭性が大きく低下する。また、Pは0.025mass%を超えて添加すると、タンク油槽内の耐食性にも悪影響を及ぼす。よって、Pは0.025mass%以下とする。好ましくは0.015mass%以下である。
Sは、非金属介在物(non−metal inclusion)であるMnSを形成して局部腐食の起点となり、耐局部腐食性を低下させる有害な元素であり、できる限り低減するのが望ましい。特に、0.010mass%を超える添加は、耐局部腐食性の顕著な低下を招く。よって、Sの上限は0.010mass%とする。好ましくは、0.005mass%以下である。
Alは、脱酸剤として添加される元素であり、本発明では0.005mass%以上添加する。しかし、0.10mass%を超えて添加すると、鋼の靭性が低下するので、Alの上限は0.10mass%とする。好ましくは、0.01~0.05mass%の範囲である。0.02~0.04mass%の範囲がより好ましい。
Nは、靭性を低下させる有害な元素であり、できる限り低減するのが望ましい。特に、0.008mass%を超えて添加すると、靭性の低下が大きくなるので、上限は0.008mass%とする。好ましくは0.006mass%以下、より好ましくは0.004mass%以下である。
Crは、腐食の進行に伴って錆層中に移行し、Cl−の錆層への侵入を遮断することによって、錆層と地鉄の界面へのCl−の濃縮を抑制する。また、Zn含有プライマー(primer)を塗布したときには、Feを中心としたCrやZnの複合酸化物を形成して、長期間にわたり鋼板表面にZnを存続させることができるため、飛躍的に耐食性を向上することができる。上記効果は、特に、タンカー油槽の底板部のように、原油油分から分離された高濃度の塩分を含む液と接触する部分において顕著であり、Crを含有した上記部分の鋼材にZn含有プライマー処理を施すことにより、Crを含有しない鋼材と比較して、格段に耐食性を向上することができる。上記Crの効果は、0.1mass%以下の添加では十分ではなく、一方、0.5mass%を超える添加は、溶接部の靭性を劣化させる。よって、Crは0.1mass%超0.5mass%以下の範囲とする。0.11~0.3mass%の範囲がより好ましい。0.12~0.2mass%の範囲がさらに好ましい。
Cuは、鋼の強度を高める元素であるとともに、鋼の腐食によって生成した錆中に存在して耐食性を高める効果がある。これらの効果は、0.03mass%未満の添加では十分に得られず、一方、0.4mass%を超えて添加すると、耐食性向上効果が飽和するほか、熱間加工時に表面割れなどの問題を引き起こす。よって、本発明の鋼材を安定して製造する観点から、Cuは0.03~0.4mass%の範囲で添加する必要がある。なお、Cu添加の効果は、添加量の増加にともない飽和していくため、費用対効果の点からは、0.008~0.15mass%の範囲が好ましい。0.01~0.14mass%の範囲がより好ましい。
W:0.01~1.0mass%
Wは、タンカー油槽部底板における孔食を抑制する効果があるほか、タンカー上甲板部の全面腐食を抑制する効果がある。上記効果は、0.01mass%以上の添加で発現する。しかし、1.0mass%を超えると、その効果が飽和する。よって、Wは0.01~1.0mass%の範囲で添加する。好ましくは0.01~0.5mass%、より好ましくは0.02~0.3mass%の範囲である。
Moは、タンカー油槽部底板における孔食を抑制するだけでなく、タンカー上甲板裏面部の耐全面腐食性や、バラストタンク(ballast tank)のように塩水浸漬と高湿潤を繰り返す腐食環境における塗装後の耐食性を向上させる効果がある。上記Moの効果は0.01mass%以上の添加で発現するが、0.5mass%を超えると、その効果は飽和する。よって、Moは0.01~0.5mass%の範囲とする。好ましくは0.02~0.5mass%、より好ましくは0.03~0.4mass%の範囲である。
なお、Moが上記のような耐食性向上効果を有する理由は、Wと同様、鋼板の腐食に伴い生成する錆中にMoO4 2−が生成し、このMoO4 2−の存在によって、塩化物イオンの鋼板表面への侵入が抑制されるからと考えられる。
SnおよびSbは、タンカー油槽部底板における孔食を抑制する効果を有する他、タンカー上甲板部の全面腐食を抑制する効果がある。上記効果は、Sn:0.001mass%以上、Sb:0.001mass%以上の添加で発現する。一方、Sn:0.2mass%超えおよびSb:0.4mass%超え添加しても、その効果は飽和する。さらに、Snの多量の添加は、Cuによる熱間加工時の表面割れを助長する。よって、SnおよびSbは、それぞれ上記範囲で添加するのが好ましい。
Ni:0.005~0.4mass%、Co:0.01~0.4mass%
NiおよびCoは、生成した錆粒子を微細化して、裸状態での耐食性およびジンクプライマー(zinc primer)にエポキシ系塗装(epoxy coating)が施された状態での耐食性を少なからず向上する効果を有する。したがって、これらの元素は、耐食性をより向上したい場合に、補助的に添加するのが好ましい。上記効果は、Ni:0.005mass%以上、Co:0.01mass%以上の添加で発現する。一方、Ni:0.4mass%超え、Co:0.4mass%超え添加しても、その効果が飽和する。また、Niは、CuやSnを含有する鋼において発生する熱間加工時の表面割れを抑制する効果がある。よって、NiおよびCoは、それぞれ上記範囲で添加するのが好ましい。
X値=(1−0.8×Cu0.5)×{1−(0.8×W+0.4×Mo)0.3}×{1−(Sn+0.4×Sb)0.3}×{1−(0.05×Cr+0.03×Ni+0.03×Co)0.3}×{1+2×(S/0.01+P/0.025)} ・・・(1)
ここで、上記式中の元素記号は、その元素の含有量(mass%)を示しており、含有していない元素は0(ゼロ)として計算する。
で定義されるX値が0.5以下を満たすよう含有している必要がある。
Z値=(1+10×Sn)×(Cu−0.7×Ni) ・・・(2)
ここで、上記式中の元素記号は、その元素の含有量(mass%)を示しており、含有していない元素は0(ゼロ)として計算する。
で定義されるZ値が0.15以下となるように含有していることが必要である。その理由は、Cuは、熱間加工時の表面割れを引き起こす元素であり、また、Snは、上記Cuによる割れを助長する元素である。一方、Niは、上記元素による弊害を防止するのに有効な元素であるが、Niの上記効果を発現させるためには、上記(2)式を満たしてNiを添加する必要があるからである。
Nb:0.001~0.1mass%、Ti:0.001~0.1mass%、Zr:0.001~0.1mass%およびV:0.002~0.2mass%
Nb,Ti,ZrおよびVは、いずれも鋼材強度を高める元素であり、必要とする強度に応じて適宜選択して添加することができる。上記効果を得るためには、Nb,Ti,Zrはそれぞれ0.001mass%以上、Vは0.002mass%以上添加するのが好ましい。しかし、Nb,Ti,Zrはそれぞれ0.1mass%を超えて、Vは0.2mass%を超えて添加すると、靭性が低下するため、Nb,Ti,Zr,Vは、それぞれ上記範囲で添加するのが好ましい。
Ca:0.0002~0.01mass%、REM:0.0002~0.015mass%およびY:0.0001~0.1mass%
Ca,REMおよびYは、いずれも、溶接熱影響部の靭性向上に効果があり、必要に応じて添加することができる。上記効果は、Ca:0.0002mass%以上、REM:0.0002mass%以上、Y:0.0001mass%以上の添加で得られるが、Ca:0.01mass%、REM:0.015mass%、Y:0.1mass%を超えて添加すると、却って靭性の低下を招くので、Ca,REM,Yは、それぞれ上記範囲で添加するのが好ましい。
B:0.0002~0.003mass%
Bは、鋼材の強度を高める元素であり、必要に応じて添加することができる。上記効果を得るためには、0.0002mass%以上添加するのが好ましい。しかし、0.003mass%を超えて添加すると、靭性が低下する。よって、Bは0.0002~0.003mass%の範囲で添加するのが好ましい。
すなわち、本発明の鋼材は、本発明に適合する成分組成に調整した鋼を、転炉(steel converter)や電気炉(electric furnace)、真空脱ガス(vacuum degassing equipment)等、公知の精錬プロセス(refinery process)を用いて溶製し、連続鋳造法(continuous casting process)あるいは造塊−分塊圧延法で鋼素材(スラブ(steel slab))とし、次いで、この素材を再加熱してから熱間圧延(hot rolling)し、厚鋼板、薄鋼板および形鋼等の鋼材とするのが好ましい。
上記適正成分に調整して製造した鋼板同士を溶接して形成した原油タンクの溶接継手は、溶接金属中のMoおよびWが下記(3)式;
3<{溶接金属中の(Mo+W)}/{母材中の(Mo+W)}≦8 ・・・(3)
を満たして含有していることが必要である。
一般に、溶接金属の耐食性が母材(鋼材)の耐食性より劣る場合には、後述するタンカー底板の孔食内部を模擬した酸浸漬試験では、溶接部の金属の溶解が促進されてしまう。また、溶接金属の耐食性を母材(鋼材)並みに高めるため、溶接金属中にSnやSbを添加すると、溶接継手の低温靭性を確保できなくなる。そのため、溶接金属にSnやSbを含有させることなく、溶接金属の耐食性を母材(鋼材)並みに向上させることが必要となる。そこで、本発明は、溶接金属の耐食性向上手段として、{溶接金属中の(Mo+W)}/{母材中の(Mo+W)}の値を上記(3)式で規定した範囲に制限するところに特徴がある。
0.2≦(溶接金属中のCu)/{溶接金属中の(Mo+W)}≦3 ・・・(4)
を満たして含有していることが必要である。
発明者らは、CuとMoあるいはWを併用して添加すると、それらの元素の相乗効果によって、溶接継手の耐食性が大幅に向上することを見出した。しかし、(4)式中の(溶接金属中のCu)/{溶接金属中の(Mo+W)}が0.2未満の場合には、溶接金属中のMoあるいはWの含有量に比してCuの含有量が著しく低く、上記相乗効果が期待できないため、溶接継手の耐食性が低下する。一方、(4)式中の(溶接金属中のCu)/{溶接金属中の(Mo+W)}が3を超える場合には、溶接金属中のCu含有量に比してMoあるいはWの含有量が低すぎて、やはり上記相乗効果(synergy effect)が期待できないため、溶接継手の耐食性が低下してしまう。よって、溶接金属中のCu,MoおよびWは上記(4)式を満たすことが必要である。なお、溶接金属中におけるMoおよびWの含有量は、その合計含有量が上記式を満たす範囲内であれば、MoおよびWのうちのいずれか一方を含まなくても構わない。(溶接金属中のCu)/{溶接金属中の(Mo+W)}は、0.5~3の範囲がより好ましい。0.5~2.5の範囲がさらに好ましい。
1≦(溶接金属中のCu)/(母材中のCu)≦8 ・・・(5)
を満たす場合には、母材(鋼材)および溶接継手を含む全ての部位における耐食性がさらに向上することを見出した。
前述したように、本発明は、溶接継手の低温靭性を低下させることなく溶接金属の耐食性を母材(鋼材)並みに向上するため、溶接金属中にSnやSbを含有させずに、{溶接金属中の(Mo+W)}/{母材中の(Mo+W)}の値を(3)式で規定した適正範囲に制御することがポイントである。しかし、溶接金属の耐食性をさらに安定して向上させるためには、上記(3)式に加えて、(溶接金属中のCu)/(母材中のCu)の値が1以上であることがより好ましい。一方、(溶接金属中のCu)/(母材中のCu)が8を超えると、Cuの必要以上の添加によって、溶接材料(溶接ワイヤ)のコストが上昇するだけでなく、溶接金属の耐食性が母材のそれを大きく上回るため、母材の選択腐食が発生するおそれがある。よって、溶接金属中と母材(鋼材)中のCuは、(5)式を満たすことが好ましい。
なお、ここでFAB溶接とは、(株)神戸製鋼所の溶接方法に関する登録商標で、ガラステープ、固形フラックス等で構成した裏当て材を鋼板裏面に直接当てて、1パス溶接で裏波ビードを形成する方法を言う。また、FCB溶接とは、(株)神戸製鋼所の溶接方法に関する登録商標で、銅板の上に裏当てフラックスを散布して、鋼板裏面に押し当てて、1パス溶接で裏波ビードを形成する方法を言う。また、RF法とは、(株)神戸製鋼所の溶接方法に関する登録商標で、溶接熱硬化性樹脂を含んだ裏当てフラックスの下に下敷きフラックスを重ねた冶具枠を鋼板裏面に当て、枠中のフラックスを押し当てて、1パス溶接で裏波ビードを形成する方法を言う。
かくして得られたNo.1~36の厚鋼板について、磁粉探傷試験で鋼板表面の割れの有無を調査し、割れが検出されなかったものを○、割れが検出されたものを×と判定した。
なお、前述したようにFCB溶接とは、銅板の上に裏当てフラックスを散布して、鋼板裏面に押し当てて、1パス溶接で裏波ビードを形成する方法を言う。また、FAB溶接とは、ガラステープ、固形フラックス等で構成した裏当て材を鋼板裏面に直接当てて、1パス溶接で裏波ビードを形成する方法を言う。
さらに、以下の要領で、上甲板裏を模擬した全面腐食試験と、タンカー底板環境を模擬した局部耐食試験を行った。
(1)タンカー上甲板環境を模擬した全面腐食試験
タンカー上甲板裏面における全面腐食に対する耐食性を評価するため、上記No.1~36の厚鋼板溶接継手の板厚1/4の位置から、溶接金属が試験片の幅方向と並行かつ中央に位置するよう、幅25mm×長さ60mm×厚さ5mmの矩形の小片を切り出し、その表面を600番手のエメリー紙(emery paper)で研磨した。裏面および端面は腐食しないよう、テープ(tape)でシールし、図1に示した腐食試験装置を用いて全面腐食試験を行った。
この腐食試験装置は、腐食試験槽(corrosion test bath)2と温度制御プレート3とから構成されており、腐食試験槽2には温度が36℃に保持された水6が注入されており、また、その水6中には、12vol%CO2、5vol%O2、0.01vol%SO2、0.3vol%H2S、残部N2からなる混合ガス(導入ガス4)を導入して腐食試験槽2内を過飽和の水蒸気(supersaturated water vapor)で充満し、原油タンク上甲板裏の腐食環境を再現したものである。そして、この試験槽の上裏面にセットした腐食試験片1に、ヒーター(electric heater)と冷却装置(cooling system)を内蔵した温度制御プレート(temperature−controlled plate)3を介して25℃×3時間+50℃×21時間を1サイクルとする温度変化を180日間繰り返して付与し、試験片1の表面に結露水を生じさせて、全面腐食を起こさせるようにしたものである。図1中、5は試験槽からの排出ガスを示す。
上記試験後、各試験片表面の錆を除去し、試験前後の質量変化から、腐食による質量の減を求め、この値から1年当たりの板厚減量(片面の腐食速度)に換算した。その結果、腐食速度が0.08mm/year以下でかつ母材部および溶接部のいずれにも局部腐食が認められない場合を耐全面腐食性が良好(○)、0.08mm/year超、あるいは、母材部か溶接部のいずれか一方にでも局部腐食が目視で認められる場合を耐全面腐食性が不良(×)と評価した。
タンカー油槽部底板における孔食に対する耐食性を評価するため、上記No.1~36の厚鋼板溶接継手の板厚1/4の位置から、溶接金属が試験片の幅方向と並行かつ中央に位置するよう、幅25mm×長さ60mm×厚さ5mmの矩形の小片を切り出し、その全面を600番手のエメリー紙で研磨した。
次いで、10mass%NaCl水溶液を、濃塩酸(concentrated hydrochloric acid)を用いてClイオン濃度10mass%、pH0.85に調製した試験溶液を作製し、試験片の上部に開けた3mmφの孔にテグスを通して吊るし、1試験片につき2Lの試験溶液中に168時間浸漬する腐食試験を行った。なお、試験溶液は、予め30℃に加温・保持し、24時間毎に新しい試験溶液と交換した。
上記腐食試験に用いた装置を図2に示す。この腐食試験装置は、腐食試験槽8、恒温槽9の二重型の装置で、腐食試験槽8には上記試験溶液10が入れられ、その中に試験片7がテグス11で吊るされて浸漬されている。試験溶液10の温度は、恒温槽(constant−temperature bath)9に入れた水12の温度を調整することで保持している。
上記腐食試験後、試験片表面に生成した錆を除去した後、試験前後の質量差を求め、この差を全表面積で割り戻し、1年当たりの板厚減少量(両面の腐食速度)を求めた。その結果、腐食速度が0.8mm/year以下でかつ母材部および溶接部に局部腐食が目視で認められない場合を耐局部腐食性が良好(○)、腐食速度が0.8mm/year超え1.0mm/year以下で、かつ母材部および溶接部に局部腐食が目視で認められない場合を耐局部腐食性が概ね良好(△)、腐食速度が1.0mm/year超、あるいは、母材部および溶接部のいずれかにでも局部腐食が目視で認められる場合を耐局部腐食性が不良(×)と評価した。
また、図3は、No.1~36の厚鋼板について、タンカー耐食鋼溶接継手の耐食性に及ぼす溶接金属組成の影響(底板および上板)を示した図で、横軸を(溶接金属中のCu)/{溶接金属中の(Mo+W)}とし、縦軸を{溶接金属中の(Mo+W)}/{母材中の(Mo+W)}として、プロットしたものである。
図3から(溶接金属中のCu)/{溶接金属中の(Mo+W)}が、0.2~3の範囲でかつ、{溶接金属中の(Mo+W)}/{母材中の(Mo+W)}が、3~8の範囲を満足した場合に、上甲板裏を模擬した耐食性試験およびタンカー底板環境を模擬した耐食性試験のいずれにおいても良好な耐食性(表2−1および表2−2の○印及び△印)を示した(図3中の◆印)。なお、図3中の■は、上甲板裏を模擬した耐食性試験およびタンカー底板環境を模擬した耐食性試験のいずれかが×を示した。
2、8:腐食試験槽
3:温度制御プレート
4:導入ガス
5:排出ガス
6、12:水
9:恒温槽
10:試験液
11:テグス
Claims (6)
- C:0.03~0.16mass%、
Si:0.05~1.50mass%、
Mn:0.1~2.0mass%、
P:0.025mass%以下、
S:0.010mass%以下、
Al:0.005~0.10mass%、
N:0.008mass%以下、
Cr:0.1mass%超0.5mass%以下、
Cu:0.03~0.4mass%を含有し、かつ、
W:0.01~1.0mass%、Mo:0.01~0.5mass%、Sn:0.001~0.2mass%およびSb:0.001~0.4mass%のうちから選ばれる1種または2種以上を含有し、
さらに上記成分が、下記(1)式で定義されるX値が0.5以下、下記(2)式で定義されるZ値が0.15以下となるよう含有し、残部がFeおよび不可避的不純物からなる鋼材同士を溶接して形成される原油タンクの溶接継手において、母材中および溶接継手における溶接金属中のCu,MoおよびWが、下記(3)式および(4)式を満たして含有する溶接継手。
記
X値=(1−0.8×Cu0.5)×{1−(0.8×W+0.4×Mo)0.3}×{1−(Sn+0.4×Sb)0.3}×{1−(0.05×Cr+0.03×Ni+0.03×Co)0.3}×{1+2×(S/0.01+P/0.025)} ・・・(1)
Z値=(1+10×Sn)×(Cu−0.7×Ni) ・・・(2)
3<{溶接金属中の(Mo+W)}/{母材中の(Mo+W)}≦8 ・・・(3)
0.2≦(溶接金属中のCu)/{溶接金属中の(Mo+W)}≦3 ・・・(4)
ここで、上記式中の元素記号は、その元素の含有量(mass%)を示す。 - 上記鋼材は、上記成分組成に加えてさらに、Ni:0.005~0.4mass%およびCo:0.01~0.4mass%のうちから選ばれる1種または2種を含有する請求項1に記載の溶接継手。
- 上記鋼材は、上記成分組成に加えてさらに、Nb:0.001~0.1mass%、Ti:0.001~0.1mass%、Zr:0.001~0.1mass%およびV:0.002~0.2mass%のうちから選ばれる1種または2種以上を含有する請求項1または2に記載の溶接継手。
- 上記鋼材は、上記成分組成に加えてさらに、Ca:0.0002~0.01mass%、REM:0.0002~0.015mass%およびY:0.0001~0.1mass%のうちから選ばれる1種または2種以上を含有する請求項1~3のいずれか1項に記載の溶接継手。
- 上記鋼材は、上記成分組成に加えてさらに、B:0.0002~0.003mass%を含有する請求項1~4のいずれか1項に記載の溶接継手。
- 請求項1~5のいずれか1項に記載の溶接継手を有する原油タンク。
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