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
  • C22C38/60—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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
• • 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
• • 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
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/06—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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • 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
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten

Definitions

• the present invention relates to structural steel which can preferably be used for the members of storage equipment and transportation equipment for ethanol. That is, the steel according to aspects of the present invention can preferably be used as steel for the members of storage equipment for ethanol and the members of transportation equipment for ethanol. In addition, the steel according to aspects of the present invention relates to structural steel excellent in terms of ethanol corrosion resistance which can be used in a corrosive environment in ethanol containing carboxylic acid, chloride ions, and water, in particular, in bioethanol.
• Bioethanol is manufactured mainly by degrading and refining the sugar of, for example, corn or wheat.
• bioethanol is widely used in the world as an alternative fuel to petroleum (gasoline) or as a fuel to be mixed with gasoline, and the amount of bioethanol used tends to increase year by year. Therefore, in a process of, for example, storing and transporting bioethanol or in a process of mixing bioethanol with gasoline, there is an increase in the amount of bioethanol handled.
• bioethanol is highly corrosive, that is, since pitting corrosion, in particular, stress corrosion cracking (SCC) progresses, it is difficult to handle bioethanol.
• SCC stress corrosion cracking
• Patent Literature 1 proposes, as a measure to handle bio-fuel, a method in which a zinc-nickel coating layer as formed on a 5 mass % to 25 mass % of Ni containing steel material for a tank or in which a chemical conversion coating film containing no hexavalent chromium is formed on the above-mentioned coating layer. Patent Literature 1 states that this method provides good corrosion resistance in gasoline containing ethanol.
• Patent Literature 2 proposes a steel material for a pipe excellent in terms of corrosion resistance for handing the vapor of a fuel such as bioethanol, which is manufactured by coating the surface of a steel plate with a Zn—Co—Mo coating layer in which the ratio of the Co content to the Zn content is 0.2 at % to 4.0 at %.
• Patent Literature 3 reports a steel material excellent in terms of alcohol corrosion resistance containing, by mass %, Cr: 0.01% to 1.0% and two or all selected from Cu: 0.05% to 1.0%, Sn: 0.01% to 0.2%, and Ni: 0.01% to 1.0%.
• Non Patent Literature 1 the inhibitor effect of ammonium hydroxide against the SCC (stress corrosion cracking) of a steel material in a simulated solution of bioethanol has been discussed.
• Non Patent Literature 1 states that there is a decrease in the degree of SCC as a result of inhibiting crack growth by adding ammonium hydroxide in the simulated solution.
• NPL 1 F. Gui, J. A. Beavers, and N. Sridhar: Evaluation of ammonia hydroxide for mitigating stress corrosion cracking of carbon steel in fuel grade ethanol, NACE Corrosion. Paper, No. 11138 (2011)
• SCC originally means a cracking phenomenon caused by the coaction of a corrosive environment and static stress.
• bioethanol SCC is often observed in equipment which is subjected to a fluctuating load environment, bioethanol SCC is fundamentally regarded as a corrosion fatigue phenomenon.
• Corrosion fatigue which occurs under dynamic stress is a severe fracturing phenomenon in which crack growth occurs more rapidly under lower stress than in the case of SCC which occurs under static stress. That is, the present inventors considered that it is necessary to increase corrosion fatigue resistance in an ethanol environment in order to prevent bioethanol SCC.
• the Zinc-Nickel coating disclosed in Patent Literature 1 is effective for increasing corrosion resistance.
• a Zn—Ni coating requires an electroplating treatment, there is no problem in the case of, for example, a small fuel tank for an automobile.
• a thick steel material having a thickness of 3 mm or more which is used for a large structure such as a storage tank having a capacity of 1000 kL or more or a line pipe, since there is a vast increase in treatment costs, it is not possible to use an electroplating treatment.
• a coating defect occurs, since pitting corrosion tends to progress in the defected portion on the contrary, corrosion fatigue tends to occur. Therefore, the Zinc-Nickel coating is not sufficient from the viewpoint of pitting corrosion resistance and corrosion fatigue resistance.
• the Zn—Co—Mo coating disclosed in Patent Literature 2 also requires an electroplating treatment, and therefore it is not possible to use the coating for a thick steel material for a large structure for the same reason as in the case of Patent Literature 1. Also, the Zn—Co—Mo coating is not sufficient from the viewpoint of pitting corrosion resistance and corrosion fatigue resistance for the same reason as in the case of Patent Literature 1.
• Patent Literature 3 Although the steel material in Patent Literature 3 effective from the viewpoint of pitting corrosion resistance, corrosion fatigue resistance has not been discussed. Therefore, it is difficult to say that the steel material in Patent Literature 3 has satisfactory ethanol corrosion resistance which is required for a practical structure.
• Non Patent Literature 1 states that the addition of an inhibitor certainly decreases the degree of a corrosion phenomenon such as corrosion fatigue, it is difficult to say that such an effect is sufficient. This is because an inhibitor develops its effect by being adsorbed onto the surface of a steel material and the adsorption behavior strongly depends on, for example, the pH of the environment. Therefore, in the case where corrosion occurs locally, there may be a case of insufficient adsorption. In addition, since there is a risk of environmental pollution due to the leakage of an inhibitor, it is difficult to say that the addition of an inhibitor is a preferable countermeasure against corrosion.
• a method for preventing corrosion through the coating is not suitable for a large structure, and the addition of an inhibitor has an insufficient effect due to a variation in the effect of decreasing the degree of corrosion on the surface of structural steel. Therefore, there is a strong demand for steel for storage equipment and transportation equipment for ethanol excellent in terms of corrosion resistance, in particular, corrosion fatigue resistance in a bioethanol environment which includes carboxylic acid, chloride ions, and water as impurities.
• An object according to aspects of the present invention is, by solving the problems with the conventional techniques, to provide structural steel for the members of storage equipment and transportation equipment for ethanol such as a steel pipe excellent in terms of ethanol corrosion resistance which can be used even in a bioethanol environment.
• excellent in terms of ethanol corrosion resistance here means a case where steel is excellent in terms of corrosion fatigue resistance in an ethanol environment which contains carboxylic acid, chloride ions, and water as impurities.
• the present inventors in order to solve the problems described above, diligently conducted investigations for the purpose of the development of steel for storage equipment and transportation equipment for ethanol excellent in terms of corrosion fatigue resistance in a bioethanol environment and, as a result, found that adding Mo and W is effective for decreasing the degree of corrosion fatigue in a bioethanol environment and that adding Sb and/or Sn and further Al in addition to Mo and W is effective.
• the present inventors found that there is a significant increase in corrosion fatigue resistance by decreasing the N content.
• such effects are also effectively realized even in the case of SCC in a static load environment under a milder stress condition.
• the present invention it is possible to obtain steel for storage equipment and transportation equipment for ethanol excellent in terms of ethanol corrosion resistance which can be used even in a bioethanol environment which contains carboxylic acid, chloride ions, and water.
• the present invention is used as steel for a storage tank, a transportation tank, and a pipeline structure for bioethanol, it is possible to use them for a longer time than ever, it is possible to avoid an accident caused by the leakage of bioethanol due to a corrosion fatigue phenomenon, and it is possible to provide them at low cost. Therefore, there is a significant effect on the industry.
• C is a chemical element which is necessary for achieving satisfactory strength of steel, and the C content is set to be at least 0.02% in order to achieve preferable yield strength (350 MPa or more) and tensile strength (400 MPa or more) in accordance with aspects of the present invention. It is preferable that the C content be 0.03% or more. On the other hand, in the case where the C content is more than 0.3%, since there is a deterioration in weldability, there are limitations added when welding is performed. Therefore, the upper limit of the C content is set to be 0.3%. It is preferable that the C content be 0.20% or less. In accordance with aspects of the present invention, it is more preferable that the C content be 0.10% or less in order to obtain good corrosion fatigue resistance.
• the Si is added for the purpose of deoxidation, and there is an insufficient deoxidizing effect in the case where the Si content is less than 0.01%.
• the Si content is set to be 0.01% to 1.0%.
• the lower limit of the Si content be 0.03%, more preferably 0.05%, or even more preferably 0.20%.
• the upper limit of the Si content be 0.7%, or more preferably 0.5%.
• the Mn content is added for the purpose of increasing strength and toughness, and it is not possible to sufficiently realize such an effect in the case where the Mn content is less than 0.1%.
• the Mn content is set to be 0.1% to 2.0%.
• the lower limit of the Mn content be 0.3%, or more preferably 0.5%.
• the upper limit of the Mn content be 1.6%, more preferably 1.3%, or even more preferably 1.0%.
• the P content is limited to 0.03% or less. Since there is a disadvantage from the viewpoint of dephosphorization cost in the case where the P content is excessively decreased, the lower limit of the P content is set to be 0.003%. Here, it is preferable that the P content be in the range of 0.003% to 0.025%, or more preferably 0.003% to 0.015%.
• S is an important chemical element which has an influence on the corrosion resistance of the steel according to aspects of the present invention. S is inevitably contained. In the case where the S content is large, there is a deterioration in toughness and weldability, and there is a decrease in corrosion fatigue resistance due to an increase in the number of inclusions such as MnS, which become the starting points at which corrosion fatigue occurs. Also, since inclusions, which become the starting points at which corrosion fatigue occurs, become preferential anode sites, pitting corrosion is promoted. Therefore, it is preferable that the S content be as small as possible, and it is acceptable that the S content be 0.01% or less, preferably 0.005% or less, or more preferably 0.003% or less. On the other hand, for the reasons described above, there is no particular limitation on the lower limit of the S content.
• Al is added as a deoxidzing agent, and there is a decrease in toughness due to an insufficient deoxidizing effect in the case where the Al content is less than 0.005%.
• the Al content is more than 0.100%, there is a decrease in the toughness of a weld metal in the case where welding is performed. Therefore, the Al content is limited to 0.100% or less.
• Al has a function of further increasing the effect of increasing acid resistance through the use of Sb and Sn described below. That is, since Al 3+ ions which have been eluted due to the anodic dissolution of a base metal undergo a hydrolysis reaction with water which exists in a small amount in bioethanol, the formation of Sb oxides and Sn oxides described below is promoted due to a decrease in pH at the anode site. Such an effect becomes noticeable in the case where the Al content is 0.005% or more.
• the lower limit of the Al content be 0.010%, more preferably 0.015%, or even more preferably 0.020%.
• the upper limit of the Al content be 0.070%, more preferably 0.060%, or even more preferably 0.050% or less.
• N 0.0010% to 0.010% and 2.0 ⁇ Al/N ⁇ 70.0
• N is an important chemical element which has an influence on the corrosion fatigue resistance of the steel according to aspects of the present invention.
• the N content is more than 0.010%, since the formation of coarse AlN is promoted, it is not possible to sufficiently realize the above-described effect of increasing corrosion fatigue resistance through the use of Al, and there is an increase in corrosion fatigue sensitivity due to coarse AlN functioning as a starting point at which corrosion fatigue occurs. Therefore, the N content is limited to be 0.010% or less, preferably 0.007% or less, or more preferably 0.005% or less.
• N has an important function of stably realizing the above-described effect of increasing corrosion fatigue resistance through the use of Al. That is, while a decrease in pH due to the hydrolysis of Al 3+ ions contributes to an increase in corrosion fatigue resistance through the promoted formation of Sb oxides and Sn oxides, there is a risk of a decrease in corrosion fatigue resistance eventually in the case where there is an excessive decrease in pH.
• N in steel has a buffering function of inhibiting an excessive decrease in pH by consuming H + to form NH 4 + through anodic dissolution. In order to realize such a buffering function, it is necessary that the N content be at least 0.0010% or more. Therefore, the lower limit of the N content is set to be 0.0010%, or preferably 0.0015%.
• Al and N are strongly related t.o each other, for example, in the formation of AlN and the realization of the effect of increasing corrosion fatigue resistance through the use of Al as described above, it is also important to appropriately control (the Al content)/(the N content) (mass ratio) in a steel material.
• the Al content is excessively large with relation to the N content, that is, in the case, where (the Al content)/(the N content) is more than 70.0, since there is a significant increase in the formation rate of AlN, there is a coarsening of AlN, and the buffering function through the formation of NH 4 + does not catch up with the formation of AlN.
• the upper limit of (the Al content)/(the N content) is set to be 70.0, preferably 50.0, or more preferably 20.0.
• the lower limit of (the Al content)/(the N content) is set to be 2.0, preferably 3.0, or more preferably 5.0.
• W is a chemical element which is effective for increasing corrosion fatigue resistance. Since W, like Mo, forms corrosion products such as oxoacid ions, and such corrosion products are rapidly adsorbed onto a crack tip to decrease anode reaction activity in the case where a crack, which becomes a starting point at which stress corrosion cracking occurs, is formed, W has a function of inhibiting the growth of the crack. In addition, as a result of W being taken in an oxide film on the surface of a steel material, since there is an increase in the dissolution resistance of the oxide film in an acidic environment of carboxylic acid, which is contained as an impurity in bioethanol, W is also effective both for decreasing the degree of inhomogeneous corrosion and for decreasing pitting corrosion resistance.
• the W content is set to be 0.010% to 0.5%. It is preferable that the lower limit of the W content be 0.05%, or more preferably 0.08%. In order to prevent an increase in cost, it is preferable that the upper limit of the W content be 0.3%, or more preferably 0.2%.
• Mo is a chemical element which is effective for increasing corrosion fatigue resistance. Since Mo forms corrosion products such as oxoacid ions, and such corrosion products are rapidly adsorbed onto a crack tip to decrease anode reaction activity in the case where a crack, which becomes a starting point at which corrosion fatigue occurs, is formed, Mo has a function of inhibiting he growth of the crack. In addition, as a result of Mo being taken an oxide film on the surface of a steel material, since, there is an increase in the dissolution resistance of the oxide film in an acidic environment of carboxylic acid, which is contained as an impurity in bioethanol, Mo is also effective both for decreasing the degree of inhomogeneous corrosion and for decreasing pitting corrosion resistance.
• the Mo content is set to be 0.010% to 0.5%. It is preferable that the lower limit of the Mo content be 0.05%, or more preferably 0.08%. Moreover, in order to prevent an increase in cost, it is preferable that the upper limit of the Mo content be 0.4%, or more preferably 0.3%.
• W and Mo described above be added in order to achieve good corrosion fatigue resistance.
• Sb is a chemical element which increases acid resistance and is an important chemical element which increases the corrosion fatigue resistance of the steel according to aspects of the present invention.
• Sb is a chemical element which is effective for inhibiting the growth of a crack at a corrosion fatigue crack tip, which is in a low-pH environment.
• Sb is retained and concentrated at an anode site in the form of oxides due to the anodic dissolution of a base metal.
• the progress of a dissolution reaction is strongly inhibited, which results in an increase in corrosion fatigue resistance.
• the Sb content is less than 0.01%, such an effect is insufficiently realized.
• the Sb content is set to be in the range of 0.01% to 0.5%.
• the lower limit of the Sb content be 0.02%, or more preferably 0.05%.
• the upper limit of the Sb content be 0.4%, or more preferably 0.30%.
• Sn is, like Sb, a chemical element which increases acid resistance and is an important chemical element which increases the corrosion fatigue resistance of the steel material according to aspects of the present invention.
• Sn is a chemical element which is effective for inhibiting the growth of a crack at a corrosion fatigue crack tip, which is in a low-pH environment.
• Sn is retained and concentrated at an anode site in the form of oxides through the anodic dissolution of a base metal.
• the progress of a dissolution reaction is strongly inhibited, which results in an increase in corrosion fatigue resistance.
• the Sn content is less than 0.01%, such an effect is insufficiently realized.
• the Sn content is set to be in the range of 0.01% to 0.3%.
• the lower limit of the Sn content be 0.02%, or more preferably 0.05%.
• the upper limit of the Sn content be 0.30%, or more preferably 0.15%.
• Cu, Cr, and Ni are chemical elements which are effective for increasing corrosion fatigue resistance in an acidic environment of carboxylic acid, which is contained as an impurity in bioethanol.
• the contents of these chemical elements are small, there is no such effect.
• the content of each of these chemical elements is more than 1.0%, a limitation is imposed from the viewpoint of the manufacture of a steel material. Therefore, the Cu content is set to be 0.05% to 1.0%, the Cr content is set to be 0.01% to 1.0%, and the Ni content is set to be 0.01% to 1.0%.
• the upper limit of the Cu content be 0.5%, or more preferably 0.2%.
• the upper limit of the Cr content be 0.5%, or more preferably 0.2%.
• the upper limit of the Ni content be 0.5%, or more preferably 0.2%.
• MnS since MnS becomes a starting point at which pitting corrosion and corrosion fatigue occur, MnS has a negative effect.
• Ca, Mg, and REM are chemical elements which are effective for decreasing such a negative effect through the control of the shape and dispersion of sulfides in steel. It is not possible to sufficiently realize such an effect in the case where the contents of these chemical elements are small.
• Ca, Mg, and REM become coarse inclusions, which become the starting points at which pitting corrosion and corrosion fatigue occur. Therefore, the Ca content is set to be 0.0001% to 0.02%, the Mg content is set to be 0.0001% to 0.02%, and the REM content is set to be 0.001% to 0.2%.
• the lower limit of the Ca content be 0.001%. It is preferable that the upper limit of the Ca content be 0.005%. It is preferable that the lower limit of the Mg content be 0.001%. It is preferable that the upper limit of the Mg content be 0.005%. It is preferable that the upper limit of the REM content be 0.030%.
• One, two or more selected from Ti, Zr, Nb, and V may also be added in order to improve the mechanical properties of steel.
• the content of each of these chemical elements is less than 0.005%, it is not possible to sufficiently realize such an effect.
• the content of each of these chemical elements is set to be in the range of 0.005% to 0.1%, or preferably 0.005% to 0.05%.
• the constituent chemical elements of the steel material according to aspects of the present invention other than those described above are Fe and inevitable impurities. Moreover, a constituent chemical element which is inevitably contained in addition to those described above may also be contained as long as it is in the range that there is no decrease in the effects of aspects of the present invention.
• a pitting corrosion portion and a crack tip are particularly exposed to a low-pH environment in an ethanol solution containing 0.02 mmol/L or more of carboxylic acid, 0.02 mg/L or more of chloride ions, and 0.05 vol % or more of water. Therefore, embrittlement cracking due to secondarily generated hydrogen may occur in addition to the occurrence of pitting corrosion and cracking.
• the tensile strength and yield strength of the steel according to aspects of the present invention be respectively 825 MPa or less and 705 MPa or less.
• the steel according to aspects of the present invention can suitably be used for storage equipment and transportation equipment for ethanol.
• the steel according to aspects of the present invention is steel excellent in terms of ethanol corrosion resistance which can be used in a corrosive environment in ethanol containing carboxylic acid, chloride ions, and water, in particular, in bioethanol.
• carrier equipment and transportation equipment for ethanol in accordance with aspects of the present invention means equipment for, for example, storing, transporting, conveying, accumulating, distributing, recovering, or blending ethanol. Examples of such equipment include a tank, a steel pipe, a tanker, pipework, a pipeline, a nozzle, and a valve.
• shape of the steel for storage equipment and transportation equipment for ethanol according to aspects of the resent invention may be selected as needed, it is preferable that the steel according to aspects of the present invention have a plate shape. It is preferable that the steel according to aspects of the present invention have a thickness (wall thickness) of 1 mm to 50 mm, more preferably 3 mm to 50 mm, or even more preferably 5 mm to 50 mm.
• molten steel having the chemical composition described above After having prepared molten steel having the chemical composition described above by using a known furnace such as a converter or an electric furnace, steel such as a slab or a billet is manufactured by using a known method such as a continuous casting method or an ingot-making method.
• a known furnace such as a converter or an electric furnace
• steel such as a slab or a billet is manufactured by using a known method such as a continuous casting method or an ingot-making method.
• vacuum-degassing refining may be performed.
• the chemical composition of molten steel may be controlled by using a known steel-refining method.
• the steel described above when the steel described above is hot-rolled into desired size and shape, it is preferable that the steel be heated to a temperature of 1000° C. to 1350° C.
• the heating temperature In the case where the heating temperature is lower than 1000° C., since there is an increase in resistance to deformation, hot rolling tends to be difficult.
• the heating temperature In the case where the heating temperature is higher than 1350° C., there is a risk of surface defects, scale loss, or an increase in specific fuel consumption. It is preferable that the heating temperature be in the range of 1050° C. to 1300° C.
• hot rolling may be directly performed without heating the steel.
• the finishing delivery temperature of hot rolling is usually optimized. It is preferable that the finishing delivery temperature of hot rolling be 600° C. or higher and 850° C. or lower. In the case where the finishing delivery temperature of hot rolling is lower than 600° C., since there is an increase in rolling load due to an increase in resistance to deformation, there is a risk of difficulty in rolling operation. On the other hand, in the case where the finishing delivery temperature of hot rolling is higher than 850° C., it may be impossible to achieve the desired strength. It is preferable that cooling after finish rolling of hot rolling be performed by using a natural cooling method or an accelerated cooling method at a cooling rate of 150° C./s or less. In the case where accelerated cooling is performed, it is preferable that a cooling stop temperature be 300° C. to 750° C. Here, a reheating treatment may be performed after the cooling.
• Table 1-1 and Table 1-2 are referred to as Table 1.
• Table 2-1 and Table 2-2 is referred to as Table 2.
• a uniaxial round-bar-shaped tensile test piece (with a parallel part having a length of 25.4 mm and a diameter of 3.81 mm ⁇ ) was taken from the steel plate, and the parallel part was then polished to #2000 finish. Subsequently, the test piece was subjected to ultrasonic degreasing in acetone for 5 minutes, subjected to air drying, and then fitted to a low-strain-rate tensile testing machine. A solution which had been prepared by adding 10 ml of water, 5 ml of methanol, 56 mg of acetic acid, and 13.2 mg of NaCl to 985 ml of ethanol was used as a simulated solution of bioethanol.
• the simulated solution of bioethanol was charged into a cell which covered the uniaxial round-bar-shaped tensile test piece, and a variable stress whose maximum value was 110% of yield strength (YS), which had been determined before the fatigue test was performed, and whose minimum value was 10% of the yield strength was appled the tensile axis direction of the uniaxial round-bar-shaped tensile test piece at a frequency, of 8.3 ⁇ 10 ⁇ 4 Hz for a maximum of 240 hours.
• YS yield strength
• the present invention produces distinct improvement.
• a surface layer which was composed of distinct two layers that is, a layer in which oxoacid-ion-forming chemical elements (W and Mo) were concentrated and a layer in which oxide-forming chemical elements (Sn and Sb) were concentrated was formed at the crack tip. That is, in the case of the examples of the present invention, the crack tip was protected by a strong protection layer.

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• 2016
  • 2016-06-20 WO PCT/JP2016/002938 patent/WO2016208172A1/ja active Application Filing
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