Classifications

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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • 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
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  • 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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  • 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
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  • 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
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C—ALLOYS
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C—ALLOYS
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • 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
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  • 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
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  • 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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • 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
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
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  • 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
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • 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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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  • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the present invention relates to a ferrite-austenite duplex stainless steel material and a method for producing the same, and a structure for crude phosphoric acid.
• Patent Literature 3 proposes a ferrite-austenite duplex stainless steel material having good corrosion resistance in environments containing a trace amount of hydrogen sulfide.
• the crude phosphoric acid is a basic raw material for fertilizers, detergents, feeds, pharmaceuticals, and the like.
• Japan has few phosphorus resources and most of them are imported by marine transport.
• the crude phosphoric acid contains corrosive substances such as F - and Cl - derived from phosphate ore, it is known that transporting crude phosphoric acid causes corrosion (including discoloration such as blackening) on inner surfaces of storage tanks of the tankers. Therefore, after transporting the crude phosphoric acid, maintenance such as repairs and cleaning is performed to the inner surfaces of the storage tanks from the viewpoint of preventing contamination, and new chemicals are loaded. Such maintenance is very time-consuming and frequent, which is problematic in terms of cost and time for chemical transport.
• Patent Literature 4 proposes an austenite stainless steel material with adjusted contents of Cr, Ni, Mo, and Cu, which would otherwise affect the corrosion resistance against crude phosphoric acid.
• the inventors have examined the elements related to the preferential dissolution of the austenite phase to crude phosphoric acid and found that amounts of five elements (Cr, Ni, Mo, Cu, and N) in the austenite phase are closely related to the preferential dissolution of the austenite phase.
• the inventors have found that preferential dissolution of the austenite phase can be suppressed by controlling the amount of each of the above elements in the austenite phase in a well-balanced manner.
• the inventors have also found that corrosion resistance against crude phosphoric acid can be improved by controlling the composition of the ferrite-austenite duplex stainless steel itself in a well-balanced manner.
• the present invention was completed on the basis of these findings.
• this invention relates to a ferrite-austenite duplex stainless steel material having a composition comprising, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities,
• this invention relates to a method for producing a ferrite-austenite duplex stainless steel material, the method comprising:
• this invention relates to a structure for crude phosphoric acid, the structure comprising the ferrite-austenite duplex stainless steel material.
• ferrite-austenite as used herein means that the metallographic structure is mainly made of two phase of a ferrite phase and an austenite phase at ordinary temperature. Therefore, the "ferritic-austenite” includes those containing minor amounts of phases other than the ferrite phase and the austenite phase (for example, martensite phases, etc.).
• C is preferably reduced as much as possible.
• the lower limit of the C content is not particularly limited, but since the refining costs are higher for decarburization of steel types containing a relatively large amount of Cr, such as duplex stainless steel materials, it is sufficient to reduce the C content to, for example, 0.001%.
• C promotes the deposition of Cr carbides, so that if the C content is too high, intergranular corrosion occurs and the corrosion resistance decreases. Therefore, from the point of view for reducing the corrosion resistance, the Cr content should be 0.100% or less. From the viewpoint of stably ensuring this effect, the C content is preferably 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.
• the Si is an element that is used as a deoxidizing element and is added to improve oxidation resistance. From the viewpoint of obtaining these effects, the Si content should be 0.05% or more. From the viewpoint of stably ensuring these effects, the Si content is preferably 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, if the Si content is too high, the duplex stainless steel material becomes hard, and the toughness and workability decrease. Therefore, from the viewpoint of suppressing the deterioration of toughness and workability, the Si content should be 1.50% or less. From the viewpoint of stably ensuring this effect, the Si content is preferably 1.45% or less, 1.40% or less, 1.35% or less, or 1.30% or less.
• N When N dissolves, it forms NH 4 + (ammonium ion), which raises the pH of acidic condensation water and helps prevent Fe dissolution.
• Nitrogen (N) is also an element that is dissolved into the austenite phase to enhance strength and corrosion resistance. Its contribution also helps reduce the need for other alloying elements. From the viewpoint of obtaining these effects, the N content should be 0.100% or more. From the viewpoint of stably ensuring these effects, the N content is preferably 0.120% or more, 0.130% or more, or 0.140% or more. Nitrogen is a significant factor in the deposition of chromium nitrides. If the nitrogen content is high, the quantity of deposited chromium nitrides increases, which can result in reducing toughness and corrosion resistance. From the view point of controlling them, the N content should be 0.250% or less. For more stable effects, the N content is preferably 0.230% or less, or even 0.220% or less.
• Ti has effects of preventing coarsening of the heat-affected zone during welding of the duplex stainless steel material and of forming fine equiaxed crystals in the solidified structure, and therefore can be contained as necessary.
• the Ti content should be 0.050% or less.
• the Ti content is preferably 0.045% or less, or 0.040% or less.
• the lower limit of the Ti content is not particularly limited, but from the viewpoint of ensuring the above effects, the Ti content is preferably 0.001% or more.
• the B has an effect of improving hot workability and can optionally be added.
• an excessively high B content will significantly deteriorate the corrosion resistance.
• the B content should be 0.0050% or less.
• the B content is preferably 0.0040% or less, or 0.0030% or less.
• the lower limit of the B content is not particularly limited, but from the viewpoint of obtaining the above effects, the B content is preferably 0.0003% or more.
• Ca is an element that enhances corrosion resistance, and is presumed to reduce inclusions that serve as starting points for corrosion in an environment where crude phosphoric acid is present, and to suppress the dissolution of Fe, and therefore, Ca can optionally be contained.
• the Ca content should be 0.0010% or more.
• the Ca content is preferably 0.0015% or less, or 0.0020% or less.
• an excessively high content of Ca tends to generate heat work cracks and deteriorates the corrosion resistance.
• the Ca content should be 0.0100% or less.
• the Ca content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.
• Mg has effects of not only deoxidizing but also refining the solidification structure, and can be thus added as needed.
• the Mg content should be 0.0020% or less.
• the Mg content is preferably 0.0019% or less, or 0.0018% or less.
• the Mg content should be 0.0001% or more.
• the Zr has an effect of forming carbides and nitrides in the duplex stainless steel material to refine the crystal grains, and therefore can optionally be added.
• the Zr content should be 0.090% or less.
• the Zr content is preferably 0.085% or less.
• the lower limit of the Zr content is not particularly limited, but from the viewpoint of obtaining the above effects, the Zr content is preferably 0.001% or more, or 0.005% or more.
• Co is an austenite stabilizing element and can optionally be contained.
• the Co content should be 3.00% or less. From the viewpoint of cost reduction, the Co content is preferably 2.80% or less, or 2.60% or less.
• the lower limit of the Co content is not particularly limited, but from the viewpoint of obtaining the above effects, the Co content is preferably 0.01% or more.
• V is an element that improves corrosion resistance and can optionally be contained.
• the V content is preferably 0.950% or less.
• the lower limit of the V content is not particularly limited, but from the viewpoint of obtaining the above effects, the V content is preferably 0.010% or more, or 0.050% or more.
• Ta has an effect of forming carbides and nitrides in the duplex stainless steel material to additionally enhance the corrosion resistance, so it can optionally be contained.
• the Ta content should be 0.200% or less.
• the Ta content is preferably 0.170% or less, or 0.150% or less.
• the lower limit of the Ta content is not particularly limited, but from the viewpoint of obtaining the above effects, the Ta content is preferably 0.001% or more, or 0.005% or more.
• Sn is an element that improves corrosion resistance and can optionally be contained. However, if the Sn content is too high, the hot workability decreases, so the Sn content should be 0.100% or less. From the viewpoint of stably ensuring this effect, the Sn content is preferably 0.050% or less, 0.030% or less, or 0.010% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of obtaining the above effects, the Sn content is preferably 0.0001% or more, or 0.0003% or more.
• the O content should be 0.0050% or less. From the viewpoint of stably ensuring this effect, the O content is preferably 0.0048% or less.
• the lower limit of the O content is not particularly limited, but reduction of the O content leads to higher production costs. Therefore, the O content is preferably 0.001% or more.
• W is an element that improves corrosion resistance and can optionally be contained. However, if the W content is too high, the load during rolling increases and manufacturing defects are easily generated, so the W content should be 1.000% or less. From the viewpoint of stably ensuring this effect, the W content is preferably 0.900% or less, or 0.800% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of obtaining the above effects, the lower limit of W is 0.005% or more, or 0.010% or more.
• the REM (rare earth elements) has an effect of improving hot workability and can optionally be contained. However, a higher REM content impairs producibility and increases costs, so the REM content should be 0.100% or less. From the viewpoint of stably ensuring this effect, the REM content is preferably 0.095% or less, or 0.090% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of obtaining the above effects, the REM content is 0.001% or more, 0.005% or more, or 0.010% or more.
• REM is the generic term for 17 elements in total: Sc, Y, and 15 elements (lanthanoides) from La to Lu, and the REM content means the total content of these elements. These elements can be used alone or in combination of two or more. Moreover, the lanthanides are industrially added in the form of misch metals.
• the austenite phase satisfies a relational expression of the following equation (2): 0.8 Cr 2 + 2 Ni + 7 Mo + 30 Cu + 30 N ⁇ 340
• the equation (2) represents the relationship between the contents of five elements (Cr, Ni, Mo, Cu, and N) that affect the preferential dissolution of the austenite phase in crude phosphoric acid.
• the value of the left side of the equation (2) is preferably 345 or more, more preferably 350 or more, and even more preferably 355 or more.
• the upper limit of the value on the left side of the equation (2) is not particularly limited, but it is, for example, 800 or 700.
• the measured CPT tends to decrease when chromium nitrides are deposited, and so if the CPT difference (expected CPT - measured CPT) is 10°C or less, it can be considered that the amounts of chromium nitrides deposited are low. That is, when the relational expression of the equation (3) as described above is satisfied, the deposition of chromium nitride is suppressed, so that the corrosion resistance can be sufficiently ensured. On the other hand, if the relational expression of the equation (3) as described above is not satisfied, there is a risk of the corrosion resistance decreasing due to the deposition of chromium nitrides.
• the duplex stainless steel material according to the embodiment of the present invention may be either a hot rolled material or a cold rolled material, but when it is used for storage tanks of chemical tankers and the like, the hot-rolled material is preferable.
• the thickness of the duplex stainless steel material according to the embodiment of the present invention may be adjusted as needed depending on applications, and is not particularly limited, but it may generally be 20.0 mm or less, preferably 15.0 mm or less, and more preferably 10.0 mm or less. If the duplex stainless steel material is in a form of a bar, the thickness means the circular equivalent diameter of the cross section. If the duplex stainless steel material is a shaped steel, the thickness means the thickness at any point in the cross section.
• the soaking temperature is lower than 950°C, it cannot sufficiently achieve the solid solution of the chromium nitride. Furthermore, if the soaking temperature is higher than 1150°C, the amount of the ferrite phase will increase and it will be difficult to control the composition of the austenite phase within a predetermined range. From the viewpoint of stably suppressing these problems, the soaking temperature is preferably 1000 to 1100°C.
• the first cooling step is to slowly cool the rolled material obtained in the soaking step to a rapid cooling initiation temperature at a cooling rate of 5°C/s or more and less than 10°C/s.
• the cooling rate in the first cooling step is 10°C/s or more, it will be difficult to control the composition of the austenite phase within a predetermined range.
• the cooling rate in the first cooling step is preferably 9°C/s or less, more preferably 8°C/s or less, and even more preferably 7°C/s or less.
• the cooling rate in the first cooling step is less than 5°C/s, the amount of chromium nitride deposited increases, and in some cases, the ⁇ phase may be deposited, resulting in a decrease in corrosion resistance.
• the cooling method in the first cooling step is not particularly limited, but it may be, for example, air cooling.
• the second cooling step is to rapidly cool (at a cooling rate of 10°C/s or more) the rolled material obtained in the first cooling step from the rapid cooling initiation temperature.
• the rapid cooling initiation temperature is 950 to 990°C, and is lower than the soaking temperature as described above. If the rapid cooling initiation temperature is lower than 950°C, the amount of the ferrite phase produced increases, resulting in a decrease in corrosion resistance. Furthermore, if the rapid cooling initiation temperature is higher than 990°C, it will be difficult to control the composition of the austenite phase within a predetermined range. From the viewpoint of stably suppressing these problems, the rapid cooling initiation temperature is preferably 960 to 980°C.
• the rapid cooling termination temperature is not particularly limited, and it can be, for example, room temperature.
• the cooling method in the second cooling step is not particularly limited, but it may be, for example, water cooling.
• the structure for crude phosphoric acid according to an embodiment of this invention includes the duplex stainless steel material described above.
• the structure for crude phosphoric acid may further include members other than the duplex stainless steel material described above.
• the structure for crude phosphoric acid according to an embodiment of this invention can be produced by processing the above duplex stainless steel into a predetermined shape and then assembling it by welding or the like.
• the structure for crude phosphoric acid includes, but not limited to, preferably phosphoric acid production plants, phosphoric acid storage tanks, or phosphoric acid transport pipes.
• Stainless steels having the compositions shown in Tables 1-1 and 1-2 were smelted in a MgO crucible in a 50 kg vacuum induction furnace in a laboratory and cast into flat steel ingots each having a thickness of about 100 mm.
• the main body of each flat steel ingot was processed into a material for hot rolling, heated at 1180°C, maintained at that temperature for 1 hour, and then hot rolled into a hot-rolled sheet having a thickness of 12 mm.
• Tables 1-1 and 1-2 the values of DF and expected CPT were calculated based on the content of each element.
• the soaking step, the first cooling step and the second cooling step were then carried out under the conditions shown in Table 2 to obtain a duplex stainless steel sheet.
• the cooling in the first cooling step was carried out by air cooling, and the cooling rate was controlled by controlling the flow rate of the cooling gas.
• the material was rapidly cooled to room temperature by water cooling. [Table 2] Nos.
• the content of each element in the austenite phase was be measured by EPMA (electron probe micro-analyzer). Specifically, qualitative analysis was carried out by EPMA using a specimen obtained by mirror-polishing a cross section of the duplex stainless steel material in the thickness direction parallel to the rolling direction. The EPMA was performed at an acceleration voltage of 15 kV, and a 300 ⁇ m ⁇ 300 ⁇ m region was measured in a lattice pattern at 0.76 ⁇ m intervals to obtain data for a total of 140,625 points. In the qualitative analysis, Ni qualitative mapping was performed on the entire cross section to identify portions that could be clearly determined to be the austenite phase based on the Ni content.
• EPMA electron probe micro-analyzer
• a sample having 10 mm ⁇ 80 mm ⁇ 2 mm was taken from the duplex stainless steel sheet, and the entire surface was wet-polished with a No. 600 grindstone.
• An aqueous solution simulating crude phosphoric acid which contained 70% by mass of phosphoric acid, 0.4% by mass of F ions, and 0.04% by mass of Cl ions, was then prepared.
• the container containing the aqueous solution was maintained at 50°C, and half of the sample was immersed therein for 6 hours.
• the corrosion rate (mm/y) was then calculated from the change in mass (corrosion weight loss) before and after immersion. If the corrosion rate is 0.15 mm/y or less, it can be determined that the corrosion resistance is improved. Since the corrosion generated in both the gas and liquid phases, the corrosion rate was calculated using the entire area of the sample.
• the measured CPT was determined in accordance with ASTM G48E by taking a sample having 50 mm x 25 mm x 2 mm thickness (thickness from a position of a depth of 1 mm from the outermost surface) from the duplex stainless steel sheet, and then conducting a pitting corrosion test on this sample at an aqueous solution temperature of 30°C, and measuring the CPT (Critical Pitting Temperature). In measuring the CPT, pitting corrosion generated on the end face (cross section in the thickness direction) of the sample was not counted, and only pitting corrosion generated on a surface of 50 mm x 25 mm was counted.
• the CPT difference (expected CPT - measured CPT) was calculated using the measured CPT thus obtained and the expected CPT calculated above.
• the CPT difference is expressed as circle if it was 10°C or lower, and as ⁇ if it was higher than 10° C.
• No. 27 (Comparative Example) could not control the element concentrations in the austenite phase and did not satisfy the relational expression of the equation (2), because the soaking temperature was too high. As a result, corrosion resistance against crude phosphoric acid was not sufficient.
• No. 28 (Comparative Example) could not control the element concentrations in the austenite phase and did not satisfy the relational expression of the equation (2), because of the excessively high cooling rate in the first cooling step. As a result, corrosion resistance against crude phosphoric acid was not sufficient.
• Nos. 30 and 31 could not control the element concentrations in the austenite phase and did not satisfy the relational expression of the equation (2), because of the excessively low cooling rate in the first cooling step.
• the corrosion resistance against crude phosphoric acid was not sufficient, because chromium nitrides, the ⁇ phases and the like were deposited.
• No. 33 (Comparative Example) had insufficient corrosion resistance against crude phosphoric acid as a result of reduced resistance against the acid because of the higher Mn content.
• No. 40 (Comparative Example) had insufficient corrosion resistance against crude phosphoric acid, because a larger amount of chromium nitride was deposited in the ferrite phase as a result of the excessively high N content.
• No. 41 (Comparative Example) did not obtain a sufficient effect of suppressing Fe elution due to the generation of NH 4 + in the crude phosphoric acid environment, resulting in insufficient corrosion resistance against crude phosphoric acid, because of the excessively low N content.
• the present invention can be the following aspects:

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