EP2199421B1 - Matériau pour fil d'acier inoxydable duplex, fil d'acier, boulon et procédé de fabrication associé - Google Patents

Matériau pour fil d'acier inoxydable duplex, fil d'acier, boulon et procédé de fabrication associé Download PDF

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EP2199421B1
EP2199421B1 EP08837204.0A EP08837204A EP2199421B1 EP 2199421 B1 EP2199421 B1 EP 2199421B1 EP 08837204 A EP08837204 A EP 08837204A EP 2199421 B1 EP2199421 B1 EP 2199421B1
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Prior art keywords
less
cold
steel wire
bolt
strength
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German (de)
English (en)
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EP2199421A1 (fr
EP2199421A4 (fr
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Kohji Takano
Shinji Tsuge
Masayuki Tendo
Yuji Mori
Yoshinori Tada
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Nippon Steel Stainless Steel Corp
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Nippon Steel Stainless Steel Corp
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Priority claimed from JP2007264992A external-priority patent/JP5171197B2/ja
Priority claimed from JP2007264993A external-priority patent/JP5171198B2/ja
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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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D6/02—Hardening by precipitation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0093—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
    • 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
    • C22C38/02—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
    • C22C38/18—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
    • 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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/02—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for springs
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • C21D9/54—Furnaces for treating strips or wire
    • C21D9/56—Continuous furnaces for strip or wire

Definitions

  • the present invention relates to a duplex stainless steel wire which has excellent cold forgeability and can provide, at low cost, for example, high strength bolts having corrosion resistance equivalent to SUS304.
  • the present invention also relates to a soft duplex stainless steel wire material which is magnetizable and has excellent cold workability, and to a soft duplex stainless steel wire material which has excellent cold workability, is magnetizable, and is capable of providing, at low cost, strong cold worked components such as screws, pins, wire gauze, wire, rope, and springs having corrosion resistance equivalent to SUS304 or SUS316.
  • an SUS304 wire material has generally been used for high strength, highly corrosion resistant bolts having strength at the 700 N/mm 2 level.
  • the automobile and consumer electronics industries have spurred demand for stronger (and more lightweight) bolts.
  • the high content of expensive Ni in SUS304 bolts and the resulting high cost has led to strong demand for a lower cost product.
  • Patent Documents 4 to 6 low-Ni duplex stainless steels which limit the use of expensive Ni have been proposed.
  • Products for which corrosion resistance is required such as screws, pins, wire netting and wire, have heretofore been manufactured using austenite stainless steel wire material such as SUS304 or SUSXM7, by severe cold processes such as drawing, cold forging and bending.
  • austenite stainless steel wire material such as SUS304 or SUSXM7
  • severe cold processes such as drawing, cold forging and bending.
  • cold working of a wire material requires softness and a high reduction of area at tensile fracture (high elongation characteristics are not a requirement).
  • this softness requires a value of 700 N/mm 2 or less, and preferably 650 N/mm 2 or less.
  • austenite-based stainless steel products are expensive despite the inexpensive manufacturing process.
  • austenite-based stainless steel leads to further inconvenience, including poor workability due to tools not being able to adhere to close a fastener, or the inability of magnetic sensors to detect when the material used in a wire netting or mesh (particularly in a conveyor belt or the like used with foodstuffs) falls into and contaminates a food item.
  • Patent Documents 10 to 12 a number of varieties of modestly priced duplex stainless steels with reduced Ni have been proposed.
  • Patent Document 10 a high strength duplex stainless steel is disclosed that has an excellent Young's modulus and low Ni content, and contains 0.04% or more nitrogen to enhance strength.
  • a high strength steel exceeding 80 kg/mm 2 , showing that no consideration is given to achieving softness and a high reduction of area at tensile fracture, meaning that in real terms, cold working of the wire material is problematic.
  • Patent Document 11 a duplex stainless steel containing low Ni and not less than 0.05% of nitrogen is disclosed which is corrosion resistant and has favorable weldability.
  • cold workability and with the preferred range for the nitrogen content reported as 0.06% to 0.12% in order to enhance the strength, and the steel (low Si steel) disclosed in an example containing not less than 0.13% of nitrogen, no consideration is given to achieving softness and a high reduction of area at tensile fracture, meaning that in real terms, cold working of the wire material is problematic.
  • Patent Document 12 a low-Ni high strength duplex stainless steel which contains not less than 0.05% nitrogen and has excellent relaxation properties is disclosed.
  • the examples disclose steels containing not less than 0.13% nitrogen in order to enhance the strength, showing that no consideration is given to achieving softness and a high reduction of area at tensile fracture, meaning that in real terms, cold working of the wire material is problematic.
  • Patent Document 13 a low-Ni duplex stainless steel which contains not less than 0.05% of nitrogen and has excellent ductility and deep drawability is disclosed.
  • the examples disclose steels that contain not less than 0.08% of strength-enhancing nitrogen in order to enhance stretching and improve the deep drawability of the steel sheet, showing that no consideration is given to achieving softness and a high reduction of area at tensile fracture, meaning that in real terms, cold working of the wire material is problematic.
  • An object of the present invention is to provide a low-cost austenite-ferrite duplex steel wire for producing high strength and highly corrosion resistant bolts, to provide steel wire and bolts and a method for their manufacture, and to impart cold forgeability and enhance the strength of bolt products by controlling the composition, constituent elements, and material properties of a low-cost corrosion resistant duplex stainless steel wire .
  • Another object of the present invention is to provide a low-cost duplex stainless steel wire material with magnetic properties which offers excellent cold workability and corrosion resistance, to significantly lower the production costs of cold-worked products in comparison to conventional austenite stainless steel wire materials, and to impart magnetizability.
  • the inventors of the present invention discovered that by reducing the amount of expensive Ni contained in a highly corrosion resistant duplex stainless steel, performing component adjustment to stabilize the composition (at a low M value), controlling the ferrite phase to yield a high volume fraction, and optimizing the tensile strength of the wire material and steel wire through heat treatment and drawing, favorable cold forgeability and a high strength bolt product could both be achieved at a low cost.
  • a corrosion resistant duplex stainless steel comprising a magnetic ferrite phase and an austenite phase
  • M value control the amount of expensive Ni and performing component adjustment to control the composition
  • C+N the amount of expensive Ni and performing component adjustment to control the composition
  • the present invention is based on these findings, and adopts the aspects described below.
  • austenite-ferrite duplex stainless steel wire material containing, in terms of mass %:
  • a first aspect of the present invention is an austenite-ferrite duplex stainless steel wire consisting of, in terms of mass %:
  • a second aspect of the present invention is a high strength and highly corrosion resistant bolt composed of the steel wire material according to the first aspect, wherein the tensile strength is within a range from 700 to 1,200 N/mm 2 .
  • a third aspect of the present invention is a method of manufacturing a high strength and highly corrosion resistant bolt, the method including subjecting an austenite-ferrite duplex stainless steel wire composed of the steel wire material according to the first aspect and having a tensile strength of 700 to 1,000 N/mm 2 to cold bolt forming, and then performing an aging heat treatment at 300 to 600°C for 1 to 100 minutes.
  • a fourth aspect of the present invention is a magnetizable soft duplex stainless steel wire material containing, in terms of mass %:
  • the duplex stainless steel wire of the present invention which exhibits excellent cold forgeability and is used for forming high strength and highly corrosion resistant bolts, despite not containing expensive Ni in large quantities, allows excellent cold forgeability to be secured while offering high corrosion resistance and high strength equivalent to or better than SUS304, and thus enables high strength and highly corrosion resistant bolts to be provided at low cost.
  • the soft duplex stainless steel wire material with excellent cold workability despite not containing expensive Ni in large quantities, allows excellent cold workability to be secured while offering magnetizability as well as corrosion resistance equivalent to austenite stainless steel such as SUS304 and SUS316, thus enabling a magnetizable highly corrosion resistant product to be provided at low cost.
  • the C content is not less than 0.005% to ensure the strength of the bolt product. However, if the C content exceeds 0.05%, then not only are Cr carbonitrides produced which adversely affect the corrosion resistance, but cold forgeability also deteriorates, and consequently the C content is restricted to not more than 0.05%. Preferably, the C content does not exceed 0.03%.
  • the N content is not less than 0.005% to ensure strengthening of the bolt product by solid solution strengthening and age hardening. However, if the N content exceeds 0.20%, cold forgeability deteriorates significantly. Therefore, the maximum N content is 0.20%. Preferably, the N content is less than 0.05%.
  • the C+N content for the above reasons related to cold forgeability, is restricted to not exceeding 0.20%.
  • C+N content is not exceeding 0.10%.
  • the Si content is not less than 0.1 %, for deoxidation purposes. However, if the Si content exceeds 1.0%, cold forgeability deteriorates. Therefore, the maximum Si content is 1.0%.
  • the preferred range is from 0.2 to 0.6%.
  • the Mn content is not less than 0.1%, for deoxidation purposes and as an adjustment to obtain a stable austenite structure. However, if the Mn content exceeds 10.0%, rust resistance and the ferrite volume fraction are reduced, tensile strength increases, and cold forgeability deteriorates. Therefore, the maximum Mn content is restricted to 10.0%. The preferred range is from 0.5 to 5.0%.
  • Ni content is not less than 1.0% to stabilize the austenite structure and secure cold forgeability. However, if the Ni content exceeds 6.0%, then the effects achieved reach saturation, the volume fraction of the ferrite phase reaches 45% or less, and the cold forgeability (tool life) deteriorates. The economic viability also suffers due to the high cost of Ni. Therefore, the maximum Ni content is restricted to 6.0%. The preferred range is more than 3.0% but not exceeding 5.0%.
  • the Cr content is not less than 19.0%. However, if the Cr content exceeds 30.0%, then because the effects thereof reach saturation and the volume fraction of the ferrite phase conversely exceeds 85%, the strength of the bolt product is reduced. Therefore, the maximum Cr content is restricted to 30.0%. The preferred range is from 22.0 to 26.0%.
  • the Cu is effective in stabilizing the austenite structure, improving the cold forgeability by suppressing work hardening, and promoting age hardening of the ferrite phase during aging treatment after cold forging, thereby increasing the strength of the bolt product. Therefore, the Cu content is not less than 0.2% but less than 1.0%.
  • the M value represented by formula (a) below is an indicator that contributes to the stability of the austenite phase, and is disclosed in " Testu to Hagane", vol. 63(1977 ), page 772.
  • M value is high, a rigid work-induced martensite phase is produced.
  • the M value exceeds 60, then a rigid work-induced martensite phase is produced during cold forging, and the cold forgeability deteriorates markedly (tool life is adversely affected and cracking occurs during cold forging). Therefore, the M value is restricted to 60 or less, and in a preferred range does not exceed 40.
  • M 551 ⁇ 462 C + N ⁇ 9.2 Si ⁇ 8.1 Mn ⁇ 29 Ni + Cu ⁇ 13.7 Cr ⁇ 18.5 Mo
  • the F value represented by formula (b) below is an indicator that contributes to the volume fraction of the ferrite phase, and is disclosed in Japanese Examined Patent Application, Second Publication No. Hei 7-74416 .
  • a higher F value indicates an increased ferrite phase.
  • FIG. 1 examines the volume fraction of the ferrite phase of the duplex stainless steel wire material product at various F values. If the F value is 45 or higher, then the volume fraction of the ferrite phase reaches 45 vol.% or more, which indicates high yield strength and low work hardening characteristics ( FIG. 2 ), allows a high product strength (the tensile strength of the bolt shaft) in the order of 700 to 1,200 N/mm 2 to be obtained, and ensures favorable cold forgeability of the head portion.
  • the F value is restricted to not less than 45.
  • the processing ratio (%) and the compressive deformation stress (N/mm 2 ) with respect to F values in FIG. 2 show that when the F value is less than 45, significant work hardening occurs, and the cold forgeability (in terms of rolling cracks and tool damage) deteriorates markedly.
  • the F value exceeds 85 then the soft ferrite phase exceeds 85%, and the high-strength austenite phase is reduced, which conversely decreases the strength of the bolt product. Therefore, the maximum F value is 85.
  • the preferred range is from 50 to 80.
  • F 5.6 Cr ⁇ 7.1 Ni + 2.4 Mo + 15 Si ⁇ 3.1 Mn ⁇ 300 C ⁇ 134 N ⁇ 26.6
  • the tensile strength of the herein disclosed - but not claimed - wire material contributes significantly to its cold forgeability, and when the tensile strength of the wire material is less than 550 N/mm 2 , the strength of cold-forged products such as bolts is low, giving a less worthwhile high strength product. Therefore, the minimum tensile strength is restricted to 550 N/mm 2 . On the other hand, when the tensile strength of the wire material exceeds 750 N/mm 2 , the cold forgeability (in terms of rolling cracks and tool damage) tends to deteriorate markedly. Therefore, the maximum tensile strength is restricted to 750 N/mm 2 . The preferred range is from 600 to 700 N/mm 2 .
  • Mo is an element that is effective in improving corrosion resistance, and this effect can be achieved in a stable manner by adding 0.1% or more of Mo.
  • the Mo content exceeds 1.0%, not only do material costs rise, but hardening of the materials occurs and the cold forgeability deteriorates. Accordingly, the maximum Mo content is restricted to 1.0%.
  • the preferred range is not less than 0.2% but less than 0.5%.
  • B is an element that is effective in improving hot workability, and this effect can be achieved in a stable manner by adding 0.001% or more of B.
  • the maximum B content is restricted to 0.01%.
  • the preferred range is from 0.002% to 0.006%.
  • Al, Mg, and Ca are effective for deoxidation, and this effect can be achieved in a stable manner by adding one or more of Al: not less than 0.005%, Mg: not less than 0.001%, and Ca: not less than 0.001%.
  • the Al, Mg and Ca content exceeds 0.1%, 0.01% and 0.01% respectively, then the effects thereof reach saturation, and adversely coarse oxides (inclusions) are produced, which can cause cracking during cold forging. Therefore, the maximum Al, Mg and Ca content is restricted to 0.1%, 0.01%, and 0.01%, respectively.
  • the preferred ranges for these elements are one or more of Al: 0.01 to 0.06%, Mg: 0.002 to 0.005%, and Ca: 0.002 to 0.005%.
  • Nb, Ti, V, and Zr are effective for ensuring corrosion resistance by suppressing the formation of Cr carbonitrides, and this effect can be achieved in a stable manner by adding one or more of Nb: not less than 0.05%, Ti: not less than 0.02%, V: not less than 0.05%, and Zr: not less than 0.05%.
  • Nb, Ti, V, and Zr content exceeds 1.0%, 0.5%, 1.0 and 1.0% respectively, the effects thereof reach saturation, and adversely coarse precipitates are produced, which can cause cracking during cold forging. Therefore, the maximum amount of each element is restricted.
  • the preferred ranges for these elements are one or more of Nb: 0.1 to 0.6%, Ti: 0.05 to 0.5%, V: 0.1 to 0.6%, and Zr: 0.1 to 0.6%.
  • steel contains oxygen inherent to the manufacturing process as an unavoidable impurity, but in the present invention, the steel preferably contains not more than 0.01% oxygen as an unavoidable impurity.
  • a drawn steel wire is produced by subjecting the wire material to wire drawing, but the tensile strength of the steel wire contributes significantly to the cold forgeability and the strength of the bolt product, and if the tensile strength of the steel wire is less than 700 N/mm 2 , then the strength of the bolt product is reduced, giving a less worthwhile high strength product. Therefore, the minimum tensile strength is restricted to 700 N/mm 2 .
  • the tensile strength of the steel wire exceeds 1,000 N/mm 2 , the cold forgeability deteriorates markedly (tool life is adversely affected and cracking occurs during cold forging). Therefore, the maximum tensile strength is restricted to 1,000 N/mm 2 .
  • the preferred range is from 700 to 900 N/mm 2 .
  • the tensile strength of the high strength bolt of the present invention is strengthened by the aging heat treatment performed after wire drawing and cold forging. At this time, if the tensile strength of the bolt product is less than 700 N/mm 2 , the bolt product is less worthwhile as a high strength bolt product. On the other hand, if the tensile strength of the bolt product exceeds 1,200 N/mm 2 , costs related to cold forging increase markedly due to cracking during cold forging and tool damage and the like. Therefore, the maximum tensile strength of the bolt product is restricted to 1,200 N/mm 2 . The preferred range for demonstrating economic effectiveness is from 800 to 1,000 N/mm 2 .
  • an effective means of improving the tensile strength of the bolt product is to perform aging heat treatment at not less than 300°C for not less than 1 minute.
  • temperatures exceeding 600°C result in overaging, which reduces the tensile strength of the bolt product. Therefore the maximum temperature is limited to 600°C.
  • the preferred temperature range is from 400 to 550°C.
  • the maximum aging time is restricted to 100 minutes.
  • the preferred range for the aging time is from 5 to 60 minutes.
  • C is added in an amount of not less than 0.005% to ensure the strength of the steel.
  • the C content exceeds 0.05%, not only does the cold workability deteriorate, but Cr carbonitrides are also produced which adversely affect the corrosion resistance. Consequently, the maximum C content is restricted to 0.05%.
  • the preferred range is from 0.01 to 0.03%.
  • N is added in an amount not exceeding 0.005% to ensure the strength of the cold-worked product by solid solution strengthening.
  • the maximum amount of N is less than 0.06%.
  • not less than 0.06% of N is added to duplex stainless steels in order to minimize the use of expensive alloying elements, but a characteristic of the steel of the present invention is that the composition and component balance are controlled and the N content is kept low to dramatically improve the cold workability of the soft wire material.
  • the preferred range is not less than 0.02% but less than 0.05%.
  • the C+N content for the above reasons related to cold workability, is restricted to 0.09% or less. Preferably, the C+N content does not exceed 0.07%.
  • Si is added in an amount of not less than 0.1% to effect deoxidation. However, if Si is added in an amount exceeding 1.0%, the steel hardens and the cold workability deteriorates. Therefore, the maximum Si content is 0.1%. The preferred range is from 0.2% to 0.6%.
  • Mn is added in an amount of not less than 0.1% in order to effect deoxidation and obtain a duplex ferrite-austenite structure, and as an adjustment to stabilize the austenite structure.
  • Mn is added in an amount exceeding 10.0%, the corrosion resistance and strength rise which adversely affects the cold workability. Therefore, the maximum Mn content is 10.0%.
  • the preferred range is from 0.5% to 5.0%.
  • Ni is added in an amount of not less than 1.6% in order to lower the M value and obtain a ferrite-austenite structure, and to stabilize the austenite structure to ensure favorable cold workability.
  • the amount of added Ni exceeds 6.0%, the effects thereof reach saturation, and economic viability suffers because Ni is an expensive element. Therefore, the maximum Ni content is restricted to 6.0%.
  • the preferred range is from 2.0% to 5.0%.
  • Cr is added in an amount of not less than 19.0% in order to ensure corrosion resistance and obtain a ferrite-austenite duplex structure, and to stabilize the austenite structure to ensure favorable cold workability.
  • the maximum Cr content is restricted to 30.0%.
  • the preferred range is from 20.0% to 26.0%.
  • Cu is added in an amount of not less than 0.05% in order to lower the M value and obtain a ferrite-austenite structure, stabilize the austenite structure, and suppress work hardening, thereby improving the cold workability.
  • the maximum Cu content is restricted to 3.0%. The preferred range is less than 1.0%.
  • the M value represented by formula (a) below is an indicator that contributes to the stability of the austenite phase, and is disclosed in " Testu to Hagane", vol. 63(1977 ), page 772.
  • M value is high, a rigid work-induced martensite phase is produced.
  • M value is restricted to 60 or less, and in a preferred range does not exceed 40.
  • M 551 ⁇ 462 C + N ⁇ 9.2 Si ⁇ 8.1 Mn ⁇ 29 Ni + Cu ⁇ 13.7 Cr ⁇ 18.5 Mo
  • the tensile strength of the wire material contributes significantly to its cold workability, and when the tensile strength of the wire material exceeds 700 N/mm 2 , the cold workability deteriorates markedly. Therefore, the maximum tensile strength is restricted to 700 N/mm 2 .
  • the tensile strength of the wire material is less than 500 N/mm 2 , the strength of the cold-forged product is too low, making the resulting product less viable. Therefore, preferably, the minimum tensile strength is restricted to 500 N/mm 2 .
  • the preferred range is from 500 to 650 N/mm 2 .
  • the reduction of area at tensile fracture properties of the wire material contribute significantly to the cold workability of the wire material, and when the reduction of area at tensile fracture is less than 70%, the workability of cold processes such as cold drawing and cold forging deteriorates. Therefore, the reduction of area at tensile fracture is limited to not less than 70%.
  • the preferred range is not less than 75%.
  • Magnetizability is a feature not inherent to austenite stainless steel, and for reasons including improving workability by offering magnetizability with respect to the magnetic tools used to close fasteners, and allowing magnetic sensors to detect when a material used as a wire netting or mesh (particularly in a conveyor belt or the like used with foodstuffs) falls into and contaminates a food product, magnetizability is a significant feature in industrial terms. Therefore, the degree of magnetizability is specified in the present invention.
  • the relative magnetic permeability is preferably not less than 3.0.
  • Mo is an element that is effective in improving the corrosion resistance, and this effect can be achieved in a stable manner by adding 0.1% or more of Mo. However, if Mo is added in an amount exceeding 3%, then the material undergoes hardening, and sigma phase precipitation occurs, causing a marked deterioration in the cold workability. Accordingly, the maximum Mo content is restricted to 3%. The preferred range is from 0.3% to 1.0%.
  • B is an element that is effective in improving the hot workability, and this effect can be achieved in a stable manner by adding 0.001% or more of B.
  • the maximum B content is restricted to 0.01%.
  • the preferred range is from 0.002% to 0.006%.
  • Al, Mg, and Ca are effective for deoxidation, and this effect can be achieved in a stable manner by adding one or more of Al: not less than 0.005%, Mg: not less than 0.001%, and Ca: not less than 0.001%.
  • the Al, Mg and Ca content exceeds 0.1%, 0.01% and 0.01% respectively, the effects thereof reach saturation, and adversely coarse oxides (inclusions) are produced, resulting in poor cold workability. Therefore, the maximum content of Al, Mg, and Ca is restricted to 0.1%, 0.01% and 0.01%, respectively.
  • the preferred ranges for these elements are one or more of Al: 0.008 to 0.06%, Mg: 0.001 to 0.005%, and Ca: 0.001 to 0.005%.
  • Nb, Ti, V, and Zr are effective for ensuring corrosion resistance by suppressing the formation of Cr carbonitrides, and this effect can be achieved in a stable manner by adding one or more of Nb: not less than 0.01%, Ti: not less than 0.01%, V: not less than 0.01%, and Zr: not less than 0.01%.
  • Nb, Ti, V and Zr content exceeds 1.0%, 0.5%, 1.0 and 1.0% respectively, the effects thereof reach saturation, and adversely coarse precipitates are produced, resulting in poor cold workability. Therefore, the maximum amount of each element is restricted.
  • the preferred ranges for these elements are one or more of Nb: 0.05 to 0.6%, Ti: 0.05 to 0.5%, V: 0.1 to 0.6%, and Zr: 0.05 to 0.6%.
  • steel contains oxygen inherent to the manufacturing process as an unavoidable impurity, but in the present invention, the steel preferably contains not more than 0.01% oxygen as an unavoidable impurity.
  • Example 1 of the present invention is described below.
  • Tables 1 through 4 show the chemical composition of the steels according to example 1.
  • the steel wires of the examples of the present invention were all within a range from 650 to 1,000 N/mm 2
  • the bolt products of the examples of the present invention were all within a range from 700 to 1,000 N/mm 2 , indicating a high level of strength.
  • the ferrite fraction in the steel wire of the examples of the present invention was within a range from 45% to 85% by volume.
  • the cold forgeability was evaluated by using three-stage heading equipment to form 5000 hexagonal heads, and checking for the presence of cracking or tool damage. A symbol O was recorded in the tool life column if no tool damage occurred, and a symbol ⁇ was recorded if tool damage occurred. With the steel wire of the examples of the present invention, no cold cracking was observed and the tool life was evaluated using the symbol O, indicating excellent cold forgeability.
  • the corrosion resistance of the bolt product was evaluated by subjecting 10 units of each bolt product to the salt spray test prescribed in JIS Z 2371 for 100 hours, and determining whether or not rusting occurred. If rust was absent, or present only in the form of minor rust spots, a symbol O was recorded in the corrosion resistance column. If outflow rust was present or rust appeared over the entire surface, a symbol x was recorded in the corrosion resistance column.
  • the bolt products of the examples of the present invention all achieved an evaluation of O for the corrosion resistance.
  • comparative examples 38 to 61 which were outside the scope of the present invention, were inferior in terms of properties such as the cold forgeability, bolt product strength and/or corrosion resistance, clearly demonstrating the superiority of the present invention.
  • Table 9 and Table 10 show the chemical composition (in terms of mass %) of the steels (sample materials) used in example 2.
  • Steels having these chemical compositions were each melted in a 150 kg vacuum melting furnace and cast into a ⁇ 180 mm steel slab.
  • the steel slab was then subjected to a hot wire rod rolling process to a diameter of ⁇ 5.5 mm, and after completing hot rolling at 1050°C, the resulting product was maintained at 1050°C for 5 minutes, subjected to continuous water-cooled heat treatment, and then acid-washed to obtain a wire material. Thereafter, heavy cold drawing was performed by a standard process to a diameter of ⁇ 2.0 mm, and the resulting steel wire was subjected to bending by a cold process to obtain a wire netting mesh for use in a conveyor.
  • the tensile strength and reduction of area at tensile fracture for the wire materials were evaluated by using the tensile test method prescribed in JIS Z 2241 to evaluate the tensile strength and the reduction of area at fracture.
  • the wire materials of examples No. 62 to 85 (which are not within the claims) all exhibited a tensile strength within the range from 500 to 700 N/mm 2 , and a reduction of area at fracture of ⁇ 70%.
  • the cold workability was evaluated in terms of the cold drawing process and subsequent wire workability. If an unbroken and unbent wire netting was produced, a symbol O was recorded in the cold forgeability column, and if the wire netting could not be formed due to wire breakage or bending or the like, a symbol x was recorded.
  • the wire materials of examples No. 62 to 85 (which are not within the claims) exhibited no breakage or bending, and displayed excellent cold workability.
  • the corrosion resistance was evaluated by polishing the surface layer of the acid-washed wire material with a #500 sandpaper, and then performing the salt spray test prescribed in JIS Z 2371 for 100 hours and determining whether or not rusting occurred. If rust was absent, or present only in the form of minor rust spots, a symbol O was recorded in the corrosion resistance column. If outflow rust was present or rust appeared over the entire surface, a symbol x was recorded.
  • the steels of Examples No. 62 to 85 (which are not within the claims) all achieved an evaluation of O for the corrosion resistance.
  • the magnetizability was determined by using a ferrite meter (a simple instrument for measuring magnetic permeability) to measure the relative magnetic permeability of the wire netting. If the wire netting had a relative magnetic permeability of 3.0 or higher, at which magnetizability is clearly demonstrable, the wire netting was adjudged to be magnetizable, whereas at less than 3.0, the wire mesh was adjudged non-magnetizable.
  • comparative examples 86 to 107 which are outside the scope of the present invention, were inferior in terms of properties such as the cold workability, corrosion resistance, cost, and magnetizability, clearly demonstrating the superiority of the present invention.
  • the highly corrosion resistant duplex stainless steel wire material of the present invention which contains only a small amount of expensive Ni, provides excellent cold forgeability and the ability to increase the strength of a bolt product, thus enabling a high strength and highly corrosion resistant bolt to be provided at low cost, while also being applicable to nuts, and is therefore extremely useful in industrial terms.
  • the present invention can also be used to manufacture a soft and magnetizable low-cost duplex stainless steel wire material, can impart excellent cold workability as well as corrosion resistance equivalent to austenite stainless steels such as SUS304 and SUS316, and is capable of providing magnetizable and highly corrosion resistant cold forged components such as screws, pins, wire netting, wire, rope and springs, and is therefore extremely useful in industrial terms.

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Claims (3)

  1. Fil en acier duplex austénite-ferrite ayant une excellente forgeabilité à froid pour produire un boulon ayant une résistance mécanique élevée et une résistance à la corrosion élevée, le fil d'acier étant constitué, en termes de pourcentages en masse, de
    C : 0,005 à 0,05 %,
    Si : 0,1 à 1,0 %,
    Mn : 0,1 à 10,0 %,
    Ni : 1,0 à 6,0 %,
    Cr : 19,0 à 30,0 %,
    Cu : au moins 0,2 %, mais moins de 1,0 %,
    N : 0,005 à 0,20 %,
    éventuellement un ou plusieurs choisis parmi Mo : au plus 1,0 %, B : au plus 0,01 %, Al : au plus 0,1 %, Mg : au plus 0,01 %, et Ca : au plus 0,01 %, Nb : au plus 1,0 %, Ti : au plus 0,5 %, V : au plus 1,0 %, et Zr : au plus 1,0 %, le reste étant du fer et des impuretés inévitables,
    dans lequel C+N ne vaut pas plus de 0,20 %, la valeur M représentée par la formule (a) ne vaut pas plus de 60, la valeur F représentée par la formule (b) vaut de 45 à 85, et la résistance à la traction est située dans la plage allant de 700 à 1 000 N/mm2, M = 551 − 462 C + N − 9,2 Si − 8,1 Mn − 29 Ni + Cu − 13,7 Cr − 18,5 Mo
    Figure imgb0025
     F = 5,6 Cr − 7,1 Ni + 2,4 Mo + 15 Si − 3,1 Mn − 300 C − 134 N − 26,6
    Figure imgb0026
  2. Boulon ayant une résistance mécanique élevée et une résistance à la corrosion élevée, le boulon ayant une composition chimique décrite dans la revendication 1, et une résistance à la traction située dans la plage allant de 700 à 1 200 N/mm2.
  3. Méthode de fabrication d'un boulon ayant une résistance mécanique élevée et une résistance à la corrosion élevée, la méthode comprenant :
    la soumission d'un fil en acier duplex austénite-ferrite ayant une composition chimique décrite dans la revendication 1 et ayant une résistance à la traction située dans la plage allant de 700 à 1 000 N/m2 à une formation de boulon à froid, et ensuite la mise en œuvre d'un traitement de vieillissement à la chaleur à une température de 300 à 600 °C pendant 1 à 100 minutes.
EP08837204.0A 2007-10-10 2008-10-10 Matériau pour fil d'acier inoxydable duplex, fil d'acier, boulon et procédé de fabrication associé Active EP2199421B1 (fr)

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JP2007264993A JP5171198B2 (ja) 2007-10-10 2007-10-10 冷間加工性に優れた着磁性を有する軟質２相ステンレス鋼線材
PCT/JP2008/068467 WO2009048137A1 (fr) 2007-10-10 2008-10-10 Matériau pour fil d'acier inoxydable duplex, fil d'acier, boulon et procédé de production du boulon

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