EP1484424B1 - Rostfreier ferritischer Bandstahl mit hervorragenden Press- sowie sekundären Verformungseigenschaften und Verfahren zu seiner Herstellung - Google Patents

Rostfreier ferritischer Bandstahl mit hervorragenden Press- sowie sekundären Verformungseigenschaften und Verfahren zu seiner Herstellung Download PDF

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EP1484424B1
EP1484424B1 EP04012345A EP04012345A EP1484424B1 EP 1484424 B1 EP1484424 B1 EP 1484424B1 EP 04012345 A EP04012345 A EP 04012345A EP 04012345 A EP04012345 A EP 04012345A EP 1484424 B1 EP1484424 B1 EP 1484424B1
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mass
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steel sheet
precipitates
annealing
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EP1484424A1 (de
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Yasutoshi Steel & Tech. Dev. Labs. Hideshima
Naoto Steel & Tech. Dev. Labs. Hiramatsu
Kouki Steel & Tech. Dev. Labs. Tomimura
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Nippon Steel Nisshin Co Ltd
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Nisshin Steel Co Ltd
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0405—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0468—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment between cold rolling steps
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0463—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment following hot rolling
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
    • C21D8/0473—Final recrystallisation annealing

Definitions

  • the present invention relates to a ferritic stainless steel sheet, which can be press-formed to a predetermined profile without defects such as poor circularity and torsion and then secondary-formed to a final profile with good hot-extruding, and also relates to a method of manufacturing thereof.
  • Ferritic stainless steels represented by SUS430 or SUS430LX, have been used so far in various fields, e.g. durable consumers' goods, due to good corrosion-resistance and cheapness compared with austenitic stainless steels, which contain Ni as an expensive element.
  • Conditions for press-forming a ferritic stainless steel sheet to a product profile become severe and severe as development of its application.
  • a press-formed steel sheet is often secondarily formed for extrusion of a hole, for instance.
  • there is a strong demand for provision of a new ferritic stainless steel sheet which is fairly excellent in formability compared with conventional ferritic stainless steel sheets and so formed to a product profile without defects even under severe conditions.
  • JP2000-192199A teaches distribution of magnesium inclusions, which are effective on anti-ridging property, in a ferritic stainless steel containing both of Ti and Nb.
  • JP8-26436B JP-A-51086015 ) teaches combination of hot-rolling conditions, which are designed for improvement of Lankford value (r) as an index of formability, with addition of Ti and Nb.
  • Shape-freezability and secondary formability of a primary-formed steel sheet, which will be formed to a final profile, are also important factors as well as to Lankford value (r) and anti-ridging property.
  • a ferritic stainless steel sheet is generally inferior in formability to an austenitic stainless steel sheet. Especially, it significantly reduces thickness in a primary-formed state, and thickness reduction is anisotropic. Consequently, dimensional accuracy such as circularity becomes worse as severer forming conditions, when the ferritic stainless steel sheet is press-formed to a cylindrical profile. Thickness deviation in the primary-formed state leads to serious degradation of secondary formability such as hole-extruding.
  • a ferritic stainless steel sheet maintains high dimensional accuracy (e.g. circularity, straightness and anti-torsion) as well as secondary formability in a press-formed state
  • a cheap ferritic stainless steel sheet can be used as parts or members, instead of an expensive austenitic stainless steel sheet, which have been unavoidably used in view of severe forming conditions.
  • EP-A2-1308532 discloses a ferritic steel sheet manufactured by hot rolling, annealing, optionally double cold rolling and intermediate annealing and finish annealing suitable to be deep-drawn and having brittle resistance to secondary processing, the steel possesses a controlled grain size and carbonitrides of Nb and Ti.
  • the present invention aims at provision of a ferritic stainless steel sheet improved in dimensional accuracy and secondary formability in a press-formed state by controlling particle size and distribution of precipitates, which are dispersed in a steel matrix.
  • the present invention as defined in claim 1, proposes a new ferritic stainless steel sheet, which consists of 0.02 mass % or less of C, 0.8 mass % or less of Si, 1.5 mass % or less of Mn, 0.050 mass % or less of P, 0.01 mass % or less of S, 8.0-35.0 mass % of Cr, 0.05 mass % or less of N, 0.05-0.40 mass % of Ti, 0.10-0.50 mass % of Nb, optionally one or more selected from the group consisting of 0.5 mass % or less of Ni, 3.0 mass % or less of Mo, 2.0 mass % or less of Cu, 0.3 mass % or less of V, 0.3 mass % or less of Zr, 0.3 mass % or less of Al and 0.0100 mass % or less of B, and the balance being Fe except inevitable impurities with a product of (%Ti x %N) less than 0.005.
  • Its metallurgical structure is defined by distribution of precipitates of 0.15 ⁇ m or
  • the ferritic stainless steel sheet is manufactured as disclosed in claim 2 and in particular, as follows:
  • a molten steel with a predetermined composition is cast to a slab.
  • the slab is hot-rolled to a steel sheet at a finish-temperature of 800°C or lower and annealed at 450-1080°C.
  • the annealed hot-rolled steel sheet is pickled and cold-rolled in accompaniment with at least one intermediate-annealing within a temperature range of from (a recrystallization-finishing temperature - 100°C) to (a recrystallization-finishing temperature).
  • the cold-rolled steel sheet is finally subjected to finish-annealing at a temperature of 1080°C or lower.
  • the hot-rolled steel sheet may be box-annealed for a predetermined time period of one hour or shorter.
  • the inter-mediate annealing and the finish-annealing may be performed in a continuous annealing furnace for one minute or shorter.
  • Fig. 1 is a view for explaining circularity of a steel sheet, which is cylindrically formed by a multiplaten press.
  • the inventors have researched and examined on manufacturing conditions of ferritic stainless steel sheets for improvement of dimensional accuracy (e.g. circularity, straightness and torsion) from various aspects, and discovered that circularity and secondary formability of a press-formed steel sheet are fairly affected by shape and distribution of TiN and other precipitates in an annealed state.
  • the inventors presume on the basis of the discovery that objective properties are imparted to a ferritic stainless steel sheet by properly controlling shape and distribution of the precipitates.
  • Formation of the precipitates to the shape and distribution suitable for the purpose is realized by adding both of Ti and Nb to a ferritic stainless steel at a value more than a stoichiometric ratio for stabilization of C and N as carbonitrides and subjecting the ferritic stainless steel to optimum thermo-mechanical treatment.
  • C and N in a ferritic stainless steel are mostly precipitated as carbonitrides by addition of Ti and Nb.
  • the precipitated carbonitrides except TiN are substantially reformed to very fine particles in the manufacturing process of from annealing a hot-rolled steel sheet through cold-rolling to finish-annealing.
  • the fine particles allow predominant growth of recrystallized grains with a certain orientation without pinning action, when the manufactured steel sheet is annealed for recrystallization, resulting in formation of an anisotropic grain-mixed structure.
  • the anisotropy causes concentration of strains along a certain direction during primary-forming a steel sheet and so worsens press formability and dimensional accuracy of the steel sheet.
  • the pinning action during recrystallization annealing is expected by distribution of precipitates having a particle size bigger than a certain value.
  • the pinning action suppresses orientative grain growth or growth to coarse grains, so as to improve isotropy and dimensional accuracy of a press-formed steel sheet.
  • the effects of the pinning action on press formability and dimensional accuracy are typically noted by distribution of precipitates of 0.15 ⁇ m or more in particle size except TiN at a rate of 5000-50000/mm 2 , as recognized in the later examples.
  • TiN is disadvantageous for press formability and dimensional accuracy.
  • a steel sheet which has a product of (%Ti ⁇ %N) above 0.005
  • Coarse TiN particles which have grown to a cubic shape, are observed at starting points of cracks. The observation result means that strains concentrate at cubic apices during press-forming and induce micro-cracks. Concentration of strains and formation of micro-cracks around the TiN particles are also unfavorable for hole-extruding in a secondary-forming step.
  • the inventive ferritic stainless steel sheet contains alloying components at predetermined ratios, as follows:
  • a C content is controlled at a lowest possible level, i.e. 0.02 mass % or less for formability and corrosion-resistance.
  • the C content is preferably reduced to 0.015 mass % or less for improvement of secondary formability.
  • a lower limit of the C content is preferably determined at 0.001 mass %. Definition of the lower limit also assures the effect of carbides on random growth of recrystallized ferritic grains in a finish-annealing step.
  • Si is an alloying component, which is added as a deoxidizing agent during a steel-making process, but has a strong solid-solution hardening effect. Excess Si above 0.8 mass % unfavorably hardens a steel sheet, resulting in poor ductility.
  • An upper limit of a Si content is preferably determined at 0.5 mass % for ductility and secondary formability.
  • Mn does not harden a steel sheet so much due to its solid-solution hardening effect weaker than Si. However, excess Mn above 1.5 mass % causes discharge of manganese fumes during a steel-making process, resulting in poor productivity.
  • P is harmful on hot-workability, so that its upper limit is determined at 0.050 mass %.
  • S is a harmful element, which segregates at grain boundaries and embrittles the grain boundaries. Such defects can be suppressed by controlling a S content to 0.01 mass % or less.
  • a Cr content is controlled to 8.0 mass % or more for assuring corrosion-resistance of a stainless steel.
  • toughness and formability of the stainless steel becomes worse as an increase of the Cr content, so that an upper limit of the Cr content is determined at 35.0 mass %.
  • the Cr content is preferably controlled to 20.0 mass % or less for further improvement of ductility and secondary formability.
  • N is converted to nitrides effective for random growth of recrystallized ferritic grains in a finish-annealing step, but has a hardening effect. Since excess N degrades ductility of a steel sheet, a N content is controlled to a lowest possible level, i.e. 0.05 mass % or less. The N content is preferably controlled to 0.02 mass % or less for further improvement of ductility and secondary formability. However, reduction of the N content to an extremely lower level necessitates a long-term refining operation and raises a steel-manufacturing cost. Therefore, a lower limit of the N content is preferably determined at 0.001 mass %. Definition of the lower limit also assures the effect of nitrides on random growth of recrystallized ferritic grains in a finish-annealing step.
  • Ti is an alloying component, which stabilizes C and N as carbonitrides for formability and corrosion-resistance. Such effect is apparently noted at a Ti content of 0.05 mass % or more. However, excess Ti above 0.40 mass % leads to rising of a steel cost and induces surface defects originated in titanium inclusions.
  • Nb which has the same effect as Ti on stabilization of C and N, is an essential component for precipitation of niobium inclusions of 0.15 ⁇ m or more in particle size except TiN.
  • the niobium inclusions are probably composed of carbides and Fe 2 Nb.
  • a Nb content of 0.10 mass % or more is necessary for precipitation of such niobium inclusions.
  • excess Nb above 0.50 mass % causes exaggerated precipitation and unfavorably raises a recrystallizing temperature of a ferritic stainless steel.
  • Ni is an optional element for toughness of a hot-rolled steel sheet and corrosion-resistance. But excess addition of Ni raises a material cost and hardens a steel sheet, so that an upper limit of a Ni content is determined at 0.5 mass %.
  • Mo is an optional element for corrosion-resistance, but excess Mo above 3.0 mass % is unfavorable for hot-workability.
  • Cu is an optional element, which is often included in a stainless steel from scraps during a steel-making process. Since excess Cu causes poor toughness and degradation of hot-workability, a Cu content is controlled to 2.0 mass % at most.
  • V and Zr are optional elements. V fixes free C as carbide in a steel matrix for formability, while Zr captures free O for formability and toughness. However, excess addition of V or Zr is necessarily avoided for productivity. In this sense, an upper limit each of V and Zr is determined at 0.3 mass %.
  • Al is an optional element, which is added as a deoxidizing agent in a steel-making process.
  • excess Al above 0.3 mass % causes an increase of nonmetallic inclusions, resulting in poor toughness and surface defects.
  • B is an optional element, which stabilizes N and improves corrosion-resistance and formability of a stainless steel.
  • the effects of B are apparently noted at 0.0010 mass % or more, but excess B above 0.0100 mass % is disadvantageous for hot-workability and weldability.
  • Ca, Mg, Co, REM (rare earth metals), etc. other than the above elements may be included from scraps during steel-making. Such elements do not put significant effects on circularity of a deeply-drawn steel sheet or dimensional accuracy of a press-formed steel sheet, unless they are included at extraordinary ratios.
  • TiN grows to coarse particles or forms clusters as an increase of (%Ti ⁇ %N).
  • the coarse TiN particles or cluster promotes accumulation of strains during primary-forming, resulting in formation of micro-cracks at an early stage of a drawing step.
  • Such harmful effects of the coarse TiN particles or cluster are eliminated by control of (%Ti ⁇ %N) to a value less than 0.005, as recognized in the later examples.
  • Carbide and nitride precipitates of 0.15 ⁇ m or more in particle size have a pinning action to suppress orientative grain growth and also growth to coarse grains, so as to improve isotropy of a stainless steel sheet, circularity in a cylindrically-drawn state and dimensional accuracy in a press-formed state.
  • Precipitates are carbides and nitrides of Ti and Nb, Laves phase and mixtures thereof.
  • TiN particles which are precipitated in a cubic shape, are excluded from the precipitates effective for press formability and dimensional accuracy, since the cubic TiN particles are likely to concentrate strains at their apices and act as a starting point of micro-cracks.
  • Distribution of the precipitates of 0.15 ⁇ m or more in particle size except TiN at a rate of 5000-50000/mm 2 assures a pinning action effective for press formability and dimensional accuracy of a press-formed steel piece, as noted in the later examples.
  • the effects of the precipitates on press formability and dimensional accuracy of a press-formed steel piece are noted at a particle size of 0.15 ⁇ m or more, and become bigger as an increase of the particle size.
  • coarse precipitates above 1.0 ⁇ m in particle size are unfavorable, since the coarse particles promote accumulation of strains and formation of micro-cracks during press-forming, resulting in poor shape-freezability
  • the pinning action of the precipitates is apparently noted at a distribution rate of 5000/mm 2 or more, but excess distribution of the precipitates above 50000/mm 2 rather degrades ductility and deep-drawability of a steel sheet.
  • the excess distribution unfavorably raises a recrystallizing temperature of the steel sheet, so that the steel sheet is hardly annealed to a recrystallized state.
  • a ferritic stainless steel sheet is hot-rolled at a relatively lower finish-temperature in order to induce nuclear site for precipitates, which will be distributed in a finish-annealed steel sheet. Boundaries of ferritic grains and internal strains in a hot-rolled state serve as the nuclear site.
  • a finish-temperature of hot-rolling is determined at 800°C or lower in order to induce the nuclear sites as many as possible.
  • Precipitates in a hot-rolled steel sheet are moderated to a shape suitable for controlling the precipitates, which will be distributed in a finish-annealed steel sheet, to 0.15 ⁇ m or more in particle size, by annealing the hot-rolled steel sheet at 450-1080°C. If the annealing temperature is lower than 450°C, effective precipitates are scarcely formed. If the hot-rolled steel sheet is heated at a temperature above 1080°C on the contrary, the precipitates except TiN are unpreferably re-dissolved in a steel matrix.
  • the annealing is completed in one hour for properly controlling distribution number of precipitates without growth to coarse particles.
  • a steel sheet is annealed at a relatively lower temperature in order to inhibit re-dissolution of the precipitates, which have been formed by annealing the hot-rolled steel sheet.
  • a temperature for intermediate-annealing just below a recrystallization-finishing temperature is preferable for relief of stress, which is introduced into the steel sheet by cold-rolling.
  • the steel sheet can be softened without re-dissolution of precipitates regardless somewhat remains of rolling texture, which is not recrystallized yet, as far as the annealing temperature is held within a range of from (a recrystallizing temperature - 100°C) to (a recrystallizing temperature).
  • the intermediate annealing period is completed in one minute in order to avoid re-dissolution of the precipitates, accounting faculty of a conventional continuous annealing furnace.
  • a rolling texture is eliminated by finish-annealing. But, a heating temperature above 1080°C is not only disadvantageous for mass-productivity but also promotes re-dissolution of the precipitates and growth to coarse grains, resulting in poor toughness.
  • the finish-annealing is completed in one minute, accounting faculty of a conventional continuous annealing furnace.
  • the inventors have investigated effects of TiN, which is often precipitated in a ferritic stainless steel matrix, as well as shape effects of precipitates on dimensional accuracy and secondary formability of a press-formed steel piece under the following conditions.
  • Table 1 shows the Nb, Ti and N contents together with a product of (%Ti ⁇ %N) and a recrystallization-finishing temperature.
  • Table 1 Nb, and N contents (mass %) together with (%Ti ⁇ %N) and a recrystallization-finishing temperature T rf (°C) Steel No.
  • Each slab was hot-rolled to thickness of 4 mm at a finish-temperature of 750°C.
  • the hot-rolled steel sheets Nos. 1-7 were annealed at 800°C for 60 seconds, pickled and then cold-rolled to thickness of 2 mm.
  • the steel sheets were further cold-rolled to final thickness of 0.5 mm, in accompaniment with intermediate-annealing at a temperature of (a recrystallization-finishing temperature -50°C) for 60 seconds.
  • the cold-rolled steel sheets were finish-annealed at 1000°C for 60 seconds.
  • the hot-rolled steel sheets Nos. 8 and 9 were annealed, pickled and then cold-rolled to thickness of 2 mm.
  • the steel sheets were intermediately annealed and further cold-rolled to final thickness of 0.5 mm.
  • the cold-rolled steel sheets were subjected to finish-annealing.
  • Table 2 shows conditions of annealing the hot-rolled steel sheets, intermediate-annealing and finish-annealing.
  • Table 2 Conditions of annealing hot-rolled steel sheets, intermediate-annealing and finish-annealing Steel No. Annealing No.
  • annealing hot-rolled steel sheets intermediate-annealing finish-annealing (°C) (seconds) (°C) (seconds) (°C) (seconds) 8 Y1 1090 60 950 60 950 60 Y2 700 60 850 60 950 60 Y3 700 28800 850 60 950 60 9 Y4 1100 60 950 60 1000 60 Y5 700 60 1000 60 1000 60 Y6 700 60 900 60 1000 60 Y7 750 1500 900 60 1000 60 Y8 700 3000 900 60 1000 60 Y9 700 28800 900 60 1000 60
  • a test piece sampled from each annealed steel sheet was etched in a nonaqueous electrolyte of 10% acetylacetone-1% tetramethyl ammonium chloride-methyl alcohol under a potentiostatic condition and then observed by a scanning electron microscope to investigate distribution of precipitates.
  • a cross section in parallel to a rolling direction was inspected at arbitrary 50 points, and maximum length of each precipitate was measured and evaluated as particle size.
  • a blank sampled from each annealed steel sheet was press-formed to a cylindrical profile (shown in Fig. 1 ) by a multiplaten press.
  • Maximum and minimum radii of a cylindrical part C at a position apart 5mm from a flanged part F were measured by a laser displacement meter.
  • a ratio of (the maximum diameter - the minimum diameter)/(the minimum diameter) was calculated and regarded as circularity to evaluate dimensional accuracy of a press-formed steel sheet.
  • Another test piece was bulged to a height of 10 mm, using a punch of 103 mm in diameter with a curvature radius of shoulder being 10 mm and a die of 105 mm in diameter with a curvature radius of shoulder being 8 mm under the condition a flange of the test piece was fixed with a bead.
  • a blank of 92 mm in diameter was sampled from a bottom of the bulged test piece, and a hole of 10 mm in diameter was formed at a center of the blank with a clearance of 10%. The blank was then subjected to a secondary formability test as follows:
  • the blank was held between a flathead punch of 40 mm in diameter with a curvature radius of shoulder being 3 mm and a die of 42 mm in diameter with a curvature radius of shoulder being 3 mm, in the manner that burrs around the hole faced to the die.
  • the hole was extruded by the punch until occurrence of cracks at its edge, while a flange of the blank was fixed with a bead.
  • a diameter of the hole was measured at the crack initiation.
  • Results are shown in Table 3. It is noted that steel sheets were cracked during press-forming at a product of (%Ti ⁇ %N) above 0.005. Steel sheets with a Nb content less than 0.02 mass % were formed with poor circularity, regardless manufacturing conditions. Observation results on any of cracked steel sheets and formed steel sheets with poor circularity prove that precipitates of 0.15 ⁇ m in particle size except TiN were distributed in a steel matrix only a few.
  • Each slab was hot-rolled to thickness of 4.0 mm, annealed, pickled and cold-rolled to thickness of 2 mm.
  • the cold-rolled steel sheet was intermediately annealed, further cold-rolled to final thickness of 0.5 mm and then finish-annealed.
  • Table 5 shows conditions of a finish-temperature of hot-rolling, annealing hot-rolled steel sheets, intermediate-annealing and finish-annealing.
  • Example 2 Each steel sheet was examined by the same way as Example 1, to investigate shape and distribution of precipitates as well as dimensional accuracy and secondary formability of a press-formed steel sheet.
  • Results shown in Table 6 prove that ferritic stainless steel sheets, wherein precipitates of 0.15 ⁇ m or more in particle size except TiN were distributed in steel matrix at a rate of 5000-50000/mm 2 , were press-formed to a good profile with circularity of 2.5 % or less.
  • comparative steel sheets (Example Nos. A2, B2, C2 and D2), which satisfied compositional conditions of the present invention but were manufactured under improper conditions, had poor dimensional accuracy and secondary formability in a press-formed state due to the metallurgical structure that distribution number of the precipitates except TiN were out of 5000 - 50000/mm 2 .
  • the steel sheet I was too hard due to excess C and cracked during press-forming.
  • the steel sheet K was too strong due to excess Nb and cracked during press-forming.
  • the steel sheet L with a product of (%Ti ⁇ %N) above 0.005 was also cracked during press-forming, wherein the cracks were initiated near coarse TiN particles.
  • the steel sheet J with shortage of Nb was press-formed with poor circularity.
  • ferritic stainless steel sheets which can be press-formed with high dimensional accuracy and excellent secondary formability, are provided by distribution of precipitates of 0.15 ⁇ m or more in particle size except TiN at a rate of 5000 - 50000/mm 2 in a steel matrix with controlled composition. Shape and distribution of such precipitates suitable for the purpose are realized by properly controlling a finish-temperature of hot-rolling and conditions of heat-treatment for annealing a hot-rolled steel sheet, intermediate-annealing during cold-rolling and finish-annealing a cold-rolled steel sheet.
  • the ferritic stainless steel sheets manufactured in this way are useful as members or parts, which demand for strict dimensional precision, in various fields, e.g.
  • sealing members for organic electro-luminescent devices, precision pressed parts, sinks, utensils, burners of stoves, oil filler tubes of fuel tanks, motor casings, covers, caps of sensors, injector tubes, thermostat valves, bearing seals, flanges and so on, instead of expensive austenitic stainless steel sheets.

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Claims (2)

  1. Ferritisches rostfreies Stahlblech, ausgezeichnet in Preßformbarkeit und sekundärer Formbarkeit, welches eine Zusammensetzung, bestehend aus 0,02 Masse-% oder weniger C, 0,8 Masse-% oder weniger Si, 1,5 Masse-% oder weniger Mn, 0,050 Masse-% oder weniger P, 0,01 Masse-% oder weniger S, 8,0-35,0 Masse-% Cr, 0,05 Masse-% oder weniger N, 0,05-0,40 Masse-% Ti, 0,10 - 0,50 Masse-% Nb, gegebenenfalls einem oder mehreren, ausgewählt aus der Gruppe, bestehend aus 0,5 Masse-% oder weniger Ni, 3,0 Masse-% oder weniger Mo, 2,0 Masse-% oder weniger Cu, 0,3 Masse-% oder weniger V, 0,3 Masse-% oder weniger Zr, 0,3 Masse-% oder weniger Al und 0,0100 Masse-% oder weniger B, wobei der Rest Fe ausgenommen unvermeidbare Verunreinigungen mit einem Produkt von (%Ti × %N) weniger als 0,005 ist, und
    eine metallurgische Struktur aufweist, bei welcher Ausscheidungen von 0,15 - 1,0 µm in der Teilchengröße, ausgewählt aus der Gruppe, bestehend aus TiC, NbC, NbN, Laves-Phasen und Gemischen davon, aber ausschließend TiN, mit einer Rate von 5000-50000/mm2 in einer Stahlmatrix verteilt sind.
  2. Verfahren zur Herstellung eines ferritischen rostfreien Stahlblechs, ausgezeichnet in der Preßformbarkeit und sekundären Formbarkeit, welches die Schritte umfaßt:
    das Bereitstellen eines Stabes eines ferritischen rostfreien Stahls mit der Zusammensetzung, bestehend aus 0,02 Masse-% oder weniger C, 0,8 Masse-% oder weniger Si, 1,5 Masse-% oder weniger Mn, 0,050 Masse-% oder weniger P, 0,01 Masse-% oder weniger S, 8,0-35,0-Masse-% Cr, 0,05 Masse-% oder weniger N, 0,05-0,40 Masse-% Ti, 0,10 - 0,50 Masse-% Nb, gegebenenfalls einem oder mehreren, ausgewählt aus der Gruppe, bestehend aus 0,5 Masse-% oder weniger Ni, 3,0 Masse-% oder weniger Mo, 2,0 Masse-% oder weniger Cu, 0,3 Masse-% oder weniger V, 0,3 Masse-% oder weniger Zr, 0,3 Masse-% oder weniger Al und 0,0100 Masse-% oder weniger B, wobei der Rest Fe ausgenommen unvermeidbare Verunreinigungen mit einem Produkt von (%Ti × %N) weniger als 0,005,
    das Heißwalzen des Stabes bei einer Finish-Temperatur von 800°C oder niedriger,
    das Glühen bzw. Ausglühen des heißgewalzten Stahlbleches bei einer Temperatur innerhalb eines Bereichs von 450 - 1080°C für eine Zeitdauer, welche das Wiederauflösen der Ausscheidungen vermeidet,
    das Kaltwalzen des geglühten Stahlblechs in Begleitung mit mindestens einem intermediären Glühen bei einer Temperatur innerhalb eines Bereiches von (einer Rekristallisationsfinishtemperatur - 100°C) zu (einer Rekristallisationsfinishtemperatur) für eine Zeitdauer, welche die Auflösung der Ausscheidungen vermeidet, und anschließend
    das Finishglühen des kaltgewalzten Stahlblechs bei einer Temperatur von 1080°C oder niedriger für eine Zeitdauer, welche die Auflösung der Ausscheidungen vermeidet.
EP04012345A 2003-06-04 2004-05-25 Rostfreier ferritischer Bandstahl mit hervorragenden Press- sowie sekundären Verformungseigenschaften und Verfahren zu seiner Herstellung Expired - Lifetime EP1484424B1 (de)

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