Classifications

• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
  • B22D11/0622—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two casting wheels
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/12—Accessories for subsequent treating or working cast stock in situ
  • B22D11/124—Accessories for subsequent treating or working cast stock in situ for cooling
• • 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
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
• • 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
• • 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
  • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
  • C21D8/0215—Rapid solidification; Thin strip casting
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
• • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • 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
  • C21D8/0247—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling

Definitions

• the invention relates to a method for producing a cold strip having a ferritic microstructure, in which a molten steel forming a ferritic microstructure upon cooling is cast into a cast strip in which the cast strip is hot rolled inline, if necessary, in which the hot rolled strip is unwound and in which from the hot rolled strip in one or more steps, the cold strip is cold rolled.
• the cast strip is produced by cooling a molten steel present in a casting vessel on a rotating roll that delimits the casting vessel on one side so that the melt already solidifies on the roll and is withdrawn from the roll as a solidified strip can.
• the cast strip can also be produced in a two-roll casting machine in which the molten steel is likewise solidified on the counter-rotating casting rolls and pressed in the casting gap into a cast strip.
• the resulting cast strip is coiled from a temperature of 800-1100 ° C. and held in the coil to achieve self-annealing in the coil. This annealing has the purpose of eliminating the residual stresses present in the cast strip.
• the cast strip is cooled, during which cooling it is avoided that the strip is kept in the austenite-ferrite transformation zone.
• the strip is reeled at a temperature between 600 ° C and the temperature of the martensite transformation.
• the reeled tape is fed at a speed of max. 300 ° C / h cooled to a temperature between 200 ° C and the room temperature.
• a known per se bell annealing the coiled tape is performed.
• the surface of the slab is first processed, then reheated the slab, then the slab in hot rolled hot strip to hot strip and then coiled to a coil.
• the hot strip thus obtained is then annealed, pickled and cold rolled in several passes. Finally, the cold strip is usually bright annealed and dressed.
• the hot strip consists of ferritic stainless steel with a Cr content in the range of 17% cold-rolled strips
• the problem that in the course of a subsequent cold forming, especially during deep drawing, Switzerlandrillmaschine or orange peel can occur With ridging, pronounced linear surface defects are referred to, which are aligned in the rolling direction in the case of ferritic chromium steels.
• the surface defect referred to as "orange peel” appears undirected and is characterized by a scarred form of the surface.
• the invention was therefore based on the object to call a method with which cold tapes ferritic stainless steels in which the risk of the formation of orange peel or Switzerlandrillmaschine is minimized in a cold forming.
• Particularly suitable for carrying out the process according to the invention are steels which are known per se and belong to the class of stainless steels containing 10 to 18% by weight Cr and which form a ferritic microstructure and not completely austenite and then again in the course of their cooling from the ferrite convert to ferrite.
• Such steels typically contain (in wt.%), In addition to iron and unavoidable impurities, up to 0.08% C, 10-18 wt.% Cr, up to 1% Si, up to 1.5% Mn, up to 1 % Ni, up to 0.04% P and up to 0.015% S.
• Ni content of steels processed according to the invention is in the range of 0.7-0.8% by weight.
• Crucial for the effect of the process according to the invention is the combination of strip casting, rapid cooling of the cast strip and holding the strip for a sufficient time of at least 10 seconds at a temperature in the range of 950 ⁇ 50 ° C, especially 950 ⁇ 20 ° C, lies.
• Steel alloys used according to the invention initially solidify ferritically in the course of strip casting. During the cooling of the solidified strip ferrite then converts between 1200 ° C and 800 ° C partly in austenite.
• the thermodynamic cause lies in the low and decreasing temperature solubility of carbon in the ferrite.
• the idea underlying the invention is now to cool very quickly to about the temperature with the maximum austenite content (950 ° C. ⁇ 50 ° C., in particular 950 ° C. ⁇ 20 ° C.). In this way, austenite formation at the crest boundaries is minimized since the diffusion path lengths for carbon, and even more so for the substitutional elements (Cr, Ni, Mn,...), Are insufficient in the short time of cooling.
• the force promoting austenite formation is the largest and the temperature-dependent diffusion coefficient is so low that austenite particles form via nucleation in the interior of the grain.
• the distribution of the substitutional elements contained in the respective alloy does not change or only slightly (para-equilibrium).
• the carbon supersaturation is reduced. If, according to the invention, the cast strip material is held at this temperature for a time of at least ten seconds, preferably twenty seconds, austenite particles therefore begin to precipitate out of the grain inside structure defects. It creates new grains in a ferritic matrix, which break up the original cast structure. The particle density increases the longer the holding time is. This precipitation mechanism causes a grain refining, which leads to the insensitivity inventively produced cold tapes against Switzerlandrillmaschine and orange peel formation.
• a particularly rapid reheating to the holding temperature has a positive effect on the work result. If heated too slowly, unwanted austenite forms at the grain boundaries, while the C supersaturation in the interior of the grain is degraded by carbon diffusion, as a result of which the original cast structure is fixed. Furthermore, one should not spend too long in the temperature range of 800-900 ° C, because practical test prove that in this temperature range after about 100 seconds, the carbon supersaturation is reduced by carbide formation.
• the cast strip is thus cooled in its intensive cooling according to the invention to an intermediate temperature of 900 - 1000 ° C, so that the critical temperature for the invention is achieved quickly by a direct route.
• a strip having a thickness of 1 to 5 mm, in particular 2 to 3 mm is preferably cast directly, and the cast strip is then inline with a reduction of 5 to 60%, in particular 10%. 40%, hot rolled.
• the mode of operation of the method according to the invention makes it possible to choose the temperature control of the strip with regard to the hot rolling as required, in such a way that optimally adjusted temperature conditions prevail during hot rolling on the deformation behavior of the respective processed steel or the desired combination of properties of the strip obtained.
• the cast strip in the course of Cooling according to the invention to a lower than 900 ° C, in particular less than 800 ° C lying intermediate temperature to cool, said cooling can go to room temperature. Later, the cast strip is then reheated to the hold temperature. Later in this context means that between the cooling and the holding further work steps, such as a hot rolling at a certain temperature, storage, cutting into sheets, etc. can be performed.
• an advantageous embodiment of the invention provides that the emanating from the respective intermediate temperature re-heating to the holding temperature in 1 - 5 seconds, in particular in 2 - 3 seconds, takes place.
• the strip for hot rolling must be reheated sufficiently quickly for the reasons already mentioned. Therefore, the invention envisages that the strip will start from the low intermediate temperature within of 200 s, in particular within 100 s, is heated to the respective hot-rolling temperature, which will typically be 700-800 ° C. If heating up to 800 ° C is too slow, unwanted carbides may precipitate. These lead to a premature reduction in supersaturation and thus to a significantly reduced density of austenite particles, with the result that the desired grain refining according to the invention is not achieved.
• the invention thus provides a method which, while avoiding expensive production steps, makes it possible to produce a product which is competitive both in terms of its properties and in terms of its price.
• the particular advantage of the method according to the invention is that can be produced with him cold strips, which are characterized by a homogeneous appearance and a zundernarbenbuild surface. The latter is achieved in that already in the intensive cooling according to the invention the adhering to the cast strip scale is largely removed, so that if necessary carried out hot rolling minimized surface damage is caused as a result of still present on the belt scale.
• a cast strip was produced from an appropriately molten steel melt, the cast strip was hot rolled into a hot strip, and finally the hot strip was rewound.
• the strip caster comprised a two-roll casting machine, a hot rolling stand arranged inline to the casting machine in the conveying direction of the cast strip, and a coiling device arranged behind the hot rolling stand in the conveying direction.
• an intensive water cooling device inductively operating strip heating devices and electric holding furnaces have been used.
• the microstructure is accordingly finer overall, but in itself also inhomogeneous than conventional fine-grained structure. Characteristic of the structure according to the invention produced bands is therefore the high number of particles per grain.
• sample parts have been produced with a total degree of deformation of 70%. None of the samples showed orange peel or tearing.
• the thickness of the cast strip was 3 mm. After the emerging from the casting gap of the two-roll casting machine cast strip had reached a strip temperature of 1180 ° C, carried out an intensive cooling with water. The cast strip was cooled within 2 s to an intermediate temperature of 950 ° C.
• the thus cooled cast strip was then maintained in a continuous, uninterrupted process flow in an inductive heating system for a period of 10 s at a holding temperature, which in this case was equal to the intermediate temperature.
• the cast strip thus heat-treated in accordance with the invention was then hot-rolled to a strip thickness of 2.5 mm.
• the belt cooled to a coiler temperature of about 550 ° C before reaching the reel in which it was wound into a coil.
• the hot strip obtained in this way had a columnar grain structure (about 100 ⁇ m wide and 500 ⁇ m long) with an equiaxed strip center region (grain size 150 ⁇ m).
• the grain boundaries were covered with a thin margin of martensite and carbides.
• In the interior of the grain were recrystallized areas with a grain size of 20 microns.
• the microstructure contained finely distributed island-like particles consisting of carbides, martensite and retained austenite. The particle density was typically 15-25 particles per grain.
• a cast, 2.8 mm thick strip was first produced from the molten steel with the above alloy.
• the cast strip was held in an inductive heating plant at a temperature of 1200 ° C and then hot rolled at this temperature to a strip thickness of 2.1 mm.
• the belt conveyed with about 1 m / s was cooled within 1 s to an intermediate temperature of 950 ° C.
• the strip then passed on to a run-out roller table whose first section associated with the hot rolling mill over a length of 15 meters was provided with a cover which ensured that the strip maintained a substantially constant temperature for 15 seconds in this first section. Subsequently, the strip was still cooled on the outlet roller table to a reel temperature of about 500 ° C, with which it has finally been wound into a coil.
• the microstructure of the hot strip obtained in the second test had the same columnar grain structure (about 100 ⁇ m wide and 500 ⁇ m long) with an equiaxed strip center region (grain size 150 ⁇ m) as the microstructure of the hot strip obtained in the first test. Also in this case, the grain boundaries showed a thin seam topped with martensite and carbides. In the interior of the grain were also recrystallized areas with a grain size of 20 microns average. Likewise, there were finely distributed island-like particles in the microstructure, which also existed as in the band of carbides, martensite and retained austenite contained after the first experiment. The particle density was typically 20-30 particles per grain.
• a 3 mm thick strip was first cast. After the cast strip had reached a temperature of 1180 ° C, intensive cooling with water began, in which the strip was cooled to an intermediate temperature of 780 ° C within 3 seconds. The cast and thus cooled strip was then kept warm in an induction heating system, heated to a hot rolling temperature of 800 ° C, and then hot rolled to a strip thickness of 2.5 mm at this hot rolling temperature. The belt then cooled on the outfeed roller table to a reel temperature of about 550 ° C and has been reeled at this temperature.
• test panels were divided at room temperature. These were then inductively heated to 800 ° C. and then to 950 ° C. within a period of 15 s. The time for heating between 800 ° C and 950 ° C was 2 s.
• the band was then held for 20s at a holding temperature of 950 ° C. This was followed by cooling in air.
• the microstructure of the heat-treated hot-rolled test panels likewise showed a columnar grain structure (about 100 ⁇ m wide and 500 ⁇ m long) with an equiaxed mid-band area (particle size 150 ⁇ m). At the grain boundaries there was also a thin seam covered with martensite and carbides. Recrystallized areas with a grain size of 20 ⁇ m were located in the interior of the grain and in the microstructure there were finely distributed island-like particles consisting of carbides, martensite and retained austenite. The In this case, particle density was typically 40-60 particles per grain.
• panels were separated from the cast strip after it had cooled to room temperature, and these panels were inductively heated from room temperature to a hot rolling temperature of 800 ° C within 30 seconds, during which they were hot rolled to a strip thickness of 2.4 mm , After re-cooling the hot-rolled sheets, they were reheated to a holding temperature of 950 ° C within 3 seconds.
• the reheated band was held at the holding temperature for 20 seconds. Subsequently, the tape has been cooled in air.
• the microstructure of the hot-rolled sheets after holding at the holding temperature showed a stalked grain structure (about 100 microns wide and 500 microns long) with an equiaxed mid-band range (grain size 150 microns), where again the grain boundaries occupied a thin hem with Martensite and carbides and were in the interior of the grain recrystallized areas with a grain size of 20 microns were.
• a stalked grain structure about 100 microns wide and 500 microns long
• an equiaxed mid-band range grain size 150 microns

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Continuous Casting (AREA)
• Metal Rolling (AREA)

Applications Claiming Priority (2)

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• 2005
  • 2005-12-29 DE DE102005063058A patent/DE102005063058B3/de not_active Expired - Fee Related
• 2006
  • 2006-12-27 CN CN2006800500453A patent/CN101365812B/zh not_active Expired - Fee Related
  • 2006-12-27 JP JP2008547960A patent/JP2009522106A/ja active Pending
  • 2006-12-27 EP EP06847041A patent/EP1966399B1/de not_active Expired - Fee Related
  • 2006-12-27 WO PCT/EP2006/070223 patent/WO2007074157A2/de active Application Filing
  • 2006-12-27 US US12/159,343 patent/US20090065104A1/en not_active Abandoned
  • 2006-12-27 BR BRPI0620748-0A patent/BRPI0620748A2/pt not_active Application Discontinuation
  • 2006-12-27 KR KR1020087015484A patent/KR101362388B1/ko active IP Right Grant

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