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 with a ferritic structure, in which molten steel which forms a ferritic structure on cooling is cast into a cast strip, wherein if necessary the cast strip is hot-rolled in-line, the hot-rolled strip is coiled or wound and in one or more steps is cold-rolled to form the cold strip.
• ferritic stainless steel Due to the high price of nickel, austenitic stainless steel is increasingly being substituted worldwide by ferritic stainless steel, which contains Ni typically only as a production-related companion element.
• a method of the type initially indicated, which is to make this possible, is known for example from the European Patent EP 0 881 305 B1.
• a stainless, ferritic steel which contains (in wt. %) max. 0.12% C, max. 1% Mn, max. 1% Si, max. 0.04% P, max. 0.030% S, 16-18% Cr and as remainder iron and unavoidable impurities, is cast into strip in the casting gap formed between the rolls of a twin roll casting machine.
• the cast strip is cooled down, during which cooling process holding the strip in the austenite-ferrite transformation range is avoided.
• the strip is coiled or wound at a temperature, which lies between 600° C. and the martensite transformation temperature.
• the coiled or wound strip is cooled to a temperature, which lies between 200° C. and ambient temperature, at a maximum rate of 300° C. per hour. Finally, batch type annealing, known per se, of the wound strip is carried out.
• the surface of the slab is shaped, then the slab is re-heated, afterwards the slab is hot-rolled in the hot strip mill into hot strip and then wound to a coil.
• the hot strip obtained in this way is thereupon annealed, pickled and cold-rolled in several passes.
• the cold strip is normally bright-annealed and skin-pass rolled.
• the problem with cold-rolled strip made from ferritic stainless steel with a Cr content in the range of 17% is that ridging or orange peel can form in the course of subsequent cold working, particularly deep-drawing.
• Ridging here describes strongly marked linear surface defects, which in the case of ferritic chromium steel are aligned in the rolling direction.
• the surface defect described as “orange peel” is non-directional and is marked by rough appearance of the surface.
• the invention features a method with which a cold-rolled strip can be produced from ferritic stainless steel, in which the risk of formation of orange peel or ridging is minimized during a cold-forming process.
• a strip cast from molten metal is cooled between a casting process and a coiling process from a starting temperature not lower than 1180° C., at a cooling rate of at least 150° C. per second, to a maximum intermediate temperature of 1000° C. and is then held for at least 10 seconds at a maintenance temperature of between 900 and 1000° C.
• Typical starting temperatures of high-intensity cooling typically range from 1180 to 1270° C., particularly 1200 to 1250° C. If the minimum temperature of 1180° C. is not reached, austenite can occur at the strip edge already in quantities, which would impair the success of the method according to the invention.
• Such steel which belongs to the category of stainless steel containing 10-18 wt % Cr forming a ferritic structure and in the course of cooling starting from the ferrite does not completely transform into austenite and then again into ferrite.
• Such steel beside iron and unavoidable impurities, typically contains (in wt. %) up to 0.08% C, 10-18% 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 possibly present here is not a metallurgically purposeful addition but [[is]] rather can be the result of the production process and enters the molten steel via the Ni-containing foundry ladles, converters or furnaces.
• Ni-content of steel processed according to the invention ranges from 0.7 to 0.8 wt. %.
• the effectiveness of the method according to the invention can be enhanced by the combination of strip casting, rapid cooling and holding of the cast strip over a sufficient period of 10 seconds at the minimum at a temperature in the range of 950 ⁇ 50° C., particularly 950 ⁇ 20° C.
• 950 ⁇ 50° C. particularly 950 ⁇ 20° C.
• Steel alloys used according to the invention initially solidify ferritically during the course of strip casting.
• ferrite When the solidified strip cools down, ferrite then transforms between 1200° C. and 800° C. partly into austenite.
• the thermodynamic cause lies in the very low solubility, decreasing with the temperature, of carbon in the ferrite.
• the solubility of carbon in the austenite is substantially higher.
• austenite forms at the grain boundaries, since carbon in the ferrite again rapidly diffuses and can migrate to the edges from the grain interior. Thus, the ferrite grain boundaries are marked with austenite. As soon as carbides form at temperatures of less than 900° C., the austenite portion at the grain boundaries again decreases. Due to the relatively slow process of carbide formation, this however does not take place completely, so that austenite residues remain, which in the temperature range of 200-300° C. subsequently transform into martensite. The residual austenite remaining at the grain boundaries with conventional working methods thus produces the rough cast structure.
• the force driving the austenite formation is greatest in the maintenance temperature range of approximately 950° C. and the temperature-dependent diffusion coefficient is so low that austenite particles form in the grain interior by way of nucleation. Due to the substantially reduced diffusion coefficient, the distribution of the substitutional elements contained in the respective alloy does not vary or varies to a small degree (para-equilibrium). At the same time the carbon supersaturation is diminished.
• the cast strip is thus cooled down during high-intensity cooling in accordance with the invention to an intermediate temperature of 900-1000° C., so that the temperature is rapidly reached in a direct way.
• Rapid cooling to the intermediate temperature in accordance with the invention and holding at the maintenance temperature between the casting process and the coiling process have a positive effect, also with such methods, on the insensitivity of the cold strip obtained to ridging and orange peel formation, wherein hot-rolling between the casting process and the coiling process is omitted.
• the cast strip which is produced, for example via a twin roll casting machine
• the cast strip however with respect to the homogeneity of its micro-structure formation and its characteristic distribution is hot-rolled between the strip casting process and the coiling process in at least one pass.
• the density and precipitation rate of austenite particles are increased by the hot deformation, since structural defects are introduced into the micro-structure therewith.
• a strip is cast to a thickness of 1-5 mm, particularly 2-3 mm, and then the cast strip is hot-rolled in-line with a reduction per pass of 5-60%, particularly 10-40%.
• the method according to the invention in this case enables temperature control of the strip to be selected with respect to hot-rolling carried out if necessary, so that temperature conditions, which are matched to the ductile behavior of the steel processed in each case or the desired characteristic combination of the strip obtained, can prevail during hot-rolling.
• hot-rolling the strip at a hot-rolling temperature of not less than 1180° C., typically
• cooling according to the invention and holding of the strip are carried out after hot-rolling.
• cooling should begin as soon as possible after hot-rolling, in practice therefore within less than three, particularly within one second, after leaving the final hot-rolling stand.
• the casting heat of the cast strip can be conducted (e.g., directly conducted) into the hot-rolling stage, so that not only high hot-rolling temperatures are possible, but also the energy consumption, necessary for controlling the temperature of the strip, is reduced to a minimum.
• a maintenance temperature which also lies between 900 and 1000° C., and particularly is substantially equal to the intermediate temperature or amounts to 950° C. ⁇ 20° C., for at least 10 seconds.
• a maintenance temperature which also lies between 900-1000° C., and particularly is substantially equal to the intermediate temperature or amounts to 950° C. ⁇ 20° C., for at least 10 seconds;
• cooling to the intermediate temperature and holding at the maintenance temperature are carried out before the cast strip is hot-rolled.
• Hot-rolling the micro-structure with the high density, produced by holding at the maintenance temperature carried out beforehand, of the austenite grains in the ferritic matrix leads to high dislocation density which on subsequent recrystallization results in a fine-grained structure.
• Such recrystallization can be due to suitable recrystallization annealing treatment, as it is performed as standard in the production of cold-rolled strip of the type under discussion.
• cooling at a cooling rate of at least 150° C. per second to an intermediate temperature below 900° C., particularly in the range of 800° C.;
• a higher deformation degree can be achieved with reduced energy consumption and less roll wear, if this is necessary and desirable.
• High-intensity cooling in accordance with the invention in temperature ranges below 900° C. offers the possibility of rolling the cast strip at temperatures substantially below 800° C. or carrying out further thermal treatment at temperatures of less than 500° C., particularly less than 400° C.
• the method according to the invention in the variants described above can be implemented particularly economically on such strip casting machines, wherein casting, hot-rolling carried out if necessary and coiling as well as the steps according to the invention carried out between the casting process and the coiling process of cooling to the intermediate temperature and holding at the maintenance temperature are performed in a sequence of steps continuously one after the other.
• the cast strip in the course of cooling in accordance with the invention to be cooled to an intermediate temperature of less than 900° C., particularly less than 800° C., this cooling possibly going as low as ambient temperature. Subsequently the cast strip is then re-heated to the maintenance temperature. Subsequently in this context means that further work steps, such as for example hot-rolling at a specific temperature, storage, dividing into plates etc., may be carried out between the cooling and the holding steps.
• the invention provides, starting from the low intermediate temperature, for the strip to be re-heated within 200 seconds, particularly within 100 seconds, to the specific hot-rolling temperature, which will typically be 700-800° C. If heating to 800° C. is too slow, unwanted carbides may precipitate. These lead to a premature decrease in supersaturation and thus to a substantially reduced density of austenite particles with the result that the grain refinement aimed at by the invention is not achieved.
• the method according to the invention permits the hot deformation degree to be increased in the strip casting line, due to the generally freely variable temperature control, via for example a second roll stand or a smaller working roll diameter, as a result of which cold strip produced according to the invention can possess better deep/stretch-drawing capacity compared to such cold strip which has been made via the usual production route.
• the rapid temperature changes necessary for implementing the method according to the invention can be realized in this case by using the strip-casting technology, since the minimum thickness of the cast strip allows sufficiently rapid temperature changes over the entire cross section of the strip.
• a cast strip was produced in each case from a suitable molten steel, the cast strip hot-rolled to form a hot strip and the hot strip coiled.
• the strip-casting plant used for this comprised a twin roll casting machine, a hot-rolling stand arranged in the conveyance direction of the cast strip following the casting machine in-line and a coiling device arranged in the conveyance direction behind the hot-rolling stand.
• high-intensity water cooling equipment, inductively-operating ovens and electric temperature maintenance furnaces were also used.
• Each hot strip produced according to the invention in trials I-IV using the casting rolling plant was subsequently processed in a conventional way including batch type annealing, pickling, cold-rolling without intermediate annealing, bright annealing and skin-pass rolling.
• Test specimens were produced from the cold strip obtained in such a way with a total deformation degree of 70%. None of the test specimens showed orange peel or ridging.
• the thickness of the cast strip was 3 mm. After the cast strip leaving the casting gap of the twin roll casting machine had reached a strip temperature of 1180° C., high-intensity water cooling took place. The cast strip was cooled within 2 seconds to an intermediate temperature of 950° C.
• the cast strip cooled in such a manner was then held without interruption in a continuous production sequence at a maintenance temperature, which in this case was equal to the intermediate temperature, in an inductive heating oven for a period of 10 seconds.
• the strip cooled down on the run-out roller table following the hot-rolling stand to a coiling temperature of approximately 550° C., before it reached the coiling device, where it was coiled to a coil.
• the hot strip obtained in such a way had a columnar grain structure (approx. 100 ⁇ m wide and 500 ⁇ m long) with an equiaxed strip central area (grain size 150 ⁇ m).
• the grain boundaries were occupied by a thin seam of martensite and carbides. Recrystallized areas with a grain size of 20 ⁇ m were found in the grain interior.
• finely distributed isolated particles which consisted of carbides, martensite and residual austenite, were present in the micro-structure. The particle density was typically 15-25 particles per grain.
• the micro-structure of the hot strip obtained in the second trial had the same columnar grain structure (approx. 100 ⁇ m wide and 500 ⁇ m long) with an equiaxed strip central area (grain size 150 ⁇ m) as the micro-structure of the hot strip obtained in the first trial. Also, in this case the grain boundaries showed a thin seam occupied by martensite and carbides. Likewise, recrystallized areas with an average grain size of 20 ⁇ m were found in the grain interior. Also, finely distributed isolated particles, which also as with the strip obtained in the first trial consisted of carbides, martensite and residual austenite, were present in the micro-structure. The particle density was typically 20-30 particles per grain.
• a 3 mm thick strip was cast. After the cast strip had reached a temperature of 1180° C., high-intensity water cooling began, the strip being cooled down within 3 seconds to an intermediate temperature of 780° C. The cast strip cooled in this way was then kept hot in an inductive heating oven, heated to a hot-rolling temperature of 800° C. and then hot-rolled at this hot-rolling temperature to a strip thickness of 2.5 mm. The strip then cooled down on the run-out roller table to a coiling temperature of approx. 550° C. and coiled at this temperature.
• Specimen plates were divided at ambient temperature from the strip obtained in such a way. These were then heated inductively to 800° C. and then to 950° C. within a period of 15 seconds. The period for heating between 800° C. and 950° C. lasted 2 seconds.
• the strip was thereupon held for 20 seconds at a maintenance temperature of 950° C. by means of a maintenance furnace. Subsequently, air cooling took place.
• micro-structure of such thermally treated hot strip specimen plates likewise showed a columnar grain structure (approx. 100 ⁇ m wide and 500 ⁇ m long) with an equiaxed strip central area (grain size 150 ⁇ m). At the grain boundaries also here a thin seam was occupied by martensite and carbides. Again recrystallized areas with a grain size of 20 ⁇ m were found in the grain interior and finely distributed isolated particles, which consisted of carbides, martensite and residual austenite, were present in the micro-structure. The particle density here was typically 40-60 particles per grain.
• plates were divided from the cast strip after it had cooled down to ambient temperature and these plates were inductively heated from ambient temperature within 30 seconds to a hot-rolling temperature of 800° C., wherein they were hot-rolled to a strip thickness of 2.4 mm. After repeated cooling of the hot-rolled plates they were re-heated within 3 seconds to a maintenance temperature of 950° C.
• the re-heated strip was held for 20 seconds at the maintenance temperature by means of a maintenance furnace. Subsequently, the strip was air cooled.
• the micro-structure of the hot-rolled plates showed a columnar grain structure (approx. 100 ⁇ m wide and 500 ⁇ m long) with an equiaxed strip central area (grain size 150 ⁇ m) after being held at the maintenance temperature, wherein the grain boundaries also here had a thin seam occupied by martensite and carbides and recrystallized areas with a grain size of 20 ⁇ m were found in the grain interior. Similar to other trials, finely distributed isolated particles, consisting of carbides, martensite and residual austenite, were also present in the micro-structure. The particle density was typically 40-60 particles per grain.

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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)

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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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