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
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
• • 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
  • C21D8/021—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
  • 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
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T29/00—Metal working
  • Y10T29/49—Method of mechanical manufacture
  • Y10T29/4998—Combined manufacture including applying or shaping of fluent material
  • Y10T29/49988—Metal casting
  • Y10T29/49991—Combined with rolling

Definitions

• the invention relates to a method for producing high-strength, cold-formable steel strip or sheet comprising TWIP properties from an Fe—C—Mn lightweight structural steel, a method for manufacturing components as well as a high-strength steel strip or sheet, which comprises TWIP properties.
• Halfield steels which apart from iron contain, as the main alloying elements, 11-14 mass % Mn and 1.1-1.4 mass % C, have already been known for a long time. Steels with such a high manganese content are marked by very high tensile strength and work-hardening due to the effect of repeated impact or friction.
• TWIP “Twinning Induced Plasticity”.
• the ductility of TWIP steel is possibly additionally assisted by a martensitic ⁇ / ⁇ transformation generally accompanying the twinning formation.
• a method for producing steel strips from Fe—C—Mn alloys of the type described above is known from EP 1 067 203 B1.
• a molten material which contains 0.001-1.6 mass % C, 6-30 mass % Mn, up to 10 mass % Ni, wherein the total content of Mn and Ni is 16 mass % up to 30 mass %, up to 2.5 mass % Si, up to 6 mass % Al, up to 10 mass % Cr, as well as P, Sn, Sb and As, provided that the total content of these elements is maximum 0.2 mass %, S, Se and Te provided that the total of these elements is maximum 0.5 mass %, V, Ti, Nb, Zr and rare earth metals (REM) provided that the total of these elements is maximum 3 mass %, Mo and W provided that the total of these elements is limited to maximum 0.5 mass %, the rest being iron and melting conditioned unavoidable impurities, is cast in a conventional twin-roll strip casting machine into a thin strip of 1.5
• cast strip can also be produced by the so-called “Direct Strip Casting” process, for which the abbreviation “DSC process” is normally used.
• DSC process the abbreviation “DSC process” is normally used.
• the molten material to be cast is poured from the foundry ladle, into a dispensing vessel, by which it is applied to a continuously revolving conveyor belt.
• the molten material is cooled intensively, so that it is solidified into a hard pre-strip on reaching the end of the conveyor belt.
• the pre-strip normally passes through a secondary cooling stage before it is heat-rolled likewise without interruption immediately after this cooling stage.
• Heat-rolling can take place in one or more rolling stands. After heat-rolling further controlled cooling takes place, before the finished hot strip is wound into a coil.
• TRIP steels (“Transformation Induced Plasticity”) have particularly high strength with a degree of elongation comparable to conventional two-phase steels or a high stretch capability with a strength comparable to the conventional two-phase steels.
• TWIP steel has a more balanced combination of properties with optimum transformation behaviour during the shaping of the component and in the event of sudden mechanical stress.
• the object of the invention consisted in creating, on the basis of the prior art described above, a method for producing steel strips and sheets having TWIP properties with high manganese content, which enables products with optimum combination of properties and equally optimum utility value to be made available at reduced cost. Furthermore, a method for producing high-strength components from a steel of the type initially described was to be indicated. Finally, a steel strip or sheet was also to be created, which possesses particularly good deformation behaviour.
• the invention achieves the object specified above in that, by using the method according to the invention, hot or cold strip is produced, from which a pre-product is then possibly produced, which afterwards is finally cold-formed into the component.
• steel strip or sheet produced by means of the method according to the invention comprises a unique optimum combination of properties right down to temperatures which lie far below 0° C. Accordingly, steel strip or sheet produced according to the invention is characterized in that its brittle/ductility transition temperature T ue lies under ⁇ 40° C.
• the transition temperature T ue concerned is normally determined with the cupping test or notched bar impact test.
• the invention is based on the realization that steels with an Mn-content of 18 mass % and above can be processed using the presently known DSC process in a particularly advantageous way, if at the same time the final hot-rolling temperature and winding temperature are adjusted in a way according to the invention. Due to the fact that the hot-rolling temperature is at least 700° C., typically at least 850° C., a completely re-crystallized hot strip is obtained after hot-rolling, which is extremely suitable for subsequent cold-forming. Because the winding temperature of maximum 750° C., typically maximum 550° C. is also selected, so that grain boundary oxidation of the finished hot strip is avoided as far as possible, surface defects only appear to a minimum extent on the hot strip obtained after winding. Therefore, hot strip produced according to the invention or cold strip made therefrom can be protected particularly satisfactorily with metal coatings, in order to improve its corrosion resistance for example.
• a particular advantage of the method according to the invention is that during the hot phase of the production process used according to the invention, the strip does not need to be diverted from a vertical to a horizontal direction. Instead the pre-strip cast from the molten material according to the invention, both during its solidification on the conveyor belt and during subsequent hot-rolling, as well as heat treatment preceding hot-rolling if required, runs exclusively in a horizontally-aligned direction with the consequence that any critical bending of the strip can be avoided in the hot phase of the production process.
• This makes it possible to produce steel strip from particularly heat resistant steel materials without problems occurring due to the still poor transformation capacity of these materials.
• the risk of having to abort the casting operation for example, due to the breaking of only insufficiently ductile cast strip does not exist when using the DSC process according to the invention.
• a further advantage of the method according to the invention consists in that pre-strip can be cast in a thickness, which is far greater than that attainable with conventional strip casting.
• pre-strip whose thickness is typically more than 10 mm, in particular more than 12 mm, can be produced without difficulty with the method according to the invention.
• Pre-strip of this kind of more than 15 mm or more than 20 mm in thickness, for example, is formed during subsequent hot-rolling using high strain degrees into a thin hot strip, which is typically less than 3 mm, in particular less than 2 mm in thickness.
• hot-forming of the cast pre-strip is preferably carried out using the method according to the invention so that high degrees of deformation of preferably more than 60%, in particular up to 95%, are attained.
• hot strips of 1 mm in thickness which at low cost can afterwards be cold-rolled into cold strips directly suitable for use in motor vehicle body construction, can be produced from pre-strip of large thickness, despite the fact that the steel alloys processed according to the invention as standard possess high heat resistance.
• a further substantial advantage of the method according to the invention lies in the fact that it is substantially more tolerant in the processed molten material in relation to the presence of alloying elements, which are problematic in the conventional process.
• molten materials which apart from considerable contents of phosphorus, sulphur and copper can have impurities in the form of relatively high contents of Sn, Sb, Zr, TA and As in total of up to 0.30 mass %, can also be cast with high success. This enables higher contents in accompanying elements to be tolerated without the possibility of producing a correspondingly alloyed steel strip according to the invention being impaired as a result.
• the invention thus allows economical production of molten material using the electric-arc furnace route employing cheaper inferior scrap iron. It is therefore possible to move way from using blast furnaces responsible for high CO 2 -emissions.
• the strength and ductility of the finished steel strip or sheet are higher with the production method according to the invention than in cases, where a comparable alloy is processed by conventional continuous casting.
• the method according to the invention can be used on production lines, which require a substantially lower capital investment than conventional continuous casting plant. Accordingly, capital outlay is less than for a conventional continuous casting wide hot strip plant. Also, the method according to the invention enables the width to be adjusted coil by coil. The output attainable with a production line operating according to the invention is comparable with conventional continuous casting plants.
• the C-content of the alloy processed according to the invention can be 0.003 mass % to 1.6 mass %. Preferably, this lies in the range of 0.2 mass % to 0.8 mass %. If the C-content is at least 0.2 mass % the risk of carbon depletion in the molten material is minimized. Carbon content of more than 0.8 mass % can make it more difficult to optimise the content of other alloying elements with regard to achieving advantageous mechanical properties.
• the preferably selected carbon content of 0.2-0.8% ensures the improved possibility of producing steel sheet and strip according to the invention. Tears and instabilities in the strip edge region are substantially reduced, the instabilities in particular becoming less with increasing carbon content.
• the carbon content proposed according to the invention opens up a wide spectrum of hot-rolling parameters.
• the manganese content of the alloy processed according to the invention is at least 18 mass %, in particular at least 20 mass %. Steels possessing such high Mn-content of the type processed according to the invention reliably have TWIP properties.
• the nickel content is limited up to 10 mass %.
• the silicon content of a molten material processed according to the invention can be up to 8 mass %, this element being added if especially lightweight steel is required. Furthermore, a higher Si content can be used, in order to substitute correspondingly reduced C and Mn contents while still maintaining the TWIP properties.
• aluminium in amounts of up to 10 mass % can be optionally added to the molten material processed according to the invention.
• Chrome can be added to the steel processed according to the invention in order to improve corrosion resistance.
• a limitation of the Cr content to maximum 10 mass % is expedient with regard to cost criteria, since above this limit only small characteristic improvements are to be observed.
• V, Ti, Nb and REM amounts can be included, in order to benefit from the positive effect, known per se, of these micro-alloying elements with regard to the mechanical properties of steels of the type processed according to the invention.
• the molten material cast into the pre-strip contains a total of at least 0.01 mass % of V, Ti, Nb and/or REM.
• the property-improving effect (isotropy) of B however already occurs, if B is present in an amount of at least 0.001 mass %.
• the total content of molybdenum, tungsten and cobalt can be up to 1.5 mass %, in order to benefit from the known property-improving effects of these elements. Also, Ca and Mg amounts in a total of 0.5 mass % can be proposed, if the effects, likewise known per se, of these elements are to be exploited in the case of steels of the type processed according to the invention.
• Nitrogen amounts of up to 0.6 mass % can be added, in order to exploit the strength-increasing and anti-corrosive effect of nitrogen in steels of the type under discussion.
• steel sheet produced according to the invention is suitable for producing wheels for vehicles, in particular motor vehicles, for producing internal high pressure or external high pressure formed components, for producing high-strength engine parts, such as cam shafts or piston rods, for producing components designed to protect against pulse-type striking pressures, i.e. bombardment, such as armour plate as well as protective elements, which are intended to protect humans, in particular against bombardment.
• Steel sheets according to the invention with purely austenitic structure are also especially suitable for producing non-magnetic components.
• steel strips or sheets produced according to the invention maintain their tensile strength even at particularly low temperatures. So it can be guaranteed, as mentioned, that transition from the ductile to the brittle behaviour in the case of steel strip or sheet produced according to the invention only takes place at a transition temperature of below ⁇ 40° C. Accordingly, steel products produced according to the invention are particularly suitable for fabricating components used in cryogenic technology such as vessels or pipes for refrigeration purposes.
• the hot strip is hot-rolled according to the invention at a final hot-rolling temperature of at least 700° C., apart from avoiding grain boundary oxidation, already mentioned, the positive effect of carbon is exploited to the full.
• carbon brings about higher tensile strength and yield point values with still acceptable degrees of elongation.
• the final hot-rolling temperature increases, the tensile strength and yield strength decrease, while the degrees of elongation rise.
• the desired properties of the yielded steel strip can therefore be influenced in a controlled and simple manner.
• the heat treatment possibly carried out between the solidification of the pre-strip on the conveyor belt and hot-rolling is intended to bring the temperature of the pre-strip to a level on the basis of which optimum hot-rolling results are achieved.
• the heat treatment in the way known per se may comprise additional controlled cooling, wherein the pre-strip is brought to a hot-rolling start temperature, which is optimum for hot-rolling.
• it is just as conceivable to carry out heat treatment by heating up the pre-strip whenever the structure of the pre-strip should be influenced by such heat treatment or a rise in the temperature of the pre-strip to the optimum hot-rolling start temperature is necessary.
• hot strip produced according to the invention is marked by good usage properties. If thinner sheets or strips are to be produced, then the hot strip can be cold-rolled into cold strip after winding, wherein cold-rolling is advantageously carried out with a cold-rolling strain degree of 10% to 90%, preferably 30% to 75%.
• the hot strip can be pickled before cold-rolling.
• the cold strip obtained after one stage or multi-stage cold-rolling can be subjected to annealing, wherein the annealing temperatures should lie between 600° C. and 1,100° C.
• Annealing can take place in a stationary furnace within the temperature range of 600° C. to 750° C. or on the run at temperatures of 700° C. to 1,100° C.
• a first advantageous use of steel strips or sheets produced according to the invention lies in producing cold-formed components by flow-turning pressing. To this end blanks are made from the steel, which are then formed by flow-turning. Due to its special characteristic profile steel strip or sheet produced according to the invention or sheet metal blanks made therefrom are especially suitable for this purpose.
• Good ductile steel with higher strengths of the type produced according to the invention can be used for manufacturing components, which are equipped with toothing or comparable shaped elements. These components are typically transmission parts equipped with internal or external toothing. These can be produced economically and with high dimensional precision by flow-turning.
• a method for manufacturing transmission parts by flow-turning is known from DE 197 24 661. In accordance with this known method a blank is formed from a metal sheet made of a micro-alloyed high-strength structural steel, which possesses a lower yield point of at least 500 N/mm 2 . This blank is then cold-formed into gearing by flow-turning. While the toothing is being produced, the metal sheet is formed to the limit of its transforming capacity. Finally, a surface of the work-piece equipped with toothing is hardened substantially while maintaining the temperature and causing no thermal warping.
• a purely austenitic or a structure consisting of a mixture of ferrite and austenite with percentages of martensite can be obtained in steel strip or sheet produced according to the invention.
• the steels according to the invention can therefore be transformed substantially better.
• they solidify substantially more strongly than high-strength micro-alloyed or multi-phase steels used, as is known, for producing components by flow-turning.
• component strengths in the range of 1,400 N/mm 2 to 2,200 N/mm 2 can be obtained in every case after cold-forming. Additional hardening of the components being produced can be dispensed with therefore after the cold-forming.
• the method according to the invention facilitates the economic production of light-weight, highly stressable steel strips and sheets, which form the base product for the possible production, requiring low capital investment, of dimensionally-precise components by cold-forming.
• all variants of steel sheet according to the invention are especially suitable for producing vehicle body components, particularly the external panels of a motor vehicle body or load-bearing components for vehicle bodies, wheels for vehicles, in particular motor vehicles, non-magnetic components, vessels, used in cryogenic technology, internal high pressure or external high pressure-formed components, tubes which are designed particularly for producing high-strength engine parts, such as cam shafts or piston rods, components designed for protecting against pulse-type striking pressures, such as bombardment, or protective elements, such as armour plate, or body armour for the human or animal body.
• highly stressable gear components which are characterized by minimum weight and good performance properties, can be made of steel sheet according to the invention without additional heat treatment needed for this purpose.
• Table 1 shows the composition of steels A, B, C, D, E and V1, of which steels A-E belong to the steels processed in a way according to the invention, while steel V1 is indicated for comparison purposes only.
• the steels are molten in each case and cast into pre-strip using the DSC process.
• the molten material was poured by means of a dispensing spout onto a revolving, heavily cooled conveyor belt, on which it has been intensively cooled down additionally by liquid cooling working from above.
• the molten material being solidified in such a way on the conveyor belt into the pre-strip was then removed from the conveyor belt and again subjected to secondary cooling in the directly adjoining stage.
• the hot strip obtained in this way was then wound at a winding temperature of 500° C. into a coil.
• Winding was followed by cold-rolling, wherein the hot strip was formed with a strain degree of approx. 62.5% into cold strip, which was 0.75 mm in thickness.
• the cold strips were then annealed while running to recrystallization at temperatures of 950° C.
• the steel strips A-E produced from the steels A-E in a way according to the invention possess outstanding cold ductility at the same time with high strengths and high elongation at rupture. At the same time in each case they comprise a pronounced isotropic behaviour. As such they are especially suitable for producing cold-formed components, which are exposed to high stress in service.
• the characteristic profile of KC indicated in Table 2 is worse than that of KV1, which is due to the only weak TWIP effect.
• the advantage of KC relative to KV1 lies in the high density reduction as a result of the high Al content.
• the comparison steel V1 comprising TRIP properties possesses high strengths with comparatively low characteristic values A80 and AG, which represent a substantially worse transformation capacity. This substantially worse deformation behaviour is also evident from the substantially worse r and ⁇ r values relative to the steels A-E.

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• Chemical & Material Sciences (AREA)
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• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
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• 2004
  • 2004-11-03 WO PCT/EP2004/012407 patent/WO2006048034A1/fr active Application Filing
  • 2004-11-03 BR BRPI0419185-4A patent/BRPI0419185A/pt not_active IP Right Cessation
  • 2004-11-03 US US11/718,498 patent/US20090010793A1/en not_active Abandoned
  • 2004-11-03 CN CN200480044461.3A patent/CN101065503A/zh active Pending
  • 2004-11-03 JP JP2007539465A patent/JP2008519160A/ja active Pending
  • 2004-11-03 EP EP04797547A patent/EP1807542A1/fr not_active Ceased

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