Patent Application
20090010793
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.
So-called “Hadfield 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.
In addition, austenitic steels with even higher Mn-contents are known, which possess so-called “TWIP” properties (“TWIP”=“Twinning Induced Plasticity”). The steels concerned together with low weight and good strength possess high ductility when mechanically loaded in consequence of a twinning formation of the grains of the structure arising in the course of mechanical loading. This twinning formation directly facilitates the deformation of the steel. The twinning also contributes thereto because it limits the mobility of dislocations, in order to increase the flow stress of the steel in the event of mechanical loading. The ductility of TWIP steel is possibly additionally assisted by a martensitic γ/α 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. In accordance with the known method 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 mm to 10 mm in thickness. The thin strip obtained in this way is then directly, or possibly after intermediate hot-rolling with subsequent winding, cold-rolled to 10%-90% reduction in one or more stages into cold strip and afterwards subjected to re-crystallization annealing.
Apart from the use of twin-roll casting machines, in technical parlance also called “Double Roller” or “Twin Roller”, cast strip can also be produced by the so-called “Direct Strip Casting” process, for which the abbreviation “DSC process” is normally used. With this method 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. Within the area of the 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. Subsequently, 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.
A possibility of producing steel strips from Fe—Mn—Al—Si alloys using the DSC process is described in the essay “DEFORMATION AND MECHANICAL PROPERTIES OF HIGH MANGANESE TRIP ALLOYS” by Renata Vi{hacek over (s)}{hacek over (c)}orová et al. published in Proceedings at IDDRG International Deep Drawing Research Group 2004 Conference, 24-26 May 2004, Sindelfingen, Verlag Stahleisen GmbH, 2004, ISBN 3-514 00708-X, pages 261-269. Apart from a general reference to the possibility of producing TWIP steel using the DSC process, in this publication as a specific example of an Fe—Mn—Al—Si-alloy cast in this way there is a steel possessing TRIP properties, which apart from iron and melting conditioned impurities comprises (in mass %) 16.2% Mn, 2.36% Al, 2.47% Si, 0.084% C, 0.007% S and 0.0093% N.
Dependent on its composition TRIP steels (“TRIP”=“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. In contrast 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.
But all variants of known metal sheet produced from lightweight structural steel of this type, although they possess high strength, have specific characteristic disadvantages. Thus, for example, wide ranging of the brittle-ductile transition temperature, heavy dependence of the properties on temperature or more anisotropic deformation behaviour occur.
In addition, steels with a high Mn-content can only be hot and cold-rolled with difficulty due to their intrinsic high strength. This is shown to be particularly critical in the case of the high-strength TWIP steels of the type discussed here. Thus with such steels instabilities or tears frequently appear at the edges of the strip, which in practice make large-scale production and processing of strip or sheet from such steels difficult. Also due to the extreme hardness, which steel with Mn-contents of 18 mass % and more possesses even in the just cast condition before heat-rolling, large capital investment in production plant is necessary, in order to produce thin hot strip from such steels, from which cold strip of narrow thickness can then be produced at reasonable cost. However, especially in the field of motor vehicle body construction there is increased demand for such thin cold-rolled metal sheets, which have low weight with high strength and good deformation and hardening behaviour in the event of an accident.
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.
With respect to the method for producing cold-formable, high-strength steel strips or sheets with TWIP properties, this object was achieved in that the following successive working steps are carried out according to the invention without interruption:
With regard to the method for producing a high-strength component 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.
Due to the special way in which it is produced 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 Tue lies under −40° C. The transition temperature Tue concerned is normally determined with the cupping test or notched bar impact test.
Thus, it can be ensured when using steel strips or sheets according to the invention, for example when producing motor vehicle body panels or comparable applications that the superior deformation capacity of these steel strips and sheets is constant over the entire temperature range in which such applications are normally used.
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. In contrast to casting strip with the known strip casting machines, therefore, 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. Thus, 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.
The heavy deformation during hot-rolling leads to the fact that, in contrast to conventional strip casting by means of a twin-roll casting machine, the original casting structure of the pre-strip is, as far as possible, completely eliminated and a hot strip structure is produced which, due to its particularly homogeneous, completely re-crystallized structure and due to the most extensive elimination of cavities, is marked by particularly good ductility. Accordingly, 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. In this way for example 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. Thus, such 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 CO2-emissions.
Processing, possible through the invention, of molten materials whose composition can be varied to high tolerances, renders the possibility of using non-optimum alloying materials with corresponding impurities and thus additionally reduces the costs for alloying materials. The high cost of blast furnace coke can be avoided.
The segregation profile, problematic with conventional vertical continuous casting, is substantially reduced when processing, according to the invention, steel of the type under discussion. Also, irregular casting structures, which arise with conventional continuous casting is homogenized using a method according to the invention.
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.
Finally, 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.
Additionally, the carbon content proposed according to the invention opens up a wide spectrum of hot-rolling parameters. Thus, it has been found that the characteristic values of steels according to the invention obtained when selecting high final hot-rolling temperatures and winding temperatures are substantially the same as those which are obtained at low final hot-rolling temperatures and winding temperatures. Also, this insensitivity favours the simple and sure feasibility of the method according to the invention.
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.
Since the total content of Mn and Ni in the case of the steel under discussion should not exceed 30 mass %, 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.
For the same purpose 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.
Surprisingly, it has been shown that the presence of selenium and tellurium results in improvement of the wetting behaviour when the composition is applied to the conveyor belt, on which the molten material is afterwards solidified into the pre-strip. An advantageous embodiment of the invention accordingly proposes that the total Te and Se contents in the molten material are at least 0.01 mass %.
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. In accordance with a further embodiment of the invention it is therefore proposed that 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.
As a result, when using the method according to the invention and exploiting the possibilities of the alloying concept according to the invention a particularly well cold-formable lightweight structural steel strip or sheet is obtained, which is suitable, in particular due to its comparatively high strength, for producing motor vehicle body panels. Likewise, 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.
Moreover, it has been shown that the 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 isotropic deformation behaviour of steel strips and sheets produced according to the invention is particularly remarkable. Thus, steel strips and sheets, whose average r-value rm is 1.0+/−0.15 and whose Δr value is −0.2 to 0.2, can be easily made available by means of the invention.
Because 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. Thus, in the case of strip hot-rolled in this range, carbon brings about higher tensile strength and yield point values with still acceptable degrees of elongation. As the final hot-rolling temperature increases, the tensile strength and yield strength decrease, while the degrees of elongation rise. As a result of varying the final hot-rolling temperatures within the limits specified by the invention, 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. Accordingly, 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. However, 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.
Already 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%.
Due to the possibility provided by the method according to the invention of producing thin hot strip whose structure is completely recrystallized from relatively thick pre-strip using a high strain degrees, it is easily possible when cold-rolling to produce cold strip in a thickness of 0.8 mm or less, for example 0.6 mm. Such thickness of metal sheet is demanded especially for motor vehicle body construction.
In order to avoid impairment of the surface quality through scale adhering to the hot strip during cold-rolling, the hot strip can be pickled before cold-rolling.
Preferably, 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.
If scale forms during annealing, then in order to improve the surface quality of the final cold strip it may be expedient to also subject the annealed hot strip to acidic pickling. This applies in particular if the cold strip unfinished in order to achieve optimum surface quality and dimensional precision as well as optimum mechanical properties.
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/mm2. 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.
Depending on the composition 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. In the course of cold-forming they solidify substantially more strongly than high-strength micro-alloyed or multi-phase steels used, as is known, for producing components by flow-turning. Thus, component strengths in the range of 1,400 N/mm2 to 2,200 N/mm2 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.
When using steel composed and produced according to the invention heat treatment or surface hardening of the component by flow-turning is therefore no longer necessary. The risk of warping and scale-formation, caused by these additional process stages in the case of the prior art, does not exist with production according to the invention. This is positively noticeable, particularly in the production of toothed components subjected to locally heavy stress in service. Thus, the steel according to the invention facilitates the economic production of light-weight high stressable and dimensionally-precise components by cold-forming, in particular flow-turning.
As a result 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.
Also, 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.
Likewise 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.
The invention is described in detail below on the basis of exemplary embodiments.
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. In this case 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 steel strips emerging from the secondary cooling, still possessing a sufficiently high temperature, were then again hot-rolled directly afterwards while exploiting the heat retained therein to a thickness of 2 mm, wherein the hot-rolling temperature was 900° C.
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 mechanical properties: yield point Re, tensile strength Rm, extension A80, uniform elongation Ag, n−, r and Δr values of the cold strips KA-KE produced in this way from the steels A-E and the strip KV1 produced from the comparison steel V1 are indicated in Table 2.
It is shown that 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.
On the contrary 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 Δr values relative to the steels A-E.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2004/012407 | 11/3/2004 | WO | 00 | 2/19/2008 |
