METHOD FOR PRODUCING METAL BAND MATERIAL WITH DIFFERENT MECHANICAL PROPERTIES ACROSS THE WIDTH OF THE BAND
A method for producing metal band material with different mechanic& properties across the width of the band, in which the hand is heated zonally with regard to the width so that zones with a different heating state are temporarily produced and the band is then cooled in, order to produce regions with a different metal structure and thus different mechanical properties, wherein one or more zones that are to be heated are acted on with a heating device, while the other zones are isolated from the heating or are freed of the heating or are actively cooled or else a beat flow, which is introduced into the zones that are not to be heated, is diverted to contact masses that are resting against the zones that are not to be heated.
The invention relates to a method and devices for producing metal band material with different mechanical properties across the width of the band.
BACKGROUND OF THE INVENTIONEP 2 529 038 B1 has disclosed a method for heat treating metal band material in which the metal band material is supposed to have different properties that vary across the width of the band. This method is particularly intended for use in a continuous annealing system and its stated object is to heat and/or cool the band differently. EP 2 529 038 B1 does not indicate a technical attainment of any of these objects. The objects mentioned in this document are the different heating of the steel band material across the width, in particular above or below the Ac1 temperature and in particular above or below the Ac3 temperature so that within the band, a variation in the absolute maximum temperatures should be present and then, the different-temperature regions should be cooled at different speeds or at the same speed. In this connection, it is disadvantageous that basically different temperatures in the band result in the fact that the steel band assumes different thermal expansions or thermal expansion states. This results in the fluttering of the band that is to be feared in continuous annealing systems.
This fluttering of the band has long been known and in conventionally operated continuous annealing, stems from the fact that the band is not uniformly heated across the width and thus particularly at the edges, is cooler than in the middle, causing the band to arch.
In order to counteract this, EP 0 155 753 has disclosed measuring the different temperatures of the band across the width and compensating for them across the width of the band by means of cooling gas flows so that the temperature is distributed uniformly, thus achieving a uniform thermal expansion state, permitting the band to be flat.
DE 29 52 670 A1 has disclosed a cooling method for steel bands in a continuous annealing with an overaging zone; the cooling in this case takes place in two stages with different temperatures.
DE 10 2008 010 062 A1 has disclosed a method for hot rolling and heat treating a band of steel. In this method, in order to economically produce high-tensile and ultra-high-tensile bands with sufficient toughness in a band system, a plate slab that is to be rolled is first heated, the plate slab is rolled to the desired band thickness, after this, the band is cooled, with the band having a temperature greater than the ambient temperature after the cooling, and then the band is wound into a coil. This band is then heated again, cooled, and transported.
DE 10 2008 049 537 A1 has disclosed a method for cooling a rough band or band of a metal strand in a hot-rolling plant. In this case, a coolant is sprayed onto the rough band or band; the rough band or band is shielded in the region of the band edges by being acted on with the coolant, for which purpose shielding devices are provided. Another shielding device can be positioned in the region of a looper or in a cooling device between the roughing stand and the finishing mill. This is supposed to reduce temperature differences in the band, particularly in cooler band edges, in order to reduce the wear on the rolling mill.
US 2009/0090437 A1 relates to a method for producing metal plates in which a temperature gradient is set across at least one direction of the plane of the plate (width or thickness and/or length or width).
JP 2004 11 5830 A has disclosed a method whose purpose is to achieve a control and monitoring of the cooling and in particular, to achieve a harmonization across the width of the band by preventing excessive cooling of the edge regions of the steel sheet. This method also uses different so-called “air boxes” by means of which cooling liquid is blown at different speeds.
Particularly in automotive engineering, there has long been a need for sheet steel blanks and/or components that have different mechanical properties across their span its both the length and width directions.
The mechanical properties that are mainly required in this context are tensile strength (Rm) and ductility. These two properties have a direct influence on the safety of the vehicle because they influence the crash properties. If the components of a passenger compartment are very strong, but have a low ductility, then although they do in fact resist very powerful forces, they nevertheless break relatively quickly, without absorbing a sufficient amount of energy. If the components are ductile but insufficiently strong, then they can in fact absorb energy, but they may permit too powerful a deformation. The solution to these problems lies, for example, in zonally different mechanical properties of such components or of sheets that are processed to produce such components.
There are a multitude of approaches for implementing these zonal differences. One such approach is for steel material with different mechanical properties to be welded to one another so that so-called tailor-welded blanks are produced. If such sheets are used, the component therefore likewise has zonally different mechanical properties. In this connection, it is also possible for the entire component, after the welding, to be subjected to a heat treatment and then cooled or quenched so that because of the different steel qualities, different structures form, making one part harder while the other part is more ductile. Such tailor-welded blanks have in principle proven their value.
Another approach is to form a blank out of a homogeneous steel material, which can, for example, be quench-hardened. For example, steel materials of this kind are boron-manganese steels with carbon contents of between 0.10 and 0.4%. In order to perform a quench hardening, they are heated to a temperature above the austenitization temperature (Ac3) and are then quenched, i.e. cooled at a cooling speed that lies above the so-called critical hardening speed. For example, the critical hardening speed is 20° C./s. Under these conditions, the austenite is transformed into martensite; the transformation of the steel phases results in a distortion in the metal lattice, which dictates the high degree of hardness. Other phase transformations within the lattice of steel materials also lead to comparable hardening effects, but these may not be quite as pronounced.
In summary, however, it can be said that the manner of the heating, the absolute height of the treatment temperature, and the manner of cooling of steel materials can be used in the broadest sense to exert influence on the structure, which in turn determines the influence on the hardness and ductility.
In order to influence the mechanical properties in a blank composed of a homogeneous steel material, it is thus necessary for parts of the blank to be either differently heated or differently cooled.
One possibility is to not heat regions of the blank to the austenitization temperature (or to other desired transformation temperatures), but to have them remain below this transformation temperature. In a subsequent uniform cooling of the component, the transformations into other phases in these regions do not take place or do not take place in this form so that different hardnesses and ductilities are achieved.
Another possibility is to evenly heat the blank to a uniform temperature, but to only quench-harden the regions that are supposed to have a high degree of hardness, while the remaining regions are cooled more slowly and therefore do not achieve this hardness and remain more ductile. This also affects other transformations, not just austenite-martensite transformation; in other transformations, different maximum temperatures and different cooling speeds are required. A third possibility is to heat the blank uniformly and then to quench harden it uniformly, but then to re-anneal regions that are supposed to remain soft, and thus to partially undo the hardening effect.
In summary, it can be stated that even with uniform blanks, there are a large number of possibilities for exerting influence on the hardness and ductility or the tensile strength and flexural strength.
Hardening effects, however, are known not only in boron-manganese steels, which are in particular referred to as press-hardened steels. Other steels such as dual-phase and multi-phase steels such as DP steels and TRIP steels can be adjusted with regard to the mechanical properties mentioned above by means of heat treatments that are known from the basic execution and from the corresponding limiting temperatures and cooling temperatures.
The mechanical properties of such steel blanks do not have to first be set at the processor by means of heating and cooling processes like press-hardening; band material can also be correspondingly heat treated. As is known from the prior art, band steel, which passes through a continuous annealing system, can be cooled with different cooling rates in order, for example, to ensure a uniform temperature in the band.
As explained in the prior art, one object is also to embody the heating of a band in a zonally different way across the width.
Regardless of the type of heating or the cooling rate, it is problematic here that different temperatures—because of the waviness occurring due to different thermal expansion states across the width of the band—can lead to so-called band flutter.
The object of the invention, therefore, is to create a method with which steel bands can be heat treated in a continuous heat treating system, with better control of the process.
SUMMARY OF THE INVENTIONThe inventors have realized that although the simple concept that it could be advantageous to produce a band with different temperatures across the width is in fact appealing, it creates a multitude of problems that in the end, result in the fact that with conventional systems, either regions of the band crack or the band is deflected and/or wobbles and/or flutters and the fact that on the one hand, the system cannot be operated reliably and on the other hand, the system can be damaged. In particular, the guide rollers become damaged and for example ridges form.
In the method according to the invention, a band is heated in a zonally different way, the heating in this case being carried out so that a high dividing precision of the heated zones can be achieved.
A first measure according to the invention seeks to match the guidance of the bands inside the system to the different thermal expansion state.
This means that the waviness of the band as a function of the heated zone is accepted and the guide elements, namely the rollers, are correspondingly matched to the waviness of the band and are embodied as correspondingly convex or concave. For example, if an inner zone of the band is heated more intensely than the outer zones, the band will then arch in a relatively harmoniously uniform fashion. A correspondingly convex roller or two correspondingly convex rollers guide the band in the region in which it has assumed this state.
For example, if the band is heated more powerfully at the edge so that only an edge arches or retracts, then the rollers are correspondingly matched to this, in particular, are embodied as partially concave or partially convex.
It is thus possible to provide different rollers for different bands and different heating states across the width.
Also according to the invention, the zonal heating of the band is carried out so that a high dividing precision between the zones is achieved.
In one embodiment of the invention, a high dividing precision in the zonal heating of the band is achieved in that above the desired heating zones of the band, inductors are provided, which selectively heat the corresponding zones of the band to the predetermined temperature. In this case, the band speed and/or the intensity of the induction and/or the size or length of the inductors in relation to the transport direction of the band is/are matched to the desired induction power.
In another advantageous embodiment, the surface of the band is acted on by means of radiators, radiant tubes, flames, or the like; in order to increase the zonal dividing precision between the corresponding zones, partition walls are provided in order to avoid a heating of the other zones or of the edge regions.
In another embodiment, the emissivity of the band can be influenced, either through adaptation of the coating (e.g. Z or ZF) and by means of paints, or through influencing by means of burners.
Particularly when using flames or burners, but also when using other heating media, a gas flow against the surface of the band due to suction or blowing can be adjusted so that hot gases from one zone cannot cross over into another, here, too, the distance of the partition walls from the band surface can be adjusted correspondingly.
To increase the dividing precision, in regions in which the band or certain band zones is/are not supposed to he heated, in parallel to the heating, the band can be kept at the desired low temperature.
According to the invention, this can happen in that particularly with the presence of partition walls, a cooling gas flow can be directed onto the band surface, but this flow advantageously does not reach the heating zones.
In another advantageous embodiment, this is implemented in that in these regions, on one or two sides, cooling rollers or rollers with a high thermal capacity are provided, which absorb the heat flow acting on the band and thus counteract an increase in the temperature. In this case, a heating device can act on the entire band.
Preferably, the different thermal states of the band and the corresponding subsequent cooling are achieved over a relatively short distance, for example after a deflection by means of a lower roller in the ascending run of the band until the upper roller, within the distance between the lower roller and upper roller (or vice versa) so that the thermal expansion state across the width of the band is uniform again when the upper roller is reached, as a result of which, the deflecting rollers—which are subjected to powerful loads—can guide a flat band while between the deflecting rollers, guide rollers can be provided, which, by means of a corresponding convexity, limit the fluttering of the band between the two deflecting rollers.
For example, in order to achieve the heating and cooling in one pass, it is possible that with a conventional pass length of 20 m and a band travel speed of approx. 120 m/min—this corresponds to 2 m/sec—the band needs about 10 seconds to travel through a pass. If the local heating is introduced at one end and the cooling takes place at the other end, then when a band reaches the deflection roller, it has a homogenous temperature.
This would also have the advantage that any temperature compensation across the band would be reduced by the short duration of the local temperature action. It is thus possible to achieve advantageously sharper dividing regions with regard to the mechanical properties.
One particularly important mechanical property is the tensile strength Rm.
When it comes to the tensile strength, the difference in the mechanical properties of the treated zone(s) in comparison to the rest of the band is at least 20 MPa, preferably 50 MPa.
In steels with a tensile strength of greater than 1000 MPa, the difference amounts to at least 5% of the tensile strength, i.e. in a steel band in which Rm=1600 MPa, for example, this difference amounts to 80 MPa.
The invention will be explained by way of example below based on the drawings. In the drawings:
According to the invention, a metal band 1 and in particular a metal band 1, which is conveyed through a device for heat treating a metal band, is heated in a zonally different way across its width B (
This means that across its band width, the metal band has different mechanical properties such as the tensile strength Rm.
The zone relates to a width section, which, across its entire height (band thickness), has this different property in comparison to the rest of the band. This is particularly true of steel belts with a maximum thickness of 5 mm, particularly preferably 0.5 mm to 3 mm.
For this different, zonal heating, preferably a band width of at most ⅓ of the total band width is carried out.
The zone(s), however, can also be divided from one another on the band, i.e. there can also be an untreated width section between 2 heat-treated width sections. The minimum width for the zonal heating can preferably be greater than 100 mm in order to adjust the mechanical properties in a more efficient way.
The device for heating or heat-treating the metal band, in particular a steel band, can be a so-called continuous annealing system, but can also be any other tempering device in which a steel band is tempered, for example it can also be the tempering device or heat treating device before the steel band undergoes a metallic hot-dip coating (zinc-based or aluminum-based coating) or a after passage through a corresponding hot-dip coating.
In order to be able to perform zonally different tempering of the metal band with a sufficient dividing precision and in particular, in order to be able to carry out tempering in accordance with predetermined zones Z1, Z2, and Z3, a heating device 2 is provided in one or more zones in which a higher temperature of the band 1 is specified (Z2 in
In order to avoid heating adjacent zones (Z1 and Z3 in
The cooling devices 3 are depicted as blowers in
Depending on the cooling power required, the blowers direct a cooling gas flow—which is metered in terms of its quantity, speed, and temperature—onto the surface of the metal band 1. With the regulation of the temperature of the gas flow and/or the quantity of the gas flow and/or the speed of the gas flow, it is possible to establish a desired necessary cooling rate.
The cooling gas flow applied to the surface is advantageously applied in the region in which heat develops in the heated zone Z2; the cooling gas flow is also advantageously withdrawn again at the end of the heating section in the zone Z2 in order to avoid an overflow of the gas flow into the heating zone Z2.
Instead of corresponding gas flows and the corresponding device for producing and directing them, the cooling can also take place in a different way, in particular by means of spraying with fluids or through contact with solid objects.
Particularly with cooling gas flows, but also with heating gas flows, in particular flame treatment, in order to also be able to increase the dividing precision in the region of zones Z1, Z2, and Z3 and particularly in the region of the zonal heating, partition walls or partition curtains 4 can be provided in order to define the respective zones.
This significantly improves the introduction of heat, but also significantly improves the withdrawal of cooling gas flows or cooling liquid flows. The ends 5 of the respective curtains or partition walls 4 oriented toward the steel band can thus be spaced a short distance away from the steel or metal band 1 and in particular, an arching of the band caused by the heating can be compensated for by changing the length and the walls 4, which means that the distance of the ends 5 from the steel surface 1 is always as uniform as possible.
Particularly with flowing cooling media or heating media, in the vicinity of the ends 5 of the curtains or walls 4, gas outlet openings can be provided, from which a gas flows, which forms a gas curtain for dividing the atmospheres of the zones Z1, Z2, and Z3. For this purpose, the walls 4 are then correspondingly embodied so that they can absorb a corresponding gas flow and convey it to the ends or end edges 5.
The ends 6 of the outer walls 4 in this case can extend laterally beyond the steel band and can thus also cover the free edges.
This arrangement can also be doubled and can act across the width of the band from both sides.
In another advantageous embodiment (
In this embodiment, the heating device 2 is, for example, an inductor, which can in particular be moved in the direction of arrow 7 toward the surface of the steel band and away from it.
On the opposite side of the steel band 1, in the zones (Z1 and Z3) that are not supposed to be heated and that are supposed to have a temperature that lies below the temperature in the heated zone Z2, devices 8 are provided, which maintain a constant heat in the zones Z1 and Z3 that are not supposed to be heated or that are supposed to be heated less. This happens by virtue of the fact that in these regions, heat, which has been brought by the heating device 2 into the zone Z2 and is discharged into the zones Z1 and Z3, is introduced into the devices 8 due to thermal conduction and due to a thermal capacity that is as high as possible so that a heat flow from the zone Z2 and/or the heating device 2 that flows into the zones Z1 and Z3 is absorbed by the devices 8.
The devices 8 in this case can, for example, be solid objects, which rest against the sheet metal band 1 from beneath, in particular cooling elements made of suitable alloys such as an amco alloy; it is also possible for a cooling liquid to flow through these cooling elements. In addition, the devices 8 can also be embodied as cooling rollers 8, which rotate along with the steel sheet band 1 and in this way, on the one hand, absorb the heat and on the other hand, are able to possible dissipate it to suitable cooling devices on the underside.
In this instance, it is advantageous that there is a very high dividing precision between the zone Z2 to be heated and the zones with a low temperature Z1 and Z3.
If it is not the central zone Z2 that is heated, but rather one or both of the edge zones, then the arrangement is modified in a corresponding way.
In order to differently embody the mechanical properties of the steel sheet band or metal band 1 in accordance with the zones, it is necessary, after a corresponding heat treatment, to cool these regions with corresponding cooling rates, thus producing a desired metal structure that possesses a corresponding hardness, ductility, tensile strength, or flexural strength.
There are already sufficient solutions known from the prior art for zonal cooling of such sheet metal bands.
A zonal heating of steel sheet bands, however, inevitably results in the fact that the thermal expansion state of the steel sheet band also changes zonally.
Non-homogeneous temperature distributions in steel sheet bands are to be feared because they result in corresponding occurrences of waviness, and fluttering of the band.
According to the invention, these occurrences of waviness can be compensated for—particularly after the zonal heating and before the zonal cooling—by providing corresponding; guides, the band can be stabilized, and band flutter can thus be counteracted, without suppressing the waviness by means of pressure with smooth rollers, which can result in cracks in the edge regions of the steel sheet band 1.
Consequently, a heating in zone Z1 (
In
By contrast, if the other edge zone Z3 is acted on with heat (
If a middle region, i.e. a zone Z2, is acted on with heat, then an arching of the entire steel band 1 occurs so that a convex roller 10 according to
In order to achieve the heating state or in order not to influence the heating state of the respective steel band 1, the rollers 10 can be composed of a material that retains the heat if need be after a start-up phase and keeps an outflow of heat to a minimum. In particular, the rollers 10 can be composed of a ceramic material or of a steel material with a ceramic coating; they can also be provided with a heating medium that flows through them.
The different rollers 10 for reacting to a different number of heated zones Z1, Z2, and Z3 or different positions thereof can be positioned in revolver stand fashion in the corresponding device for continuous heating or heat-treatment so that these rollers can be pivoted from an idle position into a working position and can be immobilized there, while the previous roller is pivoted out of the working position and into the idle position.
According to the invention, these convex or formed guide rollers for wavy bands can be positioned after a heating device 2 and before a cooling device (not shown) in order to support the wavy band and also to control the waviness as needed and to act on the band in corresponding fashion.
This requires that this heating device and the cooling device be situated between the deflection rollers of a corresponding loop-forming system. If the waviness extends across a deflection roller of such a system, then according to the invention, the deflection roller that deflects the still-wavy band, is also embodied as correspondingly convex.
EXAMPLEThe invention will be explained below based on an example.
A band steel with the following composition (all values in percent by weight)
C=0.21
Mn=1.3
Si=0.3
Cr=0.3
Ti=0.03
B=0.0025
remainder=iron and melting-related impurities has a fully martensitic structure and is to be processed in a profiling system.
In order to achieve an easier formability in the region of the profiling, this band is zonally heated and cooled again, as a result of which, the band is annealed in the zone that has been heated and cooled again and is cooled in such a way that it becomes more ductile in this region.
To this end, the band is guided through a heating device according to the invention; the band first leaves a guide roller in loop-former, is then zonally subjected to inductive heating and subsequently cooled.
The band travels through a pass, i.e. two deflection rollers, for 20 meters at a speed of 2 m/s, i.e. in 10 seconds; in this case, the band is heated from 20° C. to 370° C. and is then cooled again.
After the exit from the pass in the corresponding zone, the tensile strength has decreased by 200 MPa.
The steel band has a thickness of 2 mm and consequently, this reduction in the tensile strength will be present across the entire band thickness.
The method according to the invention can also be used for other steel alloys, see the sample compositions in Table 1.
For the alloys A through E from Table 1, Table 2 shows the respective tensile strength at an annealing temperature (in the band) of 350° C. to 500° C.,
In this case, it is clear that by varying the annealing temperature by even 50° C. significant changes occur in the mechanical property, in this case, the tensile strength.
In the example of alloy E, for example, at an annealing temperature of 350° C., a tensile strength of 1550 MPa is achieved.
If a zonal width region of for example 250 mm of the right band region is heated with induction to the 500° C. annealing temperature, then in this band region, a tensile strength of 1520 MPa, i.e. a reduction of 130 MPa, can be achieved.
Naturally, it is also conceivable tot only a certain section to experience this annealing temperature and for the rest of the band to not be annealed.
Another exemplary embodiment is the intentional annealing, of three narrow strips of 20 mm each in order to enable a selective deformation of the component produced from the band (usual band width approx. 1800 mm) for the sake of improved energy absorption in the event of a crash.
In this case, induction coils are used to anneal each of the narrow regions across the entire band thickness. The heating and cooling take place within one band pass.
With the invention, it is advantageous that a method for zonally heating steel sheet bands is created with which the bands, even other metallic bands, can be effectively and reproducible zonally heated and quenched with high dividing precision and can be embodied with zonally different mechanical properties, where a high zonal dividing precision is achieved.
This invention can advantageously be used not only with three zones, but also with a larger or smaller number of zones with different heating states and mechanical properties.
The invention relates to a method in which in at least one zone that is to be heated, the hand has a difference in the tensile strength of at least 30 MPa and in particular 50 MPa.
The invention also relates to a method in which in the one or more zones to be heated, the band has a difference in the tensile strength of at least 5%, in particular 10%, of the tensile strength of the untreated region.
The invention also relates to a method in which the one or more zones to be heated has/have a width of at least 20 mm.
Claims
1. A method for producing a metal band material with different mechanical properties across a width of the band, comprising:
- heating the band is zonally with regard to the width to temporarily produce zones with a different heating state by acting on one or more zones with a heating device; while any remaining zones that are not to be heated are isolated from the heating or are free of the heating or are actively cooled, or diverting a heat flow to contact masses that are resting against the remaining zones that are not to be heated; and
- cooling, the band in order to produce regions with a different metal structure and thus different mechanical properties.
2. The method according to claim 1, wherein the heating of the band from a starting temperature to a target temperature takes place within one pass, between two deflection rollers of a band looping apparatus.
3. The method according to claim 1, wherein the cooling of the band takes place after the zonal heating within one pass of a band looping apparatus.
4. The method according to claim 1, wherein both the heating and the cooling of the band take place within the same pass of a baud looping apparatus.
5. The method according to claim 1, wherein in the region of a zonal heating and re-cooling, the band is guided with rollers, which are embodied as convex or concave depending on thermal expansion-dictated arching of the band.
6. The method according to claim 1, wherein in at least one zone that is to be heated, the band has a difference in tensile strength of at least 30 MPa.
7. The method according to claim 1, wherein in the one or more zones that are to be heated, the band has a difference in tensile strength of at least 5%, of a tensile strength of the untreated region.
8. The method according to claim 1, wherein the one or more zones that is/are to be heated has a respective width of at least 20 mm.
Type: Application
Filed: Sep 23, 2015
Publication Date: Oct 19, 2017
Inventors: Peter Atzmüller (Linz), Andreas Pichler (Vöcklabruck)
Application Number: 15/515,972
