Patent Application
20090165986
The invention relates to a method of producing a metal strip using continuous casting, a strand, preferably a thin strand, being initially cast in a casting machine and being diverted from vertical downward travel to horizontal travel, and, in the travel direction of the strand, the strand being subjected, downstream of the casting machine, to a milling operation in a milling machine in which at least one surface of the strand is milled off and preferably two opposing surfaces are milled off. The invention furthermore relates to a apparatus for producing a metal strip using continuous casting.
In continuous casting of strands in a continuous-casting system, surface flaws can occur, such as for instance oscillation marks, casting powder errors, or surface cracks that run longitudinally or transversely. These flaws can occur with conventional and thin strand casting machines. The conventional strands are sometimes descaled depending on the purpose of the finished strip. Some strands are generally descaled at customer request. The demands on surface quality for thin strand systems are becoming increasingly stringent.
Descaling, grinding, or milling are options for surface machining.
Descaling suffers from the disadvantage that the material removed cannot be melted down again without further preparation due to its high oxygen content. During grinding, metal splinters mix with the grinding wheel dust so that the abraded material must be disposed of. Both methods are difficult to adapt to the prevailing transport speed.
Milling is therefore the primary type of surface machining used. The hot milled cuttings are collected and can be packetized and re-melted with no problem and without further preparation, and can be re-introduced into the production process in this manner. Moreover, it is easy to set the milling speed to the transport speed (casting speed, finishing train speed, advancing speed). The inventive method and the associated apparatus therefore apply primarily to milling.
A method and a apparatus of the above-described type are known that have a milling operation that takes place, or a milling machine that is disposed, downstream of a continuous-casting system. See CH 584 085 [U.S. Pat. No. 4,047,468] and DE 199 50 886.
DE 71 11 221 discloses a similar solution. This document depicts the machining of aluminum strips using the casting heat, in which the machine is connected to the casting system.
In-line milling of the surface of a thin strand (descaling, milling, etc.), on the upper and lower faces or even on only one side, just upstream of a rolling mill has also already been suggested; see EP 1 093 866.
DE 197 17 200 shows another embodiment of a surface milling machine. It describes, inter alia, the variability of the milling contour of the milling apparatus that is provided downstream of the continuous-casting system or upstream of a rolling train.
EP 0 790 093 [U.S. Pat. No. 4,436,937], EP 1 213 076 [U.S. Pat. No. 6,195,859], and EP 1 213 077 [U.S. Pat. No. 6,192,564] suggest another arrangement of an in-line milling machine in a conventional hot-strip mill for machining a rough strip and the embodiment of this arrangement.
In contrast, JP 1031 4908 describes descaling the continuous cast strip downstream of the casting machine.
In DE 199 53 252 [U.S. Pat. No. 6,436,205], the strand cast in a casting machine is initially guided through a transverse separating apparatus and then through various ovens before it is subjected to a rolling operation.
During surface machining of the thin strands in a so-called CSP system, approx. 0.1-2.5 mm are to be removed from the hot strand surface, on one side or on both sides, in the machining line (“in-line”) as a function of detected surface flaws. A thin strand that is as thick as possible is recommended (H=60-120 mm) in order not to reduce output too much.
Surface machining and the apparatuses associated therewith are not limited to thin strands, but rather can also be used in-line downstream of a conventional thick strand casting system and with strands that are cast with a thickness of more than 120 mm to up to 300 mm.
As a rule an in-line milling machine is not used for all products in a rolling program, but rather only for those for which stringent demands are made in terms of surface quality. This is advantageous for output reasons and reduces wear and tear on the milling machine and therefore is reasonable.
There is a desire to employ the technology that is already known even more efficiently and therefore with greater cost efficiency. It should be possible to produce, although not exclusively, high quality thin strands at high mass throughput.
The following should be noted regarding operational parameters for a continuous-casting system:
The casting parameters for a few exemplary parameters that can typically be attained for steels that are simple to cast are shown in the following table:
These are speeds that as a rule are at the upper end of the operational range. For high-strength materials where C>0.3%, silicon steel, and micro-alloyed steel, the speeds are typically 20% lower, i.e. 350 m/min×mm−20%=280 m/min×mm.
It has proven disadvantageous that strand surface quality suffers at high mass flow or casting speed.
The underlying object of the invention is therefore to improve a method and an apparatus of the above-described type such that an improved production process or machining process can occur with high efficiency. This should include in particular optimizing with a focus on the required addition of heat into the casting strand and into the production process, and also and in particular as concerns the rolling process that follows casting.
This object is attained by the invention using a method characterized in that the strand is milled as a first mechanical machining step after the strand has been diverted to horizontal travel, the strand being cast with a thickness of at least 50 mm and the strand being cast with a mass flow, as the product of casting speed and strand thickness, of at least 350 m/min×mm.
Alternatively, the strand is cast with a mass flow—as the product of casting speed and strand thickness—of at least 280 m/min×mm, the material for the strand being a high-strength material having a carbon content of C>0.3%, silicon steel, or micro-alloyed steel. With these materials the mass flow is thus 20% less than described above.
The strand is preferably milled immediately after the strand is diverted to horizontal travel. The strand can also be milled after the strand is diverted to horizontal travel and has passed through a thermal equalization section and/or an oven.
Upstream or downstream of the milling machine at least one surface parameter of the strand can be measured and the machining parameters during milling can be set as a function of the one measured surface parameter. Milling depth preferably is carried out as a function of the measured surface parameter. Moreover, as a function of the measured surface parameter, at least one milling cutter of the milling machine can be bent about a horizontal axis that is perpendicular to its longitudinal axis.
The strand can be cleaned prior to the measurement of the surface parameter.
In accordance with one embodiment of the invention, the strand is milled in the milling machine such that the strand upper face and the strand lower face are milled off at the same location in the travel direction. Alternatively, however, the strand is milled in the milling machine such that the strand upper face and the strand lower face are milled at two successive locations in the travel direction.
The apparatus for producing a metal strip using continuous casting, having a casting machine in which a strand, preferably a thin strand, is cast, at least one milling machine being provided downstream of the casting machine in the travel direction of the strand, in which milling machine at least one surface of the strand, preferably two opposing surfaces, can be milled off, is inventively embodied such that in the travel direction upstream and/or downstream of the milling machine means are provided with which at least one surface parameter of the strand can be measured, setting means being present with which at least one milling cutter of the cutting machine can be displaced as a function of the measured surface parameter.
These setting means can be embodied for adjusting the milling depth of the milling cutter. It is also possible for the setting means to be embodied for actuating the milling cutter with a bending moment about a horizontal axis that is perpendicular to the milling-cutter longitudinal axis. This results in advantages that will be described in greater detail later.
The means for measuring at least one surface parameter can include a camera for determining the depth of cracks on the strand surface. Furthermore, the means for measuring can permit the geometric shape of the strand to be determined across its width transverse to the travel direction.
The means for measuring at least one surface parameter can be provided immediately downstream of the milling machine. They can also be provided downstream of a finishing train that is disposed downstream, in the travel direction, of the milling machine. It has furthermore proven useful when the means for measuring are provided downstream of a cooling section that is disposed downstream, in the travel direction, of the milling machine.
With the suggested solution it becomes possible to run at a high casting speed and to operate the immediately following rolling process in an optimum manner. In particular acceptable strip output temperatures out of the finishing train are attained in this manner.
This leads to qualitatively improved production of strands, in particular thin strands.
Specifically, by means of the invention it is possible to increase the casting speed from the current level, at v×d>350 m/min×mm, to approx. 480-650 m/min×mm, i.e. to increase it by approx. 30% to 75%. Thus the following advantageously result:
Advantageously, high-quality strands result when the milling machine, or where necessary, even a different surface machining unit, is provided downstream of the casting system, in that surface flaws are removed by milling.
Cooperation between a high-speed casting system and the surface material removal, in particular milling, is critically important for quality, especially the surface quality of the product produced.
Illustrated embodiments of the invention are shown in the drawings.
Connected to the means 8 in the travel direction F is a milling machine 4 in which the strand 3 can be milled off on its upper and lower faces.
It is essential that the strand 3 is milled as the first mechanical machining step after the strand 3 is diverted to horizontal travel H at high casting speed. It is specially provided here that the strand 3 is milled immediately after it is diverted to horizontal travel H.
As will be seen, specifically adding the milling process directly after casting results in technological advantages when producing strands as high-speed thin strands. Specifically, casting errors increase as casting speed increases such that milling immediately after casting produces efficient preparation of the strand for the subsequent process steps so that overall a very economic process becomes possible.
Consequently it is desired that the strand 3 is cast with a thickness of at least 50 mm. For mass flow (expressed as the product of casting speed and strand thickness) a value of at least 350 m/min×mm has proven itself. The cooperation between these process parameters and the milling of the strand that takes place very far upstream results in great advantages in terms of attainable strand quality and efficiency during finishing.
In the solution in accordance with
The system shown in
The alternative system shown in
As in the solution of
Instead of the descaler 14 upstream of the finishing train, a milling machine 4 is provided upstream of the finishing train 9 for the purpose of optimizing temperature, it being possible to provide inductive heaters 16 between the individual roller units thereof. Finally, the cooling section 10 again follows in the travel direction F.
The solution in accordance with
The system shown in
As can be seen in
The strand 3 travels from an oven 13 into the milling machine 4, the means 8 for profile measuring and/or surface inspection being provided upstream of the milling machine.
In this case the strand 3 is again machined, i.e. milled, on its upper and lower faces in the milling machine 4, machining occurring however on the upper and on the lower faces at two locations that are somewhat spaced from one another in the travel direction F. The milling cutters 6 cooperate with support rollers 17. Measuring means 8 are again provided downstream of the milling machine 4. After the surface machining, the high-temperature strand 3 travels into a finishing train 9, measuring means 8 again being provided downstream thereof.
The means 8 can have elements for optically determining the strip shape (ski), which is indicated at reference 8′ for the means 8 farthest upstream in the travel direction. They can also have strand profile and temperature measuring elements.
It is primarily the milling amount that is considered, i.e. the depth of the roller-like milling cutters 6, that defines the quantity of the material to be removed from the strand 3. This can occur separately and differently for the upper face and the bottom, as a function of the measured values.
The amount to be milled off derives from the surface inspection of the strand, cracks and the geometric shape being primary factors. A different reduction (depth) along the length of the strand can result from this.
When the milling depth is being determined, the computed milling wear is also taken into account in a cutting wear model that determines the wear as a function of wear path, milling volume, milling speed, material strength, etc.
A fixed milling amount can also be established using the measured values.
Another option is to adapt the milling shape and bending as a function of the measured profile (see also
The surface result can be examined downstream of the milling machine 4 and where necessary an adjustment can be made if the measured values are not yet satisfactory.
Casting errors increase sharply when the casting speed or the product of casting thickness and speed increase further.
A roller-shaped milling cutter 6 is shown with schematically indicated cutters 19. The milling contour, which is created on the strand 3 using the milling process, can be influenced in that a bending moment M is applied to the milling cutter 6. The bending moment M is centered on a horizontal axis that is perpendicular to a milling-cutter longitudinal axis 7.
The moment M can be produced by double forces FF that can be applied to the shaft journals of the milling cutter 6. While the line 7 marks the milling-cutter longitudinal axis when not deformed, the bending curve 20 results when the forces FF are applied. Then the milling cutter bends as shown. Since the bending behavior of the milling cutter 6 as a function of the forces FF is known, it is possible to intentionally influence the milling results if certain convexities are measured across the strand width that can be intentionally influenced, i.e. eliminated, by acting on the milling cutter 6 with the bending moment M.
Thus it is also possible to dynamically adapt the milling process to the measured strand profile or to the measured strand shape.
References 7 and 20 illustrate the neutral axes for the milling cutter 6 for the two loads.
The milling reduction, i.e. the depth, can be adjusted differently across the strand width or can be adapted to the starting strand shape. The bending in the milling cutter can act as the actuating element for the adjustment across this width.
This can be summarized as follows:
Since the output of a CSP system can be determined by the casting machine, the invention suggests designing a casting machine with a high casting speed. Given an extreme increase in casting speed, instead of one CSP system with two strands with conventional casting systems alternatively a one-strand CSP system with a high-speed casting machine is preferred.
A high casting speed is also particularly necessary for coupled casting and rolling (casting/rolling system) so that the strip output temperature out of the finishing train is acceptable.
As casting speed increases, however, surface flaws (e.g. scale, etc.) increase disproportionately (see
It is in particular suggested that thin strand surface machining that is provided in the line downstream of the casting system, within the oven, or upstream of the rolling mill be performed for thin strands having a thickness greater than 50 mm and/or having a mass flow (speed×thickness) greater than 350 m/min×mm. For example, the thin strand thickness to be sought is approx. 60-110 mm at a casting speed of 6-9 m/min. The typical mass flow is lower.
An increase in casting speed is reasonable not only for thin strand systems. An advantageous application for thick strand systems (H>110 mm) is also conceivable. In this case, the milling machine should be provided as close as possible downstream of the continuous-casting system or the area between leaving the casting system (last section roller) to the milling machine should be closed by a roller conveyor housing so that the milling process can occur at high casting speed at a high strand temperature to the extent possible.
When needed, the milling process can be omitted at the leading strand end and/or at the trailing strand end for the purpose of protecting against milling damage. If a disadvantageous surface shape (crossbow, ski, or other irregularity) is detected optically, the milling amount, milling starting point, milling ending point, and milling profile setting are optionally made a function thereof.
In order to minimize the milling material removal and to adapt to the strand input profile, the milling cutter arrangement forms a “milling crown” (analogous to the “roller crown”) across the width. The above-described milling roller journal bending in accordance with
During in-line milling of the surface, the strand speed vstrand is provided according to milling machine arrangement either by the casting machine or the rolling mill. That is, travel speed cannot be influenced by the milling machine. In order always to set the optimum milling conditions, the milling cutter rotary speed nmiller is preferably adapted according to the formula
n
miller
=K×v
strand
where K is an empirically determined factor that depends on the material.
The milling cutter rotary speed is controlled using the milling model that is shown in
The top and bottom of a milling roller can be seen in the illustrated embodiments. At high required milling reductions per side or given very hard materials it is conceivable to arrange two milling cutter units, one after the other, on both the top and bottom.
Instead of using roller milling cutters, it is also possible to use other milling cutters, such as face cutters or even grinding tools or other surface removal tools (such as descaling machines), at the provided locations.
The following in particular can be used as the cutting material for the cutting plates of the milling cutters: HSS; uncoated or preferably coated hard metals; ceramic; polycrystalline cutting materials. As a rule conventional indexable inserts can be used.
As explained, a surface inspection (camera, test for cracks, roughness test) is recommended upstream and/or downstream of the oven or upstream of the milling machine. The measured signals are used for optimum employment of the milling. It is possible to derive from them whether milling should be performed on one or more sides or only in some longitudinal areas and what extent of milling should be set. Preferably descaling or cleaning of the strand is performed upstream of the inspection in order to be able to do a precise and reliable surface analysis.
The usefulness of in-line strand inspection is also a function of monitoring the effect of the casting system; monitoring the effect of the electromagnetic brake; optimizing mold oscillation curves; monitoring the surface at high speed; and detecting cracks, casting powder errors, and other casting errors in the early stages of the production process.
In addition, it is possible to examine the milling result or the general surface condition by surface inspection immediately downstream of the milling machine, downstream of the finishing train, or downstream of the cooling section. The result is monitored there and the amount milled off is optimized or minimized adaptively by means of a milling model (algorithm) and is thus included in the overall system.
The milling cutter or the milling machine can be provided at different locations. It can be downstream of the casting system, within the oven, or upstream of the rolling mill.
Preferably it is used immediately upstream of the reshaping instead of a descaler in order to maintain a high strip temperature in the rolling mill, especially during continuous direct strand reduction, which is particularly advantageous.
Preferably a milling model is used for controlling the milling reduction, the beginning of milling, the end of milling, and to adjust the milling cutter rotary speed. The milling model takes the following into account when determining depth: set points, actual values determined by the measuring means, computed cutting wear, values found during previous milling (adaptation).
It is also possible to have an arrangement of a plurality of milling cutters per side, one after the other, for greater milling reduction.
Face cutters can also be used as an alternative to the use of cylindrical cutters. However, basically other material-removal methods can also be used, e.g. grinding tools or other mechanical or melting material-removal tools (such as e.g. descaling machines). Descaling is of particular interest for high-speed continuous casting.
The first mechanical machining step addressed inventively, which the milling is intended to represent, should be understood such that in any case prior to the milling there is no mechanical machining that is typically used during continuous casting. If for instance upstream of the milling there is minor mechanical machining that is not typical for the method in terms of its scale (e.g. minimal rolling with a reduction in thickness of a few millimeters in a small frame or in a driver that is normally present anyway), this shall not be construed as the first mechanical machining in the sense of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 024 586.5 | May 2006 | DE | national |
| 10 2007 022 932.3 | May 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP07/04560 | 5/23/2007 | WO | 00 | 11/25/2008 |
