Patent Application
20130205855
The invention relates to a method for producing a cold-rolled steel flat product from a hot strip with scale adhering to it and consisting of a non-corroding steel, in which the hot strip with scale adhering to it is cold-rolled into the steel flat product, in which the cold-rolled steel flat product is annealed, and in which the cold-rolled steel flat product is descaled.
In addition, the invention relates to a production line for producing a cold-rolled steel flat product consisting of a non-corroding steel, wherein the production line comprises at least one cold-rolling stand, at least one annealing furnace and at least one unit for mechanically removing scale adhering to the cold-rolled steel flat product.
Here, “steel flat products” are understood as steel strips or sheets produced from a non-corroding steel or a so-called “stainless steel”.
In the course of producing cold-rolled stainless steel flat products an appropriately formed steel melt is cast into a semi-finished product, such as slabs, thin slabs or cast strip, from which a hot strip is hot rolled and wound up into a coil. The hot strip obtained, which if required is subjected to additional heat treatment, is subsequently with the scale adhering to it cold rolled into a cold strip in a single or in multiple stages.
In the course of cold rolling, the rolled steel strip hardens, as a result of which the cold strip obtained after cold rolling can at best only be formed to a much reduced extent. In order to eliminate this so-called “cold-work hardening”, the cold strip is usually subjected to heat treatment, in which it is annealed at sufficiently high annealing temperatures. The fine scale particles adhering firmly to the surface which are still present afterwards can be eliminated in a pickling unit, through which the cold strip is directed after the scale has been mechanically removed.
A typical example for the type of production line mentioned above is known from DE 691 26 699 T2 (EP 0 509 177 B1). To increase the effectiveness with which the scale adhering to the annealed strip is removed, according to this prior art the pickling unit can be combined with a mechanically effective descaling unit.
In practice, a scale breaker is used for this purpose, in which coarser pieces of the scale adhering to the annealed strip are removed. In addition, a shot blasting unit or a brushing unit is usually provided which mechanically separates larger scale particles still remaining from the cold strip when it exits the scale breaker. To complete manufacture, the cold strip is, if required, additionally skin-pass rolled after leaving the pickling unit, in order to improve its yield strength and dimensional stability.
Practical experience has shown that although large quantities of cold-rolled steel flat products can be produced from non-corroding steel cost-effectively with the above mentioned type of production line, the surface quality of steel flat products produced using such production lines in many cases does not meet present-day requirements.
Against this background, the object forming the basis of the invention was to specify a method and a production line which is particularly suitable for carrying out such a method, by means of which cold-rolled steel flat products could be produced cost-effectively from non-corroding steel with an improved surface finish.
With regard to the method, this object is achieved according to the invention by the production steps specified in claim 1 being passed through when producing cold-rolled steel flat products from non-corroding steel.
With regard to the production line, the above mentioned object is achieved according to the invention by such a line containing the features specified in claim 13.
Advantageous embodiments of the invention are specified in the dependent claims and are explained, as with the general concept of the invention, in more detail below.
As with the prior art explained at the outset, the method for producing a cold-rolled steel flat product according to the invention proceeds on the assumption that hot strip with scale adhering to it and consisting of non-corroding steel, so-called “black sheet”, is cold rolled without prior descaling. The method according to the invention additionally comprises an annealing step to soften the cold-rolled material. Descaling also takes place according to the invention to free the cold-rolled steel flat product from the scale adhering to it.
According to the invention, the cold-rolled steel flat product obtained after cold rolling the hot strip is now subjected to descaling treatment in the hard-rolled state before annealing, in which the scale present on the cold-rolled steel flat product is mechanically removed. Therefore, with the method according to the invention, the hard-rolled cold strip is mechanically descaled before it is annealed. Here, the invention, on the one hand, takes advantage of the fact that a part of the scale is already broken during cold rolling and can accordingly be easily separated from the cold-rolled steel flat product.
The advantage of this procedure is that the hard-rolled cold strip is comparably insensitive to the forces acting on its free surfaces during mechanical descaling. Thus, the surface deformations which occur when the hard-rolled material is mechanically descaled are far less than those which arise during conventional descaling of comparably soft steel strips, softened again after annealing.
The steel flat product, which is freed to the greatest possible extent from coarser scale according to the invention, then passes through a conventional annealing unit, in which annealing is carried out in a conventional manner, in order to eliminate hardening which occurs in the course of cold rolling and is possibly further increased during mechanical descaling.
On the other hand, unlike when steel flat products susceptible to corrosion are annealed, only small amounts of new scale form on the surface of the respective steel flat product during the subsequent annealing treatment of steel strips or sheets consisting of non-corroding steel. These can subsequently be easily removed in a conventional pickling unit. Further mechanical descaling does not, therefore, according to the invention, expressly take place between exiting the annealing furnace and entering the pickling unit.
After annealing, the stainless steel strip can instead optionally pass through a unit for improving surface evenness. In the prior art, stretch-bending units or suchlike are, for example, used for this purpose.
Corresponding to the above explained features of the method according to the invention, a production line according to the invention for producing a cold-rolled steel flat product consisting of a non-corroding steel comprises at least one cold-rolling stand, at least one annealing furnace and at least one unit for mechanically removing scale adhering to the cold-rolled steel flat product, wherein the at least one unit for mechanically removing scale, viewed in the conveying direction of the cold-rolled steel flat product, is arranged before entry to the annealing furnace.
The steel strips and sheets produced according to the invention and consisting of a non-corroding steel have an optimised surface finish owing to the minor damage which occurs during the descaling carried out according to the invention after cold rolling and before annealing. This optimised surface finish is only achieved by carrying out the mechanical descaling before the annealing. The units required for this purpose are already available in conventional production lines, but are employed at a different place according to the invention. Consequently, the procedure according to the invention incurs no additional costs while distinctly improving the quality of the product obtained.
Of course, the mechanical descaling which takes place according to the invention upstream of the annealing furnace can be carried out by a plurality of processes possibly passed through one after another or interacting in combination. The mechanical descaling can, therefore, be carried out by particle blasting, such as sand blasting or shot blasting or by brushing, wherein these descaling means can be particularly effective if they are carried out combined with one another in a corresponding unit or in separate blasting and brushing units one after the other.
Especially if descaling is to be carried out by particularly gentle blasting using fine particles, it is advisable to as far as possible fully dry or remove any liquids present on the surface of the steel flat product before the blasting treatment. In this way, the particles striking the surface of the hard-rolled steel flat product are prevented from intermixing with the liquids, such as rolling oil residues or suchlike, which are otherwise present on the surface and originate from the preceding processing stages, to form a substance which is difficult to remove and which impedes the desired removal of scale. The liquid residues can be removed by thermal treatment. Flame drying, for example, is suitable for this purpose, in which the surface to be cleaned is exposed to an open flame, so that the liquids present on the surface rapidly evaporate or burn. Flame drying can be carried out highly efficiently by using a so-called “booster” which produces an enveloping flame which envelops the strip to be freed from liquid. Such a booster is, for example, described in DE 10 2006 005 063 A1.
If the scale is to be removed from the cold-rolled, hard-rolled steel flat product by blasting treatment, then this treatment can be carried out by liquid jets, for example high-pressure water jets. In order to improve the effectiveness of the scale removal, the liquid blasting agent can carry particles with it, which abrasively remove the scale adhering to the cold-rolled, hard-rolled steel flat product. These particles can be scale particles which are obtained from the scale removed from the cold-rolled steel flat product.
A conventional scale breaker can also be used to mechanically descale the cold-rolled, hard-rolled steel flat product, in which the scale adhering to the cold-rolled, hard-rolled steel flat product is broken by diverting the steel flat product on at least one roll.
In order to optimise the dimensional stability, the surface roughness and the mechanical properties of steel flat products produced according to the invention, the cold-rolled steel flat product can be optionally skin-pass rolled or temper rolled after annealing.
The method according to the invention can be executed discontinuously or in a continuous pass, depending on the number of cold-rolling steps to be carried out and the plant technology available. If the method is discontinuously carried out, the cold rolling can, for example, take place in multiple stages in a reversing stand operated independently from the operation of the annealing furnace. The advantage of this is that a high overall level of deformation of the cold strip can be obtained with a correspondingly high level of cold-work hardening and thereby an accompanying particularly hard surface which is not sensitive to mechanical removal of scale.
Carrying out the method in a continuous pass is appropriate, for example, if large quantities of steel flat product are to be produced with the aim of saving time and a sufficient reduction in thickness can be obtained via the one or more available cold-rolling stands passed through in the pass. Variations in the conveying speed between the individual work stations of such a continuously passed through production line according to the invention can be compensated by arranging at least one strip accumulator in the conveying path of the steel flat product.
A steel flat product produced according to the invention and consisting of a non-corroding steel, has a surface roughness Ra which is consistently in the range of 0.1-1 μm, without requiring measures to be taken for this purpose which are outside the procedure according to the invention. Particularly low surface roughnesses are produced if the stainless steel strip is temper rolled after annealing at degrees of deformation of up to 10%, in particular 1-7%.
The invention is explained in more detail below with the help of a drawing illustrating an exemplary embodiment. The single FIGURE shows a production line 1 for producing a cold-rolled stainless steel strip E which is passed through in a continuous pass in the conveying direction F.
The production line 1 comprises in the conveying direction F, standing in line one after another,
The hot strip W which is unwound from the respectively in use coils C1 or C2, has scale adhering to it, is where appropriate joined in the welding unit 3 to the end of the previously processed hot strip W and is produced from a non-corroding steel in a conventional way, is fed into the first cold-rolling stand 5 via the strip accumulator 4 and is then cold rolled into the cold-rolled stainless steel E in the subsequently passed through cold-rolling stands 6, 7 in a total of three stages. As a result of the stainless steel strip E being reduced in thickness, which occurs via the cold-rolling stands 5, 6, 7, the stainless steel strip E is elongated, so that the scale adhering to it, owing to the surface tensions which arise, flakes off from the hot strip W in coarse pieces or is loosened.
After exiting the last cold-rolling stand 7, the scale still adhering to the stainless steel strip E now obtained, which has been cold rolled to the desired thickness and hardened as a result of cold rolling, is mechanically descaled in the first descaling unit 9 by directing a particle blast with high kinetic energy at the hardened surfaces of the stainless steel strip E. The particle blast can be a shot blast, for example, as is already used in the prior art for descaling, but at another place in the process.
In the second descaling unit 10, the stainless steel strip E, which has been mechanically descaled by blasting in such a way, has the scale still present there brushed off it by means of brushes. Thus, when it leaves the descaling unit 10 and enters the continuous annealing furnace 11 only minor, fine scale residues adhere to the surface of the stainless steel strip E.
As an alternative to or in addition to the units 9, 10, a descaling unit, which is not illustrated here, can be provided, in which the scale adhering to the stainless steel strip E is removed by means of a high-pressure liquid jet. The liquid jet, output under high pressure and striking the surface of the strip to be descaled with high kinetic energy, can in the process carry particles with it, in order to increase the effectiveness of the scale removal. These particles can be collected scale particles of a certain grain size which are mixed with the liquid coming out of the jet, for example water.
When it passes through the continuous annealing furnace 11, the stainless steel strip E is heated to a temperature, in which the hardened microstructure of the stainless steel strip E is softened again. The length of the continuous annealing furnace 11, the speed the stainless steel strip passes through it and the annealing temperature are coordinated in such a way that the microstructure is softened as required at the end of the continuous annealing furnace 11. Of course, the continuous annealing furnace 11 can be divided into a plurality of zones for this purpose, in which different temperatures prevail, so that an optimum annealing result can be achieved. Only a small amount of new scale forms on the stainless steel strip E, despite the comparably high annealing temperature, owing to its alloying and, as a consequence, the minimised susceptibility to corrosion.
The amount of scale adhering to the stainless steel strip E when it exits from the annealing furnace is so small that it is as far as possible fully removed from the surface of the stainless steel strip E by the effect of the pickling agent acting on the stainless steel strip E in the pickling unit 12. Before entering the pickling unit 12, the stainless steel strip E still, however, passes through the stretch-bending unit 17 arranged in the conveying direction F behind the annealing furnace 11. In this stretch-bending unit 17, the stainless steel strip E is deformed in a known way by multiple changes in direction, such that when it directly afterwards enters the pickling unit 12 which is then passed through it has an overall improved surface evenness. The skin-pass rolling carried out after exiting the pickling unit 12 in particular serves to improve the surface roughness of the stainless steel strip E, wherein additionally the dimensional stability and the mechanical properties are optimised before it is wound into a coil 3 or 4 in the coiling unit 16. The surface roughness values can be almost halved by temper rolling carried out at degrees of deformation of typically 5-7%, so that after temper rolling an optimally smooth product is available.
1 Production line
2 Uncoiling unit
3 Welding unit
4 Strip accumulator
5, 6, 7 Cold-rolling stand
8 Strip accumulator
9 First descaling unit
10 Second descaling unit
11 Continuous annealing furnace
12 Pickling unit
13 Strip accumulator
14 Skin-pass rolling stand
15 Strip accumulator
16 Coiling unit
17 Unit for improving surface evenness
C1, C2 Coils from hot strip W
E Stainless steel strip
F Conveying direction
W Hot strip
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 026 757.0 | Jul 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/061155 | 7/1/2011 | WO | 00 | 3/21/2013 |
