GB2255301A

GB2255301A - Rolling steel strip

Info

Publication number: GB2255301A
Application number: GB9203990A
Authority: GB
Inventors: Oskar Noe
Original assignee: BWG Bergwerk und Walzwerk Maschinenbau GmbH
Current assignee: BWG Bergwerk und Walzwerk Maschinenbau GmbH
Priority date: 1991-04-27
Filing date: 1992-02-25
Publication date: 1992-11-04
Anticipated expiration: 2012-02-25
Also published as: DE4113876C1; GB9203990D0; GB2255301B; FR2675719A1; FR2675719B1

Description

22553,31 % 25 1 A PROCESS FOR ELIMINATING THE TENDENCY IN HOT-ROLLED STEEL

STRIPS HAVING A PRONOUNCED YIELD POINT TO DEVELOP YIELD PATTERNS AND ROLLING BUCKLES WHEN PICKLED, AND A PICKLING LINE FOR EFFECTING THIS PROCESS.

The invention relates to a process for eliminating the tendency in hotrolled steel strips having a pronounced yield point to develop yield patterns and rolling buckles when pickled, whereby the steel strip is paid out from a coil unwinder, descaled and pickled, and also to a pickling line for effecting this process.

Hot-rolled strips of deep-drawing and structural steels having a pronounced yield point, in common with coldrolled and annealed steel strips, tend to develop yield patterns and rolling buckles when plastically deformed to the critical yield strain, also known as the Luder's strain limit. The same problem arises with ferritic stainless steels. The basic sources of these phenomena are dislocations in the crystal lattices. Treated two- dimensionally, one speaks of dislocation densities, whilst in three-dimensional treatment the term "dislocation field" is used. Rolling buckles are usually confined to the tensile bending stress zone set' up when the steel strip is bent under tension round deflection rolls. The term "deflection rolls" embraces for example anti- coilbreak and anti-crimping rolls as well as conventional guide rolls. Yield patterns and rolling buckles once produced in strip steel can only be removed by reduced cold rolling. Although the tendency to develop yield patterns and rolling buckles can be eliminated to some extent by finish rolling, it is only removed from the layers near the surface. The yield patterns and rolling buckles in the midlayers are persistent, 2 at depths which depend on the penetration of the critical strain zone. If this zone is reached during subsequent working, for example during deep drawing operations, the yield patterns and rolling buckles will reappear at the surface. Particularly marked rolling buckles develop when steel strips are passed under tension round deflection rolls of small diameter, and what are known as flow kinks are formed. Because the steel strip resists bending, it only makes contact with the deflection roll beyond the tangent line, and produces leverage for the tensile force. This gives rise to a bending moment, and when the latter reaches a magnitude at which plastic Strain is produced in the OUter fibres, they yield at the upper yield stress; the stress level suddenly falls to the lower yield stress, producing Luder's strain in the form of a flow kink. Such flow kinks are particularly pronounced. The magnitude of the Luder's strain E, indicated during the L tensile test is a measure of the tendency towards developing yield patterns and rolling buckles. If E- > 1%, one must L expect yield patterns and rolling buckles to occur. The invention provides a remedy.

A basic object of the invention is to provide a process and a pickling line whereby and wherein the tendency in hot-rolled steel strips having a pronounced yield point to develop yield patterns and rolling buckles is completely and permanently eliminated over the entire cross-section, even in those cases in which the strip material will subsequently be subjected to major deformations.

According to a first aspect of the present invention, 1 1 b p 3 0 r 25 there is provided a process for eliminating the tendency in hot-rolled steel strips having a pronounced yield point to develop yield patterns and rolling buckles when pickled, whereby the steel strip is paid out from a coil unwinder, descaled and pickled, and immediately after unwinding, the steel strip is presented without bending to a finishing roll gap, finish rolled and then stretch-bend straightened prior to pickling.

The term "finishing roll gap" refers to the gap betweeen the finishing rolls in a finish-rolling stand following the coil unwinder. The invention arises primarily from the discovery that the tendency of the surface layers of hot-rolled steel strips having a pronounced yield point to develop yield patterns and rolling buckles can be eliminated, after small finishing reductions, strips of limited thickness no longer develop yield patterns or rolling buckles, even after subsequent deep-drawing or like operations involving much plastic strain. On the other hand, yield patterns and rolling buckles steel reappear in steel strips of greater thickness if locally large strains are applied in subsequent operations such as deep drawing and the production of pressed sheet steel baths or the like. However, if the finish-rolled steel strip is then subjected to stretch-bend straightening, and tendency towards or incidence of the development of yield patterns and/or rolling buckles is surprisingly suppressed. The apparent reason is the plastic deformation of the entire cross-section associated with stretch-bend straightening. The Luder's fronts or dislocations in the crystal lattices are not 4 merely displaced into the midlayers - as in the finish-rolling case - but rather they are completely eliminated from the entire cross-section by nonuniform but fully plastic deformation. At the same time the strip is made outstandingly flat. It is surprising that the strip is also satisfactorily descaled for the pickling process. One might expect that finish rolling would embed the scale in the surfaces of the hot-rolled steel strip and thus produce surface flaws and inclusions. However, the surprising fact is that such inclusions are removed in the course of the ensuing stretchbend straightening stage and the surface condition is optimised for subsequent pickling.

Further significant features of the invention will now be described. Thus, the invention teaches that steel strips of thickness between 1.0 and 6.0 mm are finish-rolled with a reduction of 0.5 to 1.2% and stretch-bend straightened with an elongation of 0.5 to 1.2% The tendency to develop yield patterns and rolling buckles in the surface layers can in fact be eliminated if the finishing reduction is 1% and the strip thickness between 0.8 and 1.2 mm. On the other hand, thicker stock in the 1.0 to 6. 0 mm range still requires the additional stretch-bend straightening stage, at elongations up to 1.2% depending of the actual strip thickness, in order to remove the Luder's fronts or dislocations in the midlayers as well. The essential feature is always to present the steel strip to the finishing roll gap without deflection, for example in the line of advance. It is however possible to present the steel 0 strip to the finishing roll gap at entry angles up to 10 1 1 1 i 1 1 0 25 0 0 from 7 maximum above the line of advance to 3 maximum below the line of advance.

According to a second aspect of the invention, there is provided a pickling line for effecting the above defined process, having at least a coil unwinder, a finish-rolling stand, a stretchbend unit and a pickling bath, in which the coil unwinder leads directly into the finish-rolling stand without any intermediate deflection rolls. The finish-rolling stand can be followed by a straightening machine, shears and a strip joining device, for example a welding machine, to maintain uninterrupted strip processing. The straightening machine is only used to trim the start and end of steel strips before joining together, so that they feed satisfactorily into the strip joining device or welding machine. According to the invention, the stretch-bend unit is invariably preceded and followed by at least on tension roll stand fore and aft, to produce the requisite elongation.

The invention will now be described in more detail with reference to a drawing of an embodiment thereof, purely by way of example, which drawing depicts the entry section of a continuous pickling line.

This entry section incorporates an unwinder 1 for a coil of hot-rolled strip 2 of steel having a pronounced yield point. The unwinder 1 is followed by a two-high finishrolling stand 3, and the finish-rolling stand 3 by a straightening machine 4 to trim each strip start and end. Next come shears 5, to form satisfactory joint faces on the strip starts and ends to be joined together. Joints are made 6 in a following strip joining device 6, usually a welding machine. The endless strip advances from the welding machine 6 into a stretch-bend straightening unit 7 having preceded and following tension-roll stands 8 and 9, before finally entering a pickling bath 10.

The unwinder 1 is set up immediately ahead of the finish-rolling stand 3, with no intermediate deflection rolls, so that the steel strip 2 can be presented to the finishingroll gap 11 and there be finish rolled directly and without bending after unwinding. In the course of stretch-bend straightening prior to pickling, the Luder's front or crystal lattice dislocations are removed throughout the cross-section of the steel strip 2 by nonuniform but fully plastic deformation, with elongations up to 1. 2%. At the same time, the steel strip 2 is satisfactorily descaled and flattened.

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Claims

1. A process for eliminating the tendency in hotrolled steel strips having a pronounced yield point to develop yield patterns and rolling buckles when pickled, whereby the steel strip is paid out from a coil unwinder, descaled and pickled, and immediately after unwinding, the steel strip is presented without bending to a finishing roll gap, finish rolled and then stretchbend straightened prior to pickling.

2. A process as in Claim 1, wherein steel strips of thickness between 1.0 and 6.0 mm are finish-rolled with a reduction of 0.5 to 1.2% and stretchbend straightened with an elongation of 0.5 to 1.1.2%.

3. A process as in Claim 1 or 2, wherein the steel strip to be processed is presented to the finishing roll gap in the line of advance.

4. A process as in Claim 1 or 2, wherein the steel strip is presented to the finishing roll gap at entry angles 0 0 up to 10 i.e., from 7 maximum above the line of advance to 0 3 maximum below the line of advance.

5. A process for eliminating the tendency in hotrolled steel strips having a pronounced yield point to develop yield patterns and rolling buckles when pickled, substantially as hereinbefore described with reference to the accompanying drawing.

6. A pickling line for carrying out the process as in any of Claim 1 to 5, having at least a coil unwinder, a finish-rolling stand, a stretch-bend straightening unit and a pickling bath, in which the coil unwinder leads directly into 1 i 8 the finish-rolling stand without any intermediate deflection rolls.

7. A pickling line as in Claim 6, wherein the finish-rolling stand is followed by a straightening machine, shears and a strip joining device, for example a welding machine.

8. A pickling line as in Claim 6 or 7, wherein the stretch-bend unit is preceded and followed by at least one tension roll stand fore and aft.

9. A pickling line substantially as hereinbefore described with reference to the accompanying drawing.

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