EP2044234B1 - Method for flexibly rolling coated steel strips - Google Patents

Description

The invention relates to a method for flexible rolling of coated steel strips.

From the DE 10 2004 023 886 A1 For example, a method and apparatus for refining flexibly rolled strip material is known. In the production of flexibly rolled strip material, the material thickness of this strip is changed periodically to produce in a continuous process, the starting material for single boards, which has adapted material thicknesses according to the requirements of the sheet metal parts to be produced therefrom in the tape longitudinal direction. After successful rolling, the strip material is first wound on a coil. A heat treatment usually takes place on the wound coil. The strip is then unwound from the coil, subjected to a surface treatment and wound up again on a coil. Only then, in a further process, the separation of individual boards and the further processing to individual sheet metal components. Since this process is complex and the surface treatment is not optimized due to the different material thicknesses of the strip material, it is the task of DE 10 2004 023 886 A1 to propose an improved method and a device adapted thereto with which the finishing of the flexibly rolled strip material can be simplified and improved. The task is to be achieved in that the tape is produced as a flexible rolled strip material, is wound and then unwound from the coil and heat treated in a single continuous pass through a treatment line of annealing, quenching, preheating unit and zinc bath and hot dip galvanized. For this purpose, a continuous furnace with an annealing section and a quenching section, a preheating and a zinc bath is provided and a final Abblasdüse. The galvanizing takes place at 470 ° to 500 ° C, with part of the energy used for the previous heat treatment for the galvanizing process to be exploited. Optionally, in a Abblaseinheit behind the zinc bath excess adhesive zinc be blown off the strip material to achieve a precisely set layer thickness, which also determines the strip thickness and should serve to control the nozzle spacing.

From the DE 10 2005 031 461 A1 is a method for producing a microalloyed cold strip with a matched to the thickness profile property profile known, wherein a hot strip of steel is rolled with a substantially homogeneous thickness and strength to a cold strip having a substantially constant strip thickness with Abwalzgraden in the range between 5 and 60% , an annealing of the cold strip at a temperature between 500 ° and 600 ° C is performed and a second rolling of the cold strip is performed, wherein the rolling is performed flexibly such that predefined thickness profiles are set with a region of higher thickness and a region of lesser thickness and then a second annealing treatment is performed.

From the EP 1 074 317 B1 a method for flexible rolling of a metal strip is known, wherein the metal strip during the

Rolling process performed by a nip formed between two work rolls and the roll gap is selectively moved during the rolling process to achieve over the length of the metal strip different thicknesses. This flexible rolling should be characterized in that during the rolling process, the nip is selectively moved, whereby different lengths of tape sections are rolled with different tape thicknesses that can be connected to each other via different slopes. The aim of flexible rolling is to produce rolled products with load and weight optimized cross-sectional shapes. The EP 1 074 317 B1 proposes improved process control for flexible rolling to achieve a metal strip with improved flatness even with wide belts.

From the EP 1 080 800 B1 Also, a method for flexible rolling is known, which substantially corresponds to the aforementioned, wherein an acting on the metal strip temperature influence during rolling is compensated to avoid deviations from the nominal thickness and / or desired length of the individual band sections at a given final temperature of the metal strip.

From the EP 1 181 991 A2 In addition, a method and a device for flexible rolling of a metal strip are known in which a simple asymmetric strip thickness profile should be achieved.

From the document EP1861517 B1 discloses a process for hot dip steel strip hot dip coating in which a hot strip is hot dip coated and the finished thickness and thickness tolerance of the hot dip coated steel strip is achieved by controlled thickness reduction in a process line mill scale wherein at least one thickness gauge in the outlet of the mill stand is reached the finished thickness is controlled and deviations from this up or down as a control signal to the employment of the rolling stand are returned to increase or reduce the thickness reduction accordingly.

The object of the invention is to provide flexibly rolled and provided with a corrosion protection sheets for the press hardening process available, which are considerably cheaper than previously produced and uniform despite the flexible rolling in their remaining properties.

The object is achieved by a method having the features of claim 1.

Further advantageous developments are characterized in subclaims.

As already known from the prior art, the flexible rolling is carried out with different rolling pressures in order to produce different thicknesses of the steel strip. So far, no galvanized or otherwise coated sheets have been used for this, since the flexible rolling also affects the coating thickness of the coating. In addition, it was recognized in the invention as a problem that the different sheet thicknesses after flexible rolling in subsequent heating, in particular for press hardening, in which a heated board is placed in a hot forming tool and formed there, or a component is formed, then heated and in a mold is hardened, different heating curves results for the different Bleckdicken. This is problematic, since the different heating curves also result in different temperatures, as a result of which the material properties vary depending on the sheet thickness.

The possible coatings with which the flexible rolls to be rolled can be coated according to the invention are hot-dip coatings and electrolytic coatings. Possible hot-dip coatings are z. As hot-dip galvanized layers or feueraluminierte layers but also mixed forms thereof, ie alloys of zinc and aluminum but also of zinc and other metals or aluminum and other metals.

Possible electrolytic coatings are, for example, electrolytically applied zinc layers, but of course other electrolytically applied metal layers are also possible.

When speaking of zinc layers or hot-dip galvanized layers below, this is merely representative of the other possible layers mentioned above.

The inventively recognized problems that the different sheet thicknesses give different heating curves over the tape length and that different zinc layer thicknesses arise as a result of the flexible rolling process are inventively achieved in that a hot strip is hot dip galvanized before flexible rolling and / or the emissivity or the degree of absorption is influenced by mechanical or chemical treatment of the zinc surface. By this adjustment of the emissivity / the degree of absorption, a different heat absorption capacity over the strip length can be achieved. For example, the degree of absorption in a region in which the tape and / or the coating is particularly thin is set poorly, while being particularly well adjusted in the region in which the tape and / or the coating is particularly thick. Of course, appropriate intermediate levels are met.

To compensate for different zinc layer thicknesses as a result of the flexible rolling process, so that all sheet metal parts are covered by a zinc layer of uniform thickness after flexible rolling and thus also have uniform corrosion protection properties, the zinc layer thickness is preset by means of variable adjustment of the stripping pressure or additional electromagnetic fields during hot-dip galvanizing of the hot strip , Areas that are then very flexible very flexible Thus, after being hot-dip galvanized, they have a thicker layer of zinc, while areas that remain thicker have a thinner layer of zinc. Of course, the corresponding different intermediate areas are also set here or are readily adjustable.

The procedure according to the invention makes it possible to achieve a significantly more cost-efficient production of flexible rolled sheet metal automotive parts, since the transport costs of the coils for the post-galvanizing and the bell annealing usual in the prior art are eliminated. In addition, instead of a piece galvanizing or a narrow strip galvanizing by the Wuppermann method (WM-FVZ, see Fig. 2 ) use the significantly less expensive continuous hot dip galvanizing process on the belt, so that also saved here in a considerable way.

Figur 1:

ein Ablaufdiagramm zeigend schematisch die möglichen erfindungsgemäßen Verfahrensabläufe;

Figur 2:

ein Ablaufdiagramm zeigend schematisch den Verfahrensablauf nach dem Stand der Technik.

The invention will be explained by way of example with reference to a drawing. It shows:

FIG. 1:

a flow chart showing schematically the possible inventive procedures;

FIG. 2:

a flow chart showing schematically the procedure according to the prior art.

In the prior art, hitherto uncoated hot strip was usually calibrated from ordinary automotive steels, then flexibly rolled and then subjected to recrystallization annealing to reverse the microstructural changes produced by rolling. This recrystallization annealing usually takes place in a crucible annealing furnace, wherein the strip is previously unwound into a so-called coil and annealed as a whole coil. Subsequently, these annealed coils are transported to a galvanizing plant and there galvanized and then transported back again, cut boards and shaped components, which then yield the final product.

According to the invention, a hot strip or cold strip is fed to a hot-dip galvanizing plant in which the strip is unwound from the coil, welded to the preceding strip and then passed through the galvanizing plant. In the galvanizing plant, a heating of the strip and then the known per se implementation by the galvanizing takes place.

It is known per se to provide after the hot-dip galvanizing so-called wiping, which adjust the zinc layer on the freshly galvanized strip, with air or another gas is blown through a slot die on the still liquid zinc layer, so that a pressure on the zinc layer is formed which presses the liquid zinc counter to the strip running direction, so that after the coating nozzle is present a predetermined layer thickness. Subsequently, the tape is optionally subjected to a heat treatment or cooling.

According to the invention, the stripping device produces a zinc layer with a thickness that can be applied flexibly. The stripping device can also be a system which acts on the zinc layer by stripping off an electromagnetic field.

For a given length of the strip, which is guided by the galvanizing, it is known from the beginning, which strip lengths are thinly rolled during subsequent flexible rolling and which are thicker rolled. Since the rollers, which later take on the flexible rolling, are also controlled according to the length of the tape and is known at any time, which exact band section or which exact band length of the exact coil is running straight through the rollers, the same control is used to change the stripping nozzle pressure. Thus, different layer thicknesses of the galvanizing can be ensured in different areas of the strip. For example, in a region which is later rolled out particularly strongly, ie, thinly, the nozzle pressure is selected to be lower and thus less material is stripped off than in areas in which a lower degree of rolling is subsequently carried out. By this procedure, a uniform zinc layer thickness over the entire later flexible rolled strip can be ensured at different thicknesses there.

The abovementioned hot-dip galvanizing process can, of course, be used equally well in other hot-dip galvanizing applications, for example, fire aluminizing or hot-dip coatings of aluminum-based alloys or zinc-based alloys and alloys of other metals or more metals than zinc and aluminum.

In the case of electrolytic coatings, the layer thickness to be applied is controlled by the strength of the electrolytically active stream and / or the strip speed in the electrolytic coating bath, the control system basically also being used as the control with which the positionally accurate deviation of the different sheet thicknesses during flexible rolling is also carried out.

After coating, such. As galvanizing, the flexible rollers can follow, with the flexible rollers, as already stated, different thicknesses can be achieved exactly positioned with respect to the tape length. From the flexible rolled sheet blanks are then cut in a conventional manner, which accordingly also the predetermined Have thickness over the length or width. These boards with different thickness profiles are then used according to the invention for a press hardening.

The press hardening can be realized by two different methods.

In a first possibility, the cut boards are austenitized, d. H. subjected to a heat treatment in which austenite conversion takes place depending on the steel. Subsequently, the hot board is inserted into a hot forming tool, formed in the hot forming tool to the component and cooled at the same time. The cooling takes place here with a temperature which is above the critical hardening temperature, so that the curing takes place simultaneously in the forming tool. Subsequently, the hardened formed board leaves the press and can optionally be reworked or is already the final product.

In a second embodiment, mold hardening is performed instead of hot working. When hardening the board is cold formed. Preferably, this cold transformation in all three spatial directions is already complete, as well as the trimming of the edges and the creation of a hole pattern. Preferably, the board is formed with an undersize of 0.5 to 2% in all three spatial directions and then austenitized. During austenitization, the thermal expansion compensates for the under-forming by 0.5 to 2%, so that after complete heating, the formed blank has its final geometry. This now final geometry or final contour corresponding board is inserted into a mold hardening tool, which also has exactly the contour or geometry of the desired end component. The component is in the mold hardening tool at least in the field of particularly strongly shaped areas held positively, preferably held completely positive fit, cooled and cured by cooling.

Subsequently, the component is removed as an end product from the mold hardening tool.

As already stated, austenitization of the boards takes place. In this case, the board is heated to about 900 ° to 950 ° C in the preferably used hardenable steels type 22MnB5. Because the board has different sheet thicknesses, different heat profiles or heat treatment profiles also result in the sheet, which ultimately result in different temperatures over the length or width of the board. Since a complete hardening is desired, therefore, the areas with larger sheet thicknesses must at least have the austenitizing temperature. However, this leads to the fact that the thinner areas are virtually overheated. As a result of these different temperature or heat treatment profiles of the different sheet thickness ranges of the board over the entire treatment process, different hardnesses or material properties can arise.

In order to prevent or reduce this, the surface of the strip is influenced after galvanizing and before the flexible rolling or after the flexible rolling and cutting in front of the sinker.

The surface treatment of the tape can be done in different ways. The aim of the surface treatment is to influence the emissivity or the absorption of heat or heat radiation. As a result, a different zinc layer thickness order before rolling can also be avoided and only by the surface treatment quasi-same properties were achieved during annealing.

According to the invention, this can be achieved by a matting treatment, a skin pass, d. H. a micro-contouring of the surface, or an additional coating done.

It is thus possible to form the regions which are particularly strongly rolled during flexible rolling and later produce a thinner region of the strip, in particular highly reflective or emitting form, in order to absorb a small amount of heat when heating for austenitizing.

The areas which are later thicker after flexible rolling or even after flexible rolling can be given a matted, low-reflecting or dressed surface or be provided with a temporary dark protective lacquer or with a metal oxide surface which provides a particularly good absorption of Thermal radiation and thus a good heating of the thicker areas allows.

For the surface treatment, the same control as for the flexible rolling or the flexible galvanizing is basically used, so that the corresponding areas in their surface condition can be changed precisely and very correctly.

The advantage of the invention is that it is possible to flexibly roll hardenable steels, which have to be subjected to a heat treatment for the hardening, and nevertheless to form them with a corrosion protection layer, whereby products having a high homogeneity with respect to the material properties are obtained. In addition, sheet metal components can be obtained in a considerably more favorable manner with this method.

The invention is not limited to hardenable steels, for example of the type 22MnB5. The flexible galvanizing or galvanizing with flexible layer thicknesses is also applicable to steels that should not undergo further heat treatment, with success.

In addition, it is possible for steels which are to be subjected to an annealing after flexible rolling in order to restore original material properties, to guide the flexibly rolled steels with the flexible galvanizing according to the invention by a continuous annealing, wherein the differently adjusted emissivities of the surfaces also in the continuous annealing a very exact homogeneous material properties distribution is achieved.

Claims (11)

1. Method of producing a sheet metal part, wherein a hot- or cold-rolled strip is hot-dip coated or electrolytically coated and the hot- or cold-rolled strip thus coated is subjected to a flexible rolling process wherein, during the flexible rolling process, different roll pressures are used to create different sheet thicknesses of the flexibly rolled steel strip, characterised in that, geared to the sheet thickness after the flexible rolling and/or geared to the roll pressure during flexible rolling, the coating is applied with differing thickness, and depending on the roll pressure, with anticipated rising roll pressure the coating thickness during the coating process is greater before rolling and/or before or after flexible rolling the coating is subjected to mechanical or chemical surface treatment in the form of matting, cold-finishing or an additional coating to set a desired emissivity or heat absorption capacity.
2. Method according to claim 1, characterised in that the coating thickness is set by the level of pressure of the gas flow in jet finishing nozzles of a hot-dip coating station.
3. Method according to claim 1, characterised in that the coating thickness is set by a change in the electrolytically effective current strength and/or the line speed in the electrolytic bath.
4. Method according to claim 1 or 2, characterised in that the coating thickness is set by means of an electromagnetic process or additionally by an electromagnetic process.
5. Method according to any of the preceding claims, characterised in that the surface is coated so as to be matt or reflective or coloured or is cold-finished.
6. Method according to any of the preceding claims, characterised in that, to control the coating thickness on the strip and/or to control the surface treatment, that control is used which is necessary to carry out the flexible rolling with exact positioning on the strip.
7. Method according to any of the preceding claims, characterised in that a hardenable steel is used as steel material.
8. Method according to any of the preceding claims, characterised in that a steel of type 22MnB5 is used.
9. Method according to any of the preceding claims characterised in that, from the flexibly rolled steel material, formed depending on the steel material thickness with different coating thickness and/or surface treatments, circuit boards are cut and then austenitised, the austenitised hot circuit boards are hot-worked and cooled in the hot working tool and hardened by the cooling.
10. Method according to any of claims 1 to 8, characterised in that circuit boards are cut from the flexibly rolled steel strip provided with a coating geared to the sheet thickness and if applicable with a surface treatment, and the circuit boards are then cold-worked, wherein the cold-worked circuit boards are then austenitised and the austenitised hot-worked circuit boards are placed in a press hardening tool, wherein the press hardening tool corresponds substantially to the contour or geometry of an end component and the hot-worked circuit board is held positively, cooled and hardened in the press hardening tool, at least in heavily worked areas.
11. Method according to any of the preceding claims, characterised in that the coating applied is a hot-dip coating of zinc or based on zinc or a hot-dip coating of of aluminium or based on aluminium or an electrolytic coating of zinc or based on zinc.

Images