Patent Application
20240390963
The invention relates to a method for temper rolling a steel sheet, to a temper-rolled steel sheet and to a component manufactured therefrom.
In the temper rolling of steel sheet, use is made of two temper rolls in a temper rolling stand, through which the steel sheet is guided, such that during this process the upper temper roll acts on the upper side and the lower temper roll acts on the lower side of the steel sheet and, depending on the texture of the temper rolls, a corresponding structure is imparted to the surfaces on the upper and lower side of the steel sheet. The pair of temper rolls is usually produced by the same texturing methods and with the same texture parameters in order to make the upper and lower sides of the steel sheet as uniform as possible. However, the identical configuration of the topographies on the upper and lower sides does not in all cases suit the circumstances and requirements of the further processing processes that follow at the OEM. It is conceivable that different topographies on the upper and lower sides will bring advantages in the further processing of the sheets, in particular coated steel sheets, since the stresses on the sides of the sheet during processing are different. Possible examples of processes at the motor vehicle manufacturers during which the sides of the sheet are subjected to different stresses would include the forming process:—during forming, different stresses on the steel sheet occur in the different regions of the tool. Depending on the component, the demands on the friction behavior of the steel sheet in the locally stressed regions are different. To produce outer skin parts, the steel sheet must be held in the flange region, for example, to ensure that the steel sheet can undergo sufficient stretching in the punch region, which may generally be up to 5%. At the same time, stretching should take place as uniformly as possible across the punch in order to avoid localization or constriction, which may lead to cracking. In general, such a demand on the surface applies to all components where the material flow is to be reduced in the flange region; and the phosphatizing process:—during the phosphatizing process, the vehicle body travels along a cleaning/phosphatizing/painting line, on which the vehicle body can be dipped, sprayed, or dipped and sprayed during the cleaning and/or phosphatizing process. In the case of spray treatment, e.g. by means of an alkaline cleaner, activating or phosphatizing solution, only the outer side of the vehicle body is subjected to kinetic energy, which stems from the spray nozzles, and this may be reflected in poorer cleaning/activation and phosphatizing results on the inner side of the vehicle body.
The topography of a steel sheet side, generally represented using characteristic values for roughness, thus has a decisive effect on the further processing properties. Rougher topographies have a positive effect on the reactivity of the surface in the cleaning and bonding process. With regard to forming behavior too, rougher surfaces generally exhibit lower friction coefficients since the oil retention capacity is promoted. In order to be able to ensure optimum results during the forming or phosphatizing of steel-sheet/component surfaces, it may therefore be worthwhile to provide the upper and lower sides of a steel sheet with different topographies.
It is therefore the object of the invention to specify not only a method for temper rolling a steel sheet but also a temper-rolled steel sheet in order to allow different configurations of the upper and lower sides of the steel sheet, which are associated with a positive effect both on the forming and the phosphatizing process.
A first teaching of the invention relates to a method for temper rolling a steel sheet, wherein the steel sheet is passed between two temper rolls in a temper stand, wherein the first temper roll acts on the upper side of the steel sheet, and the second temper roll acts on the lower side of the steel sheet and the temper rolling impresses a surface structure on the upper and lower sides of the steel sheet, such that the surface structure on the upper side of the steel sheet differs from the surface structure on the lower side of the steel sheet in such a way that the characteristic values of the structure, selected from the group comprising the arithmetic roughness average Ra, skew Rsk, peak count Rpc, roughness depth Rz, developed interfacial area ratio Sdr, material ratio T and void volume L, of the respective surface structures, have a maximum relative deviation in comparison with one another, which are defined as follows:
wherein the condition for at least one of these characteristic values of the structure is satisfied.
Thus, use is made of temper rolls (roll pairs) which have different textures and are capable of producing different topographies on the upper and lower sides of the steel sheet, wherein at least one, in particular a plurality, or preferably all, of the abovementioned conditions in respect of the setting of the different characteristic values of the structure must be satisfied to enable a positive effect both on the forming and the phosphatizing process to be provided in the subsequent process.
Among those skilled in the art, it is known that full transfer of the surface topography of the temper roll to the surface of the steel sheet is not possible; it is essentially only the significant parts, such as the peaks of the surface profile of the temper roll, which penetrate into the surface of the steel sheet and thus, after temper rolling, give the surface of the steel sheet a new appearance or characteristics which differs/differ from the state before temper rolling. Through the action of a force on the surface of the steel sheet, which is, in particular, coated, the surface (positive form) of the temper roll forms a surface structure which is thus only partially imprinted in the surface (negative form) of the steel sheet.
A defined surface topography on a temper roll can be produced, in particular, by laser texturing. Methods and apparatus for producing laser-textured temper rolls are prior art, cf. inter alia EP 2 892 663 B1. Preferably, a laser is used to introduce a deterministic or pseudo-stochastic topography into the surface of the temper roll or temper roll blank by means of material removal. A deterministic surface topography/surface structure should be understood to refer to repeated surface structures which have a defined shape and/or configuration, cf. also EP 2 892 663 B1. In particular, these also include surfaces with a pseudo-stochastic appearance but which have been applied by means of a deterministic texturing method and are thus made up of deterministic shape elements. To produce a stochastic surface topography on a temper roll, known EDT methods are used in the prior art.
According to a second teaching, the invention relates to a temper-rolled steel sheet having a respective surface structure on the upper side and the lower side of the steel sheet, such that the surface structure on the upper side of the steel sheet differs from the surface structure on the lower side of the steel sheet in such a way that the characteristic values of the structure, selected from the group comprising the arithmetic roughness average Ra, skew Rsk, peak count Rpc, roughness depth Rz, developed interfacial area ratio Sdr, material ratio T and void volume L, of the respective surface structures, have a maximum relative deviation in comparison with one another, which are defined as follows:
A steel sheet should in general be understood to mean a flat steel product which can be supplied in sheet form (sheet) or in plate bar form (plate) or in strip form (steel strip).
At least one side of the steel sheet has a deterministic surface structure. In particular, the upper side and lower side can each have a deterministic surface structure. For example, one of the sides of the steel sheet can have a deterministic surface structure, and the other side can have a stochastic surface structure, cf. EP 2 006 037 B1, for example.
To determine the arithmetic roughness average Ra, a roughness profile is filtered out of the (actual) surface profile of the surface topography of a laser-textured temper roll, use being made, for example, of a Gaussian filter according to ISO 166210-21 (e.g. June 2013 edition) with a cut-off λGC=2.5 mm. The cut-off λGC is determined from DIN EN ISO 4288 (e.g. April 1998 edition), Point 7 Table 1 since the starting point is an aperiodic profile with a roughness of the texture of between 2 and 10 μm and, as a result, an individual measured section of 2.5 mm, which thus corresponds to λGC, is required. Even if it were possible to assume periodic profiles in the case of a deterministic texture in the narrower sense and, as a result, it might be possible to take into consideration a groove shape such as that in DIN EN ISO 4288, Point 7 Table 3 “RSm”, this is not considered here since, in the third section of the introduction to DIN EN ISO 4288, it is stated that the distinction between aperiodic and periodic profiles is subject to subjective assessment and it is accordingly left to the user, and therefore the assessment is made below on the basis of aperiodic profiles and this is permissible according DIN EN ISO 4288. A surface profile is understood to mean a shape or profile of the surface structure in cross section along a defined measured section. The roughness profile is filtered out of the surface profile, with longwave components with a wavelength greater than 2.5 mm being cut off.
The characteristic values of the structure, Ra, Rsk, Rpc, Rz and Sdr, can be ascertained or determined from the temper-rolled steel sheet, as indicated in DIN EN ISO 4288.
The material ratio T is obtained from the ratio of the areas made up of hills and valleys on each side of the steel sheet. The void volume L is the volume of the valleys on each side of the sheet. T and L can be determined by confocal microscopy. The surface to be analyzed can be scanned with a lateral resolution of less than 3.3 μm, for example, and each scanned point can be assigned a height value with a resolution of less than 150 nm in the z direction, for example, the topography of the surface thus being measured by means of the scan. The measured surface topography can be represented as a grayscale image by converting the scales of the height values fully into grayscales with the values 0 to 255, where the lowest point of the topography with the smallest height value is defined as black (value=0) and the largest height value is defined as white (value=255). The grayscale image img obtained, is converted by means of software, e.g. by means of Octave (Version 5.2.0) using the function bw=im2bw (img, “moments”), into a black and white image bw, in which the black regions represent the valleys and the white regions represent the hills. This black and white image is used to measure the areas of the hills and valleys. In conjunction with the height values, it is also possible to determine the volumes of the valleys.
In order to determine the characteristic values of the structure on the upper and lower sides of the steel sheet, measurement is preferably performed by means of confocal microscopy, wherein a measured area of, for example, 4×4 mm2 or larger can be considered. Determination of the characteristic values of a structure is part of the prior art.
At least one of the conditions of the abovementioned characteristic values of the structure must be satisfied. In particular, it is also possible for two or more conditions of the abovementioned characteristic values of the structure to be satisfied. Preferably, all of the conditions of the abovementioned characteristic values of the structure can be satisfied.
The relative deviations in % are obtained from the difference (magnitude) between the characteristic values of the structure on the upper and lower sides normalized to the larger characteristic value of the structure. That is to say that the relative deviation is determined from: I small characteristic value of the structure—large characteristic value of the structure I/large characteristic value of the structure.
The maximum relative deviations of the characteristic values of the respective surface structures in comparison with one another can furthermore be limited as follows:
in particular Ra>10%, preferably Ra>20%, preferably Ra>30% and/or
The steel sheet temper rolled according to the invention can be uncoated or, preferably, coated. If the steel sheet is coated, the coating of the coated steel sheet comprises a metallic coating.
According to one embodiment of the steel sheet according to the invention, the steel sheet is coated with a zinc-based coating which is applied by hot dip coating, wherein, in addition to zinc and unavoidable impurities, the coating may contain additional elements such as up to 5% by weight of aluminum, and/or up to 5% by weight of magnesium. Steel sheets with a zinc-based coating have very good cathodic corrosion protection, which has been used for many years in the construction of motor vehicles. If improved corrosion protection is provided, the coating additionally contains at least 0.3% by weight, in particular at least 0.6% by weight, preferably at least 0.9% by weight, of magnesium. At least 0.1% by weight, in particular at least 0.3% by weight, of aluminum may be present as an alternative or in addition to magnesium in order, for example, to improve bonding of the coating to the steel sheet and, in particular, to substantially prevent diffusion of iron from the steel sheet into the coating when the coated steel sheet is heat treated in order, for example, to enable a good capacity for adhesion to be ensured. Here, a thickness of the coating can be between 1.5 and 15 μm, in particular between 2 and 12 μm, preferably between 3 and 10 μm. Below the minimum limit, it is not possible to ensure sufficient cathodic corrosion protection and, above the maximum limit, joining problems when connecting the steel sheet according to the invention or a component produced therefrom to some other component may occur, and, especially when the maximum limit for the thickness of the coating is exceeded, it is not possible to ensure a stable process during thermal joining or welding.
Temper rolling when providing a steel sheet with a coating applied by hot dip coating generally takes place after coating, i.e. the imposition of respective surface structures on the upper and lower sides of the steel sheet is performed on the coated steel sheet.
According to an alternative embodiment of the steel sheet according to the invention, the steel sheet is coated with a zinc-based coating which is applied by electrolytic coating. Here, a thickness of the coating can be between 1.5 and 15 μm, in particular between 2 and 12 μm, preferably between 3 and 10 μm.
Temper rolling when providing a steel sheet with a coating applied by electrolytic coating generally takes place before coating, i.e. the imposition of respective surface structures on the upper and lower sides of the steel sheet is performed on the uncoated steel sheet.
According to a third teaching, the invention relates to a component having an outer side and an inner side, which, in particular, is produced from a steel sheet temper rolled according to the invention, wherein the outer side has a higher value in comparison with the inner side in respect of at least two, in particular at least of the, preferably at least four, preferably at least five, of the following characteristic values of the structure: Ra, Rsk, Rpc, Rz, Sdr, T and L.
According to an alternative teaching, the invention relates to a component having an outer side and an inner side, which, in particular, is produced from a steel sheet temper rolled according to the invention, wherein the outer side has a lower value in comparison with the inner side in respect of at least two, in particular at least of the, preferably at least four, preferably at least five, of the following characteristic values of the structure: Ra, Rsk, Rpc, Rz, Sdr, T and L.
Components produced, e.g. dish-shaped parts or deep-drawn parts, have an outer side and an inner side. It should be understood here that the outer side may be the visible side, and the inner side may be the correspondingly averted side.
Components according to the invention are thus used particularly preferably in vehicle construction, and it is therefore possible to explain, with reference to the example of a component according to the invention which is an outer skin part, which side is the outer side, and is thus the visible side on the vehicle in the installed state. The inner side is correspondingly averted, facing in the direction of the vehicle interior and generally concealed by add-on parts and thus not visible, e.g. in the case of moving parts such as doors and flaps but also vehicle roofs, wheel arches etc.
According to one embodiment of the component according to the invention, the component is phosphatized.
According to one embodiment of the component according to the invention, the component is painted.
Experiments have shown that, for example, deterministic surface structures respond, in particular, more sensitively to the applied quantity of oil than stochastic surface structures. This is due to the fact that the fundamental character of stochastic surface structures is that of an open structure, and they have less steep surface profile shapes. Defined deterministic surface structures, in contrast, generally have a closed structure with considerably steeper surface profile shapes. In the case of deterministic surface structures, therefore, the transition to underlubrication is more abrupt in character than in the case of stochastic surface structures, in which this transition is smoother. Below a predetermined limiting quantity of lubricant (limiting quantity of oil), the friction coefficients in the case of deterministic surface structures rise sharply since, on account of closed structure, the lubricant pockets thus produced the medium (oil) is absorbed locally or bound more strongly. By means of the deterministic surface structures or specific profile configuration, it is possible to adjust the transition to underlubrication. By means of the invention and deterministic surface structures formed on at least one side, preferably on both sides, it is possible to produce steel sheets which thus also have different tribological properties on the upper and lower side of the steel sheet. A steel sheet according to the invention can be used to particular advantage in sheet metal forming since it can have friction coefficient characteristics that are appropriate in the various tool regions (punch versus flange region), these characteristics being surface-pressure-dependent. Thus the demands that are made on the tribology in these tool regions, e.g. by a drawing process, can be met in an optimum manner. Thus, for example, the side on which the punch acts can be the inner side on the component, which has the lower value, when compared with the outer side, in respect of at least two, in particular at least the, preferably at least four, preferably at least five, of the following characteristic values of the structure: Ra, Rsk, Rpc, Rz, Sdr, T and L.
In the production of components, particularly vehicle body components, preferably outer skin components, stretching in the region of the punch of 3-5% is aimed for, with drawing tools that have a punch, die or drawing ring and a hold-down device being used. For this purpose, a low friction coefficient in the region of the punch, in particular in the region of the punch base, is advantageous since in this way it is possible to ensure uniform stretching of the material. In contrast, the steel sheet in the flange region should be held between the hold-down device and the die/drawing ring. Once again, a high friction coefficient is advantageous for this purpose. In the flange region and in the punch region, the prevailing surface pressures are in the comparable pressure range of about 3 to 8 MPa, for example. The surface pressures achieved at the drawing edge may be 12 MPa and above. Accordingly, there would be a requirement for a steel sheet which had different friction coefficients on the upper side and the lower side in the different regions. Since such a surface configuration of the steel sheet would have to be matched to the component geometry, however, this would be associated with a very high outlay in terms of manufacture. Moreover, the lateral surface areas of the temper rolls would impose a limit in respect of component size.
The steel sheet according to the invention exploits the fact that the side of the sheet which faces the punch is embodied in such a way that the friction coefficient in the surface pressure range prevailing there is as low as possible. In contrast, the side of the sheet which faces away from the punch is configured in such a way that a friction coefficient which is as high as possible is achieved. In this way, it is possible to ensure that the material can be stretched in the punch region and held in the flange region.
Surprisingly, pseudo-stochastic surface structures with repeated deterministic structures, for example, have a more open sheet topography in comparison with deterministic surface structures and thus have a friction coefficient that is lower in relative terms in the region of low surface pressures. Moreover, open sheet topographies may be advantageous for wetting by aqueous process media, and therefore the effect of different topographies on the upper side and the lower side of the steel sheet or component (e.g. deterministic surface structure on the upper side and pseudo-stochastic surface structures on the inner side) may be a positive one in the sector of automotive pretreatment (cleaning and phosphatizing). As in the forming process described above, the upper and lower sides of the steel sheet are treated differently in the pretreatment process. Thus, during a spray cleaning or phosphatizing process, for example, there is a high energy input to the outer sides of the vehicle body, leading in turn to an improved cleaning result. The inner sides, in contrast, are cleaned only by dipping or rinsing, and this may lead to poorer cleaning results, which is then reflected in poor phosphatizing results. The vehicle manufacturer can avoid this poor cleaning result by means of a longer treatment time for the inner sides, for example, or, according to the invention, by means of a surface with pseudo-stochastic surface structures, which can be wetted more effectively, on the inner sides of the components.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 125 889.8 | Oct 2021 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/077116 | 9/29/2022 | WO |
