Method for Manufacturing a Product from a Flexibly Rolled Strip Material

Abstract

A method for manufacturing a product from a flexibly rolled strip material includes the steps of: providing a strip material made from sheet steel; flexibly rolling the strip material such that a variable thickness is produced along the length of the strip material; electrolytically coating the strip material with a metallic coating material containing at least 93% of zinc by mass after the flexible rolling; heat treating at temperatures above 350° C. and below a solidus line of the coating material after the electrolytic coating; working a blank from the flexibly rolled strip material; and hot forming the blank.

Claims

1. A method for manufacturing a product from a flexibly rolled strip material comprising the steps of: providing a strip material made from hardenable sheet steel, flexible rolling the strip material, wherein a variable thickness is produced along the length of the strip material, electrolytically coating the strip material with a metallic coating material that contains at least 93% by mass of zinc, wherein the electrolytic coating is carried out after the flexible rolling, heat treating the strip material at temperatures above 350° C. and below a solidus line of the coating material, wherein the heat treatment is carried out after the electrolytic coating, working a blank from the flexibly rolled strip material, and hot forming the blank.

2. The method according to claim 1 wherein the metallic coating material has a minimum of 5% by mass of iron and a maximum of 7% by mass of iron.

3. The method according to claim 2 wherein the proportions of zinc and iron in the coating material are selected such that at least partially δ1-phase is present after the step of electrolytically coating the strip of material.

4. The method according to claim 2 wherein the temperature is increased during the heat treatment.

5. The method according to claim 2 wherein the heat treatment is carried out inductively or by means of annealing in a bell-type annealing furnace, wherein the annealing is carried out with a holding time of 10 hours to 80 hours.

6. The method according to claim 1 wherein before the electrolytically coating step, the strip material is coated with an intermediate layer.

7. The method according to claim 6 wherein the intermediate layer contains nickel or aluminum or manganese.

8. The method according to claim 1 wherein after the electrolytically coating step, a scaling prevention is deposited.

9. The method according to claim 1 wherein the hot forming includes the steps of: cold pre-forming of the blank to a cold pre-formed component, heating at least a partial area of the cold pre-formed component up to austenitization temperature, and hot post-forming of the cold pre-formed component for producing a final contour.

10. The method according to claim 1 wherein the hot forming step includes the steps of: heating at least a partial area of the blank up to the austenitization temperature, and hot forming of the blank for producing a final contour.

11. The method according to claim 1 wherein at a point of time when initiating the hot forming step, the coating material is in a solid state.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 a method according to the invention as a flow chart schematically in a first embodiment,

[0032] FIG. 2 a method according to the invention as a flow chart schematically in a second embodiment,

[0033] FIG. 3 a method according to the invention as a flow chart schematically in a third embodiment, and

[0034] FIG. 4 a zinc-iron-phase diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0035] FIG. 1 shows a method according to the invention for manufacturing a product from a flexibly rolled strip material 2 according to a first processing embodiment. In the method step V1, the strip material 2, which is wound onto a coil 3 in the starting condition, is worked by rolling, more particularly by means of flexible rolling. For this, the strip material 2, which before the flexible rolling has a more substantially constant sheet thickness along the length, is rolled by rolls 4, 5 such, that a variable sheet thickness is produced in longitudinal direction of the rolling direction. During rolling, the process is monitored and controlled, wherein the data, determined from a sheet thickness measurement 6, are used as input signals for adjusting the rolls 4, 5. After the flexible rolling, the strip material 2 has varying thicknesses in rolling direction. The strip material 2 is wound again to a coil 3 after the flexible rolling, so that it can be transferred to the next method step.

[0036] After the flexible rolling, the strip material 2 is smoothed in the method step V2, which is carried out in a strip straightening device 7. The method step of smoothing is optional and can be omitted.

[0037] After the flexible rolling (V1) or the smoothing (V2), respectively, the strip material 2 is provided with an anticorrosive coating in the method step V3. For this, the strip material 2 passes through an electrolytic strip coating device 8. It is visible, that the strip coating is produced in through-feed method, this means, that the strip material 2 is wound from the coil 3, passes through the coating device 8 and after the coating process is again wound to a coil 3. This method process is especially advantageous, as the handling expenditure is small for depositing the anticorrosive coating onto the strip material 2 and the process velocity is high. The strip coating device 8 comprises an immersion tank 9, which is filled with an electrolytic liquid 10, through which the strip material 2 runs. Guiding of the strip material 2 is achieved by means of sets of rolls 11, 12.

[0038] The electrolytic coating is achieved in the present method embodiment with a metallic coating material, which contains at least 93% by mass zinc. Because of the high zinc content, an especially good resistance to corrosion is achieved. It is understandable, that the zinc proportion could also be higher, for example larger than 95% by mass, especially larger than 97% by mass and can even be 100% by mass (pure zinc). For the coating process for example anodes made from zinc can be used, which release during a current feed zinc ions to the electrolyte. The zinc ions are deposited as zinc atoms and form a zinc layer on the strip material 2, which is connected as a cathode. Alternatively, also inert anodes and a zinc electrolyte can be used.

[0039] Besides the mentioned zinc proportion, the coating can still contain further alloying elements, as for example aluminum, chromium, manganese, molybdenum, silicon. The proportion of the added alloying elements, if necessary, are less than 7% by mass. Manganese has a good solubility in iron, which has an advantageous effect on the alloy formation during heating.

[0040] After the electrolytic coating (V3), the strip material 2, wound to a coil 3, is heat treated in method step V4. The heat treatment can be carried out in principle in any technically suitable manner, for example in an annealing furnace such as a bell-type annealing furnace or also by means of inductive heating, to only name two methods for example. In the present case the heat treatment is shown in a furnace 13.

[0041] The heat treatment is carried out at temperatures larger than 350° C. and below the solidus line of the coating material. The temperature profile of the solidus line depends on the proportional composition of the alloy. At the temperature within the stated range, a diffusion of iron is triggered into the zinc layer, so that with progressing holding time of the heat source a diffusion layer is produced.

[0042] During the heat treatment a diffusing of iron from the to be coated strip material into the metallic coating takes place. In this case, zinc of the coating converts into a zinc-iron alloy, which offers a cathodic corrosion protection system. Because the temperature range is above 350° C. and below the solidus line, the diffusion takes place relatively quickly. The holding time for the heat treatment in an annealing furnace is preferably 10 to 80 hours, preferably 30 to 60 hours, so that sufficient time is available, so that a zinc-iron alloy is formed by diffusion.

[0043] A further effect of the heat treatment is, that hardenings of the material, produced during the rolling, are reduced or disappear, so that the rolled strip material 2 takes up again a higher ductility and elasticity. The strip material can be easier further processed in the following method steps, wherein furthermore the material properties of the to be manufactured finished product can be positively influenced.

[0044] After the heat treatment (V4) the individual sheet blanks 20 are worked from the strip material 2 in the next method step V5. The working of the sheet blanks 20 from the strip material 2 takes place preferably by means of stamping or cutting. Depending on the shape of the to be manufactured sheet blanks 20, these can be stamped from the strip material 2 as a shape cut, wherein a strip of the strip material remains, which is not further used, or the strip material 2 can simply be cut to length into partial pieces. A sheet blank 20, worked from the strip material 2, which also could be characterized as three-dimensional sheet blanks (3D-TRB), is shown schematically.

[0045] After producing the blanks 20 from the strip material 2, a forming of the blanks 20 to the required finished product takes place in method step V5. According to a first possibility the blanks 20 are hot formed or according to a second possibility cold formed.

[0046] The hot forming can be carried out as a direct or indirect process. During the direct process, the blanks 20 are heated to the austenitization temperature before the forming, which heating can for example be done by means of induction or in a furnace. Austenitization temperature is, in this case, a temperature range, in which at least a partial austenitization (structure in the binary phase region ferrite and austenite) are present. However, also partial areas of the blanks can be austenitized, to enable for example a partial hardening. After the heating to the austenitization temperature, the heated blank is formed in a shape-giving tool 14 (forming tool) and at the same time is cooled with a high cooling velocity, wherein the component 20 receives its final profile and is hardened at the same time.

[0047] During the indirect hot forming, the blanks 20 are pre-formed before the austenitization. The pre-forming takes place in the cold condition of the blank, i.e. without prior heating. During the pre-forming the component receives its profile, which however still does not correspond to the final shape, however this is approximated. Then, after the pre-forming an austenitization and hot forming takes place, like during the direct process, wherein the component receives its final shape and is hardened.

[0048] The steel material should, insofar as a hot forming (direct or indirect) is provided, contain a proportion of carbon of at least 0.1% by mass up to 0.35% by mass.

[0049] Alternatively to the hot forming as a shape giving process, the blanks can also be cold formed. The cold forming is especially suitable for soft body steels or components, which do not have special requirements in view of strength. During the cold forming, the blanks are formed at room temperature.

[0050] A special feature of the method according to the invention is, that the electrolytic coating (V3) is carried out after the flexible rolling (V1). The coating deposited on the strip material 2 has a constant thickness along the length, i.e. independent of the respective thickness of the strip material 2. Also the areas, which have been rolled more intensely to a smaller thickness, have a sufficient thick coating, which protects reliably against corrosion. A further special feature is the step of heat treatment after the electrolytic coating at a temperature range between 350° C. and below the solidus line of the coating material. Because of the heat treatment, zinc diffuses from the coating into the base material and iron from the base material into the coating. With increasing iron proportion in the coating, the temperature during the heat treatment can slowly be increased because of the displacement of the solidus line towards higher temperatures. A zinc-iron alloy is produced as coating, which withstands also higher temperatures of a subsequent hot forming process if needed and offers a reliable corrosion protection.

[0051] It is understood, that the method sequence according to the invention can also be modified. For example, between the named steps, also intermediate steps, not shown here separately, can be provided. For example, the strip material can be provided with an intermediate layer, especially a nickel, aluminum, or manganese layer, before the electrolytic coating. This intermediate layer forms an additional protection of the surface and improves the adhesion capability of the subsequently deposited coating containing zinc. It can also be provided, that the strip material or the blanks manufactured therefrom, are provided with a scale protection after the electrolytic coating (V3) and before or after the heat treatment (V4). This is especially advisable, when the austenitization for a later heat forming does not take place in an inert atmosphere. The deposition of the scale protection can be carried out by means of spraying or calendar coating. Besides the protection against oxidization, a further advantage of the scale protection layer is, that the surface has a high quality. Furthermore, the friction coefficient during the hot forming as well as the heat absorption behavior is positively influenced by the scale protection. A further advantage of the scale protection is, that the adhesion of the cathodic anti-corrosion layer arranged below is improved. Furthermore, a widening of the temperature-time-window during the austenitization is possible, for example by means of alloy formation of the scale protection material with the layer arranged below. An example for this is aluminum fins in a scale protection lacquer.

[0052] Further it is understood, that the processing according to the invention can also be modified concerning the sequence of the carried out steps. For example, the working of blanks can also be carried out at another point, for example before the electrolytic coating or if necessary before or after the deposition of a scale protection.

[0053] FIG. 2 shows a method according to the invention for manufacturing a sheet blank from a strip material 2 according to a second processing embodiment. This corresponds in wide parts to the method of FIG. 1, so that in view of the similarities it is referred to the above description. At the same time, the same or modified components or steps are provided with the same reference numerals as in FIG. 1. In the following particularly the differences of the present methods are described.

[0054] The method steps V1 (rolling), V2 (strip straightening), V5 (stamping) and V6 (forming) are identical to the corresponding method steps V1, V2, V5 and V6 of FIG. 1.

[0055] A first difference of the present embodiment to the method of FIG. 1 is the method step V3 of the electrolytic coating. In the present method processing of FIG. 2, the strip material is coated with a metallic coating material, which contains at least zinc and iron. The zinc-iron-alloy layer is produced by the electrolytic deposition of a zinc-iron-layer. The proportions of zinc and iron are in this case selected according to an advantageous method processing such, that the alloy layer contains at least 5 mass percent and/or at a maximum 80 mass percent, or that the alloy layer contains at least 20 mass percent and/or at a maximum 95 mass percent of zinc.

[0056] Especially advantageous is, when the proportions of zinc and iron are selected such, that in the deposited state at least partially δ1, especially δ1-phase and Γ-phase, or only intermetallic Γ-phase is present. For this, for example a proportion of iron in the coating can be selected between 10% by mass to 30% by mass, or a proportion of zinc of 70% by mass to 90% by mass. With these proportions at least partially an intermetallic phase is formed in the deposited state.

[0057] It is advantageous for carrying-out a direct hot forming, when the content of Γ-phase is relative high and the content of δ1-phase is as small as possible. To prevent solder fissuring or cracking, the melting temperature of the coating for the hot forming should be relative high. With the increase of the proportion of iron and thus with increasing proportion of Γ-phase, the solidus line is displaced in the binary phase diagram of zinc-iron (see FIG. 4) towards higher temperatures.

[0058] After the electrolytic coating (V3) blanks are worked from the strip material 2 in method step V5, wherein it is obvious, that the blanks could also be cut-out in a modified method processing before the coating.

[0059] A further feature of the present method sequence of FIG. 2 is, that between the step of coating (V3) and the step of forming (V6) no interconnected heat treatment is carried out below the solidus temperature. The method of FIG. 2 is thus time-wise especially short.

[0060] The subsequently carried-out step of forming corresponds to that of FIG. 1, so that concerning this it is referred to the above description. The blank 20 can be cold or hot formed (directly or indirectly).

[0061] It is understood, that also in the present method sequence modifications, especially additional intermediate steps or subsequent method steps, can be carried out. It is, concerning this, referred to the above description for preventing repetitions.

[0062] FIG. 3 shows a method according to the invention for manufacturing a sheet blank from a strip material 2 according to a third method processing embodiment. This corresponds essentially to a combination of the methods of FIGS. 1 and 2, so that concerning the similarities it is referred to the above description. At the same time, the same or modified components or steps are provided with the same reference numerals.

[0063] Steps V1 (rolling), V2 (strip straightening), V3 (electrolytic coating), V5 (stamping) and V6 (forming) are identical to the corresponding method steps of FIG. 2. The only difference to the method of FIG. 2 is, that after the electrolytic coating (V3) a heat treatment is carried out in method step V4, as in the method of FIG. 1.

[0064] As in the method processing of FIG. 1, also in the present method processing of FIG. 4, the special feature is the temperature control for forming a zinc-iron-alloy layer. The respective alloy temperature is selected during the heat treatment (V4) such, that at no point of time of the formation of the alloy, the solidus line of the binary zinc-iron-phase diagram (compare with FIG. 4) or the solidus line of a layer structure, consisting of more than two alloy elements, is reached or exceeded.

[0065] An example for such a layer structure would be for example a ternary alloy from zinc, iron and manganese, wherein the manganese stems from the steel substrate and reaches by means of the diffusion during the above named heating into the electrolytically deposited zinc layer or zinc-iron-alloy layer and does not form part of an electrolytic deposition. Instead of manganese it is also possible, that for example chromium or aluminum or silicon or molybdenum diffuses into the electrolytically deposited layer. It is understood, that for the coating also steel alloy elements can be provided, which have not been named up to now and which are suitable, to diffuse by the above named heating process into the electrolytic deposited layer.

[0066] Also in the present method sequence modifications, especially additional intermediate steps or subsequent method steps, can be carried out. Concerning this, for preventing repetitions it is referred to the above description.

[0067] FIG. 4 shows the phase diagram for zinc-iron. On the x-axis, the proportions of iron (Fe) and zinc (Zn) are shown, respectively. In this case, on the left edge, a material with 100% by mass iron and 0% by mass zinc is present, while at the right edge, inversely 0% by mass iron and 100% by mass zinc is present. Between the edges, respectively, the percentaged composition, which is stated on the x-axis, is found. S characterizes the molten mass, α and γ are iron-zinc-mixed crystal systems (rich in iron), ζ and δ or δ1 and Γ are intermetallic phases, and η is a zinc-iron mixed crystal (rich in zinc).

[0068] In the following, by means of the zinc-iron phase diagram, different possibilities of the electrolytic deposition according to one of the methods according to the invention are exemplary described.

[0069] During the deposition of a pure zinc layer, as it can be produced in a method processing of FIG. 1, at the beginning an alloying temperature above 350° C. and below the melting temperature (solidus line) of 419.5° C. is selected, for example 400° C. At this temperature, a diffusion of iron into the zinc layer takes place, so that with continuing holding time during the heat treatment (V4) a diffusion layer is formed, for example a 6-phase. The further temperature processing is such, that the respective temperature is always below the solidus line of the binary zinc-iron-phase diagram.

[0070] During an electrolytic deposition of a coating, which already contains iron in the zinc layer, as it can be produced in a method processing of FIG. 3, the starting temperature can be selected above the melting temperature of pure zinc. For example, in a composition of the electrolytic deposited layer of 85% by mass zinc and 15% by mass iron, a starting temperature of 600° C. can be selected. This temperature lies in fact above the melting temperature of zinc, however below the solidus line of the two-phase-range F+61.

[0071] For an electrolytic deposition of a zinc-iron layer, which consists of 60% by mass of zinc and of 40% by mass iron, a starting temperature smaller than 782° C. is possible. An increase above this temperature is only then possible, when the layer is enriched during a following heat treatment so far with iron, that only an austenitic iron mixed crystal would be present (for example 70% by mass percent iron and 850° C.).

[0072] The type of heat treatment is, as above described, not prescribed. For example, it can be an inductive heating or a heating in an annealing furnace or a heating by means of contact with a hot body, for example a thick steel plate, which delivers its heat to the blank or the profile cut.

[0073] In a special embodiment of the invention, an electrolytic zinc-iron alloy with an iron proportion of 8% by mass to 12% by mass is provided. In this case, it is a composition, as it is used for steels with a so-called “galvannealed” coating. The advantage of this composition is that the elements zinc and iron have a distance in the range of nanometers so that a drawn-out diffusion treatment can be waived. Rather, by means of a short heat treatment in the method step V4, an intermetallic δ1-phase can be produced from an electrolytic deposited zinc-iron alloy with an iron proportion of 8% by mass to 12% by mass. Such a composition can be used for the cold forming as well as for the hot forming.

[0074] In a further special embodiment of the invention, an electrolytic zinc-iron alloy is deposited, which stoichiometric composition corresponds to the Γ-phase. Alternatively, this composition can also be reached by a deposition of a zinc-iron layer with a low iron proportion and a subsequent heat treatment, at which end the Γ-phase is present. This layer starts only to melt at a temperature of 782° C., so that this layer is especially suitable for the hot forming, as in this case the formation of a melting phase can be restricted or can be prevented by means of stabilizing the layer with elements from the steel substrate as manganese (ternary system iron-zinc-manganese).

[0075] In a further embodiment, which is also provided for the hot forming (V6), a layer is electrolytically deposited, which itself is not present in a molten state even during the heating to the maximum austenitizing temperature for the hot forming (for example at 900° C.). Such a coating would for example have a composition of 20 weight percent of zinc and 80 weight percent of iron. In this case, it is an iron based alloy of the binary iron-zinc system.

[0076] Altogether with the method according to the invention products with a reliable cathodic corrosion protection can be manufactured, which are especially suitable for a hot forming process. By means of the at least as far as possible prevention of the formation of a liquid phase in the coating during the process, the susceptibility to cracking of the solder of the product is minimized in an advantageous manner.

REFERENCE NUMERALS LIST

[0077] 2 strip material [0078] 3 coil [0079] 4 rolls [0080] 5 rolls [0081] 6 thickness control [0082] 7 smoothing device [0083] 8 coating device [0084] 9 immersion tank [0085] 10 electrolyte [0086] 11 set of rolls [0087] 12 set of rolls [0088] 13 furnace [0089] 14 molding tool [0090] 20 blank [0091] V1-V6 method steps