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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation patent application of U.S. patent application Ser. No. 14/078,025, filed Nov. 12, 2013, which claims priority from German Patent Application No. 10 2012 110 972.9, filed Nov. 14, 2012. The disclosures of both applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for manufacturing coated steel sheets made from a flexibly rolled strip material. The steel sheet should be protected against corrosion by means of the coating.

Different methods for coating components made from steel with a zinc or zinc alloy layer are known, like hot galvanization (hot-dip galvanization) or galvanic (electrolytic) galvanization. Hot galvanization means plating of steel parts with a solid metallic zinc coating by means of dipping of the pretreated steel parts into a melt of liquid zinc. Galvanic galvanization is carried out by dipping the workpieces into a zinc electrolyte. Electrodes of zinc serve, because of their less precious metal, as a “sacrificial anode”. The workpiece to be galvanized serves as a cathode, because of which the coating is also characterized as a cathodic corrosion protection.

From DE 10 2007 013 739 B3 a method for the flexible rolling of coated steel strips is known. A hot or cold strip is electrolytically coated and subsequently flexibly rolled, wherein the coated steel strips receive different sheet thicknesses along the length. The coating is adjusted to the sheet thickness after flexible rolling or to the rolling pressure during the flexible rolling. For this, the coating is formed varyingly thick.

From DE 10 2009 051 673 B3 a method for manufacturing steel strips with a cathodic corrosion protection layer is known. For this, the steel strip is hot rolled, subsequently cold rolled and is electrolytically galvanized. After the electrolytic galvanization, the steel strip is heat treated in a bell-type annealing furnace at temperatures from 250° C. to 350° C. for a time of 4 to 48 hours, whereby a zinc-iron layer is produced.

From DE 10 2007 019 196 A1 a method for producing flexibly rolled strip material with a cathodic corrosion layer is known. The method comprises the steps of providing a rolled strip as a hot or cold strip with a cathodic corrosion layer, and flexibly cold rolling of the coated rolled strip with a rolling gap adjustable during the rolling process.

From DE 601 19 826 T2 a method for achieving a workpiece with very high mechanical properties is known, which starting from a steel sheet strip is formed by means of deep-drawing. The workpiece is hot rolled and coated with a metallic alloy made from zinc. For this, the sheet is cut to size, heated up to a temperature of 800° C. to 1200° C. and subsequently a hot deep drawing process is carried out. Then, the sheet excesses, necessary for the deep drawing process, are removed by means of cutting.

SUMMARY OF THE INVENTION

The present invention is based on the object to provide a method for manufacturing coated steel sheets from a flexibly rolled strip material, which offers an especially good corrosion protection.

A first solution consists of a method for manufacturing a product from a flexibly rolled strip material comprising the steps: providing a strip material made from sheet steel, flexible rolling of the strip material, wherein a variable thickness is produced along the length of the strip material, electrolytic coating with a metallic coating material, which contains at least 93% by mass of zinc, wherein the electrolytic coating is carried out after the flexible rolling, heat treatment 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 cold or hot forming of the blank.

A second solution is a method for manufacturing a product from a flexibly rolled strip material comprising the steps: providing a strip material from sheet steel, flexible rolling of the strip material, wherein a variable thickness is produced along the length of the strip material, electrolytic coating with a metallic coating material, which at least contains zinc and iron, working a blank from the flexibly rolled strip material and cold or hot forming of the blank.

An advantage of the two above named methods is, that the electrolytic coating is carried out after the flexible rolling. Thus, it is achieved, that the deposited coating has a constant thickness along the length of the flexibly rolled strip material. Insofar also the areas of the strip material, which are stronger rolled-out, have a layer thickness, which reliably protects against corrosion. Altogether the process time for manufacturing the product can be shortened and less coating material is necessary, which again has an advantageous effect on the manufacturing costs.

A flexibly rolled product is, in connection with the present invention, understood to be a steel strip with varying thickness as well as a rectangular blank or a form-cut (profile cut), respectively, which is produced from a flexibly rolled steel strip by means of mechanical cutting or laser cutting. As strip material for the flexible rolling, a hot strip or cold strip can be used, wherein these terms should be understood in the sense of the technical terminology. A hot strip is here seen to be a rolling steel finished product (steel strip), which is produced by means of rolling after preliminary heating. A cold strip is here meant to be a cold rolled steel strip (flat steel), at which the last thickness reduction is carried out by means of rolling without a preceding heating. The strip material which is provided for being rolled can also be referred to as band material.

In both of the above named solutions it is understood, that between the individual method steps, further steps could be interposed. For example, after the flexible rolling, a strip straightening can be provided. The working of the blanks from the strip material can be carried out before or after the electrolytic coating. Conceptually, “working a blank from a strip” is supposed to include, that the sheet blank can be stamped from the strip material, which means an edge remains at the strip, which is not further used, as well as, that a simple cutting of the strip material into partial pieces can be carried out, especially by means of a cutting process. Working a blank from a strip can also be referred to as producing a blank from a strip.

In the first solution, a coating consisting at least of 93% by mass of zinc is deposited on the strip material, wherein the proportion of zinc may especially be larger than 95% by mass, 97% by mass, or 99% by mass and can even be 100% (pure zinc coating). For the electrolytic coating, anodes made from pure zinc or from zinc and other alloy elements are used, which, during feeding of current, deposit metal ions on the electrolyte. The zinc ions and possible ions of further alloy elements are deposited as atoms on the strip material, which is connected as a cathode, and form a coating. In a deposition of a coating with a high proportion of zinc of more than 93% by mass as it is provided in the first solution, the following heat treatment leads in an advantageous manner to an alloy formation between the deposited zinc and the iron contained in the strip material, so that altogether a zinc-iron coating is produced.

In the second solution, from the start, a zinc-iron-alloy layer is produced by means of electrolytic deposition. The proportions of zinc and iron are preferably selected such that at least one of the following conditions is valid: the alloy layer contains at least 5% by mass of iron, the alloy layer contains at a maximum 80% by mass of iron, the alloy layer contains at a minimum 20% by mass of zinc and/or the alloy layer contains at a maximum 95% by mass of zinc. It is especially advantageous when the proportions of zinc and iron are selected such that in the deposited state, at least partially δ1-phase, especially δ1-phase and Γ-phase, or only intermetallic Γ-phase is present. This is, for example, achieved with an iron proportion of 10% by mass to 30% by mass percent or a zinc proportion of 70% by mass to 90% by mass, wherein the addition of further alloy elements is not excluded. In this embodiment, a subsequent heat treatment is not necessary, as the coating itself already contains zinc and iron. The zinc and iron atoms are arranged at a distance of few nanometers from each other so that especially short diffusion paths are produced. It can, however, be understood that also with an electrolytic deposition of a zinc-iron alloy, the named heat treatment can be carried out. By means of the short diffusion paths, a very short heat treatment is sufficient, for example by means of induction heating. Altogether, by means of the named method process, a shortening of the process time can be achieved in an advantageous manner.

The method according to the second solution can be carried out according to a first possibility without heat treatment after the electrolytic coating and before forming. According to a second possibility of the second solution, a heat treatment at a temperature range above 350° C. and below the melting temperature of the coating material (solidus line) can be provided as a further step after the electrolytic coating. The solidus line marks in the finite state diagram for the coating material that line, below which only solid phase is present. Above the solidus line the coating material is at least present partially as melt.

With progressing heating time, the iron proportion in the coating increases, as iron atoms diffuse from the base material into the coating material. Because of the increasing iron proportion in the coating, the heat treatment temperature can then be increased, without reaching the solidus line or exceeding it. This is possible with suitable process control up to a temperature of 781° C. The possibility of the temperature increase during the heat treatment is obviously also valid for the first solution. The temperature can be step-wise or continuously increased with increasing iron proportion.

The liquidus line marks in the finite state diagram for the coating material that line, below which a two-phase or multi-phase range, solid-liquid, is present. Above the liquidus line, the coating material is in the liquid form. The lower limit of the two-phase range is characterized as the solidus line. The temperature of the solidus line depends on the proportional composition of the alloy. For pure zinc, the solidus line lies at 419.5° C., for a zinc-iron alloy it is maximal 782° C., insofar as still parts of Γ-phase are present. With a corresponding proportion of iron it is, thus, possible, to electrolytically coat the flexibly rolled strip material in a full hard (hard as rolled) condition and subsequently to carry out a heat treatment at a relatively high temperature of more than 500° C. up to maximal 782° C., without that a liquid phase is produced.

A heat treatment in a temperature range of 500° C. up to 782° C. is, furthermore, suitable to carry out a re-crystallization annealing, so that the produced material is especially suited for an indirect hot forming. An otherwise necessary re-crystallization annealing can, thus, be omitted after the flexible rolling and before the coating. For example, in the first named solution with the use of pure zinc (coating material 100% zinc), the heat treatment process can be started at an annealing temperature of 380° C. and, with increasing iron proportions due to diffusion processes, can then be step-wise increased up to a temperature of maximal 781° C.

For both solutions it applies, that the coating material can also contain further alloy elements, like for example manganese, chromium, silicon or molybdenum. Independent of the type and number of alloy elements, a feature of the invention is the temperature control for the purpose of forming the zinc-iron alloy layer. The respective alloy temperature is selected such, that the solidus line of the coating material in the composition, currently present during the process, is reached or exceeded at no point of time of the alloy formation of the binary zinc-iron phase diagram or of a layer structure, containing more than two alloy elements, respectively. The alloy is thus formed by solid phase diffusion.

During the heat treatment, a diffusion of iron from the coated 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. The stated temperature range above 350° C. and below the solidus line is especially advantageous, as the diffusion takes place relatively quickly. Because of the iron content, the affinity to solder cracking of the coating is reduced, so that the fatigue limit of the component is increased.

The phase conversion can be achieved, as mentioned above, according to a first possibility by means of inductive heating. This process method is especially suitable in an electrolytic deposition of zinc and iron, as here short diffusion paths are present, so that a short heat treatment can lead already to the required phase conversion. According to a second possibility, the heat treatment can be carried out by annealing in a bell-type annealing furnace. This annealing is especially suitable for the electrolytic deposition of pure zinc. Preferably, during the annealing in an annealing furnace a holding time of 10 to 80 hours, preferably 30 to 60 hours, is provided, so that sufficient time is available, so that by means of diffusion a zinc-iron alloy is produced. The holding time (dwell time) characterizes preferably the whole time, in which the blanks or the strip material is heat treated, and can also comprise a heating-, holding, and cooling phase. A further possibility is the conductive heating, but other technically possible heat treatment methods are obviously not excluded.

As a further method step it can be provided before the electrolytic coating that the strip material is coated with an intermediate layer. As intermediate layer, especially a nickel or aluminum containing layer can be used. These are layers, which contain at least partially nickel or aluminum, which also includes a pure nickel layer or a pure aluminum layer. The nickel layer forms an additional protection of the surface and improves the adhesion of the coating, subsequently deposited and containing zinc. The nickel coating can be formed, for example, by electrolytic deposition or deposition without a current from an external source. It is obvious, that other materials are not excluded for the intermediate layer. For example, also a coating containing manganese or chromium can be used. Manganese and chromium have both a cubic lattice and have a good solubility in iron, which has advantageous effects on the alloy behavior.

According to a possible embodiment, the strip material can be provided with a scaling protection after the electrolytic coating. This is especially applicable, when the austenitization for a later hot forming is not carried out in an inert gas atmosphere. Scaling are mainly oxidic corrosion products, produced during the reaction of metallic materials in air or other oxygen containing gases at a high temperature. The deposition of the scaling protection layer can be carried out by spraying or rolling, respectively coating. Besides the protection against oxidation, a further advantage of the scaling protection layer is, that the surface has a high quality. Especially, before a later vanishing of the sheet, no cleaning treatment like shot-blasting is necessary. Furthermore, because of the scaling protection, the friction value is positively influenced during the hot forming as well as the heat absorption behavior. A further advantage of the scaling protection is, that the adhesion of the cathodic corrosion protection layer arranged below is improved. Furthermore, a widening of the temperature-time window in the frame of the austenitization is possible, for example by means of alloy formation of the scaling protection material with the below arranged layer. The scaling protection can be deposited before or after the heat treatment carried out below the solidus line.

At a suitable position of the process, blanks or form cuts are produced from the flexibly rolled strip material, which can be carried out by means of mechanically cutting or by means of laser cutting. Blanks are understood to be especially rectangular sheet plates, which are cut from the strip material. Form cuts means in particular sheet elements, cut from the strip material, which outer contour is already adapted to the form of the final product. Predominantly the term blanks is used uniformly for rectangular blanks as well as for form cuts. The manufacture of blanks can be carried out before or after the electrolytic coating and if necessary before or after the deposition of a scaling protection.

According to a possible process embodiment which is valid for both solutions, the sheet blanks are hot formed. Hot forming means forming processes in which the workpieces are heated up to a temperature in the range of the hot forming, before being formed. The heating is carried out in a suitable heating device, for example in a furnace. The hot forming can be carried out according to a first possibility as an indirect process, which comprises the partial steps cold pre-forming of the blanks to a pre-formed component, subsequent heating at least of partial areas of the cold pre-formed component up to an austenitization temperature, as well as subsequent hot forming for producing the final contour of the product. Austenitization temperature is understood to be a temperature range, in which at least a partial austenitization (structural conditions in the two-phase area of ferrite and austenite) is present. Furthermore, it is also possible, to austenitize only partial areas of the blank, to enable for example a partial hardening. The hot forming can be carried out according to a second possibility also as a direct process, which is characterized in that at least partial areas of the blank are directly heated to austenitization temperature and are subsequently hot formed to the required final contour in one step. An earlier (cold) pre-forming does not take place here. Also during the direct process, a partial hardening can be achieved by means of austenitization of partial areas. For both processes it is valid, that a hardening of partial areas of the component is also possible by means of varyingly tempered tools or by using several tool materials, which enable different cooling velocities. In the latter case, the whole blank or the whole component can be completely austenitized.

According to a process embodiment, which is valid for both hot forming processes, the coating material at the point of time of initiating the hot forming is preferably in the solid state, i.e. the temperature has cooled down to an area below the solidus line of the coating material. After the hot forming, the iron content in the boundary layer is below 80% by mass, preferably below 60% by mass, especially preferred below 30% by mass.

According to an alternative process embodiment, which is in principle valid for both above named solutions, the sheet blanks can also be cold formed. Cold forming are forming processes, in which the blank is not specifically heated before forming. The forming thus takes place at room temperature, the blanks are heated by the dissipation of the fed energy. Cold forming is especially used as a process for forming soft car body steels.

The solution of the above named object is further a sheet blank made from flexibly rolled sheet steel, which is electrolytically coated after the flexible rolling with a metallic coating and is hot formed after the coating. Thus, the above named advantages of a constant layer thickness along the length of the flexibly rolled strip or of the blanks produced therefrom is achieved. The blanks are produced according to one or more of the above named method steps, so that concerning the steps and the advantages connected therewith it is referred to the above description.

Following, preferred embodiments are described by using the figures. It shows

BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

FIG. 4 a zinc-iron-phase diagram.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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.).

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.

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.

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).

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.

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

• • 2 strip material
  • 3 coil
  • 4 rolls
  • 5 rolls
  • 6 thickness control
  • 7 smoothing device
  • 8 coating device
  • 9 immersion tank
  • 10 electrolyte
  • 11 set of rolls
  • 12 set of rolls
  • 13 furnace
  • 14 molding tool
  • 20 blank
  • V1-V6 method steps

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.