Patent Application
20230366056
The invention relates to a method for producing a sheet metal component by hot-forming a flat steel product which is provided with an anti-corrosion coating, in particular by hot dip galvanizing, and which, by means of flexible cold rolling, is provided with at least one portion which has a different thickness than another adjoining portion of the flat steel product, wherein the transition between the portions of the flat steel product of different thicknesses takes place abruptly.
“Flat steel products” are understood here to be rolled products the length and width of which are each substantially greater than their thickness. These include in particular steel strips and steel sheets.
In the present text, unless explicitly stated otherwise, information about the contents of alloying constituents is always provided in wt %.
The proportions of certain constituents in an atmosphere, in particular an annealing atmosphere, are given in vol. %, unless otherwise stated.
From JP 2004-083988 A, a method is known by means of which a component is formed from a hot-dip galvanized steel sheet which has an Al-based anti-corrosion coating and is intended for use at high temperatures of 450 to 650° C., which component should have improved oxidation resistance at the high operating temperatures. The anti-corrosion coating of the sheet metal consists of up to 13 wt % Si, 0.5-8 wt % Mg and, if necessary, of one or more metals from the group “0.001-1 wt % Sr, 0.001-1 wt % Ca, 0.0001-0.1 wt % Be, 0.001-1 wt % Ba”. In the high-temperature use of the components formed in this way, an alloy layer is formed between the steel substrate and the anti-corrosion coating of the flat steel product. The Mg present in the anti-corrosion coating causes Mg or Mg oxides to accumulate on the exposed surfaces of the coating in the region of cracks that arise in the anti-corrosion coating. At the same time, up to 50 vol. % iron oxides can be found in a transition layer between the anti-corrosion coating and the steel substrate.
EP 2 993 248 A1 discloses a further method in which flat steel products of the type discussed here are hot-formed. The starting material used for this method is a flat steel product the steel substrate of which consists of so-called “MnB steel”. Steels of this type are standardized in DIN EN 10083-3 and have good hardenability. In the case of hot pressing, they allow reliable process control, by means of which it is possible, in a commercially viable way, to achieve martensite hardening in the tool during the hot forming, without additional cooling. A typical example of such a steel is the steel known as 22MnB5, which is to be found in the Key to Steel—Stahlschlüssel 2004 under material number 1.5528. Typically, in addition to iron and unavoidable impurities, fully killed 22MnB5 steel available on the market contains in wt. %, 0.10-0.250% C, 1.0-1.4% Mn, 0.35-0.4% Si, up to 0.03% P, up to 0.01% S, up to 0.040% Al, up to 0.15% Ti, up to 0.1% Nb, in total up to 0.5% Cr+Mo, and up to 0.005% B. In order to protect the flat steel products consisting of such composite steel against corrosive attacks and, at the same time, minimize the risk of hydrogen absorption in the heating required for hot forming, the flat steel products according to the known method are provided with an Al-based anti-corrosion coating, which contains, as an additional alloying component, effective contents of 0.005-0.7 wt % of at least one alkaline earth metal or transition metal. In addition, an Si content of 3-15 wt % and Fe content of up to 5 wt % can also be present in the coating. Mg is preferably used as the at least one alkaline earth metal or transition metal of the protective coating in a content of 0.1-0.5 wt. %, wherein calcium, strontium, sodium or barium are also possible as a substitute or in addition. The Al-based protective coating can be applied to the steel substrate by hot dip galvanizing, also called “hot dip aluminizing” in technical terminology, or by a gas separation method, for example the known PVD (physical vapor deposition) or CVD (chemical vapor deposition) methods.
Particular requirements in terms of the manner in which the anti-corrosion coating is applied to the steel substrate consisting of an MnB steel are not mentioned in the prior art explained above. Due to the presence of the alkaline earth metal or transition metal in the coating, when a blank which is coated in the manner explained above is heated in a conventional manner in a normal atmosphere over a period of 360 to 800 s to a temperature of 900′C, at most minimal oxygen absorption occurs in the steel substrate.
“Flexible rolling” is a method for the production of metal strips with different strip thicknesses defined over their length. As explained for example in DE 198 46 900 A1 or DE 100 41 280 C2, the height of the roll gap provided between two work rolls of a roll stand, which gap the flat steel product to be rolled has to pass, is usually varied during rolling for this purpose. In this way, consecutive sections having a greater thickness (wider roll gap) and smaller thickness (narrower roll gap) can be produced on the flat steel product over the length of the flat steel product.
Due to the possibility of specifically producing particular thicknesses on a flat steel product, the flexible rolling is ideally suited to producing a flat steel product the properties of which are adapted, for example, to the loads acting locally thereon during use or the requirements imposed on its deformation behavior. Thus, flat steel products can be formed by flexible rolling in such a way that different sheet metal thicknesses are present at the required locations on a component obtained from such a flat steel product by reforming, said sheet thicknesses toughening the component with minimized weight to absorb high loads.
If, as in the case of the present invention, flat steel products which are provided with an anti-corrosion coating are to be processed by flexible cold rolling, that is to say flexible rolling carried out on a flat steel product that is not separately preheated, the high rolling forces that occur regularly in the process can cause damage to the anti-corrosion coating in the form of detachments. As a result of the holes thus formed in the anti-corrosion coating, diffusible hydrogen can get into the flat steel product, which can be triggered by cracks in the steel substrate. In order to avoid this risk, in workplace practices today, the rolling degree, i.e. the relative thickness reduction achieved via a rolling step, are limited to certain maximum values at which, experience has shown, the anti-corrosion coating is not damaged.
Against this background, the object is to specify a method which makes it possible to flexibly hot-roll a flat steel product of the type explained above with high rolling degrees, without having to accept the risk of hydrogen penetrating the steel substrate.
In order to achieve this object, the invention proposes that at least the method steps specified herein are completed during flexible cold rolling of a flat steel product provided with an anti-corrosion coating.
It is self-evident that, when carrying out the method according to the invention, the person skilled in the art does not only carry out the method steps mentioned and explained herein, but also carries out all other steps and activities that are regularly carried out in the prior art upon the practical implementation of such methods, if the necessity arises for this.
Advantageous embodiments of the invention are specified and are explained in detail below, as is the general inventive concept.
According to the invention, in the production of a sheet metal component by hot-forming a flat steel product which is provided with an anti-corrosion coating and which, by means of flexible cold rolling, is provided with at least one portion which has a different thickness than another adjoining portion of the flat steel product, wherein the transition between the portions of the flat steel product of different thicknesses is abrupt, the following steps are completed:
a) Providing a flat steel product comprising a steel substrate produced from a steel which, in wt %, consists of 0.07-0.4% C, 1.0-2.5% Mn, 0.06-0.9% Si, up to 0.03% P, up to 0.01% S, up to 0.1% Al, up to 0.15% Ti, up to 0.6% Nb, up to 0.005% B, up to 0.5% Cr, up to 0.5% Mo, wherein the sum of the content of Cr and Mo is at most 0.5%, and, as the remainder, of iron and unavoidable impurities, and comprising an anti-corrosion coating applied to the steel substrate, which coating is formed from, in wt %, up to 15% Si, up to 5% Fe, optionally up to 5 wt % of at least one alkaline earth metal or transition metal, and, as the remainder, from Al and unavoidable impurities, then
b) in the event that the anti-corrosion coating contains no or less than 0.1 wt % of the at least alkaline earth metal or transition metal: applying a solution containing at least one alkaline earth metal or transition metal to the anti-corrosion coating of the flat steel product,
c) flexible cold rolling of the flat steel product in order to produce the portions of different thickness on the flat steel product, then
d) heating the flexibly cold-rolled flat steel product to a hot-forming temperature of 800-1000° C. in an atmosphere containing more than 15 vol. % oxygen, over a holding period which is sufficient to introduce a thermal energy quantity Js of more than 44,000 kJs and at most 400,000 kJs into the flat steel product, so that, after heating, the surface of the anti-corrosion coating of the flat steel product is densely coated with a layer consisting of a primary oxide of the at least one alkaline earth metal or transition metal contained in the anti-corrosion layer and/or optionally additionally applied in step b), then
e) hot-forming the flat steel product to form the sheet metal component.
According to the invention, a flat steel product is thus provided which comprises an MnB steel substrate composed in a certain way and an Al-based anti-corrosion coating applied thereto, in particular by hot dip galvanizing. In hot dip galvanizing carried out in a conventional manner for purposes of the invention, the flat steel product is passed through a melt bath alloyed in accordance with the invention, and from the flat steel product emerging from the melt bath the thickness of the protective layer is adjusted by means of stripping nozzles. Air is used as the stripping medium. By applying the air jet and the resulting rapid temperature reduction, the oxide layer present on the anti-corrosion layer is “frozen”, i.e., it cannot develop in accordance with the chemical equilibrium rules.
The anti-corrosion coating of the flat steel product has a content of at least one alkaline earth metal or transition metal or, in step b), which is carried out if necessary, is wetted with a solution containing at least one such alkaline earth metal or transition metal. The solution used for this purpose in accordance with the invention is preferably an aqueous solution whose “water” is simple in terms of process engineering and is harmless with respect to the environment.
Step b) is necessarily carried out if the anti-corrosion coating contains an excessively low content of the at least one alkaline earth metal or transition metal. However, the wetting with the aqueous solution containing the at least one alkaline earth metal or transition metal can, of course, also take place as a supplementary measure if, although a basically sufficient amount of alkaline earth metal or transition metal is present in the anti-corrosion coating, further amounts of the at least one alkaline earth metal or transition metal are to be applied to the surface of the anti-corrosion coating in order to ensure the occurrence of the effect, which is used according to the invention, of the presence of these metals in or on the anti-corrosion layer.
The alkaline earth metal and transition metals alloyed to the anti-corrosion layer and/or applied to the surface of the anti-corrosion coating in the form of a solution for the purposes of the invention include, in particular, magnesium (“Mg”) and calcium (“Ca”) but also beryllium (“Be”), strontium (“Sr”) and barium (“Ba”).
The application of the solution containing the at least one alkaline earth metal or transition metal, which is carried out optionally necessarily or optionally additionally, can take place before or after the flexible rolling. It is essential that, prior to heating to the hot-forming temperature, a sufficient amount of the respective alkaline earth or transition metal is present in or on the anti-corrosion coating.
In step c), the flat steel product provided and optionally coated with the layer containing the at least one alkaline earth metal or transition metal is flexibly cold-rolled in a conventional manner at room temperature in order to provide it with the portions of different thicknesses.
In the case of flexible rolling, the flat steel product is rolled with rolling degrees W of 0.1 to 80%. The rolling degree W is determined according to the formula W=((U/Xn)−1)*100%, in which “U” denotes the starting thickness of the respectively rolled portion n before rolling and Xn denotes the thickness of the respective portion n after rolling. That is to say, with a starting thickness U of 2.75 mm in each case, a rolling degree W of 48.64% is required to produce a first portion with a thickness X1 of 1.85 mm, a rolling degree W of 10.00% is required to produce a second portion with a thickness X2 of 2.5 mm, a rolling degree W of 27.90% is required to produce a third portion with a thickness X3 of 2.15 mm, and a rolling degree W of 22.22% is required to produce a fourth portion with a thickness X4 of 2.25 mm. Rolling degree W which are particularly suitable in practice are 0.1-60%, in particular 0.1-50%. By varying the rolling degree W in the ranges mentioned, the portions of different thicknesses are produced on the flat steel product. The respective specifically set rolling degree W depends on the respectively desired extent of the reduction in the thickness of the flat steel product compared to the initial state. The range indicated here for the rolling degree W thus defines only the limits within which the respectively set rolling degrees are set according to the invention.
The flexible rolling reduces the thickness of the flat steel product in limited length portions in a targeted manner. Due to the constant volume, this decrease in thickness is inevitably accompanied by elongation of the flat steel product. The aluminum alloy of the anti-corrosion coating on a flat steel product processed according to the invention is so ductile that it can follow this deformation of the flat steel product taking place in the longitudinal and thickness direction even in the boundary regions at which the portions of different thickness abut one another.
However, the protective oxide layer on the anti-corrosion coating is substantially more brittle with the result that it tears locally due to the deformation of the flat steel product. The resulting cracks are rapidly closed again by means of newly forming oxides. Since this process takes place in an ambient atmosphere and without separate temperature supply or removal, the new oxide layer can form such that it corresponds to the chemical equilibrium at the location of the crack, taking into account the respective ambient conditions. Damage to the originally present oxide layer occurring during the flexible rolling is closed by new oxides which emerge in the course of the cold rolling, so that a tightly closed oxide layer is again present on the finished flexibly rolled flat steel product. This is characterized by regions in which the original oxide layer remained and regions in which a new oxide layer was formed.
According to the findings of the invention, there is a direct relationship between the respectively set rolling degree W and the proportions in which the flat steel product obtained after the flexible cold rolling is coated by original and newly formed oxide layers. Thus, the percentage area ratio A of the original oxide can be estimated with an accuracy of ±5% according to the formula A=100%−W. Accordingly, the percentage area ratio B of the surface of the flat steel product B obtained after the flexible rolling, which is covered by the newly formed oxide, is 100%−A±5%. For example, if rolled with a rolling degree W of 15%, the surface of a flat steel product which is hot-rolled according to the invention is 80-90% covered with the original oxide layer formed before the flexible rolling, while the remaining surface is covered with the new oxide layer formed in the course of the flexible rolling.
In the case of a flat steel product which is rolled flexibly in a manner according to the invention, there also exists a dependence of the ratio of the Si and Al contents of the oxide layer and of the ratio of the Al, Si and Mg contents of the oxide layer on the rolling degree W adjusted via the flexible rolling in each case. Thus, in the event, for example, that Mg as the at least one alkaline earth element or transition element is alloyed to the anti-corrosion coating of the flat steel product processed according to the invention or is applied onto said anti-corrosion coating, % A/% Si≥6.4×W−0.1 applies to the total oxide layer present on the flat steel product after the flexible cold rolling, whilst to the ratio % Al/% Mg≥(2.66×W0.11)±1 applies (where % Al=Al content of the total oxide layer in atomic %, % Si=Si content of the total oxide layer in atomic %, % Mg=Mg content of the total oxide layer in atomic %).
The original oxide layer present on the flat steel product processed according to the invention prior to the flexible cold rolling typically consists of silicon, magnesium and aluminum oxides, wherein the proportion of Si is substantially smaller than the proportion of Mg, which in turn is smaller than the proportion of Al. Thus, typically present in the oxide layer, and given in atomic %, are 10-40% C, 30-60% O, 4-30% Al, 0-5% Si and 1-20% of the at least one alkaline earth metal or transition metal, in particular Mg. In addition, small proportions of Fe of up to 10 atomic % can be present in the oxide layer. This applies in particular when the anti-corrosion coating has been applied by hot dip galvanizing. The thickness of the original oxide layer is typically 5-600 nm, in particular 5-300 nm, particularly preferably 5-150 nm. In this case, the original oxide layer covers the surface of the anti-corrosion coating completely, that is to say 100%.
The oxide layer which is newly formed via flexible cold rolling and which can form in equilibrium, likewise consists substantially of oxides of silicon, magnesium and aluminum. The quantitative distribution of the Si, Mg and Al oxides here corresponds to their distribution in the primary oxide layer. The secondary oxide layer typically consists of, in atomic %, 10-40% C, 40-60% O, 20-30% AI, 0-5% Si and 1-20% of the at least one alkaline earth metal or transition metal, in particular Mg, wherein small traces of iron of up to 10 atomic % can also be contained in the secondary oxide layer. The thicknesses of the secondary oxide layer are 1-100 nm, in particular 1-80 nm or 1-50 nm, wherein thicknesses of up to 30 nm have proven to be particularly favorable. The percentage area ratio Fox of the secondary oxide layer on the overall oxide layer, which covers the anti-corrosion coating of the flat steel product processed according to the invention after flexible cold rolling, is related to the rolling degree W, wherein Fox<W.
The compositions of the oxide layers can be determined by means of X-ray photoelectron spectroscopy (XPS). For this purpose, the respective sample of the flat steel product to be investigated, for which the composition and thickness are to be determined, is degreased with n-heptane, rinsed with propanol and blown off in air. The sample is then attached in each case to a sample carrier, introduced into the measurement chamber of the X-ray photoelectron spectroscope and investigated in a high vacuum. The tank pressure is typically less than 5×108 mbar. Argon is typically used as the bombardment gas. The radiation was excited as Al K with a bombardment voltage of 2 or 4 kV. At least one measurement is carried out on each sample with respect to the composition and oxide layer thickness. Typically, a plurality of samples of a blank are investigated and the results of all samples of the relevant blank are arithmetically averaged in each case. The composition and thickness thus determined of the oxide layer present on the respectively investigated blank is therefore also referred to as “average composition” or “average thickness”.
After the flexible rolling, the flat steel product is heated to a hot-forming temperature, wherein, if necessary, from the flat steel product present beforehand, for example in the form of a steel strip or larger metal sheet, at least one is partitioned off which is then further processed as a flat steel product according to the invention.
The composition of the anti-corrosion coating selected according to the invention and/or the additional application of the alkaline earth metal or transition metal to the anti-corrosion coating by means of the aqueous solution ensures that, as a result of the heat treatment carried out prior to the hot forming, a primary oxide layer formed from the at least one alkaline earth metal or transition metal is produced on the anti-corrosion coating.
The invention is based on the finding that, on a flat steel product which is provided with an aluminum-based (“Al-based”) anti-corrosion coating doped with at least one alkaline earth metal or transition metal according to the invention, during the heating carried out for hot forming an oxide layer (“primary oxide layer”) is formed on the anti-corrosion coating which protects the underlying layers of the anti-corrosion coating and thus the steel substrate of the flat steel product against exposure to the ambient atmosphere. The primary oxide layer in question is formed in such a way that it is in chemical equilibrium under the conditions prevailing during the heating and determined in particular by the respective hot-forming temperature. This process continues even during and after the hot forming. Any damage to the oxide layer present before heating and hot forming is thus closed very quickly. Due to the oxygen affinity of the elements Al, Mg and Si of the anti-corrosion layer as well, an oxide layer is formed immediately as soon as the surface of the anti-corrosion layer is also exposed to even the smallest amounts of oxygen. In this case, the reactivity of the alkaline earth metals or transition metals provided in accordance with the invention in and/or on the anti-corrosion layer guarantees that the oxides of the newly formed oxide layer are produced within such a short time that penetration of harmful substances from the surroundings is reliably prevented.
In this way, in the case of a flat steel product heated according to the invention to the respective hot-forming temperature, it is not just its steel substrate that is generally protected against corrosive attack. The oxide layer present on the anti-corrosion coating, and in particular formed from the alkaline earth metals or transition metals provided according to the invention, covers the underlying aluminum of the anti-corrosion coating, so that contact of the Al with the ambient moisture and, consequently, a separation of a larger quantity of hydrogen during the heating to the hot-forming temperature or the hot forming itself, is prevented. The penetration of larger amounts of hydrogen into the anti-corrosion coating and the steel substrate of a flat steel product processed according to the invention can thus be effectively suppressed.
The effects utilized by the invention occur particularly reliably when the alkaline earth metal or transition metal which is additionally present in the anti-corrosion coating or additionally applied to the anti-corrosion coating, is magnesium (“Mg”), i.e., if Mg alone or in combination with further elements belonging to the group of alkaline earth metals or transition metals is present in the contents provided according to the invention in the anti-corrosion coating provided according to the invention of a flat steel product processed according to the invention or is additionally applied by means of the aqueous solution when the content of alkaline earth metal or transition metal in the anti-corrosion coating is too small.
The method according to the invention is suitable for processing flat steel products with a large thickness spectrum. Thus, flat steel products whose thickness is 0.6-7 mm can be processed with the method according to the invention.
The production of the flat steel product provided in step a) in each case can be carried out in any manner known from the prior art. The method according to the invention is thus suitable in particular for processing flat steel products having a thickness of 0.8 to 4 mm, in particular 0.8 to 3 mm.
For the method according to the invention, flat steel products can also be provided in step a) which are formed from a stack of sheet metal comprising three to five sheet metal layers, for example, which have been joined to form a uniform flat steel product in a manner known per se, for example in the manner of roll cladding. Likewise, in step a) for the method according to the invention, flat steel products and steel strips composed from different sheet metal blanks welded together or the like in the manner of tailored blanks, which flat steel products and steel strips are welded together and together form the flat steel product to be processed, can be provided for the process according to the invention.
The flat steel product provided according to the invention consists of a steel which has a composition typical for MnB steels. Steels of this kind typically have yield limits in the delivered condition of 250-580 MPa and tensile strengths of 400-720 MPa.
Due to their property profile, in particular their potential to develop high strengths, of particular interest in practice are flat steel products whose steel substrate consists, in a manner known per se, of 0.07-0.4 wt % C, 1.0-2 wt % Mn, 0.06-0.4 wt % Si, up to 0.03 wt % P, up to 0.01 wt % S, up to 0.1 wt % Al, up to 0.15 wt % Ti, up to 0.6 wt % Nb, up to 0.005 wt % B, up to 0.5 wt % Cr, up to 0.5 wt % Mo, wherein the sum of the contents of Cr and Mo is at most 0.5 wt %, remainder iron and unavoidable impurities.
This includes steels already in series production which consist of 0.07-0.4 wt % C, 1.0-1.5 wt % Mn, 0.3-0.4 wt % Si, up to 0.03 wt % P, up to 0.01 wt % S, up to 0.05 wt % Al, up to 0.15 wt % Ti, up to 0.6 wt % Nb, up to 0.005 wt % B, up to 0.5 wt % Cr, up to 0.5 wt % Mo, wherein the sum of the contents of Cr and Mo is at most 0.5 wt %, and, as the remainder, iron and unavoidable impurities. Such composite steels achieve tensile strengths of up to 2000 MPa after the hot forming and cooling.
The prerequisite for the effects achieved according to the invention is the presence of at least one alkaline earth metal or transition metal in or on the Al-based anti-corrosion coating provided according to the invention. Thus, a sufficient amount of alkaline earth metal or transition metal can be alloyed to the anti-corrosion coating. The minimum contents of alkaline earth metal or transition metal required for this purpose in the anti-corrosion coating are 0.1 wt % and can range up to 5 wt %. In this case, alkaline earth or transition metal contents of the anti-corrosion coating of at least 0.11 wt % have proven to be particularly favorable with regard to the reliability with which the positive effects of the presence of the at least one alkaline earth metal or transition metal in the coating applied according to the invention can be utilized. If the alkaline earth metal or transition metal contents exceed 5 wt %, the oxide layer thickens and dust is formed, which should be avoided. In order to avoid this result particularly reliably, the content of alkaline earth metal or transition metal in the anti-corrosion coating applied in step a) can be limited in total to at most 1.5 wt %, in particular at most 0.6 wt %. In the case that, for the purposes of the invention, sufficiently effective alkaline earth metal contents or transition metal contents are contained in the alloy of the anti-corrosion coating present on the steel substrate of a flat steel product processed according to the invention, they are thus 0.1-5 wt. %, in particular 0.11-1.5 wt. % or, especially, 0.11-0.6 wt. %.
The optional application of the solution containing the respective alkaline earth metal or transition metal (step b)) can be carried out directly after the application of the anti-corrosion layer by means of spraying and squeezing or also by conventional coil coating. For this purpose, salt solutions with up to 200 g/l are used in practice.
The alkaline earth metals or transition metals can be present as sulfates, phosphates and nitrates or in oxide form as a dispersion of alkaline earth metal or transition metal oxide particles. Chloride should not be used due to the possibility of corrosive attack. Silicates can also be used. However, it should be noted here that these compounds can impede the further processing due to possible silicon bonding. Fluorine compounds are not suitable since they can react to form hydrofluoric acid when heated to the hot-forming temperature. It is also possible to use mixtures formed from compounds of the type described here and/or different alkaline earth metals or transition metals. In order to support the formation of the oxide layer to be produced according to the invention, the solution applied according to the invention to the surface of the anti-corrosion layer, if necessary, can additionally contain a network former, such as bismuth nitrate, and/or a wetting agent, such as a surfactant.
A separately performed drying treatment (“baking”) is normally not necessary.
The solution, which is applied if necessary, is preferably dried by utilizing the process heat. If, for example, step b) provided according to the invention is to be carried out inline in a plant for hot dip galvanizing, the application of the aqueous solution containing the at least one alkaline earth metal or transition metal can take place after the flat steel product has left the melt bath and the coating thicknesses have been set at a point at which the flat steel product treated in each case is still warm enough for the solvent of the solution to evaporate rapidly after contact with the surface of the flat steel product, i.e., the applied layer dries quickly.
As an alternative to an application incorporated into the process, the solution can also be applied, in an additional method step, on a conventional coil coating system.
A separate drying treatment can be expedient if it is to be ensured that the solution is dried before further processing. This is particularly true when water is used as solvent.
When using water as solvent, it should be ensured prior to coiling or stacking the flat steel product treated according to the invention that no residual water remains on the surface. For one thing, residual water could initiate corrosion processes. Moreover, there would be a risk that water coming into intensive contact with the aluminum surface would be split into oxygen and hydrogen, thereby increasing the risk of hydrogen absorption.
In order to bring about effective drying, either the flat steel product itself can be 100-250° C., in particular 100-180° C. warm when the solution containing the at least one alkaline earth metal or transition metal is being applied, or it can be subjected to a drying treatment at these temperatures. Typical drying times here are 0-300 s, in particular 10-60 s. Drying times of “0 s” are achieved when the flat steel product or its surroundings are so hot when the solution is applied that the respective solvent evaporates spontaneously, i.e., without a waiting time, when hitting the surface of the anti-corrosion layer.
In practice, the rule is that at least steps a) and c) are completed by the producer of the flat steel product and steps d) and e) of the method according to the invention are completed by the end processor, i.e., the customer of the producer of the flat steel product, wherein before or after step c), step b) can also be carried out in the factory of the manufacturer of the flat steel product. With regard to process economy, it can be expedient in this case to apply the solution containing the at least one alkaline earth metal or transition metal directly before the flat steel product enters the furnace provided for heating to the hot-forming temperature. In this variant, care should be taken to ensure that no solvent, in particular no water, enters the furnace. It should thus be ensured that the flat steel product coated according to the invention is completely dry when it enters the furnace. Otherwise, the moisture introduced into the furnace by the water could lead to a sharp increase in the moisture of the furnace atmosphere and thus to an undesired increase in dew point, which in turn would bring about the risk of increased hydrogen absorption via the hot forming process.
Optionally, silicon (“Si”), in contents of up to 15 wt %, in particular up to 11 wt %, can be present in the anti-corrosion coating of the flat steel product provided according to the invention in order to reduce the formation of an iron-aluminum phase. In this regard, Si contents of at least 3 wt %, in particular at least 8.5 wt %, prove to be particularly favorable, so that in the case of Si contents of 3-15 wt %, in particular 3-11 wt %, especially 8.5-11 wt %, the positive influences of Si can be utilized particularly reliably in practice. With contents of at least 3 wt % of Si, it is ensured that the alloy layer between the steel substrate and the anti-corrosion layer of a flat steel product according to the invention does not become too thick and optimal further processing properties are maintained.
Likewise optionally, Fe can be present in the anti-corrosion coating provided on a steel flat product according to the invention in contents of up to 5 wt %, in particular up to 4 wt %, especially up to 3.5 wt %. The Fe content is substantially achieved by diffusion of Fe from the steel substrate and contributes to optimal adhesion of the protective layer on the substrate. In this regard, Fe contents of at least 1 wt % prove to be particularly favorable, so that, in the case of Fe contents of 1-5 wt %, in particular 1-4 wt %, especially 1-3.5 wt %, the positive influences of the presence of Fe can be utilized particularly reliably in practice.
The anti-corrosion coating can be applied to the steel substrate of a flat steel product according to the invention in any known manner. For this purpose, hot dip galvanizing, also called “hot dip aluminizing”, is particularly suitable, in which the respective flat steel product is passed through a suitably heated melt bath which is composed according to the requirements of the invention relating to the composition of the anti-corrosion coating. Such hot dip galvanizing is particularly suitable for strip-shaped flat steel products having a thickness of up to 3 mm. With larger thicknesses, it is also possible to use one of the above-mentioned vapor deposition methods (PVD, CVD) in order to apply the anti-corrosion coating.
The coating weight of an anti-corrosion coating present on a flat steel product processed according to the invention is typically 30-100 g/m2, in particular 40-80 g/m2 per side of the flat steel product.
As already mentioned, the group of alkaline earth metals or transition metals, particularly Mg, has proven particularly suitable for the purposes of the invention. In this respect, Mg can be present alone or in combination with other alkaline earth metals or transition metals, such as the elements beryllium, calcium, strontium and/or barium, which are also mentioned above, in the coating applied according to the invention, in order to utilize the effects intended by the invention.
The flat steel product provided according to the invention is heated in step d) to a hot-forming temperature of 800-1000° C., in particular 850-950° C., and kept at this temperature until a sufficient amount of heat is introduced into the flat steel product or a blank separated therefrom. Hot-forming temperatures of 850-930° C. have been found to be particularly favorable in this case. The holding period and annealing temperature specifically required in each case can be estimated based on the requirement that the amount of heat energy Js introduced into the flat steel product or the blank in step d) should be more than 40,000 kJs and at most 400,000 kJs, wherein Js can be calculated according to the following known equation:
Js[kJs]=[(T2−T1)×c×t×m]/1000;
where T2: Final temperature of the component at the end of heating in K
Heating can be performed in any suitable manner. In the event that a conventional continuous furnace is used for this purpose, in which the flat steel product or the blank is heated by radiant heat, the suitable holding period is typically 100-900 s, in particular 100-600 s or, in a particularly practical manner, 180-600 s. Especially in the case where a hot-forming temperature of 850-930° C. is selected, holding periods of 180-600 s generally prove to be sufficient in practice.
Optionally, before the hot forming in combination with the heating to the hot-forming temperature or as a separate treatment step, pre-alloying of the anti-corrosion layer can be carried out. For this purpose, the flat steel product can be kept at temperatures of 650-1100° C. for a duration of 10-240 s, in particular 30-90 s.
The flat steel product heated in the manner according to the invention is fed, within a transfer time that is customary in practice, to a hot-forming apparatus in which the flat steel product is hot-formed to form the component (step e)).
The invention is explained in more detail below with reference to exemplary embodiments.
For nine tests V1-V9, conventionally alloyed MnB steel sheets A-F were provided, the compositions of which are indicated in Table 1.
Steel sheets each had a thickness D and had been provided in a conventional manner by hot dip galvanizing with an Al-based anti-corrosion coating. Five variants Z1-Z5 of such an anti-corrosion coating were used, the compositions of which are indicated in Table 2. As the alkaline earth metal or transition metal added according to the requirements of the invention, each of the anti-corrosion coatings Z1-Z5 contained the Mg content reported in Table 2.
The steel sheets A-F provided with one of the anti-corrosion coatings Z1-Z5 in each case were flexibly cold-rolled in a conventional manner, wherein a rolling degree W was achieved in each case via this cold rolling.
After the flexible rolling, the steel sheets A-F provided with one of the anti-corrosion coatings Z1-Z5 in each case were heated in a conventional continuous furnace to a hot-forming temperature of 850-930° C., wherein the holding period at the respective hot-forming temperature was varied such that a sufficient amount of energy EE was introduced into the respective sheet. In tests V4 and V6, the heating was carried out in two stages in order to initially bring about pre-alloying of the anti-corrosion coating. In all other tests V1-V3, V5 and V7-V9, heating was carried out in a single stage.
The sheet-metal samples A-F heated to the respective hot-forming temperature in this way were hot-formed in a conventional manner in a tool provided for this purpose to form a sheet metal component.
After the hot forming, the steel sheets obtained were cooled to room temperature at a cooling rate of 20-1000 K/s.
For tests V1-V9, Table 3 gives the steel of the steel substrate of the sheet steel used in tests V1-V9 in each case, the coating applied to the respective steel sheet in each case, the thickness D of the examined sheet-metal samples, the coating weight of the coating before heating to the hot-forming temperature, the amount of heat introduced during heating to the hot-forming temperature and the rolling degree W achieved via the flexible cold-rolling.
On the steel sheets obtained after the flexible cold rolling, the area ratio % OB of the newly formed oxide layer OB, which was produced on the anti-corrosion coating of the steel sheet processed in each case during the flexible cold rolling, was determined by means of XPS analysis on the oxide layer densely coating the surface of the steel sheet.
The remaining oxide layer present on the samples consisted in each case of the original oxide layer OA already present before the flexible cold rolling, the area ratio % GA of which was thus % OA=100%−% OB on the entire surface of the samples A-F densely covered by the oxide layer.
Likewise, the thicknesses D_OA of the original oxide layers OA present before the flexible rolling, the thicknesses D_OB of the oxide layers OB newly formed by the flexible rolling and present after the flexible rolling, and the thickness D_OP, present after the hot forming, of the oxide layer formed during the heating to the hot-forming temperature and present after the hot forming on the component obtained in each case, were determined by XPS measurement in each case. The relevant measurement results are summarized in Table 4.
Likewise, the compositions of the oxide layer present on the anti-corrosion coating prior to the flexible rolling, between the flexible rolling and the heating to the hot-forming temperature and after the hot forming, were determined for samples A-F by XPS measurement in each case.
Finally, the increase in the hydrogen content which passed into the steel sheet during the course of heating and cold rolling was also determined by XPS analysis.
The results of these investigations are summarized in Table 5. The small increase in the content of diffusible hydrogen proves the effectiveness of the oxide layer, which, as a result of the doping, with Mg, of the Al-based anti-corrosion coating provided according to the invention, is established on the one hand during flexible rolling and on the other hand with the heating to the hot-forming temperature, and damage sites which arise due to the flexible rolling or the hot forming close again within a very short time due to immediate renewed reaction of the Mg with the ambient oxygen, so that at most minimal amounts of hydrogen can penetrate into the anti-corrosion coating.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20194103.6 | Sep 2020 | EP | regional |
This application is the United States national phase of International Application No. PCT/EP2021/073759 filed Aug. 27, 2021, and claims priority to European Patent Application No. 20194103.6 filed Feb. 9, 2020, the disclosures of which are hereby incorporated by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/073759 | 8/27/2021 | WO |
