Patent Application
20100294400
The invention relates to a method for producing a steel component provided with a metallic coating which protects against corrosion, in particular by means of a cathodic protective effect, by hot forming a steel flat product produced from a low-alloy hot-treated steel. In addition, the invention relates to a steel component produced by hot forming a steel flat product and provided with a metallic corrosion protection coating which protects against corrosion, in particular by means of a cathodic protective effect.
When steel flat products are mentioned here, what are meant are steel strips, steel sheets or blanks obtained from these, as well as the steel substrate of the steel component obtained from such strips, sheets or blanks.
Currently in vehicle construction, increasingly demanding requirements are being set for the rigidity and strength of components. At the same time, however, a body weight which is as low as possible and correspondingly narrow material thicknesses are being aimed for in the interest of optimising the energy consumption required for driving the respective vehicle. These requirements, which seem at first sight to be contradictory, can be fulfilled by high-strength and ultra high-strength steel materials which, by applying suitable process steps, permit the production of components having a very high strength with a narrow material thickness.
A method which permits the production of correspondingly high-strength and at the same time thin-walled steel components is hot-press hardening. In hot-press hardening, firstly a blank is cut from a steel strip. This blank is then heated to a hot-forming temperature which generally is above the Ar3 temperature of the steel material being processed in each case. The blank heated in this way is then placed in the heated state into a forming tool and therein made into the component shape desired. Subsequently or meanwhile the formed component is cooled, in which a tempered or martensitic structure forms in the processed steel.
Low-alloy steels are considered for press mould hardening. However, these steels are sensitive to corrosive attack, to which they are particularly exposed when they are used for the construction of vehicle bodies.
More recently, various attempts have been made to make it possible for the advantages of hot forming high-strength steels which are suitable for hot-press hardening to be directly used for these areas of application too. The prior art described in EP 0 971 044 B1 must be cited as a precursor to this development. According to this known method, a hot-rolled steel sheet is processed which besides iron and unavoidable impurities contains (in % wt.) between 0.15-0.5% C, between 0.5-3% Mn, between 0.1-0.5% Si, between 0.01-1% Cr, less than 0.2% Ti, in each case less than 0.1% Al and P, less than 0.05% S and between 0.0005-0.08% B. The steel made according to this specification is in practice known by the name 22MnB5.
The steel strip made in this way is provided with a coating according to EP 0 971 044 B1, which is based on aluminium or an aluminium alloy. In particular, this coating is an AlSi coating which has Fe contents. The steel strip coated in this way is heated to a temperature of more than 750° C., formed into a component and then cooled at a cooling speed at which a martensitic structure forms.
The steel component produced in the way known from EP 0 971 044 B1 exhibits, in addition to good strength properties, fundamentally good resistance to corrosion. At the same time, steel components can be produced from the steel sheets provided according to this prior art in only one, single hot-forming step without the Al coating becoming damaged.
The steels processed in the known way and provided with an Al-based coating lack one significant property which cathodically protects the steel against corrosion when damaged. This susceptibility has proved to be problematic in particular when using the steels processed according to the known method for car bodies in the automotive industry.
In order to eliminate this disadvantage, it has been proposed in WO 2005/021820 A1, WO 2005/021821 A1 and WO 2005/021822 A1 to apply onto the steel substrate a coating based on zinc instead of an Al-based coating. Although sheet steel components produced from steel flat products coated in this way do have cathodic corrosion protection, it has to be accepted that the deforming of the respective steel flat product into the component has to be carried out in two stages, wherein the first stage is a cold deformation, in which by far the greatest part of the forming operation is carried out and in the course of the hot deformation stage only one more calibration of the component is possible followed by quenching. This results in this known process only being able to be used to a limited extent commercially.
An alternative attempt to enable components produced from steels of the kind described in EP 0 971 044 B1 to be used more effectively for use in car body construction is described in DE 103 33 166 A1. According to the method known from this published application, a steel component is produced in a way known from EP 0 971 044 B1 and subsequently coated with an additional zinc layer. Although the problem of cathodic corrosion protection is solved by means of this subsequent, single piece galvanising, an additional, subsequent coating step has to be accepted for that purpose, which not only leads to an increased expenditure of time in producing body components but also to increased costs.
Against the background of the previously explained prior art, the object forming the basis of the invention is to specify an economic method for producing high-strength steel components which have optimum corrosion protection and are particularly suitable for use in car bodies. In addition, a correspondingly obtained steel component should be created.
With regard to the method, this object is achieved according to the invention by performing the production steps specified in Claim 1 when producing a steel component. Advantageous embodiments of this method are specified in the claims referring back to Claim 1.
With regard to the steel component, this object is achieved according to the invention by such a steel component being formed according to Claim 27. Advantageous embodiments of this component are mentioned in the claims dependent on Claim 27.
According to the invention, a metallic coating is produced on a steel flat product produced from a low-alloy heat-treated steel, which metallic coating is formed from two layers applied in two process steps following one another. The heat-treated steel can for example be an Mn—B-steel, as is already used in many cases in the prior art.
According to the invention, in the first production step the steel flat product produced from the heat-treated steel made in a suitable manner is provided with an Al coating which contains at least 85% wt. Al, wherein additionally contents of up to 15% wt. in the Al coating applied according to the invention can be present. Typical variants of the Al coating applied according to the invention are a coating consisting almost entirely of Al, or an AlSi variant in which the Si content of the AlSi coating applied is 8-12% wt. Si.
Subsequently, a Zn coating is applied onto this Al coating which consists of at least 90% wt. of zinc.
Before being formed into the respective component, the steel flat product coated with two layers in this way is then heated to a hot-forming temperature which is at least 750° C. A base layer is formed alloyed from Al, Fe, Zn and Si, in which the greatest proportion is Al, but Fe, Zn and Si also emerge as significant constituents. In practice, according to the invention, hot-forming temperatures of 850-950° C., in particular 850-900° C. are typically set.
The steel flat product heated to the hot-forming temperature is hot formed into the respective component in a further production step in a way known per se and cooled in a in an accelerated manner for the desired tempered or martensitic structure to form.
The production steps listed previously are the procedures which are at least required to achieve the success obtained according to the invention. Of course, additional steps can be provided if this proves necessary from the production-related point of view.
In this way, for example, splitting the flat product, present as strip beforehand and coated with two layers in the way according to the invention, into blanks can precede heating to the hot-forming temperature. Moreover, cleaning the surface of the steel flat product, and the coating applied onto it, can in each case precede the individual coating steps.
Surprisingly, it has firstly been shown that the steel flat product coated in the way according to the invention can be formed without any difficulty into a steel component. Thus, steel flat product coated according to the invention has proved itself suitable both for a direct hot-forming operation, i.e. carried out as a single-stage production step without the preceding cold-deformation, and for an indirect, i.e. at least two-stage, forming operation characterised by cold deformation followed by hot deformation.
After the hot-forming operation, carried out in each case, with a steel component according to the invention a zinc-alloyed surface having a zinc content of at least 60% wt., in particular at least 80% wt. is present. Cathodic corrosion protection results from this, which can be proved electrochemically in unequivocal terms. Thus, in the accelerated corrosion test (salt spray test) it can be proved that coatings produced according to the invention have a resistance to corrosion which are comparable to pure zinc coatings.
Additional advantages arise in relation to corrosion protection from the fact that up to 30% Al can be present in the top layer of the steel component obtained according to the invention.
The base layer of the total metallic coating produced according to the invention, which base layer has a high proportion of Al and is arranged between the Zn dominated top layer and the respective steel substrate, protects this coating from an excessive diffusion of zinc and iron during the heat treatment at the hot-forming temperatures chosen according to the invention preferably in the range from 750 to 900° C., in particular 850 to 900° C. Advantages of the barrier effect of the base layer are, on the one hand, delayed red rust formation on the surface and, on the other hand, the base layer preventing zinc from being able to reach the grain boundaries of the steel substrate, which would result in the risk of crack formation during hot forming. The Al—Fe—Zn—Si-containing base layer of the total coating produced according to the invention in addition protects the steel substrate particularly effectively against oxidation with the ambient oxygen.
With the approach according to the invention, a particularly commercially feasible opportunity is thus provided for producing components, which are corrosion-protected in an optimum way, from high-strength steels which can be hot-press formed.
A steel component obtained in the way according to the invention comprises the previously summarised findings following a metallic coating which is formed by a base layer lying on the steel flat product and by a top layer lying on the base layer, wherein the base layer contains at least 30% wt. Al, at least 20% wt. Fe and at least 3% wt. Si and the top layer has at least 60% wt. Zn, in particular at least 80% wt., and at least 5% wt. Al, as well as up to 10% wt. Fe and up to 10% wt. Si.
The Al coating can be applied by hot-dip aluminising as the first coating layer onto the respective steel flat product particularly economically and with an optimum coating effect at the same time.
The Zn coating can then also be applied by hot-dip galvanising onto the Al layer, which was previously applied onto the steel flat product, particularly economically in a comparable way known per se and proven and tested in practice.
Particularly successful coating results can, moreover, be obtained if the Zn coating, as an alternative to hot-dip galvanising, is deposited electrolytically on the Al coating. With electrolytic galvanising, preferably a layer having a Zn content of at least 99% wt. is deposited.
A further alternative possibility for applying the Zn layer is depositing the Zn coating on the Al coating in a PVD process. The use of the PVD process (PVD=Physical Vapour Deposition) for applying the Zn layer permits the thickness of the layer to be set particularly precisely.
With hot-dip galvanising and with application by means of the PVD process, in addition to Zn at least one further element from Al, Mg or Fe can be included. Advantageously, the contents of 5% wt. Al, 5% wt. Mg and/or 0.5% wt. Si should not be exceeded.
The contents of further accompanying elements in the Zn coating, such as for example, Pb, Bi, Cd, Ti, Cu, Cr or Ni, should not in total exceed 1% wt.
In order to adjust a surface roughness, improving the wettability and binding of the Zn coating applied subsequently, it can be advantageous to subject the steel flat product provided with the Al coating to a skin-pass rolling before the Zn coating is applied.
For the same purpose, it can be advantageous to pickle the steel flat product provided with the Al coating before the Zn coating is applied. With pickling, the flat products coated according to the invention are routed through an acid bath which rinses off the oxide layer from them without attacking the surface of the steel flat product itself. By means of the pickling step carried out in a targeted manner, the oxide deposition is controlled, so that a surface is obtained which is advantageously adjusted for electrolytic strip galvanising.
In some cases, in particular when carrying out the production steps in a discontinuous way, it is advantageous to additionally carry out alkaline cleaning before the pickling.
The method according to the invention can be carried out particularly economically if the Al coating and subsequently the Zn coating and all production steps required between the respective coating steps are completed in a sequence of operations which are performed following continuously one after the other.
If a corresponding systems engineering is not available, or if it proves advantageous for special reasons, then it also possible, however, to apply the Al coating and subsequently the Zn coating in a broken, discontinuous operation.
The particular advantage of the invention, as already mentioned, is that the steel flat product can be formed into the steel component in a single hot-forming step. Thus, steel strip coated according to the invention proves to be particularly unsusceptible to stresses and strains occurring in a pass during hot forming, even when the respective component obtains a complex form.
It is also, however, equally possible to carry out forming of the flat product coated according to the invention in multiple stages, wherein in each case at least one forming stage is carried out as a hot-forming step which follows on from heating to hot-forming temperature. Correspondingly, when this proves advantageous from the production-related point of view, the steel flat product can pass through at least one cold-forming step before heating to the hot-forming temperature. In doing so, deformation can almost completely take place during cold forming, so that in this case the hot-forming step carried out after cold forming rather acts in the tool as a hot-calibration step with subsequent quenching.
Particularly good results arise if, before heating to the hot-forming temperature, the Al coating applied onto the steel flat product has a thickness of 5-25 μm, in particular 5-15 μm, and if the Zn coating applied onto the AlSi coating, before heating to the hot-forming temperature, has a thickness of 2-10 μm. Tests have shown thereby that, especially if the AlSi coating is applied by hot-dip aluminising, before heating to the hot-forming temperature a 2-5 μm thick alloy barrier layer containing Al, Si and Fe is present between the steel flat product and the correspondingly applied AlSi coating. By taking the previously mentioned thicknesses of its individual layers into account, a metallic coating applied according to the invention in two production steps onto the flat product to be deformed typically has a total thickness of 7-35 μm.
As explained, with a component finish-formed or obtained according to the invention, a base layer is present, lying directly on the steel flat product and predominantly consisting of Al and additional contents of Fe, Zn and Si, on which a top layer predominantly consisting of Zn and additional contents of Al, Si and Fe lies. The base layer has, thereby, at least 30% wt. Al, at least 20% wt. Fe, at least 3% wt. Si and at most 30% wt. Zn, while in the top layer at least 60% wt., in particular at least 80% wt. Zn, at least 5% wt. Al, as well as a maximum of 10% wt. Fe and a maximum of 10% wt. Si are present.
The thickness of the base layer of the finish-formed component according to the invention is typically 10-50 μm, in particular 15-25 μm, while the thickness of the top layer is typically in the range from 5-20 μm, in particular 3-10 μm.
Subsequently, the invention will be explained in more detail by means of exemplary embodiments. These show in:
In
In the method specified in
In the example specified in
Finally,
The left half of
According to this, an alloy layer is formed between the steel substrate and the AlSi layer lying above it which typically contains 90% wt. and 10% wt. Si, which alloy layer contains Al, Si and Fe. The AlSi layer (“first layer”) and the alloy layer together form the “base layer” of the total coating. The Zn layer (“second layer”) is applied onto the “base layer” as the “top layer”, which typically consists of 99% wt. Zn and less than 1% wt. Al.
In the right half of
The cathodic protective effect of the coating produced according to the invention and obtained after heating to a hot-forming temperature of 880° C. has been substantiated by means of equilibrium rest potential measurements, the results of which are summarised in
A large number of tests were carried out to verify the method according to the invention, three of which, by way of example, are explained below:
Test 1:
A steel strip consisting of a temperable steel having a carbon content of 0.22%, an Mn content of 1.2%, a Cr content of 0.20% and a B content of 0.003% was annealed as a cold-rolled strip in a way known per se in a continuous hot-dip coating line and coated with an AlSi melt. In addition, the strip was firstly cleansed in a cleaning section of the dirt residues from the cold-rolling process and then passed through an annealing oven as it was heated to 750° C.
At this temperature, the strip was annealed in a re-crystallising way in the annealing oven in a protective gas atmosphere with 10% H2 and remainder N2.
After cooling to a temperature of 680° C. (also still under protective gas 10% H2, remainder N2) the strip entered an aluminium bath having a temperature of 660° C. In addition to Al, the aluminium bath also contained approx. 10% wt. silicon.
After withdrawing the strip from the melt bath, a coating thickness of 18 μm was set by means of a jet wiping system.
After cooling the strip to <50° C., the surface roughness of the strip provided with the AlSi coating was set by skin-pass rolling in a skin-pass mill stand.
In a subsequent section of the production line, the strip was then firstly chemically treated in an aqueous solution with 80 g/l HCl (hydrochloric acid) for 10 s at 40° C.
Afterwards, the electrolytic deposition of 7 μm zinc from a zinc sulphate electrolyte was carried out in electrolysis cells at a current density of approx. 50 A/dm2 and at an electrolyte temperature of approx. 60° C. on the surface of the AlSi coating.
Finally, the strip was wound into a finished coil.
Blanks were cut from the coated strip and firstly cold preformed in a forming press. The preformed parts were then heated in an oven to a hot-forming temperature of 880° C. for 5 min. The coating composition present before heating to the hot-forming temperature is illustrated in
Subsequently, the heated blanks were conveyed into a hot-forming press by means of a manipulator and formed there into a finished component and quickly cooled in a known way in the tool. In
Test 2:
A steel strip consisting of a temperable steel was annealed and coated as a cold-rolled strip in a continuous hot-dip coating line. The strip was firstly, as in Example 1, cleaned and annealed. Subsequently, it passed through an aluminium-silicon bath (Si proportion 10%) the temperature of which was 660° C. The thickness of the AlSi coating obtained, subsequently set by means of wiping jets, was 15 μm. After a cooling section, over which the strip is cooled to 480° C., the strip was dipped into a second melt bath of zinc, which was provided with a supplement of 0.2% Al. With the wiping jets that followed, a zinc coating thickness of 5 μm was set. After cooling the strip to <50° C., the surface roughness was set in a skin-pass mill stand. Finally, the strip was wound into a finished coil.
From the strip coated in this way with a first AlSi layer and a second Zn layer applied onto it, blanks were cut for the hot-forming process and heated in an oven to 900° C. for 5 min. Subsequently, the blanks were conveyed into a forming press by means of a manipulator and formed here into a component and cooled in the tool.
Corrosion protection properties could also be substantiated for the component obtained in such a way, which corresponded to the properties which were determined for the component produced according to Test 1.
Test 3:
A steel strip consisting of a temperable steel was annealed and coated as a cold-rolled strip in a continuous hot-dip coating line. The strip was firstly, as in Example 1, cleaned, annealed and provided with an AlSi coating. The coating thickness set by the wiping jets in this case was 20 μm. After cooling the strip to <50° C., the surface roughness was set by skin-pass rolling in a skin-pass mill stand.
In a subsequent section passed through, the strip was firstly given an alkaline cleaning, so that it could be subsequently coated with a zinc coating of 3 μm in a PVD module. Finally, the strip was wound into a finished coil.
From the strip coated in this way, blanks were cut for the hot-forming process and heated in an oven to 900° C. for 5 min. Subsequently, the blanks were conveyed into a forming press by means of a manipulator and formed here into a component and cooled in an accelerated manner in the tool.
Corrosion protection properties could also be substantiated for the component obtained in such a way, which corresponded to the properties which were determined for the component produced according to Test 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07117719.0 | Oct 2007 | EP | regional |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/63139 | 10/1/2008 | WO | 00 | 8/4/2010 |
