EP4636119A1 - Method for producing hardened steel components having a conditioned zinc corrosion protection layer - Google Patents

Abstract

The invention relates to a method for producing hardened steel components, wherein a blank is cut out of a galvanized strip made of a hardenable steel alloy and the blanks are then cold formed into a component blank and then heated to a temperature which causes a structural change towards austenite, wherein the austenitized component blank is then fed to a form-hardening tool in which the component blank is held in a form-fitting manner by means of an upper and lower tool which have a shape which essentially corresponds to the component blank, wherein the contact of the material of the component blank against the particularly cooled tools extracts the heat from the steel material so quickly that martensitic hardening occurs, wherein after the galvanizing of the metal strip and before the temperature is increased for the purpose of austenitization, an aqueous potassium hydroxide solution and/or lithium hydroxide solution is applied to the surface of the strip or the blank or the component blank.

Description

The invention relates to a method for producing hardened steel components with a conditioned zinc alloy corrosion protection layer.

It has long been known to provide metallic sheets, especially metallic strips, which could corrode under normal conditions of use, with protective coatings.

In general, corrosion protection layers on metal strips can be organic coatings, for example paints, although these paints may also contain corrosion-inhibiting agents.

In addition, it is known to protect metal strips with metal coatings. Such metal coatings can consist of an electrochemically more noble metal or a less electrochemically noble metal.

A coating made of an electrochemically nobler metal or a self-passivating metal, such as aluminum, is referred to as a barrier protective layer. For example, when aluminum is applied to steel, the steel material will suffer corrosion if this barrier protective layer is no longer present in some areas, for example, due to mechanical damage. A common barrier protective layer for steel is the aforementioned aluminum layer, which is usually applied by hot-dip coating.

If an electrochemically less noble metal is applied as a protective layer, this is referred to as a cathodic corrosion coating because, if the corrosion protection coating is mechanically damaged down to the steel material, the electrochemically less noble metal is corroded first before the steel material itself is exposed to corrosion.

The most commonly used cathodic protective coating on steel is a zinc coating or a zinc-based alloy.

Various galvanizing processes are known. A common galvanizing process is hot-dip galvanizing (also known as hot-dip galvanizing). This involves immersing steel continuously (e.g., strip and wire) or piece by piece (e.g., components) into a melt of liquid zinc at temperatures of approximately 450°C to 600°C (the melting point of zinc is 419.5°C). The molten zinc, for example, has a zinc content of at least 98.0 wt.% according to DIN EN ISO 1461. A resistant alloy layer of iron and zinc forms on the steel surface, covered by a firmly adhering pure zinc layer whose composition corresponds to the molten zinc. In a continuously galvanized strip, the zinc layer has a thickness of 5 µm to 40 µm. For a piecewise galvanized component, the zinc layer can have thicknesses of 50 µm to 150 µm.

In electrolytic galvanizing (galvanic galvanizing), steel strips or steel plates are immersed in a zinc electrolyte rather than in a molten zinc bath. The steel to be galvanized is placed in the solution as the cathode, and an electrode made of the purest possible zinc is used as the anode. A current is passed through the electrolyte solution. The zinc, which is present in ionic form (oxidation state +II), is reduced to metallic zinc and deposited on the steel surface. Compared to hot-dip galvanizing, thinner zinc layers can be applied with electrolytic galvanizing. The zinc layer thickness is proportional to the strength and duration of the current flow, resulting in a layer thickness distribution across the entire workpiece, depending on the workpiece and anode geometry.

To ensure the adhesion and uniformity of the zinc coating, careful surface pretreatment is required. This may include degreasing, alkaline cleaning, pickling, rinsing, and/or pickling. After galvanizing, one or more post-treatments can be performed, such as phosphating, oiling, or the application of organic coatings (e.g., cataphoretic dip painting, or KTL for short).

Usually, not only pure metal layers are deposited. There are also a variety of known alloys that are deposited. In addition to pure aluminum coatings, there are also coatings that contain aluminum and zinc, and coatings that contain small amounts of aluminum in addition to a predominantly zinc content. Contains zinc, nickel, chromium, magnesium, and other elements, as well as mixtures thereof. If the application refers to zinc corrosion protection coatings or galvanized steel strip, zinc-based alloys are also included.

Furthermore, it has long been known that, particularly to reduce the weight of vehicle bodies, at least parts of them can be made high-strength to ensure sufficient strength in the event of a crash. The weight savings are achieved by using high-strength steel grades with comparatively thin wall thicknesses, thus resulting in low weight.

Even when using high-strength steel grades, there are different approaches and a wide variety of steel grades that can be used.

Steel grades that become high-strength through quench hardening are often used. Quench hardening means selecting a cooling rate above the respective critical cooling rate to adjust the microstructure. This critical cooling rate is approximately 15 to 20 Kelvin per second, but can also be lower depending on the alloy composition. A common steel grade that can be hardened through quench hardening is the so-called boron-manganese steels, such as the most commonly used 22MnB5, but also derivatives of this steel, such as 20MnB8 and 30MnB5. Non-hardenable steels, such as microalloyed steels, can also be hot-formed using the direct or indirect process.

Such steel grades can be easily formed and cut in the unhardened state.

There are essentially two different processes for shaping and hardening such steel grades, particularly in car body construction.

The first and somewhat older process is press hardening. In press hardening, a flat blank is cut from a steel sheet strip made of a quench-hardenable steel alloy, such as 22MnB5 or a similar manganese-boron steel. This flat blank is then heated to such a high temperature that the steel microstructure takes on the appearance of gamma iron, or austenite. To achieve this microstructure, the so-called austenitizing temperature Ac ₃ must be exceeded, at least if complete austenitization is desired.

Depending on the steel, this temperature can be between 820°C and 900°C, with such steel blanks, for example, being heated to approximately 900°C to 930°C and kept at this temperature until the structure has completely changed.

Such a steel blank is then transferred in its hot state to a press, where, using an upper tool and a lower tool, each of which is shaped accordingly, the hot steel blank is formed into the desired shape with a single press stroke. The contact of the hot steel material with the comparatively cool, particularly cooled, press tools (i.e., forming tools) extracts energy from the steel very quickly. In particular, the heat must be extracted so quickly that the so-called critical hardening rate is exceeded, which is typically between 15° and 25° Kelvin per second.

If cooled this quickly, the austenite structure does not revert to its original ferritic structure, but rather a martensitic structure is achieved. Due to the fact that austenite can dissolve considerably more carbon in its lattice than martensite, carbon precipitation leads to lattice distortion, which leads to the high hardness of the final product. The rapid cooling stabilizes the martensitic state, so to speak. This makes hardnesses and tensile strengths R _m of more than 1500 MPa achievable. Hardness profiles can also be adjusted using suitable measures, which will not be discussed in detail here, such as complete or partial reheating.

Another, somewhat newer method for producing hardened steel components, particularly for car body construction, is hot stamping, developed by the applicant. In hot stamping, a flat steel blank is cut from a steel strip, and this flat steel blank is then cold-formed. This forming process is not performed with a single press stroke, but rather, as is usual in conventional press lines, for example, in a five-stage process. This process allows for significantly more complex shapes, so that the final product can be a complexly shaped component, such as the B-pillar or a longitudinal member of a motor vehicle.

To subsequently harden such a finished component, this component is also austenitized in a furnace and transferred in the austenitized state into a mold, whereby the mold has the contour of the final component. Preferably, the preformed component is shaped before heating in such a way that after heating and thus also after thermal expansion, this component already largely corresponds to the final dimensions of the hardened component. This austenitized blank is
The component is placed in the austenitized state into the mold, and the mold is closed. Preferably, the component is contacted and clamped by the mold on all sides, and heat is also removed through contact with the mold, creating a martensitic structure.

In the clamped state, shrinkage cannot occur, so that the hardened final component with the corresponding final dimensions can be removed from the mold after hardening and cooling.

Since motor vehicle bodies usually have a corrosion protection coating, with the corrosion protection layer closest to the metal material forming the body, in particular steel, being a metallic coating, corrosion protection coatings for hardened components have also been sought and developed in the past.

However, corrosion protection coatings for components to be hardened are subject to different requirements than corrosion protection coatings for components that are not hardened. The corrosion protection coatings must be able to withstand the high temperatures generated during hardening. Since it has long been known that hot-dip aluminized coatings can withstand high temperatures, press-hardened steels with a protective layer of aluminum were initially developed. Such coatings are capable of withstanding not only the high temperatures but also hot-dip galvanizing. A disadvantage, however, is that hot-dip galvanizing is not typically used on conventional steel grades in motor vehicles, and it is fundamentally problematic to use different corrosion protection systems, especially when there is a risk of contact corrosion.

Therefore, the applicant has developed processes that make it possible to provide zinc coatings that also withstand such high temperatures.

In general, zinc coatings are considerably easier to form than aluminum coatings, as aluminum coatings tend to chip or crack at conventional forming temperatures. This does not happen with zinc.

However, it was initially expected that zinc coatings would not be able to withstand the high temperatures. However, special zinc coatings containing a certain proportion of oxygen-affine elements are capable of being processed even at high temperatures, as the oxygen-affine elements quickly diffuse to the air-side surface, where they oxidize and form a glassy protective film for the zinc coating. Such zinc coatings have now become established, particularly for hot stamping. Such zinc coatings can also be used with great success in press hardening.

In order to ensure optimal paint adhesion and optimal weldability, it is known to clean the finished formed and hardened components in such a way that the glass-hard, protective film layer is leveled or removed.

From the DE 10 2010 037 077 B4 A method for conditioning the surface of hardened, corrosion-protected components made of sheet steel is known. The steel sheet is coated with a metallic coating and is hardened by heating and quenching. After hardening, the oxides present on the corrosion protection coating as a result of heating are removed. The component is subjected to vibratory grinding to condition the surface of the metallic coating, i.e., the corrosion protection layer. The corrosion protection coating is a zinc-based coating, and the surface conditioning is carried out in such a way that oxides lying on or adhering to the corrosion protection layer are ground away and, in particular, microporosity is exposed.

From the DE 10 2007 022 174 B3 A method for producing and removing a temporary protective layer for a cathodic coating is known, whereby a steel sheet made of a hardenable steel alloy is provided with a zinc coating using a hot-dip process, whereby the aluminum content in the zinc bath is adjusted so that a superficial oxide skin of aluminum oxide forms during the melt hardening, whereby this thin skin is blasted off or leveled after hardening by irradiating the sheet metal component with dry ice particles. An example of this is the EP 1 630 244 B1 or EP 2 233 508 B1 .

Such protective layers usually only occur with zinc coatings, while aluminum coatings often do not require cleaning or only require less complex cleaning.

From the WO 2018/126471 A1 Sol-gel preconditioning of the coating to reduce oxide layer formation and increase weldability is known. This is intended to create an oxidation protection coating for press-hardening steel materials, based on silane- and titanium-containing binders and oxide pigments, which are apparently applied in a sol-gel process. In particular, solvents such as methanol are used here, which are not suitable for use in steel production plants. The coating is intended to After press hardening, the coatings fall off on their own, although tests with titanium and silicon-based coatings conducted in 2015/16 were unsuccessful with either thick or thin wet films. The coating neither falls off on its own, nor is it suitable for industrial welding.

From the EP 2 536 857 B1 A ceramic-based coating with a thickness of ≤ 25 µm is known, which is said to consist essentially of SiO2, _Al2O3 , and MgO2, possibly containing _metallic tin fibers. It was discovered that such a coating renders the sheet metal unweldable and also causes paint delamination.

From the EP 4 110 972 B1 A coating of the galvanized surface with tin is known, which makes removal unnecessary due to the formation of a mixture of the oxide layer of tin and zinc oxide during the hardening process.

The object of the invention is to create a method for producing hardened steel components in which an existing zinc corrosion protection layer is conditioned in such a way that cleaning of the surface, and in particular cleaning with fluid and/or particle jets after hardening, can be eliminated. Particularly preferably, the method should be able to dispense with the use of tin or tin-containing solutions.

The problem is solved by a method having the features of claim 1.

Advantageous further training is indicated in the dependent subclaims.

A further task is to create a galvanized steel strip which is designed in such a way that the removal of an oxide layer is unnecessary.

The problem is solved with a galvanized metal strip having the features of claim 13.

Advantageous further training is indicated in the dependent subclaims.

According to the invention, it was recognized that under certain circumstances, cleaning of the surface of a metal strip that is galvanized and has been subjected to a temperature increase for the purpose of a structural change can be dispensed with. In particular, Mechanical cleaning of a galvanized steel sheet and a hardened component produced from it can be eliminated.

While post-treatment cleaning is a manageable and well-established process, it does involve increased labor. There is also a risk of additional surface defects, which also results in higher overall costs. For very thin components, it has been shown that, under certain circumstances, the dimensional accuracy of the components can be compromised.

If there are interconnected process sequences which require these cleaning steps to be arranged inline within an entire production process, an adjustment of the cycle time may be necessary.

According to the invention, a steel sheet plate or a steel strip is used which has a zinc-based coating.

This layer can advantageously have a thickness of 5 µm to 20 µm per side. This can ensure good corrosion protection. In particular, the coating can be a Z40, Z60, Z80, Z120, Z140, or Z180 according to DIN EN 10346.

Zinc-based corrosion protection layers can have a comparatively high zinc content of 85 wt.% to 99.8 wt.%, in particular 95 wt.% to 99.5 wt.%, preferably 98 wt.% to 99.5 wt.%, and in addition to unavoidable impurities also contain aluminum in the range of 0.2 to 2 wt.%.

Particularly preferably, the zinc-based metallic corrosion protection layer can be applied using a hot-dip galvanizing process. This can be a simple and robust application method.

According to the invention, it has been found that a surface treatment of the galvanized surface prior to the hot stamping process is successful in adjusting the phosphating, paintability, and weldability. According to the invention, the oxide growth during the hardening process can be designed in such a way that subsequent mechanical surface conditioning, such as centrifugal blasting, vibratory grinding, or Dry-egg blasting is unnecessary, as is treatment with tin or tin-containing solutions.

According to the invention, it has been found that, surprisingly, lithium and/or potassium apparently modify the surface in such a way that any kind of cleaning is unnecessary.

In particular and surprisingly, it has been shown that if the concentration of the solution to be applied satisfies the following relationship in g/l: [c] = [KOH] + 2 * [LiOH · H ₂ O], where the concentration of the lithium solution [LiOH · H ₂ O] is between 0 and 100 g/l and the potassium solution [KOH] is between 0 and 200 g/l, where the total concentration [c] is between 30 and 200, a particular effectiveness develops. Preferably, the total concentration [c] can be set between 50 and 120 to further increase the effectiveness.

According to the invention, in particular an aqueous alkaline solution is applied by means of, for example, a roll coater to a galvanized surface after skin rolling and before cold forming or annealing and hardening process.

Very thin layer thicknesses are used, which are 0.5-3 µm in aqueous solution, in particular 0.5 - 1.5 µm, and 50-300 nm thick when dried, in particular 75 - 125 nm, in particular 80 - 100 nm.

The solution can also be applied by dipping and squeezing or spraying or other application methods.

Of course, all other methods that allow liquid ionic solutions to be applied to a surface are also suitable.

For a layer thickness in the aqueous state of 1 µm, the coverage meets the following relationship in mg/m ² :

[c] = [KOH] + 2 * [LiOH H2O],

[LiOH ·H2O] = 0-100 mg/m ² ; [KOH] = 0-200 mg/ ^m2

[c] = 30 to 200, preferably 50 to 120

The potassium occupancy when using potassium hydroxide KOH is 1-140 mg potassium hydroxide per m ² , in particular 30-100 mg/m ² potassium hydroxide. The lithium occupancy when using LiOH is 1-40 mg lithium hydroxide per m ² , in particular 15-35 mg/m ² , and in particular 20-30 mg/m ² lithium hydroxide.

Surprisingly, it has been shown that a combination of potassium hydroxide and lithium hydroxide can exhibit particularly favorable properties with regard to the weldability of the treated steel sheet or steel component. According to the invention, it has been found that, with a typical annealing time for sheets to be hardened, the surface resistance is very low, and even a paint penetration test revealed only a very low tendency for paint penetration. Visually, considerably fewer oxides are detectable, which is indicated by a metallic luster of the annealed sheet. Such silveriness is usually problematic, as it indicates a lack of complete reaction. Investigations have shown that the zinc-iron crystals of the zinc layer have completely reacted. Furthermore, good formation of the phosphate crystals was observed during the phosphating process.

Treatment with potassium and/or lithium hydroxide causes an increase in emissivity in the first 780°C until the gamma phase decays.

Overall, it is not yet possible to say how the K/Li solution works in detail, but the effect is surprising and absolutely clear.

The invention thus relates in particular to a method for producing hardened steel components, wherein a blank is cut out of a strip of a hardenable steel alloy coated with a zinc-based coating, and the blanks are then cold formed into a component blank and subsequently heated to a temperature that causes a structural change towards austenite, wherein the austenitized component blank is then fed to a form-hardening tool in which the component blank is held in a form-fitting manner by means of an upper and lower tool which have a shape substantially corresponding to the component blank, wherein the contact of the material of the component blank against the particularly cooled tools extracts heat from the steel material so quickly that cooling at a cooling rate above the critical cooling rate leads to martensitic hardening, wherein after the galvanizing of the metal strip and before the temperature increase for the purpose of austenitization, an aqueous potassium hydroxide solution and/or lithium hydroxide solution is applied to the surface of the strip or the blank or the component blank. becomes.

An advantageous further development provides that the concentration of the solution to be applied satisfies the following relationship in g/l:

[c] = [KOH] + 2 * [LiOH H2O],

[LiOH·H ₂ O] = 0-100 g/l; [KOH] = 0-200 g/l

[c] = 30 to 200, preferably 50 to 120

An advantageous further development provides that the coverage with a layer thickness of the solution to be applied of 1 µm satisfies the following relationship in mg/m ² :

[c] = [KOH] + 2 * [LiOH H2O] ,

[LiOH ·H2O] = 0-100 mg/m ² ; [KOH] = 0-200 mg/m ²

[c] = 30 to 200, preferably 50 to 120

An advantageous further development provides that the potassium and/or lithium is applied from an alkaline solution.

An advantageous further development provides for the application of an aqueous potassium and/or lithium solution which is alkaline.

An advantageous further development provides that the alkaline or basic aqueous solution is applied with a layer thickness of 0.5 - 3 µm, in particular 0.5 - 1.5 µm, wherein the dried layer thickness is 50 - 300 nm, in particular 75 - 125 nm, in particular 80 - 100 nm.

An advantageous further development provides that the potassium occupancy is 1 - 140 mg/m2 potassium hydroxide, and in particular 50 - 100 mg/m2 potassium hydroxide.

An advantageous further development provides that the lithium coverage is 1 - 40 mg/m2 lithium hydroxide, in particular 15 - 35 mg/m2 lithium hydroxide, and in particular 20 - 30 mg/m2 lithium hydroxide.

An advantageous further development provides that the alkaline or basic solution is used with a solution concentration of 50 - 200 g/l KOH.

An advantageous further development provides that the alkaline or basic solution is used with a solution concentration of 5 - 100 g/l LiOH.H2O.

An advantageous further development provides for a solution concentration of 30-200 g/l KOH with 4-50 g/l LiOH.H2O. The combination of KOH and LiOH can advantageously ensure particularly good weldability properties of the component.

An advantageous further development provides for the use of a solution having a pH value of 10 - 14.

A further aspect of the invention relates to a galvanized metal strip, in particular steel strip, coated with 30 - 100 mg/m2 potassium.

An advantageous further development provides that the galvanized metal strip, in particular steel strip, is coated with 5 - 30 mg/m2 lithium.

The combination of potassium and lithium as a coating on the steel strip can be particularly advantageous.

An advantageous further development provides for the zinc-based coating to have a zinc content of 85 wt.% to 99.8 wt.%, preferably 98 wt.% to 99.5 wt.%, aluminum in the range of 0.2 to 2 wt.%, and unavoidable impurities. A high zinc content can ensure cathodic corrosion protection. Furthermore, good processability can be ensured if the layer consists predominantly of zinc and the remainder of aluminum. Other elements, such as magnesium, may affect the emissivity and thus influence the heating rate.

An advantageous further development provides that the zinc-based coating was applied by means of a hot-dip galvanizing process, in particular by hot-dip galvanizing.

An advantageous further development provides that the steel strip is formed from a hardenable steel alloy, in particular a boron-manganese steel and particularly preferably a 22MnB5 or 20MnB8 or 34MnB5.

• Kohlenstoff bis 0,4, vorzugsweise 0,10 bis 0,30 und
• Silizium bis 1,9, vorzugsweise 0,11 bis 1,5 und
• Mangan bis 3,0, vorzugsweise 0,8 bis 2,5 und
• Chrom bis 1,5, vorzugsweise 0,1 bis 0,9 und
• Molybdän bis 0,9, vorzugsweise 0,001 bis 0,1 und
• Nickel bis 0,9, vorzugsweise bis 0,2 und
• Titan bis 0,2 vorzugsweise 0,02 bis 0,1 und
• Vanadin bis 0,2 und
• Wolfram bis 0,2 und
• Aluminium bis 0,2, vorzugsweise 0,02 bis 0,07 und
• Bor bis 0,01, vorzugsweise 0,0005 bis 0,005 und
• Schwefel max. 0,01, vorzugsweise max. 0,008 und
• Phosphor max. 0,025, vorzugsweise max. 0,01 und

An advantageous further development provides that a steel strip with the following composition is used (all data in % by weight):

• Carbon up to 0.4, preferably 0.10 to 0.30 and
• Silicon up to 1.9, preferably 0.11 to 1.5 and
• Manganese up to 3.0, preferably 0.8 to 2.5 and
• Chromium up to 1.5, preferably 0.1 to 0.9 and
• Molybdenum up to 0.9, preferably 0.001 to 0.1 and
• Nickel up to 0.9, preferably up to 0.2 and
• Titanium up to 0.2 preferably 0.02 to 0.1 and
• Vanadium up to 0.2 and
• Tungsten up to 0.2 and
• Aluminium up to 0.2, preferably 0.02 to 0.07 and
• Boron up to 0.01, preferably 0.0005 to 0.005 and
• Sulphur max. 0.01, preferably max. 0.008 and
• Phosphorus max. 0.025, preferably max. 0.01 and

Rest iron and smelting-related impurities.

A further aspect of the invention relates to the use of such a steel strip, which is produced by an aforementioned method, in a process in which a steel sheet is heated for the purpose of austenitization and is then formed and quench-hardened.

Figur 1:

der Herstellungsweg beim Formhärteprozess bzw. phs-ultraform Prozess nach dem Stand der Technik;

Figur 2:

den Herstellungsweg beim Warmumformprozess bzw. Presshärten bzw. phsdirectform Prozess nach dem Stand der Technik;

Figur 3:

den Herstellungsweg einer Variante des mehrstufigen Warmumformprozess bzw. mehrstufigen Presshärten bzw. phs-multiform Prozess nach dem Stand der Technik;

Figur 4:

ein Anlagenschema einer Feuerverzinkungsanlage nach dem Stand der Technik;

Figur 5:

ein Anlagenschema einer elektrolytischen Verzinkungsanlage nach dem Stand der Technik;

Figur 6:

eine Tabelle mit unterschiedlichen Ausführungsbeispielen mit unterschiedlichen Werten an Kaliumhydroxid und/oder Lithiumhydroxid;

Figur 7:

eine Aufheizkurve am Beispiel 22MnB5 Z140 bei 1,5 mm Blechdicke mit einer erfindungsgemäßen Konditionierung im Vergleich zu einer Aufheizkurve nach dem Stand der Technik;

Figur 8:

die Heizraten der Figur 7 am Beispiel 22MnB5 Z140 bei 1,5 mm Blechdicke mit einer erfindungsgemäßen Konditionierung im Vergleich zu einer Aufheizkurve nach dem Stand der Technik.

The invention is explained by way of example with reference to a drawing. It shows:

Figure 1:

the manufacturing process for the form hardening process or phs-ultraform process according to the state of the art;

Figure 2:

the manufacturing process for the hot forming process or press hardening or phsdirectform process according to the state of the art;

Figure 3:

the manufacturing process of a variant of the multi-stage hot forming process or multi-stage press hardening or phs-multiform process according to the state of the art;

Figure 4:

a plant diagram of a state-of-the-art hot-dip galvanizing plant;

Figure 5:

a plant diagram of a state-of-the-art electrolytic galvanizing plant;

Figure 6:

a table with different embodiments with different values of potassium hydroxide and/or lithium hydroxide;

Figure 7:

a heating curve using the example of 22MnB5 Z140 at 1.5 mm sheet thickness with a conditioning according to the invention compared to a heating curve according to the prior art;

Figure 8:

the heating rates of the Figure 7 using the example of 22MnB5 Z140 at 1.5 mm sheet thickness with a conditioning according to the invention in comparison to a heating curve according to the state of the art.

According to the invention, the surface of a galvanized metal sheet, in particular steel sheet, which is first cold formed in a form hardening process in several stages and then heated as a component blank, transferred to a form hardening tool and hardened therein, is conditioned with lithium and/or potassium solution, the conditioning being discussed below.

The potassium and lithium solutions that can be used have already been listed, but a conditioning solution with a concentration that follows the following formula is particularly suitable: [c] = [KOH] + 2 * [LiOH·H ₂ O], where [c] is between 30 and 200 g/l.

The concentration of the lithium solution [LiOH·H ₂ O] is between 0 and 100 g/l and the potassium solution [KOH] is between 0 and 200 g/l.

This involves working with a basic solution which has a pH value of 10-14.

Very thin layer thicknesses are used, which are 0.5-3 µm in aqueous solution, in particular 0.5 - 1.5 µm, and 50-300 nm thick when dried, in particular 75 - 125 nm, in particular 80 - 100 nm.

In particular, an aqueous layer thickness of 0.5 - 3 µm is desired, with a dried layer thickness of 50 - 3000 nm and a tin content of 30 - 90 mg/m ² in the form of K ₂ [SnO ₃ ]. The potassium coverage when using potassium hydroxide KOH is 0-140 mg potassium hydroxide per m ² , in particular 50 - 100 mg/m ² potassium hydroxide. The lithium coverage when using lithium hydroxide is 10 - 40 mg lithium hydroxide per m ² , in particular 15 - 35 mg/m ² , and in particular 20 - 30 mg/m ² lithium hydroxide.

In the Figures 1 to 3 Conventional processes are used in which a galvanized steel sheet, in which the zinc layer contains an oxygen-affine element, such as aluminum, is either austenitized before forming or austenitized after forming and quench-hardened in a press. This corresponds to the phs-ultraform process ( Figure 1 ) where, after cold forming, the formed part is then austenitized over Ac3 and subsequently form hardened. In Figure 2 The phs-directform process is presented, in which the blank is first austenitized and formed in the warm state and then trimmed. Figure 3 shows a variant of this, the so-called phs-multiform process, in which, after austenitization and optional pre-cooling to temperatures ranging from 450 °C to 650 °C, a multi-stage process with several forming steps, or cutting and punching operations, subsumed under the term "hot forming steps," takes place. After hardening, the sheets heat-treated in this way have a surface layer, particularly of aluminum oxide and zinc oxide, which is preferably cleaned off.

According to the invention, it was found that conditioning the surface with very small amounts of potassium hydroxide and/or lithium hydroxide obviously intervenes so strongly in the formation of the oxide layer that it does not form in this form or is conditioned to such an extent that it does not need to be cleaned off.

The Figures 4 and 5 show a hot-dip galvanizing plant and an electrolytic galvanizing plant. Here, the application of the stannate can preferably be carried out in the area of chemical passivation (in Figure 4 ) or the station "Passivation" (in Figure 5 ) can be made.

In Figure 6 A table with different embodiments with different values of potassium hydroxide and/or lithium hydroxide is shown.

The effectiveness limits of KOH and LiOH can be seen in various examples. For this example, an aqueous solution with the corresponding values in g/l of KOH or LiOH was applied to a 22MnB5 steel strip with a Z140 zinc coating on a 1.5 mm thick steel sheet. This aqueous solution was applied using a roll coater, and a 1 µm thick aqueous solution was created with the appropriate addition of KOH and/or LiOH.

Therefore, the values in mg/ ^m² of potassium and/or lithium in the table are calculated. For 1 µm thick aqueous solution layers, analogous values in g/l are obtained. For other layer thicknesses, this would have to be converted accordingly for the solution in g/l. In addition, the concentration factor [c] is shown, which corresponds to the formula c = KOH + 2 * LiOH·H ₂ 0 was calculated. Depending on whether the factor [c] lies within the value of 30 to 200, the example is in accordance with the invention or not. Furthermore, the table shows the average heating rate in K/s when heating the steel sheet, blank, or blank from room temperature to the austenitizing temperature Ac3.

It can be seen that the examples according to the invention have a significantly increased heating rate, so that heating can be carried out more quickly, which can enable energy savings or CO2 savings.

Furthermore, the properties of the finished steel component are shown with regard to phosphatability (paintability), paint penetration, weldability, and contact resistance. "--" indicates a significantly negative effect, "-" indicates a slightly negative effect, "~" indicates a comparable effect, "+" indicates a slightly improved effect, and "++" indicates a significantly improved effect compared to the state of the art, i.e., in this case, compared to a 22MnB5 with a 1.5 mm sheet thickness and a Z140 coating without the inventive conditioning.

In Figure 7 A heating curve is shown using 22MnB5 Z140 at a sheet thickness of 1.5 mm, using conditioning according to the invention, compared to a heating curve according to the prior art. The aqueous solution used was 70 mgK/m ² and 4 mgLi/m ² .

In Figure 8 the heating rates of the Figure 7 using the example of 22MnB5 Z140 at 1.5 mm sheet thickness with a conditioning according to the invention in comparison to a heating curve according to the state of the art.

Both figures ( Figure 7 and Figure 8 ) clearly show that the heating rate can be increased by the conditioning according to the invention and the time until the Ac3 temperature is reached can be shortened.

The coating can be applied inline on the strip before it is cut into individual blanks. Furthermore, the blanks cut from the strip can also be coated accordingly.

The circuit boards are then formed into a component blank in a particularly multi-stage process. It is also conceivable to first coat the component blank with the potassium and/or lithium-containing solution. However, it has been shown that the potassium and/or lithium coating also tolerates the forming processes well.

The resulting component blank is then heated to a temperature that causes a structural change toward austenite. The austenitized component blank is then fed to a hot stamping tool, where the component blank is hardened in a single stroke by the contact of an upper and lower tool, which essentially have the same shape as the blank or correspond to it. The contact of the component blank material against the cooled tools, in particular, extracts heat from the steel so quickly that martensitic hardening occurs.

The invention has the advantage that it is possible to condition the surface of a steel sheet intended for hot-dip or press hardening in such a way that a final mechanical cleaning to remove oxide surface layers can be omitted, so that such sheets can be processed in the same way as, for example, hot-dip aluminized sheets, but with the advantage that a high cathodic corrosion protection effect is achieved compared to hot-dip aluminized sheets.

Claims (15)

A method for producing hardened steel components, wherein a blank is cut out of a strip of a hardenable steel alloy coated with a zinc-based coating, and the blanks are then cold-formed to form a component blank and then heated to a temperature which causes a structural change towards austenite, wherein the austenitized component blank is then fed to a form-hardening tool in which the component blank is held in a form-fitting manner by means of an upper and lower tool which have a shape substantially corresponding to the component blank, wherein the contact of the material of the component blank against the, in particular, cooled tools extracts heat from the steel material so quickly that cooling at a cooling rate above the critical cooling rate leads to martensitic hardening, characterized in that after the galvanizing of the metal strip and before the temperature is increased for the purpose of austenitizing, an aqueous potassium hydroxide solution and/or lithium hydroxide solution is applied to the surface of the strip or the blank or the component blank. Method according to claim 1,
characterized in that the concentration of the solution to be applied satisfies the following relationship in g/l:

[c] = [KOH] + 2 * [LiOH H ₂ O] ,

[LiOH H ₂ O] = 0-100 g/l; [KOH] = 0-200 g/l

[c] = 30 g/l to 200 g/l, preferably 50 g/l to 120 g/l.

Method according to one of claims 1 or 2,
characterized in that the coverage at a layer thickness of the solution to be applied of 1 µm satisfies the following relationship in mg/m ² :

[c] = [KOH] + 2 * [LiOH H ₂ O] ,

[LiOH·H ₂ O] = 0-100 mg/m ² ; [KOH] = 0-200 mg/m ²

[c] = 30 mg/m ² to 200 mg/m ² , preferably 50 mg/m ² to 120 mg/m ² .

Method according to one of the preceding claims,
characterized in that
the potassium and/or lithium is applied from an alkaline solution. Method according to one of the preceding claims,
characterized in that
an aqueous potassium and/or lithium solution is applied which is made basic. Method according to one of the preceding claims,
characterized in that
the alkaline or basic aqueous solution is applied with a layer thickness of 0.5 - 3 µm, in particular 0.5 - 1.5 µm, wherein the dried layer thickness is 50 - 300 nm, in particular 75 - 125 nm, in particular 80 - 100 nm. Method according to one of the preceding claims,
characterized in that
the potassium occupancy is 1 - 140 mg/m2 potassium hydroxide and in particular 50 - 100 mg/ ^m2 potassium hydroxide. Method according to one of the preceding claims,
characterized in that
the lithium coverage is 1 - 40 mg/m2 lithium hydroxide, in particular 15 - 35 mg/ ^m2 lithium hydroxide and in particular 20 - 30 mg/ ^m2 lithium hydroxide. Method according to one of the preceding claims,
characterized in that
the alkaline or basic aqueous solution with a solution concentration of 50 - 200 g/l KOH is used. Method according to one of the preceding claims,
characterized in that
the alkaline or basic aqueous solution with a solution concentration of 5 - 100 g/l LiOH ·H ₂ O is used. Method according to one of the preceding claims,
characterized in that
the solution concentration is 30-200 g/l KOH with 4-50 g/l LiOH·H ₂ O. Method according to one of the preceding claims,
characterized in that a solution is used which has a pH value of 10 - 14. Galvanized metal strip, especially steel strip, coated with 30 - 100 mg/m ² potassium. Galvanized metal strip, in particular steel strip, in particular according to claim 13, coated with 5 - 30 mg/m ² lithium. Use of a steel strip according to one of claims 12 - 13, produced by a method according to one of claims 1 - 11, in a process in which a steel sheet is heated above the Ac3 temperature for the purpose of austenitizing and is then formed and cooled at a cooling rate above the critical cooling rate.