Method of producing hardened steel components with a conditioned zinc Anti-corrosion layer

Abstract

The invention relates to a method of producing hardened steel components, wherein a sheet metal blank is cut out of a galvanized strip of a hardenable steel alloy and the sheet metal blanks are then heated to a temperature that brings about a change in microstructure toward austenite, wherein the austenitized sheet metal blanks are then fed to a press hardening tool in which the sheet metal blanks are hot-formed by means of an upper and lower tool in one stroke or several strokes, wherein the shaped sheet metal blank is cooled at the tools, which are cooled in particular, at a rate above the critical cooling rate, resulting in martensitic hardening, wherein the galvanizing of the metal strip is followed and the increase in temperature for the purpose of austenitization is preceded by applying of an aqueous solution consisting of potassium hydroxide and / or lithium hydroxide, with water as the balance, to the surface of the strip or of the sheet metal blank.

Description

[0001] International patent application

[0002] Voestalpine Stahl GmbH

[0003] 250030WO

[0004] 1

[0005] Method for producing hardened steel components with a conditioned zinc corrosion protection layer

[0006] The invention relates to a method for producing hardened steel components with a conditioned zinc alloy corrosion protection layer.

[0007] It has long been known to provide metallic sheets, especially metallic strips, which could corrode under ordinary operating conditions, with protective coatings.

[0008] In general, corrosion protection layers on metal strips can be organic coatings, such as paints, which may also contain corrosion-inhibiting agents.

[0009] Furthermore, it is known to protect metal strips with metal coatings. Such metal coatings can consist of a more electrochemically noble metal or a less electrochemically noble metal.

[0010] A coating made of a more electrochemically noble metal, or a self-passivating metal such as aluminum, is called a barrier coating. For example, when aluminum is applied to steel, the steel will corrode if this barrier coating is partially removed, such as by mechanical damage. A common barrier coating for steel is the aforementioned aluminum layer, which is typically applied by hot-dip coating.

[0011] If a less electrochemically noble metal is applied as a protective layer, it is called a cathodic corrosion coating because, in the event of mechanical damage to the corrosion protection coating down to the steel material, the less electrochemically noble metal is corroded first before the steel material itself is exposed to corrosion.

[0012] The most commonly used cathodic protective coating on steel is a zinc coating or a zinc-based alloy.

[0013] Several galvanizing processes are known. One common galvanizing process is hot-dip galvanizing (also known as hot-dip galvanizing). [International patent application]

[0014] Voestalpine Stahl GmbH

[0015] 250030WO

[0016] 2

[0017] Steel is continuously (e.g., strip and wire) or in sections (e.g., components) immersed in a molten zinc bath at temperatures of approximately 450 °C to 600 °C (zinc has a melting point of 419.5 °C). The zinc bath 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, and above this lies a firmly adhering layer of pure zinc, the composition of which corresponds to the zinc bath. For a continuously galvanized strip, the zinc layer has a thickness of 5 pm to 40 pm. For a section galvanized, the zinc layer can have a thickness of 50 pm to 150 pm.

[0018] Typically, not only pure metal layers are deposited in this process. A variety of alloys are also deposited, including pure aluminum coatings, coatings containing aluminum and zinc, and coatings with a predominant zinc content and small amounts of aluminum. These coatings may also contain other elements such as zinc, nickel, chromium, magnesium, and other elements, as well as mixtures thereof. When zinc corrosion protection coatings or galvanized steel strips are mentioned in the application, zinc-based alloys are also included.

[0019] To ensure the adhesion and uniformity of the zinc coating, careful surface pretreatment is required. This may include, for example, degreasing, alkaline cleaning, pickling, rinsing, and / or descaling. After galvanizing, one or more post-treatments can be carried out, such as phosphating, oiling, or the application of organic coatings (e.g., cathodic dip coating, or KTL).

[0020] Furthermore, it has long been known that, particularly to reduce the weight of vehicle bodies, at least parts of the body should be made of high-strength steel to ensure sufficient strength in the event of a crash. The weight savings result from the use of high-strength steel grades with comparatively thin walls, thus resulting in a low weight.

[0021] Even when using high-strength steels, there are different approaches and a wide variety of steel types that can be used. International patent application

[0022] Voestalpine Stahl GmbH

[0023] 250030WO

[0024] 3

[0025] Steel grades that achieve high strength through quench hardening are frequently used. Quench hardening means that a cooling rate above the respective critical cooling rate for microstructure adjustment is selected. This critical cooling rate is around 15 to 20 Kelvin per second, but can also be lower depending on the alloy composition. A common steel grade that can be hardened by quench hardening is the so-called boron-manganese steel, such as the most widely used 22MnB5, but also derivatives of this steel, such as 20MnB8 and 30MnB5. Non-hardenable steels, such as micro-alloyed steels, can also be hot-formed using direct or indirect methods.

[0026] Such steel grades can be easily deformed and cut in their unhardened state.

[0027] To bring such steel grades into the desired shape and harden them, especially in car body construction, there are essentially two different processes.

[0028] The first and somewhat older method is called press hardening. In press hardening, a flat blank is cut from a steel strip made of a quench-hardenable steel alloy, for example, 22MnB5 or a similar manganese-boron steel. This flat blank is then heated to such a high temperature that the steel microstructure exhibits the appearance of gamma iron, or austenite. To achieve this microstructure, the so-called austenitizing temperature Acs must be exceeded, at least if complete austenitization is desired.

[0029] This temperature can range between 820°C and 900°C depending on the type of steel, with such steel sheets, for example, being heated to approximately 900°C to 930°C and held at this temperature until the microstructure has completely changed.

[0030] The hot steel blank is then transferred to a press, where, using a correspondingly shaped upper and lower die, the hot steel blank is formed into the desired shape with a single press stroke. Through contact between the hot steel and the comparatively cool, especially cooled, press tools, energy is extracted from the steel very rapidly. In particular, the heat must be extracted so quickly that the so-called critical hardening rate is exceeded, which is typically between 15°C and 25°C per second.

[0031] If cooled so rapidly, the austenite structure does not revert to a ferritic structure, but instead a martensitic structure is reached. (International patent application)

[0032] Voestalpine Stahl GmbH

[0033] 250030WO

[0034] 4

[0035] The fact that austenite can dissolve significantly more carbon in its lattice than martensite leads to lattice distortion through carbon precipitation, resulting in the high hardness of the final product. Rapid cooling effectively stabilizes the martensitic state. This makes hardnesses and tensile strengths (Rm) exceeding 1500 MPa achievable. Through suitable measures, which will not be discussed in detail here, such as complete or partial reheating, specific hardness profiles can also be achieved.

[0036] Another, somewhat more recent method for producing hardened steel components, particularly for car body construction, is the cold forming process developed by the applicant. In cold forming, a flat steel blank is cut from a steel strip and then cold formed. This forming process is not carried out with a single press stroke, but rather, as is common in conventional press lines, for example, in a five-stage process. This process allows for considerably more complex shapes, so that ultimately a complexly shaped component, such as the B-pillar or a longitudinal member of a motor vehicle, can be produced.

[0037] To subsequently harden such a pre-formed component, it is also austenitized in an oven and, in its austenitized state, transferred to a mold, the mold having the contour of the final component. Preferably, the pre-formed component is shaped before heating in such a way that, after heating and subsequent thermal expansion, it already largely corresponds to the final dimensions of the hardened component. This austenitized blank is placed in the mold in its austenitized state, and the mold is closed. Preferably, the component is in contact with the mold on all sides and held in place by clamping action, and heat is extracted through this contact with the mold to such an extent that a martensitic microstructure is formed.

[0038] When clamped, shrinkage cannot occur, so the hardened final component with the corresponding final dimensions can be removed from the mold after hardening and cooling.

[0039] Since motor vehicle bodies typically have a corrosion protection coating, with the corrosion protection layer closest to the metal material forming the body, especially steel, being a metallic coating, corrosion protection coatings for hardened components have also been pursued and developed in the past. International patent application

[0040] Voestalpine Stahl GmbH

[0041] 250030WO

[0042] 5

[0043] However, corrosion protection coatings for components that are to be hardened are subject to different requirements than corrosion protection coatings for components that are not hardened. The high temperatures necessary for hardening must be withstood by the corrosion protection coatings. Since it has long been known that hot-dip aluminized coatings also withstand high temperatures, press-hardened steels were initially developed that possess a protective layer of aluminum. Such coatings are able to withstand not only the high temperatures but also the forming process while hot.However, a disadvantage is that hot-dip aluminizing is not usually used on conventional steel grades in motor vehicles, but rather hot-dip galvanized material, and it is generally problematic to use different corrosion protection systems, especially if there is a risk of contact corrosion due to different electrochemical potentials of the coatings.

[0044] Therefore, the applicant developed processes that make it possible to provide zinc coatings that can also withstand such high temperatures.

[0045] In general, zinc coatings are considerably simpler to form than aluminum coatings, as aluminum coatings tend to chip or crack at conventional forming temperatures. This does not happen with zinc.

[0046] Initially, it was expected that zinc coatings would not be able to withstand high temperatures. However, special zinc coatings containing a certain proportion of oxygen-affine elements are able to be processed even at high temperatures. This is because the oxygen-affine elements diffuse quickly to the air-facing surface, where they oxidize and form a protective ceramic film for the zinc coating. Such zinc coatings have now become particularly prevalent in die hardening. They can also be used with great success in press hardening.

[0047] To ensure optimal paint adhesion and weldability, it is known to clean the finished formed and hardened components in such a way that the ceramic, protective film layer is leveled or removed.

[0048] From DE 102010 037077 B4, a method for conditioning the surface of hardened, corrosion-protected components made of sheet steel is known, wherein the sheet steel is a sheet steel coated with a metallic coating and is heated and quenched for hardening. After hardening, the surface areas created by the heating process are treated.

[0049] Voestalpine Stahl GmbH

[0050] 250030WO

[0051] 6

[0052] Existing oxides are removed from the corrosion protection coating, whereby the component is subjected to vibratory finishing to condition the surface of the metallic coating, i.e. the corrosion protection layer, wherein 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 off and, in particular, microporosity is exposed.

[0053] From DE 102007 022 174 B3, a method for producing and removing a temporary protective layer for a cathodic coating is known, wherein a steel sheet made of a hardenable steel alloy is coated with zinc by hot-dip galvanizing, the aluminum content in the zinc bath being adjusted such that a surface oxide layer of aluminum oxide forms during hot-dip galvanizing, and this thin layer being removed or leveled after hardening by blasting the sheet component with dry ice particles. Examples of this are EP 1 630244 B1 and EP 2 233 508 B1.

[0054] Such oxide layers usually only occur on zinc coatings, while aluminum coatings often do not require cleaning or only require less extensive cleaning.

[0055] WO 2018 / 126471 Al discloses a sol-gel preconditioning process for reducing oxide layer formation and increasing weldability. This process aims to create an oxidation protection coating for press-hardening steels, based on silane- and titanium-containing binders and oxide pigments, apparently applied using a sol-gel process. In particular, solvents such as methanol are used, which are unsuitable for use on steel production equipment. The coating is intended to self-detach after press hardening; however, trials with titanium- and silicon-based coatings conducted in 2015 / 16 were unsuccessful with both thick and thin wet films. The coating neither detaches self-detaching after annealing nor is its weldability suitable for industrial applications.

[0056] From EP 2 536 857 Bl, a ceramic-based coating with a thickness of < 25 pm is known, which is said to consist essentially of Si2, Al2O3 and Mg2, optionally containing metallic tin fibers. It was found that such a coating renders the sheet metal unweldable, and the surface also exhibits unsuitable adhesion of ceramic coatings. International patent application

[0057] Voestalpine Stahl GmbH

[0058] 250030WO

[0059] 7

[0060] From EP 4 110 972 Bl, a coating of the galvanized surface with tin salts is known which, through the formation of a mixture of the oxide layer of tin and zinc oxide during the hardening process, makes its removal unnecessary.

[0061] The object of the invention is to provide a method for producing hardened steel components in which an existing zinc corrosion protection layer is conditioned in such a way that surface cleaning, and in particular cleaning with fluid and / or particle jets, after the hardening process can be omitted. The method is particularly preferably intended to be able to do without the use of tin or tin-containing solutions.

[0062] The problem is solved by a method having the features of claim 1.

[0063] Advantageous further training courses are indicated in the dependent sub-claims.

[0064] Another task is to create a galvanized steel strip that is designed in such a way that the removal of an oxide layer is unnecessary.

[0065] The problem is solved with a galvanized metal strip having the features of claim 12.

[0066] Advantageous further training courses are indicated in the dependent sub-claims.

[0067] According to the invention, it has been recognized that under certain circumstances, cleaning the surface of a galvanized metal strip that has been subjected to an increase in temperature for the purpose of structural modification can be omitted. In particular, the mechanical cleaning of a galvanized steel sheet and a hardened component produced therefrom can be eliminated.

[0068] While a cleaning post-treatment is a manageable and well-established process, it nevertheless generates increased labor. Furthermore, there is a risk of additional surface defects, resulting in higher overall costs. With very thin components, it has been found that under certain circumstances, the dimensional accuracy of the components can be limited. International patent application

[0069] Voestalpine Stahl GmbH

[0070] 250030WO

[0071] 8

[0072] If there are interconnected process sequences that stipulate these cleaning steps are arranged inline within an entire production process, an adjustment of the cycle time may be necessary.

[0073] According to the invention, a sheet steel plate or a steel strip is used which has a zinc-based coating.

[0074] Advantageously, this layer can have a thickness of 5 pm to 20 pm 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.

[0075] Zinc-based corrosion protection coatings can have a comparatively high zinc content of 93 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 aluminium in the range of 0.2 to 2 wt.%.

[0076] Particularly advantageous is the application of a zinc-based metallic corrosion protection layer using a hot-dip galvanizing process. This can be a simple and robust application method.

[0077] According to the invention, it has been found that surface treatment of the galvanized surface prior to the die hardening process is successful in adjusting the phosphating, painting, and weldability. According to the invention, oxide growth during the hardening process can be controlled in such a way that subsequent mechanical surface conditioning, such as centrifugal blasting, vibratory finishing, or dry ice blasting, becomes unnecessary. Likewise, treatment with tin or tin-containing solutions can be omitted.

[0078] According to the invention, it has been found that lithium and / or potassium surprisingly modify the surface in such a way that any kind of purification is unnecessary.

[0079] 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 * [UOH.H2O], International patent application

[0080] Voestalpine Stahl GmbH

[0081] 250030WO

[0082] 9

[0083] wherein the concentration of the lithium solution [UOH.H2O] is between 0 and 100 g / L and the potassium solution [KOH] is between 0 and 200 g / L, wherein the total concentration [c] develops a particular efficacy between 10 and 200 g / L. Preferably, the total concentration [c] can be adjusted between 30 and 120 g / L to further increase the efficacy.

[0084] According to the invention, in particular an aqueous alkaline solution is applied to the zinc surface by means of, for example, a roll coater after the dressing process and before the annealing and hardening process.

[0085] This involves working with very thin layer thicknesses, which are 0.5-3 pm when aqueous, in particular 0.5 - 1.5 pm, and are 50-300 nm thick when dried, in particular 75 - 125 nm, in particular 80 - 100 nm.

[0086] The solution can also be applied by dipping and squeezing, spraying, or other application methods.

[0087] Of course, all other methods by which liquid ionic solutions can be applied to a surface are also suitable.

[0088] By multiplying the solution concentration (g / L) by the layer thickness (pm) and the respective mass fraction of the element in the compound. For a 1 pm thick aqueous solution layer, this corresponds to a direct conversion from g / L to mg / m³. 2 , where the elemental fraction is taken into account via the molar composition (molar mass element / molar mass compound)., the coverage [C] of the surface with the respective element is obtained.

[0089] At a layer thickness of 1 pm in the aqueous state, the coating meets the following relationship in mg / m². 2 :

[0090] [C] = [K] + 2 * [Li] ,

[0091] [Li] = 0-17 mg / m 2; [K] = 0-140 mg / m 2

[0092] [c] = 10 to 175, preferably 30 to 120 mg / m³ 2

[0093] The potassium content when using potassium hydroxide (KOH) is 1-140 mg potassium per m². 2 , especially 30 - 100 mg / m² 2 Potassium. The lithium content when using LiOH is 1–17 mg lithium per m³. 2 , especially 5 - 15 mg / m³ 2 , and especially 7-13 mg / m³ 2 Lithium.

[0094] Surprisingly, it has been shown that a combination of potassium hydroxide and lithium hydroxide exhibits particularly favorable properties with regard to the weldability of the International Patent Application

[0095] Voestalpine Stahl GmbH

[0096] 250030WO

[0097] 10

[0098] The invention demonstrates that, with a typical annealing time for sheets undergoing hardening, the surface resistance is very low, and even in a paint undercutting test, only a very slight tendency for paint undercutting was observed. Visually, significantly fewer oxides are detectable, which is evident from the metallic luster of the annealed sheet. Such a silvery sheen usually presents a problem, as it indicates insufficient complete reaction. Investigations have shown that the zinc-iron crystals of the zinc layer have fully reacted. Furthermore, good formation of phosphate crystals during phosphating was observed.

[0099] Treatment with potassium and / or lithium hydroxide increases the emissivity in the first 780°C until the gamma phase decays.

[0100] Overall, it is not yet possible to say in detail how the K / Li occupancy works during austenitizing, but the effect is surprising and absolutely clear.

[0101] The invention thus relates in particular to a method for producing hardened steel components, wherein a blank is cut from a strip of hardenable steel alloy coated with a zinc-based coating, and the blanks are then heated to a temperature that causes a microstructure change towards austenite, wherein the austenitized blanks are subsequently fed to a press hardening tool in which the blanks are hot-formed by means of an upper and lower tool in one or more strokes, wherein the formed blank is cooled at the tools, which are in particular cooled, at a rate above the critical cooling rate, so that martensitic hardening occurs, wherein after the zinc plating of the metal strip and before the temperature increase for the purpose of austenitization, an aqueous solution consisting of potassium hydroxide and / or lithium hydroxide, the remainder being water, is added.applied to the surface of the tape or circuit board.

[0102] An advantageous further development stipulates that the concentration of the solution to be applied must meet the following relationship in g / L:

[0103] [C] = [KOH] + 2 * [UOH.H2O],

[0104] [UOH.H2O] = 0-100 g / L ; [KOH] = 0-200 g / L

[0105] [c] = 10 to 200, preferably 30 to 120 g / L

[0106] An advantageous further development stipulates that the zinc surface coverage should follow the following relationship in mg / m². 2 fulfilled:

[0107] [C] = [K] + 2 * [Li], International patent application

[0108] Voestalpine Stahl GmbH

[0109] 250030WO

[0110] 11

[0111] [Li] = 0-17 mg / m 2 ; [K] = 0-140 mg / m 2

[0112] [C] = 10 to 175, preferably 30 to 120 mg / m³ 2

[0113] An advantageous further development involves applying the potassium and / or lithium from an alkaline solution.

[0114] An advantageous further training involves applying an aqueous potassium and / or lithium solution that is alkaline.

[0115] An advantageous further development provides that the aqueous solution is applied with a layer thickness of 0.5 - 3 pm, in particular 0.5 - 1.5 pm, wherein the layer thickness when dried is 50 - 300 nm, in particular 75 - 125 nm, in particular 80 - 100 nm.

[0116] A beneficial further training program stipulates that the potassium level should be 1 - 140 mg / m². 2 Potassium is, and in particular 50 - 100 mg / m² 2 Potassium is present.

[0117] An advantageous further training program stipulates that the lithium saturation should be 1 - 17 mg / m². 2 Lithium levels are, in particular, 5 - 15 mg / m³ 2 Lithium is, and in particular 7 - 13 mg / m³2 Lithium is.

[0118] An advantageous further training method involves using a solution with a solution concentration of 50 - 200 g / l KOH.

[0119] An advantageous further development involves using a solution with a solution concentration of 5 - 100 g / l UOH.H2O.

[0120] A beneficial further development involves a solution concentration of 30-200 g / l KOH with 4-50 g / l UOH / H2O. The combination of KOH and LiOH can advantageously ensure particularly good weldability properties of the component.

[0121] An advantageous further training method involves using a solution with a pH value of 10-14.

[0122] Another aspect of the invention relates to a galvanized steel strip with a surface concentration [C] which fulfills the following relationship:

[0123] [C] = [K] + 2 * [Li], International patent application

[0124] Voestalpine Stahl GmbH

[0125] 250030WO

[0126] 12

[0127] [Li] = 0-17 mg / m 2 ; [K] = 0-140 mg / m 2

[0128] [C] = 10 to 175, preferably 30 to 120 mg / m³ 2

[0129] An advantageous further development method stipulates that the galvanized steel strip is coated with 10-140 mg / m². 2 Potassium is.

[0130] An advantageous further training method stipulates that the galvanized steel strip should contain 5 - 17 mg / m². 2 It is coated with lithium.

[0131] The combination of potassium and lithium as a coating on the steel strip can be particularly advantageous.

[0132] An advantageous further development provides that the zinc-based coating has a zinc content of 95 wt.% to 99.8 wt.%, preferably 98 wt.% to 99.5 wt.%, aluminum in the range of 0.2 wt.% 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.

[0133] An advantageous further development provides that the zinc-based coating was applied using a hot-dip process, in particular by hot-dip galvanizing.

[0134] An advantageous further development provides that the steel strip is made of a hardenable steel alloy, in particular a boron-manganese steel and especially preferably a 20MnB8 or 34MnB5.

[0135] An advantageous further development provides that a steel band with the following composition is used (all values ​​in wt.%):

[0136] Carbon up to 0.4, preferably 0.10 to 0.30 and

[0137] Silicon up to 1.9, preferably 0.11 to 1.5 and International Patent Application

[0138] Voestalpine Stahl GmbH

[0139] 250030WO

[0140] 13

[0141] Manganese up to 3.0, preferably 1.25 to 2.5 and

[0142] Chromium up to 1.5, preferably 0.1 to 0.9 and

[0143] Molybdenum up to 0.9, preferably 0.001 to 0.1 and

[0144] Nickel up to 0.9, preferably up to 0.2 and

[0145] Titanium up to 0.2, preferably 0.02 to 0.1 and

[0146] Vanadium up to 0.2 and

[0147] Tungsten up to 0.2 and

[0148] Aluminium up to 0.2, preferably 0.02 to 0.07 and

[0149] Boron up to 0.01, preferably 0.0005 to 0.005 and

[0150] Sulfur max. 0.01, preferably max. 0.008 and

[0151] Phosphorus max. 0.025, preferably max. 0.01 and

[0152] Residual iron and impurities resulting from the smelting process.

[0153] Surprisingly, it has been shown that a higher manganese content in the steel strip increases the effectiveness of applying potassium and / or lithium.

[0154] Another aspect of the invention relates to the use of such a steel strip which is produced by a aforementioned method in a process in which a steel sheet is heated for the purpose of austenitizing and subsequently formed and quenched and hardened.

[0155] The invention is explained by way of example with the aid of a drawing. The drawing shows:

[0156] Figure 1: the manufacturing process for the mold hardening process or phs-ultraform process according to the state of the art;

[0157] Figure 2: the manufacturing process in the hot forming process or press hardening or phs-directform process according to the state of the art;

[0158] 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 prior art; International patent application

[0159] Voestalpine Stahl GmbH

[0160] 250030WO

[0161] 14

[0162] Figure 4: a diagram of a hot-dip galvanizing plant according to the state of the art;

[0163] Figure 5: a diagram of an electrolytic galvanizing plant according to the state of the art;

[0164] Figure 6: a table with different embodiments with different values ​​of potassium and / or lithium content;

[0165] Figure 7: a heating curve using the example of 20MnB8 Z140 at 1.5 mm sheet thickness with conditioning according to the invention compared to a heating curve according to the prior art;

[0166] Figure 8: the heating rates of Figure 7 using the example 20MnB8 Z140 at 1.5 mm sheet thickness with conditioning according to the invention compared to a heating curve according to the prior art.

[0167] According to the invention, the surface of a galvanized metal strip, in particular steel strips, which is first heated in a press hardening process and is formed and hardened in a press hardening tool, is conditioned with lithium and / or potassium solution, the conditioning of which is described below.

[0168] The potassium and lithium solutions that can be used have already been listed; in particular, a conditioning solution with a concentration that follows the formula: [c] = [KOH] + 2 * [LiOH.HzO] is suitable, where [c] is between 10 and 200 g / L.

[0169] The concentration of the lithium solution [UOH.H2O] is between 0 and 100 g / L and the potassium solution [KOH] is between 0 and 200 g / L.

[0170] This process uses a basic solution with a pH value of 10-14.

[0171] This process involves working with very thin layers, which are 0.5–3 pm in aqueous solution, particularly 0.5–1.5 pm, and 50–300 nm thick when dried, particularly 75–125 nm, and particularly 80–100 nm. International patent application

[0172] Voestalpine Stahl GmbH

[0173] 250030WO

[0174] 15

[0175] The potassium content when using potassium hydroxide (KOH) is 1-140 mg potassium per m². 2 , especially 50 - 100 mg / m² 2 Potassium. The lithium content when using [this product] is 1–17 mg lithium per m². 2 , especially 5 - 15 mg / m³ 2 , and especially 7 - 13 mg / m³ 2 Lithium.

[0176] Figures 1 to 3 show conventional processes in which a galvanized steel sheet, where the zinc layer contains an oxygen-affine element, for example, aluminum, is either austenitized before or after forming and subsequently quenched and hardened in a press. This corresponds to the phs-ultra-form process (Figure 1), in which, after cold forming, the formed part is austenitized over Ac3 and then die-hardened. Figure 2 shows the phs-directform process, in which the blank is first austenitized and formed while warm, followed by trimming. Figure 3 shows a variant of this process, the so-called phs-multiform process, in which, after austenitization and optional pre-cooling to, in particular, 450 °C to 650 °C, a multi-stage process with several forming steps is carried out.Cutting and punching processes, which are subsumed under the term "hot forming steps," are carried out. After hardening, the heat-treated sheets have a surface layer, in particular of aluminum oxide and zinc oxide, which is preferably cleaned off.

[0177] According to the invention, it was found that conditioning the surface with very small amounts of potassium hydroxide and / or lithium hydroxide obviously interferes so strongly with 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.

[0178] Figures 4 and 5 show a hot-dip galvanizing plant and an electrolytic galvanizing plant, respectively. The application of the treatment solution containing potassium and / or lithium hydroxide can preferably take place in the chemical passivation area (in Figure 4) or at the "passivation" station (in Figure 5).

[0179] Figure 6 shows a table with different embodiments with different values ​​of potassium hydroxide and / or lithium hydroxide.

[0180] The effectiveness limits of KOH and LiOH can be demonstrated using various examples. For this purpose, an aqueous solution with corresponding values ​​in g / L of KOH and LiOH, respectively, was applied to a 20MnB8 grade steel strip with a Z140 zinc coating on a 1.5 mm thick steel sheet. This aqueous solution was applied using a roll coater. (International patent application)

[0181] Voestalpine Stahl GmbH

[0182] 250030WO

[0183] 16

[0184] and a 1 pm thick aqueous solution is produced with appropriate addition of KOH and / or LiOH.

[0185] Therefore, the values ​​in mg / m³ are obtained. 2The concentration of potassium and / or lithium in the table can be determined by multiplying the solution concentration (g / L) by the layer thickness (pm) and the respective mass fraction of the element in the compound. For a 1 pm thick aqueous solution layer, this corresponds to a direct conversion from g / L to mg / m². 2 , where the elemental proportion is taken into account via the molar composition

[0186] (Mole mass element / MO I IT connection) ■

[0187] In addition, the occupancy value [C] is shown, which was calculated according to the formula C = K + 2 * Li. Depending on whether the occupancy value [C] lies within the range of 10 to 175, it is an example according to the invention or not. Furthermore, the table shows the average heating rate in K / s when heating the steel sheet, circuit board, or blank from room temperature to the austenitizing temperature Ac3.

[0188] It can be seen that the examples according to the invention have a significantly increased heating rate, therefore the heating can be carried out more quickly, which can enable energy savings or CO2 savings.

[0189] Furthermore, the properties of the finished steel component are described with regard to phosphating (paintable), paint penetration, weldability, and contact resistance. Here, "+" represents a significantly negative effect, "+" a slightly improved effect, and "++" a significantly improved effect compared to the prior art, i.e., in this case, compared to a 20MnB8 at 1.5 mm sheet thickness with Z140 coating without conditioning according to the invention.

[0190] Figure 7 shows a heating curve for 20MnB8 Z140 at a sheet thickness of 1.5 mm with conditioning according to the invention, compared to a heating curve according to the prior art. An aqueous solution with 200 g / L KOH was used, corresponding to a potassium coating of 139 mg / m³. 2 .

[0191] Figure 8 illustrates the heating rates of Figure 7 using the example of 20MnB8 Z140 at a sheet thickness of 1.5 mm with conditioning according to the invention, in comparison to a heating curve according to the prior art. International patent application

[0192] Voestalpine Stahl GmbH

[0193] 250030WO

[0194] 17

[0195] 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 to reach the Ac3 temperature can be reduced.

[0196] The coating can be applied inline on the conveyor belt before it is cut into individual circuit boards. Furthermore, the circuit boards cut from the belt can also be coated accordingly.

[0197] The blanks are then heated, without prior cleaning, to a temperature that causes a microstructure change towards austenite. The austenitized blanks are then fed into a press hardening tool, where they are re-formed in one or more strokes using an upper and lower die. This can be carried out in the aforementioned phs directform or phs multiform process with multiple punching or trimming operations, and with or without pre-cooling. The contact of the formed blank material with the cooled dies rapidly dissipates heat from the steel, resulting in martensitic hardening.

[0198] The invention has the advantage that it makes it possible to condition the surface of a steel sheet intended for die hardening 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

International patent application Voestalpine Stahl GmbH 250030WO 18 Patent claims 1.A method for producing hardened steel components, wherein a blank is cut from a strip of hardenable steel alloy coated with a zinc-based coating, and the blanks are subsequently heated to a temperature that causes a microstructure change towards austenite, wherein the austenitized blanks are then fed to a press hardening tool in which the blanks are hot-formed in one or more strokes by means of an upper and lower tool, wherein the formed blank is cooled at a rate above the critical cooling rate on the tools, in particular cooled, so that martensitic hardening occurs, characterized in that after zinc plating of the metal strip and before the temperature increase for the purpose of austenitization, an aqueous solution consisting of potassium hydroxide and / or lithium hydroxide, the remainder being water, is applied to the surface of the strip or the blank.

2. 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 * [UOH.H2O], [UOH.H2O] = 0-100 g / L; [KOH] = 0-200 g / L [c] = 10 to 200, preferably 30 to 120 g / L 3. Method according to one of claims 1 or 2, characterized in that the zinc surface coverage follows the following relationship in mg / m² 2 fulfilled: [C] = [K] + 2 * [Li], [Li] = 0-17 mg / m 2 ; [K] = 0-140 mg / m 2 [C] = 10 to 175, preferably 30 to 120 mg / m³ 2 4. Method according to any one of the preceding claims, characterized by the fact that the potassium and / or lithium is applied from an alkaline solution. International patent application Voestalpine Stahl GmbH 250030WO 19 5. Method according to any one of the preceding claims, characterized by the fact that the aqueous solution is applied with a layer thickness of 0.5 - 3 pm, in particular 0.5 - 1.5 pm, wherein the layer thickness when dried is 50 - 300 nm, in particular 75 - 125 nm, in particular 80 - 100 nm.

6. Method according to any one of the preceding claims, characterized by the fact that Potassium saturation 1 - 140 mg / m² 2 Potassium is, and in particular 50 - 100 mg / m² 2 Potassium is present.

7. Method according to any of the preceding claims, characterized by the fact that the lithium concentration 1 - 17 mg / m 2 Lithium levels are, in particular, 5 - 15 mg / m³ 2 Lithium is, and in particular 7 - 13 mg / m³ 2 Lithium is.

8. Method according to any one of the preceding claims, characterized by the fact that A solution with a solution concentration of 50 - 200 g / L KOH is used.

9. Method according to any one of the preceding claims, characterized by the fact that A solution with a solution concentration of 5 - 100 g / L UOH.H2O is used.

10. Method according to any one of the preceding claims, characterized by the fact that The solution concentration is 30-200 g / L KOH with 4-50 g / L UOH.H2O.

11. Method according to any of the preceding claims, characterized by the use of a solution having a pH value of 10 - 14.

12. Galvanized steel strip, with a surface concentration [C], which satisfies the following relationship: [C] = [K] + 2 * [Li], [Li] = 0-17 mg / m 2 ; [K] = 0-140 mg / m 2 International patent application Voestalpine Stahl GmbH 250030WO 20 [C] = 10 to 175, preferably 30 to 120 mg / m³ 2 13. Galvanized steel strip, in particular according to claim 12, coated with 5 - 17 mg / m 2 Lithium.

14. Galvanized steel strip, in particular according to claim 12, coated with 10 - 140 mg / m 2 Potassium.

15. Use of a steel strip according to one of claims 12-14, manufactured by a method according to one of claims 1-11

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