DE102020120580A1 - METHOD OF MAKING COATED STEEL STRIP, AND METHOD OF MAKING A HARDENED STEEL PRODUCT - Google Patents

Abstract

The invention relates to a method for producing coated steel strip, comprising: heating a steel strip (2) made of hardenable steel to a target strip temperature (T3) of less than 625° C., and hot dip coating the steel strip (2) heated to the target strip temperature (T3). in a melt (4) containing at least aluminum and silicon, the melt (4) having a temperature (T4) of less than 620°C. The invention also relates to a method for producing a hardened steel product from the method mentioned and a hot-formed part (42) produced therefrom.

Description

The invention relates to a method for producing coated steel strip, in particular for producing hot-formed steel products, and products correspondingly produced according to the method.

It is known to coat metallic components for corrosion protection and to form them into molded parts by means of hot forming. In practice, for example, aluminium-silicon-coated high-strength and ultra-high-strength tempered steels, in particular manganese-boron-containing tempered steels such as 22MnB5 or 34MnB5, are used for safety-relevant body components of motor vehicles.

For example, from the WO 2009/090555 A1 a method of producing a hot stamped coated steel section is known, comprising the steps of: pre-coating a steel strip with aluminum or aluminum alloy by hot dipping, the thickness of the pre-coating being 20 to 33 microns on each side, cutting the pre-coated steel strip into a steel section, heating the steel section in a furnace, transferring the heated steel section to a die, hot stamping the steel section in the die, and cooling the steel section.

From the WO 2020 020644 A1 a method for producing a hardened steel product is known, comprising the steps of: providing a steel substrate with a base material made of a hardenable steel; coating the steel substrate with an aluminum-containing pre-coating having a thickness of at least 34 microns; flexible rolling of the precoated steel substrate; Working out a circuit board from the flexibly rolled strip material; heating the circuit board such that the base material of the circuit board is austenitized; Hot forming of the heated blank wherein the heated blank is formed and rapidly cooled to produce a hardened coated steel product.

From the WO 2006/097237 A1 a method and a system for hot-dip coating of hot-rolled steel strip are known. The steel strip goes through a pickling station, a rinsing station, a drying station, a heating furnace and then a molten bath. The finished thickness and the thickness tolerance of the hot-dip coated steel strip are achieved by a controlled reduction in thickness in a roll stand in the process line, in that the finished thickness is checked by a thickness gauge at the outlet of the roll stand and deviations from the target thickness are fed back as a control signal to the adjustment of the roll stand.

The present invention is based on the object of proposing a method for producing coated steel strip and a correspondingly produced steel strip which has improved processability for downstream production processes, in particular for flexible rolling or hot forming.

To solve the problem, a method for producing coated steel strip is proposed, in which steel strip made of hardenable steel is brought to a strip target temperature of less than 625° C., and the steel strip, heated to strip target temperature, in a melt containing aluminum and silicon and a temperature less than 620°C, is hot dip coated.

Due to the hot dip coating at the specified relatively low temperature, the diffusion processes between the melt and the steel strip are relatively short or low, so that a relatively thin diffusion layer is formed. This results in improved processability for downstream production processes, in particular for flexible rolling and/or hot forming of the steel strip coated according to the invention. In particular, the rollability or formability can be improved by lower rolling forces or forming forces. There is less friction between the tool and the coated steel strip, which has a positive effect on wear and the service life of the tool.

A steel strip is used as the starting material, in particular made from a martensitically hardenable steel material, which can be hot strip or cold strip. Hot strip is a finished rolled steel product (steel strip) that is produced by rolling after prior heating. Cold strip means a cold-rolled steel strip (flat steel). Flat steel is referred to as cold-rolled if the last reduction in thickness is carried out by rolling without prior heating. Hot-rolled steel strip is preferably used, since this saves one manufacturing step, namely cold rolling before coating, and thus the corresponding costs. Insofar as the term steel strip is used within the scope of this disclosure, hot strip and cold strip are to be included accordingly.

• Kohlenstoff (C) mit mindestens 0,15 % und höchstens 0,5 %, insbesondere höchstens 0,4 %; Mangan (Mn) mit mindestens 0,5 % und höchstens 5,0 %, insbesondere mindestens 0,8 % und höchstens 2,5 %; Aluminium (AI) mit höchstens 0,1 %; Silizium (Si) mit mindestens 0,1 % und höchstens 0,9 %, insbesondere höchstens 0,5 %; Chrom (Cr) mit mindestens 0,01 % und höchstens 1,0 %; Titan (Ti) mit höchstens 0,05 %,
• insbesondere mindestens 0,02 %; Bor (B) mit mindestens 0,0005 und höchstens 0,080 %, insbesondere mindestens 0,002 % und höchstens 0,006 %; Phosphor (P) mit höchstens 0,1 %, insbesondere höchstens 0,01 %; Schwefel (S) mit höchstens 0,05 %, insbesondere höchstens 0,01; optional weitere Legierungselemente mit einem Anteil von bis zu 1,55 % (1550 ppm); den Rest Eisen (Fe) und unvermeidbare Verunreinigungen.

The steel strip can, for example, be made of a steel material that can be hardened or tempered, in particular that contains manganese. the In addition to manganese, this can contain other micro-alloying elements. The steel material can, for example, contain the following proportions of alloying elements in percent by weight:

• Carbon (C) with at least 0.15% and at most 0.5%, in particular at most 0.4%; manganese (Mn) with at least 0.5% and at most 5.0%, in particular at least 0.8% and at most 2.5%; Aluminum (Al) with a maximum of 0.1%; Silicon (Si) with at least 0.1% and at most 0.9%, in particular at most 0.5%; Chromium (Cr) with a minimum of 0.01% and a maximum of 1.0%; Titanium (Ti) with a maximum of 0.05%,
• especially at least 0.02%; Boron (B) with at least 0.0005% and at most 0.080%, in particular at least 0.002% and at most 0.006%; Phosphorus (P) with a maximum of 0.1%, in particular a maximum of 0.01%; Sulfur (S) with at most 0.05%, in particular at most 0.01%; optionally other alloying elements with a proportion of up to 1.55% (1550 ppm); the balance iron (Fe) and unavoidable impurities.

As optional further alloying elements, the substrate can contain in particular at least one of: copper (Cu) with a maximum of 0.1%, nickel (Ni) with a maximum of 0.1%, niobium (Nb) with a maximum of 0.1%, molybdenum ( Mo) with a maximum of 1.0%, vanadium (V) with a maximum of 0.25%, without being limited thereto. The mass fraction of the optional alloying elements can also be lower, for example molybdenum can also be contained with at most 0.8%, 0.5% or 0.25%. The mass fraction of the optional alloying elements is a maximum of 1.55% in total, in particular a maximum of 1.0%, in particular a maximum of 0.8%. The alloying element niobium advantageously brings about a fine-grained structure of a component hot-formed from the alloy. In particular in combination with molybdenum, which can inhibit grain growth, a particularly fine-grained structure results, which in turn has a favorable effect on the strength of the component made from it. Examples of usable boron-manganese steel materials are 17MnB5, 20MnB5, 20MnB8, 22MnB5, 26MnB5 or 34MnB5.

The hot strip is preferably cleaned before coating, which can be done chemically, mechanically or electrochemically in particular. During cleaning, the solid burn-off (scale) that has occurred on the surface as a result of the hot-rolling process is removed. A common chemical or electrochemical process is pickling, which can be done, for example, in a continuous system in which the steel strip is conveyed through an acid bath. As already mentioned above, according to an alternative method, a hot strip can first be cold-rolled to a constant thickness in an intermediate step. The cold strip produced in this way is then cleaned, as described above, and coated, it being possible for the coating to be applied when the steel strip is in the as-rolled state. Before coating, the steel strip can have an initial thickness of 1.5 to 4.0 mm, for example.

The strip target temperature to which the steel strip is brought before it is immersed in the molten bath is, for example, above 580.degree. C., in particular above 600.degree. As an upper limit, the steel strip can in particular be brought to a strip target temperature of less than 620°C or also less than 610°C. In particular, the strip target temperature also includes a state immediately before or during immersion in the molten bath, ie the immersion temperature can be between 580 and 620° C., for example. The strip target temperature can be achieved directly by heating to the desired temperature, or indirectly by heating to a higher temperature and then cooling to the strip target temperature.

The steel strip is preferably heated in a continuous process, for example by means of an induction furnace or a radiant tube furnace. In an induction furnace, the steel strip can be heated to the strip target temperature (T3) at a speed (v) along a run length (L), preferably in an atmosphere with a hydrogen content (H2) of at least 30% by volume, preferably at least 50% by volume. -%. The heating is carried out in particular in such a way that the ratio of the product of the strip target temperature (T3), passage length (L) of the induction furnace and hydrogen content (H2) in the induction furnace, and the product of the heating rate (t') and speed (v) of the steel strip is greater than 96 s ² and/or less than 192 s ² [96 s ² < (T3 * L * H2)/(t' * v) < 192 s ² ]. In the case of inductive heating, the steel strip can be heated at a heating rate of more than 30 K/s, in particular more than 50 K/s. This applies above all to the temperature range from room temperature to 600°C. The heating time to the strip target temperature (T3) can be done in less than 60 seconds. The inductive heating carried out with one or more of the parameters mentioned produces a surface that can be coated very well. Together with the low coating temperature of in particular 580 to 620° C., preferably 600 to 610° C., a particularly thin diffusion zone is produced, which then contributes to an improvement in downstream manufacturing processes. The process control based on inductive heating in combination with the subsequent melt exchange coating can also be used as a heat-to- Coat method are referred to. Inductive heating has the advantage of a very short glow time, which can be less than 60 s. This shortens the time it takes to activate or clean the surface. Therefore, annealing is preferably carried out using atmospheres with a comparatively high reduction potential in order to be able to ensure adequate wettability of the melt.

However, it is also possible to carry out the process according to the Sendzimir process, which is used in particular for cold-rolled steel strips. Since the as-rolled structure produced by cold rolling must be recrystallized, heating preferably takes place in a radiant tube furnace that is large enough to recrystallize the material at more than 700°C for several minutes. Due to the appropriately adjusted atmosphere in this oven (hydrogen content, dew point, etc.) and a relatively long annealing time, the surface can be sufficiently cleaned to make it wettable for the coating process. The steel strip can be heated to the strip target temperature (T3) by means of a radiant tube furnace, for example, at a heating rate of 10 K/s to 30 K/s. The glow time can be more than 120 seconds. In particular, the manganese and silicon contents of the substrate to be coated have a major influence on the coatability of the material, since oxides of the alloying elements can form during annealing, which impede wetting of the aluminum-silicon melt on the surface. The necessary recrystallization requires a temperature of over 700°C, which means that cooling between the furnace and immersion in the molten bath is necessary in order to keep the temperature of the molten bath below the target 620°C. In this respect, when using the Sendzimir process, it is provided that cooling then takes place in order to bring the steel strip to an immersion temperature of below 620°C.

After heating, the steel strip runs through the melt. The melt preferably contains at least 85% by weight aluminum, 5 to 15% by weight silicon, up to 5% by weight iron and optionally one or more other alloying elements in minor proportions. An exemplary composition of the molten bath or the applied coating can contain up to 3 percent by weight iron, 12 to 13 percent by weight silicon, optionally one or more other alloying elements together up to 1.5 percent by weight, and the remainder aluminum. It goes without saying that unavoidable impurities may also be present. The proportion of the other alloying elements, for example at least one from the group of Mn, Cr, Ti, B, P, S, Sr, Sb, Cu, Ni, Nb, Mo, V, can together be up to 1.5 percent by weight, for example. By adding additional alloy components, the coating alloy can be modified in such a way that a layered structure of intermetallic phases is created, with either a silicon-rich phase embedded in an aluminum-rich phase in addition to the interdiffusion zone, or a three-layer layer structure consisting of the interdiffusion zone and two aluminum phases that differ in iron and silicon content.

The pre-coating is preferably applied to the steel strip with a thickness of at least 26 micrometers (µm), in particular at least 34 micrometers (µm). Alternatively or additionally, the pre-coating is preferably applied to the steel strip with a thickness of at most 50 micrometers (μm), in particular at most 46 micrometers (μm). During the hot-dip coating, diffusion processes take place between the melt material and the hot-rolled strip material, with a diffusion layer having a thickness of preferably less than seven micrometers being formed. With a thickness of the pre-coating of 26 to 50 microns, the coating weight per side of the steel strip is about 78 to 150 g/m ² .

According to one possible embodiment, a steel strip is used that has a relatively high roughness, for which hot strip is particularly well suited since it has a higher initial roughness than cold strip. Preferably, the steel strip used has a roughness for which at least one of the following applies: the roughness Ra is at least 0.8 and/or at most 2.0 microns; the roughness Rz is at least 4 and/or at most 12 microns; the roughness Rmax is at least 5 and/or at most 15 microns. The thickness of the pre-coating to be applied can be increased by a relatively high roughness. This has a positive effect against a later reduction in layer thickness in a possibly downstream manufacturing process, such as flexible rolling. The roughness changes slightly as a result of the coating, so that the following roughness values can result for the coated steel strip, for example: Ra: 0.4 to 3.0; Rz: 5 to 15; Rmax: 8 to 25.

The object is further achieved by a method for producing a hardened steel product, comprising: providing a precoated steel strip which is produced according to at least one of the above-mentioned embodiments; Flexible rolling of the precoated steel strip, with successive sections of the precoating th steel strips are rolled out to different degrees; Carving out a blank from the flexibly rolled steel strip; Heating the circuit board in such a way that the base material of the circuit board is at least partially austenitized, diffusion processes taking place between the base material and the pre-coating as a result of the heating; and hot working the heated blank, wherein the heated blank is worked and rapidly cooled to produce a hardened coated steel product.

Because strip material that is precoated according to the invention is used, the thickness of the diffusion layer of the precoated steel strip is relatively thin. In this way, the growth of the diffusion zone is also reduced during the subsequent hot forming. As already mentioned above, the thin diffusion zone results in improved processability for the downstream manufacturing processes. The friction between the coated steel strip and the respective tool is reduced. With flexible rolling, rollability improves due to lower rolling forces, which has a positive effect on wear and service life. The same applies analogously to hot forming, where reduced friction between the blank and the forming tool analogously leads to lower forming forces and reduced tool wear.

In flexible rolling, strip material with a substantially uniform sheet thickness is rolled out to strip material with variable sheet thickness over the length by changing the roll gap during the process. The sections of different thickness produced by flexible rolling extend transversely to the longitudinal direction or rolling direction of the strip material. After flexible rolling, the strip material can be wound up again into a coil and fed to another point for further processing, or it can be further processed directly, for example by cutting the strip material to length into individual sheet metal elements.

The flexible rolling can be carried out with reduction ratios of at least 1% and/or at most 60% based on the initial thickness of the precoated steel strip, in particular with reduction ratios between 3% and 55%. Due to the flexible rolling, the thickness of the pre-coating is reduced accordingly with the steel substrate. Depending on the desired component geometry, there can be any other desired thickness ranges or transitional regions between the thinnest sections and the thickest sections of the strip material. It goes without saying that between the coating and the flexible rolling further steps, such as heating, winding to the coil or unwinding from the coil, straightening, cleaning or the like can be interposed.

The object is further achieved by a hot-formed steel product with a base material and an aluminium-silicon coating, which is produced in particular according to one of the above-mentioned methods, the steel product having a thickness of 1.5 to 4 mm; wherein the base material contains the above proportions of alloying elements; wherein the coating contains aluminum and silicon and has a thickness of at least 20 micrometers (µm) and at most 50 micrometers (µm); and wherein the coating comprises a diffusion layer less than 14 micrometers (14 µm) thick in the interface to the base material, an intermetallic layer thereover, and a surface layer overlying the intermetallic layer. The intermetallic layer may have a silicon content of 10 to 18% by weight and the surface layer 5 to 10% by weight.

• 1 schematisch ein erfindungsgemäßes Verfahren zum Herstellen von beschichtetem Stahlband, insbesondere zum Herstellen von Warmformteilen, in einer ersten Ausführungsform;
• 2 einen Ausschnitt des beschichteten Stahlbands nach dem Vorbeschichten in vergrößerter schematischer Darstellung;
• 3 schematisch ein erfindungsgemäßes Verfahren zum Herstellen eines beschichteten, gehärteten Formteils;
• 4 einen Ausschnitt des beschichteten und flexibel gewalzten Stahlsubstrats nach dem Warmumformen in vergrößerter schematischer Darstellung; und
• 5 schematisch ein erfindungsgemäßes Verfahren zum Herstellen von beschichtetem Stahlband, insbesondere zum Herstellen von Warmformteilen, in einer zweiten Ausführungsform.

Preferred exemplary embodiments are explained below with reference to the drawing figures. Herein shows:

• 1 schematically shows a method according to the invention for producing coated steel strip, in particular for producing hot-formed parts, in a first embodiment;
• 2 a section of the coated steel strip after pre-coating in an enlarged schematic representation;
• 3 schematically a method according to the invention for producing a coated, cured molded part;
• 4 a section of the coated and flexible rolled steel substrate after hot forming in an enlarged schematic representation; and
• 5 schematically shows a method according to the invention for producing coated steel strip, in particular for producing hot-formed parts, in a second embodiment.

the 1 and 2 are described together below. 1 shows a method according to the invention for producing a coated steel substrate. In strip form, the steel substrate is also referred to as steel strip or generally as strip material. In its isolated state, the steel substrate is also referred to as a blank.

The coating method S10 comprises the sub-steps of heating the steel strip 2 to a target strip temperature T3 of less than 625° C. and hot-dip coating the strip target Temperature-heated steel strip 2 in a melt 4 made of an aluminum-silicon alloy, which is set to a temperature T4 of less than 620°C.

Within the scope of the present disclosure, a hardenable steel flat product is to be included as the steel substrate 2, which can contain, for example, the following proportions of alloying elements in percent by weight: carbon (C) with at least 0.15% and at most 0.5%, in particular at most 0, 4%; manganese (Mn) with at least 0.5% and at most 5.0%, in particular at least 0.8% and at most 2.5%; Aluminum (Al) with a maximum of 0.1%; Silicon (Si) with at least 0.1% and at most 0.9%, in particular at most 0.5%; chromium (Cr) with at least 0.01% and at most 1.0%; titanium (Ti) with at most 0.05%, in particular at most 0.01%; Boron (B) with at least 0.0005% and at most 0.080%, in particular at least 0.002% and at most 0.006%; Phosphorus (P) with a maximum of 0.1%, in particular a maximum of 0.01%; Sulfur (S) with at most 0.05%, in particular at most 0.01%; optionally other alloying elements with a proportion of up to 1.55% (1550 ppm); the balance iron (Fe) and unavoidable impurities.

This alloy composition includes, for example, boron-manganese steel materials such as 17MnB5, 20MnB5, 20MnB8, 22MnB5, 26MnB5 and 34MnB5. In its initial state, the steel material can have a yield point of, for example, 150 to 1100 MPa and/or a tensile strength of at least 450 MPa. The optional further alloying elements can be selected from the group: copper (Cu) with a maximum of 0.1%; nickel (Ni) with a maximum of 0.1%; niobium (Nb) with at most 0.1%; molybdenum (Mo) with a maximum of 1.0%; vanadium (V) with a maximum of 0.25%; without being restricted thereto, the stated percentages in each case relating to percentages by mass of the steel substrate. One or more of the mentioned optional alloying elements can be used. The total mass fraction of the optional alloying elements is at most 1.55%, in particular at most 1.0%, preferably at most 0.8%.

Hot-rolled strip or cold-rolled strip can be used as the steel substrate 2, with hot-rolled steel strip being particularly suitable for an efficient manufacturing process, since an additional manufacturing step, namely cold rolling before coating, can be omitted. Before coating, the steel strip used preferably has a roughness of at least one of the following values: Ra of at least 0.8 and/or at most 2.0 microns; a roughness Rz of at least 4 and/or at most 12 microns; a roughness Rmax of at least 5 and/or at most 15 microns. Due to a relatively high roughness, the melt can adhere particularly well during the subsequent hot-dip coating, so that pre-coatings 8 with greater thicknesses can also be applied well.

The steel substrate 2 can, for example, be unwound from a coil 6 by means of a reel 5 . The steel substrate can then optionally be cleaned in a cleaning device 7 before coating. The cleaning can be done chemically, mechanically or electrochemically, for example.

It can be seen that the coating process S10 is continuously performed, and in the present embodiment, heating is performed in an induction furnace 3, but not limited thereto. In the induction furnace 3, the steel substrate passes through different furnace areas. The steel substrate is heated to the strip target temperature T3 at a speed v along a passage length L, preferably in an atmosphere with a hydrogen content (H2) of at least 30% by volume, preferably at least 50% by volume. The heating is carried out in particular in such a way that the ratio of the product of the strip target temperature T3, the length L of the induction furnace and the hydrogen content (H2) in the induction furnace 3, and the product of the heating rate t' and the speed v of the steel strip 2 is greater than 96 s ² and /or is less than 192 s ² , i.e. 96 s ² < (T3 * L * H2)/(t' *v ) < 192 s ² . The passage length L can be 10 to 30 m, for example, and the speed v can be between 0.6 to 2 m/s, for example. In the case of inductive heating, the steel strip can be heated at a heating rate of more than 30 K/s, in particular more than 50 K/s. This applies above all to the temperature range from room temperature to 600°C. The heating time to the strip target temperature T3 can take place in less than 60 seconds.

The strip target temperature to which the steel strip 2 is heated before it is immersed in the molten bath 4 is, for example, above 580°C, in particular above 600°C and/or less than 620°C, in particular less than 610°C.

Immediately after heating, the steel strip 2 runs through the molten bath 4 in which it is provided with a pre-coating 8 . The steel strip 2 can have at least approximately the strip target temperature when it is immersed, that is to say the immersion temperature can in particular be above 580° C. and/or below 620° C. The alloy composition of the melt 4 determines the composition of the pre-coating 8 applied to the steel substrate 2. The melt 4 or the pre-coating layer 8 contains aluminum (Al) with at least 85 percent by weight and silicon (Si) with up to 15 percent by weight. It goes without saying that other alloying elements can also be included, for example iron and/or other alloying elements with a total of up to 5 percent by weight. In the hot-dip process, the steel substrate 2 runs through the molten bath of liquid coating material in the coating plant 10, which adheres to the surface of the substrate 2, so that a pre-coated steel substrate 2' is produced. The melt of the coating material can contain, for example, 5 to 15 percent by weight silicon, 2 to 4 percent by weight iron, optionally one or more other alloying elements, such as at least one from the group consisting of Mn, Cr, Ti, B, P, S, Sr, Sb, Cu , Ni, Nb, Mo, V, together up to 1.5 percent by weight, and the remainder aluminum and unavoidable impurities.

It has been shown that the use of silicon with a proportion of 12 to 13 percent by weight is particularly advantageous. The lowest melting point of an aluminium-silicon alloy with a eutectic composition is around 12.5-12.7 percent by weight silicon and is 577 °C. If the temperature of the coating bath in the process is kept as low as possible and at the same time effective cooling is used, the growth of the Fe-Al diffusion zone can be slowed down, since the formation of these phases is based on the principle of diffusion, which is subject to a strong temperature dependence. A prior reduction of oxides on the steel surface can ensure good wettability of the steel strip 2 with the coating alloy 8 . The reduction of the surface is subject to a thermal dependence: Due to a lower temperature in the reducing part of the furnace 3 at approx. 300 to 625 °C and a relatively short furnace length and the associated short residence time at these temperatures, there is relatively little time left for the surface of oxides to reduce. It is therefore favorable to set the furnace atmosphere appropriately and to anneal in the inlet area of furnace 3 at very high heating rates of more than 20 K/s, preferably more than 50 K/s, in order to avoid oxidation in this area of furnace 3 as far as possible. A furnace atmosphere with O2 contents of less than 10 ppm, in particular less than 6 ppm, and H2O contents of less than 500 ppm, in particular less than 200 ppm, is preferably set. Furthermore, high H2 contents can be blown into the furnace nozzle 12 in order to achieve good reduction conditions.

The melt 4 has a temperature T4 of below 620°C. Due to the hot-dip coating below the mentioned temperature T4, the diffusion processes between the melt 4 and the steel strip 2 are relatively short or low, so that a relatively thin pre-diffusion layer is formed. 2 FIG. 12 schematically shows a section of a correspondingly precoated strip material 2', in which a thin diffusion zone 9 of less than seven micrometers is formed in the transition between the substrate 2 and the precoating 8. The layer thickness of the pre-coating 8 can be adjusted, for example, in such a way that the reduction in the layer thickness due to a downstream production process, such as flexible rolling, is compensated for in order to improve the protection against corrosion. On the outside is the pre-coating 8, which can have a minimum thickness of 26 micrometers, in particular at least 34 micrometers, and/or a maximum thickness of 50 micrometers, in particular up to 46 micrometers. With a thickness of the pre-coating of 26 to 50 microns, the coating weight per side of the steel strip is about 78 to 150 g/m ² . The combination of steel substrate 2 with pre-coating is provided with reference number 2'.

The inductive heating carried out using the above parameters contributes to the creation of a surface that can be coated very well. Together with the low coating temperature of in particular 580 to 620° C., preferably 600 to 610° C., a particularly thin diffusion zone is produced, which then contributes to an improvement in downstream manufacturing processes.

After running through the molten bath 4 through the coating unit 10, the coated steel strip 2' can optionally run through a nozzle unit 13, with which excess liquid coating material is blown off the surface. In a downstream cooling unit 14, the coated steel substrate can be cooled in what is known as an air cooling section. The steel strip 2′ can then optionally be skin-passed in a skin-passing unit 15 to improve the surface. The skin-passing unit comprises several skin-passing rolls 16, 16'. Furthermore, the coated steel substrate 2 ′ can be straightened in a straightening unit 17 by means of a plurality of rollers 18 . The coated steel strip 2' can then be coiled into a coil 5' in a coiler 6'.

3 shows a method according to the invention for producing a hardened molded part from a coated steel substrate 2 ', which according to in 1 and 2 described method has been prepared.

After the pre-coating 8 has been applied in method step S10, the coated steel substrate 2' is flexibly rolled in step S20 by means of a rolling unit 19. For this purpose, the coated steel strip 2', which before flexible rolling has a largely constant sheet thickness along its length, is rolled by means of rollers 20, 21 in such a way that it has a variable sheet thickness along the rolling direction. The coated and flexibly rolled steel substrate is provided with the reference number 22 .

During rolling, the process is monitored and controlled, with the data determined by a sheet thickness measurement 23 being used as an input signal for controlling the rolls 20, 21. The flexible rolling is carried out according to the desired target thickness profile of a blank to be cut from the strip material 22 or a component to be produced from it. The flexible rolling can be carried out with a rolling degree of at least 1% and/or a maximum of 60%, starting from the initial thickness of the precoated steel substrate 2', in particular with a rolling degree of between 3% and 55%. After flexible rolling, the pre-coating 8 can have a reduced first thickness of in particular less than 20 micrometers in thinner first sections and a reduced second thickness of in particular more than 33 micrometers, preferably more than 36 micrometers in thicker second sections.

After the flexible rolling, the strip material 22 can be wound up again into the coil 5' so that it can be transported to a subsequent processing station. After the rolling process, the steel strip 22 can be smoothed in a subsequent process step, which takes place in a strip straightening device 26 . The smoothing step is optional and can also be omitted.

After the flexible rolling (S20) or smoothing (if provided), the coated and flexibly rolled steel strip 22 is separated in method step S30. In this case, individual sheet metal blanks 32 are worked out of the steel strip 22, for example by means of a punching and/or cutting device 24. Depending on the shape of the sheet metal blanks 32 to be produced, these can be punched out of the strip material 22 as a shaped cut, with an edge that is not used being discarded as scrap. or the strip material 22 can simply be cut into sections.

The blanks 32 are hot-formed in a subsequent step S40, which can also be referred to as press hardening. During hot forming or press hardening, the blank 32 is heated to a temperature that is generally above the AC1 or AC3 temperature of the material, for example between 750° C. and 1000° C. Heating may be accomplished by any suitable method, such as inductive heating, conductive heating, roller hearth heating, contact hot plate heating, infrared, or other known methods. The holding time for austenitizing the coated board 32 depends on the temperature chosen and can be between 4 and 10 minutes. After heating to the austenitization temperature, the blank 32 is then placed in a hot forming tool 25 and shaped therein and cooled or quenched so quickly that at least partially a martensitic hardened structure is produced in the molded part 42 produced in this way.

According to a first option, hot forming (S40) can be carried out as a direct process. In this case, the blank 32 is heated directly to the austenitization temperature and then hot-formed to the desired final contour in one step. A previous (cold) preforming does not take place here. According to a second possibility, hot forming can also be carried out as an indirect process, which includes the partial steps of cold preforming, subsequent heating of the cold preformed component to austenitization temperature and subsequent hot forming to produce the final contour of the molded part.

As a result of the heating of the blank 32 carried out as part of the hot forming, further diffusion processes take place between the base material of the steel substrate 2 and the pre-coating material 8 . In the process, iron diffuses from the steel substrate 2 into the coating material 8, so that the thickness of the coating 8 increases overall compared to the thickness present after flexible rolling, with different layers 28a, 28b, 28c, 28d forming. The layer structure of a finished molded part 42 is shown schematically in 4 shown. The following layers in particular can form from the outside in by way of hot forming: a surface layer 28a, in particular with a proportion of iron of 34 to 45%, aluminum of 50 to 60% and silicon of 5 to 10%; an intermetallic layer, in particular with an iron content of 45 to 60%, aluminum of 25 to 40% and silicon of 10 to 18%; an intermediate layer 28c, in particular with an iron content of 39 to 47%, aluminum of 53 to 61% and silicon of 0 to 2%; and an interdiffusion layer 28d, in particular with a proportion of α-iron or iron of 86 to 95%, aluminum of 4 to 10%, silicon of 0 to 5%, each in percent by weight.

Preferably, the coating 28 of the hot worked product 42 in the thinner first portions has a final coating thickness greater than 15 microns, more preferably greater than 20 microns. In the thicker second sections, the coating 28 can have a second final thickness after heating or hot forming, in particular of more than 30 micrometers, preferably more than 35 micrometers. The diffusion layer 28d of molding 42 preferably has a thickness of less than 14 microns (14 µm).

5 shows a method according to the invention for producing a coated steel substrate in a modified embodiment. The present method largely corresponds to the method according to 1 and 2 , to the description of which reference is made in this respect. Identical or mutually corresponding components are provided with the same reference symbols as in the above figures.

A special feature of the embodiment according to 5 is that the heating takes place by means of a so-called Sendzimir process, which is particularly suitable for cold-rolled steel strips 2. The heating takes place in a radiant tube furnace 3, which is large enough to recrystallize the material at more than 700° C. for several minutes. In the radiant tube furnace 3, an atmosphere (hydrogen content, dew point, etc.) and a relatively long annealing time are set such that the surface is sufficiently cleaned to make the steel strip 2 wettable for the coating process. The steel strip 2 is heated to the strip target temperature T3 by means of the radiant tube furnace 3, in particular at a heating rate of 10 K/s to 30 K/s. The glow time can be more than 120 seconds. The necessary recrystallization requires a temperature of over 700°C. Therefore, a cooling system 27 is provided between furnace 3 and immersion in the molten bath 4 in order to cool the steel strip to the strip target temperature, which prevents the molten bath from heating up to over 620°C. For the rest, the procedure corresponds to 5 the method described above, to the description of which reference is made for abbreviation.

Reference List

2

steel substrate

3

oven

4

melt

5, 5'

coil

6, 6'

reel

7

cleaning unit

8th

pre-coating

9

pre-diffusion layer

10

coating unit

12

furnace trunk

13

nozzle unit

14

cooling unit

15

dressing unit

16, 16'

temper rolls

17

straightening unit

18

roll

19

rolling unit

20

rolls

21

rolls

22

tape material

23

thickness measurement unit

24

cutting device

25

hot forming tool

26

straightening device

27

cooling

28

coating

28a,b,c,d

layers

32

circuit board

42

molding

S10-S40

steps

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2009/090555 A1 [0003]
• WO 2020020644 A1 [0004]
• WO 2006/097237 A1 [0005]

Claims (15)

A method of making coated steel strip, comprising: heating a steel strip (2) of hardenable steel to a target strip temperature (T3) of less than 625°C, and Hot dip coating of the steel strip (2) heated to the strip target temperature (T3) in a melt (4) containing at least aluminum and silicon, the melt (4) having a temperature (T4) of less than 620°C. procedure after claim 1 , characterized in that the steel strip (2) is a steel hot strip. procedure after claim 1 or 2 , characterized in that the steel strip (2) is heated to a target strip temperature (T3) of less than 620°C and/or to a target strip temperature (T3) of more than 580°C, in particular more than 600°C . Procedure according to one of Claims 1 until 3 , characterized in that the heating of the steel strip (2) to the strip target temperature (T3) in the continuous process at a speed (v) in an induction furnace along a run length (L) in an atmosphere with a hydrogen content (H ₂ ) of at least 30 vol %, the heating being carried out in such a way that the ratio of the product of strip target temperature (T3), run length (L) of the induction furnace and hydrogen content (H ₂ ) in the induction furnace, and the product of heating rate (t') and speed (v) the steel strip () is greater than 96 s ² and/or less than 192 s ² . [96 s ² < (T3 * L * H ₂ )/(t' * v) < 192 s ² ] procedure after claim 4 , characterized in that the steel strip (2) is heated to the strip target temperature at a heating rate (t') of more than 30 K/s, in particular more than 50 K/s, and/or with a heating time of less than 60 seconds (T3) is heated. Procedure according to one of Claims 1 until 3 , characterized in that the steel strip (2) is heated to the strip target temperature (T3) in a continuous process by means of a radiant tube furnace, in which the steel strip (2) is heated at a heating rate (t') of 10 K/s to 30 K/s and/or an annealing time of more than 120 seconds. Procedure according to one of Claims 1 until 6 , characterized in that the steel strip (2) is heated in an atmosphere containing at least 30 vol .-% hydrogen (H ₂ ) in a last third of the run length (L). Procedure according to one of Claims 1 until 7 , characterized in that during the hot-dip coating, diffusion processes take place between the melt (4) and the steel strip (2), a diffusion layer (28d) with a thickness of less than seven micrometers (7 µm) being formed. Procedure according to one of Claims 1 until 8th , characterized in that the pre-coating (8) is applied to the steel strip (2) with a thickness (d8) of at least 26 micrometers (µm), in particular at least 34 micrometers (µm), and/or in that the pre-coating (8 ) with a thickness (d8) of at most 50 micrometers (µm), in particular at most 46 micrometers (µm), is applied to the steel strip (2). Procedure according to one of Claims 1 until 9 , characterized in that steel strip (2) with a surface roughness Ra of at least 0.8 micrometers and/or Rz of at least 4 micrometers and/or Rmax of 5 to 15 micrometers is used. Procedure according to one of Claims 1 until 10 , characterized in that the steel strip (2) contains the following proportions of alloying elements, each in percent by weight: carbon (C) with more than 0.15% and less than 0.5%; manganese (Mn) greater than 0.5% and less than 5.0%; Aluminum (Al) with less than 0.1%; silicon (Si) with more than 0.1% and less than 0.9%; chromium (Cr) with more than 0.01% and less than 1.0%; titanium (Ti) with less than 0.05%; boron (B) with more than 0.0005 and less than 0.080%; phosphorus (P) with less than 0.1%; sulfur (S) with less than 0.05%; optionally further alloying elements with a proportion of up to 1.55%; the balance iron (Fe) and unavoidable impurities. Procedure according to one of Claims 1 until 11 , characterized in that the melt (4) contains the following proportions of alloying elements in percent by weight: 5 to 15 percent by weight silicon, up to 5 percent by weight iron, optionally one or more other alloying elements together with up to 1.5 percent by weight, and remainder aluminum and unavoidable impurities, at least one from the group consisting of Mn, Cr, Ti, B, P, S, Cu, Ni, Nb, Mo, V, Sr and Sb being used as further alloying elements. Procedure according to one of Claims 1 until 12 , characterized in that the melt (4) contains 12.0 to 13.0% silicon. A method of manufacturing a hardened steel product comprising: providing a precoated steel strip (2) according to any one of Claims 1 until 13 is manufactured; Flexible rolling of the pre-coated steel strip (2), successive sections of the pre-coated steel strip (2) being rolled out to different extents; Machining a blank (32) from the flexibly rolled steel strip (22); Heating the circuit board (32) in such a way that the base material of the circuit board (32) is at least partially austenitized, diffusion processes between the base material and the pre-coating (8) taking place as a result of the heating; and hot working the heated blank (32), wherein the heated blank (32) is worked and rapidly cooled to produce a hardened steel product (42) having a coating (28). Hot-formed steel product, in particular manufactured according to the method Claim 14 , with a base material and an aluminium-silicon coating (28), the steel product (42) having a thickness of 1.5 to 4 mm, the base material containing the following proportions of alloying elements, each in percentage by weight: carbon (C) with more less than 0.15% and less than 0.5%; manganese (Mn) greater than 0.5% and less than 5.0%; Aluminum (Al) with less than 0.1%; silicon (Si) with more than 0.1% and less than 0.9%; chromium (Cr) with more than 0.01% and less than 1.0%; titanium (Ti) with less than 0.05%; boron (B) with more than 0.0005 and less than 0.080%; phosphorus (P) with less than 0.1%; sulfur (S) with less than 0.05%; optionally other alloying elements with a proportion of up to 1.55%; the balance iron (Fe) and unavoidable impurities; the coating (28) containing aluminum and silicon and having a thickness of at least 20 micrometers (µm) and at most 50 micrometers (µm); characterized in that the coating (28) has a diffusion layer (28d) with a thickness of less than 14 microns (14 µm).

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