Method of hot press forming, with improved properties

Abstract

The invention relates to a method for producing a pressed flat steel sheet part by way of a special heating method prior to forming. The invention also relates to a method for reducing refuse during the production of pressed flat steel sheet parts, in particular in the production method according to the invention.

Description

[0001] Hot press forming process with improved properties

[0002] The invention relates to a method for producing a sheet steel shaped part by hot forming a sheet steel blank.

[0003] "Steel sheet blanks" here refers to blanks from flat steel products, such as blanks. When we talk about a "steel flat product" or "sheet metal product," we mean rolled products, such as steel strips or sheets, from which "sheet metal blanks" (also called blanks) are cut for the production of car body components, for example. "Formed sheet metal parts" or "sheet metal components" are made from such sheet metal blanks, whereby the terms "formed sheet metal part" and "sheet metal component" are used synonymously here.

[0004] All information regarding the contents of the steel compositions specified in this application is based on weight, unless expressly stated otherwise. All unspecified "%" data relating to a steel alloy are therefore to be understood as data in "wt%." With the exception of the data relating to the residual austenite content of the microstructure of a sheet metal part according to the invention, which is based on volume (specified in "vol%"), data on the contents of the various microstructure constituents refer to the area of ​​a microsection of a sample of the respective product (specified in area percent, "area%"), unless expressly stated otherwise. Information provided in this text regarding the contents of the constituents of an atmosphere refers to the volume (specified in "vol%").

[0005] Where formulas or conditions are mentioned in this text in which values ​​are calculated or formed on the basis of contents of certain alloying elements, the respective contents of alloying elements are inserted in these formulas or conditions in wt.%, unless otherwise stated.

[0006] Mechanical properties reported here, such as tensile strength, yield strength, and elongation, were determined in tensile tests according to DIN-EN ISO 6982-1, specimen shape 2 (Appendix B, Table B1) (as of June 2020), unless explicitly stated otherwise. The bending angle is determined according to VDA standard 238-100. The microstructure was determined on longitudinal sections subjected to etching with 3% Nital (alcoholic nitric acid). The content of retained austenite was determined by X-ray diffraction.

[0007] The steels used to make up the steel substrates of sheet steel blanks processed according to the state of the art and the invention explained here include, in particular, the so-called "MnB steels," which are standardized in EN 10083-3. A typical example of such a steel is the steel known as 22MnB5, which can be found in the 2004 Steel Code under the material number 1.5528.

[0008] Steels of the type specified above enable reliable process control during the hot forming of steel sheet blanks made from them into formed steel sheet parts. Due to their composition, they have the special feature that the steel sheet parts produced from them by hot forming can be given high strength through heat treatment. For this purpose, the component obtained through hot forming can be selectively cooled while still in the hot forming tool. At the same time, however, steel sheet blanks and the steel sheet parts hot-formed from them, which are made from steels of the type in question here, are sensitive to corrosive attack due to the high Mn content of their steel substrate. Therefore, such steel sheet blanks are usually coated with metallic protective coatings before hot forming into the respective steel sheet part, which are intended to protect the steel substrate against corrosion.

[0009] EP 2086755 B1 discloses a method for producing a hot-formed, coated steel component, in which a protective layer consisting of aluminum or an aluminum alloy containing, by mass, 8-11% Si and 2-4% Fe, is applied to a steel strip with a thickness of 20 to 33 μm by hot-dip coating. The steel strip consists of a steel containing, by mass, between 0.15-0.5% C, between 0.5-3% Mn, between 0.1-0.5% Si, between 0.01-1% Cr, less than 0.2% Ti, less than 0.1% Al, less than 0.1% P, less than 0.05% S, and between 0.0005-0.08% B, the remainder being iron and unavoidable impurities. Blanks are cut from the coated steel strip and then heated in a furnace at a constant temperature for a specific annealing time, with the respective annealing time and furnace temperature being selected depending on the thickness of the blank.For a blank that is 0.7 mm to 1.5 mm thick, the furnace temperatures and annealing times are located in a furnace temperature-annealing time coordinate system in a field with the following corner points: Point A - 930 °C, 3 min / Point B - 930 °C, 6 min / Point C - 880 °C, 13 min / Point D - 880 °C, 4 min. For a blank that is 1.5 - 3 mm thick, on the other hand, furnace temperatures and annealing times should be selected that are arranged in the furnace temperature-annealing time coordinate system in a field determined by the corner points E 940 °C, 4 min / F - 940 °C, 8 min / G - 900 °C, 6.5 min / H - 900 °C, 13 min. The blanks heated in this way are hot-formed into a steel component, removed from the forming tool and cooled from the hot-forming temperature to 400 °C at a cooling rate of at least 50 °C / s.

[0010] DE 10 2017 120 128 A1 also discloses a method for producing a hot-formed, coated steel component. In this case, a roller-hearth furnace is used to heat the coated steel sheet blanks. The roller-hearth furnace has several zones with different furnace temperatures. In particular, a peak heating zone with temperatures of 1080°C and above is set up to achieve rapid heating.

[0011] Heating coated steel sheet blanks on an industrial scale presents several challenges:

[0012] Generally, the steel sheet blanks must be heated for a specific time to ensure sufficient diffusion of iron from the steel substrate into the aluminum-based protective coating. This creates an aluminum-iron alloy with a higher melting point in the protective coating, which protects the subsequent steel sheet part against corrosion. Sufficient diffusion of iron into the protective coating is achieved by appropriately adjusting the furnace temperature and annealing time. Greater diffusion can be achieved by both a higher furnace temperature and a longer annealing time.

[0013] However, both the higher furnace temperature and the longer annealing time also have disadvantages. Longer annealing times lead to longer overall process times, which negatively impacts efficiency in an industrial environment. In addition, heating typically takes place in a roller-hearth furnace, through which the steel sheet blanks are moved at a constant speed. Longer annealing times therefore lead to larger dimensions of the roller-hearth furnaces. Consequently, it is advantageous to keep the total time a steel sheet blank spends in the roller-hearth furnace as short as possible. Higher furnace temperatures, on the other hand, have the disadvantage that energy consumption and effort increase disproportionately. For example, the installation of a peak heating zone with temperatures of 1080 °C, as in DE 10 2017 120 128 Al, requires stronger thermal insulation and significantly higher energy consumption. It is therefore advantageous if the steel sheet blanks are annealed for a particularly long time at lower temperatures.This has the further advantage of reducing scrap. For example, during the ongoing production process, a flow interruption may occasionally occur, meaning that the steel sheet blanks, which should actually be moving through the roller hearth furnace at a constant speed, are either stopped or moved at a slower rate. Consequently, the steel sheet blanks are stored in an area with a specific temperature for longer than planned. This is generally not critical if the area is low-temperature.However, if the steel sheet blank is in an area with high temperature for too long, the diffusion process is too advanced and the produced sheet metal component cannot be used because the further processing properties, such as weldability, are impaired. The proportion of scrap is therefore correspondingly smaller if the proportion of high temperature zones can be reduced.

[0014] It is also important to consider the furnace temperatures and when the steel sheet blanks are exposed to them. High furnace temperatures at the beginning of the heating process lead to a high heating rate of the steel sheet blanks and thus to a fast production process, but they can also have the disadvantage that material of the aluminum-based protective coating liquefies. As already explained, iron diffuses into the aluminum-based protective coating during heating, which increases the melting point of the protective coating. If the steel sheet blank is exposed to a very high furnace temperature too early, the melting point of the protective coating has not yet risen sufficiently due to diffusion, so that an increasing number of liquid phases form. These liquid phases lead to roller adhesion of material of the protective coating to the rollers of the roller hearth furnace.These roll deposits then have to be removed regularly, which leads to short maintenance cycles and thus an inefficient manufacturing process.

[0015] Another boundary condition is the temperature of the steel sheet blank at the end of the heating process, i.e., upon leaving the roller hearth furnace. For example, to achieve a predominantly martensitic structure during forming, the steel sheet blank must have a temperature above the ACl temperature when placed in the forming tool, at which the formation of austenite begins during a heating process. To achieve a fully martensitic structure, the steel sheet blank must even have a temperature above the AC3 temperature when placed in the forming tool. Depending on the length of the transfer time between leaving the roller hearth furnace and placing it in the forming tool, the temperature of the steel sheet blank must be correspondingly higher at the end of the heating process, as a certain amount of cooling occurs during the transfer time.

[0016] The object of the present invention is to provide an improved method for producing a shaped steel sheet part under these conditions. This object is achieved by a method for producing a shaped steel sheet part comprising the following work steps: a) Providing a steel sheet blank with a thickness d of at least 0.7 mm and a maximum of 3.5 mm, comprising a steel substrate consisting of a steel containing 0.04-0.45 wt.% C, 0.1-3 wt.% Mn, and optionally up to 0.01 wt.% B, and wherein the steel sheet blank has an aluminum-based anti-corrosion coating on at least one side; b) Heating the steel sheet blank in a furnace, i. wherein the steel sheet blank passes through the following steps in the furnace:

[0017] 1. optionally in a first time period tl a first heating zone with a first temperature Tl

[0018] 2. optionally in a second time period t2 a second heating zone with a second temperature T2

[0019] 3. in a third time period t3 a third heating zone with a third temperature T3

[0020] 4. in a fourth time period t4 a fourth heating zone with a fourth temperature T4

[0021] 5. optionally, in a fifth time period t5, a fifth heating zone with a fifth temperature T5 ii. For steel sheet blanks with a thickness d < 1.5 mm, the following parameters are set:

[0022] 45s < tl < 580s and 700°C < Tl < 850°C

[0023] 30s < t2 < 90s and 930°C < T2 < 980°C

[0024] 60s < t3 < 700s and 700°C < T3 < 880°C

[0025] 30s < t4 < 180s and 930°C < T4 < 980°C

[0026] 0s < t5 < 45s and 700°C < T5 < T4 iii. and for steel sheet blanks with a thickness d > 1.5 mm the following parameters are set:

[0027] 60s < tl < 640s and 710°C < TI < 840°C

[0028] 30s < t2 < 90s and 940°C < T2 < 980°C

[0029] 60s < t3 < 760s and 720°C < T3 < 900°C

[0030] 30s < t4 < 200s and 940°C < T4 < 980°C

[0031] Os < t5 < 60s and 700°C < T5 < T4 c) Inserting the heated sheet metal blank into a forming tool, whereby the transfer time t required for removing the blank from the furnace and inserting it T rans is at most 20 s, preferably at most 15 s; d) hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is cooled during the hot-press forming process over a period twz of more than 1 s at a cooling rate r which is at least partially more than 27 K / s W z is cooled to the target temperature Tziei and optionally maintained there; e) removing the sheet metal part cooled to the target temperature from the tool.

[0032] In the method according to the invention, a steel sheet blank is thus provided (work step a)) consisting of a steel composition suitably composed according to the following explanations. The steel sheet blank has an aluminum-based anti-corrosive coating on at least one side. The anti-corrosive coating is preferably applied to both sides of the steel sheet blank. It is therefore a one-sided or two-sided anti-corrosive coating. The two opposite large surfaces of the steel sheet blank are referred to as the two sides of the steel sheet blank. The narrow surfaces are referred to as the edges.

[0033] During heating step b), the steel sheet blank then passes through at least two heating zones, designated as the third and fourth heating zones. Additional heating zones, such as the first, second, or fifth heating zones, can be added optionally. The numbering of the heating zones is to be understood as a chronological sequence, so that the steel sheet blank passes through the heating zones in ascending order. Optional heating zones are of course omitted if they are not present. If, for example, the first of the optional heating zones is present but the second or fifth heating zones are not, the steel sheet blank passes through the first, third, and fourth heating zones one after the other (in this order). In this sense, step b) ii) should also be understood as meaning that the parameters tl, TI, t2, T2, t5, and T5 are only set if the corresponding optional heating zones are implemented.If one of the optional heating zones under b) i) is not implemented, its corresponding parameters are also not set. The entire step b) can alternatively be described as follows: b) Heating the steel sheet blank in a furnace, i., whereby the steel sheet blank passes through the furnace:

[0034] 1. optionally in a first time period tl a first heating zone with a first temperature Tl, whereby for steel sheet blanks with a thickness d < 1.5mm the following parameters are set:

[0035] 45s < tl < 580s and 700°C < Tl < 850°C and for steel sheet blanks with a thickness d >1.5 mm the following parameters must be set:

[0036] 60s < tl < 640s and 710°C < Tl < 840°C

[0037] 2. optionally in a second time period t2 a second heating zone with a second temperature T2, whereby for steel sheet blanks with a thickness d < 1.5mm the following parameters are set:

[0038] 30s < t2 < 90s and 930°C < T2 < 980°C and for steel sheet blanks with a thickness d >1.5 mm the following parameters are set:

[0039] 30s < t2 < 90s and 940°C < T2 < 980°C

[0040] 3. in a third time period t3, a third heating zone with a third temperature T3, whereby for steel sheet blanks with a thickness d < 1.5 mm the following parameters are set:

[0041] 60s < t3 < 700s and 700°C < T3 < 880°C and for steel sheet blanks with a thickness d >1.5 mm the following parameters must be set:

[0042] 60s < t3 < 760s and 720°C < T3 < 900°C

[0043] 4. in a fourth time period t4, a fourth heating zone with a fourth temperature T4, whereby for steel sheet blanks with a thickness d < 1.5 mm the following parameters are set:

[0044] 30s < t4 < 180s and 930°C < T4 < 980°C and for steel sheet blanks with a thickness d >1.5 mm the following parameters are set:

[0045] 30s < t4 < 200s and 940°C < T4 < 980°C

[0046] 5. optionally in a fifth time period t5 a fifth heating zone with a fifth temperature T5, whereby for steel sheet blanks with a thickness d < 1.5 mm the following parameters are set:

[0047] Os < t5 < 45s and 700°C < T5 < T4 and for steel sheet blanks with a thickness d >1.5 mm the following parameters are set:

[0048] Os < t5 < 60s and 700°C < T5 < T4

[0049] In a preferred variant, heating zones arranged one after the other are arranged directly adjacent to one another, which means that a transfer time from one heating zone to an immediately adjacent heating zone is not more than 5 seconds, in particular not more than 2 seconds. In preferred embodiments, the

[0050] - first heating zone and the second heating zone arranged directly next to each other

[0051] - and / or the second heating zone and the third heating zone are arranged directly adjacent to each other

[0052] - and / or the third heating zone and the fourth heating zone are arranged directly adjacent to each other

[0053] - and / or the fourth heating zone and the fifth heating zone are arranged directly adjacent to one another.

[0054] In variants with a first heating zone but no second heating zone, the first and third heating zones are preferably arranged directly adjacent to one another.

[0055] In a preferred variant, the steel sheet blank passes through no further heating zones than those mentioned. The steel sheet blank therefore passes through exactly the first, second, third, fourth, and fifth heating zones. In the case of optional heating zones, these can, of course, be omitted.

[0056] For the purposes of this application, a heating zone with a temperature within a temperature interval is understood to mean a furnace area with a temperature within this interval. The temperature does not necessarily have to be constant across the entire heating zone. It is only important that the temperature at all points within the heating zone lies within the temperature interval. If, for example, the first heating zone is required to have a temperature TI in the range of 700°C - 850°C, this also includes cases in which the first heating zone consists of two areas, for example, with temperatures of 760°C and 820°C.

[0057] Furthermore, the total time in the furnace for steel sheet blanks with a thickness d < 1.5 mm is preferably a maximum of 840 s, in particular a maximum of 720 s, preferably a maximum of 420 s, in particular a maximum of 300 s. Likewise, the total time in the furnace for steel sheet blanks with a thickness d > 1.5 mm is preferably a maximum of 900 s, in particular a maximum of 720 s, preferably a maximum of 480 s, in particular a maximum of 420 s. This ensures that the desired layer structure is achieved. At the same time, the energy consumption for heating is not too high. In the preferred variant, in which the steel sheet blank does not pass through any further heating zone than the heating zones mentioned, the total time in the furnace corresponds to the sum of the time periods t1, t2, t3, t4, t5. The time periods of optional heating zones that are not implemented are included as zero in the sum.

[0058] For sheet steel blanks with a thickness d < 1.5 mm, the time period tl is preferably at least 90 s and a maximum of 360 s, in particular a maximum of 240 s, preferably a maximum of 150 s. For sheet steel blanks with a thickness > 1.5 mm, the time period tl is preferably at least 90 s, in particular at least 120 s, and a maximum of 580 s, in particular a maximum of 360 s, preferably a maximum of 180 s.

[0059] For sheet steel blanks with a thickness d < 1.5 mm, the temperature Tl is preferably at least 720 °C, in particular at least 750 °C, preferably at least 770 °C and a maximum of 830 °C, in particular a maximum of 820 °C, preferably a maximum of 800 °C. For sheet steel blanks with a thickness > 1.5 mm, the temperature Tl is preferably at least 730 °C, in particular at least 760 °C and a maximum of 840 °C, in particular a maximum of 830 °C, preferably a maximum of 800 °C.

[0060] For sheet steel blanks with a thickness d < 1.5 mm, the time period t2 is preferably at least 30 s, preferably at least 40 s, in particular at least 45 s, and a maximum of 90 s, in particular a maximum of 80 s, preferably a maximum of 70 s. For sheet steel blanks with a thickness > 1.5 mm, the time period t2 is preferably at least 30 s, in particular at least 40 s, preferably at least 60 s, and a maximum of 90 s, in particular a maximum of 360 s, preferably a maximum of 80 s.

[0061] For sheet steel blanks with a thickness d < 1.5 mm, the temperature T2 is preferably at least 930 °C, in particular at least 940 °C and a maximum of 980 °C, in particular a maximum of 970 °C, preferably a maximum of 960 °C. For sheet steel blanks with a thickness > 1.5 mm, the temperature T2 is preferably at least 940 °C, in particular at least 950 °C and a maximum of 980 °C, in particular a maximum of 970 °C.

[0062] For sheet steel blanks with a thickness d < 1.5 mm, the time period t3 is preferably at least 60 s and a maximum of 540 s, in particular a maximum of 300 s, preferably a maximum of 180 s. For sheet steel blanks with a thickness > 1.5 mm, the time period t3 is preferably at least 60 s and a maximum of 600 s, in particular a maximum of 360 s, preferably a maximum of 180 s.

[0063] For sheet steel blanks with a thickness d < 1.5 mm, the temperature T3 is preferably at least 720 °C, in particular at least 750 °C, preferably at least 800 °C and a maximum of 880 °C, in particular a maximum of 860 °C. For sheet steel blanks with a thickness > 1.5 mm, the temperature T3 is preferably at least 720 °C, in particular at least 750 °C, preferably at least 800 °C and a maximum of 900 °C, in particular a maximum of 880 °C, preferably a maximum of 860 °C.

[0064] For sheet steel blanks with a thickness d < 1.5 mm, the time period t4 is preferably at least 30 s, preferably at least 45 s, in particular at least 50 s and a maximum of 120 s, in particular a maximum of 90 s, preferably a maximum of 70 s. For sheet steel blanks with a thickness > 1.5 mm, the time period t4 is preferably at least 30 s, in particular at least 40 s, preferably at least 50 s and a maximum of 150 s, in particular a maximum of 120 s, preferably a maximum of 90 s.

[0065] For sheet steel blanks with a thickness d < 1.5 mm, the temperature T4 is preferably at least 930 °C, in particular at least 940 °C and a maximum of 980 °C, preferably a maximum of 970 °C, in particular a maximum of 960 °C. For sheet steel blanks with a thickness > 1.5 mm, the temperature T4 is preferably at least 940 °C, in particular at least 950 °C and a maximum of 980 °C, in particular a maximum of 970 °C.

[0066] The time period t5 for steel sheet blanks with a thickness d < 1.5 mm is preferably at least 5 s, preferably at least 10 s, in particular at least lös and a maximum of 30 s, in particular a maximum of 20 s. For steel sheet blanks with a thickness > 1.5 mm, the time period tö is preferably at least 10 s, in particular at least lös, preferably at least 20 s and a maximum of 50 s, in particular a maximum of 40 s, preferably a maximum of 30 s. The temperature T5 for steel sheet blanks with a thickness d < 1.5 mm is preferably at least 750 °C, in particular at least 800 °C, preferably at least 830 °C and a maximum of T4-30 °C, preferably a maximum of T4-60 °C, in particular a maximum of T4-80 °C. For steel sheet blanks with a thickness > 1.5 mm, the temperature T5 is preferably at least 750 °C, in particular at least 800 °C, preferably at least 830 °C and a maximum of T4-30 °C, in particular a maximum of T4-60 °C, preferably a maximum of T4-80 °C.

[0067] In a particularly preferred embodiment, the following parameters are set for steel sheet blanks with a thickness d < 1.5mm:

[0068] 90s < tl < 150s and 770 °C < Tl < 800 °C

[0069] 45s < t2 < 70s and 940 °C < T2 < 960 °C

[0070] 60s < t3 < 180s and 800 °C < T3 < 860 °C

[0071] 50s < t4 < 70s and 940 °C < T4 < 960 °C lös < tö < 20s and 830 °C < Tö < T4-80 °C and for steel sheet blanks with a thickness d > 1.5 mm the following parameters are set: 120s < tl < 180s and 760 °C < Tl < 800 °C 60s < t2 < 80s and 950 °C < T2 < 970 °C 60s < t3 < 180s and 800 °C < T3 < 860 °C 50s < t4 < 90s and 950 °C < T4 < 970 °C 20s < tö < 30s and 830 °C < Tö < T4-80 °C, where also here the first, The second and fifth heating zones are therefore optional.

[0072] A special variant of all the previously listed designs, which includes precisely the third and fourth heating zones, has the advantage of being easy to implement, as only one furnace with two heating zones is required. Furthermore, the adhesion of the protective coating material to the rollers of the roller-hearth furnace is reduced, as the protective coating material does not melt as quickly.

[0073] A special variant of all the previously listed designs, which includes exactly the first, third and fourth heating zones, has the advantage that heating can be achieved even more slowly.

[0074] A special variant of all the previously listed designs, which includes precisely the second, third, and fourth heating zones, has the advantage that the relatively short second heating zone allows for rapid initial heating. At the same time, the time period t2 is short enough that the blank does not yet reach the temperature for molten phases. Thus, the temperature at which significant diffusion occurs is quickly reached. Subsequently, heating continues slowly, allowing sufficient time for diffusion before the blank reaches the temperature at which molten phases appear. This allows the total time in the furnace to be shortened, while simultaneously reducing roll adhesion.

[0075] A special variant of all the previously listed designs, which includes precisely the first, second, third, and fourth heating zones, has the advantage that, on the one hand, the temperature of the blank is gradually increased due to the upstream first heating zone. The subsequent combination of the second, third, and fourth heating zones results in the blank maintaining a relatively constant temperature as it passes through these three zones due to the transition from a hot to a cooler to a hot heating zone. This makes the process particularly stable.

[0076] Adding the fifth zone to one of the three variants described above offers further advantages. For example, the temperature of the blank is lowered before it leaves the furnace. This reduces the cooling capacity required in the subsequent forming process in a forming tool, as the blank does not need to be cooled as drastically. This allows for a reduction in the cooling water supply and also reduces the holding time in the tool. This saves costs and increases the efficiency of the process.

[0077] In a preferred embodiment, the steel sheet blank at least partially exceeds the AC3 temperature of the steel sheet blank. In addition, the temperature T E ini g of the steel sheet blank when inserted into the forming tool (work step c)) at least partially above Ms+100 °C, where Ms denotes the martensite start temperature.

[0078] For the purposes of this application, partial exceeding of a temperature (here AC3 or Ms+100 °C) means that at least 30%, in particular at least 60%, of the volume of the blank exceeds a corresponding temperature.

[0079] When placed in the forming tool, at least 30% of the blank has an austenitic structure, i.e. the transformation from a ferritic to an austenitic structure does not necessarily have to be complete when placed in the forming tool. In fact, up to 70% of the volume of the blank when placed in the forming tool can consist of other microstructure components, such as tempered bainite, tempered martensite and / or non- or partially recrystallized ferrite. For this purpose, certain areas of the blank can be deliberately kept at a lower temperature level than others during heating. To do this, the heat can be specifically directed at certain sections of the blank, or the parts that are to be heated less can be shielded from the heat supply.In the part of the blank material whose temperature remains lower, no or significantly less martensite is formed during forming in the tool, so the microstructure there is significantly softer than in the other parts where a martensitic microstructure is present. In this way, a softer area can be specifically adjusted within the formed sheet metal part, for example, by achieving optimal toughness for the respective application, while the other areas of the sheet metal part have maximized strength.

[0080] Maximum strength properties of the obtained sheet metal part can be achieved by ensuring that the temperature reached at least partially in the sheet metal blank is between Ac3 and 1000 °C, preferably between 850 °C and 950 °C.

[0081] The minimum temperature AC3 to be exceeded is determined according to the formula given by HOUGARDY, HP in Werkstoffkunde Stahl Volume 1: Grundlagen, Verlag Stahleisen GmbH, Düsseldorf, 1984, p. 229.

[0082] Ac3 = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni + 55*%V) °C with %C = respective C content, %Si = respective Si content, %Mn = respective Mn content, %Cr = respective Cr content, %Mo = respective Mo content, %Ni respective Ni content and %V = respective V content of the steel from which the blank is made.

[0083] An optimally uniform distribution of properties can be achieved by heating the blank completely in step b).

[0084] In a preferred embodiment, the average heating rate r OThe temperature of the sheet metal blank during heating in step b) is at least 3 K / s, preferably at least 5 K / s, in particular at least 10 K / s, preferably at least 15 K / s. The average heating rate is to be understood as the average heating rate from 30°C to 700°C. The dew point of the furnace atmosphere in the furnace is preferably at least -25 °C, preferably at least -20 °C, preferably at least -15 °C, in particular at least -5 °C, particularly preferably at least 0 °C, in particular at least 5 °C and a maximum of +25 °C, preferably a maximum of +20 °C, in particular a maximum of +15 °C.

[0085] The heated blank is removed from the oven and within a transfer time t Tr The workpiece is placed into a forming tool within a time of preferably no more than 20 seconds, in particular no more than 15 seconds. Such rapid transport is necessary to avoid excessive cooling prior to forming.

[0086] Preferably the temperature T Eini g The steel sheet blank is heated during insertion into the forming tool (step c)) at least partially above Ms+100 °C, preferably above 600 °C, in particular above 650 °C, and particularly preferably above 700 °C. Ms denotes the martensite start temperature. In a particularly preferred variant, the temperature is at least partially above the ACl temperature. In all of these variants, the temperature is, in particular, a maximum of 900 °C. These temperature ranges ensure good formability of the material overall.

[0087] When inserting the blank, the tool typically has a temperature between room temperature (RT) and 200 °C, preferably between 20 °C and 180 °C, in particular between 50 °C and 150 °C. Optionally, in a special embodiment, the tool can be heated at least partially to a temperature T Wz of at least 200°C, in particular at least 300°C, in order to only partially harden the component. Furthermore, the tool temperature Twz is preferably a maximum of 600°C, in particular a maximum of 550°C. It is only necessary to ensure that the tool temperature Twz is below the desired target temperature Tziei. The residence time in the tool twz is preferably at least 2s, in particular at least 3s, particularly preferably at least 5s. The maximum residence time in the tool is preferably 25s, in particular a maximum of 20s.

[0088] The target temperature Tziei of the sheet metal part is at least partially below 400 °C, preferably below 300 °C, in particular below 250 °C, preferably below 200 °C, particularly preferably below 180 °C, in particular below 150 °C. Alternatively, the target temperature Tziei of the sheet metal part is particularly preferably below Ms-50 °C, where Ms denotes the martensite start temperature. Furthermore, the target temperature of the sheet metal part is preferably at least 20 °C, particularly preferably at least 50 °C. The martensite start temperature of a steel within the scope of the inventive specifications is according to the formula:

[0089] Ms [°C] = (490.85 — 302.6 %C — 30.6 %Mn - 16.6 %Ni — 8.9 %Cr + 2.4 %Mo — 11.3 %Cu + 8.58 %Co + 7.4 %W — 14.5 %Si) [°C / wt.%], where C% is the C content, %Mn is the Mn content, %Mo is the Mo content, %Cr is the Cr content, %Ni is the Ni content, %Cu is the Cu content, %Co is the Co content, %W is the W content and %Si is the Si content of the respective steel in wt.%.

[0090] The ACl temperature and the AC3 temperature of a steel within the scope of the invention specifications are according to the formulas:

[0091] AC1[°C] = (739 — 22*%C - 7*%Mn + 2*%Si + 14*%Cr + 13*%Mo - 13*%Ni + 20*%V )[°C / wt.-%]

[0092] AC3[°C] = (902 - 225*%C + 19*%Si - 11*%Mn - 5*%Cr + 13*%Mo - 20*%Ni +55*%V)[°C / wt.%], where %C denotes the C content, %Si the Si content, %Mn the Mn content, %Cr the Cr content, %Mo the Mo content, %Ni the Ni content and +%V the vanadium content of the respective steel (Brandis H 1975 TEW-Techn. Ber. 1 8-10).

[0093] In the tool, the blank is not only formed into the sheet metal part, but is also simultaneously quenched to the target temperature. The cooling rate in the tool (rwz) to the target temperature is in particular at least 27 K / s, preferably at least 30 K / s, in particular at least 50 K / s, and in special designs at least 100 K / s.

[0094] After removing the sheet metal part in step e), the sheet metal part is preferably cooled to a cooling temperature TAB of less than 100 °C within a cooling time t AB from 0.5 to 600 s. This is usually done by air cooling.

[0095] The steel substrate of the steel sheet blank used in the process is made of a steel containing 0.04–0.45 wt.% C, 0.1–3 wt.% Mn, and optionally up to 0.01 wt.% B. In particular, the microstructure of the steel can be converted into a martensitic or partially martensitic microstructure by hot forming. The microstructure of the steel substrate of the steel sheet blank is therefore preferably a martensitic or at least partially martensitic microstructure, as this exhibits particularly high hardness.

[0096] Particularly preferably, the steel substrate is a steel which, in addition to iron and unavoidable impurities (in wt%), consists of

[0097] C: 0.04 - 0.45 wt%,

[0098] Si: 0.02 - 1.2 wt.%,

[0099] Mn: 0.5 - 2.6 wt%,

[0100] AI: 0.02 - 1.0 wt.%,

[0101] P: < 0.05 wt%,

[0102] S: < 0.02 wt%,

[0103] N: < 0.02 wt%,

[0104] Sn: < 0.03 wt%

[0105] As: < 0.01 wt%

[0106] Ca: < 0.01 wt.% and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents

[0107] Cr: 0.08 - 1.0 wt%,

[0108] B: 0.001 - 0.005 wt.%

[0109] Mo: <0.5 wt%

[0110] Ni: <0.5 wt%

[0111] Cu: <0.2 wt%

[0112] Nb: 0.02 - 0.08 wt.%,

[0113] Ti: 0.01 - 0.08 wt%

[0114] V: <0.3 wt%.

[0115] The elements P, S, N, Sn, and As are impurities that cannot be completely avoided during steel production. In addition to these elements, other elements may also be present as impurities in the steel. These additional elements are summarized under the term "unavoidable impurities." The total content of unavoidable impurities is preferably a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The optional alloying elements Cr, B, Nb, and Ti, for which a lower limit is specified, may also be present as unavoidable impurities in the steel substrate in amounts below the respective lower limit. In this case, they are also counted as unavoidable impurities, with their total content limited to a maximum of 0.2 wt.%, preferably a maximum of 0.1 wt.%. The individual upper limits for the respective impurities of these elements are preferably as follows:

[0116] Cr: < 0.050 wt%,

[0117] B: < 0.0005 wt%

[0118] Nb: < 0.005 wt%,

[0119] Ti: < 0.005 wt%

[0120] These preferred upper limits should be considered alternatively or jointly. Preferred steel variants therefore meet one or more of these four conditions.

[0121] In a preferred embodiment, the C content of the steel is a maximum of 0.37 wt.% and / or at least 0.06 wt.%. In particularly preferred embodiments, the C content is in the range of 0.06-0.09 wt.%, or in the range of 0.12-0.25 wt.%, or in the range of 0.33-0.37 wt.%.

[0122] In a preferred embodiment, the Si content of the steel is a maximum of 1.00 wt.% and / or at least 0.06 wt.%.

[0123] In a preferred variant, the Mn content of the steel is a maximum of 2.4 wt.% and / or at least 0.75 wt.%. In particularly preferred embodiments, the Mn content is in the range of 0.75-0.85 wt.% or in the range of 1.0-1.6 wt.%.

[0124] In a preferred variant, the Al content of the steel is a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%, preferably a maximum of 0.25 wt.%. Alternatively or additionally, the Al content is preferably at least 0.02%. In addition, it has been shown that it can be helpful if the sum of the silicon and aluminum contents is limited. In a preferred variant, the sum of the Si and Al contents (usually referred to as Si+Al) is therefore a maximum of 1.5 wt.%, preferably a maximum of 1.2 wt.%. Additionally or alternatively, the sum of the Si and Al contents is at least 0.06 wt.%, preferably at least 0.08 wt.%.

[0125] Calcium (Ca) is used in steels to form non-metallic inclusions, particularly manganese sulfides. The rounded shape significantly reduces the negative effect of these inclusions on hot formability, fatigue strength, and toughness. The maximum Ca content is 0.01 wt.%, in particular a maximum of 0.007 wt.%, and preferably a maximum of 0.005 wt.%. Excessively high Ca contents increase the likelihood of non-metallic inclusions containing Ca forming, which impair the purity of the steel and also its toughness. For this reason, an upper limit of the Ca content of no more than 0.005 wt.%, preferably a maximum of 0.003 wt.%, should be maintained.

[0126] The elements P, S, and N are typical impurities that cannot be completely avoided during steel production. In preferred variants, the P content is a maximum of 0.03 wt.%. Irrespective of this, the S content is preferably a maximum of 0.012%. Additionally or supplementarily, the N content is preferably a maximum of 0.009 wt.%.

[0127] Optionally, the steel also contains chromium at a content of 0.08–1.0 wt.%. The Cr content is preferably a maximum of 0.75 wt.%, in particular a maximum of 0.5 wt.%.

[0128] In the case of an optional alloying of chromium, the sum of the chromium and manganese contents is preferably limited. The sum is a maximum of 3.3 wt.%, in particular a maximum of 3.15 wt.%. Furthermore, the sum is at least 0.5 wt.%, preferably at least 0.75 wt.%.

[0129] Preferably, the steel optionally also contains boron in a content of 0.001–0.005 wt.%. In particular, the boron content is a maximum of 0.004 wt.%.

[0130] Optionally, the steel can contain molybdenum with a content of maximum 0.5 wt.%, in particular maximum 0.1 wt.%. Furthermore, the steel can optionally contain nickel with a content of maximum 0.5 wt.%, preferably maximum 0.15 wt.%. Optionally, the steel can also contain copper with a content of maximum 0.2 wt.%, preferably maximum 0.15 wt.%. In addition, the steel can optionally contain one or more of the microalloying elements Nb, Ti and V. The optional Nb content is at least 0.02 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional Ti content is at least 0.01 wt.% and at most 0.08 wt.%, preferably at most 0.04 wt.%. The optional V content is a maximum of 0.3 wt.%, preferably a maximum of 0.2 wt.%, in particular a maximum of 0.1 wt.%, preferably a maximum of 0.05 wt.%.

[0131] In the case of an optional alloying of several of the elements Nb, Ti, and V, the sum of the contents of Nb, Ti, and V is preferably limited. The sum is a maximum of 0.1 wt.%, in particular a maximum of 0.068 wt.%. Furthermore, the sum is preferably at least 0.015 wt.%.

[0132] The aforementioned corrosion protection coating is preferably produced by hot-dip coating the flat steel product. The flat steel product is passed through a liquid melt consisting of 0.1-15 wt.% Si, preferably more than 1.0 wt.% Si, optionally 2-4 wt.% Fe, optionally up to 5 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, and optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally further constituents, the total contents of which are limited to a maximum of 2.0 wt.%, with aluminum as the remainder. The optional content of alkali or alkaline earth metals is preferably at least 0.1 wt.%.

[0133] In a preferred variant, the Si content of the melt is 0.4-3.5 wt.%. In this variant, the Si content of the melt is preferably at least 0.5 wt.%, in particular at least 0.7 wt.%. Further preferably, the Si content of the melt is a maximum of 2.5 wt.%, in particular a maximum of 2.0 wt.%. It has been shown that the process according to the invention is particularly suitable for coatings in which the diffusion of iron into the coating occurs relatively quickly, i.e. a high diffusion rate is present. These are particularly coatings with a low Si content, since Si hinders the diffusion of iron into the coating.

[0134] In an alternative variant, the Si content of the melt is 7-12 wt.%, in particular 8-10 wt.%. In a preferred variant, the optional content of alkali or alkaline earth metals in the melt comprises 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the melt can comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca. Further preferably, the optional content of alkali or alkaline earth metals in the melt consists of 0.1-1.0 wt.% Mg, in particular 0.1-0.7 wt.% Mg, preferably 0.1-0.5 wt.% Mg and optionally at least 0.0015 wt.% Ca, in particular at least 0.01 wt.% Ca.

[0135] During hot-dip coating, iron diffuses from the steel substrate into the liquid coating, so that the corrosion protection coating of the flat steel product has, in particular, an alloy layer and an Al base layer upon solidification.

[0136] The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer is essentially formed from aluminum and iron. The alloy layer preferably consists of 25-50 wt.% Fe, 5-20 wt.% Si, optional further components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, and the remainder being aluminum. The optional further components include, in particular, the remaining components of the melt (i.e., optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining components of the steel substrate in addition to iron. In a further variant (variant with an Si content in the melt of 0.5-3.5 wt.%), the alloy layer consists of 25-50 wt.% Fe, 0.5-5.0 wt.% Si, optional further components whose total content is limited to a maximum of 5.0 wt.%, preferably 2.0 wt.%, and the remainder being aluminum.The optional additional components also include in particular the remaining components of the melt (i.e. alkali or alkaline earth metals, in particular Mg or Ca) and the remaining components of the steel substrate in addition to iron.

[0137] The Al base layer lies on top of the alloy layer and directly adjoins it. The composition of the Al base layer preferably corresponds to the composition of the melt of the molten bath. This means that it consists of up to 15 wt.% Si, optionally 2-4 wt.% Fe, optionally 5.0 wt.% alkali or alkaline earth metals, preferably up to 1.0 wt.% alkali or alkaline earth metals, optionally up to 15 wt.% Zn, preferably up to 10 wt.% Zn, and optionally further constituents whose total contents are limited to a maximum of 2.0 wt.%, with aluminum as the remainder. Preferred compositions of the Al base layer correspond to the preferred melt compositions.

[0138] In a preferred variant, the Si content of the Al base layer is 0.4-3.5 wt.%. The Si content of the Al base layer in this variant is preferably at least 0.5 wt.%, in particular at least 0.7 wt.%. Further preferably, the Si content of the Al base layer is a maximum of 2.5 wt.%, in particular a maximum of 2.0 wt.%. It has been shown that the method according to the invention is particularly suitable for coatings in which the diffusion of iron into the coating occurs relatively quickly, i.e. a high diffusion rate is present. These are particularly coatings with a low Si content, since Si hinders the diffusion of iron into the coating.

[0139] In an alternative variant, the Si content of the Al base layer is 7-12 wt.%, in particular 8-10 wt.%.

[0140] In a preferred variant of the Al base layer, the optional content of alkali or alkaline earth metals comprises 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg. Furthermore, the optional content of alkali or alkaline earth metals in the Al base layer can in particular comprise at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca. Further preferably, the optional content of alkali or alkaline earth metals consists of 0.1 - 1.0 wt.% Mg, in particular 0.1 - 0.7 wt.% Mg, preferably 0.1 - 0.5 wt.% Mg and optionally at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca.

[0141] In a further preferred variant of the corrosion protection coating, the Si content in the alloy layer is lower than the Si content in the Al base layer.

[0142] The corrosion protection coating preferably has a thickness of 5 - 60 μm, in particular 10 - 40 μm. The coating weight of the corrosion protection coating is in particular 30 - 360 μm for double-sided corrosion protection coatings or 15 - 180 μm in the one-sided variant. The coating weight of the corrosion protection coating is preferably 100-200^ for double-sided coatings or 50-100^ for single-sided coatings. The coating weight of the corrosion protection coating is particularly preferably 120-180^ for double-sided coatings or 60^ for single-sided coatings.

[0143] The thickness of the alloy layer is preferably less than 20 μm, more preferably less than 16 μm, particularly preferably less than 12 μm, and especially less than 10 μm. The thickness of the Al base layer results from the difference between the thicknesses of the anti-corrosive coating and the alloy layer. The thickness of the Al base layer is preferably at least 1 μm, even with thin anti-corrosive coatings. In a preferred variant, the flat steel product comprises an oxide layer arranged on the anti-corrosive coating. The oxide layer lies in particular on the Al base layer and preferably forms the outer edge of the anti-corrosive coating.

[0144] The oxide layer consists in particular of more than 80 wt.% oxides, with the majority of the oxides (i.e., more than 50 wt.% of the oxides) being aluminum oxide. Optionally, in addition to aluminum oxide, hydroxides and / or magnesium oxide are present in the oxide layer, alone or as a mixture. Preferably, the remainder of the oxide layer not occupied by the oxides and optionally present hydroxides consists of silicon, aluminum, iron, and / or magnesium in metallic form. For the optional embodiment with zinc as a constituent of the aluminum base layer, zinc oxide components are also present in the oxide layer.

[0145] Preferably, the oxide layer of the flat steel product has a thickness greater than 50 nm.

[0146] In particular, the thickness of the oxide layer is a maximum of 1 pm, preferably a maximum of 500 nm.

[0147] The provided steel sheet blanks are preferably obtained by coating a flat steel product in the manner explained above and cutting it into steel sheet blanks. Consequently, the preferred variants of the anti-corrosive coating on the flat steel product explained above apply analogously to the anti-corrosive coating on the steel sheet blank.

[0148] The invention further relates to a method for reducing waste in a method for producing a shaped steel sheet part, in particular a method as previously explained. The method for producing a shaped steel sheet part comprises the following steps: a) Providing a steel sheet blank with a thickness d of at least 0.7 mm and a maximum of 3.5 mm, comprising a steel substrate consisting of a steel containing 0.04-0.45 wt.% C, 0.1-3 wt.% Mn, and optionally up to 0.01 wt.% B, and wherein the steel sheet blank has an aluminum-based anti-corrosion coating on at least one side; b) Heating the steel sheet blank in a furnace, i. wherein the steel sheet blank passes through the following steps in the furnace:

[0149] 1. optionally in a first time period tl a first heating zone with a first temperature Tl, 2. optionally in a second time period t2 a second heating zone with a second temperature T2, where T2>T1,

[0150] 3. in a third time period t3, a third heating zone with a third temperature T3, where optionally T3 <T2,

[0151] 4. in a fourth time period t4, a fourth heating zone with a fourth temperature T4, where T4>T3 applies,

[0152] 5. optionally in a fifth time period t5 a fifth heating zone with a fifth temperature, where T5 <T4; c) Einlegen des erwärmten Blechzuschnitts in ein Umformwerkzeug, wobei die für das Entnehmen aus dem Ofen und das Einlegen des Zuschnitts benötigte Transferdauer t T rans is at most 20 s, preferably at most 15 s; d) hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is cooled during the hot-press forming process over a period twz of more than 1 s at a cooling rate r which is at least partially more than 27 K / s Wz is cooled to the target temperature Tziei and optionally maintained there; e) removing the sheet metal part cooled to the target temperature from the tool.

[0153] The scrap in such a process is reduced by the following steps: i. for each heating zone, a maximum period of time is defined for which a steel sheet blank can be additionally stored in this heating zone due to an interruption in the process, ii. if the passage of the steel sheet blank through the furnace is interrupted, it is determined in which heating zone the steel sheet blank is stored for the duration of the interruption, iii. after the interruption for the steel sheet blank, it is checked whether the additional period of time that the steel sheet blank was stored in the heating zone due to the interruption exceeds the maximum period for this heating zone.

[0154] During the ongoing production process, interruptions in the flow can occasionally occur. Typically, the method for producing a shaped steel sheet part is implemented by moving the sheet metal blank through a roller hearth furnace at a constant speed in step b). The roller hearth furnace has several heating zones with different temperatures. If an interruption in the production process occurs, the movement of the sheet metal blanks through the roller hearth furnace stops or slows down. Until now, sheet metal blanks that were stored in the roller hearth furnace for too long during the interruption were completely sorted out and not used. In a 30 - 50 m long roller hearth furnace, this can affect a larger number of sheet metal blanks. The method according to the invention is based on the knowledge that not all of the sheet metal blanks that were usually disposed of are no longer suitable for further processing.Due to the different heating zones with different temperatures, the sheet metal blanks are affected differently depending on which heating zone the blanks are stored in during the interruption. In the particularly hot heating zones, even short additional periods of time cause the diffusion process in the coating to progress to such an extent that the sheet metal blank is no longer suitable for forming and use as a formed sheet metal part. In contrast, the sheet metal blanks in the less hot heating zones can withstand a longer additional period of time without any significant effects. In other words: the process windows are much narrower in terms of duration for the hot heating zones than for the heating zones with lower temperatures.For this reason, a maximum period of time is specified for each heating zone for which a steel sheet blank can be additionally stored in this heating zone due to an interruption in the process. The process window in terms of time (the maximum period of time) is therefore specified for each heating zone. If the passage of the steel sheet blank through the furnace is interrupted, it is then determined in which heating zone the steel sheet blank is stored for the duration of the interruption. For each sheet blank in the furnace, it is therefore determined which process window should be used by establishing in which heating zone the sheet blank is stored during the interruption. After the interruption, a check is then carried out for the steel sheet blank to determine whether the additional period of time the steel sheet blank was stored in the heating zone due to the interruption exceeds the maximum period for this heating zone.It is therefore checked whether the sheet metal blank was still processed within the relevant process window.

[0155] Subsequently, the steel sheet blank for which the check according to step iii) was positive (i.e., the maximum time period was exceeded) is sorted out. This way, the number of rejected steel sheet blanks (i.e., scrap) is lower than with the usual procedure, which sorts out all steel sheet blanks that have remained in the furnace for too long during the interruption.

[0156] In a preferred embodiment, the maximum duration of the third heating zone is greater than the maximum duration of the fourth heating zone and / or the maximum duration of the first heating zone is greater than the maximum duration of the second heating zone and / or the maximum duration of the third heating zone is greater than the maximum duration of the second heating zone and / or the maximum duration of the fifth heating zone is greater than the maximum duration of the fourth heating zone.

[0157] Heating zones with lower temperatures therefore have a longer maximum duration than heating zones with higher temperatures.

[0158] In particularly preferred embodiments of the method for reducing waste, the heating zones are designed as in the previously explained method for producing a sheet steel molded part.

[0159] The two processes described above are further developed in special variants so that the steel sheet blank has areas of different thicknesses. The resulting sheet metal part also has areas of different thicknesses.

[0160] Areas of different thickness of the sheet metal blank (so-called “tailored blanks”) can be created in various ways:

[0161] One or more special cold rolling passes, in which individual areas are rolled more intensively or more frequently, result in a lower material thickness and thus a lower thickness in these areas (so-called “tailor rolled blanks”);

[0162] By welding together (typically by laser welding), sheet metal blanks of different thicknesses are joined to create a continuous sheet metal blank with areas of different thicknesses (so-called “tailor welded blanks”);

[0163] Using resistance spot welding or laser welding, patches are applied to an existing sheet metal blank to thicken it in certain areas. Alternatively, the patches can extend beyond the existing sheet metal blank or overlap only a fraction of the sheet metal blank and be joined using resistance spot welding or laser welding, resulting in a partial or complete variation of tailor-welded blanks using resistance spot welding or laser welding. Alternatively, the patches can also be applied using structural adhesives. In the latter two cases, sheet metal blanks made of different materials can also be used and joined together.

[0164] Areas of varying thickness have the advantage that specific areas of the final sheet metal part (see below) can be specifically reinforced or enhanced with increased ductility. This makes it possible to design those sections subject to particular stresses (e.g., during a crash) with increased stability, while making other sections thinner to reduce the weight of the component. The result is a weight-optimized component with targeted reinforcements in the areas subject to high stress. At the same time, more ductile areas of the component absorb energy over a greater distance in a crash, reducing the impact on passengers.

[0165] In both processes, a distinction is made for the various annealing parameters (tl, TI, t2, T2, ...) depending on whether the thickness of the steel sheet blanks is greater than 1.5 mm or less than or equal to 1.5 mm. For the steel sheet blanks with different thicknesses described here, it may happen that all areas have a thickness greater than 1.5 mm or all areas have a thickness less than or equal to 1.5 mm. In these cases, the respective parameter sets must be used. However, it may also be the case that there are areas with a thickness less than or equal to 1.5 mm and, at the same time, areas with a thickness greater than 1.5 mm. In this case, the parameters are preferably set so that both sets of relations are fulfilled, i.e. so that

[0166] 45s < tl < 580s and 700 °C < Tl < 850 °C

[0167] 30s < t2 < 90s and 930 °C < T2 < 980 °C

[0168] 60s < t3 < 700s and 700 °C < T3 < 880 °C 30s < t4 < 180s and 930 °C < T4 < 980 °C 0s < t5 < 45s and 700 °C < T5 < T4 and simultaneously

[0169] 60s < tl < 640s and 710 °C < Tl < 840 °C

[0170] 30s < t2 < 90s and 940 °C < T2 < 980 °C

[0171] 60s < t3 < 760s and 720 °C < T3 < 900 °C

[0172] 30s < t4 < 200s and 940 °C < T4 < 980 °C

[0173] 0s < t5 < 60s and 700 °C < T5 < T4 are met. The same applies to the respective preferred ranges of these parameters. By adhering to both sets of relationships, it is ensured that both the thin areas less than or equal to 1.5 mm and the thicker areas greater than 1.5 mm receive optimal annealing treatment.

[0174] In the following, the invention is explained in more detail using exemplary embodiments.

[0175] To demonstrate the effectiveness of the invention, several tests were conducted. For this purpose, corresponding steel sheets were produced in a conventional manner by producing slabs with the compositions specified in Table 1, with a thickness of 200–280 mm and a width of 1000–1200 mm. These were heated in a pusher furnace to a temperature between 1250°C and 1300°C and held for between 30 and 450 minutes until the temperature in the core of the slabs was reached and the slabs were thus thoroughly heated. The slabs were discharged from the pusher furnace at their thoroughly heated temperature and subjected to hot rolling. The tests were carried out as continuous hot strip rolling.For this purpose, the slabs were first pre-rolled into an intermediate product with a thickness of 40 mm. At the end of the pre-rolling phase, the intermediate products, which in hot strip rolling can also be referred to as pre-strips, each had an intermediate product temperature in the range of 1050 °C to 1150 °C. Immediately after pre-rolling, the pre-strips were fed to the finish rolling stage so that the intermediate product temperature corresponds to the initial rolling temperature for the finish rolling phase. The pre-strips were rolled into hot strips with a final thickness in the range of 3 - 7 mm and final rolling temperatures in the range of 800 °C to 950 °C, cooled to a coiling temperature in the range of 550 °C to 660 °C, wound into coils at the coiling temperature and then cooled in still air. The hot strips were then descaled in the conventional way by pickling before being subjected to cold rolling.The cold-rolled flat steel products were heated in a continuous annealing furnace to an annealing temperature between 650 °C and 850 °C and held at annealing temperature for 100 s each before being cooled at a cooling rate of 1 K / s to an immersion temperature in the range of 650 °C to 800 °C. The cold-rolled strips were passed through a molten coating bath with a temperature range of 650 °C to 730 °C at their respective immersion temperatures. The composition of the coating bath is given in Table 2. After coating, the coated strips were blown off using conventional methods, creating coatings with varying layer thicknesses (see Table 2). The strips were then cooled using conventional methods. Blanks were cut from the steel strips thus produced and used for further tests.In these tests, sheet metal parts samples 1 to 46 in the form of profile-shaped components (hat profiles) with a blank area of ​​approximately 200 x 400 mm were hot-pressed from the respective blanks. Table 3 shows the thickness of the flat steel product, the steel grade used according to Table 1 and the coating used according to Table 2 for each test. During the tests, the blanks were heated in a roller hearth furnace with five separately controllable zones. The five zones of the furnace were set so that the blanks passed through the heating zones specified in Table 3. For this purpose, of course, several zones of the furnace were set to the same temperature where necessary in order to represent a correspondingly longer heating zone. For example, for Test 1 all zones of the furnace were set to the same temperature. The total time in the furnace, which includes heating and holding, is designated as toten.Table 3 shows for each test how long the blank was heated in which zone. Empty cells in Table 3 mean that the respective heating zone was not present. For the comparative tests not according to the invention, the example number in Table 3 is preceded by a "V". The dew point of the furnace atmosphere was -5 °C in all cases. The blanks were then removed from the heating device and placed in a forming tool which had been cooled to room temperature. The transfer time consists of the removal from the heating device, transport to the tool and insertion into the tool. T rans was about 10s. The temperature T E ini gThe temperature of the blanks when placed in the forming tool was in all cases above the respective martensite starting temperature +100 °C. In the forming tool, the blanks were formed into the respective sheet metal parts, with the sheet metal parts being cooled in the tool at a cooling rate of about 50 K / s to a target temperature T Z iei were cooled to below 200°C. The dwell time in the mold was approximately 6 to 10 seconds. Finally, the samples were cooled in air to room temperature.

[0176] The steel sheet parts produced in this way were then tested for their processability. For this purpose, the weldability of the sheet metal parts was checked according to SEP 1220-2. The results are entered in Table 3. If the weldability is sufficient, the example is marked "iO" (okay), otherwise "niO" (not okay). It is clearly visible that longer times in the first and third zones are generally unproblematic, as these zones have a lower temperature. On the other hand, the steel sheet blanks should not be stored in the second and fourth zones for too long, as this significantly impairs weldability. This is due to the thickness of the alloy layer. This was determined on cross-sections of the produced sheet metal parts and is also shown in Table 3. Weldability is guaranteed up to an alloy layer thickness of 15 pm, whereas weldability is no longer possible at thicknesses above 15 pm.

[0177] Table 1 (steel grades)

[0178] The remainder is iron and unavoidable impurities. All values ​​are in wt.%.

[0179] Table 2 (coating variants)

[0180] Table 3 (Process parameters)

[0181]

Claims

Patent claims 1. A method for producing a steel sheet shaped part comprising the following steps: a) providing a steel sheet blank with a thickness d of at least 0.7 mm and a maximum of 3.5 mm comprising a steel substrate consisting of a steel containing 0.04 - 0.45 wt.% C, 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and wherein the steel sheet blank has an aluminum-based anti-corrosive coating on at least one side; b) heating the steel sheet blank in a furnace, i. wherein the steel sheet blank passes through the following steps in the furnace:

1. optionally in a first time period tl a first heating zone with a first temperature TI 2. optionally in a second time period t2 a second heating zone with a second temperature T2 3. in a third time period t3 a third heating zone with a third temperature T3 4. in a fourth time period t4 a fourth heating zone with a fourth temperature T4 5. optionally, in a fifth time period t5, a fifth heating zone with a fifth temperature T5 ii. For steel sheet blanks with a thickness d < 1.5 mm, the following parameters are set: 45s < tl < 580s and 700°C < Tl < 850°C 30s < t2 < 90s and 930°C < T2 < 980°C 60s < t3 < 700s and 700°C < T3 < 880°C 30s < t4 < 180s and 930°C < T4 < 980°C Os < t5 < 45s and 700°C < T5 < T4 iii. and for steel sheet blanks with a thickness d >1.5 mm the following parameters are set: 60s < tl < 640s and 710°C < Tl < 840°C 30s < t2 < 90s and 940°C < T2 < 980°C 60s < t3 < 760s and 720°C < T3 < 900°C 30s < t4 < 200s and 940°C < T4 < 980°C Os < t5 < 60s and 700°C < T5 < T4. c) inserting the heated sheet metal blank into a forming tool, wherein the transfer time t-Frans required for removing the blank from the furnace and inserting the blank is at most 20 s, preferably at most 15 s; d) hot-press forming the sheet metal blank to form the sheet metal part, wherein the blank is heated during the hot-press forming process for a duration t wz of more than 1 s with a cooling rate r at least partially exceeding Tl K / s wz to the target temperature T Ziei cooled and optionally held there; e) removing the sheet metal part cooled to the target temperature from the tool.

2. Method according to claim 1, wherein for steel sheet blanks with a thickness d < 1.5mm the following parameters are set: 90s < tl < 150s and 770°C < Tl < 800°C 45s < t2 < 70s and 940°C < T2 < 960°C 60s < t3 < 180s and 800°C < T3 < 860°C 50s < t4 < 70s and 940°C < T4 < 960°C 15s < t5 < 20s and 830°C < T5 < T4-80°C and for steel sheet blanks with a thickness d >1.5 mm the following parameters are set: 120s < tl < 180s and 760°C < Tl < 800°C 60s < t2 < 80s and 950°C < T2 < 970°C 60s < t3 < 180s and 800°C < T3 < 860°C 50s < t4 < 90s and 950°C < T4 < 970°C 20s < t5 < 30s and 830°C < T5 < T4-80°C.

3. Method according to one of claims 1 to 2, wherein in step b) the steel sheet blank at least partially exceeds the AC3 temperature of the steel sheet blank and the temperature T Einig of the steel sheet blank when inserted into the forming tool (work step c)) is at least partially above Ms+100°C, where Ms denotes the martensite start temperature.

4. The method according to any one of claims 1 to 3, wherein the temperature at least partially reached in the steel sheet blank in step b) is between AC3 and 1000 °C, preferably between 850 °C and 950 °C.

5. Method according to one of claims 1 to 4, wherein the target temperature T Ziei of the sheet metal part is at least partially below 400 °C, preferably below 300 °C.

6. Process according to one of claims 1 to 5, characterized in that the steel, in addition to iron and unavoidable impurities (in % by weight), consists of C: 0.04 - 0.45 wt.%, Si: 0.02 - 1.2 wt.%, Mn: 0.5 - 2.6 wt.%, Al: 0.02 - 1.0 wt.%, P: < 0.05 wt.%, S: < 0.02 wt.%, N: < 0.02 wt.%, Sn: < 0.03 wt.%, As: < 0.01 wt.%, Ca: < 0.01 wt.%, and optionally one or more of the elements “Cr, B, Mo, Ni, Cu, Nb, Ti, V” in the following contents Cr: 0.08 - 1.0 wt.%, B: 0.001 - 0.005 wt.% Mo: <0.5 wt.% Ni: <0.5 wt% Cu: <0.2 wt% Nb: 0.02 - 0.08 wt.%, Ti: 0.01 - 0.08 wt.% V: <0.1 wt.% consists.

7. A method according to any one of claims 1 to 6, wherein the anti-corrosive coating comprises an alloy layer and an Al base layer.

8. The method according to claim 7, characterized in that the alloy layer consists of 25-50 wt.% Fe, 5-20 wt.% Si, optional further components whose total content is limited to a maximum of 5.0 wt.%, and the remainder being aluminum.

9. Method according to one of claims 7 to 8, characterized in that the Al base layer consists of up to 15 wt.% Si, optionally 2 - 4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 15 wt.% Zn and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder of aluminum.

10. The method according to claim 9, characterized in that the Al base layer consists of 0.4 - 3.5 wt.% Si, optionally 2 - 4 wt.% Fe, optionally up to 5.0 wt.% alkali or alkaline earth metals, optionally up to 15% Zn and optional further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum.

11. A method for reducing waste in a method for producing a steel sheet shaped part, in particular a method according to one of claims 1 to 10, wherein the method for producing a steel sheet shaped part comprises the following steps: a) providing a steel sheet blank with a thickness d of at least 0.7 mm and a maximum of 3.5 mm, comprising a steel substrate consisting of a steel containing 0.04-0.45 wt.% C, 0.1-3 wt.% Mn, and optionally up to 0.01 wt.% B, and wherein the steel sheet blank has an aluminum-based anti-corrosion coating on at least one side; b) heating the steel sheet blank in a furnace, i. wherein the steel sheet blank passes through the following steps in the furnace:

1. optionally in a first time period tl a first heating zone with a first temperature TI 2. optionally in a second time period t2 a second heating zone with a second temperature T2, where T2>T1 3. in a third time period t3, a third heating zone with a third temperature T3, where optionally T3 <T2 4. in a fourth time period t4, a fourth heating zone with a fourth temperature T4, where T4>T3 applies 5. optionally in a fifth time period t5 a fifth heating zone with a fifth temperature, where T5 <T4 c) Einlegen des erwärmten Blechzuschnitts in ein Umformwerkzeug, wobei die für das Entnehmen aus dem Ofen und das Einlegen des Zuschnitts benötigte Transferdauer t-Frans höchstens 20 s, bevorzugt höchstens 15 s, beträgt; d) Warmpressformen des Blechzuschnitts zu dem Blechformteil, wobei der Zuschnitt im Zuge des Warmpressformens über eine Dauer t W z of more than 1 s with a cooling rate r at least partially exceeding Tl K / s W z to the target temperature T Zieicooled and optionally held there; e) removing the sheet metal part cooled to the target temperature from the tool, characterized in that i. for each heating zone a maximum period of time is specified for which a steel sheet blank can be additionally stored in this heating zone due to an interruption in the process; ii. in the event of an interruption in the process of the steel sheet blank passing through the furnace, it is determined in which heating zone the steel sheet blank is stored for the duration of the interruption; iii. after the interruption, a check is carried out for the steel sheet blank to determine whether the additional period of time for which the steel sheet blank was stored in the heating zone due to the interruption exceeds the maximum period for this heating zone.

12. Method according to claim 11, characterized in that the maximum duration of the third heating zone is greater than the maximum duration of the fourth heating zone and / or the maximum duration of the first heating zone is greater than the maximum duration of the second heating zone and / or the maximum duration of the third heating zone is greater than the maximum duration of the second heating zone and / or the maximum duration of the fifth heating zone is greater than the maximum duration of the fourth heating zone.

13. Method according to one of claims 11 to 12, characterized in that for steel sheet blanks with a thickness d < 1.5 mm the following parameters are set: 45s < tl < 580s and 700°C < Tl < 850°C 30s < t2 < 90s and 930°C < T2 < 980°C 60s < t3 < 700s and 700°C < T3 < 880°C 30s < t4 < 180s and 930°C < T4 < 980°C Os < t5 < 45s and 700°C < T5 < T4 and for steel sheet blanks with a thickness d >1.5 mm the following parameters are set: 60s < tl < 640s and 710°C < Tl < 840°C 30s < t2 < 90s and 940°C < T2 < 980°C 60s < t3 < 760s and 720°C < T3 < 900°C 30s < t4 < 200s and 940°C < T4 < 980°C Os < t5 < 60s and 700°C < T5 < T4.

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