WO2024179867A1

WO2024179867A1 - Method of hot press forming, with improved properties

Info

Publication number: WO2024179867A1
Application number: PCT/EP2024/054127
Authority: WO
Inventors: Dirk Rosenstock; Thomas Struppek; Janko Banik; David Hoffmann; Stefan BIENHOLZ
Original assignee: Thyssenkrupp Steel Europe Ag
Priority date: 2023-03-02
Filing date: 2024-02-19
Publication date: 2024-09-06
Also published as: DE102023105207A1

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

Process for hot press forming with improved properties

The invention relates to a method for producing a sheet steel molded part by hot forming a sheet steel blank.

“Steel sheet blanks” are understood here to mean blanks from flat steel products, such as blanks. When we talk about a “steel flat product” or a “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 manufacture of, for example, bodywork components. “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.

All information on the contents of the steel compositions specified in the present application is based on weight, unless expressly stated otherwise. All unspecified "% data" in connection with a steel alloy are therefore to be understood as data in "wt. %". With the exception of the data on the residual austenite content of the structure of a sheet metal molding according to the invention, which is based on the volume (data in "vol. %), data on the contents of the various structural components refer to the area of a cross-section of a sample of the respective product (data in area percent "area %"), unless expressly stated otherwise. Data provided in this text on the contents of the components of an atmosphere refer to the volume (data in "Vol. %").

Where formulae or conditions are used in this text in which values are calculated or formed on the basis of contents of certain alloying elements, the relevant contents of alloying elements are entered in these formulae or conditions in % by weight, unless otherwise stated.

Mechanical properties such as tensile strength, yield strength, elongation reported here were determined in tensile tests according to DIN-EN ISO 6982-1, specimen form 2 (Appendix B Tab. Bl) (as of 2020-06), unless expressly stated otherwise. The bending angle is determined according to VDA standard 238-100. The microstructure was determined on longitudinal sections that had been etched with 3% Nital (alcoholic nitric acid). The amount of retained austenite was determined by X-ray diffraction.

The steels from which the steel substrates of sheet steel blanks are made, which are 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 key under the material number 1.5528.

Steels of the type specified above allow for reliable process control during hot forming of the steel sheet blanks made from them to form a steel sheet molded part. Due to their composition, they have the special feature that the steel sheet molded part made from them by hot forming can be given high strengths through heat treatment. For this purpose, the component obtained by hot forming can be cooled in a targeted manner in the hot forming tool. At the same time, however, steel sheet blanks and the steel sheet molded parts hot formed from them, which consist of steels of the type in question here, are sensitive to corrosive attacks 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 to form the respective steel sheet molded part, which are intended to protect the steel substrate against corrosion.

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, in 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 which consists of, in 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 furnace 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 in a furnace temperature-annealing time coordinate system are 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 1.5 - 3 mm thick blank, however, 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 to form a steel component, removed from the forming tool and cooled from the hot-forming temperature to 400 °C with a cooling rate of at least 50 °C/s.

A method for producing a hot-formed, coated steel component is also known from DE 10 2017 120 128 Al. 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 more is set up in order to achieve rapid heating.

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

Basically, the steel sheet blanks must be heated for a certain time to ensure sufficient diffusion of iron from the steel substrate into the aluminum-based protective coating. This creates an aluminum-iron alloy with an increased melting point in the protective coating, which protects the subsequent steel sheet molded part against corrosion. Sufficient diffusion of iron into the protective coating is achieved by setting the appropriate furnace temperature and annealing time. Greater diffusion can be achieved by using a higher furnace temperature or a longer annealing time.

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 affects efficiency in the industrial environment. In addition, heating is typically carried out 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 that a steel sheet blank spends in the roller hearth furnace as low as possible. Higher furnace temperatures, on the other hand, have the disadvantage that energy consumption and effort increase disproportionately. For example, setting up 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 be annealed for a particularly long time at lower temperatures. This has the further advantage that scrap can be reduced. For example, the flow can occasionally be interrupted during the ongoing production process, i.e. the steel sheet blanks, which should actually be moved through the roller hearth furnace at a constant speed, are no longer moved or are moved more slowly. As a result, the steel sheet blanks are stored in an area with a certain temperature for longer than planned. This is not particularly critical if the area is low temperature. If, on the other hand, the steel sheet blank is in an area with a high temperature for too long, the diffusion process has progressed too far and the sheet metal component produced cannot be used because the further processing properties, such as weldability, are impaired as a result. The proportion of scrap is therefore correspondingly smaller if the proportion of high temperature zones can be reduced.

It is also important to consider at what point the steel sheet blanks are exposed to which furnace temperatures. On the one hand, 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 on the other hand they can 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 through diffusion, so that more liquid phases form. These liquid phases lead to roller adhesions of material from the protective coating to the rollers of the roller hearth furnace. This roller adhesion then has to be removed regularly, which leads to short maintenance cycles and thus an inefficient production process.

Another boundary condition is the temperature of the steel sheet blank at the end of the heating process, i.e. when leaving the roller hearth furnace. For example, in order 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. In order to achieve a completely 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 in the forming tool, the The temperature of the steel sheet blank at the end of the heating process may be correspondingly higher, as a certain amount of cooling occurs during the transfer time.

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

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

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. whereby 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

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

60s < tl < 640s and 710°C < TI < 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 _T rans 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 during the hot press forming the blank is cooled to the target temperature Tziei over a period twz of more than 1 s at a cooling rate r _W z which is at least partially more than 27 K/s and is optionally held there; e) removing the sheet metal part cooled to the target temperature from the tool.

In the method according to the invention, a steel sheet blank is thus provided which consists of a steel suitably composed in accordance with the explanations below (work step a)). The steel sheet blank has an aluminum-based anti-corrosion coating on at least one side. The anti-corrosion coating is preferably applied to both sides of the steel sheet blank. It is therefore a one-sided or a two-sided anti-corrosion coating. The two large surfaces of the steel sheet blank that are opposite one another are referred to as the two sides of the steel sheet blank. The narrow surfaces are referred to as edges.

During the heating step b), the steel sheet blank then passes through at least two heating zones, which are 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. The optional heating zones are of course omitted if they are not present. If, for example, the first heating zone of the optional heating zones is present, but the second or fifth heating zone is not, the steel sheet blank passes through Steel sheet blanks are heated in the first, third and fourth heating zones (in this order) one after the other. In this sense, step b) ii) is also to be understood as meaning that the parameters tl, TI, t2, T2, t5 and T5 are only set if the associated optional heating zones are implemented. In the event that one of the optional heating zones under b) i) is not implemented, its associated parameters are not set either. The entire step b) can alternatively be described as follows: b) Heating the steel sheet blank in an oven, i. whereby the steel sheet blank passes through the oven:

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 < 1.5 mm the following parameters are set:

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:

60s < tl < 640s and 710°C < Tl < 840°C

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 < 1.5 mm the following parameters are set:

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

30s < t2 < 90s and 940°C < T2 < 980°C

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

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:

60s < t3 < 760s and 720°C < T3 < 900°C

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

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

30s < t4 < 200s and 940°C < T4 < 980°C

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:

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

Os < t5 < 60s and 700°C < T5 < T4

In a preferred variant, heating zones arranged one after the other are arranged directly next 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

- first heating zone and second heating zone arranged directly next to each other

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

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

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

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.

In a preferred variant, the steel sheet blank does not pass through any other 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.

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

Furthermore, the total time in the oven for sheet steel 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 oven for sheet steel 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 sheet steel blank does not pass through any further heating zones than those mentioned, the total time in the oven corresponds to the sum of the time periods tl, t2, t3, t4, t5. The time periods of unrealized, optional heating zones are included in the sum as zero.

Preferably, the time period tl for sheet steel blanks with a thickness d < 1.5 mm is 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.

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.

Preferably, the time period t2 for sheet steel blanks with a thickness d < 1.5 mm is 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.

The temperature T2 for sheet steel blanks with a thickness d < 1.5 mm is preferably at least 930 °C, in particular at least 940 °C and maximum 980 °C, in particular maximum 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.

Preferably, the time period t3 for sheet steel blanks with a thickness d < 1.5 mm is 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.

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.

Preferably, the time period t4 for sheet steel blanks with a thickness d < 1.5 mm is 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.

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.

Preferably, the time period t5 for sheet steel blanks with a thickness d < 1.5 mm is 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 sheet steel 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. For sheet steel blanks with a thickness d < 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, preferably a maximum of T4-60 °C, in particular a maximum of T4-80 °C. For sheet steel 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.

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

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 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, whereby here too the first, The second and fifth heating zones are therefore optional.

A special variant of all the previously mentioned designs, which includes exactly the third and fourth heating zones, has the advantage that it is easy to implement, since only one furnace with two heating zones is required. In addition, the adhesion of the protective coating material to the rollers of the roller hearth furnace is reduced, since the protective coating material does not melt as quickly.

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.

A special variant of all the previously mentioned designs, which includes exactly the second, third and fourth heating zones, has the advantage that the relatively short second heating zone allows for rapid heating at the beginning. At the same time, however, the time period t2 is short enough that the The blank has not yet reached the temperature for molten phases. The temperature at which significant diffusion occurs is therefore quickly reached. Heating is then continued slowly so that there is sufficient time for diffusion before the blank reaches the temperature at which molten phases appear. This means that the total time in the oven can be shortened, while at the same time roll adhesion is reduced.

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

Adding the fifth zone to one of the three variants described above results in further advantages. The temperature of the blank is lowered before it leaves the furnace. This means that less cooling power is required in the subsequent forming process in a forming tool, as the blank does not have to be cooled down as much. The cooling water supply can therefore be reduced and the holding time in the tool can also be reduced. This saves costs and increases the economic efficiency of the process.

In a preferred development, the steel sheet blank at least partially exceeds the AC3 temperature of the steel sheet blank. In addition, the temperature T _E _inig 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.

For the purposes of this application, partially exceeding 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.

When placed in the forming tool, at least 30% of the blank has an austenitic structure, i.e. the transformation from ferritic to austenitic structure does not have to be completed when placed in the forming tool. Rather, up to 70% of the volume of the blank when placed in the forming tool can consist of other structural components, such as tempered bainite, tempered martensite and/or not or partially recrystallized ferrite. For this purpose, certain areas of the blank can be kept at a lower temperature than others during heating. To do this, the heat supply can be directed only 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 only significantly less martensite is formed during the forming process in the tool, so that the structure there is significantly softer than in the other parts that have a martensitic structure. In this way, a softer area can be specifically set in the respective formed sheet metal part, for example by ensuring optimum toughness for the respective intended use, while the other areas of the sheet metal part have maximized strength.

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

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.

Ac3 = (902 - 225*%C + 19*%Si - ll*%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.

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

In a preferred embodiment, the average heating rate r 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 r is to be understood as the average heating rate from 30°C to 700°C. Preferably, the dew point of the furnace atmosphere in the furnace is 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.

The blank heated in this way is removed from the oven and placed in a forming tool within a transfer time t _Tr ans of preferably no more than 20 s, in particular no more than 15 s. Such rapid transport is necessary in order to avoid excessive cooling before deformation.

Preferably, the temperature T _E ini _g of the steel sheet blank when it is placed in the forming tool (work step c)) is at least partially above Ms+100 °C, preferably above 600 °C, in particular above 650 °C, particularly preferably above 700 °C. Here, 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.

When the blank is inserted, 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 tempered at least in some areas to a temperature T _W z 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 only has to be ensured 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.

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:

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.%.

The ACl temperature and the AC3 temperature of a steel within the scope of the inventive specifications are according to the formulas:

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

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

In the tool, the blank is not only formed into the sheet metal part, but is also quenched to the target temperature at the same time. 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, in a special design at least 100 K/s.

After removal of 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 of 0.5 to 600 s. This is usually done by air cooling.

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 structure of the steel can be converted into a martensitic or partially martensitic structure by hot forming. The structure of the steel substrate of the sheet steel molded part is therefore preferably a martensitic or at least partially martensitic structure, since this has a particularly high hardness.

Particularly preferably, the steel substrate is a steel which, in addition to iron and unavoidable impurities (in wt. %), 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: <0.3 wt.%.

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 be present as impurities in the steel. These other elements are summarized under the "unavoidable impurities". The total content of unavoidable impurities is preferably a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The optional alloying elements Cr, B, Nb and Ti, for which a lower limit is specified, can also be present in the steel substrate as unavoidable impurities in contents below the respective lower limit. In this case, they are also counted as unavoidable impurities, the total content of which is limited to a maximum of 0.2% by weight, preferably a maximum of 0.1% by weight. The individual upper limits for the respective contamination of these elements are preferably as follows:

Cr: < 0.050 wt.%,

B: < 0.0005 wt.%

Nb: < 0.005 wt.%,

Ti: < 0.005 wt.%

These preferred upper limits should be considered alternatively or together. Preferred variants of the steel therefore meet one or more of these four conditions.

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.%.

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

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 variants, the Mn content is in the range of 0.75 - 0.85 wt.% or in the range of 1.0 - 1.6 wt.%.

In a preferred variant, the Al content of the steel is a maximum of 0.75% by weight, in particular a maximum of 0.5% by weight, preferably a maximum of 0.25% by weight. Alternatively or additionally, the Al content is preferably at least 0.02%. It has also been shown that it can be helpful if the sum of the contents of silicon and aluminum is limited. In a preferred variant, the sum of the contents of Si and Al (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 contents of Si and Al is at least 0.06 wt.%, preferably at least 0.08 wt.%.

Calcium (Ca) is used in steels to form non-metallic inclusions, particularly manganese sulphides. The rounded shape significantly reduces the negative effect of the inclusions on hot formability, fatigue strength and toughness. The maximum Ca content is 0.01 wt.%, in particular a maximum of 0.007 wt.%, preferably a maximum of 0.005 wt.%. If the Ca content is too high, the probability increases that non-metallic inclusions involving Ca will form, which will 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 observed.

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% by weight. Irrespective of this, the S content is preferably a maximum of 0.012%. In addition or in addition, the N content is preferably a maximum of 0.009% by weight.

Optionally, the steel also contains chromium with 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.%.

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

Preferably, the steel optionally also contains boron in a content of 0.001 - 0.005 wt.%. In particular, the B content is a maximum of 0.004 wt.%.

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 a maximum of 0.08 wt. %, preferably a maximum of 0.04 wt. %. The optional Ti content is at least 0.01 wt. % and a maximum of 0.08 wt. %, preferably a maximum of 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. %.

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% by weight, in particular a maximum of 0.068% by weight. Furthermore, the sum is preferably at least 0.015% by weight.

The corrosion protection coating mentioned is preferably produced by hot-dip coating the flat steel product. The flat steel product is passed through a liquid melt which consists 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 components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder being aluminum. The optional content of alkali or alkaline earth metals is preferably at least 0.1 wt.%.

In a preferred variant, the Si content of the melt is 0.4-3.5 wt.%. The Si content of the melt in this variant is preferably at least 0.5 wt.%, in particular at least 0.7 wt.%. The Si content of the melt is also preferably at most 2.5 wt.%, in particular at most 2.0 wt.%. It has been shown that the method according to the invention can be used particularly well with coatings in which the diffusion of iron into the coating occurs relatively quickly, i.e. where there is a high diffusion rate. These are in particular coatings with a low Si content, since Si hinders the diffusion of iron into the coating.

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. Furthermore, the optional content of alkali or alkaline earth metals in the melt preferably 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.

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 when it solidifies.

The alloy layer lies on the steel substrate and is directly adjacent to it. The alloy layer is essentially made of 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 aluminum. The optional further components include in particular the other components of the melt (i.e. optionally alkali or alkaline earth metals, in particular Mg or Ca) and the remaining parts of the steel substrate in addition to iron. In a further variant (variant with 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 aluminum. The optional additional components here also include in particular the other 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.

The Al base layer lies on the alloy layer and is directly adjacent to it. The composition of the Al base layer preferably corresponds to the composition of the melt of the melt bath. This means that it consists of up to 15% by weight Si, optionally 2-4% by weight Fe, optionally 5.0% by weight alkali or alkaline earth metals, preferably up to 1.0% by weight alkali or alkaline earth metals, optionally up to 15% by weight Zn, preferably up to 10% by weight Zn and optionally further components, the total contents of which are limited to a maximum of 2.0% by weight, and the remainder aluminum. Preferred compositions of the Al base layer correspond to the preferred melt compositions.

In a preferred variant, the Si content of the Al base layer is 0.4-3.5 wt.%. Preferably, the Si content of the Al base layer in this variant is at least 0.5 wt.%, in particular at least 0.7 wt.%. Furthermore, the Si content of the Al base layer is preferably at most 2.5 wt.%, in particular at most 2.0 wt.%. It has been shown that the method according to the invention can be used particularly well for coatings in which the Diffusion of iron into the coating occurs relatively quickly, meaning that there is a high diffusion rate. This is particularly the case with coatings with a low Si content, since Si hinders the diffusion of iron into the coating.

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

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 comprise in particular at least 0.0015 wt.% Ca, in particular at least 0.1 wt.% Ca. Furthermore 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.

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.

The corrosion protection coating preferably has a thickness of 5 - 60 pm, in particular 10 - 40 pm. The coating weight of the corrosion protection coating is in particular 30 - 360^ for corrosion protection coatings on both sides or 15 - 180

in the one-sided variant.

The coating weight of the anti-corrosive coating is preferably 100 - 200^ for coatings on both sides or 50 - 100^ for coatings on one side. The coating weight of the anti-corrosive coating is particularly preferably 120 - 180^ for coatings on both sides or 60 - for coatings on one side.

The thickness of the alloy layer is preferably less than 20 pm, particularly preferably less than 16 pm, particularly preferably less than 12 pm, in particular less than 10 pm. 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 lpm, 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 finish of the anti-corrosive coating.

The oxide layer consists in particular of more than 80% by weight of oxides, with the majority of the oxides (i.e. more than 50% by weight 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 taken up 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 component of the Al base layer, zinc oxide components are also present in the oxide layer.

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

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

The steel sheet blanks provided 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 of the steel sheet blank.

The invention further relates to a method for reducing waste in a method for producing a sheet steel molded part, in particular a method as previously explained. The method for producing a sheet steel molded part comprises the following steps: a) providing a sheet steel 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 that has 0.04 - 0.45 wt.% C, 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and wherein the sheet steel blank has an aluminum-based anti-corrosion coating on at least one side; b) heating the sheet steel blank in an oven, i. wherein the sheet steel blank passes through the oven:

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,

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) inserting the heated sheet metal blank into a forming tool, wherein the transfer time t _T rans 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 during the hot press forming the blank is cooled to the target temperature Tziei over a duration twz of more than 1 s at a cooling rate r _W z which is at least partially more than 27 K/s and is optionally held there; e) removing the sheet metal part cooled to the target temperature from the tool.

The scrap in such a process is reduced by the following steps: 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. 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 steel sheet blank has been interrupted, it is checked whether the additional period of time the steel sheet blank was stored in the heating zone due to the interruption exceeds the maximum period of time for this heating zone.

During the ongoing production process, there may occasionally be interruptions in the flow. Typically, the process for producing a sheet steel molded part is carried out 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 there is an interruption in the production process, the movement of the sheet metal blanks through the roller hearth furnace stops or slows down. Sheet metal blanks that were stored for too long in the roller hearth furnace during the interruption are completely sorted out and not used. In a roller hearth furnace that is 30 - 50 m long, this can affect a large 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, different effects on the sheet metal blanks arise depending on which heating zone the sheet metal blanks are stored in during the interruption. In the particularly hot heating zones, even short additional periods of time lead to the diffusion process in the coating progressing to such an extent that the sheet metal blank is no longer suitable for forming and use as a sheet metal molded part. In contrast, the sheet metal blanks in the heating zones that are not so hot can survive a longer additional period of time without this having any significant effects. Or in other words: The process windows are much narrower in terms of time for the hot heating zones than for the heating zones with lower temperatures. Therefore, 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 flow. The process window in terms of time (the maximum period of time) is therefore specified for each heating zone. If the flow 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, the steel sheet blank is then checked to see whether the additional period of time the steel sheet blank was stored in the heating zone due to the interruption exceeds the maximum period of time for this heating zone. It is therefore checked whether the sheet blank was still processed within the relevant process window.

Subsequently, the steel sheet blank for which the check in step iii) was positive (i.e. the maximum time period was exceeded) is rejected. In this way, the number of steel sheet blanks rejected (i.e. scrap) is lower than in the usual procedure, which involves rejecting all steel sheet blanks that have been in the furnace for too long during the interruption.

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.

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

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.

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 then also has areas of different thicknesses.

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

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”);

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

Using resistance spot welding or laser welding, patches (so-called "patches") are applied to an existing sheet metal blank in order to thicken it in certain areas. Alternatively, the patches can also protrude beyond the existing sheet metal blank, or only overlap a fraction of the sheet metal blank and be connected using resistance spot welding or laser welding, so that a variant of welding together ("tailor welded blanks") using resistance spot welding or laser welding is partially or essentially created. Alternatively, the patches can also be applied using structural adhesives. In the latter two cases, sheet metal cuts made from different materials can also be used and joined together.

Areas of different thicknesses have the advantage that individual areas of the final sheet metal part (see below) can be specifically reinforced or given a higher ductility. In this way, it is possible to make those parts that are subject to particular stress (for example during a crash) more stable, while other parts are made thinner to reduce the weight of the component. The result is a weight-optimized component that has targeted reinforcements in the areas of high stress. At the same time, more ductile areas of the component absorb the energy over a greater distance in the event of a crash and reduce the stress on the passengers.

In the two 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

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 0s < t5 < 45s and 700 °C < T5 < T4 and simultaneously

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

0s < t5 < 60s and 700 °C < T5 < T4 are met. The same applies to the preferred ranges of these parameters. By adhering to both sets of relations, 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.

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

Several tests were carried out to demonstrate the effect of the invention. For this purpose, corresponding steel sheets were produced in a conventional manner by producing slabs with the compositions given 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 kept for between 30 and 450 minutes until the temperature in the core of the slabs was reached and the slabs were thus heated through. The slabs were discharged from the pusher furnace at their heating through 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 to an intermediate product with a thickness of 40 mm, whereby the intermediate products, which can also be referred to as pre-strips in hot strip rolling, each had an intermediate product temperature in the range of 1050 °C to 1150 °C at the end of the pre-rolling phase. The pre-strips were fed to the finish rolling immediately after pre-rolling so that the intermediate product temperature corresponds to the initial rolling temperature for the finish rolling phase. The pre-strips were rolled out to 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 and 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 strips were passed through a molten coating bath with a temperature in the 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 in a conventional manner, producing coatings with different layer thicknesses (see Table 2). The strips were then cooled in a conventional manner. Blanks were cut from the steel strips produced in this way and used for further tests. In these tests, sheet metal part samples 1 to 46 in the form of profile-shaped components (hat profiles) with a blank surface of approx. 200 x 400 mm were hot-pressed from the respective blanks. Table 3 shows the thickness of the flat steel product, the type of steel used according to Table 1 and the coating used according to Table 2 for each test. In 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 dead. 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 _T rans, which consists of the removal from the heating device, transport to the tool and placement in the tool, was around 10 s. The temperature T _E _inig of the blanks when placed in the forming tool was in all cases above the respective martensite start temperature +100 °C. The blanks were formed into the respective sheet metal parts in the forming tool, whereby the sheet metal parts were cooled in the tool at a cooling rate of about 50 K/s to a target temperature T _Z iei of less than 200°C. The dwell time in the tool was about 6 to 10 s. Finally, the samples were cooled in air to room temperature.

The sheet steel parts produced in this way were then tested for their processability. The weldability of the sheet metal parts was tested in accordance with SEP 1220-2. The results are shown in Table 3. If the weldability is sufficient, the example is marked with "OK" (OK), otherwise with "NOK" (NOT OK). It is clear that longer periods in the first and third zones are not problematic, as these zones have a lower temperature. On the other hand, the sheet steel blanks should not be stored for too long in the second and fourth zones, as this significantly impairs weldability. This is due to the thickness of the alloy layer. This was measured on cross sections of the produced sheet metal parts and is also given in Table 3. Weldability is guaranteed up to an alloy layer thickness of 15 pm, while weldability is no longer possible for thicknesses above 15 pm.

Table 1 (steel grades)

Remainder iron and unavoidable impurities. Values in % by weight

Table 2 (Coating variants)

Table 3 (Process parameters)

Claims

Patent claims

1. Method for producing a sheet steel molded part comprising the following steps: a) providing a sheet steel 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 having 0.04 - 0.45 wt.% C, 0.1 - 3 wt.% Mn and optionally up to 0.01 wt.% B, and wherein the sheet steel blank has an aluminum-based anti-corrosion coating on at least one side; b) heating the sheet steel blank in an oven, i. wherein the sheet steel blank passes through the oven:

1. optionally in a first time period tl a first heating zone with a first temperature TI

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

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 cooled during the hot press forming over a period t _wz of more than 1 s at a cooling rate r _wz which is at least partially more than Tl K/s to the target temperature T _Ziei and is 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 sheet steel blanks with a thickness d >1.5 mm the following parameters must be 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 (working step c)) is at least partially above Ms+100°C, where Ms denotes the martensite start temperature.

4. Process according to 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 1OOO °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. Process 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 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 optionally further components, the contents of which are limited in total 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 optionally further components, the total contents of which are limited to a maximum of 2.0 wt.%, and the remainder aluminum.

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

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) placing the heated sheet metal blank in a forming tool, the transfer time t-Frans required for removing the blank from the furnace and inserting the blank being at most 20 s, preferably at most 15 s; d) hot press forming the sheet metal blank to form the sheet metal part, the blank being cooled to the target temperature T _Ziei over a period t _W z of more than 1 s at a cooling rate r _W z which is at least partially more than Tl K/s and optionally being held there during the hot press forming; 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 sheet steel blank can additionally be stored in this heating zone due to an interruption in the throughput; 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 during the duration of the interruption; iii. after the interruption of 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 of time 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

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.