DE102021214024A1 - Process for manufacturing a hot-formed and press-hardened sheet steel component - Google Patents

Abstract

The invention relates to a method for producing a hot-formed and press-hardened sheet steel component (3), which has the following process steps: an austenitizing step (ΔtA), in which a still unhardened blank (17), in particular coated with an anti-scale layer, is at least partially removed except for the material-specific austenitization temperature is heated; and a press hardening step (ΔtPH) in which the blank (17) placed in a forming die (23) in the hot state is hot worked and press hardened, thereby forming the steel sheet component (3). According to the invention, the method has a tempering step (ΔtT) with partial pre-cooling (ΔtVK) between the austenitization step (ΔtA) and the press-hardening step (ΔtPH), in which an area (N) of the blank (17) is pre-cooled to a pre-cooling temperature (ϑVK). , so that the tensile strength of the pre-cooled region (N) in the hardened state is reduced compared to the tensile strength of the non-pre-cooled region (T).

Description

The invention relates to a method for producing a hot-formed and press-hardened sheet steel component according to the preamble of claim 1 and such a sheet steel component according to claim 8.

In the prior art, such a sheet steel component is produced in a process sequence in which a circuit board coated on both sides with an anti-scale layer is provided. In a heat treatment step, the coated blank is at least partially heated to above the material-specific austenitization temperature, for example in a roller hearth furnace. The heat-treated blank is then placed in a forming tool while still hot. There, a press hardening step takes place to form the sheet steel component.

The hydrogen permeability of the scale protection layer is increased at high process temperatures and reduced at low process temperatures. Therefore, in the heat treatment step (that is, in the roller hearth furnace), the hydrogen barrier effect of the anti-scale layer is reduced and hydrogen is introduced from the furnace atmosphere into the steel substrate of the blank. In the further course of the process after the heat treatment, the hydrogen entry leads to hydrogen embrittlement. High-strength steels, such as 34MnB5, are particularly sensitive to hydrogen embrittlement, with hydrogen-induced cracking, which can occur when the finished sheet steel component is installed in the vehicle body. The hydrogen-induced cracking leads to stress corrosion cracking, especially when the sheet steel component is installed in a wet environment, in which hydrogen is produced, which locally damages the material structure of the sheet steel component. Mechanical stress on the sheet steel component therefore leads to premature component failure.

Against this background, it is common practice not to use high-strength hot-formed steels, such as 34MnB5, in vehicle bodies in a wet environment, as there is a risk of stress corrosion cracking. Instead of such ultra-high strength steels, a steel substrate of a material grade is used in a wet environment, the maximum attainable tensile strength of which is 1500 MPa or less in the hardened condition. In contrast to high-strength steels, steel substrates of such a material grade are less susceptible to stress corrosion cracking in wet environments.

From the U.S. 9,708,685 B2 a reinforcement element for a B-pillar of a vehicle is known, which is designed as a hot-formed and press-hardened sheet steel component. From the US20190366407A1 a method for producing a vehicle component from a high-strength steel substrate is known. From the US20110030442A1 a forming process is known in which a plate made of hardened material is formed.

The object of the invention is to provide a method for producing a hot-formed and press-hardened sheet steel component which, compared to the prior art, has a higher tensile strength and is less susceptible to component failure when installed in a wet environment.

The object is solved by the features of claim 1 or 8. Preferred developments of the invention are disclosed in the dependent claims.

The invention is based on a method for producing a hot-formed and press-hardened sheet steel component, which has the following process steps: First, in an austenitization step, a circuit board that is still unhardened, in particular with an anti-scale layer (for example an aluminum-based or zinc-based coating), is coated at least partially except for heated above the material-specific austenitization temperature. This is followed by a press-hardening step, in which the blank, which is placed in a forming tool while hot, is hot-formed and press-hardened. In this way, the sheet steel component is formed. According to the invention, the sheet steel component is divided into at least one wet area for the blank that can be installed in a wet environment and dry area for the blank that can be installed in a dry environment. According to the invention, the method has a tempering step between the austenitization step and the press-hardening step, in which a partial pre-cooling of the blank is carried out. With partial pre-cooling, the wet area of the board is pre-cooled to a pre-cooling temperature. In this way, the tensile strength of the blank wet area in the hardened state is reduced compared to the tensile strength of the non-precooled blank dry area. Due to the reduced tensile strength, the wet area of the blank is significantly less susceptible to stress corrosion cracking when the sheet steel component is installed in a wet environment than the dry area of the blank, whose tensile strength has not been reduced during the tempering step. When installed, the sheet steel component is therefore significantly less susceptible to stress corrosion cracking in wet environments. In the austenitizing step, the blank can be completely or partially processed in a first process route a second process route are only partially heated to above the material-specific austenitization temperature.

For a simpler understanding of the invention, the sheet steel component is divided into a dry area and a wet area by way of example in the description above and in the following. It should be emphasized that such a division is merely an exemplary, non-limiting embodiment variant of the invention. Rather, the method according to the invention according to the characterizing part of claim 1 generally enables a strength distribution that is adapted/tailor-made to the requirements/load of the sheet steel component. For this purpose, the method has a tempering step with partial pre-cooling between the austenitizing step and the press-hardening step, in which an area of the blank is pre-cooled to a pre-cooling temperature, so that the tensile strength of the pre-cooled area is reduced in the hardened state, compared to the tensile strength of the not pre-cooled area.

The tempering step can be designed as a tailored tempering process or as a thermal printer process, in which the temperature-time profile of the heat treatment before press hardening can be controlled in a targeted and local manner. For example, the tailored tempering process can either be completely austenitized and/or annealed in the 2-phase area between Ac1 and Ac3.

A core of the invention is therefore the targeted selection of the component strength/mechanical parameters for the corresponding local corrosive requirements of the sheet steel component. The background is that due to stress corrosion cracking, it is not possible to use a hot-forming grade with a tensile strength greater than 1500 MPa in a potentially wet environment, since the corrosion produces hydrogen, which locally damages the material structure. Further component stress can therefore lead to component failure.

For this reason, the component strength in corrosively stressed areas (that is, in the present exemplary embodiment, for example the column base section) is set to a maximum tensile strength of 1500 MPa. Especially when using 22MnB5, the tensile strength is up to 1650MPa. The rest of the sheet steel component is in the dry area and can therefore be designed with a strength of 1900 MPa, for example.

In a technical implementation, the steel substrate of the plate can be assigned to a material quality class for the production of high-strength sheet steel components. Their maximum achievable tensile strength in the hardened state can be more than 1650 MPa, in particular more than 1800 MPa. The steel substrate can preferably be a manganese-containing heat-treatable steel with a carbon content greater than 0.22 wt%. It is particularly preferred if the steel substrate consists of 34MnB5, whose maximum tensile strength that can be achieved in the hot forming process is 1900 MPa overall. With the method according to the invention, even a high-strength steel, such as 34 MnB5, can therefore be used when installing the sheet steel component in a wet environment, without this leading to premature component failure due to stress corrosion cracking.

In one embodiment variant, the steel substrate can consist of a manganese-boron heat-treated steel with an adapted alloy concept, which is therefore particularly well suited for the purpose according to the invention. In particular by increasing the silicon content from >0.8 to 2.2% by weight, preferably >1.4 to 2.0% by weight, particularly preferably >1.4-1.7% by weight, the intermediate structure bainite and retained austenite in the steel material can be stabilized, whereby the ductility, ie, safety against brittle failure, of the steel material can be improved. In particular, a process control in which a tempering step Δt _T with partial pre-cooling Δt _VK to pre-cooling temperature ϑ _VK takes place so quickly that neither the material-specific ferrite nor the pearlite area F, P is passed through and the pre-cooling temperature ϑ _VK is between the bainite start temperature and bainite finish temperature is advantageous for the formation of a mixed structure consisting of martensite M and additionally bainite B and/or residual austenite RA in the pre-cooled area of the sheet steel component.

The greatest possible tensile strength of the hot-formed and press-hardened sheet steel component is of great importance. Against this background, the tensile strength of the non-pre-cooled blank dry area in the hardened state corresponds to the maximum tensile strength that can be achieved in the hot forming process. In contrast, the tensile strength of the pre-cooled blank wet area is reduced in the hardened state. For example, its tensile strength can be less than 1650 MPa, in particular less than 1500 MPa. Alternatively and/or in addition to this, the tensile strength of the pre-cooled blank wet area can be in a range between 500 MPa and 1650 MPa.

In a preferred embodiment variant, the sheet steel component can be implemented as a reinforcement element for a B-pillar of a two-track vehicle. In this case, the B-pillar can be designed as a hollow beam. This consists of one, in the vehicle transverse direction inner inner sheet metal part and a contrast outer outer sheet metal part. The reinforcement element is arranged between the sheet metal inner part and the sheet metal outer part. The reinforcement element has a head section which can be connected to the vehicle roof structure and a foot section which can be connected to a vehicle rocker panel and a central pillar section which extends in between. In particular, the pillar foot section is in a potentially wet environment when installed. The column foot section thus forms the blank wet area, which is pre-cooled during the hot forming process in the tempering step. In contrast, the central column section and the head section of the reinforcement element form a blank dry area that is not pre-cooled during the hot forming process in the tempering step.

The anti-scale coating of the circuit board can be any suitable coating overlay. For example, the board's anti-scale coating may be an AISi (90-10) coating, a zinc-based GA/GI coating, or a paint-based coating that minimizes scaling. In an alternative process route, the still unhardened blank can be provided without an anti-scale layer. In this case, the uncoated board is not subjected to compressed air during the heat treatment. The design of the process is adapted to the possible formation of scale.

• 1 und 2 jeweils ein Verstärkungselement für eine B-Säule im verbauten Zustand (1) und in Alleinstellung (2);
• 3a bis 3c jeweils Ansichten einer Warmumformanlage, anhand derer eine Prozessabfolge zur Herstellung des Verstärkungselements veranschaulicht ist; und
• 4 ein Zeit-Temperatur-Profil, das den zeitlichen Temperaturverlauf in der Platine während des Warmumformungsprozesses zeigt.

An exemplary embodiment of the invention is described below with reference to the accompanying figures. Show it:

• 1 and 2 one reinforcement element each for a B-pillar when installed ( 1 ) and alone ( 2 );
• 3a until 3c in each case views of a hot-forming plant, on the basis of which a process sequence for the production of the reinforcement element is illustrated; and
• 4 a time-temperature profile that shows the course of temperature over time in the blank during the hot forming process.

In the 1 a vehicle side structure with a B-pillar 1 of a two-lane vehicle is indicated insofar as it is necessary for understanding the invention. The B-pillar 1 is designed as a hollow beam, which is constructed from an inner sheet metal part in the vehicle transverse direction y and an outer sheet metal part in the vehicle transverse direction y 1 are not specifically highlighted. A reinforcement element 3 is arranged between the two sheet metal inner and sheet metal outer parts. The reinforcement element 3 has a head section 7 connected to a vehicle roof structure 5, a foot section 11 connected to a vehicle rocker panel 9, and a central column section 13 extending therebetween.

The reinforcement element 3 is a hot-formed and press-hardened sheet steel component made from 34MnB5. How from the 2 As can further be seen, the sheet steel component is divided into a blank wet area N, which corresponds to the column foot section 11, and a blank dry area T, which comprises the central column section 13 and the column head section 7.

The reinforcing element 3 is in a below based on the 3 and 4 hot forming process shown: As a result, in the 3a provided in a circuit board stack 15 circuit boards 17, which are coated with an AlSi anti-scale layer. The blanks 7 are transferred to a hot forming system, which, viewed in the production direction F, consists of a roller hearth furnace 19 as a heat treatment device, a temperature control station 21 and a forming tool 23 . In the hot forming process takes place according to the 4 first an austenitization step Δt _A , in which the still unhardened blank 17 is heated to a process temperature above a material-specific austenitization temperature Ac3, at which point the transformation of the ferrite into austenite is complete. For example, the process temperature in the roller hearth furnace 11 is greater than 880°C.

It should be emphasized that the 4 shows only an exemplary process route to which the invention is not limited. Independently of this, other process routes are also included within the scope of the invention, in which the heating occurs between the temperature Ac1 at which austenite forms and the temperature Ac3.

The roller hearth furnace 11 has a furnace zone 25 on the outlet side, which is part of the tempering station 21 . Partial pre-cooling Δt _VK takes place in the furnace zone 25 . For this purpose, the furnace zone 25 according to the 3b divided transversely to the production direction F into a pre-cooling section 27 with compressed air nozzles 28 and a warming section 29 shielded therefrom, which is thermally shielded from the pre-cooling section 27 via a partition 31 . In the pre-cooling section 27 the later wet area N of the board is pre-cooled, while in the warm-keeping section 29 the dry area T of the board is thermally shielded from the pre-cooling and can be kept warm with the aid of a heating element 32 .

By way of example, three compressed air nozzles 18 ( 3a) provided which act on the circuit board 17 with compressed air. In the furnace zone 25, the pre-cooling takes place without a special protective gas or without atmospheres regulation. Therefore, the AlSi coating layer (that is, the scale protection layer) of the circuit board 17 is of great importance in order to avoid scale formation.

In addition to the furnace zone 25, the temperature control station 21 has a downstream heat treatment device 33 in which a partial heat treatment step Δt _W takes place, in which the pre-cooled blank wet area N is reheated with the aid of a heating element 37.

How from the 4 shows that the partial pre-cooling Δt _VK in the blank 17 sets the wet area of the blank N and the dry area of the blank T to different temperatures. In this way arise in the 4 in the two circuit board areas N, T locally changed time-temperature curves. In the 4 the time-temperature profile in the pre-cooled blank wet area N is shown with a solid line, while the time-temperature profile in the non-pre-cooled blank dry area T is shown with a dotted line.

In the 4 the tempering step Δt _T consists of the pre-cooling step Δt _VK and the heat treatment sub-step Δtw. After the tempering step Δt _{T ,} an insertion step Δt _E is carried out, in which the blank 17 is inserted into the forming tool 23 in the hot state. The insertion step Δt _E includes the following: The blank 17 is conveyed over an outlet section a to a centering station 37 . In the centering station 37, the circuit board 17 is positioned in a predefined removal position, so that a process-reliable gripping of the circuit board 17 by a transfer unit 38 is ensured. The transfer unit 38 brings the circuit board 17 to the forming tool 23 and inserts it there. Correspondingly, the blank wet area N and the blank dry area T have different insertion temperatures θ _N ,in and θ _T,in .

The press-hardening step Δt _PH takes place in the forming tool 13, in which the blank 17 is hot-formed and press-hardened, as a result of which the sheet steel component is formed. The reinforcement element 3 is removed from the forming tool 23 at a removal temperature θ _off . For example, the removal temperature ϑ _from in the 4 be about 180°C. The extraction temperature ϑ _from is in the 4 by a temperature offset below the martensite finish temperature Mf.

In the hot forming process according to the invention, the material is completely austenitized and hardened in the central column section 13 and in the head section 7 (each having high strength requirements). This creates a martensitic structure with a strength of 1900 MPa. A strength of between 500 and 1650 MPa can be set in the adjoining column foot section 11 through targeted temperature-time management. A core of the invention is that the steel substrate of the circuit board 17 is 34MnB5 or CR1900T-MB.

The Indian 4 The temperature-time profile shown as an example can be adjusted as required if required. As a design variant, joint flanges can also be specifically set using the thermal printer process, since the high-strength material condition (1900 MPa strength) is disadvantageous with regard to the suitability for joining.

The manufactured reinforcement element 3 therefore has both tailor-made strength properties and tailor-made corrosion requirements, without a tailor-made molded blank (TRB, TWB or patch) necessarily being necessary.

A tailor-made sheet steel component with a maximum strength of 1900 MPa can thus be produced with the invention. In addition, during the hot forming process in the tempering station 21, tempering states can be set in order to set strengths of locally less than 1900 MPa, in particular analogously to the local corrosion and strength requirements. For example, the blank wet area can have a strength of less than 1500 MPa, a blank deformation area can have a strength of less than 1250 MPa and a blank strength area can have a strength of greater than 1500 MPa.

The blanks used for the hot forming process according to the invention can be of any design, for example as a TRB blank, as a TWB blank, as a patch blank or as a blank with an increase in sheet thickness.

The invention is not limited to the embodiment of 1 until 4 limited. By way of example, in a second exemplary embodiment, the reinforcing element 3 in the middle column area 13 can have a fully hardened material state with a strength greater than 1650 MPa. In contrast, property-optimized areas are defined in the head and foot areas 7, 11, which have strengths greater than 1650 MPa.

It should be emphasized that, according to the invention, it is not primarily the differentiation between dry and wet areas T, N that is decisive. In the head area 7 of the reinforcement element 3, for example, a lower strength has a positive effect on the component performance in a crash, whereas in the foot area 11 (ie in the wet area) a strength greater than 1650 MPa is not permitted.

Reference List

1

B pillar

3

reinforcement element

5

vehicle roof structure

7

head section

9

vehicle rocker

11

Column foot section

13

middle column section

15

board stack

17

boards

19

Oven

21

temperature control station

23

forming tool

25

furnace zone

27

pre-cooling section

29

warming section

31

partition wall

32

heating element

33

heat treatment device

35

heating element

37

centering station

38

transfer unit

H

outlet section

N

board wet area

T

board drying area

ϑVK

pre-cooling temperature

ϑN,in, ϑT,in1

insertion temperatures

Ms

Martensite Start Temperature

Mf

Martensite Finish Temperature

ϑoff

withdrawal temperature

ΔtA

austenitizing step

ΔtT

tempering step

ΔtVK

pre-cooling step

Δtw

heat treatment step

ΔtpH

press hardening step

QUOTES INCLUDED IN DESCRIPTION

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

Patent Literature Cited

• US 9708685 B2 [0005]
• US 20190366407 A1 [0005]
• US20110030442A1 [0005]

Claims (12)

Method for producing a hot-formed and press-hardened sheet steel component (3), which has the following process steps: - an austenitization step (Δt _A ), in which a still unhardened blank (17), in particular one coated with an anti-scale layer, is at least partially heated to above the material-specific austenitization temperature and - a press-hardening step (Δt _PH ), in which the blank (17) placed in a forming tool (23) in the hot state is hot-formed and press-hardened, as a result of which the sheet steel component (3) is formed, characterized in that in order to provide a Load of the sheet steel component (3) adapted / tailor-made strength distribution, the process between the austenitizing step (Δt _A ) and the press hardening step (Δt _PH ) has a tempering step (Δt _T ) with partial pre-cooling (Δt _VK ), in which a region (N) of the blank (17) is pre-cooled to a pre-cooling temperature (ϑ _VK ), so that the tensile strength of the pre-cooled area (N) in the hardened state is reduced, compared to the tensile strength of the non-pre-cooled area (T). procedure after claim 1 , characterized in that the pre-cooled area (N) of the sheet steel component (3) is a board wet area that can be installed in a wet environment and the non-pre-cooled area (T) is a board dry area that can be installed in a dry environment, and that in particular with partial pre-cooling (Δt _VK ) the blank wet area (N) of the blank (17) is pre-cooled to a pre-cooling temperature (ϑ _VK ) so that the tensile strength of the blank wet area (N) is reduced in the hardened state, compared to the tensile strength of the non-pre-cooled blank dry area (T). procedure after claim 1 or 2 , characterized in that the steel substrate of the blank (17) is assigned to a material quality class for the production of high-strength sheet steel components, the maximum achievable tensile strength of which in the hardened state is Rm > 1650 MPa, in particular Rm > 1800 MPa, and/or that the steel substrate contains manganese Tempered steel with a carbon content greater than 0.22 wt%, in particular 34MnB5, whose maximum achievable tensile strength in the hardened state is 1900 MPa. procedure after claim 1 , 2 or 3 , characterized in that the tensile strength of the non-pre-cooled area (T) in the hardened state corresponds to the maximum achievable tensile strength, and/or that the tensile strength of the pre-cooled area (N) in the hardened state is Rm ≤ 1650 MPa, in particular Rm ≤ 1500 MPa, is, or is in a range between 500 MPa and 1650 MPa. Method according to one of the preceding claims, characterized in that the sheet steel component (3) is a reinforcing element of a vehicle, in particular for a vehicle body pillar (1), and in that in particular the body pillar (1) is designed as a hollow beam, consisting of a sheet metal inner part and a Outer sheet metal part, between which the reinforcing element is arranged, and in that in particular the reinforcing element has a head section (7) that can be connected to a vehicle roof structure (5), a foot section (11) that can be connected to a vehicle rocker panel (9) and a central column section ( 13), and that in particular the column foot section (11) forms the board wet area (N), while the middle column section (13) and the top section (5) form the board dry area (T). Method according to one of the preceding claims, characterized in that the anti-scale layer of the circuit board (17) is any suitable coating layer, and that in particular the anti-scale layer of the circuit board (17) has an AlSi coating (90-10), a GA/GI coating Zinc-based, or a paint-based coating that minimizes scaling. Method according to one of the preceding claims, characterized in that the blank (17) which has not yet been hardened is not coated with an anti-scale layer and in that in particular the uncoated blank (17) is not subjected to compressed air during the heat treatment. Method according to one of the preceding claims, characterized in that the pre-cooling temperature (ϑ _VK ) and/or the tool insertion temperature (ϑ _N,ein ) lies between the material-specific bainite start temperature and bainite finish temperature. Method according to one of the preceding claims, characterized in that the steel substrate is a manganese-boron tempering steel with a silicon content of >0.8 to 2.2% by weight, preferably >1.4 to 2.0% by weight, particularly preferably > is 1.4 to 1.7% by weight. procedure after claim 8 or 9 , characterized in that the tempering step (Δt _T ) with partial pre-cooling (Δt _VK ) to the pre-cooling temperature (ϑ _VK ) takes place so quickly that neither the material-specific ferrite nor the pearlite area (F, P) is passed through, and that in the pre-cooled Area can set a mixed structure of martensite (M) and bainite (B) and / or retained austenite (RA) in the sheet steel component. Hot-formed and press-hardened sheet steel component (3), which is produced using a method according to one of the preceding claims, characterized in that in order to provide a strength distribution adapted/tailored to the load on the sheet steel component (3), the tensile strength of the pre-cooled area (N) in the hardened state is reduced compared to the tensile strength of the non-precooled region (T). Hot-formed and press-hardened sheet steel component (3). claim 11 , characterized in that the pre-cooled area (N) of the sheet steel component (3) is a board wet area that can be installed in a wet environment and the non-pre-cooled area (T) is a board dry area that can be installed in a dry environment.

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