DE102021201845A1 - Process for the production of 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 (1) from a still unhardened steel substrate strip (3) made from a heat-treatable hot-forming alloy. According to the invention, a targeted decarburization of the surface layers takes place in the heat treatment step of the method, in which the sheet steel component is formed with soft structural layers (21) near the surface and a hard core layer (23).

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 a sheet steel component according to claim 10.

In a generic method, a hot-formed and press-hardened sheet steel component is manufactured from a steel substrate strip that is not yet hardened and is made of a heat-treatable hot-forming alloy. In the process, the steel substrate strip is coated with an anti-scale layer in a coating process. This is followed by a blank cut, in which the coated steel substrate strip is cut to form a blank. The subsequent hot forming process is divided into a heat treatment step and a forming step. In the heat treatment step, the blank is heated above the material-specific austenitization temperature. In the subsequent forming step, the hot blank is placed in a forming tool and hot-formed and press-hardened there, resulting in the sheet steel component being formed.

Depending on the hot-forming alloy used, such hot-formed and press-hardened sheet steel components can have an increased tensile strength in the hardened state, for example in the range of 1900 MPa, which is associated with a correspondingly reduced ductility. Accordingly, the bending angle of the sheet steel component is at a relatively low level, for example at 45° to 50° for hot-forming steel with a tensile strength in the hardened state of 1900 MPa. The bending angle is a crucial variable for crash-relevant components. Among other things, this indicates how much kinetic energy is absorbed by the sheet steel component in the event of a crash before the component fails due to brittle fracture. The bending angle is determined in a plate bending test according to VDA 238-100 with the aim of drawing conclusions about the deformation behavior and the susceptibility to failure of metallic materials in forming processes with dominant bending components or in the event of crash loads.

When using coated hot-forming material (e.g. AISi or ZnFe-coated), there is currently no applicable solution to significantly increase the bending angle of the sheet steel component.

When using uncoated hot forming material, on the other hand, there is the possibility of increasing the bending angle by decarburizing the surface layer and by controlling the furnace atmosphere (as well as the process temperature and time). This is based on the fact that softer microstructure layers (e.g. ferritic or pearlitic microstructure components) are formed near the surface, which counteract brittle component failure and crack initiation near the surface. In this case, however, scale forms on the component surface, which significantly worsens the tribology in the forming tool. In addition, tool wear is increased as a function of scale formation. In the hot forming process, the coefficient of friction changes gradually with the degree of tool contamination, so that regular and time-consuming cleaning of the forming tool is necessary.

In addition, there is the following problem with coated hot-forming material: conventionally, an AlSi layer with a comparatively large layer thickness of 20 to 30 μm is applied to the hot-forming material as an anti-scale layer. This advantageously reduces tool wear in the hot forming process. However, the AlSi layer acts as a hydrogen barrier, the blocking effect of which is temperature-dependent. This means that the blocking effect is increased at room temperature, while the blocking effect is reduced at high process temperatures in the heat treatment step. Due to the hydrogen barrier effect of the AlSi layer, hydrogen effusion is impaired after the heat treatment step, in which case the proportion of hydrogen introduced into the hot-forming material is intended to be reduced. This circumstance leads to hydrogen-induced cracking, particularly in the case of high-strength or ultra-high-strength steels (e.g. tempered steel, in particular 34MnB5, with a tensile strength in the hardened state of >1700 MPa). Against this background, for example, such a tempered steel is usually used uncoated in the hot forming process in order to avoid such hydrogen-induced cracking.

From the US 2020/0149129 A , from the U.S. 2020/0224295 A1 and from the WO 2018/099819 A1 methods are known for producing hot-formed and press-hardened sheet steel components.

The object of the invention is to provide a method for producing a hot-formed and press-hardened sheet steel component which, on the one hand, has high tensile strength in the hardened state and, on the other hand, enables an increased bending angle in the forming process compared to the prior art.

The object is achieved by the features of claim 1 or claim 10. 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 is manufactured from a steel substrate strip that is not yet hardened and is made of a heat-treatable hot-forming alloy. Before the hot forming process, the steel substrate strip is subjected to a coating process in which the steel substrate strip is coated with an anti-scale layer.

According to the characterizing part of claim 1 takes place in the heat treatment step during the hot forming process a targeted surface layer decarburization, in which the sheet steel component is formed with near-surface soft microstructure layers, such as ferrite or pearlite, and a hard core layer, such as martensite. The sheet steel component according to the invention therefore has a softer microstructure near the surface on both sides with an increase in hard components in the direction of the center or core of the sheet.

In the sheet steel component, viewed in the sheet thickness direction, the proportion of ferritic and pearlitic microstructure components increases towards the surface and the proportion of martensitic microstructure decreases towards the substrate surface, resulting in the formation of the microstructure layers close to the surface and the hard core layer.

During the hot forming of a material grade coated according to the invention, a targeted decarburization of the surface layer and an increase in the bending angle can be achieved with greatly reduced scale formation, compared to the hot forming of an uncoated heat-treated steel. This has the advantage that, compared to hot forming of an uncoated material quality, complex components can be manufactured (constant coefficient of friction and process parameters) and there is significantly less tool wear (this is due to the reduction in scale formation).

• - Bereitstellen eines Stahlsubstrat-Bands als ein Coil einer vergütbaren Warmumformlegierung, zum Beispiel Vergütungsstahl, insbesondere 34MnB5, mit einer Zugfestigkeit im gehärteten Zustand von >1700 MPa, oder alternativ einer warmgewalzten Werkstoffgüte (HotRolled);
• - Optional ist ein Weichglühen/Anlassen des Stahlsubstrat-Bands notwendig/möglich;
• - Abhaspeln des zu einem Coil gewickelten Stahlsubstrat-Bands sowie anschließendes Beschichten (insbesondere Bandbeschichten) des Stahlsubstrat-Bands mit einer Al-haltigen Zunderschutzbeschichtung, insbesondere eines Lacks mit Zunderschutzpartikeln in einer anorganischen Matrix; alternativ kann ein dünner Auftrag einer Schmelztauchschicht und/oder ein Abscheiden aus der Gasphase auf das Stahlsubstrat-Band erfolgen. Hierbei werden ebenfalls primär AI- und Zn-haltige Schichten aufgetragen;
• - Optional (wenn notwendig) kann ein Einbrennen des Zunderschutzlacks oder der Beschichtung erfolgen;
• - Bereitstellen des beschichteten Coils an einen Platinenschnitt (ein Laserschnitt ist ebenfalls denkbar);
• - Bereitstellen der Formplatinen an eine Warmumformlinie; Erwärmung erfolgt in der Regel im Rollenherdofen. Alternativ ist auch eine Schnellerwärmung insbesondere bei dünnen Schichten durchführbar. Dies kann induktiv, konduktiv oder über Kontakt durchgeführt werden. Insbesondere kann der Sauerstoffgehalt in der Atmosphäre sowie der Taupunkt gezielt reguliert werden, um das oberflächennahe Gefüge zu beeinflussen. Auch die Durchlaufzeit und die Erwärmungstemperatur kann bewusst reguliert werden (das heißt eine längere Zeitdauer sowie eine höhere Temperatur erhöhen in der Regel den Grad der Randschichtentkohlung).

The method according to the invention can have the following process steps:

• - Providing a steel substrate strip as a coil of a heat-treatable hot-forming alloy, for example heat-treatable steel, in particular 34MnB5, with a tensile strength in the hardened state of >1700 MPa, or alternatively a hot-rolled material grade (HotRolled);
• - Optionally, soft annealing/tempering of the steel substrate strip is necessary/possible;
• - Uncoiling the steel substrate strip wound into a coil and subsequent coating (in particular strip coating) of the steel substrate strip with an Al-containing anti-scale coating, in particular a paint with anti-scale particles in an inorganic matrix; alternatively, a thin application of a hot dip layer and/or vapor deposition onto the steel substrate strip can take place. In this case, layers containing primarily Al and Zn are also applied;
• - Optionally (if necessary) the anti-scale paint or the coating can be baked;
• - Providing the coated coils to a blank cut (a laser cut is also conceivable);
• - Providing the blanks to a hot forming line; Heating usually takes place in a roller hearth furnace. Alternatively, rapid heating can also be carried out, particularly in the case of thin layers. This can be done inductively, conductively or by contact. In particular, the oxygen content in the atmosphere and the dew point can be specifically regulated in order to influence the near-surface structure. The throughput time and the heating temperature can also be deliberately regulated (i.e. a longer period of time and a higher temperature usually increase the degree of surface layer decarburization).

Preferred process parameters in the heat treatment step are: heating temperature of the board: 850-950°C; Oven residence time/heating time: When heating in a roller hearth furnace, the minimum furnace residence times are between 100s and 300s (for sheet thicknesses between 1.0 and 3.0 mm). The maximum furnace dwell time is primarily based on the desired surface layer decarburization. In the case of rapid heating, significantly lower residence times in the oven or residence times in the heating system are conceivable. Combined heating, for example inductive and roller hearth furnaces, can also be carried out. Furnace atmosphere is typically standard atmosphere with dew point control via dry air injection. In addition, a partial addition of reducing gases is conceivable. A complete protective gas atmosphere N2 + NH4 (as in the case of uncoated hot forming) is not required or sought. Compared to a conventional AISi coating, no additional diffusion time (to form the layer) is required. The sheet metal coated according to the invention therefore heats up significantly faster.

The thickness (measured across the sheet thickness) of the decarburized surface layer, i.e. the soft structural layer, is reduced to less than 0.15 mm and/or a maximum of 10% of the sheet thickness (per side of the coating tion) limited. The thickness of the surface layer is determined by hardness measurement and runs down to the core depth at which a standardized martensite hardness is reached. For tempering steel, especially 34MnB5, with a tensile strength in the hardened condition of 1900 MPa. For example, the standard hardness is between 520 and 640 HV10.

The sheet steel component according to the invention (for example made of heat-treated steel, in particular 34MnB5, with a tensile strength in the hardened state of >1700 MPa) with an anti-scale coating has a bending angle greater than 55°. A bending angle increase of more than 10% is possible compared to a fully hard, i.e. primarily martensitic microstructure (e.g. an uncoated and hot-formed CR1900 hot-forming material) over the entire sheet thickness.

The sheet steel component according to the invention can be installed in a vehicle body, either completely blasted and/or with a remaining anti-scale coating (this can generally be phosphated). An application for TWB, as well as patch boards and TRB is also conceivable. Here, too, there are the advantages of the increased bending angle due to a deliberately adjusted ductile edge phase. Tailored tempering of the blanks is also conceivable, with at least one component area undergoing a deliberate microstructure adjustment of more ductile phases.

Aspects of the invention are highlighted again in detail below: In the coating process, a thin layer containing Al and/or a thin layer containing Zn can be applied to the steel substrate strip as an anti-scale layer. The layer thickness of the anti-scale layer can preferably be less than 5 μm and in particular be in a range from 1 to 3 μm. As a result, on the one hand scale formation can be greatly reduced compared to hot forming of uncoated heat-treated steel, and on the other hand targeted surface layer decarburization can still take place in the heat treatment step. Compared to a conventional AISi or ZnFe anti-scale layer applied as a thick layer (layer thickness between 20 and 30 μm), the Al-containing and/or Zn-containing thin layer according to the invention allows increased surface layer decarburization in the coating process.

The surface layer decarburization according to the invention reduces the tensile strength of the sheet steel component locally and increases its ductility accordingly. In this way, the bending angle of the sheet steel component that can be determined in the platelet bending test according to VDA238-100 is increased, compared to a microstructure that is fully hard, i.e. primarily martensitic, over the entire sheet thickness. The increase in the bending angle is greater than 10% compared to a conventional, hot-formed and press-hardened sheet steel component made from an uncoated or hot-dip-coated steel substrate strip of the same hot-forming alloy. Each of the structural layers close to the surface at the edge can preferably have a material thickness of less than 0.15 mm. As an alternative to this, the respective microstructure layer thickness can be up to a maximum of 10% of the sheet metal thickness.

The method according to the invention can be applied particularly preferably to a high-strength or higher-strength steel, in particular heat-treated steel, in particular 34MnB5, with a tensile strength in the hardened state of >1700 MPa.

To regulate the surface layer decarburization, the sheet thickness-dependent process parameters in the heat treatment step can be dimensioned as follows: The heating temperature of the blank can be 850°C to 950°C, for example. The minimum dwell time of the shaped blank in the heat treatment device can be 100 s and 300 s, for example, with a sheet metal thickness in the range of 1.0 mm to 3.0 mm. Under certain circumstances, longer oven residence times can also be selected.

It is particularly preferred if the anti-scale layer, which is thin compared to a conventional AISi or ZnFe anti-scale layer applied as a thick layer (layer thickness between 20 and 30 μm), is applied to the steel substrate strip as a paint in a coil coating process. The paint can preferably consist of an inorganic matrix containing anti-scale particles. After the paint coating, there is a partial baking step in which the paint is baked. Following the hot forming process, an additional blasting process can take place, in which the paint is removed from the sheet steel component in order to ensure that the sheet steel component can be welded.

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

• 1 in einem Blockschaltdiagramm eine Prozesskette zur Herstellung eines warmumgeformten und pressgehärteten Stahlblechbauteils; und
• 2 und 3 jeweils ein schematisches Querschliffbild des beschichteten Stahlsubstrat-Bands (2) und des warmumgeformten und pressgehärteten Stahlblechbauteils (3).

Show it:

• 1 in a block circuit diagram, a process chain for producing a hot-formed and press-hardened sheet steel component; and
• 2 and 3 a schematic cross-section of the coated steel substrate strip ( 2 ) and the hot stamped one and press-hardened sheet steel component ( 3 ).

In the 1 a process route for producing a hot-formed and press-hardened sheet steel component 1 is described. Accordingly, a still uncoated steel substrate strip 3 is provided as a coil. The still uncoated steel substrate strip 1 has, for example, a strength in the hardened state of 1900 MPa, with the hot-forming alloy being able to be 34MnB5, for example.

The still uncoated steel substrate strip 3 is first subjected to a coating process that is implemented in a painting station I as a coil coating process, in which the coil is unwound in a winding station and conveyed through the painting station I. The painting station I has an application unit 5 in which paint is applied to both sides of the steel substrate strip 1 . The paint is then baked in a baking unit 7 . The paint applied preferably has an inorganic matrix containing anti-scale particles.

In the painting station I, the steel substrate strip 3 is coated on both sides with a thin layer of paint 9, which acts as anti-scale agent and has a layer thickness d _L ( 2 ) in the range of 1 to 3 µm.

The coated steel substrate strip 3 is wound up again into a coil in a further winding station. The coil with the coated steel substrate strip 3 is transported to a blanking system II, in which a blank is cut. The shaped blanks 11 produced in the blanking line II are stacked on a blank stack 13 and fed from there to a hot-forming line. The hot stamping plant has in the 1 a roller hearth furnace 15 as a heat treatment device, a forming tool 17 for hot forming and press hardening, and a post-processing station 19 for a blasting process.

In the heat treatment device 15, the shaped blank 11 is heated above the material-specific austenitization temperature and then placed in the hot state in the forming tool 17, in which the sheet steel component 1 is hot-formed and press-hardened.

The essence of the invention is that both the anti-scale layer 9 and the process parameters in the heat treatment device 15 are dimensioned in such a way that a targeted decarburization of the surface layers takes place in the heat treatment step, in which the sheet steel component 1 is coated on both sides with a soft microstructure layer 21 near the surface, such as ferrite or pearlite. and a hard core layer 23 such as martensite. How from the 3 shows that the soft structural layers 21 close to the surface each have a layer thickness d _G of less than 0.15 mm, while the total thickness d of the sheet steel component 1 can be 1.5 mm, for example.

Compared to a conventional, hot-formed and press-hardened sheet steel component made from an uncoated steel substrate strip of the same hot-forming alloy, the sheet steel component 1 produced with such a microstructure enables a bending angle increase in the range of greater than 10 in a platelet bending test according to VDA 238-100 %.

Reference List

1

sheet steel component

3

substrate tape

5

order unit

7

burn-in unit

9

paint layers

11

molded board

13

stack

15

heat treatment facility

17

hot stamping tool

19

post-processing station

21

soft texture layers

23

hard core layer

I

painting station

II

blanking line

dL

paint film thickness

dG

microstructure thickness

i.e

sheet thickness

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 2020/0149129 A [0007]
• US 2020/0224295 A1 [0007]
• WO 2018/099819 A1 [0007]

Claims (10)

Method for producing a hot-formed and press-hardened sheet steel component (1) from a still unhardened steel substrate strip (3) made from a heat-treatable hot-forming alloy, with - a coating process (I) in which the steel substrate strip (3) is coated with an anti-scale layer (9 ) is coated, - a blank cutting (II), in which the coated steel substrate strip (3) is cut to form a blank (11), and - a hot forming process with a heat treatment step, in which the blank (11) except for the material-specific austenitization temperature, and with a forming step in which the shaped blank (11) is placed in a forming tool (17) while hot and is hot-formed and press-hardened therein to form the sheet steel component (1), characterized in that in the heat treatment step a targeted surface layer decarburization takes place , In which the sheet steel component (1) with near-surface soft microstructures high (21) and a hard core layer (23) is formed. procedure after claim 1 , characterized in that in the sheet steel component (1), viewed in the sheet thickness direction, the proportion of ferritic and pearlitic structural components increases in the direction of the surface or the proportion of martensitic structural structure decreases in the direction of the substrate surface, with the formation of the near-surface structural layers (21) and the hard core layer ( 23). procedure after claim 1 or 2 , characterized in that in the heat treatment step in the absence of a protective gas atmosphere, the surface layer decarburization takes place without severe Fe _x O _y formation, ie without severe scale formation, compared to hot forming of an uncoated heat-treated steel. procedure after claim 1 , 2 or 3 , characterized in that an Al-containing and/or Zn-containing thin layer is applied as an anti-scale layer (9) which, in comparison to a conventional AISi or ZnFe anti-scale layer applied as a thick layer, allows greater surface layer decarburization, and/or that in particular the layer thickness (d _L ) of the anti-scale layer (9) is less than 5 μm and is preferably in a range from 1 to 3 μm. Method according to one of the preceding claims, characterized in that the decarburization of the surface layers locally reduces the tensile strength of the sheet steel component (1) and correspondingly increases its ductility, so that the bending angle of the sheet steel component ( 1) is increased, compared to a fully hard, i.e. primarily martensitic microstructure over the entire sheet thickness (d), and/or that in particular each of the soft microstructure layers (21) near the surface has a layer thickness (d _G ) of less than 0 ,15 mm, and/or that the structural layer thickness (d _G ) is up to a maximum of 10% of the sheet thickness. Method according to one of the preceding claims, characterized in that the steel substrate (1) is a high-strength or higher-strength tempered steel, in particular 34MnB5, with a tensile strength in the hardened state of greater than 1700 MPa. Method according to one of the preceding claims, characterized in that to regulate the decarburization of the surface layers, the process parameters dependent on the sheet thickness are designed in the heat treatment step in such a way that in particular the heating temperature of the shaped blank (11) is 850°C to 950°C, and/or that the minimum dwell time of the blank (11) in the heat treatment device (15) is between 100 s and 300 s, for example, in particular with a sheet thickness (d) in the range of 1.0 mm to 3.0 mm, and/or that in particular the oxygen content in the atmosphere of the heat treatment device (15) and the dew point are regulated with regard to a targeted decarburization of the surface layers, and/or that the atmosphere of the heat treatment device (15) in particular is preferably a standard atmosphere with a dew point control via dry air feed and, if necessary, partial addition of reducing gases is. Method according to one of the preceding claims, characterized in that the anti-scale layer (9) is preferably applied to the steel substrate strip (3) as a paint in a coil coating process, and in that in particular the paint consists of an inorganic matrix with contained therein anti-scale particles, and that after the paint coating (5) there is a partial baking step (7) in which the paint is baked, and that in particular after the hot forming process there follows a blasting process (19) in which the paint is removed from the sheet steel component ( 1) is removed in order to ensure the weldability of the sheet steel component (1). Procedure according to one of Claims 1 until 7 , characterized in that the scale protection layer (9) is a thin application of a hot dip layer, in comparison to a conventional functional AISi or ZnFe anti-scale layer applied as a thick layer, and/or that the anti-scale layer (9) is applied to the steel substrate strip (1) in a gas separation process. Sheet steel component which is produced in a method according to one of the preceding claims, wherein the sheet steel component (1), in particular in a small plate bending test according to VDA 238-100, enables a bending angle increase in the range of greater than 10%, in particular in comparison to a conventional hot-formed and press-hardened sheet steel component with a fully hard, i.e. primarily martensitic microstructure over the entire sheet thickness (d), which is produced from an uncoated steel substrate strip or a steel substrate strip that is hot-dip refined with AISi and/or ZnFe of the same hot-forming alloy, and that in particular the in the platelet - The bending angle of the sheet steel component (1) determined in the bending test according to VDA238-100 is greater than 55°.

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