DE102024104924A1 - Process for producing a press-hardened sheet metal part from a hot-rolled and coated steel sheet - Google Patents

Abstract

The invention relates to a method for producing a press-hardened sheet metal part (T), comprising the steps:
- Providing a hot-rolled and coated steel sheet (S);
- Re-rolling of the coated steel sheet (S) to reduce its roughness;
- Carrying out a press-hardening process in which the steel sheet (S) is heated to austenitizing temperature and formed and hardened in a press-hardening tool (60) to form a sheet metal part (T), or in which the steel sheet (S) is preformed, heated to austenitizing temperature, and finish-formed and hardened in a press-hardening tool (60) to form a sheet metal part (T). The invention further relates to the use of a hot-rolled, coated, and temper-rolled steel sheet (S) for producing a press-hardened sheet metal part (T).

Description

The invention relates to a method for producing a press-hardened sheet metal part from a hot-rolled and coated steel sheet according to patent claim 1. The invention further relates to the use of a steel sheet designed with special properties for producing a press-hardened sheet metal part according to the independent patent claim.

Press hardening is a process in which a steel sheet made from a steel sheet material suitable for press hardening is austenitized by heating and then quench-cooled in a press-hardening tool while simultaneously forming. Due to martensitic hardening, high strength is achieved. In so-called direct press hardening, a flat steel sheet is heated and then formed in a press-hardening tool and simultaneously quench-cooled. In indirect press hardening, the steel sheet is first pre-formed, then heated and then finally formed in a press-hardening tool and simultaneously quench-cooled. The steel sheet used can have a coating, which in particular means a metallic coating that functions as a corrosion protection coating and/or scale protection coating. Regarding the state of the art, reference is made to the EP 3 265 365 B1 , in particular the introductory explanations and citations.

For press hardening, cold-rolled steel sheet (also called cold strip) or hot-rolled steel sheet (also called hot strip) can be used. Hot-rolled steel sheet is advantageous for greater sheet thickness in combination with a coating, especially with a metallic coating applied by hot-dip coating. The closest DE 699 33 751 T3 describes a hot-rolled steel sheet strip with a thickness of 0.2 mm to 20 mm and a metallic coating applied by hot dipping, which is further used for press hardening (hot press forming and quench hardening).

The invention is based on the object of providing an improved method for producing a press-hardened sheet metal part from a hot-rolled and coated steel sheet.

This object is achieved by the inventive method of patent claim 1. The independent patent claim also extends to the use of a steel sheet pretreated in accordance with the invention and thus developed with special properties for the production of a press-hardened sheet metal part. Advantageous developments and refinements for both subject matters of the invention are evident from the dependent patent claims, the following description of the invention (this expressly includes features described as examples and optionally), and the figure.

• - Bereitstellen eines warmgewalzten und beschichten, insbesondere metallisch beschichteten, Stahlblechs;
• - Nachwalzen des beschichten Stahlblechs, um dessen Rauheit, womit die Oberflächenrauheit der Blechoberfläche gemeint ist, zu verringern;
• - Durchführen eines Presshärtevorgangs, bei dem das Stahlblech, womit das nachgewalzte Stahlblech gemeint ist, auf Austenitisierungstemperatur erwärmt und in einem Presshärtewerkzeug zum Blechformteil umgeformt und gehärtet, d. h. abschreckgehärtet wird (direktes Presshärten) oder bei dem das Stahlblech vorgeformt, auf Austenitisierungstemperatur erwärmt und in einem Presshärtewerkzeug zum Blechformteil fertiggeformt und gehärtet, d.h. abschreckgehärtet wird (indirektes Presshärten).

The method according to the invention for producing a press-hardened sheet metal part comprises the following steps:

• - Providing a hot-rolled and coated, in particular metallically coated, steel sheet;
• - Re-rolling of the coated steel sheet to reduce its roughness, which means the surface roughness of the sheet surface;
• - Carrying out a press hardening process in which the steel sheet, which means the post-rolled steel sheet, is heated to austenitizing temperature and formed into a sheet metal part and hardened, i.e. quench hardened, in a press hardening tool (direct press hardening) or in which the steel sheet is pre-formed, heated to austenitizing temperature and finish-formed into a sheet metal part and hardened, i.e. quench hardened, in a press hardening tool (indirect press hardening).

The steel sheet provided is suitable for press hardening (e.g., 22MnB5 or 20MnB8) and is produced by a hot rolling process. A hot-rolled steel sheet is, in particular, a steel sheet produced in a hot rolling mill or the like at high rolling temperatures, preferably several hundred degrees Celsius or even above the recrystallization temperature, preferably by exclusively hot rolling (i.e., without additional cold rolling at low temperatures, especially room temperature). Hot-rolled steel sheet typically exhibits high or severe roughness and a larger sheet thickness tolerance. In contrast to cold-rolled steel sheet, hot-rolled steel sheet is also available in larger sheet thicknesses. Furthermore, hot-rolled steel sheet (hot strip) is usually more cost-effective than cold-rolled steel sheet (cold strip).

The hot-rolled steel sheet provided can have a sheet thickness of ≥ 2.4 mm, preferably ≥ 2.6 mm, in particular ≥ 2.8 mm. With such large sheet thicknesses, which are rather atypical for press hardening, press-hardened sheet metal parts can be produced that are very stable and resilient and are particularly suitable for vehicle components with high component requirements (e.g., for electric cars). The steel sheet provided preferably has despite the sheet thickness tolerance (see above) being present, the sheet thickness is at least approximately homogeneous.

The coating is applied after hot rolling. The coating is preferably applied to both sides, i.e., to both sides of the sheet. The coating is preferably a metallic coating, in particular made of zinc, a zinc alloy (e.g., a zinc-iron alloy), or an aluminum alloy (e.g., an aluminum-silicon alloy). The metallic coating is preferably applied by hot-dip coating. In hot-dip coating, the steel sheet is cleaned, heated, typically in a continuous furnace, passed through a molten bath, and blown off. The high roughness of the previously hot-rolled steel sheet (see above) proves advantageous because it enables rapid heating of the steel sheet even with greater sheet thicknesses, thereby saving energy and costs. The metallic coating, in particular if it is a coating made of zinc or a zinc alloy, can have a layer thickness of 7 µm to 13 µm, in particular of 9 µm to 11 µm.

The temper rolling of the coated steel sheet is carried out in particular in the cold state, preferably at room temperature, and can therefore also be referred to as cold temper rolling. The temper rolling is carried out to reduce the roughness of the coated steel sheet, i.e. to smooth the surface, in particular on both sides of the sheet. During temper rolling, an R _a value (average roughness) of ≤ 2.5 µm, preferably ≤ 2.0 µm, in particular ≤ 1.5 µm, is preferably achieved, which means the R _a value of the coated sheet surface, in particular on both sides of the sheet. During temper rolling, the sheet thickness can also be homogenized or the sheet thickness tolerance can be reduced and/or a specific surface texture can be created. Preferably, temper rolling is carried out without any significant reduction in the sheet thickness (at most only ≤ 2%) and is therefore fundamentally different from variable rolling for the production of so-called tailored rolled blanks.

Re-rolling is preferably carried out with a pair of rolls (top roll and bottom roll) or, if necessary, with several pairs of rolls. The roughness achieved during re-rolling depends on the condition, in particular the surface condition, of the rolls (work rolls). The rolls in question can be designed with a specific surface structure (e.g. EDT, LBT, SBT and EBT) in order to achieve a desired roughness value and/or a specific surface texture during re-rolling. Preferably, the two rolls of a roll pair (for re-rolling) are essentially identical, in particular with an identical surface structure. The rolls of a roll pair (for re-rolling) can also be designed differently, in particular with different surface structures, so that different roughness values and/or surface textures result on the opposite sides of the sheet.

For press hardening, the hot-rolled, coated, and temper-rolled steel sheet, optionally after pre-forming (or re-forming), is heated to an austenitizing temperature (≥ Ac1 or ≥ Ac3 temperature), particularly in a furnace or similar facility. Heating can also be carried out in specific areas or at different temperatures in specific areas. The heated steel sheet is formed or finish-formed and hardened in a press-hardening tool, whereby several press-hardening tools arranged one after the other can be provided with the same effect. Before insertion into the press-hardening tool, pre-cooling or intermediate cooling can optionally be provided, which can also be carried out in specific areas or at different temperatures in specific areas. Further steps, such as trimming and/or piercing, can also be provided. Trimming and/or piercing can particularly be carried out after pre-forming.

During the temper rolling process provided by the invention, the micro-surface of the coated steel sheet is leveled and thereby smoothed, which subsequently also reduces the roughness. The steel sheet is thus significantly smoother than a conventionally hot-rolled and coated steel sheet. The smoothing can involve only the coating or the coating and the actual steel sheet or substrate. In any case, the smoothing or reduction of the roughness leads to significantly less oxide or scale forming on the surface of the steel sheet when it is heated to austenitizing temperature, thus protecting the press hardening tool and simplifying any cleaning, welding, and/or painting processes that may follow press hardening. The temper rolling process provided by the invention is carried out with the aim of reducing the formation of oxide or scale during the subsequent austenitizing of the steel sheet or when the steel sheet is heated to austenitizing temperature (during press hardening).

Furthermore, from the roughness of the surface of the steel sheet measured before or after the final rolling, a “true” or “actual” surface including the unevenness or roughness profile can be determined, which can then be used as a parameter to characterise the oxide film. tendency and can be used to adjust process parameters and/or to describe the process.

The invention takes into account, to a certain extent, the fact that the inherently high roughness of a hot-rolled steel sheet is advantageous for coating, particularly for hot-dip coating, but disadvantageous for press hardening. Furthermore, the inherently larger sheet thickness tolerance of a hot-rolled steel sheet can also be disadvantageous for press hardening. The temper rolling provided according to the invention enables the use of a hot-rolled, preferably exclusively hot-rolled, and coated, preferably metallically coated, steel sheet for press hardening, particularly with regard to greater sheet thicknesses, which results in, among other things, cost advantages. The invention is also particularly suitable for the series production of press-hardened sheet metal parts.

The hot-rolled and coated steel sheet can be provided as sheet metal strip (steel sheet strip) or in the form of sheet metal panels (steel sheet panels). The sheet metal strip or sheets can then be temper-rolled, with particular provision for temper-rolling to take place over the entire strip length and width or over the entire sheet length and width. The temper-rolling of the sheet metal strip or sheets can take place at the steel sheet manufacturer's site, at a specially commissioned contract manufacturer or job shop, or only at the steel sheet processor's site (where the press-hardening process is also carried out). Preferably, after temper-rolling, blanks, in particular so-called shaped blanks, are cut from the sheet metal strip or sheets (this is also referred to as blank cutting), and the press-hardening process is thus carried out. It can also be provided for the blank cutting to take place before temper-rolling or for the temper-rolling to take place after temper-rolling, with the blanks then preferably being temper-rolled over their entire length and width.

The roughness achieved during temper rolling depends on the nature of the rolls used (see above). Furthermore, the roughness achieved during temper rolling can also be influenced by the roll contact pressure or rolling pressure and/or by a tensile force acting on the steel sheet. The latter refers in particular to the rolls used for temper rolling applying a tensile force acting opposite to the inlet direction on the inlet side and/or a tensile force acting in the outlet direction on the outlet side. This also creates tensile stress in the steel sheet during temper rolling, creating a multi-axial stress state that promotes smoothing. (In the case of a sheet metal strip that is drawn off a coil or a reel, a so-called strip or reel tension can be set, whereby a tensile force or tensile stress can be applied to the sheet metal strip on the inlet side. A pair of draw rollers can be used to apply a tensile force or tensile stress to the sheet metal strip on the outlet side.) A higher roll contact pressure and/or a higher tensile force can achieve greater smoothing or a lower roughness. The roughness achieved during re-rolling can therefore be adjusted or set by changing the roll contact pressure and/or by changing the tensile force, in particular in such a way that a certain R _a value or maximum R _a value (e.g. R _a ≤ 2.5 µm, ≤ 2.0 µm or ≤ 1.5 µm) is achieved. The roller contact force and/or the tensile force can be adjusted during the pause between two rolling passes or during a rolling pass. This can also be done in a control loop.

The invention also extends to the use of a hot-rolled, coated, in particular metallically and/or double-sidedly coated, and (after coating) temper-rolled steel sheet for producing a press-hardened sheet metal part by heating the hot-rolled, coated and temper-rolled steel sheet to austenitizing temperature and forming and hardening it in a press-hardening tool to form a sheet metal part or by pre-forming the hot-rolled, coated and temper-rolled steel sheet, heating it to austenitizing temperature and finish-forming and hardening it in a press-hardening tool to form a sheet metal part.

For this purpose, the hot-rolled, coated, and temper-rolled steel sheet can be provided as a sheet strip or in the form of sheet metal panels. As already explained, the temper-rolling of the hot-rolled and coated steel sheet can be carried out by the steel sheet manufacturer, by a specially commissioned contract manufacturer or contract manufacturer, or only by the steel sheet processor (where the press-hardening process is also carried out). The hot-rolled, coated, and temper-rolled steel sheet can have an R _a value of ≤ 2.5 µm, preferably ≤ 2.0 µm, in particular ≤ 1.5 µm. The hot-rolled, coated, and temper-rolled steel sheet is coated, in particular, by hot-dip coating. The coating of the hot-rolled, coated, and temper-rolled steel sheet may have a layer thickness of 7 µm to 13 µm, preferably 9 µm to 11 µm. The hot-rolled, coated, and temper-rolled steel sheet may have a sheet thickness of ≥ 2.4 mm, preferably ≥ 2.6 mm, in particular ≥ 2.8 mm. These values refer to the state before Austenitizing and press hardening. All previous and subsequent explanations apply analogously.

The press-hardened sheet metal part to be produced is preferably a vehicle component, in particular for an electric vehicle (BEV). Such a vehicle component can be a body longitudinal member or body cross member. Such a vehicle component can also be a battery support element or battery protection element for a traction battery, which is designed, for example, as a floor longitudinal member or floor cross member. Due to the relatively high vehicle weight, these vehicle components are subject to particularly high strength requirements, particularly with regard to crashes.

• 1 veranschaulicht in einer schematischen Abfolgedarstellung ein erfindungsgemäßes Verfahren zur Herstellung eines pressgehärteten Blechformteils.

The invention is explained in more detail below in a non-limiting manner with reference to a figure. The features described and/or shown in the figure may, even independently of specific combinations of features, be general features of the invention and may further develop the invention accordingly.

• 1 illustrates in a schematic sequence representation a method according to the invention for producing a press-hardened sheet metal part.

In the 1 In the process illustrated, a hot-rolled and coated steel sheet S is provided in the form of a sheet metal strip B, also referred to as hot strip. The sheet metal strip B is unwound from a coil or drawn off by means of a pair of draw-off rolls 10 (a so-called strip or reel tension can be set here) and re-rolled with a specially provided pair of rolls 20 in order to reduce the roughness of the steel sheet S. With regard to the direction of travel D of the sheet metal strip B, after the pair of rolls 20, virtually downstream of the pair of rolls 20, there is an optional pair of draw-rolls 30 which can be brought into contact with the sheet metal strip B and with which a tensile force or tensile stress influencing the achievable roughness can be applied to the sheet metal strip B, as described above. The re-rolled sheet metal strip B or steel sheet S can be wound back onto a coil or directly processed further.

Blanks P are cut from the hot-rolled, coated, and temper-rolled sheet metal strip B or steel sheet S. To perform a press-hardening process, the steel sheet S or a blank P is heated to austenitizing temperature in a furnace 50 and then formed and hardened in a press-hardening tool 60, producing a press-hardened sheet metal part T (direct press hardening). Alternatively, the steel sheet S or a blank P is first preformed into a preformed part in one or more preforming tools 40, typically preformed cold, then heated to austenitizing temperature in a furnace 50, and then finish-formed and hardened in a press-hardening tool 60, producing a press-hardened sheet metal part T (indirect press hardening).

Further features and alternative implementations of the process are described above.

LIST OF REFERENCE SYMBOLS

10

Take-off roller pair

20

Pair of rollers (for re-rolling)

30

Pair of pull rollers

40

Preforming tool

50

Oven

60

Press hardening tool

B

Sheet metal strip

D

Direction of flow

P

Circuit board(s)

S

sheet steel

T

Sheet metal part

QUOTES CONTAINED IN THE DESCRIPTION

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

Cited patent literature

• EP 3 265 365 B1 [0002]
• DE 699 33 751 T3 [0003]

Claims (10)

A method for producing a press-hardened sheet metal part (T), comprising the steps of: - providing a hot-rolled and coated steel sheet (S); - re-rolling the coated steel sheet (S) to reduce its roughness; - performing a press-hardening process in which the steel sheet (S) is heated to austenitizing temperature and formed and hardened in a press-hardening tool (60) to form the sheet metal part (T), or in which the steel sheet (S) is pre-formed, heated to austenitizing temperature, and finish-formed and hardened in a press-hardening tool (60) to form the sheet metal part (T). Procedure according to Claim 1 , characterized in that during the subsequent rolling an R _a value ≤ 2.5 µm, preferably ≤ 2.0 µm, in particular ≤ 1.5 µm, is achieved. Method according to one of the preceding claims, characterized in that the provided steel sheet (S) has a sheet thickness ≥ 2.4 mm, preferably ≥ 2.6 mm, in particular ≥ 2.8 mm. Method according to one of the preceding claims, characterized in that a pair of rollers (20) is used for the re-rolling and that the roughness achieved during the re-rolling is adjusted by changing the roller contact pressure and/or by changing a tensile force acting on the steel sheet (S). Use of a hot-rolled, coated and temper-rolled steel sheet (S) for producing a press-hardened sheet metal part (T) by heating the steel sheet (S) to austenitizing temperature and forming and hardening it in a press-hardening tool (60) to form the sheet metal part (T) or by preforming the steel sheet (S), heating it to austenitizing temperature and finish-forming and hardening it in a press-hardening tool (60) to form the sheet metal part (T). Use according to Claim 5 , characterized in that the steel sheet (S) has an R _a value ≤ 2.5 µm, preferably ≤ 2.0 µm, in particular ≤ 1.5 µm. Use according to Claim 5 or 6 , characterized in that the steel sheet (S) is coated by hot dipping. Use according to any of the preceding Claims 5 until 7 , characterized in that the coating has a layer thickness of 7 µm to 13 µm, preferably of 9 µm to 11 µm. Use according to any of the preceding Claims 5 until 8 , characterized in that the steel sheet (S) has a sheet thickness ≥ 2.4 mm, preferably ≥ 2.6 mm, in particular ≥ 2.8 mm. Use according to any of the preceding Claims 5 until 9 , characterized in that the press-hardened sheet metal part (T) to be produced is a vehicle component, in particular a battery support or protection element for a traction battery.