Method for producing a press-hardened shaped sheet-metal part from a hot-rolled and coated steel sheet

Abstract

The invention relates to a method for producing a press-hardened shaped sheet-metal part (T), comprising the steps: - providing a hot-rolled and coated steel sheet (S); - re-rolling the coated steel sheet (S) in order to reduce the roughness thereof; - carrying out a press-hardening operation, in which the steel sheet (S) is heated to an austenitization temperature and formed and hardened in a press-hardening tool (60) to form the shaped sheet-metal part (T), or in which the steel sheet (S) is preformed, heated to an austenitization temperature and formed to a finished state and hardened in a press-hardening tool (60) to form the shaped sheet-metal part (T). The invention also relates to the use of a hot-rolled, coated and re-rolled steel sheet (S) for producing a press-hardened shaped sheet-metal part (T).

Description

[0001] Process for producing a press-hardened sheet metal part from a hot-rolled and coated steel sheet

[0002] DESCRIPTION

[0003] The invention relates to a method for producing a press-hardened sheet metal part from a hot-rolled and coated steel sheet according to 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 claim.

[0004] 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 shaping it. Due to martensitic hardening, a high level of 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 (and then 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 acts as an anti-corrosion coating and / or anti-scaling coating. For the prior art, reference is made to EP 3 265 365 B1, in particular to the introductory explanations and citations.

[0005] For press hardening, either cold-rolled steel sheet (also known as cold strip) or hot-rolled steel sheet (also known as hot strip) can be used. Hot-rolled steel sheet is advantageous for greater sheet thickness in combination with a coating, particularly a metallic coating applied by hot dipping. The closest document, 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 subsequently used for press hardening (hot press forming and quench hardening).

[0006] 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.

[0007] 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 arise analogously from the dependent patent claims, the following description of the invention (this expressly includes features described as examples and optionally), and the figure.

[0008] The method according to the invention for producing a press-hardened sheet metal part comprises the following steps:

[0009] - Providing a hot-rolled and coated, in particular metallically coated, steel sheet;

[0010] - Re-rolling of the coated steel sheet to reduce its roughness, which means the surface roughness of the sheet surface;

[0011] - Carrying out a press hardening process in which the steel sheet, which refers to the temper-rolled steel sheet, is heated to austenitizing temperature and formed and hardened, i.e. quench-hardened, in a press-hardening tool to form a sheet metal part (direct press hardening), or in which the steel sheet is preformed, heated to austenitizing temperature and finish-formed and hardened, i.e. quench-hardened, in a press-hardening tool to form a sheet metal part (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 that has been produced in a hot-rolling mill or the like at high rolling temperatures, preferably of 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).

[0012] The hot-rolled steel sheet provided can have a sheet thickness of > 2.4 mm, preferably > 2.6 mm, in particular > 2.8 mm. Such large sheet thicknesses, which are rather atypical for press hardening, can be used to produce press-hardened sheet metal parts that are very stable and resilient, making them particularly suitable for vehicle components with high component requirements (e.g., for electric cars). The steel sheet provided preferably has an at least approximately homogeneous sheet thickness while still maintaining a sheet thickness tolerance (see above).

[0013] 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 to be 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 pm to 13 pm, in particular of 9 pm to 11 pm.

[0014] 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 in order 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 Ra value (average roughness) of < 2.5 pm, preferably < 2.0 pm, in particular < 1.5 pm, is preferably achieved, which means the Ra 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.

[0015] 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 re-rolled steel sheet, if necessary after pre-forming (or re-forming), is heated to an austenitizing temperature (> Ac1 or > Ac3 temperature), in particular in a furnace or the like. The heating can also take place only in certain areas or at different temperatures in certain areas. The heated steel sheet is formed or finally 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 take place only in certain areas or at different temperatures in certain areas. In addition, further steps, e.g. trimming and / or piercing, can be provided. Trimming and / or piercing can in particular take place after pre-forming.

[0016] During the temper rolling process provided according to the invention, the micro-surface of the coated steel sheet is leveled and thereby smoothed, which in turn also reduces the roughness. The steel sheet is therefore considerably smoother than a conventionally hot-rolled and coated steel sheet. The smoothing can cover only the coating or the coating and the actual steel sheet or substrate. In any case, the smoothing or reduction in roughness results in significantly less oxide or scale forming on the surface of the steel sheet when it is heated to austenitizing temperature, so that the press hardening tool is protected and any cleaning, welding and / or painting processes following press hardening are simplified. The temper rolling process provided according to the invention is carried out with the aim of ensuring that the surface is smoother during the later austenitizing of the steel sheet or substrate.to reduce oxide or scale formation when heating the steel sheet to austenitizing temperature (during press hardening).

[0017] Furthermore, the roughness of the surface of the steel sheet measured before or after temper rolling can be used to determine a "true" or "actual" surface including the unevenness or roughness profile, which can then be used as a parameter to characterize the tendency to oxide formation and to adjust process parameters and / or to describe the process.

[0018] 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 greater 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 cost advantages, among other things. The invention is also particularly suitable for the series production of press-hardened sheet metal parts.

[0019] The hot-rolled and coated steel sheet can be provided as a sheet strip (steel sheet strip) or in the form of sheet plates (steel sheet plates). The sheet 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 strip or sheets can take place by the manufacturer of the steel sheet, by a specially commissioned contract manufacturer or job shop, or by the processor of the steel sheet (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 strip or sheets (this is also referred to as blank cutting) and the press hardening process is carried out. It can also be provided for the blank cutting to take place before temper-rolling orthat the temper rolling only takes place after the blanks have been cut, with the blanks then preferably being temper rolled over their entire length and width. The roughness achieved during temper rolling depends on the quality of the rolls used (see above). Furthermore, the roughness achieved during temper rolling can also be influenced by the roll contact pressure or the rolling pressure and / or by a tensile force acting on the steel sheet. The latter means in particular that, with regard to the rolls used for temper rolling, a tensile force acting opposite to the direction of entry is applied to the steel sheet on the inlet side and / or a tensile force acting in the direction of exit on the outlet side. As a result, tensile stress and a multi-axial stress state arise in the steel sheet during temper rolling, which promotes smoothing. (In the case of a sheet metal strip that is drawn off a coil or reel, a so-called strip or reel tension can be used.(The coiler tension can be adjusted, 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 Ra value or maximum Ra value (e.g. Ra < 2.5 pm, < 2.0 pm or < 1.5 pm) is achieved. The roll contact pressure and / or the tensile force can be changed in the break between two rolling passes or during a rolling pass. This can in particular also be done in a control loop.

[0020] 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 metal 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 manufacturer of the steel sheet, by a contract manufacturer specifically commissioned for this purpose, orThe coating can be carried out either by a contract manufacturer or by the processor of the steel sheet (where the press hardening process is also carried out). The hot-rolled, coated and re-rolled steel sheet can have an Ra value of < 2.5 pm, preferably < 2.0 pm, in particular < 1.5 pm. The hot-rolled, coated and re-rolled steel sheet is coated in particular by hot-dip coating. The coating of the hot-rolled, coated and re-rolled steel sheet can have a layer thickness of 7 pm to 13 pm, preferably 9 pm to 11 pm. The hot-rolled, coated and re-rolled steel sheet can have a sheet thickness of > 2.4 mm, preferably > 2.6 mm, in particular > 2.8 mm.

[0021] These values ​​refer to the condition before austenitizing and press hardening. All previous and subsequent explanations apply analogously.

[0022] 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.

[0023] 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 can be general features of the invention, even independently of specific combinations of features, and can further develop the invention accordingly. Figure 1 illustrates a schematic sequential representation of a method according to the invention for producing a press-hardened sheet metal part.

[0024] In the process illustrated in Fig. 1, 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 take-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, i.e. downstream of the pair of rolls 20, there is an optional pair of pull-up 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.

[0025] 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).

[0026] Further features and alternative embodiments of the method are described above. LIST OF REFERENCE SYMBOLS

[0027] 10 pairs of take-off rollers

[0028] 20 pairs of rollers (for re-rolling)

[0029] 30 pairs of pull rollers

[0030] 40 preforming tool

[0031] 50 oven

[0032] 60 press hardening tool

[0033] B sheet metal strip

[0034] D Flow direction

[0035] P board(s)

[0036] S sheet steel

[0037] T sheet metal part

Claims

PATENT CLAIMS 1 . 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 pre-formed, heated to austenitizing temperature and finish-formed and hardened in a press hardening tool (60) to form a sheet metal part (T).

2. Method according to claim 1, characterized in that during the subsequent rolling an Ra value < 2.5 pm, preferably < 2.0 pm, in particular < 1 ,5 pm, is achieved.

3. 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.

4. 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).

5. 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 pre-forming 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).

6. Use according to claim 5, characterized in that the steel sheet (S) has an Ra value < 2.5 pm, preferably < 2.0 pm, in particular < 1.5 pm.

7. Use according to claim 5 or 6, characterized in that the steel sheet (S) is coated by hot-dip coating.

8. Use according to one of the preceding claims 5 to 7, characterized in that the coating has a layer thickness of 7 pm to 13 pm, preferably of 9 pm to 11 pm.

9. Use according to one of the preceding claims 5 to 8, characterized in that the steel sheet (S) has a sheet thickness > 2.4 mm, preferably > 2.6 mm, in particular > 2.8 mm.

10. Use according to one of the preceding claims 5 to 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.

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