Method of manufacturing inner panels for vehicle doors

Abstract

The present disclosure relates to methods for manufacturing an inner panel of a vehicle door. The method comprises heating a steel blank to above an austenization temperature, and cooling the steel blank and deforming in a first press step while further cooling to form an intermediate blank. The method further comprises deforming the intermediate blank to form the inner panel of the vehicle door in a second press step and cooling the obtained inner panel of the vehicle door. The inner panel of the vehicle door includes a bottom perimeter of the inner panel including a bottom edge, front and rear edges and further includes a window frame. The present disclosure further relates to inner panels and vehicle doors obtained with such methods.

Description

METHOD OF MANUFACTURING INNER PANELS FOR VEHICLE DOORS

[0001] The present application claims the benefit of EP23383304.5 filed on December 15th, 2023

[0002] The present disclosure relates to vehicle doors and more particularly relates to inner panels for vehicle doors. The present disclosure further relates to methods for manufacturing such inner panels.BACKGROUND

[0003] Vehicles such as cars incorporate a structural skeleton designed to withstand the loads that the vehicle may be subjected to during its lifetime. The structural skeleton is further designed to withstand and absorb impacts, in case of e.g. collisions with other cars or road structures.

[0004] The demand for weight reduction in the automotive industry has led to the development and implementation of lightweight materials or components, and related manufacturing processes and tools. The demand for weight reduction is especially driven by the goal of a reduction of CO2 emissions. The growing concern for occupant safety also leads to the adoption of materials which improve the integrity of the vehicle during a crash while also improving the energy absorption.

[0005] A process known as Hot Forming Die Quenching (HFDQ) typically uses boron steel sheets to create stamped components with Ultra High Strength Steel (UHSS) properties, with tensile strengths of e.g. 1.500 MPa or 2.000 MPa or even more. The increase in strength allows for a thinner gauge material to be used, which results in weight savings over conventionally cold stamped mild steel components. Throughout the present disclosure UHSS may be regarded as a steel having an ultimate tensile strength of 1.000 MPa or more after a press hardening process.

[0006] In a HFDQ process, a blank to be hot formed may be heated to a predetermined temperature e.g. austenization temperature or higher (and particularly between Ac3 and anevaporation temperature of e.g. a coating of the blank). A furnace system may be used for this purpose. Depending on the specific needs, a furnace system may be complemented with additional heaters, e.g. induction heaters or infrared heaters. By heating the blank, the strength of the blank decreases, and deformability increases i.e. to facilitate the hot stamping process.

[0007] There are several known Ultra High Strength steels (UHSS) for hot stamping and hardening. The blank may be made e.g. of a boron steel, coated or uncoated, such as Usibor® 1500 (22MnB5) commercially available from ArcelorMittal.

[0008] More recently, hot stamping of even higher grades of steel has been intensively investigated. E.g. 37MnB5 steels or similar have a higher carbon content than 22MnB5 and can achieve ultimate tensile strength after hot stamping of 2.000 MPa or even more. Usibor® 2000 which is commercially available from ArcelorMittal™ and MBW-K® 1900 (a 34MnB4 steel) commercially available from ThyssenKrupp™ are two of such steels.

[0009] Such steels may be called “second generation” press hardening steels. This newer generation of press hardening steels may herein be regarded boron-manganese steel suitable for hot stamping with a higher carbon content than 22MnB5, and which obtain an ultimate tensile strength of more than 1.500 MPa, specifically at least 1.700 MPa when fully martensitic after press hardening. The silicon content may typically be lower than for the first generation press hardening steels. Examples include 20MnCr, 28MnB5 steel (e.g. Docol PHS1800), 30MnB5 (e.g. SQ1800), 34MnB5 or 34MnB4 (e.g. MBW 1900), 37 MnB5 or 37 MnB4 (e.g. Usibor 2000 HPF 2000, Docol PHS2000, phs ultraform 2000). The new generation of steels may also be described as PHS 1 .700 or higher, i.e. PHS 1 .700, PHS 1.800, PHS 1.900 or PHS 2.000, more specifically PHS 1.900 or higher.

[0010] The steel compositions of the second generation press hardening steels can be summarized by focusing on the most important elements of their composition, which for most second generation steels tend to be as follows: a carbon content of 0.32 - 0.45 % by weight, specifically 0.32 - 0.4% of carbon by weight, and more specifically 0.32 - 0.38%, a content of manganese of 0.6 - 1.5%, specifically 0.6 - 1.4% and a boron content of 0.003 - 0.006% by weight, specifically 0.004 - 0.005% by weight. In practice, there may be slight variations between different coils of steels, even when made with the same manufacturing process by the same steel supplier. In practice, there may even be slight variations between blanks cut from the same steel coil. Suitable steels include e.g. 37MnB5 steel, 38MnB5 steel, 34MnB5 steel and 34 MnB4 steel.

[0011] One typical composition of a 34MnB4 steel that has been tested is summarized below in weight percentages (rest is iron (Fe) and impurities):Carbon (C) (%) 0.35Silicon (Si) (%) 0.8Manganese (Mn) (%) 0.96Chromium (Cr) (%) 0.19Molybdenum (Mb) (%) 0.18Phosphor (P) 0.012%Sulphur (S) (%) 0.0002Titanium (Ti) + niobium (Nb) 0.055%Aluminium (Al) 0.03%Boron (B) (%) 0.002%Nitrogen (N) 0.003%Nickel (Ni) 0.01 %

[0012] llsibor® 2000 may be generally described as 37MnB5 steel. The composition of llsibor® 2000, is summarized below in weight percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.36Maximum silicon (Si) (%): 0.8Maximum manganese (Mn) (%): 0.8Maximum phosphorus (P) (%): 0.03Maximum sulphur (S) (%): 0.01Aluminium (Al) (%): 0.01 - 0.06Maximum titanium (Ti) (%): 0.07Maximum niobium (Nb) (%): 0.07Maximum copper (Cu) (%): 0.20Maximum boron (B) (%): 0.005Maximum chromium (Cr) (%): 0.50Maximum molybdenum (Mb) (%): 0.50

[0013] MBW-K® 1900 may generally be described as a 34MnB4 steel. The composition ofMBW-K® 1900 is summarized below in weight percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.38Maximum silicon (Si) (%): 0.4Maximum manganese (Mn) (%): 1.4Maximum phosphorus (P) (%): 0.025Maximum sulphur (S) (%): 0.01Minimum Aluminium (Al) (%): 0.015Maximum titanium (Ti) (%): 0.05Maximum niobium (Nb) (%): 0.07Maximum boron (B) (%): 0.005Maximum chromium (Cr) + molybdenum (Mb) (%): 0.50

[0014] MBW® 1900 is another manganese-boron steel from ThyssenKrupp™ which may have an ultimate tensile strength of 1900 MPa. It is commercially available with aluminium-silicon coatings and suitable for hot stamping and examples of the methods disclosed herein. The chemical composition of MBW® 1900 is summarized below in weight in percentages (rest is iron (Fe) and impurities):Maximum carbon (C) (%): 0.38Maximum silicon (Si) (%): 0.40Maximum manganese (Mn) (%): 1.40Maximum phosphor (P) (%): 0.025Maximum sulphur (S) (%): 0.010Minimum aluminium (Al) (%): 0.1Maximum niobium (Nb) (%): 0.05Maximum titanium (Ti) (%): 0.05Maximum chromium and molybdenum (Cr + Mo) (%): 0.50Maximum boron (B) (%): 0.005

[0015] B1800HS is yet another boron steel which may have an ultimate tensile strength of about 1800 MPa and is suitable for hot stamping and examples of the methods disclosed herein. The chemical composition of B1800HS is summarized below in weight in percentages (rest is iron (Fe) and impurities):Carbon (C) (%): 0.28 - 0.35Maximum silicon (Si) (%): 0.5Manganese (Mn) (%): 1.0 - 1.8Maximum phosphor (P) (%): 0.025Maximum sulphur (S) (%): 0.010Aluminium (Al) (%): 0.01 - 0.06Maximum titanium (Ti) (%): 0.05Maximum boron (B) (%): 0.0050Maximum chromium and molybdenum and niobium (Cr + Mo + Nb) (%): 0.80

[0016] A further suitable LIHSS for examples of the present disclosure is CR1900T-MB-DS. The chemical composition is summarized below in weight in percentages (rest is Fe and impurities):Carbon (C) (%): 0.30 - 0.38Maximum silicon (Si) (%): 0.8Manganese (Mn) (%): 2.0Maximum chromium (Cr) : 0.25%Maximum phosphor (P) (%): 0.03Maximum Molybdenum (Mo): 0.25Maximum sulphur (S) (%): 0.005Aluminium (Al) (%): 0.01 - 0.08Maximum Titanium (Ti) + Niobium (%): 0.20Boron (B) (%): 0.0010 - 0.0050Maximum nickel (Ni): 0.10Maximum nitrogen (N): 0.01

[0017] In addition to the Ultra High Strength Steels mentioned before, more ductile steels may be used in parts of the structural skeleton requiring energy absorption. Examples of ductile steels include Ductibor® 500, Ductibor ® 1000 and CRL-340LA.

[0018] Hot Forming Die Quenching may also be called “press hardening” or “hot stamping”. These terms will be used interchangeably throughout the present disclosure.

[0019] Typical vehicle components that may be manufactured using the HFDQ process include door beams, bumper beams, cross / side members, A / B pillar reinforcements, front and rear rails, seat crossmembers and roof rails.

[0020] The door structure of a car is an integral part of the vehicle structural system. A vehicle door structure may perform inter alia the following functions: the door provides structural resistance against intrusion from other vehicles or objects, and may be configured to absorb energy and transfer loads in an appropriate manner to other areas of the vehicle and is generally designed to keep occupants safely inside the vehicle. At the same time, the door provides attaching surfaces for mechanisms, wiring, sensing devices, seals and interior trim. In addition, the door of course keeps the vehicle closed to the outside environment.

[0021] Structurally, vehicle doors such as car doors may comprise an inner panel (a panel arranged to face an “inside” of the vehicle), an outer panel (a panel facing the outside of the vehicle), and a “shield” in between the outer and inner panels. The shield may be regarded as formed by a plurality of reinforcements. These are generally denominated shield, since (like a shield) they protect the vehicle and its occupants.

[0022] The shield may comprise an inner shield, and an outer shield. The inner shield may be considered to comprise all elements between the inner panel and window glass. The outer shield may be considered to comprise all elements between the outer panel and the window glass. The inner shield may carry and guide the window glass, as well as electronic systems (e.g. motor for lowering and raising the window). The outer shield in particular may include a Side Impact Protection Beam (SIPB).

[0023] A SIPB can be manufactured by hot stamping of UHSS material and is typically arranged diagonally across the vehicle door. The SIPB may have a U-shaped (or “hat-shaped”) cross-section. Parts of the inner panel and outer panel may be hot stamped, but significant parts of the panels may be cold formed and may have a lower ultimate tensile strength than e.g. the SIPB. The SIPB may be regarded as the most important structural element inside the vehicle door to protect vehicle passengers in the case of a crash or impact. In case of a side impact, also the B-pillar, and the rocker are particularly important.

[0024] Recently, the requirements and expectations from OEM’s and others for vehicles in terms of side impact crashworthiness have increased. As a specific example, the requirements of crashworthiness have become more demanding with respect to a moving deformable barrier (MDB). In particular, both impact velocity and mass of the MDB used in tests that the structure needs to withstand have been increased. Unless specific measures are taken it can be estimated that the intrusion of the door to the inside of the vehicle increases by e.g. 20 - 30% due to the increased mass and velocity used in the reference tests.

[0025] Car manufacturers and car part manufacturers have to deal with the more demanding requirements. These requirements are potentially even more demanding in the case of electric or hybrid vehicles comprising a traction battery. Intrusion into the area of the battery is to be avoided because of e.g. fire risk in case of damaged battery cells.

[0026] One solution that has been explored is to increase the strength of the SIPB. In particular, the dimensions, specifically the thickness, of the part may be increased. Alternatively, the cross-section of the SIPB can be changed, e.g. by changing the cross-section from an open cross-section (U-shape or double U-shape) to a closed cross-section (either by providing a cover plate to close the cross-section or by changing the manufacturing process to obtain a closed cross-section). It may further be attempted to reinforce the SIPB by choosing a higher grade material, if available.

[0027] Yet other solutions focus on increasing the weight and strength of the rocker, e.g. by adding reinforcements or by increasing thickness. These solutions may lead to a significant weight increase which counteracts the overall general goal of weight reduction to avoid CO2 emissions.

[0028] It is an object of the present disclosure to provide door structures that are able to withstand increasingly high requirements in terms of crashworthiness without unduly increasing the overall weight of the structure.SUMMARY

[0029] In a first aspect, a method for manufacturing an inner panel of a vehicle door is provided. The method comprises providing a steel blank, heating the steel blank to above an austenization temperature. The method then further comprises cooling the steel blank to a temperature between 600 - 750°C and deforming the heated steel blank in a first press step while further cooling to form an intermediate blank. The method further comprises deforming the intermediate blank to form the inner panel of the vehicle door in a second press step andcooling the obtained inner panel of the vehicle door. The inner panel of the vehicle door includes a bottom perimeter of the inner panel including a bottom edge, front and rear edges and a lower portion of a window frame, and further includes an upper part of the window frame. The steel blank may have a composition in weight percentages of 0.17 - 0.25 % C, less than 1.4% Si, 1.7 - 2.4 % Mn and 0.0015 - 0.005% B.

[0030] In accordance with this aspect, an inner panel with increased strength is provided which can reduce intrusion in the case of a side impact without a significant increase in weight. In order to achieve the desired complicated shape of the inner panel, particularly in the corner areas, the window frame, and the areas of the hinges, it has been found that pressing is to be performed in a two-step process. I.e. the necessary deformation is difficult to achieve in a single deformation step. Using the two-step process, the first step may be such that material is provided in the most critical areas such that the second step can effectively provide the desired shape.

[0031] Suitable steel blanks have a composition that is suitable for press hardening and allows obtaining a high ultimate tensile strength, e.g. 1.200 MPa or more, specifically 1.400 MPa or more. At the same time, the critical cooling rate for some of these steels, particularly for the zinc coated steels, may be lower than for boron steels such as 22MnB5 or 37MnB5. This allows a slower pressing process and the aforementioned two step process while still obtaining high ultimate tensile strength. Also, other processes are provided herein in which forming is relatively fast.

[0032] In some examples, the steel blank may be coated with a Zinc based coating. Zinc coatings or zinc based coatings are known to be used in hot stamping process to provide protection against corrosion. The zinc based coatings may be provided in a galvannealing or galvanizing process. In other examples, the steel blanks may be coated with an AlSi coating.

[0033] In some examples, the steel blank may be made of 22MnSiB9-5. In other examples, the steel blank may be made of 20MnB8. In further examples, the steel blank may be made of 36SiB6.

[0034] In some examples, the whole inner panel may be made of hot stamped UHSS. In other examples, the central portion of the inner panel (“carrier portion”) may be made of a different material e.g. a plastic material or fiber-reinforced plastic material.

[0035] In some examples, the press steps of the method may be caried out in a multi-step apparatus. That is, both the first press step and the second press step may be carried out inthe same apparatus comprising first and second pairs of dies. In some examples, also the step of cooling may be integrated in the same multi-step apparatus.

[0036] In a further aspect, the present disclosure provides an inner panel for a vehicle door as obtainable by any of the herein disclosed methods. The inner panel includes a bottom perimeter including a bottom edge, front and rear edges and a lower portion of a window frame, and further includes an upper part of the window frame. The bottom perimeter and upper part of the window frame are made as an integral piece by press hardening a steel blank. The steel blank may have a composition in weight percentages of 0.17 - 0.25 % C, less than 1.4% Si, 1.7 - 2.4 % Mn and 0.0015 - 0.005% B. These process parameters may be used generally for the zinc coated steels such as 20MnB8 steel, 22MnSiB9-5 or similar.

[0037] The present disclosure also provides a vehicle door comprising such an inner panel, and a vehicle comprising such a door.BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Non-limiting examples of the present disclosure will be described in the following, with reference to the appended figures, in which:Figure 1 schematically illustrates an exploded view of a prior art vehicle door;Figure 2A schematically illustrates a method of manufacture of an inner panel of a vehicle door according to an example;Figures 2B and 2C schematically illustrate examples of production lines for manufacturing inner panels;Figures 3A - 3C schematically illustrate different deformation steps of a blank being formed into an inner panel according to an example; andFigures 4A - 4D schematically illustrate examples of inner panels.

[0039] The figures refer to example implementations and are only be used as an aid for understanding the claimed subject matter, not for limiting it in any sense.DETAILED DESCRIPTION OF EXAMPLES

[0040] Figure 1 schematically illustrates an exploded view of a vehicle door. The vehicle door 100 of this example has an inner panel 10. The inner panel 10 may be regarded as a structural member of the vehicle door and is the panel that is facing the inside of the vehicle. The innerpanel 10 may be manufactured using a variety of techniques and materials. A known inner panel 10 may be made by cold stamping of steel.

[0041] A bottom part of the inner panel 10 may be adapted to the shape of the rocker. Although not shown in figure 1A, in a zone 8, the rocker may be arranged.

[0042] The vehicle door 100 in this example further comprises an inner shield 30, a window glass 40, a side impact protection beam 50, and an outer panel 60. The inner shield 30 may serve to hold and support the window glass and electronic components such as e.g. a motor to drive movement of the window glass 40, or a loudspeaker. The side impact protection beam 50 may form the most important component to limit intrusion in the case of a side impact and to protect vehicle occupants. The outer panel 60 is a structural member facing an outside of the vehicle, and like the inner panel may typically be made of cold stamped steel.

[0043] Figure 2A schematically illustrates a method of manufacture of an inner panel of a vehicle door according to an example. Figures 2B and 2C schematically illustrate examples of production lines for manufacturing inner panels according to such a method.

[0044] A method 100 for manufacturing an inner panel of a vehicle door is provided. The method comprises, at block 102, providing a steel blank. The steel blank may have a composition in weight percentages of 0.17 - 0.25 % C (specifically 0.21 - 0.25%), less than 1 .4% Si (specifically 1.05 - 1 .33%), 1.7 - 2.4 % Mn (specifically 2 - 2.4%) and 0.0015 - 0.005% B (specifically 0.0015 - 0.004%). Suitable steels include steels that can obtain a high ultimate tensile strength in the stamping process but have a relatively low critical cooling rate.

[0045] Two suitable steels include 22MnSiB9-5 and 20MnB8. 22MnSiB9-5 is a material that is commercially available from Kobe Steel. As quenched, the material has similar mechanical properties as 22MnB5 including an ultimate tensile strength of 1.500 MPa or more. A critical cooling rate for the material is however much lower than for 22MnB5. The critical cooling rate is about 3 - 5°C / s.

[0046] The composition of 22MnSiB9-5 is summarized below in weight percentages (the rest is iron (Fe) and impurities):Carbon (C) (%) : 0.21 - 0.25 (around 0.22)Silicon (Si) (%) : 1.05 - 1.33 (around 1.15)Manganese (Mn) (%): 2.06 - 2.34 (around 2.2)Maximum phosphorus (P) (%): 0.015 (around 0.01)Maximum sulphur (S) (%): 0.003Aluminium (Al) (%): 0.02 - 0.08 (around 0.04)Titanium (Ti) (%): 0.02 - 0.03Boron (B) (%): 0.0015 - 0.004 (around 0.002)Maximum nitrogen (N) (%): 0.006Sulphur (S) (%): 0.003

[0047] Further properties of the steel include yield strength of about 1.050 MPa, and an ultimate tensile strength of 1500 MPa after stamping. A martensite start temperature is around 386°C and a martensite finish temperature is about 270°C. Austenization temperature Ac1 is about 730°C and Ac3 temperature is about 840°C.

[0048] 22MnSiB9-5 may be provided with a zinc coating of 80 g / m2, which may be applied in a galvanizing process or in galvannealing process. The zinc coating may have the following composition (by weight percentages): about 15% iron (Fe) and 85% zinc (Zn), and 0.2% of aluminium (Al) and 0.11% of silicon (Si) in the case of galvannealing process. In the case of a galvanizing process, the zinc coating may have the following composition (by weight percentages): 1.0% iron (Fe), 98% zinc (Zn), 0.6% of aluminum (Al), 0.07% of silicon (Si) and 0.3 % of manganese (Mn).

[0049] 20MnB8 has been designed for a “direct process with pre-cooling”. The critical cooling rate of 20MnB8 is approximately 10 °C / s. This allows the part to be transferred from the precooling stage to the forming die. As press hardened, the material has approximately 1000- 1050 MPa yield strength and 1500 MPa tensile strength. Once bake hardened (170 °C, 20 minutes), yield strength may exceed 1100 MPa.

[0050] The composition of 20MnB8 is summarized below in weight percentages (the rest is iron (Fe) and impurities):Carbon (C) (%) : 0.17 - 0.23 (around 0.195)Silicon (Si) (%): max 0.5 (around 0.19)Manganese (Mn) (%): 1.7 - 2.5 (around 1.98)Maximum phosphorus (P) (%): 0.015 (around 0.012)Maximum sulphur (S) (%): around 0.03Chromium (Cr) (%): around 0.03 Aluminium (Al) (%): around 0.05 Titanium (Ti) (%): around 0.003 Boron (B) (%): 0.002 - 0.004, around 0.003 Nitrogen (N) (%): around 0.054

[0051] Steel commercialized under the name Sibora ® is 36SiB6 and is also suitable for examples of the processes described herein. The composition of 36SiB6 is summarized below in weight percentages (the rest is iron (Fe) and impurities):Carbon (C) (%): 0.36 - 0.38 (around 0.37)Silicon (Si) (%): 1.4 - 1.5 (around 1.45)Manganese (Mn) (%): 0.55 - 0.95 (around 0.85) Maximum sulphur (S) (%): around 0.03 Chromium (Cr) (%): less than 0.25 (around 0.2) Aluminium (Al) (%): less than 0.04 Titanium (Ti) (%): less than 0.03 (around 0.02) Boron (B) (%): 0.002 - 0.003 (around 0.003) Nickel (N) (%): less than 0.10Niobium (Nb) (%): less than 0.04 (around 0.005 - 0.006)

[0052] Other suitable steels may also be used. In the processes according to examples of the present disclosure, it is preferred that steels are used with a relatively low critical cooling rate, e.g. 15°C / s or less, preferably 10°C / s or less. Even with relatively low cooling rates, a martensite microstructure with very high strength can be obtained.

[0053] The blank may be a combined blank comprising areas of different thickness, wherein the combined blank is formed by joining several blanks. The combined blank may be a Tailor Welded Blank (TWB), wherein multiple blanks are welded edge-to-edge. The multiple blanks may be made from the same steel, or from different steels. The multiple blanks may have different thicknesses. E.g. a first blank may have a thickness of 0.6 - 1.4 mm specifically of 0.8 - 1.4 mm, a second blank for forming a hinge reinforcement area may have a thickness of 1.8 - 3mm.

[0054] Additionally or alternatively, the combined blank may be formed by partially overlapping a first blank with a second blank to form an overlapping region. E.g. a first blank may have a thickness of 0.8 - 1.4 mm, and a second blank may e.g. have a thickness of 0.8 - 1.4 mm. The overlapping region may form the upper or lower hinge reinforcement and may e.g. have a thickness of 1.6 - 2.8 mm. The other hinge reinforcement area, if present, may be formed by another partial overlap. The lower and / or upper hinge reinforcement have an increased thickness, and the remainder of the inner panel may have a uniform thickness.

[0055] In yet further examples, the combined blank may be formed by completely overlapping a patch blank with another blank. The patch blank may have e.g. a thickness of 0.6 - 2.5 mm,specifically 0.8 - 2.5 mm, and the main blank may have e.g. a thickness of 0.6 - 1.4 mm, specifically 0.8 - 1.4 mm.

[0056] The method 100 further comprises heating the steel blank to above an austenization temperature at block 104. Heating may typically occur in a furnace, and heating may comprise heating the steel blank to above an Ac3 temperature, e.g. around 900 - 920°C. In further examples, other types of heating including e.g. infrared heaters may be used or may be combined with a furnace.

[0057] The method then comprises, at block 106, cooling the steel blank, optionally to a temperature between 600 - 750°C. Before a first deformation step, the temperature of the steel blank may be reduced in particular to avoid microcracks in the zinc coating.

[0058] This cooling, e.g. cooling to a temperature of 600 - 750°C may include passive cooling during transport from the oven to a press apparatus. The cooling may also include active cooling in a cooling tool. In an example, a cold gas flow may impinge upon the blank to reduce the temperature of the blank.

[0059] At block 108, the heated steel blank may be deformed in a first press step while further cooling to form an intermediate blank. In some examples, the blank has a temperature of 630 - 690°C, specifically 650 - 680°C, when the first press step starts. In examples, in the first press step between 40-60%, specifically about 50% of the total deformation occurs, i.e. about 50% of the overall elongation required from a substantially flat blank to end product may occur in the first press step.

[0060] In the first press step, a deformation may be provided that forces more material to the most difficult areas for pressing. Most difficult may herein be understood as those areas that undergo most deformation (elongation) and for which the material thickness is reduced most in the deformation process. The areas that undergo most deformation are mostly the corner areas of the lower perimeter of the inner panel.

[0061] At block 110, the intermediate blank thus obtained may be formed into the inner panel of the vehicle door in a second press step. The second press step may provide the final shape to the inner panel. In a specific example, the intermediate blank has a temperature of 350 - 550°C, specifically 400 - 500°C, and more specifically 430 - 470°C when the second press step starts.

[0062] After the second press step, the obtained inner panel of the vehicle door may be further cooled, at block 112. The inner panel of the vehicle door includes a bottom perimeter of theinner panel including a bottom edge, front and rear edges and a lower portion of a window frame, and further includes an upper part of the window frame.

[0063] Figure 2B schematically illustrates an example of a production line that may be used in the aforementioned process. A transfer system 200 may provide blanks to a furnace 210. The transfer system may comprise one or more industrial robots with grippers. The furnace 210 heats the blanks to above an austenization temperature.

[0064] A further transfer system 220 may pick up the blanks at the exit of the oven 210 and deliver the blanks to multistep apparatus 230. The multistep apparatus comprises a static lower body 232, and a mobile upper body 234. The mobile upper body may perform a press progression towards and away from the lower body.

[0065] The multistep apparatus 230 in this example comprises three stations: a cooling station 240, a first press station 250 downstream from the cooling station, and a second press station 260 downstream from the first press station. With every press movement, three blanks simultaneously are subjected to the three different operations.

[0066] The cooling tool in this example has an upper gas cooling tool connected to the mobile upper body 234 and / or a lower gas cooling tool connected to the fixed lower body 232, and the upper gas cooling tool and / or the lower gas cooling tool are configured to provide a pressurized cooling gas flow to impinge on the blank. The cooling gas may be air. In other examples, other types of cooling may be employed e.g. with contact plates.

[0067] The first press tool comprises an upper pressing die connected to the upper body 234 and a lower pressing die connected to the lower body 232. Similarly the second press tool comprises an upper pressing die connected to the upper body 234 and a lower pressing die connected to the lower body 232.

[0068] In further examples, the multistep apparatus 230 may comprise further stations for cutting, trimming or other.

[0069] Figure 2C schematically illustrates another example of a production line to carry out the method 100 illustrated in figure 2A. The production line in this case comprises a furnace 210, a transfer system 220 to transfer the heated blanks to cooling tool 240. After cooling, the partially cooled down blanks are transferred with transfer system 245 to a first press tool 250. A further transfer system 255 then transfers the intermediate blanks to the second press tool 260. In the second press tool, the inter mediate blank is deformed to obtain the inner panel of the vehicle door. In this illustrated example, the cooling tool, first press tool, and the second press tool are all provided in separate apparatus.

[0070] In an alternative process, particularly for the aforementioned second generation press hardening steels, e.g. incorporating an AlSi Coating, a process may be devised as follows. The methods and systems as described in pending PCT / EP2024 / 072326 may be used. Such a method for hot forming a structural component comprises heating the blank to above an austenization temperature, e.g. 900 - 920°C. The method further comprises drawing the heated blank in the press tool and transferring the blank from the press tool to the first postpress tool, which herein is another press tool.

[0071] As for the previously described processes, pressing of the blank to form the inner panel takes place in a two-step process. In the first press step about 80% of the total deformation may be accomplished, whereas about 20% of the overall deformation is carried out in the second press step.

[0072] A temperature of the blank before drawing the blank is at least 600°C, specifically at least 650°C. At temperatures below 600°C, for drawing, stripes may be formed in the coating. At temperatures of 600°C or higher, the coating has been found to be satisfactory after the hot forming process.

[0073] The temperature of the blank before drawing the blank may specifically be between 700°C and 800°C. The blanks may cool down from the moment they exit the furnace and during the transfer from the furnace to the press tool. The transfer from the furnace to the press tool may take about 4 seconds in an example.

[0074] A temperature of the blank before the next press operation may be between 500° and 600 °C, e.g. approximately 550°C. And a temperature of the blank at the end of the second press operation may be between 400°C and 500°C, e.g. approximately 450°C.

[0075] The more gradual cooling of the steel as illustrated herein (compared to a typical process for first generation AlSi coated press hardening steels) has been found to provide a high ultimate tensile strength, in combination with increased ductility. At the same time, the manufacturing process is highly efficient. In these examples, an average cooling rate after the initial press forming step to the Ms temperature (e.g. around 350°C) may be lower than 20°C / s, specifically 15°C / s or lower. The final cooling from Ms temperature to Mf temperature may be even lower, e.g. 10°C / s or lower.

[0076] Experimental results with steels including 37MnB5 and 34MnB4 steel yielded the following results: the structural component obtained with such a process may have an ultimate tensile strength of more than 1.600 MPa, specifically 1.600 MPa - 1.900 MPa, more specifically 1.600 MPa - 1.800 MPa, more specifically 1.700 - 1.800 MPa, or about 1.700 MPa. At thesame time, an A50 elongation of 5% or more may be obtained. The structural component obtained may have a yield strength of 1.000 MPa or more, specifically 1.100 - 1.400 MPa or 1.100 - 1.300 MPa. In a three-point bend test, a bending angle of 40 - 55°, specifically about 50° may be found, in both directions.

[0077] Figure 3A illustrates an example of an inner panel. The four areas indicated in figure 3A comprise areas which require most deformation in a stamping process. These areas are the most difficult to form. Figure 3C illustrates a final cross-sectional shape of a portion of one of the areas to be obtained. In order to obtain that shape, in a first press step, the flat blank may be deformed to a shape similar to the shape shown in in figure 3B. In the first press step, material of the blank may be pushed towards the areas that require most deformation. In this manner, excessive thinning of the blank can be avoided in the most challenging areas.

[0078] Figures 4A - 4D schematically illustrate examples of inner panels. In the example of figure 4A, the whole inner panel is formed by hot stamping in accordance with the example processes herein described. The inner panel of figure 4A includes a bottom perimeter of the inner panel including a bottom edge 13, a front edge 11 and a rear edge 12 and a lower portion 14 of a window frame. The inner panel further includes an upper part 15 of the window frame. In this example, the inner panel further includes a carrier portion inside the bottom perimeter of the inner panel which is integrally formed with the rest of the inner panel and formed in the same hot stamping process. The carrier portion of the inner panel may be regarded as the central portion of the bottom half of the inner panel, i.e. the portion of the inner panel inside the bottom perimeter.

[0079] In the example of figure 4B, the carrier portion 19 may be made separately, e.g. from a plastic material or composite material. In a specific example, fiber reinforced polypropylene may be used. The remainder of the inner panel including the bottom perimeter surrounding the carrier portion and the upper side of the window frame is formed in a hot stamping process.

[0080] In figures 4B-4D certain parts of the inner panels have been indicated with interrupted line. This has only been done to highlight the other portions of the inner panel. The portions shown in interrupted lines may generally be made using the hot stamping processes herein described.

[0081] In examples, the blank that is deformed to the inner panel may have a constant thickness. In other examples, the blank may include a different thickness in different areas of the blank. In some examples, the steel blank for pressing may be a Tailor Welded Blank (TWB) combining blanks of different thickness. In other examples, the steel blank may be a Tailor Rolled Blank (TRB).

[0082] In the example of figure 4B, the blank that is deformed to form the inner panel may be a combined blank comprising a main blank and two patches. A patch should be understood as a piece of material that, when attached to a blank, is entirely encompassed within the boundaries of the blank. I.e. a patch may completely overlap a main blank of material or vice versa. The overlapping of the main blank with an entire patch enables the patch and the main blank to work as a single piece after they have been welded together. Specifically, laser welding might be used to join the patches to the main blank.

[0083] In the example of figure 4B, an upper front patch 24 and a lower front patch 22 are provided to provided local reinforcements for the hinge areas of the inner panel.

[0084] Figures 4C and 4D illustrate further examples of inner panels including local reinforcements provided by such patches. It has been found that the use of patches is both efficient in terms of manufacturing and can provide the desired performance in terms of crashworthiness and local reinforcement of specific areas.

[0085] Ultimate tensile strength and yield strength mentioned throughout the present disclosure may be determined in standardized tensile strength tests, using e.g. A50 or A80 specimens in a quasi-static load test. The comparison between mechanical properties should be made using the same test conditions and specimen size. To compare yield strengths of different portions, specimens formed with the same materials as portions of the structural component may be prepared and tested in a Universal Testing Machine (UTM).

[0086] Although only a number of examples have been disclosed herein, other alternatives, modifications, uses and / or equivalents thereof are possible. Furthermore, all possible combinations of the described examples are also covered. Thus, the scope of the present disclosure should not be limited by particular examples, but should be determined only by a fair reading of the claims that follow.

Claims

CLAIMS1 . A method for manufacturing an inner panel of a vehicle door, comprising providing a steel blank; heating the steel blank to above an austenization temperature; cooling the steel blank; and deforming the heated steel blank in a first press step while further cooling to form an intermediate blank; deforming the intermediate blank to form the inner panel of the vehicle door in a second press step; and cooling the obtained inner panel of the vehicle door, wherein the inner panel of the vehicle door includes a bottom perimeter of the inner panel including a bottom edge, front and rear edges and a lower portion of a window frame, and further includes an upper part of the window frame.

2. The method of claim 1 , wherein cooling the steel blank prior to deforming comprises cooling to a temperature between 450 - 750 °C, specifically 600 - 750°C.

3. The method of claim 1 or 2, wherein the blank has a temperature of 630 - 690°C, specifically 650 - 680°C, when the first press step starts.

4. The method of any of claims 1 - 3, wherein the intermediate blank has a temperature of 350 - 550 °C, specifically 430 - 470°C when the second press step starts.

5. The method of any of claims 1 - 4, wherein the steel blank is coated with a Zinc based coating.

6. The method of claim 5, wherein the Zinc based coating is provided in a galvanizing or galvannealing process.

7. The method of any of claims 1 - 6, wherein the first press step performs between 40 and 60% of the total deformation of the blank to form the inner panel, and wherein the second press step performs between 60 and 40% of the total deformation.

8. The method of any of claims 1 — 7, wherein the steel blank is made of 22MnSiB9-5 or20 MnB8.

9. The method of claim 1 , wherein the steel blank is made of one or 37MnB5, 34MnB4, 20MnCr, 28MnB5, 30MnB5, 34MnB5 or 37 MnB4.

10. The method of claim 9, wherein a temperature of the blank when the first press step starts of 600°C or more, specifically 650° or more, more specifically 700 - 800°C.

11. The method of claim 9 or 10, wherein a temperature of the intermediate blank when the second press steps starts is 500 - 600°C.

12. The method of any of claims 9 - 11 , wherein a temperature of the inner panel at the end of the second press step is between 400 - 500°C.

13. The method of any of claims 9 - 12, wherein the first press step performs between 60 and 90% of the total deformation of the blank to form the inner panel, and wherein the second press step performs between 40 and 10% of the total deformation.

14. The method of any of claims 1 - 13, wherein the inner panel further includes a carrier portion inside the bottom perimeter of the inner panel.

15. The method of any of claims 1 - 14, wherein the steel blank for pressing is a Tailor Welded Blank or Tailor Rolled blank.

16. The method of any of claims 1 - 14, wherein the steel blank for pressing is a combined blank of a main blank with one or more patches.

17. The method of any of claims 1 - 16, wherein the first press step and the second press step occur in the same press apparatus.

18. The method of claim 17, wherein the same press apparatus includes a cooling station to cool down the steel blank, specifically wherein the cooling station employs a cooling gas flow.

19. The method of any of claims 1 - 18, wherein substantially the whole inner panel is fully hardened.

20. The method of any of claims 1 - 19, wherein substantially the whole inner panel has an ultimate tensile strength of 1.400 MPa or more, specifically 1.500 MPa or more.

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