Press methods for steels and uses of steels

Abstract

Examples of methods of hot forming structural components are provided. The methods include heating a blank made from an Ultra High Strength Steel with an aluminum coating and forming the heated blank in a multi-step production line or apparatus.

Description

Press methods for steels and uses of steels

[0001] The present application claims the benefit of n° 23 382 824.3 filed on August 7th, 2023.

[0002] The present disclosure relates to methods for manufacturing hot formed structural components and uses of ultra high strength steels in hot forming processes.BACKGROUND

[0003] In the field of vehicle construction, the development and implementation of lightweight materials or components is becoming more and more important in order to satisfy criteria for lightweight construction. The demand for weight reduction is especially driven by the goal of 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.

[0004] A process known as Hot Forming Die Quenching (HFDQ) (also known as hot stamping or press hardening) uses e.g. boron steel sheets to create stamped components with Ultra High Strength Steel (UHSS) properties, with tensile strengths of e.g. 1.500 MPa or more. The increase in strength as compared to other material allows for a thinner gauge material to be used, which results in weight savings over conventionally cold stamped mild steel components.

[0005] In order to improve corrosion protection before, during or after a hot stamping process, coatings may be applied. For example the use of Al-Si coatings or Zn coatings is known.

[0006] Depending on the composition of the base steel material, blanks may need to be quenched (i.e. be cooled down rapidly) to achieve the high tensile strengths. Examples of steel materials which can harden by leaving them to cool to room temperature by air cooling with relatively low cooling speed are also known. These steels may be referred to as “air hardenable” steels.

[0007] The hot stamping process may be performed in a manner such that a blank to be hot formed is heated to a predetermined temperature e.g. to or above anaustenization temperature by, for example, a furnace system so as to decrease the strength of the blank i.e. to facilitate the hot stamping process. The heated blank may be formed by, for example, a press system having a low temperature compared to the blank (e.g. room temperature) and a temperature control, thus a shaping process and a heat treatment using the temperature difference may be performed.

[0008] A hot stamping process may include a conveyor or a transferring device which transfers the heated blank from the furnace to a press tool which is configured to press the blank. Upstream from the furnace system, a cutting system for cutting blanks directly from a steel coil can be provided.

[0009] The use of multistep press apparatus for manufacturing hot formed elements is known. The multistep press apparatus may comprise a plurality of tools configured to perform different operations on different blanks simultaneously. With such arrangements, a plurality of blanks can undergo different manufacturing steps simultaneously during each stroke of the press apparatus. The efficiency and performance of a multistep apparatus may be higher than systems employing a plurality of different machines or apparatuses for different manufacturing steps, such as, laser trimming or hard cutting.

[0010] When zinc coated steel blanks are used, the blanks need to be cooled down to a certain temperature before a hot forming process to reduce or minimize problems such as microcracks. Once the blank is cooled down, it is transferred from the external pre-cooling tool to the multistep press apparatus.

[0011] US 2022 / 0258223 discloses press apparatus and methods for manufacturing hot formed structural components. The apparatus comprise a fixed lower body, and a mobile upper body. The apparatus comprises a cooling tool and a press tool which is arranged downstream from the cooling tool, and a blank transfer mechanism to transfer the blank from the cooling tool to the press tool. The cooling tool has an upper gas cooling tool connected to the mobile upper body and / or a lower gas cooling tool connected to the fixed lower body. The press tool comprises an upper pressing die connected to the upper body and a lower pressing die is connected to the lower body.

[0012] EP 3 437 750 A1 discloses examples of methods of hot forming structural components. The methods include heating a blank made of an Ultra High Strength Steel with an aluminum coating and forming the heated blank in a multi-step apparatus.

[0013] EP3067129 A1 discloses press systems for manufacturing hot formed structural components. The system comprises a fixed lower body, a mobile upper body and a mechanism configured to provide upwards and downwards press progression of the mobile upper body with respect to the fixed lower body. The system further comprises a cooling I heating tool configured to cool down and I or heat a previously heated blank having locally different microstructures and mechanical properties which comprises: upper and lower mating dies , and the upper and lower dies comprising two or more die blocks adapted to operate at different temperatures corresponding to zones of the blank having locally different microstructures and mechanical properties, and a press tool configured to draw the blank, wherein the press tool is arranged downstream the cooling I heating tool. This system is particularly aimed at creating “soft zones” in order to improve the ductility and energy absorption in specific areas of a component made from llsibor® 1500 (22MnB5). This use of 22MnB5 boron steel requires a specific temperature control between different die blocks of the cooling / heating tool and downstream post-processing tools to achieve the different microstructures and corresponding different characteristics.

[0014] EP3067128 A1 discloses a multistep press system for manufacturing hot formed structural components. The system comprises a fixed lower body, a mobile upper body and a mechanism configured to provide upwards and downwards press progression of the mobile upper body with respect to the fixed lower body. The system further comprises a cooling tool configured to cool down a previously heated blank which comprises: upper and lower mating dies, the lower die connected to the lower body with one or more lower biasing elements and / or the upper die connected to the upper body with one or more upper biasing elements. The system further comprises a press tool configured to draw the blank, wherein the press tool is arranged downstream from the cooling tool. This system is particularly aimed at the use of zinc coated ultra high strength steels.

[0015] One disadvantage related to the use of zinc coated steels is that a zinc oxide layer can form on the blanks. In many applications, the zinc oxide layer needs to be removed or reduced after the manufacturing process. For example shot blasting may be used to remove the zinc oxide layer partially or completely. Also, components with an AlSi coatingcan generally be welded better than components with a Zn coating.

[0016] The most commonly used steel in press hardening processes is coated 22MnB5 or similar. For example, llsibor® 1500 is commercially offered byArcelorMittal™, MBW-W®1500 is commercially offered by ThyssenKrupp, and other steel manufacturers offer further steels. After heating to above Ac3 temperature (e.g. after heating to about 900°C), a heated blank may be formed and quenched. Quenching may occur above a critical cooling rate of about 27°C / s to obtain a substantially fully martensitic microstructure and an ultimate tensile strength of about 1.500 MPa.

[0017] 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.

[0018] 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.

[0019] Another characteristic of such steels is however its brittleness and very low ductility after hot stamping. This means that post-processing operations including e.g. joining are more complicated and that energy absorption in case of an impact or crash is limited. This is one of the reasons why, in spite of the high tensile strength and potential to even further reduce the weight of automotive structures, these steels have actually had few practical applications in automotive structures to this day.

[0020] The present disclosure seeks to provide improvements in hot stamping process of high grade steels, and specifically in multistep processes and apparatuses.SUMMARY

[0021] In a first aspect, a method for hot forming a structural component in a multi-step production line is provided. The multi-step production line comprises a press tool configured to draw blanks, wherein the press tool comprises an upper press die and a lower press die. The production line further comprises a first post-press tool arranged downstream from the press tool and configured to perform a first post-press operation and comprising an upper first post-press die and a lower first post-press die. The upper press die and upper first post-press die are configured to operate in unison. The production line further comprises a transfer system to transfer blanks from the press tool to the first post-press tool.

[0022] The method comprises providing a press hardenable boron steel blank preferably with a carbon content by weight of 0.32 - 0.45%, a manganese content of 0.6 - 1.5%, and a boron content of 0.003 - 0.006% and further comprises heating the blank to above an austenization temperature. The method further comprises drawing the heated blank in the press tool and transferring the formed blank from the press tool to the first post-press tool. A temperature of the blank before drawing the blank is at least 600°C, and a temperature of the formed blank before the first post-press operation is between 500° and 650 °C, and a temperature at the end of the first post-press operation is between 400°C and 550°C.

[0023] In accordance with this aspect, an efficient method for manufacturing structural steel components with increased carbon content in which one or more post-processes may be incorporated is provided. The throughput of the production line may be increased or maximized. Furthermore, structural components with desirable mechanical properties may be obtained: an ultimate tensile strength of more than 1 .600 MPa e.g. 1 .700 MPa - 1 .900 MPa can be obtained, in combination with an A50 elongation of 5% or more, specifically an A50 elongation of 5 - 6%. The high ultimate tensile strength provides the potential for reducing thickness of the components and thereby their weight. The increased ductility as compared to conventional press hardening of such steels can improve the behaviour in the case of impact / crash and facilitate further post-processing.

[0024] Yield strength, ultimate tensile strength and A50 elongation may be measured in accordance with standard tensile strength test for metallic specimens as defined e.g. in ISO 6892-1. The A50 elongation refers to the elongation at break for a test specimen with an initial length of 50 mm (to be distinguished from e.g. A80).

[0025] In any of the herein disclosed examples, the press hardenable boron steel blank may be coated, specifically with an aluminum-silicon (AlSi) coating. Since an Ultra High Strength Steel blank with an aluminum silicon coating can be used, shot blasting to remove the zinc oxide layer partially or completely is not necessary.

[0026] A multi-step production line may herein be regarded as a production line in which several processes are carried out simultaneously on different blanks / components. The operation of the different processes are carried out in unison: the movement of the different tools is harmonized in that the cycle time of each stroke is the same. In particular, the movement of the different tools may be synchronized. In some examples, the different processes and tools may be integrated into a single press apparatus, or in separate apparatus.

[0027] Reference is herein made to press hardenable boron steels, with 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.

[0028] 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 %

[0029] 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

[0030] MBW-K® 1900 may generally be described as a 34MnB4 steel. The composition of MBW-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

[0031] 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 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

[0032] B1800HS is yet another boron steel which may have an ultimate tensile strength of about 1800 MPa and suitable for hot stamping and 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

[0033] A further suitable steel 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

[0034] In examples, the blank may be heated to above Ac3 temperature, and particularly to 870 - 930°C, more specifically 900 - 930°C. Heating may occur particularly in a furnace arranged upstream from the multi-step production line.

[0035] In some examples, the multi-step production line further comprises a second post-press tool arranged downstream from the first post-press tool and configured to perform a second post-press operation and comprising an upper second post-press die and a lower second post-press die, wherein the upper second post-press die is configured to operate in unison with the upper press die and upper first post-press, and wherein the transfer system is further configured to transfer the formed blank from the first post press-tool to the second post-press tool. In examples, the temperature of the blank at the end of the second post-press operation may be between 350° and 450°C. The second post-press operation tool may comprise a temperature control system including e.g. thermocouples and heaters and / or cooling channels in the tool to controlthe temperature of the blank, and in particular to ensure that the temperature of the blank does not drop too much or too quickly. A temperature of 350 - 450°C also enables further operations.

[0036] In some examples, the multi-step production line further comprises one or more additional tools arranged downstream from the second post-press operation tool, the additional tools being configured to operate in unison with the press tool and first and second post-press tools. Several different tools may be provided in the multi-step production line, including one or more tools for trimming, one or more tools for making holes, for cutting and others. In some examples, an additional tool may include a tool for restriking, i.e. a tool that performs a further step of shaping an already deformed blank.

[0037] In some examples, the last additional tool may be a restriking tool. A temperature of the formed blank before restriking may be e.g. between 250 °C and 350°C. A temperature of the formed blank may be high enough to maintain malleability of the material and avoid unnecessary wear of the tool.

[0038] In examples, the operational cycle of the press tool may be between 2 and 5 seconds, specifically between 2,5 and 4 seconds, and more specifically approximately 4 seconds. An operational cycle of about 4 seconds maintains a high throughput and allows for a closing time of e.g. 0.5 seconds at the press’ bottom dead center. Such a closing time allows for increased temperature control during the process.

[0039] In examples, a transfer time between the press tool and the first post-press tool may be between 1 and 3 seconds, specifically between 1 ,5 and 2,5 seconds, more specifically approximately 2 seconds. A transfer of the blanks may be carried out or at least initiated before the upper dies have reached their upper position.

[0040] In some examples, the obtained component may further be submitted to a bake hardening process. E.g. a structural component may be submitted to a temperature of about 180°C during about 20 minutes. A bake hardening process may further increase the yield strength of the obtained component, whereas ultimate tensile strength remains substantially the same.

[0041] In some examples, the multi-step production line comprises a multi-step tool comprising a fixed lower body, a mobile upper body, and a mechanism configured to provide upwards and downwards press progression of the mobile upper body with respect to the lower body, and wherein the upper press die is connected to the mobileupper body and the lower press die is connected to the fixed lower body, and the first upper post-press die is connected to the mobile upper body, and the first lower postpress die is connected to the fixed lower body. The multi-step production line may be incorporated in a multi-step apparatus comprising the press tool and further post-press tools in a single press apparatus.

[0042] With the integration of the tools in the same apparatus by connecting the upper dies of the press tool and the additional tool to the mobile upper body, the transfer time from between the press tool and the additional tool(s) may be reduced, thus the process may be optimized and the productivity may be improved. Also the temperature of the blanks during the different steps of the process can be improved.

[0043] In some examples, the dies of the cooling tool may comprise channels conducting cooling water. The dies of the cooling tool may alternatively or additionally comprise channels conducting air.

[0044] In some examples, the austenization temperature to which a blank may be heated may be an Ac3 temperature, and cooling down the complete heated blank comprises cooling down the blank to a temperature between 600 - 800°C, specifically between 650° - 700 °C.

[0045] In some examples, the multi-step apparatus may further comprise a first post operation tool downstream from the press tool, the first post operation tool comprising upper and lower first post operation dies comprising one or more working surfaces that in use face the formed blank, and the lower first post operation die being connected to the lower body and the upper first post operation die being connected to the upper body.

[0046] In some examples, the first post-press tool may comprise a temperature control system for controlling the temperature of the formed blank during the first post operation, the temperature control system optionally including thermocouples in the upper and lower first post operation dies.

[0047] In some examples, the dies of the first post-press tool may comprise channels conducting cooling water or cooling air. In some examples, the dies of the first postpress tool may comprise one or more heaters or channels conducting a hot liquid or conductive heating.

[0048] In yet a further aspect, a component obtainable by any of the methods herein disclosed is provided.BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Non-limiting examples of the present disclosure will be described in the following, with reference to the appended drawings, in which:

[0050] Figure 1 schematically represents a multistep production line according to an example;

[0051] Figure 2 schematically illustrates an example of a temperature of a blank in a production process according to an example of the present disclosure; and

[0052] Figure 3 schematically illustrates a multi-step apparatus which may be used in examples of methods according to the present disclosure.DETAILED DESCRIPTION OF EXAMPLES

[0053] Figure 1 schematically represents a multistep production line according to an example. An example of a method for hot forming a structural component in a multi- step production line is schematically illustrated in figure 1.

[0054] A steel coil 10 of a press hardenable boron steel. A press hardenable boron steel having a composition by weight of 0.32 - 0.45%, a manganese content of 0.6 - 1.5%, and a boron content of 0.003 - 0.006% may be used. A blanking apparatus 20 may cut suitably sized blanks from coil 100. The steel has an aluminium-silicon coating.

[0055] The blanks may preferably have a thickness of 0,8 mm - 2 mm, specifically 1 - 1.6 mm. In some examples, the steel may be 34MnB4 of 37MnB5 steel.

[0056] In specific examples, the steel may have a content by weight of 0.34 - 0.4% carbon, specifically 0.34 - 0.39%, and more specifically 0.34 - 0.38%. The steel may further have a content of 0.8 - 1 .4% of manganese. The steel may have a content by weight of boron of 0.003 - 0.005%, and specifically 0.004 - 0.005%. In specific examples, the steel may have a content by weight of 0.3 - 0.9%, specifically 0.4 - 0.8% of silicon.

[0057] The blanks may be provided to furnace 30. The blanks may be heated to above an austenization temperature, and in particular to above an Ac3 temperature. In specific examples, the blanks may be heated in the furnace for between 5 and 10 minute and may be heated to about 870 - 930°C, specifically 900 - 930°C.

[0058] After exiting the furnace 30, the heated blanks may be transferred to production line 100. The production line 100 of the example of figure 1 comprises a press tool 50 configured to draw blanks.

[0059] The press tool 50 comprises an upper press die 52 and a lower press die 54. A blank may be positioned in between the upper press die 52 and lower press die 54. The upper press die 52 may perform an upward-downwards progression with respect to lower press die 54 to deform a previously heated blank.

[0060] The production line 100 further comprises a first post-press tool 60 arranged downstream from the press tool 50 and configured to perform a first post-press operation on the formed blank (a blank that has undergone a drawing process in the press tool). The first post-press tool comprises an upper first post-press die 62 and a lower first post-press die 64. The upper press die 52 and upper first post-press die 62 are configured to operate in unison. Particularly, the upwards-downwards movement of the upper first post-press die 62 may be synchronized with the upward-downwards movement of the upper press die 52.

[0061] The production line 100 further comprises a transfer system to transfer formed blanks from the press tool 50 to the first post-press tool 60. In this particular example, a plurality of transfer robots with grippers or suction units is provided. A transfer robot 44 may grip the deformed blank after it has undergone the drawing operation in press tool 50 and place it in the first post-press tool 60.

[0062] The terms “post-press tool” and “post-press operation” respectively refer to any tool or operation that is arranged or takes place downstream from the press tool. The first post-press tool 60 may for example be a trimming tool to trim one or more edges of the deformed blank.

[0063] The production line 100 of this example further comprises a second post-press tool 70 comprising a second post-press upper die 72 and a second post-press lower die 74. The post-press upper die 72 is configured to operate in unison with the first post-press upper die 62 and the press upper die 52, in particular the movements of the upper dies may be synchronized.

[0064] A further transfer robot 46 may grip the blank after the first post-press operation and place the blank in the next post-press tool. An example of a second post-press tool may be a tool for cutting or a tool for piercing or making holes.

[0065] In this particular example, an additional tool 80 comprising an upper die 82 and a lower die 84 is provided. The upper die 82 may be configured to operate in unison, and in particular be synchronized with the other upper dies. A transfer robot 48 may transfer the blank from the second post-press tool 70 to the additional tool 80. After the final operation at tool 80, the blanks may be transferred and stacked by a suitable robot 49.

[0066] It should be clear that in other examples, different tools and different numbers of tools may be provided. In a specific example, a production line may include 6 tools including the press tool. The last tool of the production line may be a restriking tool to provide a final shaping of the component.

[0067] The production line illustrated in figure 1 may be used for the manufacture of a variety of structural components. In particular examples, structural components with a length and width of more than 1 meter may be manufactured in such a production line. In some examples, unitary door rings (door rings made of a single integrally formed body) may be manufactured in such a production line. Unitary door rings may be e.g. a single door rings (extending from A-pillar to B-pillar, or from B-pillar to C-pillar) or double door rings (extending from A-pillar to C-pillar).

[0068] Further large structural components which may be manufactured in a production line 100 according to figure 1 include e.g. unitary roof rings, bumper beam assemblies including a bumper beam and a pedestrian beam, a unitary ring surrounding a battery box, a rear framework structure including rear rails and a transverse beam as a unitary structure and others.

[0069] Such large structural components may be manufactured efficiently in a hot stamping process. Due to the size of such components, the different tools are difficult to integrate in a single press apparatus. However, the methods and systems may also be used for other components that may be press formed e.g. B-pillar, A-pillar, hinge pillar, bumper crossbeams and others.

[0070] In some examples, a blank may be composed of multiple smaller blanks or subblanks. Some of the sub-blanks may be joined to other sub-blanks in a Tailor Welded Blank (TWB) edge-to-edge butt joint. Alternatively, or additionally, the sub-blanks may be partially overlapped with each other to form an overlapping region with an increased thickness compared to other areas. The areas of increased thickness may be selected to locally reinforce the structural component. The increased thickness in these areas may provoke an increased heating time in a furnace. In the drawing operation, and insubsequent processes, the overlapping areas may cool down more slowly unless specific measures are taken.

[0071] In some examples, biasing elements such as e.g. springs may be integrated in the upper and / or lower press dies such that certain mating die blocks of the press tool (a pair of a die block of the upper press tool and a die block of the lower press tool that face each other) enter into contact with the blank before other die blocks enter into contact. In other words, a selection of the die blocks maybe closed earlier than other die blocks.

[0072] In those die blocks that contact is established earlier, cooling down may start earlier, and may be quicker than in other die blocks. If overlapping regions are to be cooled down more quickly, a biased die block may be used for that area. Additionally or alternatively, one or more die blocks may integrate heating means in order to avoid a too rapid cooling down.

[0073] The 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 50 and transferring the blank from the press tool 50 to the first post-press tool 60.

[0074] A temperature of the blank before drawing the blank is at least 600, specifically at least 650°C. At temperatures below 600°C, for drawing, stripes were formed in the coating. At temperatures of 600°C or higher, the coating was satisfactory after the hot forming process.

[0075] 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.

[0076] A temperature of the blank before the first post-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 first post-press operation may be between 400°C and 500°C, e.g. approximately 450°C.

[0077] In the illustrated example, a temperature of the blank at the end of the second post-press operation is between 350°C and 450°C.

[0078] An operational cycle of the press tool (and the other tools) may be between 2 and 5 seconds, specifically between 2,5 and 4,5 seconds, and more specificallyapproximately 4 seconds. A transfer time between the press tool and the first postpress tool (and between the other tools) may be between 1 and 3 seconds, specifically between 1 ,5 and 2,5 seconds, more specifically approximately 2 seconds.

[0079] The more gradual cooling of the steel as illustrated herein 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.

[0080] The last tool 80 may be a restriking tool. A temperature of the blank before restriking may be between 250 °C and 350°C. In the restriking tool, the blanks may be cooled down to below 200°C. Restriking may be used to avoid or reduce tolerance and geometric deviations from the intended design and provide a structural component with a precisely defined geometry. In the press tool, and all other tools, suitable temperature control systems including heaters and / or cooling systems may be provided. Such systems may be provided in the upper and / or the lower dies.

[0081] In particular, any of these tools may include channels for conducting a heat transfer medium. In particular, such channels may be cooling channels for conducting cold water. After cooling down the corresponding dies, the heated up water may pass through a heat exchanger to cool down again. Any of these tools may alternatively or additionally comprise heaters, e.g. electric heaters, to make sure the blanks do not cool down too quickly.

[0082] Figure 2 schematically illustrates a temperature profile of a blank from exiting the furnace and through different tools of a production line. In the example of figure 2, the temperature of the blank may decrease from about 920- 930°C to about 750°C before the drawing operation in the press tool 50.

[0083] The temperature is decreased to about 550°C before the first post-press operation in tool 60. A cooling rate for the press operation may be about 50°C / s.

[0084] The temperature may then be decreased during the first post-press operation to about 450°C before the second post-press operation. A cooling rate in this particular portion may be about 25°C / s.

[0085] In the second post-press operation tool 70 and before the additional tool 80, the temperature may decrease to about 400°C, at a cooling rate of about 12.5°C / s. Inadditional tool 110, the temperature may further decreased to 350°C at a cooling rate of about 12.5°C / s.

[0086] The last tool 120 in this particular example may be a restriking tool. The temperature of the blank may be further cooled down in the restriking tool.

[0087] 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 the same 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.

[0088] In some examples, the obtained structural component may further be submitted to a bake hardening process. A temperature of bake hardening may be between 170°C and 200°C, and a bake hardening time may be between 15 and 25 minutes. With bake hardening, an increase in yield strength of the structural component may be obtained to 1.050 MPa or more, specifically 1.200 MPa or more. Energy absorption and toughness of the material may thereby be increased. These results have been experimentally confirmed for the above-referenced steels.

[0089] Figure 3 schematically illustrates a multi-step apparatus which may be used in examples of methods according to the present disclosure. The multi-step tool 90 comprises a fixed lower body 88, a mobile upper body 86, and a mechanism (not further illustrated) configured to provide upwards and downwards press progression of the mobile upper body 86 with respect to the lower body 88.

[0090] The upper press die 52 is connected to the mobile upper body 86 and the lower press die 54 is connected to the fixed lower body 88, and the first upper post-press die 62 is connected to the mobile upper body 86, and the first lower post-press die 64 is connected to the fixed lower body 88.

[0091] Similarly, upper die 72 of the second post-press tool in this example is connected to mobile upper body 86 and the lower die 74 of the second post-press tool is connected to the fixed lower body 88.

[0092] Similarly, upper die 82 of the additional tool may be connected to the mobile upper body 86 and the lower die 84 of the additional tool may be connected to the fixed lower body 88. All upper dies 52, 62, 72 and 82 thus move in unison towards and away from the lower dies 54, 64, 74 and 84 respectively.

[0093] The fixed lower body 88 may be a large block of metal. In this particular example, the fixed lower body 88 may be stationary. In some examples, a die cushion (not shown) integrated in fixed lower body 88 may be provided. The cushion may be configured to receive and control blank holder forces. The mobile upper body 86 may also be a solid piece of metal. The mobile upper body 86 may provide the stroke cycle (up and down movement).

[0094] The press system may be configured to perform e.g. approximately 15 strokes per minute, thus each stroke cycle may be of approximately 4 seconds. The stroke cycle could be different in further examples. In a multistep press system all operations to be formed on a blank need to have the same cycle time. Note that the stroke cycle of 4 seconds may mean that the blanks or formed blanks undergo the several operations during a cycle of about 2 or 2,5 seconds, and transferring the blanks between station may take about 1 ,5 - 2 seconds.

[0095] The mechanism of the press may be driven mechanically, hydraulically or servo mechanically. The progression of the mobile upper body 86 with respect to the fixed lower body 88 may be determined by the mechanism. In this particular example, the press may be a servo mechanical press, thus a constant press force during the stroke may be provided. The servo mechanical press may be provided with infinite slide (ram) speed and position control. The servo mechanical press may also be provided with a good range of availability of press forces at any slide position, thus a great flexibility of the press may be achieved. Servo drive presses have capabilities to improve process conditions and productivity in metal forming. The press may have a press force of e.g. 2000 Tn.

[0096] In some examples, the press may be a mechanical press, thus the press force progression towards the fixed lower body 88 may depend on the drive and hinge system. Mechanical presses therefore can reach higher cycles per unit of time. Alternatively, hydraulic presses may also be used.

[0097] In some examples, one or more of the lower dies 54, 64, 74, 84 may be connected to the lower body 88 with a lower biasing element configured to bias the lower die to a position at a predetermined first distance from the lower body 88. Insome examples, a single lower biasing element may be provided, or more than two lower biasing elements can be provided. The biasing elements may comprise, for example, a spring e.g. a mechanical spring or a gas spring although some other biasing elements may be possible e.g. hydraulic mechanism.

[0098] In some examples, one or more of the upper dies 52, 62, 72 and 82 may also be connected to the upper body 86 with one or more upper biasing elements configured to bias the upper die in a position at a predetermined second distance from the upper body.

[0099] With the insertion of the upper and I or lower biasing elements, the contact time between the upper dies and the lower dies may be regulated and increased during a stroke cycle (up and down movement of the mobile upper body 86 with respect to the lower body 88).

[0100] The use of such biasing elements allows the cooling tool to have a different cycle time than the other tools integrated in the same apparatus. This is explained in more detail in EP3067128. However, within the scope of the present disclosure, the use of biasing elements is merely optional. Depending on the steel of the blanks and their coating, biasing elements may not be needed at all. As mentioned herein, such biasing elements may be used for closing of complete tools or “dies”. In examples, biasing elements may be used only for a selection of die blocks.

[0101] A press tool configured to form or draw the blank is also integrated in the same press apparatus. The upper press die 52 may comprise an upper working surface that in use faces the blank to be hot formed. The lower die 54 may comprise a lower working surface that in use faces the blank to be hot formed. A side of the upper die opposite to the upper working surface may be fastened to the upper body 86 and a side of the lower die opposite to the lower working surface may be fastened to the lower body 88.

[0102] The upper 52 and lower 54 mating dies may comprise channels with cold fluid e.g. water and / or cold air passing through the channels provided in the dies. In the water channels, the speed circulation of the water at the channels may be high, thus the water evaporation may be avoided. A control system may be further provided that may control fluid temperature and flow rate based on temperature measurements, thus the temperature of the dies may be controlled.

[0103] In examples, the press tool may be provided with a blank holder configured to hold a blank and to positioning the blank onto the lower die 54. The blank holder mayalso be provided with e.g. springs to bias the blank holder to a position at a predetermined distance from the lower die 22.

[0104] In this example, a first post-press tool configured to perform trimming and I or piercing operations is provided in the same multi-press apparatus. The first post-press tool is arranged downstream of the press tool. The first post-press tool comprises upper 62 and lower 64 mating dies. The upper mating die 62 may comprise an upper working surface and the lower mating die 64 may comprise a lower working surface. Both working surfaces in use face the blank. A side of the upper die 62 opposite to the upper working surface may be fastened to the upper body 86 and a side of the lower die 64 opposite to the lower working surface may be fastened to the lower body 88. The dies may comprise one or more knives or cutting blades (not shown) arranged on the working surfaces.

[0105] The first post-press tool may further also comprise one or more electrical heaters or channels conducting hot liquid and temperature sensors to control the temperature of the dies. The sensors may be thermocouples. In some examples, the upper 62 and lower 64 mating dies may comprise channels with cold fluid e.g. water and / or cold air passing through the channels provided in the dies.

[0106] In examples, the first post-press tool may be provided with a blank holder (not shown) configured to hold a blank and to position the blank onto the lower die 62. The blank holder may also be provided with one or more biasing elements configured to bias the blank holder to a position at a predetermined distance from the lower die.

[0107] In this example, a second post-press tool and additional tool with upper and lower mating dies are provided. The descriptions of the dies of the press tool and first post-press tool are generally applicable to these tools as well. The second post-press tool 40 may be configured to perform further trimming and I or piercing operations.

[0108] The dies may comprise one or more knives or cutting blades arranged on the working surfaces.

[0109] It should be understood that although the figures describe dies having a substantially square or rectangular shape, the blocks may have any other shape and may even have partially rounded shapes.

[0110] An automatic transfer device (not shown) e.g. a plurality of industrial robots or a conveyor, or beams with gripping elements may also be provided to perform the transfer of blanks between the tools. Since the transfer devices may be integrated inthe same press system, there is less transfer time, and the temperature control is better.

[0111] In all examples, temperature sensors and control systems in order to control the temperature may be provided in any tools or in the transfer system. The tools may also be provided with further cooling systems, blanks holders, etc.

[0112] In some examples, a centering element e.g. pins and I or guiding devices may be provided upstream the cooling tool, thus the blank may be properly centered.

[0113] For reasons of completeness, various aspects of the present disclosure are set out in the following numbered clauses:Clause 1. A method for hot forming a structural component in a multi-step production line comprising a press tool configured to draw blanks, wherein the press tool comprises an upper press die and a lower press die, a first post-press tool arranged downstream from the press tool and configured to perform a first post-press operation and, and comprising an upper first post-press die and a lower first post-press die, the upper press die and upper first post-press die being configured to operate in unison; and a transfer system to transfer formed blanks from the press tool to the first postpress tool, and the method comprising: providing a press hardenable boron steel blank heating the blank to above an austenization temperature; and drawing the heated blank in the press tool and transferring the blank from the press tool to the first post-press tool, wherein a temperature of the blank before drawing the blank is at least 600°C, specifically at least 650°C, and wherein a temperature of the blank before the first post-press operation is between 500° and 650 °C, and wherein a temperature at the end of the first post-press operation is between 400°C and 550°C.Clause 2. The method according to clause 1 , wherein the temperature of the blank before drawing the blank is between 700°C and 800°C.Clause 3. The method according to clause 1 or 2, wherein the blank is heated to870 - 930°C, specifically 900 - 930°C.Clause 4. The method according to any of clauses 1 - 3, wherein the temperature of the formed blank before the first post-press operation is between 500 and 600°C.Clause 5. The method according to any of clauses 1 - 4, wherein the temperature at the end of the first post-press operation is between 400 - 500°C.Clause 6. The method according to any of clauses 1 - 5, wherein the press hardenable boron steel blank has a content by weight of 0.32 - 0.45%, a manganese content of 0.6 - 1 .5%, and a boron content of 0.003 - 0.006%.Clause 7. The method according to clause 6, wherein the press hardenable boron steel blank has a content by weight of 0.32 - 0.4 %, a manganese content of 0.6 - 1 .4%, and a boron content of 0.004 - 0.005%.Clause 8. The method according to any of clauses 1 - 7, wherein the steel blank is made of PHS 1 .900 or PHS 2.000.Clause 9. The method according to any of clauses 1 - 8, wherein the press hardenable boron steel blank is coated.Clause 10. The method according to clause 9, wherein the press hardenable boron steel blank has an AlSi coating.Clause 11. The method according to any of clauses 1 - 10, wherein the multi-step production line further comprises a second post-press tool arranged downstream from the first post-press tool and configured to perform a second post-press operation and comprising an upper second post-press die and a lower second post-press die, and the upper second post-press die being configured to operate in unison with the upper press die and upper first post-press, and wherein the transfer system is further configured to transfer the blank from the first post press-tool to the second post-press tool.Clause 12. The method according to clause 11 , wherein a temperature of the blank at the end of the second post-press operation is between 350 - 450°.Clause 13. The method according to clause 11 or 12, wherein the multi-step production line further comprises one or more additional tools arranged downstream from the second post-press operation tool, the additional tools being configured to operate in unison with the press tool and first and second post-press tools.Clause 14. The method according to clause 13, wherein the last additional tool is a restriking tool.Clause 15. The method according to clause 14, wherein a temperature of the blank before restriking is between 250 °C and 350°C.Clause 16. The method according to any of clauses 1 - 15, wherein an operational cycle of the press tool is between 2 and 5 seconds, specifically between 2,5 and 4,5 seconds, and more specifically approximately 4 seconds.Clause 17. The method according to any of clauses 1 - 16, wherein a transfer time between the press tool and the first post-press tool is between 1 and 3 seconds, specifically between 1 ,5 and 2,5 seconds, more specifically approximately 2 seconds.Clause 18. The method according to any of clauses 1 - 17, wherein the first postpress tool is a tool for cutting or trimming or making holes.Clause 19. The method according to any of clauses 1 - 18, wherein the structural component obtained has an ultimate tensile strength of more than 1.600 MPa, specifically 1.600 MPa - 1.800 MPa, more specifically 1.700 MPa - 1.800 MPa.Clause 20. The method according to any of clauses 1 - 19, wherein the structural component obtained has an A50 elongation of 5% or more.Clause 21. The method according to any of clauses 1 - 20, wherein the structural component obtained has a yield strength of 1.000 MPa or more, specifically 1.100 - 1.300 MPa.Clause 22. The method according to any of clauses 1 - 21 , further comprising bake hardening the structural component obtained, wherein a temperature of bake hardening is between 170°C and 200°C, and wherein a bake hardening time is between 15 and 25 minutes.Clause 23. The method according to clause 22, wherein a yield strength of the structural component obtained is 1.050 MPa or more, specifically 1.200 MPa or more.Clause 24. The method according to any of clauses 1 - 23, wherein a length of the structural component after forming is at least 1 meter, specifically 1 - 2 meters, and a width of the structural component after forming is at least 1 meter, specifically 1 - 2 meters.Clause 25. The method according to clause 24, wherein the structural component is a unitary door ring, and wherein the unitary door ring is one of a front door ring extending from hinge-pillar and A-pillar to B-pi liar, a rear door ring extending from B- pillar to C-pillar or a double door ring extending from hinge-pillar and A-pillar to C-pillar.Clause 26. The method according to clause 24, wherein the structural component is a unitary roof rings, a bumper beam assembly including a bumper beam and a pedestrian beam, or a unitary reinforcement ring surrounding a battery box, or a rear framework structure including rear rails and a transverse beam as a unitary structure.Clause 27. The method according to any of clauses 1 - 26, wherein the blank has a thickness of 0,8 mm - 2 mm, specifically 1 - 1.6 mm.Clause 28. The method according to any of clauses 1 - 27, wherein the blank is made of 34MnB4 or 34MnB5 steel.Clause 29. The method according to any of clauses 1 - 27, wherein the blank is made of 37MnB5 or 37MnB4 steel.Clause 30. The method according to any of clauses 1 - 29, wherein the steel blank has a content by weight of 0.34 - 0.38% carbon and 0.8 - 1 .4% of manganese.Clause 31. The method according to clause 30, wherein the steel blank has a content by weight of boron of 0.004 - 0.005%.Clause 32. The method according to clause 30 or 31 , wherein the steel blank has a content by weight of 0.3 - 0.9%, specifically 0.4 - 0.8% of silicon.Clause 33. The method according to any of clauses 1 - 32, wherein the multi-step production line comprises a multi-step tool comprising a fixed lower body, a mobile upper body, and a mechanism configured to provide upwards and downwards pressprogression of the mobile upper body with respect to the lower body, and wherein the upper press die is connected to the mobile upper body and the lower press die is connected to the fixed lower body, and the first upper post-press die is connected to the mobile upper body, and the first lower post-press die is connected to the fixed lower body.Clause 34. A structural component obtainable by any of the methods according to any of clauses 1 - 33. 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 hot forming a structural component in a multi-step production line comprising: a press tool configured to draw blanks, wherein the press tool comprises an upper press die and a lower press die, a first post-press tool arranged downstream from the press tool and configured to perform a first post-press operation and, and comprising an upper first post-press die and a lower first post-press die, the upper press die and upper first post-press die being configured to operate in unison; and a transfer system to transfer blanks from the press tool to the first post-press tool, and the method comprising: providing a press hardenable boron steel blank, preferably with a content by weight of 0.32 - 0.45% carbon, a manganese content of 0.6 - 1.5%, and a boron content of 0.003 - 0.006%, and wherein the boron steel blank optionally has an AlSi coating; heating the blank to above an austenization temperature; and drawing the heated blank in the press tool and transferring the formed blank from the press tool to the first post-press tool, wherein a temperature of the blank before drawing the blank is at least 600°C, specifically at least 650°C, and wherein a temperature of the formed blank before the first post-press operation is between 500° and 650 °C, and wherein a temperature of the formed blank at the end of the first post-press operation is between 400°C and 550°C.

2. The method according to claim 1 , wherein the press hardenable boron steel blank has a content by weight of 0.32 - 0.38%, a manganese content of 0.6 - 1.4% by weight and a boron content of 0.004 - 0.005% by weight.

3. The method according to claim 1 or 2, wherein the temperature of the blank before drawing the blank is between 650°C and 900°C, specifically between 700°C and 800°C.

4. The method according to any of claims 1 - 3, wherein the blank is heated to 870 - 930°C, specifically wherein the blank is heated to 900 - 930°C, and optionallywherein the blank is heated in a furnace.

5. The method according to any of claims 1 — 4, wherein the multi-step production line further comprises a second post-press tool arranged downstream from the first post-press tool and configured to perform a second post-press operation and comprising an upper second post-press die and a lower second post-press die, and the upper second post-press die being configured to operate in unison with the upper press die and upper first post-press, and wherein the transfer system is further configured to transfer the formed blank from the first post press-tool to the second postpress tool.

6. The method according to claim 5, wherein a temperature of the formed blank at the end of the second post-press operation is between 350°C and 450°C.

7. The method according to claim 5 or 6, wherein the multi-step production line further comprises one or more additional tools arranged downstream from the second post-press operation tool, the additional tools being configured to operate in unison with the press tool and first and second post-press tools.

8. The method according to claim 7, wherein the last additional tool is a restriking tool and wherein a temperature of the formed blank before restriking is between 250 °C and 350°C.

9. The method according to any of claims 1 - 8, wherein an operational cycle of the press tool is between 2 and 5 seconds, specifically between 2,5 and 4,5 seconds, and more specifically approximately 4 seconds, and wherein a transfer time between the press tool and the first post-press tool is between 1 and 3 seconds, specifically between 1 ,5 and 2,5 seconds, more specifically approximately 2 seconds, and optionally wherein transfer of the blanks occurs before the upper dies have reached their uppermost position.

10. The method according to any of claims 1 - 9, wherein the structural component obtained has an ultimate tensile strength of more than 1.600 MPa, specifically 1.600 MPa - 1.800 MPa, more specifically about 1.700 MPa, and wherein the structural component obtained has a yield strength of 1.000 MPa or more, specifically 1.100 - 1.300 MPa11. The method according to any of claims 1 - 10, wherein the structural componentobtained has an A50 elongation of 5% or more.

12. The method according to any of claims 1 - 11 , further comprising bake hardening the structural component obtained, wherein a temperature of bake hardening is between 170°C and 200°C, and wherein a bake hardening time is between 15 and 25 minutes, particularly wherein a yield strength of the structural component obtained is 1.050 MPa or more, specifically 1.200 MPa or more.

13. The method according to any of claims 1 - 12, wherein a length of the structural component after forming is at least 1 meter, specifically 1 - 2 meters, and a width of the structural component after forming is at least 1 meter, specifically 1 - 2 meters, and particularly wherein the structural component is a unitary door ring.

14. The method according to any of claims 1 - 13, wherein the press hardenable boron steel blank is made of 34MnB4 steel or 37MnB5 steel.

15. The method according to any of claims 1 - 14, wherein the multi-step production line comprises a multi-step tool comprising a fixed lower body, a mobile upper body, and a mechanism configured to provide upwards and downwards press progression of the mobile upper body with respect to the lower body, and wherein the upper press die is connected to the mobile upper body and the lower press die is connected to the fixed lower body, and the first upper post-press die is connected to the mobile upper body, and the first lower post-press die is connected to the fixed lower body.

Images