Pressing methods for steel and use of steel

Abstract

An example of a method for hot forming structural components is provided. This method includes heating a blank made from ultra-high-strength steel with an aluminum coating, and forming the heated blank in a multi-stage production line or apparatus.

Description

[[TECHNICAL FIELD]]

[0001] This application claims the benefit of European Patent Application No. 23382824.3 filed on August 7, 2023.

[0002] The present disclosure relates to a method for manufacturing hot-formed structural components and to the use of ultra-high strength steel in a hot forming process. [[BACKGROUND ART]]

[0003] In the field of vehicle structures, the development and implementation of lightweight materials or components are becoming increasingly important to meet the standards for lightweight structures. Weight reduction is sought especially due to the goal of reducing CO₂ emissions. Increasing attention to occupant safety also leads to the adoption of materials that improve energy absorption while improving the crashworthiness of vehicles during a collision.

[0004] In a process known as hot forming die quenching (HFDQ, also known as hot stamping or press hardening), for example, boron steel sheets are used to produce stamped parts having the properties of ultra-high strength steel (UHSS) with a tensile strength of, for example, 1500 MPa or more. The increased strength compared to other materials enables the use of thinner gauge materials, which results in weight savings compared to mild steel parts cold stamped by conventional methods.

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

[0006] Depending on the composition of the base steel material, the blank may need to be quenched (i.e., rapidly cooled) to achieve high tensile strength. Examples of steel materials that can be hardened by being left to cool to room temperature by air cooling at a relatively low cooling rate are also known. These steels can be referred to as "air-hardening" steels.

[0007] The hot stamping process can be carried out in such a way that the blank to be hot-formed is heated to a predetermined temperature, for example, above the austenitizing temperature, by a furnace system, thereby reducing the strength of the blank and thus facilitating the hot stamping process. The heated blank can then be formed by a press system and temperature control that is at a lower temperature (e.g., room temperature) compared to the blank, and thus a forming process and heat treatment using a temperature difference can be performed.

[0008] The hot stamping process may include a conveyor or transfer device that moves the heated blank from the furnace to a press tool configured to press the blank. Upstream of the furnace system, a cutting system may be provided for cutting the blank directly from the steel coil.

[0009] It is known to use multi-stage presses to manufacture hot-formed elements. A multi-stage press may have multiple tools configured to perform different operations on different blanks simultaneously. This arrangement allows multiple blanks to undergo different manufacturing steps simultaneously during each stroke of the press. The efficiency and performance of a multi-stage press can be higher than a system employing multiple different machines or devices for different manufacturing steps, such as laser trimming or hard cutting.

[0010] When zinc-coated steel blanks are used, the blanks must be cooled to a specific temperature before the hot forming process to mitigate or minimize problems such as microcracks. Once cooled, the blanks are transferred from an external pre-cooling tool to a multi-stage press.

[0011] U.S. Patent Application Publication No. 2022 / 0258223 discloses a press apparatus and method for manufacturing hot-formed structural components. The apparatus comprises a fixed lower body and a movable upper body. The apparatus comprises a cooling tool, a press tool located downstream of the cooling tool, and a blank transfer mechanism for transferring a blank from the cooling tool to the press tool. The cooling tool has an upper gas cooling tool connected to the movable upper body and / or a lower gas cooling tool connected to the fixed lower body. The press tool comprises an upper press die connected to the upper body and a lower press die connected to the lower body.

[0012] European Patent Application Publication No. 3437750 (A1) discloses an example of a method for hot forming structural components. This method includes heating a blank made of ultra-high-strength steel and having an aluminum coating, and forming the heated blank in a multi-stage apparatus.

[0013] European Patent Application Publication No. 3067129 (A1) discloses a press system for manufacturing hot-formed structural parts. The system comprises a fixed lower body, a movable upper body, and a mechanism configured such that the movable upper body advances vertically relative to the fixed lower body for pressing. The system further comprises a cooling / heating tool configured to cool and / or heat a preheated blank having locally different microstructures and mechanical properties, and comprising a meshing upper and lower die with two or more die blocks adapted to operate at different temperatures corresponding to areas of the blank having locally different microstructures and mechanical properties, and a press tool configured to draw the blank and positioned downstream of the cooling / heating tool. The system is, in particular, intended to create “soft areas” to improve ductility and energy absorption at specific locations in a part made from Usibor® 1500 (22MnB5). To use 22MnB5 boron steel in this way, specific temperature control is required between different die blocks of the cooling / heating tool and the downstream post-processing tool to achieve different microstructures and corresponding different properties.

[0014] European Patent Application Publication No. 3067128(A1) discloses a multi-stage press system for manufacturing hot-formed structural components. The system comprises a fixed lower body, a movable upper body, and a mechanism configured such that the movable upper body advances vertically relative to the fixed lower body for pressing. The system further comprises a cooling tool configured to cool a preheated blank, which comprises a meshing upper die and a lower die, the lower die being connected to the lower body by one or more downward biasing elements, and / or the upper die being connected to the upper body by one or more upward biasing elements. The system further comprises a press tool configured to draw the blank and located downstream of the cooling tool. The system is, in particular, intended for use with zinc-coated ultra-high-strength steel.

[0015] One disadvantage associated with using zinc-coated steel is that a zinc oxide layer can form on the blank surface. In many applications, this zinc oxide layer needs to be removed or reduced after the manufacturing process. For example, shot blasting can be used to partially or completely remove the zinc oxide layer. Also, parts with AlSi coatings can generally be welded better than parts with Zn coatings.

[0016] The most commonly used steel in the press hardening process is coated 22MnB5 or similar. For example, Usibor® 1500 is commercially available from ArcelorMittal®, MBW-W® 1500 is commercially available from ThyssenKrupp, and other steel manufacturers offer further steels. After heating above the Ac3 temperature (e.g., after heating to about 900°C), the heated blank can be formed and quenched. Quenching can be carried out at a critical cooling rate exceeding about 27°C / s to obtain a microstructure that is substantially entirely martensite and a final tensile strength of about 1500 MPa.

[0017] More recently, hot stamping of higher grade steels has been the subject of intensive research. For example, 37MnB5 steel or similar steels have a higher carbon content than 22MnB5 and can achieve a final tensile strength of 2000 MPa or even higher after hot stamping. Usibor® 2000, commercially available from ArcelorMittal®, and MBW-K® 1900 (34MnB4 steel), commercially available from ThyssenKrupp®, are two such steels.

[0018] Such steels can be called “second-generation” press-hardened steels. This newer generation of press-hardened steels may be considered, as herein, boron-manganese steels suitable for hot stamping, having a higher carbon content than 22MnB5 and achieving a final tensile strength exceeding 1500 MPa, particularly at least 1700 MPa, when fully formed into martensite after press hardening. The silicon content is usually lower than that of first-generation press-hardened steels. Examples include 20MnCr, 28MnB5 steels (e.g., Docol PHS1800), 30MnB5 (e.g., SQ1800), 34MnB5 or 34MnB4 (e.g., MBW1900), and 37MnB5 or 37MnB4 (e.g., Usibor2000HPF2000, Docol PHS2000, phs ultraform2000). New generations of steel can also be described as PHS1700 and later, i.e., PHS1700, PHS1800, PHS1900 or PHS2000, or more specifically, PHS1900 and later.

[0019] However, another characteristic of these steels is that they become brittle and have very low ductility after hot stamping. This means that post-processing operations, such as joining, become more troublesome, and energy absorption in the event of a collision or crash is limited. This is one reason why, despite having high tensile strength and the potential to further reduce the weight of automotive structures, these steels have not been practically applied to automotive structures to this day. [Overview of the Initiative]

[0020] This disclosure aims to provide improvements in hot stamping processes for high-grade steel, particularly in multi-stage processes and apparatus.

[0021] In a first embodiment, a method is provided for hot forming structural components in a multi-stage production line. The multi-stage production line comprises a press tool configured to draw a blank, the press tool comprising an upper press die and a lower press die. The production line further comprises a first post-press tool located downstream of the press tool and configured to perform a first post-press operation, the post-press tool comprising a first post-press upper die and a first post-press lower die. The upper press die and the first post-press upper die are configured to operate integrally. The production line further comprises a transfer system for transferring a blank from the press tool to the first post-press tool.

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

[0023] According to this embodiment, an efficient method is provided for manufacturing steel structural components with increased carbon content, which can incorporate one or more post-processing steps. The throughput of the production line can be increased or maximized. Furthermore, structural components with desired mechanical properties can be obtained, with a final tensile strength exceeding 1600 MPa, for example, 1700 MPa to 1900 MPa, along with an A50 elongation of 5% or more, particularly 5-6%. The high final tensile strength makes it possible to reduce the thickness and, consequently, the weight of the components. The increased ductility of such steel compared to conventional press hardening improves its behavior in the event of impact / crush and facilitates further post-processing.

[0024] Yield strength, final tensile strength, and A50 elongation can be measured according to standard tensile strength tests for metallic specimens as defined, for example, ISO 6892-1. A50 strength refers to the elongation at fracture (to distinguish it from, for example, A80) for a specimen with an initial length of 50 mm.

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

[0026] A multi-stage production line may be defined herein as a production line in which several processes are performed simultaneously on different blanks / parts. The work of the different processes is performed in a coordinated manner, i.e., the movements of different tools are harmonized in such a way that the cycle times for each stroke are identical. In detail, the movements of different tools can be synchronized. In some examples, the different processes and tools may be integrated into a single press device or into separate devices.

[0027] This specification refers to press-hardened boron steel having a carbon content of 0.32–0.45% by weight, particularly 0.32–0.4% by weight, more specifically 0.32–0.38%, a manganese content of 0.6–1.5%, particularly 0.6–1.4%, and a boron content of 0.003–0.006%, particularly 0.004–0.005% by weight. In practice, there may be some variation between different steel coils, even when produced by the same steel supplier and using the same manufacturing process. In practice, there may even be some variation between blanks cut from the same steel coil. Suitable steels include, for example, 37MnB5 steel, 38MnB5 steel, 34MnB5 steel, and 34MnB4 steel.

[0028] One typical composition of the tested 34MnB4 steel is summarized below in weight percent, with the balance being iron (Fe) and impurities. Carbon (C) (%): 0.35 Silicon (Si) (%): 0.8 Manganese (Mn) (%): 0.96 Chromium (Cr) (%): 0.19 Molybdenum (Mo) (%): 0.18 Phosphorus (P) (%): 0.012 Sulfur (S) (%): 0.0002 Titanium (Ti) + Niobium (Nb) (%): 0.055 Aluminum (Al) (%): 0.03 Boron (B) (%): 0.002 Nitrogen (N) (%): 0.003 Nickel (Ni) (%): 0.01

[0029] Usibor® 2000 can generally be described as 37MnB5 steel. The composition of Usibor® 2000 is summarized below in weight percent, with the balance being iron (Fe) and impurities. Maximum carbon (C) (%): 0.36 Maximum silicon (Si) (%): 0.8 Maximum manganese (Mn) (%): 0.8 Maximum phosphorus (P) (%): 0.03 Maximum sulfur (S) (%): 0.01 Aluminum (Al) (%): 0.01 to 0.06 Maximum titanium (Ti) (%): 0.07 Maximum niobium (Nb) (%): 0.07 Maximum copper (Cu) (%): 0.20 Maximum boron (B) (%): 0.005 Maximum chromium (Cr) (%): 0.50 Maximum molybdenum (Mo) (%): 0.50

[0030] MBW-K(registered trademark) 1900 can generally be described as 34MnB4 steel. The composition of MBW-K(registered trademark) 1900 is summarized below in weight percentages (the remainder being iron (Fe) and impurities). Maximum carbon (C) content (%): 0.38 Maximum value of silicon (Si) (%): 0.4 Maximum value of manganese (Mn) (%): 1.4 Maximum value of phosphorus (P) (%): 0.025 Maximum sulfur (S) content (%): 0.01 Maximum value of aluminum (Al) (%): 0.015 Maximum value (%) for titanium (Ti): 0.05 Maximum value of niobium (Nb) (%): 0.07 Maximum value of boron (B) (%): 0.005 Maximum value (%) of chromium (Cr) + molybdenum (Mb): 0.50

[0031] MBW® 1900 is another manganese-boron steel manufactured by ThyssenKrupp®, capable of having a final tensile strength of 1900 MPa. This steel is commercially available with an aluminum-silicon coating and is suitable for hot stamping and the methods disclosed herein. The chemical composition of MBW® 1900 is summarized below in weight percent (the remainder being iron (Fe) and impurities). Maximum carbon (C) content (%): 0.38 Maximum value of silicon (Si) (%): 0.40 Maximum value of manganese (Mn) (%): 1.40 Maximum value of phosphorus (P) (%): 0.025 Maximum sulfur (S) content (%): 0.010 Maximum value of aluminum (Al) (%): 0.1 Maximum value of niobium (Nb) (%): 0.05 Maximum value (%) for titanium (Ti): 0.05 Maximum value (%) for chromium and molybdenum (Cr+Mo): 0.50 Maximum value of boron (B) (%): 0.005

[0032] B1800HS is yet another boron steel that can have a final tensile strength of approximately 1800 MPa and is suitable for hot stamping and the methods disclosed herein. The chemical composition of B1800HS is summarized below in weight percent (the remainder being iron (Fe) and impurities). Carbon (C) (%): 0.28~0.38 Maximum value of silicon (Si) (%): 0.5 Manganese (Mn) (%): 1.0~1.8 Maximum value of phosphorus (P) (%): 0.025 Maximum sulfur (S) content (%): 0.010 Aluminum (Al) (%): 0.01~0.06 Maximum value (%) for titanium (Ti): 0.05 Maximum value of boron (B) (%): 0.0050 Maximum value (%) of chromium, molybdenum, and niobium (Cr+Mo+Nb): 0.80

[0033] A further preferred steel is CR1900T-MB-DS. Its chemical composition is summarized below in weight percentages (the remainder being Fe and impurities). Carbon (C)(%):0.30~0.38 Maximum value of silicon (Si) (%): 0.8 Manganese (Mn) (%): 2.0 Maximum chromium (Cr) value (%): 0.25 Maximum value of phosphorus (P) (%): 0.03 Maximum value of molybdenum (Mo) (%): 0.25 Maximum sulfur (S) content (%): 0.005 Aluminum (Al) (%): 0.01~0.08 Maximum value (%) for titanium (Ti) + niobium: 0.20 Boron (B) (%): 0.0010~0.0050 Maximum value of nickel (Ni) (%): 0.10 Maximum nitrogen (N) value (%): 0.01

[0034] In several examples, the blank may be heated above the Ac3 temperature, specifically to 870–930°C, and more specifically to 900–930°C. The heating can be carried out in detail in a furnace located upstream of a multi-stage production line.

[0035] In some examples, a multi-stage production line is located downstream of a first post-press tool and configured to perform a second post-press operation, further comprising a second post-press tool having a second post-press upper die and a second post-press lower die, the second post-press upper die being configured to operate integrally with the upper press die and the first post-press upper, and a transfer system further configured to transfer the formed blank from the first post-press tool to the second post-press tool. In several 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 include a temperature control system that controls the temperature of the blank and, in particular, ensures that the temperature of the blank does not drop too much or too quickly, for example, thermocouples and heaters and / or cooling conduits within the tool. The temperature being between 350 and 450°C allows for further work.

[0036] In some examples, the multi-stage production line further comprises one or more additional tools located downstream of the second post-press work tool and configured to operate integrally with the press tool and the first and second post-press tools. Several different tools may be provided in the multi-stage production line, including one or more tools for trimming, one or more tools for drilling, cutting, etc. In some examples, the additional tools may include tools for reforging, i.e., tools that perform further steps to shape the already deformed blank.

[0037] In some examples, the final additional tool may be a reforging tool. The temperature of the formed blank before reforging may be, for example, between 250°C and 350°C. The temperature of the formed blank may be high enough to maintain the malleability of the material and avoid unnecessary wear of the tool.

[0038] In several examples, the press tool's work cycle may be between 2 and 5 seconds, particularly between 2.5 and 4 seconds, and more specifically, around 4 seconds. A work cycle of approximately 4 seconds maintains high throughput and allows for a clamping time of, for example, 0.5 seconds at the press's bottom dead center. This clamping time allows for increased temperature control during the process.

[0039] In several examples, the transfer time between the press tool and the first post-press tool may be between 1 and 3 seconds, particularly between 1.5 and 2.5 seconds, and more specifically, about 2 seconds. The transfer of the blank may be performed or at least started before the upper die reaches its upper position.

[0040] In some cases, the resulting parts may be subjected to a further baking hardening process. For example, structural parts may be exposed to a temperature of approximately 180°C for approximately 20 minutes. The baking hardening process can further increase the yield strength of the resulting parts while keeping the final tensile strength substantially the same.

[0041] In some examples, a multi-stage production line comprises a multi-stage tool having a fixed lower body, a movable upper body, and a mechanism configured such that the movable upper body advances vertically relative to the lower body for pressing, with the upper press die connected to the movable upper body, the lower press die connected to the fixed lower body, the first post-press upper die connected to the movable upper body, and the first post-press lower die connected to the fixed lower body. A multi-stage production line can be incorporated into a multi-stage device with press tools and into further post-press tools in a single press device.

[0042] By integrating the tools within the same device by connecting the upper die and additional tools of the press tool to a movable upper body, the transfer time between the press tool and additional tools can be reduced, thus optimizing the process and improving productivity. Furthermore, the temperature of the blank can be improved during different steps of the process.

[0043] In some examples, the die of a cooling tool may be provided with conduits for passing cooling water. The die of a cooling tool may also be provided with conduits for passing air, either alternatively or additionally.

[0044] In some examples, the austenitizing temperature at which the blank can be heated may be the Ac3 temperature, and cooling a fully heated blank involves cooling the blank between 600 and 800°C, particularly between 650 and 700°C.

[0045] In some examples, a multi-stage apparatus may further include a first post-working tool downstream of the press tool, the first post-working tool comprising a first post-working upper die and a first post-working lower die, each having one or more working surfaces that face the formed blank during use, the first post-working lower die being connected to a lower body and the first post-working upper die being connected to an upper body.

[0046] In some examples, the first post-press tool may include a temperature control system for controlling the temperature of the blank formed during the first post-processing, the temperature control system optionally including thermocouples on the first post-processing upper die and the first post-processing lower die.

[0047] In some examples, the die of the first post-press tool may be equipped with conduits for passing cooling water or cooling air. In some examples, the die of the first post-press tool may be equipped with one or more heaters or conduits for passing hot liquid, or conductive heating devices.

[0048] In a further embodiment, a component is provided which can be obtained by any of the methods disclosed herein.

[0049] Non-limiting examples of this disclosure are described below with reference to the attached drawings. [Brief explanation of the drawing]

[0050] [Figure 1] This is a schematic diagram of a multi-stage production line as an example. [Figure 2] This is a schematic graph of an example of the blank temperature in the manufacturing process according to an example of this disclosure. [Figure 3] This is a schematic diagram of a multi-stage apparatus that can be used in an example of the method described herein. [Modes for carrying out the invention]

[0051] Figure 1 schematically shows an example of a multi-stage production line. An example of a method for hot forming structural components in a multi-stage production line is schematically shown in Figure 1.

[0052] A steel coil 10 of press-hardened boron steel. Press-hardened boron steel with a composition of 0.32-0.45% by weight, a manganese content of 0.6-1.5%, and a boron content of 0.003-0.006% may be used. A punching machine 20 can cut blanks of appropriate dimensions from the coil 100. The steel has an aluminum-silicon coating.

[0053] The blank may preferably be 0.8 to 2 mm thick, particularly 1 to 1.6 mm thick. In some examples, the steel may be 34MnB4 or 37MnB5 steel.

[0054] In specific examples, steel may have a carbon content of 0.34–0.4% by weight, particularly 0.34–0.39%, and more specifically 0.34–0.38%. Steel may also have a manganese content of 0.8–1.4%. Steel may have a boron content of 0.003–0.005% by weight, particularly 0.004–0.005%. In specific examples, steel may have a silicon content of 0.3–0.9% by weight, particularly 0.4–0.8%.

[0055] The blank can be supplied to furnace 30. The blank may be heated above the austenitizing temperature, more specifically above the Ac3 temperature. In a specific example, the blank may be heated in the furnace for 5 to 10 minutes, between approximately 870 and 930°C, and particularly between 900 and 930°C.

[0056] After leaving the furnace 30, the heated blank can be transferred to the production line 100. The production line 100 in the example shown in Figure 1 includes a press tool 50 configured for drawing the blank.

[0057] The press tool 50 comprises an upper press die 52 and a lower press die 54. The blank can be positioned between the upper press die 52 and the lower press die 54. The upper press die 52 moves vertically relative to the lower press die 54, allowing the preheated blank to be deformed.

[0058] The production line 100 further comprises a first post-press tool 60 located downstream of the press tool 50 and configured to perform a first post-press operation on a formed blank (a blank that has undergone a drawing process in the press tool). The first post-press tool comprises a first post-press upper die 62 and a first post-press lower die 64. The upper press die 52 and the first post-press upper die 62 are configured to operate integrally. In detail, the vertical movement of the first post-press upper die 62 can be synchronized with the vertical movement of the upper press die 52.

[0059] The production line 100 further includes a transfer system for transferring the formed blanks from the press tool 50 to the first post-press tool 60. In this particular example, there are multiple transfer robots having gripping or suction units. The transfer robot 44 can grip the blanks that have been deformed after the drawing process in the press tool 50 and place them in the first post-press tool 60.

[0060] The terms “post-press tool” and “post-press operation” refer to any tool or operation that is placed or occurs downstream of the press tool, respectively. The first post-press tool 60 may be, for example, a trimming tool for trimming multiple edges of a deformed blank.

[0061] The production line 100 in this example further includes 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 integrally with the first post-press upper die 62 and the press upper die 52, and in detail, the movement of the upper die can be synchronized.

[0062] A further transfer robot 46 can grasp the blank after the first post-press operation and place it in the next post-press tool. An example of the second post-press tool may be a cutting tool or a drilling or hole-making tool.

[0063] In this particular example, an additional tool 80 is provided, comprising an upper die 82 and a lower die 84. The upper die 82 can be configured to operate in a coordinated manner, specifically in synchronization with the other upper die. A transfer robot 48 can transfer the blank from the second post-press tool 70 to the additional tool 80. After the final operation in tool 80, the blank may be transferred and stacked by a suitable robot 49.

[0064] In other examples, it should be clear that different tools and different numbers of tools may be provided. In one specific example, a production line may include six tools, including a press tool. The last tool on the production line may be a reforging tool for finally shaping the part.

[0065] The production line shown in Figure 1 may be used for manufacturing a variety of structural components. In certain examples, structural components with a length and width greater than 1 meter can be manufactured on such a production line. In some examples, integrated door rings (door rings made from a single, integrally formed body) can be manufactured on such a production line. An integrated door ring may be, for example, a single door ring (extending from pillar A to pillar B, or from pillar B to pillar C) or a double door ring (extending from pillar A to pillar C).

[0066] Further large structural components that may be manufactured on production line 100 as shown in Figure 1 include, for example, a roof ring unit, a bumper beam assembly including a bumper beam and a pedestrian beam, a ring unit surrounding a battery box, and a rear framework structure including a rear rail and a transverse beam as a unit structure.

[0067] Such large structural components can be efficiently manufactured in a hot stamping process. Due to the size of these components, it is difficult to integrate different tools into a single press machine. However, this method and system can also be used for other press-formed parts such as B-pillars, A-pillars, hinge pillars, and bumper crossbeams.

[0068] In some cases, a blank may consist of multiple smaller blanks or subblanks. Some of the subblanks can be joined to other subblanks by butt joints between the edges of the tailored blank (TWB). Alternatively, subblanks can be partially overlapped to form overlapping regions with increased thickness compared to other areas. These increased-thickness regions may be selected to locally reinforce the structural component. The increased thickness in these regions may result in increased heating time in the furnace. In the drawing process and subsequent processes, the overlapping regions may cool more slowly unless specific measures are taken.

[0069] In some cases, biasing elements, such as springs, may be integrated into the upper and / or lower press dies so that some of the interlocking die blocks of the press tool (pairs of the upper and lower press tool die blocks facing each other) come into contact with the blank before other die blocks make contact. In other words, this die block selection can be closed earlier than other die blocks.

[0070] In these die blocks where contact is established more quickly, cooling may begin earlier and be more rapid than in other die blocks. If overlapping areas cool more rapidly, a die block biased for those areas can be used. In addition, or separately, one or more die blocks may have integrated heating means to avoid excessively rapid cooling.

[0071] A method for hot forming a structural component includes heating a blank above its austenitizing temperature, for example, to 900-920°C. This method further includes drawing the heated blank in a press tool 50 and transferring the blank from the press tool 50 to a first post-press tool 60.

[0072] The temperature of the blank before drawing was at least 600°C, and especially at least 650°C. If the temperature was below 600°C during drawing, streaks were formed in the coating. If the temperature was 600°C or higher, the coating was acceptable after the hot forming process.

[0073] The temperature of the blank before drawing may be between 700°C and 800°C. The blank may cool from the moment it leaves the furnace and during its transfer from the furnace to the press tool. The transfer from the furnace to the press tool may take approximately 4 seconds in one example.

[0074] The temperature of the blank before the first post-press operation may be between 500 and 600°C, for example, about 550°C. The temperature of the blank at the end of the first post-press operation may be between 400 and 500°C, for example, about 450°C.

[0075] In the illustrated example, the temperature of the blank at the end of the second post-press operation is between 350°C and 450°C.

[0076] The press tool (and other tool) work cycle may be between 2 and 5 seconds, particularly between 2.5 and 4.5 seconds, more specifically about 4 seconds. The transfer time between the press tool and the first post-press tool (and other tools) may be between 1 and 3 seconds, particularly between 1.5 and 2.5 seconds, more specifically about 2 seconds.

[0077] As disclosed herein, it has been found that slower cooling of the steel increases ductility and ultimately leads to higher tensile strength. At the same time, this manufacturing process is highly efficient. In these examples, the average cooling rate from the initial press forming step to the Ms temperature (e.g., about 350°C) can be less than 20°C / s, and in particular less than 15°C / s. The final cooling from the Ms temperature to the Mf temperature can be even lower, for example less than 10°C / s.

[0078] The last tool 80 may be a restriking tool. The temperature of the blank before restriking may be between 250°C and 350°C. Inside the restriking tool, the blank can be cooled to below 200°C. Resriking can be used to avoid or reduce tolerances or geometric deviations from the intended design, resulting in structural parts with precisely defined geometry. Press tools, and all other tools, may be provided with a suitable temperature control system including a heater and / or cooling system. Such a system may be provided in the upper die and / or lower die.

[0079] In detail, any of these tools may include conduits for passing a heat transfer medium. In detail, such conduits may be cooling conduits for passing chilled water. After the corresponding die has been cooled, the heated water can pass through a heat exchanger for further cooling. Any of these tools may optionally or additionally include a heater, such as an electric heater, to ensure that the blank does not cool too quickly.

[0080] Figure 2 schematically shows a graph of the temperature of a blank after it leaves the furnace and passes through various tools on the production line. In the example in Figure 2, the temperature of the blank can decrease from approximately 920-930°C to approximately 750°C before the deep drawing operation in the press tool 50.

[0081] The temperature is reduced to approximately 550°C before the first post-press operation on tool 60. The cooling rate for the press operation can be approximately 50°C / s.

[0082] Subsequently, the temperature may decrease to approximately 450°C during the first post-press operation, before the second post-press operation. The cooling rate in this particular section may be approximately 25°C / s.

[0083] At the second post-press work tool 70 and before the additional tool 80, the temperature may decrease to approximately 400°C with a cooling rate of approximately 12.5°C / s. At the additional tool 110, the temperature may decrease further to 350°C with a cooling rate of approximately 12.5°C / s.

[0084] In this particular example, the last tool 120 may be a reforging tool. The temperature of the blank may be further cooled in the reforging tool.

[0085] Experimental results concerning steels including 37MnB5 and 34MnB4 steel yielded the following results: Structural components obtained through the above process may have a final tensile strength exceeding 1600 MPa, particularly 1600 MPa to 1900 MPa, more specifically 1600 MPa to 1800 MPa, more specifically 1700 to 1800 MPa, or approximately 1700 MPa. Furthermore, an A50 elongation of 5% or more can be obtained. The resulting structural components may have a yield strength of 1000 MPa or more, particularly 1100 to 1400 MPa or 1100 to 1300 MPa. Three-point bending tests may reveal bending angles of 40 to 55° in both directions, particularly approximately 50°.

[0086] In some cases, the resulting structural components can be subjected to a further bake-hardening process. The bake-hardening temperature may be between 170°C and 200°C, and the bake-hardening time may be between 15 and 25 minutes. Baking-hardening can increase the yield strength of the structural components to 1050 MPa or higher, particularly to 1200 MPa or higher. Thus, the energy absorption and toughness of the material can be increased. These results have been experimentally confirmed for the steels mentioned above.

[0087] Figure 3 schematically shows a multi-stage apparatus that may be used in an example of the method according to the present disclosure. The multi-stage tool 90 comprises a fixed lower body 88, a movable upper body 86, and a mechanism (not shown further) configured to cause the movable upper body 86 to advance vertically relative to the lower body 88 for pressing.

[0088] The upper press die 52 is connected to a movable upper body 86, the lower press die 54 is connected to a fixed lower body 88, the first post-press upper die 62 is connected to a movable upper body 86, and the first post-press lower die 64 is connected to a fixed lower body 88.

[0089] Similarly, in this example, the upper die 72 of the second post-press tool is connected to a movable upper body 86, and the lower die 74 of the second post-press tool is connected to a fixed lower body 88.

[0090] Similarly, the upper die 82 of an additional tool may be connected to a movable upper body 86, and the lower die 84 of an additional tool may be connected to a fixed lower body 88. Thus, all the upper dies 52, 62, 72, and 82 move in a coordinated manner toward or away from the lower dies 54, 64, 74, and 84, respectively.

[0091] The fixed lower body 88 may be a large metal block. In this particular example, the fixed lower body 88 may be stationary. In some examples, a die cushion (not shown) may be integrated with the fixed lower body 88. The cushion may be configured to receive and control the force gripping the blank. The movable upper body 86 may also be a solid piece of metal. The movable upper body 86 may provide a stroke cycle (up and down movement).

[0092] The press system may be configured to perform approximately 15 strokes per minute, for example, and therefore each stroke cycle may be approximately 4 seconds. The stroke cycle may vary in further examples. In a multi-stage press system, all operations performed on the blank must have the same cycle time. Note that a stroke cycle of 4 seconds means that the blank or the formed blank may undergo several operations during a cycle of approximately 2 or 2.5 seconds, and that transferring the blank between stations may take approximately 1.5 to 2 seconds.

[0093] The press mechanism may be driven mechanically, hydraulically, or servo-driven. The movement of the movable upper body 86 relative to the fixed lower body 88 is determined by the mechanism. In this particular example, the press may be a servo-driven press, and thus can provide a constant pressing force during the stroke. A servo-driven press can provide unlimited sliding (impact) speed and position control. A servo-driven press can also provide that a good range of pressing force is available at any sliding position, thus enabling greater flexibility of the press. A servo-driven press has the potential to improve process conditions and productivity in metal forming. This press may have, for example, a pressing force of 2000 tons.

[0094] In some examples, the press may be a mechanical press, and therefore the movement of the pressing force toward the fixed lower body 88 may depend on the drive and hinge system. Thus, a mechanical press may reach more cycles per unit time. Alternatively, a hydraulic press may be used.

[0095] In some examples, one or more lower dies 54, 64, 74, 84 may be connected to a lower body 88 having a downward biasing element configured to bias the lower dies to a predetermined first distance from the lower body 88. In some examples, there may be a single downward biasing element, or there may be more than two biasing elements. The biasing element may consist of, for example, a spring, such as a mechanical spring or a gas spring, but several other biasing elements, such as a hydraulic mechanism, may be possible.

[0096] In some examples, one or more upper dies 52, 62, 72, 82 may also be connected to an upper body 86 having one or more upward biasing elements configured to bias the upper dies and position them at a predetermined second distance from the upper body.

[0097] By inserting upward biasing elements and / or downward biasing elements, the contact time between the upper die and the lower die can be adjusted and increased during the stroke cycle (the vertical movement of the movable upper body 86 relative to the lower body 88).

[0098] By using such biasing elements, the cooling tool can have a different cycle time than other tools integrated in the same apparatus. This is described in detail in European Patent Application Publication No. 3067128. However, within the scope of this disclosure, the use of biasing elements is merely optional. Depending on the blank steel and its coating, biasing elements may not be necessary at all. As mentioned herein, such biasing elements can be used to close the entire tool or “die.” In several examples, biasing elements can be used solely to select the die block.

[0099] Press tools configured for forming or drawing blanks are also integrated into the same press apparatus. The upper press die 52 may have an upper working surface that faces the blank to be hot-formed during use. The lower press die 54 may have a lower working surface that faces the blank to be hot-formed during use. The side of the upper die opposite the upper working surface can be fastened to the upper body 86, and the side of the lower die opposite the lower working surface can be fastened to the lower body 88.

[0100] The meshing upper die 52 and lower die 54 may be provided with conduits through which a cold fluid, such as water and / or cold air, passes. In the case of water conduits, the velocity circulation of the water in the conduit may be high so as to avoid evaporation of the water. A control system may be further provided that can control the temperature and flow velocity of the fluid based on temperature measurements so as to control the temperature of the die.

[0101] In several examples, the press tool may be provided with a blank holder configured to grip the blank and position the blank on the lower die 54. The blank holder may also be provided with a spring, for example, to bias the blank holder to a predetermined distance from the lower die 22.

[0102] In this example, a first post-press tool configured to perform trimming and / or perforation operations is provided on the same multi-stage press machine. The first post-press tool is located downstream of the press tool. The first post-press tool comprises a meshing upper die 62 and a meshing lower die 64. The meshing upper die 62 may have an upper working surface, and the meshing lower die 64 may have a lower working surface. Both working surfaces in use face the blank. The side of the upper die 62 opposite the upper working surface can be fastened to an upper body 86, and the side of the lower die 64 opposite the lower working surface can be fastened to a lower body 88. The die may have one or more knives or cutting edges (not shown) positioned on the working surface.

[0103] The first post-press tool may further include one or more heaters or conduits for passing hot liquid, and temperature sensors for controlling the die temperature. The sensors may be thermocouples. In some examples, the meshing upper die 62 and lower die 64 may be provided with conduits through which a cold fluid, such as water and / or cold air, passes in the conduits provided in the die.

[0104] In several examples, the first post-press tool may be provided with a blank holder (not shown) configured to grip the blank and position the blank on 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 predetermined distance from the lower die.

[0105] In this example, a second post-press tool and additional tools are provided, having interlocking upper and lower dies. The descriptions of the dies of the press tool and the 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 / or perforation work.

[0106] The die may have one or more knives or cutting edges (not shown) positioned on the work surface.

[0107] Although the diagrams show dies that are substantially square or rectangular in shape, it should be understood that the blocks may have any other shape, and may even be partially circular.

[0108] An automated transfer system (not shown), such as multiple industrial robots or conveyors, or a beam section with gripping elements, may also be provided to transfer blanks between tools. Since the transfer devices can be integrated within the same press system, transfer time is reduced and temperature control is improved.

[0109] In all examples, the temperature sensor and the control system for controlling the temperature may be located within any tool or transfer system. The tool may also be equipped with an additional cooling system, a blank holder, etc.

[0110] In some cases, a centering element, such as a pin and / or induction device, may be provided upstream of the cooling tool so that the blank can be properly centered.

[0111] For completeness, various aspects of this disclosure are described in the following numbered clauses.

[0112] Clause 1: A method for hot forming structural components in a multi-stage production line, wherein the multi-stage production line is A press tool configured for drawing a blank, comprising an upper press die and a lower press die, A first post-press tool positioned downstream of a press tool and configured to perform a first post-press operation, comprising a first post-press upper die and a first post-press lower die, wherein the upper press die and the first post-press upper die are configured to operate integrally; A transfer system for transferring the formed blank from the press tool to the first post-press tool. The method comprises, To provide press-hardened boron steel blanks, Heating the blank to a temperature above the austenitization temperature, The heated blank is drawn using a press tool, and the blank is transferred from the press tool to a first post-press tool. Includes, The temperature of the blank before drawing is at least 600°C, and especially at least 650°C. The temperature of the blank before the first post-press operation was between 500°C and 650°C. The first method involves ensuring the temperature at the end of the post-press operation is between 400°C and 550°C.

[0113] Clause 2: The method according to Clause 1, wherein the temperature of the blank before drawing is between 700°C and 800°C.

[0114] Clause 3: The method according to Clause 1 or 2, wherein the blank is heated to 870-930°C, particularly to 900-930°C.

[0115] Clause 4: The method according to any one of Clauses 1 to 3, wherein the temperature of the formed blank before the first post-press operation is between 500 and 600°C.

[0116] Clause 5: The temperature at the end of the first post-press operation is between 400 and 500°C, as described in any one of Clauses 1 to 4.

[0117] Clause 6: A press-hardened boron steel blank according to any one of Clauses 1 to 5, wherein the content is 0.32–0.45% by weight, the manganese content is 0.6–1.5%, and the boron content is 0.003–0.006%.

[0118] Clause 7: The press-hardened boron steel blank according to Clause 6, wherein the content is 0.32-0.4% by weight, the manganese content is 0.6-1.4%, and the boron content is 0.004-0.005%.

[0119] Clause 8: The method described in any one of Clauses 1 to 7, wherein the steel blank is made by PHS1900 or PHS2000.

[0120] Clause 9: The method according to any one of Clauses 1 to 8, wherein a press-hardened boron steel blank is covered.

[0121] Clause 10: The method according to Clause 9, wherein the press-hardened boron steel blank has an AlSi coating.

[0122] Clause 11: The method according to any one of Clauses 1 to 10, wherein a multi-stage production line is located downstream of a first post-press tool and configured to perform a second post-press operation, further comprising a second post-press tool having a second post-press upper die and a second post-press lower die, wherein the second post-press upper die is configured to operate integrally with the upper press die and the first post-press upper, and a transfer system is further configured to transfer a blank from the first post-press tool to the second post-press tool.

[0123] Clause 12: The method according to Clause 11, wherein the temperature of the blank at the end of the second post-press operation is between 350 and 450°C.

[0124] Clause 13: The method according to Clause 11 or 12, wherein the multi-stage production line further comprises one or more additional tools located downstream of the second post-press work tool and configured to operate integrally with the press tool, as well as the first and second post-press tools.

[0125] Clause 14: The method described in Clause 13, wherein the last additional tool is a reforging tool.

[0126] Clause 15: The method according to Clause 14, wherein the temperature of the blank before reforging is 250°C to 350°C.

[0127] Clause 16: The method described in any one of Clauses 1 to 15, wherein the working cycle of the press tool is between 2 and 5 seconds, particularly between 2.5 and 4.5 seconds, and more specifically, about 4 seconds.

[0128] Clause 17: The method according to any one of Clauses 1 to 16, wherein the transfer time between the press tool and the first post-press tool is between 1 and 3 seconds, particularly between 1.5 and 2.5 seconds, and more specifically, about 2 seconds.

[0129] Clause 18: The method described in any one of Clauses 1 to 17, wherein the first post-press tool is a cutting, trimming, or hole-making tool.

[0130] Clause 19: The method according to any one of Clauses 1 to 18, wherein the resulting structural component has a final tensile strength greater than 1600 MPa, particularly between 1600 MPa and 1800 MPa, and more specifically between 1700 MPa and 1800 MPa.

[0131] Clause 20: The method according to any one of Clauses 1 to 19, wherein the resulting structural component has an A50 elongation of 5% or more.

[0132] Clause 21: The obtained structural component having a yield strength of 1000 MPa or more, particularly 1100 to 1300 MPa, according to the method described in any one of Clauses 1 to 20.

[0133] Clause 22: The method according to any one of Clauses 1 to 21, further comprising a bake-curing step of the obtained structural component, wherein the bake-curing temperature is between 170°C and 200°C and the bake-curing time is between 15 and 25 minutes.

[0134] Clause 23: The method according to Clause 22, wherein the yield strength of the resulting structural component is 1050 MPa or more, and in particular 1200 MPa or more.

[0135] Clause 24: The method according to any one of Clauses 1 to 23, wherein the length of the formed structural component is at least 1 meter, particularly 1 to 2 meters, and the width of the formed structural component is at least 1 meter, specifically 1 to 2 meters.

[0136] Clause 25: The method according to Clause 24, wherein the structural component is a single door ring, and the single door ring is one of the following: a hinged pillar and a front door ring extending from the A-pillar to the B-pillar, a rear door ring extending from the B-pillar to the C-pillar, or a hinged pillar and a double door ring extending from the A-pillar to the C-pillar.

[0137] Clause 26: The method according to Clause 24, wherein the structural component is a roof ring unit, a bumper beam assembly including a bumper beam and a pedestrian beam, or a reinforcing ring unit surrounding a battery box, or a rear framework structure including a rear rail and a transverse beam as a unit structure.

[0138] Clause 27: The method according to any one of Clauses 1 to 26, wherein the blank is 0.8 mm to 2 mm thick, particularly 1 to 1.6 mm thick.

[0139] Clause 28: The method according to any one of Clauses 1 to 27, wherein the blank is made of 34MnB4 steel or 34MnB5 steel.

[0140] Clause 29: The method according to any one of Clauses 1 to 27, wherein the blank is made of 37MnB5 steel or 37MnB4 steel.

[0141] Clause 30: A steel blank having a carbon content of 0.34–0.38% by weight and a manganese content of 0.8–1.4% by weight, as described in any one of Clauses 1 to 29.

[0142] Clause 31: The steel blank is made according to the method of Clause 30, wherein the boron content is 0.004-0.005% by weight.

[0143] Clause 32: The steel blank according to Clause 30 or 31, wherein the silicon content by weight is 0.3–0.9%, particularly 0.4–0.8%.

[0144] Clause 33: The method according to any one of Clauses 1 to 32, wherein a multi-stage production line comprises a multi-stage tool comprising a fixed lower body, a movable upper body, and a mechanism configured such that the movable upper body advances vertically relative to the lower body for pressing, wherein an upper press die is connected to the movable upper body, a lower press die is connected to the fixed lower body, a first post-press upper die is connected to the movable upper body, and a first post-press lower die is connected to the fixed lower body.

[0145] Clause 34: A structural component that can be obtained by the method described in any one of Clauses 1 to 33.

[0146] Although only a few examples are disclosed herein, other alternative examples, variations, uses, and / or equivalents thereof are possible. Furthermore, all possible combinations of the examples described are also covered. Accordingly, the scope of this disclosure should not be limited by any particular example, but should be determined solely by a fair reading of the following claims.

Claims

1. A method for hot forming structural components in a multi-stage production line, wherein the multi-stage production line is A press tool configured to draw a blank, comprising an upper press die and a lower press die, A first post-press tool positioned downstream of the press tool and configured to perform a first post-press operation, comprising a first post-press upper die and a first post-press lower die, wherein the upper press die and the first post-press upper die are configured to operate integrally; A transfer system for transferring a blank from the press tool to the first post-press tool, The method comprises, The objective is to provide a boron steel blank having preferably 0.32 to 0.45% carbon, 0.6 to 1.5% manganese, and 0.003 to 0.006% boron by weight, and optionally having an AlSi coating. Heating the blank above the austenitizing temperature, The heated blank is drawn using the press tool, and the formed blank is transferred from the press tool to the first post-press tool. Includes, The temperature of the blank before the drawing process is at least 600°C, and more particularly at least 650°C. The temperature of the blank formed before the first post-press operation is between 500°C and 650°C. A method wherein the 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-hardened boron steel blank has a content of 0.32 to 0.38% by weight, a manganese content of 0.6 to 1.4% by weight, and a boron content of 0.004 to 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, particularly between 700°C and 800°C.

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

5. The method according to any one of claims 1 to 4, wherein the multi-stage production line is located downstream of the first post-press tool and configured to perform a second post-press operation, further comprising a second post-press tool having a second post-press upper die and a second post-press lower die, wherein the second post-press upper die is configured to operate integrally with the upper press die and the first post-press upper, and the transfer system is further configured to transfer the formed blank from the first post-press tool to the second post-press tool.

6. The method according to claim 5, wherein the 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-stage production line further comprises one or more additional tools positioned downstream of the second post-press work tool, the additional tools being configured to operate integrally with the press tool and the first post-press tool and the second post-press tool.

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

9. The working cycle of the aforementioned press tool is between 2 and 5 seconds, particularly between 2.5 and 4.5 seconds, and more specifically, about 4 seconds. The method according to any one of claims 1 to 8, wherein the transfer time between the press tool and the first post-press tool is between 1 and 3 seconds, particularly between 1.5 and 2.5 seconds, more specifically about 2 seconds, and optionally the transfer of the blanks occurs before the upper dies reach their uppermost positions.

10. The resulting structural component has a final tensile strength exceeding 1600 MPa, particularly 1600 MPa to 1800 MPa, and more specifically, about 1700 MPa. The method according to any one of claims 1 to 9, wherein the obtained structural component has a yield strength of 1000 MPa or more, particularly 1100 to 1300 MPa.

11. The method according to any one of claims 1 to 10, wherein the obtained structural component has an A50 elongation of 5% or more.

12. The method according to any one of claims 1 to 11, further comprising baking-curing the obtained structural component, wherein the baking-curing temperature is between 170°C and 200°C, the baking-curing time is between 15 and 25 minutes, and in particular, the yield strength of the obtained structural component is 1050 MPa or more, specifically 1200 MPa or more.

13. The method according to any one of claims 1 to 12, wherein the length of the formed structural component is at least 1 meter, particularly 1 to 2 meters, and the width of the formed structural component is at least 1 meter, specifically 1 to 2 meters, and in particular the structural component is an integrated door ring.

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

15. The method according to any one of claims 1 to 14, wherein the multi-stage production line comprises a multi-stage tool comprising a fixed lower body, a movable upper body, and a mechanism configured such that the movable upper body advances vertically relative to the lower body for pressing, the upper press die being connected to the movable upper body, the lower press die being connected to the fixed lower body, the first post-press upper die being connected to the movable upper body, and the first post-press lower die being connected to the fixed lower body.

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