Process for heat treating a portion of steel and assembly for forming a steel plate

Abstract

A process and assembly for heat treating a section of steel, such as Advanced High Strength Steel (AHSS) or Gen3 steel, to form hard and transition zones in a nominally hard steel stamping, such as a vehicle structural stamping including a pillar, for designing deformation zones during a crash event. In an initial operation, the present invention involves placing heating elements at locations along the blank-shaped steel. Following the initial heating, the steel is bent or shaped using a suitable forming or stamping operation, with the heated zones defining bend axes or bend points within the steel. Following the initial heating and bending / stamping operations, additional heating elements can be placed, for example, at outer flange locations of an already stamped member, before a subsequent trimming or final processing step is used to complete the member.

Description

[Technical Field]

[0001] The present invention relates generally to locally enhancing the material properties of steel stampings, and more particularly to processes and assemblies for forming transition and hard zones in steel stampings, particularly those used as vehicle pillars, to engineer deformation zones during a crash event.

[0002] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Patent Application No. 18 / 382,624, filed October 23, 2023, which claims the benefit of U.S. Provisional Patent Application No. 63 / 418,673, filed October 24, 2022. The contents of both applications are incorporated herein in their entireties. [Background technology]

[0003] Advanced High Strength Steel (AHSS), also known as Gen3 steel, is known in the art to enable downgauging of automotive panels and therefore weight reduction, taking advantage of the material's higher strength.

[0004] During a crash event, both strategic absorption and transfer of energy is required to ensure occupant safety. To accomplish this task, the strongest materials are required. The risk in selecting the strongest materials is that they can simply transfer loads, not absorb them. Some of the energy typically associated with a crash event may be transferred to the occupants, potentially causing undue harm.

[0005] To absorb some of the energy into the body structure, the base zone, transition zone, and rigid zone are each designed into the steel pillar to allow for a controlled collapse of the structure, thereby optimizing both energy absorption and transmission.

[0006] The state-of-the-art for energy-absorbing body-in-white detail panels and subsequent subassemblies revolves around tailored material properties in press-hardenable stampings (PHS). For example, in a side impact, the BP1r outer panel must first be strong enough to keep the crumple zone intact for occupant survivability. Furthermore, the deformation or yield areas designed into the steel pillar must deform or yield in predictable areas at predetermined levels of force.

[0007] This often leads to the use of tailored properties associated with press hardened steel (PHS) parts, due to their strength, and the use of tailored soft zones to engineer the kinematics associated with a crash event.

[0008] As is well known, press hardening is an expensive process, however OEMs have paid a premium for the performance that such panels offer. Next generation steels (again, AHSS or Gen3 steels) offer a similar level of strength with a level of adjustability not found in traditional cold stamped panels by adding tailored hard zones.

[0009] Other prior art documents include U.S. Patent No. 6,299,629, which is representative of existing technology and teaches a B-pillar for a motor vehicle manufactured as a hot stamped and press hardened part made of a hardenable steel alloy, wherein the lower length section extends over less than 40% of the longitudinal length (L) of the B-pillar and has at least regionally different strength values ​​relative to the upper length section, at least one cross section of the lower length section being formed with mutually different strengths and a soft material structure being formed regionally in the lower length section.

[0010] The '666 patent teaches, in pertinent part, a side panel for a vehicle that includes providing a sheet metal blank made of high strength steel, hardening at least one region of the sheet metal blank, and pressing the sheet metal blank in a single step to form a unitary structure with a sill member and a roof panel portion with interconnecting pillars. The sheet metal blank can thus be formed and heat treated in a single operating cycle, with the regions most likely to be subjected to high loads being hardened.

[0011] Patent document 3 (Ford Global) teaches a method and apparatus for molding a part having zones of different intensity, the method and apparatus including upper and lower mold elements (tools) together with actuators and a controller, the controller being programmed to activate and operate the actuators based on separate pressure commands corresponding to first and second sets to contact and compress a plurality of portions based on a processing schedule to configure heating of the part to establish the different intensity zones, and to instruct a coolant distributor to supply coolant to coolant channels to affect austenitization and pressure application for microstructure formation. Patent Document 4 (BMW) teaches a press-hardened formed metal sheet, such as a pillar reinforcement, having at least two adjacent zones with different thicknesses and different strengths, one of which is press-hardened and the other not or only slightly hardened. A transition zone, simultaneously designated as a thickness transition zone and a strength transition zone, is located between these zones. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] U.S. Patent No. 9,884,653 [Patent Document 2] U.S. Patent No. 7,396,072 [Patent Document 3] U.S. Patent No. 11,141,769 [Patent Document 4] U.S. Patent No. 10,981,602 Summary of the Invention [Means for solving the problem]

[0013] The present invention discloses a process and assembly for forming hard zones and transition zones in steel stampings, such as those intended for use as vehicle pillars, to design deformation zones during a crash event, having nominal material properties. In an initial operation, the present invention involves placing heating elements at locations along a blank-shaped steel product. Following the initial heating, the product is bent or shaped using a suitable forming or stamping operation, with the heated zones defining bending axes or bending points within the product. Following the initial heating and bending / stamping operations, heating elements can optionally be repositioned or ideally added, such as at outer flange locations of already stamped steel, before a secondary processing step is used to complete the product.

[0014] The process and assembly also provide a more efficient method of forming hard and transition zones in materials having a base or nominal hardness to optimize the deformation behavior of the steel pillar to protect vehicle occupants during a crash event. [Brief explanation of the drawings]

[0015] Reference is now made to the accompanying drawings, in which: When read in conjunction with the following detailed description, like reference numerals refer to like parts throughout the several views.

[0016] [Figure 1A-1B] 1A and 1B show top and end views of an arrangement of heating elements positioned at specific locations below a steel plate prior to a stamping operation.

[0017] [Figure 2A-2B]1C shows top and end views of an edge heating zone resulting from the arrangement of the elements of FIGS. 1A and 1B in relation to the steel sheet after stamping.

[0018] [Figure 3A-3B] 2C illustrates further subsequent placement of heating elements along the opposed flange edges of the stamped steel sheet of FIGS. 2A and 2B during a targeted secondary heating and forming operation.

[0019] [Figure 4] 1A-1C are diagrams of press-hardened vehicle pillars incorporating hard zones, transition zones, and soft zones, respectively.

[0020] [Figure 5] Environmental diagram of a pillar as shown in Figure 4, integrated into a vehicle frame, showing both an ultra-high strength zone, typically at the top of the pillar to minimize intrusion / deformation, and a soft zone located at the very bottom, providing high ductility to maximize energy absorption.

[0021] [Figure 6] As further shown in Figure 7, a graph of stress (MPa) versus strain (mm / mm) is shown for each of the hard, transition, and soft zones shown in the press hardened pillars. [Figure 7] 1 shows the press-hardened pillars.

[0022] [Figure 8A-8B] 1A-1C show first and second views of a hybrid stamped pillar showing the hard and nominal zones, respectively.

[0023] [Figure 9] A tabular representation comparing the measurement quality increase for both unheated and heated samples for mean diagonal length, force, and hardness (measured in Vickers hardness) is shown. The average sample hardness is 374 HK for the heated samples compared to 307 HK for the unheated samples. DETAILED DESCRIPTION OF THE INVENTION

[0024] With reference to the accompanying drawings, the present invention discloses a process and assembly for heat treating a section of steel material, particularly any type of Gen 3 or advanced high strength steel, to create hard zones and transition zones, respectively, in a steel stamping having a nominal base hardness, particularly for use as a vehicle pillar, in order to engineer deformation zones during a crash event.

[0025] As will be further explained with reference to the accompanying drawings, the present invention contemplates the strategic use of thermal energy, such as the placement of heating elements to preheat locations on a steel blank (typically a flat AHSS plate) prior to the stamping operation, as well as subsequent targeted heating steps associated with any end-stage trimming or bending operations to produce a finished part that may often be closer to the desired shape than would otherwise be achievable. The present process and assembly also allows for reduced actuation forces in the stamping and finished part production steps.

[0026] 1A and 1B, there are shown top and end views of an arrangement of heating elements (see 10 and 12) positioned in specific locations beneath a steel sheet or blank 14 prior to a stamping operation. The heating elements may be of any known type or construction, including, but not limited to, either direct heating elements or other induction heating elements for passing current through a material by exposing it to an alternating magnetic field. As is further known, the alternating magnetic field is typically in the kHz range and is generated using a resonant coil, resulting in heat generation due to resistive losses and hysteresis losses in ferromagnetic materials such as iron.

[0027] Once selected areas of the steel blank are heated, a typical stamping operation (not shown) is used to bend or form the blank into the three-dimensional cross-sectional shape 14' (including base surface 16 and outer folded flange surfaces 18 / 20, respectively) shown in Figures 2A and 2B as top and end views, respectively, of the stamped steel sheet. The bending locations associated with the stamping process coincide with the edge heating zones resulting from the placement of heating elements 10 and 12 in Figures 1A and 1B relative to the stamped steel sheet.

[0028] Proceeding to FIGS. 3A and 3B, further subsequent placement of heating elements is shown, which may involve repositioning (changing the position of) the original heating elements or (more typically) placing additional heating elements at 22 and 24 (which define additional heating zones) along the opposing flange edge of the stamped steel sheet to perform a subsequent or second heating operation following stamping of the steel sheet or blank to further process the already stamped or formed blank as indicated at 14".

[0029] FIG. 4 is a diagram of a vehicle pillar generally designated 26, incorporating a hard zone 28 (generally at the top), a transition zone 30 (generally in the middle), and a soft zone 32 (generally at the bottom), respectively. As further reflected in FIG. 5, the environmental diagram of the pillar as shown in FIG. 4 is again shown incorporating both an ultra-high strength zone (again 28), typically at the top of the pillar to minimize intrusion / deformation, and a soft zone (again 32), located at the bottom, that provides high ductility to maximize energy absorption in response to an impact event. FIG. 6 shows a graph of stress (indicated in MPa along the vertical axis at 34) versus strain (indicated in mm / mm along the horizontal axis at 36) for each of the hard zone 28, transition zone 30, and soft zone 32, as shown in FIG. 7.

[0030] 8A and 8B show a pair of first and second views, respectively, of a hybrid stamping BP1r (B-pillar) with tailor hardened zones, designated 38. The pillar shows hardened zones 40 and nominal zones 42 (FIG. 8B), respectively, defined in the nominally hard pillar.

[0031] Finally, Figure 9 shows a tabular representation comparing the increase in measurement quality for the unheated and heated samples for average diagonal length, force, and hardness (measured in Vickers hardness). Of note is the average sample hardness of 374 HK for the heated samples compared to 307 HK for the unheated samples.

[0032] Aspects and advantages of the process and assembly include each of the following:

[0033] Microstructural refinement – ​​Strategic application of heat allows for locally tempered microstructures that enable advanced high strength materials to be stamped into more complex shapes with a range of strengths.

[0034] Improved resistance to localized necking - The resulting microstructure is more resistant to localized necking.

[0035] Higher strength - the resulting microstructure is harder and therefore stronger.

[0036] Superior "drawability" - Strategic heating allows for more complex shapes with greater draw depth.

[0037] Smaller Radii - Adding heat during the process allows for smaller radii.

[0038] Improved resistance to edge fracture - The application of heat allows for material properties that are more resistant to edge fracture.

[0039] Reduced forming tonnage - Higher temperatures during the forming process reduce the forming force required to form the same shape as a traditional stamped product.

[0040] Increased production speed - Because the heating element is in line with the punching and / or stamping die, production speed is lower than oven heating processes.

[0041] Higher Quality, Less Springback / Less Sidewall Curl - Heating the material allows the stamping material to have less springback and sidewall curl than traditional stamping processes.

[0042] Laser welded blank - Heating the laser weld joint allows stress relaxation in the joint, improving formability during the forming process.

[0043] Having described the present invention, other and additional preferred embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from the scope of the appended claims. It is further understood that the detailed description and drawings support this disclosure, the scope of which is defined by the appended claims. While some of the best modes and other embodiments for carrying out the claimed teachings have been described in detail, there are numerous alternative designs and embodiments for carrying out the disclosure as defined in the appended claims.

[0044] It is further understood that the above disclosure is not intended to limit the disclosure to the precise form or particular field of use disclosed. Thus, various alternative embodiments and / or modifications to the disclosure, whether expressly described or implied herein, are contemplated in light of the present disclosure. While embodiments of the present disclosure have been described in this manner, those skilled in the art will recognize that changes can be made in form and detail without departing from the scope of the present disclosure. Accordingly, the present disclosure is limited only by the claims.

[0045] In the foregoing specification, the present disclosure has been described with reference to specific embodiments. However, as those skilled in the art will understand, the various embodiments disclosed herein can be modified or otherwise embodied in various other ways without departing from the spirit and scope of the present disclosure. Accordingly, the present description is to be considered illustrative and is intended to teach those skilled in the art how to make and use various embodiments of the present disclosure. It is to be understood that the forms of the disclosure shown and described herein are to be construed as representative embodiments. Equivalent elements, materials, processes, or steps may be substituted for those typically shown and described herein. Also, certain features of the present disclosure may be utilized independently of the use of other features, all of which will be apparent to those skilled in the art after having the benefit of this description of the present disclosure. The terms "including," "comprising," "incorporating," "consisting of," "having," "being," and the like, used to describe and claim the present disclosure, are intended to be non-exclusive, i.e., allowing for the presence of items, components, or elements not expressly described. References to the singular are also to be construed as relating to the plural.

[0046] Furthermore, the various embodiments disclosed herein should be construed in an illustrative and explanatory sense, and should not be construed as limiting the present disclosure in any way. All connector references (e.g., attached, affixed, coupled, connected, etc.) are used solely to aid the reader's understanding of the present disclosure and do not imply any limitations with respect to the position, orientation, or use of the systems and / or methods disclosed herein, among other things. Accordingly, connector references, if any, should be interpreted broadly. Furthermore, such connector references do not necessarily imply that two elements are directly connected to each other.

[0047] Furthermore, all numerical terms, such as, but not limited to, "first," "second," "third," "primary," "secondary," "primary," or other general and / or numerical terms, should also be construed merely as identifiers to aid the reader's understanding of the various elements, embodiments, variations and / or modifications of the present disclosure, and are not intended to create any limitations, particularly with regard to the order or priority of any element, embodiment, variation and / or modification relative to or over other elements, embodiments, variations and / or modifications.

[0048] It will also be understood that one or more of the elements shown in the drawings / figures may be implemented in a more separate or integrated manner, or even removed or rendered inoperable in some cases, as may be useful depending on the particular application. Furthermore, symbol hatching in the drawings / figures should be considered illustrative only and not limiting, unless otherwise specified.

Claims

1. 1. A process for heat treating a section of steel to be incorporated into a vehicle pillar or similar structural panel, comprising: providing a blank steel sheet; positioning at least one heating element at a location along the steel blank; heating a portion of the blank steel plate corresponding to the arrangement of the heating elements; forming the heated blank into a pillar shape or other structural part by either a pressing or stamping operation to define a hard zone, a transition zone, and a soft deformation zone, respectively; A process involving:

2. 10. The process of claim 1, further comprising the step of either repositioning at least one heating element or placing at least one additional heating element at another location on the formed blank, including but not limited to along an outer flange location of the blank, after which a subsequent trimming or final machining step is used to complete the steel product.

3. 1. An assembly for forming a steel blank into a vehicle pillar or other structural component, comprising: at least one heating element disposed along the extended portion of the blank for heating the extended portion; a pressing or stamping operation to form the blank into a three-dimensional vehicle pillar stock defining each of a hard zone, a transition zone, and a nominal hardness zone; The assembly containing:

4. The assembly of claim 3 further comprising: the heating element being incorporated into a die setup forming part of the stamping operation.

5. The assembly of claim 4 , wherein the stamping operation further comprises each of a primary and a secondary machining operation to form the vehicle pillar.

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