SYSTEMS, METHODS AND DEVICES FOR HOT FORMING OF STEEL ALLOY PARTS

Abstract

Disclosed are hot forming systems and apparatuses for metalworking components from micro-alloyed press hardened steel (PHS), methods for operating such systems/apparatuses, processes for hot forming components from micro-alloyed PHS, and components formed from such processes. A method of hot forming components from steel is disclosed. The method includes transferring a workpiece formed from a PHS micro-alloyed with niobium (e.g., 0.02-0.1 wt % Nb) to a furnace, e.g., via material handling robot. The workpiece is then heated to a peak furnace temperature and during a furnace time (e.g., total ramp and soak time) selected from a pentagon having heating time and temperature coordinates of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.). The heated workpiece is then transferred to a hot forming apparatus.

Description

The present disclosure relates generally to metalworking processes for manufacturing steel components. More specifically, aspects of this disclosure relate to systems, methods and devices for hot forming steel alloy parts.

Steel and steel derivatives are used for manufacturing a vast array of modern products. Many tools, automobiles, bridges, trains, airplanes, buildings, and boats all depend on steel parts to make them robust and economical. During the early metal processing stages of manufacturing, large metal slabs and bar stock (known as “billets”) are formed from molten smelted iron, for example, using continuous casting and other applicable metalworking processes. To become steel, the raw cast iron is processed prior to casting to reduce carbon content through further refinement. For sheet stock, the slab/billet of steel generally undergoes “hot rolling” to obtain a continuous metal sheet of a relatively medium thickness, which is then rolled into spools or coils. Hot rolling is a process by which raw purified metal is passed, pressed, or drawn through a set of work rolls in a continuous and generally linear fashion, where the temperature of the metal is above its recrystallization temperature. Hot rolling permits large deformations of the metal to be achieved with a relatively low number of rolling cycles.

For ferrous metals, the spool may then be unrolled for chemical descaling—referred to colloquially as “pickling”—during which the surface is treated with a hydrochloric acid solution (known as “pickle liquor”) in order to remove impurities, contaminants, scale, stains, and rust. After chemical pretreatment, the sheet may then be subjected to “cold rolling” to obtain sheets with a desired final thickness. During cold rolling, the metal sheet stock is passed, pressed, or drawn through rollers without purposeful reheating (e.g., with the metal at a temperature below its recrystallization temperature). The cold rolling process may be employed to increase the yield strength and hardness of the material by introducing defects into the metal's crystal structure. Upon completion of the cold rolling operation, the metal sheet is often heat treated through annealing, tempering, etc., to obtain certain desired mechanical characteristics, machinability, etc., and again rolled into a spool for packaging and shipping.

Press hardened steel (PHS), also referred to as “hot stamped” or “hot-press formed” steel, is one of the strongest metallic materials used for automotive powertrain and body structure applications—having tensile strength properties on the order of 1,500 megapascals (MPa) or greater. PHS alloys are used to fabricate many vehicle components, including chassis frame segments (e.g., cross-members, side rails, cradles, etc.), body panels, and body-in-white (BIW) sections (bumper crossbeams, center pillars, hinge pillars, and the like). Vehicle structural components made of PHS are often produced via a “hot forming” process, such as hot stamping, vacuum forming, draw forming, die forming, etc., which are temperature-sensitive and time-dependent processes in which parts of simple and complex shapes are plastically deformed when in a softened state at elevated temperatures. During hot die forming, one or more blanks may be cut from a metal coil of sheet stock. These cut blanks, which may or may not be preformed at ambient temperatures, are heated to elevated temperatures, e.g., around 600-800 degrees Celsius (° C.), and thereafter transferred to a hot forming die. Subsequently, the hot blanks are formed and quenched in the dies to achieve a desired final shape.

SUMMARY

Disclosed herein are hot forming systems and apparatuses for fabricating components from heat-treatable micro-alloyed press hardened steel, control methods for operating such hot forming systems, hot forming processes for making components from micro-alloyed press hardened steel, and components hot formed from micro-alloyed press hardened steel. By way of example, and not limitation, there is presented a novel hot forming process with shortened cycle time for fabricating parts from heat-treatable niobium (Nb) micro-alloyed press hardened steel. In a particular example, boron containing press hardened steel is modified by Nb micro-alloying (approximately 0.02-0.1 weight percent (wt. %)) to retard grain growth (approximately 10-40 microns final equivalent average austenite grain diameter) during a blank ramp up and soaking stage with a peak furnace temperature of approximately 900 to 1100° C. for a total furnace time of approximately 120 to 300 seconds (sec). In a more specific example, 22MnB5 grade PHS is modified by Nb micro-alloying (approximately 0.05 wt. %) to retard grain growth (15 microns or less final austenite grain size) during blank ramp up and soaking with a peak furnace temperature of approximately 950° C. or higher for a total furnace time of approximately 150 sec. While not per se limited, the resultant hot formed PHS workpiece has particular applicability to vehicle structural components, such as a-pillar, b-pillar, front and rear bumper beams, door beams, etc.

Attendant benefits for at least some of the disclosed concepts include reduced hot forming process cycle times and, hence, reduced cost of manufacturing PHS components. Other attendant benefits include improved hot forming process robustness against furnace temperature fluctuation. Disclosed hot forming processes can help to reduce bottlenecking at the stage of heating up a steel blank in a furnace to a pre-set temperature—reduced ramp-up time and, hence, shorter overall hot stamping process cycle time. Disclosed hot forming processes and systems also provide for higher soaking temperatures without significant grain coarsening. The foregoing helps to achieve reductions in piece cost due to reduced furnace time (higher production through-put), and improved process robustness—Nb micro-alloying helps to prevent grain growth. Use of disclosed PHS components can help to achieve significant reductions in vehicle mass. Disclosed PHS components may provide attending improvements in energy absorption of PHS components during impact events due to refined microstructure

Aspects of the present disclosure are directed to hot forming processes for fabricating components from heat-treatable micro-alloyed PHS. In an example, there is disclosed a method of hot forming a component from steel. The method includes, in any order and in any combination with any disclosed options: transferring a workpiece to a furnace, the workpiece being formed from a press hardened steel alloyed with niobium (e.g., 22MnB5 grade PHS with 0.02 to 0.1 wt. % Nb); heating the workpiece in the furnace to a furnace temperature and during a furnace time selected from a pentagon having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.); and, transferring the heated workpiece to a hot forming apparatus. Final austenite grain size for the heated workpiece prior to quenching may be limited to approximately 10-40 microns. In this example, furnace time may include ramp up and soaking for a total furnace time of approximately 3 minutes. In this regard, furnace temperature includes a heating rate, e.g., of approximately 10° C./s, to a peak furnace temperature, e.g., of approximately 980° C.

Other aspects of the present disclosure are directed to metalworking systems, apparatuses, and devices for fabricating components from heat-treatable micro-alloyed PHS. Disclosed, for example, is a metalworking system for hot forming a component from steel. The system includes a transfer device, such as an automated material handling robot, that is operable to transfer workpieces between stations of the metalworking system. Each workpiece is formed from a PHS alloyed with niobium (Nb). A furnace, which is operable to receive the workpiece from the transfer device, is configured to heat the workpiece to a furnace temperature and during a furnace time selected from a pentagon having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.). A hot forming apparatus, which may be in the nature of a water-cooled die-forming press, is operable to receive the heated workpiece from the furnace, and to mechanically deform and quench the heated workpiece. It may be desirable that the final steel component have a ductility of between approximately 6 to 10% and a tensile strength of approximately 1,500 megapascals (MPa) or greater.

Additional aspects of this disclosure are directed to control methods for operating hot forming systems for fabricating components from heat-treatable micro-alloyed PHS. For instance, a method is disclosed for operating a metalworking system that is composed of multiple metalworking stations, including a furnace and a hot forming apparatus. The method includes, in any order and in any combination with any disclosed options: commanding a transfer device to transfer a workpiece, e.g., from a pallet of stacked blanks, to the furnace, the workpiece being formed from a boron-alloyed quenched and tempered PHS micro-alloyed with niobium (Nb); commanding the furnace to heat the workpiece to a furnace temperature (e.g., ramp to and soak at a peak furnace temp) during a furnace time (e.g., total ramp up time plus total soak time) selected from a pentagon having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.); commanding the transfer device to transfer the heated workpiece from the furnace to the hot forming apparatus; and commanding the hot forming apparatus to mechanically deform and quench the heated workpiece.

The above summary is not intended to represent every embodiment or every aspect of the present disclosure. Rather, the foregoing summary merely provides an exemplification of some of the novel aspects and features set forth herein. The above features and advantages, and other features and advantages of the present disclosure, will be readily apparent from the following detailed description of representative embodiments and representative modes for carrying out the present disclosure when taken in connection with the accompanying drawings and the appended claims. Moreover, this disclosure expressly includes any and all combinations and subcombinations of the elements and features presented above and below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagrammatic illustration of a representative metalworking system for making a component from a micro-allowed PHS blank using a hot forming process in accordance with aspects of the present disclosure.

FIG. 2 is a front perspective-view illustration of a representative motor vehicle with an inset view of a door assembly with a door fascia panel mounted to a structural door frame with a reinforcement door beam formed from a micro-allowed PHS blank using a hot forming process in accordance with aspects of the present disclosure.

FIG. 3 is a graph showing total time in furnace (seconds (sec)) versus furnace temperature (° C.) for a process window for hot forming a micro-alloyed PHS workpiece in accordance with the present disclosure contrasted with a process window for hot forming a benchmark comparison PHS workpiece.

FIG. 4 is a graph showing furnace residence time (sec) versus furnace temperature (° C.) for hot forming a micro-alloyed PHS alloy workpiece at various ramp and soak times in accordance with the present disclosure to reduce cycle time.

FIG. 5 is a graph showing austenitizing temperature (° C.) versus austenite grain size (micrometers (μm)) for a micro-alloyed PHS part made by a hot forming process in accordance with the present disclosure contrasted with a benchmark comparison PHS alloy part.

FIG. 6 is a flowchart for a representative hot forming processes or system control methodology that may correspond to instructions executed by control-logic circuitry or other computer-based device of a hot forming mill in accord with aspects of the present disclosure.

The present disclosure is susceptible to various modifications and alternative forms, and some representative embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the novel aspects of this disclosure are not limited to the particular forms illustrated in the appended drawings. Rather, the disclosure is to cover all modifications, equivalents, combinations, subcombinations, permutations, groupings, and alternatives falling within the scope and spirit of the disclosure.

DETAILED DESCRIPTION

This disclosure is susceptible of embodiment in many different forms. There are shown in the drawings and will herein be described in detail representative embodiments of the disclosure with the understanding that these representative embodiments are to be considered an exemplification of the principles of the disclosure and are not intended to limit the broad aspects of the disclosure to the illustrated embodiments. To that extent, elements and limitations that are disclosed, for example, in the Abstract, Summary, and Detailed Description sections, but not explicitly set forth in the claims, should not be incorporated into the claims, singly or collectively, by implication, inference or otherwise. For purposes of the present detailed description, unless specifically disclaimed: the singular includes the plural and vice versa; the words “and” and “or” shall be both conjunctive and disjunctive; the word “all” means “any and all”; the word “any” means “any and all”; and the words “including” and “comprising” and “having” mean “including without limitation.” Moreover, words of approximation, such as “about,” “almost,” “substantially,” “approximately,” and the like, may be used herein in the sense of “at, near, or nearly at,” or “within 3-5% of,” or “within acceptable manufacturing tolerances,” or any logical combination thereof, for example.

Referring now to the drawings, wherein like reference numbers refer to like features throughout the several views, there is shown in FIG. 1 a representative metalworking system, which is designated generally at 10 and represented herein for purposes of description by select metalworking stations for hot forming micro-alloyed PHS parts. Many of the novel aspects and features of the present disclosure will be described herein with reference to the architecture illustrated in FIG. 1 as an exemplary application with which these aspect and features can be practiced. It will be understood, however, that the disclosed concepts are by no means limited to the particular constructions illustrated in the drawings. Rather, aspects and features of the present disclosure may be implemented for forming steel workpieces of other configurations, and may be incorporated into other metal forming apparatuses and operations without departing from the intended scope and spirit of this disclosure. Lastly, the drawings presented herein are not necessarily to scale and are provided purely for instructional purposes. Thus, the specific and relative dimensions shown in the drawings are not to be construed as limiting.

FIG. 1 illustrates a metalworking system 10 operable for fabricating an ultra-high strength steel component 12 from a micro-alloyed PHS sheet blank 14 via one or more hot forming operations. By way of non-limiting example, FIG. 2 provides an exemplary application for the steel component 12—the component 12 is shown as having been hot formed into an impact-energy absorbing and attenuating door beam (shown with hidden lines) of a structural door frame 112 of a passenger-side door assembly 114 for a representative motor vehicle 110. Alternative applications for the steel component 12 may include any other vehicle frame, body and powertrain application, such as a bumper crossbeam, a chassis frame rail or cross member, a pillar, a body panel, etc. It should also be appreciated that the component 12 may be used for any other logically relevant design application, including, but not limited to, an aircraft, a building, a house, a bridge, a boat, machinery, and the like.

Referring again to FIG. 1, the metalworking system 10 is represented herein by select elements, including a furnace 16, a transfer device 18, and a stamping press 20. Only select components of the system 10 have been shown and will be described in detail herein. Nevertheless, the system 10 can include numerous additional and alternative features, and other well-known peripheral components, such as a blank punching press, a blank stacking station, a series of hot stamping presses, etc., for carrying out the various methods and functions disclosed herein without departing from the intended scope of this disclosure. As shown in the illustrated example, the sheet blank 14 (also referred to herein as “workpiece”) is fed into and passed through the hot forming furnace 16 having an average furnace temperature of 900° C. or higher. The furnace 16 can take on various forms of industrial steel heating furnaces, including a roller hearth furnace, a walking beam furnace, a stacked furnace, an induction coil, microwave, or infrared furnace, etc., that is configured to heat and soak the sheet blank 14 at a furnace temperature of approximately 900 to 1100° C. or at least approximately 950° C.

One or more transfer devices, such as the automated material handling robot 18 shown in FIG. 1, operate to transfer workpieces between stations of the metalworking system 10. By way of example, and not limitation, the transfer device 18 of FIG. 1 is operable to lift, extract and transfer the heated sheet blank 14A from the furnace 16 to the stamping press 20 after the sheet blank 14 has been soaked, e.g., for a sufficient amount of time to fully austenitize the steel of the sheet blank 14. The heating operation may help to achieve a fully austenitic condition that provides an intermediate workpiece 14B that is “forgiving” in terms of achieving a full martensite microstructure everywhere on the final workpiece. While shown as an automated material handling robot, the transfer device may be a conveyor system, a slide mechanism, an internal quick-transfer device, and the like.

Stamping press 20 performs a hot press forming operation that allows ultra-high strength steels (e.g., 22MnB5 grade PHS) to be formed into complex shapes, which is not otherwise possible with many regular cold stamping operations. As shown, the stamping press 20 includes a precision die assembly 22A, 22B constructed with a cooling mechanism, such as a water-cooled quenching device. The die assembly is designed to form a desired final shape of the component 12 from the austenitized sheet blank 14. The die assembly may include a first (male) forming die 22A juxtaposed with a second (female) forming die 22B that are brought together, e.g., via an electric, hydraulic, or pneumatic driving piston or power-screw assembly, to form a desired final shape for the component 12, such as that shown in the inset view of FIG. 2. While portrayed as a single, standalone apparatus, the stamping press 20 may comprise a series of metal forming machines to achieve the final desired shape for the component 12.

With continuing reference to FIG. 1, stamping press 20 is equipped to provide cooling to the sheet blank 14 as the two forming die 22A, 22B are brought together to form the component 12. For instance, the first and second forming die 22A, 22B may each define at least one water cooling channel (not shown) therein. Each cooling channels may be configured to control the injection of quenching fluid provided by the first and second forming die 22A, 22B such that the formed component 12 is quenched in a controlled manner. Using known means, the quenching fluid may be applied consistent across surfaces of the formed component 12 to cause a phase transformation from austenite to martensite. In this manner, the first and second die 22A, 22B may cooperate to function as a heat sink that draws heat from, and otherwise quench, the formed component 12. The workpiece may be cooled in the hot forming apparatus 20 at a predetermined cooling rate until a predetermined workpiece temperature is achieved. It may be desirable that the predetermined cooling rate is not less than a critical cooling rate. For instance, the predetermined workpiece temperature is between approximately 150 to 200° C., and the critical cooling rate is not greater than 35 Kelvin per second. It may be desirable that the final steel component have a ductility of between approximately 6 to 10% or 8 to 12%, and a tensile strength of at least approximately 1.5 gigapascals (GPa) or at least about 1.8 GPa or greater.

It is desirable, for at least some applications, that the furnace 16 operate within a predefined window of operating conditions for achieving the final formed product 12. FIG. 3, for example, graphically illustrates total time in furnace t_TOT (seconds (sec)) versus furnace temperature T_Furn (° C.) for hot forming a niobium (Nb) micro-alloyed PHS workpiece. In a non-limiting example, preferred treatment conditions for hot forming the PHS workpiece can be determined from a hot forming process window as defined by a pentagon having heating time and temperature coordinates ABCDE of: A (about 1.5 minutes, about 930° C.), B (about 1.5 minutes, about 1100° C.), C (about 4.0 minutes, about 1100° C.), D (about 5.5 minutes, about 1000° C.), and E (about 5.5 minutes, about 930° C.). Within the graph of FIG. 3 there is provided an exemplar hot forming process window that is defined by cycle time and temp pentagon 120 having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1010° C.), C (about 3.5 minutes, about 1010° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.). Any point within or along the boundary line defined by these coordinates can be selected. For some intended applications, total time in furnace t_TOT includes heat ramp up time and heat soak time. As an example, heat ramp time may require 100-150 seconds, while heat soak time may require 150-200 seconds, for a total furnace time t_TOT of 250-350 seconds.

The proposed hot forming process window 120 of FIG. 3 is juxtaposed against a hot forming process window 122 for a benchmark comparison PHS workpiece without niobium micro-alloy addition. In the latter instance, the hot forming process window 122 is limited to a furnace temperature range of 880-950° C., since heating these PHS workpieces at above 950° C. tends to cause excessive grain growth in the microstructure and, hence, lower energy absorbing “impact toughness”—i.e., energy absorbed per cross-sectional area to fracture. Hot forming process window 122 also requires a total furnace time of 5.5 to 10 minutes per workpiece, which results in an undesirably long processing time for hot forming PHS blanks (i.e., creates a process bottleneck). Hot forming an Nb micro-alloyed PHS workpiece in accordance with the hot forming process window 120 demonstrates an approximate 10-30% increase in impact energy absorption relative to the non-Nb PHS following the processing conditions of window 122. This enhanced impact performance is attributed to grain refinement due to Nb micro-alloying.

Impact toughness (resistance to fracture) is an important material property for press hardened steel when used in the construction of structural elements for automotive body chassis frames and BIW underbody constructions. Austenite grain size, as primarily controlled by the incoming microstructure and austenitization process, is an important microstructural feature that influences the impact toughness of PHS. Both ductile-to-brittle transition temperature and upper shelf energy can be used to establish a correlation between impact toughness and prior austenite grain size. Within tested conditions, grain refinement via Nb micro-alloying of GPa PHS can significantly increase impact toughness. Conversely, for non-micro-alloyed PHS, prolonged heating or “over baking” typically must be avoided, because it may lead to excessive grain growth which, in turn, reduces impact toughness.

One way to ameliorate the process bottleneck caused by unduly long cycle times is to reduce the heating time required for ramp and soak of each workpiece. FIG. 4, for example, shows furnace residence time t_FURES (sec) versus furnace temperature T_FURN (° C.) for hot forming a micro-alloyed PHS workpiece at various cycle times to achieve a desired austenite grain size. In this example, each micro-alloyed PHS workpiece is a 150 mm×900 mm blank, with a Type 1 aluminum-silicon (AlSi) coating of approximately 60 g/m, and a chemical composition of 0.30 wt. % carbon (C), 0.30 wt. % silicon (Si), 1.30 wt. % manganese (Mn), 0.001 wt. % boron (B), and 0.05 wt. % niobium (Nb), the balance being iron (Fe) and unavoidable impurities. First, second and third workpieces 130A, 130B and 130C, respectively, are shown with respective peak temperatures of 880° C., 930° C. and 950° C. at a heating rate of approximately 10° C./s. The total furnace times, which include both ramp time t_RAMP and soak time t_SOAK at peak temperatures, were 330 sec, 390 sec, and 540 sec, respectively. FIG. 4 evinces proof of concept—setting a higher peak furnace temperature, without compromising structural integrity, leads to reduced furnace time during ramp up and soaking time, e.g., due to increased heat exchange, which results in comparably faster solute diffusion within the steel.

Total furnace heating time and/or peak furnace temperature may be selectively varied based on the chemical makeup of the workpiece (including any added surface coatings). By way of example, furnace time and peak temp can depend on the solubility and coarsening of carbonitrides at the times and temperatures of interest. A rule of thumb is that coarsening temperatures are typically 50-100° C. less than a calculated micro-alloy precipitate dissolution temperature at a soak time of 30 min. Because micro-alloyed PHS soak times are significantly less, grain coarsening temperatures may be approximated as closer to precipitate dissolution temperature, although this may depend heavily on other factors such as processing before heat treatment. Precipitate dissolution temperature may vary with an equilibrium constant for precipitation/dissolution as the alloy concentration in the steel, which are fixed. This relationship varies as follows:

T _dissolution =Q/(F−log(M*C))

Where Q and F are equilibrium constants, and M and C are concentration of metals niobium and carbon, respectively.

FIG. 5 is a graph showing austenitizing temperature T_AUST (° C.) versus austenite grain size GS_AUST (micrometers (μm)) for a micro-alloyed PHS part made by a hot forming process in accordance with the present disclosure. A uniform, fine-grain microstructure may be essential in some PHS applications, e.g., that are to meet demands for high strength and toughness. To ensure such microstructures, steps of high-temperature processing of PHS must be carefully controlled, to minimize coarsening that occurs during heating for hot forming. In the illustrated example, the resultant austenite grain size for an Nb-added 22MnB5 PHS workpiece 14 with a soaking temperature of 980° C. and total furnace time (ramp+soak) of approximately 3 min is less than 20 microns, as contrasted with a benchmark 22MnB5 PHS workpiece 14 (no niobium) where grain size exceeds 40 microns at peak temperatures of 980° C. or higher. This graph helps to show the effectiveness of adding trace amounts of Nb in the PHS workpiece to retard grain growth during hot forming. In this manner, hot stamping at higher temperatures (e.g., above 950° C.) can be achieved without having detrimental effects on end product performance (e.g., impact toughness). In other words, FIGS. 3-5 help to demonstrate a high efficiency hot forming process in which peak furnace temperature is intentionally set at 950° C. or higher, which is 30-80° C. higher than some standard industry practices, without significant grain growth (which causes lower toughness) by using niobium micro-alloyed PHS grades. Nb-microalloying leads to refined grain size and hence better impact toughness, while improved process throughput helps to dramatically reduce furnace time. The foregoing hot forming process can be contrasted to known methods of hot forming PHS alloys where a hot stamping plant is generally instructed to not heat a blank above 900-950° C. for fear of over baking the blank and degrading toughness performance. Because of these heat restrictions, known hot forming processes must employ increased furnace residence times (e.g., 6-11 minutes or higher) to achieve commensurate forming and toughness characteristics.

With reference now to the flow chart of FIG. 6, an improved metalworking method or metalworking system control algorithm for hot forming a component, such as reinforcement door beam of door frame 112 of FIG. 2, for a motor vehicle, such as the automobile 110, from a micro-alloyed PHS workpiece, such as sheet blank 14 of FIG. 1, is generally described at 200 in accordance with aspects of the present disclosure. FIG. 6 can be representative of an algorithm that corresponds to at least some instructions that can be stored, for example, in main or auxiliary memory, and executed, for example, by a local or remote system controller, ECU, CPU, control logic circuit, or other device, to perform any or all of the above and/or below described functions associated with the disclosed concepts.

The method 200 commences at block 201 with providing a PHS workpiece, which may be in the form of a die cut sheet blank 14 of Nb micro-alloyed 22MnB5 ultra-high strength press hardened steel (PHS) with a microstructure consisting of primarily (auto-tempered) martensite after being formed at elevated temperature and subsequently quenched in water-cooled dies. Micro-alloying is not merely a superficial film deposition process or a post-casting surface treatment; rather, a trace amount of Nb is added during the early metal processing stages (e.g., smelting of iron process). Block 201 may be automated, e.g., via a material handling robot 18 pulling individual blanks from a pallet of stacked blanks, operator controlled, e.g., via a quick-transfer sheet feeder and conveyor system, or performed manually. The method 200 proceeds to block 203 where the micro-alloyed PHS workpiece 14 is transferred to a furnace 16 for heat processing. At the start of heating the workpiece 14 in the furnace 16, the workpiece may be at ambient temperature, e.g., approximately 25° C.

Continuing to block 205, the method includes heating the workpiece 14 in the furnace 16 in accordance with a predefined window of operating conditions, e.g., as described in the discussion above with respect to FIG. 3. For instance, the workpiece 14 is heated (ramp plus soak) to a peak furnace temperature for a total furnace time selected from a pentagon having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.). In a particular example, a boron containing PHS blank, modified by Nb micro-alloying (approximately 0.02-0.1 wt.%) to retard grain growth (approximately 10-40 microns final grain size), is subjected to a peak furnace temperature of approximately 900 to 1100° C. for a total furnace time of approximately 120 to 300 seconds (sec). In a more specific example, 22MnB5 grade PHS modified by Nb micro-alloying (approximately 0.05 wt. %) to retard grain growth (15 microns or less final grain size), is subjected to a peak furnace temperature of at least 950° C. or at least 980° C. for a total furnace time of approximately 150 sec or approximately 180 sec. Once the austenitization temperature of the sheet blank is achieved, the heated blank can be soaked in the furnace 16 for a predetermined amount of time, e.g., at or slightly above the austenitization temperature. By way of a non-limiting example, the sheet blank 14 may be soaked for between 1 and 4 minutes, depending on ramp up time.

At block 207, the heated sheet blank 14A is transferred from the furnace 16 to a hot forming apparatus, such as stamping press 20 of FIG. 1, to form the PHS component 12. When the heated sheet blank 14A is removed from the furnace 16, the heated sheet blank 14A may be at the austenitization temperature, e.g., 950° C. and above. However, once the heated sheet blank 14 exits the furnace 16, the heated sheet blank 14 may immediately begin to cool in the ambient temperature surrounding the furnace 16 to provide an intermediate workpiece 14B. For at least some embodiments, the intermediate workpiece 14B is inserted into the stamping press 20 at an intermediate temperature that is less than the austenitization temperature of the heated sheet blank 14A that was removed from the furnace 16.

Method 200 proceeds to block 209 where the intermediate workpiece 14B is hot stamped and quenched between the opposing die 22A, 22B. More specifically, the first forming die 22A and the second forming die 22B are brought together, with the intermediated workpiece 14B disposed therebetween to form, e.g., a desired final shape of the PHS component 12. The temperature of the intermediate workpiece 14B at the beginning of step 209 may be between about 700 and 850° C. Concurrently, the intermediate workpiece 14B is cooled in the stamping press 20—water is injected to flow through channels in the die 22A, 22B to promote a heat exchange between the intermediate workpiece 14B and the water to cool the workpiece 14 at a desired cooling rate until a temperature of the intermediate workpiece 14B that is not greater than a finished temperature is achieved. It should be appreciated that the quenching medium is not limited to water, as other media may also be used, such as oil and other applicable fluids. The cooling rate may be typified as the time it takes to cool the workpiece 14 from the intermediate temperature to the finished temperature. In an example, the finished temperature is a temperature corresponding to when the workpiece 14 has sufficiently achieved microstructure changes to martensite (e.g., any temperature that is greater than or equal to least a martensite start temperature and less than or equal to a martensite finish temperature of the PHS). For instance, the finished temperature may be between about 150 and 200° C. At step 211, the die 22A, 22B are separated and the press 20 may be opened to release the component 12. Upon completion of the hot forming operation, if the component 12 is formed from niobium (Nb) micro-alloyed press hardened steel, the component 12 may have a ductility of between about 6% and 12%, with a tensile strength of at least about 1,500 MPa.

Aspects of this disclosure may be implemented, in some embodiments, through a computer-executable program of instructions, such as program modules, generally referred to as software applications or application programs executed by an on-board vehicle computer. The software may include, in non-limiting examples, routines, programs, objects, components, and data structures that perform particular tasks or implement particular abstract data types. The software may form an interface to allow a computer to react according to a source of input. The software may also cooperate with other code segments to initiate a variety of tasks in response to data received in conjunction with the source of the received data. The software may be stored on any of a variety of memory media, such as CD-ROM, magnetic disk, bubble memory, and semiconductor memory (e.g., various types of RAM or ROM).

Moreover, aspects of the present disclosure may be practiced with a variety of computer-system and computer-network configurations, including multiprocessor systems, microprocessor-based or programmable-consumer electronics, minicomputers, mainframe computers, and the like. In addition, aspects of the present disclosure may be practiced in distributed-computing environments where tasks are performed by remote-processing devices that are linked through a communications network. In a distributed-computing environment, program modules may be located in both local and remote computer-storage media including memory storage devices. Aspects of the present disclosure may therefore, be implemented in connection with various hardware, software or a combination thereof, in a computer system or other processing system.

Any of the methods described herein may include machine readable instructions for execution by: (a) a processor, (b) a controller, and/or (c) any other suitable processing device. Any algorithm, software, or method disclosed herein may be embodied in software stored on a tangible medium such as, for example, a flash memory, a CD-ROM, a floppy disk, a hard drive, a digital versatile disk (DVD), or other memory devices, but persons of ordinary skill in the art will readily appreciate that the entire algorithm and/or parts thereof could alternatively be executed by a device other than a controller and/or embodied in firmware or dedicated hardware in a well-known manner (e.g., it may be implemented by an application specific integrated circuit (ASIC), a programmable logic device (PLD), a field programmable logic device (FPLD), discrete logic, etc.). Further, although specific algorithms are described with reference to flowcharts depicted herein, persons of ordinary skill in the art will readily appreciate that many other methods of implementing the example machine readable instructions may alternatively be used. For example, the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, or combined.

While aspects of the present disclosure have been described in detail with reference to the illustrated embodiments, those skilled in the art will recognize that many modifications may be made thereto without departing from the scope of the present disclosure. The present disclosure is not limited to the precise construction and compositions disclosed herein; any and all modifications, changes, and variations apparent from the foregoing descriptions are within the spirit and scope of the disclosure as defined in the appended claims. Moreover, the present concepts expressly include any and all combinations and subcombinations of the preceding elements and features.

Claims

1. A method of hot forming a component from steel, the method comprising: transferring a workpiece to a furnace, the workpiece being formed from a press hardened steel alloyed with niobium (Nb);heating the workpiece in the furnace to a furnace temperature and during a furnace time selected from a pentagon having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.); andtransferring the heated workpiece from the furnace to a hot forming apparatus.

2. The method of claim 1, wherein the press hardened steel is micro-alloyed with approximately 0.02 to 0.1 weight percent (wt. %) Nb.

3. The method of claim 2, wherein the press hardened steel is micro-alloyed with approximately 0.05 wt. % Nb.

4. The method of claim 1, wherein a final austenite grain size for the heated workpiece is approximately 10-40 microns.

5. The method of claim 1, wherein a final austenite grain size for the heated workpiece is approximately 15 microns or less.

6. The method of claim 1, wherein the furnace time includes heat ramp and soaking for a total furnace time of approximately 3 minutes, and the furnace temperature includes a heating rate of approximately 10° C./s to a peak furnace temperature of approximately 980° C.

7. The method of claim 1, wherein the press hardened steel includes an aluminum silicon (AlSi) coating.

8. The method of claim 1, wherein the press hardened steel includes a boron-alloyed press hardened steel.

9. The method of claim 8, wherein the boron-alloyed quenched and tempered press hardened steel is 22MnB5 grade press hardened steel.

10. The method of claim 1, further comprising: hot forming the heated workpiece via the hot forming apparatus; andconcurrent to the hot forming of the heated workpiece, cooling the workpiece in the hot forming apparatus at a predetermined cooling rate until a predetermined workpiece temperature is achieved,wherein the predetermined cooling rate is not less than a critical cooling rate.

11. The method of claim 11, wherein the predetermined workpiece temperature is between approximately 150 to 200° C., and wherein the critical cooling rate is not greater than 35 Kelvin per second.

12. The method of claim 1, wherein the hot forming apparatus is a stamping press with a pair of opposing die, the method further comprising: hot stamping the heated workpiece in the die to form an intermediate workpiece; andcooling the intermediate workpiece in the die at a predetermined cooling rate to form a final steel component.

13. The method of claim 12, wherein the final steel component has a ductility of between approximately 6 to 12% and a tensile strength of approximately 1,800 megapascals (MPa) or greater.

14. A metalworking system for hot forming a component from steel, the metalworking system comprising: a transfer device operable to transfer a workpiece between stations of the metalworking system, the workpiece being formed from a press hardened steel alloyed with niobium (Nb);a furnace operable to receive the workpiece from the transfer device, the furnace being configured to heat the workpiece to a furnace temperature and during a furnace time selected from a pentagon having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.); anda hot forming apparatus operable to receive the heated workpiece from the furnace and mechanically deform the heated workpiece.

15. A method of operating a metalworking system for hot forming a component from steel, the metalworking system including multiple metalworking stations, including a furnace and a hot forming apparatus, the method comprising: commanding a transfer device to transfer a workpiece to the furnace, the workpiece being formed from a press hardened steel alloyed with niobium (Nb);commanding the furnace to heat the workpiece to a furnace temperature and during a furnace time selected from a pentagon having heating time and temperature coordinates ABCDE of: A (about 2 minutes, about 940° C.), B (about 2 minutes, about 1100° C.), C (about 3.5 minutes, about 1100° C.), D (about 5 minutes, about 975° C.), and E (about 5 minutes, about 940° C.);commanding the transfer device to transfer the heated workpiece from the furnace to the hot forming apparatus; andcommanding the hot forming apparatus to mechanically deform the heated workpiece.

16. The method of claim 15, wherein the press hardened steel is micro-alloyed with approximately 0.02 to 0.1 wt. % Nb.

17. The method of claim 15, wherein a final austenite grain size for the heated workpiece is approximately 10-40 microns.

18. The method of claim 15, wherein the press hardened steel includes a boron-alloyed press hardened steel.

19. The method of claim 15, further comprising, concurrent to the hot forming apparatus mechanically deforming the heated workpiece, commanding the hot forming apparatus to cool the workpiece at a predetermined cooling rate until a predetermined workpiece temperature is achieved, wherein the predetermined workpiece temperature is between approximately 150 to 200° C., and wherein the critical cooling rate is not greater than 35 Kelvin per second.

20. The method of claim 19, wherein the furnace time includes heat ramp and soaking for a total furnace time of approximately 3 minutes, and the furnace temperature includes a heating rate of approximately 10° C./s to a peak furnace temperature of approximately 980° C.