HYBRID STAMPING PROCESS INCORPORATING TRADITIONAL COLD STAMPING WITH SELECTIVE THERMAL FORMING/FLANGING/TRIMMING OPERATIONS

Abstract

A process and assembly for heat treating portions of a steel article, such as an advanced high strength (AHSS) steel. In an initial operation, the invention provides for positioning of heating elements at locations along such as a blank shaped steel article. Following an initial heating, a suitable forming or stamping operation is employed to bend or reform the article with the heated zones defining the bend axles or points within the article. Following the initial heating and bending/stamping operations, the heating elements can be added, such as against outer flange locations of the previously stamped article, following which a subsequent trimming or final fabricating step is employed to complete the article.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Ser. No. 63/417,126 filed Oct. 18, 2022.

FIELD OF THE INVENTION

The present invention relates generally to enhancing forming of steel articles. More specifically, the present invention teaches a process and assembly for heat treating portions of a steel article prior to a stamping operation in order to enhance material properties of the stamping in order to increase formability and/or trim-ability.

BACKGROUND OF THE INVENTION

Advanced High Strength Steels (AHSS) are known in the relevant art which allow the down gaging, and therefore lightweighting, of automotive panels that take advantage of that material's higher strength. Those same material properties also make it challenging to form, flange and trim into a desired shape.

Stretch flanges are particularly troublesome to form utilizing these new materials. The higher strength and hardness associated with the newer high strength steels also are associated with the premature wear of the tooling edges used to trim the panel. The coupling of those issues can lead to production quality concerns that are challenging to resolve.

Other examples drawn from the relevant prior art include the press system and method of U.S. Pat. No. 10,618,094, to Martin Gonzalez, which teaches manufacturing hot formed structural components and includes a fixed lower body, a mobile upper body and a mechanism configured to provide upwards and downwards press progression of the mobile upper body with respect to the fixed lower body. A cooling/heating tool is configured to cool down and/or heat a previously heated blank having locally different microstructures and mechanical properties, The tool includes each of upper and lower mating dies including two or more die blocks adapted to operate at different temperatures corresponding to zones of the blank having locally different microstructures and mechanical properties. A press tool is configured to draw the blank and is arranged downstream the cooling/heating tool.

Also disclosed is the method of forming a metallic article set forth in US 2012/0067100 to Stefansson et al., which teaches directly and/or indirectly inductively heating a localized region of a metallic article to a forming temperature. The metallic article may include materials selected from titanium alloys, nickel-base alloys, and specialty steels, e.g., stainless steel, high-strength low-alloy steel, armor steel alloys, and the like. The forming temperature may be in a forming temperature range of 0.2 to 0.5 of a melting temperature of the metallic article, which is then formed in the localized region.

SUMMARY OF THE INVENTION

The present invention discloses a process and assembly for heat treating portions of a steel article, such as an advanced high strength (AHSS) steel, which is also referenced in the relevant industry as third generation or Gen3 steel. In an initial operation, the invention provides for positioning of heating elements at locations along a blank shaped steel article. Following an initial heating, a suitable forming or stamping operation is employed to bend or reform the article with the heated zones defining bend axles or points within the article. Following the initial heating and bending/stamping operations, the heating elements are repositioned, such as against outer flange locations of the previously stamped article, following which a subsequent trimming or final fabricating step is employed to complete the article.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the attached drawings, when read in combination with the following detailed description, wherein like reference numerals refer to like parts throughout the several views, and in which:

FIGS. 1A and 1B depict top and end views of an arrangement of heating elements positioned at specific locations underneath a steel sheet prior to a stamping operation;

FIGS. 2A and 2B depict top and end views of edge heated zones resulting from the placement of the elements in FIGS. 1A and 1B associated with the post-stamped sheet;

FIGS. 3A and 3B depict a further succeeding arrangement of heating elements arranged along opposite flange edges of the stamped sheet of FIGS. 2A and 2B;

FIGS. 4A and 4B depict a final arrangement of relocated heated zones at the flange edges associated with a further trimming operation;

FIG. 5A presents a graphical illustration of a heated steel blank in comparison to a conventional unheated blank prior to a stamping operation and which illustrates a reduction in the force required to form a stamping into a desired shape;

FIG. 5B presents a succeeding graphical illustration to that shown in FIG. 5A recalibrated for depicting identical press tonnages applied to each of the conventional and hybrid blanks;

FIGS. 6A and 6B provide respective illustrations of both unheated and heated samples of a steel blank and which creates a tempered Martensite and Ferrite microstructure which is both finer and stronger in the final formed product, and with a greater number of martensitic plates evidenced in the heated sample of FIG. 6B;

FIG. 7 is a tabular presentation of each of mean diagonal length, force and hardness comparing each of the un-heated and heated samples of FIGS. 6A and 6B;

FIG. 8 is a schematic illustration comparing the increase in measured quality of both a heated and conventional blank of a Gen3 980 material pressed with a high press tonnage and high nitrogen pad pressure; and

FIG. 9 is an illustration of a stamped article produced by the hybrid stamping process according to the present invention, and presented in side by side comparison with a cold stamped and fabricated part.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the attached illustrations, the present invention discloses a process and assembly for heat treating portions of a steel article, such as in particular advanced high strength steels, order to enhance the material properties of an associated stamping process, this in order to increase formability, flange-ability and/or trim-ability. In particular, the AHSS material is strategically heated in order to provide for tighter bend radii, and as compared to cold deformation of the steel in order to avoid instances of edge cracking or splitting inside of the trim line of the article.

As will be further described in reference to the attached illustrations, the present invention envisions the strategic application of thermal energy, such as in the placement of heating elements, in order to pre-heat locations of the steel blank (typically a flattened AHSS sheet) prior to a stamping operation, this followed by subsequent targeted heating steps associated with any downstream trimming or bending operation for producing a finished part, which can be achieved closer to a desired shape than is often otherwise attainable. The present process and assembly also allows for reduced application force in the stamping and finished fabrication steps.

With reference to FIGS. 1A and 1B, depicted are top and end views of an arrangement of heating elements, see at 10 and 12 positioned at specific locations underneath a steel sheet or blank 14, prior to a stamping operation. The heating elements can be of any known type or construction and can include, without limitation, any of direct heating or other inductive heating elements for introducing a current to flow in 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 created using a resonating coil, resulting in heat being generated through resistance losses, as well as hysteresis losses in ferromagnetic materials like iron.

Upon the selected areas of the steel blank being heated, a typical stamping operation (not shown) is employed in order to bend or reform the blank into a three dimensional cross sectional shape 14′ depicted in FIGS. 2A and 2B (including each of a base surface 15 and outer angled flange surfaces 16/19) as each of top and end views of the post-stamped sheet. The bend locations associated with the stamping process align with the edge heated zones depicted at 16 and 18, resulting from the placement of the elements 10 and 12 in FIGS. 1A and 1B associated with the post-stamped sheet.

Proceeding to FIGS. 3A and 3B, depicted are a further succeeding arrangement of heating elements, see at 20 and 22, arranged along opposite flange edges of the stamped sheet 14′ of FIGS. 2A and 2B, and in order to execute a succeeding or second heating operation following the initial stamping operation performed on the steel sheet or blank. Finally, FIGS. 4A and 4B depict a final arrangement of relocated heated zones (24 and 26) at the flange edges of the previously stamped and reheated sheet, this associated with a further trimming operation in order to produce the completed article as depicted at 14″.

Proceeding to FIG. 5A, presented is a graphical illustration, generally at 28 of a heated steel blank 30, this shown in comparison to a conventional unheated blank 32 prior to conducting the stamping operations previously described. FIG. 5A depicts each of Force (kN) 34 and deformation distance of Bottom (mm) 36, these both plotted relative to shifted time (sec) 38, and which illustrates a reduction in the force required to form a stamping into a desired shape. In particular, the depiction illustrates a significant reduction in required bending forces (kN) over shifted time (sec) for the heated blank 30, such as at the point of introduction of load cells (at 40), and in comparison to the conventional blank 32.

FIG. 5B presents a succeeding graphical illustration, generally at 42, to that shown in FIG. 5A, which includes the same representations recalibrated for depicting identical press tonnages, further at 44, applied to each of the conventional and hybrid blanks;

Proceeding to FIGS. 6A and 6B, provided are respective illustrations of both unheated 46 and heated 48 samples of a steel blank, and which creates a tempered Martensite and Ferrite microstructure. As is specifically shown in FIG. 6B, the Martensitic phases of the heated blank 48 are more numerous and more pronounced than as compared to the unheated blank 46, resulting in being both finer and stronger in the final formed product. As is known, ultrafine-grained dual-phase ferrite/martensite steel is produced through inter-critical annealing at 765, 775 and 795° C., in which the microstructures at all temperatures consist of particles of ultrafine ferrite, martensite and carbides.

FIG. 7 provides a tabular presentation of each of mean diagonal length, force and hardness (measured in Vickers hardness) comparing each of the un-heated 46 and heated 48 samples of FIGS. 6A and 6B. Of note, the sample Vickers hardness average for the heated sample was 374 HK, as compared to 307 HK for the unheated sample.

FIG. 8 presents, as generally shown at 50, a schematic illustration comparing both the heated 48 and room temperature (unheated) 46 blanks of a Gen3 steel. Finally, FIG. 9 is an illustration of a stamped article, see at 52, produced by the hybrid stamping process according to the present invention, and which are presented in side by side comparison with a cold stamped and fabricated part 54, the latter depicting undesirable edge cracking or splitting inside of the trim line (further at 56).

Aspects and benefits of the process and assembly of the present invention include each of the following:

• • Refined Microstructure—The strategic addition of heat allows for a localized tempered microstructure that enables Advanced High Strength Materials to be stamped with more complex shapes of varied strengths.
  • Greater Resistance to Localized Necking—The resulting microstructure is more resistant to localized necking.
  • Higher Strength—The resulting microstructure is of higher hardness and therefore higher strength.
  • Greater “Draw-Ability”—Strategic heating allows for more complex shapes of greater depth of draw.
  • Smaller Radii—Smaller radii are possible with the addition of heat in the process.
  • Greater Resistance to Edge Fractures—The addition of heat allows material properties with greater resistance to edge fractures.
  • Lower Forming Tonnages—Elevated temperatures during the forming process lowers the forming forces required to form the same shape with a conventionally stamped product.
  • Higher Production Rates—With the heating elements being in-line with the Blanking and/or Stamping dies, the production rates are lower than oven heating processes.
  • Higher Quality Less Spring-Back/Less Sidewall Curl—The heating of the material enables the stamping material to have less spring-back and sidewall curl than conventional stamping processes.
  • Laser Welded Blank—The heating of a laser welded joint allows for the stress relieves that joint, allowing for greater formability during the forming process.

Having described my invention, other and additional preferred embodiments will become apparent to those skilled in the art to which it pertains, and without deviating from the scope of the appended claims. The detailed description and drawings are further understood to be supportive of the disclosure, the scope of which being defined by the claims. While some of the best modes and other embodiments for carrying out the claimed teachings have been described in detail, various alternative designs and embodiments exist for practicing the disclosure defined in the appended claims.

The foregoing disclosure is further understood as not intended to limit the present disclosure to the precise forms or particular fields of use disclosed. As such, it is contemplated that various alternate embodiments and/or modifications to the present disclosure, whether explicitly described or implied herein, are possible in light of the disclosure. Having thus described embodiments of the present disclosure, a person of ordinary skill in the art will recognize that changes may be made in form and detail without departing from the scope of the present disclosure. Thus, the present disclosure is limited only by the claims.

In the foregoing specification, the disclosure has been described with reference to specific embodiments. However, as one skilled in the art will appreciate, various embodiments disclosed herein can be modified or otherwise implemented in various other ways without departing from the spirit and scope of the disclosure. Accordingly, this description is to be considered as illustrative and is for the purpose of teaching those skilled in the art the manner of making and using various embodiments of the disclosure. It is to be understood that the forms of disclosure herein shown and described are to be taken as representative embodiments. Equivalent elements, materials, processes or steps may be substituted for those representatively illustrated and described herein. Moreover, certain features of the disclosure may be utilized independently of the use of other features, all as would be apparent to one skilled in the art after having the benefit of this description of the disclosure. Expressions such as “including”, “comprising”, “incorporating”, “consisting of”, “have”, “is” used to describe and claim the present disclosure are intended to be construed in a non-exclusive manner, namely allowing for items, components or elements not explicitly described also to be present. Reference to the singular is also to be construed to relate to the plural.

Further, various embodiments disclosed herein are to be taken in the illustrative and explanatory sense, and should in no way be construed as limiting of the present disclosure. All joinder references (e.g., attached, affixed, coupled, connected, and the like) are only used to aid the reader's understanding of the present disclosure, and may not create limitations, particularly as to the position, orientation, or use of the systems and/or methods disclosed herein. Therefore, joinder references, if any, are to be construed broadly. Moreover, such joinder references do not necessarily infer that two elements are directly connected to each other.

Additionally, all numerical terms, such as, but not limited to, “first”, “second”, “third”, “primary”, “secondary”, “main” or any other ordinary and/or numerical terms, should also be taken only as identifiers, to assist the reader's understanding of the various elements, embodiments, variations and/or modifications of the present disclosure, and may not create any limitations, particularly as to the order, or preference, of any element, embodiment, variation and/or modification relative to, or over, another element, embodiment, variation and/or modification.

It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. Additionally, any signal hatches in the drawings/figures should be considered only as exemplary, and not limiting, unless otherwise specifically specified.

Claims

1. A process for heat treating portions of a steel article, comprising the steps:

providing a blank steel sheet;

positioning at least one heating element at a locations along the blank;

heating the blank; and

forming the heated blank according to any of a press or stamping operation and to exhibit any of a tighter bend radii or higher formability.

2. The process as described in claim 1, further comprising the step of either repositioning the at least one heating element or positioning at least one additional heating element at a further location of the formed blank, not limited along an outer flange locations of the blank, following which a subsequent trimming or final fabricating step is employed to complete the article.

3. The process as described in claim 1, further comprising the step of providing the blank steel sheet as an advanced high strength (AHSS) steel.

4. An assembly for forming a blank sheet steel article, comprising:

at least one heating element placed along an extending location of the blank sheet steel article in order to heat the extending location;

a press or stamping assembly for forming the blank article into a three dimensional article including a base surface and angled flange surfaces;

at least one additional heating element being added or the heating element being relocated to a further location of the formed blank not limited to the angled flange surfaces; and

a trimming assembly being employed to finish fabricate the steel article along the further locations of the formed blank and to exhibit a tighter bend radii.

5. The assembly as described in claim 4, blank sheet steel article further comprising an advanced high strength (AHSS) steel.