Method for producing tubular uncoated punch-hardened steel

Abstract

A method of producing a tubular component using uncoated punch hardened steel (CFPHS) is provided. The method includes feeding the CFPHS continuous tubular component into a heating unit and heating the CFPHS continuous tubular component with the heating unit. The method then includes bending the CFPHS continuous tubular component using a bending unit prior to cooling of the continuous tubular component. After the bending step, the method includes cooling the CFPHS continuous tubular member with ambient air using a cooling unit.

Description

Technical Field

[0001] The present disclosure relates to tubular uncoated press-hardened steel for vehicle structures and body components. Background Art

[0002] Press-hardened steel is used in vehicles including automobiles, trucks, vans, sport utility vehicles, autonomous vehicles, battery electric vehicles, agricultural or construction equipment, railway vehicles, etc. to provide increased material strength in load-bearing components and reduce material collapse in body impact areas such as door anti-intrusion beams and body pillars. Current press-hardened steel (PHS) is fully formed in a furnace. Before heating, a coating such as aluminum-silicon is applied to the steel to prevent corrosion and prevent the formation and decarburization of surface scale during the hot forming process. In the furnace, the heating rate and final heating temperature must be carefully controlled to avoid coating melting and to control the thickness of the interdiffusion layer between the substrate and the coating to achieve the required strength after hot forming. For uncoated press-hardened steel (PHS), processes such as shot peening must be used to remove the scale generated during heating after hot forming.

[0003] The material properties of currently prepared PHS are consistent throughout the blank and the finished form of the entire article. The uniform material properties are improved to maximize material strength, for example, in vehicle extrusion areas and pillars.

[0004] Therefore, although the current systems and methods for producing and using press-hardened steel achieve their intended purposes, there is a need for a new and improved system and method for producing and using press-hardened steel. Summary of the Invention

[0005] According to several aspects of the present disclosure, a method for producing a tubular component using uncoated press-hardened steel (CFPHS) is provided. The method includes feeding a CFPHS continuous tubular component into a heating unit and heating the CFPHS continuous tubular component with the heating unit. The method then includes bending the CFPHS continuous tubular component using a bending unit before cooling the continuous tubular component. After the bending step, the method includes cooling the CFPHS continuous tubular component using ambient air with a cooling unit.

[0006] According to another aspect of the present disclosure, the method includes a heating unit that includes at least one of an induction heating coil, a laser-based heater, or a flame-based heater.

[0007] According to another aspect of the present disclosure, the method includes a heating unit that includes a plurality of induction heating coils arranged in series along the CFPHS continuous tubular component.

[0008] According to another aspect of the present disclosure, the method includes a heating unit that includes a plurality of induction heating coils arranged in parallel along a CFPHS continuous tubular member.

[0009] According to another aspect of the present disclosure, the method includes a CFPHS continuous tubular member that includes at least one of concentric CFPHS tubing or square CFPHS tubing.

[0010] According to another aspect of the present disclosure, the method includes heating a CFPHS continuous tubular member, including heating a portion of the CFPHS continuous tubular member to a uniform temperature using at least one induction heating coil.

[0011] According to another aspect of the present disclosure, the method includes heating a CFPHS continuous tubular member, including heating the CFPHS continuous tubular member to different temperatures along the length of the CFPHS continuous tubular member to produce variable mechanical properties.

[0012] According to another aspect of the present disclosure, the method includes heating a CFPHS continuous tubular member, including heating the CFPHS continuous tubular member to different temperatures along the circumference of the CFPHS continuous tubular member to produce variable mechanical properties.

[0013] According to another aspect of the present disclosure, the method includes heating a CFPHS continuous tubular member, including employing a variable heating rate to achieve a specific temperature distribution.

[0014] According to another aspect of the present disclosure, the method further includes holding the CFPHS continuous tubular member for less than 30 seconds after heating the CFPHS continuous tubular member and before cooling the CFPHS continuous tubular member.

[0015] According to another aspect of the present disclosure, the method further includes holding the CFPHS continuous tubular member at a temperature between 875 °C and 1200 °C after heating the CFPHS continuous tubular member and before cooling the CFPHS continuous tubular member to produce a fully austenitized microstructure.

[0016] According to another aspect of the present disclosure, the method further includes holding the CFPHS continuous tubular member at a temperature between 750 °C and 875 °C after heating the CFPHS continuous tubular member and before cooling the CFPHS continuous tubular member to achieve partial austenitization.

[0017] According to another aspect of the present disclosure, the method includes bending the CFPHS continuous tubular member, including at least one of roll bending or mandrel bending.

[0018] According to another aspect of the present disclosure, the method includes cooling a CFPHS continuous tubular member, including using forced air cooling.

[0019] According to another aspect of the present disclosure, the method includes cooling a continuous tubular member, including using a tunnel furnace for cooling to slow down the cooling rate.

[0020] According to another aspect of the present disclosure, the method includes using a quenching medium including cooling air.

[0021] According to another aspect of the present disclosure, the method includes a CFPHS continuous tubular member having a tube elongation greater than 4% after cooling the CFPHS continuous tubular member.

[0022] According to several aspects of the present disclosure, a method of achieving variable properties of a tubular member using uncoated press hardenable steel (CFPHS) is provided. The method includes feeding a CFPHS continuous tubular member into a heating unit having an induction heating coil and energizing the induction heating coil using a variable power supply. The method then includes heating the CFPHS continuous tubular member by generating a varying local current intensity within the heating unit and passing through the CFPHS continuous tubular member by operating a predetermined coil in the induction heating coil. The method then continues by bending the CFPHS continuous tubular member using a bending unit and cooling the CFPHS continuous tubular member using ambient air.

[0023] According to another aspect of the present disclosure, the method further includes holding the CFPHS continuous tubular member for less than 30 seconds after heating the CFPHS continuous tubular member and before cooling the CFPHS continuous tubular member.

[0024] According to several aspects of the present disclosure, a method of producing an induction hardened tubular member using uncoated press hardenable steel (CFPHS) is provided. The method includes feeding a CFPHS continuous tubular member into a heating unit having at least one induction heating coil. The CFPHS continuous tubular member is then heated using the heating unit having at least one induction heating coil. The at least one induction heating coil provides a varying temperature along the length of the CFPHS continuous tubular member to produce customized properties. The method then includes holding the CFPHS continuous tubular member at a temperature between 875 °C and 1200 °C for less than 30 seconds to achieve a fully austenitized microstructure. After the holding step, the method then includes bending the CFPHS continuous tubular member using a bending unit before cooling the CFPHS continuous tubular member. The bending unit includes roll bending. After the bending step, the method includes cooling the CFPHS continuous tubular member using ambient air to transform the fully austenitized microstructure into martensite.

[0025] Further application areas will become apparent from the description provided herein. It should be understood that these descriptions and specific examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way.

[0027] Figure 1 is a schematic view of a system for producing induction hardened continuous tubular components using uncoated press hardened steel in accordance with the present disclosure; and

[0028] Figure 2 is a flowchart of a method for producing induction hardened continuous tubular components using uncoated press hardened steel by a system utilizing Figure 1 in accordance with an exemplary embodiment. DETAILED DESCRIPTION

[0029] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

[0030] When an element, component, or layer is referred to as being "on," "engaged to," "connected to," or "coupled to" another element or layer, it can be directly on, engaged, connected, or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," "directly connected to," or "directly coupled to" another element or layer, no intervening elements or layers may be present. Other words used to describe the relationship between elements should be interpreted in a like manner, e.g., "between" versus "directly between," "adjacent" versus "directly adjacent," etc. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0031] Reference Figure 1, a system 10 for producing a continuously formed press-hardened steel (CFPHS) tubular component 12 is shown. The CFPHS tubular component 12 is used for structural components of a vehicle (e.g., a door impact beam, an A-pillar that connects a windshield to the top of an automobile, etc.). Currently produced PHS increases the weight and cost of the entire component and may not allow the component to undergo a desired deformation under impact loads, which allows for local absorption and dissipation of the impact loads. The CFPHS tubular component 12 is continuous in that a portion or length of the CFPHS tubular component 12 is uninterrupted or continuous and, for example, a portion or length of the CFPHS tubular component 12 is fed into the system 10 using a series of rollers. As used herein, a vehicle can include, for example, an automobile, a truck, a van, a sport utility vehicle, an autonomous vehicle, a battery electric vehicle, an agricultural or construction equipment, a railway vehicle, etc. The CFPHS tubular component 12 is in a tubular form and can include a variety of cross-sectional geometries, such as circular, concentric, square, etc.

[0032] CFPHS is a low-carbon (C) content uncoated steel with chromium and silicon added, and a dense oxide layer is formed on the surface of CFPHS after hot forming. CFPHS is an alternative to traditional aluminum-silicon (Al-Si) coated press-hardened steel, where the aluminum-silicon (Al-Si) coating is applied to the steel sheet before hot forming. In addition, CFPHS is an uncoated PHS that forms an antioxidant layer during heating to protect the surface, thus eliminating the need for coating or shot peening, and in the case of traditional uncoated (bare) PHS, post-treatment is performed to maintain the surface quality.

[0033] The material composition of the CFPHS tubular component 12 includes an alloy matrix that includes carbon (C) in a concentration greater than or equal to 0.05% to less than or equal to about 0.35% by weight (wt.%), chromium (Cr) in a concentration greater than or equal to about 1 wt.% to less than or equal to about 9 wt.%, silicon (Si) in a concentration greater than or equal to about 0.5 wt.% to less than or equal to about 2 wt.%, manganese (Mn) in a concentration greater than or equal to about 0.5 wt.% to less than or equal to about 2.5 wt.%, and the balance being iron (Fe). As used herein, the term "about" is known to those skilled in the art. Alternatively, the term "about" can be understood to mean plus or minus 0.5% by weight.

[0034] Reference Figure 1, the system 10 includes a heating unit 14, a bending unit 16, and a cooling unit 18. The heating unit 14 preferably includes at least one induction heating coil 20. The induction heating coil 20 uses electromagnetic induction to heat the CFPHS continuous tubular member 12 through heat transfer passing through the inductor, which generates an electromagnetic field within the induction heating coil 20 to heat the CFPHS continuous tubular member 12. The heat from the induction heating coil 20 is generated inside the CFPHS continuous tubular member 12, rather than being generated by an external heat source through heat conduction. According to several aspects, the CFPHS continuous tubular member 12 is inductively heated from room temperature to a range including about 750 °C to about 1200 °C within the heating unit 14. Herein, the term "about" is known to those skilled in the art. Alternatively, the term "about" can be understood to mean plus or minus 10 °C. According to several aspects, a heating rate of about 20 °C / second to up to about 500 °C / second is used. Herein, the term "about" is known to those skilled in the art. Alternatively, the term "about" can be understood to mean plus or minus 5 °C / second.

[0035] In the heating unit 14, the CFPHS continuous tubular member 12 is heated to transform the crystal structure of the steel from ferrite to austenite. Austenite is a more open and flexible structure that can absorb more carbon from iron carbide in carbon steel. Transforming ferrite to austenite or austenitizing enables subsequent transformation to martensite upon cooling, which changes the mechanical properties of the CFPHS continuous tubular member 12, making it suitable for use in, for example, vehicle structural components.

[0036] The induction heating coil 20 is powered by a variable power supply 22, and the variable power supply 22 may include a frequency conversion device 24. By exciting or changing the power supplied to a predetermined coil among the plurality of induction heating coils 20, the variable power supply 22 and the frequency conversion device 24 generate a varying local current intensity on the CFPHS continuous tubular member 12. Using the variable power supply 22, a variable heating rate and a zoned holding temperature can be provided to the CFPHS continuous tubular member 12. For example, the variable heating rate may include heating the CFPHS continuous tubular member 12 at a rate of 500 °C / second for a first time period and then at a rate of 100 °C / second for a second time period. It should be understood that other variable heating rates may be used.

[0037] In some aspects, the heating unit 14 may include other heating devices, for example, a laser-based heating device or a flame-based heating device. When using a laser-based heating device, the device directs a laser onto the CFPHS continuous tubular member 12 for heating. When using a flame-based heating device, a flame directed at the CFPHS continuous tubular member 12 is used for heating.

[0038] Induction heating achieves the target surface oxide layer characteristics within the CFPHS continuous tubular component 12. The mechanical properties can be altered by induction heating through the thickness of the CFPHS continuous tubular component 12. Using induction heating across the entire surface of the CFPHS continuous tubular component 12 also results in different mechanical properties. Using induction heating to target different locations within the CFPHS continuous tubular component 12 further enables variable surface characteristics. Induction heating allows for variable mechanical properties to be achieved for different locations within the CFPHS continuous tubular component 12.

[0039] The heating unit 14 can include multi-stage heating. For example, a first induction heating coil can heat the CFPHS continuous tubular component 12, which can then be cooled and then reheated, or heated at a different rate by a second induction heating coil. The heating unit 14 can include multiple induction heating coils 20 in series and / or in parallel.

[0040] Within the heating unit 14, the CFPHS continuous tubular component 12 is heated, and when multi-stage heating is used, the CFPHS continuous tubular component 12 can include different characteristics after heating and hardening compared to the uniform characteristics of the pre-heat treated CFPHS continuous tubular component 12. For example, induction heating can provide different characteristics across different regions, lengths, or thicknesses of the CFPHS continuous tubular component 12. In a specific example, after heating, the first side of the CFPHS continuous tubular component 12 includes a first set of modified surface characteristics, and the second side of the CFPHS continuous tubular component 12 includes a second set of modified surface characteristics. In another specific example, the radially inner portion of the CFPHS continuous tubular component 12 has a first set of modified characteristics, and the radially outer portion of the CFPHS continuous tubular component 12 has a second set of modified characteristics.

[0041] After the heating and / or soaking step using the heating unit 14, the CFPHS continuous tubular component 12 is fed into the bending unit 16 while still being plastic. The bending unit 16 includes a plurality of rollers 26. For example, the CFPHS continuous tubular component 12 is fed and guided through the plurality of rollers 26 to achieve the desired bend or shape. In Figure 1 the example shown, the CFPHS continuous tubular component 12 is shown bent into a curve. However, it should be understood that the CFPHS continuous tubular component 12 can be bent or formed into any configuration (e.g., angled, an irregular pattern with curved and linear portions, etc.).

[0042] After being bent by the bending unit 16, the cooling unit 18 immediately receives and cools the CFPHS continuous tubular member 12 using a quenching medium for cooling. Preferably, ambient air is used to cool the CFPHS continuous tubular member 12. For example, the cooling unit 18 may include a forced air blower cooling unit or the like.

[0043] The CFPHS continuous tubular member 12 of the present disclosure has minimal oxidation or no oxidation and no coating, and can be induction hardened without the risk of melting the coating material or generating excessive surface oxides (which form scale). The present disclosure provides a method and system for using local heating to adjust the mechanical quality and surface quality of the finished CFPHS continuous tubular member 12. The local heating method can employ a uniform heating rate that remains substantially constant over time, or a customized heating rate that changes at a predetermined heating rate over time, heating to a desired temperature profile. Exemplary temperature profiles may include holding the CFPHS continuous tubular member 12 at a temperature of 1000 °C for 20 seconds. The heating can be carried out uniformly over the entire CFPHS continuous tubular member 12 or within a predetermined area within the CFPHS continuous tubular member 12 to achieve a defined distribution of the selective mechanical properties and surface characteristics of the CFPHS continuous tubular member 12 that are heat treated and induction hardened.

[0044] Figure 2 A method 100 for producing a tubular member using CFPHS with the system 10 shown is illustrated. Starting at block 102, the method 100 includes feeding the CFPHS continuous tubular member 12 into the heating unit 14 using an automatic tube loader (not shown). Then, the method can proceed to block 104. Figure 1 At block 104, the method 100 includes energizing the induction heating coil in the heating unit 14 using the variable power supply 22. The variable power supply 22 can be initialized by a controller (not shown) or other automated means for determining the power supply to the variable power supply 22. Then, the method proceeds to block 106.

[0045] At block 106, the method 100 includes using the bending unit 16 to bend the CFPHS continuous tubular member 12 to a desired shape. Then, the method proceeds to block 108.

[0046] At block 106, method 100 includes heating CFPHS continuous tubular member 12 with heating unit 14. Heating CFPHS continuous tubular member 12 can include providing a uniform and / or varying temperature along the length of CFPHS continuous tubular member 12 using at least one induction heating coil to produce customized properties. Additionally, heating CFPHS continuous tubular member can include using at least one of an induction heating coil, a laser-based heater, or a flame-based heater. Further, heating CFPHS continuous tubular member can include using a plurality of induction heating coils arranged in series and / or parallel along CFPHS continuous tubular member. In one example, heating CFPHS continuous tubular member includes heating the CFPHS continuous tubular member to different temperatures along the circumference of the CFPHS continuous tubular member to produce variable mechanical properties. Then, method 100 can proceed to block 108.

[0047] At block 108, method 100 includes holding CFPHS continuous tubular member 12. Holding CFPHS continuous tubular member 12 includes maintaining the temperature of the heated CFPHS continuous tubular member 12 for a duration to achieve desired properties. In a specific example, CFPHS continuous tubular member 12 can be held at a temperature between 875 °C and 1200 °C for less than 30 seconds to achieve a fully austenitized microstructure that transforms to martensite upon cooling. In another specific example, holding CFPHS continuous tubular member 12 includes holding the CFPHS continuous tubular member at a temperature between 750 °C and 875 °C to achieve partial austenitization. Then, method 100 proceeds to block 110.

[0048] At block 110, method 100 includes bending CFPHS continuous tubular member 12 using bending unit 16. Bending CFPHS continuous tubular member 12 can include using at least one of, for example, roll bending (e.g., three-roll push bending) and / or mandrel bending. As Figure 1 shown, roll bending includes pressing an upper roll against CFPHS continuous tubular member 12 and rotating at least one lower roll to move CFPHS continuous tubular member 12, thereby producing a bent CFPHS continuous tubular member 12. Then, method 100 proceeds to block 112.

[0049] At block 112, method 100 includes cooling the CFPHS continuous tubular member 12 using a cooling unit 18. The cooling unit 18 can use ambient air, a quenching medium (e.g., cooling air), and / or forced air cooling to cool the CFPHS continuous tubular member 12. In some cases, a tunnel furnace cooling unit can be used to slow down the cooling rate of the CFPHS continuous tubular member 12. Slowing down the cooling rate can develop a microstructure from a fully austenitized state, where the martensite content is less than 95%, and the content of ferrite, pearlite, bainite, and / or retained austenite increases. In one example, the CFPHS continuous tubular member 12 after the cooling step has a tube elongation greater than 4%. The CFPHS continuous tubular member 12 after the cooling step can have a local tube ultimate tensile strength (UTS) greater than 1300 MPa. In some cases, the CFPHS continuous tubular member 12 can have a UTS below 1300 MPa due to the presence of phases or microconstituents other than martensite in an amount greater than 5% by volume. Then, method 100 ends.

[0050] According to several aspects, the method of using CFPHS to achieve variable properties of a component of the present disclosure uses a locally variable heating method such as induction heating to provide variable properties within the CFPHS continuous tubular member 12. The method of using CFPHS to achieve variable properties of a component of the present disclosure allows the CFPHS oxide layer to be customized to an optimal thickness from a non-oxide layer to the maximum allowable oxide layer in different regions of the CFPHS continuous tubular member 12. Using the CFPHS material of the present disclosure enables induction heating to be used on the CFPHS to achieve desired press hardening properties. Since induction heating is used to achieve the required CFPHS heat treatment, the method of using CFPHS to achieve variable properties of a component of the present disclosure makes conventional furnace heating unnecessary. It is noted that the desired press hardening performance of the present disclosure cannot be achieved using any base grade of conventional aluminum-silicon (Al-Si) coated press hardening steel or using bare (uncoated) press hardening steel without having an adverse effect on the surface quality or lacking other necessary subsequent or pre-operations (e.g., shot peening for bare materials and pre-diffusion for Al-Si coated materials).

[0051] The method of the present disclosure for achieving variable properties of the CFPHS continuous tubular component 12 allows for customized mechanical properties to be achieved by induction heating, e.g., reaching the austenitizing temperature in one or more regions of the component, and wherein in other regions the component is heated to subcritical temperature to achieve customized mechanical properties within the component. This cannot be achieved using only a conventional furnace. As used herein, austenitizing defines a heat treatment process for steel and other ferrous alloys, in which the material is heated above its critical temperature and transformed from ferrite to austenite crystal structure, which allows the austenite to absorb carbon from the iron carbide in carbon steel. When subsequently quenched, the austenitized material becomes hardened due to the transformation of austenite to martensite.

[0052] Using induction heating, multiple regions with different mechanical properties can be achieved within the component using induction coils. Current dedicated non-induction coil furnaces typically achieve this effect, but the number of possible regions is limited. This effect cannot be achieved using only a conventional furnace.

[0053] Using induction heating, target regions with desired surface characteristics can be achieved by varying the induction heating pattern. Varying the induction heating pattern cannot be achieved using only a conventional furnace. The CFPHS continuous tubular component 12 without a coating (and having a stable oxide layer formed during heating and holding at high temperature) can be induction hardened without the risk of melting or excessive surface oxides (scale).

[0054] The method of using the CFPHS continuous tubular component 12 of the present disclosure to achieve variable characteristics of the component provides several advantages. These advantages include using induction heating to heat treat the CFPHS continuous tubular component 12 to reduce capital investment costs due to the more compact nature of induction hardening equipment and the reduced complexity of induction tools relative to furnace heating. Maintenance of induction equipment may be less costly and involve less compared to furnace equipment, thus reducing the costs for component producers. Different mechanical properties within the CFPHS continuous tubular component 12 reduce the complexity of assemblies of multiple components with different material grades. Using induction heating on the CFPHS continuous tubular component 12 can reduce product costs and can improve the performance and reduce the complexity of the component. Controlling the oxide layer in selected regions of the CFPHS component can also improve the connection in the predetermined regions, thus making the assembly stronger and better integrated.

Claims

1. A method for producing a tubular component using uncoated press-hardened steel CFPHS, comprising: Feed the CFPHS continuous tubular member into a heating unit; Heat the CFPHS continuous tubular member using the heating unit; Before the continuous tubular member cools, bend the CFPHS continuous tubular member using a bending unit; and Cool the CFPHS continuous tubular member using a cooling unit with ambient air.

2. The method according to claim 1, wherein, The heating unit includes at least one of an induction heating coil, a laser-based heater, or a flame-based heater.

3. The method according to claim 1, wherein, The heating unit includes a plurality of induction heating coils arranged in series along the CFPHS continuous tubular member.

4. The method according to claim 1, wherein, The heating unit includes a plurality of induction heating coils arranged in parallel along the CFPHS continuous tubular member.

5. The method according to claim 1, wherein, The CFPHS continuous tubular member includes at least one of concentric CFPHS tubing or square CFPHS tubing.

6. The method according to claim 1, wherein, Heating the CFPHS continuous tubular member includes heating a portion of the CFPHS continuous tubular member to a uniform temperature with at least one induction heating coil.

7. The method according to claim 1, wherein, Heating the CFPHS continuous tubular member includes heating the CFPHS continuous tubular member to different temperatures along the length of the CFPHS continuous tubular member to produce variable mechanical properties.

8. The method according to claim 1, wherein, Heating the CFPHS continuous tubular member includes heating the CFPHS continuous tubular member to different temperatures along the circumference of the CFPHS continuous tubular member to produce variable mechanical properties.

9. The method according to claim 1, wherein, Heating the CFPHS continuous tubular member includes employing a variable heating rate to achieve a specific temperature profile.

10. The method according to claim 1, further comprising: After heating the CFPHS continuous tubular member and before cooling the CFPHS continuous tubular member, hold the CFPHS continuous tubular member for less than 30 seconds.

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