Rapid preheating to improve surface quality on uncoated press-hardened steel structural elements

Abstract

A method for manufacturing a coating-free press-hardened steel (CFPHS) component comprises heating a CFPHS blank to a first predetermined temperature in a first predetermined temperature range using a heating device at a first predetermined heating rate; transferring the CFPHS blank to a furnace; heating the CFPHS blank in the furnace at a second predetermined temperature in a second predetermined temperature range for a predetermined time; and pressing and forming the CFPHS blank in a die / press to form a CFPHS component.

Description

INTRODUCTION

[0001] The information provided in this section serves the purpose of providing a general overview of the context of the disclosure. Neither the work of the inventors currently named, to the extent described in this section, nor those aspects of the description that could not otherwise qualify as prior art at the time of filing, are expressly or implicitly recognized as prior art against the present disclosure.

[0002] The present disclosure relates to the manufacture of press-hardened steel components (PHS) and in particular to methods for improving the surface quality of coating-free press-hardened steel components (CFPHS).

[0003] Press hardening of steel (PHS) increases the strength of the steel. PHS is frequently used for load-bearing components in vehicles, construction equipment, and / or equipment for other industries. In press hardening, a steel blank is heated to an austenitizing temperature range, transferred while hot into a punch / press, and formed and pressed between two or more dies of the punch / press. In some cases, the steel blank is cold-formed into an intermediate die before austenitizing and forming and pressing.

[0004] If the steel blank is uncoated, an oxide layer forms on its surface after austenitizing during transfer to the die / press. This oxide layer must be removed after forming and pressing. One method of descaling the oxide layer is shot peening, which increases costs and production cycle time. To prevent oxide layer formation, the steel blank can be coated with a coating that inhibits its formation. For example, an aluminum-silicon alloy (AlSi alloy) can be used to prevent oxide coating and avoid the associated costs and time required for oxide layer removal. SUMMARY

[0005] A method for manufacturing a coating-free press-hardened steel component (CFPHS component) comprises heating a CFPHS blank to a first predetermined temperature in a first predetermined temperature range using a heating device at a first predetermined heating rate; transferring the CFPHS blank to a furnace; heating the CFPHS blank in the furnace at a second predetermined temperature in a second predetermined temperature range for a predetermined time; and pressing and forming the CFPHS blank in a die / press to form a CFPHS component.

[0006] According to other specifications, the first preset temperature is between 500 °C and 950 °C. The first preset heating rate is between 20 °C / s and 300 °C / s. The heating device is selected from a group consisting of an induction heater, a resistance heater, a laser heater, or combinations thereof. The second preset temperature is between 750 °C and 1200 °C. The preset time is in the range of 5 s to 500 s. The preset time is in the range of 5 s to 50 s.

[0007] According to other characteristics, after preheating in the heating device, the CFPHS device exhibits an oxide layer with a thickness in the range of greater than or equal to 0.1 microns to less than or equal to 1.5 microns. The oxide layer covers at least 90% of the exposed surface of the CFPHS device. After preheating in the heating device, the CFPHS device exhibits an oxide layer with a thickness in the range of greater than or equal to 0.1 microns to less than or equal to 1.0 microns.

[0008] A method for manufacturing a coating-free, press-hardened steel (CFPHS) component comprises heating a CFPHS ingot using a heating device to a first predetermined temperature in a range between 500 °C and 950 °C at a first predetermined heating rate in a range between 20 and 300 °C / s; transferring the CFPHS ingot to a furnace; heating the CFPHS ingot in the furnace for a predetermined time at a second predetermined temperature in a range between 750 °C and 1200 °C; and pressing and forming the CFPHS ingot in a die / press to form a CFPHS component. After preheating in the heating device, the CFPHS component has an oxide layer with a thickness in a range greater than or equal to 0.1 microns to less than or equal to 1.5 microns.

[0009] According to other specifications, the heating device is selected from a group consisting of an induction heater, a resistance heater, a laser heater, or combinations thereof. The specified duration is in the range of 5 s to 500 s. The specified duration is in the range of 5 s to 50 s. The oxide layer has a thickness in the range of 0.1 microns greater than or equal to 1.0 micron less than or equal to the oxide layer. The oxide layer covers at least 90% of the exposed surface of the CFPHS device.

[0010] A method for manufacturing a coating-free press-hardened steel component (CFPHS component) comprises heating a CFPHS blank to a first predetermined temperature in a range between 500 °C and 950 °C at a first predetermined heating rate in a range between 20 and 300 °C / s using one or more of the following devices: an induction heating device, a resistance heating device, a laser heating device, or combinations thereof; transferring the CFPHS blank to a furnace; heating the CFPHS blank in the furnace for a predetermined time in a range between 5 s and 50 s at a second predetermined temperature in a range between 750 °C and 1200 °C; transferring the CFPHS blank to a die / press; and pressing and forming the CFPHS blank in the die / press to form a CFPHS component.The CFPHS device has an oxide layer with a thickness in the range of greater than or equal to 0.1 microns to less than or equal to 1.5 microns.

[0011] According to other characteristics, the oxide layer has a thickness in the range of greater than or equal to 0.1 microns to less than or equal to 1.0 microns. The oxide layer covers at least 90% of the exposed surface of the CFPHS device.

[0012] Further applications of the present disclosure will become apparent from the detailed description, the claims, and the drawings. The detailed description and the specific examples serve only for illustration and are not intended to limit the scope of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present revelation is more fully understood from the detailed description and the accompanying drawings; they show: Fig. 1 a functional block diagram of an example of a method for manufacturing a coating-free press-hardened steel structural element (CFPHS) according to the present disclosure; Fig. 2 a flowchart of an example of a process for manufacturing a CFPHS device according to the present disclosure; and Fig. 3 a cross-section showing an example of an oxide layer of a CFPHS device according to the present disclosure.

[0014] Reference symbols can be used multiple times in the drawings to identify similar and / or identical elements. DETAILED DESCRIPTION

[0015] Although the process for manufacturing coating-free press-hardened steel (CFPHS) according to the present disclosure is described in connection with structural components for vehicles, the process for manufacturing CFPHS can also be used for other applications.

[0016] In the press hardening of steel, a steel blank is heated to a temperature range suitable for austenitization, transferred while hot into a punch / press, and formed and pressed between two or more dies of the punch / press. In some cases, the steel blank is cold-formed into an intermediate die before austenitization and forming and pressing.

[0017] To prevent oxide layer formation, the steel blank can be coated with a coating that inhibits oxide layer formation. For example, the steel blank can be coated with an aluminum-silicon alloy (AlSi alloy) to prevent oxide coating and avoid the cost and time required for oxide layer removal after pressing and forming. To reduce production time, the steel blank can be rapidly preheated before furnace heating using alternative heating methods (to shorten the time required in the furnace). However, AlSi-coated PHS is not compatible with rapid preheating. Due to the high heating rate, the AlSi coating layer melts without alloying.

[0018] In the press hardening of steel (PHS) according to the present disclosure, a steel blank comprising uncoated PHS (CFPHS) is rapidly preheated. After rapid preheating, the steel blank is transferred to a furnace for a heating period at a predetermined temperature and then transferred to a die / press for pressing and forming the CFPHS component. No descaling is required after pressing and forming the CFPHS component, which reduces costs.

[0019] A method for manufacturing a coating-free, press-hardened steel component (CFPHS component) is described in jointly transferred US patent publication 2021002746, which was published on January 7, 2021, and is incorporated herein in its entirety by reference. As described therein, the CFPHS blank is hot-stamped into a press-hardened component with a predetermined shape without coatings (and without the need for descaling).

[0020] The steel alloy used for the CFPHS blank comprises carbon (C), chromium (Cr), silicon (Si), and iron (Fe). During a hot stamping process, some of the Cr and Si combine with atmospheric oxygen to form a first layer, which consists of an oxide enriched with Cr and Si. If sufficient oxygen is present in the atmosphere, some of the Fe combines with atmospheric oxygen to form a second layer, which consists of an oxide enriched with Fe. The first and second layers prevent, inhibit, or minimize further oxidation, thus eliminating the need for descaling steps such as shot peening or sandblasting.

[0021] Carbon (C) is present in the steel alloy at a concentration of at least approximately 0.01 wt.% and at most approximately 0.35 wt.%. Cr (Cr) is present in the steel alloy at a concentration of at least approximately 1 wt.% and at most approximately 9 wt.%. Si (Si) is present in the steel alloy at a concentration of at least approximately 0.5 wt.% and at most approximately 2 wt.%. The remainder of the steel alloy consists of iron (Fe) (unless additional elements, some of which are described below, are added).

[0022] Furthermore, the CFPHS component contains manganese (Mn) in various embodiments at a concentration of at least 0 wt.% to at most 3 wt.%. Furthermore, the steel alloy contains nitrogen (N) in various embodiments at a concentration of at least approximately 0 wt.% to at most approximately 0.01 wt.%. Furthermore, the steel alloy contains molybdenum (Mo) in various embodiments at a concentration of at least approximately 0 wt.% to at most approximately 0.8 wt.%. Furthermore, the steel alloy contains boron (B) in various embodiments at a concentration of at least approximately 0 wt.% to at most approximately 0.005 wt.%. Furthermore, the steel alloy contains niobium (Nb) in various embodiments at a concentration of at least approximately 0 wt.% to at most approximately 0.3 wt.%. Furthermore, the steel alloy contains vanadium (V) in various embodiments at a concentration of at least approximately 0 wt.% to at most approximately 0.3 wt.%.

[0023] Rapid preheating of the CFPHS blank improves the surface quality of the CFPHS component and eliminates the need for descaling. The CFPHS manufacturing process combines rapid preheating and oven heating to achieve optimal surface quality. During rapid preheating, a pre-formed oxide protection layer is created. A shortened oven heating time is used to reduce the oxidation time. The CFPHS component is used in several applications for crash-resistant structural components such as vehicle pillars, door supports, and bumper supports.

[0024] With reference to Fig. 1. A CFPHS blank 110 is transferred to a heating device 114, which performs rapid preheating of the CFPHS blank 110. In some examples, one or more conveyor belts 118 and / or robots 130 (with one or more end effectors 134) are used to transfer the CFPHS blank during the manufacturing of the CFPHS device.

[0025] The heating device 114 heats the CFPHS blank 110 to a first predetermined temperature range at a first predetermined heating rate. In some examples, the first predetermined heating rate is in a range between 20 °C / s and 300 °C / s. In some examples, the first predetermined temperature is in a range between 500 °C and 950 °C. The final temperature of the CFPHS blank 110 depends on the thickness and size of the CFPHS blank 110A. In some examples, the heating device 114 is selected from a group consisting of an induction heater, a resistance heater, a laser heater, or combinations thereof.

[0026] After rapid preheating, the CFPHS blank 110 is transferred to an oven 120 for heating. In some examples, the oven 120 heats the CFPHS blank 110 at a temperature between 750 °C and 1200 °C. In other examples, the oven heats the CFPHS blank 110 at a temperature between 900 °C and 950 °C. In some examples, the CFPHS blank 110 is heated for a predetermined heating time between 5 s and 500 s. In other examples, the CFPHS blank 110 is heated for a predetermined time between 5 s and 50 s.

[0027] After heating in the furnace 120, the CFPHS blank 110 is transferred to a punch / press 140 with two or more dies 144 for pressing and forming to produce a pressed and formed CFPHS component 150.

[0028] The combination of using CFPHS, preheating, and furnace heating (for a shorter heating time) enables the achievement of the desired surface quality and the mechanical properties of the base metal. In some examples, the oxide layer of the CFPHS device has a thickness greater than or equal to 0.1 microns and less than or equal to 1.5 microns. In some examples, the oxide layer covers at least 90% of the exposed surface of the CFPHS device. In other examples, the oxide layer of the CFPHS device has a thickness greater than or equal to 0.1 microns and less than or equal to 1.0 microns.

[0029] With reference to Fig.Figure 2 shows a process 200 for manufacturing CFPHS components. In Figure 210, a blank made of uncoated press-hardened steel (CFPHS) is transferred to a rapid heating device. In Figure 220, the rapid heating device performs rapid preheating of the CFPHS blank. The rapid heating device preheats the CFPHS blank to a first predetermined temperature range at a first predetermined heating rate. In some examples, the first predetermined heating rate is in a range between 20 °C / s and 300 °C / s. In some examples, the first predetermined temperature is in a range between 500 °C and 950 °C.

[0030] In 230, the CFPHS blank is fed into an oven after rapid preheating. In 232, the CFPHS blank is heated in the oven at a second predetermined temperature for a specified duration. In some examples, the second predetermined temperature is between 750 °C and 1200 °C. In other examples, the second predetermined temperature is between 900 °C and 950 °C. In some examples, the CFPHS blank 110 is heated in the oven for a specified duration between 5 s and 500 s. In other examples, the CFPHS blank 110 is heated in the oven for a specified duration between 5 s and 50 s.

[0031] In step 240, the CFPHS blank is transferred from the furnace to a die / press with two or more dies after heating. In step 242, the CFPHS blank is formed and press-hardened to create the CFPHS component.

[0032] The method for manufacturing the CFPHS device according to the present disclosure produces devices with a consistently desired surface quality. The method simplifies the requirements for the furnace atmosphere (in other words, the furnace can be operated using atmospheric air and not with nitrogen or another gas environment). The method for manufacturing the CFPHS device reduces energy consumption by decreasing the heat treatment time for CFPHS devices.

[0033] After spot welding, CFPHS components have a better appearance, and after painting, they exhibit improved corrosion resistance. The CFPHS manufacturing process reduces costs by using CFPHS instead of coated PHS (e.g., AlSi-coated PHS) (eliminating the need for descaling). Furthermore, the shorter furnace heating cycle, which reduces lead time, improves the production efficiency of hot stamping.

[0034] The preceding description is merely illustrative and is in no way intended to limit the disclosure, its application, or uses. The comprehensive teachings of the disclosure can be implemented in various forms. While this disclosure contains specific examples, the true scope of protection of the disclosure should therefore not be so limited as to reveal other modifications upon study of the drawings, the description, and the following claims. It is to be understood that one or more steps within a process may be carried out in a different order (or overlapping in time) without altering the principles of the present disclosure.Although each of the embodiments described above has been described with specific features, one or more of these features described with respect to any embodiment of the disclosure may also be implemented in any of the other embodiments and / or combined with its features, even if this combination is not explicitly described. In other words, the described embodiments are not mutually exclusive, and permutations of one or more embodiments with each other remain within the scope of protection of this disclosure.

[0035] Spatial and functional relationships between elements (e.g., between modules, circuit elements, semiconductor layers, etc.) are described using various terms, such as "connected," "interlocking," "coupled," "adjacent," "next to," "on top of," "above," "below," and "arranged." Unless explicitly described as "direct," when a relationship between a first and a second element is described in the disclosure above, this relationship can be a direct relationship in which no further intervening elements exist between the first and the second element, or it can be an indirect relationship in which one or more intervening elements (either spatial or functional) exist between the first and the second element.As the expression "at least one of A, B and C" is used here, it should be interpreted as meaning a logical (A OR B OR C) using a non-exclusive logical OR, and should not be interpreted as meaning "at least one of A, at least one of B and at least one of C".

[0036] In the diagrams, the direction of an arrow, indicated by its tip, generally demonstrates the flow of information (such as data or commands) that is relevant for illustration. For example, if element A and element B exchange various pieces of information, but the information transferred from element A to element B is relevant for the illustration, the arrow may point from element A to element B. This unidirectional arrow does not imply that no further information is sent from element B to element A. Furthermore, element B may send requests or acknowledgments of the information sent from element A to element B to element A.

Claims

[1] A method for manufacturing a coating-free press-hardened steel structural element (CFPHS structural element), comprising: Heating a CFPHS blank at a first predetermined heating rate to a first predetermined temperature in a first predetermined temperature range using a heating device; Transferring the CFPHS blank into a furnace; Heating the CFPHS blank in the furnace at a second predetermined temperature in a second predetermined temperature range for a predetermined period of time; and Pressing and forming the CFPHS blank in a punch / press to form a CFPHS component. [2] The method of claim 1, wherein the first predetermined temperature is between 500 °C and 950 °C. [3] The method of claim 1, wherein the first predetermined heating rate is between 20 °C / s and 300 °C / s. [4] The method of claim 1, wherein the heating device is selected from a group consisting of an induction heating device, a resistance heating device, a laser heating device, or combinations thereof. [5] The method of claim 1, wherein the second predetermined temperature is between 750 °C and 1200 °C. [6] The method according to claim 1, wherein the predetermined time period is in a range between 5 s and 500 s. [7] The method according to claim 1, wherein the predetermined time period is in a range between 5 s and 50 s. [8] The method of claim 1, wherein the CFPHS device, after preheating in the heating device, has an oxidation layer having a thickness in a range of greater than or equal to 0.1 micron to less than or equal to 1.5 micron. [9] The method of claim 8, wherein the oxidation layer covers at least 90% of an exposed surface of the CFPHS device. [10] The method of claim 1, wherein the CFPHS device, after preheating in the heating device, has an oxidation layer having a thickness in a range of greater than or equal to 0.1 micron to less than or equal to 1.0 micron.

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