TWO-STEP HOT FORMING OF STEELS

Abstract

Methods for press hardening steel alloys comprised of medium-Mn are provided. The press-hardened steel alloy may have an ultimate tensile strength (UTS) of at least 1,700 MPa and a tensile elongation of at least 8%. The press-hardened steel alloy may be formed in two forming steps above the martensitic finish temperature. The press-hardened steel may have a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

Description

INTRODUCTION

This section provides background information related to the present disclosure which is not necessarily prior art.

The present disclosure relates to two-step forming methods of press-hardening steel alloys to form high-strength press-hardened components.

Press-hardened steel (PHS), also referred to as “hot-stamped steel,” is one of the strongest steels used for automotive body structural applications, having tensile strength properties on the order of about 1,500 mega-Pascal (MPa) and total elongation on the order of about 5% to 6%. Such steel has many desirable properties and uses, including forming steel components with significant increases in strength-to-weight ratios. Further, PHS components have become increasingly prevalent in various industries and applications, including general manufacturing, construction equipment, automotive or other transportation industries, home or industrial structures, and the like. For example, in automotive manufacturing applications, continual improvement in fuel efficiency and performance is desirable; PHS components have therefore been increasingly used. PHS components are often used for forming load-bearing components, like door beams, which usually require high strength materials. Thus, the finished state of these steels are designed to have high strength and enough ductility to resist external forces, for example, to resist intrusion into the passenger compartment without fracturing so as to provide protection to the occupants. Moreover, galvanized PHS components may provide cathodic protection.

Typical PHS processes involve austenitization in a furnace of a sheet steel blank immediately followed by pressing and quenching of the sheet in dies. Steels used to manufacture PHS components typically contain boron; one well-known steel used to manufacture PHS components is commercially known as 22MnB5 (which comprises 0.22% C, 1.2% Mn, 0.2% Si, 0.001-0.005% B, trace elements including P, N, S and O as impurities by weight, and a balance of Fe). There are two main types of PHS processes: indirect and direct. Austenitization is typically conducted in the range of about 900° C. Under the direct method, the PHS component is formed and pressed simultaneously between dies, which quenches the steel. Under the indirect method, the PHS component is cold formed to an intermediate partial shape before austenitization and the subsequent pressing and quenching steps. The quenching of the PHS component hardens the component by transforming the microstructure from austenite to martensite. The PHS component must be quenched at a rate of at least 27° C./s to ensure the austenite transfers to martensite and not to an undesirable microstructure, such as ferrite and bainite. To the extent the PHS component is uncoated, an oxide layer forms during the transfer from the furnace to the dies. After quenching, therefore, the oxide must be removed from the PHS component and the dies. The oxide is typically removed by shot blasting. After shot blasting, further forming of the PHS component is typically accomplished with laser cutting, as die cutting is typically not effective given the high strength of the PHS component. There is a continuing need to increase the strength and elongation of PHS components and reduce the processing time and concomitant cost associated with manufacturing them.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

In certain aspects, the present disclosure contemplates a method of press-hardening a steel alloy. A blank of a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. The press-hardened component is further formed at a temperature of less than or equal to about the martensitic start temperature to greater than or equal to about the martensitic finish temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In other embodiments, the blank is heated to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. In yet other embodiments, the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. In further embodiments, the medium-Mn steel alloy blank is galvanized before heating. In yet further embodiments, the press-hardened component has a tensile elongation of greater than or equal to about 8%. In even further embodiments, the press-hardened component is air cooled to less than or equal to about room temperature after further forming the press-hardened component. In additional embodiments, the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

In other aspects, a method of press-hardening a steel alloy is provided that comprises heating a blank of a medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. The press-hardened component is further formed at a temperature of greater than or equal to about the martensitic start temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In other embodiments, the blank is heated to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. In yet other embodiments, the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. In further embodiments, the medium-Mn steel alloy blank is galvanized before heating. In yet further embodiments, the press-hardened component has a tensile elongation of greater than or equal to about 8%. In even further embodiments, the press-hardened component is air cooled to less than or equal to about room temperature after further forming the press-hardened component. In additional embodiments, the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

In yet other aspects, a method of press-hardening a medium-Mn steel alloy is provided that comprises heating a blank of a medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component. The press-hardened component is formed a second time at a temperature of greater than or equal to about the martensitic finish temperature. The further forming includes at least one of trimming, punching, or re-striking the press-hardened component. The press-hardened component is cooled at a controlled rate between the martensitic start temperature and the martensitic finish temperature such that the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. In other embodiments, the blank is heated to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. In yet other embodiments, the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. In further embodiments, the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In even further embodiments, the press-hardened component is air cooled to less than or equal to about room temperature after further forming the press-hardened component. In additional embodiments, the medium-Mn steel alloy blank is galvanized before heating.

In certain variations, the method further consists essentially of pre-treating the medium-Mn steel alloy.

Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

FIG. 1 shows a representative automotive A-pillar manufactured according to an aspect of the present invention;

FIG. 2 shows a representative automotive B-pillar manufactured according to an aspect of the present invention;

FIG. 3 shows a conventional process for forming a press hardened steel component;

FIG. 4 shows an exemplary process for providing a press hardened steel component in accordance with certain aspects of the present disclosure; and

FIG. 5 shows an exemplary process for providing a press hardened steel component in accordance with certain other aspects of the present disclosure.

Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference to the accompanying drawings.

Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed, unless otherwise indicated.

When a component, element, or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other component, 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, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (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.

Although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially or temporally relative terms, such as “before,” “after,” “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.

It should be understood for any recitation of a method, composition, device, or system that “comprises” certain steps, ingredients, or features, that in certain alternative variations, it is also contemplated that such a method, composition, device, or system may also “consist essentially of” the enumerated steps, ingredients, or features, so that any other steps, ingredients, or features that would materially alter the basic and novel characteristics of the invention are excluded therefrom.

Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term “about” whether or not “about” actually appears before the numerical value. “About” indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value; approximately or reasonably close to the value; nearly). If the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. If, for some reason, the imprecision provided by “about” is not otherwise understood in the art with this ordinary meaning, then “about” as used herein may indicate a possible variation of up to 5% of the indicated value or 5% variance from usual methods of measurement.

As used herein, the term “composition” refers broadly to a substance containing at least the preferred metal elements or compounds, but which optionally comprises additional substances or compounds, including additives and impurities. The term “material” also broadly refers to matter containing the preferred compounds or composition.

In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints and sub-ranges given for the ranges.

The present disclosure provides methods of press-hardening a medium-Mn steel alloy to form a press-hardened component having high strength. Referring first to FIGS. 1 and 2, automotive structural components, such as A-pillar 10 and B-pillar 20 are shown that can be produced from a press-hardened steel component comprising by two-step forming a medium-Mn steel alloy blank. The press-hardened component is formed by shaping the medium-Mn steel alloy at a high temperature (e.g., at less than or equal to about 850° C.; however, in certain embodiments, the temperature may be greater than or equal to about 850° C.) followed by forming (e.g., trimming, punching, or re-striking) the press-hardened component a second time at a lower temperature that is less than or equal to the martensitic start temperature for the medium-Mn steel alloy blank. In other aspects, the further forming at a lower temperature occurs at a temperature greater than or equal to the martensitic finish temperature to a temperature less than or equal to the martensitic start temperature for the medium-Mn steel alloy blank. In other aspects, the medium-Mn steel alloy blank comprises a galvanic coating comprising zinc. It will be appreciated by those skilled in the art that numerous other components may be fabricated by the methods of the present invention, and that such additional components are deemed to be within the scope of the present invention. Thus, while exemplary components are illustrated and described throughout the specification, it is understood that the inventive concepts in the present disclosure may also be applied to any structural component capable of being formed of medium-Mn steel alloy, including those used in vehicles, like automotive applications including, but not limited to, pillars, such as hinge pillars, panels, including structural panels, door panels, and door components, interior floors, floor pans, roofs, exterior surfaces, underbody shields, wheels, storage areas, including glove boxes, console boxes, trunks, trunk floors, truck beds, lamp pockets and other components, shock tower cap, control arms and other suspension, undercarriage or drive train components, and the like. Specifically, the present disclosure is particularly suitable for any piece of hardware subject to loads or impact (e.g., load bearing).

As mentioned above, in certain embodiments, the use of galvanic coatings on press-hardened steel is contemplated. Such components have a number of advantages over uncoated steel. Such a galvanic coating (e.g., comprising zinc) provides cathodic protection to the underlying steel. In addition to providing an additional measure of corrosion-resistant benefits as a barrier coating, subsequent cleaning operations following press hardening to remove scale from the die surfaces and parts are not necessary.

In various aspects, a particularly suitable, non-limiting steel is medium-Mn steel alloy, which comprises manganese at greater than or equal to about 5 weight % to less than or equal to about 12 weight % and greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % carbon. Optionally, the medium-Mn steel alloy may further comprise less than or equal to about 1.6 wt. % aluminum, less than or equal to about less than 1.8 wt. % silicon, less than or equal to about 0.25 wt. % molybdenum, less than or equal to about 0.05 wt. % niobium, less than or equal to about 0.01 wt. % phosphorus, less than or equal to about 0.005 wt. % sulfur, and less than or equal to about less than or equal to about 0.006 wt. % nitrogen. In yet other embodiments, the medium-Mn steel alloy may be pre-treated. More specifically, the medium-Mn steel alloy may be pre-treated by hot-rolling. In yet other aspects, the medium-Mn steel alloy may be pre-treated by hot-rolling, followed by cold rolling. In even other aspects, the medium-Mn steel alloy may be pre-treated by hot-rolling, followed by cold-rolling, followed by annealing.

In certain aspects, the present disclosure further contemplates modifying such steel alloy compositions so that they have zinc galvanic coatings, yet be processed via press hardening process to form components with high strength and negligible liquid metal (e.g., zinc) embrittlement (LME). In such aspects, the medium-Mn steel alloy is press-formed at a temperature of less than or equal to about 782° C. In accordance with such aspects, the PHS components are substantially free of LME. The term “substantially free” as referred to herein means that the LME microstructures and defects are absent to the extent that undesirable physical properties and limitations attendant with their presence are avoided (e.g., cracking loss of ductility, and/or loss of strength). In certain embodiments, a PHS component that is “substantially free” of LME defects comprises less than about 5% by weight of the LME species or defects, more preferably less than about 4% by weight, optionally less than about 3% by weight, optionally less than about 2% by weight, optionally less than about 1% by weight, optionally less than about 0.5% and in certain embodiments comprises 0% by weight of the LME species or defects.

Referring to FIG. 3, a flowchart showing the steps of a conventional direct press-hardening process 100 is shown. A rolled coil 110 of a conventional steel alloy is uncoiled and sheared to form blank 120. Blank 120 is heated in an oven 130 having a temperature of about 930° C. for a predetermined period (e.g., about 360 seconds) to austenitize blank 120. Blank 120 is then press hardened between dies 140 and 150 to form and simultaneously quench PHS component 160. Notably, forming and pressing a galvanized blank 120 between dies 140 and 150 to form and simultaneously quench PHS component 160 would result in LME and therefore an unsatisfactory PHS component. For conventional steel alloys, quenching at a rate of at least 27° C./s to a temperature below the 280° C. martensitic finish temperature is required to prevent the formation of bainitic or pearlitic microstructures. To arrive at a galvanized PHS component, indirect press-hardening processes, which introduces an extra step of cold forming blank 120 to an intermediate partial shape before austenitization, must be used to prevent LME. PHS component 160 is then cleaned, for example, with shot blasting 170, to remove scale as necessary. PHS component 160 is further trimmed and/or punched as necessary by laser cutter 180 to achieve formed PHS component 190.

In accordance with certain aspects of the present disclosure, the methods of press-hardening a steel component comprised of the steel alloys contemplated herein provide the ability to eliminate laser cutting and traditional quenching while still achieving a formed PHS component. Furthermore, the PHS component has higher strength and better ductility than conventional steels made according to conventional PHS processes. Thus, the overall process according to the present disclosure desirably reduces processing time, energy requirements, and cost.

As used herein, the term “pre-treated” means any of (1) hot-rolling, (2) hot-rolling, followed by cold rolling, or (3) hot rolling, followed by cold-rolling, followed by annealing. In any aspect, the pre-treated alloy may then be galvanized. Generally, when pre-treating involves cold-rolling, the cold rolling may be accomplished by methods typically known in the art to increase the steel alloy's strength by strain hardening. Generally, when pre-treating involves austenitizing, the pre-treatment involves heating the steel alloy to a temperature greater than or equal to about 900° C. to less than or equal to about 950° C. to promote austenite formation. Alternatively, the process may make use of austenite that is present from other methods known in the art involving high temperature (e.g., hot-rolling). The steel alloy comprised of austenite may then be quenched, rapidly cooled, or slowly cooled such that the steel alloy undergoes a microstructure transformation to at least one of martensite, bainite, pearlite, austenite, ferrite, and the like, including combinations thereof. In certain preferred aspects, the austenitized steel alloy is quenched to allow for martensitic transformation. Such a pre-treated alloy may then be formed into a blank for processing in accordance with certain aspects of the present disclosure.

Referring to FIG. 4, the process 200 according to one aspect of the present disclosure is shown. Optionally, a rolled coil 210 of a steel alloy is pre-treated. After any optional pre-treat, blank 220 is formed by shearing a section of rolled coil 210 of a steel alloy. Blank 220 may be sheared with trim dies, alligator shears, bench shears, guillotines, power shears, throatless shears, or the like after any necessary uncoiling having previously occurred. Optionally, before or after shearing, continuous hot-dip galvanizing is used to coat the steel alloy. The coating is applied by passing the steel alloy through a zinc galvanizing bath (not shown) held above about 420° C., and, more preferably, at a temperature of greater than or equal to about 420° C. up to about 480° C., followed by cooling to solidify the zinc into a surface coating. Continuous hot-dip galvanizing provides a relatively pure zinc coating with high cathodic corrosion resistance. Alternatively, aluminum may be added to the zinc galvanizing bath, promoting formation of a layer that prevents extensive diffusion between the zinc and iron. In some aspects, the steel alloy may be galvannealed following the hot-dipped galvanizing by heating the hot-dipped galvanized steel alloy to greater than or equal to about 500° C. to less than or equal to about 565° C. and holding for a few seconds. If shearing has not yet occurred, the steel alloy may then optionally be coiled into a coil for easier transportability.

After any optional galvanizing and/or coiling and resultant uncoiling, sheared blank 220 is placed in an oven 230 (e.g., austenitizing furnace). Blank 220 is heated to less than or equal to about 850° C., so that as recognized by those in the art, the temperature in the oven 230 may potentially exceed 850° C. By way of example, blank 220 is placed in a furnace for at last 5 minutes, so blank 220 reaches a temperature of about 850° C. The heated blank is immediately transferred to dies 240 and 250 and is press hardened into PHS component 260. No rapid quenching process is required; dies 240 and 250 may remain closed or be opened on PHS component 260, and the temperature of PHS component may decrease (e.g., on the order of about 10° C./s) as a result of heat transferring to the ambient surroundings or air cooling.

Before PHS component 260 reaches the martensitic start temperature, trimmers, punchers, or additional dies (collectively shown as 270 and 280) may further form (e.g., trim, punch, and/or re-strike) PHS component 260 to further process PHS component 260.

Notably, the steel alloys disclosed herein do not require cooling PHS component 260 at the critical cooling rate of 27° C./s, as is required in typical conventional steel alloy PHS processes to ensure full martensitic transformation. Rather, the steel alloys disclosed herein do not transform to bainite or pearlite as quickly as conventional steel alloys. Therefore, it is possible to undertake further forming of PHS component 260 while PHS component 260 is yet above the martensitic start temperature but falling below the austenitization temperature without risking inadvertent bainitic or pearlitic transformation. Further, it is possible to control the amount of retained austenite formed by controlling the ending temperature of the resulting PHS component 260 undergoing process 200.

If necessary, surface cleaning of PHS component 260 and dies 240 and 250 occurs to remove superficial oxides formed during the process.

Referring to FIG. 5, the process 300 according to another aspect of the present disclosure is shown. Optionally, a rolled coil 310 of a steel alloy is pre-treated. After any optional pre-treat, blank 320 is formed by shearing a section of rolled coil 310 of a steel alloy. Blank 320 may be sheared with trim dies, alligator shears, bench shears, guillotines, power shears, throatless shears, or the like. Optionally, before or after shearing, continuous hot-dip galvanizing is used to coat the steel alloy. The coating is applied by passing the steel alloy through a zinc galvanizing bath (not shown) held above about 420° C., and, more preferably, at a temperature of greater than or equal to about 420° C. up to about 480° C., followed by cooling to solidify the zinc into a surface coating. Continuous hot-dip galvanizing provides a relatively pure zinc coating with high cathodic corrosion resistance. Alternatively, aluminum may be added to the zinc galvanizing bath, promoting formation of a layer that prevents extensive diffusion between the zinc and iron. In some aspects, the steel alloy may be galvannealed following the hot-dipped galvanizing by heating the hot-dipped galvanized steel alloy to greater than or equal to about 500° C. to less than or equal to about 565° C. and holding for a few seconds. The steel alloy may then optionally be coiled into a coil for easier transportability.

After any optional galvanizing and/or coiling and resultant uncoiling, sheared blank 320 is placed in an oven 330 (e.g., austenitizing furnace). Blank 320 is heated to less than or equal to about 850° C., so that as recognized by those in the art, the temperature in the oven 330 may potentially exceed 850° C. By way of example, blank 320 is placed in a furnace for at last 5 minutes, so blank 320 reaches a temperature of about 850° C. The heated blank is immediately transferred to dies 340 and 350 and is press hardened into PHS component 360. No rapid quenching process is required; dies 340 and 350 may remain closed or be opened on PHS component 360, and the temperature of PHS component may decrease (e.g., on the order of about 10° C./s) as a result of heat transferring to the ambient surroundings or air cooling.

PHS component 360 is held within the die for a time period to reach the martensitic start temperature but before reaching the martensitic finish temperature, trimmers, punchers, or additional dies (collectively shown as 370 and 380) may further form (e.g., trim, punch, and/or re-strike) PHS component 360 to further process PHS component 360.

Notably, the steel alloys disclosed herein do not require cooling PHS component 360 at the critical cooling rate of 27° C./s, as is required in typical conventional steel alloy PHS processes to ensure full martensitic transformation. Rather, the steel alloys disclosed herein do not transform to bainite or pearlite as quickly as conventional steel alloys. Therefore, it is possible to undertake further forming of PHS component 360 while PHS component 360 is yet between the martensitic start temperature and martensitic finish temperature without undue cracking or resulting in a finished PHS component yielding inadequate strength. Further, it is possible to control the amount of retained austenite formed by controlling the ending temperature of the resulting PHS component 360 undergoing process 300.

If necessary, surface cleaning of PHS component 360 and dies 340 and 350 occurs to remove superficial oxides formed during the process.

Re-striking the press hardened steel alloys produced according to the present disclosure allows for more aggressive designs. By way of non-limiting example, while a first forming may result in a suitable PHS component, the re-striking may allow for more intricate featuring and/or designs tailored to better resist external forces, such as a collision.

The press hardened steel alloys produced according to the present disclosure provide a PHS component having a multi-phase microstructure, including martensite and retained austenite. Upon cooling, the PHS component undergoes a diffusionless martensitic transformation at a temperature of around 250° C. The martensitic transformation continues as the PHS component is cooled to less than or equal to about room temperature. Notably, the press hardened steel alloys do not require a cooling rate exceeding 27° C./s to ensure adequate martensitic transformation, distinguishing them from conventional steel alloys. Rather, air cooling, thereby providing cooling at a rate of about 10° C./s, is sufficient to prevent bainitic transformation of the press hardened steel alloys according to the present disclosure. That said, other cooling methods, such as quenching, may be used to control the rate of cooling the PHS components contemplated under the present disclosure. In certain aspects, the martensitic transformation provides high strength to the PHS component, and the retained austenite provides better ductility and impact toughness than conventional steel alloys. The amount of carbon present and the austenitizing temperature in the press hardened steel alloys determines the amount of ferrite that is transformed to austenite and subsequently to martensite. In certain aspects, a PHS component may have a multi-phase microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% martensite by volume and less than or equal to about 20% to greater than or equal to about 2% retained austenite.

The press hardened steel alloys produced according to the present disclosure provide excellent strength and tensile elongation compared to conventional steel alloys. In one example, a sample of a 22MnB5 alloy was subject to heating to 930° C. and holding that temperature for about 360 s, followed by quenching to below the martensitic finish temperature. The UTS of the sample comprised of 22MnB5 was about 1,500 MPa. A sample comprised of medium-Mn, was subject to heating to 850° C. and holding that temperature for 240 s, followed by air cooling the sample to about 620° C., where the sample was held at that temperature for 180 s, followed by additional air cooling to room temperature. The UTS of the sample comprised of medium-Mn yielded a superior UTS of about 1,719 MPa. Further, a second sample of a medium-Mn alloy that differed only in that it was heated to a temperature of about 800° C., rather than 850° C., resulted in an even better UTS of about 1,807 MPa. The tensile elongation of each of the samples was also assessed. The tensile elongation of the sample comprised of 22MnB5 was about 6%. The sample comprised of medium-Mn, however, yielded a superior tensile elongation of about 8%. The second medium-Mn sample, which differed only in that it was heated to a temperature of about 800° C., rather than 850° C., resulted in a slightly better tensile elongation of 8.2%. The better tensile elongation is believed to arise from the higher amount of retained austenite present in PHS components made according to the present disclosure. Further, it is believed that additional forming of the medium-Mn alloys according to the present disclosure would result in even better tensile elongation as additional forming would ultimately yield more retained austenite. The retained austenite is soft and tough compared to the formed martensite, and therefore bestows greater tensile elongation to a PHS component.

The samples were further analyzed to determine their notched tensile toughness. A 1.4 mm sample comprised of 22MnB5 and prepared according to the method in the preceding paragraph was found to exhibit a nominal engineering stress of about 1,700 MPa and a nominal engineering strain at break of about 1.2% and roughly 30-50 J/cm² toughness, as measured by a Charpy V-notch impact test at room temperature. A 1.4 mm sample comprised of medium-Mn and prepared according to the method in the preceding paragraph (where the medium-Mn alloy was heated to 800° C.) was found to exhibit a nominal engineering stress of about 2,000 MPa and a nominal engineering strain at break of about 2.1% and 70 J/cm² toughness, as measured by a Charpy V-notch impact test at room temperature.

In one exemplary method, the process comprises heating a blank of a medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. The press-hardened component is further formed at a temperature of less than or equal to about the martensitic start temperature to greater than or equal to about the martensitic finish temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In other variations, the blank is heated to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. The medium-Mn steel alloy may comprise carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. In yet other variations, the medium-Mn steel alloy blank is galvanized before heating. In other aspects, the press-hardened component has a tensile elongation of greater than or equal to about 8%. In yet other aspects, the press-hardened steel component may be air cooled to the martensitic finish temperature after further forming the press-hardened component. The press-hardened component may have a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

In another example, the process may consist essentially of the following steps. A blank of a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. The press-hardened component is further formed at a temperature of less than or equal to about the martensitic start temperature to greater than or equal to about the martensitic finish temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In certain other variations, such a process may be further limited as further consisting essentially of, any combination of, or all of the following: (1) heating the blank to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy; (2) having a steel alloy comprised of carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %; (3) galvanizing the medium-Mn steel alloy balnk before heating; (4) a PHS component having a tensile elongation of greater than or equal to about 8%; (5) air cooling the press-hardened component to the martensitic finish temperature after further forming the press-hardened component; and (6) a PHS component having a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process excludes any laser cutting that are often required in conventional processes and quenching the press-hardened component, which can result in time, energy, and cost savings benefits.

In yet another example, the process may consist of the following steps. A blank of a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. The press-hardened component is further formed at a temperature of less than or equal to about the martensitic start temperature to greater than or equal to about the martensitic finish temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In certain other variations, such a process may be further limited as further consisting of, any combination of, or all of the following: (1) heating the blank to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy; (2) having a steel alloy comprised of carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %; (3) galvanizing the medium-Mn steel alloy blank before heating; (4) a PHS component having a tensile elongation of greater than or equal to about 8%; (5) air cooling the press-hardened component to the martensitic finish temperature after further forming the press-hardened component; and (6) a PHS component having a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process excludes any laser cutting that are often required in conventional processes and quenching the press-hardened component, which can result in time, energy, and cost savings benefits.

In another exemplary method, the process comprises heating a blank comprising a medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. After press hardening, the press-hardened component is further formed at a temperature of greater than or equal to about the martensitic start temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In other variations, the blank is heated to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. The medium-Mn steel alloy may comprise carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. In yet other variations, the medium-Mn steel alloy blank is galvanized before heating. In other aspects, the press-hardened component has a tensile elongation of greater than or equal to about 8%. In yet other aspects, the press-hardened steel component is air cooled to the martensitic finish temperature after further forming the press-hardened component. The press-hardened component may have a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

In another example, the process may consist essentially of the following steps. A blank comprising a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. After press hardening, the press-hardened component is further formed at a temperature of greater than or equal to about the martensitic start temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In certain other variations, such a process may be further limited as further consisting essentially of, any combination of, or all of the following: (1) heating the blank to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy; (2) having a steel alloy comprised of carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %; (3) galvanizing the medium-Mn steel alloy blank before heating; (4) a PHS component having a tensile elongation of greater than or equal to about 8%; (5) air cooling the press-hardened component to the martensitic finish temperature after further forming the press-hardened component; and (6) a PHS component having a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process excludes any laser cutting that are often required in conventional processes and quenching the press-hardened component, which can result in time, energy, and cost savings benefits.

In yet another example, the process may consist of the following steps. A blank comprising a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component at a temperature greater than or equal to about the martensitic start temperature. After press hardening, the press-hardened component is further formed at a temperature of greater than or equal to about the martensitic start temperature, and the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In certain other variations, such a process may be further limited as further consisting of, any combination of, or all of the following: (1) heating the blank to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy; (2) having a steel alloy comprised of carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %; (3) galvanizing the medium-Mn steel alloy blank before heating; (4) a PHS component having a tensile elongation of greater than or equal to about 8%; (5) air cooling the press-hardened component to the martensitic finish temperature after further forming the press-hardened component; and (6) a PHS component having a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process excludes any laser cutting that are often required in conventional processes and quenching the press-hardened component, which can result in time, energy, and cost savings benefits.

In an additional exemplary method, a process comprises heating a blank comprised of a medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component. The press-hardened component is formed a second time at a temperature of greater than or equal to about the martensitic finish temperature. The further forming includes at least one of trimming, punching, or re-striking the press-hardened component. The press-hardened component is cooled at a controlled rate between the martensitic start temperature and the martensitic finish temperature such that the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. In other variations, the blank is heated to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. The medium-Mn steel alloy may comprise carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. In yet other variations, the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. In yet other aspects, the press-hardened steel component is air cooled to the martensitic finish temperature after further forming the press-hardened component. In further variations, the medium-Mn steel alloy blank is galvanized before heating.

In another example, the process may consist essentially of the following steps. A blank comprised of a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is ten press hardened to form a press-hardened component. The press-hardened component is formed a second time at a temperature of greater than or equal to about the martensitic finish temperature. The further forming includes at least one of trimming, punching, or re-striking the press-hardened component. The press-hardened component is cooled at a controlled rate between the martensitic start temperature and the martensitic finish temperature such that the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. In certain other variations, such a process may be further limited as further consisting essentially of, any combination of, or all of the following: (1) heating the blank to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy; (2) having a steel alloy comprised of carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %; (3) a PHS component having an ultimate tensile strength of greater than or equal to about 1,700 MPa; (5) air cooling the press-hardened component to the martensitic finish temperature after further forming the press-hardened component; and (6) galvanizing the medium-Mn steel alloy blank before heating. Notably, such a process excludes any laser cutting that are often required in conventional processes and quenching the press-hardened component, which can result in time, energy, and cost savings benefits.

In yet another example, the process may consist of the following steps. A blank comprised of a medium-Mn steel alloy is heated to a temperature of less than or equal to about 850° C. to austenitize the steel alloy. The blank is ten press hardened to form a press-hardened component. The press-hardened component is formed a second time at a temperature of greater than or equal to about the martensitic finish temperature. The further forming includes at least one of trimming, punching, or re-striking the press-hardened component. The press-hardened component is cooled at a controlled rate between the martensitic start temperature and the martensitic finish temperature such that the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. In certain other variations, such a process may be further limited as further consisting of, any combination of, or all of the following: (1) heating the blank to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy; (2) having a steel alloy comprised of carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %; (3) a PHS component having an ultimate tensile strength of greater than or equal to about 1,700 MPa; (5) air cooling the press-hardened component to the martensitic finish temperature after further forming the press-hardened component; and (6) galvanizing the medium-Mn steel alloy blank before heating. Notably, such a process excludes any laser cutting that are often required in conventional processes and quenching the press-hardened component, which can result in time, energy, and cost savings benefits.

The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

1. A method of press-hardening a medium-Mn steel alloy, the method comprising:

heating a blank of medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the medium-Mn steel alloy;

press hardening the blank of the medium-Mn steel alloy to form a press-hardened component at a temperature greater than or equal to about a martensitic start temperature; and

further forming the press-hardened component at a temperature greater than or equal to about a martensitic finish temperature to less than or equal to about the martensitic start temperature, wherein the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa.

2. The method of claim 1, wherein the heating of the blank is to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy.

3. The method of claim 1, wherein the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %.

4. The method of claim 1, wherein the medium-Mn steel alloy blank is galvanized before heating.

5. The method to claim 1, wherein the press-hardened component has a tensile elongation of greater than or equal to about 8%.

6. The method of claim 1, wherein the press-hardened component is air cooled to the martensitic finish temperature after further forming the press-hardened component.

7. The method of claim 1, wherein the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

8. A method of press-hardening a medium-Mn steel alloy, the method comprising:

heating a blank of medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the medium-Mn steel alloy;

press hardening the blank of the medium-Mn steel alloy to form a press-hardened component at a temperature greater than or equal to about a martensitic start temperature; and

further forming the press-hardened component at a temperature of greater than or equal to about the martensitic start temperature to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,700 MPa.

9. The method of claim 8, wherein the heating of the blank is to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy.

10. The method of claim 8, wherein the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %.

11. The method of claim 8, wherein the medium-Mn steel alloy blank is galvanized before heating.

12. The method to claim 8, wherein the press-hardened component has a tensile elongation of greater than or equal to about 8%.

13. The method of claim 8, wherein the press-hardened component is air cooled to the martensitic finish temperature after further forming the press-hardened component.

14. The method of claim 8, wherein the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

15. A method of press-hardening a medium-Mn steel alloy, the method comprising:

heating a blank of medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the medium-Mn steel alloy;

press hardening the blank of the medium-Mn steel alloy to form a press-hardened component;

further forming the press-hardened component at a temperature greater than or equal to a martensitic finish temperature, wherein the further forming includes at least one of trimming, punching, or re-striking the press-hardened component; and

controlling the rate at which the press-hardened component cools between a martensitic start temperature and the martensitic finish temperature, wherein the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

16. The method of claim 15, wherein the heating the blank is to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy.

17. The method of claim 15, wherein the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %.

18. The method to claim 15, wherein the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa.

19. The method of claim 15, wherein the press-hardened component is air cooled to the martensitic finish temperature after further forming the press-hardened component.

20. The method of claim 15, wherein medium-Mn steel alloy blank is galvanized before heating.