DE102018101735A1 - TWO-STAGE HOT-FORMING OF STEEL - Google Patents

Abstract

Methods for press-hardening steel alloys consisting of medium Mn are provided. The press-hardened steel alloy may have a tensile strength of at least 1700 MPa and a tensile elongation of at least 8%. The press-hardened steel alloy can be formed in two forming steps above the martensitic end temperature. The press-hardened steel may have a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite less than or equal to about 20% to greater than or equal to about 2%.

Description

INTRODUCTION

The following section provides background information for the present disclosure, which is not necessarily the prior art.

The present disclosure relates to two-stage molding processes for press-hardening steel alloys to form high-strength press-hardened components.

Press-hardened steel (PHS), also referred to as "hot-pressed steel", is one of the strongest steels used for body structure applications and has tensile properties of the order of about 1500 megapascals (MPa) and a total strain of the order of about 5% to about 6%. Such a steel has many desirable properties and uses, including the formation of 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 machinery, automotive or other transportation industries, home or industrial structures, and the like. For example, in automotive manufacturing applications, continuous improvement in fuel efficiency and performance is desirable; PHS components were therefore increasingly used. PHS components are often used for the manufacture of structural components, such as door panels, which typically require high strength materials. Thus, the finished state of these steels is designed to have high strength and sufficient ductility to withstand external forces, for example, to resist intrusion into the passenger compartment to protect the occupants without resisting breakage. In addition, galvanized PHS components can provide cathodic protection.

Typical PHS processes involve austenitizing in a steel sheet blank furnace immediately followed by pressing and quenching the sheet into molds. Steels used to make PHS components typically contain boron; One known steel used to make PHS components is known commercially as 22MnB5 (the 0.22% C, 1.2% Mn, 0.2% Si, 0.001-0.005% B, trace elements including P, N, S and O as impurities per unit weight and Fe balance). There are two main types of PHS procedures: indirect and direct. Austenitization is typically carried out in the range of about 900 ° C. Following the direct method, the PHS component is simultaneously molded and pressed between the molds, which deters the steel. As part of the indirect process, the PHS component is cold worked to an intermediate part before austenitizing and then pressing and quenching. Quenching the PHS component cures the component by converting the microstructure from austenite to martensite. The PHS component must be cooled at a rate of at least 27 ° C / s to ensure that austenite passes to martensite and not to unwanted microstructures such as ferrite and bainite. To the extent that the PHS component is uncoated, an oxide layer is formed during the transfer from the oven to the molds. After cooling, therefore, the oxide must be removed from the PHS component and molds. The oxide is typically removed by shot peening. After shot peening, further molding of the PHS component is typically achieved by laser cutting, as cutting is typically not effective given the high strength of the PHS component. There remains a need to increase the strength and elongation of PHS components and to reduce processing time and cost associated with their manufacture.

SUMMARY

This part provides a general summary of the disclosure and is not a complete disclosure of the full scope or all features.

In certain aspects, the present disclosure provides a method for press hardening a steel alloy. A blank of a medium-alloyed Mn steel 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-cured component at a temperature higher than or equal to the martensitic initial temperature. The press-cured component is further molded at a temperature from lower than or equal to the martensitic initial temperature to higher than or equal to the final martensitic temperature, and the press-cured component has a maximum tensile strength greater than or equal to about 1700 MPa. In other embodiments, the blank is heated to a temperature of less than or equal to about 800 ° C to austenitize the center-alloyed Mn steel. In yet other embodiments, the medium-alloyed Mn steel comprises carbon at greater than or equal to about 0.1 wt% to less than or equal to about 0.4 wt% and manganese greater than or equal to about 5 wt%. to less than or equal to about 12% by weight. In further In embodiments, the medium alloyed Mn steel blank is galvanized prior to heating. In still other embodiments, the press-cured component has a tensile elongation greater than or equal to about 8%. In still other embodiments, after further molding the press-cured component, the press-cured component is air-cooled to less than or equal to about room temperature. In additional embodiments, the press-cured component has a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite less than or equal to about 20% to greater than or equal to about 2%.

In other aspects, there is provided a method of press-hardening a steel alloy comprising heating a blanket of medium-alloyed Mn steel to a temperature less than or equal to about 850 ° C to austenitize the steel alloy. The blank is then press-hardened to form a press-cured component at a temperature higher than or equal to the martensitic initial temperature. The press-cured component is further molded at a temperature greater than or equal to the initial martensitic temperature, and the press-cured component has a maximum tensile strength greater than or equal to about 1700 MPa. In other embodiments, the blank is heated to a temperature of less than or equal to about 800 ° C to austenitize the center-alloyed Mn steel. In yet other embodiments, the medium-alloyed Mn steel comprises carbon at greater than or equal to about 0.1 wt% to less than or equal to about 0.4 wt% and manganese greater than or equal to about 5 wt%. to less than or equal to about 12% by weight. In further embodiments, the medium alloyed Mn steel blank is galvanized prior to heating. In still other embodiments, the press-cured component has a tensile elongation greater than or equal to about 8%. In still other embodiments, after further molding the press-cured component, the press-cured component is air-cooled to less than or equal to about room temperature. In additional embodiments, the press-cured component has a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite less than or equal to about 20% to greater than or equal to about 2%.

In still other aspects, there is provided a method of press-hardening a medium-alloyed Mn steel comprising heating a blank of a medium-alloyed Mn steel 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-cured component is shaped a second time at a temperature greater than or equal to about the final martensitic temperature. The further molding involves at least one of trimming, punching, or re-striking the press-cured component. The press-cured component is cooled at a controlled rate between the initial martensitic temperature and the final martensitic temperature such that the press-cured component has a microstructure, the martensite greater than or equal to about 80% to less than or equal to about 98%, and retained austenite less than or from about 20% to more 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 center-alloyed Mn steel. In yet other embodiments, the medium-alloyed Mn steel comprises carbon at greater than or equal to about 0.1 wt% to less than or equal to about 0.4 wt% and manganese greater than or equal to about 5 wt%. to less than or equal to about 12% by weight. In further embodiments, the press-cured component has a tensile strength of greater than or equal to about 1700 MPa. In still other embodiments, after further molding the press-cured component, the press-cured component is air-cooled to less than or equal to about room temperature. In additional embodiments, the medium alloyed Mn steel blank is galvanized prior to heating.

In certain variations, the process further consists essentially of the pretreatment of the medium-alloyed Mn steel.

Other applications will be 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.

list of figures

• 1 zeigt eine repräsentative Kraftfahrzeug-A-Säule, die gemäß einem Aspekt der vorliegenden Erfindung hergestellt ist;
• 2 zeigt eine repräsentative Kraftfahrzeug-B-Säule, die gemäß einem Aspekt der vorliegenden Erfindung hergestellt ist;
• 3 zeigt ein herkömmliches Verfahren für das Formen einer pressgehärteten Stahlkomponente;
• 4 zeigt ein exemplarisches Verfahren für das Bereitstellen einer pressgehärteten Stahlkomponente gemäß bestimmten Aspekten der vorliegenden Offenbarung; und
• 5 zeigt ein exemplarisches Verfahren für das Bereitstellen einer pressgehärteten Stahlkomponente gemäß bestimmten anderen Aspekten der vorliegenden Offenbarung.

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

• 1 shows a representative automotive A-pillar made in accordance with one aspect of the present invention;
• 2 shows a representative automotive B-pillar made in accordance with one aspect of the present invention;
• 3 shows a conventional method for molding a press-hardened steel component;
• 4 FIG. 3 illustrates an exemplary method for providing a press-hardened steel component in accordance with certain aspects of the present disclosure; FIG. and
• 5 FIG. 5 illustrates an exemplary method for providing a press-hardened steel component according to certain other aspects of the present disclosure.

Like reference numerals indicate similar construction portions throughout the several views of the drawings.

DETAILED DESCRIPTION

Exemplary embodiments will now be described in more detail with reference to the accompanying drawings.

Exemplary embodiments are provided so that this disclosure will be thorough and fully convey the scope of those skilled in the art. Numerous specific details are set forth, such as examples of specific compositions, components, devices and methods described to provide a thorough understanding of embodiments of the present disclosure. Those skilled in the art will recognize that specific details may not be required, that exemplary embodiments may be embodied in many different forms, and that neither of the embodiments is to be construed to limit the scope of the disclosure. In some exemplary embodiments, well-known methods, well-known device structures, and well-known techniques are not described in detail.

The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting in any way. The singular forms "a" and "the" used herein may also include plurals, unless the context clearly excludes them. The terms "comprising," "including," "including," and "hating" are not exclusive and, therefore, indicate the presence of the specified functions, integral entities, steps, acts, elements, and / or components, but do not exclude the presence or otherwise Adding additional functions, holistic units, steps, operations, elements, parts and / or groups thereof. The method steps, processes and processes described here are not to be interpreted as meaning that the order described or shown is absolutely necessary, unless this is specified specifically as the order of execution. It should also be understood that additional or alternative steps may be used unless otherwise stated.

When a component, element or layer is described as being "on, on, in, engaged with, or coupled with" another component or element, it may either directly on / on the other component, the other element, or the other layer, in engagement with, connected to or coupled to, or there may be intervening elements or layers. In turn, when an element is described as "directly on," "directly in connection with," or "directly coupled with" other elements or planes, there may be no intermediate elements or planes. Other words used to describe the relationship between elements are equally understood (eg, "between" and "directly between," "adjacent" and "directly adjacent," etc.). The term "and / or" includes all combinations 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 are not intended to be these expressions are restricted. These terms are only used to distinguish one step, item, component, region, layer, or section from another step, another element, another region, another layer, or another section. Terms such as "first," "second," and other numerical terms, as used herein, do not imply any sequence or order unless clearly indicated by the context. Thus, a first step discussed below, a discussed first element, a discussed component, a discussed region, a discussed layer, or a discussed section could be referred to as a second step, a second element, a second component, a second region, a second layer, or a second region. without deviating from the teachings of the exemplary embodiments.

Spatial terms, such as "inner,""outer,""lower,""lower,""above,""upper," and the like, may be used herein to better describe the relationship of one element or equipment to other elements or the like Properties, as shown in the figures used. Spatial or time related terms may be intended to refer to various arrangements of the application in use or operation Device or the system to circumscribe, in addition to the orientation shown in the figures.

It should be considered for each recitation of a method, composition, device, or system that "includes" certain steps, components, or properties that in certain alternative variations it is also conceivable that these methods, a said composition, device or device may also "consist essentially" of the enumerated steps, components or properties, so that any other steps, components or properties materially altering the basic and novel characteristics of the invention are excluded therefrom.

In this disclosure, the numerical values basically represent approximate measurements or boundaries of ranges, such as minor deviations from the particular values and embodiments having approximately the stated value, as well as those having exactly that value. In contrast to the application examples provided at the end of the detailed description, all numerical values of the parameters (eg, sizes or conditions) in this specification, including the appended claims, are to be understood in all instances by the term "about," whether or not "Approximately" actually appears before the numerical value. "Approximately" indicates that the disclosed numerical value allows for some inaccuracy (with some approximation to accuracy in value, approximately or realistic close to value, approximate). If the inaccuracy provided by "about" is not otherwise understood by those of ordinary skill in the art to be ordinary, then "about" as used herein will at least indicate variations resulting from ordinary measurement techniques and the use of such parameters. If for some reason the inaccuracy provided by the term "about" is not otherwise understood by those of ordinary skill in the art, the term "about" as used herein means a possible variation of up to 5% of the stated value or 5% variance the usual measuring method.

As used herein, the term "composition" broadly refers to a substance that contains at least the preferred metallic elements or compounds, but may also include additional substances or compounds, including additives or impurities. The term "material" broadly refers to material containing the preferred compounds or composition.

In addition, the disclosure of areas involves the disclosure of all values and further subdivided areas within the entire area, including the endpoints and subareas specified for the areas.

The present disclosure provides methods for press-hardening a medium-alloyed Mn steel to form a high-strength press-hardened component. With first reference to 1 and 2 there are shown automotive structural components such as A-pillar 10 and B-pillar 20, which can be made from a press-hardened steel component consisting of two-stage molding of a medium-alloyed Mn steel blank. The press-cured component is formed by molding the medium-alloyed Mn steel at a high temperature (eg, below or equal to about 850 ° C, but in certain embodiments, the temperature may be above or equal to about 850 ° C). followed by a second shaping (e.g., trimming, punching, or reattaching) of the press-hardened component at a lower temperature that is below or equal to the martensitic start temperature for the medium-alloyed Mn steel blank. In other aspects, further molding occurs at a lower temperature at a temperature above or equal to the final martensitic temperature to a temperature below or equal to the martensitic initial temperature for the medium-alloyed Mn steel blank. In other aspects, the medium alloyed Mn steel blank comprises a zinc-containing electroplated coating. Those skilled in the art will recognize that numerous other components can be made by the methods of the present invention and that such additional components are considered to be within the scope of the present invention. Thus, while exemplary components have been illustrated and described throughout the description, it is to be understood that the inventive concepts in the present disclosure may be applied to any structural component that may be formed from a medium alloyed Mn steel, including those used in vehicles , such as automotive applications, may be used including, but not limited to, columns such as hinge pillars, liners, including structural panels, door panels and door components, interior floors, floor pan, roofs, exterior surfaces, underbody protection, wheels, storage surfaces including glove boxes, consoles, luggage compartments, trunk floors , Truck beds, lamp recesses and other components, shock absorber caps, control arms and other suspension, suspension or powertrain components, and the like. In particular, the present disclosure is particular to each piece of Hardware that is subject to stress or shock (eg load capacity).

As noted above, in certain embodiments, the use of electroplated coatings on press-hardened steel is contemplated. Such components have a number of advantages over uncoated steel. Such a galvanic coating (eg with zinc) provides cathodic protection for the underlying steel. In addition to providing an added measure of corrosion resistance benefits as a barrier coating, post-cure post-cure cleaning operations are not necessary to remove deposits from the tool surfaces and parts.

In various aspects, a particularly suitable non-limiting steel is a medium-alloyed Mn steel, the manganese having greater than or equal to about 5 wt% to less than or equal to about 12 wt%, and greater than or equal to about 0.1 Wt% to less than or equal to about 0.4 wt% carbon. Optionally, the medium alloyed Mn steel may further comprise less than or equal to about 1.6 weight percent aluminum, less than or equal to about less than 1.8 weight percent silicone, less than or equal to about 0.25 weight percent. Molybdenum, less than or equal to about 0.05 weight percent niobium, less than or equal to about 0.01 weight percent phosphorus, less than or equal to about 0.005 weight percent sulfur, and less than or equal to slightly less than or equal to about 0.006% by weight of nitrogen. In still other embodiments, the medium alloyed Mn steel may be pretreated. In particular, the medium-alloyed Mn steel can be pretreated by hot rolling. In still other aspects, the medium alloyed Mn steel may be pretreated by hot rolling followed by cold rolling. In still other aspects, the medium alloyed Mn steel may be pretreated by hot rolling followed by cold rolling followed by annealing.

In certain aspects, the present disclosure further contemplates modifying such compositions of steel alloys to include electrogalvanizing coatings but by press hardening processes to form high strength and negligible liquid metal embrittlement (LME) components (eg, liquid metal zinc) , In such aspects, the medium alloyed Mn steel is press molded at a temperature of less than or equal to about 782 ° C. In accordance with these 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 lacking to the extent that undesirable physical properties and limitations associated with their presence are avoided (e.g., cracking, loss ductility and / or strength loss). 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, preferably less than about 4% by weight, optionally less than about 3 wt%, optionally less than about 2 wt%, optionally less than about 1 wt%, optionally less than about 0.5 wt%, and in certain embodiments 0 Wt% of LME species or defects.

Referring to 3 FIG. 3 is a flow chart showing the steps of a conventional direct press hardening process 100 shows. A wound coil 110 from a conventional steel alloy is unwound and sheared to the blank 120 to build. The blank 120 is in an oven 130 at a temperature of about 930 ° C for a predetermined duration (eg, about 360 seconds) to heat the blank 120 to austenitize. The blank 120 will then be between the forms 140 and 150 press-hardened to the PHS component 160 to shape and at the same time deter. In particular, the molding and pressing of a galvanized blank would 120 between the forms 140 and 150 for forming and simultaneously quenching the PHS component 160 lead to LME and therefore to an unsatisfactory PHS component. In conventional steel alloys, quenching at a rate of at least 27 ° C / s to a temperature below the martensitic end temperature of 280 ° C is required to prevent the formation of bainitic or pearlitic microstructures. In order to arrive at a galvanized PHS component, indirect press hardening methods, prior to austenitizing, require an additional step of cold forming the blank 120 insert into an intermediate part mold, to be used to prevent LME. The PHS component 160 then, for example, with shot peening 170 cleaned to remove any deposits if necessary. The PHS component 160 will continue through the laser cutter 180 trimmed and / or punched to obtain the molded PHS component 190.

In accordance with certain aspects of the present disclosure, the processes for press-hardening a steel component comprised of the steel alloys contemplated herein provide the ability to eliminate laser cutting and conventional quenching while still achieving a molded PHS component. In addition, the PHS component has higher strength and better ductility than conventional steels made by conventional PHS processes. Thus, the overall method according to the present disclosure desirably reduces processing time, power requirements, and costs.

As used herein, the term "pretreated" means one of ( 1 ) Hot rolling, ( 2 ) Hot rolling, followed by cold rolling or ( 3 ) Hot rolling followed by cold rolling followed by annealing. In each aspect, the pretreated alloy can then be electroplated. When the pretreatment involves cold rolling, cold rolling may generally be carried out by methods typically known in the art to increase the strength of the steel alloy by strain hardening. When the pretreatment involves austenitizing, the pretreatment generally 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 require austenitization. Alternatively, the process may use austenite present from other processes known in the art involving high temperature (eg, hot rolling). The austenite-made steel alloy may then be quenched, rapidly cooled, or slowly cooled so that the steel alloy undergoes microstructure conversion to at least one of martensite, bainite, perlite, austenite, ferrite, and the like, including combinations thereof. In certain preferred aspects, the austenitized steel alloy is quenched to facilitate martensite transformation. Such a pretreated alloy may then be formed into a blank for processing in accordance with certain aspects of the present disclosure.

Referring to 4 is the procedure 200 according to one aspect of the present disclosure. Optionally, a wound coil 210 pretreated from a steel alloy. After any optional pretreatment, the blank becomes 220 by shearing a section of the wound coil 210 formed from a steel alloy. The blank 220 can be sheared off after any necessary unwinding with trim shapes, alligator scissors, bank scissors, guillotines, power shears, thinness shears or the like. Optionally, a continuous hot dip galvanizing is used before or after shearing to coat the steel alloy. The coating is applied by passing the steel alloy through a zinc plating bath (not shown) maintained above about 420 ° C, and more preferably at a temperature greater than or equal to about 420 ° C to about 480 ° C. followed by cooling to solidify the zinc to a surface coating. Continuous hot dip galvanizing provides a relatively pure zinc coating with high cathodic corrosion resistance. Alternatively, aluminum can be added to the zinc plating bath, promoting the formation of a layer that prevents extensive diffusion between zinc and iron. In some aspects, the steel alloy may be galvanized following hot dip galvanizing by heating the hot dip galvanized steel alloy to greater than or equal to about 500 ° C to less than or equal to about 565 ° C and held at that temperature for a few seconds. If no shearing has taken place, the steel alloy can then optionally be wound into a spool to facilitate transportability.

After any optional plating and / or winding and the resulting unwinding, the sheared blank becomes 220 in an oven 230 (eg austenitizing oven). The blank 220 is heated to less than or equal to about 850 ° C, so that, as recognized by those skilled in the art, the temperature in the oven 230 may exceed 850 ° C. For example, the blank 220 is placed in a furnace for at least 5 minutes, so that the blank 220 reached a temperature of about 850 ° C. The heated blank is immediately transferred to molds 240 and 250 and becomes the PHS component 260 press hardened. There is no need for a fast quenching process; the forms 240 and 250 can stay closed or on the PHS component 260 and the temperature of the PHS component may decrease as a result of heat transfer to the environment or air cooling (eg, on the order of about 10 ° C / s).

Before the PHS component 260 When reaching the martensitic onset temperature, trimmers, punches or additional shapes (collectively shown as 270 and 280) may be the PHS component 260 reshape (for example, crop, punch, and / or re-apply) to the PHS component 260 continue to process.

Remarkably, the steel alloys disclosed herein do not require cooling of the PHS component 260 at the critical 27 ° C / s cooling rate required by typical conventional steel alloy PHS processes to ensure complete martensitic transformation. Rather, the steel alloys disclosed herein are not so rapidly converted to bainite or perlite as conventional steel alloys. Therefore, it is possible to further educate the PHS component 260 while the PHS component 260 is still above the martensitic start temperature but falls below the austenitizing temperature without risking unintentional bainitic or pearlitic transformation. Further, it is possible to control the amount of retained austenite obtained by controlling the final temperature of the resulting PHS component passing through the process 200 260 is formed.

If necessary, a surface cleaning of the PHS component takes place 260 and forms 240 and 250 to remove superficial oxides formed during the process.

Referring to 5 is the procedure 300 according to another aspect of the present disclosure. Optionally, a wound coil 310 pretreated from a steel alloy. After any optional pretreatment, the blank becomes 320 by shearing a section of the wound coil 310 formed from a steel alloy. The blank 320 can be sheared with trim shapes, alligator scissors, bank scissors, guillotines, power scissors, thinness scissors, or the like. Optionally, a continuous hot dip galvanizing is used before or after shearing to coat the steel alloy. The coating is applied by passing the steel alloy through a zinc plating bath (not shown) maintained above about 420 ° C, and more preferably at a temperature greater than or equal to about 420 ° C to about 480 ° C. followed by cooling to solidify the zinc to a surface coating. Continuous hot dip galvanizing provides a relatively pure zinc coating with high cathodic corrosion resistance. Alternatively, aluminum can be added to the zinc plating bath, promoting the formation of a layer that prevents extensive diffusion between zinc and iron. In some aspects, the steel alloy may be galvanized following hot dip galvanizing by heating the hot dip galvanized steel alloy to greater than or equal to about 500 ° C to less than or equal to about 565 ° C and held at that temperature for a few seconds. The steel alloy can then optionally be wound into a spool to facilitate transportability.

After any optional plating and / or winding and the resulting unwinding, the sheared blank becomes 320 in an oven 330 (eg austenitizing oven). The blank 320 is heated to less than or equal to about 850 ° C, so that, as recognized by those skilled in the art, the temperature in the oven 330 may exceed 850 ° C. For example, the blank 320 is placed in a furnace for at least 5 minutes so that the blank 320 reached a temperature of about 850 ° C. The heated blank is immediately transferred to the molds 340 and 350 and becomes the PHS component 360 press hardened. There is no need for a fast quenching process; the forms 340 and 350 can stay closed or on the PHS component 360 and the temperature of the PHS component may decrease as a result of heat transfer to the environment or air cooling (eg, on the order of about 10 ° C / s).

The PHS component 360 is held in the mold for a period of time to reach the initial martensitic temperature, but before it reaches the final martensitic temperature, trimmers, punches, or additional shapes (collectively shown as 370 and 380) may be the PHS component 360 reshape (for example, crop, punch, and / or re-apply) to the PHS component 360 continue to process.

Remarkably, the steel alloys disclosed herein do not require cooling of the PHS component 360 at the critical 27 ° C / s cooling rate required by typical conventional steel alloy PHS processes to ensure complete martensitic transformation. Rather, the steel alloys disclosed herein are not so rapidly converted to bainite or perlite as conventional steel alloys. Therefore, it is possible to further educate the PHS component 360 take place while the PHS component 360 is still between the martensitic start temperature and the martensitic end temperature, without excessive cracking or the result of a finished PHS component which gives insufficient strength. Further, it is possible to control the amount of retained austenite by controlling the final temperature of the process 300 passing through the resulting PHS component 360 is formed.

If necessary, a surface cleaning of the PHS component takes place 360 and forms 340 and 350 to remove superficial oxides formed during the process.

Re-striking the press-hardened steel alloys made in accordance with the present disclosure allows for more aggressive designs. As a non-limiting example, while a first molding may result in a suitable PHS component, repositioning may allow for more sophisticated features and / or designs that are tailored to better resist external forces such as a collision.

The press-hardened steel alloys prepared 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 diffusionless martensite transformation at a temperature of about 250 ° C. Martensite transformation continues when the PHS component is cooled to less than or equal to about room temperature. Remarkably, The press-hardened steel alloys do not require a cooling rate in excess of 27 ° C./s to ensure sufficient martensitic transformation, which distinguishes them from conventional steel alloys. Rather, air cooling, providing cooling at a rate of about 10 ° C / sec, is sufficient to prevent bainitic transformation of the press-hardened steel alloys of the present disclosure. That is, other cooling methods, such as. Quenching may be used to control the cooling rate of the PHS components contemplated in the present disclosure. In certain aspects, the martensite transformation imparts high strength to the PHS component, and the retained austenite provides better ductility and impact resistance 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 converted to austenite and then martensite. In certain aspects, a PHS component may have a multi-phase microstructure comprising martensite with greater than or equal to about 80% to less than or equal to about 98% martensite by volume and retained austenite less than or equal to about 20% to greater than or equal to about 2%.

The press-hardened steel alloys produced according to the present disclosure offer excellent strength and tensile elongation compared to conventional steel alloys. In one example, a sample of a 22MnB5 alloy was subjected to heating at 930 ° C and this temperature was held for approximately 360 s, followed by quenching below the martensitic end temperature. The tensile strength of the 22MnB5 sample was about 1500 MPa. A middle Mn-containing sample was heated to 850 ° C and this temperature was held for 240 seconds, followed by air cooling the sample to about 620 ° C, the sample 180 s was maintained at this temperature, followed by additional air cooling to room temperature. The tensile strength of the sample comprising average Mn gave a superior tensile strength of about 1719 MPa. Further, a second sample of an average Mn alloy, differing only in that it was heated to a temperature of about 800 ° C instead of 850 ° C, gave an even better tensile strength of about 1,807 MPa. The tensile elongation of each of the samples was also evaluated. The tensile elongation of the 22MnB5 sample was about 6%. However, the middle Mn-containing sample gave 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 instead of 850 ° C, gave a slightly better tensile elongation of 8.2%. It is believed that the better tensile strain results from the higher amount of retained austenite present in PHS components prepared in accordance with the present disclosure. Further, it is believed that additional shaping of the Mn middle alloys in accordance with the present disclosure would result in even better tensile strain because additional molding would ultimately result in more retained austenite. The retained austenite is soft and tough in comparison to the formed martensite and therefore gives a greater tensile elongation to a PHS component.

The samples were further analyzed to determine their notch toughness. A 1.4 mm sample made of 22MnB5 prepared according to the method of the previous paragraph showed a nominal engineering stress of about 1700 MPa and a nominal elongation at break of about 1.2% and about 30-50 J / cm ² Toughness measured by a Charpy (V-notch) impact test at room temperature. A 1.4 mm sample consisting of medium Mn prepared according to the procedure of the preceding paragraph (in which the Mn middle alloy was heated to 800 ° C) showed a nominal engineering stress of about 2000 MPa and a nominal engineering Elongation at break of about 2.1% and 70 J / cm ² toughness as measured by a Charpy (V-notch) impact test at room temperature.

In an exemplary method, the method comprises heating a blank of a medium-alloyed Mn steel 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-cured component at a temperature higher than or equal to the martensitic initial temperature. The press-cured component is further molded at a temperature from less than or equal to the martensitic initial temperature to higher than or equal to the final martensitic temperature, and the press-cured component has a maximum tensile strength greater than or equal to about 1700 MPa. In other variations, the blank is heated to a temperature of less than or equal to about 800 ° C to austenitize the center-alloyed Mn steel. The medium-alloyed Mn steel may contain 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% by weight. In further embodiments, the medium alloyed Mn steel blank is galvanized prior to heating. In other aspects, the press-cured component has a tensile elongation of greater than or equal to about 8%. In still other aspects, the press-hardened steel component may after further molding the press-hardened component to the martensitic final temperature air-cooled. The press-cured component may have a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite having less than or equal to about 20% to greater than or equal to about 2%.

In another example, the method may consist essentially of the following steps. A blank of a medium-alloyed Mn steel 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-cured component at a temperature higher than or equal to the martensitic initial temperature. The press-cured component is further molded at a temperature from less than or equal to the martensitic initial temperature to higher than or equal to the final martensitic temperature, and the press-cured component has a maximum tensile strength greater than or equal to about 1700 MPa. In certain other variations, such method may be further limited if it consists essentially of the following, of any combination of the following 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-alloyed Mn steel; ( 2 ) has a steel alloy consisting of carbon with greater than or equal to about 0.1% by weight to less than or equal to about 0.4% by weight and manganese with greater than or equal to about 5% by weight to less than or equal to about 12% by weight; ( 3 ) Plating the medium-alloyed Mn steel 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 final martensitic temperature after further molding the press-hardened component; and ( 6 ) a PHS component having a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite having less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process eliminates any laser cutting that is often required in conventional processes, as well as the quenching of the press-cured component, which can provide benefits in terms of time, energy, and cost savings.

In yet another example, the method may consist of the following steps. A blank of a medium-alloyed Mn steel is heated to a temperature of less than or equal to about 850 ° C in order to austenitize the steel alloy. The blank is then press-hardened to form a press-cured component at a temperature higher than or equal to the martensitic initial temperature. The press-cured component is further molded at a temperature from less than or equal to the martensitic initial temperature to higher than or equal to the final martensitic temperature, and the press-cured component has a maximum tensile strength greater than or equal to about 1700 MPa. In certain other variations, such method may be further limited if it further consists of the following, any combination of any 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-alloyed Mn steel; ( 2 ) has a steel alloy consisting of carbon with greater than or equal to about 0.1 wt% to less than or equal to about 0.4 wt% and manganese greater than or equal to about 5 wt% to less than or equal to about 12% by weight; ( 3 ) Plating the medium-alloyed Mn steel 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 final martensitic temperature after further molding the press-hardened component; and ( 6 ) a PHS component having a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite having less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process eliminates any laser cutting that is often required in conventional processes, as well as the quenching of the press-cured component, which can provide benefits in terms of time, energy, and cost savings.

In another exemplary method, the method includes heating a blanket of medium-alloyed Mn steel to a temperature less than or equal to about 850 ° C to austenitize the steel alloy. The blank is then press-hardened to form a press-cured component at a temperature greater than or equal to the martensitic initial temperature. Further, after press-hardening, the press-hardened component is molded at a temperature higher than or equal to the martensitic start temperature, and the press-hardened component has a maximum tensile strength of more than or equal to about 1700 MPa. In other variations, the blank is heated to a temperature of less than or equal to about 800 ° C to austenitize the center-alloyed Mn steel. The medium-alloyed Mn steel may contain 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% by weight. In further embodiments, the medium alloyed Mn steel blank is galvanized prior to heating. In other aspects, the press-hardened Component has a tensile elongation of greater than or equal to about 8%. In still other aspects, the press-hardened steel component is air-cooled to the final martensitic temperature after further molding the press-hardened component. The press-cured component may have a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite having less than or equal to about 20% to greater than or equal to about 2%.

In another example, the method may consist essentially of the following steps. A blank comprising a medium-alloyed Mn steel 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-cured component at a temperature greater than or equal to the martensitic initial temperature. Further, after press-hardening, the press-hardened component is molded at a temperature higher than or equal to the martensitic start temperature, and the press-hardened component has a maximum tensile strength of more than or equal to about 1700 MPa. In certain other variations, such method may be further limited if it consists essentially of the following, of any combination of the following 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-alloyed Mn steel; ( 2 ) has a steel alloy consisting of carbon with greater than or equal to about 0.1 wt% to less than or equal to about 0.4 wt% and manganese greater than or equal to about 5 wt% to less than or equal to about 12% by weight; ( 3 ) Plating the medium-alloyed Mn steel 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 final martensitic temperature after further molding the press-hardened component; and ( 6 ) a PHS component having a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite having less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process eliminates any laser cutting that is often required in conventional processes, as well as the quenching of the press-cured component, which can provide benefits in terms of time, energy, and cost savings.

In yet another example, the method may consist of the following steps. A blank comprising a medium-alloyed Mn steel 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-cured component at a temperature greater than or equal to the martensitic initial temperature. Further, after press-hardening, the press-hardened component is molded at a temperature higher than or equal to the martensitic start temperature, and the press-hardened component has a maximum tensile strength of more than or equal to about 1700 MPa. In certain other variations, such method may be further limited if it further consists of the following, any combination of any 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-alloyed Mn steel; ( 2 ) has a steel alloy consisting of carbon with greater than or equal to about 0.1 wt% to less than or equal to about 0.4 wt% and manganese greater than or equal to about 5 wt% to less than or equal to about 12% by weight; ( 3 ) Plating the medium-alloyed Mn steel 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 final martensitic temperature after further molding the press-hardened component; and ( 6 ) a PHS component having a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite having less than or equal to about 20% to greater than or equal to about 2%. Notably, such a process eliminates any laser cutting that is often required in conventional processes, as well as the quenching of the press-cured component, which can provide benefits in terms of time, energy, and cost savings.

In an additional exemplary method, a method comprises heating a blank of a medium-alloyed Mn steel to a temperature 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-cured component is shaped a second time at a temperature greater than or equal to about the final martensitic temperature. The further molding involves at least one of trimming, punching or re-striking the press-hardened component. The press-cured component is cooled at a controlled rate between the initial martensitic temperature and the final martensitic temperature such that the press-cured component has a microstructure, the martensite greater than or equal to about 80% to less than or equal to about 98%, and retained austenite less than or from about 20% to more than or equal to about 2%. In other variations, the blank is at a temperature of less than or equal to heated to about 800 ° C to austenitize the medium-alloyed Mn steel. The medium-alloyed Mn steel may contain 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% by weight. In further embodiments, the press-cured component has a tensile strength of greater than or equal to about 1700 MPa. In still other aspects, the press-hardened steel component is air-cooled to the final martensitic temperature after further molding the press-hardened component. In further embodiments, the medium alloyed Mn steel blank is galvanized prior to heating.

In another example, the method may consist essentially of the following steps. A blank made of a medium-alloyed Mn steel is heated to a temperature of less than or equal to about 850 ° C in order to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component. The press-cured component is shaped a second time at a temperature greater than or equal to about the final martensitic temperature. The further molding involves at least one of trimming, punching, or re-striking the press-cured component. The press-cured component is cooled at a controlled rate between the initial martensitic temperature and the final martensitic temperature such that the press-cured component has a microstructure, the martensite greater than or equal to about 80% to less than or equal to about 98%, and retained austenite less than or from about 20% to more than or equal to about 2%. In certain other variations, such method may be further limited if it consists essentially of the following, of any combination of the following 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-alloyed Mn steel; ( 2 ) has a steel alloy consisting of carbon with greater than or equal to about 0.1 wt% to less than or equal to about 0.4 wt% and manganese greater than or equal to about 5 wt% to less than or equal to about 12% by weight; ( 3 ) a PHS component having a tensile strength greater than or equal to about 1700 MPa; ( 5 Air-cooling the press-hardened component to the final martensitic temperature after further molding the press-hardened component; and ( 6 ) Electroplating the medium-alloyed Mn steel blank before heating. Notably, such a process eliminates any laser cutting that is often required in conventional processes, as well as the quenching of the press-cured component, which can provide benefits in terms of time, energy, and cost savings.

In yet another example, the method may consist of the following steps. A blank made of a medium-alloyed Mn steel is heated to a temperature of less than or equal to about 850 ° C in order to austenitize the steel alloy. The blank is then press hardened to form a press-hardened component. The press-cured component is shaped a second time at a temperature greater than or equal to about the final martensitic temperature. The further molding involves at least one of trimming, punching, or re-striking the press-cured component. The press-cured component is cooled at a controlled rate between the initial martensitic temperature and the final martensitic temperature such that the press-cured component has a microstructure, the martensite greater than or equal to about 80% to less than or equal to about 98%, and retained austenite less than or from about 20% to more than or equal to about 2%. In certain other variations, such method may be further limited if it further consists of the following, any combination of any 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-alloyed Mn steel; ( 2 ) has a steel alloy consisting of carbon with greater than or equal to about 0.1% by weight to less than or equal to about 0.4% by weight and manganese with greater than or equal to about 5% by weight to less than or equal to about 12% by weight; ( 3 ) a PHS component having a tensile strength greater than or equal to about 1700 MPa; ( 5 Air-cooling the press-hardened component to the final martensitic temperature after further molding the press-hardened component; and ( 6 ) Electroplating the medium-alloyed Mn steel blank before heating. Notably, such a process eliminates any laser cutting that is often required in conventional processes, as well as the quenching of the press-cured component, which can provide benefits in terms of time, energy, and cost savings.

The foregoing description of the embodiments is merely illustrative and descriptive. It is not exhaustive and is not intended to limit the revelation in any way. Individual elements or features of a particular embodiment are generally not limited to this particular embodiment, but may be interchangeable and optionally usable in a selected embodiment, although not separately illustrated or described. Also various variations are conceivable. These variations are not deviations from the disclosure, and all modifications of this nature are part of the disclosure and are within its scope.

Claims (10)

A method of press-hardening a medium-alloyed Mn steel, the method comprising: heating a blank of a medium-alloyed Mn steel to a temperature of less than or equal to about 850 ° C to austenitize the medium-alloyed Mn steel; press-curing the blank of the medium-alloyed Mn steel to form a press-hardened component at a temperature greater than or equal to the martensitic initial temperature; and further molding the press-cured component at a temperature of less than or equal to the martensitic initial temperature to higher than or equal to the martensitic end temperature, wherein the press-cured component has a maximum tensile strength greater than or equal to about 1700 MPa. Method according to Claim 1 wherein the heating of the blank to a temperature of less than or equal to about 800 ° C to austenitize the medium-alloyed Mn steel. Method according to Claim 1 wherein the medium-alloyed Mn steel has carbon at greater than or equal to about 0.1% by weight to less than or equal to about 0.4% by weight and manganese at greater than or equal to about 5% by weight to less than or equal to about 12% by weight. Method according to Claim 1 in which the medium-alloyed Mn steel blank is galvanized before heating. The method of claim 1, wherein the press-cured component has a tensile elongation greater than or equal to about 8%. Method according to Claim 1 wherein the press-cured component is air-cooled to the final martensitic temperature after further molding the press-cured component. Method according to Claim 1 wherein the press-cured component has a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite less than or equal to about 20% to greater than or equal to about 2%. A method of press-hardening a medium-alloyed Mn steel, the method comprising: heating a blank of a medium-alloyed Mn steel to a temperature of less than or equal to about 850 ° C to austenitize the medium-alloyed Mn steel; press-curing the blank of the medium-alloyed Mn steel to form a press-hardened component at a temperature greater than or equal to the martensitic initial temperature; and further molding the press-cured component at a temperature less than or equal to about the initial martensitic temperature to form a press-cured component having a tensile strength greater than or equal to about 1700 MPa. Method according to Claim 8 wherein the heating of the blank to a temperature of less than or equal to about 800 ° C to austenitize the medium-alloyed Mn steel and the medium-alloyed Mn-steel carbon with greater than or equal to about 0.1 wt .-% to less is equal to or greater than about 0.4 wt.% and manganese greater than or equal to about 5 wt.% to less than or equal to about 12 wt.%. Method according to Claim 8 wherein the medium-alloyed Mn steel blank is electroplated before heating, the PHS component has a tensile elongation of greater than or equal to about 8%, the press-cured component is air-cooled to the final martensitic temperature after further molding the press-hardened component; and the PHS component has a microstructure comprising martensite greater than or equal to about 80% to less than or equal to about 98% and retained austenite having less than or equal to about 20% to greater than or equal to about 2%.

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