MITIGATING LIQUID METAL EMBRITTLEMENT IN ZINC-COATED PRESS HARDENED STEELS

Abstract

Methods of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel alloys are provided. In one variation, the method includes decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer. The decarburized surface layer has a thickness of less than or equal to about 50 micrometers. The decarburized surface layer may have greater than or equal to about 80 volume % ferrite. The method also includes applying a zinc-based coating to the decarburized surface layer. A blank is formed from the high-strength steel alloy. The method also includes heating and press hardening the blank to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,100 MPa that is substantially free of liquid metal embrittlement.

Description

INTRODUCTION

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

Press-hardened steel (PHS), also referred to as “hot-stamped steel” or “boron-steel” (e.g., 22MnB5 alloy), is one of the strongest steels used for automotive body structural applications, typically having tensile strength properties on the order of about 1,400 megapascals (MPa) or higher. Such steel has desirable properties, including forming steel components with significant increases in strength-to-weight ratios. PHS components have become ever more 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, when manufacturing vehicles, especially automobiles, continual improvement in fuel efficiency and performance is desirable, thus PHS components have 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 while maintaining load 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. 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. When the PHS component is uncoated, an oxide layer forms during the heating of the blank and 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.

The PHS component may be coated prior to applicable pre-cold forming (if the indirect process is used) or austenitization. PHS components may require cathodic protection. The PHS component may be coated prior to applicable pre-cold forming (if the indirect process is used) or austenitization. Coating the PHS component provides a protective layer (e.g., galvanic protection) to the underlying steel component. Such coatings typically include an aluminum-silicon alloy and/or zinc. Zinc-based coatings offer cathodic protection; the coating acts as a sacrificial layer and corrodes instead of the steel component, even where the steel is exposed.

However, liquid metal embrittlement (LME) may occur when a metallic system is exposed to a liquid metal, such as zinc, during forming at high temperature, resulting in potential cracking and a reduction of total elongation or diminished ductility of a material. LME may also result in decreased ultimate tensile strength. To avoid LME in conventional PHS processes, numerous additional processing steps are conducted. Thus, there is an ongoing need for high-strength hot-formed press-hardened steel components having necessary hardness and strength levels, while providing galvanic protection substantially free of LME.

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.

The present disclosure relates to methods of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel alloys. In one variation, the method includes decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer. The decarburized surface layer has a thickness of less than or equal to about 50 micrometers. The decarburized surface layer may have greater than or equal to about 80 volume % ferrite. The method also includes applying a zinc-based coating to the decarburized surface layer. A blank is formed from the high-strength steel alloy. The method also includes heating and press hardening the blank to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,100 MPa that is substantially free of liquid metal embrittlement.

In one aspect, the decarburizing occurs at a temperature of greater than or equal to about 700° C. in an environment that is non-oxidizing to iron. The environment may comprise one or more of nitrogen, hydrogen, carbon monoxide, carbon dioxide, and water.

In one aspect, the zinc-based coating process includes passing the blank through a zinc galvanization bath at a temperature of greater than or equal to about 420° C. to less than or equal to about 480° C.

In one aspect, the decarburized surface layer has a thickness of greater than or equal to about 20 micrometers to less than or equal to about 50 micrometers.

In one aspect, the decarburized surface layer includes greater than or equal to about 90 volume % ferrite.

In one aspect, the steel alloy includes carbon at less than or equal to about 0.4 weight %.

In one aspect, the steel alloy includes:

manganese at greater than or equal to about 0.2 weight % to less than or equal to about 2.0 weight %;

carbon at greater than or equal to about 0.15 weight % to less than or equal to about 0.4 weight %; and

silicon at greater than 0.1 weight % to less than or equal to about 1 weight %.

In one aspect, the heating occurs at a temperature of greater than or equal to about 800° C. to less than or equal to about 950° C. for austenitization of the high-strength steel alloy.

In one aspect, the method further includes quenching the press-hardened component to room temperature or below after the press hardening.

In one aspect, the high-strength steel alloy has a first side and a second side opposite to the first side. After the decarburizing, the first side has a first decarburized surface layer and the second side has a second decarburized surface layer that sandwich a central region.

In one aspect, the method further includes quenching the high-strength steel alloy blank after the heating and press hardening so that the central region includes greater than or equal to about 95 volume % martensite and the first decarburized surface layer and the second decarburized surface layer have less than or equal to about 20 volume % martensite.

In one aspect, the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,300 MPa to less than or equal to about 2,000 MPa.

In another variation, the present disclosure further provides a method of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel. The method includes decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer. The decarburized surface layer has a thickness of less than or equal to about 50 micrometers. The decarburized surface layer includes greater than or equal to about 80 volume % ferrite. The high-strength steel alloy is then hot-dip galvanized in a heated zinc galvanization bath to form a zinc-based coating over the decarburized surface layer. A blank is formed from the high-strength steel alloy. The blank is heated for austenitization and press hardened to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,300 MPa to less than or equal to about 2,000 MPa. The press-hardened component is substantially free of liquid metal embrittlement.

In one aspect, the decarburizing occurs at a temperature of greater than or equal to about 700° C. in an environment that is non-oxidizing to iron. The environment may comprise one or more of nitrogen, hydrogen, carbon monoxide, carbon dioxide, and water.

In one aspect, the zinc-based coating includes passing the blank through a zinc galvanization bath at a temperature of greater than or equal to about 420° C. to less than or equal to about 480° C.

In one aspect, the decarburized surface layer has a thickness of greater than or equal to about 20 micrometers to less than or equal to about 50 micrometers.

In one aspect, the decarburized surface layer includes greater than or equal to about 90 volume % ferrite.

In one aspect, the steel alloy includes:

manganese at greater than or equal to about 0.2 weight % to less than or equal to about 2.0 weight %;

carbon at greater than or equal to about 0.15 weight % to less than or equal to about 0.4 weight %; and

silicon at greater than 0.1 weight % to less than or equal to about 1.0 weight %.

In one aspect, the heating occurs at a temperature of greater than or equal to about 800° C. to less than or equal to about 950° C. for austenitization of the high-strength steel alloy.

In one aspect, the high-strength steel alloy has a first side and a second side opposite to the first side, wherein after the decarburizing, the first side has a first decarburized surface layer and the second side has a second decarburized surface layer that sandwich a central region. The central region predominantly includes ferrite and pearlite before the heating and press hardening. After heating, press hardening, and cooling, the blank has a central region that includes greater than or equal to about 95 volume % martensite. The first decarburized surface layer and the second decarburized surface layer have less than or equal to about 20 volume % martensite.

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 cross section of a high-strength steel alloy blank formed according to certain methods of the present disclosure having decarburized surface layers and zinc-based coating layers;

FIG. 2 shows a representative press-hardening process for a high-strength steel including a continuous decarburization process according to certain aspects of the present disclosure; and

FIG. 3 shows another representative press-hardening process for a high-strength steel including a batch decarburization process according to certain aspects of the present disclosure.

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

DETAILED DESCRIPTION

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, elements, compositions, steps, integers, operations, 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. Although the open-ended term “comprising,” is to be understood as a non-restrictive term used to describe and claim various embodiments set forth herein, in certain aspects, the term may alternatively be understood to instead be a more limiting and restrictive term, such as “consisting of” or “consisting essentially of.” Thus, for any given embodiment reciting compositions, materials, components, elements, features, integers, operations, and/or process steps, the present disclosure also specifically includes embodiments consisting of, or consisting essentially of, such recited compositions, materials, components, elements, features, integers, operations, and/or process steps. In the case of “consisting of,” the alternative embodiment excludes any additional compositions, materials, components, elements, features, integers, operations, and/or process steps, while in the case of “consisting essentially of,” any additional compositions, materials, components, elements, features, integers, operations, and/or process steps that materially affect the basic and novel characteristics are excluded from such an embodiment, but any compositions, materials, components, elements, features, integers, operations, and/or process steps that do not materially affect the basic and novel characteristics can be included in the embodiment.

Any 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.

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. For example, “about” may comprise a variation of less than or equal to 5%, optionally less than or equal to 4%, optionally less than or equal to 3%, optionally less than or equal to 2%, optionally less than or equal to 1%, optionally less than or equal to 0.5%, and in certain aspects, optionally less than or equal to 0.1%.

As used herein, all amounts are weight % (or mass %), unless otherwise indicated.

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.

As referred to herein, the word “substantially,” when applied to a characteristic of a composition or method of this disclosure, indicates that there may be variation in the characteristic without having a substantial effect on the chemical or physical attributes of the composition or method.

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

The present disclosure provides methods of press-hardening a galvanized, pre-treated steel alloy to form a press-hardened component having high strength and significantly reduced susceptibility to liquid metal embrittlement during press forming. Press hardened structural components comprising a galvanic coating comprising zinc can be formed from a galvanized steel alloy blank prepared in accordance with the present technology. In certain variations, the steel may be galvanized by hot-dipping the blank in a galvanization bath. In certain variations, such a press-hardened steel component comprises a galvanic coating comprising zinc that is formed from a hot-dipped galvanized steel alloy blank.

Such a high-strength three-dimensional component may be incorporated into a device, such as a vehicle. While the high-strength structures are particularly suitable for use in components of an automobile or other vehicles (e.g., motorcycles, boats, tractors, buses, motorcycles, mobile homes, campers, and tanks), they may also be used in a variety of other industries and applications, including aerospace components, consumer goods, office equipment and furniture, industrial and construction equipment and machinery, farm equipment, or heavy machinery, by way of non-limiting example. Non-limiting examples of components and vehicles that can be manufactured by the current technology include automobiles, tractors, buses, motorcycles, boats, mobile homes, campers, and tanks. Other exemplary structures that have frames that can be manufactured by the current technology include construction and buildings, such as houses, offices, bridges, sheds, warehouses, and devices. 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 disclosure.

Structural components capable of being formed of galvanized steel alloy include those used in vehicles, like automotive applications including, but not limited to, rocker rails, engine rails, structural pillars, A-pillars, B-pillars, C-pillars, D-pillars, bumper, hinge pillars, cross-members, body panels, panels, including structural panels, door panels, and door components, interior floors, floor pans, roofs, hoods, exterior surfaces, underbody shields, wheels, storage areas, including glove boxes, console boxes, trunk lids, 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. 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 galvanized steel alloy. Specifically, the present disclosure is particularly suitable for any piece of hardware subject to loads or impact (e.g., load bearing) or requiring cathodic protection.

As mentioned above, the use of such galvanic coatings on press-hardened steel has 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 may be optional. For example, strong post-forming cleaning processes, such as shot blasting, may not be needed. Nonetheless, some cleaning may be desirable for subsequent welding and painting processes.

Initially, a blank is formed of a high-strength steel. A high-strength steel for automotive body structure applications is one that has an ultimate tensile strength of greater than or equal to about 1,000 megapascals (MPa), for example, optionally greater than or equal to about 1,100 MPa, optionally greater than or equal to about 1,200 MPa, optionally greater than or equal to about 1,300 MPa, optionally greater than or equal to about 1,400 MPa, optionally greater than or equal to about 1,500 MPa, optionally greater than or equal to about 1,700 MPa, and in certain variations, optionally greater than or equal to about 2,000 MPa. In certain aspects, a high-strength steel has an ultimate tensile strength greater than or equal to about 1,100 MPa to less than or equal to about 2,200 MPa, optionally greater than or equal to about 1,300 MPa to less than or equal to about 2,200 MPa, optionally greater than or equal to about 1,400 MPa to less than or equal to about 2,100 MPa.

In various aspects, particularly suitable, non-limiting steel compositions may include a high-strength steel alloy composition comprising carbon at greater than or equal to 0.15 weight %. For example, carbon may be present at optionally greater than or equal to about 0.15 weight % to less than or equal to about 0.4 weight % carbon, or optionally greater than or equal to about 0.2 weight % to less than or equal to about 0.4 weight %. In certain aspects, suitable high-strength steel alloys may have manganese at greater than 0.2 weight % to less than or equal to about 2 weight % and optionally at greater than or equal to about 0.2 weight % to less than or equal to about 1.5 weight %. Silicon may be present at greater than or equal to about 0.1 weight % to less than or equal to about 1 weight %, optionally greater than or equal to about 0.2 weight % to less than or equal to about 0.5 weight %. Aluminum is optionally present at less than or equal to about 0.01 weight %. One or more other alloying elements and/or impurities in the steel alloy are cumulatively present at less than or equal to about 1 weight % and optionally at less than or equal to about 0.5 weight %. Other elements, such as chromium (Cr) and molybdenum (Mo), can also be present (typically at less than about 0.5 weight %) for hardenability, while others like titanium (Ti), niobium (Nb) and vanadium (V) (typically at less than about 0.5 weight %) may be added for grain refinement purpose and hence better performance. A balance of such a steel composition is iron.

Non-limiting exemplary high-strength steels include 22MnB5, which comprises about 0.21% to about 0.25% carbon (C), about 1.1% to about 1.35% manganese (Mn), about 0.15% to about 0.4% silicon (Si), about 0.0015% to about 0.004 boron (B), a maximum of 0.16% chromium (Cr), a maximum of 0.01% sulfur (S), a maximum of 0.023% phosphorus (P), impurities cumulatively less than about 0.1%, and a remainder iron (Fe). Other high-strength steel alloys include 30MnB5 and 35MnB5. 30MnB5 comprises about 0.27% to about 0.33% carbon (C), about 1.15 to about 1.45% manganese (Mn), a maximum of 0.4% silicon (Si), about 0.0004 to about 0.005% boron (B), a maximum of 0.035% sulfur (S), a maximum of 0.025% phosphorus (P), impurities cumulatively less than about 0.1%, and a remainder iron (Fe). 35MnB5 comprises about 0.32% to about 0.4% carbon (C), about 1.2 to about 1.5% manganese (Mn), a maximum of 0.5% silicon (Si), about 0.0008 to about 0.005% boron (B), a maximum of 0.035% sulfur (S), a maximum of 0.035% phosphorus (P), impurities cumulatively less than about 0.1%, and a remainder iron (Fe).

In certain aspects, the present disclosure contemplates modifying conventional steel alloy compositions so that they may have zinc galvanic coatings, yet can be processed via press hardening (PHS) to form components with high strength and negligible liquid metal (e.g., zinc) embrittlement (LME). Zinc melts around 420° C. and reacts to form an intermetallic compound with iron around 782° C. Thus, zinc-based coatings may cause LME when exposed to high temperatures, especially at temperatures of greater than 782° C. However, PHS components formed from conventional steel alloys processed with a decarburizing step in accordance with the present disclosure are able to be galvanized and subsequently heated to temperatures of greater than or equal to about 782° C. during press hardening, while successfully minimizing or avoiding LME formation and exhibiting good strength.

In various aspects, the present disclosure thus provides a method of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel. First, the method may include decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer. Next, the high-strength steel alloy may be galvanized. Thus, after the decarburizing step, a zinc-based coating may be applied to the decarburized surface layer. The high-strength steel alloy may be formed into a blank. Notably, the high-strength steel alloy may be formed into a blank before the decarburizing and galvanizing step to apply zinc-based coating or after these steps. A blank of the high-strength steel alloy may be heated and press hardened to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,100 MPa or any of the ultimate tensile strength levels described previously above.

In accordance with various aspects of the present disclosure, press hardened steel (PHS) components formed by such processes are substantially free of liquid metal embrittlement (LME). The term “substantially free of LME” 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 exhibits LME cracking with an average depth of less than or equal to about 20 micrometers (μm) following press forming, optionally with an average depth of less than or equal to about 12 μm, optionally with an average depth of less than or equal to about 9 μm, optionally with an average of less than or equal to about 6 μm, and in certain variations, optionally with an average depth of less than or equal to about 3 μm.

The method includes decarburizing the blank. The decarburizing may include disposing the blank in an environment that is non-oxidizing to iron. For example, the environment may comprise one or more of the following: nitrogen (N₂), hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), and water (H₂O). In one variation, a suitable wet non-oxidizing atmosphere includes nitrogen and water and has a dew point of greater than about −5° C. In one variation, the decarburizing occurs at a temperature of greater than or equal to about 700° C. in an environment comprising nitrogen and water. The decarburizing may include heating the blank with peak metal temperatures above about 700° C. in a wet non-oxidizing (to Fe) atmosphere (e.g., N₂—H₂—CO—CO₂—H₂O; dew point greater than about -5 ° C.) to produce a 10 μm to 20 μm thick decarburized layer on a hot-dip coated steel intended for subsequent press hardening. In other variations, the decarburizing may also occur coupled with a batch or inline annealing method of decarburization in a wet non-oxidizing (to Fe) atmosphere (having a dew point greater than about −5° C.) and heat treatment with peak metal temperatures above 700° C. to produce a 10 μm to 20 μm thick decarburized layer on a bare steel surface intended for subsequent hot-dip coating or electro-coating and subsequent press hardening.

Previously, decarburizing of high-strength steels was avoided due to loss of strength that occurs when carbon is removed from the alloy. However, in the context of the present disclosure, only a thin decarburized surface layer is formed on one or more exposed surfaces of the high-strength steel blank. In certain aspects, the decarburized surface layer has a thickness of less than or equal to about 50 micrometers, for example, greater than or equal to about 20 micrometers to less than or equal to about 50 micrometers, optionally greater than or equal to about 10 micrometers to less than or equal to about 20 micrometers. Thus, a controlled decarburization process creates a thin surface layer having a reduced carbon content compared to a bulk carbon content in the body of the press hardenable steel blank. The decarburized thin surface layer is optionally formed on the steel blank prior to zinc-based coating or the hot forming process. The decarburized surface layer has a larger grain size and hence less grain boundary area for liquid metal embrittlement to occur during the hot stamping process.

After decarburizing, the steel blank may have an oxide layer that forms on the surface. Such an oxide may be removed in a cleaning step prior to applying the galvanic zinc-based coating. In certain variations, the oxide is removed by a pickling process known in the art that uses acid to dissolve oxide from the surface, which is then neutralized and rinsed.

In one variation, the steel alloy blank may optionally be annealed prior to the zinc-based coating being applied. The zinc-based coating may be applied by conventional methods, such as hot dip galvanizing of the blank. The zinc-based coating may be applied by passing the blank through a zinc galvanization bath. Continuous hot-dip galvanizing may be used to coat the steel alloy. A degree of heating occurs for hot dipping to bring the steel alloy to a zinc bath temperature. The coating is applied by passing the uncoiled steel alloy through the zinc galvanizing bath held above about 420° C., optionally 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-based 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 other aspects, the zinc-based coating may include iron or other constituents. In certain aspects, a zinc-based coating comprises zinc or an alloy of zinc, where the coating predominantly comprises zinc at greater than about 90%. A zinc-based coating may be applied on one or both exposed sides of the blank. In certain aspects, the process may further include galvannealing the decarburized steel alloy. 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.

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

FIG. 1 shows a cross-sectional view of a sheet blank 50 that may be formed from a metal stock or coil in a blanking operation, for example, by cutting. The sheet blank 50 includes a main body 52 formed of a high-strength steel like the compositions previously discussed above. After decarburization, a first decarburized layer 54 may be formed on a first side 56 of the main body 52, while a second decarburized layer 58 may be formed on a second opposite side 60 of the main body 52.

Prior to the decarburizing step, the sheet blank 50 formed of high-strength steel may have a microstructure comprising predominantly pearlite and ferrite, for example, cumulatively greater than or equal to about 95 volume % of these phases, optionally greater than or equal to about 96 volume %, optionally greater than or equal to about 97 volume %, optionally greater than or equal to about 98 volume %, optionally greater than or equal to about 99 volume % and up to about 100 volume % of ferrite and austenite. The decarburizing step that forms the first decarburized layer 54 and the second decarburized layer 58 removes carbon and thus changes the microstructure within the decarburized surface region.

After heating and press hardening, but prior to a quenching process, the central region or main body 52 comprises greater than or equal to about 95 volume % austenite, optionally greater than or equal to about 96 volume % austenite, optionally greater than or equal to about 97 volume % austenite, optionally greater than or equal to about 98 volume % austenite, optionally greater than or equal to about 99 volume % austenite and up to about 100 volume % austenite. When the core or main body 52 of the blank 50 is fully austenitized, the surface will have some ferrite phases due to its lower carbon level. Thus, each decarburized surface layer may comprise greater than or equal to about 80 volume % ferrite or optionally greater than or equal to about 90 volume % ferrite. After quenching, the central region or main body 52 comprises greater than or equal to about 95 volume % martensite, optionally greater than or equal to about 96 volume % martensite, optionally greater than or equal to about 97 volume % martensite, optionally greater than or equal to about 98 volume % martensite, optionally greater than or equal to about 99 volume % martensite and up to about 100 volume % martensite. However, the levels of martensite in the decarburized surface layers are minimized. In this manner, the sheet blank 50 is capable of maintaining its high strength properties in the bulk body with highly retained levels of austenite that transform to martensite, while removing reactive carbon from the surface regions on which a zinc-based coating will be applied.

Zinc has a melting temperature of 420° C. and, at 782° C., begins to react with iron via a eutectoid reaction and forms a brittle phase that results in liquid metal embrittlement (LME). Where temperatures are favorable (e.g., above 782° C. for certain high-strength steels) and the zinc is a liquid metal, during deformation processes, the zinc can wet freshly exposed grain boundaries (of the phase in the substrate) and cause de-cohesion/separation along the grain boundary. The zinc thus attacks grain boundaries, especially where austenite is present, which can undesirably form cracks associated with LME. As such, three factors together lead to LME, namely tensile stress, liquid zinc, and grain boundary area. In removing the relatively high levels of carbon in the surface regions after decarburizing, the microstructure transforms to having low levels of austenite so that the sheet blank 50 has a significantly reduced grain boundary area and reduced grain boundary energy. In this manner, the sheet blank 50 can be galvanized and later heated to relatively high temperatures during press hardening, while avoiding LME.

As shown in FIG. 1, a first zinc-based coating 62 is applied over the first decarburized layer 54 on the first side 56. A second zinc-based coating 64 is applied over the second decarburized layer 58 on the second side 60. The first zinc-based coating 62 and the second zinc-based coating 64 comprise zinc; for example, such coatings may be zinc or an alloy of zinc and thus predominantly comprise zinc at greater than about 90%. It should be appreciated, however, that the composition of the first zinc-based coating 62 and the second zinc-based coating 64 is not limited to comprising zinc, but may further include additional elements as discussed previously above. The sheet blank 50 thus undergoes the hot forming process to provide a three-dimensionally formed component.

During hot forming, the sheet blank may be heated, for example, by being introduced into a furnace or other heat source. The amount of heat applied to the sheet blank heats and soaks the sheet blank to a temperature of at least the austenitization temperature. In certain aspects, the high-strength steel has an austenitization temperature (T₁) of greater than or equal to about 800° C. to less than or equal to about 950° C. In this manner, pearlite can be transformed to austenite while the high-strength steel can be hot formed/stamped. In one aspect, the heating for austenitization occurs at a temperature of greater than or equal to about 800° C. for a specified time to fully austenitize the steel through its thickness. The sheet blank is soaked for a period long enough to austenitize the high-strength steel to a desired level. However, due to the decarburized surface layer(s), despite heating the blank to temperatures in excess of 800° C., LME described above can be significantly reduced or eliminated. Full austenitization of the core thus may occur, while the surface is only partially austenitized due to lower initial pearlite or carbon content. As such, an increased zinc concentration on the hot formed component results in improved corrosion protection, while maintaining strength.

The high-strength steel alloy can then be hot formed (e.g., stamped in a die) and then cooled, for example, by a quenching process having a high rate of cooling to facilitate transformation of the austenite formed during heating to martensite. Thus, after exiting the furnace, the sheet blank can be transferred into a stamping press. The stamping press may include a die having a cooling system or mechanism. For example, the die(s) may have a liquid-cooling system, which are well known in the art. The die is designed to form a desired final three-dimensional shape of the component from the austenitized sheet blank. The die may include a first forming die and a second forming die that are brought together to form the desired final shape of the three-dimensional component therebetween.

The cooled dies thus may quench the formed sheet blank in a controlled manner across surfaces of the formed component to cause a phase transformation from austenite to martensite (e.g., within the central body region shown as main body 52 in FIG. 1). Therefore, the first and second die may cooperate to function as a heat sink to draw heat from, and otherwise quench, the formed component. In certain aspects, the press-hardened component is quenched to a temperature at or below a martensite finish temperature and allowed to cool in air to room temperature (e.g., 21° C.) after press hardening.

Referring to FIG. 2, a flowchart showing the steps of a press-hardening process 100 is shown, where the decarburizing is conducted continuously. For brevity, the process conditions and steps described previously above will not be repeated herein. A rolled coil 110 of a steel alloy is unwound and annealed in an annealing chamber 120 or furnace at a temperature of at least about 680° C. The annealed steel alloy is then passed into a decarburizing chamber 122 having a decarburizing environment. The annealing chamber 120 and decarburizing chamber 122 may be partitioned parts or chambers of the same furnace or may be distinct furnaces. The steel alloy thus has a carburized surface layer formed on at least one exposed surface. An optional cleaning process (not shown) may be used to remove surface residue and/or oxides from the carburized surface layer on the steel alloy. However, a cleaning process can be omitted where the decarbonizing atmosphere is not oxidizing to iron. Thus, as shown in FIG. 2, the steel alloy passes from the decarburizing chamber 122 directly into a zinc galvanizing bath 130. The zinc galvanizing bath 130 may be a hot-dipping process.

In certain variations, while not shown, a galvannealing furnace (e.g., induction furnace) may be used after the zinc galvanizing bath 130 for galvannealing the galvanic coating. Notably, annealing (e.g., as shown in the annealing furnace 120) prior to hot-dip galvanizing in the zinc galvanizing bath 130 is not required.

An annealed, decarburized, hot-dipped galvanized steel alloy coil 140 is thus collected. The steel alloy may then be optionally coiled again into coil 140 for easier transportability. When coiled, the steel alloy coil 140 is then subsequently uncoiled and sheared to form a blank 150 by shearing sections of the steel alloy. The blank 150 may be sheared with trim dies, alligator shears, bench shears, guillotines, power shears, throatless shears, or the like.

The preformed blank 150 is then heated in an oven 170 (e.g., the heating may austenitize the blank, so that the oven has a temperature of about 800-950° C. for a predetermined period, such as about 300 to about 1,000 seconds). For example, representative conditions could be heating in an oven at about 850° C. for about 400 seconds, heating at about 930° C. for about 360 seconds, or heating at about 950° C. for about 300 seconds. The preformed blank is then press hardened between dies 180 and 190 to form and simultaneously quench the PHS component 195. The PHS component 195 may be quenched in the dies 180 and 190, for example, quenched at an exemplary rate of more than 27° C./s to transform the austenite into martensite. However, the quench rates may be greater or less depending on the specific alloy composition.

The zinc-based coating protects PHS component 195 from oxidation that would otherwise occur between the austenitizing and press-hardening steps. There is, therefore, little to no need for surface cleaning of PHS component 195 after the press hardening. If necessary, the PHS component 195 may then then cleaned, for example, with shot blasting 200 or acids, to remove scale.

Referring to FIG. 3, a flowchart showing the steps of a press-hardening process 202 is shown where the decarburizing is conducted in a batch process. For brevity, the process conditions and steps described previously above will not be repeated herein. A rolled coil 210 of a steel alloy is first placed in a decarburizing chamber 220. A plurality of spacers 222 is disposed between respective layers of the rolled alloy steel in the coil to ensure exposure of the surfaces to the gases in the decarburizing chamber 220. Decarburizing gases may be introduced into the chamber and exhaust gases exit as effluent during the batch operation. After the batch carburization, the spacers 222 may be removed from the rolled coil 210 of the steel alloy.

The rolled coil 210 is then unwound and annealed in an annealing chamber 230 or furnace at a temperature of at least about 680° C. It should be noted that the annealing process may be conducted prior to the batch decarburization conducted in the decarburizing chamber 220; however, such a step is optional. An optional cleaning process (not shown) may be used to remove surface residue and/or oxides from the carburized surface layer on the steel alloy. However, a cleaning process can be omitted where the decarbonizing atmosphere is not oxidizing to iron. Thus, the steel alloy passes from the annealing chamber 230 into a zinc galvanizing bath 232. The zinc galvanizing bath 232 may be a hot-dipping process.

In certain variations, while not shown, a galvannealing furnace (e.g., induction furnace) may be used after the zinc galvanizing bath 232 for galvannealing the galvanic coating. Notably, annealing (e.g., as shown in the annealing furnace 230) prior to hot-dip galvanizing in the zinc galvanizing bath 230 is not required. In such a variation, the decarburized steel on the rolled coil 210 may be unrolled and passed directly to a zinc galvanizing bath 232.

As shown in FIG. 3, a decarburized, hot-dipped galvanized steel alloy coil 240 is thus collected. The steel alloy may then be optionally coiled again into coil 240 for easier transportability. When coiled, the steel alloy coil 240 is then subsequently uncoiled and sheared to form a blank 250 by shearing sections of the steel alloy. The blank 250 may be sheared with trim dies, alligator shears, bench shears, guillotines, power shears, throatless shears, or the like.

The preformed blank 250 is then heated in an oven 270 (e.g., the heating may austenitize the blank, so that the oven has a temperature of about 950° C. for a predetermined period, such as about 300 seconds or any of the representative temperatures and times discussed previously above). The preformed blank is then press hardened between dies 280 and 290 to form and simultaneously quench the PHS component 295. The PHS component 295 may be quenched in the dies 280 and 290, for example, quenched at a non-limiting rate of more than 27° C./s to transform the austenite into martensite. The quenching rate may be more or less depending on the specific alloy composition.

The zinc-based coating protects PHS component 295 from oxidation that would otherwise occur during or between the austenitizing and press-hardening steps. There is, therefore, little to no need for surface cleaning of PHS component 295 after the press hardening. If necessary, the PHS component 295 may then then cleaned, for example, with shot blasting 300 or acid treatment, to remove scale.

A method of press-hardening a high-strength steel alloy is thus provided that comprises creating a high-strength steel alloy blank having a zinc-coated decarburized surface layer. The decarburized surface layer has a thickness of less than or equal to about 50 micrometers and comprises greater than or equal to about 80 volume % ferrite. The blank is then heated, press hardened, and quenched in a die to form a press-hardened component with an ultimate tensile strength of greater than or equal to about 1,100 MPa that is substantially free of liquid metal embrittlement. Before the heating and press hardening, a central body region predominantly comprises pearlite and ferrite, for example, at greater than or equal to about 95 volume % cumulatively. During the heating process, pearlite is transformed to austenite. The method further comprises quenching the blank after the heating and press hardening so that austenite is transformed to martensite in the central region. The central region may comprises greater than or equal to about 95 volume % martensite, optionally greater than or equal to about 98 volume % martensite. Notably, after the press-hardening and quenching, the decarburized surface layer may have less than or equal to about 20 volume % martensite, optionally less than or equal to about 10 volume % martensite, optionally less than or equal to about 5 volume % martensite, and in certain variations, optionally less than or equal to about 1 volume % martensite.

In another variation, a press-hardened high-strength steel component is thus provided. After heating to austenitize, press-hardening, and quenching, the press-hardened high-strength steel component has a decarburized surface layer with a thickness of less than or equal to about 50 micrometers. The decarburized surface layer may have greater than or equal to about 80 volume % ferrite and less than or equal to about 20 volume % martensite. The central region includes greater than or equal to about 90 volume % martensite, optionally greater than or equal to about 95 volume % martensite, optionally greater than or equal to about 98 volume % martensite, after heating, press hardening, and quenching. The press-hardened component has an ultimate tensile strength of greater than or equal to about 1,000 MPa or any of the strengths discussed previously above. Further, the press-hardened component is substantially free of liquid metal embrittlement.

The mechanical performance of the hot stamped component is significantly improved. For example, after processing, the decarburized surface layer forms a soft decarburization zone that can significantly enhance sheet bendability, which can provide enhanced crash performance for vehicle. For example, structural pillars, like a B-pillar, should have extreme strength in certain sections, but a balance of strength, ductility, and bendability in other sections. The combination of these different properties promotes buckling at a desired location when a force or impact is applied to the B-pillar, which may correspond to seat level within the interior of the vehicle to protect the occupant(s) after the force or impact is applied.

Thus, press-hardened components formed according to the methods of the present disclosure provide a greater bending angle that enhances bendability. The bending angle is dependent on thickness. In certain aspects, a blank formed according to certain aspects of the present disclosure that has the soft decarburization surface zone can have a bending angle that is at least about 1.2 times greater than a bending angle of a comparative blank, such as a blank formed from the same processes with the same conditions, but lacking a decarburizing step. In certain variations, depending on the blank thickness, a bending angle may optionally be at least about 1.5 times greater of a bending angle, optionally at least about 1.6 times greater of a bending angle, optionally at least about 1.7 times greater of a bending angle, optionally at least about 1.8 times greater of a bending angle, optionally at least about 1.9 times greater of a bending angle, and in certain variations, at least about 2 times greater of a bending angle of a comparative blank formed from the same process and conditions except for the decarburizing process. In one variation, the bending angle of the press-hardened component formed according to certain aspects of the present disclosure may be greater than or equal to about 85°, optionally greater than or equal to about 90° and in certain variations, optionally greater than or equal to about 93°. Such a bending angle can be measured by the Verband der Automobilindustrie (VDA) 238-100 test procedure “Plate Bending Test for Metallic Materials.”

In various aspects, the present disclosure significantly reduces or eliminates the liquid metal embrittlement which tends to happen when hot forming a zinc-coated press hardenable steel blank by using a controlled decarburization process (prior to the hot forming process) to create a thin surface layer having reduced carbon content. While decarburization is typically undesirable for use with high-strength steel, by using a controlled process, a thin decarburized surface layer is formed having a larger grain size and reduced grain boundary energy. The larger grain size results in less grain boundary area for liquid metal embrittlement to occur during the hot stamping process and the reduced carbon results in reduced boundary energy that limits zinc intrusion. Additionally, lower carbon content in the surface layer reduces transformational stresses associated with martensite formation from austenite. In this manner, an in-situ “layered” steel microstructure (decarburized zone/steel core) helps to prevent LME.

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 reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel, the method comprising:

decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer having a thickness of less than or equal to about 50 micrometers and comprising greater than or equal to about 80 volume % ferrite;

applying a zinc-based coating to the decarburized surface layer;

forming a blank from the high-strength steel alloy; and

heating and press hardening the blank to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,100 MPa that is substantially free of liquid metal embrittlement.

2. The method of claim 1, the decarburizing occurs at a temperature of greater than or equal to about 700° C. in an environment comprising nitrogen and water.

3. The method of claim 1, wherein the zinc-based coating comprises passing the blank through a zinc galvanization bath at a temperature of greater than or equal to about 420° C. to less than or equal to about 480° C.

4. The method of claim 1, wherein the decarburized surface layer has a thickness of greater than or equal to about 20 micrometers to less than or equal to about 50 micrometers.

5. The method of claim 1, wherein the decarburized surface layer comprises greater than or equal to about 90 volume % ferrite.

6. The method of claim 1, wherein the steel alloy comprises carbon at less than or equal to about 0.4 weight %.

7. The method of claim 1, wherein the steel alloy comprises:

manganese at greater than or equal to about 0.2 weight % to less than or equal to about 2.0 weight %;

carbon at greater than or equal to about 0.15 weight % to less than or equal to about 0.4 weight %; and

silicon at greater than 0.1 weight % to less than or equal to about 1 weight %.

8. The method of claim 1, wherein the heating occurs at a temperature of greater than or equal to about 800° C. to less than or equal to about 950° C. for austenitization of the high-strength steel alloy.

9. The method of claim 1, further comprising quenching the press-hardened component to below room temperature after the press hardening.

10. The method of claim 1, wherein the high-strength steel alloy has a first side and a second side opposite to the first side, wherein after the decarburizing, the first side has a first decarburized surface layer and the second side has a second decarburized surface layer that sandwich a central region.

11. The method of claim 10, wherein the heating austenitizes the high-strength steel alloy and the method further comprises quenching the high-strength steel alloy blank after the heating and press hardening so that the central region comprises greater than or equal to about 98 volume % martensite.

12. The method of claim 1, wherein the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,300 MPa to less than or equal to about 2,000 MPa.

13. A method of reducing liquid metal embrittlement (LME) in zinc-coated high-strength steel, the method comprising:

decarburizing an exposed surface of a high-strength steel alloy to form a decarburized surface layer having a thickness of less than or equal to about 50 micrometers and comprising greater than or equal to about 80 volume % ferrite;

hot dip galvanizing the high-strength steel alloy in a heated zinc galvanization bath to form a zinc-based coating over the decarburized surface layer;

forming a blank from the high-strength steel alloy;

heating the blank to austenitize the high-strength steel alloy; and

press hardening the blank to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,300 MPa to less than or equal to about 2,000 MPa that is substantially free of liquid metal embrittlement.

14. The method of claim 13, wherein the decarburizing occurs at a temperature of greater than or equal to about 700° C. in an environment comprising nitrogen and water.

15. The method of claim 13, wherein the zinc-based coating comprises passing the blank through a zinc galvanization bath at a temperature of greater than or equal to about 420° C. to less than or equal to about 480° C.

16. The method of claim 13, wherein the decarburized surface layer has a thickness of greater than or equal to about 20 micrometers to less than or equal to about 50 micrometers.

17. The method of claim 13, wherein the decarburized surface layer comprises greater than or equal to about 90 volume % ferrite.

18. The method of claim 13, wherein the steel alloy comprises:

manganese at greater than or equal to about 0.2 weight % to less than or equal to about 2.0 weight %;

carbon at greater than or equal to about 0.15 weight % to less than or equal to about 0.4 weight %; and

silicon at greater than 0.1 weight % to less than or equal to about 1 weight %.

19. The method of claim 13, wherein the heating occurs at a temperature of greater than or equal to about 800° C. to less than or equal to about 950° C.

20. The method of claim 13, wherein the high-strength steel alloy has a first side and a second side opposite to the first side, wherein after the decarburizing, the first side has a first decarburized surface layer and the second side has a second decarburized surface layer that sandwich a central region, wherein the method further comprises quenching the blank after the heating and press hardening so that the central region comprises greater than or equal to about 98 volume % martensite.