CN110050084B

CN110050084B - Hot-formed part containing non-coating press hardening steel and method thereof

Info

Publication number: CN110050084B
Application number: CN201680091625.0A
Authority: CN
Inventors: 卢琦; 王建锋
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2016-12-16
Filing date: 2016-12-16
Publication date: 2021-05-14
Anticipated expiration: 2036-12-16
Also published as: DE112016007444T5; CN110050084A; WO2018107446A1; US20200063232A1

Abstract

The alloy composition includes carbon in a concentration of greater than or equal to about 0.15 wt% to less than or equal to about 0.5 wt% of the alloy composition; manganese at a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 3 wt.% of the alloy composition; silicon in a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 0.5 wt.% of the alloy composition; any of the following: chromium at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% of the alloy composition and aluminum at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition, or aluminum at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% of the alloy composition and chromium at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition, and the balance of the alloy composition being iron. Alloy compositions and methods of making press hardened rigid bodies are also disclosed.

Description

Hot-formed part containing non-coating press hardening steel and method thereof

Background

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

Press Hardened Steel (PHS) (also known as "hot stamped steel" or "hot formed steel") is one of the toughest steels for automotive body structural applications, having tensile strength properties on the order of about 1,500 megapascals (MPa). Such steels have desirable properties, including forming steel components with significantly improved strength to weight ratios. PHS components are becoming more common 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, continuous improvements in fuel efficiency and performance are required, and thus PHS components have been increasingly used. PHS components are commonly used to form load bearing components, such as door beams, which typically require high strength materials. As such, the finished state of these steels is designed to have high strength and sufficient ductility to resist external forces, such as intrusion into the passenger compartment, without cracking to provide protection to the occupants. In addition, the galvanized PHS component may provide cathodic protection.

Many PHS processes involve austenitization of a steel sheet blank in a furnace followed immediately by pressing and quenching the steel sheet in a die. There are two main types of PHS methods: indirect and direct processes. Austenitization is generally carried out in the range of about 880 ℃ to 950 ℃. In the direct method, the PHS component is simultaneously formed and pressed between dies, which quenches the steel. In the indirect process, the PHS component is cold formed into an intermediate component shape, followed by austenitization and subsequent pressing and quenching steps. Quenching of the PHS component hardens the component by transforming the microstructure from austenite to martensite. An oxide layer is often formed during the transfer from the furnace to the mold. Therefore, after quenching, the oxide must be removed from the PHS component and the mold. The oxides are typically removed by shot blasting, i.e., descaling.

The PHS part may be plated prior to pre-chill forming (if indirect methods are used) or austenitization as applicable. The plated PHS component provides a protective layer (e.g., galvanic corrosion protection) to the underlying steel component. Such coatings typically comprise aluminum-silicon alloys and/or zinc. The zinc coating provides cathodic protection; the plating acts as a sacrificial layer and is corroded in place of the steel component, even if the steel is exposed. Such plating also generates oxides on the surface of the PHS component, which are removed by shot blasting.

Summary of The Invention

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 novel plateless press hardened steels and methods of making press hardened steel parts. The novel non-coating press-hardened steel contains chromium, aluminum or both chromium and aluminum.

The present technology provides an alloy composition comprising carbon in a concentration of greater than or equal to about 0.15 wt.% to less than or equal to about 0.5 wt.% of the alloy composition; manganese at a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 3 wt.% of the alloy composition; silicon in a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 0.5 wt.% of the alloy composition; any of the following: chromium at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% and aluminum at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition, or aluminum at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% and chromium at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition; and the balance of the alloy composition is iron.

In various aspects, the alloy composition is a high Cr alloy composition including chromium at a concentration of greater than or equal to about 2.25 wt.% to less than or equal to about 5 wt.% of the alloy composition.

In various aspects, the high Cr alloy composition includes aluminum in a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition.

In various aspects, the high Cr alloy composition has been oxidized at a temperature of greater than or equal to about 400 ℃ to less than or equal to about 700 ℃ for a time of greater than or equal to about 1 minute to less than or equal to about 60 minutes and subsequently cooled.

In various aspects, the high Cr alloy composition is configured to be hardened into a press hardened steel by subjecting the high Cr alloy composition to a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃ for a time period of greater than or equal to about 1 minute to less than or equal to about 15 minutes and then cooling.

In various aspects, the high Cr alloy composition does not require shot blasting after use in hot forming.

In various aspects, the high Cr alloy composition is in the form of a sheet, and at least one surface of the sheet comprises a Cr-rich oxide layer. In some aspects, the Cr-rich oxide layer has a thickness of greater than or equal to about 1 μm to less than or equal to about 40 μm.

In various aspects, the alloy composition is a high-Al alloy composition including aluminum in a concentration of greater than or equal to about 2 wt.% to less than or equal to about 6 wt.% of the alloy composition.

In various aspects, the high Al alloy composition includes chromium in a concentration of greater than or equal to about 0.05 wt.% to less than or equal to about 0.3 wt.% of the alloy composition. In some aspects, the high Al alloy composition includes aluminum in a concentration of about 4 wt.% of the alloy composition.

In various aspects, the high Al alloy composition has a density 5% less than a second alloy composition having the same composition but an aluminum concentration less than or equal to about 0.05 wt.% of the second alloy composition.

In various aspects, the high Al alloy composition is configured to be hardened into a press hardened steel by subjecting the alloy composition to a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃ for a time of greater than or equal to about 1 minute to less than or equal to about 15 minutes and then cooling. In some aspects, the press hardened steel has a predetermined shape and at least one surface of the predetermined shape comprises an Al-rich oxide layer. In other aspects, the press hardened steel does not require shot blasting after use in hot forming.

The present technology also provides an alloy composition comprising carbon in a concentration of greater than or equal to about 0.2 wt.% to less than or equal to about 0.3 wt.% of the alloy composition; boron at a concentration of greater than or equal to about 0.001 wt.% to less than or equal to about 0.005 wt.% of the alloy composition; manganese at a concentration of greater than or equal to about 0.5 wt.% to less than or equal to about 3 wt.% of the alloy composition; silicon in a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 0.5 wt.% of the alloy composition; any of the following: chromium at a concentration of greater than or equal to about 2.25 wt.% to less than or equal to about 5 wt.% of the alloy composition and aluminum at a concentration of greater than or equal to about 0.02 wt.% to less than or equal to about 5 wt.% of the alloy composition, or aluminum at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 5 wt.% of the alloy composition and chromium at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition; and the balance of the alloy composition is iron.

The present technique also provides a method of manufacturing a press hardened steel object. The method includes heating the alloy billet to a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃ for a time of greater than or equal to about 1 minute to less than or equal to about 15 minutes to produce a heated alloy billet. The alloy billet is composed of an alloy composition comprising carbon in a concentration of greater than or equal to about 0.15 wt.% to less than or equal to about 0.5 wt.% of the alloy composition; manganese at a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 3 wt.% of the alloy composition; silicon in a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 0.5 wt.% of the alloy composition; chromium at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% of the alloy composition and aluminum at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition, or aluminum at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% of the alloy composition and chromium at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition, and the balance of the alloy composition being iron. The method further includes transferring the heated alloy blank to a die to form the heated alloy blank into an object having a predetermined shape, and quenching the object having the predetermined shape to produce an object comprised of press hardened steel. The press hardened steel object has at least one surface comprising an oxide layer. The method does not require shot blasting.

In various aspects, quenching the object includes cooling the object at a rate of greater than or equal to about 15 ℃/sec.

In various aspects, the alloy composition includes chromium at a concentration of greater than or equal to about 2.25 wt.% to less than or equal to about 5 wt.% of the alloy composition, and the method further comprises pre-oxidizing the alloy composition by heating the alloy composition to a temperature of greater than or equal to about 400 ℃ to less than or equal to about 700 ℃ for a time of about greater than or equal to about 1 minute to less than or equal to about 60 minutes and subsequently cooling the alloy composition prior to heating.

In various aspects, the alloy composition includes aluminum in a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% of the alloy composition, and the method further includes soaking the heated alloy billet in the presence of niobium and vanadium at a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃.

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. 1A is a photomicrograph of the surface of a press hardened steel made from a high Cr alloy composition that has not been pre-oxidized;

FIG. 1B is a photomicrograph of the surface of a press hardened steel made from a pre-oxidized high Cr alloy composition;

FIG. 2A is a photomicrograph showing the surface of a steel containing 3 wt.% Cr prepared by heating a steel containing 3 wt.% Cr composition to 900 ℃ for 10 minutes and cooling;

FIG. 2B is a photomicrograph showing the surface of a steel containing 9 wt.% Cr made by heating a steel containing 9 wt.% Cr composition to 900 ℃ for 10 minutes and cooling;

FIG. 3A is a photomicrograph showing the surface of a steel containing 3 wt.% Cr prepared by heating a steel containing 3 wt.% Cr composition to 500 ℃ for 20 minutes and cooling;

FIG. 3B is a photomicrograph showing the surface of a 9 wt.% Cr containing steel made by heating 9 wt.% of the Cr composition to 600 ℃ for 10 minutes and cooling;

FIG. 4A is a photomicrograph showing the surface of a steel containing 3 wt.% Cr made by heating a steel containing 3 wt.% Cr composition to 500 ℃ for 20 minutes and cooling, followed by heating to 900 ℃ for 10 minutes and cooling;

FIG. 4B is a photomicrograph showing the surface of a steel containing 9 wt.% Cr made by heating a steel containing 9 wt.% Cr composition to 600 ℃ for 10 minutes and cooling, followed by heating to 900 ℃ for 10 minutes and cooling;

FIG. 5A is a first photomicrograph showing the thickness of a Cr-rich oxide layer formed on a high Cr alloy composition after pre-oxidation by heating at 500 ℃ for 20 minutes and cooling and hardening by heating at 900 ℃ for 10 minutes followed by cooling;

FIG. 5B is a second photomicrograph showing the thickness of a Cr-rich oxide layer formed on a high Cr alloy composition after pre-oxidation by heating at 500 ℃ for 20 minutes and cooling and hardening by heating at 900 ℃ for 10 minutes followed by cooling;

FIG. 6A is a photograph showing the surface of bare 22MnB5 steel heated at 900 ℃ for 6 minutes and hot stamped;

FIG. 6B is a photograph showing the surface of AlSi-plated 22MnB5 steel after hot stamping;

fig. 6C is a photograph showing the surface of Zn-plated steel after hot stamping;

FIG. 6D is a photograph of the surface of an unplated/bare steel containing 3 wt.% Cr, pre-oxidized by heating at 500 ℃ for 20 minutes and cooling, and hardened by heating at 900 ℃ for 10 minutes followed by cooling;

FIG. 6E is a photograph of the surface of an uncoated/bare steel containing 9 wt.% Cr, pre-oxidized by heating at 600 ℃ for 10 minutes and cooling, and hardened by heating at 900 ℃ for 10 minutes followed by cooling;

FIG. 7 is a graph showing the thermodynamics of a 0.22% C-1.5% Mn + XCr (by weight) system, where the x-axis is chromium concentration (wt%) and the y-axis is temperature (. degree. C.);

FIG. 8 is a graph showing the kinetics of a 0.22% C-1.5% Mn + XCr (by weight) system, where the x-axis is time and the y-axis is temperature;

FIG. 9 is a flow chart showing a method of producing a press hardened steel object;

FIG. 10A is a photomicrograph of a high Al hardened steel at a first magnification;

FIG. 10B is a photomicrograph at a second magnification of the high Al hardened steel shown in FIG. 10A;

FIG. 11A is a photograph showing the surface of bare 22MnB5 steel heated at 900 ℃ for 6 minutes and hot stamped;

FIG. 11B is a photograph showing the surface of AlSi-plated 22MnB5 steel after hot stamping;

fig. 11C is a photograph showing the surface of Zn-plated steel;

FIG. 11D is a photograph of a surface of uncoated/bare high Al steel hardened by heating at 900 ℃ for 10 minutes followed by cooling; and

fig. 11E is a photograph of the surface of an uncoated/bare high Al steel hardened by heating at 1000 ℃ for 3 minutes followed by cooling.

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

Detailed Description

The exemplary embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods in order 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 should not be construed as limiting the scope of the disclosure. In some exemplary embodiments, well-known methods, 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, components, 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. While the open-ended term "comprising" should be understood as a non-limiting term used to describe and claim the various embodiments described herein, in certain aspects the term may alternatively be understood as a more limiting and limiting term, such as "consisting of … …" or "consisting essentially of … …. Thus, for any given embodiment reciting a composition, material, component, element, feature, integer, operation, and/or process step, the disclosure also specifically includes embodiments that consist of, or consist essentially of, such recited composition, material, component, element, feature, integer, operation, and/or process step. In the case of "consisting of … …", alternative embodiments exclude any additional components, materials, components, elements, features, integers, operations, and/or process steps, and in the case of "consisting essentially of … …", exclude from such embodiments any additional components, materials, components, elements, features, integers, operations, and/or process steps that substantially affect the basic and novel characteristics, but may include any components, materials, components, elements, features, integers, operations, and/or process steps that do not substantially affect the basic and novel characteristics.

Unless specifically identified as an order of performance, 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. 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 component, element, or layer, it can be directly on or engaged, connected, or coupled to the other component, element, or layer, or intervening components 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 similar manner (e.g., "between … …" versus "directly between … …", "adjacent" versus "directly adjacent", etc.). As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

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 specified. 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, as 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 related terms, such as "before", "after", "inside", "outside", "under", "below", "lower", "above", "upper" and the like, may be used herein to facilitate describing one element or feature as it relates to another element or feature as illustrated in the figures. Spatially and 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, numerical values represent approximate metrics or boundaries of a range to encompass minor deviations from the given values, and embodiments having values that are substantially the same as those embodiments having values that are exactly the same. Other than in the operating examples provided at the end of this detailed description, all numerical values in this specification, including parameters such as amounts or conditions in 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" means that the numerical value allows some degree of slight imprecision (with some approach to exactness of the value; approximately or reasonably close to the value; approximately). As used herein, "about" means a variation that would result, at least in part, from a common method of measuring and using such parameters, if the imprecision provided by "about" is not otherwise understood in the art with this ordinary meaning. For example, "about" can include 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%.

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

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

Alloy composition

The present technology provides an alloy composition configured to form a protective oxide layer prior to and/or during a hot stamping process. The alloy composition has at least one of a high chromium content or a high aluminum content and does not require plating (which can be expensive) or shot blasting to remove scale (which is time consuming and requires additional cost). In various aspects, alloy compositions having high chromium content can be oxidized prior to and during hot stamping. In various other aspects, an alloy composition having a high aluminum content is lighter than another alloy composition having the same composition but without the high aluminum content.

In various aspects of the present technique, the alloy composition is in the form of a billet for use in a hot stamping process. Thus, the blank forms a press hardened steel after the hot stamping process. The components (e.g., boron and chromium) in the alloy composition reduce the critical cooling rate during hot stamping relative to the critical cooling rate employed in the absence of such components.

The alloy composition includes carbon in a concentration of greater than or equal to about 0.15 wt.% to less than or equal to about 0.5 wt.%, greater than or equal to about 0.15 wt.% to less than or equal to about 0.4 wt.%, greater than or equal to about 0.15 wt.% to less than or equal to about 0.3 wt.%, greater than or equal to about 0.15 wt.% to less than or equal to about 0.25 wt.%, or greater than or equal to about 0.15 wt.% to less than or equal to about 0.2 wt.% of the alloy composition.

The alloy composition also includes chromium and aluminum, wherein the alloy composition has a high chromium content and a relatively low aluminum content, or has a high aluminum content and a relatively low chromium content.

In various aspects, the balance of the alloy composition is iron.

When the alloy composition has a high chromium content, i.e., when the composition is a high chromium (Cr) alloy composition, chromium is included at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% of the alloy composition, greater than or equal to about 2 wt.% to less than or equal to about 8 wt.% of the alloy composition, greater than or equal to about 2 wt.% to less than or equal to about 6 wt.% of the alloy composition, greater than or equal to about 2 wt.% to less than or equal to about 4 wt.% of the alloy composition, or about 3 wt.% of the alloy composition, and in a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition, greater than or equal to about 0.1 wt.% to less than or equal to about 4.5 wt.% of the alloy composition, greater than or equal to about 1 wt.% to less than or equal to about 4 wt.% of the alloy composition, Greater than or equal to about 2 wt.% to less than or equal to about 3 wt.% of the alloy composition, greater than or equal to about 0 wt.% to less than or equal to about 0.1 wt.% of the alloy composition, greater than or equal to about 0.015 wt.% to less than or equal to about 0.075 wt.% of the alloy composition, or greater than or equal to about 0.02 wt.% to less than or equal to about 0.05 wt.% of the alloy composition comprises aluminum. In the high chromium alloy composition, the carbon concentration is greater than or equal to about 0.15 wt% to less than or equal to about 0.5 wt% of the alloy composition, or greater than or equal to about 0.2 wt% to less than or equal to about 0.3 wt% of the alloy composition.

When the alloy composition has a high aluminum content, i.e., when the composition is a high aluminum (Al) alloy composition, from greater than or equal to about 2 wt.% to less than or equal to about 10 wt.%, from greater than or equal to about 3 wt.% to less than or equal to about 8 wt.%, from greater than or equal to about 3.5 wt.% to less than or equal to about 6 wt.%, from greater than or equal to about 4 wt.% to less than or equal to about 5 wt.%, or about 4 wt.% of the alloy composition, and comprises aluminum at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.%, from greater than or equal to about 0.1 wt.% to less than or equal to about 4.5 wt.%, from greater than or equal to about 1 wt.% to less than or equal to about 4 wt.%, or a concentration of aluminum in the alloy composition, Greater than or equal to about 2 wt.% to less than or equal to about 3 wt.% of the alloy composition, greater than or equal to about 0.075 wt.% to less than or equal to about 0.25 wt.% of the alloy composition, or greater than or equal to about 0.1 wt.% to less than or equal to about 0.2 wt.% of the alloy composition comprises chromium. In the high aluminum alloy composition, the carbon concentration is greater than or equal to about 0.15 wt.% to less than or equal to about 0.5 wt.% of the alloy composition, greater than or equal to about 0.2 wt.% to less than or equal to about 0.5 wt.% of the alloy composition, greater than or equal to about 0.3 wt.% to less than or equal to about 0.5 wt.% of the alloy composition, or about 0.4 wt.% of the alloy composition.

In various aspects of the present technique, the alloy composition further comprises boron at a concentration of greater than or equal to about 0.001 wt.% to less than or equal to about 0.005 wt.% of the alloy composition, manganese at a concentration of greater than or equal to about 0.5 wt.% to less than or equal to about 3 wt.% of the alloy composition, silicon at a concentration of greater than or equal to about 0.1 wt.% to less than or equal to about 0.5 wt.% of the alloy composition, titanium at a concentration of greater than or equal to about 0.001 wt.% to less than or equal to about 0.1 wt.% of the alloy composition, or a combination thereof.

In other aspects, the alloy composition further comprises molybdenum, nickel, or a combination thereof at a concentration of greater than 0 wt.% to less than or equal to about 0.2 wt.% of the alloy composition, or greater than 0 wt.% to less than or equal to about 0.1 wt.% of the alloy composition.

High Cr alloy composition

With respect to the high Cr alloy composition, in various aspects, it is configured to be pre-oxidized and cooled to produce a pre-oxidized high Cr alloy composition. The pre-oxidation is carried out at a temperature of greater than or equal to about 400 ℃ to less than or equal to about 700 ℃, such as at a temperature of about 400 ℃, about 450 ℃, about 500 ℃, about 550 ℃, about 600 ℃, about 650 ℃ or about 700 ℃ for a time of greater than or equal to about 1 minute to less than or equal to about 60 minutes, such as a time of about 5 minutes, about 10 minutes, about 15 minutes, about 20 minutes, about 25 minutes, about 30 minutes, about 35 minutes, about 40 minutes, about 45 minutes, about 50 minutes, about 55 minutes or about 60 minutes. By way of non-limiting example, cooling is performed by air cooling, water cooling, oil cooling, or in die cooling. The pre-oxidized high Cr alloy composition may be rolled into a coil or provided in sheet form and stored for future use.

The pre-oxidized high Cr alloy composition is configured to press harden at a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃, such as at a temperature of greater than or equal to about 850 ℃, about 900 ℃, about 950 ℃, about 1000 ℃, or about 1050 ℃, for a time of greater than or equal to about 1 minute to less than or equal to about 15 minutes, such as for a time of about 1 minute, about 2 minutes, about 3 minutes, about 4 minutes, about 5 minutes, about 6 minutes, about 7 minutes, about 8 minutes, about 9 minutes, about 10 minutes, about 11 minutes, about 12 minutes, about 13 minutes, about 14 minutes, or about 15 minutes, followed by cooling, for example, by air cooling, water cooling, oil cooling, or mold cooling.

FIG. 1A shows a photomicrograph of the surface of a press hardened steel made from a high Cr alloy composition containing 3 wt.% Cr of the composition in accordance with the present techniques. Here, the high Cr alloy composition is not pre-oxidized. Instead, the high Cr alloy composition is heated to 900 ℃ for 10 minutes and transferred to a water or oil cooled die for press forming and quenching. Fig. 1B shows a second micrograph of a surface of press hardened steel made from a high Cr alloy composition containing 3 wt.% Cr of the composition. Here, the high Cr alloy composition was pre-oxidized at 500 ℃ for 20 minutes, cooled, and then press hardened at 900 ℃ for 10 minutes, and then cooled. From the micrographs it can be determined that the pre-oxidized press hardened steel of fig. 1B has superior surface quality compared to the non-pre-oxidized press hardened steel of fig. 1A.

Fig. 2A is a micrograph showing the surface of steel containing 3 wt% of Cr, and fig. 2B is a micrograph showing the surface of steel containing 9 wt% of Cr. These steels were produced without shot blasting. In fig. 2A and 2B, the steel is heated at 900 ℃ for 10 minutes and then cooled. Fig. 3A is a micrograph showing the surface of steel containing 3 wt% of Cr, and fig. 3B is a micrograph showing the surface of steel containing 9 wt% of Cr. In fig. 3A and 3B, the steel is pre-oxidized by heating at 500 ℃ for 20 minutes and at 600 ℃ for 10 minutes, respectively, and then cooling. Fig. 4A is a micrograph showing the surface of steel containing 3 wt% of Cr, and fig. 4B is a micrograph showing the surface of steel containing 9 wt% of Cr. In fig. 4A and 4B, the steel is pre-oxidized by heating at 500 ℃ for 20 minutes and at 600 ℃ for 10 minutes and cooling, respectively, and then hardened by heating at 900 ℃ for 10 minutes and then cooling. The micrographs of FIGS. 2A, 2B, 3A, 3B, 4A and 4B show that the optimal surfaces are those obtained by pre-oxidation prior to hardening. Furthermore, when performed in a die during hot stamping, the cooling rate will be much faster than air cooling, which will result in an even better surface quality.

Heat treating the high Cr alloy composition after pre-oxidation results in the formation of a Cr-rich oxide layer on the hardened high Cr alloy composition. The Cr-rich oxide layer is a layer with Cr and O or Cr, O and Fe. In various aspects, Cr, O, and optionally Fe are present as, for example, CrFeO, CrO, Cr₂O₃、CrO₂、CrO₃、CrO₅Or Cr₈O₂₁Exist in the form of (1). The Cr-rich oxide layer has a Cr content of greater than or equal to about 1A thickness of [ mu ] m to less than or equal to about 40 [ mu ] m. Fig. 5A and 5B are photomicrographs showing the thickness of the Cr-rich oxide layer formed on the high Cr alloy composition after pre-oxidation by heating at 500 ℃ for 20 minutes and cooling and hardening by heating at 900 ℃ for 10 minutes followed by cooling. The average thickness of the Cr-rich oxide layer was about 10 μm.

Fig. 6A is a photograph showing the surface of bare 22MnB5 steel heated at 900 ℃ for 6 minutes and hot stamped. Fig. 6B and 6C show photographs of the surfaces of AlSi-plated 22MnB5 steel and Zn-plated steel after hot stamping. Fig. 6D and 6E show photographs of the surfaces of an uncoated/bare steel containing 3 wt% of Cr and an uncoated/bare steel containing 9 wt% of Cr, respectively, in which the steel containing 3 wt% of Cr was pre-oxidized by heating at 500 ℃ for 20 minutes and cooling, and hardened by heating at 900 ℃ for 10 minutes followed by cooling. The steels according to the present technique shown in fig. 6D and 6E have a significantly better surface quality than the bare 22MnB5 steel shown in fig. 6A. Furthermore, the steels according to the present technique shown in fig. 6D and 6E have comparable surface quality to the plated steels shown in fig. 6B and 6C.

Hardened steels made from the high Cr alloy compositions have an Ultimate Tensile Strength (UTS) of greater than or equal to about 1200 MPa, such as a UTS of about 1200 MPa, about 1250 MPa, about 1300 MPa, about 1350 MPa, about 1400 MPa, about 1450 MPa, about 1500 MPa, about 1550 MPa, about 1600 MPa, about 1650 MPa, about 1700 MPa, about 1750 MPa, about 1800 MPa, about 1850 MPa, about 1900 MPa, about 1950 MPa, about 2000 MPa, or greater. Moreover, hardened steels made from the high Cr alloy compositions have a ductility (elongation) in the hardened condition of greater than or equal to about 4% (elongation) to less than or equal to about 10% (elongation), such as a ductility of about 4% (elongation), about 5% (elongation), about 6% (elongation), about 7% (elongation), about 8% (elongation), about 9% (elongation), or about 10% (elongation).

Table 1 shows various exemplary characteristics of 22MnB5 and 2.25-5Cr according to the present technique. This table is provided for comparison purposes and may not reflect all acceptable property ranges for the 2.25% ‒ 5% Cr alloys provided herein.

Table 1: process parameters of high Cr alloy composition

Alloy (I)	Composition of	Soaking heat	Shaping by& Quenching	Properties of	Coating layer
						22Mn B5	0.22%C-1.5%Mn	900℃	>30℃/ Second cooling Everywhere	1500 MPa　6%EL	Exposed or exposed Plating AlSi
High Cr Alloy (I)	0.22%C-2.25-5% Cr-Mn-Si	20 minutes at 500 DEG C Clock +900 deg.C 10 minutes	>15℃/ Second cooling Everywhere	1600 MPa　6% EL	Without coating

Without being bound by theory, the addition of high levels of Cr to the alloy composition, for example about 3 wt% Cr of the composition, lowers the austenitizing temperature. FIG. 7 shows a thermodynamic diagram in which the x-axis 10 represents the Cr concentration of 0.22% C ‒ 1.5.5% Mn + xCr steel and the y-axis 12 represents temperature in degrees Celsius. The first region 14 shows body centered cubic (bcc) + face centered cubic (fcc) 0.22% C ‒ 1.5.5% Mn + xCr steel, and the second region 16 shows bcc + M₇C₃(carbide) steel, third region 18 shows bcc + fcc + M₇C₃(carbide) steel, fourth region 20 shows fcc + M₇C₃(carbide) steel, fifth region 22 shows bcc + M₂₃C₆(carbide) steel. The baseline temperature 24 is shown at about 800 deg.C, and the hot stamped region 26 is shown for 0.22% C ‒ 1.5.5% Mn + xCr. According to this figure, the inclusion of Cr at a concentration of about 3 wt.% of the alloy composition reduces the temperature required for hot stamping from a baseline temperature 24 of about 800 ℃ to the fcc point of about 780 ℃.

Also without being bound by theory, the addition of high levels of Cr to the alloy composition, for example about 3 wt% Cr of the composition, increases the time required for the critical cooling rate, which widens the process time window. The cooling rate in the mold was greater than 15 deg.C/sec and the baseline rate was greater than 30 deg.C/sec. FIG. 8 shows a kinetic diagram wherein the x-axis 30 represents time and the y-axis 32 represents temperature. Baseline temperature 34, M of A3 (alpha-ferrite or bcc ferrite)_s(martensite Start formation) baseline temperatures 36 and M_fThe (martensite formation complete) baseline temperature 38 is shown in the y-axis 32. A first curve 40 representing the critical cooling rate of the steel, a second curve 42 representing ferrite, a third curve 44 representing pearlite and a fourth curve 46 representing bainite transformation are shown on the graph. By adding Cr to the composition at a concentration of 3 wt% of the composition, the critical cooling rate is slid to the right in the figure, as represented by the dashed curves 40 ', 42', 44 ', 46' corresponding to the first, second, third and

fourth curves

40, 42, 44, 46, respectively. Such an increase in critical cooling rate provides a wider process window.

Thus, in various aspects, the high Cr alloy composition is configured to be quenched, i.e., cooled, at a rate of greater than or equal to about 15 ℃/sec, greater than or equal to about 20 ℃/sec, or greater than or equal to about 25 ℃/sec.

Any alloy composition known in the art may be modified by including the above-described chromium content to impart the advantageous properties provided by chromium. Non-limiting examples of such alloy compositions include 22Mn, 22MnB5, 25MnB5, 30MnB5, 22MnB5+ Nb/V, 25MnB5+ Nb/V, and 30MnB5+ Nb/V.

The present technique also provides a method of manufacturing a press hardened high Cr steel object. The steel object may be any object that is typically manufactured by hot stamping, such as a vehicle part. Non-limiting examples of vehicles having parts suitable for manufacture by the present method include bicycles, cars, motorcycles, boats, tractors, buses, mobile homes, campers, gliders, airplanes, and tanks. The method includes heating the alloy billet to a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃ for a time of greater than or equal to about 1 minute to less than or equal to about 15 minutes to produce a heated alloy billet. The billet is a high Cr alloy composition as described above. For example, the billet is composed of an alloy composition including carbon at a concentration of greater than or equal to about 0.15 wt.% to less than or equal to about 0.5 wt.% of the alloy composition, chromium at a concentration of greater than or equal to about 2 wt.% to less than or equal to about 10 wt.% of the alloy composition, and aluminum at a concentration of greater than or equal to about 0 wt.% to less than or equal to about 5 wt.% of the alloy composition.

In some aspects, the billet is provided in the form of a pre-oxidized sheet, coil, or coil of a high Cr alloy composition. In other aspects, the billet is not pre-oxidized, and the method comprises pre-oxidizing the high Cr alloy composition by heating the high Cr alloy composition to a temperature of greater than or equal to about 400 ℃ to less than or equal to about 700 ℃ for a time of about greater than or equal to about 1 minute to less than or equal to about 60 minutes prior to the hot forming process, and subsequently cooling the alloy composition.

After pre-oxidation and heating the blank to austenitizing temperature, the method includes transferring the heated alloy blank into a die to form the heated alloy blank into an object having a predetermined shape, and quenching the object to produce an object comprised of high Cr press hardened steel. Quenching the object includes cooling the object at a rate of greater than or equal to about 15 ℃/sec. The press hardened steel object has at least one surface comprising a Cr-rich oxide layer. In this method, shot blasting is not required after hot forming. However, the method optionally comprises trimming the object consisting of high Cr press hardened steel, for example by laser trimming.

FIG. 9 is a flow chart providing an exemplary method 50 of manufacturing a press hardened steel object. Here, the method includes obtaining a blank 52 composed of a high Cr alloy composition, shearing the blank 52 into a sheet material 54 having dimensions substantially corresponding to dimensions of the press hardened steel object, and forming the press hardened steel object 56 by hot stamping. In various aspects, the blank 52 is pre-oxidized or not pre-oxidized. When the blank 52 is not pre-oxidized, the method includes pre-oxidizing the blank or plate 54 by heating to a temperature of greater than or equal to about 400 ℃ to less than or equal to about 700 ℃ for a time of about greater than or equal to about 1 minute to less than or equal to about 60 minutes and then cooling the high Cr alloy composition. Hot stamping includes heating the sheet material 54 to a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃ for a time of greater than or equal to about 1 minute to less than or equal to about 15 minutes and transferring the heated sheet material 54 into a mold. After quenching, the press hardened steel object 56 is removed from the die, as described above.

High Al alloy composition

With respect to the high Al alloy composition, the density is 5% lower than a second alloy composition having the same composition but having an aluminum concentration less than or equal to about 0.05 wt.% of the second alloy composition. For example, adding 4 wt.% Al to the steel may result in a 5% weight reduction. Likewise, the high Al alloy composition may be rolled into a coil or provided in sheet form.

The high Al alloy composition is configured at greater than or equal to about 850 ℃ to less than or equal to about 1050 DEG CE.g., about 850 ℃, about 900 ℃, about 950 ℃, about 1000 ℃, or about 1050 ℃ for a time period of greater than or equal to about 1 minute to less than or equal to about 15 minutes, e.g., about 1 minute, about 2 minutes, about 3 minutes, about 4 minutes, about 5 minutes, about 6 minutes, about 7 minutes, about 8 minutes, about 9 minutes, about 10 minutes, about 11 minutes, about 12 minutes, about 13 minutes, about 14 minutes, or about 15 minutes, and then cooling. By way of non-limiting example, cooling is performed by air cooling, water cooling, oil cooling, or cooling with the mold. In various aspects, the heating comprises soaking in the presence of at least one of niobium and vanadium, the niobium concentration being greater than or equal to about 0.01 wt.% to less than or equal to about 0.1 wt.% of the alloy composition, the vanadium concentration being greater than or equal to about 0.05 wt.% to less than or equal to about 0.5 wt.% of the alloy composition. In one embodiment, the soaking is conducted in the presence of about 0.05 weight percent niobium and 0.2 weight percent vanadium. The hardened high-Al alloy has at least one surface comprising an Al-rich oxide layer. The Al-rich oxide layer is a layer having Al and O or Al, O and Fe. In various aspects, Al, O, and optionally Fe are present as, for example, AlFeO, AlO, Al₂O or Al₂O₃Exist in the form of (1). The Al-rich oxide layer has a thickness of greater than or equal to about 1 [ mu ] m to less than or equal to about 40 [ mu ] m.

Fig. 10A and 10B are photomicrographs at different magnifications of high Al hardened steels made from the high Al alloy compositions. In particular, high Al-hardened steels are produced from Fe-3-5% Al-C-Mn alloy compositions in accordance with the present techniques. The alloy composition is heated at 900 ℃ for about 10 minutes and then cooled to produce a hardened steel. The hardened steel has good surface quality.

Fig. 11A is a photograph showing the surface of bare 22MnB5 steel heated at 900 ℃ for 6 minutes. Fig. 11B and 2C show photographs of the surfaces of AlSi-plated 22MnB5 steel and Zn-plated steel after hot stamping. FIGS. 11D and 11E show photographs of the surface of uncoated/bare Fe-3-5% Al-C-Mn steel hardened at 900 ℃ for 10 minutes and 1000 ℃ for 3 minutes, respectively. The steels according to the present technique shown in fig. 11D and 12E have a significantly better surface quality than the bare 22MnB5 steel shown in fig. 11A. Furthermore, the steels according to the present technique shown in fig. 11D and 11E have comparable surface quality to the plated steels shown in fig. 11B and 11C.

Hardened steels made from the high Al alloy compositions have an Ultimate Tensile Strength (UTS) of greater than or equal to about 1100 MPa, such as a UTS of about 1100 MPa, about 1150 MPa, about 1200 MPa, about 1250 MPa, about 1300 MPa, about 1350 MPa, about 1400 MPa, about 1450 MPa, about 1500 MPa, about 1550 MPa, about 1600 MPa, about 1650 MPa, about 1700 MPa, about 1750 MPa, about 1800 MPa, or greater. Also, hardened steels made from the high Al alloy compositions have a ductility (elongation) in the hardened condition of greater than or equal to about 4% (elongation) to less than or equal to about 20% (elongation), such as about 4% (elongation), about 5% (elongation), about 6% (elongation), about 7% (elongation), about 8% (elongation), about 9% (elongation), about 10% (elongation), about 12% (elongation), about 14% (elongation), about 16% (elongation), about 18% (elongation), or about 20% (elongation).

Table 2 shows various exemplary properties of 22MnB5 and high Al steels according to the present technique. The table is provided for comparative purposes and may not reflect all acceptable property ranges for the high Al steel alloys provided herein.

Table 2: process parameters of high Al alloy composition

Combination of Chinese herbs Gold (Au)	Composition of	Soaking heat	Shaping by&Quenching device Fire(s)	Properties of	Coating layer
						22M nB5	0.22%C1.5%Mn‒Si	900℃	>30℃/ Second cooling Everywhere	1500 MPa6% EL	Exposed or exposed Plating AlSi
Height of Al Steel	0.4%C3.5%Al‒Mn‒Si	880-1000 ℃ accompanied by Nb/V	>15℃/ Second cooling Everywhere	1400 MPa10% EL	Without coating

In various aspects, the high Al alloy composition is configured to be quenched, i.e., cooled, at a rate of greater than or equal to about 15 ℃/sec, greater than or equal to about 20 ℃/sec, or greater than or equal to about 25 ℃/sec.

Any alloy composition known in the art may be modified by including the above-described aluminum content to impart the advantageous properties provided by the aluminum. Non-limiting examples of such alloy compositions include 22Mn, 22MnB5, 25MnB5, 30MnB5, 22MnB5+ Nb/V, 25MnB5+ Nb/V, 30MnB5+ Nb/V, 35MnB5+ Nb/V.

The present technique also provides a method of manufacturing a press hardened high Al steel object. The steel object may be any object that is typically manufactured by hot stamping, such as a vehicle part. Non-limiting examples of vehicles having parts suitable for manufacture by the present method include bicycles, cars, motorcycles, boats, tractors, buses, mobile homes, campers, gliders, airplanes, and tanks. The method includes heating the alloy billet to a temperature of greater than or equal to about 850 ℃ to less than or equal to about 1050 ℃ for a time of greater than or equal to about 1 minute to less than or equal to about 15 minutes to produce a heated alloy billet. The billet is a high Al alloy composition as described above. For example, a billet is composed of an alloy composition comprising carbon at a concentration of greater than or equal to about 0.15 wt% to less than or equal to about 0.5 wt% of the alloy composition, manganese at a concentration of greater than or equal to about 0.1 wt% to less than or equal to about 0.3 wt% of the alloy composition, silicon at a concentration of greater than or equal to about 0.1 wt% to less than or equal to about 0.5 wt% of the alloy composition, aluminum at a concentration of greater than or equal to about 2 wt% to less than or equal to about 10 wt% of the alloy composition, chromium at a concentration of greater than or equal to about 0 wt% to less than or equal to about 5 wt% of the alloy composition, and the balance of the alloy composition being iron. The billet is provided in the form of a sheet, coil or coil of high Al alloy composition.

After heating, the method includes transferring the heated alloy blank into a die to form the heated alloy blank into an object having a predetermined shape, and quenching the object to produce an object comprised of high Al press hardened steel. Quenching the object includes cooling the object at a rate of greater than or equal to about 15 ℃/sec. The press hardened steel object has at least one surface comprising an Al-rich oxide layer. Shot blasting is not required in this method. However, the method optionally includes trimming the object composed of the high Al press hardened steel, for example by laser trimming.

The foregoing description of the embodiments has been presented 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. It can 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. An alloy composition comprising the components:

carbon at a concentration of greater than or equal to 0.15 wt% to less than or equal to 0.5 wt% of the alloy composition;

manganese in a concentration of greater than or equal to 0.1 wt.% to less than or equal to 3 wt.% of the alloy composition;

silicon in a concentration of greater than or equal to 0.1 wt% to less than or equal to 0.5 wt% of the alloy composition;

chromium at a concentration of greater than or equal to 2.25 wt.% to less than or equal to 5 wt.% of the alloy composition;

aluminum at a concentration of greater than or equal to 0.02 wt.% to less than or equal to 0.05 wt.% of the alloy composition; and

the balance of the alloy composition is iron.

2. The alloy composition of claim 1, wherein the alloy composition has been oxidized at a temperature of greater than or equal to 400 ℃ to less than or equal to 700 ℃ for a time of greater than or equal to 1 minute to less than or equal to 60 minutes, and subsequently cooled.

3. The alloy composition of claim 2, wherein the alloy composition is configured to be hardened into a press hardened steel by subjecting the alloy composition to a temperature of greater than or equal to 850 ℃ to less than or equal to 1050 ℃ for a time of greater than or equal to 1 minute to less than or equal to 15 minutes and subsequently cooling.

4. The alloy composition of claim 3, wherein the press hardened steel does not require shot blasting after use in hot forming.

5. The alloy composition of claim 1, wherein the alloy composition is in the form of a sheet and at least one surface of the sheet comprises a Cr-rich oxide layer.

6. The alloy composition of claim 5, wherein the Cr-rich oxide layer has a thickness of greater than or equal to 1 μm to less than or equal to 40 μm.

7. An alloy composition comprising the components:

chromium at a concentration of greater than or equal to 0.05 wt.% to less than or equal to 0.3 wt.% of the alloy composition;

aluminum in a concentration of greater than or equal to 2 wt.% to less than or equal to 6 wt.% of the alloy composition; and

the balance of the alloy composition is iron.

8. The alloy composition of claim 7, wherein the aluminum has a concentration of 4 wt.% of the alloy composition.

9. The alloy composition of claim 7, wherein the alloy composition is configured to be hardened into a press hardened steel by subjecting the alloy composition to a temperature of greater than or equal to 850 ℃ to less than or equal to 1050 ℃ for a time of greater than or equal to 1 minute to less than or equal to 15 minutes and subsequently cooling.

10. The alloy composition of claim 9, wherein the press hardened steel has a predetermined shape and at least one surface of the predetermined shape comprises an Al rich oxide layer.

11. The alloy composition of claim 10, wherein the press hardened steel does not require shot blasting after use in hot forming.

12. A method of manufacturing a press hardened steel object, the method comprising:

heating an alloy billet to a temperature of greater than or equal to 850 ℃ to less than or equal to 1050 ℃ for a time of greater than or equal to 1 minute to less than or equal to 15 minutes to produce a heated alloy billet, wherein the alloy billet consists of an alloy composition having the composition:

carbon at a concentration of greater than or equal to 0.15 wt.% to less than or equal to 0.5 wt.% of the alloy composition,

manganese in a concentration of greater than or equal to 0.1 wt.% to less than or equal to 3 wt.% of the alloy composition,

silicon in a concentration of greater than or equal to 0.1 wt.% to less than or equal to 0.5 wt.% of the alloy composition,

chromium at a concentration of greater than or equal to 2.25 wt.% to less than or equal to 5 wt.% of the alloy composition,

aluminum in a concentration of greater than or equal to 0.02 wt.% to less than or equal to 0.05 wt.% of the alloy composition, and

the balance of the alloy composition being iron;

transferring the heated alloy blank to a die to form the heated alloy blank into an object having a predetermined shape; and are

Quenching the object having the predetermined shape to produce an object consisting of press hardened steel,

wherein the press hardened steel object has at least one surface comprising an oxide layer, and wherein the method does not require shot blasting.

13. The method of claim 12, wherein quenching the object comprises cooling the object at a rate of greater than or equal to 15 ℃/sec.

14. The method of claim 12, wherein the method further comprises, prior to heating:

pre-oxidizing the alloy composition by heating the alloy composition to a temperature of greater than or equal to 400 ℃ to less than or equal to 700 ℃ for a time of greater than or equal to 1 minute to less than or equal to 60 minutes and then cooling the alloy composition.