US20210087661A1

US20210087661A1 - Steel for hot stamping with enhanced oxidation resistance

Info

Publication number: US20210087661A1
Application number: US16/958,362
Authority: US
Inventors: Qi Lu; Jiachen PANG; Jianfeng Wang
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2017-12-28
Filing date: 2017-12-28
Publication date: 2021-03-25
Also published as: CN111542635B; CN111542635A; WO2019127240A1

Abstract

An alloy composition is provided. The alloy composition includes chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %, carbon (C) at a concentration of greater than or equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %, manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %, silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %, and a balance of the alloy composition being iron. Methods of making shaped steel objects from the alloy composition are also provided.

Description

INTRODUCTION

This section provides background information related to the present disclosure which is not necessarily prior art.
Press-hardening steel (PHS), also referred to as “hot-stamped steel” or “hot formed steel,” is one of the strongest steels used for automotive body structural applications, having tensile strength properties on the order of about 1,500 mega-Pascal (MPa). 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 without fracturing so as to provide protection to the occupants. Moreover, galvanized PHS components may provide cathodic protection.
Many 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 880° C. to 950° 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. An oxide layer often forms during the transfer from the furnace to the dies. After quenching, therefore, the oxide must be removed from the PHS component and the dies. The oxide is typically removed, i.e., descaled, by shot blasting.
The PHS component may be made from bare or aluminum-silicon (Al—Si) alloy using the direct method or from zinc (Zn) coated PHS using the direct method or an indirect method. Coating the PHS component provides a protective layer (e.g., galvanic protection) to the underlying steel component. Zinc coatings offer cathodic protection; the coating acts as a sacrificial layer and corrodes instead of the steel component, even where the steel is exposed. Such coatings also generate oxides on PHS components' surfaces, which are removed by shot blasting. Accordingly, alloy compositions that do not require coatings or other treatments are desired.

SUMMARY

This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
In various aspects, the present technology provides an alloy composition including chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %; carbon (C) at a concentration of greater than or equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %; manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %; silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %; and a balance of the alloy composition being iron.
In one aspect, the alloy composition includes Si at a concentration of greater than or equal to about 0.6 wt. % to less than or equal to about 1.5 wt. %.
In one aspect, the alloy composition includes Cr at a concentration of greater than or equal to about 2 wt. % to less than or equal to about 3 wt. %.
In one aspect, the alloy composition further includes aluminum (Al) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %.
In one aspect, the alloy composition further includes nitrogen (N) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %.
In one aspect, the alloy composition further includes at least one of: molybdenum (Mo) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %; nickel (Ni) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %; boron (B) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %; niobium (Nb) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. %; and vanadium (V) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. %.
In one aspect, the alloy composition is in the form of an alloy coil.
In one aspect, the alloy coil includes ferrite, martensite and retained austenite (RA).
In one aspect, the alloy composition has been subjected to a quench and partitioning process.
In one aspect, a hot stamping method of forming a shaped steel object is provided. The hot stamping method includes austenitizing a blank having the alloy composition, stamping the austenitized blank to form a shaped object, and quenching the shaped object to form the shaped steel object.
In one aspect, a cold stamping method of forming a shaped steel object is provided. The cold stamping method includes cutting a blank from a coil having the alloy composition, wherein the alloy composition has been subjected to a quench and partitioning process; and stamping the blank into a predetermined shape at ambient temperature to form the shaped steel object.
In various aspects, the present technology also provides a method of forming a shaped steel object; the method including cutting a blank from a coil including an alloy composition having chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %, carbon (C) at a concentration of greater than or equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %, manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %, silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %, and a balance of the alloy composition being iron; heating the blank to a temperature above an upper critical temperature (Ac3) of the alloy composition to form a heated blank having austenite; stamping the heated blank into a predetermined shape to form a stamped object; and quenching the stamped object to form the shaped steel object, wherein the shaped steel object includes martensite.
In one aspect, the quenching includes decreasing the temperature of the stamped object at a rate of greater than or equal to about 15° C./s until the stamped object reaches a temperature below a martensite finish (Mf) temperature of the alloy composition.
In one aspect, the method is free from pre-oxidizing the alloy composition, coating the shaped steel object, and shot blasting.
In one aspect, the quenching has a quench and partitioning process, wherein the quench and partitioning process includes decreasing the temperature of the stamped object until the stamped object has a temperature between a martensite start (Ms) temperature of the alloy composition and a martensite finish (Mf) temperature of the alloy composition; incubating the stamped object at a partitioning temperature wherein carbon (C) is partitioned from martensite into austenite; and decreasing an austenite Mf temperature to a temperature below room temperature.
In one aspect, the quench and partitioning process forms the shaped steel object, wherein the shaped steel object includes ferrite, martensite and retained austenite (RA).
In one aspect, the shaped steel object is substantially free of cementite.
In various aspects, the present technology yet further provides a method of forming a shaped steel object; the method including cutting a blank from a coil of an advanced high strength steel (AHSS); and stamping the blank into a predetermined shape at ambient temperature to form the shaped steel object, wherein the AHSS is made by subjecting an alloy composition to a quench and partitioning process, the alloy composition having chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %, carbon (C) at a concentration of greater than or equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %, manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %, silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %, and a balance of the alloy composition being iron.
In one aspect, the AHSS is substantially free of an oxide layer.
In one aspect, the shaped steel object is bare of zinc (Zn) coated.
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 is a graph showing a temperature versus time for a traditional hot stamping method and a hot stamping method including quench and partitioning.

FIG. 2A is an image of a steel made from a high chromium content, low silicon content alloy without pre-oxidation.

FIG. 2B is an image of a steel made from a high chromium content, low silicon content alloy with pre-oxidation.

FIG. 3A is an image of a steel made from an alloy composition according to various aspects of the current technology (2% Cr, 0.6% Si) without pre-oxidation.

FIG. 3B is an image of a steel made from an alloy composition according to various aspects of the current technology (3% Cr, 0.6% Si) without pre-oxidation.

FIG. 3C is an image of a steel made from an alloy composition according to various aspects of the current technology (3% Cr, 1.5% Si) without pre-oxidation.

FIG. 4 shows cross sectional images of a steel made from an alloy composition according to various aspects of the current technology (2% Cr, 0.6% Si) without pre-oxidation.

FIG. 5A is a cross-sectional image of a steel made from an alloy composition according to various aspects of the current technology (2% Cr, 0.6% Si) without pre-oxidation.

FIG. 5B is a cross-sectional image of a steel made from an alloy composition according to various aspects of the current technology (3.1% Cr, 0.61% Si) without pre-oxidation.

FIG. 5C is a cross sectional image of a steel made from an alloy composition according to various aspects of the current technology (3.2% Cr, 1.46% Si) without pre-oxidation.

FIG. 6A is a graph showing thermodynamics of an alloy system that does not comprise silicon.

FIG. 6B is a graph showing thermodynamics of an alloy system that comprises silicon according to various aspects of the current technology.

FIG. 7 shows aspects of a method for making a shaped steel object according to various aspects of the current technology.

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%.
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.
Example embodiments will now be described more fully with reference to the accompanying drawings.
To overcome the necessity to coat PHS alloys, an alloy with a high chromium concentration is described. The high chromium concentration alloy comprises 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, 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, carbon at a concentration of greater than or equal to about 0.15% by weight to less than or equal to about 0.5 wt. % of the alloy composition, and a balance of the high chromium concentration alloy being iron. Although the high chromium concentration alloy does not require coating or shot blasting, it does require pre-oxidation by incubating the high chromium concentration alloy at a temperature of greater than or equal to about 400° C. to less than or equal to about 700° C. for a time of greater than or equal to about 1 minute to less than or equal to about 60 minutes.
Accordingly, the current technology relates to an alloy composition having a high chromium content that is suitable for hot and cold stamping applications, that does not require coating or shot blasting for hot stamping applications, and that is resistant to oxidation, i.e., does not require pre-oxidation prior to being press hardened. The alloy composition has a high chromium content to preclude a coating requirement, and also includes a high silicon (Si) content for improving oxidation resistance. The high silicon content also permits the chromium concentration to be decreased.
In various aspects of the current technology, the alloy composition is in a form of a blank for hot stamping processes. Here, the blank forms a press hardening steel after hot stamping processes. Components within the alloy composition, such as, for example, boron and chromium, lower a critical cooling rate in hot stamping processes relative to critical cooling rates employed without such components. In other aspects of the current technology, the alloy composition is in a form of a blank for cold stamping processes. Here, the blank is an advance high strength steel (AHSS) for cold stamping.
The alloy composition of the current technology comprises silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %, greater than or equal to about 0.6 wt. % to less than or equal to about 1.8 wt. %, or greater than or equal to about 0.8 wt. % to less than or equal to about 1.5 wt. %. For example, in various embodiments the alloy composition comprises Si at a concentration of about 0.5 wt. %, about 0.6 wt. %, about 0.7 wt. %, about 0.8 wt. %, about 0.9 wt. %, about 1 wt. %, about 1.1 wt. %, about 1.2 wt. %, about 1.3 wt. %, about 1.4 wt. %, about 1.5 wt. %, about 1.6 wt. %, about 1.7 wt. %, about 1.8 wt. %, about 1.9 wt. %, or about 2 wt. %. This high amount of Si in the alloy composition improves oxidation resistance, permits a lower amount of chromium to be added while still not requiring coating or shot blasting after forming, and prevents, inhibits, or decreases cementite formation during a quench and partitioning process.
The alloy composition also comprises chromium (Cr). Without the high levels of Si, Cr would have to be added at a level of from about 2 wt. % to about 10 wt. % to prevent the need to coat and/or shot blast. Because of the high levels of Si; however, the alloy composition comprises Cr at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %, greater than or equal to about 1.5 wt. % to less than or equal to about 8 wt. %, greater than or equal to about 1.75 wt. % to less than or equal to about 5 wt. %, greater than or equal to about 2 wt. % to less than or equal to about 4 wt. %, or greater than or equal to about 2 wt. % to less than or equal to about 3 wt. %. For example, in various embodiments the alloy composition comprises Cr at a concentration of about 0.5 wt. %, about 1 wt. %, about 1.5 wt. %, about 2 wt. %, about 2.5 wt. %, about 3 wt. %, about 3.5 wt. %, about 4 wt. %, about 4.5 wt. %, about 5 wt. %, about 5.5 wt. %, about 6 wt. %, about 6.5 wt. %, about 7 wt. %, about 7.5 wt. %, about 8 wt. %, about 8.5 wt. %, or about 9 wt. %.
The alloy composition also comprises carbon (C) at 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 0.45 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. %. For example, in various embodiments the alloy composition comprises C at a concentration of about 0.15 wt. %, about 0.2 wt. %, about 0.25 wt. %, about 0.3 wt. %, about 0.35 wt. %, about 0.4 wt. %, about 0.45 wt. %, or about 0.5 wt. %.
The alloy composition can also include manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %, greater than or equal to about 0.25 wt. % to less than or equal to about 2.5 wt. %, greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %, greater than or equal to about 0.75 wt. % to less than or equal to about 1.5 wt. %, or greater than or equal to about 1 wt. % to less than or equal to about 1.5 wt. %. In some embodiments, the alloy composition is substantially free of Mn. As used herein, “substantially free” refers to trace component levels, such as levels of less than or equal to about 1.5%, less than or equal to about 1%, less than or equal to about 0.5%, or levels that are not detectable. In various embodiments, the alloy composition is substantially free of Mn or comprises Mn at a concentration of less than or equal to about 0.5 wt. %, less than or equal to about 1 wt. %, less than or equal to about 1.5 wt. %, less than or equal to about 2 wt. %, less than or equal to about 2.5 wt. %, or less than or equal to about 3 wt. %.
In various embodiments, the alloy composition further comprises aluminum (Al) 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. %, from greater than or equal to about 2 wt. % to less than or equal to about 3 wt. %, from greater than or equal to about 0 wt. % to less than or equal to about 0.1 wt. %, from greater than or equal to about 0.015 wt. % to less than or equal to about 0.075 wt. %, or from greater than or equal to about 0.02 wt. % to less than or equal to about 0.05 wt. %. For example, in various embodiments the alloy composition is substantially free of Al or comprises Al at a concentration of about less than or equal to 0.5 wt. %, less than or equal to about 1 wt. %, less than or equal to about 1.5 wt. %, less than or equal to about 2 wt. %, less than or equal to about 2.5 wt. %, less than or equal to about 3 wt. %, less than or equal to about 3.5 wt. %, less than or equal to about 4 wt. %, less than or equal to about 4.5 wt. %, or less than or equal to about 5 wt. %.
In various embodiments, the alloy composition further comprises nitrogen (N) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %. For example, in various embodiments the alloy composition is substantially free of N or comprises N at a concentration of less than or equal to about 0.001 wt. %, less than or equal to 0.002 wt. %, less than or equal to 0.003 wt. %, less than or equal to 0.004 wt. %, less than or equal to 0.005 wt. %, less than or equal to 0.006 wt. %, less than or equal to 0.007 wt. %, less than or equal to 0.008 wt. %, less than or equal to 0.009 wt. %, or less than or equal to 0.01 wt. %.
In various embodiments, the alloy composition further comprises molybdenum (Mo) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %, or less than or equal to about 0.8 wt. %. For example, in various embodiments the alloy composition is substantially free of Mo or comprises Mo at a concentration of less than or equal to about 0.1 wt. %, less than or equal to about 0.2 wt. %, less than or equal to about 0.3 wt. %, less than or equal to about 0.4 wt. %, less than or equal to about 0.5 wt. %, less than or equal to about 0.6 wt. %, less than or equal to about 0.7 wt. %, less than or equal to about 0.8 wt. %, less than or equal to about 0.9 wt. %, or less than or equal to about 1.0 wt. %.
In various embodiments, the alloy composition further comprises nickel (Ni) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %, or less than or equal to about 0.8 wt. %. For example, in various embodiments the alloy composition is substantially free of Ni or comprises Ni at a concentration of less than or equal to about 0.1 wt. %, less than or equal to about 0.2 wt. %, less than or equal to about 0.3 wt. %, less than or equal to about 0.4 wt. %, less than or equal to about 0.5 wt. %, less than or equal to about 0.6 wt. %, less than or equal to about 0.7 wt. %, less than or equal to about 0.8 wt. %, less than or equal to about 0.9 wt. %, or less than or equal to about 1.0 wt. %.
In various embodiments, the alloy composition further comprises boron (B) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %, or less than or equal to about 0.005 wt. %. For example, in various embodiments the alloy composition is substantially free of B or comprises B at a concentration of less than or equal to about 0.001 wt. %, less than or equal to about 0.002 wt. %, less than or equal to about 0.003 wt. %, less than or equal to about 0.004 wt. %, less than or equal to about 0.005 wt. %, less than or equal to about 0.006 wt. %, less than or equal to about 0.007 wt. %, less than or equal to about 0.008 wt. %, less than or equal to about 0.009 wt. %, or less than or equal to about 0.01 wt. %.
In various embodiments, the alloy composition further comprises niobium (Nb) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. %, or less than or equal to about 0.3 wt. %. For example, in various embodiments the alloy composition is substantially free of Nb or comprises Nb at a concentration of less than or equal to about 0.1 wt. %, less than or equal to about 0.2 wt. %, less than or equal to about 0.3 wt. %, less than or equal to about 0.4 wt. %, or less than or equal to about 0.5 wt. %.
In various embodiments, the alloy composition further comprises vanadium (V) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. %, or less than or equal to about 0.3 wt. %. For example, in various embodiments the alloy composition is substantially free of V or comprises V at a concentration of less than or equal to about 0.1 wt. %, less than or equal to about 0.2 wt. %, less than or equal to about 0.3 wt. %, less than or equal to about 0.4 wt. %, or less than or equal to about 0.5 wt. %.
In various embodiments, the alloy composition comprises at least one of Mn, Al, N, Mo, Ni, B, Nb, and V, or at least one of Mo, Ni, B, Nb, and V.
A balance of the alloy composition is iron.
Table 1 shows the composition of the alloy composition relative to a baseline high chromium press hardened steel (PHS).

TABLE 1

Composition of baseline high chromium PHS and an alloy
composition according to the present technology.

Chemical Composition (wt. %)

Grade	Coating	C	Mn	Cr	Si	N	Others

Baseline Cr PHS	Free	0.1-0.45	0-3.0	2-10	0-0.5	<0.006
Cr PHS	Free	0.1-0.45	0-3.0	0.5-9	0.5-2	<0.006	Mo < 0.8,
(current technology)							B < 0.005,
							Nb/V < 0.3

The alloy composition can include various combinations of Si, Cr, C, Mn, Al, N, Mo, Ni, B, Nb, V, and Fe at their respective concentrations described above. In some embodiments, the alloy composition consists essentially of Si, Cr, C, Mn, and Fe. As described above, the term “consists essentially of” means the alloy composition precludes additional compositions, materials, components, elements, and/or features, that materially affect the basic and novel characteristics of the alloy composition, such as the alloy composition not requiring pre-oxidation, coating, or shot blasting when formed into a shaped object, but any compositions, materials, components, elements, and/or features, that do not materially affect the basic and novel characteristics can be included in the embodiment. Therefore, when the alloy composition consists essentially of Si, Cr, C, Mn, and Fe, the alloy composition can also include any combination of Al, N, Mo, Ni, B, Nb, and V that does not materially affect the basic and novel characteristics of the alloy composition. In other embodiments, the alloy composition consists of Si, Cr, C, Mn, Fe in their respective concentrations described above and at least one of Al, N, Mo, Ni, B, Nb, and V in no more than trace amounts, such as levels of less than or equal to about 1.5%, less than or equal to about 1%, less than or equal to about 0.5%, or levels that are not detectable. Other elements that are not described herein can also be included in trace amounts with the proviso that they do not materially affect the basic and novel characteristics of the alloy composition.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mn, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mn, and Fe.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mn, Al, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mn, Al and Fe.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mn, Al, Mo, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mn, Al, Mo, and Fe
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mn, Al, Mo, Nb, V, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mn, Al, Mo, Nb, V, and Fe.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mn, Al, Mo, Ni, Nb, V, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mn, Al, Mo, Ni, Nb, V, and Fe.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mn, N, Ni, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mn, N, Ni, and Fe.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mn, Al, N, Mo, Ni, B, Nb, V, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mn, Al, N, Mo, Ni, B, Nb, V, and Fe.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, and Fe.
In one embodiment, the alloy composition consists essentially of Si, Cr, C, Mo, B, Nb, V, and Fe. In another embodiment, the alloy composition consists of Si, Cr, C, Mo, B, Nb, V, and Fe.
In various aspects of the current technology, the alloy composition is in the form of a coil of the metal. In this form, the coil can be unrolled and cut into predetermined shapes or blanks. The blanks can be hot stamped using a traditional quenching method or by a quench and partitioning method. FIG. 1 shows a graph 10 having a y-axis 12 representing temperature and an x-axis 14 representing time. A first line 16 on the graph 10 represents a traditional process. Here, the blank is austenitized, i.e., heated to a final temperature 18 that is above an upper critical temperature (Ac3) 20 of the alloy composition. The final temperature 18 is greater than or equal to about 880° C. to less than or equal to about 950° C. The austentized blank is then stamped or hot formed into a shaped object at a temperature 22 between the final temperature 18 and Ac3 20 and then cooled at a rate of greater than or equal to about 1° C.s⁻¹, greater than or equal to about 5° C.s⁻, greater than or equal to about 10° C.s⁻¹, greater than or equal to about 15° C.s⁻¹, or greater than or equal to about 20° C.s⁻¹, such as at a rate of about 1° C.s⁻¹, about 3° C.s⁻¹, about 5° C.s⁻¹, about 10° C.s⁻¹, about 15° C.s⁻¹, about, 20° C.s⁻¹about, about 25° C.s⁻¹, about 30° C.s⁻¹or faster until the temperature decreases below a martensite start (Ms) temperature 24, and below a martensite finish (Mf) temperature 26, such that the shaped object comprises a fully or substantially fully martensite microstructure.
The graph 10 also includes a second line 28 representing a quench and partition process. Here, the blank is austenitized at the final temperature 18, which is above the Ac3 temperature 20 of the alloy composition. The austentized blank is then stamped or hot formed into a shaped object at a temperature 22 between the final temperature 18 and Ac3 20 and then cooled at the rate described above for the traditional process. However, when the temperature is decreased to a temperature between the Ms temperature 24 and the Mf temperature 26, i.e., after martensite begins to form, but before the structure is fully martensite, the temperature is held constant, increased, or decreased slowly, such that a partitioning temperature is obtained in which carbon (C) is partitioned from martensite into austenite. The temperature is then decreased to a temperature below the Mf temperature 26. The resulting shaped object has a microstructure comprising martensite and retained austenite (RA) and a surface comprising a thin layer of oxide of chromium (Cr) and silicon (Si). This oxide layer has a thickness of less than or equal to about 30 less than or equal to about 25 less than or equal to about 20 less than or equal to about 15 μm, less than or equal to about 10 μm, less than or equal to about 5 μm, or less than or equal to about 1 μm. The high silicon concentration in the alloy composition prevents, inhibits, or decreases the formation of cementite in the final microstructure when the quench and partitioning process is used. Neither the traditional process nor the quench and partitioning process requires a pre-oxidation step or descaling step (such as by shot blasting).
In various aspects of the current technology, the alloy composition is austentized and subjected to a quench and partitioning process to form an advanced high strength steel (AHSS), and then formed into a coil of the metal material. Here, the AHSS coil comprises ferrite, martensite and retained austenite (RA) and is substantially free of an oxide layer. Being “substantially free” of an oxide layer means that the AHSS comprises an oxide layer with a thickness of less than or equal to about 5 μm, less than or equal to about 2.5 μm, or less than or equal to about 1 μm. This AHSS is suitable for making shaped objects by cold stamping at ambient temperature. The shaped objects can be bare or zinc (Zn) coated.
FIG. 2A is an image of a control alloy (3% Cr and 0.3% Si) that was heated at 900° C. for 10 minutes and then quenched traditionally. Here, the control alloy is not pre-oxidized. Rather, the high Cr alloy composition is heated to 900° C. for 10 minutes and transferred to water or oil cooled die for press forming and quenching. FIG. 2B shows a second micrograph of a surface of a press hardening steel made from the control alloy. Here, the control alloy is pre-oxidized at 500° C. for 20 minutes, cooled, and then press hardened at 900° C. for 10 minutes, and then cooled. As can be determined from the micrographs, the control alloy comprising 0.3% Si requires peroxidation in order to achieve a high surface quality.
FIG. 3A, FIG. 3B, and FIG. 3C show alloy compositions comprising 2% Cr and 0.6% Si, 3% Cr and 0.6% Si, and 3% Cr and 1.5% Si respectively. These are alloy compositions according to the present technology that are not pre-oxidized and that are heated to 900° C. for 4-10 minutes and then cooled. The surface quality is good for each of the alloy compositions with the quality increasing from FIG. 3A to FIG. 3B to FIG. 3C. FIGS. 2A-2B and FIGS. 3A-3C show that the control alloy comprising only 0.3% Si requires pre-oxidation and the alloy composition of the current technology does not require pre-oxidation.
FIG. 4 shows cross-section images of the quenched alloy composition in FIG. 3A. A first image 30 shows a thin surface layer on the quenched alloy composition. A second image 32 shows an iron (Fe) distribution in the surface layer. A third image 34 shows an oxygen (O) distribution in the surface layer. A fourth image 36 shows a silicon (Si) distribution in the surface layer. A fifth image 38 shows a chromium (Cr) distribution in the surface layer. A high segregation of O, Si, and Cr in these images 30, 32, 34, 36, 38 show that the surface layer comprises a dense oxide of Cr and Si.
FIGS. 5A, 5B, and 5C show additional alloy compositions according to the present technology. The alloy compositions comprises 2% Cr and 0.6% Si, 3.1% Cr and 0.61% Si, and 3.2% Cr and 1.46% Si in FIGS. 5A, 5B, and 5C, respectively, which are not pre-oxidized and that are heated to 900° C. for 10 minutes and then cooled. Each of the alloy compositions results in a high surface quality. Notably, whereas the alloy compositions of FIGS. 5A and 5B generate thin oxide layers of about 20 μm thick, the alloy composition of FIG. 5C has an oxide layer of less than about 1 μm thick.
Without being bound by theory, adding high levels of Cr to the alloy composition, such as, for example, about 3% Cr by weight of the composition decreases the austenitization temperature. FIG. 6A shows a thermodynamics graph 40, wherein the x-axis 42 represents Cr concentration (from 0-12 wt. %) for a 0.22% C-1.5% Mn-xCr steel (without Si) and the y-axis 44 represents temperature (from 500-1000° C.). A first region 46 is shown for body-centered cubic (bcc)+face-centered cubic (fcc) 0.22% C-1.5% Mn-xCr steel, a second region 48 is shown for bcc+M₇C₃(carbide) steel, a third region 50 is shown for bcc+fcc+M₇C₃(carbide) steel, a fourth region 52 is shown for fcc+M₇C₃(carbide) steel, and a fifth region 54 is shown for bcc+M₂₃C₆(carbide) steel. A hot stamping area 56 is shown for 0.22% C-1.5% Mn-xCr. According to the graph, including Cr at a concentration of about 3% by weight of the alloy composition lowers the temperature required for hot stamping from the baseline temperature of about 800° C. to a fcc point of about 780° C. FIG. 6B shows a thermodynamics graph 60, wherein the x-axis 62 represents Cr concentration (from 0-12 wt. %) for a 0.22% C-1.5% Mn-1.6% Si-xCr steel and the y-axis 64 represents temperature (from 500-1000° C.). A first region 66 is shown for bcc+fcc 0.22% C-1.5% Mn-xCr steel, a second region 68 is shown for bcc+M₇C₃(carbide) steel, a third region 70 is shown for bcc+fcc+M₇C₃(carbide) steel, and a fourth region 72 is shown for bcc+M₂₃C₆(carbide) steel. A hot stamping area 74 is shown for 0.22% C-1.5% Mn-1.6% Si-xCr. The graphs 40, 60 show that adding Si has a minimal effect on the Ac3 temperature of the alloy.
Hardened steel made from the alloy composition has 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. Also, the hardened steel made from the alloy composition has a ductility (elongation) 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) in the hardened condition.
With reference to FIG. 7, the current technology also provides a method 80 of forming a shaped steel object. The shaped steel object can be any object that is generally made by hot stamping, such as, for example, a vehicle part. Non-limiting examples of vehicles that have parts suitable to be produced by the current method include bicycles, automobiles, motorcycles, boats, tractors, buses, mobile homes, campers, gliders, airplanes, and tanks.
The method 80 comprises obtaining a coil 82 of a metal material having an alloy composition according to the present technology and cutting a blank 84 from the coil 82. The method also comprises austenitizing the blank by heating the blank in a furnace 86 to a temperature above its Ac3 temperature to form a heated blank comprising austenite. Optionally by a robotic arm 88, the heated blank is transferred to a press 90. Here, the method 80 comprises stamping the heated blank into a predetermined shape to form a stamped object, and quenching the stamped object to form a shaped steel object 92, wherein the shaped steel object 92 comprises martensite. The method 80 is free of a pre-oxidation step, of a coating step, and of a descaling step (e.g., shot blasting).
In one embodiment, the quenching is performed traditionally by cooling the shaped object at a rate described above until the stamped object reaches a temperature below an Mf temperature of the alloy composition. Here, the shaped steel object has a microstructure that is fully martensite or substantially fully martensite. By “substantially fully” it is meant that greater than or equal to about 80%, greater than or equal to about 85%, greater than or equal to about 90%, or greater than or equal to about 95% of the microstructure is martensite.
In another embodiment, the quenching comprises a quench and partitioning process as described above. Here, the method comprises decreasing the temperature of the stamped object until the stamped object has a temperature between an Ms temperature of the alloy composition and a Mf temperature of the alloy composition, incubating the stamped object at a partitioning temperature wherein carbon (C) is partitioned from martensite into austenite, and then decreasing austenite's Mf temperature below room temperature. The partitioning temperature can be the temperature between the Ms and Mf temperatures at which the cooling is stopped, a temperature higher than the temperature between the Ms and Mf temperatures at which the cooling is stopped, or a temperature lower than the temperature between the Ms and Mf temperatures at which the cooling is stopped. Partitioning is performed at the partitioning temperature for a time of greater than or equal to about 0.01 min to less than or equal to about 20 min. After the quench and partitioning process, the shaped steel object has a microstructure comprising martensite and RA. Due to the high Si content of the alloy composition, the microstructure of the shaped steel object is substantially free of cementite. As used herein, “substantially free” refers to less than or equal to about 10%, less than or equal to about 5%, or less than or equal to about 1%.
In one variation of the method 80, the coil 82 comprises AHSS for a cold stamping. Here, as shown by the dotted line, after the blank 84 is cut from the coil 82, it is transferred to the press 90, optionally by way of the robotic arm 88. The method 80 comprises stamping the blank 84 into a predetermined shape at ambient temperature to form the shaped steel object 92. Although the shaped steel object can be bare, in various embodiments, the method also includes disposing a zinc (Zn) coating on the shaped steel object.
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

What is claimed is:

1. An alloy composition comprising:

chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %;

carbon (C) at a concentration of greater than or equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %;

manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %;

silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %; and

a balance of the alloy composition being iron.

2. The alloy composition according to claim 1, wherein the alloy composition comprises Si at a concentration of greater than or equal to about 0.6 wt. % to less than or equal to about 1.5 wt. %.

3. The alloy composition according to claim 1, wherein the alloy composition comprises Cr at a concentration of greater than or equal to about 2 wt. % to less than or equal to about 3 wt. %.

4. The alloy composition according to claim 1, wherein the alloy composition further comprises:

aluminum (Al) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 5 wt. %.

5. The alloy composition according to claim 1, wherein the alloy composition further comprises:

nitrogen (N) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %.

6. The alloy composition according to claim 1, wherein the alloy composition further comprises at least one of:

molybdenum (Mo) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %;

nickel (Ni) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 1 wt. %;

boron (B) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.01 wt. %;

niobium (Nb) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. %; and

vanadium (V) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 0.5 wt. %.

7. The alloy composition according to claim 1, wherein the alloy composition is in the form of an alloy coil.

8. The alloy composition according to claim 7, wherein the alloy coil comprises ferrite, martensite and retained austenite (RA).

9. The alloy composition according to claim 7, wherein the alloy composition has been subjected to a quench and partitioning process.

10. A hot stamping method of forming a shaped steel object, the hot stamping method comprising:

austenitizing a blank comprising an alloy composition according to claim 1;

stamping the austenitized blank to form a shaped object; and

quenching the shaped object to form the shaped steel object.

11. A cold stamping method of forming a shaped steel object, the cold stamping method comprising:

cutting a blank from a coil comprising an alloy composition according to claim 1, wherein the alloy composition has been subjected to a quench and partitioning process; and

stamping the blank into a predetermined shape at ambient temperature to form the shaped steel object.

12. A method of forming a shaped steel object; the method comprising:

cutting a blank from a coil of an alloy composition comprising:

chromium (Cr) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 9 wt. %,

carbon (C) at a concentration of greater than or equal to about 0.15 wt. % to less than or equal to about 0.5 wt. %,

manganese (Mn) at a concentration of greater than or equal to about 0 wt. % to less than or equal to about 3 wt. %,

silicon (Si) at a concentration of greater than or equal to about 0.5 wt. % to less than or equal to about 2 wt. %, and

a balance of the alloy composition being iron;

heating the blank to a temperature above an upper critical temperature (Ac3) of the alloy composition to form a heated blank comprising austenite;

stamping the heated blank into a predetermined shape to form a stamped object; and

quenching the stamped object to form the shaped steel object, wherein the shaped steel object comprises martensite.

13. The method according to claim 12, wherein the quenching comprises decreasing the temperature of the stamped object at a rate of greater than or equal to about 15° C./s until the stamped object reaches a temperature below a martensite finish (Mf) temperature of the alloy composition.

14. The method according to claim 12, wherein the method is free from pre-oxidizing the alloy composition, coating the shaped steel object, and shot blasting.

15. The method according to claim 12, wherein the quenching comprises a quench and partitioning process, wherein the quench and partitioning process comprises:

decreasing the temperature of the stamped object until the stamped object has a temperature between a martensite start (Ms) temperature of the alloy composition and a martensite finish (Mf) temperature of the alloy composition;

incubating the stamped object at a partitioning temperature wherein carbon (C) is partitioned from martensite into austenite; and

decreasing an austenite Mf temperature below room temperature.

16. The method according to claim 15, wherein the quench and partitioning process forms the shaped steel object, wherein the shaped steel object comprises ferrite, martensite and retained austenite (RA).

17. The method according to claim 16, wherein the shaped steel object is substantially free of cementite.

18. A method of forming a shaped steel object; the method comprising:

cutting a blank from a coil of an advanced high strength steel (AHSS); and

stamping the blank into a predetermined shape at ambient temperature to form the shaped steel object,

wherein the AHSS is made by subjecting an alloy composition to a quench and partitioning process, the alloy composition comprising:

a balance of the alloy composition being iron.

19. The method according to claim 18, wherein the AHSS is substantially free of an oxide layer.

20. The method according to claim 18, wherein the shaped steel object is bare or zinc (Zn coated).