Method of forming a steel sheet having improved flatness

Abstract

Disclosed herein are methods of forming a steel sheet having improved flatness. The disclosure provides a method of making a steel alloy sheet to improve flatness. The method includes, among other things, heating a steel alloy material to a first temperature above a full austenitization point of the steel alloy material; holding the steel alloy material at the first temperature for a period of time greater than or equal to about 1 second to less than or equal to about 10,000 seconds to form a precursor steel sheet; air quenching the precursor steel sheet to a second temperature that is lower than the first temperature and higher than a martensitic transformation start temperature of the steel alloy material; and cooling the precursor steel sheet to room temperature to produce the steel alloy sheet. The steel alloy material includes greater than or equal to about 0.5 wt% to less than or equal to about 6 wt% chromium.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for preparing a steel alloy plate for improved flatness and a method for preparing a steel alloy plate for improved flatness. BACKGROUND

[0002] This section provides background information relating to the present disclosure, which is not necessarily prior art.

[0003] Roll forming includes continuously bending (e.g., using a plurality of rollers) a relatively long metal strip (typically a coil of steel) into a desired cross-section. In particular, roll forming is well suited for making parts having a long length and a large amount of constant cross-section. The flatness of the metal strip used during roll forming ensures continuous flow between the rollers and also reduces tool wear. During the formation of the coil, a direct quenching method utilizing a rapid cooling medium, such as water, is typically used. These rapid cooling methods often result in thermal distortion and, thus, the creation of undesirable waviness in the as-formed steel sheet that defines the coil. Accordingly, it is desirable to develop a steel alloy that eliminates or minimizes undesirable thermal distortion and, thus, improves the roll forming process, as well as a method of producing a steel sheet and coil. SUMMARY

[0004] This section provides a general summary of the present disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0005] The present disclosure provides a steel alloy having high strength, high ductility, high bendability, and a steel sheet including the steel alloy and having improved flatness. The present disclosure also provides a method for heat treatment of the steel sheet, including a direct quenching method, a quenching and partitioning / tempering method, and an austenite isothermal quenching method.

[0006] In various aspects, the present disclosure provides a method for preparing a steel alloy plate for improved flatness. The method can include heating a steel alloy material to a first temperature above a full austenitization point of the steel alloy material; holding the steel alloy material at the first temperature for a period of time greater than or equal to about 1 second to less than or equal to about 10,000 seconds to form a precursor steel plate; air quenching the precursor steel plate to a second temperature, the second temperature being lower than the first temperature and higher than a martensitic transformation start temperature of the steel alloy material; cooling the precursor steel plate to room temperature to produce a steel alloy plate. The room temperature can be greater than or equal to about 15 °C to less than or equal to about 25 °C.

[0007] In one aspect, the first temperature can be greater than or equal to about 800 °C to less than or equal to about 950 °C. The second temperature can be greater than or equal to about 300 °C to less than or equal to about 500 °C. The cooling rate during air quenching can be greater than or equal to about 2 °C / s to less than or equal to about 15 °C / s.

[0008] In an aspect, the air quenching can be a first air quenching step, and the method can further include a second air quenching step. The second air quenching step can include air quenching the precursor steel plate to a third temperature that is lower than the second temperature.

[0009] In an aspect, the second air quenching step can be a continuation of the first air quenching step.

[0010] In an aspect, the cooling rate during the second air quenching step can be greater than or equal to about 0.1 °C / s to less than or equal to about 15 °C / s. The third temperature can be less than or equal to about 400 °C.

[0011] In an aspect, the steel alloy plate can have a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1600 MPa, and a total elongation greater than or equal to about 3%. The steel alloy plate can have a microstructure comprising: greater than or equal to about 80 vol.% to less than or equal to about 99 vol.% of a martensite phase; greater than or equal to about 1 vol.% to less than or equal to about 10 vol.% of a retained austenite phase; greater than or equal to about 0 vol.% to less than or equal to about 10 vol.% of a bainite phase; and greater than or equal to about 0 vol.% to less than or equal to about 10 vol.% of a ferrite phase.

[0012] In an aspect, the method can include holding the precursor steel plate at the third temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

[0013] In an aspect, the method can further include heating the precursor steel plate from the third temperature to a fourth temperature that is lower than the first temperature.

[0014] In an aspect, the fourth temperature can be greater than or equal to about 300 °C to less than or equal to about 500 °C.

[0015] In an aspect, the method can further include holding the precursor steel plate at the fourth temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

[0016] In an aspect, the steel alloy plate can have a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1500 MPa, a total elongation greater than or equal to about 7%, and a bend angle greater than or equal to about 50 degrees. The steel alloy plate can have a microstructure comprising: greater than or equal to about 50 vol.% to less than or equal to about 95 vol.% of a martensite constituent; greater than or equal to about 5 vol.% to less than or equal to about 17 vol.% of a retained austenite phase; greater than or equal to about 0 vol.% to less than or equal to about 25 vol.% of a bainite phase; and greater than or equal to about 0 vol.% to less than or equal to about 10 vol.% of a ferrite phase.

[0017] In an aspect, the method can further include holding the precursor steel plate at the second temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

[0018] In an aspect, the steel alloy plate can have a yield strength greater than or equal to about 1100 MPa, an ultimate tensile strength greater than or equal to about 1550 MPa, a total elongation greater than or equal to about 7%, and a bend angle greater than or equal to about 50 degrees. The steel alloy can have a microstructure comprising: a martensite constituent greater than or equal to about 30 volume % to less than or equal to about 97 volume %; a retained austenite phase greater than or equal to about 3 volume % to less than or equal to about 15 volume %; a bainite phase greater than or equal to about 0 volume % to less than or equal to about 45 volume %; and a ferrite phase greater than or equal to about 0 volume % to less than or equal to about 10 volume %.

[0019] In an aspect, the steel alloy material can include greater than or equal to about 0.05 wt % to less than or equal to about 0.45 wt % carbon; greater than or equal to about 0.5 wt % to less than or equal to about 6 wt % chromium; greater than or equal to about 0.5 wt % to less than or equal to about 2.5 wt % silicon; greater than 0 wt % to less than or equal to about 4.5 wt % manganese; greater than 0 wt % to less than or equal to about 2 wt % aluminum, wherein a ratio of chromium to aluminum is greater than or equal to about 1.7, and a sum of aluminum and silicon is greater than or equal to about 0.7 wt %; and a balance of iron.

[0020] In an aspect, the steel alloy material can further include greater than or equal to 0 wt % to less than or equal to about 0.5 wt % vanadium; greater than or equal to 0 wt % to less than or equal to about 0.2 wt % niobium; and greater than or equal to 0 wt % to less than or equal to about 0.3 wt % titanium.

[0021] In various aspects, the disclosure provides a method for preparing a steel alloy plate for improved flatness. The method can include heating a steel alloy material to a first temperature; holding the steel alloy material at the first temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to form a precursor steel plate; first air quenching the precursor steel plate from the first temperature to a second temperature at a first cooling rate greater than or equal to about 2 °C / s to less than or equal to about 15 °C / s; second air quenching the precursor steel plate from the second temperature to a third temperature at a second cooling rate greater than or equal to about 0.1 °C / s to less than or equal to about 15 °C / s; and cooling the precursor steel plate to room temperature to prepare the steel alloy plate. The first temperature can be greater than or equal to about 800 °C to less than or equal to about 950 °C. The second temperature can be greater than or equal to about 300 °C to less than or equal to about 500 °C. The third temperature can be less than or equal to about 400 °C. The room temperature can be greater than or equal to about 15 °C to less than or equal to about 25 °C.

[0022] In an aspect, the method can further include maintaining the precursor steel plate at a third temperature for a holding period of greater than or equal to about 1 second to less than or equal to about 10,000 seconds. The method can further include heating the precursor steel plate from the third temperature to a fourth temperature after the holding period and maintaining the precursor steel plate at the fourth temperature for a period of greater than or equal to about 1 second to less than or equal to about 10,000 seconds. The fourth temperature can be greater than or equal to about 300 °C to less than or equal to about 500 °C.

[0023] In an aspect, the steel alloy material can include greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % manganese; greater than 0 wt. % to less than or equal to about 2 wt. % aluminum, wherein a ratio of chromium to aluminum is greater than or equal to about 1.7, and a sum of aluminum and silicon is greater than or equal to about 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % niobium; greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % titanium; and a balance of iron.

[0024] In various aspects, the present disclosure provides a method for preparing a steel alloy plate for improved flatness. The method can include heating a steel alloy material to a first temperature; maintaining the steel alloy material at the first temperature for a period of greater than or equal to about 1 second to less than or equal to about 10,000 seconds to form a precursor steel plate; first air quenching the precursor steel plate from the first temperature to a second temperature at a first cooling rate of greater than or equal to about 2 °C / s to less than or equal to about 15 °C / s; maintaining the precursor steel plate at the second temperature for a period of greater than or equal to about 1 second to less than or equal to about 10,000 seconds; and cooling the precursor steel plate from the second temperature to room temperature to prepare the steel alloy plate. The first temperature can be a first temperature of greater than or equal to about 800 °C to less than or equal to about 950 °C. The second temperature can be a second temperature of greater than or equal to about 300 °C to less than or equal to about 500 °C. The room temperature can be greater than or equal to about 15 °C to less than or equal to about 25 °C.

[0025] In one aspect, a steel alloy material can include greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % carbon; greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % chromium; greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % silicon; greater than 0 wt. % to less than or equal to about 4.5 wt. % manganese; greater than 0 wt. % to less than or equal to about 2 wt. % aluminum, wherein the ratio of chromium to aluminum is greater than or equal to about 1.7, and the sum of aluminum and silicon is greater than or equal to about 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % vanadium; greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % niobium; greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % titanium; and a balance of iron.

[0026] The following embodiments are disclosed:

[0027] 1. A method for preparing a steel alloy sheet for improved flatness, the method comprising:

[0028] heating a steel alloy material to a first temperature above the full austenitization point of the steel alloy material;

[0029] maintaining the steel alloy material at the first temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to form a precursor steel sheet;

[0030] air quenching the precursor steel sheet to a second temperature that is lower than the first temperature and above the martensitic transformation start temperature of the steel alloy material; and

[0031] cooling the precursor steel sheet to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to about 15 °C to less than or equal to about 25 °C.

[0032] 2. The method of embodiment 1, wherein the first temperature is greater than or equal to about 800 °C to less than or equal to about 950 °C, the second temperature is greater than or equal to about 300 °C to less than or equal to about 500 °C, and the cooling rate during the air quenching is greater than or equal to about 2 °C / s to less than or equal to about 15 °C / s.

[0033] 3. The method of embodiment 1, wherein the air quenching is a first air quenching step, and the method further comprises a second air quenching step comprising air quenching the precursor steel sheet to a third temperature that is less than the second temperature.

[0034] 4. The method of embodiment 3, wherein the second air quenching step is a continuation of the first air quenching step.

[0035] 5. The method of embodiment 3, wherein the cooling rate during the second air quenching step is greater than or equal to about 0.1 °C / s to less than or equal to about 15 °C / s, and the third temperature is less than or equal to about 400 °C.

[0036] 6. The method of embodiment 3, wherein the steel alloy plate has a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1600 MPa, and a total elongation greater than or equal to about 3%, and

[0037] wherein the steel alloy plate has a microstructure comprising greater than or equal to about 80 vol% to less than or equal to about 99 vol% of a martensite phase; greater than or equal to about 1 vol% to less than or equal to about 10 vol% of a retained austenite phase; greater than or equal to about 0 vol% to less than or equal to about 10 vol% of a bainite phase; and greater than or equal to about 0 vol% to less than or equal to about 10 vol% of a ferrite phase.

[0038] 7. The method of embodiment 3, wherein the method comprises holding the precursor steel plate at the third temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

[0039] 8. The method of embodiment 6, wherein the method further comprises heating the precursor steel plate from the third temperature to a fourth temperature that is lower than the first temperature.

[0040] 9. The method of embodiment 8, wherein the fourth temperature is greater than or equal to about 300 °C to less than or equal to about 500 °C.

[0041] 10. The method of embodiment 8, wherein the method further comprises holding the precursor steel plate at the fourth temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

[0042] 1. The method of embodiment 10, wherein the steel alloy plate has a yield strength greater than or equal to about 1150 MPa, an ultimate tensile strength greater than or equal to about 1500 MPa, a total elongation greater than or equal to about 7%, and a bend angle greater than or equal to about 50 degrees, and

[0043] wherein the steel alloy plate has a microstructure comprising greater than or equal to about 50 vol% to less than or equal to about 95 vol% of a martensite constituent; greater than or equal to about 5 vol% to less than or equal to about 17 vol% of a retained austenite phase; greater than or equal to about 0 vol% to less than or equal to about 25 vol% of a bainite phase; and greater than or equal to about 0 vol% to less than or equal to about 10 vol% of a ferrite phase.

[0044] 12 The method of embodiment 1, wherein the method further comprises holding the precursor steel sheet at a second temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds.

[0045] 13 The method of embodiment 12, wherein the steel alloy sheet has a yield strength greater than or equal to about 1100 MPa, an ultimate tensile strength greater than or equal to about 1550 MPa, a total elongation greater than or equal to about 7%, and a bend angle greater than or equal to about 50 degrees, and

[0046] wherein the steel alloy has a microstructure comprising greater than or equal to about 30 volume percent to less than or equal to about 97 volume percent of a martensite constituent; greater than or equal to about 3 volume percent to less than or equal to about 15 volume percent of a retained austenite phase; greater than or equal to about 0 volume percent to less than or equal to about 45 volume percent of a bainite phase; and greater than or equal to about 0 volume percent to less than or equal to about 10 volume percent of a ferrite phase.

[0047] 14 The method of embodiment 1, wherein the steel alloy material comprises:

[0048] greater than or equal to about 0.05 weight percent to less than or equal to about 0.45 weight percent of carbon;

[0049] greater than or equal to about 0.5 weight percent to less than or equal to about 6 weight percent of chromium;

[0050] greater than or equal to about 0.5 weight percent to less than or equal to about 2.5 weight percent of silicon;

[0051] greater than 0 weight percent to less than or equal to about 4.5 weight percent of manganese;

[0052] greater than 0 weight percent to less than or equal to about 2 weight percent of aluminum, wherein a ratio of chromium to aluminum is greater than or equal to about 1.7, and a sum of aluminum and silicon is greater than or equal to about 0.7 weight percent; and

[0053] a balance of iron.

[0054] 15 The method of embodiment 1, wherein the steel alloy material further comprises:

[0055] greater than or equal to 0 weight percent to less than or equal to about 0.5 weight percent of vanadium;

[0056] greater than or equal to 0 weight percent to less than or equal to about 0.2 weight percent of niobium; and

[0057] greater than or equal to 0 weight percent to less than or equal to about 0.3 weight percent of titanium.

[0058] 16 A method for preparing a steel alloy sheet for improved flatness, the method comprising:

[0059] heating a steel alloy material to a first temperature greater than or equal to about 800 °C to less than or equal to about 950 °C;

[0060] maintaining the steel alloy material at the first temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds to form a precursor steel sheet;

[0061] first air quenching the precursor steel sheet from the first temperature to a second temperature greater than or equal to about 300 °C to less than or equal to about 500 °C at a first cooling rate greater than or equal to about 2 °C / s to less than or equal to about 15 °C / s;

[0062] second air quenching the precursor steel sheet from the second temperature to a third temperature less than or equal to about 400 °C at a second cooling rate greater than or equal to about 0.1 °C / s to less than or equal to about 15 °C / s; and

[0063] cooling the precursor steel sheet to room temperature to produce a steel alloy sheet, the room temperature greater than or equal to about 15 °C to less than or equal to about 25 °C.

[0064] 17 The method of embodiment 16, wherein the method further comprises maintaining the precursor steel sheet at the third temperature for a holding time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, heating the precursor steel sheet from the third temperature to a fourth temperature after the holding time period, and maintaining the precursor steel sheet at the fourth temperature for a time period greater than or equal to about 1 second to less than or equal to about 10,000 seconds, the fourth temperature greater than or equal to about 300 °C to less than or equal to about 500 °C.

[0065] 18 The method of embodiment 16, wherein the steel alloy material comprises:

[0066] greater than or equal to about 0.05 wt% to less than or equal to about 0.45 wt% carbon;

[0067] greater than or equal to about 0.5 wt% to less than or equal to about 6 wt% chromium;

[0068] greater than or equal to about 0.5 wt% to less than or equal to about 2.5 wt% silicon;

[0069] greater than 0 wt% to less than or equal to about 4.5 wt% manganese;

[0070] greater than 0 wt% to less than or equal to about 2 wt% aluminum, wherein a ratio of chromium to aluminum is greater than or equal to about 1.7, and a sum of aluminum and silicon is greater than or equal to about 0.7 wt%;

[0071] greater than or equal to 0 wt% to less than or equal to about 0.5 wt% vanadium;

[0072] Niobium greater than or equal to 0% by weight and less than or equal to about 0.2% by weight;

[0073] Titanium greater than or equal to 0% by weight and less than or equal to about 0.3% by weight; and

[0074] The remaining amount of iron.

[0075] 19. A method for preparing a steel alloy plate to improve flatness, the method comprising:

[0076] The steel alloy material is heated to a first temperature greater than or equal to about 800°C and less than or equal to about 950°C;

[0077] The steel alloy material is held at the first temperature for a period of time greater than or equal to about 1 second to less than or equal to about 10,000 seconds to form a precursor steel sheet;

[0078] The precursor steel plate is first air-quenched from the first temperature to a second temperature of greater than or equal to about 300°C to less than or equal to about 500°C at a first cooling rate greater than or equal to about 2°C / s to less than or equal to about 15°C / s.

[0079] The precursor steel plate is held at the second temperature for a period of time greater than or equal to about 1 second and less than or equal to about 10,000 seconds; and

[0080] The precursor steel plate is cooled from the second temperature to room temperature to prepare the steel alloy plate, wherein the room temperature is greater than or equal to about 15°C and less than or equal to about 25°C.

[0081] 20. The method according to embodiment 19, wherein the steel alloy material comprises:

[0082] Carbon, greater than or equal to about 0.05% by weight and less than or equal to about 0.45% by weight;

[0083] Chromium greater than or equal to about 0.5% by weight and less than or equal to about 6% by weight;

[0084] Silicon, greater than or equal to about 0.5% by weight and less than or equal to about 2.5% by weight;

[0085] Manganese greater than 0% by weight to less than or equal to about 4.5% by weight;

[0086] Aluminum greater than 0% by weight and less than or equal to about 2% by weight, wherein the ratio of chromium to aluminum is greater than or equal to about 1.7, and the total amount of aluminum and silicon is greater than or equal to about 0.7% by weight.

[0087] Vanadium greater than or equal to 0% by weight and less than or equal to about 0.5% by weight;

[0088] greater than or equal to 0.2 wt. % to less than or equal to about 0.3 wt. % of titanium; and

[0089] greater than or equal to 0.2 wt. % to less than or equal to about 0.3 wt. % of titanium; and

[0090] the balance being iron.

[0091] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in the disclosure are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF DRAWINGS

[0092] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure.

[0093] Figure 1 is an illustration showing a direct quenching method for forming a steel sheet having improved flatness in accordance with various aspects of the present disclosure;

[0094] Figure 2 is an illustration showing a quenching and partitioning / tempering method for forming a steel sheet having improved flatness in accordance with various aspects of the present disclosure;

[0095] Figure 3 is an illustration showing an austempering method for forming a steel sheet having improved flatness in accordance with various aspects of the present disclosure;

[0096] Figure 4 is an illustration showing tensile strength and ductility of exemplary steel sheets prepared using a direct quenching method in accordance with various aspects of the present disclosure;

[0097] Figure 5 is an illustration showing tensile strength and ductility of exemplary steel sheets prepared using a quenching and partitioning / tempering method in accordance with various aspects of the present disclosure; and

[0098] Figure 6 is an illustration showing tensile strength and ductility of exemplary steel sheets prepared using an austempering method in accordance with various aspects of the present disclosure.

[0099] In the several views of the drawings, like reference numerals designate corresponding parts throughout the several views. DETAILED DESCRIPTION

[0100] The exemplary embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who come to this disclosure. Numerous specific details are set forth such as examples of specific 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 exemplary embodiments can be practiced in many different

[0101] 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" are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "includes," "including," "contains," "containing," and "has," "having," are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, components, and / or groups thereof, 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-limiting term used to describe and claim various embodiments set forth herein, in certain aspects the term or can be understood to be the more restrictive and limiting term "consisting of" or "consisting essentially of." As such, the present disclosure also specifically includes embodiments in which any given embodiment reciting a composition, material, component, element, feature, integer, operation, and / or method step is "consisting of" or "consisting essentially of" such recited composition, material, component, element, feature, integer, operation, and / or method step, as the case can be. In the case of "consisting of," the alternative embodiment excludes any additional composition, material, component, element, feature, integer, operation, and / or method step, while in the case of "consisting essentially of," any additional composition, material, component, element, feature, integer, operation, and / or method step that does not materially affect the basic and novel characteristic(s) is excluded from such embodiment.

[0102] Any of the method steps, processes, and operations described herein are not to be interpreted as necessarily requiring them to be performed in the specific order discussed or illustrated, unless expressly specified in a form requiring an order of performance. It is also to be understood that additional or alternative steps can be employed, unless otherwise indicated.

[0103] When an element or layer is referred to as being "on", "engaged", "connected", or "coupled" to another element or layer, it can be directly on, engaged, connected, or coupled to the other element or layer or intervening elements or layers can be present. In contrast, when an element is referred to as being "directly on", "directly engaged", "directly connected", or "directly coupled" to another element or layer, there are 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.

[0104] Although the terms first, second, third, etc. can 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. These terms can be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Unless the context clearly indicates otherwise, terms like "first", "second", and the like can be used herein to describe a variety of parameters. Thus, a first step, element, component, region, layer or section discussed below can be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.

[0105] For ease of description, spatial or temporal terms, such as "front", "back", "inner", "outer", "under", "below", "lower", "above", "upper", and the like, can be used with respect to the orientation of one element or feature to another element(s) or feature(s) as illustrated in the figures. The spatial or temporal terms can be intended to encompass different orientations of the device or system in use or operation, unless otherwise specified.

[0106] Throughout this disclosure, numerical values represent approximate measurement or range limits to encompass slight deviations and embodiments generally having the recited value as well as embodiments exactly having the recited value. Except in the working examples provided at the end of the detailed description, all numerical values in this specification (including the appended claims) that are by way of example or that are not otherwise qualified by the term "about" are understood as being modified in all instances by the term "about" no matter how the numerical value is presented. By "about" is meant both the exact or precise value and the value allowing for some slight inaccuracy (an exact value to within a degree of approximation; approximately or reasonably near the value; nearly). If the inaccuracy provided by "about" is not otherwise understood in the art, then "about" as used herein means at least the inaccuracy that can result from ordinary methods of measuring and using such parameters. For example, "about" can include an inaccuracy 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%.

[0107] Further, the disclosure of ranges includes disclosure of all values and further subdivisions between the stated range limits, including endpoints and disclosure of sub-ranges given the ranges.

[0108] Exemplary embodiments will now be described more fully with reference to the accompanying drawings.

[0109] In various aspects, the present disclosure provides high-strength, high-ductility, high-bendable steel alloys, and steel sheets containing the steel alloys and having improved flatness. The steel sheets can be used to form parts or articles using, for example, a roll forming process. The steel sheets can be used to form parts or articles for automobiles or other vehicles (e.g., motorcycles, boats, tractors, buses, motorcycles, mobile homes, recreational vehicles, and tanks), but they can also be used in a variety of other industries and applications, including, by way of non-limiting examples, aerospace parts, consumer goods, devices, buildings (e.g., houses, offices, sheds, warehouses), office equipment and furniture, and industrial equipment machinery, agricultural or farm equipment, or heavy machinery. Non-limiting examples of automotive parts or articles include hoods, pillars (e.g., A-pillars, hinge pillars, B-pillars, C-pillars, etc.), panels, including structural panels, door panels and door parts, interior floor panels, floor pans, roofs, outer surfaces, underbody shields, wheels, control arms and other suspension, crush cans, bumpers, structural beams and frames, instrument panel beams, chassis or driveline components, and the like.

[0110] According to various aspects of the present disclosure, the steel alloy can include carbon (C), chromium (Cr), silicon (Si), and iron (Fe). In certain variations, the steel alloy can also include manganese (Mn) and / or aluminum (Al). In yet further variations, the steel alloy can include vanadium (V), niobium (Nb), and / or titanium (Ti).

[0111] In certain variations, the steel alloy can include greater than or equal to about 0.05 wt. % to less than or equal to about 0.45 wt. % carbon. For example, the steel alloy can include greater than or equal to about 0.05 wt. %, optionally greater than or equal to about 0.1 wt. %, optionally greater than or equal to about 0.15 wt. %, optionally greater than or equal to about 0.2 wt. %, optionally greater than or equal to about 0.25 wt. %, optionally greater than or equal to about 0.3 wt. %, optionally greater than or equal to about 0.35 wt. %, and in certain aspects optionally greater than or equal to about 0.4 wt. % carbon. The steel alloy can include less than or equal to about 0.45 wt. %, optionally less than or equal to about 0.4 wt. %, optionally less than or equal to about 0.35 wt. %, optionally less than or equal to about 0.3 wt. %, optionally less than or equal to about 0.25 wt. %, optionally less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.15 wt. %, and in certain aspects optionally less than or equal to about 0.1 wt. % carbon.

[0112] In certain variations, the steel alloy can include greater than or equal to about 0.5 wt. % to less than or equal to about 6 wt. % chromium. For example, the steel alloy can include greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.5 wt. %, optionally greater than or equal to about 2 wt. %, optionally greater than or equal to about 2.5 wt. %, optionally greater than or equal to about 3 wt. %, optionally greater than or equal to about 3.5 wt. %, optionally greater than or equal to about 4 wt. %, optionally greater than or equal to about 4.5 wt. %, optionally greater than or equal to about 5 wt. %, and in certain aspects optionally greater than or equal to about 5.5 wt. % chromium. The steel alloy can include less than or equal to about 6 wt. %, optionally less than or equal to about 5.5 wt. %, optionally less than or equal to about 5 wt. %, optionally less than or equal to about 4.5 wt. %, optionally less than or equal to about 4 wt. %, optionally less than or equal to about 3.5 wt. %, optionally less than or equal to about 3 wt. %, optionally less than or equal to about 2.5 wt. %, optionally less than or equal to about 2 wt. %, optionally less than or equal to about 1.5 wt. %, and in certain aspects optionally less than or equal to about 1 wt. % chromium. The presence of chromium reduces the cooling rate and allows for high hardenability to be achieved during air quenching, which includes lower and more uniform cooling rates compared to conventional direct quenching methods that use a rapid cooling medium such as water, as described in detail below.

[0113] In certain variations, the steel alloy can include greater than or equal to about 0.5 wt. % to less than or equal to about 2.5 wt. % silicon. For example, the steel alloy can include greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 0.75 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.25 wt. %, optionally greater than or equal to about 1.50 wt. %, optionally greater than or equal to about 1.75 wt. %, optionally greater than or equal to about 2 wt. %, and in certain aspects, optionally greater than or equal to about 2.25 wt. % silicon. The steel alloy can include less than or equal to about 2.5 wt. %, optionally less than or equal to about 2.25 wt. %, optionally less than or equal to about 2 wt. %, optionally less than or equal to about 1.75 wt. %, optionally less than or equal to about 1.5 wt. %, optionally less than or equal to about 1.25 wt. %, optionally less than or equal to about 1 wt. %, and in certain aspects, optionally less than or equal to about 0.75 wt. % silicon.

[0114] In certain variations, the steel alloy can include greater than or equal to about 0 wt. % to less than or equal to about 4.5 wt. % manganese. For example, the steel alloy can include greater than or equal to about 0 wt. %, optionally greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.5 wt. %, optionally greater than or equal to about 2 wt. %, optionally greater than or equal to about 2.5 wt. %, optionally greater than or equal to about 3 wt. %, optionally greater than or equal to about 3.5 wt. %, and in certain aspects, optionally greater than or equal to about 4 wt. % manganese. The steel alloy can include less than or equal to about 4.5 wt. %, optionally less than or equal to about 4 wt. %, optionally less than or equal to about 3.5 wt. %, optionally less than or equal to about 3 wt. %, optionally less than or equal to about 2.5 wt. %, optionally less than or equal to about 2 wt. %, optionally less than or equal to about 1.5 wt. %, optionally less than or equal to about 1 wt. % manganese, and in certain aspects, optionally less than or equal to about 0.5 wt. % manganese.

[0115] In certain variations, the steel alloy can include greater than or equal to about 0 wt. % to less than or equal to about 2 wt. % aluminum. For example, the steel alloy can include greater than or equal to about 0 wt. %, optionally greater than or equal to about 0.25 wt. %, optionally greater than or equal to about 0.5 wt. %, optionally greater than or equal to about 0.75 wt. %, optionally greater than or equal to about 1 wt. %, optionally greater than or equal to about 1.25 wt. %, optionally greater than or equal to about 1.5 wt. %, and in certain aspects, optionally greater than or equal to about 1.75 wt. % aluminum. The steel alloy can include less than or equal to about 2 wt. %, optionally less than or equal to about 1.75 wt. %, optionally less than or equal to about 1.5 wt. %, optionally less than or equal to about 1 wt. %, optionally less than or equal to about 0.75 wt. %, optionally less than or equal to about 0.5 wt. %, and in certain aspects, optionally less than or equal to about 0.25 wt. % aluminum. In certain variations, the ratio of chromium to aluminum can be greater than or equal to about 1.7 in order to facilitate hardenability during quenching. In still further variations, the sum of aluminum and silicon can be greater than or equal to about 0.7 wt. % in order to stabilize retained austenite at room temperature after a thermal treatment process.

[0116] In certain variations, the steel alloy can include greater than or equal to 0 wt. % to less than or equal to about 0.5 wt. % vanadium. For example, the steel alloy can include greater than or equal to 0 wt. %, optionally greater than or equal to 0.05 wt. %, optionally greater than or equal to 0.1 wt. %, optionally greater than or equal to 0.15 wt. %, optionally greater than or equal to 0.2 wt. %, optionally greater than or equal to 0.25 wt. %, optionally greater than or equal to 0.3 wt. %, optionally greater than or equal to 0.35 wt. %, optionally greater than or equal to 0.4 wt. %, and in certain aspects, optionally greater than or equal to 0.45 wt. % vanadium. The steel alloy can include less than or equal to about 0.5 wt. %, optionally less than or equal to about 0.45 wt. %, optionally less than or equal to about 0.4 wt. %, optionally less than or equal to about 0.35 wt. %, optionally less than or equal to about 0.3 wt. %, optionally less than or equal to about 0.25 wt. %, optionally less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.15 wt. %, optionally less than or equal to about 0.1 wt. %, and in certain aspects, optionally less than or equal to about 0.05 wt. % vanadium.

[0117] In certain variations, the steel alloy can include greater than or equal to 0 wt. % to less than or equal to about 0.2 wt. % niobium. For example, the steel alloy can include greater than or equal to about 0 wt. %, optionally greater than or equal to about 0.02 wt. %, optionally greater than or equal to about 0.04 wt. %, optionally greater than or equal to about 0.06 wt. %, optionally greater than or equal to about 0.08 wt. %, optionally greater than or equal to about 0.1 wt. %, optionally greater than or equal to about 0.12 wt. %, optionally greater than or equal to about 0.14 wt. %, optionally greater than or equal to about 0.16 wt. %, optionally greater than or equal to about 0.18 wt. %, and in certain aspects, optionally greater than or equal to about 0.2 wt. % niobium. The steel alloy can include less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.18 wt. %, optionally less than or equal to about 0.16 wt. %, optionally less than or equal to about 0.14 wt. %, optionally less than or equal to about 0.12 wt. %, optionally less than or equal to about 0.1 wt. %, optionally less than or equal to about 0.08 wt. %, optionally less than or equal to about 0.06 wt. %, optionally less than or equal to about 0.04 wt. %, and in certain aspects, optionally less than or equal to about 0.02 wt. % niobium.

[0118] In certain variations, the steel alloy can include greater than or equal to 0 wt. % to less than or equal to about 0.3 wt. % titanium. For example, the steel alloy can include greater than or equal to 0 wt. %, optionally greater than or equal to 0.02 wt. %, optionally greater than or equal to about 0.04 wt. %, optionally greater than or equal to about 0.06 wt. %, optionally greater than or equal to about 0.08 wt. %, optionally greater than or equal to about 0.1 wt. %, optionally greater than or equal to about 0.12 wt. %, optionally greater than or equal to about 0.14 wt. %, optionally greater than or equal to about 0.16 wt. %, optionally greater than or equal to about 0.18 wt. %, optionally greater than or equal to about 0.2 wt. %, optionally greater than or equal to about 0.22 wt. %, optionally greater than or equal to about 0.24 wt. %, optionally greater than or equal to about 0.26 wt. %, and in certain aspects, optionally greater than or equal to about 0.28 wt. % titanium. The steel alloy can include less than or equal to about 0.3 wt. %, optionally less than or equal to about 0.28 wt. %, optionally less than or equal to about 0.26 wt. %, optionally less than or equal to about 0.24 wt. %, optionally less than or equal to about 0.22 wt. %, optionally less than or equal to about 0.2 wt. %, optionally less than or equal to about 0.18 wt. %, optionally less than or equal to about 0.16 wt. %, optionally less than or equal to about 0.14 wt. %, optionally less than or equal to about 0.12 wt. %, optionally less than or equal to about 0.1 wt. %, optionally less than or equal to about 0.08 wt. %, optionally less than or equal to about 0.06 wt. %, optionally less than or equal to about 0.04 wt. %, and in certain aspects, optionally less than or equal to about 0.02 wt. % titanium.

[0119] In each variation, the steel alloy includes a balance of iron. For example, the steel alloy can include greater than or equal to about 80 wt%, optionally greater than or equal to about 81 wt%, optionally greater than or equal to about 82 wt%, optionally greater than or equal to about 83 wt%, optionally greater than or equal to about 84 wt%, optionally greater than or equal to about 85 wt%, optionally greater than or equal to about 86 wt%, optionally greater than or equal to about 87 wt%, optionally greater than or equal to about 88 wt%, optionally greater than or equal to about 89 wt%, optionally greater than or equal to about 90 wt%, optionally greater than or equal to about 91 wt%, optionally greater than or equal to about 92 wt%, optionally greater than or equal to about 93 wt%, optionally greater than or equal to about 94 wt%, optionally greater than or equal to about 95 wt%, optionally greater than or equal to about 96 wt%, optionally greater than or equal to about 97 wt%, and in certain aspects, optionally greater than or equal to about 98 wt% iron.

[0120] As described above, the steel alloys according to various aspects of the present disclosure can provide steel sheets having improved flatness. In various aspects, the present disclosure provides a heat treatment method or process for steel sheets. After heat treatment, the steel sheets can be coiled and moved to, for example, a roll forming machine for forming. The exemplary method includes various air quenching or cooling steps, and thus avoids thermal distortion that is typically caused by the use of a rapid cooling medium such as water during direct quenching (e.g., at steps 130, 230, 330 as described in detail below). Air quenching includes natural cooling in the atmosphere, or in certain cases, forcing air or gas through the steel alloy.

[0121] The steel sheet so produced can have high strength, and also high ductility and high bendability. For example, the microstructure of the exemplary steel sheet 400 can include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength, the retained austenite is associated with high ductility and bendability. In certain variations, the produced steel sheet can include greater than or equal to about 30 vol% to less than or equal to about 99 vol% of the martensite phase; greater than or equal to about 1 vol% to less than or equal to about 17 vol% of the retained austenite phase; greater than or equal to about 0 vol% to less than or equal to about 45 vol% of the bainite phase; and greater than or equal to about 0 vol% to less than or equal to about 10 vol% of the ferrite phase. Further, the produced steel sheet can have a yield strength (YS) of greater than or equal to about 1100 MPa, and in certain aspects, optionally, greater than or equal to about 1100 MPa to less than or equal to about 1500 MPa. The produced steel sheet can have an ultimate tensile strength (UTS) of greater than or equal to about 1500 MPa. The total elongation (TEL) of the produced steel sheet can be greater than or equal to about 3%, and in certain aspects, optionally, greater than or equal to about 6%. The produced steel sheet can have a bend angle of greater than or equal to about 45 degrees, and in certain aspects, optionally, greater than or equal to about 50 degrees.

[0122] Figure 1 is a graphical illustration summarizing an exemplary direct quenching method 100 for forming a steel sheet having high flatness, where the x-axis 102 represents time in seconds and the y-axis 104 represents temperature in degrees Celsius. As shown, the method 100 includes heating 110 a precursor sheet comprising a steel alloy to a first temperature. Although not shown, one skilled in the art will appreciate that in certain variations, the precursor sheet is uncoiled from a steel coil.

[0123] The first temperature is above the austenitization point of the steel alloy, which is represented by line 112. For example, in certain variations, the first temperature can be greater than or equal to about 800 °C to less than or equal to about 950 °C, optionally, greater than or equal to about 850 °C to less than or equal to about 950 °C, and in certain aspects, optionally, about 930 °C. The precursor sheet can be heated to the first temperature at a rate of greater than or equal to about 0.1 °C s -1 to less than or equal to about 100 °C s -1 The method 100 includes holding or soaking 120 the first temperature for a period of time of greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally, greater than or equal to about 200 seconds to less than or equal to about 500 seconds.

[0124] The holding 120 can be followed by a first air quench 130. For example, the precursor plate can be cooled to a second temperature that is less than the first temperature. The second temperature can be between about 500°C (which is represented by line 132) and the martensite transformation start temperature (which is represented by line 134). For example, the second temperature can be greater than or equal to about 300°C to less than or equal to about 500°C, and in certain aspects, optionally greater than or equal to about 400°C to less than or equal to about 500°C. The cooling rate (which depends on the method) should be higher than or close to the critical cooling rate required to obtain a martensitic transformation for high strength. For example, in certain variations, a cooling rate of greater than or equal to about 2°C / s to less than or equal to about 15°C / s can be applied. The first air quench 130 can be a natural air cooling process and / or a forced air / gas cooling process.

[0125] The first air quench 130 can be followed by a second air quench 140. For example, the precursor plate can be cooled to a third temperature that is less than the second temperature. The third temperature is less than the martensite transformation start temperature. For example, the third temperature can be less than or equal to about 400°C, and in certain aspects, optionally less than or equal to about 300°C. In certain variations, the third temperature can be less than or equal to about 400°C, and in certain aspects, optionally less than or equal to about 300°C. The third temperature can be greater than or equal to room temperature (e.g., greater than or equal to about 15°C to less than or equal to about 25°C). A cooling rate of greater than or equal to about 0.1°C / s to less than or equal to about 15°C / s can be employed.

[0126] After the second air quench 140, the method 100 can further include cooling 150 the precursor plate to room temperature (e.g., greater than or equal to about 15°C to less than or equal to about 25°C) to obtain a steel plate having improved flatness. For example, the steel plate can be air cooled to room temperature. As will be appreciated by one of skill in the art, the cooling rate is slower at lower temperatures due to the smaller temperature difference between the steel plate and the atmosphere (considering the differences between the first air quench 130, the second air quench 140, and the cooling 150).

[0127] As described above, the prepared steel sheet has improved flatness. For example, when the prepared steel sheet has a thickness of greater than or equal to about 0.8 mm to less than or equal to about 1.3 mm, the maximum distance between the normal plane in the sheet and each peak (i.e., height) can be less than or equal to about 7 mm, and in certain aspects, optionally less than or equal to about 6 mm. When the prepared steel sheet has a thickness of greater than or equal to about 1.3 mm to less than or equal to about 1.8 mm, the maximum distance between the normal plane in the sheet and each peak (i.e., height) can be less than or equal to about 6 mm, and in certain aspects, optionally less than or equal to about 5 mm. When the prepared steel sheet has a thickness of greater than or equal to about 1.8 mm, the maximum distance between the normal plane in the sheet and each peak (i.e., height) can be less than or equal to about 5 mm, and in certain aspects, optionally less than or equal to about 4 mm.

[0128] Although not shown, in certain variations, the method 100 can further include coiling the steel sheet having improved flatness and moving the formed coil to a roll forming machine for forming.

[0129] Figure 2 is a graphical illustration of an exemplary quenching and partitioning / tempering method 200 for forming a steel sheet having high flatness, where the x-axis 202 represents time in seconds and the y-axis 204 represents temperature in degrees Celsius. As shown, the method 200 includes heating 210 a precursor sheet comprising a steel alloy to a first temperature. Although not shown, one skilled in the art will appreciate that, in certain variations, the precursor sheet is uncoiled from a steel coil.

[0130] The first temperature is above the austenitization point of the steel alloy (represented by line 212). For example, in certain variations, the first temperature can be greater than or equal to about 800 °C to less than or equal to about 950 °C, optionally greater than or equal to about 850 °C to less than or equal to about 950 °C, and in certain aspects, optionally about 930 °C. The precursor sheet can be heated to the first temperature at a rate of greater than or equal to about 0.1 °C s -1 to less than or equal to about 100 °C s -1 The method 200 includes maintaining or soaking 220 the first temperature for a period of time of greater than or equal to about 1 second to less than or equal to about 10,000 seconds, greater than or equal to about 200 seconds to less than or equal to about 500 seconds, and in certain aspects, optionally about 340 seconds.

[0131] The holding 220 can be followed by a first air quench 230. For example, the precursor plate can be cooled to a second temperature that is less than the first temperature. The second temperature can be between about 500 °C (which is represented by line 232) and the martensite transformation start temperature (which is represented by line 234). For example, the second temperature can be greater than or equal to about 300 °C to less than or equal to about 500 °C, and in certain aspects, optionally greater than or equal to about 400 °C to less than or equal to about 500 °C. The cooling rate (which depends on the method) should be higher than or close to the critical cooling rate required to obtain a martensite transformation for high strength. For example, in certain variations, a cooling rate of greater than or equal to about 2 °C / s to less than or equal to about 15 °C / s can be employed. The first air quench 230 can be a natural air cooling process and / or a forced air / gas cooling process.

[0132] The first air quench 230 can be followed by a second air quench 240. For example, the precursor plate can be cooled to a third temperature that is less than the second temperature. Similar to the first air quench 230, the second air quench 240 can be a natural air cooling process and / or a forced air / gas cooling process. A cooling rate of greater than or equal to about 0.1 °C / s to less than or equal to about 15 °C / s can be employed. The third temperature is less than the martensite transformation start temperature. For example, the third temperature can be less than or equal to about 400 °C, and in certain aspects, optionally less than or equal to about 300 °C. In certain variations, the third temperature can be less than or equal to about 400 °C, and in certain aspects, optionally less than or equal to about 300 °C. The third temperature can be greater than or equal to room temperature (e.g., greater than or equal to about 15 °C to less than or equal to about 25 °C). The method 200 can further include holding 250 the third temperature for a period of time of greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally greater than or equal to about 20 seconds to less than or equal to about 100 seconds.

[0133] After the second air quench 240 and the holding 250, the method 200 can include heating 260 the precursor plate to a fourth temperature that is greater than the third temperature. Similar to the second temperature, the fourth temperature can be between about 500 °C (which is represented by line 232) and the martensite transformation start temperature (which is represented by line 234). For example, the fourth temperature can be greater than or equal to about 300 °C to less than or equal to about 500 °C, and in certain aspects, optionally greater than or equal to about 400 °C to less than or equal to about 500 °C. The fourth temperature can be the same as or different from the second temperature. The heating 260 can be performed at a rate of greater than or equal to about 0.1 °C / s to less than or equal to about 100 °C / s. -1 -1 ​the precursor sheet to a fourth temperature. The method 200 can include maintaining 270 the fourth temperature for a period of time greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally greater than or equal to about 20 seconds to less than or equal to about 100 seconds. After the third maintenance period, the method 200 can further include cooling 280 the precursor sheet to room temperature (e.g., greater than or equal to about 15 °C to less than or equal to about 25 °C) to obtain a steel sheet having improved flatness. For example, the steel sheet can be air cooled to room temperature.

[0134] As described above, the prepared steel sheet has improved flatness. For example, when the prepared steel sheet has a thickness greater than or equal to about 0.8 mm to less than or equal to about 1.3 mm, the maximum distance (i.e., height) between a normal plane in the sheet and each peak can be less than or equal to about 7 mm, and in certain aspects, optionally less than or equal to about 6 mm. When the prepared steel sheet has a thickness greater than or equal to about 1.3 mm to less than or equal to about 1.8 mm, the maximum distance (i.e., height) between a normal plane in the sheet and each peak can be less than or equal to about 6 mm, and in certain aspects, optionally less than or equal to about 5 mm. When the prepared steel sheet has a thickness greater than or equal to about 1.8 mm, the maximum distance (i.e., height) between a normal plane in the sheet and each peak can be less than or equal to about 5 mm, and in certain aspects, optionally less than or equal to about 4 mm.

[0135] Although not shown, in certain variations, the method 200 can further include coiling the steel sheet having improved flatness and moving the formed coil to a roll forming machine for forming.

[0136] Figure 3 is a graphical illustration summarizing an example austenitic isothermal quenching method 300 for forming a steel sheet having high flatness, where the x-axis 302 represents time in seconds and the y-axis 304 represents temperature in degrees Celsius. As shown, the method 300 includes heating 310 a precursor sheet including a steel alloy to a first temperature. Although not shown, one skilled in the art will appreciate that, in certain variations, the precursor sheet is uncoiled from a steel coil.

[0137] The first temperature is above the austenitization point of the steel alloy (which is represented by line 112). For example, in certain variations, the first temperature can be greater than or equal to about 800 °C to less than or equal to about 950 °C, optionally greater than or equal to about 850 °C to less than or equal to about 950 °C, and in certain aspects, optionally about 930 °C. The precursor sheet can be heated at a rate greater than or equal to about 0.1 °C s -1 to less than or equal to about 100 °C s -1to a first temperature. The method 100 includes holding or soaking 120 the first temperature for a period of time greater than or equal to about 1 second to less than or equal to about 10,000 seconds, optionally greater than or equal to about 200 seconds to less than or equal to about 500 seconds, and in certain aspects, optionally about 340 seconds.

[0138] The first holding step 120 can be followed by a first air quench 330. For example, the precursor plate can be cooled to a second temperature that is less than the first temperature. The second temperature can be between about 500 °C (indicated by line 132) and the martensite transformation start temperature (indicated by line 134). For example, the second temperature can be greater than or equal to about 300 °C to less than or equal to about 500 °C, and in certain aspects, optionally greater than or equal to about 400 °C to less than or equal to about 500 °C. The cooling rate (which depends on the method) should be higher than or close to the critical cooling rate required to obtain a martensitic transformation for high strength. For example, in certain variations, a cooling rate greater than or equal to about 2 °C / s to less than or equal to about 15 °C / s can be employed. The first air quench 330 can be a natural air cooling process and / or a forced air / gas cooling process.

[0139] The method can further include holding 340 the second temperature for a period of time greater than or equal to about 1 second to less than or equal to about 10,000 seconds, and in certain aspects, optionally greater than or equal to about 20 seconds to less than or equal to about 100 seconds. After the second holding period, the method 300 can further include cooling 350 the precursor plate to room temperature (e.g., greater than or equal to about 15 °C to less than or equal to about 25 °C) to obtain a steel plate having improved flatness. For example, the steel plate can be air cooled to room temperature.

[0140] As described above, the prepared steel plate has improved flatness. For example, when the prepared steel plate has a thickness greater than or equal to about 0.8 mm to less than or equal to about 1.3 mm, the maximum distance (i.e., height) between a normal plane in the plate and each peak can be less than or equal to about 7 mm, and in certain aspects, optionally less than or equal to about 6 mm. When the prepared steel plate has a thickness greater than or equal to about 1.3 mm to less than or equal to about 1.8 mm, the maximum distance (i.e., height) between a normal plane in the plate and each peak can be less than or equal to about 6 mm, and in certain aspects, optionally less than or equal to about 5 mm. When the prepared steel plate has a thickness greater than or equal to about 1.8 mm, the maximum distance (i.e., height) between a normal plane in the plate and each peak can be less than or equal to about 5 mm, and in certain aspects, optionally less than or equal to about 4 mm.

[0141] Although not shown, in certain variations, the method 300 can further include coiling the steel sheet having improved flatness and moving the formed coil to a roll forming machine for forming.

[0142] Certain features of the current technology are further illustrated in the following non-limiting examples.

[0143] Example 1

[0144] Example steel sheets can be prepared in accordance with various aspects of the present disclosure. For example, an example steel sheet 400 can be prepared from a steel alloy using a direct quenching method, such as the direct quenching method shown in Figure 1 Figure 4 is a graphical illustration showing the tensile strength and ductility of the example steel sheet 400, where the x-axis 402 represents tensile strain (mm / mm) and the y-axis 404 represents tensile stress (MPa).

[0145] The example steel sheet 400 can have high strength, and also high ductility and high bendability. For example, the microstructure of the example steel sheet 400 can include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength and the retained austenite is associated with high ductility and high bendability. In certain variations, the example steel sheet 400 can include greater than or equal to about 80 vol% to less than or equal to about 99 vol% of the martensite phase; greater than or equal to about 1 vol% to less than or equal to about 10 vol% of the retained austenite phase; greater than or equal to about 0 vol% to less than or equal to about 10 vol% of the bainite phase; and greater than or equal to about 0 vol% to less than or equal to about 10 vol% of the ferrite phase.

[0146] The example steel sheet 400 can have a yield strength (YS) of greater than or equal to about 1150 MPa, with a standard deviation of about 3 MPa.

[0147] The example steel sheet 400 can have an ultimate tensile strength (UTS) of greater than or equal to about 1600 MPa, with a standard deviation of about 3 MPa.

[0148] The example steel sheet 400 can have a total elongation (TEL) of greater than or equal to about 3%, with a standard deviation of about 3%.

[0149] Example 2

[0150] Example steel sheets can be prepared in accordance with various aspects of the present disclosure. For example, an example steel sheet 500 can be prepared from a steel alloy using a quenching and partitioning / tempering method, such as the quenching and partitioning / tempering method shown in Figure 2 Figure 5 ​​is a graphical illustration showing tensile strength and ductility of example steel sheet 500, where the x-axis 502 represents tensile strain (mm / mm) and the y-axis 504 represents tensile stress (MPa).

[0151] Example steel sheet 500 can have high strength, and also high ductility and high bendability. For example, the microstructure of example steel sheet 500 can include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength and the retained austenite is associated with high ductility and bendability. In certain variations, example steel sheet 500 can include greater than or equal to about 50 vol% to less than or equal to about 95 vol% of a martensite constituent; greater than or equal to about 5 vol% to less than or equal to about 17 vol% of a retained austenite phase; greater than or equal to about 0 vol% to less than or equal to about 25 vol% of a bainite phase; and greater than or equal to about 0 vol% to less than or equal to about 10 vol% of a ferrite phase.

[0152] Example steel sheet 500 can have a yield strength (YS) of greater than or equal to about 1150 MPa, with a standard deviation of about 3 MPa.

[0153] Example steel sheet 500 can have an ultimate tensile strength (UTS) of greater than or equal to about 1500 MPa, with a standard deviation of about 3 MPa.

[0154] Example steel sheet 500 can have a total elongation (TEL) of greater than or equal to about 7%, with a standard deviation of about 3%.

[0155] Example steel sheet 500 can have a bendability (i.e., bend angle) of greater than or equal to about 50 degrees, with a standard deviation of about 3 degrees, for example, measured using the standard VDA 238-100.

[0156] Example 3

[0157] Example steel sheets can be prepared in accordance with various aspects of the present disclosure. For example, example steel sheet 600 can be prepared from a steel alloy using an austempering method, such as the austempering method shown in Figure 3 Figure 6 is a graphical illustration showing tensile strength and ductility of example steel sheet 600, where the x-axis 602 represents tensile strain (mm / mm) and the y-axis 604 represents tensile stress (MPa).

[0158] ​The example steel sheet 600 can have high strength, and also high ductility and high bendability. For example, the microstructure of the example steel sheet 600 can include a mixture of martensite, retained austenite, bainite, and ferrite phases, where the martensite phase is associated with high strength, the retained austenite is associated with high ductility and high bendability. In certain variations, the example steel sheet 600 can include greater than or equal to about 30 vol% to less than or equal to about 97 vol% of a martensite constituent; greater than or equal to about 3 vol% to less than or equal to about 15 vol% of a retained austenite phase; greater than or equal to about 0 vol% to less than or equal to about 45 vol% of a bainite phase; and greater than or equal to about 0 vol% to less than or equal to about 10 vol% of a ferrite phase.

[0159] The example steel sheet 600 can have a yield strength (YS) of greater than or equal to about 1100 MPa, with a standard deviation of about 3 MPa.

[0160] The example steel sheet 600 can have an ultimate tensile strength (UTS) of greater than or equal to about 1550 MPa, with a standard deviation of about 3 MPa.

[0161] The example steel sheet 600 can have a total elongation (TEL) of greater than or equal to about 7%, with a standard deviation of about 3%.

[0162] The example steel sheet 600 can have a bendability (i.e., bend angle), for example measured using the standard VDA 238-100, of greater than or equal to about 50 degrees, with a standard deviation of about 3 degrees.

[0163] The foregoing description of implementations has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form disclosed. Various elements or features of a specific implementation can be modified or combined in various ways. Such variations are not to be interpreted as a departure from the scope of the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

1. A method for preparing a steel alloy plate for improved flatness, the method comprising: heating a steel alloy material to a first temperature above the full austenitization point of the steel alloy material; holding the steel alloy material at the first temperature for a period of time greater than or equal to 1 second to less than or equal to 10,000 seconds to form a precursor steel plate; subjecting the precursor steel plate to a first air quenching step to a second temperature that is lower than the first temperature and above the martensitic transformation start temperature of the steel alloy material; followed by subjecting the precursor steel plate to a second air quenching step to a third temperature that is less than the second temperature, and holding the precursor steel plate at the third temperature for a period of time greater than or equal to 1 second to less than or equal to 10,000 seconds; followed by cooling the precursor steel plate to room temperature to prepare the steel alloy plate, the room temperature being greater than or equal to 15 °C to less than or equal to 25 °C.

2. The method of claim 1, wherein the first temperature is greater than or equal to 800 °C to less than or equal to 950 °C, the second temperature is greater than or equal to 300 °C to less than or equal to 500 °C, and the cooling rate during the air quenching is greater than or equal to 2 °C / s to less than or equal to 15 °C / s.

3. The method of claim 1, wherein the cooling rate during the second air quenching step is greater than or equal to 0.1 °C / s to less than or equal to 15 °C / s, and the third temperature is less than or equal to 400 °C.

4. The method of claim 1, wherein the steel alloy plate has a yield strength greater than or equal to 1150 MPa, an ultimate tensile strength greater than or equal to 1600 MPa, and a total elongation greater than or equal to 3%, and wherein the steel alloy plate has a microstructure comprising greater than or equal to 80 vol.% to less than or equal to 99 vol.% of a martensite phase; greater than or equal to 1 vol.% to less than or equal to 10 vol.% of a retained austenite phase; greater than or equal to 0 vol.% to less than or equal to 10 vol.% of a bainite phase; and greater than or equal to 0 vol.% to less than or equal to 10 vol.% of a ferrite phase.

5. The method of claim 4, wherein the method further comprises heating the precursor steel plate from the third temperature to a fourth temperature that is lower than the first temperature.

6. The method of claim 5, wherein the fourth temperature is greater than or equal to 300 °C to less than or equal to 500 °C.

7. The method of claim 5, wherein the method further comprises holding the precursor steel plate at the fourth temperature for a period of time greater than or equal to 1 second to less than or equal to 10,000 seconds.

8. The method of claim 7, wherein the steel alloy plate has a yield strength greater than or equal to 1150 MPa, an ultimate tensile strength greater than or equal to 1500 MPa, a total elongation greater than or equal to 7%, and a bend angle greater than or equal to 50 degrees, and wherein the steel alloy plate has a microstructure comprising greater than or equal to 50 volume % to less than or equal to 95 volume % of a martensite constituent; greater than or equal to 5 volume % to less than or equal to 17 volume % of a retained austenite phase; greater than or equal to 0 volume % to less than or equal to 25 volume % of a bainite phase; and greater than or equal to 0 volume % to less than or equal to 10 volume % of a ferrite phase.

9. The method of claim 1, wherein the method further comprises holding the precursor steel plate at a second temperature for a time period greater than or equal to 1 second to less than or equal to 10,000 seconds.

10. The method of claim 9, wherein the steel alloy plate has a yield strength greater than or equal to 1100 MPa, an ultimate tensile strength greater than or equal to 1550 MPa, a total elongation greater than or equal to 7%, and a bend angle greater than or equal to 50 degrees, and wherein the steel alloy has a microstructure comprising greater than or equal to 30 volume % to less than or equal to 97 volume % of a martensite constituent; greater than or equal to 3 volume % to less than or equal to 15 volume % of a retained austenite phase; greater than or equal to 0 volume % to less than or equal to 45 volume % of a bainite phase; and greater than or equal to 0 volume % to less than or equal to 10 volume % of a ferrite phase.

11. The method of claim 1, wherein the steel alloy material comprises: greater than or equal to 0.05 wt. % to less than or equal to 0.45 wt. % carbon; greater than or equal to 0.5 wt. % to less than or equal to 6 wt. % chromium; greater than or equal to 0.5 wt. % to less than or equal to 2.5 wt. % silicon; greater than 0 wt. % to less than or equal to 4.5 wt. % manganese; greater than 0 wt. % to less than or equal to 2 wt. % aluminum, wherein a ratio of chromium to aluminum is greater than or equal to 1.7, and a sum of aluminum and silicon is greater than or equal to 0.7 wt. %; and a balance of iron.

12. The method of claim 11, wherein the steel alloy material further comprises: greater than or equal to 0 wt. % to less than or equal to 0.5 wt. % vanadium; greater than or equal to 0 wt. % to less than or equal to 0.2 wt. % niobium; and greater than or equal to 0 wt. % to less than or equal to 0.3 wt. % titanium.

13. A method for preparing a steel alloy plate for improved flatness, the method comprising: heating a steel alloy material to a first temperature greater than or equal to 800 °C to less than or equal to 950 °C; holding the steel alloy material at the first temperature for a time period greater than or equal to 1 second to less than or equal to 10,000 seconds to form a precursor steel plate; first air quenching the precursor steel plate from the first temperature to a second temperature greater than or equal to 300 °C to less than or equal to 500 °C at a first cooling rate greater than or equal to 2 °C / s to less than or equal to 15 °C / s; second air quenching the precursor steel plate from the second temperature to a third temperature less than or equal to 400 °C at a second cooling rate greater than or equal to 0.1 °C / s to less than or equal to 15 °C / s; and cooling the precursor steel plate to room temperature to prepare the steel alloy plate, the room temperature greater than or equal to 15 °C to less than or equal to 25 °C.

14. The method of claim 13, wherein the steel alloy material comprises: greater than or equal to 0.05 wt. % to less than or equal to 0.45 wt. % carbon; greater than or equal to 0.5 wt. % to less than or equal to 6 wt. % chromium; greater than or equal to 0.5 wt. % to less than or equal to 2.5 wt. % silicon; greater than 0 wt. % to less than or equal to 4.5 wt. % manganese; greater than 0 wt. % to less than or equal to 2 wt. % aluminum, wherein a ratio of chromium to aluminum is greater than or equal to 1.7, and a sum of aluminum and silicon is greater than or equal to 0.7 wt. %; and a balance of iron.

15. The method of claim 14, wherein the steel alloy material further comprises: greater than or equal to 0 wt. % to less than or equal to 0.5 wt. % vanadium; greater than or equal to 0 wt. % to less than or equal to 0.2 wt. % niobium; and greater than or equal to 0 wt. % to less than or equal to 0.3 wt. % titanium.

14. The method of claim 13, wherein the method further comprises holding the precursor steel sheet at a third temperature for a hold period of greater than or equal to 1 second to less than or equal to 10,000 seconds, heating the precursor steel sheet from the third temperature to a fourth temperature after the hold period, and holding the precursor steel sheet at the fourth temperature for a period of greater than or equal to 1 second to less than or equal to 10,000 seconds, the fourth temperature being greater than or equal to 300 °C to less than or equal to 500 °C.

15. The method of claim 13, wherein the steel alloy material comprises: greater than or equal to 0.05 wt. % to less than or equal to 0.45 wt. % carbon; greater than or equal to 0.5 wt. % to less than or equal to 6 wt. % chromium; greater than or equal to 0.5 wt. % to less than or equal to 2.5 wt. % silicon; greater than 0 wt. % to less than or equal to 4.5 wt. % manganese; greater than 0 wt. % to less than or equal to 2 wt. % aluminum, wherein a ratio of chromium to aluminum is greater than or equal to 1.7, and a sum of aluminum and silicon is greater than or equal to 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to 0.5 wt. % vanadium; greater than or equal to 0 wt. % to less than or equal to 0.2 wt. % niobium; greater than or equal to 0 wt. % to less than or equal to 0.3 wt. % titanium; and a balance of iron.

16. A method for preparing a steel alloy sheet for improved flatness, the method comprising: heating a steel alloy material to a first temperature of greater than or equal to 800 °C to less than or equal to 950 °C; holding the steel alloy material at the first temperature for a period of greater than or equal to 1 second to less than or equal to 10,000 seconds to form a precursor steel sheet; first air quenching the precursor steel sheet from the first temperature to a second temperature of greater than or equal to 300 °C to less than or equal to 500 °C at a first cooling rate of greater than or equal to 2 °C / s to less than or equal to 15 °C / s; holding the precursor steel sheet at the second temperature for a period of greater than or equal to 1 second to less than or equal to 10,000 seconds; followed by cooling the precursor steel sheet from the second temperature to room temperature to prepare the steel alloy sheet, the room temperature being greater than or equal to 15 °C to less than or equal to 25 °C.

17. The method of claim 16, wherein the steel alloy material comprises: greater than or equal to 0.05 wt. % to less than or equal to 0.45 wt. % carbon; greater than or equal to 0.5 wt. % to less than or equal to 6 wt. % chromium; greater than or equal to 0.5 wt. % to less than or equal to 2.5 wt. % silicon; greater than 0 wt. % to less than or equal to 4.5 wt. % manganese; greater than 0 wt. % to less than or equal to 2 wt. % aluminum, wherein a ratio of chromium to aluminum is greater than or equal to 1.7, and a sum of aluminum and silicon is greater than or equal to 0.7 wt. %; greater than or equal to 0 wt. % to less than or equal to 0.5 wt. % vanadium; greater than or equal to 0 wt. % to less than or equal to 0.2 wt. % niobium; greater than or equal to 0 wt. % to less than or equal to 0.3 wt. % titanium; and a balance of iron.

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