High strength steel having retained austenite

Abstract

A method for processing a steel sheet includes heating the steel sheet to a first temperature (T1), cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, re-heating the steel sheet to a carbon redistribution temperature, stabilizing austenite by holding the steel sheet at the carbon redistribution temperature, and cooling the steel sheet to room temperature. The step of heating the steel sheet to T1 including T1 being at least above the temperature at which the steel sheet transforms to austenite and ferrite. The step of cooling the steel sheet to T2 includes T2 being below the martensite start temperature (Ms). The cooling rate to T2 is sufficiently rapid to transform austenite to martensite. The carbon redistribution temperature is 500°C or more.

Description

HIGH STRENGTH STEEL HAVING RETAINED AUSTENITEPRIORITY

[0001] This application claims priority to U.S. Provisional Application Serial No. 63 / 657,213, entitled “High Strength Steel Having Retained Austenite,” fded June 7, 2024, the disclosure of which is incorporated by reference herein.BACKGROUND

[0002] The present application relates to improved high strength steel products. High strength steels are produced by incorporating certain alloying elements into the steel composition, which may increase strength by a variety of mechanisms. Increased strength is desirable in a variety of circumstances. For instance, in automotive applications, higher strengths can promote reduced part thicknesses, which can promote weight reductions in such parts. However, some alloying elements used for increasing strength may also reduce ductility of the steel, which may impede formability and toughness characteristics. This may lead to limitations on the complexity of parts produced from such steels and reduced performance in certain circumstances such as crash worthiness.

[0003] It is therefore desirable to produce a steel having both high strength and improved formability characteristics. Such a combination of characteristics may allow for reduced gauge size of steels used in parts, while still maintaining strengths that may be associated with other materials. Additionally, such a combination of characteristics may allow for forming characteristics suitable for more complex parts. Furthermore, such a combination of characteristics may provide increases in energy absorption through improved elongation.

[0004] The steels of the present disclosure seek to achieve both high strength and improved formability through the presence of certain microstructural constituents such as austenite.While various kinds of steels have been made and used, it is believed that no one prior to the inventor(s) has made or used the invention described in the appended claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0005] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention.

[0006] The Figure depicts a schematic view of a temperature profile with a carbon redistribution step performed.

[0007] The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the invention may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention; it being understood, however, that this invention is not limited to the precise arrangements shown.DETAILED DESCRIPTION

[0008] The following description of certain examples of the invention should not be used to limit the scope of the present invention. Other examples, features, aspects, embodiments, and advantages of the invention will become apparent to those skilled in the art from the following description, which is by way of illustration, one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different and obvious aspects, all without departing from the invention. Accordingly, the drawings and descriptions should be regarded as illustrative in nature and not restrictive.

[0009] It is understood that any one or more of the teachings, expressions, embodiments, examples, etc. described herein may be combined with any one or more of the otherteachings, expressions, embodiments, examples, etc. that are described herein. The abovedescribed teachings, expressions, embodiments, examples, etc. should therefore not be viewed in isolation relative to each other. Various suitable ways in which the teachings herein may be combined will be readily apparent to those of ordinary skill in the art in view of the teachings herein. Such modifications and variations are intended to be included within the scope of the claims.

[0010] I. Exemplary Steel Composition

[0011] In some circumstances, it may be desirable to use aluminum-based alloys over steel. For instance, in circumstances where reduced weight is desired, aluminum-based alloys may be substituted for steel for reduced weight. However, such aluminum-based alloys may come with certain tradeoffs such as reduced strength, increased costs, increased energy requirements for production, and / or etc. Thus, in such circumstances, it may be desirable to avoid the substitution entirely by using a steel material with weight characteristics comparable to aluminum-based alloys, but strength and cost characteristics of steel.

[0012] Steels with high strength characteristics may generally be susceptible to decreased formability. Where steels have decreased formability, parts may need to be thicker to accommodate the stresses applied during production, increasing the overall weight of the parts. By contrast, where steels have increased formability, parts may be thinner or down- gauged for lighter overall weight.

[0013] In some circumstances, formability may be increased at the cost of decreased strength. However, compositions and material processing operations described herein achieve improved formability without substantially reducing strength. As will be described in greater detail below, such improves may be achieved through compositions and processing steps that promote the structure of retained austenite.

[0014] An exemplary steel composition can include certain alloying additions to improve the propensity of a steel sheet to form a primarily austenitic and martensitic microstructure and / or to improve the mechanical properties of the steel sheet. Suitable compositions of the steel sheet may include one or more of the following, by weight percent: 0.12% or more carbon; 0.5% or more silicon + aluminum; 1.5% or more manganese, other incidental elements, and the balance being iron. In still other examples, suitable compositions of the steel sheet may include one or more of the following, by weight percent: 0.15 to 0.25% carbon; 1.5 to 3.0% manganese, 0.5 to 2.0% silicon + aluminum, other incidental elements, and the balance being iron.[00015J In addition, in some examples, suitable compositions of the steel sheet may additionally include other elements such as molybdenum, niobium, vanadium, and / or titanium. Additions of such other elements are entirely optional and may be omitted in some examples.

[0016] In some examples, carbon may be used to stabilize austenite. For instance, increasing carbon can act to lower the martensite start temperature (the Ms temperature), lower transformation temperatures for other non-martensitic constituents (e g., bainite, ferrite, pearlite), and increase the time required for non-martensitic products to form (e.g., carbides). Additionally, carbon additions may improve the hardenability of the steel sheet thus retaining formation of non-martensitic constituents near the core of the steel sheet where cooling rates may be locally depressed. However, it should be understood that carbon additions may be limited as significant carbon additions may lead to detrimental effects on weldability.

[0017] Manganese can be added to provide additional stabilization of austenite by lowering the transformation temperatures of other non-martensitic constituents, thereby delaying or preventing formation of such non-martensitic constituents. Manganese may further improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure by increasing hardenability as similarly described above with respect to carbon.

[0018] Silicon and / or aluminum may be added to reduce the formation of carbides. It should be understood that a reduction in carbide formation may be desirable in some examples because the presence of carbides may decrease the levels of carbon available for diffusion into austenite. Thus, silicon and / or aluminum additions may be used to further stabilize austenite by preventing segregation of carbon into other constituents such as carbides.

[0019] Molybdenum may be optionally added in some examples. Where added, molybdenum may be used to increase hardenability. Suitable molybdenum concentrations may include, for example 0.0 to 0.5% by percent weight.

[0020] In some embodiments, nickel, copper, and chromium may be optionally added to stabilize austenite. For instance, such elements may lead to a reduction in the Mstemperature. Additionally, nickel, copper, and chromium may further increase the hardenability of the steel sheet.

[0021] In some embodiments niobium (or other micro-alloying elements, such as titanium, vanadium, and / or the like) may be optionally used to increase the mechanical properties of the steel sheet. For instance, niobium may increase the strength of the steel sheet through grain boundary pinning resulting from carbide formation.

[0022] In other embodiments, variations in the concentrations of elements and the particular elements selected may be made. Of course, where such variations are made, it should be understood that such variations may have a desirable or undesirable effect on the steel sheet microstructure and / or mechanical properties in accordance with the properties described above for each given alloying addition.

[0023] II. Exemplary Thermal Cycle[00024J The Figure shows a schematic representation of a thermal cycle that may be used in combination with a steel sheet having one or more of the compositions described above to achieve high strength a improved formability. In particular, the Figure shows a thermalprofile (10) with certain process steps configured to promote stabilization of austenite at room temperature. In one example, the process generally involves austenitization followed by a rapid or semi-rapid cooling to a specified quench temperature to partially transform austenite to martensite, and the holding at an elevated temperature, a carbon redistribution temperature, to allow carbon to diffuse out of the martensite and into the remaining austenite, thus stabilizing the austenite at room temperature. In some examples the thermal profile (10) shown in the Figure may be implemented in a continuous heat treatment production line, although such a production line is not required.

[0025] As best seen in the Figure, the steel sheet is first heated to a peak metal temperature (12). The peak metal temperature (12) in the present example is shown as being at least above the austenite transformation temperature (Ai) (e.g., the dual phase, austenite + ferrite region). Thus, at the peak metal temperature (12), at least a portion of the steel will be transformed to austenite. Although the Figure shows the peak metal temperature (12) as being solely above Ai, it should be understood that in some examples, the peak metal temperature (12) can also include temperatures above the temperature at which ferrite completely transforms to austenite (A3) (e.g., the single phase, austenite region). In merely one example, the peak metal temperature (12) corresponds to a temperature near or above the A3 temperature. Such a peak metal temperature (12) may be desirable in such examples to maximize austenite formation in the steel sheet prior to cooling.

[0026] Next, the steel sheet undergoes rapid or semi-rapid cooling. As the steel sheet is cooling, the rate of cooling may be continuous. Alternatively, in some examples, the rate of cooling may be interrupted (e.g., briefly zero). In still other examples, the rate of cooling may be variable. It should be understood that a variety of variable cooling rate profiles may be used as will be appreciated by those of ordinary skill in the art in view of the teachings herein. For instance, in some examples, such variation may be imparted by the steel sheet moving from one section of the processing line to another (e.g., moving between various thermal processing sections).

[0027] The rapid cooling of the steel sheet is shown to continue below the martensite start temperature (Ms) for the composition of the steel sheet to a predetermined quench temperature (18). It should be understood that the cooling rate to Msmay be high enough to transform at least some of the austenite formed at the peak metal temperature (12) to martensite. In other words, the cooling rate may be high enough, or rapid enough, to transform austenite to martensite instead of other non -martensitic constituents such as ferrite, pearlite, or bainite which transform at relatively lower cooling rates.

[0028] As is shown in the Figure, the quench temperature (18) is below the Ms. and above the martensite finish temperature (Mf) The difference between the quench temperature (18) and Ms may vary depending on the particular composition of the steel sheet being used. However, in many examples the difference between quench temperature (18) and Msmay be sufficiently great to form an adequate amount of martensite to act as a carbon source to stabilize the austenite and avoid creating excessive “fresh” martensite upon final cooling. Additionally, quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench (e.g., to avoid excessive carbon enrichment of austenite greater than that required to stabilize austenite for the given example).

[0029] In some examples, quench temperature (18) varies from about 191 °C to about 281 °C, although different temperatures may be used in other examples. Additionally, quench temperature (18) can be calculated for a given steel composition. For such a calculation, quench temperature (18), for example, can correspond to the retained austenite having an Mstemperature of room temperature after carbon redistribution. Examples of methods for calculating quench temperature (18) are described in J. G. Speer, A. M. Streicher, D. K. Matlock, F. Rizzo, and G. Krauss, “Quenching And Partitioning : A Fundamentally New Process to Create High Strength Trip Sheet Microstructures,” Austenite Formation and Decomposition, pp. 505-522, 2003; and A. M. Streicher, J. G. J. Speer, D. K. Matlock, and B. C. De Cooman, “Quenching and Partitioning Response of a Si-Added TRIP Sheet Steel,” in Proceedings of the International Conference on Advanced High Strength SheetSteels for Automotive Applications, 2004, the subject matter of which is incorporated by reference herein.

[0030] The quench temperature (18) is set sufficiently low with respect to the Msto form an adequate amount of martensite, with the martensite acting as a carbon source to stabilize austenite and avoid creating excessive “fresh” martensite upon final quench. Alternatively, quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench and creating circumstances where the potential carbon enrichment of the retained austenite is greater than that required for austenite stabilization at room temperature. In some embodiments, a suitable quench temperature (18) may correspond to the retained austenite having an Mstemperature of room temperature after carbon redistribution. Speer and Streicher et al. (above) include calculations that provide guidelines with respect to processing options that may result in desirable microstructures. Such calculations assume idealized full carbon redistribution. In some examples, such calculations may be optionally performed by applying the Koistinen-Marburger (KM) relationship twice (fm= 1 -c 'L'1- first to the initial quench temperature (18) and then to the final quench at room temperature (as further described below). The Mstemperature in the KM expression can be optionally estimated using empirical formulae based on austenite chemistry (such as Andrew’s linear expression, for example):

[0031] s(°C) = 539 - 423C - 30.4M? - 7 ,5Si + 3 QAl

[0032] The result of the calculations described by Speer et al. may indicate a quench temperature (18) that may lead to a maximum amount of retained austenite. Of course, the calculations described herein are for example only, and various alternative calculations may be used as will be appreciated by those of ordinary skill in the art in view of the teachings herein. For quench temperatures (18) above the temperature having a maximum amount of retained austenite, significant fractions of austenite are present after the initial quench; however, there is not enough martensite to act as a carbon source to stabilize this austenite. Therefore, when higher quench temperatures are used, increasing amounts offresh martensite form during the final quench. For quench temperatures below the temperature having a maximum amount of retained austenite, an unsatisfactory amount of austenite may be consumed during the initial quench and there may be an excess amount of carbon that may partition from the martensite.

[0033] Once quench temperature (18) is reached, the temperature of the steel sheet is generally increased to a carbon redistribution temperature. In examples with a relatively high quench temperature (18), the temperature of the steel can optionally remain at quench temperature to achieve the carbon redistribution temperature. Regardless, the temperature of the steel sheet is maintained at the carbon redistribution temperature for a given period of time. In some examples, this stage may be referred to as a carbon redistribution stage. In such a stage, the temperature of the steel sheet is at least maintained at the quench temperature or above to permit carbon diffusion from martensite formed during the rapid cooling and into any remaining austenite. Such diffusion may permit the remaining austenite to be stable (or meta-stable) at room temperature, thus improving the mechanical properties of the steel sheet. In some examples, the microstructure may include 5% or more retained austenite. In other examples, the microstructure may include 12% or more retained austinite. In yet other examples, the microstructure may include about 10 to 15% retained austinite. In still other examples, the microstructure may include about 12 to 13% retained austenite.

[0034] In some examples, the steel sheet is heated above Ms, but below Ai, to a relatively high carbon redistribution temperature (20) and thereafter held at the high carbon redistribution temperature (20). A variety of methods may be utilized to heat the steel sheet during this stage. By way of example only, the steel sheet may be heated using induction heating, furnace heating, torch heating, and / or etc. In one example, the carbon redistribution temperature (20) is from about 500 °C to about 625 °C. In other examples, the carbon redistribution temperature (20) is from about 500 C to a temperature below AL Generally, the upper limit for the carbon redistribution temperature is optionally set to Ai to avoid transformation of the steel, which may form new or fresh austenite.

[0035] A relatively high carbon redistribution temperature (20) can be used in the present example due to the steel sheet being an uncoated product or in an uncoated condition. The term “uncoated” used herein refers to the absence of a coating such as a metallic coating including zinc coatings formed using galvanizing or galvannealing processes, aluminum coatings formed using aluminizing processes, or other coatings. In the uncoated condition, the coating itself or processes associated with such coatings, provides no limit to the carbon redistribution temperature (20). Thus, a higher carbon redistribution temperature (20) can be used relative to other contexts where the coating may impart some limits (e.g., the temperature of the zinc or aluminum bath).[00036J After the steel sheet has reached carbon redistribution temperature (20) as described above, the steel sheet is maintained at the desired carbon redistribution temperature (20) for a sufficient time to permit diffusion of carbon from martensite to austenite. The steel sheet may then be cooled to room temperature.

[0037] III. Examples

[0038] Example 1

[0039] A first trial was performed with two steel sheets. The steel sheets were prepared into individual coils. The coils were processed using a continuous thermal processing line. The two coils of steel sheet were prepared with the compositions listed below in Table 1 (compositions shown in weight percent).Table 1

[0040] As described above, each coil was subjected to processing on a continuous thermal processing line. The processing was performed with each steel sheet in an uncoated condition. No coating (e.g., no galvanizing, aluminumizing, etc.) was performed during the processing. The processing parameters used are shown below in Table 2.Table 2

[0041] In the processing parameters shown above in Table 2, the columns “Zone 1” through “SC Zone” correspond to the zone where the steel sheet is heated to the peak metal temperature (12) described above. Similarly, the column “Strip Quench” corresponds to the zone where the steel sheet is cooled to quench temperature (18) described above. Finally, the columns “TRI” and “TR2” correspond to the carbon redistribution step described above where the steel sheet is heated to a carbon redistribution temperature (20).

[0042] After processing, additional testing was performed. Specifically, mechanical testing was performed to identify the Yield Strength (YS), Tensile Strength (TS) and Total Elongation of each steel sheet after processing. The resulting mechanical testing is shown below in Table 3.Table 3

[0043] Example 2

[0044] A second trial was performed with two steel sheets. The steel sheets were prepared into individual coils. The coils were processed using a continuous thermal processing line. The two coils of steel sheet were prepared with the compositions listed below in Table 4 (compositions shown in weight percent).Table 4

[0045] As described above, each coil was subjected to processing on a continuous thermal processing line. The processing was performed with each steel sheet in an uncoated condition. No coating (e.g., no galvanizing, aluminumizing, etc.) was performed during the processing. The processing parameters used are shown below in Table 5.Table 5

[0046] In the processing parameters shown above in Table 5, the columns “Zone 1” through “SC Zone” correspond to the zone where the steel sheet is heated to the peak metal temperature (12) described above. Similarly, the column “Strip Quench” corresponds to the zone where the steel sheet is cooled to quench temperature (18) described above. Finally, the columns “TRI” and “TR2” correspond to the carbon redistribution step described above where the steel sheet is heated to a carbon redistribution temperature (20).

[0047] After processing, additional testing was performed. Specifically, mechanical testing was performed to identify the Yield Strength (YS), Tensile Strength (TS) and Total Elongation of each steel sheet after processing. The resulting mechanical testing is shown below in Table 6.Table 6

[0048] Example 3

[0049] A third trial was performed with multiple steel sheets. Each steel sheet was prepared into an individual coil. Each coil was processed using a continuous thermal processing line.The coils of steel sheet were prepared with the compositions listed below in Table 7 (compositions shown in weight percent).Table 7

[0050] As described above, each coil was subjected to processing on a continuous thermal processing line. The processing was performed with each steel sheet in an uncoated condition. No coating (e.g., no galvanizing, aluminumizing, etc.) was performed during the processing. The processing parameters used are shown below in Table 8.Table 8

[0051] In the processing parameters shown above in Table 8, the columns “Zone 1” through “SC Zone” correspond to the zone where the steel sheet is heated to the peak metal temperature (12) described above. Similarly, the column “Strip Quench” corresponds to the zone where the steel sheet is cooled to quench temperature (18) described above.Finally, the columns “TRI” and “TR2” correspond to the carbon redistribution step described above where the steel sheet is heated to a carbon redistribution temperature (20).

[0052] After processing, additional testing was performed. Specifically, mechanical testing was performed to identify the Yield Strength (YS), Tensile Strength (TS) and Total Elongation of each steel sheet after processing. The resulting mechanical testing is shown below in Table 9.Table 9

[0053] The following examples relate to various non-exhaustive ways in which the teachings herein may be combined or applied. It should be understood that the following examples are not intended to restrict the coverage of any claims that may be presented at any time in this application or in subsequent filings of this application. No disclaimer is intended. The following examples are being provided for nothing more than merely illustrative purposes. It is contemplated that the various teachings herein may be arranged and applied in numerous other ways. It is also contemplated that some variations may omit certain features referred to in the below examples. Therefore, none of the aspects or features referred to below should be deemed critical unless otherwise explicitly indicated as such at a later date by the inventors or by a successor in interest to the inventors. If anyclaims are presented in this application or in subsequent filings related to this application that include additional features beyond those referred to below, those additional features shall not be presumed to have been added for any reason relating to patentability.

[0054] Example 4

[0055] A method for processing a steel sheet, the method comprising: heating the steel sheet to a first temperature (Tl), wherein T1 is at least above the temperature at which the steel sheet transforms to austenite and ferrite; cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is below the martensite start temperature (Ms), wherein the cooling rate is sufficiently rapid to transform austenite to martensite; re-heating the steel sheet to a carbon redistribution temperature, wherein the carbon redistribution temperature is 500 °C or more; stabilizing austenite by holding the steel sheet at the carbon redistribution temperature for a holding time; and cooling the steel sheet to room temperature.

[0056] Example 5

[0057] The method of Example 4, the step of stabilizing austenite being performed while the steel sheet is in an uncoated condition.

[0058] Example 6

[0059] The method of Examples 4 or 5, the step of cooling the steel sheet to room temperature being performed while the steel sheet is in an uncoated condition.

[0060] Example 7

[0061] The method of any of Examples 4 through 6, wherein the T2 is below the carbon redistribution temperature.

[0062] Example 8

[0063] The method of any of Examples 4 through 7, the step of re-heating the steel sheet to the carbon redistribution temperature being performed after the step of cooling the steel sheet to T2.

[0064] Example 9

[0065] The method of any of Examples 4 through 8, wherein the carbon redistribution temperature is below the Ai temperature of the steel sheet.

[0066] Example 10

[0067] The method of any of Examples 4 through 9, wherein the portioning temperature is between 506 °C and 624 °C.

[0068] Example 11

[0069] The apparatus of any of Examples 4 through 10, the steel sheet having an elongation of at least 13 % after the step of cooling the steel sheet to room temperature.

[0070] Example 12

[0071] The apparatus of any of Examples 4 through 10, the steel sheet having a tensile strength of at least 1180 MPa and an elongation of at least 13 % after the step of cooling the steel sheet to room temperature.

[0072] Example 13

[0073] The method of any of Examples 4 through 12, the steel sheet comprising the following elements by weight percent: 0.12% or more carbon; 0.5% or more silicon, aluminum, or a combination thereof; 1.5% or more manganese; and the balance being iron and other incidental impurities.

[0074] Example 14

[0075] The method of any of Examples 4 through 12, the steel sheet comprising the following elements by weight percent: 0.15 to 0.25% carbon; 0.5 to 2.0% silicon, aluminum, or a combination thereof; 1.5 to 3.0% manganese; and the balance being iron and other incidental impurities.

[0076] IV. Miscellaneous

[0077] It should be appreciated that any patent, publication, or other disclosure material, in whole or in part, that is said to be incorporated by reference herein is incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as explicitly set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein will only be incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material.

[0078] Having shown and described various embodiments of the present invention, further adaptations of the methods and systems described herein may be accomplished by appropriate modifications by one of ordinary skill in the art without departing from the scope of the present invention. Several of such potential modifications have been mentioned, and others will be apparent to those skilled in the art. For instance, the examples, embodiments, geometries, materials, dimensions, ratios, steps, and the like discussed above are illustrative and are not required. Accordingly, the scope of the present invention should be considered in terms of the following claims and is understood not to be limited to the details of structure and operation shown and described in the specification and drawings.

Claims

I / We claim:

1. A method for processing a steel sheet, the method comprising:(a) heating the steel sheet to a first temperature (Tl), wherein T1 is at least above the temperature at which the steel sheet transforms to austenite and ferrite;(b) cooling the steel sheet to a second temperature (T2) by cooling at a cooling rate, wherein T2 is below the martensite start temperature (Ms), wherein the cooling rate is sufficiently rapid to transform austenite to martensite;(c) re-heating the steel sheet to a carbon redistribution temperature, wherein the carbon redistribution temperature is 500 °C or more;(d) stabilizing austenite by holding the steel sheet at the carbon redistribution temperature for a holding time; and(e) cooling the steel sheet to room temperature.

2. The method of claim 1, the step of stabilizing austenite being performed while the steel sheet is in an uncoated condition.

3. The method of claims 1 or 2, the step of cooling the steel sheet to room temperature being performed while the steel sheet is in an uncoated condition.

4. The method of any of claims 1 through 3, wherein the T2 is below the carbon redistribution temperature.

5. The method of any of claims 1 through 4, the step of re-heating the steel sheet to the carbon redistribution temperature being performed after the step of cooling the steel sheet to T2.

6. The method of any of claims 1 through 5, wherein the carbon redistribution temperature is below the Ai temperature of the steel sheet.

7. The method of any of claim 1 through 6, wherein the portioning temperature is between 506 °C and 624 °C.

8. The apparatus of any of claims 1 through 7, the steel sheet having an elongation of at least 13 % after the step of cooling the steel sheet to room temperature.

9. The apparatus of any of claims 1 through 7, the steel sheet having a tensile strength of at least 1180 MPa and an elongation of at least 13 % after the step of cooling the steel sheet to room temperature.

10. The method of any of claims 1 through 9, the steel sheet comprising the following elements by weight percent:0.12% or more carbon;0.5% or more silicon, aluminum, or a combination thereof;1.5% or more manganese; and the balance being iron and other incidental impurities.

11. The method of any of claims 1 through 9, the steel sheet comprising the following elements by weight percent:0.15 to 0.25% carbon;0.5 to 2.0% silicon, aluminum, or a combination thereof;1.5 to 3.0% manganese; and the balance being iron and other incidental impurities.

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