CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63/283,090, filed November 24, 2021, which is incorporated herein by reference.

BACKGROUND

[0002] Typically, a direct quenching approach is employed in plate products and hot strip mill products. Tempering is applied in direct quenching of the plate and aging, for example, precipitation strengthening, have been employed after direct quenching in laboratory settings when developing models of precipitation hardening kinetics. Most of the steel produced on hot- strip mills is produced using coiling temperatures exceeding 500°C. This condition restricts the strength attainable in low alloy steels, demands higher alloy content to achieve higher strength levels of interest, or requires additional processing and cost through off-line heat treatment.

[0003] In some processes, the process of manufacturing high strength steels with good local formability, for example, bending and hole expansion in the 800MPa and lOOOMPa tensile strength class is produced without the need for cold rolling.

[0004] The present disclosure includes a method of producing high strength steel directly on a hot-strip mill without further thermomechanical processing, for example, cold-rolling and annealing. In some embodiments, the process disclosed includes utilizing low coiling temperature, or “direct quenching,” in a hot strip mill to manufacture high strength steels. In some embodiments, the process described herein includes direct quenching, with reduced or eliminated subsequent thermal treatment, to achieve high strength steels having fine and tough microstructures, for example, acicular ferrite suitable for applications requiring high local formability. In some embodiments, high strength steel is produced, for example, bainite or martensite, directly after quenching. In some embodiments, the strength, ductility, or toughness balance may be modified by subsequent tempering operations, for example, through batch annealing, continuous annealing, or hot-dip coating lines. In some embodiments, steel having fine microstructures is produced while preserving precipitation strengthening elements in the dissolved state for subsequent aging treatment, similar to tempering, for example, through batch annealing, continuous annealing, or hot-dip coating lines. In some embodiments, an aging treatment will be utilized to produce a desired balance of strength, ductility, or toughness.

SUMMARY

[0005] A method of making high strength steel sheet with a tensile strength of 800 to 1100 MPa and a hole expansion ratio of at least 50%, comprising the steps of reheating a previously cast slab, or retaining the heat from a directly cast slab, above Ar3; hot rolling the slab to final desired thickness; cooling the steel sheet at a rate of 50°C per second to a temperature less than 400°C; and winding the steel sheet into a coil.

[0006] A method of making high strength steel sheet having a tensile strength of approximately 800MPa and a composition of 0.06 weight percent of Carbon, 1.0 weight percent of Mn and 0.1 weight percent of Si, 0.03 weight percent of Ti and 0.0020 weight percent of Boron, comprising the steps of: reheating a previously cast slab, or retaining the heat from a directly cast slab, above Ar3; hot rolling the slab to final desired thickness; cooling the steel sheet at a rate of 50°C per second to a temperature less than 400°C; and winding the steel sheet into a coil.

[0007] A method of making high strength steel sheet a tensile strength of approximately lOOOMPa and a composition 0.06 weight percent of C, 1.0 weight percent of Mn, 0.1 weight percent of Si, 0.03 weight percent of Ti and 0.0020 weight percent of B, comprising the steps of: reheating a previously cast slab, or retaining the heat from a directly cast slab, above Ar3; hot rolling the slab to final desired thickness; cooling the steel sheet at a rate of 50°C per second to a temperature less than 400°C; and winding the steel sheet into a coil.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 provides a graph showing modeling prior methods of cooling at various positions in a coil.

[0009] Fig. 2 provides a graph showing hole expansion as a function of tensile strength for the experimental steels without subsequent annealing.

[0010] Fig. 3 provides a graph showing hole expansion as a function of tensile strength for the experimental steels after applying different annealing cycles. [0011] Fig. 4 provides graphs showing the aging response via hardness testing to determine if there was a match to P* modeling, batch annealing paradigm.

[0012] Fig. 5 provides graphs showing the aging treatments conducted to determine sensitivity to annealing.

[0013] Fig. 6 provides a graph of low temperature aging treatment conducted to temper the microstructure with the goal of improving hole expansion.

[0014] Fig. 7 provides a graph showing the batch annealing simulations to determine sensitivity to annealing temperature.

[0015] Fig. 8 shows graphs of aging response based on batch annealing.

[0016] Fig. 9 shows a graph the aging study results conducted batch annealing simulations with hot spot and cold spot.

[0017] Fig. 10 shows graphs of annealing screening to see sensitivity to batch annealing temperatures.

[0018] Fig. 11 provides a graph of the annealing simulation with hot spot and cold spot cycle.

[0019] Fig. 12 shows graphs of lower anneal temperatures.

DETAILED DESCRIPTION

[0020] In some embodiments, the process of manufacturing high strength steels requires developing a ferritic microstructure that is substantially strengthened by precipitation hardening. The principal precipitation hardening prior processes are either titanium based, or vanadium based. These technologies employ common hot-strip mill (HSM) processing, with coiling temperatures of at least 600°C (1112°F). Free cooling of a hot coil, as is the conventional practice, inherently results in varying time-temperature history for the different positions in the coil. The extremities of the coil (edges, outer wraps in particular) cool more rapidly than the coil interior. Such variations can be estimated in the prior methods, as shown in Fig. 1. This example is based on initial coiling temperature of 1325°F, 30-inch coil inner diameter, 65.6-inch outer diameter, and 0.371-inch thickness. This has significant implications for the precipitation hardening reactions upon which this material design relies and induces undesirable mechanical property variability. [0021] In some prior methods, the addition of molybdenum may mitigate the variability for titanium carbide precipitates and in some examples vanadium-based precipitates. It is known that acicular ferrite microstructures offer combinations of strength and toughness. These microstructures are the underpinning of line pipe products. Local formability is effectively a measure of toughness and high strength steels as disclosed herein.

[0022] Acicular ferrite can be developed by quenching low carbon steels, and quenching strip to a low temperature before winding in the coiler mitigates the variability in post-coiling cooling. The present disclosure discusses direct quenching steel after hot strip mill processing. In some embodiments, direct quenching may preserve precipitation hardening species in a dissolved state (unprecipitated).

[0023] The primary purpose of a hot strip mill is to reheat thick steel slabs into thin sheets with varying thickness. The thick steel slab passes through several rolling mill stands that are driven by powerful motors. The rolled sheets then pass through coilers, thereafter these coils move on to the next process in the plant. From the startup to the end, the steel material undergoes several treatments through each stage that are the main features of a hot strip mill.

[0024] The disclosure includes a method of inducing precipitation strengthening reactions under controlled thermal conditions, such as batch annealing, continuous annealing, or adjusting properties with improved uniformity. Additionally, the disclosure includes data showing that annealing quench-and-tempered products are shown to achieve a combination of strength and toughness.

[0025] The disclosure is applicable to a broad range of steel hot rolling processes. In some embodiments, the steel is hot rolled while the steel is primarily in its austenitic state and that the rolled strip is subsequently cooled to a temperature low enough, and at a sufficient rate, to achieve acicular ferrite or bainitic structures. In some embodiments, a precursor for final hot rolling can be produced in tandem with final rolling sequence (direct casting and rolling technologies with or without intermediate reheating in advance of the final rolling) or can be produced in an independent facility with the slabs or transfer bars reheated for processing in a hot strip mill.

[0026] In some embodiments, the temperature of the final rolling step should be such that the steel is in the austenitic start. This causes the last rolling pass to be completed at a temperature greater than the austenite-to-ferrite transformation temperature, also known as temperature Ar3.

[0027] In some embodiments, upon completion of the final rolling step, the steel is rapidly cooled to achieve the desired acicular ferrite and/or bainite microstructure (depending on strength class). The rapid cooling continues until the steel is less than 400°C. The steel strip is then wound into a coil. The rate of rapid cooling should be greater than 50°C/second.

[0028] In some embodiments, a secondary treatment process is applied to the steel strip to promote precipitation reactions for strength preservation or increase. In this embodiment, the hot rolled strip should be reheated to a temperature above 500°C and below the ferrite-to- austenite phase transformation temperature, for example, a temperature Acl. The appropriate temperature depends on the time duration anticipated for the process employed. For example, continuous annealing of the steel strip will result in shorter heating times than batch annealing of coils. The shorter duration of continuous annealing operations (for hot-dip coated or uncoated strip) allows the strip to approach the Acl temperature while achieving the desired properties.

[0029] In some embodiments, the steel composition includes carbon. In some embodiments, the steel composition includes carbon in a range of approximately 0.03 to 0.07 weight percent. Carbon levels below approximately 0.03 weight percent will risk the ability to achieve the desired strength level. Higher levels of carbon risk low hole expansion performance and can make the steel prone to the adverse peritectic reaction during continuous casting.

[0030] In some embodiments, the steel composition includes manganese. In some embodiments, the steel composition includes manganese in a range of approximately at most 2.0 weight percent. Manganese is one of the more economical strengthening elements that also sequesters sulfur prevent the formation of damaging iron sulfide. A minimum practical level for higher strength steels is approximately 0.5 weight percent, and economics often dictate higher levels to preclude the use of more costly elements. Elevated levels of manganese lead to chemical segregation patterns that can be damaging to performance.

[0031] In some embodiments, the steel composition includes molybdenum. In some embodiments, the steel composition includes molybdenum in a range of at most approximately 0.5 weight percent. Molybdenum is a potent strengthening element, but often expensive to employ. It may be chosen to limit the maximum manganese content employed or to add thermal stability to precipitation hardening species. If not technically required, a residual level would be employed for economic reasons.

[0032] In some embodiments, the steel composition includes chromium. In some embodiments, the steel composition includes chromium less than approximately 2.0 weight percent. Chromium is a potent strengthening element. Economics often suggest its use after manganese but before molybdenum. It can be used to limit the maximum manganese employed. For this technology, additions less than approximately 2.0 weight percent are appropriate.

[0033] In some embodiments, the steel composition includes silicon. In some embodiments, the steel composition includes silicon less than up to approximately 1 weight percent silicon is an efficient strengthening element. Higher levels of silicon can induce surface features on the hot strip that may be objectionable depending on the application. Higher silicon levels can also interfere with galvanizing operations.

[0034] In some embodiments, the steel composition includes boron. In some embodiments, the steel composition includes boron less than up to approximately in the range of 10 to 30 parts per million. The strengthening effect can only be assured with use of a nitrogen sequestering element, most typically titanium. The sequestering of nitrogen results in coarse nitride particles that can be damaging to the toughness of the steel. As such, the use of boron alloying may not be appropriate for the most toughness critical applications.

[0035] In some embodiments, the steel composition includes titanium. In some embodiments, the steel composition includes titanium as a potent strengthening element. In the context of this disclosure, titanium is principally utilized as a nitrogen sequestering element to facilitate the use of boron, or as a precipitation strengthener for secondary thermal operations. The appropriate level for use in nitrogen sequestration is at a level of 3.4 time the nitrogen content of the steel. A practical maximum addition for the precipitation strengthening consideration would be 0.2 weight percent.

[0036] In some embodiments, the steel composition includes vanadium. In some embodiments, the steel composition includes vanadium at approximately 0.2 weight percent. Vanadium can be a potent strengthening element. In the context of this disclosure, the use of vanadium is as a precipitation strengthener for secondary thermal operations.

[0037] In some embodiments, the steel composition includes copper. In some embodiments, the steel composition includes copper at approximately in the range of 0.3 to 0.5 weight percent where atmospheric corrosion resistance is desired. Copper is not considered a critical strengthening element. In the context of the disclosure, copper would only be employed when atmospheric weathering resistance is desired. Suitable mechanical properties can be achieved without the need for this costly alloying element. The use of copper must be judicious as it can result in low ductility during hot rolling operations (hot shortness). Depending on the hot rolling process employed, a concurrent nickel addition may be mandatory to mitigate the hot ductility reduction.

[0038] In some embodiments, the steel composition includes nickel. In some embodiments, the steel composition includes nickel at approximately the level of one-half the copper addition. This level has been found suitable for mitigating the low ductility at hot rolling temperatures. Nickel additions can be employed for strengthening, toughening, or to mitigate low ductility during hot rolling. In the context of the disclosure, nickel would only be employed as a companion to copper additions when atmospheric weathering resistance is desired. Suitable mechanical properties can be achieved without the need for this costly alloying element.

[0039] In some embodiments, the steel composition includes a tensile strength of approximately 800MPa, a very economical steel composition would be approximately (all values in weight percent): 0.06C-1.0Mn-0.1 Si-0.03Ti-0.0020B; with no additional intentional additions. Using a similar alloy design for nominally lOOOMPa tensile strength, the composition would be approximately (all values in weight percent): 0.06C-1.0Mn-0.1 Si-0.03Ti-0.0020B; with no additional intentional additions. Alternative designs without boron additions can be considered. For example, 800MPa tensile strength steel would be expected with a composition of approximately (all values in weight percent): 0.06C-1 ,5Mn-0.1 Si; with no additional intentional additions. To reach the lOOOMPa tensile strength level the manganese level would be increased to its practical maximum of 2.0 weight percent and chromium would be added at a level of 0.5 weight percent.

[0040] As described herein, direct-quenching is a first step of the heat treatment operation with subsequent tempering occurring in a different process step (batch annealing, continuous annealing). This approach does not rely on precipitation hardening reactions and is an alternative implementation of known quench-and-temper concepts.

[0041] Fig. 2 illustrates the combination of two key properties of primary interest: hole expansion and tensile strength. Fig. 2 illustrates a graph showing hole expansion as a function of tensile strength. The graphs show properties of the steel manufactured according to an embodiment of the process of the present disclosure in the as-direct-quenched condition and after annealing. The present disclosure is configured to produce steel having at least 800 MPa tensile strength, with hole expansion of at least 50%. The graphs shown in Fig. 2 show properties without subsequent annealing, the second graph shows properties after applying different annealing cycles. The graphs in Fig. 2 illustrate that there are various batches that produced good hole expansion at tensile strength greater than 800MPa.

[0042] The following examples are intended to illustrate various aspects of the present disclosure and are not intended to limit the scope of the disclosure. Many different steel alloys were considered. The strength of the direct-quenched product can be expected to vary as a function of composition. Table 1 shows a regression model for tensile strength as a function of composition. The data is sorted by ascending P-Value, placing the elements of most significance to the regression at the top of the list (higher P-Value means higher probability of random contribution).

[0043] Table 1 illustrates regression results of tensile strength vs composition for as- direct-quenched plates.

Table 1

[0044] The inclusion of C, B, Mn, Mo, and Cr are utilized to increase the hardenability of the steel. In some embodiments, the contribution of Cb, particularly as indicated by a negative coefficient, reflects this element’s contribution to grain size refinement and a negative contribution to hardenability. In some embodiments, Cu and Ni additions, while expected to contribute to hardenability, were not reported as reliable contributors (quite high P-Value). Similarly, in some embodiments, V and Ti had high P-Values. This condition is reasonable for V and Ti since their contributions in these steels is primarily through precipitation hardening, which is not expected to be active in the as-quenched condition. It is through subsequent aging treatments that V and Ti, as well as Cb, will contribute to strength preservation or increase.

[0045] Tables 2 and 3 illustrate data from Campaign 1. Plates were hot rolled and direct quenched to room temperature. Heat compositions (all values in weight percent).

Table 2

Table 3

* plates were ground to 5mm thick prior to hole expansion testing.

[0046] Tables 4 and 5 illustrate data from Campaign 2. Plate was hot rolled and direct quenched to room temperature. Subsequently did aging treatments to determine sensitivity to annealing.

Table 4

Table 5

No hole expansion testing conducted.

[0047] Fig. 4 provides graphs of an aging response via hardness testing to determine if there was a match to P* modeling. Batch annealing paradigm times were 1 hour through 48hours, 3600s to 172800s. Hardness tests conducted using HRA scale, converted to HRC.

[0048] Tables 6 and 7 illustrate data from Campaign 3. Plates were hot rolled and direct quenched.

Table 6

Table 7

* 0.250” thick plates ground to 5mm prior to hole expansion testing.

[0049] Tables 8 and 9 illustrate data from Campaign 4. Hot rolled to heavy gauge and initial testing conducted.

Table 8

Table 9

[0050] Table 10 provides data for plates that were ground to 5mm thick to facilitate hole expansion tests. Subsize longitudinal tensile specimens extracted from edges and tested.

Table 10

[0051] Fig. 5 shows graphs of an aging treatment conducted to determine sensitivity to annealing. All tests were conducted in salt pots and hardness measure by HRA. Converted to VHN to allow more direct estimation of tensile strength. Figs. 6 and 7 simulate batch annealing of hot spots and cold spots subjected to a low temperature aging treatment conducted to temper the microstructure with the goal of improving hole expansion.

[0052] Table 11 illustrates hole expansion after low temperature temper treatments. Table 11

[0053] Tables 12 and 13 illustrate data from Campaign 5. Plates hot rolled and direct quenched to room temperature.

Table 12

Table 13

[0054] Tables 14 and 15 illustrate data from Campaign 6. Plates were hot rolled and quenched to room temperature.

Table 14

Table 15

* Not properly direct quenched, speed under sprays too high.

** Delay during rolling resulted in ultra-low finishing temperature.

[0055] Tables 16 and 17 illustrate a direct quench portion of 780 development bainitic approach. Lab heats hot rolled with two different finishing temperatures and direct quenched to room temperature.

Table 16

Table 17

[0056] Fig. 8 provides graphs showing an aging response based on batch annealing paradigm. In this embodiment, batch annealing was conducted to determine sensitivity to the annealing temperature, for example, heat at 100°F/hr., hold 24 hours, furnace cool.

[0057] Fig. 9 shows results based on the aging study results, conducted batch annealing simulations with hot spot and cold spot. In some embodiments, the hot spot temperature was 1100°F, around peak aging with Mo steels, overaging without Mo. In some embodiments, the cold spot was 1000°F, below peak aging, if peak aging was 24 hours at 1000°F.

[0058] Table 18 shows results based on the aging study results, conducted batch annealing simulations with hot spot and cold spot, a hot spot of 1100°F was tested at approximately peak aging with Mo steels, overaging without Mo. In addition, a cold spot of 1000°F was tested. This is below peak aging if peak aging was 24 hours at 1000°F.

Table 18

[0059] Tables 19 and 20 show results of direct quench portion of HR780 development, hybrid of FM13 and KSL 780R. The data includes lab heats for CAL/HD grades. Plates were hot rolled with different finishing temperatures and direct quenched to room temperature. Table 19

Table 20

[0060] Fig. 10 shows graphs of an annealing screening to determine sensitivity to batch annealing temperatures of 900, 1000, 1100, 1200°F at 24 hours. Fig. 11 shows a batch annealing simulation with hot spot and cold spot cycle. Table 21 includes data from a batch annealing simulation with hot spot and cold spot cycle.

Table 21

[0061] Fig. 12 shows data for lower anneal temperatures in Heat 30, 900°F for 24 hours results in approximately 755MPa. Table 22 illustrates a direct quench portion of HR780 development including Ti with different N levels. These heats were for tuning FM13/FM44. Lab heats hot rolled and direct quenched to room temperature. Table 22

[0062] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard variation found in their respective testing measurements.

[0063] Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

[0064] In this application, the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise. In addition, in this application, the use of “or” means “and/or” unless specifically stated otherwise, even though “and/or” may be explicitly used in certain instances. In this application and the appended claims, the articles “a,” “an,” and “the” include plural referents unless expressly and unequivocally limited to one referent.

[0065] As used herein, “including,” “containing” and like terms are understood in the context of this application to be synonymous with “comprising” and are therefore open-ended and do not exclude the presence of additional undescribed or unrecited elements, materials, phases or method steps. As used herein, “consisting of’ is understood in the context of this application to exclude the presence of any unspecified element, material, phase, or method step. As used herein, “consisting essentially of’ is understood in the context of this application to include the specified elements, materials, phases, or method steps, where applicable, and to also include any unspecified elements, materials, phases, or method steps that do not materially affect the basic or novel characteristics of the disclosure.

[0066] Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.