PROCESS FOR MANUFACTURING HIGH STRENGTH STEEL

A method of making high strength steel sheet with a tensile strength of 800 to 1000 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.

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Description
CROSS-REFERENCE TO RELATED APPLICATION

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

BACKGROUND

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.

In some processes, the process of manufacturing high strength steels with good local formability, for example, bending and hole expansion in the 800 MPa and 1000 MPa tensile strength class is produced without the need for cold rolling.

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

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.

A method of making high strength steel sheet having a tensile strength of approximately 800 MPa 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.

A method of making high strength steel sheet a tensile strength of approximately 1000 MPa 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

FIG. 1 provides a graph showing modeling prior methods of cooling at various positions in a coil.

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

FIG. 3 provides a graph showing hole expansion as a function of tensile strength for the experimental steels after applying different annealing cycles.

FIG. 4 provides graphs showing the aging response via hardness testing to determine if there was a match to P* modeling, batch annealing paradigm.

FIG. 5 provides graphs showing the aging treatments conducted to determine sensitivity to annealing.

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

FIG. 7 provides a graph showing the batch annealing simulations to determine sensitivity to annealing temperature.

FIG. 8 shows graphs of aging response based on batch annealing.

FIG. 9 shows a graph the aging study results conducted batch annealing simulations with hot spot and cold spot.

FIG. 10 shows graphs of annealing screening to see sensitivity to batch annealing temperatures.

FIG. 11 provides a graph of the annealing simulation with hot spot and cold spot cycle.

FIG. 12 shows graphs of lower anneal temperatures.

 DETAILED DESCRIPTION

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.

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.

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).

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.

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.

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.

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.

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.

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 Ac1. 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 Ac1 temperature while achieving the desired properties.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

In some embodiments, the steel composition includes a tensile strength of approximately 800 MPa, a very economical steel composition would be approximately (all values in weight percent): 0.06C-1.0Mn-0.1Si-0.03Ti-0.0020B; with no additional intentional additions. Using a similar alloy design for nominally 1000 MPa tensile strength, the composition would be approximately (all values in weight percent): 0.06C-1.0Mn-0.1Si-0.03Ti-0.0020B; with no additional intentional additions. Alternative designs without boron additions can be considered. For example, 800 MPa tensile strength steel would be expected with a composition of approximately (all values in weight percent): 0.06C-1.5Mn-0.1Si, with no additional intentional additions. To reach the 1000 MPa 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.

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.

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.

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).

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

TABLE 1 Standard Error Term Coefficient of the Coefficient T-Value P-Value VIF Constant 322.2 80.1 4.02 0 C 3688 665 5.55 0 2.35 B 82830 16566 5.00 0 2.48 Mn 130.1 38.9 3.35 0.002 1.87 Mo 256 126 2.03 0.048 3.40 Cr 118.7 67.7 1.75 0.086 3.90 Cb −435 320 −1.36 0.180 2.38 Si 64.9 59.2 1.10 0.278 2.90 Cu 158 492 0.32 0.749 74.05 Ni −192 850 −0.23 0.822 64.92 V −14 443 −0.03 0.975 1.68 Ti 1 208 0 0.997 2.27

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.

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 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 8774A HIC Steel 0.046 1.07 0.009 0.0031 0.20 0.30 0.20 0.50 0.01 0.005 0.015 0.03 0.0051 0.030 0.0002 0.151 8774B Development: 0.047 1.08 0.008 0.0026 0.20 0.30 0.20 0.51 0.01 0.005 0.015 0.04 0.0052 0.059 0.0002 0.153 8774C Based on 0.047 1.08 0.008 0.0026 0.20 0.30 0.20 0.51 0.01 0.005 0.015 0.04 0.0049 0.090 0.0002 0.153 8775B lower Mn 0.043 0.80 0.009 0.0030 0.19 0.02 0.01 0.24 0.01 0.005 0.014 0.03 0.0048 0.061 0.0002 0.105 8775C 0.046 0.80 0.009 0.0023 0.19 0.02 0.01 0.48 0.01 0.005 0.014 0.03 0.0048 0.062 0.0002 0.120

TABLE 3 Uni. Total Hole Thickness YS UTS Elong. Elong. Expansion* Heat (in.) (MPa) (MPa) (%) (%) (%) 8774A 0.250 502 684 10.0 28.4 69 8774B 0.250 488 685 10.1 30.7 69 8774C 0.250 541 730  9.5 28.3 52 8775B 0.250 455 631 12.7 30.9 94 8775C 0.250 482 641 11.7 32.5 79 *plates were ground to 5 mm thick prior to hole expansion testing.

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 Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm Stock slab .064 1.82 .013 .0032 .06 .01 .04 .03 .00 .020 .112 .02 .0047 .063 .0001 .162 from HSMM Studies

TABLE 5 Uni. Thickness (in.) YS (MPa) UTS (MPa) Elong. (%) Total Elong. (%) 0.250 577 784 7.6 23.2 No hole expansion testing conducted.

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 48 hours, 3600 s to 172800 s. Hardness tests conducted using HRA scale, converted to HRC.

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

TABLE 6 Heat Note C Mn p S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 8709 USSC 0.048  1.65  0.010 0.0050 0.28  0.30  0.15  0.30  0.20 0.02  0.020  0.034 0.0071 0.085  0.0003 0.189 8710 X70 0.044  1.68  0.010 0.0043 0.29  0.30  0.15  0.45  0.01 0.02  0.021  0.036 0.0061 0.084  0.0002 0.182 8711 Lab 0.050  1.64  0.010 0.0045 0.29  0.28  0.15  0.45  0.15 0.02  0.021  0.030 0.0071 0.089  0.0002 0.194 8712 Heats 0.035  1.93  0.010 0.0043 0.30  0.31  0.15  0.46  0.01 0.02  0.022  0.030 0.0054 0.087  0.0001 0.185 8713 0.047  1.91  0.010 0.0045 0.30  0.30  0.15  0.45  0.15 0.02  0.020  0.042 0.0070 0.085  0.0003 0.206 8714 0.048  1.88  0.010 0.0043 0.30  0.30  0.15  0.44  0.01 0.02  0.026  0.032 0.0070 0.086  0.0016 0.202 9183 LTO 0.093 1.412 0.0131 0.0042 0.394 0.031 0.011 0.0637 0.07 0.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate B X65 9183 Lab 0.093 1.412 0.0131 0.0042 0.394 0.031 0.011 0.0637 0.07 0.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate D Heats 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate B 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate C 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate D

TABLE 7 Uni. Total Hole Thickness YS UTS Elong. Elong. Expansion* Heat (in.) (MPa) (MPa) (%) (%) (%) 8709 0.25 697 861 6.0 21.9 54 8710 0.25 613 793 6.7 23.6 67 8711 0.25 674 861 6.8 22.8 46 8712 0.25 622 798 5.5 14.3 79 8713 0.25 670 849 6.2 60 8714 0.25 863 1000  3.8 16.8 46 9183 Plate B 0.25 577 851 10.3 23.8 25 9183 Plate D 0.178  568 859 7.9 14.3 34 9184 Plate B 0.25 726 882 3.6 17.7 42 9184 Plate C 0.17 524 792 9.0 16.2 42 9184 Plate D 0.18 540 798 9.6 20.8 43 *0.250″ thick plates ground to 5 mm prior to hole expansion testing.

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

TABLE 8 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 8709 USSC 0.048 1.65  0.010  0.0050 0.28  0.30  0.15  0.30  0.20  0.02  0.020  0.034 0.0071 0.085  0.0003 0.189 8710 X70 0.044 1.68  0.010  0.0043 0.29  0.30  0.15  0.45  0.01  0.02  0.021  0.036 0.0061 0.084  0.0002 0.182 8711 Lab 0.050 1.64  0.010  0.0045 0.29  0.28  0.15  0.45  0.15  0.02  0.021  0.030 0.0071 0.089  0.0002 0.194 8712 Heats 0.035 1.93  0.010  0.0043 0.30  0.31  0.15  0.46  0.01  0.02  0.022  0.030 0.0054 0.087  0.0001 0.185 8713 0.047 1.91  0.010  0.0045 0.30  0.30  0.15  0.45  0.15  0.02  0.020  0.042 0.0070 0.085  0.0003 0.206 8714 0.048 1.88  0.010  0.0043 0.30  0.30  0.15  0.44  0.01  0.02  0.026  0.032 0.0070 0.086  0.0016 0.202 9183 LTO 0.093 1.412 0.0131 0.0042 0.394 0.031 0.011 0.0637 0.07  0.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate B X65 9183 Lab 0.093 1.412 0.0131 0.0042 0.394 0.031 0.011 0.0637 0.07  0.0552 0.0072 0.036 0.0082 0.0319 0.19  Plate D Heats 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate B 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate C 9184 0.082 1.362 0.0095 0.0021 0.405 0.03 0.01 0.032 0.081 0.0647 0.0006 0.028 0.0072 0.020 0.18  Plate D

TABLE 9 YS UTS Uni. Total Heat Thickness (in.) (MPa) (MPa) Elong. (%) Elong. (%) 9081a 0.268 620 863 7.6 19.4 9081b 0.265 643 879 7.0 22.2 9082a 0.259 633 816 6.1 20.1 9082b 0.257 809 978 3.7 14.9 9083a 0.263 581 789 7.7 24.2 9083b 0.261 610 805 6.8 18.6 9084a 0.261 567 792 7.1 22.9

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

TABLE 10 Uni. Hole Thickness YS UTS Elong. Total Elong. Expansion Heat (in.) (MPa) (MPa) (%) (%) (%) 9081a 0.197 715 908 5.5 19.8 46 9081b 0.197 606 823 7.1 19.9 50 9082a 0.198 717 896 5.0 16.8 66 9082b 0.202 698 872 5.3 17.9 67 9083a 0.200 701 880 5.4 17.2 58 9083b 0.198 647 828 6.7 19.8 66 9084a 56

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.

Table 11 illustrates hole expansion after low temperature temper treatments.

TABLE 11 Hole Expansion (%) Heat Before tempering Hot Spot, 272° F. Cold Spot, 185° F. 9081a 46 41 40 9081b 50 48 54 9082a 66 68 69 9082b 67 78 68 9083a 58 60 66 9083b 66 56 58 9084a 56 60 53

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

TABLE 12 Heat Note Alloy C Mn P S Si Cu Ni Cr 8976 Existing FW11 0.062 1.648 0.009 0.0033 0.181 0.017 0.099 0.2 8977 Line Pipe FM25 0.054 1.52 0.015 0,0037 0.233 0.016 0.139 0.202 9026 Grades XM02 0.053 1.51 0.011 0.0025 0.198 0.021 0.02 0.03 9027 FM22 0.055 1.507 0.012 0.0044 0.21 0.02 0.019 0.19 9028 FW18 0.053 1.494 0.011 0.0043 0.197 0.099 0.201 0.25 Heat Note Mo V Ti Al N Cb B Pcm 8976 Existing 0.141 0.039 0.012 0.031 0.0059 0.051 0.1762 8977 Line Pipe 0.149 0.0025 0.0118 0.033 0.0055 0.075 0.1612 9026 Grades 0.011 0.0021 0.0149 0.033 0.0048 0.0861 0.1389 9027 0.013 0.0023 0.0166 0.031 0.0044 0.0837 0.1493 9028 0.151 0.04 0.016 0.03 0.005 0.05 0.1691

TABLE 13 Finishing Uni. Total Hole Temp. Thickness YS UTS Elong. Elong. Expansion Heat Note (º F.) (in.) (MPa) (MPa) (%) (%) (%) 8976 Plate A 1515 0.172 573 797 8.8 22.3 57 Plate B 1605 0.178 597 813 8.2 20.7 55 8977 Plate A 1525 0.172 562 767 9.5 22.8 60 Plate B 1600 0.177 572 770 8.8 22.9 56 9026 Plate A 1520 0.170 534 715 11.0 22.9 68 Plate B 1610 0.176 541 713 10.7 24.9 77 9027 Plate A 1520 0.170 563 728 10.8 26.1 67 Plate B 1625 0.175 543 734 10.3 26.0 57 9028 Plate A 1525 0.169 578 782 8.7 20.8 72 Plate B 1595 0.175 590 785 8.9 23.3 65

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

TABLE 14 Heat Note C Mn P S S Cu Ni Cr Mo V Ti Al N Cb B Pcm 8709 USSC 0.048 1.645 0.010 0.0050 0.282 0.296 0.146 0.299 0.196 0.0220 0.020 0.034 0.0071 0.085 0.0003 0.189 8710 X70 Trials 0.044 1.677 0.010 0.0043 0.293 0.296 0.148 0.451 0.014 0.0220 0.021 0.036 0.0061 0.084 0.0002 0.182 8711 0.050 1.644 0.010 0.0045 0.292 0.278 0.147 0.453 0.146 0.0210 0.021 0.030 0.0071 0.089 0.0002 0.194 8712 0.035 1.930 0.010 0.0043 0.295 0.308 0.145 0.455 0.011 0.0230 0.022 0.030 0.0054 0.087 0.0001 0.185 8713 0.047 1.910 0.010 0.0045 0.300 0.299 0.150 0.446 0.152 0.0220 0.020 0.042 0.0070 0.085 0.0003 0.206 8714 0.048 1.884 0.010 0.0043 0.304 0.299 0.150 0.444 0.011 0.0200 0.026 0.032 0.0070 0.086 0.0016 0.202 8681A Lab HTP- 0.037 1.582 0.011 0.0056 0.195 0.030 0.024 0.283 0.016 0.0020 0.015 0.028 0.0066 0.087  0.000 0.140 8681B Type Heats, 0.057 1.564 0.011 0.0054 0.194 0.030 0.024 0.282 0.016 0.0020 0.015 0.026 0.0065 0.085 0.0001 0.159 8682A C-Cr Study 0.032 1.614 0.012 0.0041 0.202 0.031 0.008 0.501 0.016 0.0030 0.015 0.031 0.0050 0.088 0.0001 0.148 8682B 0.056 1.643 0.010 0.0040 0.209 0.031 0.008 0.503 0.015 0.0030 0.016 0.028 0.0046 0.090 0.0001 0.174 8683A 0.027 0.572 0.011 0.0050 0.199 0.031 0.008 0.705 0.013 0.0030 0.015 0.029 0.0057 0.090 0.0001 0.151 8683B 0.050 1.561 0.010 0.0050 0.198 0.030 0.008 0.700 0.013 0.0030 0.015 0.026 0.0057 0.088 0.0001 0.173

TABLE 15 Uni. Total Hole Thickness YS UTS Elong. Elong. Expansion Heat (in.) (MPa) (MPa) (%) (%) (%) 8709 0.173 626 819 7.2 19.3 42 8710 0.171 618 799 8.1 22.1 52 8711 0.171 643 848 7.5 19.8 40 8712 0.170 611 785 7.6 21.6 61 8713 0.169 661 843 7.0 19.4 52 8714* 0.170 573 758 7.4 19.5 49 8681A** 0.174 568 703 9.5 23.9 63 8681B 0.172 557 742 10.1 24.6 49 8682A 0.170 567 701 8.3 22.2 78 8682B 0.170 580 778 9.1 23.6 55 8683A 0.169 555 701 8.1 20.4 71 8683B 0.167 592 784 8.3 21.5 49 *Not properly direct quenched, speed under sprays too high. **Delay during rolling resulted in ultra-low finishing temperature.

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 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 9304 FM37 0.058 1.548 0.012  0.0034 0.245 0.02  0.01  0.033 0.217 0.0016 0.023 0.031 0.0067 0.09  0.002  0.171 Base 9305 Low N 0.062 1.617 0.011  0.0034 0.248 0.02  0.011 0.034 0.22 0.0016 0.024 0.033 0.0035 0.092  0.0019 0.178 FM37 9306 FM 37, 0.059 1.58 0.011  0.0036 0.241 0.019 0.01  0.033 0.218 0.0015 0.024 0.032 0.0037 0.045  0.0018 0.172 low N, Nb 9307 Mesplont 0.063 1.585 0.012  0.0031 0.253 0.02  0.01  0.033 0.149 0.0015 0.023 0.031 0.0036 0.045  0.0019 0.173 low N 9308 Babbit 0.04  1.779 0.0099 0.0037 0.247 0.019 0.011 0.034 0.294 0.0018 0.023 0.031 0.0067 0.06  0.002  0.170 1992 9309 Nunakawa 0.11  1.604 0.011  0.0029 0.519 0.02  0.01  0.486 0.011 0.002  0.071 0.032 0.0034 0.0022 0.0004 0.236 1985

TABLE 17 Uni. Total Hole Finishing Thickness YS UTS Elong. Elong. Expansion Heat Note Temp. (º F.) (in.) (MPa) (MPa) (%) (%) (%) 9304 Plate A 1545 0.185 714 901 6.7 15.5 40 Plate H 1650 0.184 816 995 5.0 11.1 34 9305 Plate A 1555 0.182 663 878 6.7 14.8 41 Plate H 1665 0.184 779 1013 5.5 12.1 37 9306 Plate A 1520 0.182 746 911 3.6 10.8 35 Plate H 1635 0.181 1023 4.5 11.0 38 9307 Plate A 1555 0.179 787 945 4.6 11.4 55 Plate H 1640 0.180 850 1022 5.2 12.6 35 9308 Plate A 1540 0.180 756 898 4.5 11.3 44 Plate H. 1650 0.177 861 976 4.7 11.2 46 9309 Plate A 1530 0.178 796 1068 5.5 10.8 24 Plate H 1625 0.175 862 1162 5.1 11.8 30

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.

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.

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 Alloy Condition YS UTS TE HE 9304 DQ 714 901 15.5 40 Cold Spot 803 864 35 Hot Spot 737 786 39 9305 DQ 663 878 14.8 41 Cold Spot 833 894 35 Hot Spot 785 803 40 9306 DQ 746 911 10.8 35 Cold Spot 832 879 36 Hot Spot 769 804 35 9307 DQ 787 945 11.4 55 Cold Spot 823 866 38 Hot Spot 737 780 42 9308 DQ 756 898 11.3 44 Cold Spot 823 871 42 Hot Spot 780 810 39 9309 DQ 796 1068 10.8 24 Cold Spot 899 960 32 Hot Spot 751 801 33

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 Heat Note C Mn P S Si Cu Ni Cr Mo V Ti Al N Cb B Pcm 9324 FM13 0.053 1.819 0.012 0.0024 0.117 0.022 0.01  0.044 0.005  0.003 0.109  0.031 0.0068 0.034  0.0004 0.154 9325 FM13 + Si 0.055 1.827 0.011 0.0023 0.52  0.021 0.01  0.044 0.005  0.003 0.109  0.034 0.0071 0.034  0.0004 0.170 9326 FM13 + 0.055 1.792 0.011 0.0022 0.52  0.021 0.011 0.042 0.0056 0.003 0.111  0.032 0.0065 0.053  0.0004 0.168 Si + Nb 9327 FM13 + Si + 0.056 1.788 0.012 0.002 0.513 0.021 0.01  0.042 0.0054 0.004 0.175  0.034 0.0064 0.053  0.0004 0.169 Nb + Ti 9328 KSL780R + 0.084 1.793 0.012 0.0021 0.508 0.021 0.011 0.042 0.0055 0.004 0.175  0.033 0.0065 0.053  0.0004 0.197 Mn + Si 9329 KY06 0.085 1.855 0.012 0.0021 0.873 0.021 0.01  0.042 0.0051 0.002 0.0016 0.034 0.0067 0.0044 0.0004 0.213 9330 KT24 0.075 1.899 0.012 0.002  0.101 0.02  0.011 0.198 0.171  0.002 0.0008 0.03  0.0067 0.0038 0.0004 0.198

TABLE 20 Uni. Total Hole Finishing Thickness YS UTS Elong. Elong. Expansion Heat Note Temp. (º F.) (in.) (MPa) (MPa) (%) (%) (%) 9324 Plate C 1520 0.185 600 789 7.8 17.8 61 Plate J 1630 615 788 7.6 14.6 47 9325 Plate C 1505 622 832 7.9 17.4 54 Plate I 1630 639 834 6.9 15.2 42 9326 Plate C 1505 618 828 8.0 18.1 52 Plate H 1630 562 828 6.0 14.7 41 9327 Plate C 1515 639 834 8.2 17.1 52 Plate H 1650 482 835 6.5 16.2 47 9328 Plate C 1580 635 886 8.6 16.9 34 Plate H 1635 615 868 7.0 14.3 37 9329 Plate C 1480 512 866 11.5 19.9 19 Plate H 1615 591 973 7.4 15.0 29 9330 Plate C 1615 756 975 3.6 10.2 46 Plate H 1750 846 1037 4.3 11.5 42

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 Alloy Condition YS (MPa) UTS (MPa) TE (%) HE (%) 24 DQ 600 789 17.8 61 CS 783 838 21.1 42 HS 747 794 21.0 58 25 DQ 622 832 17.4 54 CS 815 876 20.9 37 HS 773 820 23.2 38 26 DQ 618 828 18.1 52 CS 819 875 20.7 37 HS 765 816 23.2 37 27 DQ 639 834 17.1 52 CS 852 914 21.7 40 HS 821 862 22.7 48 28 DQ 635 886 16.9 34 CS 848 911 20.1 40 HS 793 846 20.2 42 29 DQ 512 866 19.9 19 700CS 593 758 20.8 42 800HS 579 705 23.3 47 650CS 648 840 19.5 80 750HS 639 783 22.5 65 30 DQ 756 975 10.2 46 700CS 725 793 12.9 69 800HS 710 779 16.1 72 650CS 803 911 14.4 93 750HS 842 904 16.4 86

FIG. 12 shows data for lower anneal temperatures in Heat 30, 900° F. for 24 hours results in approximately 755 MPa. 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 Heat Note C Mn P S Si Cu Ni Cr 9420A Low Ti, 0.0676 1.93735 0.01192 0.00153 0.49234 0.02033 0.01034 0.04169 Mid N 9420B Mid Ti 0.0672 1.94201 0.01166 0.0015 0.49923 0.02077 0.0103 0.04158 Mid N 9420C High Ti, 0.0682 1.9289 0.01144 0.00154 0.49305 0.02023 0.01018 0.04111 Mid N 9435A Low Ti 0.0601 1.86559 0.01151 0.0014 0.47612 0.01718 0.00998 0.04111 Low N 9435B Mid Ti, 0.0603 1.86451 0.01147 0.00349 0.47401 0.01716 0.00997 0.04111 Low N 9435C High Ti, 0.0587 1.87687 0.01097 0.00136 0.48274 0.01764 0.00945 0.04096 Low N Heat Note Mo V Ti Al N Cb B Pcm 9420A Low Ti, 0.0055 0.00991 0.07823 0.04935 0.0066 0.03283 0.00028 0.187 Mid N 9420B Mid Ti 0.0056 0.01044 0.10243 0.05149 0.00643 0.0328 0.0003 0.187 Mid N 9420C High Ti, 0.00532 0.01051 0.12744 0.05083 0.0059 0.0318 0.00028 0.187 Mid N 9435A Low Ti 0.00509 0.00906 0.07126 0.03824 0.00425 0.03242 0.00026 0.175 Low N 9435B Mid Ti, 0.00517 0.00937 0.09294 0.0376 0.00449 0.03256 0.00027 0.175 Low N 9435C High Ti, 0.00484 0.00993 0.1167 0.03667 0.00443 0.03167 0.00026 0.174 Low N

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.

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.

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.

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.

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.

Claims

1. 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.

2. The method of making high strength steel according to claim 1, wherein the temperature Ar3 is a temperature greater than the austenite-to-ferrite transformation temperature.

3. The method of making high strength steel according to claim 1, further comprising cooling the steel sheet to an acicular ferrite structure.

4. The method of making high strength steel according to claim 1, further comprising cooling the steel sheet to a bainitic structure.

5. The method of making high strength steel according to claim 1, further comprising the application of a secondary treatment to the steel sheet to promote precipitation reactions for strength preservation or an increase in strength.

6. The method of making high strength steel according to claim 5, further comprising reheating the steel coil to a temperature below Ac1.

7. The method of making high strength steel according to claim 6, wherein the temperature Ac1 is a temperature above 500° C. and below the ferrite-to-austenite phase transformation temperature.

8. The method of making high strength steel according to claim 6, wherein the temperature depends on a time duration anticipated for a process employed.

9. The method of making high strength steel according to claim 6, further comprising continuous annealing of the steel sheet to achieve reduced heating times.

10. The method of making high strength steel according to claim 9, wherein the reduced duration of the heating time during the continuous annealing allows for the steel sheet to approach a temperature Ac1 temperature while achieving the desired properties.

11. A method of making high strength steel sheet having a tensile strength of approximately 800 MPa 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;
than 400° C.; and
hot rolling the slab to final desired thickness;
cooling the steel sheet at a rate of 50° C. per second to a temperature less winding the steel sheet into a coil.

12. The method of making high strength steel according to claim 11, further comprising cooling the steel sheet to an acicular ferrite.

13. The method of making high strength steel according to claim 11, further comprising a applying a secondary treatment to the steel sheet to promote precipitation reactions for strength preservation or an increase in strength.

14. The method of making high strength steel according to claim 13, further comprising reheating the steel coil to a temperature below Act.

15. The method of making high strength steel according to claim 14, wherein the temperature Ac1 is a temperature above 500° C. and below the ferrite-to-austenite phase transformation temperature.

16. The method of making high strength steel according to claim 11, wherein the temperature depends on a time duration anticipated for a process employed.

17. The method of making high strength steel according to claim 11, further comprising continuous annealing of the steel sheet to achieve reduced heating times.

18. The method of making high strength steel according to claim 17, wherein the reduced duration of the heating time during the continuous annealing allows for the steel sheet to approach a temperature Ac1 temperature while achieving the desired properties.

19. A method of making high strength steel sheet a tensile strength of approximately 1000 MPa 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:

slab, above Ar3;
than 400° C.; and
reheating a previously cast slab, or retaining the heat from a directly cast hot rolling the slab to final desired thickness;
cooling the steel sheet at a rate of 50° C. per second to a temperature less winding the steel sheet into a coil.
Patent History
Publication number: 20230272499
Type: Application
Filed: Nov 23, 2022
Publication Date: Aug 31, 2023
Inventor: Matthew J. Merwin (Sewickley, PA)
Application Number: 17/993,700
Classifications
International Classification: C21D 9/46 (20060101); C21D 8/02 (20060101); C21D 6/02 (20060101);