ULTRA-HIGH STRENGTH WEATHERING STEEL PILES AND STRUCTURAL FOUNDATIONS
Disclosed herein is a light-gauge, ultra-high strength weathering steel pile for use as a steel foundation in structures such as, for example, solar arrangements. The light-gauge, ultra-high strength weathering steel pile comprises a thickness of 2.5 mm or less that has been cold roll formed into a steel pile having a plurality of sidewalls. The steel pile further comprises a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%.
This patent application claims priority to and benefit of U.S. Provisional Application No. 62/802,900, filed Feb. 8, 2019, U.S. Provisional Application No. 62/811,153, filed Feb. 27, 2019, U.S. Provisional Application No. 62/830,000, filed Apr. 5, 2019, U.S. Provisional Application No. 62/830,021, filed Apr. 5, 2019, and U.S. Provisional Application No. 62/902,825, filed Sep. 19, 2019, which are all incorporated herein by reference.
BACKGROUND AND SUMMARYThis invention relates to thin cast steel strips, methods for high friction rolling a thin cast steel strips, and steel products made therefrom and thereby.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated, internally cooled casting rolls so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal is poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal, supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
To obtain a desired thickness the thin steel strip may pass through a mill to hot roll the thin steel strip. While performing hot rolling, the thin steel strip is generally lubricated to reduce the roll bite friction, which in turn reduces the rolling load and roll wear, as well as providing a smoother surface finish. The lubrication is used to provide a low friction condition. A low friction condition is defined as one where the coefficient of friction (u) for the roll bite is less than 0.20. After hot rolling, the thin steel strip undergoes a cooling process. In a low friction condition, after undergoing a pickling or acid etching process to remove oxidation scale, large prior austenite grain boundary depressions have been observed on the hot rolled exterior surfaces of cooled thin steel strips. In particular, while the thin steel strips tested using dye penetrant techniques appeared defect free, after acid pickling of the same thin steel strips, the prior austenite grain boundaries are etched by the acid to form prior austenite grain boundary depressions. This etching may further cause a defect phenomenon to occur along the etched prior austenite grain boundaries and the resulting depressions. The resulting defects and separations, which are more generally referred to as separations, can extend at least 5 microns in depth, and in certain instances 5 to 10 microns in depth.
Also applicable to the present disclosure, weathering steels are typically high strength low alloy steels resistant to atmospheric corrosion. In the presence of moisture and air, low alloy steels oxidize at a rate that depends on the level of exposure to oxygen, moisture and atmospheric contaminants to the metal surface. When the steel oxidizes it can form an oxide layer commonly referred to as rust. As the oxidation process progresses, the oxide layer forms a barrier to the ingress of oxygen, moisture and contaminants, and the rate of rusting slows down. With weathering steel, the oxidation process is initiated in the same way, but the specific alloying elements in the steel produce a stable protective oxide layer that adheres to the base metal, and is much less porous than the oxide layer typically formed in a non-weathering steel. The result is a much lower corrosion rate than would be found on ordinary, non-weathering structural steel.
Weathering steels are defined in ASTM A606, Standard Specification for Steel, Sheet and Strip, High Strength, Low-Alloy, Hot Rolled and Cold Rolled with Improved Atmospheric Corrosion Resistance. Weathering steels are supplied in two types: Type 2, which contains at least 0.20% copper based on cast or heat analysis (0.18% minimum Cu for product check); and Type 4, which contains additional alloying elements to provide a corrosion index of at least 6.0 as calculated by ASTM G101, Standard Guide for Estimating the Atmospheric Corrosion Resistance of Low-Alloy Steels, and provides a level of corrosion resistance substantially better than that of carbon steels with or without copper addition.
Prior to the present invention, weathering steels were typically limited to yield strengths of less than 700 MPa and tensile strengths of less than 1000 MPa. Also, prior to the present invention, the strength properties of weathering steels typically were achieved by age hardening. U.S. Pat. No. 10,174,398, incorporated herein by reference, is an example of a weathering steel achieved by age hardening.
Due to the strength limitations and corrosion limitations, steels, such as G100 or Gr70 steels, have not been well suited for many products such as, for example, piles or steel foundations driven into the ground for use in solar arrangements and/or the highway industry such as, for example, support guardrails, signage, or the like. As used herein, a solar arrangement is a structure for supporting solar cells, such as on a solar farm of photovoltaic power stations designed for the supply of solar power for use in an electric grid or the like. The corrosive nature of ground water and the soil compositions required material thicknesses well in excess of 2.5 mm to maintain the integrity required for these structural members. Accordingly, hot-dipped galvanized steels were turned to for such uses. The hot-dipped galvanized steels are zinc coated to improve corrosion resistance of the underlying material properties. Accordingly, it has been the solar industry's convention to rely on piles designed from zinc-plated 50 ksi W6 or W8 I-beams for structural piles. The zinc coating, however, negatively reacts with ground water and soil compositions creating the potential for contaminating the same. The zinc coating also provides only a limited degree of protection. Once zinc oxidation deteriorates the zinc coating metal oxidation still sets in deteriorating the structural integrity of the underlying material and/or requiring increased material thicknesses to maintain the integrity required for these structural members.
Accordingly, the present disclosure sets out to provide a pile or steel foundation design produced from a light-gauge, ultra-high strength weathering steel that replaces the current material relied on for piles or steel foundations. Specifically, the present disclosure sets out to provide a light weight pile or steel foundation having shapes produced from a thin cast metal strip. The shapes of the present disclosure set out to increase the strength and durability of the pile or steel foundation to withstand deformation resulting from the force required to drive the structural members into the ground and/or to serve as structural members for above-ground exterior structures such as, for example, solar arrangements, guardrails, signage, or the like. In particular, the present disclosure sets out to provide a pile or steel foundation cold roll formed from a thin cast steel strip having a thickness of 2.5 mm or less, 2.0 mm or less, or 1.6 mm or less or a pile or steel foundation cast as a thin cast steel shape with a material thickness of 2.5 mm or less, 2.0 mm or less, or 1.6 mm or less. The pile or steel foundation is produced from a thin cast steel strip that has been cold roll formed using one or more roll stands. Additionally, a punch system, a CNC plasma system, and/or a roll system, or the like, may also be relied on to provide thru-holes, slots, and/or spot welds, as noted below. The present disclosure also sets out to provide a pile or steel foundation cold roll formed from a thin cast steel strip that does not require a separately applied protective coating such as, for example, a zinc coating as provided on hot-dipped galvanized structural members. As used herein, separately applied coatings are protective coatings that may be a surface protectant that is independent of the composition of the steel. Examples of such separately applied protective coatings include a zinc coating, a galvanized coating (e.g. a hot dipped galvanized coating), an aluminum-silicon corrosion resistant coating, or the like. More importantly, the piles or steel foundations of the present disclosure produce the corrosion resistance, as set forth below, without the aid of a separately applied coating. Inherently, by definition, the ultra-high strength weathering steel disclosed herein possesses the requisite corrosion resistance that hot-dipped galvanizing would otherwise be relied on for. Thereby, the weathering steel of the present disclosure would not require or possess a zinc coating, a hot-dipped galvanized coating, or the like nor would one be applied.
In one set of examples, the present disclosure sets out to provide a light-gauge, ultra-high strength weathering steel formed by shifting of the peritectic point away from the carbon region and/or increasing a transition temperature of the peritectic point of the composition. Specifically, shifting the peritectic point away from the carbon region and/or increasing a transition temperature of the peritectic point of the composition appears to inhibit defects and results in a high strength martensitic steel sheet that is defect free. In the present example, the addition of nickel is relied on for this wherein the addition of nickel must be sufficient enough to shift the ‘peritectic point’ away from the carbon region that would otherwise be present in the same composition without the addition of nickel. Also disclosed are products produced from an ultra-high strength weathering steel being of various shapes, as additionally disclosed herein, and having improved strength properties that were not previously available.
In another set of examples, the present disclosure sets out to eliminate the prior austenite grain boundary depressions but maintain a smear pattern. In the present set of examples, the thin cast steel strip undergoes a high friction rolling condition where grain boundary depressions form a smear pattern at, at least, the surface of the thin cast steel strip. Specifically, the present example sets out to form the smear pattern of the prior austenite grain boundary depressions upon eliminating the prior austenite grain boundary depressions from the surface and improving the formability of the steel strip or steel product. By improving formability of the steel strip products being of various shapes, as additionally disclosed herein, and having improved strength properties become available that were not previously available. The present example is not only applied with the above-mentioned ultra-high strength weathering steel but may additionally be applied with martensitic steels, other weathering steels, and/or steel strips or products which exhibit prior austenite grain boundary depressions.
Still yet, in another set of examples, the present disclosure sets out to eliminate grain boundary depressions and smear patterns formed therefrom. In the present set of examples, the thin cast steel strip undergoes surface homogenization, thereby, eliminating the smear pattern. As a result, the thin cast steel strip has a surface not only free of prior-austenite grain boundary depressions but additionally free of the smear pattern produced as a result of the high friction rolling condition, to provide, in some examples, a thin cast steel strip surface having a surface roughness (Ra) that is not more than 2.5 μm. The present examples are not only applied with the above-mentioned ultra-high strength weathering steel but may additionally be applied with martensitic steels, other weathering steels, and/or steel strips or products which exhibit prior austenite grain boundary depressions.
Ultra-High Strength Weathering Steel
First, presently disclosed is a light-gauge, ultra-high strength weathering steel sheet made by the steps comprising: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) solidifying at a heat flux greater than 10.0 MW/m2 into a steel sheet less than or equal to 2.5 mm in thickness and cooling the sheet in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled; and (c) rapidly cooling to form a steel sheet with a microstructure having by volume at least 75% martensite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%.
Here and elsewhere in this disclosure elongation means total elongation. “Rapidly cooling” means to cool at a rate of more than 100° C./s to between 100 and 200° C. Rapidly cooling the present compositions, with an addition of nickel, achieves up to more than 95% martensitic phase steel strip. In one example, rapidly cooling forms a steel sheet with a microstructure having by volume at least 95% martensite. The addition of nickel must be sufficient enough to shift the ‘peritectic point’ away from the carbon region that would otherwise be present in the same composition without the addition of nickel. Specifically, the inclusion of nickel in the composition is believed to contribute to the shifting of the peritectic point away from the carbon region and/or increases a transition temperature of the peritectic point of the composition, which appears to inhibit defects and results in a high strength martensitic steel sheet that is defect free. In one example, the light-gauge, ultra-high strength weathering steel sheet may also be hot rolled to between 15% and 50% reduction before rapidly cooling.
Carbon levels in the present sheet steel are preferably not below 0.20% in order to inhibit peritectic cracking of the steel sheet. The addition of nickel is provided to further inhibit peritectic cracking of the steel sheet, but does so independent of relying on the carbon composition alone. The impact of nickel on the corrosion index is reflected in the following equation for determining the corrosion index calculation: Cu*26.01+Ni*3.88+Cr*1.2+Si*1.49+P*17.28−Cu*Ni*7.29−Ni*P*9.1−Cu*Cu*33.39 (where each element is a by weight percentage).
The molten melt may be solidified at a heat flux greater than 10.0 MW/m2 into a steel sheet less than 2.5 mm in thickness, and the sheet may be cooled in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled. A non-oxidizing atmosphere is an atmosphere typically of an inert gas such as nitrogen or argon, or a mixture thereof, which contains less than about 5% oxygen by weight. In another example, the sheet may be cooled in a non-oxidizing atmosphere to below 1100° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled.
In some examples, the martensite in the steel sheet may form from an austenite grain size of greater than 100 μm. In other examples, the martensite in the steel sheet may form from an austenite grain size of greater than 150 μm.
The steel sheet is rapidly cooled to form a steel sheet with a microstructure having at least 75% martensite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. In other examples, the steel sheet is rapidly cooled to form a steel sheet with a microstructure having at least 75% martensite plus bainite. In one specific example, rapidly cooling forms a steel sheet with a microstructure having by volume at least 95% martensite plus bainite.
In some examples, the steel sheet may be hot rolled to between 15% and 35% reduction before rapidly cooling. In other examples, the steel sheet may be hot rolled to between 15% and 50% reduction before rapidly cooling.
The molten steel used to produce the ultra-high strength weathering steel sheet is silicon killed (i.e., silicon deoxidized) comprising between 0.10% and 0.50% by weight silicon. The steel sheet may further comprise by weight less than 0.008% aluminum or less than 0.006% aluminum. The molten melt may have a free oxygen content between 5 to 70 ppm or between 5 to 60 ppm. The steel sheet may have a total oxygen content greater than 50 ppm. The inclusions include MnOSiO2 typically with 50% less than 5 μm in size and have the potential to enhance microstructure evolution and, thus, the strip mechanical properties.
Also disclosed is a method of making a light-gauge, ultra-high strength weathering steel sheet comprising the steps of: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) forming the molten melt into a casting pool supported on casting surfaces of a pair of cooled casting rolls having a nip there between; (c) counter rotating the casting rolls and solidifying at a heat flux greater than 10.0 MW/m2 producing a steel sheet less than 2.5 mm in thickness and cooling the sheet in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled, and (d) rapidly cooling to form a steel sheet with a microstructure having at least 75% martensite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. In one specific example, rapidly cooling forms a steel sheet with a microstructure having by volume at least 95% martensite plus bainite. The sheet may be cooled in a non-oxidizing atmosphere to below 1100° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled. The steel sheet composition cannot be made with carbon levels below 0.20% because it is inoperative with peritectic cracking of the steel sheet. In one example, the light-gauge, ultra-high strength weathering steel sheet may be hot rolled to between 15% and 50% reduction before rapidly cooling.
Further, the method of making a light-gauge, ultra-high strength weathering steel sheet may comprise the step of tempering the steel sheet at a temperature between 150° C. and 250° C. for between 2 and 6 hours.
The molten melt may have a free oxygen content between 5 to 70 ppm or between 5 to 60 ppm. The steel sheet may have a total oxygen content greater than 50 ppm. The molten melt may be solidified at a heat flux greater than 10.0 MW/m2 into a steel sheet less than 2.5 mm in thickness, and cooled in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled. In another example, the sheet may be cooled in a non-oxidizing atmosphere to below 1100° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled.
In some embodiments, the martensite in the steel sheet may come from an austenite grain size of greater than 100 μm. In other embodiments, the martensite in the steel sheet may come from an austenite grain size of greater than 150 μm.
The method of making the light-gauge, ultra-high strength weathering steel sheet may further comprise hot rolling the steel sheet to between 15% and 35% reduction and, thereafter, rapidly cooling to form a steel sheet with a microstructure having at least 75% by volume martensite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. In some embodiments, the method of making light-gauge, ultra-high strength steel sheet may further comprise hot rolling the steel sheet to between 15% and 50% reduction and, thereafter, rapidly cooling to form a steel sheet with a microstructure having at least 75% by volume martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. Furthermore, the method of making hot rolled light-gauge, ultra-high strength steel sheet may comprise hot rolling the steel sheet to between 15% and 35% reduction and, thereafter, rapidly cooling to form a steel sheet with a microstructure having at least 75% by volume martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. In specific examples of the above, hot rolling the steel sheet and, thereafter, rapidly cooling forms a steel sheet with a microstructure having by volume at least 95% martensite plus bainite.
Also disclosed is a steel pile comprising a web and one or more flanges cold roll formed from a carbon alloy steel sheet having a composition comprising, by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum where the carbon alloy steel sheet has a microstructure having at least 75% by volume martensite or martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, an elongation of between 1% and 10%, and having a corrosion index of 6.0 or greater.
High Friction Rolled High Strength Weathering Steel
Second, in one set of examples, presently disclosed is a carbon alloy thin cast steel strip having an as cast thickness of less than or equal to 2.5 mm. These examples are not only applied with the above-mentioned ultra-high strength weathering steel but may additionally be applied with martensitic steels, other weathering steels, and/or steel strips or products which exhibit prior austenite grain boundary depressions. The carbon alloy thin cast steel strip may comprise, by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminium, and the remainder iron and impurities resulting from melting. After high friction hot rolling the thickness of the carbon alloy thin cast steel strip is reduced by 15% to 50% of the as cast thickness. The hot rolled steel strip comprises a pair of opposing high friction hot rolled surfaces primarily free, substantially free, or free of prior austenite grain boundary depressions and having a smear pattern. In some embodiments, the steel strip comprises a microstructure having by volume at least 75% martensite or at least 75% martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%. In some examples, the steel strip is a weathering steel with a corrosion index of 6.0 or greater.
In some examples, the pair of opposing high friction hot rolled surfaces are substantially free of prior austenite grain boundary depressions. In some examples, the pair of opposing high friction hot rolled surfaces are primarily free of prior austenite grain boundary depressions.
Also disclosed is a method of making hot rolled carbon alloy steel strip comprising by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and the remainder iron and impurities resulting from melting, the method comprising the steps of:
(a) preparing a molten steel melt;
(b) forming the melt into a casting pool supported on casting surfaces of a pair of cooled casting rolls having a nip there between;
(c) counter rotating the casting rolls and solidifying at a heat flux greater than 10.0 MW/m2 the molten melt into a steel strip of less than or equal to 2.5 mm in thickness delivered downwardly from the nip and cooling the strip in a non-oxidizing atmosphere to below 1080° C. and above the Ar3 temperature at a cooling rate greater than 15° C./s;
(d) high friction hot rolling the thin cast steel strip to a hot rolled thickness of between a 15% and 50% reduction of the as cast thickness producing a hot rolled steel strip primarily free, substantially free, or free of prior austenite grain boundary depressions and having a smear pattern.
The high friction hot rolled thin cast steel strip primarily free, substantially free, or free of prior-austenite grain boundary depressions and having a smear pattern may be a weathering steel with a corrosion index of 6.0 or greater. Also, the high friction hot rolled steel strip may comprise a microstructure having, by volume, at least 75% martensite or at least 75% martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%.
High Friction Rolled High Strength Martensitic Steel
Third, in yet another set of examples, presently disclosed is a carbon alloy thin cast steel strip comprising a pair of opposing high friction hot rolled surfaces that have been surface homogenized, upon having been high friction rolled. These present examples are not only applied with the above-mentioned ultra-high strength weathering steel but may additionally be applied with martensitic steels, other weathering steels, and/or steel strips or products which exhibit prior austenite grain boundary depressions. Upon being surface homogenized, the pair of opposing high friction hot rolled surfaces are free of the smeared grain boundary depressions which were previously formed as a result of the high friction rolling process. In some embodiments, the carbon alloy thin cast steel strip may further comprise a microstructure having, by volume, at least 75% martensite or at least 75% martensite plus bainite with a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%. In some embodiments, the steel strip comprises a microstructure having, by volume, at least 90% martensite or at least 90% martensite plus bainite. In some embodiments, the steel strip of claim 1 comprises a microstructure having, by volume, at least 95% martensite or at least 95% martensite plus bainite.
Exemplary homogenized steel strips within the scope of this disclosure may comprise, by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and the remainder iron and impurities resulting from melting.
Also disclosed are methods of making hot rolled carbon alloy steel strip. The method may comprise the steps of:
(a) preparing a molten steel melt;
(b) forming the melt into a casting pool supported on casting surfaces of a pair of cooled casting rolls having a nip there between;
(c) counter rotating the casting rolls and solidifying at a heat flux greater than 10.0 MW/m2 the molten melt into a steel strip of less than or equal to 2.5 mm in thickness delivered downwardly from the nip and cooling the strip in a non-oxidizing atmosphere to below 1080° C. and above the Ar3 temperature at a cooling rate greater than 15° C./s;
(d) high friction rolling the thin cast steel strip to a hot rolled thickness of between a 15% and 50% reduction of the as cast thickness producing a hot rolled steel strip free of prior-austenite grain boundary depressions and having a smear pattern; and
(e) surface homogenizing the high friction hot rolled steel strip to eliminate the smear pattern.
The high friction hot rolled homogenized thin cast steel strip may comprise a microstructure having, by volume, at least 75% martensite or at least 75% martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%, thereby, providing a high strength martensitic steel. Further, the high friction hot rolled homogenized steel strip may comprise, by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and the remainder iron and impurities resulting from melting.
The present disclosure further sets out as to how each of the thin cast steel strips, the above compositions, and/or the above properties, as described, may be relied on for an ultra-high strength weathering steel pile. Specifically, in one example an ultra-high strength weathering steel pile comprises a plurality of sidewalls, each sidewall having a thickness of about 2.5 mm or less, 2.0 mm or less, or 1.6 mm or less. The pile may be formed from a steel strip. Specifically, the pile may be formed from an as cast steel strip. The pile may be formed from a hot rolled as cast steel strip. Moreover, the pile may be cold roll formed. The pile may have a composition comprising, by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and the remainder iron and impurities resulting from melting. The pile may further comprise, or have, a corrosion index of 6.0 or greater, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and/or an elongation of between 1% and 10%.
In some examples, the composition of the pile includes an amount of nickel sufficient for shifting a peritectic point away from the carbon region and/or increasing a transition temperature of the peritectic point to form a carbon alloy steel strip having a microstructure of at least 75% by volume martensite or martensite plus bainite. In some examples, the pile may be formed from a steel strip where the as cast thickness of the steel strip is hot rolled having a hot rolled thickness of between a 15% and 50% reduction of the as cast thickness. The hot rolled material may be high friction rolled to provide a high friction rolled thickness.
Various features and shapes for the ultra-high strength weathering steel are further described herein. These features may be provided in combination or independent of one another. In some examples the ultra-high strength weathering steel pile may be a C-channel where the plurality of sidewalls are a web and one or more flanges. More specifically, the ultra-high strength weathering steel pile may be a hemmed C-channel and/or a corrugated C-channel where the plurality of sidewalls are a web and one or more flanges. In some examples the ultra-high strength weathering steel pile may be a tube where the plurality of sidewalls form the tube. More specifically, the ultra-high strength weathering steel pile may be a square tube or a rectangular tube. Further, the ultra-high strength weathering steel pile may be a square tube or a rectangular tube, generally, where one or more of the plurality of sidewalls further comprise one or more corrugations. The plurality of sidewalls do not comprise a separately applied coating. The plurality of sidewalls are ungalvanized. At least one sidewall of the plurality of sidewalls may be a hem. More specifically, the one or more flanges of the hemmed C-channel may be a single hem. A first layer and a second layer of each single hem of the one or more flanges may be secured together by one or more spot welds. The first layer of the one or more flanges may transition to the second layer through a teardrop transition. Additionally, or alternatively, at least one of the plurality of sidewalls may comprise one or more corrugations. In some examples, the web of a C-channel may comprise one or more corrugations. Additionally, or alternatively, the one or more flanges of a C-channel may comprise one or more corrugations.
In some examples an ultra-high strength weathering steel pile comprises a thickness of about 2.5 mm or less, 2.0 mm or less, or 1.6 mm or less. The pile may be formed from a thin cast steel strip that is cold roll formed into the steel pile having a plurality of sidewalls with a corrosion index of 6.0 or greater. The ultra-high strength weathering steel pile may further comprise a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%. The composition of the ultra-high strength weathering steel pile may include an amount of nickel sufficient for shifting a peritectic point away from the carbon region and/or increasing a transition temperature of the peritectic point to form a carbon alloy steel strip having a microstructure of at least 75% by volume martensite or martensite plus bainite. In an example, the ultra-high strength weathering steel pile has a material composition comprising, by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and the remainder iron and impurities resulting from melting.
Also discussed herein is a solar arrangement. The solar arrangement may comprise an ultra-high strength weathering steel pile comprising a plurality of sidewalls, each sidewall having a thickness of about 2.5 mm or less, 2.0 mm or less, or 1.6 mm or less. The ultra-high strength weathering steel pile of the solar arrangement may comprise, by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and the remainder iron and impurities resulting from melting. The ultra-high strength weathering steel pile of the solar arrangement may comprise a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%. In this example, a partial length of the ultra-high strength weathering steel pile is driven into a ground surface and one or more solar cells are supported above the ground surface by the ultra-high strength weathering steel pile. Accordingly, the ultra-high strength weathering steel pile of the solar arrangement may additionally, or alternatively, have the many features as described above and in the remainder of the disclosure.
The invention may be more fully illustrated and explained with reference to the accompanying drawings in which:
Described herein, in one example, is a light-gauge, ultra-high strength weathering steel sheet. A light-gauge, ultra-high strength weathering steel sheet may be made from a molten melt. The molten melt may be processed through a twin roll caster. In one example, the light-gauge, ultra-high strength weathering steel sheet may be made by the steps comprising: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) solidifying at a heat flux greater than 10.0 MW/m2 producing a steel sheet less than 2.5 mm in thickness and cooling in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled; and (c) rapidly cooling to form a steel sheet with a microstructure having at least 75% by volume martensite or martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. In one example, the light-gauge, ultra-high strength weathering steel sheet may also be hot rolled to between 15% and 50% reduction before rapid cooling. The sheet may be cooled in a non-oxidizing atmosphere to below 1100° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled. The Ar3 temperature is the temperature at which austenite begins to transform to ferrite during cooling. In other words, the Ar3 temperature is the point of austenite transformation. In each example, the nickel shifts the peritectic point away from the carbon region and/or increases a transition temperature of the peritectic point of the composition of the steel sheet to provide a steel sheet that is defect free. The impact of nickel on the corrosion index is reflected in the following equation for determining the corrosion index calculation: Cu*26.01+Ni*3.88+Cr*1.2+Si*1.49+P*17.28−Cu*Ni*7.29−Ni*P*9.1−Cu*Cu*33.39 (where each element is a by weight percentage).
Also described herein are thin cast steel strips having hot rolled exterior side surfaces characterized as being primarily free, substantially free, or free of prior austenite grain boundary depressions but having smears, or elongated surface structures, such as in the examples of a high friction rolled high strength martensitic steel. Also described herein are methods or processes for producing same. These examples are not only applied with the above-mentioned ultra-high strength weathering steel but may additionally be applied with martensitic steels, other weathering steels, and/or steel strips or products which exhibit prior austenite grain boundary depressions.
Further described herein are thin steel strips having hot rolled exterior side surfaces characterized as being primarily free, substantially free, or free of prior austenite grain boundary depressions and free of smears, or elongated surface structures, such as in the examples of a high friction rolled high strength weathering steel. Also described herein are methods or processes for producing same. These examples are not only applied with the above-mentioned ultra-high strength weathering steel but may additionally be applied with martensitic steels, other weathering steels, and/or steel strips or products which exhibit prior austenite grain boundary depressions.
As used herein, primarily free means less than 50% of each opposing hot rolled exterior side surface contains prior austenite grain boundaries or prior austenite grain boundary depressions after acid etching (pickling). At least substantially free of all prior austenite grain boundaries or prior austenite grain boundary depressions means that 10% or less of each opposing hot rolled exterior side surface contains prior austenite grain boundary depressions or prior austenite grain boundary depressions after acid etching (pickling). Said depressions form etched grain boundary depressions after acid etching (also known as pickling) to render the prior austenite grain boundaries visible at 250× magnification. In other instances, free connotes that each opposing hot rolled exterior side surface is free, that is, completely devoid, of prior austenite grain boundary depressions, which includes being free of any prior austenite grain boundary depressions after acid etching. It is stressed that prior austenite grain boundaries may still exist within the material of the strip after hot rolling where the grain boundary depressions and separations on the surface have been removed by way of the techniques described herein (e.g. where hot rolling occurs at a temperature above the Ar3 temperature using roll bite coefficients of friction equal to or greater than 0.20).
As shown in
The twin roll caster may be of the kind that is illustrated and described in some detail in U.S. Pat. Nos. 5,184,668, 5,277,243, 5,488,988, and/or U.S. patent application Ser. No. 12/050,987, published as U.S. Publication No. 2009/0236068 A1. Reference is made to those patents and publications which are incorporated by reference for appropriate construction details of a twin roll caster that may be used in an example of the present invention.
After the thin steel strip is formed (cast) using any desired process, such as the strip casting process described above in conjunction with
Referring back to
In particular instances, the methods of forming a thin steel strip further include hot rolling the thin steel strip using a pair of opposing work rolls generating a heightened coefficient of friction (p) sufficient to generate opposing hot rolled exterior side surfaces of the thin steel strip characterized as being primarily free substantially free, or free of prior austenite grain boundary depressions, and being characterized as having elongated surface structure associated with surface smear patterns formed under shear through plastic deformation. In certain instances, the pair of opposing work rolls generate a coefficient of friction (p) equal to or greater than 0.20 0.25, 0.268, or 0.27, each with or without use of lubrication at a temperature above the Ar3 temperature. It is appreciated that the coefficient of friction may be increased by increasing the surface roughness of the surfaces of the work rolls, eliminating the use of any lubrication, reducing the amount of lubrication used, and/or electing to use a particular type of lubrication. Other mechanisms for increasing the coefficient of friction as may be known to one of ordinary skill may also be employed—additionally or separately from the mechanisms previously described. The above process is referred to herein, generally, as high friction rolling.
As mentioned above, it is appreciated that high friction rolling may be achieved by increasing the surface roughness of the surfaces of one or more of the work rolls. This is referred to herein, generally, as work roll surface texturing. The work roll surface texturing may be modified and measured by various parameters for use in a high friction rolling application. By example, the average roughness (Ra) of the profile of a work roll may provide a point of reference for generating the requisite coefficient of friction for the roll bite as noted in the examples above. To achieve high friction rolling by way of work roll surface texturing in one example newly ground and textured work rolls may have a Ra between of between 2.5 μm and 7.0 μm. Newly ground and textured work rolls are referred to herein more generally as new work rolls. In a specific example, new work roll(s) may have a Ra of between 3.18 μm and 4.0 μm. The average roughness of a new work roll may decrease during use, or upon wear. Therefore, used work roll(s) may also be relied on to produce the high friction rolling conditions noted above so long as the used work roll(s) have, in one example, a Ra of between 2.0 μm and 4.0 μm. In a specific example, used work roll(s) may have a Ra of between 1.74 μm and 3.0 μm while still achieving the high friction rolling conditions noted above.
Additionally, or alternatively, the average surface roughness depth (Rz) of the work roll profile may also be relied on as an identifier to achieve the high friction rolling conditions noted above. New work roll(s) may have a Rz of between 20 μm and 41 μm. In one specific example, new work roll(s) may have a Rz of between 21.90 μm and 28.32 μm. Used work roll(s) may be relied on for the high friction rolling conditions noted above in one example so long as they maintain a Rz of between 10 μm and 20 μm before being removed from service. In one specific example, used work roll(s) have a Rz of between 13.90 μm and 20.16 μm before being removed from service.
Still yet, the above parameters may be further defined by the average spacing between the peaks across the profile (Sm). New work rolls(s) relied on to produce the high friction rolling condition may comprise a Sm of between 90 μm and 150 μm. In one specific example, new work roll(s) relied on to produce the high friction rolling condition comprise a Sm of between 96 μm and 141 μm. Used work roll(s) may be relied on for the high friction rolling conditions noted above in one example so long as they maintain a Sm of between 115 μm and 165 μm.
Table 1, below illustrates measured test data for work roll surface texturing relied on to produce a high friction rolling condition, by position on the work roll, and further provides a comparison between the new work roll parameters and the used work roll parameters, before the used work roll is to be removed from service:
To determine whether high friction rolling is applicable for examples of the present disclosure may be dependent upon whether thermal etching has occurred in the hot box. Thermal etching is a byproduct, or consequence, of the casting process which exposes the prior austenite grain boundary depressions at the surface of steel strip. As indicated above, the prior austenite grain boundary depressions may be susceptible to causing the above-mentioned defect phenomenon along etched prior austenite grain boundary depressions upon further acid etching. Specifically, thermal etching reveals prior austenite grain boundary depressions in a steel strip by formation of grooves in the intersections of the prior-austenite grain boundary depressions and the surface when the steel is exposed to a high temperature in an inert atmosphere, such as the hot box. These grooves make the prior austenite grain boundary depressions visible at the surface. Accordingly, examples of the present process identify high friction rolling as the step for producing the desired steel properties upon thermal etching in the hot box. Irrespective of the presence of thermal etching and evidence of prior austenite grain boundary depressions, high friction rolling may be provided to increase recrystallization of the thin steel strip.
Ultra-High Strength Weathering Steel
In some embodiments, a light-gauge, ultra-high strength weathering steel sheet may be made from a molten melt. The molten melt may be processed through a twin roll caster. In one example, the light-gauge, ultra-high strength weathering steel sheet may be made by the steps comprising: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) solidifying at a heat flux greater than 10.0 MW/m2 producing a steel sheet less than 2.5 mm in thickness and cooling in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled; and (c) rapidly cooling to form a steel sheet with a microstructure having at least 75% by volume martensite or martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. In one example, the light-gauge, ultra-high strength weathering steel sheet may also be hot rolled to between 15% and 50% reduction before rapid cooling. The sheet may be cooled in a non-oxidizing atmosphere to below 1100° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling and/or before hot rolling, when hot rolled. The Ar3 temperature is the temperature at which austenite begins to transform to ferrite during cooling. In other words, the Ar3 temperature is the point of austenite transformation. In each example, the nickel shifts the peritectic point away from the carbon region and/or increases a transition temperature of the peritectic point of the composition of the steel sheet to provide a steel sheet that is defect free. The impact of nickel on the corrosion index is reflected in the following equation for determining the corrosion index calculation: Cu*26.01+Ni*3.88+Cr*1.2+Si*1.49+P*17.28−Cu*Ni*7.29−Ni*P*9.1−Cu*Cu*33.39 (where each element is a by weight percentage).
The present steel sheet examples provide an addition of nickel to further prevent peritectic cracking while maintaining or improving hardenability. In particular, between 0.5% and 1.5%, by weight, nickel is added. The addition of nickel is believed to prevent the strip shell from buckling caused by the volume change in the peritectic region during phase transformation on the casting rolls and therefore enhances the even heat transfer during the strip solidification. It is believed that the addition of nickel shifts the peritectic point away from the carbon region and/or increases the transition temperature of the peritectic point of the composition to form a steel sheet that is defect free. The phase diagram of
The impact of nickel on the corrosion index is reflected in the following equation for determining the corrosion index calculation: Cu*26.01+Ni*3.88+Cr*1.2+Si*1.49+P*17.28−Cu*Ni*7.29−Ni*P*9.1−Cu*Cu*33.39 (where each element is a by weight percentage).
Table 2, below, shows several compositional examples of a light-gauge, ultra-high strength weathering steel sheet of the present disclosure.
In Table 2, LecoN is the measured, percent by weight, nitrogen (N2) and CEAWS is the measured, percent by weight, carbon equivalent (CE).
Other elements relied on for hardenability produce the opposite effect by shifting the peritectic point closer the carbon region. Such elements include chromium and molybdenum which are relied on to increase hardenability but ultimately result in peritectic cracking. Through the addition of nickel, hardenability is improved and peritectic cracking is reduced to provide a fully quenched martensitic grade steel strip with high strength.
In the present compositions the addition of nickel may be combined with limited amounts of chromium and/or molybdenum, as described herein. As a result, nickel reduces any impact these hardening elements may have to produce peritectic cracking. In one example, however, the additional nickel would not be combined with a purposeful addition of boron. A purposeful addition is 5 ppm of boron, or more. In other words, in one example the addition of nickel would be used in combination with substantially no boron, or less than 5 ppm boron. Additionally, the light-gauge, ultra-high strength weathering steel sheet may be made by the further tempering the steel sheet at a temperature between 150° C. and 250° C. for between 2 and 6 hours. Tempering the steel sheet provides improved elongation with minimal loss in strength. For example, a steel sheet having a yield strength of 1250 MPa, tensile strength of 1600 MPa and an elongation of 2% was improved to a yield strength of 1250 MPa, tensile strength of 1525 MPa and an elongation of 5% following tempering as described herein.
The light-gauge, ultra-high strength weathering steel sheet may be silicon killed containing by weight less than 0.008% aluminum or less than 0.006% aluminum. The molten melt may have a free oxygen content between 5 to 70 ppm or between 5 to 60 ppm. The steel sheet may have a total oxygen content greater than 50 ppm. The inclusions include MnOSiO2 typically with 50% less than 5 μm in size and have the potential to enhance microstructure evolution and, thus, the strip mechanical properties.
The molten melt may be solidified at a heat flux greater than 10.0 MW/m2 into a steel sheet less than 2.5 mm in thickness, and cooled in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s. A non-oxidizing atmosphere is an atmosphere typically of an inert gas such as nitrogen or argon, or a mixture thereof, which contains less than about 5% oxygen by weight.
In some embodiments, the martensite in the steel sheet may form from an austenite grain size of greater than 100 μm. In other embodiments, the martensite in the steel sheet may form from an austenite grain size of greater than 150 μm. Rapid solidification at heat fluxes greater than 10 MW/m2 enables the production of an austenite grain size that is responsive to controlled cooling to enable the production of a defect free sheet.
The steel sheet additionally may be hot rolled to between 15% and 50% reduction and, thereafter, rapidly cooled to form a steel sheet with a microstructure having at least 75% martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. Further, the steel sheet may be hot rolled to between 15% and 35% reduction and, thereafter, rapidly cooled to form a steel sheet with a microstructure having at least 75% martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. In one example, the steel sheet is hot rolled to between 15% and 50% reduction and, thereafter, rapidly cooled to form a steel sheet with a microstructure having at least 90% by volume martensite or martensite and bainite. In still yet another example, the steel sheet is hot rolled to between 15% and 50% reduction and, thereafter, rapidly cooled to form a steel sheet with a microstructure having at least 95% by volume martensite or martensite and bainite.
Many products may be produced from the light-gauge, ultra-high strength weathering (“UHSW”) steel sheet of the type described herein. One example of a product that may be produced from a light-gauge, ultra-high strength weathering steel sheet includes a steel pile. More specifically, piles, or foundations, for solar arrangements, are examples of uses for a product produced from the light-gauge, ultra-high strength weathering steel sheet. As used herein, a solar arrangement is a structure for supporting solar cells, such as on a solar farm of photovoltaic power stations designed for the supply of solar power for use in an electric grid. The highway industry has similar demand for foundations such as, for example, to support guardrails, signage, or the like. The pile or steel foundation may be produced from a thin cast steel strip that has been cold roll formed using one or more roll stands. Additionally, a punch system, a CNC plasma system, and/or a roll system, or the like, may also be relied on to provide thru-holes, slots, continuous welds, partial welds, and/or spot welds, as noted below.
In one example, a steel pile comprises a web and one or more flanges cold roll formed from the carbon alloy steel strip of the varieties described above.
In
Still referring to
Still referring to
In the UHSW steel pile 100 example of
Turning now to
The web corrugations 213, 214 of the UHSW steel pile of
Still referring to
Like the first flange 220, the second flange 230 may also comprise one or more corrugations. In the example of
In the UHSW steel pile 200 example of
Turning now to
The single web corrugations 313 of the UHSW steel pile of
Still referring to
Like the first flange 320, the second flange 330 may also comprise one or more corrugations. In the example of
In the UHSW steel pile 300 example of
Turning now to
One or more of the sidewalls of steel pile 400 of
In the UHSW steel pile 400 example of
Turning now to
One or more of the sidewalls of steel pile 500 of
In the UHSW steel pile 500 example of
The shapes described above provide additional structural integrity for withstanding loads incurred by piles or steel foundations as further described below. Further, by increasing the structural integrity by way of the shape a much thinner material may be relied on for producing the steel pile than a traditional galvanized I-beam. Accordingly, a much thinner material also requires less force to be driven into the ground, while maintaining the requisite strength and integrity, because the cross-section of the present UHSW steel pile is reduced in comparison to prior piles and structural foundations.
In use, a partial length of the steel pile is driven into the earth or soil to provide a structural foundation. The steel pile is driven into the earth or soil using a ram, such as a piston or hammer. The ram may be a part of and is, at least, driven by a pile driver. The ram strikes or impacts the steel pile forcing the steel pile into the earth or soil. Due to the impact, prior steel piles may buckle or become deformed under the impact of the ram. To avoid buckling, or damage, to prior steel piles the RPM or force of the pile driver is maintained below a damaging threshold. The present steel pile has illustrated an ability for an increase in the RPM or force being applied to the steel pile without buckling, or damaging, the steel pile, as reflected by the strength properties of the steel pile, comparatively to prior steel piles. Specifically, as tested, prior steel piles of comparable dimensional characteristics were driven and structurally failed wherein the steel pile of the present disclosure provide an increase of RPM of 25%. Moreover, the prior steel piles were additionally not a weathering steel absent a galvanized, or zinc, surface. Thereby, prior steel piles are susceptible to corrosion due to their placement in exterior conditions, including earth and soil conditions, or require additional treatment such as, for example, galvanizing. Again, the present steel pile provides the necessary corrosion index for withstanding these conditions. The present strength properties and corrosion properties have not before been seen in combination for such a product.
The hemmed flanges of the hemmed C-channel, as described above and illustrated by
Steel piles formed of a light-gauge, ultra-high strength weathering steel of the present disclosure have been comparatively tested for operational life and corrosion resistant potential with prior steel pile materials. Prior steel pile materials include hot-dipped galvanized (“HDG”) piles such as, for example, G235 grade steel, as well as ungalvanized steel such as, for example, G100, Gr 70, or the like. In solar arrangements, it has been the solar industry's convention to use piles designed from zinc-plated 50 ksi steel W6 I-Beams for structural purpose. Often a redox analysis is performed on soil types to identify the corrosive characteristics of soils. These characteristics are then relied on to determine the corrosive rates of materials being placed in the soils. Soil conditions may additionally, or alternatively, be analyzed for resistivity, pH, chlorides, and sulfates. Steel piles must be designated to withstand their load requirements regardless of corrosion. In order to comparatively test the operational life and corrosion resistant potential of the present material for a light-gauge, ultra-high strength weathering (“UHSW”) steel pile a UHSW steel coupon was tested in direct comparison with steel coupons of varying material, such as G235, G100, and Gr70, using a salt-spray test. Specifically, testing was performed according to ASTM B117-18 standard specification on the four steel coupons of varying materials: “G235”, “G100”, “Gr70”, and “UHSW”. The test document specifies a 1000-hour salt spray test with inspection intervals at 250 hours. Random coupons (1 of 4) from each material type were chosen for inspection, and
To compare the relative corrosion rates of the materials tested, the difference in material loss per hour was measured for each sample, and the initial data readings were excluded due to the appearance of oxidation generally skewing the results. In other words, the appearance of oxidation increased the measured thicknesses. After 250 hours, the results were generally more linear. The averages of this exercise, shown in Table 4, below, are used to create a relative relationship between the corrosion rate of the UHSW steel coupon with the other coupons:
Using this correlation alone, the UHSW coupon performed better than the galvanized coupon. It is notable that the measurements of the UHSW greatly outperformed the other steel coupons. The ultra-high strength weathering steel of the present disclosure exhibits a resistance to corrosion where one of ordinary skill would otherwise rely on a separately applied metallic coating or galvanizing to achieve the same. Thereby, the ultra-high strength weathering steel exhibits the combination of the requisite strength and corrosion properties, in combination with the benefits of the above shapes, for use as a pile or structural foundation where a separately applied coating would otherwise be required on a steel to achieve the same strength properties not otherwise present in prior weathering steel.
Table 5, below, illustrates the steel grade and chemistry for the UHSW steel coupon relied on in the above results of Tables 3-4.
Additional testing was performed to evaluate the UHSW steel's corrosion rate in comparison to that of a galvanized (“HDG”) steel for varied geometry, duration of burial, and simulated aging. Tables 6-7, below, illustrate the results from these tests. The materials were tested in moderately salty, low resistivity soil that was also designated as “very corrosive.” Material geometry tested included small angle-shaped stakes, and full-sized cold roll formed C-piles. The material designated as “current applied” received a voltage high enough to artificially induce corrosion for approximately 24 hours, in an attempt to simulate the effects of longer-term installation. Under this comparative analysis, the UHSW steel material measurement rates varied from 77% to 99% the measured rates for the HDG steel material.
The above comparative testing and structural capacity calculations illustrate steel piles produced from a thin cast steel strip outperformed hot-dip galvanized (“HDG”) steel piles as well as prior steel piles. The UHSW steel pile of the present disclosure provides greater resistance to corrosion and at much thinner material thicknesses. These improvements are maintained while also maintaining desirable strength and elongation properties that allow the UHSW steel piles to resist deformation while being driven into the ground. As also illustrated by the material thicknesses, the UHSW steel piles are produced at much lower weights than the prior steel piles. Specifically, in comparison to a steel pile constructed from a W6×7 I-beam, weighting 7 pounds (lbs) per foot, or from a W6×9 I-beam, weighing 9 pounds (lbs) per foot, the UHSW hemmed C-channel, or NW pile, of a comparative overall cross-section weighs 5 pounds (lbs) per foot and the UHSW corrugated C-channel, or NCW pile, of comparative cross-section weighs 3.5 pounds (lbs) per foot. The UHSW steel piles of the present disclosure are also provided without a hot-dip galvanized coating or zinc coating. The UHSW steel piles of the present disclosure, thereby, eliminate any undesirable interaction between the soil or groundwater and a zinc coating that is otherwise present with a HDG steel pile. Other alternatives to a steel pile not having a separately applied coating, such as the other ungalvanized steel piles tested herein, were significantly outperformed by the UHSW steel pile of the present disclosure.
Even with a galvanized coating, the HDG steel pile structural capacity and service life fails to outperform the structural capacity and service life of a thinner UHSW steel pile. The galvanic layer's time to completely corrode is estimated by using the thickness divided by the corrosion rate. Then the remaining time is multiplied by the steel corrosion rate to determine the final material thickness. For example, consider a 0.124″ thick G235 sheet metal component: if the standard ratio of corrosion rate between the Zinc layer and base metal in corrosive soil condition is applied, and a G235 (2.1 mils/side) galvanized steel zinc coating corrosion rate is estimated to be 0.0003″/y, then a base steel corrosion rate should be around 0.0021″/y, with a service life of 30 years, the total reduction, per side, is calculated as follows:
The total metal loss will be around 0.1008″ and this would leave a 0.0232″ thick component at the end of the service life.
Assuming the UHSW material with 0.062″ thickness corrodes at the same rate as zinc, the final thickness is more simply calculated as follows after the first two years:
0.3 mils/year/side×30 years×2 sides=18 mils
resulting in a material thickness of 0.0440″ at the end of 30 years. This mild corrosivity case demonstrates how the material can outperform zinc+carbon steel structures for longevity, and the greatly increased strength compared to carbon steel allows for significantly larger capacities in virtually any loading scenario.
In some examples an ultra-high strength weathering steel pile comprises an as cast material having a thickness of less than or equal to 2.5 mm, less than or equal to 2.0 mm, or less than or equal to 1.6 mm. The as cast material thickness is a thin cast steel strip cold roll formed into a steel pile having a web and one or more flanges with a corrosion index of 6.0 or greater. The ultra-high strength weathering steel pile may further comprise a material yield strength of between 700 and 1600 MPa, a material tensile strength of between 1000 and 2100 MPa, and a material elongation of between 1% and 10%. The material composition of the ultra-high strength weathering steel pile may include an amount of nickel sufficient for shifting a peritectic point away from the carbon region and/or increasing a transition temperature of the peritectic point to form a carbon alloy steel strip having a microstructure of at least 75% by volume martensite or martensite plus bainite.
An UHSW steel pile may be a steel pile comprising a web and one or more flanges, or of one of the shapes described above, formed from a carbon alloy steel strip having a composition comprising, by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum where the carbon alloy steel strip has a microstructure having at least 75% by volume martensite or martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, an elongation of between 1% and 10%, and has a corrosion index of 6.0 or greater. In one example, the steel pile may be cold roll formed from a carbon alloy steel strip cast at a cast thickness less than or equal to 2.5 mm. In another example, the steel pile may be cold roll formed from a steel strip less than or equal to 2.0 mm or less than or equal to 1.6 mm. In still yet, another example, the steel pile may be cold roll formed from a steel sheet that is between 1.4 mm to 1.5 mm or of 1.4 mm or 1.5 mm in thickness. The steel piles may be channels, such as C-channels, box channels, double channels, or the like. The steel piles may, additionally or alternatively, be I-shaped members, angles, structural tees, hollow structural sections, double angles, S-shapes, tubes, or the like. Moreover, many of these members may be connected together, e.g. welded together, to form a single steel pile. It is appreciated herein, additional products may be made from a light-gauge, ultra-high strength weathering steel sheet. Additionally, it is appreciated herein, additional products may be made from an ultra-high strength weathering steel that is not produced through a twin roll caster but, instead, an ultra-high strength product may be produced through other methods.
Additional examples of an ultra-high strength weathering steel are provided below:
A light-gauge, ultra-high strength steel sheet comprising: a carbon alloy steel strip cast at a cast thickness less than or equal to 2.5 mm having a composition comprising:
(i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and
(ii) the remainder iron and impurities resulting from melting;
wherein in the composition the nickel shifts a peritectic point away from the carbon region and/or increases a transition temperature of the peritectic point to form the carbon alloy steel strip having a microstructure having at least 75% by volume martensite or martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10% that is defect free.
In an example of the above, the light-gauge, ultra-high strength steel sheet has a microstructure having at least 75% by volume martensite. In another example of the above, the light-gauge, ultra-high strength steel sheet has a microstructure having at least 90% by volume martensite. In yet another example of the above, the light-gauge, ultra-high strength steel sheet has a microstructure having at least 95% martensite.
In an example of the above, the light-gauge, ultra-high strength steel sheet comprises less than 5 ppm boron.
In an example of the above, the light-gauge, ultra-high strength steel sheet comprises between 0.05% and 0.12% niobium.
In an example of the above, the martensite in the steel sheet comes from an austenite grain size of greater than 100 μm.
In an example of the above, the martensite in the steel sheet comes from an austenite grain size of greater than 150 μm.
In an example of the above, the steel sheet may additionally be hot rolled to between 15% and 50% reduction before rapidly cooling.
In an example of the above, the carbon alloy steel sheet is hot rolled to a hot roll thickness of between a 15% and 35% reduction of the cast thickness before rapidly cooling.
In an example of the above, the steel sheet is a weathering steel having a corrosion index of 6.0 or greater.
A method of making a light-gauge, ultra-high strength weathering steel sheet comprising the steps of:
(a) preparing a molten steel melt comprising:
-
- (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, silicon killed with less than 0.01% aluminum, and
- (ii) the remainder iron and impurities resulting from melting;
(b) forming the melt into a casting pool supported on casting surfaces of a pair of cooled casting rolls having a nip there between;
(c) counter rotating the casting rolls and solidifying at a heat flux greater than 10.0 MW/m2 the molten melt into a steel sheet to less than 2.5 mm in thickness delivered downwardly from the nip and cooling the sheet in a non-oxidizing atmosphere to below 1100° C. and above the Ar3 temperature at a cooling rate greater than 15° C./s; and
(d) rapidly cooling to form a steel sheet with a microstructure having at least 75% by volume martensite or martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10% wherein the nickel shifts the peritectic point away from the carbon region and/or increases a transition temperature of the peritectic point for inhibiting crack, or defect, formation in a high strength martensitic steel sheet.
In an example of the above, the microstructure has at least 75% by volume martensite. In another example of the above, the microstructure has at least 90% by volume martensite. In yet another example of the above, the microstructure has at least 95% by volume martensite.
In an example of the above, the carbon alloy steel sheet is formed with less than 5 ppm boron.
In an example of the above, the carbon alloy steel sheet comprises between 0.05% and 0.12% niobium.
In an example of the above, the martensite in the steel sheet comes from an austenite grain size of greater than 100 μm.
In an example of the above, the martensite in the steel sheet comes from an austenite grain size of greater than 150 μm.
In an example of the above, the steel sheet is hot rolled to a hot roll thickness of between a 15% and 50% reduction of the cast thickness before rapidly cooling.
In an example of the above, the steel sheet is hot rolled to a hot roll thickness of between a 15% and 35% reduction of the cast thickness before rapidly cooling.
In an example of the above, the high strength steel sheet is defect free.
Also disclosed is a steel pile comprising a web and one or more flanges cold roll formed from a carbon alloy steel sheet cast at a cast thickness less than or equal to 2.5 mm having a composition comprising, by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum where the carbon alloy steel sheet has a microstructure having at least 75% by volume martensite or martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, an elongation of between 1% and 10% and is defect free.
In an example of the above, the light-gauge, ultra-high strength steel sheet has a microstructure having at least 75% by volume martensite. In another example of the above, the light-gauge, ultra-high strength steel sheet has a microstructure having at least 90% by volume martensite. In yet another example of the above, the light-gauge, ultra-high strength steel sheet has a microstructure having at least 95% martensite.
In an example of the above, the carbon alloy steel sheet of the steel pile comprises less than 5 ppm boron.
In an example of the above, the carbon alloy steel sheet of the steel pile comprises between 0.05% and 0.12% niobium.
In an example of the above, the martensite in the steel pile comes from an austenite grain size of greater than 100 μm.
In an example of the above, the martensite in the steel pile comes from an austenite grain size of greater than 150 μm.
In an example of the above, the steel sheet may additionally be hot rolled to between 15% and 50% reduction before rapidly cooling.
In an example of the above, the carbon alloy steel sheet is hot rolled to a hot roll thickness of between a 15% and 35% reduction of the cast thickness before rapidly cooling.
In an example of the above, the carbon alloy steel sheet is a weathering steel having a corrosion index of 6.0 or greater.
High Friction Rolled High Strength Weathering Steel
In the following examples, a high friction rolled high strength weathering steel sheet is disclosed. An example of an ultra-high strength weathering steel sheet is made by the steps comprising: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) solidifying at a heat flux greater than 10.0 MW/m2 into a steel sheet less than or equal to 2.5 mm in thickness and cooling the sheet in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling; (c) high friction rolling the thin cast steel strip to a hot rolled thickness of between a 15% and 50% reduction of the as cast thickness producing a hot rolled steel strip primarily free, substantially free, or free of prior austenite grain boundary depressions and having a smear pattern; and (d) rapidly cooling to form a steel sheet with a microstructure having by volume at least 75% martensite or at least 75% martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa and an elongation of between 1% and 10%. Here and elsewhere in this disclosure elongation means total elongation. “Rapidly cooling” means to cool at a rate of more than 100° C./s to between 100 and 200° C. Rapidly cooling the present compositions, with an addition of nickel, achieves up to more than 95% martensitic phase steel strip. In one example, rapidly cooling forms a steel sheet with a microstructure having by volume at least 95% martensite or at least 95% martensite plus bainite. The addition of nickel must be sufficient enough to shift the ‘peritectic point’ away from the carbon region that would otherwise be present in the same composition without the addition of nickel. Specifically, the nickel in the composition is believed to contribute to the shifting of the peritectic point away from the carbon region and/or increases a transition temperature of the peritectic point of the composition, which appears to inhibit defects and results in an ultra-high strength weathering steel sheet that is defect free.
High friction rolling an ultra-high strength weathering steel further improves the formability of the ultra-high strength weathering steel. A measure for formability is set forth by the ASTM A370 bend tests standard. In embodiments, the ultra-high strength weathering steel of the present disclosure will pass a 3T 180-degree bend test and will do so consistently. In particular, the high friction rolling generates smears from the prior austenite grain boundary depressions under shear through plastic deformation. These elongated surface structures, characterized as the smear pattern, are desirous for the properties of an ultra-high strength weathering steel. Specifically, the formability of the ultra-high strength weathering steel is improved by the smear pattern.
The steel strip may further comprise by weight greater than 0.005% niobium or greater than 0.01% or 0.02% niobium. The steel strip may comprise by weight greater than 0.05% molybdenum or greater than 0.1% or 0.2% molybdenum. The steel strip may be silicon killed containing by weight less than 0.008% aluminum or less than 0.006% aluminum. The molten melt may have a free oxygen content between 5 to 70 ppm. The steel strip may have a total oxygen content greater than 50 ppm. The inclusions include MnOSiO2 typically with 50% less than 5 μm in size and have the potential to enhance microstructure evolution and, thus, the strip mechanical properties.
The molten melt may be solidified at a heat flux greater than 10.0 MW/m2 into a steel strip less than 2.5 mm in thickness, and cooled in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s. A non-oxidizing atmosphere is an atmosphere typically of an inert gas such as nitrogen or argon, or a mixture thereof, which contains less than about 5% oxygen by weight.
In some embodiments, the martensite in the steel strip may come from an austenite grain size of greater than 100 μm. In other embodiments, the martensite in the steel strip may come from an austenite grain size of greater than 150 μm. Rapid solidification at heat fluxes greater than 10 MW/m2 enables the production of an austenite grain size that is responsive to controlled cooling after subsequent hot rolling to enable the production of defect free strip.
As indicated above, the steel strip of the present set of examples may comprise a microstructure having martensite or martensite plus bainite. Martensite is formed in carbon steels by the rapid cooling, or quenching, of austenite. Austenite has a particular crystalline structure known as face-centered cubic (FCC). If allowed to cool naturally, austenite turns into ferrite and cementite. However, when the austenite is rapidly cooled, or quenched, the face-centered cubic austenite transforms to a highly strained body-centered tetragonal (BCT) form of ferrite that is supersaturated with carbon. The shear deformations that result produce large numbers of dislocations, which is a primary strengthening mechanism of steels. The martensitic reaction begins during cooling when the austenite reaches the martensite start temperature and the parent austenite becomes thermodynamically unstable. As the sample is quenched, an increasingly large percentage of the austenite transforms to martensite until the lower transformation temperature is reached, at which time the transformation is completed.
Martensitic steels, however, are susceptible to producing the large prior austenite grain boundary depressions observed on the hot rolled exterior surfaces of cooled thin steel strips formed of low friction condition rolled steel. The step of acid pickling or etching amplifies these imperfections resulting in defects and separations. High friction rolling is now introduced as an alternative to overcome the problems identified for a low friction condition rolled martensitic steel. High friction rolling produces a smeared boundary pattern. Smeared boundary patterns may more generally be referred to herein as smear patterns. Additionally, smeared boundary patterns may alternatively be descriptively referred to as fish scale patterns.
Just as the ultra-high strength weathering steel above is relied on to produce product shapes and configurations such as the piles described above many products may be produced from a high friction rolled high strength weathering steel sheet of the type described herein. Like above, one example of a product that may be produced from a high friction rolled high strength weathering steel sheet includes a steel pile. In one example, a steel pile comprises a web and one or more flanges cold roll formed from the carbon alloy steel strip of the varieties described above. The steel pile may further comprise a length where the web and the one or more flanges extend the length. In use, the length of the steel pile is driven into the earth or soil to provide a structural foundation. The steel pile is driven into the earth or soil using a ram, such as a piston or hammer. The ram may be a part of and is, at least, driven by a pile driver. The ram strikes or impacts the steel pile forcing the steel pile into the earth or soil. Due to the impact, prior steel piles may buckle or become deformed under the impact of the ram. To avoid buckling, or damage, to prior steel piles the RPM or force of the pile driver is maintained below a damaging threshold. The present steel pile has illustrated an ability for an increase in the RPM or force being applied to the steel pile without buckling, or damaging, the steel pile, as reflected by the strength properties of the steel pile, comparatively to prior steel piles. Specifically, as tested, prior steel piles of comparable dimensional characteristics were driven and structurally failed wherein the steel pile of the present disclosure provide an increase of RPM of 25%. Moreover, the prior steel piles were additionally not weathering steel. Thereby, prior steel piles are susceptible to corrosion due to their placement in exterior conditions, including earth and soil conditions. Again, the present steel pile provides the necessary corrosion index for withstanding these conditions. The present strength properties and corrosion properties have not before been seen in combination for such a product.
In one example, the steel pile may be formed from a carbon alloy steel strip cast of the present examples at a cast thickness less than or equal to 2.5 mm. In another example, the steel pile may be formed from a steel strip of the present examples less than or equal to 2.0 mm. In still yet, another example, the steel pile may be formed from a steel sheet of the present examples that is between 1.4 mm to 1.5 mm or of 1.4 mm or 1.5 mm in thickness. The steel piles may be channels, such as C-channels, box channels, double channels, or the like. The steel piles may, additionally or alternatively, be I-shaped members, angles, structural tees, hollow structural sections, double angles, S-shapes, tubes, or the like. Moreover, many of these members may be connected together, e.g. welded together, to form a single steel pile. It is appreciated herein, additional products may be made from a high friction rolled ultra-high strength weathering steel sheet.
High Friction Rolled High Strength Martensitic Steel
In embodiments of the present disclosure, a high strength martensitic steel sheet is also disclosed. The high strength martensitic steel sheet examples that follow may additionally comprise weathering characteristics. Thereby, the high strength martensitic steel sheet examples herein may also be referred to as an ultra-high strength weathering steel sheet for such properties. Martensitic steels are increasingly being used in applications that require high strength, for example, in the automotive industry. Martensitic steel provides the strength necessary by the automotive industry while decreasing energy consumption and improving fuel economy. Martensite is formed in carbon steels by the rapid cooling, or quenching, of austenite. Austenite has a particular crystalline structure known as face-centered cubic (FCC). If allowed to cool naturally, austenite turns into ferrite and cementite. However, when the austenite is rapidly cooled, or quenched, the face-centered cubic austenite transforms to a highly strained body-centered tetragonal (BCT) form of ferrite that is supersaturated with carbon. The shear deformations that result produce large numbers of dislocations, which is a primary strengthening mechanism of steels. The martensitic reaction begins during cooling when the austenite reaches the martensite start temperature and the parent austenite becomes thermodynamically unstable. As the sample is quenched, an increasingly large percentage of the austenite transforms to martensite until the lower transformation temperature is reached, at which time the transformation is completed.
Martensitic steels, however, are susceptible to producing the large prior austenite grain boundary depressions observed on the hot rolled exterior surfaces of cooled thin steel strips formed of low friction condition rolled steel. The step of acid pickling or etching amplifies these imperfections resulting in defects and separations. High friction rolling is now introduced as an alternative to overcome the problems identified for a low friction condition rolled martensitic steel, however, high friction rolling has also been observed to produce an undesirable surface finish. In particular, high friction rolling produces smeared boundary pattern in combination with an uneven surface finish. Smeared boundary patterns may more generally be referred to herein as smear patterns. Additionally, smeared boundary patterns may alternatively be descriptively referred to as fish scale patterns. The uneven surface finish, having the smear patterns, then becomes susceptible to trapping acid and/or causing excessive corrosion, such as when the thin steel strip undergoes subsequent acid etching, thereby, resulting in excessive amounts of pitting. In view of this, for some steel strips or products, such as a martensitic steel sheet for use in an automotive application, additional surface treatment is warranted to provide a surface where the smear patterns and/or uneven surface finishes are removed from the surface.
To reduce or eliminate the smear pattern, and/or the uneven surface finish, the thin steel strip undergoes a surface homogenization process after the hot rolling mill. Examples of a surface homogenization process include abrasive blasting such as, for example, through use of an abrasive wheel, shot blasting, sand blasting, wet abrasive blasting, other pressurized application of an abrasive, or the like. One specific example of a surface homogenization process includes an eco-pickled surface (referred herein as “EPS”). Other examples of a surface homogenization process include the forceful application of an abrasive media onto the surface of the steel strip for homogenizing the surface of the steel strip. A pressurized component may also be relied on for the forceful application. By example, a fluid may propel an abrasive media. A fluid, as used herein, includes liquid and air. Additionally, or alternatively, a mechanical device may provide the forceful application. The surface homogenization process occurs after the thin cast steel strip reaches room temperature. In other words, the surface homogenization process does not occur in an in-line process with the hot rolling mill. The surface homogenization process may occur at a location separate from, or off-line from, the hot rolling mill and/or the twin cast rollers. In some examples, the surface homogenization process may occur after coiling.
As used herein, the surface homogenization process alters the surface to be free of a smear pattern or eliminates the smear pattern. A surface of a thin steel strip that is free of a smear pattern or wherein the smear pattern has been eliminated is a surface that passes a 120-hour corrosion test without any surface pitting corrosion. Test samples which did not undergo a surface homogenization process fractured after 24 hours during a 120-hour corrosion test due to surface corrosion.
After hot rolling, the hot rolled thin steel strip is cooled. In each of the embodiments, the steel strip undergoes the surface homogenization process after cooling. It is appreciated that cooling may be accomplished by any known manner. In certain instances, when cooling the thin steel strip, the thin steel strip is cooled to a temperature equal to or less than a martensite start transformation temperature Ms to thereby form martensite from prior austenite within the thin steel strip.
An embodiment of a high strength martensitic steel sheet is made by the steps comprising: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) solidifying at a heat flux greater than 10.0 MW/m2 into a steel sheet less than or equal to 2.5 mm in thickness and cooling the sheet in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling; (c) high friction rolling the thin cast steel strip to a hot rolled thickness of between a 15% and 50% reduction of the as cast thickness producing a hot rolled steel strip free of prior-austenite grain boundary depressions; (d) rapidly cooling to form a steel sheet with a microstructure having by volume at least 75% martensite or at least 75% martensite plus bainite, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 Mpa and an elongation of between 1% and 10%; and I surface homogenizing the high friction hot rolled steel strip producing a high friction hot rolled steel strip having a pair of opposing high friction hot rolled homogenized surfaces free of the smear pattern. Here and elsewhere in this disclosure elongation means total elongation. “Rapidly cooling” means to cool at a rate of more than 100° C./s to between 100 and 200° C. Rapidly cooling the present compositions, with an addition of nickel, achieves up to more than 95% martensitic phase steel strip. In one example, rapidly cooling forms a steel sheet with a microstructure having by volume at least 95% martensite or at least 95% martensite plus bainite. The addition of nickel must be sufficient enough to shift the ‘peritectic point’ away from the carbon region that would otherwise be present in the same composition without the addition of nickel. Specifically, the nickel in the composition is believed to contribute to the shifting of the peritectic point away from the carbon region and/or increases a transition temperature of the peritectic point of the composition, which appears to inhibit defects and results in a high strength martensitic steel sheet that is defect free.
Additional variations of the examples of a high friction rolled high strength martensitic steel follow. In some examples, the steel strip may comprise a pair of opposing high friction hot rolled homogenized surfaces substantially free of prior austenite grain boundary depressions and smear pattern. In yet another example, the steel strip may further comprise a pair of opposing high friction hot rolled homogenized surfaces primarily free of prior austenite grain boundary depressions and a smear pattern. In each of these examples, the surfaces may have a surface roughness (Ra) that is not more than 2.5 μm.
In some examples the thin steel strip may be further tempered at a temperature between 150° C. and 250° C. for between 2 and 6 hours. Tempering the steel strip provides improved elongation with minimal loss in strength. For example, a steel strip having a yield strength of 1250 MPa, tensile strength of 1600 MPa and an elongation of 2% was improved to a yield strength of 1250 MPa, tensile strength of 1525 MPa and an elongation of 5% following tempering as described herein.
The steel strip may further comprise by weight greater than 0.005% niobium or greater than 0.01% or 0.02% niobium. The steel strip may comprise by weight greater than 0.05% molybdenum or greater than 0.1% or 0.2% molybdenum. The steel strip may be silicon killed containing by weight less than 0.008% aluminum or less than 0.006% aluminum. The molten melt may have a free oxygen content between 5 to 70 ppm. The steel strip may have a total oxygen content greater than 50 ppm. The inclusions include MnOSiO2 typically with 50% less than 5 μm in size and have the potential to enhance microstructure evolution and, thus, the strip mechanical properties.
The molten melt may be solidified at a heat flux greater than 10.0 MW/m2 into a steel strip less than 2.5 mm in thickness, and cooled in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s. A non-oxidizing atmosphere is an atmosphere typically of an inert gas such as nitrogen or argon, or a mixture thereof, which contains less than about 5% oxygen by weight.
In some embodiments, the martensite in the steel strip may come from an austenite grain size of greater than 100 μm. In other embodiments, the martensite in the steel strip may come from an austenite grain size of greater than 150 μm. Rapid solidification at heat fluxes greater than 10 MW/m2 enables the production of an austenite grain size that is responsive to controlled cooling after subsequent hot rolling to enable the production of a defect free strip.
A high friction rolled steel sheet may be provided for use in hot-stamping applications. Generally, steel sheets relied on for use in hot-stamping applications are of stainless-steel compositions or require an aluminum-silicon corrosion resistant coating. In a hot-stamping application a corrosion resistant protective layer is desired while maintaining high-strength properties and favorable surface structure characteristics. The present high friction rolled compositions have achieved the desired properties without relying on stainless steel compositions or otherwise providing an aluminum-silicon corrosion resistant coating. Instead, the present high friction rolled compositions rely on a mixture of nickel, chromium, and copper, as illustrated in the various examples above, for improved corrosion resistance. In the hot-stamping application the high friction rolled steel sheet undergoes an austenitizing condition at between 900° C. and 930° C. for a period of between 6 minutes and 10 minutes. In one example, the high friction rolled steel sheet undergoes an austenitizing condition at 900° C. for a period of 6 minutes. In another example, the high friction rolled steel sheet undergoes an austenitizing condition at 900° C. for a period of 10 minutes. In yet another example, the high friction rolled steel sheet undergoes an austenitizing condition at 930° C. for a period of 6 minutes. In still yet another example, the high friction rolled steel sheet undergoes an austenitizing condition at 930° C. for a period of 10 minutes. Table 8, below, illustrates the properties of a high friction rolled steel sheet are maintained above a minimum tensile strength of 1500 MPa, a minimum yield strength of 1100 MPa, and a minimum elongation of 3% for a hot-stamping application.
In these examples, a steel sheet provided for use in a hot-stamping application may comprise a composition of any one of the examples of the steel sheets disclosed above, but, is a steel sheet which may remain unquenched. Specifically, a steel sheet provided for use in a hot-stamping application may be made by the steps comprising: (a) preparing a molten steel melt comprising: (i) by weight, between 0.20% and 0.40% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting; (b) solidifying at a heat flux greater than 10.0 MW/m2 into a steel sheet less than or equal to 2.5 mm in thickness and cooling the sheet in a non-oxidizing atmosphere to below 1080° C. and above Ar3 temperature at a cooling rate greater than 15° C./s before rapidly cooling; (c) high friction rolling the thin cast steel strip to a hot rolled thickness of between a 15% and 50% reduction of the as cast thickness producing a hot rolled steel strip primarily free, substantially free, or free of prior austenite grain boundary depressions and having a smear pattern; and (d) cooling at less than 100° C./s to form a steel sheet having a microstructure of primarily bainite. In other words, a steel sheet provided for use in a hot-stamping application may be any one of the examples of the steel sheets disclosed above with the exception that the steel sheet is not rapidly cooled and, thereby, the microstructure having primarily or substantially martensite or martensite plus bainite is not formed. Instead, the steel sheet provide for use in a hot-stamping application is cooled at less than 100° C./s.
Hot Rolling, Including Low Friction Hot Rolling and High Friction Hot Rolling
Hot rolling and, more specifically, low friction rolling and high friction rolling, as relied on in the above examples of the present disclosure, is further described below. The concepts as described below may be applied to the examples provided above as necessary to achieve the properties of each respective example. Generally, in each of the hot rolled examples, the strip is passed through the hot mill to reduce the as-cast thickness before the strip is cooled, such as to a temperature at which austenite in the steel transforms to martensite in particular embodiments. In particular instances, the hot solidified strip (the cast strip) may be passed through the hot mill while at an entry temperature greater than 1050° C., and in certain instances up to 1150° C. After the strip exits the hot mill, the strip is cooled such as, in certain exemplary instances, to a temperature at which the austenite in the steel transforms to martensite by cooling to a temperature equal to or less than the martensite start transformation temperature Ms. In certain instances, this temperature is <600° C., where the martensite start transformation temperature Ms is dependent on the particular composition. Cooling may be achieved by any known methods using any known mechanism(s), including those described above. In certain instances, the cooling is sufficiently rapid to avoid the onset of appreciable ferrite, which is also influenced by composition. In such instances, for example, the cooling is configured to reduce the temperature of the strip at the rate of about 100° C. to 200° C. per second.
Hot rolling is performed using one or more pairs of opposing work rolls. Work rolls are commonly employed to reduce the thickness of a substrate, such as a plate or strip. This is achieved by passing the substrate through a gap arranged between the pair of work rolls, the gap being less than the thickness of the substrate. The gap is also referred to as a roll bite. During hot working, a force is applied to the substrate by the work rolls, thereby applying a rolling force on the substrate to thereby achieve a desired reduction in the substrate thickness. In doing so, friction is generated between the substrate and each work roll as the substrate translates through the gap. This friction is referred to as roll bite friction.
Traditionally, the desire is to reduce the bite friction during hot rolling of steel plates and strips. By reducing the bite friction (and therefore the friction coefficient), the rolling load and roll wear are reduced to extend the life of the machine. Various techniques have been employed to reduce roll bite friction and the coefficient of friction. In certain exemplary instances, the thin steel strip is lubricated to reduce the roll bite friction. Lubrication may take the form of oil, which is applied to rolls and/or thin steel strip, or of oxidation scale formed along the exterior of the thin steel strip prior to hot rolling. By employing lubrication, hot rolling may occur in a low friction condition, where the coefficient of friction (p) for the roll bite is less than 0.20.
In one example, the friction coefficient (p) is determined based upon a hot rolling model developed by HATCH for a particular set of work rolls. The model is shown in
In certain exemplary instances, the coefficient of friction is equal to or greater than 0.20. In other exemplary instances, the coefficient of friction is equal to or greater than 0.25, equal to or greater than 0.268 or equal to or greater than 0.27. It is appreciated that these friction coefficients are sufficient, under certain conditions for austenitic steel (which is the steel alloy employed in the examples shown in the figures), where during hot rolling, the steel is austenitic but after cooling martensite is formed having prior austenite grains and prior austenite grain boundary depressions present, to at least primarily or substantially eliminate prior austenite grain boundary depressions from hot rolled surfaces and to generate elongated surface features plastically formed by shear. As noted previously, various factors or parameters may be altered to attain a desired coefficient of friction under certain conditions. It is noted that for the coefficient of friction values previously described, for substrates having a thickness of 5 mm or less prior to hot rolling the normal force applied to the substrate during hot rolling may be 600 to 2500 tons while the substrate and enters the pair of work rolls and translates, or advances, at a rate of 45 to 75 meters per minute (m/min) where the temperature of the substrate entering the work rolls is greater than 1050° C., and in certain instances, up to 1150° C. For these coefficients of friction, the work rolls have a diameter of 400 to 600 mm. Of course, variations outside each of these parameter ranges may be employed as desired to attain different coefficients of friction as may be desired to achieve the hot rolled surface characteristics described herein.
In one example, hot rolling is performed under a high friction condition with a coefficient of friction of 0.25 at 60 meters per minute (m/min) at a reduction of 22% with a work roll force of approximately 820 tons. In another example, hot rolling is performed under a high friction condition with a coefficient of friction of 0.27 at 60 meters per minute (m/min) at a reduction of 22% with a work roll force of approximately 900 tons.
As relied on in the examples of the present disclosure, hot rolling of the thin steel strip is performed while the thin steel strip is at a temperature above the Ar3 temperature. The Ar3 temperature is the temperature at which austenite begins to transform to ferrite during cooling. In other words, the Ar3 temperature is the point of austenite transformation. The Ar3 temperature is located a few degrees below the A3 temperature. Below the Ar3 temperature, alpha ferrite forms. These temperatures are shown in an exemplary CCT diagram in
Still referring to
In the exemplary CCT diagram shown in
While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described, and that all changes and modifications that come within the spirit of the invention described by the following claims are desired to be protected. Additional features of the invention will become apparent to those skilled in the art upon consideration of the description. Modifications may be made without departing from the spirit and scope of the invention.
Claims
1. An ultra-high strength weathering steel pile comprising:
- a plurality of sidewalls, each having a thickness of about 2.5 mm or less and a composition comprising, by weight, (i) between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and (ii) the remainder iron and impurities resulting from melting;
- the pile having a corrosion index of 6.0 or greater, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and an elongation of between 1% and 10%.
2. The ultra-high strength weathering steel pile of claim 1 wherein the thickness is about 2.0 mm or less.
3. The ultra-high strength weathering steel pile of claim 1 wherein the thickness is about 1.6 mm or less.
4. The ultra-high strength weathering steel pile of claim 1 cold formed from a steel strip where the thickness is a hot rolled thickness that is between a 15% and 50% reduction of the thickness.
5. The ultra-high strength weathering steel pile of claim 4 wherein the hot rolled thickness is a high friction rolled thickness.
6. The ultra-high strength weathering steel pile of claim 1 that is a hemmed C-channel wherein the plurality of sidewalls are a web and one or more flanges.
7. The ultra-high strength weathering steel pile of claim 1 that is a corrugated C-channel wherein the plurality of sidewalls are a web and one or more flanges.
8. The ultra-high strength weathering steel pile of claim 1 that is a corrugated tube.
9. The ultra-high strength weathering steel pile of claim 8 wherein the corrugated tube is generally square.
10. The ultra-high strength weathering steel pile of claim 8 wherein the corrugated tube is generally rectangular.
11. The ultra-high strength weathering steel pile of claim 1 wherein the plurality of sidewalls do not have a separately applied coating.
12. The ultra-high strength weathering steel pile of claim 1 wherein the plurality of sidewalls are ungalvanized.
13. The ultra-high strength weathering steel pile of claim 1 wherein at least one sidewall of the plurality of sidewalls comprises one or more corrugations.
14. The ultra-high strength weathering steel pile of claim 1 wherein each sidewall of the plurality of sidewalls comprise one or more corrugations.
15. A solar arrangement comprising:
- an ultra-high strength weathering steel pile comprising:
- a plurality of sidewalls, each having a thickness of about 2.5 mm or less and a composition comprising:
- (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, between 0.5% and 1.5% nickel, and silicon killed containing less than 0.01% aluminum, and
- (ii) the remainder iron and impurities resulting from melting;
- the ultra-high strength weathering steel pile having a corrosion index of 6.0 or greater, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and a elongation of between 1% and 10%; and
- wherein a partial length of the ultra-high strength weathering steel pile is driven into a ground surface and one or more solar cells are supported above the ground surface by the ultra-high strength weathering steel pile.
16. The solar arrangement of claim 15 wherein the plurality of sidewalls are a web and one or more flanges and form a hemmed C-channel.
17. The solar arrangement of claim 15 wherein the plurality of sidewalls are a web and one or more flanges and form a corrugated C-channel.
18. The solar arrangement of claim 15 wherein the plurality of sidewalls form a corrugated tube.
19. An ultra-high strength weathering steel pile comprising:
- a plurality of sidewalls, each having a thickness of about 2.5 mm or less, a corrosion index of 6.0 or greater, a yield strength of between 700 and 1600 MPa, a tensile strength of between 1000 and 2100 MPa, and a elongation of between 1% and 10%.
20. The ultra-high strength weathering steel pile of claim 19 having a composition comprising:
- (i) by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than or equal to 0.12% niobium, less than 0.5% molybdenum, silicon killed containing less than 0.01% aluminum, and an amount of nickel sufficient for shifting a peritectic point away from the carbon region and/or increasing a transition temperature of the peritectic point to form a carbon alloy steel strip having a microstructure of at least 75% by volume martensite or martensite plus bainite, and
- (ii) the remainder iron and impurities resulting from melting.
Type: Application
Filed: Feb 7, 2020
Publication Date: Aug 13, 2020
Inventors: Tao WANG (Germantown, TN), Kishlay MISHRA (Memphis, TN), Brian Hansen BOGH (Calimesa, CA)
Application Number: 16/784,826
