Patent Description:
Weathering steel is a high strength low alloy steel resistant to atmospheric corrosion. In the presence of moisture and air, low alloy steels oxidize, the rate of which depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. As the 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. The result is a much lower corrosion rate than would be found on ordinary 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 <NUM>, which contains at least <NUM>% copper based on cast or heat analysis (<NUM>% minimum Cu for product check); and Type <NUM>, which contains additional alloying elements to provide a corrosion index of at least <NUM> 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.

Weathering steel's yield strength allows cost reduction through the ability to design lighter sections into structures. In the past, high strength weathering low-carbon thin strip has been made by recovery annealing of cold rolled strip. Cold rolling was required to produce the desired thickness. The cold rolled strip was then recovery annealed to improve ductility without significantly reducing the strength. However, the final ductility of the resulting strip still was relatively low and the strip would not achieve total elongation levels over <NUM>%, which is required for structural steels by building codes. Such recovery annealed cold rolled, low-carbon steel was generally suitable only for simple forming operations, e.g., roll forming and bending. To produce this steel strip with higher ductility was not technically feasible in these final strip thicknesses using the cold rolled and recovery annealed manufacturing route.

High strength weathering low-carbon steel strip has also been made by microalloying with elements such as niobium (Nb), vanadium (V), titanium (Ti) or molybdenum (Mo), and hot rolling to achieve the desired thickness and strength level. Additions of nickel (Ni), copper (Cu) and silicon (Si) to the microalloying were used to obtain the corrosion-resistance properties. Microalloying required expensive and high levels of niobium, vanadium, titanium or molybdenum and resulted in formation of a bainite-ferrite microstructure typically with <NUM>% to <NUM>% bainite. Alternately, the microalloying could result in formation of a ferrite microstructure with <NUM>% to <NUM>% pearlite.

Hot rolling the strip resulted in the partial precipitation of these alloying elements. As a result, relatively high alloying levels of the Nb, V, Ti or Mo elements were required to provide enough precipitation hardening of the predominately ferritic transformed microstructure to achieve the required strength levels. These high microalloying levels significantly raised the hot rolling loads needed and restricted the thickness range of the hot rolled strip that could be economically and practically produced.

As such, making of high strength low-carbon steel strip less than <NUM> in thickness with microalloying additions of Nb, V, Ti and/or Mo to the base steel chemistry has been very difficult, particularly for wide strip due to the high rolling loads, and not always commercially feasible. For thinner thicknesses of strip, cold rolling was required; however, the high strength of the hot rolled strip made such cold rolling difficult because of the high cold roll loadings required to reduce the thickness of the strip. These high alloying levels also considerably raised the recrystallization annealing temperature needed, requiring expensive to build and difficult to operate annealing lines capable of achieving the high annealing temperature needed for full recrystallization annealing of the cold rolled strip.

Addition of phosphorus is also currently used to improve machining characteristics and atmospheric corrosion resistance in steels. For example, Chinese Patent Application Publications Nos. <CIT>, <CIT>, and <CIT>, all show purposeful addition of phosphorus between <NUM>% to <NUM>% to improve corrosion resistance of steel composition. However, phosphorus causes embrittlement which reduces toughness and ductility. For example, phosphorus causes temper embrittlement in heat-treated low-alloy steels resulting from segregation of phosphorus and other impurities at prior austenite grain boundaries. Additionally, phosphorus content greater than <NUM>% makes weld brittle and increases the tendency to crack. The surface tension of the molten weld metal is lowered, making it difficult to control.

In short, the application of previously known microalloying practices with Ni, V, Ti and/or Mo elements and the purposeful addition of phosphorus to produce high strength weathering low-carbon thin strip are not practicable methods. The high alloying costs, difficulties with high rolling loads in hot rolling and cold rolling, the high recrystallization annealing temperatures required, and phosphorus harmful effects are problems with the existing process for manufacturing high strength weathering steel. As such, there is still a need for developing an economically feasible and effective method to produce high strength weathering or corrosion-resistant thin steel.

<CIT>, which is considered to represent the closest prior art, discloses a thin steel strip including, by weight, less than <NUM>% carbon, between <NUM> and <NUM>% manganese, between <NUM> and <NUM>% silicon, less than <NUM>% aluminium, niobium between <NUM> and <NUM>%, and vanadium between <NUM> and <NUM>%, the strip having a tensile strength between <NUM> and 600MPa after age hardening at a peak temperature of between <NUM> and <NUM> degrees centigrade.

According to a first aspect of the invention there is provided a method of making weathering steel as defined in claim <NUM>.

The age hardened steel strip may be batch annealed at a temperature greater than <NUM> between <NUM> and <NUM> hours. The age hardened steel strip by batch annealing may have a yield strength of at least <NUM> MPa and a total elongation of at least <NUM>%.

Alternatively, the age hardened cast strip may be in-line annealed at a temperature between <NUM> and <NUM> for less than <NUM> minutes. The age hardened steel strip by in-line annealing may have a yield strength of at least <NUM> MPa and a total elongation of at least <NUM>%.

In one embodiment, weathering steel is continuously cast and the method further comprises the steps of: assembling a pair of counter-rotatable casting rolls to form a nip there between through which a thin strip can be casted, and capable of supporting a casting pool of molten metal formed on casting surfaces of the casting rolls above the nip with a pair of confining side dams adjacent the ends of the casting rolls; assembling a delivery system with metal delivery nozzle or nozzles disposed axially above the nip and capable of discharging molten metal to form the casting pool supported on the casting rolls above the nip with a pair of confining side dams adjacent the ends of the casting rolls; assembling a delivery system with metal delivery nozzle or nozzles disposed axially above the nip and capable of discharging molten metal to form the casting pool supported on the casting rolls; solidifying the molten metal delivered from the casting pool on the casting surfaces of the casting rolls in a non-oxidizing atmosphere and forming at the nip between the casting rolls a cast strip delivered downwardly that is less than <NUM> in thickness with a corrosion index of at least <NUM> comprising, by weight, of between <NUM>% and <NUM>% carbon, less than <NUM>% silicon, between <NUM>% and <NUM>% manganese, less than <NUM>% phosphorus, less than <NUM>% sulfur, less than <NUM>% nitrogen, between <NUM>% and <NUM>% copper, between <NUM>% and <NUM>% niobium, between <NUM>% and <NUM>% vanadium, between <NUM>% and <NUM>% chromium, between <NUM>% and <NUM>% nickel, less than <NUM>% aluminum, and the remainder iron and impurities from melting.

The present invention further provides a twin roll cast and hot rolled weathering thin cast steel strip according to claim <NUM>.

The steel strip may be age hardened and may have a yield strength of at least <NUM> MPa and a total elongation of at least <NUM>%.

In order that the invention may be described in more detail, some illustrative examples will be given with reference to the accompanying drawings in which:.

The following description of the embodiments is in the context of high strength thin cast strip with microalloy additions made by continuous casting steel strip using a twin roll caster.

Referring now to <FIG>, <FIG>, and <FIG>, a twin roll caster is illustrated that comprises a main machine frame <NUM> that stands up from the factory floor and supports a pair of counter-rotatable casting rolls <NUM> mounted in a module in a roll cassette <NUM>. The casting rolls <NUM> are mounted in the roll cassette <NUM> for ease of operation and movement as described below. The roll cassette <NUM> facilitates rapid movement of the casting rolls <NUM> ready for casting from a setup position into an operative casting position as a unit in the caster, and ready removal of the casting rolls <NUM> from the casting position when the casting rolls <NUM> are to be replaced. There is no particular configuration of the roll cassette <NUM> that is desired, so long as it performs that function of facilitating movement and positioning of the casting rolls <NUM> as described herein.

The casting apparatus for continuously casting thin steel strip includes the pair of counter-rotatable casting rolls <NUM> having casting surfaces 12A laterally positioned to form a nip <NUM> there between. Molten metal is supplied from a ladle <NUM> through a metal delivery system to a metal delivery nozzle <NUM> (core nozzle) positioned between the casting rolls <NUM> above the nip <NUM>. Molten metal thus delivered forms a casting pool <NUM> of molten metal above the nip <NUM> supported on the casting surfaces 12A of the casting rolls <NUM>. This casting pool <NUM> is confined in the casting area at the ends of the casting rolls <NUM> by a pair of side closure plates, or side dams <NUM> (shown in dotted line in <FIG>). The upper surface of the casting pool <NUM> (generally referred to as the "meniscus" level) may rise above the lower end of the delivery nozzle <NUM> so that the lower end of the delivery nozzle <NUM> is immersed within the casting pool <NUM>. The casting area includes the addition of a protective atmosphere above the casting pool <NUM> to inhibit oxidation of the molten metal in the casting area.

The ladle <NUM> typically is of a conventional construction supported on a rotating turret <NUM>. For metal delivery, the ladle <NUM> is positioned over a movable tundish <NUM> in the casting position to fill the tundish <NUM> with molten metal. The movable tundish <NUM> may be positioned on a tundish car <NUM> capable of transferring the tundish <NUM> from a heating station (not shown), where the tundish <NUM> is heated to near a casting temperature, to the casting position. A tundish guide, such as rails <NUM>, may be positioned beneath the tundish car <NUM> to enable moving the movable tundish <NUM> from the heating station to the casting position.

The movable tundish <NUM> may be fitted with a slide gate <NUM>, actuable by a servo mechanism, to allow molten metal to flow from the tundish <NUM> through the slide gate <NUM>, and then through a refractory outlet shroud <NUM> to a transition piece or distributor <NUM> in the casting position. From the distributor <NUM>, the molten metal flows to the delivery nozzle <NUM> positioned between the casting rolls <NUM> above the nip <NUM>.

The side dams <NUM> may be made from a refractory material such as zirconia graphite, graphite alumina, boron nitride, boron nitride-zirconia, or other suitable composites. The side dams <NUM> have a face surface capable of physical contact with the casting rolls <NUM> and molten metal in the casting pool <NUM>. The side dams <NUM> are mounted in side dam holders (not shown), which are movable by side dam actuators (not shown), such as a hydraulic or pneumatic cylinder, servo mechanism, or other actuator to bring the side dams <NUM> into engagement with the ends of the casting rolls <NUM>. Additionally, the side dam actuators are capable of positioning the side dams <NUM> during casting. The side dams <NUM> form end closures for the molten pool of metal on the casting rolls <NUM> during the casting operation.

<FIG> shows the twin roll caster producing the cast strip <NUM>, which passes across a guide table <NUM> to a pinch roll stand <NUM>, comprising pinch rolls 31A. Upon exiting the pinch roll stand <NUM>, the thin cast strip <NUM> may pass through a hot rolling mill <NUM>, comprising a pair of work rolls 32A, and backup rolls 32B, forming a gap capable of hot rolling the cast strip <NUM> delivered from the casting rolls <NUM>, where the cast strip <NUM> is hot rolled to reduce the strip to a desired thickness, improve the strip surface, and improve the strip flatness. The work rolls 32A have work surfaces relating to the desired strip profile across the work rolls 32A. The hot rolled cast strip <NUM> then passes onto a run-out table <NUM>, where it may be cooled by contact with a coolant, such as water, supplied via water jets <NUM> or other suitable means, and by convection and radiation. In any event, the hot rolled cast strip <NUM> may then pass through a second pinch roll stand <NUM> having roller 91A to provide tension of the cast strip <NUM>, and then to a coiler <NUM>.

At the start of the casting operation, a short length of imperfect strip is typically produced as casting conditions stabilize. After continuous casting is established, the casting rolls <NUM> are moved apart slightly and then brought together again to cause this leading end of the cast strip <NUM> to break away forming a clean head end of the following cast strip <NUM>. The imperfect material drops into a scrap receptacle <NUM>, which is movable on a scrap receptacle guide. The scrap receptacle <NUM> is located in a scrap receiving position beneath the caster and forms part of a sealed enclosure <NUM> as described below. The enclosure <NUM> is typically water cooled. At this time, a water-cooled apron <NUM> that normally hangs downwardly from a pivot <NUM> to one side in the enclosure <NUM> is swung into position to guide the clean end of the cast strip <NUM> onto the guide table <NUM> that feeds it to the pinch roll stand <NUM>. The apron <NUM> is then retracted back to its hanging position to allow the cast strip <NUM> to hang in a loop beneath the casting rolls <NUM> in enclosure <NUM> before it passes to the guide table <NUM> where it engages a succession of guide rollers.

An overflow container <NUM> may be provided beneath the movable tundish <NUM> to receive molten material that may spill from the tundish <NUM>. As shown in <FIG>, the overflow container <NUM> may be movable on rails <NUM> or another guide such that the overflow container <NUM> may be placed beneath the movable tundish <NUM> as desired in casting locations. Additionally, an optional overflow container (not shown) may be provided for the distributor <NUM> adjacent the distributor <NUM>.

The sealed enclosure <NUM> is formed by a number of separate wall sections that fit together at various seal connections to form a continuous enclosure wall that permits control of the atmosphere within the enclosure <NUM>. Additionally, the scrap receptacle <NUM> may be capable of attaching with the enclosure <NUM> so that the enclosure <NUM> is capable of supporting a protective atmosphere immediately beneath the casting rolls <NUM> in the casting position. The enclosure <NUM> includes an opening in the lower portion of the enclosure <NUM>, lower enclosure portion <NUM>, providing an outlet for scrap to pass from the enclosure <NUM> into the scrap receptacle <NUM> in the scrap receiving position. The lower enclosure portion <NUM> may extend downwardly as a part of the enclosure <NUM>, the opening being positioned above the scrap receptacle <NUM> in the scrap receiving position. As used in the specification and claims herein, "seal," "sealed," "sealing," and "sealingly" in reference to the scrap receptacle <NUM>, enclosure <NUM>, and related features may not be a complete seal so as to prevent leakage, but rather is usually less than a perfect seal as appropriate to allow control and support of the atmosphere within the enclosure <NUM> as desired with some tolerable leakage.

A rim portion <NUM> may surround the opening of the lower enclosure portion <NUM> and may be movably positioned above the scrap receptacle <NUM>, capable of sealingly engaging and/or attaching to the scrap receptacle <NUM> in the scrap receiving position. The rim portion <NUM> may be movable between a sealing position in which the rim portion <NUM> engages the scrap receptacle <NUM>, and a clearance position in which the rim portion <NUM> is disengaged from the scrap receptacle <NUM>. Alternately, the caster or the scrap receptacle <NUM> may include a lifting mechanism to raise the scrap receptacle <NUM> into sealing engagement with the rim portion <NUM> of the enclosure <NUM>, and then lower the scrap receptacle <NUM> into the clearance position. When sealed, the enclosure <NUM> and scrap receptacle <NUM> are filled with a desired gas, such as nitrogen, to reduce the amount of oxygen in the enclosure <NUM> and provide a protective atmosphere for the cast strip <NUM>.

The enclosure <NUM> may include an upper collar portion <NUM> supporting a protective atmosphere immediately beneath the casting rolls <NUM> in the casting position. When the casting rolls <NUM> are in the casting position, the upper collar portion <NUM> is moved to the extended position closing the space between a housing portion <NUM> adjacent the casting rolls <NUM>, as shown in <FIG>, and the enclosure <NUM>. The upper collar portion <NUM> may be provided within or adjacent the enclosure <NUM> and adjacent the casting rolls <NUM>, and may be moved by a plurality of actuators (not shown) such as servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, and rotating actuators.

The casting rolls <NUM> are internally water cooled as described below so that as the casting rolls <NUM> are counter-rotated, shells solidify on the casting surfaces 12A, as the casting surfaces 12A move into contact with and through the casting pool <NUM> with each revolution of the casting rolls <NUM>. The shells are brought close together at the nip <NUM> between the casting rolls <NUM> to produce a thin cast strip product <NUM> delivered downwardly from the nip <NUM>. The thin cast strip product <NUM> is formed from the shells at the nip <NUM> between the casting rolls <NUM> and delivered downwardly and moved downstream as described above.

A strip thickness profile sensor <NUM> may be positioned downstream to detect the thickness profile of the cast strip <NUM> as shown in <FIG> and <FIG>. The strip thickness sensor <NUM> may be provided between the nip <NUM> and the pinch rolls 31A to provide for direct control of the casting roll <NUM>. The sensor may be an x-ray gauge or other suitable device capable of directly measuring the thickness profile across the width of the strip periodically or continuously. Alternatively, a plurality of non-contact type sensors may be arranged across the cast strip <NUM> at the roller table <NUM> and the combination of thickness measurements from the plurality of positions across the cast strip <NUM> are processed by a controller <NUM> to determine the thickness profile of the strip periodically or continuously. The thickness profile of the cast strip <NUM> may be determined from this data periodically or continuously as desired.

Currently disclosed is a high strength weathering thin cast strip produced using a twin roll caster and overcoming the shortcomings of conventional light gauge steel products. The currently claimed invention utilizes the elements such as niobium (Nb), vanadium (V), copper (Cu), nickel (Ni), or molybdenum (Mo), or a combination thereof, without the purposeful addition of phosphorus. The residual amount of phosphorus present in the steel composition may be due to, for example, from scrap metal used to charge an electric arc furnace. The currently disclosed high strength thin cast strip and method to produce thereof combine several attributes to achieve a high strength light gauge cast strip by microalloying with these elements.

The currently disclosed high strength weathering thin cast strip is produced by hot rolling without the need for cold rolling to further reduce the strip to the desired thickness. Thus, the high strength thin cast strip overlaps both the light gauge hot rolled thickness ranges and the cold rolled thickness ranges desired. Strip thicknesses may be less than <NUM>, less than <NUM>, less than <NUM>, or less than <NUM>, and may be in a range of <NUM> to <NUM>. The strip may be hot rolled in an austenitic temperature range above Ar<NUM> to between <NUM>% and <NUM>% reduction. The strip may be cooled at a rate <NUM> per second and above, and still form a microstructure that is a majority and typically predominantly bainite and acicular ferrite with more than <NUM>% niobium in solid solution and having a yield strength of at least <NUM> MPa and a total elongation of at least <NUM>%.

After hot rolling, the hot rolled steel strip may be coiled below <NUM>. The thin cast steel strip may also be further processed by age hardening the steel strip by batch annealing at a temperature greater than <NUM> in less than <NUM> hours. The age hardened steel may have a yield strength of at least <NUM> MPa and a total elongation of at least <NUM>%. Alternatively, the thin cast steel strip may also be further processed by age hardening the steel strip by in-line annealing at a temperature between <NUM> and <NUM> in less than <NUM> minutes. The age hardened steel may have a yield strength of at least <NUM> MPa and a total elongation of at least <NUM>%.

For example, a steel composition was prepared by the currently disclosed method comprising <NUM>% by weight carbon, <NUM>% by weight copper, <NUM>% by weight niobium, <NUM>% by weight vanadium, <NUM>% by weight silicon, <NUM>% by weight chromium, <NUM>% by weight nickel, <NUM>% by weight manganese, <NUM>% by weight aluminum and a residual amount of <NUM>% by weight phosphorus. The cast strip was hot rolled at a temperature <NUM> to a reduction between <NUM>% and <NUM>%. The hot rolled cast strip was coiled at coiling temperatures between <NUM> and <NUM> and age hardened. This composition produced a calculated corrosion index of <NUM> following the procedure of ASTM G101, Standard Guide for Estimating the Atmospheric Corrosion Resistance of Low Alloy Steels.

Claim 1:
A method of making weathering steel comprising the steps of:
a. preparing a molten melt for producing an as-cast carbon alloy steel strip less or equal to <NUM> in thickness with a corrosion index of at least <NUM> calculated in accordance with the formula defined in paragraph <NUM>.<NUM> of ASTM G101 comprising, by weight, between <NUM>% and <NUM>% carbon, less than <NUM>% silicon, between <NUM>% and <NUM>% manganese, less than <NUM>% phosphorus, less than <NUM>% sulfur, less than <NUM>% nitrogen, between <NUM>% and <NUM>% copper, between <NUM>% and <NUM>% niobium, between <NUM>% and <NUM>% vanadium, between <NUM>% and <NUM>% chromium, between <NUM>% and <NUM>% nickel, less than <NUM>% aluminum, and the remainder iron and impurities from making the molten melt;
b. solidifying and cooling the molten melt into a cast strip less than or equal to <NUM> in thickness in a non-oxidizing atmosphere;
c. hot rolling the cast strip in an austenitic temperature range above Ar<NUM> to between <NUM>% and <NUM>% reduction;
d. cooling the hot rolled cast strip at above <NUM> per second and coiling the cast strip below <NUM> to form a steel strip with a microstructure comprising bainite and acicular ferrite with more than <NUM>% niobium in solid solution; and
e. age hardening the steel strip forming an age hardened steel strip having a yield strength of at least <NUM> MPa and a total elongation of at least <NUM>%.