Alloys or metallic compositions – Ferrous – Chromium containing – but less than 9 percent
Reexamination Certificate
1999-10-21
2001-11-13
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Chromium containing, but less than 9 percent
C420S109000, C420S091000, C148S332000, C148S333000, C148S335000
Reexamination Certificate
active
06315946
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention includes ultra low carbon steel compositions. More particularly, the ultra low carbon steel compositions are weathering steels having ultra low carbon compositions with copper, chromium, nickel, titanium, niobium and boron components. Most particularly, the steel of the present invention exhibits good weldability and low temperature fracture toughness at a minimum yield strength of 65,000 psi in plate sections of two inch thickness. The ultra low carbon compositions are useful in structural steel applications, such as ship and highway bridge construction.
2. Brief Description of the Related Art
Ultra low carbon bainitic (ULCB) steels rely on extremely low carbon contents and high hardenability to obtain good combinations of strength and toughness due to the fine bainitic ferrite that forms and the limited, and possibly nonexistent, amount of carbides. High strength and toughness are obtained in a ULCB steel with proper plate processing. Typical ULCB steels have high levels of nickel and molybdenum to obtain the required hardenability while other ULCB steels rely on boron microalloying.
Weathering steels are known, as are the advantages inherent in their use. Weathering steels develop a thin film of oxide that protects the underlying metals from further oxidation. As such, the weathering steel forms its own protective coating and does not require a separate covering. Paint protection against corrosion is not required of the weathering steels, allowing for ease of maintenance once incorporated into a structure. The thickness of the metal oxide layer generally is less than 0.1 mm, requiring between two and four years to form and stabilize. Weathering steels are economically desirable despite the higher cost relative to plain carbon steel. Weathering steels possess several drawbacks in addition to initial cost. After two or four years, the final appearance of the exposed surface may look rusty or otherwise unattractive. Additionally, the toughness of weathering steels is generally not dependably reproducible, limiting their use.
There is a need in the art to provide ultra low carbon weathering steel having good weldability and low temperature fracture toughness for structural steel applications. The present invention addresses this and other needs.
SUMMARY OF THE INVENTION
The present invention includes an ultra-low carbon weathering steel comprising a steel having a carbon content of from about 0.015 wt % to about 0.035 wt %; a copper content of from about 0.20 wt % to about 0.40 wt %; a chromium content of from about 0.40 wt % to about 0.70 wt %; a nickel content of from about 0.20 wt % to about 0.50 wt %; a titanium content of from about 0.01 wt % to about 0.05 wt %; a niobium content of from about 0.03 wt % to about 0.06 wt %; a boron content of from about 0.0015 wt % to about 0.003 wt %; a manganese content of from about 2.0 wt % or less; a phosphorous content of from about 0.012 wt % or less; a sulphur content of from about 0.005 wt % or less; a silicon content of from about 0.40 wt % or less; a molybdenum content of from about 0.50 wt % or less; a vanadium content of from about 0.10 wt % or less; an aluminum content of from about 0.03 wt % or less; and a nitrogen content of from about 0.006 wt % or less.
The invention also includes an ultra-low carbon weathering steel product made by the process comprising the steps of austenitizing a steel slab comprising a steel having a carbon content of from about 0.015 wt % to about 0.035 wt %; a copper content of from about 0.20 wt % to about 0.40 wt %; a chromium content of from about 0.40 wt % to about 0.70 wt %; a nickel content of from about 0.20 wt % to about 0.50 wt %; a titanium content of from about 0.01 wt % to about 0.05 wt %; a niobium content of from about 0.03 wt % to about 0.06 wt %; a boron content of from about 0.0015 wt % to about 0.003 wt %; a manganese content of from about 2.0 wt % or less; a phosphorous content of from about 0.012 wt % or less; a sulphur content of from about 0.005 wt % or less; a silicon content of from about 0.40 wt % or less; a molybdenum content of from about 0.50 wt % or less; a vanadium content of from about 0.10 wt % or less; an aluminum content of from about 0.03 wt % or less; and a nitrogen content of from about 0.006 wt % or less; conditioning the austenite microstructure of the steel slab at a deforming temperature between the austenitizing temperature and the austenite recrystallization stop temperature followed by deforming the austenite microstructure at a temperature below the austenite recrystallization stop temperature and above the Ar
3
temperature of the slab; deforming the slab to a minimum reduction ratio below the austenite recrystallization stop temperature of from about 2.5:1 or more to form a steel plate; and, cooling the steel plate to ambient temperature.
Furthermore, the present invention includes a method of forming an ultra-low carbon weathering steel comprising the steps of austenitizing a steel slab, conditioning the austenite microstructure of the steel slab at a deforming temperature between the austenitizing temperature and the austenite recrystallization stop temperature followed by deforming the austenite microstructure at a temperature below the austenite recrystallization stop temperature and above the Ar
3
temperature of the slab, deforming the slab to a minimum reduction ratio below the austenite recrystallization stop temperature of from about 2.5:1 or more to form a steel plate; and, cooling the steel plate to ambient temperature.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides an ultra low carbon weathering steel composition, and a method of its manufacture, that is useful in structural applications, such as Navy and commercial ships, buildings, automobiles, offshore rigs, pipelines, highway bridges, and other similar constructions requiring particular structural integrity. The ultra low carbon steel compositions contain copper, chromium, nickel, titanium, niobium and boron components that impart good weldability and low temperature fracture toughness.
The ultra-low carbon weathering steel of the present invention includes a steel having components of carbon (C), copper (Cu), chromium (Cr), nickel (Ni), titanium (Ti), niobium (Nb) and boron (B). Steel within the meaning of the present invention includes an iron based composition having substantially the balance of the steel composition comprising iron. Impurities may be present within the steel composition that do not interfere with the properties imparted by the carbon, copper, chromium, nickel, titanium, niobium and boron.
Carbon is very effective at imparting strength to steel. The amounts retained in the present invention are considered ultra-low compared to mild steels. Thus, other means such as alloying and processing are utilized to strengthen the steel. The ultra-low carbon level retained in the present invention improves weldability and low-temperature toughness by lowering the hardenability of the steel and reducing carbide formation, respectively. The preferred amount of carbon ranges from about 0.015 wt % to about 0.035 wt %, with more preferred amounts from about 0.015 wt % to about 0.025 wt %, and most preferred amounts from about 0.015 wt % to about 0.020 wt %.
Copper improves hardenability and provides an effective precipitation hardening element. When copper is incorporated in an amount of less than 0.20%, there is no substantial effect of the copper addition. Copper is effective to improve the weathering characteristics as well as to raise the strength level of the steel, but tends to develop hot shortness. However, for purposes of the present invention, additions of copper greater than about 0.40 wt % are not effecti
The United States of America as represented by the Secretary of
Yee Deborah
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