Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-07-16
2004-11-16
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C148S335000, C148S653000
Reexamination Certificate
active
06818072
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to high-strength heat-resistant steels, which are suitable for use in a medium-to-high temperature range up to 540° C. and which can be produced at a low cost, processes for producing the high-strength heat-resistant steels, and processes for producing high-strength heat-resistant pipes.
2. Description of the Related Art
Large portions of materials for pressure-sealed parts of piping for used in the highest temperature sections of subcritical-pressure boilers and supercritical-pressure boilers in power plants and waste heat recovery boilers in combined cycle power plants, and semi-high temperature sections of ultra supercritical-pressure boilers, are carbon steels and low alloy steels such as 1Cr steel, 2Cr steel.
Specific examples of low alloy steels which have been used are 0.5Mo steel, (JIS STBA 12), 1Cr-0.5Mo steel, (JIS KA STBA 21, STBA 22, STBA 23), and 2.25Cr-1Mo (JIS STBA 24).
Since large portions of the materials for pressure-tight parts of piping are carbon steels and low alloy steels such as 1Cr steel, 2Cr steel, achievement of sufficient strength of the materials for the parts in which they are used, without increasing the use of alloying elements, would largely contribute to reducing the cost for constructing a power plant.
In Japanese Unexamined Patent Application, First Publication (Kokai), No. Hei 10-195593, the present inventors proposed a steel, having high temperature strength as a material suitable for the above uses, comprising, in % by weight, C in an amount of 0.01 to 0.1%, Si in an amount of 0.15 to 0.5%, Mn in an amount of 0.4 to 2%, V in an amount of 0.01 to 0.3%, Nb in an amount of 0.01 to 0.1%; and the balance being iron and unavoidable impurities. In addition, in Japanese Unexamined Patent Application, First Publication (Kokai), No. 2000-160280, the present inventors also proposed another steel, having high temperature strength comprising, in % by weight, C in an amount of 0.06 to 0.15%, Si in an amount of 0.15% or less, Mn in an amount of 0.5 to 1.5%, V in an amount of 0.05 to 0.3%, at least one selected from Nb, Ti, Ta, Hf, and Zr in an amount of 0.01 to 0.1%, and the balance being iron and unavoidable impurities.
The heat resistant steel proposed as above is a useful steel, which possesses an enhanced high temperature strength in comparison with conventional steels, but which can be produced at a low cost. However, further enhancement of the high temperature strength is desired without increasing the cost.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a high-strength heat-resistant steel comprising; carbon in an amount of 0.06 to 0.15% by weight, silicon in an amount of 1.5% by weight or less, manganese in an amount of 1.5% by weight or less, vanadium in an amount of 0.05 to 0.3% by weight, chromium in an amount of 0.8% by weight or less, molybdenum in an amount of 0.8% by weight or less, at least one selected from niobium, titanium, tantalum, hafnium, and zirconium in an amount of 0.01 to 0.2% by weight, nitrogen in an amount of 20 to 200 ppm, and the balance being iron and unavoidable impurities; and comprising a bainite structure.
Although the high-strength heat-resistant steel contains a small amount of alloying elements, it possesses excellent properties, such a creep rupture strength extrapolated to 10
4
hours at 550° C. being 130 MPa or greater, due to a dispersion of fine carbonitrides which are stable in an operation temperature range into the metallic structure.
In the high-strength heat-resistant steel, when oxidation resistance is regarded as important, it is preferable for silicon to be contained in an amount of 0.6% by weight or greater.
It is preferable for the high-strength heat-resistant steel to comprise at least one selected from cobalt in an amount of 0.5% by weight or less, nickel in an amount of 0.5% by weight or less, and copper in an amount of 0.5% by weight or less. According to the high-strength heat-resistant steel, hardenability thereof is improved.
In the high-strength heat-resistant steel, it is preferable to comprise phosphorous in an amount of 0.03% by weight or less, sulfur in an amount of 0.01% by weight or less, arsenic in an amount of 0.03% by weight or less, antimony in an amount of 0.01% by weight or less, tin in an amount of 0.01% by weight or less, and oxygen in an amount of 0.01% by weight or less. According to the high-strength heat-resistant steel, a creep ductility thereof is improved.
In the high-strength heat-resistant steel, it is preferable to comprise aluminum in an amount of 0.01% by weight or less and calcium in an amount of 0.01% by weight or less. According to the high-strength heat-resistant steel, a creep ductility thereof is improved.
It is preferable for the high-strength heat-resistant steel to comprise at least one selected from lanthanoid containing lanthanum, cerium, yttrium, ytterbium, and neodymium, and for the total content of the lanthanoid to be in an amount of 0.001 to 0.05% by weight. According to the high-strength heat-resistant steel, a creep ductility thereof is further improved.
The high-strength heat-resistant steel can be produced by a process for producing a high-strength heat-resistant steel, the process comprising the steps of: normalizing a steel at a temperature in the range from 1,100 to 1,250° C., the steel comprising carbon in an amount of 0.06 to 0.15% by weight, silicon in an amount of 1.5% by weight or less, manganese in an amount of 1.5% by weight or less, vanadium in an amount of 0.05 to 0.3% by weight, chromium in an amount of 0.8% by weight or less, molybdenum in an amount of 0.8% by weight or less, at least one selected from niobium, titanium, tantalum, hafnium, and zirconium in an amount of 0.01 to 0.2% by weight, nitrogen in an amount of 20 to 200 ppm, and the balance being iron and unavoidable impurities in a temperature; after that hot working such that a final reduction ratio is 50% or greater the steel at a temperature within the range in which austenite recrystallizes; and then cooling the hot worked product to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed.
In addition, the high-strength heat-resistant steel can be also produced by a process for producing a high-strength heat-resistant steel, the process comprising the steps of: preparing an ingot comprising carbon in an amount of 0.06 to 0.15% by weight, silicon in an amount of 1.5% by weight or less, manganese in an amount of 1.5% by weight or less, vanadium in an amount of 0.05 to 0.3% by weight, chromium in an amount of 0.8% by weight or less, molybdenum in an amount of 0.8% by weight or less, at least one selected from niobium, titanium, tantalum, hafnium, and zirconium in an amount of 0.01 to 0.2% by weight, nitrogen in an amount of 20 to 200 ppm, and the balance being iron and unavoidable impurities; hot working such that a final reduction ratio is 50% or greater the ingot at a temperature within the range in which austenite recrystallizes during the process of cooling the ingot; and then cooling the hot worked product to room temperature.
In the process for producing a high-strength heat-resistant steel, after the step of hot working at a temperature in which the range in which austenite recrystallizes, it is possible to heat work in a temperature range from 950° C. to the temperature of the Ar
3
point and cool to room temperature.
In the process for producing a high-strength heat-resistant steel, it is possible to cool to room temperature and normalize the cooled product in an austenite temperature range, or to temper the cooled product at the temperature of the A
1
point or lower temperature. Furthermore, it is also possible to perform both the normalizing treatment and the temper treatment.
In the process for producing a high-strength heat-resistant steel, it is preferable for the steel or the ingot to comprise at least one selected from cobalt in an amount of 0.5% by weight or less, nickel in an amount of
Kobayashi Masahiro
Kondo Masayuki
Nishimura Nobuhiko
Ozaki Masashi
Shiibashi Akira
Mitsubishi Heavy Industries Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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