Ferritic Heat-Resistant Steel
a heat-resistant steel and ferritic technology, applied in the field of ferritic heat-resistant steel, can solve the problems of high thermal stress applied easy damage to headers and piping, material cost and process cost, etc., to improve oxidation resistance, increase si, and improve the effect of steam oxidation resistan
Inactive Publication Date: 2009-01-08
BABCOCK HITACHI KK
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Benefits of technology
[0019]Carbon (C) is an element important for forming carbide (M23C6, M6C, M7C3, etc.) that contributes to strengthening of a high-Cr ferritic heat-resistant steel. Conventional practical steels require about 0.1-0.12% of carbon, however, if the carbon content exceeds 0.10%, it promotes aggregation and coarsening and lowers the creep rupture strength, so that in the present invention, the carbon content is set to 0.10% or less to stabilize long-term creep strength. The lower the C content, the higher the creep rupture strength, however, if the C content is less than 0.01%, the toughness deteriorates, so that the C content is set to 0.01-0.10% as a practical steel. Fine control of the C content requires advanced techniques in steelmaking, however, in particular, by reducing the C content from about 0.1% of a conventional steel to 0.08% or less, the Ac1 point (transformation point) is greatly raised, and the long-term creep rupture strength can be further improved.
[0043]In addition, the ferritic heat-resistant steel of the present invention has creep rupture strength remarkably improved in comparison to the conventional ferritic heat-resistant steel, and has strength and ductibility stable even during long-term use. Therefore, by applying this steel to a high-temperature pressure resistant portion of an ultra supercritical pressure boiler, the steam temperature can be raised to about 650° C., and the plant efficiency of a thermal power plant can be improved. Further, growth and exfoliation of steam oxidized scale and damage of devices due to scattering of steam oxidized scale can be reduced. Therefore, the durability of the plant is also improved, and a remarkable effect in fuel consumption reduction such as coal and CO2 emissions reduction in the thermal power plant can be obtained.
Problems solved by technology
Therefore, when the plant starts or stops, a great thermal stress is applied to these header and piping and the header and piping are easily damaged due to thermal fatigue.
In addition, the increase in material cost and process cost causes an economic problem.
Therefore, it has been found that the creep rupture strength lowers during long-term use for several tens of thousands of hours or longer.
Particularly, at a high temperature around 650° C. much higher than 600° C., so-called fluting that involves sudden lowering in creep strength after about several tens of thousands of hours is a great obstacle in the development of a high-Cr steel (for example, Non-Patent Document 1).
To solve these problems, a special method by addition of noble metals (Non-Patent Document 2) has also been proposed, however, this greatly increases the material cost, so that it has not been made practicable.
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[0048]Hereinafter, examples / embodiments of the present invention will be described by using actual examples.
[0049]The heat-resistant steels having the chemical compositions shown in Table 1 of the present examples and comparative steels were molten in a vacuum induction melting furnace, and forged into 50 kg ingots, respectively. The comparative steel A is nominal 9Cr1MoNbV steel, comparative steels B and C are nominal 9Cr0.5Mo1.8WNbV steels, and all of these have been made practicable as boiler steels. After forming steel sheets with a thickness of 20 millimeters by hot forging, the steel sheets were normalized at 1,050° C. for 60 minutes and tempered at 780° C. for 60 minutes, and then subjected to a creep rupture test. In addition, small-sized sheet-like test specimens were processed from the steel sheets, and subjected to an oxidation test by using steam at 650° C.
TABLE 1(wt %)CSiMnPSCrMoWVNbNiSteel A of the present invention0.0430.510.500.0030.0018.90.451.800.190.0510.005Steel ...
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Abstract
Disclosed is a ferritic heat-resistant steel which has the following chemical composition (by weight): C: 0.01-0.10%; Si: 0.30-1.0%; P: 0.02 or less; S: 0.010% or less; Mn: 0.2-1.2%; Ni: 0.3% or less; Cr: 8.0-11.0%; Mo: 0.1-1.2%; W: 1.0-2.5%; V: 0.10-0.30%; Nb: 0.02-0.12%; Co: 0.01-4.0%; N: 0.01-0.08%; B: not less than 0.001% and less than 0.010%; Cu: 0.3% or less; and Al: 0.010% or less, provided that the chemical composition satisfies the following equations: Mo(%)+0.5×W(%)=1.0-1.6, and C(%)+N(%)=0.02-0.15%, and which comprises a tempered martensite single-phase tissue produced by thermal refining. The steel shows an excellent long-term creep rupture strength even when used at a steam temperature around 650˚C and also has excellent steam oxidizability. When the value represented by the equation: Al(%)+0.1×Ni(%) is adjusted to 0.02 or less, the creep strength can be more stabilized.
Description
TECHNICAL FIELD[0001]The present invention relates to a ferritic heat-resistant steel, and more specifically, to a high-strength steel for boiler steel pipes suitable for an ultra supercritical pressure thermal power plant with improved power generation efficiency.BACKGROUND ART[0002]Recently, in thermal power plants, the temperature and pressure of steam conditions have been raised for improvement in plant efficiency in view of global environmental issues such as CO2 emissions reduction. Now, plants which can raise the steam temperature from the current maximum main steam temperature around 600° C. to 650° C., ultimately, to 700° C. have been developed and studied domestically and internationally. According to such a steam temperature rise, a heat-resistant steel with high creep rupture strength is required at high-temperature and pressure resistant portions of a boiler. Therefore, for heat transfer pipes of a boiler, an austenitic heat-resistant steel having excellent corrosion re...
Claims
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Login to View More IPC IPC(8): C22C38/32C22C38/22C22C38/30
CPCC21D8/105C22C38/54C21D9/085C21D2211/005C21D2211/008C22C38/001C22C38/02C22C38/04C22C38/22C22C38/24C22C38/26C22C38/30F22B37/04F22B37/10C22C38/46C22C38/44C22C38/42C22C38/06C22C38/52C22C38/48C21D9/08
InventorSATO, TAKASHITAMURA, KOHJIFUJITA, TOSHIO
OwnerBABCOCK HITACHI KK