Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-11-27
2002-12-17
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S335000, C148S328000, C148S330000, C148S548000, C148S663000, C420S111000, C420S109000
Reexamination Certificate
active
06494970
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-212916, filed Jul. 13, 2000, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a heat resistant steel casting useful as a material of a steam turbine casing and as a material of a steam turbine valve body and to a method of manufacturing the same.
A low alloy heat resistant steel casting such as a 1.25Cr-0.5Mo cast steel or a 1Cr-1Mo-0.25V cast steel is widely used as a heat resistant steel casting material used for forming a steam turbine casing or a steam turbine valve body in a thermal power station.
On the other hand, in the thermal power station in recent years, the temperature elevation of the steam proceeds rapidly. In accordance with the temperature elevation of the steam, the change of the material of the high temperature member to a high Cr heat resistant steel casting is being vigorously promoted. The high Cr heat resistant steel casting of this kind is disclosed in, for example, Japanese Patent Publication (KOKOKU) No. 4-53928 and Japanese Patent Publication (KOKOKU) No. 3-80865. Since the high Cr heat resistant steel casting exhibits a high mechanical strength and an excellent resistance to the high temperature environment, it is possible to suppress the increase in the thickness of the high temperature member in spite of the elevation of the steam temperature. Also, since it is possible to suppress the thermal stress in the start-up and stop of the steam turbine, the steam turbine can be operated efficiently.
In recent years, the thermal power station is required to exhibit an excellent economical advantage in addition to a high thermal efficiency. Therefore, it is absolutely necessary for the material of the thermal power station to exhibit mechanical properties and manufacturing properties equal to or higher than those of the conventional material and to be excellent in economy. The material meeting these requirements includes, for example, the steel disclosed in Japanese Patent Disclosure (KOKAI) No. 2-217438 and Japanese Patent Disclosure (KOKAI) No. 8-269616.
However, the material of a high temperature member manufactured as a thick cast article is required to exhibit high temperature strength characteristics and economic properties superior to those of the steels disclosed in JP '438 and JP '616 quoted above.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention, which has been achieved in view of the situation described above, is provide a heat resistant steel casting exhibiting mechanical properties excellent under an environment in which a high temperature steam flows and excellent in economical properties and a method of manufacturing the particular heat resistance cast steel.
As a result of an extensive research on a low alloy heat resistant steel casting fully comparable to the high Cr steel casting in the high temperature strength characteristics and advantageous in economy, the present inventors have arrived at the present invention summarized below.
According to an aspect of the present invention, there is provided a heat resistant steel casting, comprising C in an amount of 0.15 to 0.3 mass %, Si in an amount of 0.1 to 0.30 mass %, Mn in an amount of 0.01 to 0.1 mass %, Cr in an amount of 2.0 to 2.5 mass %, Mo in an amount of 0.3 to 0.8 mass %, V in an amount of 0.23 to 0.3 mass %, W in an amount of 1.6 to 2.6 mass %, N in an amount of 0.005 to 0.03 mass %, B in an amount of 0.001 to 0.004 mass %, impurity elements including Ni not larger than 0.2 mass %, P not larger than 0.03 mass % and S not larger than 0.01 mass %, B equivalent determined by formula (1) given below being not larger than 0.02 mass %, Mo equivalent determined by formula (2) given below falling within a range of between 1.4 mass % and 2.0 mass %, and C equivalent determined by formula (3) given below being not smaller than 0.65 mass %, and balance of iron and unavoidable impurities:
B equivalent=B+0.5N (1)
Mo equivalent=Mo+0.5W (2)
C equivalent=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/14+V/14 (3)
wherein a precipitated phase consisting of a M
23
C
6
type carbide, a M
7
C
3
type carbide, and MX type carbonitride is a texture finely precipitated in a matrix phase, and a ratio of the precipitated phase to the matrix phase falls within a range of between 0.6 and 1.0 mass %.
In this case, it is possible for the Nb equivalent determined by formula (4) given below to be not larger than 0.15%, with the V content set at 0.23 to 0.27 mass %, and with Nb content set at 0.01 to 0.06 mass %:
Nb equivalent=Nb+0.4C (4)
It is also possible to set the V content at 0.23 to 0.27 mass % and to set the Ti content at 0.005 to 0.01 mass %.
Further, it is possible to set the V content at 0.25 to 0.3%.
According to another aspect of the present invention, there is provided a heat resistant steel casting, comprising C in an amount of 0.15 to 0.3 mass %, Si in an amount of 0.1 to 0.30 mass %, Mn in an amount of 0.4 to 0.7 mass %, Cr in an amount of 2.0 to 2.5 mass %, Mo in an amount of 0.3 to 0.8 mass %, V in an amount of 0.23 to 0.3 mass %, W in an amount of 1.6 to 2.6 mass %, N in an amount of 0.005 to 0.03 mass %, B in an amount of 0.001 to 0.004 mass %, impurity elements including Ni in an amount not larger than 0.5 mass %, P in an amount not larger than 0.03 mass % and S in an amount not larger than 0.01 mass %, B equivalent determined by formula (1) given below being not larger than 0.02 mass %, Mo equivalent determined by formula (2) given below falling within a range of between 1.4 mass % and 2.0 mass %, and C equivalent determined by formula (3) given below being not smaller than 0.65 mass %, and balance of iron and unavoidable impurities,
wherein a precipitated phase consisting of a M
23
C
6
type carbide, a M
7
C
3
type carbide, and MX type carbonitride is a texture finely precipitated in a matrix phase, and a ratio of the precipitated phase to the matrix phase falls within a range of between 0.6 and 1.0 mass %:
B equivalent=B+0.5N (1)
Mo equivalent=Mo+0.5W (2)
C equivalent=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/15+V/14 (3)
In this case, it is possible for the Nb equivalent determined by formula (4) given below to be not larger than 0.15%, with the V content set at 0.23 to 0.27 mass %, and with Nb content set at 0.01 to 0.06 mass %:
Nb equivalent=Nb+0.4C (4)
It is also possible to set the V content at 0.23 to 0.27 mass % and to set the Ti content at 0.01 to 0.025 mass %.
Further, it is possible to set the V content at 0.25 to 0.3%.
The function of each of the components described above and the reasons for specifying the composition are as described in items (a) to (p) described below. In the following description, “%” represents the mass % unless otherwise specified.
(a) C: 0.15 to 0.3%
Carbon (C) serves to ensure the hardenability, to suppress the ferrite formation, and to precipitate as a carbide or carbonitride contributing to reinforcement of precipitation. In ensuring the mechanical properties of a thick portion in casting, particularly, a large lump, it is important to ensure the hardenability and to suppress the ferrite formation. If the C content is less than 0.15%, these functions are unlikely to be performed sufficiently. On the other hand, if the C content exceeds 0.3%, the agglomeration of the precipitated carbide tends to be promoted and the welding properties tend to be lowered.
(b) Si: 0.1 to 0.3%
Silicon (Si) serves to perform the function of a deacidifying agent, to ensure a good casting properties, and to enhance the resistance to the steam oxidizing characteristics. If the Si content is lower than 0.1%, these functions tend to fail to be performed sufficiently. On the other hand, if the Si content exceeds 0.3%
Ishii Ryuichi
Tsuda Yoichi
Yamada Masayuki
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Kabushiki Kaisha Toshiba
Yee Deborah
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