Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-08-10
2002-06-18
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S334000, C148S335000, C148S909000, C420S104000, C138S171000
Reexamination Certificate
active
06406564
ABSTRACT:
TECHNICAL FIELD
This invention relates to steel for a boiler and an electric welded boiler steel pipe using the boiler steel. More particularly, this invention relates to steel, for use in a high-temperature/high-pressure environment, that is excellent in creep rupture strength and electric weldability, and an electro-unite boiler steel pipe that has excellent properties at the electrically welded portions.
BACKGROUND ART
An austenite type stainless steel, a high Cr ferrite steel having a Cr content of 9 to 12% (the term “%” means “% by weight”; hereinafter the same), a low Cr ferrite steel having a Cr content of not greater than 2.25% or a carbon steel has been generally used for high-temperature- and high-pressure-resistant members for boilers and for chemical industry and nuclear facilities. These steels are selected appropriately in consideration of the environment of use of the members such as the temperature, the pressure, etc, and economy.
Among these materials, a low Cr ferrite steel having the Cr content of not greater than 2.25% has the following features. Since this steel contains Cr, it is superior to carbon steel in oxidation resistance, high-temperature corrosion resistance and high-temperature strength. A low Cr ferrite steel is far more economical than an austenite type stainless steel. It has a small coefficient of thermal expansion and does not undergo stress corrosion cracking. It is also more economical and more excellent in toughness, heat conductivity and weldability than a high Cr ferrite steel.
Typical examples of such a low Cr ferrite steel are STBA20, STBA22, STBA23, STBA24, etc, that are stipulated by JIS. These low Cr ferrite steels are ordinarily called generically “Cr—Mo steels”. The low Cr ferrite steels, to which V, Nb, Ti, Ta or B is added as a precipitation hardening element to improve the high-temperature strength, are proposed in Japanese Unexamined Patent Publication (Kokai) Nos. 57-131349, 57-131350, 61-166916, 62-54062, 63-18038, 63-62848, 1-68451, 1-29853, 3-64428, 3-87332, and so forth.
A 1Cr-1Mo-0.25V steel as a turbine material and a 2.25Cr-1Mo—Nb steel as a structural material of a fast breeder reactor are well known as the precipitation hardening type low Cr ferrite steel. However, these low Cr ferrite steels are inferior to the high Cr ferrite steel and the austenite type stainless steel in the oxidation resistance and the corrosion resistance at high temperatures, and have lower high-temperature strength. Therefore, these steels involve the problems when used at a temperature higher than 550° C.
To improve the creep strength at a temperature of 550° C. or above, Japanese Unexamined Patent Publications (Kokai) No. 2-217438 and No. 2-217439 propose low Cr ferrite steels to which large amounts of W are added or Cu and Mg are added compositely. Japanese Unexamined Patent Publication (Kokai) No. 4-268040 proposes low Cr ferrite steel to which a trace amount of B is added after limiting the N content in order to improve the creep strength at a temperature of 550° C. or above and to restrict the drop of toughness resulting from the increase of the strength.
When these materials are electrically welded, a large number of high-melting-point oxides are formed at the electric welded portion and are entrapped into the inner surface at the time of electric welding. Consequently, a defect area ratio of the electric welded portion, as one of the properties of the electric welded portion, is high, and the properties of the electric welded portion, such as the creep rupture strength, toughness, etc., cannot be satisfied in a high-temperature environment of 550° C. or above. Therefore, these materials cannot be said to be suitable materials for electric welded steel pipes. For these reasons, the low Cr ferrite steel which is capable to use at a temperature of 550° C. or above can be nominated a seamless steel pipe. However, the production cost of the seamless steel pipe is high, and this material is not a useful material from the aspect of economy.
In view of the technical background described above, it is an object of the present invention to provide a steel for a boiler that is an ordinary steel not containing Cr (ordinary boiler steel) and a low Cr ferrite steel having a Cr content of not greater than 3.5% (low Cr ferrite type boiler steel), exhibits a high creep rupture strength after use at a high temperature for a long time, is particularly excellent in electric weldability with fewer defects formed at an electric welded portion, and an electric welded boiler steel pipe having fewer defects at the electric welded portion and produced by using the steel.
DISCLOSURE OF THE INVENTION
The present invention relates to an electric welded boiler steel pipe that can be used at a temperature of 550° C. or above, can be produced at a lower cost of production but has a better economical effect than conventional seamless steel pipes.
The inventors of the present invention have conducted intensive studies to obtain a steel and a steel pipe having fewer defects generated at an electric welded portion and having better properties, such as creep rupture strength and toughness, then in ordinary boiler steels and low Cr ferrite type boiler steels. As a result, the present inventors have found that a binary system mixed oxide of SiO
2
and MnO formed at the time of electric welding exerts a great influence on the welding defects in ordinary boiler steels, and a ternary system mixed oxide of SiO
2
, MnO and Cr
2
O
3
exerts a great influence on the occurrence of the welding defects in low Cr ferrite type boiler steels. The present inventors have clarified further that when the melting points of the respective mixed oxides are lowered, the oxides are molten at the time of electric welding and can be squeezed out as slag components from the weld portion, and this reduces the welding defects of the electric welded portion resulting from the mixed oxides.
The present invention was completed on the basis of the finding described above. As to the ordinary boiler steels, the relational formula of Si and Mn is derived on the basis of the binary system phase diagram, and the respective contents are stipulated to lower the melting point of the binary system mixed oxide of SiO
2
and MnO. As to the low Cr ferrite type boiler steels, the relational formula of Si, Mn and Cr is derived on the basis of the ternary system phase diagram of SiO
2
, MnO and Cr
2
O
3
, and the respective contents are stipulated to lower the melting points of the ternary system mixed oxide of SiO
2
, MnO and Cr
2
O
3
. In this way, the present invention reduces number of the welding defects in the electric welded portion, and prevents deterioration of the creep characteristics and toughness of the electric welded portion.
In other words, the gist of the present invention resides in the following points.
(1) A boiler steel excellent in electric weldability, containing, in terms of wt %:
C: 0.01 to 0.20%,
Si: 0.01 to 1.0%, and
Mn: 0.10 to 2.0%, and limiting the following elements:
P: to not greater than 0.030%,
S: to not greater than 0.010%, and
O: to not greater than 0.020%,
wherein a weight ratio of Si and Mn ((Si %)/(Mn %)) is from 0.005 to 1.5;
the balance Fe and unavoidable impurities; and
a melting point of a mixed oxide of SiO
2
and MnO formed at the time of electric welding is not higher than 1,600° C.
(2) A boiler steel excellent in electric weldability, containing, in terms of wt %:
C: 0.01 to 0.20%,
Si: 0.01 to 1.0%,
Mn: 0.10 to 2.0%,
Nb: 0.001 to 0.5%,
V: 0.02 to 1.0%,
N: 0.001 to 0.08%,
B: 0.0003 to 0.01%, and
Al: not greater than 0.01%, containing further at least one of the following elements:
Mo: 0.01 to 2.0%, and
W: 0.01 to 3.0%, and limiting the following elements:
P: to not greater than 0.030%,
S: to not greater than 0.010%, and
O: to not greater than 0.020%;
wherein a weight ratio of Si and Mn ((Si %)/(Mn %)) is from 0.005 to 1.5;
the balance Fe and unavoidable impurities; and
a melting point of a mixed oxide of SiO
2
and MnO formed at the time of electric welding is not higher
Hasegawa Yasushi
Muraki Taro
Okamoto Junichi
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