Metal treatment – Stock – Ferrous
Reexamination Certificate
2003-02-05
2004-03-30
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S335000
Reexamination Certificate
active
06712913
ABSTRACT:
TECHNICAL FIELD
This invention relates to a ferritic heat-resisting steel showing a low level of softening in the welding heat affected zone.
BACKGROUND ART
Among high-temperature materials for use in heat and pressure resisting piping systems in boilers, chemical plants and so forth, there are low Cr ferritic steels, typically 2·¼Cr-1Mo steel, high Cr ferritic steels, typically 9Cr-1Mo steel, and austenitic stainless steels, typically 18Cr-8Ni steel.
Among them, high Cr ferritic steels are superior to low Cr ferritic steels in strength and corrosion resistance in the temperature range of 500-600° C. High Cr ferritic steels are also superior to austenitic stainless steels in price and stress corrosion cracking resistance. Furthermore, high Cr ferritic steels have a low coefficient of thermal expansion and show smaller strains in response to temperature changes. Thus, high Cr ferritic steels, which have many advantages as materials for use at high temperatures, are currently in wide use.
In recent years, the environment for use thereof has become increasingly severe, and accordingly, the use performance requirements that are imposed on heat-resisting ferritic steels, in particular the creep strength requirement, have become much larger. Therefore, a number of improvements have been proposed. Those are new heat-resisting ferritic steels based on ferritic steels containing 8-13% of Cr and improved in strength at elevated temperatures by adjusting the content of Mo, W, Nb, V as well as Co, Ta, Nd, Zr, B and so forth, and a number of methods for heat treatment thereof (cf. e.g. Japanese laid-open patent specifications (JP Kokai) Nos. H02-310340, H04-6213, H04-350118, H04-354856, H05-263196 and H05-311342 to 311346).
It is known that when heat-resisting ferritic steels are used in welded structures, the creep strength of welded joints declines by 20% or more in the heat affected zone (HAZ). The phenomenon is called “HAZ softening”, which is described for example in “Science and Technology of Welding and Joining, 1996, Vol. 1, No. 1, pp. 36-42”.
However, as for the ferritic steels and the methods of production thereof as disclosed in the above-cited publications, the main objective is to improve the creep strength and/or toughness of the base metals. No attention has been paid at all to the decreases in creep strength of welded joints as a result of the HAZ softening phenomenon.
For suppressing the HAZ softening phenomenon, a number of heat-resisting ferritic steels and methods of production thereof have also been proposed (cf e.g. JP Kokai H05-43986, H06-65689, H07-242935, H08-85848, H08-337813, H09-13150, H09-71845 and H111-106860).
However, the ferritic steels and methods of production as disclosed in those publications require a special melting and/or thermo-mechanical treatment, as shown in JP Kokai H07-242935 or JP Kokai H08-337813 for example and therefore problems arise such as an increase in production cost and/or a decrease in production efficiency. Steels disclosed in JP Kokai H06-65689, H08-85848 and H09-71845 contain Ta oxide particles and such expensive elements as Ta, Nd and/or Hf as essential components and therefore there is the problem of an increase in production cost.
DISCLOSURE OF THE INVENTION
An objective of the present invention is to provide a ferritic heat-resisting steel that is inexpensive and shows only a slight decrease in creep strength in the heat affected zone of welded joints. The steel requires no particular melting or thermo-mechanical treatment and does not always require the addition of expensive Ta oxide particles, Ta, Nd, Hf and the like.
The ferritic heat-resisting steel of the invention is characterized by the following features (A) and (B):
(A) The chemical composition consists of, by mass %, C: less than 0.05%, Si: not more than 1.0%, Mn: not more than 2.0%, P: not more than 0.030%, S: not more than 0.015%, Cr: 7-14%, V: 0.05-0.40%, Nb: 0.01-0.10%, N: not less than 0.001% but less than 0.050%, sol. Al: not more than 0.010%, and O (oxygen): not more than 0.010%, with the balance being Fe and impurities.
(B) The density of carbide and carbonitride precipitates, contained in the steel and having a diameter of not less than 0.3 &mgr;m, is not more than 1×10
6
/mm
2
.
The ferritic heat-resisting steel of the invention may contain at least one component selected from one or more groups given below, in lieu of part of Fe in the composition (A) mentioned above.
First group: a total content of 0.1-5.0 mass % of Mo and W.
Second group: a total content of 0.02-5.00 mass % of Cu, Ni and Co.
Third group: a total content of 0.1-0.20 mass % of Ta, Hf, Nd and Ti.
Fourth group: a total content of 0.0005-0.0100 mass % of Ca and Mg.
Fifth group: 0.0005-0.0100 mass % of B.
The inventors paid attention to micro-structural changes due to thermal cycles in welding and carried out repeated experiments and investigations. As a result, they obtained the following new findings and have now completed the present invention.
First, it was revealed that HAZ softening occurs according to the following mechanisms. In the production of base metals, M
23
C
6
type carbides (in this case, M being such a metal element as Cr, Mo or W) or MX type carbonitrides (in this case, M being such a metal element as V or Nb, and X representing C and N) precipitate. Among them the M
23
C
6
type carbides, containing a large amount of Cr as a solid solution, are coarse as compared with the MX type carbonitrides, and they are partly decomposed by thermal cycles at the welding stage and dissolved and contained as a solid solution in the matrix. During the subsequent heat treatment (post-welding heat treatment) and in the earlier stage of creep, the Cr contained as a solid solution in a supersaturated condition again finely precipitates from the matrix regions, wherein said part of M
23
C
6
type carbides have become solid solution. Therefore, compared with the base metal (where the partial dissolution of carbides as solid solution does not occur) which is not subjected to welding thermal cycles or the part where the HAZ softening does not occur (where the partial dissolution of carbides as solid solution does not occur, or the carbides are completely decomposed and dissolved as solid solution), the density and size of M
23
C
6
type carbide precipitates which contain Cr as a main component become uneven or irregular in the HAZ. During use, the precipitation of the above-mentioned Cr solid-soluted in a supersaturated condition becomes complete, and after arrival of the Cr concentration in the base metal at an equilibrium concentration, the particles become coarse due to the disappearance of finer particles. Thus, Cr-based fine M
23
C
6
type carbides disappear, and the Cr is fed to the surrounding M
23
C
6
type carbides to promote the growth thereof, or the Cr re-precipitates and grows utilizing MX type carbonitrides as nuclei. The rate of growth of M
23
C
6
type carbides and MX type carbonitrides increase. As a result, the effect of dispersion strengthening by fine MX type carbonitrides, which greatly contribute to strengthening, is impaired at an early stage, whereupon the strength decreases.
Based on the above findings, the inventors made detailed investigations in search of a method of preventing the HAZ softening, and as a result, it was confirmed that the following measures are effective in preventing the HAZ softening.
(a) Reducing the amount of coarse precipitates (mainly Cr-containing M
23
C
6
type carbides) existing in the steel before welding, and thereby increasing uniformity in the size of precipitates as resulting from said partial solid solution due to welding thermal cycles.
(b) For reducing the amount of coarse M
23
C
6
type carbide precipitates, it is very effective to reduce the contents of C and N, which lower the activity of Cr.
(c) Reductions in C and N content are effective in increasing the equilibrium Cr concentration in the base metal, and retarding the rate of growth of precipitates (M
28
C
6
type carbides and MX type carbonitrides) in the process
Hirata Hiroyuki
Ogawa Kazuhiro
Clark & Brody
Sumitomo Metal Industries Ltd.
Yee Deborah
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