Metal treatment – Stock – Ferrous
Reexamination Certificate
1998-12-23
2003-02-11
King, Roy (Department: 1742)
Metal treatment
Stock
Ferrous
C148S909000
Reexamination Certificate
active
06517643
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a low alloy steel on which so-called outer surface SCC (stress corrosion cracking) taking place on a steel-made pipeline buried in soil under cathodic protection hardly occurs. The low alloy steel can be widely used for line pipes for the transportation of crude oil and natural gas and as a structural steel which are used under similar conditions.
DESCRIPTION OF THE RELATED ART
The outer surface SCC of pipelines, as discussed herein, is most often reported in cases related to corrosion in pipeline accidents. Only countermeasures such as making the coating sound and early replacement of pipelines subsequent to the formation of outer surface SCC have been taken conventionally, and no countermeasures have been taken regarding steel pipe materials. “The effects of alloying additions of ferritic steels upon stress corrosion cracking resistance” (by R. N. Parkins, P. W. Slattery and B. S. Poulson, Corrosion, vol. 37 (1981) No. 11, pp 650-664) discloses that a steel shows an improvement of resistance to outer surface SCC as a pipeline when the steel contains 0.86% by mass of Ti, 1.75% by mass of Cr, 6.05% by mass of Ni and 5% by mass of Mo. A steel containing such large amounts of alloying elements hardly satisfies other important properties such as weldability and cannot be put into practical use because the steel is costly.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide a steel excellent in resistance to outer surface SCC when used for a pipeline, without impairing the fundamental requirements of the pipeline.
The present inventors have conducted tests reproducing resistance to outer surface SCC of steels used for pipelines which steels have such chemical compositions that the steels have strength, low temperature toughness and weldability necessary for the line pipes. As a result, they have found the conditions of a steel which improve the resistance to outer surface SCC when the steel is used for a pipeline. That is, they have discovered that the resistance to outer surface SCC of a pipeline can be improved by making the surface of the steel smooth on the average and the magnitudes of the roughness smaller than a certain level, and lowering the C content with regards to the chemical composition of the steel composition. Moreover, they also have found that the resistance to outer surface SCC of the pipeline is further improved by shot-blasting the steel so that the steel satisfies a roughness to a certain level. The outer surface SCC of a pipeline is thought to take place when magnetite thinly formed on the surface is cracked by stress fluctuation and iron is dissolved from the resultant cracks. Accordingly, when the microscopic plastic deformation of the steel is suppressed to inhibit the cracking of magnetite, the outer surface SCC hardly takes place. Furthermore, when the microstructure of the steel is uniform, the properties are further improved.
The present invention has been constituted based on the discoveries as mentioned above.
That is, the present invention provides steels as mentioned below.
A steel excellent in resistance to outer surface SCC when used for a pipeline, wherein said steel has a surface adjusted to have a mean line roughness Ra of up to 7 &mgr;m and a maximum height Rmax of up to 50 &mgr;m.
A steel excellent in resistance to outer surface SCC when used for a pipeline, wherein said steel has a surface adjusted by shot blasting to have a mean line roughness Ra of up to 7 &mgr;m and a maximum height Rmax of up to 50 &mgr;m. The steel further comprising, based on mass, 0.03 to 0.16% of C, 0.5 to 2.0% of Mn, up to 0.5% of Si, up to 0.02% of P, up to 0.01% of S, up to 0.10% of Al, up to 0.1% of N, one or more kinds of the following elements in the following contents: 0.005 to 0.1% of Nb, 0.005 to 0.1% of Ti, 0.001 to 0.1% of V, 0.03 to 0.5% of Mo, 0.1 to 0.6% of Cr, 0.1 to 0.8% of Ni, 0.1 to 0.8% of Cu, 0.0003 to 0.003% of B and 0.001 to 0.01% of Ca and the balance being substantially Fe and unavoidable impurities.
Furthermore, the steel having, as the principal microstructure, acicular ferrite, bainitic ferrite or bainite.
In addition, the display of a surface roughness in the present invention is based on the specification of JIS B0601, and Ra and Rmax represent a mean line roughness and a maximum height, respectively.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be explained below in detail.
First, reasons for restricting the surface roughness of the steel will be explained. The importance of a surface roughness of a steel to the resistance to outer surface SCC has not been recognized. As a result of examining several arbitrarily selected steel pipes, Ra and Rmax have been found to vary from 5 to 30 &mgr;m and from 20 to 300 &mgr;m, respectively.
It is expected from the mechanism of the outer surface SCC as described above that a smooth surface of a steel is desirable for improving the resistance thereto. In fact, a steel having a mechanically ground surface hardly suffers outer surface SCC. Therefore, various steels mainly including steels used for line pipes were prepared. Steels having a surface roughness ranging widely among them were prepared therefrom by changing rolling and processing procedures, and the resistance to outer surface SCC of the steels was evaluated. As a result, it has been found that both the center line mean roughness Ra and the maximum height Rmax of a steel which are the indexes of a surface roughness of the material influence the resistance to outer surface SCC. That is, it has been found that a steel is likely to suffer outer surface SCC when Ra and Rmax of the steel exceed 7 &mgr;m and 50 m, respectively. Accordingly, the surface roughness of the steel is defined as follows: Ra≦7 &mgr;m and Rmax≦50 &mgr;m. In order to further improve the resistance to outer surface SCC of the steel, it is particularly desirable that Ra≦5 &mgr;m and Rmax≦35 &mgr;m.
Furthermore, the following phenomenon has been found: a steel which is shot-blasted on the surface shows improved resistance to outer surface SCC compared with the same steel which is treated otherwise to have the same surface roughness as that of the steel mentioned above. The results are thought to be brought about because the worked layer and the compressive residual stress formed by shot blasting contribute to the improvement. Shot blasting is, therefore, particularly preferred as a surface-adjusting method.
Such control of the surface shape of the steel improves the resistance to outer surface SCC. Restriction of the chemical composition of the steel to a specific range in addition to the control further improves the resistance to outer surface SCC.
Reasons for restricting the chemical composition of the steel of the present invention will be explained below.
The content of C is restricted to 0.03 to 0.16%. C is extremely effective in improving the strength of the steel. In order to obtain a strength as a structural steel, a minimum content of at least 0.03% is necessary. However, since the nonuniformity of the microstructure is increased and the resistance to outer surface SCC is lowered as the C content is increased, the upper limit of the C content is defined to be 0.16%. When the C content exceeds 0.10%, a ferrite-pearlite microstructure is not formed, and a proper strength of the steel becomes difficult to obtain. The upper limit of the C content should preferably be restricted to 0.10%.
Si is an element which is added to the steel to effect deoxidization and improve the strength, and Si is not directly related to the resistance to outer surface SCC. Since addition of Si in a large amount impairs the fundamental properties of the steel as a line pipe such as HAZ toughness and field weldability, the upper limit of the Si content is defined to be 0.5%. However, the steel can also be deoxidized with other elements such as Al, and addition of Si is not necessarily required.
Mn is an element necessary for highly strengthening the steel while a low C content of t
Asahi Hitoshi
Fukai Hideaki
Kushida Takahiro
Okano Shigeo
Yamane Yasuyoshi
King Roy
Nippon Steel Corporation
Wilkins, III Harry D.
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