Metal treatment – Stock – Ferrous
Patent
1995-04-04
1997-06-03
Yee, Deborah
Metal treatment
Stock
Ferrous
428682, 148654, C21D 804, B32B 1518, C22C 3802
Patent
active
056349882
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a high tensile welded steel plate, having excellent fatigue strength at its weld and weldability, for shipbuilding, offshore structures, bridges, and the like and a process for producing the same.
BACKGROUND ART
Recently, with an increase in size of structures, a reduction in weight of structural members has become important. In order to realize this, an effort has been made to increase the tensile strength of a steel plate used in the structures. Since, however, ships, offshore structures, bridges, and the like repeatedly undergo loading during use, consideration should be given to the prevention of fatigue failure. Welds are sites where a fatigue fracture is most likely to occur, which has led to a demand for an improvement in fatigue strength at the weld.
Up to now, the factors governing the fatigue strength at the weld and an improvement in the fatigue strength have been studied, and an improvement in fatigue strength at the weld has been primarily attempted by mechanical factors, such as a reduction in stress concentration through an improvement in the shape of the toe of the weld such as shaping of the toe of weld by grinding using a grinder or heat-remelting of the final layer of the weld bead, or shot peening treatment or other treatments for creating compressive stress at the toe of weld (Japanese Unexamined Patent Publication (Kokai) Nos. 59-110490 and 1-301823 and the like). Further, it is well known that the effect of reducing the residual stress can be attained by post-weld heat treatment.
On the other hand, a proposal has been made wherein the fatigue strength at a weld is improved by taking advantage of chemical compositions of steel products without use of the above special execution and post-weld heat treatment.
In Japanese Unexamined Patent Publication (Kokai) No. 62-10239, in order to prevent a deterioration in fatigue properties at a spot weld even in the case of high C and high Mn levels by increasing the Si content and specifying the amounts of C and P added, a high-strength thin steel sheet having excellent fatigue properties in spot welding, comprising C: not more than 0.3%, Si: 0.7 to 1.1%, Mn: not more than 2.0%, P: not more than 0.16%, and sol. Al: 0.02 to 0.1%, is disclosed.
In Japanese Unexamined Patent Publication (Kokai) No. 3-264645, in order to attain good stretch-flange ability, fatigue properties, and resistance weldability by advantageously forming clean polygonal ferrite by Si, strengthening and improving the hardenability of a steel by B, a high-strength thin steel sheet having excellent stretch-flange ability and other properties, comprising C: 0.01 to 0.2%, Mn: 0.6 to 2.5%, Si: 0.02 to 1.5%, B: 0.0005 to 0.1%, and the like, is disclosed.
In Japanese Examined Patent Publication (Kokoku) No. 3-56301, in order to advantageously improve the fatigue strength of a joint at its spot weld by optimizing the chemical compositions in the steel and the proportion of unrecrystallized structure in the steel sheet by adding B or the like, a very low carbon steel plate having a good spot weldability, comprising C: not more than 0.006%, Mn: not more than 0.5%, Al: not more than 0.05%, and 0.001 to 0.100% in total of at least one member selected from Ti and/or Nb in a solid solution form exclusive of a nitride and a sulfide, is disclosed.
Among the above techniques, the techniques disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 59-110490 and 1-301823 requires special execution after welding and cannot improve the fatigue strength of the as-welded steel. The technique where heat treatment is carried out after welding requires additional steps and unfavorably complicates welding procedure. Further, the effect attained by the technique is limited.
The thin steel sheets disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 62-10239 and 3-264645 are those of which the applications are mainly limited to base materials of wheels and disks for automobiles, and these steel sheets are quite different from steel pla
REFERENCES:
patent: 4279647 (1981-07-01), Giflo
patent: 4299621 (1981-11-01), Giflo
patent: 5312493 (1994-05-01), Masui et al.
Aihara Shuji
Kurebayashi Katsumi
Seto Atsushi
Nippon Steel Corporation
Yee Deborah
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