Metal treatment – Stock – Ferrous
Patent
1997-10-03
1999-10-12
Yee, Deborah
Metal treatment
Stock
Ferrous
22826241, C22C 3804, C22C 3806
Patent
active
059649640
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a welded joint having an excellent fatigue strength which is mainly used for welded structures such as ships, offshore structures, bridges, building machines, and so forth. More particularly, the present invention relates to a welded joint having an improved fatigue strength by increasing the area ratio of a ferrite micro-structure capable of retarding a propagation speed of a fatigue crack in a weld heat affected zone (hereinafter abbreviated as the "HAZ") of a welded joint.
2. Description of the Related Art
As the sizes of buildings have become greater, a reduction in the weight of structural members has become an important subject in recent years, and the strength of steels used for such structures has become higher and higher for accomplishing this object. However, since a load is repeatedly applied to the welded joint during use in ships, offshore structures, bridges, building machines, and so forth, counter-measures must be taken to prevent fatigue destruction in such structures. It is at the welded joint portion that this fatigue destruction is most likely to develop. Therefore, an improvement in the fatigue strength of the welded joint has been required.
Intensive studies have been made to this date on the factors deciding the fatigue strength of the welded joint and an improvement of the fatigue strength. However, most of the proposals include an improvement by additional execution after welding such as grinding by a grinder, shaping the end portion of the weld bead final layer by heating and refusing, shot peening to generate compressive stress at the weld end portion (Japanese Unexamined Patent Publication (Kokai) No. 59-110490 and No. 1-301823), and so forth. The reduction of the residual stress by post heat-treatment after welding is also well known in the art.
A method which improves the fatigue strength of the weld portion even in the as-welded state by means of components of the steel material without using a specific execution or post heat-treatment after welding as described above has also been proposed.
In order to make the formation of clean polygonal ferrite advantageous by an addition of Si, to strengthen the steel by an addition of B and to improve hardenability to thereby obtain an excellent elongation flange property, fatigue characteristics and resistance weldability, Japanese Unexamined Patent Publication (Kokai) No. 3-264645 discloses a high strength steel sheet having excellent elongation flange property, etc., which contains 0.01 to 0.2% of C, 0.6 to 2.5% of Mn, 0.02 to 1.5% of Si and 0.0005 to 0.1% of B.
Japanese Examined Patent Publication (Kokoku) No. 3-56301 discloses a ultra-low carbon steel sheet having excellent spot weldability which contains not greater than 0.006% of C, not greater than 0.5% of Mn,. not greater than 0.05% of Al and 0.001 to 0.100% of at least one of Ti and/or Nb, in total, with the proviso that nitrides and sulfides are not calculated, so as to advantageously improve the joint fatigue strength of the spot welded portion by elaborating the steel components, especially by an addition of B, and the proportion of un-recrystallized structure in the steel.
Japanese Unexamined Patent Publication (Kokai) No. 6-207245 discloses a multi-layered steel plate having excellent fatigue characteristics by means of an addition of Ni to a steel material surface layer to generate a residual stress of compression at a weld end portion so as to increase the time until the occurrence of a fatigue crack, and wherein the addition amount of Ni is at least 3% inside the region spaced apart by at least 0.2 mm from both surfaces of the steel plate and being not greater than 25% of the thickness.
Japanese Unexamined Patent Publication (Kokai) No. 6-228707 discloses a structural steel having excellent weld joint fatigue characteristics, and a welding method therefor, which contains 0.001 to 0.01% of C, 0.005 to 0.05% of Si and 0.5 to 2% of Cu and having a Ceq of not greater than 0.2. This reference i
REFERENCES:
Journal of The Iron and Steel Institute of Japan, vol. 71, No. 13, Sep. 1985, p. 1511.
Journal of The Iron and Steel Institute of Japan, vol. 61, No. 11, Sep. 1975, pp. 2589-2603.
Aihara Syuji
Kurebayashi Katsumi
Mabuchi Hidesato
Saito Naoki
Nippon Steel Corporation
Yee Deborah
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