Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-04-03
2001-07-10
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S335000, C148S332000
Reexamination Certificate
active
06258181
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a structural steel excellent in wear resistance and fatigue resistance property for use as a structural member of a bridge, pylon or the like erected in a shore region where there is concern about corrosion of steel and metallic fatigue at weld joint owing to scattering of sea salt particles or in a region where snow-melting salt is used, and a method of producing the same.
BACKGROUND TECHNOLOGY
The service life of a bridge, pylon or the like is determined by the corrosion and fatigue of its steel. Extremely long service life is possible depending on corrosion resistance and fatigue resistance. Even in the case of current wear resistant steels, however, prevention of corrosion without a coating is difficult in regions near the seashore where salinity is high and regions where snow-melting salt is used. Painting, plating or other anti-corrosion treatment must be carried out regularly. In addition, joints, such as weld joints, experience metallic fatigue over long periods owing to the vibration produced by vehicles. The problem of having to carry out regular, large-scale repair work therefore arises.
FIG. 1
shows the results of atmosphere-exposure tests conducted on carbon steel and wear resistant steel in Japan. The data represent the results of atmosphere-exposure tests particularly at coastal industrial zones were corrosion is particularly great. Over a long test period of 10 years during which the SO
x
concentration increased, the amount of thickness loss, an index of amount of corrosion, reached 0.5 mm per side in the case of carbon steel. On the other hand, wear resistant steel turned in excellent results of under 0.2 mm. The need for this type of steel is therefore increasing more and more, and calls for further improvements have been heard.
Various solutions for these problems have been proposed. As typical examples, Japanese Unexamined Patent Publications No. 8(1996)-134587 and No. 9(1997)-165647 teach welded structural steels excellent in wear resistance that contain not greater than 0.15% of C and are further added with strengthening elements such as Mn, Ni and Mo to adjust to Ni+3Mo≧1.2%, or Ni+Cu+3Mo≧1.2%, Ceq: 0.5 or less. Japanese Unexamined Patent Publication No. 8(1986)-277439, teaches a steel composed of lath-like ferrite and cementite whose weld heat affected regions have high fatigue strength owing to a metallic texture containing as-transformed martensite at an area ratio of not less than 0.5% and not greater than 5%. Japanese Unexamined Patent Publication No. 9(1997)-249915 teaches elevation of the fatigue limit by adding appropriate amounts of Mn, Ti and B to make the texture bainite single phase without depending on cooling rate, while also achieving structural strengthening, utilizing precipitation of Cu and solid solution strengthening to enhance tensile strength and improve fatigue resistance, and also conducting rolling at a reduction ratio of not less than 30% in the unrecrystalized low-temperature zone or in the temperature range of the dual-phase region.
However, none of these conventional technologies enables endurance, without coating, against use in regions near the seashore where salinity is high and regions where snow-melting salt is used. As in the past, joints, such as weld joints, experience metallic fatigue over long periods owing to the vibration produced by vehicles, so that regular, large-scale repair work is necessary.
DISCLOSURE OF THE INVENTION
This invention was accomplished to overcome the foregoing problems. In a steel for use as a structural steel used in a bridge, a pylon or the like erected in a shore region where there is concern about corrosion resistance and fatigue resistance at weld joint owing to scattering of sea salt particles or in a region where snow-melting salt is used, its object is to provide a hot rolled structural steel excellent in wear resistance and fatigue resistance property, and a method of producing the same.
In the aforesaid steel for use as a structural steel of a bridge, pylon or the like erected in a shore region where there is concern about corrosion of steel and metallic fatigue at weld joint owing to scattering of sea salt particles or in a region where snow-melting salt is used, the present invention successfully provides a hot rolled structural steel excellent in wear resistance and fatigue resistance property by, in order to suppress generation of internal oxides that act as a source of corrosion starting points and, depending on the steel type, prevent grain boundary oxidation by addition of Cr, additionally adjusts Ni/Cu concentration ratio, adds Ni, Cu and Mo, and controls thickness of a steel surface internal oxide layer, thickness of a Ni, Cu and Mo concentrated layer formed on the internal oxide layer, and total amount of element concentration of these layers. Specifically, the present invention focuses on 1) reducing amount of Si, Mn and Cr addition to suppress generation of internal oxides, i.e., reducing internal oxides that become corrosion and/or fatigue starting points, 2) adding Ni, Cu and Mo to form an alloy-concentrated layer and suppress corrosion and/or fatigue, and 3) adding Cr and reducing Si to suppress grain boundary oxidation, reduce stress concentration portions, reduce corrosion starting points and suppress internal oxide layer enlargement. Its gist is as follows.
(1) A structural steel excellent in wear resistance and fatigue resistance property containing, in percentage by weight, C: 0.02~0.20%, and further added with small amounts of Ni, Cu and Mo as essential elements, characterized in having a Ni/Cu concentration ratio of not less than 0.8, a steel surface internal oxide layer of not greater than 2 &mgr;m, and a Ni, Cu and Mo concentrated layer of a thickness of not less than 2 &mgr;m on the internal oxide layer.
(2) A structural steel excellent in wear resistance and fatigue resistance property containing, in percentage by weight, C: 0.02~0.20% and Cr: 0.1~0.5%, and further added with small amounts of Ni, Cu and Mo as essential elements, characterized in having a Ni/Cu concentration ratio of not less than 0.8, a steel surface internal oxide layer of not greater than 2 &mgr;m, and a Ni, Cu and Mo concentrated layer of a thickness of not less than 2 &mgr;m on the internal oxide layer.
(3) A structural steel excellent in wear resistance and fatigue resistance property characterized in comprising, in percentage by weight,
C: 0.02~0.20%,
Mn: ≦0.1%,
Si: ≦0.1%,
Cr: ≦0.1%,
Al: ≦0.1%,
Ti: ≦0.1%,
Ni: 0.8~3.0%,
Cu: 0.8~2.0%,
Mo: 0.4~0.7%,
N: 0.001~0.01%,
P: ≦0.1%, and
S: ≦0.006%,
the Ni/Cu concentration ratio being not less than 0.8 and the balance being Fe and unavoidable impurities, and having a steel surface internal oxide layer of not greater than 2 &mgr;m, and a Ni, Cu and Mo concentrated layer of a thickness of not less than 2 &mgr;m on the internal oxide layer, the total amount of element concentration of Ni, Cu and Mo being not less than 7.0 wt %.
(4) A structural steel excellent in wear resistance and fatigue resistance property characterized in comprising, in percentage by weight,
C: 0.02~0.20%,
Mn: 0.4~2.0%,
Si: ≦0.1%,
Cr: 0.1~0.5%,
Al: 0.001~0.10%,
Ti: ≦0.1%,
Ni: 0.3~3.0%,
Cu: 0.3~1.5%,
Mo: 0.1~0.7%,
N: 0.001~0.010%,
P: ≦0.1%, and
S: ≦0.006%,
the Ni/Cu concentration ratio being not less than 0.8 and the balance being Fe and unavoidable impurities, and having a Ni, Cu and Mo concentrated layer of a thickness of not less than 2 &mgr;m on a steel surface internal oxide layer, the total amount of element concentration of Ni, Cu and Mo being not less than 4.0 wt %.
(5) A structural steel excellent in wear resistance and fatigue resistance property set out in any of (1)~(4) above, characterized in further comprising, in percentage by weight, at least one of Nb: 0.005~0.10%, V: 0.01~0.20% and B: 0.0003~0.0030%.
(6) A structural steel excellent in wear resistance and fatigue resistance property set out in any of (1)~(4) above, characterized in further comprising, in pe
Satoh Hironori
Yamamoto Kouichi
Kenyon & Kenyon
Nippon Steel Corporation
Yee Deborah
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