Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-03-30
2002-04-02
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S333000, C148S334000, C148S335000, C148S330000, C148S648000, C148S653000, C148S654000
Reexamination Certificate
active
06364967
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a high-tensile rolled steel shape, excellent in toughness, for use as a building structural member.
BACKGROUND TECHNOLOGY
Owing to the trend toward super high-rise buildings, stricter building safety standards and the like, steel materials used for columns, e.g., especially thick, large-sized H-shapes (hereinafter called “super-thick H-shapes”), are required to have enhanced high-strength, high-toughness and low-yield-ratio properties. The conventional practice for achieving these desired properties has been to conduct annealing or other such heat treatment after rolling. However, imparting heat treatment degrades energy-cost performance and production efficiency. It therefore considerably increases cost and is a problem from the aspect of economy. Solving this problem required the development of a slab with a new alloy design enabling achievement of high-performance material properties and of a method of producing the slab.
When a steel shape having a flange, e.g., an H-shape, is produced by universal rolling, differences in the rolling finishing temperature, reduction ratio and cooling rate generally arise among the web, flange and fillet portions owing to restrictions on the rolling conditions (temperature and draft), from the aspect of roll shaping, and to the shape. As a result, differences in strength, ductility and toughness occur among the different portions so that portions may arise that, for example, fail to meet the criteria for rolled steels for welded structures (JIS G3106) and the like. In particular, when a super-thick H-shape is produced by rolling using a continuously cast slab as starting material, the rolling must be conducted at a low reduction ratio because the limited maximum slab thickness obtainable by production with a continuous casting machine makes it impossible to obtain a slab of sectional area sufficient for shaping. In addition, since high-temperature rolling is desired in order to obtain the required dimensional precision of the product by roll shaping, the thick flange portion is rolled at a high temperature and cooling of the steel material after rolling proceeds slowly. This results in a coarse microstructure that degrades strength and toughness.
Although TMCP (Thermo-Mechanical-Control Process) is available as a texture refining method in the rolling process, low-temperature, large-reduction-ratio TMCP such as applied to steel plate is hard to apply in steel shape rolling because of the restrictions on the rolling conditions. In the steel plate field, technologies have been introduced for production of high-strength, high-toughness steels that utilize the precipitation effect of VN. See, for example, Japanese Patent Publication Nos. 62(1987)-50548 and 62(1987)-54862. When these methods are applied to 590-MPa-class production, however, the presence of solid-solution N at high concentration causes high-carbon island-like martensite (hereinafter designated as “M*”) in the produced bainite texture. Since this markedly degrades toughness, a problem arises of not being able to meet the standards. On the other hand, Japanese Unexamined Patent Publication No. 10-147835 teaches a method for producing a high-strength rolled steel shape by adding minute amounts of Nb, V and Mo, reducing carbon and nitrogen to low levels, imparting texture refinement by fine dispersion of Ti oxides and TiN, and conducting accelerated cooling type controlled rolling. Owing to the utilization of C reduction and TMCP, however, this method increases production cost and complicates the production process.
In order to overcome the forgoing problems, the texture of the rolled steel shape must be refined by producing low-carbon bainite that generates little M*. For this, refinement of the &ggr; grain diameter at the time of rolling and heating must be ensured by, in the steelmaking process, producing the slab by finely crystallizing Ti—O in the slab beforehand, finely precipitating TiN with the Ti—O as nuclei, and, in addition, lowering the carbon content by adding a minute amount of a microalloy that imparts high-strength at a very low content. Moreover, the fillet portion at the joint between the flange and web of an H-shape coincides with the central segregation zone of a CC slab. The MnS in this segregation zone is drawn markedly by rolling. In some cases, the high-concentration element segregation zone and the drawn MnS in this region markedly degrade reducibility and toughness in the thickness direction and further cause lamellar tear during welding. Preventing generation of MnS having these harmful effects is a major issue. Existing technologies are thus not capable of online production and inexpensive supply of the desired high-reliability, high-strength and high-toughness rolled steel shapes.
DISCLOSURE OF THE INVENTION
An object of the present invention is to enable production of a high-tensile rolled steel shape at low cost without conducting conventional heat treatment such as annealing, thereby providing a 590-MPa-class rolled steel shape of high-strength and excellent toughness for use as a building structural member, and a method of producing the same.
The characterizing feature of the present invention resides in the point that, in a departure from conventional thinking, a high-strength and high-toughness rolled steel shape is realized through texture refinement achieved by addition of Ti, fine dispersion of fine Ti oxides and TiN produced as a result, and generation of a low-carbon bainite structure by addition of a microalloy.
In addition, the TMCP adopted is characterized in being a method of effecting water cooling between rolling passes and repeating rolling and water cooling, thereby enabling effective texture grain refinement even by low-reduction hot rolling during steel shape rolling instead of the high-reduction rolling utilized for steel plate.
The present invention is characterized in casting a slab to obtain a fine texture of low-carbon bainite of small M* content and conducting effective TMCP during steel shape rolling of this slab to produce a steel shape having high-strength and high-toughness.
The slab is produced so as to achieve &ggr; grain refinement during rolling and heating by, during the steelmaking process, adding Ti to the slab to crystallize fine Ti—O and finely disperse TiN, adding an alloying element which secures strength and toughness with the aim of reducing M* in the texture after rolling, and making the B content very low.
The slab is then roll-shaped to produce a steel shape. In this rolled steel shape rolling process, the steel is imparted with a temperature difference between the surface layer portion and the interior by water cooling the steel between hot rolling passes so as to heighten penetration of reduction into the hot steel interior even under mild reduction conditions, thereby introducing working dislocations that act as bainite formation nuclei in the &Ggr; grains and thus increasing the number of formation nuclei thereof. In addition, refinement of the microstructure can be achieved by the method of effecting cooling control of the &ggr;/&agr; transformation temperature after rolling so as to suppress growth of the bainite whose nuclei were formed, whereby control-rolled steel shape with a low production cost can be produced at high efficiency. The aforesaid problems were overcome based on this knowledge, the gist of which is as follows.
(1) A high-strength, high-toughness rolled steel shape having mechanical properties of a tensile strength of not less than 590 MPa, a yield strength or 0.2% proof strength of not less than 440 MPa and a Charpy impact absorption energy at 0° C. of not less than 47J, characterized in comprising, in percentage by weight,
C: 0.02-0.06%,
Si: 0.05-0.25%,
Mn: 1.2-2.0%,
Cu: 0.3-1.2%,
Ni: 0.1-2.0%,
Ti: 0.005-0.025%,
Nb: 0.01-0.10%,
V: 0.04-0.10%,
N: 0.004-0.009%, and
O: 0.002-0.004%,
the balance being Fe and unavoidable impurities,
having a chemical composition wherein among the impurities B is limited to not more than 0.0003% and Al conte
Hasegawa Hiroyuki
Satoh Hironori
Sugiyama Hirokazu
Yamamoto Kouichi
Yoshida Suguru
Kenyon & Kenyon
Nippon Steel Corporation
Yee Deborah
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