Shaped Steel Excellent in Fire Resistance and Producing Method Therefor

Abstract

A shaped steel, such as H-shaped steel, excellent in fire resistance, where a flange portion has 50% or more of a ratio of strength, 80% or less of yield ratio, and 100 J or more of impact strength of Charpy impact test at 0° C., wherein the ratio of strength=(proof stress of 0.2% at 600° C.) / (yield strength at room temperature), and a producing method thereof are provided. The shaped steel comprises in mass percent (%): C: 0.03-0.15; Mo: 0.1-0.6; V≦0.35; and N: 0.002-0.012, and the balance being iron and residual impurities, wherein (x) a mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600° C. is 0.3% or more, and (y) the ratio of the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 300° C. to the mol fraction of precipitate of alloy carbides and alloy carbonitrides at 600° C. is 2.0 or less.

Description

[0001]This application claims priority to Japanese patent application No. 2004-220337, filed in Japan on Jul. 28, 2004, Japanese patent application No. 2004-220454, filed in Japan on Jul. 28, 2004, and Japanese patent application No. 2005-207185, filed in Japan on Jul. 15, 2005, the entire contents of which are herein incorporated by reference.[0002]The present invention relates to a novel shaped steel, such as H-shaped steel, I-shaped steel, angle steel, channel steel, etc., which is used as a building construction material, with a low yield ratio and having excellence in toughness and fire resistance, and a method for producing the same.[0003]The legislation about “new code to design fire-resistant structures” was enacted on March, 1987 in Japan. After that, fire-resistant steel materials, which secure high strength at high temperature without or with less fire-resistant wrapping, have been desired.[0004]As for shaped steels, according to the trend, a lot of technologies for secur...

AI Technical Summary

Benefits of technology

[0005]For example, Japanese laid open No. 9-104944 proposes a fire-resistant shaped steel using precipitation strengthening technology and oxide metallurgy technology. Oxide metallurgy technology is a technology, which can control the number of oxides, which oxides are obtained by deoxidization of the dissolved oxygen in steel using Ti, B and Mg. Oxide metallurgy technology provides the following effects.

[0007]The flange portion, especially, the fillet part (see FIG. 1), at which a flange portion and web portion meet, is less strained by rolling and forced to be processed at higher temperature compared with other portions.

[0009]The degree of finishing temperature dependency in the formation of microstructure during the hot rolling process can be reduced by dispersing the transformation nuclei, such as Ti oxide, in the ferrite grains and thereby expediting transformations in the grains. This leads to uniformity / homogenization of fine-grained microstructure, i.e., uniformity of mechanical properties. Furthermore, since the grain of the microstructure is not only homogenized but also finely grained, this leads to improved toughness.

[0011]At first, when the shaped steel excellent in fire-resistance is produced using oxide metallurgy technology, it is required for the processes, especially for casting the slab, billet, bloom, near net-shape slab, ingot, etc., to have complicated steps. Such complicated steps include oxygen content control before Ti is added and further Ti addition thereafter. Such steps may cause low productivity and high cost in the manufacture of the shaped steels.

[0016]From such an analysis of technical issues, the inventors have recognized: i) it is preferred that the alloy carbides and alloy carbonitrides have not only thermodynamic stability, but also have the characteristic of being solid-soluted in the steel at the reheating temperature but not being solid-soluted at temperatures such as 600-650° C., and ii) alloy carbides and alloy carbonitrides, which are capable of being soluted in the reheating process once and subsequently being precipitated in the cooling process of the hot rolling, can effectively contribute to precipitation strengthening.

[0046]According to the present invention, a shaped steel, such as H-shaped steel having excellent strength at high temperature and excellent mechanical properties at room temperature can be obtained by forming alloy carbides and alloy carbonitrides containing mainly V and Mo in proper amounts by the specific process such as hot rolling a casted steel containing specific components. According to the present invention “casted steel” includes a slab, bloom, billet, near-net shape slab, ingot, etc.

Problems solved by technology

In the production process of the shaped steel, different portions of the shaped steel are subjected respectively to different temperatures of finish-rolling and to different cooling rates resulting from the unevenness of the shape on the section of the steel, which causes lack of uniformity the microstructure of the section of the steel.

This lack of uniformity of the microstructure, such as non-uniformity of grain size, leads to a lack of uniformity in the mechanical properties in the section of steel.

Such steps may cause low productivity and high cost in the manufacture of the shaped steels.

Accordingly, the strength and yield ratio at the room temperature may be increased excessively even under the same thermal condition, such as the same temperature decrease, if the content of C is high and the mol fraction of precipitate of alloy carbides and alloy carbonitrides is increased excessively.

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