Metal treatment – Stock – Ferrous
Patent
1997-12-04
1999-11-16
Yes, Deborah
Metal treatment
Stock
Ferrous
148334, 148333, 148320, 420126, 420110, C22C 3814, C22C 3828, C22C 3850
Patent
active
059850531
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a steel having excellent low temperature toughness at a welding heat affected zone (HAZ), and can be applied to structural steel materials to which arc welding, electron beam welding, laser welding, etc, are applied.
More particularly, this invention relates to a steel having excellent HAZ toughness by adding Ti and Mg to a steel, controlling an O quantity and finely dispersing oxides and composite oxides of these elements.
BACKGROUND ART
One of the most important characteristics required for steel materials used for structures such as ships, buildings, pressure containers, linepipes, etc, is HAZ toughness. Recently, heat-treating technologies, controlled rolling and machining heat-treating method have made a remarkable progress, and the improvement of low temperature toughness of the steel material itself has become easy. Because the welding HAZ is reheated to a high temperature, however, the fine structure of the steel material is completely lost, and its microscopic structure becomes extremely coarse to thereby invite drastic deterioration of HAZ toughness. Therefore, as means for refining the HAZ structure, (1) a technology for limiting coarsening of austenite grains by TiN and (2) a technology for forming intergranular ferrite by Ti oxides, have been examined and put into practical application. For example, CAMP-ISIJ Vol. 3 (1990) 808 describes the influences of N on intergranular ferrite transformation in Ti oxide type steels, and Vol. 79 (1993) No. 10 describes the effect of B on intergranular ferrite transformation in Ti-containing oxide steels. Nonetheless, the level of HAZ toughness produced by these technologies is not yet entirely satisfactory. From the aspect of execution of welding, therefore, a steel material which has higher strength and can be used at a low temperature and with a large heat input has been strongly desired.
DISCLOSURE OF THE INVENTION
The present invention provides a steel material having excellent HAZ toughness (such as a thick steel plates, a hot coil, a shape steel, a steel pipe, etc).
The inventors of the present invention have conducted intensive studies on the chemical components (compositions) of steel materials and their microscopic structures in order to improve their HAZ toughness, and have invented a novel steel having high HAZ toughness.
The gist of the present invention resides in a steel which contains, in terms of wt %: component: least 40 pcs/mm.sup.2 of oxides and composite oxides of Ti and Mg having a grain size of 0.001 to 5.0 .mu.m.
When the steel described above is molten, metallic Mg wrapped by an iron foil is used as a Mg addition element.
Hereinafter, the content of the present invention will be explained.
The term "%" used in the following description means "wt %".
The feature of the present invention resides in that trace Ti and Mg are simultaneously added to a low carbon steel and oxides and composite oxides containing Ti and Mg (containing additionally MnS, CuS, TiN, etc) are finely dispersed into the steel by controlling the (oxygen) quantity.
Here, the term "oxides and composite oxides containing Ti and Mg (containing additionally MnS, CuS, TiN, etc)" mainly means compounds such as Ti oxides, Mg oxides or composite oxides of Ti and Mg in the steel, oxides and composite oxides of other elements such as Mn, Si, Al, Zr, etc, and compounds such as sulfides and composite sulfides of Mn, Cu, Ca, Mg, etc. These compounds may further contain nitrides such as TiN.
It has been clarified that the finely dispersed Ti and Mg composite oxides restrict (1) formation of fine intergranular ferrite in the austenite grains that have become coarse and/or (2) coarsening of the austenite grains, make the HAZ structure fine and drastically improve the HAZ toughness. Moreover, the improvement of the HAZ toughness can be attained by the Mg quantity in the steel and the kind of the Mg addition elements. In other words, it has been found out that when pure Mg metal (at least 99%) is wrapped by an iron foil and is added, th
Asahi Hitoshi
Hara Takuya
Saito Naoki
Tamehiro Hiroshi
Uemori Ryuji
Nippon Steel Corporation
Yes Deborah
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