Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-07-16
2004-02-03
LaVilla, Michael (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S627000, C428S638000, C428S704000, C148S540000, C148S579000, C148S639000, C148S320000, C228S262410, C029S017200
Reexamination Certificate
active
06686061
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structural steel product suitable for use in constructions, bridges, ship constructions, marine structures, steel pipes, line pipes, etc. More particularly, the present invention relates to a weldable structural steel product which is manufactured using fine complex precipitates of TiN and CuS, thereby being capable of simultaneously exhibiting improved toughness and strength in a heat-affected zone of the weld. The present invention also relates to a method for manufacturing the welding structural steel product, and a welded construction using the welding structural steel product.
2. Description of the Prior Art
Recently, as the height or size of buildings and other structures has increased, steel products having an increased size have been increasingly used. That is, thick steel products have been increasingly used. In order to weld such thick steel products, it is necessary to use a welding process with a high efficiency. For welding techniques for thick steel products, a heat-input submerged welding process enabling a single pass welding, and an electro-welding process have been widely used. The heat-input welding process enabling a single pass welding is also applied to ship constructions and bridges requiring welding of steel plates having a thickness of 25 mm or more. Generally, it is possible to reduce the number of welding passes at a higher amount of heat input because the amount of welded metal is increased. Accordingly, there may be an advantage in terms of welding efficiency where the heat-input welding process is applicable. That is, in the case of a welding process using an increased heat input, its application can be widened. Typically, the heat input used in welding process are in the range of 100 to 200 kJ/cm. In order to weld steel plates further thickened to a thickness of 50 mm or more, it is necessary to use super-high heat input ranging from 200 kJ/cm to 500 kJ/cm.
Where high heat input is applied to a steel product, the heat affected zone, in particular, its portion arranged near a fusion boundary, is heated to a temperature approximate to a melting point of the steel product by welding heat input. As a result, growth of grains occurs at the heat affected zone, so that a coarsened grain structure is formed. Furthermore, when the steel product is subjected to a cooling process, fine structures having degraded toughness, such as bainite and martensite, may be formed. Thus, the heat affected zone may be a site exhibiting degraded toughness.
In order to secure a desired stability of such a welding structure, it is necessary to suppress the growth of austenite grains at the heat affected zone, so as to allow the welding structure to maintain a fine structure. Means known for meeting this requirement are techniques in which oxides stable at a high temperature or Ti-based carbon nitrides are appropriately dispersed in steels in order to delay growth of grains at the heat affected zone during a welding process. Such techniques are disclosed in Japanese Patent Laid-open Publication No. Hei. 12-226633, Hei. 11-140582, Hei. 10-298708, Hei. 10-298706, Hei. 9-194990, Hei. 9-324238, Hei. 8-60292, Sho. 60-245768, Hei. 5-186848, Sho. 58-31065, Sho. 61-79745, and Sho. 64-15320, and Journal of Japanese Welding Society, Vol. 52, No. 2, page 49.
The technique disclosed in Japanese Patent Laid-open Publication No. Hei. 11-140582 is a representative one of techniques using precipitates of TiN. This technique has proposed structural steels exhibiting an impact toughness of about 200 J at 0° C. (in the case of a base metal, about 300 J). In accordance with this technique, the ratio of Ti/N is controlled to be 4 to 12, so as to form TiN precipitates having a grain size of 0.05 &mgr;m or less at a density of 5.8×10
3
/mm
2
to 8.1×10
4
/mm
2
while forming TiN precipitates having a grain size of 0.03 to 0.2 &mgr;m at a density of 3.9×10
3
/mm
2
to 6.2×10
4
/mm
2
, thereby securing a desired toughness at the welding site. In accordance with this technique, however, both the base metal and the heat affected zone exhibit substantially low toughness where a heat-input welding process is applied. For example, the base metal and heat affected zone exhibit impact toughness of 320 J and 220 J at 0° C. Furthermore, since there is a considerable toughness difference between the base metal and heat affected zone, as much as about 100 J, it is difficult to secure a desired reliability for a steel construction obtained by subjecting thickened steel products to a welding process using super-high heat input. Moreover, in order to obtain desired TiN precipitates, the technique involves a process of heating a slab at a temperature of 1,050° C. or more, quenching the heated slab, and again heating the quenched slab for a subsequent hot rolling process. Due to such a double heat treatment, an increase in the manufacturing costs occurs.
Generally, Ti-based precipitates serve to suppress growth of austenite grains in a temperature range of 1,200 to 1,300° C. However, where such Ti-based precipitates are maintained for a prolonged period of time at a temperature of 1,400° C. or more, a considerable amount of TiN precipitates may be dissolved again. Accordingly, it is important to prevent a dissolution of TiN precipitates so as to secure a desired toughness at the heat affected zone. However, there has been no disclosure associated with techniques capable of achieving a remarkable improvement in the toughness at the heat affected zone even in a super-high heat input welding process in which Ti-based precipitates are maintained at a high temperature of 1,350° C. for a prolonged period of time. In particular, there have been few techniques in which the heat affected zone exhibits toughness equivalent to that of the base metal. If the above mentioned problem is solved, it would then be possible to achieve a super-high heat input welding process for thickened steel products. In this case, therefore, it would then be possible to achieve a high welding efficiency while enabling an increase in the height of steel constructions, and secure a desired reliability of those steel constructions.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a welding structural steel product in which fine complex precipitates of TiN and CuS exhibiting a high-temperature stability within a welding heat input range from an intermediate heat input to a super-high heat input are uniformly dispersed, thereby improving the toughness and strength (or hardness) of both the base metal and the heat affected zone while minimizing the toughness difference between the base metal and the heat affected zone, a method for manufacturing the welding structural steel product, and a welded structure using the welding structural steel product.
In accordance with one aspect, the present invention provides a welding structural steel product having fine complex precipitates of TiN and CuS, comprising, in terms of percent by weight, 0.03 to 0.17% C, 0.01 to 0.5% Si, 0.4 to 2.0% Mn, 0.005 to 0.2% Ti, 0.0005 to 0.1% Al, 0.008 to 0.030% N, 0.0003 to 0.01% B, 0.001 to 0.2% W, 0.1 to 1.5% Cu, at most 0.03% P, 0.003 to 0.05% S, at most 0.005% O, and balance Fe and incidental impurities while satisfying conditions of 1.2≦Ti/N≦2.5, 10≦N/B≦40, 2.5≦Al/N≦7, 6.5≦(Ti+2Al+4B)/N≦14, and 10≦Cu/S≦90, and having a microstructure essentially consisting of a complex structure of ferrite and pearlite having a grain size of 20 &mgr;m or less.
In accordance with another aspect, the present invention provides a method for manufacturing a welding structural steel product having fine complex precipitates of TiN and CuS, comprising the steps of:
preparing a steel slab containing, in terms of percent by weight, 0.03 to 0.17% C, 0.01 to 0.5% Si, 0.4 to 2.0% Mn, 0.005 to 0.2% Ti, 0.0005 to 0.1% Al, 0.008 to 0.030% N, 0.0003 to 0.01% B, 0.001 to 0.2% W, 0.1 to 1.5% Cu, at most 0.03% P, 0.003 to 0.05% S,
Choi Hae-Chang
Jeong Hong-Chul
LaVilla Michael
Posco
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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