1. Field of the Art
This invention relates to niobium-containing weldable structural steel having good weldability. More specifically, the present invention relates to a niobium-containing weldable structural steel having yield strength of 40-70 kg/mm.sup.2 prepared by providing a niobium-containing steel with a predetermined composition so as to improve toughness in the weld heat affected zone and resistance to weld cracking.
2. Description of the Prior Art
When added in a small amount to steel, niobium improves the strength and toughness of steel, and is an economically advantageous element. It is for this reason that a niobium-containing steel (hereinafter referred to as "Nb-containing steel") has gained a wide application as a weldable structural steel for pipe lines, ship-building, pressure containers, bridges and the like. Among numerous applications of the Nb-containing steel, the detailed description is hereby presented about line pipe steels used in specifically great quantities. Nb-containing non-quenched tempered high tensile strength steel has conventionally been used in great quantities for pipe lines for transporting crude oil and natural gas. However, the pipe lines that have so far been laid down have a small pipe diameter and low inner pressure and are used at a relatively high temperature of not lower than about 0.degree. C. Hence, requirement for toughness in the weld heat affected zone has not been much serious.
As laying of pipe lines has gradually been concentrated on the ultra-cold region such as in U.S.S.R., Canada, U.S.A., etc. and the material transported has also been changed gradually from crude oil to natural gas, however, an unexpected problem has come to the front. The conventional Nb-containing steel for pipe line causes embrittlement of the weld heat affected zone (HAZ) in the seam weld portion at the time of steel making and hence, fails to ensure sufficient toughness to withstand the use in such regions. As countermeasures, a multiple-electrode submerged arc welding process of a low restricted weld heat input or a MIG welding process has been examined on one hand in the aspect of the welding process, in order to mitigate the influence of the welding heat. Research and development has been made on the other hand in the aspect of the steel material to be used, in order to obtain a steel having such a composition of economical components that does not cause degradation of the weld heat affected zone. With regard to the properties of the steel material especially, the most desirable steel would be one that does not cause embrittlement of the weld heat affected zone and yet exhibits good notch toughness even when welding of a large weld heat input such as one side welding or both side welding with one or two layers (not more than three passes in each weld groove) is carried out in order to improve the welding efficiency and reduce the cost of production of the pipe. At the same time, the steel must satisfy a specific requirement of low weld crack sensitivity for the purpose of preventing cracks of the weld portion because when the on-the-site welding is effected in the cold zone, a high cellulose type electrode of a high hydrogen content is generally employed without preheating of the pipe in the low temperature atmosphere.
It has been a pressing need for those concerned in the art to develop a steel strip for a line pipe which simultaneously satisfies the abovementioned requirements, i.e., to have excellent toughness in the weld heat affected zone and to be less hardenable and excellent in its resistance to weld cracking.
Incidentally, the following U.S. patents are located as the prior art most relevant to the present invention;
U.S. Pat. Nos. 3,592,633, 3,807,990, 3,619,303, 3,725,049, 3,721,587 and 3,132,025.