Steel for welded structures excellent in high temperature strength and low temperature toughness and method of production of same

Abstract

By heating a steel material comprising C: 0.003 to 0.05%, Si: 0.60% or less, Mn: 0.6 to 2.0%, P: 0.020% or less, S: 0.010% or less, Cr: 0.20 to 1.5%, Nb: 0.005 to 0.05%, Al: 0.060% or less, and N: 0.001 to 0.006%, further limiting, as an impurity, Mo to 0.03% or less, having a balance of iron and unavoidable impurities, and having a weld cracking parameter PCM value defined by PCM=C+Si / 30+Mn / 20+Cu / 20+Ni / 60+Cr / 20+Mo / 15+V / 10+5B of 0.22% or less, to 1000 to 1300° C. in temperature, finishing the hot rolling at 800° C. or more in temperature, then cooling, steel for welded structures excellent in high temperature strength and low temperature toughness can be inexpensively provided.

Description

TECHNICAL FIELD[0001]The present invention mainly targets fire-resistant steel for building structures aimed at maintaining the proof strength at the time of fires and other high temperature conditions, but is not limited to building applications and can also be applied to steel for welded structures for offshore structures, ships, bridges, various storage tanks, and a broad range of other applications. Note that the strength level of the steel plate mainly covered is a yield strength of 235 to 475 MPa and a tensile strength of 400 to 640 MPa, i.e., the classes generally called “40 kg” and “50 kg steels”.PRIOR ART[0002]So-called “fire-resistant steel” is disclosed in Japanese Patent Publication (A) No. 2-77523 and numerous other publications. However, almost all contain Mo. It is true that Mo is an element extremely effective in securing the high temperature proof strength of steel, but at the same time it is an expensive element.[0003]In this regard, steel for general structures fo...

AI Technical Summary

Benefits of technology

[0009]That is, the inventors discovered that by addition and inclusion of a suitable amount of Cr in an Mo-free composition, the hardenability of the steel is improved, the transformation temperature falls, and the hard structure including cementite becomes bainitic.

[0093]However, addition of too much conversely causes the cleanliness of the steel to deteriorate and raises the base material toughness and hydrogen induced cracking (HIC, SSC, and SOHIC) susceptibility under a wet hydrogen sulfide environment, so the upper limits of the amounts of addition were respectively made, for Ca and REM, 0.004% and 0.008%. Preferably, the limits may be made 0.003% and 0.006% or less. Note that Ca and REM have substantially equivalent effects, so it is sufficient to add either of these in the above range. Addition of both is also possible.

Problems solved by technology

It is true that Mo is an element extremely effective in securing the high temperature proof strength of steel, but at the same time it is an expensive element.

However, the market for Mo greatly fluctuates.

While depending on the amount of addition as well, in many cases it results in a higher cost compared with the cost of fire-resistant coating.