High-strength thick steel plate excellent in low temperature toughness at heat affected zone resulting from large heat input welding

Abstract

The present invention provides a high-strength thick steel plate having a plate thickness of 50 to 80 mm and a tensile strength of 490 to 570 MPa which is able to realize an excellent HAZ toughness even when welding with a heat input of 20 to 100 kJ / mm is conducted and is characterized by containing, by wt %, 0.03-0.14% of C, 0.30% or less of Si, 0.8-2.0% of Mn, 0.02% or less of P, 0.005% or less of S, 0.8-4.0% of Ni, 0.003-0.040% of Nb, 0.001-0.040% of Al, 0.0010-0.0100% of N, and 0.005-0.030% of Ti, where Ni and Mn satisfy equation [1], and the balance of iron and unavoidable impurities:Ni / Mn≧10×Ceq−3 (0.36<Ceq<0.42)  [1]where,  Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a high strength thick steel plate excellent in low temperature toughness at heat affected zone (hereinafter referred to as an “HAZ”) used for ships, offshore structures, medium / high rise buildings, bridges, and so forth, more particularly relates to a steel plate having a thickness of 50 mm or more and a tensile strength of 490 to 570 MPa and having an excellent welded joint even in a case where welding with a welding heat input of 20 to 100 kJ / mm is conducted. [0003] 2. Description of the Related [0004] In recent years, the demands on the material properties of steel materials for welding used in large sized structures such as ships, offshore structures, medium / high rise buildings, and bridges have become increasingly tough. Especially, among these structures, the use of steel plates having large thicknesses exceeding 50 mm and having tensile strength of around 570 MPa has been incr...

AI Technical Summary

Benefits of technology

[0012] Therefore, an object of the present invention is to provide a high-strength thick steel plate excellent in the low temperature toughness of the heat affected zone resulting from large heat input welding, which can realize excellent HAZ toughness even in a case of welding with a heat input of 20 to 100 kJ / mm for steel plate having a thickness of 50 to 80 mm, and a tensile strength of 490 to 570 MPa.

[0016] (2) A high-strength thick steel plate excellent in low temperature toughness at a heat affected zone resulting from large heat input welding as set forth in (1), characterized by further containing, by wt %, one or more of 0.0003-0.0050% of Ca, 0.0003-0.0050% of Mg, 0.001-0.030% of an REM and containing at least 100 / mm2 of grains of an oxide containing 0.0010-0.0050% of O and having an equivalent circle diameter of 0.005 to 0.5 μm.

Problems solved by technology

Therefore, a sufficient HAZ toughness such as the E grade (−20° C. guarantee) in for example the shipbuilding field cannot be stably secured.

In addition, when the tensile strength becomes more than 570 MPa, the required HAZ toughness cannot be obtained.

Images