High strength thick steel material and high strength giant H-shape excellent in toughness and weldability and methods of production of same

22 The present invention provides a high strength thick steel material excellent in toughness and weldability reduced in amount of C and amount of N, containing suitable amounts of Si, Mn, Nb, Ti, B, and O, having contents of C and Nb satisfying C-Nb/7.74≦0.004, having a density of Ti-containing oxides of a particle size of 0.05 to 10 μm of 30 to 300/mm, and having a density of Ti-containing oxides of a particle size over 10 μm of 10/mmor less, produced by treating steel by preliminary deoxidation to adjust the dissolved oxygen to 0.005 to 0.015 mass %, then adding Ti and, furthermore, vacuum degassing the steel for 30 minutes or more, smelting it, then continuously casting it to produce a steel slab or billet, heating the steel slab or billet to 1100 to 1350° C., hot rolling the slab or billet to a thickness of 40 to 150 mm, then cooling it.