THICK STEEL PLATE AND MANUFACTURING METHOD THEREOF

To provide a steel plate having yield strength of 500 MPa or more, excellent host toughness, and when performed multilayer welding, having excellent joint CTOD characteristics.SOLUTION: A thick steel plate has a plate thickness of 60 to 100 mm, the yield strength of 500 MPa or more, the tensile strength of 570 MPa or more, comprises C, Si, Mn, Ti, Cu, Ni, Nb, N, O, P, S, Al, Mg, Ca, B in a predetermined range, the balance being Fe and impurities, has the ferrite fraction in the plate thickness t/4 part of 0 to 15 area%, the balance composed of one or more types of bainite and martensite, and the plate thickness center Ti-containing steel whose maximum hardness HVmax satisfying formula (3), contains one or more of Ti, Al, Mg, Ca, and Mn in the steel structure, and has a circular equivalent diameter of 0.5 μm or more and 5.0 μm or less, wherein a Ti content ratio of each Ti-containing inclusion particle is calculated using equation (4) for inclusion particles, and the number density of Ti-containing inclusion particles effective for intragranular transformation shown in equation (5) is 20 pieces/mm2 or more.SELECTED DRAWING: Figure 3 【課題】降伏強度が５００ＭＰａ以上で、良好な母材靭性を有し、多層盛り溶接した際に、良好な継手ＣＴＯＤ特性を有する鋼板を提供することを目的とする。【解決手段】板厚が６０～１００ｍｍ、降伏強度が５００ＭＰａ以上、引張強さが５７０ＭＰａ以上であり、Ｃ、Ｓｉ、Ｍｎ、Ｔｉ、Ｃｕ、Ｎｉ、Ｎｂ、Ｎ、Ｏ、Ｐ、Ｓ、Ａｌ、Ｍｇ、Ｃａ、Ｂを所定範囲、残部がＦｅ及び不純物からなる成分を有し、板厚ｔ／４部分のフェライト分率が０～１５面積％で残部がベイナイト、マルテンサイトの１種類以上からなり、板厚中心部の最大硬度ＨＶｍａｘが式（３）を満足し、鋼組織中にＴｉ、Ａｌ、Ｍｇ、Ｃａ、Ｍｎを１種以上含有し、円相当径で０．５μｍ以上５．０μｍ以下であるＴｉ含有介在物の粒子について式（４）でＴｉ含有介在物の粒子毎のＴｉ含有割合を算出し、式（５）に示す粒内変態に有効なＴｉ含有介在物の粒子の個数密度が２０個／ｍｍ２以上となる。【選択図】図３