| 摘要 |
PROBLEM TO BE SOLVED: To provide a thick steel plate having an improved fatigue characteristics both before and after crack generation, and a tensile strength of 490-640 MPa, and to provide a production method of the thick steel plate.SOLUTION: The thick steel plate has a specific chemical composition and of which the metal texture satisfies the following requirements (a)-(d) in measuring the metal texture at an observation position located at a depth of 3 mm from the surface of the steel plate on the longitudinal cross section parallel to the rolling direction, and of which the deposit satisfies the following requirement (A): (a) the metal texture is composed of a bainite structure and a balance structure harder than bainite and has a bainite ratio of 80% or more by area; (b) A bainite crystal grain has an average length of 7 μm or less in the plate thickness direction when measured based on a high-angle grain boundary having an orientation difference between adjacent crystals of 15° or more; (c) the hard structure of the balance has a circle-equivalent diameter of 3 μm or less; (d) the difference of hardness between the bainite and the hard balance structure is Hv 100 or more; and (A) there are 100/μmor more of deposits that contain at least any one of Nb, Ti, and V and have a circle-equivalent diameter of 20 nm or less. |
