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Steel plate for low yield ratio thick circular steel pipe having a tensile strength of 780 MPa or more, manufacturing method thereof, and low yield ratio thick circular steel pipe having a tensile strength of 780 MPa or more
Steel plate for low yield ratio thick circular steel pipe having a tensile strength of 780 MPa or more, manufacturing method thereof, and low yield ratio thick circular steel pipe having a tensile strength of 780 MPa or more
PROBLEM TO BE SOLVED: To provide a steel plate for a circular steel pipe having a thick-wall thickness with a maximum plate thickness of 80 mm, achieving both a high strength of 780 MPa and a low yield ratio of 90% in strong bending work such as D/t=10-20, and further stably having satisfactory toughness even after worked in the steel pipe.SOLUTION: The steel plate has a prescribed chemical componential composition, has a hardenability index DI of 8 inch regulated by a prescribed relational formula, and satisfies the following requirements (A), (B), and (C); wherein (A) bainite has an area% of 90 in the microstructure at 1/4 portion in a plate thickness; (B) an average circle equivalent diameter in the regions surrounded by large-angle grain boundaries having an orientation difference of 15° is 4 m, in the microstructure at 1/4 portion in the plate thickness; and (C) island martensite having an average circle equivalent diameter of 0.5-3 m and having a Vickers hardness Hv of 700 is included by 3-10 area% in the microstructure at 1/4 portion in the plate thickness.
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机译：要解决的问题：提供一种用于圆形钢管的钢板，该钢板的厚壁厚度最大为80 mm，在强弯工作中既要达到780 MPa的高强度又要达到90％的低屈强比例如：D / t = 10-20，甚至在钢管中加工后也具有稳定的令人满意的韧性。解决方案：钢板具有规定的化学成分，由规定的关系式调节的淬透性指数DI为8英寸，并且满足以下要求（A），（B）和（C）;其中（A）贝氏体在板厚的1/4部分的显微组织中具有90％的面积％; （B）在板厚的1/4部分的组织中，被取向差为15°的大角度晶界包围的区域的平均圆当量直径为4m。 （C）在组织中，在板厚的1/4处，以3〜10面积％包含平均圆当量直径为0.5〜3m，维氏硬度Hv为700的岛状马氏体。
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