The metallographic photograph of Q550 steel base material was analyzed. Metallographic microscope 500 times, 4% nitric acid alcohol corrosion. What is the structure and function?

Q550 is a microalloyed high strength steel with controlled rolling and controlled cooling, which is generally supplied according to properties and does not need heat treatment. From the organization shown in the figure, the organization is uniform, but I don't know the magnification (no ruler! ), so its level of detail is not easy to evaluate; In addition, there is no austenitizing condition, which also brings suspense to organizational judgment; Also, what is the quenching and tempering hardness? Judging from the present situation, the tempered structure should contain carbide-free bainite, upper bainite and low carbon martensite.