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Lamination crack resistant steel, which is also called "Z-direction steel", is a type of high-strength low-alloy steels. This kind of steels is produced by special smelting process, and the section shrinkage in thickness direction (Z-direction) is normally above15%. Steel structure building has been the future trend of construction industry due to its high strength, light weight, low construction time. Because of the low lamination crack resistent property of the H-beams produced in our country, the H-beams used in the high structure building in our country are produced by welding the lamination crack resistant steel plates. Compared with the integral forming H-beams, not only the cost of construction is improved, but the welding defects reduce the performance of steels. Therefore, research on the lamination crack resistant property and mechanism of hot-rolled H-beams has important significance in the economic benefit and application range.The main thought of this subject is the contrast experiment. Through analyzing the differences of Q345B and Q345EL in compositions, properties, microstructures, inclusions, second-phase particles, etc, summarized the influence factors on the Z-direction property of H-beams finally. The process of this research began with the microstructure observation by optical microscope and Z-direction performance measurement. Then, take advantage of the Scanning Electron Microscope(SEM), Transmission Electron Microscope(TEM), Energy Dispersive Spectroscopy(EDS), X-ray diffraction(XRD), Diffraction pattern analysis to study the effect of inclusions and second-phase particles on lamination crack resistant property.In this paper, conclusions can be summarized as follows. The angular position, which contains most inclusions, has the best Z-direction performance because of its unobvious banded structure and three dimensional stress. The experiment steel of Q345EL, which contains Nb, B, Ni, lower inclusions and experienced refining process, performs lower Z-direction property than the steel of Q345B, which does not contain Nb, B, Ni and does not experience the refining process, and the Z-direction section shrinkage of Q345B has exceeded the highest national standards(35%). Mechanism of this phenomenon is mainly because of the effect of Nb, B, Ni on dislocations obstruction inside and outside the grain. Precipitated phase particles which contain Nb and B play play a main role in the Z-direction decrease of Q345EL, while Ni only has a partial influence.
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