S355J2-steel_4448.html target=_blank class=infotextkey>S355JR is a low-alloy structural steel plate widely used in welding and parts conforming to European standards. With the development of China's steel structure market overseas, overseas projects are increasing, but overseas projects have strict requirements on product performance. A large number of S355JR steel plates produced by our factory have unqualified elongation, which seriously affects product quality and economic benefits. In this paper, the reasons for the low elongation pass rate of S355JR steel plate are analyzed in detail and corresponding improvement measures are given.
The Mn content of S355JR steel plate is 1.35/1.50, which belongs to high Mn steel; the segregation of Mn element is easy to occur during the solidification process of the slab, that is, the content of Mn in the center of the slab is higher than that of the matrix.
According to thermodynamic analysis, Mn is easy to combine with S to form MnS during the solidification process of the slab, and MnS has a strong ductility relative to the matrix, and can extend along the rolling direction during the rolling of the slab, thereby affecting the continuity of the matrix. At the same time, it is easy to cause the accumulation of hydrogen in the MnS inclusions, which leads to hydrogen-induced cracks.
When the S355JR steel plate is stretched, it is easy to generate stress concentration at the sharp corner of the sulfide, coupled with the existence of hydrogen-induced cracks, it is easy to break from the sharp corner of the sulfide and affect the elongation.
Banded structure, as a defect structure, often appears in hot-rolled low-carbon steel. Due to the existence of dendrite segregation during continuous casting and solidification, elements that are prone to segregation such as C, Mn, and S are unevenly distributed inside the slab. , during the rolling process, a manganese-rich zone and a manganese-poor zone appear along the rolling direction. Due to the difference in the mechanical properties of ferrite and pearlite during the stretching process of S355JR steel plate, cracks are generated on the grain boundaries adjacent to ferrite and pearlite, resulting in S355JR The fracture of the steel plate during stretching affects the elongation.
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