Bohai Steel Structure Co., Ltd.

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    Practice of staircase with steel structure

    author:Bohai Steel Structure Co., Ltd.

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    time:2019-10-07 09:56:03

    From the above chemical composition characteristics, it can be seen that the strength of the tested steel is mainly realized by the gap strengthening of carbon atom, the dissolution strengthening of manganese atom, the precipitation strengthening of niobium atom and the fine grain strengthening of these three elements. Among them, precipitation strengthening and fine grain strengthening have less plastic damage to the material while the strength of the material is added. Manganese and a small amount of niobium are mainly used as strengthening elements, and the corresponding production cost is not high. In addition, the low content of silicon and aluminum in steel is beneficial to improve the adhesion of high strength steel coatings. The mechanical properties of test steel are shown in Table 2. From the data in the table, it can be seen that the yield strength, tensile strength and elongation of the test steel are 47560 MPa and the corresponding 180 degree coating cold bending test also meet the design standard table 2. The metallographic structure of the test steel for mechanical properties is shown in Figure 1. From the observation of the microstructure, it can be seen that the metallographic structure of the test steel is composed of pre-eutectoid ferrite + bainitic ferrite. The average grain size of the ferrite is about 5 micron. This grain structure has remarkable grain strengthening effect, which is consistent with the fine grain strengthening effect of the carbon, manganese and niobium elements analyzed above. At the same time, the ferrite grain size of the test steel is similar to that of the carbon, manganese and niobium elements. It has non-equiaxed characteristics and belongs to bainitic ferrite. This ferrite is formed by shearing and diffusion during austenite slow cooling. Its forming temperature is lower than that of ferrite + pearlite, and the corresponding dislocation density is higher, which has obvious strengthening effect on test steel.

    Therefore, the test steel also has obvious dislocation strengthening. Figures 2 (a) and (b) show the low-power morphology of the coating and the corresponding chemical composition distribution of the test steel, respectively. From the distribution curve of iron content in Garden 2 (b), it can be seen that the iron content of the test steel coating is low, and there is only a small amount of iron content in the transition zone between the coating and the steel base. This indicates that the content of Fe-zn brittle phase in the coating is low, and there is only a small amount of Fe-zn brittle phase in the transition zone between the coating and the steel base. This result is consistent with the test results of adhesion of 180 cold bending coatings, that is, the coatings remain intact after cold bending test, and no cracking is found. Fig. 2 Macro-coating morphology and micro-chemical composition analysis results of test steel. Macro-coating morphology and iron content distribution curve of B coating 3. Main conclusions 1) Through chemical composition design and structure analysis, hot-dip galvanized high strength steel with yield strength, tensile strength and elongation of 470 MPa, 560 MPa and 23% respectively was developed, corresponding 180 degree plating. The layer cold bending test also meets the design standard.

    2) The analysis of chemical composition and structure shows that the strength of test steel is mainly achieved by the strengthening of carbon atoms, the re-dissolution strengthening of manganese atoms, the precipitation strengthening of niobium atoms and the fine grain strengthening of these three elements. The non-equiaxed bainitic ferrite structure also has a good strengthening effect on test steel. 3) Microstructure analysis of the coatings showed that the iron content of the coatings was low, and only a small amount of iron was found in the transition zone between the coatings and the steel base. This indicated that the content of Fe-zn brittle phase in the coatings was low, and only a small amount of Fe-zn brittle phase was found in the transition zone between the coatings and the steel base. The fewer Fe-Zn brittle phases in the coatings and the transition layer ensured the good performance of the test steel. The adhesion of the coating.

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