Experimental Investigation On Cold-Forming Effect And Residual Stress Of Middle-Walled Square Tube

Cold-formed steel has several advantages on uniform across-sectional shape, high quality, low energy consumption and good economic benefit; in addition, it has high stability and substitutability. These advantages play an important role on construction, automobile, mining machinery, bridges and roads construction, etc. With the increasing amount of cold-formed steel, people have a growing concern about the properties of cold-formed steel. Based on ten square tubes with different standards, this thesis is dedicated to researching the impact of cold-formed effect on mechanical parameters of middle-walled square tube and making a summary of residual stress distribution outside the square tube profile.In this thesis, square tubes have some differences in terms of steel grades, cross-sectional dimensions and sheet thickness, so the results can be more fully representative of the properties of cold-formed and middle-walled square tube. The main results are as follows:(1) compared with the raw material, the strength of flat and corner areas of square tube has been improved through cold-formed process; besides, the increasing value is correlated to the square tube thickness, cross-sectional dimensions, etc. (2) The yield strength and ultimate strength of the weld zone is higher than the opposite side and the other two sides. Considering different forming process, the difference in strength is also relatively large. (3) Cold-forming effect reduces the elongation of different areas of square tube. Moreover, the effect is more obvious, the decreasing value is higher. So, there is an inverse relationship between the elongation and the strength.Subsequently, the microstructure test of square tubes, which were formed by two processes named Flat-strip-to-Square Forming (CRF) and Circle-to-Square Forming with 100mm× 100mm× 6mm dimension, was carried on. Then, the microstructure of flat areas, corner areas and welded areas has been obtained. In the view of microscopic, in order to explain the change of macroscopic properties of square tube, the size of crystalline grain was measured and the phase transition of welding areas was analyzed. The microstructure of welding zone was observed. And the results show that heat-affected zone of CRF tube is smaller than that of FRF tube. The pressure of press rolls restrains the growth of austenite, so the process of CRF gets fine crystalline grain after cooling.Finally, blind hole was taken to study residual stress on the outer surface of square tube. The result shows that forming process can affect the longitudinal residual stress seriously, but affect the horizontal residual stress less obviously. Moreover, with the size of square tube increasing, the residual stress value of square tube decreases.