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Title

DEVELOPMENT OF 600MPA GRADE HIGH DUCTILITY HOT-ROLLED SHEET STEEL FOR AUTOMOTIVE SQUARE TUBE

Authorship

Hui, Yajun; Liu, Kun; Pan, Hui; Xiao, Baoliang; Zhang, Dawei; Bai, Fengxia

Hui, Yajun

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Liu, Kun

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Pan, Hui

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Xiao, Baoliang

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Zhang, Dawei

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Bai, Fengxia

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DOI

10.5151/9785-9785-32245

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Abstract

The microstructure, properties, strengthening mechanism and work hardening mechanism of 600MPa grade high ductility hot-rolled sheet steel for automotive square tube were investigated by utilizing optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the microstructures of low C-low Mn-Nb and Ti microalloyed steel (Steel A) and high C-high Mn-Nb microalloyed steel (Steel B) are composed of ferrite and a small amount of pearlite. Compared with steel B, the size of ferrite and pearlite of steel A is finer, while the size of the second phase precipitates is larger. The mechanical properties of the two tested steels are similar before tube forming, except that the yield ratio of steel A is higher. Steel A has/exhibits significant work hardening after tube forming; the yield strength and tensile strength increase by 45 MPa and 26 MPa, respectively, while the elongation decrease by 6.0 percent. The yield strength and tensile strength of steel B increase by 22 MPa and 10 MPa, respectively, and the elongation decrease by 4.0 percent. The grain refinement strengthening is the dominant strengthening mechanism, accounting for 52.9 to 61.8 percent of the total yield strength; the solid solution strengthening is the second strengthening mechanism, accounting for 17.2 to 25.3 percent, while the precipitation strengthening and dislocation strengthening contribute little to the yield strength. The dislocation strengthening increase by 82 MPa of steel A after tube forming, which is higher than that of steel B. Therefore, steel B has a low work hardening rate that is more suitable for manufacturing automotive square tube for secondary tube forming.

Keywords

6600MPa grade, Square tube steel; Strengthening mechanism; Work hardening mechanism.