On structure-property relationship in hot-rolled low-carbon carbide-free bainitic steel
Carbide-free bainitic steel(CFB steel)exhibits a good balance of strength and ductility due to the presence of a fine bainite microstructure combined with residual austenite.However,these superior mechanical properties are usually achieved by adding a high carbon content in combination with additional bainitic isothermal treatment,which not only deteriorates the weldability,but also discourages economic costs and productivity.To overcome this draw-back,instead of the conventional bainitic isothermal treatment,two hot rolled low carbon CFB steels with ferrite + bainite microstructures(refer as F-B steels)and complete bainite(refer as B steels)were designed by the curling process after hot rolling.The relationship between the microstructure and mechanical properties of these two steels was investigated.The yield strength,ultimate tensile strength,total elongation,and fracture strain of F-B steel are(575±8)MPa,(853±12)MPa,19.0%±0.1%and 0.63±0.03,respectively.Compared to F-B steel,B steel exhibits a lower total elongation of 14.7%±0.2%,while its yield strength,ultimate tensile strength,and fracture strain are significantly improved to(772±11)MPa,(1 160±10)MPa,and 0.78±0.02,respectively.The higher yield strength and ultimate tensile strength exhibited by B steel are mainly attributed to the complete bainite struc-ture,which can achieve effects similar to dislocation strengthening,fine grain strengthening and second-phase strengthening.Moreover,their deformation and damage behavior were further revealed and the related results showed that the presence of ferrite and more stable residual austenite in F-B steel contributes to the more durable work-hardening ability of F-B,which is the key to its higher total elongation.For B steel,a more homogeneous microstructure and lower carbon content result in better-coordinated deformation,which significantly inhibits the deformation damage formation and thus contributes to higher fracture strain.
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