Effect of Microstructural Characteristics on Mechanical Properties of Ferritic Stainless Steel

To improve the mechanical properties of hot-rolled ferritic stainless steel (SUS430), the microstructural characteristics of SUS430 were changed using a heating process under various conditions. The hardness of SUS430 decreased upon the increase in the heating temperature to 900 °C, and the hardness increased when the sample was heated to temperatures greater than 900 °C. The high hardness of the sample heated at 1000 °C (H_1000 °C) is attributed to the heating time: A high hardness was obtained for a H_1000 °C sample that was heated for 1 h (H_1000 °C-1h), but this decreased when the heating time was increased to more than 1 h. The high hardness of H_1000 °C-1h is caused by the fine Cr₂₃C₆ precipitates that are distributed in the sample around the grain boundaries. On the other hand, the large precipitates of Cr₂₃C₆ in H_{1000 °C-12h} decrease the hardness. The hardness value of SUS430 is directly attributed to the mechanical properties and the ultimate tensile strength. The tensile strength of H_1000 °C-1h was found to be about 200% and 20% higher than the as-received and H_{1000 °C-12h} samples, respectively. Despite the increase in the tensile strength of the H_1000 °C-1h sample, the ductility was not found to decrease significantly, for example, the fracture strain was approximately 25%. This occurrence is affected by a severe slip in the ferrite base grain, and the high strength of H_1000 °C-1h is influenced by the interruption of the slip by the Cr₂₃C₆ precipitates. Unlike the tensile strength, similar fatigue properties were observed for both H_1000 °C-1h and H_{1000 °C-12h}, which is associated with the low crack driving force of H_{1000 °C-12h}, caused by the roughness-induced crack closure arising from the large Cr₂₃C₆ precipitates.