Advanced high strength steel(AHSS) is one of the attractive material systems in automobile and construction industries, since it offers significantly high strength and elongation materials properties. Recently, transformation-induced plasticity(TRIP) steel has been extensively studied, in which the strengthening mechanism is originated from strain-induced martensitic transformations. However, the applications of TRIP steel are limited due to the high cost of Mn and Ni alloy elements. Here, we suggest austempered high carbon high silicon steel(ACSS) as one of the next-generation AHSS systems due to its outstanding ultimate tensile strength(UTS) and low cost alloying materials. We examined the composition of the alloying element of ACSS and found out that C 0.8 wt.%, Si 2.5 wt.%, Mn 0.3 wt% has the best-record mechanical properties of tensile strength 1.6 GPa and 15 % elongation, under 380 ℃ austempring process for 30 minutes, at least, in our heat-treatment system.
To reveal mechanical strengthening mechanism of ACSS, we carried out microstructural analysis including electron back scattering diffraction(EBSD), transmission electron microscopy(TEM), and X-ray diffraction(XRD). EBSD shows that the ferrite inside the ausferrite has the same lattice directions, which is also confirmed by TEM analysis. In addition, adjacent (111) plane of austenite and (110) plane of ferrite has the same directions and d-spacing, implying the coherency between two phases. We interpret it as the well-aligned austenite and ferrite structure which leads a tightly-bound interface by enhancing mechanical properties of ACSS. In particular, the comparative analysis of EBSD and TEM both before and after tensile test shows that, while the samples are deformed by tensile force, the ferrite of the whole microstructures become aligned by local lattice rotations, which is the major factor to improve the elongation property. In XRD data, no difference between carbon-supersaturated ferrite and normal ferrite was identified, implying that the compressive residual stress is existing inside the ferrite structure. During the tensile deformation, the residual stress is, therefore, released by contributing to the enhanced elongation properties. Furthermore, Kernel average misorientation(KAM) map of EBSD shows the presence of strain-field inside the austenite, which is also observed by TEM bright-field(BF) images. The strain-field created in the austenite explains the high strength property of ACSS.
To understand the change of mechanical properties according to the austempering temperature, we carried out microstructural analysis and mechanical properties of 320 ℃ austempered ACSS material. By comparing the mechanical properties between 380 ℃ and 320 ℃ ACSS, the tensile strength was increased from 1.6 GPa to 2.0 GPa, respectively. On the other hand, the elongation was significantly decreased from 15 % to 3 %. Inverse pole figure(IPF) of EBSD and diffraction pattern(DP) of TEM show that the orientation of ferrite and the austenite-ferrite interface is identical irrespective of the austempering temperature. The strain-field inside the austenite was also identified by TEM. However, XRD data shows different characteristics for 320 ℃ austempered ACSS. The d-spacing for the carbon-supersaturated ferrite is longer than that of normal ferrite, thereby no further lattice-expansion occurs as the tensile deformation takes place. Therefore, the elongation was degraded, yet the increase of carbon contents in austenite, which is calculated from the lattice parameter, increases the mechanical strength. Our study shows that the orientation-alignment and the residual stress in ferrite improves the elongation property, whereas the strain field in austenite enhances the mechanical strength.