High-strength low-alloy (HSLA) steel is widely used in automotive industry for reduction of consumption and emissions. The microstructure and mechanical properties of two automotive HSLA steels with different strength grades were systematically investigated in present study. Microstructural characterization was conducted using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD), while mechanical properties were evaluated with Vickers hardness tester and tensile tests. Both steels exhibited a ferrite matrix with spheroidized carbides/pearlites. However, Sample A displayed equiaxed ferrite grains with localized pearlite colonies, while Sample B featured pronounced elongated ferrite grains with a band structure. Tensile testing revealed that Sample B had higher ultimate tensile stress and yield stress compared to Sample A. Texture analysis indicated that both steels were dominated by α-fiber and γ-fiber textures, with minor θ-fiber texture, resulting in minimal mechanical anisotropy between the rolling direction (RD) and transverse direction (TD). The quantitative assessment of strengthening mechanisms, based on microstructural parameters and experimental data, revealed that grain boundary strengthening dominates, with dislocation strengthening also contributing significantly. This work provides the first comprehensive quantification of individual strengthening contributions in automotive HSLA steels, offering critical guidance for developing further higher-strength automotive steels.

Nanoscale Al₃Sc precipitates strongly influence dislocation transport and heterogeneous plastic deformation in Al-Sc alloys. Here, the environment-dependent evolution of precipitate–dislocation interactions in an Al–0.3 wt.% Sc alloy was investigated by in-situ TEM tensile testing, assisted by EBSD, TEM, EDS, and atomic-resolution HAADF-STEM. The ordered Al₃Sc precipitates exhibit an average equivalent diameter of 27.0 ± 6.3 nm, an areal number density of ~27.5 μm⁻², and an average center-to-center spacing of ~102 nm, corresponding to an effective obstacle spacing of ~75 nm. The modified Orowan model gives a critical shear-stress increment of ~67 MPa and a macroscopic strengthening contribution of ~205 MPa. During intragranular deformation, dislocations undergo repeated bowing, pinning, depinning, and renewed glide. Near grain boundaries, precipitate arrangements promote directional dislocation transport and heterogeneous strain accommodation. At the crack tip, selective pinning redistributes plastic flow, promotes crack-tip blunting, and activates alternative slip paths, contributing to crack-path deflection. These observations establish a continuous mechanistic link between nanoscale precipitate characteristics, dislocation transport, heterogeneous plasticity, and crack propagation in Al-Sc alloys.