Effects of Partial Replacement of Si by Al on Cold Formability in Two Groups of Low-Carbon Third-Generation Advanced High-Strength Steel Sheet: A Review

Abstract

Partial replacement of Si by Al improves the coatability (or galvanizing property) of Si-Mn advanced high-strength steel (AHSS) sheets. In this paper, the effects of the partial replacement on the microstructure, tensile property, and cold formability are reported for the low-carbon third-generation AHSS sheets, which are classified into two groups, “Group I” and “Group II”. The partial replacement by 1.2 mass% Al increases the carbon concentration or mechanical stability of retained austenite and decreases its volume fraction in the AHSSs, compared to Al-free AHSSs. The partial replacement deteriorates the tensile ductility and stretch formability in the AHSSs with a tensile strength above 1.2 GPa. On the other hand, it achieves the same excellent stretch-flangeability as Al-free AHSSs. A complex addition of Al and Nb/Mo further enhances the stretch-flangeability. The cold formabilities are related to the heat treatment condition and microstructural and tensile properties, and the stress state.

Effects of Partial Replacement of Si by Al on Impact Toughness of 0.2%C-Si-Mn-Cr-B TRIP-aided Martensitic Steel

“…The strain-induced transformation factor (k) defined by the following equation means the mechanical stability of retained austenite [4],…”

“…According to Sugimoto et al [4,33], true flow stress (true plastic strain), σT(εT), of the AHSS containing the retained austenite of 4 to 30 vol.% is formulated by where σ M (εT) and Δσh (εT) are the flow stress of the matrix structure and strain hardening increment of the steel, respectively. The Δσh (εT) can be estimated by where Δσi (εT), Δσt (εT), and Δσf (εT) represent "the long-range internal stress hardening", "the straininduced transformation hardening", and "the forest dislocation hardening", respectively, which can be formulated by where ν is the Poisson's ratio, μ is the shear modulus, εp u is "the eigenstrain" [60], f is the volume fraction of the second phase, g (Δfαm) is a function of the strain-induced martensite fraction, ζ is a material constant, b is the Burgers vector, and r is particle radius of the second phase.…”

“…The third-generation advanced ultrahigh-and high-strength steels (AHSSs) have been developed for automotive applications to sheet forming components and bar forging parts [1][2][3][4]. The AHSSs are classified into the following two groups, "Group I" and "Group II", by the kind of matrix structure and/or tensile strength level [4], as follows.…”

“…Al also remarkably increases the maximum carbon concentration of retained austenite by raising the critical temperature (T0) at which austenite and martensite have the same Gibbs free energy in steel [39]. This is because Al retards the carbide formation and resultantly increases the volume fraction of retained austenite like Si and P [4,40,[44][45][46][47].…”

“…To promote the application of galvanized third-generation low-carbon AHSS sheets and bars to automotive parts, many researchers investigated the effect of partial replacement of Si by Al on the microstructural and mechanical properties [4,35,36,[47][48][49][50][51]. Unfortunately, most of the mechanical properties were focused on the tensile properties and formabilities, not impact toughness, in the third-generation AHSSs.…”