Steel Sheet Microstructure for LME Resistance and Formability

The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.30%, Si: 0.30-1.50%, Mn: 1.40-3.49%, Al: 0.30-1.50%, Ti: 0.001-0.100%) and microstructure parameters (ferrite volume fraction 1-50%, retained austenite 5-30%, tempered martensite 10-60%, bainite 10-40%) to achieve both high tensile strength (980 MPa or more) and excellent press-formability (total elongation 10% or more, hole expansion ratio 20% or more). This multi-parameter optimization resolves the contradiction between strength and formability.

The patent creates a composite microstructure consisting of four phases: ferrite, retained austenite, tempered martensite, and bainite. Each phase contributes different properties: ferrite provides ductility, retained austenite provides TRIP effect for work hardening, tempered martensite provides strength, and bainite provides a balance of strength and ductility. This composite microstructure simultaneously achieves high tensile strength and excellent press-formability.

The patent applies local quality by creating a non-uniform Al/Si distribution at the steel sheet surface. By controlling the area ratio of regions with AlS/SiS ratio of 0.2 or less to be 50% or less, the surface develops localized Al-enriched regions that provide LME cracking resistance while Si-enriched regions maintain press-formability. This spatial differentiation of composition resolves the contradiction between the two opposing requirements.

The patent changes the compositional parameters by adding Al (0.30-1.50%) in addition to Si (0.30-1.50%) and controlling their surface concentration ratio. This parameter modification allows the steel to achieve both press-formability (through Si-induced retained austenite) and LME cracking resistance (through Al-enriched surface regions) simultaneously.