Ultra-High-Strength Steel Sheet Tri-Phase Microstructure
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Solution Overview
Problem
Current high-strength steel sheets for vehicles face limitations in achieving both high formability and strength, with existing dual-phase and transformation-induced plasticity steels struggling to balance yield strength, tensile strength, elongation, and weldability, while also being difficult to manufacture in continuous galvanizing lines.
Innovation Solution
A steel sheet composition of 0.12-0.22% C, 1.6-2.4% Si, 2.0-3.0% Mn, 0.01-0.05% Al, and 0.01-0.05% (Ti+Nb+V) with a microstructure of 11-20% ferrite, 65%+ tempered martensite, and 10-20% retained austenite, achieved through a manufacturing process involving hot-rolling, cold-rolling, primary heat treatment, slow cooling, quenching, and secondary heat treatment, allowing for a yield strength of 850 MPa or greater, tensile strength of 1180 MPa or greater, and elongation of 14% or greater.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is increased to secure impact toughness, then the safety of passengers is improved, but the weight of vehicle body increases and fuel efficiency deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.25%, Si: 1.50-2.50%, Mn: 1.50-3.00%, Al: 0.01-0.10%, Ti: 0.01-0.06%, Nb: 0.01-0.06%, V: 0.01-0.06%) and heat treatment parameters (primary heat treatment temperature: Ac3-20°C to Ac3°C, secondary heat treatment temperature: 400-460°C) to achieve ultra-high strength (tensile strength ≥1180 MPa) while maintaining low density of steel material
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, tempered martensite, and retained austenite) with specific volume fractions (ferrite: 11-20%, tempered martensite: 65% or more, retained austenite: 10-20%). This composite microstructure provides both high strength and high elongation (≥14%), achieving superior mechanical properties without increasing material density
2Strength
If the thickness of steel sheet is increased to secure sufficient impact toughness, then the safety of passengers is improved, but the weight of vehicle body increases
Solution Approach 1:
The patent changes the strength parameters through compositional control and heat treatment, achieving tensile strength ≥1180 MPa and elongation ≥14%, which allows using thinner sheets while maintaining required impact toughness and safety performance
Solution Approach 2:
The multi-phase composite microstructure (ferrite + tempered martensite + retained austenite) provides exceptional strength-to-weight ratio, enabling thin-gauge high-strength steel sheets that meet safety requirements without increasing weight
3Strength
If the strength of steel sheet is continuously increased, then the safety performance is improved, but the formability deteriorates
Solution Approach 1:
The patent employs a composite microstructure with ferrite (11-20%, provides ductility and formability), tempered martensite (65% or more, provides strength), and retained austenite (10-20%, provides elongation through TRIP effect). This combination achieves yield strength ≥850 MPa and tensile strength ≥1180 MPa while maintaining excellent formability with elongation ≥14% and hole expansion ratio ≥30%
Solution Approach 2:
The patent applies local quality by creating different phases in specific volume fractions within the microstructure, where each phase contributes different properties: ferrite for formability, tempered martensite for strength, and retained austenite for elongation. The controlled distribution of these phases throughout the material provides both high strength and good formability
4Strength
If the strength of steel sheet is increased, then the safety performance is improved, but the weldability deteriorates
Solution Approach 1:
The patent carefully controls the carbon content (0.15-0.25%) and carbon equivalent through precise compositional parameters and heat treatment (secondary heat treatment at 400-460°C), achieving ultra-high tensile strength (≥1180 MPa) while maintaining weldability by preventing excessive hardening and cracking susceptibility in the heat affected zone
5Ease of operation
If a dual-phase steel structure is used to secure strength and elongation, then the formability is improved, but the tensile strength is insufficient to achieve ultra-high strength
Solution Approach 1:
The patent extends the dual-phase concept to a tri-phase composite microstructure by adding retained austenite (10-20%) to the ferrite-tempered martensite combination. The retained austenite contributes to elongation through transformation-induced plasticity (TRIP effect) during deformation, while the tempered martensite provides ultra-high strength. This tri-phase structure achieves both excellent formability (elongation ≥14%) and ultra-high tensile strength (≥1180 MPa)
6Ease of operation
If a transformation induced plasticity steel is used to secure strength and elongation, then the formability is improved, but the yield strength is insufficient to achieve ultra-high strength
Solution Approach 1:
The patent combines transformation-induced plasticity (TRIP) steel characteristics with high-strength martensitic structure by retaining 10-20% austenite in the microstructure alongside 65% or more tempered martensite and 11-20% ferrite. The retained austenite provides TRIP effect for high elongation and formability, while the dominant tempered martensite phase provides ultra-high yield strength (≥850 MPa) and tensile strength (≥1180 MPa), overcoming the limitation of conventional TRIP steels with lower yield strength
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables the production of ultra-high-strength steel sheets with excellent formability, stable high tensile strength, and improved weldability, while maintaining a high hole expansion ratio, suitable for vehicle components requiring both strength and processability.
Implementation Method 1
a final microstructure of the steel sheet may include ferrite, tempered martensite, and retained austenite
Implementation Method 2
sequentially performing slow cooling and quenching on the cold-rolled sheet subjected to the primary heat treatment
Implementation Method 3
performing a primary heat treatment on the cold-rolled sheet at a temperature of (AC3-20) to AC3° C.
Data Source
AI summary
A steel sheet having high strength and high formability according to an aspect of the present invention includes: % by weight, an amount of 0.12-0.22% of carbon (C); an amount of 1.6-2.4% of silicon (Si); an amount of 2.0-3.0% of manganese (Mn); an amount of 0.01-0.05% of aluminum (Al); an amount greater than 0 and less than or equal to 0.05% of the sum of one or more of titanium (Ti), niobium (Nb) and vanadium (V); an amount of 0.015% or less of phosphorus (P); an amount of 0.003% or less of sulfur (S); an amount of 0.006% or less of nitrogen (N); and the reminder of Fe and inevitable impurities, and has a yield strength (YS) of 850 MPa or greater, a tensile strength (TS) of 1180 MPa or greater, an elongation ratio (EL) of 14% or greater, and a hole expansion ratio (HER) or 30% of greater.