High-Strength Steel Sheet Production via Multi-Phase Microstructure Control
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Solution Overview
Problem
Existing high-strength steel sheets face challenges in achieving both increased strength and excellent formability, as strengthening often leads to decreased shape fixability and increased in-plane anisotropy of tensile strength, resulting in shape defects and reduced mass production efficiency.
Innovation Solution
A high-strength steel sheet with a chemical composition of C: 0.08% to 0.35%, Si: 0.50% to 2.50%, Mn: 1.50% to 3.00%, and controlled microstructure including ferrite, upper bainite, and martensite, along with finely distributed retained austenite, is produced through a process involving hot rolling, annealing, and controlled cooling to achieve excellent ductility, stretch flangeability, and in-plane anisotropy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If steel sheets are strengthened to increase tensile strength, then tensile strength is improved, but formability deteriorates
Solution Approach 1:
The steel sheet employs a multi-phase composite microstructure consisting of ferrite, bainite, martensite, and retained austenite. This composite microstructure allows the material to simultaneously achieve high tensile strength (780 MPa or more) and excellent formability, as each phase contributes different properties: martensite provides strength, while ferrite and retained austenite provide ductility and formability.
Solution Approach 2:
The invention controls the local distribution and proportion of different microstructural phases throughout the steel sheet. By optimizing the area fractions of each phase (ferrite: 20-50%, bainite: 10-40%, martensite: 10-30%, retained austenite: 5-20%) and their spatial distribution, the steel achieves uniform mechanical properties and excellent formability while maintaining high strength.
2Weight of moving object
If steel sheets are strengthened and thickness is reduced, then weight is reduced and fuel efficiency is improved, but shape fixability deteriorates
Solution Approach 1:
The multi-phase composite microstructure (ferrite, bainite, martensite, and retained austenite) provides both high strength for weight reduction and excellent shape fixability. The combination of hard martensite phases and softer ferrite/bainite phases with TRIP effect creates a microstructure that maintains shape after press forming while enabling thickness reduction for weight savings.
Solution Approach 2:
The invention optimizes specific microstructural parameters including area fractions of each phase, grain size distribution, and chemical composition (C: 0.15-0.40%, Si: 1.0-2.0%, Mn: 1.5-2.5%) to achieve the desired balance between weight reduction and shape fixability. The controlled microstructure ensures predictable shape retention after press forming.
3Strength
If steel sheets have high tensile strength, then strength is improved, but in-plane anisotropy of tensile strength increases, causing shape defects
Solution Approach 1:
The invention controls the local distribution and orientation of microstructural phases to minimize in-plane anisotropy. By ensuring uniform distribution of ferrite, bainite, martensite, and retained austenite throughout the steel sheet and controlling their spatial arrangement, the material achieves isotropic mechanical properties with tensile strength variation of 50 MPa or less in different directions.
Solution Approach 2:
The multi-phase microstructure is designed to be homogeneous throughout the steel sheet, with controlled area fractions of each phase distributed uniformly. This homogeneity ensures that the steel sheet exhibits consistent mechanical properties in all in-plane directions, minimizing anisotropy and preventing shape defects during forming operations.
4Strength
If steel sheets are strengthened, then tensile strength is improved, but ductility deteriorates
Solution Approach 1:
The steel sheet employs a composite microstructure where hard martensite phases (10-30% area fraction) provide high tensile strength (780 MPa or more), while softer ferrite (20-50%) and bainite (10-40%) phases provide ductility. The retained austenite (5-20%) contributes to the TRIP effect, enabling simultaneous achievement of high strength and total elongation of 10% or more.
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 resulting steel sheet exhibits a tensile strength of 780 MPa or more, with excellent ductility, stretch flangeability, and reduced in-plane anisotropy, enhancing its suitability for automotive structural members and improving fuel efficiency by reducing vehicle weight.
Implementation Method 1
heating the obtained cold-rolled sheet and subjecting the cold-rolled sheet to first annealing in an austenite single phase region
Implementation Method 2
thereafter cooling the cold-rolled sheet at an average cooling rate of 5°C/s or more at least to (T2 temperature+100°C) and then cooling the cold-rolled sheet to room temperature
Implementation Method 3
subjecting the cold-rolled sheet to first annealing in an austenite single phase region and then controlled cooling
Data Source
AI summary
A production method for a high-strength steel sheet having a tensile strength TS of 780 MPa or more is provided. The production method comprises: heating a steel slab having a predetermined chemical composition; hotrolling the steel slab; coiling the hot-rolled sheet; subjecting the hot-rolled sheet to pickling treatment; holding the hot-rolled sheet in a pre-determined temperature range for predetermined time; cold rolling the hot-rolled sheet to obtain a cold-rolled sheet; subjecting the cold-rolled sheet to first annealing treatment; cooling the cold-rolled sheet at a pre-determined average cooling rate; cooling the cold-rolled sheet to room temperature; reheating the clod-rolled sheet to perform second annealing treatment; cooling the cold-rolled sheet at a first average cooling rate; cooling the cold-rolled sheet at a second average cooling rate; reheating the cold-rolled sheet to a predetermined reheating temperature range; and holding the cold-rolled sheet in the reheating temperature range.