Complex Phase Steel Microstructure Strength Formability
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Solution Overview
Problem
Advanced high-strength steel sheets (AHSS) face limitations in formability and weldability due to their strong multi-phase structure and phase transformations during deformation, making them difficult to apply to complex automotive parts, and existing solutions for achieving high tensile strength compromise elongation and galvanisability.
Innovation Solution
A steel grade with a complex phase structure comprising ferrite, carbide-free bainite, martensite, and retained austenite, optimized chemical composition, and processing methods that include hot-rolling and cold-rolling with tailored annealing to achieve high yield and tensile strength while maintaining good elongation and hole expansion ratios, along with improved weldability and galvanisability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel grades are used to reduce weight and fuel consumption, then strength increases, but formability deteriorates
Solution Approach 1:
The patent applies composite material principles by creating a multi-phase steel microstructure consisting of martensite, bainite, and retained austenite. This composite microstructure combines the high strength of martensite with the ductility and formability contributions from bainite and retained austenite, resolving the contradiction between strength and formability
Solution Approach 2:
The patent employs parameter changes by precisely controlling chemical composition parameters (C: 0.23-0.40%, Si: 0.70-1.50%, Mn: 1.00-3.00%, Al: 0.015-0.100%, B: 0.0005-0.0060%) and processing parameters (hot rolling temperature, cooling rate, annealing temperature and time) to achieve the desired balance between strength and formability in the final steel product
2Strength
If carbon content is increased to achieve high tensile strength over 1200 MPa, then strength improves, but weldability deteriorates
Solution Approach 1:
The patent optimizes the carbon content parameter to a specific range (0.23-0.40%) that enables achieving tensile strength over 1200 MPa while maintaining acceptable weldability. This is accomplished by combining controlled carbon levels with other alloying elements and heat treatment parameters to achieve the desired strength without excessive carbon content that would harm weldability
3Strength
If silicon content is increased to improve strength, then tensile strength increases, but galvanisability deteriorates
Solution Approach 1:
The patent controls silicon content within the range of 0.70-1.50%, which provides sufficient strength contribution through solid solution strengthening and austenite stabilization while limiting the formation of excessive Si-O-Mn compounds during continuous annealing that would compromise galvanisability. This balanced silicon level achieves the desired strength without severely affecting galvanizing quality
4Strength
If manganese content is increased to improve strength and elongation, then mechanical properties improve, but cold rolling forces increase and edge cracking occurs
Solution Approach 1:
The patent optimizes manganese content to the range of 1.00-3.00%, which provides adequate solid solution strengthening and austenite stabilization to achieve yield strength of at least 600 MPa after temper rolling, while avoiding excessive manganese levels that would cause excessively high cold rolling forces and edge cracking during cold rolling processes
5Strength
If high strength steel grades are used, then tensile strength increases, but elongation and hole expansion ratio deteriorate
Solution Approach 1:
The patent creates a composite microstructure containing martensite (for strength), bainite (for ductility), and retained austenite (for elongation and hole expansion ratio). This multi-phase composite structure achieves tensile strength over 1200 MPa while maintaining elongation of at least 10% and excellent hole expansion ratio, resolving the contradiction between strength and ductility
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 provides steel grades with yield strengths of at least 600 MPa and tensile strengths of at least 1200 MPa, enhanced formability, and excellent weldability and galvanisability, overcoming the limitations of existing AHSS grades by balancing strength and ductility.
Implementation Method 1
complex phase structure comprising one or more of ferrite, carbide free bainite, martensite and/or retained austenite in its microstructure
Implementation Method 2
cold rolled and annealed steel
Implementation Method 3
cold rolled and annealed steel
Data Source
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AI summary
The invention relates to a steel strip or sheet having a complex phase microstructure comprising one or more of ferrite, carbide free bainite, martensite and/or retained austenite in its microstructure comprising: - 0.16 – 0.25 wt.% C; - 1.50 - 4.00 wt.% Mn; - 5 - 50 ppm B; - 5 - 100 ppm N; - 0.001 – 1.10 wt.% Al_tot; - 0.05 - 1.10 wt.% Si; - 0 - 0.04 wt.% Ti; - 0 – 0.10 wt.% Cu; - 0 - 0.10 wt.% Mo; - 0 – 0.10 wt.% Ni; - 0 - 0.20 wt.% V; - 0 – 0.05 wt.% P; - 0 – 0.05 wt.% S; - 0 – 0.10 wt.% Sn; - 0 – 0.025 wt.% Nb - 0 – 0.025 wt% Ca; remainder iron and inevitable impurities.