High-Strength Steel Sheet Microstructure for Better Workability
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Solution Overview
Problem
Existing high strength steel sheets fail to achieve a balance of superior ductility, bending formability, and hole expansion ratio while maintaining high strength, as previous technologies do not satisfy the tensile strength and elongation requirements of 22,000 MPa % or more.
Innovation Solution
A high strength steel sheet composition comprising specific weight percentages of C, Si, Mn, Al, P, S, N, and other elements, with microstructures of ferrite, tempered martensite, and retained austenite, optimized by nanohardness ratios and volume fractions to enhance workability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of the steel sheet is increased, then the tensile strength is improved, but the workability (ductility, bending formability, hole expansion ratio) deteriorates
Solution Approach 1:
The steel sheet employs a composite microstructure consisting of multiple phases (martensite for strength, retained austenite for ductility through TRIP effect, and bainite for balanced properties) rather than a single phase structure. This composite microstructure allows the material to simultaneously achieve high tensile strength (1300 MPa or higher) and superior workability including ductility, bending formability, and hole expansion ratio by distributing different functional characteristics across different microstructural components.
Solution Approach 2:
The invention optimizes specific compositional parameters (C: 0.15-0.75%, Si: 0.01-4.0%, Mn: 1.50-5.00%, Al: 2.00-6.00%, P: 0.005-0.150%, S: 0.005-0.050%, N: 0.005-0.050%) and microstructural parameters (retained austenite volume fraction: 5-50%, martensite volume fraction: 30-80%, nanohardness ratio [H]F/[H]TM+B+γ: 0.40-0.95) to simultaneously improve strength and workability. By precisely controlling these parameters, the steel achieves both high tensile strength and excellent formability characteristics.
2Ease of operation
If tempered martensite is formed by tempering hard martensite, then the workability is improved through softening, but the tensile strength decreases due to the strength difference between tempered and untempered martensite
Solution Approach 1:
The invention creates local quality differentiation within the microstructure by controlling the nanohardness ratio between ferrite and the combined tempered martensite, bainite, and retained austenite phases. By optimizing the nanohardness ratio [H]F/[H]TM+B+γ to 0.40-0.95, softer ferrite regions provide ductility and workability while harder tempered martensite, bainite, and retained austenite regions maintain high strength, achieving both workability improvement and tensile strength retention (1300 MPa or higher).
Solution Approach 2:
The steel sheet employs a composite microstructure consisting of multiple phases (martensite for strength, retained austenite for ductility through TRIP effect, and bainite for balanced properties) rather than a single phase structure. This composite microstructure allows the material to simultaneously achieve high tensile strength (1300 MPa or higher) and superior workability including ductility, bending formability, and hole expansion ratio by distributing different functional characteristics across different microstructural components.
3Ease of operation
If TRIP steel using transformation-induced plasticity of retained austenite is used, then both high strength and superior workability are obtained, but the balance of tensile strength and elongation does not satisfy 22,000 MPa % or more when bainite is used as main phase
Solution Approach 1:
The invention optimizes specific compositional parameters (C: 0.15-0.75%, Si: 0.01-4.0%, Mn: 1.50-5.00%, Al: 2.00-6.00%, P: 0.005-0.150%, S: 0.005-0.050%, N: 0.005-0.050%) and microstructural parameters (retained austenite volume fraction: 5-50%, martensite volume fraction: 30-80%, nanohardness ratio [H]F/[H]TM+B+γ: 0.40-0.95) to simultaneously improve strength and workability. By precisely controlling these parameters, the steel achieves both high tensile strength and excellent formability characteristics.
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 steel sheet achieves a balance of tensile strength and elongation of 22,000 MPa % or more, a balance of tensile strength and hole expansion ratio of 7*106 (MPa2%1/2) or more, and bendability of 0.5 to 3.0, ensuring superior workability for automotive applications.
Implementation Method 1
transformation induced plasticity (TRIP) steel using transformation-induced plasticity of retained austenite
Implementation Method 2
a method of utilizing tempered martensite... Since the tempered martensite made by tempering hard martensite is softened martensite
Data Source
AI summary
Provided is a method for manufacturing a steel steel sheet having superior workability. The method includes: providing a cold-rolled steel sheet; primarily heating the cold-rolled steel sheet to Ac1-Ac3, and primarily maintaining the primarily heated steel sheet for 50 seconds or more; primarily cooling the primarily heated steel sheet to 600-850° C. at an average cooling rate of 1° C./s or more; secondarily cooling the primarily cooled steel sheet to 300-500° C. at an average cooling rate of 2° C./s or more, and secondarily maintaining the secondarily cooled steel sheet for 5 seconds or more; tertiarily cooling the secondarily cooled steel sheet to 100-300° C. at an average cooling rate of 2° C./s or more; secondarily heating the tertiarily cooled steel sheet to 300-500° C. at an average temperature increase rate of 5° C./s or more, and tertiarily maintaining the secondarily heated steel sheet for 50 seconds or more; and quaternarily cooling to room temperature.