High-Strength Steel Sheet Composition for Strength-Workability Balance
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Solution Overview
Problem
Existing steel sheets struggle to achieve a balanced combination of high tensile strength, elongation, hole expansion ratio, and yield ratio evaluation index, as previous techniques fail to satisfy the required ranges of 3.0*106 to 6.2*106 (MPa2%1/2) for tensile strength and elongation, 6.0*106 to 11.5*106 (MPa2%1/2) for tensile strength and hole expansion ratio, and 0.15 to 0.42 for the yield ratio evaluation index.
Innovation Solution
A high strength steel sheet composition comprising specific weight percentages of elements like C, Si, Mn, Al, P, S, N, B, and others, with microstructures of bainite, tempered martensite, fresh martensite, and retained austenite, controlled within certain fractions and boron content ratios, is manufactured through a multi-step heating and cooling process to optimize mechanical properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of the steel sheet is increased, then high strength characteristics are achieved, but the workability of the steel sheet is lowered
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.35%, Si: 0.01-1.50%, Mn: 1.50-3.50%, Al: 0.01-1.50%, B: 0.0005-0.0050%) and microstructural parameters (retained austenite content: 3-15%, tempered martensite: 40-70%, fresh martensite: 10-30%) to achieve optimal balance between strength and workability. The multi-stage heat treatment process parameters (heating rates: 5-50°C/s, holding temperatures: Ac3-920°C, cooling rates: 2-100°C/s) are also controlled to transform the microstructure and achieve the desired property balance.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, bainite, martensite, retained austenite) within the steel sheet. This composite microstructure allows different phases to contribute different properties: martensite provides strength, while retained austenite provides ductility and workability through transformation-induced plasticity. The synergistic combination of these phases resolves the contradiction between strength and workability.
2Ease of operation
If tempered martensite is formed to improve workability, then ductility increases, but the balance of tensile strength and elongation does not satisfy the required range
Solution Approach 1:
The patent merges multiple microstructural phases (tempered martensite for ductility, fresh martensite for strength, and retained austenite for transformation-induced plasticity) into a unified composite structure. This combination allows the steel to simultaneously achieve good ductility from tempered martensite and high strength from fresh martensite, while retained austenite provides additional elongation through TRIP effect during deformation, satisfying the required balance range.
Solution Approach 2:
The patent applies local quality by creating different microstructural regions with distinct properties within the steel sheet. Tempered martensite regions provide local ductility and toughness, while fresh martensite regions provide local strength. The boron segregation to martensite boundaries creates localized hardening effects that enhance overall strength while maintaining ductility in other regions.
3Strength
If boron is added to induce bainite generation, then strength and workability coexist, but the balance of tensile strength and elongation, hole expansion ratio, and yield ratio cannot be secured simultaneously
Solution Approach 1:
The patent optimizes the boron content parameter to a precise range (0.0005-0.0050%) to induce sufficient bainite generation for strength while avoiding excessive boron that would harm ductility. The boron content is controlled to achieve optimal hardenability and martensite boundary segregation without over-hardening the steel, thereby maintaining workability. This precise parameter control allows simultaneous achievement of strength and workability balances.
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 a steel sheet with excellent balances of tensile strength and ductility, tensile strength and hole expansion ratio, and yield ratio evaluation index, meeting the desired ranges of 3.0*106 to 6.2*106 (MPa2%1/2), 6.0*106 to 11.5*106 (MPa2%1/2), and 0.15 to 0.42, respectively.
Implementation Method 1
transformation induced plasticity (TRIP) steel, using transformation-induced plasticity of retained austenite
Implementation Method 2
tempered martensite made by tempering hard martensite is softened martensite
Data Source
AI summary
Provided is a steel sheet that may be used for automobile parts and the like, and to a steel sheet having an excellent balance of strength and ductility, an excellent balance of strength and hole expansion ratio, and an excellent yield ratio evaluation index, and a method for manufacturing the same. The steel sheet includes: by wt %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures.