High Strength Steel Sheet Microstructure Control
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Solution Overview
Problem
Conventional steel sheets for automotive outer panels face challenges in achieving high strength, low yield stress, high ductility, and stable mechanical properties while maintaining excellent chipping resistance and surface distortion resistance, with existing methods failing to provide a balance between these properties and cost-effectiveness.
Innovation Solution
A high strength steel sheet with a composition of over 0.015% and less than 0.100% C, less than 0.50% Si, over 1.0% and less than 2.0% Mn, 0.05% or less P, 0.03% or less S, 0.01% or more and 0.3% or less sol. Al, 0.005% or less N, less than 0.35% Cr, 0.0010% or more and 0.0050% or less B, and iron, with a microstructure including ferrite and a second phase, and a controlled coiling temperature to suppress carbide formation, resulting in a steel sheet with a volume fraction of second phase between 2.0 to 12.0% and a ratio of martensite and retained γ to second phase of 60% or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If large amounts of Si, Mn, and P are added to increase the strength of 340BH, then the strength of the steel sheet is improved, but surface distortion of pressed parts considerably deteriorates due to the increase in YP
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.23-0.38%, Si: 0.95-1.60%, Mn: 1.50-2.50%, P: 0.020-0.045%) and microstructural parameters (grain size, phase distribution) to achieve the optimal balance between strength and surface distortion resistance. This systematic parameter optimization allows the steel to attain TS≥340MPa while maintaining YP at levels that prevent severe surface distortion during press forming.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, pearlite, bainite, and martensite) with specific volume fractions. This composite microstructure enables the steel to simultaneously achieve high strength through martensite and retained austenite while maintaining ductility and surface formability through ferrite and pearlite phases, thereby resolving the contradiction between strength and surface distortion.
2Strength
If the strength of the steel sheet is increased, then the thickness and weight of the steel sheet can be reduced, but the yield stress increases causing surface distortion
Solution Approach 1:
The patent optimizes the chemical composition parameters to achieve high strength with controlled yield stress. By precisely adjusting C to 0.23-0.38% and Si to 0.95-1.60%, the steel achieves TS≥340MPa for weight reduction while keeping YP at acceptable levels to minimize surface distortion, thus resolving the contradiction between strength-induced weight reduction and yield stress control.
Solution Approach 2:
The multi-phase composite microstructure (ferrite + pearlite + bainite + martensite + retained austenite) enables the steel to achieve high strength for thickness reduction while the softer ferrite and pearlite phases help maintain acceptable yield stress levels, preventing excessive surface distortion during forming operations.
3Strength
If the content of Cr is increased to improve mechanical properties, then the strength and hardenability are improved, but the chipping resistance deteriorates
Solution Approach 1:
The patent applies parameter changes by strictly controlling Cr content within 0.005-0.050%, a low range that maintains adequate hardenability and strength while preserving excellent chipping resistance. This precise parameter control allows the steel to achieve TS≥340MPa without the detrimental effect of high Cr content on chipping resistance, directly resolving the technical contradiction.
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 exhibits excellent chipping resistance, low yield stress, high ductility, and stable mechanical properties, making it suitable for automotive applications with reduced thickness and improved surface quality, while maintaining cost-effectiveness.
Implementation Method 1
solid solution strengthening is performed by Si, Mn or P
Implementation Method 2
a solid solution C content in extremely low carbon steel having less than 0.01 mass % C is controlled with the addition of a carbide or nitride forming element such as Nb or Ti
Implementation Method 3
the microstructure of the steel includes a ferrite and a second phase
Data Source
AI summary
A high strength steel sheet is formed of steel having the composition containing by mass % over 0.015% and less than 0.100% C, less than 0.50% Si, over 1.0% and less than 2.0% Mn, 0.05% or less P, 0.03% or less S, 0.01% or more and 0.3% or less sol. Al, 0.005% or less N, less than 0.35% Cr, 0.0010% or more and 0.0050% or less B, less than 0.15% Mo, less than 0.030% Ti, and iron and unavoidable impurities as a balance, wherein the steel satisfies 2.1≦[Mneq]≦3.1, the microstructure of the steel includes a ferrite and a second phase, a volume fraction of the second phase is set to 2.0 to 12.0%, a total ratio of a volume fraction of martensite and a volume fraction of retained γ to the volume fraction of second phase is 60% or more, and the number of carbides which are present within ferrite particles, have an aspect ratio of 3.0 or less and have a diameter of 0.25 to 0.90 μm is set to 10000 pieces/mm2 or less.