High-Strength Steel Sheet Microstructure for Stable Shape Fixability
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Solution Overview
Problem
Existing high-strength steel sheets with tensile strengths of 980 MPa or more face challenges in achieving both high ductility and stability of mechanical properties, leading to variations in springback during press forming, which complicates mass production and reduces efficiency.
Innovation Solution
A high-strength steel sheet with a specific chemical composition and microstructure, including 30% to 70% ferrite, bainitic ferrite, tempered martensite, and retained austenite, with a maximum ferrite grain diameter of 2.5 μm or less, is manufactured through controlled rolling, annealing, and cooling processes to achieve TS of 980 MPa or more and TS×El of 17000 MPa·% or more, with a TS variation of less than 45 MPa.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheet with TS of 980 MPa or more is used to reduce weight, then fuel efficiency is improved, but springback increases and shape fixability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.05-0.20%, Si: 0.01-0.6%, Mn: 1.6-3.5%, P: 0.05% or less, S: 0.01% or less, Al: 1.5% or less, N: 0.01% or less) and heat treatment parameters (annealing temperature, cooling rate) to achieve a balanced microstructure that reduces springback while maintaining high strength
Solution Approach 2:
The patent creates a composite microstructure containing multiple phases (ferrite, bainite, martensite, retained austenite) with specific area ratios and characteristics. The ferrite area ratio is controlled at 30-70%, with bainitic ferrite and martensite forming a low-temperature transformation structure that provides both strength and reduced springback
2Strength
If TS of steel sheet is increased to achieve high strength, then weight reduction is enabled, but TS variation among steel sheets increases and mass production efficiency decreases
Solution Approach 1:
The patent controls TS variation by precisely adjusting chemical composition parameters and heat treatment parameters. The annealing temperature is controlled to produce specific microstructures, and the cooling rate is optimized to ensure consistent transformation characteristics, resulting in TS variation of 40 MPa or less across mass-produced sheets
Solution Approach 2:
The patent achieves homogeneity in mechanical properties by ensuring uniform microstructure distribution throughout the steel sheet. The controlled composition and heat treatment process produce consistent ferrite-bainite-martensite- retained austenite structures across the entire sheet, eliminating TS variation and ensuring uniform springback behavior
3Weight of moving object
If wall thickness is reduced using high-strength steel sheet, then fuel efficiency is improved, but ductility and stability of mechanical properties become difficult to maintain
Solution Approach 1:
The patent employs a composite microstructure with ferrite (30-70% area ratio) providing ductility and toughness, and bainitic ferrite plus martensite forming a low-temperature transformation structure providing strength. This composite structure maintains mechanical property stability even at reduced wall thicknesses
Solution Approach 2:
The patent controls the area ratio parameters of different phases and the maximum ferrite grain diameter (2.5 μm or less) through precise composition and heat treatment control, achieving a balance between ductility (TS×El ≥ 17000 MPa·%) and strength stability
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 results in a high-strength steel sheet with excellent ductility and stability, ensuring consistent shape fixability and improved mass production efficiency by minimizing TS variation.
Implementation Method 1
a steel structure that contains ferrite and a harder phase and in which the ferrite accounts for 30% to 70% in terms of area ratio, and the harder phase contains bainitic ferrite, tempered martensite, martensite, and retained austenite
Implementation Method 2
cold rolling with a maximum value of applied tension of 98 MPa or more
Data Source
AI summary
There is provided a high-strength steel sheet in which content amounts of a specific steel composition are limited and manufacturing conditions are optimized and that has a steel structure that contains ferrite and a harder phase and in which ferrite accounts for 30% to 70% in terms of area ratio, the harder phase contains bainitic ferrite, tempered martensite, martensite (including 0%), and retained austenite (including 0%), the maximum grain diameter of ferrite dmax is 2.5 μm or less, and the area ratio of ferrite AF and the average grain diameter of ferrite dF satisfy AF [%]/dF [βm]≥15.