Steel Sheet Suppressing End Face Cracking via Solid-Solution C
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Solution Overview
Problem
Conventional dual phase (DP) and transformation induced plasticity (TRIP) steel sheets used in automotive applications often experience end face cracking during punching, leading to decreased collision properties and energy absorption, which affects safety and fatigue performance.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure, including controlled levels of solid-solution C, crystal grain orientations, and reduced retained austenite, which enhances grain boundary strength and suppresses end face cracking, thereby improving yield strength and collision properties after coating and baking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If DP steel sheet or TRIP steel sheet is used to achieve high strength, then yield strength is improved, but end face cracking occurs during punching which decreases collision property
Solution Approach 1:
The invention changes the chemical composition parameters (C: 0.15-0.35%, Si: 0.05-3.0%, Mn: 1.5-3.5%, Al: 1.5% or less) and microstructural parameters (ferrite area fraction: 15% or less, retained austenite area fraction: 15% or less, first martensite area fraction: 20-95%) to achieve both high yield strength and suppressed end face cracking, thereby improving collision property
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (ferrite, first martensite with iron carbides, retained austenite, and bainite or second martensite) with specific area fractions and characteristics, where each phase contributes to both strength and crack suppression, achieving improved yield strength and collision property simultaneously
2Strength
If DP steel sheet or TRIP steel sheet is used to achieve high strength, then yield strength is improved, but energy absorption amount decreases due to end face cracking
Solution Approach 1:
The invention optimizes chemical composition (C, Si, Mn, Al contents) and microstructural parameters (phase area fractions, martensite lath thickness, iron carbide distribution) to suppress end face cracking, thereby maintaining the ability to absorb collision energy while achieving high yield strength
3Strength
If DP steel sheet or TRIP steel sheet is used to achieve high strength, then yield strength is improved, but reaction force characteristic becomes insufficient
Solution Approach 1:
The invention carefully controls the chemical composition parameters and microstructural parameters to achieve a balance where the material exhibits both high yield strength and adequate reaction force characteristic during collision, by optimizing the area fractions of different phases and the distribution of iron carbides
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 proposed steel sheet effectively suppresses end face cracking and achieves excellent yield strength and collision properties by optimizing the chemical composition and microstructure, ensuring better energy absorption and reaction force characteristics.
Implementation Method 1
Solid-solution C contained in the steel sheet segregates to grain boundaries to strengthen the grain boundaries
Implementation Method 2
the content of solid-solution C is 0.44 ppm or more
Data Source
AI summary
A steel sheet includes: a predetermined chemical composition; and a steel structure represented by, in area %, first martensite in which two or more iron carbides each having a circle-equivalent diameter of 2 nm to 500 nm are contained in each lath: 20% to 95%, ferrite: 15% or less, retained austenite: 15% or less, and the balance: bainite, or second martensite in which less than two iron carbides each having a circle-equivalent diameter of 2 nm to 500 nm are contained in each lath, or the both of these, in which the total area fraction of ND//<111> orientation grains and ND//<100> orientation grains is 40% or less, and the content of solid-solution C is 0.44 ppm or more.