Hot-Stamped Steel Microstructure for Strength and Embrittlement Resistance
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Solution Overview
Problem
Existing high-strength steel sheets used in vehicle manufacturing face challenges in formability and hydrogen embrittlement resistance, particularly when subjected to hot stamping processes.
Innovation Solution
A hot-stamping formed body with a specific chemical composition and microstructure, including 20-30% residual austenite, 70-80% bainite and tempered martensite, and a high angle grain boundary ratio of 30%, enhances strength and hydrogen embrittlement resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheet is used to reduce vehicle body weight, then strength is improved, but formability deteriorates
Solution Approach 1:
The patent applies temperature parameter changes by heating the steel sheet to austenite region temperatures before press forming. This temporary parameter change allows the high-strength steel to become more formable during the forming process, resolving the contradiction between high strength and formability.
Solution Approach 2:
The patent utilizes phase transition of steel from austenite to martensite through controlled cooling after heating. The steel sheet is heated to transform to austenite phase for improved formability, then rapidly cooled in the die to transform to martensite phase for high strength, thus resolving the strength-formability contradiction.
2Strength
If high-strength steel sheet is used to reduce vehicle body weight, then strength is improved, but hydrogen embrittlement resistance deteriorates
Solution Approach 1:
The patent controls the cooling temperature parameter after press forming to achieve a specific microstructure with 10% or more residual austenite. This parameter control of the final temperature and cooling rate creates a microstructure that provides both high strength and improved hydrogen embrittlement resistance.
Solution Approach 2:
The patent creates a composite microstructure containing multiple phases (martensite for strength and residual austenite for hydrogen embrittlement resistance). This composite microstructure combines the advantages of different phases to simultaneously achieve high strength and good hydrogen embrittlement resistance.
3Ease of operation
If residual austenite is increased to improve deformability, then uniform deformability is improved, but strength may be compromised
Solution Approach 1:
The patent precisely controls the cooling temperature and rate parameters after press forming to achieve optimal residual austenite content (10% or more). This parameter optimization ensures that enough residual austenite remains to provide uniform deformability while the majority martensite structure maintains high strength.
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 provides a hot-stamping formed body with improved strength and hydrogen embrittlement resistance, suitable for vehicle components, while maintaining formability.
Implementation Method 1
press forming is performed after a steel sheet is heated up to a high temperature of an austenite range where the steel sheet softens
Implementation Method 2
performing the hardening of the steel sheet in a die at the same time as press working
Data Source
AI summary
A hot-stamping formed body has a predetermined chemical composition and includes microstructure which includes residual austenite of which an area ratio is in a range of 20% to 30%. Among grain boundaries of crystal grains of bainite and tempered martensite in the microstructure, a ratio of a length of a grain boundary having a rotation angle in a range of 55° to 75° to a total length of a grain boundary having a rotation angle in a range of 4° to 12°, a grain boundary having a rotation angle in a range of 49° to 54°, and a grain boundary having a rotation angle in a range of 55° to 75° to the <011> direction as a rotation axis is 30% or more.