Hot-Stamped Steel Grain Refinement for Mn Segregation Control
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Solution Overview
Problem
Existing hot-stamping techniques face challenges in achieving both high strength and toughness in vehicle components due to issues like Mn segregation, coarse carbides, and increased manufacturing costs, particularly in high-strength steel sheets above 2,000 MPa.
Innovation Solution
A hot-stamping process involving a pre-heat treatment to re-dissolve coarse carbides and concentrate Mn at austenite grain boundaries, followed by a thermo-mechanical treatment to diffuse Mn to high-angle grain boundaries, resulting in a microstructure with an average austenite grain size of 5.0 μm or less and reduced Mn concentration at grain boundaries, thereby enhancing toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If spheroidizing annealing is performed at lower than Ac3 point to spheroidize carbides, then carbide spheroidization is achieved, but Mn is not sufficiently diffused and coarse carbides are generated, causing toughness deterioration
Solution Approach 1:
The patent applies parameter changes by performing spheroidizing annealing at a temperature of Ac1 to Ac1+50°C (just below the Ac3 point) rather than at significantly lower temperatures. This specific temperature range enables sufficient Mn diffusion while achieving carbide spheroidization, and the subsequent rapid cooling at 10°C/s to 500°C/s suppresses carbide coarsening, thereby improving toughness while maintaining carbide spheroidization
Solution Approach 2:
The patent employs periodic action through a two-stage thermal process: first performing spheroidizing annealing to spheroidize carbides and diffuse Mn, then rapidly cooling to suppress carbide coarsening and transform the microstructure. This periodic heating and cooling sequence achieves both carbide spheroidization and fine microstructure, preventing toughness deterioration
2Strength
If high strength steel sheet is used to increase load bearing capacity, then strength is improved, but formability into complex shapes deteriorates
Solution Approach 1:
The patent utilizes phase transitions by heating the steel sheet to the austenite region where the steel softens, improving formability for complex shapes. After forming, rapid cooling transforms the austenite into martensite, achieving high strength. This phase transition approach allows the steel to exhibit good formability during processing and high strength in the final product
Solution Approach 2:
The patent applies parameter changes by controlling the heating temperature to the austenite region (above Ac3 point) during forming to soften the steel and improve formability, then rapidly cooling at 10°C/s to 500°C/s to transform the microstructure and achieve high strength. This dynamic parameter control enables both good formability and high strength
3Strength
If high strength steel sheet is used to increase load bearing capacity, then strength is improved, but toughness decreases and cracks are likely to occur
Solution Approach 1:
The patent applies parameter changes by performing spheroidizing annealing at Ac1 to Ac1+50°C to spheroidize carbides and reduce stress concentration points, followed by rapid cooling at 10°C/s to 500°C/s to create a fine microstructure with small prior austenite grain size. These parameter changes simultaneously improve toughness while maintaining high strength, preventing crack occurrence
Solution Approach 2:
The patent applies local quality by creating a uniform distribution of fine carbides throughout the microstructure through spheroidizing annealing and rapid cooling. This uniform local structure eliminates coarse carbide aggregates that would act as crack initiation sites, thereby improving toughness while maintaining high strength
4Strength
If Mn concentration is increased to improve hardenability, then hardenability is improved, but Mn segregation occurs causing toughness deterioration
Solution Approach 1:
The patent employs periodic action by first performing spheroidizing annealing at Ac1 to Ac1+50°C to uniformly diffuse Mn throughout the steel, then rapidly cooling to lock in this uniform distribution. This periodic thermal treatment prevents Mn segregation while maintaining high hardenability, thereby improving toughness
Solution Approach 2:
The patent applies parameter changes by performing spheroidizing annealing at a specific temperature range (Ac1 to Ac1+50°C) that enables sufficient Mn diffusion without excessive segregation, followed by rapid cooling to preserve this uniform distribution. This controlled parameter change achieves both high hardenability and uniform Mn distribution, preventing toughness deterioration
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 process achieves a hot-stamping formed body with excellent strength and toughness by suppressing crack occurrence and propagation, improving the overall mechanical properties of high-strength steel components.
Implementation Method 1
a pre-heat treatment to re-dissolve coarse carbides and concentrate Mn at austenite grain boundaries
Implementation Method 2
followed by a thermo-mechanical treatment to diffuse Mn to high-angle grain boundaries
Implementation Method 3
A hot-stamping process involving a pre-heat treatment... followed by a thermo-mechanical treatment
Data Source
AI summary
A hot-stamping formed body has a predetermined chemical composition, in which an average grain size of prior austenite grains in a microstructure is 5.0 μm or less, and an average Mn concentration at grain boundaries of the prior austenite grains is 1.0 mass % or less. The hot-stamping formed body may be provided with a plating layer on the surface thereof, or may have a softened region in a portion thereof.