Hot-Forming Steel Composition for High-Strength Bendability
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Solution Overview
Problem
Current high-strength steel materials for hot forming face challenges in achieving both high strength and excellent bendability, which is crucial for impact resistance and structural integrity, particularly in vehicle components like the B-pillar, where traditional methods like ferrite structure control and tailor welded blanks have limitations in improving bendability and impact resistance.
Innovation Solution
A steel material composition with specific alloying elements (C, Si, Mn, P, S, Al, Cr, N, and optional Mo, Ni, Nb, Ti, B) and a manufacturing process involving cold-rolling, skin pass rolling, and controlled heating and hot forming to achieve a surface roughness factor of 1.8 μm or less, ensuring high strength and improved bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If the thickness of steel material is reduced to improve fuel efficiency, then weight decreases, but strength and stability deteriorate
Solution Approach 1:
The patent applies parameter changes by controlling the chemical composition parameters (C: 0.15-0.45%, Si: 1.5-3.0%, Mn: 1.50-3.50%, P: 0.02-0.05%, S: 0.005-0.020%) and processing parameters (heating temperature, holding time, cooling rate) to achieve both high strength and good workability in thin steel sheets
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite (for strength) and controlled amounts of retained austenite or ferrite (for ductility and workability), effectively combining the benefits of different phases in a single steel material system
2Strength
If high-strength steel materials are used to maintain stability, then strength increases, but workability deteriorates
Solution Approach 1:
The patent applies preliminary action by performing hot forming (heating to Ac3 transformation point or higher, holding, then rapid cooling) before final assembly, which temporarily improves workability during forming while ensuring high strength in the final product after cooling
Solution Approach 2:
The patent changes the temperature parameter during processing (heating to austenite region, then rapid cooling) to temporarily improve workability during forming, followed by transformation to high-strength martensite structure after cooling
3Reliability
If ferrite structure control is optimized to improve impact resistance, then energy absorption capacity increases, but bendability deteriorates
Solution Approach 1:
The patent applies local quality by creating a surface layer with controlled ferrite structure (5-20% area ratio) that provides impact resistance, while the bulk material maintains a microstructure optimized for bendability through controlled alloying and hot forming parameters
4Reliability
If tailor welded blanks are used to supplement energy absorption capacity, then impact resistance improves, but bendability deteriorates due to welded part deterioration
Solution Approach 1:
The patent extracts the welded joint problem from the system by developing a base material with inherently superior comprehensive properties (high strength, good bendability, excellent impact resistance) that does not require welding to achieve energy absorption, thereby eliminating the bendability deterioration caused by welded parts
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 solution enables the production of hot-formed members with enhanced strength, impact resistance, and superior bendability, as demonstrated by increased yield and tensile strengths and improved crack initiation energy absorption capacity.
Implementation Method 1
rapidly cooling the same to a low temperature to form a low-temperature structure such as martensite
Implementation Method 2
heating the blank to a temperature of Ac3 to 980° C., and then maintaining the temperature
Data Source
AI summary
The present invention relates to: a steel material for hot forming, used for a vehicle and the like; a hot-formed member; and a method for manufacturing same. In the method, when the formed steel material is skin pass rolled, rolling pressure and roughness of skin pass rolling rolls are controlled so that bendability can be improved.