Galvanized Steel Sheet Multiphase Microstructure for Formability
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Solution Overview
Problem
High tensile strength galvanized steel sheets with excellent anti-crush properties and formability are needed for automotive applications, as current materials suffer from poor TS-E1 balance, stretch-flangeability, and anti-crush properties.
Innovation Solution
A galvanized steel sheet with an optimized microstructure containing 20-87% ferrite, 3-10% martensite and residual austenite, and 10-60% tempered martensite, produced through a process involving rapid heating, cooling, and annealing to achieve fine austenite formation and subsequent galvanizing treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of the steel sheet is increased to reduce car body weight, then the crush safety is improved, but the ductility and formability of the steel sheet deteriorate
Solution Approach 1:
The patent creates a composite microstructure consisting of multiple phases (ferrite, martensite, and retained austenite) within the steel sheet. This multiphase structure combines the high strength of martensite with the ductility and formability of ferrite and retained austenite, resolving the contradiction between tensile strength and formability by integrating multiple material phases with complementary properties
Solution Approach 2:
The patent applies local quality by creating a specific microstructure with controlled distribution of different phases throughout the steel sheet. The martensite provides localized strength enhancement while ferrite and retained austenite maintain overall ductility and formability, allowing different regions of the microstructure to contribute different properties to the overall material performance
2Strength
If high tensile strength steel sheets are used to improve crush safety, then the anti-crush properties are enhanced, but the stretch-flangeability required for hole expansion processes deteriorates
Solution Approach 1:
The patent employs a composite microstructure with ferrite, martensite, and retained austenite phases that work synergistically. The ferrite matrix provides ductility and stretch-flangeability, while the dispersed martensite and retained austenite enhance anti-crush properties, thereby resolving the contradiction between these two opposing requirements through multi-phase material design
3Ease of manufacture
If the steel sheet is subjected to conventional heat treatment processes, then the manufacturing simplicity is maintained, but the TS-E1 balance and anti-crush properties remain insufficient
Solution Approach 1:
The patent applies parameter changes by precisely controlling heat treatment parameters including heating temperature (Ac3 transformation point or higher), heating time (10 seconds or more), and cooling rate. These parameter adjustments enable the formation of the desired multiphase microstructure with fine austenite grains, achieving high TS-E1 balance and anti-crush properties while maintaining manufacturing feasibility through a streamlined process
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 high TS-E1 balance, excellent stretch-flangeability, and enhanced anti-crush properties, enabling reduced car weight, improved corrosion resistance, and enhanced crush safety.
Implementation Method 1
the steel sheet is rapidly cooled to the Ms point or lower thereby forming martensite partially or wholly in the steel sheet
Implementation Method 2
heated to a temperature not lower than the Ms point and at least equal to the galvanizing bath temperature and galvannealing furnace temperature thereby forming partially or completely tempered martensite
Implementation Method 3
the steel sheet is heated and maintained at a temperature not lower than the recrystallization temperature and Ac 1 transformation point
Data Source
AI summary
Provided are a high tensile strength galvanized steel sheet with excellent formability and anti-crush properties and a method for making the same. The high tensile strength galvanized steel sheet with excellent formability and anti-crush properties contains, in terms of % by mass, 0.05 to 0.3% of C, 0.01 to 2.5% of Si, 0.5 to 3.5% of Mn, 0.003 to 0.100% of P, 0.02% or less of S, 0.010 to 1.5% of Al, and 0.01 to 0.2% in total of at least one element selected from Ti, Nb, and V, and optionally one or more of 0.005 to 2.00% of Cr, 0.005 to 2.00% of Mo, 0.005 to 2.00% of Ni, 0.005 to 2.00% of Cu, 0.0002 to 0.005% of B, 0.001 to 0.005% of Ca and 0.001 to 0.005% of REM, the remainder being Fe and unavoidable impurities, the steel sheet having a microstructure composed of, in terms of area fraction, 20 to 87% of ferrite, 3 to 10% in total of martensite and residual austenite, and 10 to 60% of tempered martensite, and a second phase composed of the martensite, residual austenite, and tempered martensite having an average crystal grain diameter of 3 µm or less.