Hot Dip Galvanized Steel Sheet Soft Layer Microstructure
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Solution Overview
Problem
High strength hot dip galvanized steel sheets face challenges in maintaining press formability and bending deformation load due to excessive softening of the surface layer, leading to a potential drop in deformation load during collisions.
Innovation Solution
A method involving controlled heat treatment and cold rolling conditions to form a decarburized surface layer with a moderate increase rate of martensite, stabilizing retained austenite, and tempering martensite to balance strength and ductility, while maintaining a soft layer with specific microstructural composition and thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the surface layer of steel sheet is softened by decarburization to improve bendability, then bendability is improved, but the deformation load during collision may fall due to excessive softening
Solution Approach 1:
The patent applies local quality by creating a soft layer with specific microstructure (tempered martensite with controlled area ratio) at the surface region while maintaining different microstructural characteristics in the interior. This localized microstructural control allows the surface to exhibit improved bendability while the overall sheet maintains sufficient deformation load capacity.
Solution Approach 2:
The patent utilizes parameter changes by controlling the area ratio of tempered martensite in the soft layer and its increase rate in the thickness direction. By adjusting these microstructural parameters through heat treatment processes, the patent achieves optimal balance between bendability and deformation load without excessive softening.
2Strength
If high strength steel sheet is used to maintain deformation load, then strength is improved, but press formability deteriorates
Solution Approach 1:
The patent creates a soft layer at the surface region with tempered martensite microstructure having controlled area ratio (5% or more) and increase rate (5.0%/μm or less). This localized microstructural design provides the dual benefit of maintaining high strength in the bulk material while improving press formability through the softer surface layer that facilitates forming operations.
Solution Approach 2:
The patent effectively creates a composite microstructure within the steel sheet, where the soft layer with tempered martensite combines with the underlying microstructure to form a multi-phase system. This composite approach allows the material to exhibit both high strength and improved press formability by leveraging the different properties of adjacent microstructural regions.
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 method results in hot dip galvanized steel sheets with enhanced press formability, ductility, and bendability, suppressing the drop in load during bending deformation and maintaining mechanical properties suitable for automotive applications.
Implementation Method 1
a soft layer having a thickness of 10 μm or more at the base steel sheet side from the interface, the soft layer contains tempered martensite
Implementation Method 2
a steel microstructure at a range of 1⁄8 thickness to 3⁄8 thickness centered about a position of 1⁄4 thickness from a surface of the base steel sheet
Implementation Method 3
the soft layer contains tempered martensite, and an increase rate in a thickness direction of an area % of tempered martensite from the interface to the inside of the base steel sheet inside the soft layer is 5.0%/μm or less
Data Source
AI summary
Provided are a hot dip galvanized steel sheet comprising a base steel sheet wherein the base steel sheet has a predetermined composition and contains ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, remaining structures consist of bainite, when defining a region having a hardness of 90% or less of the hardness at a position of ¼ thickness to the base steel sheet side from an interface of the base steel sheet and a hot dip galvanized layer as a “soft layer”, there is a soft layer having a thickness of 10 μm or more at the base steel sheet side from the interface, the soft layer contains tempered martensite, and an increase rate in a thickness direction of an area % of tempered martensite from the interface to the inside of the base steel sheet inside the soft layer is 5.0%/μm or less, and a method for producing the same.