High-Strength Steel Sheet Microstructure for Weld Fatigue Resistance
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Solution Overview
Problem
High-strength steel sheets with yield strength of 550 MPa or more, used in automotive components, face challenges in maintaining fatigue strength at welded portions, particularly after shape change, which limits weight reduction and collision strength.
Innovation Solution
A high-strength steel sheet with a specific microstructure composition and processing method, including a component composition of C, Si, Mn, P, S, Al, N, B, Ti, Nb, and optional elements like Mo and Cr, with a microstructure containing 2-30% fresh martensite and retained γ, average grain size of 5 μm or less, and a controlled ratio adjacent to ferrite, combined with a hot-rolling, cold-rolling, and annealing process, and optionally coated with a hot-dip galvanized layer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheet with yield strength of 550 MPa or more is used, then weight reduction and collision strength are improved, but fatigue strength of welded portions decreases after shape change
Solution Approach 1:
The patent applies local quality by creating distinct microstructure regions with different properties: the base material has a controlled mix of ferrite, fresh martensite, and retained γ for high strength, while the heat-affected zone is engineered to contain primarily ferrite and bainite with minimal fresh martensite to ensure toughness and fatigue resistance after welding and shape change
Solution Approach 2:
The patent creates a composite microstructure at the metallurgical level by combining multiple phases (ferrite, fresh martensite, retained γ, bainite) in specific proportions and distributions. This composite structure allows the material to simultaneously achieve high yield strength through the martensite content and good fatigue strength through the ferrite-rich heat-affected zone
2Strength
If alloying elements are increased for reinforcement, then yield strength is improved, but toughness of heat-affected zone deteriorates
Solution Approach 1:
The patent uses local quality by controlling the spatial distribution of alloying effects: bulk alloying elements (C, Si, Mn, Mo, Cr, B) provide high yield strength in the base material, while the heat-affected zone is controlled to have lower fresh martensite content (5% or less) and higher ferrite content (60% or more), ensuring toughness despite the presence of alloying elements
Solution Approach 2:
The patent applies parameter changes by precisely controlling the composition ranges of alloying elements (e.g., C: 0.07-0.25%, Si: 0.01-1.80%, Mn: 1.80-3.20%, Mo: 0.03-0.50%, Cr: 0.10-1.00%, B: 0.0005-0.0050%) and the microstructure phases to achieve the optimal balance between yield strength and heat-affected zone toughness
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 significantly enhances the fatigue strength of resistance spot welded portions, preventing cracking and maintaining yield strength of 550 MPa or more, thereby supporting effective weight reduction and collision strength in automotive applications.
Implementation Method 1
a microstructure containing 2-30% fresh martensite and retained γ, average grain size of 5 μm or less
Implementation Method 2
combined with a hot-rolling, cold-rolling, and annealing process
Data Source
AI summary
A high-strength steel sheet that has a predetermined component composition, that has a steel microstructure in which an area percentage of ferrite ranges from 5% to 50% in a thickness cross-section in a rolling direction, a total area percentage of fresh martensite and retained γ ranges from 2% to 30%, each of the fresh martensite and the retained γ has an average grain size of 5 μm or less, and a ratio of the fresh martensite and the retained γ adjacent only to ferrite with respect to the fresh martensite and the retained γ from a surface to 200 μm in the thickness direction is 30% or less in total area percentage, and that has a yield strength of 550 MPa or more.