Ultra-high strength steel sheet with optimized alloying for phosphatability
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Solution Overview
Problem
Ultra-high strength steel sheets face challenges in achieving both high bendability and phosphatability due to issues with phase fractions, weldability, and surface oxide formation during annealing, which affect their application in vehicle manufacturing.
Innovation Solution
Optimizing the composition of alloying components such as carbon, silicon, manganese, phosphorus, sulfur, aluminum, chromium, boron, titanium, and nitrogen, along with specific manufacturing conditions like hot rolling, cold-rolling, annealing, and rapid cooling, to control phase fractions and surface oxide removal, ensuring excellent phosphatability and bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high amounts of carbon are added to secure the fraction of retained austenite, then elongation is improved, but weldability deteriorates
Solution Approach 1:
The patent changes the chemical composition parameters by limiting carbon to 0.15% or less and adding specific alloying elements (Ti: 0.02-0.06%, Nb: 0.02-0.06%, V: 0.03-0.10%, B: 0.0005-0.0050%) to achieve the desired microstructure without excessive carbon, thereby improving weldability while maintaining elongation through controlled phase transformation
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, martensite, bainite, and retained austenite) with specific volume fractions, where each phase contributes different properties: ferrite provides ductility, martensite provides strength, and controlled retained austenite (5-20%) provides elongation through TRIP effect, while the composite structure achieves both high elongation and good weldability
2Strength
If alloying elements such as silicon or chromium are added in large amounts to guarantee elongation and bendability, then elongation and bendability are improved, but phosphatability deteriorates due to oxide formation
Solution Approach 1:
The patent optimizes the parameters of alloying elements by limiting Si to 0.01-1.50% and Cr to 0.01-2.00%, and introduces Ti, Nb, V, and B as alternative strengthening elements that do not form problematic oxides, thereby maintaining mechanical properties while improving phosphatability by reducing surface oxide formation during annealing
Solution Approach 2:
The patent uses Ti, Nb, V, and B as intermediary elements that provide strengthening and microstructure control without the oxide formation problem of Si and Cr, acting as substitutes that mediate between the need for mechanical properties and the need for good phosphatability
3Strength
If transformation phases are utilized to achieve ultra-high strength, then strength is improved, but bendability becomes difficult to secure
Solution Approach 1:
The patent creates a composite microstructure with multiple phases where soft ferrite (30-70% volume fraction) provides bendability and ductility, while hard martensite and bainite provide ultra-high strength, and controlled retained austenite (5-20% volume fraction) provides both strength through TRIP effect and ductility, achieving the balance between strength and bendability
Solution Approach 2:
The patent applies local quality by having different phases distributed throughout the microstructure, with each phase providing specific local properties: ferrite regions provide ductility and bendability, while martensite and bainite regions provide strength, and the overall composite achieves both ultra-high strength and good bendability
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 an ultra-high strength steel sheet with enhanced phosphatability, suitable for press forming and roll forming, while maintaining ultra-high strength and ductility, overcoming previous limitations in phase control and surface treatment.
Implementation Method 1
TRIP steel is a type of steel having strength and ductility by processing finely and uniformly distributed retained austenite at room temperature to induce martensitic transformation
Implementation Method 2
transformation induced plasticity (TRIP) steel
Implementation Method 3
annealing the cold-rolled steel sheet at 770° C. to 850° C.
Implementation Method 4
rapidly cooling and maintaining the annealed cold-rolled steel sheet at a temperature within a range of Ms (martensitic transformation starting temperature) to Bs (bainitic transformation starting temperature)
Data Source
AI summary
Provided is an ultra-high strength steel sheet having excellent phosphatability and bendability. The ultra-high strength steel sheet includes, by weight percentage (wt %), carbon (C): 0.08% to 0.2%, silicon (Si): 0.05% to 1.3%, manganese (Mn): 2.0% to 3.0%, phosphorus (P): 0.001% to 0.10%, sulfur (S): 0.010% or less, aluminum (Al): 0.01% to 0.1%, chromium (Cr): 0.3% to 1.2%, boron (B): 0.0010% to 0.0030%, titanium (Ti): 0.01% to 0.05%, nitrogen (N): 0.001% to 0.01%, a remainder of iron (Fe) and other inevitable impurities, satisfying: 3.4≤Ti/N≤10, 1.0≤Mn/(Si+Cr), and 0.7≤Mn*/(Si*+Cr*)≤Mn/(Si+Cr), where Ti, N, Mn, Si and Cr refer to a weight percentage (wt %), and Mn*, Si* and Cr* refer to an average of values obtained by GDS component analysis from a surface to a 0.1 μm position in a thickness direction.