Coated High-Strength Steel Sheet Balancing Formability and Spot Weldability
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Solution Overview
Problem
Current high-strength steel sheets with high ductility and formability often compromise weldability, particularly spot weldability, due to high carbon and silicon content, leading to cracks in welds.
Innovation Solution
A method for producing a coated steel sheet with a specific chemical composition and microstructure, involving cold-rolling, annealing, quenching, and partitioning, followed by hot-dip coating, to achieve a balance of high yield and tensile strength, ductility, and improved weldability, including spot weldability, by controlling the microstructural composition and cooling rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high carbon and silicon content is added to achieve high strength and ductility, then yield strength and tensile strength are improved, but weldability deteriorates due to crack formation in spot welds
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.23%, Si: 0.6-1.3%, Mn: 1.4-2.6%, Al: 0.4-1.0%, Nb: 0.010-0.035%, Mo: 0.1-0.5%) and heat treatment parameters (annealing temperature 860-900°C, quenching temperature Ms-10°C to Ms-60°C, partitioning temperature 410-470°C, partitioning time 60-130 s) to achieve high strength properties while maintaining weldability through optimized microstructure
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite, retained austenite, intercritical ferrite, and lower bainite) with specific area fractions, where each phase contributes different properties: martensite provides strength, retained austenite provides ductility through TRIP effect, and the controlled composition ensures weldability by limiting carbon and silicon content
2Stability of the object's composition
If high carbon and silicon content is added to achieve high ductility and formability, then total elongation and hole expansion ratio are improved, but spot weldability deteriorates due to crack formation
Solution Approach 1:
The patent optimizes the silicon content parameter (0.6-1.3%) and carbon content (0.15-0.23%) to achieve the right balance between ductility/formability and weldability, while using heat treatment parameters (partitioning temperature 410-470°C, partitioning time 60-130 s) to control the microstructure that provides both high total elongation (≥13%) and high hole expansion ratio (≥30%) while maintaining spot weldability
Solution Approach 2:
The patent creates a composite microstructure with specific phase fractions where retained austenite (10-15% area fraction) provides ductility through the TRIP (Transformation Induced Plasticity) effect during deformation, while the controlled carbon and silicon content and presence of other phases ensure good spot weldability by preventing excessive crack formation
3Stability of the object's composition
If annealing temperature is increased to obtain high austenite content, then ductility is improved, but manufacturing complexity increases due to precise temperature control requirements
Solution Approach 1:
The patent optimizes the annealing temperature parameter to a specific range (860-900°C) that ensures formation of at least 90% austenite with controlled intercritical ferrite (2-10% area fraction) in the final microstructure, achieving high ductility (total elongation ≥13%) while maintaining manageable manufacturing complexity through well-defined process parameters
Solution Approach 2:
The patent applies preliminary action by performing batch annealing as a separate preparatory step before final heat treatment, allowing the steel to develop the desired austenite-rich microstructure in advance, which simplifies the subsequent quenching and partitioning steps and reduces the complexity of real-time temperature control during final processing
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 steel sheets with yield strength of at least 800 MPa, tensile strength of at least 1180 MPa, total elongation of at least 13%, and a hole expansion ratio of at least 30%, while maintaining high spot weldability with reduced crack formation.
Implementation Method 1
annealing the cold-rolled steel sheet at an annealing temperature TA comprised between 860° C. and 900° C. so as to obtain an annealed steel sheet having a structure consisting of at least 90% of austenite and at least 2% of intercritical ferrite
Implementation Method 2
quenching the annealed steel sheet from the annealing temperature TA down to a quenching temperature QT comprised between Ms-10° C. and Ms-60° C. at an average cooling rate Vc higher than 30° C./s
Implementation Method 3
heating the quenched sheet from the quenching temperature QT to a partitioning temperature PT comprised between 410° C. and 470° C., and maintaining the sheet at the partitioning temperature PT for a partitioning time Pt comprised between 60 s and 130 s
Implementation Method 4
hot-dip coating the steel sheet in a bath
Data Source
AI summary
A method for producing a high-strength steel sheet having high ductility, formability and weldability includes providing a cold-rolled sheet, with a composition containing: 0.15% ≤C≤0.23%, 1.4% ≤Mn≤2.6%, 0.6% ≤Si≤1.3%, with C+Si/10≤0.30%, 0.4% ≤Al≤1.0%, with Al≥6(C+Mn/10)−2.5%, 0.010% ≤Nb≤0.035%, 0.1% ≤Mo≤0.5%, annealing the sheet at 860° C.-900° C. to obtain a structure consisting of at least 90% austenite and at least 2% intercritical ferrite, quenching to a temperature between Ms-10° C. and Ms-60° C. at a rate Vc higher than 30° C./s, heating to a temperature PT between 410° C. and 470° C. for 60 s to 130 s, hot-dip coating the sheet, and cooling to room temperature. The microstructure includes 45% to 68% of martensite, consisting of 85% to 95% partitioned martensite having a C content of at most 0.45%, and fresh martensite; 10% to 15% retained austenite; 2% to 10% intercritical ferrite; 20% to 30% lower bainite.