Annealed Steel Sheet Microstructure for Strength and Weldability
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Solution Overview
Problem
Current high strength steel sheets face challenges in achieving a combination of high tensile strength, yield strength, ductility, and weldability while maintaining resistance to liquid metal embrittlement and avoiding brittleness, particularly in the automotive industry where weight reduction is crucial for fuel efficiency without compromising safety.
Innovation Solution
A cold-rolled and annealed steel sheet with a specific composition and microstructure, including carbon, manganese, aluminum, molybdenum, and boron, is developed to achieve tensile strength above 1050 MPa, yield strength above 780 MPa, and uniform elongation above 13%, while maintaining good weldability and resistance to liquid metal embrittlement, through a process involving hot rolling, annealing, and intercritical annealing to stabilize austenite and control manganese distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If manganese content is increased to improve ductility and mechanical properties, then tensile strength and elongation are improved, but brittleness increases and weldability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the manganese content within a specific range (3.5-12%) rather than simply increasing it indefinitely. This optimized parameter range achieves the desired balance between mechanical properties and weldability. Additionally, the patent controls the carbon content (0.03-0.18%) and carbon equivalent (Ceq < 0.4%) to further optimize the material properties while maintaining ease of manufacturing.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (austenite, ferrite, and martensite) with specific volume fractions. This composite microstructure, achieved through controlled rolling and annealing processes, provides both high strength and good weldability by combining the advantages of different microstructural components.
2Ease of manufacture
If batch annealing is used to soften the hot band for further processing, then processability is improved, but toughness is lost
Solution Approach 1:
The patent changes the annealing parameters by conducting intercritical annealing at a controlled temperature range (Ac1 to Ac3 transformation temperatures) with specific holding times (10-1800 seconds). This optimized annealing regime achieves the desired softening for processability while preserving toughness through controlled microstructural transformation, avoiding the toughness loss associated with conventional batch annealing.
3Strength
If carbon or silicon content is increased to improve strength, then tensile strength is improved, but resistance to liquid metal embrittlement deteriorates
Solution Approach 1:
The patent applies parameter changes by strictly limiting the carbon content (0.03-0.18%) and silicon content (≤1.20%) within optimized ranges. This controlled composition achieves the required tensile strength (≥1050 MPa) while maintaining low LME index (<0.36) and good resistance to liquid metal embrittlement, resolving the contradiction between strength and reliability.
4Weight of moving object
If high strength steel sheets are used to reduce vehicle weight, then fuel efficiency is improved, but achieving the required combination of strength, ductility, and weldability becomes more difficult
Solution Approach 1:
The patent achieves high strength (tensile strength ≥1050 MPa, yield strength ≥780 MPa) with good ductility (uniform elongation ≥13%, total elongation ≥15%) and weldability by optimizing multiple parameters simultaneously: composition (C, Mn, Al, Mo, B contents) and processing parameters (rolling reduction rate 20-80%, annealing temperature and time). This coordinated parameter optimization simplifies manufacturing by achieving multiple performance targets through a controlled sequence of standard industrial processes.
Solution Approach 2:
The patent creates a composite microstructure with controlled phase distribution (austenite 30-55%, ferrite 45-70%, martensite <5%) that delivers high strength and good formability. This engineered microstructure enables the steel to meet all performance requirements (strength, ductility, weldability, LME resistance) simultaneously, reducing vehicle weight while maintaining manufacturability through conventional processing techniques.
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 steel sheet achieves the desired mechanical properties with improved weldability and resistance to liquid metal embrittlement, as evidenced by a low LME index and high hole expansion ratio, ensuring both safety and fuel efficiency in automotive applications.
Implementation Method 1
The presence of manganese helps to increase ductility of steels thanks to the stabilization of austenite
Implementation Method 2
The steel sheet is annealed at a temperature Tsoak between Ac1 and Ac3 of the cold rolled steel sheet and maintained at said temperature for a holding time tsoak
Implementation Method 3
said steel sheet having a microstructure comprising, in surface fraction, from 30% to 55% of retained austenite, from 45% to 70% of ferrite
Data Source
AI summary
A cold rolled and annealed steel sheet, made of a steel having a composition including, by weight percentC: 0.03-0.18%Mn: 6.0-11.0%Al: 0.2-3%Mo: 0.05-0.5%B: 0.0005-0.005%S≤0.010%P≤0.020%N≤0.008%and including optionally one or more of the following elements, in weight percentage:Si≤1.20%Ti≤0.050%Nb≤0.050%Cr≤0.5%V≤0.2%the remainder of the composition being iron and unavoidable impurities resulting from the smelting, the steel sheet having a microstructure including, in surface fraction,from 30% to 55% of retained austenite,from 45% to 70% of ferrite,less than 5% of fresh martensitea carbon [C]A and manganese [Mn]A content in austenite, expressed in weight percent, satisfying[C]A*[Mn]A/((0.1+C %2)*(Mn %+2))≥1.10and an inhomogeneous repartition of manganese characterized by a manganese distribution with a slope above or equal to −30.