Heat-Treated Hot-Rolled Steel Sheet with Mn Gradient for LME Resistance
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Solution Overview
Problem
High strength steel sheets with improved weldability and resistance to liquid metal embrittlement (LME) are needed to reduce vehicle weight while maintaining safety and mechanical properties, as existing steels are brittle and prone to LME-induced cracking during welding.
Innovation Solution
A hot rolled and annealed steel sheet with a specific composition and microstructure, including 0.03-0.18% C, 6.0-11.0% Mn, 0.05-0.5% Mo, 0.0005-0.005% B, and optional elements like Al and Ti, with a microstructure of 10-60% austenite, 40-90% ferrite, and less than 5% martensite, and an inhomogeneous manganese distribution, is developed to enhance toughness and weldability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If manganese content is increased to improve ductility and toughness, then the steel becomes too hard and brittle for further processing, but reducing manganese content compromises mechanical properties
Solution Approach 1:
The patent applies parameter changes by precisely controlling the manganese content within a specific range (6.0-11.0%) and combining it with controlled amounts of boron (0.0005-0.005%) and molybdenum (0.05-0.5%). This optimized composition parameter set achieves the desired balance between toughness and processability, allowing the steel to maintain high strength while remaining workable through standard manufacturing processes.
2Strength
If carbon or silicon content is increased to improve strength, then LME resistance deteriorates, but reducing these elements compromises mechanical properties
Solution Approach 1:
The patent implements parameter changes by strictly limiting carbon content to 0.03-0.18% and silicon content to 1.20% or less, while compensating for strength through optimized manganese (6.0-11.0%) and molybdenum (0.05-0.5%) additions. This revised parameter set maintains adequate strength levels while ensuring LME index remains at 0.36 or less, thereby achieving both mechanical performance and LME resistance.
3Ease of manufacture
If batch annealing is used to soften the hot band for further processing, then the steel becomes easier to process, but toughness is significantly reduced
Solution Approach 1:
The patent applies preliminary action by optimizing the chemical composition parameters before the hot rolling process. By pre-configuring the steel with controlled manganese (6.0-11.0%), boron (0.0005-0.005%), and molybdenum (0.05-0.5%) content, the steel achieves appropriate softness and ductility during hot rolling without requiring subsequent batch annealing. This preliminary compositional optimization eliminates the need for toughness-reducing annealing treatments while maintaining excellent processability.
4Weight of moving object
If high strength steel is used to reduce vehicle weight, then fuel efficiency improves, but weldability and resistance to LME deteriorate
Solution Approach 1:
The patent implements comprehensive parameter changes by optimizing the complete chemical composition: carbon (0.03-0.18%), manganese (6.0-11.0%), boron (0.0005-0.005%), molybdenum (0.05-0.5%), and silicon (≤1.20%). This optimized parameter set enables the production of high strength steel with adequate weldability and LME resistance, allowing vehicle weight reduction while maintaining manufacturing quality and safety requirements.
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 a Charpy impact energy of over 0.4 J/mm² and a low LME index of 0.36 or less, ensuring high toughness and good weldability, while maintaining resistance to LME and brittleness, thus addressing the challenges of weight reduction and safety 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
A hot rolled and heat-treated steel sheet, made of a steel having a composition comprising... said steel sheet having a microstructure comprising...
Data Source
AI summary
A hot rolled and heat-treated steel sheet, made of a steel having a composition including, by weight percent: C: 0.03-0.18%, Mn: 6.0-11.0%, 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: Al<3%, 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 10% to 60% of retained austenite, from 40% to 90% of ferrite, less than 5% of martensite, carbides below 0.8%, and an inhomogeneous repartition of manganese, characterized by a manganese distribution with a slope above or equal to −40.