Steel Sheet Composition and Annealing for Bake Hardenability
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Solution Overview
Problem
Traditional 340BH steel sheets face challenges with ductility, formability, and surface quality issues, such as press cracking, line patterns, and ghost bands, which affect their practical application in automotive components.
Innovation Solution
A steel sheet composition with controlled C, Mn, Nb, P, Ti, and B contents, combined with specific rolling and annealing processes, including low-temperature finish rolling and controlled dew point annealing, to produce finer ferrite grains and improve bake hardenability and aging resistance while reducing surface defects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If traditional 340BH steel sheet composition and processing methods are used, then bake hardenability and aging resistance are achieved, but ductility and formability are insufficient causing press cracking
Solution Approach 1:
The patent applies parameter changes by precisely controlling the composition ranges of alloying elements (C: 0.0010-0.0040%, Si: 0.05% or less, Mn: 0.1-1.0%, Nb: 0.005-0.025%, P: 0.030% or less, Ti: 0.009% or less) and processing parameters (heating rate, soaking temperature, cooling rate) to achieve optimal balance between bake hardenability and ductility, preventing press cracking while maintaining strength
Solution Approach 2:
The patent uses composite material principles by combining multiple alloying elements (Mn, Nb, Ti, B, P) in specific proportions to create a synergistic effect where Mn provides solid solution strengthening, Nb and Ti provide precipitation hardening, and their combinations create a complex microstructure that simultaneously improves both bake hardenability and ductility
2Strength
If traditional 340BH steel sheet composition is used, then strength is achieved, but surface quality deteriorates with line patterns and ghost bands
Solution Approach 1:
The patent applies parameter changes by strictly controlling the composition of surface-affecting elements (P: 0.030% or less, Ti: 0.009% or less, Nb: 0.005-0.025%) and processing parameters (heating rate: 0.1×([% Nb]/[% C])° C./s or more, soaking temperature, cooling rate) to prevent the formation of line patterns and ghost bands, achieving both strength and high surface quality
Solution Approach 2:
The patent applies the extraction principle by removing or reducing harmful elements (P, Ti, Nb within specific ranges) that cause surface defects, and by extracting control measures specifically targeting surface quality improvement through compositional optimization and processing parameter control
3Strength
If heating rate is increased to improve bake hardenability, then BH characteristic improves, but uniform elongation and ductility decrease
Solution Approach 1:
The patent applies parameter changes by establishing an optimized heating rate formula (0.1×([% Nb]/[% C])° C./s or more) that balances bake hardenability improvement with uniform elongation maintenance, and by controlling soaking temperature and cooling rate to achieve both high BH characteristic and desirable formability
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 results in a steel sheet with enhanced ductility, formability, and surface quality, maintaining high bake hardenability and aging resistance, suitable for automotive applications, and contributing to improved weight reduction and appearance of automobile bodies.
Implementation Method 1
low-temperature finish rolling and controlled dew point annealing, to produce finer ferrite grains
Data Source
AI summary
A steel sheet has a composition comprising, in mass %, C: 0.0008 to 0.0024%, Si: less than 0.15%, Mn: more than 0.55% and less than 0.90%, P: more than 0.025% and less than 0.050%, S: 0.015% or less, sol. Al: 0.01% or more and 0.1% or less, N: 0.01% or less, B: more than 0.0003% and less than 0.0035%, Nb: more than 0.005% and less than 0.016%, Ti: 0.009% or less, and Sb: 0.002 to 0.030%, in which C and Nb satisfy the following formula (1), and the balance is Fe and unavoidable impurities, and in which a ds/d ratio of an average crystal grain diameter, at a ¼ thickness position of the sheet (d), to that at a steel sheet surface layer (ds), is 0.40 to 1.20, where d is 8 to 18 μm, and where−10≤([% C]−([% Nb]/93)×12)×10,000≤14. Formula (1)