Steel Sheet Bake Hardening via Sb Segregation Control
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Solution Overview
Problem
Current high-strength steel sheets for automobile exterior panels face challenges in achieving excellent bake hardening, plating adhesion, and aging resistance while maintaining formability and ductility, often requiring excessive additions of elements that increase manufacturing costs and reduce surface quality.
Innovation Solution
A steel sheet composition with specific ranges of carbon, manganese, phosphorus, sulfur, nitrogen, aluminum, chromium, antimony, silicon, molybdenum, and boron, along with a microstructure of 1 to 5% martensite and remaining ferrite, and a method involving reheating, hot rolling, cold rolling, continuous annealing, and hot-dip galvanizing to control the Sb segregation at grain boundaries, ensuring excellent bake hardening and plating adhesion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel is used to improve weight reduction and dent resistance, then strength and lightweighting are improved, but formability deteriorates during processing
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.005-0.08%, Mn: 1.3-2.3%, Cr: 0.01-1.0%, Mo: 0.01-0.2%, B: 0.0005-0.005%) and processing parameters (cooling rates, annealing temperatures) to achieve a microstructure with 1-5% martensite and 95-99% ferrite, resulting in tensile strength of 340-490 MPa with elongation of 10% or more, thus improving both strength and formability simultaneously
Solution Approach 2:
The patent creates a composite microstructure consisting of two distinct phases: martensite (1-5% area ratio) providing strength and ferrite (95-99% area ratio) providing ductility and formability. This composite structure at the microstructural level allows the steel to exhibit both high strength and excellent formability, resolving the contradiction between these two properties
2Ease of operation
If excessive Cu is added to precipitate fine Cu particles and improve workability, then workability is improved, but manufacturing cost increases and red brittleness occurs
Solution Approach 1:
The patent extracts Cu from the alloy composition entirely, setting Cu content to 0.01% or less. Instead of using Cu precipitation to improve workability, the patent achieves excellent workability (total elongation of 10% or more) through a ferrite-martensite composite microstructure with controlled composition parameters, thereby eliminating red brittleness while maintaining improved workability
Solution Approach 2:
The patent replaces expensive Cu (which causes red brittleness) with cheaper and safer elements like Mn (1.3-2.3%), Cr (0.01-1.0%), Mo (0.01-0.2%), and B (0.0005-0.005%) to achieve the desired microstructure and workability. This substitution eliminates the harmful effects of Cu while maintaining or improving workability at lower cost
3Stability of the object's composition
If a large amount of Si and Al is added to secure retained austenite phase and improve ductility, then ductility is improved, but plating quality deteriorates and manufacturing cost increases
Solution Approach 1:
The patent changes the composition parameters by strictly limiting Si to 0.01-0.3% and Al to 0.01-0.06%, and achieves ductility (total elongation of 10% or more) through a different mechanism: a composite microstructure of 1-5% martensite and 95-99% ferrite formed by controlled cooling rates (10-70°C/sec) and annealing processes, rather than through retained austenite. This approach maintains excellent plating quality while achieving the required ductility
Solution Approach 2:
The patent copies the beneficial effect of retained austenite (improved ductility) through a different microstructural approach. Instead of relying on retained austenite phase which requires high Si and Al additions, the patent creates a ferrite-martensite composite structure that achieves similar or better ductility (total elongation ≥10%) without compromising plating quality or requiring excessive alloying
4Ease of operation
If a large amount of Ti and Mo is added to improve elongation and r value, then workability is improved, but manufacturing cost increases
Solution Approach 1:
The patent optimizes the parameters by limiting Ti to 0.01% or less and Mo to 0.01-0.2%, and achieves improved workability (total elongation of 10% or more, r value of 0.65 or more) through a ferrite-martensite composite microstructure formed by controlled cooling rates (10-70°C/sec) and annealing temperatures (Ac1+20°C to Ac3-20°C), rather than through excessive Ti and Mo additions. This parameter optimization maintains excellent workability while significantly reducing manufacturing cost
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 a steel sheet with improved bake hardening, plating adhesion, and aging resistance, suitable for automotive exterior panels, while maintaining strength and ductility, and reducing manufacturing costs by optimizing the composition and processing conditions.
Implementation Method 1
A relationship (Cgb/Cf) between an Sb average area occupancy ratio (Cgb,%) of grain boundaries of a martensite phase and a ferrite phase and an average Sb area occupancy ratio (Cf,%) in a ferrite phase within 1 μm around the martensite phase
Implementation Method 2
The bake hardening phenomenon is a phenomenon in which the solid solution carbon and nitrogen activated when coating is baked are adhered to the dislocations generated during the pressing to increase the yield strength
Implementation Method 3
continuously annealing the cold rolled steel sheet under a hydrogen concentration of 3 to 30% by volume at a temperature in a range of Ac1+20° C. to Ac3−20° C.
Data Source
AI summary
Provided is a steel sheet used as a material for an automotive exterior panel, etc., and a method for manufacturing the same. More particularly, provided is a cold-rolled steel sheet and a hot-dip galvanized steel sheet, which have excellent bake hardening properties, plating adhesion, and anti-aging properties, and a method of manufacturing the same.