High-Strength Galvanized Sheet Grain Control for Painted Image Clarity
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Solution Overview
Problem
Existing high-strength steel sheets used for vehicle exteriors face challenges in achieving excellent distinctness of image after painting due to surface waviness, which is influenced by the surface waviness of the substrate and the forming process, and there is a need for improved formability and reduced vehicle weight.
Innovation Solution
A high-strength hot-dip galvanized steel sheet is developed with specific alloy compositions and manufacturing processes, including controlled addition of elements like P, Nb, and Ti, to achieve a microstructure with fine grain size and reduced surface waviness, ensuring excellent distinctness of image after painting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheet is used to reduce vehicle weight, then vehicle weight is reduced, but surface waviness increases leading to poor distinctness of image after painting
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.003-0.005%, Si: 0.05% or less, Mn: 0.4-1.0%, P: 0.04-0.06%, Ti: 0.005-0.03%, Nb: 0.02-0.035%, Mo: 0.05-0.08%, Cu: 0.06-0.1%) and processing parameters (hot-rolling temperature 920-970°C, cooling rate 10-50°C/s, annealing temperature 760-830°C) to achieve a microstructure with ferrite grain size of 15 μm or less and ultra-fine grains occupying 7-10% of the area, thereby reducing surface waviness (ΔWsa ≤ 0.1) while maintaining high strength
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite matrix with controlled grain size distribution (average 15 μm or less with 7-10% ultra-fine grains ≤5 μm) through the synergistic effect of multiple alloying elements (Ti, Nb, Mo, Cu, P) that work together to control grain growth and precipitation, achieving both high strength and excellent surface quality
2Manufacturing precision
If grain size is reduced to improve distinctness of image, then surface waviness is reduced, but manufacturing complexity increases
Solution Approach 1:
The patent applies preliminary action by adding Ti and Nb elements during steelmaking that form fine precipitates during hot-rolling and cooling, which act as grain growth inhibitors from the beginning of the process. The alloying elements are pre-distributed in the molten steel before rolling, ensuring uniform grain refinement throughout the material without requiring complex post-processing
Solution Approach 2:
The patent uses parameter changes by optimizing the hot-rolling finish temperature (920-970°C) and cooling rate (10-50°C/s) to control the phase transformation and grain growth kinetics, achieving the desired grain size distribution through controlled thermal processing rather than mechanical means
3Manufacturing precision
If alloying elements are added to control grain size, then grain size distribution is controlled, but material cost increases
Solution Approach 1:
The patent applies parameter changes by precisely specifying narrow ranges for each alloying element (Ti: 0.005-0.03%, Nb: 0.02-0.035%, Mo: 0.05-0.08%, Cu: 0.06-0.1%, P: 0.04-0.06%) to achieve the desired grain size control with minimal amounts of expensive elements, avoiding excessive additions that would increase cost without providing additional benefit
Solution Approach 2:
The patent creates a composite alloying strategy where multiple elements work synergistically: Ti and Nb form carbonitride precipitates for grain refinement, Mo enhances strength and controls hardenability, Cu improves surface quality, and P contributes to strength, allowing each element to contribute optimally at low concentrations rather than relying on large amounts of a single element
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 exhibits high tensile strength, excellent formability, and reduced surface waviness, enabling its use as an exterior panel in vehicles while potentially reducing vehicle weight.
Implementation Method 1
adding P, Nb, and Ti to ultra-low carbon steel applied to an external plate of a vehicle requiring formability to control grain size distribution
Implementation Method 2
adding P, Nb, and Ti to ultra-low carbon steel applied to an external plate of a vehicle requiring formability to control grain size distribution
Implementation Method 3
the ferrite has a grain average size of 15 μm or less, an ultra-fine grain of 5 μm or less has an occupancy ratio to 7 to 10%
Implementation Method 4
a high-strength hot-dip galvanized steel sheet
Implementation Method 5
performing alloying heat treatment on the hot-dip galvanized steel sheet within a temperature range of 500 to 560° C.
Implementation Method 6
annealing the cold-rolled steel sheet within a temperature range of 760 to 830° C.
Implementation Method 7
annealing the cold-rolled steel sheet within a temperature range of 760 to 830° C. and then performing hot-dip galvanizing
Implementation Method 8
manufacturing a hot-rolled steel sheet by hot-rolling the heated steel slab to a finish-rolling temperature of 920 to 970° C.
Data Source
AI summary
Provided are a high-strength galvanized steel sheet having excellent distinctness of image after painting and a manufacturing method therefor. The steel sheet includes: by mass %, 0.003 to 0.005% of C, 0.05% or less of Si, 0.4 to 1.0% of Mn, 0.04 to 0.06% of P, 0.01% or less of S, 0.005% or less of N, 0.1% or less of S. Al, 0.05 to 0.08% of Mo, 0.005 to 0.03% of Ti, 0.02 to 0.035% of Nb, 0.06 to 0.1% of Cu, 0.0015% or less of B, and a balance of Fe and inevitable impurities. The alloy microstructure has ferrite in an amount of 95% by area fraction or greater, and the ferrite has a grain average size of 15 μm or less, with the ultra-fine grain 5 μm or less having an occupancy ratio to 7 to 10% within 1 mm×1 mm area.