Composite-Phase Thick Steel Cooling for Uniform Fatigue Durability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional high-strength thick hot-rolled steel sheets used in commercial vehicle components suffer from non-uniform microstructures and increased fatigue crack propagation due to excessive alloy components, leading to reduced durability and formability during shear and punching processes.
Innovation Solution
A composite-phase steel with a mixed phase of ferrite and bainite as the base structure, controlled alloy composition (C: 0.05 to 0.15%, Si: 0.01 to 1.0%, Mn: 1.0 to 2.3%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, N: 0.001 to 0.01%, Nb: 0.005 to 0.07%, Ti: 0.005 to 0.11%, Fe, and inevitable impurities) and specific cooling rates (Relational Expressions 1 to 4) to ensure uniform tensile and fatigue strengths across different regions of the coil.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If alloy components (Si, Mn, Al, Mo, Cr) are added to improve strength, then tensile strength increases, but microstructure non-uniformity increases leading to easier crack propagation
Solution Approach 1:
The patent applies parameter changes by precisely controlling the composition ranges of alloying elements (C: 0.05-0.15%, Si: 0.01-1.0%, Mn: 1.0-2.3%, Cr: 0.005-1.0%, etc.) and processing parameters (finishing temperature: 850-950°C, cooling rates: 10-50°C/s) to achieve a balanced microstructure with uniform distribution of ferrite and bainite phases, preventing both excessive strength and microstructure non-uniformity
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite phase (30-70% area ratio) and bainite phase (30-70% area ratio) with controlled precipitate distributions. This composite-phase structure achieves tensile strength of 650 MPa or more while maintaining microstructure uniformity and durability by combining the ductility of ferrite with the strength of bainite
2Strength
If coiling is carried out at high temperature (500-700°C) to form precipitates, then precipitation strengthening is achieved, but coarse carbides form in the center thickness deteriorating shear surface quality
Solution Approach 1:
The patent changes the coiling temperature parameter to a lower range of 200-400°C and adjusts the cooling rate to 10-50°C/s, which prevents coarse carbide formation in the center thickness while still achieving adequate precipitation strengthening through controlled cooling, thereby maintaining both strength and shear surface quality
3Shape
If 40% pressure reduction is applied in non-recrystallization region to refine austenite grains, then grain refinement is achieved, but shape quality of rolled sheet deteriorates and equipment load increases
Solution Approach 1:
The patent changes the processing approach by controlling the finishing temperature to 850-950°C and applying controlled cooling rates of 10-50°C/s, which achieves austenite grain refinement and uniform microstructure formation without requiring excessive pressure reduction, thereby avoiding shape quality deterioration and excessive equipment load
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 high-strength thick composite-phase steel with uniform material and durability, achieving tensile strengths of 650 MPa or more, with improved cross-sectional quality and reduced crack propagation during shear and punching, ensuring excellent formability and durability.
Implementation Method 1
a mixed phase of ferrite and bainite as a base structure
Implementation Method 2
specific cooling rates (Relational Expressions 1 to 4) to ensure uniform tensile and fatigue strengths
Data Source
AI summary
Provided is a manufacturing method of composite-phase steel. The method includes: reheating a steel slab including, by wt %, C:0.05 to 0.15%, Si:0.01 to 1.0%, Mn:1.0 to 2.3%, Al:0.01 to 0.1%, Cr:0.005 to 1.0%, P:0.001 to 0.05%, S:0.001 to 0.01%, N:0.001 to 0.01%, Nb:0.005 to 0.07%, Ti 0.005 to 0.11%, Fe and unavoidable impurities at a temperature of 1200 to 1350° C.; finish hot rolling the reheated steel slab; primarily cooling the hot-rolled steel sheet to a mid-temperature range of 550 to 650° C.; and secondarily cooling a region of the head part and the tail part corresponding to an outer wound portion of a coil during winding to a temperature range from 450 to 550° C., and secondarily cooling a region of the mid part corresponding to an inner wound portion of the coil to the temperature range from 400 to 500° C., and coiling the secondly cooled steel sheet.