Cold Rolled Steel Sheet Multiphase Microstructure for Low Yield Point
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Solution Overview
Problem
Current high-strength cold rolled steel sheets for automotive applications face challenges in achieving low yield point (YP) with minimal variation in material properties, while maintaining excellent corrosion resistance and surface distortion resistance, which is crucial for automotive outer panels to ensure durability and appearance quality.
Innovation Solution
A high-strength cold rolled steel sheet composition with specific alloying elements and microstructure, including ferrite and martensite phases, is developed, where the content of silicon, chromium, and molybdenum is controlled to reduce chromium below 0.30% and phosphorus is utilized to enhance hardenability and corrosion resistance, forming a multiphase structure with retained γ to minimize surface distortion and variation in material properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If larger amounts of manganese and phosphorus are added to conventional 340BH for increased strength, then the strength of pressed products is improved, but the surface distortion resistance deteriorates because YP increases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the composition ranges of multiple alloying elements (carbon: 0.005-0.100%, silicon: 0.010-0.400%, manganese: 1.00-1.90%, phosphorus: 0.015-0.050%, chromium: 0.005-0.300%, molybdenum: 0.005-0.150%) and processing parameters (annealing temperature: 750-850°C, cooling rates) to achieve a balanced microstructure that provides both high strength and low YP. This systematic parameter optimization resolves the contradiction by finding the optimal composition space where strength and surface distortion resistance coexist.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, martensite, and retained γ) with specific volume fractions (ferrite: 85-95%, martensite: 3-12%, retained γ: 1-5%). This multiphase composite structure combines the low YP characteristic of ferrite with the high strength of martensite and the ductility contribution of retained γ, thereby resolving the contradiction between strength and surface distortion resistance.
2Strength
If steel sheets with higher strengths than 340BH are used, then the strength is improved, but variations in material properties such as YP, TS, and El increase, making the steel liable to surface distortion and breakage
Solution Approach 1:
The patent stabilizes material properties by optimizing composition parameters within narrow ranges and controlling processing parameters (annealing temperature: 750-850°C, cooling rates: 3-40°C/sec). This parameter control ensures consistent formation of the desired multiphase microstructure, reducing variations in YP, TS, and El while maintaining high strength.
Solution Approach 2:
The patent promotes microstructure homogeneity by controlling the uniform distribution of alloying elements and achieving a consistent phase composition throughout the steel sheet. The controlled annealing and cooling processes ensure homogeneous transformation to the desired ferrite-martensite-retained γ structure, minimizing local variations in material properties.
3Stability of the object's composition
If chromium content is increased to improve microstructure control, then the microstructure stability is improved, but the corrosion resistance deteriorates
Solution Approach 1:
The patent optimizes chromium content within a specific range (0.005-0.300%) to achieve the right balance between microstructure stability and corrosion resistance. This controlled chromium level, combined with other alloying elements, provides sufficient microstructure control during annealing while maintaining adequate corrosion resistance for automotive applications.
Solution Approach 2:
The patent compensates for reduced chromium content by creating a composite microstructure with multiple phases (ferrite, martensite, retained γ) and utilizing synergistic effects of multiple alloying elements (manganese, phosphorus, molybdenum, silicon). This composite approach maintains microstructure stability through phase interactions rather than relying solely on chromium.
4Weight of moving object
If the thickness of steel sheets is reduced to achieve lightweight car bodies, then the weight is reduced, but the strength and corrosion resistance may be compromised
Solution Approach 1:
The patent enables thin-gauge steel sheets by creating a high-strength composite microstructure consisting of ferrite (85-95%), martensite (3-12%), and retained γ (1-5%). This multiphase composite provides exceptional strength-to-weight ratio, allowing reduced thickness while maintaining required strength levels for automotive safety and structural integrity.
Solution Approach 2:
The patent achieves high strength in thin sheets through optimized composition parameters (ultralow carbon: 0.005-0.100%, controlled manganese: 1.00-1.90%, phosphorus: 0.015-0.050%) and precise thermal processing parameters (annealing: 750-850°C, controlled cooling rates). These parameter optimizations maximize strength development while enabling reduced thickness for weight reduction.
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 conversion treatment properties, corrosion resistance, and reduced surface distortion, making it suitable for automotive outer panels with increased strength and reduced thickness, addressing the need for stable mechanical properties and corrosion resistance.
Implementation Method 1
a microstructure that is a multiphase structure containing, in percent by volume, ferrite and 3% to 12% of a second phase, the multiphase structure containing, as the second phase, 1.0% to 10% of martensite and 1.0% to 5.0% of retained γ
Implementation Method 2
strengthening the steel with manganese and phosphorus by solid solution strengthening
Data Source
AI summary
A method of manufacturing a high strength cold rolled steel sheet includes hot-rolling and cold-rolling a steel slab annealing the steel sheet at an annealing temperature of 750° C. to 830° C.; subjecting the steel sheet to first cooling at an average cooling rate of 3° C./sec to 40° C./sec in a temperature range from the annealing temperature to 480° C.; subjecting the steel sheet to second cooling at an average cooling rate of 8° C./sec to 80° C./sec in a temperature range from 480° C. to Tc (° C.) given by formula (6):Tc=435−40×[% Mn]−30×[% Cr]−30×[% V] (6)wherein [% A] is the content (% by mass) of alloying element A; and subjecting the steel sheet to third cooling at an average cooling rate of 0.3° C./sec to 30° C./sec in a temperature range from Tc (° C.) to 200° C.