Cold-Rolled Steel Sheet Composition for Heat Resistance and Moldability
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Solution Overview
Problem
Existing cold-rolled steel sheets face challenges in maintaining strength at high temperatures due to inferior heat resistance and reduced moldability, especially when exposed to environments like zinc and aluminum plating baths, and methods to enhance strength often require expensive alloying elements, affecting economic feasibility.
Innovation Solution
A cold-rolled steel sheet composition with controlled amounts of C, Mn, N, Nb, and Ti, along with a microstructure of limited recrystallized grains and dislocation density, combined with a manufacturing process involving heating, hot rolling, cold rolling, and annealing at specific temperatures, to achieve excellent heat resistance and moldability without excessive alloying.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If work hardening is used to improve strength without alloying elements, then economic feasibility is improved, but heat resistance deteriorates due to high dislocation density and recrystallization at high temperatures
Solution Approach 1:
The invention changes the chemical composition parameters by adding specific amounts of Ti (0.01-0.08 wt%) and Nb (0.01-0.05 wt%) alloying elements to the steel sheet. These parameter changes enable precipitation hardening which increases recrystallization temperature, thereby improving heat resistance while maintaining economic feasibility through controlled, minimal alloying rather than extensive alloying
Solution Approach 2:
The invention creates a composite microstructure by forming precipitates of TiC, NbC, and TiN within the steel matrix. These precipitates act as reinforcement phases that increase strength and raise the recrystallization temperature, transforming the simple work-hardened steel into a composite material with enhanced heat resistance
2Strength
If a large amount of alloying elements are added for solid solution strengthening, then strength is improved, but manufacturing cost increases reducing economic feasibility
Solution Approach 1:
The invention optimizes the alloying element parameters by adding Ti and Nb in very small, controlled amounts (0.01-0.08 wt% Ti and 0.01-0.05 wt% Nb). This minimal alloying approach achieves precipitation hardening and strength enhancement without the excessive cost associated with large amounts of alloying elements, thereby maintaining economic feasibility while improving yield strength
3Strength
If precipitation strengthening is used to form precipitates, then strength is improved, but cold rollability deteriorates when precipitates are formed in excess
Solution Approach 1:
The invention precisely controls the composition parameters of Ti (0.01-0.08 wt%) and Nb (0.01-0.05 wt%) to optimize precipitate formation. By maintaining these elements within specific ranges, the invention achieves sufficient precipitation hardening for strength improvement while preventing excessive precipitate formation that would harm cold rollability, thus balancing both requirements
4Strength
If high dislocation density is generated by cold rolling to improve strength, then yield strength is improved, but moldability deteriorates due to reduced ductility
Solution Approach 1:
The invention changes the microstructural parameters by controlling the area fraction of recrystallized grains (5 area% or less) and dislocation density (1×10^15/m² or less) through specific heat treatment parameters. This optimization maintains high strength from work hardening while preserving sufficient ductility for moldability by preventing excessive recrystallization that would reduce dislocation density too much
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 maintains high strength and moldability even at elevated temperatures, ensuring economic feasibility by avoiding excessive alloy additions, with a yield strength of 450 MPa and elongation of 4% or more, suitable for structural applications.
Implementation Method 1
work hardening may be utilized as an economic method because no alloying element is added and the strength may be improved by the generation of high dislocations by simple cold rolling
Implementation Method 2
performing recovery annealing at a temperature less than the recrystallization temperature
Implementation Method 3
strength is again reduced by recrystallization during heat treatment at a temperature equal to or higher than the recrystallization temperature
Implementation Method 4
forming fine precipitates to increase the recrystallization temperature and performing recovery annealing... TiN, NbC, and TiC are precipitated
Implementation Method 5
Precipitation strengthening also requires the addition of expensive alloying elements to form precipitates
Data Source
AI summary
A cold-rolled steel sheet having excellent heat resistance and moldability according to an exemplary embodiment of the present invention includes 0.002 to 0.01 wt % of C, 0.1 to 1.0 wt % of Mn, less than 0.01 wt % (except for 0 wt %) of P, 0.01 wt % or less (except for 0 wt %) of N, 0.01 to 0.05 wt % of Nb, and 0.01 to 0.08% of Ti, with the balance being Fe and inevitable impurities, and has a microstructure in which the area fraction of recrystallized grains is 5 area % or less, and the dislocation density is 1×1015/m2 or less.