Cold-Rolled Steel Sheet Composition for Bendability and Hole Expandability
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Solution Overview
Problem
Existing cold-rolled steel sheets face challenges in achieving high yield ratio, bendability, and hole expandability due to limitations in strength, elongation, and formability, particularly in high-strength applications for vehicle components, with existing methods leading to material defects and reduced workability.
Innovation Solution
A cold-rolled steel sheet composition and manufacturing process involving specific alloying elements (C, Si, Mn, Cr, Ti, Nb, B, P, S, N) and controlled microstructures (martensite and bainite fractions, particle sizes, and cooling rates) to enhance strength, ductility, and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is increased, then yield strength and impact resistance are improved, but elongation is decreased and forming processability is lowered
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.23-0.38%, Si: 0.03-0.35%, Mn: 1.50-3.00%, Cr: 0.50-2.00%, Ti: 0.03-0.10%, Nb: 0.01-0.05%, B: 0.0005-0.0050%) and processing parameters (cooling rates, annealing temperatures) to achieve a balanced microstructure that provides both high strength and good formability
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite, bainite, and ferrite) with specific volume ratios (martensite: 60-80%, bainite: 10-30%, ferrite: 5-15%). This multi-phase composite structure combines the high strength of martensite with the ductility contribution from bainite and ferrite, resolving the contradiction between strength and formability
2Strength
If high-strength steel is used for structural members, then impact resistance is improved, but bendability and hole expandability are reduced
Solution Approach 1:
The patent applies local quality by creating a heterogeneous microstructure where different phases are distributed throughout the material. The martensite provides local high-strength zones for impact resistance, while dispersed bainite and ferrite regions provide local ductility for bendability and hole expandability. This spatial distribution of different properties resolves the contradiction between impact resistance and formability
Solution Approach 2:
The patent controls the volume ratio parameters of different microstructural phases (martensite 60-80%, bainite 10-30%, ferrite 5-15%) and interphase hardness ratio (≤1.4) to optimize the balance between impact resistance and formability. By adjusting these parameters, the material achieves both high strength and good local deformability
3Strength
If yield ratio is increased for energy absorption, then impact energy absorption capacity is improved, but elongation and ductility are decreased
Solution Approach 1:
The patent employs a composite microstructure with multiple phases (martensite, bainite, ferrite) where each phase contributes differently to mechanical properties. The high-strength martensite phase (60-80% volume ratio) provides high yield strength and yield ratio for energy absorption, while the softer bainite (10-30%) and ferrite (5-15%) phases provide elongation capacity. This composite approach resolves the contradiction between yield ratio and elongation
Data Source
AI summary
Provided is a method of manufacturing a cold-rolled steel sheet. The method includes: hot-rolling a steel slab under a finish rolling outlet temperature condition of Ar3 to Ar3+50° C. after reheating the steel slab; coiling the hot-rolled steel sheet at a temperature of 600° C. to 750° C.; by cold-rolling the hot-rolled steel sheet at a reduction rate of 40% to 70%; and overaging the cold-rolled steel sheet after continuous annealing, primary cooling at a cooling rate of 1 to 10° C./sec to 650° C. to 700° C., and secondary cooling at a cooling rate of 5 to 20° C./sec to a temperature of Ms−100° C. to Ms° C. The steel slab includes, by wt %, 0.03 to 0.07 of carbon, 2.0 to 3.0 of manganese, 0.01 to 0.10 of soluble aluminum, 0.3 to 1.2 of chromium, 0.03 to 0.08 of titanium, 0.01 to 0.05 of niobium, 0.0010 to 0.0050 of boron, 0.001 to 0.10 of phosphorous.