Cold-Rolled Steel Composition Balancing Weldability and Formability
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Solution Overview
Problem
High-strength steel sheets with tensile strength of 980 MPa or higher face challenges in maintaining ductility and formability due to the addition of Si and Al, which causes liquid metal embrittlement during welding, restricting the use of plated and cold-rolled steel sheets.
Innovation Solution
A cold-rolled steel sheet composition with specific weight percentages of C, Si, Al, Mn, Cr, Mo, B, Nb, Ti, P, S, and N, along with controlled microstructures and manufacturing processes, including hot-rolling, coiling, cold-rolling, annealing, and reheating, to achieve excellent weldability, strength, and formability while minimizing liquid metal embrittlement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si and Al are added to introduce retained austenite to improve formability, then elongation and formability are improved, but liquid metal embrittlement occurs during spot welding
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.10-0.16%, Si: 0.3-0.8%, Al: 0.01-0.5%, Mn: 2.0-3.0%, Cr: 0.001-0.5%, Mo: 0.001-0.5%, B: 0.0001-0.001%, Nb: 0.001-0.05%, Ti: 0.001-0.05%) to achieve the desired microstructure while preventing liquid metal embrittlement. This quantitative parameter optimization resolves the contradiction between formability enhancement and welding quality maintenance.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, retained austenite, martensite, and bainite) with specific area percentages. This composite microstructure achieves both high formability through retained austenite (1-5% area %) and high strength (980 MPa tensile strength) while maintaining weldability by controlling the overall composition to prevent LME.
2Strength
If high-strength steel sheet with tensile strength of 980 MPa or higher is produced, then strength is improved, but ductility and formability deteriorate
Solution Approach 1:
The patent creates a composite microstructure consisting of multiple phases (ferrite, retained austenite, martensite, and bainite) with specific area percentages. This composite microstructure achieves both high formability through retained austenite (1-5% area %) and high strength (980 MPa tensile strength) while maintaining weldability by controlling the overall composition to prevent LME.
Solution Approach 2:
The patent applies local quality by creating different microstructural phases distributed throughout the material, each contributing specific properties. The martensite phase (25-50% area %) provides high strength, while the retained austenite phase (1-5% area %) provides ductility and formability through TRIP effect. This local differentiation of microstructural properties resolves the contradiction between strength and ductility.
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 cold-rolled steel sheet with enhanced weldability, strength, and formability, achieving a tensile strength of 980 MPa or higher, improved hole expansion ratio, and resistance to liquid metal embrittlement, ensuring excellent mechanical properties and manufacturing feasibility.
Implementation Method 1
a method of utilizing a transformation induced plasticity (TRIP) phenomenon by introducing retained austenite as a method of increasing elongation
Implementation Method 2
heating a steel slab... Hot-rolling the heated slab... coiling the hot-rolled steel sheet... cold-rolling the coiled hot-rolled steel sheet... continuously annealing the cold-rolled steel sheet... primary cooling... secondary cooling... reheating
Data Source
AI summary
Provided is a cold-rolled steel sheet including: by weight %, C: 0.10 to 0.16%, Si: 0.3 to 0.8%, Al: 0.01 to 0.5%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.5%, B: 0.0001 to 0.001%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), a remainder of Fe, and other unavoidable impurities, as a microstructure, by area %, ferrite: 10% or less (excluding 0%), retained austenite: more than 1% to 5% or less, martensite: 25% or more but less than 50%, and bainite: 35% or more but less than 70%. An average size of martensite-austenite (MA) in the bainite is 0.35 to 0.55 μm.