Cold-Rolled Steel Sheet Multi-Phase Microstructure
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Solution Overview
Problem
Existing high-strength cold rolled steel sheets lack sufficient ductility and workability, particularly in achieving a balance of high tensile strength, total elongation, and yield ratio, which is crucial for automotive applications requiring crashworthiness and fuel efficiency.
Innovation Solution
A high-strength cold rolled steel sheet with a chemical composition of C: 0.15% to 0.35%, Si: 1.0% to 2.0%, Mn: 1.8% to 3.5%, P: 0.020% or less, S: 0.0040% or less, Al: 0.01% to 0.1%, and N: 0.01% or less, featuring a multi-phase structure of ferrite, martensite, and retained austenite, with controlled crystal grain circularity of retained austenite, achieved through specific hot rolling and annealing processes, including coiling temperature control and overaging treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength cold rolled steel sheets are designed with high tensile strength, then strength is improved, but ductility and workability deteriorate
Solution Approach 1:
The patent employs a multi-phase composite microstructure consisting of ferrite, martensite, and retained austenite. This composite structure combines the high strength of martensite with the ductility contribution from retained austenite (via TRIP effect) and the workability of ferrite, thereby achieving both high tensile strength and excellent ductility simultaneously
Solution Approach 2:
The patent precisely controls chemical composition parameters (C: 0.15-0.35%, Si: 1.0-2.0%, Mn: 1.8-3.5%, Al: 0.01-0.1%) and microstructure parameters (area ratios of each phase, crystal grain circularity of retained austenite) to optimize the balance between strength and ductility. By adjusting these parameters, the steel achieves tensile strength ≥900 MPa while maintaining total elongation ≥20%
2Strength
If steel sheets with high yield ratio are used for automotive parts, then crashworthiness is improved, but workability deteriorates
Solution Approach 1:
The multi-phase composite microstructure (ferrite + martensite + retained austenite) enables the steel to achieve a high yield ratio of 60% or more while maintaining excellent workability. The retained austenite undergoes strain-induced transformation (TRIP effect) during deformation, providing continuous energy absorption and delaying necking, thus preserving workability even with high yield ratio
Solution Approach 2:
The patent controls the area ratio of retained austenite (10-30%) and its crystal grain circularity (mode ≤0.6) to optimize the yield ratio while maintaining workability. The specific composition ranges (particularly Si: 1.0-2.0% and Mn: 1.8-3.5%) are designed to stabilize retained austenite and enable the TRIP effect, achieving yield ratio ≥60% with total elongation ≥20%
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 results in a steel sheet with tensile strength of 900 MPa or more, total elongation of 20% or more, and yield ratio of 60% or more, enhancing both strength and workability while securing occupant space in collisions and improving fuel efficiency through reduced automotive body weight.
Implementation Method 1
to utilize the TRIP effect by retained austenite
Implementation Method 2
form the steel structure as a multi-phase structure that has ferrite, martensite, and retained austenite
Implementation Method 3
performing annealing in the ferrite-austenite dual phase region and performing overaging treatment
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
A high-strength cold rolled steel sheet has both high strength, i.e. a tensile strength of 900 MPa or more, and high workability, i.e. a total elongation of 20 % or more, and also has a high yield ratio of 60 % or more. The high-strength cold rolled steel sheet comprises: a predetermined chemical composition; and a steel structure having, in area ratio, ferrite: 30 % to 75 %, martensite: 15 % to 40 %, and retained austenite: 10 % to 30 %. In the case where a distribution of crystal grain circularity of the retained austenite is represented by a histogram with a class range of more than 0.1 × (n - 1) and 0.1 × n or less and a class value of 0.1 × n (n is an integer from 1 to 10), a mode of the crystal grain circularity of the retained austenite is 0.6 or less.