Cold-Rolled Steel Sheet Grain Refinement via Annealing
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Solution Overview
Problem
Conventional methods for producing cold-rolled steel sheets with refined structures face challenges in achieving both fine grain diameters and manufacturing stability, often requiring the addition of costly elements like Ti and Nb, and struggle with maintaining stability during annealing due to rapid grain growth.
Innovation Solution
A cold-rolled steel sheet with a specific chemical composition and microstructure, including a main phase of ferrite and a low temperature transformation phase, is produced through rapid heating during annealing to suppress recrystallized grain growth, maintaining a fine structure even with extended holding times, and featuring a texture that enhances workability and strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional methods add large amounts of Ti, Nb, and Mo to refine austenite grains, then ferrite grain diameter is reduced, but manufacturing cost increases and resource consumption increases
Solution Approach 1:
The patent changes the chemical composition parameters by strictly limiting the contents of Ti (0.01-0.06%), Nb (0.003-0.03%), and Mo (0.01-0.3%), and introduces a specific relationship between C and Mn contents (1.5 ≤ %C × 100/%Mn ≤ 6) to achieve grain refinement without excessive alloying. This resolves the contradiction by achieving fine ferrite grains through optimized composition rather than large amounts of expensive elements.
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite as the main phase (50-90% by area) with low temperature transformation phases (martensite, bainite, pearlite, or cementite) as secondary phases (10-50% by area). This composite structure achieves both fine grain diameter and high strength without requiring large amounts of alloying elements.
2Manufacturing precision
If heating to single-phase austenitic region is carried out by rapid heating followed by holding for extremely short time, then grain coarsening is prevented, but manufacturing stability deteriorates due to difficulty in controlling the process
Solution Approach 1:
The patent changes the heating rate parameter to a moderate range (10-100°C/s) rather than extremely rapid heating, and sets the holding time in the austenite region to a practical range (0.1-10 seconds). Combined with the specific chemical composition constraints, this creates a process that is both effective at preventing grain coarsening and stable enough for manufacturing.
Solution Approach 2:
The patent takes preliminary action by strictly controlling the chemical composition (especially C: 0.03-0.20%, Mn: 1.50-3.50%, and the relationship between them) before the heating process. This preliminary compositional control creates conditions that prevent grain coarsening during heating, making the process more stable and easier to control.
3Ease of operation
If cold rolling and annealing are carried out on hot-rolled steel sheet with fine structure, then workability is improved, but grain growth occurs during annealing reducing the refined structure
Solution Approach 1:
The patent optimizes the annealing parameters by setting the heating rate to 10-100°C/s and the holding time in the austenite region to 0.1-10 seconds. Combined with the specific chemical composition, these parameter changes allow the steel to achieve fine ferrite grains (3.5 μm or less) after annealing while maintaining good workability.
Solution Approach 2:
The patent applies dynamic control to the annealing process by using a moderate heating rate (10-100°C/s) rather than static or extremely rapid heating. This dynamic approach allows the microstructure to transform controllably from austenite to fine ferrite, preventing grain coarsening while achieving the desired fine structure for improved workability.
4Reliability
If holding time at annealing temperature is extended to ensure manufacturing stability, then material property stability improves, but ferrite grain diameter increases reducing strength
Solution Approach 1:
The patent changes the holding time parameter to an optimized range (0.1-10 seconds) that is long enough to ensure manufacturing stability and material property consistency, but short enough to prevent excessive grain growth. This is made possible by the specific chemical composition constraints, particularly the controlled C and Mn contents and their relationship.
Solution Approach 2:
The patent takes preliminary action by optimizing the chemical composition (C: 0.03-0.20%, Mn: 1.50-3.50%, and the relationship 1.5 ≤ %C × 100/%Mn ≤ 6) before annealing. This preliminary compositional optimization creates a microstructure that is resistant to grain growth during annealing, allowing longer holding times for stability without significant grain coarsening.
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 process results in a cold-rolled steel sheet with a refined structure comparable to hot-rolled steel, offering excellent workability and high strength without the need for rare metals, while ensuring stable material properties and improved manufacturing stability.
Implementation Method 1
heating to a single-phase austenitic region in the above-described annealing is carried out by rapid heating followed by holding for an extremely short length of time to prevent coarsening of the structure
Implementation Method 2
a cold-rolled steel sheet having a steel structure primarily comprising ferrite with an average grain diameter of at most 3.5 μm
Implementation Method 3
causing transformation from unrecrystallized ferrite to fine austenite, from which fine ferrite is formed at the time of cooling
Data Source
Figure 1~2
Figure 3
AI summary
A cold-rolled steel sheet having a refined structure in which grain growth during annealing is suppressed has a chemical composition containing, in mass percent, C: 0.01 - 0.3%, Si: 0.01 - 2.0%, Mn: 0.5 - 3.5%, Nb: 0 - 0.03%, Ti: 0 - 0.06%, V: 0 - 0.3%, sol. A1: 0 - 2.0%, Cr: 0 - 1.0%, Mo: 0 - 0.3%, B: 0 - 0.003%, Ca: 0 - 0.003%, and REM: 0.003%, and a microstructure which contains at least 50% by area of ferrite as a main phase and has a second phase containing at least 10% by area of a low temperature transformation phase and 0 - 3% by area of retained austenite and which satisfies the following Equations (1) - (3), in addition to a particular texture. dm<2.7+10000/5+300×C+50×Mn+4000×Nb+2000×Ti+400×V2 dm < 4.0 ··· (2), and ds ≤1.5 ... (3), wheren dm is the average grain diameter (µm) of ferrite defined by a high angle grain boundary having a tilt angle of at least 15°, and ds is the average grain diameter (µm) of the second phase.