High Strength Strip Steel Deep Drawability
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Solution Overview
Problem
Advanced High Strength Steels (AHSS) suffer from low r-value, which results in poor deep drawability, a critical issue for complex automotive stamped parts.
Innovation Solution
A method involving hot-rolling, batch recrystallisation annealing, and continuous intercritical annealing to produce a microstructure comprising ferrite and second phases like martensite and austenite, with specific temperature and cooling rate controls to enhance crystallographic texture and prevent AlN precipitation, thereby improving the r-value.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If AHSS are produced with high strength and good formability through alloying and processing, then strength and ductility are improved, but r-value decreases leading to poor deep drawability
Solution Approach 1:
The patent applies parameter changes by precisely controlling alloying element concentrations (Ti: 0.01-0.05%, B: 0.0005-0.005%, Nb: 0.02-0.06%, V: 0.01-0.05%) and processing parameters (annealing temperature between Ac1 and Ac3, holding time 1-600 seconds, cooling rates) to achieve a microstructure with controlled austenite content (10-40%) and specific precipitate distribution, which simultaneously improves strength and deep drawability
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, martensite, retained austenite, bainite) and precipitates (TiC, TiN, NbC, NbN, VC, VN) within the steel matrix. This composite structure at the microstructural level provides both high strength from the hardened phases and good deep drawability from the controlled austenite content and fine precipitate distribution
2Productivity
If conventional hot rolling and continuous annealing are used, then production efficiency is maintained, but deep drawability remains poor due to low r-value
Solution Approach 1:
The patent segments the continuous annealing process into distinct stages with different temperature ranges and holding times: a first stage at lower temperature (Ac1- Ac3) to form controlled austenite, and a second stage with faster cooling to achieve the desired microstructure. This segmented approach within continuous processing improves deep drawability while maintaining productivity
Solution Approach 2:
The patent implements periodic action through controlled heating and cooling cycles during continuous annealing, with specific holding times (1-600 seconds) at intermediate temperatures to allow precipitate formation and phase transformation, creating a periodic microstructural evolution that enhances deep drawability
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 method achieves a high r-value and maintains the excellent strength-ductility relationship, enhancing deep drawability and cost-effectiveness by eliminating one cold rolling step and optimizing crystallographic texture for improved formability.
Implementation Method 1
reheating the precursor material to a temperature of at least 1100°C to dissolve all, or at least most, AlN-precipitates or in case of a thin slab cast precursor or strip-cast precursor to be rolled immediately after casting, maintaining the temperature at a temperature of at least 1100°C to prevent, or at least substantially reduce, AlN from forming
Implementation Method 2
cooling the hot-rolled strip to a temperature so as to substantially prevent precipitation of AlN in the hot-rolled strip
Implementation Method 3
continuous intercritical annealing of the recrystallised strip by reheating the strip to a temperature between Ac1 and Ac3, holding the strip between Ac1 and Ac3 for at most 600 seconds to obtain a microstructure comprising at most 50% austenite, preferably at most 30%
Implementation Method 4
batch recrystallisation annealing the cold-rolled strip at a temperature of between 550°C and Ac1 to achieve a favourable crystallographic texture for deep-drawing
Implementation Method 5
followed by cooling from the holding temperature to an intermediate temperature of 350-500°C at a cooling rate of between 1 and 100°C/s and followed by a. cooling without delay from the intermediate temperature to a temperature between ambient temperature and 200°C at a cooling rate of between 1 and 100°C/s
Data Source
AI summary
This invention relates to a method for producing a high strength strip steel with a good deep drawability and a high strength steel produced thereby.