Multiphase Steel Strip Heat Treatment for Edge Crack Resistance
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Solution Overview
Problem
Existing multiphase steels face challenges in achieving a high elasticity limit ratio and elongation at fracture simultaneously, leading to issues with edge crack sensitivity and reduced energy absorption capacity, particularly in complex component geometries.
Innovation Solution
A method for producing a steel strip with a multiphase microstructure involving specific alloy compositions and controlled annealing and cooling processes, including multiple annealing stages and varying cooling rates to optimize the Rp0.2 elasticity limit and tensile strength ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional annealing methods are used to produce multiphase steel strips, then the production process is simple, but the elasticity limit ratio is low and elongation at fracture is compromised
Solution Approach 1:
The annealing process is divided into multiple distinct stages: initial annealing at 750-950°C to form austenite, controlled cooling to 200-500°C, further cooling to supercooling temperature below 100°C, and final annealing with specific Hollomon-Jaffe parameter. Each stage produces specific microstructural phases that collectively achieve the target elasticity limit ratio and elongation properties.
Solution Approach 2:
The patent precisely controls thermal processing parameters including temperature ranges, cooling rates, and holding times to achieve specific microstructural transformations. The use of Hollomon-Jaffe parameter (Hp = TH*(ln(τ)+20) > 7.5×10³) as a controlling metric allows optimization of the final annealing process to achieve the desired balance between elasticity limit and elongation at fracture.
2Weight of moving object
If high strength steels are used to reduce vehicle weight, then weight reduction is achieved, but weldability and resistance to liquid metal embrittlement deteriorate
Solution Approach 1:
The patent optimizes chemical composition parameters including carbon content (0.15-0.30 wt.%) and alloying elements to achieve the desired strength while maintaining weldability. The controlled thermal processing parameters (temperature, time, cooling rate) transform the microstructure to reduce susceptibility to liquid metal embrittlement and hydrogen embrittlement, enabling high-strength steel to be welded reliably.
Solution Approach 2:
The patent creates a multiphase microstructure comprising multiple phases (ferrite, bainite, martensite, and/or pearlite) with controlled proportions. This composite microstructure combines the high strength of martensite with the ductility and toughness of ferrite and bainite, while the specific phase distribution and morphology improve weldability and resistance to embrittlement mechanisms.
3Ease of manufacture
If dual-phase steel is used for formability, then cold-formability is improved, but edge crack resistance and energy absorption capacity are reduced
Solution Approach 1:
The patent creates a complex-phase microstructure that extends the dual-phase concept by incorporating additional phases (bainite, martensite, pearlite) in controlled proportions. The ferritic-bainitic matrix provides good formability similar to dual-phase steel, while the distributed martensite and pearlite phases enhance edge crack resistance through stress distribution and the overall energy absorption capacity through the synergistic behavior of multiple phases with different mechanical properties.
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 enhances the elasticity limit ratio and elongation at fracture, improving weldability and energy absorption capacity while maintaining high tensile strength, suitable for complex component geometries.
Implementation Method 1
First annealing, in particular continuous annealing, of the steel strip, in particular of the cold-rolled steel strip, at a temperature between 750° C. to 950° C.
Implementation Method 2
subsequently first cooling of the steel strip to a temperature between 200° C. to 500° C. inclusive with an average cooling rate of 2 K/s to 150 K/s
Implementation Method 3
Further cooling of the steel strip to a supercooling temperature below 100° C.
Implementation Method 4
The multiphase microstructure is characterised by a predominantly ferritic-bainitic basic matrix, wherein proportions of martensite, tempered martensite, residual austenite and/or pearlite can also be present
Data Source
AI summary
A method for producing a steel strip with a multiphase structure by which the production of complex geometries with a high energy-absorption capacity and high resistance to edge cracking is provided achieving a high yield strength or high yield-strength ratio and a high elongation at break, comprising producing a rolled steel strip of particular elements, and first annealing the steel strip at a temperature of between 750° C. and 950° C., and subsequently first cooling of the steel strip to a temperature of between 200° C. and 500° C. at an average cooling rate of 2 K/s to 150 K/s, further cooling of the steel strip to a supercooling temperature below 100° C. at an average cooling rate of 1 K/s to 50 K/s, final annealing of the steel strip with a Hollomon-Jaffe parameter, and final cooling of the steel strip to room temperature at an average cooling rate of 1 K/s to 160 K/s.