Bainitic Steel Composition for Continuous Cooling Without Isothermal Holding
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Solution Overview
Problem
Existing high strength bainitic steel compositions require costly alloying elements like Ni and Mo, and prior manufacturing methods are energy-intensive and not suitable for continuous production, struggling to achieve a balance of high strength and elongation without compromising environmental sustainability.
Innovation Solution
A bainitic steel composition with C: 0.25-0.55 wt%, Si: 0.5-1.8 wt%, Mn: 0.8-3.8 wt%, and other elements, which allows for bainite transformation during cooling without isothermal holding, reducing energy consumption and eliminating the need for costly alloying elements, while maintaining high strength and elongation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength bainitic steel is produced using conventional methods with costly alloying elements like Ni and Mo, then strength is improved, but manufacturing cost increases and environmental sustainability deteriorates
Solution Approach 1:
The patent replaces expensive alloying elements (Ni, Mo) with cheaper alternatives (Si, Mn, Cr, Ti, Cu, V, Nb, Al) to achieve the same high strength properties. The composition uses Si (0.5-1.8 wt%), Mn (0.8-3.8 wt%), Cr (0.2-2.0 wt%), and other microalloying elements to substitute for costly Ni and Mo, thereby reducing manufacturing cost while maintaining UTS ≥1300 MPa
Solution Approach 2:
The patent changes the chemical composition parameters by eliminating Ni and Mo entirely and using alternative elements with optimized content ranges. This parameter change achieves cost reduction while maintaining or improving strength through different strengthening mechanisms (solid solution strengthening, precipitation hardening, grain refinement)
2Stability of the object's composition
If isothermal holding is used to maximize bainitic transformation, then bainitic microstructure is improved, but energy consumption increases and production time extends
Solution Approach 1:
The patent extracts or removes the isothermal holding step from the conventional manufacturing process. By using the specified composition with Si, Mn, Cr, and microalloying elements, the steel achieves complete bainitic transformation during continuous cooling without requiring separate isothermal holding, thereby eliminating the energy-intensive and time-consuming step
Solution Approach 2:
The patent enables continuous cooling transformation instead of interrupted isothermal holding. The bainitic transformation occurs continuously during cooling from austenite region to room temperature, making the process more energy-efficient and suitable for continuous production in hot strip mills
3Stability of the object's composition
If isothermal holding is used to maximize bainitic transformation, then bainitic microstructure is improved, but productivity decreases
Solution Approach 1:
The patent removes the prolonged isothermal holding step from the process, allowing bainitic transformation to occur during continuous cooling. This extraction of the time-consuming step directly increases production rate and enables continuous manufacturing
Solution Approach 2:
The patent skips the intermediate isothermal holding step by using a composition that enables direct transformation during cooling. The steel composition allows bainitic transformation to proceed through the cooling range without pausing at a specific temperature, thereby rushing through the transformation process more quickly
4Strength
If strength is increased in high strength steel grades, then passenger safety is improved, but elongation decreases
Solution Approach 1:
The patent creates a composite microstructure consisting of nano-structured bainite (70-80%) and retained austenite (20-30%). This composite structure combines the high strength of bainite with the ductility contribution from retained austenite, achieving UTS ≥1300 MPa with elongation ≥20%
Solution Approach 2:
The patent creates local quality differences in the microstructure by forming nano-structured bainite plates (50-100 nm thickness) interspersed with C-enriched retained austenite regions. The bainite provides local strength while the retained austenite provides local ductility, achieving high strength-elongation combination
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 achieves a minimum UTS of 1300 MPa and at least 20% elongation with 70-80% nano-structured bainite and 20-30% C-enriched austenite, enabling energy-efficient, continuous production of high-strength, carbide-free bainitic steel with improved ductility and reduced environmental impact.
Implementation Method 1
bainite transformation during cooling without isothermal holding
Implementation Method 2
continuous cooling transformation
Data Source
AI summary
The invention relates to a bainite steel consisting of the following elements in weight %: C: 0.25-0.55 Si: 0.5-1.8 Mn: 0.8-3.8 Cr: 0.2-2.0 Ti: 0.0-0.1 Cu: 0.0-1.2 V: 0.0-0.5 Nb: 0.0-0.06 Al: 0.0-2.75 N: <0.004 P: <0.025 S: <0.025 and a method for manufacturing a bainite steel strip that comprises the step of cooling the coiled strip of such composition to ambient temperature, during which the bainite transformation takes place.