Hot-Rolled Bainitic Steel for Weldable 1100 MPa Structural Sheet
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Solution Overview
Problem
Ultra-high strength steel sheets face limitations in formability and weldability due to high alloy content, long production times, and low elongation, making them unsuitable for various automotive components, while attempts to improve these aspects often result in increased silicon content, which complicates the rolling process and surface quality.
Innovation Solution
A hot rolled advanced high strength bainitic steel with a composition of C: 0.15-0.25, Mn: 0.8-2.1, Si: 0.4-1.1, Cr: 0.8-1.5, Al: 0.05-0.3, Mo: 0.05-0.25, Nb: 0.018-0.035, Ti: 0.01-0.1, S: 0.008 max, P: 0.025 max, N: 0.005 max, produced using a method involving soaking above the martensitic start temperature but below the bainitic start temperature, followed by air cooling to achieve a microstructure of 80-85% bainite, 10-14% martensite, and 5-6% austenite, enhancing tensile strength and elongation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If ultra-high strength steel is developed to increase tensile strength, then strength increases, but elongation decreases significantly
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.25%, Si: 0.40-1.10%, Mn: 0.80-2.10%, Cr: 0.80-1.50%, Mo: 0.05-0.25%, Nb: 0.018-0.035%, Ti: 0.01-0.10%) and processing parameters (soaking temperature above Ms but below Bs, air cooling rate) to achieve a microstructure with 80-85% bainite, 10-14% martensite, and 5-6% austenite, resulting in tensile strength ≥1100 MPa with elongation ≥21%
Solution Approach 2:
The patent creates a composite microstructure combining three phases (bainite, martensite, and austenite) where each phase contributes different properties: bainite provides strength, martensite provides hardness, and retained austenite provides ductility and elongation through TRIP effect, achieving the rare combination of ultra-high strength and high elongation
2Strength
If high alloy content is used to increase strength, then tensile strength increases, but formability and weldability deteriorate
Solution Approach 1:
The patent optimizes alloying parameters by limiting carbon to 0.15-0.25% (lower than conventional UHSS) and using specific ranges for Si, Mn, Cr, Mo, Nb, and Ti, achieving ultra-high strength through controlled microstructure rather than excessive alloying, thereby maintaining formability and weldability
Solution Approach 2:
The patent applies local quality by creating different microstructural phases in specific proportions (80-85% bainite, 10-14% martensite, 5-6% austenite) where each phase is distributed throughout the microstructure to provide localized properties: bainite for strength, martensite for hardness, and austenite for ductility, achieving overall high strength with good formability
3Productivity
If conventional production methods are used to reduce production time, then productivity increases, but elongation and formability decrease
Solution Approach 1:
The patent changes the processing parameters by using a simplified two-step process (soaking above Ms but below Bs temperature followed by air cooling) instead of complex multi-step controlled rolling and cooling, achieving the target microstructure and high elongation (≥21%) while maintaining fast production speed suitable for continuous casting and rolling
Solution Approach 2:
The patent applies self-service by utilizing air cooling instead of controlled cooling systems, allowing the steel to cool naturally in ambient air while achieving the desired microstructure through the combination of controlled soaking temperature and composition, reducing equipment complexity and production time
4Strength
If silicon content is increased to improve strength, then tensile strength increases, but surface quality and rolling process complexity worsen
Solution Approach 1:
The patent optimizes silicon content to a moderate range of 0.40-1.10% rather than high levels, achieving ultra-high strength through the synergistic combination of multiple alloying elements and controlled microstructure, thereby avoiding surface scaling and quality issues associated with high silicon content while maintaining good formability
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 developed steel achieves a tensile strength of at least 1100 MPa and elongation of 21%, with improved weldability and formability, suitable for automotive components, while maintaining commercial viability and producing thicker sheets using conventional hot rolling mill facilities.
Implementation Method 1
produced using a method involving soaking above the martensitic start temperature but below the bainitic start temperature, followed by air cooling to achieve a microstructure of 80-85% bainite, 10-14% martensite, and 5-6% austenite
Data Source
AI summary
Present invention discloses a high strength hot rolled steel product with tensile strength at least 1100 MPa and elongation not less than 21%. The steel further has uniform elongation not less than 10% and yield and tensile ratio 0.6-0.7. The steel further has tensile toughness in the range 19-23.5 GPa %. The developed steel is primarily aimed for automotive structural applications and also for many other such as defence where good combination of strength and ductility required is very high. The developed steel product has following composition C: 0.15-0.23, Mn: 0.8-2.1, Si: 0.3-1.1, Cr: 0.8-1.3, Mo: 0.08-0.25, Nb: 0.018-0.035, Ti—0.01-0.1 S—0.008 max, P—0.025 max, Al—0.05 to 0.3, N—0.005 max. The liquid metal was continuous cast into slab casting. The cast slab was soaked above 1150° C. for few hours and subsequently the cast structure was broken by deformation prior to hot rolling. The slab was then hot rolled into strip with thickness not less than 10 mm with finish rolling temperature in austenite region and subsequently cooled to above Ms (martensite temperature) but below Bs (Below Bainite start temperature) to avoid polygonal ferrite. The steel with above mentioned properties was developed using existing hot rolling.