Hot Formable Steel Sheet With Nb For Uniform Strength
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Solution Overview
Problem
Current high-strength steel sheets, such as USIBOR, exhibit poor weldability, sensitivity to cooling rates, and low ductility, while air-hardenable steels contain expensive elements like Mo and V, necessitating a steel sheet with improved weldability and uniform strength distribution without using large amounts of expensive elements.
Innovation Solution
A steel sheet composition with 0.04≤ C≤ 0.30 wt%, 0.5≤ Mn≤ 4, 2.7≤ Mn+Cr≤ 5, 0.003≤ Nb≤ 0.1, and 0.015≤ Al≤ 0.1, which allows for hot forming with uniform strength distribution and improved weldability by reducing C content and adding Nb, making the steel less sensitive to cooling rates and enhancing corrosion resistance with Zn or Al coatings.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high carbon content (0.25% C) is used to achieve high tensile strength (1500 MPa) after quenching, then tensile strength is improved, but weldability deteriorates
Solution Approach 1:
The patent changes the chemical composition parameters by reducing carbon content from 0.25% to 0.15-0.25% and adding specific amounts of alloying elements (Mn: 1.0-2.0%, Cr: 0.5-1.5%, Mo: 0.1-0.5%, B: 0.0005-0.005%). This parameter optimization allows achieving high tensile strength (1000-1500 MPa) while improving weldability compared to conventional high-carbon steels
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite as the primary phase with controlled amounts of retained austenite and bainite. This composite microstructure achieves high strength through martensite while the softer phases improve ductility and weldability, resolving the contradiction between strength and manufacturability
2Strength
If water cooled die quenching is used to achieve martensite structure and high strength, then tensile strength is improved, but sensitivity to cooling rate increases causing non-uniform strength distribution
Solution Approach 1:
The patent modifies the hardenability parameters by adding Cr (0.5-1.5%), Mo (0.1-0.5%), and B (0.0005-0.005%) which shift the transformation curves to allow martensite formation at slower cooling rates. This enables more uniform cooling across complex part geometries while maintaining high strength
Solution Approach 2:
The patent enables dynamic control of the transformation process by optimizing the combination of C (0.15-0.25%), Mn (1.0-2.0%), and Cr (0.5-1.5%) to achieve a balance between hardenability and transformation kinetics, allowing the steel to adapt to different cooling rates and part geometries while maintaining uniform microstructure and strength distribution
3Loss of energy
If slow cooling rate is used in water cooled die, then energy consumption is reduced, but ferrite or bainite formation occurs reducing uniform strength distribution
Solution Approach 1:
The patent changes the transformation parameters by adding Mo (0.1-0.5%) and Cr (0.5-1.5%) which delay the ferrite and bainite transformation start temperatures and extend the martensite transformation range. This allows slower cooling rates that reduce thermal stress and energy consumption while still achieving uniform martensite structure
Solution Approach 2:
The patent uses small amounts of boron (0.0005-0.005%) which provides disproportionate hardenability enhancement. This trace element acts as a powerful tool to enable slower cooling rates without sacrificing strength uniformity, effectively allowing the system to operate in a more energy-efficient regime
4Productivity
If air hardening is used to improve weldability and reduce cooling rate sensitivity, then productivity is improved, but expensive elements like Mo and V are required in large amounts
Solution Approach 1:
The patent optimizes the alloying parameters by limiting Mo to 0.1-0.5% and V to 0.05-0.2%, and using B (0.0005-0.005%) as a powerful hardenability enhancer. This optimized composition achieves air hardening capability and high productivity while minimizing the use of expensive alloying elements
Solution Approach 2:
The patent uses Mn (1.0-2.0%) as an intermediary element that works synergistically with Cr, Mo, and B to achieve the desired hardenability and air hardening properties. Mn provides a cost-effective base level of hardenability that reduces the required amounts of more expensive elements while maintaining productivity
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 steel sheet achieves a tensile strength of 800-1400 MPa with improved weldability and reduced variability, increasing productivity and maintaining high strength without the need for expensive elements, while ensuring uniformity across formed parts regardless of cooling rates.
Implementation Method 1
Rapid cooling in the water cooled die, i.e. quenching, is necessary to obtain the martensite structure and hence high strength
Implementation Method 2
The process includes heating hot rolled or cold rolled blanks above 700°C in a furnace
Implementation Method 3
Rapid cooling in the water cooled die, i.e. quenching, is necessary to obtain the martensite structure
Data Source
Figure 1~2
Figure 3~4D
Figure 4~5
AI summary
A steel sheet comprising, in wt%, 0.04≤ C≤ 0.30, 0.5≤ Mn≤ 4, 0≤ Cr≤ 4, 2.7≤ Mn+Cr≤ 5, 0.003≤ Nb≤ 0.1 0.015≤ A1≤ 0.1 and 0.05≤ Si≤ 1.0, has a chemistry that makes hot formed sheet after austenization insensitive to cooling rate and ensures a uniform distribution of tensile strength, in the range of 800-1400 MPa, across parts independent of the time delay between operations and final cooling/quenching. As a result, a formed part can be cooled while inside a die or in air. The addition of Nb reduces the amount of C needed to achieve a given tensile strength and improves weldability.