Bainite-Ferrite Hot Rolled Steel for Formability and Weldability
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Solution Overview
Problem
Existing high-strength steel sheets for automotive parts face challenges in achieving a balance between high formability, strength, and hole expansion ratio, while also considering factors like weldability and coatability, which are not adequately addressed by previous inventions such as EP 1138796 and EP 2171112.
Innovation Solution
A hot rolled steel sheet composition with specific elemental ranges (0.11%≤C≤0.16%, 1%≤Mn≤2%, 0.1%≤Si≤0.7%, 0.02%≤Al≤0.1%, 0.15%≤Mo≤0.4%, 0.15%≤V≤0.4%, 0.002%≤P≤0.02%, 0.01%≤N≤0.01%, optional Cr, Nb, Ca, B, Mg, Ti) and a microstructure of 70% to 90% Bainite and 10% to 25% Ferrite, produced through a three-step cooling process to achieve tensile strength ≥940 MPa, total elongation ≥8%, and hole expansion ratio ≥40%, with optional zinc coating for corrosion resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheets is increased, then vehicle crashworthiness and durability are improved, but formability decreases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (Carbon: 0.11-0.16%, Manganese: 1-2%, Silicon: 0.1-0.7%, Mo: 0.15-0.4%, V: 0.15-0.4%) and thermal processing parameters (reheating temperature: 1200-1300°C, rolling temperature: 850-975°C, cooling rates: 40-150°C/s in step 1, 1-10s in step 2) to achieve a microstructure of 70-90% Bainite and 10-25% Ferrite, resulting in tensile strength ≥940 MPa and total elongation ≥8%
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (Bainite and Ferrite) with specific proportions. The Bainite phase provides high strength while the Ferrite phase maintains formability, achieving a synergistic effect that resolves the contradiction between strength and formability
2Loss of substance
If the amount of material is reduced, then fuel consumption is improved, but strength must be increased to compensate
Solution Approach 1:
The patent changes the material parameters by optimizing the chemical composition and microstructure to achieve higher strength (≥940 MPa) with reduced material usage, enabling weight reduction while maintaining structural integrity for improved fuel efficiency
3Strength
If high strength steel is used, then vehicle durability is improved, but weldability and coatability deteriorate
Solution Approach 1:
The patent controls the carbon content (0.11-0.16%) and carbon equivalent through precise composition design and cooling rate control (40-150°C/s in step 1, 1-10s in step 2) to achieve high strength while maintaining weldability and coatability by preventing excessive hardening and cracking
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 provides steel sheets with enhanced strength, formability, and hole expansion ratio, suitable for automotive parts, maintaining good weldability and coatability, while being compatible with conventional industrial processes.
Implementation Method 1
the microstructure of said steel sheet comprising in area fraction, 70% to 90% Bainite, 10% to 25% Ferrite
Implementation Method 2
cooling the hot rolled strip in three step cooling wherein: the step one of cooling the hot rolled steel sheet starts from a temperature range between 850° C. and 975° C. to a temperature range between 650° C. and 725° C., with a cooling rate between 40° C./s and 150° C./s
Data Source
AI summary
A method of production of a hot rolled steel sheet having a composition including the elements, expressed in percentage by weight 0.11%≤Carbon≤0.16%, 1%≤Manganese≤2%, 0.1%≤Silicon≤0.7%, 0.02%≤Aluminum≤0.1%, 0.15%≤Molybdenum≤0.4%, 0.15%≤Vanadium≤0.4%, 0.002%≤Phosphorus≤0.02%, 0%≤Sulfur≤0.005%, 0%≤Nitrogen≤0.01%, and can contain one or more of the following optional elements 0%≤Chromium≤0.5%, 0%≤Niobium≤0.05%, 0.0001%≤Calcium≤0.005%, 0%≤Boron≤0.001%, 0%≤Magnesium≤0.0010%, 0%≤Titanium≤0.01%, with 0.3%≤Mo+V+Nb≤0.6%, the remainder composition being composed of iron and unavoidable impurities, the microstructure of steel sheet including in area fraction, 70% to 90% Bainite, 10% to 25% Ferrite wherein the cumulated amounts of Bainite and Ferrite is at least 90% and a cumulated amount of Residual Austenite and Martensite is between 0% and 10%.