Hot-Rolled Steel Sheet Composition for Strength and Ductility
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Solution Overview
Problem
Existing methods for producing ultra high strength hot-rolled steel sheets with excellent ductility are costly and often compromise weldability and material uniformity, particularly when using additives like Cr, Ti, and Al.
Innovation Solution
A hot-rolled steel sheet composition of 0.15% to 0.25% C, 0.6% to 2.0% Si, 1.5% to 3.0% Mn, 0.01% to 0.5% Cr, 0.005% to 0.2% Mo, 0.001% to 0.05% P, 0.001% to 0.01% S, 0.001% to 0.01% N, 0.003% to 0.1% Nb, 0.003% to 0.1% Ti, 0.003% to 0.1% V, and 0.0005% to 0.005% B, with specific relational expressions governing element ratios, is used to achieve the desired properties without significantly altering the hot-rolling process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Cr and Al are additionally used to obtain Q&P steel during cooling process after ordinary hot-rolling, then tensile strength of 1 GPa or more is achieved, but weldability deteriorates and material uniformity becomes severe due to abnormal increase in hardenability
Solution Approach 1:
The patent applies parameter changes by precisely controlling the content ranges of Cr (0.01-0.5%) and Al (0.01-0.1%) within specific limits, rather than using large amounts. This controlled parameter adjustment achieves the required tensile strength of 1 GPa or more while preventing abnormal hardenability increase that would harm weldability. The balanced composition parameters resolve the contradiction between strength enhancement and manufacturability.
Solution Approach 2:
The patent creates a composite material system by combining multiple alloying elements (C, Si, Mn, Cr, Al, Mo, P, S, Nb, Ti, V, B) in specific proportions. This composite approach allows the steel to achieve ultra-high strength through synergistic effects of different elements while maintaining weldability through balanced composition design, preventing any single element from causing excessive hardenability.
2Strength
If Cr and Al are additionally used to obtain Q&P steel during cooling process after ordinary hot-rolling, then tensile strength of 1 GPa or more is achieved, but material uniformity becomes severe due to abnormal increase in hardenability
Solution Approach 1:
The patent controls the Cr content at 0.01-0.5% and Al content at 0.01-0.1%, using precise parameter adjustments to achieve uniform material properties. This controlled composition prevents abnormal hardenability increase that would cause severe material uniformity issues, while still achieving the required tensile strength through balanced alloy design.
3Strength
If Ti is additionally used to obtain Q&P steel, then strength is improved, but ductility and bending properties become insufficient due to formation of carbide at high temperature which decreases retained austenite phase fraction
Solution Approach 1:
The patent controls Ti content within 0.003-0.1% and combines it with specific C (0.15-0.25%) and Si (0.6-2.0%) content to prevent excessive carbide formation. This parameter control ensures that retained austenite phase fraction remains sufficient (5-20%) to provide ductility and bending properties, while still achieving ultra-high strength through the composite alloying effect.
Solution Approach 2:
The patent creates a composite material system where Ti works synergistically with C, Si, Mn, Cr, Al, Mo, P, S, Nb, V, and B elements. This composite approach allows Ti to contribute to strength enhancement while the combined effect of all elements prevents excessive carbide formation that would deplete retained austenite, thereby maintaining ductility and bending properties.
4Ease of manufacture
If conventional hot-rolling process is used without additional alloying elements, then manufacturing cost is reduced, but tensile strength of 1 GPa or more cannot be achieved simultaneously with elongation of 10% or more
Solution Approach 1:
The patent uses parameter changes by optimizing the content ranges of essential elements (C: 0.15-0.25%, Si: 0.6-2.0%, Mn: 1.5-3.0%) and adding small amounts of alloying elements (Cr: 0.01-0.5%, Al: 0.01-0.1%, Mo: 0.005-0.2%, Nb: 0.003-0.1%, Ti: 0.003-0.1%, V: 0.003-0.1%, B: 0.0005-0.005%). This controlled parameter adjustment achieves ultra-high strength of 1 GPa or more with elongation of 10% or more through Q&P microstructure formation, while keeping manufacturing costs reasonable compared to conventional methods.
5Ease of manufacture
If conventional hot-rolling process is used without additional alloying elements, then manufacturing cost is reduced, but ductility of 10% or more cannot be achieved simultaneously with tensile strength of 1 GPa or more
Solution Approach 1:
The patent controls the composition parameters to achieve a balanced microstructure with 80-95% martensite and 5-20% retained austenite. The specific content ranges of C (0.15-0.25%), Si (0.6-2.0%), Mn (1.5-3.0%), and alloying elements (Cr, Al, Mo, Nb, Ti, V, B) enable this microstructure to form during cooling, providing both ultra-high strength of 1 GPa or more and excellent ductility of 10% or more, while keeping manufacturing costs reasonable.
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 produces steel sheets with a tensile strength of 1200 MPa or more and elongation of 10% or more, maintaining excellent ductility and weldability while controlling microstructure and phase stability.
Implementation Method 1
a hot-rolled quenching and partitioning (Q&P) steel utilizing a transformation induced plasticity (TRIP) phenomenon caused by the retained austenite phase
Implementation Method 2
quenching the heated steel to a temperature between a martensite transformation start temperature (Ms) and a martensite transformation end temperature (Mf) thereby forming a martensite phase and a retained austenite phase
Implementation Method 3
maintaining the quenched steel at a cooling end temperature or heating the quenched steel to a temperature slightly higher than Ms to perform a heat-treatment thereby stabilizing the retained austenite phase
Data Source
AI summary
Provided is an ultra high strength hot-rolled steel sheet including, in percentage by weight: 0.15-0.25% of C, 0.6-2.0% of Si, 1.5-3.0% of Mn, 0.01-0.1% of Al, 0.01-0.5% of Cr, 0.005-0.2% of Mo, 0.001-0.05% of P, 0.001-0.05% of S, 0.001-0.01% of N, 0.003-0.1% of Nb, 0.003-0.1% of Ti, 0.003-0.1% of V, and the remainder being Fe and other unavoidable impurities, and satisfying following relational expressions (1) and (2). [Relational Expression 1] 4.5≤[Mn]+12[sol.C]+2.5[Mo]+2[Cr]+300[B]+[V]≤5.3. [Relational Expression 2] [sol.C]=[C]−(0.25[Ti]+0.13[Nb]+0.125[Mo]), 0.17≤[sol.C]≤0.22. (In relational expressions 1 and 2, each element symbol represents the content of each element in wt %).