Hot-Rolled Steel Sheet Microstructure for Strength-Ductility Balance
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Solution Overview
Problem
Current methods fail to produce a hot-rolled steel sheet with a tensile strength x elongation value of 20,000 MPa% or more, while maintaining excellent resistance to hydrogen penetration and ultra-high strength after heat treatment, particularly for vehicle components.
Innovation Solution
A hot-rolled steel sheet with specific alloy compositions and microstructures, including 0.40 to 0.60% C, 0.7 to 1.5% Mn, and 0.9 to 1.5% Ni + Cu, with a Mn/Si ratio of 3 or more and Ni/Si ratio of 1 or more, is manufactured through heating, hot-rolling, and quenching-tempering processes to achieve the desired properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of vehicle component material is increased, then the tensile strength increases, but the ductility or elongation tends to decrease
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.25-0.45%, Si: 0.03-0.35%, Mn: 1.50-3.00%, P: 0.030% or less, S: 0.005% or less, Al: 0.005-0.100%, Ti: 0.030% or less, B: 0.0005-0.0050%) and processing parameters (hot rolling temperature, cooling rate, quenching temperature, tempering temperature) to achieve a microstructure containing 7-30% ferrite and 70-93% pearlite, resulting in tensile strength of 600-1000 MPa with elongation of 15-25%, giving a tensile strength x elongation value of 20,000 MPa% or more
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite and pearlite) with specific volume fractions. The dual-phase microstructure combines the ductility contribution from ferrite (7-30%) with the strength contribution from pearlite (70-93%), achieving both high strength and high elongation simultaneously, which resolves the contradiction between strength and ductility
2Ease of manufacture
If conventional hot-rolled steel sheets are used, then the manufacturing process is simple, but the tensile strength x elongation value cannot reach 20,000 MPa% or more with ultra-high strength after heat treatment
Solution Approach 1:
The patent applies preliminary action by establishing the optimal chemical composition and microstructure (7-30% ferrite, 70-93% pearlite) during the hot-rolling process before final heat treatment. The controlled cooling rate (5-50°C/s) and specific finishing temperature range create a pre-conditioned microstructure that responds optimally to subsequent quenching and tempering, enabling ultra-high strength (1800 MPa or more) while maintaining high elongation without requiring complex additional processing steps
3Strength
If the steel sheet is designed for ultra-high strength, then the tensile strength increases, but the resistance to hydrogen penetration from external sources deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition, particularly limiting P to 0.030% or less and S to 0.005% or less, while optimizing C (0.25-0.45%), Si (0.03-0.35%), and Mn (1.50-3.00%). These compositional parameters directly influence both the mechanical strength and the resistance to hydrogen penetration. The resulting microstructure with controlled ferrite and pearlite phases provides both ultra-high strength after heat treatment and excellent resistance to hydrogen penetration from corrosive environments
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 resulting steel sheet exhibits a tensile strength x elongation value of 20,000 MPa% or more, excellent resistance to hydrogen penetration, and ultra-high strength after heat treatment, effectively addressing the limitations of existing technologies.
Implementation Method 1
heating a steel slab to a temperature within a range of 1150 to 1300°C; hot-rolling the heated steel slab by using the hot-rolling operation of a rough rolling and a finish rolling at an Ar3 temperature or higher
Implementation Method 2
quenching-tempering processes
Implementation Method 3
austempering process may be performed by adding a large amount of Si, Al, and Mn to low-carbon steel to form austenite during a continuous annealing operation, maintaining the steel at the constant range of a bainite temperature during a cooling operation
Data Source
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AI summary
A preferable aspect of the present invention provides: an ultra-high-strength hot-rolled steel sheet containing, by weight, one or two of 0.40-0.60% of C, 0.7-1.5% of Mn, 0.3% or less (excluding 0%) of Si, 0.03% or less (including 0%) of P, 0.004% or less (including 0%) of S, 0.04% or less (excluding 0%) of Al, 0.3% or less (excluding 0%) of Cr, 0.3% or less (excluding 0%) of Mo, 0.9-1.5% of Ni, and 0.9-1.5% of Cu, 1.1% or more of Cu+Ni, 0.04% or less (excluding 0%) of Ti, 0.005% or less (excluding 0%) of B, 0.006% or less (excluding 0%) of N, and the balance Fe and other impurities, the alloy elements satisfying relational formulas 1 and 2 below, wherein a microstructure of the hot-rolled steel sheet comprises, by volume, 7% or more of ferrite and 93% or less of perlite; a steel pipe and a member each using the same; and manufacturing methods therefor. Mn/Si≥3weightratio Ni/Si≥1weightratio