High-Strength Steel Sheet Microstructure Control
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Solution Overview
Problem
Existing high-strength steel sheets for automotive structural members lack control over in-plane anisotropy of yield stress, requiring multiple annealing processes and high Ti content, which affects ductility, stretch-flangeability, and mass production efficiency.
Innovation Solution
A high-strength steel sheet with a microstructure comprising 75% tempered martensite, 1-20% fresh martensite, and 5-20% retained austenite, along with controlled grain size ratios and hardness ratios, and optional additional elements like Ti, Nb, and Mo, produced through specific hot rolling and annealing processes to achieve tensile strength of 1180 MPa or more, good ductility, and reduced in-plane anisotropy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If multiple annealing processes are used to control microstructure, then tensile strength and ductility are improved, but manufacturing complexity and production time increase
Solution Approach 1:
The patent applies preliminary action by controlling the microstructure during the hot rolling process itself, rather than requiring subsequent annealing treatments. Specifically, the steel sheet is hot-rolled at controlled temperatures (Ar3 transformation point or higher) to achieve the desired microstructure of 5-20% retained austenite and 70-90% ferrite directly during manufacturing, eliminating the need for multiple post-processing annealing steps while still achieving the required tensile strength of 1180 MPa or more
Solution Approach 2:
The patent employs parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.50-2.50%, Mn: 2.00-3.50%, Al: 0.010-1.000%) and hot rolling process parameters (temperature, reduction ratio) to achieve the target microstructure. This allows the steel to develop the required strength properties through controlled phase transformation during hot rolling, avoiding complex post-manufacturing heat treatment processes
2Strength
If high Ti content is added to improve strength, then tensile strength increases, but ductility and stretch-flangeability deteriorate
Solution Approach 1:
The patent applies parameter changes by strictly limiting the Ti content to 0.001-0.100% and instead achieving high tensile strength (1180 MPa or more) through controlled hot rolling parameters and chemical composition (C: 0.15-0.35%, Si: 0.50-2.50%, Mn: 2.00-3.50%). This composition and process control produces a microstructure with 5-20% retained austenite and 70-90% ferrite that simultaneously delivers high strength and good ductility/elongation of 10% or more, along with acceptable stretch-flangeability
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (retained austenite, ferrite, and controlled amounts of martensite/bainite) rather than relying on a single phase or heavy alloying. This multi-phase microstructure, achieved through controlled hot rolling, provides a synergistic effect where the soft ferrite matrix ensures ductility while the dispersed harder phases contribute to strength, eliminating the need for high Ti additions that would compromise formability
3Strength
If yield stress is increased to improve collision energy absorption, then collision resistance improves, but in-plane anisotropy increases and shape control after forming deteriorates
Solution Approach 1:
The patent applies parameter changes by controlling the yield ratio (YS/TS) to be 0.75 or more through precise chemical composition (particularly C: 0.15-0.35%, Si: 0.50-2.50%, Mn: 2.00-3.50%) and hot rolling parameters. This yields a microstructure with 5-20% retained austenite and 70-90% ferrite that provides high collision energy absorption while maintaining yield stress variability of 150 MPa or less in the plane perpendicular to the rolling direction, ensuring good shape control after press forming
Solution Approach 2:
The patent applies local quality by creating a microstructure where the ferrite phase forms the continuous matrix (70-90% area ratio) providing uniform mechanical properties and good formability, while retained austenite (5-20%) and controlled martensite/bainite provide localized strength enhancement. This spatial distribution of phases with different properties ensures both high collision energy absorption and minimal in-plane anisotropy
4Weight of moving object
If steel sheet thickness is reduced to decrease vehicle weight, then fuel efficiency improves, but strength and shape fixability deteriorate
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition (C: 0.15-0.35%, Si: 0.50-2.50%, Mn: 2.00-3.50%, Al: 0.010-1.000%) and hot rolling parameters (temperature at Ar3 or higher, reduction ratio) to achieve a microstructure containing 5-20% retained austenite and 70-90% ferrite. This produces a steel sheet with tensile strength of 1180 MPa or more and elongation of 10% or more, enabling significant thickness reduction for weight savings while maintaining adequate strength and collision energy absorption characteristics
Solution Approach 2:
The patent creates a composite microstructure with multiple phases (ferrite matrix with dispersed retained austenite and controlled martensite/bainite) that provides high strength-to-weight ratio. The soft ferrite matrix ensures good ductility and formability even at reduced thickness, while the harder retained austenite and martensite/bainite phases provide strength reinforcement, enabling thin-gauge steel sheets to maintain adequate strength and shape fixability
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 a high-strength steel sheet with improved ductility, stretch-flangeability, and controllable yield stress, reducing in-plane anisotropy and enhancing mass production efficiency while maintaining high tensile strength, suitable for lightweight automotive applications.
Implementation Method 1
a steel microstructure containing, by area, 75.0% or more tempered martensite, 1.0% or more and 20.0% or less fresh martensite, and 5.0% or more and 20.0% or less retained austenite
Implementation Method 2
performing annealing, in which letting a temperature defined by formula (1) be temperature T1 (° C.) and letting a temperature defined by formula (2) be temperature T2 (° C.), the annealing includes, in sequence, retaining heat at a heating temperature equal to or higher than temperature T1 for 10 s or more
Implementation Method 3
performing coiling at a coiling temperature of 600° C. or lower, performing cold rolling, and performing annealing, in which letting a temperature defined by formula (1) be temperature T1 (° C.)
Implementation Method 4
the annealing includes, in sequence, retaining heat at a heating temperature equal to or higher than temperature T1 for 10 s or more, performing cooling to a cooling stop temperature
Data Source
AI summary
There is provided a high-strength steel sheet and a method for producing the same. The high-strength steel sheet has a specified chemical composition and a steel microstructure including, by area fraction, 75.0% or more tempered martensite, 1.0% or more and 20.0% or less fresh martensite, and 5.0% or more and 20.0% or less retained austenite. A hardness ratio of the fresh martensite to the tempered martensite is 1.5 or more and 3.0 or less, the ratio of the maximum KAM value in the tempered martensite in the vicinity of the heterophase interface between the tempered martensite and the fresh martensite to the average KAM value in the tempered martensite is 1.5 or more and 30.0 or less, and the average of ratios of grain sizes of prior austenite grains in the rolling direction to those in the thickness direction is 2.0 or less.