Hot-Rolled Steel Sheet Microstructure for Strength and Hole Expansion
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Solution Overview
Problem
Existing high-strength hot-rolled steel sheets struggle to simultaneously achieve satisfactory toughness, hole expansion properties, and tensile strength of 1,470 MPa or more, due to limitations in texture development and phase composition during the rolling process.
Innovation Solution
A hot-rolled steel sheet with a chemical composition including C: 0.12% to 0.25%, Si: 0.01% to 2.0%, Mn: 0.5% to 3.0%, and B: 0.0005% to 0.005%, along with a metal structure comprising more than 90% tempered martensite, refined carbides, and controlled prior austenite grain size and aspect ratio, is developed. The manufacturing process involves heating, hot-rolling, rapid cooling, coiling, and low-temperature tempering to achieve the desired microstructure and properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If the steel sheet is thinned to reduce weight, then weight reduction is achieved, but strength and toughness are compromised
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by precisely controlling the content ranges of C (0.15-0.35%), Si (0.5-2.0%), Mn (1.0-3.0%), and other elements. This parameter optimization enables the steel to achieve high tensile strength (1,470 MPa or more) while maintaining thin dimensions, thus resolving the contradiction between weight reduction and strength preservation
Solution Approach 2:
The invention creates a composite microstructure consisting of tempered martensite (90% or more by area ratio) with refined carbides dispersed throughout. This composite phase structure provides both high strength and adequate toughness, allowing the steel sheet to be thinned without sacrificing mechanical properties
2Strength
If high strength is achieved through increased carbon content, then tensile strength is improved, but toughness and hole expansion properties deteriorate
Solution Approach 1:
The invention optimizes the carbon content parameter within a specific range (0.15-0.35%) rather than using high carbon content alone. This parameter optimization, combined with controlled Si (0.5-2.0%) and Mn (1.0-3.0%) contents, achieves tensile strength of 1,470 MPa or more while maintaining toughness and hole expansion properties above required thresholds
Solution Approach 2:
The invention creates local quality differences through the formation of tempered martensite as the dominant phase (90% or more by area ratio) with uniformly distributed refined carbides. This microstructural arrangement provides high strength locally while maintaining adequate toughness and ductility at the macro level, enabling simultaneous achievement of high strength and acceptable toughness
3Strength
If rolling is performed in the unrecrystallized region to reduce grain size, then strength is improved, but texture development increases aspect ratio and hole expansion properties deteriorate
Solution Approach 1:
The invention changes the rolling temperature parameter to perform finish rolling in the recrystallized region (above Ac3 transformation temperature) rather than the unrecrystallized region. This temperature parameter adjustment prevents excessive texture development and controls the aspect ratio of prior austenite grains, thereby maintaining hole expansion properties while still achieving high strength through the resulting tempered martensite microstructure
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 hot-rolled steel sheet exhibits improved strength, hole expansion properties, and toughness, with a tensile strength of 1,470 MPa or more, a hole expansion ratio of 60% or more, and a brittle-to-ductile transition temperature of −40° C. or lower, making it suitable for automotive and mechanical structural applications.
Implementation Method 1
heating a steel material with the composition comprising... at a temperature of 1,100 to 1,250° C.
Implementation Method 2
subjecting the heated steel material to rough rolling at a rough rolling outlet temperature RDT of 900 to 1,100° C., and finish rolling
Implementation Method 3
cooling to a cooling stop temperature of 300° C. or lower at an average cooling rate of 100° C./second or more after completion of finish rolling
Implementation Method 4
coiling at a temperature of 300° C. or lower
Data Source
AI summary
To provide a hot-rolled steel sheet comprising by mass %: C: 0.12% or more and 0.25% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.5% or more and 3.0% or less, Al: 0.001% or more and 0.10% or less, and B: 0.0005% or more and 0.0050% or less, and comprising one or both of: Nb: 0.001% or more and 0.020% or less, and Ti: 0.001% or more and 0.20% or less, wherein a metal structure at a position of ¼ thickness from a surface contains more than 90%, in area ratio, of tempered martensite, carbides in the tempered martensite have an average grain size of 10 nm or less, prior austenite grains have an average grain size of less than 40 μm, the prior austenite grains have an aspect ratio of 3.5 or less, and an X-ray random intensity ratio of {112}<110> orientation at a position of ½ thickness from a surface is 4.0 or less, and a method for manufacturing the same.