Hot-Rolled Steel Sheet Microstructure Optimization
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Solution Overview
Problem
Existing high-strength hot-rolled steel sheets face challenges in achieving excellent formability, bendability, and low temperature toughness, while maintaining sufficient strength and hole expansibility.
Innovation Solution
A hot-rolled steel sheet with a chemical composition and microstructure optimized to include 90.0% to 98.0% bainite as a primary phase, 2.0% to 10.0% martensite or martensite-austenite mixed phase as a secondary phase, controlled grain sizes, and a specific texture to enhance ductility, bendability, and low temperature toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strengthening of steel sheets is implemented to ensure collision safety, then strength is improved, but formability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.40%, Si: 0.70-1.50%, Mn: 1.50-3.00%, P: 0.050% or less, S: 0.010% or less, Al: 0.01-0.10%, Ti: 0.010-0.060%, B: 0.0005-0.0050%, Cr: 0.10-1.00%, Nb: 0.005-0.060%, V: 0.050-0.50%, Mo: 0.050-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%) and microstructural parameters (bainite area ratio: 85-95%, martensite area ratio: 5-10%, average grain size of secondary phase: 3.0 μm or less, aspect ratio of prior austenite grains: 1.3-5.0, area ratio of recrystallized to unrecrystallized prior austenite grains: 15% or less) to achieve both high strength and good formability
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (bainite as primary phase, martensite or martensite-austenite mixed phase as secondary phase) with specific proportions and characteristics. This composite structure combines the high strength of martensite with the good formability of bainite, resolving the contradiction between strength and formability
2Strength
If high-strengthening is implemented to secure impact characteristics, then strength is improved, but low temperature toughness deteriorates
Solution Approach 1:
The patent controls specific parameters including limiting P content to 0.050% or less and S content to 0.010% or less to prevent embrittlement, while optimizing the microstructure with 85-95% bainite and 5-10% martensite, and controlling the average grain size of the secondary phase to 3.0 μm or less. These parameter changes ensure both high strength and excellent low temperature toughness
3Ease of manufacture
If ductility, hole expansibility, and bendability are improved, then formability is improved, but strength deteriorates
Solution Approach 1:
The patent optimizes parameter combinations including C content (0.15-0.40%) for strength, Si content (0.70-1.50%) for formability, and Mn content (1.50-3.00%) for both strength and ductility. The microstructural parameters are precisely controlled: bainite area ratio (85-95%) for formability, martensite area ratio (5-10%) for strength, and average grain size of secondary phase (3.0 μm or less) for both properties, achieving a balance between formability and strength
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a ¼ position of a sheet thickness in a sheet thickness direction from a surface, a primary phase is bainite, a secondary phase is martensite or a martensite-austenite mixed phase, an average grain size of the secondary phase is 1.5 μm or less, an average grain size of particles having grain diameters that are largest 10% or less out of all particles in the secondary phase is 2.5 μm or less, a pole density in a (110)<112> orientation is 3.0 or less, and, in a microstructure from the surface to a 1/16 position of the sheet thickness in the sheet thickness direction from the surface, a pole density in a (110)<1-11> orientation is 3.0 or less.