Hot-Rolled Steel Sheet Texture Control for Toughness Isotropy
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Solution Overview
Problem
Current high-strength steel sheets face a challenge in achieving high strength without compromising toughness, and they exhibit anisotropy in toughness, which affects vehicle fuel efficiency and collision safety.
Innovation Solution
A hot-rolled steel sheet with a specific chemical composition and microstructure is developed, featuring a texture control in the austenite region and martensite, with a tensile strength of 1,180 MPa or more, to reduce anisotropy in toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strengthening is applied to improve vehicle fuel efficiency and collision safety, then strength increases, but toughness deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.40%, Si: 0.10-3.00%, Mn: 0.50-3.00%, P: 0.05% or less, S: 0.01% or less, Al: 0.01-2.00%, Ti: 0.01-0.20%, Nb: 0.01-0.10%, Ca: 0.0001-0.0060%, Mo: 0.01-0.50%, Cr: 0.01-1.00%, V: 0.01-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Sn: 0.001-0.050%) and processing parameters (rolling temperature, cumulative strain, cooling rate) to achieve a microstructure containing 90% or more martensite with average grain size of 12.0 μm or less, thereby obtaining both high strength (1180 MPa or more) and excellent toughness
Solution Approach 2:
The patent creates a composite microstructure by combining martensite (for strength) with controlled amounts of other phases and specific inclusion characteristics (total area ratio of tempered martensite, martensite, and lower bainite is 85% or more; average grain size 12.0 μm or less), achieving a material that simultaneously exhibits high strength and improved toughness properties
2Reliability
If rolling reduction in unrecrystallized austenite is increased to improve toughness, then toughness improves, but aspect ratio of prior austenite grains increases and anisotropy in toughness increases
Solution Approach 1:
The patent applies parameter changes by optimizing the cumulative strain parameter during rolling in the unrecrystallized austenite region and controlling the rolling temperature to maintain austenite in an unrecrystallized state, thereby achieving improved toughness while limiting the aspect ratio of prior austenite grains and reducing anisotropy in the final microstructure
Solution Approach 2:
The patent applies local quality by creating a uniform fine-grained martensitic microstructure throughout the steel sheet with controlled texture, ensuring that the toughness properties are consistent in different directions and reducing anisotropy while maintaining high overall toughness
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition and a microstructure, in which, in a texture of a surface layer region, pole densities of {001}<110>, {111}<110>, and {112}<110> orientation groups are 2.0 or more, in a texture of an internal region, a pole density of a {110}<112> orientation is 5.0 or less, and a tensile strength is 1,180 MPa or more.