Hot-Rolled Steel Texture Control for 980 MPa Bendability
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Solution Overview
Problem
Current technologies face challenges in achieving a hot-rolled steel sheet with a tensile strength of 980 MPa or more, while maintaining excellent bending workability, stretch flangeability, and high proof stress simultaneously.
Innovation Solution
A hot-rolled steel sheet with a chemical composition of C: 0.02% to 0.12%, Si: 0.01% to 2.00%, Mn: 1.00% to 3.00%, and specific amounts of Ti, Nb, V, and other elements, featuring a microstructure with 80% or more of tempered martensite and 5×10^9 pieces/mm^3 or more of precipitates containing Ti with an equivalent circle diameter of 5 nm or less. The surface layer region has a controlled texture with a sum of average pole densities of specific crystal orientations limited to 6.0 or less.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheets is increased to achieve high strength, then tensile strength is improved, but formability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.02-0.12%, Si: 0.01-2.00%, Mn: 1.00-3.00%, Ti: 0.01-0.20%, Nb: 0-0.10%, V: 0-0.100%) and microstructural parameters (tempered martensite volume percentage: 80% or more, precipitate number density: 5×10^9 pieces/mm^3 or more, precipitate size: equivalent circle diameter of 5 nm or less) to achieve both high tensile strength (980 MPa or more) and excellent formability. This systematic parameter optimization resolves the contradiction between strength and formability.
Solution Approach 2:
The patent creates a composite microstructure consisting of tempered martensite (80% or more by volume) with dispersed precipitates containing Ti, Nb, and V. This composite structure combines the high strength characteristics of tempered martensite with the strengthening effect of fine precipitates, achieving tensile strength of 980 MPa or more while maintaining formability through the refined microstructure.
2Strength
If the strength of steel sheets is increased to achieve high strength, then tensile strength is improved, but bending workability deteriorates
Solution Approach 1:
The patent controls specific parameters including the ratio of tempered martensite (80% or more), the number density of precipitates (5×10^9 pieces/mm^3 or more), and the size of precipitates (equivalent circle diameter of 5 nm or less). These parameter controls enable the steel to achieve tensile strength of 980 MPa or more while suppressing inside bend cracks and maintaining bending workability.
Solution Approach 2:
The patent introduces local quality by controlling the surface layer region texture characteristics (sum of average pole densities of specific crystal orientations: 6.0 or less) and the distribution of fine precipitates. This local optimization of microstructure and texture in the surface layer improves bending workability and suppresses inside bend cracks while maintaining overall high strength.
3Strength
If the strength of steel sheets is increased to achieve high strength, then tensile strength is improved, but stretch flangeability deteriorates
Solution Approach 1:
The patent controls the microstructural parameters including tempered martensite volume percentage (80% or more) and precipitate characteristics (number density: 5×10^9 pieces/mm^3 or more, size: equivalent circle diameter of 5 nm or less). This parameter control achieves tensile strength of 980 MPa or more while maintaining stretch flangeability through the refined microstructure.
Solution Approach 2:
The patent creates a composite microstructure of tempered martensite with fine precipitates that provides both high strength and good stretch flangeability. The tempered martensite matrix (80% or more by volume) provides strength, while the fine dispersed precipitates provide strengthening without excessive brittleness, enabling stretch flangeability to be maintained.
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 achieves a tensile strength of 980 MPa or more, suppresses inside bend cracks, exhibits excellent bending workability and stretch flangeability, and maintains a high proof stress, thereby addressing the limitations of existing technologies.
Implementation Method 1
the tempered martensite includes 5×10^9 pieces/mm^3 or more of precipitates containing Ti and having an equivalent circle diameter of 5 nm or less per unit volume
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, a microstructure includes 80% or more of tempered martensite by a volume percentage and a remainder consisting of one or more of ferrite, pearlite, bainite, fresh martensite, and residual austenite, the tempered martensite includes 5×109 pieces/mm3 or more of precipitates containing Ti and having an equivalent circle diameter of 5 nm or less per unit volume, in a surface layer region that is a range from a surface to a 1/10 position of a sheet thickness, a sum of an average pole density of a crystal orientation group consisting of {211}<111> to {111}<112> and a pole density in a crystal orientation of {110}<001> is 6.0 or less, and a tensile strength is 980 MPa or more.