Hot-Rolled Steel Sheet Texture Control for Formability
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Solution Overview
Problem
High-strength steel sheets exhibit reduced formability and local deformability, limiting their application in vehicle components due to increased strength, and existing methods for improving elongation and local deformability are limited by structural constraints and anisotropy issues.
Innovation Solution
A hot-rolled steel sheet with controlled texture and grain size, specifically by managing pole densities and Lankford values, and incorporating elements like Nb and Ti, to enhance elongation and local deformability while minimizing anisotropy, achieved through a multi-pass hot rolling process and controlled cooling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of a steel sheet is increased to reduce vehicle weight, then the weight reduction goal is achieved, but the formability and local deformability are reduced
Solution Approach 1:
The patent applies parameter changes by controlling chemical composition parameters (C: 0.15-0.40%, Si: 0.01-2.50%, Mn: 0.50-4.00%, P: 0.015-0.100%, S: 0.005-0.050%, Al: 0.01-2.00%, Ti: 0.001-0.100%, Nb: 0.001-0.100%, V: 0.001-1.000%, B: 0.0005-0.0050%) and processing parameters (hot rolling temperature range, cooling rate, waiting time before cooling) to achieve a balance between strength and formability. The controlled composition and process parameters produce a metallographic structure that simultaneously provides high strength and improved local deformability.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, bainite, and martensite) with specific volume fractions (ferrite: 30-80%, bainite: 10-50%, martensite: 5-20%). This composite structure at the microscale allows the material to exhibit both high strength from the hard martensite phase and good formability from the ductile ferrite and bainite phases, resolving the contradiction between strength and formability.
2Ease of manufacture
If structure control methods are used to improve elongation and local deformability, then formability is improved, but the metallographic structure formation is greatly affected and limited
Solution Approach 1:
The patent controls the metallographic structure by precisely managing parameters including chemical composition (specific ratios of alloying elements), hot rolling temperature (950-1200°C), rolling reduction rates, and cooling characteristics. These parameter controls guide the formation of a specific multi-phase structure with predetermined volume fractions, achieving both improved local deformability and structural stability.
Solution Approach 2:
The patent utilizes phase transitions during hot rolling and cooling to achieve the desired metallographic structure. By controlling the temperature profile and cooling rate, the austenite phase transforms into a mixture of ferrite, bainite, and martensite phases with specific volume fractions. This controlled phase transition process enables simultaneous achievement of improved formability and structural stability.
3Strength
If high-strength steel sheets are used for vehicle components, then weight reduction is achieved, but uniform elongation deteriorates
Solution Approach 1:
The patent creates a composite microstructure with multiple phases (ferrite, bainite, martensite) where each phase contributes different properties. The soft ferrite phase (30-80% volume fraction) provides ductility and uniform elongation, while the hard martensite phase (5-20% volume fraction) provides strength. The intermediate bainite phase (10-50% volume fraction) bridges the properties of the two extremes, enabling simultaneous achievement of high strength and good uniform elongation.
Solution Approach 2:
The patent applies local quality by creating a multi-phase microstructure where different phases are distributed throughout the material to provide different local properties. The ferrite regions provide local ductility for uniform deformation, while martensite regions provide local strength, creating a material that exhibits both high overall strength and good uniform elongation characteristics.
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 results in a steel sheet with superior elongation and local deformability, reduced orientation dependence of formability, and improved mechanical properties, suitable for high-strength applications without compromising anisotropy.
Implementation Method 1
cooling after hot rolling is controlled to control a metallographic structure; and a precipitate and a transformation structure are controlled to obtain appropriate fractions of ferrite and bainite
Implementation Method 2
large reduction is performed in an austenite region in a lowest possible temperature range to transform non-recrystallized austenite into ferrite and thus to facilitate the grain refinement of ferrite
Data Source
AI summary
In a hot-rolled sheet, an average value of pole densities of an orientation group {100}<011> to {223}<110>, which is represented by an arithmetic mean of pole densities of orientations {100}<011>, {116}<110>, {114}<110>, {112}<110>, and {223}<110> in a thickness center portion of a thickness range of ⅝ to ⅜ from a surface of the steel sheet, is 1.0 to 6.5 and a pole density of a crystal orientation {332}<113> is 1.0 to 5.0; and a Lankford value rC in a direction perpendicular to a rolling direction is 0.70 to 1.10 and a Lankford value r30 in a direction that forms 30° with respect to the rolling direction is 0.70 to 1.10.