Bainitic Steel with High Local Cold Formability
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Solution Overview
Problem
Current high-strength hot-rolled steel products with bainitic microstructure face limitations in achieving both high local cold-formability and economic viability, as they often compromise on either strength or cold-formability, and existing methods are cost-intensive and inefficient in producing steel with optimal microstructure and alloy composition.
Innovation Solution
A high-strength, hot-rolled flat steel product with a microstructure consisting of more than 50% bainite and a specific alloy composition, including elements like C, Si, Mn, Nb, and Ti, is produced using a process that involves controlled final rolling and reeling temperatures to achieve a balance of strength and cold-formability, with a hyperstoichiometric ratio of carbon and nitrogen, and optional additions of Cr, Ni, V, or Ca for improved properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength hot-rolled steel products with bainitic microstructure are produced using conventional methods, then tensile strength is improved, but local cold-formability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the alloy composition (C: 0.18-0.35%, Si: 0.05-0.60%, Mn: 1.50-2.50%, Ti: 0.030-0.100%, Nb: 0.030-0.100%, V: 0.030-0.200%, Mo: 0.10-0.50%, Cr: 0.10-0.60%, Ni: 0.10-0.60%, Al: 0.01-0.06%) and processing parameters (cooling rates, reeling temperatures, finishing temperatures) to achieve a bainitic microstructure with optimized properties that simultaneously provides high tensile strength and improved local cold-formability
Solution Approach 2:
The patent creates a composite microstructure consisting of bainite as the primary phase with controlled amounts of other phases, achieved through multi-element alloying. This composite microstructure combines the strength benefits of bainite with improved formability characteristics, resolving the contradiction between strength and cold-formability
2Reliability
If existing production methods are used to achieve optimal microstructure and alloy composition, then steel properties are improved, but production cost increases
Solution Approach 1:
The patent optimizes production cost through parameter changes by controlling alloying elements within specific ranges that balance performance and cost. The method uses controlled cooling rates and reeling temperatures to achieve the desired bainitic microstructure without requiring expensive additional processing steps, thereby maintaining production efficiency while achieving optimal material properties
3Strength
If high-strength steel with tensile strength of at least 760 MPa is produced, then strength is improved, but elongation at fracture deteriorates
Solution Approach 1:
The patent resolves the contradiction between strength and elongation through parameter changes in the alloy composition, specifically optimizing the content of microalloying elements (Ti, Nb, V) and cooling parameters. These changes produce a refined bainitic microstructure with appropriate grain size and phase distribution, enabling the steel to achieve both high tensile strength (≥760 MPa) and adequate elongation at fracture (≥10%)
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 steel product achieves a combination of high tensile strength, high local cold-formability, and economic viability, with enhanced elongation at fracture, hole expansion ratio, and a favorable ratio of local to global cold-formability, while maintaining cost-effectiveness and process stability.
Implementation Method 1
a multiphase microstructure which contains substantially, i.e. a proportion of more than 50 vol. %, bainite
Implementation Method 2
The flat steel product in accordance with the invention has, along the rolling direction, a tensile strength Rm of at least 760 MPa, a yield strength ratio of at least 0.8
Data Source
AI summary
The present disclosure provides a high-strength hot-rolled flat steel product and a method of producing such a product. This is achieved by a high-strength hot-rolled flat steel product with high local cold formability, having a tensile strength Rm of at least 760 MPa, a yield point ratio of at least 0.8 and a hole expansion ratio of at least 30%, an elongation at break of at least 10, a measure of cold formability of at least 0.12, and a ratio of local and global cold formability of at least 5 and at most 13, and a microstructure consisting of more than 50% by volume of bainite and up to 10% by volume, of carbon-rich microstructure constituents