Nitrided Packaging Steel Two-Stage Process
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Solution Overview
Problem
Existing nitriding methods for packaging steel are limited by a maximum nitrogen content of 100 ppm, restricting the achievable strength of steel sheets to below 600 MPa, and fail to maintain formability and elongation to fracture at higher nitrogen levels.
Innovation Solution
A two-stage nitriding process is employed, where molten steel is initially nitrided to a maximum of 160 ppm, followed by a second stage of nitriding during recrystallization annealing in an annealing furnace using ammonia gas, allowing for a total nitrogen content of up to 500 ppm, ensuring high strength and formability in cold-rolled steel products.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the nitrogen content in steel is increased to achieve higher strength, then the ultimate tensile strength increases, but the steel loses formability and cannot be cold-rolled
Solution Approach 1:
The nitriding process is divided into two distinct stages: (1) nitriding molten steel to achieve 5-160 ppm nitrogen content before casting and rolling, and (2) nitriding cold-rolled steel strips to achieve additional 50-450 ppm nitrogen content. This segmentation allows the steel to gain strength through nitrogen while maintaining formability during cold rolling, as the nitrogen is introduced at different stages when the steel has different physical states and properties.
Solution Approach 2:
Nitrogen is introduced into the molten steel during the steelmaking process before casting and cold rolling. This preliminary nitriding establishes a base nitrogen content (5-160 ppm) that provides initial strength enhancement while the steel is still in a state suitable for forming operations.
2Strength
If nitrogen is introduced into molten steel during steelmaking, then the steel achieves baseline strength, but additional nitriding after cold rolling creates surface hardening that may affect formability
Solution Approach 1:
The patent applies different nitrogen concentrations and distribution patterns to different stages: the first stage introduces nitrogen uniformly into the bulk molten steel (5-160 ppm), while the second stage introduces additional nitrogen (50-450 ppm) into the cold-rolled steel strip. This creates a composition gradient that provides both bulk strength and surface properties suitable for packaging applications.
Solution Approach 2:
The patent changes the nitrogen concentration parameter across different stages: from 5-160 ppm in molten steel to an additional 50-450 ppm in cold-rolled steel. This parameter change allows optimization of both formability (lower nitrogen during rolling) and final strength (higher nitrogen in finished product).
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 method produces packaging steel with ultimate tensile strengths exceeding 650 MPa and elongation to fracture of at least 5%, while maintaining excellent formability and preventing defects, even at high nitrogen levels.
Implementation Method 1
the molten steel is nitrided to a nitrogen content of at most 160 ppm by introducing a nitrogen-containing gas and/or a nitrogen-containing solid into the molten steel
Implementation Method 2
a flat steel product produced from the nitrided molten steel by cold rolling is treated with a nitrogen-containing gas in order to increase further the amount of uncombined nitrogen in the flat steel product
Implementation Method 3
The second stage is performed in an annealing furnace, in which the flat steel product is at the same time annealed in a recrystallizing manner
Data Source
AI summary
A nitrided packaging steel in the form of a flat steel product and method for producing a nitrided packaging steel with a carbon content of 10-1000 ppm and uncombined nitrogen, dissolved in the steel, of more than 100 ppm. The nitriding is performed in two stages: a first stage, in which a molten steel is nitrided to a nitrogen content of at most 160 ppm by introducing a nitrogen-containing gas and/or a nitrogen-containing solid into the molten steel, and a second stage, in which a flat steel product produced from the nitrided molten steel by cold rolling is treated with a nitrogen-containing gas in order to increase further the amount of uncombined nitrogen in the flat steel product.