Steel Sheet Two-Layer Crystallization Structure
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Solution Overview
Problem
Thin steel sheets used for packaging face challenges in maintaining strength and formability during deep drawing processes, leading to roughening issues due to coarse grain structures, which affect stability and corrosion resistance.
Innovation Solution
A steel sheet with a two-layer crystallization structure is produced by applying a nitrogen-impermeable barrier layer on one side, allowing nitrogen to diffuse only on the opposite side during annealing, creating a gradient that prevents complete recrystallization and maintains high strength and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the steel sheet is completely recrystallized after cold rolling to restore formability, then the steel sheet becomes softer and more formable, but the grain structure becomes coarser leading to roughening during severe forming
Solution Approach 1:
The patent applies local quality by creating a two-layer crystallization structure where the inner layer is completely recrystallized to provide formability, while the outer layer is partially recrystallized to maintain fine grain structure and prevent roughening. This is achieved by controlling the annealing process parameters including temperature (550-800°C), time, and nitrogen atmosphere to create different recrystallization degrees at different depths from the surface.
2Loss of substance
If thinner steel sheets are used to improve resource efficiency, then material usage is reduced, but the strength and stability of the packaging decreases
Solution Approach 1:
The patent applies composite materials principle by creating a composite microstructure within the steel sheet consisting of two distinct crystallization layers. The inner completely recrystallized layer provides ductility and formability, while the outer partially recrystallized layer provides strength and resistance to roughening. This internal composite structure allows thin steel sheets to maintain high strength despite reduced thickness.
3Strength
If the steel sheet is nitrided to increase strength, then the strength increases, but the grain structure coarsens and roughening susceptibility increases
Solution Approach 1:
The patent applies local quality by spatially separating the effects of nitriding: the outer layer is nitrided to increase strength while being partially recrystallized to maintain fine grains, whereas the inner layer is completely recrystallized for formability. The controlled nitriding process introduces nitrogen atoms that strengthen the outer layer through solid solution strengthening and precipitation hardening, while the partial recrystallization prevents excessive grain growth that would cause roughening.
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 sheet achieves high strength over 500 MPa with 5% elongation, preventing roughening during severe forming with small bending radii, ensuring stability and corrosion resistance in packaging applications.
Implementation Method 1
whereby during heating of the steel sheet nitrogen diffuses from the nitriding gas atmosphere at least into a region near the surface on the second side of the steel sheet and is deposited in this region
Implementation Method 2
the steel sheet is heated in a continuous annealing furnace to a (maximum) heating temperature which is at least as high as the recrystallization temperature
Data Source
AI summary
A method for producing a steel sheet having a two-layer recrystallization structure includes: providing a cold-rolled steel sheet made of a steel with a carbon content of 10 to 1000 ppm, based on the weight, and a specified recrystallization temperature; applying a barrier layer which is at least partly impermeable to nitrogen onto a first face; and heating the steel sheet to a heating temperature. The heating is carried out at least temporarily in a nitrogenizing gas atmosphere at least until the recrystallization temperature is reached, whereby nitrogen from the gas atmosphere is diffused at least in a region near the surface on a second face of the steel sheet upon heating and is stored in the region, whereby the recrystallization temperature of the steel is raised in the region. The heating temperature is higher than or equal to the specified recrystallization temperature and lower than the raised recrystallization temperature.