Steel Sheet Crack Resistance via Inclusion Control
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Solution Overview
Problem
Conventional carbon steel sheets with complex shapes face issues of cracking and reduced toughness due to the presence of elongated MnS inclusions and coarse carbonitrides, which deteriorate workability and forgeability, especially when forming complex shapes like gears.
Innovation Solution
A carbon steel sheet with a chemical composition of C: 0.25-0.50%, Si: 0.10-0.60%, Mn: 0.40-0.90%, and controlled amounts of Ca, REM, and Ti, where the number density of A-type, B-type, and C-type inclusions, as well as coarse carbonitrides, is reduced to enhance workability and toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If the steel sheet is punched, expanded, or forged to form a complex shape, then the manufacturing cost is reduced and the number of components is reduced, but the steel sheet may become cracked and the dimensional accuracy deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.15-0.50%, Si: 0.10-0.60%, Mn: 0.40-0.90%, Al: 0.003-0.070%, Ca: 0.0005-0.0040%, REM: 0.0003-0.0050%) and processing parameters (finishing temperature 850-950°C, coiling temperature 500-650°C) to optimize the steel's microstructure and inclusion characteristics, thereby improving both workability and cracking resistance simultaneously
Solution Approach 2:
The patent converts the harmful effect of inclusions by controlling their type, size, and distribution through specific alloying additions. The controlled inclusions act as nucleation sites for austenite grains, refining the grain structure and improving toughness, thereby transforming potential defect sources into beneficial microstructural features that enhance both formability and crack resistance
2Ease of operation
If the grain size of carbide is increased to enhance notched tensile elongation ratio and punchability, then the punchability is improved, but the steel may have reduced strength and toughness
Solution Approach 1:
The patent applies local quality by creating different microstructural zones and controlling the local distribution of inclusions and carbides. The steel exhibits refined grain structures in critical areas while maintaining appropriate carbide distribution, allowing localized ductility improvements without compromising overall strength
Solution Approach 2:
The patent creates a composite microstructure combining ferrite, pearlite, and controlled carbide distributions. The multi-phase microstructure with specific inclusion characteristics (number density ≤6 pieces/mm², average size 5-20 μm) provides both the ductility needed for punching and the strength required for structural integrity
3Ease of manufacture
If Ca and REM are added to control oxide inclusions and prevent elongation during rolling, then the forgeability is improved, but the cost of steel production increases
Solution Approach 1:
The patent applies parameter changes by optimizing the small additions of Ca (0.0005-0.0040%) and REM (0.0003-0.0050%) to achieve the desired inclusion control and forgeability improvement with minimal cost impact. The precise parameter control ensures maximum benefit from these expensive alloying elements
Data Source
AI summary
A steel sheet according to the present invention includes a predetermined chemical composition, in which amounts of each elements by mass % in the chemical composition satisfy both of expression “0.3000≤{Ca/40.88+(REM/140)/2}/(S/32.07)” and expression “Ca≤0.0058−0.0050×C”, and a number density of carbonitrides including Ti which exists independently and has a long side of 5 μm or more is limited to 5 pieces/mm2 or less.