Low-Expansion Cast Steel Composition for Etch-Pit-Free Plating
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Solution Overview
Problem
Low thermal expansion cast steel products suffer from etch pits during plating pretreatment, leading to deteriorated plating properties and potential pinhole formation.
Innovation Solution
A low thermal expansion cast steel product with specific chemical composition and microstructure, including controlled amounts of C, Si, Mn, S, Ni, Co, O, and Al, is produced through cryogenic treatment and recrystallization processes to suppress etch pit formation, resulting in a product with a thermal expansion coefficient of 0.4 × 10^-6/°C or less and improved plating properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional low thermal expansion steel is used for plating, then the thermal expansion coefficient is low, but etch pits occur on the surface during pretreatment causing poor plating properties
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting S content to 0.010% or less and O content to 0.0100% or less, while optimizing Ni (31.00-34.00%) and Co (4.90-6.00%) contents. It also changes microstructural parameters by controlling austenite grain size to 200μm or less through specific heat treatment processes, thereby eliminating etch pit formation while maintaining low thermal expansion
Solution Approach 2:
The invention applies local quality control by specifically targeting the surface quality through compositional optimization. The restricted S and O content directly addresses surface etch resistance, while the bulk composition maintains low thermal expansion. The austenite grain size control creates a uniform fine-grained structure that prevents localized etch pit formation on the surface
2Stability of the object's composition
If alloy composition is optimized for low thermal expansion, then thermal expansion coefficient decreases, but plating property deteriorates due to etch pit formation
Solution Approach 1:
The invention optimizes multiple compositional parameters simultaneously: Ni content (31.00-34.00%), Co content (4.90-6.00%), S content (≤0.010%), and O content (≤0.0100%). This multi-parameter optimization achieves both low thermal expansion coefficient (0.4×10^-6/°C) and good plating properties by preventing etch pit formation through ultra-low S and O content
Solution Approach 2:
The invention creates a composite alloy system combining Fe, Ni, and Co with precisely controlled trace elements. The specific composition ratio of Ni (31-34%) and Co (4.90-6.00%) creates a synergistic effect that achieves low thermal expansion, while the ultra-pure base metal (low S and O) ensures good plating properties without etch pits
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 effectively suppresses etch pit formation, ensuring good plating properties and a low thermal expansion coefficient, thereby enhancing the product's performance in applications requiring stable thermal expansion characteristics.
Implementation Method 1
PLT 2 discloses a method of transforming a part of an austenite phase of a low thermal expansion line into a deformation-induced martensite phase of by cold working
Implementation Method 2
PLT 3 discloses a method comprising annealing the alloy at 650 to 900°C and then cooling at a rate of less than 20°C/min in a furnace
Data Source
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AI summary
The present invention addresses the problem of providing a low thermal expansion cast steel product having excellent plating properties. A low thermal expansion cast steel product according to the present invention has a chemical composition containing, in mass%, 0.040% or less of C, 0.30% or less of Si, 0.50% or less of Mn, 0.010% or less of S, 31.00-34.00% of Ni, 4.90-6.00% of Co, 0.0100% or less of O, and 0.010-0.100% of Al, the remaining portion being Fe and unavoidable impurities, has an austenite structure having an average grain size of 200 µm or less, and has an average thermal expansion coefficient of 0.4×10- 6 /°C or less at 25-100°C.