High-Strength Steel Plate Phosphating via Internal Oxide Layer
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Solution Overview
Problem
High-strength cold-rolled steel plates with high silicon content face challenges in phosphatability and formability due to silicon oxide formation during annealing, leading to poor coating quality and corrosion resistance, which limits their application in automobiles.
Innovation Solution
A cold-rolled high-strength steel plate with a chemical composition of C 0.15-0.25%, Si 1.50-2.50%, Mn 2.00-3.00%, and a surface layer comprising an inner oxide layer with specific thickness and particle distribution, preventing silicon and manganese enrichment, ensuring excellent phosphatability and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high silicon content (1.50-2.50%) is added to improve strength and formability, then tensile strength reaches ≥1180 MPa and elongation reaches ≥14%, but silicon oxides form during continuous annealing that hinder uniform phosphating reaction, causing poor phosphating coverage and large phosphated crystal size
Solution Approach 1:
The steel plate surface is pre-treated with a specific oxide layer (1-5 μm thick containing Fe, Si, Mn oxides) before phosphating. This preliminary oxide layer prevents excessive silicon enrichment and ensures uniform phosphating reaction, solving the problem of poor phosphating quality that would otherwise occur with high silicon content steel
Solution Approach 2:
The patent controls the thickness and composition of the oxide layer on the steel surface through precise control of annealing parameters (temperature 700-900°C, atmosphere composition, time). By changing these parameters, the oxide layer prevents silicon oxide formation that would hinder phosphating, while maintaining the high silicon content (1.50-2.50%) needed for strength and formability
2Stability of the object's composition
If high silicon content (1.50-2.50%) is added to improve formability, then elongation reaches ≥14%, but silicon enrichment at the surface during annealing causes deteriorated phosphatability and substandard coating quality
Solution Approach 1:
A controlled oxide layer is formed on the steel surface before phosphating treatment. This preliminary action prevents silicon from enriching at the surface during annealing, thereby maintaining good phosphatability while preserving the high silicon content (1.50-2.50%) necessary for excellent formability (elongation ≥14%)
Solution Approach 2:
The oxide layer acts as an intermediary between the high silicon steel matrix and the phosphating solution. It prevents direct harmful interaction between silicon enrichment and phosphating, allowing the steel to maintain both high formability and good phosphating quality
3Reliability
If traditional pickling process is used after annealing to remove oxides, then phosphating quality improves, but production cost increases and production efficiency decreases
Solution Approach 1:
Instead of removing the oxide layer formed during annealing through costly and time-consuming pickling, the patent converts this potentially harmful oxide layer into a beneficial pre-treatment layer for phosphating. By controlling the oxide layer composition and thickness (1-5 μm), it actually improves phosphating quality while eliminating the need for additional pickling processes, thereby maintaining high production efficiency
4Manufacturing precision
If traditional pickling process is used after annealing, then phosphating coverage improves, but production cost increases due to additional process steps and waste acid treatment
Solution Approach 1:
The oxide layer that would normally require removal through pickling is instead utilized as a beneficial pre-treatment for phosphating. By controlling its composition and thickness, the patent achieves uniform phosphating coverage without needing the complex and costly pickling process, thereby simplifying production and reducing costs
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 plate achieves a tensile strength of ≥1180 MPa and elongation of ≥14%, with improved phosphatability and formability, suitable for automobile structural parts, while being cost-effective and environmentally friendly.
Implementation Method 1
When the steel plate is continuously annealed in a heating furnace
Implementation Method 2
the Si element is enriched in a surface of the steel plate to form silicon oxides
Data Source
AI summary
A cold-rolled steel plate (1) and a manufacturing method therefor. The chemical composition of the steel plate (1) in percentage by weight is: C 0.15-0.25%, Si 1.50-2.50%, Mn 2.00-3.00%, P≤0.02%, S≤0.01%, Al 0.03-0.06%, N≤0.01%, with the balance being Fe and impurities. The surface layer has an inner oxide layer (2) with a thickness of 1-5 μm, and there is no enrichment of Si or Mn on the surface. The steel plate (1) has good phosphating performance and formability, with a tensile strength of ≥1180 MPa and an elongation of ≥14%, and has a complex-phase structure of ferrite, martensite, and retained austenite, the content of the retained austenite being not lower than 5%. A dew point is at −25° C. to 10° C. in continuous annealing, such that external oxidation transitions to internal oxidation.