High-Strength Steel Plate Phosphating via Inner Oxide Layer
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Solution Overview
Problem
High-strength steel plates face challenges in achieving both high phosphatability and formability due to surface enrichment of Si and Mn elements during annealing, leading to poor phosphating coverage and corrosion resistance, and existing methods are costly and difficult to implement in continuous annealing production lines.
Innovation Solution
A cold-rolled high-strength steel plate with a specific chemical composition and a controlled inner oxide layer, comprising iron as a matrix with oxide particles of Si and Mn, prevents surface enrichment of these elements, ensuring excellent phosphatability and formability, and a manufacturing method involving hot rolling, pickling, and continuous annealing with a N2-H2 mixed gas atmosphere.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Si element is added to improve formability of high-strength steel plate, then formability is improved, but surface enrichment of Si and Mn occurs during annealing which deteriorates phosphatability
Solution Approach 1:
The patent changes the chemical composition parameters by strictly controlling Si content to 1.0-2.0% and Mn content to 2.0-3.0%, while also controlling the ratio of Si/Mn ≤ 0.5. This parameter optimization prevents excessive surface enrichment during annealing while maintaining adequate formability through sufficient Si addition.
Solution Approach 2:
The patent creates a non-uniform distribution of Si and Mn elements between the bulk material and surface. By controlling the overall composition and annealing process, the bulk material maintains high Si content for formability while the surface maintains lower Si content for phosphatability, achieving different local qualities in different regions.
2Reliability
If oxidation treatment is performed in continuous annealing to improve phosphatability, then phosphatability is improved, but additional equipment and production cost are required
Solution Approach 1:
The patent enables the steel plate to self-regulate its surface composition during standard continuous annealing without additional oxidation equipment. The controlled Si/Mn ratio and annealing parameters allow the material to naturally achieve optimal surface phosphatability through diffusion and oxidation processes that occur in the existing annealing atmosphere.
Solution Approach 2:
The patent extracts the need for additional oxidation equipment from the production line by incorporating oxidation control into the existing annealing process parameters. The solution removes the requirement for separate oxidation treatment apparatus while achieving the same phosphatability improvement through compositional control.
3Strength
If Si content is increased to enhance formability, then formability is improved, but phosphating coverage becomes poor and crystal size increases
Solution Approach 1:
The patent optimizes the Si content parameter to a specific range of 1.0-2.0% and controls the Si/Mn ratio to ≤ 0.5. This precise parameter control ensures sufficient Si for formability while preventing excessive Si that would cause poor phosphating coverage and large crystal size.
Solution Approach 2:
The patent creates a composite chemical composition system where Si and Mn work together in a controlled ratio. The combination of Si (1.0-2.0%) and Mn (2.0-3.0%) with Si/Mn ≤ 0.5 creates a synergistic effect that improves formability through Si while Mn helps control surface enrichment and maintains uniform phosphating.
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 results in a steel plate with a tensile strength of 980 MPa or more, elongation of 20% or more, and a room temperature structure suitable for complex automobile parts, with improved phosphatability and formability, while being producible on existing continuous annealing lines without additional equipment costs.
Implementation Method 1
In continuous annealing, a surface of the steel plate is exposed to an atmosphere in which iron is oxidized, such that the surface thereof is oxidized
Implementation Method 2
After pickling at an outlet side of an annealing furnace
Data Source
AI summary
A cold-rolled high-strength steel plate having excellent phosphating performance and formability and a manufacturing method therefor. The chemical composition of the steel plate is, in percentage by weight, C 0.01-0.20%, Si 1.50-2.50%, Mn 1.50-2.50%, P≤0.02%, S≤0.02%, Al 0.03-0.06%, N≤0.01%, the remainder being Fe and impurities. The surface layer of the steel plate has an inner oxide layer with a thickness of 1-5μm, and there is no enrichment of Si and Mn on the surface of the steel plate. The steel plate has tensile strength of ≥980 MPa and an elongation of ≥20%. The structure at the room temperature contains retained austenite, ferrite, and martensite and/or bainite.