Galvanized Steel Sheet Surface Oxide Management
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Solution Overview
Problem
High strength galvanized steel sheets with tensile strength of 780 MPa or higher face challenges in achieving excellent bendability, fatigue resistance, and surface appearance quality due to increased oxidation of elements like Si and Mn, leading to issues with coatability and surface defects during the coating process.
Innovation Solution
A method involving soft reduction and pickling after primary annealing to introduce strain only in the surface layer, followed by secondary annealing to control grain growth and hardness distribution, resulting in a high strength galvanized steel sheet with a specific microstructure and composition that enhances bendability, fatigue resistance, and surface appearance quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets are produced by increasing C, Mn, and Si content to achieve tensile strength of 780 MPa or higher, then strength is improved, but oxidation of Si and Mn occurs during annealing, leading to degraded coatability and surface appearance quality
Solution Approach 1:
The patent applies preliminary action by conducting pickling treatment before the final annealing process to remove surface oxides formed during hot rolling and intermediate annealing. This preliminary removal of oxides prevents them from interfering with the subsequent zinc coating process, thereby resolving the contradiction between achieving high strength (requiring Si and Mn addition) and preventing oxidation that degrades coatability.
Solution Approach 2:
The patent employs parameter changes by optimizing the annealing atmosphere composition (using reducing atmosphere with controlled oxygen potential), temperature (700-850°C), and time parameters. These parameter adjustments create conditions that minimize oxidation of Si and Mn while still achieving the desired microstructure and strength properties, thus resolving the contradiction between strength enhancement and oxidation prevention.
2Strength
If high strength steel sheets with tensile strength of 780 MPa or higher are produced, then crash safety is improved, but bendability deteriorates making press-forming difficult
Solution Approach 1:
The patent applies local quality by creating a non-uniform microstructure with different phases distributed throughout the material. Specifically, it produces a mixed microstructure containing martensite (for strength), bainite, and ferrite phases, where each phase contributes differently to overall properties. The martensite provides high strength for crash safety, while the softer ferrite and bainite phases maintain bendability, thus resolving the contradiction between strength and formability.
Solution Approach 2:
The patent employs composite materials principle by creating a multi-phase steel microstructure that combines different crystalline phases (martensite, bainite, ferrite) within a single material system. This composite microstructure allows the steel to exhibit both high strength from martensite and good ductility from softer phases, resolving the contradiction between achieving 780 MPa tensile strength and maintaining bendability for press-forming operations.
3Strength
If Si and Mn are added in large quantities to achieve high strength through martensite formation, then tensile strength is improved, but wettability of molten zinc deteriorates due to oxide formation
Solution Approach 1:
The patent applies preliminary action by implementing pickling treatment before the final annealing and coating processes. This preliminary step removes surface oxides that would otherwise form during hot rolling and intermediate annealing, preventing them from interfering with zinc wettability. By addressing oxide removal in advance, the patent enables successful coating despite the presence of high Si and Mn content necessary for achieving 780 MPa tensile strength.
Solution Approach 2:
The patent converts the harmful effect of oxide formation into a beneficial process by using controlled oxidation during hot rolling, then removing these oxides through pickling. The oxides that form during hot rolling are not entirely eliminated but are converted into a manageable intermediate state that can be selectively removed. This approach allows the steel to achieve high strength through Si and Mn addition while the oxide management process ensures good coatability.
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 method produces a high strength galvanized steel sheet with excellent bendability, fatigue resistance, and surface appearance quality, suitable for automotive frame parts, improving crash safety and enabling weight reduction while maintaining high tensile strength.
Implementation Method 1
the steel sheet is preliminarily heated in an oxidizing atmosphere so as to rapidly generate an Fe oxide film on a surface at a particular oxidation rate or higher to thereby prevent oxidation of additive elements on the steel sheet surface
Implementation Method 2
the Fe oxide film is reduced by reduction annealing to improve wettability to molten zinc
Implementation Method 3
Si and Mn contained in the steel undergo selective oxidation even in a non-oxidizing atmosphere or a reducing atmosphere used in a typical annealing furnace and thereby form oxides by surface segregation
Implementation Method 4
Si and Mn contained in the steel undergo selective oxidation even in a non-oxidizing atmosphere or a reducing atmosphere used in a typical annealing furnace and thereby form oxides by surface segregation
Implementation Method 5
The presence of these oxides of Si and Mn degrades wettability of molten zinc to the steel sheet in the coating process
Data Source
AI summary
A high strength galvanized steel sheet that has a tensile strength (TS) of 780 MPa or higher and excellent bendability, fatigue resistance, and surface appearance quality and a method for producing the high strength galvanized steel sheet. The steel sheet includes, at a position ½ of a thickness of the steel sheet, 5% or more and 80% or less of a ferrite phase in terms of area fraction, 20% or more and 70% or less of a martensite phase in terms of area fraction, and 0% or more and 25% or less of a bainite phase in terms of area fraction.