High-Strength Galvanized Steel Sheet Annealing
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Solution Overview
Problem
High-strength galvanized steel sheets with tensile strength of 1180 MPa or more face challenges in maintaining consistent mechanical properties due to annealing temperature variations, leading to instability in press forming and coating defects from oxidation of Si and Mn elements.
Innovation Solution
A method involving hot rolling, cold rolling, first annealing, pickling, and second annealing processes with controlled heat treatment and cooling rates to form a microstructure with 10-60% ferrite and 40-90% martensite, along with a Si—Mn-depleted layer to inhibit surface oxidation and enhance zinc coatability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is increased by adding Si and Mn, then tensile strength reaches 1180 MPa or more, but Si and Mn are easily oxidized and concentrated on the surface forming oxides that cause coating defects
Solution Approach 1:
The patent applies preliminary action by performing pickling treatment before annealing to remove surface oxides of Si and Mn in advance. This prevents the oxidation problem from occurring during subsequent galvanizing, allowing the steel to achieve high strength through Si and Mn addition without suffering from surface oxidation defects.
2Strength
If multi-phase steel sheet is manufactured by ordinary continuous annealing, then good ductility and strength-ductility balance are achieved, but mechanical properties widely vary due to annealing temperature variation
Solution Approach 1:
The patent applies parameter changes by precisely controlling the annealing temperature range (750-850°C) and holding time (10-500 seconds) to achieve consistent mechanical properties. This controlled parameter approach ensures that the steel sheet maintains stable tensile strength (1180 MPa or more) and elongation (10% or more) despite variations in the continuous annealing process.
3Reliability
If Fe oxide film is formed rapidly by heating in oxidizing atmosphere to improve zinc wettability, then adhesiveness of galvanizing layer is improved, but pressing flaw occurs due to oxidized iron sticking to rolls
Solution Approach 1:
The patent applies preliminary action by performing pickling treatment before annealing to remove surface oxides in advance. This eliminates the need for rapid oxide formation during annealing, preventing pressing flaws while still achieving good galvanizing adhesiveness through the clean surface prepared by pickling.
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 high-strength galvanized steel sheets with reduced annealing temperature dependency and improved surface appearance quality, enabling stable press forming and enhanced crashworthiness while maintaining high tensile strength.
Implementation Method 1
cooling the held steel sheet from the annealing temperature to a cooling stop temperature of 500° C. or lower at an average cooling rate of 5° C./s or more to obtain a steel sheet having a steel microstructure including ferrite in an amount of 10% or more and 60% or less in terms of area ratio, and martensite, bainite, and retained austenite in a total amount of 40% or more and 90% or less in terms of area ratio
Implementation Method 2
Si and Mn contained in steel are subjected to selective oxidation even in a non-oxidizing atmosphere or a reducing atmosphere used in a general annealing furnace, Si and Mn are concentrated and form oxides on the surface of the steel
Data Source
AI summary
A method for manufacturing a high-strength galvanized steel sheet includes performing hot rolling, cold rolling, first annealing, pickling, and second annealing. The first annealing is performed to obtain a steel sheet having a steel microstructure including ferrite in an amount of 10% or more and 60% or less in terms of area ratio, and martensite, bainite, and retained austenite in a total amount of 40% or more and 90% or less in terms of area ratio. The second annealing includes heating to an annealing temperature of 750° C. or higher and 850° C. or lower, holding at the annealing temperature for 10 seconds or more and 500 seconds or less, cooling at an average cooling rate of 1° C./s or more and 15° C./s or less, performing a galvanizing treatment, and cooling to a temperature of 150° C. or lower at an average cooling rate of 5° C./s or more and 100° C./s or less.