Hot-Dip Galvanized Steel Sheet for Low-Hydrogen Edge Formability
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Solution Overview
Problem
Existing hot-dip galvanized steel sheets used in automotive framework structural parts face challenges with high diffusive hydrogen content leading to poor stretch flangeability, bendability, and trimmed edge quality, especially immediately after production, which affects their suitability for high-strength automotive applications.
Innovation Solution
A hot-dip galvanized steel sheet with a specific chemical composition and microstructure, including controlled amounts of elements like C, Si, Mn, and a targeted microstructure of martensite and ferrite, along with a hot-dip galvanized layer, is produced through a controlled manufacturing process to reduce low-temperature diffusive hydrogen and enhance stretch flangeability and bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets with high yield ratio are used to increase part strength and absorbable impact energy, then crashworthiness is improved, but stretch flangeability and bendability deteriorate due to high diffusive hydrogen content
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by precisely controlling the content ranges of C, Si, Mn, P, S, Al, N, Ca, and Cr elements. This parameter optimization enables the steel to achieve high yield ratio (80% or more) while maintaining low diffusive hydrogen content (0.80 ppm or less), thereby resolving the contradiction between strength and formability
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (bainite, ferrite, tempered martensite, and retained austenite) with specific area fractions. This composite microstructure provides both high strength properties and good ductility, enabling the steel to exhibit high yield ratio while maintaining adequate stretch flangeability
2Strength
If high strength steel sheets with high yield ratio are used to increase part strength, then absorbable impact energy increases, but bendability deteriorates due to high diffusive hydrogen content immediately after production
Solution Approach 1:
The invention optimizes chemical composition parameters to control hydrogen trapping sites in the steel microstructure. By controlling element contents (particularly Si and Mn) and microstructure phases, the diffusive hydrogen content is reduced to 0.80 ppm or less, preventing hydrogen-induced embrittlement that would otherwise deteriorate bendability in high-strength steel
Solution Approach 2:
The multi-phase composite microstructure (bainite, ferrite, tempered martensite, retained austenite) provides a balanced combination of strength and ductility. The softer phases (ferrite, retained austenite) provide ductility for bending operations, while the harder phases (tempered martensite, bainite) provide strength, resolving the contradiction between yield ratio and bendability
3Reliability
If hot-dip galvanized steel sheets are used to improve rust proof performance, then corrosion resistance is enhanced, but trimmed edge quality deteriorates due to high diffusive hydrogen content remaining for 24 hours after production
Solution Approach 1:
The invention changes the chemical composition parameters to reduce the total amount of diffusive hydrogen in the steel sheet to 0.80 ppm or less. This parameter optimization ensures that even 24 hours after production when hydrogen content is highest, the steel maintains adequate trimmed edge quality without hydrogen-induced cracking or defects
Solution Approach 2:
The composite microstructure with controlled phase distribution provides a matrix that traps and immobilizes hydrogen atoms. The combination of bainite, ferrite, tempered martensite, and retained austenite creates a microstructure that minimizes hydrogen diffusion pathways, thereby maintaining trimmed edge quality over time while preserving corrosion resistance
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 sheet with high yield ratio, excellent stretch flangeability, and improved trimmed edge quality, suitable for high-strength automotive framework structural parts, contributing to reduced vehicle weight and improved crashworthiness.
Implementation Method 1
hot-dip galvanized steel sheets obtained through hot-dip galvanization are sometimes used for steel sheets used as materials of automotive framework structural parts
Implementation Method 2
immediately after the production of the hot-dip galvanized steel sheet, i.e., when the hot-dip galvanized steel sheet is wound into a coil on the exit side of the continuous annealing line, a large amount of diffusive hydrogen is contained in the steel sheet, and the amount of diffusive hydrogen decreases with time at room temperature
Data Source
AI summary
It is an object to provide a hot-dip galvanized steel sheet that has a high YR, exhibits high stretch flangeability immediately after production and high bendability immediately after production, and provides improved trimmed edge quality and to provide a method for producing the hot-dip galvanized steel sheet and a member. The hot-dip galvanized steel sheet includes: a base steel sheet; and a hot-dip galvanized layer on a surface of the base steel sheet. The base steel sheet has a prescribed chemical composition and has, at a position 1/4 of the thickness of the base steel sheet, a microstructure including martensite at an area fraction of 30% or more, ferrite at an area fraction of 70% or less, and retained austenite at a volume fraction of 20.0% or less. The amount of low-temperature diffusive hydrogen is 0.015 ppm by mass or less. Here, the amount of low-temperature diffusive hydrogen is an amount of hydrogen released from the base steel sheet when the base steel sheet is heated from room temperature to 50°C 24 hours after production of the hot-dip galvanized steel sheet. In the hot-dip galvanized layer, the number density of cracks penetrating through the hot-dip galvanized layer is 30 cracks/mm or more, and the full width at half maximum of the 51 phase in the hot-dip galvanized layer is 0.100 degrees or more.