Hot-Dip Galvanized Steel Sheet Balancing Strength and Formability
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Solution Overview
Problem
Hot dip galvanized steel sheets for automobiles face challenges in achieving high strength, press formability, and hydrogen embrittlement resistance, particularly when subjected to plastic strain during press forming, with existing methods failing to adequately improve these properties.
Innovation Solution
A hot dip galvanized steel sheet with a specific chemical composition and microstructure is developed, including a balance of ferrite, retained austenite, tempered martensite, and boron segregation at austenite grain boundaries, achieved through controlled hot rolling and galvanization processes to enhance strength, ductility, and hydrogen embrittlement resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is increased to 980 MPa or more, then collision safety is improved, but press formability deteriorates
Solution Approach 1:
The steel sheet employs a composite microstructure consisting of multiple phases (martensite for strength, retained austenite for ductility, and ferrite) to simultaneously achieve ultra-high strength of 980 MPa or more and excellent press formability. This multi-phase composite structure allows the material to exhibit both high strength and good formability that cannot be achieved with a single-phase structure
Solution Approach 2:
The invention precisely controls the volume fractions of different microstructural phases (martensite: 5-80%, retained austenite: 5-50%, ferrite: 0-30%) and chemical composition parameters (C: 0.15-0.40%, Si: 1.50-3.00%, Mn: 1.50-3.50%, B: 0.0005-0.0100%) to optimize the balance between strength and press formability, achieving tensile strength of 980 MPa or more while maintaining elongation of 10% or more
2Strength
If the strength of steel sheet is increased to 980 MPa or more, then collision safety is improved, but hydrogen embrittlement resistance deteriorates
Solution Approach 1:
Boron is selectively segregated to austenite grain boundaries at a concentration of 2.0 atm% or more, creating a localized protective zone at the grain boundaries that effectively prevents hydrogen embrittlement cracking. This local concentration of boron at critical locations (grain boundaries) provides excellent hydrogen embrittlement resistance while maintaining the overall ultra-high strength of 980 MPa or more
Solution Approach 2:
Boron acts as an intermediary element that segregates to austenite grain boundaries and forms a protective barrier against hydrogen penetration. The boron enrichment at grain boundaries (2.0 atm% or more) creates a protective interface that prevents hydrogen embrittlement, allowing the steel to achieve both ultra-high strength and excellent hydrogen embrittlement resistance
3Ease of manufacture
If conventional production methods are used with multiple heating and cooling steps, then hot dip galvanization is achieved, but material quality deteriorates due to excessive tempering
Solution Approach 1:
The steel sheet is heated to Ac1 or higher and held for a predetermined time before hot dip galvanization to ensure complete austenite transformation and dissolution of carbides. This preliminary heating action ensures that the subsequent galvanization process proceeds smoothly and prevents excessive tempering, maintaining material quality while achieving proper hot dip galvanization
Solution Approach 2:
The invention optimizes the heating and cooling rates to quickly pass through temperature regions that would cause excessive tempering (460-650°C). By controlling the heating rate to 0.5-10.0°C/s and cooling rate to 10-100°C/s, the process rushes through the problematic tempering region, minimizing excessive tempering while still achieving proper hot dip galvanization
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 excellent press formability, ductility, and hydrogen embrittlement resistance, meeting the requirements for automotive applications with a tensile strength of 980 MPa or more and improved mechanical properties.
Implementation Method 1
a steel microstructure at a range of 1⁄8 thickness to 3⁄8 thickness centered about a position of 1⁄4 thickness from a surface of the base steel sheet contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, fresh martensite: 0% to 10%
Implementation Method 2
a concentration of B atoms at prior austenite grain boundaries is 2.0 atm % or more
Implementation Method 3
in the heating of the steel sheet before the first soaking, an average heating rate from 650° C. to a maximum heating temperature of Ac1+30° C. or more and 950° C. or less is 0.5° C./s to 10.0° C./s, the steel sheet is held at the maximum heating temperature for 1 second to 1000 seconds
Implementation Method 4
the first cooling is performed by an average cooling rate in a temperature range of 700 to 600° C. at the first cooling is 10 to 100° C./s, the second cooling is performed down to Ms-50° C. or less
Implementation Method 5
the second cooled steel sheet is heated to a temperature region of 200 to 420° C., then held in the temperature region for 5 to 1000 seconds
Data Source
AI summary
A hot dip galvanized steel sheet includes a base steel sheet and a hot dip galvanized layer on at least one surface of the base metal steel sheet, wherein the base steel sheet has a predetermined chemical composition, and contains, by volume fraction, ferrite: 0% to 50%, retained austenite: 0% to 30%, tempered martensite: 5% or more, fresh martensite: 0% to 10%, and pearlite and cementite in total: 0% to 5%, when there are remaining structures, the remaining structures consist of bainite, a concentration of B atoms at prior austenite grain boundaries is 2.0 atm % or more, and an average effective crystal grain size is 7.0 μm or less, and a method for producing the same.