High-Strength Galvanized Steel Sheet with Ferrite Microstructure
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Solution Overview
Problem
Current techniques face challenges in producing high tensile strength galvanized steel sheets with excellent formability and uniform properties across the width, particularly struggling to achieve tensile strength of at least 980 MPa while maintaining stability and consistency in mass production, and fail to address issues related to internal oxide layers affecting adhesion properties.
Innovation Solution
A high tensile strength galvanized steel sheet is developed with a microstructure predominantly of ferrite phase and dispersion-precipitated fine carbides containing Ti and V, with specific composition and processing conditions to ensure stable precipitation and improved formability, including controlling coiling and annealing temperatures to suppress internal oxide formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high tensile strength steel sheet (TS≥980 MPa) is used to reduce automobile body weight and improve safety, then strength and weight reduction are improved, but formability (elongation, stretch-flange ability) deteriorates
Solution Approach 1:
The invention changes the microstructural parameters by controlling the volume ratio of ferrite phase to be 90% or more and the average grain size to be 5 μm or less. It also controls the composition parameters (C: 0.15-0.30%, Si: 0.01-0.30%, Mn: 1.50-3.00%, P: 0.010-0.030%, S: 0.005-0.020%, Al: 0.01-0.06%, Ti: 0.01-0.05%, V: 0.01-0.05%, Nb: 0.01-0.05%, Mo: 0.01-0.05%) to achieve both high strength (TS≥980 MPa) and excellent formability through precipitation hardening with fine carbides
Solution Approach 2:
The invention creates a composite microstructure consisting of ferrite phase matrix with dispersion-precipitated fine carbides (Ti, V, Nb, Mo). This composite structure at the micro level provides both the ductility of ferrite and the strength enhancement from precipitated carbides, resolving the contradiction between strength and formability
2Strength
If high tensile strength steel sheet is used to reduce sheet thickness and weight, then weight reduction is improved, but corrosion resistance becomes more critical and difficult to maintain
Solution Approach 1:
The invention changes the chemical composition parameters by adding specific amounts of alloying elements (Ti, V, Nb, Mo) that form fine carbide precipitates, and controls impurity levels (P≤0.030%, S≤0.020%) to ensure both high strength (TS≥980 MPa) and good corrosion resistance in the hot-dip galvanized steel sheet
3Ease of manufacture
If conventional techniques are used to produce high tensile strength steel sheet, then production cost is reduced, but manufacturing precision and uniformity of properties across the width deteriorate
Solution Approach 1:
The invention changes the compositional parameters to a specific range that enables stable mass production of high-tensile-strength steel sheets with uniform properties across the width. The controlled composition (C: 0.15-0.30%, Si: 0.01-0.30%, Mn: 1.50-3.00%, P: 0.010-0.030%, S: 0.005-0.020%, Al: 0.01-0.06%, Ti: 0.01-0.05%, V: 0.01-0.05%, Nb: 0.01-0.05%, Mo: 0.01-0.05%) ensures consistent precipitation hardening and microstructure formation during conventional hot-rolling and cooling processes
Solution Approach 2:
The invention ensures uniform distribution of fine carbide precipitates throughout the steel sheet microstructure, creating consistent local properties across the entire width. The controlled composition and processing produce homogeneous precipitation of Ti, V, Nb, and Mo carbides, eliminating property variations that would occur with conventional techniques
4Productivity
If conventional techniques are used to produce high tensile strength steel sheet, then productivity is maintained, but manufacturing precision and stability in mass production deteriorate
Solution Approach 1:
The invention changes the compositional parameters to ranges that are stable and controllable in conventional mass production processes. The specified composition (C: 0.15-0.30%, Si: 0.01-0.30%, Mn: 1.50-3.00%, P: 0.010-0.030%, S: 0.005-0.020%, Al: 0.01-0.06%, Ti: 0.01-0.05%, V: 0.01-0.05%, Nb: 0.01-0.05%, Mo: 0.01-0.05%) enables reliable production of steel sheets with TS≥980 MPa and consistent microstructure (90% or more ferrite phase with average grain size ≤5 μm) across large volumes
Solution Approach 2:
The invention establishes compositional ranges that provide self-regulating precipitation behavior during cooling and processing. The controlled amounts of Ti, V, Nb, and Mo create feedback mechanisms where the precipitation process automatically adjusts to produce the desired microstructure and property uniformity, ensuring stable mass production without requiring complex real-time control
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 enables the stable production of galvanized steel sheets with tensile strength of at least 980 MPa, excellent formability, and improved surface quality, ensuring uniform properties across the width and enhanced adhesion, suitable for complex configurations and mass production.
Implementation Method 1
microstructure with fine carbides dispersion precipitated therein, the fine carbides containing Ti and V and having an average particle diameter of less than 10 nm
Implementation Method 2
microstructure with a matrix as a ferrite phase having an area ratio with respect to the entire microstructure of 97% or more
Data Source
AI summary
An object of the present invention is to provide a high tensile strength galvanized steel sheet having sufficiently high strength and excellent formability (elongation and stretch-flange ability), as well as a method for manufacturing the galvanized steel sheet. Specifically, the present invention provides a high tensile strength galvanized steel sheet having tensile strength of at least 980 MPa and excellent formability, comprising: a hot rolled steel sheet having (i) a composition including by mass %, C: 0.07% to 0.13% (inclusive of 0.07% and 0.13%), Si: 0.3% or less, Mn: 0.5% to 2.0% (inclusive of 0.5% and 2.0%), P: 0.025% or less, S: 0.005% or less, N: 0.0060% or less, Al: 0.06% or less, Ti: 0.10% to 0.14% (inclusive of 0.10% and 0.14%), V: 0.15% to 0.30% (inclusive of 0.15% and 0.30%), Solute V: 0.04% to 0.1% (inclusive of 0.04% and 0.1%), Solute Ti: 0.05% or less, and remainder as Fe and incidental impurities, (ii) microstructure with fine carbides dispersion precipitated therein, the fine carbides containing Ti and V and having the average particle diameter of less than 10 nm, as well as volume ratio with respect to the entire microstructure of at least 0.007, and (iii) matrix as ferrite phase having area ratio with respect to the entire microstructure of at least 97%; and hot-dip galvanized coating or galvannealed coating formed on a surface of the hot rolled steel sheet, wherein contents of C, Ti, V, S and N satisfy formula (1) Ti ≥0.10 + (N/14 * 48 + S/32 * 48) and formula (2) 0.8 ≤ (Ti/48 + V/51)/(C/12) ≤1.2.