Steel Sheet Surface Layer for Ductility and Hole Expansibility
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Solution Overview
Problem
High strength steel sheets used in vehicle components face challenges in achieving a balance between ductility, hole expansibility, fatigue resistance, bendability, and plating adhesion, with existing technologies falling short in providing a comprehensive solution.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure is developed, featuring a lath-like martensite structure inside and a soft ferrite surface layer with dispersed retained austenite, achieved through two heat treatments and hot-dip galvanizing, which enhances ductility, hole expansibility, fatigue resistance, and plating adhesion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If dual phase steel containing ferrite and martensite is used to improve press formability, then ductility is improved, but hole expansibility deteriorates due to hard phase serving as void formation origin
Solution Approach 1:
The patent applies local quality by creating a surface layer with a different microstructure (ferrite and retained austenite) from the base material (bainite or bainitic ferrite). This surface layer with thickness of 5μm to 50μm provides improved ductility and hole expansibility while the bainitic base material maintains high strength, thus resolving the contradiction between ductility and hole expansibility.
Solution Approach 2:
The patent creates a composite microstructure consisting of a surface layer containing ferrite and retained austenite on a base material of bainite or bainitic ferrite. This composite structure combines the benefits of different phases: the ferrite-austenite surface layer provides ductility and hole expansibility, while the bainitic base material provides high strength, thereby resolving the technical contradiction.
2Stability of the object's composition
If TRIP steel containing austenite is used to improve ductility, then ductility is improved, but hole expansibility deteriorates and chemical convertibility and plating adhesion worsen due to large amount of Si alloy addition
Solution Approach 1:
The patent concentrates the austenite phase in a surface layer of thickness 5μm to 50μm, rather than distributing it throughout the entire material. This localized austenite layer provides the necessary ductility through TRIP effect, while the bulk material maintains bainitic structure that ensures good hole expansibility and plating adhesion, avoiding the need for excessive Si alloy addition.
Solution Approach 2:
The patent controls the volume fraction of retained austenite in the surface layer to be 5% to 50%, and the surface layer thickness to be 5μm to 50μm. By precisely controlling these parameters, the patent achieves improved ductility through TRIP effect while maintaining adequate hole expansibility and plating adhesion, avoiding the drawbacks of conventional TRIP steel with excessive Si content.
3Ease of operation
If decarburization treatment is performed to improve bending workability, then bendability is improved, but fatigue resistance deteriorates
Solution Approach 1:
The patent creates a localized soft surface layer with ferrite and retained austenite structure that is 5μm to 50μm thick. This surface layer provides excellent bendability and formability, while the underlying bainitic base material maintains high strength and fatigue resistance. This resolves the contradiction by providing different properties at different depths of the material.
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 steel sheet exhibits excellent ductility, hole expansibility, fatigue resistance, and plating adhesion, making it suitable for various vehicle components formed through press working.
Implementation Method 1
achieved through two heat treatments and hot-dip galvanizing
Implementation Method 2
a steel structure in a range from 1⁄8 thickness to 3⁄8 thickness centered on a 1⁄4 thickness position from a surface contains, by volume fraction, soft ferrite: 0% to 30%, retained austenite: 3% to 40%, fresh martensite: 0% to 30%
Implementation Method 3
form an internal oxide layer containing Si oxides at a predetermined depth
Implementation Method 4
achieved through two heat treatments and hot-dip galvanizing, which enhances ductility, hole expansibility, fatigue resistance, and plating adhesion
Data Source
AI summary
A steel sheet has a chemical composition with a steel structure containing, by volume fraction, soft ferrite: 0-30%, retained austenite: 3-40%, fresh martensite: 0-30%, pearlite and cementite: 0-10%, and a remainder including hard ferrite. In the steel sheet, a number proportion of retained austenite having an aspect ratio of 2.0 or more in the total retained austenite is 50% or more, and a soft layer having a thickness of 1-100 μm is present. In the soft layer, a volume fraction of ferrite grains having an aspect ratio of less than 3.0 is 50% or more, and a volume fraction of retained austenite is 50% or more of the volume fraction of the retained austenite of the inside of the steel sheet. A peak of an emission intensity at a wavelength indicating Si appears in a range of more than 0.2 μm to 5 μm or less from the surface.