Hot-Pressed Steel Microstructure for Delayed Fracture Resistance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High-strength steel sheets with tensile strength of 1780 MPa or more face challenges in delayed fracture resistance and cross tensile strength after hot pressing, particularly due to low ductility and residual stress, which are exacerbated by alloying elements that reduce joint strength during resistance spot welding.
Innovation Solution
The dispersion of fine Ti-based precipitates on the surface layer and the precipitation of cementite in martensite as hydrogen trapping sites, combined with optimized alloy compositions and processing methods, enhance both delayed fracture resistance and cross tensile strength by refining microstructure and reducing residual stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheets with tensile strength of 1780 MPa or more are used, then strength is improved, but delayed fracture resistance deteriorates due to residual stress and low ductility
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.23-0.38%, Si: 0.01-1.50%, Mn: 1.50-3.00%, P: 0.005-0.050%, S: 0.005-0.030%, Al: 0.01-0.50%, Ti: 0.005-0.050%) and microstructural parameters (prior austenite grain size: 5-15 μm, martensite volume fraction: 80-100%, cementite grain size: 0.05-1.0 μm) to achieve a balance between high tensile strength and delayed fracture resistance. The hot pressing process parameters (heating temperature: 800-1000°C, holding time: 5-120 seconds, cooling rate: 10-100°C/s) are also optimized to control residual stress and microstructure, resolving the contradiction between strength and reliability.
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite as the matrix phase with dispersed cementite particles (0.05-1.0 μm) and Ti-based precipitates. This composite structure combines the high strength of martensite with the hydrogen-trapping capability of cementite and Ti precipitates, achieving both tensile strength of 1780 MPa or more and improved delayed fracture resistance by preventing hydrogen embrittlement.
2Strength
If cold press forming is used to achieve high strength, then strength is improved, but formability deteriorates due to low ductility and large spring back
Solution Approach 1:
The patent utilizes phase transitions by heating the steel sheet to the austenite region (800-1000°C) where the material becomes soft and formable, performing the forming operation in the austenite phase, and then rapidly cooling to transform to martensite, achieving high strength. This phase transition approach allows easy forming followed by strength enhancement, resolving the contradiction between formability and strength.
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
This approach stabilizes high tensile strength and improves delayed fracture resistance and cross tensile strength, ensuring no cracking even in hydrochloric acid immersion and maintaining high cross tensile strength after resistance spot welding, with a tensile strength of 1780 MPa or more and cross tensile strength of 5 kN or more.
Implementation Method 1
Ti-based precipitates which serve as a hydrogen trapping site for hydrogen entering from the surface
Implementation Method 2
precipitation of cementite in martensite as hydrogen trapping sites
Implementation Method 3
heating a steel sheet to the temperature range of austenite single phase and then forming (processing) the steel sheet at the high temperature, and that enables increase of the strength through quenching by cooling the steel sheet after the forming
Implementation Method 4
increase of the strength through quenching by cooling the steel sheet after the forming
Implementation Method 5
since the residual stress after press forming is reduced as compared with cold pressing, the delayed fracture resistance is also improved
Data Source
AI summary
Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing of 1780 MPa or more, excellent delayed fracture resistance, and high cross tensile strength after resistance spot welding by properly adjusting its chemical composition and its microstructure such that a prior austenite average grain size is 8 μm or less, a volume fraction of martensite is 90% or more, and at least 10 cementite grains having a grain size of 0.05 μm or more are present on average per 200 μm2 of a cross section parallel to a thickness direction of the member, and such that at least 10 Ti-based precipitates having a grain size of less than 0.10 μm are present on average per 100 μm2 of the cross section parallel to the thickness direction of the member in a range of 100 μm in the thickness direction from a surface of the member.