Hot-Pressed Steel Microstructure for Weld Crack Resistance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High-strength steel sheets with tensile strength of 1780 MPa or more used in automobile parts face issues with cracking during cold press forming due to low ductility and high yield strength, and resistance welding cracking and delayed fracture due to liquid metal embrittlement and hydrogen ingress.
Innovation Solution
Control the microstructure of hot-pressed members by dispersing fine Nb-based precipitates within 100 μm of the surface and forming a Ni diffusion layer to refine austenite grain size and improve toughness, thereby reducing resistance welding cracking and delayed fracture resistance.
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 ductility deteriorates causing cracking during cold press forming
Solution Approach 1:
The invention changes the microstructural parameters by controlling prior austenite grain size to 7 μm or less through specific heat treatment processes, and by controlling the volume fraction of martensite and retained austenite. This enables achieving tensile strength of 1780 MPa or more while maintaining sufficient ductility to prevent cracking during cold press forming.
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (martensite, retained austenite, and bainite) with specific volume fractions. This multi-phase composite structure provides both the required high strength and adequate ductility, resolving the contradiction between strength and formability.
2Ease of manufacture
If resistance spot welding is performed on steel sheets with Zn coating or plating, then ease of manufacture is improved, but resistance welding cracking occurs due to liquid metal embrittlement
Solution Approach 1:
The invention applies preliminary anti-action by controlling the microstructure before welding to have fine prior austenite grain size (7 μm or less) and specific phase distribution. This pre-conditioned microstructure resists liquid metal embrittlement during subsequent resistance spot welding, preventing cracking even when Zn coating is present.
Solution Approach 2:
The invention changes the microstructural parameters (prior austenite grain size, martensite volume fraction, retained austenite volume fraction) to specific ranges that inherently resist liquid metal embrittlement. This enables resistance spot welding of Zn-coated sheets without cracking while maintaining high strength.
3Ease of operation
If resistance spot welding is performed in the presence of sheet gap, then ease of operation is improved, but delayed fracture occurs due to hydrogen ingress at nugget ends
Solution Approach 1:
The invention applies preliminary anti-action by creating a microstructure with fine prior austenite grain size and specific phase distribution before welding. This pre-conditioned microstructure resists hydrogen ingress and delayed fracture even when welding is performed with sheet gap, maintaining reliability while improving operational flexibility.
4Productivity
If cold press forming is performed on high-strength steel sheets, then productivity is improved, but dimensional accuracy deteriorates due to large spring back
Solution Approach 1:
The invention changes the microstructural parameters (prior austenite grain size, phase composition) to achieve a balance between strength and ductility. This enables cold press forming to proceed with adequate dimensional accuracy while maintaining high productivity, reducing the need for post-forming adjustments.
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 effectively enhances the resistance to resistance welding cracking and delayed fracture while maintaining high tensile strength, preventing cracking even under varying hot pressing conditions and in corrosive environments.
Implementation Method 1
it is effective to control the prior austenite grain boundaries as the microstructure of the member, to disperse fine Nb-based precipitates in the surface layer of the member
Implementation Method 2
to have a Ni diffusion layer on the surface layer of the member
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
Implementation Method 4
enables increase of the strength through quenching by cooling the steel sheet after the forming
Data Source
AI summary
Disclosed is a hot-pressed member that can exhibit very high tensile strength after hot pressing as high as TS: 1780 MPa or more, excellent resistance to resistance welding cracking, and excellent delayed fracture resistance after resistance welding by having a specific chemical composition, and a microstructure such that a prior austenite average grain size is 7 μm or less, a volume fraction of martensite is 90% or more, and at least 5 Nb-based precipitates having a grain size of less than 0.08 μm are present on average per 100 μm2 of a cross section parallel to a thickness direction of the member within a range of 100 μm in the thickness direction from a surface of the member, and such that a Ni diffusion region having a thickness of 0.5 μm or more is present in a surface layer of the member.