Hot-Pressed Steel Surface Microstructure for Weld Crack Resistance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional techniques fail to improve the delayed fracture resistance of hot-pressed members with tensile strength of 1780 MPa or more at projection welds, particularly in the interface between nuts and steel sheets, due to residual stress and low ductility.
Innovation Solution
Incorporating Ti-based precipitates and cementite as hydrogen trapping sites in the surface layer of cold-rolled steel sheets before hot pressing, with specific chemical compositions and microstructural features to enhance toughness and resist delayed fracture.
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 temperature parameter during forming operations. By performing hot press forming instead of cold press forming, the steel sheet maintains high ductility at elevated temperatures while achieving the required high tensile strength of 1780 MPa or more after cooling, thereby preventing cracking during forming
Solution Approach 2:
The invention utilizes phase transition of steel between austenite and martensite. During hot press forming, the steel sheet is heated to the austenite region where it has high ductility for forming, then rapidly cooled to transform to martensite phase which provides the required high strength and prevents cracking
2Strength
If high-strength steel sheets are used, then strength is improved, but spring back increases due to high yield strength
Solution Approach 1:
The invention changes the temperature parameter during forming. By performing hot press forming, the steel sheet exhibits reduced yield strength at elevated temperatures, allowing for forming with minimal spring back and high dimensional accuracy, while achieving the required high strength properties after cooling
3Ease of manufacture
If cold press forming is performed on high-strength steel sheets, then forming is achieved, but residual stress remains causing delayed fracture
Solution Approach 1:
The invention changes the temperature parameter from cold to hot during press forming. Hot press forming reduces residual stress in the steel sheet, thereby improving delayed fracture resistance and reliability while maintaining the ability to achieve the required forming geometry
Solution Approach 2:
The invention converts the potentially harmful residual stress generated during forming into a beneficial low-residual-stress state by performing hot press forming. The high temperature during forming allows plastic deformation with minimal residual stress, and the subsequent cooling to martensite structure provides both strength and improved delayed fracture resistance
4Ease of manufacture
If projection welding is performed on hot-pressed members with high strength, then assembly is achieved, but delayed fracture occurs at the weld interface
Solution Approach 1:
The invention changes the microstructural parameters of the steel sheet by controlling composition (C: 0.28-0.40%, Si: 1.5-3.0%, Mn: 1.0-2.5%) and heat treatment to achieve a martensite volume fraction of 90-100% with fine grain size. This microstructure provides both high strength for assembly and improved delayed fracture resistance at projection weld interfaces
Solution Approach 2:
The invention creates a composite microstructure consisting of martensite as the primary phase with controlled grain size and distribution. This composite microstructure provides both the high strength required for assembly and the toughness needed to resist delayed fracture at projection weld interfaces
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 approach effectively traps hydrogen, preventing crack formation and improving delayed fracture resistance, even under hydrochloric acid immersion and bolt loading, while maintaining high tensile strength.
Implementation Method 1
it is effective to have Ti-based precipitates as hydrogen trapping sites and at the same time improve the toughness after projection welding, and to have fine cementite as hydrogen trapping sites in the prior austenite grains of the surface layer of the member
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 and excellent delayed fracture resistance after projection welding by properly adjusting its chemical composition and its microstructure such that at least 5 Ti-based precipitates having a grain size of 0.10 μm or less 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 a thickness direction from a surface of the member, a volume fraction of martensite is 95% to 100% within a depth range of 20 μm to 100 μm in the thickness direction from the surface of the member, and at least 10 cementite grains having a grain size of less than 0.20 μm are present on average in a prior austenite grain.