High Strength Steel Sheet Delayed Fracture Resistance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High strength steel sheets with high ductility and delayed fracture resistance are needed for vehicle components like bumper reinforcing members and shock absorbers, but existing methods increase the fraction of retained austenite, leading to delayed fracture due to internal stress and hydrogen diffusion.
Innovation Solution
A cold rolled steel sheet and hot dip galvanized steel sheet with a composition of 0.05 to 0.3% C, 0.3 to 1.6% Si, 4.0 to 7.0% Mn, 0.5 to 2.0% Al, 0.01 to 0.1% Cr, 0.02 to 0.1% Ni, 0.005 to 0.03% Ti, 5 to 30 ppm B, and 0.01 to 0.03% Sb, processed through hot finish rolling, coiling, pickling, and either continuous annealing or batch annealing to achieve a tensile strength of 980 MPa or more and elongation of 28% or more, with improved delayed fracture resistance by adding Al to stabilize retained austenite.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength and high elongation are increased by increasing the fraction of retained austenite, then tensile strength and ductility are improved, but delayed fracture resistance deteriorates due to internal stress and hydrogen diffusion
Solution Approach 1:
The patent changes the chemical composition parameters by adding specific elements (Al: 0.01-2.0%, Ti: 0.003-0.06%, B: 0.003-0.03%, Nb: 0.003-0.06%, V: 0.003-0.06%) to modify the properties of retained austenite. This allows achieving high tensile strength (980 MPa or more) and elongation (28% or more) while improving delayed fracture resistance through controlled parameter changes in the steel composition
Solution Approach 2:
The patent creates a composite microstructure consisting of retained austenite (20-40% by area ratio) combined with martensite and/or bainite. This composite structure at the micro level provides both high strength and improved delayed fracture resistance by combining the ductility benefits of retained austenite with the strength characteristics of martensite/bainite
2Weight of moving object
If high strength steel is used for vehicle components, then vehicle weight is reduced, but delayed fracture occurs due to hydrogen diffusion in retained austenite
Solution Approach 1:
The patent converts the harmful effect of retained austenite (which normally promotes delayed fracture) into a beneficial feature by controlling its fraction (20-40%) and stabilizing it through alloying elements. The retained austenite continues to provide ductility through TRIP effect while the harmful delayed fracture is suppressed by the optimized composition and microstructure
Solution Approach 2:
The patent introduces intermediary elements (Al, Ti, B, Nb, V) that act as mediators between the retained austenite and hydrogen. These elements modify the chemical and physical properties of the steel matrix, reducing hydrogen diffusion rates and trapping hydrogen, thereby preventing delayed fracture while maintaining the benefits of retained austenite
3Ease of operation
If conventional manufacturing methods are used to produce steel with high retained austenite, then strength and ductility are improved, but delayed fracture resistance remains poor
Solution Approach 1:
The patent optimizes multiple parameters simultaneously: chemical composition (adding Al, Ti, B, Nb, V), microstructure (controlling retained austenite fraction at 20-40%), and heat treatment conditions (cooling rates, annealing temperatures). This multi-parameter optimization achieves both excellent formability through drawing and high delayed fracture resistance, overcoming the limitations of conventional single-parameter approaches
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 sheets exhibit enhanced tensile strength, elongation, and delayed fracture resistance, allowing for increased ductility and stability, making them suitable for vehicle components that require bending and drawing without significant crack formation, thus improving vehicle safety and lightweight body stability.
Implementation Method 1
adding Al to stabilize retained austenite
Implementation Method 2
the retained austenite formed as above is caused to transform into martensite during plastic deformation, thereby increasing strength as well as ductility by alleviating stress concentration
Implementation Method 3
processed through hot finish rolling, coiling, pickling, and either continuous annealing or batch annealing
Implementation Method 4
The delayed fracture is generally in the form of cracks, which are caused by the diffusion of hydrogen atoms or molecules under high residual stress
Data Source
AI summary
A cold rolled steel sheet and a hot dip galvanized steel sheet, which have high strength and elongation, such as a tensile strength of 980 MPa or more and an elongation of 28% or more, and excellent delayed fracture resistance, and manufacturing methods thereof. The cold rolled steel sheet has a composition including 0.05 to 0.3 weight percent C, 0.3 to 1.6 weight percent Si, 4.0 to 7.0 weight percent Mn, 0.5 to 2.0 weight percent Al, 0.01 to 0.1 weight percent Cr, 0.02 to 0.1 weight percent Ni and 0.005 to 0.03 weight percent Ti, 5 to 30 ppm B, 0.01 to 0.03 weight percent Sb, 0.008 weight percent or less S, balance Fe and impurities. The hot dip galvanized steel sheet has a hot dip galvanized layer or a hot dip galvannealed layer on the cold rolled steel sheet.