High Strength Steel Plate Formability and Galvanizing Compatibility
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High tensile steel sheets with high strength face challenges in formability and galvanizing treatment compatibility, particularly in achieving sufficient elongation and preventing delayed fracture, while maintaining suitable yield stress and weldability.
Innovation Solution
A high tensile steel sheet with specific composition ranges of C, Si, Mn, Al, and other elements, along with a method involving hot rolling, acid pickling, cold rolling, continuous annealing, and temper rolling, to establish a relation between Al and Si content and hardness distribution, ensuring compatibility of formability and galvanizing treatment properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of the steel sheet is increased to achieve weight reduction, then the tensile strength is improved, but the formability deteriorates due to increased yield stress and reduced elongation
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by strictly controlling the content ranges of C (0.15-0.35%), Si (0.01-1.50%), Mn (1.50-3.50%), and Al (0.01-1.50%), along with establishing specific relational expressions between these elements. This parameter optimization enables the steel to achieve high tensile strength (≥980 MPa) while maintaining adequate formability by balancing the competing effects of strength and ductility.
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases including martensite, bainite, and retained austenite. This multi-phase composite structure allows the steel to simultaneously achieve high strength from the martensite phase and good formability through the ductile retained austenite phase that can undergo TRIP effect during deformation.
2Strength
If a large amount of C is contained in the steel sheet to increase strength, then the tensile strength is improved, but welding quality deteriorates due to nugget cracking
Solution Approach 1:
The invention optimizes the carbon content parameter to a specific range (0.15-0.35%) that is high enough to achieve the required tensile strength (≥980 MPa) but controlled to prevent excessive carbon from causing severe nugget cracking during welding. This balanced parameter selection resolves the contradiction between strength and weldability.
3Strength
If the yield stress is increased to achieve high strength, then the tensile strength is improved, but shape fixability deteriorates during press forming
Solution Approach 1:
The invention adjusts the chemical composition parameters, particularly the Si content (0.01-1.50%) and Mn content (1.50-3.50%), along with establishing relational expressions between these elements and the strength parameters. This composition optimization enables the steel to achieve high tensile strength while maintaining adequate shape fixability during press forming by controlling the yield stress within an appropriate range.
4Ease of operation
If a large amount of retained austenite is contained to improve elongation, then the formability is improved, but delayed fracture occurs due to hydrogen accumulation at interfaces
Solution Approach 1:
The invention optimizes the chemical composition parameters, particularly the Al content (0.01-1.50%) and Si content (0.01-1.50%), and establishes specific relational expressions between these elements and the retained austenite content. This composition control enables the steel to achieve adequate elongation (≥10%) while minimizing delayed fracture by controlling the amount and distribution of retained austenite to reduce hydrogen trapping sites.
5Strength
If the cooling speed after recrystallization annealing is increased to manufacture DP steel, then the tensile strength is improved, but application to common manufacturing lines becomes difficult
Solution Approach 1:
The invention changes the chemical composition parameters, particularly increasing the Si content (0.01-1.50%) and Mn content (1.50-3.50%), and establishes relational expressions that enable the steel to achieve high tensile strength (≥980 MPa) through a more gradual cooling process after annealing. This composition optimization allows the use of conventional cooling speeds (≥3°C/s) that are compatible with common continuous annealing manufacturing lines, eliminating the need for extremely high cooling speeds (≥30°C/s) required by traditional DP steel.
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 enables compatible formability and galvanizing treatment properties, enhancing elongation and tensile strength while reducing yield stress and preventing delayed fracture, as demonstrated by improved performance in side bend tests and tensile strength measurements.
Implementation Method 1
next, performing continuous annealing of the cold-rolled steel strip in a continuous annealing line
Implementation Method 2
a metal structure includes a ferrite and a martensite
Implementation Method 3
performing acid pickling of the hot-rolled steel strip
Data Source
Figure 1~2
Figure 3~4
Figure 5
AI summary
With regard to an Al content (%) and a Si content (%), a relation of a formula (A) is established, and an average value Yave defined by a formula (B) regarding hardnesses measured at 100 points or more with a nanoindenter is equal to or more than 40. 0.3≤0.7×Si+Al≤1.5 Yave=Σ180×Xi-3-2/n ([A1] indicates the Al content (%), [Si] indicates the Si content (%), n indicates a total number of the measuring points of the hardnesses, and Xi indicates the hardness (GPa) at the i-th measuring point (i is a natural number equal to or less than n).