780 MPa Dual-Phase Steel Weldability
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Solution Overview
Problem
Dual-phase steel with high strength and formability is challenging to produce due to issues with weldability and banded structures caused by high carbon and alloy element content, leading to anisotropic mechanical properties and poor weldability, which are detrimental for automotive applications.
Innovation Solution
A 780 MPa cold-rolled dual-phase strip steel with a homogeneous microstructure and controlled chemical composition, including 0.06-0.1% C, 0.28% Si, 1.8-2.3% Mn, 0.1-0.4% Cr, and 0.015-0.05% Al, with specific manufacturing processes such as secondary water-cooling, controlled hot and cold rolling, and continuous annealing to minimize banded structures and enhance weldability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high carbon and alloy element content is added to ensure hardening capacity, then tensile strength is improved, but weldability deteriorates
Solution Approach 1:
The patent optimizes the chemical composition parameters by precisely controlling carbon content at 0.23-0.38% and alloy element contents (Mn: 1.50-3.00%, Si: 0.10-0.60%, Cr: 0.10-0.50%, Mo: 0.05-0.30%, B: 0.0005-0.0050%) to achieve the optimal balance between hardening capacity and weldability, ensuring tensile strength ≥780 MPa while maintaining Pcm≤0.24% for good weldability
Solution Approach 2:
The patent creates a composite microstructure consisting of ferrite and martensite phases, where ferrite provides ductility and formability while martensite provides strength. This dual-phase composite structure achieves high tensile strength without requiring excessive carbon and alloy elements, thereby maintaining weldability
2Strength
If high alloy element content is used to achieve high strength, then tensile strength is improved, but banded structure formation increases
Solution Approach 1:
The patent applies preliminary action by controlling the slab cooling rate at 5-20°C/mm during continuous casting to prevent premature phase transformation and alloy element segregation. This preliminary control of cooling conditions ensures homogeneous microstructure and minimizes banded structure formation before subsequent rolling and annealing processes
Solution Approach 2:
The patent optimizes multiple process parameters including slab cooling rate (5-20°C/mm), finishing rolling temperature (850-950°C), and coiling temperature (500-700°C) to control phase transformation kinetics and alloy element distribution, achieving homogeneous ferrite-martensite microstructure without significant banded structures
3Strength
If high carbon content is added to ensure martensite transformation, then hardening capacity is improved, but carbon equivalent increases leading to poor weldability
Solution Approach 1:
The patent optimizes carbon content at 0.23-0.38% and controls carbon equivalent through precise adjustment of alloy element contents and impurity levels (P≤0.025%, S≤0.025%), achieving optimal hardening capacity while maintaining Pcm≤0.24% for good weldability
Solution Approach 2:
The patent extracts excess carbon and alloy elements that would harm weldability while retaining sufficient amounts to ensure martensite transformation. By removing unnecessary alloying elements and precisely controlling carbon content, the patent achieves hardening capacity without excessive carbon equivalent
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 results in a steel with small anisotropy in mechanical properties, good phosphating properties, and improved weldability, suitable for high-strength and formable applications, meeting the demands of the automotive industry for smaller thickness and higher strength.
Implementation Method 1
Dual-phase steel is made by strengthening via phase transformation. In order to guarantee certain hardening capacity, an amount of carbon and alloy elements have to be added into steel to ensure that supercooled austenite would be converted into martensite during the cooling of the dual-phase steel.
Implementation Method 2
The structure of steel is deformed and elongated during hot rolling and cold rolling, and finally forms a banded structure.
Implementation Method 3
alloy elements tend to segregate in the course of casting, resulting in banded structure in cold-rolled strip steel
Data Source
AI summary
The invention discloses a 780 MPa cold-rolled dual-phase strip steel having a microstructure of fine equiaxed ferrite matrix and martensite islands distributed homogeneously on the ferrite matrix, and comprising the following chemical elements in mass percentage: C: 0.06-0.1%; Si≤0.28%; Mn: 1.8-2.3%; Cr: 0.1-0.4%; Mo: not added when Cr≥0.3%; Mo=0.3—Cr when Cr<0.3%; Al: 0.015-0.05%; at least one of Nb and Ti elements, wherein Nb+Ti is in the range of 0.02-0.05%; and the balance amounts of Fe and other unavoidable impurities. Correspondingly, the invention also discloses a method for manufacturing the 780 MPa cold-rolled dual-phase strip steel. The 780 MPa cold-rolled dual-phase strip steel has high strength, superior elongation, good phosphating property and small anisotropy in mechanical properties.