Dual-Phase Steel Composition for 780 MPa Strength Without Cr or Mo
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Solution Overview
Problem
Existing dual-phase steels with high strength, such as 780 MPa grade, often have high contents of elements like Mn and Si, which lead to nonuniform mechanical properties, poor weldability, and increased costs, while also affecting surface quality and phosphatability.
Innovation Solution
A cold-rolled and annealed dual-phase steel with a composition of C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, and Ti: 0.01-0.03%, without Mo and Cr, designed to achieve a strength of 780 MPa with a fine and uniform martensite+ferrite structure, ensuring excellent elongation and cold bending performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high Mn content is added to achieve 780 MPa strength, then tensile strength is improved, but severe banded structure occurs leading to nonuniform mechanical properties
Solution Approach 1:
The patent optimizes the Mn content parameter to a specific range (1.5-2.5%) rather than using high Mn content, and combines it with controlled Si content (0.5-2.0%) and microalloying elements (Ti, Nb, V) to achieve the desired strength while preventing severe banding and ensuring uniform mechanical properties
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite and ferrite phases through controlled composition and processing, where the combination of multiple alloying elements (Mn, Si, Ti, Nb, V) works synergistically to achieve both high strength and uniformity without relying on high Mn content alone
2Strength
If high Si content is added along with high Mn to achieve 780 MPa strength, then tensile strength is improved, but surface quality and welding performance deteriorate
Solution Approach 1:
The patent controls Si content within a moderate range (0.5-2.0%) rather than using high Si content, and compensates for strength through optimized Mn content and microalloying elements, thereby maintaining both strength requirements and surface quality while improving welding performance
3Strength
If high total alloy content is used to achieve 780 MPa strength, then tensile strength is improved, but weldability deteriorates
Solution Approach 1:
The patent optimizes the total alloy content by controlling individual element ranges (C: 0.05-0.13%, Si: 0.5-2.0%, Mn: 1.5-2.5%, Cr: 0.05-1.0%, Mo: 0.05-0.6%) and using microalloying elements (Ti, Nb, V) in small amounts, achieving 780 MPa strength while maintaining good weldability through balanced composition rather than high total alloy content
Solution Approach 2:
The patent uses small amounts of microalloying elements (Ti: 0.005-0.05%, Nb: 0.005-0.05%, V: 0.005-0.2%) that provide significant strength contribution through precipitation hardening, reducing the need for higher total alloy content and thereby improving weldability
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 achieves a yield strength of ≥420 MPa, tensile strength of ≥780 MPa, elongation at break of ≥18%, and a 90-degree cold bending parameter R/t≤1, while maintaining economic efficiency and improved weldability and surface quality.
Implementation Method 1
a fine and uniform martensite+ferrite dual-phase structure is obtained
Implementation Method 2
annealing at a temperature of 700-900° C.
Data Source
AI summary
Disclosed is a cold-rolled and annealed dual-phase steel having a tensile strength of greater than 780 MPa. A matrix structure thereof is fine and uniform martensite+ferrite. The cold-rolled and annealed dual-phase steel contains the following chemical elements in the following mass percentages: C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, and Ti: 0.01-0.03%. Furthermore, the cold-rolled annealed dual-phase steel does not contain the elements Cr or Mo. In addition, also disclosed is a method for manufacturing the cold-rolled and annealed dual-phase steel, comprising smelting and continuous casting, hot rolling, cold rolling, annealing, tempering and flattening. The cold-rolled and annealed dual-phase steel of the present invention is not only economical, but also has the characteristics of high strength, excellent elongation and cold bending properties.