Galvannealed Steel Sheet With Ultrafine Ferrite And Retained Austenite
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Solution Overview
Problem
Existing high-strength steel sheets for vehicles face limitations in achieving ultra-high strength and high formability due to the constraints of the Rule of Mixture (ROM) and the low formability of transformation induced plasticity steels, particularly with dual-phase structures of ferrite and martensite, and transformation induced plasticity steels.
Innovation Solution
A hot-dip galvannealed steel sheet with a microstructure composed of ultrafine-grained ferrite and retained austenite, controlled through specific alloying elements and heat treatment processes, including primary and secondary heat treatments, to achieve a yield strength of 800 MPa or greater, tensile strength of 980 MPa or greater, elongation of 25% or greater, and a hole expansion ratio of 20% or greater.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If dual-phase steel structure (ferrite and martensite) is used to increase strength, then yield strength is improved, but elongation and formability deteriorate due to the Rule of Mixture limits
Solution Approach 1:
The patent applies parameter changes by precisely controlling the composition ranges of alloying elements (C: 0.05-0.15 wt%, Si: 0.01-1.00 wt%, Mn: 4.00-9.00 wt%, Al: 0.01-0.60 wt%) and heat treatment parameters (primary heat treatment at Ac3+5 to Ac3+20°C, secondary heat treatment at Ac1+10 to Ac3-10°C) to transform the microstructure from conventional dual-phase to ultrafine-grained ferrite with retained austenite, achieving both high strength and improved formability
Solution Approach 2:
The patent creates a composite microstructure consisting of ultrafine-grained ferrite matrix with dispersed retained austenite particles. This composite structure combines the high strength of ferrite with the ductility and formability enhancement from retained austenite, overcoming the limitations of conventional dual-phase steel
2Ease of operation
If transformation induced plasticity steel with retained austenite is used to improve elongation, then ductility is enhanced, but strength is reduced because the main matrix is composed of bainite
Solution Approach 1:
The patent changes the matrix phase from bainite to ultrafine-grained ferrite through precise control of heat treatment parameters (primary heat treatment temperature at Ac3+5 to Ac3+20°C, secondary heat treatment temperature at Ac1+10 to Ac3-10°C) and cooling rates, while maintaining high retained austenite content (10-30%), thereby achieving both high strength and high elongation simultaneously
3Ease of operation
If heat treatment processes are optimized to increase retained austenite content, then elongation and hole expansion ratio are improved, but manufacturing complexity increases
Solution Approach 1:
The patent applies preliminary action by performing primary heat treatment before secondary heat treatment to establish the initial microstructure with austenite formation, then followed by secondary heat treatment to achieve the final ultrafine-grained ferrite with retained austenite structure. This sequential approach simplifies the overall process control compared to attempting to achieve the target microstructure in a single heat treatment step
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 sheet exhibits enhanced strength and formability, with improved elongation and hole expansion ratio, overcoming the limitations of existing technologies by securing a large amount of retained austenite and minimizing cementite formation through controlled process conditions.
Implementation Method 1
a transformation induced plasticity steel that secures strength and an elongation through phase transformation of retained austenite in a final structure during plastic deformation
Implementation Method 2
the base steel sheet has a microstructure composed of ferrite and retained austenite
Implementation Method 3
performing a galvannealing treatment on the cold-rolled sheet on which the hot-dip galvanized layer is formed
Data Source
AI summary
Disclosed are a hot-dip galvannealed steel sheet with ultra-high strength and high formability, and a manufacturing method therefor. In an exemplary embodiment, a hot-dip galvannealed steel sheet include: a base steel sheet; and a hot-dip galvannealed layer formed on the surface of the base steel sheet. The base steel sheet includes an amount of 0.05 to 0.15 wt % of carbon (C), an amount greater than 0 and less than or equal to 1.0 wt % of silicon (Si), an amount of 4.0 to 9.0 wt % of manganese (Mn), an amount greater than 0 and less than or equal to 0.6 wt % of aluminum (Al), an amount greater than 0 and less than or equal to 0.02 wt % of phosphorus (P) in, an amount greater than 0 and less than or equal to 0.005 wt % of sulfur (S), an amount greater than 0 and less than or equal to 0.006 wt % of nitrogen (N), and the balance of iron (Fe) and other inevitable impurities. The base steel sheet has a microstructure consisting of ferrite and retained austenite; the grain size of the microstructure is 3 μm or less; and the hot-dip galvannealed steel sheet has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.