Hot-Dip Galvanized Steel Sheet With Mn Interface Control
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Solution Overview
Problem
Hot dip galvanized steel sheets used in automobiles lack sufficient press formability and low temperature toughness, particularly in cold regions, as existing production methods lead to excessive tempering of martensite and bainite, compromising material quality.
Innovation Solution
A method involving continuous hot dip galvanization with isothermal holding between 480°C to 600°C before coating, controlling Mn concentration at phase interfaces, and optimizing casting conditions to segregate Mn, thereby stabilizing retained austenite and tempering martensite, enhancing both strength and ductility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the steel sheet is heated to Ac1 or more, rapidly cooled down to Ms or less, then reheated to the bainite transformation temperature region and held to stabilize austenite, and then reheated to the coating bath temperature or alloying treatment temperature for galvannealing, then the austenite is stabilized and the coating process can be completed, but the martensite and bainite are excessively tempered in the coating and alloying step, causing poor material quality
Solution Approach 1:
The patent applies preliminary action by performing the isothermal holding treatment at 480-600°C to stabilize retained austenite and control Mn concentration distribution before the coating and alloying treatment. This preliminary microstructure control prevents excessive tempering during subsequent coating processes, thereby maintaining material quality while completing the necessary manufacturing steps.
2Strength
If high strength steel sheet is used to lighten the weight of car bodies and secure collision safety, then the tensile strength increases to 980 MPa or more, but the press formability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the microstructure parameters (volume fractions of ferrite, retained austenite, martensite, and bainite) and chemical composition parameters (C, Si, Mn, P, S, Al, Ti, B contents) to achieve a balance between high tensile strength (980 MPa or more) and press formability. The specific microstructure composition and Mn concentration distribution enable both high strength and excellent elongation and hole expansion rates.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, retained austenite, martensite, and bainite) with specific volume fractions. This composite microstructure combines the high strength of martensite with the ductility and press formability contributions from ferrite and retained austenite, achieving both high tensile strength and excellent press formability.
3Reliability
If the Mn concentration at the interfaces of different phases is increased to improve low temperature toughness, then the low temperature toughness improves, but the press formability may be affected
Solution Approach 1:
The patent applies local quality by creating a non-uniform Mn concentration distribution where the Mn concentration at the interfaces of different phases is specifically enriched (with a ratio of Mn concentration at interfaces to average Mn concentration of 1.05 or more). This local Mn enrichment at phase interfaces improves low temperature toughness by preventing brittle fracture, while the overall microstructure composition maintains press formability.
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 approach results in hot dip galvanized steel sheets with improved press formability, ductility, and low temperature toughness, meeting the requirement of tensile strength of 980 MPa or more while maintaining excellent hole expandability.
Implementation Method 1
a number ratio of tempered martensite with a Mn concentration profile satisfying the following formulas (1) and (2) is 0.2 or more with respect to the total number of the tempered martensite: wherein [Mn] is the Mn content in mass% in the base steel sheet, [Mn]a is the average Mn concentration in mass% in the tempered martensite, and [Mn]b is the Mn concentration in mass% at the interfaces of different phases of the tempered martensite and ferrite phase and bainite phase
Implementation Method 2
PTL 5 describes that the steel sheet is heated to Ac1 or more, is then rapidly cooled down to the martensite transformation start temperature (Ms) or less, is then reheated to the bainite transformation temperature region and held at the temperature region to stabilize the austenite (austemper it)
Implementation Method 3
PTL 5 describes that the steel sheet is heated to Ac1 or more, is then rapidly cooled down to the martensite transformation start temperature (Ms) or less
Implementation Method 4
is then reheated to the bainite transformation temperature region and held at the temperature region to stabilize the austenite (austemper it)
Data Source
Figure 1
Figure 2
AI summary
Provided are a hot dip galvanized steel sheet comprising a base steel sheet wherein the base steel sheet has a predetermined chemical composition, and contains ferrite: 50% or less, retained austenite: 30% or less, tempered martensite: 5% or more, fresh martensite: 10% or less, and pearlite and cementite in total: 5% or less, remaining structures consist of bainite, and a number ratio of tempered martensite with a Mn concentration profile satisfying [Mn]b /[Mn]a >1.2 and [Mn]a/[Mn]<2.0 ([Mn] is the Mn content in the base steel sheet, [Mn]a is the average Mn concentration in the tempered martensite, and [Mn]b is the Mn concentration at the interfaces of different phases of the tempered martensite and ferrite phase and bainite phase) is 0.2 or more with respect to the total number of tempered martensite, and a method for producing the same.