High-Strength Galvanized Steel Sheet With Balanced Martensite-Austenite
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Solution Overview
Problem
Existing high strength galvanized steel sheets face challenges in achieving yield strengths of 1,000 MPa while maintaining good workability, collision proof stress, and crack stopping properties, which are essential for automotive applications.
Innovation Solution
A high strength galvanized steel sheet with specific chemical composition and microstructure, including controlled amounts of elements like C, Si, Mn, P, S, Al, O, N, and others, along with a tailored heat treatment process involving multiple cooling stages and a galvanizing treatment, to achieve yield strength of 1,000 MPa, improved workability, and enhanced crack stopping properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the yield strength of the steel sheet is increased to 1,000 MPa or higher, then the strength is improved, but the workability deteriorates
Solution Approach 1:
The invention applies parameter changes by precisely controlling chemical composition parameters (C: 0.150-0.450%, Si: 0.50-3.00%, Mn: 1.50-4.00%, etc.) and microstructure parameters (area fraction of retained austenite: 5-30%, total area fraction of tempered martensite and bainite: 55-95%) to achieve yield strength of 1,000 MPa or higher while maintaining adequate workability through the balanced microstructure
2Strength
If the yield strength of the steel sheet is increased to 1,000 MPa or higher, then the strength is improved, but the collision proof stress deteriorates
Solution Approach 1:
The invention creates a composite microstructure consisting of multiple phases (tempered martensite, bainite, and retained austenite) with specific area fractions. This composite microstructure provides both high yield strength (1,000 MPa or higher) and adequate collision proof stress by combining the strength of martensite with the toughness contribution from retained austenite (5-30% area fraction)
3Strength
If the yield strength of the steel sheet is increased to 1,000 MPa or higher, then the strength is improved, but the crack stopping property deteriorates
Solution Approach 1:
The invention controls the microstructure parameters, specifically maintaining the area fraction of retained austenite at 5-30% and the total area fraction of tempered martensite and bainite at 55-95%, along with controlling the presence ratio X/Y between 0.5-2.5. This balanced microstructure achieves high yield strength (1,000 MPa or higher) while maintaining adequate crack stopping property through the toughness contribution of retained austenite and the crack-arresting capability of the tempered martensite and bainite phases
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 sheet with excellent workability, collision proof stress, and crack stopping properties, suitable for automotive parts, ensuring sufficient strength and durability against collisions.
Implementation Method 1
in the heat treatment, the cold rolled steel sheet is heated at a heating temperature T1 of 800° C. to 950° C., in the first cooling, the cold rolled steel sheet is cooled from the heating temperature T1 to a temperature T2 not lower than (Ms point+100)° C. and not higher than (Ms point+600)° C.
Implementation Method 2
in the first cooling, the cold rolled steel sheet is cooled from the heating temperature T1 to a temperature T2 not lower than (Ms point+100)° C. and not higher than (Ms point+600)° C. at an average cooling rate v1 of not lower than 10° C./s
Implementation Method 3
in the second cooling, the cold rolled steel sheet is cooled while an average cooling rate v2 in a temperature range T3 of not lower than Ms point and not higher than (Ms point+100)° C. is 0.1 to 5.0° C./s
Implementation Method 4
a galvanizing layer disposed on at least one surface of the steel sheet
Data Source
AI summary
A high strength galvanized steel sheet having a yield strength of not less than 1,000 MPa is disclosed. An amount of diffusible hydrogen is not more than 0.60 mass ppm. The chemical composition of the steel sheet includes, by mass %, C: 0.150 to 0.450%, Si: 0.50 to 3.00%, Mn: 1.50 to 4.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 0.100%, O: not more than 0.0100%, and N: not more than 0.0100%, with a balance being Fe and inevitable impurities. In the microstructure of the steel sheet, a total area fraction of tempered martensite and bainite is 55 to 95%, an area fraction of retained austenite is 5 to 30%, and a presence ratio X/Y of a structure X having a nanohardness of 7.0 GPa or more to a structure Y having a nanohardness of 6.5 GPa or less is 0.5 to 2.5.