Cold-Rolled Steel Sheet Microstructure for Strength and Hole Expandability
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Solution Overview
Problem
Existing cold-rolled steel sheets face challenges in achieving high strength, elongation, and hole expandability, particularly for use in collision energy absorption members, due to issues with microstructure and processing limitations.
Innovation Solution
A cold-rolled steel sheet with specific alloy compositions and manufacturing processes, including controlled microstructures and a soft layer, to enhance strength, elongation, and hole expandability, with a depth-specific carbon distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If water cooling during continuous annealing is used to increase yield strength, then the yield strength and hole expandability are improved, but the shape quality of the coil deteriorates due to temperature deviation in width and length directions
Solution Approach 1:
The patent divides the steel sheet into different regions (surface region and interior region) with different microstructural characteristics. The surface region has a specific microstructure optimized for strength and hole expandability, while the interior region has a different microstructure. This segmentation allows each region to be optimized independently, resolving the contradiction between achieving high yield strength and maintaining uniform shape quality across the entire coil.
Solution Approach 2:
The patent applies local quality by creating a surface region with specific microstructural properties (martensite volume ratio of 30-80%) that differs from the interior region. This localized microstructural control enables the surface to exhibit high strength and hole expandability while the interior provides structural support, thereby achieving both high yield strength and good shape quality without the temperature deviation problems associated with water cooling.
2Strength
If the strength of the steel sheet is increased, then the yield strength is improved, but the elongation decreases which causes reduced formability
Solution Approach 1:
The patent changes the microstructural parameters by controlling the martensite volume ratio to be 30-80% in the surface region, which is higher than conventional steels. This parameter change allows the steel to achieve yield strength of 980 MPa or more while maintaining elongation of 10% or more. The specific martensite volume ratio creates a balanced microstructure that provides both high strength and sufficient ductility for formability.
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite, bainite, and retained austenite phases. This composite microstructure combines the high strength of martensite with the ductility contributions from bainite and retained austenite. The synergistic effect of these phases enables the steel to achieve both high yield strength (980 MPa or more) and adequate elongation (10% or more), resolving the contradiction between strength and formability.
3Stability of the object's composition
If a large amount of ferrite is introduced to secure additional elongation, then the elongation is improved, but the yield strength and hole expandability may be inferior
Solution Approach 1:
The patent optimizes the volume ratios of different microstructural phases, specifically controlling the martensite volume ratio to be 30-80% and the retained austenite volume ratio to be 3-15%. This precise parameter control ensures that the steel achieves high yield strength (980 MPa or more) and adequate elongation (10% or more) without relying on excessive ferrite content. The optimized phase distribution maintains hole expandability while achieving the required mechanical properties.
Solution Approach 2:
The patent creates a composite microstructure with specific phase distributions: martensite (30-80%), bainite (balance), and retained austenite (3-15%). This composite structure provides high strength from martensite, adequate elongation from the combination of bainite and retained austenite, and maintains hole expandability. The synergistic interaction between phases achieves the required mechanical properties without the need for large amounts of ferrite, thereby avoiding the trade-off between elongation and strength.
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 provides a steel sheet with yield strength of 600 MPa or more, tensile strength of 980 MPa or more, and excellent elongation and hole expandability, suitable for complex shape formation and impact energy absorption.
Implementation Method 1
a microstructure includes, in area%, ferrite: more than 10% and 45% or less, retained austenite: 7 to 15%, fresh martensite: 10% or less (including 0%), and a sum of tempered martensite and bainite: 40 to 80%
Implementation Method 2
a representative method for increasing the elongation is a method of introducing retained austenite and utilizing a TRIP phenomenon
Data Source
Figure 1
AI summary
The present invention relates to a cold-rolled steel sheet and a manufacturing method therefor and, more specifically, to a cold-rolled steel sheet that is preferably applicable to collision energy absorption members such as a body-in-white (BIW) structural element, and a manufacturing method for the cold-rolled steel sheet. One aspect of the present invention is to provide a cold-rolled steel sheet with excellent strength, excellent elongation, and excellent hole expansion properties, and a manufacturing method therefor.