Cold-Rolled Steel Sheet Microstructure Balancing Strength and Formability
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing cold-rolled steel sheets for vehicle body components face challenges in achieving high tensile strength, uniform elongation, and hydrogen embrittlement resistance while maintaining excellent formability and bendability, as previous technologies either fail to meet these criteria or compromise one or more properties.
Innovation Solution
A cold-rolled steel sheet with a specific chemical composition and controlled metallographic structure, primarily containing tempered martensite with a predetermined amount of retained austenite, ferrite, and bainite, along with controlled grain sizes, is developed to enhance strength, formability, and hydrogen embrittlement resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a structure containing ferrite is provided to secure ductility such as uniform elongation, then uniform elongation is improved, but tensile strength cannot reach 1,310 MPa or more
Solution Approach 1:
The steel sheet employs a composite microstructure consisting of ferrite, bainite, and martensite phases. The ferrite matrix provides ductility and uniform elongation, while the dispersed bainite and martensite phases contribute to high tensile strength. This multi-phase composite structure enables simultaneous achievement of both ductility and high strength.
Solution Approach 2:
The invention applies local quality by creating a non-uniform microstructure where different phases are distributed throughout the material. The ferrite phase dominates the matrix to ensure ductility, while localized regions contain hardened bainite and martensite phases to provide high strength. This spatial distribution of different microstructural qualities resolves the contradiction between overall ductility and localized strength.
2Strength
If a hard secondary phase is added to obtain high strength of 1,310 MPa or more, then tensile strength is improved, but hole expansibility deteriorates
Solution Approach 1:
The invention changes the parameters of the hard phases by controlling their volume fraction to be 20-60% and their hardness to be 450-550 HV. Additionally, the grain size is controlled to be 5-20 μm. These parameter optimizations ensure that the hard phases provide sufficient strength while maintaining adequate hole expansibility by preventing excessive brittleness.
3Strength
If high-strengthening is applied to achieve tensile strength of 1,310 MPa or more, then tensile strength is improved, but susceptibility to hydrogen embrittlement increases
Solution Approach 1:
The invention optimizes the chemical composition parameters to balance strength and hydrogen embrittlement resistance. The carbon content is controlled at 0.18-0.35%, silicon at 0.90-1.80%, manganese at 1.50-3.00%, and boron at 0.0005-0.0050%. These parameter ranges achieve high tensile strength while limiting hydrogen embrittlement susceptibility by avoiding excessive carbon content and incorporating elements that improve both strength and embrittlement resistance.
4Strength
If the steel sheet is cooled to near room temperature during cooling during quenching to achieve high strength, then tensile strength is improved, but volume percentage of retained austenite becomes small and high uniform elongation cannot be obtained
Solution Approach 1:
The invention changes the cooling parameters by controlling the cooling rate to be 3-30°C/s and the final temperature to be 150-450°C. This controlled cooling regime allows sufficient retained austenite (3-15% volume fraction) to remain in the microstructure, which provides the TRIP effect for high uniform elongation, while still achieving high tensile strength through the formation of bainite and martensite 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 achieves a tensile strength of 1,310 MPa or more, uniform elongation of 4.0% or more, and a limit bend radius to sheet thickness ratio of 5.0 or less, while maintaining excellent hydrogen embrittlement resistance.
Implementation Method 1
Patent Document 3 proposes a steel sheet using a transformation induced plasticity (TRIP) effect caused by retained austenite as a technique for achieving both high-strengthening and high formability
Implementation Method 2
a steel sheet containing tempered martensite as a primary phase has been proposed
Data Source
AI summary
This cold-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure at a ¼ depth position, which is a ¼ thickness position from a surface, contains, by volume percentage, retained austenite: more than 1.0% and less than 10.0%, tempered martensite: 80.0% or more, ferrite and bainite: 0% or more and 15.0% or less in total, and martensite: 0% or more and 3.0% or less, and an average grain size of a first grain counted in a sheet thickness direction from the surface when viewed at a cross section parallel to a rolling direction and parallel to the sheet thickness direction is 20.0 μm or less, and an average grain size of the first grain when the surface is viewed in a plan view is 30.0 μm or less.