High-Strength Steel Sheet Composition for Formability and Yield Ratio
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing technologies face challenges in simultaneously achieving excellent formability, high yield ratio, and strength in high-strength steel sheets, particularly for applications requiring tensile strength of 610 MPa or more.
Innovation Solution
A high-strength steel sheet composition including 0.05 to 0.25% C, 0.7% or less Si, 0.46 to 1.8% Mn, 0.7% or less Al, 0.05% or less P, 0.03% or less S, 0.03% or less N, and a total of 0.22% or less of Ti, Nb, and V, with a microstructure comprising 1 to 13% unrecrystallized ferrite, 67 to 98% recrystallized ferrite, and 1 to 20% cementite, manufactured through a process involving hot rolling, coiling, heat treatment, cold rolling, reheating, and tertiary cooling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If solid solution strengthening elements (Mn, Si, Cr) are added to increase yield strength, then yield strength is improved, but surface oxidation and formation of low-temperature transformation phases occur which deteriorate formability and yield ratio
Solution Approach 1:
The patent extracts and removes harmful elements (Si ≤ 0.7%, Cr ≤ 0.7%, Mn ≤ 1.8%) that cause surface oxidation and low-temperature transformation phase formation, while retaining only the necessary amount for strength. This extraction approach eliminates the side effects of excessive element addition while maintaining the primary function of yield strength enhancement.
Solution Approach 2:
The patent changes the compositional parameters by strictly controlling the content ranges of Mn (0.15-1.8%), Si (0.01-0.7%), and Cr (0.01-0.7%), and by controlling the C content (0.10-0.25%). This parameter optimization ensures that strengthening elements provide sufficient yield strength without causing surface oxidation or harmful phase transformation, thereby resolving the contradiction between strength and formability.
2Reliability
If unrecrystallized ferrite is introduced to improve formability and yield ratio, then formability and yield ratio are improved, but it becomes difficult to simultaneously secure excellent strength, elongation, and high yield ratio
Solution Approach 1:
The patent applies local quality by creating a non-uniform microstructure with specific phases distributed in particular proportions: unrecrystallized ferrite (1-13%) for formability, recrystallized ferrite (67-98%) for ductility, and cementite (1-20%) for strength. This localized phase distribution allows different regions to contribute different properties, simultaneously achieving excellent formability, strength, and high yield ratio.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (unrecrystallized ferrite, recrystallized ferrite, and cementite) with controlled proportions. This composite structure combines the advantages of each phase: unrecrystallized ferrite provides formability and high yield ratio, recrystallized ferrite provides elongation, and cementite provides strength, thereby simultaneously achieving all three properties.
3Strength
If Mn content is increased to strengthen ferrite, then yield strength is improved, but low-temperature transformation phases (bainite or martensite) form which lower the yield ratio
Solution Approach 1:
The patent optimizes the Mn content parameter within a specific range (0.15-1.8%) and combines it with controlled C content (0.10-0.25%) and heat treatment parameters (cooling rates of 1-100°C/s). This parameter optimization ensures that Mn strengthens ferrite without promoting excessive formation of low-temperature transformation phases, thereby maintaining high yield ratio while achieving sufficient yield 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 proposed steel sheet achieves a tensile strength of 610 MPa or more, a yield ratio of 0.8 to 0.95, and excellent formability and hole expandability, effectively addressing the challenges of simultaneously securing strength, elongation, formability, and high yield ratio.
Implementation Method 1
precipitation strengthening steel using Nb, Ti, V, and the like is a steel sheet which improves yield strength by precipitating fine carbides in ferrite
Implementation Method 2
a microstructure includes, by area: 1 to 13% of unrecrystallized ferrite, 67 to 98% of recrystallized ferrite, and 1 to 20% of cementite
Data Source
AI summary
The present invention relates to a high-strength steel sheet having excellent formability and a high yield ratio, and a manufacturing method therefor, and more particularly, to a high-strength steel sheet having excellent formability and a high yield ratio, and a manufacturing method therefor, the steel sheet having various uses such as that of vehicle parts.