High-Strength Steel Sheet Microstructure for Formability
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Solution Overview
Problem
Existing high strength steel sheets with tensile strengths of 1320 MPa or more face challenges in achieving sufficient elongation (El ≥ 8%) and a wide range of appropriate clearances for hole expanding deformation without causing delayed fracture, which are not adequately addressed by existing technologies.
Innovation Solution
The steel composition is optimized with limited ferrite and bainitic ferrite to 10% or less, retained austenite to 15% or less, carbon concentration in retained austenite to 0.50% or more, and specific hardness and kernel average misorientation ratios, combined with controlled microstructural features to achieve the desired properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets with tensile strength of 1320 MPa or more are used to reduce vehicle weight and enhance crashworthiness, then strength is improved, but elongation becomes insufficient and formability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling microstructural parameters including the volume fraction of retained austenite (5-15%), carbon concentration in retained austenite (0.50% or more), and the ratio of Kernel Average Misorientation (KAM) values. These parameter optimizations enable the steel to achieve tensile strength of 1320 MPa or more while maintaining elongation of 8% or more, thereby resolving the contradiction between strength and formability
2Ease of operation
If shear clearance is reduced to improve hole expanding deformation, then formability is improved, but delayed fracture resistance deteriorates
Solution Approach 1:
The patent employs parameter changes by optimizing the microstructural parameters including retained austenite volume fraction (5-15%) and carbon concentration in retained austenite (0.50% or more). These parameter optimizations create a microstructure that can accommodate hole expanding deformation with appropriate shear clearance while maintaining delayed fracture resistance, thus resolving the contradiction between formability and reliability
3Reliability
If shear clearance is increased to improve delayed fracture resistance, then reliability is improved, but hole expanding deformation capability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the volume fraction of retained austenite (5-15%) and carbon concentration in retained austenite (0.50% or more). These optimized parameters create a microstructure that simultaneously provides good delayed fracture resistance and accommodates hole expanding deformation with appropriate shear clearance, resolving the contradiction between reliability and formability
Data Source
AI summary
Objects are to provide a high strength steel sheet having a TS of 1320 MPa or more and an El of 8% or more and having a wide range of appropriate clearances for hole expanding deformation and a wide range of appropriate clearances not leading to delayed fracture; and to provide a method for manufacturing the same. A high strength steel sheet includes a specific microstructure having a specific chemical composition and satisfying the formulas (1) and (2) defined below: KAMS/KAMC<1.00 wherein KAM (S) is a KAM (Kernel average misorientation) value of a superficial portion of the steel sheet, and KAM (C) is a KAM value of a central portion of the steel sheet, HvQ−HvS≥8 wherein Hv (Q) indicates the hardness of a portion at 1/4 sheet thickness and Hv (S) indicates the hardness of a superficial portion of the steel sheet.