High-Strength Steel Sheet Microstructure for Energy Absorption
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Solution Overview
Problem
High-strength steel sheets with a tensile strength of 980 MPa or more have not been effectively used in energy-absorbing automotive members due to fracture issues and inadequate energy absorption during collisions, which compromises safety and weight reduction goals.
Innovation Solution
A steel sheet with a chemical composition and microstructure optimized for high tensile strength, including an equivalent carbon content of 0.60% to 0.85%, a microstructure of less than 40% ferrite, 40% or more tempered martensite and bainite, and 3% to 15% retained austenite, and a specific grain size change upon bending, ensuring improved collision characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength steel sheets with a tensile strength of 980 MPa or more are used, then the strength and weight reduction are improved, but the collision characteristics and energy absorption are worsened due to member fracture
Solution Approach 1:
The invention changes the microstructural parameters of the steel sheet by controlling the area fractions of different phases (ferrite <40%, tempered martensite + bainite ≥40%, retained austenite 3-15%) and the grain size ratio of tempered martensite. This parameter optimization enables the steel to achieve both high tensile strength (980 MPa or more) and good collision characteristics by preventing member fracture while maintaining strength.
2Weight of moving object
If the thickness of steel sheets is reduced to reduce weight, then the weight reduction is improved, but the strength and safety are worsened
Solution Approach 1:
The invention changes the material parameters by optimizing the chemical composition (equivalent carbon content Ceq: 0.60-0.85%) and microstructure (phase area fractions and grain size distribution). This enables the steel sheet to achieve high tensile strength (980 MPa or more), allowing for thickness reduction and weight reduction while maintaining the required strength and safety levels.
3Strength
If high-strength steel sheets with a tensile strength of 980 MPa or more are applied to energy-absorbing members, then the strength is improved, but the energy absorption consistency is worsened due to member fracture from primary processing portions
Solution Approach 1:
The invention optimizes the microstructural parameters, specifically controlling the grain size ratio of tempered martensite in the primary processing portion (change of 0.40 or more in (a grain size in a thickness direction)/(a grain size in a direction perpendicular to the thickness)). This parameter control prevents member fracture in primary processing portions, ensuring consistent energy absorption while maintaining high strength (980 MPa or more) in energy-absorbing members.
Data Source
AI summary
A steel sheet including a chemical composition satisfying an equivalent carbon content of 0.60% or more and less than 0.85%, and a steel microstructure with an area fraction of ferrite: less than 40%, tempered martensite and bainite: 40% or more in total, retained austenite: 3% to 15%, and ferrite, tempered martensite, bainite, and retained austenite: 93% or more in total. A 90-degree bending at a curvature radius/thickness ratio of 4.2 in a rolling (L) direction with respect to an axis extending in a width (C) direction causes a change of 0.40 or more in (a grain size in a thickness direction)/(a grain size in a direction perpendicular to the thickness) of the tempered martensite in an L cross section in a 0- to 50-μm region from a surface of the steel sheet on a compression side. The steel sheet has a tensile strength of 980 MPa or more.