High-Strength Steel Sheet Microstructure Control
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Solution Overview
Problem
Current techniques fail to simultaneously enhance collision safety and moldability in high-strength steel sheets, as improving one property often compromises the other, making it difficult to achieve both high tensile strength and ductility for automotive applications.
Innovation Solution
A high-strength steel sheet with a specific chemical composition (C: 0.10% to 0.35%, Si+Al: 0.5% to 3.0%, Mn: 1.0% to 3.0%, P: 0% to 0.05%, S: 0% to 0.01%, and a microstructure of 0% to 10% ferrite, 0% to 30% MA, 70% to 100% hard phase, and 5% to 30% retained austenite, along with a controlled IQ skewness of −1.2 to −0.3, is developed, combined with a manufacturing process involving hot rolling, pickling, cold rolling, and controlled heat treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If tensile strength is increased to improve collision safety, then collision safety is improved, but moldability (ductility and hole expansion property) deteriorates
Solution Approach 1:
The invention changes the microstructural parameters by controlling the phases present (martensite, bainite, retained austenite) and their proportions, along with controlling inclusion characteristics (oxide inclusions with specific size distribution and aspect ratios). This allows achieving high tensile strength (≥780 MPa) while maintaining good moldability through specific microstructural parameter control
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (martensite, bainite, retained austenite) and controlled inclusions (oxide inclusions). This composite structure enables simultaneous achievement of high strength and good moldability by combining the advantages of different microstructural components
2Strength
If yield ratio is increased to improve collision safety, then collision safety is improved, but total elongation and hole expansion ratio deteriorate
Solution Approach 1:
The invention optimizes the microstructural parameters by controlling the phases (martensite, bainite, retained austenite) and their proportions, along with inclusion characteristics. This enables achieving high yield ratio (≥0.70) while maintaining good total elongation (≥13000 MPa·%) and hole expansion ratio (≥40000 MPa·%)
Solution Approach 2:
The invention controls the local characteristics of inclusions (oxide inclusions with specific size distribution and aspect ratios) and the local microstructural phases to achieve uniform deformation behavior. This local quality control allows high yield ratio while maintaining good elongation and hole expansion properties
3Force
If load for initial deformation is increased to improve collision safety, then collision safety is improved, but moldability deteriorates
Solution Approach 1:
The invention changes the microstructural parameters (phases and inclusions) to achieve high load for initial deformation (≥3.0 kN/mm) while maintaining good moldability. The controlled microstructure allows the material to exhibit high initial strength while still being formable
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 both excellent collision safety and moldability, with tensile strength of 780 MPa or more, yield ratio of 0.70 or more, TS×EL of 13000 MPa·% or more, and load/sheet thickness of 3.0 kN/mm or more in the VDA bending test, effectively addressing the dual requirements of strength and ductility.
Implementation Method 1
heating the steel material to a temperature T1 of an Ac3 transformation point of the steel or more and 950° C. or less and holding the steel material for a time t1 of 5 seconds or more and 1800 seconds or less in this temperature range for austenitization
Implementation Method 2
a step of performing cooling from a rapid cooling start temperature T2 of 700° C. or more to a cooling stop temperature T3a in a temperature range of 300° C. or more and 500° C. or less at an average cooling rate CR2 of 10° C./sec or more
Implementation Method 3
a step of performing heating from the cooling stop temperature T4 to a reheating temperature T5 that satisfies the following requirement in a temperature range of 300° C. or more and 500° C. or less and performing holding for a time t5 of 350 seconds or more and 1800 seconds or less in the temperature range of the reheating temperature T5
Data Source
AI summary
The present invention provides a high-strength steel sheet, which can be used in various applications including automobile parts and exhibits excellent collision safety and excellent moldability, and a method for manufacturing the high-strength steel sheet. The high-strength steel sheet according to an aspect of the present invention satisfies a predetermined chemical composition and has a metallographic microstructure having ferrite fraction: 0% to 10%, MA fraction: 0% to 30%, hard phase other than ferrite and MA: 70% to 100% in terms of area proportion and retained austenite fraction: 5% to 30% in terms of volume proportion, and in the high-strength steel sheet, the skewness of IQ as analyzed by the EBRD method is −1.2 to −0.3 when the skewness is expressed by a predetermined relational expression in a case where crystal grains having a bcc structure and a bct structure are regarded as an aggregation of regions having an area of 0.05 μm2.