High-Strength Steel Sheet With Uniform Boron Grain-Boundary Segregation
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Solution Overview
Problem
Existing automotive steel sheets have tensile strengths less than 1180 MPa and lack sufficient stretch flangeability and toughness, particularly in high-strength applications.
Innovation Solution
A steel sheet composition with specific elements and microstructural controls, including a high fraction of martensite and bainite, controlled austenite grain size, and uniform boron segregation, achieved through a multi-step annealing process, to enhance strength, flangeability, and toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the area fraction of ferrite is increased to improve ductility, then toughness is improved, but tensile strength decreases below 1180 MPa
Solution Approach 1:
The invention changes the microstructural parameters by controlling the area fractions of different phases (martensite 90-98%, bainite 2-10%, ferrite 0-5%) and adjusting chemical composition parameters (C: 0.15-0.40%, Si: 0.01-2.50%, Mn: 1.50-4.00%, B: 0.0005-0.0050%) to achieve both high tensile strength (1180 MPa or more) and excellent toughness (brittle-to-ductile transition temperature of -40°C or lower).
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (martensite, bainite, and ferrite) where each phase contributes different properties: martensite provides high strength, bainite provides toughness, and controlled ferrite improves ductility. This multi-phase composite structure enables simultaneous achievement of tensile strength ≥1180 MPa and excellent toughness.
2Strength
If the area fraction of martensite is increased to achieve tensile strength of 1180 MPa or more, then strength is improved, but stretch flangeability deteriorates due to void formation at ferrite-martensite interfaces
Solution Approach 1:
The invention optimizes the ferrite area fraction parameter to 0-5% (particularly 1% or less) to minimize void formation at ferrite-martensite interfaces while maintaining tensile strength of 1180 MPa or more. This precise parameter control improves stretch flangeability (hole expansion ratio of 30% or more) without sacrificing strength.
3Reliability
If prior-austenite grain size is reduced to improve toughness, then brittle-to-ductile transition temperature decreases, but manufacturing complexity increases due to multi-step annealing process
Solution Approach 1:
The invention segments the heat treatment process into distinct stages: first annealing to form austenite and segregate B to grain boundaries, followed by cooling to form martensite-bainite microstructure, then second annealing to form retained austenite nuclei, and final cooling to achieve the target microstructure. This segmented process enables precise control of prior-austenite grain size (10 μm or less) and uniform B segregation.
Solution Approach 2:
The invention performs preliminary B segregation to austenite grain boundaries during the first annealing step before final microstructure formation. This preliminary action ensures uniform B distribution at grain boundaries, which significantly improves toughness and lowers brittle-to-ductile transition temperature to -40°C or lower.
4Reliability
If B concentration at prior-austenite grain boundaries is increased to improve toughness, then brittle-to-ductile transition temperature decreases, but manufacturing precision requirements increase to ensure uniform segregation
Solution Approach 1:
The invention maintains continuous B segregation action during the first annealing step by holding at the first heating temperature for a sufficient period (not less than 60 seconds). This continuous process ensures uniform B distribution at prior-austenite grain boundaries with variation less than 0.010% in mass%, achieving brittle-to-ductile transition temperature of -40°C or lower.
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 provides a steel sheet with a tensile strength of 1180 MPa or more, excellent stretch flangeability, and a brittle-to-ductile transition temperature below -40°C, addressing the limitations of prior technologies.
Implementation Method 1
addition of B causes B to segregate to the prior-austenite grain boundaries and strengthen the grain boundaries, thereby improving the toughness
Implementation Method 2
B segregates nonuniformly to prior-austenite grain boundaries in some cases while B segregates at a uniform concentration in other cases
Implementation Method 3
Annealing a cold rolled sheet in an austenite region once causes B to segregate to the austenite grain boundaries
Implementation Method 4
at this stage, diffusion of B is insufficient and segregation of B is nonuniform
Implementation Method 5
Cooling this steel once to form a martensite-bainite microstructure
Implementation Method 6
the main phase is preferably martensite or bainite
Implementation Method 7
addition of Nb decreases the prior-austenite grain size to 10 μm or less, and the toughness is improved
Implementation Method 8
martensite and bainite contain numerous dislocations, and B that has been dissolved therein diffuses rapidly into austenite grain boundaries through the dislocations during the second annealing; thus, uniform segregation of B occurs
Data Source
AI summary
Provided is a steel sheet and a member having high strength and excellent stretch flangeability and toughness, and methods for producing the same. A steel sheet has a chemical composition containing, in mass%, C, Si, Mn, P, Al, N, Ti, Nb, and B in specified contents, in which the total area fraction of martensite and bainite is 95% or more, the retained austenite area fraction is 5% or less, the ferrite area fraction is 1% or less, the prior-austenite grain size is 10 µm or less, the C concentration in a prior-austenite grain boundary is equal to or more than 1.5 times the C content in steel, the B concentration in the prior-austenite grain boundary is 0.05% or more in mass%, and the variation in B concentration within the same prior-austenite grain boundary is less than 0.010% in mass%.