Hot-Rolled Steel Sheet Microstructure for Shearing and Fatigue Resistance
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Solution Overview
Problem
Existing hot-rolled steel sheets lack high strength, adequate sheet thickness reduction at critical fracture, excellent ductility, shearing property, and fatigue resistance, which are essential for vehicle body components requiring press forming and shearing accuracy.
Innovation Solution
A hot-rolled steel sheet with specific chemical composition and microstructure, including controlled residual austenite, ferrite, and pearlite ratios, along with defined concentrations of elements like C, Mn, and Cr, enhances strength, ductility, and shearing properties, and improves fatigue resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheet is used to reduce vehicle body weight, then weight reduction is achieved, but formability (sheet thickness reduction at critical fracture and ductility) deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters (C: 0.15-0.35%, Si: 2.00-3.00%, Mn: 1.50-3.00%, Cr: 1.00-2.00%, Ti: 0.050-0.200%, Nb: 0.050-0.200%, V: 0.050-0.200%, B: 0.0005-0.0050%) and microstructural parameters (ferrite 10-40%, bainite 50-80%, martensite 5-20%, residual austenite 0-10%) to achieve high tensile strength (1320-1580 MPa) while maintaining adequate sheet thickness reduction at critical fracture (15% or more), resolving the contradiction between strength and formability
Solution Approach 2:
The invention creates a composite microstructure consisting of multiple phases (ferrite, bainite, martensite, and residual austenite) with specific area ratios. This multi-phase composite structure provides both high strength from martensite and good formability from ferrite and bainite, achieving tensile strength of 1320-1580 MPa with sheet thickness reduction at critical fracture of 15% or more
2Weight of moving object
If high-strength steel sheet is used to reduce vehicle body weight, then weight reduction is achieved, but ductility deteriorates
Solution Approach 1:
The invention optimizes chemical composition parameters (particularly Si: 2.00-3.00% and Mn: 1.50-3.00%) and microstructural parameters (ferrite 10-40%, bainite 50-80%, martensite 5-20%, residual austenite 0-10%) to achieve a balance between strength and ductility, obtaining tensile strength of 1320-1580 MPa with total elongation of 10% or more
Solution Approach 2:
The invention creates a composite microstructure where soft phases (ferrite and bainite comprising 60-90% of total area) provide ductility and strength phases (martensite and residual austenite comprising 5-30% of total area) provide high strength, achieving tensile strength of 1320-1580 MPa with total elongation of 10% or more
3Weight of moving object
If high-strength steel sheet is used to reduce vehicle body weight, then weight reduction is achieved, but shearing property deteriorates
Solution Approach 1:
The invention optimizes chemical composition parameters (C: 0.15-0.35%, Si: 2.00-3.00%, Mn: 1.50-3.00%, Cr: 1.00-2.00%, B: 0.0005-0.0050%) and microstructural parameters (ferrite 10-40%, bainite 50-80%, martensite 5-20%, residual austenite 0-10%) to achieve high strength (1320-1580 MPa) while maintaining adequate shearing property (secondary sheared surface occurrence rate of 20% or less), enabling weight reduction without compromising shearing quality
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure at a depth position of 1/4 from a surface in a sheet thickness direction, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value is 10.7 or more, an I value is 1.020 or more, a CS value is −8.0×105 to 8.0×105, a standard deviation of Mn concentrations is 0.60 mass % or less, a solute Cr concentration in an outermost layer region is 0.10 mass % or more, a number density of Cr oxides having a sphere equivalent radius of 0.1 μm or more at the surface is 1.0×104 pieces/cm2 or less.