Multi-Phase Steel Composition for Low-Temperature Burring
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Solution Overview
Problem
Existing multi-phase steels face challenges in achieving high strength and excellent burring properties at low temperatures due to issues such as microstructural uniformity, segregation of alloy components, increased rolling loads, and decreased bake hardenability, which are exacerbated by excessive use of alloying elements like Ti, Nb, and V.
Innovation Solution
A high strength multi-phase steel composition with controlled alloying elements (C, Si, Mn, Al, Cr, Mo, P, S, N, Nb, Ti, V, B) and a specific microstructure (97-99% ferrite and bainite, 1-3% martensite-austenite) is produced through a controlled reheating, hot-rolling, and cooling process, ensuring [C]* is between 0.022 and 0.10, and adhering to Relationship 1 (Mn + 2.8[Mo] + 1.5[Cr] + 500[B] ≤ 4.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If alloying elements (Ti, Nb, V) are excessively used to obtain high strength, then tensile strength is improved, but rolling load increases due to delay of recrystallization and formability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the content ranges of alloying elements (Ti: 0.003-0.06%, Nb: 0.003-0.06%, V: 0.003-0.06%) to optimize the balance between strength and formability. This quantitative parameter control ensures sufficient precipitation strengthening while preventing excessive recrystallization delay that would deteriorate formability.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, bainite, martensite, and austenite) through controlled alloying and cooling. This composite structure provides both high strength from the hard phases and good formability from the ductile phases, resolving the contradiction between strength and manufacturability.
2Strength
If alloying elements (Si, Mn, Al, Mo, Cr) are excessively added to improve strength and stretch flangeability, then strength is improved, but segregation of alloy components and ununiformity of microstructure occur causing stretch flangeability to deteriorate
Solution Approach 1:
The patent applies parameter changes by establishing specific content ranges for multiple alloying elements (Si: 0.01-1.0%, Mn: 1.0-3.0%, Al: 0.01-0.1%, Mo: 0.003-0.3%, Cr: 0.005-1.0%) and controlling their synergistic effects. This parameter optimization prevents excessive segregation while maintaining microstructural uniformity through balanced alloy composition.
Solution Approach 2:
The patent applies local quality by creating different microstructural phases (ferrite, bainite, martensite, austenite) with distinct properties in specific regions. The controlled distribution of these phases ensures uniform overall microstructure while allowing local variations in phase composition to optimize both strength and formability.
3Stability of the object's composition
If C and N content is decreased to reduce segregation, then microstructural uniformity is improved, but bake hardenability decreases making it economically disadvantageous
Solution Approach 1:
The patent applies parameter changes by precisely controlling carbon (0.05-0.14%) and nitrogen (0.001-0.01%) content within optimal ranges that balance microstructural uniformity and bake hardenability. This parameter optimization ensures sufficient C and N for hardenability while preventing excessive segregation through the controlled alloy composition.
Solution Approach 2:
The patent creates a composite microstructure where retained austenite phases provide bake hardenability through transformation-induced plasticity, while the overall uniform distribution of phases ensures microstructural uniformity. This composite approach resolves the contradiction between uniformity and hardenability.
4Strength
If low temperature transformed structure phase is formed ununiformly to achieve high strength, then strength is improved, but stretch flangeability deteriorates
Solution Approach 1:
The patent applies parameter changes by controlling cooling rates and temperature profiles during hot-rolling to achieve uniform low-temperature phase transformation. This parameter control ensures consistent formation of martensite and bainite phases throughout the material, providing both high strength and good stretch flangeability through uniform microstructure.
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 high tensile strength, excellent burring properties at low temperatures, and improved bake hardenability, with a Hole Expanding Ratio (HER) at -30°C of 30,000 MPa% or more and bake hardenability (BH) of 40 MPa or more.
Implementation Method 1
the sum of area ratios of ferrite and bainite is 97% to 99%, an area ratio of martensite and austenite (MA) is 1% to 3%
Implementation Method 2
precipitate forming elements such as titanium (Ti), niobium (Nb), and vanadium (V)
Data Source
AI summary
Provided is a method for producing high strength multi-phase steel. the method includes: reheating a slab; hot-rolling the reheated slab to obtain a hot-rolled steel sheet; firstly cooling the hot-rolled steel sheet to a first cooling end temperature of 500° C. to 700° C. at a rate of 10° C./sec to 70° C./sec; air-cooling the firstly cooled hot-rolled steel sheet at the first cooling end temperature for 3 to 10 seconds; secondly cooling the air-cooled hot-rolled steel sheet to a second cooling end temperature of 400° C. to 550° C. at a rate of 10° C./sec to 70° C./sec; coiling the secondly cooled hot-rolled steel sheet at the second cooling end temperature; and thirdly cooling the coiled hot-rolled steel sheet to 200° C. or less at a rate of 25° C./hour or less, excluding 0° C./hour.