High-Strength Multi-Phase Steel 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 ununiformity, increased rolling loads, and deteriorated formability and weldability caused by excessive alloying elements and cooling conditions.
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 controlled reheating, hot-rolling, and multi-stage cooling processes to maintain optimal phase ratios and dislocation density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If alloying elements (Si, Mn, Al, Mo, Cr) are added in excessively amounts 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 precisely controlling the content ranges of alloying elements (Si: 0.01-1.0%, Mn: 1.0-3.0%, Al: 0.01-0.1%, Cr: 0.005-1.0%, Mo: 0.003-0.3%) and implementing multi-stage cooling processes with specific temperature rates to achieve uniform microstructure and maintain stretch flangeability while achieving high strength
Solution Approach 2:
The patent applies local quality by creating different microstructural phases (ferrite, bainite, martensite-austenite) with specific area ratios (ferrite+bainite: 97-99%, MA: 1-3%) and controlling austenite grain size distribution to achieve both high strength and excellent burring properties in different regions of the steel microstructure
2Strength
If precipitate forming elements (Ti, Nb, V) are excessively used to obtain high strength, then strength is improved, but rolling load increases due to delay of recrystallization during hot-rolling, making it difficult to produce thin products and formability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the content of precipitate forming elements (Ti: 0.005-0.13%, Nb: 0.005-0.06%, V: 0.003-0.2%) and implementing multi-stage cooling processes to achieve high strength without excessive rolling load and maintain formability
Solution Approach 2:
The patent applies preliminary action by performing air-cooling at the first cooling end temperature (500-700°C) for 3-10 seconds before the second cooling stage, which preliminarily controls the microstructure formation and prevents excessive rolling load during hot-rolling while maintaining subsequent formability
3Reliability
If C and N content are increased to obtain high bake hardenability, then BH value is improved, but microstructural ununiformity and susceptibility to microstructural changes occur depending on cooling conditions
Solution Approach 1:
The patent applies parameter changes by precisely controlling C (0.05-0.14%) and N (0.001-0.01%) content and calculating the modified carbon equivalent [C]* to achieve high bake hardenability while maintaining microstructural uniformity and stability against cooling condition changes
Solution Approach 2:
The patent applies feedback by using the calculated [C]* value (0.022-0.10) and Relationship 1 ([Mn]+2.8[Mo]+1.5[Cr]+500[B]≤4.0) as control criteria to feedback-adjust the alloy composition and cooling parameters, ensuring consistent microstructure and bake hardenability
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 tensile strengths of 590 MPa or more with excellent burring properties at low temperatures, demonstrated by a Hole Expanding Ratio (HER) of 30,000 MPa% or more at -30°C, and improved bake hardenability.
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 high strength multi-phase steel having excellent burring properties at low temperature, and a method for producing the same. More specifically, provided are a high strength multi-phase steel having excellent burring properties at low temperature, and a method for producing the same, wherein the multi-phase steel can be appropriately used as a member, a lower arm, a reinforcement material, a connection material, or the like for a vehicle chassis component.