Hot-Rolled Steel Sheet Microstructure for Tough Welded Suspension Parts
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Solution Overview
Problem
Conventional high-strength hot-rolled steel sheets for automotive applications lack sufficient toughness and impact resistance, particularly for suspension system members, where durability and press formability are critical.
Innovation Solution
A hot-rolled steel sheet with a chemical composition of C: 0.02-0.20%, Si: 0.01-1.50%, Mn: 0.10-3.00%, and a microstructure comprising 60-80% ferrite and 90% or more of ferrite and bainite, with an average crystal grain size of 7.0 μm or less and a standard deviation of 2.0 μm or less, along with a weld joint process that includes specific heating and cooling protocols to ensure uniform grain size and precipitate distribution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strength hot-rolled steel sheets with ferritic-martensitic or ferritic-bainitic structures are used to improve strength and hole expandability, then tensile strength and hole expandability are enhanced, but toughness and impact resistance are insufficient
Solution Approach 1:
The invention changes the microstructural parameters by controlling the area ratio of ferrite (60-80%) and bainite (10-40%), and by controlling the crystal grain size (average 7.0 μm or less with standard deviation 2.0 μm or less). This parameter control achieves both high strength (tensile strength 780 MPa or more) and improved toughness compared to conventional high-strength steels
Solution Approach 2:
The invention applies local quality control by ensuring uniform distribution of Ti carbo-nitride precipitates with small standard deviation (10 nm or less), creating a homogeneous microstructure that simultaneously provides strength and toughness throughout the steel sheet
2Strength
If high-strength steel sheets are used for suspension system members to improve strength, then load-bearing capacity is enhanced, but durability under impact and press forming conditions is insufficient
Solution Approach 1:
The invention optimizes microstructural parameters including ferrite area ratio (60-80%), bainite area ratio (10-40%), and crystal grain size (average 7.0 μm or less), which provides both high load-bearing capacity and improved durability for suspension members subjected to repeated impacts and press forming operations
Solution Approach 2:
Instead of prioritizing only strength through martensitic structures, the invention inverts the approach by using a predominantly ferritic-bainitic structure with controlled grain refinement, achieving both strength and durability simultaneously
3Ease of manufacture
If conventional hot-rolled steel sheets are used to maintain ease of manufacture, then production simplicity is preserved, but hole expandability and press workability are insufficient
Solution Approach 1:
The invention changes the microstructural parameters (ferrite 60-80%, bainite 10-40%, grain size 7.0 μm or less) through controlled hot rolling and cooling processes, achieving excellent hole expandability while maintaining compatibility with conventional hot-rolled steel sheet manufacturing
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 resulting steel sheet exhibits enhanced toughness, high strength, and excellent hole expandability, while the weld joint achieves improved low-temperature toughness, addressing the limitations of previous technologies.
Implementation Method 1
Ti carbo-nitride precipitates
Implementation Method 2
steel micro-structure includes, in area %, ferrite: 60 to 80%, and a total of ferrite and bainite: 90% or more
Data Source
AI summary
Hot-rolled steel sheet having a chemical composition including, in mass %, C: 0.02 to 0.20%, Si: 0.01 to 1.50%, Mn: 0.10 to 3.00%, P: 0.10% or less, S: 0.010% or less, Al: 0.005 to 0.100%, Ti: 0.02 to 0.20%, N: 0.001 to 0.010%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Cr: 0 to 1.00%, Mo: 0 to 0.40%, Nb: 0 to 0.060%, V: 0 to 1.00%, B: 0 to 0.0100%, Ca: 0 to 0.0050%, O: 0.0100% or less, and the balance: Fe and impurities; in which: a steel micro-structure includes, in area %, ferrite: 60 to 80%, and a total of ferrite and bainite: 90% or more; an average of the crystal grain size of ferrite and bainite is 7.0 μm or less, a standard deviation of the crystal grain size is 2.0 μm or less; and a standard deviation of a diameter of Ti carbo-nitrides is 10 nm or less.