Ultra-High Strength Steel Sheet With Bainite for Shear Workability
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Solution Overview
Problem
Existing methods for manufacturing high-strength steel sheets for automotive friction plates face challenges in achieving ultra-high strength and shear workability while minimizing cracking and manufacturing costs, due to issues with alloy composition segregation, non-uniform microstructures, and excessive addition of alloying elements.
Innovation Solution
A steel sheet with a specific alloy composition (C: 0.06 to 0.14%, Si: 0.1 to 0.8%, Mn: 1.6 to 2.2%, Cr: 0.005 to 0.8%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, Al: 0.01 to 0.1%, N: 0.001 to 0.01%, Ti: 0.01 to 0.06%, Nb: 0.005 to 0.06%, B: 0.0003 to 0.0050%) and a manufacturing process involving heating, hot-rolling, controlled cooling, and cold-rolling, with specific cooling rates and temperatures to form a bainite phase-dominated microstructure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If alloying elements (C, Si, Mn, Mo, Cr) are added to improve strength through solid-solution strengthening, then the strength of the steel sheet increases, but excessive addition causes segregation of alloy composition and formation of non-uniform microstructures, leading to delayed ferrite phase transformation and formation of low-temperature phases (martensite) that increase cracking during shearing
Solution Approach 1:
The patent optimizes the content ranges of alloying elements (C: 0.15-0.35%, Si: 0.01-0.60%, Mn: 1.00-3.00%, Cr: 0.005-1.000%, Mo: 0.005-0.500%) to achieve the desired balance between strength and microstructure uniformity. By precisely controlling these parameters, the patent prevents excessive addition that would cause segregation and non-uniform microstructures while still achieving ultra-high strength through solid-solution strengthening.
2Strength
If precipitation strengthening elements (Ti, Nb, V) are added to improve physical properties, then strength is enhanced, but coarse carbides, nitrides, and precipitates form in grain boundaries, increasing occurrence of cracking and facilitating propagation of cracking during shearing
Solution Approach 1:
The patent precisely controls the content of precipitation strengthening elements (Ti: 0.010-0.050%, Nb: 0.005-0.050%, V: 0.010-0.050%) to achieve strength enhancement while preventing the formation of coarse carbides, nitrides, and precipitates in grain boundaries. This parameter optimization ensures that fine precipitates form throughout the matrix rather than coarse ones at grain boundaries, reducing cracking susceptibility.
3Strength
If recovery annealing method is used to manufacture high-strength cold-rolled steel sheet, then hardness is guaranteed, but it is difficult to manufacture ultra-high strength steel sheet
Solution Approach 1:
The patent achieves ultra-high strength (tensile strength 1350 MPa or more, hardness 360 Hv or more) through optimized alloy composition and controlled rolling processes without requiring recovery annealing. The combination of solid-solution strengthening from optimized alloying and precipitation strengthening from controlled Ti, Nb, and V addition enables ultra-high strength to be achieved through conventional cold rolling alone, simplifying the manufacturing process.
4Strength
If spheroidizing heat treatment is performed on high carbon steel or cold rolling is performed twice, then high strength is achieved, but manufacturing costs increase significantly
Solution Approach 1:
The patent achieves ultra-high strength through a single cold rolling process with optimized alloy composition, eliminating the need for spheroidizing heat treatment or double cold rolling. The optimized composition (particularly C: 0.15-0.35% combined with Si, Mn, Cr, Mo, and precipitation elements) enables strength enhancement through conventional processing, significantly reducing manufacturing costs while achieving tensile strength of 1350 MPa or more.
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 ultra-high strength, hardness, and excellent shear workability, reducing crack formation to less than 0.1 mm, while avoiding costly annealing processes and maintaining economic viability.
Implementation Method 1
Alloying elements, such as carbon (C), silicon (Si), manganese (Mn), molybdenum (Mo), and chromium (Cr), are mainly added as an alloy composition to manufacture a cold-rolled high-strength steel sheet. In this case, solid-solution strengthening is improved to result in an effect of increasing strength of a steel sheet.
Implementation Method 2
ferrite phase transformation is significantly delayed because of an increase in the hardenability of steel during cooling, so that low-temperature phases (martensite, and the like) are formed
Data Source
AI summary
The present invention relates to a steel sheet which is suitable to be used as the material of a part such as a friction plate for a vehicle automatic transmission. More specifically, the present invention relates an ultra-high strength steel sheet having excellent shear workability and a method for manufacturing same.