Variable-Thickness Steel Blank Composition for Strength and Recrystallization
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Solution Overview
Problem
The challenge lies in producing high-strength tailor-rolled blanks (TRBs) for automotive chassis components with variable thickness, where the balance between recrystallization, formability, and strength is compromised due to high precipitate density from micro-alloying elements like Nb and Ti, particularly at low cold-rolling reductions, leading to insufficient recrystallization and strength loss during batch annealing.
Innovation Solution
A variably rolled steel strip or sheet composition with 0.05-0.20% C, 0.30-0.60% Si, 0.80-2.50% Mn, 0.01-0.10% Al, 0.07-0.25% Ti, 0.10-0.35% V, 0.05-0.40% Mo, 0.02-0.10% Nb, and optional Cr, P, and Ca, featuring a high thickness variation of at least 35%, where V is added in solid solution to accelerate precipitation and recrystallization, and Ti and Mo enhance thermal stability to maintain strength and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If increased levels of micro-alloying elements (Nb and Ti) are used to achieve increased strength in hot-rolled HSLA steel, then precipitation strength is improved, but recrystallization during batch annealing is hindered, leading to insufficient formability
Solution Approach 1:
The patent changes the chemical composition parameters by adding V (0.05-0.50 wt%) alongside controlled amounts of Nb (0.01-0.10 wt%) and Ti (0.07-0.25 wt%). This parameter change modifies the precipitation behavior during batch annealing, allowing sufficient recrystallization while maintaining strength through a different precipitation mechanism that is less hindering to grain boundary migration.
Solution Approach 2:
The patent creates a composite microstructure by combining multiple alloying elements (V, Nb, Ti, Mo) that work synergistically. V provides precipitation strengthening through VC precipitates that are less harmful to recrystallization than Nb/Ti precipitates, while Nb and Ti provide additional strengthening. This composite approach allows both strength and formability to be achieved.
2Ease of manufacture
If a higher top temperature and extended holding time are used during batch annealing to achieve sufficient recrystallization, then formability is improved, but precipitation strengthening is reduced due to increased precipitate coarsening
Solution Approach 1:
The patent modifies the batch annealing temperature parameter to a specific range (650-750°C) that optimizes the balance between recrystallization and precipitation strengthening. At this temperature range, VC precipitates form and strengthen the matrix without excessive coarsening, while providing sufficient driving force for recrystallization to achieve the required formability.
Solution Approach 2:
V acts as an intermediary element that mediates between the conflicting requirements of recrystallization and precipitation strengthening. V-based precipitates form during batch annealing and provide strengthening without as severely hindering grain boundary migration as Nb/Ti precipitates, thus enabling both formability and strength to be achieved simultaneously.
3Strength
If increased levels of Nb and Ti are used to achieve high strength, then yield strength is improved, but the density of precipitates increases, which seriously hinders recrystallization during batch annealing
Solution Approach 1:
The patent changes the alloy composition parameters by introducing V at 0.05-0.50 wt% and limiting Nb to 0.01-0.10 wt% and Ti to 0.07-0.25 wt%. This parameter change results in a lower overall precipitate density compared to high-Nb/Ti alloys, while V-based precipitates have different characteristics that are less detrimental to recrystallization, thus improving microstructure stability during batch annealing.
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 a balance between recrystallization, strength, and ductility, ensuring high yield strength and tensile elongation across varying cold-rolling reductions, preventing delamination and splitting during shearing operations, and maintaining strength through grain refinement and precipitation strengthening.
Implementation Method 1
V is added in solid solution to accelerate precipitation and recrystallization
Implementation Method 2
batch annealed to promote recrystallization of the work-hardened microstructure
Implementation Method 3
Ti and Mo enhance thermal stability to maintain strength and formability
Data Source
Figure 1
AI summary
The invention relates to a variably rolled steel strip, sheet or blank, having at least one portion having a high thickness and at least one portion having a low thickness, wherein the variation in thickness between the high thickness and the low thickness is at least 35%, and wherein in the portion with high thickness the yield strength is 350 MPa or higher and the tensile elongation A50 (in %) and sheet thickness t (in mm) satisfy the equation of A50 / t0.2 ≥ 14, and in the portion with low thickness the yield strength is 450 MPa or higher and the tensile elongation A50 (in %) and sheet thickness t (in mm) satisfy the equation A50 / t0.2 ≥ 17.