Hot-Rolled Flat Steel Composition for Uniform Strength and Low Springback
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Solution Overview
Problem
Existing hot-rolled flat steel products exhibit significant fluctuations in mechanical properties across their length and width due to non-uniform temperature distribution during cooling, leading to inconsistent strength, formability, and increased springback, making them unsuitable for complex component manufacturing.
Innovation Solution
A hot-rolled flat steel product with a specific composition and microstructure, primarily containing niobium as the sole microalloying element, is produced through a controlled thermo-mechanical rolling and cooling process to achieve uniform mechanical properties and low springback, featuring a fine-grained structure with aligned fiber orientations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional multi-element microalloying (Ti, V, Nb) is used to achieve high strength, then yield strength increases, but mechanical property uniformity across length and width deteriorates due to non-uniform temperature distribution during cooling
Solution Approach 1:
The patent extracts titanium and vanadium from the microalloying element combination, using niobium as the sole microalloying element. This simplification eliminates the problem of non-uniform precipitate formation that occurs with multiple elements, thereby achieving both high strength and uniform mechanical properties across the steel product length and width
Solution Approach 2:
The patent changes the chemical composition parameter by specifying a niobium content range of 0.04-0.12 wt.% and limiting titanium to ≤0.12 wt.% and vanadium to ≤0.2 wt.%. This parameter optimization ensures uniform precipitate distribution and consistent mechanical properties while maintaining high yield strength
2Strength
If high titanium content is used to achieve high strength in titanium-based microalloyed steels, then yield strength increases, but forming properties deteriorate due to sharp-edged titanium nitride precipitates
Solution Approach 1:
The patent removes titanium's dominant role in strength enhancement by limiting its content to ≤0.12 wt.% and replacing it with niobium as the primary microalloying element. This eliminates the formation of sharp-edged titanium nitride precipitates that harm forming properties, while niobium provides equivalent or superior strength enhancement with more favorable precipitate morphology
Solution Approach 2:
The patent optimizes the alloy composition by setting niobium content at 0.04-0.12 wt.%, which provides sufficient strength through niobium carbonitride precipitates that have more favorable morphology for forming, while keeping titanium content low (≤0.12 wt.%) to avoid harmful precipitate formation
3Strength
If vanadium is used to enhance strength through precipitation hardening, then yield strength increases, but manufacturing cost increases significantly
Solution Approach 1:
The patent replaces expensive vanadium with more cost-effective niobium and titanium as the primary microalloying elements. By limiting vanadium content to ≤0.2 wt.% and relying on the synergistic effect of niobium (0.04-0.12 wt.%) and titanium (≤0.12 wt.%), the patent achieves the required strength at lower manufacturing cost
Solution Approach 2:
The patent creates a composite microalloying system based on niobium and titanium, where niobium provides the primary strength enhancement through precipitation hardening and titanium contributes to grain refinement and secondary strengthening. This composite approach achieves vanadium-level strength at lower cost
4Productivity
If non-uniform temperature distribution occurs during cooling of hot-rolled steel, then production efficiency is maintained, but mechanical property consistency across the product deteriorates
Solution Approach 1:
The patent changes the microalloying composition parameter to use niobium as the dominant element with optimized content (0.04-0.12 wt.%). Niobium's precipitation behavior is less sensitive to temperature variations compared to titanium and vanadium, allowing uniform mechanical properties to be achieved even with the inherent non-uniform temperature distribution during rapid cooling, thereby maintaining both productivity and precision
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 results in a steel product with high yield strength, excellent toughness, and low springback, enabling the production of complex components with uniform mechanical properties across its length and width, suitable for applications in automotive and truck construction.
Implementation Method 1
The fine grain structure and the high density of fine and very fine precipitates, together with the solid solution strengthening caused by the presence of manganese and silicon, lead to the high strength of such flat steel products
Implementation Method 2
The fine grain structure and the high density of fine and very fine precipitates, together with the solid solution strengthening caused by the presence of manganese and silicon, lead to the high strength of such flat steel products
Implementation Method 3
By processing the precursor cast from an alloyed steel melt into a hot-rolled strip in a controlled thermo-mechanical rolling and cooling process, the hot-rolled flat steel product obtained in this way has a very fine-grained microstructure. This typically consists predominantly of ferrite and/or bainite and small amounts of pearlite or ferrite.Cementite
Data Source
AI summary
The invention provides a hot-rolled flat steel product with a particularly uniform distribution of its mechanical properties over its length and width, which also has good formability and a low tendency to spring back. The steel flat product consists of (in wt.%) C: 0.02 - 0.1%, Mn: 0.1 - 2.5%, Al: 0.02 - 0.1%, Nb: 0.02 - 0.12%, and optionally one or more elements from the group "Si, Ti, V, Cr, B, Ca, Mo" with the stipulation that the Si content is ≤ 0.6%, the Ti content ≤ 0.12%, the V content ≤ 0.2%, the Cr content ≤ 0.2%, the B content ≤ 0.0025%, the Ca content ≤ 0.01% and the Mo content ≤ 0.3%, and as a remainder of iron and unavoidable impurities, wherein the impurities include ≤ 0.05% P, ≤ 0.03% S, ≤ 0.01% N, ≤ 0.2% Ni, ≤ 0.15% Cu count.The microstructure of the steel flat product consists of ≥ 60% of its surface area of ferrite and/or bainite, with the remainder consisting of pearlite, carbide or carbonitride precipitates, and ≤ 2% of its surface area of other microstructural constituents, and exhibits a grain size distribution of 0.2–0.7. The steel flat product also possesses a yield strength Re, for which Re > RE_BER, where Re_BER = (400 + 2243 * %Nb) / (d0.15), where %Nb is the Nb content in wt.% and d is the respective thickness of the steel flat product in mm. The invention also describes a method for producing such a steel flat product.