Hot-Rolled Steel Sheet Coiling Window for Surface Defect Control
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Solution Overview
Problem
The fabrication of high-strength steel sheets for automotive applications is challenged by the formation of surface defects during high-temperature coiling, which increases production costs and reduces mechanical properties, and existing steels with molybdenum and vanadium are expensive and unsuitable for all deformation modes.
Innovation Solution
A hot rolled steel sheet composition with specific ranges of C, Mn, Si, Ti, Cr, Mo, Nb, Al, N, S, P, and optionally V, coiled between 525° C. and 635° C., followed by pickling, to prevent surface defects and achieve high mechanical strength, elongation, and hole-expansion ratio without requiring high rolling forces or additional cooling processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-temperature coiling is performed to precipitate titanium carbides and achieve maximum hardness, then mechanical strength is improved, but surface defects occur due to oxidation of elements like silicon, manganese, chromium and aluminum
Solution Approach 1:
The patent optimizes the coiling temperature parameter to a specific range (525-635°C) that balances two competing requirements: it is high enough to ensure adequate precipitation of titanium carbides for achieving maximum hardness and mechanical strength, but low enough to prevent excessive oxidation of alloying elements that would cause surface defects. This parameter optimization resolves the contradiction between strength improvement and surface quality.
2Object-affected harmful factors
If rapid cooling is performed to prevent surface defects, then surface quality is improved, but production cost increases due to additional processes
Solution Approach 1:
The patent applies preliminary action by carefully controlling the coiling temperature within the optimal range of 525-635°C before any cooling or pickling operations. This preliminary temperature control prevents the formation of excessive surface oxidation defects during coiling, thereby eliminating the need for additional rapid cooling processes or extensive pickling treatments, which reduces production costs while maintaining surface quality.
3Object-affected harmful factors
If coiling temperature is reduced to prevent surface defects, then surface quality is improved, but precipitation of titanium is reduced, affecting hardness
Solution Approach 1:
The patent identifies and applies an optimal coiling temperature range (525-635°C) that simultaneously satisfies two opposing requirements: it is low enough to prevent excessive oxidation and surface defects, but high enough to ensure adequate precipitation of titanium carbides for achieving maximum hardness. This optimized parameter range resolves the contradiction between surface quality and hardness.
4Strength
If high rolling forces are used to achieve desired microstructure, then mechanical properties are improved, but fabrication complexity and cost increase
Solution Approach 1:
The patent optimizes the composition parameters (specific ranges of C, Mn, Si, Ti, Cr, Mo, Nb, Al, N, S, and P) and processing parameters (coiling temperature, reduction rates) to achieve the desired microstructure (granular bainite with area percentage >70%, ferrite <20%, and controlled lower bainite, martensite and residual austenite) without requiring excessively high rolling forces. This parameter optimization simplifies the fabrication process and reduces equipment requirements while maintaining excellent mechanical properties.
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 cost-effective steel sheet with yield stress between 680 MPa and 840 MPa, tensile strength between 780 MPa and 950 MPa, elongation at failure greater than 10%, and a hole-expansion ratio greater than 45%, while preventing surface defects and ensuring compatibility with continuous hot dip zinc coating processes.
Implementation Method 1
this operation makes it possible, among other things, to precipitate the titanium carbides and to confer maximum hardness to the sheet
Implementation Method 2
for certain steels that include elements that are more oxidizable than iron, such as silicon, manganese, chromium and aluminum, certain sheets, once coiled at high temperature, exhibit surface defects
Data Source
AI summary
A method for the fabrication of a hot rolled steel includes providing a liquid metal comprising a certain chemical composition; carrying out a vacuum or SiCa treatment, the chemical composition including, expressed by weight 0.0005%≤Ca≤0.005%, if a SiCA treatment is carried out; dissolving quantities of Ti and N in the liquid metal so as to satisfy (% [Ti])×(% [N])<6.10−4%2; casting the steel to obtain a cast semi-finished product; rolling the cast semi-finished product with an end-of-rolling temperature between 880° C. and 930° C., a reduction rate of the penultimate pass being less than 0.25, and a start-of-rolling temperature of the penultimate pass being less than 960° C. to obtain a hot-rolled product, then cooling the hot rolled product at a rate between 20 and 150° C./s to obtain a hot rolled steel sheet; and coiling the hot rolled product to obtain a hot rolled steel sheet.