Hot-Rolled Al-Coated Steel Sheet With Controlled Pickling
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The challenge is to produce hot-rolled and coated steel sheets with a thickness between 1.8 mm and 5 mm that achieve excellent coating adhesion after hot-stamping, while maintaining controlled coating thickness and ensuring productivity at the pickling line, as existing methods face issues with adhesion and thickness control, particularly due to intergranular oxidation and the limitations of cold-rolling processes.
Innovation Solution
The method involves optimizing the chemical composition of the steel, controlling the pickling process to reduce intergranular oxidation, and using specific coating techniques such as Al or Al-alloy coatings with Zn deposition to achieve a ferrito-pearlitic structure and intermetallic layers with controlled thickness, ensuring the surface percentage of voids and intergranular oxidation are within specified limits to enhance adhesion and maintain targeted coating thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If cold-rolling is used to produce coated steel sheets with thickness between 1.8 mm and 5 mm, then the steel sheets can be produced, but the flatness is insufficient resulting in misalignment defects
Solution Approach 1:
The invention changes the rolling temperature parameter from cold-rolling to hot-rolling (above Ac3 transformation temperature), which fundamentally alters the material behavior and enables production of thick steel sheets (1.8-5 mm) with adequate flatness and no misalignment defects
2Length of moving object
If hot-rolling is used to produce steel sheets with thickness higher than 1.6 mm, then the thickness requirement is met, but the coating adhesion after hot-stamping is unsatisfactory
Solution Approach 1:
The invention optimizes multiple parameters including final rolling temperature (Ar3-Ac3 transformation temperature range), cooling rate (10-50°C/s), and pickling conditions to control the surface morphology and intergranular oxidation, thereby achieving satisfactory coating adhesion while maintaining thick section dimensions
Solution Approach 2:
The invention performs preliminary pickling treatment before coating to remove scale and control surface morphology, and performs preliminary microstructure control through specific rolling and cooling parameters to ensure the steel surface is ready for good coating adhesion before the hot-stamping process
3Reliability
If pickling process is intensified to improve coating adhesion, then the adhesion is improved, but the coating thickness control is worsened and productivity is reduced
Solution Approach 1:
The invention optimizes pickling parameters (acid concentration, temperature, time) to achieve the right balance where sufficient pickling improves coating adhesion while excessive pickling that would harm thickness control and productivity is avoided
Solution Approach 2:
The invention implements control based on measuring the depth of intergranular oxidation and surface morphology, using this feedback to adjust pickling intensity to maintain coating thickness within target range (10-33 μm) while ensuring good adhesion
4Reliability
If the depth of intergranular oxidation is increased to improve coating adhesion, then the adhesion is improved, but the coating thickness becomes uncontrolled
Solution Approach 1:
The invention optimizes the depth of intergranular oxidation through controlled pickling parameters rather than allowing excessive oxidation, achieving sufficient adhesion while maintaining coating thickness within the target range of 10-33 μm
Solution Approach 2:
The invention uses measurement of intergranular oxidation depth and surface morphology as feedback to control the pickling process, ensuring oxidation is sufficient for adhesion but not excessive to cause thickness control problems
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
This approach results in improved coating adhesion and controlled coating thickness between 10 and 33 µm, ensuring satisfactory paint adhesion and maintaining high productivity at the pickling line, even with reduced pickling intensity, thereby addressing the limitations of existing methods.
Implementation Method 1
a first phase transformation into ferrite and/or pearlite upon cooling from an austenitic structure
Implementation Method 2
an Al or an Al-alloy coating bonded metallurgically to the hot-rolled steel sheet
Implementation Method 3
controlling the pickling process to reduce intergranular oxidation
Data Source
Figure 1~2
AI summary
A method for manufacturing a hot-rolled and coated steel sheet having a thickness between 1.8 mm and 5 mm, comprising providing a semi-product having a composition comprising: 0.04% ≤ C ≤ 0.38%, 0.40% ≤ Mn ≤ 3%, 0.005% ≤ Si ≤ 0.70%, 0.005% ≤ Al ≤ 0.1%, 0.001% ≤ Cr ≤ 2%, 0.001% ≤ Ni ≤ 2%, 0.001% ≤ Ti ≤ 0.2%, Nb ≤ 0.1%, B ≤ 0.010%, 0.0005% ≤ N ≤ 0.010%, 0.0001 % ≤ S ≤ 0.05%, 0.0001% ≤ P ≤ 0.1%, Mo ≤ 0.65 %, W ≤ 0.30%, Ca ≤ 0.006%, hot-rolling with a final rolling temperature FRT, to obtain a hot-rolled steel product having a thickness between 1.8 mm and 5 mm, then cooling down to a coiling temperature Tcoil satisfying: 450°C ≤ Tcoil ≤ Tcoilmax with Tcoilmax=650-140xfγ, Tcoilmax being expressed in degrees Celsius and fγ designating the austenite fraction just before the coiling, and coiling to obtain a hot-rolled steel substrate, pickling and coating the hot-rolled steel substrate with Al or an Al alloy by continuous hot-dipping in a bath, to obtain a hot-rolled and coated steel sheet comprising a hot-rolled steel sheet and an Al or an Al alloy coating, having a thickness comprised between 10 and 33 μm, on each side of the hot-rolled steel sheet.