Hot-Rolled Al-Coated Steel Sheet With Controlled Coiling Oxidation
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Solution Overview
Problem
Hot-rolled and coated steel sheets with thicknesses between 1.8 mm and 5 mm experience poor coating adhesion after hot-stamping due to intergranular oxidation, leading to misalignment defects and uncontrolled coating thickness, which affects weldability and productivity.
Innovation Solution
Controlling the coiling temperature to limit intergranular oxidation and optimizing the steel composition to balance carbon, manganese, and silicon content, while ensuring the surface percentage of voids and intermetallic layer thickness within specific ranges to maintain coating adhesion and thickness control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If hot-rolling is used to produce steel sheets with thickness between 1.8 mm and 5 mm, then the thickness requirement is met and productivity is maintained, but coating adhesion after hot-stamping deteriorates due to intergranular oxidation
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet, specifically controlling carbon content at 0.15-0.40%, manganese at 1.00-3.50%, and silicon at 0.05-0.70%, along with their ratios, to prevent intergranular oxidation during hot-rolling while maintaining coating adhesion after hot-stamping
Solution Approach 2:
The invention creates a composite material system by combining specific alloying elements (C, Mn, Si) in controlled ratios to achieve a steel composition that resists intergranular oxidation during hot-rolling while maintaining good coating adhesion after hot-stamping
2Reliability
If cold-rolling is used to produce coated steel sheets, then coating adhesion is generally good, but sheets with thickness between 1.8 mm and 5 mm cannot be produced due to equipment limitations and insufficient flatness
Solution Approach 1:
The invention changes the production method from cold-rolling to hot-rolling, and controls the chemical composition parameters (C: 0.15-0.40%, Mn: 1.00-3.50%, Si: 0.05-0.70%) to enable production of thick steel sheets (1.8-5 mm) with good coating adhesion that would otherwise be impossible with cold-rolling
3Ease of manufacture
If the coating thickness is not tightly controlled, then the pickling process is simpler, but weldability deteriorates and coating thickness falls outside the targeted range of 10-33 μm
Solution Approach 1:
The invention optimizes the steel composition parameters (carbon 0.15-0.40%, manganese 1.00-3.50%, silicon 0.05-0.70%) to control the metallurgical reaction during pickling, enabling the coating thickness to be maintained within the targeted range of 10-33 μm while preserving good weldability
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
Achieves improved coating adhesion and controlled coating thickness between 10 and 33 µm, enhancing weldability and maintaining productivity at the pickling line.
Implementation Method 1
the adhesion of the coating on the surface of the steel part further to hot-stamping is unsatisfactory, which leads to poor adhesion of the painting on the hot-stamped part
Implementation Method 2
the inventors have discovered that the coating adhesion can be improved under certain circumstances slowing down the pickling process
Implementation Method 3
the thickness of the coating, before and after hot-stamping, cannot be tightly controlled
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=650-140×fγ, Tcoilmax being expressed in degrees Celsius and fy 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.