Color-coated steel features excellent corrosion resistance, good formability, and bright colors. Due to these advantages, it is widely used in construction, home appliances, light industry, transportation, and other industries. In order to control the quality of pre-painted steel, there are a series of performance tests, including appearance tests, physical property tests, anti-aging and anti-corrosion performance tests.
colored steel sheets
Physical Property Tests
Physical property tests include coating thickness test, hardness test (pencil hardness, scratch hardness), flexibility test (impact test, shaft bending test, T-bending test, cupping test, tensile test), adhesion test (cross-cut test, scratch resistance test), curing degree test (glass transition test, solvent resistance test, dry heat test), and other tests.
Paint Thickness Test
The magnetic coating thickness gauge is commonly used to measure the paint thickness of prepainted steel with cold-rolled sheets and hot-dip galvanized substrates. When the paint thickness is below 50 μm, it should be accurate to 1 μm, and when it is above 50 μm, accurate to 2 μm. Also, the micrometer is used to measure the coating thickness, that is, the difference between the thickness of the color-coated steel before and after the paint is removed. The other one is DJH method, which uses an optical microscope to observe and position the painting through the drill hole and then calculates the paint thickness according to the taper and the horizontal distance.
Bending Test
The T bending test aims to measure the flexibility of coating. The first step is to bend the sample around itself by 180° and then observe the cracking or peeling of the coating. And the minimum multiple values that will not cause paint cracking or peeling is the result of the test (denoted as nT ). T bending test evaluates the anti-cracking or anti-peeling ability of the coating when the sample is bent.
Impact Test
The hammer of the tester impacts the sample heavily so that the sample is rapidly deformed to form a convex area. Then check the coating in the impacted area to evaluate the resistance to cracking or peeling of the coating. There are 5 different levels.
L5: The coating has no peeling and cracking;
L4: The coating does not peel off, but there are cracks;
L3: The coating has tiny cracks;
L2: The coating has tiny peeling;
L1: The coating is peeled off on the bent area and impacted area.
L3, L4, and L5 are qualified products.
Hardness Test
The pencil hardness test is designed to measure the hardness of the coating. It uses a set of pencils with a known hardness to plow the coating to measure the relative hardness of the coating. There are two methods: using the instrument and the manual method. In the instrumental test, the constant load of the scratch is (7.5+0.1) N, and the moving speed is 0.5 mm/s. If the pencil is pushed manually, the lead moves 6.5mm forward at 45° along the surface. The test starts with the hardest pencils and tests each pencil in turn until at least 4 pencils are found that cannot plow through the coating.
Cupping Test
The cupping test also aims to evaluate the resistance to cracking or peeling of the coating. First, push the drift plug of the tester from the back of the sample at a constant speed to a specified depth. Then check if the coating is cracked or peeled off from the substrate. The cupping test is to evaluate the strength, elasticity, and adhesion of the coating with elongation of the substrate.
Cross-cut Test
The cross-cut test is designed to evaluate the adhesion of the coating to the substrate. The first step is to cut through the coating to the substrate out a lattice pattern with a cross-hatch cutter. Then stick the tape on the cut area and tear it off to evaluate the adhesion of the coating according to the peeling area.
Solvent Resistance Test
