1.What are the effects of uneven hot rolling temperature?
The raw material for cold-rolled coils is hot-rolled coils. Studies have found that during the cooling process of hot-rolled coils, the cooling rate at the beginning and end is typically faster than in the middle. For steels like DP980, this results in significantly higher strength at the beginning and end compared to the middle, and this difference is inherited by subsequent cold-rolled products.
2.What are the effects of temperature fluctuations in a continuous annealing furnace?
The final properties of cold-rolled coils are largely determined by the annealing process.
Too fast an annealing speed: Insufficient heating of the strip in the furnace may lead to incomplete recrystallization or insufficient grain growth, resulting in high strength but low elongation, meaning the material becomes hard and brittle.
Too slow an annealing speed: Excessively high strip temperature, potentially exceeding the austenitizing temperature of pearlite, causes spheroidization of pearlite in the microstructure, significantly reducing the material's strength.
3.What are the characteristics of "carbon gain" in bell-type annealing?
For thin-gauge IF steel (interstitial atomless steel, a type of deep-drawing steel) annealed in a bell-type furnace, this is a typical cause of uneven performance.
Principle: The emulsion remaining on the strip surface after pickling and rolling will crack during annealing heating. The cracked carbon reacts with hydrogen to produce methane. When the temperature exceeds 700℃, the methane decomposes to release active carbon atoms.
Consequence: These active carbon atoms adsorb onto the strip surface and diffuse inward, causing surface carburization. Because the edges of the steel coil have more contact with the atmosphere, the carbon gain at the edges is much higher than in the middle, resulting in a much higher edge strength. Studies have shown that the resulting yield strength difference can reach an astonishing 89 MPa.
4.What are the genetic effects of hot rolling coiling temperature?
The coiling temperature in the hot rolling process affects not only the longitudinal direction but also the transverse direction of the microstructure uniformity. Higher coiling temperatures (such as 650℃) can lead to finer grains or even mixed grains in the hot-rolled sheet, and the precipitation of coarse cementite at the grain boundaries. These non-uniform microstructures will be inherited by the finished sheet after subsequent cold rolling and annealing, resulting in differences in mechanical properties (such as the plastic strain ratio r value) in different parts.
5.What are the consequences of improper sample size design?
According to the latest research, if the ratio of the clamping section width to the parallel section width of the tensile specimen is not designed properly, the stress state will be affected during the test, causing abnormal fluctuations in the tensile curve during the uniform plastic deformation stage. This will distort the measured yield point elongation and other indicators, leading to the mistaken conclusion that the material itself is unstable.