1.What are the chemical composition and internal quality of the material itself?
Chemical composition deviation: Insufficient content of reinforcing elements such as carbon and manganese will lead to insufficient strength; excessive content of impurity elements such as sulfur and phosphorus will reduce plasticity, resulting in unacceptable elongation (material becomes brittle).
Residual element influence: Excessive levels of residual elements such as copper and tin in steel may cause hot brittleness and also affect the final tensile properties.
Non-metallic inclusions: The presence of large amounts of oxides and sulfides in steel is equivalent to forming crack initiation sites in the matrix. During tensile testing, these sites are prone to premature fracture, leading to unacceptable strength or elongation.
2.What are the effects of cold rolling reduction rate?
The cold rolling process hardens and embrittles the material (work hardening). If the total reduction during cold rolling is too high, the plasticity may not be fully restored during annealing, resulting in a lower elongation of the finished product.
3.What are the effects of temperature variations in the annealing process?
Annealing temperature too low or holding time insufficient: The steel is not sufficiently softened, resulting in material that is too hard (excessively high strength) and insufficient elongation.
Annealing temperature too high: The grains grow too large. Although the elongation may be acceptable, it will lead to a significant decrease in yield strength and tensile strength, failing to meet standards.
Annealing atmosphere issues: If the atmosphere inside the annealing furnace is not well controlled, decarburization of the steel plate surface will occur, resulting in low surface strength and affecting overall tensile properties.
4.What impact will problems with the testing machine or its operation have?
Slippage during clamping can lead to false yielding.
Excessive stretching rate usually results in an overestimation of strength and an underestimation of elongation (especially for materials with insignificant yield strength).
Improper installation of the extensometer will result in an underestimation of elongation.
5.What other external factors are there?
Aging Effects: If cold-rolled coils are left for an extended period after rolling (especially mild steel), room temperature aging will occur, leading to an increase in yield strength and a decrease in plasticity. It is recommended to complete the test within the performance stabilization period.
Specimen Orientation: Cold-rolled coils exhibit anisotropy. Generally, the elongation of transverse specimens is lower than that of longitudinal specimens. If the standard specifies transverse specimens but longitudinal specimens are mistakenly used, the results may not be comparable.