At present, cold-rolled steel plates usually come from international technology leaders, and the material grades are also produced in Europe, the United States, Japan and other places with leading technologies, although the ingredients are roughly the same. There are still certain differences, resulting in differences in forming, welding, surface treatment, fatigue, etc. When measuring material elements of St12, St13, St14 and other brands, whether it is direct reading spectroscopy or chemical analysis, it is easily affected by factors such as the surface condition of the sample (flatness, oil stains, coating residues, etc.) and the introduction of foreign matter. This This results in distortion of test data for the content of elements such as carbon.
There are certain differences in the carbon content range of the same material in different steel mills, and there is a certain overlap in the carbon content of different grades. It should be pointed out that due to the continuous advancement of metallurgical technology, the St14 grades of Baosteel, Anshan Iron and Steel, Benxi Iron and Steel, Shougang and other steel plants have basically developed into interstitial-free steel containing effective Ti elements, and the carbon element content is usually controlled to ≤0.003%. However, some steel plants do not yet have this control feature. St12: C≥0.02% and does not contain effective Ti and Nb elements, the typical value of C content is 0.05%~0.08%; St13: C<0.05% and does not contain effective Ti and Nb elements, the typical value of C content is 0.01%~0.03 %; St14: C≤0.01% and does not contain effective Ti and Nb elements, which usually meets but sometimes is less than the effective Ti content value calculated by the formula; St16: C≤0.006% and does not contain effective Ti and Nb elements, which meets the formula to calculate the effective Ti content content value.
Determine according to the technical requirements of relevant standards. When it cannot be clearly divided, other factors can be referred to, such as metallographic structure. There is no clear technical standard for the metallographic structure of St12, St13, St14, and St16 series cold-rolled steel plates, but usually the change pattern of the St12-St16 metallographic structure is: ferrite + a very small amount of pearlite or ferrite + tertiary cementite → Ferrite + tertiary cementite → Ferrite, as the stamping level of the material continues to increase, pearlite gradually disappears, and tertiary cementite gradually decreases or rarely exists.
Cold-rolled steel plates St12-St16 series, 08, SPHC, SAPH400, SAPH440, P210, LA420, etc. are widely used for flat washers and special-shaped fasteners in automobiles, home appliances, construction, packaging and other fields due to their high strength and excellent stamping performance. pieces. With the increasingly fierce market competition, people have higher and higher requirements for the surface quality of cold-rolled steel plates, mainly for the surface treatment quality of flat washers and special-shaped fasteners. Therefore, it is very necessary to study the linear defects on the surface of cold-rolled steel plates.
The defect of black discoloration on the surface of cold-rolled steel plates is a frequently encountered quality problem. The surface of the black-discolored steel plate is obviously darker than the bright normal plate. The blackened defect material is firmly bonded to the substrate and cannot be removed by degreasing with a strong alkaline degreasing agent. After phosphating treatment, the morphology of the phosphating film was observed under a scanning electron microscope. The phosphating morphology of the normal steel plate was normal, and the weight of the phosphating film was 1.43g/m2; while the phosphating crystals on the surface of the defective steel plate were not completely covered, and the phosphating film The weight is only 0.78g/m2. Black discoloration defects on the surface will cause the steel plate to be degreased and have a low coverage rate of the phosphating film, seriously affecting the adhesion and corrosion resistance of subsequent coatings.
The surface composition of the steel plate was analyzed with a scanning electron microscope energy spectrometer. The results showed that the peak value of C element in the black discoloration sample was stronger, while the peak value of C element in the normal sample was weaker. There was no obvious difference in other aspects. Further confirmation of the structural analysis of the C element in the black discoloration defect on the surface shows that the black discoloration on the surface of the steel plate is caused by the enrichment of a large amount of C element in the form of graphite on the surface of the strip. The sources of C element can be divided into two major categories: one is exogenous, that is, from the cracking and distillation of organic residues such as rolling oil on the surface of the steel plate; the other is endogenous, that is, seeping into the steel plate matrix. After the carbon body (Fe3C) decomposes, it diffuses to the surface.
The influence of the composition and content of steel on the graphitization of C element on the surface of strip steel is mainly reflected in two aspects: first, whether the element can generate stable carbides, thereby inhibiting the decomposition of Fe3C and the precipitation of C element; second, whether the element It also has the tendency of surface segregation and forms a competitive segregation relationship with the C element.
The annealing process of cold-rolled steel plates has a great impact on the surface state. During bell furnace annealing, especially during nitrogen and hydrogen bell furnace annealing, due to the weak reducing atmosphere in the furnace and insufficient methanol cracking, slight cracks will be formed on the surface. The oxide layer becomes rough, even sponge-like, loose and porous. Studies have shown that as long as there are pores on the surface, there is a great danger of graphite. Insulation between 570-680°C must be avoided. The process temperature is exactly in the temperature range where graphitization is most strongly reflected.
There is no significant difference in the surface composition of the black discolored defective steel plate and the normal steel plate, and the C element content on the surface of the defective steel plate is significantly higher than that of the normal steel plate. The black discoloration defect on the surface of the steel plate is caused by the enrichment of a large amount of C element in the form of graphite on the surface of the strip. The formation of graphitization defects of C element on the surface of the steel plate is closely related to the chemical composition of the steel plate, the surface condition of the steel plate, and annealing and other production.
In short, the control of the surface quality of cold-rolled steel plates is very necessary for the production of fasteners and can only be strengthened, not weakened.
