Analysis of Reasons for Unqualified Reinforcement Quality Project
Weight deviation refers to the allowable deviation between the actual weight of steel bar and the theoretical weight. Some small and medium-sized enterprises do not organize production according to the standard and roll with excessive negative deviation, which seriously affects the sampling qualified rate. At the same time, unqualified weight deviation may also lead to unqualified size, process and mechanical properties. Table 10-5 is the statistical results of unqualified national spot checks of hot-rolled ribbed steel bars in recent years, which shows that the weight deviation of spot checks is the main unqualified item. The lowest deviation of weight exceeded the standard value was - 14.30%, the highest was - 180%, most exceeded the range of - 20% to - 40%.
The main reasons for this phenomenon are:
(l) Competition in the market is fierce. In order to reduce production costs, enterprises adopt improper means to improve efficiency and organize production according to the lower limit of weight deviation in management, resulting in weight deviation as the main unqualified item;
(2) In order to meet the requirements of middlemen, enterprises pursue profit maximization and deliberately produce negative deviation and ultra-negative deviation products.
(3) Weight deviation items are not included in the sampling inspection items of construction sites, which is one of the reasons for the existence of super negative deviation products.
Previous national supervision spot checks attach great importance to weight deviation. Data show that in recent years, the number of unqualified weight deviation declines year by year. In 2011, no unqualified items of weight deviation appeared in the results of spot checks, which shows that good results have been achieved in recent years by taking weight deviation as a key inspection item.
The unqualified dimensions and shapes are mainly rib spacing, transverse rib height and inner diameter. The shape of steel bar affects the bond strength between steel bar and concrete. The main reason for this phenomenon is that in order to save costs, production enterprises generally organize production according to negative deviation of inner diameter, which easily results in smaller inner diameter and transverse rib height and larger rib spacing. On the other hand, some enterprises have low awareness of product quality management, fail to use standard rolls and strictly implement the roll change slot system.
The main chemical constituents are C.P.S content exceeding the standard. The higher the content of C in steel, the lower the plasticity and impact, the worse the weldability of steel, the greater the cold brittleness and aging sensitivity, and the lower the atmospheric corrosion resistance; the higher the content of P, the greater the cold brittleness, the worse the cold working and weldability of steel; the higher the content of S, the more serious the hot brittleness, and the lower the various mechanical properties of steel. It also reduces the weldability, impact toughness, fatigue resistance and corrosion resistance of steel. Increasing C content in steel helps to increase yield point and tensile strength. In order to ensure yield point and tensile strength, some enterprises tend to exceed the standard C content. In addition, the content of P and S exceeded the standard mainly due to improper control in steelmaking process or lack of strict implementation of billet acceptance system.
The main mechanical properties are unqualified yield strength. In addition to the influence of chemical composition, size and weight deviation on mechanical and technological properties, improper control of steelmaking, continuous casting and rolling process is also one of the reasons for unqualified mechanical properties.
The unqualified items of surface marks are mainly due to the revision of product standards and the failure of enterprise management to follow up in time.
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