GB/T 3274-1988, Hot-rolled Carbon Structural Steels and Low-alloy Structural Steels Plates and Strips, specifies the standard for hot-rolled carbon structural steels and low-alloy structural steels plates and strips with a thickness below 5mm. The standard includes chemical composition, metallurgical quality, mechanical properties and other technological requirements. In addition, GB/T 3274-1988 also specifies processes for testing and inspection of steel plates and strips for dimensional accuracy, flatness and surface quality.
GB/T 3274-1988 is applicable to the hot-rolled steel plates and strips with a minimum thickness of not less than 5 mm. It is divided into five series according to the minimum tensile strength of steels, namely Q235 series (σb ≤ 235MPa), Q275 series (σb ≤ 275MPa), Q285 series (σb ≤ 285MPa), Q295 series (σb ≤ 295MPa) and Q345 series (σb ≤ 345MPa), among which the Q235 series is the lowest level and the Q345 series is the highest level.
The chemical composition of steels specified in GB/T 3274-1988 mainly includes the content of carbon, phosphorus, sulfur, smelting reductants, residual element ratio and other elements. According to its chemical composition, the hot-rolled plates and strips are divided into two types, namely low-alloy steel and small-alloy steel.
The metallurgical quality of steel plates and strips refers to their surface quality, allowable thickness-to-diameter ratio, allowable deviation from flatness and allowable deviation from straightness. For the steel plates and strips specified in GB/T 3274-1988, the surface quality requirements are that the surface should be free of defects caused by the steelmaking process, such as scale, adhesion, cratering, folds, etc.; the allowable thickness-to-diameter ratio is ≤ 4.0; the allowable deviation from flatness is ≤ 0.30mm/m, and the allowable deviation from straightness is ≤ 0.15mm/m.
The mechanical properties of steel plates and strips involve the thickness direction and the width direction of steel plates and strips, as well as their performance in tension, bending and flattening tests. The tensile strength, yield strength and elongation of GB/T 3274-1988 are presented as a series of values corresponding to steel plates of different thickness and strength grades.
Testing requirements for the inspection of steel plates and strips involve the mechanical tests, chemical analysis tests and other tests. For mechanical tests, tensile, bending and flattening tests are performed on each plate and strip. For chemical analysis tests, chemical analysis of the material is performed to check for inclusion elements, residual elements and impurity elements, and to determine the chemical composition of steel. For other tests, the surface quality, dimensional accuracy, flatness and straightness, etc. of steel plates and strips also need to be tested.
To sum up, GB/T 3274-1988 specifies the standard for hot-rolled carbon structural steels and low-alloy structural steels plates and strips with a thickness below 5mm, including chemical composition, metallurgical quality, mechanical properties, testing and inspection requirements, etc. which provides a guide for the production and purchase of steel plates and strips.
