High strength ship steel plate ABS AH36’s Reason analysis for unqualified mechanical property

When ABS AH36 was tested, it was found that the tensile strength of the batch of steel plates was slightly higher than the required range. The elongation of ABS AH36 material was lower than the technical requirements, and the margin of impact toughness was not large. As a high-strength hull structure, the final inspection was carried out. The party rejected the shipment as a non-conforming product. For this reason, the cause analysis of the batch of ABS AH36 steel plate was carried out.

1.1 ABS AH36 steel plate acceptance results of mechanical properties

1.1.1 ABS AH36 steel plate tensile test
According to GB/T2975-1998, two samples were taken and the steel plate was subjected to tensile test according to GB/T228-2002 standard. The sample type is P10, and the results are shown in Table 1. The tensile strength of the steel plate is slightly higher than that of the classification society for the strength of the ABS AH36 high-strength steel plate, and the elongation is lower than required.
1.1.2 ABS AH36 steel plate bending test
According to the GB/T 232-1999 standard, the bending test is carried out on the steel plate. The diameter of the sample core is 45 mm and the bending angle is 180°. There is no visible crack on the outside of the material, and the bending performance meets the requirements.
1.1.3 Impact test The material shall be subjected to low temperature impact according to GB/T229-1994 standard, longitudinal specimen, test temperature 0 °C, notched type V-2 mm, test results are shown in Table 2. Basically meet the technical requirements of the ABS AH36 steel plate, but the margin is not large.

1.2 ABS AH36 steel plate chemical composition

According to the GB/T 4336-2002 standard, the material of the steel plate was inspected by direct reading spectrometer. The chemical composition of the material complies with the composition requirements of ABS AH36 high-strength steel in GB/T712-2000. The test results are shown in Table 3. The chemical composition meets the requirements.

1.3 Metallographic analysis of ABS AH36 steel plate

The cut surface of the sample is taken from the longitudinal direction of the material. After grinding and polishing, it is made into a standard metallographic sample. According to the GB/T13298-1991 standard, the microstructure of the ABS AH36 steel plate is iron after being etched by 4% nitric acid. The body + pearlite, which is normal tissue, has an average grain size of 10 (see Figure 1), but has a distinct band-like distribution with a banded structure of 2.5.

ABS AH36 steel plate Result analysis

As a high-strength ABS AH36 steel plate, microalloying is generally used to refine the grain to increase the strength of the ABS AH36 steel plate. From the chemical composition, Nb and Al are added to the material, and the grain size reaches 10 (requires level 5 or higher). The strength of the material is achieved and slightly above the requirements. At the same time, ABS AH36 steel is a low-alloy structural steel, which is a hypoeutectoid steel. It is generally considered that when the steel ingot is heated or the continuous casting billet is heated, the diffusion coefficient of the alloying elements and other impurity elements is low, it is difficult to homogenize, and the composition or dendrite segregation is easy to occur. . When hot rolling, the hypoeutectoid steel is cooled from the austenite state during the finish rolling. First, the pro-eutectoid ferrite is precipitated from the Ar3 point temperature, and pearlite is formed when cooled to Ar1. Due to the presence of dendrite segregation, After thermal deformation, it becomes a band-like structure, which causes different components of dendrites to be different, resulting in different time of precipitation of pro-eutectoid ferrite, thereby forming a strip-like structure in which pearlite and ferrite are alternated. The presence of banded structure seriously affects the mechanical properties of ABS AH36 steel plate