Analysis and Research on the Abnormal Tensile Fracture of Ship Plate F36

Using spectrometer, metallographic microscope and scanning electron microscope to study the phenomenon of fracture separation of ship plate F36 during the stretching process,analyze. The results show that the difference in plasticity of the bainite structure, ferrite and pearlite structure in the thickness direction of the sample causes the uncoordinated deformation of the sample during stretching, and there is a Z -direction in the center of the thickness of the necking part during the stretching process. The stress concentration causes the center separation of the specimen fracture.
Key words : ship plate; tensile fracture; separation; stress concentration

With the rapid development of the domestic shipbuilding industry and the increasingly fierce competition in the steel market, more and more attention has been paid to marine steel plates, and the demand for steel plates with high strength, toughness, weldability, and high surface quality is also increasing. As one of the important medium and heavy plate varieties, the annual production volume is quite high.
Due to the particularity of the shipbuilding industry, the internal quality of the ship plate is directly related to the safe use of the ship.

• Ship Plate F36 Experimental materials and methods

sample material is F36-Z25 steel plate with a thickness of 40mm ; the thickness of the tensile sample is 20mm , and the sampling site is from the surface of one side of the steel plate to 1/2 of the thickness , and there is a layered feature at nearly 1/2 of the tensile sample .
After ultrasonic cleaning the fracture , observe and analyze it with a JSM-6490 scanning electron microscope, intercept the longitudinal section of the fracture of the tensile sample and the normal clamping part far away from the fracture, grind and polish, corrode with 4% nitric acid alcohol solution and observe the structure , and perform spectrochemical composition analysis on the sample at the normal clamping position.

• Ship Plate F36 Experimental results

Ship Plate F36 Chemical composition
of the intercepted sample was analyzed with a spectrometer ARL 4460OES Metals Analyzer , the results are shown in the table below, and the chemical composition meets the requirements of the classification society.

Table 1 Chemical composition of F36 ship plate /%

( C )	en ( Mn )	en ( P )	en ( S )	en ( And )	ro ( Ni )	ro ( Cr )	en ( With )	en ( Al )	ro ( Nb )	en ( V )	ro ( Ti )
0.072	1.47	0.009	< 0.002	0.207	0.027	0.150	0.036	0.030	0.033	0.005	0.009

Macroscopic morphology of fracture
Macroscopic longitudinal section morphology of tensile fracture delaminated specimen. It can be seen that there is an obvious separation in the center of the thickness of the sample, and there are obvious signs of cracking at the extension of the center separation, and the fracture is dull and dull without obvious metallic luster; the morphology of the layered fracture is observed with a scanning electron microscope JSM-6490 , The central crack is deep, and there is no inclusion inside the crack.

This separation phenomenon is significantly different from the delamination phenomenon of the steel plate. There are obvious metal separation phenomena on the layered section of the steel plate, unrolled gaps, inclusions, air bubbles in the steel, large non-metallic inclusions, unremoved residual shrinkage cavities or rolling folds can be seen at the defects. It may cause delamination of the steel plate, which can lead to deterioration of the mechanical properties of the steel plate, especially a significant decrease in the mechanical properties along the thickness direction, which will lead to the scrapping of the steel plate. The delamination or separation mentioned in the ship plate inspection refers to the separation and tearing of the fracture of the tensile specimen during the tensile test, which only refers to the fracture of the tensile specimen and does not represent the entire steel plate. defective. Therefore, the fracture " delamination " in ship plate survey should be called " separation " more accurately.

Stress state analysis
During the steel tensile test, the deformation process of the sample has three stages: elastic deformation, plastic deformation and local concentrated plastic deformation caused by necking. When the tensile sample does not shrink, under the action of tensile stress, the sample undergoes uniform elastic and plastic deformation, and each part of the sample is subjected to uniform axial stress. As the tensile stress continues to increase, the sample will produce necking phenomenon.
Some mechanical studies have also shown that: Although the engineering stress-strain curve shows a downward trend after reaching the tensile strength, the true stress-strain curve of the necked part shows an upward trend. The Z -direction stress along the thickness direction is also increasing, and the increase is faster than the true stress. That is to say, during the stretching process, there is a Z -direction stress concentration in the center of the plate thickness at the necked part , and it reaches the maximum value at the moment of tensile fracture, which is conducive to the continued initiation and growth of cracks, resulting in macroscopic separation cracks in the center of the sample. appear.

• Ship Plate F36 Result analysis

In most cases, the tensile fracture separation is caused by the segregation of elements such as C and Mn in the center to form brittle structures such as martensite and bainite, or the presence of a large number of inclusions such as sulfide in the center [6,8- 9] . During this test, no hard structures such as martensite or a large number of inclusions were found; in the mechanical performance test of the finished product, full-thickness tension was used, that is, the sample thickness was 40mm , and fracture separation was not found. However, when the user divides and thins the sample, when the thickness is 20mm , there is a separation feature at nearly 1/2 of the thickness of the tensile sample, that is , separation occurs at 1/4 of the original sample thickness. Through the metallographic structure and scanning electron microscope analysis of the fracture and the normal clamping part, it can be seen that the thickness center of the tensile sample is the limit, the upper half of the tissue and the lower half of the tissue are significantly different, and the upper half is close to The surface of the sample is a bainite structure. The thickness of the bainite layer is about 10mm , which is exactly half of the thickness of the tensile sample. The structure in the lower half near the center of the original sample thickness is ferrite + pearlite. During the tensile test, plastic deformation occurs along the tensile direction, and due to the large difference in the plastic deformation resistance of bainite and ferrite + pearlite, the upper and lower regions will undergo uncoordinated deformation. To a certain extent, there is a significant difference in the amount of deformation of the two parts in different tissue regions, and when the stretching is continued, the difference in the amount of deformation further increases, eventually causing an abnormal phenomenon of separation from the center of the thickness of the sample.

For the steel plate with controlled rolling and controlled cooling, it is a normal phenomenon that there is a certain thickness of bainite structure layer on the surface, and it does not affect the normal use. Literature [10] found that in the 60 mm Q345 thick plate of controlled rolling and controlled cooling steel , the thickness of the bainite structure layer on the upper and lower surfaces of the steel plate at the head, middle and tail reaches about 10 mm and 6 mm respectively . , due to the fast cooling rate on the surface, the structure transforms into mesothermal structure bainite. In addition, it can be seen from the stress analysis that with the progress of the tensile test, the transverse stress of the necked part has a change process from small to large, and reaches the maximum value at the moment of fracture, and the plate at the necked part during the stretching process There is a Z -direction stress concentration in the center of the thickness , and it reaches the maximum value at the moment of tensile fracture. The difference in tissue deformation and stress concentration work together to cause the macro-fracture separation in the center of the sample.

---