Cause Analysis of Tensile Fracture Delamination of DH36 Ship Plate Steel

DH36 ship plate steel were analyzed by means of metallographic examination and scanning electron microscope examination .
The results show that the delamination defects in tensile fracture of DH36 ship plate steel are mainly caused by the strip-shaped MnS inclusions in the center of the plate thickness and the wide bainite and martensite band structures.
Key words: DH36 ship plate; fracture delamination; inclusions; banded structure

DH36 ship plate steel belongs to high-strength hull structural steel, which has the characteristics of high strength, good low-temperature impact toughness and good welding performance, and is widely used in the shipbuilding industry. When the classification society inspects the DH36 hull structural steel, it often occurs that the mechanical properties are all qualified, but the fracture of the tensile test sample is delaminated and it is judged as a waste product. In view of this, this paper analyzes the reasons for the fracture delamination of the tensile specimens of the DH36 high-strength hull structural steel produced by a certain company , in order to find out the factors that affect the fracture delamination of the structural steel for the hull, and to optimize the rolling process and strictly control the steel. Purity has a certain guiding role and practical significance.

DH36 Ship Plate Steel
DH36 ship plate steel used in the experiment is: molten iron — 1300t iron mixing furnace — hot metal desulfurization — 150t converter — 150t ladle refining furnace — 1600mm x 220mm continuous casting machine — slab heating — rolling — straightening — cooling bed— Steel plate inspection - mark storage.

Chemical composition of DH36 ship plate steel :
Table 1 Chemical composition of DH36 ship plate steel ( mass fraction, % )
Tab.1 Chemical composition of DH36 hull plate (wt, % )

C	Si	mn	P	S	V	Ti	Nb	Alt
0.14	0.40	1.51	0.015	0.007	0.057	0.014	0.02	0.028

conclusion and suggestion of DH36 ship plate steel
The crack in the brass nut of the fuel conduit is a stress corrosion crack. In the surrounding atmospheric environment, the critical torque value of the stress corrosion crack is 8.1 to 8.6 Nm , and the recommended tightening torque value in actual maintenance is 6.3 Nm . Larger size leads to the expansion of the short axis direction of the brass nut body and martensite band. When the crack expands in the direction of the long axis and the short axis of the bainite and martensite bands, a step will be formed. If the sample is broken at this position, a step parallel to the rolling surface will be formed on the fracture .

Experiments have found that when the width of the bainite and martensite bands is larger than 20 "m , cracks are easy to occur on the bainite and martensite bands, and the probability of cracks increases with the increase of the width , the probability of delamination at the tensile fracture also increases.

MnS inclusions and AbO3 inclusions in the DH36 ship plate steel sample used in the experiment , and there are wide bei martensite and pearlite band structures in the center of the steel plate thickness, resulting in excessive differences in the local mechanical properties of the ship plate, which are prone to defects. Cracking and delamination occur at a concentrated microstructural level.

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