Analysis of Tensile Delamination Causes of High- strength Ship Plate Steel EH36

The severe delamination phenomenon of the high- strength ship plate EH36 near the 1/2 thickness of the sample during the tensile test, the fracture and section of the sample were analyzed and studied by means of metallographic inspection and scanning electron microscope observation, and the delamination occurred clearly. reasons and put forward corresponding control measures.
is concluded that the direct cause of the central delamination of the steel plate is the banded structure and the lamellar MnS inclusions in the center, and the root cause is the central segregation of C , Mn and S in the continuous casting process .

Key words: ship plate steel EH36 ;

The high-strength ship plate steel EH36 has been widely used due to its high strength , good comprehensive performance, reduced hull weight, and increased load capacity. Many domestic steel mills are developing and producing high-strength ship plate steel . However, the phenomenon of tension delamination of medium and thick plates occurs from time to time, which has become a problem for ship plate steel mills.
one of the most important issues at home .
Based on the phenomenon of tensile delamination of EH36 medium and thick plates, the fracture and cross-section of tensile samples were observed and detected by means of metallographic inspection and scanning electron microscope observation, and the reasons for tensile delamination were systematically analyzed and corresponding control measures were proposed. .

Tensile delamination phenomenon of EH36 plate
Tensile delamination phenomenon of EH36 steel plate . During the stretching process, cracks parallel to the surface of the steel plate are generated near 1/2 thickness of the steel plate sample, and the cracks propagate along the rolling direction and two directions perpendicular to the rolling direction, resulting in the peeling of the upper and lower parts of the steel plate, forming delamination .

Cause analysis and control measures of tension delamination of ship plate steel EH36

Through the metallographic and scanning electron microscope analysis of the tensile sample of the ship plate steel EH36 , it can be inferred that the direct cause of the tensile delamination is the banded structure, the lamellar MnS inclusions, holes and low melting point metal Pb in the core of the EH36 steel plate. . In the banded structure, ferrite and pearlite bands are alternately distributed, and the elongation of ferrite and pearlite is different, which promotes the generation of delamination during stretching; the existence of MnS destroys the continuity of the substrate, making the center plasticity poor, The bonding force between the two parts above and below the center line becomes weaker, and there are still some holes in MnS, and MnS becomes the origin and propagation direction of cracks during stretching ; the low melting point metal Pb exists in the center of the steel plate, making the bonding force between grains deteriorating, promoting stratification .
ship plate steel EH36 is the center segregation of C , Mn and S in the continuous casting process . Severe center segregation produces severe banded structure near the center of the steel plate, and forms a large number of lamellar MnS inclusions .
Therefore, in order to control the tension delamination of ship plate steel EH36 , we should mainly start with continuous casting, supplemented by optimization of rolling. The specific measures are as follows:

• the content of impurity elements in steel, especially S and Pb .
• Optimize the continuous casting process, such as controlling the superheat of molten steel , controlling the amount of secondary cooling water , using electromagnetic stirring and light reduction at the end of solidification, etc., to reduce interdendritic segregation and central segregation, thereby reducing the band structure and reducing the amount of MnS in the center of the slab .
• Optimize the slab heating and controlled rolling and controlled cooling process, including slab heating temperature and time , starting rolling temperature , finishing rolling temperature , post-rolling cooling rate and cooling method, etc., to reduce the band structure and improve the performance of the steel plate core .

conclusion of ship plate steel EH36 :

• ship plate steel EH36 is the band structure, lamellar MnS inclusions , holes and low melting point element Pb in the center of the steel plate .
• The root cause of tensile delamination of ship plate steel EH36 is the central segregation of C , Mn and S during continuous casting .
• Tensile delamination can be controlled by continuous casting and rolling process optimization . 880 °C . At the same time, adjust the dew point in the SF section to 20 T to ensure that the specific concentration is less than 19% , so that Ph 2 o / P h 2 >0-12 , can reduce the occurrence of such defects .

  2 ) The longitudinal cracking defect of W600 chilled strip steel and finished strip steel is formed by the aluminate composite inclusions in the continuous casting slab due to deformation during hot rolling and cold rolling and being crushed and pressed out to the upper surface . Reducing the mixing of non-metallic inclusions during casting can alleviate the occurrence of such defects . Avoid secondary oxidation of molten steel during casting, and ensure that the liquid level in the crystallizer fluctuates within ±3 mm - ±5 mm , which can reduce slag inclusion on the surface of the slab.

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