Analysis of the Causes of Cracks in the AH32 Ship Plate

MnS is a common non-metallic plastic inclusion in steel. For most steel grades, its size , shape and distribution seriously affect the properties of steel . Low w ( Mn ) / w ( S ) molten steel can form interdendritic low-melting FeS phases during solidification, and these low-melting point phases aggravate internal cracks during continuous casting and intergranular cracks during hot rolling . The effect of sulfide in steel on the plasticity of steel has been done a lot of research before. The more mature theory is that during the solidification process, sulfide precipitates and enriches at the grain boundary, which becomes a source of stress concentration, promotes grain boundary slip, and produces microcracks. .

MnS in steel was simulated and calculated by using Thermo-Calc thermodynamic software , and the effect of the change of element content in the equilibrium state was studied.
MnS precipitation law and solidification process characteristics of AH32 ship plate steel . Using the Quanta-400 scanning electron microscope to conduct microscopic observation and energy spectrum analysis of the internal cracks in the 1/4 of the thickness direction of the AH32 ship plate , the cause of the internal cracks was found out, and the measures to eliminate defects were formulated. Obvious effect .
1 Analysis of MnS precipitation law in AH 32 steel
The main process route of AH32 ship plate steel is: molten iron - desulfurization pretreatment - 100 t converter - LF - continuous casting - heating - rolling . The effect of precipitation temperature and chemical elements on it was simulated 1 . 1 Thermodynamic analysis of MnS precipitation .
The chemical composition of the test steel is shown in Table 1 .
Using Thermo-Calc software to analyze MnS under equilibrium state
Table 1 Main chemical composition of AH32 steel ( w B ) %
C Si Mn P S Nb Al SI
0. 10 ~0. 14 0. 10 -0.22 1.30 ~1.45 W 0.018 W 0.008 0.012 ~0.030 0.018 ~ 0.045

The liquidus temperature calculation formula [ 1 ] for carbon steel is: T L = 1536 - 78 w ( C ) - 7.6 w ( Si ) - 4.9 w ( Mn ) - 34.4 w ( P ) - 33 w ( S ) - 4.7w ( Cu ) - 3.1w ( Ni ) - 1.3w ( Cr ) - 3.6w ( Al ) ( 1 ) _ _ _ _
to formula ( 1 ) , the liquidus temperature of AH32 steel is about 1 520 T. According to the equilibrium phase diagram calculated by thermodynamic software, it can be seen that for low carbon steel with w( C ) =0.074 % , the Y ^ S transition temperature is 1 480 ~ 1 500 T , which is higher than the thermodynamically calculated MnS precipitation temperature ( 1 438 ° C ) nearly 42 higher . At this time, the precipitation temperature has fallen in the austenite region, and the liquidus temperature of AH32 steel is 1520 C , so it can be judged that MnS is not precipitated in the liquid phase, but precipitated below the solidification temperature of the steel, which is due to the solidification front Sulfur content increases, sulfur solubility decreases , Mn-S reaction equilibrium shifts, and sulfur segregates and enriches at grain boundaries, which is formed by Mn-S reaction, and mainly occurs during the precipitation process in austenite .
According to the thermodynamic theory , the precipitation of MnS is determined by the temperature , manganese-sulfur content and the activity of MnS . During the solidification process, various components segregate to the solidification front, the concentration of various impurities in the liquid phase increases, and the liquidus As the temperature decreases, the equilibrium concentration product also decreases .
The change of the precipitation amount at different temperatures calculated by the thermodynamic equation is shown in Figure 1. It can be seen that the precipitation has begun at about 1430 C , and the precipitation amount increases as the temperature drops, and tends to be flat after 1100 C, reaching 800 C The amount of precipitation left and right has reached the maximum .
Since the content of other elements in the steel also affects the precipitation of MnS phase, therefore, w ( C ) = 0.074 % , w ( Mn ) = 1.44 % , w ( Si ) = 0.31 % , w ( S ) = 0.005 % , w ( P ) =0.011 % , w ( Nb ) =0.036 % as the basis, using the Ploy- module to calculate w(C ) <0.2 % , w ( Si ) <0.5 % , w ( Mn ) <2.0 % , w(P ) <0. 1 % , w(S ) <0.01 % , w (Nb ) <0.1 % , the equilibrium precipitation of MnS phase changes with temperature .
1 ) Through the range analysis of the influence of various factors on the characteristics of the flow field in the orthogonal test, it can be seen that the primary and secondary order of the influence on the average residence time of molten steel in the tundish and the volume fraction of the dead zone is: diversion hole diameter > inclination angle > Height, which can provide ideas for the structural design of the diversion baffle .
2 ) After R&D and design, the parameters of the diversion holes of the diversion baffle are: the diameters of the diversion holes are 93 , 72 and 36 mm, and the positions are ( 220 mm , 140 mm ) , ( 350 mm , 280 mm ) , ( 700 mm, 300 mm ), the inclination angles are ( 21°, 15° ), ( 8° , 23° ) , ( 10° , 28° ) respectively . After setting the diversion baffle, the flow field of the tundish is the most reasonable .
3 ) Under the research conditions, it can be known from the process of hydraulic simulation and CFD simulation that after setting reasonable diversion baffles, the molten steel in the casting area of the tundish can be redistributed . Utilizing this technology and rationally designing the diversion baffle can extend the life of the tundish from 13.8 h to 24.5 h, an increase of 77.5 % .
4 ) Through the industrial test on the tundish, it can be seen that after installing the designed diversion baffle, the maximum temperature difference of the average temperature of each flow is only 3.76 °C , which is reduced by 72.7 % . The number of large-grain microscopic inclusions larger than 50 Jim in the tundish slab decreased from 4 to 8 to 0 to 2 , and w(T. 0 ) also decreased from the original average of 39.5 X 10" to 27.8 X 10 ' 6 , a decrease of 29.6% .

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