Study On Secondary Cooling System For Beam Blank Continuous Casting Of Nb-bearing Steel

As an economic section material, it has been used widely in machinery manufacturing, high-rise building, bridge, offshore drilling platform and large ship because of its significant advantages, such as large section modulus, light weight and, saving metal. However, compared with conventional strand casting, there will be more crack defects during Nb-bearing beam blank casting because of its complex cross-section. Moreover, it can improve the austenite recrystallization temperature of steel with Nb, thus grain refinement can be realized, which can greatly improve the properties of steel. However, with the increasing of niobium, high temperature plastic of steel decreases. By optimizing the cooling conditions to adjust the cooling rate of beam blank casting and improve the temperature uniformity of cross section, thereby the state of cooling of beam blank can be improved and the quality defects can be reduced. In this paper, based on a domestic continuous casting beam blank with size of 450mm×350mm×90mm, the effects of secondary cooling system for beam blank continuous casting on the quality of beam blank was studied. The main conclusions are as follows:(1) A two-dimensional beam blank continuous casting solidification process of heat transfer mathematical model and secondary cooling iterative calculation model were established. According to the requirements of high temperature mechanical properties of steel and metallurgy criteria, casting secondary cooling target temperature curve was determined. The target temperature of secondary cooling can be set as boundary conditions for the continuous casting solidification model. According to secondary cooling iterative calculation model, a parameters in the heat transfer coefficient h=1.57W0.55 (1-0.0075Tw)/? formula can be calculated. The values of a in five secondary cooling segments are 3.2912,2.8656,1.8139, 2.8373,3.9475 for inner and outer arc side, respectively and 3.1127,2.5004,2.0954,1.1421, 1.5462 for narrow side, respectively. According to the empirical relationship between heat transfer coefficient and water flow density, the spray water distributing of secondary cooling for three typical kind of Ni-bearing steel beam blank can be obtained.(2) Combined with spray water distributing of secondary cooling for Ni-bearing steel of Q345B, a two-dimensional thermo-mechanical coupled finite element model was established, under the condition of casting speed at 1.0m·min-1 and superheat of 40?. The temperature, shell thickness and thermal strain distribution were studied. According to the beam blank metallurgical criteria for secondary cooling, the temperature distribution was evaluated. The temperature rally speeds in the secondary cooling segments are 99.3?/m,18.4?/m,17.1? /m,11.5?/m,4.0?/m, respectively. All of them are less than 100?/m, which can meet metallurgical criteria for secondary cooling. With the use of surface temperature standard deviation of beam blank, the uniformity of surface temperature in cross-section of beam blank was investigated. The temperature standard deviation of mold exit, five secondary cooling segments and end of air cooling are 105.81?,81.12?,104.07?,142.11?,141.22?, 136.38?,79.97?, respectively. Shell thickness of web center line at mold exit is 15mm. Beam blank web center has completely solidified when it moved to the position of 3.88m below the meniscus. Throughout the casting process, the thermal strain at flange corners was always the largest.(3) With single variable method, the effect of the process parameters, such as casting speed and superheat were discussed. It indicates that at position before straightening area, with the increase of casting speed from 0.8 m/min to 1.2 m/min, as for an increase of 0.1 m/min, the temperature of beam blank web increases about 25? and completely solidified position of the web center increases about 0.80m from meniscus. With the increase of casting speed, thermal strain of web center surface gradually becomes smaller. Thermal strain of web center surface before straightening area was -0.0122,-0.0110,-0.0101 under the casting speed of 0.8m·min-1, 1.0m·min-,1.2m·min-1, respectively.(4) At the same distance from meniscus, as the superheat increases, the surface temperature and shell thickness at beam blank web center increase gradually, while thermal strain of becomes smaller. After the air cooling area, with the increase of superheat from 20? to 40?, as for an increase of 10?, the surface temperature of beam blank web center increases about 5? and completely solidified position of the web center increases about 0.09m below meniscus. Based on analysis above, it is concluded that the influence of casting speed on thermo-mechanical behavior is greater than that of superheat.