This article describes the influence of different copper tube materials on the heat transfer of the mold.
Key words: copper tube material; crystallizer; performance
The mold is the core component of continuous casting, and the copper tube of the mold is an important heat-conducting part for continuous casting from liquid steel to solidified billet shell. Its quality directly affects the surface quality of the slab, the casting speed of the continuous casting machine and other indicators. In order to maximize the service life of the copper tube. reduce manufacturing cost. Guarantee the quality of billet. This paper analyzes and compares the thermal and mechanical properties of copper pipe materials. Combined with the study of heat transfer between molten steel and cooling water in the crystallizer. The influence of copper tube material on heat transfer is analyzed by numerical simulation.
Thermal and Mechanical Properties of Copper Alloys Used in Molds and Their Application Status
In the history of continuous casting development, it has been tried to use brass, low carbon steel, aluminum, stainless steel, etc. as crystallizers. But they are not ideal enough and have not been popularized while copper has good thermal conductivity. The earliest used slab crystallizer base material is pure copper. Although it has good thermal conductivity, it is not resistant to high temperature, and has poor wear and corrosion resistance. Copper plates with a single copper element cannot meet the requirements of the crystallizer. New copper alloys need to be developed to improve the properties of copper sheets. Commonly used copper alloys are silver copper, phosphorus deoxidized copper and chromium zirconium copper.
The following lists the general application of mold copper plate and tube materials of different casting machines:
Billet continuous casting generally uses more tubular molds of deoxidized phosphorous copper, and blooms generally use chrome-zirconium copper mold copper tubes. In particular, chromium-zirconium copper is used for copper tubes of unequal rectangular billets, and better round billet mold copper tubes are made of beryllium-drilled copper for horizontal continuous casting. Silver copper is generally used for large sections, and chrome-zirconium copper is used for larger sections. Silver copper and cold-rolled silver copper are generally used for combined billets and small-section strip steel crystallizer copper plates; chrome-zirconium copper is generally used for large-section slab molds.
Table 1 shows the chemical composition and physical properties of several different copper plate materials, and Table 2 shows the mechanical properties of various copper alloys.
Mathematical model of solidification heat transfer
1. The method of expressing the thermal resistance between molten steel and cooling water 1/h represents the total thermal resistance of the heat transfer from molten steel in the crystallizer to cooling water, which can be expressed as:
The formula for calculating the radiation heat transfer coefficient of the air gap between the molten steel and the copper pipe wall is as follows:
Ts—casting temperature, ℃
Tm—temperature of hot surface of copper plate, ℃
σ—radiative heat transfer coefficient, W/(m²·℃4)
ℇ—darkness
h₀—Convective heat transfer resistance between molten steel and copper tube
Ec/λc is the heat conduction resistance Rc of the copper pipe wall
λ—copper tube material thermal conductivity, W/m k
ec—copper pipe wall thickness, m
The heat transfer coefficient h at the interface between the cooling water and the copper plate is generally calculated using the following formula:
he—heat transfer coefficient at the interface between cooling water and copper plate, cal/(cm² s ℃)
De—the equivalent diameter of the tank, cm
Ke—thermal conductivity of cooling water, cal/(cm²·s·℃)
Ve—water velocity in the tank, m/s
Ce——Specific heat of water, cal/(kg·℃)
μe——dynamic viscosity of water, g/(cm s)
ρe—density of water, g/cm²
2. Calculation assumptions for the heat transfer process of the slab. Simplifying assumptions for the strand cooling process:
(1) Since the pouring speed of molten steel is fast, and the thermal conductivity of molten steel is much lower than that of copper in the mold, the heat transfer rate in the casting direction (vertical direction) is much lower than the transverse heat transfer rate. Therefore, the heat transfer of the slab along the pouring direction is negligible:
(2) The mold slag film at the meniscus is thin, and the influence of the mold slag on the meniscus on the temperature distribution of the mold calculated in this paper is ignored;
(3) Neglect the influence of copper tube taper on heat transfer;
(4) In the calculation area, the initial temperature of molten steel is the same;
(5) The heat transfer on the geometrically symmetrical surface of the crystallizer is the same, and the calculation area can be treated as geometrically symmetrical, so the calculation selects 1/4 copper tube as the analysis object;
(6) Neglect the influence of vibration on heat transfer during solidification;
(7) The thermophysical parameters of steel are only related to temperature, not to the spatial position.
Calculation scheme and related parameters
1. Calculation scheme. Comparing the influence of materials: Calculate the temperature of copper tubes of seven materials when the thickness is 22.5mm.
2. The main process parameters of the continuous casting machine and the cast steel.
Simulation results and analysis
Compare the influence of the material on the temperature of the copper tube; select the temperature of three special parts of the copper tube to analyze and compare the influence of the material on the temperature field. Figure 1 takes the Al-10 copper tube as an example to show that 1/4 copper at the meniscus of the crystallizer The temperature distribution of the pipe section.
Fig.1 Temperature distribution of Al-10 crystallizer copper tube at the meniscus
Figure 2 Histogram of temperature comparison at special locations of copper pipes of different materials
Figure 2 is a comparison of temperature distributions in the order of increasing thermal conductivity of copper tube materials. It is easy to see that the temperature of the copper tube with high thermal conductivity is relatively low. The temperature rise of the copper tube is caused by the accumulation of heat inside the copper tube due to insufficient heat transfer from the molten steel, while the copper tube with relatively high thermal conductivity is caused by the accumulation of copper tube. There is less heat, so the temperature is lower. When the temperature of the cold surface of the copper tube exceeds the boiling point of water, the cooling mechanism of the cold surface of the copper plate is nucleate boiling, which is the heat transfer condition of the meniscus of the crystallizer; at the same time, considering the thermal deformation of the crystallizer on its working life and Influenced by the quality of the slab, in the selection of the material of the mold copper tube, special attention must be paid to its recrystallization temperature being higher than the maximum temperature of its hot surface.
In conclusion
Under the condition of certain process parameters, using the method of numerical simulation, the crystallizer meniscus positions of seven kinds of copper tubes with a thickness of 22.5 mm and seven different thickness values of chromium-zirconium copper tubes between 15 mm and 30 mm were calculated respectively. 1/4 copper pipe section temperature distribution. The conclusions are as follows:
The maximum temperature of the hot surface of the copper tube of different materials will increase significantly with the decrease of the thermal conductivity of the material, and the maximum temperature of the cold surface and the minimum temperature of the corner will not change significantly. Considering the impact of the thermal deformation of the mold on its working life and the quality of the slab, special attention must be paid to the recrystallization temperature being higher than the maximum temperature of the hot surface in the selection of the material of the mold copper tube.
