Proposal and Analysis of Critical Thickness of Secondary Cold Blank Shell

1.The concept of secondary cooling critical thickness

After the first cooling of the mold is completed, the solid billet shell formed in the mold envelops the high-temperature molten steel to enter the secondary cooling process. In order to ensure the smooth flow and cooling of the suspended cast slab during the secondary cooling process, the solid billet leaving the copper tube of the mold The shell must have a safe thickness. For example, the thickness of the safety shell of the 150mm billet leaving the mold copper tube is 8 ~ 10mm. If it is lower than this value, it is easy to tear and remelt at the weakest part of the shell, resulting in leakage. Steel accident. The main purpose of a cold is to ensure that all sides of the solid blank shell after leaving the copper tube reach or exceed the safe thickness, and there is no internal stress that can produce defects. Generally speaking, a well-designed cooling process can reach a thickness of 13mm even at a pulling speed of 3m/min. The solid shell in the copper tube has been well cooled in all directions. At this time, the thickness of the shell is one A more uniform value. At present, many companies are pursuing high-speed production. For example, the drawing speed is above 5m/min. In order to maintain the slab's circumference, more foot rollers are used. The foot roller section plays a role equivalent to a cold, which can also be regarded as a cold. In the extended section of the cooling process, the cast blank after leaving the foot roll can reach the corresponding thickness of the safe shell and continue to receive the baptism of the secondary cooling process. For a large-section cast slab with a very low drawing speed, the thickness of the solid shell of the cast slab outside the confinement of the mold copper tube is often more than 25mm, so there is no need to worry about the thickness of the solid shell in a cold process.

In order to ensure the internal quality of the cast slab, it is necessary to control the generation and propagation of cracks inside the cast slab. The four walls of the cast slab suspended in the second cold chamber should be uniformly cooled. Under the action of radiation, convection and water mist particle conduction, vaporization and heat transfer, the heat from the interior of the cast slab to the surface is released in time, and as time goes by , The solid billet shell is continuously increasing its thickness. When the cooling effect of the water mist particles leaving the secondary cooling nozzle, that is, when the secondary cooling ends, the cast slab only relies on two heat transfer methods: radiation and convection to dissipate heat, that is, dry cooling. The slab is cooled.

Here is a concept of the critical thickness of the secondary cooling blank shell, that is, at which point should the arrangement of the secondary cooling nozzle end? How long does the second cold spray pipe arrangement need to be to ensure normal slab heat transfer? What are the factors that determine the angle of the secondary cooling nozzle? How to design the secondary cooling to ensure the good internal and external quality of the slab and eliminate or reduce the internal cracks of the slab?

The operation of the cast slab in the second cold chamber is essentially a process in which the solid slab with increasing thickness under cooling conditions contains the molten steel descending. The corners of the billet and the rectangular slab are two-dimensional heat transfer methods, and the heat transfer efficiency is higher than that of the billet. The middle of the shell. From the color temperature of the cast slab, it can be clearly seen that the temperature at the corners of the cast slab is lower than that of the center, as shown in Figure 1. The same is true for observing the casting billet in the second cold room. The heat transfer inside the cast slab can be regarded as the one-dimensional heat transfer mode of the solidus temperature along the solid shell. The difference between the solidus temperature and the surface temperature of the cast slab is the driving force for conduction heat transfer. One-dimensional conduction heat transfer The heat flux formula is expressed as

In the formula: λ is the thermal conductivity. According to calculations and actual data, the thermal conductivity of the solid shell at high temperature is about 30 ~33W/m·K. Assuming that the surface temperature of the cast slab leaving the secondary cooling spray is about 1050℃, if the thickness of the solid shell is 50mm at this time, it is known that the heat flux density of the 20 steel cast slab is 231.6KW/m2, and the sum of radiation and convection heat dissipation It is 201.24KW/m2. The heat flux density of the solid shell at this time has accumulated a difference of -30KW/m2, indicating that the heat transfer from the inside of the cast slab to the surface is not completely taken away, and the accumulation of heat causes the solid shell to heat up. When the billet shell reaches a thickness of 60mm, the difference is 8.24KW/m2, which means that only the heat transfer by radiation and convection is greater than the heat generated inside the billet, and the heat on the surface of the billet can be completely taken away. It will accumulate in the solid shell, and there is no solid shell reheating condition. As we all know, the excessive re-temperature of the slab will cause internal cracks in the slab. A good secondary cooling design is to ensure the internal quality of the slab. Therefore, the arrangement of the secondary cooling nozzle must make the slab reach and exceed this critical slab Shell thickness. When the cast slab enters the dry cooling stage, the cast slab will only continue to cool down under the action of radiation and convection. The thickness of the blank shell of 60mm given here is the critical thickness of the secondary cooling. For the corresponding casting machine radius, there is a design for the critical angle of the secondary cooling. For example, a 9-meter-radius casting machine produces 150mm billets at a drawing speed of 2.8m/min. The critical angle of the secondary cooling should be 60 degrees, so as to ensure the normal heat transfer process. The secondary cooling design needs to abide by this principle.

2.Critical shell thickness calculation

The solid shell conduction heat transfer heat flux Qc is shown in formula 1

Here W is the thermal conductivity, h is the thickness of the solid shell, and Tsold and Tsurface are the solidus temperature and the surface temperature of the cast slab, respectively.

The radiation heat transfer heat flux Qradiation is shown in formula 2.

Where ε is the surface emissivity, σ is the Stefan-Boltzmann constant with a value of 5.669×10-8W/(m2 K4), T is the surface temperature of the cast slab (absolute temperature K), and T 1 is the environment Temperature (absolute temperature K).

The convective heat transfer heat flux Qconvection is shown in formula 3.

Here K is the convective heat transfer coefficient, and T2 is the ambient temperature.

The critical thickness of the solid shell, that is, the heat transferred from the inside to the outside of the slab and the outside world can be completely taken away only by radiation heat transfer and convection heat transfer. At this time, the thickness of the solid shell becomes the critical thickness of the secondary cold solid shell:

More than ten years ago, the author produced round billets and billets of various specifications in a steel plant. In order to achieve a good secondary cooling process, this must be studied and practiced. All the steel grades produced at that time were calculated and JMatPro was used to calculate the steel grades. The liquidus temperature is shown in Figure 4. The solidus temperature and the thermal conductivity of the high-temperature solid shell are considered to be the internal hot surface temperature of the solid shell. The result of this calculation is that when the surface temperature of the cast slab is 1100℃ At that time, the critical shell thickness of the second cold was about 60mm. According to different steel grades, different liquidus and solidus two-phase zone widths, and drawing speed, work out the correct surface temperature of the cast slab during the secondary cooling process, calculate the critical secondary cooling thickness of the cast slab, and then determine the required secondary cooling Length of cold section.

3.Second cold section of large section low-speed casting billet

For particularly large cast slabs, due to the low drawing speed and the cooling of the foot roll section, the remaining secondary cooling section is often unnecessary, or only a short secondary cooling section is needed. The purpose is to prevent excessive cooling of the cast slab. , As long as the balance of heat transfer and heat dissipation is reached, the second cold spray arrangement is ended, and heat dissipation is only required by radiation heat transfer. If the basic radius of the casting machine is too large and the casting speed is very low, the cooling of the water nozzle must be stopped after the solid shell reaches the critical thickness. When necessary, heat preservation measures must be taken when entering the secondary cold room before the stretching and leveling machine. , In order to prevent the too low surface temperature of the cast slab from entering the interweaving area. Figure 2 shows that the large-section rectangular billet only has a second cooling stage.

For example, the solidification coefficient is K = 20, the drawing speed V = 0.2m/min, and the critical shell thickness of the secondary cooling is 60mm, so only 1.8m of cooling distance is needed to cool without the secondary cooling. Generally, the length of the mold copper tube is 800mm, 700mm is effectively used, the foot roller section is estimated to be 300 ~ 500mm, and the second cooling section is only about 1 meter to meet the requirements. Then the cast slab does not have to be subjected to the baptism of water, so as to reduce the heat dissipation of the cast slab as much as possible, and keep the surface grains of the cast slab in austenite state into the straightening deformation.

4.The second cold section of the super high-speed billet continuous caster

The drawing speed of modern plain carbon steel billet production is getting higher and higher. Foreign steel equipment manufacturers advertise that their ultra-high drawing speed billet can reach a drawing speed of 6m/min or more for normal production, achieving good benefits, wait and see . The layout of the secondary cooling section of this ultra-high drawing speed billet must be longer. According to the drawing speed of 6m/min to produce 150mm billet, if the secondary cooling critical thickness of 60mm is to be achieved, the length of the entire cooling section must be The arrangement of the secondary cooling nozzle can be lifted after reaching 20 meters. In this way, a large radius must be used to arrange the secondary cooling section of this ultra-high-speed continuous casting machine. For example, a casting machine with a radius of R12m has an arc length of approximately 19 meters, the second cold arc length can be about 15 meters, in order to provide sufficient secondary cooling for the high-speed moving cast slab. Figure 3 shows the second and third sections of the secondary cooling of the high-speed continuous casting machine.

According to the calculation of the commonly used Q235 steel grade, the liquidus temperature of the steel is 1513℃ and the solidus temperature is 1460℃, as shown in Figure 4. When the surface temperature of the cast slab in the second cold chamber is 1130℃, the second cold cast slab The critical billet shell thickness is 60mm. If 20 ℃ is increased, the surface temperature of the cast slab will be increased to 1150°C. At this time, the critical billet shell thickness is 55mm, so the surface temperature of the cast slab is also very high during the secondary cooling process with high drawing speed. , The arc length of the corresponding secondary cooling section is reduced correspondingly, and the balance of heat dissipation and heat transfer is achieved under the condition of the limited basic radius of the continuous casting machine.

5.Production of cast slabs of excellent special steel

Due to the use of more alloying elements, the temperature of the liquidus and solidus of special steel is quite different. For example, the liquidus temperature of bearing steel is 1455℃, but the solidus temperature is only 1330℃. At temperature, the driving force △t for the heat transfer of the solid shell is only 230℃; while the solidus temperature of the plain carbon steel Q235 is as high as 1460℃, △t is 370℃, which shows the conduction heat transfer of the solid shell of the bearing steel The efficiency is 60% lower than the heat transfer of the solid billet shell of ordinary carbon steel, and the heat transfer from the inside of the cast slab to the surface is greatly reduced. Compared with ordinary carbon steel, the secondary cooling is designed as a weak cooling mode. If a stronger secondary cooling is used , Once the cooling is uneven, it will cause various defects in the casting billet, such as de-square rhombus, internal cracks and so on.

Due to the low heat transfer driving force of the solid shell of high carbon steel and alloy steel structural steel, the growth rate of the solid shell is correspondingly small, and it takes a long time to reach the critical thickness of the secondary cooling of the cast slab. In weak cooling mode, a longer secondary cooling section is required. Generally, secondary cooling section 1, secondary cooling section 2 and secondary cooling section three are configured, and the arc radius of the continuous caster used is correspondingly larger, which is why the special steel needs One of the reasons for the larger radius of the casting machine. At the same time, USpecial Steel's secondary cooling uses gas-water atomizing nozzles to dominate, as shown in Figures 5 and 6.

Due to the relatively low drawing speed of the special steel continuous casting machine, it takes a long time to form the critical billet thickness, and the length of the secondary cooling section also needs to be extended. The secondary cooling design of the special steel continuous casting machine needs to be calculated. According to the author’s own practice, we don’t want too strong secondary cooling. As long as the blank shell reaches the critical secondary cooling thickness of the steel, the nozzle can stop functioning, and only rely on radiation heat to dissipate. In addition to the heat transferred from the inside of the slab, it can keep the slab at a higher slab temperature, reduce and eliminate internal cracks in the slab, thereby ensuring the inherent quality of the slab.

The critical secondary cooling thickness of the slab has nothing to do with the specifications of the slab. Whether it is a small-section carbon steel or a large-section special steel, as long as the solid slab shell reaches this thickness, regardless of the thickness of the liquid steel in the slab, its casting The heat conducted from the inside to the outside of the billet can be carried away by the radiant heat, achieving a balance between heat transfer and heat dissipation. For example, when 150mm billet and 165mm billet are produced at high drawing speed, the critical secondary cold billet shell thickness is the same when the steel grades are the same. If the drawing speeds of these two sections are the same, the length of the two cold sections can also be the same.

However, many continuous casters produce a large number of steel grades, from low-carbon steel to high-carbon steel, from high-speed normal carbon steel to low-speed high-quality special steel, from small billet to bloom, and some continuous caster radius The billet is used for both, so the design of the secondary cooling is particularly important. Understanding the critical secondary cooling thickness of the cast slab is helpful to optimize the design of the secondary cooling process.