Solidification in Continuous Casting of Steel

Dear LinkedInners,

In this last edition of my series " Steel Processes ", we are going to talk about solidification, continuous casting and lamination. In ref. to the instrumentation for the control of the process, we will focus on the temperature control of the surface of the treated pieces when they come out of the oven, such as slabs or rolls. During the years in the industry I noticed an important impact on the quality of the final product, if the distribution of heat in the oven is not uniform. The quality of the final product suffers.

But we´ll start with the process:

1. Solidification

The Solidification is a phenomenon of nucleation and growth at a suitable temperature, that, when reaching mentioned temperature, a set of adjacent atoms takes a fixed position called nucleus. In the case of Iron (Fe), the arrangement of the atoms forms a cubic crystal lattice, like the one represented below. The Carbon (C), given its smaller atomic diameter, is located in the interatomic interstices forming the Fe-C alloy (name of the alloy).

The phenomenon of nucleation and growth occurs in different points of the liquid mass. From each nucleus, crystals arise that form a crystalline network that increases in size. These portions of crystals grow until they are surrounded by others of the same origin. These parts of the Steel mass is called Material Grains. The mechanical properties of steel will depend on the shape of the grains and their crystallographic structure.

2. Continuous casting

The Continuous Casting process consists in solidifying the liquid steel in a continuous way, from the surface to the center, obtaining a slab in the case of flat products or a billet in the case of long products. Mex reactor to the rolling of the hot rolling mill.

Traditionally, the liquid steel obtained from the furnaces, either from BOF or Siemens-Martin, was cast in molds for solidification, hence ingots are obtained which it is necessary to reheat to be able to be rolled in the grinding mill and obtain intermediate products from suitable dimensions that can be used in the finishing mills to obtain a final product. These intermediate products are: billet, bloom, or slab.

 In order to increase the yield of liquid steel to the finished product, and to avoid the steps of reheating and roughing, tests were carried out to obtain the i ico has been characterized by the technological development of its processes. The incorporation of modern technologies to manufacture steel such as the Minimill Continuous Casting of billet, bloom, or slab (Thin Slab) for flat products is a continuous process from the direct reduction ntermediate products directly from the casting. The result of these tests was the birth of continuous casting.

Main parts:

DISTRIBUTOR. Artesa or box covered with refractory material that has a steel exit hole in each of the ends, and at the bottom of which by means of manual mechanisms of bar, plug and submerged discharge nozzle, the casting molds are fed continuous and where the steel is stored to allow the change of pots during the operation of a sequence.

MOLD. Piece made of copper plates (material of high thermal conductivity) that has internal cooling and receives the liquid steel from the distributor.

ROLLERS GUIDE. A whole series of these are placed underneath the molds to form a thread.

SECTION OF CUT. Equipment necessary to size the slab by cutting normally by the oxyacetylene type.

Before starting a casting, it is necessary to prepare the machine. This preparation consists of sealing the copper mold on the bottom. To carry out this operation, a piece of steel similar in dimensions to the slab to be obtained is introduced through the lower part of the guide rollers. This piece is called a false bar. By having the bar in the lower part of the mold, small pieces of steel (burrs, shavings, etc.) that serve as cooling agents and allow the steel that is cast into the mold, solidify and adhere to the false bar, with which it will be possible to pull the mold slab through the guide rollers.

Once the machine is ready, the steel pot provided by the BOF workshop is placed in the turret; which in turn is placed above the distributor, it receives the steel from the pot and leads it in the molds that make up the machine.

  The mold is continuously cooled externally. The solidification of the steel starts with the one that is in contact with the mold, and as time passes, the solid layer increases towards the center. The mold is filled to the level of operation and the vertical oscillation system of the same mold starts or goes to work; these oscillations allow and facilitate the slide of the slab down the mold and is driven by the rollers that are curved to leave it horizontal when finishing the thread. During the passage of the slab through the guide rollers, it is cooled by water until its complete solidification.

  At the exit of the guide rollers are the oxycutting machines to obtain slabs of the required lengths for subsequent processes. The already cut slab is stored in the yard and then sent to the corresponding section or rolling department.

3. Lamination

Flat hot rolling involves passing a metallic material between two cylinders, which rotate at the same speed and in the opposite direction, to reduce their thickness by the pressure exerted by them. The metal is compressed, reduced in its section and changed shape. The deformation by rolling is plastic, that is to say that the dimensions of the material obtained are maintained after the force of the cylinders has ceased.

4. Thermographic system for temperature measurement inside the oven

As I mentioned at the beginning, I want to share with you a way to measure the temperature inside the hot rolling mill to control the process before the material comes out as a preventive control measure for your process.

The goal is to control the temperature inside the oven to take measurements at the time of detecting a temperature drop in certain areas due to eg. clogged injectors.

What does this look like?

This is the image of the interior of an oven working at 920 ⁰C. The problem so far was the high temperatures damaging the thermal imager.

Our business partner Mytec works with the company VisionTIR using Optris cameras. These camera solutions are prepared to work in an environment of up to 2200⁰C without suffering from temperatures:

The interesting thing about this solution:

• System based on high resolution thermographic cameras, able to withstand the high temperatures generated inside the oven and cooler.
• Resists up to 2,200⁰C.
• It offers a real thermographic vision of what happens inside.
• Each point measures temperature
• Economizes the production Process

The complete system consists of:

•       Thermal imaging camera

•       Enclosure with water and air cooling system

•       Control cabin

•       Protection system, retractable system and / or fixed system

•       Connections Box

•       Air purge system

For more information contact me.

Thank you for your interest and a successful week.

Best Regards

hanniganquistrace.blogspot.com

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