28/11/2018

Petr Kotrbáček finished his habilitation and became associate professor on 28th November. Petr Kotrbáček operates in the field of heat transfer. Mainly, he is specified in optimization of heat transfer processes based on measured boundary conditions. His habilitation is called ‘Optimalizace tepelných procesů na základě experimentálně stanovených okrajových podmínek’.

His work is devided in three parts: semi-solid state of steel, cooling of working rolls, and in-line heat treatment.

Semi-solid state of steel

The work is focused on the experimental research of the boundary conditions. Real boundary conditions are necessary for numerical simulation and can be used for the cooling optimization process, especially in heavy industry applications. Typical applications are continuous casting, hydraulic descaling, roll cooling, interstand cooling, product cooling and heat treatment. For this purpose a special methodology was developed. Input data for the inverse task are obtained from laboratory experiments. For the operating conditions of steel plant, it is very complicated and often impossible to measure cooling conditions directly on the lines. Therefore, it is necessary to design and implement laboratory measurements. Laboratory stands simulate the real plant conditions as accurately as possible. For this purpose, unique laboratory equipment for cooling simulation in steel works is used.

The aim of the basic research is to describe material properties of steel in semi-solid state (i.e. between solidus and liquids). Two experimental techniques are used: measuring the steel resistance of a partially melted cylindrical steel sample against upsetting and also measuring the steel resistance to the tool indentation in semi-solid state in dependence on the tool position. The influence of the temperature and that of the rate of loading in the process of forces acting on the tool were evaluated.

A very interesting phenomenon of an expressive decrease of force at loading is evident while seeing measured dependency force – deformation. Further interesting phenomenon is the relaxation stress of the material. It appears immediately after the end of a loading period while force is falling very quickly from the maximum value to approx. 1/5 of the maximum value.

The hot upsetting tests of tool carbon steel were conducted. Typical resisting force curves were obtained with a local maximum at the beginning, decreasing in the central part and finally increasing in the last stage of the upsetting process. This time period of resisting force has been until now only observed in studies dealing with relatively low-melting temperature materials at mushy states, including lead-tin alloys.

To simulate the experimental results by a relative simple numerical model, a viscoplastic constitutive equation according to Perzyna was applied. A procedure how to identify the material parameters was suggested, based on minimization of difference between the numerical and experimental results of resisting force. Within the research an efficient correspondence between the measured and simulated behaviour of the tested specimens was achieved.

Cooling of working rolls

As the demands on rolling mill performance and the quality of the rolled material increase, the demands on cooling the rollers of rolling mills are also increasing. An appropriately designed cooling system must ensure a sufficiently intensive heat dissipation from the working rolls. The temperature of the rollers has a significant effect on the quality of their surface, service life and, above all, the temperature of the rollers. Thermal balicity is one of the parameters affecting the dimensional accuracy of the roll. This is particularly evident in the rolling of a narrow range on wide rolling mills. At the point of contact of the roll with the roll, the roll is heated, the average temperature increases, and the roll diameter is increased locally due to thermal expansion. On most rolling mills, balicity is compensated by the pre-bending of the working rollers. This method causes a high load on the bearing housings and racks and does not allow for the high thermal load of the cylinders to be compensated. Therefore it is necessary to combine the roll bending with suitable rolling cadence and suitable cooling mode (zone cooling). Several individually controlled cooling sections are designed along the width of the cylinder. The cooling intensity is controlled to ensure the desired temperature profile across the roll width. Thus, controlled temperature balicity can be achieved in individual sections. The temperature profile of the working rollers creates their thermal balicity, which has a major impact on the flatness of the rolled strips. The temperature profile and, in particular, the maximum working cylinder temperature achieved also indicate the efficiency of the cylinder cooling system.

Currently, water jets are almost exclusively used to cool the working rollers. Typically, the nozzle manufacturer specifies the following parameters:

  • Nozzle type designation
  • Spraying angles (or change in spray angle depending on pressure)
  • Equivalent nozzle hole diameter
  • Flow characteristic of the nozzle
  • Density of water distribution across beam width

However, based on these data, it is not possible to reliably determine the cooling effects. Relationships can be found in the literature where the heat transfer intensity (usually specified by the heat transfer coefficient) is calculated from the distribution of the incident water density. The main disadvantage of these relationships is the fact that they cannot respect the dependence of the heat transfer coefficient on the cooled surface temperature. Another disadvantage is the difficult to define area of ​​overlap of individual nozzles and the impossibility to reliably describe their interaction.

The choice of cooling system nozzles should be such that the desired range of cooling effects is achieved with minimal cooling water consumption. This can only be achieved with detailed knowledge of the cooling characteristics. In this case, reliable research is a reliable way.

Most of the realized research projects were solved on the basis of experimental data. The aim was always to create experimental conditions that would be as close as possible to the conditions in real operation. From this requirement, there was a need to develop a variety of experimental devices suitable for simulation of observed events.

In-line heat treatment

At the exit of the rolling mill, the rolling stock must be cooled before further technological operations. This is done in several ways, depending on the type of rolled product range and track layout. In the case of profile rolling, most technologies use cooling on the cooler by natural or forced air convection. Flat products are wound at the end of the track by winders. The coiling temperature is usually precisely defined and affects the resulting mechanical properties of the rolls. Therefore, cooling systems are installed in front of the reels on the run-out section of the track. These can be of different design. Previously, so-called laminar walls were often used. In this case, the water flows out of the overflow troughs, forms a water wall and falls on the cooled surface. The advantage of this arrangement is the relatively simple construction, resistant to mechanical damage and clogging. The big drawback is the discontinuous regulation (it is controlled only by the number of on and off sections) and the possibility of cooling only horizontal, upwardly oriented surfaces. Another design variant is the use of water jets. The advantages of this arrangement are the relatively wide control range and the possibility of cooling an arbitrarily oriented surface. This can be used for cooling both flat and profiled rolls.

Modern technologies require not only cooling of the roll on the run-off section from the rolling mill necessary for other processing technologies, but also continuous heat treatment that will affect the resulting material structure and thus its mechanical properties. In-line heat treatment brings an increase in the utility value of products when, due to improved mechanical properties, it is possible to reduce the weight of steel structures or to extend the lifetime of exposed parts of railway tracks. At the same time, steels with a lower alloying content can be used. Advantageously, a sufficiently intensive cooling of the surface layer causes it to become cloudy and then to be tempered by utilizing the internal heat from the subsurface layers. To apply this procedure, knowledge of the quenching and tempering curves of the material is necessary. The cooling system is controlled to the desired cooling intensity. Due to the required cooling range, water jets are suitable for this purpose.

Main benefits of in-line heat treatment technology:

  • Increasing the product performance
  • savings of alloying elements
  • a direct link to the forming process without re-heating,
  • the versatility of the solution with the possibility of modification for a wide range of manufactured products,
  • use of environmentally friendly water cooling
  • relatively easy to connect to water management and track control systems.

Congratulations to associate professor Petr Kotrbáček.

Associate professor Petr Kotrbáček (right) with Dr. Martin Chabičovský