1. Introduction
The steel industry in the global market is booming. Due to increased market competition, the demand for quality steel products has attracted a high level of attention in recent decades. In order to become more competitive in the steel industry, improving the ironmaking process and increasing the quality control in manufacturing have become a focus of steel plants.
Most steel products are manufactured after high temperature slabs are rolled by rolling mills. In the hot strip rolling process shown in
Figure 1, the high temperature slabs are roughly rolled into the transfer bars and then transported into the finishing rolling through the original heat retention panel. During the transport process, in addition to self-radiation heat dissipation, the convective heat dissipation of air, the contact heat conduction cooling of the rollers beneath the conveyors, and the change in the speed of the conveyors will cause a temperature difference between the head and the tail of the transfer bar. When this temperature difference is too large, it will make the finishing rolling more difficult and also cause the production line to shut down.
There are three types of equipment that can improve decreases in the temperature difference in the transfer bars: an inducting heater, an acting-type heat retention panel, and a heat coil box. This study mainly focuses on the acting-type heat retention panel. An acting-type heat retention panel involves installation of a radiation plate inside a traditional passive heat retention panel and a heat source, which can be supplied by burning natural gas or an electric heater, which is used to heat the radiation plate. Eventually, the radiation plate will heat the head and the tail of the transfer bar by radiation heat transfer so that the temperature difference can be decreased. In response to global energy conservation and carbon reduction, as well as goals of green production, numerous numerical models and methods for the prediction of the hot strip rolling process have been developed and successfully applied to many steel plants but there have been few studies focusing on the heat retention panel.
According to a hot rolling plant layout, Wang et al. [
1] designed a space for installing heat retention panels. Bu [
2] analyzed the hot rolling assembly line without a heat retention panel and discussed the influence of temperature at the finishing entrance. Furthermore, he analyzed the hot rolling assembly line with an installed heat retention panel. The results indicated that a heat retention panel can effectively decrease the temperature difference between the head and the tail of the transfer bar. Zhang et al. [
3] utilized a two-dimensional simplified model to simulate a hot rolling assembly line, both with a heat retention panel and without a heat retention panel. They assumed the heat transfer mode of transfer bar in the heat retention panel to be based on radiation. Thus, they ignored the convective effect of air, and in order to simplify the computational calculation, the influence of the rollers beneath the conveyor was also ignored. Compared with the in-situ data in a steel plant, the results worked well for the prediction of the temperature history of the transfer bar. Zhang et al. [
4] utilized a two-dimensional symmetry finite element method (FEM) to simulate the transfer bar inside a heat retention panel with a high radiation coating. They sprayed a high radiation ceramic coating on the inner wall of the panel. Their results showed that a high radiation ceramic coating can significantly decrease the temperature gradient in sections of the transfer bar.
In addition, a significant amount of data on the hot strip rolling process, such as the size of the slab before rolling, the temperature of the slab at the exit of the reheating furnaces, and the running speed of conveyors, has been found to be important parameters to affect the quality of steel products. Bu et al. [
5] obtained numerous in-situ data from a steel plant, including the size and properties of slab, the temperature at the exit of the slab reheating furnaces, and the size and the temperature distribution of the transfer bar after rough rolling. They used the Microsoft Visual Basic program to design a computer-aided design program to calculate effect of the temperature of the transfer bar on the finishing rolling force. Ling [
6] analyzed the advantages and disadvantages of a traditional passive heat retention panel, an acting-type heat retention panel, and a heat coil box. The results indicated that a traditional passive heat retention panel consumes the least energy but it is inefficient for heat retention. Although an acting-type heat retention panel consumes the most energy, it is the most effective for retaining heat in the transfer bar. A heat coil box exhibits a good performance for heat retention and consumes less energy than an acting-type heat retention panel, but the cost of installation is the most expensive. Speicher et al. [
7] used the finite difference method (FVM) combined with heat conduction and heat convection to investigate the impact of the rollers beneath conveyors on slab. Legrand et al. [
8] studied the thermal fatigue effect of the rollers beneath the conveyors on slab.
Chielo et al. [
9] used the nonlinear heat transfer equation to predict the temperature of the steel on the run-out table (ROT) process. The results show that the performance with a cooling stop temperature concept to the temperature and property of the steel was greatly improved. Mei et al. [
10] investigated the strip steel, which was heated by induction heater by the finite element method. They found that the temperature difference became more and more obvious with the increase of thickness. Shulkosky et al. [
11] developed a program which allows users to set-up their hot strip mill configuration and simulate the mechanical properties of the steel in the hot rolling process. The program includes reheating furnace, roughing mill stands, heat retention equipment (panels and coil box), finishing mill stands, the run-out table, and the mill exit area. Panjkovic [
12] designed a model to predict strip temperature from the roughing mill exit to the finishing mill exit. The results were compared to those from the plant measurements, and it was shown that this model worked very well. Grajcar et al. [
13] used a semi-industrial physical model to simulate thermomechanical rolling and controlled cooling of advanced high-strength steels with increased Mn and Al content. The results indicated that the high-quality strip samples with a thickness up to 3.3 mm could be obtained by using heat retention panels. Tudball and Brown [
14] developed a transient 3D finite element model to obtain thermal variations during the hot rolling process. The numerical model showed that the temperature results can provided a relatively accurate prediction with less than 10% deviation. Delpature et al. [
15] studied the active tunnel furnace in order to minimize heat losses of the transfer bar on the roller table between roughing mill and finishing mill. With traditional passive heat retention panels, the temperature difference in the head and the tail of the carbon transfer bar was about 20 °C. They found that the active heat retention panels was able to compensate for this drop in temperature.
Based on the studies referenced above, it is important to decrease the temperature difference between the head and the tail of the transfer bar to enhance the quality of steel products. The traditional passive heat retention panel is widely used in many steel plants, but its ability to hold temperature is inefficient. In order to solve this problem, it is necessary to develop other heat retention equipment as soon as possible. Based on the theory of fluid mechanics and the theory of heat transfer, in this study, the commercial software ANSYS-FLUENT combined with UDF (User-Defined Functions) are first used to simulate the traditional passive heat retention panel used in the China Steel Corporation (CSC), Taiwan. After the above results were proved to work well in terms of prediction, an acting-type heat retention panel model was constructed to investigate the performance of both types of heat retention panels.
4. Conclusions
In this study, the traditional passive heat retention panel numerical model combined with UDF was adopted to simulate the temperature distributions and temperature differences when the transfer bars pass through the heat retention panel at different running speeds.
For the traditional passive heat retention panel, the deviation in the temperature difference in the transfer bar at FET position between the numerical simulation and the in-situ data was about 1.53%. Moreover, it was found that the temperature difference was induced by variations in the running speed of the transfer bar.
Table 3 shows that the temperature deviations between the numerical simulation and the in-situ data for the inner and outer walls of the heat retention panel were 8.80% and 5.33%, respectively. The temperature deviations between the numerical simulation and the in-situ data of the inner and outer walls of the rollers were 4.31% and 4.80%, respectively. Since the performance of the numerical results worked well compared with the in-situ data, the corresponding parameters, including the initial temperature, convective heat transfer coefficient of the surroundings, and emissivity of the solid surfaces, could be used in the acting-type heat retention panel numerical model.
According to the numerical results of the acting-type heat retention panel model, providing the two heat fluxes on the upper surface of the radiation plate is an effective method by which to replace the burning process or the electric heating process. The numerical simulation indicates that the transfer bars can be heated by the radiation plate. When the heat flux increases, the temperature difference between the head and the tail of the transfer bar will be reduced. In contrast, the heat flux will cause the temperatures of the heat retention panel and the rollers to increase. Eventually, by converting the heat flux to the amount of fuel required, steel plants can obtain the relevant information about their energy consumption.