1. Field of the Invention
The present invention relates to a transport roll and a train of transport rolls for transporting a metal body in a hot state while the heat radiation therefrom is being prevented, and more particularly, to a transport roll and a train of transport rolls for transporting a hot slab or ingot emerging from a mold or a heating furnace.
2. Description of the Prior Art
The conventional transport roll for the transport of a steel ingot or slab heretofore used in steel making and rolling works has been of a heat-resistant, solid or hollow cylindrical type roll.
In transport rolls which transport a very hot and heavy material such as a hot slab emerging from the continuous casting process or a hot slab or bloom emerging from a heating or slabbing step to a subsequent processing step, several difficult problems are encountered. The train of transport rolls tends to incur thermal fatigue from the heat conducted directly from the hot material and from the radiant heat absorbed thereby; the rolls draw off heat from the hot material causing it to drop considerably in temperature; and the roll surfaces undergo severe deterioration.
Quite recently, with a view to attaining energy saving as well as high productivity, much work has gone into the development of a direct rolling process wherein the hot casting emerging from a continuous casting mold or the hot slab or bloom emerging from a slabbing step is delivered directly to a subsequent rolling step.
Such a hot casting, slab or bloom (referred to as "hot material" hereinafter) is continuously transported by the transport apparatus between prior and subsequent steps and, for carrying out the direct rolling step, it is of primary importance that the temperature drop of the hot material during its travel through the transport apparatus should be held to the minimum possible.
Since the hot material comes in direct touch with the transport rolls employed in the transport apparatus, its heat is conducted to the low temperature cold transport rolls and is radiated in all directions. As a result, the hot material falls in temperature so sharply that it is very difficult to transport it as far as the rolling stand before its temperature drops below the minimum required for the rolling step. No transport roll satisfactory for such a direct rolling process has been proposed heretofore.
Thus, the inventors of the present invention attempted to make the circumferential area of the transport rolls which comes in direct touch with the hot material as small as possible by providing grooves on the circumferential surface of the roll. The result was not, however, so remarkable as we had expected. We consider that this result occurred probably because cold air currents are generated in the grooves around the transport roll when the rolls rotate and these currents act to cool the hot material. Moreover, another unexpected problem came to light. It was found that cracks tend to occur at the bottom of the grooves, reducing the service life of the transport rolls. No doubt, these cracks occur because the bottom of the groove is subject to repeated thermal stresses. Next, we tried adhering heat resistant tape to the bottom of the grooves with a view to lessening the thermal shock. Although this was somewhat effective for preventing the occurrence of cracks, it did not eliminate the air currents which cool the hot material.