METHOD FOR THE REDUCTION OF LOCAL TEMPERATURE IRREGULARITY IN STEEL SLABS AT ROLLING

Abstract: In order to eliminate the drawbacks from colder zones on steel slabs from water cooled carrying pipes in reheating furnaces, the steel slab is heated locally, preferably by induction heaters in a following roller bed.

Description: TITLE OF THE INVENTION Method for the reduction of local temperature ir¬ regularity in steel slabs at rolling. TECHNICAL AREA The present invention concerns a way to influence the temperature irregularity in slabs which are heated for hot-rolling i.a. to achieve a better endproduct, which is more even, has better limit gauge and material- -characteristics and to balance loading pikes in the mill stand. The possibility to achieve the attained quality with more simple control equipment in the mill also is achieved. BACKGROUND TECHNIC Before rolling, the steel slab is heated up to a desired temperature usually in a so called reheating fur¬ nace. For bigger slabs it is usual that the steel slab is heated from both sides at the same time. This usually is managed by resting the steel slab during the heating upon a pipe structure which mostly is water cooled. The desi- red temperature in the slab is reached by supplying ener- gi from e.g. combustion of fuel to both of the over-side and the under-side of the steel slab. With the technics till now existing the steel slab with great accuracy can be heated up to the desired temperature level. But where the steel slab has been in contact with the water-cooled pipe construction, a lower temperature level than in the other parts of the steel slab is present. If e.g. a so called reheating furnace is used for heating, the lower temperature level mostly is present as strips across the underside of the slab in the locations, where the steel slab has been in contact with the carrying pipes in the furnace.These strips causes at the succeeding hot-rolling i.a. dimension deviations, lo¬ wer uniformity in quality and e.g. bad flatness if the end product is a steel strip. The disadvantages at the heating partly can be compensated in the following rol¬ ling mills with very exact control equipment, but can not be fully eliminated. The techics to get slabs with an uniform heat are that the reheating furnaces have been provided with a me¬ tallic, heat resistant material, so called travellers, between the slab and the carrying pipe structure. It is also known, during the rolling, to arrange heat equaliza¬ tion stations, often heat insulated and also heat sto¬ ring, where the slab is stopped for temperature equaliza¬ tion. The result has not been satisfactory when the loss in temperature and remaining irregularities in temperatu¬ re still have caused bad limit gauges in the finish rol¬ ling and a reduced capacity in the rolling mill. Not even equalization stations whith supply of heat have given sa¬ tisfying results, while as well areas with colder spots as adjacent areas have been supplied with heat. Irregula¬ rities in temperature still have remained even if the cold spots have been eliminated. The different pairs of rolls in the rolling mill also have been provided with a more and more advanced control equipment. Still remaining however, irregularities in temperature in the slab con¬ stitute a considerable inconvenience in quality with the present technics of furnaces and rolling. CONCLUSION OF THE INVENTION With this invention colder, limited sections in a steel slab for rolling, which remain after heating in an e.g. reheating furnace, can be equalized at a desired temperature level. The temperature equlization is made by a local heat supplying directed to the colder sections in the steel slab. Heating and rolling by that can be done with an encreased degree of freedom and with an improved quality as a result. The invention will be described closer in claims, in the following sector and in an example of performance and by help of figures enclosed of which figure 1 shows a view of a rolling mill for wide steel strips and figure 2 how a slab is in close contact with a water-cooled carrying pipe. CLOSER DESCRIPTION OF THE INVENTION The equalization of the temperature is brought about by delimimited heating the steel slab at a suitable occation at the rolling at the section where the steel slab has an irregularity in temperature. The difficulty to locate and control this heating, during the rolling or when the steel slab is moving, makes the proposed inven¬ tion especially applicable if the heated slab is put in a fix position before the rolling, in that the colder sec¬ tions are placed straight in front of the firmly instal¬ led heating source. In this position of waiting the col- der sections of the steel slab is heated from one side during a period of time necessary to get the wished in¬ crease in temperature. The proposed method is, when hea¬ ting of steel slabs in a so called reheating furnace, especially applicable in the position which becomes natu- ral to the slab when pulled out from the furnace and put down upon the roller bed leading to the rolling mill. The local supplying of heat can be generated in many ways. As an example, induction heating, gas and oil burning and radiation of electrical heating can be men- tioned. Induction heating is preferred why this can be concentrated in an easy way just to the areas which are needed to be heated without affecting surrounding sec¬ tions worth mentioning. The induction heating also can be computer-controlled to be further precised to the right areas. It also can be freely used in the room without af¬ fecting the surrounding environment. EXAMPLE Steel slabs 4, with a length of 11 m and a cross- -section with a width of 1200 mm and a thichness of 220 ram as an average, is heated in a oil-fired furnace 1 up to about 1250°C. The maximum capacity of heating amounts to 300 ton/hour. In the hearth the steel slab 4 is heated both from its upper and lower side with a great number of oil burners. This is brought about by resting the slab 4 upon eleven water-cooled carrying pipes 2. In order to, if possible, avoid a too great depression of temperature in the steel slab 4 above the carrying pipes 2, these are provided with a heat insulating material 13. Despite of that the depression of temperature in the slab 4 will be obvius and gives for the rolling unfavourable gradients 14 of temperature. At the outlet-end of the furnace 1 the heated slab will be put in a fix position upon a so called rol¬ ler bed 5 for further transporting to the mill. At the put dow point at the roller bed 5, the steel slab 4 is heated during a short period of time by help of a heating source 6, which consists of inductors, which are placed in the roller bed 5 in that way that they are heating the slab 4 only at that side and in those places where the carrying pipes 2 in the furnace 1 have caused unfavoura¬ ble gradients 14 of temperature. The inductors are supp- lied from a for the purpose especially arranged electri¬ cal interlocking post 7 and can be controlled separately. By this well defined additional heat, as well as quantity as area, together with the subsequent rolling in an uni¬ versal mill 9, remaining variations in temperature in the rolled slab are to be neglected at the arrival to the fi¬ nishing pairs 11. After the inductive heating, scales on the surfa¬ ce of the slab will be separated in a spraying chamber 8. In universal mill 9, coiling box 10, and finishing pairs 11, the slab is rolled to a strip which after cooling is coiled to a coil in a coiling capstan 12.

Claims: CLAIMS 1. Method for the reduction of temperature irre¬ gularity in a steel slab (4) at hot rolling, c h a ¬ r a c t e r i s e d in that the steel slab (4), after an ordinary heating, is heated by firmly mounted inductive heaters against the sections of the slab (4) where the temperature is unfavourable differing and that the slab (4) thereby is stationary at a predetermined place upon the roller bed (5) direct after the ordinary heating furnace ( 1) .

Inventors: ANDERSSON SVEN GOERAN, NORBERG PER-OLOF, ANDERSSON, SVEN, GOERAN, NORBERG, PER-OLOF
Applicants: SSAB SVENSKT STAL AB, SSAB SVENSKT STAOL AB
Application year: 1985
Main classification: B21B47
IPC codes: B21B1, B21B45