EP0208679B1

EP0208679B1 - Method for the reduction of local temperature irregularity in steel slabs at rolling

Info

Publication number: EP0208679B1
Application number: EP19850901132
Authority: EP
Inventors: Sven Göran ANDERSSON; Per-Olof Norberg
Original assignee: SSAB Svenskt Stal AB
Current assignee: SSAB AB
Priority date: 1984-02-29
Filing date: 1985-02-27
Publication date: 1989-08-30
Also published as: SE8401102L; SE8401102D0; SE447069B; DE3572608D1; WO1985003891A1; AU3995885A; EP0208679A1

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

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 end product, 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 furnace. 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 desired temperature in the slab is reached by supplying energy from e.g. combustion of fuel to both of the over-side and the under-side of the steel slabs.
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, lower unformity 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 rolling mills with very exact control equipment, but can not be fully eliminated.
The technics to get slabs with an uniform heat are that the reheating furnaces have been provided with a metallic, heat resistant material, so called travellers, between the slab and the carrying pipe structure. It is also known, during the rolling, to arrange heat equalization stations, often heat insulated and also heat storing, where the slab is stopped for temperature equalization. The result has not been satisfactory when the loss in temperature and remaining irregularities in temperature still have caused bad limit gauges in the finish rolling and a reduced capacity in the rolling mill. Not even equalization stations with supply of heat have given satisfying results, while as well as areas with colder spots as adjacent areas have been supplied with heat. Irregularities 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 constitute a considerable inconvenience in quality with the present technics of furnace and rolling.
The patent publication US-A-3 587 268 discloses the provision of hot-strip rolling, at the holding table preceding the finishing train, with temperature-sensing means and fast-acting electric-induction heating means responsive thereto for eliminating cold spots in the workpiece to permit it to be processed through the finishing train. However, the purpose of this method is to make it possible to continue with an already started rolling process when cold spots appear on the plate or strip, but not the elimination of cold spots before the start of the rolling. Because of the raised requirements in quality of the finished product this is no more a possible way to achieve a satisfactory result and that is why this method cannot be used.

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 equilization 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 increased 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 delimited heating the steel slab at a suitable location 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 invention especially applicable if the heated slab is put in a fix position before the rolling, in that the colder sections are placed straight in front of the firmly installed heating source. In this position of waiting the colder sections of the steel slab is heated from one side during a period of time necessary to get the wished increase in temperature. The proposed method is, when heating of steel slabs in a so called reheating furnace, especially applicable in the position which becomes natural 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 mentioned. 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 sections 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 affecting the surrounding environment.

Example

Steel slab 4, with a length of 11 m and a cross- section with a width of 1200 mm and a thickness of 220 mm 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 orderto, 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 obvious and gives for 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 roller bed 5 for further transporting to the mill. Atthe put down 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 unfavourable gradients 14 of temperature. The inductors are supplied from a for the purpose especially arranged electrical interlocking post 7 and can be controlled separately. By this well defined additional heat, as well as a quantity as area, together with the subsequent rolling in an universal mill 9, remaining variations in temperature in the rolled slab are to be neglected at the arrival to the finishing pairs 11.
Afterthe inductive heating, scales on the surface 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

1. Method for the reduction of temperature irregularity in a steel slab (4) at hot rolling, at which the slab is heated to eliminate cold spots by inductive heaters, characterised in that the steel slab (4), after an ordinary heating, is heated by stationary mounted inductive heaters at the sections of the slab (4) where the temperature is unfavourable differing and that the stab (4) thereby is stationary at a predetermined place upon the roller bed (5) direct after the ordinary heating furnace (1).