EP1134296A2 - Process and device for surface treatment of hot rolled metal sheets or strips - Google Patents

Abstract

Process for surface treating hot rolled metal sheets or strips (6) comprises homogeneously cooling the rolled material over its width and length directly after the last deformation step in the mill train to a temperature at which scale formation is reduced or suppressed, and annealing the weakly scaled rolling material under reducing conditions to completely descale its surface and to adjust the required mechanical properties. Preferred Features: The rolling material is fed through a continuous annealing oven (8) having heating (13), holding (14) and cooling zones (15) to heat the material to 500-800 degrees C, hold the material at this temperature and cool it to 100-700 degrees C.

Description

The invention relates to a method for the surface treatment of hot-rolled Metal sheets or strips - especially steel - for removal of scale or surface oxides, with the rolling stock over an outlet section is promoted to a reel.

In the production of thin sheets or strips of steel - whether after conventional hot wide strip rolling or when casting and rolling thin slabs - the material is exposed to the air during the forming in the rolling stands as well as on the subsequent runout or the cooling roller table. so that the surface of the rolling stock is inevitably scaled due to contact with the atmospheric oxygen. The growth rate as well as the type of scale layers that are formed depend essentially on the alloy composition of the rolled material and the temperature / time cycle used. At temperatures above 570 ° C, for example, the main product in the oxidation of iron is FeO layers, below 570 ° C Fe ₂ O ₃ and Fe ₃ O ₄ layers, and oxides of the respective alloy components in alloyed steels.

The rolled strip usually runs over the cooling roller table with water cooling elements to a reel device, often in the cooling roller table as well the mechanical properties via the temperature selection in the reel device of the band are adjustable. Here, however, it is disadvantageous that in the Coiler device influences the temperature on scale formation and setting the mechanical properties in the opposite direction by adjusting the structure behavior. In addition, the scale layer in the wound coil grows in the area of Band edges stronger than in the because of the better contact to the atmospheric oxygen Middle of the strip, especially at high reel temperatures. A disadvantage of the setting The mechanical properties in the hot strip mill is that the Temperature distribution over coil length and width undefined and especially on Coil end and beginning is inhomogeneous. Accordingly, the mechanical Properties of the finished products must be inhomogeneous.

The coiled, scaled, hot strip of a pickling line is usually used fed to scale and surface oxides by means of passage in salt or remove sulfuric acid media from the belt surface. However, for the economic operation of such a pickling line a certain amount Rolling volume to be conveyed by the pickling liquid is necessary. The rolling stock volume increases with decreasing hot strip thickness with the same surface throughput from. The increase in surface throughput by increasing the Throughput speeds are nevertheless limits with the known pickling techniques set. The surface treatment of increasingly thin hot strip pickling plants cannot be followed arbitrarily.

Accordingly, the invention has for its object a method and a System for the surface treatment of hot-rolled sheets or strips to provide with the above all thin tape with high surface quality homogeneous mechanical properties across belt length and width economically can be manufactured.

This object is achieved in a generic method in that a first step, the rolling stock immediately after the last shaping step in the rolling mill - i.e. after leaving the finishing stand - homogeneous is cooled to a temperature at the rolling stock width and length at which scaling of the rolling stock is restricted or suppressed, and that in a second step to complete the possibly slightly scaled rolling stock Descaling its surface and setting the desired one mechanical properties subjected to annealing in a reducing atmosphere becomes.

This solution is based on the knowledge that a corresponding abrupt cooling of the rolling stock which emerges directly from the rolling train, Temperatures are set at which scaling on the Band surface can be restricted or prevented. By the chosen Temperatures and cooling rates is the setting of the desired Structure possible. The scale formation depends on the material and takes with it increasing temperature. With iron, scale formation is at temperatures below noticeably reduced from 450 ° C. In a second step, this is such cooled and therefore only slightly scaled material, which if necessary still has scale formed during rolling, one Subject to annealing in a reducing atmosphere. During this step on the one hand, the surface of the rolling stock is completely descaled, on the other hand, the Annealing for setting a homogeneous structure and thus more homogeneous mechanical Properties used in the rolling stock and thus in the finished product.

This annealing preferably takes place with a simultaneous reduction of scale layers in a continuous annealing furnace sealed from the outside atmosphere rather than with a heating, holding and cooling zone, with the initial Heating zone the belt to temperatures in a range of 500-800 ° C, preferably about 550 ° C, is heated.

In a particularly preferred method variant, the rolling stock is in one the second heating zone directly adjoining the first heating zone Temperatures in the range of 700 to 800 ° warmed.

The holding zone is mainly used for descaling the surfaces and setting of the desired structure under the influence of time.

The continuous furnace consists of chambers that are gastight from the air atmosphere and contains reducing gases, preferably 100% hydrogen. Other reducing gases such as carbon monoxide or hydrocarbons and their Mixtures or mixtures of hydrogen and nitrogen are also conceivable. However, hydrogen has two advantages. It reacts with the oxygen the in a continuous furnace by the continuous conveyance of the rolling stock through the introduced air, which cannot be completely avoided, into water. In addition, the decarburization of the rolling stock surface by hydrogen is very low. A 100% hydrogen atmosphere is particularly recommended at high or higher alloyed steels, as the alloy metals are usually more sensitive to oxidation are as iron.

On the one hand, the proposed method can end with the annealing step, whereby Make sure that in the cooling zone of the sealed continuous furnace the rolling stock is already cooled to a temperature of about 100 ° C by one Reoxidation when the rolling stock emerges from the furnace and comes into contact again to prevent atmospheric oxygen. The tape is wound up immediately afterwards and is a glow product with a descaled surface and a homogeneous finish Structure available.

Alternatively, the continuous annealing process can immediately be followed by a surface coating connect, preferably in the form of a hot-dip coating line for coating with zinc or aluminum alloys. For such an embodiment the rolling stock is brought to a temperature in one in the cooling zone Cooled temperature range between 400 to 700 ° C and then immediately the surface treatment subject.

To combine the proposed method with known methods in a conventional hot strip mill or a CSP system is recommended to promote the cooled rolling stock over the existing runout sections and reel, then in the bundle to a separately arranged continuous annealing furnace to transport, reel here, rewind after annealing or immediately following surface coating systems feed.

According to the invention, a suitable one for carrying out the method is also used Attachment proposed. This is characterized in that in addition to known Units for rolling strips and sheets require a cooling device is that in the transition between the finishing mill and the run-out section is arranged, and a continuous annealing furnace is provided, which under reducing Atmosphere works.

A significant advantage of the proposed method is that hot strip, the dimensions due to the known processes with regard to its thickness of cold rolled strip, also with regard to its surface quality as well the mechanical properties reach the range of cold strip. Can too Hot-rolled strip can be provided surface-coated without any negative Influence on the structure.

Figur 1

die schematische Abfolge des erfindungsgemäßen Verfahrens mit dem ersten Schritt einer Abkühlung nach der Walzstraße sowie dem nachgeschalteten Glühen;

Figur 2

die schematische Darstellung einer Dünnbrammengieß- und walzanlage.

Further details and advantages of the invention emerge from the patent claims and the following description of exemplary embodiments of the subject matter of the invention according to the accompanying figures, which show:

Figure 1

the schematic sequence of the inventive method with the first step of cooling after the rolling mill and the subsequent annealing;

Figure 2

the schematic representation of a thin slab casting and rolling plant.

Figure 1 shows schematically the sequence of the proposed method as well the necessary investment units. The two embodiments of the Processes differ in the treatment of the rolling stock in the cooling zone of the continuous annealing furnace, which will be discussed later.

The method according to the invention can be used in conventional hot wide strip mills as well as in plants for the production of thin strip (see FIG. 2) Find. Both types of plant have a rolling mill 1 - the finishing line the rolling mill is here with three stands 2a, 2b, 2c, each with two work rolls and two backup rolls shown schematically - to which there is an outlet section 3 and connect a reel device 4. Immediately after the last rolling stand 2c, a cooling device 5 is arranged according to the invention, which Hot strip 6 cools down at an adjustable speed. The temperatures to be reached should be below 450 ° C. With the help of the cooling rate and the cooling temperature is the desired structure - pure martensite, bainite or ferritic-pearlitic structure or mixed structure - adjustable. That so frightened Volume 6 is without further major formation of scale or surface oxides transported to the reel device 4. Located on the belt surface only the scale or surface oxides that form during the rolling process have formed.

The coil 7 rolled up in the reel device 4 is then separated arranged continuous annealing furnace 8 supplied. According to an embodiment not shown it is also conceivable that the annealing furnace in the outlet section of the Rolling mill is integrated.

The already completely cooled or still warm coil 7 is in an unwinding device 9 unrolled, and via a welding device 10 in an inlet memory 11 funded. These three system elements form the inlet group 12 of the continuous annealing furnace. From there it continuously enters the annealing furnace 8 preferably has a 100% hydrogen atmosphere. The seals of the annealing furnace on both sides can be realized with dynamic gas seals. The furnace is divided into three basic zones 13 (heating zone), 14 (holding zone), 15 (cooling zone), the heating zone 13 being divided into a first (13a) and divides a second (13b) heating zone.

The first heating zone 13a is preferably provided with an induction furnace, which quickly heats the belt to temperatures of around 550 ° C. The follows second heating zone 13b to set temperatures in the band from 700 to 800 ° C. Then follows a relatively long holding area 14, in which essentially under the reducing conditions the descaling process and the setting of the desired structure takes place.

For the first process variant, a cooling process then closes Reeling on. For this purpose, the band in the cooling zone 15 to a Temperature cooled below 100 ° C, with the cooling rate and the cooling medium, the desired structure is adjustable. The Continuous annealing furnace 8 is a discharge group 16 downstream with discharge storage 17, skin pass mill and / or stretching judge 18 and a reel device 19.

Alternatively, the strip 6 in the continuous annealing furnace 8 is only at a temperature of 400 to 700 ° C cooled and in the warm state of a hot-dip coating plant 20 supplied, shown here only schematically.

The rolling mill shown only partially in FIG. 1 is shown in FIG schematic representation of a combined casting and rolling plant concretized. Such a plant essentially consists of a continuous casting plant 21 (here schematically indicated with a mold) for a strand 22 of 30 to 250 mm Thickness, preferably from 30 to 130 - this indication of thickness is the protected area of the invention not limited -, a cross-section device 23, a device 24 for heating or equalizing the temperature, for example a walking beam furnace or a roller hearth furnace, and a rolling mill 1 (here schematically indicated with two roll stands 2a, 2b). Depending on the type of execution, the separation device can be dispensed with.

According to the proposed method, a hot strip with very good results can be obtained Generate surface quality with good mechanical properties. The belt surface is reduced for further processing steps or surface treated, preferably hot-dip coated, as a finished strip product.

Claims (11)

Process for the surface treatment of hot-rolled metal sheets or strips (6) for removing scale or surface oxides, the rolling stock being conveyed to a reel device (4) via an outlet section (3) after the rolling process,
characterized, that in a first step the rolling stock (6) is cooled homogeneously over the width and length of the rolling stock immediately after the last shaping step to a temperature at which scaling of the rolling stock is restricted or suppressed, and that in a second step the possibly slightly scaled rolling stock is subjected to annealing under reducing conditions for complete descaling of its surface and for setting the desired mechanical properties. Method according to claim 1,
characterized in that the rolling stock for the second process step is fed to a continuous continuous annealing furnace (8) with a heating (13), holding (14) and cooling zone (15) for heating the rolling stock to temperatures in the range of approximately 500 to 800 ° C, keeping the rolled product at the heating temperature while reducing the remaining scale layer and cooling to a temperature in the range of 100 to 700 ° C. Method according to claim 2,
characterized in that after passing through a first heating zone (13a), in which it is preheated to a temperature of about 550 ° C , the rolling stock passes through an immediately following second heating zone (13b) for the purpose of further heating the rolling stock to temperatures in the range of 700 up to 800 ° C. Method according to claim 2 or 3,
characterized by
that in the cooling zone (15) of the continuous annealing furnace, the rolling stock is cooled to temperatures below 100 ° C in order to prevent reoxidation and is then wound up immediately. Method according to claim 2 or 3,
characterized in that in the cooling zone (15) of the continuous annealing furnace the rolling stock is cooled to temperatures in a temperature range between 400 to 700 ° C and then immediately surface-coated in the still heated state. Method according to one of claims 1 to 5,
characterized in that the rolling stock is cooled immediately after the last rolling pass, is coiled in the coiler device (4) following the outlet section, the coiled bundle (7) is fed to a continuous annealing furnace (8) separate from the rolling train, and is unwound before entering the continuous annealing furnace , annealed under reducing conditions and then cooled and coiled or immediately afterwards fed to a hot-dip finishing line (20). Method according to one of claims 1 to 6, characterized in that
that the reducing atmosphere is set by hydrogen, carbon monoxide, hydrocarbons, their mixtures or by mixtures of hydrogen and nitrogen. Plant for surface treatment of hot-rolled metal sheets or strips for removing scale or surface oxides for carrying out the method according to one of claims 1 to 7, wherein the rolling stock (6) emerges from the finishing stand (2a, b, c) of a rolling mill (1) and is conveyed via a discharge path (3) to a reel device (4),
marked by
a cooling device (5) in the transition between the finishing roll stand (2c) and the outlet section (3) and a continuous annealing furnace (8) for the cooled rolling stock, which works in a reducing atmosphere. System according to claim 8,
characterized in that the continuous annealing furnace (8) is arranged separately from the rolling train (1). System according to claim 9,
marked by Means for transporting the cooled coiled strip (7) to the continuous annealing furnace, an inlet group (12) which is arranged upstream of the continuous annealing furnace (8), comprising a device for unwinding (9), optionally a welding device (10) and optionally an inlet store (11) and an outlet group (16), which is arranged downstream of the continuous annealing furnace (8), optionally comprising an outlet store (17), a skin pass mill and / or a stretch leveling device (18) as well as a device for reeling (19) the strip or sheet or one directly hot-dip coating line (20) connected to the continuous annealing furnace. Plant for the production of metal sheets and strips with a Hot strip end thickness preferably less than 2 mm, comprising one Continuous caster (21) for casting slabs (22), optionally one Cross-section device (23), a heating device and / or temperature compensation device (24), a hot rolling mill (1) and a surface treatment plant according to one of claims 8 to 10.

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