EP0702608B1 - Process and device for making semi-finished products - Google Patents

Abstract

PCT No. PCT/DE94/00656 Sec. 371 Date Jan. 25, 1996 Sec. 102(e) Date Jan. 25, 1996 PCT Filed Jun. 3, 1994 PCT Pub. No. WO94/29048 PCT Pub. Date Dec. 22, 1994An apparatus and a process for making semi-finished products in the form of thin metal bars having a width-gauge ratio of over 60 and a maximum sheet metal gauge tolerance of 2%. A metal profile is fed continuously and upwardly through a pool of melt material having the same composition as the metal profile so as to form a coated metal profile. The metal profile is fed at a rate which would result in a coated metal profile having a thickness of at least three times that of the uncoated metal profile. The coated metal profile is subjected to a smoothing pass between a pair of smoothing rolls when the mean temperature in the crystallized layer of the coated metal profile meets a given condition. The smoothing rolls are adjustably disposed inside a housing at a distance of 0.5 to 5 m from the melt pool surface.

Description

The invention relates to a method for producing semi-finished products in the form of thin metal strands according to the preamble of claim 1 and a device for performing the method.

A method and a device for producing thin metal strands are known from EP 0 311 602 B1, from which the preambles of claims 1 and 9 are based. A metal profile cleaned on the surface, for example in the form of a band-shaped steel sheet (mother band) with a thickness of 0.1-1.4 mm, is continuously guided through the bottom of a melt container filled with a similar steel melt. For this purpose, a slot-like opening is provided in the bottom of the melt container, which is provided with a sealing device in order to prevent melt from escaping. The temperature of the melt is close to the liquidus temperature T _liq . The steel strip is moved through the melt at a constant speed and led upwards out of the melt. Due to its low heat content (strip temperature approximately equal to room temperature), an adherent layer of crystallized and still forms on the surface of the steel strip liquid melt. The thickness of this layer can be a multiple of the thickness of the original mother tape. It depends in particular on the residence time in the melt (speed of the mother tape), on the melt temperature (temperature difference to the solidus temperature T _sol ), on the heat of fusion and the specific heat of the material used and on the mother tape thickness. The process must be carried out in such a way that re-melting of crystals that are already adhering is avoided. Under this condition there is a temperature gradient across the strip thickness. During the movement through the melt pool, the temperature inside the mother tape is the lowest and rises towards the edge. A temperature curve of the same quality is also present in the adhering layer. The liquidus temperature T _liq is precisely present in the outermost region of the layer.

The adhesive layer initially has a mixed composition of the crystals formed and the molten phase in between (mushy zone). The proportion of molten phases increases towards the outside. After leaving the molten bath, the adhering layer cools down, whereby the temperature gradient that existed up to that point is reversed. The adherent layer solidifies completely.

From EP 0 311 602 B1 it is also known to keep a semifinished product to be produced in the manner described above after leaving the molten bath in an atmosphere protecting against oxidation until it cools or enters a forming machine in which the semifinished product is heated - And / or cold forming process is subjected. Part of the quantity of finished product produced is then returned to the beginning of the process as a mother tape and again passed through the molten bath.

With regard to the production of steel strip material, the practical application of this method has hitherto stood in the way of a decisive obstacle. The buyers of high-quality cold or hot strip require, among other things, the manufacturer to adhere to a fluctuation range in the sheet thickness that is at most 2% of the nominal thickness. Such a narrow tolerance cannot be reliably maintained with the previous method. Existing irregularities in the thickness of the strip, which exist after leaving the molten pool and exceed the prescribed maximum limit, can practically no longer be eliminated by the subsequent forming processes. This is due to the fact that due to the extreme flatness of the semi-finished product used in the rolling process (width / ceiling ratio at least 60), the forming (with decreasing thickness) takes place practically only in the longitudinal direction and no significant spreading occurs. Existing thickness differences along a line transverse to the longitudinal direction of the strip therefore remain relatively unchanged.

EP 0 311 602 B1 describes a second method variant in which the mother tape is introduced in the reverse manner into the melt bath from above and is pulled off again through the bottom of the melt vessel. In this embodiment, the problem of the floor sealing is particularly serious, since the directions of exit of the melt and the strip material are the same and, as a result, not only is there no dynamic sealing effect, but moreover a negative "entrainment effect" which supports the tendency of the melt to exit can also be found. For this reason, a special sealing device in the form of a pair of sealing rollers is required in the bottom region of the melt vessel. This pair of sealing rollers drastically compresses the "mushy zone" and thus squeezes out large parts of the liquid phase from the "sponge-like" crystallizate structure already formed. This has the consequence that the thickness of the adhesive layer that can be achieved is considerably less than that of the first method variant. For economic reasons alone, such a procedure can hardly be considered for practical application.

The object of the invention is to develop a generic method in such a way that the required sheet thickness tolerance of at most 2% can be reliably maintained and to provide an apparatus for carrying out the method.

This object is achieved with regard to the method by the characterizing features of patent claim 1. Advantageous developments of the invention are specified in subclaims 2 to 8. A device for carrying out the method, which is in principle also suitable for producing different types of profiles (e.g. round or any polygonal cross-sectional shapes), has the features of patent claim 9 and can be configured in an advantageous manner by the characterizing features of subclaims 10 to 14.

The invention is explained in more detail below with reference to the exemplary embodiment of a device suitable for the method, which is shown schematically in the single figure.

A sheet coil 12 is used as the mother sheet, which is unwound at a certain speed. Reference number 11 designates a strip welding system which connects the end of an already unwound coil to a new coil 12 in order to enable a continuous process sequence. At 7, a strip storage system is indicated, which stops the supply of strip during the welding process at a short time Coil change can catch, so that the production operation is not interrupted. In the production flow behind the belt storage system 7, a belt cleaning 6 is arranged, in which the surface of the mother belt used is made metallically clean. A pair of transport rollers 2 ensures that the mother tape, which asked for a width / thickness ratio of at least 60, preferably at least 100, is guided into the melt 3 at a constant preselected speed through a corresponding slot-like opening in the bottom of the melt container 1. The mother tape has a very low heat content, since it has room temperature, for example. The melt 3 (eg steel) consists of the same material as the mother tape. A seal, which is arranged on the bottom of the melt container 1, is not shown separately in the figure. While the mother tape is passed through the melt 3 from bottom to top, a layer which grows with increasing dwell time (ie with an approach to the melt pool level) crystallizes, since the mother tape draws heat from the melt 3 in its immediate vicinity, whereby it heats up. The melt 3 is otherwise kept at a temperature of, for example, 10 K above the liquidus temperature. The level of the weld pool level is kept at a constant value by means of a feed, not shown. Taking these and other parameters into account (in particular solidus temperature, heat of fusion, specific heat of the melt material), the belt speed via the transport rollers 2 is preferably set such that the mother belt with the adhering layer when leaving the melt 3 is 3 to 7 times as thick has like the original mother band.

A smoothing roller device in the form of a pair of smoothing rollers 4 arranged next to one another is positioned above the melt pool level. The distance of this pair of smoothing rollers 4 from the melt pool level is variable in that the height of the pair of smoothing rollers 4, for example, by a Electromechanical or hydraulic adjustment device, which is indicated by the arrows, is adjustable. The minimum distance of the pair of smoothing rollers 4 from the melt pool level is about 0.5 m, the maximum distance 5 m. The altitude is chosen so that the smoothing stitch takes place at a point where the layer adhering to the mother tape is already relatively solidified on the one hand, but on the other hand still has sufficient proportions of liquid phase in its outer zone which also have a problem-free material flow transversely to the longitudinal direction of the Enable mother band. It is therefore a question of the most favorable quantitative ratio of the solid to the liquid phase. The average temperature in the crystallized layer can be used as a control variable for this. According to the invention, the smoothing should take place at a temperature T _gl which satisfies the following relationship: $${\text{T}}_{\text{gl}}{\text{= T}}_{\text{Sol}}{\text{+ ax (T}}_{\text{liq}}{\text{- T}}_{\text{Sol}}\text{)}$$

Therein, a means a factor in the range of 0.1-0.8, preferably in the range 0.2-0.4. The lower a is, the higher the solidified part. The lower limit is to be regarded as critical in that, in the case of malfunctions, complete or almost complete solidification can easily occur, which would make it impossible to compensate for any larger strip thickness differences. The upper limit of value a is primarily economic. Due to the high proportion of molten phase, a considerable part was squeezed down because of the vertical guidance of the strip material, so that the output would decrease accordingly. In order to facilitate the adjustment work, a strand surface temperature measuring device (not shown) can be provided in the adjustment range of the pair of smoothing rollers 4. The smoothing roller pair 4 is expediently with an internal fluid cooling (e.g. water cooling). The desired reduction in the thickness of the metal strand as a result of the smoothing stitch should be in a range of 5-15%.

In order to avoid an oxidation of the strand surface which is disruptive for the subsequent further processing of the semi-finished product produced, the adhesive layer of the mother tape is protected against the entry of atmospheric oxygen by a housing 5 which can be flooded with an inert atmosphere. The housing 5 directly adjoins the melt container 1 and also envelops the pair of smoothing rollers 4. In order to avoid undesirably rapid cooling of the adhering layer and thus excessive solidification, if necessary, in particular in the area of the adjustment of the smoothing roller device 4, it can be provided that at least parts of the walls of the housing 5 are provided with thermal insulation. Moreover, it is expedient to design the walls of the housing 5 as cooling walls, in particular as walls that are fluid-cooled from the inside (for example water cooling). Controlling the coolant temperature then allows controlled cooling of the semifinished product produced in the cooling zone 8 downstream of the smoothing roller device 4, which leads to particularly favorable material properties. Similar to a continuous annealing process, the band-shaped material is guided in loops in a central section of the cooling zone 8 by corresponding deflection rollers, so that a correspondingly longer dwell time occurs in this zone. After the metal strand produced has cooled sufficiently, it leaves the housing 5 with its inert atmosphere and can be oiled, for example, by an electrostatic oiling device 9 and protected against corrosion. The material is then continuously wound into a coil 13. After reaching a certain weight, the coil 13 is cut off from the rest of the strand by means of a pair of scissors 10 and is further processed into a warm or Cold rolling mill transported away.

It is of course also possible, as already described in EP 0 311 602 B1, to connect the further processing directly. In this case, the cooling can be interrupted far above room temperature if necessary to save thermal energy, and the enclosure can be guided to the subsequent forming machine with an inert atmosphere.

The invention is explained in more detail with reference to the following exemplary embodiment, in which reference is made to the system diagram shown in the figure.

(hier a = 0,5 gewählt) mit einer Durchschnittstemperatur von T = 1497°C + 0,5 x (1507°C - 1497°C) = 1502°C in der aufgewachsenen Schicht in das vertikal verschiebbare Glättwalzwerk 4, das in einer mit z.B. Argon gefüllten und kontrolliert gekühlten Einhausung 5 angeordnet war, eingeführt, wo seine maximale Dicke um ca. 17 % (0,5 mm) reduziert und seine Oberflächenrauhigkeit weitestgehend abgebaut wurde. Für die vorliegenden Verhältnisse erwies sich zum Erreichen des angestrebten Zieles eine integrale Temperatur von 1502°C für die Durchführung des erfindungsgemäßen Glättstichs als besonders günstig. Das Glättwatzwerk 4 wurde daher in seiner vertikalen Position so eingestellt, daß diese Temperatur auf der Eintrittsseite in das Glättwalzwerk unter den vorliegenden Abkühlbedingungen gegeben war. Der durchgeführte Glättstich führte zu einem vollständig lunkerfreien und in seiner Schichtung optimal verschweißten Stahlband mit einer gleichförmigen Dicke von ca. 2,5 mm. Die vorhandene Abweichung der tatsächlichen Banddicke von der Sollbanddicke lag mit nur 1,6 % noch deutlich unter dem maximal zulässigen Wert von 2 % für Warmband, das kalt weiterverarbeitet werden soll. Nach dem Austritt aus dem Glättwalzwerk 4 wurde das Stahlband, das weiterhin durch eine Argonatmosphäre vor Oxidation geschützt war, in dem wassergekühlten Dom der Einhausung 5 einer kontrollierten Abkühlung unterzogen und nach Durchlaufen eines ebenfalls gekühlten und mit Argon gefüllten Pufferraums (Kühlzone 8) einer Wickelstation 13 zugeführt. Anschließend wurde das Stahlband in einem nicht dargestellten Kaltwalzwerk auf eine Dicke von wiederum 0,5 mm ausgewalzt. Das so erzeugte Kaltband wies ausgezeichnete mechanisch-technologische Eigenschaften auf und erfüllte alle gestellten Qualitätsanforderungen. Etwa 20 % der laufend erzeugten Produktionsmenge wurden wieder als Eingangsmaterial in den Prozeß zurückgeführt.A cold strip made of a steel X60
0.16% C
0.35% Si
1.30% Mn
0.013% P
0.003% S
0.041% Al
0.025% Nb
0.0092% N
Remainder iron and usual impurities,
which had a thickness of 0.5 mm and a width of 1000 mm, entered after degreasing in a pickling bath 6 with the aid of a pair of drive rollers 2 vertically through the bottom of a melt vessel 1 filled with liquid steel. The melt showed an analysis comparable to the steel strip. The melt vessel 1 became liquid steel continuously from a distributor, not shown fed. The height of the molten bath 3 and the speed of the steel strip are the control variables for setting the desired contact tent between the steel strip and the molten bath 3, which should be about 2 seconds in the present case. Since the belt speed was 1 m / s, a melt pool height of 2 m was therefore maintained at all times. In the steel melt 3, which had a temperature of approx. 1512 ° C, a crystallization of a total thickness of approximately 2.5 mm occurred during the passage of the steel strip, so that the total thickness of the steel strip as it emerged from the steel melt 3 was approximately 3 mm was. This steel strip with a "pasty" surface (two phases: melt and crystals) was then according to the formula ${\text{T = T}}_{\text{Sol}}{\text{+ ax (T}}_{\text{liq}}{\text{- T}}_{\text{Sol}}\text{)}$

(here a = 0.5 selected) with an average temperature of T = 1497 ° C + 0.5 x (1507 ° C - 1497 ° C) = 1502 ° C in the grown layer in the vertically displaceable smoothing mill 4, which in one with, for example, argon-filled and controlled-cooled housing 5 was introduced, where its maximum thickness was reduced by approximately 17% (0.5 mm) and its surface roughness was largely reduced. For the present conditions, an integral temperature of 1502 ° C. for carrying out the smoothing stitch according to the invention proved to be particularly favorable in order to achieve the desired goal. The smoothing unit 4 was therefore adjusted in its vertical position so that this temperature was given on the entry side into the smoothing unit under the present cooling conditions. The smoothing stitch carried out resulted in a completely void-free steel strip with an optimally welded layering and a uniform thickness of approx. 2.5 mm. The existing deviation of the actual strip thickness from the target strip thickness was still only 1.6%, which is significantly below the maximum permissible value of 2% for hot strip, which is to be processed cold. After exiting the smoothing mill 4, the steel strip, which was further protected from oxidation by an argon atmosphere, was checked in the water-cooled dome of the housing 5 Subsequent cooling and after passing through a likewise cooled and filled with argon buffer space (cooling zone 8) fed to a winding station 13. The steel strip was then rolled out to a thickness of again 0.5 mm in a cold rolling mill, not shown. The cold strip produced in this way had excellent mechanical-technological properties and met all the quality requirements. About 20% of the current production volume was returned to the process as input material.

With the present invention it is possible in a surprisingly simple manner to produce a strip-shaped metal strand which is extraordinarily precise with regard to its shape and surface tolerance (deviation of the profile and the thickness over the strip length is less than 2%). At the same time, this process ensures that the adhering layer is securely welded to the nut plate throughout. Due to the possibility of controlled cooling, a strip material with excellent material properties can be achieved.

Claims (14)

1. A method for the production of semi-finished product in the form of thin metal billets, in particular of steel, having thicknesses of below 20 mm, in which an uncooled, cleaned metal profile (12) of low heat content is moved continuously through from bottom to top through a melt bath (3) of the same type of material, the speed of the metal billet (12) being set dependent on the height of the melt bath (3) such that by depositing crystals and melt on the metal profile (12) a billet thickness is produced which is at least three times the original thickness of the metal profile (12) and wherein furthermore an inert atmosphere is maintained during the production of the metal billet, characterised in that in order to produce semi-finished product having a width/thickness ratio of more than 60 and a fluctuation in billet thickness of at most 2% the metal billet after leaving the melt bath is subjected to a smoothing pass when the average temperature in the initially crystallised layer of the metal billet T_gl satisfies the relationship: $${\text{T}}_{\text{gl}}{\text{= T}}_{\text{sol}}{\text{+ a x (T}}_{\text{liq}}{\text{- T}}_{\text{sol}}\text{)}$$

   

      in which a = 0.1 - 0.8
      T_sol = solidus temperature
      T_liq = liquidus temperature.
2. A method according to Claim 1, characterised in that the factor a lies in the range of values 0.2 - 0.4.
3. A method according to one of Claims 1 to 2, characterised in that the reduction in thickness during the smoothing pass lies in the range of 5 - 15%.
4. A method according to one of Claims 1 to 3, characterised in that when producing steel billets the speed when guiding the metal profile through the melt bath (3) is set such that the ratio of the billet thickness to the original steel profile thickness is in the range of 3 - 7.
5. A method according to one of Claims 1 to 4, characterised in that the cooling of the metal billet until the smoothing pass is controlled by influencing the wall temperature of a housing (5) for the environment of the melt bath (3) and the metal billet which is guided out.
6. A method according to Claim 5, characterised in that the control is effected by decelerating the natural cooling.
7. A method according to Claim 5, characterised in that the control is effected by accelerating the natural cooling.
8. A method according to one of Claims 1 to 7, characterised in that the metal billet is subjected to controlled cooling once the smoothing pass has been performed.
9. An apparatus, in particular for performing the method according to Claim 1, with a melt vessel (1), in the base of which an opening, provided with a sealing means which prevents the emergence of melt, is arranged for introducing a metal profile, and also with a transport means (2) for continuously guiding the metal profile through the apparatus, and with a housing (5) which covers the region in which the metal profile emerges from the melt (3) and a succeeding cooling zone (8) for the metal billet and can be filled with an inert atmosphere, characterised in that a smoothing rolling means (4) is arranged within the housing (5) at a distance from 0.5 - 5 m from the level of the melt bath (3) and that the distance of the smoothing rolling means (4) from the level of the bath is changeable.
10. An apparatus according to Claim 9, characterised in that the opening is slot-shaped for introducing a strip-shaped metal sheet with a width/thickness ratio of at least 60 and that the smoothing rolling means (4) is designed as a pair of smoothing rolls arranged next to one another.
11. An apparatus according to one of Claims 9 to 10, characterised in that the vertical position of the smoothing rolling means (4) is adjustable electro-mechanically or hydraulically.
12. An apparatus according to one of Claims 9 to 11, characterised in that the housing (5) in the region of the vertical adjustment zone of the smoothing rolling means (4) is designed at least partially with thermally isolating walls.
13. An apparatus according to one of Claims 9 to 12, characterised in that the walls of the housing (5) are designed at least in partial regions as fluid-cooled cooling walls.
14. An apparatus according to one of Claims 9 to 13, characterised in that at least one means for measuring the surface temperature of the metal billet is arranged in the region of the vertical adjustment zone of the smoothing rolling means (4).

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