ES2134427T5 - FOUNDING OF A STAINLESS STEEL BAND ON A DEFAULT RUGOSITY SURFACE. - Google Patents

Abstract

Method of continuously casting metal strip (20) from a casting pool of molten metal supported on chilled casting rolls (16) such that metal solidifies onto moving casting surfaces of the rolls. The metal is austenitic stainless steel containing chromium and nickel in a ratio (Cr/Ni)eq of less than 1.60 (preferably no greater than 1.55) and the casting surface of each roll has an Arithmetical Mean Roughness Value (Ra) of more than 2.5 microns (preferably in the range 2.5 to 15 microns).

Description

Casting of a stainless steel band on a predetermined roughness surface.

The present invention relates to casting continuous of an austenitic stainless steel band. Have particular application, although not exclusive in continuous casting of a stainless steel band in a roller smelter Twins.

It is known to cast a metal band in continuous operation in a twin roller smelter. The metal cast is introduced between a pair of cast rollers horizontal that rotate in opposite directions subjected to cooling in such a way that on the surface of the rollers rotating solidify metal plates that join in the passage between the rollers to produce a solidified band that peel down from between the rollers. The term "step between rollers "is used herein to make reference to the area in general in which the rollers are located  closer. The molten metal can be poured from a cauldron of pouring into a smaller container from which the metal flows through a metal feed nozzle located above the rollers in order to direct it to the passage between them, forming thus a wash bath that is supported by the surfaces of Roller casting immediately above the passage between same. This wash bath can be confined between plates laterals or dikes that are in sliding coupling with the extreme surfaces of the rollers.

These twin casting rollers have been used with some success with non-ferrous metals that solidify quickly when cooling, for example, aluminum. The patent Australian No. 631728 on behalf of this applicant announces a procedure and apparatus that make continuous casting possible of a ferrous band between 0.5 mm and 5 mm and a apparatus of this type until the stage in which it is possible to produce continuously a smooth steel band of good quality. Without However, there have been particular problems in the foundry of an austenitic stainless steel band because of the marked tendency of such steels to suffer cracks and the appearance of repeated superficial depressions as superficial defect generally known as "crocodile skin". The solicitor He has done extensive experimental work in which we have certain factors that make it possible to melt continuously austenitic stainless steel band of good surface quality No significant cracking defects.

In the description that follows it will be necessary refer to a quantitative measurement of the roughness of the molten surfaces A specific measurement used in our experimentation work that helps to define the Scope of the present invention is the known standard measurement as "arithmetic mean value of roughness" that is generally represented by the symbol R_ {a}. This value is defined as the arithmetic mean of the absolute values of the roughness profile distances from the centerline of the profile  within the measurement distance l_ {m}. The center line of profile is the line on which roughness is measured and it is about a line parallel to the general direction of the profile within the boundaries of the roughness width section such that the sums of the areas contained between it and the parts of the Profile left on each side are the same. Average value arithmetic roughness can be defined as

R_ {a} = \ frac {1} {l_ {m}} \ \ int \ limits ^ {x \ = \ l_ {m}} _ {x \ = \ 0} \ | y | dx

Exhibition of the invention

According to the invention, a method as defined in claim 1. The surface of this texture can be obtained by machining regular ridges on the surface.

Preferably, the ratio of chromium to Nickel must not exceed 1.55.

More specifically, the invention provides a procedure as defined in claim 7.

Roller casting surfaces can have a regular texture of circumferential grooves with a texture depth in the range of 10 microns to 60 microns and a passage of grooves in the range of 100 microns to 200 microns.

In an alternative embodiment, the roller can possess a texture of projections regularly distanced, which can take the form of pyramids or cones with a distance of distance in the range of 100 to 200 microns and a depth in the range of 10 to 60 microns.

It is preferable that the carbon contents, Chrome and nickel steel are within the following ranges:

Carbon - 0.04 - 0.06% by weight Chrome - 17.5 - 19.5% by weight Nickel - 8.0 - 10.0% in weight.

Brief description of the drawings

For a more complete explanation of the invention, its application will be exposed to band production of stainless steel in a continuous twin roller smelter referring to the attached drawings, in which:

Figure 1 is a plan view of a smelter Continuous band of twin rollers according to the present invention;

Figure 2 is a side elevation of the melter of band depicted in Figure 1;

Figure 3 is a vertical cross section on line 3-3 of Figure 1;

Figure 4 is a vertical cross section on line 4-4 of Figure 1;

Figure 5 is a vertical cross section on line 5-5 of Figure 1;

Figure 6 illustrates the textured surface of a foundry surface used in a series of operations casting test; Y

Figures 7 to 9 illustrate the results of the foundry tests using steels of various compositions.

Detailed description of the preferred embodiment

The smelter illustrated comprises a frame Main machine 11 resting on the factory floor 12. The frame 11 supports a laundry roller carriage 13 which is horizontally movable between a mounting station 14 and a casting station 15. Carriage 13 supports a pair of rollers of parallel casting 16 to which molten material is supplied during the casting operation from a casting cauldron 17 to through a hopper 18 and a feeding nozzle 19. The casting rollers 16 are water cooled so that the layers of metal that solidify on the surfaces of rotating rollers and join in the passage between them to produce a solidified band 20 at the exit of the rollers. This product is collected in a standard 21 tape drive and can subsequently transferred to a second tape drive 22. In the Machine frame, adjacent to the casting station is a receptacle 23 is mounted and the molten metal can deviate towards this receptacle through a piquera of overflow 24 existing in the hopper or by extraction of an emergency plug 25 located on the side of the hopper if a significant malformation of the product or other will be detected severe dysfunction during the laundry operation.

The roller carriage 13 comprises a frame of the carriage 31 mounted on wheels 32 that roll on rails 33 that are extend along part of the main frame of the machine 11, whereby the carriage 13 as a whole is mounted to move along rails 33. Carriage 31 supports a pair of roller cradles 34 in which they are mounted swivel the rollers 16. The roller cradles 34 are mounted on the carriage frame 31 through sliding elements complementary 35, 36 to allow the cribs to move on the car under the influence of cylinder units Hydraulics 37, 38 to regulate the passage between the casting rollers 16. The carriage is movable as a whole along the rails 33 by the action of a hydraulic cylinder and piston unit of double acting 39, connected between the carrier 40 located in the roller carriage and the main frame of the machine and whose function is to move the roller carriage between the station assembly 14 and the casting station 15, and reciprocally.

The casting rollers 16 are driven in their reverse rotation through drive shafts 41 by a electric motor and transmission mounted on carriage frame 31. The rollers 16 have peripheral copper walls provided of a series of cooling water passages that extend longitudinally and are circumferentially spaced which they receive cooling water through the ends of the rollers by means of water supply conduits located in the drive shafts of rollers 41 connected to hoses of water supply 42 through rotating glands 43. The rollers can normally be about 500 mm in diameter and up to 1300 mm in length in order to produce a 1300 band mm wide

The cauldron of laundry 17 is of construction completely conventional and is supported by a fork 45 by an elevated crane with which it can be brought into position from a reception station of molten metal. He pouring cauldron is provided with a locking rod 46 which is operated by a servo cylinder to allow molten metal flow from the pouring cauldron through a piquera 47 and a refractory duct 48 to the inside of the hopper 18.

Hopper 18 is also construction conventional. It presents the shape of a large bucket constructed of a refractory material such as magnesium oxide (MgO). A side from the hopper receives the molten metal from the cauldron of laundry and is provided with the overflowing spout 24 above cited and emergency plug 25. The other side of the hopper is equipped with a series of 52 spaced metal outlet openings longitudinally. The bottom of the hopper has some mounting brackets 53 for placing the hopper on the frame of the roller carriage 31 said supports being provided with a openings to receive position 54 pythons existing in the cart to position the hopper exactly.

The feeding nozzle 19 is constituted by an elongated body constructed of such refractory material as graphite to aluminum. Its bottom is conical so that converges inward and downward so that it can project into the passage between the casting rollers 16. It is equipped with a mounting bracket 60 that supports it on the frame of the roller carriage and its upper part has some tabs projected lateral sides 55 that place it on the support mounting

The nozzle 19 can present a series of steps horizontally spaced flow that extend in the direction vertical for convenient download speed lower the metal across the width of the rollers and supply the molten metal in the passage between the rollers without impact directly on the surfaces of the rollers where it occurs initial solidification. Alternatively, the nozzle can present a single continuous output slot to provide a low speed cast metal curtain directly in step between the rollers and / or can be submerged in the metal bath molten.

The metal bath is confined at the ends of the rollers by means of a pair of closing plates 56 which held against the staggered ends 57 of the rollers when The roller carriage is in the casting station. The side closure plates 56 are constructed of a material resistant refractory, for example boron nitride, and present its toothed side edges 81 to engage the curvature of the staggered ends 57 of the rollers. Side plates they can be mounted on plate holders 82 that are movable in the station in the casting station by the performance of a couple of hydraulic cylinder units 83 to carry the plates lateral to its coupling with the staggered ends of the casting rollers to constitute extreme closures for the bath of metal casting that forms on the casting rollers during the laundry operation.

During a casting operation the closing rod 46 of the casting cauldron to allow the molten metal sneaks from the cauldron to the hopper through the feed nozzle and from it to the casting rollers. He clean head end of the product band 20 is guided by the  action of an apron table 96 to the jaws of the tape drive 21. The apron table 96 hangs on mounting pivots 97 located on the main frame and can swing towards the tape drive through the action of a hydraulic cylinder unit 98 once it has been formed the clean end of head. Table 96 can work against a top band guide fin 99 driven by a unit of piston and cylinder 101 and the band produced 20 can be confined between a pair of vertical side rollers 102. Once you head end has been guided to the jaws of the tape drive, the winder is rotated to wind the produced band 20 and the apron table can return to its resting position where it hangs simply from the machine frame separated from the product that is taken directly by the tape drive 21. The band product resulting 20 can be subsequently transferred to a tape drive 22 to obtain a final coil for transport outside the founder.

It has been checked in the operation of the device It has been described that it is possible to continuously produce a good austenitic stainless steel band for a successful regulation of the chemical composition of steel in combination with the use of rollers presenting textured surfaces to minimize segregation in initial cooling regimes Quick.

In the casting of stainless steel band austenitic, the way solidification can occur can play an important role in determining surface quality from the band. The primary mode of austenitic solidification, which is produced when the Cr / Ni ratio is less than 1.60 approximately, normally not recommended since it is provided segregation which leads to an increase in the tendency to crack formation Previously, it was believed necessary to ensure a Cr / Ni ratio within the range of 1.7 to 1.9 to minimize the formation of cracks due to the reduction of severity in the segregation and to which tortuous roads are obtained that make difficult The spread of cracks. However, our work Experimental has shown that the casting of a continuous band with steel of this composition it is very prone to produce bands with depressions of "crocodile skin" and severity in the Depression may be enough to produce cracks. Steel with a Cr / Ni ratio of less than 1.55 is more prone to segregation and can therefore increase the presence of cracks. If solidification occurs on an initial smooth substrate, the heat transmission rates are low and the structure of solidification is enough, resulting in segregation and cracking. However, we have determined that this tendency to segregation and cracking can be overcome ensuring a high initial heat transmission rate and this can be achieved more easily using a substrate textured, for example, by machining highlights in the substrate surface.

The initial experimental work was carried out in a solidification testing equipment in which a block 40mm x 40mm chilled is immersed in a molten metal bath at a speed that simulates the conditions of The casting surfaces of a twin roller smelter. He steel solidifies on the refrigerated block as it scroll through the molten bath to produce a layer of steel solidified on the block surface. The thickness of this layer can be measured at certain points on its surface to establish a distribution of variations in the regime of solidification and therefore the effective transmission regime of heat at different points. In this way it is possible to produce a complete solidification constant, usually indicated by the K symbol, as well as a distribution of individual values in The whole band solidified. It is also possible to examine the microstructure of the surface of the band to correlate the changes in the solidification microstructure with changes in the observed heat transmission values.

The nature of the experimental work and the results obtained.

Experimental conditions

Tests were conducted on three substrates of copper with different surface characteristics; a surface smooth and a textured copper surface and a surface ground coated with Cr (100 µm thick). The texture is introduced into the copper block by machining grooves and projections lengths of a geometry such as the one shown schematically in Figure 6. Each of these blocks is given provided with thermocouples to determine the transmission rates of prevalent heat during solidification. To keep the constant casting conditions throughout the experiments, they remained constant, within reasonable limits, the variables such as bath overheating and block temperature The bath temperature was tried to keep at about 1525 ° C corresponding to an overheating of 75 ° C. The introduction of argon gas in the oven proved very effective for the prevention of a chemical interaction between the bath and the atmosphere ambient. The chemical composition of the bath was adjusted to achieve the desired proportions (Cr / Ni) eq, mainly by additions of Cr, Ni, C and N2. To determine the Cr_ {eq} and Ni_ {eq} the following were used expressions:

(1) Cr_ {eq} = Cr + 1.37 Mo + 1.50 Si + 2.0 Nb + 3.0 You

(2) Ni_ {eq} = Ni + 0.31 nm + 22.0 C + 14.2 n + Cu

Table 1 shows a summary of the test conditions The experimental program included approximately 45 tests with proportions (Cr / Ni) eq variables between 1.55 and 1.74. The main characteristics of the Different tests are summarized in Table 2.

TABLE 1

10

TABLE 2

one

Results Effect of proportion (Cr / Ni) eq on quality shallow band

Visual examination of the samples revealed that the proportion (Cr / Ni) eq has a direct influence on the surface quality of the band obtained with a substrate textured; however, no effects could be observed appreciable with the smooth substrate. The molten specimens with Variable proportion (Cr / Ni) eq revealed a progression gradual from a type of texture of severe crocodile skin to a smooth surface texture as the proportion decreases (Cr / Ni) eq. The effect of the proportion (Cr / Ni) eq on the severity of the crocodile skin, which is represented in Figure 9, suggests that they can be obtained Substantial improvements in surface quality of the band keeping the ratio (Cr / Ni) eq below 1.60.

Effect of the ratio (Cr / Ni) eq on the heat transfer during solidification i) Textured substrate

The heat transmission regimes from the surface of the band towards the substrate were determined from of substrate temperatures. Figure 7 shows the influence of the proportion (Cr / Ni) eq in the bath on Heat flows for a textured substrate. Can be seen that profiles are characterized by a peak in heat flow which manifests initially followed by a rapid reduction of this peak, and with increasing time the heat flow tends to a constant value The highest heat transmissions (about 30 MW / m2) that are in the initial stages of Solidification can be attributed to intimate contact.

The experimental program determined that the proportion (Cr / Ni) eq that proves to produce the best surface texture (on a textured substrate) is less than 1.60.

ii) Smooth substrate

Figure 8 reveals the influence of the proportion (Cr / Ni) eq on heat transmission for a smooth surface. It can be seen that heat flows are relatively constant throughout the solidification and, what more importantly, the magnitudes of the flow peaks are much lower than the measures with the textured substrate (Figure 7). This finding is in accordance with the structure of observed solidification that is enough on the surface. Although produce certain variations in heat flow with different proportions (Cr / Ni) eq, a trend is not appreciated defined. However, as time increases heat flows will approximate regardless of the ratio (Cr / Ni) eq. This apparent lack of dependence on heat transmission with with respect to the ratio (Cr / Ni) eq with a substrate smooth agrees with the texture observations superficial of the band that were not influenced by that proportion.

The experimental program showed that the range normal operating for the proportions (Cr / Ni) eq of 1.7-1.9 is not optimal in relation to the surface texture of the band. The use of a proportion (Cr / Ni) eq lower than 1.60 produces better quality superficial.

Claims (11)

1. Procedure for continuous casting of a metal band of the type in which a casting bath is formed in contact with a moving casting surface of such so that the metal solidifies from the bath on the surface of casting in motion, in which the metal is a stainless steel austenitic containing chromium and nickel in a proportion (Cr / Ni) eq less than 1.60 and the casting surface has an arithmetic mean roughness value (R_ {a}) between 2.5 and 15 microns and heat is transmitted from said austenitic stainless steel on the casting surface with an initial peak of the heat transmission rate of more than 15 MW / m2 within the first 20 ms of contact, for enable solidification of said steel on the surface of laundry without harmful segregations or cracking superficial. 2. Method according to claim 1, in the that the casting surface has a grooved texture and regular highlights. 3. Method according to claim 2, in the that the depth of the texture is within the range of 10 microns to 60 microns and the passage of the grooves is inside from the range of 100 microns to 200 microns. 4. Method according to claim 1, in the that the casting surface has a texture of projections Discrete regularly distanced with a distance step within the range of 100 to 200 microns and a depth of texture within the range of 10 to 60 microns. 5. Procedure according to any of the previous claims, wherein the proportion of chromium with respect to nickel it is not higher than 1.55. 6. Procedure according to any of the previous claims, wherein the carbon contents, chrome and nickel steel are within the ranges following: Carbon - 0.04 - 0.06% by weight Chrome - 17.5 - 19.5% by weight Nickel - 8.0 - 10.0% in weight. 7. Procedure for continuous casting of a metal band of the type in which molten metal is introduced within the passage between a pair of casting rollers through a metal feed nozzle arranged above the passage between rollers to create a supported molten metal casting bath on the roller casting surfaces immediately above the passage between rollers, in which the metal is a steel austenitic stainless containing a proportion of (Cr / Ni) less than 1.60, the roller casting surfaces have an arithmetic mean roughness value (R_ {a}) between 2.5 and 15 microns and heat is transmitted from said austenitic stainless steel on the casting surface with an initial peak of the heat transmission rate of more than 15 MW / m2 within the first 20 ms of contact, for enable solidification of said steel on the surface of laundry without harmful segregations or cracking superficial. 8. Method according to claim 7, in the that the roller casting surfaces have a texture of regular circumferential grooves with a depth of texture within the range of 10 microns to 60 microns and a step between the slots in the range of 100 microns to 200 microns. 9. Method according to claim 7, in the that the rollers have a texture of discrete projections regularly distanced with a distancing step within the range of 100 to 200 microns and a depth in the range of 10 to 60 microns 10. Procedure according to any of the claims 7 to 9, wherein the ratio of chromium to nickel is not greater than 1.55. 11. Procedure according to any of the claims 7 to 10, wherein the contents of carbon, chromium and nickel of steel are within the ranges following: Carbon - 0.04 - 0.06% by weight Chrome - 17.5 - 19.5% by weight Nickel - 8.0 - 10.0% in weight.