FI101944B - Casting of steel strips - 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

101944

Steel strip casting - Gjutning av stälband 5

The invention relates to the casting of steel strip. It refers specifically, but not exclusively, to applications where stainless steel strip is cast by a double casting machine.

10

The casting of a metal strip is known as a method in which the casting takes place with a double-roll device. The molten metal is fed between a pair of casting rollers with counter-rotating rollers and the rollers are cooled so that the metal-15 shells solidify on the surface of the moving rolls and coalesce at the roll pressing point to form a solid strip product to be moved downward between the rolls. The term "nip" is used herein to refer to the general area in which the rollers are closest to the second-20. The molten metal can be poured from the ladle into a smaller vessel from which it flows through a metal distribution nozzle above the nip so as to be directed to the nip between the rolls, forming a molding pool containing the molten metal supported by the casting surfaces immediately above the nip. This basin can be defined between side plates or dams that are kept in sliding contact with the ends of the rollers.

Double casting has been used with considerable success in hand-30 filling of non-ferrous metals that solidify rapidly, such as aluminum. Our Australian Patent No. 631728 discloses a method and apparatus for continuously casting an iron-containing strip in the range of 0.5 mm to 5 mm, and this type of apparatus is welded to a level at which it is possible to consistently produce a good quality cast steel strip. However, there have been particular problems in casting austenitic stainless steel strip, as such steel 2 101944 has a considerable tendency to suffer from cracking and repeated surface depressions, which generally appear as surface defects known as "crocodile brick". We have embarked on extensive experimental work in which we have determined the 5 factors that allow the casting of a stainless austenitic steel strip to achieve good surface quality without significant fracture defects.

In the following description, it is necessary to refer to the quantitative measure of the smoothness of the casting surfaces. The specific measure used in our experimental work, which has also been helpful in determining the range covered by this invention, is a standard measure known as the arithmetic mean roughness value. It is usually denoted by the symbol Re. This value is defined as the arithmetic mean of the roughness profile-15 Iin from the center line of the profile of all absolute distances within the measurement length lm. The center line of the profile is the line against which the roughness is measured and is a line parallel to the general direction of the profile, within the contours of the width of the roughness, so that the sums between it and the parts of the profile on either side are the same. The arithmetic mean roughness value can be determined as follows: x = 1_ n 25

Ra = --L- I | y | dx

K

x = 0 30

According to the invention, there is provided a method of casting a continuous metal strip in which a casting pool containing molten metal is brought into contact with a movable casting surface so that the metal solidifies from a casting to a movable casting surface, the metal being austenitic stainless steel containing chromium and nickel in ratio (Cr / Ni) eq less than 1.60 and the arithmetic mean roughness (R i) of the casting surface is greater than 2.5 microns. More specifically, the arithmetic mean roughness value Ra of the casting surface may be between 2.5 and 15 microns. The patterned surface of the roll 40 can be produced by machining protrusions on the surface at regular intervals.

3 101944

The chromium to nickel ratio should preferably not exceed 1.55.

More specifically, the invention provides a method of continuously casting a metal strip in which molten metal is fed to a compression point through a metal dispensing nozzle located above a pair of casting rolls to provide a molten metal-containing casting base supported on the casting surfaces of the rolls immediately and the casting rolls are rotated to feed the solidifying metal strip downwards from the nip, the metal being of austenitic stainless steel containing chromium and nickel in a ratio (Cr / Ni) of less than 1.60 and the arithmetic mean roughness value (R.) of the casting surfaces of the rolls being higher than 2.5 microns.

The patterning of the casting surfaces of the rolls may comprise regular circumferential grooves with a depth between 10 microns and 60 microns and a groove spacing between 100 microns and 200 microns.

20

In an alternative embodiment, the patterning of the roll may comprise regularly spaced protrusions, which may be pyramidal or conical in shape, with intervals in the range of 100 to 200 microns and a depth of 10 to 60 microns.

It is best that the content of carbon, chromium and nickel in the steel is within the following limits: 30 carbon - 0.04 - 0.06% by weight chromium - 17.5-19.5% by weight nickel - 8.0-10.0% by weight -%.

For a detailed explanation of the invention, we will now explain its application to the production of stainless steel strip in a two roll continuous casting apparatus, with reference to the accompanying drawings, in which: Figure 1 is a plan view of a two roll continuous strip casting apparatus which may be used in accordance with the present invention; Fig. 2 is a side plan view of the strip casting apparatus of Fig. 1; Fig. 3 is a vertical cross-sectional view taken along line 3-3 of Fig. 1; Fig. 4 is a vertical cross-sectional view taken along line 4-4 of Fig. 1; Fig. 5 is a vertical cross-sectional view taken along line 5-5 of Fig. 1; Fig. 6 shows the surface structure of the casting surface used in the test casting series; and Figures 7-9 show the results of test castings using steels of different compositions.

The casting device shown in the figure comprises the main body 11 of the machine, which rises 20 from the floor 12 of the factory. The body 11 supports a casting roller carriage 13 which is movable in a horizontal direction between the assembly station 14 and the casting station 15. The carriage 13 conveys a pair of parallel casting rollers 16 to which molten metal is fed during casting operation from the ladle 17 through the intermediate nozzle 19 of the intermediate vessel 18 and 25. The casting rolls 16 are water-cooled so that the shells solidify on the surfaces of the moving rolls and come into contact at the point of compression between them, producing a solidified strip product 20 at the exit of the rolls. This product is fed to a standard winder 21 30 and can then be transferred to another winder 22. A vessel 23 is mounted on the machine body, next to the casting station, to which molten metal can be guided from the intermediate vessel overflow spout 24 or by pulling the emergency plug 25 on the other side of the vessel. serious shape error or 35 if any serious disturbance occurs in the casting operation.

The roller carriage 13 comprises a carriage body 31 mounted on wheels 32 on rails 33, extending to a part of the main body 5 of the machine 101944 11, whereby the roller carriage 13 is mounted as a whole for movement on the rails 33. The carriage body 31 carries a pair of roll housings 34 in which the rollers 16 are mounted for rotation. The roll housings 34 are mounted on the carriage body 31 by attaching complementary sliders 35, 36 to allow the housings to move into the carriage by the hydraulic cylindrical units 37, 38 to adjust the compression point between the casting rollers 16. The carriage travels along rails 33 using a double-acting hydraulic piston and cylinder cylinder unit 39 connected between a trolley 40 on a roll carriage and the main body of the machine so that it can be used to move a roll carriage between assembly station 14 and casting station 15 and vice versa.

The casting rollers 16 rotate in opposite directions with the drive shafts 41 coming from the electric motor and gearbox mounted on the carriage body 31. The rollers 16 have copper circumferential walls made to include longitudinal and circumferentially spaced cooling passages 20; these must have water supply channels inside the roll drive shafts 41, which are connected through rotating sealing flanges 43 to the water supply hoses 42. The rolls can typically be 500 mm in diameter and up to 1300 mm in length to produce a 1300 mm wide strip product.

The ladle 17 has a completely conventional construction and is supported by a crane 45 above it, by means of which it can be moved to its position from the hot metal receiving station. Mounted on the ladle is a stop rod 46 which can be operated by a servo cylinder to allow molten metal to flow from the ladle through the outlet nozzle 47 and the refractory cover 48 to the intermediate vessel 18. 1 The intermediate vessel 18 is also of conventional construction. It is made of a refractory material like magnesium oxide (MgO) into a wide plate. One side of the intermediate vessel receives molten metal from the ladle and has the aforementioned overflow spout 24 and emergency stopper 25. The other side of the intermediate vessel is provided with a plurality of longitudinally spaced metal outlets 52. At the bottom of the intermediate vessel are mounting brackets 53 for mounting the intermediate vessel there are openings for receiving the alignment protrusions 54 in the carriage body to accurately position the intermediate container.

The dispensing nozzle 19 is made of an alumina-like refractory material as an elongate part. Its lower part is tapered so that it tapers inwards and downwards so that it can protrude at the pressing point between the casting rollers 16. It includes a mounting bracket 60 by means of which it rests on the frame of the roller carriage and its upper part 15 is provided with outwardly projecting side flanges 55 located in the mounting bracket.

The nozzle 19 may have a series of horizontally spaced, generally vertically extending flow channels to provide a suitably low metal discharge rate across the entire width of the rolls and to distribute the molten metal at the nip between the rolls without directly touching the surface of the rolls. Alternatively, the nozzles may have a single continuous slot outlet to distribute the Mata-25 slurry molten metal curtain directly to the nip between the rolls and / or may be embedded in the molten metal substrate.

At the end of the rollers, the pool is bounded by a pair of side baffles 56, 30 which are held against the stepped ends 57 of the rollers when the roll carriage is in the casting position. The side baffles 56 are made of strong; of a refractory material, for example boron nitride, and their side edges 81 are knurled to fit the arcs of the stepped ends 57 of the rollers. The side plates can be mounted 35 on plate holders 82 movable in a casting station using a pair of hydraulic cylinder units 83 to connect the side plates to the stepped ends of the casting rolls to close the end of the molten metal pool formed on the casting rolls during the casting operation.

During the casting operation, the ladle stop rod 46 is used to allow molten metal to flow from the ladle to the intermediate vessel 5 through a metal distribution nozzle from which it flows to the ladles. The clean front end of the strip product 20 is guided to the jaws of the winding device 21 using a chute table 96. The chute table 96 depends on the pivotable mounting parts 97 of the main body and can be pivoted towards the winding device by using the hydraulic cylinder unit 10 98 after the clean front end has been formed. The table 96 may act against the upper belt guide flap 99 operated by the piston and cylinder unit 101, and the belt product 20 may be defined by a vertical side between the pair of rollers 102. After the front end is guided to the jaws of the winder, the ke-15 winder is rotated to wind the strip product 20 and the chute table is allowed to return to its waiting position, where it simply depends on the machine body, away from the product fed directly to the winder 21. The resulting strip product 20 can then be transferred. 20 to a winding device 22 to produce a final coil for removal from the casting device.

In connection with the use of the device described above, it has been found that it is possible to produce a good austenitic stainless steel strip by carefully adjusting the chemistry of the steel when using rolls whose surface is shaped to minimize separation due to initial rapid cooling. 1 2 3 4 5 6

In the casting of austenitic stainless steel strip, the method of solidification 2 may play an important part in the surface 3 of the strip; in terms of quality. The primary austenitic 4 solidification mode, which occurs when the Cr / Ni ratio is less than about 1.60, is not usually recommended because it improves 6 separation, leading to an increased tendency to break. In the past, it has been considered necessary to ensure that the Cr / Ni ratio remains in the range of 1.7-1.9 in order to minimize fractures due to a decrease in separation severity and to make winding pathways that make fracture propagation difficult. However, experimental work has shown that casting a continuous strip on steel having this composition is quite prone to producing strips with "crocodile leather depressions" and the severity of the depressions may be so great as to cause fracture. Steel with a C / Ni ratio of less than 1.55 is the most prone to separation and can thus increase fracture. If solidification occurs with a soft substrate, the initial heat transfer rates are low and the solidification structure is rough, resulting in separation and fracture. However, we have determined that this tendency to separate and break can be overcome by ensuring a high initial heat transfer rate and this can be most conveniently obtained using a patterned substrate, for example by machining ridges on the substrate surface.

The initial test operation was performed on a metal solidification test bench where a 40 mm x 40 mm cooled body was immersed in a molten steel bath at a rate that closely simulated conditions on the casting surfaces of a twin casting machine. The steel solidifies in the cooled body as it passes through the molten bath and produces a layer of solidified steel on the surface of the body. The thickness 25 of this layer can be measured from points throughout its area to map variations in the rate of solidification and to determine the effective rate of heat transfer resulting therefrom at various locations. Thus, it is possible to arrive at an overall solidification constant, which is generally indicated by the symbol K, 30, as well as a mapping of individual values over the entire area of the solidified strip. It is also possible to study as well. the microstructure of the strip surface to correlate the changes in solidification in the microstructures with the changes observed in the heat transfer values.

35

The following explains the nature of the pilot activity and the results obtained from it.

9 101944

The experiments were performed on three copper substrates, all with different surface properties; smooth and textured copper surface and chrome-plated (thickness 100 pm) bottom surface. The pattern was distributed on the surface of the copper body by machining 5 longitudinal grooves and ridges according to the geometry shown schematically in Fig. 6. Each piece was treated with thermocouples to characterize the heat transfer rates prevailing during solidification. To maintain consistently similar casting ratios throughout the experiment, variables such as melting superheat and body temperature were kept constant within reasonable limits. The melting temperature was designed to be 1525 ° C, which corresponded to a superheat of 75 ° C. The argon gas fed to the melting furnace was effective enough to prevent chemical mixing of the melt with the surrounding atmosphere. The melting chemistry was adjusted to achieve the desired (Cr / Ni) eq ratios, mainly using Cr, Ni, c and N2 as additions. The following expressions were used to determine Creq and Nieq: 20 Creq = Cr + 1.37 Mo + 1.50 Si + 2.0 Nb + 3.0 Ti (1)

Nieq = Ni + 0.31 Mn + 22.0 C + 14.2 N + Cu (2)

A summary of the experimental conditions is shown in Table 1. The entire experimental program consisted of approximately 45 experiments (Cr / Ni) with eq ratios ranging from 1.55 to 1.74. The features that emerged from the various experiments are summarized in Table 2.

10 101944

Table 1: Experimental conditions

Substrate surface Smooth copper 5 Cr-coated (base) copper

Textured copper (150 pm division, 20 pm depth

Chassis cleaning method Brushing and blowing 10

Melting point 1525 ° C

Unit temperature 125 ° C

15

Table 2: Details of different experiments

CONDITIONS (Cr / Ni) MELT N2 GAS ATM TOTAL LOSS

20 1 1.56-1.71 0.047 Ar 9 2 1.58-1.71 0.037 Ar 9 3 1.57-1.61 <0.062 N2 7 4 1.59 0.062 Ar 7 25 5 1.74 0.054-0.059 With 15

Ar + He

SCORE

Effect of 30 (Cr / Ni) eq ratio on strip surface quality.

Superficial inspection of the samples revealed that the (Cr / Ni) eq ratio had a direct effect on the surface quality of the strips obtained on the patterned substrate, that no observable effect was seen in those made on smooth substrates.

Samples cast with a variable (Cr / Ni) eq ratio showed a transition from a severe crocodile skin-type pattern to a smooth texture with respect to a decrease in the (Cr / Ni) eq ratio. The effect of the (Cr / Ni) eq ratio on the severity of the crocodile skin shown in Figure 9 tends to suggest that significant improvements in the surface quality of the strip can be obtained by keeping the (Cr / Ni) eq ratio below 1.60.

Effect of (Cr / Ni) eq ratio on heat transfer during solidification. 45 i) Patterned substrate The heat transfer rates from the surface of the strip to the substrate were determined from the measured substrate temperatures. Figure 7 shows the effect of the melting (Cr / Ni) eq ratio on the heat fluxes for the patterned substrate. We can see that the profiles are characterized by an early peak of heat flow, followed by a rapid decrease of 5 of this peak, and as time increases, the heat flow approaches a steady value. The higher heat transfer rates (approximately 30 MW / m2) found in the early stages of solidification can be derived from close contact.

The experimental program determined that the (Cr / Ni) eq ratio found to provide the best surface texture (on the textured substrate) was less than 1.60.

ii) Smooth substrate Figure 8 reveals the effect of the (Cr / Ni) eq ratio on heat transfer on a smooth substrate. We can see that the heat fluxes are relatively uniform throughout the solidification period and, most importantly, the size of the peak fluxes is much smaller than that measured with the patterned surface (Fig. 7). This finding is consistent with the observed solidification structure that is rough in surface. Although some variation in heat flow is observed for the different (Cr / Ni) eq ratios, there is no definite trend here. However, as time increases, the heat fluxes approach similar values, regardless of the (Cr / Ni) eq ratio. This apparent lack of dependence on the heat transfer (Cr / Ni) eq ratio in the case of a smooth surface is consistent with the observations on the surface pattern of the strip that were not affected by the (Cr / Ni) eq ratio.

30

The experimental program showed that the normal (Cr / Ni) eq ratio of 1.7-1.9 is not the best for the texture of the strip. A (Cr / Ni) eq ratio of less than 1.60 results in better surface quality than usual.

35

Claims (13)

101944 A method for continuous casting of a metal strip, wherein the molten metal casting pool is brought into contact with a movable casting surface so that the metal solidifies upon entering the movable casting surface from 5 basins, characterized in that the metal is austenitic stainless steel containing chromium and nickel in a ratio of (Cr / Ni) eq , which is less than 1.60, and that the arithmetic mean roughness value (Ra) of the casting surface is greater than 2.5 microns. 10 Method according to Claim 1, characterized in that the arithmetic mean roughness value (Ra) of the casting surface is between 2.5 and 15 microns. Method according to Claim 2, characterized in that the casting surface has a regular pattern formed by grooves and ridges. A method according to claim 3, characterized in that the depth of the pattern is between 10 and 60 microns and the distance between the grooves is between 100 and 200 microns. A method according to claim 3, characterized in that the patterning of the casting surface comprises fine protrusions at regular intervals, the intervals of which are between 100 and 200 microns and the depth of the patterning is between 10 and 60 microns. According to any one of the preceding claims. 30 method, characterized in that the ratio of chromium to nickel does not exceed 1.55. Process according to any one of the preceding claims, characterized in that the content of carbon, chromium and nickel in the steel is within the following limits: 101944 carbon - 0.04 to 0.06% by weight chromium - 17.5 to 19.5% by weight -% nickel - 8.0-10.0% by weight. A method of continuously casting a metal strip, wherein molten metal is fed to a compression point between a pair of casting rollers through a metal distribution nozzle located above the compression point to form a molten metal casting base resting on the roll casting surfaces immediately above the extrusion point , characterized in that the metal is an austenitic stainless steel containing chromium and nickel in a ratio (Cr / Ni) of less than 1.60 and that the arithmetic mean roughness value (Ra) of the casting surfaces of the rolls is greater than 2.5 microns. Method according to Claim 8, characterized in that the arithmetic mean roughness value (Ra) of the casting surfaces of the rolls is between 2.5 and 15 microns. A method according to claim 9, characterized in that the patterning of the casting surfaces of the rollers comprises regular circumferential grooves with a pattern depth of 25 It has 10 and 60 microns and the groove spacing is between 100 and 200 microns. A method according to claim 9, characterized in that the patterning of the rolls comprises fine protrusions at regular intervals 30 between 100 and 200 microns and between 10 and 60 microns deep. Process according to any one of Claims 8 to 11, characterized in that the ratio of chromium to nickel is not greater than 1.55. Process according to any one of Claims 8 to 12, characterized in that the content of carbon, 101944 chromium and nickel in the steel is within the following limits: carbon - 0.04 to 0.06% by weight 5 chromium - 17.5 to 19, 5% by weight of nickel - 8.0-10.0% by weight.