WO1995013156A1

WO1995013156A1 - Continuously cast steel strip

Info

Publication number: WO1995013156A1
Application number: PCT/AU1994/000689
Authority: WO
Inventors: Leslie George Gore; Massoud Assefpour-Dezfully
Original assignee: Ishikawajima-Harima Heavy Industries Company Limited; Bhp Steel (Jla) Pty. Ltd.
Priority date: 1993-11-11
Filing date: 1994-11-10
Publication date: 1995-05-18
Also published as: KR960705646A; EP0728052A1; JPH09504830A; EP0728052A4

Abstract

Steel strip (120) produced by a continuous strip caster (40) is treated as it comes from the caster and while it is in line with the caster by applying to the hot strip exiting the caster by means of a coater (201) an oxidation protective coating of a material which is fluid at the temperature of application and susequently removing the coating by a coating removal treatment which includes cooling of the coating by a cooler (204) to cause it to solidify and fracture. The coated strip may be hot rolled by rolls (203) before removal of the coating.

Description

CONTINUOUSLYCASTSTEELSTRIP TECHNICAL FIELD

This invention relates to the production of steel strip by continuous casting. It has particular application to the production of steel strip by twin roll casting but may also be applied to production by other continuous strip casting techniques such as by a single roll caster.

In a twin roll caster molten metal is introduced between a pair of contra-rotated chilled casting rolls so as to form a casting pool of molten metal above the nip between the rolls. Metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product at the outlet from the roll nip. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be introduced into the nip between the rolls via a tundish and a metal delivery nozzle located beneath the tundish so as to receive a flow of metal from the tundish and to direct it into the nip between the rolls.

Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals. Some of these problems have been addressed by the inventions disclosed in our previous Australian Patent Specifications 631728, 634896, 634429 and 645296. These developments have permitted steel strip to be cast continuously without breakages and without major structural defects. However, twin roll casters produce strip at very high temperatures, typically in excess of 1300°C, and it is found that oxidation of the strip to form surface scale is a significant problem. About 2% of the cast metal can be lost to scale. Moreover, the strip frequently exhibits a tenacious oxide layer which is difficult and expensive to remove prior to further processing of the strip. By the present invention it is possible to minimise the formation of scale. The invention also enables very effective descaling of the strip when it has cooled to temperatures at which further scaling is not a significant problem. DISCLOSURE OF THE INVENTION According to the invention there is provided method of producing steel strip, comprising continuously casting a solidified strip product from a casting pool of molten steel in a continuous strip caster and treating the solidified strip as it comes from the caster and while it is in line with the caster by

(a) applying to the hot metal strip an oxidation protective coating of a material which is fluid at the temperature of the strip to which it is applied; and

(b) subsequently removing the coating by a coating removal treatment including cooling of the coating to cause it to solidify and fracture.

Preferably, the coating is applied to the strip in the region where it exits the caster.

The coating material may be applied in the form of a powder which melts/flows on application of heat from the strip.

The coating material may be an oxygen containing glassy material, for example a vitreous enamel.

Preferably the coating is applied to the strip when its temperature is in excess of 1100°C, and more preferably when it is above 1300°C.

Preferably the coating material has a melting/softening point in the range 600°C to 1400°C.

Preferably, further the coating material has a viscosity of less than 1000 Pa.s at the temperature of the strip to which it is applied. More particularly, it is preferred that the viscosity of the coating material be less than 100 Pa.s under those conditions.

The hot coated strip may be subjected to working so as to alter its size and/or shape prior to removal of the coating.

The invention further provides apparatus for producing steel strip, comprising a continuous caster for continuously casting a solidified strip product from a casting pool of molten steel; a coating applicator operable to apply to the hot steel strip as it is delivered from the continuous caster an oxidation protective coating; and coating removal means to remove the coating from the strip while it is in line with the continuous caster and comprising cooling means to cool the coating whereby to cause it to solidify and fracture.

The continuous caster may be a twin roll caster and the hot apparatus may further include rolling means to roll the coated hot strip prior to removal of the coating. More specifically the invention provides apparatus for producing steel strip comprising a pair of casting rolls forming a nip between them; a metal delivery nozzle disposed above the nip to deliver molten steel into the nip to create a casting pool of molten steel supported on the casting surfaces of the rolls immediately above the nip; roll drive means to contra-rotate the rolls so as to deliver a solidified hot metal strip downwardly from the nip; a coating applicator operable to apply to the hot metal strip as it is delivered from the nip between the casting rolls an oxidation protective coating of a glassy material which is fluid at the application temperature; strip rolling means operable to roll the coated hot strip so as to reduce its thickness; and coating removal means to remove the coating from the reduced strip which removal means includes cooling means to cool the strip to cause it to solidify and fracture. The coating removal means may also comprise mechanical means to assist in the fracture and removal of the coating. Tests carried out on coating rigs which simulate the conditions in the twin roll casting of steel strip have shown that coherent protective coatings of borosilicate glassy material can be applied to steel strip at temperatures of the order of 1300°C to 1350°C either by immersing the hot strip in a fluidised bed of the coating powder or by directly spraying the powder onto the strip. Moreover, hot rolling tests on the coated strips have shown that the coating remains effective throughout hot rolling and provides the additional benefit of reducing rolling mill loads due to the high degree of lubrication provided by the glass coating.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more fully explained results of experimental work carried out to date and apparatus constructed in accordance with the invention will be described with reference to the accompanying drawings in which:

Figures 1 and 2 illustrate experimental apparatus for applying coatings to metal samples under conditions simulating those in a twin roll caster for casting steel strip;

Figure 3 illustrates diagrammatically a twin roll caster and hot rolling mill constructed in accordance with the invention;

Figure 4 is a side elevation of the strip caster;

Figure 5 is a vertical cross-section on the line 5-5 in Figure 4;

Figure 6 is a vertical cross-section on the line 6-6 in Figure 4;

Figure 7 is a vertical cross-section on the line 7-7 in Figure 4;

Figure 8 is a cross-section on the line 8-8 in Figure 4; and Figures 9 to 13 show the cross-section and a series of elemental x-ray maps of a cross-section of a piece of glass coating detached from a steel surface. Figures 1 and 2 illustrate coating rigs in which steel strip specimens can be coated with glass enamel coatings under conditions simulating those in a twin roll strip caster. The rig shown in Figure 1 comprises an infra-red imaging furnace 11 having a gas inlet 12 and a gas outlet 13 whereby the furnace can be charged with an inert gas such as nitrogen. A steel strip specimen 14 is mounted on the end a pneumatic cylinder unit 15 whereby it can be held within the furnace so as to be heated up to temperature within the inert atmosphere within the furnace and then moved out of the furnace to positions above or below the furnace. The temperature of the steel strip is monitored by a thermocouple 16. A chamber 17 is located directly beneath the furnace and can be opened to the interior of the furnace by operation of a sliding gate valve 18. Chamber 17 is provided with a gas inlet 19 and a gas outlet 20 whereby it can be charged with an inert gas such as nitrogen. In the apparatus illustrated in Figure 1 there is installed within chamber 17 a vessel 22 to receive the enamel coating powder under test. A gas permeable diaphragm forms an effective floor for vessel 22 and enables the powder in the vessel to be fluidised by supply of an inert gas such as nitrogen through an inlet 24 into a chamber 25 in the bottom of vessel 22 whence the gas passes up through the permeable diaphragm 23 into the powder.

When the test strip 14 has been heated to test temperature, sliding gate valve 18 is operated and the strip is lowered by operation of pneumatic cylinder unit 15 so that it enters into the fluidised bed of coating powder in vessel 22 at a controlled speed chosen to simulate the conditions of a strip steel caster. The strip is then withdrawn outside the assembly and allowed to cool to room temperature.

The apparatus illustrated in Figure 2 is the same as that illustrated in Figure 1 except that the vessel for holding a fluidised bed of powder has been removed and chamber 17 is instead fitted with a dry powder spray gun 31 (conventional or electrostatic) which is operable to spray a stream of powder 32 directly onto the test strip as it is advanced into the chamber.

TESTS USING OXIDATION/COATING RIG

The rig shown schematically in Figure 1 and Figure 2 was used for these tests. The furnace chamber was purged with high purity N₂ for all the tests to avoid oxidation of the specimen during heating. The specimen was heated to 1300°C to 1350°C for all the tests. The chemical composition of glass powders used in the tests are given in Table 1. Powders EPP283 and PC53 are commercial glasses supplied by Ferro Corporation, with the former having high adhesion properties and the latter being a low adherence powder. Powders EXP.l to EXP.4 are low adherence experimental grades tailored to give varying viscosities at the application temperature. Silica was also used for its very high viscosity at 1350°C as an upper limit case. Powders EPP283 and PC53 were tested in the fluidised bed arrangement (Figure 1), whereas EXP.l to EXP.4, silica and PC53 were tested using the powder spray arrangement shown in Figure 2.

Table 1 Chemical composition of glass powders (wt %)

TYPE/ Si0₂ Na₂0 CaO B₂0₃ A1₂0₃ K₂0 MgO F Others CODE

EPP283 42.3 13.8 3.98 20.5 2.82 1.85 1.56 0.74 Ti0₂, 2.8 Zr, 1.4 Ni, 1.4 Co, 1.2 Li,

PC53* 45.6 7.9 0.04 15.9 0.54 7.23 0.19 1.38 20.3 Ti0₂, 0.28 MgO

EXP.l 57.5 6.5 13.6 13.6 6.6 1.32 0.28 0.04 - I

EXP.2 72.2 12.4 13.9 0.7 0.12 - - - -

EXP.3 66.8 11.0 3.7 17.0 1.13 - - - -

EXP.4 64.0 11.4 2.81 9.3 12.5 - - - -

SILICA 100.0 - - - - - - - -

* Powder particle size (particle diameter) = 2 to 150 μm (mean about 40 μm)

The powders were applied to the hot specimen by either dipping into a fluidised bed as shown in Figure 1 or by electrostatic spraying as shown in Figure 2 at speeds chosen to simulate the conditions of a steel strip caster. The atmosphere of the lower chamber and the fluidised bed or the powder spray system was high purity N₂ for some of the tests and air for others.

Tests using fluidised bed arrangement (Figure 1) :

Test 1 The specimen was heated in N₂ to 1300°C to 1350°C and then driven into a fluidised bed of glass powder in N₂ for about

0.3 s. It was then retracted into air (above the furnace) and allowed to cool to room temperature

Outcome: • For the high adherence powder (EPP283) a very adherent coating about 100 to 200 microns thick was formed on the sample which protected it from oxidation.

• For the low adherence powder (PC53), the coating remained on the specimen during cooling but began to flake off at a temperature of about 120°C to 150°C, revealing a scale free shiny surface underneath. The coating thickness was 70 to

120 microns.

Test 2

As in test 1, but the atmosphere of the lower chamber and the fluidising bed was air rather than N₂

Outcome:

• Only the low adherence powder (PC53) was tested, and again the coating remained on the specimen during cooling but began to flake off at a temperature of about 120°C to 150°C, revealing a scale free shiny surface underneath.

Test 3

The specimen heated in N₂ to 1300°C to 1350°C and retracted into air (above furnace) for 0.3 s to allow some oxidation.

It was then driven into a fluidised bed of glass powder in N₂ for about 0.3 s, following which it was retracted into air (above the furnace) and allowed to cool to room temperature Outcome :

Test 4

As in test 3, but the specimen was retracted into air for

1 s for pre-oxidation before coating in fluidised bed

Outcome: • Only the low adherence powder (PC53) was tested, and again the coating remained on the specimen during cooling but began to flake off at a temperature of about 120°C to

150°C, revealing a scale free shiny surface underneath.

Test 5 As in test 3, but the specimen retracted into air for 10 s for pre-oxidation before coating in the fluidised bed

Outcome:

• Only the low adherence powder (PC53) was tested, and again the coating remained on the specimen during cooling but began to flake off in isolated patches at a temperature of about 120°C to 150°C, revealing a scale free shiny surface underneath these. The coating continued to peel off slowly over a number of hours at room temperature. The steel surface beneath the coating was scale free and shiny, however a dark layer of scale was visible on the back of the glass flakes, suggesting that the glass had reacted with the iron oxides and had pulled it away from steel (ie de-scaled the steel) .

Tests 3, 4 and 5 confirm that the coating adheres to a hot pre-oxidised surface and retards further oxidation during cooling to room temperature. Furthermore, on peeling from the steel, it pulls the scale from the steel surface and essentially de-scales the steel. This is confirmed by referring to Figures 9 to 13 which comprise elemental maps carried out using the x-ray analysis facilities of a scanning electron microscope on a piece of glass which flaked off from the steel surface upon cooling to room temperature. The image in Figure 9 shows the cross-section of the detached piece and the elemental maps of Figures 10 to 13 indicat glass containing Si, Na and Al on the outermost (c) with the inner most part of the section (a) being mainly iron (ie the iron oxide which the glass pulled off) and a reaction zone in between where the glass has reacted with the scale (b) . Tests using spray coating arrangement (Figure 2)

Powders PC53, silica and EXP.l to EXP.4 were tested using the experimental set up of Figure 2, with the coating chamber purged with N₂. A summary of the test results is given in Table 2. The results indicate that for all the powders tested, those with a viscosity at the application temperature (1350°C) in the range 0.4 - 250 Pa.s, protected the steel if they were applied in sufficient quantity (coating thickness of about 100 μm or more) . EXP.l applied at 900°C (viscosity = 625 Pa.s) and silica applied at 1350°C (viscosity = 10⁹ Pa.s) did not melt/flow to cover the specimen and therefore did not offer any protection against oxidisation. The samples from these tests were therefore heavily oxidised. Powder EXP.3 with a viscosity of 20 Pa.s at 1350°C gave the best performance in that it protected the steel when applied at a thickness as low as 30 to 70 μm.

Table 2 Summary of oxidation tests using spray coating rig (Figure 2)

Coating Type Application Viscosity at Range of Coating Oxidation Temperature (°C) Application Thickness (μm) Protection

Temperature (Pa.s) Effectiveness

PC53 1350 0.4 35-70 F

PC53 1350 0.4 80-110 P

EXP.l 1350 6 20-30 F

EXP.l 1350 6 80-100 P

EXP.l 1350 6 300-400 P

EXP.l 900 625 would not coat F

EXP.2 1350 10 40-60 P

EXP.2 1350 10 90-150 P

EXP.3 1350 20 30-70 P

EXP.3 1350 20 40-90 P

EXP.4 1350 250 20-75 F

EXP.4 1350 250 60-105 P

SILICA 1350 10⁹ would not coat F

P = Pass (steel surface completely protected against oxidation or partially protected with patches of oxide being descaled by the glass coating) .

F = Fail (steel totally or significantly oxidised) .

HOT ROLLING TESTS

In all of the hot rolling tests, the glass powder was applied to a ground or sand blasted strip surface by electrostatic spraying at room temperature. The coated strip was then placed inside a furnace operating at 1250°C for -about five minutes after which it was removed and hot rolled in 1 pass at 900°C to 1100°C, followed by either air cooling or water quenching to room temperature. The mill settings were kept constant for all the tests with a roll separation of 1 mm for tests 1 to 15 and zero for tests 16 to 18 (Table 3). Optical pyrometers at mill entry and exit monitored the strip temperature. In addition, for some of the tests the strip temperature was recorded from thermocouples embedded in the specimen. Three different strip sizes and two different steel grades F 08 and A06 were used. The strip dimensions were: Series 1 tests: Thickness = 2.5 mm, Width = 30 mm,

Length = 120 mm Series 2 tests: Thickness = 2.5 mm, Width = 127 mm, Length = 127 mm

Series 3 tests: Thickness = 2.03 mm, Width = 100 mm,

Length = 400 mm The results of these tests are summarised in Table 3.

Table 3 Summary of hot rolling results

Test Strip Coating Rolling Roll Final % Cooling After Surface No Dimensions Type Temp Force Thickness Reduction Rolling Finish of End Product

(mm) (°C) (kN) (mm)

1 2.5x30.120 PC53 1065 52 1.2 52 air cooled large scale free areas

2 2.5x30x120 uncoated 1100 -100 1.4 44 water quenched thick rolled in scale

3* 2.5x30x120 PC53

4 2.5x30x120 PC53 975 45 1.4 44 water quenched large scale free areas

5 2.5x30x120 PC53 990 51 1.45 42 water quenched large scale free areas

6 2.5x30x120 uncoated 1075 80 1.6 36 water quenched heavy scale

7 2.5x127x127 PC53 1025 195 1.6 36 water quenched large scale free areas

8 2.5x127x127 uncoated 1100 275 1.8 28 water quenched heavy scale

9 2.5x127x127 EPP2830 950 220 1.7 32 water quenched scale free + coated part

10 2.03x100x400 PC53 950 # 1.53 25 air cooled thin scale

11 2.03x100x400 PC53 940 105 1.52 25 water quenched large scale free areas

12 2.03x100x400 PC53 975 90 1.58 22 air cooled scale free regions + glass covered areas

13 2.03x100x400 uncoated 1025 135 1.86 8 air cooled thick scale

14 2.03x100x400 PC53 925 115 1.50 26 water quenched large scale free areas

15 2.01x100x400 EXP.l 980 130 1.29 36 air cooled large scale free areas

16 2.05x100x400 uncoated 950 333 1.31 36 air cooled thick rolled in scale

17 2.02x100x400 PC53 890 185 0.98 52 air cooled large scale free areas

18 2.08x100x382 EXP.l 920 200 0.98 53 air cooled large scale free areas

* Coating too thin to provide coverage and the specimen was oxidised. # Mill load not logged Steel for tests 1 to 9 was F 08, and for tests 10 to 18 it was A06. Coating thickness for most tests estimated to be about 100 to 200 μm.

The following key points can be outlined from the hot rolling trials:

• the mill loads were significantly lower for the coated samples (25 to 45% on average) due to the high degree of lubrication provided by the glass coating

• for the same mill settings, the thickness reductions achieved in hot rolling were higher for the coated samples compared to the uncoated ones, again due to the lubricating effect of the coating reducing frictional forces

• there was a tendency for some of the coated samples to slide sideways in the mill, however, none was too slippery not to roll. It should be noted that appropriate mill tension and side guides will control the tendency to slip. Furthermore, a small amount of lateral slip is considered beneficial to the development/modification of the strip profile during hot rolling of strip cast steel.

• layers of solidified glass peeled off the surface on the exit side of the rolls and although small patches did adhere to the rolls occasionally, they were loose and could be brushed off

• there was no evidence of coating adhesion to the rolls of the roller table over which the coated strip travelled before entry to the mill

• the non-adherent glasses (PC53, and EXP.l) flaked off upon quenching in water after hot rolling or on air cooling and slight bending after hot rolling

• the adherent glass (EPP283) peeled off only in patches upon quenching after rolling, and significant coated areas remained intact on the strip

• in general the coating was very effective in protecting the strip from oxidation.

The above tests show that commercially available borosilicate glasses chosen to have a suitable melting/softening point and low viscosity (less than 1000 Pa.s at application temperature) can produce a stable coating on direct application to hot steel strip. The coating will remain in a fluid state through subsequent hot rolling so as not only to provide very good protection against oxidation but also to decrease rolling loads by providing effective lubrication during rolling. Borosilicate glass can be tailored to provide an appropriate combination of particle size, low melting/softening point, sufficient but not excessive lubrication, viscosity and wettability, depending on the particular casting and working conditions. Typically, an appropriate borosilicate glass may have the composition: Na₂0 - 6.6% K₂0 - 1.3% CaO - 14.2% B ^•"2,0^*-^*3, - 14.4% ^•

Al₂0₃ - 6.0%

It is to be understood that the invention may employ a coating material other than a borosilicate glass. Other glassy materials may be employed. The main criteria is that the coating should provide oxidation resistance. It will therefore usually contain oxygen, it should preferably have a melting/softening point above 600°C and it must have appropriate viscosity and wettability in the working temperature range for the particular application.

The viscosity at the application temperature should be less than 1000 Pa.s and preferably less than 100 Pa.s.

Figure 3 illustrates application of the invention to the production of steel strip from a twin roll caster and the hot rolling of the steel strip as it comes from the caster. The caster which is denoted generally as 40 may be constructed and operated in the manner illustrated in Figures 4 to 7. This caster comprises a main machine frame 111 which stands up from the factory floor 112. Frame 111 supports a casting roller carriage 113 which is horizontally movable between an assembly station 114 and a casting station 115. Carriage 113 carries a pair of parallel casting rollers 116 to which molten metal is supplied during a casting operation from a ladle 117 via a tundish 118 and delivery nozzle 119. Casting rollers 116 are water cooled so that shells solidify on the moving roller surfaces and are brought together at the nip between them to produce a solidified strip product 120 at the roller outlet. A receptacle 123 is mounted on the machine frame adjacent the casting station and molten metal can be diverted into this receptacle via an overflow spout 124 on the tundish or by withdrawal of an emergency plug 125 at one side of the tundish if there is a severe malformation of product or other severe malfunction during a casting operation.

Roller carriage 113 comprises a carriage frame 131 mounted by wheels 132 on rails 133 extending along part of the main machine frame 111 whereby roller carriage 113 as a whole is mounted for movement along the rails 133. Carriage frame 131 carries a pair of roller cradles 134 in which the rollers 116 are rotatably mounted. Roller cradles 134 are mounted on the carriage frame 131 by interengaging complementary slide members 135, 136 to allow the cradles to be moved on the carriage under the influence of hydraulic cylinder units 137, 138 to adjust the nip between the casting rollers 116 and to enable the rollers to be rapidly moved apart for a short time interval when it is required to form a transverse line of weakness across the strip as will be explained in more detail below. The carriage is movable as a whole along the rails 133 by actuation of a double acting hydraulic piston and cylinder unit 139, connected between a drive bracket 140 on the roller carriage and the main machine frame so as to be actuable to move the roller carriage between the assembly station 114 and casting station 115 and vice versa.

Casting rollers 116 are contra rotated through drive shafts 141 from an electric motor and transmission mounted on carriage frame 131. Rollers 116 have copper peripheral walls formed with a series of longitudinally extending and circumferentially spaced water cooling passages supplied with cooling water through the roller ends from water supply ducts in the roller drive shafts 141 which are connected to water supply hoses 142 through rotary glands 143. The roller may typically be about 500 mm diameter and up to 1300 mm long in order to produce 1300 mm wide strip product.

Ladle 117 is of entirely conventional construction and is supported via a yoke 145 on an overhead crane whence it can be brought into position from a hot metal receiving station. The ladle is fitted with a stopper rod 146 actuable by a servo cylinder to allow molten metal to flow from the ladle through an outlet nozzle 147 and refractory shroud 148 into tundish 118. Tundish 118 is also of conventional construction.

It is formed as a wide dish made of a refractory material such as magnesium oxide (MgO) . One side of the tundish receives molten metal from the ladle and is provided with the aforesaid overflow 124 and emergency plug 125. The other side of the tundish is provided with a series of longitudinally spaced metal outlet openings 152. The lower part of the tundish carries mounting brackets 153 for mounting the tundish onto the roller carriage frame 131 and provided with apertures to receive indexing pegs 154 on the carriage frame so as to accurately locate the tundish.

Delivery nozzle 119 is formed as an elongate body made of a refractory material such as alumina graphite. Its lower part is tapered so as to converge inwardly and downwardly so that it can project into the nip between casting rollers 116. It is provided with a mounting bracket 160 whereby to support it on the roller carriage frame and its upper part is formed with outwardly projecting side flanges 155 which locate on the mounting bracket. Nozzle 119 may have a series of horizontally spaced generally vertically extending flow passages to produce a suitably low velocity discharge of metal throughout the width of the rollers and to deliver the molten metal into the nip between the rollers without direct impingement on the roller surfaces at which initial solidification occurs. Alternatively, the nozzle may have a single continuous slot outlet to deliver a low velocity curtain of molten metal directly into the nip between the rollers and/or it may be immersed in the molten metal pool.

The pool is confined at the ends of the rollers by a pair of side closure plates 156 which are held against stepped ends 157 of the rollers when the roller carriage is at the casting station. Side closure plates 156 are made of a strong refractory material, for example boron nitride, and have scalloped side edges 181 to match the curvature of the stepped ends 157 of the rollers. The side plates can be mounted in plate holders 182 which are movable at the casting station by actuation of a pair of hydraulic cylinder units 183 to bring the side plates into engagement with the stepped ends of the casting rollers to form end closures for the molten pool of metal formed on the casting rollers during a casting operation.

During a casting operation the ladle stopper rod 146 is actuated to allow molten metal to pour from the ladle to the tundish through the metal delivery nozzle whence it flows to the casting rollers. In accordance with the present invention the hot strip product 120 passes through a coating unit 201 which may comprise a pair of powder spray guns or a fluidised chamber for coating powder onto the hot metal surface. The coating process may be carried out in a protective atmosphere, such as a nitrogen atmosphere. The coating powder melts/flows to form a viscous coating and the coated hot strip is then passed through a pair of pinch rolls 202 and a hot rolling mill 203 which works the hot metal so as to reduce its thickness. After passing through the hot rolling mill 203 the coated metal strip is quenched by passing through a cooler 204 in which it is subjected to water sprays. Cooling of the coating causes it to solidify and fracture. Coating fragments accordingly peel from the strip in passing through the cooler and are separated from the water circulation system through filtration for re-use after recycling.

After removal of the coating, the strip passes through a further pair of pinch rolls 205 onto a coiler 206. The strip so produced is either bright strip or strip with a very thin layer of scale. It therefore may not need any further de-scaling or pickling prior to further processing. Indeed the strip may be passed immediately onto further processing apparatus for in line processing, for example coating of the hot rolled product.

Claims

CLAIMS :

1. A method of producing steel strip, comprising continuously casting a solidified strip product from a casting pool of molten steel in a continuous strip caster and treating the solidified strip as it comes from the caster and while it is in line with the caster by (a) applying to the hot metal strip an oxidation protective coating of a material which is fluid at the temperature of the strip to which it is applied; and (b) subsequently removing the coating by a coating removal treatment including cooling of the coating to cause it to solidify and fracture.

2. A method as claimed in claim 1, wherein the coating is applied to the strip in the region where it exits the caster.

3. A method as claimed in claim 1 or claim 2, wherein the coating material is applied in the form of a powder which melts on application of heat from the strip.

4. A method as claimed in any one of the preceding claims, wherein the coating is applied to the strip when its temperature is in excess of 1100°C.

5. A method as claimed in claim 4, wherein the coating is applied to the strip when its temperature is in excess of 1300°C.

6. A method as claimed in any one of the preceding claims, wherein the oxidation protective coating reacts with iron oxide on the strip surface so that the iron oxide is removed with the coating so as to descale the strip.

7. A method as claimed in any one of the preceding claims, wherein the coating material has a melting/softening point in the range 600°C to 1400°C.

8. A method as claimed in any one of the preceding claims, wherein the coating material is an oxygen containing glassy material.

9. A method as claimed in claim 8, wherein the coating material is a vitreous enamel.

10. A method as claimed in any one of the preceding claims, wherein the coating material has a viscosity of less than 1000 Pa.s at the temperature of the strip to which it is applied.

11. A method as claimed in claim 10, wherein the coating material has a viscosity of less than 100 Pa.s at the temperature of the strip to which it is applied.

12. A method as claimed in any one of the preceding claims, wherein the coating is cooled to cause it to solidify and fracture by the application of water sprays.

13. A method as claimed in any one of the preceding claims, wherein the hot coated metal strip is subjected to working so as to alter its size and/or shape prior to removal of the coating.

14. A method as claimed in claim 13, wherein the hot coated strip is subjected to working by hot rolling whereby to reduce its thickness.

15. A method as claimed in any one of the preceding claims, wherein the continuous caster is a twin roll caster comprising a pair of chilled casting rolls forming a nip between, said casting pool of molten steel is supported on the casting surfaces of the casting rolls immediately above the nip, the casting rolls are rotated to deliver said solidified strip downwardly from the nip, and the coating material is applied to the strip beneath the nip.

16. Apparatus for producing steel strip, comprising a continuous caster for continuously casting a solidified strip product from a casting pool of molten steel; a coating applicator operable to apply to the hot steel strip as it is delivered from the continuous caster an oxidation protective coating; and coating removal means to remove the coating from the strip while it is in line with the continuous caster and comprising cooling means to cool the coating whereby to cause it to solidify and fracture.

17. Apparatus as claimed in claim 16, wherein the coating applicator is such as to apply the coating material in powdered form.

18. Apparatus as claimed in claim 16 or claim 17, and further comprising rolling means operable to roll the hot coated strip so as to reduce its thickness prior to removal of the coating.

19. Apparatus as claimed in any one of claims 16 to 18, wherein the continuous caster comprises a pair of casting rolls forming a nip between them, a metal delivery nozzle disposed above the nip to deliver molten steel into the nip to create a casting pool of molten steel supported on the casting surfaces of the rolls immediately above the nip, and roll drive means to contra-rotate the rolls so as to deliver a solidified hot metal strip downwardly from the nip.