WO2024245395A1

WO2024245395A1 - Method for controlling foreign material embedded in surfaces of thin steel strip

Info

Publication number: WO2024245395A1
Application number: PCT/CN2024/096656
Authority: WO
Inventors: Yihai ZHU; Yujun Liu; Rongzhou WU; Bin Qian; Ting Zhang; Yiqing ZHAI; Xiaojia Li
Original assignee: Zhangjiagang Zhongmei Ucs Technology Co Ltd; Jiangsu Shagang Group Co Ltd; Jiangsu Shagang Iron and Steel Research Institute Co Ltd
Current assignee: Zhangjiagang Zhongmei Ucs Technology Co Ltd; Jiangsu Shagang Group Co Ltd; Jiangsu Shagang Iron and Steel Research Institute Co Ltd
Priority date: 2023-05-31
Filing date: 2024-05-31
Publication date: 2024-12-05
Anticipated expiration: 2025-11-30
Also published as: CN116851453A

Abstract

A method for controlling foreign material embedded in the surfaces of the thin steel strip comprises any one or more than one of the following steps: (1) controlling production of molten steel: controlling an oxygen blowing amount of a refining furnace to be at 15-18 m³ and controlling a quantity of the molten steel in a tundish to be above 15 tonnes; (2) controlling a hot box atmosphere between a twin roll caster and a hot rolling mill: controlling a nitrogen flow rate in the hot box to be not less than 2500 m³/h, and controlling a total flow rate of compressed air in the hot box to be as follows: when rolling thin steel strip with a gauge over 1.2 mm, the total flow rate of the compressed air is controlled to be less than 100 m³/h; or, when rolling thin steel strip with a gauge of 1.2 mm and below, the total flow rate of the compressed air is controlled not to exceed 350 m³/h.

Description

METHOD FOR CONTROLLING FOREIGN MATERIAL EMBEDDED IN SURFACES OF THIN STEEL STRIP

TECHNICAL FIELD

The present invention belongs to the technical field of steel production, and in particular relates to a method for controlling foreign material, such as scale, embedded in the surface of the thin steel strip.

BACKGROUND

The applicant has found that there is often a serious problem of foreign material, such as scale, embedded in the surfaces of thin steel strip that is produced in a twin roll strip caster and then hot rolled and coiled. As a consequence, some steel coils can only be sold as a degraded product because of the excessive amount and the extent of embedded foreign materials, such as scale. The end result is lower economic benefits.

The amount of embedded foreign material can result in up to 15%of product degraded.

Therefore, minimizing the amount of foreign material embedded in the surfaces of thin twin roll cast steel strip is very important for improving the quality and economic benefits of the thin steel strip.

SUMMARY

In order to solve the above technical problem, according to a first aspect of the present invention, a method for controlling the foreign material embedded in the surfaces of twin roll cast thin steel strip comprises:

(1) controlling production and treatment of molten steel:

controlling an oxygen blowing amount of a refining furnace, i.e., a ladle metallurgical furnace (LMF) , to be at 15-18 m³; and

controlling a quantity of the molten steel in a tundish to be above 15 tonnes;

(2) controlling an atmosphere in a hot box between a twin roll strip caster and a hot rolling mill:

controlling a nitrogen flow rate in the hot box to be not less than 2500 m³/h; and

controlling a total flow rate of compressed air introduced into the hot box around the solid strip to achieve controlled oxidation of the strip surfaces:

(2.1) when hot rolling thin steel strip with a gauge over 1.2 mm, the total flow rate of the compressed air is controlled to be less than 100 m³/h; or

(2.2) when hot rolling thin steel strip with a gauge of 1.2 mm and below, the total flow rate of the compressed air is controlled not to exceed 350 m³/h.

The method for controlling foreign material embedded in the surface of thin steel strip according to the present invention may further comprise:

(3) controlling the flow rate of roll bite lubrication oil:

to be less than 150 cc/min.

Beneficial technical effects

The above method steps are the result of the inventors analyzing the appearance of surfaces of cast and rolled strip and considering the causes of foreign material, such as scale, being embedded into the strip surfaces.

The above method steps make it possible to reduce the rate of process yield reduction caused by foreign material embedded into the strip surfaces by up to 15%to less than 1%.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the technical solutions of embodiments of the present disclosure more clearly, the accompanying drawings are briefly introduced hereinafter. It is noted that the drawings in the following description are only related to some embodiments of the present disclosure, and do not limit the present disclosure.

Fig. 1 is a schematic diagram of an example of a method and an apparatus for twin roll strip casting and hot rolling steel strip;

Fig. 2 is an enlarged partial sectional view of a portion of the twin roll caster that is a part of the apparatus of Figure 1;

Fig. 3 is a photograph of calcium oxide slag (originating from refractory material) embedded in a surface of cast and hot rolled strip;

Fig. 4 is a photograph of embedded scale (originating from oxidation of strip) in a surface of cast and hot rolled strip; and

Fig. 5 is a photograph of embedded slag (originating from any one or more of a steel ladle, tundish covering agent, and oxide precipitation from molten steel in molten pool) in a surface of cast and hot rolled strip.

DETAILED DESCRIPTION OF EMBODIMENTS

In order to make the purpose, technical solutions and advantages of embodiments of the present disclosure clearer, the technical solutions of the embodiments of the present disclosure will be described clearly and completely below.

It is noted that the described embodiments are only a part of the embodiments of the present disclosure, but not all of them. Based on the described embodiments of the present disclosure, all other embodiments obtained by those of ordinary skills in the art without creative labor are within the protection scope of the present disclosure.

Unless otherwise defined, the technical terms or scientific terms used in the present disclosure shall have their ordinary meanings as understood by those of ordinary skills in the field to which this disclosure belongs.

In general terms, the invention is a method for controlling foreign material embedded in the surfaces of the thin steel strip that comprises any one or more than one of the following steps: (1) controlling production of molten steel: controlling an oxygen blowing amount of a refining furnace to be at 15-18 m³; controlling a quantity of the molten steel in a tundish to be above 15 tonnes; (2) controlling a hot box atmosphere between a twin roll caster and a hot rolling mill: controlling a nitrogen flow rate in the hot box to be not less than 2500 m³/h; and controlling a total flow rate of compressed air in the hot box to be as follows: when rolling thin steel strip with a gauge over 1.2 mm, the total flow rate of the compressed air is controlled to be less than 100 m³/h; or, when rolling thin steel strip with a gauge of 1.2 mm and below, the total flow rate of the compressed air is controlled not to exceed 350 m³/h.

The purpose of Figs. 1 and 2 is to provide context for the embodiments of the invention. The Figs. discloses typical twin roll steel strip casting apparatus for producing coils of thin twin roll cast and hot rolled steel strip.

Referring now to Figs. 1 and 2, a twin roll caster of the apparatus comprises a main machine frame 10 that supports a pair of counter-rotatable casting rolls 12 mounted in a module in a roll cassette (not shown) . The casting rolls 12 are mounted in the roll cassette for ease of operation and movement.

The twin roll caster includes the pair of counter-rotatable casting rolls 12 having casting surfaces 12A laterally positioned to form a nip 18 there between. Molten steel is supplied from a ladle 13 through a metal delivery system (14, 16) to a metal delivery nozzle 17 (core nozzle) positioned between the casting rolls 12 above the nip 18. Molten steel thus delivered forms a casting pool 19 of molten steel above the nip 18 supported on the casting surfaces 12A of the casting rolls 12. This casting pool 19 is confined in the casting area at the ends of the casting rolls 12 by a pair of side closure plates, or side dams 20 (shown in partial outline in Fig. 2) . The upper surface of the casting pool 19 may rise above the lower end of the delivery nozzle 17 so that the lower end of the delivery nozzle 17 is immersed within the casting pool 19. The casting area includes a protective atmosphere above the casting pool 19 to inhibit oxidation of the molten steel in the casting area.

The ladle 13 typically is of a conventional construction holding 100 tonnes (or any other suitable amount) of molten steel supported on a rotating turret 40. For molten steel delivery, the ladle 13 is positioned over a movable tundish 14 in a casting position to fill the tundish 14 with molten steel.

The movable tundish 14 is fitted with a slide gate 25 (or other suitable steel flow control option) , actuable by a servo mechanism, to allow molten steel to flow from the tundish 14 through the slide gate 25, and then through a refractory outlet shroud 15 to a transition piece or distributor 16 in the casting position. From the distributor 16, the molten steel flows to the delivery nozzle 17 positioned between the casting rolls 12 above the nip 18.

The side dams 20 may be made from a refractory material. The side dams 20 have a face surface capable of physical contact with the casting rolls 12 and molten steel in the casting pool 19. The side dams 20 form end closures for the molten pool of steel on the casting rolls 12 during the casting operation.

Fig. 1 shows the twin roll caster producing cast thin strip 21 that moves initially downwardly and then loops upwardly to a guide table 30 through a hot box 27 that contains a controlled, protective atmosphere, for example containing nitrogen, to minimize strip oxidation, and move across a guide table 30 to a pinch roll stand 31, comprising pinch rolls 31A. Upon exiting the pinch roll stand 31, the cast thin strip 21 passes through a hot rolling mill 32, comprising a pair of work rolls 32A, and backup rolls 32B, forming a gap capable of hot rolling the cast thin strip 21 delivered from the casting rolls 12, where the cast thin strip 21 is hot rolled to reduce the strip to a desired thickness, improve the strip surface, and improve the strip flatness.

The hot rolled cast thin strip 21 then passes onto a run-out table 33 within cooling station 97, where it is cooled by contact with a coolant, such as water, supplied via spray nozzles 90 or other suitable means, and by convection and radiation. In any event, the cooled hot rolled cast thin strip 21 passes through a second pinch roll stand 91 having a pair of rollers 91A that provide tension to the cast thin strip 21 during strip cuts. Finally, the cooled hot rolled cast thin strip 21 is then coiled, with a shear at a shear station 98 cutting the strip periodically upstream of the coiler to form required length of strip for each coil.

The casting rolls 12 are internally water cooled so that as the casting rolls 12 are counter-rotated, shells solidify on the casting surfaces 12A, as the casting surfaces 12A move into contact with and through the casting pool 19 with each revolution of the casting rolls 12.The shells are brought close together at the nip 18 between the casting rolls 12 to produce a cast thin strip product 21 delivered downwardly from the nip 18. The cast thin strip product 21 is formed from the shells at the nip 18 between the casting rolls 12 and delivered downwardly and moved downstream as described above.

In operation, the cast strip leaves the nip at temperatures of the order of 1400℃and greater. To prevent oxidation and scaling of the strip, the strip is cast downwardly into the hot box 27 supporting a protective atmosphere immediately beneath the casting rolls in the casting position. The hot box 27 extends along the path of the cast thin strip until the first pinch roll stand 31 and also extends along the path of the cast thin strip until the hot rolling mill 32 to reduce oxidation and scaling.

The hot box 27 can be described as comprising 3 sections, namely hot box No. 1 enclosing the section of hot rolled cast thin strip 21 between the casting rolls 12 and the pinch roll stand 31, hot box No. 2 enclosing the pinch roll stand 31, and hot box No. 3 between the pinch roll stand 31and the hot rolling mill 32.

After leaving the hot rolling mill 32, the rolled thin strip then passes into a cooling station 97 where the strip is cooled by water that is delivered by spray nozzles 90 of a plurality of rows of water spray assemblies extending across the run-out table 33 as the strip moves over the run-out table 33 in the cooling station 97.

Finally, the cooled, hot rolled strip is coiled in a coiler 92.

Further details of the twin roll caster described in relation to Figs. 1 and 2 can be found in the specification of Chinese Patent Application No. 201780029304.2 in the name of the applicant and the disclosure in that specification is incorporated herein by cross-reference.

According to the inventors’ research and development work in the thin strip casting process, foreign material is often embedded in the surfaces of thin steel strip that has been cast in a twin roll strip caster and hot rolled, and the embedded foreign material mainly comes from the following four sources:

(1) oxides in the molten steel supplied to the twin roll strip caster;

(2) material of the delivery nozzle extending into the molten pool between casting rolls in the twin roll strip caster;

(3) scale formed in the hot box and other equipment downstream of the twin roll strip caster;

(4) oxides in the hot rolling mill.

Based on the above research, in embodiments of the present invention, the following technical means are adopted in order to control the foreign material embedded in the surfaces of cast and rolled thin steel strip. The embodiments comprise any one or more of the following technical means.

1. Molten steel production:

Firstly, the oxygen blowing amount of the refining furnace, i.e., a ladle metallurgical furnace (LMF) , is preferably controlled at 15-18 m³ (noting that different ladle sizes are used in steelmaking plants, typically ranging from 100 tonnes to 140+ tonnes) , so as to avoid a problem that oxides in the molten steel exceed the standard due to excessive oxygen blowing.

Secondly, the quantity of molten steel in the tundish should be controlled at above 15 tonnes, so as to avoid a problem of slag entrapment due to low liquid levels.

Finally, controlling the slag discharge of the ladle, and when 15 tonnes is retained in the ladle for casting, allowing ladle personnel conduct on-site inspection and close a ladle slide plate in time when slag discharge is found.

2. Hot box atmosphere control:

The nitrogen flow rate and the air consumption in the hot box is strictly controlled during production:

the nitrogen flow rate in the hot box should not be less than 2500 m³/h; and

the total flow rate of the compressed air on the upper surface and the lower surface of the thin steel strip in No. 3 hot box is as follows:

when rolling with a gauge over 1.2 mm, the total flow rate is less than 100 m³/h; and

when rolling with a gauge of 1.2 mm and below, the total flow rate does not exceed 350 m³/h.

3. Roll bite lubrication oil flow rate:

The flow rate of roll bite lubrication oil is strictly controlled during production:

the flow rate of roll bite lubrication oil is less than 150 cc/min on the upper surface and the lower surface of mill rolls adjacent to the thin steel strip respectively, noting that typically the roll lubrication oil is introduced to back up mill rolls, and the oil then transfers onto work rolls thus lubricating the roll bite.

Such means can effectively avoid the problem of excessive roll bite lubrication oil sludge at inlet and outlet of the rolling mill caused by excessive flow rate of the roll bite lubrication oil and improve the quality of the thin steel strip.

As an option, the control of foreign material embedded in the surface of thin steel strip can also be optimized from aspects of equipment material cleaning and management.

4. Equipment cleaning:

Firstly, every time a casting campaign is completed, the sealing frame at the entrance of the hot rolling mill, the anti-crimp rolls, and the rolling mill rolls are checked and cleaned.

Secondly, every time a casting campaign is completed, the hot box is checked for water leakage, and if water leakage is detected, maintenance work is undertaken as soon as possible.

Thirdly, every time it is necessary to change rolls or conduct an overhaul or similar, the iron oxide under the roller table of No. 3 hot box is cleared out.

5. Process management:

Firstly, in the production process, the condition of the upper surface of steel strip on the roller platform is checked after rolling, as well as making spot checks for the occurrence of foreign material embedded into the strip surface and of pits in the surfaces of cast and hot rolled strip.

Secondly, conducting spot checks on the surface quality, and, if consecutive coils are found with obvious foreign material embedded into the strip surface or pits in the surfaces of cast and hot rolled strip, the casting sequence is concluded when the current ladle is emptied, and an inspection of strip handling equipment is carried out.

What have been described above are only specific embodiments of the present invention. It should be pointed out that for those of ordinary skills in the art, without departing from the principle of the present disclosure, several improvements and embellishments can be made, and these improvements and embellishments should also be regarded as the scope of protection of the present disclosure.

Claims

A method for controlling the foreign material embedded in the surfaces of twin roll cast thin steel strip comprising:

(1) controlling production and treatment of molten steel:

controlling an oxygen blowing amount of a refining furnace, i.e., a ladle metallurgical furnace (LMF) , to be at 15-18 m³; and

controlling a quantity of the molten steel in a tundish to be above 15 tonnes;

(2) controlling an atmosphere in a hot box between a twin roll strip caster and a hot rolling mill;

controlling a nitrogen flow rate in the hot box to be not less than 2500 m³/h; and

controlling a total flow rate of compressed air introduced into the hot box around the solid strip to achieve controlled oxidation of the strip surfaces:

(2.1) when hot rolling thin steel strip with a gauge over 1.2 mm, the total flow rate of the compressed air is controlled to be less than 100 m³/h; or

(2.2) when hot rolling thin steel strip with a gauge of 1.2 mm and below, the total flow rate of the compressed air is controlled not to exceed 350 m³/h.
The method for controlling foreign material embedded in the surface of thin steel strip defined in claim 1 further comprising:

(3) controlling the flow rate of roll bite lubrication oil:

to be less than 150 cc/min.