JP4495455B2 - Steel strip casting - Google Patents

Abstract

A method of producing strip comprising the steps of assembling a pair of casting rolls with a nip between them, introducing between the casting rolls to form a casting pool of molten carbon steel having a total oxygen content of at least 100 ppm and usually less than 250 ppm, counter rotating the casting rolls, solidifying the molten steel on the rolls to form metal shells with levels of oxide inclusions reflected by the total oxygen content of the molten steel, and forming thin steel strip through the nip between the casting rolls from the solidified shells. A unique steel strip may be obtained using the method having ductile properties.

Description

    The present invention relates to steel strip casting and is particularly applicable to continuous casting of thin steel strips in a twin roll caster.

    In twin-roll casting, molten metal is introduced between a pair of cooled horizontal casting rolls that rotate in the opposite directions, so that the metal shell solidifies on the moving roll surface and fits in the roll gap between the rolls. This produces a solidified strip product fed downward from the roll gap. In this specification, the term “roll gap” is used to indicate the entire region where the rolls are closest to each other. Molten metal is poured from the ladle into a small container and flows from there through a metal supply nozzle located above the roll gap, directed to the roll gap between the rolls, and directly above the roll gap. A molten metal casting pool supported on a roll casting surface extending along the length direction can be formed. Usually, the casting reservoir is defined by a side plate or a weir that is held by sliding engagement with the roll end face and blocks the two ends of the casting reservoir so as not to overflow, but alternative means such as an electromagnetic barrier may also be used. Proposed.

US Pat. No. 5,720,336 US Patent No. 5,934,359 US Patent No. 6,059,014 US Pat. No. 5,184,668 US Pat. No. 5,277,243 Document submitted to METEC Conference 99 in Düsseldorf, Germany (June 13-15, 1999) "Recent development in Project M, joint development of low carbon steel strip casting by BHP and IHI"

  In the case of thin steel strip casting in a twin roll caster, the molten steel in the casting pool is generally at a temperature of about 1500 ° C. or higher, and therefore it is necessary to cool the entire roll casting surface at a very high speed. Of particular importance is the achievement of high heat flux and extensive nucleation during the initial solidification of steel at the casting surface to form a metal shell. According to US Pat. No. 5,720,336, most of the metal oxide formed as deoxidized product is at the initial solidification temperature so that an essential liquid layer is formed at the interface between the molten metal and each casting surface. A method is disclosed in which the heat flux during initial solidification can be increased by adjusting the molten steel component to be liquid. As disclosed in US Pat. Nos. 5,934,359 and 6,059,014 and international application AU99 / 00641, steel nucleation during initial solidification affects the texture of the casting surface. Can be done. In particular, International Application AU99 / 00641 discloses that random textured textures can provide nucleation sites that are distributed over the entire casting surface to enhance initial solidification. We believe that nucleation is also dependent on the presence of oxide inclusions in the molten steel, and surprisingly the number of inclusions formed during deoxidation of the molten steel prior to casting in twin roll strip casting. This time, I found out that casting with “clean” steel with the least amount of iron was not advantageous.

Steel for continuous casting is deoxidized in a ladle before pouring. In twin-roll casting, steel is generally subjected to deoxidation of a silicon manganese ladle. By using aluminum deoxidation with addition of calcium, the narrow metal flow path of the metal supply system that delivers molten metal to the casting pool is used. It is possible to control the formation of solid Al ₂ O ₃ inclusions that can be clogged. In the past, it has been considered desirable to seek an optimum clean of the steel by ladle treatment to minimize the total oxygen level in the molten steel. However, what we have found this time is that the volume of inclusions is reduced by lowering the oxygen level of the steel, and if the total oxygen content of the steel is reduced below a certain level, The nature of the initial contact is adversely affected and can result in insufficient nucleation to produce rapid initial solidification and high heat flux. The molten steel is trimmed by deoxidation in the ladle so that the total oxygen content is in a range that ensures sufficient solidification on the casting roll and sufficient strip production. The molten steel includes a distribution of sufficient oxidizing inclusions (typically MnO, CaO, SiO ₂ and / or Al ₂ O ₃ ) to provide a density nucleation site on the roll surface suitable for initial solidification. The resulting strip product shows a characteristic distribution of solidified inclusions.

Assembling a pair of cooling cast rolls having a roll gap in between and having a confinement closure adjacent to the end of the roll gap;
Steel is introduced between the casting rolls to form a casting pool between the casting rolls,
The casting rolls are mutually rotated, the molten steel is solidified, and a metal shell is formed on the casting roll surface,
In the steel strip manufacturing method by continuous casting, comprising the steps of forming a solidified thin steel strip from the solidified shell through a roll gap of the casting roll,
The molten steel has a carbon content of 0.0001 to 0.01 wt%, a manganese content of 0.01 to 2.0 wt%, a silicon content of 0.01 to 10 wt%, free oxygen and deoxidation inclusions A low carbon steel with a total oxygen content of at least 100 ppm consisting of oxygen in the middle, in a steel with an oxide inclusion level at an inclusion density of 2-4 g / cm ³ reflecting the total oxygen content of the molten steel by solidifying the molten steel Forming the metal shell having deoxidized inclusions consisting of any one or more of MnO, SiO ₂ , Al ₂ O ₃ and oxidation inclusions consisting of solidified inclusions , A method of manufacturing a steel strip by continuous casting that facilitates formation is provided.

  The total oxygen content of the cast pool molten steel can be 100-250 ppm. More specifically, it can be about 200 ppm. The low carbon steel can have a carbon content of 0.001 to 0.1 wt%, a manganese content of 0.1 to 2.0 wt%, and a silicon content of 0.01 to 10 wt%. The steel can have an aluminum content on the order of 0.01% by weight or less. Aluminum may be a small amount, for example, 0.008% by weight or less. The molten steel can be silicon / manganese killed steel.

Oxidation inclusions are solidification inclusions and deoxidation inclusions. Solidification inclusions are formed during cooling and solidification of steel in casting, and deoxidation inclusions are formed during deoxidation of molten steel before casting. Solidified steel contains oxidative inclusions usually composed of any one or more of MnO, SiO ₂ , Al ₂ O ₃ distributed in the steel at inclusion density of 2-4 g / cm ^3. be able to.

  Molten steel is refined in a ladle before being introduced between casting rolls, and the steel charge and slag formation material are heated in the ladle to form molten steel covered with slag containing silicon, manganese, and calcium oxide. By doing so, a casting pool can be formed. Molten steel is agitated by blowing inert gas to cause desulfurization, and steels such as silicon / manganese killed steel can then be blown to produce steel with the desired total oxygen content of at least 100 ppm, usually 250 ppm or less. . By desulfurization, the sulfur content of the molten steel can be reduced to 0.01% by weight or less.

A thin steel strip produced by twin roll continuous casting as described above has a thickness of 5 mm or less, and is formed as a solidified steel containing solidified oxide inclusions. The distribution of inclusions can be such that two surface areas of a 2 micron deep strip from the outer surface contain solidification inclusions with a density per unit area of at least 120 inclusions / mm ² .

The solidified steel can be a silicon / manganese killed steel and the oxidation inclusions can consist of one or more of MnO, SiO ₂ and Al ₂ O ₃ inclusions. Inclusions can typically be 2 to 12 microns in size, so that at least a majority of the inclusions are within that size range.

  The above-described method produces a unique steel with a high oxygen content distributed in the oxidized inclusions. Specifically, the combination of the high oxygen content of the molten steel and the short residence time of the molten steel in the casting pool produces a thin steel strip with improved ductility characteristics.

In order that the invention may be described in more detail, several embodiments are provided with reference to the accompanying drawings.

  We produce steel strips of about 1 mm thickness and less using a twin roll caster of the type fully described in US Pat. Nos. 5,184,668 and 5,277,243 A casting test was conducted. Such a casting test using silicon manganese killed steel has demonstrated that the melting point of the oxide inclusions in the molten steel has an effect on the heat flux obtained during solidification of the steel, as shown in FIG. The low melting point oxide improves the heat transfer contact between the molten metal and the casting roll surface in the upper region of the reservoir and produces a high heat transfer rate. If the melting point is higher than the steel temperature of the casting pool, no liquid inclusions are produced. Therefore, when the inclusion melting point is about 1600 ° C. or higher, there is a dramatic decrease in the heat transfer rate.

Cast tests on aluminum killed steel showed that it was necessary to have calcium treatment and liquid CaO.Al ₂ O ₃ inclusions to avoid the formation of high melting alumina inclusions (melting point 2050 ° C.). .

  Oxidized inclusions formed in the solidified metal shell, and thus in the thin steel strip, consist of inclusions formed during steel cooling and solidification and deoxidized inclusions formed during refining in a ladle.

Free oxygen levels in the steel are dramatically reduced at the meniscus when cooled, resulting in solidification inclusions near the strip surface. Most of these solidified inclusions are made of MnO.SiO _{2 according} to the following formula.
Mn + Si + 3O = MnO · SiO ₂

  The state of the solidified inclusions on the strip surface obtained from the energy dispersive spectroscopy (EDS) map is shown in FIG. It can be seen that the solidified inclusions are extremely fine (typically 2 to 3 μm or less) and are located in a band shape within 10 to 20 μm from the surface. Our document “Recent Developments in Project M, BHP” submitted to METEC Conference 99 (June 13-15, 1999) in Dusseldorf, Germany, shows the typical size distribution of inclusions in the strip. 3 from "Joint development of low carbon steel strip casting by IHI and IHI".

The comparative level of solidified inclusions depends mainly on the manganese and silicon levels in the steel. FIG. 3 shows that the manganese: silicon ratio has a significant effect on the liquidus temperature of the inclusions. Carbon content of 0.001 to 0.1% by weight, manganese content of 0.1 to 2.0% by weight, silicon content of 0.1 to 10% by weight and aluminum content of about 0.01% by weight or less. Manganese silicon killed steel can produce such oxidized inclusions when the steel is cooled in the upper casting pool. In particular, a steel named M06 can have the following composition:
Carbon 0.06 wt%
Manganese 0.6 wt%
Silicon 0.28% by weight
0.002% by weight of aluminum.

Deoxidation inclusions are produced in aluminum, silicon and manganese during deoxidation of molten steel in the ladle. Therefore, the composition of the oxidized inclusion formed during deoxidation is mainly based on MnO.SiO ₂ .Al ₂ O ₃ . These deoxidation inclusions are randomly located in the strip and are coarser than the solidification inclusions near the strip surface.

The alumina content of inclusions has a strong effect on the free oxygen level of the steel. FIG. 4 shows that free oxygen in the steel decreases as the alumina content increases. As can be seen from the following reaction, the introduction of alumina results in dilution of the MnO.SiO ₂ inclusions, resulting in decreased activity, which in turn reduces free oxygen levels.
MnSi + 3O + Al ₂ O ₃ ⇔ (Al ₂ O ₃ ) · MnO · SiO ₂

For inclusions based on MnO—SiO ₂ —Al ₂ O ₃ , the effect of inclusion composition on liquidus temperature can be obtained from the three-phase diagram shown in FIG. Analysis of the oxidized inclusions in the thin steel strip shows that the MnO / SiO ₂ ratio is typically in the range of 0.6 to 0.8, in this regime, as shown in FIG. It was found that the alumina content of the oxidized inclusion had the strongest effect on the inclusion melting point (liquidus temperature).

The casting according to the invention has solidification inclusions and deoxidation inclusions that are liquid at the initial solidification temperature of the steel, and the molten steel in the casting pool has an oxygen content of at least 100 ppm and is reflected in the total oxygen content of the molten steel. We have determined that it is important to produce metal shells with oxidized inclusion levels to promote nucleation and high heat flux during the initial solidification of steel on the casting roll surface. Both solidification inclusions and deoxidation inclusions are oxidative inclusions that provide a nucleation site and contribute significantly to nucleation during the metal solidification process. However, it is speed control. Deoxidation inclusions are very large, typically 4 microns or more, but solidification inclusions are generally less than 2 microns, are based on MnO.SiO ₂ and do not contain Al ₂ O ₃ acid inclusions have also Al ₂ O _3.

  In a casting test using the above M06 grade silicon / manganese killed steel, if the total oxygen content of the steel is reduced to a level lower than 100 ppm in the ladle refining process, the heat flux is reduced and casting is impaired. It has been found that good casting results can be achieved if the content is at least 100 ppm or more, typically around 200 ppm. The total oxygen content can be measured with a “Leco” instrument, the degree of “rinsing” during ladle processing, ie the amount of argon bubbled into the ladle via a porous plug or top lance and the amount of treatment. Controlled by duration. The total oxygen content can be obtained from the TC436 Nitrogen / Oxygen Analyzer Instruction Manual (Form No. 200-403 Revised, April 1996, p.7-1, available from Luco using a Luco TC-436 Nitrogen / Oxygen Analyzer. It was measured by the conventional procedure described in Chapter 7 on page 7-4.

  Steel to perform ladle deoxidation with calcium silicide (Ca-Si) to know if the increased heat flux obtained with high total oxygen content is due to the availability of oxidation inclusions as nucleation sites A casting test was conducted and the results were compared to casting with a low carbon silicon killed steel known as M06 grade steel. The results are shown in the following table.

Manganese and silicon levels were similar to normal silicon kill grades, but the free oxygen level of the Ca-Si set was low and the oxide inclusions contained relatively much calcium oxide. The heat flux of the Ca—Si group was low despite the low inclusion melting point (see Table 2).

  The free oxygen level of the Ca-Si grade was lower than the M06 grade of 40-50 ppm, typically 20-30 ppm. Oxygen is a surface active element, and therefore a decrease in oxygen level is expected to cause a decrease in heat transfer rate by reducing wetting between the molten steel and the casting roll. However, it can be seen from FIG. 7 that a 40-20 ppm oxygen reduction may not be sufficient to increase the surface tension to account for the observed heat flux reduction.

It can be concluded that lowering the oxygen level of the steel reduces the volume of inclusions and hence the number of oxidative inclusions for initial nucleation. This has the potential to adversely affect the nature of the initial contact between the steel and the roll surface. Due to the dip testing operation, a density per nucleation unit area of about 120 / mm ² is necessary to generate sufficient heat flux in the initial solidification in the meniscus area, which is the upper casting pool area. Indicated. The dip test involves advancing the cooling block into the molten steel bath at a speed that closely simulates the condition at the casting surface of the twin roll caster. As the steel moves through the molten bath, it solidifies on the cooling block, creating a layer of solidified steel on the block surface. This layer thickness can be measured at various points throughout the area to map the variation in solidification rate, and thus the effective rate of heat transfer at various locations. Therefore, it is possible to measure the entire solidification rate and total heat flux. It is also possible to examine the microstructure of the strip surface and correlate the change in solidification microstructure with the observed solidification rate and change in heat transfer value to investigate the structure associated with nucleation during initial solidification on the cooling surface. The dip test apparatus is described in relatively detail in US Pat. No. 5,720,336.

The relationship between oxygen content and heat transfer in liquid steel during initial nucleation was investigated using the model described in Appendix 1. This model assumes that all inclusions are spherical and are evenly distributed throughout the steel. Assuming that the surface layer is 2 μm, it was assumed that only the inclusions present in the surface layer could participate in the nucleation process during the initial solidification of the steel. The model inputs were steel total oxygen content, inclusion diameter, strip thickness, casting speed, and surface layer thickness. The output was the percent of inclusions in the overall steel needed to achieve a target nucleation unit area density of 120 / mm ² .

Figure 8 shows the participation in the nucleation process, assuming a strip thickness of 1.6 mm and a casting speed of 80 / min, per target nucleation unit area at various steel clean levels expressed in total oxygen content. FIG. 5 is a graph of the percentage of oxidized inclusions in the surface layer required to achieve density. This means that with an inclusion size of 2 μm and a total oxygen content of 200 ppm, 20% of the total oxidation inclusions available in the surface layer are necessary to achieve a target density of 120 / mm ² per unit nucleation unit area. Is shown. However, at a total oxygen content of 80 ppm, as much as 50% of the inclusions are necessary to achieve the critical nucleation rate, and a total oxygen level of 40 ppm is insufficient oxidation to achieve the target density per nucleation unit area. At the inclusion level. Thus, when preparing steel by deoxidation in a ladle, the oxygen content of the steel can be controlled to produce a total oxygen content of 100-250 ppm, typically about 200 ppm. As a result, the 2 micron deep layer adjacent to the casting roll during initial solidification contains oxidized inclusions with a density per unit area of at least 120 / mm ² . These inclusions are present in the outer surface layer of the final solidified strip product and can be detected by a suitable test such as energy dispersive spectroscopy (EDS).

(Example)
[input]
Density per unit area of critical nucleation (necessary to obtain sufficient heat transfer rate) (number / mm ² ) 120
(This value was obtained from the experimental dip test work.)
Roll width (m) 1
Strip thickness (mm) 1.6
Ladle ton (t) 120
Steel density (kg / m ³⁾ 7800
Total oxygen (ppm) 75
Inclusion density (kg / m ³ ) 3000

[output]
Amount of intervening substance (kg) 21.42857
Inclusion diameter (m) 2.00E-06
Inclusion volume (m ³ ) 0.0
Total number of inclusions 1706096451319381.5
Surface layer thickness μm (one side) 2
Total number of surface inclusions 42652411288.45.4536
(Only these inclusions can participate in the initial nucleation process)
Casting speed (m / min) 80
Strip length (m) 9615.38462
Strip surface area (m ² ) 19230.76923
Total number of nucleation sites required 23076992.30760
% Of available inclusions required to participate in the nucleation process 54.1462

(Appendix 1)
[Symbol list]
w = roll width (m)
t = strip thickness (mm)
m _s = steel weight in ladle (tons)
ρ _s = steel density (kg / m ³ )
ρ _i = inclusion density (kg / m ³ )
O _t = total oxygen in steel (ppm)
d = Inclusion diameter (m)
v _i = 1 inclusion volume (m ³ )
m _i = mass of inclusion (kg)
N _t = total number of inclusions t _s = surface layer thickness (μm)
N _s = total number of inclusions in the surface (can participate in the nucleation process) u = casting speed (m / min)
L _s = strip length (m)
A _s = strip surface area ^(m 2)
N _req = total number of inclusions necessary to achieve the target nucleation density NC _t = target nucleation density per unit area (obtained from dip test) (number / mm ² )
N _av =% of total inclusions available from the molten steel on the casting roll surface for the initial nucleation process
[formula]
_{(1) m i = (O} t × m s × 0.001) /0.42
Note: In manganese silicon killed steel, 0.42 kg of oxygen is required to produce 1 kg of inclusions with a composition of 30% MnO, 40% SiO ₂ and 30% Al ₂ O ₃ .
In aluminum killed steel (calcium injected), 0.38 kg of oxygen is required to produce 1 kg of inclusions with 50% Al ₂ O ₃ and 50% CaO composition.
(2) v _i = 4.19 × (d / 2) ³
_{(3) N t = m i} / (ρ i × v i)
_{(4) N s = (2.0} t s × 0.001 × N t / t)
(5) L _s = (m _s × 1000) / (ρ _s × w × t / 1000)
_{(6) A s = 2.0 ×} L s × w
(7) N _req = A _s × 10 ⁶ × NC _t
(8) N _av % = (N _req / N _s ) × 100.0
Equation 1 calculates the amount of inclusions in the steel.
Equation 2 calculates the volume of one inclusion assumed to be spherical.
Equation 3 calculates the total number of inclusions available in the steel.
Equation 4 calculates the total number of inclusions available in the surface layer (assuming 2 μm on each side). Note that these inclusions can only participate in early nucleation.
Equations 5 and 6 are used to calculate the total surface area of the strip.
Equation 7 calculates the number of inclusions required on the surface to achieve the target nucleation rate.
Equation 8 is used to calculate the percentage of total inclusions available on the surface that must participate in the nucleation process. Note that if this number is greater than 100%, the number of inclusions on the surface is not sufficient to achieve the target nucleation rate.

Figure 3 shows the effect of inclusion melting point on heat flux obtained in a twin roll casting test using silicon / manganese killed steel. 2 is an energy dispersive spectroscopy (EDS) map of manganese showing a group of finely solidified inclusions in a solidified steel strip. It is a graph which shows the effect of changing manganese: silicon content to inclusion liquid phase temperature. Fig. 4 shows the relationship between alumina content (measured from strip inclusions) and deoxidation effect. It is a three-phase diagram of MnO.SiO ₂ .Al ₂ O ₃ . The relationship between an alumina content inclusion and a liquidus temperature is shown. The effect of oxygen in molten steel on the surface tension is shown. It is a graph of the calculation result about the inclusion which can be used by nucleation in different steel clean levels.

Claims (3)

Assembling a pair of cooling cast rolls having a roll gap in between and having a confinement closure adjacent to the end of the roll gap;
Steel is introduced between the casting rolls to form a casting pool between the casting rolls,
The casting rolls are mutually rotated, the molten steel is solidified, and a metal shell is formed on the casting roll surface,
In the steel strip manufacturing method by continuous casting, comprising the steps of forming a solidified thin steel strip from the solidified shell through a roll gap of the casting roll,
The molten steel in the ladle before the formation of the casting pool is refined by heating the charged steel in the ladle and the material for forming the slag to form a molten steel covered with iron, which contains silicon, manganese, and calcium oxide. The molten steel in the ladle is stirred and desulfurized by blowing gas into the molten steel, and then oxygen is blown to produce molten steel having a total oxygen content of 100 ppm or more.
It consists of further stages
Molten sump molten steel, the carbon content of 0.001 to 0.1 wt%, 0.1 to 2.0 wt% of manganese content, silicon content of from 0.1 to 10 wt%, about 0.01% or A manganese silicon killed steel having a lower aluminum content and a total oxygen content of 100 ppm to 250 ppm consisting of free oxygen and oxygen in deoxidation inclusions ;
A metal shell formed on the casting roll surface,
Having oxidized inclusions of any one or more of MnO, SiO ₂ , Al ₂ O ₃ distributed in the steel at an inclusion density of 2-4 g / cm ³ to form a thin steel strip A method of producing a steel strip by continuous casting, characterized by promoting. The method for producing a steel strip according to claim 1 , wherein the sulfur content of the molten steel at the time of casting is 0.01% by weight or less . 3. A method according to claim 1 or claim 2 , wherein the solidified steel has a total oxygen content of 100 to 250 ppm .

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