CN116057195B - Steel continuous casting method and steel experimental solidification device - Google Patents

Abstract

The present invention simply determines whether the molten steel to be continuously cast is a steel type with a risk of cracking, and based on this, optimizes the operating conditions of continuous casting, thereby preventing the cracking of the cast piece and the adverse conditions of continuous casting, and achieving improved productivity. The molten steel to be continuously cast is injected into a test solidification device and cooled to produce a test cast piece, and the surface roughness of the lower surface of the test cast piece is measured. When the surface roughness is above a given threshold, the molten steel is determined to be a steel type that is prone to cracking during continuous casting, and the continuous casting is performed using a slow cooling protective slag suitable for preventing the cracking. When the surface roughness is less than a given threshold, the molten steel is determined to be a steel type that is not prone to cracking during continuous casting, and the continuous casting is performed using a strong cooling protective slag suitable for increasing the casting speed of continuous casting.

Description

Steel continuous casting method and steel experimental solidification device

Technical Field

The present invention relates to a continuous casting method of steel and a test solidification device of steel for preventing cracking and breakout of cast pieces during continuous casting.

Background technique

When a hypoperitectic medium carbon steel with a C content of 0.09 to 0.17% by mass is continuously cast, cracks are likely to occur on the surface of the casting. Specifically, due to the solidification shrinkage caused by the δ-γ phase transformation during solidification on the molten steel side of the solidifying shell, the part of the solidifying shell with a high cooling rate is warped convexly relative to the mold surface, resulting in bumps on the surface of the casting, and uneven growth occurs in the solidifying shell. In the concave part of the surface of the casting, the thermal resistance increases due to the air gap, and the thickness of the solidifying shell decreases, thereby generating strain in the solidifying shell and solidification cracks on the surface of the casting. The solidification cracks expand during the secondary cooling of continuous casting and grow into longitudinal cracks and transverse cracks. When the degree of solidification cracks of the casting is large, there is also a risk of steel leakage due to the cracks.

Therefore, the following operations are usually performed in the continuous casting process: for steel types in the sub-peritectic carbon region that are prone to solidification fracture during the first cooling in the mold (hereinafter referred to as "fracture risk steel types"), slow cooling protective slag is used to achieve slow cooling in the mold, thereby preventing the fracture of the casting and the occurrence of steel leakage.

If the continuous casting is performed using the slowly cooled mold slag, the thickness of the solidified shell in the mold becomes smaller, so the risk of the solidified shell breaking directly below the mold and causing steel leakage increases. Therefore, when the slowly cooled mold slag is used, the casting speed of the continuous casting needs to be reduced so that the thickness of the solidified shell in the mold does not decrease.

For steel grades other than the fracture risk steel grade, when the slow cooling mold slag is unnecessarily used for continuous casting, the casting speed of the continuous casting must still be reduced, and the productivity of the continuous casting is reduced. Therefore, from the perspective of preventing the fracture of the cast piece and the occurrence of continuous casting defects and achieving improved productivity, it is important to properly determine whether the molten steel is a fracture risk steel grade and to perform continuous casting only for the fracture risk steel grade using the slow cooling mold slag.

It is known that the range of carbon concentration corresponding to the hypoperitectic region on the Fe-C binary system equilibrium state diagram actually changes due to the influence of other alloy components. Considering these aspects, it is important to properly determine whether the molten steel is a fracture risk steel grade and optimize the operating conditions of continuous casting.

As described above, when continuous casting is performed on steel grades with a risk of cracking, unevenness is generated on the surface of the cast piece. As an indicator for evaluating the unevenness, for example, the shape of the unevenness on the surface of the cast piece, such as the depth of the vibration mark, can be used. The vibration mark of the cast piece is formed when the protective slag is pressed into the cast piece when the mold descends, and its depth increases due to the solidification shrinkage occurring inside the solidified shell. Therefore, if the continuous casting conditions are the same, the depth of the vibration mark of the steel grade with a risk of cracking will become larger.

Patent document 1 discloses a method for preventing the occurrence of cracking steel leakage of a cast piece by measuring the depth of a vibration mark online. Specifically, at a position downstream of the mold, a laser rangefinder arranged opposite to the thickness surface of the cast piece continuously detects the profile of the surface of the cast piece, and when the measured depression depth is greater than a reference value, it is determined that there is a risk of cracking steel leakage of the cast piece, and the operating conditions are changed.

In addition, Non-Patent Document 1 discloses a method comprising: immersing a water-cooled plate in molten steel offline to form a solidified shell on the plate, directly measuring the thickness difference and interval between concave and convex parts of the solidified shell, and evaluating the non-uniformity of the solidified shell.

In addition, non-patent document 2 discloses a method for predicting whether a steel grade is at risk of cracking based on alloy composition. Specifically, for various steel grades, a simulated Fe-C binary system equilibrium state diagram is calculated as a function of carbon concentration using a thermodynamic program. Then, based on the hypoperitectic region in these simulated Fe-C binary system equilibrium state diagrams, the changes in the carbon concentration lower limit (C _a ) and carbon concentration upper limit (C _b ) of the hypoperitectic region based on other alloy components are formulated. Whether the molten steel is at risk of cracking is determined based on whether the carbon concentration of the steel grade is within the range of _Ca to C _b .

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Laid-Open No. 9-57413

Non-patent literature

Non-patent document 1: Hiroshi Murakami et al., "Control of non-uniform solidification of hypoperitectic carbon steel in continuous casting mold", Iron and Steel, 1992, Vol. 78, No. 1, pp. 105-112

Non-patent document 2: K. Blazeck and 3 others, "Calculation of the Peritectic Range for Steel Alloys", AISTech 2007 Conference Proceedings, 2007, pp. 81-88

Non-patent document 3: Hanao Fumi and two others, "Effect of mold slag on initial solidification of hypoperitectic steel in continuous casting mold", Iron and Steel, 2014, Vol. 100, No. 4, pp. 581-590

Summary of the invention

Problems to be solved by the invention

However, in the method disclosed in Patent Document 1, it is difficult to prevent the occurrence of casting fractures by changing the type of protective slag accordingly according to the depth of the vibration mark measured during continuous casting. For steel grades with severe fracture risk and unevenness, there is a risk of untimely measures to prevent the occurrence of casting fractures.

Furthermore, the method disclosed in Non-Patent Document 1 is a complicated test for immersing a water-cooled plate in molten steel to form a solidified shell on the plate, and therefore is not suitable for evaluating the non-uniformity of the solidified shell for many types of steel.

Furthermore, in the method disclosed in Non-Patent Document 2, steel grades that are empirically known to cause longitudinal cracking and transverse cracking may not necessarily be appropriately determined as steel grades with cracking risks.

The present invention is completed to solve the above-mentioned problems. That is, the subject of the present invention is to provide a continuous casting method of steel and a test solidification device of steel, which considers that the sub-peritectic region of the molten steel to be continuously cast is affected by the alloy composition and changes, and simply determines whether the molten steel to be continuously cast is a cracking risk steel grade, and optimizes the operating conditions of continuous casting based on this, thereby preventing the cracking of the casting piece and the occurrence of continuous casting defects, and achieving improved productivity.

way of solving the problem

In view of the above problems, the present inventors conducted intensive research and development from a unique perspective and found that by making test slabs from molten steel and evaluating their surface roughness, it is possible to simply and accurately predict whether the molten steel is a steel grade with a risk of fracture, thereby completing the present invention.

The continuous casting method of steel and the experimental solidification apparatus of steel according to the present invention are as follows.

[1] A method for continuous casting of steel, the method comprising: preparing a test casting by injecting molten steel to be continuously cast into a test solidification device and cooling the casting, measuring the surface roughness of the lower surface of the test casting, and when the surface roughness is above a given threshold, performing the continuous casting using a slowly cooled protective slag suitable for preventing the casting from cracking when the molten steel is continuously cast; and when the surface roughness is less than the given threshold, performing the continuous casting using a strongly cooled protective slag suitable for increasing the casting speed of the continuous casting.

[2] The method for continuous casting of steel according to [1], wherein:

The threshold value is 60 μm in terms of the arithmetic mean height of the surface roughness obtained by the method specified in ISO 25178.

[3] A method for continuous casting of steel, the method comprising: preparing a test cast piece by injecting molten steel to be continuously cast into a test solidification device and cooling the test cast piece, measuring the surface roughness of the lower surface of the test cast piece, and for a plurality of molten steels M having the surface roughness being above a given threshold, respectively determining the influence coefficients αa _,M and αb,M of the composition of the molten steel M on the carbon concentration lower limit value _Ca (mass %) and the carbon concentration upper limit value _Cb (mass %) in the hypoperitectic region on the Fe-C binary system equilibrium diagram, calculating the sum of the influence coefficients αa _,M and αb _{,M of the plurality of molten steels M ,} determining the carbon concentration lower limit value _Ca (mass %) and the carbon concentration upper limit value _Cb (mass %) in the hypoperitectic region of the plurality of molten steels M by the following equations (1) and (2), and determining the carbon concentration lower limit value _Ca and the carbon concentration upper limit value Cb in the hypoperitectic region of the new molten steel by the following equations (1) and (2) based on the composition of a new molten steel different from the plurality of molten steels _M. , the carbon equivalent _Cp (mass %) of the new steel liquid is obtained by the following formula (3) based on the obtained carbon concentration lower limit _Ca , the obtained carbon concentration upper limit _Cb , and the carbon concentration C (mass %) of the new steel liquid; when the carbon equivalent _Cp is within the range of 0.09 to 0.17, the new steel liquid is continuously cast using a slowly cooled mold slag suitable for preventing the casting from breaking when the new steel liquid is continuously cast; when the carbon equivalent _Cp is not within the range of 0.09 to 0.17, the new steel liquid is continuously cast using a strongly cooled mold slag suitable for increasing the casting speed of continuous casting;

[Mathematical formula 1]

[Mathematical formula 2]

C _p =0.09+{(CC _a )/(C _b -C _a )}×(0.17-0.09)···(3).

[4] The method for continuous casting of steel according to any one of [1] to [3], wherein:

The slow cooling mold flux contains _SiO2 and CaO as main components, the mass ratio of CaO to _SiO2 (CaO/ _SiO2 ) is 1.0 or more and less than 2.0, the crystallization temperature is 1100°C or more, and lanceolite is crystallized as the primary crystal.

[5] The method for continuous casting of steel according to any one of [1] to [4], wherein:

The strong cooling mold powder contains SiO ₂ and CaO as main components, the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) is 0.7 or more and less than 1.0, and the crystallization temperature is less than 1100°C.

[6] The method for continuous casting of steel according to any one of [1] to [5], wherein:

The test solidification apparatus has a cooling capacity such that the cooling rate at a depth of 1 mm from the surface layer of the solidified shell of the molten steel is 10 ² to 10 ⁵ °C/min.

[7] The method for continuous casting of steel according to any one of [1] to [6], wherein:

The injection speed (unit: kg/s) when injecting the above-mentioned molten steel into the above-mentioned test solidification device is 3 times or more of the solidification speed (unit: kg/s) of the molten steel.

[8] The method for continuous casting of steel according to any one of [1] to [7], wherein:

The test coagulation device has a bottom surface with a width and a depth of 10 mm or more, respectively.

[9] A steel test solidification device, which is a steel test solidification device that produces a test cast piece by injecting molten steel and cooling it.

The test solidification apparatus for the steel includes a mold having a cooling rate of 10 ² to 10 ⁵ °C/min at a depth of 1 mm from the surface layer of the solidified shell of the injected molten steel.

[10] The experimental solidification device for steel according to [9] further comprises an injection device for injecting the molten steel into the mold, wherein the injection speed (unit: kg/s) of the molten steel by the injection device is at least 3 times the solidification speed (unit: kg/s) of the molten steel in the mold.

[11] The steel solidification test device according to [9] or [10], wherein:

The casting mold has a bottom surface with a width and a depth of 10 mm or more, respectively.

Effects of the Invention

According to the continuous casting method for steel and the test solidification device for steel of the present invention, by using the surface roughness or carbon equivalent of the lower surface of the test slab produced by injecting molten steel to be continuously cast into the test solidification device and cooling it, it is possible to easily determine whether the molten steel is a type of steel that is prone to cracking in the slab during continuous casting.

Furthermore, when it is determined that the cast piece is prone to cracking, continuous casting can be performed using a slowly cooled mold slag suitable for preventing cracking, thereby reliably preventing the cast piece from cracking and leaking. In addition, when it is determined that the cast piece is not prone to cracking, continuous casting can be performed using a strongly cooled mold slag suitable for increasing the casting speed of continuous casting, thereby improving the productivity of continuous casting without reducing the casting speed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of a test solidification apparatus used in the continuous casting method of steel according to the present invention.

FIG. 2( a ) and FIG. 2( b ) are photographs showing examples of the surface roughness of the lower surface of a test piece produced by a test solidification apparatus for steel of the present invention.

Symbol Description

1 Experimental solidification device for steel

2. Casting

21 Bottom

3 Injection device

31 Crucible

32 High frequency induction coil

33 Tilt table

W Width

D Depth

H Height

S steel sample (molten steel)

Detailed ways

Hereinafter, embodiments of a continuous casting method for steel and a test solidification apparatus for steel according to the present invention will be described with reference to the drawings.

＜Steel solidification test equipment＞

FIG. 1 shows an outline of a test solidification apparatus 1 used in the continuous casting method of steel according to the present embodiment.

As shown in FIG. 1 , a test solidification apparatus 1 for steel of the present embodiment includes a mold 2 for producing a test cast piece by injecting molten steel S into the mold and cooling and solidifying the molten steel S, and an injection device 3 for injecting the molten steel S into the mold 2 .

The casting mold 2 is a copper container of a substantially rectangular parallelepiped shape, and a water cooling device (not shown) is provided on its bottom surface 21. The thickness of the casting mold 2 and the capacity of the water cooling device are designed to obtain the following cooling capacity: when the molten steel S is poured into the casting mold 2 and cooled and solidified, the cooling rate of the bottom surface 21 side of the casting mold 2 cooled by the water cooling device at a depth of 1 mm from the surface layer of the solidified shell is 10 ² to 10 ⁵ ° C/min.

In the present invention, the shape of the mold 2 of the test solidification device 1 is not particularly limited. It is preferred that the width W and the depth D of the bottom surface 21 of the mold 2 are 10 mm or more, and it is further preferred that the width W and the depth D are 40 mm or more and 60 mm or less. This is based on the following situation: it is known that the size of the lower surface of the test casting made by the test solidification device 1 is the same size as the bottom surface 21 of the mold 2. When the surface roughness of the lower surface of the test casting is measured as described later, the interval of the concave and convex that can be confirmed by naked eye observation is within the range of 10 mm to 40 mm. In addition, the surface roughness of the bottom surface of the mold 2 in contact with the lower surface of the test casting is preferably less than 30 μm by the arithmetic mean height of the surface roughness obtained by the method specified in ISO25178 "Three-dimensional surface properties (surface roughness)". This is because, as described later, when the surface roughness of the lower surface of the test slab is evaluated using the arithmetic mean height of the surface roughness obtained by the method specified in ISO25178, the shape of the bottom surface 21 of the mold 2 affects the surface roughness of the lower surface of the test slab.

The injection device 3 includes: a bottomed _cylindrical crucible 31 made of _Al2O3 or MgO, a high-frequency induction coil 32 that covers the content in the crucible 31 in a manner that covers the outer circumference of the crucible 31 and heats it to melt it, a tilting table 33 that tilts the crucible 31 in a fixed state to inject the molten material in the crucible 31 into the mold 2, a plurality of thermocouples (not shown) for measuring the temperature of the molten steel in the crucible 31, and a temperature display device (not shown) that converts the output voltage of each thermocouple into a temperature and displays it.

Next, a method for continuous casting of steel using the above-mentioned test solidification apparatus 1 for steel will be described.

＜Preparation of test castings＞

In the present embodiment, a steel sample (molten steel) S having the same composition as the target composition of the molten steel to be continuously cast is placed in a crucible 31, and the crucible 31 is fixed on a tilting table 33. A high-frequency induction coil 32 is further provided in a manner covering the outer circumference of the crucible 31, and the steel sample S in the crucible 31 is heated to be melted. At this time, the heating of the steel sample S is continued until the operator visually confirms that the steel sample S has melted, and confirms that the temperature of the molten steel sample S displayed on the temperature display device has reached a range of 1590 to 1610°C. Here, the output value from the above-mentioned thermocouple can also be input into a computer to automatically determine whether the temperature of the molten steel sample S has reached a range of 1590 to 1610°C, thereby replacing the operator's visual observation.

Next, the high-frequency induction coil 32 is moved away from the crucible 31, and the tilting table 33 is tilted to tilt the crucible 31, and the steel sample S melted in the crucible 31 is poured into the mold 2. Then, the water cooling device of the mold 2 is operated to cool the molten steel (steel sample) S poured into the mold 2 and solidify it to produce a test cast. At this time, the operation of the water cooling device is adjusted so that the cooling rate at a depth of 1 mm from the surface of the solidified shell reaches 10 ² to 10 ⁵ ° C/min.

This cooling rate is based on the following report in Non-Patent Document 3: when a crack risk steel grade is continuously cast by a practical continuous casting machine, the occurrence of non-uniform solidification becomes obvious at a stage where the thickness of the solidified shell exceeds 1 mm, and the cooling rate at this position is 10 ³ to 10 ⁵ ° C/min. That is, in the cooling of the molten steel (steel sample) S in the test solidification device 1, the cooling rate at the position where the occurrence of non-uniform solidification becomes obvious in a practical continuous casting machine is reproduced.

In addition, if the tilting speed of the tilting table 33 is coordinated with the action of the above-mentioned water cooling device, and the injection speed (unit: kg/s) of the steel sample S injected into the mold 2 using the tilting table 33 is set to be more than 3 times the solidification speed (unit: kg/s) of the molten steel S in the mold 2, then when the molten steel S is in the sub-eutectic region, it is easy to produce bumps and depressions on the surface of the solidified shell, and it is possible to determine with better accuracy whether it is a steel type with a risk of fracture, so it is preferred.

FIG2 shows an example of the lower surface of the test casting produced by the test solidification apparatus 1 in the form of a photograph. FIG2(a) is an example of the case where the steel sample S is a steel grade with a risk of cracking, and FIG2(b) is an example of the case where the steel sample S is not a steel grade with a risk of cracking. When the steel sample S is a steel grade with a risk of cracking, it can be clearly confirmed that the lower surface of the test casting has unevenness.

＜Production of test slabs using molten steel in the steelmaking process＞

In the actual steelmaking process, the composition of the molten steel during continuous casting may also deviate from the target value. Therefore, in order to improve the accuracy of determining whether the molten steel is a steel grade with a risk of fracture, the molten steel can be collected from the ladle into which the molten steel to be continuously cast is added using a sampler, and the molten steel is directly injected into the mold 2 of the test solidification device 1 and cooled, thereby making a test cast. In this case, if the sampler for collecting molten steel from the ladle has the function of the mold 2, there is no need to prepare the test solidification device 1 separately.

<Measurement of surface roughness>

Next, the height of the concavities and convexities on the bottom surface of the test slab produced as described above was measured by a measuring device such as a laser rangefinder, and the surface roughness of the surface roughness was calculated using the arithmetic mean height specified in ISO25178.

As the calculation conditions of the surface roughness, the size of the measurement evaluation area, the interval of the measurement points and the wavelength of the cut-off can be cited. For the continuous casting method of steel and the test solidification device of steel of the present invention, the size of the measurement evaluation area, the interval of the measurement points and the wavelength of the cut-off is not particularly limited, and is preferably as follows.

First, for the measurement and evaluation area, the center is set to the center of the lower surface of the test slab, and the lengths of the vertical and horizontal directions are preferably set to 10 mm or more, and more preferably to 40 mm or more and 60 mm or less. This is based on the fact that the interval between the concave and convex areas that can be confirmed by naked eye observation is known to be in the range of 10 mm to 40 mm. The interval between the measurement points is preferably set to 10 mm or less. The size of the cut-off wavelength is preferably set to 800 μm.

＜Determination of steel grades with risk of cracking＞

Next, when the surface roughness of the lower surface of the test slab calculated as described above (arithmetic mean height of surface roughness) is greater than 60 μm, the molten steel having the same composition as the steel sample S is judged to be a crack risk steel grade (a steel grade in which the slab is prone to cracking during continuous casting).

As described above, for crack risk steel, due to the solidification shrinkage caused by the δ-γ phase transformation during solidification on the molten steel side of the solidified shell, the portion with a high cooling rate in the solidified shell warps convexly relative to the mold surface, resulting in unevenness on the surface of the cast piece. Therefore, the surface roughness of the test cast piece becomes an indicator of whether the molten steel having the same composition as the steel sample S is a crack risk steel.

Furthermore, for a plurality of types of molten steel, a relational expression of carbon equivalent _Cp can be established as follows using the result of determining whether each molten steel is a crack risk steel type based on whether the surface roughness of the test slab is equal to or greater than a given threshold.

That is, when the surface roughness of the test casting made from the molten steel reaches a given threshold value or more and is determined to be a steel grade with a risk of cracking, the influence coefficients αa,M and αb,M of the carbon concentration lower limit ( _Ca ) (mass %) and the carbon concentration upper limit ( _Cb ) (mass %) in the hypoperitectic region on the Fe _- C binary system equilibrium diagram of each component element of the steel grade _M are calculated. Then, for a plurality of steel grades M, considering that the range of carbon concentration in the hypoperitectic region changes due to the influence of other alloy components, the relationship between _Ca and _Cb is established as shown in the following formulas (1) and (2).

[Mathematical formula 3]

[Formula 4]

Then, when determining whether the new molten steel (target steel) is a crack risk steel grade, _Ca and _Cb are obtained from the component composition of the target steel by the above-mentioned formula (1) and formula (2), and the carbon equivalent _Cp (mass %) of the target steel is obtained from the obtained _Ca and _Cb and the carbon concentration C (mass %) of the target steel by the following formula (3), instead of making the determination based on the surface roughness of the test cast piece.

C _p = 0.09 + {(CC _a )/(C _b -C _a )} × (0.17 - 0.09) (3)

When the carbon equivalent C _p is within the range of 0.09 to 0.17 mass %, the target steel is in the hypoperitectic region and can be determined as a crack risk steel grade.

＜Selection of mold slag＞

Next, based on the above-mentioned determination of whether or not the steel grade is at risk of cracking, it is selected whether to use the slowly cooled mold slag or the strongly cooled mold slag for continuous casting.

The slow cooling effect of the solidified shell brought about by the mold slag can be obtained as follows: the powder slag flowing into the gap between the mold of the continuous casting machine and the solidified shell is cooled and solidified on the surface of the mold, thereby forming a slag film, and the heat transfer resistance is increased by the crystals in the slag film. The components of the mold slag are _SiO2 and CaO as the main components, and _Li2O , _Na2O , F, MgO, _Al2O3 , _etc. are added to adjust the viscosity of the mold slag and the precipitation of crystals. The usual seed crystal precipitated in the slag film is cuspidine (3CaO· _2SiO2 · _CaF2 ).

In order to suppress surface cracking of the casting, it is effective to achieve slow cooling of the solidified shell near the surface of the molten steel. Therefore, in order to give the protective slag the effect of suppressing longitudinal cracking, it is necessary to precipitate crystals instantly after the powder slag flows into the gap between the mold and the solidified shell, and slowly cool the solidified shell.

Since it is believed that the protective slag with a high crystallization temperature and crystallized lanceolite as the primary crystal has the function of slowly cooling the inside of the mold, for steel types with a risk of cracking, using such a slowly cooling protective slag and reducing the casting speed can reliably prevent the occurrence of cracking and steel leakage. For steel types without a risk of cracking, productivity can be maintained by not using a slowly cooling protective slag and not reducing the casting speed.

Specifically, when the surface roughness of the lower surface of the test casting calculated as described above is 60 μm or more, the molten steel having the same composition as that of the steel sample S is determined to be a crack risk steel grade, and continuous casting is performed using a slow cooling mold slag suitable for preventing cracks. As the slow cooling mold slag, specifically, a slow cooling mold slag containing SiO ₂ and CaO as main components, a mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) of 1.0 or more and less than 2.0, a crystallization temperature of 1100°C or more, and crystallizing lanceolite as a primary crystal can be used.

The reasons for setting the constituent components of the mold slag as described above are as follows. When the mass ratio of CaO to _SiO2 (CaO/ _SiO2 ) is less than 1.0, the amount of spar precipitated in the slag film is insufficient, and the crystallization temperature becomes too low, so that the mold slag cannot be given a slow cooling function to prevent longitudinal and transverse cracks. In addition, when the mass ratio of CaO to _SiO2 (CaO/ _SiO2 ) is 2.0 or more, the crystallization temperature of the mold slag rises, the crystallization of the mold slag is excessively promoted, the friction between the mold and the cast piece increases, and steel leakage is likely to occur.

In addition, when the surface roughness of the lower surface of the test cast piece calculated as described above is less than 60 μm, it is determined that the molten steel having the same composition as the steel sample S is not a crack risk steel grade (a steel grade in which the cast piece is not prone to cracking during continuous casting), and continuous casting is performed using a strong cooling mold slag suitable for increasing the casting speed of continuous casting. As the strong cooling mold slag, a strong cooling mold slag containing SiO ₂ and CaO as main components, a mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) of 0.7 or more and less than 1.0, and a crystallization temperature of less than 1100°C can be used.

The reason for setting the composition of the mold flux as described above is as follows. When the mass ratio of CaO to _SiO2 (CaO/ _SiO2 ) is 1.0 or more, the amount of spar precipitated in the slag film increases, and the crystallization temperature becomes too high. Therefore, the mold flux is given a slow cooling function, and the casting speed needs to be reduced. In addition, when the mass ratio of CaO to _SiO2 (CaO/ _SiO2 ) is less than 0.7, the melting point of the mold flux rises, the amount of inflow into the mold decreases, and there is a risk of sticking breakout.

Example

Steel types a to d (medium carbon steel) shown in Table 1 were respectively subjected to 1 to 2 times of feeding and melting by a converter and vacuum degassing equipment (secondary refining), and the steel was poured into a water-cooled mold of a vertical bending type continuous casting machine through a tundish. Then, while strongly cooled mold slag A or slowly cooled mold slag B having the composition shown in Table 2 was supplied to the surface of the molten steel in the mold, continuous casting was performed at the casting speed shown in Table 3 to produce cast pieces.

The surface of each cast piece obtained as a result of the above was visually observed to confirm whether the surface cracks of the cast piece occurred. Specifically, the length of the cracks was measured, and when cracks with a length of 10 mm or more were confirmed, it was determined that the surface cracks of the cast piece occurred.

At the same time, by the continuous casting method of steel of the present invention, it was determined whether steel types a to d were crack risk steel types (inventive example) based on whether the surface roughness of the lower surface of the test cast piece was 60 μm or more. In addition, by the method disclosed in the above-mentioned non-patent document 2, it was determined whether steel types a to d were crack risk steel types (comparative example).

In the present invention, a sampler is used to collect molten steel from a ladle into which molten steel to be continuously cast is added, and a test slab is made from the molten steel. The height of the concavities and convexities on the lower surface of the test slab is measured, and the surface roughness is calculated using the arithmetic mean height Sa specified in ISO25178.

In the comparative example, as disclosed in Non-Patent Document 2, the carbon concentration lower limit (C _a ) (mass %) and carbon concentration upper limit (C _b ) (mass %) in the hypoperitectic region of each of steel types a to d were obtained by the following equations (4) and (5).

_Ca ＝0.0896+0.0458×Al-0.0205×Mn-0.0077×Si+0.0223× ^Al2-0.0239 ×Ni+0.0106×Mo+0.0134×V-0.0032×Cr+0.00059× ^Cr2 +0.0197×W···(4)

C _b = 0.1967 + 0.0036 × Al - 0.0316 × Mn - 0.0103 × Si + 0.14 × 11 Al ² + 0.05 × (Al × Si) - 0.0401 × Ni + 0.03255 × Mo + 0.0603 × V + 0.0024 × Cr + 0.00142 × Cr ² - 0.00059 × (Cr × Ni) + 0.0266 × W ··· (5)

In the formula (4) and the formula (5), Al, Mn, Si, Ni, Mo, V, Cr and W are the contents (mass %) of the above elements.

Then, the carbon equivalent Cp0 (mass %) was calculated from the carbon concentration lower limit (Ca) (mass %) and carbon concentration _upper limit ( _Cb ) (mass %) and the carbon concentration C (mass %) of each of steel types a to d by the following formula (6).

C _p0 = 0.17 + {(CC _a )/(C _b -C _a )} × (0.17 - 0.09) (6)

In the comparative example, when the carbon equivalent C _p0 was within the range of 0.09 to 0.17 mass %, the steel was determined to be in the hypoperitectic region and to be a crack risk steel.

[Table 1]

Steel Type C (mass %) Si (mass%) Mn (mass%) P (mass %) S (mass %) a 0.12 1.20 2.4 0.016 0.0013 b 0.08 0.01 0.3 0.010 0.0139 C 0.08 1.0 2.2 0.009 0.0009 d 0.07 0.01 2.2 0.011 0.0010

[Table 2]

[table 3]

The surface roughness Sa of the test castings of steel types a and b is greater than 60 μm, and they are judged as steel types with a risk of cracking in the present invention. Based on this judgment, it is confirmed that if slow cooling mold slag B is used and the casting speed Vc is set to 1.6 m/min for continuous casting, the cracking of the casting can be suppressed. On the other hand, the carbon equivalent _Cp of steel types a and b calculated by the above formula (6) is outside the range of 0.09 to 0.17 mass%, and in the comparative example, steel types a and b are judged as steel types with no risk of cracking. Based on this judgment, it is confirmed that when strong cooling mold slag A is used and the casting speed Vc is set to 2.0 m/min for continuous casting, the casting will crack.

In addition, the surface roughness Sa of the test castings of steel types c and d is less than 60 μm, and they are judged as non-fracture risk steel types in the present invention example. Based on this judgment, when the strong cooling mold slag A is used and the casting speed Vc is set to 2.0 m/min for continuous casting, the casting does not break, and productivity can be improved without reducing the casting speed Vc. On the other hand, the carbon equivalent _Cp of steel types c and d calculated by the above formula (6) is in the range of 0.09 to 0.17 mass%, and in the comparative example, steel types c and d are judged as fracture risk steel types. Based on this judgment, it is necessary to use the slow cooling mold slag B and set the casting speed Vc to 1.6 m/min for continuous casting. However, as described above, for steel types c and d, even if the strong cooling mold slag A is used and the casting speed Vc is set to 2.0 m/min for continuous casting, the casting does not break, and it is confirmed that if the slow cooling mold slag B is used and the casting speed Vc is reduced based on the judgment of the comparative example, the productivity will be unnecessarily damaged.

Claims (12)

1. A method of continuously casting steel, the method comprising:

A test cast piece is produced by injecting molten steel to be continuously cast into a test solidification apparatus and cooling the molten steel, the injection speed of the molten steel when the molten steel is injected into the test solidification apparatus is 3 times or more the solidification speed of the molten steel, the injection speed is in kg/s, the solidification speed is in kg/s,

The surface roughness of the lower surface of the test piece was measured,

The continuous casting is performed using slow cooling mold flux suitable for preventing breakage of a cast piece at the time of continuous casting of the molten steel in the case where the surface roughness is a given threshold or more, and the continuous casting is performed using strong cooling mold flux suitable for increasing the casting speed of continuous casting in the case where the surface roughness is less than a given threshold,

The threshold value is 60 μm in terms of the arithmetic mean height of the surface roughness obtained by the method specified in ISO 25178.

2. The continuous casting method of steel according to claim 1, wherein,

The slow cooling mold flux contains SiO ₂ and CaO as main components, the mass ratio CaO/SiO ₂ of CaO to SiO ₂ is more than 1.0 and less than 2.0, the crystallization temperature is more than 1100 ℃, and the gun spar is crystallized as primary crystal.

3. The continuous casting method of steel according to claim 1 or 2, wherein,

The strongly cooling mold flux contains SiO ₂ and CaO as main components, the mass ratio CaO/SiO ₂ of CaO to SiO ₂ is more than 0.7 and less than 1.0, and the crystallization temperature is less than 1100 ℃.

4. The continuous casting method of steel according to claim 1 or 2, wherein,

The test solidification device has a cooling capacity such that the cooling rate at a depth of 1mm from the surface layer of the solidification shell of the molten steel is 10 ²~10⁵ ℃/min.

5. The continuous casting method of steel according to claim 1 or 2, wherein,

The test solidification device has a bottom surface with a width and a depth of 10mm or more.

6. A method of continuously casting steel, the method comprising:

a test cast piece is produced by injecting molten steel to be continuously cast into a test solidification apparatus and cooling the molten steel,

The surface roughness of the lower surface of the test piece was measured,

For a plurality of kinds of molten steel M having the surface roughness of a predetermined threshold value or more, the influence coefficients alpha _a,M、α_b,M of the components of the molten steel M on a carbon concentration lower limit value C _a and a carbon concentration upper limit value C _b of a sub-peritectic region on a Fe-C binary system equilibrium state diagram are respectively obtained, the carbon concentration lower limit value C _a is expressed by mass percent, the carbon concentration upper limit value C _b is expressed by mass percent,

Calculating the sum of the influence coefficients α _a,M、α_b,M of the molten steel M, obtaining a carbon concentration lower limit value C _a and a carbon concentration upper limit value C _b of the peritectic region of the molten steel M, the carbon concentration lower limit value C _a being represented by mass%, the carbon concentration upper limit value C _b being represented by mass%, by the following formulas (1) and (2),

The carbon concentration lower limit value C _a and the carbon concentration upper limit value C _b of the peritectic region of the new molten steel are obtained by the following formulas (1) and (2) based on the components of the new molten steel different from the plurality of molten steels M, the carbon equivalent C _p of the new molten steel is obtained by the following formula (3) based on the obtained carbon concentration lower limit value C _a, the carbon concentration upper limit value C _b and the carbon concentration C of the new molten steel, the carbon concentration C of the new molten steel is expressed as mass%, the carbon equivalent C _p of the new molten steel is expressed as mass%,

Continuous casting of the new molten steel using slow cooling mold flux suitable for preventing breakage of a cast piece at the time of continuous casting of the new molten steel in the case where the carbon equivalent C _p is in the range of 0.09 to 0.17, and continuous casting of the new molten steel using strong cooling mold flux suitable for increasing the casting speed of continuous casting in the case where the carbon equivalent C _p is not in the range of 0.09 to 0.17,

C_p＝0.09+{(C-C_a)/(C_b-C_a)}×(0.17-0.09) ···(3)

The threshold value is 60 μm in terms of the arithmetic mean height of the surface roughness obtained by the method specified in ISO 25178.

7. The continuous casting method of steel according to claim 6, wherein,

The slow cooling mold flux contains SiO ₂ and CaO as main components, the mass ratio CaO/SiO ₂ of CaO to SiO ₂ is more than 1.0 and less than 2.0, the crystallization temperature is more than 1100 ℃, and the gun spar is crystallized as primary crystal.

8. The continuous casting method of steel according to claim 6 or 7, wherein,

The strongly cooling mold flux contains SiO ₂ and CaO as main components, the mass ratio CaO/SiO ₂ of CaO to SiO ₂ is more than 0.7 and less than 1.0, and the crystallization temperature is less than 1100 ℃.

9. The continuous casting method of steel according to claim 6 or 7, wherein,

The test solidification device has a cooling capacity such that the cooling rate at a depth of 1mm from the surface layer of the solidification shell of the molten steel is 10 ²~10⁵ ℃/min.

10. The continuous casting method of steel according to claim 6 or 7, wherein,

The test solidification device has a bottom surface with a width and a depth of 10mm or more.

11. A steel test solidification apparatus for producing a test cast piece by injecting molten steel and cooling, which is used for the continuous casting method of steel according to any one of claims 1 to 10,

The steel test solidification device is provided with a casting mold with a cooling speed of 10 ²~10⁵ ℃/min at a depth of 1mm from the surface layer of the solidification shell of the injected molten steel,

The steel test solidification device further comprises an injection device for injecting the molten steel into the casting mold, wherein the injection speed of the molten steel by the injection device is more than 3 times of the solidification speed of the molten steel in the casting mold, the injection speed is in kg/s, and the solidification speed is in kg/s.

12. The test solidification device of steel according to claim 11, wherein,

The mold has a bottom surface with a width and a depth of 10mm or more, respectively.