CN116057195A - Continuous casting method for steel and test solidification device for steel - Google Patents

Abstract

The invention can simply judge whether the molten steel to be continuously cast is a fracture risk steel grade, and optimize the operation condition of continuous casting based on the judgment, thereby preventing the fracture of cast pieces and the bad condition of continuous casting, and realizing the improvement of productivity. 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 cast piece is measured, when the surface roughness is equal to or higher than a predetermined threshold value, it is determined that the cast piece is a steel type which is likely to break when the molten steel is continuously cast, the continuous casting is performed using slow cooling mold flux suitable for preventing the break, and when the surface roughness is lower than the predetermined threshold value, it is determined that the cast piece is not likely to break when the molten steel is continuously cast, and the continuous casting is performed using strong cooling mold flux suitable for increasing the casting speed of the continuous casting.

Description

Continuous casting method for steel and test solidification device for steel

technical field

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

Background technique

When continuously casting a hypoperitectic medium carbon steel with a C content of 0.09 to 0.17% by mass, cracks tend to occur on the surface of the cast slab. Specifically, due to the solidification shrinkage caused by the δ-γ phase transformation at the time of solidification on the molten steel side of the solidification shell, the portion with a high cooling rate in the solidification shell is warped protrudingly relative to the surface of the mold, and the surface of the slab Asperities are produced, and uneven growth occurs in the solidified shell. In the concave portion on the surface of the cast slab, the thermal resistance increases due to the air gap, and the thickness of the solidified shell becomes small. As a result, strain occurs in the solidified shell, and solidification cracks occur on the surface of the cast slab. This solidification crack expands during the secondary cooling of the continuous casting, and grows into a longitudinal crack and a transverse crack. When the degree of solidification cracking of the cast slab is large, there is a risk of breakout due to the cracking.

Therefore, the following operations are usually carried out in the continuous casting process: for steel grades in the sub-peritectic carbon region that are prone to solidification and cracking during primary cooling in the mold (hereinafter referred to as "cracking risk steel grades"), slow cooling mold powder is used. Slow cooling in the mold is realized, thereby preventing the cracking of the cast slab and the occurrence of breakouts.

If continuous casting is performed using slowly cooled mold flux, the thickness of the solidified shell in the mold becomes smaller, so the risk of breaking the solidified shell right below the mold and causing breakouts increases. Therefore, in the case of using slow cooling mold flux, it is necessary to reduce the casting speed of continuous casting so that the thickness of the solidified shell inside the mold does not decrease.

For steel grades other than crack risk steel grades, in the case of continuous casting using slow cooling mold flux unnecessarily, the casting speed of continuous casting still has to be reduced, and the productivity of continuous casting decreases. Therefore, from the viewpoint of preventing slab cracking and continuous casting failures and improving productivity, it is important to properly determine whether the molten steel is a cracking risk steel grade, and to use it slowly only for the cracking risk steel grade. Cool the mold flux for continuous casting.

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

As described above, when continuous casting of crack risk steel grades is performed, irregularities are formed on the surface of the cast slab. As an index for evaluating the unevenness, for example, the shape of the unevenness on the surface of the slab, such as the depth of vibration marks, can be used. The vibration marks of the cast piece are formed by mold powder being pressed into the cast piece when the mold is lowered, and its depth is increased by the solidification shrinkage that occurs inside the solidified shell, so if the conditions of continuous casting are the same, there is a risk of cracking The depth of the vibration marks of the steel type becomes larger.

Patent Document 1 discloses a method of measuring the depth of vibration marks on-line to prevent the occurrence of breakouts in cast slabs. Specifically, at the position downstream of the casting mold, the laser rangefinder installed opposite to the thickness surface of the slab continuously detects the surface profile of the slab, and when the measured depth of the sag is greater than the reference value, it is determined that there is a In order to avoid the hidden danger of cracked steel breakout in the cast slab, the operating conditions should be changed.

In addition, Non-Patent Document 1 discloses a method that includes: immersing a water-cooled plate in molten steel off-line, forming a solidified shell on the plate, and directly measuring the difference in thickness and the interval between the concavo-convex portions of the solidified shell. The inhomogeneity of the solidified shell was evaluated.

In addition, Non-Patent Document 2 discloses a method of predicting whether or not the alloy is a crack risk steel grade based on the alloy composition. Specifically, for various steel grades, a thermodynamic program was used to calculate a simulated Fe-C binary system equilibrium state diagram as a function of carbon concentration. Then, according to the hypoperitectic regions in these simulated Fe-C binary system equilibrium state diagrams, the carbon concentration lower limit (C _a ) and carbon concentration upper limit (C _b ) of the subperitectic regions are based on other alloys Compositional changes were formulated. According to whether the carbon concentration of the steel grade is within the range of C _a to C _b , it is determined whether the molten steel is a fracture risk steel grade.

prior art literature

patent documents

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

non-patent literature

Non-Patent Document 1: Hiroshi Murakami and others, "Control of Non-uniform Solidification of Hypoperitectic Carbon Steel in a Continuous Casting Mold", Iron and Steel (鉄と钢), 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 Katashi and 2 others, "Effect of Mold Flux on Initial Solidification of Hypoperitectic Steel in Continuous Casting Mold", Iron and Steel (鉄と钢), 2014, Vol.100, No.4,pp.581-590

Contents of the invention

The problem to be solved by the invention

However, in the method disclosed in Patent Document 1, it is difficult to change the type of mold flux according to the depth of vibration marks measured in continuous casting to prevent the occurrence of slab cracking. There is a possibility that measures for preventing the occurrence of slab cracking may not be timely.

In addition, in the method disclosed in Non-Patent Document 1, the test of forming a solidified shell on the plate by immersing the water-cooled plate in molten steel is complicated, so it is not suitable for measuring the unevenness of the solidified shell for many types of steel. evaluate.

In addition, in the method disclosed in Non-Patent Document 2, it may not always be possible to appropriately determine a steel type with a crack risk for a steel type that is empirically known to cause longitudinal cracking or transverse cracking.

The present invention has been made to solve the above problems. That is, the subject of the present invention is to provide a continuous casting method for steel and a test solidification device for steel, which consider that the subperitectic region of molten steel to be continuously cast is changed by the influence of alloy components, and can easily determine whether the continuous casting is to be performed. Whether the molten steel is a crack risk steel grade, and based on this, optimize the operating conditions of continuous casting, thereby preventing the cracking of cast slabs and the occurrence of continuous casting failures, and improving productivity.

way of solving the problem

In view of the above-mentioned problems, the present inventors have carried out in-depth research and development from a unique point of view. As a result, it has been found that by making a test cast piece from molten steel and evaluating its surface roughness, it is possible to easily and accurately predict whether the molten steel is The present invention has been accomplished thus for crack risk steel grades.

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

[1] A continuous casting method for steel, the method comprising: making a test cast piece by injecting molten steel intended to be continuously cast into a test solidification device and cooling it, and adjusting the surface roughness of the lower surface of the test cast piece It is measured that when the above-mentioned surface roughness is above a given threshold value, the above-mentioned continuous casting is carried out using slowly cooling mold flux suitable for preventing the slab cracking when the above-mentioned molten steel is continuously cast, and the above-mentioned surface roughness is less than In the case of a given threshold value, the above-mentioned continuous casting is carried out using strong cooling mold flux suitable for increasing the casting speed of continuous casting.

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

The said threshold value is 60 micrometers by the arithmetic mean height of the surface roughness obtained by the method prescribed|regulated by ISO25178.

[3] A method for continuous casting of steel, the method comprising: making a test cast piece by injecting molten steel intended for continuous casting into a test solidification device and cooling it, and adjusting the surface roughness of the lower surface of the test cast piece Measurement is performed to obtain the carbon concentration of the composition of the molten steel M with respect to the subperitectic region on the Fe-C binary system equilibrium state diagram for the plurality of types of molten steel M whose surface roughness is equal to or greater than a predetermined threshold value. Influence coefficients α a _,M and α _b,M of the lower limit value C _a (mass %) and the upper limit value of carbon concentration C _b (mass %), and calculate the above-mentioned influence coefficients α _{a,M of the above-mentioned molten steel M} , α _{b, the sum of M} , the carbon concentration lower limit C _a (mass %) and the carbon concentration upper limit of the sub-peritectic region of a plurality of above-mentioned molten steel M are obtained by the following formula (1) and formula (2) The value C _b (mass %) is obtained from the above-mentioned carbon concentration in the hypoperitectic region of the above-mentioned new molten steel according to the following formula (1) and formula (2) from the composition of the new molten steel different from the above-mentioned plurality of kinds of the above-mentioned molten steel M The lower limit value C _a and the upper limit value C _b of the above-mentioned carbon concentration are based on the obtained lower limit value C _a of the above-mentioned carbon concentration, the upper limit value C _b of the above-mentioned carbon concentration, and the carbon concentration C (mass %) of the above-mentioned new molten steel Obtain the carbon equivalent C _p (mass %) of above-mentioned new molten steel by following formula (3), under the situation that above-mentioned carbon equivalent C _p is in the scope of 0.09～0.17, use suitable Slow cooling mold flux for slab cracking during continuous casting Perform continuous casting of the above-mentioned new molten steel, and if the above-mentioned carbon equivalent C _p is not in the range of 0.09 to 0.17, use strong cooling suitable for increasing the casting speed of continuous casting Mold slag carries out the above-mentioned continuous casting of above-mentioned new molten steel,

[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 above slow cooling mold flux contains SiO ₂ and CaO as main components, the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) 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 flux 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 of 10 ² to 10 ⁵ °C/min at a cooling rate at a depth of 1 mm from the surface layer of the solidified shell of the molten steel.

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

The injection rate (unit: kg/s) when injecting the above-mentioned molten steel into the above-mentioned test solidification device is more than 3 times the solidification rate (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 said test coagulation apparatus has the bottom surface whose width and depth are each 10 mm or more.

[9] A test solidification device for steel, which is a test solidification device for making test cast steel by injecting molten steel and cooling it,

The experimental solidification apparatus for the above-mentioned steel was equipped with 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 comprising an injection device for injecting the molten steel into the casting mold, and the injection rate (unit: kg/s) of the molten steel by the injection device is as follows: The solidification rate (unit: kg/s) of the above-mentioned molten steel in the mold is more than 3 times.

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

The said mold has the bottom surface whose width and depth are each 10 mm or more.

The effect of the invention

According to the continuous casting method of steel and the test solidification device of steel according to the present invention, the surface roughness or carbon equivalent of the lower surface of the test slab produced by pouring molten steel to be continuously cast into the test solidification device and cooling it is used. , it can be easily determined whether the molten steel is a steel type that tends to crack the slab during continuous casting.

Furthermore, when it is determined that the slab is prone to cracking, continuous casting using slow cooling mold flux suitable for preventing cracking can reliably prevent the slab from cracking and breakout. In addition, in the case of steel grades that are determined to be less prone to cracking, continuous casting can be performed using strongly cooled mold flux suitable for increasing the casting speed of continuous casting, so that the productivity of continuous casting can be improved without reducing the casting speed.

Description of 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 an example of the surface roughness of the lower surface of the test cast piece produced by the test solidification apparatus of steel according to the present invention.

Symbol Description

1 Test solidification device for steel

2 molds

21 Bottom

3 injection device

31 Crucible

32 high frequency induction coil

33 tilting table

W width

D depth

H height

S steel sample (liquid steel)

Detailed ways

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

＜Steel experimental solidification equipment＞

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

As shown in FIG. 1 , a test solidification apparatus 1 for steel according to this embodiment includes: a casting mold 2 for producing test slabs by injecting molten steel S and cooling it to solidify; and an injection device for injecting molten steel S into the casting mold 2 3.

The mold 2 is a substantially rectangular parallelepiped copper container, and a water cooling device (not shown) is provided on the bottom surface 21 thereof. The thickness of the 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 mold 2 and cooled and solidified, the bottom surface 21 side of the mold 2 cooled by the water cooling device is separated from the solidification shell. The cooling rate at a depth of 1 mm in the surface layer is 10 ² -10 ⁵ °C/min.

In the present invention, the shape of the mold 2 of the test solidification device 1 is not particularly limited, but the width W and the depth D of the bottom surface 21 of the mold 2 are preferably 10 mm or more, and more preferably the width W and the depth D are 40 mm or more and 60 mm or less. This is based on the fact that it is known that the size of the lower surface of the test cast piece made by the test solidification device 1 is the same size as the bottom surface 21 of the mold 2, and the surface roughness of the lower surface of the test cast piece as described later When the degree is measured, the interval between the concavities and convexities that can be confirmed by naked eyes 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 cast piece is preferably less than 30 μm in terms of the arithmetic mean height of the surface roughness obtained by the method specified in ISO25178 "Three-dimensional Surface Texture (Surface Roughness)". This is because, as will be 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 ISO 25178, the shape of the bottom surface 21 of the mold 2 will vary. The surface roughness of the lower surface of the test cast piece was affected.

The injection device 3 includes a bottomed cylindrical crucible 31 made of Al ₂ O ₃ or MgO, and a high-frequency induction coil that covers the outer periphery of the crucible 31 and heats and melts the content in the crucible 31 32. A tilting table 33 for pouring the melt in the crucible 31 into the mold 2 by tilting the crucible 31 in a fixed state, 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 temperature and displays it.

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

＜Preparation of test slabs＞

In this embodiment, a steel sample (molten steel) S having the same composition as the target composition of the molten steel to be continuously cast is put into the crucible 31 , and the crucible 31 is fixed on the tilt table 33 . Furthermore, a high-frequency induction coil 32 was installed so as to cover the outer periphery of the crucible 31, and the steel sample S in the crucible 31 was heated and melted. At this time, the heating of the steel sample S is continued until the operator confirms that the steel sample S has melted through visual observation, and confirms that the temperature of the molten steel sample S displayed by the temperature display device has reached 1590-1610°C. within the range of °C. Here, the output values from the above thermocouples may be input into a computer to automatically determine whether the temperature of the molten steel sample S has reached the range of 1590 to 1610° C., instead of visual inspection by the operator.

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

This cooling rate is based on the report in Non-Patent Document 3 that when continuous casting of crack risk steel grades is carried out by a practical continuous casting machine, the occurrence of uneven solidification becomes conspicuous at the stage where the thickness of the solidified shell exceeds 1 mm. 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 apparatus 1, the cooling rate at the position where occurrence of uneven solidification becomes conspicuous in a practical continuous casting machine was reproduced.

In addition, if the tilting speed of the tilting table 33 is coordinated with the operation of the above-mentioned water cooling device, the injection rate (unit: kg/s) of the steel sample S injected into the mold 2 by using the tilting table 33 is set to be If the solidification rate of molten steel S (unit: kg/s) is more than 3 times, then when the molten steel S is in the subperitectic region, it is easy to produce unevenness on the surface of the solidified shell, and it can be judged with better accuracy whether it is Crack risk steel grades are therefore preferred.

An example of the lower surface of a test slab produced in this way by means of the test solidification device 1 is shown in photographic form in FIG. 2 . FIG. 2( a ) is an example of a case where the steel sample S is a fracture-risk steel type, and FIG. 2( b ) is an example of a case where it is not a fracture-risk steel type. In the case of the steel sample S being a fracture-risk steel type, it was clearly confirmed that unevenness was generated on the lower surface of the test cast piece.

＜Preparation of test slabs using molten steel from the steelmaking process＞

In the actual steelmaking process, the composition of molten steel during continuous casting may deviate from the target value. Therefore, in order to improve the judgment accuracy of whether the molten steel is a fracture risk steel type, a sampler can be used to collect the molten steel from the ladle that is filled with the molten steel that wants to be continuously cast, and directly inject the molten steel into the mold 2 of the test solidification device 1 And it cooled, and the test cast piece was produced by this. In this case, if the sampler for collecting molten steel from the ladle has the function of the casting mold 2, it is not necessary to prepare the test solidification device 1 separately.

＜Measurement of Surface Roughness＞

Next, the height of the unevenness on the lower surface of the test cast piece produced as described above was measured with 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.

Examples of calculation conditions for the above-mentioned surface roughness include measurement and evaluation area, interval between measurement points, and magnitude of cutoff wavelength. In the continuous casting method for steel and the experimental solidification apparatus for steel of the present invention, the measurement and evaluation area, the interval between measurement points, and the size of the cutoff wavelength are not particularly limited, but are preferably as follows.

First, in measuring the evaluation area, the center thereof is defined as the center of the lower surface of the test slab, and the vertical and horizontal lengths are preferably 10 mm or more, more preferably 40 mm or more and 60 mm or less. This is based on the fact that it is known that the interval between the concavities and convexities that can be confirmed by naked eyes is in the range of 10 mm to 40 mm. The interval between measurement points is preferably 10 mm or less. The magnitude of the cutoff wavelength is preferably set to 800 μm.

<Determination of whether it is a steel grade with a risk of cracking>

Next, when the surface roughness (arithmetic mean height of the surface roughness) of the lower surface of the test slab calculated as described above is 60 μm or more, it is judged that the molten steel having the same composition as the steel sample S is Crack risk steel grades (steel grades that tend to crack slabs during continuous casting).

As mentioned above, for crack risk steel grades, due to the solidification shrinkage caused by the δ-γ phase transformation during solidification on the molten steel side of the solidification shell, the part where the cooling rate is high in the solidification shell protrudes from the surface of the mold The ground is warped, and unevenness occurs on the surface of the cast sheet. Therefore, the surface roughness of the test slab serves as an indicator of whether the molten steel having the same composition as the steel sample S is a fracture-risk steel type.

In addition, for a plurality of types of molten steel, the carbon equivalent can be established as follows using the results of determining whether each molten steel is a fracture risk steel grade based on whether the surface roughness of the test slab is equal to or greater than a predetermined threshold value The relational expression of C _p .

That is, in the case where the surface roughness of the test slab produced from the molten steel reaches a predetermined threshold value or more and is judged to be a crack risk steel grade, the Fe-C binary system balance of each component element of the steel grade M is obtained. Influence coefficients α _{a,M ,} α _b,M of the carbon concentration lower limit (C _a ) (mass %) and the carbon concentration upper limit (C _b ) (mass %) of the subperitectic region on the state diagram. Then, for a variety of steel types M, considering that the range of carbon concentration in the subperitectic region is affected by other alloy components, Ca _and C are established as shown in the following formulas (1) and (2). The relational expression of _b .

[mathematical formula 3]

[mathematical formula 4]

Then, when judging whether the new molten steel (target steel) is a fracture risk steel type, C _a and C _b are obtained by the above formula (1) and formula (2) according to the composition of the target steel, and according to the obtained C _a , C _b and the carbon concentration C (mass %) of the target steel are obtained from the following formula (3) to obtain the carbon equivalent C _p (mass %) of the target steel, instead of the surface roughness based on the test slab. The decision.

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

When the carbon equivalent C _p is in the range of 0.09 to 0.17% by mass, the target steel is in the subperitectic region, and can be determined as a fracture risk steel type.

＜Selection of Mold Flux＞

Next, based on the above-mentioned determination of whether the steel is a fracture risk steel grade, it is selected which of the slow cooling mold flux and the strong cooling mold flux is used for continuous casting.

The slow cooling effect of the solidified shell by the mold flux can be obtained by forming a slag film by cooling the powder slag (powder slag) that has flowed into the gap between the mold of the continuous casting machine and the solidified shell on the surface of the mold , heat transfer resistance increases due to crystallization in the slag film. The components of the mold flux are SiO ₂ and CaO as the main components, and Li ₂ O, Na ₂ O, F, MgO, Al ₂ O ₃ etc. added to adjust the viscosity of the mold flux and the precipitation of crystals. The usual seed crystals precipitated in the slag film are lanceolite (Cuspidine: 3CaO·2SiO ₂ ·CaF ₂ ).

In order to suppress the surface cracking of the cast slab, it is effective to realize the slow cooling of the solidified shell near the molten steel surface. Therefore, in order to impart the effect of suppressing longitudinal cracking to the mold slag, after the powder slag flows into the gap between the mold and the solidified shell, Crystals are precipitated instantly, and the solidified shell is cooled slowly.

It is believed that the mold flux with high crystallization temperature and the crystallization of lance spar as the primary crystal has the function of slowly cooling the mold. Therefore, for steels with a risk of cracking, using such slowly cooling mold flux and reducing the casting speed can Reliably prevent the occurrence of cracks and breakouts, and maintain productivity by not using slow-cooling mold powder and reducing casting speed for steel grades that are not at risk of cracking.

Specifically, when the surface roughness of the lower surface of the test slab calculated as described above is 60 μm or more, it is determined that the molten steel having the same composition as the steel sample S is a crack risk steel type, and a suitable Continuous casting with slow cooling mold flux to prevent cracking. Specifically, as the slow cooling mold flux, one 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 The lance spar is crystallized as the slow cooling mold flux of the primary crystal.

The reason for setting the constituent components of the mold flux as described above is as follows. When the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) is less than 1.0, the precipitation of lancet spar in the slag film is insufficient, and the crystallization temperature becomes too low. Cracked slow cool down feature. In addition, when the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) is 2.0 or more, the crystallization temperature of the mold flux rises, the crystallization of the mold flux is excessively promoted, the friction between the mold and the cast piece increases, and breakouts are likely to occur.

In addition, when the surface roughness of the lower surface of the test slab 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 fracture risk steel type (the slab during continuous casting Steel grades that are not prone to cracking), continuous casting is carried out using strong cooling mold slag suitable for increasing the casting speed of continuous casting. As the strong cooling mold flux, strong cooling mold flux containing SiO ₂ and CaO as the 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 strong cooling mold flux with a crystallization temperature lower than 1100°C can be used .

The reason for setting the constituent components of the mold flux as described above is as follows. When the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) is 1.0 or more, the precipitation of lancet spar in the slag film increases, and the crystallization temperature becomes too high. Reduce casting speed. Also, when the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) is less than 0.7, the melting point of mold flux rises, the amount of inflow into the mold decreases, and sticking breakout may occur.

Example

Through the converter and vacuum degassing equipment (secondary refining), the steel types a~d (medium carbon steel) shown in Table 1 are smelted for 1~2 times respectively, and the steel is injected into the vertical bending type through the tundish. Water-cooled molds for continuous casting machines. Then, continuous casting was carried out at the casting speed shown in Table 3 while supplying strongly cooled mold flux A or slowly cooled mold flux B having the composition shown in Table 2 to the surface of the molten steel in the mold, and cast slabs were manufactured. .

The surface of each slab obtained as the above-mentioned result was visually observed to confirm whether or not surface cracking of the slab occurred. Specifically, the length of the crack was measured, and when a crack with a length of 10 mm or more was confirmed, it was determined that the cast slab had surface cracking.

At the same time, by the continuous casting method of steel according to the present invention, it was determined whether the steel grades a to d were fracture risk steel grades (examples of the present invention) based on whether the surface roughness of the lower surface of the test slab was 60 μm or more. In addition, by the method disclosed in the above-mentioned Non-Patent Document 2, it was determined whether or not steel types a to d were crack risk steel types (comparative example).

In the example of the present invention, utilize sampler to collect molten steel from the ladle that adds the molten steel that wants to carry out continuous casting, make test cast piece by this molten steel, measure the height of the concavities and convexities of the lower surface of this test cast piece, The surface roughness of 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 lower limit value of the carbon concentration (C _a ) (mass %) and carbon concentration upper limit (C _b ) (mass %).

C _a =0.0896+0.0458×Al-0.0205×Mn-0.0077×Si+0.0223×Al ² -0.0239×Ni+0.0106×Mo+0.0134×V-0.0032×Cr+0.00059×Cr ² +0.0197×W ( 4)

C _b ＝0.1967+0.0036×Al-0.0316×Mn-0.0103×Si+0.14×11Al 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)+0.0266W···(5)

However, Al, Mn, Si, Ni, Mo, V, Cr, and W in the formulas (4) and (5) are the contents (% by mass) of the above elements.

Then, based on the carbon concentration lower limit (Ca) (mass %) and carbon concentration upper limit (C _b ) (mass %), and the carbon concentration C (mass %) of each of the steel types a to d, the following The carbon equivalent C _p0 (mass %) was obtained from the formula (6).

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

In the comparative example, when the carbon equivalent C _p0 is in the range of 0.09 to 0.17% by mass, it is determined that the steel type is in the hypoperitectic region and is a fracture risk steel type.

[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 slabs of steel types a and b was 60 μm or more, and they were judged as crack risk steel types in the examples of the present invention. Based on this determination, it was confirmed that cracking of the cast slab can be suppressed when continuous casting is performed using the mold flux B which is slowly cooled and the casting speed Vc is set to 1.6 m/min. On the other hand, when the carbon equivalent _Cp of the steel types a and b obtained by the above formula (6) is outside the range of 0.09 to 0.17% by mass, in the comparative example, the steel types a and b are judged to have no risk of cracking. steel type. Based on this determination, it was confirmed that when continuous casting was performed using the strongly cooled mold flux A and the casting speed Vc was set to 2.0 m/min, the slabs were cracked.

In addition, the surface roughness Sa of the test slabs of steel types c and d was less than 60 μm, and was judged not to be a crack risk steel type in the example of the present invention. Based on this determination, when continuous casting was performed using the strongly cooled mold flux A and the casting speed Vc was set to 2.0 m/min, the slabs were not cracked, and productivity could be improved without lowering the casting speed Vc. On the other hand, the carbon equivalents C _p of the steel types c and d obtained by the above formula (6) are in the range of 0.09 to 0.17% by mass, and in the comparative example, the steel types c and d are judged to be fracture risk steel types . Based on this determination, it is necessary to perform continuous casting using slowly cooling mold flux B and setting the casting speed Vc to 1.6 m/min. However, actually, as mentioned above, for steel types c and d, even if continuous casting is carried out using strongly cooled mold flux A at a casting speed Vc of 2.0 m/min, the cast slabs do not break. In the judgment of the comparative example, if the mold flux B is slowly cooled and the casting speed Vc is lowered, the productivity will be unnecessarily impaired.

Claims (11)

1. A continuous casting method for steel, the method comprising: Test slabs are produced by pouring molten steel to be continuously cast into a test solidification device and cooling it, The surface roughness of the lower surface of the test slab was measured, In the case where the surface roughness is above a predetermined threshold value, the continuous casting is performed using slowly cooled mold flux suitable for preventing slab cracking during continuous casting of the molten steel, wherein the surface roughness If it is less than a given threshold, the continuous casting is carried out using strong cooling mold flux suitable for increasing the casting speed of continuous casting. 2. The continuous casting method of steel according to claim 1, wherein, The threshold value is 60 μm in terms of the arithmetic mean height of the surface roughness obtained by the method prescribed in ISO25178. 3. A continuous casting method for steel, the method comprising: Test slabs are produced by pouring molten steel to be continuously cast into a test solidification device and cooling it, The surface roughness of the lower surface of the test slab was measured, For the plurality of molten steels M whose surface roughness is above a given threshold, the composition of the molten steel M is calculated for the carbon concentration in the sub-peritectic region on the Fe-C binary system equilibrium state diagram. Influence coefficients α _a,M , α _b,M of limit value C _a (mass %) and carbon concentration upper limit value C _b (mass %), Calculate the sum of the influence coefficients α _{a, M} , α _{b, M} of a variety of molten steel M, and obtain the subpackages of a variety of molten steel M by following formula (1) and formula (2) The carbon concentration lower limit C _a (mass %) and the carbon concentration upper limit C _b (mass %) of the crystal region, The carbon concentration lower limit _{C a} of the hypoperitectic region of the new molten steel is obtained by the following formula (1) and formula (2) based on the composition of the new molten steel different from the plurality of kinds of molten steel M. and the carbon concentration upper limit C _b , based on the obtained carbon concentration lower limit C _a , the carbon concentration upper limit C _b , and the carbon concentration C (mass %) of the new molten steel pass Following formula (3) obtains the carbon equivalent C _p (mass %) of described new molten steel, In the case where the carbon equivalent _Cp is in the range of 0.09 to 0.17, the continuous casting of the new molten steel is carried out using slowly cooling mold flux suitable for preventing the slab from cracking when the new molten steel is continuously cast , when the carbon equivalent C _p is not in the range of 0.09 to 0.17, the continuous casting of the new molten steel is carried out using strong cooling mold slag suitable for increasing the casting speed of continuous casting,

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 claims 1 to 3, wherein: The slow cooling mold flux contains SiO ₂ and CaO as main components, the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) is 1.0 or more and less than 2.0, the crystallization temperature is 1100° C. or more, and lanceolite is crystallized as Primary crystal. 5. The method for continuous casting of steel according to any one of claims 1 to 4, wherein: The strong cooling mold flux contains SiO ₂ and CaO as main components, the mass ratio of CaO to SiO ₂ (CaO/SiO ₂ ) is not less than 0.7 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 claims 1 to 5, wherein: The test solidification device has a cooling capacity of 10 ² to 10 ⁵ °C/min at a cooling rate at a depth of 1 mm from the surface layer of the solidification shell of the molten steel. 7. The method for continuous casting of steel according to any one of claims 1 to 6, wherein: The injection rate (unit: kg/s) when injecting the molten steel into the test solidification device is more than 3 times the solidification rate (unit: kg/s) of the molten steel. 8. The method for continuous casting of steel according to any one of claims 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 test solidification device for steel, which is a test solidification device for making test cast steel by injecting molten steel and cooling, The test solidification apparatus for steel includes a mold with a cooling rate of 10 ² to 10 ⁵ °C/min at a depth of 1 mm from the surface layer of the solidification shell of the injected molten steel. 10. The experimental solidification device for steel according to claim 9, further comprising an injection device for injecting the molten steel into the mold, and the injection speed (unit: kg/s) of the molten steel by the injection device It is more than 3 times the solidification rate (unit: kg/s) of the molten steel in the casting mold. 11. The experimental solidification device of steel according to claim 9 or 10, wherein, The casting mold has a bottom surface with a width and a depth of 10 mm or more, respectively.

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