KR101949351B1 - Method of producing continuous-cast steel product using continuous casting machine - Google Patents

Abstract

Provided is a casting method using a continuous casting machine capable of obtaining a cast steel in which surface cracks are suppressed without lowering productivity even when molten steel has a predetermined component composition.
The present invention relates to a method of casting a cast steel, comprising the steps of: first cooling molten steel containing 0.13 mass% or more and 0.20 mass% or more of C and 0.50 mass% or more of C in a mold; drawing the cast product from the mold at a drawing speed of 1.0 m / And a step of secondary cooling the cast slab, wherein the surface temperature of the cast slab is lower than the Ar ₃ transformation point and then returned to a temperature higher than the Ac ₃ transformation point, The negative strip time Tn is in the range of 0.08 second or more and 0.20 second or less and the negative strip time ratio R _NS is in the range of 0.30 or more and 0.38 or less and the surface temperature of the cast steel reaches the Ar ₃ transformation point The time is over 60 seconds.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a continuous casting machine,

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing a cast steel using a continuous casting machine.

In continuous casting of steel, prevention of surface cracking of the cast steel is very important in order to maintain good surface quality of the product after rolling the cast steel. Here, Patent Documents 1 to 3 are known as technologies for suppressing surface cracking of cast steel by making austenite grains in the crystal structure in the cast steel finer by using? -? Transformation in the secondary cooling step of cast steel. When the austenite grains are made finer, the surface area of the austenitic grains is relatively increased, and the stress acting on the austenitic grains of the cast steel during the casting of the continuous casting machine (upper correction, lower correction) So that surface cracks are less likely to occur. Further, when the austenite grains are small, cracks are hardly propagated even if cracks occur once.

Patent Literature 1 discloses that in the secondary cooling step of casting using a continuous casting machine of a curved or vertical bending type, the surface temperature of the cast steel is made lower than the Ar ₃ transformation point within 2 minutes after leaving the casting mold, A technique of preventing lateral cracking of the cast steel by restoring the cast steel to 850 DEG C or more is disclosed.

Patent Document 2 discloses a method in which, in a secondary cooling step of a casting using a curved or vertical bending type continuous casting machine, the casting is once cooled so that the surface temperature of the casting becomes lower than the A ₃ transformation point, the surface temperature of the cast steel is returned to a temperature higher than the A ₃ transformation point by performing mild cooling for 0.5 to 2.0 minutes at a density of 0.003 to 0.015 liters / .

Patent Document 3 discloses that continuous casting of low alloy carbon steels having a high crack susceptibility and a carbon equivalent Cp as defined by the following formula (1) of not less than 0.10 and less than 0.18 is performed by using a continuous casting machine of a curved or vertical bending type, The drawing time from the meniscus portion of the molten steel to the lower end of the cast steel is set to be within 1 minute, the secondary cooling is immediately performed, and the surface temperature of the cast steel is lowered to the A ₃ transformation point or less within 1 minute, A technique for preventing surface cracking of the cast steel is described.

Cp = [C] + [Mn] / 33 + [Ni] / 25 + [Cu] / 44 + [N] / 1.7. (One)

Cp represents carbon equivalent, and [] represents the content (mass%) of each element in the steel.

Japanese Patent Application Laid-Open No. 9-225607 Japanese Patent Application Laid-Open No. 11-197809 Japanese Patent Application Laid-Open No. 9-47854

The techniques of Patent Documents 1 to 3 prevent the surface cracks of the cast steel by controlling the casting temperature in the secondary cooling and making the austenite grains in the cast steel micro-fine.

In recent years, it has been demanded to increase the drawing speed of the casting from the mold, from the request to increase the productivity. However, if the drawing speed is increased, the surface crack of the casting is likely to occur. According to the study by the inventors of the present invention, in the case of continuous casting of a high-C high Mn steel containing 0.13 mass% or more and 0.20 mass% or more of Mn and 0.50 mass% or more of Mn at a drawing speed of 1.0 m / min or more, It has been found that the techniques of Patent Documents 1 to 3 do not sufficiently prevent surface cracks such as lateral cracks and longitudinal cracks of cast steel.

Even in the case of such a high-C and high-Mn steel having a carbon equivalent Cp of not less than 0.10 and less than 0.18, the surface cracks sometimes occur even when the surface temperature of the cast steel is controlled as in Patent Document 3. Further, even in the case of a steel having a carbon equivalent Cp of more than 0.18, surface cracks sometimes occurred in the case of a high-C high Mn steel as described above.

In view of the above problems, the present invention provides a casting method using a continuous casting machine capable of obtaining a cast steel in which surface cracks are suppressed without lowering the productivity even when molten steel has the predetermined compositional composition .

Means for Solving the Problem As a result of intensive studies to solve the above problems, the present inventors have found that a high-C high Mn steel containing 0.13 mass% or more and 0.20 mass% or more of Mn and 0.50 mass% or more of Mn at a drawing rate of 1.0 m / In continuous casting, it has been found that surface cracks such as lateral cracks and longitudinal cracks can be sufficiently prevented by simultaneously satisfying the following two conditions.

[1] It is insufficient to make the austenite grains finer by the secondary cooling as in Patent Documents 1 to 3, and the time from the passing of the cast slab through the casting mold to the surface temperature of the cast steel becomes the Ar ₃ transformation point is set to exceed 60 seconds Needs to be.

[2] The optimization of the secondary cooling conditions is insufficient, and it is necessary to optimize the conditions of the primary cooling (initial solidification in the mold). That is, as the vibration condition of the mold, it is necessary to set the negative strip time Tn and the negative strip time ratio R _NS to a predetermined numerical value range.

The present invention has been completed by the above recognition and conception. That is, the present invention provides a casting method using a continuous casting machine of a curved or vertical bending type,

C: 0.13 mass% or more and 0.20 mass% or less, and Mn: 0.50 mass% or more,

A step of drawing the cast steel from the casting mold at a drawing speed of 1.0 m / min or more,

A step of secondary cooling the cast slab, wherein the surface temperature of the cast slab is lower than the Ar ₃ transformation point and thereafter the temperature is returned to a temperature higher than the Ac ₃ transformation point,

Lt; / RTI &

The vibration condition of the mold satisfies the condition that the negative strip time Tn is 0.08 second or more and 0.20 second or less and the negative strip time ratio R _NS is 0.30 or more and 0.38 or less,

When the time from the passing of the cast strip through the mold to the surface temperature of the cast steel reaches the Ar ₃ transformation point is more than 60 seconds

.

According to the casting method using the continuous casting machine of the present invention, even when the molten steel has the predetermined composition, the cast steel can suppress the surface cracks without lowering the productivity.

1 is a schematic view of a vertical bending type continuous casting machine used in an embodiment of the present invention.
Fig. 2 is a cross-sectional view in the casting direction of the casting of the casting machine taken out from the casting mold in the vertical bending type continuous casting machine shown in Fig. 1;
Fig. 3 is a cross-sectional view in the casting direction of a cast steel casting from a mold in a curved continuous casting machine used in another embodiment of the present invention. Fig.
Fig. 4 is a perspective view showing the lateral cracks and longitudinal cracks occurring on the surface of the cast steel. Fig.

(Mode for carrying out the invention)

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

First, the configuration of a two-strand type vertical bending type continuous casting machine 100 used in an embodiment of the present invention will be described. The continuous casting machine 100 includes a ladle 10, a tundish 11, a mold 12, a spray nozzle 13, a plurality of rolls 14, a cutting device 15, and an electromagnetic stirring device 16 ).

In the ladle 10 located at the top of the continuous casting machine, molten steel M is accommodated. The molten steel M is poured from the bottom of the ladle 10 into the tundish 11 located below the ladle 10. [ Thereafter, the molten steel M is poured into the mold 12 from the bottom of the tundish 11 through the immersion nozzle, and primary cooling of the molten steel is performed in the mold 12.

In order to guide the cast steel S withdrawn from the casting mold 12 in the horizontal direction from the vertical direction and to prevent deformation of the cast steel S by ferrostatic pressure along a curve such as an arc or a hyperbola A plurality of pairs of rolls 14 are arranged. A part of the roll 14 has a function as a pinch roll for pulling out the slab S. 2, the cast steel S pulled out vertically downward from the casting 12 is bent in the upper calibrating table 20B after passing through the vertical table 20A, and in the curved table 20C, After the curved state is maintained, the lower calibrating table 20D is not bent in a flat plate shape and passes through the horizontal table 20E. There is an uncooled portion of the molten steel in the inside of the cast steel S from the bottom of the cast steel through the horizontal bar and the roll 14 is disposed so as to support the surface of the cast steel S from just below the casting mold to almost the entire length of the horizontal band. The spray nozzles 13 are positioned between the adjacent rolls in the casting direction and the cooling water is sprayed from the spray nozzles 13 to the cast steel S and secondary cooling of the cast steel is performed. Although a plurality of spray nozzles are actually arranged between the rolls, in FIG. 1, a part thereof is schematically represented by a line segment connecting a plurality of nozzles.

On the downstream side of the horizontal stand, there is provided a cutting device 15 such as a gas torch for cutting the solidified cast steel S, and a hydraulic cutter. The slab (slab, bloom, or billet) cut by the cutting device 15 is discharged from the continuous casting machine 100 and is conveyed to the rolling device.

Referring to Fig. 2, bending stress is applied to the cast steel S in the upper calibrating table 20B and the lower calibrating table 20D. By this bending stress, tensile stress is applied to the bottom side of the slab S in the upper calibrating table 20B and tensile stress is applied to the upper side of the slab S in the lower calibrating table 20D. As a result, as shown in Fig. 4, a lateral crack C ₁ may occur on the upper surface side or the lower surface side (mainly the corner portion) of the slab S (slab). As used herein, the term " transverse crack " means a surface crack along the direction perpendicular to the casting direction.

On the other hand, when the surface of the cast steel is subjected to severe cooling immediately after being pulled out from the casting mold 12, a vertical crack C ₂ is generated in the cast steel S (slab) as shown in FIG. 4 due to the unevenness of the solidified shell . As used herein, the term " longitudinal crack " means a surface crack of the cast steel along the casting direction.

One embodiment of the present invention relates to prevention of surface cracking of cast steel during continuous casting, and more particularly to a method of preventing lateral cracking and longitudinal cracking of cast steel (low alloy carbon steel of apolygeotate and superphosphate).

A curved continuous casting machine shown in Fig. 3 can also be used in the present invention. In the vertical bending type continuous casting machine, since the casting is pulled out vertically downward from the casting mold, the inner wall face of the casting mold 12 is flat. However, in the case of the curved continuous casting machine, the casting mold 21 is used because the casting S is pulled out from the casting mold into an arc shape. Since the inner wall surface of the mold 21 is curved, the curved main stanchion is fed out, and calibration is performed so as to eliminate bending in the lower calibrating table 20D. In the case of the curved type, there is no bending process in the upper calibration zone, unlike the case of the vertical bending type.

Next, the composition of molten steel will be described.

In the case of a steel having a carbon equivalent Cp defined by the above formula (1) of 0.10 or more and less than 0.18, an austenitic grain system having a cornerstone ferrite film is often clearly observed in the surface layer portion of the slab. When a tensile stress acts on the cast steel in such a state, as shown in Figs. 2 and 3, cracks easily occur in the austenitic system and transverse cracks occur. On the contrary, the surface of the poured cast steel is cooled to a temperature just below the casting to lower the surface temperature of the cast steel to a temperature lower than the Ar ₃ transformation point, and then to return to a temperature higher than 850 ° C or higher than the Ac ₃ transformation point to refine the austenite lips An effect of reducing lateral cracks can be obtained.

However, in the case of steel containing 0.13 mass% or more and 0.20 mass% or less of C and 0.50 mass% or more of Mn, there is a tendency as follows.

(A) The depth of the oscillation mark is deepened, the stress is concentrated on the concave portion of the mark, and horizontal cracks are likely to occur.

(B) the Ar ₃ transformation point and the Ac ₃ transformation point tend to decrease, and when the drawing speed is increased, the ductility improving effect by the slab steel cooling tends to be difficult to manifest.

(C) Even in the case of the same carbon equivalent, it is easy to embrittle over a wide temperature range, and the susceptibility to cracks is higher than that of a steel having a low C content or a low Mn content.

(D) The thickness of the solidified shell is uneven due to uneven cooling, and longitudinal cracks are likely to occur in the mold.

Therefore, as shown in the following examples, high-C high Mn steel containing 0.13 mass% or more and 0.20 mass% or more of Mn and 0.50 mass% or more of Mn is continuously cast at a drawing speed of 1.0 m / min or more It has been found that even when the surface temperature of the cast steel is controlled as described above, surface cracks such as lateral cracks and longitudinal cracks can not be sufficiently prevented.

As described above, the composition of the molten steel to which the present invention is applied is assumed to contain 0.13 mass% or more and 0.20 mass% or less of C and 0.50 mass% or more of Mn. When the C content is less than 0.13 mass% or the Mn content is less than 0.50 mass%, surface cracking can be sufficiently prevented by controlling the surface temperature of the cast steel as in the prior art, and the problem of the present invention is not realized. When the C content exceeds 0.20 mass%, the Ar ₃ transformation point and the Ac ₃ transformation point decrease, and the surface temperature of the cast steel is lower than the Ar ₃ transformation point in the vertical zone between the lower end of the casting mold and the upper calibrating belt in the continuous casting machine And thereafter, it may become difficult to perform the secondary cooling condition in the present embodiment in which the temperature is returned to a temperature higher than the Ac ₃ transformation point.

When the Mn content exceeds 2.5 mass%, the Ar ₃ transformation point and the Ac ₃ transformation point decrease in the same manner as in the case where the C content increases, and it may be difficult to perform the secondary cooling conditions in the present embodiment Further, MnS tends to precipitate easily, and the susceptibility to cracking tends to increase. Therefore, the Mn content is preferably 2.5% by mass or less.

The composition of the molten steel may optionally contain not more than 0.8 mass% Si, not more than 0.10 mass% P, not more than 0.05 mass% S, not more than 0.05 mass% Al, not more than 0.5 mass% Cu, not more than 1.0 mass% Ni, 0.6% by mass of Cr, 0.14% by mass or less of V, 0.09% by mass or less of Nb, 0.4% by mass or less of Ti and 0.02% or less by mass of N, It is an inevitable impurity.

Cast manufacturing method according to an embodiment of the present invention, in the step of cooling the first molten steel in the mold and 1.0m / min drawing speed above step, the surface temperature of the cast slab to a pull-out from the mold Ar ₃ And then returning the temperature to a temperature higher than the Ac ₃ transformation point. When the drawing speed is less than 1.0 m / min, surface cracking can be sufficiently prevented by controlling the surface temperature of the cast steel as in the prior art, and the problem of the present invention is not realized at present. The upper limit of the drawing speed is not particularly limited, but may be 2.5 m / min. At a drawing speed exceeding 2.5 m / min, the surface temperature of the cast steel is made lower than the Ar ₃ transformation point at the vertical stand between the lower end of the casting mold and the upper calibrating table in the continuous casting machine, and then the temperature is higher than the Ac ₃ transformation point It may be difficult to perform the secondary cooling condition in this embodiment.

In the primary cooling process, the mold vibrates at regular intervals up and down in the vertical direction. In this embodiment of the present invention, it is important that the vibration condition of the mold satisfies the condition that the negative strip time Tn is 0.08 second or more and 0.20 second or less and the negative strip time ratio R _NS is 0.30 or more and 0.38 or less. When the Tn is more than 0.20 seconds or the R _NS is more than 0.38, the oscillation marks are deepened and serve as notches on the surface of the cast steel, and the generation of the lateral cracks becomes remarkable. When the Tn is less than 0.08 sec, the lubrication between the mold and the solidification shell becomes insufficient, resulting in unstable operation and break-out. When R _NS is less than 0.30, the inflow amount and the consumption amount of the mold powder are reduced, and break-out is caused. Thus, by satisfying the condition that Tn is 0.08 second or more and 0.20 second or less and R _NS is 0.30 or more and 0.38 or less, generation of lateral cracks can be prevented without generating breakout. Further, Tn is less than 0.20 seconds, and also, in the R _NS is 0.38 or less at, Tn is greater than 0.20 seconds, or, R _NS is 0.38 as compared to the case of excess, the particle size of the initially formed in the sphere (舊) γ lip after solidification . This phenomenon is also believed to contribute to the reduction of the lateral cracks.

Here, the period in which the falling speed of the mold is faster than the drawing speed of the casting is the negative strip period, and the time of this period in one cycle is called " negative strip time Tn (second) ". On the other hand, the period in which the falling speed of the mold is equal to or lower than the drawing speed of the strip is the positive strip period, and the time of this period in one cycle is called " positive strip time Tp (second) ". Assuming that the vibration frequency of the mold is f (Hz), Tn + Tp = 1 / f. The negative strip time ratio R _NS is defined as T n / (T n + T p). The waveform of the vibration is not limited, and either a sinusoidal waveform or a non-sinusoidal waveform may be used.

Next, in one embodiment of the present invention, it is important that the time taken for the surface temperature of the cast steel to reach the Ar ₃ transformation point after passing through the casting mold as a secondary cooling condition is more than 60 seconds. If the time is less than 60 seconds, the main part is rapidly cooled immediately after being drawn out from the mold, and as a result, the solidified shell becomes uneven and longitudinal cracks are generated. By exceeding this time for 60 seconds, occurrence of longitudinal cracks can be prevented.

As described above, according to the continuous casting method of steel according to one embodiment of the present invention, it is possible to obtain a high-quality cast steel (slab, bloom, or billet) suppressing lateral cracks and longitudinal cracks. As a result, it is unnecessary to carry out a step of grinding the surface of the slab, and the cracks do not hinder the operation during the rolling process, thereby improving the yield. As the yield is improved, it is possible to reduce the portion corresponding to the improvement of the yield among the enormous amount of energy consumption required for the iron-making process from the iron ore as raw material to the steel making process, and it is industrially and energetically effective.

Example

Using the continuous casting machine shown in Fig. 1, the steels (the remainder Fe and inevitable impurities) of the respective component compositions shown in Table 1 were continuously cast under the continuous casting conditions shown in Table 2. The lower line in Table 2 indicates that it is out of the scope of the present invention. Table 1 shows the carbon equivalent CE calculated by the following formula (2) and the carbon equivalent Cp calculated by the following formula (3), the Ar ₃ transformation point calculated by the following formula (4) The Ac ₃ transformation point calculated by the equation (5) is also shown.

CE = [C] + [Mn] / 33 (2)

(3) Cp = [C] + [Mn] / 33 + [Ni] / 25 + [Cu] / 44 +

Ar ₃ = 910-273 x [C] -74 x [Mn] -16 x [Cr]

( ₃₎ : Ac ₃ = 937-476.5 [C] + 56 x [Si] -19.7 x [Mn] -4.9 x [Cr] + 124.8 x [V] -19 x [Nb] + 198 x [Al]

Here, [] represents the content (mass%) in the steel of each element. Steel B, C and D have a carbon equivalent CE and Cp of greater than 0.18, and others have carbon equivalent CE and Cp in the range of 0.10 to 0.18.

Table 2 out of 2 "cooling temperature" of the primary cooling conditions, indicates the primary minimum value of the comfort surface temperature in the riser between the at the bottom of the mold to the upper calibrated against, "time needed to the Ar ₃ point" is Represents the time until the surface temperature of the cast steel reaches the Ar ₃ transformation point for the first time after the cast steel has passed through the casting mold and the "double heat temperature" represents the time from when the surface temperature of the cast steel reaches the above- Represents the maximum value of the surface temperature of the cast steel. The cooling temperature, the time required up to the Ar ₃ point, and the recuperative temperature were changed by adjusting the drawing rate (casting speed) and the water density distribution of the secondary cooling water in each test condition. The surface temperature of the cast steel refers to the surface temperature of the corner portion of the cast steel, which is calculated by a solidification / heating analysis described below.

The transition of the surface temperature of the cast steel was obtained by two-dimensional solidification and heat transfer analysis by numerical calculation disclosed in JP-A-4-231158. That is, data depending on the cooling water quantity and the surface temperature of the cast steel of the heat transfer coefficient distribution by each spray nozzle were obtained by an off-line test. The boundary condition of the surface of the cast steel in the secondary cooling zone was set in accordance with the cooling water amount of each spray nozzle, the position of the cast steel surface from each spray nozzle, and the temperature of the cast steel surface at each point and each position. Cooling by contact with the support roll was evaluated by setting the heat transfer rate in the same manner as in the method disclosed in JP-A-4-231158. Since the boundary condition of the surface of the cast steel changes in accordance with the position in the casting direction, in the two-dimensional solidification and electrothermal analysis in the section perpendicular to the casting direction, the position in the casting direction is divided by the casting speed and converted into the elapsed time , And boundary conditions at each elapsed time were set. Since the temperature distribution in the cross section perpendicular to the casting direction in each elapsed time is obtained as a result of the analysis, the elongation time is multiplied by the elongation speed and converted into the position in the casting direction, A temperature distribution in a section perpendicular to the casting direction is obtained. The "cooling temperature", "time required up to the point of Ar ₃ ", "time required for heating to the point", and "time required for heating" are shown in Table 2 from the transition of the distribution of the surface temperature of the cast steel in the casting direction, Quot; double temperature "

In each test example, the formation of the slab surface layer and the occurrence of cracks were evaluated, and the results are shown in Table 2. In Table 2, " texture of the slab surface layer " indicates that the structure of the end of the slab recovered after the test was observed to observe clearly the spherical? -Phosphate system and the coarse spherical? In the case of a tissue in which the? lip system is unclear, it is described as " tissue refinement ". "Occurrence of cracks" means that the surface of the cast steel immediately after casting was visually observed and then the surface of the cast steel after the rolling was visually observed with naked eyes and that there was no transverse crack and longitudinal crack on both sides thereof was described as " , And other cases are described as " present ".

In Test Nos. 8 and 10 where Tn was more than 0.20 seconds or R _NS was more than 0.38, transverse cracks occurred even though the secondary cooling condition was within the scope of the present invention. On the other hand, in Test No. 12 in which the drawing speed was less than 1.0 m / min, no transverse cracks occurred even when Tn was more than 0.20 seconds and R _NS was more than 0.38. On the other hand, when the drawing speed is 1.0 m / min or more, the lateral cooling may occur even in the secondary cooling condition within the scope of the present invention, so that Tn is 0.20 seconds or less and R _NS is 0.38 or less It can be seen that lateral cracking can be prevented.

This tendency was also the same in the steels B, C and D in which the C content was 0.13 mass% or more and 0.20 mass% or less and the Mn content was 0.50 mass% or more (Test Nos. 16, 17, 21, 22, 26 , 27). On the other hand, the C content is less than 0.13% by weight, or, Mn content is less than 0.50% by weight steel E, F, G, when the drawing speed than 1.0m / min, Tn is greater than 0.20 seconds, or, R _NS is Even if it exceeds 0.38, no transverse cracks occurred (see Test Nos. 31, 32, 36, 37, 41 and 42). The reason for this is not clear, but it is considered that the C content or the Mn content is low and the susceptibility to cracks is low. In this regard, in the case of continuously casting a high-C high Mn steel containing 0.13 mass% or more and 0.20 mass% or more of C and 0.50 mass% or more of Mn at a drawing speed of 1.0 m / min or more, The transverse cracks may be generated even within the scope of the present invention. As a result, transverse cracks can be prevented by forming the template vibration condition in which Tn is 0.20 sec or less and R _NS is 0.38 or less.

In the case of the steels A, B, C and D having a C content of not less than 0.13 mass% and not more than 0.20 mass% and having a Mn content of not less than 0.50 mass%, the time required until the Ar ₃ point was 60 seconds or less, Vertical cracks occurred at the center in the width direction (see Test Nos. 6, 15, 20 and 25). On the other hand, in the steels E, F, and G having a C content of less than 0.13 mass% or a Mn content of less than 0.50 mass%, longitudinal cracks did not occur even under the condition that the time required up to the Ar ₃ point was 60 seconds or less Test Nos. 30, 35 and 40). The reason for this is not clear, but it is considered that the C content or the Mn content is low and it is possible to prevent the thickness of the solidified shell from becoming uneven. In this regard, when continuous casting is carried out at a drawing speed of 1.0 m / min or more at a high-C high Mn steel containing 0.13 mass% or more and 0.20 mass% or more of C and 0.50 mass% or more of Mn, vertical cracks And it is understood that longitudinal cracks can be prevented by setting the time required for Ar ₃ point to be longer than 60 seconds.

In Test Nos. 5, 7, 14, 19, 24, 29, 34 and 39 in which the cooling temperature was not lower than the Ar ₃ transformation point or the double heat temperature was not higher than the Ac ₃ transformation point, , Horizontal cracks occurred.

From the above examples, it can be seen that the present invention makes it possible to obtain a cast product with good quality without surface cracks while using a continuous casting machine rationally and efficiently.

[Industrial applicability]

According to the casting method using the continuous casting machine of the present invention, even when molten steel has a predetermined component composition, it is possible to obtain a cast steel in which surface cracks are suppressed without lowering the productivity.

100: Continuous casting machine (vertical bending type)
10: ladle
11: Tundish
12: Mold
13: Spray nozzle
14: roll
15: Cutting device
16: Electronic stirring device
20A: Vertical stand
20B: Upper calibration unit
20C: Curved band
20D: Lower calibration unit
20E: Horizontal
21: Curved mold
M: Molten steel
S: Casting
C ₁ : transverse crack
C ₂ : longitudinal crack

Claims (1)

A method for producing a casting slab using a continuous casting machine of a curved or vertical bending type,
0.13 mass% or more and 0.20 mass% or less of C, 0.50 mass% or more and 2.5 mass% or less of Mn and 0.8 mass% or less of Si, 0.10 mass% or less of P, 0.05% or less of Cu, 1.0% or less of Cu, 1.0% or less of Ni, 0.6% or less of Cr, 0.14% or less of V, 0.09% or less of Nb, 0.4% By mass or less, and the remainder being Fe and inevitable impurities, in a mold;
A step of drawing the cast steel from the casting mold at a drawing speed of 1.0 m / min or more,
A step of secondary cooling the cast slab, wherein the surface temperature of the cast slab is lower than the Ar ₃ transformation point and thereafter the temperature is returned to a temperature higher than the Ac ₃ transformation point,
Lt; / RTI &
Wherein the vibration condition of the template satisfies a condition that a negative strip time Tn according to the following definition is 0.08 second or more and 0.20 second or less and a negative strip time ratio R _NS satisfying the following definition is 0.30 or more and 0.38 or less,
When the time from the passing of the cast strip through the mold to the surface temperature of the cast steel reaches the Ar ₃ transformation point is more than 60 seconds
≪ / RTI >
Negative strip time Tn: Vibration of the mold During one cycle, the time period during which the falling speed of the mold is faster than the drawing speed of the casting
Positive Strip Time Tp: Temporal Vibration During one cycle, the time period during which the falling speed of the mold is lower than the pulling speed of the casting
Negative strip time ratio R _NS = T n / (T n + T p)

Images