JP2003117643A - Method for continuous casting of steel blooms and billets - Google Patents

Abstract

(57) [Summary] [PROBLEMS] Ar₃  Steel with a lot of ferrite phase during transformation
Also prevents the occurrence of center cavities and component segregation.
Of steel casting and billet continuous casting method
Provided. In a first step A1 of secondary cooling, directly under a mold
In the second step A2 after the secondary cooling,
A method of performing secondary cooling in the vicinity of the end of solidification, wherein the second work
By secondary cooling of the slab 2 in the middle,
From the position of the slab where the solid fraction is 0.4, the slab is further cooled.
Of the slab where the temperature at the center of the thickness is 1200 ° C.
At any position in the casting direction up to the position,
Area S of solidified shell in the cross section of slab₁  Ar in the inside
₃  Area S of the portion below the transformation point ₂  Of 3% or less
To do.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel blooms and billets.

[0002]

2. Description of the Related Art In a process for producing a seamless steel pipe from a continuously cast slab through a rolling or forging process by the Eugene Sejournet method, Mannesmann method, etc. When internal defects such as component segregation are present and the degree thereof is large, internal surface flaws occur in the seamless steel pipe manufactured from the slab, and this easily causes defects in product quality.

For the purpose of preventing the occurrence of these internal defects in the cast slab, there is known a secondary cooling method utilizing heat shrinkage during cooling of the cast slab. For example, in Japanese Unexamined Patent Publication No. 7-1096, the water amount density is 25 to 100 liters / (min · m) when the solid fraction of the central portion of the slab becomes 0.1 to 0.3.
² ) A method for cooling a cast piece is proposed, in which surface cooling of the cast piece by water cooling is started and water cooling is continued by this water amount density until the solid fraction of the central portion of the cast piece becomes 0.8 or more. There is. However, in steels such as low carbon steels and Cr-containing low alloy steels that often generate ferritic phases during Ar ₃ transformation in the cooling process of the slab, as described in the above-mentioned publication, the slab of the slab in the final stage of solidification is Increasing the secondary cooling may rather promote the generation of internal defects such as center cavities and component segregation.

[0004]

DISCLOSURE OF THE INVENTION The present invention provides a center cavity or a steel, such as a low carbon steel or a low alloy Cr-containing steel, which often produces a ferrite phase during Ar ₃ transformation in the cooling process of a slab. It is an object of the present invention to provide a continuous casting method for steel blooms and billets that can prevent the occurrence of internal defects such as component segregation such as macro segregation and semi-macro segregation.

[0005]

The gist of the present invention lies in the continuous casting method shown in the following (1) and (2). (1) The step of performing the secondary cooling of the slab consists of two steps. In the first step, the secondary cooling of the slab is performed immediately below the mold,
In the subsequent second step, which is a continuous casting method in which the slab is secondarily cooled in the vicinity of the final stage of solidification, the slab is secondarily cooled in the second step, so that the solid fraction of the central portion of the thickness is increased. At any position in the casting direction from the position of the slab that is 0.4 to the position of the slab where the temperature of the center of thickness becomes 1200 ° C when the slab is further cooled, the transverse cross section of the slab is obtained. The continuous casting method of steel blooms and billets in which the ratio of the area S ₂ of the portion below the Ar ₃ transformation point in the area S ₁ of the solidified shell in ₃ is 3% or less.

(2) Carbon steel having a C content of 0.1% by mass or less, or a Cr-containing low alloy having a C content of 0.2% by mass or less and a Cr content of 0.5 to 3.0% by mass. The method for continuously casting a bloom or billet of steel according to (1) above, in which molten steel is cast.

The "first step in the secondary cooling of the slab" defined in the present invention means the secondary cooling of the slab that is usually adopted in the continuous casting of the bloom or billet of steel. To do. That is, it is the secondary cooling of the slab to increase the thickness of the solidified shell inside the slab that has escaped from the mold outlet and prevent the slab from bulging. At that time, usually, the slab is secondarily cooled at a water amount density just below the mold so that the slab does not bulge.

The "second step in the secondary cooling of the slab" defined in the present invention means to restart the secondary cooling of the slab after the first step, especially near the end of solidification. To do. By performing secondary cooling of the slab near the end of solidification, the slab is positively contracted, the center cavity is made smaller, and the occurrence of component segregation such as macro segregation and semi-macro segregation is suppressed.

The secondary cooling of the slab may be interrupted once between the "first step in the secondary cooling of the slab" and the "second step in the secondary cooling of the slab". Secondary cooling of the slab may be performed continuously.

"Solid phase fraction", "Temperature of thickness center of cast piece", and "Area of solidified shell area S ₁ in cross section of cast piece below Ar ₃ transformation point" defined in the present invention The “S ₂ ratio” can be obtained by a commonly used unsteady heat transfer calculation.

In the case of a steel in which a ferrite phase is formed from an austenite phase at the Ar ₃ transformation point in the process in which a continuously cast slab is secondarily cooled and cooled, the volume difference is caused by the crystal structure of these phases. Expansion occurs.

In continuous casting of blooms and billets of steel used in ordinary seamless steel pipes having a high Cr content and a C content of up to about 0.6% by mass, a process in which a slab is secondarily cooled and cooled In, the ferrite phase is generated from the austenite phase at the Ar ₃ transformation point. If excessive secondary cooling is performed at the end of solidification during continuous casting of such steel, in an extreme case, all of the solidified shells that have completed solidification will be A
The r ₃ transformation point is reached, all the austenite phase transforms to the ferrite phase, and large volume expansion occurs.

The Ar ₃ transformation point is usually around 900 ° C., and when the solidified shell of the slab undergoes a large volume expansion at such a temperature and the solidified shell undergoes large expansion deformation, the temperature is higher than around 900 ° C. Deformation due to volume expansion concentrates on the solidified shell near the center of thickness of the slab with low deformation resistance. Therefore, the portion where the unsolidified molten steel exists and the center cavity become large. This is because the steel pipe is heated
This is the same as the increase in the outside diameter due to thermal expansion, the increase in the inside diameter. When the portion where the unsolidified molten steel is present and the center cavity become large, the micro-segregated molten steel that exists between the solidifying dendrite trees flows, then accumulates in the enlarged center cavity, and new macro segregation, semi-macro segregation, etc. Is generated.

Therefore, as a method of suppressing a large volume expansion, Ar _{3 is used in the} secondary cooling of the slab at the final stage of solidification.
A method of reducing the proportion of solidified shell that reaches the transformation point, that is, a method of making the slab temperature as high as possible before starting secondary cooling at the final stage of solidification, is possible, but if the temperature is excessively increased, it will occur at the final stage of solidification. Secondary cooling of the slab,
The effect of shrinking the slab becomes smaller. Furthermore, it is necessary to dispose a heat retention or heating device for the slab in the continuous casting machine, which is an excessive device and is not a realistic method.

A at the time of secondary cooling of the slab at the final stage of solidification
By appropriately adjusting the proportion of the solidified shell reaching the r ₃ transformation point, it is possible to effectively prevent the occurrence of center segregation and component segregation such as macro segregation and semi-macro segregation.

That is, even if the solidified shell undergoes a large volume expansion, it is possible to suppress the expansion of the center cavity depending on the proportion of the solidified shell that has been solidified. If there is sufficient unsolidified molten steel in the center of the thickness of the slab and the unsolidified molten steel is in a flowable state, even if the solidified shell surrounding it expands in volume, the amount will not be large enough. The solidified molten steel flows in from the upstream side in the casting direction, and the volume-expanded portion is filled with the unsolidified molten steel.

Further, depending on the proportion of the solidified shell that has been solidified, the strength of the solidified shell becomes an appropriate size,
Even if the solidified shell on the outer peripheral side of the slab expands in volume, the solidified shell on the inner peripheral side can be resistant to the expansion of the center cavity.

On the other hand, there is not enough unsolidified molten steel in the center of the thickness of the slab, the unsolidified molten steel is hard to flow, and the temperature of the solidified shell that has been solidified is high. Even if the strength is small, it is sufficient to cause the solidified shell to undergo a large volume expansion, and to cause an appropriate volume expansion, that is, if the method shown below is adopted, the center cavity, the macro segregation, It was found that the occurrence of component segregation such as semi-macro segregation can be effectively prevented.

At a position on the downstream side in the casting direction from the position of the slab having a solid fraction of 0.4 in the thickness center, there is not enough unsolidified molten steel in the thickness center of the slab. Unsolidified molten steel is in a state where it is difficult to flow. It was confirmed by the following test that the solidification rate of 0.4 or more makes it difficult for the unsolidified molten steel to flow. That is, when casting steels of various chemical compositions, by using a pinch roll provided in a continuous casting machine, a slab having different solid fractions in the center of thickness is pressed down, and a cross-section of the slab after casting A sample was taken. This sample was macro-etched to investigate the flow condition of molten steel with concentrated component segregation during rolling. As a result, it was confirmed that when the solid fraction is 0.4 or more, the unsolidified molten steel becomes difficult to flow. Further, at the position on the upstream side in the casting direction from the position of the slab where the temperature at the center of the thickness after casting becomes 1200 ° C, the temperature of the solidified shell that has completed solidification is high and its strength is low.

In the method of the present invention, the slab is further cooled from the position of the slab having a solid fraction of 0.4 at the center of thickness, and the temperature of the center of thickness becomes 1200 ° C. Since the ratio of the area S ₂ of the portion below the Ar ₃ transformation point in the area S ₁ of the solidified shell in the cross section of the cast piece to 3% or less at any position in the casting direction up to the position of It is possible to effectively prevent the occurrence of cavities and component segregation such as macro segregation and semi-macro segregation.

[0021]

1 is a schematic view showing an example of a continuous casting machine when the method of the present invention is applied. An example of a curved continuous casting machine with a round billet having a circular cross-sectional shape and a diameter of about 200 to 300 mm will be shown.

The molten steel 3 supplied into the mold 2 through the immersion nozzle 1 forms a solidified shell 4 in the mold. The slab 6 composed of the solidified shell and the unsolidified molten steel 5 is secondarily cooled by the cooling spray 7 immediately after leaving the mold. As the first step A1, from immediately below the mold, for example, in the casting direction 2
During the m length, the slab is secondarily cooled. Secondary cooling is once interrupted, and, for example, as the second step A2, at the end-solidification position in the range of 30 to 36 m from the meniscus,
The slab is secondarily cooled.

Secondary cooling of the slab is preferably performed by using an air mist spray. This is because the secondary cooling of the slab can be performed effectively. The water density at that time (liter / (min · m ² )) is defined by the water density of the water that constitutes the air mist.

In the method of the present invention, the secondary cooling of the slab is performed in two steps. In the first step, the secondary cooling of the slab is performed immediately below the mold, and in the subsequent second step. ,
Secondary cooling of the slab is performed at the end of solidification.

In the first step, the slab is subjected to secondary cooling at a water density such that the slab does not bulge. The water amount density at that time may be determined depending on the size of the slab, the casting speed, etc., but in the case of a round billet having a diameter of about 200 to 300 mm, about 100 to 300 liters / (min · m ² ) is desirable.

In the second step, the slab near the final stage of solidification is appropriately secondarily cooled to heat-shrink the slab. At that time, the area of the solidified shell that transforms from the austenite phase to the ferrite phase is appropriately suppressed, and the volume expansion of the solidified shell is suppressed to an appropriate expansion. The water density at that time is the size of the slab,
The diameter may be 200-
In the case of a round billet having a size of about 300 mm, it is desirable that the billet has a capacity of about 50 to 300 liter / (min · m ² ).

In the method of the present invention, the slab is further cooled from the position of the slab having a solid fraction of 0.4 at the center of thickness and the temperature of the center of thickness becomes 1200 ° C. The ratio of the area S ₂ of the portion below the Ar ₃ transformation point in the area S ₁ of the solidified shell in the cross section of the cast slab at any position in the casting direction up to the position is 3% or less.

From the time when there is not enough unsolidified molten steel in the center of the thickness of the slab and it is difficult for the unsolidified molten steel to flow, the temperature of the solidified shell that has completed solidification is high and its strength is low. Between the Ar ₃ transformation points, the ratio of the area of the solidified shell area below the Ar ₃ transformation point is appropriately small, that is, the area S ₂ of the portion below the Ar ₃ transformation point in the area S ₁ of the solidified shells in the cross section of the slab. Since the proportion is 3% or less, the slab can be effectively heat-shrinked while suppressing the volume expansion of the solidified shell to an appropriate expansion. Therefore, it is possible to effectively prevent the occurrence of the center cavity and component segregation such as macro segregation and semi-macro segregation.

An example of a method of secondary cooling of the cast slab to make the ratio of the area S ₂ of the portion below the Ar ₃ transformation point occupying in the area S ₁ of the solidified shell in the cross section of the cast slab to 3% or less,
The details will be described below.

The slab is cooled by heat removal from the mold (primary cooling), secondary cooling immediately below the mold, secondary cooling near the end of solidification, radiant heat release to the atmosphere, contact cooling with guide rolls, etc. Etc. Among these conditions, it is the degree of secondary cooling of the slab and the casting speed that determine the temperature of the slab. Usually, the target casting speed is constant because of the size and quality of the slab. Therefore, the temperature of the slab changes depending on the degree of secondary cooling of the slab.

During the casting of steel blooms or billets,
When the secondary cooling of the slab is changed, since the ratio of the solidified shell in the outer peripheral portion to the slab cross-sectional area is large, the temperature change at the center of the slab thickness is usually small, and the outer peripheral portion of the slab is small. The temperature in the vicinity changes greatly. That is, the temperature in the vicinity of the outer peripheral portion of the cast piece can be changed without changing the temperature in the vicinity of the central portion of the thickness of the cast piece.

When the secondary cooling of the slab in the first step immediately below the mold is strong, the temperature of the outer peripheral portion of the slab further decreases due to the cooling of the slab in the second step near the end of solidification,
Of the solidified shells on the outer peripheral portion of the cast slab, the proportion of the solidified shells that reach a temperature equal to or lower than the Ar ₃ transformation point increases. In that case, the water amount density in the secondary cooling of the slab in the first step may be lowered to weaken the cooling.

By using the above method, the area S ₂ of the portion below the Ar ₃ transformation point occupying in the area S ₁ of the solidified shell in the transverse section of the slab corresponding to the size of the slab, the casting speed, etc. The specific water amount density of the secondary cooling of the slab in the first step and the second step in which the ratio is 3% or less can be obtained in advance.

Further, at that time, the surface temperature of the slab is measured in advance using a radiation thermometer or the like at the inlet side position and the outlet side position of the second step of cooling to obtain the standard surface temperature. Can be kept.

During actual casting, the measured surface temperature of the slab is input to the process computer and unsteady heat transfer calculation is performed to determine the temperature distribution of the solidified shell and the temperature at the center of the thickness of the slab. An estimate can be made. By monitoring the result, the secondary cooling of the slab can be performed more accurately by adjusting the water amount density of the secondary cooling of the slab.

The "steel" targeted by the method of the present invention means a steel in which a ferrite phase is formed from an austenite phase at the Ar ₃ transformation point in the process of continuously cooling and cooling a continuously cast slab. .

Further, carbon steel having a C content of 0.1% by mass or less, or Cr-containing low alloy steel having a C content of 0.2% by mass or less and a Cr content of 0.5 to 3.0% by mass. It is desirable to apply the method of the present invention when casting the molten steel.

Here, carbon steel having a C content of 0.1% by mass or less means C: 0.02 to 0.1% by mass% and Si:
0.08-0.4%, Mn: 0.6-1.6%, Al:
0.008 to 0.02%, and if necessary, C
It means steel containing one or two of a: 0.008% or less and B: 0.003% or less, with the balance being Fe and impurities. C, Si and Mn are mainly added to secure the strength of steel and Al is mainly added to deoxidize molten steel. Further, Ca is added to control the morphology of non-metallic inclusions, and B is added to improve the hardenability of steel.

Further, the C content is 0.2 mass% or less, and the Cr content is
The Cr-containing low alloy steel having a content of 0.5 to 3.0% by mass,
% By mass, C: 0.02 to 0.2%, Si: 0.08 to
0.4%, Mn: 0.3 to 1.0%, Al: 0.008
.About.0.02%, Cr: 0.5 to 3.0%, and if necessary, Mo: 1.0% or less, Ni: 1.0% or less, Cu: 1.0% or less, Ti. : 0.2% or less, Nb:
0.2% or less, V: 0.2% or less, Ca: 0.006%
Hereinafter, B: steel containing one or more of 0.003% or less, and the balance being Fe and impurities. C, Si, Mn, and Cr are mainly added to secure the strength and toughness of steel, and Al is added mainly to deoxidize molten steel. Further, Mo, Ni, Cu, Ti, Nb and V are added to improve the strength and toughness of steel, Ca is added to control the morphology of non-metallic inclusions, and B is added to improve the hardenability of steel. It

Since the carbon steel and the Cr-containing low alloy steel described above have a large amount of ferrite phase generated from the austenite phase at the Ar ₃ transformation point in the cooling process of the slab, the effect of applying the method of the present invention is remarkable, therefore The effect of preventing internal defects such as center cavities and component segregation such as macro segregation and semi-macro segregation is great.

[0041]

EXAMPLES Molten steels shown in Table 1 were cast using a continuous casting machine having the structure shown in FIG. The Ar ₃ transformation points of these steels were measured by thermal analysis, and it was confirmed that carbon a steel a was 845 ° C., steel b was 850 ° C., and Cr-containing low alloy steel c was 880 ° C.

Steel a is a steel in which a ferrite phase is formed from the austenite phase upon Ar ₃ transformation, and steels b and steel c are steels in which a lot of ferrite phase is formed from the austenite phase upon Ar ₃ transformation.

[0043]

[Table 1]

The billet is a round billet with a diameter of 200 mm,
The first step of the secondary cooling of the slab was from immediately below the mold to the length of 2 m in the casting direction, and the second step was the range of 6 m length from the meniscus at a distance of 30 to 36 m. An air mist spray is used as a water spray device for secondary cooling, and in the first step, the air-water ratio is 30 and the maximum water amount density is 300.
Liter / (min · m ² ), and in the second step, the air-water ratio is 50 and the maximum water density is 500 liters /
(Min · m ² ), and the water content density was changed within the range of the water content density.

By changing various combinations of the water amount densities in the first step and the second step, from the time when the solid fraction of the thickness center of the cast becomes 0.4, the thickness center of the cast becomes The ratio of the area S ₂ of the portion below the Ar ₃ transformation point in the area S ₁ of the solidified shell in the cross section of the cast piece at an arbitrary position in the casting direction until the temperature of the part reaches 1200 ° C. (described later) The test was performed by changing the value of (b) of S ₂ / S ₁ in Table 2 below.

Further, as described above, by changing various combinations of the water amount densities in the first step and the second step, the temperature of the thickness central portion of the cast piece changes from the time when it becomes the molten steel temperature to the cast piece. The value of S ₂ / S ₁ (S ₂ / S ₂ in Table 2 to be described later) at any position in the casting direction until the solid phase ratio at the thickness center of 0.4 (not included) is reached.
S value of (a) of S ₁ ), and S at any position in the casting direction from the time when the temperature at the center of the thickness of the cast becomes 1200 ° C (not included) to the time when it reaches 700 ° C. ₂ / S ₁ value (S ₂ / S ₁ in Table 2 described later)
The value of c) was changed and tested.

At that time, the temperature distribution of the solidified shell forming the cast piece and the solid fraction in the thickness center portion of the cast piece were determined by unsteady heat transfer calculation. It was confirmed in advance that the accuracy of this calculation was good by measuring the surface temperature of the slab and by a tack test of the slab.

Based on this calculation, at any casting time, from the time when the solid fraction in the thickness center of the cast becomes 0.4 to the time when it reaches 1200 ° C. The temperature distribution of the solidified shell on the cross section of the slab at the position was determined. Also, based on the temperature distribution of the solidified shell,
The area S ₂ of the portion of the solidified shell having a temperature equal to or lower than the Ar ₃ transformation point is obtained by specifying and numerically integrating the portion of the solidified shell having a temperature equal to or lower than the r ₃ transformation point. By dividing by the area S ₁ of
The value of ₂ / S ₁ (values of a, b, and c in Table 2 described later) was obtained.

FIG. 2 is a diagram schematically showing an example of the temperature distribution of the solidified shell in the cross section of the cast slab. Reference numeral B1 indicates a high temperature solidified shell whose temperature exceeds the Ar ₃ transformation point, reference numeral B2 indicates a solidified shell whose temperature is equal to or lower than the Ar ₃ transformation point, and reference numeral 5 indicates an unsolidified molten steel. The area of the region of the code B2 is _{S 2,} the total area of the region of the code B1 and B2 are _{S 1.}

The casting speed is 2.5 m / m for steel a.
in, steel b was 2.8 m / min, and steel c was 2.6 m / min. 21 and No. 21. In the tests of other comparative examples except for 22, the water amount density in the first step was increased, so that the solidification completion position was set to the same position as the other tests, and therefore the casting speed was set to steel a, steel b, steel c. Both were cast at a speed of 0.1 m / min faster than the target speed. The solidification completion position is set to a fixed position in order to ensure a fixed and good internal quality of the cast slab.

After casting, a slab having a length of 2 m was sampled from the slab portion in a steady casting state, and further, three transverse cross-section samples having a thickness of 50 mm were sampled at intervals of 0.5 m to obtain a center cavity. And the situation of component segregation were investigated.

The size and number of the center cavities visually observed within a diameter of 50 mm of the axial center portion of the cross-section sample were investigated, the total area of the center cavities was obtained, and divided by the observed area to obtain the percentage value. Then, the center cavity area ratio So (%) was obtained. The degree of formation of the center cavity was evaluated by averaging the survey results of the three cross-section samples.

Further, in the vicinity of the shaft center portion, chips were collected from 5 positions within 50 mm in the diameter direction including the shaft center portion with a drill blade having a diameter of 4 mm, a chemical analysis of C was performed, and an average value was obtained. was the C content _{C 1} (mass%), the ratio by dividing the C content _{C 0} of the ladle _(wt%), C 1 / _{C 0}
The component segregation was evaluated by obtaining (-) as the degree of component segregation.
Table 2 shows the test conditions and the test results.

[0054]

[Table 2]

Test No. of the present invention example 1-No. In 6,
Carbon with a C content of 0.18% by mass or 0.05% by mass
Steel a or steel b, which is steel, is used in the first step.
The water density of the secondary cooling is 150 liters / (min · m
^Two  ) As the second step.
The water density of the next cooling is 50 to 300 liters / (min.
m ^Two ) Within the range. Both of the above S_Two  
/ S₁  The value of (b in Table 2) is 0.3 to 3%.
Within the range, the value was within the range specified in the present invention. Above
Of S_Two  / S₁  Of the value of (the value of c in Table 2),
Some had values above 3%.

Test No. 1 and No. 2, the center cavity area ratio So is 0.05% or 0.09%.
The C component segregation degree C ₁ / C ₀ was 1.11 or 1.12. The center cavity area ratio and the C component segregation degree are both test No. 3 to No. Although it was slightly worse than that of No. 6, it was a better result than that of Comparative Example described later. In addition, the test No. 3 to No. 6, the center cavity area ratio So is 0.0 to 0.04.
%, The component segregation degree C ₁ / C ₀ of C is 1.03 to ₁ .
It was 10, and both were good results.

Test No. of the present invention example 7-No. In No. 10, steel c, which is a Cr-containing low alloy steel, is used, and the water amount density of the secondary cooling in the first step is 200 liter / (min.
m ² ), weak cooling is performed, and the water amount density of the secondary cooling in the second step is 50 to 280 liters / (min.
It was changed within the range of m ² ). Both are S ₂ /
The value of S ₁ (value of B in Table 2) was in the range of 0 to 2.9%, which was within the range specified in the present invention. Among the values of S ₂ / S ₁ described above (values of C in Table 2), some were values exceeding 3%.

Test No. 7-No. In No. 10, the center cavity area ratio So was 0.0 to 0.04%, and the C component segregation degree C ₁ / C ₀ was 1.05 to 1.08, which were both good results.

Test No. of the comparative example. 11-No. In No. 20, test No. of the example of the present invention. 1-No. Corresponding to 10, the water quantity density of the secondary cooling in the first step is 3
Strong cooling was performed at 50 liters / (min · m ² ) or 400 liters / (min · m ² ). The water amount density of the secondary cooling in the second step is the test No. of the example of the present invention. 1-No. Same as 10. In all cases, the value of S ₂ / S ₁ (value of (b) in Table 2) was within the range of 3.1 to 7.4%, which was a high value outside the conditions specified in the present invention. Of the values of S ₂ / S ₁ (values of “a” or “c” in Table 2), only slightly exceeded 3%.

Test No. 11-No. In No. 20, the center cavity area ratio So is 1.1 to 12.3%, and the C component segregation degree C ₁ / C ₀ is 1.24 to 1.53. 1-No. It was worse than 10.

Test No. of the comparative example. 21 and No. 22
Then, the secondary cooling in the second step was not performed. The center cavity area ratio So is 9.5% or 8.5%, and the C component segregation degree C ₁ / C ₀ is 1.44.
Or at 1.46, both were bad.

[0062]

EFFECTS OF THE INVENTION By applying the method of the present invention, even in steels such as low carbon steels and low alloy steels containing Cr, which often generate ferrite phase during Ar ₃ transformation in the cooling process of cast slabs, the center cavity and It is possible to prevent the occurrence of internal defects such as component segregation such as macro segregation and semi-macro segregation.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a continuous casting machine when the method of the present invention is applied.

FIG. 2 is a diagram schematically showing an example of a temperature distribution of a solidified shell in a cross section of a cast piece.

[Explanation of symbols]

1: Immersion nozzle 2: Mold 3: Molten steel 4: Solidified shell 5: Unsolidified molten steel 6: Cast slab 7: Cooling spray 8: Guide roll 9: Pinch roll A1: First step A2: Second step B1: High temperature solidified shell whose temperature exceeds the Ar ₃ transformation point, B2: Solidified shell whose temperature is equal to or lower than the Ar ₃ transformation point

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B22D 11/124 B22D 11/124 P // C22C 38/18 C22C 38/18 38/54 38/54

Claims (2)

[Claims] 1. A step of performing secondary cooling of a cast piece comprises two steps. In the first step, the secondary cooling of the cast piece is performed immediately below the mold, and in the subsequent second step, casting is performed near the end of solidification. A continuous casting method for secondary cooling of a piece, wherein the second step cools the slab so that the solid phase fraction at the center of thickness is 0.4, Ar ₃ occupying in the area S ₁ of the solidified shell in the transverse section of the slab at any position in the casting direction up to the position of the slab where the temperature of the thickness center part becomes 1200 ° C. when the slab is cooled. A continuous casting method for steel blooms and billets, characterized in that the proportion of the area S ₂ of the portion below the transformation point is 3% or less. 2. A carbon steel having a C content of 0.1% by mass or less, or a C content of 0.2% by mass or less and a Cr content of 0.5.
The method for continuous casting of a bloom or billet of steel according to claim 1, characterized in that molten steel of Cr-containing low alloy steel of ˜3.0 mass% is cast.