JPH09253814A - Method for restraining surface crack on cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To promote a restraining method of the surface cracking on a cast slab which reduces or prevents the surface cracking of traverse crazing, etc., developed on the cast slab surface at the time of executing continuous casting. SOLUTION: In a second cooling pattern immediately executed after drawing the cast slab from a mold at the time of producing the cast slab, in which a carbon equivalent satisfies relations of formulas I and II, by using a bending type or a vertical-curving type continuous caster, the cooling is executed so that an average water quantity density (d) 1/(cm<2> .min) of the cooling water at an arbitrary time zone in the range of the time (t) (min) after passing the cast slab through the mold in the formula III, satisfies the range shown with the formula IV and in the range of (t), (d) is to satisfy the formula V. (I) Cp=C (%)+Mn(%)/33+Ni(%)/25+Cu(%)/44+N(%)/1.7 (II) Cp<0.18 (III) 0.5<=t<=2.5 (IV) (-0.009t+0.043)<d (V) d<(-0.02t+0.11) By this method, the surface crack of the traverse crazing, etc., can be reduced or prevented.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for reducing or preventing slab surface cracking during continuous casting of steel.

[0002]

2. Description of the Related Art In recent years, Nb, V, and Ni have been demanded due to the requirement for material characteristics.
The production of low alloy steels containing various alloying elements such as Cu and Cu is increasing. However, with the addition of these alloy elements, lateral cracks and surface cracks called lateral cracks (hereinafter referred to as surface cracks) may occur on the surface of the slab during continuous casting, which is a manufacturing problem. .

Surface cracking occurs when the surface temperature of the slab becomes close to the γ → α transformation temperature (about 600 to 850 ° C.) at which the hot ductility decreases during secondary cooling of the continuously cast slab, at which time the slab is straightened. It is known that it is caused by being subjected to a correction stress due to. As a countermeasure against this, a method is usually adopted in which the surface temperature during straightening of the cast slab is avoided on the low temperature side or high temperature side of the temperature range in which the hot ductility decreases (hereinafter referred to as the embrittlement temperature range) to suppress surface cracking. There is.

However, it is impossible to prevent the surface cracks only by controlling the surface temperature at the time of slab straightening, and various methods have been proposed.

For example, Japanese Patent Publication No. 58-3790 discloses a cooling pattern which can avoid the temperature range where the surface temperature at the above-mentioned correction point is low in ductility on the low temperature side, and the upper part of the secondary cooling zone. Forcibly cooling the slab surface temperature from 650 to
By setting the temperature to 700 ° C, once γ (austenite) →
A method of α (ferrite) transformation is disclosed. Japanese Unexamined Patent Publication No. 5-329505 discloses a method of cooling and holding the surface layer of the cast slab at a temperature of 350 to 500 ° C. for 1 minute or more before charging the heating furnace.

[0006] In all of these methods, the surface temperature of the slab is once lowered to cause the phase transformation of most or all of the slab to structurally weaken the crack susceptibility. However, once the surface temperature of the slab is raised to 7
If the temperature is lowered to 00 ° C. or less, it is thermally difficult to avoid the embrittlement temperature range on the high temperature side even if it is subjected to multiple heating thereafter.

On the other hand, it is difficult to avoid the embrittlement temperature range at the low temperature side during slab straightening due to changes in the cooling characteristics of steel types having a high alloy content and high crack susceptibility.

Further, since surface cracks occur at the γ grain boundary, there are many proposals for paying attention to the γ grain size and refining it. In order to suppress the growth of γ grains, the applicant has
Japanese Patent Laid-Open No. 63-63559 discloses a method of setting the cooling rate from the austenite single crystallization temperature to 10 ° C./sec or more, and Japanese Laid-Open Patent Publication No. 61-195742 defines a relational expression of the mold length, which should be set earlier. A method was proposed in which the slab was pulled out and immediately secondarily cooled. However, since the solidified portion near the slab surface usually passes the austenite single crystallization temperature in the mold,
Since it is difficult to control the cooling rate, and making the mold length extremely shorter than usual tends to cause operational troubles, it was difficult to put the two methods into practical use.

On the other hand, AlN is present at the grain boundary where surface cracking occurs.
Is deposited, and it is known that stress concentration accompanying this promotes cracking. On the other hand, in order to suppress AlN precipitation in steel, TiN is often added to precipitate TiN, and a high effect is obtained. For example, Shoko 5
According to Japanese Patent Laid-Open No. 5-7106, AlN precipitation is controlled by controlling cooling conditions. However, there are many steel grades in which Ti cannot be added due to the requirement of material properties, and there is a problem that the control of AlN precipitation by cooling conditions lacks stability.

As described above, many methods for preventing the surface cracking of cast slabs have been proposed, but all have advantages and disadvantages, and the current situation is that surface cracks occur frequently.

[0011]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention focuses on the relationship between the microstructure of the cast slab (formation of grain boundary ferrite) and the susceptibility to cracking (presence or absence of cracks). It was made. An object of the present invention is to provide a method for controlling the microstructure of a continuously cast slab by performing secondary cooling under appropriate conditions to reduce or prevent surface cracks such as lateral cracks.

[0012]

The gist of the present invention resides in the following method for suppressing surface cracking of cast slabs.

A curved or vertical bending continuous casting machine for casting steel slabs in which the respective contents of C, Mn, Ni, Cu and N and the carbon equivalent Cp satisfy the relations of the following formulas (1) and (2). When manufacturing using, the secondary cooling is performed immediately after the slab is pulled out from the mold, and in the secondary cooling pattern, the following formula is used.
Average water volume density d of cooling water in an arbitrary time zone within the elapsed time t (min) after passing through the mold of the slab specified in (3)
(Liter / (cm ² · min)) satisfies the regions represented by the following formulas (4) and (5), and within the range of the elapsed time t defined by the following formula (3), the average water volume density d is Equation (6) and
A method for suppressing slab surface cracking, which comprises cooling so as to fill a region represented by (7).

Cp = C (%) + Mn (%) / 33 + Ni (%) / 25 + Cu (%) / 44 + N (%) / 1.7 (1) Cp <0.18・ (2) 0.5 ≦ t <2.5 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (3) A <d ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (4) A = −0.009 t + 0.043 ・ ・・ ・ ・ ・ (5) d <B ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (6) B = −0.02t ＋ 0.11 ・ ・ ・ ・ ・ ・ (7) However, X (%) is ,% By mass of the X component.

The above-mentioned "average water amount density d" means the total amount of cooling water (liter) during the elapsed time t (min) after the slab has passed through the mold, the corresponding mold surface area (cm ² ) and the above t (min). )
It is the value divided by.

The inventor of the present invention has investigated in detail the microstructure of the surface crack occurrence portion of the continuously cast slab under various casting conditions. As a result, it was found that there is a clear correlation between the microstructure of the slab (generation of grain boundary ferrite) and the susceptibility to surface cracking (presence or absence of surface cracking), and control of the surface microstructure by the above secondary cooling conditions. It was found that

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION A continuous casting machine to which the method of the present invention is applied is a curved type or a vertical bending type in which a straightening stress is generated in a slab. As described above, the surface cracks are caused by the straightening strain of the slab, and therefore the method of the present invention is performed when using a continuous casting machine of an oath type or a vertical bending type equipped with a straightening part of the slab. Will be effective.

As a device for secondary cooling, an ordinary device used immediately after the cast slab is pulled out from the mold is used.

The chemical composition of the cast steel slab is C, Mn, N
The respective contents of i, Cu and N and the carbon equivalent Cp are represented by the following formula (1)
It satisfies the relationship of and (2).

Cp = C (%) + Mn (%) / 33 + Ni (%) / 25 + Cu (%) / 44 + N (%) / 1.7 ・ ・ ・ (1) Cp <0.18 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・-(2) In the above formula (1), (%) is% by mass.

<Reason for limiting carbon equivalent Cp> According to FIG.
The reason why the carbon equivalent Cp is limited as described above will be explained. FIG. 1 is a diagram schematically showing a solidification mechanism in various chemical compositions. (a) is a phase diagram of Fe-C system, (b) is a schematic diagram showing solidification in the case where carbon equivalents are A, B, C and D. In the composition that corresponds to the sub-peritectic (in the case of B) or δ solidification (in the case of A) on the left side of the peritectic composition, the final solidification position coincides with the γ grain boundary because the phase transformation occurs after complete solidification and the γ phase is produced. . Therefore, segregation of constituent elements to the grain boundaries,
The pinning effect of grain boundary migration due to precipitation is small, and the γ grain size grows after solidification, resulting in coarse γ grains. It is in this region where coarse γ grains are formed that the control of the microstructure described above has an important influence on the suppression of surface cracking.

The above formula (1) is known as a formula for obtaining the carbon equivalent Cp of the peritectic reaction from the contents of C, Mn, Ni, Cu and N in steel. Cp calculated by this formula
When the value is smaller than 0.18, δ solidification or hypoperitectic solidification occurs as shown in FIG. 1, so Cp is set to less than 0.18 as shown in the equation (2).

On the other hand, actual manufacturing experience has revealed that there is a correlation between the Cp value and the occurrence of surface cracks, and surface cracks hardly occur when Cp is less than 0.10. Therefore, the effect of suppressing surface cracking by the method of the present invention has a substantial meaning when Cp is 0.10.

In the method of the present invention, in the secondary cooling under the above conditions, the average amount of cooling water in any time zone within the elapsed time t (min) after passing through the mold of the slab defined by the following formula (3) If the density d (liter / (cm ² · min)) satisfies the region represented by the following formulas (4) and (5) and within the elapsed time t defined by the following formula (3), the average water volume density d But the following formula
The cooling pattern is set so as to fill the areas represented by (6) and (7).

0.5 ≦ t <2.5 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (3) A <d ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (4) A = −0.009 t + 0.043 ・ ・ ・・ ・ ・ (5) d <B ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (6) B = −0.02t ＋ 0.11 ・ ・ ・ ・ ・ ・ (7) However, X (%) is It represents the mass% of the X component.

The above-mentioned "average water amount density d" means the total amount of cooling water (liter) during the elapsed time t (min) after the slab has passed through the mold, the corresponding mold surface area (cm ² ) and the above t (min). )
It is the value divided by.

<Reason for limiting water amount density d and elapsed time t> As described above, there is a clear correlation between the microstructure of the slab (generation of grain boundary ferrite) and the surface cracking susceptibility (presence or absence of cracking). . This example will be described with reference to FIG.

FIG. 2 is a copy of a photograph showing a typical microstructure of a cracked part and a non-cracked part on the surface of the cast slab.
(a) is the case where surface cracking occurs and (b) is the case where surface cracking does not occur.

As shown in the drawing, the microstructures on the surface of the slab are all ferrite-pearlite structures. But,
As shown in Fig. 2 (a), the microstructure when surface cracks occur is characterized in that γ grain boundaries are clear, ferrite at grain boundaries is formed in a film shape, and is coarse. On the other hand, when surface cracking does not occur, the γ grain boundary becomes unclear as shown in FIG. 2 (b), and the microstructure is relatively fine. As described above, the surface crack is a γ grain boundary crack, and if the γ grain boundary is unclear, the origin of the crack does not exist, and as a result, the surface crack susceptibility also decreases.

Since the structure of steel is generally determined by the temperature history, if the secondary cooling conditions are properly controlled, a microstructure with a low susceptibility to surface cracking can be obtained, and surface cracking of the cast slab may be reduced. . For this reason, various temperature histories on the surface of the slab were changed, and basic tests were conducted to investigate the effect on the microstructure.

In this test, 200 kg of molten steel was statically cast into slabs of approximately 400 x 400 x 200 mm, which were removed from the mold before complete solidification and cooled by controlled spraying. The mist spray which mixed air and water was used for cooling. The temperature history of the surface of the slab was measured with a radiation thermometer or a thermocouple which was previously cast, and changes in the microstructure of the surface of the slab with respect to various temperature histories were investigated.

FIG. 3 shows the effect on the microstructure of the surface of the slab,
It is a figure which shows the influence of the elapsed time t after a slab passes a mold, and the average water amount density d of secondary cooling. In the secondary cooling process, the water amount density is changed. Therefore, the average water amount density d changes with the elapsed time t after passing through the mold, but in FIG. 3, when the average water amount density d becomes maximum in the secondary cooling process. The value of The microstructure of the slab surface is such that the γ grain boundary is clear (marked with □ in FIG. 3), partially unclear (marked with Δ), unclear (marked with ○), and The bainite structure (also marked with x) obtained when the secondary cooling was strong and did not sufficiently reheat was classified into four types.

In FIG. 3, the straight line A represents the equation (5) d =
-0.009 t + 0.043, the straight line B represents the formula (7) d = -0.02
t + 0.11, which are empirical formulas.

As shown in FIG. 3, if the elapsed time t after the slab has passed through the mold and the average water content density d of the secondary cooling are set to appropriate conditions, a desirable microscopic state where the γ grain boundary is unclear is set. The organization is obtained.

First, the relationship between the microstructure of the surface of the cast slab and the average water content density d and the straight lines A and B will be described.

For example, after the slab has passed through the mold, it is cooled for 1 minute under the condition that the average water amount density d is 0.034 liter / (cm ² · min) or more, and then reheated to be cooled at an appropriate cooling rate. The obtained structure is a ferrite-pearlite structure, and the γ grain boundary becomes unclear. When the average water amount density d is extremely increased to 0.09 liters / (cm ² · min) or more when the slab is cooled for 1 minute after passing through the mold, the slab surface temperature will be extremely reduced and will not reheat. , Becomes a bainite structure. Further, if the average water content density d is extremely increased, the occurrence of temperature unevenness on the surface of the slab cannot be avoided, and thermal stress is generated.
This may cause surface cracks. That is, when cooled at a high water density d equal to or higher than the straight line B, the slab does not sufficiently reheat and has a bainite structure. Similar investigations were conducted for other steel grades and the same results were obtained.

On the other hand, in order to obtain a ferrite-pearlite structure with unclear γ grain boundaries, γ →
It is necessary to perform α-transformation, and for that purpose, the average water content density d needs to exceed the straight line A.

Next, the reason why the elapsed time tmin after the slab has passed through the mold is limited as in the following formula (3) will be explained.

0.5 ≦ t <2.5 (3) In order to obscure the γ grain boundary, it is necessary to transform the γ into α immediately after the slab has passed through the mold. Is as described above. However, in actual continuous casting, the strength of the solidified shell is insufficient at a time of less than 0.5 minutes after passing through the mold, and it is difficult to perform strong cooling to complete the γ → α transformation promptly by this time. Therefore, the time t
Was 0.5 min or more.

On the other hand, as shown in FIG. 3, under the condition that the average water amount density d after the passage through the mold for 2.5 minutes or more exceeds the straight line A, the bainite structure or the structure with clear γ grain boundaries is obtained even if d does not exceed the straight line B. And both of them have high surface cracking susceptibility. Therefore, the time t is set to less than 2.5 min.

As described above, the average water volume density d (liter / (cm ² ) of the cooling water in any time zone within the elapsed time t (min) after the slab passed through the mold defined by the above formula (3) is determined.・ Mi
n)) satisfies the regions represented by the formulas (4) and (5), and within the range of the elapsed time t defined by the formula (3), the average water density d falls within the region of the straight line B or more. By adopting a cooling pattern that must not exist, the temperature of the surface of the slab is temporarily reduced in the initial stage of secondary cooling, then reheated by the latent heat of solidification of unsolidified molten steel, and then an appropriate cooling rate is set. Then, the microstructure of the cast slab is a ferrite-pearlite structure, and the γ grain boundaries are not clear. Hereinafter, the region in which the γ grain boundaries are unclear shown in FIG. 3 is referred to as the present invention region for convenience.

In actual continuous casting, a cooling pattern as shown in FIG. 4 may be adopted.

FIG. 4 is a diagram for explaining an example of the cooling pattern of the method of the present invention. Is the elapsed time t after passing through the mold
Examples in which the average water density d falls within the range of the present invention in the range of 0.5 to about 1.25 min and the maximum average water density d is relatively low,
Is an example in which the average water density d falls within the range of the present invention in the entire range of the time t from 0.5 to less than 2.5 min. Is a pattern similar to, but the average water density d is relatively high, and This is an example in which the average water amount density d falls within the range of the present invention in the latter half range of t from 1.5 to less than 2.5 min and is low and substantially constant.

Since sprays and rolls are alternately arranged in the secondary cooling zone of actual continuous casting, secondary cooling is intermittent cooling. Some secondary cooling methods use a mist spray in which air and water are mixed and some use a water spray. However, the amount of air is taken into consideration because the contribution of water amount density is greater to the cooling capacity. There is no big problem if you do not include it.

When the casting conditions are different, the time pitch between spray cooling and non-cooling due to the presence of rolls,
In addition, the temperature history of cooling and recuperation received from the one-stage spray or mist nozzle changes, but the cooling conditions for forming a predetermined microstructure on the surface of the slab can be arranged using the average water content density d. It doesn't matter.

When the method of the present invention is applied, the desirable casting speed range is about 0.4 to 5.0 m / min, and the desirable slab size is about 700 to 2500 mm in width and 100 to 350 mm in thickness.

<Mechanism of Obscuring γ Grain Boundary> In the ferrite-pearlite structure, film-like ferrite is generated in the γ grain boundary portion as shown in FIG. 2 (a), so that the γ grain boundary can be observed. Becomes Generally, when the γ grain size is coarse and the grain boundary ferrites coalesce under the condition that the γ grain boundary is linear,
Alternatively, it is known that the grain boundary ferrite becomes a film when the grain boundary ferrite is generated by dynamic precipitation. However, in the as-cast slab, there is no large difference in the γ grain size, and strain that causes dynamic precipitation of grain boundary ferrite does not occur. According to the results of observation of the microstructure of the slab, the shape of the grain boundary ferrite in the as-cast slab is determined by the supercooling effect of the secondary cooling.

FIG. 5 is a diagram schematically showing the generation mechanism of grain boundary ferrite. When the slab is pulled out of the mold and then rapidly cooled, it becomes a supercooled state, and ferrite is generated at the γ grain boundary as shown on the left of FIG. 5, regardless of the crystal orientation of the adjacent γ grains. For this reason, the matching between the grain boundary ferrite and the γ grains is poor, and the ferrite grows into grains, so that the γ grain boundary becomes unclear.

On the other hand, when the cooling rate is slowed and the material is gradually cooled, it is gradually cooled in the latter half of continuous casting, and the grain boundary ferrite corresponding to the crystal orientation of the γ grains precipitates. The growth of ferrite does not proceed to the other γ grain side. For this reason, the original γ grain boundary remains and becomes a clear γ grain boundary. Therefore, as can be seen from FIG. 3, when the average water amount density d is reduced to a straight line A or less, or when cooling is performed for 2.5 minutes or more after passing through the mold, a sufficient cooling rate cannot be obtained and the γ grain boundary It becomes a clear organization.

As described above, when the average water content density is extremely increased or when it is cooled at a relatively high average water content density for a long time, a bainite structure is formed and the γ grain boundary becomes clear.

In order to prevent this, it is necessary to reheat the slab that has been cooled until the grain boundary ferrite is precipitated in the secondary cooling process. Although γ-single-crystallized during recuperation, the diffusion rate of elements in the ferrite phase is remarkably higher than that in the γ phase, so it is considered that some traces of proeutectoid ferrite remain near the grain boundaries.

Even when the cooling rate for the γ → α transformation again after the heat recovery is increased, the microstructure on the surface of the cast slab becomes a bainite structure, and the γ grain boundary becomes clear. However, in the continuous casting process, this transformation is generally at the end or after the end of continuous casting, the cooling rate is slow, and the microstructure on the surface of the cast piece becomes a ferrite-pearlite structure. Therefore, it is not necessary to specify the cooling conditions for the slab after the secondary cooling by the method of the present invention is completed.

As described above, by controlling the secondary cooling conditions, it is possible to obscure the γ grain boundary and reduce the surface cracking susceptibility, but the slab surface temperature at the bending and straightening points of continuous casting is Needless to say, high effects can be obtained by avoiding the high embrittlement temperature range of steel. In the method of the present invention, since a curved or vertical bending type continuous casting machine having a slab straightening portion is used, it is preferable that the slab surface temperature at the bending / straightening point is 850 ° C. or higher.

As described above, in order to obscure the γ grain boundary near the surface of the slab, it is necessary to reheat the second half of the secondary cooling. For that purpose, the unsolidified molten steel needs to remain in the slab during the secondary cooling, and the method of the present invention is effective in the slab thickness of 100 mm or more. In the range of 100 mm or more, the recuperation behavior is mainly governed by the thermal resistance of the solidified shell, so it is not necessary to change the above relational expression even if the thickness of the slab is changed.

It should be noted that when the time of holding at high temperature from casting to the start of secondary cooling becomes long, the γ grain size in the vicinity of the surface of the slab remarkably increases, and the effect of suppressing surface cracking by the above-mentioned structure control can be obtained. Disappear. However, the time required for the slab to pass through the mold is within 2 min in normal continuous casting, and even under this condition, if the predetermined secondary cooling is carried out immediately after the mold has passed, there is no adverse effect on the microstructured surface of the slab.

[0056]

Example Using a steel having the chemical composition shown in Table 1 and a curved continuous casting machine of an actual production line, the average water amount density d with respect to the elapsed time t of the slab after passing through the mold was changed under the conditions shown in Table 2. A slab was manufactured and the microstructure and cracks on the surface of the slab were investigated. The steel types shown in Table 1 have high crack susceptibility. Casting speed is 0.7m / min, slab dimensions are width 2200mm, thickness
It was 240 mm. For measuring the surface temperature of the slab, a method of engaging a thermocouple on the surface of the slab just below the mold was used.

[0057]

[Table 1]

[0058]

[Table 2]

The cooling patterns of Examples 1 to 4 of the present invention correspond to the items 1 to 4 shown in FIG. The cooling patterns of Comparative Examples 1 to 4 are shown together in FIG. 4 for convenience.

The average water amount density d under the cooling conditions shown in Table 2 is
As a representative value thereof, the values were obtained when the elapsed time t after passing through the mold was 1 min and 2 min.

As described above, in order to suppress surface cracking, it is important that the surface temperature of the cast slab at the bending and straightening points avoids the embrittlement temperature range. In each of the inventive examples and the comparative examples in Table 2, the brittle temperature range was avoided. That is,
In order to clarify the effect of the secondary cooling conditions, the total amount of cooling water after passing through the mold to the straightening point was controlled so that the range of the slab surface temperature at the straightening point was approximately 850 ° C to 900 ° C.

The evaluation was carried out based on the microstructure of the obtained slab surface and the degree of surface cracking. In the microstructure, the case where the γ grain boundary was not clear was marked with ◯, and the case where it was clear was marked with x. When observing the occurrence of cracks, the surface of the slab is scarfed to remove the oxide film on the surface and then visually observed. When no cracks occur using the crack generation code, it is 0, and cracks with a depth of 30 mm or more The index was indexed into 6 levels, with 5 being the case. Table 2 also shows the results.

Inventive Example 1 is a case in which the average water content density within about 1.25 min after passing through the mold was made to satisfy the range of the present invention, and the subsequent average water content density was reduced to outside the range of the present invention. In, the γ grain boundary became unclear and surface cracking did not occur.

Inventive Example 2 changes the water amount density distribution,
This is the case where the average water content density is made to satisfy the range of the present invention for a longer time, but even under this condition, the γ grain boundary becomes unclear and surface cracking does not occur.

Example 3 of the present invention is a case where the average water density was further increased under the same conditions as Example 2 of the present invention.
Although the grain boundaries became unclear, several minor surface cracks with a depth of 5 mm or less occurred near the corners over almost the entire length of the slab. However, the degree was acceptable. This is because the recuperation was small because the average water density was increased over a long period of time, and the surface temperature near the corner at the straightening point of the slab was further reduced.

Inventive Example 4 is a case in which the average water amount density after passing through the mold is further lowered as compared with Inventive Example 1 and the average water amount density exceeds the straight line A after about 1.5 minutes, so that the present invention range is satisfied. Although the γ grain boundary was unclear, some minor surface cracks with a depth of 5 mm or less occurred near the corners of the slab. Under these conditions, it was considered that the surface temperature near the corner at the straightening point was lowered, and the surface quality of the slab was deteriorated as compared with Inventive Examples 1 and 2.

In Comparative Example 1, since the average water content density is low and all the conditions are outside the range of the present invention, the γ grain boundary is clear because the surface portion of the slab does not undergo γ → α transformation immediately below the mold. It became a different organization. Further, surface cracks having a depth of about 10 mm occurred over the entire surface of the slab, and the crack code was markedly deteriorated to 4. In Comparative Example 2, the average water amount density exceeds the straight line A after 2.5 minutes or more, but like Comparative Example 1, all of the conditions are outside the range of the present invention, so the cooling rate is slow and the γ grain boundaries are microscopic. It became an organization and the crack code became 4. Comparative Examples 3 and 4 are cases in which the average water density is extremely increased, but even if there is a difference before and after in the time zone after passing, the average water density in a certain time zone shows the straight line B in any case. Since it entered the region beyond the limit, the microstructure on the surface of the cast slab became bainite, and surface cracking which is considered to be caused by thermal stress occurred.

[0068]

According to the method of the present invention, it is possible to reduce or prevent surface cracks such as lateral cracks that occur on the surface of a slab during continuous casting.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a solidification mechanism. (a) is Fe
-C phase diagram, (b) shows carbon equivalents of A, B, C and D
It is a schematic diagram which shows the coagulation in the case of.

FIG. 2 is a copy of a photograph showing a typical microstructure of a cracked part and a non-cracked part on the surface of a cast slab. (a) shows the case where cracks occur, and (b) shows the case where cracks do not occur.

FIG. 3 is a diagram showing the influence of the elapsed time after the slab has passed through the mold and the average water density of the secondary cooling on the microstructure of the slab.

FIG. 4 is a diagram illustrating an example of a cooling pattern of the method of the present invention.

FIG. 5 is a diagram schematically showing a generation mechanism of grain boundary ferrite.

Claims (1)

[Claims] 1. A steel slab in which the contents of C, Mn, Ni, Cu and N and the carbon equivalent Cp satisfy the relations of the following formulas (1) and (2) are continuously bent or bent. When manufacturing using a casting machine, secondary cooling is performed immediately after pulling out the slab from the mold, and in this secondary cooling pattern, the following formula
Average water volume density d of cooling water in an arbitrary time zone within the elapsed time t (min) after passing through the mold of the slab specified in (3)
(Liter / (cm ² · min)) satisfies the regions represented by the following formulas (4) and (5), and within the range of the elapsed time t defined by the following formula (3), the average water volume density d is Equation (6) and
A method for suppressing slab surface cracking, which comprises cooling so as to fill a region represented by (7). Cp = C (%) + Mn (%) / 33 + Ni (%) / 25 + Cu (%) / 44 + N (%) / 1.7 ・ ・ ・ (1) Cp <0.18 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (2 ) 0.5 ≦ t <2.5 ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (3) A <d ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (4) A = −0.009 t + 0.043 ・ ・ ・ ・ ・・ (5) d <B ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (6) B = -0.02t ＋ 0.11 ・ ・ ・ ・ ・ ・ (7) However, X (%) is the X component Represents the mass% of