JP3008825B2 - Slab surface crack suppression method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing surface cracks of a slab during continuous casting of steel.

[0002]

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

[0003] These cracks are caused when the surface temperature of the slab during the secondary cooling of continuous casting is close to the γ → α transformation temperature (about 600 to 850).
° C) and the hot ductility is reduced, and it is known that the hot ductility is caused at this time by receiving a correcting stress due to the correction of the slab.

[0004] On the other hand, there is a method of suppressing cracking by avoiding a temperature range in which the surface temperature at the time of slab correction is reduced in hot ductility (hereinafter referred to as embrittlement temperature range) on a low temperature side or a high temperature side. Usually taken. However, it is impossible to prevent cracking only by controlling the surface temperature during slab correction, and the following various methods have been proposed.

[0005] For example, Japanese Patent Publication No. 58-3790 discloses a cooling pattern in which a temperature range in which the surface temperature at the correction point is reduced in ductility can be avoided to a lower temperature side, and an upper part of a secondary cooling zone is provided. Forcibly cool the slab surface temperature to 650 ~
A method of once transforming to 700 ° C. is disclosed.

Japanese Patent Application Laid-Open No. 5-329505 discloses a method of cooling and maintaining the surface layer of a slab at a temperature of 350 to 500 ° C. for one minute or more.

[0007] In any of these methods, once the surface temperature of the slab is lowered, phase transformation occurs in most or the whole of the slab, and the cracking sensitivity is systematically reduced. However, once the slab surface temperature reaches 700 ° C
When the temperature is lowered to the following, it is thermally difficult to avoid the embrittlement temperature range on the high temperature side even if the steel sheet is subsequently subjected to double heating. On the other hand, it is difficult to avoid the brittle temperature range at the time of straightening of the slab to a lower temperature side due to a change in cooling characteristics of a steel type having a high cracking sensitivity due to a large alloy content.

Further, since the surface cracks occur at the γ grain boundaries, there have been many proposals to pay attention to the γ grain size and reduce the size. In order to suppress the growth of γ grains, the applicant of the present invention disclosed in Japanese Patent Application Laid-Open No. 63-63559 a method in which the cooling rate from the austenite single crystallizing temperature was set to 10 ° C./sec or more. In the gazette, a relational expression of a mold length is defined, and a method of drawing out a slab early and immediately performing secondary cooling is proposed. However, the austenite single crystallization temperature near the slab surface is usually inside the mold, making it difficult to control the cooling rate, and making the mold length extremely short than usual is likely to cause operational problems. Therefore, it was difficult to commercialize any of them.

On the other hand, it is known that AlN is precipitated at the grain boundary where the crack has occurred, and the stress concentration accompanying the AlN promotes the crack. On the other hand, in order to suppress the precipitation of AlN in steel, Ti is often added to precipitate TiN, and a high effect is obtained. In addition, Tokubo Sho 5
In the method for preventing surface cracks disclosed in Japanese Patent Application Laid-Open No. 5-7106, AlN precipitation is controlled by controlling cooling conditions. However, there are many steel types for which Ti cannot be added due to material property requirements, and there is a problem that the control of AlN precipitation by cooling conditions lacks stability.

As described above, a number of methods have been proposed for preventing the surface cracks of cast slabs, but all have their advantages and disadvantages, and at present, surface cracks frequently occur.

[0011]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems by focusing on the relationship between the microstructure of the slab (the state of formation of grain boundary ferrite) and the susceptibility to cracking, in addition to controlling the γ grain size. It was done to do so.

An object of the present invention is to provide a method for suppressing surface cracks such as lateral cracks by controlling the microstructure of a continuous cast slab.

[0013]

Means for Solving the Problems The present inventors have investigated in detail the microstructure of a portion where a surface crack has occurred in a continuously cast slab.

As a result, it was found that there was a clear correlation between the microstructure of the slab (the state of formation of grain boundary ferrite) and the susceptibility to cracking (the presence or absence of cracking). The present invention has been made.

The gist of the present invention resides in the following methods (1) and (2) for suppressing surface cracks of a slab during continuous casting of steel.

(1) When producing a steel slab having a content of each of C, Mn, Ni, Cu and N satisfying the following formula using a curved or vertical bending type continuous casting machine, The time required for drawing the slab from the meniscus part of the molten steel to the lower end of the mold (hereinafter referred to as “mould passing time”) is set to within 1 minute. After drawing from the mold, the secondary cooling is immediately performed, and the slab surface temperature is reduced within 1 minute. cast slab surface cracks suppression method in the continuous casting of steel, characterized by cooling to below a ₃ transformation temperature. Hereinafter, this is referred to as a first method of the present invention.

Cp = C (%) + Mn (%) / 33 + Ni (%) / 25 + Cu (%) / 44 + N (%) / 1.7 Cp <0.18 Where Cp represents carbon equivalent and (%) represents mass%.

(2) In accordance with the above (1), after cooling to the A ₃ transformation temperature or less, the steel sheet is heated to 850 ° C. or more at the bending point and the straightening point by double heating. A method for suppressing slab surface cracking during continuous casting of steel, wherein slab straightening is completed within 20 minutes after passing. Hereinafter, this is referred to as a second method of the present invention.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the process and theory leading to the completion of the present invention will be described.

As described above, there is a clear correlation between the microstructure of the slab (the state of formation of grain boundary ferrite) and the susceptibility to cracking (the presence or absence of cracking). This will be described with reference to FIG.

FIG. 1 is a simulated view of a photograph showing a typical example of a microstructure in a portion where a crack has occurred and a portion where no crack has occurred on the surface of a slab obtained in actual production. FIG. 1 (a) shows a case where cracks occur, and FIG. 1 (b) shows a case where cracks do not occur. The structures are all ferrite-pearlite structures.

As shown in FIGS. 1 (a) and 1 (b), the structure when a crack occurs has a clear γ grain boundary, whereas when no crack occurs, the γ grain boundary is unclear. Becomes
As described above, this crack is a γ grain boundary crack, and when the γ grain boundary is clear, stress concentration on the grain boundary portion becomes remarkable,
It is considered that the crack sensitivity increased and the cracks occurred.

In the structure as shown in FIG. 1A in which the γ grain boundary is clear, ferrite at the grain boundary is formed in a film form.
It has the characteristic of being coarse.

From these results, it was considered that if the microstructure of the slab (the state of formation of grain boundary ferrite) is appropriately controlled, the susceptibility of the slab to cracking is reduced, and the cracking of the slab can be reduced.

When cracking does not occur, the structure is finer than when cracking occurs, and traces of dendrite remain. In other words, it can be seen that when no crack occurs, the holding time at a high temperature is short, and cooling has been applied to make the structure finer.

The following basic tests were conducted to elucidate the control mechanism of the microstructure of the slab based on this finding.

[0027] 200kg of molten steel is about 400mm x by static casting.
A slab of 400 mm x 200 mm was taken out of the mold before complete solidification of the slab and cooled by controlled spraying.
The temperature history of the surface of the slab was measured by a thermocouple or a radiation thermometer which had been previously cast, and changes in the structure of the slab with respect to various temperature histories were investigated.

From the test results, it was found that the following a. And b. Were important in order to obtain a microstructure having low crack susceptibility, and the first method of the present invention was completed.

A. Do not enlarge the gamma particle size.

B. The γ grain boundary ferrite should not be formed into a film.

Next, the reasons for limiting the manufacturing conditions in the first method of the present invention will be described.

[Principle of Control of γ Particle Size] As described above, it is well known that miniaturization of γ particle size is effective for suppressing cracking. In order to suppress cracking by reducing the γ grain size, γ is required near the surface layer of the slab where cracking is a problem.
It is necessary that the particle size is remarkably reduced to 1 mm or less.

To realize this, the above-mentioned Japanese Patent Application Laid-Open No.
A method that is difficult to apply to actual operations, such as the methods disclosed in JP-A-3-63559 and JP-A-61-195742, is required.

Since there is a correlation between the γ grain size and the morphology of the γ grain boundary ferrite, the combination of the γ grain boundary ferrite shape control and the γ grain boundary ferrite morphology control allows the susceptibility to cracking without extremely miniaturizing the γ grain size. It is possible to have a low organization.

FIG. 2 is a schematic view showing a γ grain morphology. FIG.
2A shows an example of a cell, and FIG. 2B shows an example of a network. When the γ grain size increases, the slab surface becomes a cellular form as shown in FIG. 2 (a), and the radius of curvature of the grain boundary increases. At this time, since the crystal orientations of the adjacent pro-eutectoid ferrites formed at the grain boundaries match, the pro-eutectoid ferrites are united during the growth process, resulting in a film-like shape, resulting in an increase in cracking susceptibility. .

On the other hand, if the enlargement of the γ grain size is prevented, the γ grain boundary becomes a network from the surface of the slab as shown in FIG. 2 (b), and the crystal orientation of the ferrites formed at the grain boundary. Is not consistent and does not form a film. That is, in order to prevent the grain boundary ferrite from becoming a film, it is necessary to prevent the gamma grain size from becoming too large and becoming a cell.

Further, in order to prevent the γ grain size from becoming too large to form a cell, it is necessary to shorten the time of holding at a high temperature after the γ single crystal. That is, the γ grains in the vicinity of the surface layer of the slab generated in the form of a fine network at the time of γ single crystal growth grow while being maintained at a high temperature to become a cell shape. It is known that the growth of γ grains is caused by the diffusion of component elements in steel after γ single crystallization, and the diffusion is extremely rapid in a high temperature region as represented by an Arrhenius type. Therefore, the growth of γ grains can be suppressed by shortening the time of maintaining at high temperature after γ single crystallization in the continuous casting process.

In the above-described basic test, as a result of investigating the change of γ grains by variously changing the holding time at a high temperature, the mold passing time was set to within 1 minute in the actual manufacturing process, and immediately after the mold was pulled out of the mold, the following conditions were satisfied. It was found that secondary cooling in any of the steel types could prevent the γ grains from becoming cellular. As long as the above-mentioned mold passage time is within 1 minute, the faster the better, the more practically desirable lower limit is 1
It is about 0 seconds.

The temperature history of the cast slab in the mold is slightly different depending on the shape, material, surface plating method, etc. of the mold, and the growth behavior of the γ grains is also changed accordingly. However, the change is small, and the above conditions can be applied under any template conditions.

Next, the reason for limiting the carbon equivalent Cp will be described.

FIG. 3 is a diagram schematically showing the mechanism of solidification in various compositions. FIG. 3 (a) is a phase diagram of the Fe—C system showing the relationship between the carbon equivalent Cp and the temperature, and FIG. 3 (b) schematically shows the solidification process when the carbon equivalent Cp is A, B, C and D. FIG. As shown by C and D in FIG. 3 (b), in a composition corresponding to hyperperitetic or γ single crystal solidification on the right side of the peritectic composition, the liquid phase (L) and γ phase coexist immediately before complete solidification. , The final solidification position coincides with the γ grain boundary. Therefore, the pinning effect is large due to segregation and precipitation of the component elements at the γ grain boundary, and the growth of the γ grain size after solidification is small. That is, even if the time required until the start of the secondary cooling at the lower end of the mold changes, the γ particle size hardly changes, and it is not necessary to define the time required to reach the lower end of the mold.

The above equation is known as an equation for obtaining the carbon equivalent of the peritectic reaction from the respective contents of C, Mn, Ni, Cu and N in steel. The Cp value obtained by this equation is
When it is smaller than 0.18, for example, when the carbon equivalent Cp is A or B as shown in FIG. 3 (b), δ solidification or subperitectic solidification occurs, and the growth of γ grain size after complete solidification becomes important. Therefore, the carbon equivalent Cp was limited to less than 0.18 as in the formula.

On the other hand, it has been found from the experience of actual production that there is a correlation between the Cp value and the occurrence of cracks.
At 10, the surface cracks hardly occur. Therefore, the effect of suppressing cracking by the method of the present invention has a substantial meaning when the Cp value is 0.10 or more.

Next, the reasons for limiting the secondary cooling conditions will be described.

[Principle of Form Control of Grain Boundary Ferrite] As described above, whether or not a γ grain boundary can be confirmed depends on the form of γ grain boundary ferrite. The γ grain boundary ferrite is one in which proeutectoid ferrite precipitated during the cooling process of the slab remains. As a result of a series of basic tests, it was found that the morphology of the γ grain boundary ferrite changes depending on the slab temperature history after drawing from the mold. FIG. 4 shows the temperature history of the slab surface, and FIG. 5 shows the morphology of the structures corresponding to these.

FIG. 4 is a diagram showing an example of the temperature history of the slab surface for controlling the microstructure of the slab. FIG. 5 is a mimetic diagram of a photograph showing the structure near the surface layer of the slab when the slab is strongly cooled by secondary cooling after the slab is pulled out from the mold and when the slab is gradually cooled. FIG. 5 (a) shows that when the mold was pulled out from the mold so that the holding time at a high temperature was shortened and then cooled strongly,
Is the case of slow cooling.

When strong cooling is applied immediately after being pulled out of the mold, the ferrite at the γ grain boundary does not form a film as shown in FIG. 5A, and the γ grain boundary is not clear. On the other hand, when slowly cooled after being pulled out of the mold, a film-like grain boundary ferrite is formed as shown in FIG.
It becomes a clear γ grain boundary.

In this test, only the cooling conditions immediately after being pulled out of the mold were changed, and it was found that cooling in this portion determines the form of grain boundary ferrite. It has been confirmed that the same effect can be obtained even if the cooling pattern in the latter half after 5 minutes or more from the start of cooling is changed. The secondary cooling method here is not particularly limited.

FIG. 6 is a diagram showing the results of investigation of the change in the structure in various cooling patterns. This figure 6
Is a table summarizing the minimum temperature until reheating after being pulled out of the mold and the time required until the temperature reaches the minimum temperature after being pulled out of the mold. The evaluation of the structure was performed in three stages: when the γ grain boundary was clear, when it was partially unclear, and when it was unclear. Subject steel species, and 2 steel species A ₃ transformation temperature of about 805 ° C., either after casting, the time required for pulling out the mold 1
Within minutes.

From the results shown in FIG. 6, it is clear that if the slab surface temperature is rapidly cooled to the A ₃ transformation temperature or less within 1 minute after being extracted from the mold, the γ grain boundary becomes unclear. Was. Further, the same investigation was performed for other steel types, and after the slab was drawn out of the mold, the slab surface temperature was adjusted to A within one minute.
It was confirmed that the γ grain boundaries became unclear in any case when the temperature was cooled to ₃ or less. Therefore, the aforementioned Japanese Patent Publication No. 58-
As in the method of 3790, the surface temperature of the slab is 650-
There is no need to cool to temperatures as low as 700 ° C.

FIG. 7 is a diagram schematically showing the mechanism of γ grain boundary ferrite formation. After withdrawal from FIG. 7 (a) template, the billet surface temperature when rapidly cooled to A ₃ transformation temperature or less, 7 (b) is a time required for the A ₃ transformation temperature of more than 1 minute within a minute This is the case where cooling is performed slowly. When the slab is rapidly cooled after the slab is drawn out of the mold, ferrite is generated at the grain boundary irrespective of the crystal orientation of the adjacent γ grains as shown in FIG. Therefore, the coherence between the grain boundary ferrite and the γ grains is poor, and the grains grow so that the γ grain boundaries become unclear. On the other hand, after being pulled out of the mold, if gradually cooled as described above, as shown in FIG. 7 (b), γ grain boundary ferrite having an orientation corresponding to the crystal orientation of γ grains precipitates during the cooling process, and The growth of ferrite does not progress on the γ grain side, and the original γ grain boundary remains, forming a clear γ grain boundary.

In this case, the surface temperature of the slab was cooled to the A ₃ transformation temperature or lower, and then returned to the γ single crystal temperature by recuperation. In each case, a good structure was obtained. This is once A ₃
It is considered that if the temperature is cooled below the transformation temperature, some traces of pro-eutectoid ferrite remain at the grain boundaries even if the γ-single crystal is formed by reheating, and it is considered that it does not depend on the subsequent cooling conditions.

For the A ₃ transformation temperature, relational expressions which can be easily calculated with respect to the component content have been reported, and these relational expressions may be used or may be measured by a basic test or the like. Absent. The above is the first method of the present invention.

According to the second method of the present invention, after cooling to the A ₃ transformation temperature or lower according to the first method of the present invention, the slab surface temperature is double-heated, and the slab surface temperature at the bending point and the straightening point is reduced to 850. C. or more, and the straightening of the slab is completed within 20 minutes after the molten steel in the mold passes through the meniscus.

The above series of basic tests revealed a method of systematically reducing the susceptibility of a slab to cracking. However, as described above, in a low alloy steel having a large alloy component content, it is difficult to avoid the brittle temperature range at the time of slab bending and straightening to a lower temperature side due to a change in cooling characteristics. For this reason, a specific method is required that can avoid the embrittlement temperature range on the high temperature side.

This condition is to limit the surface temperature of the slab at the bending point and the straightening point. Although the ductility differs depending on the steel type, the hot ductility of many steel types almost recovers at around 850 ° C, so the surface temperature at the bending and straightening points of the slab is 850 ° C
What is necessary is just to secure more than ° C. A desirable upper limit for this temperature is 10
It is around 50 ° C.

However, since the corners of the slab are cooled in two directions, the long side and the short side, it is difficult to secure 850 ° C. or more, especially at the correction point. In order to prevent the slab corner portion from being excessively cooled, a method called “width cutting” is usually performed in the secondary cooling. This is a method of preventing spray water from hitting the corner portion of the slab in order to prevent overcooling of the corner portion of the slab by secondary cooling.
A number of devices and methods have been proposed in, for example, Japanese Patent Publication No. However, even if the "width cutting" is performed, the corner portion cools faster than other portions due to cooling by a roll and radiant cooling.

As a result of studying the operating conditions of various continuous casting machines by the present inventor, it has been found that even if countermeasures such as “width cutting” are taken, the time required from the passage of the meniscus of molten steel in the mold to the correction of the slab is 20 minutes. It was found that if the length was longer than a minute, the corner portion could not be prevented from being overcooled, and the brittle temperature range could not be avoided on the high temperature side.

On the other hand, as a result of further detailed investigation of the operation of continuous casting, it was found that if the difference in surface temperature between the corner portion and the center portion of the width at the correction point of the slab increases, surface cracks are promoted by thermal stress. Also got. In other words, as a result of actually measuring the surface temperature of the slab, the difference in surface temperature between the corner and the center of the width increased with time under any operating conditions, and surface cracks occurred when the required time exceeded 20 minutes. Easier to do. For the above two reasons, it was decided to finish bending and straightening the slab within 20 minutes after passing the meniscus.

As described above, it is difficult to avoid the brittle temperature range at the time of bending and straightening of the cast slab to a low temperature side due to a change in the cooling characteristics of the low alloy steel. On the other hand, the method of avoiding the embrittlement temperature range on the high temperature side is not limited by such a steel type. When avoiding the embrittlement temperature range on the high temperature side, there is a problem that the heat load of the continuous casting machine is large because the slab temperature is maintained at a high temperature. However, there is no problem if the design is made in advance to cope with the heat load. Therefore, the second method of the present invention is a continuous casting method applicable to all steel types satisfying the above formulas and

Since the surface cracks of the slab are caused by the straightening distortion of the slab, the method of the present invention is applicable to the production of a slab using a curved or vertical bending type continuous casting machine having a slab straightening part. It is effective for However, it goes without saying that reducing the susceptibility of the slab structure to cracking is also effective in suppressing flaws generated in the manufacturing process, even in a continuous casting machine having no slab straightening section.

[0062]

EXAMPLE Using a curved or vertical bending type continuous casting machine of an actual production line, a casting test was conducted in which the length under the meniscus of the mold and the casting speed were variously changed in order to change the passage time in the mold. . The surface temperature of the slab was controlled by variously changing the amount of secondary cooling water just below the mold, and the surface temperature of the slab was measured using a thermocouple biting into the surface of the slab just below the mold.

(Test 1: First Method Example and Comparative Example of the Present Invention) Table 1 shows the chemical composition and A ₃ transformation temperature of the steel used.

[0064]

[Table 1]

In order to clarify the effect of suppressing the surface cracking by the method of the present invention, these steels were made to have high crack susceptibility. A ₃ transformation temperature was determined by the calculation formula which is known conventionally composition of the steel. Table 2 shows the casting conditions and evaluation.

[0066]

[Table 2]

The evaluation was made on the basis of the structure of the obtained slab and the degree of occurrence of cracks. Regarding the structure of the cast slab, 場合 indicates that the γ grain boundary was unclear, X indicates that it was clear, and Δ indicates that it was partially unclear. The occurrence of cracks was evaluated by visual observation after removing a surface oxide film by applying a scarf to the surface of the slab. The obtained results were indexed into six stages of crack occurrence codes, where 0 when no crack occurred and 5 when a crack having a depth of 30 mm or more was present.

As described above, in order to suppress surface cracking, the surface temperature of the slab at the bending point and the straightening point must be set so as to avoid the embrittlement temperature range. In each of the present invention examples and comparative examples in Table 2, the brittle temperature range was avoided, and in order to clarify the influence of the casting conditions, the secondary temperature was set so that the surface temperature of the slab at the correction point was almost equal. Controlling cooling.

As shown in Table 2, in Example 1 of the present invention in which the mold passing time was set within 1 minute and the A ₃ transformation temperature was set within 1 minute after passing through the mold, cracks having a depth of about 5 mm were formed on the surface of the slab. Are scattered over almost the entire length.
Was evaluated. In Example 2 of the present invention in which the mold length was shortened, the casting speed was increased, and the mold passage time was significantly shortened, several cracks having a depth of 5 mm or less were generated on the surface of the slab. It was evaluated, and the surface crack was further reduced.

On the other hand, Comparative Example 1 which required one minute or more for the mold passing time, and Comparative Example 2 in which the cooling immediately below the mold was remarkably weakened
In each case, a surface crack having a depth of about 10 mm occurred over the entire surface, and the crack generation code was evaluated as 4, which was clearly worse than that of Example 1 of the present invention. The microstructure of the surface layer of the slab also
In Example 1 of the present invention, the γ grain boundaries were unclear, while in Comparative Examples 1 and 2, the γ grain boundaries were clearly observed.

In Comparative Example 3 in which the cooling immediately below the mold was slightly weaker than that of Example 1 of the present invention, and the cooling was such that the A ₃ transformation temperature was not reached slightly, the evaluation of the surface cracking code was 3;
It was somewhat improved over Comparative Example 2. However, in any of the extent of tissue and surface cracking also, the present invention Example 1 there is a large difference, it can be seen that to be cooled to below A ₃ transformation temperature in the mold immediately below are important.

(Test 2: Second Method Example and Comparative Example of the Invention) Table 3 shows the chemical composition and A ₃ transformation temperature of the steel used.

[0073]

[Table 3]

This steel type contains Ni, and its cooling characteristics at about 1000 ° C. or less change, so that it is difficult to avoid the embrittlement temperature range on the low temperature side. The A ₃ transformation temperature was determined in the same manner as in Table 1. Table 4 shows the casting conditions, the surface temperature of the slab at the correction point, and the evaluation.

[0075]

[Table 4]

The measurement of the temperature at the correction point was performed by using a contact-type thermocouple or a radiation thermometer on the corner of the slab. The evaluation was performed according to the test 1.

As shown in Table 4, in Example 3 of the present invention, the slab surface temperature at the straightening point was reduced to 850 ° C. or more and the time required for the straightening point to be shortened to within 20 minutes. An extremely high effect was found in suppressing cracking.

On the other hand, Comparative Example 4, in which the time required to correct the point was extended to 28 minutes, was evaluated as 2 in the surface crack generation code. In Comparative Example 4, although the corner temperature at the correction point was substantially equal to that of Example 3 of the present invention, the required time to the correction point was long. Was encouraged. It is apparent from the comparison with the above-described present invention 3 that shortening the time required for the correction point has a high effect.

In Comparative Example 5, the slab surface temperature at the correction point was 690 ° C. so that the surface temperature of the slab at the correction point could be avoided to the lower side of the embrittlement temperature range. However, as described above, the cooling characteristics of this steel type varied depending on the alloy components, the temperature of the slab surface was largely uneven, and a severe crack of 5 occurred in the crack occurrence code. Therefore, in this steel type, it is necessary to avoid the embrittlement temperature range toward the high temperature side.

In Comparative Example 6, as a result of setting the surface temperature of the cast slab at the correction point to 820 ° C., a rating of 4 was obtained in the crack occurrence code. This is presumably because the surface temperature of the slab at the correction point could not sufficiently avoid the embrittlement temperature range.
Therefore, it is necessary that the surface temperature of the slab at the correction point can reliably avoid the brittle temperature range, and the surface temperature is 85%.
It is essential to maintain a temperature of 0 ° C or higher.

In each of Comparative Examples 4 to 6, since the mold passing time was set to 1 minute or less and the slab surface temperature was cooled to the A ₃ transformation temperature or less within 1 minute after passing the mold, systematically. In each case, the γ grain boundaries were unclear. in this way,
Even if the surface temperature of the slab cannot sufficiently avoid the embrittlement temperature range, or severe surface cracking may occur depending on the type of steel, even if the structure is low in crack sensitivity by defining the conditions from the mold to the portion immediately below the mold . The second method of the present invention is to prevent this and obtain the effect of suppressing surface cracks more reliably.

[0082]

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

[Brief description of the drawings]

FIG. 1 is a simulated view of a photograph showing a typical example of a microstructure in a crack occurrence portion and a non-incidence portion on a slab surface.
(a) is the case where cracks occur, and (b) is the case where cracks do not occur.

FIG. 2 is a schematic diagram showing a γ grain morphology. (a) is a cellular example, and (b) is a network example.

FIG. 3 is a diagram schematically showing a mechanism of solidification in various compositions. (a) is a graph showing the relationship between the carbon equivalent Cp and the temperature.
FIG. 3 (b) is a diagram showing the solidification process when the carbon equivalent Cp is A, B, C and D. FIG.

FIG. 4 is a diagram showing an example of a temperature history of a slab surface for controlling a microstructure of the slab.

FIG. 5 is a schematic view of a photograph showing the structure near the surface layer of a slab when the slab is strongly cooled by secondary cooling after the slab is pulled out from the mold and when the slab is gradually cooled. If (a) is pulled out of the mold so that the holding time at high temperature is short, and then cooled strongly,
(b) is the case of slow cooling.

FIG. 6 is a diagram showing the results of investigating changes in the structure in various cooling patterns.

FIG. 7 is a diagram schematically showing the mechanism of γ grain boundary ferrite formation. (a) is after withdrawal from the mold, when the cast slab surface temperature was rapidly cooled to A ₃ transformation temperature or lower within 1 minute, (b) if the gradual cooling of the time required for the A ₃ transformation temperature of more than 1 minute It is.

Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI C21D 9/00 101 C21D 9/00 101W (56) References JP-A-61-276747 (JP, A) JP-A-61-195742 (JP, A) JP-A-63-63559 (JP, A) JP-A-53-106335 (JP, A) JP-A-61-189851 (JP, A) JP-A-2-37941 (JP, A) JP-A-58 JP-A-5-329505 (JP, A) JP-B-58-3790 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) B22D 11/16 B22D 11/00 B22D 11/20 B22D 11/22 C21D 1/00 118 C21D 9/00 101

Claims (2)

(57) [Claims] 1. A method for producing a steel slab in which each content of C, Mn, Ni, Cu and N satisfies the following formula by using a curved or vertical bending type continuous casting machine: withdrawal duration of slab to the mold bottom to a maximum of 1 minute from the meniscus portion of, after withdrawal from the mold, immediately subjected to secondary cooling, cooling the billet surface temperature to below a ₃ transformation temperature within 1 minute A method for suppressing slab surface cracks during continuous casting of steel. Cp = C (%) + Mn (%) / 33 + Ni (%) / 25 + Cu (%) / 44 + N (%) / 1.7 Cp <0.18 ······································································% 2. A method for producing a steel slab in which each content of C, Mn, Ni, Cu and N satisfies the following formula by using a curved or vertical bending type continuous casting machine: the withdrawal time required billet to mold the lower end and within 1 minute from the meniscus portion, after withdrawal from the mold, immediately subjected to secondary cooling, the billet surface temperature is cooled to a ₃ transformation temperature or lower within 1 minute , After which the steel sheet is heated to 850 ° C or more at the bending point and the straightening point, and the straightening of the slab is completed within 20 minutes after the molten steel in the mold passes through the meniscus. A method for suppressing slab surface cracks during casting. Cp = C (%) + Mn (%) / 33 + Ni (%) / 25 + Cu (%) / 44 + N (%) / 1.7 Cp <0.18 ······································································%