JPH08132207A - Method for suppressing surface crack at the time of continuously casting steel - Google Patents

Abstract

PURPOSE: To provide a method for suppressing the surface crack at the time of continuously casting a steel, particularly Ni-containing steel. CONSTITUTION: In a secondary cooling at the time of continuously casting the steel containing 0-50% Ni, the cooling is executed under condition in which the relation of Ni content MNi in a cast slab, water quantity density W (l/min.m<2> ) of the secondary cooling and the surface temp. T ( deg.C) of the cast slab at the secondary cooling zone satisfy the following inequality to suppress the surface crack at the time of continuously casting the steel. The inequality is: T>(650+5W<1/2> .MNi <1/3> ). By this method, the surface crack of traversed crazing caused on the cast slab surface of the continuous casting can be prevented or reduced.

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 such as lateral cracks during continuous casting of steel, especially Ni-containing steel.

[0002]

2. Description of the Related Art When Ni is added to steel, low temperature toughness is improved. Therefore, steel containing Ni in an amount of 2 to 10% is used as a low temperature steel. Above all, steel containing about 9% Ni
It is used for tank materials such as liquefied natural gas because it withstands use below -160 ° C. However, when the Ni content is 5.5-10%, compared to ordinary carbon steel and low alloy steel,
Surface lateral cracks, subepidermal cracks, and corner cracks are severely generated, making continuous casting difficult.

On the other hand, not only the steel materials for special applications such as the 9% Ni steel described above but also the general thick plate materials are required to have material properties, so that the production amount of steel types containing 0.1 to 1% Ni increases. ing. However, lateral cracks may also occur during continuous casting of this steel, which hinders improvement in the orthogonality ratio to the hot rolling process.

[0004] These cracks become the slab surface temperature near the γ → α transformation temperature (about 600 to 850 ° C) which lowers the hot ductility at the time of the secondary cooling of continuous casting, at which time the thermal stress and the correction stress are reduced. It occurs by receiving.

Several methods have been proposed for improving the cooling method and for limiting the chemical composition of steel in order to continuously cast steel containing 5.5 to 10% of Ni.

As a method for improving the cooling method, for example, Japanese Patent Laid-Open No. 57-32862 discloses a cooling pattern of weak cooling that can avoid a temperature range where the surface temperature at the correction point is low in ductility on the high temperature side. A secondary cooling method is shown which aims at taking and uniformizing the surface temperature of the slab.

According to the present invention, by using an oval type nozzle or a steam nozzle (mist nozzle) as the secondary cooling water nozzle, the surface temperature of the slab is made uniform and the thermal stress generated on the surface of the slab is reduced, As a result, it is also described that it is possible to prevent surface defects on the slab. However, even if these measures are taken, the end portion in the width direction of the slab (the slab corner portion) is easily cooled, and the effect of recuperating heat from the inside of the slab is small, so that the surface temperature at the straightening point is less ductile. There is an inherent problem that it is difficult to maintain a stable temperature above the decreasing temperature.

Similarly, Japanese Examined Patent Publication No. 5-4169 discloses that 1150-
Based on the knowledge that if the cooling rate is controlled to 20 ° C / min or less in the temperature range of 950 ° C, the ductility is improved, the cooling rate during continuous casting should be 20 ° C / min or less in the temperature range of 1150 to 950 ° C. A method of preventing surface cracks during continuous casting by suppressing the amount has been proposed.

On the other hand, as a method for limiting the chemical composition, there is a method disclosed in JP-B-60-8134. This method improves the ductility of the material by adding Ca and Ti,
It is intended to prevent flaws generated on the surface of the slab. As for P and S, if these elements segregate at the grain boundaries, they tend to crack. For example, in JP-B-5-4169, in addition to suppressing the cooling rate, S: 0.003% or less, P: 0.010%
Hereinafter, it is said that it is desirable to highly purify N: 0.004% or less.

However, in low temperature steel containing 5.5 to 10% of Ni, even if continuously cast and cooled by the method as proposed above, the generation of cracks has not been completely prevented yet.

As a means for preventing lateral cracks occurring in continuous casting of general steel, for example, Japanese Patent Publication No. 58-
In Japanese Patent No. 3790, a method is proposed in which, immediately after being immediately cooled under the mold, the structure is refined, and then at the correction point, the high temperature embrittlement region near the γ → α transformation temperature is avoided. A method of controlling the secondary cooling is generally performed so as to avoid the high temperature embrittlement region on the high temperature side or the low temperature side in a place where the correction that causes the strain that occurs in the slab and causes the crack generation occurs. However, the Ni-containing steel is not sufficiently effective.

[0012]

Even with the above-mentioned conventional method, the occurrence of cracks on the surface of the slab cannot be avoided, and a slab with sufficient quality cannot be obtained. Further, as described above, in the conventional method, in order to prevent lateral cracks that occur during continuous casting of general steel or steel containing 0.1 to 1% of Ni, the high temperature embrittlement region is set at the high temperature side or low temperature at the correction point. It just controls the cooling to avoid it. Therefore, quantitative cooling conditions for continuously cast slabs that can be applied to steels with high Ni content have not been clarified.

The present invention solves these problems and provides a Ni
The present invention provides a cooling method capable of preventing or reducing lateral cracking of a slab by controlling secondary cooling conditions during continuous casting according to the content.

[0014]

DISCLOSURE OF THE INVENTION The gist of the present invention is a method for suppressing surface cracks in the following continuous casting.

In secondary cooling when continuously casting a steel containing Ni in an amount of 0 to 50% by mass, the Ni content of the slab M _Ni , 2
Continuous steel characterized in that cooling is carried out under the condition that the relationship between the water quantity density W (liter / min · m ² ) of the secondary cooling and the surface temperature T (° C) of the slab in the secondary cooling zone satisfies the following formula: Method of suppressing surface cracks during casting.

T> (650 + 5W ^1/2 · M _Ni ^1/3 ) ... The water amount density here means that the water collides with a unit area and a unit time at an arbitrary point on the surface of the slab in the secondary cooling zone. It means the amount of water.

The present inventor, in the process of developing a surface flaw prevention technique which occurs during continuous casting of steel, has a cooling characteristic of Ni when cooling the steel with water spray or steam mist.
It was found that it changes depending on the content, and the change of the surface temperature at this time can be arranged by the Ni content and the cooling water density.

[0018]

The reason for setting the cooling conditions of the method of the present invention as described above will be explained by the experimental results. % Means mass%.

When cooling the slab, factors affecting the surface temperature include the surface heat transfer coefficient, the components of the slab (particularly the Ni content) related to this coefficient, the presence or absence of scale on the surface, and the cooling medium. The density is considered. These are also factors that have an important influence on the surface cracking of the slab. Hereinafter, the surface heat transfer coefficient will be simply referred to as the heat transfer coefficient.

First, the relationship between the surface temperature T (° C.) of the slab and the cooling medium and the surface heat transfer coefficient h (W / m ² · K) will be described.

The inventors of the present invention have found that "temperature unevenness" on the surface of the slab is likely to occur during continuous casting of Ni-containing steel, and in order to clarify this mechanism, the Ni content and the cooling medium are changed to cool. , Its cooling characteristics were investigated.

Table 1 shows the composition of the test material. Only the Ni content was changed from 0 to 9%, and the other components were kept almost constant.

[0023]

[Table 1]

The experiment was conducted in advance by using high frequency wave at 1200-1300 ° C.
The test piece that had been heated above was cooled by a water spray or a mist of water and air. The temperature change during cooling was measured by a thermocouple embedded in the test piece in advance. Table 2 shows the dimensions of the test pieces and the cooling conditions.

[0025]

[Table 2]

Numerical analysis was carried out from the obtained cooling curve to determine the relationship between the surface temperature T of the test piece and the heat transfer coefficient h. Examples of these are shown in FIGS. 1 and 2.

FIG. 1 is a diagram showing the effects of the Ni content and the surface temperature T on the heat transfer coefficient h.

FIG. 2 is a diagram schematically showing an example of the relationship of FIG. 1 and explaining the meaning of a heat transfer coefficient inflection point Tc described later.

From FIG. 2, when the surface temperature T decreases,
It can be seen that the heat transfer coefficient h changes abruptly at a certain temperature Tc and rises steeply below Tc. Below this temperature Tc
Is called the heat transfer coefficient inflection point.

That is, the cooling conditions on the surface of the slab suddenly change with the inflection point Tc of the heat transfer coefficient as a boundary, and this causes the occurrence of cracks on the surface of the slab. This mechanism is explained as follows.

According to the conventional research, this phenomenon corresponds to the change of the boiling state of the cooling water, and the high temperature side and the low temperature side of the heat transfer coefficient inflection point Tc correspond to the film boiling region and the transition boiling region, respectively. There is. As shown in FIG. 2, the heat transfer coefficient increases as the surface temperature becomes lower. Therefore, if a part having a low surface temperature occurs for some reason, the part has a high heat transfer coefficient, so that the cooling is strengthened and the temperature is further increased. Is reduced. Since the change in the heat transfer coefficient is rapid in the transition boiling region, this cooling enhancement effect is promoted.

Therefore, when the surface temperature of the slab during continuous casting is in the transition boiling region, unevenness of the surface temperature is promoted, which causes "temperature unevenness" on the surface of the slab, causing thermal stress. , Which contributes to the occurrence of surface cracks.

Therefore, in order to suppress cracks on the surface of the slab during secondary cooling in continuous casting, control is performed so that the surface temperature of the slab does not fall in the transition boiling region, that is, the surface temperature of the slab is constantly heated. It is necessary to maintain the transfer coefficient at or above the inflection point Tc.

FIG. 1 is a diagram showing the surface temperature and the heat transfer coefficient h for each Ni content. For each Ni content, it is desirable that the temperature range is above the heat transfer coefficient inflection point Tc. Becomes As is clear from FIG. 1, even if the Ni content is increased, the heat transfer coefficient in the film boiling region does not change significantly. However, as the Ni content increases, the heat transfer coefficient inflection point Tc largely moves to the high temperature side.

Therefore, in order to suppress the cracking of the surface of the slab of Ni-containing steel, cooling conditions are required so that the surface temperature of the slab can always be maintained above the heat transfer coefficient inflection point Tc in accordance with the Ni content. It is essential to find out.

Next, the effect of the Ni content and the conditions on the cooling medium side on the heat transfer coefficient inflection point Tc will be described.

In the Ni-containing steel, subscale is formed on the surface of the slab. The structure of the subscale is different from that of general steel,
In addition to being difficult to peel off, its thickness increases with increasing Ni content. When such a scale exists on the surface of the slab, the heat transfer coefficient inflection point Tc moves to the high temperature side. However, there is no report clarifying the relationship between the Ni content, the heat transfer coefficient inflection point Tc, and the cooling conditions.

Therefore, when a water spray and a mist of water and air are used as the cooling medium, and the water amount density W (liter / min · m ² ) is changed, the heat transfer coefficient inflection point Tc is determined according to the Ni content. To see how much it changes. Depending on the cooling conditions, there was a recess due to the latent heat of transformation of γ → α, and there were cases where it was difficult to determine the heat transfer coefficient inflection point Tc, but the determination was made to eliminate this effect as much as possible. As a result, it was found that the heat transfer coefficient inflection point Tc showed good reproducibility within ± 30 ° C. and had a good linear relationship with the 1/3 power of the Ni content M _Ni .

FIG. 3 is a diagram showing an example of the influence of the cooling conditions and the Ni content M _Ni on the heat transfer coefficient inflection point Tc. Plots are measured values. The curve is an empirical formula obtained by performing a numerical analysis, and is obtained by multiplying the Ni content M _{Ni to} the 1/3 power by a coefficient.

Further, in order to properly determine the cooling condition, the water amount density W (liter / min) which affects the heat transfer coefficient inflection point Tc
・ It is necessary to quantify the effect of m ² ). Therefore, in FIG.
This was examined based on the data obtained from

It is known that the heat transfer coefficient (cooling strength) in the film boiling region is mainly related to the water amount density W in the case of spray cooling, and the water amount density W and the collision flow velocity V in the case of mist cooling. Has been. Therefore, even when the boiling state of the cooling water changes from film boiling to transition boiling, it can be considered from the mechanism that the same factors are dominant.

According to the experimental results, the heat transfer coefficient inflection point Tc
Moved to the high temperature side when the collision flow velocity V was high. However, in the range of the collision flow velocity V: 10 to 50 m / s which is usually used in the secondary cooling condition of continuous casting, the influence is as small as about 20 ° C., and the influence of the collision flow velocity V can be ignored. We also examined other factors such as droplet size and collision pressure,
Their effects are not recognized.

From the above examination and numerical analysis, the heat transfer coefficient inflection point Tc is influenced by the water amount density W in both cases of mist cooling and spray cooling,
The result is that it has a good linear relationship with 1/2 power.

From the above findings, the heat transfer coefficient inflection point Tc
The relationship between (° C.), Ni content M _Ni (mass%), and water amount density W (liter / min · m ² ) can be expressed as an empirical formula shown by the following formula (1).

Tc = 650 + 5W ^1/2 · M _Ni ^1/3 (1) Here, 650 of the first term is the inflection point of the heat transfer coefficient which is the base when Ni is not contained, and the second term is Represents the influence of Ni content and water density on the heat transfer coefficient inflection point.

The value of 650 in the first term was determined in consideration of the following. That is, it is conceivable that the boiling state of the cooling water changes depending on the cooling conditions and the heat transfer coefficient inflection point Tc moves even when it does not originally contain Ni. However, in the above experimental results, the influence is small at about 50 ° C., so it is ignored and set to 650, which is a higher value in the range of fluctuation of the measured value of the heat transfer coefficient inflection point Tc.

The coefficient of the second term is the relationship between the above-mentioned heat transfer coefficient inflection point Tc and the Ni content M _Ni and the heat transfer coefficient inflection point T.
It is determined from the coefficient obtained in the process of deriving the relationship between c and the water amount density W. However, the coefficient of the second term is also the first
Similar to the constant value of the term, considering the temperature fluctuation in the actual continuous casting, the Tc value determined by the equation (1) was set to be larger than the actually measured value in the experiment.

FIG. 4 is a diagram showing the influence of the water content density W and the Ni content M _Ni on the heat transfer coefficient inflection point Tc obtained by using the equation (1).

As described above, if the surface temperature of the slab during continuous casting is in the transition boiling region, it will cause the generation of surface cracks. Therefore, the surface temperature T of the slab is always expressed by the equation (1). By maintaining the transfer coefficient higher than the inflection point Tc, surface cracks can be prevented or reduced. Therefore, in the continuous casting of Ni-containing steel, the surface temperature T (° C) of the slab in the secondary cooling zone, the Ni content M _Ni (mass%) of the slab, and the water content density W (liter / min. If the cooling is performed under the condition that the relationship between m ² ) satisfies the following formula, surface defects that occur during continuous casting can be prevented or suppressed.

T> (650 + 5W ^1/2 · M _Ni ^1/3 ) ... That is, in FIG. 4, the slab surface temperature T is the heat transfer coefficient inflection obtained from the Ni content M _Ni and the water amount density W. Point Tc
Below that, the surface temperature T enters the transition boiling region and contributes to the occurrence of surface cracks.

The water amount density usually refers to the average water amount obtained from the slab surface area of the entire secondary cooling zone or the portion obtained by dividing the cooling zone and the corresponding water amount. It means the amount of water that collides with a unit area and unit time at any point on the surface of the slab in the cooling zone. Ie 2
At the time of the next cooling, there is a water amount density distribution depending on the characteristics and arrangement of the nozzles, but any part of the secondary cooling zone must satisfy the relationship shown by the above formula.

When the method of the present invention is applied to an actual machine, the surface temperature T of the slab is predicted in advance to determine the water content density W, and the relationship between the surface temperature T measured online and the water content density W is shown. The water amount density W is adjusted so as to satisfy the above formula in any arbitrary part of the secondary cooling zone.

The reason why the method of the present invention is applicable in the Ni content range of 0 to 50% is as follows.

As described above, the change in the cooling characteristics due to the inclusion of Ni also depends on the oxide scale that is formed on the surface and is difficult to peel off. When the Ni content increases, the thickness of the adhered oxide scale increases, and the heat transfer coefficient inflection point Tc increases depending on the thickness.
Moves. This fixed oxide scale has a Ni content of 9%
Even in the range of more than 50% and less than 50%, it has a structure in which a metal enriched in Ni and an oxide are intricate, and this structure is similar to the case of steel having Ni of 9% or less.

Since the thickness of the scale also increases in the same tendency in steel having a Ni content of more than 9%, the method of the present invention can be applied in the Ni content range of 0 to 50%.

Further, a cooling test was conducted in the same manner for steel types in which the contents of components other than Ni were changed, and steels to which Ca, Ti and Mo were added, but no significant difference was found in the cooling characteristics. If the Ni content is in the range of 0-50%,
It shows that the scale structure and thickness do not change significantly even when the components other than Ni are changed, and it can be judged that the scale structure and the thickness depend only on the Ni content. Therefore, the method of the present invention is also effective for steel types in which the content of components other than Ni is changed within the above range.

The effect of preventing or reducing the cracks of the method of the present invention can be similarly obtained even if the casting conditions such as the type of the continuous casting machine, the drawing speed or the cooling conditions for avoiding the ductility lowering temperature region at the straightening point are changed. You can Further, if a method for improving secondary cooling at the time of continuous casting as proposed hitherto and a method for limiting the composition of the target steel are also carried out, a greater effect can be obtained.

[0058]

[Example] Steel having the chemical composition shown in Table 3 was melted, and the machine length was 23
A test for suppressing surface cracks such as lateral cracks of the slab was conducted using a curved continuous casting machine with three-point straightening. The four steel types selected were low alloy steel with a Ni content of 0.5%, low temperature steels with 3.5% and 9%, and high alloy steel with 41%. The dimensions of the slab were constant with a width of 1800 mm and a thickness of 230 mm.

[0059]

[Table 3]

The experimental conditions are shown in Table 4. Under cooling conditions γ →
There were two types of cooling conditions: strong cooling conditions to avoid the high temperature embrittlement region near the α transformation temperature (about 600 to 850 ° C) on the low temperature side, and weak cooling conditions to avoid the high temperature embrittlement region on the high temperature side. That is, the low alloy steels have only the conditions of strong cooling and weak cooling, and the other steel types have only the conditions of weak cooling.

In the present invention example, the cooling water amount density was controlled so as to satisfy the condition of the following formula defined in the present invention over the entire length of the surface of the slab of the secondary cooling zone.

T> (650 + 5W ^1/2 · M _Ni ^1/3 ) ... FIG. 5 is a diagram showing the surface temperature transition of the slab and the water amount density distribution in the case of the present invention example 1 and comparative example 1. Is.

That is, in the case of the comparative example, FIG.
Increase the water density like the hatched part of
A part where the water amount density does not satisfy the above formula was intentionally made. In all of the examples of the present invention and the comparative examples, except the secondary cooling conditions,
The conditions were almost the same.

[0064]

[Table 4]

The surface temperature of the slab during continuous casting was measured by a radiation thermometer on the surface of the slab near the straightening point. For controlling the water content density, a method was used in which the surface temperature of the slab was predicted in advance to set the water content density, and it was confirmed by the surface temperature measured during casting that the above formula was satisfied.

One of the evaluations was carried out by the difference (nonuniformity) between the maximum value and the minimum value of the surface temperature in the width direction obtained from the measured values of the surface temperature distribution. The smaller this temperature difference is, the smaller the surface temperature unevenness is, which means that cracks are less likely to occur.

The occurrence of cracks on the surface of the slab was evaluated by cooling the slab and grinding the surface, and then visually observing the cracks, and using the "crack code index" which indicates the degree of occurrence of cracks. The crack code index is 0 when no crack occurs at all,
A 6-level index is used, which is 5 when deep cracks occur on the entire surface and maintenance is impossible. The results are also shown in Table 4.

As shown in Table 4, according to the method of the present invention,
As a result of promoting the uniformity of the slab surface temperature in the width direction, it is clear that surface cracks are eliminated or reduced. Furthermore, it can be seen that the method of the present invention is effective in reducing surface cracks regardless of the Ni content. From Table 4, it is not always necessary to combine the above-mentioned strong cooling and weak cooling in the cooling conditions, and simply satisfy the conditions defined in the present invention.
It is also clear that the subsequent cooling is effective in improving surface cracks.

According to another test, the more the water density deviates from the curve shown in FIG. 4, or the more the deviation deviates, the more uneven the surface temperature of the slab,
The surface quality deteriorated.

From the above results, according to the method of the present invention for determining the water amount density of the secondary cooling of continuous casting according to the above formula,
It is obvious that the effect of preventing or reducing the surface crack of the slab can be obtained. This effect can be obtained similarly even if the type of casting machine is changed to a vertical bending type continuous casting machine or the like, or if the casting conditions such as the drawing speed are changed.

[0071]

According to the method of the present invention, it is possible to prevent or reduce surface cracks such as lateral cracks which are generated on the surface of a slab during continuous casting of Ni-containing steel.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of a Ni content and a surface temperature T on a heat transfer coefficient h.

FIG. 2 is a diagram schematically showing an example of the relationship of FIG. 1 and explaining the meaning of a heat transfer coefficient inflection point Tc.

FIG. 3 is a diagram showing an example of the influence of cooling conditions and Ni content M _Ni on the heat transfer coefficient inflection point Tc.

FIG. 4 is a diagram showing an influence of a water content density W and a Ni content M _Ni on a heat transfer coefficient inflection point Tc based on the equation (1).

FIG. 5 is a diagram showing the surface temperature transition of the slab and the water amount density distribution in the case of the present invention example 1 and comparative example 1.

Claims (1)

[Claims] 1. A Ni content M _{Ni of} a cast slab in secondary cooling during continuous casting of a steel containing Ni in an amount of 0 to 50% by mass.
Continuous steel characterized in that cooling is carried out under the condition that the relationship between the water quantity density W (liter / min · m ² ) of the secondary cooling and the surface temperature T (° C) of the slab in the secondary cooling zone satisfies the following formula: Method of suppressing surface cracks during casting. T> (650 + 5W ^1/2・ M _Ni ^1/3 ) ...