JPH05200514A - Continuous casting method - Google Patents

Abstract

PURPOSE:To prevent the development of corner crack caused by excess cooling at the corner parts in a continuous casting method for executing in order to obtain a cast slab having the corner parts in the cross section. CONSTITUTION:On each of rolls 2 for guiding while rolling contacting with the cast slab 1, an annular groove (recessed part) 2a arranged, and by flowing out staying water 4a produced at the rolling.contact position between the cast slab 1 and the roll 2 downward from the gap formed between the annular groove 2a and the cast slab 1, the water 4b flowed down along the corner parts 1a, 1b in the cast slab 1 is relatively reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method, and more particularly to continuous casting carried out by using a high carbon steel containing 0.55 to 1.20% of carbon to obtain a slab having a corner in a cross section such as a slab or a bloom. Regarding the casting method.

[0002]

2. Description of the Related Art In a continuous casting method, molten steel injected into a mold is brought into contact with a water-cooled inner wall of the mold to solidify it, and a slab coated with a solidified shell on the outside is obtained, and a lower opening of the mold is opened. This slab sent out from is guided along the parallel path of a large number of rolls arranged in parallel at the bottom of the mold, and is continuously drawn out by the rotation of the pinch rolls arranged at the end of this parallel path. While further cooling by spraying cooling water ejected by a spray nozzle arranged between the rolls, cut into appropriate dimensions at the stage where solidification has progressed to the inner depth, and the material in the post-process such as rolling process. It is a method of obtaining a product cast.

However, in the continuous casting method for obtaining a slab for slabs, blooms, etc., which has corners in the cross section, the corners of the slab are supercooled by the cooling water sprayed as described above. It is easy, especially in continuous casting of high carbon steel and alloy steel, cracks (corner cracks) due to supercooling occur at the corners of product slabs, and post-processing to ensure the prescribed product quality is strong. There is a problem.

FIG. 1 is an explanatory diagram of a process of supercooling occurring at a corner of a cast slab. In the drawing, reference numeral 1 denotes a slab for a slab having a rectangular cross section, and rolls 2, 2 ... For guiding the slab are rollingly contacted to both long sides of the slab 1 at appropriate intervals, The cooling water 4 ejected from each of the plurality of spray nozzles 3, 3 arranged between the rolls 2 and 2 is sprayed onto the cast piece 1.

The cooling water 4 sprayed on the slab 1 flows down along the surface of the slab 1, and this downflow is a roll 2 which rolls on the surface of the slab 1 at appropriate intervals as described above. Part of the spilled water becomes pooled water 4a at the rolling contact point between the slab 1 and the roll 2, and then branches into both sides in the longitudinal direction of the roll 2 to flow in the longitudinal direction of the slab 1. After reaching the corners 1a, 1a at both ends, the water 4b flows down along these corners. Therefore, 4 of the slab 1 having a rectangular shape
Each of the corners 1a, 1a ... Of each location is in constant contact with much more cooling water 4 than the other portions, and the heat removal associated with this contact causes supercooling.

Conventionally, the following three methods have been adopted as a method for preventing the supercooling of the corners 1a, 1a ... Which causes corner cracking. The first method is a method of reducing the absolute amount of the cooling water 4 sprayed on the slab 1, and the second method is that air is sprayed on the rolling contact portion between the slab 1 and the roll 2.
The third method is to blow away the accumulated water 4a. The third method is to bring the draining plates into contact with both short sides of the cast slab 1 and to make corners on both sides of the short sides.
This is a method of cutting the water 4b that travels along 1a and 1a.

[0007]

FIG. 2 is a graph showing the correlation between the occurrence rate of internal cracks and corner cracks and the amount of cooling water. As is clear from this figure, a decrease in the amount of cooling water reduces the incidence of corner cracking, but on the other hand, it delays the growth of the solidified shell that covers the outside of the slab 1 and reduces the shell thickness, thus increasing bulging between rolls. , Increase the incidence of internal cracks caused by this. Therefore, when carrying out the first method, it is necessary to maintain a water amount that can achieve a certain reduction effect in the vicinity of the intersection of the curves showing the occurrence rates of internal cracks and corner cracks. However, there is a problem in that corner cracks and internal cracks cannot be sufficiently eliminated even if the above is correctly performed.

In the second method, a large number of rolls 2, 2, ...
Water 4a, 4a generated at the rolling contact point between the slab and the slab 1, respectively
A large number of air nozzles are required to blow away ..., And it takes a lot of time and labor to maintain them, and there is a problem that a large amount of compressed air for ejecting from each air nozzle is consumed.

Further, the third method has a problem in the durability of the draining plate which is constantly in contact with both short sides of the cast slab 1, which requires frequent replacement, and has a width change widely used for slabs. In a continuous casting facility equipped with a possible mold, a mechanism for changing the position of the draining plate according to the change in the position of the short side of the slab 1 due to this width change is required, and there is a drawback that it is not practical. It was

The present invention has been made in view of such circumstances, and can effectively eliminate supercooling of a corner portion of a cast piece with a simple structure and prevent occurrence of corner cracks and internal cracks of the cast piece. An object is to provide a continuous casting method.

[0011]

A continuous casting method according to the present invention guides a cast piece fed from a mold along a large number of rolls arranged in parallel below the mold, and separates between the rolls. In a continuous casting method in which cooling water is continuously drawn out while spraying cooling water, a recess is formed on a part or all of the peripheral surface of the roll, and the cooling is performed through a gap between the recess and the surface of the cast piece. The feature is that the water is made to flow downward.

In addition to the above method, the weight ratio of the molten steel in the mold to the content of sulfur S in the molten steel is 0.
Addition of calcium Ca in the range of 3 ≦ Ca / S ≦ 1.5, and the nitrogen content [N] in the molten steel is [N]
Adjust to ≦ 30ppm, and the specific water amount W of the cooling water
Is performed at least one of W ≦ 0.70 liter / kg.

[0013]

According to the present invention, the cooling water sprayed on the slab between the rolls is formed on the peripheral surface of the roll while it is collected at the rolling contact portion between the rolls and the slab and flows toward both sides in the longitudinal direction. It flows downward through the gap formed between the formed recess and the surface of the cast slab. As a result, the amount of cooling water propagating through the corners of the slab is relatively reduced, and supercooling of the corners and the occurrence of corner cracks resulting from this are prevented.

Further, in the present invention, the addition of an appropriate amount of calcium in the molten steel inside the mold prevents the occurrence of internal cracks, and the nitrogen content [N] in the molten steel is set to an upper limit so that the acid-soluble aluminum ( sol.Al) and nitrogen (N), which suppresses precipitation of Al-N, which is a factor that reduces ductility, enhances the effect of preventing corner cracks, and sets an upper limit on the specific water amount of cooling water. To increase the surface temperature of the slab and promote solid solution of Al—N precipitated near the surface, thereby preventing corner cracking.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing the embodiments. FIG. 3 is a schematic side view showing an implementation state of the continuous casting method according to the present invention (hereinafter referred to as the present invention method), and FIG.
Is also a front view.

In the figure, reference numeral 1 denotes a slab slab having a rectangular cross section, and the slab 1 is delivered from the lower opening of the mold M, and is then provided on both long sides of the slab 1 at appropriate intervals. .. are guided along a parallel installation path of a large number of rolls 2, 2, ... Which roll-contact, and are continuously pulled out downward by rotation of a pinch roll (not shown) located at the end of this parallel installation path.

A plurality of spray nozzles 3, 3 ... Are arranged between the rolls 2, 2 ... Within a predetermined range below the mold M, and the slab 1 that is pulled out from the mold M is By spraying the cooling water 4 ejected from the spray nozzles 3, 3, ..., The cooling water 4 is gradually cooled during the drawing.

As shown in FIG. 4, the rolls 2, 2 ... Within the range where the spray nozzles 3, 3 ... Are arranged have an appropriate width on their peripheral surfaces, and each one annular groove 2a, 2a. Is equipped with. The formation position of the annular groove 2a in each roll 2 is set so as not to be aligned in the longitudinal direction between the rolls 2 and 2 which are vertically adjacent to each other.

FIG. 5 is an explanatory view of a flow state of the cooling water 4 at the rolling contact portion between the roll 2 having the annular groove 2a as described above and the cast piece 1. Most of the cooling water 4 sprayed on the slab 1 is evaporated by contact with the surface of the slab 1, and the rest is
It flows down along the surface of the slab 1. Then, this flow-down is caused by the roll 2 rolling on the surface of the slab 1 below the spraying position.
Therefore, a part of the run-down water becomes pooled water 4a at the rolling contact portion between the roll 2 and the slab 1, and the branched water flows along the rolling contact portion on both sides in the longitudinal direction of the roll 2.

However, since the roll 2 has an annular groove 2a and a gap is formed between the roll 2 and the slab 1 at the portion where the annular groove 2a is formed, part of the accumulated water 4a It is possible to flow downward through this gap.
As a result, the amount of transfer water 4b that reaches the corners 1a, 1a at both longitudinal ends of the cast slab 1 and flows down through these is relatively reduced, and the amount of heat removal due to contact with the transfer water 4b is reduced. Therefore, the risk of supercooling of the corners 1a, 1a is greatly reduced.

As described above, the method of the present invention is characterized in that the roll 2 having the concave portion on the surface is used, and the accumulated water 4a is flowed out from the gap between the concave portion and the cast piece 1.
The concave portion is an annular groove formed in the manner shown in FIG.
Not limited to 2a.

FIG. 6 is a front view showing another embodiment of the roll 2 usable in the method of the present invention. The roll 2 shown in FIG. 6 (a) is provided with a plurality of narrow annular grooves 2b, 2b ... Arranged side by side in the longitudinal direction, and the roll 2 shown in FIG. 6 (b) has a peripheral surface. The roll 2 shown in FIG. 6 (c) has a recess 2d of a predetermined width formed over approximately half the circumference at different positions in the longitudinal direction and the circumferential direction. , 2d, and even when these rolls 2 are used, the risk of supercooling of the corners 1a, 1a is similarly reduced.

As another embodiment, a roll 2 having a large number of dimple-shaped depressions on its peripheral surface, a roll 2 having concave grooves formed according to a predetermined pattern such as a tire surface for an automobile are used. You can also Further, it is not necessary to form the recesses as described above on all of the rolls 2, 2 ... Within the range where the spray nozzles 3, 3 ... Are arranged, but they may be formed on only a part thereof. The same recess may be formed in the rolls 2, 2 ... Which are located below the range of arrangement of the rolls 3, 2.

FIG. 7 is a diagram showing the results of comparing the areas of the quenching structures generated at the corners of the cast pieces obtained by the method of the present invention and the conventional method. This comparison is from 960
Carbon tool steel (JIS SK-3) with a width of 1100 mm and a thickness of 210 to 230 mm, manufactured at a casting speed of 0.75 m / min.
It is for cast slab slabs.

As shown in FIG. 7, in the case of the conventional method, the area of the quenching structure generated at the corners of the slab was 3700 mm ² , whereas in the case of the method of the present invention, the same area was 100 mm ^2.
Therefore, it became clear that the implementation of the method of the present invention can significantly reduce the supercooling of the corners and the occurrence of corner cracks accompanying it.

The results shown in FIG. 7 are obtained by setting the cooling water amount (specific water amount W) per 1 kg of the slab to 0.90 liters / kg in the conventional method and 0.66 liters / kg in the method of the present invention. is there. As described above, in the method of the present invention, the cooling water 4 sprayed on the surface of the slab 1 flows down through the gap between the concave portion of the roll 2 and the slab 1,
Since the entire surface of the cooling water 4 is spread, the specific water amount W of the cooling water 4 can be reduced without causing the occurrence of internal cracks.

The main cause of corner cracking is acid-soluble aluminum (sol. A) contained in the molten steel inside the mold M.
Al-N generated from 1) and nitrogen (N) may precipitate during the solidification process, leading to a decrease in ductility of the slab 1. FIG. 8 is a diagram showing the relationship between the surface temperature of the cast slab 1 and the precipitation amount of Al—N.

As is clear from this figure, the amount of Al-N deposited is
It decreases as the surface temperature of the slab 1 increases. This is because the increase in the surface temperature of the cast slab 1 promotes the solid solution of Al—N that precipitates near the surface. On the other hand, reducing the specific water amount W of the cooling water 4 has the effect of increasing the surface temperature of the cast slab 1, and suppresses the decrease in ductility of the cast slab 1 that leads to the occurrence of corner cracks due to the decrease in the precipitation amount of Al-N. be able to.
In the method of the present invention using the roll 2 provided with the recesses as described above, the specific water amount W of the cooling water 4 can be reduced without causing internal cracking in the slab 1, so that the effect of preventing corner cracking should be improved. , The specific water amount W of the cooling water 4 is 0.
Limit to 70 liters / kg or less. As a result of performing actual continuous casting under such a limitation of the specific water amount W, as will be described later, the spray nozzle 3, which becomes a region where a corner crack occurs,
The surface temperature of the slab 1 in the arrangement area of 3 ... (Secondary cooling zone)
It can be kept around 1000 ℃, and Al-
It can be seen from FIG. 8 that the precipitation amount of N is extremely small.

In order to prevent internal cracking of the slab 1, it is effective to add calcium (Ca) to the molten steel inside the mold M. The amount of Ca added may be determined according to the content of sulfur (S) in the molten steel. Figure 9 shows Ca
It is a figure which shows the result of having investigated the generation | occurrence | production state of an internal crack by changing the addition amount of various, and the horizontal axis of a figure is a ratio of the addition amount of calcium (Ca) with respect to the content of sulfur (S) (= Ca /
S).

As is apparent from this figure, the rate of occurrence of internal cracking decreases with increasing Ca addition amount, and Ca / S
In the region of 0.3 or more, the occurrence of internal cracks can be almost completely prevented. However, since excessive addition of Ca causes an increase in CaO clusters in the cast slab 1, the appropriate addition amount of Ca is limited to the range of the following formula.

0.3 ≦ Ca / S ≦ 1.5 (1)

The results shown in FIG. 9 show that the carbon (C) content is 0.
80-1.10%, sulfur (S) content 0.003-0.006
% Of high carbon steel, the specific water volume is 0.60 to 0.69 liters /
The results are obtained by investigating a slab having the above-mentioned dimensions, which was produced by the method of the present invention, in kg and the casting speed was 0.75 m / min.

That is, the roll 2 provided with the concave portion as described above.
When the method of the present invention using is carried out while adding Ca within the range shown in the formula (1) to the molten steel inside the mold M, it is possible to prevent the occurrence of internal cracks as well as corner cracks, and stabilize a high quality product slab. Can be obtained. When Ca is added alone into molten steel, the reaction accompanying this addition is too strong. Therefore, the actual addition of Ca is performed by adding a wire made of an alloy of Ca and Si (silicon) to the molten steel inside the mold M. It is done by throwing in.

As is clear from FIG. 8, the amount of precipitation of Al—N, which causes corner cracking, depends on the content of acid-soluble aluminum [sol. Al] and the content of nitrogen [N] in the molten steel.
It is effective to reduce [sol.Al] and [N] in order to prevent corner cracking. The former of these is difficult to adjust from the viewpoint of controlling the size of austenite grains, but the latter is possible, and in the method of the present invention, the roll 2 having a recess is used.
In carrying out the continuous casting method using, the occurrence of corner cracking can be prevented more effectively by adjusting the nitrogen content [N] of the molten steel inside the mold M.

FIG. 10 and FIG. 11 are diagrams showing the results of high temperature tensile tests for three types of test pieces having different acid-soluble aluminum contents [sol.Al] and nitrogen contents [N].
This test is performed after heating up to 1300 ℃, then about 100 ℃ / m
For each test piece cooled to a temperature close to the temperature of the cast piece 1 in the secondary cooling zone at a cooling rate of in, a strain rate (= 10 ^{− 3} / s)
The abscissa of both figures is the tensile test temperature, the ordinate of FIG. 10 is the ductile fracture surface ratio, and the ordinate of FIG. 11 is the internal stress of the test piece.

The three kinds of test pieces shown by ●, ○ and × in the figure are the contents of acid-soluble aluminum [sol.
Al] and nitrogen content [N] are different, [sol.
Regarding l], only the test piece marked with ● is 0.006%, the other 2
For both types of test pieces, 0.002% is used, and for [N], the test pieces marked with ● are 0.028% (28ppm) and the test pieces marked with ○ are 0.027% (27ppm). Yes, only the test piece marked with x is 0.037% (37ppm). Further, in the test pieces marked with ●, the ratio to the S content is 0.
6, that is, contains Ca that satisfies the condition of the formula (1).
The composition of components other than sol. Al and N is almost the same in all three test pieces.

As shown in FIGS. 10 and 11, at 700 ° C. to 900 ° C., which is the average temperature of the secondary cooling zone, the strength of the cross-marked test piece having a large [N] is higher than that of the other two test pieces. It is also clear from this result that the occurrence of corner cracks can be prevented by adjusting the nitrogen content [N] in the molten steel. Further, from these figures and FIG. 8, it can be seen that the occurrence of corner cracks can be effectively prevented by setting the nitrogen content [N] to be [N] ≦ 30 ppm. The [sol.Al] in each test piece is determined so as to form austenite grains of a sufficient size and improve the hardenability.

The adjustment of [N] with respect to the molten steel inside the mold M is achieved by using electrode powder as a carburizing material used for adjusting the amount of carbon in the molten steel. Fig. 12 shows the nitrogen in the product in each case when various carburizing materials were used to adjust the carbon content during continuous casting of slab slabs made of carbon tool steel (JIS SK-3 or SK-5). It is a figure which shows the result of having investigated content [N]. The horizontal axis of the figure is the percentage of the carbon content in the product with respect to the standard content, the vertical axis is the nitrogen content (ppm) in the product, and the carbon content when electrode powder is used as the carburizing material. Regardless of [C], the nitrogen content [N] is maintained at 30 ppm or less, and it can be seen that the condition of [N] ≦ 30 ppm in the method of the present invention can be satisfied.

Finally, the results of a test in which the continuous casting method was carried out under various conditions and the occurrence of corner cracks and internal cracks of the test slab in each case were investigated are shown. The composition of the four types of test slabs (A, B, C, D) obtained by this test is shown in Table 1, the casting conditions for each, and the occurrence of corner cracks and internal cracks are shown in Table 2. FIG. 13 is a diagram showing how the surface temperature of each slab changes.

[0040]

[Table 1]

[0041]

[Table 2]

Among the test slabs, each of the above-mentioned conditions, that is, addition of Ca in the range of 0.3≤Ca / S≤1.5,
Nitrogen content [N] ≤ 30 ppm, and specific water amount of cooling water 4 W ≤
Only the test slab C satisfies all three conditions of 0.70 liter / kg, the test slabs A and B do not satisfy all three conditions, and the test slab D has a nitrogen content [N] ≦ The two conditions of 30 ppm and the specific water amount of cooling water W ≦ 0.70 liter / kg are satisfied, but the amount of Ca added is outside the above conditions.

As shown in Table 2, in the test slab D, corner cracks and slight internal cracks were observed at three places as a whole, 0.01 places per 1 m in length. Further, in the test slabs A and B, 0.14 corner cracks and slight internal cracks were observed per 1 m in length, and the depth of the corner cracks was larger than that in the test slab D. all right. On the other hand, in the test slab C satisfying all of the above three conditions, no corner cracks or internal cracks were generated over the entire length thereof, and the slab having good quality was stable by the method of the present invention. It became clear that it was obtained.

Further, as shown in FIG. 13, the specific water amount W of the cooling water 4
The surface temperature of the test slabs C and D that are under the above limits is about 150 ° C higher than that of the test slabs A and B that are also outside the above limits, and the spray nozzle 3, which becomes a region where corner cracks occur, It is stably maintained at 1000 ° C or higher over almost the entire area where 3 ... On the other hand, as is clear from FIG. 8, the amount of precipitation of Al—N, which is a cause of corner cracking, is
It rapidly decreases as the temperature exceeds 1000 ° C, and it is understood that limiting the specific water amount W of the cooling water 4 to 0.70 liter / kg or less is effective in preventing the occurrence of corner cracks.

[0045]

As described in detail above, in the method of the present invention, the roll for guiding the slab below the mold is provided with a recess, and the gap between the recess and the slab is the gap between the roll and the slab. Since it functions as a discharge passage for the cooling water that collects in the rolling contact area, the cooling water does not concentrate at the corners of the slab, and it is possible to effectively prevent the occurrence of corner cracks accompanying it.
The above effects can be obtained by the small improvement of the roll. Furthermore, by adding calcium to the molten steel inside the mold, the occurrence of internal cracks can be suppressed by limiting the heating degree of the molten steel, and by limiting the nitrogen content in the molten steel and limiting the specific amount of cooling water. Since the precipitation of Al—N, which is a factor of corner cracking, is suppressed and the effect of preventing the occurrence of corner cracking is further enhanced, the present invention is excellent in that a cast piece having good quality can be stably obtained. Play.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a process of occurrence of supercooling in a corner portion of a cast slab.

FIG. 2 is a graph showing the relationship between the incidence of corner cracks and internal cracks and the amount of cooling water.

FIG. 3 is a side view showing an implementation state of the method of the present invention in a continuous casting facility.

FIG. 4 is a front view showing an implementation state of the method of the present invention.

FIG. 5 is an explanatory view of a flow state of cooling water at a rolling contact portion between a roll and a cast in the method of the present invention.

FIG. 6 is a front view showing another embodiment of a roll that can be used in the method of the present invention.

FIG. 7 is a diagram showing a corner crack reducing effect by the method of the present invention.

FIG. 8 is a diagram showing the relationship between the surface temperature of the cast slab and the precipitation amount of Al—N.

FIG. 9 is a diagram showing an appropriate range of a calcium addition amount required to prevent the occurrence of internal cracking.

FIG. 10 is a diagram showing the influence of the nitrogen content during tensile at high temperature.

FIG. 11 is a diagram showing the influence of the nitrogen content when pulling at high temperature.

FIG. 12 is a diagram showing the nitrogen content in the product when various carburizing materials are used.

FIG. 13 is a diagram showing changes in the surface temperature of a slab under different specific water contents.

[Explanation of symbols]

 1 cast piece 1a corner part 2 roll 2a annular groove 2b annular groove 2c spiral groove 2d hollow 3 spray nozzle 4 cooling water 4a pooled water 4b transmission water M mold

Claims (4)

[Claims] 1. A continuous casting method in which a slab sent out from a mold is guided along a number of rolls arranged in parallel below the mold and continuously drawn while cooling water is sprayed between the rolls. In the casting method, a concave portion is formed on the peripheral surface of a part or all of the roll, and the cooling water is caused to flow downward through a gap between the concave portion and the surface of the slab. Continuous casting method. 2. The weight ratio of the molten steel in the mold to the content of sulfur S in the molten steel is 0.3 ≦ Ca / S ≦ 1.
The continuous casting method according to claim 1, wherein calcium Ca in a range of 5 is added. 3. The nitrogen content [N] in the molten steel is
The continuous casting method according to claim 1 or 2, wherein [N] ≤ 30 ppm is adjusted. 4. The continuous casting method according to claim 1, wherein the specific water amount W of the cooling water is W ≦ 0.70 liter / kg.