JPH0577013A - Manufacture of continuously cast slab excellent in ultrasonic flaw detection property - Google Patents

Abstract

(57) [Abstract] [Purpose] Continuous casting in which the solidification shrinkage hole generated at the center of the slab and the internal crack generated at the ridge are simultaneously pressed to prevent harmful noise during ultrasonic inclusion inspection. Method of manufacturing slab. [Structure] The slab surface temperature is 1050 ° C or higher, the center temperature is 1370 ° C or lower, and 100 to 250 from the surface temperature.
Control to a high temperature. Roll diameter 400 mmφ or more 10
Using a rolling roll having a diameter of 00 mm or less, the reduction ratio in each direction is 1.1 or more and the total reduction ratio is 1.4 to 3.0. [Effect] It becomes possible to efficiently press-bond the solidification shrinkage hole and the internal crack simultaneously under a wide range of temperature conditions, and it becomes easy to prevent harmful noise from being generated when inspecting inclusions by ultrasonic waves.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a continuously cast slab for rolling down a continuously cast carbon steel slab. In particular, solidification shrinkage holes that occur in the center of rectangular cross-section slabs and internal cracks that occur at the edges are simultaneously crimped with good operability, and no harmful noise is generated during ultrasonic inclusion inspection. The present invention relates to a method for producing a continuously cast slab having excellent ultrasonic flaw detection.

[0002]

2. Description of the Related Art In steel bars and wire rods that are subjected to strength processing such as forging and wire drawing, coarse non-metallic inclusions in the steel cause cracks and fractures during processing, which is extremely harmful. However, at present, it is impossible to manufacture steel products containing no harmful non-metallic inclusions at the steelmaking stage.Therefore, ultrasonic inclusions are inspected to remove steel products containing harmful non-metallic inclusions. ing. For this purpose, it is advantageous to perform this inspection at a stage where processing does not proceed as much as possible, and conducting it with a fine coiled wire rod or steel bar is not only technically difficult due to its various sizes. Since the efficiency is poor and the cost is increased significantly due to the long length, it is usual to perform ultrasonic inclusion inspection on the billet which is an intermediate product.

On the other hand, steel bars and wire rods are manufactured by a process of slab-casting a slab to obtain a slab, which is then hot-rolled into a product of a predetermined size. However, since there is a waste of heat energy cost in this process,
Omission of slabbing and further direct rolling of slabs is aimed. Such straightening is extremely effective not only for effective use of thermal energy but also for improving the yield of products, but it is difficult to carry out inspection and refinement, which is essential for quality assurance such as ultrasonic inclusion inspection. There is.

If the lump-rolling is omitted, it is impossible in terms of cost and technology to perform the inclusion inspection by ultrasonic waves on the product steel bar or wire as described above.
It will be carried out with as-cast slabs. When ultrasonic inspection is performed on the as-cast slab, reflected waves from the solidification shrinkage hole and coarse crystal grains at the center of the slab are detected, and the noise from the non-metallic inclusions is much louder. Therefore,
The accuracy of the inclusion inspection is significantly reduced.

[0005] Therefore, straightening such as omission of slabbing can be carried out only in the product field where quality is not a serious problem, and for bar wire rods subjected to severe working with strict standards for inclusion of non-metallic inclusions. It was impossible to omit the slab rolling process. In the conventional agglomeration method, since it is pressed at a high total reduction ratio for a continuous casting slab with a large ingot or a large casting cross-sectional area, solidification shrinkage holes are crimped and coarse crystal grains at the time of casting are also finely divided by ultrasonic waves. No noise was generated when the inclusions were detected by.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 61-82901 and "Iron and Steel," Vol. 60, pages 875-884 and 99.
On pages 0 to 999 (1974), a function corresponding to so-called slabbing rolling was introduced in the continuous casting machine, which was intended to apply compression to the solidification shrinkage hole compression and recrystallize and refine coarse grains during the final stage of continuous casting. A process is disclosed. These technologies are
It is intended to reduce the core temperature of the slab higher than the surface temperature to secure a high substantial reduction ratio at the center of the slab and efficiently press-bond the solidification shrinkage holes. As a result, it is possible to reduce the noise from the solidification shrinkage holes and coarse crystal grains in the ultrasonic inspection without performing the heavy rolling at the level of slabbing. In addition to this, as a technique for rolling down continuously cast slabs in a casting machine, a method is also disclosed in which a sufficient soaking is performed in order to reduce P segregation to reduce the difference between the surface temperature and the center temperature. Yes (“Materials and Processes” Volume 3 11
32-1-1133 (1990)).

[0007] However, although the above-described rolling reduction immediately after casting enabled the compression of solidification shrinkage holes and the refinement of coarse crystal grains, the noise of ultrasonic inspection of inclusions did not actually decrease. As a result of detailed investigation of the source of this noise, the present inventors have found out that the cause is internal cracks generated between the trees of the columnar crystals in the surface layer portion. Inclusion detection by ultrasonic flaw detection is performed by reducing the temperature of the hot cast piece immediately after casting at a total reduction ratio of 1.4 to 3.0 or by limiting the temperature distribution of the surface layer to the center of the hot cast piece. Invented a method for producing a slab capable of achieving the following (Japanese Patent Application No. 1-172796, Japanese Patent Application No. 1-17296).
No. 302575). Although these methods can achieve the object by strictly following the limited conditions, they have a drawback that they are very narrow as manufacturing conditions.

Based on these inventions, the present invention provides a method for easily and stably producing continuously cast slabs having excellent ultrasonic flaw detection.

[0009]

In the inclusion inspection of the as-cast slab by ultrasonic waves, a solidification shrinkage hole is found at the center of the slab.
Further, since internal cracks remain just below the surface of the ridge, it becomes a source of noise in ultrasonic flaw detection. The solidification shrinkage hole at the center of the slab is crimped even at a low reduction rate when the reduction is applied in a state where the center temperature of the slab is higher than the surface temperature.

However, when the casting speed is increased with the aim of improving productivity, internal cracks immediately below the ridge surface layer increase and noise is generated. Internal cracks along the columnar crystals just below the surface layer of the ridge part are difficult to crimp by a reduction method in which the core temperature of the slab is higher than the surface temperature and the substantial reduction ratio of the slab center is relatively high. ..

In view of the above problems, the present invention provides a method for producing a carbon steel continuous cast slab having excellent ultrasonic flaw detection properties by simultaneously crimping internal cracks at the slab ridge and solidification shrinkage holes at the center. To do.

[0012]

Means for Solving the Problems The gist of the present invention is to reduce the surface temperature of a slab to 1050 ° C. or more and the temperature at the center of the slab in a step of rolling down a continuously cast slab to a predetermined size during cooling after solidification. 100 ° C or lower than 1370 ° C and surface temperature
It is characterized in that the rolling temperature is controlled to be 250 ° C. higher, a rolling diameter of 400 mmφ or more and 1000 mmφ or less is used, and the rolling reduction in each direction is 1.1 or more and the total rolling reduction is 1.4 to 3.0. It is a method for producing a continuously cast slab having excellent ultrasonic flaw detection. It is more effective to use a caliber roll having a shape that restrains the ridge of the slab as the rolling roll.

[0013]

The present inventors have conducted investigations to clarify the reason why internal cracks generated at the ridges of cast slabs are not sufficiently pressure-bonded.
As a result, the temperature is lower in the vicinity of the surface of the cast slab after solidification than in the central portion, so the amount of reduction is substantially smaller than in the central portion,
It was found that it was not possible to secure a substantial amount of reduction sufficient for crimping internal cracks on the ridge. That is, in order to prevent cracks at the ridges, the surface is rolled down to a temperature higher than that of the central portion as in the case of conventional reheated material to reduce the temperature, or at least the difference between the surface temperature and the central temperature of the solidified slab is reduced. Need to be rolled down.

On the other hand, in order to press-fit the solidification shrinkage holes of the cast piece under a light pressure, it is advantageous to make the surface temperature lower than the central temperature and increase the temperature difference as described above. That is, in order to prevent the noise of ultrasonic flaw detection by pressing down the solidified slab, it is necessary to make the solidification shrinkage hole at the center part and the internal cracking at the ridge part compatible with each other, and the temperature condition of the slab for that is Conflicting states are needed.

The present inventors have solved this contradictory condition by changing the size of the reduction roll, and completed the present invention.

The background of the completion of the present invention will be described below.

Machine structural steel S48C was made 1.8 m / mi
It was cast into a 220 mm × 220 mm square cross section at a casting speed of n. It was confirmed that the slab had internal cracks at the ridge of the slab. This continuous cast slab is heated to a surface temperature of 900 to 1150 ° C and a central temperature of 50 to 330 ° C, kept warm and cooled to have a diameter of 150 to 100 ° C.
1 each in each surface direction using a 0 mm rectangular hole type roll
I pressed down with a pass. Then, after cooling, ultrasonic flaw detection was performed to investigate the internal cracking of the ridge and the pressure bonding behavior of the solidification shrinkage hole at the center.

FIG. 1 shows the influence of the temperature difference between the surface and the center and the roll diameter on the presence or absence of noise from the ridge and the center after the reduction of each surface with a reduction ratio of about 1.2. In the figure, ◯ indicates that noise was generated below the background, and × indicates that noise was above the background. As the conventional knowledge, when the temperature difference becomes large, the noise from the central part disappears, but the noise at the ridge part does not decrease,
On the contrary, when the temperature difference becomes small, the noise from the ridge disappears but the noise at the center does not decrease. However, as is apparent from FIG. 1, when the roll diameter is increased, the temperature difference range in which the noises in the central portion and the ridge portions can be eliminated at the same time is expanded. The present invention is based on this finding.

In addition, when the rolling roll has a shape that restrains the ridge of the slab, not only the rolled shape of the ridge becomes good, but also
It is easy to crimp internal cracks. FIG. 2 shows a specific example of the caliber roll 1 having a shape to restrain the ridge.

Next, the reasons for limitation of the present invention will be described.

In order to crimp the solidification shrinkage hole at the center,
Since a total reduction ratio of 1.4 or more is required, the lower limit was set. The larger the reduction ratio is, the more advantageous it is for solidification shrinkage holes and crack compression bonding of ridges. However, the larger the reduction amount, the larger the required facility capacity and the higher the manufacturing cost. Therefore, the upper limit of the total reduction ratio is 3.
It was set to 0.

The reduction is usually performed in two directions perpendicular to each surface. Therefore, the total reduction ratio includes the reduction ratios from both directions, but when the reduction distribution is significantly biased in one direction, the noise at the ridge portion did not disappear even if the total reduction ratio was set to 1.4 or more. It is presumed that this is because the pressure bonding is relatively easy if the rolling is carried out at a right angle to the direction of propagation of the internal cracks, but the pressure bonding is difficult when the rolling is carried out in the rolling direction. That is, since the internal cracks are generated in the grain boundaries of the columnar structure during solidification, they propagate in the direction perpendicular to each surface. Therefore, when the rolling is in one direction, it is considered that the internal cracks generated just below the rolling surface cannot be crimped. When the rolling reduction was limited to one direction, the influence of the rolling reduction ratio on the pressure bonding of the internal crack generated just below the side face during rolling was examined, and it was found that at least 1.1 was necessary. From this result, the lower limit of the reduction ratio in one direction was set to 1.1.

When the temperature is below 1050 ° C., the columnar coarse crystals are not refined by recrystallization even if the temperature is below 1050 ° C., so the lower limit is set. On the other hand, when the temperature exceeds 1370 ° C., on the contrary, rolling strain does not accumulate and softens and does not recrystallize.

From the result of FIG. 1, the temperature difference between the surface and the central portion is lower than 100 ° C. because the solidification shrinkage hole in the central portion does not disappear, and when it exceeds 250 ° C., the internal cracks at the ridge portion do not disappear. The upper limit was set.

When the roll diameter during rolling is less than 400 mmφ, the range of temperature difference between the surface and the central portion is 200 to 250 ° C.
Since it becomes very narrow, the lower limit was set. Also, 1000 mm
When φ is exceeded, a large rolling mill power is required, and equipment cost increases, so the upper limit was made.

The present invention is characterized in that the temperature distribution of the continuously cast slab after the completion of solidification is properly limited and is reduced by a large-diameter roll. As a result, internal cracks and solidification contraction holes, which cause noise in ultrasonic flaw detection, are pressed at the same time even under a light pressure, and coarse crystal grains, which also cause noise in ultrasonic flaw detection, are recrystallized to be harmless. It becomes possible. In particular, by setting the lower limit of the reduction ratio in each surface direction, the pressure-bonding residue of internal cracks is eliminated.

By applying the method of the present invention, inclusions can be inspected at the stage of slabs, and the step of agglomerating the rod rod for strong working with a high tolerance for inclusions can be omitted. In addition, since coarse crystal grains at the time of casting are miniaturized and noise at the time of inspecting inclusions by ultrasonic waves disappears, it is possible to significantly relax measurement conditions such as ultrasonic frequency to be used.

In the method of the present invention, the control of the proper temperature distribution can be easily obtained by utilizing the sensible heat of casting, so that the process design advantageous in terms of cost can be performed.

[0029]

[Examples] Machine structural steel S48C was 1.8 m / min
At a casting rate of 220 mm × 220 mm. This continuously cast slab was heated, heat-retained and cooled so that the center and the surface of the slab each had a predetermined temperature, and then rectangular rolls of various diameters were used, and the slab was also pressed down in each surface direction at various reduction ratios. The test conditions are shown in Table 1. The cast piece after cooling was cooled, and then ultrasonic flaw detection was performed. further,
The presence or absence of coarse crystal grains in the surface layer of the cast piece was microscopically observed with an optical microscope. The results are also shown in Table 1.

[0030]

[Table 1]

In Examples 1 to 6 of the present invention, internal cracking and noise from solidification shrinkage holes were not detected by ultrasonic flaw detection, and coarse crystal grains were not recognized.

On the other hand, in Comparative Example 7, the temperature difference was appropriate, but the temperature of the central portion was too low, so that the coarse crystal grains were not recrystallized into fine particles, resulting in noise. Comparative Example 8
Is an as-cast slab that does not undergo reduction during solidification,
Noise due to both solidification shrinkage holes and internal cracks and noise due to coarse grains were detected. Comparative Example 9
Since the temperature difference between the surface of the slab and the center was too large, internal cracks at the ridges were not sufficiently crimped, and noise was generated due to this. In Comparative Example 10, the reduction was biased in one direction and the reduction ratio of the other was insufficient, so noise from internal cracks at the ridge was detected. In Comparative Example 11, since the temperature difference was too small, noise from the solidification shrinkage holes was recognized. In Comparative Example 12, since the temperature difference was small and the rolling temperature was low, noise from solidification shrinkage holes and noise due to coarse crystal grains during casting were generated. Comparative Example 1
In No. 3, the temperature difference and the reduction temperature were appropriate, but the rolling roll had a small diameter, so that the solidification shrinkage hole at the center was not crimped sufficiently, and noise resulting from it was detected.

[0033]

As described above, by appropriately controlling the temperature distribution in the thickness direction of the rectangular continuous cast slab of steel after completion of solidification, and applying appropriate reduction to each surface with a large diameter roll, Internal cracks and solidification shrinkage holes of the slab can be pressure-bonded efficiently with good operability. According to the present invention, internal cracking and pressure-bonding of solidification shrinkage holes can be achieved under a light pressure, and inclusions can be easily detected by ultrasonic flaw detection. As a result, the conventional agglomeration process can be omitted without deteriorating the quality, and the cost can be reduced in the production of steel products for heavy working such as bar steel wire rods.

The technique for detecting inclusions by light reduction of the cast piece can be applied not only to the cast piece that is pressed down to the steel bar wire rod but also to the cast piece that is rolled into a thin plate or a steel pipe. It is possible.

[Brief description of drawings]

FIG. 1 is a diagram showing influences of a temperature difference between a surface and a center and a roll diameter on presence or absence of noise from a ridge portion and a central portion after each surface is reduced at a reduction ratio of about 1.2.

FIG. 2 is a diagram showing an example of a caliber roll having a shape that restrains a ridge portion of a cast slab.

[Explanation of symbols]

 1 caliber roll

Claims (2)

[Claims] 1. In the step of rolling down continuously cast slabs to a predetermined slab size during cooling after solidification, the slab surface temperature is 1050 ° C. or higher, the center temperature is 1370 ° C. or lower, and 100 to 100 ° C. higher than the surface temperature. It is characterized in that the rolling temperature is controlled to be 250 ° C. higher, a rolling diameter of 400 mmφ or more and 1000 mmφ or less is used, and the rolling reduction in each direction is 1.1 or more and the total rolling reduction is 1.4 to 3.0. A method for producing a continuously cast slab having excellent ultrasonic flaw detection. 2. The method for producing a continuously cast slab having excellent ultrasonic flaw detection properties according to claim 1, wherein the rolling roll is a caliber roll having a shape that restrains a ridge portion of the slab.