JPH0569099A - Method for improving internal quality in cast slab - Google Patents

Abstract

PURPOSE:To produce a continuously cast slab preventing the development of segregation and center porosity and improving the internal quality. CONSTITUTION:At the time of continuously casting a steel, by adjusting the overheat degree of molten steel in a tundish 1 to <=50 deg.C, the molten steel is poured into a mold 2, and while stirring it by acting electromagnetic force to the molten steel in a strand with the electromagnetic stirring devices 3, 5, light rolling reduction of 5-15mm is applied to the cast slab in the range of 0.3-0.7 solid phase ratio at the center part in the cross sectional face of the cast slab by using a light rolling reduction device 6. Then the surface temp. of the cast slab is risen to >=1100 deg.C by using the heating device 7 at the rear side of the light rolling reduction zone and further, in the range of 0.8-1.0 the solid phase ratio at the center part in the cross sectional face of the cast slab or in the range of >=1200 deg.C the center temp. in the cross sectional face of the cast slab after the solidification completes, the rolling reduction is applied at least at >=30% rolling reduction ratio per one step in the uni-direction by using a large rolling reduction device 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for improving the internal quality of a slab for reducing segregation of continuously cast slabs and for pressure bonding of center porosity.

[0002]

2. Description of the Related Art As a method for improving the segregation of continuous cast slabs, a low-temperature casting method for refining a solidification structure to disperse the segregation, a low-speed casting method, or an electromagnetic stirring technique in a mold or a secondary cooling zone has been used. Have been developed and contributed to the improvement of center segregation, but the effect cannot be said to be sufficient with steel types with severe segregation or with severe segregation materials with severe segregation allowable levels, either alone or in combination.

On the other hand, Japanese Patent Publication No. 59-16862, Japanese Patent Publication No. 59-39225, Japanese Patent Publication No. 62-34460, Japanese Patent Publication No. 2-56982 and the like are based on solidification shrinkage caused by rolling down a cast piece at the end of solidification. A method of lightly reducing the final stage of solidification for suppressing the flow of concentrated molten steel and improving center segregation is disclosed.

Under these light pressures at the end of solidification, the center segregation can be greatly improved, but the amount of reduction is limited due to the formation of internal cracks, reverse V segregation and negative segregation. Is not enough.

Further, in JP-A-61-232247, JP-A-63-183765, or "Iron and Steel," 1985, pp. 875-884, a slab at the final stage of solidification is subjected to large reduction with a roll or a die. A method for improving center segregation is disclosed.

However, when it is attempted to improve the center segregation by carrying out a large reduction by these methods, internal segregation is generated depending on the reduction condition, or negative segregation is generated at the center of the slab because the concentrated molten steel is squeezed out. To do.

Further, if the squeezing is incomplete, the concentrated molten steel is captured and remarkable segregation is generated, which conversely deteriorates the internal quality of the slab.

Further, immediately after the completion of solidification, a technique for pressing down a slab to crimp the center porosity and refining the microstructure is described in "Iron and Steel", 60th pp. 875-884 and Japanese Patent Application No. -7296.

As described above, the technique of rolling down the slab after completion of solidification cannot be expected to have an effect of improving segregation formed during solidification.

Further, according to the experience of the present inventors, it is true that the center porosity is considerably improved by these reduction techniques, but when the center porosity generated in the slab is large, sufficient pressure bonding is performed. It is difficult to do so, and microporosity often remains when examined by ultrasonic flaw detection.

Further, when the internal quality is largely reduced after unsolidified or solidified as described above, surface cracking is likely to occur due to large processing in a region where the deformability of the slab is low, and in some cases it can be removed by maintenance. Some cracks may occur.

[0012]

SUMMARY OF THE INVENTION In the present invention, the conventional internal quality improvement techniques as described above cannot sufficiently or stably improve the internal quality such as segregation, center porosity, and the like. The present invention intends to solve at once a problem that internal quality deteriorates due to internal cracks, generation of negative segregation, capture of concentrated molten steel, and the like, and surface cracks that accompany the application of quality improvement technology.

[0013]

The present invention is configured as follows to solve the above problems.

That is, one of the gist of the present invention is to adjust the heating degree of molten steel in the tundish to 50 ° C. or less and inject it into the mold in continuous casting of steel, and to apply electromagnetic force to the molten steel in the strand. When the solid fraction in the central portion of the cross section of the cast piece is in the range of 0.3 to 0.7, a light reduction of 5 to 15 mm is applied to the cast piece and the cast piece is placed behind the light reduction zone. The surface temperature is raised to 1100 ° C. or higher, and a reduction rate of 30% or more per step is applied in at least one direction in the range where the solid fraction in the center of the slab cross section is 0.8 to 1.0. There is a method for improving the quality of cast slab.

Another point is that in continuous casting of steel, the degree of heating of molten steel in the tundish is adjusted to 50 ° C. or less and poured into a mold, and an electromagnetic force is applied to the molten steel in the strand. The solid content in the center of the cross section of the slab is 0.3 to 0.7,
A light reduction of 15 mm is applied and the surface temperature of the slab is raised to 1100 ° C or higher in the rear of the light reduction zone, and after completion of solidification, at least one direction per step in the range where the center temperature of the slab cross section is 1200 ° C or higher This is a method for improving the quality of cast slabs by applying a reduction of 30% or more at a reduction rate.

[0016]

[Function] The center segregation of the continuous cast slab is caused by the flow of the concentrated molten steel due to solidification shrinkage. Therefore, in order to improve the center segregation, the flow of the concentrated molten steel is suppressed and It is important to prevent the accumulation of

The present inventors have made various studies on the technical conditions for preventing the flow and accumulation of the concentrated molten steel at the final stage of solidification, and have found the technical conditions for effectively preventing them.

One of the conditions is that the molten steel heating degree in the TD is 5
It is to adjust the temperature to 0 ° C. or less, inject it into a mold, and apply an electromagnetic force to the molten steel in the strand to stir it so that the solidification structure in the central portion of the slab becomes a fine equiaxed crystal.

By making the solidification structure a fine equiaxed crystal, the amount of solidification shrinkage, the fluidity in the solid-liquid coexisting phase and the distribution of flow resistance are made more uniform in the cross section, and as a result, in the center of the slab. The flow of concentrated molten steel and its accumulation in the central part are reduced.

Still another condition is that the solid fraction at the central portion of the cross section of the slab (hereinafter referred to as the central solid fraction) is 0.3.
It is to apply a light reduction of 5 to 15 mm to the slab in the range of ~ 0.7.

The above-mentioned light reduction at the end of solidification is a center segregation countermeasure that suppresses the flow of the concentrated molten steel by compensating for the solidification contraction to reduce the driving force of the flow at the end of solidification and suppresses the flow of the concentrated molten steel. The flow of the concentrated molten steel can be significantly suppressed by rolling down the range of the solid fraction of 0.3 to 0.7 and setting the rolling down amount to 5 to 15 mm.

The reason why the reduction amount is 5 to 15 mm is that when the reduction amount is less than 5 mm, the deformation does not penetrate into the inside of the slab and does not effectively act on the shrinkage compensation in the solid-liquid coexisting phase.

The reason why the reduction amount is limited to 15 mm or less is as follows.
This is because when a reduction exceeding 15 mm in the solid fraction range is applied, the segregation level deteriorates due to the occurrence of negative segregation or reverse V segregation, or the internal quality of the slab deteriorates due to the occurrence of internal cracks.

According to the experience of the present inventors, the effect of preventing the accumulation of concentrated molten steel due to the refinement of the solidification structure and the light reduction by combining the equiaxed crystallization and refinement of the solidification structure and the light reduction at the end of solidification The flow suppressing effect of the concentrated molten steel is combined, and the segregation level is further improved, and a good level can be stably achieved.

Next, the reason why the present invention is extremely effective for improving the center porosity of continuous cast slabs will be described.

The center porosity of the continuously cast slab is generated because solidification progresses at the center of the slab and the hot water supply property in the solid-liquid coexisting phase is lost, so that the volume due to solidification shrinkage is not compensated.

The equiaxed crystallization of the solidification structure is an effective technique for improving the center porosity by dispersing the solidification shrinkage and compensating the solidification shrinkage by the reduction in the final solidification light reduction.

However, under the light pressure at the end of solidification, as described above, the amount of reduction is limited due to the occurrence of negative segregation, reverse V segregation and internal cracking, so it cannot be said to be a sufficient technique for improving center porosity.

Further, the degree of heating of the molten steel in the TD is adjusted to 50 ° C. or less, and the molten steel in the strand is stirred by applying an electromagnetic force to promote fine equiaxed crystallization of the solidified structure.
When the end-coagulation light reduction is applied, it is significantly improved as compared with the case where they are not applied, and is also improved as compared with the end-coagulation light reduction alone.

However, even in that case, the level of center porosity detected by ultrasonic waves still remains.

Further, the center porosity to be produced is kept small by making the solidification structure equiaxed by the low temperature casting and applying a light reduction at the final stage of solidification, and further, the solid phase in the central portion of the transverse section of the slab. The ratio is in the range of 0.8 to 1.0, or the center temperature of the cross section of the slab after completion of solidification is 120.
By applying a reduction of 30% or more per stage in at least one direction in the range of 0 ° C. or higher, the center porosity can be sufficiently crimped to a level that cannot be detected by ultrasonic waves.

The solid fraction at the center of the cross section of the cast slab is 0.
The reason for the large reduction in the range of 8 or more, or in the range of the center temperature of the cross section of the slab after completion of solidification is 1200 ° C or more is that solid-liquid coexistence occurs when the center solid fraction is reduced to 0.8 or less, which is the flow limit solid fraction The phase flows, negative segregation and in some cases the concentrated molten steel is not squeezed out completely, a significant positive segregation zone is formed inside the negative segregation, and the deformation applied in the solid-liquid coexisting phase is consumed for the flow. This is because the prevention of generation of center porosity is incomplete.

In the range where the solid fraction of the central portion is 0.8 to 1.0 or more, which is the critical solid fraction of the flow, the flow of the solid-liquid coexisting phase due to the reduction is not induced as in the case after the completion of solidification, and the solid phase is added to the slab. The deformation works effectively by pressing the center porosity.

The reason why the temperature of the central portion of the slab is set to 1200 ° C. or higher in the case of rolling down after completion of solidification is as follows.

The deformation resistance of steel material changes greatly with temperature, and the higher the temperature, the smaller the deformation resistance. Immediately after the completion of solidification, the slab has a temperature distribution that increases toward the inside, and correspondingly the deformation resistance decreases toward the inside.

Compared to the case where the temperature distribution in the cross section becomes uniform over time after the completion of solidification, in the temperature distribution as in the above case, the deformation on the outer surface of the slab easily reaches the inside and the center porosity is increased. This is an advantageous condition for crimping.

From this point of view, it is easier to press the center porosity by pressing down immediately before the completion of solidification when the solid fraction of the central portion is 0.8 or more, than by pressing down after the completion of solidification.

Generally, since the cooling rate after completion of solidification is higher toward the inside, the central temperature lowers as time elapses from completion of solidification, and the difference in deformation resistance between the surface of the slab and the interior decreases, resulting in the above-mentioned center porosity pressure bonding. Not only is the advantage lost, but the deformation resistance of the entire slab increases as the sensible heat of the slab decreases, so the load required to secure the same amount of reduction considerably increases.

That is, as the elapsed time from the completion of solidification to the reduction is longer, the required reduction amount and the required reduction load for pressing the center porosity increase, and accordingly the reduction ability and the required strength of the reduction equipment increase. There is a need.

In the case where the solidification state of the slab or the temperature distribution in the slab satisfies the conditions advantageous for the pressure bonding of the center porosity, the conditions for sufficiently pressing the center porosity to a level at which it cannot be detected by the ultrasonic flaw detection method. As a result of studying, the reduction rate per stage in at least one direction is 30% regardless of whether it is reduced in one stage or in several stages.
It was necessary to apply the above reduction.

As will be described later in Examples, when the end-coagulation light reduction is not applied, or when the formation of center porosity is not suppressed due to equiaxed crystallization of the solidification structure, the subsequent large reduction is performed. The reduction rate required to sufficiently press the center porosity increases considerably, and it becomes impossible to prevent the occurrence of surface cracks due to the reduction.

With respect to the direction of rolling, in the case of a flat rectangular cast slab, it can be said that it is advantageous if the flat slab is pressed in a direction in which the dimension is smaller because deformation is likely to occur in the central portion of the cast slab.

Finally, the reason why the surface temperature of the slab is raised to 1100 ° C. or higher behind the light reduction zone will be described.

Generally, in the temperature range of about 900 to 1200 ° C., the lower the temperature, the lower the hot deformability of the steel material and the higher the deformation resistance. Therefore, when pressing the slab to press the center porosity, the slab is pressed. If the temperature is low, surface cracks will occur and it will not be possible to secure a sufficient amount of reduction.

In order to prevent the occurrence of surface cracks and secure a sufficient amount of reduction, it is necessary to raise the temperature of the cast slab to 1100 ° C. or higher. In order to secure the temperature, it is necessary to install a heating zone behind the light pressure lower zone and heat the slab by heating.

As a heating device for the festival, the induction heating device is excellent in that it is relatively compact and capable of rapid heating.

The effects of the present invention will be described below with reference to Examples and Comparative Examples of the present invention.

FIG. 1 schematically shows the equipment for carrying out the method of the present invention. The degree of heating of molten steel in the tundish (TD) 1 is adjusted to 50 ° C. or less and injected into the mold 2, and the electromagnetic stirrer 5 in the mold 3 and the electromagnetic stirrer 5 installed in the secondary cooling zone 4 in the strand Stir the molten steel.

A light pressure reduction zone 6 is provided on the downstream side of the secondary cooling zone so that the solid fraction of the central portion of the cross section of the slab falls within the range of 0.3 to 0.7 in the main pressure reduction zone. The casting conditions are set, and a light reduction of 5 to 15 mm is applied to the slab in the main reduction zone.

Further, the surface of the slab is heated to 1100 ° C. or higher by the heating device 7 immediately after the light reduction zone, and then the solid fraction in the central portion of the transverse section of the slab is in the range of 0.8 to 1.0. Or after the solidification is completed, the center temperature of the cross section of the slab is 120
In the range of 0 ° C. or higher, the reduction device 8 for large reduction applies a reduction of 30% or more at a reduction rate per stage in at least one direction.

In the embodiment, S45C steel has a cross section of 220 m.
It was cast into a bloom of m × 220 mm, a straightening machine was modified and used as a light reduction device, a gas heating device was adopted as a heating device, and immediately after that, a roll type reduction device was installed.

As described above, the induction heating method is suitable as a heating device for carrying out the present invention in that it can perform quick heating and has good controllability.

Further, as a rolling down device for lightly or largely rolling down continuously drawn slabs, in addition to a roll type rolling down device, a casting system having a mechanism that moves in synchronization with the slab during rolling down, etc. A surface or bar reduction device may be used.

The results of evaluating the internal and surface qualities of the cast pieces of the examples and comparative examples of the present invention carried out under various casting, reduction and heating conditions will be described below.

FIG. 2 shows a case where the magnetic steel heating degree in the TD is 50 ° C. or less as in the present invention and the electromagnetic stirring (EMS) is not applied, or the molten steel heating degree in the TD exceeds 50 ° C. FIG. 2 shows the results of comparing the equiaxed crystal ratios of the upper surface side of the cast pieces.

From this figure, the equiaxed crystal ratio on the upper surface side of the slab is clearly increased in the present invention.

Further, as shown in FIG. 3, when the molten steel heating degree in the TD is 50 ° C. or less, electromagnetic stirring is applied, and the solid fraction of the central portion is 0.3 to 0.7. The segregation level is obviously improved and the variation is reduced in the present invention as compared with the case of the present invention in which the reduction is applied and the comparative material in which the electromagnetic stirring is not applied or the light reduction amount is 5 mm or less.

FIG. 4 shows the state of internal cracking when the solid fraction of the central portion is reduced to 0.3 to 0.7 when the reduction amount is 5 to 15 mm and the reduction amount exceeds 15 mm, which is the range of the present invention. It is the result of comparison.

When the rolling amount exceeds 15 mm, a considerable amount of internal cracks are generated, but when the rolling amount is 15 mm or less, the occurrence of internal cracking can be greatly reduced.

5 and 6 show the results of inspecting and scoring the center porosity generation state of the cast pieces manufactured under various conditions by the ultrasonic flaw detection method.

As shown in FIG. 5, TD molten steel heating degree (SH)
By electromagnetic stirring (EM
S), the center porosity score was improved by combining with the final coagulation light reduction, and in that case, a score of 0 at which the center porosity was not detected was achieved at the lowest reduction rate of 30%.

The results of FIG. 5 show that the solid fraction of the central portion is 0.8 to
Within the range of 1.0, or after the solidification is completed, the slab center temperature is 1
This is the result of reduction in the range of 200 ° C. or higher.

On the other hand, FIG. 6 shows the results of investigating the influence of the timing of large reduction on the pressure bonding behavior of center porosity at a reduction rate of 30 to 40% in the case of combining low temperature casting, electromagnetic stirring and light reduction. ..

In the state where the non-solidified portion remains, the solidification rate of the central portion was reduced by less than 0.8, and the comparative example 2 in which the cast core temperature was reduced by less than 1200 ° C. after the completion of solidification.
In the case of, the center porosity score of 0 was not achieved.

On the other hand, the solid fraction of the central portion is 0.8 to
In the range of 1.0 or after the solidification is completed, the slab center temperature is 1
In the case of the present invention in which the pressure was reduced in the range of 200 ° C. or higher, the center porosity was sufficiently pressure-bonded and all the scores were 0.

FIG. 7 shows the results of evaluation by rating the slab surface temperature during large reduction and the scale-like surface cracks generated at the corners of the slab.

According to this figure, the surface cracking becomes worse as the surface temperature of the slab becomes lower, and if the reduction ratio of 30% or more, which is the necessary condition for achieving the center porosity score of 0, is to prevent the surface cracking of the slab as in the present invention. The surface temperature of the slab needs to be 1100 ° C or higher.

[0068]

As described in detail above, the application of the present invention makes it possible to significantly improve the center segregation, center porosity, etc. generated in the continuous casting process of steel.

If the quality of these slabs is greatly improved, it is possible to perform casting with a small cross section by omitting the diffusion heat treatment that has been conventionally performed as a countermeasure against segregation and reducing the reduction ratio required for the product characteristics. ..

As a result, the agglomeration process and heating process are omitted, and the manufacturing cost is greatly reduced accordingly.

Further, in the conventional case where ultrasonic waves are used to guarantee the inspection of inclusions with a rolled material such as billet, if there is a center porosity, it is picked up as noise, so it is difficult to distinguish the inclusions from the center porosity, and it is difficult to distinguish between the center of the cross section. I had no choice but to give up the assurance of inspection of inclusions.

However, when the center porosity is pressure-bonded by the present invention, flaw detection near the center of the cross section is possible, and the inspection guarantee of inclusions becomes more reliable.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an embodiment of the present invention.

FIG. 2 is a diagram illustrating the effect of equiaxed crystallization of a solidified structure according to the present invention.

FIG. 3 is a diagram illustrating a segregation improving effect according to the present invention.

FIG. 4 is a diagram illustrating an internal crack prevention effect according to the present invention.

FIG. 5 is a diagram illustrating a center porosity improving effect according to the present invention.

FIG. 6 is a diagram illustrating a center porosity improving effect according to the present invention.

FIG. 7 is a diagram illustrating a surface crack preventing effect according to the present invention.

[Explanation of symbols]

 1 Tundish 2 Mold 3 Electromagnetic stirrer in mold 4 Secondary cooling zone 5 Electromagnetic stirrer for secondary cooling zone 6 Light reduction zone at final stage of solidification 7 Heating device 8 Reduction machine for large reduction 9 Cast slab 10 Solidification shell 11 Unsolidified part

Claims (2)

[Claims] 1. In continuous casting of steel, the molten steel heating degree in a tundish is adjusted to 50 ° C. or less and poured into a mold, and the molten steel in the strand is stirred by applying an electromagnetic force to the molten steel. The solid fraction at the center of one cross section is 0.3-
A light reduction of 5 to 15 mm is applied to the slab in the range of 0.7, and the slab surface temperature is 1100 ° C behind the light reduction zone.
Improving the internal quality of the cast slab by increasing the temperature above and further applying a reduction of 30% or more per stage in at least one direction in the range where the solid fraction in the center of the cross section of the cast is 0.8 to 1.0. Method. 2. In continuous casting of steel, the molten steel heating degree in the tundish is adjusted to 50 ° C. or less and poured into a mold, and the molten steel in the strand is cast by stirring by applying an electromagnetic force. The solid fraction at the center of one cross section is 0.3-
A light reduction of 5 to 15 mm is applied to the slab in the range of 0.7, and the slab surface temperature is 1100 ° C behind the light reduction zone.
A method for improving the internal quality of a slab, which comprises raising the temperature as described above and further applying a reduction of 30% or more at a reduction rate per step in at least one direction in a range where the center temperature of the transverse section of the slab after completion of solidification is 1200 ° C or higher.