Patent Application
20190151937
The present invention relates to a method for continuously casting steel.
Continuous casting of steel is carried out while molten metal in a tundish is supplied into a mold of continuous casting equipment via an immersion nozzle. The molten steel is discharged from an outlet port that is formed in a lower end portion of the immersion nozzle, into the mold, is cooled in the mold, and is withdrawn from a mold outlet in the state where a thickness of a solidified shell enough to prevent breakout is ensured. The solidified shell is completely solidified by secondary cooling with spray during the process of withdrawn, and is cut, to be a cast steel.
As a technique of improving the cleanliness of a cast steel, for example, Patent Literature 1 discloses that electromagnetic stirrers are oppositely arranged in the vicinity of a meniscus in long sides of a mold, so that a swirl flow is generated on the surface of molten steel in the mold; the cleaning effect of this swirl flow checks the phenomenon of adhesion of inclusions and bubbles to the surface of the mold, which is a main cause of defects in a cast steel. Patent Literature 2 discloses that an electromagnetic brake is operated on an outlet flow that is discharged from an outlet port of an immersion nozzle, so as to hold down the descending speed of molten steel, to have time for inclusions in the molten steel to float up.
In the technique of the above Patent Literature 1, an electromagnetic brake does not work on an outlet flow discharged from an outlet port of an immersion nozzle. Thus, the descending speed of the outlet flow is not held down. Therefore, bubbles and inclusions such as alumina remaining in molten steel do not float up or are not removed enough, and they infiltrate into a deep portion of the cast steel, to be a cause of internal defects, which is problematic. This problem can be avoided by operating the electromagnetic brake on the outlet flow as the above described Patent Literature 2.
When an electromagnetic brake is operated on an outlet flow, as shown in
Patent Literature 1: JP2008-183597A
Patent Literature 2: JP5245800B
An object of the present invention is to solve the above described conventional problems, and to provide a technique of avoiding occurrence of surface defects caused by an electromagnetic brake while checking internal defects with this electromagnetic brake, so that cleanliness of a cast steel can be improved compared with prior arts.
To solve the above problems, the present invention provides a method for continuously casting steel, the method comprising supplying molten steel into a mold while applying an electromagnetic brake to an outlet flow discharged from an outlet port of an immersion nozzle, wherein magnetic flux density (B) of the electromagnetic brake is within a range of the following (Formula 1):
Bmin≤B≤Bmax (Formula 1)
wherein
D0=a mold thickness (m) of the mold having short sides and the long sides on a horizontal cross-sectional shape, the mold thickness measured as a distance between the long sides facing each other in the mold at ends of the long sides,
Dmax=a maximum value of a mold thickness (m) of the mold having the short sides and the long sides on the horizontal cross-sectional shape, the maximum value measured as a distance between the long sides facing each other in the mold at a middle of each long side,
H0=a distance (m) between a surface of the molten steel and a center of an electromagnetic brake coil in a vertical direction,
HSEN=a distance (m) between a bottom surface of the immersion nozzle and the center of the electromagnetic brake coil in the vertical direction,
v=a flow velocity (m/s) of the molten steel discharged from the immersion nozzle, and
θ=an outlet flow angle (°) of the molten steel obtained as an angle formed with a horizontal line where an upward direction is a positive.
In the present invention, a rectangular mold that has short sides and long sides on a horizontal cross-sectional shape can be used as the mold.
In the present invention wherein a rectangular mold is used as the mold, preferably, the flow velocity v of the molten steel is 0.685 m/s to 0.799 m/s. Whereby, an upward flow is gently generated all over, which makes it easy to check generation of a downward flow along a solidification interface.
In the present invention, preferably, a funnel mold with short sides and long sides on a horizontal cross-section, in which a distance between the long sides facing each other in the mold at a middle of each long side is enlarged than a distance between the long sides at ends of the long sides, is used as the mold.
In the present invention wherein a funnel mold is used as the mold, preferably, Dmax/D0 is 1.16 to 1.24. Whereby, even if inclusions are carried by a downward flow, it is easy to decrease the frequency with which these inclusions are supplied to a solidification interface.
In the present invention wherein a funnel mold is used as the mold, preferably, HSEN/H0 is 0.161 to 0.327. Whereby, an upward flow is gently generated all over, which makes it easy to check generation of a downward flow along a solidification interface.
In the present invention wherein a funnel mold is used as the mold, preferably, the flow velocity v of the molten steel is 0.441 m/s to 1.256 m/s. Whereby, it is easy to stabilize a molten steel flow in the mold, and to check fluctuation on the surface of the molten steel.
In the present invention, preferably, the outlet flow angle θ of the molten steel is −45° to −5°. Whereby, it is easy to stabilize a molten steel flow in the mold, and to check fluctuation on the surface of the molten steel.
According to the present invention that employs the structure that magnetic flux density (B) of the electromagnetic brake is within a range of the above described (Formula 1) in the method for continuously casting steel, the method comprising supplying molten steel into a mold while applying an electromagnetic brake to an outlet flow discharged from an outlet port of an immersion nozzle, occurrence of surface defects caused by the electromagnetic brake can be efficiently avoided even if the mold for manufacturing a thin cast steel is used, while the effect of the electromagnetic brake which is to hold down the descending speed of the molten steel and to reduce internal defects in the cast steel is enjoyed.
That is, according to the present invention, both internal defects in the mold and surface defects can be surely reduced, and the cleanliness of the cast steel can be improved with an extremely easy method of having the electromagnetic brake of proper magnetic flux density in accordance with the above (Formula 1).
A preferred embodiment of the present invention will be described hereinafter.
In this embodiment, as shown in
In this embodiment, as shown in
An outlet port 5 from which molten steel is discharged in the mold 1 diagonally downward is formed on each portion of the immersion nozzle 2 which faces short side walls 7a and 7b of the mold 1 respectively. Bubbles of an Ar gas, and alumina and slag-type inclusions are contained in an outlet flow 6 discharged from the outlet port 5 because an Ar gas is blew into the immersion nozzle 2.
In this embodiment, the electromagnetic brake device 4 is oppositely arranged so that the mold 1 is sandwiched therein, at a position downward from the lower end part of the immersion nozzle 2 in order to avoid the phenomenon that those bubbles of Ar gas, and alumina and slag-type inclusions infiltrate into a deep portion of the cast steel, to be internal defects while not floating up or removed enough in the mold 1.
The electromagnetic brake device 4 is composed of an electromagnet etc. The electromagnetic brake device 4 can apply a DC magnetic field to the outlet flow 6 just after discharged from the outlet port 5 of the immersion nozzle 2, in the mold thickness direction (Y direction in
When an electromagnetic brake is operated on an outlet flow in a conventional art, as shown in
The above (Formula 1) was obtained through inventors' various studies. The effect of the present invention is brought about only with the combination of all the elements composing the above (Formula 1). Here, Bmin is the lower limit of a proper range of the magnetic flux density of the electromagnetic brake. If the magnetic flux density is under this lower limit, it cannot be prevented that inclusions and bubbles are carried by the outlet flow, to infiltrate downward. Bmax is the upper limit of a proper range of the magnetic flux density of the electromagnetic brake. If the magnetic flux density is over this upper limit, the upward flow along the immersion nozzle 2 becomes too strong, and thus, the downward flow turning around according to this also becomes strong. Therefore, the frequency with which inclusions and bubbles carried by this downward flow are in contact with the solidified shell 8 becomes high. As a result, surface defects are easy to occur. Bmin and Bmax are defined by the combination of some factors that influence flows in the mold.
Specifically, both internal defects in the mold and surface defects can be reduced, and the cleanliness of the cast steel can be improved only with the combination of a mold thickness (m) of the mold having short sides and the long sides on a horizontal cross-sectional shape, the mold thickness measured as a distance between the long sides facing each other in the mold at ends of the long sides (D0), a maximum value of a mold thickness (m) of the mold having the short sides and the long sides on the horizontal cross-sectional shape, the maximum value measured as a distance between the long sides facing each other in the mold at a middle of each long side (Dmax), a distance (m) between a surface of the molten steel and a center of an electromagnetic brake coil in a vertical direction (H0), a distance (m) between a bottom surface of the immersion nozzle and the center of the electromagnetic brake coil in the vertical direction (HSEN), a flow velocity (m/s) of the molten steel discharged from the immersion nozzle (v), and an outlet flow angle (°) of the molten steel (θ), so as to satisfy the above (Formula 1).
The smaller the value of HSEN is, the stronger breaking force of the electromagnetic brake to the outlet flow is. Thus, the descendent speed of the outlet flow is held down, and the velocity of the upward flow shown in
On the other hand, if the value of HSEN is large so as to approach H0, the effect of the electromagnetic brake weakens, and in addition, fluctuation of the surface of the molten steel becomes large. As a result, involvement of mold powder is easy to occur.
A larger value of θ necessitates breaking force by the larger electromagnetic brake. The upward flow also tends to be large.
As described above, increase and decrease of each variable in the above (Formula 1) brings about different effects. Thus, conventionally, it is difficult to determine proper magnetic flux density of the electromagnetic brake in continuous-casting equipment configured by the combination of them whenever the size of a mold, the casting speed, an immersion nozzle, etc. are changed. In contrast, according to the present invention, both internal defects in the mold and surface defects can be surely reduced, and the cleanliness of the cast steel can be improved with an extremely easy method of having the electromagnetic brake of proper magnetic flux density in accordance with the above (Formula 1).
In the present invention, in a case where the mold is a rectangular mold where Dmax=D0, the flow velocity of the molten steel v discharged from the immersion nozzle is preferably 0.685 m/s to 0.799 m/s. The flow velocity of the molten steel v of no less than 0.685 m/s makes it easy to obtain the molten steel flow for checking inclusions to be caught by a solidification interface. The flow velocity of the molten steel v of no more than 0.799 m/s makes it easy to check fluctuation on the surface of the molten steel.
On the other hand, in the present invention, in a case where the mold is a funnel mold, Dmex/D0 is preferably 1.16 to 1.24. Dmax/D0 of no less than 1.16 makes it easy to gently form the upward flow all over, and to check generation of the downward flow along the solidification interface. Dmax/D0 of no more than 1.24 makes it easy to reduce the drag when the solidified shell is withdrawn from the mold. In the case where the mold is a funnel mold, Dmax/D0 is more preferably 1.18 to 1.22 in view of making the above effect outstanding.
In the case where the mold is a funnel mold, preferably, HSEN/H0 is 0.161 to 0.327. HSEN/H0 of no less than 0.161 makes it easy to stabilize heat supply to the surface of the molten steel. HSEN/H0 of no more than 0.327 makes it easy to check fluctuation on the surface of the molten steel. In the case where the mold is a funnel mold, HSEN/H0 is more preferably 0.15 to 0.30 in view of making the above effect outstanding.
In the case where the mold is a funnel mold, preferably, the flow velocity of the molten steel v discharged from the immersion nozzle is 0.441 m/s to 1.256 m/s. The flow velocity of the molten steel v of no less than 0.441 m/s makes it easy to obtain the molten steel flow checking inclusions to be caught, and to supply heat to the surface of the molten steel. The flow velocity of the molten steel v of no more than 1.256 m/s makes it easy to check fluctuation on the surface of the molten steel. In the case where the mold is a funnel mold, more preferably, the flow velocity of the molten steel v is 0.500 m/s to 1.100 m/s in view of making the above effect outstanding.
In the case where the mold is a funnel mold, preferably, an outlet flow angle θ of the molten steel is −45° to −5°. The outlet flow angle θ of the molten steel of no less than −45° makes it easy to supply heat to the surface of the molten steel. The outlet flow angle θ of the molten steel of no more than −5° makes it easy to check fluctuation on the surface of the molten steel. In the case where the mold is a funnel mold, more preferably, the outlet flow angle θ of the molten steel is −45° to −15° in view of making the above effect outstanding.
Continuous casting of steel was carried out under the casting conditions shown in Table 1 below, and the quality of produced coils was evaluated. Specifically, the quality of coils was evaluated as follows: visual inspections were done on coils of no less than 50 in each Example, to count sliver defects; and evaluation was made according to the number of defects like: ⊚ (excellent: the number of defects≤0.5/a coil); ∘ (good: 0.5/a coil<the number of defects≤1.0/a coil); and x (poor: the number of defects>1.0/a coil).
In each Example 1, 2, 4, 5, 6, 7, 8, 9, 11, 13, 14, 15, 18, 20, 21, 23 and 24, the magnetic flux density of the electromagnetic brake was within a proper range, and a funnel mold was used. As shown in these Examples, it was confirmed that the quality of coils in every Example was excellent ⊚ when the magnetic flux density of the electromagnetic brake was within a proper range and a funnel mold was used, without any influence of other casting conditions (the casting speed, the casting width, the thickness of an expanding part of a funnel portion, and the conditions of the immersion nozzle).
In each Example 3 and 26, the magnetic flux density of the electromagnetic brake was within a proper range but a rectangular mold without a funnel portion was used. The quality of coils under this condition was good ∘.
En each Example 10, 17, 19 and 27, a funnel mold was used, the magnetic flux density of the electromagnetic brake was within a proper range, and the casting speed was low. The quality of coils under this condition was good ∘ in every Example.
In Example 22, a funnel mold was used, the magnetic flux density of the electromagnetic brake was within a proper range, and the casting speed was high. The quality of coils under this condition was good ∘.
In Example 25, a funnel mold was used and the magnetic flux density of the electromagnetic brake was within a proper range with a slight outlet flow angle (−5°). The quality of coils under this condition was good ∘.
In each Comparative Example 1 to 10, the magnetic flux density of the electromagnetic brake was not within a proper range. The quality of coils under this condition was poor x in every Example.
In each Comparative Example 7 and 8 and Example 12 to 16, conditions other than the magnetic flux density of the electromagnetic brake were standardized, and a proper range of the magnetic flux density of the electromagnetic brake according to the above described (Formula 1) was 657 to 4795 (Gauss).
In each Example 13 to 15, the magnetic flux density of the electromagnetic brake was within a proper range and remote from both upper and lower limits. It was confirmed that the quality of coils in every Example was excellent ⊚.
In Comparative Example 7, the magnetic flux density of the electromagnetic brake was lower than the lower limit of a proper range in 24%. In Comparative Example 8, the magnetic flux density of the electromagnetic brake was higher than the upper limit of a proper range in 4%. The quality of coils in every Example was poor x.
In Example 12 where a funnel mold was used, the magnetic flux density of the electromagnetic brake was within a proper range and close to the lower limit compared with the density in each Example 13 to 15. The quality of coils under this condition was good ∘.
In Example 16 where a funnel mold was used, the magnetic flux density of the electromagnetic brake was within a proper range and close to the upper limit compared with the density in each Example 13 to 15. The quality of coils under this condition was good ∘.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2015-072279 | Mar 2015 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 15535439 | Jun 2017 | US |
| Child | 16255904 | US |
