Patent Application
20190366423
The present invention relates to a continuous casting method for steel utilizing electro-magnetic stirrer (EMS).
As a continuous casting method for steel, a method of injecting molten steel into a mold (casting mold) with a submerged nozzle having two discharge ports has been widely employed. The molten steel discharged from the submerged nozzle unavoidably contains bubbles, non-metallic particles, and the like mixed therein. Representative examples of the bubbles include argon gas bubbles. Argon is blown into the molten steel in the process of refining, such as VOD and AOD, used as a seal gas for a tundish, or intentionally added to the molten steel flow channel for preventing clogging of the nozzle, but is substantially not dissolved in the molten steel, and thus tend to mix in the mold as bubbles. The non-metallic particles mainly include a part of such materials as a slag for refining, a deoxidation product formed in the refining process, a refractory as a constitutional material of a ladle and a tundish, and powder existing on a molten steel surface in a tundish, which are entrained into the molten steel, and flow into the mold along with the molten steel through the submerged nozzle. Separately, mold powder is added to the surface of the molten steel in the mold. The mold powder generally floats on the molten steel surface and covers the surface of the molten steel, and has functions, such as lubrication between a cast piece and the mold, heat retention, and antioxidation, and also a function trapping non-metallic particles emerging on the molten steel surface.
The bubbles and the non-metallic particles flowing into the molten steel in the mold float in the mold along with the flow of the molten steel, and those having a relatively large size tend to emerge near the molten steel surface, and may be entrained in some cases into the solidification shell (i.e., the surface layer portion of the cast piece) formed in the initial stage. The mold powder on the molten steel surface may also be entrained in some cases into the solidification shell in the initial stage. In the following description, the bubbles and the substances, such as the non-metallic particles and the mold powder, in the molten steel entrained into the solidification shell, and the substances having been entrained into the solidification shell are referred to as “foreign matters”. The incorporation of foreign matters to the solidification shell may be a factor forming a defect (flaw) on the surface of the steel sheet obtained through the process including hot rolling and cold rolling.
In the continuous casting of steel, electro-magnetic stirrer (EMS) is effective as a measure for suppressing the incorporation of foreign matters to the solidification shell, and has been widely used (see, for example, PTL 1). It has been empirically confirmed that foreign matters can be prevented from being entrained into the solidification shell by making the molten steel in the vicinity of the solidification shell to flow forcedly.
In the case where the temperature of the molten steel surface in the mold is decreased, it is considered that the initial solidification shell may be formed with an uneven thickness due to the influence of the heat removal from the molten steel surface. The uneven initial solidification shell descends along the surface of the mold while exhibiting a craw-like cross section, and becomes a factor increasing the entrainment of foreign matters into the solidification shell. Accordingly, the retention of the temperature of the molten steel surface to a high temperature is also effective for suppressing the entrainment of foreign matters into the solidification shell.
PTL 2 describes that the discharge angle of the submerged nozzle is in a range of from 5 to 30 degrees upward from the horizontal direction (PTL 2, paragraph 0013). In the case where the casting rate is as small as 0.9 m/min or less, the inverse flow directed to the submerged nozzle from the short edge is small (ditto, paragraph 0021), and thus the temperature of the molten steel in the vicinity of the meniscus cannot be retained to a high temperature by the ordinary feed of the molten steel. The problem is then solved by directing the discharge angle of the nozzle upward from the horizontal direction, so as to facilitate the supply of heat to the meniscus (ditto, paragraph 0022). It is stated that in the case where the molten steel is discharged upward from the submerged nozzle, a flow thereof directed directly to the meniscus is formed, by which the molten steel having not been cooled with the mold is fed to the meniscus, so as to increase the temperature of the meniscus (ditto, paragraph 0023).
PTL 2 also describes a method of retaining the temperature of the molten steel in the vicinity of the meniscus to a high temperature by performing electro-magnetic stirring in the same direction on the long edge surfaces on both sides to increase or decrease the velocity of the inverse flow from the short edge, in the case where the casting rate is as large as approximately from 0.9 to 1.3 m/min or approximately 1.3 m/min or more (ditto, paragraphs 0025 to 0029). In this case, it is taught that the discharge angle may be relatively small (ditto, paragraph 0029), and 5° upward is employed in the example (ditto, Table 2). With a discharge angle of 5° upward, the discharged flow from the submerged nozzle is directed to the short edge surface, and the inverse flow from the short edge flows to the molten steel surface.
PTL 1: JP-A-2004-98082
PTL 2: JP-A-10-166120
According to the description of PTL 2, it is stated that a cast piece excellent in surface cleanness without surface cracking can be obtained in such a manner that in the continuous casting, the discharge angle of the molten steel from the submerged nozzle is directed upward, and electro-magnetic stirring is performed appropriately. However, as a result of the repeated ingot experiments by the present inventors, it has been empirically found that even in the case where a good surface condition is obtained in the stage of the cast piece, the surface defects elicited in the stage where the cast piece is processed to a cold rolled steel sheet cannot be necessarily decreased significantly and stably. For example, in the method using a discharge angle of 5° upward with electro-magnetic stirrer (EMS) employed in combination, even in the case where the casting rate is as large as 0.9 m/min or more (i.e., in the case where the discharged flow amount is relatively large), the surface defects in the cold rolled steel sheet caused by the entrainment of foreign matters into the solidification shell cannot be sufficiently decreased in some cases, and the improvement in quality and the improvement in yield of the steel sheet cannot be achieved. Furthermore, it has also been found that even in the case where the discharge angle of the submerged nozzle is increased, for example, to approximately 30 degrees upward from the horizontal direction, and electro-magnetic stirrer (EMS) is employed in combination, the surface defects in the cold rolled steel sheet caused by the entrainment of foreign matters into the solidification shell cannot be necessarily decreased significantly and stably. In the case where the molten steel is a stainless steel, in particular, it is further difficult to provide a sufficient improvement effect. A stainless steel sheet has a larger number of applications attaching importance to a good surface appearance, as compared to a common steel sheet, and thus generally requires a higher standard for the improvement of the surface condition. This is considered to be one of the factors complicating the sufficient improvement effect for a stainless steel only by the application of the ordinary techniques.
An object of the invention is to provide a continuous casting technique that is capable of decreasing stably and significantly the surface defects in a cold rolled steel sheet caused by the entrainment of foreign matters to the solidification shell, even in the case where the technique is applied to continuous casting of a molten stainless steel.
It has been known that in the continuous casting of a steel, the prevention of decrease of the temperature of the surface of the molten steel in the mold is generally effective for decreasing the entrainment of foreign matters into the solidification shell. However, it is difficult to achieve the aforementioned object even though electro-magnetic stirrer is employed in combination. As a result of detailed investigations by the inventors, it has been found that in a molten steel flow discharged from a submerged nozzle by a method of discharging the molten steel from the submerged nozzle directed directly to the molten steel surface, the strict limitation of a molten steel flow that is directed to the short edge surface of the mold before reaching the molten steel surface is significantly effective for suppressing the entrainment of foreign matters into the solidification shell. At this time, it is important that the discharge condition is controlled in such a manner that the period of time of the molten steel flow discharged from the submerged nozzle until reaching the molten steel surface is prevented from becoming too long, and electro-magnetic stirrer (EMS) is employed in combination. Furthermore, the direction of the molten steel flow discharged from the submerged nozzle directly to the molten steel surface with convergence thereof while preventing the molten steel flow from being broadened is effective for ensuring the temperature of the molten steel surface.
However, in the continuous casting of steel, the operation where the direction of the discharged flow from the submerged nozzle is directed directly to the molten steel surface is difficult to perform practically in the commercially production. This is because such a discharging method may make the molten steel surface considerably wavy, and thereby there may be adverse effects that the thickness of the solidification shell formed becomes uneven, and the mold powder is entrained into the solidification shell. In this case, the wavy molten steel surface can be suppressed by decreasing the discharge velocity. However, the decrease of the discharge velocity may lead to the decrease of the temperature of the molten steel surface, and may also be a factor causing the deterioration in productivity. The inventors have found a measure capable of decreasing significantly the entrainment of foreign matters into the solidification shell while preventing the aforementioned adverse effects.
The following inventions are described for achieving the aforementioned object.
[1] The object can be achieved by a continuous casting method for steel,
assuming that in continuous casting of steel using a mold having an inner surface of the mold in a rectangular profile shape cut in a horizontal plane, two inner wall surfaces of the mold constituting long edges of the rectangular shape each are referred to as a “long edge surface”, two inner wall surfaces of the mold constituting short edges thereof each are referred to as a “short edge surface”, a horizontal direction in parallel to the long edge surface is referred to as a “long edge direction”, and a horizontal direction in parallel to the short edge surface is referred to as a “short edge direction”,
the continuous casting method including: disposing a submerged nozzle having two discharge ports, at a center in the long edge direction and the short edge direction in the mold; discharging a molten steel from each of the discharge ports under the following conditions (A) and (B); and applying electric power to the molten steel in a region having a depth providing a thickness of a solidification shell of from 5 to 10 mm at least at a center position in the long edge direction, so as to cause flows in directions inverse to each other in the long edge direction on both long edge sides, thereby performing electro-magnetic stirring (EMS):
(A) an extended line of a central axis of a discharged flow of the molten steel at an cutlet opening of the discharge port of the submerged nozzle (which is hereinafter referred to as a “discharge extended line”) intersects a molten steel surface in the mold at a point P, and the molten steel is discharged from the discharge port of the submerged nozzle in a direction upward from the horizontal direction with a position of the point P satisfying the following expression (1):
0.15≤M/W≤0.45 (1)
wherein W represents a distance (mm) between the short edges facing each other at a level of the molten steel surface, and M represents a distance (mm) in the long edge direction from a center position in the long edge direction between the short edges facing each ether to the point P; and
(B) the molten steel is discharged from the discharge ports of the submerged nozzle to satisfy the following expression (2):
0≤L−0.17 Vi≤350 (2)
wherein L represents a distance (mm) from a center position of the outlet opening of the discharge port of the submerged nozzle to the point P, and Vi represents a discharge velocity (mm/s) of the molten steel at the outlet opening of the discharge port.
[12] The continuous casting method according to the item [1], wherein the two discharge ports of the submerged nozzle each have an area of the outlet opening viewed in a discharge direction of from 950 to 3,500 mm2.
[3] The continuous casting method according to the item [1] or [2], wherein L in the expression (2) is 450 mm or less.
[4] The continuous casting method according to any one of the items [1] to [3], wherein a casting rate is 0.90 m/min or more.
[5] The continuous casting method according to any one of the items [1] to [4], wherein the steel is a stainless steel having a C content of 0.12% by mass or less and a Cr content of from 10.5 to 32.0% by mass.
[6] The continuous casting method according to any one of the items [1] to [4], wherein the steel is a ferritic stainless steel containing, in terms of percentage by mass, from 0.001 to 0.080% of C, from 0.01 to 1.00% of Si, from 0.01 to 1.00% of Mn, from 0 to 0.60% of Ni, from 10.5 to 32.0% of Cr, from 0 to 2.50% of Mo, from 0.001 to 0.080% of N, from 0 to 1.00% of Ti, from 0 to 1.00% of Nb, from 0 to 1.00% of V, from 0 to 0.80% of Zr, from 0 to 0.80% of Cu, from 0 to 0.30% of Al, from 0 to 0.010% of B, and the balance of Fe, with unavoidable impurities.
The application of the measure of the invention enables stable and significant decrease of the entrainment of foreign matters into the solidification shell, which unavoidably occurs in continuous casting of steel. In the case where argon gas is used as a seal gas for a tundish or as a gas for preventing clogging of a nozzle, bubbles of argon gas can be significantly prevented from being mixed in as foreign matters. According to the invention, therefore, a cold rolled steel sheet having high quality with significantly less surface defects caused by the foreign matters can be obtained without any particular mechanical or chemical removal treatment applied to the surface of the cast piece or the hot rolled steel sheet. The continuous casting method of the invention is particularly effective when applying to a stainless steel, which is desired to have a good surface appearance.
Electro-magnetic stirrer devices 70A and 70B are disposed behind the molds 11A and 11B, and thereby a flowing force in the long edge direction can be applied to a region having a depth providing a thickness of the solidification shell of from 5 to 10 mm formed at least along the surfaces of the long edge surfaces 12A and 12B. The “depth” herein means a depth based on the level of the molten steel surface. The molten steel surface may fluctuate during the continuous casting, and in the description herein, the average level of the molten steel surface is designated as the position of the molten steel surface. The region having a depth providing a thickness of the solidification shell of from 5 to 10 mm generally exists in a range of a depth of 300 mm or less from the molten steel surface while depending on the casting rate and the heat removal rate from the mold. Accordingly, the electro-magnetic stirrer devices 70A and 70B are disposed at positions capable of applying a flowing force to the molten steel in a depth of approximately 300 mm from the molten steel surface.
In
In the invention, the molten steel is discharged from both the two discharge ports 31 in a direction upward from the horizontal direction with the position of the intersection point P of the discharge extended line 52 and the molten steel surface 41 satisfying the following expression (1):
0.15≤M/W≤0.45 (1)
wherein W represents the distance (mm) between the short edges facing each other at the level of the molten steel surface, and M represents the distance (mm) in the long edge direction from the center position in the long edge direction between the short edges facing each other to the point P.
In the case where the expression (1) is satisfied, the position of the point P is in a range where M is 0.15 W or more and 0.45 W or less in
The application of the condition satisfying the following expression (1)′ instead of the expression (1) is more effective.
0.20≤M/W≤0.40 (1)′
It is important that the molten steel is discharged from both the two discharge ports 31 to satisfy the following expression (2):
0≤L−0.17 Vi≤350 (2)
wherein L represents a distance (mm) from the center position of the outlet opening of the discharge port of the submerged nozzle to the point P, and Vi represents a discharge velocity (mm/s) of the molten steel at the outlet opening of the discharge port. The center position of the outlet opening is the center point of the discharged flux at the position of the outlet opening 32, i.e., the starting point of the discharge extended line.
L is shown in
The value of L−0.17 Vi in the expression (2) is advantageously as small as possible for retaining the temperature of the molten steel surface to a high temperature. However, in the case where the value of L−0.17 Vi becomes less than 0, the wavy molten steel surface becomes excessive due to the discharged flow directly reaching the molten steel surface, and thereby the possibility of the entrainment of the mold powder existing on the molten steel surface into the solidification shell as foreign matters is rapidly increased. On the other hand, the condition where the value of L−0.17 Vi exceeds 350 greatly decreases the temperature of the discharged flow until reaching the molten steel surface, and the effect of suppressing the entrainment of foreign matters into the solidification shell by retaining the temperature of the molten steel surface to a high temperature is weakened even with the discharge direction satisfying the expression (1).
The application of the condition satisfying the following expression (2)′ instead of the expression (2) is more effective.
20≤L−0.17 Vi≤300 (2)′
For controlling the discharge condition to satisfy the expression (1) or the expression (1)′, the discharge angle of the submerged nozzle and the submerged depth of the submerged nozzle may be controlled. For controlling the discharge condition to satisfy the expression (2) or the expression (2)′, the discharge velocity Vi may further be controlled. The discharge velocity Vi depends on the size of the discharge opening (i.e., the area of the outlet opening viewed in the discharge direction) and the discharge amount of the molten steel per unit time.
The size of the outlet opening of the discharge port of the submerged nozzle not only influences the discharge velocity Vi but also influences the mode of broadening of the discharged flux. According to the investigations made by the inventors, it has been found that the use of the submerged nozzle having a discharge port with an outlet, opening having a small size can increase the discharge velocity Vi in ensuring a constant discharged flow amount, and in addition is advantageous for suppressing the broadening of the discharged flux. With the smaller broadening of the discharged flow velocity, the interference thereof to the molten steel flow caused by the electro-magnetic stirrer can be prevented, and the electric power of the electro-magnetic stirrer required for forming the stable eddying flow can be decreased. Accordingly, the use of the submerged nozzle with an outlet opening having a small size is significantly effective for enhancing the degree of freedom in setting the electro-magnetic stirrer condition. As a result of the various investigations, the use of the submerged nozzle having two discharge ports each having an area of an outlet opening of from 950 to 3,500 mm2 viewed in the discharge direction (i.e., the direction of the discharge extended line) is more preferred. The area of the outlet opening may be more effectively from 950 to 3,000 mm2. In the case where the area of the outlet opening is less than 950, such problems as clogging of the nozzle and the like tend to occur.
In the case where the L in the expression (2) (i.e., the distance from the center position of the outlet opening of the discharge port of the submerged nozzle to the point P) is long, the influence of the broadening of the discharged flow tends to be large. As a result of the various investigations, it has been found that in the case where the molten steel is discharged under the condition providing L of 450 mm or less, the interference thereof to the eddying flow caused by the electro-magnetic stirrer can be decreased, so as to enhance the effect of washing out the foreign matters by the electro-magnetic stirred flow, and thus the elicitation of the surface defects in the cold rolled steel sheet can be further efficiently suppressed. However, in the case where the L is too small, the degree of freedom of the discharge velocity Vi for satisfying the expression (2) becomes small. The value of L is preferably ensured to be 200 mm or more. It is more effective that the submerged nozzle with the cutlet opening having an area controlled as described above is used, and simultaneously the value of L is 450 mm or less.
It has been considered that in the case where the casting rate is large, the discharge velocity is also increased accompanied thereby, and thus it is difficult to increase the upward discharge angle, so as to direct the discharged molten steel directly to the molten steel surface. However, under the discharge condition satisfying the expression (2), the sufficient discharged amount can be ensured in such a range that the molten steel surface does not become considerably wavy. Accordingly, even in the case where the casting rate is large, the entrainment of foreign matters into the solidification shell can be significantly suppressed through the increase and homogenization of the temperature of the molten steel surface. In particular, the invention can exert the excellent effect at a casting rate of 0.90 m/min or more or exceeding 0.90 m/min. The upper limit of the casting rate may depend on the equipment capacity, and may be generally 1.80 m/min or less or may be managed to 1.60 m/min or less.
The velocity of the flow of the molten steel through the electro-magnetic stirrer may be such a value that provides an average flow velocity in the long edge direction of the molten steel in contact with the surface of the solidification shell, for example, of from 100 to 600 mm/s, in a region having a depth providing a thickness of the solidification shell of from 5 to 10 mm at the center position in the long edge direction. The velocity may be managed to be from 200 to 400 mm/s. The flow velocity in the long edge direction of the molten steel in contact with the surface of the solidification shell can be confirmed by observing the metal structure of the manufactured cast piece on the cross section perpendicular to the casting direction.
Except for the control of the discharge condition from the submerged nozzle to the aforementioned condition, and the electro-magnetic stirring (EMS) performed in the aforementioned manner, the ordinary continuous casting method can be applied. For example, a method of providing another electro-magnetic stirrer device in the lower region inside the mold to form a vertically upward flow of the molten steel may be applied. In this case, an effect of further preventing the entrainment of foreign matters into the solidification shell may be expected.
The continuous casting method of the invention is effective for various steel species that have been produced by applying a continuous casting method. The continuous casting method is more effective for a stainless steel, which is frequently required to have a good surface appearance. The stainless steel is an alloy steel having a C content of 0.12% by mass or less and a Cr content of 10.5% by mass or more, as defined in JIS G0203:2009, No. 3801. An excessive Cr content may cause deterioration of the productivity and increase of the cost, and thus the Cr content is preferably 32.0% by mass or less. More specific examples of the standard steel species of the stainless steel include the various species shown in JIS G4305:2012.
Specific examples of the component, composition thereof include a ferritic stainless steel containing, in terms of percentage by mass, from 0.001 to 0.080% of C, from 0.01 to 1.00% of Si, from 0.01 to 1.00% of Mn, from 0 to 0.60% of Ni, from 10.5 to 32.0% of Cr, from 0 to 2.50% of Mo, from 0.001 to 0.080% of N, from 0 to 1.00% of Ti, from 0 to 1.00% of Nb, from 0 to 1.00% of V, from 0 to 0.80% of Zr, from 0 to 0.80% of Cu, from 0 to 0.30% of Al, from 0 to 0.010% of B, and the balance of Fe, with unavoidable impurities. In the aforementioned ferritic stainless steel, in particular, the application of the invention is considerably effective for a so-called ferritic single phase steel species, in which the C content is restricted to from 0.001 to 0.030% by mass and the N content is restricted to from 0.001 to 0.025% by mass. For the ferritic steel with a low C content and a low N content, such an operation is employed that the molten steel in the tundish is prevented from being in contact with a nitrogen component as much as possible, and in the case where such an operation is performed that the gas phase portion in the tundish is sealed with argon gas for preventing the contact with a nitrogen component, the argon gas bubbles carried over to the mold can be effectively prevented from being entrained into the solidification shell.
The ferritic stainless steels having the chemical compositions shown in Table 1 were cast with a continuous casting apparatus to produce cast pieces (slabs).
The size of the mold for the continuous casting at the level of the molten steel surface was set to 200 mm for the short edge length and a range of from 700 to 1,650 mm for the long edge length (i.e., W in
A submerged nozzle having two discharge ports on both sides in the long edge direction was disposed at the center position in the long edge direction and the short edge direction. The submerged nozzle had an outer diameter of 105 mm. The two discharge ports were disposed symmetrically with respect to a plane passing through the center of the nozzle and in parallel to the short edge surface. The discharge direction (i.e., θ in
In consideration of comparative examples having a discharge extended line that does not intersect the molten steel surface, in Tables 2A and 2B, the “distance in the long edge direction from the center position in the long edge direction between the short edges facing each other to the intersection point of the horizontal plane including the molten steel surface and the discharge extended line” is shown as the geometric distance M, and the “distance from the center position of the outlet opening of the discharge port of the submerged nozzle to the horizontal plane including the molten steel surface” is shown as the geometric distance L. In the examples of the invention, the geometric distance M in Tables 2A and 2B corresponds to M in
A calculation example of M/W in the expression (1) and L−0.17 Vi in the expression (2) is shown by taking No. 1 in Table 2A as an example. Reference may be made to
In No. 1 in Table 2A as an example, the depth of the outlet opening H=180 mm and the discharge angle θ=30° C., from which the geometric distance M is R+130/tan θ=52.5+311.8=364.3 mm. The geometric distance L is H/sin θ=180/0.5=360 mm. The distance W between the short edges facing each other at the level of the molten steel surface is 1,250 mm, from which M/W=364.3/1,250=0.291. The value satisfies the expression (1).
In No. 1 in Table 2A as an example, the casting rate is 1.00 m/min=16.67 mm/s, the size of the mold at the level of the molten steel surface is 200 mm×1,250 mm=250,000 mm2, and the number of the discharge ports is 2, from which the discharge amount of the molten steel from one discharge port per unit time is 250,000×16.67/2=2,083,750 mm3/s. The area of the outlet opening viewed in the discharge direction (i.e., the direction of the discharge extended line) is 2,304 mm2, from which the discharge velocity Vi of the molten steel at the outlet opening is 2,083,750/2,304=904.2 mm/s. Accordingly, L−0.17 Vi=360−0.17×904.2=206.3. The value satisfies the expression (2).
The resulting cast pieces (continuous cast slabs) each were subjected to the ordinary production process of a ferritic stainless steel (including hot rolling, annealing, acid pickling, cold rolling, annealing, and acid pickling), so as to produce a coil of a cold rolled annealed steel sheet having a sheet, thickness of 1 mm. A surface inspection for the entire width on one surface was performed over the entire length of the coil, and blocks of 1 m obtained by segmenting the coil in the longitudinal direction each were inspected as to whether or not a surface defect was detected in the block. In the case where at least one surface defect was detected in the block of 1 m, the block was designated as a “block having surface defect”, and the number proportion of the “block having surface defect” occupied in the total number of blocks in the entire length of the coil is designated as the defect occurrence rate (%) of the coil. The detection of a surface defect was performed by the combination of the method of detecting a disorder of the surface profile under irradiation of the entire width of the coil in threading with laser light and the visual observation, for all the coils with the same standard. The procedure can detect a surface defect caused by foreign matters (such as non-metallic particles, bubbles, and powder) entrained into the solidification shell in the continuous casting, with high accuracy. A ferritic stainless steel cold rolled annealed steel sheet that has a defect occurrence rate of 2.5% or less can be expected to achieve a large effect of enhancing the yield of the product even in an application attaching importance to a good surface appearance. Accordingly, the case where the defect occurrence rate is 2.5% or less is evaluated as “pass”, and the others are evaluated as “fail”. The results are shown in Tables 2A and 2B.
In the examples of the invention where electro-magnetic stirrer (EMS) was employed, and the molten steel was discharged from the submerged nozzle upward from the horizontal direction to satisfy the expressions (1) and (2), the defect occurrence rate was suppressed to low in all the cold rolled annealed steel sheets, from which the effect of significantly suppressing the phenomenon that foreign matters in the molten steel were entrained into the solidification shell in the continuous casting was confirmed.
On the other hand, in Nos. 13 to 18, due to the discharge direction with M/W exceeding 0.45 and too large L−0.17 Vi, the temperature of the molten steel surface was not retained sufficiently high. As a result, the entrainment of foreign matter was increased to provide a high defect occurrence rate of the cold rolled annealed steel sheet. In No. 19, due to the small submerged depth of the submerged nozzle providing the discharge direction with M/W of less than 0.15, the temperature of the molten steel surface was largely decreased in the position near the short edge. As a result, the entrainment of foreign matter was increased. In Nos. 20 and 21, due to the large L with the relatively low discharge velocity Vi, L−0.17 Vi became excessive to fail to retain the temperature of the molten steel surface to sufficiently high. As a result, the entrainment of foreign matter was increased. In Nos. 24 and 25, due to the small L with the relatively high discharge velocity Vi, the molten steel surface was largely wavy to increase the entrainment of the mold powder. In No. 24 therein, due to the discharge direction with M/W of less than 0.15, the unevenness of the temperature of the molten steel surface was increased to increase further the entrainment of foreign matters. In No. 27, due to the discharge direction with M/W exceeding 0.45, the temperature of the molten steel surface was not retained sufficiently high. As a result, the entrainment of foreign matter was increased.
The influence of the electro-magnetic stirrer on the effect of suppressing the entrainment of foreign matters was investigated by utilizing a part of the ingot charges shown in Table 2A. The continuous casting conditions and the state of defect occurrence of the cold rolled annealed steel sheets are shown in Table 3. The items shown therein the same as in Table 2A. The numeral of the example No. in Table 3 corresponds to the numeral of the example No. in Table 2A, and the examples with the same numeral uses the same ingot charge. Only the electro-magnetic stirrer condition was changed stepwise for the same ingot charge, and coils of cold rolled annealed steel sheets were produced in the same manner as in Example 1 by using the cast pieces (continuous cast slabs) produced under the respective electro-magnetic stirrer conditions, and subjected to the surface inspection. The inspection method was the same as in Example 1. The examples with an electro-magnetic stirrer flow velocity of 300 mm/s in Table 3 are re-posting of the examples shown in Table 2A. The examples with an electro-magnetic stirrer flow velocity of 0 mm/s each mean that no electro-magnetic stirring is performed.
It is understood that the effect of suppressing the entrainment of foreign matters is not sufficiently exhibited in the case where electro-magnetic stirring is not performed even though the condition satisfying the expressions (1) and (2) is employed.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2016/077415 | 9/16/2016 | WO | 00 |
