JP2005103604A - Continuous casting method, continuous casting cast slab, and steel plate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for casting a cast slab for plate wherein central segregation or the like is reduced, a cast slab produced with the method, and also to provide a steel plate excellent in bending deformability in a plate thickness cross-section obtained by rolling the cast slab. <P>SOLUTION: (1) The continuous casting method incudes: magnetically stirring unsolidified molten steel inside the cast slab in a curved part or a bent part position where an angle which a tangent line of an arc forming the curved part or the bent part of a continuous casting machine forms with a horizontal plane, is 30° or more; bulging the cast slab; and screwing down the cast slab including the unsolidified part with a screw down roll arranged in a horizontal part on a downstream side by adjusting a ratio of a thickness of a screw down amount D1 to the unsolidified part D2 at 0.2 to 0.6 within a range of 0.1 to 0.6 in a solid phase rate of a cast slab central part. (2) In the cast slab casted with the continuous casting method described in (1), its upper surface equiaxed ratio is 10% or more, and all the segregation ratios of C, Mn, P and S in its thickness central part are 1 or less. Further, all the segregation ratios of the same components of the steel plate obtained by hot rolling the cast slab are also 1 or less. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a continuous casting method of a steel slab, a slab cast cast by the continuous casting method, and a steel plate excellent in bending deformability in a plate thickness section, and more specifically, a continuous slab reduction after bulging the cast slab. The present invention relates to a casting method, a slab with reduced component segregation cast by the casting method, and a steel plate having excellent deformability against bending deformation in a plate thickness section in which a tensile or compressive stress acts in the plate thickness direction.

In a continuous cast slab of steel, a central segregation or a bulk segregation, which is a segregation band enriched with alloy components such as C, S, P and Mn and impurity components, is formed in a V shape at the center of the slab thickness. V segregation presenting is one of the serious casting defects that causes the deterioration of mechanical properties in thick plate products. These defects are due to the fact that when the unmelted residual molten steel at the end of casting is solidified, the molten steel with microsegregation finely concentrated between the dendritic trees is sucked out between the trees due to the negative pressure generated by shrinkage. This is because macroscopic segregation is formed by accumulating and solidifying in a closed space where the solidified tissue is locally adhered.
Conventionally, there has been disclosed a method for reducing the macro segregation defect by controlling the solidification structure of the slab or by applying mechanical reduction in the thickness direction from the surface of the slab at the end of solidification.
For example, Patent Document 1 discloses that a cast structure in a region of 20% or more including the axial center portion of a slab is made equiaxed by electromagnetic stirring, and the slab is reduced in a range from the V segregation start position to the completion of solidification. A method is disclosed. The reason for equiaxed crystallization by the method disclosed here is that by controlling the solidified structure to be as close to a spherical shape as possible, it is possible to suppress crystals from adhering to each other and improve fluidity. It is. This method attempts to compensate for heat shrinkage and solidification shrinkage inside the slab from the outside by applying reduction, and to remove the root cause of macrosegregation. The slab unsolidified reduction method that compensates for solidification shrinkage inside the slab as in Patent Document 1 is called a light reduction method, and is widely applied in continuous casting methods.

By using the conventional light reduction method, a considerable effect was obtained in reducing defects due to macrosegregation. However, this light reduction method is still insufficient for the recent demand for higher product quality. I must say. In other words, the basic idea of the light reduction method is to prevent the generation of negative pressure in the slab by adding a reduction amount corresponding to the solidification shrinkage amount. However, it is not necessarily uniform in the width direction of the slab, and the distribution of floating equiaxed crystals is also non-uniform, which causes non-uniform deformation resistance of the slab during rolling. In the method of uniform reduction from the outer surface of the slab, such as the light reduction method, uniform shrinkage compensation is not provided in the width direction of the slab for solidification shrinkage, and macro segregation is also reduced in the width direction. It must be. Further, the degree of growth and shrinkage of the solidified shell differs depending on the position in the longitudinal direction of the continuous casting machine, and it is practically impossible to provide a reduction taper that can be completely adapted to these in the light reduction method.
On the other hand, the present inventors solved the problem of internal cracking of a slab that occurs when the amount of reduction in the above-described light reduction method is simply increased. Patent Document 2 proposes a method of reducing the thickness corresponding to bulging by a pair of rolls and a slab and a thick steel plate obtained by the method after ˜25% bulging and before starting the formation of equiaxed crystals. .

  However, in the casting method that is reduced after bulging proposed in Patent Document 2, in addition to reducing the segregation at the center, depending on the properties of the solidified structure at the center of the slab when unsolidified, the slab is rolled. It has been found that cracks may occur near the center of the plate thickness when the steel plate is manufactured and subjected to bending work in which a tensile or compressive stress acts in the plate thickness direction, for example, as a steel plate for building materials. That is, in order for a steel sheet obtained from a slab through rolling or the like to have a good deformability with respect to bending or the like, the segregation degree and the structure are controlled within predetermined conditions in the slab stage. There is a need.

Japanese Examined Patent Publication No. 64-4868 (Claims and column 4, line 39 to column 5, line 9)

JP 2000-94101 A (claims and paragraphs [0012] to [0015])

The present invention has been made in view of the above problems, and a method for producing a slab with controlled macrosegregation such as center segregation and V segregation, and a slab and a thickness section processed from the slab. It aims at providing the steel plate excellent in the bending deformability in.
Here, the “bending deformability in the plate thickness section” means the deformability when bending deformation in which tensile stress or compressive stress acts in the plate thickness direction is generated.

In order to solve the above-mentioned problems, the present inventor, in view of the above-described conventional problems, is a method for continuously casting a slab slab capable of reducing component segregation, and bending in a plate thickness section obtained by rolling a slab. The present invention was completed by examining the steel sheets excellent in deformability and obtaining the following knowledge (a) to (c).
(A) Maintaining a high degree of equiaxed crystal filling in the central part of the slab slab thickness, and containing each component of C, Mn, P and S in the central part of the slab thickness and molten steel at the time of casting When the steel is cast so that the ratio to the average content of each of the above components (hereinafter referred to as “segregation ratio”) is 1 or less, the steel sheet obtained by rolling the slab is excellent in the plate thickness section. Bending deformation characteristics.
(B) The slab shown in the above (a) is obtained by electromagnetically stirring the unsolidified molten steel after bulging the slab including the unsolidified portion using a curved or vertical bending type continuous casting machine, It can manufacture by the continuous casting method which rolls down the slab containing an unsolidified part using a reduction roll downstream from an electromagnetic stirring position.

  (C) In the continuous casting method of (b) above, the electromagnetic stirring is performed at a curved portion or a bent portion where an angle formed between a tangent line of a circular arc forming the curved portion or the bent portion of the continuous casting machine and a horizontal plane is 30 degrees or more. The reduction of the slab including the unsolidified part is performed at the position of the slab in the range where the solid part ratio of the center part of the slab is 0.1 to 0.6 and the reduction amount D1 (mm) and the unsolidified part thickness at the time of reduction. It is necessary to adjust the value of the rolling reduction ratio (D1 / D2), which is the ratio to the thickness D2 (mm), in the range of 0.2 to 0.6.

  The present invention has been completed based on the above findings, and the gist of the present invention is the continuous casting method shown in (1) below, the continuous cast slab shown in (2), and the steel plate shown in (3). .

(1) When casting a slab having a rectangular cross-sectional shape using a curved or vertical bending type continuous casting machine, the slab including the unsolidified part is electromagnetically agitated into the unsolidified molten steel inside the slab Is a continuous casting method in which a slab including an unsolidified portion is reduced using a reduction roll after bulging, and an angle formed between a tangent to an arc forming a curved portion or a bent portion of the continuous casting machine and a horizontal plane Electromagnetically stir the unsolidified molten steel inside the slab at the position of the bending part or bending part where the angle is 30 degrees or more, and arrange a reduction roll in the horizontal part of the continuous casting machine downstream from the position of electromagnetic stirring. In the region where the solid fraction at the center of the slab is 0.1 to 0.6, the reduction ratio (D1 / D1), which is the ratio between the reduction amount D1 (mm) and the unsolidified portion thickness D2 (mm) at the time of reduction. The value of D2) is adjusted to the range of 0.2 to 0.6 and the unsolidified part is included Continuous casting method to pressure the slab.
(2) A slab cast by the continuous casting method described in (1) above, wherein the upper surface equiaxed crystal ratio is 10% or more, and the upper surface side and the lower surface side sandwiching the thickness center portion of the slab A continuous cast slab in which the segregation ratio of C, Mn, P, and S in the range of at least 10 mm is 1 or less.

  (3) A steel plate obtained by hot rolling the continuous cast slab described in (2), wherein the segregation ratios of C, Mn, P, and S in the central portion of the plate thickness are all 1 or less. steel sheet.

In the present invention, the “center part solid phase ratio” means a fraction occupied by the solid phase in the solid / liquid phase coexisting phase at the center part of the slab.
“Unsolidified part thickness” refers to the dimension in the slab thickness direction of an unsolidified part (solid fraction is less than 0.8) in the slab.
The “upper surface equiaxed crystal ratio” refers to a value representing, as a percentage, the area ratio occupied by the equiaxed crystal portion on the upper surface side from the thickness center in the cross section in the thickness direction of the slab.

  According to the continuous casting method of the present invention, the electromagnetic stirring of the unsolidified molten steel inside the slab, and the position at which electromagnetic stirring is performed when the slab including the unsolidified portion is bulged after being squeezed using a reduction roll, It is possible to produce slab slabs for thick plates with stable reduction of macro-segregation and semi-macro segregation by optimizing the range of solid fraction in the center of the slab during electromagnetic stirring and the reduction ratio of the slab. It becomes. The thick plate obtained by rolling the above slab has excellent bending deformability in the plate thickness cross section, and can prevent the occurrence of defects such as work cracks.

As described above, in the present invention, when the unsolidified portion of the slab is subjected to electromagnetic stirring and bulging and then rolled down, the position where the electromagnetic stirring is performed, the range of the solid phase ratio at the center during electromagnetic stirring, and the rolling ratio are set appropriately. This is a continuous casting method of a slab slab for a thick plate, a continuous casting slab and a thick plate obtained by rolling the slab slab without defects such as work cracks. The reason why the present invention is limited to the above range, the preferable range, etc. will be described in detail below.
1) Model of target continuous casting machine The reason why the continuous casting method of the present invention targets a curved or vertical bending type continuous casting machine will be described. A continuous casting machine called a so-called slab for casting a slab having a rectangular cross-sectional shape is roughly classified into three types: a vertical type, a curved type, and a vertical bending type. Of these, the vertical type has a high casting machine height, and the equipment cost including the factory building is excessive, which is not common at present. The method of the present invention is also aimed at promoting the formation of equiaxed crystals on the upper surface side of the slab. From this point of view, the slab has an upper surface side and a lower surface side, and the difference in the degree of equiaxed crystal formation in these regions is that of a rectangular cross section using a curved or vertical bending type continuous casting machine. This is a case of casting a slab. On the other hand, in the case of casting by the vertical continuous casting machine, the slab has neither the upper surface side nor the lower surface side, and the generation degree of equiaxed crystals is almost uniform within the cross section of the slab.

  For the above reasons, the present invention is directed to the case where the type of the continuous casting machine is a curved type or a vertical bending type.

2) Position at which electromagnetic stirring is performed “An electromagnetic stirring is performed at a curved portion or a bent portion of a continuous casting machine upstream of a reduction roll, and an angle formed between a tangent line of a circular arc forming the curved portion or the bent portion and a horizontal plane. The reason for “execution at a position of 30 degrees or more” will be described below.
In the present invention, it is important that sufficient equiaxed crystals are formed at the center of the slab slab thickness and filled. The reason is as follows. That is, the molten steel is poured into the mold and cooled, and the portion in contact with the mold forms a solidified shell, but the molten steel between the solidified shells has a superheat (temperature difference obtained by subtracting the liquidus temperature from the molten steel temperature) due to cooling. Since it is reduced, equiaxed nuclei or primary crystals are formed in the molten steel, which are floating. The equiaxed crystal nucleus or primary crystal has a slightly higher density than the molten steel, and therefore settles in the molten steel as time passes.
If electromagnetic stirring is not performed at this time, the equiaxed crystals will settle to the lower surface side of the slab, and the degree of filling of equiaxed crystals on the upper surface of the slab will be low. Further, when the timing of electromagnetic stirring is late, the equiaxed crystal that has settled on the lower surface side of the slab is fixed to the solidified shell on the lower surface side, so that the equiaxed crystal cannot be dispersed in the residual molten steel. As a result of repeating various tests, in order to effectively improve the filling degree of the equiaxed crystal on the upper surface side in the present invention, the tangent line of the arc forming the curved portion or the bent portion of the continuous casting machine and the horizontal plane are formed. It has been found that electromagnetic stirring should be carried out at a position where the angle is 30 degrees or more.
If the angle formed by the tangent line of the arc and the horizontal plane is less than 30 degrees, the slab becomes nearly horizontal, and it is impossible to secure a sufficient degree of equiaxed crystal filling. The upper limit of the angle is 90 degrees in the vertical portion in the vertical bending type continuous casting machine, and is close to that in the curved type continuous casting machine. However, in that case, stirring is performed at a position close to the mold, and the timing of stirring after casting is too early, so that equiaxed crystals once dispersed in the slab may settle again due to electromagnetic stirring. There is. Therefore, the angle is preferably within about 60 degrees.
As will be described later, it was found that when the angle is 30 degrees or more, the equiaxed crystal ratio of the upper surface of the slab of the slab becomes 10% or more.
3) Rolling after bulging of the slab and placement of the rolling roll on the horizontal part “Reason to squeeze the slab after bulging once during casting” This is because the reduction can be performed without crushing the solidified shell, and a large reduction effect can be obtained with a relatively small reduction force.
The reason why the rolling roll is arranged in the horizontal portion in the continuous casting machine is as follows. That is, the first reason is that it is necessary to dispose at the downstream side of the continuous casting machine in order to perform the reduction at the end of solidification where the solid fraction of the central portion is 0.1 to 0.6. The second reason is that in order to support the reaction force under the slab reduction that requires a large force by the base part of the equipment through the segment, it is better to arrange the reduction roll in the horizontal part than in the curved part or the vertical part. This is because it is cheaper than the arrangement in the above-described configuration and does not cause unreasonableness in the equipment structure.
In addition, it is preferable that the rolling rolls arranged in the horizontal portion are a pair of upper and lower rolling rolls. This is because accumulation of segregation can be effectively avoided. A plurality of reduction roll pairs may be used. In that case, the reduction condition defined by the continuous casting method of the present invention is satisfied every time the reduction roll pairs are reduced.

4) Central solid phase ratio and reduction ratio at the time of unsolidified reduction "In the region where the central solid phase ratio of the slab is 0.1 to 0.6, the ratio between the reduction amount D1 and the unsolidified portion thickness D2 The reason for adjusting the value of a certain reduction ratio (D1 / D2) to the range of 0.2 to 0.6 will be described below. In addition, unsolidified part thickness says the dimension of the slab thickness direction of the area | region where the solid-phase rate in a slab is less than 0.8.

The quality of the slab of the present invention is characterized by C, Mn, P and S within a total range of at least 10 mm on the upper surface side and the lower surface side sandwiching the thickness center of the slab in order to obtain a steel plate of desirable quality. The segregation ratio is 1 or less. In order to obtain these segregation ratio values, it is necessary to squeeze out the molten steel enriched with components existing between equiaxed crystals in the final solidified portion at the center of the slab by unsolidification pressure.
When the solid fraction at the center is higher than 0.6, it becomes a state close to complete solidification, so that excessive force is required to squeeze out the remaining molten steel in the center, and the squeezing becomes practically difficult. On the other hand, when the solid fraction of the central part is less than 0.1, the fluidity of the central part is good and the reduction is easy, but component segregation occurs again in the central part of the thickness after the reduction.
On the other hand, if the reduction ratio (D1 / D2) is less than 0.2, the reduction amount is insufficient and component segregation remains. On the other hand, when the reduction ratio exceeds 0.6, the degree of negative segregation becomes too large, and as described later, the bending workability of the steel sheet is deteriorated.
Here, the range where the reduction ratio is 0.2 to 0.6 is not a condition where the solidification interface (the surface where the solid phase ratio becomes 0.8) is pressure-bonded. By setting it to 0.1 to 0.6, the equiaxed crystals dispersed in the unsolidified liquid phase play the role of a medium for stress transmission as if the upper and lower solidification interfaces approached, and both solidifications The inside of the interface is compressed and the remaining molten steel is appropriately squeezed out. This appropriate state gives good workability to the steel sheet obtained after rolling. Therefore, the appropriate range of the solid fraction at the center when the slab is reduced is set to 0.1 to 0.6, and the appropriate range of the reduction ratio is set to 0.2 to 0.6.
5) Segregation ratio of C, Mn, P, and S in the thickness center of the slab and steel plate “C, Mn, P, and S in the thickness center of the steel plate obtained by rolling the slab. The significance of the “steel plate having a segregation ratio of 1 or less” will be described.

The segregation ratio of C, Mn, P, and S in the range of at least 10 mm in the total of the upper surface side and the lower surface side sandwiching the thickness center portion of the slab is 1 or less. In the process of rolling the slab cast slab into a steel plate, the component concentration in the central portion of the thickness does not change, and the component concentration of the slab cast is maintained even in the steel plate after rolling.
In conventional ordinary steel plates, center segregation and V segregation originally present in the center of the slab are present as strip-like segregation extending in the rolling direction. For example, when bending is applied to such a steel sheet as a building material application, stress concentration may occur at the segregation boundary due to the difference in strength between the segregated portion and other portions, and cracks may occur. In addition, coarse MnS inclusions generated in the slab stage due to Mn and S in the steel remain and are likely to become the starting point of cracking.
On the other hand, when the central component is adjusted within the range specified in the present invention, the steel plate center portion is lightly negative segregation, there is no band-like segregation like ordinary steel plates, and bending workability is good. is there. The segregation ratio of these component elements being 1 or less indicates that there is no significant macrosegregation as a result of the component concentrated molten steel being discharged from between dendritic trees.
“The reason why the total of the upper surface side and the lower surface side across the thickness center portion of the slab is within a range of at least 10 mm” is that if the total value of the upper surface side and the lower surface side is less than 10 mm, the thickness center portion is This is because even if the segregation ratio of the component elements within the range sandwiched is 1 or less, the effect of negative segregation on the steel sheet is reduced when large segregation occurs at the stage of the slab. The upper limit of the total value of the upper surface side and the lower surface side sandwiching the thickness center portion is not particularly specified, but it is preferably within 30 mm because there is a restriction that uncoagulation reduction needs to be performed at the end of coagulation. .

6) The equiaxed crystal ratio of the upper surface of the slab “The reason why the upper surface equiaxed crystal ratio of the slab is 10% or more” is as follows. That is, the equiaxed crystal dispersed in the residual molten steel is effective in obscuring the center segregation and V segregation in the center part of the slab thickness in the thickness direction instead of being connected. In addition, the effect of preventing segregation of segregation in the slab is exerted by adding the effect of discharging segregation dispersed when unsolidified under the condition 4). If the equiaxed crystal ratio of the upper surface of the slab is less than 10%, the concentrated molten steel that forms center segregation and V segregation is not sufficiently dispersed and is partially accumulated, so that it is easy to form semi-macrosegregation that is segregated grains. Therefore, the above effect cannot be obtained. Therefore, the appropriate range of the equiaxed crystal ratio on the upper surface is set to 10% or more. Thus, by avoiding the accumulation of segregation in the slab, it becomes possible to avoid the concentration of strain on the segregation part during the bending of the steel sheet and the cracks based on it.

  Further, as described above, excessive squeezing in which the rolling ratio (D1 / D2) exceeds 0.6 rather deteriorates the bending workability of the steel sheet. Although the center segregation and V segregation in the slab are reduced and the macro appearance is further improved, the degree of negative segregation formed at the center increases as a result of unsolidification reduction, and the steel sheet after rolling has a thickness center. From the difference in mechanical strength between the negative segregation of the part and the positive segregation on the both outer surfaces, it has been found that a new problem of concentration of strain at the boundary occurs. When such a steel sheet was bent, a phenomenon was observed in which cracking occurred not only at the center of the thickness but along the bending arc slightly outside the thickness center.

In order to confirm the effect of the continuous casting method of the present invention, the following examples of the present invention and comparative examples were tested and the results were evaluated.
(Test method)
1) Casting method FIG. 1 schematically shows a longitudinal section of a continuous casting apparatus used in this test. For the casting test, a vertical bending type continuous casting machine was used, the slab thickness was 235 mm, the component composition was mass%, C: 0.15%, Mn: 0.5%, P: 0.018% and S: 0.004% of 500 N / mm grade ² steel was cast. The slab width was uniformly 1800 mm, the casting speed was 0.75 to 0.95 m / min, and the specific water amount for secondary cooling was 1.5 liter / kg-steel.
The molten steel 4 injected into the mold 3 through the immersion nozzle 1 is cooled by spray water sprayed from the mold 3 and a group of secondary cooling spray nozzles (not shown) below the mold 3 to form a solidified shell 5 and a slab 8. Become. At this time, the slab 8 is supported by the guide roll group 6 while holding the unsolidified portion 10 therein, and is squeezed by the squeezing roll 7 and pulled out by the pinch roll group 11. The reduction roller 7 is disposed at a position 21 m from the meniscus, the diameter of the reduction roll is 450 mm, and the reduction force is a maximum of 3.43 × 10 ⁶ N per roll. Although FIG. 1 shows the case where the continuous casting method of the present invention is applied to a vertical bending type continuous casting machine, the method of the present invention can also be applied to a curved continuous casting machine.
The guide roll group 6 is arranged so that the interval in the slab thickness direction can be increased stepwise in the drawing direction within the range indicated by the arrow between B1 and B2 in the figure (hereinafter referred to as “bulging zone”). In this section, the slab in which the unsolidified portion 10 exists is bulged. Further, the portion corresponding to the bulging of the slab is reduced by the reduction roll 7 arranged on the downstream side. The bulging amount can be adjusted by controlling the distance in the thickness direction of the guide roll group 6. In this test, the bulging start position was uniformly about 9 m from the meniscus, and the bulging amount was set to 25 mm.

  The unsolidified thickness at the time of rolling is determined by the casting speed and the bulging amount. That is, the unsolidified thickness is determined by the growth thickness of the solidified shell due to cooling of the slab up to the reduction start position and the thickness of the bulged slab. Unsteady heat transfer analysis in the slab thickness direction considering the casting speed, slab surface cooling conditions, and physical properties of the cast steel, respectively It can be obtained by calculation. From the calculation result thus obtained and the slab thickness before reduction (that is, the sum of the thickness of the slab in the mold and the bulging amount), the solid phase ratio before unsolidification reduction is 0.8. An unsolidified thickness (ie, D2) based on a certain interface is obtained.

Further, the continuous casting machine used in the test is provided with an electromagnetic stirring device 9, and the arrangement position thereof can be changed in the drawing direction of the slab as indicated by an arrow between E1 and E2 in the figure. is there. By setting it as the above facilities, it is possible to adjust the angle which the slab and the horizontal surface make in the timing which provides electromagnetic stirring, and can confirm the effect of the continuous casting method of this invention. Thus, the degree of filling of equiaxed crystals was controlled by changing the position of electromagnetic stirring. The electromagnetic stirrer 9 is a moving magnetic field system, the magnetic flux density is 300 gauss, and the stirring speed of molten steel is 300 rpm at maximum.
Moreover, the superheat degree (ΔT) of the molten steel in the tundish was substantially constant in the range of 40 to 50 ° C., and the central solid fraction and the final solidification thickness during the reduction were adjusted by changing the casting speed. .

2) Investigation Method of Macro Structure and Segregation Ratio FIG. 2 is a diagram showing a method of collecting a sample for investigation of the macro structure and component segregation status of a slab. A sample having a length of 1 m was cut out in the casting direction from the slab obtained by each casting test, and three plate samples 12 for cross-sectional macro observation were cut out from both ends and the center in the length direction and used for investigation. . About 250 mm of solidified portion from the end in the slab width direction is removed for each plate sample 12, the area ratio occupied by the equiaxed crystal portion on the upper surface side of the slab is investigated, and the average value of the area ratio is obtained to determine the upper surface. The equiaxed crystal ratio was taken.

  Further, from the plate sample 12 of each transverse section, a sample 13 for mapping analysis by EPMA (hereinafter referred to as “MA analysis”) is made the center in the width direction of the slab (indicated by “C” in the figure), width A total of 9 positions are cut out from three positions: a ¼ position in the direction (indicated by “¼W” in the figure) and a ¾ position in the width direction (indicated by “3 / 4W” in the figure). MA analysis was performed on each of the 13 samples for MA analysis. Each MA analysis was performed for a range of 10 mm in the thickness direction and 40 mm in the width direction of the center portion of the slab, and the average content of C, Mn, P and S was determined. The segregation ratio of each component is obtained by dividing the average content of each component by the average content of each component at the time of casting the molten steel, and the segregation ratio is obtained by arithmetically averaging the segregation ratio values of nine samples. The representative value.

3) Bending test method in plate thickness section A slab having a length of 8 m is collected, heated to 1200 ° C. in a normal heating furnace, and then hot-rolled at a rolling temperature of 1200 to 700 ° C. to have a thickness of 30 mm. A thick steel plate was used. The center of the plate width (hereinafter referred to as “C”), the position of 1/4 of the plate width (hereinafter referred to as “1 / 4W”), and the position of 3/4 of the plate width (hereinafter referred to as “C”). 3 / 4W "), each including a total thickness of the steel sheet in the plate thickness direction (sample length) 30 mm x plate width direction (sample width) 32 mm x plate rolling direction (sample thickness) 9 mm A total of three slice samples were cut out.
Similarly, with respect to the thickness cross section in the longitudinal direction of the steel plate, similarly, three positions on the center side (hereinafter referred to as “1 / 4L”, “1 / 2L”, and “3”) obtained by dividing the longitudinal direction of the steel plate into four equal parts. / 4L "), a total of three slice samples having a plate thickness cross section were cut out from the central portion in the plate width direction.
FIG. 3 is a diagram showing a bending test method of a sample collected from a plate thickness cross section. A steel plate 15 having a C content of 0.2% by mass having a width of 32 mm, a length of 60 mm, and a thickness of 9 mm at both ends in the plate thickness direction (sample length) of the plate thickness section slice sample 14 is obtained. Welded. Fix both ends, and press a metal fitting with a width of 32 mm having an arc with a radius of curvature of 1.5 mm against the center of the thickness of the sample, and the entire sample in a direction perpendicular to the paper surface until the sample follows the arc shape of the metal fitting. And the presence or absence of cracks at this time was investigated.
As for the presence or absence of cracks, the case where no cracks occurred was marked as “No cracks” by ○, the case where the total crack length was less than 5 mm was marked as “Minor cracks” by Δ, and The case where the total crack length was 5 mm or more was classified as “crack generation” by x mark.
(Test results)
The test conditions and test results described above are summarized in Table 1. Here, the central solid phase ratio indicates the central solid phase ratio at the start of the reduction.

  Examples 1 to 5 of the present invention in the table are tests on examples of the present invention that satisfy all the conditions specified by the casting method of the present invention, and Comparative Examples 1 to 13 are conditions specified by the method of the present invention. It is the test about the comparative example which does not satisfy | fill at least 1 of these.

  The tests of Comparative Examples 1, 2, and 3 were compared with the tests of Invention Examples 1, 2, and 4, respectively, and the angle formed by the tangent line of the arc of the slab bend at the electromagnetic stirring position and the horizontal plane (hereinafter, simply “ Only the angle of the electromagnetic stirring position ”) was changed to be less than 30 degrees defined by the method of the present invention. As a result, the concentrated molten steel could not be discharged evenly under unsolidified pressure, and segregated grains were observed on the upper surface side of the slab where the equiaxed crystal ratio was insufficient. Moreover, in the plate | board thickness cross-section bending test of the steel plate, the crack generate | occur | produced and it was confirmed in the crack part that the MnS inclusion resulting from the segregation of Mn and S has become the starting point of a crack. From this, it was found that it is effective to set the angle of the electromagnetic stirring position to 30 degrees or more.

In the tests of Comparative Examples 4, 5, 6, 7 and 8, compared with the tests of Examples 1, 2, 3, 4 and 5 of the present invention, only the amount of reduction was changed, and the reduction ratio (D1 / D2 ) Was outside the range defined by the method of the present invention. In Comparative Examples 4, 7, and 8, the value of the reduction ratio was less than 0.2. As a result, macrosegregation remained in the slab, and numerous cracks occurred in the plate thickness section bending test of the steel sheet. It was revealed that cracks occurred in the macro segregation part at the center of the thickness of the sample.
In the tests of Comparative Examples 5 and 6, the amount of reduction was increased and the value of the reduction ratio was set to exceed 0.6. As a result, no macrosegregation remained in the slab, and a clear negative segregation band was observed at the center of the plate thickness even in the plate thickness section sample of the steel plate. As a result, when a thickness cross-section bending test was performed on this sample, the negative segregation zone at the center of the thickness had high ductility, so cracking did not occur, and cracking occurred at the boundary between the negative segregation zone and its outside. Concentrated.

In the tests of Comparative Examples 9, 10, 11, and 12, the casting rate was changed, and the solid fraction of the central part at the time of unsolidified pressure was set to a value outside the range specified in the present invention. When the casting speed is changed, the thickness of the unsolidified portion is also changed at the same time. Therefore, by adjusting the amount of reduction, the value of the reduction ratio is set within the range defined by the method of the present invention. In Comparative Examples 9 and 11, since the solid fraction in the central part exceeded 0.6, the molten steel enriched with segregation components remained between the dendritic branches without being sufficiently discharged. As a result, the results of the plate thickness section bending test of the steel plate were poor, and cracks were generated starting from segregation at the center of the sample plate thickness.
In the tests of Comparative Examples 10 and 12, the segregation of the components remained because the solid fraction at the center of the slab was as small as less than 0.1. This is presumably because the liquid phase amount remained at the center of the slab at the time of reduction, so that the liquid phase portion remained even after the reduction, and component segregation occurred when it further solidified. As a result, in the plate thickness section bending test of the steel plate, a crack occurred in the center portion of the plate thickness of the sample.

  The test of Comparative Example 13 is a test in which the unsolidified portion was not reduced. In the slab, normal center segregation occurred, and in the plate thickness section bending test of the steel plate, cracks occurred in the center portion of the plate thickness due to the remaining center segregation.

On the other hand, in the tests of Examples 1 to 5 of the present invention that satisfied all the conditions defined by the method of the present invention, the equiaxed crystal ratio of the upper surface of the cast slab was 10% or more. The segregation ratio of C, Mn, P and S was 1 or less for all the steel plates, and excellent results were obtained in the bending deformability of the steel plate cross section.
From the above test results, the excellent effect of the present invention was confirmed.

  According to the continuous casting method of the present invention, when rolling down a slab containing unsolidified molten steel, the position at which electromagnetic stirring is performed, the range of the solid phase ratio at the center of the slab during electromagnetic stirring, and the amount of reduction By optimizing the reduction ratio, which is the ratio of the thickness of the unsolidified part of the slab and the slab, it is possible to manufacture a slab slab for thick plates in which macro segregation and semi-macro segregation are stably reduced. Moreover, the steel plate obtained by rolling said slab is excellent in the bending deformability in a plate | board thickness cross section, and generation | occurrence | production of defects, such as a work crack, can be prevented. Therefore, the casting method of the present invention, the slab and the steel plate obtained by rolling the slab require a deformability at the time of bending work in which a tensile or compressive stress acts in the plate thickness direction, such as a steel plate for building materials. It can be widely applied to the steel manufacturing technology field.

It is a longitudinal section showing a continuous casting device used for a test typically. It is a figure which shows the sample collection method for slab structure | tissue and segregation status investigation. It is a figure which shows the bending test method of the extract | collected sample from a plate | board thickness cross section.

Explanation of symbols

1: immersion nozzle,
2: Meniscus,
3: Mold,
4: Molten steel,
5: solidified shell,
6: Guide roll,
7: Rolling roll,
8: Slab slab,
9: Electromagnetic stirrer,
10: unsolidified part,
11: Pinch roll 12: Plate sample for cross-sectional macro observation,
13: Sample for mapping analysis (MA),
14: Plate thickness section slice sample,
15: Steel plate

Claims (3)

  When casting a slab with a rectangular cross-section using a curved or vertical bending type continuous casting machine, the solidified molten steel inside the slab is electromagnetically stirred and the slab containing the unsolidified part is bulged. Then, a continuous casting method of rolling down a slab including an unsolidified portion using a rolling roll, wherein an angle formed between a tangent line of an arc forming a curved portion or a bent portion of the continuous casting machine and a horizontal plane is 30 degrees. At the position of the bending portion or bending portion, the unsolidified molten steel inside the slab is electromagnetically stirred, and a reduction roll is disposed in the horizontal portion of the continuous casting machine downstream from the position where electromagnetic stirring is performed. In the region where the solid fraction at the center of the piece is 0.1 to 0.6, the reduction ratio (D1 / D2), which is the ratio between the reduction amount D1 (mm) and the unsolidified portion thickness D2 (mm) at the time of reduction In which the value of is adjusted to a range of 0.2 to 0.6 and the unsolidified portion is included Continuous casting method characterized by rolling the.   A slab cast by the continuous casting method according to claim 1, wherein the upper surface equiaxed crystal ratio is 10% or more, and the total of the upper surface side and the lower surface side sandwiching the thickness center portion of the slab, A continuous cast slab characterized in that the segregation ratio of C, Mn, P and S in the range of at least 10 mm is 1 or less. A steel plate obtained by hot rolling the continuous cast slab according to claim 2, wherein the segregation ratios of C, Mn, P and S in the center portion of the plate thickness are all 1 or less. Steel plate to be used.

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