JP6199218B2 - Manufacturing method of slab for thick plate - Google Patents

Description

The present invention relates to a method for producing a thick plate slab.

  When producing steel for thick plates by rolling a steel ingot obtained by a casting method with a pair of ingot rolling rolls, a double plate-like shape that folds in the thickness direction at the head and bottom of the ingot at the time of ingot rolling The overlap and the fish tail having a concave shape when viewed in the rolling direction are grown from the rolling direction to form a defective shape portion (hereinafter referred to as a crop).

  Since the crop is a portion that cannot be used in the product and is cut off, it is an important issue to improve the yield of the partial rolling to reduce the loss due to the crop to the limit. Depending on the steel type, the amount of rounding off of segregation failure occurring in the hot metal part on the head side of the steel ingot has a significant effect on the yield of segment rolling, and the effect on the yield of segment rolling by the top crop on the head side. Is small.

  Therefore, (1) a steel ingot (refer to Japanese Patent Laid-Open No. Sho 56-53802) that reduces the size of the bottom crop generated during the block rolling by making the bottom portion a drawing shape, and (2) the block rolling is performed. A steel ingot (see Japanese Patent Application Laid-Open No. 53-93158) has been devised in which the bottom of the steel ingot is formed into a drawn shape by forming a recess in the bottom of the steel ingot with a large-diameter roll (rolling back rolling). Yes.

  However, even in the case of the steel ingot of (1) above, when the outer diameter of the segment rolling roll is small or when the amount of reduction by the segment rolling roll is small, overlap is likely to occur, and the size of the bottom crop cannot be reduced sufficiently. There is a fear. The reason for this is that if the outer diameter of the rolling roll is small or the amount of reduction is small compared to the thickness of the steel ingot to be rolled, the surface layer metal rolled by the rolling roll will be in the middle in the thickness direction. This is because it is easier to flow than metal, so that overlap easily occurs at the end of the steel ingot.

  Further, even in the case of the steel ingot of (2), when the outer diameter of the split roll is small or the reduction amount is small, the overlap is likely to occur, and the size of the bottom crop may not be sufficiently reduced. In addition, since biting rolling requires strong reduction with a large-diameter roll, there is a risk that facility modification may be required, and productivity may be reduced because biting rolling is performed.

JP-A-56-53802 JP-A-53-93158

  The present invention has been made based on the above-described circumstances, and steel for split rolling that can reduce bottom crop loss without reducing productivity when split rolling to produce a slab for thick plate. The purpose is to provide lumps.

The invention made to solve the above problems is a method of manufacturing a slab for a thick plate having a mass of 20 t or more and 80 t or less and a thickness of 200 mm or more and 600 mm or less, the step of casting a steel ingot for split rolling, Hot-rolling the ingot for ingot rolling with a reduction amount d (mm) using a pair of ingot rolling rolls having a radius of r (mm). A first tapered portion having a base shape of a truncated pyramid that has a rectangular bottom and a base that is symmetrical with respect to the axial center, and that gradually increases toward the upper base with a small thickness on the basis of the base shape on a pair of long side surfaces of the rectangle; The base-shaped central axis has a partial cylindrical concave connecting portion that continues to the first tapered portion, and a second tapered portion that continues to the connecting portion and has a thickness that gradually increases to the base shape from the bottom side. In the cross section passing through the long side of the top and bottom bottom, above The inclination angle θ3 formed by the bottom bottom of the source shape and the first imaginary plane extending from the long side surface is 70 ° or more and 89 ° or less, and the thickness D (mm) at the bottom bottom is the above-mentioned block rolling roll The following formula (1) is satisfied between the radius r (mm) and the reduction amount d (mm), and the following formula (2) is satisfied with the base shape thickness D0 (mm). And the first taper portion has an inclination angle θ2 of 80 ° or more and less than 90 °, and the inclination angle θ1 formed by the lower bottom and the second virtual surface extending the second taper portion is 45 ° or more and 75 ° or less, The height H of the tapered portion is 100 mm or more and 400 mm or less, and the radius of curvature of the connecting portion based on the cross section is 10 mm or more and 70 mm or less.
3 × (r × d) ^0.5 <D <5 × (r × d) ^0.5 (1)
0.5 <D / D0 <0.8 (2)

  The ingot for rolling is in the rolling state when the thickness D (mm) at the lower bottom, the radius r (mm) of the rolling roll and the reduction amount d (mm) satisfy the above formula (1). The difference in metal flow between the surface layer portion and the center portion of the steel ingot is reduced, and the amount of overlap or thickness-shaped convex shape generated at the end of the steel ingot can be reduced, so that the yield of the ingot rolling is improved. Further, the ingot for ingot rolling includes a connecting portion having a radius of curvature equal to or greater than the lower limit and a first tapered portion having an inclination angle θ2 equal to or greater than the lower limit and a height H equal to or greater than the lower limit. Sometimes the mold is not easily chipped, so that the life of the mold can be extended and the manufacturing cost of the slab for thick plate can be reduced. Further, the ingot for ingot rolling is prevented from cracking of the ingot when the radius of curvature of the connecting portion is not more than the above upper limit, and the ingot of inclining of the second taper portion is not less than the above lower limit. Two-skin skin (bited shape) hardly occurs in the slab after rolling. Further, in the ingot for ingot rolling, the inclination angle θ1 of the second taper portion is equal to or less than the above upper limit, and the height H of the first taper portion is equal to or less than the above upper limit. It is difficult to generate, and a high-quality ingot for ingot rolling is obtained. The ingot for ingot rolling is produced by having the first tapered portion having such a configuration, the connecting portion continuing to the first tapered portion, and the second tapered portion continuing to the connecting portion in order from the lower bottom side. Therefore, the crop loss on the bottom side can be reduced and the yield of the block rolling can be improved without reducing the property.

  The “base shape” means a virtual shape that includes the outer shape of the ingot for ingot rolling, and the “upper bottom” means the base shape when casting the steel ingot by the lower steel ingot casting method. The “bottom floor” means the bottom surface of the base shape when casting a steel ingot. The “height of the first taper portion” means the shortest distance between the position where the first taper portion continues to the connecting portion and the bottom bottom. The “rolling amount” means the amount of change in the thickness of the steel ingot per one piece rolling roll when the steel ingot is rolled once through the pair of piece rolling rolls. That is, the “rolling amount” is ½ of the amount of change in thickness before and after passing the steel ingot once between a pair of split rolls.

  As explained above, the ingot for ingot rolling of the present invention can reduce bottom crop loss without reducing productivity when ingot rolling to produce a slab for thick plate.

The schematic side view seen from the width direction of the steel ingot for a bundle rolling which concerns on one Embodiment of this invention Schematic side view seen from the thickness direction of the steel for ingot rolling in FIG. 1A Enlarged view of broken line circular part A in FIG. 1A FIG. 1A is a schematic side view showing a method of performing the rolling of the steel for ingot rolling shown in FIG. 1A.

  Hereinafter, embodiments of the steel ingot for split rolling according to the present invention will be described with reference to the drawings as appropriate.

  The ingot for ingot rolling according to one embodiment of the present invention shown in FIG. 1A, FIG. 1B and FIG. 1C has a mass of 20 to 80 t and a thickness of 200 to 600 mm by rolling using a pair of ingot rolling rolls. The upper base 1 and the lower base 2 are rectangular and have a truncated pyramid-shaped base shape 3 that is symmetric about the axis. The base shape 3 is a virtual shape including the outer shape of the ingot for ingot rolling, and is indicated by a two-dot chain line in FIGS. 1A and 1B. The pair of long side faces of the rectangle are rolled on the ingot for ingot rolling by the ingot rolling roll 9 shown in FIG. In this specification, the direction in which the pair of long side surfaces face each other is referred to as a thickness direction, and the vertical direction in FIGS. 1A and 1B from the lower base 2 toward the upper base 1 is referred to as a height direction. A direction perpendicular to the direction and the height direction is referred to as a width direction. The ingot for ingot rolling is continuous with the first taper portion 4, which has a small thickness with respect to the base shape 3 and gradually increases toward the upper base 1 on the pair of long side surfaces. It has a partial cylindrical concave connection portion 5 and a second taper portion 6 that is continuous to the connection portion 5 and has a thickness that gradually increases to the base shape 3 in this order from the bottom 2 side. In addition, although the connection part 5 is curved-surface shape and continues smoothly with each of the 1st taper part 4 and the 2nd taper part 6, in order to make the connection part 5 easy to understand in FIG. A broken line is shown at the boundary between the portion 4 and the second taper portion 6.

  Moreover, as shown in FIG. 1A and FIG. 1B, the said steel ingot for partial rolling has the drawing-shaped feeder part formed in the head. The ingot for ingot rolling may be cast by a casting method other than the feeder frame method, and it is not necessary to form a feeder part on the head. The segregation generated in the lump can be collected in the feeder part, and a high-quality steel ingot can be cast. In the steel type where segregation is a problem, this feeder part is cut off as a top crop after rolling.

  In a cross-section passing through the central axis of the base shape 3 and perpendicular to the long sides of the upper base 1 and the lower base 2, that is, in a schematic side view as viewed from the width direction of FIG. The lower limit of the inclination angle θ3 formed by the first virtual surface 7 extending the side surface on the long side is 70 °, and more preferably 75 °. On the other hand, the upper limit of the inclination angle θ3 is 89 °, more preferably 88 °. If the tilt angle θ3 is less than the lower limit, the top crop may be increased during width rolling, and the crop cut-off amount may increase. On the other hand, if the inclination angle θ3 exceeds the upper limit, the steel ingot may not be removed from the mold during die cutting.

  When the distance between the lower base 2 and the upper base 3 of the base shape 3 is the height H0 of the ingot for rolling, the lower limit of the height H0 of the ingot for rolling is preferably 2000 mm, 2200 mm is more preferable. On the other hand, the upper limit of the height H0 is preferably 4000 mm, and more preferably 3500 mm. If the height H0 is less than the lower limit, the amount of cut-off increases as the height decreases, which may reduce the yield of split rolling. On the other hand, if the height H0 exceeds the upper limit, it may be difficult to handle the steel ingot in the factory.

<First taper part>
As shown in FIGS. 1A, 1B and 1C, the first taper portion 4 is a portion where the thickness gradually increases linearly from the connecting portion with the lower base 2 on the long side surface toward the upper base 1. Yes, the thickness is smaller than the thickness of the base shape 3.

  In FIG. 1A, the lower limit of the inclination angle θ2 formed by the lower base 2 and the first tapered portion 4 is 80 °, and more preferably 82 °. Further, the inclination angle θ2 of the first taper portion 4 is less than 90 °, and more preferably 89 ° or less. If the tilt angle θ2 is less than the lower limit, the mold may be lost during die cutting, and the mold life may be shortened. On the other hand, if the inclination angle θ2 exceeds the upper limit, the steel ingot may not be removed from the mold during die cutting.

  The minimum of the height H of the said 1st taper part 4 is 100 mm, and 105 mm is more preferable. On the other hand, the upper limit of the height H is 400 mm, more preferably 350 mm. If the height H is less than the lower limit, the convex portion at the bottom of the mold may be chipped due to thermal stress, and the mold life may be shortened, and depending on the relationship with the following formula (1), overlap is likely to occur. Crop loss may increase. On the other hand, when the height H exceeds the upper limit, a zack defect is likely to occur inside the steel ingot, and the quality of the steel ingot may be deteriorated. When the molten metal solidifies during casting, it gradually solidifies from the outer surface of the ingot, solidification shrinkage occurs when the molten metal remaining inside solidifies, and the shrinkage forms a zaku that is a collection of coarse porosity. Is done. Since the steel ingot is uniformly solidified from the periphery, the zaku can be gathered upward by forming a shape in which the long side surface is inclined with respect to the lower base 2 as in the base shape 3. However, since zaku can be observed in the lower part of the steel ingot, it is considered that when the lower part of the mold is changed from the base shape, the state of occurrence of zaku defects may be deteriorated. Therefore, in order to maintain the quality of the steel ingot, Height H shall be below the above-mentioned upper limit.

The thickness D (mm) at the lower bottom 2 of the ingot for rolling steel satisfies the following formula (1) when the radius and rolling amount of the rolling roll 9 are r (mm) and d (mm). .
3 × (r × d) ^0.5 <D <5 × (r × d) ^0.5 (1)

When a material such as a steel ingot is rolled by a rolling roll, the thickness at which the material is deformed by the rolling of the rolling roll is the length in the material traveling direction of the region in which the force is applied to the material (hereinafter referred to as the "rolling action length"). It is sometimes called “sa”. Therefore, if the rolling action length is small relative to the thickness of the steel ingot to be rolled, the metal in the surface layer part of the steel ingot is more likely to flow than the metal in the central part in the thickness direction. Occur. Conversely, if the rolling action length is large relative to the thickness of the steel ingot, the difference in metal flow between the surface layer and the center of the steel ingot is reduced, and a convex shape is generated at the end of the steel ingot as viewed from the rolling direction. To do. Therefore, the amount of crop truncation increases in both cases where the rolling action length is too small and too large with respect to the thickness of the steel ingot. The rolling action length can be expressed by (r × d) ^0.5 , and the thickness D at the bottom 2 of the ingot for ingot rolling satisfies the above formula (1). The occurrence of overlap and convex shape can be suppressed, and the crop cut-off amount is reduced.

Moreover, when the thickness in the lower base 2 of the base shape 3 is D0 (mm), the value of the ratio D / D0 with respect to the thickness D (mm) in the lower base 2 of the steel for ingot rolling is expressed by the following formula ( 2) is satisfied.
0.5 <D / D0 <0.8 (2)

  Even when the thickness D of the lower bottom 2 of the steel for ingot rolling satisfies the above formula (1), if the ratio D / D0 is 0.5 or less, the inclination angle of the second taper portion 6 to be described later If θ1 is small, the surface of the slab after the partial rolling tends to have two skins, which may impair the product quality of the steel for thick plates. Further, when the ratio D / D0 is 0.8 or more, there is a possibility that a sufficient yield improvement effect cannot be obtained.

<Connecting part>
As shown in FIG. 1C, the connecting portion 5 is a curved surface smoothly continuing to the first tapered portion 4 and the second tapered portion 6, and is a partial shape of a cylindrical concave surface having a radius of curvature R when viewed from the width direction. have. 1C indicates a virtual circle having the same radius of curvature R as the partial cylindrical concave surface of the connecting portion 5.

  In FIG. 1C, the lower limit of the radius of curvature R of the concave surface of the partial cylinder of the connecting portion is 10 mm, and more preferably 12 mm. On the other hand, the upper limit of the curvature radius R is 70 mm, more preferably 68 mm. If the radius of curvature R is less than the lower limit, the mold may be missing when the mold is removed, and the mold life may be shortened. On the other hand, when the curvature radius R exceeds the upper limit, the steel ingot may be broken.

<Second taper part>
As shown in FIG. 1C, the second tapered portion 6 is smoothly continuous with the connecting portion 5. The thickness of the second tapered portion 6 gradually increases from the portion continuing to the connecting portion 5 toward the base shape 3 toward the upper base 1.

  In FIG. 1A, the lower limit of the inclination angle θ1 formed by the lower base 2 and the second imaginary surface 8 obtained by extending the second tapered portion 6 is 45 °, and more preferably 46 °. On the other hand, the upper limit of the inclination angle θ1 is 75 °, more preferably 74 °. If the tilt angle θ1 is less than the lower limit, the slab surface after the ingot rolling is likely to have two skins, which may impair the product quality of the steel for thick plates. On the other hand, when the inclination angle θ1 exceeds the upper limit, a zack defect is likely to occur inside the steel ingot, and the quality of the steel ingot may be deteriorated.

  The steel type used for the ingot for ingot rolling is not particularly limited, but SS steel, CrMo steel, high Ni steel and the like can be used. For example, steel adjusted to a predetermined composition as shown in Table 1 can be used.

<Production method of steel sheet for products>
The ingot for ingot rolling is manufactured, for example, by the following molten steel processing step and casting step. Furthermore, the steel plate for thick plates of a product is manufactured from the said ingot for rolling and the said rolling ingot and a product rolling process.

(Molten steel treatment process)
Molten steel melted in a blast furnace or converter, or molten steel melted in an electric furnace using scrap, is adjusted to a predetermined composition as shown in Table 1 through a molten steel treatment process. Further, gas components such as O (oxygen) and H (hydrogen) and impure elements are also removed in the molten steel treatment process.

(Casting process)
In order to cast a steel ingot having a mass of 20 t or more and 80 t or less, a general pouring steel ingot casting method is employed in the casting process. Specifically, a mold corresponding to the ingot for ingot rolling having the shape defined in the present embodiment is prepared, and the molten metal adjusted in the molten steel treatment process is poured into this mold and left still. Thereafter, the solidified steel ingot is demolded by a conventional method. By such a process, the ingot for ingot rolling is manufactured.

(Bunch rolling process)
In the partial rolling step, the manufactured steel ingot for partial rolling is hot-rolled at a temperature of 900 ° C. or higher and 1200 ° C. or lower using a pair of partial rolling rolls. The above-mentioned ingot for ingot rolling is repeatedly rolled until a predetermined thickness is obtained to obtain a semi-finished product slab having a thickness of 200 mm to 600 mm.

(Product rolling process)
In the product rolling process, the slab obtained in the above-mentioned block rolling process is heated to 1000 ° C. or higher, hot-rolled to a predetermined thickness by a rough rolling mill and a finish rolling mill, and a steel plate for products is obtained. obtain.

<Advantages>
The ingot for ingot rolling has a base shape in the shape of a truncated pyramid whose upper and lower bases are rectangular and symmetrical with respect to the axis center, and a first thickness that is small with respect to the base shape on the pair of long side surfaces of the rectangle. A tapered portion, a partially cylindrical concave connecting portion that continues to the first tapered portion, and a second tapered portion that continues to the connecting portion and has a thickness that gradually increases to the base shape, in order from the lower bottom side. With the above-described configuration, the crop loss on the bottom side can be reduced without deteriorating the quality of the product. That is, the yield of the partial rolling can be improved by the steel ingot for the partial rolling without reducing the productivity.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to these Examples.

[Casting steel ingots]
1A, FIG. 1B and FIG. 1C have a first tapered portion, a connecting portion, and a second tapered portion, and the thickness D (mm) at the bottom of the steel ingot shown in Table 2 thickness D0 (mm), the radius of curvature R of the connecting portion (mm), the inclination angle theta ₁ of the second tapered portion (°), the inclination angle theta ₂ of the first tapered portion (°), based inclination angle shape theta ₃ ( °), and each steel ingot having a height H (mm) of the first taper portion was cast using a mold. Specifically, the molten steel was poured into the mold by the casting method of the ingot steel ingot, and then allowed to stand to cast a test ingot.

  As the test steel ingot, SS steel having the composition shown in Table 1 was used as a steel type, and a steel ingot having a mass of 20 to 80 t was cast. Since SS steel has a high temperature at the time of ingot rolling of 900 ° C. or more and 1200 ° C. or less and the difference in deformation behavior due to the steel type (component) is small, the deformation behavior of other steel types can be simulated. Since the thickness is large and the amount of rolling reduction is small, it is difficult to press-fit the zaku, and zaku defects are easily generated. Therefore, as described above, SS steel was used as the steel type of the test ingot.

[Die cutting]
After casting the test steel ingot, the feeder frame was removed from the mold using a crane, the steel ingot was lifted together with the mold with the crane, and the runner portion was cut. Thereafter, the steel ingot lifted together with the mold was placed on a steel ingot extraction table on the ground, the steel ingot was protruded from the mold, the upper portion of the protruding steel ingot was grasped with a crane, and the steel ingot was extracted from the mold.

[Rolling steel ingots]
Using an ingot-rolling roll having a roll radius r of 500 mm or more and 700 mm or less, a test steel ingot was hot-rolled at a temperature of 900 ° C. or more and 1200 ° C. or less with a reduction amount d of 30 mm or 40 mm to prepare an evaluation slab. Further, the evaluation slab was rolled to obtain a steel plate for products.

Example 1
In Example 1, as shown in Table 1, a steel ingot that satisfies the numerical range of the parameters defined in the present invention is cast as a test ingot, and the above formula (1) is expressed with respect to the thickness D at the bottom of the steel ingot. The test steel ingot was rolled with a roll with a radius r satisfying the relationship and a reduction amount d, and an evaluation slab was prepared.

(Examples 2 and 3, Comparative Examples 1 and 2)
The test steel ingots of Examples 2 and 3 and Comparative Examples 1 and 2 were different from the test steel ingot of Example 1 in thickness D at the bottom of the steel ingot. Example 2 rolls with the same rolling amount d with the same rolling r having the same radius r as in Example 1, and Example 3 and Comparative Example 1 are the rolling rolls having a radius r different from that of Example 1. Rolling was performed, and Comparative Example 2 was rolled with a reduction amount d different from that in Example 1 to prepare respective evaluation slabs.

(Examples 4, 5, and 6, Comparative Examples 3 and 4)
The test steel ingots of Examples 4, 5, 6 and Comparative Examples 3, 4 were different from the test steel ingot of Example 1 in the curvature radius R of the connecting portion. In Example 5 and Example 6, rolling was performed with a block rolling roll having a radius r different from that of Example 1, and respective evaluation slabs were prepared. In Example 4, Comparative Example 3 and Comparative Example 4, rolling was performed with the same rolling amount d with the same roll r having the same radius r as in Example 1, and each evaluation slab was created.

(Examples 7, 8, and 9 and Comparative Examples 5 and 6)
In the test steel ingot of Example 8, the inclination angle θ ₁ of the second taper portion is the same as that of the test steel ingot of Example 1, and the thickness D at the bottom of the steel ingot is the same as the test steel ingot of Example 1. Were different. The test steel ingots of Examples 7 and 9 and Comparative Examples 5 and 6 were different from the test steel ingot of Example 1 in the inclination angle θ ₁ of the second taper portion.

(Examples 10, 11, and 12, Comparative Examples 7 and 8)
The test steel ingots of Examples 10, 11, 12 and Comparative Examples 7 and 8 were different from the test steel ingot of Example 1 in the height H of the first taper portion. In Examples 10, 11, 12 and Comparative Example 7, rolling was performed with the same rolling amount d with the same roll r having the same radius as in Example 1, and respective evaluation slabs were prepared. On the other hand, Comparative Example 8 was rolled with a lump rolling roll having a radius r different from that of Example 1 to create an evaluation slab.

(Examples 13, 14, 15 and Comparative Examples 9, 10)
The test steel ingots of Examples 13 and 15 and Comparative Examples 9 and 10 were different from the test steel ingot of Example 1 in the inclination angle θ ₂ of the first taper portion. Test steel ingot of Example 14, but the inclination angle theta ₂ of the first tapered portion is the same as the test steel ingot in Example 1, the thickness of the lower base of the thickness D and the base shape of the lower bottom of the steel ingot D0 was different from the test steel ingot of Example 1.

<Slab shape evaluation>
In order to evaluate the slab shape, the cut weight of the bottom crop was measured. The ratio of the cut weight of the bottom crop to the total weight of the slab is the bottom crop rate. When the bottom crop rate is less than 2%, the evaluation is “A”, and when the bottom crop rate is 2% or more, the evaluation is “B”. .

<Ingot quality evaluation>
About the steel ingot after casting, the presence or absence of surface cracks and the occurrence state of zaku defects were evaluated as follows. When the surface crack did not occur and no defect was detected by the following ultrasonic flaw detection test, the steel ingot quality was judged to be good, and the evaluation result of the steel ingot quality was set to “A”.

(Surface crack)
The surface of the steel ingot after casting was visually observed, and when an open crack was observed, the evaluation result of the steel ingot quality was “B”.

(Zaku defect occurrence status)
About the steel sheet after product rolling, the presence of a cavity defect (hereinafter, this may be referred to as “zaku defect”) is confirmed by an ultrasonic flaw detection test based on JIS-G0801 (2008). went. Usually, there is a zaku inside a steel ingot produced by casting, and there is a case where a zaku is formed also in the lower part of the steel ingot. When the lower part of the mold is changed from the base shape as in the configuration of the present invention, it is conceivable that the state of occurrence of the zaku defect deteriorates. Therefore, the ultrasonic flaw detection is performed in the range of the height from the bottom of the steel ingot to 30%. The test was conducted. Here, when at least one light defect was detected in the ultrasonic flaw detection test, it was determined that the state of occurrence of the zaku defect was worse than before, and the evaluation result of the steel ingot quality was set to “C”.

<Slab quality evaluation>
During the partial rolling, folding (double skin) may occur on the inside starting from the connecting portion. Therefore, the surface of the slab after partial rolling is visually observed, and the case where the occurrence of folding is not recognized is evaluated as “A” as the quality of the slab is good, and the case where folding is observed is evaluated. “B”.

<Evaluation of mold reusability>
The mold was evaluated whether it could be reused after casting. If thermal stress is applied to a weak part of the mold, the mold may break. Therefore, the mold after steel ingot casting was visually observed, and the case where defects were observed was evaluated as “B”. In addition, after injecting the molten metal into the mold, when the steel ingot lifted together with the mold was placed on the steel ingot removing table to remove it from the mold, the case where the steel ingot could not be removed from the mold was evaluated as “C”. did. When the steel ingot could be removed from the mold and no mold defect was observed, it was determined that the mold could be reused, and the evaluation was “A”.

<Comprehensive evaluation>
In this test, a comprehensive evaluation was made assuming that the evaluation results of slab shape evaluation, steel ingot quality evaluation, slab quality evaluation, and mold reusability evaluation were all “A”, and that the cutting weight of the bottom crop could be greatly reduced without inconvenience. “A” was assigned, and “B” was given as the overall evaluation for other items. These evaluation results are shown in Table 2.

[Measurement result]
In each of Examples 1 to 15, the steel ingot quality, slab quality, and mold reusability were all good, and the cut weight of the bottom crop was greatly reduced as compared with the conventional case.

  In contrast, Comparative Examples 1 and 2 had good steel ingot quality, slab quality, and mold reusability, but there was no significant reduction in the cutting weight of the bottom crop compared to the conventional case, and an improvement in productivity was recognized. I couldn't. Neither Comparative Example 1 nor 2 satisfies the above formula (1). In Comparative Example 2, since the rolling amount d with respect to the thickness D at the bottom of the steel ingot is small, the rolling action length on the steel ingot at the time of rolling is small, so that the metal flow of the surface layer portion is smaller than the vicinity of the center of the steel ingot to be rolled. It can be said that it became larger, a large fold occurred, and the cutting weight of the bottom crop increased. On the contrary, the comparative example 1 has a large rolling action length because the radius r of the split rolling roll with respect to the thickness D at the bottom of the steel ingot is large, so that the metal flows near the center of the steel ingot, and the end of the slab Thus, it can be said that the convex shape in which the central portion in the thickness direction protrudes is formed, and the cutting weight of the bottom crop is increased.

  In Comparative Example 3, chipping was observed in the mold after ingot casting. Since the test steel ingot of Comparative Example 3 has a radius of curvature R of the connecting portion as too small as 7 mm, the portion of the mold corresponding to the connecting portion is weak against thermal stress, and the mold is considered to be cracked and chipped. Further, in Comparative Example 4, cracks opening on the surface of the test steel ingot were observed. This is because the radius of curvature R of the connecting portion of the test steel ingot is large, and it can be seen that the radius of curvature R of the connecting portion is too large at 75 mm.

In Comparative Example 5, two sheets of skin were observed on the prepared evaluation slab. This is because the inclination angle theta ₁ of the second taper portion of the test steel ingot is small, considered Orekomi starting occurs the connecting portion during rolling. Further, in Comparative Example 6, it was recognized that the generation situation of the zaku defect was deteriorated. This is because the inclination angle θ ₁ of the second taper portion of the test ingot is large, and it can be seen that the inclination angle θ ₁ is too large at 78 °.

  In Comparative Example 7, chipping was observed in the mold after ingot casting. This is probably because the height H of the first taper portion was too small, so that a portion weak against thermal stress was formed in the mold. Further, in Comparative Example 8, it was recognized that the state of occurrence of the zaku defect was deteriorated. This is because the height H of the first taper portion is large, and it can be seen that the height H is too large at 450 mm.

In Comparative Example 9, chipping was observed in the mold after ingot casting. This is probably because the first taper portion has an inclination angle θ _{2 that} is too small, and thus a portion that is weak against thermal stress is formed in the mold. In Comparative Example 10, the test steel ingot could not be removed from the mold. Thus, it can be seen that the die cannot be removed unless the inclination angle θ ₂ is less than 90 °.

  As described above, since the steel block for rolling in a block can be rolled into pieces to reduce bottom crop loss without reducing productivity when producing a slab for plate, the construction using steel for plate It is useful as a member for objects, bridges, ships, automobiles, railway vehicles, marine structures and the like.

DESCRIPTION OF SYMBOLS 1 Upper base 2 Lower base 3 Base shape 4 1st taper part 5 Connection part 6 2nd taper part 7 1st virtual surface 8 2nd virtual surface 9 Split roll

Claims (1)

Or mass is 20t 80t or less, the thickness is a method for manufacturing a 600mm below plank slabs least 200 mm,
A step of casting a steel ingot for split rolling;
Hot-rolling the ingot for ingot rolling using a pair of ingot rolling rolls having a radius of r (mm) at a reduction amount d (mm);
With
The ingot for ingot rolling has a base shape in the shape of a truncated pyramid that has a rectangular top and bottom and is symmetrical about the axis,
A first taper portion having a small thickness with respect to the base shape and gradually increasing toward the upper bottom on the side surfaces of the pair of long sides of the rectangle, a connecting portion having a partially cylindrical concave surface continuous with the first taper portion, and the connection The second taper part that is continuous with the part and gradually increases in thickness to the base shape, in order from the bottom side,
In a cross section passing through the central axis of the base shape and perpendicular to the long sides of the upper and lower bases, an inclination angle θ3 formed by the lower base of the base shape and the first virtual surface extending the side surface of the long side is 70 ° or more and 89 ° or less. And
The thickness D (mm) in the lower bottom is between the following formula (1) and the base shape thickness D0 (mm) between the radius r (mm) and the reduction amount d (mm) of the above-mentioned block rolling roll. Satisfies the following formula (2),
In the cross section, an inclination angle θ2 formed by the lower base and the first taper portion is 80 ° or more and less than 90 °, and an inclination angle θ1 formed by the lower virtual surface and the second virtual surface extending the second taper portion is 45 °. 75 ° or less and the height H of the first taper portion is 100 mm or more and 400 mm or less,
A method for manufacturing a slab for thick plates, wherein the radius of curvature of the connecting portion based on the cross section is 10 mm or more and 70 mm or less.
3 × (r × d) ^0.5 <D <5 × (r × d) ^0.5 (1)
0.5 <D / D0 <0.8 (2)

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