JP6624396B2 - Steel sheet manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a steel sheet, and more particularly, to a method for producing a steel sheet of a steel type strictly resistant to segregation through the production of a continuous cast slab having a small center segregation.

  In general, in continuous casting slabs, during the solidification process, solute elements such as carbon, phosphorus, sulfur, and manganese concentrate in the unsolidified liquid phase due to redistribution during solidification, and this is trapped between dendrite trees. It is known that micro segregation occurs. In other words, continuous cast slabs, due to solidification shrinkage and heat shrinkage, bulging of the solidified shell generated between the rolls of the continuous casting machine, etc., voids are formed in the center of the thickness of the slab, or negative pressure is generated. Usually, molten steel is absorbed into this part. In particular, since continuous cast slabs at the end of solidification do not have a sufficient amount of molten steel in the unsolidified portion, the molten steel concentrated by the microsegregation flows there and accumulates at the center of the continuous cast slab. To solidify. In the segregation spot formed in this way, the concentration of the solute element is much higher than the initial concentration of the molten steel, and this is generally called macrosegregation. We call it segregation.

  In recent years, for the center segregation occurring in continuous cast slabs, it has been required to reduce this, and in particular, the required level for segregation strict steel grades such as line pipe materials is becoming more severe. It is a fact. For example, line pipe materials are used for transporting crude oil, natural gas, etc., but if MnS or Nb carbide is generated at the center segregation site, it is generated by a corrosion reaction, and hydrogen that has entered inside the steel is It diffuses and accumulates around the MnS and Nb carbides inside and induces cracks due to its internal pressure. In addition, since the center segregation portion is hard, the crack propagates. This is hydrogen-induced cracking (HIC). Therefore, it is important to reduce the center segregation of the slab.

  In order to cope with this, conventionally, many measures have been proposed to reduce or detoxify the center segregation of the slab in the continuous casting process.

  For example, a method has been proposed in which a slab at the end of solidification having an unsolidified layer in a continuous casting machine is cast while gradually reducing the slab by a slab support roll at a reduction amount corresponding to the sum of the amount of solidification shrinkage and the amount of heat shrinkage. ing. (Patent Document 1, Patent Document 2). These techniques, called "light reduction" or "light reduction method", in which a slab is reduced by a reduction amount corresponding to the sum of the amount of solidification shrinkage and the amount of heat shrinkage, are arranged in the casting direction. By using a plurality of pairs of rolls, the slab is gradually reduced by a reduction amount corresponding to the sum of the amount of solidification shrinkage and the amount of heat shrinkage, and the volume of the unsolidified layer is reduced. This is a method of preventing the formation of a pressurized portion and, at the same time, preventing the flow of concentrated molten steel formed between dendrite trees, thereby reducing the center segregation of the slab.

  In recent years, continuous casters of the segment type, which are composed of segments having a plurality of roll pairs, are the mainstream, and a group of rolls for performing light reduction (hereinafter referred to as “light reduction zone”) are also used. , A plurality of segments, and the opposite roll opening is adjusted so that the entrance side is larger than the exit side between the entrance side and the exit side of the segment, thereby forming a low pressure lower zone.

  However, the light rolling method has the following problems. The reason is that although the degree of segregation can be reduced to some extent by light reduction, when the solidification completion position in the slab width direction is different, the effect of improving segregation is not sufficient. This is because if the solidification completion position extends downstream in the casting direction compared to other positions in the slab width direction, the part where solidification has already been completed becomes a resistance, making it difficult to apply light reduction, In some cases, the hydrogen-induced cracking described above may occur.

  On the other hand, as a method for solving the problems of the light reduction method, a method of improving the shape of the solidification completion (crater end) position in the width direction, that is, a method of improving the shape of the W-shaped crater end is conventionally used. It has been proposed. For example, the method described in Patent Literature 3 proposes eliminating non-uniform solidification in the slab width direction by controlling the cooling in the casting direction of the secondary cooling in order to effectively function the reduction. .

  In addition, regarding the crater end shape, Patent Literature 4 discloses a method in which the agitation intensity in a mold is adjusted according to the crater end shape during casting, and the peak-valley difference is controlled within 2 m in order to keep the final solidification position in a low pressure zone. Has been proposed. In the following description, the term “final solidification position” simply means the position (distance from the meniscus) of the final solidification position in the slab thickness direction in the slab withdrawal direction.

  As described above, in the conventional technology described above, not only the above-described problem of the application of the light reduction, but also when a difference occurs in the width direction in the solidification completion position, the liquid phase is formed from the width direction position where the solidification is completed first. Because the thickened molten steel flows to the unsolidified width direction position through, the thickened molten steel will accumulate around the width position where the final solidification position is the most downstream in the casting direction in the width direction. Partial segregation will inevitably occur. Therefore, even if the crater end shape is controlled from the W-shape to the U-shape or the V-shape, unless the solidification is simultaneously completed in the width direction, center segregation will inevitably occur, and severe segregation in recent years is particularly severe. There was a problem that steel types could not be used.

JP-A-8-132203 JP-A-8-192256 JP-A-9-192806 JP-A-2004-351481

  The present invention aims to establish a technology that solves the above-mentioned various problems of the conventional technology, and in particular, realizes the production of a continuous cast slab with a small center segregation, and a steel plate of a steel type severe against segregation. To develop and propose a method for advantageously producing

The characteristic configuration of the present invention developed to solve the above problems is as described below.
That is, the present invention is a method of manufacturing a steel sheet by hot rolling a continuously cast slab slab, when the slab slab is continuously cast by a continuous casting machine,
When the distance from the meniscus at the final solidification position at which solidification in the slab thickness direction is completed is Z, and the position at the slab width direction is X, the distance Z (X) from the meniscus at the final solidification position is both ends in the slab width direction. part side has a minimum value in the slab width direction central portion and having a maximum at, between the front Symbol maximum value the minimum value, the absolute value of the distance Z in the width direction position X (X) | dZ / dX | has a gradient of 0.5 m / m or more, and the slab width direction position of the maximum value at both ends in the slab width direction is at least 0.2 times the slab thickness from the slab width direction end face, Continuous casting so as to have a distance of 0.7 times or less, and subsequently cut in the width direction so as to have a desired drawing direction length to form a slab cast,
Then, after previously cutting and removing both ends of the slab slab in the slab width direction including the position of the maximum value in the slab width direction, heating the slab slab slab body portion on the remaining central portion side after the cut portion is removed. Hot-rolled to a steel sheet, or
The slab slab obtained by cutting in the width direction is first heated and then hot-rolled into a steel sheet, and thereafter, from the steel sheet thus obtained, including the position of the maximum value in the slab width direction. A method for manufacturing a steel sheet, characterized in that a part corresponding to both ends in the slab width direction is cut and removed so that a central part side, which is a remaining uncut part, is a product steel sheet.

  In the present invention, in the continuous casting of the slab slab, the maximum value of the slab width direction position at both ends in the slab width direction is 0.025 times or more of the slab width W from the slab width direction end face, respectively. It is preferable to perform continuous casting so that the distance is within a range of 075 times or less.

  Further, in the present invention, when cutting and removing both ends in the slab width direction including the position of the maximum value in the slab width direction from the slab slab, the both ends in the slab width direction to be cut and removed are the maximum size. Cut the portion corresponding to the slab width direction both ends including the maximum slab width direction position including the width direction position of the central value from the slab width direction position of 50 mm from the slab width direction position or the hot rolled steel sheet. When removing, the cutting is performed so that the portions corresponding to both ends in the slab width direction to be cut and removed include the portions corresponding to the width direction positions 50 mm central side from the slab width direction position of the maximum value. Is preferred.

  Further, in the present invention, when continuously casting the slab slab, secondary cooling water is injected over the entire width of the long side surface to cool until the shell thickness of the long side surface becomes at least 30 mm, and the downstream side thereof is immediately downstream. In this case, it is preferable to cool by spraying the secondary cooling water only to the center portion of the width, and then to increase the cooling width sequentially from the center portion of the width for cooling.

Further, the range in which the absolute value | dZ / dX | of the distance Z (X) indicates a gradient of 0.5 m / m or more is 100 mm or more from the width direction position of the minimum value at the center of the slab width direction ; it is preferable that the distance from the widthwise position of the maximum of the slab width direction end portion side in the width center side is located above 100 mm.

  According to the present invention configured as described above, the segregated portions are gathered near both ends in the slab width direction, so that the slab or the steel plate manufactured by hot rolling the same is mechanically removed from the portion. In addition, a slab or a hot-rolled steel sheet with extremely low segregation can be reliably manufactured, and thus a steel sheet of a steel type strictly resistant to segregation can be advantageously manufactured.

It is a figure which shows the relationship between a crater end shape and a segregation degree when light reduction is not given. It is a figure which shows the relationship between a crater end shape and a segregation degree at the time of giving light reduction. It is a figure which shows the relationship between a crater end shape and a segregation degree at the time of light pressure and water volume density distribution adjustment. It is a figure showing the relation between the final solidification position and the width direction position in an example. It is a figure which shows the relationship between Mn segregation degree and the width direction position in an Example. FIG. 4 is a diagram showing a relationship between a final solidification position and a width direction position in a comparative example. FIG. 4 is a diagram showing a relationship between Mn segregation degree and width direction position in a comparative example.

  The inventors carried out slab continuous casting under various conditions and investigated the relationship between the crater end shape and the segregation distribution in the width direction. The results of the survey are described below.

  First, the crater end shape and the segregation degree distribution in the width direction of the slab slab when the continuous casting was performed without applying the light reduction were investigated. The cast slab was obtained by continuously casting low carbon steel using a vertical bending type continuous casting machine. The size is a slab slab having a thickness of 250 mm and a width of 2100, and the casting speed is 1.4 m / min. FIG. 1 is a diagram showing a crater end shape (distance Z from a meniscus at a final solidification position in a slab thickness direction) and a segregation degree distribution in a slab width direction. In the following FIGS. 1 to 7, the width direction position X of the slab slab shows a distribution of only a half width with the center in the width direction as the origin. Regarding the crater end shape, longitudinal wave ultrasonic waves were applied to the slab at a plurality of positions (width direction) slightly upstream of the final solidification position predicted by heat transfer / solidification calculation as disclosed in JP-A-4-231158 in advance. ), And based on a comparison between the result of determining the thickness of the unsolidified portion at each application position from the propagation time and the thickness of the unsolidified portion predicted by the heat transfer / solidification calculation, the transfer is performed in advance at each width position. It was calculated by correcting the final solidification position predicted by the heat / solidification calculation.

  The plots of the final solidification position shown in FIGS. 1 to 4 and FIG. 6 are based on the results of the above measurement at a pitch of 100 mm from the center of the slab width direction in the slab width direction. The crater end shape in the middle of the points is obtained by interpolating the measurement points with a spline curve, and the crater end shape near the measurement point at the end in the width direction of the final solidification position is determined by heat transfer It is obtained by shifting and correcting based on the measured value of the end portion in the width direction predicted by a finer calculation mesh by the solidification calculation.

  Next, regarding the degree of segregation, the Mn concentration was quantitatively analyzed by EPMA over the entire width of the center in the thickness direction in a cross section perpendicular to the casting direction of the slab, and was calculated as the degree of Mn segregation. Here, the degree of Mn segregation refers to the ratio of the Mn concentration of the center segregated portion analyzed by EPMA to the average value of the Mn concentration at a position sufficiently distant from the center of the sheet thickness.

  As shown in FIG. 1, it was found that the Mn segregation degree had a distribution substantially along the crater end shape (indicated by the final solidification position). It should be noted here that the segregation degree of the slab slab in the width direction position that has previously solidified is better than that in the position that subsequently solidifies, even if no light reduction is applied. It is considered that this is because, as described above, the concentrated molten steel moved from the width direction position where solidification was completed first to the width direction position where solidification was not completed.

  Next, continuous casting was performed under the same conditions as above by applying light reduction. The rolling gradient is 0.6 mm / m, which is an amount that is considered to sufficiently compensate for the amount of coagulation shrinkage. FIG. 2 is a diagram showing the crater end shape and the segregation degree distribution in the width direction at this time. As shown in this figure, even when a slight reduction is applied, the distribution of Mn segregation degree in the width direction follows the crater end shape. Also, when viewed as a whole, the Mn segregation degree shows better results when the reduction is applied than when the reduction is not applied, but the width direction position solidified earlier when the reduction is not applied. The result is that the degree of Mn segregation at the position in the width direction that was finally solidified in the case of applying the light reduction is greater than the degree of Mn segregation of the Mn.

  Next, in addition to the above-described conditions of the application of the light reduction, the overlap of the secondary cooling and the like are eliminated so that the water density distribution becomes uniform in the width direction. Continuous casting was performed to make the shape as flat as possible. The results are shown in FIG. 3 as the relationship between the crater end shape and the distribution of Mn segregation degree in the width direction. Although the Mn segregation degree is better than that of the two examples, even if the difference in the width direction position of the final solidification position is smaller, the segregation degree of the width direction position where solidification is completed later than the periphery is higher. It is larger than the degree of segregation at the width direction position that has solidified. In each of FIGS. 1 to 3, the width direction position where the degree of Mn segregation is large corresponds to the width direction position where the distance Z from the meniscus at the final solidification position has a maximum value. In FIGS. 1 to 3, only the half width of the slab slab is shown with the width center as the origin, but the half width on the opposite side also shows a generally symmetric distribution. The distance Z from the meniscus at the solidification position had a maximum value at the center of the width.

  From these facts, the inventors intentionally add a valley to the crater end shape at the width direction position, and when the distance Z from the meniscus at the final solidification position is a function of the width direction position X of the slab, the final solidification The distance Z (X) of the position from the meniscus has a maximum value at both ends in the slab width direction, has one minimum value at the center in the slab width direction, and has the maximum value at both ends in the slab width direction. And the minimum value is a function of X that changes monotonically, and the slab width direction position of the maximum value at both ends of the slab width direction is 0.2 times the slab thickness from the slab width direction end face. As described above, if continuous casting is performed so as to have a distance of 0.7 times or less, so-called segregated portions can be concentrated near both ends in the width direction, and at the same time, a position where it can be mechanically removed. And knowledge that can be formed.

  These findings mean that solidification is sequentially completed from the center of the width of the slab slab to both short side directions. If this condition is not satisfied, for example, the distance Z from the meniscus at the final solidification position is a position X in the slab width direction between the minimum value at the center in the slab width direction and the maximum value appearing at both ends in the slab width direction. Does not monotonically change and has a local maximum value in the middle, which means that the segregation partially increases at the width direction position of the local maximum value. Furthermore, if the consolidation position (the position in the width direction at which the maximum value of Z is located) is not near the short side, the yield of the product will be significantly reduced when mechanical removal is performed later.

  In addition, the width direction position where the final solidification position is the most downstream side at both ends in the slab width direction needs to be as short as possible. The reason is that mechanical removal is performed in a later step, so that the yield is deteriorated and the cost is increased. However, if the shell thickness on the short side is too thin, bulging or breakout on the short side is caused. Therefore, the width position at which the final solidification position is the most downstream side at both ends in the slab width direction (both ends viewed in the slab width direction). (The final solidification position on the part side) is preferably at least 0.2 times and 0.7 times the slab thickness from the end face in the slab width direction. In addition, the final solidification position at both ends as viewed in the slab width direction is 0.025 times or more of the slab width W from the end face in the slab width direction from the viewpoint of ensuring both short side shell thickness and yield. The distance is preferably 075 times or less.

  On the other hand, the shell thickness on the short side can be controlled by the amount of water of the secondary cooling spray on the short side. In this case, even if the amount of water is reduced to zero in order to suppress cooling, radiation cooling occurs due to a temperature difference from the atmosphere. Therefore, radiation is controlled using an edge heater or the like as necessary.

  Also, a plurality of support rolls may be provided on the short side to reduce the risk of short side bulging and breakout.

  Further, the distance Z (X) from the meniscus at the final solidification position monotonously changes with respect to the width direction position X between the maximum value at both ends in the slab width direction and the minimum value at the width center portion. In such continuous casting, it is preferable that the absolute value | dZ / dX | of the gradient with respect to the width direction position X of the distance Z (X) be at least 0.5 m / m. The reason is that, when the absolute value of the width direction gradient of the distance Z is less than 0.5 m / m, when a change in the final solidification position occurs due to a change in the operation, a difference between the maximum value and the minimum value is obtained. This is because a local maximum value of the distance Z may be generated in the middle portion in the width direction, and the center segregation is deteriorated at that position.

  However, in the vicinity of the minimum value of the distance Z at the center of the width and the maximum value at both ends in the width direction, the width direction gradient of the distance Z, which is a smooth function of the position X in the width direction, usually has an absolute value of 0.5 m. / M must be small. However, in the vicinity of less than about 100 mm from these minimum and maximum values, even if the cooling and solidification conditions fluctuate in the slab width direction due to variations in water density, etc., the minimum and maximum values in the width direction There is little possibility that another maximum value is newly generated just by displacement, which causes central segregation. Therefore, the absolute value of the gradient in the width direction of the distance Z is 0 in a range where the distance from the local minimum value at the center in the slab width direction is 100 mm or more and the distance inward from the local maximum value at both ends in the width direction is 100 mm or more. 0.5 m / m or more is desirable.

  In order to successively complete solidification from the center of the width of the slab slab to the shorter side, it is necessary to devise a secondary cooling condition that varies in the width direction. However, just below the mold, cooling only the width center portion causes an increase in inter-roll bulging, and ultimately leads to an increase in unsteady bulging in the final stage of solidification, and as a result, the degree of segregation increases. . In order to prevent this, it is preferable to apply the secondary cooling water to the entire width of the long side surface until the shell thickness of the long side surface becomes at least 30 mm or more. Thereafter, a cooling zone for cooling only the central portion of the width is provided immediately downstream thereof, and then, the cooling width is gradually increased from the central portion of the width to obtain a desired final solidification position shape. The desired crater can also be obtained by changing the water density of the secondary cooling water gently in the slab width direction and providing a cooling zone having a high water density at the center of the width and a low water density at both ends of the width. End shape can be realized.

  It is also known that the heat transfer coefficient of the secondary cooling is affected by the spraying force (impact pressure of water droplets), and the distribution of the heat transfer coefficient becomes a desired crater end shape. As described above, the height of the spray nozzle from the slab surface may be reduced toward the center of the width.

  In carrying out the present invention, in the continuous casting of the slab slab, as described above, the crater end shape obtained by measuring the thickness of the unsolidified portion using longitudinal ultrasonic waves satisfies predetermined requirements. It is desirable to perform continuous casting while confirming that the crater end shape predicted in advance by heat transfer / solidification calculation satisfies predetermined requirements. Can also be implemented. At this time, a maximum value of Z is prevented from being generated at an intermediate width position between the minimum value of the distance Z at the center of the width and the maximum value of the distance Z at both ends in the width direction due to an error between the prediction by calculation and the actual value. Therefore, in the crater end shape predicted by the heat transfer / solidification calculation, the distance from the minimum value of the distance Z at the center of the width is 100 mm or more, and from the maximum value of the distance Z at both ends in the width direction to the center of the width. It is desirable to set the continuous casting conditions such that the absolute value of the gradient in the width direction of the distance Z is 0.5 m / m or more in the slab width direction range where the distance is 100 mm or more.

  In carrying out the present invention, there is no problem even if a slight reduction is applied as necessary. This is because the application of light pressure can provide a slab slab having a better segregation degree inside the width direction position where the distance Z from the meniscus at the final solidification position becomes a maximum value. However, since the solid phase ratio at the center in the thickness direction varies depending on the position in the width direction of the slab slab, the position in the casting direction where the reduction should be applied in the width direction also differs. Therefore, it is preferable to change the position in the casting direction where the light reduction is applied according to the center solid phase ratio at each position in the width direction. For example, if a split-type pressing roll described in Japanese Patent Application Laid-Open No. 2009-125770 is used, the position in the casting direction where light reduction is performed can be changed depending on the position in the width direction.

  The present invention preferably uses a slab slab in which center segregation is concentrated on both ends in the slab width direction, and is a method for hot rolling using such a slab slab to produce a product steel sheet. In this method, first, both ends in the width direction of the slab including the width direction position at which the distance Z (X) from the meniscus at the final solidification position shows a maximum value at both ends in the width direction of the slab slab. The portion is cut and removed, and thereafter, the remaining portion of the slab slab without the both ends in the slab width direction (the remaining portion on the central side after the removal of the cut portion) is heated and hot-rolled to produce a steel sheet product.

  In the second method, both ends in the slab width direction including the width position where the distance Z (X) from the meniscus at the final solidification position becomes a maximum value without removing the both end portions in the slab width direction at the stage of the slab slab are removed. The remaining cast slab is heated and hot-rolled to form a steel sheet first, and then the distance Z from the meniscus at the final solidification position at both ends in the slab width direction from the meniscus is obtained from the steel sheet thus obtained. This is a method of cutting and removing portions corresponding to both ends in the slab width direction including a width position to be a value to obtain a desired steel plate.

  In the present invention, since the center segregation becomes apparent in the vicinity of the maximum value of the distance Z from the meniscus at the final solidification position on both ends in the slab width direction, the range to be cut and removed is slightly centered in the slab width direction. Preferably, the slab is cut at a position closer to the center of the slab width direction by 50 mm or more than the final solidification position as viewed in the slab width direction at both ends of the slab width direction, or a steel sheet after hot rolling. It is preferable to cut and remove the corresponding site from the above. The size of the slab slab to be continuously cast is usually determined in accordance with the size and quantity of the desired steel sheet product by calculating back from the processing conditions in hot rolling and the like after the next step, but in the present invention, The cut amount at both ends in the slab width direction in the slab slab or the cut amount of the portion corresponding to both end portions in the slab direction in the rolled steel sheet is determined in advance. The width position where the distance Z from the meniscus of the final solidification position is actually the maximum is not always exactly as expected due to subtle fluctuations in operating conditions, and may include some errors. However, by allowing a margin in the cutting position as described above, such an error can be absorbed, and the influence of the portion where the segregation is concentrated can be reliably removed. Depending on the crater end shape measured during continuous casting, it is also conceivable to change the cutting length of the slab slab and the casting width together with the cutting position at both ends in the width direction. When the cutting amount is increased, there is a possibility that the desired product size and quantity cannot be satisfied due to lack of molten steel, and it is necessary to pay attention to the influence on the production plan of the product.

  In this example, a steel sheet was manufactured using a material obtained by continuously casting a low-carbon steel line pipe material using a vertical bending type continuous casting machine. The mold size is 2100 mm x 250 mm and the casting speed is 1.4 m / min. Heat transfer / solidification calculations were performed in advance, and secondary cooling water was applied to the entire width of the long side surface up to the casting direction position where the shell thickness on the long side surface was predicted to be 30 mm. From there, the distance Z (X) from the meniscus at the final solidification position has a local maximum value at both ends in the slab width direction, has only one local minimum value at the center in the slab width direction, and has a slab width direction. Secondary cooling is applied so that the maximum value and the minimum value at both ends are monotonically changed, and solidification proceeds more slowly from the center of the width toward the shorter side. The area of the long side surface of the slab to be formed was sequentially widened in the width direction from the center of the width.

  For the width direction position where the final solidification position is the most downstream at both ends in the slab width direction, the water amount on the short side and the edge heater installed in the continuous casting machine are used to measure the slab thickness from the slab width direction end face by 0.2 mm. The distance was controlled so as to be not less than twice and not more than 0.7 times. The shape of the final solidification position was predicted in advance by heat transfer / solidification calculation, and a light reduction zone was provided in the continuous casting machine, and light reduction was applied within a range of 23 to 30 m. The rolling gradient was 0.7 mm / m.

  Next, at a slightly upstream side of the predicted final solidification position, ultrasonic waves are applied over a plurality of locations in the width direction during casting, and the thickness of the unsolidified portion at the ultrasonic application position is determined from the propagation time of longitudinal waves ( For example, refer to Japanese Patent Application Laid-Open No. 2005-177860), by correcting the final solidification position predicted by the heat transfer / solidification calculation in advance at each width position based on the obtained thickness of the unsolidified portion, the shape of the final solidification position I asked. The result is shown in FIG. The width direction position X of the slab in FIG. 4 shows a distribution of only a half width from the center of the width as the origin, but the half width on the opposite side also shows a substantially symmetric distribution, and the distribution from the meniscus at the final solidification position is shown. The distance Z (X) has a local maximum at both ends in the slab width direction, has only one local minimum at the center in the slab width direction, and has the local maximum and the local minimum at both ends in the slab width direction. It was confirmed that each value monotonously changed.

  The width position where the final solidification position is the most downstream side at both ends in the slab width direction (the final solidification position at both ends as viewed from the slab width direction) is approximately 150 mm from the end face in the slab width direction on both end sides. Yes, it was confirmed that the position was about 0.6 times the slab thickness.

  Further, in FIG. 4, a straight line having a slope of dZ / dX = 0.5 (m / m) is shown by a broken line, and the distance from the minimum value at the center of the slab width is 100 mm or more and the maximum value at both ends is In the range where the distance from the inside to the inside was 100 mm or more, the width direction gradient of the distance Z was 0.5 m / m or more.

  Next, a sample was cut out from the entire width of the obtained slab slab, and the Mn concentration at the center of the thickness was quantitatively analyzed using EPMA, and the Mn segregation degree was calculated in the slab width direction. The result is shown in FIG. As is clear from this figure, the Mn segregation degree was very good except for the vicinity of the place where the segregation degree at both ends in the width direction was large. And 200 mm of both ends in the width direction of each slab cast were removed by gas cutting (fusing). Thereafter, the slab was hot-rolled, a sample was cut out from the obtained hot-rolled steel sheet, and a HIC (hydrogen-induced cracking) test was performed. The sampled width positions are a central part of the width of the steel plate, and three places of １ ／ width and ８ width from the end face. As a result of the HIC test, cracks did not occur in any case, and the CAR (the ratio of the area in which the cracks occurred) was all 0%.

  In addition, the result in this embodiment was described in the example of hot rolling after cutting and removing both ends in the width direction of the slab slab, but this was performed by first hot rolling the slab slab, A similar effect can be obtained in an example in which a portion corresponding to both ends in the slab width direction, including a portion where segregation is large, is subsequently cut and removed from the hot-rolled steel sheet thus obtained.

  Next, in a comparative example, a slab for a low carbon steel line pipe material was continuously cast using the same vertical bending type continuous casting machine as described above. The mold size is 2100 mm x 250 mm and the casting speed is 1.4 m / min. Heat transfer and solidification calculations were performed in advance, and secondary cooling water was applied to the entire width of the long side surface up to the casting direction position where the shell thickness on the long side surface was predicted to be 30 mm. Then, from the position in the casting direction, secondary cooling water was applied so that the water density in a 1/4 width became the largest. Further, the cooling water amount on the short side was also increased as compared with the embodiment. The light reduction zone is in the range of 23 to 30 m as in the example, and the reduction gradient is 0.7 mm / m. FIG. 6 shows the distribution in the slab width direction of the distance Z (X) from the meniscus of the final solidification position in the slab thickness direction measured during casting.

  As shown in FIG. 6, the position X in the width direction of the slab shows a distribution of only a half width from the center of the width as the origin, but the half width on the opposite side is also a substantially symmetric distribution. The distance Z (m) from the meniscus at the final solidification position has a minimum value near the quarter width and a maximum value at the center of the width, which does not satisfy the condition of the present invention. Furthermore, since the width position where the distance Z from the meniscus of the final solidification position is the most downstream in the casting direction is a position about 250 mm from the end face in the width direction, and is a position about 1.0 times the slab thickness, This also does not satisfy the conditions of the present invention.

  FIG. 7 shows the relationship between the Mn segregation degree of the slab slab and the width direction position in this comparative example. As shown in this figure, the Mn segregation degree was deteriorated at two places near the width center and the width direction end. Then, a range of 300 mm at both ends of the slab was removed by gas cutting, and then rolled to obtain a steel plate. Thereafter, a sample was cut out from the obtained steel plate, and an HIC test was performed. As a result, HIC occurred at the center of the width, and the CAR was 5.4%.

  In the above-described example, the use of low-carbon steel for line pipe materials has been described. However, the present invention may be practiced with segregation strict steel grades other than line pipe materials (for example, wear-resistant steel).

Claims (5)

In the method of manufacturing a steel sheet by hot rolling a continuously cast slab slab, when continuously casting a slab slab by a continuous casting machine,
When the distance from the meniscus at the final solidification position at which solidification in the slab thickness direction is completed is Z, and the position at the slab width direction is X, the distance Z (X) from the meniscus at the final solidification position is both ends in the slab width direction. part side has a minimum value in the slab width direction central portion and having a maximum at, between the front Symbol maximum value the minimum value, the absolute value of the distance Z in the width direction position X (X) | dZ / dX | has a gradient of 0.5 m / m or more, and the slab width direction position of the maximum value at both ends in the slab width direction is at least 0.2 times the slab thickness from the slab width direction end face, Continuous casting so as to have a distance of 0.7 times or less, and subsequently cut in the width direction so as to have a desired drawing direction length to form a slab cast,
Then, after previously cutting and removing both ends of the slab slab in the slab width direction including the position of the maximum value in the slab width direction, heating the slab slab slab body portion on the remaining central portion side after the cut portion is removed. Hot-rolled to a steel sheet, or
The slab slab obtained by cutting in the width direction is first heated and then hot-rolled into a steel sheet, and thereafter, from the steel sheet thus obtained, including the position of the maximum value in the slab width direction. A method for manufacturing a steel sheet, comprising cutting and removing portions corresponding to both end portions in the slab width direction to obtain a product steel sheet on a central portion side as a remaining uncut portion.   In the continuous casting of the slab slab, the slab width direction position of the maximum value at both ends of the slab width direction is within a range of 0.025 times or more and 0.075 times or less of the slab width W from the slab width direction end face. The method for producing a steel sheet according to claim 1, wherein the steel sheet is continuously cast so as to have a distance of: When cutting and removing both ends of the slab width direction including the slab width direction position of the maximum value from the slab slab, the slab width direction both end portions to be cut and removed are the slab width directions of the maximum value. Including the width direction position 50 mm center side from the position, or
When cutting and removing portions corresponding to both ends of the slab width direction including the position of the maximum value in the slab width direction from the hot-rolled steel sheet, portions corresponding to both ends of the slab width direction to be cut and removed, The method for manufacturing a steel sheet according to claim 1, further comprising a portion corresponding to a width direction position 50 mm central side from the slab width direction position of the maximum value.   When continuously casting the slab slab, secondary cooling water is injected and cooled to the entire width of the long side surface until the shell thickness of the long side surface becomes at least 30 mm. The method for producing a steel sheet according to any one of claims 1 to 3, wherein cooling is performed by injecting secondary cooling water, and thereafter cooling is performed by sequentially increasing a cooling width from a central portion of the width. Absolute value of the distance Z (X) | dZ / dX | range indicating a gradient of more than 0.5 m / m, the distance from the widthwise position of the minimum value of the slab width direction central portion is 100mm or more, slabs method for producing a steel sheet according to any one of claims 1 to 4, wherein the distance from the widthwise position of the maximum value of the width direction end portion side in the width center side is more positions 100 mm.

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