JP2005074496A - Surface conditioning method for continuously cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface conditioning method for a continuously cast slab which completes the work efficiently by distinguishing the necessity and area to be conditioned in advance when the surface conditioning is performed on the cast slab prior to charging it into a heating furnace. <P>SOLUTION: The continuously cast slab is hot-scarfed on its surface, then charged into the heating furnace, and sent to the rolling process after removal of oxide scale generated in the heating furnace. Based on a detection result of the cast slab surface irregularity after hot scarfing and the heating furnace conditions to affect the oxide scale thickness formed during heating, the cast piece surface irregularity after the removal of oxide scale is estimated. If the estimated irregularity exceeds a predetermined threshold value, the irregularity on the cast slab surface is removed before the cast slab is sent to the heating furnace. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a surface care method performed before charging a continuous cast slab into a heating furnace, and more particularly, to provide the heating furnace with an uneven shape that causes a surface defect when the slab taken out from the heating furnace is finally rolled. The present invention relates to a surface care method for predicting before removal and removing in advance.

  Slabs (slabs, blooms, billets, etc.) obtained by the continuous casting method are first subjected to hot scarfing and surface-cutting in order to improve the surface quality. It may be rolled after heating to temperature. In the above hot scarf treatment, the surface of the slab is melted by spraying fuel and oxygen on the slab surface from a plurality of torches arranged in the slab width direction. The unevenness on the surface of the product may not be completely removed, and the unevenness on the surface of the partially left slab may fall down during rolling, causing streak defects on the product surface. It will adversely affect the properties.

  In order to solve such problems, various techniques have been proposed so far. For example, in Patent Document 1, when the surface is subjected to hot cutting with a hot scarf, the surface wrinkles after the cutting are detected, and when the wrinkles are present on the surface crest or in the vicinity of the crest, the abrasive grains and the liquid There has been proposed a metal piece care method for removing wrinkles while spraying an abrasive water jet, which is a mixture of the above.

  Further, Patent Document 2 proposes a method in which a continuous cast slab is entirely welded and then subjected to test welding, and final care is performed according to the state of occurrence of fireworks observed at the time of test welding.

  Further, in Patent Document 3, a square or rectangular grinding range display frame is arranged at a surface defect portion discovered by artificial inspection, and the defect type and the grinding depth are displayed in the immediate vicinity of the grinding range display frame. The defect display plate is placed, and then the grinding range display frame and the defect display plate are photographed with a television camera, and the grinding range, defect type, and grinding depth are read with a mark reader based on the signals, and then maintained. There has been proposed a method of grinding and removing a surface defect portion by instructing a care device via a controller.

  In these methods, the surface defects can be reliably treated and a product with good surface quality can be obtained, but only the presence or absence of irregularities is judged, and the cause of surface defects in the final product However, it does not distinguish between unevenness that does not become a defect and unevenness that does not become a defect, and wasteful care is required, which is inefficient.

  On the other hand, from the viewpoint that the thickness of the scale formed at the time of charging in the heating furnace also affects the generation of surface defects, in Patent Documents 4 and 5, etc., (1) past heating slab heating data and The slab surface scale thickness is calculated from the heating schedule until the furnace extraction and (2) the in-furnace time, and the slab surface is obtained by descaling after the calculated scale thickness exceeds a certain value. A method for effectively removing the defects is proposed.

However, in these methods, it is necessary to hold the scale in the furnace until the scale thickness reaches a predetermined value, and there is a problem that the yield decreases due to an increase in the scale-off amount, which is not efficient.
Japanese Patent No. 2771461 Patent Claims etc. JP, 2003-71551, A Claims etc. JP, 7-290353, A Claims etc. JP, 7-54036, A Claims etc. Japanese Patent Laid-Open No. 11-131144, claims, etc.

  The present invention has been made under such circumstances. The purpose of the present invention is to determine the necessity of maintenance and its area in advance when cleaning the surface of the slab prior to charging into the heating furnace. An object of the present invention is to provide a slab surface care method capable of efficiently completing the work.

  The method of the present invention that can achieve the above object is that the surface of a continuous cast slab is scraped by hot scarf processing and then sent to the heating furnace, and after removing the oxide scale formed in the heating furnace, it is sent to the rolling process At the time, the uneven shape of the slab surface after removing the oxide scale is determined based on the detection result of the uneven shape of the slab surface after the hot scarf treatment and the conditions in the heating furnace that changes the thickness of the oxidized scale formed during heating. The gist of the present invention is that when the predicted uneven shape exceeds a predetermined threshold value, the unevenness on the surface of the slab is removed before being sent to the heating furnace.

  Examples of the threshold value include “inclination angle in the predicted uneven shape”, “vertical length of the inclined surface”, “height to the top of the inclined surface or depth to the bottom”, etc. When the above has the following relationship (1) or (2), defects on the surface of the slab may be removed.

(1) When the vertical length of the inclined surface where the inclination angle in the predicted uneven shape is 60 ° or more is 0.5 mm or more (2) The inclination angle in the predicted uneven shape is 60 ° or more When the height to the top or the depth to the bottom of the inclined surface becomes 0.5 mm or more

  The present invention is configured as described above, and when cleaning the surface of the slab, it is possible to determine the necessity of the maintenance and the region in advance and efficiently complete the operation, resulting in surface defects during rolling. A slab surface cleaning method that can effectively remove irregularities at the stage of charging the furnace was realized.

  It has been found that the cause of surface defects in rolled or semi-finished products, such as sliver defects, is due to the collapse of slab irregularities that remain after descaling before rolling. The present inventors investigated the relationship between the slab surface irregularities before charging the slab into the furnace, operating conditions in the furnace, sliver defects, and the like.

  First, the amount of scale loss (scale thickness) can be determined from the operating conditions in the heating furnace (slab surface temperature time history, atmosphere). At this time, a scale thickness prediction formula as shown in Patent Documents 4 and 5 can be used, but the scale thickness of the slab preheated under the same conditions is measured and converted into a scale loss amount. A value can also be used. It has also been found that the oxidation in the heating furnace is generated evenly even if the slab surface has irregularities (oxygen diffusion rate control in the scale). Furthermore, the concavo-convex shape of the slab shows a tendency that the corner of the obtuse angle becomes round after removal of the oxide scale (after heat treatment), or the convex shape becomes narrow and the concave shape type scratch changes wide by heating. It was also found out.

  And the slab surface unevenness | corrugation shape by the side of a heating furnace can be estimated from the measurement result of a prior slab surface unevenness | corrugation, the amount of scale off, the property of scale production | generation, etc. FIG. In addition, when comparing the relationship between the concavo-convex shape of the slab surface on the exit side of the heating furnace predicted above and the product surface defect, it was found that the concavo-convex defect having the following characteristics becomes a harmful defect with a certain probability or more. .

  That is, when the vertical length of the inclined surface in which the inclination angle in the predicted uneven shape is 60 ° or more is 0.5 mm or more, or (2) the inclination angle in the predicted uneven shape is 60 °. Since the product surface defect occurs when the height to the top of the inclined surface or the depth to the bottom becomes 0.5 mm or more, the surface of the region when such a concavo-convex shape exists. If the surface irregularities are removed by carrying out care, the occurrence of the defects can be avoided.

  Since the shape of the ridge (uneven shape) when rolling out of the heating furnace directly affects the generation of surface defects, defects before heating that should be removed depending on the heating conditions (heating temperature, time, and atmosphere in the heating furnace) It was thought that the removal conditions should be changed. That is, because the slab surface uneven shape that causes defects changes depending on the heating conditions, the uneven shape after descaling is predicted according to the subsequent heating conditions before the heat treatment, and the shape is changed to that shape. The surface care conditions before heat treatment should be changed accordingly.

At the stage of inspecting the surface of the slab, the minimum heating temperature and the shortest in-furnace time required for the quality and operation stability of the slab in the hot rolling furnace are determined. Further, the minimum scale loss thickness on the surface of the slab can be obtained from such information, the slab surface temperature history in the heating furnace, the typical atmosphere in the heating furnace (O ₂ , H ₂ O), and the like.

  In addition, when the surface of the slab is inspected, if the uneven shape after the heating furnace corresponds to the uneven shape causing the defect from the found surface uneven shape and the scale-off amount obtained above, it will not correspond to such a shape. Remove irregularities with a grinder or hand scarf to an extent. At this time, prepare data that clearly shows the height (depth) of the unevenness before loading in the heating furnace and the range of the inclination angle of the slope to be removed for each scarf-off amount. Judgment can be made.

  That is, according to the present invention, the unevenness shape after descaling is estimated before charging the heating furnace, and according to the shape, the necessity and area of slab surface maintenance before charging the heating furnace are determined. The operation can be completed efficiently, and irregularities that become surface defects during rolling can be effectively removed in the heating furnace charging stage.

  Next, the “threshold value” in the uneven shape predicted in the present invention will be described more specifically with reference to the drawings. FIG. 1 is an explanatory view schematically showing a representative example of the uneven shape on the surface. FIG. 1 (a) shows the case of the convex shape, and FIG. 1 (b) shows the case of the concave shape. In the figure, the portion indicated by the thick line (line A) shows the outline of the surface shape before charging the furnace, and the portion indicated by the thin line (line B) is after charging the furnace (that is, oxidation). The outline of the surface shape after removal of the scale is shown (the same applies to FIGS. 2 to 5 described later). In the figure, W is the length of the convex top flat surface or concave bottom flat surface before charging the heating furnace, L is the length of the inclined surface before charging the heating furnace, d is the oxidation case thickness, α indicates the inclination angle, I indicates the length of the inclined surface after charging the furnace (predicted uneven shape), h indicates the height of the inclined surface, and D indicates the depth of the inclined surface.

When measuring the vertical length I [FIG. 1 (a)] of the inclined surface (surface having a constant inclination angle) in the predicted convex shape based on the convex shape before charging the heating furnace. The following formulas (1) and (2) are shown. Of these, the formula (1) is expected to satisfy the requirement of [(cos (α) −1) / sin (α)] × d + W / 2> 0 (that is, a flat portion is expected to exist after the scale-off). Equation (2) is calculated from [(cos (α) −1) / sin (α)) × d + W / 2 <0 (that is, no flat portion exists after the scale-off). The calculation formulas are shown respectively.
I = sqrt [(L × cos (α) −d × tan (α)) ² + L × sin (α) −d] ² ] (1)
I = sqrt [[(W / 2) × tan (α) + L × sin (α) −d / cos (α)] ²
+ [(W / 2) + L × cos (α) −d / cos (α)] ² ] (2)

Further, the following equation (3) shows an equation for measuring the length I [the above FIG. 1 (b)] of the inclined surface in the concave shape. In the case of a concave shape, the bottom flat portion is always present because it spreads, and the equations for measuring the vertical length I of the inclined surface are summarized in the following equation (3). .
I = L−d × (1−cos (α)) / sin (α) (3)

  Then, when the value (I value) calculated by the equations (1) to (3) is 0.5 mm or more and the inclination angle α is 60 ° or more, care is taken to remove surface irregularities. You will decide to do it. When the predicted uneven shape is expected to be a complicated shape (see, for example, FIG. 5 below), the height h of the inclined surface at which the inclination angle α is 60 ° or more [FIG. ]] Or the depth of the inclined surface [FIG. 1 (b)] may be measured, and when these become 0.5 mm or more, it may be determined that care is required.

  The chemical component composition in the slab targeted by the present invention is not particularly limited and can be applied to various types, but as a representative one, steel plate for outer plate with strict surface quality And steel for cold heading.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the following examples are not intended to limit the present invention, but are modified within a range that can meet the purpose described above and below. All of these are included in the present invention.

  Produces a slab (chemical composition, Al: 0.03 mass%, Si <0.01 mass, C: 34 ppm, Mn: 0.5 mass%) having a cross-sectional shape of 230 × 1200 (mm) by continuous casting. Then, after the surface of the slab is subjected to hot scarf treatment, the surface properties are visually observed, and the oxide scale thickness is calculated according to the subsequent heating conditions and the like. Predicted surface irregularities. And in various uneven | corrugated shapes estimated, I value was calculated from either of said (1)-(3) formula, and the relationship between this calculated value and the flaw (defect) generation | occurrence | production situation after rolling was investigated. Whether or not surface flaws occurred was determined by visual inspection of the product.

  An example of the results is shown in Table 1 and Table 2 below. Table 1 shows the results when the inclination angle of the convex portion is 75 ° and the oxide layer thickness is 0.5 mm, and Table 2 shows the result when the inclination angle is 75 ° and the oxide layer thickness is 1.5 mm. This shows the results. In both Tables 1 and 2, the region where the surface flaws are generated is surrounded by a double line (‖).

  As is apparent from these results, it is understood that surface irregularities that cause surface defects are likely to remain when the I value is 0.5 or more. On the other hand, it can be seen that when the I value is less than 0.5, no defects that cause surface defects occur. Even if it is determined that the I value is 0.5 or more, it may not be a surface flaw. Just do it. As a result, at least the effectiveness of removing defects that cause surface defects can be achieved.

  As described above, in order to more accurately evaluate the defect shape, it is useful to measure the inclination angle α and the inclined surface length I (the I value), but the shape in which the inclined length I is difficult to calculate. If it is or if it is desired to evaluate relatively easily, the height h (see FIG. 1) and the depth D may be used as evaluation criteria instead of the I value.

  Based on the experiments so far, the present inventors will arrange the uneven shape that does not remain as wrinkles and the defect shape that remains as wrinkles and explain them with drawings.

FIG. 2 is an explanatory view schematically showing a typical concavo-convex shape that does not remain as wrinkles basically. Among these, FIG. 2A shows a surface shape in which the inclination angle of the convex portion is less than 60 °, and FIG. 2B shows a surface shape in which the inclination angle α of the concave portion is 60 ° or less. In such a shape, even if the height h ₁ or the depth D (or the I value) is 0.5 or more, it does not remain as a surface flaw.

  In FIG. 2C, the inclination angle α is 60 ° or more, but since the height h (expected height) after the scale-off is less than 0.5 mm, it does not remain as surface defects after rolling. .

On the other hand, FIG. 3 is an explanatory view schematically showing a typical uneven shape remaining as a ridge. Among these, FIG. 3 (a) has an inclined surface having a step in part, and the inclined angles α ₁ and α _{2 of} each inclined surface are 60 ° or more and the inclined surface length (I value) is the same. FIG. 3 (b) shows an example in which the inclination angle α of the convex portion after the scale-off is 60 ° or more and the inclined surface length is 0.5 mm or more. is there. In the case of these shapes, it is a target for defects that require surface care.

Although FIG. 4 includes a shape that is determined not to be deleted as a part of the unevenness, since it includes a part that is to be deleted in other parts, it is determined as an uneven shape that remains as wrinkles as a whole. Is. Among these, the convex shape shown in FIG. 4 (a) has an inclined surface with a step in part, and both of the inclination angles α ₁ and α ₂ are 60 ° or more, and a part thereof (α ₁ side) ) Includes a portion where the I value is determined to be less than 0.5 mm, but includes a portion where the I value is 0.5 mm or more on the other side (α ₂ side), and is therefore a target of deletion. It will be judged as an uneven shape. 4B has a convex shape including a portion with an inclination angle of less than 60 ° on a part (α ₂ side), and the other portion has an inclination angle α ₁ of 60 ° or more. Since there is a portion having an I value of 0.5 mm or more, it is determined as a surface defect to be deleted. Further, FIG. 4 (c) has a concave shape including a part with an inclination angle of less than 60 ° on a part (α ₂ side), but the other part has an inclination angle α ₁ of 60 ° or more and I. Since there is a portion having a value of 0.5 mm or more, it is determined as a surface defect to be deleted.

  That is, even if a shape that is determined to be a surface defect is included in a part of the shape, it may remain as a wrinkle after rolling, so that it is determined as a surface defect that is deleted during maintenance. However, in the concavo-convex shape shown in FIGS. 3 and 4, none of them is a removal target if its I value is less than 0.5 mm.

  FIG. 5 schematically shows an example of the convex shape, but when the convex portion formed on the surface is hemispherical as shown in the figure, the inclined surface after the scale-off is a curved surface, Measuring the I value is complicated. In such a case, the inclination angle is approximately 60 ° or more. However, if the height h is measured and the height h is 0.5 mm or more, it is determined as a surface defect to be deleted. Good.

It is explanatory drawing which showed typically the representative example of the uneven | corrugated shape of the surface. It is explanatory drawing which showed typically the typical uneven | corrugated shape which does not remain as a ridge. FIG. 3 is an explanatory view schematically showing a typical uneven shape remaining as a ridge. It is explanatory drawing which showed typically what was judged as the uneven | corrugated shape which remains as wrinkles as a whole. It is explanatory drawing which showed typically an example of the convex shape.

Claims (3)

  When the surface of a continuous cast slab is scraped by hot scarf processing and then sent to a heating furnace, the oxide scale formed in the heating furnace is removed and then sent to the rolling process. Based on the detection results and the conditions in the heating furnace that changes the oxide scale thickness formed during heating, the concavo-convex shape of the slab surface after removal of the oxide scale is predicted, and the predicted concavo-convex shape is predetermined. A surface care method for a continuous cast slab characterized by removing irregularities on the slab surface before being sent to a heating furnace when a threshold value is exceeded.   The surface care method according to claim 1, wherein a defect on the surface of the slab is removed when the vertical length of the inclined surface at which the inclination angle in the predicted uneven shape is 60 ° or more is 0.5 mm or more. .   The defect on the slab surface is removed when the height to the top of the inclined surface or the depth to the bottom of the inclined surface at which the inclination angle in the predicted concavo-convex shape is 60 ° or more is 0.5 mm or more. The surface care method according to 1.

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