JP2006021218A - Method for determining the amount of slab cutting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for determining a slab cutting amount capable of reducing yield loss while maintaining good surface quality of a slab when surface slab of a continuously cast slab before or after heating. To do.
SOLUTION: A slab surface flaw stable region for at least one of the steel component ranges, continuous casting conditions, and heating conditions (evaluation index) for each steel type, and a reference amount of cutting A corresponding to the stable region The reference amount of cutting B when the stable region is deviated is set, and the actual results (evaluation index results) of the evaluation index of the slab to be actually welded are compared with the stable region. Determination of the amount of slabs to be cut, characterized in that the reference amount of cutting A is selected when it is within the stable region and the reference amount of cutting B is selected when at least one of the evaluation index results is outside the stable region. Method. The reference cutting amount A or / and the reference cutting amount B are corrected by the surface flaw area ratio of the latest multiple charge slabs that have been cut with the same steel type and the same cutting amount.
[Selection] Figure 1

Description

  The present invention relates to a method for determining the amount of slab cutting when a continuously cast slab is subjected to surface cutting before or after heating for the purpose of removing slab surface flaws.

  Near the surface of the continuously cast slab, there may be defects due to non-metallic inclusions or cracks.If the slab containing such defects is subjected to ingot rolling or hot rolling as it is, May cause surface defects in the final product. In order to prevent the occurrence of such surface defects, the slab surface may be scraped to remove surface defects before or after the continuously cast slab is heated for subsequent rolling. In this cutting, oxygen gas and fuel gas are sprayed on the surface of the slab from a plurality of nozzle units arranged toward the surface of the slab, and at the same time, the slab moves in the longitudinal direction, thereby making the surface uniform. It is melted and removed.

  When the entire surface of the slab surface is cut as described above, the yield is lost corresponding to the amount of slab cutting (the amount of cutting in the slab depth direction). In order to reduce the yield loss, it is preferable to reduce the amount of slab cutting to the minimum necessary for removing surface defects.

  For this reason, methods for quickly evaluating the cause of soot represented by non-metallic inclusions in an intermediate process have been tried. A method is known in which total oxygen is analyzed and the amount of non-metallic inclusions in steel is simply evaluated from the results.

  In Patent Document 1, there is a method in which sampling is performed from a tundish or the like during continuous casting of steel, the concentration of non-metallic inclusions such as alumina and calcia in the steel is investigated, and subsequent processing conditions are determined for manufacturing processing. It is disclosed. Specifically, a method of cleaning the surface layer of 3 mm for a charge whose alumina and calcia values exceed a threshold value is described. For charges where the alumina and calcia values do not exceed the threshold value, only the first slab of the first charge and the slab corresponding to the pan changing part are cleaned for the surface layer of 3 mm, and the others are not processed for subsequent processes. It is flowing.

  Further, in Patent Document 2, the injection pressure can be adjusted independently for each hot scurfer cutting unit, and the surface flaws of the steel slabs obtained by the partial rolling or continuous casting are detected by the position detector, the size and the depth. A partial hot-slab method for hot slabs is described in which the hot-slab is subjected to selective hot-swing according to the degree of the position, size and depth of the defect defect. Since the depth of cutting is adjusted according to the state of wrinkles on the surface of the steel slab, the yield can be improved.

JP 2002-214222 A JP-A-53-109840

  By adjusting the amount of slab cutting based on the total oxygen of the steel sampled before continuous casting or the concentration of alumina, calcia, etc., the amount of slab can be reduced while reducing the amount of cutting and reducing yield loss. There was a limit to maintaining good quality. Moreover, about the method of detecting the surface flaw of a hot billet with a flaw detector as described in Patent Document 2, due to reasons such as insufficient flaw detection accuracy due to the scale of the hot billet surface, etc. Not yet commercialized.

  The present invention relates to a method for determining the amount of slab cutting that can reduce yield loss while maintaining good surface quality of the slab when surface slabs the continuously cast slab before or after heating. The purpose is to provide.

That is, the gist of the present invention is as follows.
(1) Depending on at least one of the results of continuous casting conditions and heating conditions (however, limited to the cast slab after heating) when surface-scraping a continuously cast slab before or after heating. A method for determining the amount of cutting of a slab, wherein the amount of cutting is determined.
(2) When surface-scraping a continuously cast slab before or after heating, the steel component range, continuous casting conditions, and heating conditions (however, limited to the slab to be heated) are preliminarily set for each steel type. A slab surface defect stable region is set for at least one condition (hereinafter referred to as “evaluation index”), and the actual performance (hereinafter referred to as “evaluation index performance”) of the evaluation index of the slab to be slab-cut actually. A method for determining a cutting amount of a slab, wherein the amount of cutting is determined according to a difference from a stable region.
(3) The amount of cutting is determined by taking into account the surface flaw record of the latest slab that has been cut with the same steel type and the same amount of cutting. Method of determining the amount of welding.
(4) When surface-scraping a continuously cast slab before or after heating, the steel component range, continuous casting conditions, and heating conditions (however, limited to the slab to be heated) for each steel type are previously set. A slab surface wrinkle stable region for at least one condition (evaluation index), a reference cutting amount A corresponding to the stable region, and a reference cutting amount B when the stable region is deviated, The actual result (evaluation index result) of the slab to be slab-cut is compared with the stable region, and when all of the evaluation index results are within the stable region, the reference amount of cutting A is evaluated. A method for determining the amount of cutting of a slab, wherein a reference amount of cutting B is selected when at least one of the index results is outside the stable region.
(5) The reference cutting amount A or / and the reference cutting amount B to be set are corrected by the surface flaw area ratio of the latest multiple charge slabs that have been cut with the same steel type and the same cutting amount. The method for determining the amount of cutting of a slab according to (4) above, characterized in that:
(6) The slab surface flaw stable region for each evaluation index is set based on the relationship between the past evaluation index results of the steel type and the surface flaw area ratio after welding. The method for determining the amount of cutting of the slab according to any one of (2) to (5) above.
(7) A method for determining the amount of slab cutting, wherein the amount of slab cutting is increased when the casting speed of continuous casting exceeds a predetermined reference value.
(8) A method for determining the amount of slab cutting, wherein the amount of slab cutting is increased when the molten steel temperature during continuous casting exceeds a predetermined reference value.
(9) A method for determining the amount of slab cutting, characterized by increasing the amount of slab cutting when the amount of free oxygen in the molten steel before continuous casting exceeds a predetermined reference value.
(10) A method for determining the amount of slab cutting, wherein the amount of slab cutting is increased when the extraction temperature for heating before the block rolling exceeds a predetermined reference value.
(11) A method for determining the amount of slab cutting, characterized by increasing the amount of slab cutting when the pre-rolling heating furnace time exceeds a predetermined reference value.
(12) The method for determining the amount of cutting of a slab according to any one of (1) to (11), wherein the determination of the amount of cutting is performed for each surface and corner of the slab.

  The present invention provides a slab by determining the amount of slicing according to at least one of the results of continuous casting conditions and the results of heating conditions when surface-scraping the continuously cast slab before or after heating. Yield loss can be reduced while maintaining good surface quality.

  In the present invention, when the surface of the continuously cast slab is subjected to surface cutting before or after heating, casting is performed in advance for at least one of the steel component ranges, continuous casting conditions, and heating conditions (evaluation index) for each steel type. By setting a single-surface wrinkle stable region and determining the amount of cutting according to the difference between the actual value (evaluation index actual result) of the slab to be slab-cut and the stable region, Yield loss can be reduced while maintaining good surface quality.

  The surface slab cutting of the slab targeted by the present invention is performed by spraying oxygen and fuel gas from a nozzle to the surface of a steel material before heating or in a heated state, thereby cutting the surface layer of the steel material to a certain depth. Targets scarves that eliminate surface wrinkles. The hot scarf aims to eliminate the surface flaws of the steel material after the partial rolling, and is for cutting the slab surface after continuous casting, after the partial rolling and during the partial rolling.

  In the method for determining the amount of slab cutting of the slab according to the present invention, first, when surface slabning is performed on a continuously cast slab before or after heating, the results of continuous casting conditions, the results of heating conditions (however, the The amount of cutting is determined according to at least one of the results.

  Examples of continuous casting condition results include molten steel temperature, casting speed, and primary and secondary cooling water amounts in the tundish. As for the molten steel temperature, if the molten steel superheat degree (ΔT) increases, solidification non-uniformity is likely to occur and cracking flaws increase. Therefore, based on the molten steel temperature measurement result in the tundish, the charge slab according to the molten steel superheat degree It is better to determine the amount of cutting for. Further, when the casting speed is increased, the powder film thickness is likely to fluctuate. In particular, cracks increase at the corners. Therefore, it is preferable to determine the amount of cutting for the slab based on the actual casting speed.

  About the heating slab which heated after continuous casting, the amount of cutting can be determined according to a heating condition performance. Examples of heating condition results include furnace time and extraction temperature. If the furnace time in the heating furnace becomes longer, cracks due to grain coarsening on the surface of the slab increase, so it is preferable to determine the amount of cutting for the slab based on the actual time in the furnace.

  In the method for determining the amount of slab cutting of the slab according to the present invention, secondly, when the surface of the continuously cast slab is subjected to surface cutting before or after heating, the steel component range, continuous casting conditions, heating conditions ( However, it is related to the evaluation index of the slab to be slab-cut in advance by setting a slab surface defect stable region for at least one condition (hereinafter referred to as “evaluation index”). The amount of cutting is determined according to the difference between the actual result (hereinafter referred to as “evaluation index actual result”) and the stable region.

  The evaluation index can be selected from steel component ranges and continuous casting conditions for each steel type. Moreover, heating conditions can also be selected for the slab to be heated. For each evaluation index, it is possible to find an evaluation index performance area where the surface flaw area ratio is low even if the amount of cutting is small, from the relationship between the past evaluation index performance of the steel type and the surface flaw area ratio after welding. . In the present invention, this region is called a slab surface defect stable region.

  For each slab where the evaluation index results are within the stable region for all evaluation indexes and for the slab where the evaluation index results are out of the stable region for at least one of the evaluation indexes It is possible to determine in advance the amount of cutting that is necessary to maintain the flaw condition well. The amount of cutting when deviating from the stable region may be changed for each evaluation index deviating from the stable region.

  In this way, by determining the amount of cutting according to the difference between the evaluation index results and the stable region, it is possible to achieve good surface quality with the minimum amount of cutting for each charge or each slab. It becomes possible.

  As an example of selecting a steel component as an evaluation index, free oxygen of molten steel can be selected. For example, 100 ppm or less of free oxygen is set as the stable region, and the amount of cutting is determined according to the difference between the free oxygen record of the charge and the stable region.

  As an example of selecting continuous casting conditions as an evaluation index, molten steel temperature and casting speed can be selected. For example, in the molten steel temperature record, the molten steel superheat degree can be set to 50 ° C. or less as the stable region and the casting speed of 1.5 m / min or less as the stable region. And the amount of cutting is determined according to the difference between the molten steel temperature record of the charge and the stable region, and the difference between the casting speed record of the slab and the stable region. As continuous casting conditions, it is also possible to select a slab part (such as a pan changing part) in the cast as an evaluation index.

  For the slab to be heated, the time in the heating furnace can be selected as an evaluation index. For example, the in-furnace time of 3 hours or less is determined as the stable region, and the amount of cutting is determined according to the difference between the in-furnace time performance of the slab and the stable region.

  In determining the amount of cutting, it is preferable to take into account the surface flaw record of the most recent slab of the same steel type and the same amount of cutting. When in the stable region or out of the stable region, even if the same steel type and the same amount of cutting are used, the amount of cutting will be insufficient or the amount of cutting will become excessive depending on the time. There is. In such a case, if the required amount of cutting is increased by determining the amount of cutting by taking into account the surface defects of the most recent slab that has been cut with the same steel type and the same amount of cutting, The amount of cutting is increased as necessary, and when the required amount of cutting decreases, the amount of cutting is reduced, and the best quality can always be achieved with the minimum amount of cutting.

  In the method for determining the amount of slab cutting of the slab according to the present invention, fourthly, when the surface of the continuously cast slab is subjected to surface cutting before or after heating, the steel component ranges, continuous casting conditions, and heating conditions are classified in advance for each steel type. (However, it is limited to the slab to be heated.) The slab surface flaw stable region for at least one of the conditions (evaluation index), the reference amount of cutting A corresponding to the stable region, and out of the stable region In this case, a reference cutting amount B is set, and a result (evaluation index result) of the evaluation index of the slab to be actually sliced is compared with the stable region, and all of the evaluation index results are in the stable region. The reference cutting amount A is selected when it is within the range, and the reference cutting amount B is selected when at least one of the evaluation index results is outside the stable region.

  Both the concept of selecting the evaluation index and the concept of the stable region for each evaluation index are the same concept as the method for determining the amount of slab cutting in the second aspect of the present invention. In the fourth aspect of the present invention, a reference welding amount A corresponding to the stable region and a reference welding amount B when the stable region is deviated are set. Thereby, it becomes possible to tabulate the amount of cutting by the stable area | region for every evaluation parameter | index, and when it deviates, and it becomes possible to simplify the logic which determines the amount of cutting.

  As for the reference amount of cutting A, the criterion “not to be cut” can be adopted. Moreover, about the reference | standard welding amount B, it is good also as determining the different amount of cutting for every evaluation parameter | index.

  For example, molten steel free oxygen, molten steel temperature in continuous casting, casting speed, and ingot furnace in-furnace time are selected as evaluation indices, the reference cutting amount A is set to “no cutting”, and the reference cutting amount B is As for the evaluation index, the amount of cutting can be determined as 3 mm. The stable region for each index can be the same as the concept of the stable region in the second aspect of the present invention.

  When determining the amount of cutting, the surface flaw area of the most recently charged slabs of the same steel type and the same amount of cutting with the reference amount of cutting A or / and the reference amount of cutting B set. It is preferable to correct by rate. The surface quality of the slab may change based on a change in an unknown index that is not recognized. In such a case, even if the evaluation index results are the same, the amount of cutting required to maintain a good surface quality increases or decreases. Therefore, the reference welding amount A and / or the reference welding amount B to be set is corrected according to the surface flaw area ratio of the latest multiple charge slabs that have been cut with the same steel type and the same cutting amount. By doing so, it becomes possible to always maintain the best amount of cutting, and to achieve the best yield while maintaining good slab quality. The reason why a plurality of charge slabs are used is that the result of single charge slabs may not reflect the trend of quality change.

  The slab surface flaw stable region for each evaluation index of the present invention is preferably set based on the relationship between the past evaluation index results of the steel type and the surface flaw area ratio after cutting. The range of the surface defect stable region may also expand or contract based on unknown indicator changes that are not recognized. If the range of the stable region is corrected based on past results, the best yield can be realized while always maintaining good slab quality.

  When determining the amount of slab cutting, the same amount of slab cutting may be set for all surfaces of the slab, or for each surface of the slab or for each surface and corner of the slab. It is also good. This is because surface flaws that occur when the evaluation index results deviate from the stable region may occur only on a specific surface or a specific corner of the slab. For example, in certain varieties, cracks may occur at the corners on the upper surface of the slab as surface defects that occur when the casting speed deviates from the stable region. In such a case, as the amount of cutting when the casting speed deviates from the stable region, the reference amount of cutting B may be determined so as to increase the amount of cutting only at the corner of the upper surface of the slab. This makes it possible to prevent the occurrence of surface flaws while reducing the amount of cutting on the slab surface where no flaws are generated.

  As is clear from the matters described above, in the method of determining the amount of slab cutting of the slab according to the present invention, the casting speed of continuous casting, the molten steel temperature during continuous casting, the amount of free oxygen in the molten steel before continuous casting, split rolling If either the preheating extraction temperature or the preheating furnace preheating time exceeds a predetermined reference value, the amount of slab cutting may be increased.

  FIG. 1 is a flowchart showing an example of a method for determining the amount of cutting of a slab according to the present invention. The slab is smelted with a hot scurfer 4 after passing through the smelting process 1, the continuous casting process 2, and the block heating furnace 3, and the surface defect is detected by the cold surface defect detection device 6 after the block rolling 5. .

  The hot scurfer control device 7 has a component record from the smelting process 1, a cooling water amount, a casting speed and a molten steel temperature record from the continuous casting process 2, and a steel material temperature from the thermometer 10 after the block heating is cold. Surface wrinkle information is sent from the surface wrinkle detection device 6. The hot scurfer control device 7 includes a region for storing the slab surface flaw stable region for each evaluation index, and a region for storing the reference cutting amount A and the reference cutting amount B in the form of the reference cutting amount table 8. Have. Based on the charge sent from the smelting process 1, the continuous casting process 2, and the block heating furnace 3, and the evaluation index results of the slab, the amount of cutting is determined corresponding to the reference amount of cutting table 8.

  The hot scurfer control device 7 also has a reference cutting amount correcting means 9. The reference cutting amount correction means 9 is a reference cutting amount A or / or set in the table 8 on the basis of the surface defect area ratio of the most recently charged slabs of the same steel type and the same cutting amount. And the reference amount of cutting B is corrected.

  As a smelting process, after passing through a converter with a molten steel amount of 270 tons, as a continuous casting process, a slab of 220 × 220 mm was cast with a curved bloom continuous casting machine, and the slab was heated with a block heating furnace, The present invention was applied to the production of a 162 × 162 mm steel slab by performing ingot rolling after performing slab cutting. In the hot scurfer, the surface cutting amount and the corner cutting amount can be set separately for each surface (upper surface, lower surface, right surface, left surface) of the slab.

  For low-carbon steel, the relationship between the extraction temperature of the block heating furnace and the soot area ratio was evaluated. When the amount of cut of the slab was set to 1 mm, the relationship between the extraction temperature and the haze area ratio resulted as shown by ◆ in FIG. Therefore, the extraction temperature ≦ 1080 ° C. is set as the stable region, and when the actual extraction temperature is out of the stable region, the amount of cutting is set to 2 mm. As a result, the wrinkle area ratio is the result indicated by ◯ in FIG. 2 (a), and a good surface quality can be obtained even if the extraction temperature record is high.

  For Mn steel, the relationship between the casting speed of continuous casting and the area ratio due to corner cracks was evaluated. When the amount of cutting of the slab was set to 1 mm, the relationship between the casting speed and the heel area ratio was as shown by ◆ in FIG. Therefore, the casting speed ≦ 1.6 m / min is set as a stable region, and when the casting speed deviates from the stable region, the amount of cutting at the corner is set to 2 mm. As a result, the wrinkle area ratio is the result indicated by ○ in FIG. 2 (b), and a good surface quality can be obtained even when the casting speed record is high.

  Regarding the V-added alloy steel, the relationship between the degree of superheated molten steel in the tundish and the area ratio of defects due to surface vertical cracks was evaluated. When the amount of cut of the slab was set to 1 mm, the relationship between the degree of superheat of molten steel and the area ratio of iron was as shown by ◆ in FIG. Therefore, the superheat degree of molten steel ≦ 40 ° C. is set as a stable region, and when the molten steel superheat degree is out of the stable region, the amount of cutting is set to 2 mm. As a result, the soot area ratio is the result indicated by ◯ in FIG. 2 (c), and a good surface quality can be obtained even if the molten steel superheat degree is high.

  In manufacturing the S45C as a product in the same process as in Example 1, the molten steel temperature, casting speed, and ingot heating furnace time in the continuous casting process were adopted as evaluation indexes. About molten steel temperature, superheat degree (DELTA) T <= 50 degreeC was made into the stable area | region. The casting speed was set to 1.4 m / min or less as the stable region. The stable ingot time was 3 hours or less.

  Initially, both the standard cutting amounts A and B were set to 1 mm for all surfaces and corners. However, in the casting where the casting speed is 1.5 m / min, which is out of the stable region, the area ratio of the four corners is significantly increased as shown in FIG. Since this situation continued for a plurality of charges, only the amount of cutting at the four corners was increased to 2 mm with respect to the reference amount of cutting B. As a result, even in the casting where the casting speed was 1.6 m / min, which was out of the stable region, it was possible to produce a good slab with a small surface area ratio on each surface as shown in FIG.

It is a flowchart which shows an example of the amount-of-cutting determination method of the slab of this invention. It is a figure which shows the relationship between the evaluation index results, the amount of cutting and the surface flaw area ratio, (a) is a case where the lump extraction temperature is adopted as an evaluation index in low-carbon steel, (b) is in Mn steel This is a case where the casting speed is adopted as the evaluation index, and (c) is a case where the molten steel superheat degree is adopted as the evaluation index in the V-added alloy steel. It is the result which evaluated the ridge area rate of S45C for each surface, (a) shows the result after increasing the reference cutting amount B only for four corners before correcting the reference cutting amount B, and (b).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Smelting process 2 Continuous casting process 3 Batch heating furnace 4 Hot scurfer 5 Batch squeeze 6 Surface flaw detection apparatus 7 Hot skar fur control device 8 Reference | standard cutting amount table 9 Reference | standard cutting amount correction means 10 Thermometer

Claims (12)

  Depending on at least one of the actual results of continuous casting conditions and actual heating conditions (but only after heated slabs) when surface slabs of continuously cast slabs before or after heating, A method for determining the amount of slab cutting, wherein the amount is determined.   When surface-scraping a continuously cast slab before or after heating, at least one of the steel component range, continuous casting conditions, and heating conditions (however, limited to the slab to be heated) in advance for each steel type. A slab surface flaw stable region is set for the conditions (hereinafter referred to as “evaluation index”), and the actual performance (hereinafter referred to as “evaluation index performance”) of the evaluation index of the slab to be slab-cut and the stable region A method for determining the amount of cutting of a slab, wherein the amount of cutting is determined according to the difference between the two.   3. Determination of the amount of cutting of the slab according to claim 2, wherein the determination of the amount of cutting includes taking into account the surface defect of the latest slab of the same steel type and the same amount of cutting. Method.   When surface-scraping a continuously cast slab before or after heating, at least one of the steel component range, continuous casting conditions, and heating conditions (however, limited to the slab to be heated) in advance for each steel type. The slab surface wrinkle stable region for the condition (evaluation index), the reference cutting amount A corresponding to the stable region, and the reference cutting amount B when the stable region is deviated are set in advance. Compare the performance of the slab with respect to the evaluation index (evaluation index performance) and the stable region, and if all of the evaluation index performance is within the stable region, the reference amount of cutting A is A method for determining the amount of cutting of a slab, wherein a reference amount of cutting B is selected when at least one of them is outside the stable region.   The reference cutting amount A and / or the reference cutting amount B to be set is corrected by the surface flaw area ratio of the most recently charged slabs that are cut with the same steel type and the same cutting amount. The method for determining the amount of slab cutting according to claim 4.   The slab surface flaw stable region for each evaluation index is set based on a relationship between a past evaluation index performance of the steel type and a surface flaw area ratio after welding. The method for determining the amount of cutting of the slab according to any one of 2 to 5.   A method for determining the amount of slab cutting, wherein the amount of slab cutting is increased when the casting speed of continuous casting exceeds a predetermined reference value.   A method for determining the amount of slab cutting, characterized by increasing the amount of slab cutting when the molten steel temperature during continuous casting exceeds a predetermined reference value.   A method for determining the amount of slab cutting, wherein the amount of slab cutting is increased when the amount of free oxygen in the molten steel before continuous casting exceeds a predetermined reference value.   A method for determining the amount of slab cutting, wherein the amount of slab cutting is increased when the extraction temperature for heating before the block rolling exceeds a predetermined reference value.   A method for determining the amount of slab cutting, wherein the amount of slab cutting is increased when the pre-rolling heating furnace in-furnace time exceeds a predetermined reference value.   The method for determining the amount of slab cutting according to any one of claims 1 to 11, wherein the amount of slicing is determined for each surface and corner of the slab.

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