JP5601055B2 - Surface care method and apparatus for continuous cast slab - Google Patents

Description

  The present invention relates to a method and apparatus for surface treatment of a continuous cast slab for removing defects on the surface of a continuous cast slab before hot rolling of a steel product, and in particular, prevents the occurrence of defects in the post-process of a welding start portion. The present invention relates to a surface care method and apparatus for a continuously cast slab that can be used. The slab is also called a steel piece.

Surface defects such as surface cracks and oscillation marks often occur on the surface of slabs produced by continuous casting. In addition, non-metallic inclusions and gas bubbles (Ar gas, N ₂ gas, etc.) mainly composed of alumina or mold powder are likely to gather on the surface layer of the continuous casting slab, and when the steel material is produced by rolling the continuous casting slab as it is, A slab surface caused by slab surface defects and a slab surface caused by inclusions on the steel slab surface layer are generated.

  In order to deal with the above problems, surface care is generally performed in which the surface layer (depth 2 to 4 mm) of the slab is scraped with a gas scarf of oxygen gas before hot rolling. In this surface care (gas scarfing), the pre-heated gas (combustion gas) is pre-injected to preheat the part to be cut and then the oxygen and oxygen oxidation combustion reaction generated by blowing oxygen gas is used. Yes. As shown in FIG. 3, since preheating flame is injected with a constant inclination angle (about 32 degrees) with respect to the slab horizontal direction as shown in FIG. 3, temperature control in the slab thickness direction is difficult and the slab temperature is high (about 700 ° C.). In the above case, there is a problem that the preheating portion is likely to be deeply digged. It is known that many oxides resulting from the oxidation reaction of iron are generated on the surface of the preheated portion where deep digging has occurred, which causes surface defects after rolling. In addition, as the gas scarf, a configuration in which about 10 scarfing units having a unit width of about 210 to 270 mm are arranged in the slab width direction is used. Is known to occur. When the slab temperature is high (about 700 ° C. or higher), the slab temperature is greatly affected by unevenness in the preheating flame. For this reason, it is necessary to grind the preheated part with a grinder (grinding wheel grinding device) after gas scarfing, which not only increases the work load, but also requires a sufficient lead time before the maintenance process to lower the slab temperature. There is a problem that HDR which directly rolls a high temperature slab without reheating and DHCR which inserts the slab into a heating furnace at a high temperature is hindered, and the fuel intensity of hot rolling is deteriorated. In order to solve such a problem, Patent Document 1 proposes a method for reducing unevenness in the widthwise cutting by making the wall shape of the cutting oxygen slit nozzle streamlined.

JP 2005-52867 A

  However, in the method described in Patent Document 1, although the unevenness in the width direction flow of oxygen gas during cutting can be improved, there is a problem that the unevenness in the width direction temperature during preheating cannot be improved. Investigation revealed. FIG. 3 shows an overview of the prior art. The scarfing unit 5B has a width of about 210 to 270 mm and is arranged in a maximum of 10 pieces in the width direction of the steel piece. The steel piece end is preheated by the preheating flame 6 shown in FIG. 3, and oxygen gas is blown onto the preheated portion to cause an oxidation combustion reaction of iron. The preheating time is, for example, about 1 minute for an 800 ° C. slab. Iron oxide is melted by reaction heat and blown off by the momentum of oxygen gas to be removed. Once the reaction is started, the vicinity is heated by the reaction heat, so after the start of the cutting, the cutting is continued by supplying only oxygen gas. In this conventional method, since the preheating of the welding start portion is performed by spraying a preheating flame with a constant inclination angle (about 32 degrees) with respect to the slab horizontal direction, it is difficult to control the temperature in the slab thickness direction. When it is high (about 700 ° C. or higher), deep digging exceeding the desired depth of cutting is likely to occur (FIG. 5). In addition, when the slab temperature is high (about 700 ° C. or higher), unevenness in the width direction is easily generated due to the influence of preheating unevenness caused by the gas flow unevenness of the unit seam of the scarfing unit 5B (FIG. 7). Since deep digging and unevenness in the width direction of the preheated part cause surface defects after hot rolling, the slab surface must be ground by a grinder after welding with a gas scarf.

  In order to solve the above-mentioned problems, the present invention aims to provide a surface care method and apparatus for a continuous cast slab that can reduce the depth of the preheated portion and the unevenness in the width direction and can omit the grinding step by a grinder. To do.

As described above, the surface care of the continuous cast slab is usually carried out with a gas scarfer. However, since the pre-heated part is deeply deep and unevenness in the width direction causes surface defects after rolling, the gas scarfer. The surface is ground with a grinder after the hot-cutting, which increases the work load and deteriorates the fuel intensity of hot rolling.
The present invention has been made in view of the above situation, and the gist thereof is as follows.

(1) A method of cleaning a continuous cast slab in which the surface of a continuous cast slab before hot rolling is preheated by gas injection and then subjected to welding , and the gas injection from the preheating gas injection nozzle at the time of preheating The angle θ formed by the direction with respect to the horizontal direction is calculated by the following equation according to the surface temperature T (° C.) of the pre-heating slab , and when the calculation result is less than 30 degrees, θ is set to 30 degrees. The method of cleaning the surface of a continuously cast slab , wherein the angle θ is changed in a range of 30 degrees to less than 35 degrees .
θ = 35−αT
α: Constant set in advance in the range of 0.004 to 0.006 depending on the heat capacity of the preheating gas, the target cutting depth, and the steel type

( 2 ) A continuous casting slab care apparatus that preheats the surface of a continuous cast slab before hot rolling by gas injection and then performs welding, and includes a preheating gas injection nozzle and a gas cutting nozzle for cutting In addition, a scarfing unit having a preheating nozzle rotating means capable of rotating the preheating gas injection nozzle to a given nozzle angle, and an angle θ formed by the gas injection direction with respect to the horizontal direction is a slab surface temperature before preheating. Calculated by the following formula using an actual measurement value or a predicted calculation value of T (° C.) , and when the calculation result is less than 30 degrees, θ is set to 30 degrees, so that the angle θ is 30 degrees or more and 35 degrees A surface care device for a continuous cast slab, characterized in that it has a nozzle angle control means which is derived within a range of less than and given to the preheating nozzle rotating means.
θ = 35−αT
α: Constant set in advance in the range of 0.004 to 0.006 depending on the heat capacity of the preheating gas, the target cutting depth, and the steel type

  ADVANTAGE OF THE INVENTION According to this invention, the surface care of the continuous casting slab which does not produce the deep digging of a preheating part and the width direction unevenness | corrugation is attained.

Schematic diagram showing the scarfing unit used in the present invention Schematic diagram showing an example of an embodiment of the present invention Schematic diagram showing an example of the prior art Slab longitudinal sectional view showing an example of slab surface properties before and after cleaning according to the present invention Slab longitudinal sectional view showing an example of slab surface properties before and after maintenance according to the prior art Slab width direction sectional view showing an example of slab surface properties before and after cleaning according to the present invention Cross-sectional view in the slab width direction showing an example of slab surface properties before and after maintenance by the prior art Graph showing defect rates in cold-rolled coils obtained from both the steel billet according to the prior art and the steel billet according to the present invention

  The present invention enables surface care of a continuously cast slab without causing the above problems. 1 and 2 show an outline of a surface care apparatus and method for a continuous cast slab according to the present invention. The surface care device of the present invention can rotate the preheating gas injection nozzles 1A and 2A to a given nozzle angle θ in addition to the preheating gas injection nozzles 1A and 2A and the cutting gas injection nozzles 3A and 4A. The scarfing unit 5 having the preheating nozzle rotating means 20 and the nozzle angle control means 30 for deriving the nozzle angle θ from the actual measurement value or the predicted calculation value of the pre-heating slab surface temperature T and giving it to the preheating nozzle rotating means 20 (FIG. 1). In this example, the preheating nozzle rotating means 20 is a system in which the driven gear provided at the nozzle base is rotated by the drive gear, but not limited to this, any system such as a rack and pinion system or a worm gear system is used. It may be.

When the slab temperature is low, the preheating portion is unlikely to be deeply dug. Therefore, it is possible to shorten the preheating time by setting the gas injection angle (= the nozzle angle θ) large. When the slab temperature is high, by setting the gas injection angle to be small, it is possible to reduce the depth of the preheating portion and to minimize the unevenness in the width direction. Here, the slab temperature refers to the average value in the width direction of the surface temperature of the portion corresponding to the preheated portion. Although the value measured with the radiation thermometer at the entrance side of the scarfing unit is used here, a predicted calculation value from the measurement result in the previous process may be used. According to the knowledge of the inventors, the variable range (changeable range) of the gas injection angle θ [°] (FIG. 1) is 30 degrees or more and less than 35 degrees, and the slab surface temperature T [° C.] is expressed by the formula (1). It is good to set like this. In the formula (1), α is a constant determined by the heat capacity of the preheating gas, the target depth of cutting, and the steel type, but 0.004 ≦ α ≦ 0.006 is preferable.

θ = 35−αT, 30 ≦ θ < 35 Equation (1)
If θ is less than 30 degrees, the preheated part is not sufficiently heated and cannot reach the temperature capable of cutting, and if it is 35 degrees or more , the preheated part is deeply dug even when the slab temperature is low. Further, as shown in FIG. 1, it is preferable to set the ejection angles of the preheating gas 2 (LPG, CO gas) and the preheating oxygen 1 to be equal. The injection angle set during preheating shall be maintained during cutting.

  By changing the gas injection angle at the time of preheating according to the slab temperature and applying gas scarfing, it is possible to perform cutting without causing deep digging of the unheated part and the preheated part as shown in FIG. is there. Further, the unevenness in the width direction can also be suppressed as shown in FIG.

(Example 1)
For a continuously cast steel grade slab for automobile outer plates (width 1500 mm, length 9000 mm, thickness 230 mm, surface temperature T = about 200 to 800 ° C.), the target depth of cutting was set to 4 mm, as shown in FIG. Preheating was performed in the embodiment, and then cutting was performed. The injection angle θ of the preheating gas is calculated from the measured value of the surface temperature T for each slab immediately before the preheating using the formula (1), and the nozzle angle of the preheating gas injection nozzle is adjusted so as to be the calculated θ value. (changed. At this time, α in the equation (1) was set to 0.006, and therefore the adjustment (change) range of θ was 30.2 to 33.8 degrees. As a result, there is no deep digging of the preheated part, and both the preheated part (steel end part) and the steady part have been cut by the target cutting depth, and the surface of the slab after the cutting has no irregularities and is beautiful. Met.
(Example 2)
For a continuously cast steel plate for cans and a slab for tinning (width 1200 mm, length 8000 mm, thickness 225 mm, surface temperature T = approximately 300 to 800 ° C.), the target cutting depth is 4 mm. In the embodiment shown in FIG. 1, preheating was performed , and then the cutting was performed. The injection angle θ of the preheating gas is calculated from the measured value of the surface temperature T for each slab immediately before the preheating using the formula (1), and the nozzle angle of the preheating gas injection nozzle is adjusted so as to be the calculated θ value. (changed. At this time, α in the equation (1) was set to 0.005, and therefore the adjustment (change) range of θ was 31 to 33.5 degrees. As a result, there is no deep digging of the preheated part, and both the preheated part (steel end part) and the steady part have been cut by the target cutting depth, and the surface of the slab after the cutting has no irregularities and is beautiful. Met.

  Further, after rolling the steel slab surface with a conventional gas scarf, the steel slab whose edge was ground with a grinder and the steel slab not grinder-polished after surface cleaning in the same manner as in Examples 1 and 2 were respectively rolled. When the surface defect occurrence rate (coil unit) in the obtained cold-rolled coil (hereinafter, also simply referred to as a coil) is compared, as shown in FIG. Whereas the defect occurrence rate was 1.2%, the surface defect occurrence rate in the coil was 0.1% in the case of the steel slab surface care method of the present invention. It was found that the grinder process can be omitted.

  As is clear from the above examples, according to the present invention, the surface of the continuously cast slab without deep digging of the preheated portion and unevenness in the width direction can be obtained, and the grinder process after gas scarfing, which has been conventionally required Further, it is possible to provide a steel slab in which surface defects after the rolling process do not occur.

1 Preheating gas (oxygen)
1A Preheating gas (oxygen) injection nozzle
2 Preheating gas (other than oxygen)
2A preheating gas (other than oxygen) injection nozzle
3 Gas for cutting (oxygen)
3A gas cutting nozzle (oxygen)
4 Gas for cutting (other than oxygen)
4A gas for cutting (other than oxygen) injection nozzle
5 Scarfing unit (according to the present invention)
5B Scarfing unit (conventional)
6 Preheating flame
7 Continuous casting slab
8 Transport roll
9 Slab before cutting
10 Slab after maintenance according to the invention
11 Conventional slab after maintenance
20 Nozzle rotation means for preheating
30 Nozzle angle control means

Claims (2)

A method of cleaning a continuous cast slab in which the surface of a continuously cast slab before hot rolling is preheated by gas injection and then subjected to welding, wherein the gas injection direction from the preheating gas injection nozzle during the preheating is horizontal. The angle θ formed with respect to the direction is calculated by the following equation according to the surface temperature T (° C.) of the pre-heated slab , and when the calculation result is less than 30 degrees, θ is set to 30 degrees, A method for surface treatment of a continuously cast slab, characterized in that θ is changed within a range of 30 degrees to less than 35 degrees .
θ = 35−αT
α: Constant set in advance in the range of 0.004 to 0.006 depending on the heat capacity of the preheating gas, the target cutting depth, and the steel type A continuous casting slab care device that preheats the surface of a continuous cast slab before hot rolling by gas injection and then performs welding, in addition to the preheating gas injection nozzle and the gas cutting nozzle for cutting, A scarfing unit equipped with a preheating nozzle rotating means capable of rotating the preheating gas injection nozzle to a given nozzle angle, and an angle θ formed by the gas injection direction with respect to the horizontal direction is a slab surface temperature T (° C.) before preheating. ) Using the actual measured value or the predicted calculated value, and when the result of the calculation is less than 30 degrees, θ is set to 30 degrees, so that the angle θ is in the range of 30 degrees to less than 35 degrees. And a nozzle angle control means derived from the above and applied to the preheating nozzle rotating means.
θ = 35−αT
α: Constant set in advance in the range of 0.004 to 0.006 depending on the heat capacity of the preheating gas, the target cutting depth, and the steel type

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