JP2003033850A - Method and equipment for manufacturing steel product having very few surface defects - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably manufacture a steel plate which has very few surface defects and has good surface conditions. SOLUTION: A process to trim the surface of the cast steel piece characteristically comprises quenching of the trimmed surface of the cast steel piece immediately upon completion of trimming. The quenching miniaturizes surface layer tissues of the trimmed steel piece and increases strength of grain boundary. Thereby, the generation of cracks on the surface layer of the steel piece is prevented, and surface defects of the steel piece are reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel product having few surface defects and equipment used for the implementation thereof, and more particularly to improvements in a process and equipment for caring for the surface of a cast slab. .

[0002]

2. Description of the Related Art In a general method for manufacturing a steel product, first, a molten steel is cast into a mold to obtain a steel ingot, or a continuous casting method in which a cast piece is cast by a continuous casting machine to obtain a slab.
Produce a slab. The obtained slab, after the slab surface is cared for by a method such as fusing or grinding, if necessary, the temperature is homogenized in a heat compensation process typified by a heating furnace,
Next, hot rolling is performed while removing the scale generated on the surface of the slab with high-pressure water or the like.

[0003] Due to the recent decline in product prices and soaring labor costs, rolling can be carried out without the need for slab care, and the in-furnace time of the heating furnace can be shortened to reduce manufacturing costs. Is being promoted. On the other hand, the demands of users for the quality of steel products are becoming more and more strict, and in particular, container materials such as automobile outer panels and steel cans are
There is a demand for quality levels that far exceed conventional standards.

In particular, steel sheets for automobile outer panels can be obtained as final products through the processes of hot rolling, cold rolling, annealing and plating. However, if surface defects are discovered after the final products are formed, Despite the great manufacturing cost, changing the user's destination or, in the worst case, scrapping it became a very economical problem. Therefore, in order to avoid such a problem, it is necessary to detect defects at an early stage without manufacturing cost or to develop a process in which the occurrence of defects is zero. In particular, eliminating defects in the casting stage is very important for obtaining steel products without surface defects.

Defects due to factors in the casting stage are roughly classified into two types. The first is that mold powder, which has various roles such as lubrication between the mold / cast piece, oxidation prevention of the molten steel surface, and heat retention of the meniscus, is trapped in the molten steel when the molten steel flow velocity in the mold is high, and the like. Rolled,
It is called a powdery defect found when plated. Conventionally, in order to prevent this powdery defect, the physical properties of the powder are changed, the shape of the immersion nozzle is optimized, the electromagnetic brake and electromagnetic stirring used for flow control in the mold are optimized, and the like. Is prevented from being caught in molten steel.

[0006] The second is an inclusion property defect that appears when the inclusions contained in the molten steel are trapped in the oscillation mark portion and rolled. Among them, alumina formed by combining aluminum added as a deoxidizer with oxygen in molten steel is detected from the surface defect portion of the product. As a measure against this inclusion property defect, the slag used in the refining stage should be modified to clean the molten steel as much as possible, and the electromagnetic stirring should be optimized in the same way as the prevention of powder entrainment so that the alumina in the initial solidified shell part It is possible to agitate so as not to be trapped and aim for the washing effect.

Further, even if various measures are taken to prevent defects due to factors in the casting stage, the slab may still need to be cared for. The following three types of methods are roughly known as methods for caring for the slab. Spraying combustion gas onto the surface of the slab to heat it and melting the thick part of the surface layer of the slab to be cleaned, and removing it (melt cutting method) Grinding the slab surface using a grinder (grinding method) Many Method to remove surface layer defects by hitting small steel balls (shots) on the slab surface with high pressure (shot blast method)

[0008]

By treating the slab surface by the method as described above, factors such as powdery defects on the slab surface can be removed to some extent. In the steel sheet obtained by rolling the slab, a surface defect may occur in a portion corresponding to approximately the center of the slab, and the state in which the cause could not be elucidated continued.

[0009]

The inventors of the present invention have compared a large number of slabs with a steel sheet obtained from the slabs, and have examined the locations and properties of the slab corresponding to the steel sheet portion where the surface defect has occurred. investigated. As a result, when surface defects occur in the steel plate portion corresponding to the steady portion where the slab care was carried out stably, when observing the surface layer portion of the slab in that portion, the crystal grains became coarse. I found out that

As a result of investigating and examining the cause, the following facts were found. First, when slab care is performed by the fusion cutting method, the surface layer of the slab momentarily rises to the vicinity of the melting point of the steel in a short time and then is mainly air-cooled, so the surface layer of the slab is gradually cooled. The result is that the crystal grains in the surface layer of the cast slab become coarser. For this reason, the surface layer of the slab has weak grain boundaries, and cracks or the like occur in the surface layer of the slab with weak grain boundary strength during rolling, resulting in surface defects of the steel sheet. Also in the grinding method and the shot blasting method, the temperature of the surface layer portion of the slab rises during maintenance and the material is gradually cooled thereafter, so that the crystal grains are likely to become coarse and similar problems occur.

From the above points, therefore, by refining the structure of the surface layer of the slab after maintenance and controlling the structure of the slab to increase the strength of the grain boundary, cracks etc. may occur in the surface layer of the slab during rolling. It was found that the surface defects of the steel sheet were reduced without occurrence. Further, as a method for refining the structure of the surface layer of the slab, it has been found that in the step of caring for the surface of the slab, the surface of the slab is rapidly cooled immediately after the end of the maintenance.

The present invention has been made on the basis of the above findings, and the features thereof are as follows. [1] In the process of caring for the surface of the cast slab,
A method for producing a steel product having few surface defects, which comprises rapidly cooling the surface of a cast slab that has been cared for immediately after the caring is completed. [2] A steel material manufacturing facility with few surface defects, characterized in that the equipment for caring for the slab is equipped with a cooling means for rapidly cooling the surface of the slab after the caring is completed.

According to the method of the present invention, the surface of the slab is treated by a fusion cutting method (a method of spraying combustion gas onto the surface of the slab to heat the slab to melt the surface layer of the slab to be maintained, and removing it). The effect is particularly large when it is carried out. This is because the temperature of the surface layer of the slab at the end of maintenance is higher in the fusing method than in other maintenance methods, and the effect of refining the structure by quenching is greater. Further, the fusing method is advantageous in terms of processing time and equipment cost as compared with other maintenance methods.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In a laboratory, the surface layer of a cast slab was abraded by a portable type abrader to a thickness of about 2 mm. As a slab, C: 0.04ma in a vacuum melting furnace
ss%, Si: 0.1 mass%, Mn: 0.7 mass%, P:
0.01 mass%, S: 0.01 mass%, sol. Al:
A low-carbon steel having a composition of 0.04 mass% was melted, and a small test continuous casting machine (mold shape: width 200 mm, thickness 100 m
m) was used, and a slab obtained by casting at a casting speed of 1 m / min was used. Acetylene gas, which has a large amount of combustion heat, was used as the cutting gas for the portable type cutting machine, and it was mixed with oxygen. The pressure of oxygen is 5 kgf / cm ² ,
The oxygen flow rate used for cutting was 3600 Nl / hr, the oxygen flow rate used for preheating was 480 Nl / hr, the acetylene gas pressure was 0.2 kgf / cm ² , and the flow rate was 550 Nl.
/ Hr.

The experiment was conducted by the following three methods. In the first method, it is allowed to cool as it is after the above-mentioned fusing, in the second method it is immediately air-cooled after the above-mentioned fusing, and in the third method it is immediately water-cooled after the above fusing. Carried out. Although air cooling, contact of a cooling body, water cooling, etc. can be considered as methods of forced cooling, water cooling and air cooling were adopted in this experiment. For water cooling, a square blow nozzle is used, and the discharge pressure from this nozzle is 4 kgf / cm ² and the flow rate is 50 l / m.
Cooling was performed by spraying water on the surface of the slab immediately after maintenance with in. For air cooling, a gas injection nozzle was used, and cooling was performed by blowing air from the nozzle onto the surface of the slab immediately after maintenance at a discharge pressure of 4 kgf / cm ² and an air flow rate of 200 Nl / min. In addition, water cooling,
The distance from the tip of the nozzle to the surface of the slab was kept constant at 100 mm with air cooling. Further, in order to measure the cooling rate of the surface layer of the slab, a thermocouple was embedded at each position of 5 mm depth, 10 mm depth, and 20 mm depth from the surface of the slab in the width center part of the slab before fusing, The temperature history was measured. The surface temperature of the slab was measured using a radiation thermometer capable of two-dimensional temperature measurement.

FIG. 1 shows an example of the temperature history of the surface of the slab of the cooling material, the water cooling material and the air cooling material, and FIG. 2 shows the average crystal grain size of the portion 5 mm deep from the surface of each slab. According to this, it is understood that the crystal grains of the surface layer of the cast slab are made finer by immediately cooling the surface of the cast slab by water-cooling or air-cooling immediately after the surface cutting of the cast slab. Further, it can be seen that water cooling has a greater effect on miniaturization. At this time, the cooling rate of the surface of the cast slab to the Ar ₁ temperature was ₁ in water cooling.
It was around 0 K / sec, around 3 K / sec during air cooling, and about 0.5 K / sec for the cooling material.

For this reason, in the present invention, in the step of caring for the surface of the cast slab, the surface of the cast slab that has been cared for is immediately cooled immediately after the caring is finished, so that the structure of the surface layer of the cast slab is refined. . The temperature of the slab during maintenance is not particularly limited, but the slab is generally maintained in a cold state or a surface temperature of 100 ° C. or lower. When the slab at such a temperature is cared for (serving by a solvent method, a grinding method, a shot blasting method, etc.), the temperature of the slab is raised by this caring. In the method of the present invention, the surface of the slab immediately after this care is quenched.

Although the surface of the cast slab is rapidly cooled at a cooling rate exceeding the cooling rate, the cooling means is not particularly limited. Typical cooling means include the air cooling and water cooling mentioned above.
Of these, water cooling is preferable in order to sufficiently obtain the effect of refining the structure of the surface layer of the cast slab, and in that case, it is preferable to use water cooling equipment having a high cooling capacity. However, considering the facility cost and the like, air cooling is advantageous over water cooling in that it is not necessary to install drainage facilities. Figure 3
The relationship between the cooling rate up to Ar ₁ point on the surface of the slab and the average crystal grain size at the surface layer of the slab (average grain size at a depth of 5 mm from the surface of the slab) is summarized and shown. When the cooling rate of one surface is 2 K / sec or more, the structure of the surface layer of the cast slab is effectively miniaturized, and particularly when the cooling rate is 5 K / sec or more, more preferably 10 K / sec or more, the structure is remarkably miniaturized. . From the above results, it is desirable that the cooling rate by quenching is 2 K / sec or more, preferably 5 K / sec or more, and particularly preferably 10 K / sec or more.
It is preferable to quench to r ₁ point.

Further, the effect of the method of the present invention is a method of slab care (for example, a fusing method, a grinding method, a shot blast method, etc.).
It can be obtained regardless of the type, but among them, the "melting method", that is, the method of spraying combustion gas onto the surface of the slab to heat it to melt the slab surface layer to be maintained and remove it The effect is particularly large when it is taken. This is because the temperature of the surface layer of the slab at the end of maintenance is higher in the fusing method than in other maintenance methods, and the effect of refining the structure by quenching is greater. Further, the fusing method is advantageous in terms of processing time and equipment cost as compared with other maintenance methods. The maintenance equipment for carrying out the method of the present invention as described above is equipped with a cooling means for quenching the surface of the cast slab after the maintenance. Examples of this cooling means include a water cooling mechanism and an air cooling mechanism.

FIG. 4 shows an equipment structure used for carrying out the present invention and an embodiment of the present invention based on the equipment structure. 1 is continuous casting equipment, 2 is slab surface maintenance equipment, and 3 is a heating furnace. ,
4 is a hot rolling facility. Mold 5 of the continuous casting facility 1
The cast slab A cast in 1. is drawn downward by a plurality of rolls 6 provided below the mold, cooled by cooling water sprayed from a secondary cooling spray 7 provided between the rolls 6, and It is transported while being supported and guided by 6, and finally cut by a cutter 8 into a slab a of a predetermined length.

Thereafter, the slab a, which requires no maintenance, is placed in the heating furnace 3 as it is and heated to a predetermined temperature at which it can be rolled. Further, the slab a requiring maintenance is sent to a shaving equipment 2 equipped with at least one of a fusing machine 9, a grinder 10 and a shot blasting equipment 11, and the slab surface is treated by one or more of the above-mentioned maintenance means. Take care of. The surface of the slab can be cared for, for example, by two methods such as the fusion cutting + grinding method, the fusion cutting + shot blasting method, the grinding method + shot blasting method.
You may implement combining a method other than one. Then, immediately after the completion of the maintenance, the quenching means 12 rapidly cools the maintenance portion. As the quenching means 12, for example, a water cooling mechanism, an air cooling mechanism or the like is used. The slab a which has undergone such a care process is charged into the heating furnace 3 and heated to a predetermined temperature at which it can be rolled.

The scale of the slab extracted from the heating furnace 3 is removed. In this scale removal, after the width reduction rolling machine 13 applies width reduction to the slab to plastically deform the scale, high-pressure water may be injected from the high-pressure water injection device 14 onto the surface of the slab. The high-pressure water injection device 14 may only inject high-pressure water without performing the width reduction by the reduction rolling mill 13. The slab a from which the scale of the surface has been removed as described above is sent to the hot rolling equipment 4 and rolled into a rough bar by the rough rolling mill 15, and this rough bar is further rolled by the finishing rolling mill 16 to a predetermined thickness. Then, a hot rolled steel sheet is manufactured. The produced hot rolled steel sheet may be used as a product as it is, or may be commercialized as a cold rolled steel sheet or a surface-treated steel sheet through post-processes such as cold rolling and plating.

As described above, according to the method of the present invention, when a steel sheet is manufactured from a cast slab, the cast slab with a refined microstructure of the surface layer of the slab after maintenance to increase the grain boundary strength of the surface layer is obtained. By controlling the microstructure of No. 2, cracks and the like do not occur in the surface layer of the cast slab during rolling, and a steel sheet with few surface defects can be manufactured.

[0024]

EXAMPLE Using a series of steel sheet manufacturing equipment shown in FIG. 4, hot rolled steel sheets were produced from the cast pieces of the two steel types of ultra-low carbon steel and low carbon steel shown in Table 1 according to the above-mentioned process. Manufactured. 250mm thickness and 1050mm width in continuous casting equipment
A slab of the above size was cast at a casting speed of 2.5 m / min.

The slab a was cared for using the fusing machine 9. A water cooling mechanism and an air cooling mechanism are provided as a quenching means after cleaning, and a square blowing nozzle is used in the water cooling mechanism, and a discharge pressure of 4 kgf / cm ² and a flow rate of 50 l / mi from this nozzle.
With n, cooling was performed by spraying water on the surface of the slab immediately after maintenance. In addition, a gas injection nozzle is used in the air cooling mechanism, and the discharge pressure from this nozzle is 4 kgf / cm ² ,
Cooling was performed by blowing air at the flow rate of 200 Nl / min onto the surface of the slab immediately after maintenance. The distance from the nozzle to the slab is 10 with water and air cooling.
It was fixed at 0 mm.

The slabs after the above-mentioned care (the same applies to the slabs left to cool after the servicing and the unmaintained slabs which are comparative examples) are placed in a heating furnace 3 having an ambient temperature of 1200 ° C. and subjected to a soaking treatment. After that, the width reduction is performed 1 pass at 50 mm / pass, and thereafter, after removing the scale by high-pressure water spraying,
A rough bar was made by a rough rolling mill, and the rough bar was rolled by a hot finishing mill to make a hot-rolled steel sheet having a plate thickness of 3.2 mm, and then cold rolling to make a cold-rolled steel sheet having a plate thickness of 0.8 mm. Then, this cold-rolled steel sheet was subjected to alloying hot-dip galvanizing with an adhesion amount of 50 g / m ^{2 on} each of the front and back surfaces in a continuous hot-dip galvanizing facility.

Regarding the alloyed hot-dip galvanized steel sheet produced in this way, the product coil 1
The number of surface defects having a size of 1 mm or more generated per book was visually inspected and evaluated as follows. ⊚: No surface defects ○: Number of surface defects is 1 to 10 Δ: Number of surface defects is more than 10, 20 or less ×: Number of surface defects is more than 20

The results are shown in Table 2 together with the results of the comparative example in which the slab was left to stand and then cooled, and the comparative example in which the slab was not maintained. According to this, although the defect occurrence rate is greatly reduced by the care of the cast slab, it is understood that the defect occurrence is almost completely suppressed by quenching after the care.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

According to the present invention described above, it is possible to manufacture a steel sheet having very few surface defects. Therefore, it becomes possible to stably manufacture a product having excellent surface properties, and it is possible to improve the yield and reduce the manufacturing cost.

[Brief description of drawings]

FIG. 1 is a graph showing an example of a temperature history of a surface layer of a cast piece when the cast piece is left to cool, water-cooled, and air-cooled after being cared for.

FIG. 2 is a graph showing the crystal grain size of a portion at a depth of 5 mm from the surface of the slab when the slab is left to cool, water-cooled, and air-cooled after the slab is cared for.

FIG. 3 is a graph showing the relationship between the cooling rate of the surface of the slab and the average grain size of the surface layer of the slab immediately after the completion of maintenance.

FIG. 4 is an explanatory diagram showing a series of equipment configurations used for implementing the present invention and an embodiment of the present invention based on the configuration.

[Explanation of symbols]

1 ... Continuous casting equipment, 2 ... Care equipment, 3 ... Heating furnace, 4 ...
Hot rolling equipment, 5 ... Mold, 6 ... Roll, 7 ... Secondary cooling spray, 8 ... Cutter, 9 ... Fusing machine, 10 ... Grinding machine, 1
DESCRIPTION OF SYMBOLS 1 ... Shot blasting apparatus, 12 ... Quenching means, 13 ... Width reduction rolling machine, 14 ... High pressure water injection apparatus, 15 ... Rough rolling machine,
16 ... Finishing rolling mill

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toru Kite             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Akio Fujibayashi             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Makoto Suzuki             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd. (72) Inventor Mikio Suzuki             1-2-1, Marunouchi, Chiyoda-ku, Tokyo             Main Steel Pipe Co., Ltd.

Claims (2)

[Claims] 1. A method for producing a steel product having few surface defects, characterized in that, in the step of caring for the surface of the cast slab, the surface of the cared slab is rapidly cooled immediately after finishing the care. 2. A facility for manufacturing a steel product having few surface defects, characterized in that the facility for caring for the slab is equipped with a cooling means for rapidly cooling the surface of the slab after the caring is completed.