JPH0335855A - Method for detecting inclusion in continuously cast slab with ultrasonic wave - Google Patents

Abstract

PURPOSE:To enable sufficient detection of inclusion omitting blooming process by immediately executing light rolling reduction of the specific rolling reduction ratio to hot continuous cast slab after completion of solidification and executing ultrasonic flaw detection under hot or cold condition. CONSTITUTION:The rolling reduction is executed to the hot steel cast slab with solidification thereof completed in continuous casting at 1.4 to <3.0 the total rolling reduction ratio. After that, the ultrasonic flaw detection is executed under hot or cold condition to detect the inclusion in the cast slab. The above rolling reduction is executed by rolling with roll or using a press machine, and the rolling reduction is divided into the initial period and the end period, and it is desirable that the rolling reduction ratio at the initial period is set to >=1.15 in the total rolling reduction ratio and the surface temp. of cast slab at the time of starting the rolling reduction at the initial period is set to 1050-1200 deg.C and the surface temp. of cast slab at the time of starting the rolling reduction at the end period is set to 850-1050 deg.C. It is desirable to keep or raise the cast slab temp. before rolling reduction at the initial period or before rolling reduction at the end period, to cool the cast slab after rolling reduction at the initial period and to execute hot scarfing to the hot cast slab before rolling reduction.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for detecting inclusions using ultrasonic waves at the stage of steel slabs, where the presence of minute nonmetallic inclusions in steel is a problem. be.

(Conventional technology) In steel materials that are cold worked to strengthen them, even the slightest presence of minute nonmetallic inclusions in the steel can lead to accidents such as cracking during processing. The size of the object is larger than a sphere with a diameter of 11IIl in a steel billet. Detecting inclusions after rolling this steel billet into a coiled wire is extremely difficult because the wire is thin and long. Furthermore, even when rolled into steel bars, careful inspection for inclusions leads to a significant increase in costs.

Therefore, it is necessary to conduct strict inspections at the steel billet stage and remove steel billets containing harmful inclusions in advance. Therefore, first, the rice cake pieces that have been made into ingots or continuously left are soaked in a heating furnace either as hot pieces or once cooled, and the reduction ratio is set to 3.0 or more as shown in Figure 3, which will be described later. The steel billets were produced by blooming rolling with a reduction of 100%. This is under crimping pressure to make noise from the solidification shrinkage holes harmless.
This steel slab is inspected using ultrasonic waves with a frequency of 2 MHz or higher that can detect inclusions of spheres with a diameter of 1-11, and after removing the steel slab containing inclusions in advance, it is rolled into wire rods, steel bars, and the like.

Also, iron and steel: 60th year, pp. 875-884, 990-
999'K (1974), etc., there are reports on the quality of directly rolled pieces.

(Problems to be Solved by the Invention) The existence of conventional blooming rolling brings about an increase in costs such as installation costs for heating furnaces and rolling mills, fuel costs and electricity costs for heating and rolling. *In addition, production management becomes complicated due to complicated processes, and FIRm points occur, such as making it difficult to respond to flexible delivery dates.

In order to solve these problems, it has been a long-standing challenge in the industry to omit the blooming process.

In order to omit blooming rolling, a piece of rice cake with a cross-sectional dimension comparable to that of a steel piece would be manufactured, but it is said that the detection of minute harmful inclusions detected by ultrasonic waves cannot be performed adequately for the following two reasons. There were drawbacks.

■Since the crystal grains of slabs are coarse, ultrasonic waves are attenuated by scattering at grain boundaries, reducing the ability to detect inclusions.

■It is difficult to distinguish between solidification shrinkage pores and minute inclusions that inevitably occur in the center.

Therefore, when inspecting a steel material with a coarse structure for which it was not possible to adopt a manufacturing process that omitted blooming for grade 4 steel using the current frequency of 2 MHz, the sound waves are scattered by the grain boundaries and noise is generated. occurs, which reduces the ability to detect inclusions. Although this noise can be reduced by using ultrasonic waves with a long tIL beam, the lower limit size of detectable inclusions increases, making it difficult to detect target inclusions. In order to solve this problem, it is necessary to make the fibers finer and to reduce noise generation even at 2MH2. *The convergence hole in the center is a reflection source of sound waves.I)1. ; It is difficult to separate and identify waves reflected from inclusions smaller than this, so it is necessary to apply sufficient pressure.

The present invention provides a method for detecting inclusions using ultrasonic waves that eliminates the blooming step.

(Means for Solving the Problems) The gist of the present invention is that immediately after the completion of solidification of a hot piece of continuously cast steel, it is compressed at a total reduction ratio of 1.4 or more and less than 3.0, and then This is a method for detecting inclusions in continuously cast slabs using ultrasonic waves, which is characterized by detecting inclusions using ultrasonic waves in a hot or cold state. Detection of inclusions in the rolled rice cake pieces using ultrasonic waves is carried out on a continuous hook making line after rolling in a hot state, and the rolling of the rice cake pieces is performed using a roll pressure fifi or a press machine, and the rolling is divided into initial rolling and final rolling. Separately, the reduction ratio of the initial reduction during the total reduction is 1.2 or more, the reduction ratio of the final reduction is 1.15 or more, and the surface temperature of the slab at the beginning of the initial reduction is 1.
050°C or more and 1200°C or less, and the surface temperature of the slab at the start of the final reduction must be 850°C or more and 050'C or less, and at this time, the slab must be kept warm or heated before the initial reduction and/or before the final pressure F. It is preferable to cool the cast piece after the initial reduction, and to hot-cut the hot cast piece before reduction.

In addition, groups of rolling devices capable of horizontal and/or vertical rolling are installed at 2111 locations on the front and rear of the rice cake making line as front and rear groups, and rolling with the rolling devices of the front group is called initial rolling. , rolling down by the rear rolling device group is referred to as final rolling, and the number of rolling reductions within each rolling device group is not limited.

For ultrasonic waves to detect inclusions, an electromagnetic ultrasonic flaw detector can be used for hot mackerel pieces, and a regular ultrasonic flaw detector with an ultrasonic transmitting probe can be used for cold mackerel pieces with insects. Desirable.

(Operation) First, as a prerequisite for determining these requirements, 2M)I
In order to obtain a detection sensitivity that can clearly detect harmful inclusions larger than a sphere equivalent to 1 mm in diameter using ultrasonic waves of It was investigated using an artificial flaw made by drilling a hole in the hole. As a result, it was found that inclusions with the target diameter lam can be detected if an S/N ratio of 10 dB or more is obtained as the ability to detect artificial defects.

Next, JISS45C steel, billet size 200mm x 20
Grain size and the ability to detect harmful inclusions detected by ultrasonic waves were measured using 0 m-long continuously rolled slabs, directly rolled slabs, and reheated rolled slabs. The detection ability was evaluated by the detection sensitivity of an artificial flaw in which a hole was made with a diameter of 1-1 and a depth of 5-mm. As a result, as shown in FIG. 2, it was found that if the crystal grain size was JIS No. 2 or higher, the S/N would be 10 dB or higher, and the target harmful inclusions could be detected.

Next, we will examine the relationship between the crimped state of solidification shrinkage holes and the rolling reduction ratio when hot continuous hot slabs of various sizes are directly rolled using the above-mentioned steel types, as well as those when cold slabs are heated and bloomed. As shown in Figure 3, in order to eliminate the influence of crystal grains, heat treatment is applied after rolling to make the crystal grain size as fine as JISS number, and the 2MHz
was examined using ultrasound. From this, it can be seen that when the reduction ratio is 1.4 or more for Sabazou directly rolled materials, and the reduction ratio is 3.0 or more for blooming rolled materials, the solidification shrinkage pores are crimped to a detection sensitivity of 5 dB or less that does not harm the detection of harmful inclusions. It turned out that it would be done.

Immediately after solidification of continuous slabs, the temperature at the center is higher than that at the surface, and the reduction is concentrated in the center, so solidification shrinkage occurs at a smaller reduction rate than in the conventional method of soaking slabs. Holes can be crimped. After the li-consolidated pores of the slab have been sufficiently crimped, targeted micro-inclusions can be easily detected using ultrasonic waves. In order to detect even minute inclusions, the crystal grains may be made sufficiently fine by ensuring a sufficient final reduction amount, and the inspection may be performed by increasing the frequency of the ultrasonic waves.

Furthermore, in the present invention, when rolling down the slab, the initial rolling ratio is 1.2 or more, the final rolling ratio is 1.15 or more, the surface temperature of the slab at the beginning of the initial rolling is 1050°C or more and 1200°C or less, and the surface temperature of the slab at the start of the final rolling is The reason why it is preferable to set the single surface temperature to 850° C. or more and 1050° C. or less is as follows.

The purpose of the initial reduction is to destroy the columnar crystals that are generated from the surface of the rice cake piece to the inside. Figure 4 is 247x300m-
These are the results of an investigation to determine whether or not the columnar crystals in the surface layer were refined after a JISS45C slab cast using the mold was hot-rolled using the sensible heat of forging. It can be seen that if rolling is carried out at a surface temperature of 1050° C. or higher at the beginning of initial rolling and a rolling force ratio of 1.2 or higher, the coarse grains in the surface layer are sufficiently destroyed. but,
The surface temperature at the beginning of the initial rolling is preferably 1200°C or lower.The reason is that rolling in this temperature layer causes the structure to become rough due to recrystallization and subsequent growth. In this case, there remains a possibility that the miniaturization target cannot be achieved remotely. The purpose of the final reduction is to sufficiently refine the grains over the entire length and cross section.

Figure 5 shows the initial reduction at a surface temperature of 1200°C and a reduction ratio of 1.2.
The graph shows the grain size after the final reduction of the JIS-o No. 5 slab obtained when the test was carried out under various conditions. From this, in order to further refine the crystal grains refined by the initial pressure reduction by recrystallization and obtain fine grains with the target grain size of JIS No. 2 or higher, the surface temperature at the start of the final reduction must be set to 1050°C or less. It turns out that it is preferable to do so. In addition, the surface temperature of the slab at the start of final rolling was set at 8
If the temperature is less than 50°C, it is difficult to further refine the crystal grains that have been refined by the initial rolling with weak rolling. Setting it above 15 seems like a shame. The same applies to the conditions for rolling down by pressing.

Both the initial reduction and the final reduction contribute to the crimping of the solidification shrinkage hole. In the process of sufficiently soaking the slab and then performing blooming rolling, a total reduction ratio of 3.0 or more was required in order to render noise from the solidification shrinkage holes harmless. However, under rolling immediately after solidification, the temperature near the surface of the slab is low and the temperature at the center is high, so the rolling applied from outside is concentrated in the center, so solidification shrinks under light rolling with a total pressure ratio of 1.4. The purpose of setting the total rolling reduction ratio to less than 3.0 is to make the noise of conventional blooming rolled materials harmless.
, 0 or more and the method of the present invention.

The reason why it is desirable to heat or keep the slab warm before rolling the slab is to ensure the rolling temperature and to reduce the temperature difference between the center and surface of the slab, so that the rolling effect is not excessively applied to the center. This is to prevent the occurrence of internal cracks concentrated on the surface and the occurrence of unrecrystallized coarse grains near the surface. In addition, the cooling performed after the initial reduction is aimed at lowering the temperature of the entire scale, increasing the effect of the final reduction performed after the destruction of surface coarse grains due to high temperature initial reduction, and making it easier to refine the scales. It is. Therefore, after initial rolling, it is possible to cool the whole slab to a predetermined temperature and then heat it or wait for the surface layer to recover before final rolling. Effective for refining L fibers.

Furthermore, by hot cutting the hot slab before rolling, it is possible to obtain a slab with good surface quality from the slab with many surface defects and pinholes that occur during hook making, and it is possible to obtain a slab with good surface quality. Object detection ability can be improved. In addition, the inclusion inspection using ultrasonic waves of the cast slab that has been subjected to light reduction treatment was performed before cutting the cast slab on continuous fins, and after cutting the slab.
Performing vI directly in a hot state is effective because slabs containing harmful inclusions can be removed in advance.

(Example) Figure 1 shows the ordinary arc-shaped continuous support rock used in this example (Rozo rock diameter: 12 m), light pressure equipment, and heating/warming/cooling) a
It is a diagram showing equipment.

First, the hot slab 2 that has been solidified is heated and/or kept warm as necessary in the heating/warming device 6 at an initial pressure T'M, and then a solvent is applied to the hot slab surface 13 in accordance with the condition of the slab surface. . After that, the slab is rolled in the horizontal direction and/or vertical direction with a pair of horizontal rolls and a pair of vertical rolls of the initial rolling Wt device, and the slab is cooled as necessary with the cooling device 8, and then
In addition, a heating/warming device 9 before the final rolling heats and/or heats the slab as necessary before the final rolling, and a pair of horizontal rolls and a pair of vertical rolls of the final rolling device 10 heat the slab in the horizontal direction or/and heat it. Rolled vertically. Furthermore, after that, the hot slab 2 is cut Il! The hot slab 2 was cut to a predetermined length using a T-machine 11, and immediately after, inclusions in the hot slab 2 were detected using an ultrasonic flaw detector 12 (here, an electromagnetic ultrasonic flaw detector).

As a heating/insulating device, we used a device with a total length of 4 rods and a heating burner attached to a cover made of refractory material on the inside, but this can be configured to suit the desired temperature of the pieces. Dota, pressure dosou! [If the reduction in steps 7 and 10 is only in one direction, in order to sufficiently reduce the ridges of slab 2 where the temperature drop is large, for example, when rolling in the horizontal direction at the initial reduction, in the vertical direction at the final reduction. It is better to use perpendicular rolling pressure, such as rolling down.

Inclusions in the slab were detected using the above continuously cast am fin under the following manufacturing conditions.

The casting steel type is JISS45C, and the casting slab size is 24.
The bloom is 7a+mX300m-, the pressure is F
The combination conditions such as ratio, reduction method, heating/insulation, and presence/absence of cooling were as shown in Table 1.

The temperature increase due to heating can raise the surface temperature of the slab to a maximum of 30°C, but in the case of only heat retention without heating, the drop in surface temperature of the slab can be suppressed to 10 to 30°C. The surface temperature drop of the slab when no heating or heat insulation is performed is 45 to 65°C.

The slab is cooled by air/water cooling at a cooling rate of 65°C/min.
The surface was cooled for 2 minutes. In addition,! pI type JISS45
Since C is a U material with good castability and is a fully cast slab with few surface flaws, hot cutting was not performed in this example.

As shown in Fig. 1, the ultrasonic inspection was carried out immediately after cutting the slab, but the ability to detect inclusions was determined by the noise detection sensitivity from the solidification shrinkage hole using ultrasonic waves at a frequency of 2 MHz and the diameter of 1 mm and depth of 5 @ So. The evaluation was based on the ability to detect artificial defects. This artificial flaw was created on the surface of the slab by making holes on the continuous fins on the opposite side of the slab that was in contact with the ultrasonic probe. @If the detection sensitivity for solid shrinkage holes is 5 dB or less and the detection power for artificial defects is S/N 10 dB or more, it is possible to detect the target inclusion with a diameter of 1 nm.

Table 1 shows the inclusion detection ability and the microscopic examination results for the presence or absence of coarse crystal grains in the surface layer, along with comparative examples.

Regarding the detection ability of inclusions caused by shrinkage hole noise and artificial defects in Examples 1 to 11, the detection values as per the target were obtained for all slabs, and there was no occurrence of coarse crystal grains in the surface layer. , the detection sensitivity of shrinkage hole noise in Examples 2.4.8.9 and 11 was reduced to about pack ground, but
This is due to the large reduction ratio of 2.8.

In Comparative Example 12, the cast slab was inspected up to the point where it was not subjected to light reduction treatment, but due to the coarse shingle combing, the detection power for artificial defects was lower than the standard 10 d for ultrasonic waves with a frequency of 2 MHz.
In Comparative Examples 13, 14, and 15, which did not reach B, the total rolling reduction ratio was made as small as 1.2, so that the fineness of the composite fibers and the compression of the solidification shrinkage holes were not sufficient. In addition, in Comparative Example 14, the initial pressure reduction temperature was 1000°C, so the columnar crystals could not be sufficiently destroyed.
Coarse particles remained on the surface, and sufficient detection ability for inclusions could not be obtained. Furthermore, in Comparative Example 15, the final rolling temperature was 80
Since the temperature is as low as 0°C, recrystallization due to final pressure is not sufficient, and the detection power for artificial flaws is low.

Fig. 6 shows the microscopic metallographic structures of Example 3 and Comparative Example 14. The cross-sectional structure of the cast slab of Example 3 has fine crystal grains of JIS Sa number or higher in the entire region, which is difficult to detect inclusions. Are better. However, in the case of Comparative Example 14, near the surface
Coarse particles remain, and sufficient inclusion detection ability cannot be obtained.

From the above comparison, it can be seen that although it is necessary to reduce at a predetermined reduction ratio in order to obtain sufficient inclusion detection ability, the detection accuracy can be improved by further limiting the reduction temperature.

(Effects of the Invention) As explained above, the present invention improves the conventional I-building process for steel products such as wire rods and steel bars, which requires guaranteeing that there are no inherent inclusions in the steel billets that have undergone the blooming and rolling process. for,
It is possible to replace the blooming rolling process with light reduction using the sensible heat of hooking, making it possible to save labor and reduce costs in the production of steel products such as wire rods and steel bars.

[Brief explanation of drawings]

Figure 1 shows the arc type continuous casting machine equipped with light reduction equipment and heating/insulating/cooling equipment used in the examples. Figure 3 shows the relationship between the S/N and the detection power for artificial defects of the equivalent ball. Figure 4 shows a comparison of the echoes from the solidification shrinkage holes, Figure 4 shows the relationship between the rolling reduction ratio required to completely recrystallize the Hozoka fiber and the surface temperature during rolling, and Figure 5 Initial rolling start surface temperature 1200°C, rolling ratio 1.
The grain size number obtained when the structure with the grain size number -0,5 at the position 174 depth from the surface of the slab obtained by the initial rolling in step 2 is subjected to the final rolling is selected using the rolling temperature and the rolling ratio as variables. The figures shown in and FIG. 6 are photographs showing the metal fibers observed under a microscope in the cross sections of the surface layer of the slabs of Example 3 and Comparative Example 14. 1... Molten steel, 2... Hot slab, 3... Hook shape, 4...
・〃Idroll, 5... Vinci roll, 6... Heating/warming device before initial rolling down, 7... Initial rolling down device, 8.
・・Cooling device, 9・−・Heating/warming device before final reduction,
10... Final stage reduction device, 11... Cutting machine, 12...
・Ultrasonic flaw detector, 13... Hot melting solvent, 14... Unsolidified part.

Claims (6)

[Claims] (1) Immediately after the continuous casting of a hot slab of steel is completed, it is rolled down at a total reduction ratio of 1.4 or more and less than 3.0, and then ultrasonic waves are used to detect inclusions in the hot or cold state. A method for detecting inclusions in continuously cast slabs using ultrasonic waves. (2) A method for detecting inclusions in a continuously cast slab using ultrasonic waves according to claim 1, wherein the detection of inclusions in the rolled slab by ultrasonic waves is carried out in a hot state after rolling on a continuous casting line. (3) The slab is rolled by roll rolling or a press machine,
The rolling reduction is divided into initial rolling and final rolling, and among the total rolling ratios, the rolling ratio of the initial rolling is 1.2 or more and the rolling ratio of the final rolling is 1.1.
5 or more, and the surface temperature of the slab at the beginning of the initial rolling is 1050
Claim 1 or 2, wherein the slab surface temperature at the start of final rolling is 850°C or more and 1050°C or less.
The described method for detecting inclusions in continuously cast slabs using ultrasonic waves. (4) The method for detecting inclusions in continuously cast slabs using ultrasonic waves according to claim 3, wherein the slab is kept warm or heated before initial rolling and/or before final rolling. (5) The method for detecting inclusions in continuously cast slabs using ultrasonic waves according to claim 3 or 4, wherein the slab is cooled after the initial reduction. (6) The method for detecting inclusions in a continuously cast slab using ultrasonic waves according to any one of claims 1 to 5, wherein the hot slab is hot-cut before being rolled down.