JPS5814053A - Flaw detector of hot rolled plate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic particle flaw detection method for the purpose of evaluating inclusions in hot rolled sheets, that is, determining quality.

Improving the inclusion cleanliness of steel plates is an important issue in steel manufacturing technology, but as an example where extremely clean steel is required,
For example, D I (Drawn and Ironin
g) Cans can be made from tin. The presence of inclusions with a diameter of approximately 50μ or more in the casting stage can cause 7-lunge cracking in tinplate for DI cans. Tin inclusions need to be controlled.

1- Currently, the most sensitive and widely used method for evaluating inclusions in tinplate for DI cans is the magnetic particle flaw detection method for tinplate plates. In the literature, it is said that in order to reduce the incidence of 7-lunge cracking in DI cans to 100 ppm or less, this can be achieved by selecting tinplates with magnetic particle detection defects of approximately 1/ld or less.

However, in the method of evaluating the tinplate, which is the final product, if the magnetic particle test results are poor, there is a large loss such as a delay in delivery or a decrease in the yield of the tinplate. In addition, cold rolled
In factories that do not have a tinplate manufacturing process and only have processes up to hot rolling, tinplate flaw detection cannot be carried out within the factory. Therefore, if it is possible to determine whether or not a hot-rolled sheet can be used as a material for tinplate for DI cans, the above-mentioned losses will be significantly reduced, and there will be great benefits in terms of quality assurance.

However, magnetic particle testing for hot-rolled steel sheets has significantly lower inclusion detection sensitivity than that for tinplate.

Conventionally, there has been no example of this being used to determine whether or not it can be used as a material for tinplate for DI cans.

2- From this point of view, the present inventors have completed the present invention as a result of various studies on improving the sensitivity of the inclusion evaluation method using magnetic particle testing of hot rolled sheets. A surface obtained by removing 5 to 35% of the plate thickness from one side of the hot-rolled plate, which is the top surface during casting, produced by a normal hot rolling process from a slab cast by a mold continuous casting machine. It consists of magnetic particle flaw detection.

Details are given below.

Even with a magnetic probe for tinplate (0.32mm thick), small inclusions are detected from the surface but not from the back, and large inclusions with a wall thickness of about 35μ or more and a length of about 1mm or more are detected. can be detected from both the front and back sides. This is because the detection sensitivity decreases as the position of the inclusion is farther away from the flaw detection surface.

Therefore, with magnetic sensors for hot-rolled steel sheets, the detection sensitivity for inclusions that exist deep from the surface is low, so the number of magnetic defects detected/unit volume of steel is smaller than that of the same hot-rolled steel sheet after cold rolling or plating. , cold-rolled sheet or 3- has significantly fewer magnetic defects than tin plate/unit volume of steel.

On the other hand, as is well known, the distribution of inclusions in continuously cast slabs is
In slabs produced by a vertical continuous caster, there is a distribution peak at the center of the slab thickness, but in slabs produced by a curved mold continuous caster, inclusions floating in the crater appear on the 5th side. That is, when the slab is placed in a horizontal position, it is trapped on the solidified surface that becomes the upper surface, so the peak of the distribution is at a position biased toward the L plane. This is schematically shown in Figure 1. The parameter X that represents the peak position of inclusions in the slab produced by the curved mold continuous casting machine is defined as: X = (distance from L surface to peak position/thickness of slab) Then, the smaller the bending radius R of the mold and continuous casting machine, the smaller the casting speed (m
It is known that the faster the casting flow (min. In the range of ~35%.

4- Therefore, trends in continuous casting equipment technology and operation technology are moving toward lower heads (that is, smaller diameter R) and higher speed casting, in which case X will be in the range of 8 to 35%.

A slab with such an inclusion distribution has some of its surface layer removed by scale loss and hot or cold treatment before hot rolling, but the thickness of this removed surface layer is = (thickness removed on one side/thickness of slab), the value of Y ranges from less than 1% in continuous casting - no maintenance - direct rolling to 3% in continuous casting - cold single hand reheat rolling. In the range below.

Therefore, the inclusion peak position X″ in the hot-rolled sheet is χ
'-(distance from surface equivalent to L plane to peak position/thickness of hot-rolled sheet), X'' is in the range of 5 to 35%, including movement of the position of inclusions due to metal flow during hot rolling. It is in.

Therefore, considering the dependence of the defect detection sensitivity of the magnetic probe on the depth from the magnetic probe surface to the inclusion described above, it is necessary to set the inclusion peak position of X'' on the magnetic probe surface. If the surface from which the thickness of X'' is removed is used as the magnetic detection surface, the defect detection sensitivity will be close to the maximum.

More specifically, if the maximum peak of the inclusion is broad (width at half maximum) and the inclusion is small, the magnetic probe surface can be set to X'' itself, but if the maximum peak of the inclusion is sharp and the inclusion is large In this case (the dependence of defect detection sensitivity in the depth direction is smaller than in the case of small inclusions), if a thickness smaller than X″ (X″-50 to 100μ) is removed to form the magnetic probe surface, The number of detected inclusion defects becomes maximum.

As a means for removing the thickness of X', well-known mechanical cutting/polishing methods, electrolytic polishing methods, chemical polishing methods, or electrical discharge machining methods may be used alone or in combination, and the location to be removed may also be It may be done offline, for example by cutting the ends of the hot rolled coil and then removing it, or online.
For example, removal equipment may be incorporated into the pickling line or coiling line. However, the WF military method, electrolytic polishing method, and chemical polishing method require 6.
First of all, it is necessary to remove black scale from the surface of the hot rolled sheet. Examples of methods for removing black skin include pickling, dry and wet shot blasting.

In addition, the surface roughness of the removed surface should be adjusted, for example, from No. 320 to
In addition, in order to shift the mechanical polishing direction from the inclusion extension direction, consideration must be given to polishing in a direction perpendicular to the hot rolling direction.

In addition, in order to avoid misidentifying mechanical surface flaws, scale, rust, etc. as inclusion defects, even in normal magnetic detection of hot-rolled sheets,
Surface polishing is sometimes performed, but in this case the polishing depth is 2
It is approximately 0μ or less, and is completely different from the present invention in its basic concept and removed thickness.

Although the above explanation has been made using tinplate for DI cans as an example, the present invention is applicable to all types of magnetic detectors for hot-rolled steel sheets for internal use, which are steel types where inclusions are harmful to the usability of the product. This is self-evident and can be applied to, for example, hot-rolled sheets for high-grade electric resistance welded pipes (for example, for sour gas), so there is no limitation on the steel type in the present invention.

As is clear from the above-mentioned reasons, the above-mentioned X'' changes depending on the continuous casting equipment conditions and operating conditions. Therefore, when applying the method of the present invention, it is sufficient to select a generally suitable removal thickness determined by these conditions.

Of course, when you want to know the approximate distribution of inclusions in the depth direction of a hot-rolled sheet, for example, after removing 5% of the sheet thickness, use a magnetic probe,
Just like the magnetic probe after removal, you can replace the removal and magnetic probe in stages.

Furthermore, the above explanation was made regarding curved mold type continuous casting material, but in vertical type continuous casting machines, there is an inclusion accumulation zone at the center of slab thickness, so in vertical type continuous casting material, the removed thickness is 50%. Degree and nine are good.

However, since the degree of inclusion accumulation (peak height/mid-half value) in the slab thickness direction in vertical continuous casting materials is smaller than that in curved mold continuous casting materials, the present invention method for vertical continuous casting materials The advantages of application are not comparable to those of curved mold continuous casting materials.

The magnetic detection results of hot-rolled sheets using the method of the present invention can be fed back to understand the level of refining and continuous casting operation conditions, detect abnormalities, improve operational methods, or change the use at the hot-rolled steel sheet stage or change the use of tinplate. It is feed forwarded to etc.

Examples will be described below.

Low-order AQ killed steel slab thickness 250+sm, slab width 81
A slab of 0 mm was cast on a continuous arm machine with a bending radius of 10.5 m using an inverted T-shaped immersion nozzle at a drawing speed of 1.6 m/min.

A hot rolling machine directly connected to a continuous casting machine allows slabs to be rolled to a thickness of 2 without any maintenance.
．． It was rolled into a hot-rolled plate with a width of 0 and a width of 785.

Hot-rolled coils are produced using the normal cold-rolling-tinplate process.

A tin plate with a thickness of 0.21 m was produced.

In the above method, the hot-rolled sheets taken from the ends of the hot-rolled coils were sequentially removed to remove black scales, pickled, and finished with No. 320 using a combination of surface grinder and puff polishing.

The aforementioned hot-rolled sheet sample with a width 'i' of 90 Itm and a length of 600 mm was tested with a magnetomotive force of 300 O using a magnetic particle flaw detector using the interpolar method.
Continuous magnetization was performed at A and T, and flaw detection was performed using 8Y9000 magnetic powder under the conditions of magnetic particle roughness of 0.30 f/l. The relationship between the removed thickness of the surface equivalent to the surface during casting and the average number of defects of 10 samples is determined by the casting number (Ca5tNα).
) separately shown in Figure 2.

The number of defects for Ca5t A was 1.9 per sample when the removal thickness was 10μ, and 19.8 per sample when the removal thickness was 200μ, which was the maximum value for the 4" nozzle, but the defect length for Ca5tA was lo
It was both inside and outside. The number of defects in Ca5t B is 8 in the removal thickness.
μ: 0.1 piece/sample, removal thickness: 250μ: 23.
The maximum value was 4 samples/sample. The number of defects of Ca5tc is 0 defects/sample when the removed thickness is lOμ, and the removed thickness is 250.
The maximum value of μ was 7.5 μ/sample. castB
In most cases, the defect length i/i of C was 57 m or less.

In this way, (maximum number of defects/number of defects at removal thickness of 2 μm or less) increases as the number of small inclusions increases, and the present invention is particularly effective when the purpose is to detect small inclusions. One thing has been proven.

From the results of a large number of Ca5t tests, it was found that the removal thickness at which the number of defects becomes maximum is an average of 245μ (12.3% of the plate thickness). Number of magnetic detection defects (average value of 10 samples) on a surface that is 250μ removed from the surface equivalent to the face surface during casting of a large number of heat-rolled sheets and the positions adjacent to the hot-rolled sheet samples. Figure 3 shows the relationship between the number of magnetic detection defects (average value of 10 samples) of tinplate samples manufactured from 763mm wide and 600mm long, stratified by operating levels A and B.

It is clear that the number of magnetic probes for hot-rolled sheets greatly exceeds that for tinplate, and especially at operation level A, where the number of defects is small, (number of magnetic probes for hot-rolled sheets/number of magnetic probes for tinplate) is larger. Therefore, the present invention was proved to be particularly effective as a method for detecting inclusions in a cleaner steel plate.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between the depth from the L plane and the number of inclusions, Figure 2 is a graph explaining the number of defects detected by magnetic probe, and Figure 3 is a graph showing the comparison between the number of defects in tinplate and hot rolled sheet. It is. 1l- L 1@Kai et al.’s evening home tF side
surface

Claims (1)

[Scope of Claims] One side of a hot-rolled sheet produced by a normal hot rolling process from a slab cast by a double-curve mold type continuous casting machine has a thickness of 5 to 35 on the side that will become the special casting surface. A hot-rolled plate flaw detection method characterized by magnetic particle flaw detection on the surface obtained by removing the clouds.