JPS62176655A - Nonmetallic inclusion grading method determining assigning products for slab - Google Patents

Abstract

PURPOSE:To improve measuring accuracy and to prevent generation of product defects by measuring the quantity of nonmetallic inclusions by ultrasonic reflectoscope after hot-rolling casting slab samples extracted from a surface and a center part in the slab, and deciding assigning products for the slab. CONSTITUTION:Before heating the casting slab steel drawn from bending type continuous casting machine, the casting slab sample including an accumulating zone of the nonmetallic inclusion existing at the middle point of a slab surface and axial center is extracted from the slab, and then, after hot-rolling these samples at >=2 roll reduction ratio, the quantity of the nonmetallic inclusions is measured by the ultrasonic reflectoscope and graded. Based on this grading result, the casting slab is decided for the assigning products. In this way, as the grade of the nonmetallic inclusions is executed with high accuracy and also the assignment of products for the slab is executed precisely, the quality of the products and yield of the production are improved.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for quickly evaluating non-metallic inclusions in steel products manufactured from continuously cast steel slabs at the slab stage and determining appropriate products for the slabs. It is.

(Prior art) Conventionally, steel slabs drawn from a bending type continuous casting machine are
After cutting to predetermined length as shown in the figure.

After cooling, cleaning, heating the slab, and rolling it to the desired thickness, the product was collected. When inspecting a product, a sample is cut out from the collected product and subjected to ultrasonic testing, such as a C-scan (
(Refer to ``Tetsu to So-called,'' No. 5, 1971, 5401), and based on this, the quality of the product was determined.

(Problems to be Solved by the Invention) However, this product inspection had the following problems. In other words, ■If non-metallic inclusions exceeding the product criteria are detected, the product will be rejected and will have to be manufactured from a slab again, which will increase manufacturing costs and shorten the manufacturing time. This will lead to a prolonged period.

■In the case of thick-walled products with a small rolling ratio from the slab, it is difficult to distinguish between unpressed bubbles in the slab and non-metallic inclusions, resulting in a lack of reliability.

■On the contrary, in the case of thin-walled products with a large rolling ratio.

Since non-metallic inclusions are dispersed, a large flaw detection area is required, which is inefficient.

The present invention solves the above-mentioned problems of the prior art, quickly measures and evaluates the size of nonmetallic inclusions at the slab stage before a product, and determines in advance the product to be applied to the slab at the slab stage. The purpose is to provide a method that can be used.

(Means for Solving the Problems) In order to achieve the above object, the present inventor has conducted intensive research on a method for collecting samples at the slab cutting stage and using them for evaluation of non-metallic inclusions. .

By pre-treating this sample before ultrasonic flaw detection, it is possible to inspect the product with high accuracy regardless of whether it is thick or thin, making it possible to determine the appropriate product with high reliability. This is the first heading that led to the present invention.

In other words, 1. The method for evaluating non-metallic inclusions for determining suitable products for slabs according to the present invention is based on the method for evaluating non-metallic inclusions that exist between the slab surface and the shaft center when cutting a steel slab drawn from a bending type continuous casting machine before heating. A slab sample containing an accumulation zone of non-metallic inclusions is taken from the slab, hot-rolled at a rolling reduction ratio of 2 or more, and the size of the non-metallic inclusions is measured and evaluated by ultrasonic flaw detection. This method is characterized by determining the appropriate product for the slab before heating the slab.

The present invention will be explained in detail below based on examples.

(Example) As shown in Fig. 1, the process outline of the present invention is as follows: A sample is taken from a steel slab when it is cut after continuous casting, quickly heated, rolled, and subjected to ultrasonic flaw detection. , the size of nonmetallic inclusions is measured and evaluated, and appropriate products are determined before heating the slab. In addition, cooling after cutting the slab,
Although it is sufficient to cut out a sample before cleaning, it is convenient and preferable to take the sample at the time of cutting the slab.

First, in the present invention, since the rolling ratio (rolling ratio) of the sample is usually different from the rolling ratio of the product, it is important to understand the relationship between those rolling ratios and nonmetallic inclusions. therefore,! The size of nonmetallic inclusions in eight samples and products rolled from adjacent slabs was investigated by varying the rolling ratio. The results are shown in FIG. As is clear from this, it can be confirmed that the size of nonmetallic inclusions is proportional to the rolling ratio, and it is possible to measure and evaluate the size of nonmetallic inclusions even in samples with a rolling ratio different from the rolling ratio of the product. It turns out.

Next, non-metallic inclusions in steel slabs produced by bending continuous casting machines physically accumulate between the slab surface and the shaft center. Figure 4 shows a slab sample with a cross section of C.
This is a chart when nonmetallic inclusions were measured by scanning, and the black dots in Figure 1 are nonmetallic inclusions. From this, it can be seen that nonmetallic inclusions are accumulated between the slab surface and the axis.

Therefore, by collecting a sample from the slab so as to include this accumulation zone, the measurement efficiency of ultrasonic flaw detection can be increased.

Further, FIG. 5 shows the results of investigating the relationship between the bubble remaining ratio and the rolling reduction ratio during hot rolling of the sample.

From this, no bubbles remain when the reduction ratio is 2 or more.

Therefore, the only defects detected by ultrasonic flaw detection of slab samples that have been hot rolled at a rolling reduction ratio of 2 or more are nonmetallic inclusions.

The length of the nonmetallic inclusions obtained in this way is, for example,
For products for welded structures, if the rolling ratio is 5, slabs with a nonmetallic inclusion length of 4 mm or less are allowed;
It is possible to evaluate and determine in advance that slabs with nonmetallic inclusions exceeding 4 mm in length cannot be used.

(Effects of the Invention) As detailed above, according to the present invention, a sample is taken when cutting a continuous n#A slab, and the sample is hot-rolled at a specific rolling reduction ratio. Measures non-metallic inclusions,
Since the method evaluates the product and determines the appropriate product for the slab before heating the slab, the following effects can be expected.

■The size of non-metallic inclusions is measured and evaluated using a sample taken from a slab, and the product to be applied to the slab is determined, making it possible to prevent product defects.

■In the sample, even with the same rolling reduction ratio as the product, air bubbles are more compressed, so there are no unpressed air bubbles or non-metallic inclusions in defects detected by ultrasonic testing of samples that have been hot-rolled at a rolling reduction ratio of 2 or higher. It can only detect objects and has extremely high measurement accuracy and reliability.

■ Ultrasonic flaw detection has high detection efficiency because the sample contains an accumulation zone of non-metallic inclusions.

[Brief explanation of drawings]

Figure 1 is a diagram showing the process outline of the present invention, Figure 2 is a diagram showing the conventional process outline, Figure 3 is a diagram showing the relationship between the length of nonmetallic inclusions in the sample and the product, and Figure 4 is the slab. A C-scan chart showing the distribution of nonmetallic inclusions in a cross section. FIG. 5 is a diagram showing the relationship between the bubble remaining rate and the rolling reduction ratio in the sample. Patent applicant Takashi Nakamura, Patent attorney representing Kobe Steel, Ltd. Figure 3 Figure 4 Figure 5 Pressure Yoshi

Claims (1)

[Claims] When cutting a steel slab drawn from a bending type continuous casting machine before heating, a slab sample containing an accumulation zone of non-metallic inclusions existing between the slab surface and the axis is taken from the slab and Non-metallic inclusions characterized in that after hot rolling at a reduction ratio of 2 or more, the size of the non-metallic inclusions is measured and evaluated by ultrasonic flaw detection, and a suitable product for the slab is determined before the slab is heated. Evaluation method.