JPH09178733A - Method for evaluating continuously casted piece center segregation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for easily, rapidly, and quantitatively evaluating center segregation for understanding the degree of center segregation. SOLUTION: The hardness of the axial center part of a continuously casted cast piece is measured and the degree of center segregation of the continuously casted cast piece is obtained from at least one type of the average value, the maximum value, and the difference between the maximum and minimum values of the measurement value of the hardness. After slightly corroding the axial center part of the continuously casted cast piece, the hardness of the regularly segregated part appearing at the axial center part is measured and the degree of central segregation of the continuously casted cast piece may be obtained from at least one type out of the average value, the maximum value, and the difference between the maximum and minimum values.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous cast slab center segregation evaluation method, and more specifically, it is a kind of internal quality of a cast slab obtained by continuous casting (hereinafter referred to as continuous cast slab). It belongs to the technical field of continuous cast slab center segregation evaluation method for grasping the degree of center segregation.

[0002]

2. Description of the Related Art As a method for evaluating the segregation of the center of continuous cast slabs, a method of visually observing after macro-corrosion, a method of drilling a shaft center portion, or a method of measuring segregated grain size after H printing (light corrosion) is adopted. Has been done.

Here, the method of visually deciding after macro-corrosion is as follows: "The continuously cast slab is cut so as to pass through the axial center portion (continuous casting direction: longitudinal direction: vertical direction), and the obtained slab is obtained. After polishing the cut surface (hereinafter referred to as axial cutting slab) and macro-corrosion, visually observing the segregation situation, the degree of segregation, etc.,
It is a method of determining the degree of center segregation ". This method
Although simple, it has the drawback of being non-quantitative.

The method of analyzing the axial center drill is as follows: "Chips are collected by cutting with a drill from the axial center of the axially cut slab obtained from the continuous cast slab, and this swarf is used as an analytical sample in C, It is a method of quantitatively analyzing segregated elements such as P and thereby grasping the degree of center segregation. " Although this method is quantitative, it has the drawback of being time consuming and costly.

The method for measuring the segregated grain size after H-printing (light corrosion) is as follows: "The axial cut slab obtained from a continuous cast slab is polished and H-printed (light erosion), and then the print. The method of measuring and determining the segregated particle size from ". Although this method is quantitative, it has the drawback of being time consuming and costly.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and its purpose is to solve the problems of the conventional continuous cast slab center segregation evaluation method and simply It is intended to provide a continuous cast slab center segregation evaluation method capable of quickly and quantitatively grasping the center segregation degree.

[0007]

In order to achieve the above object, the continuous cast slab center segregation evaluation method according to the present invention is a continuous cast slab center segregation evaluation method according to claims 1 to 3. It has the following configuration.

That is, the center segregation evaluation method for continuous cast slabs according to claim 1 measures the hardness of the axial center portion of the continuous cast slabs, and the average, maximum, maximum and minimum of the measured hardness values. The center segregation degree of a continuous cast slab is grasped from one or more of the differences from the values, which is a method for evaluating the center segregation of a continuous cast slab (first invention).

According to the method for evaluating center segregation of continuously cast slabs as set forth in claim 2, the hardness of the positive segregation portion appearing at the axial center of the continuous cast slab after lightly corroding the axial center of the continuously cast slab is measured, A center segregation evaluation method for continuous cast slabs, characterized by grasping the center segregation degree of the continuous cast slabs from at least one of the average value, the maximum value, and the difference between the maximum value and the minimum value of the measured values. (Second invention).

The continuous cast slab center segregation evaluation method according to claim 3 is the continuous cast slab center segregation evaluation method according to claim 1 or 2, wherein the indentation diameter when measuring the hardness is 1 to 5 mm. (Third invention).

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is implemented, for example, as follows. A continuous casting slab (hereinafter referred to as a measurement target continuous casting slab) whose center segregation degree is to be determined is cut so as to pass through the axial center portion in the axial direction to obtain an axial cutting slab. The hardness of the axial center of the piece is measured, and the average value, the maximum value, and / or the difference between the maximum value and the minimum value of the measured values of the hardness (that is, these average value, maximum value, maximum value and minimum value) And one or more) of the difference between the above and the value and the past data to grasp the center segregation degree of the measured continuous cast slab.

That is, the relationship between the center segregation degree and one or more of the average value, the maximum value, and the difference between the maximum value and the minimum value of the hardness in the axial center portion of the axially cut slab of the continuous cast slab. Data is obtained in advance. Then, the hardness of the axial center part of the axially cut slab of the continuous casting slab to be measured is measured, and one or more of the average value, the maximum value, and the difference between the maximum value and the minimum value of the measured values of the hardness are measured. the calculated, it matches the magnitude of these values (H _x values) and the data, the magnitude of these values on the data (H _x value)
Then, the center segregation degree is calculated, and this is grasped as the center segregation degree of the continuous cast slab to be measured.

As can be seen from the above-mentioned embodiment, the continuous cast slab center segregation evaluation method according to the present invention can easily, quickly and quantitatively grasp the center segregation degree of the continuous cast slab.

That is, the continuous cast slab center segregation evaluation method according to the present invention measures the hardness of the axial center portion of the continuous cast slab, and the average, maximum, maximum and minimum of the measured hardness values. The center segregation degree of the continuous cast slab is grasped from one or more types (hereinafter, referred to as hardness value H) of the difference between (1st invention). Here, the hardness is measured extremely simply and quickly, as compared with the analysis by the conventional method of the axial center drill analysis and the particle size measurement by the method of measuring the segregated particle size after H printing. You can Obtaining the hardness value H from this hardness measurement result and ascertaining the center segregation degree of the continuous cast slab to be measured from this hardness value H and past data can be carried out simply and quickly. Further, the average value of the hardness of the axial center portion of the continuous cast slab, the maximum value, or the difference between the maximum value and the minimum value, and the degree of center segregation, as described later, there is a close relationship, therefore, these The center segregation degree can be quantitatively grasped from one or more of the average value, the maximum value, and the difference between the maximum value and the minimum value (hardness value H) of the measured hardness values.

Therefore, according to the method for evaluating center segregation of continuously cast slabs according to the present invention, the degree of center segregation of continuously cast slabs can be grasped simply, quickly and quantitatively. That is, compared with the conventional method of analyzing the axial center portion of the drill and the method of measuring the segregated particle size after H-printing, the method can be performed very easily and quickly, and, in the same manner as these conventional methods, The center segregation degree can be grasped quantitatively. It can be said that the center segregation degree can be grasped extremely quantitatively in comparison with the above-mentioned conventional method of visually observing after macro-corrosion, and moreover, it can be carried out simply and rapidly, almost like this conventional method.

Further, the continuous cast slab center segregation evaluation method according to the present invention measures the hardness of the positive segregation portion appearing in the axial center portion after lightly corroding the axial center portion of the continuous cast slab, and the hardness thereof The center segregation degree of the continuous cast slab is understood from the average value, the maximum value, and the difference between the maximum value and the minimum value of one or more (hardness value H) (second invention). . According to this method, as will be described later, the average value of the hardness of the positive segregation portion appearing in the shaft center after lightly corroding the shaft center of the continuous cast slab, the maximum value, or the maximum value and the minimum value. Since there is a better correlation between the difference and the central segregation degree, one or more (hardness value H) among the average value, the maximum value, and the difference between the maximum value and the minimum value of the measured values of the hardness can be used to Method (1st
The center segregation degree can be grasped more accurately and quantitatively than in the case of the invention). Compared with the method (first invention), the time required for light corrosion increases, but since the increase time is short, the conventional method for axial drill analysis and segregation after H printing are performed. Compared with the method of measuring particle size, it can be performed very easily and quickly.

As described above, the hardness of the axial center of the continuous cast slab, or the average value of the hardness of the positive segregation portion appearing in the axial center of the continuous cast slab after lightly corroding the axial center, the maximum value The value, or the difference between the maximum value and the minimum value, and the center segregation degree have a close relationship.
This is a new technical knowledge obtained as a result of intensive studies, and the present invention has been completed based on such knowledge.

The details will be described below.

At the axial center portion of the continuous cast slab, C, P, Mn, S are the same as those of the slabs other than the continuous cast slab, or more.
It is very important to understand the center segregation degree (the degree of segregation at the axial center) of the continuous cast slab, as such elements segregate and this leads to deterioration of the internal quality of the continuous cast slab. . Among these segregation elements, since C constitutes iron and carbides,
It is considered that the correlation with hardness becomes high. Therefore, it is considered possible to estimate the center segregation degree by measuring the hardness. In order to confirm this idea, the following research experiments were conducted.

Usually, a simple evaluation of the internal quality of a continuous cast slab is performed by visually observing the center segregation after macro corrosion. That is, a visual determination method after macro-corrosion is adopted as a method for evaluating center segregation of continuous cast slabs. Therefore, the relationship between the segregation rank and the measured value of the hardness of the shaft center portion in the visual determination was first investigated. A slab high carbon steel (C: 0.55 mass%), which is a continuous cast slab, was used as a sample for this investigation, and the Brinell hardness at the segregation position of the axial center of the axially cut slab was measured at 10 to 20 points. (Load: 1.75Ton, steel ball diameter: 10mm). The indentation diameter is about 3 mm
Met.

FIG. 1 shows the relationship between the average value of the above hardness measurements and the segregation rank by visual judgment. Although the variation is large, it can be seen that the segregation rank deteriorates as the average hardness of the shaft center portion increases.

FIG. 2 shows the relationship between the maximum hardness measurement value and the segregation rank. Compared with the case of FIG. 1, the variation is slightly smaller and the correlation is slightly better. It is considered that this is because the segregation rank in the visual judgment is aggravated if there is even one large segregated grain, and the correlation with the maximum hardness value is slightly improved.

Brinell hardness measurement was performed on the shaft center portion (load: 3 Ton, using slab high carbon steel (C: 0.82 mass%) instead of the above slab high carbon steel (C: 0.55 mass%).
Steel ball diameter: 10 mm). Then, the relationship between the difference (hardness deviation) between the maximum value and the minimum value of the hardness measurement values and the maximum segregated particle size by visual determination was examined. The result is shown in FIG. It can be seen that there is a good correlation between the hardness deviation and the maximum segregated grain size. As described above, it was found that hardness is closely related to segregation.

Further, if the C concentration in the segregated portion can be estimated from the hardness measurement value, the segregation can be quantitatively determined without performing the time-consuming and costly drill analysis as in the conventional method of the axial core drill analysis. Becomes possible. Therefore, for the slab high carbon steel (C: 0.55 mass%), the Brinell hardness measurement of the segregated portion of the shaft center (load: 3 Ton, steel ball diameter: 10 mm, indentation diameter:
About 4 mm), and thereafter, a chip having a diameter of 5 mm was taken from the same location as this hardness measurement location, and C was analyzed to investigate the correlation between the C concentration and the hardness value. FIG. 4 shows the results. As can be seen from FIG. 4, the segregated grains are small and the C concentration is low at the position (location) where the hardness value is small. The segregated grains are large and the C concentration is high at the position where the hardness value is large, and the C concentration and the hardness measurement value show a good correlation.

In order to further improve the accuracy, the number of N is increased,
Brinell hardness measurement at the shaft center (load: 1.75Ton, steel ball diameter: 1
Chips are collected from the 0 mm) part with a 3 mm diameter drill, and C
Analysis was conducted to investigate the relationship between C concentration and hardness. The result is shown in FIG. As can be seen from FIG. 5, there is a close relationship between the C concentration and the hardness, and it is possible to estimate the C concentration from the hardness value, and therefore it can be said that the center segregation degree can be quantitatively grasped.

Further, in order to quantitatively grasp the center segregation of the continuously cast slab, a plurality of axial center drill analyzes are often performed, and the average value of the analyzed values is often used for evaluation. That is, a method of performing axial drill analysis is often adopted. Therefore, the continuous cast slab is lightly corroded to cause segregation grains to appear, and the hardness of the portion is measured and the C concentration is measured by drill analysis for about 20 points,
The relationship between the average hardness value and the average C concentration value was investigated. The conditions for hardness measurement and drill analysis are the same as in the case of FIG. The survey results are shown in FIG. As can be seen from FIG. 5, the variation becomes smaller and smoothed, and C
There is a fairly good correlation between concentration and hardness.

From the above results, it became clear that the center segregation degree can be quantitatively grasped by measuring the hardness. That is, the hardness of the axial center of the continuous cast slab is measured,
From the hardness value H (an average value, a maximum value, and one or more of the differences between the maximum value and the minimum value of the measured hardness values), it has become clear that the center segregation degree can be quantitatively grasped.

By the way, in addition to C in steel, Mn, Si, P, S
It is difficult to unambiguously estimate the C content from the hardness value H for steels having different composition values unless the effects of these on hardness are comprehended. It is difficult to accurately grasp the degree. Therefore, the Brinell hardness of the shaft center portion was measured using a continuous cast slab (average C content: 0.55 mass%) in which segregation of C did not exist and only segregation of Mn, Si, P, and S remained. Done (load: 3 Ton
, Steel ball diameter: 10 mm). FIG. 7 shows the result. From Figure 7,
Despite the segregation of Mn, Si, P, and S in the shaft center, the Brinell hardness is almost constant (about 210).
It can be seen that there is almost no difference in hardness between the segregated portions of Si, P and S and the portions without these segregation, and therefore the hardness is hardly influenced by the segregation state of Mn, Si, P and S. Further, from FIG. 5, when estimating the C content at which the Brinell hardness value is about 210,
C: Approximately 0.55% by mass, which corresponds to the average value of the C content of the continuously cast slab in the case of FIG. 7 described above. From this, the hardness value is greatly influenced by the C value in the steel, and Mn,
It can be seen that Si, P, S, etc. are hardly affected.

Further, the hardness value largely changes due to the change of the structure due to the heat history, but since the hardness of the axial center portion of the continuous cast slab is the object here, the cooling rate of the axial center portion is important. It becomes a problem. Therefore, we investigated how much the cooling rate of this axial center portion actually changes. As an example of the result, the average cooling rate until the solid phase fraction f _S = 0.2 to 1.0 of the axial center portion of the high carbon steel (C: 0.56 mass%) when the casting rate was variously changed was calculated by solidification calculation. The obtained results are shown in FIG. As can be seen from FIG. 8, even if the casting speed changes, the average cooling rate of the shaft center part hardly changes. This suggests that the structure of the shaft center part hardly changes even if the casting conditions are slightly different, and therefore the hardness value is influenced only by the C content in the steel.

From the above, if the hardness of the axial center portion of the continuous cast slab is measured, it becomes possible to quantitatively grasp the center segregation degree from the hardness value H, and moreover, C which is unique to this hardness is obtained.
It can be said that the amount can be estimated, and thus the center segregation degree can be accurately grasped. The present invention has been completed on the basis of the above new technical knowledge, and as described above, it is possible to easily, quickly, and quantitatively grasp the center segregation degree of continuous cast slabs. Play.

In the present invention, when measuring the hardness of the axial center portion of the continuously cast slab, it is desirable that the indentation diameter be 1 to 5 mm (third invention). It is C when the indentation diameter at the time of hardness measurement is almost the same as the size of the segregated grain size of the shaft center portion.
It is considered that the correlation between the amount and the hardness becomes particularly good, and generally, the segregated particle size is often about 1 to 5 mm. Therefore, when the indentation diameter is set to 1 to 5 mm, the correlation between the C amount and the hardness is This is because the center segregation degree can be more accurately grasped, and eventually, the center segregation degree can be more accurately grasped.

Further, if the position of the segregated grains in the positive segregation portion appearing in the shaft center portion after lightly corroding the shaft center portion of the continuous cast slab is specified and the hardness of that portion is measured, the C content and hardness can be further improved. And the center segregation degree can be more accurately grasped (second invention).

In the present invention, the type of material of the continuous cast slab to be measured is not particularly limited, and the present invention can be applied to material types in which center segregation poses a problem. For example, in addition to high carbon steel, low It can be applied to carbon steel, medium carbon steel, various alloy steels, stainless steel and the like.

[0034]

【Example】

(Example 1) First, slab-shaped slabs (carbon steel slab slabs) made of carbon steel of various compositions obtained by continuous casting, particularly those having different C contents, were axially cut at the center of thickness. And obtain axial cutting slab, to measure the center segregation degree by the method of measuring the hardness of the shaft center part and the shaft center part drill analysis for these axial cutting slabs, the average value of the hardness of the shaft center part, Data on the relationship between the maximum value, the difference between the maximum value and the minimum value, and the center segregation degree was created. As the carbon steel, C: 0.01 to 1.0 mass%, Mn: 0.2 to 1.3 mass%, S
i: 0.2 to 0.4 mass%, P: 0.01 to 0.03 mass%, S: 0.0
The composition range of 001 to 0.002 mass% was used. The hardness is measured with a Brinell hardness meter. The conditions at that time are: steel ball diameter: 10 mm, load: 1-3 Ton, and indentation diameter: 1
I made it 5mm.

Next, regarding the carbon steel slab slabs to be measured, which are continuous cast slabs having the compositions shown in Table 1, the axial thickness center is cut in the axial direction to obtain axially cut slabs. The hardness of the shaft center portion of the cast piece was measured, and the average value, the maximum value, and the difference between the maximum value and the minimum value of the measured values of the hardness were obtained.
Then, the magnitude of these values (H _x value) is collated with the above data, the center segregation degree at which the magnitude of these values (H _x value) is reached on the above data is obtained, and this is measured continuous casting It was grasped as the center segregation degree of the slab. Table 1 shows the center segregation degree (the center segregation degree grasping value A) thus grasped.
The hardness was measured with a Brinell hardness meter, and the conditions at that time were steel ball diameter: 10 mm, load: 1-3 Ton, and the indentation diameter was 1-5 mm.

Further, in order to confirm the validity and accuracy of the center segregation degree grasping value A obtained in this way, axial center drill analysis is carried out on the above-mentioned carbon steel slab slab to axially cut slab. The center segregation degree was measured by the method. Table 1 shows the measured value Z of the center segregation degree. As can be seen from Table 1, in any of the carbon steel slab slabs ~, the central segregation degree grasping value A and the central segregation degree measured value Z correspond well, and the difference between the two is extremely small.
The central segregation degree grasping value A was confirmed to be highly valid and highly accurate.

(Example 2) The above carbon steel slab cast piece
After axially corroding the axial center of the cast slab with picric acid, the hardness of the positive segregation part that appeared in the axial center was measured, and the average, maximum, and maximum of the measured hardness values were measured. The difference between the value and the minimum value was calculated. Then, the magnitude of these values (H _x
Value) and the above data are compared, and the degree of central segregation when the magnitude of these values (H _x value) is obtained on the above data,
This was grasped as the center segregation degree of the continuous cast slab to be measured.
The center segregation degree (center segregation degree grasped value B) thus grasped is shown in Table 1. The hardness is measured with a Brinell hardness meter, and the conditions are as follows: steel ball diameter: 10 mm, load: 1
It was set to 3 Ton and the indentation diameter was set to 1 to 5 mm.

Further, the center segregation degree grasping value B thus obtained corresponds better to the center segregation degree measuring value Z than the case of the center segregation degree grasping value A in the first embodiment. ,
It was confirmed that the difference between the two is very small and almost nonexistent. Therefore, the above-mentioned center segregation degree grasping value B is extremely highly valid and highly accurate.

(Example 3) When measuring the hardness of the axially centered slab of the carbon steel slab slab, the diameter of the steel ball was 10 mm, the load was 1 to 3 Ton, and the indentation diameter was 0.1 to. Ten
mm, and the effect of the indentation diameter was confirmed. As a result, when the indentation diameter is set to 1 to 5 mm, the obtained center segregation degree grasping value B has a good correspondence with the measured value Z of the center segregation degree and is a highly accurate value. If it exceeds 5 mm,
Negative segregation in the surrounding area is also incorporated, and the central segregation degree is generally reduced. Also, if it is less than 1 mm, the locations of the microsegregation that do not pose a problem will be widened, and the center segregation degree will vary greatly. The correspondingness of was decreased and the accuracy was decreased.

[0040]

[Table 1]

[0041]

According to the method for evaluating center segregation of continuously cast slabs according to the present invention, the degree of center segregation of continuously cast slabs can be grasped simply, quickly and quantitatively. That is, as compared with the conventional method of analyzing the axial center drill and the method of measuring the segregated particle size after H-printing, the method can be performed extremely easily and quickly, and, in the same manner as these conventional methods, The center segregation degree can be grasped quantitatively. Further, compared to the recently-developed image analysis method for measuring segregated grains, the method can be performed very simply and quickly, and the degree of central segregation can be quantitatively grasped almost like the present method. It can be said that the center segregation degree can be grasped extremely quantitatively in comparison with the above-mentioned conventional method of visually observing after macro-corrosion, and moreover, it can be carried out simply and quickly, almost like this conventional method. Thus, the continuous cast slab center segregation evaluation method according to the present invention is simple and quick,
In addition, the center segregation degree of the continuous cast slab can be quantitatively grasped with high accuracy.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between an average hardness measurement value (axial center hardness average value) of a shaft center portion of a continuously cast slab and a segregation rank by visual determination.

FIG. 2 is a diagram showing a relationship between a maximum hardness measurement value (maximum hardness of the shaft center portion) of a shaft center portion of a continuous cast slab and a segregation level by visual determination.

FIG. 3 is a diagram showing the relationship between the hardness deviation (difference between the maximum value and the minimum value) of the hardness measurement values for the axial center portion of the continuous cast slab and the maximum segregated grain size by visual determination.

FIG. 4 is a diagram showing the hardness at each position of the shaft center portion of the continuous cast slab and the C amount obtained by the method of the shaft center portion drill analysis at each same position. It is a figure for demonstrating the correspondence with C amount.

FIG. 5 is a diagram showing a relationship between a measured Brinell hardness value (axial Brinell hardness value) of a shaft center portion of a continuous cast slab and a shaft core C value obtained by a method of a shaft center portion drill analysis.

[Fig. 6] Hardness value (shaft hardness value) of the positive segregation portion that appears in the shaft center after lightly corroding the shaft center of the continuous cast slab and the shaft center obtained by the method of the shaft center drill analysis It is a figure which shows the relationship with C density.

FIG. 7 is a diagram showing the relationship between the casting direction position number and the Brinell hardness for the axial center portion of the continuous casting slab (distribution of Brinell hardness at the casting direction position for the axial center portion of the continuous casting slab). .

FIG. 8 is a diagram showing a relationship between a casting rate and an axial center average cooling rate in manufacturing a continuous cast slab.

Claims (3)

[Claims] 1. A continuous cast slab is obtained by measuring the hardness of the axial center portion of the continuous cast slab, and selecting one or more of the average value, the maximum value, and the difference between the maximum value and the minimum value of the measured hardness values. Segregation degree of continuous cast slab characterized by grasping center segregation degree of 2. The hardness of the positive segregation portion that appears on the shaft center after lightly corroding the shaft center of the continuous cast slab is measured, and the average, maximum, maximum and minimum of the measured hardness values are measured. A method for evaluating center segregation of continuous cast slabs, which comprises determining the degree of center segregation of continuous cast slabs from at least one of the differences from the values. 3. The indentation diameter when measuring the hardness is 1 to 5 mm.
The continuous segregation slab center segregation evaluation method according to claim 1 or 2.