JPS594943A - Production of continuous casting ingot having no semimacro segregation - Google Patents

Abstract

PURPOSE:To suppress the area rate of semimacro segregation in continuous casting with electromagnetic stirring by rolling down the ingot between the crater end of the liquidus line and the crater end of the solidus line at a specific draft in the thickness direction thereof. CONSTITUTION:An ingot 4 is rolled down between the crater end 5 of the liquidus line and the crater end 6 of the solidus line thereof by as much as the shrinkage on solidification thereof by means of hydraulic cylinders 8 with light rolling reduction rolls 7 in the thickness direction of the ingot 4 in the stage of casting continuously molten steel 1 under stirring with electromagnetic stirring. The area rate of semimacro segregation is suppressed to about <=0.12% by setting the light draft at >=0.5mm./m.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a casting method for improving component segregation in four pieces of continuously cast steel.

In recent years, the continuous casting method (hereinafter referred to as "continuous casting method") has been gaining popularity as a method for manufacturing steel slabs, as it improves yield and production efficiency, reduces energy consumption, etc., is easy to automate and mechanize, and improves the working environment. It has been adopted. In these continuous casting methods,
An electromagnetic stirring method is applied to the continuous casting process to eliminate the center segregation of the four continuously cast slabs, and this allows stable generation of equiaxed crystals or granular crystals within the slabs.There is no uneven electromagnetic stirring. The center segregation that occurred at the time disappears as shown in attached Figure 1. (Figure 1 shows the change in the segregation situation at the center of the slab depending on the presence or absence of electromagnetic stirring. Pouring speed: 0.9%, molten steel superheat: 25°C, slab size: 250" x 2300mm)
However, as shown in Figure 2 (the slab cast under the conditions of Figure 1 with and without electromagnetic stirring) This is a diagram showing the solidification structure in the cross section in the direction of incorporation (1K
) is absent (b) is present), the L cross section of the slab (
As can be seen from the solidification structure in the cross section (in the casting direction), products cast with equicrystalline or granular crystals always have a V-shaped segregation band (usually referred to as V-segregation). When observing the solidified structure of the slab at a magnification of about 10 times, this V
As shown in FIG. 3, it can be seen that segregation is formed by island-like segregation parts intermittently continuing in a V-shape at the grain boundaries of equiaxed crystals or granular crystals. This island-like segregation is defined as semi-macro segregation. This V segregation is said to be formed when residual molten steel is attracted to the high solid phase side and flows in the solid-liquid coexistence phase as solidification progresses, and it is said that the core part contains a high concentration of solutes. Due to the presence of Mn18, there is a microscopic solidification delay, which is the cause of void formation, and an extremely high concentration part is likely to be formed, and with the concentration of Mn18, it becomes a place where Mn+S forms. It has been known. Semi-macro segregation part #'i, which has increased demand recently, has increased I resistance (IC property) (Improve the weave to as fine a cape axis crystal as possible, and make semi-macro segregation as fine as possible. Although efforts have been made,
This problem has not yet been completely resolved.

On the other hand, in slabs subjected to conventional high-temperature casting, hydrogen-induced cracking may occur when columnar crystal problems occur.

The present invention aims to prevent the formation of semi-macro segregation, which constitutes V segregation in equiaxed crystals, which occurs when electromagnetic stirring is used in the continuous casting method, which is a problem with the conventional method.

The present inventors have obtained the following knowledge as a result of various studies on the generation of semi-macro segregation.

That is, as mentioned above, semi-macro segregation is a place where extremely high concentration of solute exists. In other words, as shown in Figure 1, the macroscopic check analysis shows that electromagnetic stirring is 1! Despite the fact that the solute distribution in the sample subjected to electromagnetic stirring was extremely flat compared to the sample without electromagnetic stirring, Pmar/
P6 = 12 + Mn max/Mn6:; , 2
．． It was confirmed that there was approximately 0 semi-macro segregation.

Figure 4 shows an X-ray microanalyzer (XMA) using an electron beam of 50 μΦ at a casting speed of 0, 8 to t OmA.
-k, molten steel superheating degree 10~62℃, slab size 250x
2300-1 P of slab cast with electromagnetic stirring
, Mn was line analyzed and the relationship of the ratio of each peak value to the raw steel value is shown.

FIG. 5 shows the relationship between the thickness and number of semi-macro segregations in the L cross section of the electromagnetically stirred slab under the casting conditions shown in FIG. 1, and the maximum thickness exists up to about 311II. In such a high-concentration portion, the solute may disappear due to diffusion of the solute during the high-temperature holding in the furnace during the rolling process. In other words, in terms of the ease of solute diffusion, the smaller the thickness of semi-macro segregation, the easier it is to disappear, and it is generally known that the limit thickness of semi-macro segregation that disappears up to the rolled product is about 300μ, so If the size is large, it will remain in the product at a high concentration.

In addition, the peak value of the degree of solute refinement during semi-macro segregation is recognized to be correlated with the magnitude of semi-macro segregation.

Figure 6 shows a casting speed of 0.8 to 1.0 my'-1, a molten steel superheat of 10 to 62°C, and a slab of 250 x 2300.
.

XM with a beam diameter of 50μφ under casting conditions with electromagnetic stirring
A is a graph showing the relationship between Pmax/P6 and the thickness of semi-macro segregation in a sample with a cape axis crystallinity of 25%; as shown in FIG. 6, as the thickness of semi-macro segregation increases, solute concentration also increases. On the other hand, the thickness of semi-macro segregation and the semi-macro segregation area ratio within the L cross section of the slab (HC of the slab cross section)
There is a correlation between the total area of semi-macro segregation observed after performing macro tests using an image processing device and dividing it by the measured slab cross-sectional area, and the area ratio is thick. It is clear from Figure 7, which shows the relationship between the area ratio of semi-macro segregation in the slab and the thickness of semi-macro segregation, that the thickness tends to increase as the thickness increases.

This tendency exists regardless of the magnitude of equiaxed crystallinity. This kind of semi-macro segregation will occur as long as the steel undergoes volumetric contraction during solidification, so in order to eliminate this, it is necessary for K to completely stop the flow of the residual molten steel in the solid-liquid coexistence phase. I'm coming. However, simply stopping the flow cannot compensate for the volumetric contraction during solidification, which promotes the formation of voids.

Based on the above, the inventors have found that a method (light reduction casting method) of reducing the slab thickness by the amount of volumetric shrinkage as solidification of the solid-liquid coexisting phase progresses is effective.

That is, the gist of the present invention is [in a continuous casting method in which molten steel is cast while being stirred by electromagnetic stirring, light reduction casting is performed and a light reduction ratio of 0.5 is applied between the liquidus crater end and the homophase crater end. Roll down in the thickness direction of the slab at a pressure of m or more,
This is a continuous slab manufacturing method that aims to prevent the formation of semi-macro segregation by casting while preventing the flow of molten steel in the solid-liquid coexistence phase at the center of the slab. ” Figure 8 is a conceptual diagram of the light reduction method. In Fig. 8, 1 is the molten steel, 2 is the liquidus line 6 of the molten steel, the solidus line 4 is the cast product 5, is the liquidus line crater end, 6 is the solidus line crater end, and 7 is the liquidus line of the molten steel.
is a light reduction roll, 8 is a hydraulic cylinder of the light reduction roll, and 9 is a pinch roll. As shown in FIG. 8, the present invention has a liquidus crater end 5 and a solidus crater end 6.
The slab 4 is reduced by a hydraulic cylinder 8 via a light reduction roll 7 by an amount commensurate with the solidification shrinkage. This is a method in which reduction by a light reduction roll 7 corresponding to the solidification shrinkage of the liquid-coexisting portion is simultaneously performed. However, the individual values of the semi-macro segregation area ratio and thickness change depending on the light reduction conditions. The light reduction condition for the slab is defined as the length of light reduction in the slab drawing direction. ) and light reduction rate (light reduction length per 1 m in the slab drawing direction), by setting the light reduction rate to 0.5 vm/m or more according to Example 3, which will be described later. The semi-macro segregation area ratio is suppressed to a value of 0.12% or less.

In addition, when the light reduction ratio is fixed at 0.5 wI/ln and the light reduction length is varied to determine the semi-macro segregation area ratio, according to Example 4 described later, if the light reduction length is 2 m or more, semi-macro segregation is detected. The area ratio becomes a value of 0.12% or less.

The semi-macro segregation area ratio is stable at a lower level in the cases where electromagnetic stirring of the present invention and the conventional method is carried out, or when light reduction is carried out at the same time as low-temperature casting to generate equiaxed crystals, compared to cases where it is not carried out.

The degree of solute formation during semi-macro segregation was measured and compared using XMA for slabs cast under the following casting conditions, and the results are shown in Figure 10.

(Measured with beam diameter = 50μφ, equiaxed crystallinity is about 2 for both sides.
5%) As shown in the figure, it is clear that the case where electromagnetic stirring was performed and light pressure was applied at the same time was within the lowest level of the case where only electromagnetic stirring was used.

Example 6 The L-section semi-macro segregation area ratio of a slab cast under the following casting conditions was determined, and the relationship with the light reduction rate was determined, and the results are shown in FIG.

Equiaxed crystallinity: 25 to 60 cm As shown in Figure 11, the relationship between the light reduction rate and the semi-macro segregation area ratio when the light reduction length is set to 2 m is that the light reduction rate is 0.5 m/ By setting m or more, the semi-macro segregation area ratio will be lowered by 0.12%U.

Example 4 The semi-macro segregation area ratio of the slab cast under the following casting conditions was determined, and the relationship with the light reduction length was determined. As a result, the equiaxed crystal ratio = 25
~30% As is clear from Fig. 12, when the light reduction ratio is fixed at 0.5~, the semi-macro segregation area ratio becomes a value of 0.12% or less if the light reduction length is 2 m or more.

Example 5 The equiaxed crystallinity and semi-macro segregation area ratio of a slab cast under the following casting conditions were determined, and a comparative test was conducted between the present invention and the conventional method, and the results are shown in FIG.

Conventional method (2) is a case of low-temperature casting.As is clear from Figure 16, the slab produced by the method of the present invention is less stable than the slab produced by conventional electromagnetic stirring or low-temperature casting to generate equiaxed crystals. are doing.

Example 6 Slabs A and B were manufactured under the following casting conditions, semi-macro segregation was determined for A, and center segregation was determined for B, and slabs A and B of the present invention were obtained.
A rolled product equivalent to APIX 52 to x 80 was manufactured from B, and a hydrogen crocodile cracking (HIC) test was conducted to obtain CL R (Cra
ck leugth ratio) was determined, and the relationship between area ratio and CLR% was investigated. The results are shown in FIG.

[Note] In the HIC test, the rolled product is divided into three parts as shown in FIG. 15(a), and six samples are examined for defects. Furthermore, three cross sections of one sample are shown in Fig. 15(b).
Find t, t, t, (crack propagation length), and calculate W (cross-sectional length)
Furthermore, the hydrogen corrosion cracking test environment under the NACE environment is based on the conditions shown in the table below.

As shown in Figure 14, the semi-macro segregation area ratio is 0.12.
- If it is below, hydrogen corrosion cracking will not occur.

Test slab B did not use electromagnetic stirring, so the HIC test results were obtained when no equiaxed crystals were present.
The relationship with LR is shown in FIG. 14.1 This result shows that both semi-macro segregation and central segregation have a critical value for defect generation at approximately the same segregation area ratio.

[Brief explanation of the drawing]

Figure 4 is a diagram showing the ratio of the peak value of P + Mn to the raw steel value when line analysis of semi-macro segregation is performed by XMA. A diagram showing the relationship between the thickness and number of semi-macro A) TL segregation in a single cross section, Figure 6 is a diagram showing the relationship between the thickness of semi-macro segregation and the results of measuring the degree of solute formation by A diagram showing the relationship between the thickness and number of segregation up to semima in one piece, Figure 10 is Example 2
Figure 11 is a diagram showing an example of measuring the degree of solute concentration during semi-macro segregation of the slab shown in Figure 11 using XMA.
The figure shown in Example 5 shows that the light rolling length is constant and the light rolling ratio is varied.
The equiaxed dimensions of the inventive slab and the conventional slab are shown in FIG. Diagram showing the relationship between crystal rate and semi-macro segregation area ratio, No. 14
The figure shows the relationship between the HIC test results shown in Example 6 and the semi-macro segregation area ratio of the corresponding slab.
The figure (,) is a crack investigation cross-sectional drawing used to explain the reason for determining the needle CLR in the HIC test shown in Example 6. In Figure 8, 1: Molten steel, 2: Liquidus line, 6: Solidus line, 5
: Liquidus crater end, 6: Solidus crater end,
7: Light reduction roll, 9: Pinch roll. Figure 1 Sashimi Figure 2 Figure (G) (b) ■ Tetsu Tetsu' JA bamboo Figure 3 Figure 4 1 2 4 6 8 10 12 14PmO
X/PO 5th IvI rmizuriu%? '+717 mm Fig. 6 Fig. 7 Yami Macro tomb width rdb bag 1 tongue 6 Fig. 8 9DA 10th country 2.0 40 6.0 &OIQO12,QJ14.
0Pmax/P. Fig. 11 Wheel size: 1,000 mm/m Fig. 12 Fig. 13 Equal shaft product skewer%

Claims (1)

[Claims] A continuous casting method in which molten steel is cast while being stirred by electromagnetic stirring, characterized by rolling down in the slab thickness direction between the liquidus crater end and the solidus crater end at a light reduction rate of 0.5'% or more. Continuously cast four-piece manufacturing method without semi-macro segregation.