JPH03114635A - Method for guiding cast slab utilizing metallurgical characteristic - Google Patents

Abstract

PURPOSE:To improve cooling effect and to eliminate bulging by beforehand predicting heat shrinkage and transformation expansion in the structure transforming in casting direction of surface layer at shell part in a cast slab, deciding short side interval of a mold based on them and executing casting. CONSTITUTION:When the molten steel having <=0.765wt.% C is cast into the mold and continuous casting is executed to make the cast slab by drawing while cooling, in the case of using B for width of the cast slab and (x) for distance in casting direction from meniscus in the cast slab, the relation of the equation I is obtd., i.e., B=ax<2>+bx+c (a<0, c>0) in gamma structure single part at the shell part in the cast slab. Further, the relation of the equation II, i.e., B=dx+e (d>0) in gamma+alpha structure part and the equation III, i.e., B=fx+g (f<=0) in alpha structure single part is obtd. By which, the short side interval at right and left sides in the cast slab is determined so as to continue the equations I, II and III related to (x). By this method, fear of the development of breakout is eliminated and the productivity is improved and the production cost is reduced, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for determining the width of a continuous casting mold using metallurgy characteristics and guiding a slab using the mold.

[Prior art] The conventional continuous casting of slabs uses a water-cooled copper mold and uses flux as a lubricant to cast slabs with a thickness of 200 to 300 mm.
Slabs with a width of around 1000 to 2000 mm,
Casting is carried out at a speed of 1 to 2 m/min, and the amount of heat removed during the casting process is approximately 7 x 105 kcal/h-m2, so that the shell within the mold is completely solidified with a γ (austenite) structure throughout the entire area.

In such continuous casting methods, vertical corner cracks are one of the causes of breakout. A method has been implemented in which the slab is guided by a taper.

For example, in Tetsu to Hagane Vol. 1.67, No. 8, page 1203, vertical corner cracks occur when the short side of the mold is tapered with a constant slope of Q, 9 mm/m during continuous casting at a casting speed of 1.6 m/min. Rate 13. : There is a report that it has been improved from 196 to 5.2 trees.

Referring to this, the present inventors used the short side tapered with a constant slope of 0.9 mm/m to increase the continuous casting efficiency, and created a casting with a thickness of 50 mm and a width of 1.2 m. When the slab was continuously cast with a heat removal amount of about 2 x 106kcal/h-m2, the solidification efficiency of the short side of the slab was poor and it did not solidify sufficiently, and the thickness of the solidified shell of the slab was the same as that of the conventional example shown in Fig. 2. It became as shown. As a result, the time required for cooling and the distance required for cooling became longer, the continuous casting efficiency decreased, bulging increased, and breakout occurred.

[Problems to be Solved by the Invention] The present invention provides a mold that increases continuous casting efficiency when the amount of heat removed from the mold during the casting process is increased, and that does not cause vertical corner cracks, short side bulging, or breakouts. The objective of this project is to develop a method for guiding slabs using the developed method. Generally, as the amount of heat removed from the mold increases, the surface temperature of the slab inside the mold decreases significantly.
It will change from 2 phase → γ single phase → γ + α 2 phase → α single phase.

[Means for Solving the Problems] The present invention solves the above problem when the amount of heat removed in the mold is increased by focusing on the structure of the solidified shell generated in the mold,
The purpose of this invention is to provide a new slab guiding force method that determines the short side spacing of a mold based on the characteristics of its structure, that is, the amount of thermal contraction and transformation expansion, and guides the slab with the mold.

The present invention employs the following means to achieve the object.

1. When continuously casting slabs by injecting molten steel into a mold and drawing it out while cooling, the above-mentioned A method for guiding a slab using metallurgy characteristics, characterized by determining the short side spacing of a mold and guiding the slab using the mold.

2. When continuously casting molten steel with a carbon content of 0.765!6 or less by weight* into a mold and drawing it out while cooling, transformation occurs in the casting direction of the surface layer of the shell part of the slab, which was assumed in advance. Based on the transformation expansion amount of the γ (austenite) → γ + α (ferrite) → α structure, the short side spacing of the mold is determined, and the slab is guided by the mold. How to guide slabs.

3. The method for guiding a slab using the metallurgy characteristics described in 2. #r, characterized in that the distance between the short sides of the mold is widened in the range of the γ+α structure described in #r.

[Operation] In order to establish the above means, the present inventors conducted repeated experiments and studies to clarify the problems of continuous casting using a constant Taber mold.

The test steel used at this time was 0.15, which is particularly susceptible to cracking.
96C-0,254ksi-0,74jiMn-0
,01%; P -0,005! hS-0,02 Zero Yadan's Nakatan is #l. On the other hand, as a means to increase the amount of heat removed from the mold, the casting speed was 4 m/min, the short side of the mold was a non-water-cooled block that moved in synchronization with the slab with a constant slope taper, as shown by the dotted line in Figure 1, and the long side of the mold was is 35m37
A continuous casting machine consisting of a 1.2 mm thick steel plate belt with a water cooling function of 1 mm was used.

As a result of our experimental analysis, the inventors found that a large cap was formed between the short side of the mold and the short side of the slab, as shown in Figure 1, and therefore the short side of the slab was indicated by the dotted line in Figure 2. As shown, a solidified shell grows up to 0.25 m from the meniscus, but solidification stagnates from 0.25 m to 0.75 m due to the formation of a large gap, and solidification progresses again after O', 75 m. , the coagulation efficiency decreased significantly throughout;
It was discovered that bulging is unavoidable and there is a risk of breakout.

Therefore, we investigated the solidification process of the slab in detail, and found that during the mold-like cooling process of continuous casting, the structure of the solidified slab is as shown in Figure 4, starting from just below the meniscus, δ (ferrite), δ + γ, γ, γ + α. , α, and from the relationship between the transformation volume change and thermal contraction, it was found that the shape was shown on the short side of the slab in FIG. However, it was confirmed that the δ→γ transformation occurs rapidly over time and can be almost ignored because the rigidity of the solidified shell is small, so it can be treated as γ, γ+α, and α transformations in practical terms. Furthermore, it can be estimated from the Fe--C system phase diagram (not shown in FIG. 5) that the phenomenon described in item 1 can occur in steel having a carbon content of 0.765% or less by weight*.

The present inventors have studied and devised based on this fact, and specifically, have developed a mold that increases continuous casting efficiency and does not cause vertical corner cracks, short side bulging, or breakout. The challenge of establishing a method for guiding the slab using it is to ensure that the distance between the surface of the slab and the inner surface of the mold is within a narrow, almost constant range in the casting direction within the mold (particularly in the high temperature region). This study was conducted based on the idea that this is the case.

Since the long side surfaces of the molds are supported by water pressure from the back side of the mold belt, there is no need to consider the spacing between the molds of the slab, and only the short side surfaces of the molds, that is, the spacing between the left and right short sides of the molds.

When the slab shell part is a γ-structured part, the temperature drop of the slab is large, so the slab width B needs to be considered as a function of the square of the distance X in the casting direction from the meniscus, mainly due to thermal contraction. .

In other words, as shown in the following equation (1), the larger X becomes, the narrower B becomes.

B = a x 2 + b x + c
(1) However, a<O, c>0 Next, in the γ+α texture part of the slab shell part, as
As the ratio of the structure increases, the change mainly depends on the amount of transformation expansion, and by combining it with thermal contraction,
The slab width B increases as X increases, as shown in equation (2) below.

B = d x + e (2
) However, d>0 Furthermore, when the slab shell part is a single phase α structure, the slab width B becomes narrower due to thermal contraction, but the temperature drop is very gradual compared to when the slab shell is a single phase γ structure. As shown in the following equation (3), the slab width B can be approximated as being narrowed by a linear function of X.

B = fx + g (3) However, f≦0 From the following -12, specifically, as the distance between the short sides of the left and right slab increases as X, the above formulas (1), (2), and (3)
All you have to do is connect the expressions together.

However, in practical terms, the region of equation (3) does not exist,
Even if it does exist, it is so narrow that ignoring it will have little effect.

Furthermore, specifically, by adjusting the amount of heat removed in the mold and the casting speed, etc. according to the steel type so that the length of the single phase part of the γ structure is almost constant, the mold shape can be made into a simple shape. It can be made large and specific.

In terms of equipment, as shown in Figure 1, the guide trajectory of the short-side block group (or its guide) that comes into direct contact with the slab is connected by connecting equations (1) and (2) above. It should be realized. Furthermore, the link between equations (1) and (2) is @
It is preferable to make the rear 20 to 30 mm smooth. FIG. 2 shows a comparison of the short side of the cast slab realized in this manner with that of the conventional tapered type, and it is clear that the example of the present invention has smooth cooling and good continuous casting efficiency.

Furthermore, when we experimented by attaching a thermometer to the short side block, we found that
As shown in FIG. 3, it was confirmed that in the example of the present invention, as in FIG. 2, cooling was smoother within the mold and the continuous casting efficiency was improved. As a result, the present invention not only improved the continuous casting efficiency, but also virtually eliminated the occurrence of vertical corner cracks, short side pulsing, and breakouts.

[Example] (1) Test steel 0.1596C-0,25*5i-0,7*Mn-o
, o1%; P -0,005!6S -0,0296
Steel (2) Continuous casting conditions Casting speed: 4 m/min Short side of the mold: Endless non-water cooled copper block Long side of the mold: 35 m3/min Water cooling function 1.
2 mm thick steel plate belt 0 Slab dimensions: Thickness! 'i 0mm, width 600mm (
3) Short side block guide trajectory The shape as shown in Figure 4 was determined.

・γ group MIi single phase portion y (mm) = Ilx 2 (m) Distance from meniscus x = O ~ 600 ・γ + α tissue portion y (mm) = for, 7 (mm/m) + hx =
600 to 1100 mm ・Extension of α structure part (5) x = 1100 to 1200 mm ・Length of mold 1200 mm (4) Casting results (invention) (Conventional example) Bulging 0.4 mvn on one side 1.
8 +nn+breakout 0 times/25ch
2 times/19ch [Effects of the Invention] As described above, the present invention is capable of shortening the length of a slab by guiding the short side of the mold in response to the structural transformation volume change and thermal contraction that appear on the slab surface during continuous casting. Since cooling is carried out locally, the cooling efficiency is high, there is no bulging, there is no risk of breakout, and this has great effects on the industry, such as improving productivity and reducing production costs.

[Brief explanation of drawings]

Fig. 1 is a comparative diagram of the short side of the inventive example and the conventional example, Fig. 2 is a comparative diagram of the generation situation of the short side shell thickness in the casting direction in the mold in the inventive example and the conventional example. The figure is a comparison diagram of the short side block temperature in the casting direction in the mold in the example of the present invention and the conventional example, Figure 4 is a shape diagram of the short side of the casting in an example of the present invention, Figure 5 is Fe-C system FIG.

Claims (1)

[Claims] 1. When continuously casting a slab by pouring molten steel into a mold and drawing it out while cooling, the thermal contraction and transformation expansion of the structure that transforms in the casting direction of the surface layer of the slab shell, which is assumed in advance. A method for guiding a slab using metallurgical properties, characterized in that the spacing between short sides of the mold is determined based on the amount of the mold, and the slab is guided by the mold. 2. When pouring molten steel with a carbon content of 0.765% or less by weight into a mold and drawing it out while cooling to continuously cast slabs, the surface layer of the slab shell undergoes transformation in the casting direction assumed in advance. γ (austenite) → γ + α (ferrite)
→A method for guiding a slab using metallurgy characteristics, characterized in that the spacing between short sides of the mold is determined based on the amount of transformation expansion of the α structure, and the slab is guided by the mold. 3. The method for guiding a slab using metallurgical properties according to claim 2, wherein the distance between the short sides of the mold increases within the range of the γ+α structure.