JPS61189850A - Continuous casting method of steel slab - Google Patents

Abstract

PURPOSE:To improve remarkably the internal quality and particularly central segregation of a slab by exerting rolling down force to the slab advancing in a continuous casting machine in the position where the thickness of the solidified shell of the slab satisfies the specific equation relating to the thickness of the solidified shell and the thickness of the slab. CONSTITUTION:The slab 1 is drawn by guide rolls 2, etc. and advances in the direction (a) while the rolling down force is exerted to the slab 1 in the thickness direction thereof by driving rolls 3 having backup rolls 4. The solidified shells 11 are formed to the slab 1 from the surface part and the molten steel remains while the molten steel is held enclosed by the shells 11, by which an unsolidified part 12 is formed. The slab 1 is rolled down in the thickness direction thereof by the rolls 3 and the upper and lower shells 11 are thereby press-welded in the position of the range where the shells 11 satisfy the equation (h is the thickness of the solidified shell on one side of the slab in the thickness direction and D is the thickness of the slab). The increased segregation owing to the flow of the unsolidified steel as a result of the rolling reduction and the worry about the press-weldability of the solidified shells to each other are thoroughly eliminated by forming a crater end 13 in the above- mentioned manner.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to continuous casting of steel, in particular to a continuous casting method for slabs, in which a crater end is forcibly formed in a slab moving through a continuous casting machine. This provides a method to improve its internal quality.

Conventional technology In recent years, there has been a remarkable shift from ingot making to continuous casting as a method for manufacturing billets and ingots from molten steel.

Currently, nearly 80% of all products are manufactured using the continuous casting method.

In continuous casting, after pouring molten steel into a mold to form a slab with a predetermined cross-sectional shape, the slab is continuously pulled out by a group of pinch rolls and guide rolls arranged following the mold, and its core is After it has completely solidified, it is cut to a predetermined length to produce steel billets for steel plates, that is, slabs and steel sections.

However, in the continuous casting method, especially the continuous casting method for slabs, it is not possible to achieve a large reduction ratio (ratio of product thickness to casting thickness) compared to the ingot making method, so the unused material remaining in the core of the slab is Center segregation occurs near the position where the molten steel in the solidified state completely solidifies (the position where the molten steel in the unsolidified state completely solidifies to the core of the slab is referred to as the crater end in the present invention). This has the problem of causing internal defects in the slab.

That is, the vicinity of the crater end is supported by guide rolls arranged above and below the slab, and the distance between the guide rolls in the vertical direction is determined by the thickness of the slab in the mold until it reaches the crater end. The dimensions are maintained by subtracting the thickness that occurs during solidification and shrinkage.

Conventionally, the roll spacing was set before starting casting based on the past average solidification shrinkage amount, and moreover, the roll spacing was set based on the past average solidification shrinkage amount, and
It was common for a group of upper and lower rolls placed with the slab in between (called a pair) to be spaced at regular intervals.

The solidification shrinkage is caused by the sequential formation of solidified shells from the surface of the slab, but in actual operation, the formation of the solidified shells is not uniform, but rather constantly changes, and solidification occurs at predetermined locations. The amount of shrinkage varies widely. For this reason, the conventional technique described above cannot provide appropriate support commensurate with the amount of solidification shrinkage of the slab in the vicinity of the crater end.

In other words, if the actual amount of solidification shrinkage is greater than the amount of solidification shrinkage assumed in advance, the roll interval will become wider than the set value, and the volume of the unsolidified portion of the slab core will increase. Conversely, if the actual amount of solidification shrinkage is small, the roll interval becomes narrower than the set value, and the volume of the unsolidified portion of the slab core becomes smaller. Therefore, the volume inside the solidified shell (unsolidified portion) near the crater end changes. When such a volume change occurs, molten steel is sucked into the area, and as a result, harmful components such as [S] and CP) concentrate and solidify near the crater end, causing central segregation. .

In order to solve the above problem, in the past, as shown in Japanese Patent Publication No. 5B-382913, for example, the solidification profile of the slab is detected or estimated, and the solidification profile of the slab is detected or estimated, and the slab is moved in the direction of movement according to the profile. On the other hand, a technique has been proposed in which a gentle gentle reduction is applied.

However, since the solidification profile near the crater end changes in a complex manner depending on the operating conditions and is also non-uniform in the width direction, it is extremely difficult to detect or estimate it. Following this and applying light pressure reduction was not expected to have any significant effect in actual purchase.

In addition, in Japanese Patent Publication No. 411i-29387, rolling forming rolls are used, and by detecting the transfer speed of the slab before and after the forming rolls and controlling the rotational speed of the forming rolls, casting in the vicinity of the crater end is controlled. A technique has been proposed for making the size and shape of the unsolidified portion remaining in one core portion constant.

However, the crater end should not be determined only by the relationship between the rotational speed of the forming roll and the cross-sectional area of the slab before and after forming, and even if all of the above relationships are satisfied,
For example, if cooling is delayed and the solidified shell thickness is thin, and if rolling is continued with forming rolls with unsolidified parts remaining in the core of the slab after forming, this will result in the flow of molten steel near the crater end, causing the slab to flow. The internal quality of the system will deteriorate significantly.

Problems to be Solved by the Invention The present invention fundamentally solves the conventional problems in the above-mentioned continuous casting method for slabs. It provides a method for making improvements possible.

Means for Solving the Problems The feature of the present invention for solving the above problems is that in the continuous casting method for steel slabs, the solidified shell thickness of the slab advancing in the continuous casting machine satisfies the following equation (1). The purpose is to apply a reduction blade in the thickness direction of the slab at a satisfactory location to forcibly form a crater end.

70≦(2h/D)X 100≦95 ・・φ (1
) However, h: Solidified shell thickness on one side in the slab thickness direction D: Slab thickness effect 1. The present inventors have proposed the above-mentioned conventional method of setting the roll spacing commensurate with the amount of solidification shrinkage of the slab, or the progress of the slab. The problem with the method is that the pressure is applied gently and gently in the direction.
Focusing on the fact that this is caused by changes in the position and shape of the crater end, we first conducted experimental research on a method for accurately controlling the position of the crater end.

In order to control the crater end position, for example, adjusting the casting speed or cooling intensity may be considered, and some attempts have been made in the past.

However, when controlling using such macroscopic operational variables, although it is possible to keep the position of the crater end within a certain range, the thickness of the unsolidified part, which has the greatest influence on the central segregation, It was found that in a range where the diameter is around 1 mm, even the slightest disturbance causes instantaneous complete solidification, so it has been found that this cannot be controlled in practice.

Therefore, the present inventors found that there were limitations in the method of controlling the position of the crater end and determining the roll interval to follow that position.As a result of further experimental research, the inventors found that the unsolidified portion We have obtained the knowledge that it is possible to fundamentally and effectively solve the above problem by adding a reduction blade to the cast slab having the following properties to forcibly form a crater end.

When the crater end is forcibly formed, there are concerns about (1) deterioration of segregation due to the flow of unsolidified molten steel due to rolling reduction, (2) poor adhesion of solidified shells to each other, etc. However, these concerns could be resolved by adjusting the area where the reduction blade is applied to the slab within the range described below.

This will be explained below based on the drawings.

FIG. 1 is an explanatory diagram for explaining the method of forming a crater end based on the present invention, and is a diagram showing a partial cross section near the crater end.

In the figure, 1 is a slab, which moves in the direction shown by arrow a. 2 is a guide roll for the slab l, 3 is a rolling drive roll (hereinafter simply referred to as a driving roll) that applies a rolling blade in the thickness direction of the slab l, and 4 is the driving roll 3.
This is a backup role. A solidified shell 11 is generated from the surface of the slab l, and molten steel remains surrounded by the solidified shell 11, forming an unsolidified portion 12.

As a result of repeated many experiments using such an apparatus, the present inventors have found that the solidified shell 1 of the slab 1 progressing inside the continuous casting machine.
1 satisfies the following formula (1), the slab 1 is rolled down in the thickness direction by the drive roll 3, and the upper and lower solidified shells 11 are crimped to form the crater end 13. It has been confirmed that the above concerns (① and ②) have been completely resolved.

70≦ (2h/D) X 100≦95---
(1) However, h: Solidified shell thickness on one side in the slab thickness direction D: Slab thickness, that is, the ratio of the thickness h of the solidified shell 11 to the thickness D of the slab l (hereinafter, this ratio is referred to as the solidification rate) Rate = ((
Within the range where 2h/D)
Add a reduction blade so that they join, and make the crater end 1.
3 is forcibly formed.

If the solidification rate is 85% or less, the reduction by the drive rolls will function as a dam to prevent the flow of molten steel, and the flow of molten steel will be dammed up in the area after the dam, and no segregation will occur, and the slab will not flow. Since an appropriate amount of molten steel remains in the core, the upper and lower solidified shells 11 are also completely adhered and integrated.

In particular, as shown in Fig. 1, the solidification rate is 85% or less, and the slab thickness D before being rolled and the slab thickness Do after being rolled satisfy the following formula (2). Adding a reduction blade completely prevents the flow of molten steel and allows crimping to occur without any defects.

(D-Do) / 2≧((D-2h)/2)+1
--- (2) However, D: Slab thickness before reduction (■) Do: Slab thickness after reduction (m) h: Solidified shell thickness on one side in the slab thickness direction (a) By the way, the solidification of slab 1 If we look at the situation in cross section in the width direction, as shown in Figure 2, solidified shells 11 are generated from the surface in the thickness direction and width direction.For example, the solidification rate in the thickness direction of the slab is 70 to 95 as described above. %, the solidified shell in the width direction (hereinafter, the solidified shell in the width direction is referred to as the end solidified shell, and the solidified shell in the thickness direction is simply referred to as the solidified shell unless otherwise specified) 11a also becomes considerably thicker.

Therefore, in order to form the crater end 13 in such a solidified state, there is a method using a drive roll 3a with protrusions that rolls down only the portion corresponding to the unsolidified portion 12, as shown in FIG. 3, and a method shown in FIG. A possible method is to use drive rolls 3b of the same diameter to apply a reduction edge to the entire slab l, as shown in FIG. However, in either method, there is an unsolidified portion 12a at the joint between the end solidified shell 11a and the solidified shell 11.
tends to remain, and the above-mentioned flow of molten steel may occur in that area, resulting in significant segregation. The incidence of such phenomena becomes extremely high when a reduction blade is added to form a crater end before the solidification rate has reached 70%, but when the solidification rate exceeds 70%, they almost never occur. It was confirmed that it does not.

In addition, in the case shown in Figure 4 above, a very large strain occurs at the center of the end of the slab (X section in Figure 4), and cracks appear in the casting direction when the solidification rate is less than 70%. It was done. This is the reason why the solidified shell is limited to 70 to 95% in the present invention.

Note that the width of the slab slab L produced in continuous casting varies, and the amount of variation may be large. When the slab width changes frequently and the amount of change is large, two pairs of drive rolls 31 a are moved in the traveling direction a of the slab 1, as shown in the perspective view of FIG. 5, for example. , 31
b is provided horizontally movably, and the two pairs of drive rolls 31a,
31b may form a crater end 13a.

Example 3 In a curved continuous casting machine with a capacity of 50 tons/Hr,
The invention was carried out during the production of -9i-killed steel.

The casting conditions are: width of slab is 1300mm, thickness is 250mm)
, the casting speed Vc was 1.8 m/sin, and the solidification constant of the solidified shell was 27.0 mm/m1n2. Given the solidification constant K, the solidified shell thickness of the slab is the distance flI from the meniscus? (m) and the casting speed Vc as shown in the following equation (3).

h=K.(L/vC)'' (3) Based on the present invention, the site where the crater end is formed can be determined by the following equation (4) using the distance as a reference.

70≦2X27X (L/L, 6)”X100/2
50≦95--- (4) In other words, the distance is 21.9
m≦L≦30.9m, and the solidified shell thickness at that time is 99
．． 9 m≦h≦118.7 mm In this example, a portion of L = 21.9 m was selected (the solidified shell thickness at that time was approximately 1 ooms), and the following (5) based on the above formula (2) was selected. ) The slab thickness D0 after rolling was set to 180 mm in order to achieve a rolling reduction that satisfied the formula.

(250-Do) / 2≧((250-200)
/2) +1.0 --- (5) Now, in this embodiment, in order to obtain the above rolling reduction amount, a drive roll 3a with projections as shown in FIG. 3 is used, and the roll diameter of the projections is 450 mm. It was set as l. Further, rolling was applied evenly in the width direction of the slab from a point that entered the center of 75II11 from the end face of the slab. The reaction force due to rolling at this time was approximately 200 tons, and no problems such as local deformation of the drive roll occurred.

Table 1 shows the center segregation of the slab manufactured by this example, which was investigated using the scoring method using sulfur print, and compared with the conventional method. In the conventional method, the roll interval in the area where the distance fllL is 18 m to 37 m is 253 mm.
It is kept at a constant value and allowed to solidify in its natural state.

Table 1 As can be seen from the above Table 1, by implementing the present invention, there is no B grade for the sulfur print, which has a significant concentration and continuity of harmful ingredients, and the grade is below C, which is the grade range that does not pose a problem when processed into products. It became possible to produce the entire amount.

Effects of the Invention As detailed above, according to the present invention, the center segregation of slabs manufactured by continuous casting can be significantly improved, and as a result, it has become possible to manufacture slabs of excellent quality.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view for explaining the crater-end forming method according to the present invention, Fig. 2 is a cross-sectional view showing the solidification state of a general slab, and Fig. 3 is a cross-sectional view showing the solidification state of a general slab. FIG. 4 is a cross-sectional view showing an example using drive rolls of the same diameter, and FIG. 5 is a perspective view showing an example using two pairs of drive rolls that are laterally movable. . l... Φ slab, 2... Guide roll, 3@... Drive roll, 4... φ backup roll, 11... Slab 1
solidified shell of 12--unsolidified portion of slab 1, 13--crater end of slab 1.

Claims (1)

[Claims] In a continuous casting method for steel slabs, a reduction blade is applied in the thickness direction of the slab at a portion where the solidified shell thickness of the slab advancing in the continuous casting machine satisfies the following formula (1), and A continuous casting method for steel slabs characterized by forming a crater end. 70≦(2・h/D)×100≦95...(1) (However, h represents the solidified shell thickness on one side in the slab thickness direction, and D represents the slab thickness.)