JPH0390259A - Continuous casting method - Google Patents

Abstract

PURPOSE:To prevent segregation of impurity elements appeared at center part of thickness direction in a continuously cast slab and to manufacture a metal having uniform quality by dividing rolls in rolling reduction zone into plural blocks and making higher average rolling reduction velocity in each block as going to the downstream side in the rolling reduction zone. CONSTITUTION:The cast slab drawn from a mold 3 is cooled with a spray and bent with plural group rolls and pinch rolls and supplied to the rolling reduction zone. in the rolling reduction zone, the block 1 is constituted with the roll R42, R43 and R44, and the block 2 with R45, R46, R47, R48, R49 and R50, and the block 3 with R51 and R52 are constituted. Then, the rolling reduction velocity in the range of 0.1-0.3 and 0.3-0.55 center solid phase ratio of thickness direction in the cast slab is limited with spacers and the rolling reduction velocity in the range of 0.55-0.7 the center solid phase ratio of thickness direction in the cast slab is made free. Further, by making higher the average rolling reduction velocity in each block as going to the downstream side in the rolling reduction zone, the segregation of impurity elements appeared at the center part of thickness direction in the continuously cast slab is prevented and the metal having uniform quality can be obtd.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention prevents the segregation of impurity elements found in the center of the thickness of continuously cast slabs, such as sulfur, phosphorus, and manganese in the case of steel slabs. This invention relates to a continuous casting method that can produce homogeneous metal.

[Prior art and problems to be solved by the invention]

In recent years, steel pipes for offshore structures, storage tanks, oil and gas transportation,
Requirements for material properties such as high-tensile wire rods are becoming increasingly strict, and providing homogeneous steel materials has become an important issue. Originally, steel should be homogeneous in its cross section, but steel generally contains impurity elements such as sulfur, phosphorus, and manganese, and these segregate and become partially concentrated during the casting process. becomes vulnerable. Particularly in recent years, continuous casting methods have become popular for purposes such as improving productivity and yield and saving energy, but noticeable component segregation is usually observed in the center of the thickness of slabs obtained by continuous casting. .

The above-mentioned component segregation significantly impairs the homogeneity of the final product.
Stress acting on steel during the product use process and the wire drawing process can cause serious defects such as cracks, so there is a strong desire to reduce this. Such component segregation occurs when the residual molten steel flows at the final stage of solidification due to solidification contraction force, washes out the concentrated molten steel near the solid-liquid interface, and the residual molten steel progressively becomes concentrated. Therefore, in order to prevent component segregation, it is important to eliminate the cause of the flow of residual molten steel.

like this? 8 Causes of steel flow include flow caused by solidification shrinkage, as well as flow caused by slab bulging between rolls and roll misalignment, but solidification shrinkage is also an important cause of these steel thicknesses, and segregation To prevent
It is necessary to reduce the slab by an amount that compensates for this.

Attempts have been made to improve segregation by compressing the slab, which involves solidifying and shrinking the slab during the period when the temperature at the center of the slab reaches from the liquidus temperature to the solidus temperature during the continuous casting process. A method is known in which the pressure is reduced at a constant rate greater than the amount to be compensated.

[Problem to be solved by the invention]

However, with conventional continuous casting methods, there is a problem that, depending on the conditions, there may be little effect on improving segregation, and in some cases, segregation may even worsen, making it difficult to sufficiently improve component segregation. Ta.

The present inventors have conducted various investigations into the causes of such problems in the conventional method, and have found that the reason why the conventional method does not have an effect on improving segregation or even worsens segregation is that the pressure It was discovered that this was caused by inappropriate coagulation timing and range.

The present inventor previously disclosed in Japanese Unexamined Patent Publication No. 62-275556 that from the point in time when the center of the slab reaches a temperature corresponding to a solid phase rate of 0.1 to 0.3, the solid phase rate corresponds to the flow limit solid phase rate. The area up to the point at which the temperature reaches the solid state is continuously reduced at a rate of 0.5 + m/min or more and 2.5 trm or less per unit time, and from the time when the center of the slab reaches the temperature corresponding to the flow limit solid fraction, the area becomes solid. We proposed a continuous casting method that does not apply substantial reduction until the phase temperature is reached.

Furthermore, the present inventor hypothesized several formulas based on numerous experimental results, and by comparing them with the experimental results, proposed a more advanced continuous casting method.

An object of the present invention is to obtain a homogeneous metal by preventing the segregation of impurity elements found in the center of the thickness of a continuously cast slab.

[Means to solve the problem]

According to the present invention, the central solid fraction of the slab is 0.1 to 0.
In a continuous casting method of molten metal, the region from the point when the temperature corresponds to 3 to the point when the temperature corresponds to the flow limit solid fraction is continuously reduced and pulled out using a plurality of roll pairs having a hydraulic pressure reduction mechanism. , dividing the rolls of the rolling band into a plurality of blocks, and limiting the rolling speed of the roll located in at least one of the upstream blocks with a spacer;
A continuous casting method is provided, characterized in that the rolling speed of the remaining blocks is not restricted and is free rolled, and the average rolling speed of each block is made larger downstream of the rolling zone.

[For production]

According to the continuous casting method of the present invention, the central solid fraction of the slab is 0.
．． It is continuously rolled down by a plurality of roll pairs having a hydraulic rolling down mechanism from the time point when the temperature corresponds to 1 to 0.3 to the time point when the temperature corresponds to the flow limit solid fraction. Further, all the rolls in the rolling zone are divided into a plurality of blocks, and the rolling speed of the roll located in at least one of the upstream blocks is limited by the space, and the rolling speed of the remaining blocks is not limited and is free rolling. It is said that The average rolling speed of each block is increased as the downstream of the rolling zone increases.

Here, all the rolls of the rolling band are divided into three blocks,
The solid fraction at the center of thickness of the slab is in the range of O01 to 0.3 and 0.
．． It is preferable to limit the reduction rate in the range of 3 to 0.55 with a spacer, and to set the free reduction in a range where the thickness center solid fraction of the slab is 0.55 to 0.7. In addition, all the rolls of the rolling zone are divided into two blocks, and the rolling speed is limited by a spacer when the solid fraction at the center of thickness of the slab is in the range of 0.1 to 0.3. is 0.3
It may be configured such that a range of 0.7 to 0.7 is set as the free pressure reduction.

Thereby, it is possible to prevent the segregation of impurity elements found in the center of the thickness of the continuously cast slab, and to obtain a homogeneous metal.

〔Example〕

First, an example of a continuous casting machine to which the continuous casting method according to the present invention is applied will be schematically explained with reference to FIG. can be,
In order to prevent solidification shrinkage flow, it is necessary to increase the reduction I (reduction speed) applied to the surface of the slab toward the downstream of the reduction zone. - When the reduction blade is constant, the amount of reduction by the direction reduction roll is larger as the upstream of the reduction zone increases, and the upstream of the reduction zone becomes overpressure and an upward flow occurs, resulting in reverse segregation of slabs and worsening of segregation. . In addition, excessive reduction at a time when solidification has not progressed sufficiently may cause internal cracks, so it is necessary to limit the amount of reduction upstream of the reduction zone. As mentioned above, it is necessary to achieve an appropriate reduction rate depending on the progress of solidification.
The present invention is provided as a method for stably realizing these.

FIG. 1 shows a continuous casting machine to which the continuous casting method according to the present invention is applied, and specifically shows an example of a twin cast circular arc type continuous casting machine. As shown in the figure, in this continuous casting machine, a ladle l filled with molten steel is placed above the tundish 2, and the molten steel in the ladle 1 is poured into the tundish 2 through the sliding nozzle 11 at the bottom. It is designed so that

Here, the opening degree of the sliding nozzle 11 is controlled according to the entire weight of the tundish 2 containing the molten steel poured from the ladle 1, and
M is kept constant.

The molten steel in the tundish 2 is controlled by vertically moving the stopper 21 that closes the bottom of the tundish.
It is injected into the mold 3 at a constant rate. The bottom of the mold 3 is also open, and the molten steel injected into the mold 3 is cooled on the side wall of the mold 3 to which cooling water is supplied, and is solidified from the outside (secondary cooling). The melt (slab) that has been primarily cooled by the mold 3 is continuously drawn out by rollers.

The slab pulled out from the mold 3 is, for example, spray-cooled in a spray zone, further bent by a plurality of group rolls and vinyl rolls, and then supplied to a rolling zone.

The rolling band consists of a plurality of rolls Rsz + Ras * R4
4,..., and the rolls of these rolling bands are divided into three blocks. That is, roll R4□
Block 1 is composed of 7R43 + R44, and roll R
4,.

R46* R4? Block 2 is composed of + R41r R49+ Rso, and furthermore, roll R% I r
Block 3 is composed of R52. Then, the rolling speed is limited by a spacer when the solid fraction at the center of thickness of the slab is in the range of 0.1 to 0.3 (corresponding to block 1) and in the range of 0.3 to 0.55 (corresponding to block 2). The free reduction is set in a range where the solid fraction at the center of thickness of the slab is 0.55 to 0.7 (corresponding to block 3). Here, the rolls R□, R1°+R]9+... upstream of the roll R42 at the unbending point
A spacer is provided between the rolls. Furthermore, a method for limiting the rolling speed during light rolling is to control the rolling speed by inserting a spacer between the bearings of the rolls and preventing the rolls from moving beyond the spacer position. In addition, free rolling is a rolling method that does not limit the rolling speed, and achieves the degree of rolling achieved by the rolling blades of the rolls and the deformation resistance of the slab. Note that the rolling speed is defined as in the following equation.

Reduction speed = (Reduction amount of 10-L/Roll spacing (m)) x Casting speed (m/m1n) (aa/
(win) Tests conducted using the continuous casting method of the present invention using the continuous casting machine shown in FIG. 1 will be described above. Here, a schematic diagram of the continuous casting machine in which this test was conducted is shown in FIG.

The characteristics of this light rolling method are that the light rolling zone is divided into three (or two) parts depending on the solidification stage, the rolling speed of the first two blocks (or one block) is limited by the application of spacers, and the rolling speed of the second half block (or two blocks) is limited by the application of spacers. ) is a free reduction that does not limit the amount of reduction, and the reduction rate is increased toward the downstream of the reduction zone.

Table 1 shows the strength of the melt I1 in which the test was conducted, and Table 2 shows the reduction zone divided into three parts according to the central solid fraction of the slab, and the reduction amount of the two upstream blocks limited by spacers. death,
This shows the case where one downstream block is under free pressure.

Table 3 (Components of molten steel tested) Table 3 shows the results of segregation, etc. measured by corroding the A-width section of slabs cast according to Tables 1 and 2 above and comparing them with conventional examples.

Table 3 (Comparison of original properties and conventional method) Solid fraction at center of slab thickness Block I H0.1 to 0.3
．． Block 2: 0.3-0.55. block 380.5
5 to 0.7, or block 1j0.1 to 0.55.
Block 2 j 0.55 to 0.7 As is clear from Table 3, the slabs obtained by the present invention have fewer inverted V (!l folds) and less segregation than those obtained by the conventional method. As shown in 9 and above, which shows a significant improvement, the present light rolling method has been proven to be superior to the conventional method.

〔Effect of the invention〕

As detailed above, the continuous casting method according to the present invention includes:
The rolls of the rolling zone are divided into a plurality of blocks, the rolling speed of the roll located in at least one of the upstream blocks is limited by a spacer, the rolling speed of the remaining blocks is not limited and is free rolling, and each block is By increasing the average rolling speed toward the downstream of the rolling zone, it is possible to prevent the segregation of impurity elements found in the center of the thickness of the continuously cast slab and obtain a homogeneous metal.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a continuous casting machine to which the continuous 1fjt casting method according to the present invention is applied. (Explanation of symbols) 1...Ladle, 2...Tandish, 3
...Mold, 11...Sliding nozzle, 21...Stopper.

Claims (1)

[Claims] 1. The area from the time when the center solid fraction of the slab reaches a temperature corresponding to 0.1 to 0.3 to the time when the temperature corresponds to the flow limit solid fraction is under hydraulic pressure. In a continuous casting method of molten metal in which a plurality of pairs of rolls having a mechanism are used to continuously reduce and draw the metal, the rolls of the reduction zone are divided into a plurality of blocks, and the reduction speed of the roll located in at least one of the upstream blocks is A continuous casting method, characterized in that the rolling speed of the remaining blocks is limited by a spacer, the rolling speed of the remaining blocks is not limited and the rolling speed is free, and the average rolling speed of each block is increased downstream of the rolling zone. 2. The roll of the rolling band is divided into three blocks,
The solid fraction at the center of thickness of the slab is in the range of 0.1 to 0.3 and 0.
．． Claim 1, wherein the reduction rate is limited by a spacer in the range of 3 to 0.55, and the free reduction is defined as a range in which the solid fraction at the center of thickness of the slab is 0.55 to 0.7. The continuous casting method described. 3. The roll of the rolling band is divided into two blocks,
The rolling speed in the range where the solid fraction at the center of thickness of the slab is 0.1 to 0.3 is limited by a spacer, and the rolling speed in the range where the solid fraction at the center of thickness of the slab is 0.3 to 0.7 is free rolling. The continuous casting method according to claim 1, wherein: