JPS60162564A - Vertical type continuous casting method - Google Patents

Abstract

PURPOSE:To provide a billet having high quality with extremely little segregation in the stage of casting continuously the billet of a specific size by providing a light screw-down device consisting of plural diametrally opposed split type roll groups having a specified roll pitch in the final solidifying region. CONSTITUTION:A molten high-purity carbon steel consisting of <=0.008% P and <=0.003% S is continuously cast to the billet size of 200-500mm. thickness and >=1,000mm. width by using a vertical type continuous casting machine. A light screw-down device of >=4m overall length consisting of plural diametrally opposed split type roll groups having <=300mm. roll pitch is disposed in the final solidifying region in this stage. A slab is successively drawn and rolled down in such a way that the space between the rolls facing each other satisfies the relation 67<=delta.D<=210 (delta; the drawing rate of the rolls mm./min, D; the thickness of the billet mm.).

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing high-quality continuous slabs with extremely low segregation using a vertical continuous caster.

Steel slabs cast by the conventional continuous casting method have a problem in that components such as C, Mn, P, and S accumulate near the center of the slab, causing a so-called segregation phenomenon. The shape of this segregated part is various, such as plate-like or granular, but for example, in the case of granular,
Its size can reach up to 2 to 3 cm in diameter. When the slab is subjected to rolling in a subsequent process, this segregated portion is stretched to form a plate-shaped or linear heterogeneous portion within the steel plate.

It is well known that these foreign parts have a negative effect on the quality of steel sheets.

When the above-mentioned segregation of continuously cast slabs occurs, when the slabs are held by the rolls of the continuous casting machine, the slabs swell to the outside of the slabs due to the static pressure of the molten steel between the rolls. , when passing through the rolls, a reduction corresponding to the bulge is applied, and thus the slab undergoes multiple expansions (bulging) and under pressure from the time it is cast into the mold until it finishes solidifying. will be repeated.

As a result, the molten steel receives a pump action inside the slab and moves within the slab. This movement is said to promote segregation.

In the past, various means have been developed to reduce this segregation. One example is tojimata and electromagnetic stirring. In continuous steel casting, when the internal substance of the slab is unsolidified, the molten steel is stirred by electromagnetic force, and the influence of the flow causes solidification to take place in an equiaxed manner, thereby dispersing segregation. This is a method of reducing the size of segregated grains.

For example, as a method for reducing segregation using electromagnetic stirring, there is a method of limiting cooling so that the surface temperature of the solidified shell is at least one Curie point (Japanese Patent Laid-Open No. 514352), and an electromagnetic stirring device is installed near the mold and in the middle of the continuous casting machine. Method of
5271), and a method of installing it in the final solidification part of the slab (Japanese Patent Application Laid-open No. 57-1115587).

In addition, a method has also been developed in which the area near the final solidification part of the slab is rolled down to compensate for the volume shrinkage associated with phase transformation during solidification of molten steel in the final solidification part, thus reducing the flow of molten steel and reducing segregation. .

For example, Japanese Patent Application Laid-Open No. 57-E12804 discloses a method of continuously casting slabs for steel materials with excellent sour gas resistance, in which an equiaxed heavy solidification structure is formed and reduction is applied. However, there are still issues that require further consideration, such as the proper roll spacing and casting speed.

On the other hand, from the perspective of improving productivity, for example,
When producing extremely thick slabs with a thickness of 200 to 500+ nm or more, especially when the thickness is about 500+a+a, vertical type continuous PI is used to avoid internal cracks, etc. as described later.
a is used.

In order to prevent internal cracks in extra-thick slabs manufactured using this vertical continuous casting machine, there is a method described in Japanese Patent Application Laid-open No. 57-84464, in which in the secondary cooling zone,
The slab support roll and spray nozzle are periodically reciprocated to ensure uniform cooling and prevent internal cracks.

However, all of the above-mentioned methods were insufficient as a method for producing high-quality, extra-thick slabs.

OBJECT OF THE INVENTION The object of the present invention is to provide optimal conditions for obtaining high-quality, extra-thick slabs using a vertical continuous casting machine.

Structure and operation of the invention That is, the configuration of the present invention is as follows.

(a) The continuous casting machine is a linear type (based on vertical casting) using upper and lower open molds.

(b) Slab thickness of 200 to 500 mm, l (100 mm
A slab having the above slab +iJ is manufactured.

(c) A dense split roll stand (light reduction device) is arranged in the final solidification area of the slab (the peripheral area including the area where molten steel in the slab has completed solidification).

A dense split roll stand is a roll stand for holding slabs.
The roll is divided into two or more parts, and the roll pitch is 3.
A roll stand with a diameter of 30 mm or less, which has a grindness of 4 m or more and is adjusted so that the distance between opposing rolls gradually decreases within the range of 67≦δ・D≦210.

(d) The structure of the steel to be cast is P≦0.008%, S≦0.
Must be 0.003% or less.

The present invention will be described below with reference to the drawings.

The present invention is based on the use of a vertical continuous casting machine.
An example elevation view is shown in Figure 1. That is, the molten steel 2 in the tundish 1 is injected into the mold 4 by the injection nozzle 3. The molten steel 2 is cooled by the mold 4 and the spray nozzle 6 in the secondary cooling zone, and is drawn out by the slab holding roll 5 while solidifying from the surface. A light rolling device consisting of a group of split rolls (7) is arranged near the solidification completion point 8 (final solidification zone).

Although the present invention is a vertical type continuous casting machine, a slightly inclined straight type casting machine, for example, one having an inclination angle of less than 30° is also included.

After the cast slab 9 is completely solidified, it is cut into a predetermined length with a gino and is carried out of the machine. In this case, since the slab does not undergo bending deformation during the solidification process unlike in a curved continuous casting machine, a healthy slab can be obtained without cracking inside the slab. FIG. 2 shows the relationship between the straight strain of the slab and the presence or absence of cracking in the relationship between the radius of curvature of a simple circular arc type continuous casting machine and the slab withdrawal speed. That is, the slab straightening strain is determined by the following formula: D: slab thickness (IIffl) S: solidified shell thickness (mm) R: continuous casting machine curvature radius (11 m1) From this figure, continuous casting without internal cracks The radius of curvature of the machine is 300mm thick slab, 400am,
13.4m and 22 respectively for 500mm
．． 2m, 28.8m. Although a curved radius that can be commonly used for these is 28.6 m, a curved radius such as this is practically impossible to implement industrially.

For this reason, as described above, a vertical continuous casting machine is used.

Next, the slab size is 200mm thick or more, 500II1
m or less and slab rtNOOOmm or more.

In the case where the thickness of the slab was less than 200 mm, the bulging welfare was large and segregation was likely to occur, so other conditions were required, and this was excluded.

It is still difficult to cast slabs with a thickness exceeding 500 m+++ using a continuous casting machine.

Slabs II] Those with a length of less than 1100 h belong to bloom,
The solidified structure of the bloom is different from that of the slab according to the present invention and was excluded from the present invention.

Next, in order to produce high-grade steel, it is necessary to improve the purity of the steel. As for the steel plate for low-temperature toughness, as shown in the test results in FIG. 3, the ranges of S≦0.003%, P≦o, ooe% are required.

The same applies to durable Melatiyar steel.

In the case of hydrogen-induced cracking resistant steel, S≦(1,002%, P≦0
．． 0.004% is preferred. Other ingredients are adjusted to the composition required by the product material.

Further, as for the composition of the steel, methods of adding other components such as Ti addition and Ca weighting can be used in combination.

Next, the most important feature of the present invention is the relationship between the roll interval and the amount of narrowing. Explanatory diagrams of this relationship are shown in Figures 4 and 5.
Shown in the figure.

FIG. 4 shows the relationship between the bulging strength of continuously cast slabs and the continuous caster roll pitch. In other words, bulging begins to occur where the roll pitch exceeds 200 mm,
At 330+nm, the bulging strength is 0.5 mm. From the viewpoint of center segregation, the allowable limit of bulging strength must be 0.5 mm or less, and for this reason, the limit of roll pitch was set to 330III+ or less.

The contraction accompanying phase transformation during solidification is the deformation of the solidified shell, i.e.
In order to compensate for this by reduction, the distance between the rolls must be reduced so as to satisfy the following condition, as shown in FIG. 5: δ・D≧67 δ: roll reduction amount mad/win D: slab thickness mm

Furthermore, if the reduction becomes too large, the pressure will be applied in excess of the amount of solidification shrinkage, and the molten steel will rather be pushed out, resulting in large negative segregation. Such slabs are also unfavorable from the standpoint of segregation. That is, δ-D≦210.

The results shown in Figure 5 were obtained when this roll stand was placed at a length longer than the shaft around the continuous caster position where the steel completes solidification.

This is necessary to ensure stable slab quality.

Now, to explain this with a specific example, when a slab passes through this continuous casting machine, the surface of the slab is subjected to static pressure of molten steel and bulges. This bulging strength increases rapidly when the roll pitch exceeds 200mm.
Segregation is rejected for roll pitches greater than m.

In a vertical continuous casting machine, for example, the roll pitch is set to 27h.
It's fine if you wear them.

After molten steel is cast, as solidification progresses, contraction occurs in response to the change in density during the phase transformation from liquid to solid. This is about 4%
It is. It is necessary to compensate for this shrinkage by squeezing the rolls of the continuous casting machine. As shown in FIG. 5, this amount varies depending on the thickness of the slab.

For example, when the thickness is 300 mm, the necessary amount of roll narrowing is 0.22 to 0.7 θ1 m/win. 300m
When casting slabs with m thickness at 0.7m/gin, the roll interval of the roll stand is (0.22~0.?Omm/win) ÷ (0.7m).
/win)=0.3~1.0mm/m, that is, 0.3~1.0m per 1m of roll stand length
It must be as small as m/m.

By solidifying the molten steel in the roll stands arranged in this way, the flow of molten steel in the slab can be reduced, and the generation of segregation can be suppressed to a slight degree.

Even in this case, the composition of the molten steel to be cast is set to P≦0.008% and S≦0.003%, so that the low temperature toughness properties below -40°C are As shown in FIG. 3, the results are extremely good.

Note that embodiments of the present invention will be described below.

(a) A continuous caster is constructed so that the slab holding part of the continuous caster is in a straight line until the steel is completely solidified, and after solidification, the slab is drawn out by bending and unbending. Continuous casting machines that are tilted slightly from the vertical, and that can be considered to be substantially vertical from the perspective of solidification phenomena, include continuous casting machines that are constructed in a straight line.

(b) Roll stands may be of roll segment type or individual roll set type.

(c) There are various methods of cooling slabs, such as methods of mixing air with water, but these methods are not restricted.

However, highly uniform and strong cooling is preferred.

(d) As a casting method, the temperature of the molten steel is raised.

This also includes the case where a method is added to adjust the coagulation structure by lowering the coagulation structure.

(e) This also includes cases in which a method for controlling the coagulation structure is added using electromagnetic stirring.

Effects of Examples and Inventions Below, using Examples, in comparison with conventional examples (comparative examples),
The effects of the present invention will be specifically explained.

The cast slab size is 400mm in mold size and 1500mm in width.
The composition of molten steel in m11 is as shown in Table 1.

This molten steel was cast using three types of continuous casting machines shown in Table 2.

The casting results are shown in Table 3 (slab quality, plate material evaluation results,
When P≦0.008%, S≦0.003%).

As shown in Table 3, the NOI continuous casting machine is a vertical type with a machine length of 1
From 5 m to 22 m at the end, we used two-part rolls with a roll diameter of 280 mm and a roll pitch of 31 h.
Roll narrowing to 1.0+u+/m (slab drawing speed 0
．． 1.0mm/trr corresponding to 35m/min
X O, 35m/5in= 0.35o+m/win
As shown in Table 3, the rejection rate from the perspective of segregation evaluation and internal evaluation is as low as 0.2% and 0.1%, respectively. Even from the viewpoint of low temperature toughness properties, the rejection rate is much lower than in other cases.

As another example, Table 4 was obtained (slab quality, plate material evaluation results).

From Table 4, even when using an NOI type continuous casting machine, if low temperature toughness is required, P≦0.0
0.08%, and S≦0.003%.

In this way, the continuous casting machine according to the present invention has S≦0.003%, P
It can be said that casting high-purity steel of ≦Q, 008% is an important condition when manufacturing high-grade steel.

As described above, the effects of the present invention are remarkable.

Table 1 (Double distribution of units) Table 2 Table 3 Table 4

[Brief explanation of drawings]

Figure 1 is a conceptual diagram of a vertical continuous casting machine, Figure 2 is a diagram showing the radius of curvature of the continuous casting machine, corrective strain, and the range where internal cracks do not occur, ttS
Figure 3 is a diagram showing the influence of P and sg degree of steel on the low-temperature toughness characteristics of thick plate, Figure 4 is an explanatory diagram of the relationship between roll pitch and bulging, and Figure 5 is an illustration of the relationship between slab thickness and required roll reduction amount. It is an explanatory diagram of a relationship. 1. Fist/tanditshu, 2. Molten steel, 3.. Injection nozzle, 4φ.. Mold, 5.. Slab holding roll, 6φ.
Number: Cooling water spray nozzle, 7-...Split roll, 8
Fist: solidification completion point, 9: cloudy slab. Patent Applicant Nippon Steel Corporation Representative Patent Attorney Masaru Inoue

Claims (1)

[Claims] High-purity carbon steel molten steel with P≦o, ooe%, and S≦0.003% is cast into slabs with a thickness of 200 to 500 mm and a width of 1000 mm or more using a vertical continuous casting machine. During continuous casting, a light reduction device with a total length of 4 m or more consisting of a group of split rolls facing each other with a roll pitch of 33 hm or less is installed in the final solidification region of the slab, and the spacing between the opposing rolls is 67≦δ・D. ≦210 (However, δ is the roll narrowing amount mll1/win, D
represents slab thickness (roll spacing) am. A vertical continuous casting method characterized in that the slabs tJj-pieces are sequentially squeezed and rolled down so as to satisfy the following relationship.