JPH03189050A - Method for continuously casting molten steel by twin roll method - Google Patents

Abstract

PURPOSE:To stably cast a high purity steel strip having good surface characteristic without variation of strip thickness by controlling degree of superheat of molten steel poured into between rolls in the range of developing no clogging of a nozzle and continuously supplying the specific carbon concn. of cold steel material into molten steel. CONSTITUTION:At the time of producing the high purity steel strip having <=0.005% carbon concn., the degree of superheat in the molten steel 3 poured into between the rolls 1, 1 is made to little in the range of developing no clogging of the nozzle 4. Then, the cold steel material 11 having <=0.005% carbon concn. is continuously supplied into the molten steel 3 between the rolls. By this method, cast strip of the high purity steel can be directly sent to cold rolling process without any treatment.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention continuously produces thin plates of high-purity steel with no thickness variation and good surface quality using a twin roll method while preventing the growth of non-uniform solidified shells. This relates to a method of casting.

(Conventional Techniques) In recent years, from the viewpoint of process and energy saving, a technology has been developed to manufacture thin sheets close to the final product at the casting stage, that is, Nyanet Shave CC, which is a thin sheet type. A promising CC method is the twin roll method (Japanese Unexamined Patent Publication No. 137562/1983).

FIG. 4 is a diagram for explaining the outline of the twin roll method, which consists of a pair of cooling rolls 1 rotating in opposite directions.
In this method, a slab 6 is cast by supplying molten steel 3 from a molten steel container 5 such as a tundish into a pool 2 defined by a glue 4. When a thin metal plate is cast by this method, stress is generated due to shrinkage during solidification, which causes vertical cracks. For this reason, by fixing the thickness of the slab 6 to be cast with twin rolls to 3-3 or less, and further suppressing the degree of superheating when pouring between the rolls to 15°C or less, the effects of stress generated during solidification and shrinkage can be reduced. (Japanese Unexamined Patent Publication No. 1-91941). Here, the degree of superheating refers to the difference between the temperature of the molten metal and its liquidus temperature.

(Problems to be Solved by the Invention) In recent years, as the environment in which steel materials are used has become stricter, improvements in the properties of steel materials have been desired. In order to improve the properties of steel,
It is necessary to cast high-purity steel with few impurities. In particular, thick plating is essential to ensure the rust prevention properties of automobile steel sheets, and steel materials suitable for this are those with a carbon concentration of o,
The production volume of high purity steel below oos% is increasing significantly.

Therefore, when high-purity steel with a carbon concentration o, oo of 5% or less was cast using the twin roll method, severe thickness fluctuations occurred in the slab 6, and cold open rolling was no longer possible in this state. As a result of a detailed study of the cause of this, it was discovered that the plate thickness variation occurs through the following mechanism.

Generally, the molten steel 3 comes into contact with the cooling roll 1 to remove heat and begins to solidify. At this time, non-uniformity occurs in the contact state between the cooling roll 1 and the solidification shelf due to solidification shrinkage, '&' distortion, etc. This increases the non-uniformity of the solidified shell thickness as solidification progresses, resulting in a delayed solidification area on the exit side of the roll.

Since this part has a thin shell, its strength is weaker than other parts, and when it communicates with the upper molten steel 3, bulging occurs and plate thickness changes. At the same time, the surface condition also deteriorates.

In 22, bulging refers to the clumping of the slab 6 due to the static pressure of the melt.However, the present inventors have discovered that in steel types with relatively high carbon concentrations, an equiaxed crystal structure grows in the center of the plate thickness. It was discovered that this alleviated the uneven growth of the solidified shelf exhibiting a columnar crystal structure and prevented plate thickness variations due to bulging.

Generally, the equiaxed weight ratio increases as the degree of superheating decreases.

However, it is known that the equiaxed crystallinity is strongly dependent on the carbon concentration, and as the carbon concentration becomes smaller, the equiaxed crystal structure no longer occurs. Here, the equiaxed product ratio refers to the equiaxed crystal thickness divided by the plate thickness. For this reason, in high-purity steel with a very low carbon concentration, even if the degree of superheating is suppressed to 15° C. or less, an equiaxed heavy structure is not generated, and plate thickness fluctuations due to bulging are likely to occur. In addition, a thin slab 6 with a plate thickness of 31 or less
In this case, the growth rate of the solidified shelf exhibiting columnar crystals is accelerated, and the chance of forming an equiaxed heavy structure in the center of the plate thickness is further reduced. Therefore, with conventional casting methods, it is difficult to stably produce high-purity steel with a low carbon concentration without fluctuations in plate thickness.

Therefore, the present invention aims to reduce the unevenness of the solidified shell thickness that occurs due to solidification shrinkage, * state distortion, etc. when casting high purity steel with a carbon concentration of 0.005% or less and in which equiaxed heavy weave is difficult to form. The purpose of this is to use the twin roll method to cast a thin sheet with high purity, which is relaxed, has good surface properties, and has no thickness variation.

(Means for Solving the Problems) The present invention aims to reduce the degree of superheating of molten steel poured between rolls when casting high-purity steel with a carbon concentration of 0.005% or less by a twin-roll method to prevent nozzle clogging. This is a molten steel continuous and traversed method using a twin roll method, which is characterized by continuously supplying cold steel material with a carbon concentration of 0.005% or less to the molten steel between the rolls.

(Function) FIG. 2 is a diagram showing how nonuniform solidification is alleviated by equiaxed crystallization at the center of the plate thickness. Figures (a) to (
e) shows the state of development of the solidification shelf in the width direction of the cooling roll in a cross section cut in the radial direction of the roll ((d) in the same figure). Figure (a) shows the development of an uneven solidification shelf from the surface of the cooling roll 1 due to uneven contact between the cooling roll 1 and the solidification shelf.

Gaps 8 are generated here and there between the roll 1 and the coagulation well 7. This non-uniformity of the solidification shelf further increases as solidification progresses, producing a solidification delayed portion 9 at the center of the plate thickness, as shown in FIG. 2(b). When the molten steel in this part communicates with the molten steel 3 in the upper part, the weak solidification delayed part 9 due to the static pressure of the molten steel causes bulging after the roll exit side, resulting in plate thickness variation. However, if equiaxed crystal nuclei 10 are generated at the center of the plate thickness and equiaxed crystal solidification begins before the uneven solidification shelf reaches the center of the plate thickness as shown in FIG. It is possible to suppress the progress of a non-uniform solidification shelf exhibiting columnar crystals #Lm grown from the surface of 1. Therefore, non-uniform solidification is alleviated on the roll exit side, and the solidification delay s9 does not occur, so that variation in plate thickness can be prevented. Therefore, even in high-purity steel with low carbon concentration and no equiaxed crystal structure,
Promoting the nucleation of equicyclic heavy structures causes equiaxed crystal solidification,
This ends with preventing plate thickness fluctuations.

However, high-purity steel with a low carbon concentration reduces the degree of superheating of molten steel by i.
It is not possible to achieve equiaxed crystallization by simply rolling the steel.As a result of various experiments, we have investigated means to enable equiaxed crystallization, and have found that the degree of superheating of molten steel with a carbon concentration of 0.005% or more, which is poured between twin rolls, is The nucleation of equiaxed crystals is promoted, and the carbon concentration is reduced by continuously supplying cold steel material with a carbon concentration of 1° or less to the molten steel between the rolls, without clogging the nozzle. We found that equiaxed loading is possible even in low-purity steel.

FIG. 1 shows an example of a method in which a thin plate as a cold steel material 11 is continuously introduced into a melt #43 between rolls. The thin plate passes through an idle roll 12 interlocked with a cooling roll 1 and is continuously charged into molten steel 3 between the rolls.

In this H method, the charging speed of the cold steel material 11 is determined by the casting speed, so the amount of cold steel material 11 introduced is adjusted by changing the thickness of the thin plate. That is, cold steel material 1 to molten steel 3
1 locally lowers the temperature of the molten steel 3, which increases the frequency of formation of equiaxed nuclei 10 in that area. Therefore, even when the plate thickness becomes thinner and the growth rate of the solidification shelf exhibiting a columnar crystal structure increases, or when the carbon concentration becomes lower and equiaxed loading becomes difficult, it is necessary to increase the input amount of the cold steel material 11. The equiaxed weighting can be controlled freely.

The cold steel material does not need to be specially cooled, and it is sufficient to use one at room temperature, and linear or strip-shaped steel materials are convenient for continuous supply.

As shown above, according to the present invention, the carbon concentration is o, o
Equicyclic island structures can be obtained even in steels with a hardness of less than os%,
This makes it possible to cast high-purity steel sheets with good surface properties and no variation in thickness by the twin-roll method.

(Example) Next, a specific operation example is shown above, component C: 0,0
03%, Si: 0.04%, Mn: 0.20%, P
: 0.02%, S: 0.02%, AI: 0.05%,
Molten steel 2 with T i: o, 015%, N b: o, 017%
00 kg was fed to a twin-roll continuous casting machine with a roll diameter of 600 mm, and slabs with a plate width of 600 cm and a plate thickness of 21 mm were produced at an S casting speed of 50 mm/s.
m1nr was cast. The method shown in Figure 1 was used to introduce the cold steel material into the molten steel between the rolls.

The casting conditions of Examples and Comparative Examples are shown in Table 1, and the thickness distribution of the resulting slabs is shown in Figure PLS3. It was measured at

Table 1 In Example 1, a cold steel material with a plate thickness of 0.25+m was continuously introduced into molten steel, and the degree of superheating of the molten steel was set to 1 when pouring between rolls.
It was cast at 8℃, and the thickness of the slab was 15
As shown in FIG. 3(a), there was no variation in plate thickness and the surface quality was good. In addition, in Example 2, a cold steel material with a plate thickness of 0.35III was continuously poured into molten steel, and the molten steel was superheated to 10°C when poured between rolls. Approximately 38% isocycle island structure is generated in the central part of the thickness.
As shown in Figure tJi3 (b), there was no variation in plate thickness at all, and the surface quality was good.

In Comparative Example 1, a cold steel material with a plate thickness of 0.25vm was continuously introduced into molten steel, and the degree of superheating of the molten steel was set to 3 when pouring the molten steel between rolls.
It was cast at 0°C. That is, in Comparative Example 1, only the degree of superheating when pouring between the rolls was higher than in Example 1, and the other casting conditions were the same. However, as shown in Figure 1'53 (c), the slab obtained here had severe variations in plate thickness.

In addition, all of the coagulated fibers were composed of columnar crystals M, and non-uniform coagulation was observed in the It deposition. Comparative example 2 did not include cold steel material,
The molten steel was cast with a superheat degree of 5°C when poured between rolls. The degree of superheating in this case was the minimum degree of superheating that allowed casting, so the nozzle was blocked and casting was abandoned midway. Even though the degree of superheating was kept as low as possible, no equiaxed crystal fibers were formed in the obtained slab, and as shown in Figure 3 (d), the plate thickness was severe. Changes were occurring.

(Effects of the Invention) As described above, according to the present invention, high-grade steel with a carbon concentration of 0.005% or less, which could not be cast by the twin roll method, is thickened by continuous injection of cold steel material and control of the degree of superheating. You will be able to build II without any fluctuation. Therefore, according to the present invention, it is possible to stably cast high-purity steel thin plates with good surface properties without plate thickness fluctuations by the twin-roll method, and it is also possible to cast high-purity steel slabs directly to the cold rolling process without any maintenance. Therefore, conventional continuous casting
Significant cost advantages can be expected compared to manufacturing processes that involve hot rolling. Furthermore, the steel types that can be applied to the twin roll process have been expanded, and the ability to cast steel with a carbon concentration of 0.005% or less, which is expected to increase in production in the future, will be a significant improvement in production and operation.

[Brief explanation of drawings]

Fig. 1 shows an example of the present invention, Fig. 2 shows that the isocyclic structure alleviates uneven solidification shell growth, and Fig. 3 shows the plate thickness distribution of the example and comparative example. The figure shown in FIG. 4 is a diagram showing continuous casting by the twin roll method. DESCRIPTION OF SYMBOLS 1... Cooling roll, 2... Hot water pool, 3... Molten steel, 4... Nozzle, 5... Molten steel container, 6... Slab, 7... Solidified shell, 8... ... void, 9... solidification delayed part, 10... core of isocycle island, 11... cold steel material, 12
...〃Idrol.

Claims (1)

[Claims] (1) When casting high-purity steel with a carbon concentration of 0.005% or less using the twin roll method, the degree of superheating of the molten steel poured between the rolls is reduced to the extent that nozzle clogging does not occur, and the carbon concentration is 0. A method for continuous casting of molten steel using a twin roll method, characterized in that cold steel material of 0.005% or less is continuously supplied into molten steel between rolls.