JPS619952A - Continuous casting method of steel - Google Patents

Abstract

PURPOSE:To obtain a billet having no internal crack, surface transverse crack and corner crack by specifying the top and bottom surface temp. of the billet in a straightening zone. CONSTITUTION:The top surface temp. of the billet in the straightening region is kept at >=900 deg.C in continuous casting of a molten steel having the process of straightening the curved billet having an unsolidified phase with a multi-point straightening and curving type continuous casting device having 5m machine height. The bottom surface temp. of the billet is maintained higher by >=150 deg.C than the top surface temp. of the billet.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention provides a curved continuous casting method for obtaining slabs free of internal cracks, surface transverse cracks, and corner cracks when continuously casting molten steel to obtain slabs. This article relates to casting methods, particularly to cooling conditions for slabs.

(Prior art) In recent years, continuous casting technology has developed to obtain slabs by continuously casting molten metal, and even in the steel industry, molten steel is poured into molds to obtain steel ingots, which are then bloomed and rolled. Instead of the process of obtaining slabs, a continuous casting process in which molten steel is continuously cast to directly obtain slabs (steel slabs) has been adopted, and the proportion of steel slabs manufactured by this continuous casting process has been increasing significantly. Continuous casting process is compared to traditional ingot-blurring process,
It is superior in that it has a high yield and low energy consumption.

The slab obtained by this continuous casting process has a large amount of sensible heat, and if it is supplied to the rolling process in the hot slab state without dissipating this sensible heat, it can be Compared to the process of heating and rolling a piece, it requires less energy and
It is advantageous in terms of cost.

In order to enable the slab obtained by continuous casting to be directly supplied to the rolling process while still at high temperature, the surface of the slab must be free of cracks, etc., in other words, no maintenance such as surface flaw removal is required. It is necessary to obtain slabs of excellent quality. High-quality slabs refer to slabs with center segregation, internal cracks, surface flaws, and inclusions, especially those with horizontal surface cracks, corner cracks, etc. after cooling the slab to room temperature. ,
It must be free of surface imperfections that require care to detect and remove defects.

The current technical issues in the continuous steel casting process, including the points mentioned above, are as follows.

(1) High-speed casting enables high productivity.

(2) Enable the so-called hot charge process, in which continuously cast slabs are directly rolled in a rolling process, or continuously cast slabs are charged into a heating furnace for rolling while still at high temperatures. , reducing or eliminating heating energy for rolling.

(3) To produce high-quality slabs that enable the direct rolling process or hot charging process of continuously cast slabs.

(4) A continuous casting machine with low equipment cost and easy maintenance.

(5) The process must be capable of stable operation.

In order to solve these technical problems, conventionally, cast slabs with unsolidified parts are straightened using curved continuous casting machines.

(2) The slab pulled out of the mold is slowly cooled, the slab is straightened with unsolidified portions, and the curved slab is bent back), and then reheated. This type of operation was adopted.

This conventional technology has the following problems.

(1) By avoiding the embrittlement region of steel that exists at 750 to 900°C as shown in Figure 2 and straightening the slab, it is possible to prevent defects such as surface cracks, thereby preventing defects on the slab. Eliminate maintenance,
Although it is possible to produce high-temperature slabs, bulging is likely to occur, which leads to internal cracking and worsening of center segregation. The above figure 2 shows the 67th year of Tetsu to Hagane (198
1) Quoted from No. 8, page 1180.

(2) For this reason, in the current operation, the powder for continuous casting has been improved to improve the casting speed, slab cooling strength, eliminate the need for cleaning the slab surface, and limit internal cracking and center segregation. We operate within the following scope. Therefore, productivity decreases.

On the other hand, in order to further stabilize the slow cooling and unsolidified operation and to suppress bulging, which is a problem in obtaining high-quality slabs, we aim to reduce the spacing in the slab traveling direction of the rolls that support and guide the slabs (roll pitch). refinement). ■ Lowering the height of the continuous casting machine (lower head) to lower the static pressure of molten steel and suppressing the increase in bulging is being implemented.However, even with such technical means, the above-mentioned
The current technical problems in the continuous steel casting process of items (1) to (5) cannot be satisfactorily solved.

That is, the distance between the rolls that support and guide the slab in the slab advancing direction is reduced. The so-called finer roll pitch is
The limit is to shorten the roll pitch to 300111111, and it is not possible to reduce the size of bulging that occurs in the slab to a level that does not cause internal cracks in the slab. On the other hand, finer roll pitches also have the disadvantage of increasing equipment costs.

In addition, lowering the height of the continuous casting machine, so-called low head, causes the curved curvature of the progress trajectory of the slab to become a dog, and the correction strain during bending correction to straighten the slab from a curved state becomes large, leading to internal cracks. There is a problem. In order to prevent internal cracks that occur during the straightening process of bending this curved slab back straight, the total strain εT, shown in equation (1) below, is currently kept at 0.40 inches or less. In this way, the roll pitch t and radius of curvature R corresponding to the slab temperature are determined, and a continuous casting machine is designed based on these.

That is, εT2ε. 1εb 1ε□ ・・・・・・・・・・・・
(1) Here, εT, total strain εU: correction strain εb: bulging strain εlTl: misalignment strain, usually as a constant ε□
= 0.05%.

D: Thickness of slab S: Solidified shell thickness of #4 piece R1: i-th radius of curvature R4+l: i+1st radius of curvature L+ t: roll pitch δB: bulging amount ao: shape factor P: molten steel static pressure ■ : Casting speed Cm/accuracy] T8+1490 TM= +273T8: Surface temperature of slab Cm: Misalignment coefficient δ: Misalignment amount The overall type εT described above is calculated for various curvature radii R, slab thickness, 250 Figure 6 shows the case of operation under slow cooling operation (solidification coefficient: 25 m/f; casting speed: V = 1.5 m).

The conditions at this time are as follows. (1) The trajectory of the slab shall be a multi-point straightening profile. (2) For strain distribution in multi-point correction, the radius of curvature is determined so as to evenly distribute surface strain. (3) Represented by continuous orthodontic profile.
(4) The roll pitch shall be a dense arrangement based on divided rolls.

Initial radius of curvature R designed based on this technical idea =
Using a continuous casting machine of 10.5 m and R = 3 m,
The above operating conditions, slab thickness: 250 m, casting speed: 1,
Continuous casting of molten steel was performed at a solidification coefficient of 577+/m and a solidification coefficient of 25 m/f;-, and the following results were obtained.

■ In the case of low carbon steel with C<0.12%, no cracking occurs on the inner or outer surfaces.

■ In the case of medium carbon steel with C>0.13%, internal cracks occur frequently. In a continuous casting machine with R=10.5m, C>0.1 is achieved by compression casting (called CPC operation), etc.
Even when casting from 3% medium carbon steel, care has been taken to prevent internal cracks from occurring.

However, in a low-head continuous casting machine where the initial radius of curvature R is 3 m, the length of the straightening band is short (due to the tension acting on the inner surface of the curved slab when it is bent back straight). There is not enough driving force generation band to generate the compressive force necessary to suppress the cracks that occur.In addition, the static pressure of molten steel required to generate the compressive force is low, so sufficient straightening is not possible. cannot bring about strain relaxation.

In the case of a low-head continuous casting machine, C>0. Internal cracks occur during continuous casting of x3% medium carbon steel, making high-speed casting impossible.

On the other hand, it is known that the straightening of the slab can be alleviated by devising a cooling method for the slab. That is, Japanese Patent Application Publication No. 1973
JP-A-25434, JP-A-50-102526, JP-A-50-102527, JP-A-52-52], JP-A-55-5115, and JP-A-55-5115, the curved slabs are straightened. When straightening the bend, the temperature of the solidified shell on the upper surface of the N frame (on the inside of the curve), that is, on the side that produces tensile stress, must be lower than the temperature of the solidified shell on the lower surface of the slab (on the outside of the curve), that is, on the side that produces compressive stress. discloses that the strength of the upper solidified shell is increased to reduce the amount of tensile strain of the upper solidified shell due to unbending straightening, thereby preventing internal cracks caused by unbending straightening. has been done.

By adopting this method of cooling the slab, the upper surface (inner side) of the curved slab is relatively strongly cooled than the lower surface (outside), and the mechanically neutral axis of the slab during straightening is It is moved to the inside of the curve rather than the geometric center axis of the cross section, thereby preventing internal cracking of the slab. ■ The appropriate temperature range for the slab is 00 to 900℃ near the inside of the slab;
Outside of slab: Temperature not exceeding 1000°C. is disclosed.

However, even with these techniques, it is still not sufficient to solve the technical problems of items (1) to (5) mentioned above.

The embrittlement temperature range of steel is 70 at all parts of the slab cross section.
In order to avoid temperatures of 0 to 900°C and supply high-quality slabs with no surface cracks or internal cracks to the rolling line, for example at temperatures above 1100°C, cooling of the short sides of the slab cross section can be omitted. This is because a process that does not cause surface cracks or internal cracks on the short sides has not been established.

(Problems to be Solved by the Invention) This invention provides a multi-point straightening curvature with a machine height of 5 m or less that can suppress short-side shear strain, which is a factor that causes surface cracks and internal cracks on the short sides of a slab cross section, to a low level. It was made for the purpose of obtaining a type of continuous casting process.

(Means for Solving the Problems) A feature of the present invention is that a curved slab having an unsolidified phase is produced by a multi-point straightening curved continuous casting machine with a machine height of 5 m or less.
Continuous casting method for molten steel that includes straightening process (
・The surface temperature of the upper surface (L side) of the slab in the straightening zone is set to 900°C or higher, and the surface temperature of the lower surface (F side) of the slab is set to 150° C. or higher than the surface temperature of the upper surface (L side) of the slab. ℃
There is a continuous casting method that maintains higher than above.

The present invention will be explained in detail below.

In order to avoid surface cracks and internal cracks in the slab when bending (straightening) a slab with a large curvature that is cast using a curved continuous casting machine with a low machine height, As already mentioned, the temperature range of 700 to 900°C, which is the embrittlement temperature range of steel shown in FIG. 2, must be avoided.

On the other hand, without dissipating the sensible heat possessed by molten steel,
In order to reduce or eliminate heating energy when rolling the slab into steel, the slab output from the continuous casting process must be as hot as possible.

However, the temperature of the corners and short sides of the cross-section of the slab is most likely to drop, and this area is often used during the straightening process of the slab.
- In order to avoid the embrittlement temperature range of 700 to 900° C., it is necessary to omit cooling of the short sides and to prevent surface cracks or internal cracks from occurring on the short sides. The place is
If cooling of the short side of the slab cross section is omitted and the short side surface temperature is maintained at a temperature exceeding the temperature range of 700 to 900°C,
Easy to cause misalignment and deformation on the short side.

According to the findings of the inventors, when straightening a slab with an extremely large curvature, the solidified shells on the upper and lower long sides of the slab's cross section correct the misalignment in the slab's advancing direction. Correction is progressing as I wake up. Therefore, the displacement deformation on the short side of the cross section of the slab is (long side strength) / (short side strength)
This occurs when the ratio of

When misalignment occurs on the short side of the cross section of the slab, not only does the corrective strain deviate from the geometrical strain, but also defects such as surface cracks and internal cracks occur on the short side. If the straightening strain deviates from the geometrical strain, it becomes impossible to plan the strain distribution when straightening a curved slab to straighten it, and defects such as internal cracks occur due to concentration of strain.

On the other hand, in order to prevent cracks from occurring inside or on the surface of the slab, it is necessary to alleviate the strain caused by straightening. The inventors obtained the knowledge shown in FIG. 5 as a result of many experiments.

That is, the surface temperature TL of the upper surface side (L side) and the lower surface side (F side) of the slab are
Surface) The thicker the difference in surface temperature TF, TF - Tt =ΔT, the smaller the strain relaxation rate β. This strain relaxation rate β is defined as follows.

Therefore, the smaller the strain relaxation rate, the more the strain is relaxed. FIG. 5 shows an example of a slab size of 250 mm thick x 1050 mm wide.

According to the experience of the inventors, the strain relaxation rate β should be 08 or less. Therefore, ΔT may be 150° C. or higher.

On the other hand, the inventors have discovered that there is an appropriate cooling pattern for the slab in order to prevent concentration of strain on the short sides of the cross section of the slab. This is shown in Figure 3.

The inventors set the surface temperature T of the short side of the slab cross section to 1100°C on the assumption that ΔT = Tp TL 〉150°C.
℃, and the surface temperature TL of the upper long side (L side) of the slab cross section in the straightening zone is 750℃ (cooling B), and 950℃.
℃ (cooling A), the shear strain occurring on the short side of the cross section was measured for two levels.

As a result, when TL is set to 750°C, significant concentration of shear strain occurs in the first stage of the straightening zone, but when TL is set to 950°C, the shear strain on the short side of the slab cross section is reduced to 750°C. It has been reduced to 1/3 of the level when expressed as °C.

The changes in the upper long side surface temperature TL, lower long side surface temperature TF, and short side surface temperature Ts of the slab cross section in the slab drawing direction at this time are shown in Figure 4(a) (cooling pattern A) and 4°
This is shown in Figure (b) (cooling pattern B).

In summary, by allowing the slab to pass through the straightening zone while maintaining the surface temperature of each part of the slab cross section within the shaded area shown in Figure 1, concentration of shear strain on the short side of the slab cross section can be prevented. In addition, high-speed casting is possible using a continuous casting machine with a low machine height of 5 m or less.

That is, the surface temperature TI on the long side (L side) of the slab cross section is 9
00℃ or higher, and the lower long side (F surface) side surface temperature TF is 1300℃ or lower, which is the solidification range of steel, and ΔT=Tr
A straightening process is performed to bend the curved slab back to straight in a region that satisfies Tt, >150°C.

By doing this, surface cracks and internal cracks due to shear deformation will not occur in the slab drawing direction, and the straightening strain will not deviate from the geometric strain. Strain distribution can be accurately scheduled.

(Example) l) Slab cross-sectional dimensions 25011111 thickness 1050 m
Width 2) Casting speed V=1.7771/Temperature 3) Cooling pattern Pattern shown in Figure 4 (a) TL-950℃, TF
= 1.100℃, T8=1100℃ 4) Casting profile 15-point correction 5) Type medium coal At-8i killed steel 6) Vertical direction (height) dimension from meniscus to slab surface in horizontal part: 310011111 Above Molten steel was continuously cast according to the specifications. A high-quality high-temperature slab with no surface flaws or internal cracks was obtained.

For comparison, TL=750°C, TF=900°C, Ts
= 1.100°C, but otherwise cast under the same conditions as in the above example, the copper piece had many surface transverse cracks and internal cracks.

(Effects of the Invention) Since the present invention is configured and operated as described above, it is possible to produce high-quality hot slabs without surface cracks, internal cracks, or other defects with high productivity. can be supplied to the rolling line.

Therefore, heating energy for rolling can be reduced or omitted, resulting in great effects in terms of energy saving and productivity.

[Brief explanation of the drawing]

Figure 1 is a chart showing the surface temperature conditions of each part of the slab in the continuous steel casting process of the present invention, and Figure 2 is
Figure 3 is a chart showing the embrittlement temperature range of steel, and Figure 4 (a) is a chart showing the concentration of short side types of slabs by cooling and ξ turns.
is a diagram showing the cooling pattern A of the slab, Figure 4 (tb) is a diagram showing the cooling pattern B of the slab, and Figure 5 is the relationship between the difference ΔT between TL and TF and the strain relaxation rate of the slab. FIG. 6 is a chart showing the relationship between the initial radius of curvature and the total strain of the slab. Fig. 1 7F ('C) Temperature in the boiling embrittlement range (0C) Fig. 3 Roll number Female engine Fig. 4 Meniscar entry distance (7n) Fig. 5 7!1l-TF-Tt Figure 6 First jJA radius R (m) Procedural amendment (spontaneous) August 2, 1980 Commissioner of the Patent Office Manabu Shiga Indication of the case 1988 Patent Application No. 13091'7 2
4. Title of the invention Continuous casting method for steel 3. Person making the amendment Relationship to the case Patent applicant Location
2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (66)
5) Nippon Steel Stock Food Coloring Representative Takeshi 1) Yutaka 4th generation Masashi Address 3-3-3-5 Nihonbashi, Chuo-ku, Tokyo Date of amendment order Showa Year, month, day (shipment date) Increased by the 6th amendment Number of inventions (1) On page 11 of the specification, line 20, "Procene" is amended to "Process".

Claims (1)

[Claims] In a continuous casting method for molten steel that involves the process of straightening a curved slab having an unsolidified phase using a multi-point straightening curved continuous casting machine with a machine height of 5 m or less, the surface of the upper surface (L side) of the slab in the straightening zone. Continuous casting is carried out at a temperature of 900°C or higher, and the surface temperature of the lower surface (F side) of the slab is higher than the surface temperature of the upper surface (L side) of 150°C or more. Continuous casting method for steel.