JPH057107B2 - - Google Patents

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 been developed to continuously cast molten steel to obtain slabs, and even in the steel industry, molten steel is poured into molds to obtain steel ingots, which are then bloomed and cast. Instead of the process of obtaining slabs, a continuous casting process has been adopted in which molten steel is continuously cast to directly obtain slabs (steel slabs), and the proportion of steel slabs manufactured by this continuous casting process has increased significantly.

Continuous casting processes are superior to conventional ingot-blanking and rolling processes in that they have higher yields and lower energy consumption.

The slab obtained by this continuous casting process has a large amount of sensible heat, and if this sensible heat is not dissipated and the slab is supplied to the rolling process in the form of a hot slab, it is possible to Compared to the process of heating and rolling a piece, it is advantageous in terms of energy and cost.

The slab obtained by continuous casting is kept at high temperature,
In order to be able to supply the slab directly to the rolling process, it is necessary to obtain a slab of excellent quality that has no cracks on the surface of the slab, in other words, does not require maintenance such as removing surface scratches. . A slab of excellent quality is one that is free of center segregation, internal cracks, surface flaws, and inclusions, especially surface horizontal cracks, corner cracks, etc., after the slab is cooled to room temperature. It must be free of surface imperfections that would require maintenance to detect and remove defects.

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

(1) Making high productivity possible through high-speed casting. (2) A 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 temperature.
To enable a so-called hot charge process and to reduce or eliminate heating energy for rolling. (3) To produce high-quality slabs that enable continuous casting slabs to be directly rolled or hot-charged. (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, the cast slabs having unsolidified parts produced by conventional curved continuous casting machines are straightened. The slab pulled out of the mold is slowly cooled, the slab is straightened (curved is bent back) while it has unsolidified parts, and then it is reheated. This type of operation was adopted.

This conventional technique has the following problems.

(1)By avoiding the embrittlement zone of steel that exists at 750 to 900℃ and straightening the slab, it is possible to prevent the occurrence of defects such as surface cracks, thereby eliminating the need for maintenance of defects on the slab. ,
Although it makes it possible to produce high-temperature slabs, bulging tends to occur, which leads to the occurrence of internal cracks and worsening of center segregation.

(2) For this reason, in the current operation, the powder for continuous casting has been improved, and the casting speed and slab cooling strength are not required to clean the surface of the slab, and are below the allowable limits for internal cracking and center segregation. The company operates within the following range. 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, (1) the distance between the rolls that support and guide the slabs in the slab traveling direction should be reduced ( Refinement of roll pitch). (2) Lower the height of the continuous casting machine (lower head)
Therefore, efforts are being made to lower the static pressure of molten steel and suppress the increase in bulging.

However, even with such technical means, the current technical problems in the continuous steel casting process of items (1) to (5) mentioned above cannot be sufficiently solved.

In other words, the so-called refinement of the roll pitch, which reduces the interval in the slab traveling direction between the rolls that support and guide the slab, has a limit of shortening the roll pitch to 300 mm, and the size of bulging that occurs in the slab can be reduced by It cannot be lowered to a level that does not cause internal cracks in the slab. On the other hand, making the roll pitch finer has the disadvantage of increasing equipment costs.

In addition, lowering the height of the continuous casting machine, so-called low head, increases the curvature of the progress trajectory of the slab.
There is a problem in that the correction strain during bending straightening, which straightens the slab from a curved state, becomes large, leading to internal cracks. In order to prevent internal cracks from occurring during the straightening process of bending this curved slab back straight, the total strain ε _T is currently set at 0.40 as shown in equation (1) below.
% or less, the roll pitch l and radius of curvature R corresponding to the slab temperature are determined, and a continuous casting machine is designed based on these.

That is, ε _T = ε _u + ε _b + ε _n ...(1) where ε _T : Total strain ε _u : Correction strain ε _b : Bulging strain ε _n : Misalignment strain, usually a constant, ε _n
Calculated as = 0.05%.

ε _u = (D/2-S) (1/R _i -1/R _i+1 )×100...
(2) D: Thickness of the slab S: Thickness of the solidified shell of the slab R _i : Radius of curvature at the i+i-th straightening point R _i+1i : Radius of curvature at the first straightening point ε _b = 1600δ _B・S /l ² ...(3) l: Roll pitch δ _B : Bulging amount a _e = 1.45×10 ³ exp (-74000/1.986.T _M ) α _p : Shape factor P : Molten steel static pressure V : Casting speed [m/min] T _M = T _S +1490/2 + 273 T _S : Sculpture Surface temperature ε _n = C _n・δ・S/l ² ...(4) C _n : Misalignment coefficient δ : Misalignment amount The total strain ε _T mentioned above is calculated for various curvature radii R Piece thickness: 250mm, casting speed: V=1.5m/
min, slow cooling operation (solidification coefficient: K=25m/√)
Figure 4 shows the case of operation under the following conditions.

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

Based on this technical idea, a continuous casting machine with an initial radius of curvature R = 10.5 m and R = 3 m was designed under the above operating conditions, slab thickness: 250 mm, casting speed: 1.5 m/min, and solidification. Coefficient K=25m/√
Then, when continuous casting of molten steel was carried out, the following results were obtained. (1) In the case of low carbon steel with C0.12%, no cracking occurs on the inner or outer surfaces. (2)C0.13%
In the case of medium carbon steel, internal cracks occur frequently.

The continuous casting machine with R = 10.5m uses compression casting (called CPC operation) to ensure that internal cracks do not occur even when casting medium carbon steel with a carbon content of 0.13%. There is.

However, in the case of a low-head continuous casting machine with an initial radius of curvature R of 3 m, the length of the straightening band in which rolls are arranged to reduce the curvature of the curved slab is short, and the curved slab can be straightened. When the curved slab is bent back, the tension acting on the inner surface of the curved slab does not create a sufficient driving force generation band to generate the compressive force necessary to prevent cracks from occurring. In addition, since the static pressure of molten steel required to generate compressive force is low, sufficient straightening strain relaxation cannot be achieved.

For this reason, internal cracks occur in low-head continuous casting machines when continuously casting medium carbon steel with a carbon content of 0.13%, making high-speed casting impossible.

On the other hand, by devising a cooling method for slabs,
It is known to alleviate the straightening strain of slabs. That is, JP-A-50-25434, JP-A-50-102526,
JP-A-50-102527, JP-A-52-52126, and JP-A-55-5115 disclose that when straightening a curved slab by bending it back straight, By lowering the temperature of the solidified shell on the side that generates tensile stress to be lower than the temperature of the solidified shell on the lower surface of the slab (on the outside of the curve), that is, the side that generates compressive stress, the strength of the solidified shell on the upper surface side is increased and the bending is straightened. It has been disclosed that the amount of tensile strain of the upper surface side solidified shell accompanying this is reduced to prevent internal cracks caused by unbending and straightening.

By adopting this method of cooling the slab, (1) the upper surface (inside) of the curved slab is relatively strongly cooled than the lower surface (outside), and the mechanical neutral axis of the slab during straightening is The curved slab is moved toward the upper surface side (inward) of the geometric center axis of the cross section of the slab, thereby preventing internal cracking of the slab.

(2) The appropriate temperature range for the slab is 700 to 900℃ for the top surface (inside) of the slab, and a temperature that does not exceed 1000℃ for the bottom surface (outside) of the slab. is disclosed.

As mentioned above, in order to prevent internal cracking of slabs and to enable continuous casting under low molten steel static pressure (low machine height) of 5 m or less, the total strain ε _T
It is necessary to distribute it uniformly in the longitudinal direction of the slab.

(Problems to be Solved by the Invention) This invention aims to uniformly reduce the substantial strain at the solidification interface of the slab over the entire straightening zone during continuous casting of molten steel using a continuous casting machine with a low machine height of 5 m or less. This was done with the aim of providing a method for continuous casting of molten steel without causing cracks inside the slab.

(Means and effects for solving the problems) The present invention is characterized by 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. In the continuous casting method of molten steel _with ⁱ⁼¹ ε _i ε Later stage = 1/n _2o 〓 ⁱ⁼ⁿ⁺¹ ε _i When the number of correction points N is an odd number (N = 2n + 1) ε First stage = 1/n _o 〓 ⁱ⁼¹ ε _i ε Later stage = 1/ n _2o+1 〓 ⁱ⁼ⁿ⁺² ε _iIn addition, if the straightening strain on a certain roll is less than 1/10 times the maximum straightening strain, that roll is not considered as a straightening point and is included in N. I don't count.

ε _ui = (D/2-S) (1/R _i -1/R _i )×100 Here, ε _ui : Correction strain at the i-th correction point D: Thickness of slab [mm] S: i Solidified shell thickness at the ith straightening point [mm] R _i : Design radius of curvature of the section immediately before the i-th straightening point R _i : Straightening of the curved slab is performed using the design radius of curvature of the section immediately after the i-th straightening point This is a multi-point straightening method for slabs in a low machine height curved continuous casting process.

Here, to explain the meanings of the first stage and the second stage, the first stage of correction refers to the first half of the correction point. In other words, when the number of correction points is an even number (2n), 1
The correction points numbered .about.n are the first stage, and the correction points numbered (n+1) to (2n) are the second stage.

Also, when the number of correction points is an odd number (2n+1),
Except for the center point (n+1), correction points 1 to n are the front stage, and correction points (n+2) to (2n+1) are the rear stage.

This invention will be explained in detail below.

According to the inventors' knowledge, the total strain ε _T = ε _u + ε _b +
Of each strain that constitutes ε _n (ε _u : correction strain, ε _b : bulging strain, ε _n : misalignment strain),
The bulging strain ε _b and the misalignment strain ε _n depend on the hardware, and therefore can be treated as constant values during continuous casting operations.

The correction strain ε _u is the geometric strain ε _ui ε _ui = (D/2−S) (1/R _i −1/R _i )×100 (ε _ui : Correction strain (geometric strain) at the i-th correction point ) D: Thickness of the slab [mm] S: Solidified shell thickness at the i-th straightening point (shell thickness) [mm] R _i : Design radius of curvature of the section immediately before the i-th straightening point R _i : i-th straightening point Even if the rolls are set in the straightening zone so that the design radius of curvature in the section immediately after the straightening point is distributed uniformly in the longitudinal direction of the slab,
Not uniformly distributed in the longitudinal direction of the slab.

This is due to the abnormal behavior of strain caused by the solidified shell of the slab being wrapped around the rolls. This pattern is shown in FIGS. 1 and 2.

In FIG. 1, the locus of the slab shown by the dashed line is the locus of the slab during pure bending deformation, and the profile shown by the solid line shows the actual state of the slab. As is clear from this figure, when a slab with a liquid phase (unsolidified phase) inside is restrained, supported and guided by rolls, the slab moves linearly between the rolls and When they come into contact with each other, the solidified shells bend and wrap around each other. In the figure, a indicates a normal strand path, and b indicates a strand path during winding.

As a result, the correction strain ε _u of the slab becomes an abnormally large value at or near the roll position, as shown in FIG. 2 (solidification interface strain distribution). This becomes a factor that causes internal cracks in the slab. In the figure εu#12,
εu#21 is i=12, 21, i.e., the 12th, 21
The correction distortion in the th roll is shown.

The ratio between the actual strain εui~ and the geometric strain _εui at the solidification interface of the slab is defined as the strain concentration coefficient α. That is, α=εui˜/ε _ui .

α changes depending on the solidified shell thickness of the slab, roll pitch, slab surface temperature, etc.

The inventors have clarified from the results of many experiments that there are relationships between α and various parameters as shown in FIGS. 3a to 3e. As a result, it became clear that the straightening strain ε _u of the slab was distributed in the longitudinal direction of the slab as shown in FIG. 3b.

That is, as is clear from Fig. 3b, when the surface temperature of the slab is 700°C, the concentration coefficient α of straightening strain in the slab at the final stage of the straightening zone (roll #23) is the same as that at the first stage of the straightening zone (roll #11). ) is now 2/5 of that in Japan. When the surface temperature of the slab is 800℃, the situation is slightly different, and the final stage α is about 5/9 of the first stage α.

Figure 3 a shows the shell thickness (casting speed) characteristics at the #11 roll position, b shows the roll position characteristics, c shows the roll pitch characteristics, d shows the slab surface temperature characteristics at the #11 roll position, and e shows the shell curvature at the # roll position. It is a radius characteristic.

Based on this knowledge, the inventors have completed a casting process that uniformly distributes the actual strain at the solidification interface of the slab and obtains a high-quality high-temperature slab without internal cracks.

By setting the roll arrangement profile through which the slab passes, that is, the distribution of the geometric strain of the slab to ε front stage ε rear stage 3×ε front stage, the actual correction strain at the solidification interface of the slab can be adjusted to the length of the slab. This made it possible to distribute the particles uniformly in the hand direction.

(Example) A curved continuous casting machine with a radius of curvature of the first arc of 3 mR and a machine height of 3.2 to 3.3 m, with a cross-sectional size of 250 mm thick x 1000 mm.
A wide slab was cast.

Casting specifications (common to both profiles) (1) Steel type: Medium coal, aluminum, silicon, killed steel (2) Casting speed: 1.7 m/min (3) Water injection ratio: 0.8/Kg Casting profile (1) Invention profile A Number of correction points: 15 points ε Second stage (average correction strain of second stage) = 1.2ε First stage (average correction strain of first stage) B Number of correction points: 15 points ε Second stage = 1.5ε First stage C Number of correction points: 15 points ε Second stage = 2.0ε First stage D Number of correction points : 15 points ε Second stage = 2.8ε First stage (2) Comparison profile E Number of correction points: 15 points ε Second stage = 0.9ε First stage F Number of correction points: 15 points ε Second stage = 3.2ε First stage As a result of casting, the following results regarding internal cracks were obtained. Ta.

Invention profile: A, B, C, D internal crack frequency 0 pieces/m Comparison profile: E internal crack frequency 12 pieces/m F internal crack frequency 8 pieces/m (Effects of the invention) As described above, this invention has the following advantages: Since it is configured and made to act, internal cracks due to concentration of correction strain will not occur in the slab.
Since high-quality hot slabs can be supplied to the rolling line with high productivity, great effects can be achieved, such as reducing or omitting heating energy for rolling.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the phenomenon of the solidified slab shell wrapping around the roll, Fig. 2 is a diagram showing the abnormal phenomenon of straightening strain caused by the wrapping of the solidified slab shell around the roll, and Fig. 3 a ~e is a chart showing the relationship between the strain concentration coefficient α and various parameters, and FIG. 4 is a chart showing the relationship between the total strain ε _T and the initial radius of curvature.

Claims (1)

[Claims] 1. A method for continuous casting of molten steel, which includes the process of straightening a curved slab having an unsolidified phase using a multi-point straightening curved continuous casting device with a machine height of 5 m or less,
The strain distribution at the slab solidification interface in the straightening zone is as follows: ε first stage ε second stage 3・ε first stage However, when the number of straightening points N is an even number (N=2n) ε first stage=1/n _o 〓 ⁱ⁼¹ ε _i ε second stage=1 /n _2o 〓 ⁱ⁼ⁿ⁺¹ ε When the number of _i correction points N is an odd number (N=2n+1) ε First stage=1/n _o 〓 ⁱ⁼¹ ε _i ε Second stage=1/n _2o+1 〓 ^{i=n+ 2} ε _i Note that if the straightening strain on a certain roll is less than 1/10 times the maximum straightening strain, that roll is not considered as a straightening point and is not included in N. ε _ui = (D/2-S) (1/R _i -1/R _i )×100 Here, ε _ui : Correction strain at the i-th correction point D: Thickness of slab [mm] S: i Solidified shell thickness at the ith straightening point [mm] R _i : Design radius of curvature of the section immediately before the i-th straightening point R _i : Straightening of the curved slab is performed using the design radius of curvature of the section immediately after the i-th straightening point A multi-point straightening method for cast slabs.