JPS6352988B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a curved continuous casting method for obtaining slabs without internal cracks, surface horizontal cracks, or corner cracks when continuously casting molten steel. Regarding cooling conditions. (Conventional technology) 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 by continuous casting of molten steel, 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 produced by this continuous casting process has increased significantly. There is. The continuous casting process is superior to the conventional agglomeration and blooming rolling process in that it has a higher yield and consumes less energy. 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, the slab can be cast at room temperature. Compared to the process of heating and rolling a piece, it is advantageous in terms of energy and cost. In order to make it possible for slabs obtained by continuous casting to be directly supplied to the rolling process while still at high temperatures, the slab surface must be free of cracks, etc. In other words, it is necessary to take care such as removing surface scratches. It is necessary to obtain slabs of excellent quality. 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 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) To enable high productivity through high-speed casting. (2) Enables a process in which continuously cast slabs are directly rolled in a rolling process, or a so-called hot charge process in which continuously cast slabs are charged into a heating furnace for rolling while still at high temperature. , reducing or eliminating heating energy for rolling. (3) To produce high-quality slabs that enable the direct rolling process or hot charge 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, conventional curved continuous casting machines have been used to straighten slabs with unsolidified parts, to slowly cool the slabs pulled from the mold, and to form slabs with unsolidified parts. The method of operation was to straighten the steel (bending back the curved material) and then reheat it. (Problems to be Solved by the Invention) This prior art has the following problems. (1) By straightening the slab while avoiding the embrittlement zone of steel that exists at 750 to 850℃, it is possible to prevent defects such as surface cracks from occurring, thereby eliminating the need to clean the slab and Although it is possible to produce cast slabs, bulging is likely to occur, which causes
This leads to the occurrence of internal cracks and worsening of center segregation. (2) For this reason, in the current operation, we have improved the powder for continuous casting, and improved the casting speed and slab cooling strength.
There is no need to clean the surface of the slab, and the operation is carried out within a range where internal cracks and center segregation are below the allowable limit. 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, the spacing in the slab traveling direction of the rolls that support and guide the slabs is reduced (reducing the roll pitch). ), measures are being taken to reduce the height of continuous casting machines (lower head) to lower the static pressure of molten steel and to suppress the increase in bulging. However, even with such technical measures, the current technical problems in the continuous steel casting process described in items (1) to (5) mentioned above cannot be sufficiently solved. In other words, the limit of the so-called finer roll pitch, which is to reduce the distance in the slab traveling direction between the rolls that support and guide the slab, is to shorten the roll pitch to 300 mm. It cannot be lowered to a level that does not cause internal cracks in the pieces. On the other hand, making the roll pitch finer has the disadvantage of increasing setup 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 correction to straighten the slab from a curved state becomes large, leading to internal cracks. In order to prevent internal cracks that occur during the straightening process of bending a curved slab back straight, we are currently adjusting the slab temperature so that the total strain ε _T shown in equation (1) below is 0.40% or less. The roll pitch l and radius of curvature R 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 ε _n = as a constant.
Calculated as 0.05%. ε _u = (D/2-S) (1/R _i -1/R _i+1 )×100...
...(2) D: Thickness of slab S: Thickness of solidified shell of slab R _i : i-th radius of curvature R _i+1 : i+1-th radius of curvature ε _b = 1600δ _B・S/l ² ... ...(3) l: Roll pitch δ _B : Bulging amount a _O = 1.45×10 ⁸ exp (-74000/1.986・T _M ) α _O : Shape factor P: Molten steel static pressure V: Casting speed [m/min] T _M = T _L +1490/2+273 T _M : Tensile stress T _L indicates the average temperature of the cross section of the solidified shell of the slab on the side where tensile stress is generated in absolute temperature: Surface temperature of the slab on the side where tensile stress occurs [℃] ε _n =C _n・δ・S/l ² ………( 4) C _n : Misalignment coefficient δ: Misalignment amount The above-mentioned total strain ε _T is calculated as follows for various curvature radii R: slab thickness: 250 mm, casting speed: V = 1.5 m/
min, slow cooling operation (solidification coefficient: K=25m/√
In the case of operation under
As shown in the figure. 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. Using a continuous casting machine with an initial radius of curvature R = 10.5 m, which was designed based on this technical idea, the above-mentioned operating conditions, slab thickness: 250 mm, and casting speed: 1.5 m/min were used.
When continuous casting of molten steel was carried out with a solidification coefficient K of 25 m/√, the following results were obtained. In the case of low carbon steel with C≦0.12%, cracks on the inner and outer surfaces do not occur at all. In the case of medium carbon steel with C≧0.13%, internal cracks occur frequently. Continuous casting machines with R = 10.5 m are designed to prevent internal cracks from occurring even when casting medium carbon steel with C≧0.13% using compression casting (called CPC operation). . On the other hand, it is known that correction strain in a slab can be alleviated by devising a cooling method for the slab. 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,
In other words, by lowering the temperature of the solidified shell on the side that produces tensile stress lower than the temperature of the solidified shell on the lower surface (curved outside) of the slab, that is, on the side that produces compressive stress, the strength of the solidified shell on the upper surface side is increased, and as a result, the strength of the solidified shell on the upper surface side is increased. It is disclosed that the amount of tensile strain of the upper solidified shell is reduced to prevent internal cracks caused by unbending and straightening. By adopting this method of cooling the slab, the upper surface (inside) of the curved slab is cooled relatively more strongly than the lower surface (outside), and the mechanically neutral axis of the slab during straightening is aligned with the cross section of the slab. The geometric center axis is moved to the inside of the curve, thereby preventing internal cracking of the slab, and the appropriate temperature range for the slab is: Inside the slab: 700-900℃ Outside: 1000℃ It is disclosed that the temperature does not exceed . However, even with these techniques, it is still not sufficient to solve the technical problems in items (1) to (4) mentioned above. Even with a critical strain of 0.40%, C≧0.13
This is because internal cracking occurs during continuous casting of medium carbon steel. This invention has an initial radius of curvature of more than 6m and a machine height of 6.5m.
The purpose of this project was to create a continuous casting technology for super continuous casting machines that would prevent internal cracks from forming in slabs, even when continuously casting steel types that are highly susceptible to cracking, such as medium carbon steel. (Means for solving the problem) The characteristic feature is the process of straightening a curved slab with an unsolidified phase using a continuous casting machine with an initial radius of curvature of over 6 m and a machine height of over 6.5 m. has,
In a continuous casting method where the casting speed is 1.5 m/min or more, when bending (straightening) a curved slab to straighten it, the surface temperature of the slab on the side where tensile stress is generated is T _L [°C]. Similarly, when the surface temperature of the slab on the side where compressive stress is generated is T _F [°C], and the surface temperature of the short side of the slab cross section is T _S [°C], the initial radius of curvature in the continuous casting device is R [ m], −14.3R+1043≧T _L ≧700 −14.3R+1143≧T _F ≧(1+a)T _L −b T _S ≧900 and T _S ≧T _L where, a=−0.0125R+0.205 b=−5.13 Continuous casting was performed under conditions that satisfied R + 81.25, and the initial radius of curvature was over 6 m, and the machine height was 6.5 m.
A continuous casting method with a casting speed of 1.5 m/min or more, which includes the process of straightening a curved slab having an unsolidified phase using a continuous casting device of more than 1.5 m/min, which straightens the curved slab Let T _L [°C] be the surface temperature of the slab surface on the side that produces tensile stress when bending it back (straightening) to make it straight, and let T _F [°C] be the surface temperature of the slab surface on the side that produces compressive stress. When the surface temperature of the short side in one cross section is T _S [℃], and the initial radius of curvature in the continuous casting device is R [m], -14.3R + 1043≧T _L ≧700 -14.3R + 1143≧T _F ≧ (1 + a) Continuous casting is performed under conditions that satisfy the following conditions: T _L −b 900>T _S ≧T _L , where a=−0.0188R+0.438 b=−1.2R+152. (Function) The present invention will be explained in detail below. As a result of research on the behavior of strain caused by unbending and straightening of curved slabs, the inventors found that unbending and straightening of curved slabs can be done in a geometrical manner as shown by the broken line in Figure 2, as previously thought. Rather than following the target profile, as shown by the solid line in Figure 2,
It was discovered that this was done locally and concentrated at the position of the support and guide rolls. As a result, it became clear that the bending straightening strain of the slab was two to three times greater than the level previously thought. Therefore, it was found that the total strain ε _T due to straightening of the slab should be expressed by the following formula. ε _T = αε _u + ε _b + ε _n ………(5) Here, α = 2.0 to 3.0 This comprehensive strain concept was discovered for the first time by the inventors, and this phenomenon is This occurs during casting when straightening force, pulling force, etc. are applied to the slab in a state where it has an unsolidified part inside, a high temperature, and a thin solidified shell. This phenomenon becomes more pronounced as the solidified shell becomes thinner, the solidified shell strength becomes lower, and the geometric distortion becomes larger. Through the elucidation of these phenomena, C≧0.13
In high-speed casting of medium carbon steel with 0.4%, cracks occurred in the slab even at the conventionally accepted critical strain of 0.40%, which is the result of corrective strain acting on α times the geometric strain ε _u . It became clear. That is, this is because (α-1) times as much extra corrective strain is concentrated. On the other hand, the inventors, in cooling the slab,
When straightening a slab by creating a temperature difference between the upper and lower surfaces of the slab, it was found that the effect of relaxing the straightening strain also changes depending on the solidified shell strength on the short side of the slab cross section.
In other words, when the temperature of the solidified shell on the short side of the slab cross section is 900°C or higher, the effect of straightening strain relaxation when bending and straightening by creating a temperature difference between the upper and lower surfaces of the slab becomes large;
Below ℃, a large temperature difference must be created between the upper and lower surfaces of the slab. When the solidified shell temperature T _S on the short side of the slab cross section is lower than the solidified shell temperature T _L on the upper surface of the slab,
The effect of creating a temperature difference on the lower surface to alleviate orthodontic strain is lost. Elucidation of the above-mentioned phenomenon was obtained through theoretical studies and experiments by the inventors. In summary, when moving the mechanically neutral axis and mitigating correction strain through continuous casting, which involves cooling with a temperature difference between the upper and lower surfaces of the slab, the relaxation due to the temperature difference between the upper and lower surfaces of the slab is at least ( α−1) ε _u is required, and therefore the amount of strain that must be alleviated is
According to equation (2), it largely depends on the radius of curvature of the slab and the solidified shell thickness during continuous casting. Based on the above conclusion, the concentration coefficient α of correction strain is set to 2, the slab thickness is 250 mm, and the casting speed V = 1.5 m/
min, solidification coefficient K = 25 m/√ under slow cooling operation conditions, the total strain ε _T according to the above equation (5) is 0.40
% or less, Figures 3 and 4 show
As shown in the figure. Here, (a) the lower limit value of the solidified shell temperature T _L on the top surface of the slab is the slab temperature required when rolling the slab obtained by continuous casting in the direct rolling process; The upper limits of the solidified shell temperature T _L on the upper surface of one side and the solidified shell temperature T _F on the lower surface are determined from the viewpoint of preventing internal cracks from occurring due to bulging of the slab between the rolls, and on the straightening band side, when R = 10 m, the upper limit values are 1000 It needs to be below ℃. The appropriate range that does not cause internal cracks as shown in Figures 3 and 4 mentioned above can be expressed mathematically (casting speed V
= 1.5m/min), if T _S ≧900℃ and T _S ≧T _L , −14.3R+1043≧T _L ≧700 ………(6) −14.3R+1143≧T _F ≧(1+a)T _L − b………(7) Here, a=−0.0125R+0.205 b=−5.13R+81.25 If 900＞T _S ≧T _L , −14.3R+1043≧T _L ≧700℃……(8) − 14.3R+1143≧T _F ≧(1+a)T _L −b……(9) Here, a=−0.0188R+0.438 b=−1.2R+152 Note that T _F is completely different with T _S =900℃ as the boundary The reason determined by the formula will be explained below.
T _F exhibits continuous physical properties, but in the case of steel, the temperature around this corresponds to the A ₃ transformation temperature, and the high temperature strength changes significantly. If this change appears synergistically with the short side strength (strongly dependent on T _S ) and the surface strength on the side that generates tensile stress (strongly dependent on T _L ), rather than being a continuous phenomenon, it appears that the values appear to be very different at first glance. As a result, the effects become clearly visible, resulting in different forms of expressions. Here, the solidified shell thickness S[ _{ε u}
=(D/2-S)(1/R _i -1/R _i+1 )] and the relationship between casting speed V is as follows. Here, K: Solidification coefficient L: Distance from the meniscus V: Casting speed. Now, if L is the bending and straightening start position, the solidified shell thickness S is uniquely determined as a function of the casting speed V. When the solidified shell thickness S changes, the unbending straightening strain ε _u changes, so the amount of strain to be alleviated (α-1) ε _u also changes, which in turn changes the required temperature difference ΔT between the top and bottom surfaces of the slab. do. In this way, the required temperature difference ΔT between the upper and lower surfaces of the slab also changes depending on the casting speed. (Example) An example of a continuous steel casting method to which the method of the present invention is applied will be described below. Casting conditions Continuous casting machine: initial radius of curvature...10.5m, machine height...10.8m,
4 point correction. Steel type: Central Al-K Slab size: 250 mm thick x 1050 mm width Casting speed: 1.6 m/min Tables 1 and 5 show the temperature conditions and internal cracking results of the slabs of Examples of the present invention.

[Table] In Figure 5, solid line b has an initial radius of curvature of 10.5.
In the case of m, this is the limit line when T _S ≧900 and T _S ≧T _L , and the broken line a is the limit line when T _L ≦T _S <900°C. Examples of the present invention when the short side temperature T _S is T _L ≦T _S <900°C are indicated by △, comparative examples are indicated by ▲, and when T _S ≧900 and T _S ≧T _L Examples of the present invention are indicated by ◯, and comparative examples are indicated by ●. In Examples of the present invention, △ and ○, no internal cracks occurred, but in Comparative Examples, ▲ and ●, internal cracks occurred. (Effects of the Invention) As is clear from the above results, according to the configuration of the present invention, the relationships among the solidified shell temperature of the short side of the slab cross section, the temperature difference between the upper and lower surfaces of the slab, and the initial radius of curvature R of the slab are satisfied. If casting is carried out using the same method, no internal cracks will occur, and no surface cracks will occur. This invention is constructed and operated as described above; (1) productivity is increased by making high-speed casting possible; and (2) continuously cast slabs are directly cast in the rolling process. (3) Since the continuous casting machine can be made compact by lowering the machine height, equipment and buildings can be made compact. It has remarkable effects such as reducing equipment costs, making the equipment easier to maintain, and (4) stabilizing casting operations because troubles such as internal cracks do not occur.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the initial radius of curvature and the total strain ε _T for each level of the number of straightening points, and FIG. 2 is a schematic diagram showing the concentration phenomenon of straightening strain in the bending and straightening of slabs discovered by the inventors. Figure 3 shows the relationship between the slab top surface temperature T _L and the required temperature difference between the top and bottom surfaces of the slab in the region where the surface temperature T _S of the solidified shell on the short side of the slab cross section is T _S ≧900 and T _S ≧ T _L. Figure 4 shows the required temperature difference between the top surface temperature T _L of the slab _and the top and bottom surfaces of the slab in the temperature range where the surface temperature T _S of the solidified shell on the short side of the slab cross section is T _L ≤ T S <900℃. FIG. 5 is a diagram showing the results of an example of the present invention.

Claims (1)

[Claims] 1. A continuous casting device with an initial radius of curvature of more than 6 m and a machine height of more than 6.5 m, which includes the process of straightening a curved slab having an unsolidified phase, with a casting speed of 1.5 m/min.
It is a continuous casting method that is more than min, and bends back (straightens) a curved slab to make it straight.
When the slab surface temperature on the side where tensile stress occurs is
Similarly, when T _L [℃] is the surface temperature of the slab on the side where compressive stress occurs, and T _F [℃] is the surface temperature of _the short side of the cross section of the slab, then The initial radius of curvature is R [m]
As, −14.3R+1043≧T _L ≧700 −14.3R+1143≧T _F ≧(1+a)T _L −b T _S ≧900 and T _S ≧T _L where a=−0.0125R+0.205 b=−5.13R+81. 25. A continuous casting method for steel, characterized in that continuous casting is carried out under conditions that satisfy the following conditions. 2 A continuous casting machine with an initial radius of curvature of over 6 m and a machine height of over 6.5 m is used to straighten a curved slab with an unsolidified phase, and the casting speed is 1.5 m/min.
It is a continuous casting method that is more than min, and bends back (straightens) a curved slab to make it straight.
When the slab surface temperature on the side where tensile stress occurs is
Similarly, when T _L [℃] is the surface temperature of the slab on the side where compressive stress occurs, and T _F [℃] is the surface temperature of _the short side of the cross section of the slab, then The initial radius of curvature is R [m]
As, −14.3R+1043≧T _L ≧700 −14.3R+1143≧T _F ≧(1+a)T _L −b 900>T _S ≧T _L where a=−0.0188R+0.438 b=−1.2R+152 The following conditions are satisfied. 1. A continuous casting method for steel, characterized in that continuous casting is carried out under such conditions.