JPS6076260A - Continuous casting method of steel - Google Patents

Abstract

PURPOSE:To cast continuously a billet having high quality without defects such as internal creck or the like at a high speed by specifying the respective surface temp. on the side of the billet where tensile stress is generated in the stage of straightening, on the side where compressive stress is generated and on short sides in the relation with the initial radius of curvature. CONSTITUTION:The billet temp. on the side where tensile stress is generated when a curved ingot having an initial radius R[m] of curvature is unbent so as to be straightened is designated as TL, the billet surface temp. on the side where compressive stress is similarly generated is designated as TF and the surface temp. on the short sides in the billet section is designated as TS in the straightening stage of the curved billet having an unsolidified phase by a continuous casting method at >=1.5m/min casting speed. The steel is cast continuously in the state satisfying the conditions -14.3R+1,043>=TL>=700, -14.3R+1,143>=TF>= (1+a) TL-b, TS>=900 where DELTAT=TF-TL, a=-0.0125R+0.205, b=-5.13R+ 81.25.

Description

Detailed Description of the Invention (Industrial Application Field) This invention provides a curved mold for obtaining slabs free of internal cracks, transverse surface cracks, and corner cracks when continuously casting molten steel. Regarding the continuous casting method, especially regarding the cooling conditions of the slab, in recent years continuous casting technology has been developed to obtain slabs by completely continuous casting of molten metal, and even in the steel industry, it is now possible to pour molten steel into a mold to produce a slab. Instead of the process of obtaining slabs by blooming and rolling the molten copper, a continuous casting process has been adopted in which molten copper is continuously cast to directly obtain slabs (@slabs). The continuous casting process, which has been increasing its production ratio significantly, is superior to the conventional ingot-blowing process in terms of higher yield and lower energy consumption. (Prior technology) The slabs obtained by this continuous casting process.

It retains a large amount of sensible heat, and if it is supplied to the rolling process in the form of a high-temperature slab without dissipating this sensible heat,
This method is advantageous in terms of energy and cost compared to the process of heating and rolling slabs at room temperature.

In order to make it possible to directly supply the slab obtained by continuous casting to the rolling process while keeping it at a high temperature, the slab must have no cracks or other defects on its surface, in other words, it does not require any maintenance such as surface flaw removal. 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, but especially surface cracks, corner cracks, etc., after cooling the slab to room temperature. It must be free of surface imperfections that require careful 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) High-speed LA construction makes it possible to achieve high productivity.

(2) If possible, use the so-called hot charging process, in which continuously cast slabs are fully rolled in a direct rolling process, or continuously cast slabs are charged into a heating furnace for high-temperature rolling. Total reduction or omission of heating energy for tightening and rolling.

(3) To produce high-quality slabs that can be subjected to a direct rolling process or a hot charging process for continuously cast slabs.

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

(5) The process allows for stable seedling production.

In order to solve all of these technical problems, conventional curved continuous casting machines have been used to: ■ Straighten the slab with unsolidified parts 5 ■ Cool the slab pulled out of the mold slowly to the state where it has unsolidified parts The cast slab, which was completely straightened and curved, is completely bent back, and then reheated.

This type of operation was adopted.

Such conventional technology has the following problems: (By straightening the slab while avoiding the embrittlement region of steel that exists in 11750 to 850C, the occurrence of defects such as surface cracks is prevented; Although this makes it possible to manufacture single-height mixed slabs by eliminating the need for maintenance of defects in the slabs, lugs are likely 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 to improve the casting speed, slab cooling strength, eliminate the need to clean the slab surface, and allow for internal cracking and center segregation. We are operating within a range that is below the limit. Therefore, productivity decreases.

On the other hand, in order to make the slow-cooled unsolidified steel more stable, it is possible to suppress the lugging that is a problem in obtaining high-quality slabs.

■ Reduce the distance between the rolls that support and guide the slab in the slab advancing direction (fine roll pitch).

■ Lower the height of the continuous casting machine (lower head) to lower the molten copper static pressure and suppress the increase in melting.

Things are being implemented.

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 distance between the rolls that support and guide one slab in the slab traveling direction is reduced, and the roll pitch is reduced to 3001111! Is there a limit to shortening in trenches and the size of bulging that occurs in slabs? ,
It cannot be lowered to a level that does not cause internal cracks in the slab. On the other hand, the finer roll pitch also has the disadvantage of increasing equipment costs.

In addition, lowering the height of the continuous casting machine, lowering the height of the continuous casting machine, and lowering the head will result in a large curvature of the progress trajectory of the slab, and the correction strain in straightening the slab from a curved state to straightening will become large. In order to completely prevent internal cracks that occur during the straightening process of bending a curved slab back to straight, there is a problem of internal cracks. The radius of curvature R of the roll pilaf 1i corresponding to the slab temperature is determined so as to be .40% or less, and a continuous casting machine is designed based on this.

That is, εT−ε. ＋εb＋ε□ ・・・・・・・・・・・・・・・
(1) where εT: total strain εU: corrective strain εb kneeling strain εm: misalignment strain, usually εm-0 as a constant
,05%.

D: Thickness of the slab S: Solidified shell thickness of the slab ■, i: Radius of curvature of the (2)th R4-4-1: i+1th ll11 eaves 16O0δ
B, S 6b E River・・・・・・・・・Fal + 2 t: 0-Lupitute δB=/molding amount 1-986, TM α0: Shape factor P: Clean can static pressure ■2 Casting speed Cm /峨] Ts+1490 T, = +273 Ts: Surface temperature of slab δ, S ε□=Om, ...................................... (4) 2 On+= Misalignment coefficient δ: Above the amount of misalignment For the total strain εTk and various radii of curvature R described in
Slab thickness: 250isb Casting speed: V=1.5m/m
, slow cooling operation (solidification coefficient: K = 2 s m /fj
7) When operating under the following conditions, the conditions shown in Figure 1 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 entire radius of curvature is determined so that surface strain is evenly distributed.

(3) 5 (4) The roll pitch represented by the continuous straightening profile shall be a dense arrangement based on divided rolls.

Initial radius of curvature 1 designed based on this technical idea
Using a continuous casting machine of 1 = 10.5 m and R = 3 m, the molten steel was Continuous casting was performed and the results were as follows.

■ In the case of a low carbon state with a 'E of 0.12%, no cracking occurs on the inner or outer surfaces.

■ In the case of medium carbon steel of C20, 13 grade, internal cracks occur frequently.In a continuous casting machine of 5 R = 10.5 m, compression casting (called OPO operation) etc. Even when casting medium carbon steel, care has been taken to prevent internal cracks from occurring.

However, in a low-head continuous casting machine with an initial radius of curvature of 3 m, the straightening band length is short, and when the curved slab is bent back straight, it acts on the inner surface of the curved slab. One driving force generation band that is sufficient to generate the compressive force necessary to completely suppress cracking caused by tension cannot be obtained.In addition, the static pressure of molten steel required to generate the full compressive force is low; It cannot provide sufficient corrective strain relaxation.

For this reason, in the case of a low-head continuous casting machine, internal cracks occur completely during continuous casting of C20.13% medium carbon steel, making high-speed casting impossible.

On the other hand, it is known that the normal strain of a slab can be alleviated by devising a cooling method for the slab.
-25434, JP-A-50-10252.6, JP-A-50-1025.27, JP-A-52-52126, and JP-A-55-5115 disclose curved slabs. When straightening the bend back to straight, the temperature of the solidified shell on the top surface (curved inside), that is, the side that produces tensile stress, is lower than the temperature of the solidified shell on the bottom surface (curved outside), that is, the side that produces compressive stress. In order to increase the strength of the upper surface side solidified shell and reduce the tensile strain of the upper surface side solidified shell due to the bending and straightening, it is possible to prevent internal cracks caused by the bending and straightening. Disclosed.

By completely exposing the slab cooling method, the upper surface (inside) of the curved slab is relatively strongly cooled than the lower surface (outside), and the mechanically neutral axis of the slab during straightening is The appropriate temperature range for the slab is as follows: 00 to 900C on the inside of the slab, 00 to 900C on the outside of the slab. : The temperature does not exceed 1000℃.

is disclosed.

However, even with these techniques, the above-mentioned problems still occur. This is not sufficient to solve the technical problems in items (1) to (5). Even with a critical strain of 0.40%, C20.1
This is because continuous casting of 3% medium carbon steel causes internal cracks.

(Object of the invention) This invention provides continuous casting machines with any radius of curvature.
Continuous casting (
This was developed with the aim of obtaining a continuous casting technology that does not generate internal cracks in the slab even when the slab is in the same state. A continuous casting method with a casting speed of 1.5 m/''n or more, which involves the process of When the temperature is TL, the surface temperature of the slab surface on the side where the compressive stress sound occurs is TF, and the surface temperature of the short side in the cross section of the slab is Ts, the initial radius of curvature in the continuous casting machine is -14 (Ram). , 3 liters + 1043〉TL≧700-14.3R+
1143≧TF(-TL+△T)Ta2900 Here, ΔT=aTj-b a = 0.0125R+0.205 b =-5,13R-! −81,25 Continuous casting was performed under conditions that satisfied the following conditions.

A continuous casting method with a casting speed of 1.5 m/wh or higher, which includes the process of straightening a curved slab having an unsolidified phase, when bending back (straightening) the curved slab to make it straight. When the surface temperature of the slab on the side where tensile stress occurs is TL, the surface temperature of the slab on the side where compressive stress is generated is TF, and the surface warmed gold 'rs on the short side in the cross section of the slab, then continuous The initial radius of curvature k (RCm) in the casting device is -14,3FL+1043≧TL≧700214
．． 3 R+ 1143≧'I'F C= TL + Δ
T ) 900) Ts≧TL where T = aTL - b a = -0,0188R, +0.438b = -1
, , 2R+152 Continuous casting is performed under conditions that satisfy the following conditions.

The present invention will be described in detail below.As a result of research into the behavior of strain associated with the unbending and straightening of curved slabs, the inventors have discovered that the unbending and straightening of curved slabs is not as simple as previously thought. , which is not carried out along a geometrical profile as shown by the dashed line in Fig. 2, but is carried out locally concentrated at the position of the support guide roll, as shown by the real roots in Fig. 2. I discovered that it was. As a result, it became clear that the bending straightening strain of the slab was two to three times greater than the level conventionally thought.

Therefore, it has been found that the total strain εT of the slab due to straightening can be expressed by the following formula.

εT = αεU+ εb+ ε□ ・・・・・・・・・
(5) Here, α = 2.0 to 3.0 This comprehensive type concept was discovered for the first time by the inventors, and this phenomenon occurs in the continuous casting of molten steel.
This occurs when straightening force, pulling force, etc. are applied to the slab in a state where it has an unsolidified part inside and a thin solidified shell at high temperature.

This phenomenon occurs as the thinner the solidified shell and the lower the solidified shell strength,
Moreover, the larger the geometric distortion, the more conspicuous it is.

Through the elucidation of these phenomena, 0≧0.13%
In high-speed casting of medium carbon steel, cracks occurred in slabs even at a critical strain of 0.40%, which was previously said to be the result of corrective strain acting on α times geometric strain ε1. became clear. In other words, this is a sign that an extra concentration of corrective distortion of (α-1) times has occurred.

On the other hand, the five inventors et al. established that when cooling a slab, there is a temperature difference between the upper and lower surfaces of the slab, and when straightening the slab,
It was revealed that the effect of relieving correction strain also changes depending on the cross section of the fishing piece and the strength of the solidified shell on the short side.

That is, when the solidified shell temperature of the short side of the slab cross section is 900C or higher, the effect of straightening strain relaxation when bending and straightening by creating a difference in salinity between the upper and lower surfaces of the slab is large;
There must be a large difference in salinity between the top and bottom surfaces of the slab.

When the solidified shell temperature T8 of the short side of the slab cross section is lower than the solidified shell temperature TL of the upper surface of the slab, the effect of creating a temperature difference between the upper and lower surfaces of the slab to alleviate correction strain is lost.

The above-mentioned current numbers can be elucidated by continuous casting, which cools the upper and lower surfaces of the slab to create a difference in salinity, according to the findings obtained through theoretical studies and experiments by the inventors. When moving the mechanical neutral axis to alleviate the correction strain, ■ the humidity due to the humidity difference between the upper and lower surfaces of the slab is at least (α-1)ε. A size equivalent to that is required.

■ Therefore, the amount of strain that must be alleviated largely depends on the radius of curvature of the slab during continuous casting and the thickness of the solidified shell.

Based on the above conclusion, the concentration coefficient α of corrective strain is set to 2, the slab thickness is 250+u+, and the casting speed V=1. ．． 57FZ
/ -, assuming the salinity conditions under which the solidification coefficient K is 0.40% or less under the slow cooling operating conditions where K = 25 m1fj, the results are as shown in Figures 3 and 4. be.

Here, (a) The lower limit value of the solidified shell temperature TL of the upper surface of the slab is the slab hot water temperature required when the slab obtained by continuous casting is directly rolled in a rolling process.

(b) The upper limits of the solidified shell temperature TL on the upper surface of the slab and the solidified shell temperature TF 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 Teijin side, when R = 3 m. , 1100° C. or less When R=1'Om, it is necessary to make it 1000 tl or less.

Converting the matters shown in Figures 3 and 4 above into a mathematical formula, (casting speed - 1.5ml =) TEI≧900
In the case of −℃, −14,3a+ 1043≧TL≧700 −・・・−
・-+61-14.3Ft+1143≧TF (-TL
+ M'r) ・” (7) Here, △T=aT1−b ・・・・・・・・・・・・・・・・
... (8) a = -0,0125R+0.205 b = -5,13R, +81.25 When 900>Ts>TL, -14,3R+ 1043≧TL≧70'O℃...
...(9) 14-3 R+ 1143≧TF(-TL
+△T) ・・・00Here, ΔT=aTL−b ・・・・・・・・・・・・・・・・・・
・・Ren a −−0,0188EL+(1,438b=
−1,21”j+152 Here, the solidified shell thickness S[εu−(
])/2-8) (1/ Rl-1,/J + i
)) and the casting speed ■: Here, K: Solidification coefficient L: Distance from the meniscus V: Casting speed.

Now, when L'ff is the bending 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 bending straightening strain ε changes.

Since this changes, the amount of strain (α-1) ε0 to be relaxed also changes, which in turn changes the required temperature difference ΔT at the bottom surface of the slab. In this way, the required temperature difference ΔT between the upper and lower surfaces of the slab varies depending on the casting speed.

(Example) An example of a continuous casting method for steel to which the method of the present invention is applied will be described below.Example 1 Casting conditions Continuous casting machine: Initial radius of curvature (member: 3 m).

Machine height = 3.5m 15 points kX correction, axis: medium coal At-8i- slab size 250mm thickness x 1500 fin width casting speed 21
．． 5 m/wh Water injection ratio 20.3 to 0.5 t/9 The results of the example of the present invention and the results of the comparative example are shown in FIG. In Fig. 5, the solid line (b) indicates the initial radius of curvature 3
In the case of m, it is the limit line when 'rs≧900°C, and broken nJi (al is the limit line when TL < Ts (900'C).

When the short side temperature 'rs is TL≦Ts (900°C, the implementation flI of the present invention is indicated by Δ, and the comparative example is indicated by M.
The example of the present invention when rs≧900C is marked as a comparative example fO
△ and ○ indicate that there was no internal crack.

Example 2 Casting conditions Continuous casting machine: initial radius of curvature...10.5 m, machine height...
10.8 m, 4-point correction.

Silver Germany: Center AA- Slab size: 25o thick x 1050U width Casting speed: 1.
．． 6m/pongee Figure 6 shows the results of the example of the present invention and the results of the comparative example. In Figure 6, fruit! b has an initial radius of curvature of 1'0
．． This is the limit line when TS≧900°C in the case of 5 m, and the broken line a is the limit line when TL≦Ts (900°C).

When the short side temperature Ts is TL≦Ts (the example of the present invention at 900°C is indicated by △, and the comparative example is indicated by M, j), T
When s≧900℃, the example of the present invention is marked with ○, and the comparative example is marked with e.
There were no internal cracks in the reeds, ∆, and ○ shown by .

(Effects of the Invention) From the above results, it is clear that according to the configuration of the present invention, the temperature of the solidified shell on the short side of the slab cross section, and the temperature on the slab top.

If the relationship between the bottom surface temperature difference and the initial radius of curvature of the slab is satisfied and casting is performed, internal cracks will not occur.
Furthermore, no surface cracks occur.

This invention is constructed and operated as described above, and therefore (1) improves productivity by making high-speed casting possible, and (2) directly rolls continuously cast slabs in the rolling process. Therefore, the material heating energy for rolling can be reduced or omitted.

(3) The continuous casting machine can be made compact by lowering the height of the machine, making the equipment more compact and reducing the total equipment cost.

Furthermore, the equipment can be easily maintained.

(4) Casting operations are stable because troubles such as internal cracks do not occur.

etc., have a remarkable effect.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the initial radius of curvature and total strain εT for each level of the number of straightening points. Figure 2 shows the concentration phenomenon of positive strain during straightening of cast slabs, which was discovered by the inventors. The schematic diagram shown in FIG. 3 is a diagram showing the relationship between the slab upper surface temperature TL and the required temperature difference between the upper and lower surfaces of the slab in a region where the surface temperature Ts of the solidified shell on the short side of the slab cross section is 182900 ° C. Figure 4 shows that the surface temperature 'rs of the solidified shell on the short side of the slab cross section is %
A diagram showing the relationship between the upper surface temperature TL of the slab and the required bulk difference between the upper and lower surfaces of the slab in the temperature range of TT, S TE+ < 900 C. FIG. 5 and FIG. 6 are diagrams showing the results of an example of the present invention. Agent Patent Attorney Masaaki Aki Sawa Koretsu 2 Narrow 77#n Curved Mustard Diameter *(m+ 21'1 Figure Piece 2 Figure R Surface Resistance ■5 (°C) Back 4 Figure Top 6 & fn51nK n (-c) 1 , Indication of the case Special application Shokaku - No. 183304 No. 2, Chomei's name in the form of Ji: A Gen & A method 3, Person making the amendment Relationship to the case Applicant's address (residence) Otemachi, Chiyoda-ku, Tokyo 2-6 No. 3 Name (Name) (6G5) "d" Nippon Steel Corporation 4, Agent Residence: Taiyo Hill Rakushogi 17-5, 12-1 Nihonbashi Kabuto-cho, Chuo-ku, Tokyo tlne, !rI, 1ilflil date Showa year, month, day (shipment) 6 Number of inventions increased by the amendment None 7. Subject of the amendment Meizushi 1. (Claims, details of the invention IyLIIl explanation) 8. Contents of the amendment Attached sheet Contents of Amendment 1 The claims of the present patent application are amended as follows: r (1) The casting speed is 1.5 gr. / ma
In the above-mentioned continuous tricking method, when bending (returning) a curved fishing piece back to straighten it,
Let the surface temperature of the slab on the side where tensile stress is generated be TL, and similarly, let the surface temperature of the slab on the side where compressive stress is generated be TF). When the initial radius of curvature in the continuous casting device is ㇠ [m] old, -14,3 几 + 10432 TL≧ 700, where a = -0,0125R + 0.205b = -5,
13R+81.25 A method for continuous casting of kabura, characterized in that continuous casting is carried out under conditions satisfying the following conditions. (2) A continuous casting method in which the casting speed is 1.5 m/ax or more, which includes the process of straightening a curved slab having an unsolidified phase, and in which the curved slab is straightened. When bending back (straightening) as much as possible, let TL be the surface roughness of the slab on the side that produces tensile strength, and let Tp be the surface roughness of the slab on the side that produces compressive stress at 1k. When the surface temperature of the side is TS, the initial radius of curvature at R [m] is -14.3 + 1043≧TL''700 - 14.3 +
1143≧TF≧(1+a)TL−b Here, a = −0,0188㇠10.438b=−1゜2㇠+152 The feature is that continuous casting is performed under conditions that satisfy the following condition. Steel reaming and casting methods. J(2) r -14,3R11143≧ stated in the following location in the specification
TF (-TL+△T) J is 4-143L+
Correct as 1143≧Ty≧(1+a) TL-b J. Page 14; line 3, line 1 from the bottom; page 19; line 2, line 9. (3) lΔT-a'f' described in the following part of the specification
L-b” and line 11.

Claims (2)

[Claims] (1) A continuous casting method in which the process of straightening a curved slab having an unsolidified phase is performed at a casting speed of 1.5 m/m or more, which involves bending back the curved slab to make it straight (straightening). When you do
Let the surface flow rate of the slab on the side where tensile stress is generated be TL, similarly, let the surface flow rate of the slab on the side where compressive stress is generated be TF, and let the surface humidity of the short side in the cross section of Ω)5 be f 'l''s. When, the initial radius of curvature in the continuous casting device is R[7711, -14,3R+1.043≧TL≧700−14.3R
+1143≧Tp(-TL+△T)Ts≧900 Here. ΔT=aTL-b a =-0,01258+0.205 b =-5,13R-1-81,25 A continuous casting method for steel, characterized in that continuous casting is performed under conditions that satisfy the following condition. (2) ``A continuous casting method with a casting speed of 1.5 m/'' or higher, which includes the process of straightening a curved slab having an unsolidified phase, and which involves bending the curved slab back to straighten it. When correcting
Continuous casting: When the surface flow rate of the slab on the side where tensile stress occurs is TL, the surface flow rate of the slab surface on the side where compressive stress is generated is TF, and the surface salinity of the short side in the cross section of the slab is Ts. Initial radius of curvature in the device 'e R[, r+]
As, −14,3R+ 1043≧TL≧700−14.3
R+ 1143≧TF(=Tj+ΔT)900)Ts≧
A continuous casting method for steel, characterized in that continuous casting is carried out under conditions that satisfy the following conditions: ΔT=aTL-ba=-0,0188B+0.438b=-1,2R+152,