JP2008073748A - Method for detecting longitudinal cracking based on thermal flux of mold, and continuous casting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for detecting longitudinal cracking capable of sharply detecting the longitudinal cracking of a mold. <P>SOLUTION: Regarding the wide face side inner wall faces AA of a mold, in a high thermal flux face region A as a face region with a meniscus distance M [mm] of 20 to 40, at the prescribed face regions a<SB>1</SB>, a<SB>2</SB>... taken at one spacing p [mm] among 10, 20, 30, 40, 50 to the width direction of the mold, the average thermal flux q [W/m<SP>2</SP>] is obtained, respectively. In a plurality of the arranged face regions a<SB>1</SB>, a<SB>2</SB>..., with the area regions a<SB>k</SB>to a<SB>k+n-1</SB>of the adjoining n regions as a pair, and, per pair, the standard deviations σ<SB>k to k+n-1</SB>[W/m<SP>2</SP>] of the average thermal flux q [W/m<SP>2</SP>] of a plurality of the face regions a<SB>k</SB>to a<SB>k+n-1</SB>belonging to the pair are obtained. The maximum standard deviation σ<SB>max</SB>[W/m<SP>2</SP>] among the obtained standard deviations σ<SB>k to k+n-1</SB>[W/m<SP>2</SP>] and a prescribed value σ<SB>0</SB>[W/m<SP>2</SP>] are compared, thus longitudinal cracking is detected. In this case, n is suitably fixed in accordance with the size of the spacing p [mm]. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a vertical detection method for detecting the occurrence of so-called vertical split by monitoring the state of a mold, and a technique for preventing this vertical split.

As this type of technology, Patent Document 1 discloses a surface flaw detection method in continuous casting. This is because “a plurality of temperature detection ends 3 are embedded” (paragraph number 0024) in the mold wall, and “temperature measurement is performed” (paragraph number 0024) at each temperature detection end. It is determined that surface wrinkles are generated when the standard deviation σ ₀ is 1.2 times or more when no surface wrinkles are generated ”(paragraph 0027).

JP 2003-10950 A

  However, according to the above-mentioned patent document 1, it will not be easy to obtain accurate thermal characteristics of the mold.

  That is, the temperature at each temperature detection end varies depending on the material of the mold, the thickness of the copper plate of the mold, the shape of the mold cooling water slit, the flow rate of the cooling water flowing through the mold cooling water slit, the temperature of the cooling water, etc. . In addition, the thickness of the copper plate of the mold and the shape of the slit may change due to repair or replacement of the mold. As long as such changes in operating conditions cannot be taken into account, it will be very difficult to accurately obtain the thermal characteristics of the mold by the method described in Patent Document 1.

  The present invention has been made in view of these points, and its main purpose is to detect the accurate thermal characteristics of the mold and to detect the vertical split of the slab sensitively based on the thermal characteristics. And a continuous casting method that is technically related to the vertical split detection method and that can prevent vertical splitting of a slab.

Means and effects for solving the problems

  The problems to be solved by the present invention are as described above. Next, means for solving the problems and the effects thereof will be described.

According to the first aspect of the present invention, the vertical detection is performed by the following method. That is, a high heat flux surface as a surface region having a meniscus distance M [mm] of 20 to 40 of the wide inner surface AA of the casting mold for continuous casting for cooling molten steel to form a solidified shell having a predetermined shape In the region A, the average heat flux q [W in a predetermined surface region a ₁ · a ₂ ... Taken at a spacing p [mm] of any one of 10, 30, 20, 40, 50 in the width direction of the mold. / m ² ] respectively. Among the plurality of surface regions a ₁ , a ₂ ... Arranged in parallel, adjacent surface regions a _{k to} a _{k + n−1} of n regions are taken as one set, and each set belongs to the plurality determination of the surface area a _k ~a _{k + n-1} of the standard deviation _σ k~k _{+ n-1} of the average heat flux ^{q [W / m 2] [} W / m 2]. Among the obtained standard deviations σ _{k to k + n-1} [W / m ² ], a maximum standard deviation σ _max [W / m ² ] and a predetermined value σ _o [W / m ² ] The vertical division is detected by comparison. However, when (a) the interval p [mm] is 10 or 20, the n is any one of 3 to 7, and (b) when the interval p [mm] is 30, the n Is any one of 3 to 5, (c) when the interval p [mm] is 40, n is 3 or 4, (d) when the interval p [mm] is 50, The n is 3.

  According to this, the vertical division of the slab can be detected sensitively.

Moreover, according to the 2nd viewpoint of this invention, continuous casting is performed by the following methods. That is, a high heat flux surface as a surface region having a meniscus distance M [mm] of 20 to 40 of the wide inner surface AA of the casting mold for continuous casting for cooling molten steel to form a solidified shell having a predetermined shape In the region A, the average heat flux q [W in a predetermined surface region a ₁ · a ₂ ... Taken at a spacing p [mm] of any one of 10, 30, 20, 40, 50 in the width direction of the mold. / m ² ] respectively. Among the plurality of surface regions a ₁ , a ₂ ... Arranged in parallel, adjacent surface regions a _{k to} a _{k + n−1} of n regions are taken as one set, and each set belongs to the plurality determination of the surface area a _k ~a _{k + n-1} of the standard deviation _σ k~k _{+ n-1} of the average heat flux ^{q [W / m 2] [} W / m 2]. Among the obtained standard deviations σ _{k to k + n-1} [W / m ² ], a maximum standard deviation σ _max [W / m ² ] and a predetermined value σ _o [W / m ² ] In comparison, the casting speed Vc [m / min] is reduced so that the _maximum standard deviation σ _max [W / m ² ] does not exceed the predetermined value σ _o [W / m ² ]. However, when (a) the interval p [mm] is 10 or 20, the n is any one of 3 to 7, and (b) when the interval p [mm] is 30, the n Is any one of 3 to 5, (c) when the interval p [mm] is 40, n is 3 or 4, (d) when the interval p [mm] is 50, The n is 3.

  According to this, the vertical division of the slab can be suppressed.

<Definition of terms>
“Menniscus distance M”: Means a distance that starts from the meniscus (molten steel surface) in the mold and is considered along the casting path.
“Vertical Split”: A crack with a length of 100 mm or more in the casting direction on the wide surface of the slab. The measurement of the length in the casting direction of the vertical division is, for example, by calipers or visual observation.
“Casting part”: A slab obtained by cutting a continuously produced slab into a predetermined length (for example, 3 m) in the casting direction. And count.

The vertical split detection method and continuous casting method according to the present invention are intended for the following operating conditions, for example.
-Carbon content C [wt%] of the slab: 0.08 to 0.18
・ Casting speed Vc [m / min]: 0.7-2.5
-Mold width (much the width of the slab's wide surface: mold width at the top of the mold) [mm]: 1000-2110
-Mold thickness (much the width of the narrow surface of the slab: mold thickness at the top of the mold) [mm]: 230-280
-Mold height [mm]: 900

  Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

  First, a continuous casting facility for continuously casting a slab (so-called slab or the like) will be outlined with reference to FIG. That is, this continuous casting equipment is a mold 1 for cooling molten steel to form a solidified shell of a predetermined shape, and a figure for temporarily storing molten steel in order to pour molten steel at a predetermined flow rate into the mold 1 A plurality of roll pairs 4, 4... (Indicated by a two-dot chain line in the figure) arranged in line with a predetermined casting path with the lower end of the mold 1 as a base point; It is composed of

  With this configuration, the molten steel poured from the tundish (not shown) into the mold 1 is cooled by the mold 1 to form a solidified shell having a predetermined shape, and this solidified shell is introduced into the casting path by appropriate means. It is conveyed while being pinched by roll pairs 4, 4... And eventually becomes a solidified slab until it reaches the inside. And the slab produced continuously is cut | disconnected by predetermined length in the casting direction, and is managed for every slab part.

<1, Detailed description of mold>
Next, the structure of the mold 1 will be described in detail with reference to the drawing. That is, in this embodiment, the mold 1 is composed of a copper plate 2 that directly contacts the molten steel poured into the mold 1 and a jacket 3 that is formed so as to surround the copper plate 2. Yes.

  The copper plate 2 is formed in a cylindrical shape having a rectangular cross section, has a predetermined thickness (see RR in FIG. 7), and extends a predetermined length (= mold height) along the casting direction. To do. A plurality of groove-like recesses 2d, 2d,... Along the casting direction are provided in the outer peripheral surface of the copper plate 2.

  The jacket 3 is made of, for example, SUS, and supports / fixes the copper plate 2 described above. As described above, the jacket 3 is formed so as to surround the copper plate 2. Therefore, the plurality of recesses 2d, 2d,. Cooling water flow paths 2d, 2d,... For flowing cooling water for cooling are formed. Each of the cooling water flow paths 2d, 2d,... Has a rectangular cross section in the present embodiment. In the present embodiment, the cooling water flow paths 2d, 2d, ... are formed only on the wide surfaces 2a, 2a side of the mold 1, but in addition to this, on the narrow surface side of the mold 1. Also possible is a configuration that is formed.

<2. Detailed explanation of average heat flux q>
Next, the heat flux on the wide surfaces 2a and 2a of the mold 1 during continuous casting will be described with reference to FIGS. That is, in the present embodiment, in order to detect whether vertical splitting has occurred on the wide surface of the slab to be cast (facing the wide surfaces 2a and 2a), the heat flow on the wide surfaces 2a and 2a of the mold 1 is determined. Focus on bundles. Specifically, it is as follows.

  That is, the vertical detection method according to the present embodiment is intended to detect whether vertical splitting has occurred on the wide surface of the slab. First, as shown in FIG. Focus on (2a).

  However, since the solidified shell formed in the mold 1 gradually increases in thickness in the casting direction, the heat flux to be noticed in this embodiment is not uniform on the above-mentioned wide-side inner wall surface AA.

Therefore, in the present embodiment, attention is paid to a high heat flux surface area A as a surface area having a meniscus distance M [mm] of 20 to 40 in the wide-surface-side inner wall surface AA, and as shown in FIG. Attention is paid to predetermined surface areas a ₁ , a _2, ... Taken in the width direction of the mold 1 at a distance p [mm] of 10 or 20, 30, 40, 50. Then, the average heat flux q [W / m ² ] is determined for each of the surface regions a ₁ , a ₂ ,.

In the present embodiment, the shape of these surface areas a ₁ , a _2, ... Is substantially rectangular with w [mm] in the width direction of the mold 1 and dz [mm] in the casting direction as shown in FIG. It is said. Further, the position in the casting direction of these surface regions a ₁ , a _2, ... Is approximately the center in the casting direction of the high heat flux surface region A.

Various computational / analytical techniques can be applied to obtain the average heat flux q [W / m ² ]. For example, the first, the temperature of the template 1 measured by the first thermocouple embedded in the mold 1 so as to correspond to each surface region a ₁ · a ₂ ···, cooling water flow path 2d · the above-mentioned Numerical methods such as a difference method (described later in detail), a finite element method, a boundary element method, and the like are executed based on the temperature of the cooling water flowing through 2d. Second, a numerical method based on the temperature pair of the mold 1 measured by a pair of thermocouples embedded in the mold 1 so as to correspond to the surface areas a ₁ , a ₂ ... Can be applied. right. Of course, other known computational / analytical methods can also be applied, and the selection thereof will be appropriately selected according to the operating conditions and the equipment environment.

<3. Variation in average heat flux q [W / m ² ] in the mold width direction>
By the way, the cause of the vertical split generated on the wide surface of the slab (for example, non-uniform flow of mold powder between the solidified shell and the mold copper plate) is caused by the boundary between the solidified shell and the wide-side inner wall surface AA of the mold 1. It is considered that the heat flux at 1 is expressed as a local change in the width direction of the mold 1. Therefore, in the longitudinal detection method according to the present embodiment, local variation in the mold width direction of the average heat flux q [W / m ² ] of each surface area a ₁ , a _2. Pay attention. Specifically, it is as follows.

That is, the surface areas a _{k to} a _{k + n-1} of the adjacent n areas among the plurality of face areas a ₁ , a ₂ ... Arranged in parallel are set as a set, and each set belongs to the set. determining the plurality of surface areas a _k ~a _{k + n-1} of the average heat flux q [W / m ^2] standard deviation as the variation _σ k~k _{+ n-1} of [W / m ^2]. This will be described in more detail with reference to FIG. This figure exemplifies the case where n (hereinafter, for convenience of explanation, this n is also referred to as “the number of regions n”) is 3, and in this case, the possible range of the subscript k is 1 ~ I-2.

[Number of areas n = 3]
(First group)
That is, the first set is formed by combining the adjacent surface areas a _1, a ₂ , and a ₃ . In other words, the surface areas a ₁ and a ₂ and a ₃ are set as the first set. Then, the standard as the standard deviation σ [W / m ² ] of the average heat flux q [W / m ² ] in the three regions (that is, the surface regions a ₁ and a ₂ , a ₃ ) belonging to the first set. Deviation σ _{1 to 3} [W / m ² ] is obtained.

(Second set)
Similarly, the second set is formed by combining the adjacent surface regions a _2, a ₃ , and a ₄ . Then, the standard as the standard deviation σ [W / m ² ] of the average heat flux q [W / m ² ] in the three regions (that is, the surface regions a ₂ and a ₃ , a ₄ ) belonging to the second set. Deviation σ _{2 to 4} [W / m ² ] is obtained.

(Third to i-2)
Similarly to the above, standard deviations σ _{k to k + n−1} [W / m ² ] are obtained from the third group to the i- _2th group.

[Number of areas n = 4]
The case where the number of regions n is 4 can be considered in the same manner as the above case where the number of regions n is 3, and will be simply exemplified below.

(First group)
In other words, the first set is formed by combining the adjacent surface areas a ₁ and a ₂ , a ₃ , a ₄ . In other words, the surface areas a ₁ and a ₂ , a ₃ , a ₄ are set as the first set. Then, the standard deviation σ [W / m ² ] of the average heat flux q [W / m ² ] in the four regions (that is, the surface regions a ₁ and a ₂ , a ₃ , a ₄ ) belonging to the first set. As the standard deviation σ _{1 to 4} [W / m ² ].

(2nd to i-3)
Similarly to the above, standard deviations σ _{k to k + n−1} [W / m ² ] are obtained from the second set to the i-3th set.

[When n = 5 or 6, 7]
The case where the number of regions n is 5 or 6, 7 can be considered in the same manner as the above-described case where the number of regions n is 3 or 4 as described above.

  In the present embodiment, the number of regions n is as follows. That is, (a) when the interval p [mm] is 10 or 20, the n is any one of 3 to 7, and (b) when the interval p [mm] is 30, the n Is any one of 3 to 5, (c) when the interval p [mm] is 40, n is 3 or 4, (d) when the interval p [mm] is 50, The n is 3.

<4, Vertical detection based on standard deviation σ _{k to k + n-1} >
In the present embodiment, the vertical detection is performed by detecting the maximum standard deviation σ _max [W / m ² ] among the standard deviations σ _{k to k + n−1} [W / m ² ] obtained above and a predetermined value. This is done by comparing σ _o [W / m ² ]. Specifically, it is as shown in FIG. This figure shows the time variation of each standard deviation σ _{k to k + n-1} [W / m ² ] (time transition per second, 140 [sec] (ie, length in the longitudinal direction is 3.0 [m] For one slab part)).

(Casting conditions in FIG. 4)
Slab carbon content C [wt%]: 0.13
Casting speed Vc [m / min]: 1.3
Mold width [mm]: 2110
Mold thickness [mm]: 280
Mold height [mm]: 900
(Other conditions in FIG. 4)
Interval p [mm]: 20
Number of areas n: 3
Width of each surface area w [mm]: 1 (see Fig. 3)

That is, as indicated by the bold line in this figure (as if tracing the boundary between the mountain range and the sky), the maximum of the standard deviations σ _{k to k + n−1} [W / m ² ] at each time T Focusing on the standard deviation σ _max [W / m ² ], the standard deviation σ _max (T) [W / m ² ] that can also be expressed as a function of this time T and the predetermined value σ _o [W / m ² ] And compare.

In the above comparison, if the standard deviation σ _max (T) [W / m ² ] slightly exceeds the predetermined value σ _o [W / m ² ], vertical splitting occurred at the time T _p that exceeded the standard deviation σ _max (T) [W / m ² ]. Assuming / estimating In actual operation, the slab produced continuously as described above is cut into a slab part in the casting direction to a slab part, and the subsequent processing is independently performed according to each slab part. To manage. Therefore, it is reasonable to manage the slab part including the slab that has passed through the mold 1 at the time T _p as the “slab part in which the vertical split is generated”. In this case, “management” means a surface treatment represented by a hot scarf, for example.

<5. An example of a method of determining the predetermined value σ _o [W / m ² ]>
Next, an example of a method for determining the “predetermined value σ _o [W / m ² ]” will be introduced. FIG. 5 shows a relationship between a cast slab portion-maximum standard deviation MAX (σ _max (ΔT)) [W / m ² ], which will be described later, and a vertical split occurrence frequency [%].

(Casting conditions in FIG. 5)
Slab carbon content C [wt%]: 0.13
Casting speed Vc [m / min]: 1.3
Mold width [mm]: 2110
Mold thickness [mm]: 280
Mold height [mm]: 900
(Other conditions in FIG. 5)
Interval p [mm]: 20
Number of areas n: 3
Width of each surface area w [mm]: 1

(Description of FIG. 5)
This figure shows whether slabs that are continuously produced are cut at about 3.0 [m] in the casting direction to form slab parts, and whether each slab part has been split vertically on its wide surface. This is based on a survey test conducted on this point. The horizontal axis of this figure represents the standard deviation σ _max (T) [W / in the time zone ΔT in which a single slab passed through the vicinity of the high heat flux surface area A of the mold 1 (see FIG. 2). As a further maximum value of m ² ], the slab portion-maximum standard deviation MAX (σ _max (ΔT)) [W / m ² ] is divided every 0.05 [MW / m ² ]. On the other hand, the vertical axis represents the ratio of the number of slab parts in which the vertical split has occurred on the wide surface in the number of slab parts as the investigation target. In this investigation test, 200 slabs were investigated.

The above-mentioned `` predetermined value σ _o [W / m ² ] '' indicates whether (1) the ability of the post-process, (2) which should be prioritized, such as productivity or quality, taking this figure into consideration. The decision may be made comprehensively. That is, if the above-mentioned “predetermined value σ _o [W / m ² ]” is set low, it is difficult to overlook the cast slab portion where the vertical split has occurred (that is, the vertical split missing rate decreases). However, the slab portion that is assumed to have been generated even though the vertical split did not occur will increase (that is, the vertical split detection rate will increase). On the other hand, if the above-mentioned “predetermined value σ _o [W / m ² ]” is set higher, the vertical miss rate will increase and the vertical excess detection rate will decrease. In short, the above-mentioned “predetermined value σ _o [W / m ² ]” may be appropriately determined in light of the actual operation conditions. For example, according to this figure, when the slab part-maximum standard deviation MAX (σmax (ΔT)) [W / m ² ] is 0.40 or less, the frequency of occurrence of vertical splitting is very good at around 10%. If it is 0.40 or more, the frequency of occurrence of vertical split greatly exceeds 50%. Therefore, it can be said that the “predetermined value σ _o [W / m ² ]” is preferably 0.40 from the various viewpoints described above.

<6. Grounds for focusing on high heat flux surface area A>
Next, in the present embodiment, the grounds for focusing on the high heat flux surface area A as the surface area where the meniscus distance M [mm] is 20 to 40 of the wide-surface inner wall surface AA are shown in FIGS. 2 and 6. This will be explained based on. In the graph shown in FIG. 6, the horizontal axis indicates the meniscus distance M [mm], and the vertical axis indicates the heat flux [MW / m ² ] in the vicinity of each meniscus distance M [mm] of the wide inner wall surface AA.

(Casting conditions in FIG. 6)
Slab carbon content C [wt%]: 0.13
Casting speed Vc [m / min]: 1.1
Mold width [mm]: 2110
Mold thickness [mm]: 280
Mold height [mm]: 900

  According to FIG. 6, it can be seen that the heat flux on the wide surface of the mold is maximized in the surface region where the meniscus distance M [mm] is 20 to 40 in the wide surface side inner wall surface AA. Thus, it is considered that the difference in the heat flux appears more noticeably as the peak value of the heat flux becomes larger, and the vertical split is easier to detect as the difference in heat flux appears more noticeably. Therefore, in the present embodiment, attention is paid to the high heat flux surface area A as a surface area having a meniscus distance M [mm] of 20 to 40 of the wide-surface inner wall surface AA.

<7. An example of how to obtain the average heat flux q [W / m ² ]>
Next, an example of how to obtain the average heat flux q [W / m ² ] will be illustrated based on FIGS. FIG. 7 is a partially enlarged view of FIG. 1, and FIG. 8 is an explanatory diagram for explaining heat transfer calculation. Here, the average heat flux q [W / m ² ] is measured using the one thermocouple 5 embedded in the mold 1 so as to correspond to each surface area a ₁ , a _2. 1) and the temperature of the cooling water flowing through the cooling water flow paths 2d, 2d... Described above by way of heat transfer calculation (two-dimensional difference method).

(Configuration for executing heat transfer calculation)
In this example, as shown in FIG. 7, in the copper plate 2 on the wide surface side of the mold 1, thermocouples 5 · 5... At a predetermined interval p [mm] (see also FIG. 3) in the width direction. (For example, K thermocouples (+ → chromel,-→ alumel)) are embedded, and the temperature t ₅ · t ₅ · in the copper plate 2 on the wide surface side of the mold 1 using these thermocouples・ ・ Measure. The measured temperature data is configured to be transmitted at predetermined time intervals (for example, 1 second) to an electronic computer appropriately provided with a CPU and a storage device (ROM, RAM, etc.). Although not shown, the temperature t _{2d of the} cooling water flowing through the cooling water flow paths 2d, 2d,... Is also measured with another thermocouple provided as appropriate, and the measured temperature data is also measured at predetermined time intervals. It is configured to be transmitted to the electronic computer. Note that the temperature t _2d of the cooling water is a temperature measurement value obtained by measuring the temperature of the cooling water coming out of the cooling water flow paths 2d, 2d,. To do.

  In the present embodiment, the thermocouples 5, 5... Are located at a position of about 8 mm from the wide surfaces 2a, 2a of the copper plate 2 of the mold 1 to the jacket 3 side (the thermocouple embedding depth dπ). [mm] = 8)

(Outline of heat transfer calculation)
The above-mentioned computer includes (1) the temperature t ₅ · t ₅ of the copper plate 2 on the wide surface side of the mold 1 and (2) the temperature t _{2d of the} cooling water, and the storage device of the computer in advance before the start of casting (3) to (6), that is, (3) the cross-sectional shape of the mold (the thickness of the copper plate 2 of the mold 1 (where the thickness of the copper plate 2 is denoted by the symbol RR in FIG. 7) ), And the cross-sectional shape of the cooling water flow paths 2d, 2d, etc.), (4) copper thermal conductivity λ, (5) interfacial heat transfer coefficient h ( For example, it is calculated based on the flow velocity of the cooling water flowing through the cooling water flow paths 2d, 2d,...), (6) Interfacial heat transfer coefficient h between the copper plate 2 and the jacket 3 (for convenience of calculation, 7) and (7) the thermocouple embedding depth dπ described above based on (1) to (7), etc., and the two-dimensional heat transfer in the copper plate 2 as shown in FIG. Solve differential equations by the difference method.

(Calculation target area of heat transfer calculation)
In this example, the thermocouple 5 is embedded in the approximate center of two adjacent cooling water flow paths 2d and 2d when viewed in the width direction of the mold 1. Therefore, the copper plate 2 corresponding to one surface area a _x is substantially line symmetric. Therefore, the heat transfer analysis in the calculation target area of the heat transfer calculation is performed in the area indicated by the thick line (hereinafter, the calculation target area). It can be replaced with the analysis of heat transfer in the thick line region). Therefore, only the heat transfer in the thick line region is analyzed using the above-described electronic computer.

(About heat input and output to the thick line area)
[Q _1: heat input]
The symbol q ₁ [W / m ² ] indicates the amount of heat per unit area (ie, average heat flux) [W / m ² ] entering the thick line region from the solidified shell side.
[Q _2, q _3: heat removal]
In addition, in this figure, the symbols q ₂ [W / m ² ] and q ₃ [W / m ² ] indicate the amount of heat per unit area that flows from the thick line region to the cooling water flowing through the cooling water flow paths 2d, 2d,. (Ie, average heat flux) [W / m ² ].
[Q _4: heat removal]
Further, in this drawing, the symbol q ₄ [W / m ² ] indicates the amount of heat per unit area (that is, average heat flux) [W / m ² ] coming from the thick line region to the jacket 3.
[Other heat input and output]
In the present embodiment, for convenience of calculation, it is assumed that there is no movement of heat between a region indicated by a thick line in this drawing and a region adjacent in the copper plate 2. Similarly, it is assumed that there is no heat transfer in the casting direction in the copper plate 2.

(Specific contents of difference method for heat transfer calculation)
When executing the difference method, a mesh having an interval of Δx (for example, 1 mm) was applied to the above-described thick line region (that is, the mesh was divided at an interval of Δx (for example, 1 mm)). Then, the balance of the amount of heat passing through the center of each mesh element is calculated. This calculation is started and executed based on the following initial conditions, boundary conditions, and calculation conditions (however, the mathematical expressions described as the following boundary conditions and calculation conditions introduce a rough concept of calculation). Then, the temperature t 5 by each thermocouple 5 is compared with the temperature of the center of the mesh element arranged at the position of the thermocouple ₅ in the calculation, and the difference is within 1%. The average heat flux q ₁ (= q _ax ) [W / m ² ] is adopted as the calculation result.

(Initial condition)
The average heat flux q ₁ [W / m ² ] and q ₂ , q ₃ , and q ₄ are set to arbitrary values.

(boundary condition)
《Boundary condition of interface between copper plate 2 and cooling water (or jacket 3)》
The above-described interfacial heat transfer coefficient h is used (see also FIG. 8).
λ / dz {t (I + 1,1) -t (I, 1)} + λ / dz {t (I-1,1) -t (I, 1)} + λ / dz {t (I, 2) -t (I, 1)} = h {t (I, 1) -tw}
Note that the variable dz is indicated by dz in FIG. 3 (depth in FIG. 7).
<Boundary condition of interface between copper plate 2 and solidified shell>
The aforementioned average heat flux q ₁ [W / m ² ] is used.
λ / dz {t (I + 1, M) -t (I, M)} + λ / dz {t (I-1, M) -t (I, M)} + λ / dz {t (I, M-1) -t (I, M)} + q ₁ = 0

(Calculation condition)
<Inside of copper plate 2>
λ / dz {t (I + 1, J) -t (I, J) + t (I-1, J) -t (I, J) + t (I, J + 1) -t (I, J ) + t (I, J-1) -t (I, J)} = 0

(Specific values for each constant used in heat transfer calculations)
Copper thermal conductivity λ: 355 [W / m / deg]
-Interfacial heat transfer coefficient h between copper plate 2 and cooling water: 15000 [W / m ² / deg]
-Interfacial heat transfer coefficient h between copper plate 2 and jacket 3: Same as above

In the heat transfer calculation described above, the temperature of the cooling water coming out of the cooling water flow paths 2d, 2d... Is used as the temperature t _2d of the cooling water, so that the absolute average heat flux q _ax [ W / m ² ] cannot be obtained. However, in the longitudinal detection method according to the present embodiment, the variation in the average heat flux q _ax [W / m ² ] in the width direction of the mold 1 at the same time (standard deviation σ _{k to k + n−1} [W / m ^2] ]), It is not particularly problematic as long as at least a variation in the relative average heat flux q _ax [W / m ² ] is required.

  In the above heat transfer calculation, since the three-dimensional heat flow is the two-dimensional heat flow as described above, the variable dz which means infinitesimal increment is substantially ignored in the calculation. Become.

<8. Grounds for setting the number of areas n>
Next, the basis for setting the number of areas n will be described in detail with reference to FIGS.

As described above, it is considered that the vertical split on the wide surface of the slab can be detected based on the standard deviation σ _{k to k + n−1} [W / m ² ]. 9 to 13 exemplify average heat flux q _ax [MW / m ² ] and standard deviations σ _{k to k + n−1} [MW / m ² ] in each surface region a _x at an arbitrary time. To do. 9 to 13 show the same data displayed in different formats. That is, for example, referring to FIG. 10, this corresponds to a thinned average heat flux q _ax [W / m ² ] in each surface region a _x shown in FIG. The interval p [mm] is added to the upper right of each figure.

(Casting conditions of FIGS. 9 to 13)
Slab carbon content C [wt%]: 0.13
Casting speed Vc [m / min]: 1.2
Mold width [mm]: 2110
Mold thickness [mm]: 280
Mold height [mm]: 900

(Other conditions in FIGS. 9 to 13)
Interval p [mm]: (Indicated in the figure)
Number of areas n: (shown in the figure)
Width of each surface area w [mm]: 1

According to these FIGS. 9 to 13, when the number of regions n is 3, it can be said that it is most preferable in that local variation of the average heat flux q _ax [W / m ² ] can be reflected sensitively. On the other hand, it can be seen that as the number of regions n is increased, the local variation in the average heat flux q [W / m ² ] is less sensitively reflected (insensitive). According to these FIG. 9 to FIG. 13, the same can be said for the interval p [mm]. That is, it can be said that the smaller the interval p [mm] is, the more sensitive it is, and the larger the interval p [mm] is, the less sensitive it is.

9 to 13, according to another aspect, the degree of difference between the maximum value and the minimum value of the standard deviation σ [W / m ² ] is closely related to the sensitivity of vertical detection. I can say. Because the difference is large, there is a clear difference between the part where the vertical split occurs in the wide surface of the slab and the part where the stable casting as the part where no vertical split occurs. It is because it can be said that it has appeared.

  Therefore, in the present embodiment, as described above, the number of areas n is set as follows (a) to (d). That is, (a) when the interval p [mm] is 10 or 20, the n is any one of 3 to 7, and (b) when the interval p [mm] is 30, the n Is any one of 3 to 5, (c) when the interval p [mm] is 40, n is 3 or 4, (d) when the interval p [mm] is 50, The n is 3.

The technical effect of setting the number of regions n will be confirmed based on FIG. FIG. 14 is a diagram related to the sensitivity of vertical detection, and more specifically, the interval p [mm] and the number of regions n and the standard deviations σ _{k to} k in each of FIGS. 9 to 13 described above. _It shows the relationship between the difference between the maximum value and the minimum value of _{+ n-1} [W / m2] at the same time.

According to FIG. 14, according to (a) to (d) as the method for setting the number of regions n, the maximum value and the minimum value of the standard deviations σ _{k to k + n−1} [W / m ² ] are obtained. It can be seen that a difference of at least 0.1 can be secured. In other words, according to the above method for setting the number of regions n, the vertical split generation site as the vertical split portion of the wide surface of the mold, and the stable casting portion as the vertical split portion as well Since the difference between and clearly appears as the above difference, it is possible to detect the vertical division with very good sensitivity.

  The distance p [mm] is preferably at least 10 or more in consideration of the size and shape of the thermocouple 5 to be embedded.

As described above, in the first embodiment, the vertical detection is performed by the following method. That is, a high heat flux surface as a surface region having a meniscus distance M [mm] of 20 to 40 of the wide inner surface AA of the casting mold for continuous casting for cooling molten steel to form a solidified shell having a predetermined shape In the region A, the average heat flux q [W in a predetermined surface region a ₁ · a ₂ ... Taken at a spacing p [mm] of any one of 10, 30, 20, 40, 50 in the width direction of the mold. / m ² ] respectively. Among the plurality of surface regions a ₁ , a ₂ ... Arranged in parallel, adjacent surface regions a _{k to} a _{k + n−1} of n regions are taken as one set, and each set belongs to the plurality determination of the surface area a _k ~a _{k + n-1} of the standard deviation _σ k~k _{+ n-1} of the average heat flux ^{q [W / m 2] [} W / m 2]. Among the obtained standard deviations σ _{k to k + n-1} [W / m ² ], a maximum standard deviation σ _max [W / m ² ] and a predetermined value σ _o [W / m ² ] The vertical division is detected by comparison. However, when (a) the interval p [mm] is 10 or 20, the n is any one of 3 to 7, and (b) when the interval p [mm] is 30, the n Is any one of 3 to 5, (c) when the interval p [mm] is 40, n is 3 or 4, (d) when the interval p [mm] is 50, The n is 3.

  According to this, the vertical division of the slab can be detected sensitively.

  From another viewpoint, the vertical detection method according to the first embodiment (hereinafter also simply referred to as the present method) is the surface flaw detection method (hereinafter simply referred to as the preceding method) described in Patent Document 1. Compared to the method, it has the following advantageous features. In short, the prior method focuses on the temperature of the mold measured by the thermocouple, whereas the present method focuses on the heat flux on the wide surface of the mold. Therefore, it is thought that accurate thermal characteristics can be obtained.

  That is, for example, the mold temperature is measured by a thermocouple depending on the material of the mold, the thickness of the copper plate of the mold, the cross-sectional shape of the cooling water flow path, the flow rate of the cooling water flowing through the cooling water flow path, the temperature of the cooling water, etc. The results will vary greatly. Moreover, the thickness of the copper plate of the mold and the cross-sectional shape of the cooling channel are frequently changed due to repair and replacement of the apparatus. Under such circumstances, it is unlikely that accurate thermal characteristics can be obtained by focusing only on the temperature of the mold measured by the thermocouple.

  On the other hand, in the said embodiment, the heat flux in the inner wall surface of a casting_mold | template is calculated with various computational / analytical methods, and it tries to detect a vertical split according to how this heat flux is. Therefore, the present method can comprehensively judge by absorbing the above-described plurality of factors different for each facility and the above-mentioned plurality of factors associated with maintenance, management, and replacement of the facility without any problem. Based on the above, it can be said that this method has extremely advantageous characteristics in that accurate thermal characteristics can be obtained as compared with the previous method.

  Although the first embodiment which is a preferred embodiment of the present invention has been described above, the first embodiment may be modified as follows.

That is, in the first embodiment, the shape of the surface areas a ₁ , a _2, ... Is substantially rectangular as shown in FIG. 3, but is not limited thereto, and may be, for example, substantially square, circular, or elliptical. The shape is arbitrary.

In the first embodiment, the surface areas a ₁ , a ₂ ... Are located in the center of the casting direction in the high heat flux surface area A. It may be positioned so as to contact the upper end or the lower end of the high heat flux surface area A.

  Next, a second embodiment of the present invention will be described with reference to FIGS.

In the first embodiment described above, when the maximum standard deviation σ _max (T) [W / m ² ] slightly exceeds the predetermined value σ _o [W / m ² ], the vertical deviation occurs at the time T _p that exceeds the predetermined value σ _o [W / m ² ]. It was assumed that the split occurred / estimated (see FIG. 4).

On the other hand, in the continuous casting according to the present embodiment, the casting speed Vc [m / m] so that the maximum standard deviation σ _max (T) [W / m ² ] does not exceed the predetermined value σ _o [W / m ² ]. Let min] be reduced. For example, when the above-mentioned “predetermined value σ _o [W / m ² ]” is set to 0.40, the standard deviation σ _max (T) [W / m ² ] continues for a predetermined time from when it reaches 0.35. The casting speed Vc [m / min] is reduced by at least 10%.

A test for confirming the technical effect of the continuous casting method according to this embodiment will be described below. FIGS. 15 to 18 are graphs showing changes in standard deviations σ _{k to k + n−1} [W / m ² ] depending on the casting speed. The test conditions for this test are as follows.

(Casting conditions of FIGS. 15 to 18)
Slab carbon content C [wt%]: 0.13
Casting speed Vc [m / min]: Mold width shown in the figure [mm]: 1230
Mold thickness [mm]: 230
Mold height [mm]: 900
(Other conditions in FIGS. 15 to 18)
Interval p [mm]: 20
Number of areas n: 3
Width of each surface area w [mm]: 1

In addition, the average heat flux q _{a15 to} q _a19 [W / m ² ] for the surface areas a ₁₅ , a ₁₆ , a ₁₇ , a _18, and a ₁₉ is obtained (see also FIG. 3), and the time transition 3 standard deviations σ _{15 to 17} · σ _{16 to 18} · σ _{17 to 19} [MW / m ² ] were obtained.

FIG. 15 shows the results of a test in which the casting speed Vc [m / min] is constant regardless of the standard deviation σ _{k to k + n−1} [MW / m ² ].

As shown in FIG. 16, when any one of the three standard deviations σ _{k to k + n−1} [MW / m ² ] reaches 0.35, the casting speed Vc [m / min] is reduced by 0.5%. In the same way, even when continuously reduced, the standard deviation σ _{k to k + n-1} [MW / m ² ] could not be suppressed to 0.40 or less.

On the other hand, as shown in FIGS. 17 and 18, when any one of the three standard deviations σ _{k to k + n−1} [MW / m ² ] reaches 0.35, at least the casting speed Vc [m / min] is set. When continuously reduced by 10% or more, the standard deviation σ _{k to k + n-1} [MW / m ² ] was always suppressed to 0.40 or less.

  Note that reducing the casting speed Vc [m / min] means lowering the productivity, lowering the surface temperature of the molten steel (so-called skinning), or the gap between the solidified shell and the mold inner wall surface. Various undesired problems may be caused, such as non-uniform inflow of mold powder. From this point of view, it is recommended that the reduction rate of the casting speed Vc [m / min] be at most about 20%.

  As described above, in the second embodiment, continuous casting is performed by the following method.

That is, a high heat flux surface as a surface region having a meniscus distance M [mm] of 20 to 40 of the wide inner surface AA of the casting mold for continuous casting for cooling molten steel to form a solidified shell having a predetermined shape In the region A, the average heat flux q [W in a predetermined surface region a ₁ · a ₂ ... Taken at a spacing p [mm] of any one of 10, 30, 20, 40, 50 in the width direction of the mold. / m ² ] respectively. Among the plurality of surface regions a ₁ , a ₂ ... Arranged in parallel, adjacent surface regions a _{k to} a _{k + n−1} of n regions are taken as one set, and each set belongs to the plurality determination of the surface area a _k ~a _{k + n-1} of the standard deviation _σ k~k _{+ n-1} of the average heat flux ^{q [W / m 2] [} W / m 2]. Among the obtained standard deviations σ _{k to k + n-1} [W / m ² ], a maximum standard deviation σ _max [W / m ² ] and a predetermined value σ _o [W / m ² ] In comparison, the casting speed Vc [m / min] is reduced so that the _maximum standard deviation σ _max [W / m ² ] does not exceed the predetermined value σ _o [W / m ² ]. However, when (a) the interval p [mm] is 10 or 20, the n is any one of 3 to 7, and (b) when the interval p [mm] is 30, the n Is any one of 3 to 5, (c) when the interval p [mm] is 40, n is 3 or 4, (d) when the interval p [mm] is 50, The n is 3.

  According to this, the vertical division of the slab can be suppressed.

Cross section of mold Vertical section of mold Partial enlarged view of FIG. Graph showing the time variation of each standard deviation σ _{k to k + n-1} Frequency distribution diagram showing the relationship between slab part-maximum standard deviation MAX (σ _max (ΔT)) and the frequency of occurrence of vertical split Graph showing the relationship between heat flux and meniscus distance on the wide mold surface Partial enlarged view of FIG. Explanatory diagram of heat transfer calculation A graph showing the relationship between the average heat flux q _ax and the standard deviation σ _{k to k + n-1} in each surface area a _x (p [mm] = 10) Figure similar to Figure 9 (p [mm] = 20) Figure similar to Figure 9 (p [mm] = 30) Figure similar to Figure 9 (p [mm] = 50) Figure similar to Figure 9 (p [mm] = 70) Figure for sensitivity of vertical detection Graph showing changes in standard deviation due to casting speed operation A figure similar to FIG. A figure similar to FIG. A figure similar to FIG.

Explanation of symbols

1 Mold
2 Copper plate
3 Jacket
5 Thermocouple
AA (2a) Wide side inner wall
A High heat flux surface area
a area

Claims (2)

High heat flux surface area A as a surface area having a meniscus distance M [mm] of 20 to 40 of the wide inner surface AA of the mold for continuous casting for cooling molten steel to form a solidified shell of a predetermined shape In the width direction of the mold, the average heat flux q [W / m in a predetermined surface area a ₁ · a ₂ ... Taken at a distance p [mm] of 10 or 20, 30, 40, 50 ² ]
Among the plurality of surface regions a ₁ , a ₂ ... Arranged in parallel, adjacent surface regions a _{k to} a _{k + n−1} of n regions are taken as one set, and each set belongs to the plurality seeking surface area a _k ~a _{k + n-1} of the standard deviation sigma _{k to k} of the average heat flux _{^{q [W / m 2] +}} n-1 [W / m 2],
Among the obtained standard deviations σ _{k to k + n-1} [W / m ² ], a maximum standard deviation σ _max [W / m ² ] and a predetermined value σ _o [W / m ² ] A vertical detection method characterized by detecting vertical division by comparison.
However,
(a) When the interval p [mm] is 10 or 20, the n is any one of 3-7,
(b) When the interval p [mm] is 30, the n is any one of 3-5,
(c) When the interval p [mm] is 40, the n is 3 or 4,
(d) When the interval p [mm] is 50, the n is 3. High heat flux surface area A as a surface area having a meniscus distance M [mm] of 20 to 40 of the wide inner surface AA of the mold for continuous casting for cooling molten steel to form a solidified shell of a predetermined shape In the width direction of the mold, the average heat flux q [W / m in a predetermined surface area a ₁ · a ₂ ... Taken at a distance p [mm] of 10 or 20, 30, 40, 50 ² ]
Among the plurality of surface regions a ₁ , a ₂ ... Arranged in parallel, adjacent surface regions a _{k to} a _{k + n−1} of n regions are taken as one set, and each set belongs to the plurality seeking surface area a _k ~a _{k + n-1} of the standard deviation sigma _{k to k} of the average heat flux _{^{q [W / m 2] +}} n-1 [W / m 2],
Among the obtained standard deviations σ _{k to k + n-1} [W / m ² ], a maximum standard deviation σ _max [W / m ² ] and a predetermined value σ _o [W / m ² ] Compare and
A continuous casting method, wherein the casting speed Vc [m / min] is reduced so that the _maximum standard deviation σ _max [W / m ² ] does not exceed a predetermined value σ _o [W / m ² ].
However,
(a) When the interval p [mm] is 10 or 20, the n is any one of 3-7,
(b) When the interval p [mm] is 30, the n is any one of 3-5,
(c) When the interval p [mm] is 40, the n is 3 or 4,
(d) When the interval p [mm] is 50, the n is 3.

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