JPH079104A - Method for continuously casting steel - Google Patents

Abstract

PURPOSE:To provide a means for fixing the oscillating condition of a mold in order to uniformly flow mold powder between the mold and a cast slab. CONSTITUTION:At the time of using the mold powder having uiscosity of 2-8 poise at 1300 deg.C solidified temp., the oscillation of the mold and/or oscillation stroke, the deviation Dp of unit consumption of the powder is used as the index, and this DP is adjusted to the range shown in the following inequalities to continuously cast a steel. The deviation Dp of unit consumption of the powder: difference between the max. value and the min. value of the mold powder during one stroke of the mold flowed into gap between the mold and the cast slab. In the case of being [C] <8X 10<-2>%, Dp<=0.28, and in the case of being 8<=[C]<=18X10<-2>%, Dp<=0.15-0.28.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method in iron making.

[0002]

2. Description of the Related Art In continuous casting of steel, from the viewpoint of process saving and energy saving, it is an important manufacturing problem that uncarried hot strip transfer between steel making / rolling is important.

Recently, the required level of cleanliness in cast slabs tends to increase day by day. From this viewpoint, in order to prevent internal defects in products caused by the inclusion of mold powder, it is necessary to increase the viscosity of the mold powder. Has become.

Further, there is an increasing need for a higher casting speed for the purpose of improving productivity in the continuous casting and conveying high-temperature slabs. From the above points, high-speed casting and use of high-viscosity powder are one of the manufacturing conditions. Is being treated as.

At this time, the technique for suppressing vertical cracks on the surface of the cast slab has an extremely important meaning in realizing the slab-free maintenance of the hot slab.

Conventionally, as the above measures, physical properties of mold powder (viscosity, melting point, freezing point, surface tension, etc.), casting speed and vibration conditions of mold (negative strip rate,
There are many examples in which vertical cracking is suppressed by indicating the lubrication performance, consumption amount, and frictional resistance of mold powder from indicators such as stroke). Alternatively, there is no example in which it is arranged by linking with mold vibration conditions.

Longitudinal cracks can be prevented by not generating an external force exceeding the hot strength of steel, and it is only necessary to secure the mold powder basic unit (in many cases, the basic unit is used as a lubricating performance index). Instead, it is realized by minimizing the fluctuation of powder inflow.

[0008]

SUMMARY OF THE INVENTION The present invention provides a means for defining mold vibration conditions for allowing mold powder to flow uniformly between a mold and a slab.

[0009]

Means for Solving the Problems Viscosity at 1300 ° C. is 2 to 8 poise and solidification temperature is 1000 to 1300.
When using mold powder at ℃, vibration of the mold and
Alternatively, a continuous casting method for steel, characterized in that the vibration stroke is used as an index for the powder unit deviation DP and is adjusted within the following range.

Powder unit deviation DP: difference between the maximum value and the minimum value of the mold powder flowing between the mold and the slab during one stroke of the mold.

[0011]

[C] <8 × 10 ⁻² % Dp ≦ 0.28 8 ≦ [C] ≦ 18 × 10 ⁻² % Dp ≦ 0.15 to 0.28

[0012]

[Action]

(1) About powder inflow index As an expression showing the mold powder lubrication between the mold and the slab,
Nippon Steel's Ansai et al. Have submitted the ceremony (Steelmaking Research / 19
87, No. 324).

[0013]

[Equation 3]

As shown in FIG. 14, η; powder viscosity ρ; powder density p ₀ ; pressure boundary condition g; weight acceleration V; strand velocity U; mold vibration velocity h _i ; powder film specifications (inflow path) Thickness) l _i ; Powder film specifications (inflow path length) σ _i ; Powder film specifications (inflow path gradient) a _i ; Powder film specifications (contact piece) Q; Powder inflow amount

From the equation, the difference between the maximum inflow amount and the minimum inflow amount of the powder during one stroke of the mold is calculated, which is defined as the powder unit deviation (hereinafter referred to as Dp).

Positive strip time (Tp) of Dp
Fig. 1 is obtained by rearranging the influence of the above with the viscosity of the powder at 1300 ° C as a parameter.

Here, a large Dp indicates that the powder inflow is non-uniform. From FIG. 1, it can be seen that the higher the powder viscosity and the shorter the Tp, the more likely the uneven flow will occur.

(2) Permissible powder basic unit It is desirable that Dp is smaller, but there is a proper range when the presence or absence of vertical cracks is used as an index. Therefore,
Verify that the Dp is within the proper range from the steel side.

I) Estimation of the degree of non-uniform solidification leading to cracks Vertical cracks occur in the non-uniform portion of the slab during the initial solidification process in the mold. A simple model of it is as shown in FIG.

It is assumed that a material with a waist is subjected to tensile forces from both sides. Here, the tensile force is a thermal stress in the solidification process of steel, and it is assumed that it is concentrated in the constricted part.
Thermal stress and stress concentration are each expressed by an equation.

[0021]

[Formula 4] σ ₀= Α (T₁-T₀) E ... ε₀= Α (T₁-T₀)

[0022]

[Equation 5] α ε ₀ = ε _MAX <ε _CRIT = 0.5% ...

Here, α: shape factor T ₁ -T ₀ ; temperature difference (= 200 ° C.) E: longitudinal elastic modulus ε _MAX , ε _CRIT ; strain critical strain (ε _CRIT ) for not breaking the constricted portion Is 0.5
%

From the above, the degree of non-uniform solidification leading to fracture (Fig. 2
B / a) can be calculated from FIG. 3 to be 0.87.

Therefore, when uneven solidification occurs, when the solidification delay portion (position b in FIG. 2) has a cross-sectional ratio difference of 13% or more with respect to the sound portion (position a in FIG. 2), It can be presumed to break.

Ii) Estimation of air gap amount leading to cracking Next, a solidification delay of 13% occurs, so that the mold / part is partially
The overall heat transfer between the molten steels must be reduced. It is evaluated by the air gap amount.

The heat transfer from the molten steel to the mold is given by the equation.

[0028]

[Equation 6] Q = U (T _L −T _W ) ...

[0029]

## EQU00007 ## U = (dc / .lamda.c + dp / .lamda.p + 1 / h _gap + dm / .lamda.m + 1 / hw) ^-1 ...

[0030] Here, Q; heat transfer amount T _L -T _W; Temperature U; overall heat transfer coefficient dc / [lambda] c, etc .; heat resistance dc / [lambda] c; slab thermal resistance dc / .lambda.p; thermal resistance of powder 1 / h _gap ; thermal resistance of air gap dm / λm; thermal resistance of mold 1 / hw; thermal resistance of cooling water

From the equation, from the case where there is no air gap at all, calculating the decrease in the overall heat transfer amount due to the air gap in the solidification delay portion, the expected air gap amount is about 0.015 mm from FIG. It is estimated to be.

Iii) Estimation of Dp leading to cracks It is assumed that an air gap is generated when the inflow of the powder becomes uneven (when the powder does not flow or when it becomes difficult). As a result, ii), iii)
For the reason shown in (1), the initially solidified slab is unevenly solidified and cracks. Dp at that time is given by an equation.

[0033]

[Equation 8] 18.7cm ³ /s/mm×0.015mm=0.28cm ³ / s ···

Here, 18.7c m³/ S / mm; calculated from actual operation data
Uder basic unit)

[C] <8 × 10 ^-2 % steel has Dp> 0.
It is considered that cracking occurs at 28 cm ³ / s.

Therefore, the numerical value of 0.28 cm ³ / s is defined as the limit unit deviation of powder (Dp _CRIT ).

[0037]

EXAMPLE Three examples in which the above results are applied to actual operation are shown in the following (1), (2) and (3).

(1) Table 1, FIG. 5 and FIG. 6 Conditions (arrow in the figure) A B Positive strip time 0.25 0.31 (seconds) Negative strip rate 71 64 (%) Frequency 155 130 (cpm) Vibration stroke 6 6 (mm)

(2) Table 2, FIG. 7, FIG. 8 Conditions (arrow in the figure) AB Positive strip time 0.25 0.29 (seconds) Negative strip rate 71 74 (%) Frequency 155 130 (cpm) Vibration stroke 68 (mm)

(3) Table 3, FIG. 9, FIG. 10 Conditions (arrow in the figure) A B Positive strip time 0.30 0.41 (seconds) Negative strip rate 70 70 (%) Frequency 130 95 (cpm) Vibration stroke 68 (mm)

It can be seen that the occurrence of vertical cracks can be sorted _out with Dp _CRIT .

It has been reported so far that the powder basic unit is improved by increasing the vibration stroke and lengthening the positive strip time, and as a result, it is generally arranged that vertical cracks are reduced.

Here, the points to be particularly noted are (1),
As shown in (2) and (3), the negative strip ratio was made almost the same, and the results of the test conducted by shaking the vibration stroke did not show the above-mentioned effect of improving vertical cracking. Rather, there is an increasing trend.

Although it is true that the longer stroke lengthens the positive strip time, at the same time, since Dp increases (Dp curve shifts upward), it can be judged that the vertical crack was not improved as a result. The above mechanism can be clearly explained in the present invention.

In continuous casting of steel, it is important to set mold vibration conditions according to the powder viscosity to be used, and the point should be controlled by the powder non-uniformity inflow index, which is the powder unit deviation (Dp). (The vertical crack index in FIGS. 6, 8 and 10 indicates the ratio of the number of vertical crack occurrence slabs).

Effect of Higher Freezing Point of Powder It has been reported that vertical cracking can be suppressed by raising the solidification temperature of the powder, which can be explained by the present invention.

As shown in FIGS. 11 and 12, increasing the solidification temperature of the powder has a role of increasing the solid phase ratio in the powder layer between the mold and the slab.

It is generally said that the slow cooling effect is improved by increasing the solid fraction, but when verified from Dp, for example, FIG.
(H '= 0.9h in FIGS. 11 and 12).

At the same Tp value, Dp shifts to a lower level, and the stable range is substantially expanded. Therefore, vertical cracking is suppressed even under the same vibration condition.

Further, by giving the powder a solidification temperature, it is possible to suppress a partial excess inflow. Due to the low solidification temperature, the powder is liquid-lubricated between the mold and the slab, making it easy for the powder to excessively flow into the mold short piece or in the vicinity of the short piece.
It is considered that there is a tendency for slab gaps to form easily, and as a result, insufficient lubrication on the wide surface side of the mold, where slab vertical cracking occurs frequently, or to promote non-uniformity of the initially solidified slab due to strong cooling. is there.

In the concept (2) -i) of the peritectic zone steel type, only the thermal stress was taken into consideration as the external force for causing the fracture of the initially solidified slab, but the peritectic zone steel type ([C] = 8)
-18 × 10 ^-2 %), the force generated during the phase transformation is added to it.

In that region, the critical powder _basic unit (Dp _CRIT ) shifts downward from (2) -i) to iii), so that the vertical crack occurrence risk region expands.

Although the estimation of the phase transformation strain (δ-γ transformation) has not been clarified, empirically, Dp _CRIT =
It varies from 0.15 to 0.28.

[0054]

[Table 1]

[0055]

[Table 2]

[0056]

[Table 3]

[0057]

EFFECTS OF THE INVENTION The present invention has provided means for determining the mold vibration conditions for uniformly injecting mold powder between a mold and a slab.

[Brief description of drawings]

FIG. 1 is a relationship diagram between Dp and positive strip time.

FIG. 2 is an explanatory diagram that models that vertical cracks occur in a non-uniform portion of a slab.

FIG. 3 is a relationship diagram between ε ₀ and b / a.

FIG. 4 is a diagram showing the relationship between the overall heat transfer coefficient and the air gap amount.

FIG. 5 is a relationship diagram between Dp and positive strip time.

FIG. 6 is a diagram showing changes in vertical cracking index.

FIG. 7 is a relationship diagram between Dp and positive strip time.

FIG. 8 is a diagram showing changes in vertical cracking index.

FIG. 9 is a relationship diagram between Dp and positive strip time.

FIG. 10 is a view showing changes in vertical cracking index.

FIG. 11 is an explanatory diagram when a low freezing temperature powder is used.

FIG. 12 is an explanatory view when using a powder having a high solidification temperature.

FIG. 13 is a relationship diagram between Dp and positive strip time.

FIG. 14 is a diagram illustrating mold powder lubrication.

Claims (1)

[Claims] 1. The viscosity at 1300 ° C. is 2 to 8 poi.
When using a mold powder having a se and a solidification temperature of 1000 to 1300 ° C., the steel is characterized by adjusting the vibration and / or the vibration stroke of the mold with the powder unit deviation DP as an index, and adjusting it within the following range. Continuous casting method. Powder unit deviation DP: The difference between the maximum value and the minimum value of the mold powder flowing between the mold and the slab during one stroke of the mold. [C] <8 × 10 ⁻² % Dp ≦ 0.28 8 ≦ [C] ≦ 18 × 10 ⁻² % Dp ≦ 0.15 to 0.28