JPH06106302A - Method for continuously casting martenstic stainless steel - Google Patents

Abstract

PURPOSE:To prevent the surface defect of a cast slab at an initial casting stage of continuous casting of a martenstic stainless steel. CONSTITUTION:On the molten steel surface in a mold, powder having 1.0-2.5 poise of viscosity at 1300 deg.C of molten state and 1130-1180 deg.C solidified temp. is added and the number of oscillating cycles Cy of the mold is set so that powder consumption PLC shown in the following expression 0.02g/cm the number of cycles to execute the continuous casting. PLC=Pch/(LM.Cy). Wherein, Pch: the powder consumption per one charge (g/charge), LM: outer peripheral length at meniscus part on the molten steel surface in the mold (cm), Cy: the number of oscillating cycles of the mold (cycle/min).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is directed to optimizing the physical properties of powder and the conditions of mold oscillation.
The present invention relates to a method for continuously casting a slab in which surface maintenance can be omitted by preventing vertical cracks and fogging defects that occur in a slab of martensitic stainless steel.

[0002]

2. Description of the Related Art In continuous casting of molten steel, in order to reduce friction between a mold and a slab, prevent seizure and secure a stable casting state, it is necessary to add a lubricant (powder) in the mold. Oscillation is applied to move the mold up and down. However, in the early stage of casting, the melting of the powder added to the surface of the molten steel in the mold proceeds unsteadily.

During that time, the powder film formed between the mold and the cast piece becomes non-uniform, and non-uniform solidification is likely to occur due to non-uniform heat transfer resistance to the mold. like this,
When solidification is non-uniform, the initial solidified shell thickness becomes non-uniform in the mold, causing microcracks due to stress concentration. Easy to crack.

On the other hand, martensitic stainless steel represented by SUS 420J1 and 420J2 steel is 13% Cr-C in FIG.
As shown in the system phase diagram, solidification shell deformation due to transformation is likely to occur due to peritectic reaction in the solidification process. In particular, SUS 420J2 steel, which has a central composition in the peritectic region, has a liquid phase L up to the low temperature region.
Therefore, when the initial solidified shell strength is low and uneven solidification occurs, the above-mentioned vertical cracks and rash defects such as breakout marks are likely to occur. Therefore, from the start of casting, where the casting conditions are unsteady and solidification unevenness is likely to occur, vertical cracks and fog defects tend to occur frequently on the surface of the cast piece up to a casting length of about 10 m.

Therefore, in the conventional continuous casting of molten martensitic stainless steel, all the slabs in the initial stage of casting are
It was necessary to maintain the planer after gradual cooling, which inevitably resulted in a decrease in yield and an increase in manufacturing cost. Moreover, since the method of directly feeding the slab to the heating furnace of the rolling line without cooling it cannot be applied, the production efficiency cannot be increased,
It was not possible to reduce the fuel consumption rate of the heating furnace.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to prevent frequent vertical cracking and fog defects on the surface of a cast slab in the early stage of casting of martensitic stainless steel, thereby making it possible to omit slab surface maintenance. An object of the present invention is to provide a continuous casting method capable of improving the yield and reducing the manufacturing cost.

[0007]

[Means for Solving the Problems] The inventors of the present invention conducted a test operation of continuous casting of martensitic stainless molten steel, and (a) regardless of whether the number of oscillation cycles of the mold was high or low, a low viscosity mold. Vertical cracking can be prevented by increasing the freezing point of the powder. (B) Vertical cracking and fog defects can be prevented by using a powder with a high freezing point and reducing the number of oscillation cycles of the mold so that the powder consumption amount exceeds a predetermined value. I learned that you can prevent both at the same time.

Therefore, the solidification temperature of the mold powder, the number of mold oscillation cycles and the appropriate values of the powder consumption were examined to complete the present invention.

The gist of the present invention is that "13
A powder having a viscosity of 1.0 to 2.5 poise in a molten state of 00 ° C. and a solidification temperature of 1130 to 1180 ° C. is added, and a powder consumption amount P _LC represented by the following formula is 0.02 g /
The continuous casting method for molten martensitic stainless steel is characterized in that the number of oscillation cycles C _y of the mold is set so as to exceed the cm · cycle number.

P _LC = P _Ch / (L _M · C _y ), where P _LC : powder consumption (g / cm · cycle number) P _Ch : powder consumption per charge (g / charge) ) L _M: the molten steel surface meniscus portion outer peripheral length of the mold (cm) C _y: oscillation number of cycles of the mold (cycle / min),
Is.

In carrying out the above method of the present invention, the number of oscillation cycles C _y of the mold may be set to be equal to or less than the value of (C _y ) _UL represented by the following formula, depending on the solidification temperature of the powder. desirable.

(C _y ) _UL = 595−0.421 · T _S (where, (C _y ) _{UL is} the upper limit of the number of mold oscillation cycles (cycles / min) T _{S is} the solidification temperature (° C.) of the powder) is there.

[0013]

[Operation] FIG. 2 shows the relationship between the powder solidification temperature and the number of mold oscillation cycles, the powder consumption amount, and the degree of occurrence of defects on the surface of the initial cast slab during continuous casting of SUS 420J2 equivalent steel. ) Figure shows vertical crack, (b)
The figure shows a blurring defect. T _S shown in the figure is the solidification temperature of the powder used. Black plots, semi-black plots, and white spots in each plot indicate that the degree of defect occurrence is large,
Medium and no defects are shown. Regarding the degree of defect occurrence, the case where the maximum value of the defect-occurring slab length from the start of casting was 10 m or more was large, and the case of 1 to 10 m was medium. The powders used are all low-viscosity powders with viscosities of 1.0 to 2.5 poise at 1300 ° C.

The method of the present invention will be specifically described below with reference to FIG.

In the method of the present invention, a powder having a physical property of a viscosity at 1300 ° C. of 1.0 to 2.5 poise and a solidification temperature of 1130 to 1180 ° C. and a powder consumption of 0.02 g / cm 3 is used.
The reason for setting the number of mold oscillation cycles so as to exceed the number of cycles is as follows.

As shown in FIG. 2 (a), the vertical cracks generated on the surface of the slab in the initial stage of casting have a powder solidification temperature of 1130 to 1180 regardless of the number of oscillation cycles of the mold and the powder consumption. It can be prevented by increasing the temperature in the range of ° C. This is because the low-viscosity powder of 1.0 to 2.5 poise at 1300 ° C is used under the condition that the powder melting in the initial stage of casting is unstable, so from the molten steel meniscus part in the mold,
The molten powder flows smoothly between the mold and the solidifying shell, and since the solidifying temperature of the powder is increased to the range of 1130 to 1180 ° C, a stable powder film is formed between the mold and the solidifying shell, It is considered that this is because the mold was cooled uniformly and slowly.

On the other hand, as shown in FIG. 2 (b), the bleeding defect raises the solidification temperature of the powder to 1130 to 1180 ° C. and reduces the number of oscillation sills of the mold, so that the powder moves between the mold and the solidification shale of the powder. This can be prevented by setting the number of mold oscillation cycles so as to promote the influx of the powder and the powder consumption to exceed 0.020 g / cm · cycle number. It is considered that this is because the powder film formed between the mold and the solidified shell became thicker due to the increase in the powder consumption, the heat transfer resistance to the mold increased, and the mold cooling was further uniformly cooled. To be

When the solidification temperature of the powder exceeds 1180 ° C., it takes time to melt the powder, the viscosity of the molten powder is high, and the flow between the mold and the solidifying shell is hindered. And In addition, the solidification temperature is 1130
When using a powder in the range of ~ 1180 ° C, if the viscosity of the powder at 1300 ° C seems to exceed 2.5 poise, the flow into the mold-solidifying shell is hindered, so 2.
5 Poise was set as the upper limit.

Next, in the method of the present invention, the reason why it is desirable to adjust the number of mold oscillation cycles to not more than the upper limit value represented by the above formula according to the solidification temperature of the powder is as follows.

As shown in FIG. 2, powder consumption P
_LC (g / cm · cycle number) is powder coagulation temperature T _S (° C) and mold oscillation cycle number C _y (cycle / min)
And the first-order inverse correlation are shown by the following equation.

P _LC = (− 17.8 · C _y −7.5 · T _S ) × 10 ⁻⁵ +0.126 ・ ・ ・ From the formula, the powder consumption P _LC is 0.020 g / cm · a mold oscillation cycle exceeding the number of cycles. Upper limit of number (C _y )
_UL will be represented by the following formula.

(C _y ) _UL ≦ 595−0.421 · T _S ... The effects of the present invention will be specifically described below with reference to examples.

[0023]

[Example] A steel equivalent to SUS 420J2 having the composition shown in Table 1 was continuously cast, and the state of occurrence of vertical cracks and fog defects on the surface of the slab from the start of casting was visually observed to investigate the powder consumption.

The continuous casting machine used is a vertical type, the mold oscillation method is a sine curve method, and the mold dimensions are 1020 mm long side width × 200 mm short side width × 700 mm length.

The casting conditions were as follows.

Casting temperature: 1502 to 1527 ° C [freezing point (14
77 ℃) or higher heating temperature 25 to 50 ℃] Casting speed: From the start of casting to 3 minutes after ... 300 mm / min From 3 minutes to 9 minutes after ... 300 mm / min to 600 mm / min
After 9 minutes ... 600 mm / min Mold powder: having the chemical composition shown by symbol A in Table 2, viscosity at 1300 ° C. is 1.48 poise, solidification temperature is 1151
Oscillation cycle number of powder mold at 85 ° C .: 85 cycles / min (the upper limit value calculated by the above formula is 110 cycles / min.) In addition to the above-mentioned examples of the present invention, 1, shown in Table 3 as a comparative example,
2, 3 and 4 were carried out. In these comparative examples, powders having the chemical compositions shown by the symbols B and C in Table 2 and having a solidification temperature lower than the lower limit value defined in the present invention were used. 130 cycles
/ min, and in Comparative Examples 2 and 4, continuous casting was performed at 85 cycles / min.

Table 3 shows the powder consumptions of the present invention example and the comparative example, and the maximum value for each charge of the length of the defective slab from the start of casting.

As shown in Table 3, when the number of oscillation cycles of the mold is 130 cycles / min, the solidification temperature of the powder becomes high. It decreased and was completely prevented in Comparative Example 5. However, in Comparative Example 5, the powder consumption is 0.020 g / cm
-The number of cycles was less than or equal to the number of cycles, and the degree of occurrence of a blurring defect was large as compared with Comparative Examples 1 and 3. That is, although the vertical cracking flaw can be prevented by increasing the freezing point of the powder, the occurrence of a fog defect increases due to the reduction of the powder consumption.

The number of mold oscillation cycles is 85
In the case of / min, the vertical cracks decreased in the order of Comparative Examples 2 and 4 in which the powder coagulation temperature was high, and the present invention example, which was completely prevented in the present invention example. Further, in the example of the present invention, the powder consumption amount increased to 0.025 g / cm · cycle number, and it was possible to completely prevent the occurrence of the fog defect, which cannot be prevented in Comparative Examples 2 and 4.

As described above, in the examples of the present invention, it was possible to prevent the generation of vertical cracks and fogging defects at the same time because the viscosity of the powder was lowered, the solidification temperature was increased, and the number of mold oscillation cycles was decreased. This is because the uniform inflow of powder between the surfaces of the slabs was promoted, the stable formation of powder film was possible, and the uniform gradual cooling of the slabs was achieved.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

According to the method of the present invention, in continuous casting of martensitic stainless molten steel, it is possible to completely prevent vertical cracks and fog defects frequently occurring on the surface of the slab in the initial stage of casting. Therefore, it is possible to omit the surface treatment of the slab, improve the yield, and reduce the manufacturing cost. Further, so-called hot charging in which hot slabs are sent directly to the heating furnace in the hot rolling step can be carried out, so that the production efficiency can be improved and the fuel consumption rate of the heating furnace can be reduced.

[Brief description of drawings]

FIG. 1 is a phase diagram of 13% Cr-C steel.

[Figure 2] Powder solidification temperature and number of mold oscillation cycles during continuous casting of SUS 420J2 equivalent steel,
The relationship between the amount of powder consumed and the degree of occurrence of defects on the surface of the initial cast slab is shown. Fig. (A) shows vertical cracks, and (b) shows a case of fog defects.

Claims (2)

[Claims] 1. A powder consumption amount P represented by the following formula, to which powder having a viscosity of 1.0 to 2.5 poise in a molten state of 1300 ° C. and a solidification temperature of 1130 to 1180 ° C. is added to a molten steel surface in a mold. A continuous casting method for martensitic stainless steel, characterized in that the number of oscillation cycles C _y of the mold is set so that _LC exceeds 0.02 g / cm · cycle number. P _LC = P _Ch / (L _M · C _y ) ・ ・ ・ ・ However, P _LC : Powder consumption (g / cm · cycle number) P _Ch : 1 Charge powder consumption (g / charge) L _M : Perimeter of molten steel surface meniscus in mold (cm) C _y : Number of mold oscillation cycles (cycles / min) 2. The continuous casting method for martensitic stainless steel according to claim 1, wherein the number of oscillation cycles C _y of the mold is set to be equal to or less than an upper limit value (C _y ) _UL represented by the following formula. . (C _y ) _UL = 595-0.421 · T _S ··· However, (C _y ) _UL : Upper limit of the number of mold oscillation cycles (cycles / min) T _S : Freezing temperature of powder (° C)