JP3025631B2 - Continuous casting method of Ni-containing steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous steel having a 5 to 10 wt% Ni-containing steel (mainly 9% Ni steel) which has been almost cast by an ingot-making method without cracks such as surface cracks and subsurface cracks. It relates to a casting method.

[0002] Steel containing 5 to 10 wt% Ni has an embrittlement zone in a temperature range of 700 to 900 ° C and easily causes surface cracks and subsurface cracks. Particularly, it was extremely difficult to continuously cast 9% Ni steel. However, with the recent increase in the use of LNG (liquefied natural gas) due to the demand for energy cleanup, the use of LNG tank materials has been increasing.
Increased demand for Ni-containing steel, making it difficult to manufacture
It is important to establish a casting technology that enables mass production of Ni-containing steel at low cost.

[0003]

2. Description of the Related Art Conventionally, as a continuous casting technique for Ni-containing steel, for example, Japanese Patent Publication No. 60-8134 (a method for preventing surface flaws in continuous casting of Ni-containing low-temperature steel) includes S: 0.0020%.
Hereinafter, N: 0.045% or less, Ca: 0.0020 to 0.0070%,
A method for continuously casting the same is disclosed in Japanese Patent Application Laid-Open No. Sho 58-77756 (a method for continuously casting a defect-free slab of nickel-containing steel), in which P: 0.007% by weight or less, S: 0.003% by weight or less, and A method of cooling the slab drawn from the mold from directly below the casting mold to the slab straightening point starting position at a secondary cooling water ratio of 0.4 to 1.0 l / kg,
Japanese Patent Publication No. 5-4169 (Method of preventing surface cracks in continuous casting of Ni-containing steel) discloses a method in which the cooling rate of the slab surface in the range of 1150 ° C to 950 ° C is 20 ° C / min or less. Are respectively proposed and disclosed.

Here, the secondary cooling water ratio (l / kg) in the above-mentioned Japanese Patent Application Laid-Open No. 58-77756 is [slab receiving water secondary cooling water amount (l / min)] / [slab thickness (m). × slab width (m) × slab density (kg / m ³⁾ × casting speed (m / min)]
It is represented by

[0005] However, according to the inventors' research, it was found that Ni-containing
The requirements for preventing the occurrence of cracks under the surface or under the surface layer of the slab in continuous casting of steel greatly differ depending on the continuous casting machine and changes in the casting conditions, and in any of the above disclosure examples, only the respective requirements, It became clear that castings without cracks on the surface or under the surface layer could not be cast stably in the process.

[0006] That is, according to the casting experiment of the inventors, it is as follows. A casting experiment was conducted under the following condition A using the method for producing a low-temperature Ni-containing steel described in Japanese Patent Publication No. 60-8184. Condition A Steel composition: C: 0.06%, Si: 0.3%, Mn: 0.6%,
Ni: 9.01%, Cu: 60 ppm, P: 110 ppm, N: 35 ppm,
Ca: 60 ppm Casting speed: 1.2 m / min Slab thickness: 220 mm Secondary cooling: Mist cooling Secondary cooling water ratio: 0.6 l / Kg Bending radius: 10 m Strain rate (dε / dt): 1 × 10 ⁻⁴ / s Straightening point temperature: 897 ℃-913 ℃ Straightening: One-point bending straightening

[0007] As a result, fine crack defects were observed on the entire front and back surfaces of the slab. EP for these defects
As a result of observation with MA, enrichment of P in addition to S was observed. Therefore, casting was attempted under the same conditions as above except that the P content of the steel composition was reduced to 80 ppm or less. As a result, cracking defects were greatly reduced and hardly observed. Therefore, it was clarified that the regulation of only S, N and Ca components was insufficient to prevent cracking defects in Ni-containing steel.
Incidentally, in the examples of the specification of Japanese Patent Publication No. 60-8134, the P content is 110 ppm.

Further, when casting is performed under the following condition B, the method disclosed in Japanese Patent Application Laid-Open No. Sho 58-7776 (P ≦ 0.
0070%, S ≦ 0.0030%, secondary cooling water ratio: 0.4 to 1.0 l /
kg), it was possible to cast a defect-free slab having no cracks on the surface or under the surface layer.
No defect-free slab could be cast under the following condition C, which was carried out with the components and the water ratio of JP-A-6-106.

Condition B Steel composition: C: 0.05%, Si: 0.25%, Mn: 0.5%,
Ni: 9.05%, P: 55 ppm, S: 30 ppm Casting speed: 1.4 m / min Slab thickness: 260 mm Secondary cooling: Cross spray cooling Secondary cooling water ratio: 0.41 / kg Bending radius: 10 m Strain rate (dε / dt): 1 × 10 ⁻⁴ / s Straightening point temperature: 910 to 930 ° C. Straightening: One-point bending straightening Condition C Steel composition: C: 0.06%, Si: 0.3%, Mn: 0.55%,
Ni: 8.95%, P: 50 ppm, S: 20 ppm Casting speed: 0.9 m / min Slab thickness: 220 mm Secondary cooling: Mist cooling Secondary cooling water ratio: 0.51 / kg Bending radius: 10 m Strain rate (dε / dt) ): 0.5 × 10 ^-5 / s Straightening point temperature: 780 to 850 ° C Straightening: One-point bending straightening

[0010] As described above, none of the disclosed examples of the prior art described above have fully extracted all the requirements for eliminating the frequently occurring slab surface and subsurface cracks in continuous casting of Ni-containing steel. There was a problem.

[0011]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and advantageously responds to changes in the continuous casting machine and its casting conditions, such as surface cracks and subsurface cracks. It is an object of the present invention to propose a continuous casting method for Ni-containing steel in which a cast slab free of defects can be obtained stably.

[0012]

SUMMARY OF THE INVENTION The present invention relates to a conventional technique for continuously casting Ni-containing steel, which only controls the temperature in the bending section and straightening section in the continuous casting machine and regulates the contents of P and S. Differently, the concept of strain rate in the bending and straightening part, which is a new finding, and embrittlement behavior due to slab temperature in the bending and straightening part were found by experiments, and their relationship was expressed by a linear equation. By casting under such conditions, continuous casting of a Ni-containing steel slab free of surface cracks and subsurface cracks is achieved.

That is, the gist of the present invention is as follows.
is there. Ni: For continuous casting of alloy steel containing 5 to 10 wt%
And the contents of P and S in the steel were respectively P: 0.006 wt
% And S: 0.003 wt% or less.
Bend roll and straightening with secondary cooling zone below
Continuous rolls with only regular rolls or straightening rolls
To the casting machine, and to the bending and straightening section in the continuous casting machine.
Rate of cast slab dε / d_t(S ^-1) And slab surface
Casting under the condition that the relationship with the temperature T (° C) satisfies the following formula (1)
It is a continuous casting method of Ni-containing steel characterized by
(First invention). [Note] T ≧ −1.018 × 10^Five・ (Dε / d_t) +907.7 ---- (1)

Ni: a series of alloy steels containing 5 to 10 wt%
In continuous casting, the P, S and Ca contents of the steel
P: 0.008 wt% or less, S: 0.005 wt% or less and
Ca: 0.0010-0.0090wt% of molten steel
Bending roll and straightening roll with secondary cooling zone below
Or a continuous casting machine with only straightening rolls
Feeding and casting in the bending and straightening section in the continuous casting machine
Piece strain rate dε / d_t(S ^-1) And the slab surface temperature T
Casting under the condition that the relationship with (℃) satisfies the following formula (1)
A continuous casting method for Ni-containing steel, characterized in that
invention). [Note] T ≧ −1.018 × 10^Five・ (Dε / d_t) +907.7 ---- (1)

[0015] In the method according to the first or second invention, each roll provided in the continuous casting machine is provided with:
This is a continuous casting method of Ni-containing steel used as a full width roll (third invention).

[0016] In the method according to the first, second, or third invention, a continuous casting method for Ni-containing steel, wherein cooling in the secondary cooling zone is performed by mist cooling (fourth invention).

Here, the respective rolls provided in the continuous casting machine refer to guide rolls (roller-aprons), support rolls and pinch rolls, as well as bending rolls and straightening rolls.

A full-width roll is a roll disposed in a continuous casting machine, as shown in FIG. 1, in order to maintain the strength of the roll and improve the flow of cooling water on the slab surface. It is common to use a so-called split roll that has a bearing part in the circumferential direction of the roll face at, for example, 1/3 or 1/2 position in the roll width direction. A roll without a part is called a full width roll.

FIG. 1 is an explanatory view showing a split type roll having a bearing portion at a position 1/3 in the roll width direction and the flow of cooling water, wherein 1 is a roll, 2 is a bearing portion, and 3 is a slab. The cooling water flows downward from the upper part of the drawing through the bearing part as shown by the arrow.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation of the present invention will be described below based on experimental examples. In order to solve the above problems, the inventors conducted a high-temperature tensile test of a Ni-containing steel ingot, a casting experiment using a test continuous casting machine, and a casting experiment using a continuous casting machine for process production. The results of these experiments and studies are described below in order.

In the test continuous casting machine and the continuous casting machine for process production used here, it is natural that the bending section by the straightening roll in the straightening section located downstream thereof is more than the bending section by the bending roll. However, since the slab temperature decreases and enters the temperature embrittlement region described later, the slab crack in the straightening section may be considered as a problem. Therefore, the following description (including the example) will be made only for the straightening section. belongs to.

1) High-temperature tensile test of Ni-containing steel slab In a small melting furnace, C: 0.052 wt%, Si: 0.32 wt%, Mn:
0.62wt%, Ni: 9.0wt%, P: 0.0050wt%, S: 0.0013
After smelting a Ni-containing steel containing 0.0030 wt% and wt% and N: 0.0030 wt%, using a test piece cut from an ingot cast by an ingot method, a high-temperature tensile test was performed by changing the test conditions, and the drawing value ( RA) was measured. These results are summarized in FIG. FIG. 2 is a graph showing the relationship between the test temperature and the aperture value with the strain rate as a parameter.

From FIG. 2, a. The brittle zone of this Ni-containing steel is 900 ° C or less. b. The drawing value in the embrittlement range largely changes mainly depending on the slab temperature (test temperature) and the strain rate. c. The lower the strain rate, the lower the aperture value. That is, surface cracks and subsurface cracks are likely to occur. I understand.

Therefore, based on these results, it is only necessary to control the temperature of the slab and the cooling rate thereof, such as the regulation of the cooling rate in the specific temperature range and the ratio of the secondary cooling water, as disclosed in the above-mentioned prior art. It was speculated that, in some cases, a defect-free slab could be cast, but crack defects could occur on the slab surface or under the surface layer depending on the strain rate of the slab in the straightening section.

2) Casting experiment using a test continuous casting machine From the relationship between the strain rate, temperature and drawing value shown in FIG. 2 above, what is the drawing value, the surface layer of the slab (slab) (surface and under the surface layer) In order to clarify whether or not cracking occurs in (1), continuous casting of 9% Ni-containing steel having the same component composition as the steel used in the high-temperature tensile test was performed using a test continuous casting machine.

The continuous casting conditions are such that the straightening section strain rate d
ε / _dt was kept constant at 5 × 10 ⁻⁴ (s ⁻¹ ), and the surface temperature of the cast slab was 800 ° C., 820 ° C., 840 ° C., 850 ° C., 86
The test was carried out at eight levels of 0 ° C., 880 ° C., 900 ° C. and 920 ° C., and each of the obtained slabs was examined for cracks in the surface layer.

Here, the bending strain rate of the straightening section is known (for example, Japan Iron and Steel Association, Iron and Steel Handbook II
Edition, Maruzen, 57.12.25, P629) The following formula (2) and
Determined by (3).

Ε _n = (D / 2) · (1 / R _n−1 −1 / R _n ) ---- (2) dε / dt = ε _n / L / V _c ---- (3) Where ε _n : strain rate at the n-th bending straightening point (−) D: slab thickness (m) R _n : bending radius at the n-th bending straightening point (m) dε / dt: straightening section strain rate (s ⁻¹⁾ ) L: Straightening part roll pitch (m) V _c : Casting speed (m / s)

The cracks on the surface layer are detected by an MT method (flaw detection method using a magnetic fluid) after the slab surface is treated with a grinder, and 500 × 500 mm on both sides of the slab.
Was measured for the total number of defects generated in the area.

The results are summarized in FIG. FIG.
3 is a graph showing the relationship between the correction part temperature on the slab surface and the defect index. The defect index in FIG. 3 is obtained by changing the total number of defects on both sides of the slab to the defect index based on Table 1.

[0031]

[Table 1]

FIG. 3 shows that the slab temperature of the bending straightening section is 84
With a threshold temperature of 0 to 850 ° C., a defect-free slab is obtained at a higher temperature. When the temperature is lower than the threshold temperature, the number of defects increases. When the temperature is extremely low, the number of individual defects (cracks) reaches several millimeters to tens of millimeters. From these results, the slab temperature T ≧ 850 ° C. and the strain rate dε / _dt = 5 × 10
It can be seen that casting a defect-free slab by passing through the bending straightening section at ^-4 (s ^-1 ) is possible.

When these results are superimposed on the relationship between the strain rate, the test temperature, and the reduction value shown in FIG. 2, the slab temperature T = 850 ° C. and the strain rate dε / _{dt are considered.}
= 5 × 10 ^-4 and aperture value is 65%, so aperture value is 65%
If it is above, it can be presumed that casting of a defect-free slab is possible.

From this, Table 2 shows the threshold temperature of the slab surface corresponding to the strain rate when the reduction value becomes 65% or more from FIG.

[0035]

[Table 2]

When the relationship between the strain rate dε / _dt (s ⁻¹ ) in Table 2 and the slab temperature T (° C.) is regressed by a linear expression, it can be expressed by the following expression (1). T ≧ −1.018 × 10 ⁵ · (dε / _dt ) +907.7 ---- (1) Therefore, in order to conform to this equation (1), the slab in the bending straightening section is determined by the secondary cooling water ratio. By adjusting the surface temperature and selecting the strain rate, it is possible to stably cast a slab of a Ni-containing steel having no defects, that is, without surface cracks and subsurface cracks.

As is apparent from Table 2, the above equation (1) is applicable when the strain rate is 1 × 10 ⁻³ (s ⁻¹ ) or less, and exceeds 1 × 10 ⁻² (s ⁻¹ ). ^-1 ) Above is 500
Since the aperture value is always 65% or more at temperatures above ℃, 50
At a temperature of 0 ° C. or more, embrittlement is avoided, and thus a defect-free slab with a slab surface temperature T of 500 ° C. or more is obtained.

The straightening unit in the continuous casting machine for process production
Practical strain rate is 2 × 10 ^-3(s^-1)
You. If the strain rate exceeds this, the internal strain
Will be increased. Field with increased internal strain
If the Ni-containing steel becomes an austenitic phase at high temperatures
Is highly sensitive to strain, and internally
Due to problems such as S and P enrichment,
It becomes easy to generate defects such as cracks. Therefore, the strain
Increasing speed increases internal defects.
You. Therefore, 2 × 10^-3(s^-1)
Is not preferred.

3) Casting test using a continuous casting machine for process production The surface of a slab when a test operation was performed on 9% Ni-containing steel using a continuous casting machine used for process production under conditions compatible with the above equation (1). The relationship between the state of occurrence of cracks and subsurface crack defects and the P and S contents of 9% Ni-containing steel was investigated.

FIG. 4 shows the results of these investigations.
FIG. 4 is a graph showing the relationship between the P and S contents in continuous casting of 9% Ni-containing steel and the surface cracks and subsurface cracks of the slab. As is apparent from FIG.
Even if conforming to (1), cracking defects occur when the contents of P and S in the steel exceed 70 wtppm of P and 40 wtppm of S, respectively. P and S as conventionally said
Is a harmful component that promotes the generation of cracking defects. In order to prevent the occurrence of cracking defects, it is necessary to satisfy the condition of the above formula (1), and to check the P and S contents in steel for safety. It is important that P ≦ 0.0060 wt% and S ≦ 0.0030 wt%, respectively. In the present invention, Ca may be added, but in this case, since Ca fixes S in the steel,
The content is acceptable up to 0.005 wt%, and the P content is acceptable up to 0.008 wt% from experimental results, although the reason is unknown.

FIG. 4 shows that the strain rate dε / _dt is 1 ×
If it exceeds 10 ^-3 (s ^-1 ), it is not included. In this case, no crack defect occurred even if the contents of P and S exceeded the above values.

The addition of Ca to fix S in steel is effective in preventing cracking defects. In this case Ca
If the content is less than 0.0010 wt%, the effect is not obtained.
If the content exceeds wt%, the excess Ca remaining in the steel is combined with S in the steel and rust is likely to occur. The characteristics will be degraded. Therefore, the content of Ca is 0.
It is preferable that the content be not less than 0010 wt% and not more than 0.0090 wt%.

The above was obtained mainly with 9% Ni-containing steel, but there was no substantial change in the phenomenon when the Ni content was in the range of 5 to 10 wt%.

Next, as a result of examination of the test operation in more detail, it was found that the roll of the continuous casting machine was not a split type as shown in FIG. It turned out that it is preferable for homogenization. That is, when the roll of the continuous casting machine is a split type, as shown in FIG. 1, the cooling water flowing on the surface of the slab selectively flows through the bearing portion of the split roll, and the portion is strongly cooled and straightened. This is because it becomes difficult to control the temperature in the slab width direction at the part.

Further, it has become clear that it is more preferable to use mist cooling than to use water spray in the method of cooling the slab.

That is, in the water spray, even if the amount of water is changed, it is difficult to largely change the heat flux removed from the slab by the cooling water, and it is difficult to control the heat flux, which is required for Ni-containing steel and the like. It is not always suitable for uniform ultra-low cooling, but if ultra-low cooling is to be achieved by water spray, the amount of water will be very small. The density becomes large at the center of the spray and becomes small at the periphery thereof, resulting in a non-uniform state. As a result, uniform cooling cannot be performed, and uneven cooling occurs in the slab width direction, which may cause cracks in the slab.

When mist cooling is used to solve these problems, it is easy to change the heat flux from the slab by changing the water / water ratio, and it is easy to achieve extremely slow cooling. Was. Further, since the water-vapor ratio may be changed, the water density corresponding to the slab did not change at each position of the slab, and a uniform water density was always obtained over the entire slab cooling surface. By using the mist cooling in this way, it is possible to suppress the heat removal heat flux from the slab, achieve extremely slow cooling, and uniformly cool the slab. Therefore, the use of mist cooling is effective in preventing the occurrence of crack defects in the slab.

[0048]

EXAMPLES Five examples of 9% Ni-containing steel slab casting using a continuous casting machine for production under conditions meeting the above equation (1) will be described below in order. Table 3 summarizes the casting conditions of the fifth embodiment.

[0049]

[Table 3]

Example 1 Hot metal was subjected to hot metal desulfurization in a pretreatment to reduce S to 30 wtppm or less, and then was blown twice in a converter to reduce P and S to reduce P ≦ 50.
wtppm, S ≦ 20 wtppm. Further, degassing was carried out by a degassing device, and premelt flux was charged to perform melting to prevent resulfurization. Continuous casting of the molten steel having the composition shown in Table 4 was performed.

[0051]

[Table 4]

In the casting, as shown in Table 3, the slab surface temperature at the straightening section was set at a strain rate of 2.3 × 10 ⁻⁴ (s ⁻¹ ) at the straightening section and a secondary cooling water ratio of 0.85 l / kg. However, it was confirmed in advance by a preliminary experiment that a value calculated by substituting the strain rate into the above equation (1) was 884 ° C. or more, and casting was performed under conditions satisfying the above equation (1).

FIG. 5 is a graph showing a calculated value and an actually measured value of the slab surface temperature. From this figure, the measured values also show that the correction section slab surface temperature is in the range of 890 to 950 ° C., which is equivalent to the calculated value.

Further, the strain rate of the straightening portion: 2.3 × 10 ⁻⁴ (s
^-1 ) is the slab thickness: 0.22m, casting speed: 0.85m / min,
Assuming that the straightening roll pitch is 0.54 m and the radius of curvature is 12 m, the other radius is infinite for one-point bending, and can be obtained from the above equations (2) and (3).

After grinding all the slabs cast in this way to 2 mm on one side and 4 mm in total on both sides with a grinder, cracking defects were examined by the MT method, but no defects were detected.

Example 2 A molten steel having the same composition as that of Example 1 was produced by performing hot metal treatment, blowing and degassing, and the strain rate of the straightening portion was 1.1 × 10 ⁻⁴ (s ^{− 1} ) Cast as

At this time, the correction part slab surface temperature obtained by substituting the above strain rate into the above equation (1) is 896 ° C. or more. Therefore, in the above casting, the slab surface temperature of the straightening section is 90
The secondary cooling water ratio was set to 0.5 l / kg so as to be 0 ° C. or higher. As a result, the measured value of the slab surface temperature of the straightening section was 900.
It was in the range of 930 ° C., and could be cast under the conditions satisfying the above formula (1).

All the slabs thus obtained were inspected for cracking defects by the MT method after the grinder treatment in the same manner as in Example 1, but no defects were detected.

Example 3 C: 0.050 wt%, S melted in the same manner as in Example 1.
i: 0.24 wt%, Mn: 0.55 wt%, Ni: 9.2 wt%, P: 0.008
0 wt%, S: 0.0050 wt%, N: 0.0032 wt% and Ca: 0.0
Strain rate of straightening section of Ca-added molten steel with composition of 025wt%
Degree 1.0 × 10^-Four(s ^-1).

At this time, the correction part slab surface temperature obtained by substituting the strain rate into the above equation (1) is 898 ° C. or more. Therefore, in the above casting, the slab surface temperature of the straightening section is 91
The secondary cooling water ratio was set to 0.49 l / kg so as to be 0 ° C. or higher. As a result, the measured value of the slab surface temperature of the straightening section was 905.
It was in the range of 920920 ° C. and could be cast under the conditions satisfying the above formula (1).

For all the slabs thus obtained, cracking was carried out by a method similar to that of Example 1 after the grinder treatment by the MT method, but no defects were detected.

Example 4 C: 0.055 wt%, smelted in the same manner as in Example 1.
i: 0.25 wt%, Mn: 0.50 wt%, Ni: 9.1 wt%, P: 0.005
Using molten steel having a composition of 0 wt%, S: 0.0015 wt% and N: 0.0031 wt%, the strain rate of the straightening portion was 4.47 × 10 ⁻⁴ (s
^-1 ).

At this time, the correction part slab surface temperature obtained by substituting the strain rate into the equation (1) becomes 862 ° C. or more. Therefore, in the above casting, the slab surface temperature of the straightening section is 89
The secondary cooling water ratio was set to 0.7 l / kg so as to be 0 ° C. or higher. As a result, the measured value of the slab surface temperature of the straightening section was 890
It was in the range of 910 ° C. and could be cast under the conditions satisfying the above formula (1).

Further, in this casting, a split type roll having bearing portions at 1/3 and 1/2 of the roll width was used as the roll of the continuous casting machine.

When all the slabs thus obtained were subjected to a grinder treatment by the same method as in Example 1 and examined for cracking defects by the MT method, a cracking defect was detected at the center of the slab width. Investigation of the cause revealed that the water flow was concentrated at the center of the slab width and that this part was locally undercooled.

Therefore, the roll of the continuous casting machine was changed to a full-width type roll having no division in the width direction, and casting was performed under the same conditions as above, and cracking defects were examined for all the obtained slabs in the same manner as above. However, no defect was detected.

Example 5 C: 0.055 wt%, S produced by melting in the same manner as in Example 1.
i: 0.23 wt%, Mn: 0.55 wt%, Ni: 9.1 wt%, P: 0.004
0 wt%, S: 0.0010 wt%, N: 0.0031 wt% and Ca: 0.0
Strain rate of straightening section of Ca-added molten steel with 030wt% composition
3.4 x 10 degrees^-Four(s ^-1).

At this time, the correction part slab surface temperature obtained by substituting the above strain rate into the above equation (1) is 873 ° C. or more. Therefore, in the above casting, the slab surface temperature of the straightening section is 89
The secondary cooling water ratio was set to 0.8 l / kg so as to be 0 ° C. or higher. As a result, the measured value of the slab surface temperature of the straightening section was 895.
It was in the range of 925925 ° C. and could be cast under the conditions satisfying the condition of the above formula (1).

All the slabs thus obtained were treated with a grinder in the same manner as in Example 1 and then examined for cracking defects by the MT method. As a result, there were places where no defect was found, and places where some cracking defects were detected. As a result of investigating the cause of the generation of the defect, it was found that the cooling method was a normal water spray (flat type), the cooling was sometimes partially uneven, and a crack defect was sometimes generated.

For this reason, casting was carried out by changing to mist cooling capable of more uniformly cooling, and as a result of examining all the obtained slabs for cracking defects, no defects were detected.

From the above, it is clear that a slab of Ni-containing steel free from cracking defects can be obtained by casting according to the present invention satisfying the above formula (1), and is sufficiently applicable to a continuous casting machine for production. You can see that.

FIG. 6 is a graph showing the relationship between the strain rate in the straightening portion and the temperature, which affects the presence or absence of crack defects in the slab slab, including the results of the experimental examples and examples. become that way. FIG. 6 shows the equation (1)
The presence or absence of a crack defect is clearly separated from this straight line, and FIG. 6 shows that the continuous casting condition of the Ni-containing steel slab free of crack defect is represented by the above formula (1).
Is valid.

[0073]

The present invention restricts the contents of P and S in the Ni-containing steel and satisfies the relationship between the surface speed and the strain rate of the slab in the bending and straightening section in the continuous casting machine represented by the primary equation. According to the present invention, Ni is free from surface cracks and subsurface cracks.
Continuous casting of slabs of steel containing steel can be performed stably, and with continuous casting, production costs can be greatly reduced by improving productivity and yield, etc. Can be supplied to

[Brief description of the drawings]

FIG. 1 is an explanatory view showing a split type roll having a bearing at a position 1/3 in the roll width direction and a flow of cooling water.

FIG. 2 is a graph showing a relationship between a test temperature and an aperture value using a strain rate as a parameter.

FIG. 3 is a graph showing a relationship between a correction portion temperature of a slab and a defect index.

FIG. 4 is a graph showing the relationship between the P and S contents in continuous casting of 9% Ni-containing steel and the surface cracks and subsurface cracks of a slab.

FIG. 5 is a graph showing a relationship between a calculated value of a slab surface temperature and an actually measured value.

FIG. 6 is a graph showing the relationship between the strain rate in the straightening section and the temperature with respect to the presence or absence of crack defects in the slab slab.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Roll 2 Bearing part 3 Slab (slab)

────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI B22D 11/128 340 B22D 11/128 340A 350 350Z C22C 38/00 302 C22C 38/00 302Z 38/08 38/08 (72) Invention Person: Mamoru Suda 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Inside the steelworks (72) Inventor Tadashi Kohinata 1-chome, Mizushima-Kawasaki-dori, Kurashiki-shi, Okayama Pref. Kawasaki Steel Corporation Mizushima Works (56) References JP-A-8-197193 (JP, A) -10919 (JP, A) JP-A-7-32092 (JP, A) JP-A-6-15412 (JP, A) JP-A-59-163063 (JP, A) JP-A-58 77756 (JP, A) JP-A-63-216924 (JP, A) JP-A-63-123556 (JP, A) JP-A-57-26141 (JP, A) JP-A-1-228644 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) B22D 11/00 B22D 11/12 B22D 11/124 B22D 11/128 B22D 11/128 340 B22D 11/128 350 C22C 38/00 302 C22C 38 / 08

Claims (4)

(57) [Claims] 1. A continuous alloy steel containing 5 to 10% by weight of Ni.
In casting, the contents of P and S in the steel were
P: 0.006 wt% or less and S: 0.003 wt% or less
Bent steel with a secondary cooling zone below the mold.
Rolls and straightening rolls or only straightening rolls
Slab in a bending and straightening section in the continuous casting machine.
Speed dε / d_t(s ^-1) And the slab surface temperature T (° C)
The feature is that casting is performed under the condition that the relationship satisfies the following formula (1)
Continuous casting method for Ni-containing steel. [Note] T ≧ −1.018 × 10^Five・ (Dε / d_t) +907.7 ---- (1) 2. A continuous alloy steel containing 5 to 10% by weight of Ni.
In casting, the contents of P, S and Ca of the steel
P: 0.008 wt% or less, S: 0.005 wt% or less and C
a: Molten steel from 0.0010 to 0.0090 wt%
Bending roll and straightening roll with secondary cooling zone below
Or a continuous casting machine with only straightening rolls
Feeding the slab strain in the bending and straightening section in the continuous casting machine
Speed dε / d_t(S ^-1) And the slab surface temperature T (° C)
The feature is that casting is performed under the condition that the relationship satisfies the following formula (1)
Continuous casting method for Ni-containing steel. [Note] T ≧ −1.018 × 10^Five・ (Dε / d_t) +907.7 ---- (1) 3. The roll according to claim 1, wherein each roll disposed in the continuous casting machine is a full width roll.
A continuous casting method for Ni-containing steel. 4. The continuous casting method for Ni-containing steel according to claim 1, wherein the cooling in the secondary cooling zone is by mist cooling.