JPS6360056A - Method and mold for continuously casting stainless steel containing titanium - Google Patents

Abstract

PURPOSE:To prevent the development of surface defect in a cast slab and to reduce the inclusion by satisfying the specific equation about the distance from a meniscus to a motor core center of stirring apparatus and specifying the horizontal direction flow speed of molten steel in a mold. CONSTITUTION:In case of casting continuously the molten stainless steel containing >=0.1wt% Ti, the setting height of motor core is adjusted by adjusting hydraulic pressure supplying into the hydraulic cylinder 11 and the electromagnetic stirring apparatus holding to the prescribed height is used. And, the distance (h) from the meniscus to the center of motor core 1 for the stirring apparatus is decided, so as to satisfy the equation. Next, the range of stirring flow speed by horizontal spiral flow is provided to 15-40cm/sec at the position of 2Xh. In this way, the development of surface defect in the cast slab is pre4vented and the inclusion caught on the surface layer of cast slab is reduced. In this equation, (h) for the distance (m) toward casting direction from the meniscus to the center of core for the linear motor K for the solidification constant mm/min<1/2> and D for solidified shell thickness mm at the distance 2h from the meniscus under consideration of 10-30mm this value range, are expressed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a Ti-containing stainless steel that prevents the occurrence of Ti mist leakage, which is always a problem when stainless steel containing a relatively large amount of Ti is produced by continuous JR production. This invention relates to a continuous casting method.

[Conventional technology]

Adding Ti has been used to improve the corrosion resistance and mechanical properties of stainless steel by converting C and N in steel into stable carbonitrides. However, there is a problem of +Ti mist leakage. In other words, since Ti is an extremely active metal, it reacts with N in molten steel when it is added.
Furthermore, when molten steel is injected, it reacts with N in the atmosphere to form nitrides.

When these nitrides try to float due to the difference in specific gravity in the molten steel in the mold, they coagulate and form clusters. Since such nitride clusters have low solubility in mold powder, they are often trapped in the surface layer of continuously cast slabs. The nitride clusters trapped in the surface layer of the slab have no plasticity because the melting point of the nitride particles that make up the clusters is extremely high, so they are only arranged in a linear shape when the slab is rolled.
Linear flaws, that is, Ti mist leaks, occur on the surface of the cold-rolled sheet. This Ti mist leak not only impairs the beauty of the product, but also adversely affects the corrosion resistance, which is the mission of stainless steel.

For this reason, conventionally, in the case of continuous casting of Ti-containing stainless steel, after reducing gas components such as Ox and Nt as much as possible using secondary refining furnaces such as VOD and AOD, and separating and removing inclusions. , a continuous casting method has been used.

However, stainless steel contains a large amount of C, which has a strong affinity with 0 There is a limit to the removal of inclusions.For this reason, non-metallic inclusions (particularly in Ti-added steels)
There is no need to cast molten steel in which residual nitrides remain.

On the other hand, in continuous casting of 1W4, a technology has been developed in which molten steel is given fluidity in the mold using a magnetic stirring device. This method provides electromagnetic stirring to the molten steel in the mold.This in-mold TL electromagnetic stirring makes it possible to reduce inclusions in the surface layer of the slab.
, 67 (1981) 5833. Furthermore, according to Japanese Patent Publication No. 59-7536, if the upper end of the beam near motor is located within 200 mm above the scene, continuous casting can be performed even with undeoxidized molten steel, and in the case of deoxidized molten steel, the occurrence of surface defects can be reduced. is taught.

In other words, if electric stirring is applied to the molten steel in the mold to appropriately give the molten steel flow to the front surface of the solidified shell that is growing in the mold, the molten steel will be trapped in the pinholes that occur in the surface layer of the slab and in the surface layer of the slab. It is known that inclusions can be reduced.

Furthermore, Japanese Patent Application Laid-Open No. 59-33060 discloses that gas blowing or i! [Disclosed is a method for reducing inclusions in the surface layer of a continuous cast slab of stainless steel by applying a flow at a predetermined flow rate to the center above the surface of molten steel in a mold using means such as stirring. This method is based on the premise that inclusions in stainless steel are localized in the surface layer within 3IIlffi from the surface.

[Problem that the invention seeks to solve]

The present invention aims to solve the problem of surface defects mainly due to Ti nitrides, which occur during continuous casting of Ti-containing stainless steel, by applying mold-like electromagnetic stirring technology. Until now, it was completely unknown whether the problem of surface flaws caused by Ti nitride could be avoided by electromagnetic stirring within the mold. The inventors have discovered that Ti
The distribution of inclusions in the thickness direction from the surface of the stainless steel slab was investigated using a large number of slabs, but unlike the case of the above-mentioned Japanese Patent Application Laid-open No. 59-33060, inclusions were found within 3 mm or more from the surface of the slab. I learned that there are many. Therefore, even if the previously proposed in-mold electric mixing method is applied to continuous casting of Ti-containing stainless steel, the area from the surface of the cast slab where inclusions are removed is small. It was concluded that the occurrence of mist leaks cannot be prevented.This is also demonstrated by, for example, test floor 2 in Table 1 below.

Furthermore, in the conventionally proposed in-mold electromagnetic stirring device, the position of the motor core is fixed within the mold. Therefore, it is also difficult to find conditions suitable for continuous casting of Ti-containing stainless steel. In other words, if the position of the 5 motor core is fixed, PXlt! ? However, as the thickness of the solidified shell at a certain distance from the meniscus changes as the casting speed changes, in some cases (for example, in the case of high-speed casting) stirring is not necessary. A situation arose in which the agitation did not reach the intended area.

For this reason, when the stirring intensity is increased to expand the area where stirring is required, a new problem has arisen in that powder entrainment occurs.

An object of the present invention is to solve the problem of surface flaws occurring in Ti-containing stainless steel slabs by improving the conventionally proposed steel casting type electromagnetic stirring device and finding appropriate operating conditions. .

[Means to solve problems]

The present invention is a continuous casting method for reducing the occurrence of surface defects in continuous cast slabs of stainless steel containing 0.1% by weight or more of Ti, and is equipped with a linear motor type stirring device whose setting position is variable in the height direction. Using a continuous casting mold.

The distance from the meniscus to the center of the motor core of the device is determined so that the following equation (11) is substantially satisfied, and the horizontal flow velocity of molten steel in the mold at a position 2 x h from the meniscus is 15 to 40 cm The present invention provides a method for continuous casting of Ti-containing stainless steel, characterized in that a horizontal swirling flow is applied to the ffI steel in the mold by the electromagnetic stirring device so that the flow rate is within the range of sec.

11-□・V・(D/K)”...(1) however.

h: Distance (m) in the casting direction from the meniscus to the offshore center of the linear motor core.

K: Coagulation fixation number (mm/min"") +D: Coagulation seal thickness (mm) at a distance of 2h from the meniscus,
The value shall be in the range of 10 to 30 degrees.

The contents of the present invention will be specifically explained below with reference to the drawings.

FIG. 1 shows a plan view of a continuous casting mold according to the present invention.

FIG. 2 shows a cross section of FIG. It is installed vertically movably inside both long sides 3. 4 is an immersion nozzle for injecting ?8W4 into the mold, 5 is molten steel in the mold, 6 is continuous casting powder, and 7 is a solidified shell. ing.

As shown in FIG. 1, a motor core 1 and a coil 2 surrounding the motor core 1 are installed in molds on both long sides at the width of the slab. By reversing the moving directions of the magnetic fields of the pair of glue near motors installed in the molds on both long sides,
The flows along both long sides are in opposite directions, and the molten steel in the mold rotates (swivels) in the horizontal direction as shown by the arrow in FIG. This stirring mode is referred to as a "horizontal swirl flow" in this specification.On the other hand, when the moving directions of the magnetic fields of a pair of linear motors are set in the same direction, the flow along both long sides becomes the same direction in the scene. , after hitting the short side, it dives downward and rotates.

This stirring mode is referred to as "horizontal unidirectional flow" in this specification.

The in-mold electromagnetic stirring device according to the present invention is characterized in that a coil 2 and a motor core 1 are integrally installed so as to be movable up and down within the mold. That is, as shown in FIG. 2, a lifting guide 9 is provided inside the mold on the long side.

A stator 10 supporting the motor core 1 is installed in the lifting guide 9 so as to be vertically slidable, and the stator IO is connected to a piston rod 12 that is reciprocated by a hydraulic cylinder 11. Thereby, by adjusting the oil pressure supplied to the hydraulic cylinder 11, the installation height of the motor core I can be adjusted and maintained at a predetermined height. Note that both motor cores 1 on both sides of the long-side mold are installed in the same structure so that they can be raised and lowered, but if the hydraulic cylinders 11 are driven independently,
Both motor cores 1 can be installed at different heights. Therefore, according to this device, by setting the two motor cores 1 at different heights, an agitation mode using an inclined flow is also possible. However, in carrying out the method of the present invention, unless otherwise specified, both motor cores are set at the same height and a stirring mode of horizontal reciprocating swirling flow is adopted. Note that the drive mechanism for raising and lowering the stator 10 may be an electric mechanism instead of the hydraulic cylinder 11.

In the present invention, Ti-containing stainless steel is rapidly cast using an electromagnetic stirring mold in which the position of the motor core can be freely set. It is necessary to remove inclusions from a wide area of 31 or more, and in some cases more than 1OIII+ from one surface.
The center position of the morph core is set so as to satisfy the relationship of equation l1. first.

The four outputs of equation (1) will be explained.

-II The relationship between the shell thickness D of stainless steel molten copper and the time t can be expressed relatively well by the quadratic equation (2), although it varies somewhat depending on the degree of superheating of the molten steel and the influence of flow.

D=to-A (2) where 2 K is the coagulation fixation number.

On the other hand, in continuous casting, the slab is drawn downward at a constant speed, so the relationship between the 1 hour period and the distance H from the meniscus is as follows, assuming that the casting speed is V.

H=Vt (3). From equations (2) and (3), the relationship between the thickness of the solidified shell and the distance H from the meniscus is expressed by the following equation (4).

H=V・(D/K)"...(4
) Therefore, in order to remove the inclusions trapped in the solidified shell up to the thickness D, it is necessary to provide the necessary agitation to remove the inclusions within a distance of H from the meniscus.

On the other hand, the horizontal swirling flow that occurs when the moving directions of the magnetic fields of a pair of linear motors are opposite to each other.

If the maximum flow velocity is at the center of the motor core, the distribution will be as shown in Figure 3. Therefore, when attempting to stir the horizontal swirling flow at a flow rate effective for reducing inclusions in the distance range from the meniscus to H, the center of the motor core is
Stirring to match 2=h is most efficient.

From the above, the optimal position of the center of the linear motor core for removing inclusions in the region D from the slab surface by horizontal swirling flow is expressed by the quadratic (112).

■ h = −−V −(D/K)2−− (1) Next, regarding the stirring mode, if stirring is continued at the same stirring intensity,
In the horizontal unidirectional flow, the surface rises and is exposed on one short side of the mold, and the continuously cast powder is drawn in on the other short side. Furthermore, when vertical upward stirring is applied, the molten metal surface rises in the center of the mold, causing continuous casting powder to be drawn in on both lamp sides. Since vertical stirring produces a large downward flow, there is a risk that large inclusions will be introduced into the slab.

On the other hand, when stirring molten steel in a mold for slab continuous casting using a horizontal swirling flow, the stirring efficiency is inferior to the above two methods, but there is almost no bulge in the scene and there is a large bottom +
Since neither +s i occurs, this horizontal swirling flow is adopted in the method of the present invention. In the present invention, the range of the stirring flow rate due to this horizontal swirling flow is set to 15 to 40 cm/sec at the 2xh position. This was determined from the results shown in FIG. The figure shows the relationship between the stirring flow rate VFa and the inclusion index of the surface layer of the slab, and it is clear that inclusions can be significantly reduced by stirring at a flow rate of at least 15 cm/sec or higher. Therefore, in order to stir so that inclusions are difficult to be trapped in the solidified shell of thickness D, the distance from the meniscus to this thickness D must be at least 15 cm.
A flow rate of 11/sec or more is required. On the other hand, as seen in FIG. 3, when stirring in the horizontal direction to give a flow velocity of a magnitude equal to that at a position 2xh from the meniscus, a flow approximately corresponding to the flow velocity of that magnitude also occurs at the meniscus. When this flow becomes large, the powder for continuous casting becomes entangled. When a horizontal swirling flow is applied, the flow velocity at which continuous casting powder is drawn in is approximately 40 cm/sec. Therefore, the range of the stirring flow rate is set to 15 to 40 cm/sec at the 2×h position.

Next, a method of calculating h will be explained.

First, D in the formula ([) is determined to be the solidified shell thickness at a position 2xh from the meniscus, and the value of D is determined to be 10 to 30 mm. The value range of D is 10 to 30 mm for the following reason. By changing the stirring time of electromagnetic stirring for Ti-containing stainless steel molten steel, samples with different thicknesses of solidified shells subjected to stirring flow were prepared, and the samples were subjected to hot rolling and cold rolling.
The relationship between the range of the thickness of the solidified shell subjected to stirring and the linear flaws observed on each cold-rolled sheet was determined, and the results shown in FIG. 5 were obtained. From the same figure, the reason why the linear flaws are significantly reduced in L3 is that the range of the solidified shell thickness subjected to PA agitation is 1011II.
This is a case of +1 or more. From this, it is necessary to ensure the flow of electromagnetic stirring necessary for removing inclusions until the solidified shell grows to at least 10 mm or more. However, when the temperature exceeds 30 a+m, the effect of reducing linear flaws is saturated, and further stirring is meaningless in achieving the purpose of the present invention. The range of the thickness of the solidified shell from which all inclusions must be removed by magnetic stirring is 10 to 30 mm, and considering the case where surface grinding of the slab is forced due to other factors, it is preferable. shall be 15 mm or more.

Next, regarding the value of coagulation fixation number, Cr-based or Ni-C
In the case of r-series stainless steel, the value of K is approximately 25n+m/
e+in'', which is almost equal to this value even in Ti-containing stainless steel. Therefore, in the method of the present invention, a value of K=25ms+/5ill'' can be adopted. However, if the cooling conditions within the mold or the casting conditions are to be significantly changed, it is necessary to obtain a more accurate K in advance through experiments such as introducing radioactive isotopes.

Using D and K obtained in this way, h can be determined according to formula +11 when the casting speed V is given. As an example, casting speed V=1. Om/min, D=15
mm.

If h is determined by equation (1) under the condition of K = 25I1m/win"", h" is 180mm.

Next, 15cm at the 2xh position from the meniscus.
A procedure for determining the stirring intensity to obtain a predetermined flow rate of /sec or more will be explained.

The stirring intensity is adjusted by the current applied to the coil of the near motor. Therefore, for example, as shown in FIG. 6, the current I and the flow velocity vF of molten steel in the mold.

If we find the relationship with the distribution of
It is possible to know the current () required to obtain a predetermined flow velocity at the position of Xh. Here, to give a convenient example of finding the relationship between I and vF, since the actual magnitude of flow depends on the magnetic flux density B at the position where the flow occurs,
First, the distribution of the magnitude of B in the casting direction with the center of the motor core as the origin at each current value (■) is measured. on the other hand,
In a range where the flow velocity is 40 cm/see or less, the relationship between the magnetic flux density B and the flow velocity v0 is expressed by the following equation (5).

v, 1l=a-Bz...(5) where 5a is a constant, and this value is 9For example, when molten steel is experimentally stirred under a current 2 with a known B distribution, the resulting cast It can be obtained by determining v9 estimated from the deflection angle of the dendrites of the mass. As a result, flow velocity distribution charts at different current values can be created as shown in FIG. Once this flow velocity distribution diagram is obtained, electromagnetic stirring may be performed using a current value (2) at which the flow velocity at the position 2×h is a predetermined value of 15 cm/sec or more.

Table 1 summarizes the results of typical examples in which Ti-containing stainless steel was continuously cast according to the method of the present invention described above, together with comparative examples.

The mold used was a linear motor type electromagnetic stirring mold in which a coil and a motor core were installed in a slab type curved continuous casting machine measuring +200 x 1030 mm so that they could be raised and lowered on both long sides as shown in Figures 1 and 2. . The Ti-containing molten stainless steel used for casting was degassed using VOD in the same way as normal molten stainless steel that does not contain Ti. The casting temperature shown in Table 1 is the molten steel temperature in the Kumbush, which was 40 to 50° C. higher than the liquidus temperature. The stirring intensity was adjusted by changing the current value while keeping the frequency constant at 1 OHZ. Regarding the installation position n (mm) of the motor core, those marked with an asterisk (*) in Table 1 correspond to the installation position determined using formula +11 above, assuming shell thickness d (mm) = D. Other than that, they do not match. In the evaluation, inclusions existing within a range of 15 mm+ from the surface of the obtained slab were examined and their area ratio was determined, and this was taken as the inclusion area ratio of the surface layer of the slab. Further, the slab was surface ground with a grinding amount of 2111111 mm or less on one side, and then subjected to hot rolling and cold rolling.

Both the occurrence rate of scraps and the occurrence rate of second-class products in cold-rolled products due to inclusion defects were summed to determine the occurrence rate of downgrading.

The following is clear from Table 1.

Test fish 1 is a control example without stirring, but in this case, inclusions in the surface layer of the cast slab and product failure rate are high.

In test patient 2, the position of the motor core center was made to match h,
Flow velocity is 15cm/see at shell thickness 5IIlvA
Comparative example, test holes 3 to 7 subjected to horizontal swirling flow
The flow velocity is 15 cm/
This is an example of the present invention which was subjected to a flow due to a horizontal swirl flow of more than . As seen in Test Floor 2, in the case of Ti-containing molten stainless steel, the failure rate is high even when flow is applied at a shell thickness of 5 mm. On the other hand, a test f in which flow was applied with a shell thickness of 10 mm or more according to the method of the present invention
For lh3 to lh7, the failure rate has been reduced to a level of 2% or less.

Test fish 8 and 9 are comparative examples in which the stirring mode is horizontal unidirectional flow. In this case, even if the position of the morph core is made to match h and sufficient flow is provided with the shell TI'1.1OIIIffi or more, the area ratio of inclusions in the surface layer of the slab and the occurrence rate of failure will increase. This is caused by the continuous casting powder being entrapped or chewed.

Tests 11kL10 and 11 are comparative examples in which the center of the motor core is aligned with h and the fluid is caused to flow in a horizontal swirling flow, and the agitation intensity is low and high. At floor 10, the stirring intensity is weak, and the flow velocity at the shell thickness of 15 mm and the flow velocity at the meniscus are low. For this reason, stirring has little effect on removing inclusions. On the other hand, when the stirring intensity was increased and the flow rate exceeded 40 cm/sec, as in Il&Lll, the amount of inclusions and the occurrence rate of failure were equal to or higher than those of Control Example No. 1.

This is caused by the inclusion of continuous casting powder.

Tests N112 to N15 are comparative examples in which the center of the motor core is located away from h. As in floors 12 and 14, when the motor core center position is above the position determined from equation 11, good results as in the example of the present invention are not obtained, and 1lh14 is the worst result. This is because if an attempt is made to apply a flow of 15 cm/sec or more to a solid shell with a shell thickness of 10 mm, the flow velocity at the meniscus will increase and the continuous casting powder will be drawn in. on the other hand.

As in tests m13 and 15, the motor core center position is f
Sufficient results are not obtained even when the position is below the position determined from the i1 formula. This is because the effect of removing inclusions in the extreme surface layer of the slab is reduced.

Tests 1m16 and 17 are examples in which molten steel with different Ti contents was cast under the same casting conditions according to the method of the present invention.

Even in the case of stainless steel containing more than 1% Ti, such as 1b17, according to the method of the present invention, the incidence of downgrading is 2.
% or less.

[Brief explanation of the drawing]

FIG. 1 is a plan view of a continuous casting mold according to the present invention. FIG. 2 is a cross-sectional view taken along the line nn' in FIG. 1, and FIG. 3 is a diagram showing an example of the velocity distribution of horizontal swirling flow caused by electromagnetic stirring.
Figure 4 shows the relationship between the stirring speed and the inclusion index of the surface layer of the slab, and Figure 5 shows the range of thickness of the solidified shell subjected to stirring flow and the linear flaws caused by inclusions in the cold rolled sheet. A diagram showing a relationship with an index. FIG. 6 is a diagram showing the relationship between induced current value and speed distribution in electromagnetic stirring. ■... Motor core, 2... Coil, 3... Long side mold, 4... Immersion nozzle for injecting molten steel. 5. Ti-containing molten stainless steel, 6. Powder for continuous casting, 7. Solidified shell, 9. Lifting guide 9.10. Stator that supports the motor core. 11... Hydraulic cylinder, 12... Piston rod.

Claims (2)

[Claims] (1) In a continuous casting method for molten stainless steel containing 0.1% by weight or more of Ti, a continuous casting mold equipped with a linear motor-type stirring device whose position can be varied in the height direction is used, and the device is The distance to the center of the motor core is determined so that the relationship of the following formula (1) is substantially satisfied, and the horizontal flow velocity of molten steel in the mold at a position 2×h from the meniscus is in the range of 15 to 40 cm/sec. A continuous casting method for Ti-containing stainless steel, characterized in that the electromagnetic stirring device imparts a horizontal swirling flow to molten steel in the mold. h=1/2・V・(D/K)^2...(1) However, h: Distance in the casting direction from the meniscus to the center of the linear motor core (m), K: Number of solidification fixation (mm/min^1 ^/^2)_1D; Solidified shell thickness at a distance of 2h from the meniscus (mm)
, and the value is in the range of 10 to 30 mm. (2) In an electromagnetic stirring mold in which a motor core surrounded by a coil is installed inside a steel continuous casting mold,
The method according to claim 1, wherein the integral body of the coil and motor core is attached so as to be able to slide up and down on a lifting guide installed in a mold, and the set height of the motor core can be freely adjusted by external power. Continuous casting mold used.