JPH0429458B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for continuous casting of stainless steel, which produces slabs with good surface properties that do not require surface care.

Maintenance-free rolling of stainless steel (that is, rolling without surface treatment steps such as grinding the surface of the stainless steel slab that is the starting material for rolling) does not require cooling of the slab for this treatment. It has been considered a major issue in stainless steel manufacturing because it achieves a large thermal economy and improves productivity by omitting maintenance steps.

When manufacturing slabs by continuous casting,
In order to realize this maintenance-free rolling of continuously cast slabs, it is necessary to reduce the depth of the oscillation marks. This is because in the case of stainless steel, the amount of scale generated during the slab heating stage is smaller than that of ordinary steel, so if the oscillation mark is deep, the convex part of the oscillation mark will not completely scale. Therefore, oscillation marks remain on the surface even after pickling, and this remains as a pattern on the cold-rolled sheet, and deep oscillation marks themselves form scales with poor descaling properties. This is because it remains on the surface of the hot-rolled sheet even after pickling, and this causes surface defects on the cold-rolled sheet. In general, it is said that oscillation marks are harmless even when rolled without care, as long as the depth of the oscillation marks does not exceed 250μ.

Oscillation marks are unavoidable in normal continuous casting methods in which continuous casting is performed while the mold is vibrated, but the depth of the oscillation marks depends on the mold vibration conditions and the physical properties of the mold powder used (especially (viscosity), so it can be made shallower by appropriately adjusting these. Figures 1 and 2 show an example of this relationship.

As these figures show, regarding the mold vibration conditions, it is recommended to adopt low stroke high vibration conditions (Fig. 1), in which the stroke is small and the vibration frequency is large.
It can be seen that selecting a highly viscous powder (Figure 2) is effective in reducing the depth of the oscillation mark. The ability to reduce the depth of the oscillation mark by optimizing mold vibration conditions and mold powder viscosity is disclosed in, for example, JP-A-59-220262;
It is described in JP-A-220261, JP-A-59-191547, etc.

However, under mold vibration conditions that produce shallow oscillation marks of 250μ or less and the use of highly viscous powder, the mold powder cannot effectively flow between the mold and the solidified shell. As the lubrication within the slab deteriorates, scratches caused by statesking may occur on the slab surface, damaging the surface quality of the slab, or vertical cracks may occur, resulting in a defective slab, or extreme statesking may occur. If this occurs, it will cause a restrictive breakout.

In order to improve lubricity within the mold, solid lubricants such as BN (boron nitride) are applied to the inner surface of the mold.
It has also been proposed to plate the inner surface of the mold with a mixture of nickel and a solid lubricant, but this method is not only expensive, but also wears out the coated and plated layers, making it difficult to use over long periods of time. I can't stand it,
Further, there are also problems such as there being a limit to reducing the coefficient of friction on the inner surface.

The purpose of the present invention is to maintain lubricity in the mold at all times even under mold vibration conditions and mold powder conditions to obtain a shallow oscillation mark depth of 250μ or less required for maintenance-free rolling of stainless steel slabs. The goal is to make it possible.

In order to achieve this objective, the present inventors have investigated various methods of casting in order to ensure sufficient lubrication in the mold under casting conditions with such a shallow oscillation mark depth. I did a lot of research. As a result, it has been found that the above objective can be effectively achieved by appropriately allowing a liquid lubricant to flow between the mold and the solidified shell below the surface of the mold. That is, in order to realize maintenance-free rolling of slabs, the present invention provides a continuous casting method for stainless steel in which the vibration conditions of the mold and the viscosity of the mold powder are adjusted so that the depth of the oscillation mark is 250μ or less. It is characterized by allowing a liquid lubricant to properly flow between the mold and the solidified shell below the surface of the mold.

The present invention will be specifically explained below.

The present invention uses mold powder in the same way as in the past, and in its use, it uses a high viscosity mold powder that reduces the depth of the oscillation mark, although it depends on the mold vibration conditions.
A liquid lubricant is also used. Suitable liquid lubricants include vegetable oils, fatty acid esters, mineral oils, and mixtures thereof. The use of such liquid lubricants during casting is used, for example, in the production of small cross-section slabs such as billets and blooms, but in this case no mold powder is used, and in both cases liquid lubricants are used. Single use. On the other hand, with regard to the combined use of mold powder and liquid lubricant, there are reports of cases in which mold powder and rapeseed oil are used together in the casting of ordinary steel, but in this case, the rapeseed oil is supplied from above the hot metal surface. Since the supply method is different from the method of the present invention in which the molten metal flows between the mold and the solidified shell below the molten metal surface of the mold, the lubricating effect as in the present invention cannot be expected.

In the present invention, it is important that the liquid lubricant flow between the mold and the solidified shell below the surface of the mold. More specifically, the mold powder is caused to flow between the mold and the solidified shell from below the layer of mold powder that is supplied on the surface of the mold. FIG. 3 and FIG. 4 show the difference in the way the liquid lubricant is supplied. In either case, a liquid lubricant supply port 2 is provided in the middle of a mold 1 that vibrates with a predetermined vertical stroke and frequency, and the liquid lubricant is allowed to flow into the mold from this supply port 2. In FIG. 3, this liquid lubricant supply port 2 is located above the molten metal level 4 of the molten steel 3 in the mold (above the mold powder layer 5), whereas in FIG. The liquid lubricant supply port 2 is located below the molten metal level 4 of the molten steel 3 in the mold (below the mold powder layer 5).

In the process shown in Figure 3, the liquid lubricant is applied to the mold 1.
Since it contributes little to the lubrication between the solidified shell 6 and the solidified shell 6, the object of the present invention cannot be achieved. It has been found that with this method, the supply of the liquid lubricant sometimes impedes the stable inflow of the mold powder 5. This is thought to be because partial combustion of the liquid lubricant occurs, which adversely affects the inflow of mold powder. Therefore, in this case, it becomes impossible to obtain a product with good surface properties.

On the other hand, it has been found that the object of the present invention can be effectively achieved by the method shown in FIG.
In this case, the distance from the molten steel surface 4 in the mold to the liquid lubricant supply port 2 (depth below the molten metal surface)
In addition to varying h, the amount of liquid lubricant supplied was also varied to test whether an appropriate range existed. As a result, it was found that it is preferable to satisfy the following relationship when h≧30 mm.

11×10 ^-7・h≦Q≦175×10 ^-7・h ………(1) Where, Q: Amount of liquid lubricant supplied per unit surface area of slab (cc/cm ² ) h: Liquid lubrication The depth below the surface of the molten metal (mm) at the agent supply position, and both the upper and lower limits of Q increase as h increases. This relationship is shown in FIG.

The relationship between the amount of liquid lubricant supplied and the depth below the surface of the liquid lubricant at the supply position shown in equation (1) and Fig. 5 is similar to that shown in Figs. (Figure) shows a particularly favorable relationship under mold vibration conditions and mold powder usage conditions.

If h is 30mm or more, h is 30mm.
This is because if the temperature is less than 1, the combustion gas of the liquid lubricant will be ejected above the surface of the mold and disturb the flow of the molten powder layer of the mold powder. The lower limit of Q, 11×10 ^-7 h, is the minimum amount required to ensure lubrication, and the upper limit of Q, 175×10 ^-7 h, is the minimum amount required to ensure lubrication.
If the supply amount of h exceeds this value, the inflow of molten powder due to the jetting of combustion gas will be inhibited, the solidification of the shell will be delayed due to an excessive lubricant film, and furthermore, carburization of the slab surface may occur. Therefore, the upper limit value is set so that such a situation does not occur. That is, it has been found that the relationship between the amount of liquid lubricant supplied and the supply position within the range shown by equation (1) and the hatched area in FIG. 5 is the appropriate range.

Although these relationships do not vary greatly depending on the type of liquid lubricant, mineral oil burns somewhat quickly, so it is more preferable to use vegetable oil such as rapeseed oil or fatty acid esters. In supplying this liquid lubricant, a solid lubricant such as BN can also be suspended in this liquid lubricant, although this increases the cost burden.

As shown in Fig. 4, the liquid lubricant supply ports are provided with a number of supply ports 2 penetrating the mold wall, and the liquid lubricant is simultaneously supplied from the multiple supply ports 2 to the inner surface of the mold and the solidified shell so as to satisfy the above relationship. 6, but instead of providing a large number of such through holes, a slit-shaped groove extending laterally is installed on the inner surface of the mold, and a passage for supplying liquid lubricant from the outside is provided in this groove. A process installed within the mold may provide liquid lubricant between the mold interior surface and the solidification shell. In this case as well, it is preferable that the equation (1) and the relationship shown in FIG. 5 be satisfied.

As described above, even under mold vibration conditions that satisfy the requirement of oscillation mark depth of 250μ or less required for maintenance-free rolling of stainless steel, and casting conditions that cause lubrication failure under mold powder usage conditions, the present invention can be applied. According to the invention, this poor lubrication can be completely avoided, and this can be achieved through the use of inexpensive liquid lubricants, so that the economic and productivity-enhancing effects of maintenance-free rolling can be achieved at low cost. .

Comparative examples and examples of the present invention are listed below.

Comparative example SUS430 steel was cast under mold vibration conditions of a casting speed of 0.7 m/min, a stroke of 6 mm, and a frequency of 120 cpm.
Continuous casting was also performed at 1300℃ using mold powder with a viscosity of 3.8 poise. At that time, the supply port should be as shown in Figure 4, and the depth h below the surface of the hot water should be 100 mm.
0.00009cc/ ^cm2 of rapeseed oil from supply port 2.
(Supplied amount per unit surface area of slab). This inflow amount is lower than the appropriate range of Q in equation (1) and FIG.

The oscillation mark depth of the obtained slab was 160μ, which is harmless even when rolled without maintenance, but according to the results of measuring the frictional force between the mold and solidification shell using a load cell installed in the mold, it was found that rapeseed Compared to the case of casting under the same conditions except that no oil was supplied, the frictional force was almost the same value. Therefore, in this case, since the amount of liquid lubricant supplied was small, a good lubrication effect could not be obtained.

Example 1 SUS430 steel was cast under mold vibration conditions of a casting speed of 0.7 m/min, a stroke of 4 mm, and a frequency of 130 cpm.
Continuous casting was also performed at 1300℃ using mold powder with a viscosity of 3.8 poise. At that time, the supply port should be as shown in Figure 4, and the depth h below the surface of the hot water should be 100 mm.
Rapeseed oil was flowed in at an amount of 0.001 cc/cm ² from the supply port 2.

The oscillation mark depth of the obtained slab was 110μ, which is harmless even when rolled without care, and the friction force measurement method described in the comparative example was followed under the same conditions as this example except that rapeseed oil was not supplied. Comparing the frictional force with that of casting, the frictional force index is 1.0, whereas
A lubrication improvement effect of up to 0.5 was observed.

In addition, the surface quality of the slab was determined by the index during casting under the same conditions as in this example, except that rapeseed oil was not supplied.
If it is 1.0, in this example, the frequency of occurrence of cracks and vertical cracks in the slab will be 0.4 each, and the frequency of occurrence of cracks and vertical cracks in the slab will be 0.4.
The frequency of flaw occurrence has decreased to 0.3.

Example 2 SUS430 steel was cast under mold vibration conditions of a casting speed of 0.7 m/min, a stroke of 6 mm, and a frequency of 120 cpm.
Continuous casting was also performed at 1300℃ using mold powder with a viscosity of 2.7 poise. At that time, the supply port should be as shown in Figure 4, and the depth h below the hot water surface should be 200 mm.
Rapeseed oil was flowed in at an amount of 0.003 cc/cm ² from the supply port 2.

The oscillation mark depth of the obtained slab was 230μ, which is harmless even when rolled without maintenance.The friction force measurement method described in the comparative example was followed under the same conditions as this example except that rapeseed oil was not supplied. Comparing the frictional force with that of casting, the frictional force index is 1.0, whereas
A lubrication improvement effect of up to 0.6 was observed.

In addition, the surface quality of the slab was determined by the index during casting under the same conditions as in this example, except that rapeseed oil was not supplied.
If it is 1.0, in this example, the frequency of occurrence of cracks and vertical cracks in the slab will be 0.6 each, and the frequency of occurrence of cracks and vertical cracks in the slab will be 0.6.
The frequency of flaw occurrence has decreased to 0.4.

[Brief explanation of drawings]

Figure 1 shows the effect of mold vibration conditions (stroke and frequency) on the depth of the oscillation mark, and Figure 2 shows the effect of mold powder viscosity (at 1300℃) on the depth of the oscillation mark. Fig. 3 is a schematic cross-sectional view of the mold part showing an example of supplying liquid lubricant into the mold;
Figure 4 is a schematic cross-sectional view of the mold part showing an example of supplying liquid lubricant between the mold and the solidified shell below the hot water surface, and Figure 5 shows the depth (h) of the liquid lubricant below the hot water surface. Amount of liquid lubricant supplied per unit surface area of slab (Q)
It is a relationship diagram showing the appropriate area with. 1... Continuous casting mold, 2... Liquid lubricant supply port, 3... Molten steel in the mold, 4... Molten metal surface in the mold, 5
... Layer of mold powder, 6... Solidified shell.

Claims (1)

[Claims] 1. In a stainless steel continuous casting method in which the vibration conditions of the mold and the viscosity of the mold powder are adjusted so that the depth of the oscillation mark is 250μ or less, A continuous casting method for stainless steel characterized by flowing a liquid lubricant between the shells so that the following relationship is satisfied, h≧30mm, 11×10 ^-7・h≦Q≦175× 10 ^-7・h However, Q: Amount of liquid lubricant supplied per unit surface area of the slab (cc/cm ² ) h: Depth below the surface of the liquid lubricant at the position where the liquid lubricant is supplied (mm).