JPH02207944A - Method for preventing surface cracking in continuous casting of sb-containing steel - Google Patents

Abstract

PURPOSE:To improve toughness and to prevent surface crack so as to relieve or omit ingot mending prior to rolling of the ensuing stage by pouring a steel contg. a specific ratio of Sb at the specific overheating degree of the molten steel and controlling the cooling of the ingot, thereby continuously casting the steel. CONSTITUTION:The overheating degree of the molten steel at the time of pouring the molten steel into a casting mold in continuous casting of the steel contg. 0.02 to 0.15wt.% Sb is specified to <=25 deg.C to avoid rapid cooling at the time of casting. The cooling rate of the ingot surface in the region where the surface temp. of the ingot is 1150 to 950 deg.C is specified <=20 deg./min. The generation of the surface crack and subsurface crack of the ingot is effectively decreased and suppressed and the precise mending stage for the ingot prior to the rolling of the ensuing stage is decreased or omitted. The stable production of the Sb- contg. steel is thus executed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for preventing surface cracking in continuous casting of Sb-containing steel.

Continuous casting, a conventional technology, improves yield, saves energy, and saves labor by omitting the ingot making and blooming processes in the steel manufacturing process, and its application fields are almost 100%, excluding physical limitations such as plate thickness. %.

Continuous casting has also been attempted for Sb-containing steels, such as sulfuric acid dew point corrosion-resistant steels.

However, these steels containing 0.02 to 0.15% Sb have a 600 to 90% Sb content compared to general low alloy steel.
Because its toughness at 0°C is extremely low (indicating brittleness) and its dimensional changes due to structural transformation are large, it is a steel with high cracking susceptibility, and is prone to surface cracking of slabs and subsurface (subcutaneous) cracking during continuous casting. This tends to occur, and a step of finishing and cleaning is required before the next rolling step, making it difficult to fully enjoy the benefits of continuous casting.

The physical causes of these cracks in continuous casting include (1) thermal stress caused by uneven temperature in the width direction of the slab during cooling with cooling water between the slab support rolls in the cooling zone; (2) Stress caused by the slab swelling between the rolls due to the static pressure of the molten steel (bulging);
In addition, (3) mechanical tensile stress (correction strain) at the correction point when bending the slab back from the curved part to the horizontal part in curved continuous casting.

Therefore, as a measure to suppress and prevent surface cracking during continuous casting of these Sb-containing steels, Cu and Ti have been added from the viewpoint of increasing the toughness and corrosion resistance of the material.

However, removing impurity elements and adding trace elements to improve the toughness of these materials requires additional steps in the refining process before casting, which leads to an increase in temperature compensation, refining materials, alloys, and other refining costs. ing.

On the other hand, in order to suppress surface cracking without adding special trace elements, etc., the amount of cooling water in the cooling zone is reduced and the cooling spray is shaped using an oval type nozzle that has a uniform water density. and Steel, 87 (1980), S
-738), or a method of using an air/water nozzle to equalize the temperature of the slab has also been adopted.

Problems to be Solved by the Invention Although surface cracking during continuous casting of Sb-containing steel has been significantly improved by the various measures described above, in the actual continuous casting process, for example, it is difficult to straighten a curved continuous casting machine. Even if a slow cooling pattern is adopted in order to pass through a point at a high temperature, there is little recuperation of heat from inside the slab and it is easy to dissipate heat, so it is necessary to always maintain one surface stably above the temperature at which toughness is weak. The problem remained that it was difficult.

The present invention improves toughness during continuous casting of Sb-containing steel, even in the temperature range where toughness has conventionally been said to decrease, reduces the occurrence of surface cracks and subsurface cracks, and improves the toughness of the cast iron before the next rolling process. The purpose is to ease or omit processes.

Means for Solving the Problems The present invention provides a method for continuous casting of steel containing 0.02 to 15 wt% of Sb, by setting the superheat (degree of superheating of molten steel) to 25°C or less when injecting it into the mold, and This is a method for preventing surface cracking in continuous casting of Sb-containing steel, characterized in that the cooling rate of the slab surface in a region where the slab surface temperature is 1150 to 950 °C is 20 °C/min or less.

It is generally known that the occurrence of surface cracks in continuous casting is due to a decrease in hot ductility (toughness).

When surface cracks occur, the surface of the slab needs to be treated before the next rolling process, but there have been reports on the relationship between this surface treatment rate and hot ductility. The aperture value (RA), which represents the shrinkage rate of the cross section obtained in the test, is 60.
% or more, it is said that the 1-care rate is greatly reduced.

Therefore, we conducted a high-temperature tensile test that simulated the effect of cooling rate under the thermal history that is thought to be experienced by the surface of a continuously cast slab as shown in Figure 1. The material components used in this test are shown in Table 1. This 600~
It has been reported that the phenomenon in which toughness decreases in the temperature range of 900°C is significant when the strain rate is very slow.
The test was conducted at a slow strain rate of 1×10 −3 /sec.

Table 1 (wt%) Figure 2 shows the relationship between the tensile temperature and the reduction of area (RA) obtained in this test. 800-90
A decrease in toughness is seen in the temperature range of 0°C, but as the cooling rate is reduced, ductility gradually recovers.

Based on Figure 2, 13 when performing a tensile test at 800℃
Cooling rate and aperture value (RA・) from 00℃ to 800℃
Figure 3 shows the relationship between
It was found that when the temperature is less than /min, the toughness decreases the most and the reduction of area (RA) at a temperature of 800° C. can be maintained at 60% or more.

Based on the test results, it is ideal to have a cooling rate of 20°C/min or less in the area where the surface temperature of the slab is 1300°C. In a casting machine, the surface temperature of a slab when it passes through the primary cooling zone of the water-cooled mold and begins the so-called secondary cooling zone, where the slab is directly cooled by cooling water, is affected by the casting speed and the cooling capacity of the water-cooled mold. However, the temperature is about 1200°C, and rapidly reducing the cooling rate directly below the mold also lacks operational stability. Therefore, the cooling rate of the surface of the slab in the region of at least 1150°C or lower must be maintained at 20°C/min or less.

On the other hand, it is important to set the surface temperature of the slab high when it passes through the straightening point of the curved continuous caster.

It is difficult to maintain the temperature of the edge portion in the width direction of the slab, where heat radiation is greater than that of a portion of the center of the slab, above 950° C., where toughness begins to deteriorate, and the present invention is effective here. That is, for parts where a high temperature of 950° C. or more cannot be stably maintained, it is important to maintain the cooling rate at 20° C./min or less up to 1350° C., where toughness begins to deteriorate.

Therefore, in order to control the cooling rate in the range where the slab surface temperature is 1150 to 950°C, a method suitable for the cooling capacity of various continuous casting machines is used. It is preferable to avoid partial cooling as much as possible; for example, it is preferable to keep the water density in the radial direction of the slab as uniform as possible so as not to cause uneven temperature distribution.

Based on this, Sb-containing steel (Sb 70.02~0.15
%) continuous casting at 20°C between 1150 and 950°C.
When cooled at less than 1 minute, cracks may still occur, and a large amount of data was analyzed to determine the cause.

As a result, we discovered that it is important not only to simply control cooling within a certain temperature range, as shown in Figure 4, but also to simultaneously limit superheat (molten steel superheating temperature) to 25°C or less during casting. Ta.

However, if the superheat exceeds 25°C, the cooling rate in the mold and in the secondary cooling zone that passes through the mold and reaches the straightening point becomes rapid cooling, resulting in a rapidly cooling structure from the surface of the slab to the inside. When cracks occur on the surface due to thermal strain, the cracks propagate deeply along the unsolidified Sb existing at the high-temperature internal grain boundaries, so cracks remain even with 4 μm of solvent per side.

On the other hand, if the superheat is small (below 25°C), the quenched structure is only on the surface of the slab, so even if a crack occurs due to thermal strain, the Sb existing at the internal grain boundaries will also solidify, causing the crack to crack. The cracks do not propagate, remain in shallow cracks, and can be removed sufficiently with a small amount of solvent.

This super heat is better if it is smaller, but FJ
It is preferable that the lower limit is 5° C. or higher in view of operational issues for production.

Next, the Sb content was limited to 0.02-0.15% because the sulfuric acid dew point corrosion resistance is poor if it is less than 0.02%, and the reason why the upper limit was set at 0.15% is that more than 0.02% is added. This is because no improvement in characteristics can be expected.

Preferred ranges of other components for the Sb-containing steel are as follows.

C: 0.05-0.20%.

Mn: 0.20-1.0096, P: 0.030% or less, Si: 0.05-0.50%, Cu: 0.10-0.50%.

AQ: 0.005-0.050%, S: 0.0
Based on 20% or less, the old 0. 10 to 0.50% may be added.

Example: The sulfuric acid dew point corrosion resistant steel shown in Table 2 produced by hot metal pre-treatment - converter - secondary refining was cast in a curved continuous casting machine with a radius of 10.5 aR and a distance of 117 m from the pouring point to the straightening point. Casting was carried out using the method of the present invention.

Table 2 (1 knowledge The casting conditions are as follows.

Superheat: 15℃ Casting speed: 1.15 (m/5in) Cooling method Niopal type spray nozzle cooling conditions
: Pouring ratio o, se (view/kg) The water density from the bottom of the mold to the end of the mold is shown in Table 3.

Figure 5 shows the results of the casting surface temperature measured by installing a thermocouple on the surface of the slab when casting was carried out under the above conditions. The cooling rate up until now was 0.1°C/sec (6°C/min), and the slab surface temperature when passing the straightening point was 880°C.

On the other hand, the surface temperature of the slab surface at the edge in the first width direction when passing through the straightening point decreased to 800℃, but it increased from 1150℃ to 950℃.
The cooling rate was 0.3°C/sec (18°C/min) until the slab was cooled naturally, and then the surface was cooled with a solvent.

As a result of surface observation, a healthy surface was obtained. That is,
Although the edge part has passed the straightening point in the temperature range where the toughness decreases, it has been
Controlling the cooling rate down to 25℃ superheats
Surface cracking was suppressed by controlling as follows.

Comparative Example Steel having the same composition as shown in Table 2 as in the above example was cast under different cooling conditions.

The casting conditions are as follows.

Superheat: 15℃ Casting speed: 0.13G (m/5in) Cooling method Niopal type spray nozzle cooling conditions
:Water injection ratio: 1.39 (1/kg) The water density from the bottom of the mold to the end of the mold is shown in Table 4.

Figure 6 shows the results of the slab surface temperature measured by installing a thermocouple on the slab surface when casting was performed under the above conditions. From this figure, the slab surface temperature when passing the straightening point is SOO The cooling rate from 1150°C to 950°C was 0.87°C/sec (40°C/min). After this slab was allowed to cool naturally, it was cut from the surface for 4s+. As a result of surface observation, austenite grain boundaries were observed at various locations along the slab radiation direction. Shallow cracks of about 1 to 1.3 layers were observed. Even if the temperature when passing through the straightening point is maintained at a high temperature in this way, if the cooling rate from 1150°C to 1350°C is 20°C/min or more, the occurrence of one crack cannot be completely suppressed.

Table 3 Table 4 Effects of the invention As described in detail above, in continuous casting of Sb-containing steel containing 0.02 to 0.15% Sb, the superheat is set to 25°C or less and the temperature is set to 1150 to 950°C. According to the method of the present invention, which controls the cooling rate of the slab in the area below 25°C, it is possible to effectively reduce and suppress the occurrence of surface cracks and subsurface cracks in the slab. It has become possible to reduce and omit the single-side finishing process, and to enjoy the benefits of continuous casting, while stably manufacturing shun-containing steel.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the thermal history of a high temperature tensile test (Greeple test) of Sb-containing steel. FIG. 2 is a graph showing the relationship between test temperature and cross-sectional area of area during a high-temperature tensile test (Greeple test) of Sb-containing steel. FIG. 3 is a graph showing the relationship between cooling rate and cross-sectional area of area during a high-temperature tensile test at 800° C. of Sb-containing steel. FIG. 4 is a graph showing the relationship between the continuous casting conditions of Sb-containing steel and the occurrence of cracks on the slab surface. FIG. 5 is a graph showing the history of slab surface temperature in Examples. FIG. 6 is a graph showing the history of slab surface temperature in a comparative example.

Claims (1)

[Claims] In continuous casting of steel containing 0.02 to 0.15 wt% Sb, the degree of superheating of the molten steel when injecting it into the mold is set to 2.
5℃ or less, and the surface temperature of the slab is 1150~9
A method for preventing surface cracking in continuous casting of Sb-containing steel, characterized in that the cooling rate of the slab surface in a region of 50°C is 20°C/min or less.