JPH04284945A - Production of non-conditioning continuously cast slab of ti-containing steel - Google Patents

Abstract

PURPOSE:To enable non-conditioning to a continuously cast slab even in the case of a Ti-containing steel and to prevent fusiform scab developed in a hot coil. CONSTITUTION:At the time of continuously casting the Ti-containing steel by continuously pouring a molten steel in a tundish into a mold while feeding gas for sealing into the tundish, the Ti-containing steel is regulated to <=0.25wt.% Ti content and <=0.009wt.% N content. Further, flow rate of the gas for sealing fed into this tundish is made to <1.1Nm<3>/T.Steel and negative strip time tN decided with casting speed, amplitude and the oscillating number of the mold, is regulated to <=0.31sec.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a continuously cast slab of Ti-containing steel that allows no slab maintenance.

0002

[Prior art] Ti-containing steel (particularly Ti-containing stainless steel) contains Ti, which has a strong affinity with O and N.
The surface of the hot-rolled steel strip (hot coil) or cold-rolled steel strip (cold-rolled coil) is coated with TiN and T streaks.
It is well known that band-shaped bald spots caused by iO2 are likely to occur. Not only do Ti streak defects significantly reduce coil yield, but if they cannot be removed, they must be reduced to scraps, so many causes and countermeasures have been taken to address these defects. For example, powders have been developed that prevent properties from changing even if TiO2 is absorbed into the mold powder during continuous casting, but it is currently extremely difficult to reduce Ti streak defects.

[0003] Furthermore, when the surface layer of a continuously cast slab of Ti-containing stainless steel is hot-rolled without polishing and removal, spindle-shaped surfaces are formed on the resulting hot coil surface, although no inclusions are observed. Scratches may occur. As for this defect, no abnormality was found in the visual inspection of the slab appearance, and there are almost no actual cases of Ti-containing stainless steel being hot-rolled without any care in other reports, so we were able to find out the cause. not present. For these reasons, there is a problem in making continuously cast slabs of Ti-containing stainless steel without any maintenance, so the slab surface has been ground before hot rolling.

0004

[Problem to be solved by the invention] Therefore, the present invention solves the following problems:
In order to achieve maintenance-free continuous casting of Ti-containing stainless steel slabs, continuous casting of Ti-containing steel can produce hot coils without Ti streak defects caused by TiN or TiO2 and without spindle-shaped heave defects. The challenge is to establish a method.

[0005]

[Means for Solving the Problems] According to the present invention, in continuous casting of Ti-containing steel, in which Ti-containing molten steel in the tundish is continuously injected into the mold while a sealing gas is supplied into the tundish, Ti content in Ti-containing steel is 0.2
In addition to regulating the N content to 5% by weight or less and the N content to 0.009% by weight or less, the flow rate of the seal gas sent to the tundish was 1.1 Nm3/T. By regulating the negative stripping time tN, which is determined by the casting speed, the amplitude and vibration frequency of the mold, to 0.31 sec or less, a continuously cast slab that can be continuously cast without maintenance even with Ti-containing steel is manufactured. However, tN=1/f-tP tP=cos-1(-v/2πaf)/πf where, t
N: Negative strip time (sec) tP: Positive strip time (sec) V: Casting speed (m/mi
n) a: amplitude (mm) f: mold vibration frequency (cycle/min). In addition,
In the present invention, the Ti-containing steel includes Ti-containing stainless steel, which particularly requires surface beauty.

[Details of the invention] As mentioned above, Ti-containing steel contains Ti, which has a strong affinity with N and O. Therefore, it is not only possible to make continuous casting slabs without maintenance, but also because of Ti-based inclusions. Compared to continuously cast slabs of other steel types, the slabs had to undergo significantly more heavy grinding. However, the amount of [Ti] and [N
] By keeping the amount below a certain level, the occurrence of Ti streak defects mainly composed of TiN was eliminated, and the possibility of making the slab maintenance-free was confirmed. However, when a continuously cast steel slab containing Ti at a level where Ti streaks do not occur was rolled without any care, new spindle-shaped flaws that were not Ti streak flaws occurred. This flaw did not contain any inclusions. Therefore, it was assumed that the cause was other than inclusions at the time of slabbing. After conducting a detailed investigation of the slab and tracing the coil, we learned that the cause was bubble defects.

[0007] First, we investigated in detail the relationship between Ti and N content levels in steel, which are the main components of Ti streak defects, and found that Ti streak defects occur when the Ti and N contents exceed a certain level. We found that this occurs. Figure 1 shows the results. As a base steel, C: 0.06% or less, Si: 0.7% by weight.
5% or less, Mn; 0.70% or less, Cr; 10.50~
12.00%, Ti; 0.15-0.5%, N; 0.
0.006 to 0.012% stainless steel is continuously cast, the surface of the slab is treated, and hot rolled to produce a hot coil. Medium Ti content and N
They are organized by content. As is clear from Figure 1, [Ti] is 0.25% or less and [N] is 0.009%.
If it is below, Ti streak flaws will not occur.

[0008] However, when a hot coil is manufactured by hot rolling a slab with a Ti and N content that should not cause these Ti streak defects without any maintenance, spindle-shaped sludge defects sometimes occur. . Therefore, a sample was cut from the untreated continuously cast slab, sanded and buffed, and then thoroughly examined under a microscope. As a result, bubble-like defects were confirmed on the surface of the slab. There were no inclusions in this bubble defect, and no inclusions were observed in the spindle-shaped henge defect itself. Therefore, a correlation between these bubble defects and spindle-shaped henge defects was presumed. Therefore, a follow-up investigation was conducted in which hot coils were manufactured by drilling holes in the slab that were similar to the bubble defects, and as a result, defects that were exactly the same as spindle-shaped sludge defects appeared on the surface of the hot coils. Based on these facts, it is presumed that the occurrence of spindle-shaped sagging defects can be prevented by reducing the number of bubble defects observed in slabs, and in order to realize this, the relationship between bubble defects and continuous casting conditions was investigated in detail. .

As a result, it was found that two factors in continuous casting conditions influence the occurrence of bubble defects. One is the amount of sealing gas supplied to the tundish, and the other is the negative stripping time of the mold.

FIG. 2 shows the argon gas supplied to the tundish in a casting operation performed on the same type of steel as in FIG. The number of occurrences (scratches + spindle-shaped henge defects) is determined. From FIG. 2, it is clear that the flow rate per ton of molten steel (Nm3/T.Steel) of the sealing gas sent to the tundish influences the occurrence of bubble defects. In other words, regardless of the size of the tundish or the casting scale, as the unit flow rate of the sealing gas that comes into contact with the molten steel increases, the number of bubble defects on the surface of the hot coil increases. The results in Figure 2 show that the gas flow rate for tundish sealing should be 1.1 N to prevent bubble defects.
This indicates that it is necessary to make the value less than m3/TSteel. However, if this seal gas flow rate is lowered too much, problems will arise in the sealing performance with the atmosphere, so it is necessary to supply seal gas, and the flow rate is 0.5Nm3/T S
steel or more, preferably 0.7Nm3/T Ste
It is better to supply air above el. Note that FIG. 2 is a collection of operational examples when the negative stripping time of the mold is set to 0.31 sec or less.

FIG. 3 shows the changes in casting speed and mold stroke (
This figure shows the relationship between negative stripping time determined by amplitude) and frequency and (bubble flaws + spindle-shaped flaws), and the gas flow rate for tundish sealing is approximately 1.0 Nm3.
/T Steel. From Figure 3,
There is a clear correlation between the negative stripping time and the number of defects, and as the negative stripping time becomes shorter, the number of bubble defects decreases and the occurrence of spindle-shaped henge defects disappears. The result in Figure 3 shows that the negative strip time is 0.31.
sec or less is important in preventing the occurrence of the flaw.

From the above findings, in order to suppress the occurrence of Ti streak defects that appear on hot coils of Ti-containing steel and spindle-shaped sludge defects that appear even after slab cleaning, the Ti and N contents should be reduced to 0. 25% by weight or less and N content 0
．． In addition to regulating the flow rate of the seal gas to the tundish to 1.1 Nm3/T.
It has become clear that it is sufficient to limit the negative stripping time of the mold to 0.31 sec or less, and if these requirements according to the present invention are met, even Ti-containing steel can be made without maintenance of the slab, and the spindle-shaped shape can be reduced. It is possible to manufacture healthy hot coils without defects.

In this way, according to the present invention, not only the occurrence of Ti streak defects but also the occurrence of spindle-shaped sludge defects can be suppressed, and the reason for this is thought to be as follows. In order to suppress the occurrence of these flaws, it is important to perform casting under casting conditions in which air bubbles are not trapped in the surface layer of the slab. For this reason, it is better to reduce the amount of sealing gas, such as Ar, brought into the molten steel in the tundish to the extent that there is no problem with sealing performance. Specifically, it is 1.1 Nm3/T.
Must be less than Steel. In addition, gas that could not float up inside the tundish and was brought into the mold (
To prevent air bubbles from being trapped in the solidified shell in the mold, it is best to slow down the solidification rate as much as the shell strength allows. The vibration conditions to slow down the solidification rate are generally a stroke of 4 to 9 mm and a vibration frequency of 70 to 120 cycles/
The negative stripping time can be reduced within the casting speed of 0.4 to 1.0 m/min, but specifically, the negative stripping time should be 0.31 sec or less due to the relationship between bubble defects and spindle-shaped heave defects on the slab surface. It is better to The reason why many bubble defects are observed in Ti-containing steel is thought to be that the presence of Ti increases the viscosity at the molten steel-slab interface, making it difficult for bubbles to come out.

[0014]

[Example] Table 1 shows that C≒0.2%, Si≒0.55%,
0 for stainless steel with Mn≒0.2% and Cr≒11.0%
．． Sealing gas (argon gas) was used to feed molten steel with Ti addition of 30% or less and N content of 0.015% or less to the tundish during the negative stripping period.
Continuous casting was performed under the indicated casting conditions with varying flow rates, and the resulting slab was hot-rolled without any treatment to produce hot coils, and the degree of occurrence of defects in the resulting hot coils was investigated. The results are shown below. As can be seen from the results in Table 1, no Ti streak defects or spindle-shaped heave defects were observed in the hot coils manufactured under the conditions according to the present invention.

[0015]

[Table 1]

[0016]

[Effects of the Invention] According to the present invention, it is possible to prevent the occurrence of Ti streak flaws even in Ti-containing steel and when slabs are rolled without maintenance.
In addition, it is possible to prevent the occurrence of spindle-shaped scratches that occur even in treated slabs, so not only can slabs of Ti-containing steel be left without maintenance.
This enables the production of high-quality hot coils, and in addition to the economic benefits of eliminating the need for slab maintenance, it also greatly contributes to quality improvement.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the Ti content and N content in steel and the occurrence of Ti streaks in a hot coil.

FIG. 2 is a diagram illustrating the relationship between the sealing gas flow rate to the tundish and the occurrence of bubble defects and spindle-shaped sludge defects in a hot coil.

FIG. 3 is a diagram showing the relationship between the mold negative stripping time and the occurrence of bubble defects and spindle-shaped heave defects in hot coils.

Claims (2)

[Claims] Claim 1: During continuous casting of Ti-containing steel, in which Ti-containing molten steel in the tundish is continuously injected into the mold while a sealing gas is supplied into the tundish, the Ti content in the Ti-containing steel is reduced to zero. In addition to regulating the N content to 0.25% by weight or less and the N content to 0.009% by weight or less, the flow rate of the seal gas supplied to the tundish was 1.1 Nm3/T.
Steel, and the negative stripping time tN determined by the casting speed, mold amplitude and vibration frequency is 0.
A method for manufacturing a maintenance-free continuous cast slab of Ti-containing steel characterized by regulating the time to 31 seconds or less, provided that tN = 1/f
-tP tP=cos-1(-v/2πaf)/πfwhere, t
N: Negative strip time (sec) tP: Positive strip time (sec) V: Casting speed (m/mi
n) a: amplitude (mm) f: mold vibration frequency (cycle/min). 2. The continuous casting method according to claim 1, wherein the Ti-containing steel is stainless steel.