JPH06344079A - Production of austenitic stainless steel having little surface flaw - Google Patents

Abstract

PURPOSE:To prevent the surface crack on a continuously cast austenitic stainless steel slab and to obtain a hot-rolled steel sheet having little fine crack of surface flaw containing scale after hot-rolling. CONSTITUTION:In the continuous casting process casting while oscillating in a continuous casting apparatus, by forcedly cooling a part concentrated with Ni called as an Ni segregated part or an Ni segregated line in the bottom part of an oscillation mark generated on the surface of the continuously cast slab by the part of the continuously cast slab at the initial stage of solidification in the cooling zone at the lower part of a mold, delta-Fe is allowed to remaine. By this method, the crack till the low temp. and the crack during heating are prevented and the hot-rolled band steel having no fine cracking flaw containing the scale on the surface layer is produced since the scale is not generated in the inner part of the crack in the heating furnace.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to continuous casting of austenitic stainless steel slabs which prevent surface cracking of austenitic stainless steel continuous cast slabs, and in particular, which contains scale after hot rolling and has few microcracked surface flaws. It is about law.

[0002]

2. Description of the Related Art One of the main surface defects which becomes a problem after hot rolling an austenitic stainless steel is a linear heald defect called a line heave (sliver). This defect is often a fine bald spot that cannot be found in hot rolled coils.
It is a serious problem in production because it is only manifested by cold rolling or product sheet. In stainless steel where surface quality is important, such defects are fatal,
This is a major impediment factor in terms of yield reduction and production planning, resulting in a significant cost increase.

The cause of such defects is hot workability, which has conventionally been a problem, and various studies have been made to improve the hot workability of austenitic stainless steels. That is, it is considered that defects are caused by embrittlement of the former austenite grain boundaries during hot rolling, especially during solidification by casting.
It is presumed that sulfur and oxygen are concentrated in the former austenite grain boundaries, which promotes embrittlement during hot rolling, causes small cracks, and causes bald defects. for that reason,
In order to prevent this, it was necessary to reduce the sulfur and oxygen contents in the steel or render it harmless.

In order to prevent the occurrence of surface defects due to cracks during hot rolling, it is important to reduce the contents of sulfur and oxygen in steel as much as possible. However, in reality, the cost is high and there is a limit to the reduction of the content. For example, Japanese Patent Laid-Open No. 57-16153 discloses δ-Fe Ca.
l (calculated value of δ ferrite amount) = 3 (Cr + Mo + 1.
5Si + 0.5Nb) -2.8 (Ni + 1 / 2Mn + 1)
/2Cu)-84(C+N)-19.8 determined δ-F
e Cal of 4.0% or less, and further, in JP-A-57-127506, the molten steel heating temperature Δ during continuous casting is described.
There is a method of adjusting the slab heating temperature of the heating furnace according to the product of T (difference between liquidus temperature and casting temperature) and N value.

[0005] A detailed investigation of the line-health flaws found in cold-rolled sheets and product sheets revealed that there were two types of flaws that contained scale and those that did not. By taking measures to improve the workability, the scale-free flaws are reduced, but the peculiar microcracked flaws containing the scale still remain.

[0006]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a method for producing an austenitic stainless steel with less surface flaws that does not cause microcracking flaws containing scale in the hot rolled steel strip after slab hot rolling. It is intended to provide.

[0007]

SUMMARY OF THE INVENTION The gist of the present invention is a continuous casting method for austenitic stainless steel, in which the Ni segregation portion or the Ni segregation line in the valley portion of the oscillation mark generated in the surface layer of the continuously cast slab is enriched with Ni. The part
By vigorous cooling at the continuous slab portion at the early stage of solidification in the lower cooling zone of the mold, the residual δ-Fe is prevented, so that microcracking due to thermal strain from the casting temperature to the constant temperature of the continuous casting slab is prevented and continuous hot rolling is performed. By preventing cracks due to thermal strain that occurs during heating, the scale does not form inside the cracks in the hot rolling heating furnace. It is a manufacturing method that does not cause cracks or flaws. In particular, δ-Fe existing in the surface layer of the slab at the early stage of solidification is strongly cooled to remain at a low temperature, and impurity elements such as S and O that promote grain boundary embrittlement are fixed so as not to move to the grain boundary. It is possible to prevent the surface layer of the slab from being cracked before the scale is grown by heating. The present invention will be described below.

[0008]

FUNCTION The scale composition in the micro cracks found in cold-rolled steel sheets and hot-rolled steel sheets is the same as the scale composition that grows on the slab surface in the slab heating furnace. Further, the generation of micro-cracking flaws containing scale becomes higher as the slab thickness of the continuously cast slab becomes thicker (143 to 250 mm) and as the slab width becomes wider (1000 to 1600 mm). When the slab surface care amount was deepened to about 2 mm to 6 mm, the defect occurrence rate decreased, and it became clear that the very surface layer portion of the slab was a problem.
Furthermore, there was a feature that a flaw was generated within 200 mm from the coil edge.

Next, as a result of cutting and polishing the cross section of the surface layer of the continuously cast slab and performing a microinvestigation, as the slab thickness of the continuously cast slab size becomes thicker and the slab width becomes wider, the mouth is opened on the surface per unit cross-sectional area. The number of cracks increased,
The change corresponding to the tendency of bald spots was shown. Furthermore, 10
As a result of a structure investigation by% oxalic acid etching, many of these cracks were found in the Ni segregation portion of the valley portion of the oscillation mark generated in the surface layer of the continuously cast slab and the Ni segregation line where Ni was particularly concentrated. It was found that δ-Fe did not remain in the Ni segregation portion of many slabs.
Regarding the crack and structure in the width direction, the slab with a large slab thickness and a wide slab width has many cracks at the width edge portion, and the edge portion 2
A large amount of δ-Fe of 00 mm also disappeared.

As described above, the occurrence of the bald defects corresponds to the size of the cast piece, and the surface layer crack of the cast piece and the change corresponding to the cast structure are observed. . An experiment was conducted in which the cooling rate of the cooling zone under the mold was changed with the thickness of 190 mm slab and δ-Fe in the Ni segregation portion of the valley portion of the oscillation mark generated on the surface of the continuously cast slab was changed to examine the occurrence rate of line bald defects. . Surface casting of continuously cast slabs after heating to 1260 ° C without maintenance 3
mm hot rolled, mechanically descaling the steel plate surface with high-pressure water containing iron sand, and then pickling with nitric hydrofluoric acid solution,
FIG. 1 shows the relationship between the number of bald defects per unit length of the coil after cold rolling to a thickness of 0.4 mm and the δ-Fe residual index of the Ni segregation portion in the valley portion of the oscillation mark. δ-Fe
As the residual index of (1) becomes higher (more δ-Fe remains), the number of beards decreases. Regarding the number of cracks per unit length obtained by cutting and polishing the surface layer of the continuously cast slab in the intermediate step of this experiment, no cracks were found in those having a high δ-Fe residual index.

Next, the cooling conditions of the continuously cast slab 1
The effect of specific water content was investigated for a slab having a thickness of 40 and a drawing speed of 0.8 m / min. As shown in FIG. 2, the slab having a large amount of specific water according to the distance from the molten metal surface has a reduced number of units for the occurrence of bald defects, and the slab has a high δ-Fe residual index (δ-Fe More remains).

The cooling conditions are as follows: S: distance from molten metal surface (m) A: unit slab weight (kg / m ² ) K: drawing speed (m / min) Q: boundary area specific water amount (1 / Min.m ² ), it can be obtained by cooling more strongly than the boundary area specific water amount under the condition of Q = K / 0.8 × A / 1144 × (55 / S + 17). Normally, the cooling water amount is changed for each zone for operation, but an effective effect can be obtained if the water amount is equal to or more than the boundary area specific water amount Q. Regarding cracks in the width direction, cooling of the edge part is slower according to the result of measuring the surface temperature of the slab.By uniformly cooling in the width direction, it is possible to eliminate slab cracks and scab defects in the edge part. It was

Regarding the mechanism by which the amount of residual δ-Fe is increased, the cracking of the continuously cast slab is reduced. The δ-Fe phase which is stable at high temperature immediately after solidification and which is stable at high temperature Change into a phase. In particular, the Ni segregation portion in the valley portion of the oscillation mark is likely to change from the δ-Fe phase to the γ phase, and easily changes to the γ phase when cooled slowly to 1150 ° C or lower. When changing from the δ-Fe phase to the γ phase, the impurity element that was uniformly dissolved in the δ-Fe phase was γ
Since it does not form a solid solution in the phase, it begins to precipitate at the grain boundaries and the grain boundaries become brittle and thermal strain causes grain boundary cracking. On the other hand, when strongly cooled, the δ-Fe phase in which the impurity element is well solid-dissolved comes to exist at a low temperature, the impurity element can be fixed in the grain, and grain boundary cracking can be prevented.

Next, the reasons for limitation in the present invention will be described. Regarding the point of focusing on the residual degree of δ-Fe in the Ni-enriched portion called the Ni segregation portion or the Ni segregation line of the oscillation mark valley portion, since δ-Fe easily changes to the γ phase when slowly cooled, It is easy to determine the degree of strong cooling. That is, it is easy to determine the residual state of δ-Fe. Since the continuous cast slab containing more δ-Fe has less surface cracks, the scale did not grow in the cracked portion in the heating furnace, and the scale was contained in the hot-rolled steel strip after hot rolling. It does not cause a slight crack.

Although the component composition of the austenitic stainless steel to which the present invention is applied is not limited,
Above all, the following ranges of components are preferable. The basis of the constituent requirements will be described below. The lower C is, the better the corrosion resistance is, and the better the heat resistance is, the better 0.20% or less. Si is more preferable in terms of corrosion resistance, but if it exceeds 4%, embrittlement becomes large. It is desirable that Mn be large in amount for stabilizing austenite, but since the effect is saturated, 4% or less is preferable.

The lower P is, the more desirable it is, and therefore 0.06% or less is preferable. If it exceeds this, the corrosion resistance deteriorates. S is an element which is a main factor in the conventional hot workability in which cracking occurs during hot rolling and also in cracking during cooling or heating of a continuously cast slab, so S is preferably as low as possible. The following is good. O, like S, promotes bald spots and is preferably low, and is reduced by refining and deoxidizing methods. Therefore, O is preferably 0.01% or less.

From the viewpoint of corrosion resistance, Cr is not less than 15% and more than 30% is difficult to work as stainless steel.
Ni is preferably 7% or more from the viewpoint of stabilizing the composition of stainless steel, and is more expensive than 28%. Mo is effective in enhancing the acid resistance and pitting corrosion resistance of stainless steel, and is a component that can be selectively added up to 5% depending on the application. Cu is also effective for acid resistance and is a component that can be selectively added up to 3% depending on the application. N is effective as an austenite former, but if it exceeds 0.15%, the hot workability tends to deteriorate. The lower limit is 0.001%, but below this is technically difficult.

Nb is effective as a carbide stabilizing element and is selectively added depending on the application. The content of 1% or less tends to cause embrittlement if it exceeds this. In addition to the above component composition, δ-F
e cal = 3 (Cr + Mo + 1.5Si + 0.5N
b) -2.8 (Ni + 1 / 2Mn + 1 / 2Cu) -84
Δ-Fe cal represented by (C + N) -19.8
It is recommended to target 1.5 to 4.0% of austenitic stainless steel. Next, examples of the present invention will be described.

[0019]

[Examples] Table 1 shows austenitic stainless steel SUS
304 steel has a δ-Fe cal of 2.5 to 4.0% and a tundish temperature of 1490 to 1530 ° C. in a continuous casting machine with an oscillation cycle of 150 to 250 c /
Min., Slab thickness 143-250 mm, slab width 1000- when continuously casting with an oscillation stroke of 2 to 7 mm
A continuous cast slab was manufactured by changing the area specific water amount for each cooling zone at a drawing speed of 1600 mm and a drawing speed of 0.6 to 1.5 m / min.

For such a continuously cast slab, cracks on the surface layer were observed, and δ-Fe at the Ni segregation portion at the valley portion of the oscillation mark was observed.
While observing the remaining degree, increase the temperature to 1200-1260 ℃
Heated in LNG combustion atmosphere for ~ 4 hours, hot rolled to 2.5
A hot rolled steel sheet having a thickness of ~ 4 mm was obtained. This hot rolled steel sheet is
After annealing at 1000 ° C for a short time or omitting annealing,
80 to 150 g / l nitric acid, 30 to 30
Nitrous hydrofluoric acid aqueous solution containing 200 g / l hydrofluoric acid 50-
Spray pickling was performed at 70 ° C., and the number of beards per coil length recognized on the surface of the hot rolled pickled plate after 5-10 μm lapping was evaluated.

The pickled steel strip thus obtained was cold-rolled together with a comparative material to a thickness of 0.4 to 0.7 mm, then annealed by light volatilization or in the air, and pickled by nitric hydrofluoric acid to produce a product plate, which was subjected to a hege The pass / fail judgment was made based on the flaw. In the case of a strongly cooled slab material cooled at a specific water content larger than the boundary area specific water content, the δ-Fe residual index of the Ni segregation part at the valley portion of the oscillation mark of the slab is also high δ.
A large amount of -Fe remains, the number of cracks on the surface of the CC slab is small, and the number of bald spots found on hot-rolled and pickled sheets is small. Was obtained.

On the other hand, in the case of a slab that is weakly cooled, the δ-Fe residual index of the Ni segregation portion in the valley portion of the oscillation mark of the slab is low, the δ-Fe residual is small, the number of cracks in the surface layer of the CC slab is large, and the hot rolling is Baldness frequently occurred on pickled boards and product boards, and the product was rejected. 10 to 25 thick with the same slab
A thick plate product was manufactured by hot rolling and heat treatment and pickling, but no strong slab defects occurred in the strongly cooled slab material.
On the other hand, with slabs that were not strongly cooled, there were many bald defects and many failed.

[0023]

[Table 1]

[0024]

As described above, according to the present invention, the pass rate in the cold rolled steel strip coil can be stably maintained at 95% or more without causing microcracking flaws containing scale in the hot rolled steel strip. This makes it possible to reduce the coil grinder implementation rate in the surface layer care process of the slab and the cold rolling process, contribute to the improvement of the yield, and solve the problem in terms of production planning. Furthermore, since bald spots do not occur even if the cooling rate is increased, high speed casting in which the cooling strength and the casting rate are balanced is also possible. This concept of flaw prevention is based on the hot charging method that directly connects continuous casting and hot rolling and CC-DR.
It can also be applied to a process (continuous casting-direct rolling process) and can remarkably increase productivity, so that its industrial effect is great.

[Brief description of drawings]

FIG. 1 shows a slab in which the residual amount of δ-Fe in the Ni segregation portion in the valley portion of the oscillation mark of the slab surface is changed by changing the CC casting cooling conditions, and the residual amount of δ-Fe is indexed and this index and hot rolling / acid Figure showing the correspondence with the number of bald spots per unit length that can be confirmed after washing

FIG. 2 shows the δ-F of the Ni segregation portion in the valley portion of the oscillation mark of the slab surface when the area specific water amount is changed according to the distance from the molten metal surface in the mold by changing the cooling condition of CC casting.
e is a diagram showing a relationship between an index of the residual amount and the number of bald spots per unit length that can be confirmed after hot rolling and pickling.

Claims (1)

[Claims] 1. A continuous casting method for austenitic stainless steel, in which a Ni-rich portion of a valley portion of an oscillation mark generated on a surface layer of a continuously cast slab is strongly cooled at a continuous slab portion in a lower cooling zone of a mold at an early stage of solidification. By leaving δ-Fe in the austenite system with less surface flaws, characterized by preventing microcracking due to thermal strain from the casting temperature of the continuous casting slab to constant temperature and cracking due to thermal strain during hot rolling heating. Manufacturing method of stainless steel.