JPH1043839A - Method for continuously casting ni-cr base stainless steel thin cast slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a good quality cast slab having no fine crack on the surface by slowing down cooling speed of solidified shell, leaving δ-ferrite phase on the surface layer part of the cast slab and reducing the segregation of P and S. SOLUTION: In a continuous caster synchronously shifting a mold surface with the thin cast slab, a mold forming a metallic oxide layer on the outermost surface thereof contacting with molten Ni-Cr base stainless steel, is used. The casting is executed under the condition satisfying <=1.2 of an Ni balance value in the molten steel defined with the following equation. Ni balance = Ni + 0.5Mn + 30(C+N) -1.1(Cr+1.5Si+Mo) + 8.2, wherein, all figures are based on wt%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method suitable for continuously casting and solidifying molten steel of Ni-Cr stainless steel to produce a thin plate slab.

[0002]

2. Description of the Related Art A continuous casting method for producing a sheet slab by a continuous casting machine in which a mold surface moves in synchronization with the sheet slab is called a strip casting method (hereinafter, referred to as an SC method). Numerous invention proposals have been made for the method. In the SC method, a roll is mainly used as a mold, and a twin roll top pouring method in which molten steel is supplied from above to a pair of rolls rotating in opposite directions. Similarly, molten steel is supplied between a pair of rolls from a horizontal direction. And a single-roll type in which molten steel is supplied to one roll from the horizontal direction.

In these systems, in order to obtain a good product, it is necessary to maintain good surface properties and internal quality of the thin plate slab. In particular, since surface cracks and porosity of the thin plate slab cannot be removed before the product is produced depending on the size, it is necessary to prevent the occurrence at the continuous casting stage.

For this reason, as described below, there has been proposed a method of adjusting and controlling the surface layer of the roll, the composition and temperature of the molten stainless steel or the pressure load of the roll to reduce surface cracks and porosity.

According to the continuous casting method disclosed in Japanese Patent Publication No. 23858/1993, the outer surface of a roll is made of metal oxide or nitride.
A film having a thickness of 51 μm to 5 mm is formed to achieve slow cooling such that the rate of heat removal from the molten steel to the roll becomes slow. Thereby, the thickness of the solidified shell is made uniform to prevent the occurrence of porosity.

In the continuous casting method disclosed in Japanese Patent Application Laid-Open No. 4-33752, a steel strip is cast by setting the amount of δ ferrite in a stainless steel molten steel component to 3% or less and the casting temperature to 1520 ° C. or more. As a result, shallow tortoise-like hot lines are generated on the surface of the steel strip to increase the critical strain with respect to fracture of the surface layer, thereby reducing surface cracking.

In the method for preventing solidification cracking of a thin steel sheet disclosed in Japanese Patent Application Laid-Open No. Hei 4-237544, the magnitude of a pressing load applied to a slab between rolls is controlled by an austenite balance value of molten stainless steel. This prevents solidification cracking caused by the vibration of the pressure load of the thin steel plate.

In the above publication, the amount of δ ferrite and the austenite balance value are the same as the Ni balance value.
It is calculated from the component content of the molten stainless steel and is an index indicating the ratio of the δ ferrite phase in the slab.

[0009]

When a thin plate slab is cast from a molten Ni-Cr stainless steel typified by SUS304 (18Cr-8Ni) by the method disclosed in each of the above publications, the scale after casting remains attached. In the cast slab, it is improved until no surface crack is observed. However, after removing the scale by pickling or the like, when observing the slab surface, a small crack having a length of 3 to 15 mm (hereinafter, referred to as
Many are described as micro-cracks. This small crack
According to the microscopic observation of the cross section, it has a depth of 0.2 to 0.5 mm and cannot be removed even in a subsequent step (for example, CG (coil fray removal)), and thus remains as a surface flaw on the product.

An object of the present invention is to provide a SUS304 (18Cr)
-8Ni) It is an object of the present invention to provide a continuous casting method capable of preventing minute cracks on the surface of a slab when a thin plate slab of Ni-Cr stainless steel represented by -8Ni) is manufactured by the SC method.

[0011]

The gist of the present invention is as follows.
-It is a continuous casting method for a Cr-based stainless steel sheet slab.

A continuous casting method for producing a Ni-Cr type stainless steel sheet slab by a continuous casting machine in which a mold surface moves in synchronization with the sheet slab. Using a mold having a metal oxide layer formed on the surface, Ni, Cr, C, Si, Mn, Mo and N
Wherein the Ni balance value of the molten stainless steel defined by the following formula according to the content (% by weight) of Ni is -1.2 or less. Continuous casting of pieces.

Ni balance = Ni + 0.5Mn + 30 (C + N) −1.1 (Cr + 1.5Si + Mo) +8.2... The above “mold” is a roll, a belt (endless metal plate), a block (metal), etc. Point to. Here, the symbol of the element in the formula represents the content (% by weight) of each element. A desirable lower limit of the Ni balance value is about -2.8.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to FIG. 1, an example of the configuration of a continuous casting machine used when carrying out the method of the present invention will be described. FIG. 1 is a vertical cross-sectional view showing a twin-roll top-pouring continuous casting machine.

Stainless steel melt 2 supplied between rolls 1
Is solidified by removing heat from the outer surface of the roll 1 in which the inside is cooled. At this time, the solidified shell 3 moves downward with the rotation of the roll 1 and is pressed near the kiss point (the closest point of the two rolls) to be conveyed as a thin plate-shaped slab 4. A refractory material called a side dam 10 for holding the molten stainless steel 2 is provided on the side surface of the roll 1.

In the continuous casting machine having the structure as shown in FIG. 1, the roll 1 corresponds to a mold, and when the roll 1, that is, the mold is rotated, its surface moves in synchronization with the thin plate-shaped slab 4.

The continuous casting machine to which the method of the present invention is applied is not limited to the above-mentioned ordinary twin-roll top-pouring system, but may be an ordinary twin-roller that supplies molten steel between a pair of rolls rotating in opposite directions from each other. A roll side pouring method and a normal single roll method in which molten steel is supplied to one roll from a horizontal direction may be used.

As a mold in a continuous casting machine to which the method of the present invention is applied, a belt or a block can be used in addition to the above-mentioned roll. The method of the present invention uses a mold having a metal oxide layer formed on the outermost surface of a mold in contact with stainless steel molten steel when producing a thin plate slab of Ni-Cr stainless steel by the continuous caster as described above. , Ni,
Casting is performed so that the Ni balance value of the molten stainless steel defined by the following formula according to the content (% by weight) of Cr, C, Si, Mn, Mo, and N satisfies -1.2 or less. is there.

Ni balance = Ni + 0.5Mn + 30 (C + N) −1.1 (Cr + 1.5Si + Mo) +8.2... FIG. 2 shows an example of the structure of a mold or roll applied when using the continuous casting machine of FIG. explain. FIG. 2 is a cross-sectional view showing a structural example of a mold (roll) used in the method of the present invention.

As shown in FIG. 2, the roll 1 is
And a flow path 9 for cooling water 8. Cooling water 8 flows through the inner surface of the sleeve 5 so that the temperature of the sleeve 5 does not rise above a certain level. The material of the sleeve 5 is a metal such as a copper alloy or steel. A metal oxide layer 6 is applied to the outermost surface of the sleeve 5.

In order to increase the bonding strength between the metal oxide layer 6 and the sleeve 5, a plating layer of Ni or a metal sprayed layer of Ni—Cr may be applied as the intermediate layer 7.

A desirable material for the metal oxide layer 6 is Z
rO ₂ , Al ₂ O ₃ , SiO ₂ , MgO or Ti
O, or a mixture thereof. These have low thermal conductivity and are excellent in heat resistance and thermal shock resistance.

The desirable range of the thickness of the metal oxide layer 6 is 3
0 to 1000 μm. If it is less than 30 μm, the effect of slow cooling described later cannot be obtained. On the other hand, if it exceeds 1000 μm, the cooling becomes too slow, and a thin plate-like cast piece (solidified shell) 4 having a sufficient thickness cannot be obtained. Considering the effect of slow cooling and the thickness of the thin slab 4 to be cast, the metal oxide layer 6
Is more preferably 80 to 200 μm.

The metal oxide layer 6 is generally applied to the surface of the sleeve 5 or the intermediate layer 7 by a thermal spraying method.
However, other construction methods do not affect the effect of the metal oxide layer 6 and therefore may be used.

Next, the target steel type and the Ni balance value will be described.

The Ni-Cr stainless steel to be subjected to the method of the present invention is SUS304 (1) specified in JIS.
8Cr-8Ni), SUS301, SUS303, and SUS308. Note that a desirable range of the thickness of the thin plate slab is about 0.3 to 10 mm.

As described above, micro-cracking was not eliminated only by applying a metal oxide layer on the mold surface and slowly cooling, so an attempt was made to improve by adjusting the components in the molten stainless steel. As a result, as shown in FIG. 3 of the examples described later, it was found that the microcracks were eliminated by setting the Ni balance value of the molten stainless steel obtained by the above equation to −1.2 or less.

The Ni balance value is a value indicating the ratio of the austenite phase and the δ ferrite phase in the thin plate slab.
The smaller the size, the smaller the austenite phase and the greater the δ ferrite phase. If this Ni balance value exceeds -1.2, microcracks occur on the surface of the thin plate-shaped slab. Therefore, from the viewpoint of preventing minute cracks, the Ni balance value is-
1.2 or less. Further, if the Ni balance is too small, a large amount of δ ferrite remains in the product, which adversely affects the product. Therefore, from the viewpoint of preventing the δ ferrite phase from remaining in the product, the Ni balance is about −2.8 or more. It is desirable to do.

As a result of a detailed investigation of the portion where the microcracks occurred in the thin plate slab, the surface layer portion of the thin plate slab was an austenitic single phase without a δ ferrite phase, and the crack portion was contained in the molten stainless steel. P and S were segregated. On the other hand, a δ ferrite phase remained in the center in the thickness direction of the thin plate slab.

Since the thin plate slab produced by the SC method is thin, the molten stainless steel is rapidly cooled by a mold to form a solidified shell and become a thin plate slab. When a metal mold such as a copper alloy or steel is used as is, or when a mold having a metal plating layer such as Ni on its surface (hereinafter referred to as a metal mold) is used, the cooling rate of the solidified shell increases.

From the results of the heat transfer analysis, the cooling rate at the center in the thickness direction of the thin plate slab is about 500 ° C./s, but the position at 10 μm from the surface is as large as about 5000 ° C./s.
As described above, since the cooling rate of the surface layer portion of the thin plate slab is large, the Ni-Cr stainless steel has no δ ferrite phase and is an austenitic single phase. Austenitic phase is P
Since the solid solubility limit of S and S is small, P and S are liable to segregate, the strength of the portion is reduced, and micro cracks are easily generated on the surface of the thin plate-shaped slab.

By providing a metal oxide layer having a large thermal resistance on the surface of the mold, slow cooling can be achieved, and the cooling rate of the surface layer of the thin plate slab can be reduced. As compared with the metal mold, the cooling rate of the surface layer of the thin slab can be reduced to about ２〜 to ３.

The Ni balance value of molten stainless steel is set to -1.
By setting the ratio to 2 or less, the amount of the δ ferrite phase in the surface layer of the thin plate slab can be increased.

As described above, by using the mold provided with the metal oxide layer, the cooling rate of the surface layer portion of the thin slab is reduced,
Further, by increasing the Ni balance value to -1.2 or less and increasing the amount of δ ferrite phase in the surface layer of the sheet-like slab, it is possible to prevent the surface layer of the sheet-like slab from becoming an austenitic single phase,
A small amount of ferrite phase can be left. Due to the presence of a small amount of the δ ferrite phase in the sheet slab, P and S become δ
The solid solution in the ferrite phase reduces these segregations. For this reason, microcracks do not occur on the surface of the Ni-Cr-based stainless steel sheet slab.

In the case where a metal roll is used, even if the Ni balance value of the molten stainless steel is reduced to -2.8 to -1.2, minute cracks are reduced but cannot be completely prevented. This is because the cooling rate of the thin slab is higher than that of the mold provided with the metal oxide layer, and the austenitic single phase is more likely to be formed. When a metal roll is used, it is necessary to further reduce the Ni balance value to −3 or less in order to leave the δ ferrite phase on the surface layer of the thin plate slab. However, this is not practical because it adversely affects the properties of the product.

[0036]

【Example】

(Test 1) Using a normal twin-roll horizontal casting continuous casting machine equipped with rolls of different diameters, a thin plate-like slab having a thickness of 1.3 mm and a width of 250 mm was cast under the following mold (roll) and operating conditions, The cracks on the slab surface were investigated.

<Mold> Roll under conditions defined by the method of the present invention Sleeve: Material: Copper alloy, wall thickness 20 mm Size: Upper roll; Body length 250 mm, diameter 500 mm Lower roll: Body length 400 mm, diameter 600 mm Intermediate layer: Ni plating with a thickness of 50 μm is applied to the surface of the sleeve as a base. Further, Ni—Cr alloy with a thickness of 50 μm is applied as a base to the above surface by thermal spraying. Metal oxide layer: material: ZrO ₂ -8 wt% Y ₂ O ₃ , Construction thickness: 120 μm Construction method: Plasma spraying method <Operating conditions> Molten steel: Ni-Cr stainless steel including composition examples shown in Table 1 Range of Ni balance value: Example of the present invention; -2.73 to -1.25 Comparative example: -1.08 to -1.03 Molten steel amount: 2 ton Casting temperature: 1520 to 1550 ° C Casting speed: 50 m / min

[0038]

[Table 1]

When the obtained slab was observed with the scale still attached, no surface crack was observed in any of the molten steel compositions.

Next, these cast slabs were pickled to remove scale, and then microcracks on the surface were observed. The evaluation of the microcracks was performed by measuring the individual lengths of the cracks and calculating the sum of the lengths of the cracks generated per 1 m of the slab length as the crack length (mm).
/ M). The results are shown in Table 1 (some examples) and FIG.

FIG. 3 is a diagram showing the influence of the Ni balance value and the mold (roll) surface on the crack length. FIG.
The symbol “〇” in FIG.

As shown in FIG. 3, the Ni balance value is-
In the case of 1.2 or less, the crack length was 0 mm / m, and a slab with a good surface without fine cracks was obtained. On the other hand, Ni
When the balance value was larger than -1.2, microcracks occurred even when a roll having a metal oxide layer formed on the outermost surface was used. The larger the Ni balance value, the longer the crack length, and the number of cracks tended to occur.

(Test 2) The same continuous casting machine as in Test 1 was used, and a metal roll under the following conditions was applied as a comparative example. Other operating conditions and investigation items were the same as those in Test 1.

<Mold> Metal roll Sleeve: Material: Copper alloy, thickness 20 mm Size: Upper roll: Body length 250 mm, diameter 500 mm Lower roll: Body length 400 mm, diameter 600 mm Surface layer: thickness 2 mm on sleeve surface Further, a hard Cr plating having a thickness of 50 μm was formed on the above surface. <Operating conditions> Molten steel: Ni-Cr stainless steel including the composition example shown in Table 1. Range of Ni balance value: -2. 9 to -0.7 In any of the molten steel compositions, a relatively large surface crack having a length of 15 to 50 mm was observed in the slab with the scale attached.

Next, the scale was removed by pickling the slab, and the surface was observed. As a result, a number of minute cracks having a length of 15 mm or less which were not observed before pickling were observed.

FIG. 3 and Table 1 (some examples) also show the results. The mark ● in FIG. As shown in FIG. 3, the crack length tended to decrease as the Ni balance value became smaller. However, even in the case of -1.2 or less, cracks having a crack length of 20 to 6000 mm / m occurred, and micro cracks could not be prevented.

[0047]

According to the method of the present invention, the cooling rate of the solidified shell is reduced, and the δ ferrite phase in the sheet slab of stainless steel is increased, so that the δ ferrite phase remains on the surface layer of the slab. Thus, segregation of P and S can be reduced. As a result, it is possible to produce a thin plate-shaped slab of good quality with no fine cracks on the surface.

[Brief description of the drawings]

FIG. 1 is a vertical cross-sectional view showing a twin roll top pouring type continuous casting machine used in carrying out the method of the present invention.

FIG. 2 is a cross-sectional view illustrating a structural example of a mold (roll) used in the method of the present invention.

FIG. 3 is a diagram showing the influence of the Ni balance value and the mold (roll) surface on the crack length.

[Explanation of symbols]

1: Roll, 2: Molten stainless steel, 3: Solidified shell, 4: Sheet slab, 5: Sleeve, 6: Metal oxide layer, 7: Intermediate layer, 8: Cooling water, 9: Channel, 1
0: Side dam

Claims (1)

[Claims] 1. A continuous casting method for producing a Ni-Cr stainless steel sheet cast by a continuous casting machine in which a mold surface moves in synchronization with the sheet cast, the method comprising: Using a mold having a metal oxide layer formed on the outermost surface, Ni, Cr, C, Si, Mn, Mo and N
Wherein the Ni balance value of the molten stainless steel defined by the following formula according to the content (% by weight) of Ni is -1.2 or less. Continuous casting of pieces. Ni balance = Ni + 0.5Mn + 30 (C + N) -1.1 (Cr + 1.5Si + Mo) +8.2 ...