JPS63183766A - Method for continuously casting two-phase stainless steel cast slab - Google Patents

Abstract

PURPOSE:To make structure of a cast slab fine and to prevent the development of surface defect by rapidly cooling solidified surface layer part of two-phase stainless steel formed from molten steel in a mold at the suitable cooling velocity from higher temp. than the solidus line. CONSTITUTION:The molten steel 7 supplied in the mold 1 is cooled by upper inner wall face 5 arranging cooling water passage 10 and lower inner wall face 2 providing nozzles 4 for injecting coolant and inducing holes 6 for sucking to form the solidified surface layer part 9 and further, cooled by cooling water nozzles 11, and the formed cast slab is drawn downward. In continuous casting method for the above two-phase stainless steel slab, the solidified surface layer part 9 of the two-phase stainless steel formed in both end opening mold 1 is cooled at high cooling velocity, such as >=10 deg.C/sec in the range of temp. from higher than solidus line to at least 1300 deg.C by measuring the temp. sensors 3 buried in the lower inner wall face 2. The above rapid cooling is better to execute to above 10 mm depth and preferable to above 20 mm depth. In this way, the structure of cast slab is made to fine, and the development of surface defect is prevented and the yield improvement under saving cleaning process is obtd. at low cost.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention is a method for treating duplex stainless steel blooms and slabs manufactured by a continuous casting method, as well as hot-rolled materials thereof. The present invention relates to a method for continuous casting of duplex stainless steel slabs, which prevents the above problems and thereby makes it possible to improve the yield and reduce the number of man-hours by omitting the flaw removal process.

[Conventional technology]

In recent years, methods have been used to continuously cast duplex stainless steel slabs using vertical or curved continuous casting machines equipped with molds that are open at both ends.

[The key point that the invention is trying to solve]

However, if you try to manufacture pieces of duplex stainless steel such as blooms or slabs using the continuous casting method described above,
Bending stress applied to the steel billet during casting and thermal stress caused by cooling may cause surface flaws, and furthermore, steel billets obtained by continuous casting may be subjected to direct rolling, hot charge rolling, or reheating. During rolling, problems such as similar surface flaws occurring on rolled materials are noticeable, and this is an obstacle to improving product yield. This is a major obstacle in promoting labor and energy saving in the manufacturing process, and there are also problems such as significant edge cracking during hot rolling.

[Means for solving problems]

Therefore, from the above-mentioned viewpoint, the present inventors developed a method to prevent surface flaws (surface cracks) from occurring in continuously cast duplex stainless steel blooms and slabs, as well as hot-rolled materials thereof. As a result of research from a scientific point of view, (a) duplex stainless steel has a harder film-like austenite phase (hereinafter referred to as γ phase) at the coarse grain boundaries of the primary δ-ferrite matrix. The precipitated structure, that is, δ-ferrite crystal grains (hereinafter referred to as δ grains), has a structure surrounded by γ phase, and therefore, the above surface flaws in duplex stainless steel are caused by the γ phase at grain boundaries, with δ grains as a unit. Furthermore, wrinkle flaws, which are one of the surface flaws that occur during hot working, are also caused by the deformation of coarse δ grains extending in the working direction.

(b) If the δ grains are refined, the occurrence of surface flaws on the upper ε will be prevented, but since the δ grains have a two-phase structure, one phase This is extremely difficult because it pins down the grain boundary movement of the other phase.

(c) During continuous casting of duplex stainless steel,
The cooling rate of the first part of the slab that is currently solidifying is at least 13
By cooling down to 00°C at a high cooling rate of 10°C/5ec12L, it is possible to refine the structure.

ldl There are no particular restrictions on the target duplex stainless steel as long as it contains Fe*Cr and Ni as the main components and has a two-phase structure of δ grains + γ phase near room temperature (, Cr can be replaced with Si * Mo + Furthermore, M or N may be contained instead of Ni, and the target range is the first
The shaded area in the Schiefler diagram shown in the figure (points in the shaded area!! The indicated area indicates the range of normal duplex stainless steel)
It is within the range shown by , and a greater effect can be obtained in steel with a primary δ single phase.

The findings shown in (al to (d)) above were obtained.

The present invention has been made based on the above findings, and includes forming a solidified surface layer of duplex stainless steel from molten steel in a mold with both ends open, and then heating the solidified surface layer from a temperature above the solidus line to a temperature of at least 1300°C. This method is characterized by a continuous casting method for duplex stainless steel slabs that cools down to 10°C at a cooling rate of 10°C/sec or more.

In the method of the present invention, the surface layer portion of the duplex stainless steel slab whose cooling rate is to be controlled refers to the portion directly related to surface flaws up to 10 days, preferably up to 20 days.

In addition, when carrying out the method of the present invention, we will examine specific means for increasing the cooling rate from the start of solidification to 1300°C to a high cooling rate of 10°C/sec or more. In normal molds used in casting machines (700 to 900 days or more in length), solidification progresses in the vicinity of the molten steel meniscus, where the solidified surface layer and mold wall are in close contact with each other through the molten powder. Although a sufficient cooling rate is ensured, below that point, the slab separates from the mold wall due to the solidification shrinkage of the molten steel and the shrinkage of the slab due to the temperature drop, creating an air gap that impairs the heat removal effect of the mold. There is a problem in that it is impossible to secure a high cooling rate at an early stage as described above, and if a mold with short dimensions is used, there will be a significant cooling delay. The surface layer of the solidified slab is very thin, so if a method is attempted to increase the cooling rate in high temperature ranges by spraying a cooling medium onto the slab that has been pulled out early from the bottom of the mold, breakout of the slab will occur. Although there are problems such as extremely high danger, as a mold for continuous casting, as shown in the schematic cross-sectional view in Fig. 2, a temperature sensor is installed on the lower inner wall surface 2 of the mold l to measure the temperature of the solidified surface layer of the slab. 3, a cooling medium injection nozzle 4, and a cooling medium suction passage hole 6 above the lower inner wall surface 2 are used. The temperature is at least 1300°C above the solidus line above.
The temperature sensor 3 can detect whether or not the temperature range is 16 (a range in which a rapid cooling effect of grain refinement can be expected), and the solidification surface layer at the above-mentioned appropriate temperature can be cooled at a rate of 10°C/sec or more. The blowing of the coolant from the coolant blowing nozzle 4 can be adjusted via the temperature measurement sensor 3 so that the cooling rate is 100%. meniscus 8
By creating a gap between the coolant and the coolant and blowing upward from there, destabilizing the cooling in the vicinity of the meniscus 8 can be prevented by smooth discharge from the coolant suction passage hole 6 (in Fig. 2, 10 is a cooling water passage.

11 indicates a cooling water spray nozzle), as a result, it is possible to easily ensure a high cooling rate of the cast molten steel at high temperature while reliably avoiding the risk of slab breakout, and the solidification of the slab can be easily achieved. 1 from the temperature of the surface layer above the solidus line
This makes it possible to stably achieve the condition of cooling at a cooling rate of 0°C/set; I2i.

That is, in FIG. 2, when molten steel 7 is poured into the mold 1, a very thin solidified surface layer is formed by the heat removal action of the upper inner wall surface 5 of the mold, but the length of this upper inner wall surface 5 For example, about 4001 (length below the meniscus: 3
If the temperature is kept very short (approximately 00+w), the part of the slab where the solidified surface layer has just been formed will be immediately lowered to the position of the lower inner wall surface 2 of the mold, and the coolant will be blown from the cooling medium injection nozzle 4. For example, He gas or a mixture of He gas and water that flows back to the cooling medium suction passage hole 6 is efficiently cooled by the cooling medium such as gas, so that "solidification" as in a conventional mold with both ends open is possible. The solidified surface layer separates from the inner wall of the mold due to shrinkage and cooling, forming an air layer between both surfaces, which causes a delay in cooling.Moreover, the temperature sensor 3 allows the slab to be rapidly cooled. It is possible to accurately check the temperature of the solidified surface layer, and it is also easy to adjust the cooling rate, thus stably ensuring a high cooling rate in the high temperature range of the solidified surface layer. If the mold has such a structure, the slab can be kept in the mold until the solidified surface layer of the desired thickness is formed, so there is no risk of breakout. FIG. 2 shows a quenching mode of the method of the present invention, and the present invention is not limited thereto.

Further, in the method of the present invention, the cooling rate in the temperature range from the solidus temperature to at least 1300'C1 is set to be 10°C/see IJ or above.

When the cooling rate in the temperature range is less than 10°C/sec, the desired structure refinement, that is, δ grain refinement, is achieved. This is based on the reason that δ grain growth is determined up to 1300° C., and at temperatures below this, the r phase begins to precipitate, inhibits δ grain boundary movement, and contributes to microstructural refinement.

More specifically, in terms of overlapping % (hereinafter % indicates overlapping %), C: 0.02%, Si: 0.30%.

Mn: 0.85%, P: 0.021%.

S: 0.003%, Ni: 6.0%.

Cr: 23.5%, Mo: 2.5%.

N: 0.11%.

A duplex stainless steel (referred to as 91-A steel) having a composition containing Fe and the remainder consisting of Fe and unavoidable impurities.

C: 0.02%* st: 1. s%.

Labor: 3.5%, P: 0.0022%.

S: O, 002% + Ni: 4.2%.

Cr: 18.4%, N: 0.005%.

A small piece of duplex stainless steel (referred to as TLL grade steel) having a composition of
50°C (= after heating and melting, 10°C at various cooling rates)
FIG. 3 shows the relationship between the six particle sizes and the cooling rate when water cooling was performed after cooling to 00°C.

As shown in FIG. 3, it can be seen that the δ grain refinement effect becomes remarkable at a cooling rate of 10° C./5 ecLl.

[Example] - Next, the method of the present invention will be specifically explained with reference to Examples.

By normal dissolution method, C: 0.04%, Si: 0.2
5%.

Mn: 0.65%, P: 0.0
21%.

S: 0.004%, Cr: 25.3%.

Ni: 6.5%, Mo: 3.02%
.

N: 0.1 4%.

Duplex stainless steel (hereinafter referred to as C steel (hereinafter referred to as C steel)), and C: 0.04%, Si: 0.31%.

Mn: 0.55%, P: 0.022%.

S: 0.003%, Cr: 19.2%.

Ni: 5.1%, N: 0.009%.

2° phase stainless steel (hereinafter referred to as C steel) having a composition containing Fe and the remainder consisting of Fe and unavoidable impurities), each having the structure shown in Fig. 2 and having a total length of 7
Using a curved continuous casting machine with a radius of 12.5 m and equipped with a casting mold 1 at both ends with dimensions of 00 × upper inner wall surface length: 3001, the cooling rate of the solidified surface layer 9 in the mold 1 was determined. 8℃/sec, 10℃/sB, respectively.
Continuous casting was performed under conditions of 20°C/sec and 30°C/SeQ to obtain a slab with cross-sectional dimensions of 250cm x 2100mm, the appearance of surface flaws on this slab was observed, and then this casting was The piece was reheated to a temperature of 1250° C. to form a hot-rolled material having a thickness of 50 cm, and the method of the present invention was carried out by visually observing the surface flaws even in this state.

In addition, for the purpose of comparison, a conventional method was carried out under the same conditions except that a conventional mold with a total length of near 00 mm and open at both ends, which was not equipped with a cooling medium injection nozzle, a cooling water spray nozzle, and a temperature sensor, was used as the mold. .

〔Effect of the invention〕

As a result, the slabs produced by the method of the present invention and the hot-rolled materials produced therefrom have no surface flaws and do not require any maintenance, whereas the conventional method The slabs of steel C and steel D had many surface flaws, and both of the hot-rolled steels had edge cracks, requiring grinder grinding.

As described above, according to the method of the present invention, it is possible to prevent surface defects from occurring in plumes and slabs of duplex stainless steel manufactured by continuous casting, as well as in hot rolled materials thereof. The scratch removal process can be omitted, which brings about industrially useful effects such as improved yield and reduced costs due to reduced man-hours.

[Brief explanation of the drawing]

Figure 1 is a Schiefler diagram showing the target range of the duplex stainless steel of this invention, Figure 2 is a schematic sectional view showing the quenching mode of the method of this invention, and Figure 3 is a cooling rate up to 1300°C and 6 grains. It is a relationship diagram with a diameter. 1... Mold, 2... Lower inner wall surface. 3...Temperature sensor. 4...Nozzle for blowing cooling medium. 5... Upper inner wall surface, 6... Cooling medium suction conduction hole. 7... Molten steel, 8... Meniscus. 9... Part 1 of the solidification table, 10... Cooling water passage. 10... Cooling water spray nozzle. Creq = Cr + Mo +1.5 Si Figure 1': $2 Figure

Claims (1)

[Claims] After forming a solidified surface layer of duplex stainless steel from molten steel in a mold with both ends open, the solidified surface layer is cooled from a temperature above the solidus line to at least 1300°C at a cooling rate of 10°C/sec or more. Continuous casting method for duplex stainless steel slabs.