JPH05237604A - Production of dual phase stainless steel ingot - Google Patents

Abstract

PURPOSE:To produce a two phase stainless steel ingot with an excellent property in material quality of a fine austenite grain uniformly distributed structure not subjecting to an additional heat treatment like a solution heat treatment, when a dual phase molten stainless steel is rapidly cooled and solidified to continuously cast to a ingots. CONSTITUTION:When a molten steel of dual phase stainless steel is rapidly cooled and continuously cast to a ingots, a molten steel is rapidly cooled with a cooling speed in the range of >=40 deg.C/sec-<=10 deg.C/sec to solidify. Subsequently, a volume ratio of the austenite phase in the obtained ingots is <=32% by continuously cooling to a normal temp.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is based on a rapid solidification method, for example, in which a molten steel of duplex stainless steel is rapidly solidified by a synchronous continuous casting machine to directly produce a thin plate-like duplex stainless steel slab. The present invention relates to a method for manufacturing a thin plate-shaped duplex stainless steel slab.

[0002]

2. Description of the Related Art In recent years, a method of directly casting a thin plate-shaped slab by rapidly solidifying molten metal has been studied. For example, Japanese Patent Laid-Open No. 58-210150 discloses iron, boron and silicon. A method of directly casting a ribbon of an amorphous alloy by rapidly quenching a melt of an amorphous alloy composed of an element and rapidly solidifying the melt is disclosed.

However, in the above-mentioned method, since the quenching rate is 10 ⁵ ° C / sec or more, the thickness of the cast ribbon must be about 500 µm or less and the width thereof must be several 100 mm or less. Therefore, it has been limited to a special purpose.

On the other hand, in order to simplify the casting process, for example, at least a pair of endless belts moving in the same direction and at the same speed are arranged facing each other at a predetermined interval to form a horizontal mold, The molten metal supplied into the mold is rapidly solidified by contact with a mold, and a continuous continuous casting method comprising continuously casting a thin plate-shaped slab by withdrawing in synchronization with the endless zone, and A twin-roll continuous casting method comprising: rapidly melting and solidifying molten metal supplied onto the surfaces of a pair of rotary cooling bodies by contact with the rotary cooling bodies, and continuously casting thin plate-shaped cast pieces. Etc., many methods have been proposed.

According to the above-described rapid solidification casting method, since the cooling rate of the molten metal is not the ultra-quick cooling as in the case of casting of the amorphous alloy ribbon, a thick and wide plate-shaped slab is continuously cast. Can be cast into.

[0006]

However, little research has been done on the structure and material of the slab when the thin plate slab of duplex stainless steel is cast by the method described above. Metallurgical Transact
Ion A. VOL.14A (1983), P.1839, describes that when molten steel of SUS 308 and 312 is cooled at a rate of 10 ⁵ K / sec or less, no phase transformation of the metal structure occurs. It is only about.

In duplex stainless steel, it is necessary to have a structure in which fine austenite grains are uniformly dispersed in the ferrite phase (Micro Duplex structure) in order to impart excellent workability and corrosion resistance to the steel. .. However, in the conventional ingot casting method and continuous casting method, since the cooling rate is slow, the slab cooled to room temperature has a structure in which ferrite and coarse austenite having a high volume ratio are mixed. Therefore, in order to enhance its workability and corrosion resistance, the cast piece cooled to room temperature is subjected to an additional heat treatment such as solution treatment, and a structure in which fine austenite is uniformly dispersed in the ferrite phase. Must be changed to As a result, due to such additional heat treatment,
There is a problem that the manufacturing cost increases.

Therefore, an object of the present invention is to add a solution treatment to a slab cooled to room temperature when rapidly casting and solidifying molten steel of duplex stainless steel to continuously cast a thin slab. A thin plate duplex stainless steel slab with excellent properties in terms of the structure, in which fine austenite grains are uniformly dispersed, by utilizing the change in the metal structure caused by rapid solidification without performing a general heat treatment. It is to provide a method that can be manufactured at low cost.

[0009]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the inventors of the present invention rapidly quench solidify molten steel of duplex stainless steel to continuously cast thin plate-shaped slabs depending on the cooling rate. We have conducted intensive studies on the resulting changes in the metal structure. As a result, the following findings were obtained.

That is, the conventional ingot casting method or continuous casting method
When the molten steel of duplex stainless steel is solidified, the cooling rate of the molten steel is slow, so the structure of the slab at the time of solidification is a ferrite single phase, but until solidification is continuously cooled to room temperature after solidification. In addition, a multiphase mixed structure is formed in which coarse austenite grains are precipitated in the ferrite phase. However, the rapid cooling of the duplex stainless steel until solidification of molten steel is performed at 40 ° C / sec or more,
When the molten steel is solidified at a cooling rate in the range of less than 10 ⁴ ° C / sec, precipitation of the austenite phase during the continuous cooling to normal temperature after solidification is suppressed, and the cooling rate is If properly selected within the range, the structure can be made into a ferrite single phase while being rapidly solidified.

The present invention has been made on the basis of the above findings, and is for producing a thin plate-shaped duplex stainless steel slab for rapidly cooling molten steel of a duplex stainless steel and continuously casting thin plate-shaped slabs. In the method, quenching of the duplex stainless steel until the solidification of the molten steel is performed at a cooling rate in the range of 40 ° C./sec or more and less than 10 ⁴ ° C./sec to solidify the molten steel, and then to normal temperature. It is characterized in that the volume ratio of the austenite phase of the thin plate-shaped slab that is continuously cooled and thus cooled to room temperature is 32% or less.

[0012]

In the present invention, the rapid cooling of the duplex stainless steel until the solidification of the molten steel is carried out at a cooling rate in the range of 40 ° C./sec or more and less than 10 ⁴ ° C./sec, so that it is continuously cooled to room temperature after solidification. Precipitation of the austenite phase is suppressed before cooling to room temperature, and the volume ratio of the austenite phase of the thin plate-shaped cast piece cooled to room temperature becomes 32% or less.

As described above, if the volume ratio of the austenite phase of the thin plate-shaped slab cooled to room temperature is 32% or less, the slab is cold-rolled and then heat-treated, so that the coarse austenite grains are sheared. Of the structure in which fine austenite grains are uniformly dispersed in the ferrite phase.
A duplex stainless steel sheet is obtained. Therefore, unlike the conventional case, it is not necessary to perform additional heat treatment such as solution treatment consisting of heating and quenching on the slab that has been cooled to room temperature, so that the manufacturing cost can be reduced. Can be reduced.

When the volume ratio of the austenite phase of the thin plate slab cooled to room temperature exceeds 32%, even if the slab is cold-rolled and then heat-treated, the coarse austenite grains in the structure are sheared. However, it cannot be dispersed, and a structure in which fine austenite grains are uniformly dispersed in the ferrite phase cannot be obtained. Therefore, it is necessary to perform additional heat treatment such as solution treatment on the cast piece cooled to room temperature.

In particular, when the rapid cooling until the solidification of the molten steel of the duplex stainless steel is carried out at a cooling rate in the range of 10 ³ ° C / sec or more and less than 10 ⁴ ° C / sec, after solidification, it is cooled to room temperature. The austenite phase does not precipitate during this period, and the structure can be a single ferrite phase. Therefore,
By cold rolling this slab and then subjecting it to heat treatment, a duplex stainless steel thin plate having an excellent structure in which fine austenite grains are uniformly dispersed in the ferrite phase can be obtained without performing solution treatment or the like.

When the rapid cooling rate until the solidification of the molten steel is less than 40 ° C./sec, the volume ratio of the austenite phase of the thin plate-shaped slab cooled to room temperature after solidification exceeds 32%, resulting in cooling to room temperature. It is necessary to subject the slab to solution treatment or the like. On the other hand, the quenching rate until solidification of the molten steel is 10 ⁴ ℃
Above / sec, the thickness of the cast ribbon has to be reduced to about 500 μm or less, so that there is a problem that it is limited to special applications.

After rapid solidification at a cooling rate within the above-mentioned range, air cooling is sufficient for cooling to room temperature when the plate thickness is 2 mm or less, but forced air cooling is used when the plate thickness is thicker than 2 mm. Or it is necessary to quench rapidly by quenching with water. In addition, in order to manufacture a thin plate product using the thin plate-shaped cast piece manufactured by the above-mentioned method as a raw material, any processing heat treatment may be performed.

[0018]

Next, the present invention will be described with reference to examples.
Several types of test material slabs having different thicknesses are obtained by rapidly cooling molten steel of duplex stainless steel having the composition shown in Table 1 by changing the cooling rate until solidification and then continuously cooling it to room temperature. Prepared. Next, the effect of the cooling rate until solidification of the test material cast thus prepared on the volume ratio of the austenite phase was investigated.

[0019]

[Table 1]

FIG. 1 is a graph showing the relationship between the cooling rate until solidification of the test material cast and the thickness of the cast, and the volume ratio of the austenite phase. As is clear from FIG.
When the cooling rate until solidification is 40 ° C / sec (thickness of the slab is 20 mm), the volume ratio of the austenite phase becomes 32%. Thereafter, the faster the cooling rate until solidification is, that is, the slab The volume ratio of the austenite phase decreased as the thickness decreased. When the cooling rate until solidification was 10 ³ ° C / sec or more (thickness of the slab was 2 mm or less), almost a ferrite single-phase structure was obtained.

FIG. 2 shows a thickness of 20 mm which has been subjected to the above-mentioned treatment.
3 is a photomicrograph showing the metallographic structure of the cast slab (magnification 400 times, the same applies below), and FIG. 3 shows the thickness 2 after the above-mentioned treatment.
It is a micrograph which shows the metallographic structure of the slab of mm. Figure 4
4 is a micrograph showing the metallographic structure of a thin plate obtained by cold rolling the slab having a thickness of 2 mm shown in FIG. 3 at a rolling rate of 75% and then heat-treating at a temperature of 1000 ° C. for 5 minutes. As shown in FIG. 4, by performing the cold rolling and heat treatment described above, an austenite microdispersion structure (Micro Duct) having fine austenite grains of about 5 to 6 μm uniformly dispersed in the ferrite phase was obtained.
plex organization) was obtained. Thus, after solidification,
In the duplex stainless steel in which precipitation of the austenite phase is suppressed before cooling to room temperature,
By subjecting to appropriate processing and heat treatment, it is possible to obtain a thin plate product having a metallographically characteristic.

[0022]

As described above, according to the method of the present invention, the molten steel of the duplex stainless steel is rapidly cooled and solidified by the synchronous continuous casting method, the twin roll continuous casting method, etc. In continuous casting, by utilizing the change of the metal structure caused by rapid solidification, fine austenite grains can be made uniform without applying additional heat treatment such as solution treatment to the slab cooled to room temperature. Of the tissue,
It is possible to produce a thin plate-like duplex stainless steel slab having excellent material properties, which brings about an excellent industrial effect.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a cooling rate until solidification of a test material slab and a thickness of the slab, and a volume ratio of an austenite phase.

FIG. 2 is a photomicrograph showing the metallographic structure of a slab having a thickness of 20 mm produced by the method of the present invention.

FIG. 3 is a micrograph showing a metallographic structure of a 2 mm-thick cast piece manufactured by the method of the present invention.

FIG. 4 is a micrograph showing the metallographic structure of a thin plate obtained by cold rolling the 2 mm-thick slab shown in FIG. 3 at a rolling rate of 75% and then heat-treating it at a temperature of 1000 ° C. for 5 minutes. Is.

Claims (1)

[Claims] 1. A method for producing a thin plate-shaped duplex stainless steel slab, which comprises rapidly cooling a molten steel of a duplex stainless steel and continuously casting a thin plate-shaped slab, comprising: Quench, 40 ℃
/ Sec or more and less than 10 ⁴ ° C / sec to solidify the molten steel by cooling at a cooling rate within a range, and then continuously cool the molten steel to room temperature. The method for producing a thin plate-like duplex stainless steel slab, characterized in that the volume ratio of the austenite phase in (1) is 32% or less.