JPS6054251A - Production of stainless steel round billet for making pipe - Google Patents

Abstract

PURPOSE:To obtain a billet which is free from thickness deviation when made into a pipe by hot extrusion straightly after external machining in the stage of producing the titled billet by a horizontal continuous casting method by stirring electromagnetically the unsolidified molten steel of the billet having the specific liquid phase rate in the stage of solidification. CONSTITUTION:The unsolidified molten steel is stirred at >=5cm/sec stirring flow rate for the molten steel by an electromagnetic stirrer disposed in the position where the liquid phase rate of the stainless steel round billet which is manufactured by a horizontal continuous casting method and is in the stage of solidification. The generation of the deviated columnar crystalline part of the billet is thus prevented and the direction where the columnar crystal grows is made axis- symmetrical approximately uniformly over the entire circumference of the cross section of the billet. The titled billet having a good characteristic is thus obtd. without requiring hot working such as blooming and rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a method for producing round billets for G-tubes directly from molten stainless steel without requiring hot working steps such as blooming and rolling.

In general, stainless steel seamless pipes, which have inferior hot workability compared to mild steel, are made from steel ingots or bloom continuous castings (obtained by fully blooming and rolling), and are made from Eugene Séjourne. It was commonly manufactured by hot extrusion.

As described above, the conventional stainless steel round billets used for making pipes were manufactured through hot working processes such as slow blooming rolling or forging, so the internal structure was changed to the solidified structure due to recrystallization. It was destroyed,
Therefore, if the tube manufacturing conditions were selected correctly, it was possible to obtain a seamless steel V with less wall thickness deviation and good surface quality by hot extrusion using the Eugene-Séjournet method.

However, in the production of round billets for pipe making, as mentioned above, the hot working process (blowing, rolling, or forging) is performed once.
This process results in a significant increase in costs, and high alloy steels are prone to cracking during the process, which has been a major problem in the past.

For these reasons, recently, a method of continuously casting round billets for pipe making directly from molten steel has attracted attention, and studies have been conducted from various angles.

Among them, the horizontal continuous casting method L1 has the advantage that the equipment cost is relatively low, and the obtained slab is not subjected to external forces such as bending or straightening. The shell is uniform because it acts on
It is also excellent in that it can produce round billets with good roundness, and therefore it can be used not only for ordinary stainless steel, but also for high alloy steel, which is difficult to cast using vertical or curved continuous casting methods. It attracted particularly high expectations because it can be directly cast into round billets.

However, when a stainless steel round billet manufactured by the horizontal continuous casting method is externally milled and then hot extruded using the Eugene-Séjournet method, the part of the pipe that corresponds to the top side of the billet (the upper side at the time of casting) The inventors of the present invention were unable to solve the problem of uneven wall thickness, which is thinner than the base side (lower side during casting), which causes pipe failure noise. From this point of view, we developed a complete round billet for pipe manufacturing that can produce high-quality seamless steel pipes with excellent dimensional accuracy from molten stainless steel without requiring hot working processes such as blooming and rolling. In order to directly manufacture pipes, we first conducted research to determine the causes of pipe manufacturing defects found in stainless steel round billets manufactured by the horizontal continuous casting method as described above, and confirmed all of the following items.

In other words, in a horizontally continuously cast round billet h, as seen in the macrostructure photograph shown in Fig. 1, the growth direction of the columnar crystals is deviated from the axis symmetry and is biased toward the upper side (top side), and the billet is lower side. Columnar crystals from the ground side pass through the center and extend to the upper side. Figure 1 shows a round billet (diameter: 212 mm) obtained by horizontal continuous casting of 5US304 stainless steel.
mm) is a cross-sectional macroscopic structure photograph. and,
The reason for this bias in the growth direction of columnar crystals is that in horizontal continuous casting, the cooling capacity is generally greater on the bottom side, so the temperature distribution of the unsolidified molten steel inside the billet during the solidification process is higher on the top side and lower on the bottom side. This is presumed to be due to the phenomenon that if the growth direction of the columnar crystals is non-uniform, the deformation resistance at the cross-sectional position of the billet will also be different. When you do
The thickness of the resulting i4ino differs depending on the part.
It was confirmed that so-called uneven thickness occurs.

Figure 2 shows the area ratio (%) of the deflected columnar crystals of a 5US304 stainless steel continuously cast round billet (diameter: 212 ym) vi: when hot extrusion tubes were made using the Eugene Séjournet method as cast. It is a graph showing the relationship with the thickness unevenness rate (%) of the obtained steel pipe.

In addition, the area ratio of the columnar crystal deflection portion is the total area of the columnar crystal deflection portion (the area of AI + Aa in FIG. 3) relative to the total area of the billet cross section in FIG. ) is expressed as the ratio of It is also clear from FIG. 3 that as the area ratio of the deflected portion of columnar crystals increases, the uneven thickness ratio of the obtained steel pipe increases.

Therefore, in order to prevent the occurrence of uneven thickness of the columnar crystals when producing stainless steel round billets by horizontal continuous casting method, the present inventors have determined that the growth direction of the columnar crystals extends over the entire circumference of the cross section of the billet in order to prevent the occurrence of uneven thickness of the columnar crystals. We found a way to achieve almost uniform axial symmetry, and as a result of further research to realize a stainless steel round billet for pipe manufacturing with less uneven thickness when pipe-manufactured using a completely cast material, we found the following (a) ) to (e). That is, (a) When manufacturing stainless steel round billets using the horizontal continuous casting method, the solidification of round billet slabs does not progress at all. When the molten steel is stirred by electromagnetic stirring in the round billet slab, the temperature distribution between the top and bottom of the unsolidified molten steel inside the round billet slab becomes uniform, and the direction of columnar crystal growth becomes axially symmetrical, almost all around the cross section of the round billet. (b) In this case, unless the electromagnetic stirring device is placed at a position where the liquid phase ratio of the billet slab in the solidification process is 30% or more, the growth direction of columnar crystals will be uniform. The liquid phase ratio in this case is defined as the radius of the unsolidified part of the billet slab divided by the slab radius and squared.

Figure 4 is a graph showing the relationship between the liquid phase ratio of a round billet slab (material: 5US304 steel, diameter: 212 mm) at the position where electromagnetic stirring is fully started and the deflection area ratio of the columnar crystals. However, as shown in Figure 4, the larger the liquid phase ratio at the electromagnetic stirring position, that is, the closer the electromagnetic stirring device is placed to the mold, the smaller the inner area ratio of columnar crystals becomes. :30%
It can be seen that stirring at the above positions is highly effective. Based on this fact, the growth direction of columnar crystals is determined by the region with a high liquid phase ratio (the part where the solidified shell thickness is large), and the direction of the columnar crystals once grown is determined by the unsolidified part in the later part. It can also be inferred that it is difficult to correct even if the molten steel is stirred.

When unsolidified molten steel is stirred in a part of a billet slab where the liquid phase ratio is 30% or more, as is clear from Figure 4, the area ratio of the columnar crystal deflection part becomes 20% or less, and especially in the vicinity of the mold. (C) While stirring, this becomes less than 5%, and as shown in Figure 2, the uneven wall thickness of the manufactured steel pipe is greatly reduced, and it becomes comparable to a normal rolled billet. Even if the stirring position of unsolidified molten steel is appropriate, it is difficult to uniformize the growth direction of columnar crystals if the strength of electromagnetic stirring is weak. When stirring, the difference between the maximum and minimum values of the angle θ between the growth direction of columnar crystals and the normal direction of the round billet slab in the entire inner circumferential direction becomes 20.
(d) As mentioned above, the unsolidified molten steel is electromagnetically stirred near the mold to reduce the temperature difference between the unsolidified part of the slab and the ground in the early stage of solidification. At the same time, the strength of the electromagnetic stirring was set to a molten steel stirring flow rate of 5 cm/sec or more, and the difference in angle between the direction of growth of columnar crystals from the skin of the round billet slab and the normal direction of the slab was adjusted over the entire circumference of the slab. If the difference between the maximum value and the minimum value is suppressed to within 20 degrees, even if the resulting stainless steel round billet slab is made into pipes using the Eugene-Séjournet method as cast, the thickness deviation rate will be normal. (e) In addition, when the fillet of cast reed is completely directly extruded, it is easy to cause surface roughness, especially in the case of high-alloy steel; In such cases, it is effective to lightly reduce the entire area near the billet surface. However, even in this case, coarse columnar crystals still exist inside the billet, which will still cause M'll defects in billets manufactured by conventional horizontal continuous casting, but as mentioned above, electromagnetic stirring with appropriate strength at the appropriate position In the horizontal continuous casting billet produced by
Even if the pipe is made of high-alloy steel with light reduction applied only near the surface to prevent surface deterioration, it is possible to manufacture pipes with minimal thickness deviation.

This invention has been made based on the above knowledge, and the present invention is based on the above-mentioned findings.The present invention is based on the above-mentioned findings.
Continuous water heater casting, which involves stirring unsolidified molten steel at a temperature of c or more, allows molten stainless steel to be continuously formed into round billet slabs, resulting in less uneven thickness even when pipes are made as cast. This method is characterized in that it manufactures stainless steel round billets for pipe making that can yield seamless steel pipes of good quality.

Note that, as mentioned above, the liquid phase ratio here is the ratio of the radius of the unsolidified part of the unsolidified slab divided by the total billet radius, and the quotient is squared.

Furthermore, the term "stainless steel" as used herein is defined in a broad sense, and refers to all alloy steels with an alloying element content of 5% or more, resistant to #f acid, oxidation resistant, or heat resistant. For example, s ty s 304m or 5US31
Class stainless steel f+M (i7) represented by No. 6 steel (i7) If the method described above is used to cast into a billet and then directly extruded from the casting mill, a steel pipe with sufficiently good dimensional accuracy and surface quality can be obtained. When a smooth surface is required or in the case of high-alloy steel, it is necessary to
By applying light reduction or the like to a depth of mm or more to make the surface layer finer, a desired good surface quality can be achieved.

As described above, the present invention achieves almost uniform deformation resistance in the entire growth direction R,j of columnar crystals by electromagnetically stirring the entire unsolidified cast slab under appropriate conditions in horizontal continuous casting C (C). Differences depending on position! This is related to manufacturing continuous cast stainless steel round pipes that do not cause uneven wall thickness during pipe manufacturing. From the viewpoint of maintenance, it is best to use a permanent magnet rotation system in which the permanent magnet rotates in the entire circumferential direction.

Next, in the method of this invention, the reason why the arrangement position of the electromagnetic stirring device and the molten steel stirring flow rate are limited to the Tic value as described above will be explained.

A) Placement position of the electromagnetic stirring device As is clear from Figure 4, if the placement position of the electromagnetic stirring device is below the liquid phase ratio of the slab of 30%, the deflected portion of the columnar crystals becomes extremely large. 1.The liquid phase ratio of the slab is 30%, whereas uneven thickness occurs during pipe manufacturing.If an electromagnetic stirring device is placed in the upper position, the uniformity of the growth direction of the columnar crystals is greatly improved. The electromagnetic stirrer was placed at a position where the liquid phase ratio of the gold and slab was 30% or more, in order to suppress the occurrence of uneven thickness in the tube.

B) Molten steel stirring flow rate When the molten steel stirring flow rate is 5 cm/sec, it is difficult to make the growth direction of columnar crystals uniform, and the difference θ between the angle between the growth direction of columnar crystals and the normal direction of the round slab is , the difference between the maximum value and the minimum value in the circumferential direction exceeds 20 degrees, which causes uneven thickness during pipe manufacturing, so the molten steel stirring flow rate k 5
on/sec or more.

Next, all embodiments of the present invention will be explained in comparison with comparative examples.

Example Casting speed: 0.7 m/mi by horizontal continuous casting machine
N5US304 steel and Incoloy 800 (Shop name:
20Cr-32Ni steel), each with a diameter of 2
12-shaped round billet 14-cast. The amount of molten steel at this time was 20 tons, and a billet of about 70 m was obtained in about 100 minutes of casting time.

A permanent magnet rotating type electromagnetic stirring device was used, and casting was carried out under the conditions shown in Table 1 with various positions and some changes in the electromagnetic stirring intensity.

Conditions A and B shown in Table 1 are mold-like electromagnetic stirring, and conditions C to F are cases in which electromagnetic stirring devices are placed in the secondary cooling zone and the cooling zone.

As shown in Table 2, SU^304 steel was cast for conditions A to F, and Incoloy 800 (trade name) was cast for conditions A and E.

The resulting billet has an outer diameter of 206 m by cutting off the black skin.
, Inner diameter: 28mm, Length = 480mm After shaping to the dimensions of the body, it was hot extruded using the Eugene-Séjournet method of pipe making, and the outer diameter: 83m, the inner diameter: 59mn, and the wall thickness:
A 12m seamless steel pipe was manufactured.

Regarding NAO and Incoloy 800 (trade name), part of the billet was subjected to light pressure reduction to make the skin layer finer, and then used for pipe production.

The area ratio of the columnar crystal deflection part of the billet measured during the grain-like treatment process, and the thickness unevenness ratio and surface texture of the obtained steel pipe were measured in the second
Shown in the table.

From the results shown in Table 2,! Billets obtained by horizontal continuous casting with the magnetic stirrer placed at a location where the liquid phase ratio of the billet is at least 30%, and the stirring strength and bath steel flow rate at least 5 cm/see, have columnar crystals that grow regularly. It is clear that the steel pipe obtained by this method shows good properties with a thickness unevenness ratio of 10% or less when the pipe is manufactured, and it can be seen that the surface quality of the steel pipe obtained from Q1 is also good.

In addition, FIG. 5 is a macrostructure photograph of the horizontal continuous casting billet obtained by the method of the present invention in test number 1.
It is clear from FIG. 5 that according to the method of the present invention, a horizontal continuous casting billet in which columnar crystals grow regularly can be obtained.

On the other hand, if the electromagnetic stirring device is placed at a liquid phase ratio of less than 30%, or if the stirring intensity is low, the resulting horizontal continuous casting pipe will have a large thickness deviation. It is also clear from Table 2 that pipe defects occur frequently.

In this way, when manufacturing stainless L/S steel round billets by horizontal continuous casting, by placing the electromagnetic stirring device as close as possible to the mold entrance, the growth direction of columnar crystals during initial solidification can be made uniform. It is possible,
Therefore, when the resulting billet metal tube is made, the uneven thickness is on the same level as that of ordinary material.

In this example, only two types of steel were selected as test steels, but other ferritic stainless steels were used.

Good results can be obtained by applying the method of the present invention to any of the so-called broadly defined stainless steels, including martensitic stainless steel and austenitic stainless steel, as well as those containing 5% or more of alloying elements. This is the general rule mentioned above. It goes without saying that the billet size can also be selected arbitrarily.

As described above, according to the present invention, it is possible to manufacture stainless steel round billets for pipe making with good properties at low cost without requiring hot working steps such as blooming and rolling, and to produce high-quality seamless billets. This can bring about industrially useful effects, such as making it possible to significantly increase the overall manufacturing efficiency of steel pipes.

[Brief explanation of drawings]

Figure 1 shows a macrostructure photograph of a cross section of a stainless steel round billet manufactured by conventional horizontal continuous casting, and Figure 2 shows the relationship between the area ratio of the columnar grain deflection area of the cast billet and the uneven thickness ratio of the manufactured steel pipe. A graph showing the relationship, Figure 3, is a schematic representation of the horizontal continuous casting billet cross-sectional macrostructure.
Figure 4 is a graph showing the relationship between the liquid phase ratio of a round billet slab and the area ratio of columnar crystal deflection areas at the location of the electromagnetic stirrer. FIG. 2 is a photograph of the macrostructure of a cross section of a stainless steel round billet obtained by the method of the invention. Applicant Sumitomo Metal Industries Co., Ltd. Agent Tomi 1) Kazuo and 1 other person Figure 1 Figure 2 Imitinto 14 Fui 2F Force 7f #I'4 Takumi (%n Figure 3 Tsutomu 4 Figure L) 'Situ I-Reyne 13P (%n year 5 Kual -゛゛j

Claims (1)

[Claims] The molten steel is stirred at a flow rate of 6 cm/5 by an electromagnetic stirring device placed at a position where the liquid phase ratio of the slab in the solidification process is 23° or more.
A method for manufacturing stainless steel round billets for pipe making, which includes the step of stirring unsolidified molten steel at a temperature of 60° C. or higher.