JP2006283115A - Method for heating billet for high chromium-based seamless steel pipe - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for heating a billet, applying the optimum heat pattern to the pipe-making of a high Cr-based seamless steel pipe having a little defect in the internal surface. <P>SOLUTION: In a pre-heating process for billet with a heating furnace before piercing-rolling the high Cr-based seamless steel pipe, the inner part of the heating furnace is divided into a preheating zone, a heating zone and a soaking zone and in the case of using Ta(°C) for targeted extracting temperature at the outlet side of the soaking zone, the heat pattern in the heating furnace is made to ≤Ta(°C) in the pre-heating zone, to Ta(°C)±10°C in the heating zone and to Ta(°C) in the soaking zone. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for heating a billet for a high Cr-based seamless steel pipe used in oil wells, gas wells, various plants or construction structural materials, and more specifically, using a billet having a Cr content of 9% by mass or more. The present invention relates to a method for heating a billet for a high Cr-based seamless steel pipe that is less likely to cause inner surface defects such as medium burrs even when producing a steelless pipe.

  Conventionally, so-called high-Cr seamless steel pipes containing 9 mass% or more of Cr for oil wells, various plants, or construction structures have been widely used. This high Cr seamless steel pipe heats the slab manufactured by the ingot-making method or continuous casting method, turns it into a billet (material for pipe making) by rolling or forging, and uses an inclined rolling punch such as a Mannesmann punch. After the center portion of the material is pierced and rolled, it is manufactured by a hot pipe manufacturing method in which it is stretched by a mandrel mill, a plug mill or the like, and finish-rolled by a reducer.

  However, the high Cr alloy steel is inferior in hot workability compared to general steel, and therefore, internal defects such as medium fogging and scabs are likely to occur during piercing and rolling that imposes severe deformation on the material. If the occurrence of this internal defect is significant, not only the yield of the product will be reduced, but the entire pipe mill consisting of a piercing mill, a drawing mill and a drawing mill may have to be suspended. In such a case, the production efficiency of the seamless steel pipe is significantly hindered.

  When making high Cr seamless steel pipes, internal defects occur in the product steel pipe because the hot workability of the material is inferior, and cracks occur in areas that are weak in the structure due to distortion during pipe making. This is because it progresses to an internal defect. The fragile part of the high Cr alloy steel in hot working is a grain boundary between austenite γ grains, which is the main structure in a high temperature state, and δ grains contained in a trace amount with the formation of δ-ferrite.

  In order to suppress the formation of δ-ferrite and prevent the occurrence of internal defects during piercing and rolling, many methods for manufacturing seamless steel pipes have been proposed.

  For example, Patent Document 1 discloses that a slab is heated and held at a temperature that becomes an austenite-ferrite two-phase region, and then rolled into a billet, and the billet is heated to a temperature that becomes an austenite single-phase region. There has been proposed a method for producing a martensitic stainless steel seamless pipe that is pierced and rolled after being held for 1 hour or longer. Furthermore, in Patent Document 2, heating and holding at a temperature that becomes the austenite-ferrite two-phase region are performed in a bloom obtained by rolling a slab, and then rolled into a billet, in the austenite single-phase region. A method of heating and holding for 30 minutes or more is described.

  According to these methods, the diffusion of Cr and P is accelerated by heating in the austenite-ferrite two-phase region, followed by heating in the austenite single-phase region. ) Is apparent, it is said that δ ferrite can be eliminated and a seamless steel pipe without internal defects can be produced.

  Further, in Patent Document 3, when high Cr steel is produced as a raw material for pipe making, δ-ferrite with a soaking time at the slab or steel slab manufacturing stage and a soaking time at the stage before pipe making of the material is disclosed. Describes a method for manufacturing a high Cr-based seamless steel pipe that suppresses the occurrence of internal flaws by introducing an F value by indexing the effect on the amount and managing the F value to be higher than a predetermined value. Has been. By managing the F value, it is not necessary to excessively reduce the impurities of the material, and a high Cr seamless steel pipe excellent in inner surface quality can be efficiently manufactured at a low manufacturing cost.

  However, in the method for manufacturing a seamless steel pipe proposed in Patent Documents 1 to 3, there is no description about the heat pattern in the billet (material) heating step before pipe making. The heating temperature during pipe production affects the production of δ ferrite, and the higher the heating temperature, the easier it is to produce δ ferrite. For example, in the proposed manufacturing method, overheating in the preheating stage of the billet or perforation There is a case where a local temperature rise occurs due to heat generated by rolling, and δ ferrite is generated in that portion, resulting in an inner surface defect.

JP-A-6-306466 JP-A-6-330170 JP 2003-3212 A

  The present invention can suppress the generation of δ ferrite due to the overheating at the time of preheating the billet and the local temperature rise of the inner surface of the tube in piercing rolling, and can produce a seamless steel tube with fewer inner surface defects. It aims at providing the heating method of the billet for high Cr type seamless steel pipes.

  In order to solve the above-mentioned problems, the present inventor investigated the heat pattern in the furnace during billet heating, and studied to obtain an appropriate heat pattern that can avoid overheating and prevent local temperature rise on the inner surface of the tube. Repeated.

  As a result of detailed examination, the furnace operation control at the maximum set temperature normally performed in the operation of the heating furnace has been revised, and the temperature immediately before the material is extracted from the heating furnace is set as the target extraction temperature Ta, and the furnace is based on the target extraction temperature Ta. It was clarified that the above-mentioned problems can be solved by managing the internal heat pattern, especially by controlling the temperature rise.

  This invention is completed based on said knowledge, and makes the summary the heating method of the following high Cr-type seamless steel pipe billet.

  That is, in the step of preheating the billet in the heating furnace before piercing and rolling the high Cr-based seamless steel pipe, the heating furnace is divided into the pre-tropical zone, the heating zone and the soaking zone, and the target extraction on the soaking zone is extracted. When the temperature is Ta (° C.), the heating pattern in the furnace is Ta (° C.) or less in the pre-tropical zone, Ta (° C.) ± 10 ° C. in the heating zone, and Ta (° C.) in the soaking zone It is a heating method of the billet for high Cr type | system | group seamless steel pipes characterized by these.

  Here, the “target extraction temperature” is the billet temperature on the soaking side immediately before the billet is extracted from the heating furnace, and in order to suppress the formation of δ ferrite, not only the billet overheating but also the perforation The target temperature also takes into account the amount of heat generated during rolling.

  According to the billet heating method of the present invention, even in the case of pipe making using high Cr alloy steel as a raw material, overheating in the billet heating process before piercing and rolling, and local temperature rise on the inner surface of the pipe due to piercing and rolling. It is possible to suppress the formation of δ ferrite due to the above, and it is possible to stably produce a high Cr alloy steel seamless steel pipe with few internal surface defects.

  The billet heating method for high Cr seamless steel pipes according to the present invention includes the step of preheating the billet in a heating furnace before piercing and rolling, and dividing the inside of the heating furnace into a pre-tropical zone, a heating zone, and a soaking zone. When the target extraction temperature on the tropical side is Ta (° C.), the in-furnace heat pattern of the heating furnace is Ta (° C.) or less in the pre-tropical zone, Ta (° C.) ± 10 ° C. in the heating zone, It is a heating method that uses Ta (° C.) in the tropics.

The high Cr seamless steel pipe targeted by the present invention has a Cr content of 9% or more by mass%. Cr is an element that improves the corrosion resistance. However, if the content is less than 9%, the desired corrosion resistance, for example, CO ₂ corrosion resistance cannot be secured. Therefore, it is intended for seamless steel pipes with a Cr content of 9% or more. . On the other hand, if the Cr content exceeds 20%, a δ-ferrite phase is likely to be formed during high-temperature heating, and the hot workability is lowered, so the upper limit is desirably 20%.

  Furthermore, regarding the chemical composition of the high Cr-based seamless steel pipe, component elements corresponding to 13% Cr steel, SUS304 steel, SUS316 steel, SUS321 steel and SUS347 steel can be added in addition to the Cr content. Table 1 specifically illustrates target materials of the present invention.

  The heating furnace targeted by the present invention is not particularly limited in its type, and may be a system in which a partition wall is provided in the furnace and divided into a pretropical zone, a heating zone and a soaking zone, for example, a rotary hearth type Either a walking beam type or a walking beam type can be adopted.

  When the heat pattern applied to the billet heating method of the present invention is compared with the heat pattern in the conventional heating method that has been conventionally performed, for example, it is as shown in Table 2 shown in the examples described later. Similarly, FIG. 1 which shows an Example illustrates this. The billet heating method of the present invention will be described with reference to Table 2 and FIG.

  In the billet heating method of the present invention, as described above, the heat pattern in the furnace is managed based on the target extraction temperature Ta. Conventionally, the heating furnace is operated at the maximum set temperature Tm determined empirically. It was.

  That is, as shown in Table 2 and FIG. 1, in the normal heating method, the furnace temperature is controlled to the maximum set temperature Tm or lower in the pre-tropical zone of the furnace, maintained at Tm in the heating zone, and the target in the soaking zone. The extraction temperature Ta is controlled. As a result, the furnace temperature becomes the target extraction temperature Ta on the soaking side in the soaking zone, but shows the maximum set temperature Tm or a temperature close thereto at the heating zone. On the other hand, in the heating method of the present invention, the target extraction temperature Ta is set to be equal to or lower than the target extraction temperature in the pretropical zone, maintained at Ta ± 10 ° C. in the heating zone, and maintained as Ta in the soaking zone.

  In the billet heating method of the present invention, the condition of “pre-tropical ≦ Ta” is provided in order to prevent overheating of the material. Conventionally, in order to increase the rate of temperature rise of the material, the temperature is controlled at the maximum set temperature Tm higher than the target extraction temperature Ta, so the target extraction temperature Ta is exceeded in the heating zone, and as shown in the examples described later, overheating It had been. However, such a situation can be surely avoided by setting the condition “pre-tropical ≦ Ta”.

  The reason for providing the condition of “heating zone: Ta ± 10 ° C.” is to reduce the fluctuation of the temperature in the furnace toward the soaking zone while avoiding overheating of the material. Therefore, here, a variation of about ± 10 ° C. is unavoidable in terms of temperature control.

  Further, the condition of “soaking tropics: Ta” is a condition necessary for setting the billet temperature from the surface to the target extraction temperature Ta. This condition setting in the soaking zone is related to the condition setting in the heating zone. As a result of the fluctuation of the furnace temperature being absorbed in the heating zone, the fluctuation of Ta is so small in the soaking zone that the billet temperature reaches the inside. The target extraction temperature Ta is completely reached.

  The target extraction temperature Ta may be set based on the conventional operation results, etc., depending on the steel type, particularly the Cr content, but as described above, the heat generation amount due to piercing rolling and the local temperature rise due thereto are considered. Need to be set.

  The target extraction temperature Ta is desirably in the range of 1100 to 1300 ° C. With the Cr content of the target steel type described above, by setting the extraction temperature to 1100 ° C. or higher, hot workability in piercing and rolling can be ensured, and the pipe making efficiency and productivity can be secured, and 1300 ° C. is the upper heating limit. By doing so, not only overheating of the billet but also local temperature increase in piercing and rolling can be suppressed, and the formation of δ ferrite can be prevented.

  Furthermore, in the billet heating method of the present invention, it is not necessary to particularly define the soaking time of the target extraction temperature Ta. When the heat pattern applied to the present invention is compared with the heat pattern in the conventional heating method conventionally performed, the in-furnace time of the billet becomes longer as the temperature rising rate decreases. This is because the fluctuation of the temperature of the billet on the soaking side of the tropical zone becomes small as a result of the fluctuation of the internal temperature being absorbed.

  The pipe making process to which the billet heating method of the present invention can be applied may be a conventional seamless steel pipe making process, as described above, manufacturing a hollow shell by billet Mannesmann drilling, press drilling, etc. Any method may be used as long as the raw pipe is rolled and then finished into a product steel pipe by drawing.

  Usually, since it is advantageous in terms of dimensional accuracy and productivity, the Mannesmann-mandrel mill method or the Mannesmann-plug mill method is applied. On the other hand, this invention prescribes | regulates the heating method before performing piercing-rolling of a billet, and the method conventionally used can be applied to the process before reaching this heating process.

  As described above, according to the heating method of the present invention, it is possible to prevent overheating up to the target extraction temperature Ta during billet heating before piercing rolling, and even when local temperature rise due to heat generation during piercing rolling occurs. Since temperature control can be performed, variation in the formation of δ ferrite for each operation opportunity can be reduced, and a high Cr seamless steel pipe with good inner surface quality can be stably produced.

  In manufacturing a seamless steel pipe of 9% Cr steel (JIS STBA28 steel), the heat pattern in the furnace is managed by applying the billet heating method of the present invention in the heating process before piercing and rolling the billet, We investigated the occurrence situation of the middle burrs. For comparison, the same investigation was performed for the case where the pipe was manufactured by the conventional heating method.

  Table 2 shows the heat pattern of the heating furnace used in the billet heating process. FIG. 1 schematically shows this heat pattern with each region in the furnace length direction (pre-tropical zone, heating zone and soaking zone) as the horizontal axis and temperature as the vertical axis.

  City gas or low sulfur C heavy oil (LSC heavy oil) was used as the fuel for the heating furnace. Table 3 shows the air-fuel ratio. The air / fuel ratio was set higher than the stoichiometric air / fuel ratio in consideration of the outer surface quality (surface oxidation of the billet) and the fuel consumption rate.

  Table 4 shows the results of investigation on the occurrence of medium fogging. In Table 4, “in-furnace time” is the time required from charging the billet into the furnace until extraction. Further, “biased heat” is the deviation of the billet temperature from the target temperature Ta in the furnace, and under various heating patterns, as shown in FIG. 2, the billet center (a), center and surface ( The temperature of the middle (b) of the outer surface) and the surface (c) was determined by actual measurement.

  As is apparent from the results shown in Table 4, when the billet heating method of the present invention is applied, the billet in-furnace time varies with each operation and becomes longer than in the case of the normal billet heating method. The incidence of medium fog was greatly reduced.

  This is because the formation of δ ferrite due to overheating in the preheating process of the billet and local temperature rise on the inner surface of the tube during Mannesmann drilling was effectively suppressed, and the number of fragile parts in the structure decreased. It is.

  In addition, when the heating method of the present invention was applied, the in-furnace time was longer than in the case of the normal heating method because temperature increase management was performed to prevent billet overheating and the like. However, due to the application of the heating method of the present invention, the product yield decreases due to the reduction of inner surface defects when the normal heating method is applied, and the production efficiency is significantly reduced due to the stop of the rolling piercing machine and the suspension of the entire pipe manufacturing line. Deterioration can be avoided reliably.

  FIG. 3 is a diagram showing the results of measuring the billet temperature in the heating furnace at the surface and center of the billet, where (a) is based on a normal heating method, and (b) is applied with the heating method of the present invention. Shows the case. The billet temperature was measured by attaching a thermocouple to the billet surface in advance and fixing it, or by embedding it in the center.

  From the results shown in FIG. 3, when the conventional heating method is applied, both the surface temperature and the center temperature of the billet settle at the target extraction temperature Ta at the time of extraction, but exceed Ta from the heating zone to the soaking zone and are overheated. You can see that On the other hand, when the heating method of the present invention was applied, both the surface temperature and the center temperature were maintained at Ta without exceeding the target extraction temperature Ta.

  According to the billet heating method of the present invention, during the production of seamless steel pipes of high Cr alloy steel, δ ferrite is caused by billet overheating in the heating furnace and local temperature rise on the inner surface of the pipe in piercing rolling. It is possible to suppress the generation of internal defects and the accompanying internal defects. Therefore, the billet heating method of this invention can be utilized suitably for manufacture of a high Cr type seamless steel pipe.

It is a figure which compares and compares the heat pattern in the furnace at the time of applying the billet heating method of this invention with the heat pattern in the case of the normal heating method. It is a figure for demonstrating the measuring method of billet's thermal deviation, and is the figure which showed the temperature measurement site | part (a, b, and c) in the cross-sectional position of a billet. It is a figure which shows the result of having measured the billet temperature in a heating furnace in the surface and center part of a billet, (a) is a case by a normal heating method, (b) is a case where the heating method of this invention is applied.

Claims (1)

In the step of pre-heating the billet in the heating furnace before piercing and rolling the high Cr-based seamless steel pipe, the heating furnace is divided into the pre-tropical zone, the heating zone and the soaking zone, and the target extraction temperature on the soaking zone is set to the target extraction temperature. In the case of Ta (° C.), the in-furnace heat pattern of the heating furnace is set to Ta (° C.) or less in the pre-tropics, Ta (° C.) ± 10 ° C. in the heating zone, and Ta (° C.) in the soaking zone. A heating method for billets for high Cr seamless steel pipes.

Images