JP2005213645A - Continuous heat treatment furnace, and steel tube and heat treating method using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous heat treatment furnace and a heat treatment method capable of easily removing deposits on inner and outer surface of a steel pipe before the heat treatment even when a cleaning step after the cold working includes only alkali decreasing and cleaning. <P>SOLUTION: In the continuous heat treatment furnace in which atmospheric gas is introduced in a heating chamber 1 having a heating zone 1a, a steel tube is continuously charged from a furnace inlet 2a along the axial direction, and the heat-treated steel tube is carried out of a furnace outlet 2b, the continuous heat treatment furnace has a front chamber 4 having a preheating zone 3 provided on the input side of the heating chamber, and seal curtains 5a and 5b on the inlet side and the outlet side of the front chamber. The heat treatment method is performed by using the heat treatment furnace. A rear chamber 6 is preferably provided on the outlet side of the heating chamber, and a seal curtain 7a is preferably mounted on the inlet side of the rear chamber. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a continuous heat treatment of a cold-worked steel pipe, and more particularly, to a steel pipe such as a stainless steel pipe that is cold-worked using a rolling oil or a lubricant containing a hydrocarbon-based component. The present invention relates to a continuous heat treatment furnace that does not cause contamination by generated gas from deposits, a steel pipe heat treated using the furnace, and a heat treatment method.

  Appropriate surface treatment on the inner and outer surfaces of the steel pipe, such as applying rolling oil during cold rolling and coating with lubricant (metal soap) during cold drawing when cold working steel pipes are cold worked Is processed into a predetermined dimension.

  When heat-treating a cold-worked steel pipe, it is necessary to remove (degrease) the rolling oil and lubricant before the heat treatment to remove deposits on the inner and outer surfaces of the steel pipe. If heat treatment is performed with deposits remaining on the surface of the steel pipe, the rolling oil and lubricant contain hydrocarbon-based components, and some contain chlorine, etc., so these components evaporate during the heat treatment. Chlorine and other polluted gases are generated, and contamination may occur on the inner surface of the steel pipe where these gases are particularly likely to stay.

  Moreover, even when chlorine and other pollutant gases are not included in the evaporative gas, the inner and outer surfaces of the steel pipe are exposed to a high-temperature gas containing a carbon source, so carburization occurs depending on the temperature conditions. There are things to do. When a steel pipe in which carburization has occurred on its surface is repeatedly used at a high temperature and high pressure, the carburized portion may be the starting point and may cause SCC (stress corrosion cracking). For this reason, when heat-treating a cold-worked steel pipe, it is necessary not to cause carburization on the inner and outer surfaces of the steel pipe.

  In order to prevent contamination and carburization of the inner surface of the steel pipe, a method of replacing the gas in the pipe with an atmospheric gas is effective, and various countermeasures have been proposed for this purpose.

  In Patent Document 1, a pair of open / close doors provided with elastic pads at opposing portions are provided so as to move up and down, respectively, above and below the inlet portion of the purge chamber. An in-pipe gas purging device has been proposed in which the pressure in the purge chamber is increased by sandwiching it from the open / close door to replace the straight pipe with the atmospheric gas.

  However, in the apparatus proposed in Patent Document 1, it is necessary to stop charging the straight pipe each time at the inlet of the purge chamber, so the heat treatment efficiency is significantly reduced. At the same time, there is a problem that the quality of the elastic pad is severely deteriorated in a heated atmosphere, and the required performance cannot be obtained, or frequent replacement is required.

  On the other hand, the heat treatment apparatus disclosed in Patent Document 2 is charged to the side of a heat treatment furnace for heat-treating a straight tube in an atmospheric gas so as to feed the straight tube toward the inlet of the straight tube. A negative pressure means is provided for placing a negative pressure at a position where the rear end of the straight pipe is located in a state where the front end of the straight pipe is in the heat treatment furnace. Is provided. As a result, the purging operation in the straight pipe can be performed very easily.

  However, since the apparatus disclosed in Patent Document 2 requires a large-capacity negative pressure means, there is a problem that a large capital investment is required, and the cost for manufacturing the steel pipe is high.

JP-A-5-320745, [Claims]

JP-A-6-128645, [Claims]

  As described above, in the measures proposed in Patent Documents 1 and 2, since the straight pipe is stopped each time, the heat treatment efficiency is lowered, and a large-scale capital investment is required, so that the manufacturing cost increases. This is not a perfect countermeasure.

  On the other hand, in order to prevent contamination and carburization on the surface of the steel pipe due to the heat treatment, when trying to remove the deposits remaining on the inner and outer surfaces of the steel pipe before the heat treatment, only alkaline degreasing and washing after cold working In addition to this, an inner surface cleaning process such as sandblasting is required. Moreover, if pickling is applied, the man-hour for that will increase, and in any case, the steel pipe manufacturing cost by cold work will increase.

  The present invention has been made in view of the problem of deposits remaining on the inner and outer surfaces of such a cold-worked steel pipe, and is a case where the cleaning process after cold working is only alkaline degreasing and washing. However, it is possible to provide a continuous heat treatment furnace that can easily remove residual deposits before heat treatment and that does not reduce heat treatment efficiency, and a steel pipe heat treated using the same and a heat treatment method. It is aimed.

  In order to solve the above-mentioned problems, the present inventor has made various studies on a heat treatment method for removing deposits remaining on the surface after washing a cold-worked steel pipe. As a result, even when the cleaning process after cold working is only alkaline degreasing and cleaning, when the steel pipe is inserted into the heat treatment furnace, the deposits remaining on the inner and outer surfaces are easily decomposed and vaporized, It was found that it can be removed.

  Almost all deposits (rolling oil during cold working, lubricant for drawing (metal soap), etc.) remaining on the surface of the steel pipe after alkali degreasing and washing are decomposed when heated to 200-600 ° C during heat treatment. Generate hydrocarbon gases (and chlorine and other polluting gases). In particular, the generation of hydrocarbon gas and the like becomes most noticeable at 400 ° C. Therefore, in order to effectively decompose the remaining deposits, it is desirable to heat the steel pipe surface to 400 ° C. or higher.

  Usually, in the steel pipe charged in the heat treatment furnace, the decomposition gas of the outer surface deposit is easily diffused by the gas flow in the furnace, but the decomposition gas of the inner surface deposit tends to stay inside the steel pipe. Decomposition gas of deposits may contain chlorine and other pollutants, and since it is hydrocarbon-based and carburizing, when the steel pipe is heated to 800 ° C or higher, contamination and carburization occur on the steel pipe surface. There is a case.

  Therefore, in order to effectively prevent the occurrence of contamination and carburization, it is necessary to manage the temperature of the steel pipe surface below 800 ° C. In an actual furnace, it is desirable to manage at 750 ° C. or lower in consideration of the management accuracy in the continuous heat treatment furnace.

  When the steel pipe, which is the material to be heat treated, is charged into the furnace, the temperature of the tip of the steel pipe previously charged rises, and when the surface temperature reaches 200 to 600 ° C., the residual deposits decompose and the hydrocarbon gas Etc. Therefore, when the atmospheric gas in the furnace is set to a positive pressure compared to the outside of the furnace, a gas flow from the front end to the rear end of the steel pipe can be formed.

  As mentioned above, the cracked gas (contaminated gas) that generates deposits on the inner surface of the steel pipe is likely to stay inside the steel pipe. To make the gas flow inside the steel pipe noticeable, cover the furnace inlet of the continuous heat treatment furnace and seal it. However, it is effective to maintain the positive pressure in the furnace by covering both ends of the furnace inlet and the furnace outlet, and to allow the atmospheric gas to enter from the tip of the steel pipe. Under such a way of thinking, the present applicant has previously stated that "a heat-resistant curtain suspended so as to cover the entire surface is provided at the furnace inlet, and the steel pipe is inserted through the heat-resistant curtain. (Application No. 2003-28616).

  According to this, when the steel pipe is inserted into the heat treatment furnace, the deposits remaining on the inner surface are decomposed and removed, and the atmosphere gas flows from the front end to the rear end inside the steel pipe. The inside can be easily replaced with atmospheric gas, and contamination and carburization due to the decomposition gas of the deposit can be prevented without lowering the heat treatment efficiency.

  The present invention further improves this technology. That is, in the continuous heat treatment furnace according to the previous application, if the rear end of the steel pipe, which is a material to be heat-treated, enters the furnace (more precisely, inside the heat-resistant curtain), the front end and the rear end of the steel pipe Since there is no pressure difference and there is no gas flow in the steel pipe, hydrocarbon gas and polluted gas are likely to stay near the rear end. For this reason, it is necessary to always manage the temperature at the furnace inlet so that the deposit on the inner surface of the steel pipe can be decomposed before the rear end of the steel pipe enters the inside of the heat-resistant curtain. Therefore, a front chamber with a pre-tropical zone is provided on the entrance side of the heating chamber, and a seal curtain is attached to the exit side of the front chamber (that is, the entrance side of the heating chamber). By setting a step-like pressure difference in the heat treatment furnace, it is not necessary to always manage the temperature at the furnace inlet, and it is easy to disassemble and remove deposits on the inner surface of the pipe. It was confirmed that it could be done reliably.

  The present invention has been made on the basis of the above-mentioned knowledge, and has the gist of the following (1) continuous heat treatment furnace, (2) steel pipe, and (3) heat treatment method.

  (1) A continuous heat treatment furnace in which an atmospheric gas is introduced into a heating chamber having a heating zone, a steel pipe is continuously inserted from the furnace inlet along the axial direction, and the heat treated steel pipe is carried out from the furnace outlet. A continuous heat treatment furnace having a front chamber with a pre-tropical zone on the entry side of the heating chamber and having seal curtains on the entry side and the exit side of the front chamber.

  In the heat treatment furnace, it is desirable to have a rear chamber on the exit side of the heating chamber and a seal curtain on the entrance side of the rear chamber.

  (2) A steel pipe manufactured in the continuous heat treatment furnace as described in (1) above.

  (3) A heat treatment method for introducing an atmospheric gas into a heating chamber having a heating zone, continuously loading the steel pipe along the axial direction from the furnace inlet, and carrying out the heat treatment to carry out the steel pipe from the furnace outlet, Set the internal pressure of the front chamber with the pre-tropical zone on the inlet side of the heating chamber to be higher than the pressure outside the furnace and lower than the pressure of the heating chamber, to a temperature at which the deposits remaining on the inner and outer surfaces of the steel pipe can be vaporized in the front chamber A heat treatment method in which a steel pipe is heated and heat treated.

  Here, “vaporization of the deposit” means that the deposit is decomposed to generate a hydrocarbon gas or the like.

  According to the continuous heat treatment furnace of the present invention, even if the cleaning step after the cold working is only alkaline degreasing and cleaning, deposits on the inner and outer surfaces of the steel pipe can be easily removed before the heat treatment. Therefore, by adopting this heat treatment method using a continuous heat treatment furnace, there is no occurrence of contamination and carburization (herein, focusing on contamination, hereinafter referred to as “contamination”) without reducing the heat treatment efficiency. Finished steel pipe can be obtained.

  As described above, in the present invention, a front chamber having a pre-tropical zone is provided on the entrance side of the heating chamber, and a seal curtain is attached to the front chamber. In this way, stepwise pressure is applied to the heat treatment furnace. Investigated whether there was a problem with doing.

For this investigation, the seal performance test apparatus shown in FIG. 1 was used. This apparatus has a duct 10 (cross-sectional shape: height 160 mm × width 800 mm) having a seal curtain mounting portion 9 in the center, and a seal curtain 11 is attached to the duct 10 to supply a supply amount of 30 in the duct 10. Gas (using air) was supplied as ˜90 Nm ³ / h, and the pressure in the duct 10 was measured (hereinafter, the pressure is described as “gauge pressure”).

(A) Seal curtain structure (number of sheets) and seal performance The seal curtain 11 was attached to a seal performance test apparatus, and the pressure in the duct in the cross section A (the portion indicated by the broken line in the figure) of the front portion of the seal curtain was measured. The installation of the seal curtain was 8 curtains (4 sheets × 2 sets) shown in FIG. 2 (a) and 16 curtains (4 sheets × 4 sets) shown in FIG. 2 (b). In addition, it was confirmed by the test (c) mentioned later that the sealing performance can be evaluated by measurement at the front part (cross section A) of the seal curtain.

  The test results are shown in FIG. As is clear from this result, the air supply amount increases and the duct internal pressure is improved (that is, the sealing performance is improved). When the number of sealing curtains is 16, the performance is about twice that of 8 sealing curtains. Show.

(B) Pressure distribution in the longitudinal direction of the seal curtain When the seal curtain is installed in the seal performance test apparatus with 8 curtains shown in FIG. 2 (a) and 16 curtains shown in FIG. 2 (b). For each of these, the pressure in the duct between the front and rear portions of the seal curtain and each set of the seal curtain was measured.

The measurement results are shown in FIGS. In these figures, the attachment position of the seal curtain is also shown, and the measurement results are shown correspondingly. From these results, the pressure in the duct increases linearly from the rear part of the seal curtain to the front part when the number of seal curtains is 8 or 16, and when the air supply amount is 60 Nm ³ / h, the seal curtain It was confirmed that the sealing performance of about 3 Pa could be secured with one set.

(C) Uniformity of pressure in the duct Width direction in the duct: 100 mm pitch, height direction: 50 mm pitch (see FIG. 6), longitudinal direction: 250 mm pitch, air supply amount 60 Nm ³ / h, seal curtain 16 Pressure measurement was performed when the number of sheets (4 sheets x 4 sets) was used.

  Table 1 shows the measurement results at the front part of the seal curtain (cross section A), and Table 2 shows the measurement results at the rear part of the seal curtain (cross section B).

  From the results of Table 1 and Table 2, the pressure distribution is uniform in the duct cross section at both the front and rear of the seal curtain, and is not shown, but is uniform within ± 0.1 Pa in the longitudinal direction. There was found. Further, since the pressure at the rear part of the seal curtain was approximately 0 Pa, it was confirmed that the seal performance could be evaluated by measuring the pressure at the front part of the seal curtain (for example, the cross section A).

  As a result of the test by the sealing performance test apparatus, even if a plurality of seal curtains are stacked and set, and even if a plurality of sets are arranged, the pressure uniformity in any cross section in the furnace is maintained, and the number of seal curtains It can be seen that the pressure decreases in proportion to. This confirmed that the internal pressure of the heat treatment furnace can be easily set in two stages.

  Therefore, a seal curtain was adopted as a means for achieving a two-stage internal pressure.

  FIG. 7 shows a cross-sectional configuration example of the continuous heat treatment furnace of the present invention (FIG. 7 (a)), material temperature pattern (same (b)), furnace pressure distribution (same (c)), and residual pollutant gas release effect ( It is a figure which shows the same (d)) typically. In FIG. 7, the horizontal lengths in (B) to (D) all correspond to those in (A).

  In the heat treatment furnace shown in FIG. 7 (a), an atmospheric gas is introduced into the heating chamber 1 having the heating zone 1a, and a steel pipe is continuously charged along the axial direction from the furnace inlet 2a to perform a predetermined heat treatment. After that, it is structured to be carried out from the furnace outlet 2b. A pipe feeding roller (not shown) is disposed on the hearth from the furnace inlet 2a to the furnace outlet 2b.

  As shown in the figure, a front chamber 4 having a pre-tropical zone 3 is provided on the entrance side of the heating chamber 1, and is defined by the present invention on the entrance side of the front chamber 4 and the exit side (that is, the entrance side of the heating chamber 1). Seal curtains 5a and 5b are attached. As a result, when the steel pipe flowing on the pipe feeding roller travels a certain distance or more, the deposit on the inner surface of the steel pipe is vaporized at the position where the pre-tropical zone 3 is installed, while the front chamber 4 and the continuous heat treatment furnace sandwiching the seal curtain 5a. Since there is a pressure difference between the outside and the atmosphere, a flow of atmospheric gas from the front end to the rear end of the steel pipe is formed, and the polluted gas generated by vaporization is exhausted out of the continuous heat treatment furnace through the rear end of the steel pipe together with the atmospheric gas. . When the tip of the steel pipe enters the heating chamber 1, there is a pressure difference between the heating chamber 1 and the front chamber 4 (or a pressure difference between the heating chamber 1 and the outside of the continuous heat treatment furnace) across the seal curtain 5b. Since this occurs, the contaminated gas is similarly discharged through the rear end of the steel pipe to the front chamber 4 (or outside the continuous heat treatment furnace).

  Further, in this example, a rear chamber 6 that is desirable in the present invention is provided on the exit side of the heating chamber 1 with a cooling zone interposed therebetween, and a seal curtain 7a is attached to the entrance side thereof. By doing so, the amount of atmospheric gas flowing into the front chamber 4 increases, and the pipe feeding speed can be increased without causing contamination.

  In this example, a seal curtain 7 b is also attached to the exit side of the rear chamber 6. This seal curtain 7b is also attached conventionally, and is intended to prevent the atmospheric gas from unilaterally flowing out from the outlet side (furnace outlet 2b) of the rear chamber 6. That is, in the past, although the seal curtain 7b for preventing the outflow of the atmospheric gas was attached, the steep internal pressure gradient of the atmospheric gas (in other words, the internal pressure of the furnace is reduced as realized in the continuous heat treatment furnace of the present invention. It was not a consideration for setting it in two steps.

  Details will be described below with reference to FIGS.

  FIG. 7 (b) is a material temperature pattern, where the solid line (indicated as “current state” in the figure) is not provided with the pre-tropical zone 3, and the broken line indicates the pre-tropical zone that is a constituent requirement of the heat treatment furnace of the present invention. This is a case where the front chamber 4 provided with 3 is provided on the entrance side of the heating chamber 1. By providing the pre-tropical zone 3, the temperature of the steel pipe is changed to vaporize the residual deposits in the pipe, as described above, and hydrocarbon gases, chlorine and other pollutant gases (here, paying special attention to pollution, It can be rapidly raised to 450 ° C., which is within the desired temperature range for generating “gas”.

  FIG. 7 (c) is a pressure distribution in the furnace (estimated pressure distribution including some actually measured values), and a solid line (indicated as “current state (estimated)” in the figure) is the front chamber 4 in the present invention. This is a case where the seal curtain 5b is not attached among the seal curtains 5a and 5b to be defined, and the rear curtain 6 is not provided with the seal curtain 7a which is desirable in the present invention. A broken line is an example of the present invention, and shows a case where a seal curtain 5 b is provided on the exit side of the front chamber 4 (that is, the entrance side of the heating chamber 1) and a seal curtain 7 a is provided on the entrance side of the rear chamber 6. As a result, the furnace pressure is increased between the seal curtain 5b and the seal curtain 7a, the furnace pressure is set in two stages for the front chamber 4 portion and the heating chamber 1 portion, and the inner pressure of the front chamber is set to the outside pressure of the furnace. The pressure in the heating chamber can be reduced to the above.

  FIG. 7 (d) is a view for explaining the effect of releasing the pollutant gas remaining in the steel pipe. The “current state” in this figure (D) is the case where the rear end 8b of the steel pipe 8 is in the entrance side portion of the front chamber 4 and the front end 8a of the steel pipe 8 is near the center of the heating chamber 1, and the unheated length is It is 13m. The “unheated length” here means that the material temperature does not reach the desired temperature for decomposition of the residual deposit (in this example, 450 ° C.), so the deposit remains (or only partially vaporized). This is the length of the part. Compared with the pressure distribution in the furnace of FIG. 7 (c), at this time, the pressure at the tip 8a is higher than that at the rear end 8b, so there is a gas flow in the pipe, but the steel pipe 8 is conveyed and the rear end 8b When the point A in FIG. 7C is reached, the pressure difference disappears between the front end 8a and the rear end 8b of the tube, so that the gas flow in the tube stops and the polluted gas stays in the tube.

  In the “pre-tropical installation” of FIG. 7 (d), as is clear from the comparison with the material temperature pattern of FIG. 7 (b), the distance from the furnace inlet 2a where the material temperature reaches 450 ° C. is short. The heating length is reduced to 5 m. However, as described above, when the rear end 8b reaches the point A in FIG. 7C, the gas flow in the pipe stops, and the contaminated gas stays in the pipe near the rear end 8b.

  In (1) of “pre-tropical + seal curtain installation” in FIG. 7 (d), the rear end 8 b of the steel pipe 8 reaches point A in FIG. 7 (C), and the front end 8 a is near the center of the heating chamber 1. In some cases, the unheated length is even shorter than that of the “pre-tropical installation”. Since the seal curtain 5b is provided on the exit side of the front chamber 4 (that is, on the entrance side of the heating chamber 1), as shown in FIG. 7C, the internal pressure of the heat treatment furnace is set in two stages. Even if the rear end 8b of the pipe reaches the point A in FIG. 7 (c), there is a pressure difference between the front end 8a and the rear end 8b of the pipe, and a gas flow in the pipe is generated. Absent. When the steel pipe 8 is transferred to the state (2), the pipe rear end 8b also reaches 450 ° C., the unheated length becomes 0 m, and all residual deposits in the pipe are decomposed and vaporized. Moreover, as is clear from the comparison with the in-furnace pressure distribution in FIG. 7C, the vaporized polluted gas is discharged from the rear end of the pipe by the in-pipe gas flow.

  There are no particular limitations on the material, shape, etc. of the seal curtain. Conventional heat-resistant curtains can be used, and as shown in the previous experimental results, if multiple sheets are stacked and then used in multiple sets, it is effective for maintaining the pressure difference before and after the seal curtain. It is.

  Thus, according to the continuous heat treatment furnace of the present invention, even if the cleaning process after cold working is only alkaline degreasing and cleaning, the deposits on the inner and outer surfaces of the steel pipe can be easily removed before the heat treatment. In addition, the required capital investment is relatively low.

  The steel pipe described in (2) is a steel pipe manufactured in the heat treatment furnace of the present invention described above. Even if the cleaning process after cold working is only alkaline degreasing and cleaning, before and after heating to a high temperature (1100 ° C. in the example shown in FIG. Since the residual deposits on the surface are removed, the steel pipe surface (in particular, the inner surface) is not contaminated.

  The heat treatment method according to (3) described above is as follows: “Atmosphere gas is introduced into a heating chamber having a heating zone, a steel pipe is continuously charged along the axial direction from the furnace inlet, and the heat treated steel pipe is subjected to heat treatment. The heat treatment method is carried out from the heating chamber, and is set so that the internal pressure of the front chamber provided with a pre-tropical zone on the inlet side of the heating chamber is higher than the furnace external pressure and lower than the pressure of the heating chamber. This is a method in which the steel pipe is heated to a temperature at which the remaining deposits can be vaporized and heat treated.

  The temperature at the time of “heating the steel pipe to a temperature at which deposits can be vaporized” is preferably such that the inner surface temperature of the steel pipe is 400 ° C. or higher and 750 ° C. or lower. In order to effectively decompose and vaporize the remaining deposits, it is appropriate to heat the surface temperature to 400 ° C or higher. To reduce the action of polluting gases and to prevent carburization, control accuracy is required. This is because it is effective to set the temperature to 750 ° C. or less in consideration of the above.

  In order to “set so that the internal pressure of the front chamber is not less than the pressure outside the furnace and not more than the pressure of the heating chamber”, it is only necessary to introduce the atmospheric gas into the heating chamber with an appropriate supply amount. The seal curtain 5b provided on the exit side of the front chamber and the seal curtain 5a provided on the entrance side effectively act, so that the internal pressure of the front chamber is higher than the furnace external pressure and lower than the pressure of the heating chamber.

  This heat treatment method can be carried out using the heat treatment furnace of the present invention described above. That is, since the furnace internal pressure can be set in two stages, that is, the front chamber part and the heating chamber part, the internal pressure in the front chamber can be made higher than the furnace external pressure and lower than the pressure in the heating chamber. Since the flow of the atmospheric gas from the front end to the rear end can be generated without difficulty, the residual deposits inside the tube can be vaporized and replaced or removed with the atmospheric gas. Since heat treatment is subsequently performed at a predetermined temperature, the heat treatment efficiency is not reduced.

Using an “isothermal flow model” that represents the gas flow in the pipe where a differential pressure of ΔP _P [Pa] is generated at both ends, the pre-tropical zone and the installation conditions of the seal curtain are changed, and the inner diameter is 6 mm and the length is 20 m. We examined whether or not the gas in the pipe could be discharged when the steel pipe was sent, and further investigated whether the inner surface of the pipe was contaminated with deposits containing chlorine in an actual furnace. In addition, the furnace pressure distribution required in the examination of whether or not to discharge the in-pipe gas was estimated by the furnace pressure distribution estimation formula described later.

Derivation of “isothermal flow model formula”:
The differential pressure ΔP _P [Pa] generated at both ends of the tube and the gas flow velocity ν _P [m / s] generated in the tube have the relationship of the following equation (1).

  For laminar flow,

Therefore, ΔP _P [Pa] is

It becomes.

On the other hand, the entrance of the front chamber is L = 0, the rear end of the seal curtain installed on the entrance side of the front chamber is L ₁ , and the front and rear ends of the seal curtain installed on the exit side of the front chamber are L ₂ and L ₃ , respectively. (L ₃ = L ₂ + “thickness of the seal curtain 5b”) (see FIG. 7A), the static pressure increases linearly before and after the seal curtain and approximates to an equal pressure between the seal curtains. The internal pressure distribution is expressed by the following equation (4).

Here, when the steel pipe reaches 450 ° C., it is assumed that pollutant gas is generated (contaminant adhering to the inner surface of the pipe is vaporized), and the position in the furnace where the steel pipe reaches 450 ° C. is L ₄₅₀ . The time at which the tip (end in the pipe feeding direction) reaches L ₄₅₀ is t ₄₅₀ , and the time at which the pressure reaches the position L ₄ (L ₄ = L ₃ + L _P , L _P is the total length of the pipe) where the differential pressure across the steel pipe is eliminated When the the t _4, the distance L _Drain (0) in which the atmosphere gas is moved through the tube at the tip position of the steel pipe during the time (t ₄ -t ₄₅₀₎ is expressed by the following equation (5).

If the pipe feeding speed is ν _t , from L = t · ν _t ,

The distance L _drain (x) that moves in the steel pipe while the gas at the pipe internal position x [m] from the tip of the steel pipe reaches 450 ° C. and is sent to the position L ₄ where the differential pressure at both ends of the steel pipe disappears. )

It is represented by Therefore, the unheated length L _res is

If L _res ≦ 0, “no heating length”, that is, the atmospheric gas can be discharged from the inside of the pipe, and the contaminated gas is also discharged from the pipe.

Furnace pressure distribution estimation formula:
The gas mass flow rate G [kg / s] and the static pressure change ΔP _j [Pa] flowing out from the j-th seal curtain are expressed by the following equations (9) and (10), respectively.

The differential pressure ΔP _total [Pa] generated when the gas passes through n seal curtains is expressed by the following equation (11).

N _en-in , N _en-out , N _ex [set], the number of seal curtain sets (1 set = 4) installed in the front room entrance, front room exit, and rear room, respectively, each G _en the amount of hydrogen gas that flows out from the rear chamber side, if G _ex [kg / s], the heating zone static pressure ΔP _{H_Zone} = cooling zone static pressure is obtained (12).

Here, if G _total = G _en + G _ex , the equations (13) and (14) are obtained.

From the formulas (12) to (14), when the number of seal curtain sets and the total hydrogen supply amount G _total are given, the heating zone static pressure (that is, the heating chamber pressure) ΔP _{H_Zone} is obtained. The front chamber pressure ΔP _{front chamber} is

It can ask for.

Simulation result (examination result of whether or not the gas in the pipe can be discharged):
Using the above-mentioned “isothermal flow model formula”, the unheated length L _res was calculated on the assumption that a steel pipe having an inner diameter of 6 mm and a length of 20 m was heat-treated. As described above, if the unheated length L _res ≦ 0, the contaminated gas in the pipe is discharged from the rear end of the pipe. Here, as the temperature inside the tube, an average temperature of 775 ° C. of 450 to 1100 ° C. in the heat treatment furnace was adopted.

  In the simulation, when there is no pre-tropical and outlet seal curtain in the front chamber and no inlet seal curtain in the rear chamber (simulation 1), when there is only the pre-tropical zone in the front chamber (simulation 2), the outlet seal in the front chamber When having only a curtain (simulation 3), having a pretropical and outlet seal curtain in the front chamber (simulation 4), and having a pretropical and outlet seal curtain in the front chamber and an inlet seal curtain in the rear chamber For a total of 5 cases (Simulation 5), the calculation was performed at a tube feeding speed of 1450 mm / min or 950 mm / min, respectively.

Table 3 shows the simulation results. In addition to the unheated length L _res , Table 3 also shows the setting conditions on the equipment in the continuous heat treatment furnace, such as the presence of a pre-tropical zone and the presence of a seal curtain, and the pressures in the front chamber and the heating chamber. The circles in the “front chamber” and “rear chamber” columns of the heat treatment furnace indicate that there is a pre-tropical zone or a seal curtain, and the _{circle in the} “unheated length L _res ” column indicates contamination on the inner surface of the pipe. X indicates that it cannot be prevented from calculation, and x indicates that it cannot be prevented.

As is clear from the results shown in Table 3, in the simulation 4 having the pre-tropical zone and the exit-side seal curtain in the front chamber, when the pipe feeding speed is slow (950 mm / min), the unheated length is 0 or less (L _res ≦ 0). That is, it is expected that the pollutant gas can be discharged from the inside of the pipe. Further, in simulation 5 having the pre-tropical and outlet seal curtains in the front chamber and the entrance seal curtain in the rear chamber, the unheated length is 0 or less even when the pipe feeding speed is high (1450 mm / min). Therefore, it is expected that more efficient heat treatment can be performed.

Investigation of tube internal contamination in actual furnace:
Following the simulation, heat treatment was performed on a steel pipe (inner diameter 6 mm, length 20 m) with a lubricant containing chlorine on the inner and outer surfaces in an actual furnace, and the presence or absence of contamination by chlorine was investigated. Hydrogen gas was used as the atmospheric gas in the heat treatment furnace, and the supply amount was 95.00 Nm ³ / h, and the gas was sent to the heating chamber. The tube feeding speed was 950 m / min or 1450 m / min.

  Table 4 shows the survey results. In Table 4, the seal curtain on the entrance side of the anterior chamber is not shown because it uses a commonly used curtain and is installed in both the comparative example and the example. In addition, regarding “presence or absence of contamination”, the rear end portion (part which becomes the rear end with respect to the traveling direction of the steel pipe) is cut out from the steel pipe after the heat treatment, and the chlorine adhering to the inner surface is removed. Extraction was performed with pure water, and ion chromatographic analysis was performed on the extracted water to investigate the amount of residual chlorine on the inner surface of the tube.

  As is clear from the results shown in Table 4, in Comparative Examples 1 to 3 that deviate from the conditions defined in the present invention, all were determined to be “contaminated”, but in Examples 1 to 3, “no contamination”, Or “Slight” (Example 2).

  Some contamination was observed in Example 2 because the pipe feeding speed was faster than that in Example 1 under the same conditions, the replacement of the pollutant gas in the pipe with the atmospheric gas was delayed, and the pollutant gas was near the rear end of the pipe. This is thought to be due to the remaining in In Example 3, although the pipe feeding speed was high, no contamination was observed. As a result of installing a seal curtain on the entrance side of the rear chamber, the pressure in the heating chamber increased from 8.73 Pa to 11.93 Pa. This is because the amount of atmospheric gas flowing into the front chamber is increased, the replacement of the gas in the pipe is promoted, and the polluted gas is removed.

  According to the continuous heat treatment furnace and the heat treatment method of the present invention, even when the cleaning process after cold working is only alkaline degreasing and cleaning, deposits on the inner and outer surfaces of the steel pipe can be easily removed before the heat treatment. it can. Therefore, the present invention can be suitably used for manufacturing steel pipes including stainless steel pipes that are cold-worked using rolling oil or lubricant containing hydrocarbon-based components.

It is a figure which shows schematic structure of the principal part of a sealing performance test apparatus. It is a figure which shows the structure of the seal curtain used for performance evaluation, (A) is the case where the seal curtain is 8 sheets (4 sheets x 2 sets), and (B) is the case of 16 sheets (4 sheets x 4 sets). . It is a figure which shows the relationship between the air supply amount and duct internal pressure (seal performance) by using the number of seal curtains as a parameter. It is a figure which shows the pressure distribution in the duct of the longitudinal direction of a seal curtain in case the number of seal curtains is 8 sheets (4 sheets x 2 sets). It is a figure which shows the pressure distribution in the duct of the longitudinal direction of a seal curtain in case a seal curtain is 16 sheets (4 sheets x 4 sets). It is a figure which shows the measurement position in the duct cross section in the uniformity evaluation test of the pressure in a duct. Cross-sectional configuration example (FIG. 7 (a)), material temperature pattern (same (b)), furnace pressure distribution (same (c)), and residual pollutant gas release effect (d) (d) ) Is a diagram schematically showing.

Explanation of symbols

1: Heating chamber 1a: Heating zone 2a: Furnace inlet 2b: Furnace outlet 3: Pre-tropical 4: Front chamber 5a, 5b: Seal curtain 6: Rear chamber 7a, 7b: Seal curtain 8: Steel pipe 8a: Tip 8b: Rear end 9: Seal curtain mounting part 10: Duct 11: Seal curtain

Claims (4)

  A continuous heat treatment furnace for introducing an atmosphere gas into a heating chamber having a heating zone, continuously charging the steel pipe from the furnace inlet along the axial direction, and carrying out the heat treatment from the furnace outlet. A continuous heat treatment furnace having a front chamber provided with a pre-tropical zone on the entrance side of the interior, and seal curtains on the entrance side and the exit side of the front chamber.   The continuous heat treatment furnace according to claim 1, further comprising a rear chamber on the exit side of the heating chamber and a seal curtain on the entrance side of the rear chamber.   A steel pipe manufactured by the continuous heat treatment furnace according to claim 1 or 2. An atmosphere gas is introduced into a heating chamber having a heating zone, a steel pipe is continuously charged from the furnace inlet along the axial direction, and the heat-treated steel pipe is carried out from the furnace outlet. The internal pressure of the front chamber with pre-tropical zone on the inlet side is set to be higher than the pressure outside the furnace and lower than the pressure of the heating chamber, and the steel tube is heated to a temperature at which the deposits remaining on the inner and outer surfaces of the steel pipe can be vaporized in the front chamber. Then, a heat treatment method characterized by heat treatment.

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