JP2016074003A - Mandrel bar cooling method and cooling device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cooling method and a cooling device capable of preventing heat checking from occurring in a mandrel bar after the mandrel bar rolled by a mandrel mill has been pulled out from a tube body, thereby preventing crack damage originating from the heat checking.SOLUTION: The mandrel bar cooling method is characterized by carrying out slow cooling with an average cooling speed set to 100°C/sec., or lower until the surface temperature of the mandrel bar drops to 200°C or lower. The mandrel bar cooling method is characterized in that the slow cooling is carried out with an average cooling speed set to 50°C/sec., or higher, the rapid cooling of the mandrel is carried out with an average cooling speed set to 100°C/sec., or higher after the slow cooling, the rapid cooling is stopped when the surface temperature of the mandrel bar is 50°C/sec., or higher, and the surface temperature of the mandrel bar during the application of a lubricant is set to 50°C/sec., or higher.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a cooling method and cooling equipment for a mandrel bar drawn from a tube rolled by a mandrel mill.

  A mandrel mill is a series of processes for manufacturing seamless steel pipes (so-called seamless steel pipes). The mandrel mill is a rolling mill in which a plurality of a pair of perforated rolls are arranged in series. In the mandrel mill, a round steel piece (so-called billet) heated in a heating furnace is rolled with a mandrel bar inserted into a base pipe obtained by punching with a punching machine (so-called piercer). The wall thickness is squeezed and discharged to the outlet side as a tubular body extending along the mandrel bar.

  Subsequently, on the exit side of the mandrel mill, the tube body with the mandrel bar is brought into contact with the stopper, and the mandrel bar is pulled out from the tube body using a drawing device. The tubular body thus obtained is fed to a subsequent process (that is, a reheating furnace, a reducer, etc.). On the other hand, the mandrel bar pulled out from the tube body is cooled, further coated with a lubricant on the surface, conveyed to the entry side of the mandrel mill, and inserted again into the raw tube. Thus, the mandrel bar is circulated and used repeatedly.

  The temperature of the mandrel bar rises by being inserted into a high-temperature raw tube on the entry side of the mandrel mill, and decreases after being pulled out of the tube on the exit side. In other words, the mandrel bar undergoes repeated heating and cooling thermal cycles as well as loads acting by rolling and conveying while being used in circulation. As a result, cracks due to loads and thermal stress (hereinafter referred to as heat check) On the surface.

When a heat check occurs on the surface of the mandrel bar,
(a) The raw tube sticks to the mandrel bar during rolling of the mandrel mill,
(b) A seizure of fraying or fraying occurs on the inner surface of the tube during rolling of the mandrel mill.
(c) After rolling, the tube is seized when the mandrel bar is pulled out.
(d) When the mandrel bar is pulled out, seizure and soot are generated on the inner surface of the tube body.
(e) It becomes a starting point for fatigue cracks during circulation of the mandrel bar, and causes problems such as breakage of the mandrel bar. Among these problems, (b) (d) seizure firewood and survivors remain in seamless steel pipes manufactured through subsequent processes (that is, reheating furnaces, reducers, etc.), leading to a decrease in yield. . In addition, (a), (c) and (e) cause not only the mandrel mill but also the entire production process of seamless steel pipes to be stopped, resulting in a decrease in productivity.

  Therefore, an attempt is made to prevent these problems from occurring by setting a reference value (for example, the number of days used, the number of circulations, etc.) of the mandrel bar in advance and discarding the mandrel bar that has reached the reference. Has been made. However, factors such as mandrel mill operating conditions and mandrel bar materials have complex effects, so the mandrel bar usage limit cannot be uniformly defined, and the above problems (a) to (e) can be solved. Is not reached.

  In addition, in recent years, seamless steel pipes containing a large amount of suitable alloying elements (such as Cr, Ni, etc.) can be used in fields where corrosion resistance and creep resistance are required as new applications of seamless steel pipes. Has been developed and its production is increasing. Seamless steel pipes with excellent corrosion resistance and creep resistance contain a large amount of alloying elements, so their hot strength has increased, and when rolling the pipe with a mandrel mill, In comparison, a large surface pressure is generated on the surface of the mandrel bar.

  In such a seamless steel pipe manufacturing process, the material of the mandrel bar used for rolling the pipe body with a mandrel mill is hot tool steel (for example, SKD6 or SKD61 defined in JIS standards). An increase in the surface pressure of the mandrel bar, and hence an increase in the load applied to the mandrel bar (ie, load load, thermal load) cannot be avoided. As a result, a heat check is likely to occur, resulting in a problem that the life of the mandrel bar is shortened. The components of SKD6 and SKD61 are as shown in Table 1, and the balance is Fe and inevitable impurities.

  Therefore, a technique for preventing the heat check of the mandrel bar has been studied.

  For example, in Patent Documents 1 and 2, a mandrel bar pulled out from a tubular body is kicked out onto a skid and cooled while being rotated to achieve uniform cooling and suppress bending that occurs during cooling of the mandrel bar. Technology is disclosed. When a mandrel bar curved by bending is inserted into a raw tube and rolled, the load concentrates on the curved portion, and therefore, a heat check and breakage of the mandrel bar are likely to occur. Therefore, if the bending of the mandrel bar can be prevented, an effect of suppressing a heat check or breakage that occurs during rolling can be obtained.

  However, since the inventions disclosed in Patent Documents 1 and 2 are techniques for preventing the bending of the mandrel bar, the cooling rate of the mandrel bar pulled out from the tube is not specified, and the cooling is stopped at the cooling rate. The temperature is not specified. Accordingly, it is inevitable that a heat check occurs while the mandrel bar is rotated and cooled, and this causes a problem that it causes a breakage. Moreover, since the mandrel bar is rotated on the skid and the cooling water is sprayed, the structure of the skid becomes complicated and a large amount of investment is required.

Japanese Unexamined Patent Publication No. 60-40927 JP 61-42405 JP

  The present invention eliminates the problems of the prior art, prevents the heat check from occurring on the cooling mandrel bar after pulling out the mandrel bar from the tube rolled by the mandrel mill, and as a result, An object of the present invention is to provide a cooling method and a cooling facility that prevent breakage starting from a check.

  The inventor has investigated the cause of heat check on the mandrel bar during cooling. And when the cooling rate of the mandrel bar pulled out from the tubular body was too large, it was found that a heat check occurred on the surface. Furthermore, the knowledge that a heat check progresses and leads to breakage by the repeated application of load and thermal stress while circulating the mandrel bar was obtained. That is, in order to prevent a heat check and thus prevent breakage, it is necessary to reduce the cooling rate of the mandrel bar after being pulled out from the tube.

  The present invention has been made based on such knowledge.

  That is, the present invention relates to a method for cooling a mandrel bar drawn from a tube rolled by a mandrel mill, and after the mandrel bar is drawn from the tube and before the lubricant is applied, the surface temperature of the mandrel bar is 200 ° C. This is a mandrel bar cooling method in which the average cooling rate is set to 100 ° C./second or less until the temperature drops below.

  In the cooling method of the present invention, it is preferable to perform slow cooling at an average cooling rate of 50 ° C./second or less. In addition, it is preferable to perform rapid cooling of the mandrel bar at an average cooling rate exceeding 100 ° C./second after performing slow cooling. Furthermore, it is preferable to stop the rapid cooling when the surface temperature of the mandrel bar is 50 ° C. or higher. The surface temperature of the mandrel bar when applying the lubricant is preferably 50 ° C. or higher.

  The present invention also relates to a cooling facility for a mandrel bar drawn from a tube rolled by a mandrel mill, wherein the mandrel bar drawn from the tube is slowly cooled to a surface temperature of 200 ° C. or less at an average cooling rate of 100 ° C./second or less. It is a cooling facility for a mandrel bar having a slow cooling device capable of cooling.

  In the cooling device of the present invention, it is preferable to have a rapid cooling device that performs rapid cooling of the mandrel bar on the outlet side of the slow cooling device. The rapid cooling device is preferably a cooling water tank.

  According to the present invention, after pulling out the mandrel bar from the tube on the outlet side of the mandrel mill, it is possible to prevent the heat check from occurring during the cooling of the mandrel bar, and to prevent the breakage starting from the heat check. Since it becomes possible to improve the service life of the mandrel bar, it has a remarkable industrial effect. Moreover, the effect which improves the inner surface property of a seamless steel pipe is also acquired.

It is a flowchart which shows the example of the arrangement | sequence of the slow cooling apparatus and the related apparatus for operating a mandrel mill by applying this invention. It is a graph which shows the example of distribution of a heat check. It is a graph which shows the example of distribution of thermal stress. It is a graph which shows the other example of distribution of thermal stress. It is a graph which shows the relationship between bar surface temperature and lubricant adhesion amount.

  FIG. 1 is a flow diagram showing an example of an arrangement of a slow cooling device and related equipment for operating a mandrel mill by applying the present invention. In a mandrel mill, a round steel piece (so-called billet) heated in a heating furnace is rolled with a mandrel bar inserted into a base pipe obtained by punching with a drilling machine (so-called piercer), and the outer diameter and thickness of the base pipe are rolled. And is discharged to the outlet side as a tubular body extending along the mandrel bar.

  Subsequently, on the exit side of the mandrel mill, the tube body with the mandrel bar is brought into contact with the stopper, and the mandrel bar is pulled out from the tube body using a drawing device. The tubular body thus obtained is fed to a subsequent process (that is, a reheating furnace, a reducer, etc.). On the other hand, the mandrel bar pulled out from the tube body is slowly cooled by a slow cooling device (see FIG. 1 (a)) and then rapidly cooled by a rapid cooling device (see FIG. 1 (b)) if necessary. Further, a lubricant is applied to the surface by a lubricant application device, conveyed to the entry side of the mandrel mill, and inserted again into the raw tube. Thus, the mandrel bar is circulated and used repeatedly.

The slow cooling device shown in FIG. 1 is not particularly limited as long as it can cool the mandrel bar at a predetermined cooling rate. For example,
(A) Water spray cooling or water shower cooling using only water as a refrigerant,
(B) Mist cooling using air-water mixed fluid as refrigerant,
(C) Air cooling by compressed air or blast,
(D) Cooling in the atmosphere
(E) Slow cooling is performed by means such as an atmospheric furnace, an induction heating furnace, and slow cooling using a hood. However, in order to uniformly and slowly cool the entire mandrel bar, it is preferable to perform the slow cooling while rotating the mandrel bar around the axis. As a means for rotating the mandrel bar, a conventionally known device (for example, a turning roller) may be used.

  When rotating the mandrel bar around the axis in the slow cooling, if the rotation speed is too slow, the effect of cooling the mandrel bar uniformly and reducing the thermal stress cannot be obtained. On the other hand, if the rotational speed is too high, the mandrel bar will bend. Therefore, the rotation speed of the mandrel bar is preferably 10 to 60 times / minute.

  In this manner, slow cooling is performed at an average cooling rate of 100 ° C./second or less until the surface temperature of the mandrel bar (hereinafter referred to as “bar surface temperature”) drops to 200 ° C. or less.

  Here, the average cooling rate in the slow cooling will be described.

  First, in order to investigate the distribution of heat check generated in the mandrel bar, after pulling out the mandrel bar from the tube on the exit side of the mandrel mill, the heat check was generated by performing rapid cooling without performing slow cooling. . After the mandrel bar was used a plurality of times, the depth of the heat check was measured at 1 m intervals from the tip. The result is shown in FIG.

  As is clear from FIG. 2, the depth of the heat check is almost constant in the range of 300 to 350 μm at the portion where the high temperature tube contacts, and the heat check of the tip and rear ends protruding from the tube is It is shallow. And from the heat check (depth 350 micrometers) with the largest depth, many fatigue cracks considered to have developed by the rolling load were generated. From this, it can be seen that the heat check of the mandrel bar is concentrated on the portion where the temperature rises by rolling with a mandrel mill.

  That is, the mandrel bar is inserted into the raw pipe on the inlet side of the mandrel mill, and the temperature rises due to heat input from the raw pipe while being rolled as it is. Then, the mandrel bar pulled out from the tube on the exit side of the mandrel mill is cooled during the conveyance and is fed to the slow cooling device. Conventionally, the cooling water immersion method and the cooling water injection method were used for rapid cooling without slow cooling. However, the rapid cooling caused a temperature difference between the surface and the inside of the mandrel bar, resulting in variations in expansion amount. Tensile stress (ie, thermal stress) is generated. This thermal stress causes a heat check and further progresses.

  Therefore, it is possible to prevent the heat check from occurring and proceeding by performing slow cooling to suppress the temperature difference between the surface and the inside of the mandrel bar.

  Therefore, the preferred average cooling rate in the slow cooling was investigated. Here, the average cooling rate is a value obtained by dividing the temperature difference (° C.) between the bar surface temperature at the start of the slow cooling and the bar surface temperature at the stop of the slow cooling by the required time (seconds) of the slow cooling.

  In order to investigate the effect of slow cooling, unsteady heat transfer analysis was performed using the finite element method, and a distribution map of tensile stress (ie, thermal stress) generated inside the mandrel bar was created. The result is shown in FIG. 3, and the distribution curve with an average cooling rate of 104 ° C./second shows the distribution of thermal stress when rapid cooling (immersion in cooling water) is performed without slow cooling. When this distribution curve is compared with FIG. 2, it can be seen that when a heat check having a depth of 300 to 350 μm occurs, a thermal stress of 180 to 220 MPa is generated.

  On the other hand, when the mandrel bar is pulled out from the tube and then slowly cooled (cooling rate 12 ° C / second, 41 ° C / second, 95 ° C / second), a thermal stress of 180 to 220 MPa induces a heat check. Occurrence occurs at a position 240 to 320 μm from the surface of the mandrel bar. In other words, by performing slow cooling at an average cooling rate of 100 ° C / sec or less, the amount of heat removed from the surface of the mandrel bar is reduced, and the recuperation due to heat transfer from the inside of the mandrel bar is balanced with the heat removal. Since the thermal stress is significantly reduced, the effect of suppressing the occurrence and progress of the heat check is remarkably exhibited.

  For this reason, the average cooling rate in the slow cooling is set to 100 ° C./second or less. Preferably it is 50 degrees C / sec or less. However, if the average cooling rate is too low, it takes a long time for slow cooling, which leads to a decrease in productivity of the seamless steel pipe. Therefore, the average cooling rate in the slow cooling is preferably 10 to 100 ° C./second, and more preferably 25 to 50 ° C./second.

  Next, the bar surface temperature when the slow cooling is stopped (hereinafter referred to as the slow cooling stop temperature) will be described.

  Perform unsteady heat transfer analysis using the finite element method in the same way as in Fig. 3, pull out the mandrel bar from the tube, and slowly cool it until the bar surface temperature reaches 200 ° C with an average cooling rate of 3.3 ° C / sec. Further, a distribution diagram of tensile stress (that is, thermal stress) generated in the mandrel bar when rapid cooling (average cooling rate of 104 ° C./second) was performed was prepared. The result is shown in FIG. The distribution curve for only strong cooling in FIG. 4 is the same as the distribution curve for the average cooling rate of 104 ° C./second shown in FIG.

  As is apparent from FIG. 4, if the bar surface temperature is slowly cooled down to 200 ° C., the thermal stress is greatly reduced even if rapid cooling with an average cooling rate exceeding 100 ° C./second is performed thereafter. I understand that. In other words, by setting the slow cooling stop temperature to 200 ° C or less, the temperature difference between the cooling water used in the rapid cooling system and the mandrel bar is reduced, and the amount of heat removed by rapid cooling is reduced. Can be reduced.

  For this reason, the slow cooling stop temperature is set to 200 ° C. or lower. However, if the slow cooling stop temperature is too low, it takes a long time for slow cooling, resulting in a decrease in productivity of the seamless steel pipe. Therefore, the slow cooling stop temperature is preferably 50 to 200 ° C. from the viewpoint of improving productivity.

  The cooling means for the mandrel bar after the slow cooling is stopped is not particularly limited. However, it is preferable to perform rapid cooling (average cooling rate exceeding 100 ° C./second) after the slow cooling is stopped, in addition to reducing thermal stress and improving productivity. When performing rapid cooling, stop rapid cooling until the bar surface temperature reaches 50 ° C.

  That is, when only slow cooling is performed, the slow cooling stop temperature is preferably 50 to 200 ° C. When the rapid cooling is performed after the slow cooling, it is preferable that the slow cooling is first stopped at a temperature exceeding 50 ° C., and then the temperature at which the rapid cooling is stopped is lower than the slow cooling stop temperature and 50 ° C. or more. The reason will be described below from the viewpoint of the lubricant adhesion amount.

  FIG. 5 is a graph showing the relationship between the bar surface temperature and the lubricant adhesion amount. As is apparent from FIG. 5, when the bar surface temperature is less than 50 ° C., the amount of lubricant adhered is significantly reduced. As a result, due to the problems (a) and (c) described above, the operation of the entire manufacturing process of not only the mandrel mill but also the seamless steel pipe is stopped, resulting in a decrease in productivity. Accordingly, the lower limit of the temperature at which the slow cooling or the rapid cooling is stopped (that is, the bar surface temperature when the lubricant is applied) is preferably 50 ° C.

  When the bar surface temperature exceeded 200 ° C., the lubricant easily flowed down, making it difficult to measure the amount of lubricant adhered. For this reason, in FIG. 5, data exceeding the bar surface temperature of 200 ° C. is not shown.

  The bar surface temperature is measured while the mandrel bar is being transported, so a non-contact type thermometer (for example, a radiation thermometer) is used. Moreover, rapid cooling is preferable because existing equipment can be utilized by immersing a mandrel bar in a cooling water tank.

  A seamless steel pipe was produced while the mandrel bar was cooled and circulated in a mandrel mill under the conditions shown in Table 2.

  Comparative Example 1 is an example in which only rapid cooling is employed without performing slow cooling. In this example, Table 2 shows the average cooling rate of the rapid cooling and the bar surface temperature when the rapid cooling is stopped. Further, since the lubricant is applied after the rapid cooling is finished, the bar surface temperature at which the rapid cooling is stopped is equal to the lubricant application temperature. The rapid cooling was performed by immersing a mandrel bar in a cooling water tank.

  Inventive Examples 1 to 3 are examples in which only slow cooling is employed (see FIG. 1A), and the average cooling rate and slow cooling stop temperature in slow cooling are as shown in Table 2. In this example, the rapid cooling is not performed, and the lubricant is applied after the slow cooling is completed. Therefore, the slow cooling stop temperature is equal to the lubricant application temperature. The slow cooling was performed by spraying cooling water while rotating the mandrel bar around the shaft core with a turning roller at a rotation speed of 20 times / minute (so-called water spray cooling).

  Inventive Examples 4 to 6 are examples in which slow cooling is performed and further rapid cooling is performed (see FIG. 1B). The average cooling rate and the slow cooling stop temperature in slow cooling are as shown in Table 2. In this example, since the quick cooling is performed after the slow cooling, the lubricant application temperature is lower than the slow cooling stop temperature. The average cooling rate in the rapid cooling was 105 ° C./second, which is the same as that in Comparative Example 1.

  Under such conditions, the mandrel bar was circulated while being cooled, the mandrel bar that reached the standard of use limit (so-called life) was hung with a crane, air-cooled to room temperature, and the depth of the heat check was measured. The standard of the mandrel bar usage limit is set based on various factors such as not only the heat check but also depressions and rods, and there are various defects on the surface of the mandrel bar suspended by the above crane. . A heat check was selected from those defects, and the depth was measured. Table 2 shows the maximum value of the data. The life of the mandrel bar is shown in Table 2 as a relative ratio with Comparative Example 1 being 100.

  As apparent from Table 2, the maximum heat check depth of Invention Examples 1 to 6 was 255 to 310 μm, whereas Comparative Example 1 did not perform slow cooling, so the heat check was extremely deep ( 355 μm).

  In Comparative Example 2, which employs only slow cooling, the maximum depth of the heat check is 150 μm, which is shallower than the inventive example. This is because the lubricant application temperature (that is, the temperature at which the rapid cooling is stopped) is below 50 ° C., so that the lubricant is not sufficiently applied and the mandrel bar is burned. In other words, compared to the invention example, the lifetime was extremely short, and the limit of use limit was reached before the heat check grew.

  As described above, according to the present invention, it was confirmed that heat check can suppress generation and growth and improve the mandrel bar life.

Claims (8)

  In the method for cooling a mandrel bar drawn from a tube rolled by a mandrel mill, the surface temperature of the mandrel bar is reduced to 200 ° C. or less after the mandrel bar is drawn from the tube and before the lubricant is applied. A method for cooling a mandrel bar, characterized by performing slow cooling at an average cooling rate of 100 ° C./second or less until the temperature decreases.   The method for cooling a mandrel bar according to claim 1, wherein the slow cooling is performed at an average cooling rate of 50 ° C / second or less.   The method for cooling a mandrel bar according to claim 1 or 2, wherein after the slow cooling, the mandrel bar is rapidly cooled at an average cooling rate exceeding 100 ° C / second.   The method for cooling a mandrel bar according to claim 3, wherein the rapid cooling is stopped when the surface temperature of the mandrel bar is 50 ° C or higher.   The method for cooling a mandrel bar according to any one of claims 1 to 4, wherein a surface temperature of the mandrel bar when the lubricant is applied is 50 ° C or higher.   A cooling facility for a mandrel bar drawn from a tube rolled by a mandrel mill, wherein the mandrel bar drawn from the tube is slowly cooled to a surface temperature of 200 ° C. or less at an average cooling rate of 100 ° C./second or less. A cooling system for a mandrel bar, characterized by having a slow cooling device capable of   The cooling facility for a mandrel bar according to claim 6, further comprising a rapid cooling device that performs rapid cooling of the mandrel bar on the outlet side of the slow cooling device.   The mandrel bar cooling facility according to claim 7, wherein the rapid cooling device is a cooling water tank.

Images