JP2016043364A - Manufacturing apparatus and manufacturing method for mandrel bar - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing apparatus line for a mandrel bar that dispenses with a rolling schedule where a low alloy steel is wedged into rolling of a steel species containing many alloy elements, invites no complication in the rolling schedule or no deterioration in profitability and can elongate the tool life of the mandrel bar by inviting no large investment with no need of plating treatment.SOLUTION: A manufacturing apparatus line for a mandrel bar 1, which heats a bar blank 10 that is a semi-product of mandrel bar shape up to a tempering temperature and succeedingly cools the same down to 100°C or less to form a mandrel bar 1, has a tempering furnace 20, which heats the bar blank 10 up to a tempering temperature and an acceleration cooling device 22 that sets the cooling rate from the tempering temperature of the bar blank extracted out of the tempering furnace to 100°C at 10.0 - 35°C/min, disposed in this order.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a mandrel bar manufacturing apparatus row and a manufacturing method. Here, the mandrel bar is an inner surface tool used in mandrel rolling related to the production of seamless steel pipes.

  One of the methods for producing seamless steel pipes is a process called a mandrel rolling method. In this mandrel rolling process, the mandrel bar, which is the inner surface tool, is inserted on the inner surface side in order to reduce the wall thickness of a raw steel tube that has been punched with a piercing machine in a round steel piece heated in a heating furnace. Then, tandem rolling is performed by a mandrel mill in which a plurality of stands equipped with a pair of left and right hole-type rolls are arranged in series, and the mandrel bar is pulled out from the steel pipe after the rolling.

  In this mandrel rolling method, the raw tube is rolled between the mandrel bar and the perforated roll to reduce the wall thickness. At that time, the mandrel bar is subjected to loads such as rolling load, thermal stress, and wear. Cracks occur in the surface circumferential direction. Circumferential cracking of the mandrel bar causes the occurrence of non-conforming products that cause seizure of the inner surface of the steel pipe and the mandrel bar and the steel pipe at the time of rolling or drawing, so when the crack progresses to some extent, the mandrel bar It is judged that the tool life has been reached and is discarded.

  In recent years, the production of steel pipes containing a large amount of alloy elements such as Cr and Ni has been increasing for the purpose of increasing the strength of steel pipes and improving corrosion resistance. Since these steel pipes have high hot strength, a large rolling load is generated during rolling compared to general steel, and the load on the inner surface tool is also increased. Hot tool steels such as SKD6 and SKD61 specified in JIS are generally used as the mandrel bar steel grade. However, due to insufficient material strength, the occurrence of the aforementioned circumferential cracks increases and the life is shortened. Has occurred. Table 1 shows the steel compositions of SKD6 and SKD61 (mass%; the balance is Fe and inevitable impurities).

  To deal with this problem, Patent Document 1 rolls low-alloy general steel before manufacturing a steel pipe containing a large amount of alloy elements, thereby attaching an oxide scale to the surface of the mandrel bar, and the friction coefficient between the mandrel bar and the steel pipe. There has been proposed a method for reducing the tool load by reducing the tool load. In Patent Document 2, a hard chromium film is formed on the surface in advance at the mandrel bar manufacturing stage to improve wear resistance.

Japanese Patent No. 4900385 Japanese Patent No. 3395715

  However, the above prior art has the following difficulties. That is, in the method described in Patent Document 1, when rolling a steel type containing a lot of alloy elements, it is necessary to roll the low alloy steel in advance, so that the rolling schedule becomes complicated, and the low alloy steel with low profitability is also required. Since it is necessary to place an order together, profitability deteriorates. Further, the method described in Patent Document 2 requires a plating process in the mandrel bar manufacturing process, and requires a great investment.

  The present invention overcomes the drawbacks of the prior art described above, and does not require a rolling schedule in which a low alloy steel is interrupted in rolling of a steel type containing a large amount of alloy elements, so that the rolling schedule is not complicated and the profitability is not deteriorated. In addition, an object of the present invention is to provide a mandrel bar manufacturing apparatus row and a manufacturing method that can extend the tool life of the mandrel bar without requiring a large amount of investment without plating.

  The present inventors diligently studied to solve the above-mentioned problems, and as a result, when performing tempering (also referred to as final heat treatment) which is the final stage of the heat treatment process (quenching-tempering) used for manufacturing the mandrel bar. In the past, the material to be treated was heated to a tempering temperature and then allowed to cool to 100 ° C. or lower. However, instead of allowing to cool, accelerated cooling at a specific cooling rate allows the mandrel bar to have a hardness (Vickers hardness). To improve the toughness value while maintaining the thickness HV), suppress fatigue progress and unstable progress in the circumferential direction, improve the tool life, and improve the inner surface quality of the steel pipe product. Made.

  That is, the present invention provides a mandrel bar manufacturing apparatus array in which a bar material, which is a semi-finished product of a mandrel bar shape, is heated to a tempering temperature and then acceleratedly cooled to 100 ° C. or less to form a mandrel bar. A tempering furnace that heats to a temperature and an accelerated cooling device that sets a cooling rate of the bar material extracted from the tempering furnace from the tempering temperature to 100 ° C to 10.0 ° C / min or more are arranged in this order. FIG.

  The cooling rate of the accelerated cooling is preferably 10.0 ° C./min or more and 35 ° C./min or less.

  Further, the present invention provides a mandrel bar manufacturing method in which a bar material which is a mandrel bar-shaped semi-finished product is heated to a tempering temperature and subsequently cooled to 100 ° C. or lower to form a mandrel bar. A method for producing a mandrel bar, wherein accelerated cooling is performed at a cooling rate of 10.0 ° C./min or more. In the production method, the cooling rate of accelerated cooling is preferably 10.0 ° C./min or more and 35 ° C./min or less.

  According to the present invention, as the apparatus row used in the tempering process, which is the final stage of the mandrel bar manufacturing process, the apparatus row that uses the accelerated cooling as the cooling from the tempering temperature is adopted, so that the profitability is deteriorated or greatly increased. Increases toughness value while maintaining mandrel bar hardness without investment, suppresses fatigue and unstable progress of circumferential cracks, improves tool life, and improves internal quality of steel pipe products There is an effect that can be made.

It is the schematic which shows an example of the manufacturing apparatus row | line | column of the mandrel bar which concerns on this invention. It is a graph which shows the relationship between the distance from the mandrel bar surface in a mandrel rolling process, and the circumferential direction tensile stress which acts at the time of a mandrel bar water cooling. It is a schematic diagram which shows the stress value in the tensile stress generation | occurrence | production part of a mandrel bar found by the mandrel rolling load analysis. It is a graph which shows the summary of the reproduction | regeneration thermal cycle provision experiment result. It is a diagram which shows the result of having analyzed the surface stress state of the bar raw material in the mist cooling from tempering temperature.

  Hereinafter, the operational effects and embodiments of the present invention will be described.

  FIG. 1 is a schematic view showing an example of a mandrel bar manufacturing apparatus line according to the present invention. In FIG. 1, 1 is a mandrel bar, 10 is a bar material, 20 is a tempering furnace, and 22 is an accelerated cooling device. The bar material 10 before being charged into the tempering furnace 20 is one that has been subjected to quenching treatment after external polishing at a bar treatment plant. The quenching process is, for example, a process of heating to 1000 to 1130 ° C. by induction heating and then cooling to room temperature.

  The mandrel bar manufacturing apparatus row according to the present invention includes a tempering furnace 20 that heats the bar material 10 after the tempering treatment to a tempering temperature, and a cooling of the bar material 10 extracted from the tempering furnace 20 from the tempering temperature to 100 ° C. An acceleration cooling device 22 that makes the speed 10 ° C./min or more is arranged in this order. The bar material 10 exiting the acceleration cooling device 22 becomes the mandrel bar 1 as the final product.

  The accelerated cooling apparatus 22 illustrated in FIG. 1 is a refrigerant spraying apparatus, but may be an underwater immersion apparatus. However, the underwater immersion type has a disadvantage that the bar material 10 is likely to be warped.

  The refrigerant spraying method may be either water spray cooling or water shower cooling using only water as the refrigerant, or may be mist cooling using an air-water mixed fluid as the refrigerant. In order to cool the bar material 10 during the coolant blowing evenly in the circumferential direction, it is preferable to rotate the bar material 10 around the axis of the bar material by a turning roller (not shown).

  The bar material 10 after leaving the tempering furnace 20 and entering the accelerated cooling device 22 is allowed to cool, but if this cooling time is too long, the tempering temperature from the tempering temperature to 100 ° C. is achieved even with the accelerated cooling device 22. Since it becomes difficult to set the cooling rate to 10.0 ° C./min or more, the tempering furnace 20 and the accelerated cooling device 22 are arranged so that the bar material transfer time from the former to the latter is 5 minutes or less. It is preferable that the bar material transfer time is 1 minute or less.

  The mandrel bar which is the production target of the present invention is an inner surface tool used in the mandrel rolling method.

  The steel type of the material forming the mandrel bar is the SKD6 or the SKD61 as in the conventional case.

  On the surface of the mandrel bar being used for the mandrel rolling, when the number of rolling is increased, cracks occur in the circumferential direction, causing the occurrence of flaws on the inner surface of the steel pipe. As a result of detailed examination of the tool damage mechanism, the present inventors have elucidated the mechanism and have come up with a method for suppressing the occurrence of cracks.

  In the mandrel rolling process, the mandrel bar receives heat when it is inserted into the raw tube formed by punching a round steel piece heated to a high temperature (about 1000 to 1300 ° C.), and then is extracted and then water Is cooled rapidly. At that time, a temperature difference is generated in the depth direction from the mandrel bar surface, and a thermal stress is generated. In order to quantitatively elucidate the load on the mandrel bar due to the thermal stress, the thermal stress during water cooling of the mandrel bar was analyzed using a finite element method.

  An example of the result of this analysis is shown in FIG. FIG. 2 is a graph showing the relationship between the distance from the mandrel bar surface and the circumferential tensile stress that acts during water cooling of the mandrel bar. As shown in FIG. 2, it was found that a tensile stress of about 690 MPa was generated on the mandrel bar surface during water cooling. On the other hand, it was found that the depth at which a high tensile stress of about 150 MPa or more occurs was as shallow as about 300 μm from the surface.

  In addition, in order to quantitatively evaluate the influence of the rolling load load in the mandrel rolling process, a mandrel rolling load load analysis was performed using a finite element method. As a result, as shown in FIG. 3, the mandrel bar 1 during the mandrel rolling process of the material 5 to be rolled in cooperation with the roll (hole type roll) 2, the vicinity of the roll flange portion 3 forming the roll bottom dead center 4. In addition, it was found that a tensile stress generating portion 6 which is a region where a high tensile stress is generated in the longitudinal direction in which the circumferential crack is developed is formed. Further, the tensile stress generating portion 6 is formed in a range from the surface of the mandrel bar 1 to a depth of about 15 mm, and the stress value there is more alloy elements than in the case of general steel (JIS SS400 equivalent steel type). Like steel grades (13Cr steel etc.), steel grades with higher deformation resistance showed larger values.

  From these facts, the mechanism of the occurrence of large cracks in the circumferential direction of the mandrel bar is that cracks originate from the surface within a range from the surface to a depth of about 300 μm. Unstable fracture in which the fracture progresses by fatigue due to the longitudinal tensile stress acting during rolling, and when the developed crack (fatigue crack) reaches a certain length, the fracture toughness value of the material cannot withstand and the brittle crack progresses It is thought that this is a mechanism that cracks greatly. Therefore, in order to suppress the occurrence of large cracks in the circumferential direction of the mandrel bar by such a mechanism, it is possible to improve the toughness value and fracture toughness value, to limit the growth rate of fatigue cracks and to limit fracture leading to unstable fracture It is necessary to improve the stress value. On the other hand, since the mandrel bar is worn due to relative sliding friction with the steel pipe during mandrel rolling, it is necessary to maintain the hardness.

  According to the present invention, in the tempering process that is the final stage of the mandrel bar manufacturing process, the cooling from the tempering temperature is the accelerated cooling, so that the toughness can be improved while maintaining the hardness.

  Next, the reason for limiting the cooling rate of the accelerated cooling according to the present invention will be described. It is the same as usual in that the SKD6 and SKD61 (see Table 1), which are hot tool steels, are used as the steel material of the bar material. These materials are made by adding V, Mo, Mn, Cr, or the like as alloy elements, and the hot strength is increased by precipitating them as carbides in the course of the final heat treatment. These carbides change the kind of precipitation depending on the tempering temperature at the time of tempering, and exhibit characteristics corresponding thereto. The mandrel bar employs a high tempering temperature of 625 ° C. or higher from the viewpoint of emphasizing the toughness value. The tempering temperature is set so that the hardness at the center of the mandrel bar constant-diameter section is the target (Vickers hardness HV400 to 550). If the tempering temperature is 625 ° C. or higher, the hardness does not exceed HV550. On the other hand, in order to avoid the hardness from being less than HV400, the tempering temperature is set to 750 ° C. or less, preferably 700 ° C. or less.

  However, in tool steels such as SKD6, 61, when S and P cause grain boundary segregation in the vicinity of 400 to 550 ° C., or the secondary hardening by precipitation of carbides increases the hardness and decreases the toughness. There is an area. In the cooling process from the tempering temperature, the mandrel bar has a low toughness because it takes a long time to pass through a temperature range of 400 to 550 ° C. in the conventional cooling (natural cooling in the atmosphere). there is a possibility. That is, it is necessary to set the cooling rate to be less than the time required for forming the factor causing embrittlement.

  In order to confirm the existence of such a lower limit of the cooling rate, a reproducible heat treatment experiment was conducted, and a plurality of tempering temperatures were selected from the normal range for the test pieces of the two steel types SKD6 and SKD61. The cooling history was evaluated for multiple levels selected from the range of cooling to water cooling. In the evaluation method, Charpy test pieces and Vickers hardness test pieces were collected from the test pieces after the reproducible heat treatment experiment and tested. In the Charpy test, the toughness value (impact absorption energy) was measured at a test temperature of 25 ° C. The results are summarized in a graph and shown in FIG.

  In FIG. 4, the horizontal axis is the cooling rate CR (= [tempering temperature−100 ° C.] / Cooling time from the tempering temperature to 100 ° C.) between the tempering temperature and 100 ° C., and the vertical axis is the toughness value E, Vickers hardness. There are two types of HV, and both vertical and horizontal axes are arbitrary scales. From Fig. 4, HV maintains approximately the same level in the CR range for both of the two steel types, while E is low E level on the low CR side and CR on the high CR side at CR = 10.0 ° C / min. Therefore, the lower limit of the cooling rate corresponding to the time required to pass through the temperature range of 400 to 550 ° C. being lower than the time required for segregation formation is CR = 10.0 ° C. within the CR region. It can be said that it exists at / min.

  Based on the experimental results, in the present invention, the cooling rate of the accelerated cooling is limited to 10.0 ° C./min or more between the tempering temperature and 100 ° C. This limiting requirement should be satisfied over the entire cross section of the bar material. The slowest cooling rate in the cross section is the center of the cross section (specifically, the center of the cross section of the constant diameter portion which is the remaining length excluding the sharp tip of the bar material). If the CR in the part is 10.0 ° C./min or more, the above-mentioned limiting requirement is satisfied.

  The accelerated cooling execution means may be water cooling (cooling only with water such as water spray, water shower, or immersion in water) or mist cooling (cooling with an air-water mixed fluid). However, if the cooling rate is excessive (for example, over 50 ° C./min, in some cases over 35 ° C./min), the desired material can be obtained, but warping or the like tends to occur due to the residual stress on the compression side after cooling is completed. Therefore, the cooling rate of the accelerated cooling is preferably CR ≦ 35 ° C./min.

  In the final tempering process at the time of manufacturing the mandrel bar, cooling from the tempering temperature (650 ° C.) to the bar material (length of constant diameter = 22000 mm, diameter = 100 mmφ) of steel type A shown in Table 2 is allowed to cool, water cool (Here, water shower cooling) and mist cooling were used. In each of the water cooling and the mist cooling, the bar material extracted from the tempering furnace is supported by a turning roller, and the coolant is blown onto the surface of the bar material from the dedicated nozzles arranged above while rotating around the bar axis. Form. A toughness value measurement by a Charpy test (test temperature = same as above) and a Vickers hardness measurement were performed using a test piece taken from the center of the cross section of the mandrel bar after the cooling. The results are shown in Table 3. Table 3 shows the toughness value E and the Vickers hardness HV, both in terms of percentages with respect to the case of cooling. The CR in Table 3 (the tempering temperature at the center of the constant diameter section to the cooling rate between 100 ° C.) is the material property value and the surface heat transfer coefficient so as to output a calculated temperature with a very good agreement with the measured temperature. It is a calculated value calculated using a heat transfer calculation model that has been optimized.

  From Table 3, it can be seen that, in both water cooling and mist cooling, which are examples of the present invention, the toughness value can be improved to 120 to 140% while maintaining the hardness as compared with the conventional cooling.

  FIG. 5 is a diagram showing the result of analyzing the surface stress state of the bar material during the mist cooling of the example of the present invention. From FIG. 5, it can be seen that in mist cooling, the stress is about 85 MPa at the maximum, and no residual stress is generated on the mandrel bar surface after cooling.

  The mandrel bars (15 in total) manufactured by the mist cooling of the example of the present invention were repeatedly used as an inner surface tool for mandrel rolling in an actual production line of seamless steel pipes. As a result of investigating their tool life (the number of repeated use from the start of use until they become unusable), it is in the range of 121-201% compared to the conventional example, and the durability of the mandrel bar is improved by the present invention than before. It was confirmed that

1 Mandrel bar 2 Roll (hole type roll)
3 Roll flange part 4 Roll bottom dead center 5 Rolled material 6 Tensile stress generating part 10 Bar material 20 Tempering furnace 22 Accelerated cooling device

Claims (4)

  A mandrel bar manufacturing apparatus line that heats a bar material, which is a semi-finished product of a mandrel bar shape, to a tempering temperature, and continues to accelerate cooling to 100 ° C. or lower to form a mandrel bar, wherein the bar material is heated to the tempering temperature. A mandrel bar characterized in that a furnace and an accelerated cooling device for setting the cooling rate of the bar material extracted from the tempering furnace from the tempering temperature to 100 ° C to 10.0 ° C / min or more are arranged in this order. Device column.   The mandrel bar manufacturing apparatus row according to claim 1, wherein a cooling rate of the accelerated cooling is 10.0 ° C./min or more and 35 ° C./min or less.   In the manufacturing method of a mandrel bar in which a bar material which is a mandrel bar-shaped semi-finished product is heated to a tempering temperature and subsequently cooled to 100 ° C. or lower to form a mandrel bar, the cooling rate from the tempering temperature to 100 ° C. is set to 10.0. A method for producing a mandrel bar, characterized by performing accelerated cooling to at least ° C / min.   The method for producing a mandrel bar according to claim 3, wherein a cooling rate of the accelerated cooling is set to 10.0 ° C / min or more and 35 ° C / min or less.

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