JP5835259B2 - Mandrel bar manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a mandrel bar, and more particularly to a method for manufacturing a mandrel bar which is an inner surface tool (= inner surface tool for mandrel rolling) used in mandrel rolling.

  One of the methods for producing seamless steel pipes is a process called a mandrel rolling method. In this process (= mandrel rolling process), the mandrel bar, which is the inner surface tool, is disposed on the inner surface side in order to reduce the wall thickness of the raw tube in which a round steel piece heated in a heating furnace is perforated with a perforator. In the inserted state, 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 rolled steel pipe.

  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, 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 the hardness and wear resistance.

Japanese Patent No. 4900385 Japanese Patent No. 3395715

  However, in the method described in Patent Document 1, when rolling a steel type containing a large amount of alloy elements, it is necessary to roll low alloy steel in advance, so that the rolling schedule becomes complicated, and low profitability low alloy Since it is necessary to place an order with steel, 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.

  As described above, in the conventional technique, there is a problem that if the tool life of the mandrel bar is extended, the profitability is deteriorated and a great investment is involved.

  The inventors have intensively studied to solve the above problems, and as a result, when performing tempering, which is the final stage of the heat treatment process (quenching-tempering) used in the manufacture of the mandrel bar, After being heated to a specific speed, accelerated cooling at a specific cooling rate improves the toughness value while maintaining the hardness of the mandrel bar, suppresses fatigue progress and unstable progress in the circumferential direction, and improves tool life. As a result, the present inventors have found that the inner surface quality of steel pipe products can be improved.

That is, the present invention is a method for producing a mandrel bar that is subjected to a final heat treatment to a bar material that is a semi-finished product of a mandrel bar shape made of SKD6 or SKD61, which is a hot tool steel . after heating the bar material to 625 ° C. or higher tempering temperature, and facilities the accelerated cooling the cooling rate is 35 ° C. / min or less 10.0 ° C. / min or more from該焼return temperature to 100 ° C., while maintaining the hardness A mandrel bar manufacturing method characterized by improving toughness and preventing warpage of a mandrel bar after completion of cooling .

  According to the present invention, in the tempering process, which is the final stage of the mandrel bar manufacturing process, the cooling from the tempering temperature is the accelerated cooling, so that the mandrel bar can be manufactured without deteriorating profitability and enormous investment. While maintaining the hardness, the toughness value is improved, the fatigue progress and unstable progress of the circumferential crack are suppressed, the tool life is improved, and the inner surface quality of the steel pipe product can be improved.

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.
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 occurs 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 calculated (analyzed) using a finite element method.

  An example of the result of the analysis is shown in FIG. FIG. 1 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. 1, 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. 2, 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, in 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, an embodiment of 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. 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. 3, 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 the vertical and horizontal axes are arbitrary scales. From Fig. 3, Hv maintains approximately the same level in the CR range for both of the two steel grades, 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 to the bar material of steel type A shown in Table 2 (the length of the constant diameter portion = 22000 mm, the diameter = 100 mmφ) is allowed to stand by cooling, water cooling (here, water It was performed by each cooling method of shower cooling) and mist cooling. 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 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. 4 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. 4, it is found that the maximum stress is about 85 MPa in the mist cooling, and no residual stress is generated on the mandrel bar surface after the 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 4 Roll bottom dead center 5 Rolled material 6 Tensile stress generator

Claims (1)

A mandrel bar manufacturing method for subjecting a bar material which is a mandrel bar-shaped semi-finished product made of SKD6 or SKD61, which is a hot tool steel, to a mandrel bar, wherein the bar material is 625 at the time of the final heat treatment. after heating ° C. above the tempering temperature, and facilities the accelerated cooling the cooling rate is 35 ° C. / min or less 10.0 ° C. / min or more from該焼return temperature to 100 ° C., to improve the toughness while maintaining the hardness, And the manufacturing method of the mandrel bar characterized by preventing the curvature of the mandrel bar after completion | finish of cooling .

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