WO2014119251A1

WO2014119251A1 - Manufacturing method and manufacturing equipment for seamless steel pipe or tube with excellent toughness

Info

Publication number: WO2014119251A1
Application number: PCT/JP2014/000297
Authority: WO
Inventors: 鈴木　健史; 江口　健一郎; 石黒　康英
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2013-01-31
Filing date: 2014-01-22
Publication date: 2014-08-07
Also published as: CN104968808B; EP2952592A1; JP2014148699A; EP2952592A4; EP2952592B1; JP5907083B2; US20150368734A1; CN104968808A

Abstract

With the prior art, it is difficult to reduce material variability caused by a non-uniform micro-structure in the thickness direction of a pipe or tube body and to maintain the productivity of an entire thermal treatment line at the same time. Whether or not a pipe or tube body is of a steel grade having an Ms point of lower than 200ºC is determined in advance, and after quenching, a pipe or tube body that is found to have the abovementioned steel grade in the determination results is separately detained in a room-temperature environment (preferably, transferred and detained on a detainment floor (6)) until the temperature difference between the highest-temperature part and the lowest-temperature part in a cross-section orthogonal to the pipe/tube axis becomes smaller than 2.0ºC, and the pipe or tube body is then subjected to a tempering treatment. In contrast, a pipe or tube body that is found not to have the abovementioned steel grade in the determination results is subjected to tempering treatment without being detained.

Description

Manufacturing method and equipment for seamless steel pipe with excellent toughness

The present invention relates to a manufacturing method and manufacturing equipment for a seamless steel pipe excellent in toughness. This is especially because the stainless steel (stainless steel) has a low Ms point (= martensitic transformation start temperature) and Mf point (= martensitic transformation finish temperature). Products with excellent toughness by applying quenching and tempering, which is a thermal refining, to pipes, which are the intermediate products (semimanufactured products) of seamless steel pipes of the following steel types It is used to obtain a tube.

Here, “excellent toughness” means, for example, a material satisfying ISO standard 13680. That is, absorbed energy at the test temperature = −10 ° C. measured in the pipe circumferential direction (C direction, reverse test piece) Charpy impact test of the central part of wall thickness of the product pipe ( absorbed energy) (symbol: vE ₋₁₀ ) is an average of 40 J or more for 3 specimens, and less than 40 J is less than 1 out of 3 and the value is 27 J or more (required value of 40 J) 2/3 or more).

The following are examples of conventional technologies related to the production of seamless steel pipes.

Patent Document 1 discloses a technique for obtaining a high-strength and high-toughness product by defining a heating temperature and a cooling rate during quenching heat treatment when manufacturing a seamless steel pipe of a thick 13Cr stainless steel. Yes.

Patent Document 2 discloses a facility for minimizing a decrease in productive efficiency when processing a steel type that cannot increase the quenching cooling rate. However, the heat treatment sequence itself is first-in first-out (first-in, first-out) unless there is a trouble.

Patent Document 3 discloses a method of manufacturing a seamless steel pipe of martensite-ferritic duplex steel.

In Patent Document 4, in the quenching method in which quenching liquid flows in one direction on the inner surface of the pipe, the flow rate of the liquid is controlled based on the measured liquid temperature on the inflow side and the outflow side. A technique for reducing the variation in the hardness of the tube in the longitudinal direction is disclosed.

JP 2008-189945 A JP 2009-242863 A JP 2005-336595 A JP 2001-032022 A

Pipes that are in the process of manufacturing seamless steel pipes of steel grades such as martensitic stainless steel are hot-worked and then subjected to tube rolling to a predetermined shape (tube rolling), followed by quenching and tempering. By performing the heat treatment, the required strength and toughness level are controlled. In the normal heat treatment, first, in the quenching process, the tube is heated to a temperature of Ac ₁ point or more and Ac ₃ point or less in a heating furnace, and then rapidly cooled to near room temperature by water cooling or the like. In the subsequent tempering process, The tube body after rapid cooling is heated to a temperature of Ac ₁ point or less in another heating furnace and then allowed to cool (for example, see Patent Document 1). Recently, facilities for performing such heat treatment are continuously linable, and treatment conditions such as heating temperature and heating time are set for various product types.

Steel types such as martensitic stainless steel (see Patent Document 1) and martensite-ferritic duplex steel (see Patent Document 3) obtain a desired amount of martensite phase by quenching and tempering. Here, the Ms point and the Mf point vary greatly depending on the steel composition specifying the steel type, and some steel types have an Ms point of less than 100 ° C. and an Mf point of less than room temperature. The temperature of the tube after quenching is usually confirmed by measuring the surface temperature. For steel grades with a low Ms point and low Mf point as described above, the temperature difference between the surface of the tube and the inside of the wall thickness (= inhomogeneous temperature distribution in the wall thickness direction) affects the martensitic transformation ratio. The effect cannot be ignored. That is, even if the surface temperature of the tube after quenching is near room temperature, the temperature distribution in the wall thickness direction (temperature distribution) reaches tempering before reaching a uniform steady state. In other words, an unintentional distribution of the structure occurs, which contributes to material variability (= mechanical property, especially toughness variability) after tempering heat treatment.

On the other hand, the steel grade (referred to as a specific steel grade for convenience) that seeks to obtain the desired martensite phase by quenching and tempering is such that the cooling rate after quenching heating (= heating in the quenching process) is allowed to cool. Since the martensitic transformation itself occurs even at a low cooling rate, the material variation can be reduced by allowing it to stand at room temperature for a sufficient time after cooling to room temperature. However, if heat treatment of a specific steel grade and other different steel grades is performed first-in first-out (for example, refer to Patent Document 2) in the same heat treatment line, it is necessary to leave the specific steel grade at room temperature for a predetermined time or more. Has the problem that the productivity of the entire heat treatment line is lowered.

After all, conventionally, there is known a quenching method and equipment for reducing the variation in the hardness in the longitudinal direction of the pipe body by controlling the flow rate of the quenching liquid (for example, see Patent Document 4). However, as described above, in the heat treatment in which a specific steel type and a different steel type are passed through the same heat treatment line, the variation in material due to the non-uniform microstructure in the tube thickness direction of the specific steel type is reduced. There has been a problem that it is difficult to achieve both the maintenance of the productivity of the entire heat treatment line.

The present inventors diligently studied to solve the above-mentioned problems, and as a result, the tube was discriminated from a steel type having an Ms point of less than 200 ° C. and other steel types. The former was separately cooled after water cooling in quenching. In addition, when placed in a room temperature environment until the temperature difference between the highest temperature portion and the lowest temperature portion in the cross section orthogonal to the tube axis direction (thickness direction) is less than 2.0 ° C., the material variation is significantly reduced, and It has been found that the average value (average value of vE- ₁₀ ) of the data within the variation range of the material is also improved. In the latter case, normal quenching and tempering may be performed. The present invention has been made based on these findings, and the gist thereof is as follows.
(1) In a seamless steel pipe manufacturing method having a step of quenching and tempering a pipe body that is an in-process product of a seamless steel pipe, it is determined in advance whether the pipe body is a steel type having an Ms point of less than 200 ° C. After the quenching, the tube body for which the determination result is correct is left in a separate room temperature environment until the temperature difference between the highest temperature portion and the lowest temperature portion in the cross section perpendicular to the tube axis is less than 2.0 ° C. A method for producing a seamless steel pipe excellent in toughness, characterized in that the tube body that is subjected to the tempering process and is subjected to the tempering process without being placed in the tube body in which the determination result is negative.
(2) In seamless steel pipe manufacturing equipment including equipment for quenching and tempering the pipe, which is an in-process product of seamless steel pipe, it is determined in advance whether the pipe is a steel type with an Ms point of less than 200 ° C. The difference between the highest temperature portion and the lowest temperature portion in the cross section perpendicular to the tube axis is 2 before the tempering of only the tube for which the determination result is correct among the tube body after quenching and the tube body after quenching. A facility for producing seamless steel pipes with excellent toughness, characterized by having a holding bed that is separately kept in a room temperature environment until the temperature becomes below 0 ° C.

According to the present invention, a steel tube having an Ms point of less than 200 ° C. is placed in an excellent toughness tube with a small variation in material by being kept in a separate room temperature environment until sufficiently uniform in the thickness direction after quenching and before tempering. Thus, other steel types are heat-treated in a first-in first-out manner as usual without being obstructed by the indwelling, so that it is possible to produce a seamless steel pipe excellent in toughness while maintaining the productivity of the entire heat treatment line.

FIG. 1 is a schematic plan view showing an example of a heat treatment line used in the present invention.

FIG. 1 is a schematic plan view showing an example of a heat treatment line used in the present invention. Among the pipes 1 that are in the process of manufacturing seamless steel pipes, pipes whose Ms point is determined to be 200 ° C. or higher (also referred to as “A pipe” for convenience) are heated to an appropriate heating temperature depending on the steel type in the quenching furnace 2. After that, the tube is immersed in cooling water in a quenching water tank 3 and is cooled with water until the temperature of the outer peripheral surface of the pipe body decreases to near room temperature. After that, it is tempered at an appropriate tempering temperature depending on the steel type in a tempering heating furnace (heating furnace for tempering) 5 via a cooling bed 4. The Ms point is obtained by calculation using the formula (1) described later.

On the other hand, a pipe body (which is also referred to as a B pipe for convenience) whose Ms point is determined to be lower than 200 ° C. is processed through the same path as the A pipe until it reaches the cooling bed 4. However, only the B pipe is moved to a detainment floor (also referred to as a buffer line), which is a path different from the path of the A pipe. It is left in a room temperature environment until the temperature difference (denoted as ΔT) between the high temperature part and the lowest temperature part becomes less than 2.0 ° C. Thereafter, it is returned to the cooling bed 4 and thereafter tempered in the same path as the A pipe.

In the present invention, the cooling floor 4 and the detention floor 6 are separate facilities. If there is room in the space of the cooling floor 4, a part thereof may be used as a detention floor.

In the present invention, the above-mentioned specific steel types (steel types to obtain a desired amount of martensite phase by quenching and tempering) are, for example, mass%, C: 0.005 to 0.05%, Si: 0.05 to 1.0%, Mn: 0.2 to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 11 to 20%, Ni: 1.5 to 10%, Mo: 1 to 5%, N: 0.15% or less, with the balance being Fe and inevitable impurities. In this composition, instead of a part of Fe, in mass%, Al: 0.002 to 0.05%, Cu: 3.5% or less, Nb: 0.5% or less, V: 0.5 %, Ti: 0.3% or less, Zr: 0.2% or less, W: 3% or less, B: 0.01% or less, Ca: 0.01% or less, REM: 0.01% or less The composition may contain one or more selected from the above.

As described above, in the steel type in which the Mf point is less than room temperature (this is one of the specific steel types), the temperature distribution in the thickness direction of the pipe body at the start of tempering (perpendicular to the pipe axis) The martensite transformation rate at each position in the thickness direction, in other words, the amount of retained austenite (amount of residual austenite) is substantially determined by the temperature distribution in the cross section. In such a temperature distribution, even if the temperature difference ΔT between the highest temperature portion and the lowest temperature portion of the temperature distribution in the thickness direction of the tube is less than 10 ° C., the difference in the amount of retained austenite depending on the thickness direction position of the tube ( The variation is not negligible. This variation in the amount of retained austenite is one of the causes of the material variation of the product.

On the other hand, in this invention, B pipe | tube is detained in room temperature environment until (DELTA) T will be less than 2.0 degreeC. Thereby, the variation in the amount of retained austenite in the tube thickness direction at the start of tempering is remarkably reduced, the material variation of the product after tempering is greatly reduced, and the average value (vE) of the data within the material variation range is reduced. The average value of ₋₁₀ ) is improved. If tempering is started before ΔT is reduced to less than 2.0 ° C., such an effect cannot be obtained. It should be noted that the determination condition for the B tube is that the Ms point: less than 200 ° C. may be practically considered to be equivalent to the fact that the Mf point: less than room temperature. This is based on the results of experiments conducted by the inventors.

In the present embodiment, the Ms point is a thermal expansion curve (thermal) measured in advance by a continuous cooling transformation experiment using thermal expansion test pieces of various compositions for the specific steel type. It is calculated by using the following formula (1) obtained by collecting the experimental data of Ms point from the expansion (curve) and regressing the experimental data with the composition component amount [mass%]. .

Ms [° C.] = 502-810 [% C] -1230 [% N] -13 [% Mn] -30 [% Ni] -12 [% Cr] -54 [% Cu] -6 [% Mo] ( 1)
In the formula (1), [% M] is the component amount of each component element M. Moreover, when there is a component element not contained in steel, 0 shall be substituted for the component element term.

As a specific preferred implementation means, a waiting time (lead time) from the completion of quenching (water cooling completion) to the start of tempering is set for each steel type entering the heat treatment. For this setting, prepare a lead time calculation means (calculation device) that combines the prior grasp of the Ms point by the above formula (1), the measurement of the ambient temperature (ambient temperature) and the surface temperature of the tube, and the heat transfer calculation. Is preferred. For steel pipes with a Ms point of less than 200 ° C. (the B tube), in the normal first-in first-out, the time required to equalize the temperature until the lead time on the cooling bed 4 becomes ΔT <2.0 ° C. Those that do not reach are temporarily released to the buffer line 6 and placed in a room temperature environment until ΔT <2.0, and then subjected to tempering again.

A steel billet showing the chemical composition and Ms point calculated by the formula (1) in Table 1 is formed by hot working, and after the forming, the steel billet is air-cooled from 100 ° C. to room temperature to obtain an outer diameter of 195 10 pipes made of a seamless steel pipe of 0.0 mm × wall thickness 27.0 mm were formed.

As an example of the present invention, the following heat treatment (quenching-tempering) was performed on five (P1 to P5) randomly extracted (P1 to P5) tubes. The heat treatment line shown in FIG. 1 was used. The quenching was performed by heating to 950 ° C. and then cooling with water. The surface temperature (actual value) of the tube at the time of completion of recuperation after water cooling was 30 to 36 ° C. The tube was left at room temperature (in the atmosphere) for 8 hours or longer. When ΔT (calculated value) reached 1.2 to 1.8 ° C., the tube was placed in a tempering furnace and tempered at 600 ° C. .

As a comparative example, the remaining five pipe bodies (P6 to P10) were quenched under the same conditions as in the present invention example, and then time management at ΔT <2.0 ° C. was not performed. First-in-first-out was charged into a tempering furnace and tempered at 600 ° C. In this case, ΔT (calculated value) at the time of charging into the tempering furnace was 6.0 ° C.

From each tube after tempering treatment, V notch test piece (collection position = thickness center of pipe, test piece thickness = 10 mm, test piece length direction = pipe circumference direction (C Direction), V notch depth direction = pipe length direction (L direction) (S1, S2, S3) sampled three times and subjected to Charpy impact test in accordance with the provisions of JIS Z 2242 to obtain vE- ₁₀ It was.

The obtained results are shown in Table 2. From Table 2, in the present invention example, the average value of the vE ₋₁₀ values (N number = 15) = 87.7 J, and there was no test piece less than 40 J. In addition, the standard deviation = 3.8 J, and results with very little variation were obtained. On the other hand, in the comparative example, the average value of vE ₋₁₀ values (N number = 15) = 81.7J. However, two test pieces less than 40 J were generated. In the comparative example, standard deviation = 17.9 J, a decrease in average value and an increase in variation were observed. Looking at the results for each tube, some comparative examples have vE- ₁₀ values that are comparable to those of the present invention. On the other hand, the vE ₋₁₀ value is greatly reduced, which leads to a decrease in average value and an increase in dispersion.

Thus, according to the present invention, stable mechanical properties can be obtained.

DESCRIPTION OF SYMBOLS 1 Tube 2 Quenching heating furnace 3 Quenching water tank 4 Cooling floor 5 Tempering heating furnace 6 Detention floor (buffer line)

Claims

In the method of manufacturing a seamless steel pipe having a step of quenching and tempering a pipe body that is an in-process product of a seamless steel pipe, it is determined in advance whether the pipe body is a steel type having an Ms point of less than 200 ° C., After quenching, the tube body for which the determination result is correct is separately placed in a room temperature environment until the temperature difference between the highest temperature portion and the lowest temperature portion in the cross section orthogonal to the tube axis is less than 2.0 ° C., and then the tempering treatment. On the other hand, a tubular body for which the determination result is negative is a method for producing a seamless steel pipe which is not placed but is subjected to the tempering treatment.
Discriminating means for discriminating whether or not a pipe body is a steel type having an Ms point of less than 200 ° C. in advance in a seamless steel pipe production facility including a facility for quenching and tempering a tubular body that is an in-process product of a seamless steel pipe And a detainment floor separately detained in a room temperature environment until the difference between the highest temperature portion and the lowest temperature portion in the cross section orthogonal to the tube axis is less than 2.0 ° C. before being subjected to the tempering. Manufacturing equipment.