JP2007321178A - Method for cooling steel tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for cooling a steel tube in which glow deflection generated when quenching a thin steel tube having e.g. ≤0.07 (t/D) ratio of thickness to outer diameter, can effectively be restrained without lowering the producing efficiency. <P>SOLUTION: The inner surface of the steel tube 2 is cooled by spraying cooling water into the inner part while rotating the steel tube 2 set horizontally in the circumferential direction and also, the outer surface thereof is cooled by flowing down the plane-like cooling waters 5a,5b from the upper part onto the outer surface along the axial direction of the steel tube 2. At this time, the cooling of the outer surface is performed by flowing down the plane-like cooling waters 5a,5b to almost symmetrical two positions 4a,4b centering the most upper part of the steel tube 2, respectively. Then, the flow of the cooling water 5a which flows down to the position at the upstream side in the rotating direction of the steel tube 2, is more than that of the cooling water 5b which flows down to the position at the downstream side in the rotating direction, and the ratio of the thickness to the outer diameter of the steel tube to be cooled, is ≤0.07 and also, the cooling of the inner surface is started by preceding ≥7 sec before the cooling of the outer surface. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for cooling a steel pipe that can effectively suppress bending that occurs when quenching a thin-walled steel pipe.

  As is well known, when a steel pipe is quenched, bending (curving in the axial direction of the steel pipe, referred to as “bending” in this specification) may occur due to uneven cooling. In particular, when a thin steel pipe having a small thickness (t / D) ratio (t / D) to the outer diameter (D) of, for example, about 0.07 or less is quenched, a large bending that is judged as a poor quality occurs. easy. For this reason, many inventions related to a method for cooling a steel pipe that can suppress this bending have been proposed.

  For example, Patent Document 1 discloses a cooling method (heat treatment) that suppresses bending by performing normal strong cooling after reducing the temperature difference of each part of the surface of the tube by performing gentle cooling at the initial stage of cooling the outer surface of the tube. Invention) is disclosed.

Further, in Patent Document 2, in a method of blowing jet water into one end of a pipe from one end and cooling the outer surface side of the pipe by jet water which is jetted from a nozzle over almost the entire length, a discharge end of inner surface jet water is disclosed. An invention that cools the entire pipe uniformly and in a short time by increasing the amount of jet water for external surface cooling, increasing the cooling start timing of external cooling, or delaying the cooling end timing of external cooling toward the side It is disclosed.
Japanese Patent Publication No.2-7372 Japanese Patent Publication No. 61-4896

  However, in the invention disclosed in Patent Document 1, since it is necessary to perform slow cooling in the initial stage of cooling and only the outer surface of the pipe is cooled, the cooling time is inevitably increased, and the manufacturing efficiency of the pipe is reduced. .

  Further, in the invention disclosed in Patent Document 2, since it is necessary to vary the amount of water for jetting outer surface cooling and the timing of jetting in the axial direction of the pipe, the configuration and control of the apparatus are complicated. Further, although it is disclosed that the entire tube can be cooled uniformly, it is not specifically disclosed whether or not the bending can be suppressed. In particular, the ratio (t / D) of the steel pipe exemplified as the object to be cooled is about 0.075 (= 8.6 / 114) (see Patent Document 2, page 2, right column, line 13). It has not been.

  As a result of intensive studies in order to solve the above-mentioned problems, the inventors of the present invention have a cooling method in which both the inner and outer surfaces of the steel pipe are cooled. (A) The cooling of the outer surface of the steel pipe is substantially centered on the uppermost portion of the steel pipe. The flow rate of the cooling water is caused to flow down to the position on the upstream side in the rotation direction of the steel pipe, and the flow rate of the cooling water to flow down to the position on the downstream side in the rotation direction. (B) The ratio of the wall thickness to the outer diameter of the steel pipe to be cooled is 0.07 or less, and (c) the cooling of the inner surface of the steel pipe is started 7 seconds or more before the cooling of the outer surface of the steel pipe. As a result, even if it is a thin-walled steel pipe having a ratio (t / D) of 0.07 or less, it has been found that it is possible to effectively suppress the bending without reducing the manufacturing efficiency of the steel pipe, and the present invention has been completed.

  The present invention cools the inner surface of the steel pipe by injecting cooling water into the steel pipe while rotating the steel pipe arranged in the circumferential direction, and at the same time, cools the outer surface of the steel pipe along the axial direction of the steel pipe. A cooling method for a steel pipe that cools the outer surface of the steel pipe by flowing water from above, and the cooling of the outer surface of the steel pipe flows down to two substantially symmetrical positions around the uppermost portion of the steel pipe. The ratio of the wall thickness to the outer diameter of the steel pipe to be cooled is set so that the flow rate of the cooling water flowing down to the upstream position in the rotation direction of the steel pipe is equal to or higher than the flow rate of cooling water flowing down to the downstream position in the rotation direction. Is 0.07 or less, and the cooling of the inner surface of the steel pipe is started 7 seconds or more before the cooling of the outer surface of the steel pipe.

  By the steel pipe cooling method according to the present invention, the bending that occurs when quenching a thin steel pipe having a ratio (t / D) of, for example, 0.07 or less is effectively suppressed without reducing the manufacturing efficiency of the steel pipe. Will be able to.

The best mode for carrying out the method for cooling a steel pipe according to the present invention will be described in detail with reference to the accompanying drawings as appropriate.
FIG. 1 is a longitudinal sectional view schematically showing a configuration of a cooling device 1 for carrying out the steel pipe cooling method of the present embodiment.

  As shown in FIG. 1, the cooling device 1 according to the present embodiment supports a horizontally disposed steel pipe 2 and rotates rollers 3 and 3 for rotating in the circumferential direction, and is disposed on one end side of the steel pipe 2. An inner surface cooling nozzle (not shown) for injecting cooling water into the inside of the steel pipe 2, and an uppermost portion of the outer peripheral surface of the steel pipe 2 along the axial direction of the steel pipe 2. An outer surface cooling nozzle 7 having discharge ports 6a and 6b for flowing down the planar cooling waters 5a and 5b is provided at two positions 4a and 4b, which are substantially targets as the center.

  In the steel pipe 2 whose inner and outer surfaces are cooled by the cooling method according to the present embodiment, the ratio of the wall thickness t to the outer diameter D (t / D) is likely to be large, which causes a problem of quality. It is a thin-walled steel pipe that is 07 or less. The cooling method according to the present embodiment can cool the inner and outer surfaces of a line pipe made of low-carbon steel that is low in strength and easily bends, or API standard X60 grade (for example, in mass%, (a) C: 0. 0.06%, Si: 0.26%, Mn: 1.24%, P: 0.013%, S: 0.001%, Cr: 0.16%, V: 0.06%, balance Fe and impurities Ceq: 0.311%, or (b) C: 0.06%, Si: 0.40%, Mn: 1.60%, P: 0.020%, S: 0.003%, Cu: 0.30%, Ni: 0.50%, Cr: 0.28%, Mo: 0.23%, V: 0.08%, balance Fe and impurities, Ceq: 0.498%) It can be particularly suitably used for cooling the inner and outer surfaces.

The cooling method according to the present embodiment is particularly suitable because it can effectively suppress the occurrence of bending, particularly for a long steel pipe 2 of 20 m or longer.
When the steel pipe 2 is cooled by the cooling device 1 according to the present embodiment, first, the steel pipe 2 is rotated in the circumferential direction by rotating the rotary rollers 3 and 3 in the arrow direction. Then, cooling of the inner surface of the steel pipe 2 is started by spraying cooling water from an inner surface cooling nozzle (not shown), and then the cooling water 5a and 5b are respectively discharged from the discharge ports 6a and 6b of the outer surface cooling nozzle 7 to the outer periphery of the steel pipe 2. The cooling of the outer surface of the steel pipe 2 is started by flowing down toward the surface.

  At this time, if the rotational speed of the steel pipe 2 is less than 30 rpm, the quenching state in the circumferential direction of the steel pipe 2 tends to fluctuate. On the other hand, the equipment is increased in size and complexity in order to make the rotational speed of the steel pipe 2 exceed 80 rpm. Therefore, it is desirable that the rotation speed of the steel pipe 2 is 30 rpm or more and 80 rpm or less.

  Further, if the amount of cooling water injected from the inner surface cooling nozzle to the inner surface of the steel pipe 2 is less than 2000 t / hr, the cooling capacity is insufficient. On the other hand, if the amount exceeds 6500 t / hr, the equipment becomes larger and complicated, resulting in a lower equipment cost. Therefore, the amount of cooling water sprayed from the inner surface cooling nozzle to the inner surface of the steel pipe 2 is desirably 2000 t / hr or more and 6500 t / hr or less.

In the cooling method of the present embodiment, the cooling of the inner surface of the steel pipe 2 is started 7 seconds or more before the cooling of the outer surface of the steel pipe 2. The reason for this will be explained.
FIG. 2 is a graph showing the results of numerical calculation of the surface temperature, yield stress YS and axial stress sz of the steel pipe 2 when the inner and outer surfaces of the steel pipe 2 are cooled, and FIG. FIG. 2 (b) shows the result when only cooling of the inner surface of the steel pipe 2 is performed (internal lead ∞ seconds). 2A and 2B, the results shown in the graph are as follows: the outer diameter of the steel pipe 2: 412.3 mm, the wall thickness: 8.30 mm, the length: 30 m, the material: low carbon steel, the inner surface cooling cooling water is sprayed from the nozzle to the inner surface of the steel pipe 2: 5400 m ³ / hr, the amount of cooling water injected from the outer surface cooling nozzle 7 to the outer surface of the steel pipe 2: 2700 m ³ / hr, and, rotational speed steel 2: 65rpm conditions Asked.

  2A, when the cooling of the inner and outer surfaces of the steel pipe P is started simultaneously, the thermal expansion of the steel pipe 2 in the initial stage after the start of cooling, that is, the stage where the surface temperature of the steel pipe 2 is 550 ° C. or higher. And axial stress caused by shrinkage (axial stress in the region indicated by symbol A in the graph of FIG. 2A), bainite transformation and martensitic transformation after the surface temperature of the steel pipe 2 drops below 550 ° C. The absolute value | sz | is greater than the absolute value | YS | of the yield stress with respect to the axial stress generated by the influence of the stress (axial stress in the region indicated by symbol B in the graph of FIG. 2A). Exists.

  On the other hand, as shown in the graph of FIG. 2 (b), when only the inner surface cooling of the steel pipe 2 is performed, the axis is always maintained from the start to the end of cooling, that is, until the surface temperature of the steel pipe 2 decreases to room temperature. The absolute value of directional stress | sz | <the absolute value of yield stress | YS |.

  The reason for this is that, compared to the outer surface cooling in which only the portions where the planar cooling waters 5a and 5b flow down are instantaneously cooled, the inner surface cooling can be performed substantially uniformly over the entire circumference of the steel pipe 2, so It is considered that temperature unevenness hardly occurs and variation in axial stress sz becomes small.

  Furthermore, as a result of actually performing a cooling test of the steel pipe 2 under the same conditions as those set for obtaining the results shown in the graphs in FIGS. 2A and 2B, simultaneous cooling of the inner surface and the outer surface is performed. In the case of performing the heat treatment, a large bending occurred, whereas in the case where only the inner surface cooling was performed, a large bending causing a problem did not occur.

  From the results shown in the graphs of FIGS. 2A and 2B described above and the results of the cooling test, the bending of the steel pipe 2 becomes the axial stress | sz |> the absolute value of yield stress | YS |. It is thought to occur. Therefore, in order to suppress the bending of the steel pipe 2, the steel pipe 2 may be cooled so that the relationship | sz | <| YS | always holds. Note that the relationship | sz | <| YS | is always established by performing only the inner surface cooling shown in FIG. 2B. However, the cooling capacity per unit time for the steel pipe 2 is insufficient due to the inner surface cooling alone. As a result, the manufacturing efficiency of the steel pipe 2 decreases, or the steel pipe 2 cannot be sufficiently cooled due to the effect of recuperation from the steel pipe 2.

  Therefore, in the present embodiment, not only the inner surface of the steel pipe 2 but also the outer surface is cooled from the viewpoint of preventing reduction in production efficiency, etc. | at least in the initial stage of cooling when the surface temperature of the steel pipe 2 is 550 ° C. or higher. In order to establish the relationship | <| YS |, it is effective to precede the inner surface cooling of the steel pipe 2 before the outer surface cooling. Specifically, by setting the preceding time to 7 seconds or more, the relationship | sz | <| YS | can be maintained in almost all the cooling processes of the steel pipe 2.

  For the reason described above, in the present embodiment, the cooling of the inner surface of the steel pipe 2 is started 7 seconds or more before the cooling of the outer surface of the steel pipe 2, that is, the cooling water starts to be injected from the inner surface cooling nozzle. The timing | sz | <| YS | is maintained in substantially all of the cooling process by making the timing 7 seconds or more earlier than the timing at which the cooling water 5a, 5b starts to flow down from the outer surface cooling nozzles 6a, 6b. Thereby, the bending of the steel pipe 2 can be effectively and reliably suppressed.

If the preceding time is more than 30 seconds, it takes a long time to cool the steel pipe 2 and the operation efficiency is lowered. Therefore, the preceding time is preferably 30 seconds or less.
Here, in order to increase the cooling efficiency of the outer surface of the steel pipe 2, it is conceivable to increase both the flow rates of the cooling waters 5a and 5b flowing down from the discharge ports 6a and 6b, respectively. However, if both the flow rates of the cooling waters 5a and 5b are excessively increased, the thickness of the water film accumulated on the outer surface of the steel pipe 2 and between the positions 4a and 4b where the cooling waters 5a and 5b flow down is more than necessary. As a result, the effective utilization rate of the cooling water (the ratio of the cooling water that truly contributes to the cooling of the steel pipe 2) decreases, and the flow of the cooling water in the rotational direction of the steel pipe 2 rather deteriorates.

  Further, the cooling water 5a that flows down to the upstream side position 4a in the rotation direction of the steel pipe 2, that is, most of the cooling water 5a that flows down from the discharge port 6a flows in the rotation direction along the outer surface of the steel pipe 2. The cooling water 5b that flows down to the position 4b on the downstream side in the rotation direction, that is, the cooling water 5b that flows down from the discharge port 6b has a portion that flows against the rotation direction of the steel pipe 2, but immediately after most of it flows down. It will flow down. That is, as the degree of contribution to the cooling capacity of the outer surface of the steel pipe 2, the cooling water 5a is larger than the cooling water 5b.

  Therefore, in the present embodiment, as the entire cooling water to be flowed down, the cooling water 5a flowed to the position 4a on the upstream side in the rotation direction of the steel pipe 2 is cooled to flow to the position 4b on the downstream side in the rotation direction of the steel pipe 2. It sets so that it may become more than the flow volume of water 5b.

  As a result, the amount of cooling water flowing in the rotation direction along the outer surface of the steel pipe 2 can be increased, and the outer surface of the steel pipe 2 is accumulated between the positions 4a and 4b where the cooling water 5a and 5b flow down. The water film can be set to an appropriate thickness, whereby the cooling efficiency of the outer surface of the steel pipe 2 can be further enhanced.

Further, when the angle θ formed by the position 4a where the two rows of cooling water 5a and 5b collide with the steel pipe 2 and the center of the steel pipe 2 is less than 12 degrees, a region where a water film is formed on the surface of the steel pipe 2 On the other hand, when the angle θ exceeds 95 degrees, it means that the outer diameter of the steel pipe 2 is extremely large and is not practical. Therefore, it is desirable that the angle θ is 12 ° or more and 95 ° or less. Thus, according to the present embodiment, the bending that occurs when quenching a thin steel pipe P with a ratio (t / D) of, for example, 7.0% or less, in the same lot without reducing the manufacturing efficiency of the steel pipe. The maximum full length bend can be effectively suppressed to 21 mm / 10 m or less, for example.

Furthermore, the present invention will be described more specifically with reference to examples.
Using the cooling device 1 shown in FIG. 1, the API standard X60 grade (mass%, C: 0.06) having the outer diameter D, the wall thickness t, the ratio (t / D) and the length shown in Table 1. %, Si: 0.26%, Mn: 1.24%, P: 0.013%, S: 0.001%, Cr: 0.16%, V: 0.06%, balance Fe and impurities, Ceq The steel pipe 2 of 0.311%) was cooled at the rotational speed of 60 rpm, and was cooled at the inner surface flow rate, the outer surface total flow rate, the inner surface leading time, the outer surface cooling water interval, and the angle θ as shown in Table 1.

Separately from this, the steel pipe 2 was cooled with a row of cooling water flowing down to the outer surface of the steel pipe 2.
And the amount of bending (mm / 10m, the bending amount in the same heat treatment lot in the same heat treatment lot was measured in the steel pipe 2 after cooling. The value converted into a quantity) and the maximum transition temperature vts in the Charpy impact test (maximum values measured at four locations in the circumferential direction of the steel pipe) were measured.

  And, when the bending amount is 10 mm or less, ◎, when the bending amount is more than 10 mm and less than 20 mm, ○, when the bending amount is more than 20 mm and less than 30 mm, Δ, and when the bending amount is more than 30 mm Was marked with x. On the other hand, the case where the maximum transition temperature vts is −40 ° C. or lower is marked as “◯”, the case where the maximum transition temperature vts is above −40 ° C. and below 0 ° C. is marked as Δ, and the case where the maximum transition temperature vts is above 0 ° C. It was. And comprehensive evaluation matched the evaluation of a bad thing among these two types of evaluation.

  The results are shown in Table 1.

  Examples 1 to 7 in Table 1 are examples of the present invention that satisfy all of the conditions defined by the present invention. These comprehensive evaluations are ○ or ◎, and the bending that occurs when quenching a thin steel pipe having a ratio (t / D) of, for example, 0.07 or less can be effectively performed without reducing the manufacturing efficiency of the steel pipe. It turns out that it can suppress.

  On the other hand, Comparative Example 1 performs outer surface cooling by flowing a row of cooling water and simultaneously starts inner surface cooling and outer surface cooling. Therefore, the amount of bending is excessive and the transition temperature is −30 ° C. It was unsatisfactory.

In Comparative Examples 2, 3, 5, and 7, since the outer surface was cooled by flowing down a row of cooling water, the transition temperature was unsatisfactory at −30 ° C.
Although the comparative example 4 performs outer surface cooling by flowing down two rows of cooling water, since the inner surface cooling and the outer surface cooling are started at the same time, the amount of bending is excessive.

  In Comparative Examples 6 and 8, the transition time was unsatisfactory at −30 ° C. because the time for the inner surface cooling to precede the outer surface cooling was as short as 5 seconds and 4 seconds.

It is a longitudinal cross-sectional view which shows typically the structure of the cooling device for enforcing the cooling method of the steel pipe of embodiment. FIG. 2A is a graph showing the results of numerical calculation of the surface temperature of the steel pipe, the yield stress YS, and the axial stress sz when the inner and outer surfaces of the steel pipe are cooled, and FIG. 2A shows the cooling of the inner and outer surfaces of the steel pipe. FIG. 2B shows the result when only the inner surface cooling of the steel pipe is performed (inner surface leading ∞ seconds).

Claims (1)

While rotating the steel pipe arranged horizontally in the circumferential direction, the cooling water is injected into the steel pipe to cool the inner surface of the steel pipe, and the planar cooling water is applied to the outer surface of the steel pipe along the axial direction of the steel pipe from above. A cooling method of a steel pipe that cools the outer surface of the steel pipe by flowing down,
The cooling of the outer surface of the steel pipe is performed by flowing down the planar cooling water at two substantially symmetrical positions around the uppermost part of the steel pipe, and the flow rate of the cooling water flowing down to the upstream position in the rotation direction of the steel pipe. Is greater than or equal to the flow rate of the cooling water flowing down to the position downstream in the rotational direction,
The ratio of the wall thickness to the outer diameter of the steel pipe to be cooled is 0.07 or less,
The method for cooling a steel pipe, wherein the cooling of the inner surface of the steel pipe is started at least 7 seconds before the cooling of the outer surface of the steel pipe.

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