JPH05171294A - Method for cooling welded part of electric resistance welded tube - Google Patents

Abstract

PURPOSE:To improve the low temp. toughness at the welded part of the resistance welded tube by resistance-welding a strip steel composed of a material specifying the thickness and the carbon equivalent and heating and cooling the welded part in a specific condition. CONSTITUTION:The strip steel composed of the material having the carbon equivalent shown in the equation I in the range of 0.25-0.45% is resistant-welded to apply this strip steel to the resistance welded tube having the thickness t=5-17mm. After the welded part of the resistant welded steel pipe is heated at the Ac3 transformation point temp. or higher and <=1050 deg.C, this part is cooled with cooling water in the range of water flow rate Qmin and Qmax shown in the equation II and the equation III. From the time, when the outer surface temp. of the resistant welded steel pipe reaches to 500-400 deg.C, this welded part is cooled with the water flow rate exceeding Qmax until the outer surface temp. becomes <=200 deg.C. The cooling water flow rate is made to l/min.m<2>.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cooling method in a heat treatment for producing an electric resistance welded steel pipe having excellent toughness at a welded portion.

[0002]

2. Description of the Related Art In order to improve the toughness of a welded portion of an electric resistance welded steel pipe, a method of quenching and tempering is used instead of the normalizing treatment which has been generally performed in the past.
-43827 or JP-A-59-153839.
It is disclosed in Japanese Patent Publication No. These heat treatments are aimed at refining the crystal grains by quenching, and are intended to improve the toughness by obtaining a fine ferrite structure. In such a conventional heat treatment method, only the heat treatment conditions are defined by the heating temperature, the cooling start temperature and the cooling rate. Since the heating temperature is generally an induction heating device arranged in series with the mill line, it can be easily realized by adjusting its output, and the cooling start temperature by adjusting the cooling start position. However, regarding the cooling rate at the time of quenching, according to the study of the inventors, since a cooling method is a cooling from one direction on the outer surface side of the pipe as a practical cooling method, a difference in cooling rate occurs between the outer surface side and the inner surface side. It was found that a uniform cooling rate was not obtained in the plate thickness direction, and therefore the target fine ferrite structure could not be obtained uniformly in the plate thickness direction.

The prior art does not show a specific cooling method, even if the cooling rate is presented.
Generally, a normal cooling method cannot obtain a uniform cooling rate in the plate thickness direction. In particular, when the plate thickness exceeds 10 mm, if the cooling water amount is increased in order to secure the cooling rate on the inner surface side of the tube, the cooling rate on the outer surface side of the tube becomes excessively high, and a bainite structure is generated on the outer surface side of the tube to deteriorate the toughness. On the other hand, if the amount of cooling water is reduced in order to suppress the cooling rate on the outer surface side of the tube, the cooling rate on the inner surface side of the tube becomes too small, the structure is not sufficiently refined, and the toughness also deteriorates. As described above, in the normal cooling method, there is a difference in the cooling rate in the plate thickness direction, which causes nonuniformity of the structure on the inner and outer surfaces, resulting in deterioration of toughness. In order to obtain a finer ferrite structure, the appropriate cooling rate differs depending on the plate thickness and the steel type, and therefore the amount of cooling water needs to be appropriately adjusted depending on the plate thickness and the steel type in order to realize the cooling rate.

[0004]

With respect to the above points, the present inventors have conducted experiments on electric resistance welded pipes of various plate thicknesses and steel types to realize a uniform cooling rate in the plate thickness direction and an appropriate cooling rate. As a result of investigating the amount of cooling water for this purpose, the following findings (a) to (d) were obtained. (A) The inner surface of the pipe is cooled by heat conduction mainly due to a decrease in temperature on the outer surface of the pipe. Therefore, the cooling rate is smaller toward the inner surface of the pipe, and there is a time difference in cooling from the outer surface of the pipe.

(B) In the temperature range below 500 ° C. near the Ar ₁ transformation point, the cooling rate has almost no effect on the structure. (C) From the above (a), even at the temperature of 500 ° C. on the outer surface of the pipe, the temperature at the center of the wall thickness and the inner surface of the pipe are higher than that. Therefore, the amount of cooling water can be increased to quickly lower the outer surface temperature. As a result, the cooling rate increases at the central portion of the wall thickness and on the pipe inner surface side, and approaches the cooling rate on the pipe outer surface side.

(D) If the amount of cooling water in the first slow cooling zone is set within the range of the minimum value Q _min or more and the maximum value Q _max or less which is a function of the plate thickness t and the carbon equivalent Ceq, the outer surface temperature is 8
The range from 00 ℃ to 500 ℃ to 400 ℃ is 10 ℃ per second.
The above and the condition of cooling at the cooling rate at which ferrite precipitates can be satisfied. The present invention has been completed based on the above findings, and by improving the cooling process of the welded portion of the electric resistance welded pipe, and by adjusting the amount of cooling water for each plate thickness and steel type, in the thickness direction of the welded portion. The purpose is to optimize the structure and improve the toughness of the weld.

[0007]

The present invention has been made on the basis of the above findings, and the tube thickness t and the carbon equivalent Ceq are t = 5 to 17 mm and Ceq = 0.25 to 0.4, respectively.
It is applied to electric resistance welded steel pipes made of materials within the range of 5%, and consists of the following technical means. That is, continuously formed band steel is electric resistance welded, and subsequently the welded portion is Ac-welded.
_It is heated to a temperature of ₃ transformation points or more and 1050 ° C. or less, and cooled with cooling water within the range of water amount Q _{min to} Q _max shown by the following formula, and when the outer surface temperature reaches 500 ° C. to 400 ° C., Q _A method for cooling a welded portion of an electric resistance welded steel pipe, which comprises cooling the outer surface to 200 ° C. or less with an amount of water exceeding _max .

Q _min = (− 0.00805t ² + 0.204t) (Ceq ^−4.4 −3) (1) Q _max = (− 0.0184t ² + 0.467t) (Ceq ^−4.4 −3) (2) ) However, Q _min , Q _max ; cooling water amount [1 / min. m ² ] t; tube thickness [mm] = 5 to 17 mm Ceq; carbon equivalent [%] = C + (Mn / 6) + (Si / 24) + (Ni / 40)
+ Cr / 5) + (Mo / 4) + (V / 14) = 0.25 to 0.45%

[0009]

The operation of the present invention will be described in detail below. In order to obtain a fine ferrite structure exhibiting high toughness, it is necessary to control the cooling rate of the material from 800 ° C. to 500 ° C. within the range of 10 ° C. or more per second and the precipitation of ferrite. First, in the preceding stage of cooling, the cooling rate of the outer surface of the tube must be within the above range.

(A) When the outer surface of the pipe was heated to 950 ° C. by an experiment, the plate thickness, the amount of cooling water and the outer surface of the pipe of 800 ° C.
To 500 ° C. to the cooling rate was investigated. (B) On the other hand, the ferrite precipitation limit cooling rate, which is the upper limit of the cooling rate, differs depending on Ceq and the heating temperature before cooling.
That is, the larger the Ceq and the higher the heating temperature before cooling, the slower the ferrite precipitation limit cooling rate. Various CC
The relationship between Ceq at the heating temperature of 950 ° C. and the ferrite precipitation limit cooling rate was determined from the T (continuous cooling transformation) curve.

Since a ferrite precipitation limit cooling rate is the most desirable cooling rate for the refinement of the structure, the heating temperature is 950 ° C. by combining the results of the above (a) and (b).
The relationship between the plate thickness, Ceq, and the amount of cooling water Q ₀ for obtaining the ferrite precipitation limit cooling rate in was obtained as an empirical formula as follows. Q ₀ = (− 0.0115t ² + 0.292t) (Ceq ^−4.4 −3) Based on this formula, the heating temperature is 105 from the Ac ₃ transformation point.
Considering the change of the ferrite precipitation limit cooling rate up to 0 ° C,
The Q _max as the heating temperature is the fastest cooling rate when the Ac ₃ transformation point is obtained and set as ^{_{follows, Q max = (- 0.0184t 2}} + 0.467t) (Ceq -4.4 -3) heating temperature When Q is 1050 ° C., Q _min was set according to the following equation so that the slowest cooling rate was obtained.

Q _min = (− 0.00805t ² + 0.204t) (Ceq ^−4.4 −3) It was confirmed that the cooling rate at Q _min was 10 ° C. or more per second regardless of the plate thickness and Ceq. .. That is, the present invention adjusts the amount of water according to the above equations (1) and (2) for each plate thickness and steel type based on the above-mentioned premise knowledge. In addition to the plate thickness and carbon equivalent, heating temperature, water temperature, etc. Considering factors, if the cooling water amount in the slow cooling zone is within this range, the outer surface temperature range from 800 ° C to 500 ° C to 400 ° C is 10 ° C or more per second and less than the cooling rate at which ferrite precipitates. The cooling rate satisfying the condition of cooling with, and a cooling rate suitable for atomization can be obtained.

However, the plate thickness for which the above equations (1) and (2) are effective,
The carbon equivalent range is t = 5 to 17 mm, Ceq =
It is 0.25 to 0.45%. Here, the reason why the plate thickness and the carbon equivalent are limited will be described. (A) The reason why the plate thickness is limited to 5 to 17 mm is that if the plate thickness is less than 5 mm, the heat capacity of the heating portion is small, so in order to obtain an appropriate cooling rate, the cooling water amount must be equal to or less than the cooling water amount shown by the mathematical formula of the present invention. However, if it exceeds 17 mm, the temperature difference between the inner and outer surfaces of the steel pipe at the time of heating becomes large, and it is difficult to make the structure of the entire plate thickness uniform even if multi-stage cooling is used.

(A) Carbon equivalent Ceq is 0.25 to 0.4
The reason for being limited to 5% is that if it is less than 0.25%, the limit cooling rate becomes higher, and the amount of cooling water shown by the mathematical formula of the present invention is required, and if it exceeds 0.45%, proper cooling is achieved. This is because the material does not become a fine ferrite structure even at the speed and the material is different. FIG. 2 shows the relationship between the plate thickness and the amount of cooling water when Ceq = 0.3%. As the plate thickness increases, the heat capacity of the heating part also increases, so the water amount increases with the plate thickness. However, the water amount decreases at the peak around t = 12 to 13 mm because the heating temperature of the outer surface increases as the plate thickness increases. Is increased, and the ferrite formation limit cooling rate is decreased.

FIG. 3 shows Ce when the plate thickness t = 12.7 mm.
The relationship between q and the amount of cooling water is shown. As the Ceq is larger, the hardenability is higher and the ferrite formation limit cooling rate is lower, so it is necessary to reduce the amount of water. Regarding the slow cooling zone length,
The position where the outer surface becomes 500 ° C. to 400 ° C. can be calculated from the cooling rate and the pipe forming rate corresponding to the heating temperature and the amount of water, and can be determined accordingly.

[0016]

EXAMPLE An outline of an electric resistance welded pipe manufacturing facility for carrying out the present invention is shown in FIG. The edge of the continuously formed steel strip 1 is heated by the welding electrode 3, pressed and joined by the squeeze roll 4, and the electric resistance welded steel pipe 9 having the electric resistance welded portion 2 is manufactured. After the electric resistance welded portion 2 is heated to a predetermined temperature by the induction heating devices 5 and 6, the water cooling device 8 cools it from the outer surface side to a predetermined temperature by a predetermined method. Further, when quenching and tempering is performed thereafter, the induction heating device 7 reheats and performs tempering.

Using steel having the chemical composition shown in Table 1,
With the equipment outlined in Fig. 1, we perform electric resistance welding and heat treatment,
Outer diameter 508 mmφ, wall thickness 9.5 mm, 12.
7 mm and 15.9 mm ERW steel pipes were manufactured. Examples of cooling patterns at this time are shown in FIGS. The water amount of the slow cooling zone is determined so that the water amount Q _min or more and Q _max or less obtained from the equations (1) and (2) with respect to each plate thickness t and carbon equivalent Ceq are determined. _The amount of water was set to exceed _max .

In Table 2, the zone is divided into 3 minutes and the amount of water Q _min ,
Q _{max, the} amount of cooling water in each zone and the zone length are shown. (1) The reason for dividing the zone into three is that the cooling rate of 800 to 500 ° C. at each position in the plate thickness direction of the material needs to be 10 ° C. or more per second and the range in which ferrite precipitates, but the outer surface is 500 If the material is rapidly cooled at a temperature of up to 400 ° C., the cooling rate of the surface layer portion of 2 to 3 mm from the outer surface becomes excessive due to the plate thickness and Ceq, which adversely affects toughness. Therefore, in order to keep the cooling rate of this portion at 10 ° C. or more per second and within the range in which ferrite precipitates, a zone having an intermediate flow rate was provided therebetween.

(2) The zone length is shown because when the cooling rate is determined by a certain amount of cooling water, the outer surface is 5 from the start of cooling.
This is because the cooling time required to reach the temperature of 00 to 400 ° C. is required, and in order to realize the cooling time, it is necessary to adjust the zone length according to the line speed. Next, from the welded portion of the electric resistance welded steel pipe manufactured in this way,
10mm x 7.5mm, 2V for wall thickness of 9.5mm
1 for notch, wall thickness 12.7mm, 15.9mm
A Charpy test piece of 0 mm × 10 mm, 2V notch was cut out to carry out the test, and the fracture surface transition temperature was investigated from the Charpy transition curve.

Table 3 shows the critical cooling rate at which ferrite precipitates, the heat treatment conditions for the weld zone, and the transition temperature in the weld zone Charpy impact test. As can be seen from Table 3, in the present invention example, if the amount of cooling water is in the range of Q _{min to} Q _max as in the present invention, the cooling rate of 800 to 500 ° C. on the outer surface of the pipe is always 10 ° C. / It is equal to or more than sec and is within the range of the ferrite formation limit cooling rate. Incidentally, it can be seen that in the conventional example, it is outside the range of the ferrite formation limit cooling rate.

In the example of the present invention, the amount of water in the slow cooling zone and the amount of water in the subsequent zones both satisfy the conditions of the present invention. Further, in the examples of the present invention, the cooling rate on the outer surface side of the pipe and the cooling rate on the inner surface side of the pipe were both below the limit cooling rate and at 10 ° C / se
Above c, this also meets the conditions of the invention. Table 2,
As is clear from the results shown in Table 3, the examples satisfying the conditions of the present invention are ERW steel pipes having extremely excellent low-temperature toughness at the welded portion, as compared with the comparative examples deviating from the conditions of the present invention.

[0022]

[Table 1]

[0023]

[Table 2]

[0024]

[Table 3]

[0025]

According to the method of the present invention, it is possible to manufacture an electric resistance welded steel pipe having excellent low temperature toughness at the welded portion.

[Brief description of drawings]

FIG. 1 is a schematic view of an electric resistance welded steel pipe manufacturing facility for carrying out the present invention.

FIG. 2 is an example graph showing the relationship between the pipe thickness and the amount of cooling water according to the method of the present invention.

FIG. 3 is an example graph showing the relationship between the carbon equivalent and the amount of cooling water in the method of the present invention.

FIG. 4 is a chart showing an example of a cooling pattern of the present invention.

FIG. 5 is a chart showing an example of a cooling pattern of the present invention.

FIG. 6 is a chart showing an example of a cooling pattern of the present invention.

[Explanation of symbols]

1 Steel strip 2 ERW welded part 3 Welding electrode 4 Squeeze roll 5, 6, 7 Induction heating device 8 Water cooling device 9 ERW steel pipe

Claims (1)

[Claims] 1. Tube thickness t = 5 to 17 mm, carbon equivalent Ceq
= 0.25 to 0.45% band steel in a range of electric resistance welding,
Subsequently, the welded portion is heated to a temperature not lower than the Ac ₃ transformation point and not higher than 1050 ° C. and cooled with cooling water within the range of water amount Q _{min to} Q _{max represented} by the following equation, and the outer surface temperature is 500 ° C. to 400 ° C.
A method for cooling a welded portion of an electric resistance welded steel pipe, which comprises cooling the outer surface to 200 ° C. or less with a water amount exceeding Q _max from the time when the temperature reaches 0 ° C. Q _min = (-0.00805t ² + 0.204t) (Ceq ^-4.4 -3) (1) Q _max = (-0.0184t ² + 0.467t) (Ceq ^-4.4 -3) (2) However, Q _min , Q _max ; cooling water amount [1 / min. m ² ] t; tube thickness [mm] Ceq; carbon equivalent [%] = C + (Mn / 6) + (Si / 24) + (Ni / 40)
+ (Cr / 5) + (Mo / 4) + (V / 14)