JPH0676624B2 - Solution heat treatment method for welded stainless steel pipe - Google Patents

Description

The present invention relates to a solution heat treatment method for preventing the occurrence of weld metal cracks in an austenitic stainless steel pipe produced by welding.

(Prior Art) Welded austenitic stainless steel pipes are manufactured by rolling steel strips in a pipe making line into a tubular shape with rolls and continuously welding the abutting portions by a welding method such as TIG welding or plasma welding. However, since the corrosion resistance and ductility deteriorate as compared with the material and the residual stress is large when the pipe is made as it is, the solution treatment after pipe forming (usually in the temperature range of 1010 ° C or higher) is applied to improve them. . Conventionally, this solution heat treatment was often done offline with a continuous annealing furnace, but in recent years, an in-line method has been increasingly performed where a solution heat treatment furnace is installed behind the welded part of the pipe making line. There is.

A major difference between the case where the solution heat treatment is performed by the off-line method and the case where the solution heat treatment is performed by the in-line method is that strain due to tension is applied or not applied in the longitudinal direction of the steel pipe. That is, in the case of the off-line method, the steel pipe is cut into a predetermined length and passed through the solution heat treatment furnace, so that the steel pipe is not subjected to any strain other than thermal expansion. However, in the case of the in-line method, a drive roll such as a pinch roll is installed behind the solution heat treatment furnace,
Since the solution-treated heat-treated steel pipe must be transferred,
Tension in the longitudinal direction is applied to the steel pipe in the solution heat treatment furnace.

When tension is applied to the steel pipe, the yield strength of the austenitic stainless steel in the solution temperature range is as low as several Kgf / mm ² or less, so strain easily occurs in the longitudinal direction depending on the magnitude of tension. On the other hand, austenitic stainless steel is a material in which hot cracking easily occurs, and when the weld is heated to the solution temperature range and a few percent of strain is applied in the welding direction, the low melting point compound concentrated in the final solidified part is added. Since strain is imparted in the state of being remelted, cracks having the same form as weld hot cracks (hereinafter referred to as hot embrittlement cracks) occur in the weld metal. For this reason, when an austenitic stainless steel pipe is conventionally subjected to solution heat treatment by an in-line method, hot embrittlement cracking may occur.

(Problems to be Solved by the Invention) Since the present invention has the problem of hot embrittlement cracking of the weld metal as described above in the in-line solution heat treatment of welded austenitic stainless steel pipe, there is a problem of high temperature embrittlement cracking. The present invention provides a solution heat treatment method that does not occur.

(Means for Solving Problems) The present inventors investigated the effect of strain type and strain amount on hot embrittlement cracks in order to prevent the occurrence of weld metal hot embrittlement cracks in welded austenitic stainless steel pipes. I just did
As for the type of strain, it was found that strain due to thermal expansion does not affect high temperature embrittlement cracking, and strain due to tension does. It was also found that the amount of strain needs to be adjusted depending on the steel type. Therefore, based on such knowledge, we investigated the relationship between the steel type and the amount of strain that does not cause high-temperature embrittlement cracking, and found that the austenitic stainless steel pipe produced by welding in a continuous pipe production line was produced in a solution heat treatment furnace. The strain amount in the longitudinal direction of the steel pipe generated by the tension applied to the steel pipe in the solution heat treatment furnace during the solution heat treatment while pulling it with the drive roll arranged on the side, and the critical strain amount that does not cause steel pipe cracking in advance for each steel type. If it is determined, the strain amount in the longitudinal direction of the steel pipe generated by the tension applied to the steel pipe in the solution heat treatment furnace during pipe forming is detected, and if the strain amount is controlled to a tension that does not exceed the critical strain amount, high temperature embrittlement will occur. It was concluded that chemical cracking will not occur.

The present invention will be described in detail below.

In order to investigate the relationship between the steel type and the strain amount at which high temperature embrittlement cracking does not occur, the inventors of the present invention used SUS304, SUS304L, SUS
While forming 316, SUS316L and SUS310 steel strips into a tube shape with a forming roll, using a steel pipe with an outer diameter of 21.7mm and a wall thickness of 2.0mm produced by TIG welding without using filler metal, the steel type and high temperature embrittlement cracking The relationship with the amount of strain that does not occur was investigated. In the investigation, the steel pipes were marked with predetermined intervals, and the parts were placed in a solution heat treatment furnace, various tensions were applied in the longitudinal direction while heating to the solution temperature, and then the steel pipes were taken out from the furnace and cooled to room temperature. Then, the measurement of the gauge length and the observation of the high temperature embrittlement crack of the weld metal were carried out. Strain amount ε
(%) Was calculated by dividing the amount of change in the gauge length after solution heat treatment by the gauge distance before solution heat treatment and multiplying by 100. The high temperature embrittlement cracking of the weld metal was observed by examining the longitudinal section of the weld metal.

Figure 1 shows the relationship between the amount of strain ε (%) and the region where high temperature embrittlement cracking occurs (upper curve) due to changes in solution heat treatment temperature. SUS316, SUS316L, and SUS310 are more sensitive to high-temperature embrittlement cracking than SUS304 and SUS304L due to the influence of the amount ε and the solution heat treatment temperature. Therefore, when the solution heat treatment is performed at the same temperature, the former must have a smaller strain amount ε than the latter.

By the way, when performing solution heat treatment by in-line method, it is desirable that the length of the furnace is short in terms of fuel cost, yield, and workability, and the solution heat treatment temperature is for corrosion resistance, recovery of ductility and coarsening of crystal grains. Considering the influence, it is optimal to set the temperature range of 1100 to 1200 ° C. But 1100 ℃ and 1200 ℃
As shown in FIG. 1, the strain amounts ε at which high temperature embrittlement cracking does not occur differ between and. Therefore, in order to prevent high-temperature embrittlement cracks from occurring in the entire temperature range of 1100 to 1200 ° C, the strain amount ε may be made smaller than the strain amount ε at 1200 ° C, which is the smallest. That is, the strain amount ε in the longitudinal direction of the steel pipe generated by the tension applied to the steel pipe in the solution heat treatment furnace is SUS3
04 steel pipe 4.0% or less, SUS304L steel pipe 3.0% or less, SUS
The tension applied to the steel pipe in the solution heat treatment furnace may be controlled so that the 316 steel pipe has 1.5% or less, the SUS316L steel pipe has 1.2% or less, and the SUS310 steel pipe has 0.5% or less.

In order to control the tension applied to the steel pipe in the solution heat treatment furnace in the pipe making line in which the solution heat treatment is an in-line method, for example, as shown in FIG. That is, in FIG. 2, reference numeral 1 denotes a welding torch, which is not shown, but in front of the welding torch 1, a steel strip carry-in device and a forming roll for rolling the steel strip supplied from the carry-in device into a tubular shape. Is provided. 2 is a squeeze roll that abuts the edges of a steel strip rolled into a tubular shape by a forming roll, 3 is a solution heat treatment furnace that performs solution heat treatment on the steel pipe (for example, a high-frequency heating furnace or electric resistance heating furnace), 4 is a solid A cooling device for cooling the steel pipe subjected to the solution heat treatment, 5 is a pinch roll for transferring the steel pipe, 6 and 6a are sizing rolls, and the steel pipe 7 is moved by the pinch roll 5 from left to right in the figure. In this pipe making line, in order to control the tension applied to the steel pipe 7 in the solution heat treatment furnace 3, roll rotation type speed measuring devices 8 and 8a are arranged in front of and behind the solution heat treatment furnace 3 to control them. 9, the strain amount ε calculated as described below by the control panel 9 is fed back to the pinch roll 5, and the rotation of the pinch roll 5 is controlled according to the strain amount ε.

ε (%) = (V ₂ −V ₁ ) / V ₁ × 100 V ₁ Steel pipe speed (m / min) by the speed measuring device 8 Steel pipe speed (m / min) by the V ₂ speed measuring device 8a The speed measuring device 8 As 8a, other contact type or non-contact type (for example, one utilizing the laser Doppler effect) may be used.

Next, the present invention will be described with reference to examples.

(Example) In the pipe making line shown in FIG. 2, SUS304, SUS304L, SUS31
6, while forming SUS316L and SUS310 steel strip by roll forming, without using filler metal with welding torch 1.
After TIG welding, the temperature of 1100 ~ 1200 in solution heat treatment furnace 3
Solution heat treatment was performed at ℃ to make a steel pipe with an outer diameter of 21.7 mm and a wall thickness of 2.0 mm. At the time of pipe making, the control panel 9 controls the rotation of the pinch roll 5 so that the amount of strain applied in the longitudinal direction falls within the range of the present invention (invention method), and the control panel 9 is not used. A method (conventional method) in which the amount of strain applied in the direction is not controlled was performed. Table 1 shows the results of an investigation of high temperature embrittlement cracking of weld metal.

As shown in Table 1, in the case of SUS316, SUS316L and SUS310 steel pipes, which have high susceptibility to high temperature embrittlement cracking, high temperature embrittlement cracking remarkably occurs in the conventional method, but it does not occur at all in the method of the present invention.

(Effects) As described above, according to the present invention, it is possible to prevent the high temperature embrittlement cracking that occurs when the solution heat treatment is performed by the conventional in-line method.

[Brief description of drawings]

Figure 1 shows the amount of strain ε applied by tension in the longitudinal direction of the steel pipe.
FIG. 2 is a graph showing the relationship between (%) and the area where high temperature embrittlement cracking occurs due to changes in the solution heat treatment temperature (upper side of the curve). 1 shows an example of a method of controlling the strain amount ε (%) applied by tension in the longitudinal direction of the steel pipe. 1 ... Welding torch, 2 ... Squeeze roll, 3 ... Solution heat treatment furnace, 4 ... Cooling device, 5 ... Pinch roll, 6, 6a ... Sizing roll, 7 ... Steel pipe, 8, 8a ... Velocity measuring device, 9 ...
control panel,

Claims (2)

[Claims] 1. A solution heat treatment when an austenitic stainless steel pipe produced by welding in a continuous pipe making line is subjected to solution heat treatment while being pulled by a drive roll arranged at the exit side of the solution heat treatment furnace. The amount of strain in the longitudinal direction of the steel pipe generated by the tension applied to the steel pipe in the furnace, the critical strain amount that does not cause steel pipe cracking is determined in advance for each steel type, and the tension applied to the steel pipe in the solution heat treatment furnace during pipe making A solution heat treatment method for a welded stainless steel pipe, comprising detecting the amount of strain in the longitudinal direction of the steel pipe generated and controlling the strain so that the strain does not exceed a critical strain amount. 2. When the solution heat treatment is performed at 1100 to 1200 ° C. in the solution heat treatment furnace, the tension applied to the steel pipe in the solution heat treatment furnace is SUS3.
04 steel pipe 4.0% or less, SUS304L steel pipe 3.0% or less, SUS
The solution heat treatment method of the welded stainless steel pipe according to claim 1, wherein the control is performed so that the 316 steel pipe has 1.5% or less, the SUS316L steel pipe has 1.2% or less, and the SUS310 steel pipe has 0.5% or less. .