JPS62192535A - Cooling method for steel pipe - Google Patents

Abstract

PURPOSE:To obtain desired mechanical property and metallographic structure without causing quench cracking by using misting jet nozzle to cooling head, independently setting cooling capacities of plural cooling heads together with moving velocity of steel pipe to cool it. CONSTITUTION:The steel pipe 2 heated to a prescribed temp. is inserted, moved and cooled in annular cooling headers 5 plurally set in series in a roller 4 conveyer capable of varying cooling capacity and velocity formed with nozzles capable of adjusting water quantity and gas water ratio. In this time, moving velocity of the pipe 2 and cooling capacities of respective cooling headers 51-58 are respectively prescribed so that cooling rate pattern and temp. difference described below are obtd. and cooling is carried out. The pattern is that satisfying temp. hysteresis peculiar to steel kind during cooling necessary for obtaining mechanical property and metallographic structure required to the pipe 2 after cooling over the whole wall thickness direction of the pipe 2. The temp. difference is that in wall thickness direction without causing quench cracking when martensite transformation is started at the vicinity of cooling surface of the pipe 2.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) There are two methods for cooling steel pipes: one using a cooling header and the other using an immersion method. The invention is directed to a method of cooling steel pipes using a single type cooling header, and more specifically, a method of cooling steel pipes that can impart desired metal structure and mechanical properties to heated steel pipes by strictly controlling the cooling rate. Regarding.

(Prior technology) Conventionally, cooling headers using spray nozzles have been mainly used for cooling steel pipes during quenching, etc., but these spray nozzles have a variable flow rate range of cooling water that can cool the steel pipe surface with uniform spray density. Because the area is narrow, it is difficult to adjust the cooling oil power according to the steel pipe dimensions and cooling specifications. Therefore, when performing cooling control, it has been common practice to keep the cooling capacity of the header constant and adjust the withdrawal speed according to the steel pipe dimensions, material, cooling specifications, etc.

On the other hand, recently, in cooling hot-rolled steel sheets, the cooling rate pattern has been controlled in addition to the coiling temperature (cooling end point temperature), but in this case, the control is within a relatively small range of cooling capacity. In many cases, the adjustment range of the cooling capacity can be narrow, and it is possible to deal with the problem by making small improvements to existing means without using any new methods or equipment.

(Problem to be solved by the invention) However, in the case of cooling steel pipes, high-speed cooling is often required, and in particular, in quenching, etc., the required specifications such as 50°C/S or more even in the slowest cooling rate part are required. There is something. In cases where the absolute value of the cooling capacity is at a high level and control of the cooling capacity over a wide range is required, the improvement of the existing means as described above cannot be used at all.

In addition, as a method for relatively strict cooling control, two or more cooling headers are connected and installed, and the cooling capacity of each header is appropriately adjusted to alleviate the temperature difference in the thickness direction of the steel pipe. It is publicly known (Japanese Unexamined Patent Publication No. 55-6417). However, this method is a cooling control method for preventing quench cracking, and does not control the cooling temperature history pattern in consideration of the quality after quenching. There is also an example of controlling the temperature at the end of cooling using a conveying speed control method to eliminate temperature differences in the longitudinal direction of steel pipes (Japanese Patent Publication No. 58-20296).
This also does not take into account cooling temperature history control.

The object of the invention is to provide a highly accurate cooling method for steel pipes that can strictly and stably provide the metallographic structure and mechanical properties required for steel pipes as products.

(Means for solving the problem) By the way, the present inventors have previously developed a new steel pipe cooling method in accordance with the above-mentioned purpose, and filed an application for the same (Japanese Patent Application No. 128834/1983). . This method was developed based on the following viewpoints.

■ In order to obtain the desired metallographic structure and mechanical properties of steel pipes, it is most important to not only cool the pipe but also to control the temperature history during cooling.

■ In order to perform this management, a new cooling header and cooling method that can strictly control cooling capacity are required.

■ As a cooling header, we replaced the misting jet nozzle, which was mostly used for slow cooling, with a strong cooling nozzle that has a lower air-water pressure (ratio of supplied static electricity to water) and a larger amount of water. Use the one incorporated as .

In other words, misting jet nozzles were conventionally used for slow cooling, such as cooling control in CC (continuous casting), and were used at an air/water ratio of 80 or higher, but with this method, they are used for strong cooling of steel pipes. It is used at an air-water ratio of 5 to 10 culm. In other words, if the air-water ratio is less than 5, the energy of collision of the mist particles with the material to be cooled decreases, resulting in a decrease in cooling ability, and if it exceeds 10, the mist particle size becomes small and sufficient cooling ability cannot be obtained.

In addition, the amount of water used is in the range of 5 to 20 V/nozzle, whereas conventionally it was about 5 to 7 V/nozzle. That is, if it is less than 517 minutes per nozzle, it is difficult to obtain stable cooling capacity at each nozzle, and if it exceeds 20 minutes per nozzle, the effect on cooling capacity when changing the amount of water decreases, and there is no point in changing the amount of water.

■ As a cooling method, a plurality of the above-mentioned cooling headers are installed in series on a steel pipe conveyor fin whose speed can be adjusted, and the moving speed when the steel pipe is inserted through these headers and the cooling capacity of each header are independently adjusted. Set.

However, subsequent research by the present inventors revealed that this method lacks consideration for quench cracking.

The invention is the addition of this measure against quench cracking to the above-mentioned method of the prior application, and its gist is that the cooling capacity is variable using a misting jet nozzle that can adjust the amount of water and the air-water ratio. When a steel pipe heated to a predetermined temperature is passed through and cooled through a plurality of annular cooling headers arranged in a row in the direction of steel pipe movement in a variable speed roller conveyor, the speed of movement of the steel pipe and the speed of each cooling header are determined. The method of cooling a steel pipe is characterized in that the cooling capacity is set respectively so as to obtain the following cooling rate pattern and temperature difference.

Cooling rate pattern: Cooling rate pattern that satisfies the temperature layer M (specific to the type of pipe steel to be treated) during cooling necessary to obtain the mechanical properties and metallographic structure required for the steel pipe after cooling throughout the pipe wall thickness direction. .

Temperature difference: The temperature difference in the wall thickness direction that does not cause quench cracking when martensitic transformation begins near the cooling surface of the steel pipe.

The effectiveness of non-invention will be discussed below.

(Function) ○ Effectiveness for the mechanical properties and metallographic structure and structure required for steel pipes after cooling When looking at quenching as cooling of steel pipes, the temperature history during cooling is It can be determined from the hardness (Vickers hardness) as the structure and mechanical properties of the steel pipe, and the CCT diagram known for each steel pipe material.

Figure 8 shows c:o, xs%, Si: 0.22%, Mn: 0
．． This is an OCT diagram for steel containing 82% P: 0.017%, showing the relationship between its cooling temperature and the elapsed time after the ACJ temperature was cut. In this OCT diagram, for example, if cooling is performed along the cooling curve (■), a mixed structure of bainite and martensite will be formed, and the Sivitkas hardness will be 23.
It becomes 4. Further, when cooling is performed along cooling curve C, a structure consisting only of martensite is obtained, and the Sivitkas hardness is 410.

Therefore, if the hardness and material of the steel pipe required for the steel pipe after quenching are given, the temperature history required during quenching and cooling will be determined.

In addition, here, Vickers strength and 9 are used as mechanical properties.
However, since toughness, tensile strength, etc. are also controlled by the temperature history during cooling, the term "mechanical properties" as used herein means all of these performances.

Furthermore, the temperature history or cooling rate pattern during cooling in the uninvented cooling method also includes the cooling end point temperature.

Effectiveness against O quench cracking When a steel pipe is quenched by cooling the outer surface with a cooling head, a temperature distribution deviation occurs in the thickness direction of the steel pipe, and the temperature history differs depending on the position in the thickness direction. If the structure after quenching is 10
If a martensitic structure is required for 0, the location with the coldest f and 11 velocity in the wall thickness direction is B (pay nai))flli! It is necessary to create a temperature history such that the cooling curve indicated by the 0 curve (indicated by a broken line) is also below the M (martensite) range in which the temperature is passed.

In this case, in the method of the prior application described above, it is conceivable to dissipate heat by water cooling from the outer surface so that the part with the slowest cooling rate (inner surface of the pipe in the case of outer surface cooling) has a cooling curve that satisfies this requirement. However, with this method alone, quench cracking may occur due to the rapid temperature gradient on the outer surface of the steel pipe and thermal stress caused by expansion due to transformation. This situation is shown in Figure 4 (a).

In Figure 4), the part with the slowest cooling rate (inner surface of the pipe) is
OC that satisfies Vickers hardness and microstructure after quenching
This is a case where cooling is controlled so that the temperature history follows the cooling curve on the T-diagram, but in this case, a rapid temperature gradient and thermal stress due to expansion due to transformation occur on the outer surface of the tube, causing quench cracking. There is a high possibility that

On the other hand, if the cooling capacity of each cooling header is reduced and the temperature difference between the inside and outside of the tube is reduced during cooling to suppress the occurrence of thermal stress, quench cracking can be prevented, but as shown in Figure 4 (b). As shown in Figure 2, there is an overall gradual change in /M degree, and the required Vickers hardness and tissue groove a cannot be obtained.

The present invention prevents both of these problems, and for this purpose, it is possible to maintain the temperature history of the steel pipe during cooling while ensuring that the entire pipe has the required temperature history during cooling to obtain the mechanical properties and microstructure required for the steel pipe after cooling. The temperature difference in the wall thickness direction near the cooling surface at the start of martensitic transformation is kept within a range that does not cause quench cracking.

@ The same figure corresponding to Figure 4 (A) (/Tsuyoshi) To explain specifically, the inner surface of the tube, which has the slowest cooling rate, satisfies the Vickers hardness and microstructure after quenching, as in the case of Figure 4 (A). A temperature history is given.

On the other hand, the outer surface of the tube, which has the fastest cooling rate, is rapidly cooled until it reaches the point where it begins to transform into martensai.
When passing through this starting point, the cooling rate is greatly suppressed, the temperature difference between the inner and outer surfaces is reduced, and the power is distributed so that thermal stress does not occur near the outer surface due to the difference in the timing of the start of martenside transformation between the inner and outer surfaces. There is. The temperature history near the outer surface of the tube at this time is naturally within a range that satisfies the required mechanical properties and metallographic structure.

Here, the term "near the tube outer surface" (near the tube cooling surface) refers to a region extending from the tube outer surface to approximately 80% in the wall thickness direction.

In the present invention, such highly accurate cooling control is possible due to the adoption of a misting jet nozzle in the cooling head, and the fact that the cooling capacity of multiple cooling heads installed in series is independently set along with the steel pipe movement speed. Needless to say, this is based on what was possible.

(Embodiments) Hereinafter, the present invention will be explained in more detail with reference to the drawings.

Figure 1 shows an example of an apparatus for carrying out the uninvented method ((a) is a plan view and (b) is a side view. According to the figure, the high temperature emitted from the heating furnace (1) The steel pipe <21 is indicated by the arrow (3
) in the direction of conveyance rollers (4), and passes through cooling headers (5/) (5 pieces)... (51). A plurality of misting jet nozzles are provided in each header in a centripetal manner toward the pass line of the steel pipe <2j. As mentioned above, misting jet nozzles have conventionally been used for specific axial slow cooling applications, but in the present invention, misting jet nozzles are Conventionally, the nozzle had a large ratio of 30 or more, but it was made smaller to increase the amount of water and was used as a nozzle for 9-strong cooling to control the cooling rate of steel pipes. in particular,
This air/water ratio was adjusted to provide a high cooling capacity and a uniform misting jet flow.
It has been found that it is best to set the value within the range of ~10. However, since this value depends on the characteristics of the nozzle, the above value is not necessarily fixed.

Figure 2 shows the cooling capacity adjustment device for each header.

According to the figure, cooling water is supplied from a cooling water supply source (6) to a cooling header (5) via a cooling water flow rate control device (7).
In addition, air is supplied from the air supply source (8) to the air flow control device (9).
) to the cooling header (5). The cooling water and air are mixed by the misting jet nozzle μ in the cooling header (5) to form a misting jet flow.
The cooling water is sprayed onto the surface of the steel pipe (21) that passes through the header.In other words, each cooling header uses a method that allows the cooling rate to be controlled by controlling the cooling water flow rate and air supply amount respectively to adjust the cooling capacity. In addition, as more cooling headers are installed, the overall cooling capacity improves, allowing steel pipes to be transferred at a faster speed and improving productivity, but due to equipment cost and equipment space constraints, in this example 8. Each header (5) and its control device (
7) (9) is the supply source (6) (81
are provided in parallel to each other.

The above-mentioned conveyance device also allows the conveyance speed to be arbitrarily set using the conveyance roller (4). As a result, for the cooling device as a whole, the conveyance speed and the cooling capacity of each cooling header can be freely selected depending on the dimensions, material, and cooling specifications of the steel pipe (2). In this example, the amount of cooling water to each cooling header is 5 to 20 m''/hr.

The air supply amount was 50 to 100 Nrrl/hr s, and the conveying speed could be freely set in the range of 0.05 to 0.4 m/s. In addition, the effective cooling length per stage of the cooling header was set to 6001EI, and a conveyor loaf (4) was provided for every two stages of the cooling header to prevent bending of the steel pipe.

By using the above device example, a desired temperature history can be given to the steel pipe while preventing quench cracking.

The method is described below. - As the cooling device, an example of eight cooling headers that can control the cooling capacity shown in the first factor shall be used. Cooling curve 0 (Fig. 8) is considered as the critical temperature history (temperature history of the part with the slowest cooling rate (inner surface in case of external cooling)). Cooling end temperature is 220°C
(TM: temperature at which martensitic transformation ends).

First, the heat transfer coefficient of the steel pipe surface necessary for cooling in each cooling header (51) (51) necessary to satisfy these conditions is determined, but this is determined by off-fin based on the steel pipe temperature calculation model. It is possible to do this by simulation. Figure 5 shows the calculation flowchart, and the details are as follows (see Figure 6).

■ Inner surface Ms point arrival time t, j-s, inner surface cooling end point temperature time tf, cooling start temperature time tB-θ, cooling start temperature pXTs (uniform in thickness direction), inner surface cooling end point temperature Tf = TMf (martensitic transformation end temperature) etc.

■ Nn1~tl! It is assumed that all cooling headers of o8 are to be cooled, and the conveyance speed (■) is determined by the following formula.

V-1c/cf・・・・・・・・・・・・
...(υHowever, tc: total length of the cooling header ■) The Ms point position t1s on the inner surface is subject to the following equation constraints depending on the transport speed V and the limit time Zj-s to reach the Ms point on the inner surface.

t1S≦V tIS・・・・・・・・・・・・
...(2) This is the temperature history of the inner surface of the 8th OCT
This is a constraint equation based on the condition that it should be below the cooling curve (■), and the initial value is assumed to be when the equality sign of equation (2) holds.

Note that if the predetermined heat transfer coefficient of each cooling header cannot be obtained with the initial value (at the time of occurrence of C/I/- in Fig. 5), 1
@Next, reduce tis by an appropriate step width and continue calculation.

■ The point of illegality is how to determine the position of M8 point on the outer surface. Although the complete mechanism of the occurrence of quenching cracks has not been elucidated, it is known that quenching cracks are likely to occur on the outer surface of steel pipes due to thermal stress caused by rapid temperature gradients during cooling and expansion due to transformation. There is. The present invention provides a cooling method that reduces the temperature distribution at the start of martensitic transformation near the outer surface and alleviates thermal stress.

It is necessary to reduce the cooling rate by slow cooling the outer surface immediately before the start of martensitic transformation, and to equalize the temperature distribution near the outer surface.In order to lengthen the slow cooling time as much as possible, it is necessary to Therefore, it is necessary to delay the start of martensitic transformation of the outer surface.

Therefore, the initial value of the M8 point arrival position tO8 on the outer surface is the Ms point arrival position tiS on the inner surface, and if the predetermined heat transfer coefficient of each cooling header cannot be obtained (1 in Fig. 5)/l
/- when a loss occurs), the loss is
Continue calculation by decreasing .

It is necessary to consider the temperature calculation steps (1) to (2) in the following 8 cases. First, we will show the points of temperature calculation in each case and explain the calculation methods for (1) to (2).

! When cooling starts, calculate the temperature up to the Ms point on the outer surface.B) Bring the time when the outer surface reaches the Ms point closer to the inner surface arrival time.

When the outer surface temperature reaches TMS+α, strong cooling is performed to create a temperature difference between the inner and outer surfaces, thereby promoting a temperature drop on the inner surface.

C. When the outer surface temperature approaches TM8, slow cooling is performed to reduce the temperature difference between the inner and outer surfaces and to reduce the temperature distribution near the outer surface.

■ Calculate /lt from the Ms point on the outer surface to the M8 point on the inner surface. Set the inner surface temperature at the position where the inner surface Ms point is reached to TM8.

When the outer surface temperature reaches Tuf+β, strong cooling is performed to create a temperature difference between the inner and outer surfaces and to promote a decrease in the inner surface temperature.

C. When the outer surface temperature approaches TMf, slow cooling is performed to reduce the temperature difference between the inner and outer surfaces, and the time when the surface reaches the Ms point is adjusted.

■ Temperature calculation from point Ms on the inner surface to point Mf on the inner surface A. Make sure that the inner surface temperature becomes the cooling end point temperature.

■ The assumption of this heat transfer coefficient is determined appropriately according to the concept of calculating the final cooling temperature in Steps to m above, and is continued until the calculated temperature and target temperature match (Fig. 5 aI to am/L/-
De).゛■ As a heat transfer model, consider a one-dimensional heat transfer model in the thickness direction.

Basic formula〉 However, C: Specific heat of steel pipe p:, # Density χ: ・ Boundary condition formula for thermal conductivity〉 Steel pipe outer surface χ-knee h (Tw-Ts) ...
・(4) However, h: Heat transfer coefficient T between cooling water and steel pipe surface
w: Cooling water temperature TS: Steel pipe outer surface temperature Steel pipe inner surface λ-knee〇〇insulation)・・・・・・(5)
Based on equations (3), (4), and (5), numerical calculations are performed to determine the temporal change in temperature distribution in the thickness direction. Numerical calculations employ a method of performing forward calculations every minute time Δ using the difference method, but since this is a well-known technique, it will be omitted here.

■Compare the total Based on this, the cooling water supply amount and air-water ratio for each cooling header are determined.

The relationship between the cooling water charge f1, the heat transfer coefficient hi, and the air/water ratio in each cooling header is as shown in Fig. 7. Therefore, once the required heat transfer coefficient is determined, the required cooling water supply amount can be determined. , the air-water ratio is determined.

(Example) The outer diameter is 150 m$, the wall thickness is 8 mt, and the component composition is 3rd.
When the above calculation was performed on the steel pipe shown in the figure using the 8-stage cooling header shown in Fig. 1, the target Vickers hardness of the inner surface of the pipe was 234, and the conveyance speed was 0.4 m/s. The results shown in Table 1 were obtained.

When we actually performed cold gradient J based on this result, the Vickers hardness distribution in the wall thickness direction was 240 or more, 32
It was within a predetermined range of 5, and no quench cracking occurred.

For comparison, the same type of steel pipe was cooled using the same equipment, the same target hardness, and the same conveyance speed, according to the method of the prior application described above. Table 2 shows the calculation results of the cooling water supply amount and air/water ratio for each header. the result,
Although the Vickers hardness distribution in the wall thickness direction was within a predetermined range of 240 to 375, quench cracking occurred at a rate of about 67X.

Table 2, FIG. 6 shows the temperature history of the inner and outer surfaces of the steel pipes in the above-mentioned non-inventive example and comparative example. As is clear from the figure, in the comparative example in which no countermeasures against quench cracking were taken, the outer surface of the tube reached the martensitic transformation point after passing through the 2nd head, and the inner surface of the tube reached the point after passing through the 5th head. The difference in time between the two to reach the martensitic transformation point is 8 head passage times.

As a result, the temperature in the direction of the tube wall near the outer surface of the tube at the start of martensitic transformation of the outer surface of the tube is represented by ΔT.

On the other hand, in the non-inventive example in which quench cracking measures were taken, the temperature history of the inner surface of the tube was the same as in the comparative example, and the specified Vickers hardness was achieved even after passing through the fifth stand when martensitic transformation started. On the other hand, the start of martensite transformation on the outer surface of the tube is delayed by one head passage, while maintaining the necessary temperature history on the inner surface.This delay causes the start of martensite transformation on the outer surface of the tube after passing through the NQ8 head. The temperature difference in the direction of the tube wall near the outer surface of the tube at the point ΔG is greatly reduced compared to ΔT in the case of the comparative example, and therefore quench cracking can be prevented.

In addition to quenching steel pipes, the present invention is also effective for rapid cooling of hot rolled steel plates and thick plates.

(Effects of the Invention) As is clear from the above description, the method of the present invention not only can ensure desired mechanical properties and metal structure without causing quench cracking through strict cooling control, but also: It also has the following effects.

Currently, when quenching that requires a completely martensitic structure to obtain a specified product quality, there is a risk of quench cracking, so expensive alloying elements are added and immersion using oil is used. The manufacturing cost of high-grade steel pipes has increased because of the quenching method. However, according to the method of the present invention, these countermeasures against quench cracking are completely unnecessary, and manufacturing costs can be reduced.

When the quenching process and tempering process are continuous, by setting the quenching end point temperature to the minimum necessary temperature, the charging temperature to the tempering furnace can be increased.
Energy saving becomes possible.

Conventionally, the average temperature of the steel pipe at the completion of penetration is 80 to 100°C.
However, by using the cooling control method of the present invention, it was possible to raise the temperature to 150°C.

It has become possible to correct the minimum cooling rate control based on the CCT diagram, and it has become possible to stabilize the product structure, that is, to stabilize the mechanical properties. For example, regarding tensile strength, the strength deviation σ-11 that previously occurred under the same manufacturing conditions
was reduced to 0.5"fi, and the overall strength deviation could be reduced to a minimum.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing an example of a device suitable for carrying out the method of the present invention, (a) is a plan view, (b) is a side view, and Fig. 2 is a cooling capacity adjustment device used in the device. An explanatory diagram, Fig. 8 is an OCT diagram of a specific steel, and Fig. 4 is a schematic explanatory diagram of the temperature history during cooling, where (a) is a conventional example, ((b) is a comparative example, (
c) is the case of the present invention example, FIG. 5 is an example of a calculation flow chart in the method of the present invention, FIG. 6 is a diagram illustrating the temperature history during cooling in detail for the comparative example and the present invention example, and FIG. 7 is is a diagram showing the relationship between the heat transfer coefficient and the cooling water supply amount f using the air-water ratio γ as a parameter. In the figure, l: heating furnace, 2: w4 tube, 3: traveling direction arrow, 4
: Conveyance roller, 51-5t: Cooling header, 6: Cooling water supply source, 7: Cooling water flow rate control device, 8: Air supply source, 9:
Air flow control device, lO: Cooling header total length tc, 11:
Branch pipe. Figure 1 Figure 2 Cause 3 Figure 7 Same ttogp xogI

Claims (1)

[Claims] (1) Inside an annular cooling header with variable cooling capacity configured using misting jet nozzles with adjustable water volume and air-water ratio, and with multiple units installed in series in the steel pipe movement direction on a variable speed roller conveyor. When a steel pipe heated to a predetermined temperature is passed through and cooled, the moving speed of the steel pipe and the cooling capacity of each cooling header are set so as to obtain the following cooling speed pattern and temperature difference. A method for cooling steel pipes. Cooling rate pattern: A cooling rate pattern that satisfies the temperature history during cooling (specific to the type of pipe steel to be treated) necessary to obtain the required mechanical properties and metallographic structure of the steel pipe throughout the thickness of the pipe after cooling. Temperature difference: The temperature difference in the wall thickness direction that does not cause quench cracking when martensitic transformation begins near the cooling surface of the steel pipe.