KR910008105B1 - Manufacturing method of oil well tube with a tensile strength 70 kgf/mm2 and over - Google Patents

Abstract

An oil well steel tube having at least 70kgf/mm2 of a tensile strength is manufactured by rolling, water cooling and winding a steel to produce a hot rolled steel plate, electric resistance welding the plate to produce a steel tube, and heating the steel at 950-1100 deg.C for t [t=(-4.4T+4900)±30; T:deg.C sec. The steel is composed of 0.25-0.35 wt.% C, 1.0-2.0 wt.% Mn, at most 0.5 wt.% Si; at most 0.03 wt.% P, at most 0.01 wt.% S, at most 0.015 wt.% N, 0.05-0.2 wt.% V and impurities. The steel tube is used for casting and tubing of an oil well.

Description

Manufacturing method of oil well steel pipe with tensile strength over 70㎏f / ㎠

1 is a graph showing the change in tensile strength according to the winding temperature.

2 is a graph showing the change of mechanical properties with homogenization heat treatment time.

Figure 3 is a microstructure showing the change of microstructure with the homogenization heat treatment time.

The present invention relates to a method for manufacturing a high tensile steel (API-N80) hot-rolled steel pipe having a tensile strength of 70 kgf / mm2 or more, and more specifically, an oil well steel pipe having a tensile strength of 70 kgf / mm2 or more and excellent in ductility and toughness. It relates to a manufacturing method of.

API-N80 steel for petroleum industry is used for casting and tubing of oil fields, and it accounts for a very large portion of the annual demand of oil pipes in the world. It is a steel grade. API-N80 steel should have high strength (tensile strength of 70.3kgf / mm2 and above, yield strength of 56.3-77.3kgf / mm2) and low distortion due to external forces. As a result, API-N80 steels must be optimized for hot rolling and alloy design to achieve low crushing properties with strength. API-N80 steel can be classified into three types according to the heat treatment method after pipe forming. There are two types of normalizing and quenching and tempering after pipe manufacturing. The three are as rolled.

However, to date, the demand for N80 steel pipes that do not undergo heat treatment is extremely small, and in the case of oil well steel pipes with relatively large pipe forming amount, the stran due to the pipe forming process is not solved by heat treatment. This is because reliability is lower than steel pipes. On the other hand, the hardened / frozen heat-treated N80 steel pipe is more satisfactory than the other two types in terms of reliability, but the heat treatment process is complicated, and thus the manufacturing cost of the product is not widely used. Therefore, the most widely produced morpheme is the semi-arid API-N80 steel pipe.

As described above, although a small amount of V, Nb, Mo, etc. was added using the medium carbon component system as the basic component system in the sub-junction API-N80 steel pipe, the range of the hot rolling manufacturing conditions was very narrow due to the large hardenability of the material. In other words, if the coiling temperature is slightly higher, the strength of the hot rolled coil is less than the specification. On the contrary, if the coiling temperature is low, the strength is excessively increased due to the formation of low temperature transformation structure, causing cracking during pipe processing. In addition, when the API-N80 steel is heat-treated after pipe forming, the strength is greatly reduced due to the coarsening of precipitates, and a long time homogenization treatment is required to prevent such a decrease in strength.

On the other hand, high-strength oil well steel pipes of medium-carbon component type require excellent low-temperature toughness with the desired strength, but the size of the final grain should be reduced, but long-term heat treatment leads to grain growth, which inevitably leads to lowered toughness. There is a need for a technology capable of manufacturing steel of the material.

Therefore, the present invention is to solve the problems of the structure and strength change before and after heat treatment by controlling the hot rolling using the low-alloy steel of the medium carbon component system and to control the heat treatment conditions to produce more economically hot-rolled steel pipe for oil wells There is a purpose.

The present invention, in weight percent, C: 0.25-0.35%, Mn: 1.0-2.0%, Si: 0.5% or less, S: 0.01% or less, N: 0.015% or less, V: 0.05-0.2%, and other unavoidable The hot rolled steel sheet wound at 580 ° C or above by rolling control and rolling the steel formed by impurity in a conventional manner is manufactured by an electric resistance welded steel tube, and then heated to a temperature range of 950-1100 ° C, at which temperature t [t (Sec) = (-4.4T + 4900) ± 30, T: temperature (° C.)] The present invention relates to a method for producing an oil well steel pipe having a tensile strength of 70 kgf / mm 2 or more.

Hereinafter, the reason for numerical limitation of the said component is demonstrated.

When the C is 0.35% or more, low-temperature toughness is lowered, and the electrical resistance weldability used in the manufacture of pipes is lowered, and when it is 0.25% or less, the desired strength cannot be obtained by the rolling method of the present invention, 0.25-0.35 % Is preferred.

The Si is added as a deoxidizer during steelmaking and also has a solid solution strengthening effect. However, if it is 0.5% or more, the weldability decreases and the oxide film is severely formed on the surface of the steel sheet, so the upper limit thereof is 0.5%.

When the Mn is 1.0% or less, it is difficult to obtain the desired strength, and when 2.0% or more, the transformation is excessively delayed from the austenite to ferrite and pearlite transformation, so that the transformation proceeds after winding. However, after winding, the cooling speed of the coil is different for each part, so if the transformation proceeds after winding, the longitudinal material deviation of the coil is increased. In addition, when the Mn amount is 2.0% or more, the micro segregation of the material is increased and the hardenability increases, so that the low temperature transformation structure is easily formed, and thus the upper limit of the Mn content is preferably limited to 2.0%.

The lower P is added as an inevitable impurity in the steel, the lower it is preferable. The upper limit of P is set at 0.03%, and the reason is that when it exceeds 0.03%, the abnormal metamorphic structure is developed at the center of the plate due to coagulation segregation. Because it promotes.

Since S is a non-metallic regeneration of MnS in the steel, the formation of manganese emulsifiers increases with increasing S content, and the lower the lower the Mn inclusions, which act as a cause of processing cracks when the material undergoes severe processing. It is preferable and the upper limit is set to 0.01%.

N is an element necessary for producing nitrides such as VN, but in many cases, the upper limit is set to 0.015% because rust is a problem on the surface of the slab after continuous casting.

Since V is combined with N in the medium carbon component system and precipitates as VN, it not only suppresses the growth of crystal grains upon reheating, but also binds to carbon and precipitates as VC, thereby adding to maximize the strength increase due to precipitation strengthening. If the added amount is 0.05% or less, the degree of contribution to the strength increase is small. If the added amount is 0.2% or more, the strength increase effect by V addition is not economical, so 0.05-0.2% is preferable.

Hereinafter, homogeneous heat processing conditions are demonstrated.

As mentioned above, the present invention regulates various elements in order to obtain the required strength and toughness as an oil refined steel. The steel of such composition is manufactured by a conventional continuous casting slab, and then reheated and hot rolled to control the rolling of austenite. The crystal grains of are refined. Water cooling immediately after the end of hot rolling suppresses the grain growth of the austenite micronized by hot rolling and increases the number of nucleated ferrites to form ferrite.

In order to prevent the formation of pearlite stably by winding in the temperature range of Ar ₁ and winding it below the pearlite transformation end temperature, the low temperature transformation structure (bainite) is formed and the strength is rapidly increased and the low temperature toughness is lowered. Shall be the pearlite transformation section temperature.

Since the temperature at which the low temperature transformation structure is formed in the steel having the composition of the present invention is 580 ° C. or less, the coiling temperature should be 580 ° C. or more.

As described above, the hot rolled steel sheet is manufactured into a predetermined pipe and then subjected to heat treatment. The heating temperature is preferably limited to the range of 950-1100 ° C., which is added to increase the strength when the heating temperature is lower than 950 ° C. The precipitates are not re-used during heat treatment and coarsened, so the strength decreases regardless of the heat treatment time.When the heating temperature is over 1100 ℃, the reverse transformation to austenite and the re-use of precipitates are advantageous in a short time. This is because the austenite is coarsened and the ductility and toughness are reduced.

Therefore, according to the conventional method, uniformly setting the homogenization time at each heating temperature results in deterioration of the mechanical properties. Therefore, in the present invention, the heating time (t: seconds) within the heating temperature range is set to "t = (-4.4). T (temperature, ° C) +4900) ± 30 "was satisfied to satisfy the range of the formula. When heat treatment with the homogenization time out of the above formula range, coarsening of precipitates or coarsening of austenite occurs, It causes a decrease in toughness.

The homogenization heat treatment temperature and time will be described in more detail below. Perlite spheroidization and coarsening of V (CN) should be considered as the cause of the strength change with the increase of heat treatment temperature. This phenomenon can be thought of as a function of heat treatment temperature and time as a matter of kinetics. First of all, the change of pearlite occurs above Ac ₁ (727 ℃), the vacancy transformation. Investigating reverse transformation behavior by heating 0.8% C steel, the vacancy steel, showed that the reverse transformation of pearlite into austenite at 750 ℃ requires about 10 seconds of incubation and about 200 seconds to complete reverse transformation. Therefore, the spheroidization of pearlite occurs up to about 800 ° C, and it is thought that pearlite is re-used and reverse transformation occurs at temperatures higher than that. If so, even when the heat treatment temperature is raised to 800 ° C or higher, the cause of the decrease in strength is that V (CN) is coarsened.

The behavior of V (CN) in austenite can be estimated and interpreted from the following equation.

log [% V] [% N] = (-7733 / T) +2.99

log [% V] [% C] = (-9500 / T) +6.72

When the composition of N80 steel is substituted and solved, the solid solution temperature of VN becomes 1036 degreeC, and the solid solution temperature of VC becomes 885 degreeC. Therefore, it can be considered that precipitates are employed at temperatures above 900 ° C., but the homogenization time is very short, such as 1 minute, so the precipitates are not reusable and only growth occurs. Therefore, V (CN) only grows during heating up to 1050 ° C, and re-use of V (CN) occurs at temperatures above 1100 ° C, increasing strength again.

On the other hand, when the homogenization time was subjected to the normalization of 15 minutes, the decrease in strength was found to be greatest at the heating temperature of about 800 ° C. The pearlite was spheroidized and V (CN) was decreased. Because it is coarsened. When the heating temperature reaches 900 ℃, the strength rises again. This is because re-precipitation and reprecipitation of pearlite occur. At 950 ℃, re-precipitation and re-precipitation of V (CN) also occur, and the strength is restored to a level similar to that of the hot rolled state. However, when heating temperature is raised to 1100 degreeC, it can be considered that reverse transformation austenite coarsens and intensity falls again.

Therefore, it is preferable that the temperature suitable for heat processing shall be 950-1100 degreeC. However, if the homogenization time is made the same without considering the heat treatment temperature, incomplete employment of V (CN) may occur according to each heat treatment temperature, or coarsening of the reverse transformation austenite is promoted, thereby deteriorating mechanical properties. That is, at the heating temperature of 750 ℃, the pearlite is spheroidized as the homogenization time increases, and at the same time, V (CN) is coarsened, so the strength decreases. When the heat treatment temperature is increased to 850 ° C, the pearlite increases with the increase of the homogenization time It is reusable and reprecipitated, but the coarsening of V (CN) is intensified, resulting in continuous strength reduction. However, when the heating temperature is further increased to 950 ° C, the reintegration and re-precipitation of V (CN) occurs as the homogenization time increases, so the strength is increased again, and the re-precipitation of V (CN) takes a considerable amount of time. Therefore, it takes more than 12 minutes of homogenization time to show the same level of strength as in the hot rolled state. After increasing the temperature to reach 1050 ℃, the strength equivalent to that of the hot rolled state appears after about 5 minutes.

From this result, the result of the homogenization time showing the optimum strength at each heat treatment temperature was found to be “t = (-4.4T + 4900) ± 30” (where t is the homogenization time and the unit is second and T is The heat treatment temperature, unit is C.)

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

The steel slab having the chemical composition of Table 1 was hot-rolled by changing the winding temperature to a temperature range of 590-650 ℃ as shown in Table 2, to prepare a hot rolled sheet, the mechanical properties of the hot-rolled sheet was measured It is shown in the figure.

TABLE 1

TABLE 2

As shown in FIG. 1, the strength increases and the ductility (elongation) decreases as the winding temperature decreases. When the winding temperature is 570 ° C., low-temperature metamorphic structure (bainite) is formed to increase the strength. Due to excessive increase, the risk of cracking in pipe piping increases, and the flatness ratio of the pipe decreases, and the ductility is also lower than the standard. Therefore, it can be seen that the proper rolling conditions of the API-N80 steel should be wound at 580 ° C or higher, which is a temperature at which low-temperature transformation tissue is not formed. In addition, the invention rolled material C of Table 2 is subjected to homogenization heat treatment while changing the homogenization time at 1050 ℃, 950 ℃, and 850 ℃, and the tensile strength is measured by measuring the mechanical properties according to the homogenization heat treatment time In (a), yield strength is shown in (b) of FIG. 2, elongation is shown in (c) of FIG. 2, and loyalty values are shown in (d) of FIG.

According to the present invention, when the heat treatment temperature is 1050 ° C., the heat treatment time is about 5 minutes, and when the heat treatment temperature is 950 ° C., about 12 minutes, as shown in FIG. 2, tensile strength, yield strength, elongation and toughness. All of the excellent heat treatment conditions are about 5 minutes at 1050 ℃, about 12 minutes at 950 ℃. In addition, the rolled invention (C) of Table 2 was subjected to homogenization treatment at 1050 ° C. for 5 minutes and 15 minutes, and then observed in a microstructure photograph for 5 minutes in FIG. 3 (a), and for 15 minutes. 3 is shown in (b). As shown in FIG. 3, in the case of homogenization treatment at 1050 ° C. for 5 minutes (FIG. 3 (a)), when re-precipitation of pearlite and V (CN) is optimally performed and reprecipitation is obtained, homogenization is achieved. In the case where the time was increased to 15 minutes (Fig. 3 (b)), coarse austenite was observed.

As described above, the present invention can easily obtain satisfactory yield strength, tensile strength and excellent impact toughness by regulating the composition of the steel and forming a proper heat treatment structure using the rolling and heat treatment method, and strict heat treatment conditions Not only can we save energy and form stable materials due to heat treatment, but also we can heat up the production air which is increased by manufacturing steel pipes and then heat-treating them into heat treatment furnaces by using a simple heat treatment facility attached to steel pipe forming machine. There is an effect that can significantly improve the production yield.

Claims (1)

By weight%, C: 0.25-0.35%, Mn: 1.0% -2.0%, Si: 0.5% or less, P: 0.03% or less, S: 0.01% or less, N: 0.015% or less, V: 0.05% -0.2% , Steel, and other unavoidable impurities are controlled by rolling in a conventional manner, and water-cooled to produce a hot rolled steel sheet wound at 580 ° C. or higher in an ordinary resistance welded steel tube, and then to a temperature range of 950-1100 ° C. A steel pipe for oil wells having a tensile strength of 70 kgf / mm 2 or more, characterized in that it is heated and homogenized at t [t (sec) = (-4.4T + 4900) ± 30, T: temperature (° C.) sec. Way.

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