JPS622009B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a method for manufacturing an electric resistance welded steel pipe that has high dipping strength and excellent sour resistance. In recent years, gas and oil wells have tended to become deeper and deeper, and the demand for high-pressure immersed oil country tubular goods has been increasing year by year. Furthermore, there is a view that all deep wells are sour gas environments. For example, at a depth of 9,000 meters, the hydrostatic pressure is approximately 900 atmospheres. According to NACE's definition, an environment with hydrogen sulfide partial pressure of 0.05 psi or more is called sour, so if it contains 5 ppm or more,
At 900 atmospheres, the partial pressure will be over 0.064 psi, creating a sour environment. Therefore, it is essential for oil country tubular goods for deep wells to have both excellent sour resistance and pressure resistance.
However, to improve sour resistance, it is necessary to lower hardness and strength, but to improve pressure soaking resistance, it is necessary to increase strength, and it has been extremely difficult to achieve both using conventional manufacturing methods. Traditionally, quenching and tempering are performed to satisfy both sour resistance and pressure soaking resistance, and then warm straightening is performed to improve the straightness and roundness of the pipe. It had the disadvantage of poor retention and high manufacturing costs. In order to eliminate the above-mentioned drawbacks, this invention does not perform quenching and tempering, but instead utilizes strain aging, which was conventionally considered to be bad, thereby achieving both sour resistance and pressure pickling resistance. The purpose of the present invention is to obtain a method for manufacturing high-strength ERW oil country tubular goods with excellent performance. This invention refines the crystal grains by hot rolling at low temperatures as described above, and then stably preserves solute carbon and nitrogen by rapid cooling and low-temperature winding, and further improves plasticity during pipe forming. A large number of dislocations are introduced by increasing the strain compared to conventional methods, and then solid solution nitrogen and solid solution carbon are fixed to these dislocations by low-temperature and short-time heat treatment. This is a method for manufacturing high-strength electric resistance welded steel pipes that have excellent both sour resistance and pressure soak resistance. In the method for manufacturing an electric resistance welded steel pipe of this invention, C: 0.08
~0.15%, Mn: 1.0~1.5%, Si: 0.10~0.25%,
P: 0.001-0.030%, S: 0.020% or less, Nb:
Steel containing 0.05% or less, V: 0.05% or less, Ti: 0.030% or less, and the balance consisting of Fe and unavoidable impurities is hot rolled at a temperature of 740 to 830℃, followed by average cooling until rolling. Speed 15℃/sec~50
°C/sec, rolled up at 100 to 500 °C, and then formed under strong pressure so that the amount of longitudinal strain is 3% to 10% during ERW steel pipe forming.
Shall be heated to ~450°C for 30 seconds and held for 30 minutes. This invention will be explained in detail below. It has been well known that in order to improve the crushing pressure, it is sufficient to increase the yield strength, but if only the yield strength is simply increased, the tensile strength also increases accordingly. Sour resistance decreases with an increase in tensile strength, so in order to achieve both sour resistance and pressure soaking resistance, it is better to have a high yield strength and as low a tensile strength as possible.In other words, the yield ratio ( It is important to increase yield strength/tensile strength x 100 (%)). Furthermore, by reducing the variation as much as possible, both sour resistance and pressure soaking resistance can be satisfied. Therefore, in this invention, by limiting the hot rolling and cooling conditions, and further limiting the amount of strain during steel pipe forming and the strain aging temperature and time, it is possible to achieve a high yield ratio and control the variation to be extremely small.
It aims to achieve both sour resistance and pressure soaking resistance. First, the ingredients of the material will be described. Although the present invention can be applied to any material ingredients, the following method is more effective. C is required to be at least 0.08% in order to secure the necessary strength; if it exceeds 0.15%, an intermediate structure will be generated due to rapid cooling in hot rolling, which will significantly deteriorate the toughness and further reduce the yield ratio. ~0.15%. Mn is also required in an amount of 1.0% or more to ensure the necessary strength, and if it exceeds 1.5%, it is undesirable in terms of an increase in pearlite band structure, an increase in segregation, and the generation of an intermediate structure. Si is also necessary to ensure the necessary strength, but due to the Mn/Si relationship, a minimum of 0.10% is required.
If it exceeds 0.25%, weldability will be impaired, so 0.10 to 0.25
%. Since P content deteriorates toughness due to segregation, it is preferable that it is low, and it is not desirable that it exceeds 0.030%. Also, P
Significant manufacturing costs are required to reduce this, and the lower limit is set at 0.001%. Since low-temperature toughness deteriorates due to elongated MnS inclusions, the S content is desirably as low as 0.020% or less. Nb,
Precipitated elements such as V and Ti are added to ensure the necessary strength, and also improve yield strength by precipitation strengthening within ferrite grains, and improve yield strength by making ferrite grains finer. It is an effective element for Therefore, Nb: 0.05% or less, V: 0.05%
Hereinafter, Ti is added at a solid solubility limit of 0.03% or less. The steel billet may be manufactured by either ingot-blushing rolling or continuous casting, but the continuous casting method is more advantageous. Next, regarding the hot rolling conditions, it is desirable that the hot rolling end temperature be as low as possible. The reason for this is that when the γ grains are refined by low-temperature rolling, intermediate structures are less likely to occur, and since this intermediate structure lowers the yield ratio, it is better to prevent it. Furthermore, rolling at a lower temperature makes the ferrite grains finer and increases the yield ratio. Therefore, the hot rolling finish temperature was set to 830°C or lower. but
Below 740°C, the yield ratio increases due to coarsening of ferrite grains. Next, the cooling conditions will be described. Cooling conditions are extremely important for minimizing strength variations and preserving solute carbon and nitrogen. Therefore, the average cooling rate should be set to 15℃/sec to 50℃/sec.
It was rapidly cooled. The reason for this is that finishing pearlite transformation at a certain speed on the runout table reduces strength variations, and by rapidly cooling during pearlite transformation, solid solution nitrogen,
It is possible to secure solid solution carbon. Also, 50℃/
If the temperature exceeds sec, the sour resistance will be significantly adversely affected due to the formation of bainite structure, so the upper limit should be set at 50℃/
sec. Next, let's talk about the winding temperature: 100℃~500℃
By rolling it up, solid solution nitrogen and solid solution carbon can be stably secured. If rolled at a temperature exceeding 500°C, nitrogen and carbon will precipitate during aging and will not work effectively for later strain aging. Also, when the temperature drops below 100℃, the strength increases rapidly,
Problems such as limited winding ability and flaws occurred, which was undesirable, so the lower limit was set at 100°C. Next, we will discuss the forming of ERW steel pipes. The meaning of ERW steel pipe forming in this invention is to introduce an appropriate amount of strain not for pipe forming but for strain aging to be performed later. This amount of strain is the strain in the longitudinal direction of the pipe. This is because tensile tests on oil country tubular goods are conducted from the longitudinal direction of the pipe. Its longitudinal strain ε ₃ is
If it is 3.0% or more, strain aging can be effectively utilized as shown in Figure 1. There is no limit to the amount of strain when it comes to introducing dislocations for use in strain aging, but if it exceeds 10%, roll flaws will occur during forming, so the appropriate range is from the minimum required 3% to the upper limit of 10%. shall be. The longitudinal elongation rate of the pipe ε ₃ is determined by determining the material width W ₀ . Therefore, if W ₀ is selected so that ε ₃ is 3% or more, an electric resistance welded steel pipe for oil wells with a stable low yield ratio can be manufactured. can. Describing the determination of the material width W ₀ , it is calculated from the following formula. That is, ε ₁ : Reduction rate in the pipe circumferential direction (%), ε ₂ : Thickness increase rate in the pipe wall thickness direction (%), ε
₃ : Pipe longitudinal elongation rate (%), D: Outer diameter,
t: wall thickness, W ₀ : material width, ε ₃ = {ε ₁ −ε ₂ +ε ₁ ε ₂ /(1−ε ₁ )(1+ε
₂ )} (×100(%)) …(1) ε ₂ =(3.97/D-0.0476/t)(×100(
%))…(2) ε ₁ = {W ₀ −π(D−t)/π(D−t)}(×100
(%))…(3) is established, and from the above formula, the material width W ₀ is ε ₃
% or more. Furthermore, ε
₃ and ε ₁ are logical formulas, but ε ₂ is an empirical formula containing Mill's unique constants. Next, conditions for strain aging treatment will be described. The strain aging treatment conditions vary depending on the amount of solute carbon, the amount of solute nitrogen, and the amount of strain, but the best temperature is 100 to 450°C and the time is 30 seconds to 30 minutes. Among these, the low temperature and long time side is desirable, but the balance between temperature and time can be determined based on economic reasons and changes in pipe roundness and straightness. Table 1 shows the conditions and results of tests conducted using sample materials with a pipe size of 51/2" x 0.034", divided into the present invention and comparative materials. It was good if both the pressure immersion value and the sour resistance were within the range of the present invention.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 shows the relationship between the strain in the longitudinal direction of the pipe and the usage characteristics of the heat-treated material.

Claims (1)

[Claims] 1 C: 0.08 to 0.15%, Mn: 1.0 to 1.5%, Si:
0.10-0.25%, P: 0.001-0.030%, S: 0.020%
Below, Nb: 0.05% or less, V: 0.05% or less, Ti:
Steel containing 0.030% or less with the remainder Fe and unavoidable impurities is hot rolled at a temperature of 740 to 830°C, and the average cooling rate until rolling is 15°C/sec to 50°C.
°C/sec, rolled up at 100 to 500 °C, and then formed under strong pressure so that the amount of longitudinal strain is 3% to 10% during ERW steel pipe forming.
A method for producing a high-strength ERW oil country pipe with excellent sour resistance and crush resistance, which comprises heating to ~450°C and holding for 30 seconds to 30 minutes.