阅读说明：本技术 一种马氏体不锈钢油套管及其制造方法 (Martensitic stainless steel oil casing and manufacturing method thereof ) 是由 张春霞 张忠铧 刘耀恒 齐亚猛 赵鹏 赵济美 于 2018-06-27 设计创作，主要内容包括：一种马氏体不锈钢油套管及其制造方法,其成分重量百分比为：C 0.15～0.22％、Si 0.1～1.0％、Mn 0.10～1.0％、P≤0.02％、S≤0.01％、Cr 11.0～13.5％、N≤0.02％、V：0.03～0.1％、Al：0.01～0.04％,其余为Fe和不可避免的杂质,且满足1.90≤Cr:30(C+N)≤2.25。本发明通过控制最后一次热加工的温度和变形量,控制冷速,使变形过程的晶格畸变的能量保留,使马氏体相变核增加,相变后的晶粒得到细化,并在Ms点和Mf点之间进行缓冷处理,使部分奥氏体得到保留提高韧性；并在冷却后通过一次短时高温回火提高回火效率和一次低温回火获得高韧性低硬度的不锈钢油套管,氧化层厚度明显小于淬火和回火的调质管,提高内表氧化皮去除效率；并提高产品的冲击韧性降低硬度,从而提高耐腐蚀性能。(A martensite stainless steel oil casing and a manufacturing method thereof comprise the following components in percentage by weight: 0.15-0.22% of C, 0.1-1.0% of Si, 0.10-1.0% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, 11.0-13.5% of Cr, less than or equal to 0.02% of N, and V: 0.03-0.1%, Al: 0.01-0.04 percent, the balance of Fe and inevitable impurities, and the balance of Cr which is more than or equal to 1.90 and less than or equal to 30(C + N) and less than or equal to 2.25. The method comprises the steps of controlling the temperature and the deformation amount of the last hot working, controlling the cooling speed, keeping the energy of lattice distortion in the deformation process, increasing martensite phase transformation nuclei, refining crystal grains after phase transformation, and performing slow cooling treatment between an Ms point and an Mf point to keep part of austenite and improve the toughness; after cooling, the tempering efficiency is improved through one-time short-time high-temperature tempering, and the stainless steel oil casing with high toughness and low hardness is obtained through one-time low-temperature tempering, the thickness of an oxide layer is obviously smaller than that of a quenched and tempered pipe, and the removal efficiency of an inner oxide skin is improved; and the impact toughness of the product is improved, and the hardness is reduced, so that the corrosion resistance is improved.)

1. A martensite stainless steel oil casing comprises the following components in percentage by weight: c: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10-1.0%, P is less than or equal to 0.02%, S is less than or equal to 0.01%, Cr: 11.0-13.5%, N is less than or equal to 0.02%, V: 0.03-0.1%, Al: 0.01 to 0.04 percent, the balance of Fe and inevitable impurity elements, and the balance of Cr is more than or equal to 1.90 and less than or equal to 30(C + N) and less than or equal to 2.25.

2. The martensitic stainless steel oil jacket as claimed in claim 1, wherein the stainless steel structure of said oil jacket has a delta ferrite content of less than 0.5%.

3. The martensitic stainless steel oil casing as claimed in claim 1 or 2, wherein the yield strength of the oil casing is 552 MPa and 655MPa, the 0 ℃ impact reaches more than 80J, and the hardness is below 22 HRC.

4. The method of manufacturing a martensitic stainless steel oil jacket according to claim 1,

1) smelting and casting

Smelting and casting the components according to the claim 1 into a tube blank;

2) the tube blank adopts common perforation and continuous rolling;

3) tension reducing-sizing

The final rolling temperature is controlled at 900-980 ℃, the deformation amount of the section area of the sizing is controlled at 25-40%, then cooling is carried out at a cooling speed of more than or equal to 10 ℃/S until the temperature is Ms-50 ℃, the temperature is kept between Ms-50 ℃ and Ms point, and the heat preservation time T1 is obtained according to the formula 1:

T1＝2.5t+20 (1)

wherein t is the wall thickness of the tube in mm; t1 units minute;

cooling the heat-preserved seamless steel pipe to the ambient temperature at the speed of not higher than 10 ℃/S;

4) one-time tempering

Heating to AC1-30 deg.C, holding time T2 formula 2 gives:

T2＝1.8t+10 (2)

wherein, T, the wall thickness of the pipe, unit mm, T2 unit minute;

cooling the heat-preserved seamless steel pipe to the ambient temperature at the speed of not higher than 10 ℃/S;

5) secondary tempering

Reheating to AC1-110 ℃ and holding time T3 is given according to formula 3:

T3＝3.5t+25 (3)

wherein, T, the wall thickness of the pipe, unit mm, T3 unit minute;

and cooling the heat-preserved seamless steel pipe to the ambient temperature at the speed of not higher than 10 ℃/S.

5. The method for manufacturing a martensitic stainless steel oil jacket pipe according to claim 3, wherein the pipe blank is manufactured by a converter, electric furnace or vacuum induction furnace melting method, a continuous casting method or an ingot bloom method.

6. The method of manufacturing a martensitic stainless steel oil jacket tube according to claim 1, wherein the stainless steel structure of the oil jacket tube has a delta ferrite content of less than 0.5%.

7. The method of claim 3, wherein the yield strength of the oil jacket pipe is 552 MPa and 655MPa, the 0 ℃ impact is more than 80J, and the hardness is less than 22 HRC.

Technical Field

The invention relates to an oil casing and a manufacturing method thereof, in particular to a martensitic stainless steel oil casing which reduces the thickness of a surface oxidation layer and has low hardness and high toughness and a manufacturing method thereof.

Background

The API 5CT standard comprises a 2Cr13 martensitic stainless steel oil casing pipe which is mainly used for containing CO₂And a small amount of H₂And S, exploiting and developing oil and gas resources. In order to ensure the corrosion resistance of the product, the requirement of high grade (PSL-2) usually involves the removal of the internal oxide scale. In addition, for martensitic stainless steels, lowering the hardness and increasing the toughness also have a beneficial effect on improving the corrosion resistance.

Generally, the 2Cr13 martensitic stainless steel oil casing is produced by free cooling after pipe making, then quenching by heating to a temperature above the AC3 austenite region and tempering heat treatment below the AC1 temperature to reach the requirements of L80 steel grade. The AC3 of 2Cr13 is between 860 ℃ and 890 ℃, and in order to ensure sufficient austenitization, the quenching heating temperature is generally above 900 ℃, so that the crystal grains are coarse, the thickness of the oxide scale is increased, and the problems of low toughness, high hardness, large difficulty in removing the oxide scale and the like are caused.

In the chinese patent 201310148137.7, a complex heat treatment process of annealing, normalizing, quenching and tempering is adopted to improve the low-temperature impact toughness. However, the normalizing and quenching temperature of the process is high, the heat preservation time is long, and the surface oxidation condition of the workpiece is inevitably serious, so that the removal difficulty of the oxide scale is very high.

Disclosure of Invention

The invention aims to provide a martensitic stainless steel oil casing and a manufacturing method thereof, which can reduce the thickness of an oxide layer of 2Cr13 and improve the removal efficiency of an inner surface oxide skin; and the impact toughness of the product is improved, and the hardness is reduced, so that the corrosion resistance is improved.

In order to achieve the purpose, the technical scheme of the invention is as follows:

the Cr content in the 2Cr13 steel is high, and after the hot working and the heat treatment process of high-temperature quenching and tempering are adopted, the Cr content in an oxide layer formed on the surface is high, the oxide layer is compact, and the removal difficulty is high. The martensitic stainless steel product is characterized in that the martensitic transformation can occur under the air cooling condition to form martensite. It is well known that direct annealing can also adjust the properties and reduce the thickness of the oxide layer. However, if the free hot working and the free cooling after hot working are adopted in combination with the direct tempering, the performance stability is poor, and the requirement of L80 in the API standard is difficult to meet.

Aiming at 2Cr13, the invention controls the temperature and the deformation amount of the last hot working and the cooling speed in the subsequent cooling process, so that the energy of lattice distortion in the deformation process is reserved, the martensite phase transformation nucleus is increased, the crystal grain after the phase transformation is refined, and slow cooling treatment is carried out between the Ms point and the Mf point, so that part of austenite is reserved and the toughness is improved. And after cooling, the tempering efficiency is improved by one-time short-time high-temperature tempering, and the stainless steel oil casing with high toughness and low hardness is obtained by one-time low-temperature tempering, and the thickness of an oxide layer is obviously smaller than that of a quenched and tempered quality-adjusted pipe.

Further, δ ferrite is a poor structure which is common in martensitic stainless steel, and generation of δ ferrite is caused by improper composition control. The presence of delta ferrite reduces the corrosion resistance and impact toughness and increases the hardness of the martensitic stainless steel. The invention reduces the precipitation of delta ferrite by controlling the chemical composition of steel.

According to the design principle, the martensitic stainless steel oil casing comprises the following components in percentage by weight: c: 0.15 to 0.22%, Si: 0.1 to 1.0%, Mn: 0.10-1.0%, P is less than or equal to 0.02%, S is less than or equal to 0.01%, Cr: 11.0-13.5%, N is less than or equal to 0.02%, V: 0.03-0.1%, Al: 0.01 to 0.04 percent, the balance of Fe and inevitable impurity elements, and the balance of Cr is more than or equal to 1.90 and less than or equal to 30(C + N) and less than or equal to 2.25.

In the component design of the martensitic stainless steel oil casing pipe of the invention:

C：0.15-0.22％

c is used as an austenite forming element in the martensitic stainless steel, and the percentage of austenitizing the stainless steel at high temperature can be increased by increasing the content of C, so that the martensite under the room temperature condition is obtained, and the strength is improved. However, when the content of C is too large, the corrosion resistance of the stainless steel is lowered and the toughness is lowered. In order to ensure the desired strength and reduce the risk of ferrite precipitation, it is preferable to contain 0.17% to 0.22%.

Si：0.1-1.0％

Si is an important deoxidizer in steel making, but Si has a risk of promoting the formation of sigma phase and ferrite phase in stainless steel having a high Cr content, and sigma phase and ferrite have adverse effects on toughness and corrosion resistance of stainless steel. Si is therefore limited to 0.1-1.0%.

Mn：0.1-1.0％

Mn can increase the strength of stainless steel, and in the present invention, Mn is added in an amount of 0.1% or more in order to ensure the strength required for use as an oil jacket pipe. However, if Mn exceeds 1.0%, the toughness is lowered. Therefore, Mn is limited to the range of 0.1 to 1.0%. And preferably 0.2 to 0.5%.

P: less than 0.02%

P is to make CO resistant at high temperatures₂Harmful elements whose corrosion properties are reduced, and adversely affect hot workability. If the content of P exceeds 0.02%, the corrosion resistance cannot meet the high temperature environment requirement, so that P is limited to 0.02% or less. Further, it is preferably 0.015% or less.

S: less than 0.01%

S is a harmful element that causes a reduction in hot workability while adversely affecting impact toughness. If the content of S exceeds 0.01%, the steel pipe cannot be normally manufactured. Therefore, S is limited to 0.01% or less. Further, it is preferably 0.005% or less.

Cr：11.0-13.5％

Cr is an important element for improving the corrosion resistance of the stainless steel, and the addition of Cr ensures that the surface of the stainless steel can quickly form a corrosion-resistant passive film even in the air, thereby improving the CO content of the oil casing in the high-temperature resistant environment₂Corrosiveness ofCan be used. In order to obtain the CO2 corrosion resistance of more than 500 ℃, the addition amount of Cr in the stainless steel system of the invention is more than 11.0 percent. On the other hand, addition of Cr element exceeding 13.5 increases the risk of ferrite precipitation, adversely affecting both hot workability and corrosion resistance of the product. Therefore, Cr is limited to the range of 11.0 to 13.5%. And preferably 11.5 to 13.0%.

N: less than 0.02%

Although N is an element that improves the pitting corrosion resistance of stainless steel, this application is mainly embodied in aqueous systems. Since N can be used as interstitial atoms to fill in the crystal lattice of the alloy, so that toughness is lowered and hardness is increased, the stainless steel of the present invention preferably contains 0.02 or less of N.

V：0.03-0.1％

V is an important microalloy element, and generally, it can improve strength by refining crystal grains by the pinning action of carbonitride precipitation. In order to achieve the above effects, the amount of V added must be 0.03 or more, while if the amount of V added exceeds 0.1%, the toughness decreases.

Al：0.01-0.04％

Al is added as a deoxidizer in the smelting process, and the addition amount of Al is more than 0.01 percent in order to achieve the deoxidizing effect. However, an Al content exceeding 0.04% causes a decrease in toughness. Therefore, Al is limited to 0.01 to 0.04%.

Meanwhile, in order to make the content of delta ferrite in the stainless steel structure of the invention less than 0.5% and improve the corrosion resistance of the product, Cr, C and N also need to meet the requirements of the following formula:

1.90≤Cr:30(C+N)≤2.25

wherein, Cr, C and N are the mass percentage content of each element.

The manufacturing method of the martensitic stainless steel oil casing pipe comprises the following steps:

1) smelting and casting

Smelting and casting the components into a tube blank;

2) the tube blank adopts common perforation and continuous rolling;

3) tension reducing (sizing)

The final rolling temperature is controlled at 900-980 ℃, the deformation amount of the section area of the sizing is controlled at 25-40%, then cooling is carried out at a cooling speed of more than or equal to 10 ℃/S until the temperature is Ms-50 ℃, and the temperature is kept between Ms-50 ℃ and Ms point, and the heat preservation time T1 is obtained according to the formula (1):

T1＝2.5t+20 (1)

wherein t is the wall thickness of the tube in mm; t1 units minute;

cooling the heat-preserved seamless steel pipe to the ambient temperature at the speed of not higher than 10 ℃/S;

4) one-time tempering

Heating to AC1-30 deg.C, keeping the temperature for T2, and obtaining the following formula (2):

T2＝1.8t+10 (2)

wherein, T, the wall thickness of the pipe, unit mm, T2 unit minute;

cooling the heat-preserved seamless steel pipe to the ambient temperature at the speed of not higher than 10 ℃/S;

5) secondary tempering

Reheating to AC1-110 ℃ and holding time T3 is given according to formula (3):

T3＝3.5t+25 (3)

wherein, T, the wall thickness of the pipe, unit mm, T3 unit minute;

and cooling the heat-preserved seamless steel pipe to the ambient temperature at the speed of not higher than 10 ℃/S.

In the manufacturing method of the stainless steel oil casing pipe of the invention:

the stainless steel pipe of the present invention is not particularly limited in smelting, rolling of a raw pipe, and rolling of a steel pipe, as long as the above-described respective composition is satisfied. Manufacturing a tube blank by adopting a conventional smelting method such as a converter, an electric furnace, a vacuum induction furnace and the like and by adopting a method such as continuous casting, ingot casting and blooming and the like; then the tube blank is made into a seamless steel tube with specified size by perforation, continuous rolling and tension reducing (sizing).

The final rolling temperature of the tension reducing (sizing) is controlled at 900-. If the finish rolling temperature is less than 900 ℃, poor plasticity results. The deformation of the sectional area of the sizing is controlled between 25 and 40 percent, if the deformation is more than 40 percent, the quality of the inner surface and the outer surface of the steel pipe is influenced, and if the deformation is less than 25 percent, the lattice distortion is insufficient, so that the performance is not controlled. After rolling, the cooling rate is controlled to be more than or equal to 10 ℃/S, and if the cooling rate is lower than the cooling rate, the lattice distortion energy retention is insufficient, so that the performance regulation is not favorable. The tempering efficiency can be improved by the primary short-time high-temperature tempering, and the stainless steel oil casing with high toughness and low hardness can be obtained by the secondary low-temperature tempering.

Compared with the prior 2Cr13 oil casing manufacturing method, the invention has the following beneficial effects:

1) the manufacturing method provided by the invention can reduce the thickness of the oxide layer of the oil sleeve and obviously improve the removal efficiency of the oxide layer;

2) the toughness of the sleeve is obviously improved, the hardness is reduced, and the yield strength performance can meet the requirements of the L80 steel grade, by adopting the process, the impact toughness can reach more than 80J, and the hardness can be stably controlled below 22HRC (the standard requirement is below 23 HRC);

3) the content of delta ferrite in the martensitic stainless steel provided by the invention is less than 0.5%.

Detailed Description

The present invention will be further described with reference to the following examples.

The ingredients of the examples of the invention are shown in Table 1. Table 2 shows the process parameters of the steel of the embodiment of the invention, and Biao shows the properties of the steel of the embodiment of the invention.

After steel of the embodiment of the invention is smelted, cast, rolled, perforated and continuously rolled, sizing temperature, deformation and final wall thickness are controlled, cooling process is controlled, AC1, Ms and the like are listed in a table 2, and oxide layer thickness and mechanical property data and the like of each embodiment are listed in a table 3.

Wherein the values of formula (1) listed in table 1 are calculated according to the following formula:

Cr：30(N+C) (1)

wherein Cr, N and C are the mass percentage contents of each element.

In the examples, the test pieces of the steel grades of the present invention not subjected to the manufacturing method of the present invention were also tested, and the test pieces of nos. 21 to 24 were subjected to the processes of air cooling after rolling and air quenching and tempering.

The following test was conducted on the pipe cut out sample after the heat treatment.

And (3) testing yield strength: the prepared steel pipe is processed into an API arc sample, and the API arc sample is obtained by taking the average number after being tested according to the API standard and listed in the table.

Charpy V-type impact absorption work (i.e. impact toughness) test: the V-shaped impact test specimens with a cut-off volume of 5 × 10 × 55(mm) were obtained on steel pipes with a wall thickness of 7.5mm, the V-shaped impact test specimens with a cut-off volume of 10 × 55(mm) were obtained on steel pipes with a wall thickness of 12.0mm, the V-shaped impact test specimens with a cut-off volume of 7.5 × 10 × 55(mm) were obtained on steel pipes with a wall thickness of 9.7mm, the V-shaped impact test specimens were averaged after testing according to ASTM E23 standard and were converted to 10 × 55(mm) full size according to API 5CT standard and are listed in table 3 at a test temperature of 0 ℃.

And (3) hardness testing: the steel pipes were sampled and tested for hardness in cross section, and the hardness in the middle of the wall thickness was measured according to ASTM E18, and the average values of the test data are shown in Table 3.

The grinding efficiency of the steel pipe obtained by the manufacturing method is compared with that of the steel pipe obtained by the conventional quenching and tempering quality-adjusting method. Compared with the inner surface of the steel pipe obtained by the conventional quenching and tempering conditioning method after grinding for three times, the inner surface of the steel pipe obtained by the manufacturing method of the invention after grinding for one time reaches the same level.

As is clear from Table 3, the stainless steel and the steel pipe obtained by the production method according to the examples of the present invention have a strength such that the yield strength YS is 552-655MPa or more, an impact at 0 ℃ of 80J or more, and a hardness of 22HRC or less. For comparative example a, the levels of hardness and impact were lower due to more ferrite.

Compared with the steel grade and the manufacturing method of the comparative example, the pipe obtained by the manufacturing method of the invention has thinner oxide layer thickness and better mechanical property.

Table 1 units: weight percent of

TABLE 2

TABLE 3