WO2017050227A1 - Seamless steel tube with high strength and toughness and manufacturing method therefor - Google Patents

Abstract

A seamless steel tube with high strength and toughness, comprising the following chemical elements by mass: 0.1-0.25% of C, 0.1-0.5% of Si, 0.01-0.1% of Al, 0.6-2% of Mn, the balance of Fe and other unavoidable impurities, wherein C+Mn/6≥0.35. Also provided is a method for preparing a seamless steel tube.

Description

High-strength and toughness seamless steel pipe and manufacturing method thereof Technical field

The invention relates to a pipe fitting and a manufacturing method thereof, in particular to a steel pipe and a manufacturing method thereof.

Background technique

Due to the product form and manufacturing method of seamless steel pipe, the performance of seamless steel pipe can only be improved by adding alloying elements and controlling the offline heat treatment process after rolling. Taking the oil well pipe as an example, the 555 MPa (80 ksi) or higher level needs to obtain the corresponding seamless steel pipe by adding more alloying elements or off-line quenching and tempering treatment. However, this obviously increases the manufacturing cost of the seamless steel pipe.

At present, the conventional process steps of hot-rolled steel pipe is to first enter the pipe stock after rolling, and then heat treatment according to the need, which not only causes waste of waste heat after rolling of the steel pipe (usually the temperature of the steel pipe after rolling is above 900 ° C), It also brings about the complexity of the process and the increase in cost. In addition, offline heat treatment can not be used to strengthen the induced phase transformation effect after deformation of the material. According to the research, the steel is directly subjected to in-line quenching after deformation, and its performance is significantly higher than that after cooling and then reheating and quenching.

As described above, since those skilled in the art have already known that the use of in-line quenching can result in better performance of seamless steel tubes, why is the prior art still not using in-line quenching? This is because the seamless steel pipe is not used for general hot-rolled steel pipes. Due to its special cross-sectional shape, the internal stress state of the seamless steel pipe is more complicated than that of the plate. Therefore, if the in-line quenching process is adopted, it is difficult to stabilize on the one hand. Controlling its performance, on the other hand, is prone to cracking of the steel pipe.

Summary of the invention

The object of the present invention is to provide a high-strength and toughness seamless steel pipe which has high strength and good toughness. Further, the seamless steel pipe according to the present invention does not add an expensive alloying element, and the alloy is economically expensive to add.

In order to achieve the above object, the present invention proposes a high-strength and toughness seamless steel pipe whose chemical element mass percentage is:

C: 0.1-0.25%,

Si: 0.1-0.5%,

Al: 0.01-0.1%,

Mn: 0.6-2%,

The balance is Fe and other unavoidable impurities; in addition, it must be satisfied: C + Mn / 6 ≥ 0.35.

The design principle of each chemical element in the high-strength and toughness seamless steel pipe according to the present invention is as follows:

Carbon: 0.1-0.25%

C is an important element to ensure the strength and hardenability of steel pipes. When the C content is less than 0.1%, on the one hand, the strength of the steel is difficult to ensure, and on the other hand, it is difficult to avoid the precipitation of the pro-eutectoid ferrite, thereby affecting the sulfur resistance of the steel. Since the material is subjected to both deformation stress and tissue stress during on-line quenching, the material is more prone to cracking than off-line quenching. According to the technical solution of the present invention, controlling the C content between 0.1 and 0.25% can significantly reduce the formation of quench cracks in the seamless steel pipe.

Silicon: 0.1-0.5%

Si is an element brought into the steel by a deoxidizer. Once the content exceeds 0.5%, the tendency of cold and brittleness of the steel is remarkably increased, and for this reason, it is necessary to limit the Si content to 0.5% or less. At the same time, in order to ensure the deoxidation effect of the steel, it is necessary to keep the Si content in the steel at 0.1% or more.

Aluminum: 0.01-0.1%

Similarly, Al is also an element brought into the steel by a deoxidizing agent. A small amount of Al has a beneficial effect of refining the steel grains. However, if the Al content is too high, the process steps such as tube casting and hot working may be adversely affected. In view of this, it is necessary to set the Al content in the high-strength toughness seamless steel pipe according to the present invention to 0.01 to 0.1%.

Manganese: 0.6-2.0%

Mn is also an element brought into the steel by a deoxidizer. Mn has a beneficial effect of expanding the austenite phase region, increasing the hardenability of the steel and refining the crystal grains. However, Mn is prone to segregation during solidification, resulting in a distinct banded structure in the seamless steel pipe. There is a significant difference between the hardness and the precipitation phase of the matrix of the banded structure and the seamless steel pipe, which in turn affects the toughness of the steel. Therefore, the high-strength toughness seamless steel pipe according to the present invention should be controlled to have a Mn content of 2.0% or less. At the same time, in order to ensure the hardenability of the steel, the Mn content in the steel should be made 0.6% or more.

C+Mn/6≥0.35

The reinforcing effect of the seamless steel pipe according to the present invention needs to be achieved by a combination of various effects such as solid solution strengthening and precipitation strengthening. In the case of no additional alloying elements, it must be guaranteed C, The Mn element has a certain content to obtain a sufficient strengthening effect. Therefore, when C and Mn satisfy the above relationship, the reinforcing effect of the steel can be effectively ensured, thereby ensuring high toughness of the steel.

Further, the microstructure of the high-strength toughness seamless steel pipe according to the present invention is mainly martensite, and the ratio of martensite is not less than 75%.

Further, the microstructure of the high-strength toughness seamless steel pipe according to the present invention further includes a small amount of ferrite and bainite.

Further, in the high-strength and toughness seamless steel pipe according to the present invention, S ≤ 0.005%, P ≤ 0.02%, and O ≤ 0.01% among other unavoidable impurities.

The main unavoidable impurities in the high-strength toughness seamless steel pipe according to the present invention are S, P and O. Among them, P and S are harmful elements in steel, S will have adverse effects on the hot workability and toughness of steel, and P will adversely affect the hot workability and toughness of steel. ≤ 0.005%, P is controlled to ≤ 0.02%. O is an element which lowers toughness and needs to be controlled to be 0.01% or less. Preferably, the content of the O element is controlled to be 0.005% or less.

Further, the high-strength and toughness seamless steel pipe according to the present invention has a yield strength of ≥ 555 MPa, and its full-scale impact energy of 0 ° C is >50 J.

Another object of the present invention is to provide a method for producing a high-strength toughness seamless steel pipe. A seamless steel pipe having high strength and good toughness can be obtained by this manufacturing method. The manufacturing method of the high-strength toughness seamless steel pipe can fully utilize the residual heat after rolling, thereby effectively reducing the waste of energy consumption, thereby reducing the input cost of the process manufacturing, and the manufacturing method can also effectively avoid the cracking of the seamless steel pipe.

In order to achieve the above object, a method for manufacturing a high-strength and toughness seamless steel pipe provided by the present invention comprises the steps of:

(1) smelting and producing a tube blank;

(2) heating the tube blank, and obtaining the waste pipe by perforation, continuous rolling, tension reduction or tension sizing, wherein the cross-sectional area ratio of the tube blank to the waste pipe is greater than 4.5 (note that although only limited here) The lower limit of the cross-sectional area ratio of the tube blank and the waste pipe is 4.5, and the upper limit is not limited. However, according to the actual equipment, the cross-sectional area ratio of the blank to the waste pipe is generally less than 10, that is, the The upper limit will be limited by the production capacity of the equipment);

(3) On-line quenching: quenching and opening temperature is 850-1100 ° C, cooling rate is 20-60 ° C / s, and the Rockwell hardness of the steel tube after quenching is greater than 40 HRC;

(4) Tempering: The tempering temperature is 500-700 °C.

The core of the method for manufacturing the high-strength and toughness seamless steel pipe according to the present invention is the in-line quenching step. As described above, the in-line quenching is to directly quench the hot-rolled steel pipe, and the quenching in the prior art is generally off-line quenching. That is, the filled steel pipe will be first introduced into the pipe stock after rolling, and then heat treated according to the subsequent production needs, on the one hand not only causes waste of waste heat after rolling (usually the temperature of the steel pipe after rolling is above 900 ° C), on the other hand, the heat treatment process It also requires a large amount of heat energy, which greatly increases the heat energy consumption of the seamless steel pipe manufacturing method. The comprehensive mechanical properties of the steel after the rapid cold quenching of the hot-rolled steel pipe directly after deformation are significantly higher than the comprehensive mechanical properties of the steel which is reheated and quenched after cooling. However, the in-line quenching of the seamless steel pipe is very prone to cracking of the steel pipe. Therefore, the technical solution also strictly controls the specific process parameters of the in-line quenching, so that the manufacturing method of the present invention not only fully utilizes the post-rolling process as compared with the prior art. The residual heat also achieves the strengthening effect of the steel pipe through the deformation-induced phase transformation benefit of the steel pipe, preventing the cracking of the seamless steel pipe, thereby realizing the strength of the steel pipe and the steel pipe without additional expensive alloying elements. toughness.

In the in-line quenching step, if the quenching and cooling temperature is lower than 850 ° C, some pro-eutectoid ferrite will be formed in the steel pipe, and the required microstructure (for example, martensite structure) cannot be obtained after quenching, so Ensure that the temperature of the steel pipe is above 850 °C. At the same time, the cooling rate is controlled between 20-60 ° C / s, because the cooling rate is slow, it is difficult to obtain the required microstructure (for example, martensite structure), and conversely, when the cooling rate is faster Since the internal stress is large after deformation of the steel pipe, it is easy to cause quenching cracking of the steel pipe.

In addition, in the tempering step, when the tempering temperature is <500 °C, the internal stress of the steel pipe cannot be effectively reduced, and the steel pipe has sufficient toughness, and when the tempering is stable >700 ° C, the microstructure in the steel pipe is The rate of decomposition and dislocation density (for example, martensite structure) is rapidly lowered, and for this reason, the high strength required for the steel pipe cannot be ensured, so the tempering temperature is controlled to 500 to 700 °C.

Further, in the method for producing a high-strength toughness seamless steel pipe according to the present invention, in the above step (2), the tube blank is heated to 1100-1250 ° C and held for 1-4 h.

Further, in the method for producing a high-strength toughness seamless steel pipe according to the present invention, in the above step (2), the tube blank before the tension reduction or the tension sizing step and the step of completing the tension reduction or the tension sizing step are performed. The cross-sectional area ratio of the subsequent blank is greater than 1.05. (It should be noted that although the lower limit of the ratio is limited to 1.05, the upper limit is not limited. However, according to the actual equipment, the upper limit of the ratio is generally about 1.3. That is to say, the upper limit value is limited by the production capacity of the equipment).

Further, in the method for manufacturing a high-strength and toughness seamless steel pipe according to the present invention, in the above steps (3) In the case of uniformly spraying water around the waste pipe or immersing the steel pipe in water for quenching.

The technical scheme of the invention fully utilizes the residual heat after rolling to realize the strengthening effect of the steel pipe by the deformation-induced phase change benefit, and saves the heat energy consumption of the production process and the steel pipe without adding expensive alloying elements. The comprehensive mechanical properties, while also effectively avoiding cracking of steel pipes.

For the present technical solution, since the reinforcing effect of the steel pipe is achieved by the deformation-induced phase change benefit, the seamless steel pipe according to the present invention has high strength and a yield strength of ≥ 555 MPa.

In addition, the seamless steel pipe of the present invention also has high toughness, and its full-scale impact work of 0 ° C is >50 J.

Further, the seamless steel pipe according to the present invention is suitable for oil and gas production or pipes for mechanical structures.

The method for manufacturing a high-strength and toughness seamless steel pipe according to the present invention can obtain a seamless steel pipe having high strength and good toughness by controlling the heat deformation amount, the quenching temperature, the cooling rate, and the tempering temperature of the steel pipe.

In addition, the manufacturing method of the high-strength and toughness seamless steel pipe according to the present invention has simple process steps, low energy consumption, low cost and high efficiency.

DRAWINGS

1 is a microstructural view of a high strength and toughness seamless steel pipe according to an embodiment A7 of the present invention.

detailed description

The high-strength and toughness seamless steel pipe and the manufacturing method thereof according to the present invention will be further explained and explained below with reference to the accompanying drawings and specific embodiments. However, the explanation and description are not intended to unduly limit the technical solutions of the present invention.

Examples A1-A8 and Comparative Examples B1-B5

The seamless steel tubes in Examples A1-A8 and Comparative Examples B1-B5 of the present invention were produced in accordance with the following procedures:

(1) Smelting and producing tube billet: molten steel smelting, controlling the mass percentage of each chemical element as shown in Table 1, directly pouring the molten steel into a round billet, or casting the billet after casting (or rolling) Made into a tube blank;

(2) heating the tube blank, and obtaining the waste pipe by perforation, continuous rolling, tension reduction or tension sizing: heating the tube blank to 1100-1250 ° C, and maintaining the size of the tube blank for 1-4 hr, in order to ensure the strengthening effect, The ratio of the cross-sectional area of the tube blank to the waste tube is greater than 4.5, and the cross-sectional area ratio of the tube blank before the tension reduction or tension sizing is completed and the tube diameter after the tension reduction or tension sizing is greater than 1.05;

(3) On-line quenching: uniformly spray water around the waste pipe or immerse the steel pipe in water for quenching, quenching and cooling temperature ≥ 850 ° C, cooling rate is 20-60 ° C / s, and the Rockwell hardness of the steel pipe after quenching is greater than 40 HRC ;

(4) Tempering: The tempering temperature is 500-700 ° C and the holding time is 1 hr.

The specific process parameters of the method for producing the seamless steel pipe in the above examples and comparative examples are shown in Table 2, wherein the Rockwell hardness of the steel pipe after the completion of the on-line quenching was measured by a Rockwell hardness tester.

It should be noted that the key to the manufacturing method of the high-strength toughness seamless steel pipe is the steps (2) to (4), and does not mean that the manufacturing method of the high-strength toughness seamless steel pipe in the actual production process only includes the above steps, and other steps. The prior art in the art may be employed, and the other technical steps are not particularly limited.

Table 1 lists the mass percentages of the respective chemical elements in the seamless steel pipes of Examples A1 to A8 and Comparative Examples B1 to B5.

Table 1. (wt.%, margin is Fe and other inevitable impurity elements other than S, P and O)

Table 2 lists specific process parameters of the manufacturing method of the seamless steel pipes of Examples A1 to A8 and Comparative Examples B1 to B5.

Table 2.

After sampling the seamless steel tubes of Examples A1-A8 and Comparative Examples B1-B5, the samples were subjected to mechanical properties tests, and the mechanical property parameters after the tests are shown in Table 3. Among them, the yield strength is obtained by processing the seamless steel pipe into an API arc sample and taking the average after the API standard test; the impact work is to process the seamless steel pipe into 10*10*55 size and V-shaped gap. Standard impact specimens, measured at 0 °C.

Table 3 lists the relevant performance parameters of the seamless steel pipes of Examples A1 to A8 and Comparative Examples B1 to B5.

table 3.

Serial number	Yield strength Rp 0.2 (MPa)	Impact work (full size, 0 ° C) (J)
A1	590	118
A2	645	97
A3	790	89
A4	610	123
A5	708	130
A6	596	105
A7	698	121
A8	714	107
B1	705	35
B2	520	72
B3	496	68
B4	472	154

B5

422

165

As can be seen from Tables 1 and 3, since the mass percentages and process parameters of the chemical elements in the seamless steel tubes of Examples A1 to A8 are within the limits defined by the technical solutions of the present invention, Examples A1-A8 The yield strength of seamless steel tubes is ≥590MPa and the impact energy is ≥89J. However, since the P and S elements in the seamless steel pipe of Comparative Example B1 were too high, the impact work of the seamless steel pipe of Comparative Example B1 was only 35 J, indicating that the toughness of the seamless steel pipe was remarkably lowered. Further, in the seamless steel pipe of Comparative Example B2, the Mn element was too low and the value of C+Mn/6 was too low, so that the hardenability of the seamless steel pipe of Comparative Example B2 was affected, and the seamless steel pipe of Comparative Example B2 was used. The yield strength is only 520 MPa, indicating that the strength of the seamless steel pipe is not high and does not meet the strength requirements of the high strength and toughness seamless steel pipe of the present invention.

It can be understood from the contents of Table 2 and Table 3 that the Mn element in the seamless steel pipe of Comparative Example B3-B5 is out of the scope defined by the technical solution of the present invention. And also because of the cross-sectional area ratio of the tube blank to the waste tube in the step (2) and the tube blank before the tension reduction or tension sizing step and the completion of the tension reduction or tension sizing step in the step B3 The cross-sectional area ratio of the tube blank exceeds the range defined by the technical solution of the present invention, thus affecting the strengthening effect of the deformation-induced phase transformation, resulting in insufficient strength of the steel pipe, and the yield strength of Comparative Example B3 is only 496 MPa. In addition, since the quenching temperature of the seamless steel pipe of Comparative Example B4 is too low, the pro-eutectoid ferrite is first generated in the microstructure in the steel pipe, thereby reducing the strength of the steel pipe, and the yield strength is 472 MPa. In addition, because the cooling rate of the seamless steel tube of Comparative Example B5 is too slow, the martensite in the microstructure of the steel pipe is insufficient, and the seamless steel pipe cannot obtain sufficient strength. Therefore, the seamless steel pipe of Comparative Example B5 is obtained. The yield strength is only 422 MPa.

Referring to Table 1, Table 2 and Table 3, it can be known that the seamless steel pipes of Examples A1 to A8 have a yield strength of ≥ 590 MPa and an impact work of ≥ 89 J, thereby indicating that the seamless steel pipes of Examples A1 to A8 have higher Yield strength and good toughness.

Figure 1 shows the microstructure of the high strength toughness seamless steel pipe of Example A7.

It can be seen from Fig. 1 that the microstructure of the high-strength toughness seamless steel pipe is mainly composed of martensite, and a small amount of ferrite and bainite are also present.

The high-strength and toughness seamless steel pipe according to the present invention has low alloy addition cost and energy consumption in the manufacturing process step, and thus the production cost of the high-strength and toughness seamless steel pipe according to the present invention is more economical and has a wider application range, and can be extended to A steel pipe production line that has strict control requirements for production costs.

The high strength and toughness seamless steel pipe according to the present invention can be used for oil and gas exploitation or mechanical structural pipes.

It is to be noted that the above is only specific embodiments of the present invention, and it is obvious that the present invention is not limited to the above embodiments, and there are many similar variations. All modifications that are directly derived or associated by those of ordinary skill in the art are intended to be within the scope of the invention.

Claims (9)

1. A high strength and toughness seamless steel pipe characterized in that the chemical element mass percentage is:

   C: 0.1-0.25%, Si: 0.1-0.5%, Al: 0.01-0.1%, Mn: 0.6-2%, the balance is Fe and other unavoidable impurities; in addition, it is necessary to satisfy: C+Mn/6≥ 0.35.
2. The high-strength and toughness seamless steel pipe according to claim 1, wherein the microstructure is mainly martensite, and the ratio of martensite is not less than 75%.
3. The high strength toughness seamless steel pipe according to claim 2, wherein the microstructure further comprises a small amount of ferrite and bainite.
4. The high-strength toughness seamless steel pipe according to claim 1, wherein S ≤ 0.005%, P ≤ 0.02%, and O ≤ 0.01% among other unavoidable impurities.
5. The high strength and toughness seamless steel pipe according to claim 1, wherein the yield strength is ≥ 555 MPa, and the full-scale impact work of 0 ° C is > 50 J.
6. The method for producing a high-strength and toughness seamless steel pipe according to any one of claims 1 to 5, which comprises the steps of:

   (1) smelting and producing a tube blank;

   (2) heating the tube blank, through the perforation, continuous rolling, tension reduction or tension sizing to obtain the waste pipe, wherein the cross-sectional area ratio of the pipe blank to the waste pipe is greater than 4.5;

   (3) On-line quenching: quenching and opening temperature of 850-1100 ° C, cooling rate of 20-60 ° C / s, the hardness of the steel tube after quenching is greater than 40HRC;

   (4) Tempering: The tempering temperature is 500-700 °C.
7. The manufacturing method according to claim 6, wherein in the step (2), the tube blank is heated to 1100-1250 ° C and held for 1-4 h.
8. The manufacturing method according to claim 6, wherein in the step (2), the tube blank before the tension reducing or tension sizing step and the tube blank after the completion of the tension reducing or tension sizing step are performed The cross-sectional area ratio is greater than 1.05.
9. The manufacturing method according to claim 6, wherein in the step (3), water is uniformly sprayed around the waste pipe or the steel pipe is immersed in water for quenching.

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