High-strength, high-toughness and low-density steel pipe and manufacturing method thereof

Abstract

The invention discloses a high-strength, high-toughness and low-density steel pipe and a manufacturing method thereof. The high-strength, high-toughness and low-density steel pipe comprises the following components in percentage by weight: 1.0-1.5% of C, 0.1-1.0% of Si, 18.0-25.0% of Mn, 8.0-14.0% of Al, 1.0-4.0% of Ni, 1.0-6.0% of Cr, 0.01-0.07% of Nb, less than or equal to 0.01% of P, less than or equal to 0.01% of S, less than or equal to 0.01% of V, less than or equal to 0.01% of Ti and the balance of Fe and inevitable impurities. The obtained steel pipe has high strength, toughness and plasticity, the density is reduced by 10% or above compared with a traditional seamless steel pipe, the yield strength of the obtained steel pipe is larger than or equal to 900 MPa, the tensile strength is larger than or equal to 1000 MPa, the ductility is larger than or equal to 44%, the impact energy at the temperature of-40 DEG C is larger than or equal to 35 J, and the density is smaller than or equal to 6.95 g / cm < 3 >.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of seamless steel pipes, in particular to a high-strength high-toughness low-density steel pipe and a manufacturing method thereof. BACKGROUND

[0002] Taking deep oil and gas exploitation, one of the key points of national energy strategy, as an example, the overall strength and toughness of the pipe material is required to be higher and higher with the increase of well depth. At present, there are two paths to improve the overall mechanical properties of the pipe material: first, to develop new pipe materials with synchronous improvement of strength and toughness, but the material development has reached a bottleneck, especially the development of high-toughness high-strength steel materials is difficult to guarantee; second, to improve the overall mechanical properties by thickening, adding layers and other measures on the basis of the existing performance of the pipe material, but this will lead to problems such as high cost of pipe material and large load of drilling machine, affecting the safety and efficiency of drilling. Steel pipe products such as high-strength mechanical structure pipe also face the contradiction between pipe weight and performance. Under the premise of guaranteeing strength and toughness, the development of low-density steel pipe can better solve the problems faced by the second path.

[0003] Chinese patent CN 108950392A discloses "a kind of ultra-high ductility low-density steel and its preparation method", the component weight percentage is: C: 0.3~0.5%, Si: 0.2~0.5%, Mn: 8~10%, Alt: 3~5%, Ce: 0.02~0.04%, P≤0.01%, S≤0.01%, N≤0.005%, the balance is Fe and trace elements, according to the conventional air cooling annealing production line, determine the air cooling annealing process matched therewith, obtain 700MPa grade ultra-high ductility low-density steel. Add appropriate content of Al to reduce density, and add appropriate amount of Mn and C elements to obtain ferrite and austenite dual-phase microstructure, through austenite TRIP effect and TWIP effect, the low-density steel has ultra-high elongation and work hardening capacity. By effectively controlling the Mn / Al value to avoid the precipitation of brittle intermetallic compounds, the cracking problem is solved.

[0004] Chinese patent CN 108396244A discloses "a kind of cold-rolled medium-manganese high-aluminum low-density steel and its preparation method", the alloy components are as follows in terms of weight percentage: C: 1.20~1.30%, Mn: 10.7~11.3%, Al: 9.5~10%, P≤0.005%, S≤0.003%, the balance is Fe and inevitable impurities, after the ingot is prepared by controlling the component ratio of medium-manganese high-aluminum, the cold-rolled plate is obtained through processes such as forging, hot rolling, solid solution and cold rolling.

[0005] Chinese patent CN 108486506A discloses "a method for preparing and applying high-performance low-density steel plate", the composition according to the following weight percentage: C: 0.2-1.0%, Mn: 4.0-16.0%, Al: 3.0-12.0%, Si: 0.2-2.0%, Ni: 0.1-1.2%, Cr: 0.1-0.8%, V: 0.01-0.5%, Mo: 0.1-0.3%, Ti: 0.01-0.2%, Nb: 0.01-0.07%, S≤0.03%, P≤0.03%, the balance of Fe and unavoidable impurities, tungsten carbide and iron powder are controlled proportion ball milled and added to the molten metal liquid, and high-performance low-density steel plate is obtained by pouring, forging, hot rolling, water quenching and heat treatment.

[0006] Chinese patent CN 108486492A discloses "1200MPa high-strength high-plasticity low-density steel plate and its manufacturing method", the composition contains C: 0.7-1%, Mn: 13-20%, Al: 8-11%, Ni: 1-4%, Si<0.1%, S<0.01%, P<0.01%, Ti: 0-0.1%, Nb: 0-0.1%, V: 0-0.15%, the balance of Fe and unavoidable impurities, by controlling the alloy composition system after smelting, through hot rolling quenching, solid solution, cold rolling process, the obtained steel plate tensile strength is 1200-1300MPa, elongation is 30-40%, density is 6.5-7g / cm 3 .

[0007] Chinese patent CN107641763A discloses "a kind of high strength plastic product of low-density high manganese steel", the chemical composition mass percentage is: Mn: 19.40-20%, Al: 9.82-10%, C: 0.98-1%, P≤0.003%, S≤0.003%, the rest is Fe and other unavoidable impurities, by controlling the sum of mass percentage of Mn and Al>28% and the mass ratio of Mn, Al 1.8-2.0, and by hot rolling, solid solution, quenching, cold rolling, annealing process, get multi-phase structure of austenite + ferrite + kappa carbide.

[0008] Chinese patent CN106521318A discloses "a high-strength Fe-Mn-Al-C low-density cast steel and a preparation method thereof", the chemical mass percentage of the material is: 0.05-0.35% of C, 2.0-10.0% of Si, 5.0-25.0% of Mn, 3.0-12.0% of Al, 0.1-0.8% of Cr, 0.1-0.5% of Mo, 0.1-1.2% of Ni, 0.01-0.8% of V, 0.6-1.0% of Ti, 0.01-0.08% of Nb, 0.01-0.5% of Re, S≤0.01%, P≤0.02%, the rest is Fe and inevitable impurities, and also needs to meet the following relationships simultaneously: 11%≤Si+Al≤20%, 2C≤Nb+V+Ti≤3C, and then through normalizing, solid solution and aging treatment, a low-density steel with high strength and good plasticity and toughness and having a matrix structure of austenite and good corrosion resistance is obtained. The product has good low-temperature impact resistance and HIC corrosion resistance.

[0009] Chinese patent CN104928569A discloses "an 800MPa grade high-ductility low-density steel and a manufacturing method thereof", the chemical composition mass percentage is: C: 0.25-0.50%, Mn: 0.25-4.0%, Al: 3.0-7.0%, P≤0.02%, S≤0.01%, N≤0.01%, the rest is Fe and inevitable impurity elements, and the above elements need to meet the following relationship simultaneously: 0.134≤(1.5C+0.1Mn) / Al≤0.30, and a low-density steel plate is prepared through hot rolling, pickling, cold rolling and continuous annealing.

[0010] In general, there are many studies on low-density steel plates at present, but since the product form and production process of seamless steel pipes are quite different from other hot-rolled products such as hot-rolled plates, the requirements for pipe blanks are also very high, therefore, the existing low-density steel plates cannot be directly used for the preparation of seamless steel pipes, and the research on high-strength and low-density seamless steel pipes has not been carried out. SUMMARY

[0011] The purpose of the present application is to provide a high-strength and high-ductility low-density steel pipe and a manufacturing method thereof, and the obtained steel pipe has higher strength, toughness and plasticity, and the density is reduced by more than 10% compared with traditional seamless steel pipes, the yield strength of the steel pipe is ≥900MPa, the tensile strength is ≥1000MPa, the elongation is ≥44%, the impact energy at-40℃ is ≥35J, and the density is ≤6.95g / cm 3 .

[0012] To achieve the above purpose, the technical scheme of the present application is as follows:

[0013] A high-strength high-toughness low-density steel pipe, which comprises the following components in percentage by weight: C: 1.0-1.5%, Si: 0.1-1.0%, Mn: 18.0-25.0%, Al: 8.0-14.0%, Ni: 1.0-4.0%, Cr: 1.0-6.0%, Nb: 0.01-0.07%, P≤0.01%, S≤0.01%, V≤0.01%, Ti≤0.01%, and the rest comprises Fe and inevitable impurities.

[0014] Preferably, the rest is Fe and inevitable impurities.

[0015] The steel pipe has an austenitic microstructure, and the grain size is 13.0-15.5 μm.

[0016] The steel pipe has a yield strength of ≥900 MPa, a tensile strength of ≥1000 MPa, an elongation of ≥44%, an impact energy at -40 ℃ of ≥35 J, and a density of ≤6.95 g / cm 3 .

[0017] In the component design of the high-strength high-toughness low-density steel pipe, the following principles are followed:

[0018] C: plays a solid solution strengthening role, and is an important element for ensuring strength and hardenability. When the content of C is less than 1.0%, the strength is difficult to guarantee. When the content of C is greater than 1.5%, cracks are prone to occur during quenching, and the tendency of coarse carbide precipitation at grain boundaries is increased. Therefore, the content of C in the steel pipe is controlled to be 1.0-1.5%.

[0019] Si: can play a solid solution strengthening role, but when the content of Si is too high, the formation of σ phase and carbide is promoted, the cold brittleness tendency of the steel is increased, and the plasticity and toughness of the steel are reduced. Therefore, the content of Si in the steel pipe is controlled to be 0.1-1.0%.

[0020] Mn: has beneficial effects such as expanding the austenite phase region, increasing hardenability, and refining grains, but too much Mn promotes the formation of σ phase and carbide, and segregation is prone to occur during solidification, resulting in the existence of banded structures in the product which have obvious differences in hardness and precipitated phase from the matrix. Therefore, the content of Mn in the steel pipe is controlled to be 18.0-25.0%.

[0021] Al: can effectively reduce the density of the steel, and the density of the steel is reduced by about 1.3% for each 1% of Al added. However, when the content of Al in the steel is too high, it has an adverse effect on the casting process, and therefore the content of Al is limited to be less than 14%. Therefore, the content of Al in the steel pipe is controlled to be 8.0-14.0%.

[0022] Ni: can improve the stability of austenite, in addition, can improve the low temperature toughness and corrosion resistance of steel. But the price of Ni is very expensive, therefore, the content of Ni in the steel pipe of the present application is controlled at 1.0-4.0%.

[0023] Cr: can inhibit the room temperature aging of kappa-carbide, reduce its mass fraction and particle size. But when the content of Cr is too high, there is a risk of precipitating sigma phase, and the hot workability of the steel will be reduced, therefore, the content of Cr in the steel pipe of the present application is controlled at 1.0-6.0%.

[0024] Nb: can effectively refine the grain, improve the strength and toughness of the steel. But the price of Nb is high, therefore, the content of Nb in the steel pipe of the present application is controlled at 0.01-0.07%.

[0025] V: is an associated element in steel, itself is a strong carbide element, the dispersion strengthening of its carbide can improve the strength of the steel, but when the content of V is too high, coarse carbides will be formed, and the price of V is relatively high, so it is not added additionally, and the content of V is controlled at ≤0.01%.

[0026] Ti: is an associated element in steel, when the content of Ti element in the steel is too high, the impact toughness of the steel will be significantly reduced, therefore, the content of Ti in the present application is controlled at ≤0.01%.

[0027] P, S: are harmful elements in steel, their existence has adverse effects on the hot workability, toughness and the like of the steel, therefore, the content of P and S needs to be limited, in the steel pipe of the present application, P is strictly controlled at ≤0.01%, and S is strictly controlled at ≤0.01%.

[0028] The present application reduces the density of the steel by adding Al, combines with the addition of a higher content of Mn element, expands the austenite zone, refines the grain, increases the hardenability, increases the solid solution strengthening and the like, and further combines with the addition of a higher content of C, utilizes the solid solution strengthening effect to ensure the strength of the steel pipe; utilizes the mutual cooperation of Mn and Ni to realize the structure of austenite matrix, ensures the toughness and plasticity, and finally realizes the purpose of low density with high strength and high toughness. The addition of Cr reduces and inhibits kappa-carbide to avoid cracking.

[0029] The manufacturing method of the high-strength high-toughness low-density steel pipe of the present application comprises the following steps:

[0030] 1) smelting, mold casting and slow cooling

[0031] The pipe blank is obtained by smelting and mold casting according to the above-mentioned composition, and the obtained pipe blank is subjected to stack slow cooling for 20-25h;

[0032] 2) heating

[0033] The heating temperature is 1200-1300℃, and the heating time is 1.5-2.5h;

[0034] 3) piercing, continuous rolling

[0035] The heated pipe blank is pierced to obtain a rough pipe, and then is rolled to obtain a hollow pipe;

[0036] 4) reheating

[0037] The hollow pipe is cooled to 500-550 DEG C, and then is reheated to 900-930 DEG C and is kept for 15-25 min;

[0038] 5) tension reducing, water quenching

[0039] The reheated hollow pipe is directly water quenched to room temperature after tension reducing;

[0040] 6) solid solution heat treatment

[0041] The solid solution heat treatment is carried out at 900-1100 DEG C, and the solid solution heat treatment time is 20-35 min.

[0042] The present application adopts higher Mn, Al and other alloy elements in the component design, and if the pipe blank is manufactured according to the continuous casting technology commonly used in the field of seamless steel pipes, there will be more smelting defects, surface defects, even cracking and the like, which affect the internal and external quality, yield and performance of the product, therefore, the present application proposes to manufacture the pipe blank by using the die casting method, to reduce the number of inclusions and banded carbides in the pipe blank, and to obtain a pipe blank with better density. Combined with the subsequent slow cooling operation, the pipe blank crack and other defects are avoided.

[0043] The pipe blank is heated to 1200-1300 DEG C and is kept for 1.5-2.5 h, in order to fully austenitize the structure in the pipe blank and to smoothly carry out the subsequent piercing and other hot rolling processes. The temperature cannot be too low or the time cannot be too short, but the temperature is too high or the time is too long, which will lead to coarse grains, overburning and affect the performance.

[0044] The obtained hollow pipe is cooled to 500-550 DEG C, and the temperature is reduced below the recrystallization temperature, so as to ensure that the steel pipe structure can recrystallize, and the temperature after cooling needs to be controlled below 550 DEG C, but the cooling temperature is not the lower the better, and the temperature is too low on the one hand will delay the production rhythm, and on the other hand will cause the increase of energy consumption. Then the temperature is reheated to 900-930 DEG C, so as to realize the recrystallization of the steel pipe structure, to refine the grains and to improve the strength and toughness of the steel pipe. The reheating temperature is controlled above 900 DEG C, to ensure that the steel pipe of the present application is austenitized, and the temperature is too high will lead to grain growth and affect the performance, therefore the reheating temperature is controlled below 930 DEG C. The holding time is controlled to 15-25 min, and the heating time is too long will lead to grain growth and affect the improvement of mechanical properties.

[0045] The steel pipe of the present application is cooled to room temperature after tension reducing and then is subjected to solid solution heat treatment, the solid solution heat treatment temperature is 900-1100 DEG C, the solid solution heat treatment time is 20-35 min, if the solid solution temperature is too low, the organization in the steel pipe cannot be completely recrystallized, in addition, the twin density is also small, the austenite grain cannot be refined, and there are more unsolved second phase and unsolved kappa-carbide in the grain boundary, which affects the final performance. If the solid solution temperature is too high, the austenite will be excessively grown, which will also affect the final performance. If the holding time is too short, the heating is not enough, the temperature cannot be reached, and if the holding time is too long, the austenite will be excessively grown.

[0046] Compared with the prior art, the present application has the following beneficial effects:

[0047] In the component design, higher C, Mn and Al contents are adopted, the solid solution strengthening effect of C is combined with the effects of expanding the austenite region, refining the grain, increasing the hardenability and increasing the solid solution strengthening of Mn, so that the steel pipe has the purposes of low density and high strength and high toughness. In the existing seamless steel pipe, the contents of C and Mn are usually low, the strength is improved through precipitation strengthening, but only high strength can be achieved, and the density cannot be reduced.

[0048] On the basis of the component design, the die casting and slow cooling process is adopted in the process, the influence of the higher Mn and Al contents on the performance of the pipe blank is reduced, the complete recrystallization of the organization is realized through the subsequent cooling, reheating and solid solution heat treatment, the grain is refined, the microstructure formed is austenite, and the grain size is controlled in 13.0-15.5 μm, so that the higher strength and toughness matching is obtained, and the steel pipe obtained has good quality and will not have cracking defects.

[0049] The yield strength of the steel pipe obtained by the present application is ≥900 MPa, the tensile strength is ≥1000 MPa, the elongation is ≥44%, the impact energy at-40 DEG C is ≥35 J, and the density is ≤6.95 g / cm 3 , which is reduced by more than 10% compared with the traditional seamless steel pipe. The existing seamless steel pipe usually only considers the strength, and does not consider the density. DETAILED DESCRIPTION

[0050] The present application will be further described below in combination with examples.

[0051] The component of the steel pipe of the examples and the comparative examples of the present application is shown in Table 1, the specific process parameters of the examples and the comparative examples of the present application are shown in Table 2, and the performance of the steel pipe obtained by the examples and the comparative examples of the present application is shown in Table 3.

[0052] The density of the steel pipe of examples 1-6 and comparative examples 1-6 is measured by using an electronic direct-reading solid density meter, and the measurement results are shown in Table 3.

[0053] According to GB / T 228.1-2010 Metal Materials Tensile Test Part 1: Room Temperature Tensile Test Method, the yield strength, tensile strength and elongation of Examples 1-6 and Comparative Examples 1-6 at room temperature were obtained, and the test results are shown in Table 3.

[0054] According to GB / T 229-2020 Metal Materials Charpy Pendulum Impact Test Method, control for 10x10x55mm full size sample, to detect the impact energy of Examples 1-6 and Comparative Examples 1-6 at-40℃, and the test results are shown in Table 3.

[0055] The average grain size of Examples 1-6 and Comparative Examples 1-6 was characterized by EBSD inverse pole figure of scanning electron microscope, and the results are shown in Table 3.

[0056] In Comparative Example 1, the Al content added in the composition design is lower than the range described in the application, which causes the density of the steel to not decrease by 10%.

[0057] In Comparative Example 2, the C content added in the composition design is lower than the range described in the application, which causes the strength of the steel to be low.

[0058] In Comparative Example 3, the Mn content added in the composition design is higher than the range described in the application, which causes the steel to have banded segregation structure, causing the mechanical properties to decrease.

[0059] In Comparative Example 4, although all the element compositions of the steel pipe meet the requirements of the application, the grain growth is caused by the high reheating temperature in the reheating stage, causing the mechanical properties to decrease.

[0060] In Comparative Example 5, although all the element compositions of the steel pipe meet the requirements of the application, the solid solution is insufficient due to the too low solid solution temperature, causing the grain of the steel pipe to be large and the mechanical properties to be poor.

[0061] In Comparative Example 6, although all the element compositions of the steel pipe meet the requirements of the application, the solid solution is insufficient due to the short solid solution time, causing the grain of the steel pipe to be large and the mechanical properties to be poor.

[0062]

[0063]

[0064]

[0065]

Claims

1. A high strength high toughness low density steel pipe, the composition of which includes, in weight percent: C： 1.0-1.5%, Si: 0.1-1.0%, Mn: 18.0-25.0%, Al: 8.0-14.0%, Ni: 1.0-4.0%, Cr: 1.0-6.0%, Nb: 0.01-0.07%, P≤0.01%, S≤0.01%, V≤0.01%, Ti≤0.01%, the balance including Fe and inevitable impurities.

2. The high-strength high-ductility low-density steel pipe of claim 1, wherein, the balance being Fe and inevitable impurities.

3. The high-strength high-ductility low-density steel pipe according to claim 1 or 2, characterized in that, The microstructure of the steel pipe is austenite, and the grain size is 13.0-15.5 μm.

4. The high-strength high-ductility low-density steel pipe according to claim 1 or 2 or 3, characterized in that, The steel pipe has yield strength ≥ 900 MPa, tensile strength ≥ 1000 MPa, elongation ≥ 44%, impact energy at -40℃ ≥ 35 J, and density ≤ 6.95 g / cm 3 .

5. The method of producing a high-strength high-ductility low-density steel pipe according to any one of claims 1 to 4, characterized by, The method comprises the following steps: 1) smelting, moulding and slow cooling The pipe blank is obtained by smelting, moulding and slow cooling according to the composition of claim 1 or 2, and the obtained pipe blank is subjected to stack slow cooling for 20-25 h; 2) heating The heating temperature is 1200-1300 °C, and the heating time is 1.5-2.5 h; 3) piercing and continuous rolling The heated pipe blank is pierced to obtain a rough pipe, and then the rough pipe is rolled to obtain a billet; 4) reheating The billet is cooled to 500-550 °C, and then reheated to 900-930 °C and kept for 15-25 min; 5) tension reducing and water quenching The reheated billet is subjected to tension reducing and then directly water quenched to room temperature; 6) solid solution heat treatment The solid solution heat treatment is carried out at 900-1100 °C, and the solid solution heat treatment time is 20-35 min.

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