890 mpa-grade cylinder pipe and manufacturing method therefor

Abstract

A cylinder pipe, comprising Fe and inevitable impurities, and further comprising the following chemical elements in percentage by mass: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; and Ca: 0.0005-0.005%. The present invention further provides a manufacturing method for a cylinder pipe. By means of a reasonable chemical element component design, preferably further in combination with a specific process, the cylinder pipe has high strength, has low residual stress and low yield strength ratio, and also has good welding performance.

Description

An 890MPa grade oil cylinder tube and its manufacturing method Technical Field

[0001] The present disclosure relates to a steel material and a manufacturing method thereof, and in particular to an oil cylinder tube and a manufacturing method thereof. Background Art

[0002] The increasing demand for lightweight construction machinery places increasing demands on the strength of cylinder tubes. However, this increased strength leads to an increase in the carbon equivalent of the tube, which in turn reduces welder performance. This increased strength also increases residual stress in the cylinder tube, leading to dimensional changes after machining and oil leakage.

[0003] The Chinese patent application with publication number CN116162849A and publication date May 26, 2023, entitled “A Cylinder Tube and Its Manufacturing Method” discloses a cylinder tube, the composition of which is as follows by weight: C: 0.16-0.3%, Si: 0.15-0.5%, Mn: 1.2-1.8%, P≤0.01%, S≤0.001%, Nb: 0.02-0.04, Mo: 0.1-0.2%, and when the wall thickness of the cylinder tube is ≥20 mm, Ti and B are added, Ti: 0.015-0.03%, B: 0.0015-0.0035%, and the rest is Fe and other unavoidable impurities. After the steel tube is stretched and quenched, different graded cooling processes are used to increase the steel tube's stiffness and straightness, control the distribution of phase transformation and thermal stress throughout the cylinder tube's wall thickness, and control the distribution of ferrite in the cylinder tube's microstructure. This results in a cylinder tube with high strength and low residual stress. The yield strength of this cylinder tube is ≥600MPa, the tensile strength is ≥730MPa, and the residual stress is ≤50MPa.

[0004] However, the strength of the steel materials disclosed in the above patent documents is still not high enough.

[0005] Summary of the Invention

[0006] One of the purposes of the present disclosure is to provide a cylinder tube, which has high strength, low residual stress and yield strength ratio, and good welding performance through reasonable chemical element composition design and preferably further combined with process design.

[0007] To achieve the above-mentioned objectives, the present disclosure provides a cylinder tube, which, in addition to containing Fe and inevitable impurities, also contains the following chemical elements in the following mass percentages: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%.

[0008] In a preferred embodiment, the cylinder tube of the present invention contains the following chemical elements in the following mass percentages: C: 0.14-0.20%; Si: 0.15-0.55%; Mn: 0.5-1.5%; Cr: 0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V: 0.05-0.1%; Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%; the balance is Fe and unavoidable impurities.

[0009] In a preferred embodiment, the mass percentage of each chemical element of the cylinder tube of the present disclosure further satisfies at least one of the following items: Si: 0.15-0.35%; Mn: 1.1-1.5%; Cr: 0.5-1%.

[0010] In a preferred embodiment, the carbon equivalent of the oil cylinder tube disclosed herein is less than 0.72, preferably 0.45-0.65, and more preferably 0.50-0.65, and the carbon equivalent is calculated by the following formula: carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the symbols of each element are substituted into the numerical value before the percentage sign of the mass percentage content of the corresponding chemical element.

[0011] In a preferred embodiment, the outer wall structure and the center structure of the oil cylinder tube disclosed herein are tempered bainite + bainite, and the inner wall structure is tempered bainite + bainite + ferrite.

[0012] In a preferred embodiment, the total volume content of bainite and ferrite in the inner wall structure is less than 20%, and the volume content of bainite in the outer wall structure and the center structure is less than 10%.

[0013] In a preferred embodiment, the performance of the cylinder tube of the present disclosure meets the following requirements: yield strength ≥ 890 MPa, tensile strength of 960 MPa to 1100 MPa, yield strength ratio ≤ 0.93 and / or -40°C impact toughness KV8 ≥ 45 J, preferably KV8 ≥ 70 J.

[0014] In a preferred embodiment, the residual stress of the cylinder tube of the present disclosure is -100 MPa to 48 MPa, preferably -30 MPa to 40 MPa.

[0015] The steel disclosed herein is designed with a low-carbon, micro-alloyed steel containing W, Cr, and Mo, and the carbon equivalent of the steel is controlled to be ≤0.72, so that it has good welding performance.

[0016] On the other hand, the present disclosure also provides a method for manufacturing a cylinder tube, comprising the following steps: (1) smelting and continuous casting to obtain a tube blank; (2) heating, perforating, rolling, and sizing; (3) online controlled cooling: the starting cooling temperature is above 800°C, and cooling is performed by spraying water on the outer wall, the average cooling rate is 20-40°C / s, and the final cooling temperature is 600°C-680°C; then air cooling to room temperature; (4) quenching: the heating temperature is 820-880°C, the holding time is 20-40min, and cooling is performed by water cooling after heating. During cooling, the steel tube is rotated, and the outer wall of the steel tube is sprayed with water by an external water spraying method. After cooling for 35-40s, the inner wall of the steel tube is sprayed with water to cool; (5) tempering.

[0017] In a preferred embodiment, in the quenching step (4), the flow rate density of the water spray cooling on the outer wall of the steel pipe is 3000-3500m 3 / (h*mm 2 ), where "h" represents hours.

[0018] In a preferred embodiment, in the tempering step (5), the tempering temperature is 500-600° C., and the holding time is 20-30 min.

[0019] In the manufacturing method disclosed herein, the online controlled cooling process utilizes the technology of preheating the tube after rolling and then rapidly cooling it to refine the rolled structure, thereby further refining the structure after quenching and tempering heat treatment and ensuring a good match between strength and toughness.

[0020] Furthermore, in the manufacturing method disclosed herein, during the quenching and tempering heat treatment process, a process is adopted in which the outer wall is cooled first, and then the inner wall is cooled by delayed water spraying. This process produces seamless pipes that are easy to weld, have a low yield ratio, and low residual stress. DETAILED DESCRIPTION

[0021] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs.

[0022] As used herein, the term "and / or" refers to and encompasses any and all possible combinations of more than one of the listed items.

[0023] In this article, carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the symbol of each element is substituted into the value before the percentage sign of the mass percentage of the corresponding chemical element.

[0024] In this paper, the volume content of the microstructure is determined by metallographic structure output and phase ratio analysis methods.

[0025] In this paper, the yield strength and tensile strength were measured according to GB / T 228.1-2000.

[0026] In this paper, -40℃ impact toughness KV8 is measured according to GB / T 229-2007 "Charpy pendulum impact test method for metallic materials".

[0027] In this paper, residual stress was measured according to ISO / TR 10400 standard.

[0028] In the oil cylinder tube disclosed herein, the design principles of the various chemical elements are as follows:

[0029] C: In the cylinder tube disclosed herein, carbon is an essential element for ensuring strength and hardenability. However, its content affects the carbon equivalent and weldability. If the carbon content in the steel is less than 0.14%, the steel's hardenability is poor and its strength does not meet requirements. If the carbon content in the steel exceeds 0.2%, this excessive carbon content can lead to poor weldability and reduced plasticity and toughness. Therefore, the present disclosure controls the mass percentage of carbon to between 0.14% and 0.20%.

[0030] Si: In the cylinder tube disclosed in the present invention, Si acts as an important deoxidizer in the steel. The Si element can be dissolved in ferrite to increase the yield strength of the steel. However, it should be noted that the content of Si element in the steel should not be too high. When the Si element content in the steel is higher than 0.55%, the processing performance and toughness of the steel will deteriorate. When the Si element content in the steel is too low, below 0.15%, the steel will become easily oxidized. Therefore, the present invention controls the mass percentage of the Si element between 0.15-0.55%.

[0031] Mn: In the cylinder tube disclosed herein, Mn acts as a deoxidizing and desulfurizing agent, significantly impacting the steel's hardenability and toughness. A Mn content of 0.5% or greater in the steel produces excellent results. However, excessive Mn can increase the steel's hardenability, reduce the toughness of the heat-affected zone (HAZ) during welding, and cause center segregation during continuous casting, deteriorating the impact toughness of the base material. Therefore, the present disclosure limits the mass percentage of Mn to between 0.5% and 1.5%.

[0032] Cr: In the cylinder tube disclosed herein, Cr increases the strength and hardenability of the steel, and its combination with Mo is particularly effective. However, it should be noted that excessive Cr content in the steel significantly increases the carbon equivalent, leading to increased weld cracking susceptibility and reduced toughness in the weld heat-affected zone. Therefore, the present disclosure limits the Cr content to between 0.5% and 1.5% by weight.

[0033] Mo: In the oil cylinder tube disclosed in the present invention, Mo is one of the main added elements, which can improve the hardenability of steel. The composite effect of Mo and Cr can better improve the hardenability. In addition, the Mo element also has good precipitation strengthening and solid solution strengthening effects. Mo has a good interaction with micro-alloying elements, which can effectively refine the precipitate phase, increase the stability and volume fraction of the precipitate phase, and improve the strength and toughness of the weld heat affected zone. However, it should be noted that the Mo element is expensive. Adding too high a content of Mo to the steel will not only lead to an excessively high carbon equivalent, but also increase the cost of the alloy. Therefore, the present invention controls the mass percentage of the Mo element between 0.25-0.65%.

[0034] W: W is the primary additive element in this steel grade. It significantly improves strength through solid solution strengthening without increasing carbon equivalent or significantly affecting weldability. It also maintains excellent post-weld strength. However, excessive W content can lead to surface defects in rolled pipes and excessively high costs. Therefore, the present disclosure limits the W content to between 0.3% and 0.8% by mass.

[0035] V: In the cylinder tube disclosed herein, V refines the steel grains, and the carbides it forms significantly enhance the steel's strength. However, when V is added to steel at a certain level, its strengthening effect is not significant. Furthermore, V is a relatively expensive alloying element and should not be added in excessive amounts. Therefore, in the present disclosure, the mass percentage of V is controlled between 0.05% and 0.1%.

[0036] Nb: The primary function of Nb is to refine phase transformations and grain size through carbide precipitation. In this disclosure, adding a small amount of Nb not only refines the microstructure and improves strength-toughness matching, but also significantly reduces post-weld softening. However, excessive Nb content increases costs without increasing the refinement effect. Therefore, this disclosure limits the Nb content to between 0.02% and 0.06% by mass.

[0037] Al: In the cylinder tube disclosed herein, Al is a preferred deoxidizing element. However, adding too much Al to the steel can easily result in aluminum oxide inclusions. To maximize the proportion of acid-soluble aluminum in the total aluminum content, an appropriate amount of Al wire is added after vacuum degassing. Therefore, the mass percentage of Al in the disclosed embodiment is controlled between 0.01-0.05%.

[0038] Ca: In the cylinder tube disclosed herein, Ca purifies the molten steel, promotes the spheroidization of MnS, and improves the material's impact toughness. However, it's important to note that the Ca content in the steel should not be too high. Excessive Ca content in steel can easily form coarse non-metallic inclusions, which can affect the steel's performance. Therefore, the mass percentage of Ca in the disclosed invention is controlled between 0.0005% and 0.005%.

[0039] In addition, it should be noted that the oil cylinder tube of the present disclosure still contains some unavoidable impurity elements, which are mainly S and P. If technical conditions permit, in order to obtain an oil cylinder tube with better performance and higher quality, the content of impurity elements should be reduced as much as possible.

[0040] Through design of the composition and process, this disclosure achieves a steel with an outer wall and core structure composed of tempered bainite with a small amount of bainite, and an inner wall structure composed of tempered bainite with a small amount of bainite and a small amount of ferrite. This is because when cooling only the outer wall, the inner wall temperature drops below Ar3, causing some ferrite to precipitate. Water spray cooling then transforms the remaining austenite into martensite and bainite, which then transforms into tempered bainite with a small amount of bainite and a small amount of ferrite during tempering.

[0041] In the manufacturing method disclosed in the present invention, in order to further refine the rolled structure, thereby utilizing the genetic characteristics of the structure and refining the final tempering heat treatment structure, a rapid cooling technology after sizing is adopted to effectively utilize the residual heat of the pipe rolling. The thermal deformation during the sizing process causes a large number of dislocations in the pipe body, and these dislocations are effectively retained after rapid cooling to a certain temperature. These dislocations can become nucleation points for phase transformation and precipitation phases. In addition, rapid cooling increases the supercooling degree of phase transformation, thereby increasing the driving force of phase transformation. These two aspects lead to a significant refinement of the rolled structure. Based on this, the specific parameters of the online controlled cooling adopted in the present invention are as follows: the starting cooling temperature is ≥800°C, cooling is carried out by spraying water on the outer wall, the average cooling rate is 20-40°C / s, and the final cooling temperature is 600°C-680°C; then air cooling to room temperature, and then subsequent tempering heat treatment is carried out. In the present disclosure, the online controlled cooling step forms a hardened layer on the outer wall by spraying water to cool the outer wall, thereby increasing the deformation resistance of the outer wall, ensuring that the pipe body is less likely to bend and deform during the subsequent cooling process, ensuring the straightness of the pipe body, and at the same time reducing the residual stress level of the entire pipe. Since the surface is a hardened layer, compressive stress is generated, which increases the tolerance of the pipe body to surface defects.

[0042] In the manufacturing method disclosed in the present invention, water cooling is used for cooling after the quenching heating step. The steel pipe is rotated during cooling, and the outer wall of the steel pipe is sprayed with water by external water spraying. After cooling for 35-40 seconds, the inner wall is sprayed with water to ensure that the positions where the cooling effect is poor during the cooling of the outer wall are fully cooled by the cooling of the inner wall. Since the entire length of the steel pipe is cooled simultaneously by external water spraying, and the steel pipe is rotated during cooling, the cooling uniformity is ensured to be better, and the rigidity of the steel pipe is increased. The better cooling uniformity ensures that the steel pipe has a better straightness level, avoiding the large residual stress caused by the subsequent bending and straightening deformation of the pipe. The above-mentioned cooling method disclosed in the present invention can obtain a better straightness level compared with the method of simultaneous cooling of the outer wall by external spraying + inner wall axial flow.

[0043] In addition, the residual stress of the steel pipe is closely related to the phase change and thermal stress during the cooling process. Through the above-mentioned cooling method disclosed in the present invention, a gradient distribution of phase change and thermal stress is obtained in the wall thickness direction. The outer wall after water cooling produces compressive stress due to thermal expansion and contraction, and the phase change stress is tensile stress. The sum of the two is within the range of 0-200MPa. The thermal stress at the center of the wall thickness is tensile stress, and the phase change stress is compressive stress. The sum of the two is negative. The thermal stress and phase change stress of the inner wall are both positive, so there is a certain risk of cracking on the inner wall, and tempering treatment is required as soon as possible after cooling.

[0044] After tempering, the residual stress in the wall thickness direction of the cylinder tube of the present disclosure is reduced, but the gradient distribution trend remains unchanged. The residual stress level of the entire tube body after tempering is -100 MPa to 48 MPa.

[0045] The oil cylinder tube and the manufacturing method thereof disclosed in the present invention have the following advantages and beneficial effects:

[0046] The oil cylinder tube disclosed herein is designed with a low-carbon, micro-alloyed steel containing W, Cr, and Mo, and the carbon equivalent of the steel is controlled to be ≤0.72, so that it has good welding performance.

[0047] The disclosed online controlled cooling process utilizes residual heat from rolling to rapidly cool the pipe, refining the as-rolled microstructure. This further refines the microstructure after quenching and tempering, ensuring a good balance of strength and toughness. Furthermore, during the quenching and tempering heat treatment, the outer wall is cooled first, followed by delayed water spray cooling of the inner wall. This process produces seamless pipes that are easy to weld, have a low yield ratio, and exhibit low residual stress.

[0048] The oil cylinder tube and the manufacturing method thereof disclosed in the present invention will be further explained and illustrated below in conjunction with specific embodiments. However, such explanation and illustration do not constitute an undue limitation to the technical solution disclosed in the present invention.

[0049] Examples 1-8 and Comparative Examples 1-8

[0050] The oil cylinder tubes of Examples 1-8 were prepared by the following steps:

[0051] (1) Smelting and continuous casting are carried out according to conventional processes to obtain a tube blank;

[0052] (2) heating, punching, rolling, and sizing the tube blank according to conventional processes to obtain a tube body;

[0053] (3) Using the residual heat from rolling, the pipe is cooled online after sizing: the cooling temperature is ≥800°C, and the outer wall is sprayed with water for cooling, with an average cooling rate of 20-40°C / s and a final cooling temperature of 600-680°C; then air-cooled to room temperature;

[0054] (4) Quenching: The heating temperature is 820-880℃, the holding time is 20-40min, and the steel pipe is cooled by water cooling after heating. The steel pipe is rotated during cooling, and the outer wall of the steel pipe is sprayed with water for cooling. After cooling for 35-40s, the inner wall of the steel pipe is sprayed with water until the pipe body is cooled to below 100℃. Optionally, the flow density of the water spraying for cooling the outer wall of the steel pipe is 3000-3500m 3 / (h*mm 2 ). The water spray pressure can be 0.5MPa;

[0055] (5) Tempering: Control the tempering temperature to 500-600℃ and the holding time to 20-30min.

[0056] The comparative oil cylinder tubes of Comparative Examples 1-8 were prepared using the same steps as above, with the only difference being the component content and specific process parameters.

[0057] Table 1 lists the mass percentage content of each chemical element in the oil cylinder tubes of Examples 1-8 and the comparative oil cylinder tubes of Comparative Examples 1-8.

[0058] Table 1. (wt%, the balance is Fe and unavoidable impurities)

[0059] Note: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15. Since Examples 1-8 and Comparative Examples 1-8 do not contain Cu and Ni, the values ​​of these two elements are substituted into 0 for calculation.

[0060] Table 2 lists the specific process parameters of the oil cylinder tubes of Examples 1-8 and the comparative oil cylinder tubes of Comparative Examples 1-8 in the online controlled cooling and tempering treatment steps (ie, quenching and tempering steps).

[0061] The oil cylinder tubes of Examples 1-8 and the control tubes of Comparative Examples 1-8 were sampled and tested. The test results are listed in Table 3. The test process of the relevant properties is as follows:

[0062] (1) Microstructure: Take a cross section of the tube for microstructure observation. Grind and polish the sample. Etch the metallographic structure with 4% nitric acid + alcohol. After etching, observe the structure using a metallographic microscope.

[0063] (2) Tensile properties: The room temperature tensile properties were measured according to GB / T 228.1-2000.

[0064] (3) Impact toughness: The impact energy at -40°C is measured according to GB / T 229-2007 “Charpy pendulum impact test method for metallic materials”.

[0065] (4) Welding performance: Welding evaluation shall be carried out in accordance with NB / T-47014-2011 Welding procedure assessment for pressure-bearing equipment.

[0066] (5) Residual stress: measured in accordance with ISO / TR 10400 standard.

[0067] Table 3 lists the test results of the oil cylinder tubes of Examples 1-8 and the comparative oil cylinder tubes of Comparative Examples 1-8.

[0068] Table 3.

[0069] As shown in Table 3, the cylinder tubes of Examples 1-8 all have yield strengths ≥900 MPa, tensile strengths ≥1025 MPa, impact energies ≥70 J at -40°C, yield ratios ≤0.91, and residual stresses ranging from -30 MPa to 40 MPa. Furthermore, they exhibit good weldability. In contrast, Comparative Examples 1-3 fail to achieve a combination of high strength, low yield ratio, good low-temperature toughness, and excellent weldability.

[0070] All publications, patent applications, patents, and other references mentioned in this disclosure are incorporated by reference in their entirety.

[0071] Although the present disclosure has been illustrated and described with reference to certain preferred embodiments thereof, it should be understood by those skilled in the art that the above description is provided to further illustrate the present disclosure in conjunction with specific embodiments thereof, and that the present disclosure is not limited to these descriptions. Those skilled in the art may make various changes in form and detail, including simple deductions or substitutions, without departing from the spirit and scope of the present disclosure.

Claims

1. A cylinder tube, which contains, in addition to Fe and unavoidable impurities, the following chemical elements in the following mass percentages: C:0.14-0.20%;Si:0.15-0.55%;Mn:0.5-1.5%;Cr:0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V：0.05-0.1％； Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%.

2. The oil cylinder tube according to claim 1, comprising the following chemical elements in the following mass percentages: C:0.14-0.20%;Si:0.15-0.55%;Mn:0.5-1.5%;Cr:0.5-1.5%; Mo: 0.25-0.65%; W: 0.3-0.8%; V：0.05-0.1％； Nb: 0.02-0.06%; Al: 0.01-0.05%; Ca: 0.0005-0.005%; the balance is Fe and inevitable impurities.

3. The oil cylinder tube according to claim 1 or 2, wherein the mass percentage of each chemical element thereof further satisfies at least one of the following items: Si: 0.15-0.35%; Mn: 1.1-1.5%; Cr：0.5-1％。 4. The cylinder tube according to any one of claims 1 to 3, wherein: The carbon equivalent of the oil cylinder tube is less than 0.72, preferably 0.45-0.65, and more preferably 0.50-0.

65. The carbon equivalent is calculated by the following formula: Carbon equivalent = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15, where the symbol of each element is substituted into the value before the percentage sign of the mass percentage content of the corresponding chemical element.

5. The cylinder tube according to any one of claims 1 to 4, wherein: The outer wall structure and the central structure of the oil cylinder tube are tempered troostite+bainite, and the inner wall structure is tempered troostite+bainite+ferrite.

6. The cylinder tube according to claim 5, wherein: The total volume content of bainite and ferrite in the inner wall structure is less than 20%, and the volume content of bainite in the outer wall structure and the central structure is less than 10%.

7. The cylinder tube according to any one of claims 1 to 6, wherein: The performance of the oil cylinder tube meets the following requirements: yield strength ≥ 890 MPa, tensile strength of 960 MPa to 1100 MPa, yield strength ratio ≤ 0.93, and / or -40°C impact toughness KV8 ≥ 45 J, preferably KV8 ≥ 70 J.

8. The cylinder tube according to any one of claims 1 to 7, wherein: The residual stress of the cylinder tube is -100 MPa to 48 MPa, preferably -30 MPa to 40 MPa.

9. A method for manufacturing the cylinder tube according to any one of claims 1 to 8, comprising the following steps: (1) Smelting and continuous casting to obtain tube blanks; (2) heating, perforating, rolling, and sizing to obtain a tube body; (3) Online controlled cooling: The starting cooling temperature is above 800°C, and the cooling is carried out by spraying water on the outer wall. The average cooling rate is 20-40°C / s, and the final cooling temperature is 600°C-680°C; then air cools to room temperature; (4) Quenching: The heating temperature is 820-880°C, the holding time is 20-40 minutes, and the steel pipe is cooled by water after heating. The steel pipe is rotated during cooling, and the outer wall of the steel pipe is sprayed with water for cooling. After cooling for 35-40 seconds, the inner wall of the steel pipe is sprayed with water for cooling; (5) Tempering.

10. The method of claim 9, wherein: In the (4) quenching step, the flow density of the water spray cooling on the outer wall of the steel pipe is 3000-3500m 3 / (h*mm 2 ).

11. The method according to claim 9 or 10, wherein: In the tempering step (5), the tempering temperature is 500-600° C. and the holding time is 20-30 min.

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