Hot-rolled steel plate for medium-break deep well mining casing and preparation method thereof

Abstract

The present invention relates to a hot-rolled steel plate for medium-break deep well mining casing, wherein the alloy components are as follows by weight percentage: 0.18-0.25% C, 1.20-1.50% Mn, 0.10-0.30% Si, ≤0.010% P, ≤0.010% S, 0.01-0.06% Als, 1.0-1.40% Cr, 0.40-0.60% Mo, and the remainder is iron and inevitable impurities. The present invention significantly improves the control level of central segregation of oil well casing and improves the internal quality of continuous casting billet by optimizing chemical composition, smelting process, process parameters of continuous casting machine, ensuring the accuracy of fan-shaped segment equipment, increasing the cooling intensity of secondary cooling zone, etc. The central segregation of the steel plate is ≤C1.0, the proportion of equiaxed crystals is 30-40% and the proportion of columnar crystals is 60-70%. It has high concentricity, uniform wall thickness, high dimensional accuracy and high elongation after fracture. The weld metallographic structure of the medium-break deep well welding casing prepared by the full-tube heat treatment process is uniform and the performance reaches the level of the parent material, realizing the optimization of weld performance and having high anti-collapse ability. Its impact toughness and flattening test results are better than those of seamless steel pipes.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy, and specifically relates to a hot-rolled steel plate for medium-break deep well mining casing and a preparation method thereof, and particularly relates to a steel plate for manufacturing oil casing, oil expansion tube and pipeline and a preparation method thereof. Background Art

[0002] With the development of the oil industry, the requirements for the performance of oil well casing are constantly increasing, and the forming process and structure of oil well casing materials are also constantly developing. Its forming process can be divided into two categories: welded oil well casing and seamless oil well casing. The advantages of welded oil well casing are that it has high geometric dimensional accuracy, uniform wall thickness, and its ability to resist collapse and crushing when serving underground is 10-15% higher than that of seamless casing of the same steel grade. The material has low carbon content, no tearing of the holes after perforation, and low price.

[0003] Oil well casing is a steel pipe that supports the well wall. The life of the casing determines the life of the oil well. The deeper the oil well, the greater the tension or pressure on the casing in the well. Deep wells, ultra-deep wells and other difficult process wells have put forward new and higher requirements for the performance of oil well casing. According to the service conditions of oil well casing and its main failure forms, the oil industry has the following requirements for oil well casing steel: high strength, high toughness, high collapse resistance, and high threaded connection strength.

[0004] At present, the C content of oil well casing produced by major steel mills is controlled within the range of 0.32-0.36%, and the Mn content is controlled within the range of 1.8-2.0%. The main problem with this traditional process is that due to the high C and Mn content in the steel, during the continuous cooling process of the continuous casting billet, defects such as cracks, segregation and looseness are easily formed in the center and between dendrites, causing the hot-rolled plate to form a serious banded structure. In order to solve the problems existing in the traditional production process and meet the performance requirements of oil well casing, many steel mills adopt the design of reducing the Mn content and adding Nb, V, and Ti micro-alloying components. This method can reduce the harm caused by high Mn content, but because Nb, V, and Ti elements have extremely active chemical properties, on the one hand, they combine with nitrogen and oxygen in steel to reduce the alloy yield in steel; on the other hand, the alloy cost is relatively high.

[0005] The steel plate is cut into two equal parts longitudinally from the center, that is, the user uses "one broken into two". As a result of selective crystallization, there will be central segregation in the middle of the steel plate during the solidification process of the continuous casting billet, which deteriorates the performance of the steel, and is accompanied by the generation of central porosity and central cracks, which can easily cause defects such as molding cracking, delamination, and inconsistent microstructure and performance between the weld and the parent material. Therefore, how to eliminate or reduce the central segregation of the oil well casing so that the various properties after pipe making meet the requirements of APIspec5CT is particularly prominent and necessary.

[0006] For example, Chinese patent CN107881428B "A production method for reducing dendritic segregation of hot-rolled coil Q345: mainly adding Cr to replace part of Mn and C, and no Mo, an element that increases hardenability, is added.

[0007] Chinese patent CN108856668B "A method for controlling the center segregation of continuous casting billets" includes high superheat control, strong secondary cooling control, pressure reduction control, etc. The composition is controlled at 0.09-0.12% C, 1.85-2.20% Mn and 0.1-0.2% Ti. Since Ti is more active in steel, it causes large fluctuations in performance. On the one hand, it combines with nitrogen and oxygen in steel to reduce the alloy yield in steel; on the other hand, the alloy cost is higher.

[0008] Chinese patent CN113198993.A "Method for reducing center segregation of low alloy steel continuous casting billet" mainly optimizes the molten steel composition and casting parameters according to the carbon equivalent range.

[0009] Chinese patent CN110541056.B "Process for reducing center segregation of ingot" mainly includes: control of molten steel purity and inclusions in steel; control of ingot shell bulging before pressing and maximum forward pressing, etc.

[0010] In summary, it is expected that steel plates with good corrosion resistance can replace seamless steel pipes under room temperature and acidic environment, but there is currently no more suitable production technology. Summary of the invention

[0011] In order to achieve the above-mentioned purpose, the present invention provides a hot-rolled steel plate for medium-break deep-well mining casing, which is a special steel material specially developed for manufacturing equipment working in an acidic environment. The so-called center segregation refers to the difference in chemical composition. On the cross-sectional sample of the continuous casting billet, samples are taken from the surface to the center at regular intervals for chemical composition analysis, and it is found that the content of carbon, phosphorus, sulfur, etc. in the center is significantly higher than that in other parts. It is caused by the flow of molten steel that has not yet solidified during the solidification process of the continuous casting billet, especially at the end of solidification. The present invention significantly improves the control level of center segregation of oil well casing and improves the internal quality of the continuous casting billet by optimizing the chemical composition, smelting process, continuous casting machine process parameters, ensuring the accuracy of the fan-shaped segment equipment, increasing the cooling intensity of the secondary cooling zone, etc.

[0012] The specific content of the invention is as follows:

[0013] A hot-rolled steel plate for medium-break deep-well mining casing, the alloy components of the steel plate are as follows by weight: 0.18-0.25% C, 1.20-1.50% Mn, 0.10-0.30% Si, ≤0.010% P, ≤0.010% S, 0.01-0.06% Als, 1.0-1.40% Cr, 0.40-0.60% Mo, and the rest are iron and inevitable impurities.

[0014] The present invention adds Cr and Mo alloy elements in the composition ratio, reduces the content of Mn and C, utilizes the effects of Cr and Mo on precipitation strengthening, solid solution strengthening and increasing hardenability in steel, replaces part of Mn, C and Nb, V and Ti, and significantly reduces the central segregation and dendrite segregation behavior in the steel without affecting the mechanical properties of the strip steel, reduces the precipitation of pearlite, promotes and refines the ferrite grains, improves the formability and toughness of the steel plate, refines the structure, reduces the banded structure of the steel plate after rolling, and obtains the mechanical properties that meet the standard requirements.

[0015] Preferably, the central segregation of the steel plate is ≤C1.0, the proportion of equiaxed crystals is 30-40% and the proportion of columnar crystals is 60-70%.

[0016] Another object of the present invention is to provide a method for preparing a hot-rolled steel plate for medium-break deep well mining casing, which is prepared according to the following process steps:

[0017] 4. Smelting: adopt top and bottom combined blowing converter smelting, apply pure steel smelting process, control the cleanliness of molten steel, oxygen blowing time: 12-15min, control the tapping hole to be round, control the oxygen content at 400-500PPm during tapping, carry out deoxidation and alloying, add molybdenum iron, chromium iron and other alloys during tapping, reach the tapping composition range, add the insufficient part during refining, and ensure that the alloy components of the molten steel meet the target requirements;

[0018] 1 Continuous casting: The continuous casting machine with electromagnetic stirring is used for production. The casting is performed at a constant pulling speed. The superheat and pulling speed of the molten steel are strictly controlled. Electromagnetic stirring devices are installed on the wide surface of the crystallizer, the fan-shaped section of the second cooling zone of the slab continuous casting machine, and the end of the slab continuous casting solidification. The electromagnetic stirring current intensity is 260-270A and the current frequency is 1-5Hz. At the same time, the two fan-shaped sections at the end of the slab continuous casting solidification and after the electromagnetic stirring in the second cooling zone are mechanically pressed down to obtain a homogenized slab. The electromagnetic stirring in the second cooling zone can stir and mix the molten steel to make the steel The water temperature and the solidified billet shell are uniform, the equiaxed crystal ratio is increased, and the surface quality is improved; mechanical pressure reduction at the end of solidification can promote the discharge of molten steel rich in solute elements upstream, improve central segregation, compensate for solidification shrinkage, and inhibit shrinkage cavity growth. At the same time, the secondary cooling electromagnetic stirring can further mix the concentrated molten steel discharged by mechanical pressure reduction. The secondary cooling electromagnetic stirring and the mechanical pressure reduction at the end of solidification are combined to achieve the purpose of improving the internal quality of the ingot; control the spacing between the nip rollers, and require the nip rollers to be strictly centered; accurately control the arc and opening of the fan-shaped segment; increase the cooling intensity of the secondary cooling zone;

[0019] 2 Rolling: The continuous casting billet is heated to 1200-1240℃ and then sent into the hot rolling mill for rolling. Rough rolling is performed for 3 passes of R1 and 3 passes of R2. The rough rolling is performed with full water to remove the primary iron oxide scale. After rough rolling, the billet is sent into the 7-stand finishing rolling mill for rolling. The final product thickness is 2.5-12.0mm.

[0020] Preferably, the spacing between the nip rollers is 170-190 mm. During the straightening process of the billet, the spacing between the nip rollers must be well controlled and the rollers must be strictly aligned to avoid internal cracks and bulging.

[0021] Preferably, the superheat of the molten steel is controlled within the range of ΔT10 to 15° C., which can effectively reduce the occurrence of center segregation.

[0022] Preferably, the pulling speed is strictly controlled between 1.6 and 1.8 m / min according to the different cross sections of the continuous casting billet.

[0023] Preferably, the curvature of the fan-shaped segment is 120-150°, the opening degree of the fan-shaped segment is 0.2-0.5 mm, the curvature error of the rollers between segments within the fan-shaped segment is ≤0.2 mm, and the opening degree error of the fan-shaped segment does not exceed 0.1 mm.

[0024] Furthermore, the mechanical pressing adopts the light pressing technology of continuous casting billet, and by changing the deformation behavior of the billet shell at the end of solidification of the billet, the molten steel rich in solute segregation elements in the two-phase region is forced to be squeezed upstream to be discharged and compensate for the solidification shrinkage. The roll spacing is adjusted, the single roll pressing amount is 4 to 5 mm, and the total pressing amount is 12 to 15 mm.

[0025] Preferably, the process for increasing the cooling intensity of the secondary cooling zone is: increasing the cooling water volume of the secondary cooling zone by 50-60%.

[0026] Another object of the present invention is to provide a medium-break deep well mining casing made of the above hot-rolled steel plate, wherein the casing is made by a full-tube heat treatment process. Since the pipe making process uses HFW welding heating tension reduction, there will be differences in the structure and performance of the pipe body and the weld. The full-tube heat treatment process, i.e., quenching and tempering process, can eliminate the adverse effects caused by center segregation on the one hand; on the other hand, the structure and performance of the pipe body and the weld are consistent, avoiding the generation of defects.

[0027] The above-mentioned method for preparing hot-rolled steel plates for medium-break deep-well mining casing only limits the key steps and parameters. The preparation process also includes conventional steps and parameters for preparing steel, such as deoxidation and alloying, refining, heating, curling, etc., which can be carried out in the manner disclosed in the prior art and meet the process requirements.

[0028] The beneficial effects of the present invention are:

[0029] (1) The hot-rolled steel plate and welded pipe forming prepared by the method of the present invention are easy to control and have high dimensional accuracy. Therefore, the diameter and wall thickness tolerance of the medium-break deep well welded casing prepared by using this steel plate are much smaller than those of the seamless steel pipe, and the concentricity is high and the wall thickness is uniform.

[0030] (2) The method of the present invention strictly controls the superheat and the pulling speed to make the center segregation of the steel plate ≤C1.0 level.

[0031] (3) The electromagnetic stirring process adopted by the method of the present invention is a homogenized slab continuous casting production method under external field coordinated control. The method adds an electromagnetic stirring process to the crystallizer, the second cooling zone of the slab continuous casting machine and the end of the slab continuous casting solidification, and simultaneously performs mechanical pressing at the end of the slab continuous casting solidification and the two fan-shaped sections of the second cooling zone to obtain a homogenized slab. Among them, the electromagnetic stirring in the second cooling zone can stir and mix the molten steel, make the molten steel temperature and the solidified slab shell uniform, increase the equiaxed crystal ratio, and improve the surface quality. The mechanical pressing at the end of solidification can promote the upstream discharge of the molten steel rich in solute elements, improve the center segregation, compensate for the solidification shrinkage, and inhibit the growth of shrinkage cavities. The electromagnetic stirring in the second cooling zone and the mechanical pressing at the end of solidification are combined with each other to achieve the purpose of improving the internal quality of the ingot, so that the equiaxed crystal ratio is 30-40% and the columnar crystal ratio is 60-70%.

[0032] (4) The hot-rolled steel plate prepared by the method of the present invention has a high elongation after fracture. The weld metallographic structure of the medium-break deep well welding casing prepared by the whole pipe body heat treatment process is uniform and the performance reaches the level of the parent material, realizing the optimization of the weld performance and having a high anti-collapse ability. Its impact toughness and flattening test results are better than those of seamless steel pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a sample morphology image of the hot-rolled steel plate for medium-break deep-well mining casing prepared by a patented method after low-magnification microstructure inspection.

[0034] Figure 2 This is the metallographic structure diagram of the deep well casing made of the hot-rolled steel plate produced by this patent after heat treatment.

[0035] Figure 3 This is a diagram showing the inclusion inspection of the deep well casing made of the hot-rolled steel plate produced by this patent after heat treatment. DETAILED DESCRIPTION

[0036] The following non-limiting embodiments may enable a person skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0037] The test methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials described are commercially available unless otherwise specified.

[0038] Example 1

[0039] The hot-rolled steel plate for medium-break deep well mining casing is manufactured by adopting the technical solution of the present invention, and the specific production steps are as follows:

[0040] 5. Smelting: adopt top and bottom combined blowing converter smelting, apply pure steel smelting process, control the cleanliness of molten steel, oxygen blowing time: 13min, control the tapping hole to be round, control the oxygen content at 450PPm during tapping, carry out deoxidation and alloying during tapping, and add molybdenum iron, chromium iron and other alloys to reach the tapping composition range, and add the insufficient part during refining to ensure that the alloy components of the molten steel meet the target requirements;

[0041] 1 Continuous casting: The continuous casting machine with electromagnetic stirring is used for production, and the casting speed is constant. The superheat of the molten steel is strictly controlled within the range of △T13℃, and the pulling speed is 1.7m / min. Electromagnetic stirring devices are installed on the wide surface of the crystallizer, the fan-shaped section of the second cooling zone of the slab continuous casting machine, and the end of the slab continuous casting solidification to carry out electromagnetic stirring. The current intensity of the electromagnetic stirring is 265A and the current frequency is 3Hz. At the same time, the two fan-shaped sections at the end of the slab continuous casting solidification and after the electromagnetic stirring in the second cooling zone are mechanically pressed down by the continuous casting billet light pressing technology. The single roller pressing amount is 4mm and the total pressing amount is 12mm. The spacing between the nip rollers is controlled to be 180mm, and the nip rollers are required to be strictly centered. The arc of the fan-shaped section is accurately controlled to be 135° and the opening degree is 0.4mm, ensuring that the arc error of the rollers between the sections in the fan-shaped section is ≤0.2mm, and the opening error of the fan-shaped section does not exceed 0.1mm. The cooling intensity of the second cooling zone is increased, and the cooling water volume of the second cooling zone is increased by 55%;

[0042] 2 Rolling: The continuous casting billet is heated to 1220℃ and then sent to the hot rolling mill for rolling. The rough rolling is performed for 3 passes of R1 and 3 passes of R2. The rough rolling is performed with full water to remove the primary iron oxide scale. After the rough rolling, the billet is sent to the 7-stand finishing mill for rolling. The final product thickness is 8.0mm.

[0043] 3. The hot-rolled steel plate is prepared into a medium-break deep well mining casing by adopting a full-tube heat treatment process.

[0044] Example 2

[0045] The hot-rolled steel plate for medium-break deep well mining casing is manufactured by adopting the technical solution of the present invention, and the specific production steps are as follows:

[0046] 6. Smelting: adopt top and bottom combined blowing converter smelting, apply pure steel smelting process, control the cleanliness of molten steel, oxygen blowing time: 12min, control the tapping hole to be round, control the oxygen content at 500PPm during tapping, carry out deoxidation and alloying during tapping, and add molybdenum iron, chromium iron and other alloys to reach the tapping composition range, and add the insufficient part during refining to ensure that the alloy components of the molten steel meet the target requirements;

[0047] 1 Continuous casting: The continuous casting machine with electromagnetic stirring is used for production, and the casting speed is constant. The superheat of the molten steel is strictly controlled within the range of △T10℃, and the pulling speed is 1.6m / min. Electromagnetic stirring devices are installed on the wide surface of the crystallizer, the second cooling zone of the slab continuous casting machine, and the end of the slab continuous casting solidification to carry out electromagnetic stirring. The current intensity of the electromagnetic stirring is 270A and the current frequency is 1Hz. At the same time, the two fan-shaped segments at the end of the slab continuous casting solidification and after the electromagnetic stirring in the second cooling zone are mechanically pressed down by the continuous casting billet light pressing technology. The single roller pressing amount is 4mm and the total pressing amount is 12mm. The spacing between the nip rollers is controlled to be 170mm, and the nip rollers are required to be strictly centered. The arc of the fan-shaped segment is accurately controlled to be 120° and the opening degree is 0.2mm, ensuring that the arc error of the rollers between the segments in the fan-shaped segment is ≤0.2mm, and the opening error of the fan-shaped segment does not exceed 0.1mm; the cooling intensity of the second cooling zone is increased, and the cooling water volume of the second cooling zone is increased by 50%;

[0048] 2 Rolling: The continuous casting billet is heated to 1200℃ and then sent to the hot rolling mill for rolling. The rough rolling is performed for 3 passes of R1 and 3 passes of R2. The rough rolling is performed with full water to remove the primary iron oxide scale. After the rough rolling, the billet is sent to the 7-stand finishing mill for rolling. The final product thickness is 2.5mm.

[0049] 3. The hot-rolled steel plate is prepared into a medium-break deep well mining casing by adopting a full-tube heat treatment process.

[0050] Example 3

[0051] The hot-rolled steel plate for medium-break deep well mining casing is manufactured by adopting the technical solution of the present invention, and the specific production steps are as follows:

[0052] 7. Smelting: adopt top and bottom combined blowing converter smelting, apply pure steel smelting process, control the cleanliness of molten steel, oxygen blowing time: 15min, control the tapping hole to be round, control the oxygen content at 400PPm during tapping, carry out deoxidation and alloying during tapping, and add molybdenum iron, chromium iron and other alloys to reach the tapping composition range, and add the insufficient part during refining to ensure that the alloy components of the molten steel meet the target requirements;

[0053] 1 Continuous casting: The continuous casting machine with electromagnetic stirring is used for production, and the casting speed is constant. The superheat of the molten steel is strictly controlled within the range of △T15℃, and the pulling speed is 1.8m / min. Electromagnetic stirring devices are installed on the wide surface of the crystallizer, the fan-shaped section of the second cooling zone of the slab continuous casting machine, and the end of the slab continuous casting solidification to carry out electromagnetic stirring. The current intensity of the electromagnetic stirring is 260A and the current frequency is 5Hz. At the same time, the two fan-shaped sections at the end of the slab continuous casting solidification and after the electromagnetic stirring in the second cooling zone are mechanically pressed down by the continuous casting billet light pressing technology, with a single roller pressing amount of 5mm and a total pressing amount of 15mm. The spacing between the nip rollers is controlled to be 190mm, and the nip rollers are required to be strictly centered. The arc of the fan-shaped section is accurately controlled to be 150° and the opening degree is 0.5mm, ensuring that the arc error of the rollers between the sections in the fan-shaped section is ≤0.2mm, and the opening error of the fan-shaped section does not exceed 0.1mm. The cooling intensity of the second cooling zone is increased, and the cooling water volume of the second cooling zone is increased by 60%;

[0054] 2 Rolling: The continuous casting billet is heated to 1240℃ and then sent to the hot rolling mill for rolling. The rough rolling is performed for 3 passes of R1 and 3 passes of R2. The rough rolling is performed with full water to remove the primary iron oxide scale. After the rough rolling, the billet is sent to the 7-stand finishing mill for rolling. The final product thickness is 12.0mm.

[0055] 3. The hot-rolled steel plate is prepared into a medium-break deep well mining casing by adopting a full-tube heat treatment process.

[0056] Product inspection and performance testing

[0057] The final products of Examples 1 to 3 were subjected to flattening test, impact toughness, macrostructure, metallographic structure and inclusion inspection.

[0058] (1) Flattening test

[0059] The casing prepared in Examples 1 to 3 was subjected to a flattening test. When the casing was flattened to 1 / 4, there was no crack at the weld. The casing was further flattened to 1 / 2 of the steel pipe, and there was no crack on the steel pipe body. All the flattening tests were qualified.

[0060] (2) Impact toughness test

[0061] The casings prepared in Examples 1 to 3 were subjected to an impact toughness test. The impact toughness of the casings prepared by the method of the present invention was greater than or equal to 150 J at -20°C, which was much higher than 70 J of the seamless steel pipe.

[0062] (3) Macroscopic examination

[0063] After low-power tissue examination, Figure 1 The center segregation of the hot-rolled steel plate is C0.5-1.0, the center porosity is 0-0.5, the corner crack is 0, the triangular crack is 0, the equiaxed crystal ratio is 30-40%, and the columnar crystal ratio is 60-70%. According to the physical inspection results, the center segregation of the hot-rolled steel plate prepared by the method of the present invention is lighter than that of Q355B steel (center segregation ≥ C2.0), and the equiaxed crystal ratio is greater than that of Q355B steel (equiaxed crystal ratio is 15-30%).

[0064] (4) Metallographic structure and inclusion inspection

[0065] Metallographic structure and inclusion inspection of casing body after heat treatment, see Figure 2 and Figure 3 It can be seen that the metallographic structure of the casing body after heat treatment is tempered troostite structure, the inclusions in the tube body are D0.5 level, and the rest are 0 level.

Claims

1. A hot-rolled steel plate for medium-break deep well mining casing, characterized in that: The alloy components of the steel plate are as follows by weight: 0.18-0.25% C, 1.20-1.50% Mn, 0.10-0.30% Si, ≤0.010% P, ≤0.010% S, 0.01-0.06% Als, 1.0-1.40% Cr, 0.40-0.60% Mo, and the rest is iron and unavoidable impurities; The steel plate is prepared according to the following process steps: ① Smelting: adopt top and bottom combined blowing converter smelting, apply pure steel smelting process, control the cleanliness of molten steel, oxygen blowing time: 12-15min, control the tapping hole to be round, control the oxygen content at 400-500ppm during tapping, carry out deoxidation and alloying, and add ferromolybdenum, ferrochrome and other alloys during tapping to reach the tapping composition range, and add the insufficient part during refining to ensure that the alloy components of the molten steel meet the target requirements; ②Continuous casting: Use a continuous casting machine with electromagnetic stirring, cast at a constant pulling speed, strictly control the superheat and pulling speed of the molten steel, install electromagnetic stirring devices on the wide surface of the crystallizer, the second cooling zone of the slab continuous casting machine, and the end of the continuous casting solidification, and perform electromagnetic stirring. The current intensity of the electromagnetic stirring is 260-270A, and the current frequency is 1-5Hz. At the same time, mechanical pressing is performed at the end of the slab continuous casting solidification and the two fan-shaped segments after the electromagnetic stirring in the second cooling zone; control the spacing between the nip rollers, and require the nip rollers to be strictly centered; accurately control the curvature and opening of the fan-shaped segments; increase the cooling intensity of the second cooling zone; ③Rolling: The continuous casting billet is heated to 1200-1240℃ and then sent to the hot rolling mill for rolling. The rough rolling is performed for 3 passes of R1 and 3 passes of R2. The rough rolling is performed with full water to remove the primary iron oxide scale. After the rough rolling, the billet is sent to the 7-stand finishing mill for rolling. The final product thickness is 2.5-12.0mm. The pulling speed is 1.6-1.8 m / min; The arc of the fan-shaped segment is 120-150°, the opening of the fan-shaped segment is 0.2-0.5mm, the arc error of the rollers between the segments within the fan-shaped segment is ≤0.2mm, and the opening error of the fan-shaped segment does not exceed 0.1mm; The mechanical pressing adopts the continuous casting billet light pressing technology, and the roller distance is adjusted, the single roller pressing amount is 4-5mm, and the total pressing amount is 12-15mm; The spacing between the nip rollers is 170 to 190 mm; The superheat of the molten steel is controlled within the range of ΔT 10 to 15°C; The process for increasing the cooling intensity of the secondary cooling zone is: increasing the cooling water volume of the secondary cooling zone by 50-60%.

2. The steel plate according to claim 1, characterized in that: The central segregation of the steel plate is ≤C1.0, the proportion of equiaxed crystals is 30-40% and the proportion of columnar crystals is 60-70%.

3. A medium-break deep well mining casing made from the hot-rolled steel plate according to claim 1, characterized in that: The sleeve is prepared by adopting a whole-tube heat treatment process.

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