An ultra-large-section directly-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acid environment and a preparation method thereof

Abstract

The application discloses an ultra-large-section directly-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by an acid environment and a preparation method thereof, and belongs to the technical field of metal materials and forming. The preparation method comprises the following steps: S01, heating continuous casting billets to 1220-1240 DEG C and keeping for 3-5 h; S02, water dephosphorization after the continuous casting billets are discharged, and rough rolling; S03, multi-pass groove rolling to become an intermediate billet after the rough rolling is finished; S04, fine continuous rolling; and S05, rapid strong water cooling after the fine continuous rolling is finished. The application preforms a dense high-copper oxide skin on the surface of a finished rolled material by improving the rolling mill roller material and part of the rolling process, simultaneously improves the Cu content of the surface of the matrix, solves the problem that the Cu content in the oxide skin generated by only relying on the 0.20%-0.55% Cu element of the steel matrix is low, and the oxide skin generation speed is slow in the service process, and remarkably improves the ability of the bar to hinder the entry of exogenous hydrogen into the bar in an acid environment to cause hydrogen embrittlement.

Description

Technical Field

[0001] This invention relates to a method for preparing ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments, belonging to the field of metal materials and forming technology. Background Technology

[0002] Direct-cut non-quenched and tempered steel achieves the desired mechanical properties directly after rolling, allowing for the direct fabrication of shafts, columns, and other components without further cutting, micro-cutting, or partial cutting. This eliminates forging and tempering processes, significantly shortening the manufacturing process and reducing energy consumption. However, as the diameter of direct-cut non-quenched and tempered steel bars increases, hydrogen embrittlement becomes a critical issue requiring resolution. Hydrogen embrittlement originates from both internal and external sources. Internal hydrogen primarily comes from moisture introduced during smelting, while external hydrogen mainly originates from H ions introduced during processing or service due to water films or acidic environments. Under stress and other influences, these hydrogen molecules aggregate within the steel, forming H2 molecules that generate significant pressure, leading to cracks or even complete fracture of the steel, posing a serious hazard.

[0003] Chinese patent ZL202211500098.8 discloses a low-cost, non-quenched and tempered steel for direct cutting of ultra-large cross-sections resistant to hydrogen embrittlement. It utilizes elements such as Cu to form precipitated phases internally, fixing endogenous hydrogen; and forms a thin, dense oxide scale on the steel bar surface, hindering the entry of exogenous hydrogen. However, the Cu content in the patent is only 0.2%-0.55%, requiring a strong oxidizing environment or a long time to form a dense copper-containing oxide scale on the steel surface. If used in an acidic environment, the oxide scale may not form in time, or the corrosion rate may exceed the formation rate, making it difficult to achieve resistance to exogenous hydrogen embrittlement. Higher Cu content can lead to cracking during steel rolling, making forming difficult. Other processes, such as coating finished components like shafts and columns with acid- and hydrogen-embrittlement-resistant coatings, increase both the number of steps and cost, and the interface between the coating and the substrate is a rapid corrosion pathway.

[0004] Therefore, there is an urgent need for a method to prepare ultra-large cross-section direct-cut non-quenched and tempered steel that can withstand hydrogen embrittlement caused by acidic environments. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to obtain ultra-large cross-section direct-cut non-quenched and tempered steel bars that can resist hydrogen embrittlement caused by acidic environment by generating a dense oxide layer on the surface of the bars in advance without changing the composition and basic production process of ultra-large cross-section direct-cut non-quenched and tempered steel bars.

[0006] Meanwhile, this invention provides a method for preparing ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments.

[0007] Meanwhile, this invention provides an application of ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments in shaft and column components.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for preparing ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments mainly includes the following steps: S01: Heat the continuous casting billet to 1220-1240℃ and hold for 3-5 hours; S02: After the continuously cast billet exits the furnace, it is descaled with water. The inner wall of the first pass of the rough rolling mill is covered with a fine mesh with raised protrusions. Then it quickly enters the second pass with a copper inner wall and the third pass with a smooth inner wall. The second and third passes have the same shape as the first pass, but the size decreases slightly in sequence. The deformation per pass from the first to the third pass is <5%. The material of the rolls not specified in this invention is generally medium-to-high carbon alloy steel, such as 55CrMnMo or 60CrMnMo, used in roughing rolling; the manufacturer is Xingtai Machinery Roll Co., Ltd. of China Steel Group. The copper inner wall of the rolls does not perform the function of conventional roll forming, but rather, through contact with the high-temperature continuously cast billet, it melts, allowing copper to enter the billet surface and increase the copper content on the billet surface. The thickness of the copper inner wall is 3-5 cm.

[0009] S03: The billet is produced by multi-pass rolling to the end of roughing. Starting from the third pass, water cooling is applied between each pass to ensure the billet surface temperature does not exceed 1050℃. The deformation per pass from the fourth to the seventh pass is >20%, while no requirements are placed on the deformation of subsequent passes. The total reduction in surface area during roughing is >60%. S04: After the intermediate billet is heated to a surface temperature of 1050-1080℃, continuous finishing rolling begins. The first finishing rolling pass has a deformation of 5%-10% to break off the oxide scale and increase the surface temperature of the billet. The second pass is a copper inner wall hole type with a deformation of <5%. Subsequent passes are rolled normally to the finished diameter, with a final rolling temperature of 850-900℃. S05: After the finishing continuous rolling is completed, it is quickly cooled with strong water, and then cooled to 550-600℃ on the surface through 6-8 sets of strong water cooling. During this period, air cooling is interspersed to ensure that the surface temperature is maintained at 550-600℃. Then it is slowly cooled to room temperature in the heat preservation pit.

[0010] Preferably, the continuously cast billet in step S01 has the following chemical composition by mass percentage: C: 0.35%-0.45%, Si: 0.30%-0.90%, Mn: 1.10%-1.60%, V: 0.10%-0.20%, Nb≤0.01%, Ti: 0.015%-0.030%, N: 0.005%-0.008%, Cu: 0.20%-0.55%, S: 0.01%-0.04%, P≤0.02%, with the balance being Fe and unavoidable impurities.

[0011] Preferably, in step S02, the water pressure for descaling is not less than 25 MPa, and annular water spraying is used to ensure that all oxide scale is removed.

[0012] Preferably, in step S02, the first perforation is made of high-speed steel, the height of the raised mesh on the inner wall is 5-10mm, the raised area accounts for 50%-60%, and the minimum mesh spacing is no more than 10mm.

[0013] Preferably, in step S02, the size of the second and third holes is reduced by 5-10 mm in turn, and the reduction in the size of the second hole is not greater than the height of the raised mesh on the inner wall of the first hole.

[0014] Preferably, in step S04, the oxide scale that has been peeled off is removed by blowing air after the first pass of finishing rolling.

[0015] The final diameter of the rolled product obtained by this invention is 100mm-300mm, preferably 160mm-300mm.

[0016] The rolled product obtained by this invention has a tensile strength of 894-973 MPa, a yield strength of 679-784 MPa, a copper oxide scale thickness of 21-37 μm, a Cu content of 64.8-72.2 wt% on the oxide scale surface, a Cu content of 0.63-1.2 wt% at 1 mm from the oxide scale surface, and no cracks in the HIC test in 5% dilute hydrochloric acid aqueous solution.

[0017] The present invention relates to the application of a type of non-quenched and tempered steel with an ultra-large cross-section that is resistant to hydrogen embrittlement caused by acidic environments in shaft and column components.

[0018] The present invention has the following beneficial effects: Based on the composition of patent ZL202211500098.8, this invention improves the rolling mill roll material and some rolling processes to pre-form a thick and dense high-copper oxide layer on the surface of the finished rolled material. Simultaneously, it increases the Cu content on the substrate surface, solving the problem of low Cu content and slow oxide layer formation during service caused by relying solely on the 0.20%-0.55% Cu element in the steel substrate. This significantly improves the bar's ability to prevent hydrogen embrittlement caused by the entry of exogenous hydrogen in acidic environments. Furthermore, the production cost is significantly lower than that of preparing a hydrogen-resistant coating.

[0019] The basic principle of this invention is as follows: Cu has a melting point of approximately 1083°C. After descaling in water, the continuously cast billet directly exposes a steel matrix at approximately 1200°C. The first pass of the rough rolling mill presses out grid grooves on the surface of the billet through raised meshes. Upon contact with the inner wall of the second copper pass, a small amount of copper melts and enters the grid grooves, diffusing with the steel matrix. The second pass is slightly smaller than the first to ensure sufficient contact between the copper and the billet and to provide a small amount of extrusion pressure. The liquid copper is evenly distributed on the surface of the billet through the capillary action of the mesh and the extrusion pressure. The third pass further reduces its size, flattening the grid grooves while further promoting the fusion and diffusion of copper with the surface matrix of the billet, forming a Cu concentration gradient layer of a certain depth, significantly increasing the copper content on the surface of the billet. Subsequent passes use normal rolling processes, with water cooling between passes to below 1050°C to prevent surface cracking caused by liquid copper and improve the surface quality of the billet. During the rough rolling process, Cu continuously diffuses inward from the surface of the billet. However, as the billet diameter decreases and oxidation occurs, the Cu-containing layer gradually thins and becomes more uniform.

[0020] After the intermediate billet is heated to a surface temperature of 1050-1080℃, continuous finishing rolling begins to ensure that Cu no longer melts. The first finishing roll removes the oxide scale, exposing the internal metal at a higher temperature. During the second finishing roll, a small amount of the copper inner wall cavities can still be melted and coated onto the surface of the intermediate billet, further increasing the surface Cu content and supplementing the copper layer. A deformation of less than 5% also plays a role in extrusion and promoting Cu fusion and diffusion to the matrix surface. Furthermore, the amount of Cu supplemented can be controlled by adjusting the deformation in the second finishing roll. Increased deformation leads to more thorough contact between the inner wall of the copper rolls and the intermediate billet, resulting in greater extrusion pressure, more melting, and thus more Cu supplementation. Subsequent normal finishing rolling follows. The final finishing rolling temperature is controlled at 850-900℃, followed by multiple water cooling cycles to maintain the surface temperature at 550-600℃. This controls the precipitation of carbonitrides in V and copper-containing phases in B2 structure, improving strength and generating sufficient hydrogen traps, while also preventing the formation of abnormal structures such as martensite. In addition, the high final rolling temperature and rapid cooling prevent the copper oxide layer from becoming too thick and causing breakage.

[0021] This invention, without altering the chemical composition, key rolling processes, and final mechanical properties of the rolled material as described in patent ZL202211500098.8, prefabricates a high-copper dense oxide scale on the surface of the rolled material. This increases the Cu content on the surface of the rolled material, enhancing its resistance to hydrogen embrittlement caused by acidic environments and its self-repairing ability after subsequent oxide scale breakage. Even in HIC experiments conducted in a more acidic 5% dilute hydrochloric acid aqueous solution, no hydrogen-induced cracking occurs.

[0022] This invention discloses a method for preparing ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement in acidic environments, belonging to the field of metal materials and forming technology. By improving the rolling mill roll material and some rolling processes, this invention pre-forms a dense high-copper oxide layer on the surface of the finished rolled product, while simultaneously increasing the Cu content on the substrate surface. This solves the problem of low Cu content and slow oxide layer formation during service caused by relying solely on the 0.20%-0.55% Cu element in the steel substrate for oxide scale formation, significantly improving the ability of the bar to prevent the entry of exogenous hydrogen and thus prevent hydrogen embrittlement in acidic environments. Furthermore, the production cost is significantly lower than that of preparing a hydrogen-resistant coating. Attached Figure Description

[0023] Figure 1 The morphology of the high-copper oxide scale formed on the surface of the rolled material in Example 1; Figure 2 The transmission electron microscope (TEM) morphology of vanadium carbonitride precipitation near the surface of the rolled material in Example 1; Figure 3 The scanning electron microscope (SEM) morphology of the copper-containing precipitates near the surface of the rolled material in Example 1; Figure 4 The morphology of the high-copper oxide scale formed on the surface of the rolled material in Comparative Example 1 is shown. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Example 1

[0025] A type of ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments is shown in Table 1.

[0026] Table 1 Chemical composition (wt%) (the remainder is Fe and impurity elements)

[0027] Continuously cast round billets are produced according to the composition described in the above embodiment. The billets are heated to 1230℃ and held for 4 hours. After exiting the furnace, the billets are descaled with water at a pressure of 30 MPa. The first pass of the roughing mill uses high-speed steel with a fine, raised mesh on the inner wall. The mesh height is 8 mm, the raised area accounts for 55%, and the minimum mesh spacing is 8 mm. The deformation in the first pass is approximately 3%. Then, the billets quickly move to the second pass with a copper inner wall and the third pass with a smooth inner wall. The radius of the second pass is 5 mm smaller than that of the first pass, and the deformation is approximately 3%. The radius of the third pass is also 5 mm smaller than that of the second pass, and the deformation is approximately 3%. From the third pass onwards, water cooling is applied during each pass to ensure the billet surface temperature does not exceed 1050℃. The deformation in the fourth to seventh passes is approximately 25% per pass. No specific deformation requirements are placed for subsequent passes. The total reduction in surface area during roughing is 65%. The billet is then finished as an intermediate billet.

[0028] After the intermediate billet reaches a surface temperature of 1060℃, continuous finishing rolling begins. The first pass of finishing rolling involves a 5% deformation to break up and remove oxide scale and raise the surface temperature of the billet, while air blowing removes the removed oxide scale. The second pass uses a copper inner wall pass with a 3% deformation. Subsequent passes roll normally to the finished diameter, with a final rolling temperature of 880℃. After finishing rolling, the billet is rapidly cooled with strong water, undergoing six sets of strong water cooling to a surface temperature of approximately 580℃. During this period, air cooling is interspersed to maintain the surface temperature at 580℃. Finally, the billet is slowly cooled to room temperature in an insulation pit.

[0029] This embodiment demonstrates the application of ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments in shaft and column components.

[0030] The finished rolled product has a diameter of 210 mm. The morphology of the longitudinal section of the oxide scale on the surface of the rolled product was photographed using a scanning electron microscope. (See attached image.) Figure 1 The morphologies of vanadium carbonitride and copper-containing precipitates near the surface of the rolled material were captured using transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. Figure 2 , Figure 3 The Cu content on the oxide scale surface and in the matrix 1 mm from the rolled material surface was determined using the attached energy dispersive spectroscopy (EDS) instrument. Tensile specimens were taken according to the national standard GB / T 228.1-2021 for tensile testing to determine tensile strength and yield strength. HIC tests were conducted according to GB / T 8650 "Evaluation Method for Hydrogen-Induced Cracking Resistance of Pipeline Steel and Pressure Vessel Steel," with the corrosive solution replaced by a stronger 5% dilute hydrochloric acid solution. The rolled material surface was exposed to the solution, and the cut surfaces were protected with a corrosion-resistant coating. The test results are shown in Table 2.

[0031] Figure 1 A dense oxide scale, approximately 28 μm thick, was observed to form on the surface of the rolled material. The copper content on the oxide scale was as high as 72.2%, indicating that the outermost surface was almost entirely composed of copper oxide. This dense copper oxide can prevent hydrogen ions from entering the interior of the rolled material in an acidic environment. The Cu content at a distance of 1 mm from the surface also increased to 1.2%, which is beneficial for the continuous formation of a copper-containing oxide scale after the copper oxide film is damaged, exhibiting a faster repair function than the substrate (containing 0.30% copper). Figure 2 , Figure 3 The vanadium carbonitride precipitate and copper-containing precipitate, which provide strengthening and hydrogen trapping effects, are visible in the sample. The number and area fraction of the copper-containing precipitate are significantly greater than those in patent ZL202211500098.8. This indicates that a higher surface copper content can generate more copper-containing precipitates, providing more hydrogen traps and Cu reservoirs, further enhancing the ability to continuously form a copper oxide scale after the copper oxide film is damaged. After the HIC test, the oxide scale thickness was still 19 μm. Therefore, no cracks appeared in the 5% dilute hydrochloric acid HIC test.

[0032] Comparative Example 1

[0033] Comparative Example 1 and Example 1 both used continuously cast billets of the same composition for rolling. The difference lies in the fact that the first three passes of rough rolling did not use high-speed steel, copper inner wall, or smooth inner wall pass patterns and rolling processes; subsequent passes were rolled directly. The first two passes of finish rolling also did not use copper inner wall pass patterns and rolling processes; subsequent passes were rolled directly and cooled under controlled conditions. All other processes were the same.

[0034] Comparative Example 1: The diameter of the finished rolled product is 210 mm. The morphology of the longitudinal section of the oxide scale on the surface of the rolled product was photographed using a scanning electron microscope. (See figure) Figure 4 The copper content, tensile properties, and HIC test results determined according to Example 1 are shown in Table 2.

[0035] Due to the low copper content in the base material, although a thicker oxide scale was formed on the surface of the rolled material than in Example 1, the scale was relatively loose and easily broken and peeled off. The surface copper content was only 26.5%, indicating that the surface oxide scale was a mixture of copper oxide and iron oxide, and its ability to prevent hydrogen ion entry was significantly weaker than that of Example 1. At a distance of 1 mm from the surface, the copper content was the same as the base material, and the repair speed after the outer oxide scale was damaged was significantly slower than that of Example 1. Although a high delayed fracture strength ratio could be obtained in the constant load test of ZL202211500098.8, indicating that it could withstand the entry of exogenous hydrogen in a normal environment, cracks appeared in the HIC test with 5% dilute hydrochloric acid, indicating that the rolled material of Comparative Example 1 could not withstand hydrogen embrittlement caused by hydrogen ion entry under acidic conditions. After the HIC test, the oxide scale thickness was only 5 μm, indicating weak repair ability.

[0036] Example 2

[0037] A method for preparing ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments mainly includes the following steps: A continuously cast round billet is produced according to the following composition: C: 0.35%, Si: 0.30%, Mn: 1.10%, V: 0.10%, Nb: 0.01%, Ti: 0.015%, N: 0.005%, Cu: 0.20%, S: 0.01%, P: 0.02%, with the balance being Fe and unavoidable impurities. The continuously cast billet is heated to 1220℃ and held for 3 hours; after exiting the furnace, the billet is descaled with water at a pressure of 25 MPa. The first pass of the roughing mill uses high-speed steel, with a fine mesh with raised sections on the inner wall. The mesh height is 5 mm, the raised area accounts for 50%, and the minimum mesh spacing is 10 mm. The deformation in the first pass is approximately 1%. The billet then rapidly enters the second pass with a copper inner wall and the third pass with a smooth inner wall. The radius of the second pass's arc is 5 mm smaller than that of the first pass, with a rolling deformation of approximately 1%. The radius of the third pass's arc is also 5 mm smaller than that of the second pass, with a rolling deformation of approximately 1%. From the third pass onwards, water cooling is applied during each pass to ensure that the billet surface temperature does not exceed 1050℃. The deformation per pass from the fourth to the seventh pass is approximately 20%, and no requirements are placed on the deformation of subsequent passes. The total reduction in surface area during rough rolling is approximately 60%, and the billet becomes an intermediate billet after rough rolling.

[0038] After the intermediate billet reaches a surface temperature of 1050℃, continuous finishing rolling begins. The first pass of finishing rolling involves a 5% deformation to break up and remove oxide scale and raise the surface temperature of the billet, while air blowing removes the removed oxide scale. The second pass uses a copper inner wall die with a 5% deformation. Subsequent passes roll normally to the finished diameter, with a final rolling temperature of 850℃. After finishing rolling, the billet is rapidly cooled with strong water, undergoing seven sets of strong water cooling to a surface temperature of approximately 550℃. During this period, air cooling is interspersed to maintain the surface temperature at 550℃. Finally, the billet is slowly cooled to room temperature in an insulation pit. The diameter of the rolled product is 160mm.

[0039] This embodiment demonstrates the application of ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments in shaft and column components.

[0040] Example 2: The rolled material had a thick and dense oxide scale on its surface, with high Cu content in the oxide scale and at a depth of 1 mm from the surface. Therefore, it showed no cracks in the HIC test with 5% dilute hydrochloric acid, demonstrating its ability to withstand hydrogen embrittlement caused by hydrogen ion intrusion in an acidic environment. After the HIC test, the oxide scale thickness remained at 16 μm.

[0041] Example 3

[0042] A method for preparing ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments mainly includes the following steps: A continuously cast round billet is produced according to the following composition: C: 0.45%, Si: 0.90%, Mn: 1.60%, V: 0.20%, Nb: 0.005%, Ti: 0.030%, N: 0.008%, Cu: 0.55%, S: 0.04%, P: 0.01%, with the balance being Fe and unavoidable impurities. The continuously cast billet is heated to 1240℃ and held for 5 hours; after exiting the furnace, the billet is descaled with water at a pressure of 35 MPa. The first pass of the roughing mill uses high-speed steel, with a fine mesh with raised sections distributed on the inner wall. The mesh height is 10 mm, the raised area accounts for 60%, and the minimum mesh spacing is 5 mm. The deformation in the first pass is approximately 5%. The billet then rapidly enters the second pass with a copper inner wall and the third pass with a smooth inner wall. The radius of the second pass's arc is 10 mm smaller than that of the first pass, with a rolling deformation of approximately 5%. From the third pass onwards, water cooling is applied between each pass to ensure the billet surface temperature does not exceed 1050℃. The deformation per pass from the fourth to the seventh pass is approximately 25%, while the deformation for subsequent passes is not specified. The total reduction in surface area during rough rolling is approximately 65%, and the billet becomes an intermediate billet upon completion of rough rolling.

[0043] After the intermediate billet reaches a surface temperature of 1080℃, continuous finishing rolling begins. The first pass of finishing rolling involves a 10% deformation to break up and remove oxide scale and raise the surface temperature of the billet, while air blowing removes the removed oxide scale. The second pass uses a copper inner wall pass with a 5% deformation. Subsequent passes roll normally to the finished diameter, with a final rolling temperature of 900℃. After finishing rolling, the billet is rapidly cooled with strong water, undergoing eight sets of strong water cooling to a surface temperature of approximately 600℃. During this period, air cooling is interspersed to maintain the surface temperature at 600℃. Finally, the billet is slowly cooled to room temperature in an insulated pit. The diameter of the rolled material is 300mm.

[0044] This embodiment demonstrates the application of ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments in shaft and column components.

[0045] Example 3: The rolled material has a thick and dense oxide scale on its surface, with high Cu content in the oxide scale and at a depth of 1 mm from the surface. Therefore, it showed no cracks in the HIC test with 5% dilute hydrochloric acid, demonstrating its ability to withstand hydrogen embrittlement caused by hydrogen ion intrusion in an acidic environment. After the HIC test, the oxide scale thickness remained at 25 μm.

[0046] Comparative Example 2

[0047] The only difference between Comparative Example 2 and Example 1 is that the inner wall of the first pass of the roughing mill is smooth. After the second pass of the roughing mill, a large amount of liquid copper dripped down and did not completely cover the surface of the rolled material, causing difficulties in equipment operation and obvious surface patches.

[0048] Comparative Example 3

[0049] The only difference between Comparative Example 3 and Example 1 is that the mesh height on the inner wall of the first pass is 2mm, and the raised area accounts for 20%. The dimensions of the second and third passes decrease by 5mm respectively. The rolled material surface shows less Cu diffusion and uneven distribution. This results in a thin local oxide scale, and almost no Cu diffusion into the substrate within 1mm of the surface, which cannot quickly repair the oxide scale after it cracks. After the HIC test, the local oxide scale thickness is only 2μm, indicating weak repair capability. This leads to cracks appearing in the HIC test.

[0050] Comparative Example 4

[0051] The only difference between Comparative Example 4 and Example 1 is that the mesh height on the inner wall of the first pass is 12 mm, and the protruding area accounts for 40%. The dimensions of the second and third passes decrease by 12 mm respectively. Numerous folds appeared on the surface of the rolled material, which were not removed during subsequent oxidation and rolling, resulting in substandard surface quality.

[0052] Comparative Example 5

[0053] The only difference between Comparative Example 5 and Example 1 is that the finishing mill did not use a copper inner wall die. The copper oxide scale on the surface of the rolled material was relatively thin, which occasionally led to cracking during the HIC test.

[0054] Table 2. Rolled material diameter, oxide scale thickness, tensile properties and HIC test

[0055] It should be understood that, in order to simplify this disclosure and aid in understanding one or more of the various aspects of the invention, features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof in the above description of exemplary embodiments of the invention. However, this method of disclosure should not be interpreted as reflecting an intention that the claimed invention requires more features than expressly recited in each claim. Rather, as reflected in the claims, inventive aspects lie in fewer than all the features of the foregoingly disclosed embodiments. Therefore, the claims, following the detailed description, are hereby expressly incorporated into that detailed description, wherein each claim itself is a separate embodiment of the invention.

[0056] Although the invention has been described with reference to a limited number of embodiments, those skilled in the art will understand from the foregoing description that other embodiments are conceivable within the scope of the invention described herein. Furthermore, it should be noted that the language used in this specification has been chosen primarily for readability and instructional purposes, and not for the purpose of interpreting or limiting the subject matter of the invention. Therefore, many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the appended claims. The disclosure of the invention is illustrative and not restrictive, and the scope of the invention is defined by the appended claims.

[0057] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing ultra-large cross-section direct-cut non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments, characterized in that, Includes the following steps: S01: Heat the continuous casting billet to 1220-1240℃ and hold for 3-5 hours; S02: After the continuously cast billet exits the furnace, it is descaled with water. The inner wall of the first pass of the rough rolling mill is covered with a fine mesh with raised protrusions. Then it quickly enters the second pass with a copper inner wall and the third pass with a smooth inner wall. The second and third passes have the same shape as the first pass, but the size decreases slightly in sequence. The deformation per pass from the first to the third pass is <5%. S03: Multi-pass rolling to the end of roughing to become an intermediate billet. Starting from the third pass, water cooling is sprayed in the middle of each rolling pass to ensure that the surface temperature of the billet does not exceed 1050℃; the deformation amount of a single pass from the fourth to the seventh pass is >20%, and there are no requirements for the deformation amount of subsequent passes. The total reduction rate of roughing is >60%. S04: After the intermediate billet is heated to a surface temperature of 1050-1080℃, continuous finishing rolling begins. The first finishing rolling pass has a deformation of 5%-10% to break off the oxide scale and increase the surface temperature of the billet. The second finishing rolling pass is a copper inner wall hole type with a deformation of <5%. Subsequent finishing rolling passes are rolled normally to the finished diameter, with a final rolling temperature of 850-900℃. S05: After the finishing continuous rolling is completed, it is quickly cooled with strong water, and then cooled to 550-600℃ on the surface through 6-8 sets of strong water cooling. During this period, air cooling is interspersed to ensure that the surface temperature is maintained at 550-600℃. Then it is slowly cooled to room temperature in the heat preservation pit.

2. The preparation method according to claim 1, characterized in that, The continuously cast billet of S01 has the following chemical composition by mass percentage: C: 0.35%-0.45%, Si: 0.30%-0.90%, Mn: 1.10%-1.60%, V: 0.10%-0.20%, Nb≤0.01%, Ti: 0.015%-0.030%, N: 0.005%-0.008%, Cu: 0.20%-0.55%, S: 0.01%-0.04%, P≤0.02%, with the balance being Fe and unavoidable impurities.

3. The preparation method according to claim 1, characterized in that, In SO2, the water pressure for descaling is not less than 25 MPa, and a ring-shaped water spray is used to ensure that all oxide scale is removed.

4. The preparation method according to claim 1, characterized in that, In S02, the first perforation is made of high-speed steel. The height of the raised grid on the inner wall of the first perforation is 5-10mm, the raised area accounts for 50%-60%, and the minimum grid spacing is no more than 10mm.

5. The preparation method according to claim 1, characterized in that, In S02, the dimensions of the second and third perforations decrease by 5-10 mm in sequence, and the reduction in the size of the second perforation is no greater than the height of the raised mesh on the inner wall of the first perforation.

6. The preparation method according to claim 1, characterized in that, In S04, after the first rolling pass of the finishing mill, the detached oxide scale is removed by blowing air.

7. The preparation method according to claim 1, characterized in that, In S04, the diameter of the finished product is 160mm-300mm.

8. The preparation method according to any one of claims 1-7 yields a large-section direct-cutting non-quenched and tempered steel resistant to hydrogen embrittlement caused by acidic environments, characterized in that... Tensile strength 894-973MPa, yield strength 679-784MPa, copper oxide scale thickness 21-37μm, Cu content on the oxide scale surface 64.8-72.2wt%, Cu content 0.63-1.2wt% at 1mm from the oxide scale surface, no cracks in HIC test in 5% dilute hydrochloric acid aqueous solution.

9. The application of the ultra-large cross-section direct-cut non-quenched and tempered steel with acid resistance and hydrogen embrittlement caused by the preparation method according to claim 8 in shaft and column components.

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