Medium-carbon chromium-nickel-molybdenum alloy structural steel and surface quality control method therefor

Abstract

The present invention belongs to the technical field of preparation of medium-carbon chromium-nickel-molybdenum alloy structural steel, and particularly relates to medium-carbon chromium-nickel-molybdenum alloy structural steel and a surface quality control method therefor. The method comprises: sequentially subjecting the raw materials of the medium-carbon chromium-nickel-molybdenum alloy structural steel to smelting, continuous casting and rolling forming, so as to obtain the medium-carbon chromium-nickel-molybdenum alloy structural steel, wherein during the smelting, a titanium microalloying treatment and aluminum and nitrogen control are performed, and the titanium content is controlled to be 0.0080-0.0120%; and the rolling forming comprises low-temperature heating rolling, and the temperature during the low-temperature heating is 1220-1240°C. Since the titanium microalloying treatment is used and aluminum and nitrogen are controlled during the smelting, and by means of low-temperature heating rolling, the structure of a continuous casting billet is fine, and surface defects are significantly reduced; the generation of a large-size defect depth is avoided; and the problem of the poor surface quality of 39NiCrMo3 large-specification quenched and tempered round steel caused by a coarse structure is solved, such that the surface defect depth of the steel is within 0.3 mm.

Description

A medium-carbon chromium-nickel-molybdenum alloy structural steel and its surface quality control method Technical Field

[0001] The invention belongs to the technical field of medium-carbon chromium-nickel-molybdenum alloy structural steel preparation, and in particular relates to medium-carbon chromium-nickel-molybdenum alloy structural steel and a surface quality control method thereof. Background Art

[0002] Medium carbon chromium-nickel-molybdenum alloy structural steel 39CrNiMo3 requires good steel surface quality, and the depth of black skin surface defects on delivery shall not exceed 0.3mm.

[0003] However, for the specifications (100mm<∮≤160mm) 39NiCrMo3 tempered round steel, due to the thin shell of the continuous casting billet and the use of traditional smelting methods, the continuous casting billet has a coarse structure, well-developed columnar crystals, and a large difference between the inner and outer arc surfaces. Longitudinal cracks with varying depths are very likely to appear on the surface of the billet, resulting in poor surface quality of the steel.

[0004] For the specifications (100mm<∮≤160mm) 39NiCrMo3 quenched and tempered round steel, due to the large specifications of the continuous casting billet, the traditional heating method is used, the heating temperature is high, and the steel surface is easily overheated, resulting in coarse grains. At the same time, the Al and N content is high, and it is easy to precipitate on the grain boundaries. The superposition effect leads to cracking.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] In view of the shortcomings and defects of the existing technology, the purpose of the present invention is to provide a medium-carbon chromium-nickel-molybdenum alloy structural steel and a surface quality control method thereof. The method of the present invention adopts titanium microalloying treatment and controls aluminum and nitrogen during smelting, so that the structure of the continuous casting billet is fine and the surface defects are significantly reduced, avoiding the generation of large-scale defect depth, so that the surface defects of the steel are within 0.3mm; and by adopting a new heating process for rolling, the heating temperature range of 1220℃~1240℃ is accurately controlled, so that the steel is completely burned through, and the grains are fine while being in the high-temperature plastic deformation range, so that the surface defect depth of the steel is within 0.3mm. It solves the problem of poor surface quality of large-size quenched and tempered round steel 39NiCrMo3 due to coarse structure.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] In a first aspect, a method for controlling the surface quality of medium-carbon chromium-nickel-molybdenum alloy structural steel comprises: smelting, continuously casting and rolling the raw materials of the medium-carbon chromium-nickel-molybdenum alloy structural steel in sequence to obtain the medium-carbon chromium-nickel-molybdenum alloy structural steel; wherein, during the smelting, titanium microalloying treatment and aluminum and nitrogen control are performed to control the titanium content to 0.0080-0.0120%; the rolling forming comprises low-temperature heating rolling, and the low-temperature heating temperature is 1220-1240°C.

[0009] Furthermore, during the smelting, vacuum degassing is used, the vacuum degree is ≤0.5 Torr, and the total vacuum time is 20-35 minutes. At the same time, argon soft blowing is used, and the soft blowing time is 20-35 minutes to control aluminum and nitrogen.

[0010] Furthermore, the aluminum control is to control the aluminum content to 0.010%-0.025%; and the nitrogen control is to control the nitrogen content to be less than or equal to 0.0070%.

[0011] Furthermore, the primary smelting adopts converter primary smelting, by adding scrap steel and molten iron, and then melting, dephosphorizing, decarburizing and alloying in a converter in an oxidizing atmosphere, the carbon content at the converter end point is ≥0.08%, the phosphorus content is controlled to be ≤0.015%, and the tapping temperature is 1620-1650°C;

[0012] And / or, after the refining outside the furnace, the sulfur content is controlled to be ≤0.005%, the titanium content is controlled to be 0.008-0.012%, and the tapping temperature is 1595-1635°C.

[0013] Furthermore, the blank obtained by the continuous casting is a square billet with a specification of 410*530mm.

[0014] Furthermore, the continuous casting temperature is 1515-1530°C, and the casting speed is 0.41-0.45 m / min;

[0015] And / or, the rolling forming includes final rolling, and the temperature of the final rolling is not lower than 860°C.

[0016] Furthermore, the medium-carbon chromium-nickel-molybdenum alloy structural steel obtained by the rolling forming is a round bar with a specification of 100mm<∮≤160mm.

[0017] In a second aspect, a medium-carbon chromium-nickel-molybdenum alloy structural steel is prepared by the surface quality control method.

[0018] Furthermore, the material of the medium carbon chromium-nickel-molybdenum alloy structural steel is 39NiCrMo3.

[0019] Furthermore, the chemical composition of the 39NiCrMo3, calculated by mass percentage, is as follows: C 0.35-0.43%, Si 0.15-0.40%, Mn 0.50-0.80%, P≤0.025%, S≤0.035%, Cr 0.60-1.00%, Ni 0.70-1.00%, Mo 0.15-0.25%, V≤0.05%, Cu≤0.30%, Ti 0.0080-0.0120%, Al 0.010-0.025%, N≤0.0070%, and the balance is Fe and unavoidable impurities.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] The surface quality control method of carbon-chromium-nickel-molybdenum alloy structural steel of the present invention effectively improves the surface quality of the continuous casting billet by adopting a new smelting method, a smelting process of micro-titanium alloying, and controlled aluminum and nitrogen. At the same time, by adopting a low-temperature heating and rolling method, the grain size of the steel is prevented from growing, and the probability of surface quality deterioration caused by excessive heating temperature is reduced. Since the solidification structure of the continuous casting billet is sequentially from the outside to the inside: a chill layer (fine equiaxed crystals), a columnar crystal region, and a central equiaxed crystal region. The internal crystal refinement is enhanced, and the finer the grains, the less likely it is to produce crack defects on the surface. Therefore, titanium micro-alloying treatment and aluminum and nitrogen control are adopted to effectively control the morphology and size of the continuous casting billet structure, so that the surface defects of the continuous casting billet are significantly reduced, and the generation of large-scale defect depth is avoided. At the same time, by adopting a new heating process rolling, the heating temperature range of 1220℃~1240℃ is accurately controlled, so that the steel is completely burned through, and the grains are fine while being in the high-temperature plastic deformation range, avoiding the steel billet staying in the high-temperature section for a long time, causing coarse grains that are harmful to the surface quality.

[0022] The method of the present invention can solve the problem of poor surface quality of large-size 39NiCrMo3 quenched and tempered round steel due to coarse structure, and is suitable for the preparation of 39NiCrMo3 quenched and tempered round steel with specifications (100mm<∮≤160mm). The surface defect depth of the prepared steel is within 0.3mm, and the surface qualification rate of the steel is greatly improved, which is not less than 90%. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0024] FIG1 is a flow chart of the method of the present invention;

[0025] FIG2 is a 500-fold metallographic photograph of the 1 / 2 radius portion of the 39NiCrMo3 round steel bar obtained in Example 1 of the present invention;

[0026] FIG3 is a 100-fold metallographic photograph of the 1 / 2 radius portion of the 39NiCrMo3 round steel bar obtained in Example 1 of the present invention;

[0027] FIG4 is a 100-fold metallographic photograph of the 1 / 2 radius portion of the 39NiCrMo3 round steel bar obtained in Comparative Example 4 of the present invention. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. The process parameters for which specific conditions are not specified in the following examples are generally in accordance with conventional conditions.

[0029] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0030] According to a first aspect of the present invention, a method for controlling the surface quality of medium-carbon chromium-nickel-molybdenum alloy structural steel is provided, wherein the raw materials of the medium-carbon chromium-nickel-molybdenum alloy structural steel are sequentially smelted, continuously cast, and rolled to obtain the medium-carbon chromium-nickel-molybdenum alloy structural steel; wherein, during the smelting, titanium microalloying treatment and aluminum and nitrogen control are performed to control the titanium content to 0.0080-0.0120%; and the rolling forming includes low-temperature heating rolling, and the low-temperature heating temperature is 1220-1240°C.

[0031] The present invention pioneers a surface quality control method for medium-carbon chromium-nickel-molybdenum alloy structural steel. This method utilizes a smelting process involving micro-titanium alloying and controlled aluminum and nitrogen content to effectively improve the surface quality of continuous casting ingots. Furthermore, by employing low-temperature heating and rolling, grain size growth in the steel is prevented, reducing the likelihood of surface quality degradation due to excessively high heating temperatures. This method addresses the problem of poor surface quality in large-scale quenched and tempered 39NiCrMo3 round steel due to coarse microstructure. Specifically, the method comprises: 1. By employing a novel smelting method and titanium micro-alloying, the titanium content is controlled to 0.0080-0.0120% (e.g., 0.0085%, 0.0090%, 0.0095%, 0.0100%, 0.0105%, 0.0115%, etc.), and the aluminum and nitrogen content is kept low, so that the depth of surface defects in the continuous casting ingots is within 0.3 mm, meeting surface quality requirements. 2. The 39NiCrMo3 continuous casting billet with a specification of 410*530mm is heated and rolled at a temperature of 1220℃~1240℃ (such as 1221℃, 1223℃, 1225℃, 1227℃, 1229℃, 1231℃, 1233℃, 1235℃, 1237℃, 1239℃, etc.) in a walking furnace to avoid the billet staying in the high temperature section for a long time, which causes coarse grains and is harmful to the surface quality, and obtains a balanced structure of pearlite and ferrite to meet the surface requirements.

[0032] In this invention, titanium microalloying is achieved by adding ferro-titanium during the refining process according to the target composition. The titanium content is controlled to 0.0080-0.0120%. If the titanium content is too high, it will affect the quality of the steel, especially the impact resistance, while if the titanium content is too low, the surface quality improvement effect will be poor.

[0033] As an optional implementation mode of the surface quality control method of the present invention, during the smelting, vacuum degassing is adopted, the vacuum degree is ≤0.5 Torr (such as 0.4 Torr, 0.3 Torr, 0.2 Torr, 0.1 Torr, etc.), the total vacuum time is 20-35min (such as 21min, 23min, 25min, 27min, 29min, 31min, 33min, etc.), and argon soft blowing is adopted at the same time, and the soft blowing time is 20-35min (such as 21min, 23min, 25min, 27min, 29min, 31min, 33min, etc.) to control aluminum and nitrogen. Furthermore, the aluminum content is controlled to be between 0.010% and 0.025% (e.g., 0.011%, 0.013%, 0.015%, 0.017%, 0.019%, 0.021%, 0.023%, etc.); and the nitrogen content is controlled to be less than or equal to 0.0070% (e.g., 0.0070%, 0.0065%, 0.0060%, 0.0055%, 0.0050%, 0.0045%, 0.0040%, 0.0035%, etc.). Excessive aluminum and nitrogen content can easily precipitate at grain boundaries and form surface cracks. Excessive aluminum content can result in coarse grain size and poor impact energy.

[0034] The present invention further provides a method for controlling the surface quality of the medium-carbon chromium-nickel-molybdenum steel, comprising the following steps:

[0035] (a) Smelting: The raw materials are subjected to primary refining, external refining, and vacuum degassing to obtain molten steel;

[0036] (b) casting: continuously casting the molten steel to obtain a continuous casting billet;

[0037] (c) Rolling: The continuous casting billet is formed into round steel.

[0038] On the basis of the above technical solution of the present invention, in step (a), a converter is used for primary smelting, scrap steel and molten iron are added, and then melting, dephosphorization, decarburization and alloying are carried out in a converter in an oxidizing atmosphere. The carbon content at the converter end point is ≥0.08% (such as 0.08%, 0.085%, 0.09%, 0.095%, 0.1%, 0.105%, 0.115%, etc.), and the phosphorus content is controlled to be ≤0.015% (such as 0.015%, 0.012%, 0.010%, 0.008%, 0.005%, 0.003%, 0.0 01%, etc.), the tapping temperature is 1620-1650℃ (such as 1625℃, 1630℃, 1635℃, 1640℃, 1645℃, etc.); after refining outside the furnace, the sulfur content is controlled to be ≤0.005% (such as 0.005%, 0.004%, 0.003%, 0.002%, 0.001%, etc.), the titanium content is controlled to be 0.008-0.012% (such as 0.0085%, 0.0090%, 0.0095%, 0.0100%, 0.0105%, 0.0115%, etc.), and the tapping temperature is 1595-1635℃ (such as 1600℃, 1605℃, 1610℃, 1615℃, 1620℃, 1625℃, 1630℃, etc.); vacuum degassing, vacuum degree ≤0.5Torr (such as 0.4Torr, 0.3Torr, 0.2Torr, 0.1Torr, etc.), total vacuum time is 20-35min (such as 21min, 23min, 25min, 27min, 29min, 31min, 33min, etc.), while using argon soft blowing, soft blowing time is 20-35min (such as 21min, 23min, 2 5min, 27min, 29min, 31min, 33min, etc.), control the aluminum content to 0.010-0.025% (such as 0.011%, 0.013%, 0.015%, 0.017%, 0.019%, 0.021%, 0.023%, etc.), and control the nitrogen content to be less than or equal to 0.0070% (such as 0.0070%, 0.0065%, 0.0060%, 0.0055%, 0.0050%, 0.0045%, 0.0040%, 0.0035%, etc.).

[0039] On the basis of the above technical solution of the present invention, in step (b), ladle induction heating is adopted, and the molten steel is poured into the tundish at a certain speed. The molten steel passes through a water-cooled crystallizer, solidifies into a hard shell, and is continuously pulled out from the outlet below the crystallizer. After being cooled by water spray, it is completely solidified. The liquidus line of 39NiCrMo3 is 1495°C, the continuous casting temperature is 1515-1530°C (such as 1516°C, 1518°C, 1520°C, 1522°C, 1524°C, 1526°C, 1528°C, etc.), the continuous casting billet is 410*530mm, and the pulling speed is 0.41-0.45m / min (such as 0.42m / min, 0.43m / min, 0.44m / min, etc.).

[0040] Based on the above technical solution of the present invention, in step (c), the continuous casting billet is heated to 1220-1240°C (e.g., 1221°C, 1223°C, 1225°C, 1227°C, 1229°C, 1231°C, 1233°C, 1235°C, 1237°C, 1239°C, etc.), and fully burned through. Then, the billet is opened by a 1350 rolling mill and then rolled by a 750 rolling mill. The final shape is round steel, and the final rolling temperature is not less than 860°C (e.g., 860°C, 865°C, 870°C, 875°C, 880°C, 885°C, 890°C, 895°C, 900°C, etc.). Furthermore, the round steel specification is 100mm<∮≤160mm (e.g., ∮110mm, ∮120mm, ∮130mm, ∮140mm, ∮150mm, etc.).

[0041] According to a second aspect of the present invention, a medium-carbon chromium-nickel-molybdenum alloy structural steel is prepared by the surface quality control method.

[0042] As an optional embodiment of the medium-carbon chromium-nickel-molybdenum alloy structural steel of the present invention, the material of the medium-carbon chromium-nickel-molybdenum alloy structural steel is 39NiCrMo3. Furthermore, the chemical composition of the 39NiCrMo3, by mass percentage, is as follows: C 0.35-0.43%, Si 0.15-0.40%, Mn 0.50-0.80%, P ≤ 0.025%, S ≤ 0.035%, Cr 0.60-1.00%, Ni 0.70-1.00%, Mo 0.15-0.25%, V ≤ 0.05%, Cu ≤ 0.30%, Ti 0.0080-0.0120%, Al 0.010-0.025%, N ≤ 0.0070%, and the balance is Fe and unavoidable impurities.

[0043] The present invention will be further described in detail below with reference to specific examples and comparative examples.

[0044] Example 1

[0045] A surface quality control method for medium-carbon chromium-nickel-molybdenum alloy structural steel comprises the following steps:

[0046] (a) Smelting: The raw materials are subjected to primary refining, external refining, and vacuum degassing to obtain molten steel;

[0047] (d) casting: continuously casting the molten steel to obtain a continuous casting billet;

[0048] (e) Rolling: The continuous casting billet is formed into round steel.

[0049] In step (a), a converter is used for primary refining, scrap steel and molten iron are added, and then melting, dephosphorization, decarburization and alloying are carried out in a converter in an oxidizing atmosphere. The carbon content of the converter is 0.012%, the phosphorus content is controlled to be 0.013%, and the tapping temperature is 1630°C; after refining outside the furnace, the sulfur content is controlled to be 0.002%, the titanium content is controlled to be 0.0092%, and the tapping temperature is 1605°C; vacuum degassing is carried out with a vacuum degree of ≤0.5 Torr and a total vacuum time of 25 minutes. Argon soft blowing is used at the same time for 30 minutes to control the aluminum content to be 0.017% and the nitrogen content to be 0.0041%.

[0050] In step (b), ladle induction heating is used to pour the molten steel into the tundish at a certain speed. The molten steel passes through a water-cooled crystallizer, solidifies into a hard shell, and is continuously pulled out from the outlet below the crystallizer. After being cooled by water spray, it is completely solidified. The liquidus line of 39NiCrMo3 is 1495°C, the continuous casting temperature is 1520°C, the continuous casting billet is 410*530mm, and the pulling speed is 0.43m / min.

[0051] In step (c), the continuous casting billet is heated to 1230°C and fully burned, and then the billet is opened by a 1350 rolling mill and then rolled by a 750 rolling mill. The final shape is round steel, and the final rolling temperature is 880°C.

[0052] The 39NiCrMo3 round steel bar obtained in Example 1 has a specification of ∮130 mm and includes the following chemical components by mass fraction: C 0.40%, Si 0.25%, Mn 0.75%, P 0.013%, S 0.002%, Cr 0.92%, Ni 0.76%, Mo 0.18%, V 0.008%, Cu 0.02%, Ti 0.0098%, Al 0.017%, N 0.0041%, and the balance is Fe and unavoidable impurities.

[0053] Example 2

[0054] A surface quality control method for medium-carbon chromium-nickel-molybdenum alloy structural steel comprises the following steps:

[0055] (a) Smelting: The raw materials are subjected to primary refining, external refining, and vacuum degassing to obtain molten steel;

[0056] (f) casting: continuously casting the molten steel to obtain a continuous casting billet;

[0057] (g) Rolling: The continuous casting billet is formed into round steel.

[0058] In step (a), a converter is used for primary refining, scrap steel and molten iron are added, and then melting, dephosphorization, decarburization and alloying are carried out in a converter in an oxidizing atmosphere. The carbon content of the converter is 0.08%, the phosphorus content is controlled to be 0.015%, and the tapping temperature is 1650°C; after refining outside the furnace, the sulfur content is controlled to be 0.005%, the titanium content is controlled to be 0.012%, and the tapping temperature is 1635°C; vacuum degassing is carried out with a vacuum degree of ≤0.5 Torr and a total vacuum time of 20 minutes. Argon soft blowing is used at the same time for 20 minutes, and the aluminum content is controlled to be 0.025% and the nitrogen content is controlled to be 0.0070%.

[0059] In step (b), ladle induction heating is used to pour the molten steel into the tundish at a certain speed. The molten steel passes through a water-cooled crystallizer, solidifies into a hard shell, and is continuously pulled out from the outlet below the crystallizer. After being cooled by water spray, it is completely solidified. The liquidus line of 39NiCrMo3 is 1495°C, the continuous casting temperature is 1530°C, the continuous casting billet is 410*530mm, and the pulling speed is 0.45m / min.

[0060] In step (c), the continuous casting billet is heated to 1240°C and fully burned, and then the billet is opened by a 1350 rolling mill and then rolled by a 750 rolling mill. The final shape is round steel, and the final rolling temperature is 890°C.

[0061] The obtained 39NiCrMo3 round steel bar has a specification of ∮160 mm and includes the following chemical compositions by mass: C 0.43%, Si 0.40%, Mn 0.80%, P 0.025%, S 0.035%, Cr 1.00%, Ni 1.00%, Mo 0.25%, V 0.05%, Cu 0.30%, Ti 0.0120%, Al 0.025%, N 0.0070%, and the balance is Fe and unavoidable impurities.

[0062] Example 3

[0063] A surface quality control method for medium-carbon chromium-nickel-molybdenum alloy structural steel comprises the following steps:

[0064] (a) Smelting: The raw materials are subjected to primary refining, external refining, and vacuum degassing to obtain molten steel;

[0065] (h) casting: continuously casting the molten steel to obtain a continuous casting billet;

[0066] (i) Rolling: The continuous casting billet is formed into round steel.

[0067] In step (a), a converter is used for primary refining, scrap steel and molten iron are added, and then melting, dephosphorization, decarburization and alloying are carried out in a converter in an oxidizing atmosphere. The carbon content of the converter is 0.011%, the phosphorus content is controlled to be 0.012%, and the tapping temperature is 1620°C; after refining outside the furnace, the sulfur content is controlled to be 0.003%, the titanium content is controlled to be 0.008%, and the tapping temperature is 1595°C; vacuum degassing is carried out with a vacuum degree of ≤0.5 Torr and a total vacuum time of 35 minutes. Argon soft blowing is used at the same time for 35 minutes, the aluminum content is controlled to be 0.010%, and the nitrogen content is controlled to be 0.0030%.

[0068] In step (b), ladle induction heating is used to pour the molten steel into the tundish at a certain speed. The molten steel passes through a water-cooled crystallizer, solidifies into a hard shell, and is continuously pulled out from the outlet below the crystallizer. After being cooled by water spray, it is completely solidified. The liquidus line of 39NiCrMo3 is 1495°C, the continuous casting temperature is 1515°C, the continuous casting billet is 410*530mm, and the pulling speed is 0.41m / min.

[0069] In step (c), the continuous casting billet is heated to 1220°C and fully burned, and then the billet is opened by a 1350 rolling mill and then rolled by a 750 rolling mill. The final shape is round steel, and the final rolling temperature is 860°C.

[0070] The obtained 39NiCrMo3 round steel bar has a specification of ∮100 mm and includes the following chemical compositions by mass: C 0.35%, Si 0.15%, Mn 0.50%, P 0.012%, S 0.003%, Cr 0.60%, Ni 0.70%, Mo 0.15%, V 0.015%, Cu 0.08%, Ti 0.0080%, Al 0.010%, N 0.0030%, and the balance is Fe and unavoidable impurities.

[0071] Comparative Example 1

[0072] This comparative example provides a surface control method for medium-carbon chromium-nickel-molybdenum alloy structural steel. Except that the Ti component is adjusted to 0.0030% during smelting, the remaining steps are the same as those in Example 1.

[0073] The 39NiCrMo3 round steel obtained in Comparative Example 1 includes the following chemical components by mass fraction: C 0.40%, Si 0.26%, Mn 0.74%, P 0.014%, S 0.003%, Cr 0.91%, Ni 0.75%, Mo 0.18%, V 0.007%, Cu 0.02%, Ti 0.0030%, Al 0.017%, N 0.0041%, and the balance is Fe and unavoidable impurities.

[0074] Comparative Example 2

[0075] This comparative example provides a surface control method for medium-carbon chromium-nickel-molybdenum alloy structural steel. Except that the Al component is adjusted to 0.03% during smelting, the remaining steps are the same as those in Example 1.

[0076] The obtained 39NiCrMo3 round steel includes the following chemical components by mass fraction: C 0.40%, Si 0.25%, Mn 0.75%, P 0.013%, S 0.002%, Cr 0.92%, Ni 0.76%, Mo 0.18%, V 0.008%, Cu 0.02%, Ti 0.0098%, Al 0.03%, N 0.0041%, and the balance is Fe and unavoidable impurities.

[0077] Comparative Example 3

[0078] This comparative example provides a surface control method for medium-carbon chromium-nickel-molybdenum alloy structural steel. Except that the nitrogen content during smelting is adjusted to 0.0080%, the remaining steps are the same as those in Example 1.

[0079] The obtained 39NiCrMo3 round steel includes the following chemical components by mass fraction: C 0.40%, Si 0.25%, Mn 0.75%, P 0.013%, S 0.002%, Cr 0.92%, Ni 0.76%, Mo 0.18%, V 0.008%, Cu 0.02%, Ti 0.0098%, Al 0.0017%, N 0.0080%, and the balance is Fe and unavoidable impurities.

[0080] Comparative Example 4

[0081] This comparative example provides a surface control method for medium-carbon chromium-nickel-molybdenum alloy structural steel. Except that the rolling temperature is adjusted to 1250° C., the remaining steps are the same as those in Example 1.

[0082] test

[0083] The 39NiCrMo3 bars obtained in the examples and comparative examples were subjected to magnetic particle inspection using a CDW-24000AT instrument. The specific test results are as follows:

[0084] Note: Defects with a depth of more than 0.3 mm are considered unqualified defects. Bars with unqualified defects on the surface are considered surface unqualified bars. The surface unqualified ratio of each embodiment and comparative example is the number of unqualified bars / the total number of tested bars × 100%. For example, the surface unqualified ratio of Example 1 = the number of unqualified bars in Example 1 / the total number of tested bars in Example 1 × 100%.

[0085] At the same time, Figures 2 and 3 show the metallographic conditions of the 1 / 2 radius portion of the 39NiCrMo3 round steel bar obtained in Example 1. It can be seen that the control method of the present invention obtains a balanced structure of pearlite and ferrite, and compared with the bar obtained by high-temperature rolling in Figure 4, the control method of the present invention greatly refines the grain size.

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for controlling the surface quality of medium-carbon chromium-nickel-molybdenum alloy structural steel, characterized in that, Including: The raw materials of the medium-carbon chromium-nickel-molybdenum alloy structural steel are successively smelted, continuously cast, and rolled into shape to obtain the medium-carbon chromium-nickel-molybdenum alloy structural steel; wherein, during the smelting, titanium microalloying treatment and aluminum and nitrogen control are carried out, and the titanium element content is controlled to be 0.0080 - 0.0120%; the rolling into shape includes low-temperature heating rolling, and the temperature of the low-temperature heating is 1220 - 1240°C.

2. The surface quality control method according to claim 1, wherein, During the smelting, vacuum degassing is adopted, the vacuum degree ≤ 0.5 Torr, the total vacuum time is 20 - 35 min, and at the same time, argon soft blowing is adopted, the soft blowing time is 20 - 35 min, and aluminum and nitrogen are controlled.

3. The surface quality control method according to claim 1 or 2, characterized in that, The aluminum control is to control the aluminum element content to be 0.010% - 0.025%; the nitrogen control is to control the nitrogen element content to be less than or equal to 0.0070%.

4. The surface quality control method according to claim 1, characterized in that The smelting includes primary smelting and secondary refining.

5. The surface quality control method according to claim 4, characterized in that The primary smelting is carried out by a converter. Scrap steel and molten iron are added, and then melting, dephosphorization, decarburization, and alloying are carried out in the converter with an oxidizing atmosphere. The carbon content at the end of the converter is ≥ 0.08%, the phosphorus element content is controlled to be ≤ 0.015%, and the tapping temperature is 1620 - 1650°C; And / or, after the secondary refining, the sulfur element content is controlled to be ≤ 0.005%, the titanium element content is controlled to be 0.008 - 0.012%, and the tapping temperature is 1595 - 1635°C.

6. The surface quality control method according to claim 1, wherein The billet obtained by the continuous casting is a square billet with a specification of 410*530 mm.

7. The surface quality control method according to claim 1, wherein The temperature of the continuous casting is 1515 - 1530°C, and the drawing speed is 0.41 - 0.45 m / min; And / or, the rolling into shape includes finish rolling, and the temperature of the finish rolling is not lower than 860°C.

8. The surface quality control method according to claim 1, characterized in that The medium-carbon chromium-nickel-molybdenum alloy structural steel obtained by the rolling into shape is a round bar with a specification of 100 mm < ∮ ≤ 160 mm.

9. A medium-carbon chromium-nickel-molybdenum alloy structural steel is prepared by the surface quality control method according to any one of claims 1 - 8.

10. The medium-carbon chromium-nickel-molybdenum alloy structural steel according to claim 9, wherein The material of the medium-carbon chromium-nickel-molybdenum alloy structural steel is 39NiCrMo3; for the 39NiCrMo3, by mass percentage, its chemical composition: C 0.35 - 0.43%, Si 0.15 - 0.40%, Mn 0.50 - 0.80%, P ≤ 0.025%, S ≤ 0.035%, Cr 0.60 - 1.00%, Ni 0.70 - 1.00%, Mo 0.15 - 0.25%, V ≤ 0.05%, Cu ≤ 0.30%, Ti 0.0080 - 0.0120%, Al 0.010 - 0.025%, N ≤ 0.0070%, and the balance is Fe and unavoidable impurities.

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