Medium-carbon bainite non-heat-treated steel for large-scale direct cutting and its production process

Abstract

This invention belongs to the technical field of alloy steels and relates to a medium-carbon bainite non-heat-treated steel for large-grade direct cutting and its production process. The composition is C 0.35~0.45%, Si 0.3~1.0%, Mn 1.5~2.0%, P≦0.012%, S≦0.006%, Cr 0.6~1.2%, V 0.04~0.2%, Nb 0.01~0.03%, Mo 0.15~0.2%, B 0.0005~0.00015%, Al 0.02~0.04%, Ti 0.015~0.025%, Ni 0.1~0.2%, O≦15ppm, N 110~180ppm, with the remainder being Fe and unavoidable impurities. In this invention, by combining optimization of components and processes, nonmetallic inclusions and macrostructure are all suitably controlled, and the yield strength is consistently 750 MPa or higher, making it promising for a wide range of applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of alloy steel, and specifically relates to medium-carbon bainite non-quenched and tempered steel for direct cutting of large specifications and its production process.

Background Art

[0002] In recent years, companies represented by Jiangsu Yonggang Company and Japanese JFE Company have carried out research and development on various pearlite-type non-quenched and tempered steels and bainite non-quenched and tempered steels. However, due to the influence of the metal structure and specifications of the materials, currently, various non-quenched and tempered steels with a diameter of φ160mm or more are mainly pearlite-type and low-carbon bainite-type non-quenched and tempered steel products, which can only meet the yield strength requirement of about 700MPa and cannot meet the requirement of 750MPa or more.

[0003] The key components of the conventional pearlite-type non-quenched and tempered steel are: C: 0.38 - 0.48%, Si: 0.17 - 0.70%, Mn: 0.80 - 1.60%, P ≤ 0.020%, S ≤ 0.035%, Cr: 0.10 - 0.20%, V: 0.05 - 0.18%, Ti: 0.010 - 0.020%, Ni ≤ 0.025%, Mo ≤ 0.015%, Al ≤ 0.030%, Cu ≤ 0.2%, N: 130 - 200ppm, H ≤ 2.0ppm, O ≤ 20ppm.

[0004] The key components of the conventional low-carbon bainite-type non-quenched and tempered steel are: C: 0.10 - 0.35%, Si: 0.30 - 1.00%, Mn: 1.50 - 3.0%, P ≤ 0.012%, S ≤ 0.006%, Cr: 0.35 - 0.95%, V: 0.04 - 0.12%, Nb: 0.010 - 0.030%, Mo: 0.05 - 0.20%, Al: 0.020 - 0.040%, Ti: 0.015 - 0.025%, Ni: 0.10 - 0.20%, O ≤ 15ppm, N: 110 - 180ppm.

[0005] Furthermore, our research revealed that current research is primarily focused on small-scale non-heat-treated steel (diameter ≤ 60 mm), and is limited by the difficulty of producing large-scale round bars. As a result, there is little research on large-scale non-heat-treated steel, and there is almost no media coverage of research addressing the problem of not being able to meet the requirement of a yield strength greater than 750 MPa. Therefore, how to solve the yield strength problem, how to improve yield strength, and how to achieve superior grain refinement strengthening and precipitation strengthening effects to further enhance the material's strength are urgent technical issues that need to be resolved. [Overview of the project] [Problems that the invention aims to solve]

[0006] The objective of the present invention is to overcome the technical shortcomings of the conventional technology and, by uniquely designing the composition for large-spec non-heat-treated steel (φ160 mm or larger) for direct cutting and combining it with corresponding process conditions, solve the problem that, as the specifications increase in pearlite non-heat-treated steel and low-carbon bainite non-heat-treated steel, microstructure transformation is limited and it is not possible to meet the requirement of a yield strength greater than 750 MPa. [Means for solving the problem]

[0007] To achieve the above objectives, the present invention first provides, in mass%, This invention provides a large-grade medium-carbon bainite unheat-treated steel for direct cutting, consisting of C:0.35~0.45%, Si:0.30~1.00%, Mn:1.50~2.00%, P≦0.012%, S≦0.006%, Cr:0.60~1.20%, V:0.04~0.20%, Nb:0.010~0.030%, Mo:0.15~0.20%, B:0.0005~0.00015%, Al:0.020~0.040%, Ti:0.015~0.025%, Ni:0.10~0.20%, O≦15ppm, N:110~180ppm, with the remainder being Fe and unavoidable impurities.

[0008] The aforementioned large-grade medium-carbon bainite unheat-treated steel for direct machining has a diameter of 160 mm or more and a yield strength of 755 MPa or more.

[0009] Furthermore, in this embodiment, the medium-carbon bainite non-heat-treated steel for large-scale direct cutting has a diameter of 300 mm and a yield strength of 755 to 832 MPa.

[0010] The production method for the above-mentioned large-size medium-carbon bainite non-heat-treated steel for direct cutting includes electric furnace steelmaking → LF (Ladle Furnace) refining → VD (Vacuum Degassing) vacuum treatment → continuous casting → heating → controlled rolling and controlled cooling → sampling → finishing → medium-temperature aging → defect detection → warehousing.

[0011] The process is required to include the following specific steps: (1) Electric furnace steelmaking: The tapping endpoint carbon (C) is ≥ 0.15%, and the target carbon content is 0.10% ≥ C ≥ 0.30%. This helps prevent peroxidation of molten steel and remove inclusions. The target phosphorus (P) is ≤ 0.008%, and the target temperature is ≥ 1620°C. This controls the tapping temperature appropriately and effectively controls the return of P to the molten steel. (2) LF refining: The slag time is ≥ 20 min and the refining time is ≥ 40 min. Sufficient slag time is used to adequately remove oxygen from the molten steel, and by controlling the slag and refining time, it is ensured that the alloy components are precisely controlled within an appropriate refining cycle, and that inclusions are sufficiently floated to the surface and removed. (3) VD vacuum treatment: The VD high vacuum level is ≤67 Pa, the high vacuum holding time is ≥10 min, and the soft blow time is ≥15 min. This helps to remove H from the molten steel and promotes sufficient flotation of inclusions. (4) Continuous casting: Casting is performed at a constant drawing speed. Cooling is divided into primary and secondary cooling, with water cooling used for each. The primary water cooling rate is 4000 L / min, and rapid primary water cooling increases the proportion of columnar crystals and improves the density of the billet. The secondary water cooling rate is 800 L / min. The crystallizer electromagnetic stirring current is 250 A and the frequency is 2 Hz, the casting stream electromagnetic stirring current is 200 A and the frequency is 8 Hz, and the terminal electromagnetic stirring current is 1200 A and the frequency is 8 Hz. Electromagnetic stirring technology helps in molten metal replenishment and improves central defects in continuously cast billets. (5) Heating and rolling: In the heating process, in order to prevent the microalloy carbonitrides from growing and causing surface defects due to prolonged heating at high temperatures during direct hot transport, this steel grade is prohibited from being hot transported and rolled. To ensure sufficient solid solution of carbonitrides, the heating temperature and heating time for this steel type are as follows: Preheating stage temperature < 900°C, target temperature 860°C, reducing the risk of billet cracking due to billet preheating, time > 1.05h; Heating stage I temperature 900~1100°C, target temperature 1000°C, time ≥ 1.0h; Heating stage II temperature 1200~1260°C, target temperature 1230°C, time ≥ 4h; Soaking stage temperature 1200~1260°C, target temperature 1240°C, time ≥ 5h, total heating time ≥ 11.0h. Two-stage heating and one-stage soaking ensures uniform internal and external temperatures of the round billet, allowing sufficient solid solution of the forming elements of the microalloy steel carbonitride. In the rolling process, a 9-stand continuous rolling mill is used, and the continuous rolling entry temperature is controlled to 800-950°C to facilitate control of grain size. (6) After rolling, the cooling rate is controlled to 3-10°C / s by rapid cooling to cool to below 500°C to control the growth of recrystallized grains, the material is placed in a pit and slowly cooled when the cooling bed temperature is >400°C, and removed from the pit when the temperature is ≤200°C, with a slow cooling time of more than 24 hours.

[0012] Furthermore, in step (1), the target temperature is controlled to 1620°C to 1630°C.

[0013] Furthermore, in step (2), the slag time is 20-30 minutes, and the smelting time is 40-50 minutes.

[0014] Furthermore, in step (3), the high vacuum holding time is 10-15 minutes, and the soft blow time is 15-25 minutes.

[0015] Furthermore, in step (4), the drawing speed for constant drawing speed casting is 0.35 m / min.

[0016] Furthermore, in step (5), the duration of heating stage I is 1-2 hours, the duration of heating stage II is 4-6 hours, and the duration of the soaking stage is 5-6 hours.

[0017] Furthermore, in step (6), the mixture is cooled to 420°C to 500°C. [Effects of the Invention]

[0018] This steel grade is based on the composition of conventional bainite non-heat-treated steel, but by reducing the Mn content, increasing the C content, adding a certain amount of B element, and increasing the content of carbide-forming elements such as Cr and Mo, the stability of the supercooled austenite is enhanced, the shape of its continuous cooling curve is altered, and a stable medium-carbon bainite can be obtained under normal air-cooling conditions.

[0019] In this invention, the Cr content is appropriately increased and limited to Cr:0.60~1.20%. This has a solid solution strengthening effect and a grain refinement strengthening effect, increasing the pearlite content, reducing the lamellar spacing of the pearlite, and ultimately increasing the strength and toughness of the material.

[0020] Manganese (Mn) has solid solution strengthening and grain refinement properties, expanding the range of the austenite phase and lowering the phase transformation temperature, thereby contributing to improved performance in controlled rolling and controlled cooling processes and enhancing strength and toughness. However, if the manganese content is too high, segregation occurs, making it difficult to control the surface quality of the cast slab, and if the manganese content is too low, the required strength and hardness cannot be achieved. Therefore, in this invention, the optimal effect is achieved by controlling the manganese content to 1.50-2.00%.

[0021] Furthermore, the main effect of adding B to steel is to improve hardenability. The advantageous effect of B on hardenability is that it delays the ferrite nucleation process without affecting the thermodynamic properties of the austenite or ferrite matrix. Combined with the addition of microalloy elements such as V and Ti and the reduction of other alloying elements, it reduces material costs. The simultaneous addition of microalloys forms and precipitates composite carbides, resulting in small carbide precipitation size and a wide precipitation temperature range. These carbides have a pinning effect on austenite grains, effectively inhibiting the growth of austenite grains during the heating process. This delays the recrystallization process during rolling, improving and increasing the material's performance, thereby significantly improving both the strength and toughness of the steel. In another embodiment, the present invention uses two-stage heating and one-stage soaking to equalize the internal and external temperatures of the round billet, allowing the forming elements of the microalloy steel carbonitride to be sufficiently dissolved. In the rolling process, a 9-stand continuous rolling mill is used, and the continuous rolling entry temperature is controlled to 800-950°C to facilitate control of grain size.

[0022] To summarize the above, in the present invention, through the design of specific components, as the components change, the carbon content of the bainite matrix increases, and further the yield strength of the material becomes higher. Also, by controlling the heating temperature, rolling deformation, and cooling after rolling, the controlled rolling and controlled cooling functions of the conventional rolling line are fully exerted, the crystal grain size of the material is further refined, better grain refinement strengthening and precipitation strengthening effects are achieved, and the effect of further improving the material strength is realized. Further, the chemical composition and non-metallic inclusions are all suitably controlled, the yield strength is all 750 MPa or more, and even reaches 832 MPa, obtaining remarkable technical effects.

Brief Description of Drawings

[0023] [Figure 1] It is a metallographic structure diagram of the steel material manufactured in Example 2. [Figure 2] It is a metallographic structure diagram of the steel material manufactured in Example 3.

Modes for Carrying Out the Invention

[0024] Here, various exemplary embodiments of the present invention will be described in detail, but these should not be regarded as limiting the present invention, and should be understood as a more detailed description of specific aspects, characteristics, and implementation means of the present invention.

[0025] It should be understood that the technical terms described in the present invention are only for explaining special embodiments and do not limit the present invention. Unless otherwise indicated, all technical terms and scientific terms used in this specification have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs.

[0026] While this invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may be used in the practice or testing of this invention. All references mentioned herein are incorporated by reference to disclose and describe methods and / or materials related to those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail. Various improvements and modifications can be made to specific embodiments of the specification of this invention without departing from the scope or spirit of this invention. The specification and examples of this invention are illustrative only. (Example 1)

[0027] The round steel in this embodiment has a diameter of 300 mm, and its chemical composition is as follows (in mass %) (unit: wt%): The composition is C:0.35%, Si:0.30%, Mn:1.50%, P:0.006%, S:0.003%, Cr:0.60%, V:0.20%, Nb:0.030%, Mo:0.20%, B:0.00015%, Al:0.020%, Ti:0.025%, Ni:0.20%, O:10 ppm, N:180 ppm, with the remainder being Fe and unavoidable impurities.

[0028] According to the above-mentioned production method for medium-carbon bainite non-heat-treated steel for direct cutting of large-scale steel, this method includes electric furnace steelmaking → LF refining → VD vacuum treatment → continuous casting (Φ800) → heating → controlled rolling and controlled cooling (Φ300) → sampling → finishing → medium-temperature aging → defect detection → warehousing.

[0029] Specifically, this includes the following steps: (1) Electric furnace steelmaking: The tapping endpoint carbon content is C≧0.15%, and the target carbon content is 0.10%≧C≧0.30%. This helps prevent peroxidation of molten steel and remove inclusions. The target phosphorus content is ≦0.008%, and the target temperature is 1620°C. This controls the tapping temperature appropriately and effectively controls the return of phosphorus to the molten steel. At the same time, basic slag is formed during tapping using 600 kg of lime / furnace, 200 kg of cleaning accelerator / furnace, and 110-130 kg of tapped aluminum / furnace, which helps in desulfurization. (2) LF refining: The slag time is 25 mins and the refining time is 45 mins. Sufficient slag time ensures that the oxygen content in the molten steel is sufficiently removed, the alloy components are precisely controlled with an appropriate refining cycle, and inclusions are sufficiently floated to the surface and removed. The auxiliary material used is silicon carbide at a rate of 100-140 kg / furnace, which is used for deoxygenation. (3) VD vacuum treatment: The VD high vacuum level is ≤ 67 Pa, the high vacuum holding time is 15 min, and the soft blow time is 20 min, to remove H from the molten steel and promote sufficient floating of inclusions. (4) Continuous casting: Casting is performed at a constant drawing speed of 0.35 m / min. Cooling is divided into primary and secondary cooling, with water cooling used for each. The primary water cooling rate is 4000 L / min, and rapid primary water cooling increases the proportion of columnar crystals to improve the density of the billet. The secondary water cooling rate is 800 L / min. The crystallizer electromagnetic stirring current is 250 A and the frequency is 2 Hz, the casting stream electromagnetic stirring current is 200 A and the frequency is 8 Hz, and the terminal electromagnetic stirring current is 1200 A and the frequency is 8 Hz. Electromagnetic stirring technology helps in molten metal replenishment and improves central defects in the continuously cast billet. (5) Heating and Rolling: In the heating process, hot rolling of this steel type is prohibited in order to prevent the growth of the microalloy carbonitrides and the occurrence of surface defects due to prolonged high-temperature heating during direct hot transport of the microalloy carbonitrides. To ensure sufficient solid solution of the carbonitrides, the heating temperature and heating time for this steel type are as follows: The preheating stage temperature is <900°C, the target temperature is 860°C, reducing the risk of billet cracking due to billet preheating, and the time is 1.05h; the heating stage I temperature is 1000°C and the time is 2h; the heating stage II temperature is 1200~1260°C, the target temperature is 1230°C and the time is 4h; and the soaking stage temperature is 1240°C and the time is 6h. Two-stage heating and one-stage soaking ensure that the internal and external temperatures of the round billet are uniform and that the forming elements of the microalloy steel carbonitrides are sufficiently dissolved. In the rolling process, a φ800 standard round billet is divided and rolled in a bloc mill, and the head and tail are cut off by hydraulic cutting. Rolling is performed using a 9-stand continuous rolling mill, and the continuous rolling entry temperature is controlled to 950°C to control the grain size. (6) After rolling, the cooling rate is controlled to 10°C / s by rapid cooling and the material is cooled to 420°C to control the growth of recrystallized grains. The material is placed in a pit at a cooling bed temperature of 410°C and slowly cooled until the temperature is ≤200°C, at which point it is removed from the pit, and the slow cooling time is greater than 24 hours. (Example 2)

[0030] The round steel in this embodiment has a diameter of 300 mm, and its chemical composition is as follows (in mass %) (unit: wt%): The composition is C:0.40%, Si:0.60%, Mn:1.80%, P:0.008%, S:0.001%, Cr:0.90%, V:0.12%, Nb:0.020%, Mo:0.18%, B:0.0010%, Al:0.028%, Ti:0.020%, Ni:0.15%, O:8ppm, N:150ppm, with the remainder being Fe and unavoidable impurities.

[0031] According to the above-mentioned production method for medium-carbon bainite non-heat-treated steel for direct cutting of large-scale steel, this method includes electric furnace steelmaking → LF refining → VD vacuum treatment → continuous casting (Φ800) → heating → controlled rolling and controlled cooling (Φ300) → sampling → finishing → medium-temperature aging → defect detection → warehousing.

[0032] Specifically, this includes the following steps: (1) Electric furnace steelmaking: The tapping endpoint carbon content is C≧0.15%, and the target carbon content is 0.10%≧C≧0.30%. This helps prevent peroxidation of molten steel and remove inclusions. The target phosphorus content is ≦0.008%, and the target temperature is 1620°C. This controls the tapping temperature appropriately and effectively controls the return of phosphorus to the molten steel. At the same time, basic slag is formed during tapping using 600 kg of lime / furnace, 200 kg of cleaning accelerator / furnace, and 110-130 kg of tapped aluminum / furnace, which helps in desulfurization. (2) LF refining: The slag time is 25 mins and the refining time is 45 mins. Sufficient slag time ensures that the oxygen content in the molten steel is sufficiently removed, the alloy components are precisely controlled with an appropriate refining cycle, and inclusions are sufficiently floated to the surface and removed. The auxiliary material used is silicon carbide at a rate of 100-140 kg / furnace, which is used for deoxygenation. (3) VD vacuum treatment: The VD high vacuum level is ≤ 67 Pa, the high vacuum holding time is 15 min, and the soft blow time is 20 min, to remove H from the molten steel and promote sufficient floating of inclusions. (4) Continuous casting: Casting is performed at a constant drawing speed of 0.35 m / min. Cooling is divided into primary and secondary cooling, with water cooling used for each. The primary water cooling rate is 4000 L / min, and rapid primary water cooling increases the proportion of columnar crystals to improve the density of the billet. The secondary water cooling rate is 800 L / min. The crystallizer electromagnetic stirring current is 250 A at a frequency of 2 Hz, the casting stream electromagnetic stirring current is 200 A at a frequency of 8 Hz, and the terminal electromagnetic stirring current is 1200 A at a frequency of 8 Hz. Electromagnetic stirring technology helps in molten metal replenishment and improves central defects in the continuously cast billet. (5) Heating and Rolling: In the heating process, hot rolling of this steel type is prohibited in order to prevent the growth of the microalloy carbonitrides and the occurrence of surface defects due to prolonged high-temperature heating during direct hot transport of the microalloy carbonitrides. To ensure sufficient solid solution of the carbonitrides, the heating temperature and heating time for this steel type are as follows: The preheating stage temperature is <900°C, the target temperature is 860°C, reducing the risk of billet cracking due to billet preheating, and the time is 1.05h; the heating stage I temperature is 1000°C and the time is 2h; the heating stage II temperature is 1200~1260°C, the target temperature is 1230°C and the time is 4h; and the soaking stage temperature is 1240°C and the time is 6h. Two-stage heating and one-stage soaking ensure that the internal and external temperatures of the round billet are uniform and that the forming elements of the microalloy steel carbonitrides are sufficiently dissolved. In the rolling process, a φ800 standard round billet is rolled in a bloc mill, and the head and tail are cut off by hydraulic cutting. Rolling is performed using a 9-stand continuous rolling mill, and the continuous rolling entry temperature is controlled to 880°C to control the grain size. (6) After rolling, the cooling rate is controlled to 5°C / s by rapid cooling to cool to 480°C to control the growth of recrystallized grains. The material is placed in a pit at a cooling bed temperature of 410°C and slowly cooled until the temperature is ≤200°C, at which point it is removed from the pit, with a slow cooling time greater than 24 hours. (Example 3)

[0033] The round steel in this embodiment has a diameter of 300 mm, and its chemical composition is as follows (in mass %) (unit: wt%): The composition is C:0.45%, Si:1.00%, Mn:2.00%, P:0.012%, S:0.006%, Cr:1.20%, V:0.04%, Nb:0.010%, Mo:0.15%, B:0.0005%, Al:0.040%, Ti:0.015%, Ni:0.10%, O:5ppm, N:110ppm, with the remainder being Fe and unavoidable impurities.

[0034] According to the above-mentioned production method for medium-carbon bainite non-heat-treated steel for direct cutting of large-scale steel, this method includes electric furnace steelmaking → LF refining → VD vacuum treatment → continuous casting (Φ800) → heating → controlled rolling and controlled cooling (Φ300) → sampling → finishing → medium-temperature aging → defect detection → warehousing.

[0035] Specifically, this includes the following steps: (1) Electric furnace steelmaking: The tapping endpoint carbon content is C≧0.15%, and the target carbon content is 0.10%≧C≧0.30%. This helps prevent peroxidation of molten steel and remove inclusions. The target phosphorus content is ≦0.008%, and the target temperature is 1620°C. This controls the tapping temperature appropriately and effectively controls the return of phosphorus to the molten steel. At the same time, basic slag is formed during tapping using 600 kg of lime / furnace, 200 kg of cleaning accelerator / furnace, and 110-130 kg of tapped aluminum / furnace, which helps in desulfurization. (2) LF refining: The slag time is 25 mins and the refining time is 45 mins. Sufficient slag time ensures that the oxygen content in the molten steel is sufficiently removed, the alloy components are precisely controlled with an appropriate refining cycle, and inclusions are sufficiently floated to the surface and removed. The auxiliary material used is silicon carbide at a rate of 100-140 kg / furnace, which is used for deoxygenation. (3) VD vacuum treatment: The VD high vacuum level is ≤ 67 Pa, the high vacuum holding time is 15 min, and the soft blow time is 20 min, to remove H from the molten steel and promote sufficient floating of inclusions. (4) Continuous casting: Casting is performed at a constant drawing speed of 0.35 m / min. Cooling is divided into primary and secondary cooling, with water cooling used for each. The primary water cooling rate is 4000 L / min, and rapid primary water cooling increases the proportion of columnar crystals to improve the density of the billet. The secondary water cooling rate is 800 L / min. The crystallizer electromagnetic stirring current is 250 A at a frequency of 2 Hz, the casting stream electromagnetic stirring current is 200 A at a frequency of 8 Hz, and the terminal electromagnetic stirring current is 1200 A at a frequency of 8 Hz. Electromagnetic stirring technology helps in molten metal replenishment and improves central defects in the continuously cast billet. (5) Heating and Rolling: In the heating process, hot rolling of this steel type is prohibited in order to prevent the growth of the microalloy carbonitrides and the occurrence of surface defects due to prolonged high-temperature heating during direct hot transport of the microalloy carbonitrides. To ensure sufficient solid solution of the carbonitrides, the heating temperature and heating time for this steel type are as follows: The preheating stage temperature is <900°C, the target temperature is 860°C, reducing the risk of billet cracking due to billet preheating, and the time is 1.05h; the heating stage I temperature is 1000°C and the time is 2h; the heating stage II temperature is 1200~1260°C, the target temperature is 1230°C and the time is 4h; and the soaking stage temperature is 1240°C and the time is 6h. Two-stage heating and one-stage soaking ensure that the internal and external temperatures of the round billet are uniform and that the forming elements of the microalloy steel carbonitrides are sufficiently dissolved. In the rolling process, a φ800 standard round billet is rolled in a bloc mill, and the head and tail are cut off by hydraulic cutting. Rolling is performed using a 9-stand continuous rolling mill, and the continuous rolling entry temperature is controlled to 800°C to control the grain size. (6) After rolling, the cooling rate is controlled to 3°C / s by rapid cooling and the material is cooled to 500°C to control the growth of recrystallized grains. The material is placed in a pit at a cooling bed temperature of 410°C and slowly cooled until the temperature is ≤200°C, at which point it is removed from the pit, with a slow cooling time greater than 24 hours.

[0036] The detection status of nonmetallic inclusions and mechanical performance data for Examples 1 to 3 described above are shown in the table below. Mechanical performance was sampled according to GB / T2975-2018, and mechanical properties were detected according to GB / T228.1-2021 and GB / T229-2020. The nonmetallic inclusion content of the steel type was detected according to GB / T10561-2023.

[0037] [Table 1] Note: Sampling position 1 / 2R.

[0038] [Table 2]

[0039] As can be seen from Tables 1 and 2, the large-sized, medium-carbon bainite unheat-treated steel for direct cutting manufactured according to this invention all had yield strengths of 750 MPa or higher, and even reached 832 MPa. Furthermore, both the chemical composition and non-metallic inclusions were suitably controlled, resulting in remarkable technical advantages.

[0040] In this invention, by designing specific components, the carbon content of the bainite matrix is ​​improved as the components change, further increasing the yield strength of the material. In addition, by controlling the heating temperature, rolling deformation, and cooling after rolling, the controlled rolling and controlled cooling functions of conventional rolling lines are fully utilized, further refining the grain size of the material and achieving superior grain refinement and precipitation strengthening effects (Figures 1-2), thereby realizing a further improvement in material strength.

[0041] The above embodiments are merely illustrative of the present invention and do not limit the technical solutions described herein. Accordingly, although this specification has described the present invention in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the present invention. Any technical solutions and improvements thereof that do not depart from the spirit and scope of the present invention should be included within the scope of the claims of the present invention.

Claims

1. In mass%, the composition is as follows: C: 0.35–0.45%, Si: 0.30–1.00%, Mn: 1.50–2.00%, P ≤ 0.012%, S ≤ 0.006%, Cr: 0.60–1.20%, V: 0.04–0.20%, Nb: 0.010–0.030%, Mo: 0.15–0.20%, B: 0.0005–0.00015%, Al: 0.020–0.040%, Ti: 0.015–0.025%, Ni: 0.10–0.20%, O ≤ 15 ppm, N: 110–180 ppm, with the remainder being Fe and unavoidable impurities. A large-grade, medium-carbon bainite non-heat-treated steel for direct cutting, characterized by having a diameter of 160 mm or more and a yield strength of 755 MPa or more.

2. A large-grade medium-carbon bainite non-heat-treated steel for direct cutting, as described in claim 1, characterized by having a diameter of 300 mm and a yield strength of 755 to 832 MPa.

3. Step (1) involves electric furnace steelmaking, where the tapping endpoint C ≥ 0.15%, the target carbon 0.10% ≥ C ≥ 0.30%, the target P ≤ 0.008%, and the target temperature ≥ 1620°C. Step (2) involves performing LF refining, where the slag time is ≥ 20 min and the refining time is ≥ 40 min, Step (3) involves performing VD vacuum treatment, where the VD high vacuum degree is ≤ 67 Pa, the high vacuum holding time is ≥ 10 min, and the soft blow time is ≥ 15 min. Step (4) involves continuous casting, casting at a constant drawing speed, dividing the cooling into primary and secondary cooling, each using water cooling, with a primary water cooling rate of 4000 L / min, increasing the proportion of columnar crystals and improving the density of the billet through primary water rapid cooling, a secondary water cooling rate of 800 L / min, a crystallizer electromagnetic stirring current of 250 A at a frequency of 2 Hz, a casting stream electromagnetic stirring current of 200 A at a frequency of 8 Hz, and a terminal electromagnetic stirring current of 1200 A at a frequency of 8 Hz. Step (5) involves heating and rolling, controlling the preheating stage temperature to <900°C, target temperature to 860°C, time >1.05h, heating stage I temperature to 900-1100°C, target temperature to 1000°C, time ≥1.0h, heating stage II temperature to 1200-1260°C, target temperature to 1230°C, time ≥4h, soaking stage temperature to 1200-1260°C, target temperature to 1240°C, time ≥5h, total heating time ≥11.0h, rolling using a 9-stand continuous rolling mill, and controlling the continuous rolling entry temperature to 800-950°C to be advantageous for controlling the grain size. A production process for large-grade medium-carbon bainite unheat-treated steel for direct cutting, according to any one of claims 1 to 2, characterized by comprising: step (6) after rolling, controlling the cooling rate to 3 to 10°C / s by rapid cooling to cool to 500°C or below to control the growth of recrystallized grains, placing the steel in a pit at a cooling bed temperature > 400°C and slowly cooling it to a temperature ≤ 200°C before removing it from the pit, and slow cooling for a period of more than 24 hours.

4. A production process for large-grade medium-carbon bainite non-heat-treated steel for direct cutting, as described in claim 3, characterized in that the target temperature is controlled to 1620°C to 1630°C in step (1).

5. A production process for large-grade medium-carbon bainite unheat-treated steel for direct cutting, as described in claim 3, characterized in that in step (2), the slag time is 20 to 30 minutes and the smelting time is 40 to 50 minutes.

6. A production process for large-grade medium-carbon bainite unheat-treated steel for direct cutting, as described in claim 3, characterized in that in step (3), the high vacuum holding time is 10 to 15 minutes and the soft blow time is 15 to 25 minutes.

7. A production process for large-grade medium-carbon bainite non-heat-treated steel for direct cutting, according to claim 3, characterized in that in step (4), the drawing speed for constant drawing rate casting is 0.35 m / min.

8. A production process for large-grade medium-carbon bainite non-heat-treated steel for direct cutting, as described in claim 3, characterized in that in step (5), the time for heating stage I is 1 to 2 hours, the time for heating stage II is 4 to 6 hours, and the time for soaking stage is 5 to 6 hours.

9. A production process for large-grade medium-carbon bainite non-heat-treated steel for direct cutting, as described in claim 3, characterized in that the temperature is cooled to 420°C to 500°C in step (6).

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