Large-size medium-carbon bainite non-quenched and tempered steel for direct cutting processing and production process thereof

Abstract

The present invention belongs to the technical field of alloy steel, and relates to a large-size medium-carbon bainite non-quenched and tempered steel for direct cutting processing and a production process thereof. 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 balance being Fe and unavoidable impurities. By combining optimized composition and process, the present invention achieves good control of non-metallic inclusions and macrostructure, with yield strength all above 750 MPa, demonstrating broad application prospects.

Description

Medium-carbon bainite non-quenched and tempered steel for large-size direct cutting and its production process TECHNICAL FIELD

[0001] The application belongs to the technical field of alloy steel, and particularly relates to a medium-carbon bainite non-quenched and tempered steel for large-size direct cutting and a production process thereof. BACKGROUND

[0002] In recent years, enterprises represented by Jiangsu Yonggang and JFE Corporation of Japan have developed various pearlite non-quenched and tempered steels and bainite non-quenched and tempered steels. The above non-quenched and tempered steels are mainly pearlite and low-carbon bainite non-quenched and tempered steel products, which can only meet the requirement of a yield strength of about 700 MPa and cannot meet the requirement of a yield strength of greater than or equal to 750 MPa.

[0003] The key components of the existing pearlite 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 130-200 ppm, H≤2.0 ppm, and O≤20 ppm.

[0004] The key components of the existing low-carbon bainite 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≤15 ppm, and N 110-180 ppm.

[0005] At the same time, through investigation, it is found that the current research is basically focused on small-size non-quenched and tempered steels (diameter≤60 mm), and the research on large-size non-quenched and tempered steels is less due to the difficulty in production of large-size round bars; and there is almost no report on solving the problem of yield strength not meeting the requirement of greater than 750 MPa. Therefore, how to solve the problem of yield strength, how to obtain better fine-grain strengthening and precipitation strengthening effect while improving the yield strength, and how to further improve the strength of the material are technical problems to be solved at present.

[0006] SUMMARY

[0007] The present application aims to overcome the technical defects existing in the prior art, and provides a large-diameter non-quenched and tempered steel for direct cutting (see the above) Through unique design of the components and in combination with corresponding process conditions, the present application solves the problem that the microstructure transformation of the pearlitic non-quenched and tempered steel and the low-carbon bainitic non-quenched and tempered steel is limited with the increase of the size, and the yield strength cannot meet the requirement of being greater than 750 MPa.

[0008] To achieve the above-mentioned purpose, the present application first provides a large-diameter medium-carbon bainitic non-quenched and tempered steel for direct cutting, which is composed of the following components in percentage by mass:

[0009] 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, and the balance being Fe and inevitable impurities.

[0010] The large-diameter medium-carbon bainitic non-quenched and tempered steel for direct cutting has a diameter of greater than or equal to 160 mm and a yield strength of greater than or equal to 755 MPa.

[0011] Further, in the present embodiment, the large-diameter medium-carbon bainitic non-quenched and tempered steel for direct cutting has a diameter of 300 mm, and the yield strength is 755-832 MPa.

[0012] The production method of the large-diameter medium-carbon bainitic non-quenched and tempered steel for direct cutting includes the following steps: electric furnace steelmaking, LF refining, VD vacuum treatment, continuous casting, heating, controlled rolling and controlled cooling, sampling, finishing, medium-temperature aging, flaw detection, and warehousing.

[0013] The process requirements specifically include the following steps:

[0014] (1) Electric furnace steelmaking: the tapping end point C≥0.15%, the target carbon 0.10%≥C≥0.30%; prevent the molten steel from being over-oxidized, which is beneficial to the removal of inclusions; the target P≤0.008%, and the target temperature≥1620℃; control the appropriate tapping temperature to effectively control the P return of the molten steel.

[0015] (2) LF refining: white slag time≥20min, and smelting time≥40min; sufficiently remove the oxygen content in the molten steel through sufficient white slag time, control the white slag and smelting time, and ensure the accurate control of the alloy components and the sufficient floating removal of inclusions through appropriate smelting cycle.

[0016] (3) VD vacuum treatment: VD high vacuum degree ≤67 Pa, high vacuum holding time ≥10 min, soft blowing time ≥15 min; beneficial to remove H in molten steel, promote inclusions to float fully;

[0017] (4) continuous casting: constant speed casting; cooling is divided into primary cooling and secondary cooling, both of which adopt water cooling; the rate of primary water cooling is 4000 L / min, through primary water fast cooling, the proportion of columnar crystal is increased, and the compactness of the billet is improved; the rate of secondary water cooling is 800 L / min; the current of crystallizer electromagnetic stirring is 250 A, the frequency is 2 Hz, the current of casting flow electromagnetic stirring is 200 A, the frequency is 8 Hz, the current of end electromagnetic stirring is 1200 A, the frequency is 8 Hz, through electromagnetic stirring technology, it is beneficial to molten steel feeding and improving the center defects of continuous casting billet;

[0018] (5) heating and rolling: in the heating, in order to prevent the micro-alloy carbonitride from growing up due to high temperature and long time holding when the micro-alloy carbonitride is directly hot sent, which causes surface defects, the hot rolling of the steel is prohibited.

[0019] In order to ensure the sufficient solid solution of carbonitride, the heating temperature and time of the steel are as follows: the preheating temperature is <900℃, the target temperature is 860℃, the billet preheating reduces the risk of cracking, the time is >1.05h, the heating I temperature is 900-1100℃, the target temperature is 1000℃, the time is ≥1.0h, the heating II temperature is 1200-1260℃, the target temperature is 1230℃, the time is ≥4h, the soaking temperature is 1200-1260℃, the target temperature is 1240℃, the time is ≥5h, the total heating time is ≥11.0h. Through two-stage heating and one-stage soaking, the temperature inside and outside the round billet is uniform, and the carbonitride forming elements of the micro-alloy steel are fully solid-solved; in rolling, 9-stand continuous rolling mill is adopted, and the continuous rolling temperature is controlled at 800-950℃, which is beneficial to control the grain size.

[0020] (6) after rolling, through rapid cooling, the cooling rate is controlled at 3-10℃ / S to cool below 500℃, in order to control the recrystallization grain growth; the cooling bed temperature is >400℃, the slow cooling is carried out in the pit, the slow cooling temperature is ≤200℃, and the slow cooling time is greater than 24 hours.

[0021] Further, in step (1), the target temperature is controlled at 1620-1630℃.

[0022] Further, in step (2), the white slag time is 20-30 min, and the smelting time is 40-50 min.

[0023] Further, in step (3), the high vacuum holding time is 10-15 min, and the soft blowing time is 15-25 min.

[0024] Further, the casting speed in step (4) is 0.35 m / min.

[0025] Further, the heating I section time is 1-2 h, the heating II section time is 4-6 h, and the soaking section time is 5-6 h in step (5).

[0026] Further, the cooling is to 420-500 DEG C in step (6). Beneficial effects:

[0027] The present steel is based on the composition of the existing bainite non-quenched and tempered steel, the Mn content is reduced, the C content is increased, a certain B element is added, and the Cr, Mo and other carbide forming element contents are increased, so that the stability of the supercooled austenite is improved, and the continuous cooling curve shape is changed, so that stable medium-carbon bainite can be obtained under normal air cooling conditions.

[0028] In the present application, the Cr content is appropriately increased and is limited to Cr: 0.60-1.20%; it has solid solution strengthening effect, fine grain strengthening effect, increases pearlite content and reduces pearlite interlamellar spacing, so that the strength and toughness of the material are improved.

[0029] Mn has solid solution strengthening, fine grain strengthening effect, expands the austenite phase region, reduces the phase transition temperature, helps to obtain excellent performance by controlled rolling and controlled cooling process, and is beneficial to the improvement of strength and toughness. However, too high manganese content will produce segregation and the surface quality of the casting blank is not easy to control, and too low manganese content will not achieve the required strength and hardness; therefore, the manganese content is selected and controlled to be 1.50-2.00% in the present application to achieve the best effect.

[0030] Meanwhile, the main role of B added to the steel is to improve the hardenability, the beneficial effect of B on the hardenability is that the ferrite nucleation process is delayed, but the thermodynamic properties of the austenite or ferrite matrix are not affected; combined with the addition of V, Ti and other micro-alloying elements, the cost of raw materials is reduced by reducing other alloying elements; the micro-alloying elements are added to form complex carbide precipitation, the carbide precipitation size is small and the precipitation temperature is wide, which has a pinning effect on the austenite grains and can effectively prevent the growth of austenite grains during the heating process, delays the recrystallization process during rolling, improves and increases the performance of the material, and greatly improves the strength and toughness of the steel. On the other hand, the present application uses two-stage heating and one-stage soaking to make the temperature of the round billet uniform inside and outside, and to make the micro-alloyed steel carbonitride forming elements fully solid-solute; in rolling, 9-stand continuous rolling mill is used for rolling, and the continuous rolling temperature is controlled at 800-950 DEG C, which is beneficial to the control of grain size.

[0031] In summary, this invention, through the design of specific components, increases the carbon content of the bainitic matrix, thereby further improving the yield strength of the material. Simultaneously, by controlling the heating temperature, rolling deformation, and post-rolling cooling, the invention fully utilizes the controlled rolling and cooling capabilities of existing rolling lines, further refining the grain size of the material and achieving better fine-grain strengthening and precipitation strengthening effects, thus further enhancing the material's strength. Moreover, the chemical composition and non-metallic inclusions are well controlled, with yield strengths exceeding 750 MPa, and even reaching 832 MPa, demonstrating significant technical advantages. Attached Figure Description

[0032] Figure 1 shows the metallographic structure of the steel prepared in Example 2.

[0033] Figure 2 shows the metallographic structure of the steel prepared in Example 3. Detailed Implementation

[0034] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0035] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art.

[0036] While only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with those documents. In the event of any conflict with any incorporated document, the content of this specification shall prevail. Various modifications and variations may be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention. This specification and its embodiments are merely exemplary.

[0037] Example 1:

[0038] The round steel bar involved in this embodiment has a diameter of 300mm, and its chemical composition by mass percentage (unit: wt%) is as follows:

[0039] 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; the balance being Fe and unavoidable impurities.

[0040] The production method of the above large-size direct cutting medium-carbon bainite non-quenched and tempered steel comprises the following steps: electric furnace steelmaking, LF refining, VD vacuum treatment, continuous casting (Φ800), heating, controlled rolling and controlled cooling (Φ300), sampling, finishing, medium-temperature aging, flaw detection, and warehousing.

[0041] Specifically, the following steps are included:

[0042] (1) Electric furnace steelmaking: the tapping endpoint C is greater than or equal to 0.15%, the target carbon is 0.10% greater than or equal to C and less than or equal to 0.30%; the molten steel is prevented from being over-oxidized, which is conducive to the removal of inclusions; the target P is less than or equal to 0.008%, and the target temperature is 1620°C; the appropriate tapping temperature is controlled to effectively control the P return of the molten steel; at the same time, according to the lime 600 kg / furnace, the deoxidizing agent 200 kg / furnace, and the tapping aluminum 110-130 kg / furnace, the tapping of the basic slag is conducive to the desulfurization.

[0043] (2) LF refining: the white slag time is 25 min, and the smelting time is 45 min; the oxygen content in the molten steel is sufficiently removed through sufficient white slag time, the appropriate smelting period ensures the accurate control of alloy composition, and the inclusions are sufficiently removed; the auxiliary material dosage: silicon carbide 100-140 kg / furnace, used for deoxidization;

[0044] (3) VD vacuum treatment: the VD high vacuum degree is less than or equal to 67 Pa, the high vacuum holding time is 15 min, and the soft blowing time is 20 min, thereby removing H in the molten steel and promoting the inclusions to float up;

[0045] (4) Continuous casting: constant speed casting, the casting speed is 0.35 m / min. The cooling is divided into primary cooling and secondary cooling, both of which adopt water cooling; the rate of the primary water cooling is 4000 L / min, the columnar crystal ratio is increased through the primary water fast cooling, and the billet is more dense; the rate of the secondary water cooling is 800 L / min; the current of the mold electromagnetic stirring is 250 A, the frequency is 2 Hz, the current of the casting flow electromagnetic stirring is 200 A, the frequency is 8 Hz, the current of the end electromagnetic stirring is 1200 A, and the frequency is 8 Hz, thereby the electromagnetic stirring technology is conducive to the feeding of the molten steel and the improvement of the center defects of the continuous casting billet;

[0046] (5) Heating, rolling: in the heating, to prevent the micro-alloy carbonitride from growing up due to the long time holding at high temperature when the micro-alloy carbonitride is directly hot sent, causing surface defects, the hot sending rolling of the steel is prohibited. To ensure the sufficient solid solution of the carbonitride, the heating temperature and time of the steel are as follows: the temperature of the preheating section < 900℃, the target temperature is 860℃, the preheating of the billet reduces the risk of cracking of the billet, the time is 1.05h, the temperature of the heating I section is 1000℃, the time is 2h, the temperature of the heating II section is 1200-1260℃, the target temperature is 1230℃, the time is 4h, the temperature of the soaking section is 1240℃, the time is 6h. Through the two-stage heating and one-stage soaking, the temperature inside and outside the round billet is uniform, and the carbonitride forming elements of the micro-alloy steel are fully solid-solved; in the rolling, the φ800 specification round billet is opened by the cogging mill, and the head and tail are cut by the hydraulic shear; the 9-stand continuous rolling mill is used for rolling, and the temperature entering the continuous rolling mill is controlled at 950℃ to control the grain size.

[0047] (6) After rolling, through rapid cooling, the cooling rate is controlled at 10℃ / S to cool to 420℃ to control the recrystallization grain growth; the temperature of the cooling bed is 410℃ to enter the pit for slow cooling, and the slow cooling is to the temperature ≤200℃ to exit the pit, and the slow cooling time is greater than 24 hours.

[0048] Example 2

[0049] The round steel diameter involved in the embodiment is 300mm, and the chemical composition is (unit, wt%) as follows:

[0050] C: 0.40%, Si: 0.60%, Mn: 1.80%, P: 0.008%, S: 0.001%, Cr: 0.90%, V: 012%, Nb: 0.020%, Mo: 018%, B: 0.0010%, Al: 0.028%, Ti: 0.020%, Ni: 0.15%, O: 8ppm, N: 150ppm; the balance is Fe and unavoidable impurities.

[0051] The production method of the above large specification directly cutting medium carbon bainite non-quenched and tempered steel includes the following steps: electric furnace steelmaking→LF refining→VD vacuum treatment→continuous casting (Φ800)→heating→controlled rolling and controlled cooling (Φ300)→sampling→finishing→medium temperature aging→flaw detection→warehousing.

[0052] Specifically includes the following steps:

[0053] (1) Electric furnace steelmaking: tapping endpoint C≥0.15%, target carbon 0.10%≥C≥0.30%; prevent over-oxidation of the molten steel, conducive to the removal of inclusions; target P≤0.008%, target temperature 1620°C; control the appropriate tapping temperature, effectively control the molten steel back P; at the same time, according to lime 600 kg / furnace, 200 kg / furnace, 110-130 kg / furnace, tapping of alkali slag is conducive to the removal of S.

[0054] (2) LF refining: white slag time 25 min, smelting time 45 min; through sufficient white slag time to fully remove the oxygen content in the molten steel, appropriate smelting cycle to ensure accurate control of alloy composition, inclusions are fully removed; auxiliary material dosage: silicon carbide 100-140 kg / furnace, for deoxidation;

[0055] (3) VD vacuum treatment: VD high vacuum degree≤67 Pa, high vacuum holding time 15 min, soft blowing time 20 min, remove H in the molten steel, promote inclusions to float fully;

[0056] (4) Continuous casting: constant speed casting, casting speed is 0.35 m / min. Cooling is divided into primary cooling and secondary cooling, both of which use water cooling; the rate of primary cold water cooling is 4000 L / min, which increases the proportion of columnar crystals through primary cold water fast cooling, and the billet is more dense; the rate of secondary cold water cooling is 800 L / min; the current of mold electromagnetic stirring is 250 A, the frequency is 2 Hz, the current of casting flow electromagnetic stirring is 200 A, the frequency is 8 Hz, the current of end electromagnetic stirring is 1200 A, the frequency is 8 Hz, which is conducive to the molten steel feeding and improves the center defects of continuous casting billet through electromagnetic stirring technology;

[0057] (5) Heating and rolling: in order to prevent the growth of micro-alloy carbonitride caused by direct hot sending at high temperature for a long time, which causes surface defects, the hot rolling of this steel is prohibited. In order to ensure the solid solution of carbonitride, the heating temperature and time of this steel are as follows: preheating temperature <900°C, target temperature 860°C, billet preheating reduces the risk of cracking, time 1.05 h, heating I temperature 1000°C, time 2 h, heating II temperature 1200-1260°C, target temperature 1230°C, time 4 h, soaking temperature 1240°C, time 6 h. Through two-stage heating and one-stage soaking, the temperature inside and outside the round billet is uniform, and the carbonitride forming elements of this micro-alloy steel are fully solid-solved; in rolling, φ800 specification round billet is opened by cogging mill and cut by hydraulic shearing head; 9-stand continuous rolling mill is used for rolling, and the entering continuous rolling temperature is controlled at 880°C to control the grain size.

[0058] (6) After rolling, through rapid cooling, the cooling rate is controlled at 5℃ / S to 480℃ to control the recrystallization grain growth; the cooling bed temperature is 410℃ to enter the pit for slow cooling, and the slow cooling is to a temperature of ≤200℃ to exit the pit, and the slow cooling time is greater than 24 hours.

[0059] Example 3:

[0060] This example relates to a round steel with a diameter of 300 mm, and the chemical composition in terms of mass percentage (unit, wt%) is as follows:

[0061] 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: 015%, B: 0.0005%, Al: 0.040%, Ti: 0.015%, Ni: 0.10%, O: 5ppm, N: 110ppm; the balance is Fe and unavoidable impurities.

[0062] The production method of the above large-size direct cutting medium-carbon bainite non-quenched and tempered steel includes the following steps: electric furnace steelmaking, LF refining, VD vacuum treatment, continuous casting (Φ800), heating, controlled rolling and controlled cooling (Φ300), sampling, finishing, medium-temperature aging, flaw detection, and warehousing.

[0063] Specifically, the following steps are included:

[0064] (1) Electric furnace steelmaking: the tapping endpoint C is ≥0.15%, the target carbon is 0.10%≥C≥0.30%; prevent the over-oxidation of the molten steel, which is conducive to the removal of inclusions; the target P is ≤0.008%, and the target temperature is 1620℃; control the appropriate tapping temperature to effectively control the molten steel back P; at the same time, according to the lime 600kg / furnace, the promoting agent 200kg / furnace, and the tapping aluminum 110-130kg / furnace, the tapping of the alkaline slag is conducive to the desulfurization.

[0065] (2) LF refining: white slag time 25min, smelting time 45min; through sufficient white slag time to fully remove the oxygen content in the molten steel, appropriate smelting period to ensure accurate control of alloy composition, and inclusions to fully float and remove; auxiliary material dosage: silicon carbide 100-140kg / furnace, for deoxidation;

[0066] (3) VD vacuum treatment: VD high vacuum degree ≤67Pa, high vacuum holding time 15min, soft blowing time 20min, to remove H in the molten steel, and promote inclusions to fully float;

[0067] (4) Continuous casting: constant speed casting, the casting speed is 0.35 m / min. The cooling is divided into primary cooling and secondary cooling, both of which adopt water cooling; the rate of primary water cooling is 4000 L / min, the columnar crystal ratio is increased by primary water fast cooling, and the steel billet is more dense; the rate of secondary water cooling is 800 L / min; the current of crystallizer electromagnetic stirring is 250 A, the frequency is 2 Hz, the current of casting stream electromagnetic stirring is 200 A, the frequency is 8 Hz, the current of end electromagnetic stirring is 1200 A, the frequency is 8 Hz, and the electromagnetic stirring technology is beneficial to the feeding of molten steel and the improvement of center defects of continuous casting billet;

[0068] (5) Heating and rolling: in the heating, in order to prevent the growth of micro-alloy carbonitride caused by direct hot sending at high temperature for a long time, which causes surface defects, the hot sending and rolling of the steel grade is prohibited. In order to ensure the sufficient solid solution of carbonitride, the heating temperature and time of the steel grade are as follows: the preheating temperature is <900℃, the target temperature is 860℃, the billet preheating reduces the risk of cracking, the time is 1.05h, the heating I temperature is 1000℃, the time is 2h, the heating II temperature is 1200-1260℃, the target temperature is 1230℃, the time is 4h, the soaking temperature is 1240℃, and the time is 6h. Through two-stage heating and one-stage soaking, the temperature inside and outside the round billet is uniform, and the carbonitride forming elements of the micro-alloy steel are fully solid-solved; in the rolling, the φ800 specification round billet is opened by an opening mill, and the head and tail are cut by a hydraulic shear; a 9-stand continuous rolling mill is adopted for rolling, and the temperature entering the continuous rolling mill is controlled at 800℃ to control the grain size.

[0069] (6) After rolling, the cooling rate is controlled at 3℃ / S to cool to 500℃ to control the recrystallization grain growth; the temperature of the cooling bed is 410℃, the slow cooling is entered into the pit, the slow cooling is out of the pit when the temperature is ≤200℃, and the slow cooling time is greater than 24 hours.

[0070] The non-metallic inclusion detection conditions and mechanical property data of the above-mentioned examples 1-3 are shown in the following table; the mechanical properties are sampled according to GB / T2975-2018, and the mechanical properties are detected according to GB / T228.1-2021 and GB / T 229-2020. The non-metallic inclusion content of the steel grade is detected according to GB / T10561-2023.

[0071] Table 1, non-metallic inclusions

[0072] Note: the sampling position is 1 / 2R.

[0073] Table 2, round steel mechanical properties

[0074] As shown in Table 1-2, the yield strength of the large-size directly-chipping medium-carbon bainite non-quenched and tempered steel prepared by the application is above 750 MPa, even reaching 832 MPa; and the chemical composition and non-metallic inclusions are well controlled, and remarkable technical effects are achieved.

[0075] Through the design of specific components, with the change of components, the increase of the carbon content of the bainite matrix, the yield strength of the material is further improved. At the same time, through the control of heating temperature, rolling deformation and cooling after rolling, the control rolling and cooling function of the existing rolling line is fully utilized, the grain size of the material is further refined, better fine-grain strengthening and precipitation strengthening effect is obtained (Fig. 1-2), and the effect of further improving the strength of the material is realized.

[0076] Description: The above examples are only used to illustrate the technical solutions described in the application and not to limit the application; therefore, although the application has been described in detail with reference to the above embodiments, those skilled in the art should understand that the application can still be modified or replaced; and all technical solutions and improvements that do not deviate from the spirit and scope of the application should be covered within the scope of the claims of the application.

Claims

1. A medium-carbon bainitic non-quenched and tempered steel for large-size direct cutting, characterized in that, It consists of the following components by mass percentage: 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, balance Fe and unavoidable impurities; The medium carbon bainitic non-quenched and tempered steel used for large-size direct cutting has a diameter greater than or equal to 160 mm and a yield strength greater than or equal to 755 MPa.

2. The medium-carbon bainitic non-quenched and tempered steel for large-size direct cutting machining according to claim 1, characterized in that, The diameter of the medium carbon bainitic non-quenched and tempered steel used for large-size direct cutting is 300 mm, and the yield strength is 755-832 MPa.

3. The production process of medium carbon bainitic non-quenched and tempered steel for large-size direct cutting according to any one of claims 1-2, characterized in that, Includes the following steps: (1) Electric arc furnace steelmaking: the final tapping C ≥ 0.15%, the target carbon 0.10% ≥ C ≥ 0.30%, the target P ≤ 0.008%, and the target temperature ≥ 1620℃; (2) LF refining: white slag time ≥ 20 min, smelting time ≥ 40 min; (3) VD vacuum treatment: VD high vacuum degree ≤67Pa, high vacuum holding time ≥10min, soft blowing time ≥15min; (4) Continuous casting: constant casting speed; cooling is divided into primary cooling and secondary cooling, both of which are water cooling; the rate of primary water cooling is 4000L / min, and the columnar crystal ratio is increased by primary water cooling to improve the density of the billet; the rate of secondary water cooling is 800L / min; the current of the electromagnetic stirring in the crystallizer is 250A and the frequency is 2Hz, the current of the electromagnetic stirring in the casting stream is 200A and the frequency is 8Hz, and the current of the electromagnetic stirring at the end is 1200A and the frequency is 8Hz. (5) Heating and rolling: The preheating section temperature is <900℃, the target temperature is 860℃, and the time is >1.05h; the heating section I temperature is 900-1100℃, the target temperature is 1000℃, and the time is ≥1.0h; the heating section II temperature is 1200-1260℃, the target temperature is 1230℃, and the time is ≥4h; the soaking section temperature is 1200-1260℃, the target temperature is 1240℃, and the time is ≥5h; the total heating time is controlled to be ≥11.0h; during rolling, a 9-stand continuous rolling mill is used, and the temperature of the continuous rolling mill is controlled at 800-950℃, which is beneficial to control the grain size; (6) After rolling, the cooling rate is controlled at 3-10℃ / S to cool to below 500℃ in order to control the growth of recrystallized grains; the temperature of the cooling bed is >400℃ and the temperature is slow cooled to ≤200℃ before leaving the cooling bed, and the slow cooling time is greater than 24 hours.

4. The production process of medium carbon bainitic non-quenched and tempered steel for large-size direct cutting machining according to claim 3, characterized in that, In step (1), the target temperature is controlled at 1620℃~1630℃.

5. The production process of medium carbon bainitic non-quenched and tempered steel for large-size direct cutting machining according to claim 3, characterized in that, In step (2), the white slag time is 20-30 min, and the smelting time is 40-50 min.

6. The production process of medium carbon bainitic non-quenched and tempered steel for large-size direct cutting machining according to claim 3, characterized in that, In step (3), the high vacuum holding time is 10-15 min and the soft blowing time is 15-25 min.

7. The production process of medium carbon bainitic non-quenched and tempered steel for large-size direct cutting machining according to claim 3, characterized in that, In step (4), the constant casting speed is 0.35 m / min.

8. The production process of medium carbon bainitic non-quenched and tempered steel for large-size direct cutting machining according to claim 3, characterized in that, In step (5), the heating period I lasts 1-2 hours, the heating period II lasts 4-6 hours, and the heat equalization period lasts 5-6 hours.

9. The production process of medium carbon bainitic non-quenched and tempered steel for large-size direct cutting machining according to claim 3, characterized in that, In step (6), the temperature is cooled to 420℃~500℃.

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