Process for producing a non-quenched and tempered round steel for direct machining

Abstract

The present application relates to the technical field of non-quenched and tempered steel, in particular to a production process of non-quenched and tempered round steel for direct cutting. The present application discloses a non-quenched and tempered round steel for direct cutting, the content of each component is calculated according to weight percentage and includes: C: 0.39~0.42%, Si: 0.20~0.30%, Mn: 0.60~0.80%, P≤0.025%, S: 0~0.005%, Cr: 0.90~1.2%, Mo: 0~0.02%, Cu: 0~0.02%, H≤1.5ppm, O≤40ppm, N≤100ppm, and the balance is Fe and inevitable impurities. Under the condition of not adding micro-alloying elements and relatively low total amount of alloy, through accurate component design and optimized controlled rolling and controlled cooling process, the present application realizes accurate control of the proportion of pearlite, ferrite and bainite three phases, so that the comprehensive performance of the non-quenched and tempered round steel reaches or approaches the level of quenched and tempered steel.

Description

Technical Field

[0001] This invention relates to the field of non-quenched and tempered steel technology, specifically to a production process for non-quenched and tempered round steel for direct cutting. Background Technology

[0002] Currently, mainstream non-quenched and tempered steels can be divided into forging non-quenched and tempered steels and direct-cutting non-quenched and tempered steels according to their uses. According to their microstructure, they mainly include pearlite + ferrite, bainitic, and martensitic types. There are two main technical approaches: one relies on microalloying elements for grain refinement and precipitation strengthening to achieve material properties at or near the level of quenched and tempered steels; the other increases the alloy content to improve austenite stability and combines this with controlled rolling and cooling processes to obtain bainitic or martensitic microstructures, thereby achieving performance comparable to quenched and tempered steels. Both of these are currently the most widely adopted technical approaches in the industry.

[0003] Chinese patent application CN202110460891.9 discloses a production process for large-size direct-cutting non-quenched and tempered steel. This patent, without increasing silicon content, eliminates the commonly used microalloying element Nb, optimizes the composition, and employs semi-continuous rolling technology to achieve controlled rolling and controlled cooling processes, resulting in large-size direct-cutting non-quenched and tempered steel. This invention proposes an innovative design concept for non-quenched and tempered steel: under conditions of no added microalloying elements and relatively low total alloy content, through precise composition design and optimized controlled rolling and controlled cooling processes, a composite microstructure composed of pearlite, ferrite, and bainite is obtained, enabling the material's comprehensive properties to reach or approach the level of quenched and tempered steel. The core challenge of this invention lies in the precise control of chemical composition and its synergistic effect with a matching controlled rolling and controlled cooling process to achieve precise control of the three-phase ratio of pearlite, ferrite, and bainite. Summary of the Invention

[0004] The purpose of this invention is to provide a production process for non-quenched and tempered round steel for direct cutting, so as to solve the problems mentioned in the background art.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A non-quenched and tempered round steel for direct cutting, comprising the following components by weight percentage: C: 0.39~0.42%, Si: 0.20~0.30%, Mn: 0.60~0.80%, P≤0.025%, S: 0~0.005%, Cr: 0.90~1.2%, Mo: 0~0.02%, Cu: 0~0.02%, H≤1.5ppm, O≤40ppm, N≤100ppm, with the balance being Fe and unavoidable impurities.

[0006] Furthermore, in non-quenched and tempered round steel, by volume fraction, it contains 70-80% pearlite, 19-25% bainite, and 1-5% ferrite.

[0007] A production process for non-quenched and tempered round steel for direct cutting includes the following steps: The steelmaking raw materials are sequentially processed through electric arc furnace steelmaking, LF refining, VD vacuum treatment, and continuous casting to obtain continuously cast billets; the continuously cast batches are sequentially heated, rolled, and pit cooled to obtain non-quenched and tempered round bars; among them... Furthermore, in the electric arc furnace steelmaking process, the final C is ≥ 0.08%, the target C is 0.10~0.24%, the target P is ≤ 0.018%, and the target temperature is T ≥ 1610℃; 600kg of lime, 300kg of cleaning agent, and 100~120kg of aluminum are added to each furnace of molten steel.

[0008] Furthermore, in the LF refining process, the white slag time is ≥20min, the smelting time is ≥35min, and the amount of ferrosilicon powder used in each furnace of molten steel is 120~250kg.

[0009] Furthermore, in the VD vacuum treatment process, the VD high vacuum degree is ≤67Pa, the high vacuum holding time is ≥10min, and the soft blowing time is ≥15min.

[0010] Furthermore, in the VD vacuum treatment process, the wire feeding sequence is aluminum wire and then silicon-calcium wire; among them, the silicon-calcium wire is 90m / furnace for the first furnace and 55m / furnace for the continuous casting furnace.

[0011] Furthermore, in the continuous casting process, the flow parameters of the two-stage water cooling are as follows: the flow rate of the first cooling water is 5600 L / min; the flow rate of the second cooling water in zone 1 is 30 L / min, and the flow rate in zone 2a is 48 L / min; the current of the electromagnetic stirring in the crystallizer is 1100 A and the frequency is 2 Hz; the current of the final electromagnetic stirring is 1000 A and the frequency is 8 Hz; and the casting speed is 0.27 m / min.

[0012] Furthermore, in the heating and rolling process, the preheating zone temperature is <880℃, the target temperature is 850℃, and the time is >1h; the heating zone I temperature is 900~1050℃, the target temperature is 980℃, and the time is ≥1h; the heating zone II temperature is 1200~1280℃, the target temperature is 1240℃, and the time is ≥2h; the soaking zone temperature is 1220~1280℃, the target temperature is 1250℃, and the time is ≥2h; the total heating time for the heating stages is ≥8h. Furthermore, in the heating and rolling process, the φ600 round billet obtained after continuous casting and heating processes is processed by the billet mill to obtain a 270mm×280mm intermediate billet. After hydraulic shearing of the head and tail, it enters the controlled cooling equipment, and the temperature of the intermediate billet entering the continuous rolling mill is controlled at 810~850℃. It is rolled through a 4-mill, with the stands arranged in a vertical and horizontal 90° alternating rolling mill configuration. A 5-stage water cooling system is used for controlled cooling, and the water flow rate settings for the 5 stages are as follows: the inlet water flow rate of stage 1 is 220~270m³ / h. 3 / h, 2-stage influent flow rate 200~220m³ / h 3 / h, 3-stage influent flow rate 180~200m³ / h 3 / h, 4-stage influent flow rate 220~240m³ / h 3 / h, 5-stage influent flow rate 160~200m³ / h 3 / h; the rolling speed is controlled within 0.45m / s.

[0013] Furthermore, in the heating and rolling process, the 5 water cooling equipment sections correspond to 5 water tanks, which are labeled as water tank #1, water tank #2, water tank #3, water tank #4, and water tank #5 in sequence; the arrangement of the 4 rolling mills and the 5 water tanks is as follows: 1 horizontal frame → 2 vertical frame → 3 horizontal frame → 4 vertical frame → water tank #1 → water tank #2 → water tank #3 → water tank #4 → water tank #5.

[0014] Compared with the prior art, the beneficial effects achieved by the present invention are: This invention provides a production process for non-quenched and tempered round steel for direct cutting. This process improves the mechanical properties of large-size round steel by precise composition design and optimized controlled rolling and cooling processes without adding microalloying elements and with a relatively low total alloy content. The mechanical properties reach or even exceed those of the quenched and tempered state, resulting in finer grains and more uniform radial mechanical properties, which is beneficial to improving the fatigue life of the material.

[0015] This invention eliminates the need for quenching and tempering of round steel. Through precise composition design and optimized controlled rolling and cooling processes, specifically: during continuous rolling, the incoming rolling temperature is controlled within a low range, allowing the round steel to store higher deformation energy, providing a nucleation driving force for subsequent phase transformations. At this stage, the water flow rate in tank #1 is increased, causing the steel surface temperature to drop rapidly, resulting in higher supercooling and a greater nucleation driving force during subsequent water cooling, further refining the core microstructure of the round steel. Precise control of the cooling rate allows for precise regulation of the proportions of pearlite, ferrite, and bainite, ultimately achieving a good balance of strength and toughness within the round steel. Compared to traditional non-quenched and tempered steel, this significantly reduces steel production costs and effectively enhances product market competitiveness. Attached Figure Description

[0016] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a metallographic image of the sample prepared in Example 1 of the present invention.

[0017] Figure 2 This is a metallographic image of the sample prepared in Example 2 of the present invention.

[0018] Figure 3This is a metallographic image of the sample prepared in Example 3 of the present invention.

[0019] Figure 4 This is a metallographic diagram of the tempering material used in the experiments of this invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] The production process of non-quenched and tempered round steel for direct cutting described in Examples 1-3 below includes the following steps: S1. Electric Arc Steelmaking: Steelmaking raw materials are smelted in an electric arc furnace to obtain molten steel. The final tapping concentration of the electric arc furnace is controlled to be C≥0.08%, with a target C of 0.10~0.24%, a target P of ≤0.018%, and a target temperature T≥1610℃. During the tapping process, 600kg of lime, 300kg of cleaning agent (the content of each component, by weight percentage, is 53.5% CaO, 3.5% SiO2, 34.3% Al2O3, 8.5% MgO), and 100~120kg of aluminum are added to each furnace of molten steel.

[0022] S2, LF refining: The molten steel after step S1 is refined by precipitation deoxidation combined with diffusion deoxidation. The molten steel is fed into the station to make white slag. The white slag holding time is ≥20min, the smelting time is ≥35min, and the amount of ferrosilicon powder used in each furnace of steel is 120~250kg. S3, VD Vacuum Treatment: The molten steel from step S2 is then vacuum treated in the VD station. The high vacuum degree is controlled to be ≤67Pa, the high vacuum holding time is ≥10min, and the nitrogen soft blowing time is ≥15min. Wire is fed in the order of aluminum wire and silicon-calcium wire. The first furnace uses 90m of silicon-calcium wire, and the continuous casting furnace uses 55m of silicon-calcium wire. Aluminum wire is fed to adjust the aluminum content in the molten steel. The calcium treatment in the later stage of VD causes the inclusions to deform and spheroidize, increasing the castability of the molten steel. The inclusions of types A, B, C, and D are controlled to be within grade 1.5, and the inclusions of type DS are ≤1.0. S4. Continuous Casting: The molten steel from step S3 is poured under full protection to prevent secondary oxidation. Secondary cooling is used in conjunction with electromagnetic stirring in the crystallizer, casting flow stirring, and end electromagnetic stirring. The primary cooling water flow rate is 5600 L / min, the secondary cooling water flow rate in zone 1 is 40 L / min, and the secondary cooling water flow rate in zone 2a is 48 L / min. The current of the electromagnetic stirring in the crystallizer is 1100 A and the frequency is 2 Hz; the current of the end electromagnetic stirring is 1000 A and the frequency is 8 Hz. The casting speed is controlled at 0.27 m / min to obtain the continuously cast round billet. S5. Heating: The continuously cast round billet obtained in S4 is sent to a heating furnace for heating. The heating program of the heating furnace is as follows: preheating section temperature < 880℃, target temperature 850℃, time > 1h; heating section I temperature 900~1050℃, target temperature 980℃, time ≥ 1h; heating section II temperature 1200~1280℃, target temperature 1240℃, time ≥ 2h; soaking section temperature 1220~1280℃, target temperature 1250℃, time ≥ 2h; total heating time for the heating stage ≥ 8h. S6. Rolling: The φ600 round billet from step S5 is passed through a billet mill to obtain a 270mm×280mm intermediate billet. The head and tail are hydraulically sheared, and the intermediate billet's temperature before entering the continuous rolling mill is controlled at 810~850℃. Rolling is then performed through a 4-mill system, with the stands operating in a vertical and horizontal 90° alternating pattern. Cooling is controlled using a 5-stage water cooling system, with the following parameters: Stage 1 water inlet flow rate 220~270m³ / h. 3 / h, 2-stage influent flow rate 200~220m³ / h 3 / h, 3-stage influent flow rate 180~200m³ / h 3 / h, 4-stage influent flow rate 220~240m³ / h 3 / h, 5-stage influent flow rate 160~200m³ / h 3 / h; the rolling speed is controlled within 0.45m / s; among them, the 5 sections of water cooling equipment correspond to 5 water tanks, which are marked as water tank 1, water tank 2, water tank 3, water tank 4, and water tank 5 in sequence; the arrangement of the 4th rolling mill and the 5th water tank is as follows: 1 horizontal stand → 2 vertical stand → 3 horizontal stand → 4 vertical stand → 1 water tank → 2 water tank → 3 water tank → 4 water tank → 5 water tank.

[0023] Example 1: The present invention provides a production process for non-quenched and tempered round steel for direct cutting. The chemical composition of the non-quenched and tempered round steel, by weight percentage, is: C: 0.39%, Si: 0.26%, Mn: 0.70%, P: 0.013%, S: 0.003%, Cr: 0.94%, V: 0.005%, H: 1.0ppm, O: 20ppm, N: 50ppm, with the balance being Fe and unavoidable impurities.

[0024] Example 2: The present invention provides a production process for non-quenched and tempered round steel for direct cutting. The chemical composition of the non-quenched and tempered round steel, by weight percentage, is: C: 0.40%, Si: 0.28%, Mn: 0.72%, P: 0.010%, S: 0.001%, Cr: 0.93%, H: 1.1ppm, O: 12ppm, N: 40ppm, with the balance being Fe and unavoidable impurities.

[0025] Example 3: The present invention provides a production process for non-quenched and tempered round steel for direct cutting. The chemical composition of the non-quenched and tempered round steel, by weight percentage, is: C: 0.395%, Si: 0.27%, Mn: 0.70%, P: 0.008%, S: 0.002%, Cr: 0.95%, V: 0.004%, H: 1.1ppm, O: 10ppm, N: 31ppm, with the balance being Fe and unavoidable impurities.

[0026] Experiments: The mechanical properties of the samples prepared in Examples 1-3 of this invention and the tempered materials were tested. The tensile strength, yield strength, elongation after fracture, and reduction of area were tested in accordance with GB / T228.1-2021. The Charpy (U-notch) impact test was conducted in accordance with GB / T 229-1984. The experimental results are shown in Table 1.

[0027] The quenched and tempered material is grade 40Cr, with the following composition: C: 0.40%, Si: 0.28%, Mn: 0.65%, P: 0.020%, S: 0.008%, Cr: 0.95%, and the balance being Fe and unavoidable impurities; its heat treatment process is: quenching at 850℃ and tempering at 530℃.

[0028] Table 1.

[0029] The samples in Examples 1-3 of this invention and the tempered material were etched with a saturated picric acid solution (the etched area was 1 / 3R below the skin). After being removed and cleaned, the austenitic grains of the steel were observed under a microscope at a magnification of 200x, and the grain size was rated (the higher the grade, the finer the grains). The results are shown in Table 2.

[0030] Table 2.

[0031] Figure 1 The image shows the metallographic structure of the sample prepared in Example 1 of this invention. According to Image-Pro analysis, the pearlite content is about 75%, the bainite content is about 22%, and the ferrite content is about 3% by volume fraction.

[0032] Figure 2 The image shows the metallographic structure of the sample prepared in Example 2 of this invention. According to Image-Pro analysis, the pearlite content is about 76%, the bainite content is about 21%, and the ferrite content is about 3% by volume fraction.

[0033] Figure 3 The image shows the metallographic structure of the sample prepared in Example 3 of this invention. According to Image-Pro analysis, the pearlite content is about 75%, the bainite content is about 22%, and the ferrite content is about 3% by volume fraction.

[0034] Figure 4 The image shows the metallographic structure of the tempering material used in the experiment of this invention. Image-Pro analysis shows that the main component is pearlite.

[0035] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-quenched and tempered round steel bar for direct cutting, characterized in that: The non-quenched and tempered round steel contains the following chemical components by weight percentage: C: 0.39~0.42%, Si: 0.20~0.30%, Mn: 0.60~0.80%, P≤0.025%, S: 0~0.005%, Cr: 0.90~1.2%, Mo: 0~0.02%, Cu: 0~0.02%, H≤1.5ppm, O≤40ppm, N≤100ppm, with the balance being Fe and unavoidable impurities.

2. The non-quenched and tempered round steel for direct cutting according to claim 1, characterized in that: In non-quenched and tempered round steel, by volume fraction, it contains 70-80% pearlite, 19-25% bainite, and 1-5% ferrite.

3. The production process for non-quenched and tempered round steel for direct cutting according to claim 1 or 2, characterized in that: Includes the following steps: The steelmaking raw materials are sequentially processed through electric arc furnace steelmaking, LF refining, VD vacuum treatment, and continuous casting to obtain continuously cast billets; the continuously cast batches are sequentially heated, rolled, and pit cooled to obtain non-quenched and tempered round bars; among them... Electric arc furnace steelmaking process: final C ≥ 0.08%, target C: 0.10~0.24%, target P ≤ 0.018%, target temperature T ≥ 1610℃; LF refining process: white slag time ≥ 20 min, smelting time ≥ 35 min; VD vacuum treatment process: VD high vacuum degree ≤67Pa, high vacuum holding time ≥10min, soft blowing time ≥15min; In the continuous casting process: two-stage water cooling is used in conjunction with electromagnetic stirring in the crystallizer, casting flow stirring, and end electromagnetic stirring; Heating and rolling process: Preheating section temperature < 880℃, target temperature 850℃, time > 1h; Heating section I temperature 900~1050℃, target temperature 980℃, time ≥ 1h; Heating section II temperature 1200~1280℃, target temperature 1240℃, time ≥ 2h; Soaking section temperature 1220~1280℃, target temperature 1250℃, time ≥ 2h; Total heating time for the heating stages ≥ 8h.

4. The production process according to claim 3, characterized in that: In the electric arc furnace steelmaking process, 600 kg of lime, 300 kg of cleaning agent, and 100-120 kg of aluminum are added to each furnace of molten steel.

5. The production process according to claim 3, characterized in that: In the LF refining process, the amount of ferrosilicon powder used in each furnace of molten steel is 120~250kg.

6. The production process according to claim 3, characterized in that: In the VD vacuum treatment process, the wire feeding sequence is aluminum wire and then silicon-calcium wire; the silicon-calcium wire is 90m / furnace for the first furnace and 55m / furnace for the continuous casting furnace.

7. The production process according to claim 3, characterized in that: In the continuous casting process, the flow parameters of the two-stage water cooling are as follows: the flow rate of the primary cooling water is 5600 L / min; the flow rate of the secondary cooling water in zone 1 is 30 L / min, and the flow rate in zone 2a is 48 L / min; the current of the electromagnetic stirring in the crystallizer is 1100 A and the frequency is 2 Hz; the current of the final electromagnetic stirring is 1000 A and the frequency is 8 Hz; and the casting speed is 0.27 m / min.

8. The production process according to claim 3, characterized in that: In the heated rolling process, the round billets obtained after continuous casting and heating processes are processed by a billet mill to obtain 270mm×280mm intermediate billets. After hydraulic shearing of the head and tail, the intermediate billets enter the controlled cooling equipment, and the temperature of the intermediate billets entering the continuous rolling mill is controlled at 810~850℃. The billets are rolled through a 4-mill system, with the stands arranged in a vertical and horizontal 90° alternating pattern. A 5-stage water cooling system is used for controlled cooling, with the water flow rate settings for the 5 stages as follows: Stage 1 water flow rate 220~270m³ / h. 3 / h, 2-stage influent flow rate 200~220m³ / h 3 / h, 3-stage influent flow rate 180~200m³ / h 3 / h, 4-stage influent flow rate 220~240m³ / h 3 / h, 5-stage influent flow rate 160~200m³ / h 3 / h; the rolling speed is controlled within 0.45m / s.

9. The production process according to claim 8, characterized in that: In the heating and rolling process, the five water-cooling equipment sections correspond to five water tanks, which are labeled as water tank #1, water tank #2, water tank #3, water tank #4, and water tank #5 in sequence. The arrangement of the four rolling mills and the five water tanks is as follows: 1 horizontal stand → 2 vertical stand → 3 horizontal stand → 4 vertical stand → water tank #1 → water tank #2 → water tank #3 → water tank #4 → water tank #5.

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