Rapidly spheroidized medium carbon steel bars and their manufacturing method

Abstract

The present invention relates to a rapid spheroidized medium-carbon steel bar and its manufacturing method. The chemical composition of the steel is, by mass percentage, 0.30-0.60% C, ≦0.40% Si, 0.50-0.90% Mn, ≦0.020% P, ≦0.030% S, ≦0.30% Cr, and typically ≦0.25% Cu and Ni. Microalloying elements such as B can be added as needed to adjust performance. To ensure grain size, one or more grain-refining elements such as Al, Nb, and N can be added, with the balance consisting of Fe and unavoidable impurities. The steel undergoes smelting, continuous casting, heating, ultra-low-temperature rolling, and rapid spheroidization. The total spheroidization time is 5.5-6.5 hours, which is similar to soft annealing but significantly shorter than conventional spheroidization annealing processes. The resulting material structure consists of dotted spheroids, ferrite, and a small amount of lamellar cementite. When the hardness is ≦160HBW, the elongation rate is ≧30%, and the rolling rate is 80%, there are no cracks during cold forging, making it suitable for cold forming processes with large deformation amounts, and realizing modern high-precision, energy-saving, emission-reducing, eco-friendly, and high-quality production.

Description

Technical Field

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[0001] The present invention relates to the field of special steel smelting to related thereto.

Background Art

[0002] The processing technology of steel parts is advancing by leaps and bounds. The cold forming processing technology is gradually replacing the conventional hot forming technology, improving the surface quality and dimensional accuracy of forged products, increasing the yield, reducing the subsequent turning and grinding processing amount, and reducing the production energy consumption, emissions, production costs, etc. Taking automobile parts as an example, the proportion of cold forming has already exceeded 20% and is still increasing year by year.

[0003] The plastic deformation ability of steel materials at room temperature is lower than that in the austenitized state at high temperature. Therefore, during steel production, it is usually necessary to perform heat treatments such as rolling cooling control, offline softening annealing, and even spheroidizing annealing to reduce the hardness of the material and improve the low-temperature deformability. As a general rule, the higher the content of alloying elements mainly composed of carbon, the higher the hardness of the steel material, and the worse the plasticity and cold forging performance. Taking carbon steel as an example, when the carbon content is less than 0.2%, the hot-rolled steel material has relatively high plasticity and can directly perform cold forming with a small amount of deformation. When the steel material undergoes spheroidization and its plasticity is significantly improved, more deformation and more complex cold forming processing can be performed. However, low-carbon steel has a rather low strength, so there are certain restrictions in its application. As the carbon content increases, the strength of the steel material is significantly improved. Taking 45 # steel as an example, its application in industry is extremely extensive.

[0004] 45 # Hot-rolled bar steel has a relatively high proportion of pearlite content and relatively low plasticity, so it is only suitable for cold forming processing of parts with a small amount of deformation such as shafts. As the parts become more complex and the amount of deformation increases, 45 #Steel becomes unsuitable for cold forming unless it has been heat-treated. A typical heat treatment process is as follows: For cold-working parts with minimal deformation, after hot rolling, off-line softening annealing (5-6 hours) is performed. Once the deformation increases, off-line spheroidizing annealing (usually 12-24 hours) is necessary to improve the plasticity of the steel. Otherwise, cracks are likely to occur during cold forming. While the above conventional heat treatment before cold forming can effectively solve the cracking problem in cold forming, the heat treatment time is relatively long, energy consumption is relatively high, and it is not suitable for modern manufacturing that emphasizes eco-friendliness and double carbon emissions.

[0005] As described above, by inventing a medium-carbon steel bar that can be applied to cold forming processes with large deformation amounts and that can be rapidly spheroidized, it is possible to shorten the processing cycle, simplify the processing process, increase production efficiency, and lower production costs. Furthermore, it is possible to save energy consumption and reduce emissions, thus having very significant economic and social value. [Overview of the project]

[0006] The technical problem that this invention aims to solve is to provide a method for manufacturing medium-carbon steel bars that can be applied to cold forming processes with a large amount of deformation compared to the conventional techniques described above, and that enables rapid spheroidization. The medium-carbon steel bars manufactured using this method have excellent plasticity, can be applied to cold forming processes with a large amount of deformation, have the advantages of short processing time, and reduce consumption and waste.

[0007] The technical method employed in this invention to solve the above problems is as follows: A rapidly spheroidized medium-carbon steel bar, the steel series being typically represented by 45# steel, the chemical composition of the steel being, by mass percentage, C 0.30-0.60%, Si ≤0.40%, Mn 0.50-0.90%, P ≤0.020%, S ≤0.030%, and Cr ≤0. 3 It is 0%, and the remainder consists of Fe and unavoidable impurity elements.

[0008] The residual elements Cu and Ni in the material, as defined in this invention, are typically ≤0.25%. Microalloy elements such as B and Ti can be added as appropriate to adjust performance. To prevent abnormal grain growth during subsequent heat treatment of the parts, one or more refined grain elements such as Al, Nb, and N can be added.

[0009] The mechanical properties of the steel material of this invention satisfy the following conditions. For application in cold forming processes with large deformation amounts, the plasticity of the material at room temperature is an important indicator, and is usually expressed by the rolling ratio in cold forging at which cracks do not occur. The limit rolling ratio at which cracks do not occur must be ≥80%. That is, when the rolling ratio in cold forging is 80%, the crack phenomenon of cold rolling does not occur. The elongation ratio of the steel material is >30%, and the reference hardness of the steel material is ≤160HB.

[0010] The central method for rapidly spheroidizing medium carbon steel bars of the present invention involves ultra-low temperature rolling to fracture the cementite in the pearlite lamellae of the rolled steel, thereby imparting a large amount of dislocation energy and strain energy to the interior. After the rolling is complete and the bar is cut, it is rapidly spheroidized by placing it in a furnace at high speed. The total time for the spheroidizing process is approximately 6 hours, similar to general softening annealing, but far less than conventional spheroidizing annealing methods. The steel of the present invention has good cold formability, and no cracks occur when the cold rolling ratio is 80%. The microstructure of the steel is point-like spheres + ferrite, or point-like spheres + ferrite + a small amount of lamellar cementite (the lamellar cementite is less than 10% of the total cementite amount and is diffusely distributed). The setting principle for each major element of the steel material of this invention is as follows.

[0011] C: 0.30~0.60%. The steel grade of the present invention is essentially a medium-carbon steel, and the C content mainly determines the proportion of cementite phase in the material, thus affecting the strength of the material. If the proportion of cementite phase is too low, the material strength cannot be guaranteed, and as the C content increases, the proportion of cementite phase increases, and the cold formability and toughness of the steel decrease. The C content range of the steel material of the present invention is set to 0.30~0.60%.

[0012] Si: ≤0.40%. Si is usually dissolved in ferrite and performs a solid solution strengthening effect, significantly improving the strength of ferrite. However, at the same time, it reduces the plasticity and toughness of ferrite, and since the Si element promotes the segregation of impurity elements, the grain boundaries become brittle. In the smelting process, Si is often added to steel as a deoxygenating element. Since the steel material of the present invention is applied to cold forming, in view of the above characteristics, the Si content is set to a lower limit, with a range of ≤0.40%, more preferably ≤0.25%.

[0013] Mn: 0.50~0.90%. Mn is one of the commonly found deoxygenating elements and is one of the most effective and economical elements for adjusting steel strength. An appropriate amount of Mn significantly increases steel strength without significantly affecting its plasticity. Furthermore, if the steel contains sulfur, Mn and S form MnS, which has excellent plasticity, improving the cutting performance of the steel. The Mn content of the steel material of this invention is set to 0.50~0.90%.

[0014] P: ≤0.020%. Except for some special-purpose steel grades where P is added to steel as an alloying element, in general steel grades, P is a typical harmful element that increases the brittleness of the steel. In terms of performance, the lower the P content in the steel of this invention, the better. However, if the requirements are too strict, it will increase the smelting cost of the steel. Therefore, as stated above, the range of P content in the steel of this invention is set to ≤0.020%.

[0015] S: ≤0.030%. The element S can improve the cutting performance of steel. When the S content in the steel is appropriate and it forms MnS with excellent deformability together with Mn, there is no significant effect on the cold formability of the steel. However, if the S content is too high, as the steel is rolled and deformed, too many elongated MnS molecules will be produced in the steel, causing various isomers in the steel and reducing the lateral performance of the steel. The S content range of the steel in this invention is set to ≤0.030%.

[0016] Cr: ≤0.30%. While Cr is also an effective and economical element for adjusting steel strength, the steel material of this invention is a medium-carbon steel. Therefore, an increase in Cr significantly affects the structural transformation curve of the steel, causing a transformation in steel performance and leading to changes in the spheroidization principle and method. The Cr content range for the steel material of this invention is set to ≤0.30%.

[0017] The residual elements Cu and Ni in the material, as defined in this invention, are typically ≤0.25%. Microalloy elements such as B can be added as appropriate to adjust performance. To prevent abnormal grain growth during subsequent heat treatment of the parts, one or more refined grain elements such as Al, Nb, Ti, and N can be added.

[0018] This invention relates to a medium-carbon steel material. By controlling the chemical composition and then implementing innovative process control in the subsequent rolling and heat treatment processes, the objective of preventing crack formation during cold forming, which involves large amounts of deformation, is achieved. Ultimately, this improves the surface quality and dimensional accuracy of cold-formed parts, reduces the amount of turning or grinding required for the parts, improves yield, and eliminates the need for pre-forming heat treatment.

[0019] Another objective of the present invention is to provide a method for rapidly spheroidizing medium carbon steel bars, which is also a key control point for the steel material of the present invention, and mainly includes the following specific processes. Smelting, continuous casting, hot rolling, and rapid spheroidizing.

[0020] During smelting, the chemical composition of this steel grade must be appropriately controlled. In particular, regarding phosphorus (P) control, when smelting in a converter or electric furnace, slag must be produced at high speed to increase the fluidity of the slag, while simultaneously controlling the furnace temperature to ≤1670°C. If the temperature is too high, it is unfavorable for dephosphorization, and in other words, there are no other dephosphorization processes in the steel production process starting from tapping. Therefore, if the phosphorus exceeds the control target, it cannot be recovered. Furthermore, it is necessary to control the purity of the molten steel, and if there are large impurities in the steel, especially on the surface and subsurface, cracks are likely to occur during cold rolling.

[0021] During continuous casting, the purity of the steel material is further controlled, and liquid level fluctuations are controlled to ≤±5mm by means of automatically monitoring the liquid level in the crystallizer online. Once liquid level fluctuations occur, protective slag can be entrained and trapped in the shell, making it easy for subsurface inclusions to form. The degree of superheating is controlled to the lower and middle limits, targeting 10-25°C, and the amount of cooling water in the crystallizer is controlled to the upper and middle limits. These two points ensure that the primary shell is cooled with high intensity, thereby increasing the thickness of the strengthened equiaxed crystal structure, which helps improve the surface quality of the billet and subsequent rolled material, and reduces the occurrence of microcracks on the surface, thus avoiding the expansion of microcracks during cold forming.

[0022] Rolling after continuous casting is one of the central processes of this invention. Achieving ultra-low temperature rolling, which causes the cementite in the pearlite lamellae of the rolled steel to break and increases the strain energy inside the steel after it has been rolled and removed from the line, is beneficial for the rapid breakage, dissolution, and spheroidal precipitation of cementite during subsequent spheroidization.

[0023] The billet is heated in a stepped heating furnace. The preheating zone temperature is 650-750°C, and the heating zone temperature is 850-950°C, ensuring even heating. zone The temperature is controlled to 950-1020°C, and the total heating time is set to ≥180 min to ensure that the billet material components are heated evenly. During the heating process, soaking is performed. zone Temperature control is extremely important, and since the steel material of this invention employs ultra-low temperature rolling, the temperature of the uniform heating zone is kept as low as possible within the rolling capacity range. When the heating temperature is high, the surface temperature of the steel material can be reduced to a limited extent by subsequent rolling cooling control, but it is difficult to control the temperature of the core of the steel material, and especially when the rolling specification is ≥Φ60mm, it is even more difficult to solve the technical problem of uneven cross-sectional temperature due to rolling cooling control. After the billet is removed from the heating furnace, it is cooled with high-pressure water. scale Remove it. scaleThe rolling start temperature after removal is 880 - 950 °C. Subsequently, continuous rolling is carried out using 25 continuous rolling units. Each of the 25 rolling mills consists of 6 roughing mills + 10 intermediate mills + 4 pre-finishing mills + 5 finishing mills. Water-cooling tanks for water passing are provided in the intermediate mills, pre-finishing mills, and finishing rolling units, enabling realization of rolling cooling control. When performing rolling cooling control, in the general process design concept, it is designed so that the steel material stores more strain energy during the rolling process and the cementite is more easily dissolved during subsequent spheroidization. During and after rolling, at least four stages of spray water tanks are sequentially arranged for cooling control. After intermediate rolling, a #1 water tank is arranged for spray cooling. After pre-finishing rolling, #2 and #3 water tanks are arranged. After three-roll finishing rolling, a #4 water tank is arranged to perform alternating strong and weak cooling on the steel material. Among them, the opening degree of the #1 water tank is 45 - 85% for strong cooling, and the #2 and #3 water tanks perform weak cooling. The opening degree of the cooling water is 5 - 15% in Yes, the material input temperature of the finishing rolling unit is controlled at 730 - 840 °C to to perform rolling in the two-phase region. After exiting the finishing rolling unit, the cooling intensity is appropriately increased, and the opening degree of the cooling water is increased to 40 - 60% Ma to ensure that the temperature when the material reaches the cooling bed is 720 - 800 °C in By the above rolling method, it is ensured that the steel material has as high internal strain energy as possible. After rolling, the steel material is quickly placed on the cooling bed and sheared or cut.

[0024] After the steel is unloaded from the line and bundled, it is quickly put into an annealing furnace for rapid spheroidizing annealing treatment. Rapid spheroidization is also one of the central processes of the present invention. Since the steel adopts ultra-low temperature rolling, a large amount of strain energy exists in the rolled steel, and the cementite lamella in pearlite is in a metastable state, and fusing is likely to occur during reheating. Therefore, the bar steel unloaded from the line is directly put into the annealing furnace for heat preservation. The heat preservation temperature is 710 - 760°C. The spheroidizing heat preservation temperature is a sensitive process, and the temperature range with the optimal process effect is very narrow. If the temperature is too low, it is difficult for the cementite lamella to fuse; if the temperature is too high, a large amount of cementite will dissolve into austenite and precipitate again during subsequent transformation, so it is not suitable for spheroidization. By adjusting the target value of the heat preservation temperature according to the steel component, the optimal effect can be achieved. To avoid a large amount of cementite lamella dissolving into austenite during heat preservation, the heat preservation time should not be too long. The heat preservation time adopted in the present invention is 120 - 150 min. Then, it is cooled to 670 - 720°C at a rate of 1°C / min for isothermal transformation, heat-preserved for 150 - 180 min, and then taken out of the furnace for natural cooling. The total processing time for spheroidization is 5.5 - 6.5 h, which is the same as that of a general softening annealing process. However, softening annealing has a lamellar pearlite structure and is not suitable for cold forming processes with a large amount of deformation. Compared with the conventional spheroidizing annealing process (12 - 24 h), heat the processing time is significantly shortened, and the energy consumption is significantly reduced. Compared with the prior art, the advantages of the present invention are as follows.

[0025] The rapidly spheroidized steel structure obtained in the present invention is a punctate spheroid + ferrite, or a punctate spheroid + ferrite + a small amount of lamellar pearlite (the lamellar cementite is within 10% of the total amount of cementite and is diffusely distributed).

[0026] The above rapidly spheroidized medium-carbon bar steel of the present invention undergoes processes such as specific chemical composition design, low-temperature heating, rolling cooling control, ultra-low temperature finishing rolling, and rapid spheroidization to achieve the spheroidization of the structure, reduce the hardness of the steel, and greatly improve the cold forming performance, which is advantageous for the cold forging of complex parts. [Brief explanation of the drawing]

[0027] [Figure 1] Figure 1 shows the microstructure of a rapidly spheroidized medium carbon steel bar in Example 1 of the present invention. [Figure 2] Figure 2 shows the microstructure of the rapidly spheroidized medium carbon steel bar in Example 2 of the present invention. [Figure 3] Figure 3 shows the microstructure of the rapidly spheroidized medium carbon steel bar in Example 3 of the present invention. [Modes for carrying out the invention]

[0028] The present invention will be further explained below, in conjunction with practical examples. Examples 1-3 Rapid spheroidizing medium carbon steel bars according to Examples 1 to 3 of the present invention are manufactured based on the following steps. 1) Smelting: After smelting in a 100-ton steelmaking furnace (converter or electric furnace), the molten steel is further refined outside the furnace and then vacuum degassed. The mass percentage of each chemical element is strictly controlled according to requirements. 2) Continuous Casting: Square billets measuring 240 mm x 240 mm are continuously cast, the tundish temperature is controlled to 10-25°C, water cooling of the crystallizer is enhanced, and the thickness of the equiaxed lamellae of the billet is ensured. Advanced equipment and processes, including terminal electromagnetic stirring and continuous casting under light pressure, are employed during continuous casting. The percentage of the chemical composition of the obtained continuously cast billets is shown in Table 1 below. [Table 1] 3) Heating: The billet material is heated in a step-type heating furnace that includes a preheating zone, a heating zone, and a soaking zone. The temperature of the preheating zone is controlled to 650-750°C, the temperature of the heating zone to 850-950°C, and the temperature of the soaking zone to 950-1020°C, with a total heating time of 180 minutes or more. 4) pressure Rolling: The rolling start temperature is set to 880-950°C and the rolling finish temperature to 710-790°C, and ultra-low temperature rolling is performed within the range of both phases. pressure By ensuring the storage of a large amount of strain energy within the rolled steel material, this is useful for subsequent cementite cutting and extraction. 5) Rapid spheroidization: pressure After being rolled out and removed from the line, the material is placed in a high-speed annealing furnace and kept warm at 710-760°C for 120-150 minutes. Then, it is cooled at 1°C / min to 670-720°C, kept warm for 150-180 minutes, and immediately removed from the furnace and air-cooled to room temperature.

[0029] The specific process parameters for rapidly spheroidizing carbon steel bars in each of the above examples are shown in Table 2 below. [Table 2]

[0030] The relevant tests were performed on the rapidly spheroidizing medium carbon steel bars of Examples 1 to 3, and the mechanical properties obtained from the measurements are shown in Table 3 below. [Table 3]

[0031] Table 3 shows that the hardness in the examples met the target requirement of ≤160 HBW, and after rapid spheroidization, the tensile strength was 545-582 MPa, and the elongation after rapid spheroidization was 32-35.5%, which is a significant improvement over the hot-rolled state. No cracking occurred during 80% cold rolling. In other words, the steel materials in the examples have good plasticity and microstructure, making them very suitable for cold forming.

[0032] Figures 1-3 show the microstructure of rapidly spheroidized medium-carbon steel bars in Examples 1-3. From Figures 1-3, it can be seen that the microstructure of these steels consists of point-like spheres + ferrite + a small amount of lamellar cementite. The proportion of lamellar cementite is <10% in all cases, and this structure is very advantageous for cold forming processes that involve a lot of deformation.

[0033] As described above, preferred embodiments of the present invention have been explained in detail, but those skilled in the art should clearly understand that the present invention is subject to various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be considered to fall within the scope of protection of the present invention.

Claims

1. In rapidly spheroidized medium carbon steel bars, the chemical composition of the steel is, by mass percentage, 0.30-0.60% C, ≤0.40% Si, 0.50-0.90% Mn, ≤0.020% P, ≤0.030% S, ≤0.30% Cr, ≤0.25% Cu and Ni, with the remainder being Fe and unavoidable impurity elements. A rapidly spheroidizing medium-carbon steel bar characterized in that the limit rolling ratio without cracking of the steel is ≥ 80%, the elongation ratio of the steel is > 30%, and the reference hardness of the steel is ≤ 160 HB.

2. The rapid spheroidizing medium carbon steel bar according to claim 1, characterized in that the structure of the steel is punctate spheres + ferrite, or punctate spheres + ferrite + a small amount of lamellar cementite, wherein the lamellar cementite is within 10% of the total cementite amount and is diffusely distributed.

3. A method for producing rapidly spheroidized medium carbon steel bars according to Claim 1, The aforementioned manufacturing method 1) Smelting The process involves smelting molten steel to match the chemical composition of the steel material, and during smelting in a converter or electric furnace, producing slag at high speed to increase the fluidity of the slag, while simultaneously controlling the furnace temperature to ≤1670°C to control the purity of the molten steel and reduce large impurities in the steel. 2) Continuous casting During casting, the purity of the steel material is further controlled, liquid level fluctuations are controlled to ≤±5 mm, the degree of superheating is controlled to the lower middle limit, and the target temperature is 10-25°C. 3) Heating The billet material is heated in a step-type heating furnace, with the temperature of the preheating zone controlled to 650-750°C, the temperature of the heating zone to 850-950°C, and the temperature of the soaking zone to 950-1020°C, and the total heating time is set to 180 min or more so that the billet material is heated sufficiently evenly. 4) Rolling After removing the billet from the heating furnace, scale is removed with high-pressure water. The rolling start temperature after scale removal is set to 880-950°C, and the finishing temperature is controlled to 710-790°C. After rolling is complete, the steel material is placed on a cooling bed at high speed and sheared or cut. 5) Rapid spheroidization The method is characterized by including the step of taking the steel materials off the line, bundling them, and then placing them again in a high-speed annealing furnace to perform rapid spheroidizing annealing, with the total spheroidizing treatment time being 5.5 to 6.5 hours. Manufacturing method.

4. The method for manufacturing rapidly spheroidized medium-carbon steel bars according to claim 3, characterized in that 25 continuous rolling mills are used in the rolling process to perform continuous rolling, the 25 rolling mills each consisting of 6 roughing mills, 10 medium rolling mills, 4 pre-finishing mills, and 5 finishing mills, and water tanks for water cooling are installed in the medium rolling mills, pre-finishing mills, and finishing mills, enabling rolling cooling control.

5. The method for rapidly spheroidizing medium-carbon steel bars according to claim 3, characterized in that the rapid spheroidizing step specifically involves annealing the steel bars taken off the line and directly placing them into a furnace, maintaining the temperature at 710 to 760°C for 120 to 150 minutes, then, after the maintenance is complete, cooling again at a rate of 1°C / min to 670 to 720°C to perform isothermal transformation, maintaining the temperature for 150 to 180 minutes, and then removing the bars from the furnace and allowing them to cool naturally.

6. A method for rapidly spheroidizing medium-carbon steel bars according to claim 4, characterized in that, during rolling cooling control, at least four spray water tanks are sequentially arranged during and after rolling to perform cooling control, the first water tank is arranged after intermediate rolling for spray cooling, the second and third water tanks are arranged after pre-finishing rolling, and the fourth water tank is arranged after three-roll finishing rolling to perform alternating strong and weak cooling of the steel material.

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