High carbon manganese steel with precisely controlled network carbides and method for producing same

Abstract

The application discloses a high-carbon silicon-manganese steel with accurate control of network carbide and a preparation method thereof, and belongs to the technical field of high-carbon steel production. The high-carbon silicon-manganese steel with accurate control of network carbide solves the problem of more network carbide, poor plasticity and poor toughness of the high-carbon silicon-manganese steel in the prior art. The preparation method of the high-carbon silicon-manganese steel with accurate control of network carbide comprises the following steps: smelting and continuous casting to obtain a continuous casting billet; wherein the pouring superheat is below 20 DEG C; the continuous casting billet is heated and kept warm, then is broken down and rolled to an intermediate billet; the intermediate billet is heated and kept warm, then is rolled into a specified size; the rolling compression ratio of the continuous casting billet is not less than 8; and the rolled steel is subjected to annealing treatment. The high-carbon silicon-manganese steel prepared by the method has excellent comprehensive performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-carbon steel production, in particular to a high-carbon silicon-manganese steel with accurate control of network carbide and a preparation method thereof. BACKGROUND

[0002] High-carbon silicon-manganese steel is often used for bearings and shaft sleeves. In order to prevent safety problems of products under accidental collision, overload and the like, the steel material is required to have certain plasticity and toughness, especially plane strain fracture toughness. For high-carbon silicon-manganese steel, network carbide is an important factor affecting its plasticity and toughness. If the network carbide in the rolled steel bar cannot be well controlled, the defect will be inherited to the product, thereby seriously reducing the plasticity and toughness of the product, and making the bearing, shaft sleeve and other structural parts prone to cracks under accidental collision, overload and the like, causing safety accidents. Therefore, from the perspective of ensuring product safety, the network carbide must be strictly controlled.

[0003] Network carbide control is one of the control difficulties in the production of large-size high-carbon steel bars. Since high-carbon steel has high carbon content, carbon is prone to precipitate at grain boundaries during the cooling process after the steel is rolled, i.e. network carbide. At present, domestic steel plants do not have cooling devices after rolling of large-size high-carbon steel bars, and the rolled steel bar is slowly cooled at 700-900℃, which is easy to lead to the formation of network carbide. In addition, the carbon composition segregation problem of large-size continuous casting steel bars is more serious, and network carbide is more likely to form in the area with high carbon concentration. Therefore, how to reduce the network carbide of high-carbon silicon-manganese steel and thereby ensure the plasticity and toughness of high-carbon silicon-manganese steel has become a problem to be solved. SUMMARY

[0004] In view of the above, the present application aims to provide a high-carbon silicon-manganese steel with accurate control of network carbide and a preparation method thereof, for solving the problems of too much network carbide and poor plasticity and toughness of the existing high-carbon silicon-manganese steel.

[0005] The purpose of the present application is mainly realized by the following technical solutions:

[0006] The present application provides a preparation method of a high-carbon silicon-manganese steel with accurate control of network carbide, and the preparation method comprises the following steps:

[0007] Step 1, smelting and continuous casting to obtain a continuous casting billet; wherein the pouring superheat is below 20℃;

[0008] Step 2, after the continuous casting billet is heated and kept, it is broken down and rolled to an intermediate billet;

[0009] Step 3, after the intermediate billet is heated and kept, it is rolled into a specified size; the rolling compression ratio of the continuous casting billet is not less than 8;

[0010] Step 4, the rolled steel material is subjected to annealing treatment.

[0011] Further, in step 1, electromagnetic stirring of the crystallizer is adopted in the continuous casting process.

[0012] Further, in step 1, dynamic reduction is adopted at the solidification end of the continuous casting blank, and the total reduction is 15-25 mm.

[0013] Further, in step 2, the continuous casting blank is heated to 1200-1250 DEG C and kept.

[0014] Further, in steps 2 and 3, the opening rolling temperature is 1100-1150 DEG C, and the final rolling temperature is 850-900 DEG C.

[0015] Further, in step 4, the steel after rolling is annealed at 770-790 DEG C.

[0016] Further, it further comprises:

[0017] Step 5, the annealed steel is quenched and tempered to obtain high-carbon silicon manganese steel.

[0018] Further, in step 5, the quenching process is: 800-860 DEG C, 0.2-2 h, oil quenching.

[0019] Further, in step 5, the tempering process is: 550-620 DEG C, 1-6 h, air cooling.

[0020] The application also provides a high-carbon silicon manganese steel, which is prepared by the above preparation method, and the reticular carbide of the high-carbon silicon manganese steel is less than grade 3.

[0021] Compared with the prior art, the application has the following beneficial effects:

[0022] a) The preparation method of the reticular carbide precisely controlled high-carbon silicon manganese steel of the application reduces the carbon segregation by adopting low superheat pouring, electromagnetic stirring of the crystallizer, controlling the total reduction, etc. during smelting, continuous casting, rolling, etc. and reduces the opening rolling temperature during continuous rolling, thereby reducing the final rolling temperature, and further ensuring the reduction of the reticular carbon grade of the high-carbon silicon manganese steel, and obviously improving the plasticity and toughness of the steel, which can better meet the safety use requirements of the product prepared by the high-carbon silicon manganese steel of the application;

[0023] b) In the preparation method of the reticular carbide precisely controlled high-carbon silicon manganese steel of the application, the reticular carbide (referred to as reticular carbon) grade of the steel obtained by annealing after rolling is not more than grade 3 at any position;

[0024] c) The high carbon silicon manganese steel of the present application has low level of network carbide in the microstructure, and excellent plasticity and toughness. The high carbon silicon manganese steel of the present application has excellent comprehensive performance, such as tensile strength of 1160 MPa or more (for example, 1180-1260 MPa), reduction of area of 26% or more (for example, 28%-45%), fracture toughness of 55 MPa.m 1 / 2 for example, 55-80 MPa.m 1 / 2 .

[0025] Other features and advantages of the present application will be set forth in the descriptions that follow, and in part will become apparent to those skilled in the art upon examination of the following or can be learned by practice of the present application. The objects and other advantages of the present application can be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this application, illustrate embodiments of the present application and together with the description serve to explain the principles of the present application.

[0027] Figure 1 Photograph of network carbide rating for the steel of the present application;

[0028] Figure 2 Photograph of network carbide rating for the comparative steel. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present application will be described in detail below with reference to the drawings, in which the same or similar components are denoted by the same reference numerals, and therefore the description will be omitted. The present application will be described in detail with reference to the drawings.

[0030] The present application provides a method for preparing a high carbon silicon manganese steel with precisely controlled network carbide, comprising the following steps:

[0031] Step 1, smelting and continuous casting to obtain a continuous casting billet; wherein the pouring superheat is 20℃ or less, electromagnetic stirring is used in the continuous casting process, and dynamic pressing is used at the solidification end of the continuous casting billet, with a total pressing amount of 15-25mm;

[0032] Step 2, after the continuous casting billet is heated and kept, it is broken down and rolled to an intermediate billet;

[0033] Step 3, after the intermediate billet is heated and kept, it is rolled to a specified size; the rolling compression ratio of the continuous casting billet is not less than 8;

[0034] Step 4, the steel after rolling is annealed.

[0035] Specifically, in the above step 1, smelting includes converter or electric furnace smelting plus secondary refining. Low superheat pouring is achieved by the way of induction heating of the tundish.

[0036] Specifically, in step 1 above, excessive superheat during casting will lead to the development of columnar crystals, a reduction in the central equiaxed crystal region, and severe compositional segregation during the solidification of the continuously cast billet (carbon segregation exacerbates network carbides), resulting in increased metallurgical defects in the center. Conversely, insufficient superheat during casting will result in poor fluidity of the molten steel, and in severe cases, blockage of the nozzle, affecting the casting process. Therefore, the superheat during casting should be controlled below 20°C, for example, between 10°C and 18°C.

[0037] Specifically, in step 1 above, considering that excessive electromagnetic stirring current can lead to severe compositional segregation in the continuously cast billet, resulting in negative compositional segregation bands and other issues that seriously affect the metallurgical quality of the steel, while insufficient electromagnetic stirring current cannot expand the equiaxed crystal zone of the continuously cast billet or reduce compositional segregation (carbon segregation exacerbates network carbides), the current for electromagnetic stirring in the crystallizer is controlled at 200–300 A.

[0038] Specifically, in step 1 above, considering that excessive total reduction can easily lead to problems such as internal cracks and severe deformation in the continuously cast billet, seriously affecting the metallurgical quality of the billet; while insufficient total reduction will not achieve the effect of reducing component segregation (reducing carbon content), the total reduction is controlled at 15–25 mm.

[0039] Specifically, in step 2 above, excessively high holding temperatures and excessively long holding times for the continuously cast billet can lead to overheating and burning. This can range from affecting the grain size of the subsequently rolled steel and reducing its ductility and toughness to, in severe cases, rendering the steel unusable. Conversely, excessively low holding temperatures and excessively short holding times fail to achieve uniform composition, resulting in severe segregation of the steel composition and an increased level of network carbides. Therefore, the continuous casting billet should be heated to 1200–1250℃ and held for a time t1, with the billet thickness d1 conforming to the following relationship: t1 = (1.1–1.6) min / mm·d1, where t1 is in minutes and d1 is in mm.

[0040] Specifically, in step 3 above, an excessively high compression ratio will lead to more severe center segregation, while an excessively low compression ratio will result in insufficient strain during the rolling process, leading to severe center porosity defects. Therefore, the compression ratio should be controlled to be no less than 8, preferably between 8 and 20.

[0041] Specifically, in step 3 above, excessively high intermediate billet holding temperature or excessively long holding time can lead to overheating and burning. This can range from affecting the grain size of subsequently rolled steel and reducing its ductility and toughness to, in severe cases, rendering the steel unusable. Conversely, excessively low temperature or excessively short holding time can result in uneven heating, affecting the quality of subsequently rolled steel. Therefore, the intermediate billet should be heated to 1200–1250℃ and held for a period of time t2. The holding time t2 should be related to the billet thickness d2 as follows: t2 = (0.3–0.5) min / mm·d2, where t2 is in minutes and d2 is in millimeters.

[0042] Specifically, in the above steps 2 and 3, considering that the too high roughing temperature can cause the finish rolling temperature to be too high, the difficulty of controlling the network carbide increases, and the too low roughing temperature can cause the rolling deformation resistance to be significantly increased, and the rolling mill is stopped due to exceeding the rolling capacity of the rolling mill. Therefore, the roughing temperature is 1100-1150℃, and the finish rolling temperature is 850-900℃.

[0043] Specifically, in the above step 4, considering that the too high annealing temperature can cause the network carbide to be more serious and increase the energy consumption, and the too low annealing temperature cannot play the role of softening annealing. Therefore, the steel after rolling is annealed at 770-790℃.

[0044] Specifically, the network carbide (referred to as network carbon) level of the steel after step 4 is not more than 3 levels (referring to GB 18254-2016 rating).

[0045] Specifically, the above preparation method further comprises:

[0046] Step 5, quenching and tempering the annealed steel to obtain high-carbon silicon manganese steel.

[0047] Specifically, in step 5, considering that the too high quenching temperature can cause the undissolved carbide to dissolve, affecting the strength and plasticity and toughness after quenching; and the too low quenching temperature can cause the structure not to be completely converted, and the expected mechanical properties cannot be reached. Therefore, the quenching process is controlled as follows: 800-860℃ for 0.2-2h, and oil quenching.

[0048] Specifically, in step 5, considering that the too high tempering temperature can significantly reduce the strength; and the too low tempering temperature can not reach the expected target of plasticity and toughness. Therefore, the tempering process is controlled as follows: 550-620℃, 1-6h, and air cooling.

[0049] Specifically, the high-carbon silicon manganese steel obtained in step 5 has a microstructure of high-temperature tempered martensite plus micron-sized carbides, a grain size of 20-40 microns, and a weight percentage of 1-3 micron-sized carbides of 2%-3%.

[0050] Specifically, the micron-sized carbides in the microstructure of the high-carbon silicon manganese steel obtained in step 5 are mainly residual carbides and network carbides, the network carbides are discontinuously distributed at the grain boundaries, and the level of the network carbides is 3 levels or less.

[0051] Specifically, the steel prepared by the preparation method of the present application has a large size, for example, when the steel is round steel, the diameter is not less than 120mm, and when the steel is square steel, the side length is not less than 120mm.

[0052] Specifically, the high-carbon silicon-manganese steel has the following components in percentage by mass: C: 0.90-1.20%, Si: 0.60-1.40%, Mn: 0.90-2.40%, P: ≤0.015%, S: ≤0.010%; and the balance of Fe and inevitable impurities.

[0053] Specifically, the high-carbon silicon-manganese steel has the following components in percentage by mass: C: 0.90-1.20%, Si: 0.60-1.40%, Mn: 0.90-2.40%, P: ≤0.015%, S: ≤0.010%; and the balance of Fe and inevitable impurities.

[0054] Specifically, the high-carbon silicon-manganese steel has the following components in percentage by mass: C: 0.90-1.20%, Si: 0.60-1.40%, Mn: 0.90-2.40%, P: ≤0.015%, S: ≤0.010%; and the balance of Fe and inevitable impurities.

[0055] The following is a specific description of the effects and dosage selection of the components contained in the application:

[0056] C: The content of carbon element is increased to 0.90-1.20%, and the high-temperature tempering after two-phase zone quenching can obtain high-temperature tempering martensite + residual carbide structure, realizing good strength and plasticity and toughness.

[0057] Ni, Mn and Cr: improve the hardenability of the steel. Too much Mn will greatly reduce the low-temperature toughness of the steel, so it is controlled below 2.4%; too high Ni addition will increase the cost and cause the content of residual austenite in the steel to be high, so it should be selected according to the requirements of the hardenability of the steel of the application and should be controlled below 0.4%. High Cr content will form carbides with C, affecting the performance, so it should be controlled below 0.5%.

[0058] Ti, Nb and V: all are strong carbonitride forming elements, which can form fine and dispersed carbon or nitride, playing a role in refining austenite grains. Too much content will weaken the fine-grain effect, so Nb is controlled below 0.10%, Ti is controlled below 0.25%, and V is controlled below 0.25%.

[0059] P: forms micro-segregation when the steel liquid solidifies, and then segregates to the grain boundary when heated at the austenitizing temperature, significantly increasing the brittleness of the steel, thereby increasing the hydrogen-induced delayed fracture sensitivity. Therefore, the content of P should be controlled below 0.015%.

[0060] S: inevitable impurities, forming MnS inclusions and segregating at grain boundaries can deteriorate the toughness of the steel, thus reducing the toughness and ductility of the steel. In order to ensure the comprehensive performance of the steel, the S content is determined based on the critical failure free path of MnS inclusions, and the S content should be controlled below 0.010%.

[0061] B: can significantly improve the hardenability of the steel and purify the grain boundaries. When the content is below 0.0005%, the effect is not obvious, and when the content is higher than 0.0050%, the effect is not obvious. Therefore, the B content should be controlled within the range of 0.0005% to 0.0050%.

[0062] Mo: effectively improves the hardenability and grain boundary purification of the steel, and improves the hydrogen brittleness sensitivity of the material. Too high content weakens the effect of improving the hardenability, and increases the cost, so it is controlled below 0.3wt%.

[0063] Si: is a solid solution strengthening element, further improves the yield ratio of the steel, and is beneficial to reduce the critical instability strain of the material. Too high Si content weakens the effect of improving the strength, so it is controlled below 1.40%.

[0064] Al: can effectively deoxidize and refine grains, and improve toughness. When the content is below 0.015%, the effect is not obvious, and when the content is higher than 0.060%, the effect is not obvious, and coarse alumina inclusions may be formed, which deteriorates the toughness of the steel. Therefore, the Al content should be controlled within the range of 0.015% to 0.060%.

[0065] RE: plays a role in deoxidation and desulfurization, and makes inclusions change, thereby improving the plasticity and toughness of the steel. When the content is below 0.003%, the effect is not obvious, and when the content is higher than 0.050%, the effect is not obvious, and reaches saturation. Therefore, if added, the RE content should be controlled within the range of 0.003% to 0.050%.

[0066] Ca: deoxidizes and desulfurizes, and makes inclusions change, thereby improving the plasticity and toughness of the steel. The ratio of the addition amount of Ca to the S content in the molten steel is 3:1. Therefore, the Ca content should be controlled within the range of 0.006% to 0.030%.

[0067] The high-carbon silicon manganese steel of the present application has excellent comprehensive performance, for example, the tensile strength is 1160 MPa or more (for example, 1180-1260 MPa), the reduction of area is 26% or more (for example, 28%-45%), and the fracture toughness is 55 MPa·m 1 / 2 (for example, 55-80 MPa·m 1 / 2 ). The plasticity and toughness of the high-carbon silicon manganese steel of the present application are excellent.

[0068] The advantages of precise control of the composition and process parameters of the steel of the present application are demonstrated below with specific examples and comparative examples.

[0069] The embodiment of the present application provides a high-carbon silicon manganese steel with accurate control of net carbide and a preparation method thereof. The chemical composition of the high-carbon silicon manganese steel of the embodiment is shown in Table 1.

[0070] According to the chemical composition requirements in Table 1, 5 batches of the steel of the present application are co-smelted by an electric furnace. The comparative steel has the same chemical composition as the embodiment, and the difference is mainly in the continuous casting and rolling process, including the rolling compression ratio of the continuous casting billet, the dynamic reduction amount at the solidification end of the continuous casting billet, the finish rolling temperature and the like.

[0071] The preparation method of the steel of the embodiment comprises:

[0072] Step 1, smelting and continuous casting to obtain a continuous casting billet; wherein the pouring superheat is below 20 DEG C, electromagnetic stirring is used in the continuous casting process, dynamic reduction is used at the solidification end of the continuous casting billet, and the total reduction amount is 15-25 mm;

[0073] Step 2, after the continuous casting billet is heated and kept, it is broken down and rolled to an intermediate billet, and the rolling compression ratio of the continuous casting billet is not less than 8;

[0074] Step 3, after the intermediate billet is heated and kept, it is rolled into a specified size;

[0075] Step 4, the steel after rolling is annealed;

[0076] Step 5, the annealed steel is quenched and tempered to obtain a high-carbon silicon manganese steel; the quenching process is: 800-860 DEG C, keeping for 0.2-2 h, oil quenching; the tempering process is: 550-620 DEG C, keeping for 1-6 h, air cooling.

[0077] The specific process parameters of the steel of the embodiment and the steel of the comparative example are shown in Tables 2-3; the net carbide grade of the steel of the embodiment and the steel of the comparative example after annealing and the microstructure after quenching and tempering are shown in Table 4, and the main performance test results of the steel of the embodiment and the steel of the comparative example are shown in Table 5.

[0078] Table 1 Chemical composition, wt%

[0079]

[0080] Table 2 Smelting and continuous casting process of the steel of the embodiment and the comparative steel

[0081]

[0082] Table 3 Rolling and heat treatment process of the steel of the embodiment and the comparative steel

[0083]

[0084] Table 4 Microstructure of the steel of the embodiment and the comparative steel

[0085]

[0086]

[0087] Table 5 Properties of example steels and comparative steels

[0088]

[0089] Figure 1 Photograph of network carbide rating for the steel of the present application; Figure 2 Photograph of network carbide rating for the comparative steel. The Figures 1-2 As can be seen from Table 4 and Table 5, the network carbide rating of the example steels of the present application is significantly reduced compared to the comparative steels, and the plasticity and toughness of the steels are obviously improved. The main reason is that the example steels of the present application are effectively controlled in terms of the rolling compression ratio of the continuous casting billet and the rolled steel, the dynamic reduction at the solidification end of the continuous casting billet, and the finishing rolling temperature. Under the condition of the same chemical composition and pouring superheat, the rolling compression ratio of the comparative steel is less than 8, and the network carbide rating of the rolled steel bar is high. After dynamic reduction at the solidification end of the continuous casting billet, the degree of carbon segregation of the steel is reduced, which is beneficial to the control of network carbide. However, the dynamic reduction amount should be within a suitable range, so as to control the network carbide rating to be less than 3. The finishing rolling temperature has a significant effect on the network carbide. Only when the finishing rolling temperature is controlled at 850-900℃, the goal of controlling the network carbide can be achieved, and the lower the finishing rolling temperature, the more obvious the effect of controlling the network carbide. As can be seen from the mechanical property results, the network carbide has an important influence on the reduction of area, the fracture toughness and the impact absorbed energy of the high-carbon silicon manganese steel. Through the preparation method of the present application, the comprehensive performance of the high-carbon silicon manganese steel can be ensured, and thus the use safety of various products manufactured by the high-carbon silicon manganese steel can be ensured.

[0090] The above description is merely preferred embodiments of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions within the technical scope disclosed by the present application can be easily conceived by those skilled in the art, and should be covered within the protection scope of the present application.

Claims

1. A method for preparing high-carbon silicon-manganese steel with precise control of network carbides, characterized in that, The high-carbon silicon-manganese steel comprises, by mass percentage: C: 0.90~1.20%, Si: 0.60%~1.40%, Mn: 0.90%~2.40%, P: ≤0.015%, S: ≤0.010%, and one or more of the following elements: Ni: 0.2%~0.4%, Cr: 0.2%~0.5%, Mo: 0.1%~0.3%, B: 0.0005%~0.005%, Nb: 0.03%~0.10%, Ti: 0.05%~0.25%, V: 0.04%~0.25%, Al: 0.015%~0.060%, RE: 0.003%~0.050%, Ca: 0.006%~0.03%; the balance being Fe and unavoidable impurity elements. The preparation method includes the following steps: Step 1: Smelting and continuous casting to obtain a continuously cast billet; wherein, the superheat during casting is below 20°C; Step 2: After heating and holding the continuously cast billet, roll it into an intermediate billet. Step 3: After heating and holding the intermediate billet at the specified temperature, roll it into the specified size; the rolling compression ratio of the continuously cast billet shall not be less than 8. Step 4: Anneal the rolled steel at 770–790℃; Step 5: Quench and temper the annealed steel to obtain high-carbon silicon-manganese steel; When the prepared steel is round, the diameter shall not be less than 120 mm; when the steel is square, the side length shall not be less than 120 mm. In step 1, electromagnetic stirring of the crystallizer is used during the continuous casting process, and the current of the electromagnetic stirring of the crystallizer is controlled to be 200~300A. In step 1, dynamic reduction is adopted at the end of the solidification of the continuous casting billet, and the total reduction is 20-25mm. In step 2, the continuously cast billet is heated to 1200-1250℃ and held at that temperature. The holding time t1 and the billet thickness d1 are related as follows: t1 = (1.1~1.6) min / mm·d1; In steps 2 and 3, the initial rolling temperature is 1100-1150℃, and the final rolling temperature is 850-900℃. In step 3, the intermediate billet is heated to 1200-1250℃ and held at that temperature. The holding time t2 and the billet thickness d2 are related as follows: t2 = (0.3~0.5) min / mm·d2; In step 5, the quenching process is: holding at 800~860℃ for 0.2~2h, oil quenching; the tempering process is: holding at 550~620℃ for 1~6h, air cooling. The microstructure of the high-carbon silicon-manganese steel obtained in step 5 is high-temperature tempered martensite plus micron-sized carbides, with a grain size of 20-40 microns and a weight percentage of 1-3 micron carbides of 2%-3%. The micron-sized carbides are mainly residual carbides and network carbides. The network carbides are discontinuously distributed at the grain boundaries and the level of the network carbides is below level 3.

2. The preparation method according to claim 1, characterized in that, In steps 2 and 3, the final rolling temperature is 850–880°C, or the final rolling temperature is 880–900°C.

3. The preparation method according to claim 1, characterized in that, In step 4, the rolled steel is annealed at 770°C or 790°C.

4. The preparation method according to claim 1, characterized in that, In step 5, the quenching process is as follows: heat treatment at 820~860℃ for 0.2~2 hours, followed by oil quenching.

5. The preparation method according to claim 1, characterized in that, In step 5, the tempering process is as follows: 550~600℃, heat preservation for 1~6 hours, and air cooling.

6. A high-carbon silicon-manganese steel, characterized in that, The high-carbon silicon-manganese steel is prepared by the preparation method described in any one of claims 1 to 5.

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