High-strength high-temperature-resistant cutting bainite non-quenched and tempered steel and preparation method thereof

Abstract

The application relates to a high-strength high-temperature-resistant cutting bainite non-quenched and tempered steel and a preparation method thereof. The non-quenched and tempered steel is prepared according to an electric furnace steelmaking-LF refining-VD vacuum treatment-continuous casting-heating-breaking down-controlling rolling and cooling-detraining tempering process flow, and is formed by Mo and W strong carbide forming elements, higher Si element content, a small amount of Co element, V and Nb micro-alloy elements to form an optimized component design. Low-temperature controlled rolling is adopted, the steel is cooled at a cooling speed of 2-5 DEG C / s to 480-510 DEG C after continuous rolling, and a bainite structure steel is obtained. The steel can be directly cut and the quenching and tempering process is omitted, so that the material has higher strength, more uniform performance and higher high-temperature creep strength. The creep strength is greater than or equal to 240 MPa under 1000 hours at 400-500 DEG C, and is especially suitable for large column components of super-large presses in high-temperature environments.

Description

Technical Field

[0001] This invention belongs to the field of cutting non-quenched and tempered steel technology, specifically relating to a high-strength, high-temperature resistant cutting bainitic non-quenched and tempered steel and its preparation method. Background Technology

[0002] With the development of the equipment manufacturing industry, higher requirements have been placed on heavy equipment, represented by ultra-large presses. The columns of ultra-large presses, as the core skeleton components of the equipment, are the foundation for the stable operation of the press. On the one hand, ultra-large presses are generally used to forge forgings with large tonnage. Due to the high temperature of the forgings and strong heat radiation to the surrounding environment, the working environment temperature of the columns is relatively high. Under long-term operation, the steel material is required to have a certain high-temperature stability to avoid significant strength reduction and increased creep at high temperatures, which could lead to deformation or even breakage of the columns and threaten equipment safety. On the other hand, the columns and other components need to be machined. In order to pursue a larger working surface to improve processing efficiency, the diameter of the columns needs to be reduced while ensuring load-bearing capacity. Therefore, the strength requirements of the column steel material are higher.

[0003] For press columns, the most common material is 42CrMo. In ultra-large press column applications, this material often needs to be of a larger size to meet strength requirements, and its overall mechanical properties are improved through overall quenching and tempering after processing. However, due to the limitations of alloy composition, the low hardenability of the steel leads to uneven internal and external properties of the material. The significant strength decay at high temperatures will cause the column to creep and crack under long-term alternating loads, threatening the safe operation of the press. Quenching and tempering also brings process bottlenecks such as increased procedures, high energy consumption, and long cycle time. Therefore, in order to meet the stringent material requirements of ultra-large press columns, it is urgent to develop a high-strength, high-temperature resistant, cutting bainitic non-quenched and tempered steel to solve this problem.

[0004] To improve the mechanical properties of 42CrMo, existing technologies may add microalloying elements such as V. For example, patent CN116904856A discloses a method for preparing large-size direct-cutting low-carbon bainitic non-quenched and tempered steel produced by continuous casting, which uses a low-carbon, high-manganese composition design of C-Si-Mn-Cr-V-Nb-Ti-Mo-Al-B, combined with semi-continuous rolling after continuous casting and slow cooling in the pit to produce bainitic non-quenched and tempered steel, in order to improve the mechanical and fatigue properties of the product. However, its application in ultra-large press columns still has the following technical problems:

[0005] I. To promote bainitic phase transformation, improve strength performance, and reduce the hardness difference from the surface to the core, elements such as high manganese and boron are used to improve the hardenability of steel. However, this can also exacerbate compositional segregation, induce temper brittleness, and affect high-temperature service performance. Reducing alloy content, under high-temperature rolling and slow cooling phase transformation, reduces the austenite stability and bainitic phase transformation driving mechanics of large-size steel. Pearlite transformation tends to precede bainitic phase transformation, causing the core to form non-strengthening structures such as pearlite. Furthermore, untransformed austenite residues are prone to appear, or coarse and uneven bainite structures are formed, which in turn affects the material strength and performance uniformity, failing to meet the overall requirements of the column for heat resistance, strength, and load-bearing uniformity.

[0006] Second, to improve the high-temperature resistance of steel, existing technologies add a certain amount of toughening, strengthening, and high-temperature resistant alloying elements such as Ni, Nb, and Ti. However, on the one hand, the high alloy content increases the difficulty of quality control and deformation resistance of the core of the continuously cast billet, which will bring the risk of cracking. Improper control can easily lead to large deviations in indicators such as hardness between the core and the surface, affecting the long-term high-temperature service safety of the column components. On the other hand, these alloying elements have limited effect on improving high-temperature resistance and cannot be directly machined. They also need to be combined with quenching and tempering heat treatment to play their role, which will increase production energy consumption and cycle, affect production efficiency, and make it difficult to break through the production bottleneck. Summary of the Invention

[0007] The present invention aims to at least partially solve one of the above-mentioned technical problems. The present invention provides a high-strength, high-temperature resistant, non-quenched and tempered bainitic steel and its preparation method, which can be directly cut, eliminating the need for quenching and tempering processes, so that the material has higher strength, more uniform performance, and higher high-temperature creep strength, and is especially suitable for large column components in the high-temperature environment of ultra-large presses.

[0008] The technical solution adopted by this invention to solve its technical problem is:

[0009] A high-strength, high-temperature resistant, non-quenched and tempered bainitic steel has the following chemical composition by mass percentage: C: 0.15%~0.25%, Si: 1.5%~2.0%, Mn: 1.5%~2.0%, P: ≤0.015%, S: ≤0.005%, Cr: 0.5%~1.0%, Ni: ≤0.25%, Cu: ≤0.25%, V: 0.05%~0.15%, Nb: 0.10%~0.20%, Al: 0.02%~0.04%, Mo: 0.30%~0.45%, W: 0.25%~0.45%, Co: 0.15%~0.25%, H: ≤1.5ppm, O: ≤40ppm, N: 0.01%~0.03%, with the balance being Fe and unavoidable impurities.

[0010] The aforementioned non-quenched and tempered steel, by adding a certain amount of strong carbide-forming elements such as Mo and W, strongly delays the transformation of pearlite and carbides, refines bainite laths, strengthens the matrix through solid solution, and improves creep resistance; adding a higher Si content inhibits cementite precipitation, promotes the formation of carbide-free bainite, and delays high-temperature tempering softening; adding a small amount of Co inhibits the transformation of austenite to ferrite, increases the bainite phase transformation temperature, and reduces dislocation density degradation; adding microalloying elements such as V and Nb, combined with controlled rolling and controlled cooling processes, precipitates nanoscale carbides that pin grain boundaries, hinders microstructure coarsening at high temperatures, and delays high-temperature microstructure degradation, ultimately yielding a non-quenched and tempered, high-strength, high-temperature resistant bainitic non-quenched and tempered steel, which is beneficial for meeting the product characteristics requirements of ultra-large press columns.

[0011] In the preferred technical solution, the microstructure of the non-quenched and tempered steel is entirely bainitic. The refined bainitic structure can not only improve the strength of the material through fine grain strengthening, but also reduce the brittleness caused by coarse structure, significantly improving the strength and toughness matching of the material, meeting the requirements of non-quenched and tempered steel and ultra-large press columns for heavy load capacity. At the same time, suppressing pearlite structure and transforming it entirely into bainitic structure, avoiding austenite residue, can make the mechanical properties of the material surface and core uniform, thereby improving reliability and extending the service life of the components.

[0012] In the preferred technical solution, the diameter of the non-quenched and tempered steel is 220~260mm, and the non-metallic inclusions at 1 / 2 radius are ≤1.5 grade. The low level of non-metallic inclusions can avoid reducing the fatigue strength, impact toughness and fracture toughness of the material, and improve the service reliability of the material under long-term load.

[0013] In the preferred technical solution, the hardness difference between the core and the surface of the non-quenched and tempered steel is ≤30HBW, indicating uniform performance across the entire cross section, suitable for stress bearing, and reducing the risk of performance fluctuations during processing.

[0014] In the preferred technical solution, the non-quenched and tempered steel maintains an impact energy of ≥20J, a high-temperature tensile strength of ≥1050MPa, a yield strength of ≥800MPa, and a high-temperature tensile strength of ≥980MPa at 1 / 2 radius under high temperature of 400~500℃. The material has higher strength and more uniform performance, and the column diameter can be made smaller under the same load, increasing the working area of ​​the press.

[0015] In the preferred technical solution, the non-quenched and tempered steel has a creep strength of ≥240MPa at 400~500℃ for 1000 hours, which can meet the requirements of ultra-large press columns for high-temperature service performance of steel and improve service safety.

[0016] The above-mentioned method for preparing high-strength, high-temperature resistant, non-quenched and tempered bainitic steel adopts a controlled rolling and cooling process, controlling the initial rolling temperature to ≤900℃, the continuous rolling temperature to ≤820℃, the cumulative reduction rate in the finishing rolling stage to ≥15%, and after exiting the continuous rolling, controlling the steel to cool to 480~510℃ at a cooling rate of 2~5℃ / s, then air-cooling to a temperature below 210℃ on the lower cooling bed, then slowly cooling to room temperature in the pit, and finally stress-relief tempering treatment after exiting the pit.

[0017] Because the aforementioned non-quenched and tempered steels have high alloy content and are bainitic, a matching controlled rolling and cooling process is required to ensure the microstructure.

[0018] On the one hand, low-temperature continuous rolling is adopted: the initial rolling temperature of ≤900℃ avoids the coarsening of austenite grains, which would result in a coarse microstructure after the subsequent bainite transformation, thus creating conditions for the subsequent finishing rolling to refine the microstructure. In order to make the deformation resistance moderate and avoid excessive equipment load, the initial rolling temperature can be further optimized to 850~900℃; the exit rolling temperature of ≤820℃, combined with a cumulative reduction rate of ≥15% in the finishing rolling stage, accumulates dislocations through deformation, refines grains and microstructure, and significantly increases the grain boundary area, so as to become the nucleation point for the subsequent bainite transformation, promoting the uniform refinement of bainite laths. In order to make the deformation resistance moderate and avoid excessive equipment load, the exit rolling temperature can be further optimized to 780~820℃.

[0019] On the other hand, compared to the existing controlled cooling treatment of direct slow cooling in the rolling mill, austenite may preferentially form pearlite during slow cooling, affecting the material strength. Controlled cooling first uses high-pressure air cooling to control the steel at a cooling rate of 2~5℃ / s to 480~510℃. This can be combined with alloying elements such as Mo, W, and Cr to delay the pearlite transformation, cooling large-diameter steel from the continuous rolling temperature to the bainite transformation zone, promoting the transformation of fine bainite, ensuring the matrix structure is bainitic, and avoiding pearlite precipitation and insufficient strength due to excessively slow cooling. At the same time, the steel grade has high alloy content, high deformation resistance, and high internal stress after rolling, avoiding the need for excessively slow cooling... Excessive speed leads to a large temperature gradient between the steel surface and core, exacerbating stress and material brittleness, resulting in cracking. Air cooling to a temperature below 210℃ on the lower cooling bed reduces the temperature difference between the core and surface, promotes the complete bainite transformation, and reduces retained austenite. Once cooled to below 210℃, the material's plasticity is somewhat restored. At this point, slow cooling in the pit results in a smaller temperature gradient, allowing internal stress to be gradually released, significantly reducing the risk of cracking and the difficulty of stress-relief tempering. After removal from the pit, stress-relief tempering is performed to remove retained austenite from the phase transformation process, eliminate phase transformation stress and thermal stress, and further improve dimensional stability and mechanical property consistency.

[0020] Based on this, the steel grade has high alloy content, high deformation resistance, and stringent requirements for rolling mill equipment. To improve the stability of continuous rolling, the preferred technical solution adopts a billet continuous rolling process. The temperature of the soaking zone in the preheating process before billet opening is controlled at 1100~1280℃, the soaking time is ≥10h, and the billet opening temperature is ≥1120℃. The selection of a higher soaking zone temperature and a longer soaking time can promote the welding of defects in the billet, the full solidification of alloying elements, and alleviate compositional segregation, thus providing a basis for the homogenization of austenite grains. The semi-continuous rolling controlled rolling process of billet continuous rolling can effectively remove small defects in the core of the billet at a higher billet opening temperature, while reducing the load on the rolling mill equipment, avoiding cracks caused by deformation concentration, and connecting with subsequent controlled rolling processes to control grain size.

[0021] Because steel has a high alloy element content, in the preferred technical solution, the heating process employs a preheating section, heating section I, heating section II, and soaking section with temperature control performed sequentially. The preheating section temperature is controlled to be <650℃, and the preheating time is >3h. Using a lower preheating section temperature can avoid surface tensile stress and core compressive stress caused by rapid heating, thus preventing surface cracks. Heating section I has a temperature of 750~900℃ and a heating time of ≥3h, using a medium-temperature transition to promote the initial homogenization of the billet composition and stress release. Heating section II has a temperature of 950~1100℃ and a heating time of ≥5h, using a high-temperature transition to the soaking section to reduce the core and surface temperatures. The total heating time is ≥22h, improving the uniformity of the entire cross-section of the billet and creating conditions for removing core defects during billet opening.

[0022] Compared with forged billets, continuously cast billets have advantages such as higher production efficiency, lower cost, better microstructure uniformity, and higher dimensional accuracy. In the preferred technical solution, continuously cast billets produced by continuous casting process are rolled and cooled under controlled conditions. Low superheat control is used during continuous casting, with the superheat of the first furnace controlled at 15~25℃ and the superheat of the continuous casting furnace controlled at 10~20℃. This can reduce segregation and columnar crystal ratio, avoid the anisotropy of mechanical properties of continuously cast billets caused by the development of columnar crystals, reduce defects such as internal porosity, shrinkage cavities and gas pores, and reduce the risk of cracks caused by uneven microstructure during subsequent billet opening and rolling.

[0023] During continuous casting, a primary cooling water is applied to the crystallizer of the continuous casting equipment, which is the initial solidification zone of the continuously cast billet. By controlling the flow rate of the primary cooling water, a uniform billet shell can be formed quickly, preventing steel leakage and surface defects. A secondary cooling water is applied to the billet after the crystallizer outlet, which is the area from the crystallizer to the straightening machine of the continuous casting equipment. By controlling the flow rate of the secondary cooling water, slow and uniform cooling can be achieved, avoiding internal stress and internal cracks. In the preferred technical solution, the flow rate of the primary cooling water is controlled at 5500~5800 L / min and the flow rate of the secondary cooling water is controlled at 50~60 L / min during continuous casting.

[0024] The continuous casting process employs a three-stage electromagnetic stirring technique to ensure the core quality of the continuously cast billet. In the preferred technical scheme, the electromagnetic stirring current in the crystallizer is controlled at 120~180A and the frequency at 2Hz. Low-frequency, low-intensity stirring within the crystallizer improves the flow of molten steel and initial solidification conditions, reducing surface defects and the risk of subsequent columnar crystal penetration into the core. The electromagnetic stirring current in the casting stream is 330~380A and the frequency at 8Hz. In the secondary cooling zone from the crystallizer outlet to the end of solidification, medium-frequency, medium-intensity stirring breaks up growing columnar dendrites, increases the number of equiaxed crystals, and reduces core segregation. At the end of solidification, the electromagnetic stirring current is 1100~1300A and the frequency is 8Hz. The casting speed is 0.15~0.18m / min. High-intensity stirring at the end of the solidification of the continuously cast billet drives the remaining molten steel to flow, filling the voids formed by core solidification shrinkage, eliminating shrinkage cavities and porosity, refining the core structure, thereby ensuring core quality and improving the core density of the billet.

[0025] To further control product performance, the quality of molten steel before controlled rolling and cooling is controlled by sequentially performing electric arc furnace steelmaking, LF refining, and VD vacuum treatment. In the preferred technical solution, the final carbon content in the electric arc furnace steelmaking stage is controlled to be ≥0.05%, and the target carbon content is ≥0.10% (0.03%). An appropriate final carbon content can reduce the oxygen content in the molten steel and reduce the risk of oxide inclusion formation. The target phosphorus content is ≤0.006%, and strictly controlling the phosphorus content at an extremely low level can avoid defects such as cracks caused by phosphorus segregation during subsequent rolling and cooling. The target temperature is ≥1620℃, and a higher final temperature can ensure that the molten steel has good fluidity, which is convenient for subsequent tapping.

[0026] In the preferred technical solution, each furnace of the electric arc furnace steelmaking stage produces 85-110 tons of molten steel, and the amount of auxiliary materials used is controlled as follows: 450-550 kg of lime per furnace, 250-350 kg of cleaning agent per furnace, and 110-130 kg of aluminum per furnace. After the lime is dissolved, it can increase the alkalinity of the slag, providing an alkaline environment for dephosphorization and desulfurization reactions. The cleaning agent is used to promote the polymerization and flotation of inclusions, enhance the removal of inclusions and the separation of slag and steel. Aluminum, as a strong deoxidizer, is used to rapidly reduce the oxygen content of the molten steel.

[0027] In the preferred technical solution, the white slag time in the LF refining stage is controlled to be ≥20 min, the smelting time is ≥40 min, and the auxiliary material dosage is: 100-140 kg / furnace of silicon carbide. Silicon carbide can promote the rapid formation of white slag and maintain its reducibility, indirectly assisting in desulfurization and inclusion removal, while appropriately supplementing the silicon element in the molten steel. The white slag time can deeply purify the molten steel, reduce the inclusion and sulfur content, and the smelting time can ensure that the LF refining is sufficient and the inclusions are fully floated.

[0028] In the preferred technical solution, the VD vacuum treatment stage controls the VD vacuum degree to be ≤67Pa, the vacuum holding time to be ≥15min, and the soft blowing time to be ≥20min; the high vacuum degree and high vacuum holding time can efficiently remove hydrogen and nitrogen, ensuring sufficient degassing and inclusion removal, and, together with the soft blowing time, achieve steel homogenization and final purification of inclusions.

[0029] In the preferred technical solution, the VD vacuum treatment stage controls the wire feeding sequence: aluminum wire is fed first, followed by silicon-calcium wire; feeding aluminum wire first can enhance final deoxidation and reduce primary inclusions, while feeding silicon-calcium wire later can modify the morphology of inclusions and improve the purity and fluidity of molten steel.

[0030] In the preferred technical solution, the amount of silicon-calcium wire used in the VD vacuum treatment stage is controlled as follows: 90~110m for the first furnace and 60~70m for the continuous casting furnace. The first furnace requires a higher amount to avoid blockage of the continuous casting nozzle or defects in the continuous casting billet due to incomplete treatment of inclusions. The amount can be reduced in the continuous casting furnace to avoid side effects such as CaS inclusions caused by excessive calcium.

[0031] The stress-relief tempering treatment can further control the heating temperature and holding time, remove residual austenite during the phase transformation process, promote the release of phase transformation stress and thermal stress, and stabilize mechanical properties. In the preferred technical solution, the stress-relief tempering treatment involves heating to 540~560℃ and holding for 10~15h.

[0032] Compared with the prior art, the beneficial effects of the present invention are at least as follows:

[0033] (1) This invention improves the lower bainite transformation temperature by using strong carbide-forming elements such as Mo and W, a high Si content, a small amount of Co, and microalloying elements such as V and Nb to form an optimized composition design, resulting in a finer full bainite structure. This gives the material higher strength, achieving the following: at high temperatures of 400-500℃, the material maintains an impact energy of ≥20J, a high-temperature tensile strength of ≥1050MPa, a yield strength of ≥800MPa, and a high-temperature tensile strength at 1 / 2 radius of ≥980MPa. Simultaneously, the material exhibits more uniform performance, achieving a hardness difference between the core and surface of ≤30HBW. The composition design and microstructure composition are crucial. It is determined that it still has relatively high strength in relatively high temperature environments. The creep strength of the steel at 400~500℃ for 1000 hours is ≥240MPa. It is especially suitable for large column components in high-temperature environments of ultra-large presses. Compared with the traditional material 42CrMo, it can be directly machined, eliminating the need for heat treatment, thereby reducing column production energy consumption, shortening cycle, improving efficiency, breaking through process bottlenecks. Under the same load, the column diameter can be made smaller, increasing the working area of ​​the press. At the same time, it has higher high-temperature creep strength than 42CrMo, which is conducive to improving the long-term service life at high temperatures and equipment safety, and has good market application prospects.

[0034] (2) Based on the composition design, this invention adopts a controlled rolling and controlled cooling preparation method. By using low-temperature rolling and reduction rate control, the grains and structure are refined. By using appropriate cooling rate, air cooling and slow cooling in the pit, the matrix structure is guaranteed to be bainitic, avoiding the formation of non-strengthening structures such as pearlite in the core and avoiding coarse or uneven bainitic structures. The stress relief treatment after exiting the pit removes the residual austenite, phase transformation stress and thermal stress during the phase transformation process, effectively controlling the phase transformation and structure state. Furthermore, it can be combined with smelting, continuous casting, heating and billet control to improve the purity of molten steel, the density of the core of the continuous casting billet, and reduce the risk of cracking, and has good industrial adaptability. Attached Figure Description

[0035] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0036] Figure 1 This is a metallographic diagram of Embodiment 1 of the present invention.

[0037] Figure 2 This is a metallographic diagram of Comparative Example 3 of the present invention. Detailed Implementation

[0038] The present invention will be further described in detail below through specific preferred embodiments. However, the present invention is not limited to the following embodiments. It should be noted that, unless otherwise specified, the chemical reagents involved in the present invention, such as purification accelerators (wherein, the mass content of key effective components is: CaO 53.5%, SiO2 3.5%, Al2O3 34.3%, MgO 8.5%), are all purchased through commercial channels.

[0039] This invention provides a high-strength, high-temperature resistant, non-quenched and tempered bainitic steel for cutting, the chemical composition of which, by mass percentage, includes: C: 0.15%~0.25%, Si: 1.5%~2.0%, Mn: 1.5%~2.0%, P: ≤0.015%, S: ≤0.005%, Cr: 0.5%~1.0%, Ni: ≤0.25%, Cu: ≤0.25%, V: 0.05%~0.15%, Nb: 0.10%~0.20%, Al: 0.02%~0.04%, Mo: 0.30%~0.45%, W: 0.25%~0.45%, Co: 0.15%~0.25%, H: ≤1.5ppm, O: ≤40ppm, N: 0.01%~0.03%, with the balance being Fe and unavoidable impurities.

[0040] The design basis for the chemical composition (mass percentage) of the above-mentioned non-quenched and tempered steel includes:

[0041] (1) Carbon: Carbon is one of the most effective strengthening elements in steel. It can promote the transformation of austenite to bainite and form a moderate solid solution in bainitic ferrite to obtain a uniform and stable bainitic structure. At the same time, it provides carbon source for the formation of carbides by alloying elements such as Mo, W, and V, providing a high strength and high temperature resistance foundation. However, excessive carbon can easily lead to the formation of coarse carbides in the bainitic structure, which will lose its strengthening effect on the matrix and reduce stability, resulting in a decrease in the high temperature strength and heat resistance of the steel. At the same time, the hardness of the steel increases, and the coarse carbides aggravate the wear of cutting tools. Therefore, a moderate carbon content can take into account both the bainitic phase transformation and the formation of high temperature stable carbides. A low carbon content is used to avoid the matrix being too hard or too many carbides, while taking into account the machinability. The mass percentage of C is controlled at 0.15%~0.25%.

[0042] (2) Silicon: Silicon has extremely low solubility in cementite. Adding a higher Si content can enrich it at the interface to form a diffusion barrier, which can hinder the migration of carbon atoms to the cementite nucleation site, inhibit cementite nucleation and growth, and promote the formation of carbide-free bainite (CFB) during the bainite transformation process, so as to balance high strength and high toughness and improve the uniformity of the structure. Si can also inhibit coarsening at high temperature and maintain the precipitation strengthening effect by hindering atomic diffusion. It can delay the softening of high temperature tempering through solid solution strengthening and maintain high strength. At the same time, under high temperature environment, Si can work synergistically with alloying elements such as Mo and W to improve the high temperature stability of carbides, thereby avoiding the rapid decline of high temperature strength and improving high temperature creep strength. However, excessive silicon content can easily lead to excessive matrix lattice distortion, excessive amount of retained austenite and decreased machinability. Therefore, it is necessary to appropriately increase the Si content and control the mass percentage of Si to 1.5%~2.0%.

[0043] (3) Manganese: Manganese is an austenite stabilizing element that can lower the transformation temperature of austenite to pearlite and ferrite and improve the hardenability of steel. This inhibits the formation of pearlite in large-size steel under cooling conditions, creating conditions for the formation of carbide-free bainite. At the same time, Mn can refine the bainite lath structure and improve the strength and toughness of steel by combining solid solution strengthening. However, excessive Mn will over-stabilize austenite, resulting in incomplete bainite transformation and excessive untransformed austenite. This can easily lead to compositional segregation in steel, causing brittle phases such as martensite to appear in the structure, deteriorating toughness, and increasing the difference in mechanical properties in different regions of the material. This manifests as local brittleness or insufficient strength, increasing the steel's high-temperature tempering brittleness sensitivity, and affecting the stability and high-temperature service performance during the cutting process. Therefore, the mass percentage of Mn should be controlled at 1.5%~2.0%.

[0044] (4) Chromium: Chromium can improve the hardenability of steel and lower the transformation temperature of austenite to pearlite. It works synergistically with elements such as Mn and Mo to inhibit the formation of low-strength structures such as pearlite and ferrite, creating conditions for the uniform transformation of bainite. This allows the steel to form a stable and tough structure during cooling, inhibits the coarsening of bainite structure and the precipitation and aggregation of carbides during high-temperature tempering, delays the softening trend of steel, and allows the steel to maintain high strength after tempering. It can also enhance the high-temperature strength of steel through solid solution strengthening. However, excessive chromium content may lead to incomplete bainite transformation, leaving too much austenite or forming coarse bainite laths. Through segregation and local enrichment, hard and brittle carbides are formed, affecting the cutting performance. Therefore, the mass percentage of Cr should be controlled at 0.5%~1.0%.

[0045] (5) Nickel: Ni has a high price and excessive content will increase material costs. At the same time, it interacts with carbon, exacerbates the segregation of carbides at grain boundaries, reduces the steel’s resistance to high-temperature tempering, and affects the stability during the cutting process. Therefore, the mass percentage of Ni should be controlled to ≤0.25%.

[0046] (6) Copper: Excessive Cu content tends to segregate at the austenite grain boundaries, which weakens the grain boundaries during rolling, causing surface cracks or internal cracks. This makes the material prone to brittle fracture under impact or cutting stress, affecting the material's formability. Therefore, the mass percentage of Cu should be controlled to be ≤0.25%.

[0047] (7) Vanadium: By adding V microalloying element, V can combine with C or N in the controlled rolling and cooling process to form nanoscale VC or V(CN) with a size <100nm, or form (V,Nb)C composite precipitates with Nb, which can hinder the coarsening of the structure at high temperature and refine the bainite structure. At the same time, the diffusion rate of V is higher than that of Nb, and it is easier to precipitate at lower temperatures. Combined with low temperature rolling, the nanoprecipitates pin grain boundaries / dislocations to form precipitation strengthening and delay the degradation of the structure at high temperature. However, V is expensive, and excessive addition is not conducive to controlling the cost of materials. At the same time, the coarsening and uneven distribution of the precipitates will weaken the strengthening effect. Therefore, the mass percentage of V should be controlled at 0.05%~0.15%.

[0048] (8) Niobium: By adding Nb microalloying elements, nanoscale NbC pinning grain boundaries with a size of <100nm can be precipitated in the controlled rolling and controlled cooling process, which hinders grain growth during heating or high-temperature deformation, making it easier to retain deformation energy during hot rolling deformation. Through fine grain strengthening and promoting the refinement of the microstructure during subsequent phase transformation, the strength and toughness of steel are improved. At the same time, the precipitated phase has high stability at high temperature, which can delay the microstructure degradation during high-temperature tempering or service, and improve the mechanical property stability of the material in high-temperature environment. However, Nb has a high cost, and excessive addition is not conducive to controlling the material cost. At the same time, excessive precipitation coarsening reduces strength and toughness. Therefore, the mass percentage of Nb is controlled at 0.10%~0.20%.

[0049] (9) Aluminum: Aluminum is a strong deoxidizing element that can reduce defects such as porosity and looseness caused by oxygen, and improve the density and purity of steel. However, excessive aluminum content can easily form coarse inclusions, deteriorate mechanical properties, and increase the brittleness of steel. Therefore, the mass percentage of Al should be controlled at 0.02%~0.04%.

[0050] (10) Molybdenum: Mo is a strong carbide-forming element that can combine with carbon to form stable high-temperature carbides such as Mo2C, which helps resist high-temperature softening and gives the material higher room temperature and high-temperature strength. At the same time, it strongly delays the formation rate of pearlite in the cooling process of steel, inhibits the premature precipitation and transformation of carbides, avoids performance degradation caused by pearlite, and can limit the growth of bainite laths, promoting the formation of finer and more uniform lath bundles. Through solid solution strengthening, it hinders the movement and slip of dislocations and directly improves the strength of the matrix. The combined action of solid solution Mo and precipitated nano carbides can effectively inhibit dislocation climb and grain boundary sliding at high temperatures, significantly improve the creep resistance of the material, and extend the service life of the material at high temperatures. However, if the Mo content is too high, it is easy to coarsen or aggregate at the grain boundary, which will affect the toughness of the material, or increase the hardness and deformation resistance of the steel, which will affect the cutting performance. Therefore, the mass percentage of Mo should be controlled at 0.30%~0.45%.

[0051] (11) Tungsten: W is a strong carbide-forming element that can combine with carbon to form stable high-temperature carbides such as WC and W2C, which help resist high-temperature softening and give the material higher room temperature and high-temperature strength. Together with Mo, it strongly delays the transformation of pearlite and carbides, refines bainite laths, strengthens the matrix through solid solution, and improves creep resistance. However, if the W content is too high, too much untransformed austenite will remain, increasing the difficulty of tempering. At the same time, the alloy cost of W is relatively high, which is not conducive to controlling the material cost. Therefore, the mass percentage of W should be controlled at 0.25%~0.45%.

[0052] (12) Cobalt: Cobalt mainly exists in austenite in solid solution form, which can inhibit the austenite → ferrite transformation, avoid premature precipitation of ferrite, provide sufficient parent phase for subsequent bainitic phase transformation, and increase the bainitic phase transformation temperature. A higher phase transformation temperature is conducive to the uniform growth and refinement of bainite laths, reduces the precipitation of coarse carbides, and reduces phase transformation stress, improves the strength and toughness matching of the structure, stabilizes the dislocation structure in the matrix through solid solution strengthening, and reduces dislocation density degradation, so as to withstand loads for a long time. However, excessive Co is not conducive to controlling material cost. Excessive Co may form brittle phases with other elements, leading to material embrittlement. Therefore, a small amount of Co is added, and the mass percentage of Co is controlled to be 0.15%~0.25%.

[0053] (13) Nitrogen: Nitrogen can combine with strong nitride-forming elements such as niobium and vanadium to form fine and dispersed nitrides, which can inhibit the growth of austenite grains and refine the grain size. However, excessive nitrogen content can easily lead to defects such as porosity and looseness, which can deteriorate the as-cast quality and processing performance. Therefore, the mass percentage of N should be controlled at 0.01%~0.03%.

[0054] The above-mentioned high-strength, high-temperature resistant, non-quenched and tempered bainitic steel is prepared according to the process flow of electric arc furnace steelmaking → LF refining → VD vacuum treatment → continuous casting → heating → billet preparation → controlled rolling and cooling → stress-relief tempering → sampling → finishing → warehousing. Specifically:

[0055] The electric arc furnace steelmaking process is used to smelt steel into steel by feeding steelmaking raw materials into an electric arc furnace. Each furnace produces 85-110 tons of steel. The final carbon content is controlled to be ≥0.05%, the target carbon content is ≥0.10%, the target phosphorus content is ≤0.006%, and the target temperature is ≥1620℃. The amount of auxiliary materials added during tapping is: 450-550 kg of lime per furnace, 250-350 kg of purifying agent per furnace, and 110-130 kg of aluminum per furnace.

[0056] The LF refining process is used to transport molten steel from electric arc furnace steelmaking to the LF station along with the ladle. Through refining, the composition is precisely controlled, white slag is made for deep desulfurization, and inclusions are removed. The white slag making and desulfurization stages are controlled to have a white slag time of ≥20min. Auxiliary material dosage: silicon carbide 100-140kg / furnace, smelting time ≥40min.

[0057] The VD vacuum treatment process is used to transport the molten steel after LF refining to the VD station along with the ladle, start the vacuum pump, blow argon gas softly, control the VD vacuum degree ≤67Pa, the vacuum holding time ≥15min, and the soft blowing time ≥20min; after breaking the vacuum, wire feeding is carried out, the wire feeding sequence is: first feed aluminum wire, then feed silicon-calcium wire, the amount of silicon-calcium wire used: 90~110m for the first furnace, 60~70m for the continuous casting furnace, to improve the inclusion morphology and stabilize the composition of the molten steel.

[0058] The continuous casting process is used to feed molten steel treated by VD vacuum into the continuous casting equipment for continuous casting. During continuous casting, the superheat of the first furnace is controlled at 15~25℃, the superheat of the continuous casting furnace is controlled at 10~20℃, the flow rate of the primary cooling water is 5500-5800L / min, and the flow rate of the secondary cooling water is 50-60L / min. Three-stage electromagnetic stirring is used to control the core quality of the continuously cast round billet and improve the core density. The electromagnetic stirring current of the crystallizer is controlled at 120~180A and the frequency is 2Hz; the electromagnetic stirring current of the casting stream is controlled at 330~380A and the frequency is 8Hz; the electromagnetic stirring current of the solidification end is controlled at 1100~1300A and the frequency is 8Hz; the casting speed is 0.15~0.18m / min, and a continuously cast round billet with a diameter of φ800mm is obtained.

[0059] The heating process is used to feed the continuously cast round billet into a heating furnace for heating into a high-temperature round billet. The heating furnace employs a preheating section, heating section I, heating section II, and soaking section for temperature control. The preheating section temperature is controlled to be <650℃, with a target temperature of 600℃ and a preheating time >3h; the heating section I temperature is 750~900℃, with a target temperature of 820℃ and a heating time ≥3h; the heating section II temperature is 950~1100℃, with a target temperature of 1050℃ and a heating time ≥5h; the soaking section temperature is 1100~1280℃, with a target temperature of 1250℃ and a soaking time ≥10h; the total heating time is ≥22h, reducing the temperature difference between the inside and outside of the high-temperature round billet.

[0060] The billet-opening process is used to feed the heated high-temperature round billet into the billet-opening machine. The billet-opening temperature is ≥1120℃. In the billet-opening process, small defects in the core of the high-temperature round billet are fully removed. After billet-opening and hydraulic shearing of the head and tail, it is processed into a middle rectangular billet with a length of 405mm and a width of 355mm.

[0061] The controlled rolling and cooling process uses a 4-stand continuous rolling mill, controlling the initial rolling temperature to ≤900℃. By using a relatively low initial rolling temperature (i.e., the entry temperature into the continuous rolling mill), the grains and microstructure are refined. The exit temperature from the continuous rolling mill is ≤820℃, and the cumulative reduction rate during the finishing rolling stage is ≥15%. After exiting the continuous rolling mill, high-pressure air cooling is used to control the steel to cool to 480~510℃ at a cooling rate of 2~5℃ / s to promote bainite transformation. Then, it is air-cooled to a lower cooling bed temperature below 210℃ and slowly cooled to room temperature in the pit. Through a suitable cooling rate, the matrix microstructure is ensured to be bainite, resulting in hot-rolled round steel with a diameter of 220~260mm.

[0062] The stress-relief tempering process involves sending the hot-rolled round steel bars out of the pit into an annealing furnace, heating them to 540~560℃ and holding them at that temperature for 10~15 hours to perform stress-relief tempering, removing residual austenite, phase transformation stress, and thermal stress from the phase transformation process, and then cooling them down before they are taken out of the furnace.

[0063] The sampling process involves taking steel that has undergone stress-relief tempering and performing chemical composition analysis according to GB / T223 standard; examining the metallographic structure according to GB / T13298 "Metallic Materials - Microscopic Examination Methods" to obtain a metallographic diagram; quantitatively rating the non-metallic inclusions in the steel according to GB / T10561 "Determination of Non-metallic Inclusion Content in Steel - Standard Rating Chart Microscopic Examination Method", with the sampling location at 1 / 2 radius; and performing sampling according to GB / T228.1 "Metallic Materials - Tensile Testing - Part 1: Room Temperature Testing". The yield strength and high-temperature tensile strength of steel at 500℃ were determined according to GB / T231.1 "Metallic Materials - Brinell Section Hardness Test - Part 1: Test Method"; the Brinell section hardness of steel at different locations on the surface (10mm, 20mm, 40mm, 60mm, 80mm, 100mm, 120mm, 130mm) was tested according to GB / T2039 "Metallic Materials - Uniaxial Tensile Creep Test Method"; and the creep strength of steel at 500℃ for 1000 hours was determined according to GB / T2039 "Metallic Materials - Uniaxial Tensile Creep Test Method".

[0064] The finishing process involves straightening, surface cleaning, length cutting, and flaw detection of the stress-relieved tempered steel according to delivery requirements, so that the surface and dimensions of the steel meet the delivery requirements. Finally, qualified products are packaged and stored in the warehouse to obtain non-quenched and tempered steel finished products.

[0065] The chemical composition and weight percentage of the non-quenched and tempered steel in each embodiment are shown in Table 1, with the balance being iron and unavoidable impurities.

[0066] Table 1. Chemical composition and weight percentage

[0067]

[0068] The process parameters for the smelting of non-quenched and tempered steel from electric arc furnace steelmaking → LF refining → VD vacuum treatment in each embodiment are shown in Table 2.

[0069] Table 2. Smelting process parameters

[0070]

[0071] The continuous casting process parameters for non-quenched and tempered steel in each embodiment and Comparative Example 4 are shown in Table 3.

[0072] Table 3. Continuous casting process parameters

[0073]

[0074] The process parameters for controlled rolling, controlled cooling, and tempering of non-quenched and tempered steel in each embodiment, from heating to billet opening, controlled rolling and controlled cooling, and stress-relief tempering, are shown in Table 4.

[0075] Table 4. Process parameters for controlled rolling, controlled cooling, and tempering

[0076]

[0077] Metallographic structures were examined from non-quenched and tempered steel samples taken from each embodiment. Taking Example 1 as an example, ... Figure 1 As shown, the microstructure is entirely bainitic; the non-metallic inclusions at the 1 / 2 radius position in the steel were quantitatively rated, and the results are shown in Table 5 below.

[0078] Table 5. Non-metallic inclusion levels

[0079]

[0080] The results of Examples 1 and 2 show that the steel has a high alloy content. Through process control of electric arc furnace steelmaking, LF refining and VD vacuum treatment, the purity of the molten steel is improved. Non-metallic inclusions at the 1 / 2 radius position in the steel are effectively controlled. Type A sulfide inclusions are ≤1.5 grade, Type B alumina inclusions can reach 0 grade, and Type C silicate inclusions and Type D spherical oxide inclusions are ≤0.5 grade. This reflects the stability and reliability of the process and can stably produce high-quality non-quenched and tempered steel.

[0081] Comparative Example 1: The same material as Example 1, 42CrMo, conforming to standard GB / T 3077-1999.

[0082] The Brinell section hardness of steel samples obtained from Comparative Example 1 and Example 1 was tested, and the results are shown in Table 6 below.

[0083] Table 6. Comparison of Brinell cross-sectional hardness

[0084]

[0085] As can be seen from the comparison results between Comparative Example 1 and Example 1, the present invention obtains a finer full bainitic structure by forming an optimized composition design through strong carbide-forming elements such as Mo and W, a higher Si element content, a small amount of Co element, and microalloying elements such as V and Nb. The hardness difference between the core and the surface is ≤30HBW, indicating that the performance uniformity of the entire cross section is better suited to stress bearing, while reducing the risk of performance fluctuations during the processing.

[0086] Comparative Example 2: The difference between it and Example 1 is that high-pressure air cooling is used after continuous rolling, and the steel is cooled to 480~510℃ at a cooling rate of 6~20℃ / s. However, due to the excessively fast cooling rate, the hot-rolled round steel cracked. As can be seen from the comparison results between Example 1 and Comparative Example 2, due to the high alloy content and large deformation resistance of the steel, the internal stress after rolling is relatively high. When controlling the cooling, high-pressure air cooling is used to control the steel to cool to 480~510℃ at a cooling rate of 2~5℃ / s. This can work with alloying elements such as Mo, W, and Cr to delay the pearlite transformation and promote the bainite transformation. At the same time, it avoids the excessively fast cooling rate, which leads to an excessive temperature gradient between the surface and the core of the steel, and exacerbates stress and material brittleness, resulting in cracking.

[0087] Comparative Example 3: The difference between it and Example 1 is that after continuous rolling, high-pressure air cooling is used to control the steel to be cooled to 480~510℃ at a cooling rate of 0.2~1.8℃ / s, and the hot-rolled round steel does not crack.

[0088] The 42CrMo steel of Comparative Example 1 was subjected to quenching at 880℃ and tempering at 560℃, while the quenching and tempering step was omitted in Examples 1-2 and Comparative Example 3. The steel properties were tested by taking samples from the steel obtained in Comparative Example 3 and each example, and the results are shown in Table 7 below.

[0089] Table 7. Comparison of Steel Properties

[0090]

[0091] A comparison of the conventional 42CrMo steel in Comparative Example 1 with that of the present invention shows that, on the one hand, the non-quenched and tempered steel of the present invention can maintain an impact energy of ≥20J, a high-temperature tensile strength of ≥1050MPa, a yield strength of ≥800MPa, and a high-temperature tensile strength of ≥980MPa at 1 / 2 radius at high temperatures of 400~500℃. The material has higher strength and more uniform performance, allowing for a smaller column diameter under the same load, which increases clearance space and thus the working area of ​​the press. Furthermore, compared to the conventional 42CrMo steel, it can be directly machined, eliminating the need for quenching and tempering, thereby reducing column production energy consumption. This invention reduces waste, shortens cycles, and improves efficiency, overcoming process bottlenecks. Furthermore, the non-quenched and tempered steel of this invention has a microstructure entirely composed of bainite. The addition of certain amounts of alloying elements such as W, Co, and Mo increases the lower bainite transformation temperature, resulting in a finer bainite structure after controlled rolling and cooling. The precipitation of microalloying elements such as V and Nb further enhances the material's excellent high-temperature strength. The composition and microstructure also ensure relatively high strength even at relatively high temperatures. Traditional material 42CrMo exhibits a significant decrease in creep strength at 500℃, with a creep strength of approximately 80-100 MPa after 1000 hours. In contrast, this invention achieves a creep strength of ≥240 MPa, reaching up to 250 MPa, at 500℃ after 1000 hours, demonstrating higher high-temperature creep strength and a greater fatigue limit. This is beneficial for meeting the high-temperature service performance requirements of ultra-large press columns and improving service safety.

[0092] The metallographic structure of Example 1 is as follows Figure 1 As shown, the bainitic structure is relatively fine; the metallographic structure of Comparative Example 3 is as follows: Figure 2 As shown, the bainitic structure is relatively coarse. As can be seen from the comparison results of Example 1 and Comparative Example 3, compared with the slow cooling rate, controlling the cooling by first using high-pressure air cooling to cool the steel to 480~510℃ at a cooling rate of 2~5℃ / s can avoid the precipitation of pearlite and the failure to meet the strength requirements due to the slow cooling rate, and ensure that the matrix structure is bainitic.

[0093] Comparative Example 4: The difference between Comparative Example 4 and Example 1 lies in the different continuous casting process parameters, which are shown in Table 3. Inspection revealed that the core of the continuously cast round billet in Comparative Example 4 had large pores, and the core density of the hot-rolled round steel could not be guaranteed after rolling. Ultrasonic testing failed, and the core was located near the center of the cross-section along its length. Based on the shape of the reflected wave from the testing, it was inferred that there were shrinkage cavities of 30-100mm in the core. All examples passed ultrasonic testing. The comparison shows that further control of continuous casting parameters can further reduce internal porosity, shrinkage cavities, and gas pores, thus reducing the risk of cracks caused by uneven microstructure during subsequent billet opening and rolling.

[0094] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-strength, high-temperature resistant, non-quenched and tempered bainitic steel, characterized in that, Its chemical composition, by mass percentage, includes: C: 0.15%~0.25%, Si: 1.5%~2.0%, Mn: 1.5%~2.0%, P: ≤0.015%, S: ≤0.005%, Cr: 0.5%~1.0%, Ni: ≤0.25%, Cu: ≤0.25%, V: 0.05%~0.15%, Nb: 0.10%~0.20%, Al: 0.02%~0.04%, Mo: 0.30%~0.45%, W: 0.25%~0.45%, Co: 0.15%~0.25%, H: ≤1.5ppm, O: ≤40ppm, N: 0.01%~0.03%, with the balance being Fe and unavoidable impurities; The preparation method follows the process flow of electric arc furnace steelmaking → LF refining → VD vacuum treatment → continuous casting → heating → billet opening → controlled rolling and cooling → stress relief tempering → sampling → finishing → warehousing. The controlled rolling and cooling process is adopted, controlling the opening rolling temperature to ≤900℃, the exit temperature of continuous rolling to ≤820℃, the cumulative reduction rate in the finishing rolling stage to ≥15%, and after exiting the continuous rolling, the steel is cooled to 480~510℃ at a cooling rate of 2~5℃ / s, then air-cooled to a temperature below 210℃ on the lower cooling bed, and then slowly cooled to room temperature in the pit. After exiting the pit, stress relief tempering treatment is performed.

2. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to claim 1, characterized in that, The billet rolling process is adopted, and the temperature of the soaking zone in the heating process before billet rolling is controlled at 1100~1280℃, the soaking time is ≥10h, and the billet rolling temperature is ≥1120℃.

3. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to claim 2, characterized in that, The heating process employs a preheating section, heating section I, heating section II, and homogenization section with temperature control performed sequentially. The preheating section temperature is controlled to be <650℃ and the preheating time is >3h; the heating section I temperature is 750~900℃ and the heating time is ≥3h; the heating section II temperature is 950~1100℃ and the heating time is ≥5h; the total heating time is ≥22h.

4. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to claim 1, characterized in that, The continuously cast billet produced by the continuous casting process is rolled and cooled under controlled conditions. During continuous casting, the superheat of the first furnace is controlled at 15~25℃, the superheat of the continuous casting furnace is controlled at 10~20℃, the flow rate of the first cooling water is 5500~5800L / min, and the flow rate of the second cooling water is 50~60L / min.

5. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to claim 4, characterized in that, The continuous casting process employs three-stage electromagnetic stirring, controlling the electromagnetic stirring current in the crystallizer to be 120~180A and the frequency to be 2Hz; the electromagnetic stirring current in the casting stream to be 330~380A and the frequency to be 8Hz; and the electromagnetic stirring current at the solidification end to be 1100~1300A and the frequency to be 8Hz; with a casting speed of 0.15~0.18m / min.

6. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to claim 1, characterized in that, The quality of molten steel before controlled rolling and cooling is controlled by sequential electric arc furnace steelmaking, LF refining, and VD vacuum treatment. The final carbon content of the electric arc furnace steelmaking stage is ≥0.05%, the target phosphorus content is ≤0.006%, and the target temperature is ≥1620℃. The auxiliary materials are: lime 450~550kg / furnace, purifying agent 250~350kg / furnace, and aluminum at the tapping point 110~130kg / furnace.

7. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to claim 6, characterized in that, The white slag time in the LF refining stage is controlled to be ≥20 min, the smelting time is ≥40 min, and the auxiliary material dosage is 100-140 kg / furnace of silicon carbide.

8. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to claim 6, characterized in that, The VD vacuum treatment stage controls the VD vacuum degree to ≤67Pa, the vacuum holding time to ≥15min, and the soft blowing time to ≥20min; the wire feeding sequence is: first feed aluminum wire, then feed silicon-calcium wire, and the amount of silicon-calcium wire used is: 90~110m for the first furnace and 60~70m for the continuous casting furnace.

9. The high-strength, high-temperature resistant, non-quenched and tempered bainitic steel according to any one of claims 2 to 8, characterized in that, The stress-relief tempering process involves heating to 540-560℃ and holding for 10-15 hours.

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