A method for producing Ti-containing martensite steel with a tensile strength ≥1700MPa and refined effective grain size.

Abstract

A method for producing Ti-containing martensite steel with a tensile strength ≥1700MPa and refined effective grain size: The process involves conventional smelting, refining, and continuous casting into billets, followed by heating; rolling in the austenite recrystallization zone; rolling in the non-recrystallization zone; ultra-rapid cooling; offline austenitizing water quenching; tempering; and then leaving the steel for use. This invention can refine the effective grain size to no more than 1.15μm, achieve a tensile strength ≥1700MPa, an elongation A ≥9%, and an impact energy ≥43J at -20℃.

Description

Technical Field

[0001] This invention relates to a steel for mechanical engineering and a production method thereof, specifically to a production method for a Ti-containing martensite-refined steel with a tensile strength ≥1700MPa, applicable to steel plates with a thickness of 10-30mm. Background Technology

[0002] Ultra-high strength steel is an important structural and protective material, widely used in engineering machinery, mining machinery, and special-purpose vehicles. Under the "dual carbon" target (carbon reduction and emission reduction), both machinery and vehicles need to reduce their weight to achieve energy conservation and emission reduction. However, steel accounts for more than 40% of the overall materials used in machinery and vehicle manufacturing; therefore, the use of ultra-high strength steel is the preferred material for weight reduction in machinery and vehicle design.

[0003] Various types of machinery and vehicles play a vital role in national economic development. To reduce weight while maintaining equipment functionality, ultra-high-strength steel plates with a thickness of less than 30mm are the preferred choice for machinery and vehicle design. The rapidly developing large machinery and specialized vehicles require extensive use of ultra-high-strength steel plates with a medium-thin matrix. Therefore, the research and development of high-quality ultra-high-strength, medium-thin matrix steel plates is of great significance for resolving the contradiction between high performance and lightweight design in machinery and vehicles.

[0004] Currently, the high-strength steel widely used in domestic machinery and vehicles mostly has a strength of around 1000 MPa. From a production practice perspective, the manufacture of high-strength and ultra-high-strength steel plates with strengths exceeding 1000 MPa has always faced technical bottlenecks, primarily manifested in the contradiction between strength and ductility. Especially when the tensile strength reaches above 1700 MPa, the ductility of the steel plate decreases sharply, making it prone to cracking and failure under impact, which seriously affects the service life of machinery and vehicles.

[0005] In traditional industrial production, quenching is widely used to produce ultra-high-strength martensitic steel. This involves heating the steel in a furnace to above the austenitic transformation completion temperature, holding it at that temperature until fully austenitized, and then rapidly cooling it in a cooling medium (water or oil) (quenching). During cooling, a martensitic phase transformation occurs, ultimately resulting in a martensitic microstructure. However, this method lacks precise control over the martensitic microstructure. With the development of thermomechanical control processes, direct quenching technology has gradually been applied to the production of high-strength martensitic steel. Direct quenching refers to the method of using the residual heat after rolling of steel materials for online direct quenching to obtain martensitic and / or bainitic microstructures. However, currently, the tensile strength of steel plates produced using direct quenching technology both domestically and internationally is generally below 1200 MPa, and research on directly quenched steels with higher strength levels is still limited.

[0006] Among various strengthening methods for metallic materials, grain refinement is currently recognized as the only method that can simultaneously improve both strength and toughness. Therefore, grain refinement is considered the main approach to developing ultra-high-strength steel. To further improve the performance of ultra-high-strength martensitic steel, materials researchers have conducted extensive research on the preparation of ultra-fine-grained martensitic steel, developing many grain refinement methods, such as cyclic heat treatment, rapid heating, and deformation heat treatment. However, these new methods can only refine austenite grains to approximately 5 μm, offering limited improvement to the performance of steel materials. Moreover, these methods suffer from drawbacks such as cumbersome processes and high costs, making them difficult to adapt to the characteristics of current industrial production equipment and processes, significantly limiting industrial applications. Therefore, it is necessary to develop simple and feasible grain refinement methods based on existing production equipment and processes to improve the comprehensive mechanical properties of ultra-high-strength martensitic steel.

[0007] For ultra-high strength steel characterized by martensitic structure, due to the complexity of the martensitic structure, the statistical standard for grain size is the effective grain, that is, the orientation difference between adjacent grains is greater than 15°. The strength, plasticity and toughness of martensitic ultra-high strength steel are directly related to the effective grain size. The finer the effective grain size, the higher the strength and the better the plasticity and toughness.

[0008] Therefore, refining the effective grain size of martensitic ultra-high strength steel has become crucial for its development. To address the complexity of the martensitic microstructure, this invention develops a method for refining the effective grain size of Ti-microalloyed martensitic ultra-high strength steel. Summary of the Invention

[0009] The present invention aims to overcome the shortcomings of the existing technology and provide a Ti-containing martensite grain-refined steel with effective grain refinement of no more than 1.15 μm, tensile strength ≥1700 MPa, elongation A ≥9%, and impact energy ≥43 J at -20℃, as well as a production method thereof.

[0010] Measures to achieve the above objectives:

[0011] A method for producing Ti-containing martensite-refined steel with a tensile strength ≥1700MPa, comprising the following steps:

[0012] 1) After conventional smelting, refining and continuous casting into a billet with a thickness of 250±10mm, it is heated. The heating temperature is controlled at 1190~1230℃ and the heating time is controlled at 1~1.3min / mm.

[0013] 2) Rolling is performed in the austenite recrystallization zone, during which:

[0014] The first stage of two-pass rolling is carried out: the temperature is controlled at 1050-1080℃, the reduction rate of each pass is controlled at 20-30%, and the rolling speed is controlled at 0.5-1.5m / s;

[0015] Cooled to 980–1010°C by air;

[0016] The second stage involves three passes of rolling, with the reduction rate of each pass controlled at 15-25%, and the pause between each two passes controlled at 5-8 seconds.

[0017] 3) Rolling is performed in the non-recrystallized austenite region, during which:

[0018] The first stage of three-pass rolling is carried out: the temperature is controlled at 910-950℃, and the reduction rate of each pass is controlled at 13-20%.

[0019] Cooled to 860–880°C by air;

[0020] The second stage involves two to three passes of rolling, with the reduction rate of each pass controlled at 10-15%.

[0021] 4) Perform ultra-rapid cooling: control the initial cooling temperature at 820-840℃, and cool to room temperature at a cooling rate of 85-100℃ / s;

[0022] 5) Offline austenitizing water quenching

[0023] First, heat to 150-180°C at a heating rate of 3-5°C / min;

[0024] Then heat to 850-880℃ at a heating rate of 15-25℃ / min and hold at this temperature for 10-25min; 6) perform tempering, controlling the tempering temperature at 190-250℃ and the tempering time at 150-200min.

[0025] 7) To be used.

[0026] Preferably: during rolling in the austenite recrystallization zone:

[0027] The rolling temperature in the first stage is controlled at 1055–1076℃, the reduction rate per pass is controlled at 23–27%, and the rolling speed is controlled at 0.8–1.3 m / s.

[0028] Cooled to 986–1003°C by air;

[0029] In the second stage of rolling, the reduction rate per pass is controlled at 18-25%.

[0030] Preferably: During rolling in the non-recrystallized austenite region, control:

[0031] The rolling temperature in the first stage is controlled at 914–946℃, and the reduction rate per pass is controlled at 15–18%.

[0032] It is cooled by air to 866–875°C.

[0033] Preferably, the ultra-fast cooling rate is 89–96 °C / s.

[0034] Preferably: During offline austenitizing water quenching: the heating temperature is controlled at 156-176℃ at a heating rate of 3-5℃ / min; then the heating temperature is controlled at 861-873℃ at a heating rate of 15-25℃ / min.

[0035] Preferably, the tempering temperature is between 197 and 235°C.

[0036] The role and mechanism of each raw material and main process in this invention

[0037] The reason why the heating temperature is controlled at 1190-1230℃ and the heating time is controlled at 1-1.3 min / mm is to reduce the dissolution and ripening of the second phase particles and avoid excessive growth of austenite grains.

[0038] The reason why this invention involves rolling in the austenite recrystallization zone is that:

[0039] During the first stage of two-pass rolling, the rolling temperature is controlled at 1050-1080℃, the reduction rate of each pass is controlled at 20-30%, and the rolling speed is controlled at 0.5-1.5m / s.

[0040] Cooled to 980–1010°C by air;

[0041] During the second stage of three-pass rolling, the reduction rate of each pass is controlled at 15-25%, and the pause between each two passes is controlled at 5-8 seconds.

[0042] This is because the austenite recrystallization zone is controlled by two stages of rolling to refine the original austenite grains: the first stage rolling temperature is 1050-1080℃, the pass reduction rate is 20-30%, and the rolling speed is 0.5-1.5m / s, which fully breaks down the columnar grains of the billet; the second stage rolling is carried out after cooling to 980-1010℃, with a pass reduction rate of 15-25%. During this stage, a large amount of Ti-containing precipitates are precipitated. The pinning effect of the second phase particles on the grain boundaries delays the growth of austenite recrystallized grains. The interval between passes is 5-8s, which makes the recrystallized grains more uniform.

[0043] The reason why this invention involves rolling in the non-recrystallized austenite region is that:

[0044] During the first stage of three-pass rolling, the rolling temperature is controlled at 910-950℃, and the reduction rate of each pass is controlled at 13-20%.

[0045] Cooled to 860–880°C by air;

[0046] During the second stage of two to three passes of rolling, the reduction rate of each pass should be controlled at 10-15%.

[0047] This is because low-temperature rolling causes high-density dislocations and other defects to form inside the flattened austenite grains, and stores a large amount of deformation energy, providing a driving force for subsequent cooling phase transformation.

[0048] The reason why this invention controls the initial cooling temperature at 820–840°C and cools to room temperature at a cooling rate of 85–100°C / s during the high-cooling stage is that the ultra-fast cooling equipment used after the four-stage rolling process cools the steel plate to room temperature, resulting in a fine lath martensite structure with deformation heat treatment effects. At the high cooling rate, the flattened grain morphology, high-density dislocations, and deformation energy stored during rolling are fully preserved, which is a favorable condition for refining the grains during subsequent austenitization reheating. Furthermore, the high cooling rate suppresses the precipitation of Ti during the phase transformation, keeping Ti in a supersaturated state.

[0049] The reason why this invention first heats the austenitizing water quenching process offline to 150–180°C at a heating rate of 3–5°C / min, and then to 850–880°C at a heating rate of 15–25°C / min, holding at this temperature for 10–25 min, is that supersaturated Ti gradually disperses and precipitates as the temperature increases, without dissolving or ripening. The nanoscale Ti precipitates are retained in the austenite region, which can more effectively prevent the growth of austenite grains during reheating. The sufficiently refined reheated austenite transforms into a fine martensite structure during the subsequent water quenching process, meaning the effective grain size is also finer.

[0050] The reason why the tempering temperature is controlled at 190-250℃ and the tempering time at 150-200min in this invention is that the low-temperature tempering heat treatment eliminates quenching stress, improves toughness and plasticity, promotes the precipitation of carbides such as Cr / Mo, and increases precipitation strengthening effect.

[0051] Compared with the prior art, the present invention can refine the effective grain size to no more than 1.15 μm and the tensile strength ≥

[0052] 1700MPa, elongation A≥9%, impact energy at -20℃≥43J. Attached Figure Description

[0053] Figure 1 This is a diagram of the effective grain size of the present invention. Detailed Implementation

[0054] The present invention will now be described in detail:

[0055] Table 1 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0056] Table 2 is a list of performance test results for each embodiment and comparative example of the present invention.

[0057] The various embodiments of the present invention are produced according to the following steps.

[0058] 1) After conventional smelting, refining and continuous casting into a billet with a thickness of 250±10mm, it is heated. The heating temperature is controlled at 1190~1230℃ and the heating time is controlled at 1~1.3min / mm.

[0059] 2) Rolling is performed in the austenite recrystallization zone, during which:

[0060] The first stage of two-pass rolling is carried out: the temperature is controlled at 1050-1080℃, the reduction rate of each pass is controlled at 20-30%, and the rolling speed is controlled at 0.5-1.5m / s;

[0061] Cooled to 980–1010°C by air;

[0062] The second stage involves three passes of rolling, with the reduction rate of each pass controlled at 15-25%, and the pause between each two passes controlled at 5-8 seconds.

[0063] 3) Rolling is performed in the non-recrystallized austenite region, during which:

[0064] The first stage of three-pass rolling is carried out: the temperature is controlled at 910-950℃, and the reduction rate of each pass is controlled at 13-20%.

[0065] Cooled to 860–880°C by air;

[0066] The second stage involves two to three passes of rolling, with the reduction rate of each pass controlled at 10-15%.

[0067] 4) Perform ultra-rapid cooling: control the initial cooling temperature at 820-840℃, and cool to room temperature at a cooling rate of 85-100℃ / s;

[0068] 5) Offline austenitizing water quenching

[0069] First, heat to 150-180°C at a heating rate of 3-5°C / min;

[0070] Then heat to 850-880℃ at a heating rate of 15-25℃ / min and hold at this temperature for 10-25min; 6) perform tempering, controlling the tempering temperature at 190-250℃ and the tempering time at 150-200min.

[0071] 7) To be used.

[0072] Table 1. List of main process parameters for each embodiment and comparative example of the present invention.

[0073]

[0074]

[0075] Table 1 (continued) lists the cooling and heat treatment process parameters for each embodiment and comparative example of the present invention.

[0076]

[0077]

[0078] Table 2. Performance testing and grain size statistics of each embodiment and comparative example of the present invention.

[0079]

[0080] Analysis of the results in Table 2 shows that the steel plate of the present invention has a thickness range of 12–30 mm, Rm ≥ 1700 MPa, elongation ≥ 9%, impact energy at -20℃ ≥ 43 J, and elongation ≥ 98%. Compared with the comparative example (conventional process), the strength and toughness are improved, mainly due to the refinement of the original austenite grain size and the refinement of the effective grain size.

[0081] The above embodiments are merely best examples and are not intended to limit the implementation of the present invention.

Claims

1. A method for producing Ti-containing martensite-refined steel with a tensile strength ≥1700MPa, comprising the following steps: 1) After conventional smelting, refining and continuous casting into a billet with a thickness of 250±10mm, it is heated. The heating temperature is controlled at 1190~1230℃ and the heating time is controlled at 1~1.3min / mm. 2) Rolling is performed in the austenite recrystallization zone, during which: The first stage of two-pass rolling is carried out: the temperature is controlled at 1050-1080℃, the reduction rate of each pass is controlled at 20-30%, and the rolling speed is controlled at 0.5-1.5m / s; Cooled to 980–1010°C by air; The second stage involves three passes of rolling, with the reduction rate of each pass controlled at 15-25%, and the pause between each two passes controlled at 5-8 seconds. 3) Rolling is performed in the non-recrystallized austenite region, during which: The first stage of three-pass rolling is carried out: the temperature is controlled at 910-918℃, and the reduction rate of each pass is controlled at 13-14%; Cooled to 860–880°C by air; The second stage involves two to three passes of rolling, with the reduction rate of each pass controlled at 10-14%. 4) Perform ultra-rapid cooling: control the initial cooling temperature at 820-840℃, and cool to room temperature at a cooling rate of 85-100℃ / s; 5) Offline austenitizing water-cooled quenching First, heat to 150-180°C at a heating rate of 3-5°C / min; Then heat to 850-866℃ at a heating rate of 15-25℃ / min, and hold at this temperature for 10-25 minutes; 6) Perform tempering, controlling the tempering temperature at 205~250℃ and the tempering time at 150~160min; 7) For later use; This production method is suitable for steel plates with a thickness of 10 to 30 mm, effective grain refinement of the steel plate to no more than 1.15 μm, tensile strength ≥ 1700 MPa, elongation A ≥ 9%, and impact energy at -20℃ ≥ 43 J.

2. The method for producing a Ti-containing martensitic steel with a tensile strength ≥1700MPa and refined effective grains as described in claim 1, characterized in that: During rolling in the austenite recrystallization zone: The first stage of rolling reduction rate is controlled at 23-27%, and the rolling speed is controlled at 0.8-1.3 m / s; Cooled to 986–1003°C by air; In the second stage of rolling, the reduction rate per pass is controlled at 18-25%.

3. The method for producing a Ti-containing martensitic steel with a tensile strength ≥1700MPa and refined effective grains as described in claim 1, characterized in that: The ultra-fast cooling rate is 89–96 °C / s.

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