Method for improving impact toughness of aluminum-containing ultrahigh-strength steel

Abstract

The invention discloses a method for improving impact toughness of aluminum-containing ultrahigh-strength steel. The method comprises the following steps: forging an aluminum-containing ultrahigh-strength steel cast ingot; carrying out two-stage hot rolling on the forged ultrahigh-strength steel, and carrying out water cooling treatment after hot rolling; the hot-rolled ultrahigh-strength steel is annealed at the annealing temperature of 800 + / -50 DEG C, heat preservation is conducted for a period of time, furnace cooling is conducted to 600 + / -50 DEG C, and discharging and air cooling are conducted; the ultrahigh-strength steel is subjected to solid solution and aging treatment, the solid solution process comprises the steps that a heat treatment furnace is heated to 950-1000 DEG C for charging, heat preservation is conducted for 30-40 min, then oil cooling is conducted, the aging process comprises the steps that heat preservation is conducted for 55-65 min at the temperature of 500-550 DEG C, and then air cooling is conducted. The technology is scientific and reasonable, the toughness of the aluminum-containing ultrahigh-strength steel is greatly improved while the strength of the ultrahigh-strength steel is guaranteed through the combination of two-stage rolling and the reasonable heat treatment technology, and therefore the service safety of the aluminum-containing ultrahigh-strength steel is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel materials, and relates to a method for improving impact toughness of aluminum-containing ultrahigh-strength steel. BACKGROUND

[0002] The ultrahigh-strength steel generally refers to high-strength alloy steel with a tensile strength value higher than 1470 MPa and a yield strength value higher than 1380 MPa at room temperature, and can be divided into low-alloy ultrahigh-strength steel, secondary hardening ultrahigh-strength steel and maraging steel according to metallurgical characteristics, and is mainly used for manufacturing high-stress structural parts, such as bulletproof steel plates, aircraft landing gears, high-end bearing steels, warships and rocket engine housings, and is the first choice of metal materials for high-end manufacturing at present, and plays an important role in national defense and new technology fields.

[0003] The ultrahigh-strength steel needs to have high strength and good toughness due to the particularity of the service environment, however, the strength and toughness of structural materials are often incompatible, and the ultrahigh-strength steel also has the problem of mismatching between strength and toughness, which limits the application and development of the ultrahigh-strength steel. With the low-costing of the ultrahigh-strength steel, the cheap alloy elements are generally used to replace the noble metals in the steel. The aluminum-containing ultrahigh-strength steel is one of the development directions, but the aluminum-containing ultrahigh-strength steel has a higher requirement for the hot working process, and therefore a suitable hot working process is needed to ensure the performance of the steel material. The hot rolling is a common process in the steel processing process, and can greatly affect the performance of the steel, and through the use of suitable rolling temperature, deformation amount and cooling mode in the rolling process, the matrix structure can be refined, and the mechanical properties of the steel can be optimized, but the ordinary hot rolling process cannot meet the requirements of the aluminum-containing ultrahigh-strength steel for matching strength and toughness.

[0004] The two-stage rolling process combines high-temperature rolling and low-temperature rolling, refines the austenite grains while retaining the deformation structure, provides a structure basis for the heat treatment process, and is helpful to the improvement of the comprehensive mechanical properties of the steel. The application utilizes the two-stage rolling process and a reasonable heat treatment process, and provides a method for improving the toughness of the aluminum-containing ultrahigh-strength steel. SUMMARY

[0005] The technical problem to be solved by the application is to provide a hot working method for improving the toughness of the aluminum-containing ultrahigh-strength steel, the two-stage rolling process and the solid solution aging process are combined, the toughness is effectively improved while the strength of the aluminum-containing ultrahigh-strength steel is ensured, and the service safety is improved.

[0006] The technical scheme adopted by the application to solve the above technical problem is that a method for improving impact toughness of aluminum-containing ultrahigh-strength steel comprises the following steps:

[0007] 1) The cast ingot of the aluminum-containing ultrahigh-strength steel is subjected to forging treatment;

[0008] 2) two-stage hot rolling of the forged ultra-high strength steel, water cooling treatment after hot rolling; the combination of one-stage high-temperature rough rolling and two-stage low-temperature finish rolling significantly refines austenite grains and martensite laths, achieving the effect of fine-grain strengthening;

[0009] 3) annealing of the hot-rolled ultra-high strength steel;

[0010] 4) heat treatment of the obtained plate.

[0011] As preferred, the mass percentage of each component of the aluminum-containing ultra-high strength steel is: C: 0.1wt%~0.3wt%, Si: 0.05wt%~0.15wt%, Mn: 0.1wt%~0.5wt%, Cr: 3wt%~5.5wt%, Ni: 5wt%~7wt%, Mo: 0.5wt%~1wt%, V: 0.4wt%~1.5wt%, Al: 0.1wt%~1wt%, and the balance is Fe and inevitable impurities.

[0012] Further, the initial temperature of the forging treatment in step 1 is 1100℃~1200℃, and the final forging temperature is 950℃~1050℃. The purpose of the forging treatment before hot rolling is to eliminate structural defects, improve hot working performance, and reduce the risk of cracking and uneven deformation of the steel during hot rolling treatment.

[0013] Further, the specific process of the two-stage hot rolling in step 2) is: heating the ingot to 1200±50℃ at 10±1℃ / s, then cooling to the rolling temperature at a cooling rate of 10±1℃ / s for the first-stage high-temperature rough rolling, the opening rolling temperature of the first-stage rolling is 1100±50℃, the final rolling temperature is 1025±25℃, the rolling passes are 3, and the reduction is 30±3%; after the first-stage rolling is completed, cooling to the rolling temperature at a cooling rate of 20±2℃ / s for the second-stage finish rolling, the opening rolling temperature of the second-stage finish rolling is 930±25℃, the final rolling temperature is 830±50℃, the rolling passes are 3, the reduction is 25±2%, and finally water cooling. After the two-stage hot rolling process, a large number of dislocations and sub-grain boundaries in the steel matrix are retained, providing a structural basis for subsequent heat treatment.

[0014] Further, the annealing temperature of step 3) is 800±50℃, and the furnace is cooled to 600±50℃ for a period of time, and then air-cooled after discharge.

[0015] The annealing time of step 3) should not be too long, and the specific time should be determined according to the size of the workpiece. Long-term annealing can completely eliminate the structural defects after hot rolling, which is not conducive to the subsequent heat treatment process.

[0016] Finally, the step 4) heat treatment of the method is a solid solution treatment process of 950-1000 ℃, holding for 30-40 min, and then oil cooling, and an aging process of 500-550 ℃ holding for 1 h, and then air cooling.

[0017] Compared with the prior art, the method has the advantages that: two-stage rolling is adopted, the combination of one-stage high-temperature rough rolling and two-stage low-temperature finish rolling can significantly refine austenite grains and martensite laths, refine the matrix structure, and improve the toughness of the steel. Since the second-stage rolling temperature is low, the matrix does not undergo dynamic recrystallization in the rolling process, a large number of dislocations and subgrain boundaries are retained, which can improve the effect of precipitation strengthening in the heat treatment process and improve the comprehensive mechanical properties of the steel. The process is scientific and reasonable, and the toughness of the aluminum-containing ultrahigh-strength steel is greatly improved by two-stage rolling combined with a reasonable heat treatment process, while the strength of the ultrahigh-strength steel is ensured, the toughness is effectively improved, and the service safety is improved. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a schematic diagram of the hot rolling process provided by the application;

[0019] Figure 2 is a grain structure morphology diagram of the hot-treated example 1 of the application;

[0020] Figure 3 is a TEM structure morphology diagram of example 1 of the application.

[0021] Figure 4 is a TEM precipitated particle structure diagram of example 1 of the application. DETAILED DESCRIPTION

[0022] The application will be further described in detail below with reference to the embodiments combined with the drawings.

[0023] Example 1:

[0024] The hot rolling process of the application is shown in Figure 1 The aluminum-containing ultrahigh-strength steel involved in this embodiment includes the following components according to the mass percentage: C: 0.28%, Si: 0.1%, Mn: 0.47%, Cr: 4.96%, Ni: 5.88%, Mo: 0.71%, V: 0.8%, Al: 0.92%, and the balance is iron and other unavoidable impurities. The method for improving the toughness of the above-mentioned aluminum-containing ultrahigh-strength steel comprises the following steps in sequence:

[0025] 1) The above-mentioned ultrahigh-strength steel is placed in a vacuum induction melting furnace, and a ingot is obtained, and then the ingot is forged, the initial temperature of forging is 1150 ℃, the final forging temperature is 1000 ℃, and then air cooling is performed to obtain a forged blank. The method for improving the toughness of the above-mentioned aluminum-containing ultrahigh-strength steel comprises the following steps in sequence:

[0026] 2) Hot rolling: the above forged blank is subjected to two-stage multi-pass hot rolling, the material is put into a heating furnace to be heated to 1200°C and kept for a while, then cooled to a rolling temperature at a cooling rate of 10°C / s, the first-stage rolling has a starting rolling temperature of 1150°C and a finishing rolling temperature of 1050°C, the rolling passes are 3 times and the reduction is 30%; after the first-stage rolling is completed, the second-stage finish rolling is performed at a cooling rate of 20°C / s to 930°C, the second-stage rolling has a starting rolling temperature of 930°C and a finishing rolling temperature of 870°C, the rolling passes are 3 times and the reduction is 25%, then water cooling is performed to obtain a hot-rolled material;

[0027] 3) The hot-rolled material is subjected to annealing, the annealing temperature is 800°C, kept for a while, furnace cooling to 600°C, and air cooling after being discharged from the furnace;

[0028] 4) Solid solution treatment: the hot-rolled material is subjected to solid solution treatment, the solid solution temperature is controlled to be 980°C, kept for 30 min, and then oil cooled;

[0029] 5) High-temperature aging: the steel subjected to the solid solution treatment is subjected to high-temperature aging treatment, the aging temperature is 525°C, kept for 1 h, and then air cooled, the higher aging temperature is helpful for the precipitation of NiAl intermetallic compounds and carbides in the matrix.

[0030] Figure 2 It is a grain structure diagram of the test steel subjected to heat treatment, which is graded according to the grain size grade standard, the grain size of the embodiment is 8, and the grains are relatively small.

[0031] Figure 3 It is a TEM and diffraction spot diagram of the test steel of the embodiment, from which it can be seen that there is residual austenite in the matrix, the existence of the residual austenite will make the crack in the matrix in a zigzag path, improve the energy required for fracture, and thus hinder the crack propagation and improve the toughness.

[0032] Figure 4 It is a morphology diagram of precipitates and dislocations in the matrix of the test steel of the embodiment, it can be seen that there are many precipitate particles in the matrix, and there are a large number of dislocations around the precipitate particles, which is because the precipitates can pin the dislocations and hinder the movement of the dislocations, thereby improving the strength of the material and ensuring that the super-high strength steel does not decrease in strength while improving the toughness. It is tested that the tensile strength of the embodiment is 1792 MPa, the yield strength is 1546 MPa, and the impact toughness at room temperature is 26.5 J.

[0033] Comparative Example 1:

[0034] The difference between the above-mentioned embodiment 1 is only that the conventional hot rolling and heat treatment process is adopted, and the conventional hot working process is as follows: the starting rolling temperature is 1200°C, the finishing rolling temperature is 950°C, the solution temperature is 1100°C, the holding time is 45 min, the aging temperature is 550°C, and the holding time is 1 h. The tensile strength of the comparative example is 1817 MPa, the yield strength is 1559 MPa, and the impact toughness at room temperature is 17.6 J.

[0035] From the above comparison, it can be seen that the impact toughness at room temperature of the embodiment 1 is obviously improved compared with the comparative example 1 which is only treated by the ordinary process, and the impact toughness of the embodiment 1 is increased by 50.5%, and at the same time, the tensile strength and the yield strength are slightly decreased, but the difference is small.

[0036] Embodiment 2:

[0037] The main difference between the embodiment and the embodiment 1 is that the process parameters are different, specifically, in step 2), the starting rolling temperature of the first stage of rolling is 1150°C, the finishing rolling temperature is 1000°C, and the reduction is 30%; the starting rolling temperature of the second stage of rolling is 920°C, the finishing rolling temperature is 840°C, and the reduction is 25%. In step 4), the solution treatment temperature is 960°C, and the holding time is 40 min.

[0038] The sample of the embodiment is tested after aging treatment at 525°C for 1 h, and the tensile strength is 1736 MPa, the yield strength is 1501 MPa, and the impact toughness at room temperature is 24.2 J.

[0039] Comparative example 2:

[0040] The difference between the above-mentioned embodiment 2 is only that the conventional hot rolling and heat treatment process is adopted. The tensile strength of the comparative example is 1771 MPa, the yield strength is 1470 MPa, and the impact toughness at room temperature is 18.9 J. From the above, it can be seen that the impact toughness at room temperature of the embodiment 2 is obviously improved compared with the comparative example 2 which is treated by the ordinary process, and the impact toughness of the embodiment 2 is increased by 28.04%, and the tensile strength and the yield strength change little.

[0041] Embodiment 3:

[0042] The main difference between the embodiment and the embodiment 1 is that the process parameters are different, specifically, in step 2), the starting rolling temperature of the first stage of rolling is 1130°C, the finishing rolling temperature is 1050°C, and the reduction is 30%; the starting rolling temperature of the second stage of rolling is 910°C, the finishing rolling temperature is 830°C, and the reduction is 25%. In step 4), the solution treatment temperature is 970°C, and the holding time is 30 min. In step 5), the aging treatment temperature is 550°C, and the holding time is 1 h.

[0043] The sample of the embodiment is tested after aging treatment, and has a tensile strength of 1806 MPa, a yield strength of 1528 MPa and a room temperature impact toughness of 22.7 J.

[0044] Comparative Example 3:

[0045] The difference between the above embodiment 3 and the comparative example 3 is only that the conventional hot rolling and heat treatment process is used. The tensile strength of the comparative example 3 is 1789 MPa, the yield strength is 1532 MPa, and the room temperature impact toughness is 17.1 J. As can be seen from the above, compared with the comparative example 3 treated by the ordinary process, the room temperature impact toughness of the embodiment 3 is obviously improved, the impact toughness of the embodiment 3 is increased by 32.75%, and the tensile strength and yield strength change little.

[0046] Embodiment 4:

[0047] The main difference between the embodiment and the embodiment 1 is that the process parameters are different. Specifically, in step 2), the opening rolling temperature of the first stage is 1150℃, the finishing rolling temperature is 1025℃, and the reduction is 30%; the opening rolling temperature of the second stage is 930℃, the finishing rolling temperature is 850℃, and the reduction is 25%. In step 4), the solid solution treatment temperature is 980℃, and the holding time is 40 min. In step 5), the aging treatment temperature is 500℃, and the holding time is 1 h.

[0048] The sample of the embodiment is tested after aging treatment, and has a tensile strength of 1745 MPa, a yield strength of 1458 MPa and a room temperature impact toughness of 25 J.

[0049] Comparative Example 4:

[0050] The difference between the above embodiment 3 and the comparative example 3 is only that the conventional hot rolling and heat treatment process is used. The tensile strength of the comparative example 3 is 1789 MPa, the yield strength is 1532 MPa, and the room temperature impact toughness is 17.1 J. As can be seen from the above, compared with the comparative example 3 treated by the ordinary process, the room temperature impact toughness of the embodiment 3 is obviously improved, the impact toughness of the embodiment 3 is increased by 32.75%, and the tensile strength and yield strength change little.

[0051] The mechanical property test of the embodiments and the comparative examples of the application is shown in the following table 1. Compared with the conventional process (opening rolling temperature of 1200℃, finishing rolling temperature of 950℃, solid solution temperature of 1100℃, holding time of 45 min, aging temperature of 550℃, and holding time of 1 h) used in the comparative examples, the impact toughness at room temperature is greatly improved, and the tensile strength and yield strength change little.

[0052] The performance test structure of the four embodiments and the four comparative examples is shown in table 1:

[0053] Table 1 Performance of the inventive examples and comparative examples

[0054] Tensile strength / MPa Yield strength / MPa Impact toughness / J Example 1 1792 1546 26.5 Comparative Example 1 1817 1559 17.6 Example 2 1736 1501 24.2 Comparative Example 2 1771 1470 18.9 Example 3 1806 1528 22.7 Comparative Example 3 1789 1532 17.1 Example 4 1745 1458 25 Comparative Example 4 1806 1490 16.5

[0055] It can be seen that the method of the present application can effectively improve the toughness of the aluminum-containing ultra-high strength steel, while having high tensile strength and yield strength.

[0056] The innovation and improvement idea of the present application is:

[0057] I. The combination of one-stage 1100-1025℃ high-temperature rough rolling and two-stage 930-830℃ lower-temperature finish rolling can significantly refine the austenite grains and martensite laths, achieve the effect of fine-grain strengthening, and improve the toughness of the steel. Since the two-stage temperature is relatively low, the matrix does not undergo dynamic recrystallization during rolling, and a large number of dislocations and subgrain boundaries are retained. These dislocations and subgrain boundaries have two main effects: (1) they pass on defects to the martensite during cooling, and transform into austenite in the form of α'→γ non-diffusion during solid solution heating, resulting in an increase in the volume fraction of residual austenite after final cooling, which hinders the propagation of cracks; (2) the maximum free energy change and the critical nucleation size of carbide precipitation on dislocations are smaller than those on grain boundaries, and the strain-induced precipitates during aging treatment will preferentially nucleate on dislocations and subgrain boundaries, thereby promoting the generation of precipitates and enhancing the precipitation strengthening effect. In addition, due to the increase in the number of precipitates, the precipitates dispersed in the matrix can limit the growth of grains, thereby improving the fine-grain strengthening effect.

[0058] II. The use of 980℃ for solid solution treatment is beneficial to the refinement of austenite grains, and the use of higher temperature for aging treatment is to utilize the precipitation strengthening effect of NiAl precipitates and carbides precipitated at higher temperatures. These second phases are alloyed during the solid solution stage and dispersedly precipitated during the heat treatment process, and are distributed in the matrix, thereby hindering the movement of dislocations through the dislocation cutting mechanism or bypassing mechanism, and improving the strength of the steel.

[0059] In summary, the interaction between NiAl precipitates and carbides in the matrix and dislocations ensures the strength of the steel, and the hindering effect of residual austenite and refined grains on cracks improves the toughness of the aluminum-containing ultra-high strength steel. The above method effectively improves the toughness while ensuring the strength of the ultra-high strength steel, thereby improving its service safety.

[0060] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several improvements and refinements without departing from the technical principles of the present application, and these improvements and refinements should also be considered within the protection scope of the present application.

Claims

1. A method of improving impact toughness of an aluminum-containing ultra-high strength steel, characterized by The method comprises the following steps: 1) forging a cast ingot of an aluminum-containing ultra-high strength steel; 2) two-stage hot rolling of the forged ultra-high strength steel, water cooling after hot rolling; the combination of one-stage high-temperature rough rolling and two-stage low-temperature finish rolling significantly refines austenite grains and martensite laths, achieving the effect of fine-grain strengthening; 3) annealing the hot-rolled ultra-high strength steel; 4) heat treatment of the obtained hot-rolled plate.

2. The method of claim 1, wherein: The aluminum-containing ultra-high strength steel comprises the following components in percentage by mass: C: 0.1wt%-0.3wt%, Si: 0.05wt%-0.15wt%, Mn: 0.1wt%-0.5wt%, Cr: 3wt%-5.5wt%, Ni: 5wt%-7wt%, Mo: 0.5wt%-1wt%, V: 0.4wt%-1.5wt%, Al: 0.1wt%-1wt%, and the balance of Fe and inevitable impurities.

3. The method of claim 1, wherein: The initial temperature of the forging treatment in step 1 is 1100-1200°C, and the final forging temperature is 950-1050°C.

4. The method of claim 1, wherein: The specific process of the two-stage hot rolling in step 2 is as follows: the cast ingot is heated to 1200±50°C at a rate of 10±1°C / s, then cooled to the rolling temperature at a cooling rate of 10±1°C / s for the first-stage high-temperature rough rolling, the opening rolling temperature of the first-stage rolling is 1100±50°C, the final rolling temperature is 1025±25°C, the rolling passes are 3, and the reduction is 30±3%; after the first-stage rolling, the second-stage finish rolling is performed at a cooling rate of 20±2°C / s, the opening rolling temperature of the second-stage finish rolling is 930±25°C, the final rolling temperature is 830±50°C, the rolling passes are 3, the reduction is 25±2%, and finally water cooling is performed.

5. The method of claim 1, wherein: The annealing temperature in step 3 is 800±50°C, the annealing time is a certain period, the furnace is cooled to 600±50°C, and the workpiece is air-cooled after being taken out of the furnace.

6. The method of claim 5, wherein: The annealing time in step 3 should not be too long, and the specific time should be determined according to the size of the workpiece.

7. The method of claim 1, wherein: The heat treatment in step 4 is solid solution treatment at 950-1000°C for 30-40 min, oil cooling, aging treatment at 500-550°C for 1 h, and then air cooling.

Images