Method for improving strength and impact toughness of cast steel 32MnMoNiCu

Abstract

The invention belongs to the field of material design, and relates to a method for improving the strength and impact toughness of cast steel 32MnMoNiCu. According to the invention, harmful inclusions such as MnS and Al2O3 in the steel are modified by adding a certain proportion of rare earth Ce. After the rare earth Ce is added, the tensile strength of the cast steel 32MnMoNiCu is improved by 12.3%, and the impact toughness is improved by 9.2%; the strengthening effect of the rare earth Ce on the cast steel 32MnMoNiCu is proved in a manner of combining calculation and test, and a reliable theoretical basis is provided for formulation of a subsequent process.

Description

Technical Field

[0001] The invention belongs to the field of material design and relates to a method for improving the strength and impact toughness of cast steel 32MnMoNiCu. Background Art

[0002] Cast steel 32MnMoNiCu has high strength, shock resistance and wear resistance, and is widely used in engineering machinery, military products and other fields. As structural parts develop towards high strength and lightweight, cast steel 32MnMoNiCu with higher strength and toughness is urgently needed, and the degree of material purity is the main factor that determines the comprehensive mechanical properties of cast steel such as strength and plasticity. At present, in order to reduce environmental pollution, a small medium frequency furnace is used to smelt track plates. The smelting process is batching-charging-melting-deoxidation-pouring. During the smelting process, slag collecting agents are continuously added to continuously discharge the impurities floating on the molten steel from the medium frequency furnace. In addition, pure aluminum wire accounting for 0.15% to 0.2% of the molten steel is used for final deoxidation, and casting is carried out immediately after final deoxidation.

[0003] The raw materials in the smelting process are return materials and scrap steel, resulting in high O, S, and P content in the melted steel. The active metal elements in the smelting and casting process of steel easily react with harmful elements such as O, S, and P in the molten steel to form metal oxide and sulfide inclusions, such as Al2O3 and MnS inclusions, which affect the mechanical properties of cast steel.

[0004] The medium frequency furnace does not have the function of slag formation, and the furnace is open during the smelting process. The surface of the molten steel constantly reacts with oxygen in the air to form oxides floating on the surface, which are gathered and salvaged out of the furnace by slag collectors. However, the high S content in the molten steel reacts with the Mn in the steel to form MnS inclusions, such as Figure 1 As shown in the figure, MnS inclusions roll in the molten steel with induction heating and are finally poured into the structural parts, reducing the impact toughness and strength of the structural parts. Summary of the invention

[0005] Aiming at the problem that the O, S and P contents in steel smelted in a medium frequency furnace are high, which reduces the impact toughness and strength of structural parts, the present invention proposes a method for improving the strength and impact toughness of cast steel 32MnMoNiCu, by redesigning the chemical composition of cast steel 32MnMoNiCu, by adding a certain proportion of rare earth Ce content, modifying the harmful inclusions of MnS and Al2O3 in the steel, and by carrying out thermodynamic calculations, Hall-Petch equation calculations and heat treatment test verifications, the optimal heat treatment strengthening process is obtained, and the tensile strength and impact toughness of cast steel 32MnMoNiCu are improved.

[0006] The scheme adopted by the present invention is as follows:

[0007] A method for improving the strength and impact toughness of cast steel 32MnMoNiCu, comprising the following steps:

[0008] 1) First, prepare the return material and waste material of cast steel 32MnMoNiCu required for smelting, wherein the ratio of the return material to the waste material is 30% to 40%: 60% to 70% in terms of the mass percentage of the total mass of the two;

[0009] 2) Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. During the heating process, the power is continuously increased to make the returned materials and waste materials melt evenly until they are heated to 1640℃~1680℃, at which time all the raw materials are melted;

[0010] 3) During the melting process, a slag collecting agent is added to the surface of the molten steel to collect the impurities inside the molten steel on the surface of the medium frequency furnace, and the impurities are fished out. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities;

[0011] 4) Before pouring, 0.2 kg to 0.3 kg of aluminum wire is inserted into the molten steel for final deoxidation. After final deoxidation, the molten steel is sampled every 1 minute to test the oxygen content concentration. When the oxygen content in the molten steel is lower than 0.0050%, 0.45 kg to 0.50 kg of rare earth alloy is added to the molten steel. The rare earth alloy contains 19% of Ce and 81% of Fe, accounting for 19% of the total mass of the rare earth alloy, and the density is about 7.8 g / cm 3 , and the density is equivalent to that of steel, so it will not float above the molten steel. Preferably, in terms of mass percentage, the amount of aluminum wire used is 0.15%-0.2% of the mass of the molten steel. The burnout rate of the rare earth alloy is calculated as 25%.

[0012] 5) After adding the rare earth alloy for 1 to 2 minutes, pour it into the mold at a pouring temperature of 1580°C to 1610°C.

[0013] 6) The cast steel is kept at a temperature of 780°C to 900°C for 2.5 to 3 hours, and then water quenched. After quenching, it is kept at a temperature of 570°C to 650°C for 2.5 to 3 hours, and then air-cooled to obtain cast steel 32MnMoNiCu with improved strength and impact toughness.

[0014] The advantages of the present invention are:

[0015] 1. After adding rare earth Ce, the tensile strength of cast steel 32MnMoNiCu increased by 12.3% and the impact toughness increased by 9.2%;

[0016] 2. The strengthening effect of rare earth Ce on cast steel 32MnMoNiCu is proved by combining calculation and experiment, which provides a reliable theoretical basis for the formulation of subsequent processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 : MnS inclusions in steel;

[0018] Figure 2 : Properties of cast steel 32MnMoNiCu after adding rare earth Ce;

[0019] Figure 3 : Cast steel microstructure: (a) (c) are the microstructure of cast steel 32MnMoNiCu, (b) (d) are the microstructure of cast steel 32MnMoNiCuRe. DETAILED DESCRIPTION

[0020] The following is combined with Figure 1 To Attachment Figure 3 The present invention is described in further detail.

[0021] Embodiment 1:

[0022] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0023] 1. First, prepare the return materials and waste materials required for smelting, of which 40% are return materials and 60% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0024] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1640℃, at which time all the raw materials are melted;

[0025] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0026] 4. Before pouring, insert 0.3 kg of aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration. When the oxygen content in the molten steel is lower than 0.0050%, add 0.45 kg of rare earth alloy into the molten steel. The rare earth alloy contains 19% of Ce and 81% of Fe, accounting for 19% of the total mass of the rare earth alloy, and the density is about 7.8 g / cm 3 , which is equivalent to the density of steel, so it will not float on the molten steel. The burnout rate of rare earth alloys is calculated as 25%.

[0027] 5. Add rare earth alloy and pour into the mold after 1min to 2min, the pouring temperature is 1580℃.

[0028] 6. The cast steel is kept at a temperature of 780°C for 2.5 hours, then water quenched, kept at a temperature of 570°C for 2.5 hours after quenching, and then air-cooled to obtain cast steel 32MnMoNiCu with improved strength and impact toughness.

[0029] Embodiment 2:

[0030] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0031] 1. First, prepare the return materials and waste materials required for smelting, of which 30% are return materials and 70% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0032] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1640℃, at which time all the raw materials are melted;

[0033] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0034] 4. Before pouring, insert 0.2 kg aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration. When the oxygen content in the molten steel is lower than 0.0050%, add 0.45 kg of rare earth alloy into the molten steel. The rare earth alloy contains 19% Ce and 81% Fe of the total mass of the rare earth alloy, and the density is about 7.8 g / cm 3 , which is equivalent to the density of steel, so it will not float on the molten steel. The burnout rate of rare earth alloys is calculated as 25%.

[0035] 5. Add rare earth alloy and pour into the mold after 1min to 2min, the pouring temperature is 1580℃.

[0036] 6. The cast steel is kept at a temperature of 780°C for 3 hours, then water quenched, kept at a temperature of 570°C for 2.5 hours after quenching, and then air-cooled to obtain cast steel 32MnMoNiCu with improved strength and impact toughness.

[0037] Embodiment three:

[0038] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0039] 1. First, prepare the return materials and waste materials required for smelting, of which 30% are return materials and 70% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0040] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1680℃, at which time all the raw materials are melted;

[0041] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0042] 4. Before pouring, insert 0.2 kg aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration. When the oxygen content in the molten steel is lower than 0.0050%, add 0.45 kg of rare earth alloy into the molten steel. The rare earth alloy contains 19% Ce and 81% Fe of the total mass of the rare earth alloy, and the density is about 7.8 g / cm 3 , which is equivalent to the density of steel, so it will not float on the molten steel. The burnout rate of rare earth alloys is calculated as 25%.

[0043] 5. Add rare earth alloy and pour into the mold after 1min to 2min, the pouring temperature is 1610℃.

[0044] 6. The cast steel is kept at a temperature of 900°C for 2.5 hours, then water quenched, kept at a temperature of 650°C for 3 hours after quenching, and then air-cooled to obtain cast steel 32MnMoNiCu with improved strength and impact toughness.

[0045] Embodiment 4:

[0046] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0047] 1. First, prepare the return materials and waste materials required for smelting, of which 30% are return materials and 70% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0048] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1680℃, at which time all the raw materials are melted;

[0049] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0050] 4. Before pouring, insert 0.3 kg of aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration. When the oxygen content in the molten steel is lower than 0.0050%, add 0.5 kg of rare earth alloy into the molten steel. The rare earth alloy contains 19% of Ce and 81% of Fe, accounting for 19% of the total mass of the rare earth alloy, and the density is about 7.8 g / cm 3 , which is equivalent to the density of steel, so it will not float on the molten steel. The burnout rate of rare earth alloys is calculated as 25%.

[0051] 5. Add rare earth alloy and pour into the mold after 1min to 2min, the pouring temperature is 1610℃.

[0052] 6. The cast steel is kept at a temperature of 900°C for 3 hours, then water quenched, kept at a temperature of 570°C for 3 hours after quenching, and then air-cooled to obtain cast steel 32MnMoNiCu with improved strength and impact toughness.

[0053] Embodiment five:

[0054] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0055] 1. First, prepare the return materials and waste materials required for smelting, of which 35% are return materials and 65% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0056] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1660°C, at which time all the raw materials are melted;

[0057] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0058] 4. Before pouring, insert 0.25 kg aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration. When the oxygen content in the molten steel is lower than 0.0050%, add 0.48 kg of rare earth alloy into the molten steel. The rare earth alloy contains 19% of Ce and 81% of Fe, accounting for 19% of the total mass of the rare earth alloy, and the density is about 7.8 g / cm 3 , which is equivalent to the density of steel, so it will not float on the molten steel. The burnout rate of rare earth alloys is calculated as 25%.

[0059] 5. Add the rare earth alloy and pour it into the mold after 1min to 2min. The pouring temperature is 1600℃.

[0060] 6. The cast steel is kept at a temperature of 900° C. for 2.8 hours, and then water quenched. After quenching, it is kept at a temperature of 600° C. for 2.8 hours, and then air-cooled to obtain cast steel 32MnMoNiCu with improved strength and impact toughness.

[0061] Comparative Example 1

[0062] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0063] 1. First, prepare the return materials and waste materials required for smelting, of which 40% are return materials and 60% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0064] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1640℃, at which time all the raw materials are melted;

[0065] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0066] 4. Before pouring, insert 0.3 kg of aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration, measured in mass percentage. When the oxygen content in the molten steel is lower than 0.0050%, pour it into the mold at a pouring temperature of 1580°C.

[0067] 5. The cast steel was kept at a temperature of 780°C for 2.5 hours, then water quenched, kept at a temperature of 570°C for 2.5 hours after quenching, and then air-cooled to obtain cast steel 32MnMoNiCu.

[0068] Comparative Example 2:

[0069] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0070] 1. First, prepare the return materials and waste materials required for smelting, of which 30% are return materials and 70% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0071] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1640℃, at which time all the raw materials are melted;

[0072] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0073] 4. Before pouring, insert 0.2 kg of aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration, measured in mass percentage. When the oxygen content in the molten steel is lower than 0.0050%, pour it into the mold at a pouring temperature of 1580°C.

[0074] 5. The cast steel was kept at a temperature of 780°C for 3 hours, then water quenched, kept at a temperature of 570°C for 2.5 hours after quenching, and then air-cooled to obtain cast steel 32MnMoNiCu.

[0075] Comparative Example 3:

[0076] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0077] 1. First, prepare the return materials and waste materials required for smelting, of which 30% are return materials and 70% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0078] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1680℃, at which time all the raw materials are melted;

[0079] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0080] 4. Before pouring, insert 0.2 kg of aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration, measured in mass percentage. When the oxygen content in the molten steel is lower than 0.0050%, pour it into the mold at a pouring temperature of 1610°C.

[0081] 5. The cast steel was kept at 900°C for 2.5 hours, then water quenched, kept at 650°C for 3 hours after quenching, and then air-cooled to obtain cast steel 32MnMoNiCu.

[0082] Comparative Example 4:

[0083] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0084] 1. First, prepare the return materials and waste materials required for smelting, of which 30% are return materials and 70% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0085] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1680℃, at which time all the raw materials are melted;

[0086] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0087] 4. Before pouring, insert 0.3 kg of aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration, measured in mass percentage. When the oxygen content in the molten steel is lower than 0.0050%, pour it into the mold at a pouring temperature of 1610°C.

[0088] 5. The cast steel is kept at a temperature of 900°C for 3 hours, and then water quenched. After quenching, it is kept at a temperature of 570°C for 3 hours, and then air-cooled to obtain cast steel 32MnMoNiCu.

[0089] Comparative Example 5

[0090] The detailed process of improving the strength and impact toughness of cast steel 32MnMoNiCu is implemented in the following six steps:

[0091] 1. First, prepare the return materials and waste materials required for smelting, of which 35% are return materials and 65% are waste materials. The weight of molten steel in a single furnace is 150Kg;

[0092] 2. Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. The power is continuously increased during the heating process until it is heated to 1660°C, at which time all the raw materials are melted;

[0093] 3. During the melting process, slag collecting agent is added to the surface of the molten steel to gather the impurities inside the molten steel on the surface of the medium frequency furnace. The impurities are fished out by tools such as spoons. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; the impurities are negligible relative to the mass of the molten steel.

[0094] 4. Before pouring, insert 0.25 kg of aluminum wire into the molten steel for final deoxidation. After final deoxidation, sample the molten steel every 1 minute to test the oxygen content concentration, measured in mass percentage. When the oxygen content in the molten steel is lower than 0.0050%, pour it into the mold at a pouring temperature of 1600°C.

[0095] 5. The cast steel was kept at 900°C for 2.8 hours and then water quenched. After quenching, it was kept at 600°C for 2.8 hours and then air-cooled to obtain cast steel 32MnMoNiCu.

[0096] The present invention redesigns the chemical composition of cast steel 32MnMoNiCu, adds rare earth Ce to cast steel 32MnMoNiCu, and the chemical compositions of the two cast steels before and after the improvement are shown in Table 1. By testing the change law of oxygen content over time after final deoxidation in a medium frequency furnace, the time period with low oxygen content is determined, and an appropriate amount of rare earth alloy is added. The Ce in the rare earth alloy reacts with the MnS inclusions formed in the molten steel to achieve the modification of the MnS inclusions, such as Figure 1 shown.

[0097] The properties of cast steel after adding rare earth are calculated using thermodynamic software Thermo-Calc. Figure 2 As shown, according to Figure 2 The quenching temperature range of cast steel is set at 780℃~1450℃, and the high temperature tempering temperature range is 506℃~690℃. Considering the recrystallization temperature range of cast steel 32MnMoNiCu, the quenching temperature range is determined to be 780℃~900℃, and the high temperature tempering temperature range is 570℃~650℃.

[0098] Table 1 Chemical composition of cast steel (%, mass fraction)

[0099]

[0100] According to the chemical composition and heat treatment process in Table 1, the two cast steels before and after the improvement were smelted, cast and heat treated to obtain the cast steel in the final heat treatment state. The microstructure analysis and mechanical property test of the two cast steels were carried out. Figure 3 As shown in Table 2, the ferrite structure size of cast steel 32MnMoNiCu is relatively large, and the ferrite structure size is significantly reduced by adding rare earth Ce. After adding rare earth, the proportion of inclusions in cast steel is reduced, and the average size is reduced from 14.8μm to 4.2μm, and the inclusions are spherical, which reduces the stress concentration around the inclusions. The average values ​​of the mechanical property test results of cast steel prepared by five comparative examples and five embodiments are shown in Table 2. It can be found that after adding rare earth Ce to cast steel 32MnMoNiCu, the strength and impact properties of cast steel are improved. In order to further verify the strengthening effect of rare earth Ce on cast steel, the Hall-Petch equation is used to analyze the strengthening mechanism.

[0101] Table 2 Average values ​​of the mechanical properties test results of the cast steel prepared in five comparative examples and five embodiments

[0102]

[0103] The chemical compositions of the two cast steels differed from each other except for a small amount of rare earth Ce, while the contents of other elements were basically the same, so the strengthening effect of solid solution strengthening on cast steel was ignored. The above results show that after adding rare earth Ce to cast steel, the grain size of cast steel decreased, which played a role in fine grain strengthening of cast steel. In addition to fine grain strengthening, the proportion and size changes of inclusions in steel had a certain influence on the properties of cast steel.

[0104] According to the Hall-Petch equation, the role of inclusions in steel is mainly precipitation strengthening. The precipitation strengthening is calculated by the Gladman-Ashby-Orown equation. Except for the difference in the trace rare earth Ce, the content of other elements in the chemical composition of the two cast steels is basically the same, so the strengthening effect of solid solution strengthening on cast steel is ignored. The above results show that after adding rare earth Ce to cast steel, the grain size of cast steel is reduced, which plays a role in fine grain strengthening of cast steel. In addition to fine grain strengthening, the proportion and size changes of inclusions in steel have a certain influence on the performance of cast steel.

[0105] According to the Hall-Petch equation, the role of inclusions in steel is mainly precipitation strengthening. The precipitation strengthening σ is calculated by the Gladman-Ashby-Orown equation. py :

[0106]

[0107] Where D is the diameter of the inclusion and L is:

[0108]

[0109] Where f is the volume fraction of the precipitate.

[0110] According to the above analysis, after adding rare earth Ce, the proportion and size of inclusions in the cast steel decreased. The inclusion diameter D and proportion f were statistically analyzed through microstructure photos, and the results are shown in Table 3. According to formulas (1) and (2), the precipitation strengthening values ​​of inclusions in the two cast steels were calculated. Through calculation, the precipitation strengthening values ​​of cast steel 32MnMoNiCu before improvement and 32MnMoNiCuRe after improvement were 98.3MPa and 110.3MPa, respectively, indicating that after adding rare earth Ce, the size of inclusions decreased, but the cast steel was strengthened.

[0111] Table 3 The data of precipitation strengthening

[0112]

[0113] The present invention accurately controls the adding time of rare earth alloy by accurately testing the changing relationship between the oxygen content concentration of the medium frequency furnace and time. Adding the alloy too early or too late will cause serious oxidation of the rare earth Ce and poor effect. The heat treatment tempering temperature is designed to be controlled. Too high a temperature will lead to reduced strength, and too low a tempering temperature will lead to reduced impact toughness.

Claims

1. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu, characterized in that: The following steps are involved: 1) First, prepare the return material and waste material of cast steel 32MnMoNiCu required for smelting, wherein the ratio of the return material to the waste material is 30% to 40%: 60% to 70% in terms of the mass percentage of the total mass of the two; 2) Add the returned materials and waste materials into the medium frequency induction furnace, and then start heating. During the heating process, the power is continuously increased to make the returned materials and waste materials melt evenly until they are heated to 1640℃~1680℃, at which time all the raw materials are melted; 3) During the melting process, a slag collecting agent is added to the surface of the molten steel to collect the impurities inside the molten steel on the surface of the medium frequency furnace, and the impurities are fished out. The slag collecting process is repeated many times until the surface of the molten steel is visually pure and free of impurities; 4) Before pouring, a certain amount of aluminum wire is inserted into the molten steel for final deoxidation. When the oxygen content in the molten steel is lower than 0.0050% by mass, a certain amount of rare earth alloy is added to the molten steel, wherein the rare earth alloy contains 19% of Ce and 81% of Fe accounting for the total mass of the rare earth alloy; 5) After adding rare earth alloy, pouring into a mold at a pouring temperature of 1580°C to 1610°C; 6) The cast steel is kept at a temperature of 780°C to 900°C, and then water quenched. After quenching, it is kept at a temperature of 570°C to 650°C, and then air-cooled to obtain cast steel 32MnMoNiCu with improved strength and impact toughness.

2. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu as claimed in claim 1, characterized in that: In step 4), the amount of aluminum wire used is 0.15%-0.2% of the mass of the molten steel in terms of mass percentage.

3. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu as claimed in claim 2, characterized in that: In step 4), the method for testing the oxygen content concentration of the molten steel is as follows: after final deoxidation, the molten steel is sampled and tested for oxygen content concentration every 1 minute.

4. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu as claimed in claim 3, characterized in that: In step 4), the amount of rare earth alloy added is 0.30% to 0.33% by mass of the molten steel.

5. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu as claimed in claim 4, characterized in that: In step 5), the rare earth alloy is added for 1 to 2 minutes and then poured into a mold.

6. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu as claimed in claim 5, characterized in that: In step 6), the cast steel is kept at a temperature of 780°C to 900°C for 2.5 to 3 hours.

7. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu as claimed in claim 6, characterized in that: In step 6), after quenching, the temperature is kept at 570°C to 650°C for 2.5 to 3 hours.

8. A method for improving the strength and impact toughness of cast steel 32MnMoNiCu as claimed in claim 7, characterized in that: After adding rare earth Ce, the tensile strength of cast steel 32MnMoNiCu increased by 12.3% and the impact toughness increased by 9.2%.

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