Nb-Ti composite microalloyed non-quenched and tempered steel and preparation method thereof

Abstract

The invention discloses Nb-Ti composite microalloyed non-quenched and tempered steel and a preparation method thereof, and belongs to the technical field of steel and iron material manufacturing. The non-quenched and tempered steel comprises the following chemical components in percentage by mass: 0.23 to 0.28 percent of C, 0.25 to 0.50 percent of Si, 1.20 to 1.60 percent of Mn, 0.06 to 0.20 percent of V, 0.015 to 0.025 percent of Nb, 0.010 to 0.015 percent of Ti, 0.01 to 0.02 percent of N, less than or equal to 0.025 percent of P, less than or equal to 0.015 percent of S and the balance of Fe and inevitable impurities. Through Nb-Ti microalloying design, the effects of precipitation strengthening and fine grain strengthening are achieved, a controlled rolling and controlled cooling process is combined to regulate and control the structure, the high strengthening and toughening effect is formed, a microscopic structure composed of ferrite and pearlite is obtained, and the excellent mechanical property is obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material manufacturing, and relates to a Nb-Ti composite microalloyed non-quenched and tempered steel and a preparation method thereof. Background Art

[0002] Non-quenched and tempered steel is a steel with mechanical properties equivalent to those of quenched and tempered steel that is obtained by adding microalloying elements to traditional medium carbon steel and controlling rolling (forging) and cooling to make the steel material without quenching and tempering after rolling (forging). Since non-quenched and tempered steel eliminates the quenching and tempering treatment (quenching + high temperature tempering), its application can save about 700 to 900 kW·h / t of electricity consumption during the production process, reduce energy consumption by 30% to 40%, reduce manufacturing costs by about 25% to 38%, and greatly improve the yield of workpieces. However, due to its poor strength and toughness, the application of non-quenched and tempered steel is limited to crankshafts, connecting rods and other parts that do not require high impact toughness. With the global shortage of resources, the development of non-quenched and tempered steel with good strength and toughness has become an inevitable development direction.

[0003] In order to improve the strength and toughness of non-quenched and tempered steel and further expand its application range, it is necessary to add microalloying elements to the steel. Non-quenched and tempered steel manufactured by microalloying and controlled forging and controlled cooling technology can not only ensure the strength and toughness of the material, but also simplify the production process and improve the material utilization rate. Therefore, non-quenched and tempered steel has broad market prospects and development prospects, and is mainly used in forgings in the automotive and mechanical equipment manufacturing industries.

[0004] The patent application with application number 202310049107.4 discloses a non-quenched and tempered high-strength steel and its preparation process. The composition of the non-quenched and tempered high-strength steel provided by the invention is: C: 0.25%~0.35%, Si: 0.05%~0.15%, Mn: 2.5%~3.0%, P: 0.03%~0.10%, S≤0.02%, Cr: 0.75%~1.25%, Ni: 0.35%~0.65%, Mo: 0.3%~0.6%, V: 0.2%~0.6%, B: 0.0005%~0.0015%, and the balance is iron and unavoidable impurities. The non-quenched and tempered high-strength steel of the invention has the advantages of high strength and hardness. The yield strength can reach 1758MPa, the tensile strength can reach 1800MPa, and the HRC hardness can reach 46, which can solve the problem of low strength of existing non-quenched and tempered steel. However, Nb and Ti elements are not added to the composition for Nb-Ti composite microalloying, and its elongation after fracture is low, and a good combination of strength and toughness cannot be achieved.

[0005] The patent application with application number 202111396800.6 discloses a non-quenched and tempered steel that replaces normalizing heat-treated 45 steel and its application. The non-quenched and tempered steel composition provided by the invention that replaces normalizing heat-treated 45 steel is: C: 0.32-0.39%, Si: 0.17-0.37%, Mn: 0.50-0.80%, Cr≤0.30%, V: 0.04-0.10%, Ti: 0.01-0.04%, P≤0.020%, S≤0.035%, Alt: 0.01-0.04%, Cu≤0.20%, Ni≤0.20%, Mo≤0.10%, O≤15×10 -4 %, and the balance is Fe and unavoidable impurities. This invention adds V and Ti microalloying elements to the C-Mn structural steel composition; VC and TiC are precipitated to play a precipitation strengthening role, and the composite addition of V and Ti can improve the strength and toughness of the steel, and improve the contradiction of insufficient toughness of ferrite pearlite non-quenched and tempered steel; and by controlling the Ti content and the range of carbon equivalent, the amount of V added is reduced while ensuring the forging performance of the material, and the performance requirements of the forgings can be met without nitrogen addition, effectively reducing the manufacturing cost. However, the Nb element is not added to the composition for Nb-Ti composite microalloying. Therefore, the tensile strength and yield strength of the non-quenched and tempered steel obtained by this invention are low, and excellent mechanical properties cannot be obtained. Summary of the invention

[0006] In order to solve the above technical problems, the present invention provides a Nb-Ti composite microalloyed non-quenched and tempered steel and a preparation method thereof. The mechanical properties of the non-quenched and tempered steel are improved by designing the composition of the Nb-Ti composite microalloying. In combination with the optimization of the preparation method, the microalloying effect is further improved, and a uniform ferrite + pearlite structure can be obtained.

[0007] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is: A Nb-Ti composite microalloyed non-quenched and tempered steel, the chemical composition and mass percentage of which are: C: 0.23-0.28%, Si: 0.25-0.50%, Mn: 1.20-1.60%, V: 0.06-0.20%, Nb: 0.015-0.025%, Ti: 0.010-0.015%, N: 0.01-0.02%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurities.

[0008] Furthermore, the microstructure of the non-quenched and tempered steel is ferrite+pearlite.

[0009] Furthermore, the non-quenched and tempered steel has a tensile strength of ≥700 MPa, a yield strength of ≥500 MPa, and an elongation after fracture of ≥20%.

[0010] For ferrite pearlite non-quenched and tempered steel, the main factors controlling mechanical properties are the phase ratio of pearlite to ferrite, ferrite grain size, pearlite interlamellar spacing, and precipitation strengthening effect of microalloying elements. The design concept for the development of the chemical composition of new high-strength and tough non-quenched and tempered steel is: using conventional medium carbon manganese steel as the matrix, adding appropriate amounts of microalloying elements, and adopting controlled rolling and controlled cooling processes during rolling to achieve fine grain strengthening and precipitation strengthening to obtain excellent mechanical properties.

[0011] The content of C element has a crucial influence on the hardness and strength of steel, but when adding C element, its content must be strictly controlled, otherwise it will have a serious impact on the plasticity and toughness of non-quenched and tempered steel. The C content of medium carbon non-quenched and tempered steel is usually not allowed to exceed 0.5%, and the C content of the steel designed by the present invention is 0.23-0.28%.

[0012] Mn is one of the most important elements in medium carbon non-quenched and tempered steel. Appropriate addition of Mn to non-quenched and tempered steel can effectively improve its strength and toughness. However, it should be noted that the Mn content in the steel should not be too high, otherwise bainite will be formed in the structure after quenching, thereby affecting the plasticity and toughness of the steel. For medium carbon non-quenched and tempered steel, its Mn content generally does not exceed 1.5%. The Mn content of the steel designed by the present invention is 1.20-1.60%.

[0013] Si exists in the form of solid solution in non-quenched and tempered steel and is an important ferrite strengthening element. Like the effect of excessive C element on steel, excessive Si will have a serious impact on the toughness of steel. The Si content of the steel designed in the present invention is 0.25-0.50%.

[0014] Compared with Nb and Ti, the precipitation temperature of V in non-quenched and tempered steel is lower, which is mainly manifested as interphase precipitation during the γ-α transformation process and precipitation in ferrite, which improves the strength level of steel by precipitation hardening. The efficient strengthening effect of V provides a basic guarantee for the high strengthening of non-quenched and tempered steel, but too high V will deteriorate the toughness. In order to meet the performance requirements of high-strength non-quenched and tempered steel, the content of alloying elements must be controlled. Depending on the strength level, the addition amount of V in non-quenched and tempered steel is generally 0.06-0.20%, and the addition amount of Ti is 0.010-0.015%.

[0015] For medium carbon non-quenched and tempered steel, the role of Nb is not limited to refining austenite grains, but also includes refining ferrite grains and improving the morphology of pearlite lamellae. The addition of Nb in non-quenched and tempered steel can also promote the transformation of pearlite lamellae from neatly parallel arrangement to spheroidization and disordered morphology, thereby improving the plastic toughness of the steel. The Nb addition amount of the steel designed by the present invention is 0.015-0.025%.

[0016] Adding N to V-containing steel can promote the precipitation of V, enhance the precipitation strengthening effect of V, and significantly improve the strength of non-quenched and tempered steel. Therefore, using cheap N elements to improve the toughness of steel is a very effective method to reduce costs. The N addition amount of the steel designed by the present invention is 0.01-0.02%.

[0017] On the other hand, the present invention also provides a method for preparing the above-mentioned Nb-Ti composite microalloyed non-quenched and tempered steel, which comprises a smelting process, a LF refining process, a VD vacuum degassing process, a continuous casting process, and a hot rolling process; In the hot rolling process, the steel is put on a cooling bed after rolling, cooled to 500-600°C at a cooling rate of 0.1-0.5°C / s, and then taken off the cooling bed, and then air-cooled in a well-ventilated place.

[0018] Furthermore, the smelting process adopts electric furnace smelting, and the molten iron ratio is controlled to be ≥40%; the smelting end point C≥0.10wt%, P≤0.010wt%; before steel tapping, 1.3-1.6kg / t of aluminum blocks are added, and 9-12kg / t of slag materials are added; 19-21kg / t of metallic manganese and 3.5-4.5kg / t of ferrosilicon are added for alloying during the steel tapping process.

[0019] Furthermore, in the LF refining process, an aluminum wire is fed for deoxidation before taking a sample for analysis; a pure calcium wire is fed for calcium treatment before tapping; the basicity of the refined slag is 3.0-4.0, and the white slag retention time is 20-25 minutes.

[0020] Furthermore, in the VD vacuum degassing process, the slag is removed before entering the VD furnace, the vacuum degree is less than 67Pa and maintained for ≥12 minutes, and the argon flow rate is 50-70NL / min; after breaking the vacuum, 3.0-5.0m / t of manganese nitride wire steel is fed, the argon flow rate is 10-20NL / min, the soft argon blowing time is not less than 15 minutes, and the steel temperature of the normal furnace is controlled at 1550-1580°C.

[0021] Furthermore, the continuous casting process adopts full-process protection pouring, the temperature of the molten steel in the tundish is controlled at 1510-1525°C, and the continuous casting speed is 0.50-0.55m / min; electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted.

[0022] Furthermore, in the hot rolling process, the uniform heating temperature of the steel billet in the heating furnace is 1130-1180°C, and the heating time is 2-2.5h; the starting rolling temperature is 1000-1100°C, and the finishing rolling temperature is 930-980°C.

[0023] The beneficial effect of adopting the above technical solution is that: the present invention adopts Nb-Ti composite microalloying design, Nb (C, N) can precipitate in austenite and at subgrain boundaries, prevent the original austenite grains from growing, and can also improve the strength of steel by refining the pearlite interlamellar spacing. Due to its high solid solution temperature, Ti can effectively refine the grains by forming carbonitrides and preventing grain growth at high temperatures. Through Nb-Ti composite microalloying, the cooling method and cooling rate of microalloyed non-quenched and tempered steel after rolling are controlled, and combined with controlled rolling and controlled cooling processes, excellent comprehensive mechanical properties are obtained. The non-quenched and tempered steel of the present invention has a tensile strength of ≥700MPa, a yield strength of ≥500MPa, and an elongation after fracture of ≥20%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 3 is a microstructure diagram of the non-quenched and tempered steel obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0025] The present invention will be further described in detail below in conjunction with specific embodiments.

[0026] Example 1 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 41%; the C content at the end of the electric furnace is 0.10%, and the P content is 0.008%. Before steel tapping, 1.35kg / t of aluminum blocks are added, and 9.3kg / t of slag materials are added; 19kg / t of metallic manganese and 3.5kg / t of ferrosilicon are added for alloying during the steel tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 3.1, and the white slag retention time is 20 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 13 minutes, and the argon flow rate is 50NL / min; after breaking the air, the amount of manganese nitride wire fed is 3.0m / t steel, the argon flow rate is 16NL / min, the soft argon blowing time is 15 minutes, and the upper steel temperature is 1553℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1512℃; the continuous casting speed is 0.50m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1130℃, the heating time is 2h, the starting rolling temperature is 1030℃, and the final rolling temperature is 930℃; after rolling, the steel is placed on a cooling bed with a cooling rate of 0.5℃ / s, and the temperature of the steel on the lower cooling bed is 500℃. After leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0027] The non-quenched and tempered steel prepared in this embodiment was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.25%, Si was 0.35%, Mn was 1.39%, V was 0.12%, Nb was 0.019%, Ti was 0.010%, N was 0.017%, S was 0.0063%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0028] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1; its metallographic structure is as follows Figure 1 As shown, it is medium carbon pearlite and ferrite.

[0029] Example 2 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 42%; the C content at the end of the electric furnace is 0.12%, and the P content is 0.008%. Before steel tapping, 1.3kg / t of aluminum blocks are added, and 9.5kg / t of slag materials are added; 19kg / t of metallic manganese and 3.5kg / t of ferrosilicon are added for alloying during the steel tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 3.0, and the white slag retention time is 22 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 13 minutes, and the argon flow rate is 53NL / min; after breaking the air, the amount of manganese nitride wire fed is 3.3m / t steel, the argon flow rate is 10NL / min, the soft argon blowing time is 15 minutes, and the upper steel temperature is 1560℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1515℃; the continuous casting speed is 0.50m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1142℃, the heating time is 2h, the starting rolling temperature is 1035℃, and the final rolling temperature is 950℃; after rolling, the steel is placed on the cooling bed with a cooling rate of 0.3℃ / s, and the temperature of the steel on the lower cooling bed is 530℃. After leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0030] The non-quenched and tempered steel prepared in this embodiment was tested for composition. The test results were as follows, in terms of mass percentage: C was 0.26%, Si was 0.40%, Mn was 1.40%, V was 0.14%, Nb was 0.020%, Ti was 0.012%, N was 0.014%, S was 0.0054%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0031] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1. Its metallographic structure is similar to that in Example 1, so it is omitted.

[0032] Example 3 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 41%; the C content at the end of the electric furnace is 0.11%, and the P content is 0.009%. Before steel tapping, 1.4kg / t of aluminum blocks are added, and 9kg / t of slag materials are added; 19.4kg / t of metallic manganese and 3.5kg / t of ferrosilicon are added for alloying during the steel tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 3.4, and the white slag retention time is 20 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 15 minutes, and the argon flow rate is 70NL / min; after breaking the air, the amount of manganese nitride wire fed is 3.0m / t steel, the argon flow rate is 10NL / min, the soft argon blowing time is 20 minutes, and the upper steel temperature is 1565℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1515℃; the continuous casting speed is 0.52m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1150℃, the heating time is 2h, the starting rolling temperature is 1050℃, and the final rolling temperature is 950℃; after rolling, the steel is placed on the cooling bed with a cooling rate of 0.4℃ / s, and the temperature of the steel on the lower cooling bed is 550℃. After leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0033] The non-quenched and tempered steel prepared in this embodiment was tested for composition. The test results were as follows, in terms of mass percentage: C was 0.23%, Si was 0.31%, Mn was 1.35%, V was 0.11%, Nb was 0.020%, Ti was 0.010%, N was 0.014%, S was 0.0060%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0034] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1. Its metallographic structure is similar to that in Example 1, so it is omitted.

[0035] Example 4 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 42%; the C content at the end of the electric furnace is 0.12%, and the P content is 0.008%. Before steel tapping, 1.3kg / t of aluminum blocks are added, and 10kg / t of slag materials are added; 21kg / t of metallic manganese and 4.5kg / t of ferrosilicon are added for alloying during the steel tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 3.0, and the white slag retention time is 20 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 13 minutes, and the argon flow rate is 50NL / min; after breaking the air, the amount of manganese nitride wire fed is 5.0m / t steel, the argon flow rate is 14NL / min, the soft argon blowing time is 15 minutes, and the upper steel temperature is 1560℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1520℃; the continuous casting speed is 0.51m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1160℃, the heating time is 2.5h, the starting rolling temperature is 1060℃, and the final rolling temperature is 950℃; after rolling, the steel is placed on the cooling bed with a cooling rate of 0.4℃ / s, and the temperature of the steel on the lower cooling bed is 540℃. After leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0036] The non-quenched and tempered steel prepared in this embodiment was tested for composition. The test results were as follows, in terms of mass percentage: C was 0.28%, Si was 0.30%, Mn was 1.45%, V was 0.15%, Nb was 0.018%, Ti was 0.015%, N was 0.014%, S was 0.0054%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0037] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1. Its metallographic structure is similar to that in Example 1, so it is omitted.

[0038] Example 5 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 40%; the C content at the end of the electric furnace is 0.10%, and the P content is 0.008%. Before steel tapping, 1.5kg / t of aluminum blocks are added, and 12kg / t of slag materials are added; 20kg / t of metallic manganese and 3.7kg / t of ferrosilicon are added for alloying during the steel tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 3.0, and the white slag retention time is 25 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 20 minutes, and the argon flow rate is 55NL / min; after breaking the air, the amount of manganese nitride wire fed is 3.0m / t steel, the argon flow rate is 13NL / min, the soft argon blowing time is 15 minutes, and the upper steel temperature is 1580℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1520℃; the continuous casting speed is 0.50m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1177℃, the heating time is 2h, the starting rolling temperature is 1100℃, and the final rolling temperature is 980℃; after rolling, the steel is placed on a cooling bed with a cooling rate of 0.5℃ / s, the temperature of the steel on the lower cooling bed is 600℃, and after leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0039] The non-quenched and tempered steel prepared in this embodiment was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.25%, Si was 0.40%, Mn was 1.50%, V was 0.16%, Nb was 0.021%, Ti was 0.015%, N was 0.018%, S was 0.0045%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0040] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1. Its metallographic structure is similar to that in Example 1, so it is omitted.

[0041] Example 6 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 43%; the C content at the end of the electric furnace is 0.10%, and the P content is 0.009%. Before steel tapping, 1.3kg / t of aluminum blocks are added, and 9kg / t of slag materials are added; 19kg / t of metallic manganese and 3.5kg / t of ferrosilicon are added for alloying during the steel tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 3.6, and the white slag retention time is 23 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 15 minutes, and the argon flow rate is 60NL / min; after breaking the vacuum, the amount of manganese nitride wire fed is 4.0m / t steel, the argon flow rate is 10NL / min, the soft argon blowing time is 20 minutes, and the upper steel temperature is 1565℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1517℃; the continuous casting speed is 0.52m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1150℃, the heating time is 2h, the starting rolling temperature is 1050℃, and the final rolling temperature is 945℃; after rolling, the steel is placed on the cooling bed with a cooling rate of 0.4℃ / s, and the temperature of the steel on the lower cooling bed is 560℃. After leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0042] The non-quenched and tempered steel prepared in this embodiment was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.23%, Si was 0.25%, Mn was 1.20%, V was 0.20%, Nb was 0.025%, Ti was 0.015%, N was 0.012%, S was 0.0061%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0043] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1. Its metallographic structure is similar to that in Example 1, so it is omitted.

[0044] Example 7 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 42%; the C content at the end of the electric furnace is 0.12%, and the P content is 0.008%. Before tapping, 1.3kg / t of aluminum blocks are added, and 10.6kg / t of slag materials are added; 20.2kg / t of metallic manganese and 4.0kg / t of ferrosilicon are added for alloying during the tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 3.0, and the white slag retention time is 20 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 13 minutes, and the argon flow rate is 50NL / min; after breaking the air, the amount of manganese nitride wire fed is 3.0m / t steel, the argon flow rate is 20NL / min, the soft argon blowing time is 17 minutes, and the upper steel temperature is 1570℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1525℃; the continuous casting speed is 0.55m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1160℃, the heating time is 2.5h, the starting rolling temperature is 1060℃, and the final rolling temperature is 960℃; after rolling, the steel is placed on the cooling bed with a cooling rate of 0.1℃ / s, and the temperature of the steel on the lower cooling bed is 550℃. After leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0045] The non-quenched and tempered steel prepared in this embodiment was tested for composition. The test results were as follows, in terms of mass percentage: C was 0.28%, Si was 0.50%, Mn was 1.60%, V was 0.06%, Nb was 0.015%, Ti was 0.011%, N was 0.010%, S was 0.0054%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0046] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1. Its metallographic structure is similar to that in Example 1, so it is omitted.

[0047] Example 8 The Nb-Ti composite microalloyed non-quenched and tempered steel of this embodiment is produced by an electric furnace continuous casting process, including an electric furnace smelting process, an LF refining process, a VD vacuum degassing process, a continuous casting process and a hot rolling process, wherein: Electric furnace smelting process: electric furnace smelting is adopted, the main raw materials are molten iron + scrap steel, and the proportion of molten iron is 41%; the C content at the end of the electric furnace is 0.10%, and the P content is 0.008%. Before steel tapping, 1.6kg / t of aluminum blocks are added, and 9kg / t of slag materials are added; 19.2kg / t of metallic manganese and 3.6kg / t of ferrosilicon are added for alloying during the steel tapping process, and V, Nb, and Ti elements are added during the electric furnace smelting process; LF refining process: feeding aluminum wire for deoxidation before taking a sample for analysis; feeding pure calcium wire for calcium treatment before tapping the LF furnace; the basicity of the refined slag is 4.0, and the white slag retention time is 21 minutes; VD vacuum degassing process: slag is removed before entering the VD furnace, the vacuum degree is 65Pa and the time is maintained for 13 minutes, and the argon flow rate is 50NL / min; after breaking the air, the amount of manganese nitride wire fed is 3.6m / t steel, the argon flow rate is 10NL / min, the soft argon blowing time is 15 minutes, and the upper steel temperature is 1555℃; Continuous casting process: The continuous casting process is protected and poured, the temperature of the molten steel in the tundish is 1512℃; the continuous casting speed is 0.50m / min; and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted; Hot rolling process: the uniform heating temperature of the steel billet in the heating furnace is 1130℃, the heating time is 2h, the starting rolling temperature is 1030℃, and the final rolling temperature is 930℃; after rolling, the steel is placed on a cooling bed with a cooling rate of 0.5℃ / s, and the temperature of the steel on the lower cooling bed is 500℃. After leaving the cooling bed, it is placed in a well-ventilated place for air cooling.

[0048] The non-quenched and tempered steel prepared in this embodiment was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.25%, Si was 0.25%, Mn was 1.41%, V was 0.15%, Nb was 0.018%, Ti was 0.014%, N was 0.016%, S was 0.0072%, and the remainder was Fe and unavoidable impurities, with P ≤ 100 ppm among the impurities.

[0049] The performance test results of the non-quenched and tempered steel in this embodiment are shown in Table 1; its metallographic structure is as follows Figure 1 As shown, it is medium carbon pearlite and ferrite.

[0050] Table 1 Test results of non-quenched and tempered steel properties in various examples

[0051] In summary, the present invention allows Ti to exist in the form of carbonitride in Nb-Ti composite microalloyed non-adjustable steel, which can prevent the growth of austenite and refine the grains. The added Nb and V can improve the strength of the steel and the toughness of the steel. This is because V has a low solid solution temperature and can play a precipitation strengthening role, while Nb is mostly insoluble at ordinary forging heating temperatures and can play a role in refining the grains. At the same time, the addition of Ti can increase the size of the precipitate (Ti, V) (C, N), thereby promoting the formation of intragranular ferrite (IGF). In this case, the formation of IGF can effectively divide and refine the structure, thereby improving the toughness of the steel.

[0052] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A Nb-Ti composite microalloyed non-quenched and tempered steel, characterized in that: Its chemical composition and mass percentage are: C: 0.23-0.28%, Si: 0.25-0.50%, Mn: 1.20-1.60%, V: 0.06-0.20%, Nb: 0.015-0.025%, Ti: 0.010-0.015%, N: 0.01-0.02%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurities.

2. The Nb-Ti composite microalloyed non-quenched and tempered steel according to claim 1, characterized in that: The microstructure of the non-quenched and tempered steel is ferrite+pearlite.

3. The Nb-Ti composite microalloyed non-quenched and tempered steel according to claim 1 or 2, characterized in that: The non-quenched and tempered steel has a tensile strength of ≥700MPa, a yield strength of ≥500MPa, and an elongation after fracture of ≥20%.

4. The method for preparing the Nb-Ti composite microalloyed non-quenched and tempered steel according to any one of claims 1 to 3, characterized in that: Including smelting process, LF refining process, VD vacuum degassing process, continuous casting process, hot rolling process; In the hot rolling process, the steel is put on a cooling bed after rolling, cooled to 500-600°C at a cooling rate of 0.1-0.5°C / s, and then taken off the cooling bed, and then air-cooled in a well-ventilated place.

5. The method for preparing the Nb-Ti composite microalloyed non-quenched and tempered steel according to claim 4, characterized in that: The smelting process adopts electric furnace smelting, controls the molten iron ratio to be ≥40%; the smelting end point C is ≥0.10wt%, and P is ≤0.010wt%; before steel tapping, adds 1.3-1.6kg / t of aluminum blocks and 9-12kg / t of slag materials; and during the steel tapping process, adds 19-21kg / t of metallic manganese and 3.5-4.5kg / t of ferrosilicon for alloying.

6. The method for preparing the Nb-Ti composite microalloyed non-quenched and tempered steel according to claim 5, characterized in that: In the LF refining process, aluminum wire is fed for deoxidation before taking a sample for analysis; pure calcium wire is fed for calcium treatment before tapping; the basicity of the refined slag is 3.0-4.0, and the white slag retention time is 20-25 minutes.

7. The method for preparing the Nb-Ti composite microalloyed non-quenched and tempered steel according to claim 6, characterized in that: In the VD vacuum degassing process, the slag is removed before entering the VD furnace, the vacuum degree is less than 67Pa and maintained for ≥12 minutes, and the argon flow rate is 50-70NL / min; after breaking the vacuum, 3.0-5.0m / t of manganese nitride wire steel is fed, the argon flow rate is 10-20NL / min, the soft argon blowing time is not less than 15 minutes, and the steel temperature of the normal furnace is controlled at 1550-1580°C.

8. The method for preparing the Nb-Ti composite microalloyed non-quenched and tempered steel according to claim 7, characterized in that: The continuous casting process adopts continuous casting full-process protection pouring, the temperature of molten steel in the tundish is controlled at 1510-1525° C., the continuous casting speed is 0.50-0.55 m / min, and electromagnetic stirring of the crystallizer and electromagnetic stirring at the end of solidification are adopted.

9. The method for preparing Nb-Ti composite microalloyed non-quenched and tempered steel according to any one of claims 4 to 8, characterized in that: In the hot rolling process, the uniform heating temperature of the steel billet in the heating furnace is 1130-1180° C., and the heating time is 2-2.5 hours.

10. The method for preparing the Nb-Ti composite microalloyed non-quenched and tempered steel according to claim 9, characterized in that: In the hot rolling process, the starting rolling temperature is 1000-1100°C and the final rolling temperature is 930-980°C.

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