High-strength and high-toughness bainite type non-quenched and tempered steel and preparation method thereof

Abstract

The invention discloses high-strength and high-toughness bainite type 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: 0.22 to 0.28 percent of C, 0.20 to 0.40 percent of Si, 1.80 to 2.10 percent of Mn, 0.08 to 0.15 percent of V, 0.40 to 0.60 percent of Cr, 0.016 to 0.030 percent of Nb, 0.010 to 0.025 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. The V, Nb and Ti elements are added, carbonitride is formed and separated out, the structure is regulated and controlled in combination with the controlled rolling and controlled cooling process, the high strengthening and toughening effect is formed, and the V opening impact energy of the obtained non-quenched and tempered steel at the room temperature of 20 DEG C can reach 50 J or above.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material manufacturing, and relates to a high-strength and toughness bainite type non-quenched and tempered steel and a preparation method thereof. Background Art

[0002] Non-quenched and tempered steel is made by adding micro-alloying elements such as V, Nb, Ti to medium-low carbon (0.2~0.5wt%C) steel, and refining the grains through controlled rolling (forging) and controlled cooling processes. At the same time, dispersed carbonitrides are obtained as strengthening phases in the microstructure of ferrite and pearlite, thereby obtaining mechanical properties similar to those of quenched and tempered steel and eliminating the modulation treatment process.

[0003] The mechanical properties of traditional non-quenched and tempered steel are close to those of quenched and tempered steel at room temperature, but still cannot reach the comprehensive mechanical properties of 42CrMo quenched and tempered steel. At present, domestic medium-carbon ferrite-pearlite non-quenched and tempered steel has been successfully applied in batches on engine crankshafts and connecting rods, but its further promotion and application in safety components such as steering knuckles, half shafts, stabilizer bars, and new energy vehicle motor shafts is hindered. The main problem is to improve and stabilize strength and toughness while controlling alloy costs. Compared with ferrite-pearlite non-quenched and tempered steel, bainite non-quenched and tempered steel has better comprehensive mechanical properties at the same strength level.

[0004] At present, all major steel production enterprises and automobile parts manufacturers are vigorously developing low-cost, high-strength and toughness non-quenched and tempered steels, especially bainitic non-quenched and tempered steels with good comprehensive mechanical properties.

[0005] The patent application with application number 202310177106.8 discloses a bainitic non-quenched and tempered steel and a preparation method thereof. The composition of the non-quenched and tempered steel is as follows by mass percentage: C is 0.22-0.29%, Si is 0.25-0.40%, Mn is 1.80-2.00%, Cr is 0.40-0.60%, V is 0.01-0.08wt%, P<0.015%, S is 0.020-0.050%, Ti is 0.020-0.050%, Zr is 0.020-0.040%, N is 0.010-0.020%, [O]≤15ppm, [H]≤2ppm, and the balance is Fe. The non-quenched and tempered steel bar adopts the design of low carbon, high manganese, trace V, Ti, and Zr, giving full play to the effects of solid solution strengthening, precipitation strengthening, and fine grain strengthening. Zr effectively improves the morphology and distribution of MnS, giving full play to the metallurgical effect of MnS; through the multi-water tank cooling technology and the design of slow cooling temperature, the strength and toughness matching, uniformity, hardness, and organization of the steel are well controlled. However, the Nb element is not added to the composition for V-Nb-Ti composite microalloying. Therefore, the non-quenched and tempered steel microstructure obtained by the invention has a relatively coarse grain size, and it is impossible to obtain excellent strength and toughness at the same time.

[0006] The patent application with application number 202111321425.9 discloses a high-strength and tough bainite non-quenched and tempered steel and a method for manufacturing the same. The composition of the high-strength and tough bainite non-quenched and tempered steel is as follows by mass percentage: C is 0.23-0.27%, Si is 0.25-0.40%, Mn is 1.80-2.00%, Cr is 0.50-0.60%, S is 0.045-0.065%, P≤0.02%, Nb is 0.021-0.030%, V is 0.11-0.14%, Mo is 0.04-0.10%, B is 0.0012-0.0025%, N is 0.010-0.020%, and the remainder is Fe and other unavoidable impurities; on the basis of the original 25Mn2CrVS, molybdenum and boron alloy elements are added to increase the hardenability of the material, promote the formation of bainite, and reduce the cooling rate required for the formation of bainite. However, Nb and Ti elements are not added to the composition for V-Nb-Ti composite microalloying. Therefore, the non-quenched and tempered steel obtained by the invention cannot achieve precipitation strengthening and further grain refinement through (Nb, Ti) (C, N), resulting in low strength and toughness. Summary of the invention

[0007] In order to solve the above technical problems, the present invention provides a high-strength and toughness bainitic non-quenched and tempered steel and a preparation method thereof. By controlling the composition design and preparation process of V-Nb-Ti composite microalloying, the finally obtained bainitic non-quenched and tempered steel has excellent comprehensive mechanical properties.

[0008] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is: A high-strength and toughness bainite non-quenched and tempered steel, the chemical composition and mass percentage of which are: C: 0.22-0.28%, Si: 0.20-0.40%, Mn: 1.80-2.10%, V: 0.08-0.15%, Cr: 0.40-0.60%, Nb: 0.016-0.030%, Ti: 0.010-0.025%, N: 0.01-0.02%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurities.

[0009] Furthermore, the non-quenched and tempered steel contains 0.23-0.26% C and 1.90-2.00% Mn.

[0010] Furthermore, the carbon equivalent Ceq of the non-quenched and tempered steel is 0.72-0.78%, and its microstructure includes 60-65% bainite and 35-40% ferrite according to the area ratio. The carbon equivalent Ceq is calculated according to the following formula: Ceq(%)=C%+(1 / 10)Si%+(2 / 11)Mn%+(1 / 5)Cr%+(1 / 3)V%, Wherein: C, Si, Mn, Cr, V are the mass percentages of the corresponding elements in the non-quenched and tempered steel.

[0011] Furthermore, the non-quenched and tempered steel has a tensile strength of ≥1000 MPa, a yield strength of ≥600 MPa, and a V-mouth Charpy impact energy of ≥50 J at 20°C.

[0012] The preparation method of the above-mentioned high-strength and toughness bainite non-quenched and tempered steel includes an electric furnace smelting process, an LF refining process, a VD vacuum refining process, a continuous casting process, a heating process, a rolling process, and a controlled cooling process; The controlled cooling process includes water tank cooling and cooling bed cooling steps in sequence; During the water tank cooling process, the temperature of the steel when entering the water tank is 920-980℃, and the temperature when leaving the water tank is 780-840℃; then it is cooled on a cooling bed, and an insulation cover is used on the upper part of the cooling bed. The return temperature of the steel on the cooling bed is 820-870℃, and the temperature of the lower cooling bed is 520-590℃.

[0013] Furthermore, in the electric furnace smelting process, the weight proportion of molten iron entering the furnace is ≥60%; the tapping temperature is 1580-1660°C, and the C in the molten steel at the end of the electric furnace is ≥0.080wt%, and P is ≤0.010wt%; before tapping, slag making material is added at 9-12kg / t; and metallic manganese and ferrosilicon are added during the tapping process for alloying.

[0014] Furthermore, in the LF refining step, the refining time is ≥45 min.

[0015] Furthermore, in the VD vacuum refining process, the vacuum degree is less than 67Pa and maintained for 5 to 10 minutes, and the argon flow rate is 70 to 90NL / min; after 10 to 15 minutes, the vacuum is broken, and manganese nitride wire and sulfur wire are fed into the steel. After breaking the vacuum, argon soft blowing is continued, and the soft blowing time is 15 to 20 minutes.

[0016] Furthermore, in the heating process, the temperature of the preheating section does not exceed 920°C, and the time is 45 to 55 minutes; the temperature of the first heating section is 1060 to 1130°C, and the time is 50 to 65 minutes; the temperature of the second heating section is 1130 to 1240°C, and the time is 50 to 65 minutes; the temperature of the equalizing section is 1180 to 1260°C, and the time is 50 to 65 minutes.

[0017] Furthermore, the rolling process includes rough rolling, intermediate rolling and finish rolling in sequence, with the starting rolling temperature being 1100-1140°C and the final rolling temperature being 920-980°C.

[0018] Furthermore, in the controlled cooling process, after being placed on the cooling bed, slow cooling is performed by stack cooling at a cooling rate of 0.1-0.3°C / min, and after stack cooling to 75-85°C, air cooling is performed to room temperature.

[0019] The beneficial effect of adopting the above technical scheme is that: the present invention adds V, Nb, and Ti microalloying elements for composite microalloying, controls the cooling method and cooling rate of the microalloyed non-quenched and tempered steel after rolling, and combines the controlled rolling and controlled cooling process to obtain excellent comprehensive mechanical properties. The non-quenched and tempered steel of the present invention has a tensile strength of ≥1000MPa, a yield strength of ≥600MPa, and a V-mouth Charpy impact energy of ≥50J at 20°C. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 3 is a microstructure diagram of the bainite non-quenched and tempered steel obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below in conjunction with specific embodiments.

[0022] In order to make the purpose, technical solution and advantages of the present invention more clear, the technical solution in the embodiment of the present invention will be clearly and completely described below. The described embodiment is only a part of the present invention. Based on the embodiment of the present invention, ordinary technicians in the field should understand that the embodiment only helps to understand the present invention and is not used to limit the present invention.

[0023] Examples 1-8 use the production process of Φ90mm rods to specifically illustrate how the present invention is implemented.

[0024] Example 1 High-quality self-circulating scrap steel is used. The weight ratio of molten iron entering the electric furnace smelting process is 69.3%. The C content at the end of the electric furnace is 0.08%, the P content is 0.009%, the tapping temperature is 1580℃, and 10kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 50min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 8min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 70NL / min. After 15min, the air is broken and manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 15NL / min, the time is 15min, and the tapping temperature is 1580℃. The preheating temperature of the billet heating is 865℃, and the preheating time is 55min; the first heating temperature is 1060℃, and the heating time is 50min; the second heating temperature is 1180℃, and the heating time is 60min; the soaking temperature is 1180℃, and the heating time is 65min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1100℃, and the steel is rolled through 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit, and the final rolling temperature is 920℃; The method adopts water tank cooling, with a water cooling inlet temperature of 920°C and an outlet temperature of 780°C, followed by cooling on an upper cooling bed with an upper cooling bed temperature of 835°C and a lower cooling bed temperature of 535°C, and then stack cooling and slow cooling with a stack cooling and slow cooling cooling rate of 0.3°C / min and an end point temperature of 80°C. After the stack cooling and slow cooling, the method is air cooled to room temperature to obtain the high strength and toughness bainitic non-quenched and tempered steel.

[0025] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 1 of the present invention 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 1.85%, V was 0.08%, Cr was 0.40%, Nb was 0.020%, Ti was 0.010%, S was 0.015%, and the balance was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.728%.

[0026] The microstructure of the high-strength and toughness bainite non-quenched and tempered steel prepared in Example 1 of the present invention is shown in FIG. Figure 1 As shown in the figure, it can be seen that the microstructure is bainite and ferrite, of which bainite accounts for 64% of the area, ferrite accounts for 35% of the area, and the rest is a small amount of retained austenite.

[0027] Example 2 High-quality self-circulating scrap steel is used. The weight ratio of molten iron entering the electric furnace smelting process is 69.3%. The C content at the end of the electric furnace is 0.08%, the P content is 0.009%, the tapping temperature is 1620℃, and 9kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 45min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 10min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 80NL / min. After 10min, the air is broken and manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 14NL / min, the time is 20min, and the tapping temperature is 1620℃. The preheating temperature of the billet heating is 920℃, and the preheating time is 45min; the first heating temperature is 1130℃, and the heating time is 65min; the second heating temperature is 1240℃, and the heating time is 50min; the soaking temperature is 1260℃, and the heating time is 50min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1140℃, and the rolling is carried out by 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit, and the final rolling temperature is 960℃; The method adopts water tank cooling, with a water cooling inlet temperature of 980°C and an outlet temperature of 840°C, followed by cooling on an upper cooling bed, with an upper cooling bed temperature of 840°C and a lower cooling bed temperature of 520°C, and then stack cooling and slow cooling, with a stack cooling and slow cooling cooling rate of 0.1°C / min and an end point temperature of 85°C. After the stack cooling and slow cooling, the method is air cooled to room temperature to obtain the high strength and toughness bainitic non-quenched and tempered steel.

[0028] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 2 of the present invention was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.23%, Si was 0.20%, Mn was 1.92%, V was 0.11%, Cr was 0.48%, Nb was 0.016%, Ti was 0.015%, S was 0.0072%, and the remainder was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.732%.

[0029] Example 3 High-quality self-circulating scrap steel is used. The weight ratio of molten iron entering the electric furnace smelting process is 68.1%. The C content at the end of the electric furnace is 0.08%, the P content is 0.007%, the tapping temperature is 1660℃, and 12kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 48min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 5min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 90NL / min, and the air is broken after 13min. Manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 15NL / min, the time is 17min, and the tapping temperature is 1650℃; The preheating temperature of the billet heating is 875℃, and the preheating time is 50min; the first heating temperature is 1080℃, and the heating time is 60min; the second heating temperature is 1130℃, and the heating time is 65min; the soaking temperature is 1200℃, and the heating time is 62min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1120℃, and the rolling is carried out by 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit, and the final rolling temperature is 940℃; The method adopts water cooling through a water tank, the inlet temperature of the water cooling through water cooling is 940°C, the outlet temperature is 835°C, and then cooling is performed on an upper cooling bed, the temperature of the upper cooling bed is 820°C, the temperature of the lower cooling bed is 580°C, and then stack cooling and slow cooling are performed, the stack cooling and slow cooling cooling rate is 0.2°C / min, the terminal temperature of the stack cooling and slow cooling is 75°C, and after the stack cooling and slow cooling is completed, air cooling is performed to room temperature to obtain the high strength and toughness bainitic non-quenched and tempered steel.

[0030] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 3 of the present invention was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.26%, Si was 0.36%, Mn was 2.10%, V was 0.12%, Cr was 0.51%, Nb was 0.021%, Ti was 0.020%, S was 0.0061%, and the remainder was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.776%.

[0031] Example 4 High-quality self-circulating scrap steel is used. The weight ratio of molten iron in the electric furnace smelting process is 69.8%. The C content at the end of the electric furnace is 0.12%, the P content is 0.007%, the tapping temperature is 1600℃, and 9.3kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 46min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 7min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 75NL / min. After 14min, the air is broken and manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 14NL / min, the time is 18min, and the tapping temperature is 1600℃. The preheating section temperature of the billet heating is 878℃, and the preheating time is 47min; the first heating section temperature is 1085℃, and the heating time is 60min; the second heating section temperature is 1197℃, and the heating time is 60min; the soaking section temperature is 1210℃, and the heating time is 60min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1130℃, and the rolling is carried out by 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit, and the final rolling temperature is 980℃; The method adopts water cooling in a water tank, with an inlet temperature of 980°C and an outlet temperature of 796°C, followed by cooling on an upper cooling bed, with an upper cooling bed temperature of 870°C and a lower cooling bed temperature of 590°C, followed by stack cooling and slow cooling, with a stack cooling and slow cooling cooling rate of 0.2°C / min and an end point temperature of 80°C. After the stack cooling and slow cooling, the method is air cooled to room temperature to obtain the high strength and toughness bainitic non-quenched and tempered steel.

[0032] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 4 of the present invention was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.28%, Si was 0.40%, Mn was 1.81%, V was 0.09%, Cr was 0.41%, Nb was 0.019%, Ti was 0.017%, S was 0.0063%, and the remainder was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.738%.

[0033] Example 5 High-quality self-circulating scrap steel is used. The weight ratio of molten iron entering the electric furnace smelting process is 72.5%. The C content at the end of the electric furnace is 0.09%, the P content is 0.010%, the tapping temperature is 1610℃, and 11kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 45min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 9min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 86NL / min. After 12min, the air is broken, and manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 16NL / min, the time is 20min, and the tapping temperature is 1610℃; The preheating section temperature of the billet heating is 895℃, and the preheating time is 52min; the first heating section temperature is 1087℃, and the heating time is 62min; the second heating section temperature is 1195℃, and the heating time is 63min; the soaking section temperature is 1200℃, and the heating time is 60min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1131℃, and the final rolling temperature is 968℃ after rolling by 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit. The method adopts water cooling in a water tank, with an inlet temperature of 962°C and an outlet temperature of 822°C, followed by cooling on an upper cooling bed, with an upper cooling bed temperature of 857°C and a lower cooling bed temperature of 546°C, followed by stack cooling and slow cooling, with a stack cooling and slow cooling cooling rate of 0.2°C / min and an end point temperature of 79°C. After the stack cooling and slow cooling, the method is air cooled to room temperature to obtain the high-strength and toughness bainitic non-quenched and tempered steel.

[0034] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 5 of the present invention was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.25%, Si was 0.39%, Mn was 1.98%, V was 0.10%, Cr was 0.49%, Nb was 0.018%, Ti was 0.025%, S was 0.0065%, and the balance was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.780%.

[0035] Example 6 High-quality self-circulating scrap steel is used. The weight ratio of molten iron entering the electric furnace smelting process is 71.2%. The C content at the end of the electric furnace is 0.08%, the P content is 0.009%, the tapping temperature is 1620℃, and 10kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 47min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 8min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 80NL / min. After 15min, the air is broken, and manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 14NL / min, the time is 19min, and the tapping temperature is 1615℃; The preheating temperature of the billet heating is 889℃, and the preheating time is 50min; the first heating temperature is 1095℃, and the heating time is 60min; the second heating temperature is 1198℃, and the heating time is 60min; the soaking temperature is 1200℃, and the heating time is 57min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1130℃, and the rolling is carried out by 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit, and the final rolling temperature is 970℃; The method adopts water cooling in a water tank, with an inlet temperature of 960°C and an outlet temperature of 831°C, followed by cooling on an upper cooling bed, with an upper cooling bed temperature of 850°C and a lower cooling bed temperature of 586°C, followed by stack cooling and slow cooling, with a stack cooling and slow cooling cooling rate of 0.2°C / min and an end point temperature of 80°C. After the stack cooling and slow cooling, the steel is air cooled to room temperature to obtain the high strength and toughness bainitic non-quenched and tempered steel.

[0036] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 6 of the present invention was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.22%, Si was 0.38%, Mn was 1.87%, V was 0.12%, Cr was 0.60%, Nb was 0.030%, Ti was 0.017%, S was 0.0063%, and the remainder was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.758%.

[0037] Example 7 High-quality self-circulating scrap steel is used. The weight ratio of molten iron entering the electric furnace smelting process is 60.0%. The C content at the end of the electric furnace is 0.08%, the P content is 0.010%, the tapping temperature is 1620℃, and 9kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 51min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 10min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 90NL / min. After 15min, the air is broken, and manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 16NL / min, the time is 18min, and the tapping temperature is 1611℃; The preheating temperature of the billet heating is 891℃, and the preheating time is 50min; the first heating temperature is 1088℃, and the heating time is 60min; the second heating temperature is 1192℃, and the heating time is 60min; the soaking temperature is 1220℃, and the heating time is 60min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1112℃, and the rolling is carried out by 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit, and the final rolling temperature is 960℃; The method adopts water cooling through a water tank, with an inlet temperature of 957°C and an outlet temperature of 821°C, followed by cooling on an upper cooling bed, with an upper cooling bed temperature of 855°C and a lower cooling bed temperature of 523°C, and then stack cooling and slow cooling, with a stack cooling and slow cooling cooling rate of 0.2°C / min and an end point temperature of 82°C. After the stack cooling and slow cooling, the method is air cooled to room temperature to obtain the high strength and toughness bainitic non-quenched and tempered steel.

[0038] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 7 of the present invention was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.26%, Si was 0.29%, Mn was 1.85%, V was 0.15%, Cr was 0.47%, Nb was 0.018%, Ti was 0.021%, S was 0.0065%, and the remainder was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.769%.

[0039] Example 8 High-quality self-circulating scrap steel is used. The weight ratio of molten iron entering the electric furnace smelting process is 68.7%. The C content at the end of the electric furnace is 0.08%, the P content is 0.007%, the tapping temperature is 1615℃, and 9kg / t of slag material is added before tapping. Metallic manganese and ferrosilicon are added for alloying during the tapping process. The refining time of the LF refining process is 46min. In the VD vacuum refining process, the vacuum degree is maintained at less than 67Pa for 6min, and argon is blown and stirred after the composition is adjusted. The argon flow rate is 83NL / min. After 15min, the air is broken, and manganese nitride wire and sulfur wire are fed into the steel. After breaking the air, argon soft blowing is continued. The soft blowing argon flow rate is 15NL / min, the time is 20min, and the tapping temperature is 1614℃; The preheating temperature of the billet heating is 875℃, and the preheating time is 50min; the first heating temperature is 1080℃, and the heating time is 60min; the second heating temperature is 1190℃, and the heating time is 60min; the soaking temperature is 1200℃, and the heating time is 60min. The high-pressure water dephosphorization water pressure is 22Pa; The starting rolling temperature is 1120℃, and the rolling is carried out by 5 rough rolling units + No. 1 flying shear + 6 intermediate rolling units + No. 2 flying shear + 6 finishing rolling units + No. 3 flying shear + sizing unit, and the final rolling temperature is 930℃; The method adopts water cooling through a water tank, the inlet temperature of the water cooling through water cooling is 929°C, the outlet temperature is 780-840°C, and then cooling is performed on an upper cooling bed, the temperature of the upper cooling bed is 852°C, the temperature of the lower cooling bed is 534°C, and then stack cooling and slow cooling are performed, the stack cooling and slow cooling cooling rate is 0.2°C / min, the terminal temperature of the stack cooling and slow cooling is 80°C, and after the stack cooling and slow cooling is completed, air cooling is performed to room temperature to obtain the high-strength and toughness bainitic non-quenched and tempered steel.

[0040] The high-strength and toughness bainite non-quenched and tempered steel product prepared in Example 8 of the present invention was tested for composition, and the test results were as follows, in terms of mass percentage: C was 0.26%, Si was 0.40%, Mn was 1.80%, V was 0.12%, Cr was 0.55%, Nb was 0.020%, Ti was 0.018%, S was 0.00654%, and the remainder was Fe and unavoidable impurities, and P in the impurities was ≤100ppm. Carbon equivalent Ceq: 0.777%.

[0041] The performance of the product prepared in the embodiment of the present invention is tested, and the testing method is as follows: the tensile strength and yield strength are tested according to GB / T 228.1 "Metallic Material Tensile Test Part 1: Room Temperature Test Method" standard, and are carried out on a CMT 4102 universal electronic tensile testing machine; the hardness is tested according to GB / T 231.1 "Metallic Material Vickers Hardness Test Part 1: Experimental Method" standard, and is carried out on a HV-10002 microhardness tester; the impact energy is tested according to GB / T 229 "Metallic Material Charpy Pendulum Impact Test Method" standard, and the Charpy impact test at room temperature of 20°C is carried out on a JB-30B impact testing machine; the test results are shown in Table 1.

[0042] Table 1 Test results of non-quenched and tempered steel properties in various examples

[0043] In summary, the present invention designs the steel composition, and forms fine and dispersed carbonitrides by adding V, Nb, and Ti microalloying elements during the smelting process, thereby forming grain refinement and solid solution strengthening effects; combined with the controlled rolling and controlled cooling process during the rolling process, the cooling method and cooling rate of the microalloyed non-quenched and tempered steel after rolling are controlled, thereby effectively achieving the purpose of controlling the microstructure of the non-quenched and tempered steel, so that the V-mouth impact energy of the finally obtained V-Nb-Ti composite microalloyed high-strength and toughness bainitic non-quenched and tempered steel at room temperature of 20°C reaches more than 50J.

[0044] 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 high-strength and toughness bainitic non-quenched and tempered steel, characterized in that: Its chemical composition and mass percentage are: C: 0.22-0.28%, Si: 0.20-0.40%, Mn: 1.80-2.10%, V: 0.08-0.15%, Cr: 0.40-0.60%, Nb: 0.016-0.030%, Ti: 0.010-0.025%, N: 0.01-0.02%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurities.

2. The high-strength and toughness bainite non-quenched and tempered steel according to claim 1, characterized in that: The non-quenched and tempered steel contains 0.23-0.26% C and 1.90-2.00% Mn.

3. The high-strength and toughness bainite non-quenched and tempered steel according to claim 1 or 2, characterized in that: The carbon equivalent Ceq of the non-quenched and tempered steel is 0.72-0.78%. Calculated by area ratio, its microstructure includes 60-65% bainite and 35-40% ferrite.

4. The high-strength and toughness bainite non-quenched and tempered steel according to claim 3, characterized in that: The non-quenched and tempered steel has a tensile strength of ≥1000MPa, a yield strength of ≥600MPa, and a V-mouth Charpy impact energy of ≥50J at 20°C.

5. The method for preparing the high-strength and toughness bainite non-quenched and tempered steel according to any one of claims 1 to 4, characterized in that: Including electric furnace smelting process, LF refining process, VD vacuum refining process, continuous casting process, heating process, rolling process, controlled cooling process; The controlled cooling process includes water tank cooling and cooling bed cooling steps in sequence; During the water tank cooling process, the temperature of the steel when entering the water tank is 920-980℃, and the temperature when leaving the water tank is 780-840℃; then it is cooled on a cooling bed, and an insulation cover is used on the upper part of the cooling bed. The return temperature of the steel on the cooling bed is 820-870℃, and the temperature of the lower cooling bed is 520-590℃.

6. The method for preparing high-strength and toughness bainite non-quenched and tempered steel according to claim 4, characterized in that: In the electric furnace smelting process, the weight ratio of molten iron in the furnace is ≥60%; the tapping temperature is 1580-1660°C, C ≥0.080wt%, P ≤0.010wt% in the molten steel at the end of the electric furnace; before tapping, slag material is added at 9-12kg / t; and metal manganese and ferrosilicon are added during tapping for alloying; The LF refining process has a refining time of ≥45 min.

7. The method for preparing high-strength and toughness bainite non-quenched and tempered steel according to claim 6, characterized in that: In the VD vacuum refining process, the vacuum degree is less than 67Pa and is maintained for 5 to 10 minutes, and the argon flow rate is 70 to 90NL / min; after 10 to 15 minutes, the vacuum is broken, and manganese nitride wire and sulfur wire are fed into the steel. After breaking the vacuum, argon soft blowing is continued, and the soft blowing time is 15 to 20 minutes.

8. The method for preparing high-strength and toughness bainite non-quenched and tempered steel according to claim 7, characterized in that: In the heating process, the temperature of the preheating section does not exceed 920°C and the time is 45 to 55 minutes; the temperature of the first heating section is 1060 to 1130°C and the time is 50 to 65 minutes; the temperature of the second heating section is 1130 to 1240°C and the time is 50 to 65 minutes; the temperature of the equalizing section is 1180 to 1260°C and the time is 50 to 65 minutes.

9. The method for preparing high-strength and toughness bainite non-quenched and tempered steel according to any one of claims 4 to 8, characterized in that: The rolling process includes rough rolling, intermediate rolling and finish rolling in sequence, with the starting rolling temperature being 1100-1140°C and the final rolling temperature being 920-980°C.

10. The method for preparing high-strength and toughness bainite non-quenched and tempered steel according to claim 9, characterized in that: The controlled cooling process comprises stack cooling and slow cooling after the cooling bed is lowered, with a cooling rate of 0.1-0.3°C / min, and stack cooling to 75-85°C and then air cooling to room temperature.

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