A non-quenched and tempered steel and a method for producing the same

Abstract

The application relates to the technical field of non-quenched and tempered steel, in particular to non-quenched and tempered steel and a preparation method thereof. The non-quenched and tempered steel provided by the application has the following chemical components and mass percentages: C: 0.32%-0.36%, Si: 0.15%-0.35%, Mn: 1.55%-1.65%, V: 0.06%-0.09%, Ti: 0.007%-0.017%, Alt: 0.015%-0.030%, N: 0.014%-0.018%, P<=0.018%, S<=0.005%, the total content of the rest impurities is <=0.035%, and the rest is Fe; wherein, 0.61%<=[C]+[Mn] / 6+[V] / 5+[Si] / 30<=0.65%. Through the optimization of the component design and the preparation method of micro-alloying, the application realizes the accurate control of the nitrogen content in the non-quenched and tempered steel and improves the micro-alloying effect.

Description

Technical Field

[0001] This invention relates to the field of non-quenched and tempered steel technology, and in particular to a non-quenched and tempered steel and its preparation method. Background Technology

[0002] Microalloyed non-quenched and tempered steel is a new type of steel. It refers to specialized structural steel made by adding trace amounts of alloying elements to carbon structural steel or low-alloy steel, directly machining it into parts after hot rolling, and achieving performance requirements without the need for quenching and tempering heat treatment. Compared to steels that undergo quenching and tempering, it has advantages in chemical composition, production process, mechanical properties, process performance, and techno-economic aspects, especially in terms of techno-economics, as it reduces the cost of heat treatment. The main type is non-quenched and tempered steel with vanadium microalloying and a small amount of nitrogen added. It typically utilizes inexpensive nitrogen, which combines with vanadium to precipitate VN, enhancing product performance.

[0003] However, during the smelting process, the use of vanadium-nitrogen alloys for nitrogen alloying presents two challenges: firstly, the high price of the alloy increases the cost of alloying; secondly, the wide range of nitrogen content in the finished product makes it difficult to control the chemical composition of nitrogen, resulting in significant fluctuations in the properties of non-quenched and tempered steel. Furthermore, the addition of elements such as Nb, Cr, and Ni to some non-quenched and tempered products further increases the production cost of non-quenched and tempered steel. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a non-quenched and tempered steel and its preparation method. Through microalloying composition design, without adding elements such as Nb, Cr, and Ni, it fully utilizes inexpensive nitrogen to improve the precipitation strengthening effect of vanadium, thereby enhancing the mechanical properties of the non-quenched and tempered steel, saving on the amount of vanadium-containing alloys used, and reducing costs. Optimization of the non-quenched and tempered steel preparation method enables precise control of the nitrogen content in the steel, further improving the microalloying effect.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0006] A non-quenched and tempered steel has the following chemical composition and mass percentage: C: 0.32%–0.36%, Si: 0.15%–0.35%, Mn: 1.55%–1.65%, V: 0.06%–0.09%, Ti: 0.007%–0.017%, Alt: 0.015%–0.030%, N: 0.014%–0.018%, P≤0.018%, S≤0.005%, with the total content of other impurities ≤0.035%, and the balance being Fe;

[0007] Among them, 0.61%≤[C]+[Mn] / 6+[V] / 5+[Si] / 30≤0.65%,[P]+[S]≤0.020%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components.

[0008] Compared to existing technologies, this invention, without adding elements such as Nb, Cr, and Ni, utilizes inexpensive nitrogen through microalloying composition design to enhance the precipitation strengthening effect of vanadium, thereby improving the mechanical properties of non-quenched and tempered steel, saving on the amount of vanadium-containing alloys used, and reducing costs. Specifically, V continuously precipitates VN in austenite, and Ti precipitates as TiC at lower temperatures, while simultaneously inhibiting the formation of MnS in the steel. Through the synergistic effect of Ti-V, grain boundaries are further strengthened, improving microstructure and properties, and the inhibitory effect on recrystallization is weakened, allowing for repeated recrystallization during rolling. Furthermore, by limiting 0.61≤[C]+[Mn] / 6+[V] / 5+[Si] / 30≤0.65, the overall strength of the non-quenched and tempered steel can be further guaranteed.

[0009] Another aspect of the present invention provides a method for preparing the above-mentioned non-quenched and tempered steel, including post-furnace deoxidation and slag formation in an electric furnace, refining and nitrogen alloying, continuous casting and rolling processes;

[0010] The refining nitrogen alloying process is as follows: during the refining process, nitrogen is blown from the bottom. When the N content in the gas analysis results meets the N content requirements of the composition design, the nitrogen blowing is stopped and the continuous casting process begins. When the N content in the gas analysis results is lower than the N content requirements of the composition design, nitrogen blowing continues for nitrogen alloying.

[0011] During the bottom-blowing nitrogen process, the nitrogen enrichment rate of non-quenched and tempered steel under different conditions is as follows:

[0012] At 1530–1550℃, the nitrogen flow rate was 180–220 NL / min, the pressure was 5.8–6.2 MPa, and the average nitrogen increase rate was 1.10–1.30 ppm / min.

[0013] At 1530–1550℃, the nitrogen flow rate was 280–320 NL / min, the pressure was 6.3–6.7 MPa, and the average nitrogen increase rate was 1.25–1.45 ppm / min.

[0014] At 1560–1575℃, the nitrogen flow rate was 280–320 NL / min, the pressure was 6.3–6.7 MPa, and the average nitrogen increase rate was 2.3–2.7 ppm / min.

[0015] At 1570–1585℃, the nitrogen flow rate was 280–320 NL / min, the pressure was 6.3–6.7 MPa, and the average nitrogen increase rate was 3.0–3.4 ppm / min.

[0016] At 1560–1585℃, the nitrogen flow rate is 380–420 NL / min, the pressure is 6.8–7.2 MPa, and the average nitrogen increase rate is 4.9–5.5 ppm / min.

[0017] Compared to existing technologies, this invention improves steel purity through post-furnace deoxidation and slag formation processes in electric furnaces, promotes the removal of inclusions, and increases effective refining time. Through a refining nitrogen alloying process, it significantly enhances the precipitation strengthening effect of vanadium, saves on vanadium-containing alloy usage, reduces costs, and improves the plasticity and toughness of the steel. Furthermore, this invention provides nitrogen enrichment rates under different conditions, achieving precise control of nitrogen content in non-quenched and tempered steel, further improving the microalloying effect of non-quenched and tempered steel.

[0018] In some embodiments, the post-furnace deoxidation and slag-forming process of the electric furnace includes the following steps:

[0019] S1. When the molten steel is tapped into the ladle, and the electric furnace tapping rate is 5% to 6%, silicon carbide is added to the ladle for pre-deoxidation, and argon gas is blown into the ladle to stir the molten steel.

[0020] S2. When the steel output is 17% to 18%, aluminum ingots are added for precipitation and deoxidation.

[0021] S3. When the steel output is 29% to 31%, alloying is carried out by adding alloys.

[0022] S4. When the steel output is 35% to 37%, lime and synthetic slag are added to form slag.

[0023] S5. When the steel reaches 49% to 51% of its output, calcium carbide is added to achieve the steel retention operation inside the electric furnace.

[0024] In some embodiments, the amount of steel retained for steel removal is 30t to 35t.

[0025] This invention employs a method of tapping molten steel into a ladle while retaining it. First, pre-deoxidation is performed using silicon carbide, followed by precipitation deoxidation with aluminum ingots to further reduce the oxygen activity of the molten steel. Then, alloying is carried out by adding an alloying agent, followed by lime and synthetic slag, and finally, calcium carbide is added to form a composite protective slag. Through the synergistic effect of these steps, the purity of the molten steel is improved, inclusions are removed more easily, and the effective refining time is increased.

[0026] In some embodiments, in step S1, the amount of silicon carbide added is 0.34 kg / t to 0.36 kg / t.

[0027] In some embodiments, in step S2, the amount of aluminum ingot added is 1.1 kg / t to 1.3 kg / t.

[0028] In some embodiments, in step S4, the amount of lime added is 4.6 kg / t to 4.8 kg / t, and the amount of synthetic slag added is 3.4 kg / t to 3.6 kg / t.

[0029] In some embodiments, in step S5, the amount of calcium carbide added is 0.40 kg / t to 0.42 kg / t.

[0030] In some embodiments, the synthetic slag comprises the following components by mass percentage: Al2O3: 45%–55%, CaO: 32%–42%, SiO2: 4%–10%, MgO ≤ 3%.

[0031] In some embodiments, after the slag-forming process, the mass percentage of nitrogen in the molten steel is 0.0055% to 0.0065%.

[0032] In some embodiments, during the rolling process, the initial rolling temperature is 1050℃~1150℃, and the final rolling temperature is 900℃~1000℃. By limiting the initial rolling temperature, burn-off and natural gas consumption are reduced; by limiting the final rolling temperature, grain size is refined, residual stress is reduced, and the mechanical properties of the non-quenched and tempered steel are further improved.

[0033] In some embodiments, the rolled steel is cooled to 20°C to 40°C at a rate of 70°C / s to 110°C / s. Increasing the cooling rate further refines the grain size and enhances the precipitation strengthening effect of microalloying elements.

[0034] In some embodiments, the initial rolling temperature X, the final rolling temperature F, and the average cooling rate R of the steel after rolling satisfy the following relationship:

[0035] The yield strength of non-quenched and tempered steel is -242.75 + 516.06 [C] + 210.1 [Mn] + 1461.1 [V] + 0.169 [N] + 0.183X + 1.722R ± 10;

[0036] The tensile strength of non-quenched and tempered steel = -215.5 + 889.8[C] + 261.17[Mn] + 1060.3[V] + 0.2339F + 1.029R - 0.0829[N] ± 10;

[0037] Wherein, the units of the initial rolling temperature X and the final rolling temperature F in the rolling process are both ℃, the unit of the average cooling rate R of the steel after rolling is ℃ / s, the unit of the yield strength of the unadjusted steel and the unit of the tensile strength of the unadjusted steel are both MPa, and [C], [Mn], [V] and [N] are the mass percentage contents of the corresponding chemical components.

[0038] The above-mentioned relationship provided by this invention can provide guidance for selecting process parameters such as the initial rolling temperature X, the final rolling temperature F, and the average cooling rate R of the steel after rolling, based on the mass percentages of C, Mn, V, and N, as well as the requirements for the yield strength and tensile strength of non-quenched and tempered steel. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0040] To better illustrate the present invention, further examples are provided below.

[0041] Example 1

[0042] This embodiment provides a non-quenched and tempered steel with the following chemical composition and mass percentage: C: 0.34%, Si: 0.25%, Mn: 1.6%, V: 0.075%, Ti: 0.01%, Alt: 0.02%, N: 0.016%, P: 0.01%, S: 0.003%, with the total content of other impurities ≤0.02%, and the balance being Fe;

[0043] Among them, [C]+[Mn] / 6+[V] / 5+[Si] / 30=0.63%, [P]+[S]=0.013%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components.

[0044] This embodiment also provides a method for preparing the above-mentioned non-quenched and tempered steel, including the following steps:

[0045] (1) Smelt the raw materials and adjust the composition and content of the steel molten except for N to meet the above requirements for non-quenched and tempered steel.

[0046] (2) Post-furnace deoxidation and slag formation in electric furnace

[0047] The molten steel is tapped into the ladle using a method of leaving steel in the ladle. When the electric arc furnace taps 5.5% of the steel, 0.35 kg / t silicon carbide is added to the ladle for pre-deoxidation, and argon gas is blown into the ladle to stir the molten steel. When the tapping rate reaches 17.5%, 1.2 kg / t aluminum ingot is added for precipitation deoxidation. When the tapping rate reaches 30%, alloy is added for alloying. When the tapping rate reaches 36%, 4.7 kg / t lime and 3.5 kg / t synthetic slag are added. When the tapping rate reaches about 50%, 0.41 kg / t calcium carbide is added, and tapping ends. After tapping, the amount of steel remaining in the electric arc furnace is 32 t.

[0048] The synthetic slag comprises the following components by mass percentage: Al2O3: 50%, CaO: 40%, SiO2: 8%, MgO: 2%.

[0049] After testing, the nitrogen content in the molten steel was found to be 0.006% by mass after the deoxidation and slag-forming processes at the end of the electric furnace.

[0050] (3) Refining nitrogen alloying

[0051] (a) After hoisting the ladle containing molten steel to the refining station, connect the gas pipeline between the ladle car and the ladle, and use nitrogen gas at a pressure of 7.0 MPa to break the top slag shell.

[0052] (b) Drive the ladle car to the refining furnace position, adjust the nitrogen flow rate to 300NL / min and the pressure to 6.5MPa, so that the exposed diameter of the molten steel is 350mm.

[0053] (c) When the slag is white and the temperature of the molten steel rises above 1560°C, the bottom-blown nitrogen gas with a pressure of 7.0 MPa and a flow rate of 450 NL / min is used to stir for 90 seconds, and spectral and gas samples are taken.

[0054] (d) After sampling is completed, the nitrogen flow rate is restored to 300 NL / min and the pressure is adjusted to 6.5 MPa. The power supply time is adjusted according to the temperature. Power is cut off when the temperature is between 1550℃ and 1560℃ to wait for the first spectral sample. During the power outage, the nitrogen flow rate is adjusted to 200 NL / min and the pressure is adjusted to 6.0 MPa to reduce the temperature drop during the process.

[0055] (e) Based on the results of the first spectral sample, the steel is carbonized and the alloy composition is fine-tuned. This process is carried out by bottom-blowing nitrogen at a pressure of 7.0 MPa and a flow rate of 450 NL / min for 40 seconds to promote uniform steel composition.

[0056] (f) After completing the first stage of carbonization and alloying, continue to supply power, adjust the nitrogen flow rate to 300NL / min and the pressure to 6.5MPa, until the temperature reaches 1560-1570℃, then stop the power supply, switch to the argon pipeline, use argon to stir to promote uniform steel composition, and take the second spectral sample and gas sample.

[0057] (g) After taking the second sample, switch to the nitrogen pipeline and determine whether to continue power supply based on the temperature. When power is supplied, adjust the nitrogen flow rate to 300 NL / min and the pressure to 6.5 MPa. When power is cut off, adjust the nitrogen flow rate to 200 NL / min and the pressure to 6.0 MPa.

[0058] (h) Fine-tune the steel composition based on the results of the second spectral sample. This process involves stirring with bottom-blown nitrogen at a pressure of 7.0 MPa and a flow rate of 450 NL / min for 40 seconds to promote uniform steel composition.

[0059] (j) The gas analysis result was 147 ppm. Using a nitrogen flow rate of 280–320 NL / min and a pressure of 6.3–6.7 MPa at 1570–1585℃, after bottom blowing nitrogen for 4 minutes, the gas sample was tested and found to be 159 ppm, which meets the composition design requirements. Bottom blowing nitrogen was stopped, and the argon pipeline was switched until refining was completed. After the composition and temperature met the process requirements, the gas was then introduced into the continuous casting process.

[0060] (4) Continuous casting

[0061] After refining, the ladle containing molten steel is hoisted to the continuous casting station for continuous casting.

[0062] (5) Rolling

[0063] The rolling process was carried out using an ACCU-ROLL rolling mill, with an initial rolling temperature of 1100℃ and a final rolling temperature of 950℃.

[0064] (6) Cooling

[0065] The rolled steel is cooled to 30°C at a rate of 80°C / s.

[0066] Example 2

[0067] This embodiment provides a non-quenched and tempered steel with the following chemical composition and mass percentage: C: 0.32%, Si: 0.15%, Mn: 1.65%, V: 0.06%, Ti: 0.017%, Alt: 0.015%, N: 0.014%, P: 0.018%, S: 0.002%, with the total content of other impurities ≤0.02%, and the balance being Fe;

[0068] Among them, [C]+[Mn] / 6+[V] / 5+[Si] / 30=0.61%, [P]+[S]=0.020%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components.

[0069] This embodiment also provides a method for preparing the above-mentioned non-quenched and tempered steel, including the following steps:

[0070] (1) Smelt the raw materials and adjust the composition and content of the steel molten except for N to meet the above requirements for non-quenched and tempered steel.

[0071] (2) Post-furnace deoxidation and slag formation in electric furnace

[0072] The molten steel is tapped into the ladle using a method of leaving steel in the ladle. When the electric arc furnace taps 5% of its steel, 0.34 kg / t silicon carbide is added to the ladle for pre-deoxidation, and argon gas is blown into the ladle to stir the molten steel. When the tapping rate reaches 17%, 1.1 kg / t aluminum ingot is added for precipitation deoxidation. When the tapping rate reaches 29%, alloy is added for alloying. When the tapping rate reaches 35%, 4.6 kg / t lime and 3.4 kg / t synthetic slag are added. When the tapping rate reaches approximately 49%, 0.40 kg / t calcium carbide is added, and tapping ends. After tapping, the amount of steel remaining in the electric arc furnace is 30 t.

[0073] The synthetic slag comprises the following components by mass percentage: Al2O3: 45%, CaO: 42%, SiO2: 10%, MgO: 3%.

[0074] After testing, the nitrogen content in the molten steel was found to be 0.0055% by mass after the deoxidation and slag-forming processes following the electric furnace deoxidation.

[0075] (4) Refining nitrogen alloying

[0076] (a) After hoisting the ladle containing molten steel to the refining station, connect the gas pipeline between the ladle car and the ladle, and use nitrogen gas at a pressure of 7.0 MPa to break the top slag shell.

[0077] (b) Drive the ladle car to the refining furnace station, adjust the nitrogen flow rate to 270NL / min and the pressure to 6.5MPa, so that the exposed diameter of the molten steel is 300mm.

[0078] (c) When the slag is white and the temperature of the molten steel rises above 1560°C, the bottom-blown nitrogen gas with a pressure of 7.0 MPa and a flow rate of 400 NL / min is used to stir for 95 seconds, and spectral and gas samples are taken.

[0079] (d) After sampling is completed, the nitrogen flow rate is restored to 270 NL / min and the pressure is adjusted to 6.5 MPa. The power supply time is adjusted according to the temperature. Power is cut off when the temperature is between 1550℃ and 1560℃ to wait for the first spectral sample. During the power outage, the nitrogen flow rate is adjusted to 170 NL / min and the pressure is adjusted to 6.0 MPa to reduce the temperature drop during the process.

[0080] (e) Based on the results of the first spectral sample, the steel is carbonized and the alloy composition is fine-tuned. This process is carried out by bottom-blowing nitrogen at a pressure of 7.0 MPa and a flow rate of 400 NL / min for 45 seconds to promote uniform steel composition.

[0081] (f) After completing the first stage of carbonization and alloying, continue to supply power, adjust the nitrogen flow rate to 270NL / min and the pressure to 6.5MPa, until the temperature reaches 1560-1570℃, then stop the power supply, switch to the argon pipeline, use argon to stir to promote uniform steel composition, and take the second spectral sample and gas sample.

[0082] (g) After taking the second sample, switch to the nitrogen pipeline and determine whether to continue power supply based on the temperature. If power is supplied, adjust the nitrogen flow rate to 270 NL / min and the pressure to 6.5 MPa. If power is cut off, adjust the nitrogen flow rate to 170 NL / min and the pressure to 6.0 MPa.

[0083] (h) Fine-tune the steel composition based on the results of the second spectral sample. This process involves stirring with bottom-blown nitrogen at a pressure of 7.0 MPa and a flow rate of 400 NL / min for 45 seconds to promote uniform steel composition.

[0084] (j) The gas analysis result was 137 ppm, and the measured temperature was 1567℃. Using a temperature range of 1560–1575℃, a nitrogen flow rate of 280–320 NL / min, and a pressure of 6.3–6.7 MPa, after bottom-blowing nitrogen for 2 minutes, the sampled gas concentration was 142 ppm, which meets the composition design requirements. The process was then switched to the argon pipeline until refining was completed. Once the composition and temperature met the process requirements, the ladle was hoisted to the continuous casting station for pouring.

[0085] (4) Continuous casting

[0086] After refining, the ladle containing molten steel is hoisted to the continuous casting station for continuous casting.

[0087] (5) Rolling

[0088] The rolling process was carried out using an ACCU-ROLL rolling mill, with an initial rolling temperature of 1150℃ and a final rolling temperature of 1000℃.

[0089] (6) Cooling

[0090] The rolled steel is cooled to 20°C at a rate of 110°C / s.

[0091] Example 3

[0092] This embodiment provides a non-quenched and tempered steel with the following chemical composition and mass percentage: C: 0.36%, Si: 0.35%, Mn: 1.55%, V: 0.09%, Ti: 0.007%, Alt: 0.03%, N: 0.018%, P: 0.01%, S: 0.005%, with the total content of other impurities ≤0.035%, and the balance being Fe;

[0093] Among them, [C]+[Mn] / 6+[V] / 5+[Si] / 30=0.65, [P]+[S]=0.015%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components.

[0094] This embodiment also provides a method for preparing the above-mentioned non-quenched and tempered steel, including the following steps:

[0095] (1) Smelt the raw materials and adjust the composition and content of the steel molten except for N to meet the above requirements for non-quenched and tempered steel.

[0096] (2) Post-furnace deoxidation and slag formation in electric furnace

[0097] The molten steel is tapped into the ladle using a method of leaving steel in the ladle. When the electric arc furnace taps 6% of the steel, 0.36 kg / t silicon carbide is added to the ladle for pre-deoxidation, and argon gas is blown into the ladle to stir the molten steel. When the tapping reaches 18%, 1.3 kg / t aluminum ingot is added for precipitation deoxidation. When the tapping reaches 30%, alloy is added for alloying. When the tapping reaches 37%, 4.8 kg / t lime and 3.6 kg / t synthetic slag are added. When the tapping reaches about 50%, 0.42 kg / t calcium carbide is added, and tapping ends. After tapping, the amount of steel remaining in the electric arc furnace is 35 t.

[0098] The synthetic slag comprises the following components by mass percentage: Al2O3: 55%, CaO: 38%, SiO2: 5%, MgO ≤ 2%.

[0099] After testing, the nitrogen content in the molten steel was found to be 0.0065% by mass after the deoxidation and slag-forming processes at the end of the electric furnace.

[0100] (5) Refining nitrogen alloying

[0101] (a) After hoisting the ladle containing molten steel to the refining station, connect the gas pipeline between the ladle car and the ladle, and use nitrogen gas at a pressure of 7.0 MPa to break the top slag shell.

[0102] (b) Drive the ladle car to the refining furnace station, adjust the nitrogen flow rate to 330NL / min and the pressure to 6.5MPa, so that the exposed diameter of the molten steel is 400mm.

[0103] (c) When the slag is white and the temperature of the molten steel rises above 1560°C, the bottom-blown nitrogen gas with a pressure of 7.0 MPa and a flow rate of 500 NL / min is used to stir for 90 seconds, and spectral and gas samples are taken.

[0104] (d) After sampling is completed, the nitrogen flow rate is restored to 330 NL / min and the pressure is adjusted to 6.5 MPa. The power supply time is adjusted according to the temperature. Power is cut off when the temperature is between 1550℃ and 1560℃ to wait for the first spectral sample. During the power outage, the nitrogen flow rate is adjusted to 230 NL / min and the pressure is adjusted to 6.0 MPa to reduce the temperature drop during the process.

[0105] (e) Based on the results of the first spectral sample, the steel is carbonized and the alloy composition is fine-tuned. This process is carried out by bottom-blowing nitrogen at a pressure of 7.0 MPa and a flow rate of 500 NL / min for 30 seconds to promote uniform steel composition.

[0106] (f) After completing the first stage of carbonization and alloying, continue to supply power, adjust the nitrogen flow rate to 330NL / min and the pressure to 6.5MPa, until the temperature reaches 1560~1570℃, then stop the power supply, switch to the argon pipeline, use argon to stir to promote uniform steel composition, and take the second spectral sample and gas sample.

[0107] (g) After taking the second sample, switch to the nitrogen pipeline and determine whether to continue power supply based on the temperature. If power is supplied, adjust the nitrogen flow rate to 330 NL / min and the pressure to 6.5 MPa. If power is cut off, adjust the nitrogen flow rate to 230 NL / min and the pressure to 6.0 MPa.

[0108] (h) Fine-tune the steel composition based on the results of the second spectral sample. This process involves stirring with bottom-blown nitrogen at a pressure of 7.0 MPa and a flow rate of 500 NL / min for 30 seconds to promote uniform steel composition.

[0109] (j) The gas analysis result was 148 ppm. Using a nitrogen flow rate of 380–420 NL / min and a pressure of 6.8–7.2 MPa at 1560–1585℃, after 6 minutes of bottom-blowing nitrogen, the gas sample was found to be 179 ppm, which meets the composition design requirements. The process was then switched to argon gas until refining was completed. Once the composition and temperature met the process requirements, bottom-blowing nitrogen was stopped, and the continuous casting process commenced.

[0110] (4) Continuous casting

[0111] After refining, the ladle containing molten steel is hoisted to the continuous casting station for continuous casting.

[0112] (5) Rolling

[0113] The rolling process was carried out using an ACCU-ROLL rolling mill, with an initial rolling temperature of 1050℃ and a final rolling temperature of 900℃.

[0114] (6) Cooling

[0115] The rolled steel is cooled to 40°C at a rate of 70°C / s.

[0116] Example 4

[0117] This embodiment provides a non-quenched and tempered steel with a yield strength ≥750MPa and a tensile strength ≥9000MPa. Its chemical composition and mass percentage content are the same as those in Example 1, and will not be repeated here.

[0118] The preparation method of the above-mentioned non-quenched and tempered steel provided in this embodiment refers to Embodiment 1, except that in steps (5) and (6), according to the guidance of the following two relationships, the initial rolling temperature in step (5) is 1140°C and the final rolling temperature is 960°C; in step (6), the rolled steel is cooled to 30°C at a rate of 95°C / s.

[0119] The two relations are:

[0120] The yield strength of non-quenched and tempered steel is -242.75 + 516.06 [C] + 210.1 [Mn] + 1461.1 [V] + 0.169 [N] + 0.183X + 1.722R ± 10;

[0121] The tensile strength of non-quenched and tempered steel = -215.5 + 889.8[C] + 261.17[Mn] + 1060.3[V] + 0.2339F + 1.029R - 0.0829[N] ± 10;

[0122] Wherein, the units of the initial rolling temperature X and the final rolling temperature F in the rolling process are both ℃, the unit of the average cooling rate R of the steel after rolling is ℃ / s, the unit of the yield strength of the unadjusted steel and the unit of the tensile strength of the unadjusted steel are both MPa, and [C], [Mn], [V] and [N] are the mass percentage contents of the corresponding chemical components.

[0123] Example 5

[0124] This embodiment provides a non-quenched and tempered steel with the same chemical composition and mass percentage as in Example 1, which will not be repeated here.

[0125] The preparation method of the above-mentioned non-quenched and tempered steel provided in this embodiment is the same as that in embodiment 1. The difference is that the final rolling temperature in step (5) is 1050℃. The other operation steps are the same as those in embodiment 1, and will not be repeated here.

[0126] Example 6

[0127] This embodiment provides a non-quenched and tempered steel with the same chemical composition and mass percentage as in Example 1, which will not be repeated here.

[0128] The preparation method of the above-mentioned non-quenched and tempered steel provided in this embodiment is the same as that in embodiment 1. The difference is that the cooling rate in step (6) is 120℃ / s, and the other operation steps are the same as those in embodiment 1, which will not be repeated here.

[0129] Comparative Example 1

[0130] This comparative example provides a non-quenched and tempered steel with the following chemical composition and mass percentage: C: 0.34%, Si: 0.25%, Mn: 1.6%, V: 0.075%, Ti: 0.01%, Alt: 0.02%, N: 0.03%, P: 0.01%, S: 0.003%, with the total content of other impurities ≤0.02%, and the balance being Fe;

[0131] Among them, [C]+[Mn] / 6+[V] / 5+[Si] / 30=0.63%, [P]+[S]=0.013%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components.

[0132] The preparation method of the above-mentioned non-quenched and tempered steel provided in this comparative example is the same as that in Example 1, except that the N content is adjusted to 0.03% in step (3) refining nitrogen alloying. The remaining operation steps are the same as those in Example 1 and will not be repeated.

[0133] Comparative Example 2

[0134] This comparative example provides a non-quenched and tempered steel with the following chemical composition and mass percentage: C: 0.34%, Si: 0.25%, Mn: 1.6%, V: 0.075%, Alt: 0.02%, N: 0.016%, P: 0.01%, S: 0.003%, with the total content of other impurities ≤0.02%, and the balance being Fe;

[0135] Among them, [C]+[Mn] / 6+[V] / 5+[Si] / 30=0.63%, [P]+[S]=0.013%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components.

[0136] The preparation method of the above-mentioned non-quenched and tempered steel provided in this comparative example is the same as that in Example 1, except that Ti is not added in steps (1) to (3), and the remaining operation steps are the same as those in Example 1, which will not be repeated here.

[0137] Comparative Example 3

[0138] This comparative example provides a non-quenched and tempered steel with the following chemical composition and mass percentage: C: 0.32%, Si: 0.15%, Mn: 1.6%, V: 0.06%, Ti: 0.017%, Alt: 0.015%, N: 0.014%, P: 0.018%, S: 0.002%, with the total content of other impurities ≤0.02%, and the balance being Fe;

[0139] Among them, [C]+[Mn] / 6+[V] / 5+[Si] / 30=0.6%, [P]+[S]=0.020%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components.

[0140] The preparation method of the above-mentioned non-quenched and tempered steel provided in this comparative example is the same as that in Example 2. The difference is that the content of Mn is adjusted to 1.6% in steps (1) to (3). The remaining operation steps are the same as those in Example 2 and will not be repeated.

[0141] The yield strength, tensile strength, and elongation of the non-quenched and tempered steels prepared in Examples 1-6 and Comparative Examples 1-3 were tested according to standard ISO 6892-1:2019. The test results are detailed in Table 1. Analysis of Table 1 shows that the non-quenched and tempered steel provided by this invention has higher yield strength, tensile strength, and elongation.

[0142] Table 1

[0143]

[0144]

[0145] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-quenched and tempered steel, characterized in that, Its chemical composition and mass percentage are as follows: C: 0.32%~0.36%, Si: 0.15%~0.35%, Mn: 1.55%~1.65%, V: 0.06%~0.09%, Ti: 0.007%~0.017%, Alt: 0.015%~0.030%, N: 0.014%~0.018%, P≤0.018%, S≤0.005%, and the total content of other impurities ≤0.035%, with the balance being Fe; Among them, 0.61%≤[C]+[Mn] / 6+[V] / 5+[Si] / 30≤0.65%,[P]+[S]≤0.020%, and [C], [Mn], [V], [Si], [P] and [S] are the mass percentage contents of the corresponding chemical components, respectively. The preparation method of the non-quenched and tempered steel includes the following processes: deoxidation and slag formation after electric furnace, refining and nitrogen alloying, continuous casting and rolling. The refining nitrogen alloying process is as follows: during the refining process, nitrogen is blown from the bottom. When the N content in the gas analysis results meets the N content requirements of the composition design, the nitrogen blowing is stopped and the continuous casting process begins. When the N content in the gas analysis results is lower than the N content requirements of the composition design, nitrogen blowing continues for nitrogen alloying. The initial rolling temperature X, the final rolling temperature F, and the average cooling rate R of the steel after rolling satisfy the following relationship: The yield strength of non-quenched and tempered steel = -242.75 + 516.06[C] + 210.1[Mn] + 1461.1[V] + 0.169[N] + 0.183X + 1.722R ± 10; The tensile strength of non-quenched and tempered steel = -215.5 + 889.8[C] + 261.17[Mn] + 1060.3[V] + 0.2339F + 1.029R - 0.0829[N] ± 10; Wherein, the units of the initial rolling temperature X and the final rolling temperature F in the rolling process are both ℃, the unit of the average cooling rate R of the steel after rolling is ℃ / s, the unit of the yield strength of the non-quenched and tempered steel and the unit of the tensile strength of the non-quenched and tempered steel are both MPa, and [C], [Mn], [V] and [N] are the mass percentage contents of the corresponding chemical components.

2. The non-quenched and tempered steel as described in claim 1, characterized in that, The post-furnace deoxidation and slag-making process of the electric furnace includes the following steps: S1. When the molten steel is tapped into the ladle, and the electric furnace tapping rate is 5%~6%, silicon carbide is added to the ladle for pre-deoxidation, and argon gas is blown into the ladle to stir the molten steel. S2. When the steel output is 17%~18%, aluminum ingots are added for precipitation and deoxidation. S3. When the steel output is 29%~31%, alloy is added for alloying. S4. When the steel output is 35%~37%, lime and synthetic slag are added to form slag. S5. When the steel reaches 49%~51%, calcium carbide is added to achieve the steel retention operation in the electric furnace.

3. The non-quenched and tempered steel as described in claim 2, characterized in that, In step S1, the amount of silicon carbide added is 0.34 kg / t to 0.36 kg / t; and / or In step S2, the amount of aluminum ingot added is 1.1 kg / t to 1.3 kg / t; and / or In step S4, the amount of lime added is 4.6 kg / t to 4.8 kg / t, and the amount of synthetic slag added is 3.4 kg / t to 3.6 kg / t; and / or In step S5, the amount of calcium carbide added is 0.40 kg / t to 0.42 kg / t.

4. The non-quenched and tempered steel as described in claim 3, characterized in that, In step S4, the synthetic slag comprises the following components in the following mass percentages: Al2O3: 45%~55%, CaO: 32%~42%, SiO2: 4%~10%, MgO≤3%.

5. The non-quenched and tempered steel according to any one of claims 1 to 4, characterized in that, After the deoxidation and slag-forming process following the electric furnace deoxidation, the mass percentage of nitrogen in the molten steel is 0.0055%~0.0065%.

6. The non-quenched and tempered steel as described in claim 1, characterized in that, In the rolling process, the initial rolling temperature is 1050℃~1150℃, and the final rolling temperature is 900℃~1000℃.

7. The non-quenched and tempered steel as described in claim 6, characterized in that, The rolled steel is cooled to 20℃~40℃ at a rate of 70℃ / s~110℃ / s.

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