Non-quenched and tempered steel for 12.9-grade wind power bolt and production method of non-quenched and tempered steel

Abstract

The invention belongs to the technical field of non-quenched and tempered steel, and relates to non-quenched and tempered steel for a 12.9-grade wind power bolt and a production method of the non-quenched and tempered steel. According to the method, the component design of steel is optimized, and the steel comprises 0.20%-0.25% of C, 1.30%-1.50% of Si, 1.90%-2.20% of Mn and 0.0090%-0.0150% of N; the key process control comprises the following steps: (1) LF refining nitrogen alloying; (2) continuous casting weak cooling and electromagnetic stirring; (3) carrying out controlled rolling at 1200 + / -15 DEG C, heating at 780-830 DEG C, and (4) quickly cooling after rolling, feeding on a cooling bed at the temperature of 580 + / -30 DEG C, and tempering at the medium temperature of 400 + / -20 DEG C; the obtained non-quenched and tempered steel directly meets the performance requirements of 12.9-grade bolts: Rm is larger than or equal to 1200 MPa,-40 DEG C KV2 is larger than or equal to 30 J, the yield ratio RP0.2 / Rm is larger than or equal to 0.9, a traditional quenching and tempering heat treatment procedure is omitted, and the non-quenched and tempered steel is suitable for manufacturing the high-strength wind power bolts with the diameter being 30-70 mm.

Description

Technical Field

[0001] The present invention belongs to the technical field of non-quenched and tempered steel, and relates to a 12.9-grade non-quenched and tempered steel for wind power bolts and a production method thereof. Background Art

[0002] Wind turbine bolts are commonly made of high-strength alloy steels, such as 42CrMoA, 42CrMoA-RD, 35VB, and 40CrNiMo, to meet the requirements for high tensile strength (e.g., Grade 10.9) and low-temperature toughness. Traditional high-strength bolts require quenching and tempering, which can lead to high energy consumption (≥500kWh per ton of steel), high costs, and significant deformation. Existing non-tempered steels only have tensile strengths of Grade 10.9 and low low-temperature impact energy (KV2 < 20J at -40°C).

[0003] Patent CN115386803A discloses a non-quenched and tempered steel for high-strength and toughness wind power bolts and a production method thereof, comprising the following chemical components: C, Si, Mn, Cr, Mo, V, Ti, Alt, and 2.3≤[Mn] / ([C]+[Si])≤6.0. The non-quenched and tempered steel for high-strength and toughness wind power bolts produced by the method of the present invention can omit quenching + tempering treatment. After low-temperature tempering at 200±10°C, the tensile strength Rm≥900MPa, the yield strength ratio RP0.2 / Rm≥0.9, the elongation after fracture A≥12%, the cross-sectional shrinkage Z≥45%, the delayed fracture strength ratio>0.8, the austenite grain size of the steel is greater than or equal to grade 10.0, and it has good strength and plasticity and toughness, and has excellent delayed fracture resistance.

[0004] Patent CN115386802A discloses a 10.9-grade non-quenched and tempered steel for large-size wind power bolts and its production method, comprising the following chemical components: C, Si, Mn, Cr, Mo, V, Nb, Ti, B, Alt, and 1.1≤1.2*[Cr]+1.5*[Mo]+2.0*[V]≤1.8; 3.0≤([V]+1.3*[Nb]) / 2.0*[Ti]≤11.0. Large-size wind power bolts produced by this method can omit quenching + tempering treatment. After low-temperature tempering at 200±10℃, the tensile strength Rm≥1000MPa, the yield strength ratio RP0.2 / Rm≥0.9, the elongation after fracture A≥12%, the cross-sectional shrinkage Z≥48%, the room temperature impact energy KV2≥50J, the fatigue strength ≥440MPa, the delayed fracture performance R≥3, and have good strength, plasticity and toughness, fatigue strength, and excellent delayed fracture resistance.

[0005] Existing non-quenched and tempered steels are micro-alloyed with V, but R mOnly 1000MPa and -40℃ KV2<20J; the nitrogen content control in conventional processes is unstable (fluctuation ±0.003%), resulting in large performance dispersion.

[0006] Patent CN111676423A has developed a 12.9-grade large-size high-toughness steel for wind power bolts and its production method, belonging to the technical field of wind power bolt steel. The main chemical composition and mass percentage content of the 12.9-grade large-size high-toughness steel for wind power bolts are: C: 0.35% to 0.45%, Si: 0.20% to 0.40%, Mn: 0.60% to 0.90%, Cr: 0.60% to 0.90%, Mo: 0.10% to 0.30%, Ni: 1.00% to 1.50%, V: 0.15% to 0.25%, Nb: 0.015% to 0.035%, Alt: 0.015% The steel contains 1% to 0.040% Mg, 0.0015% to 0.0035% P ≤ 0.020%, S ≤ 0.010%, and O ≤ 0.0015%. The remainder is Fe and unavoidable impurities. The austenite grain size of the steel is ≥ Grade 9, making it suitable for producing large-sized wind power bolts measuring 50-80mm. It exhibits high strength and toughness, with heat-treated mechanical properties exceeding Grade 12.9, a KV2 impact toughness of ≥ 50J at -101°C, and excellent fatigue properties and resistance to hydrogen-induced delayed fracture. However, it is not a non-quenched and tempered steel. Summary of the Invention

[0007] In order to solve the above technical problems, the present invention provides a 12.9 grade non-quenched and tempered steel for wind power bolts and a production method thereof. The high-performance non-quenched and tempered wind power bolt steel with a diameter of 30-70 mm is produced by a converter, refining, RH, continuous casting, rod and wire + annealing furnace process. The steel has low-temperature impact resistance and high strength. The mechanical properties of hot-rolled delivery can meet the requirements of 12.9 grade bolts: -40℃ impact absorption energy KV2 ≥ 30J, R m ≥1200MPa, yield strength ratio R P0.2 / R m ≥0.9, elongation after fracture A≥10%, section shrinkage Z≥45%, with good low temperature impact performance, strength and plastic toughness.

[0008] The technical solution adopted by the present invention is a 12.9 grade non-quenched and tempered steel for wind power bolts, the composition of which is as follows by weight: C: 0.20%-0.25%, Si: 1.30%-1.50%, M: 1.90%-2.20%, Cr: 0.50%-0.70%, P≤0.015%, Mo: 0.18%-0.22%, V: 0.050%-0.070%, S≤0.008%, Al: 0.020%-0.050 %, Nb: 0.020%-0.050%, Ni: 0.20%-0.30%, N: 0.0100%-0.0150%, and the rest are Fe and inevitable impurities; the metallographic structure of the non-quenched and tempered steel for wind power bolts is tempered bainite structure, and the performance in the delivery state is -40℃ impact absorption energy KV2≥30J, Rm≥1200MPa, yield strength ratio RP0.2 / Rm≥0.9, elongation after fracture A≥10%, and section shrinkage Z≥45%.

[0009] The present invention also provides a method for producing 12.9-grade non-quenched and tempered steel for wind power bolts, the key process steps of which include:

[0010] (1) Smelting: Using molten iron and scrap steel as raw materials, smelting is carried out in a top-bottom double-blown converter, controlling the end point of the molten steel C ≥ 0.05% and P ≤ 0.012%, tapping with slag blocking, and adding a composite deoxidizer to slag and deoxidize during the tapping process;

[0011] (2) Refining: The LF furnace adopts high basicity slag making and deoxidation, and a one-time aluminum adjustment process. In the early stage, aluminum wire is fed to adjust the aluminum content of the molten steel to 0.070%~0.090%. When the refining temperature rises to above 1580℃, nitrogen wire is fed to adjust the nitrogen content of the molten steel to 0.0130%~0.0180%. Before leaving the station, calcium wire is fed to treat the molten steel with calcium, and then a covering agent is added to protect the molten steel. After leaving the station, the LF furnace enters the RH furnace. The RH furnace is evacuated to below 67Pa and the vacuum is maintained for 15±2min. After that, the vacuum is broken to determine the hydrogen content of the molten steel and the hydrogen content is controlled to ≤1.5ppm. The RH furnace is turned on for circulation to increase nitrogen. The soft blowing time before leaving the station is 25±5min.

[0012] (3) Continuous casting: Weak cooling is used for secondary cooling, with a water content of 0.25±0.1L / KG and a target value of 20±5°C for the tundish superheat. Through a soft reduction process, combined with electromagnetic stirring at the mold and end, continuous casting segregation is reduced. The temperature of the straightening machine is controlled at 1030-1100°C, and the billet is stacked and cooled for ≥36h.

[0013] (4) Rolling: adopting segmented heating mode, rolling adopts large reduction process, the cumulative reduction rate of the first and second passes is ≥50%; final rolling is KOCKS controlled rolling, and the final rolling temperature is 780-830℃;

[0014] (5) Rapid cooling after rolling: After rolling, strong water penetration for rapid cooling, and the cooling bed insulation cover for slow cooling;

[0015] (6) Annealing furnace: After exiting the insulation cover, enter the heat treatment furnace for medium-temperature tempering, and control the temperature at 400±20℃ for 2h.

[0016] Furthermore, in the smelting step (1), the weight of the molten iron entering the furnace is 130t±2 tons, and the weight of the scrap steel is 20±2 tons.

[0017] Furthermore, in the (4) rolling step, the preheating section temperature is below 600°C for a period of ≥65 min, and the high temperature section temperature is 1200±15°C for a period of ≥70 min.

[0018] Furthermore, the (5) post-rolling rapid cooling step controls the upper cooling bed temperature to 580±30°C.

[0019] The outstanding features of the present invention are mainly reflected in:

[0020] (1) Ingredient innovation

[0021] High Si-Mn-N system (Si=1.30-1.50%, Mn=1.90-2.20%, N=0.0090-0.0150%):

[0022] -Si promotes bainite phase transformation and improves low-temperature toughness;

[0023] -Mn expands the austenite region and enhances hardenability;

[0024] -N forms nanoscale precipitation phases (V(C,N), Nb(C,N)) with V / Nb to achieve precipitation strengthening.

[0025] - microalloying (V = 0.050-0.070%, Nb = 0.020-0.050%);

[0026] - Refine the grains and improve the strength, avoiding the traditional quenching and tempering process;

[0027] The appropriate Mo (0.18-0.22%) + Cr (0.50-0.70%) content improves hardenability and suppresses temper brittleness;

[0028] The appropriate Ni (0.20-0.30%) content improves impact performance;

[0029] Auxiliary elements: Al (0.020-0.050%) to refine grains

[0030] -Low P and S control (P≤0.015%, S≤0.008%):

[0031] -Reduce grain boundary embrittlement and improve impact toughness.

[0032] (2) Process innovation

[0033] Precise nitrogen control: LF nitrogen feeding line + RH circulating nitrogen addition ensures [N] is stable at 0.0130±0.0020% to avoid performance fluctuations.

[0034] Controlled rolling and controlled cooling: KOCKS final rolling at 780-830℃ low temperature finishing rolling, grain size reaches above 8 level; rapid cooling to bainite transformation zone (580±30℃) followed by medium temperature tempering (400±20℃×2h) to eliminate internal stress, improve toughness and avoid martensite brittleness.

[0035] (3) Full process optimization

[0036] - Continuous casting uses weak cooling + electromagnetic stirring + soft reduction to reduce segregation and cracks;

[0037] - High reduction ratio (≥50%) is used in rolling to improve the uniformity of the structure;

[0038] Short-process production eliminates the quenching and tempering steps and reduces energy consumption.

[0039] The beneficial effects of the present invention are:

[0040] (1) The technical solution of the present invention does not require quenching and tempering. After direct rolling, the performance meets the 12.9 grade (Rm≥1200MPa, RP0.2 / Rm≥0.9), and the low-temperature toughness -40℃ KV2≥30J, which is 50% higher than that of traditional non-quenched and tempered steel. Moreover, because the quenching and tempering process is omitted, CO2 emissions are reduced by 0.8 tons per ton of steel, and the cost of 800-1000 yuan per ton of steel is saved.

[0041] (2) Avoiding deformation during quenching and tempering, the qualified rate of bolt processing is increased by more than 15%; it can directly process Φ30-70mm specification 12.9 grade wind power bolts to meet the requirements of extreme environment of -40℃, with the structure of tempered bainite, and has both high strength (Rm≥1200MPa) and high toughness (Z≥45%).

[0042] (3) The present invention adopts a series of new technologies such as composition optimization design, refining high N content and precise composition control, continuous casting combined with electromagnetic stirring, soft reduction, controlled rolling and controlled cooling process, rapid cooling after rolling, and medium temperature tempering to break through the limitations of existing materials and processes, give full play to the advantages of equipment, and promote the research and development of special high-strength non-tempered steel for wind power bolts. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0044] Figure 1 Metallographic diagram of the round steel according to Example 1 of the present invention. DETAILED DESCRIPTION

[0045] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0046] The technical solution to the above technical problems is to produce 12.9-grade non-tempered wind power bolt steel with a diameter of 30-70 mm in a -40°C environment and a production method thereof. The process route is smelting - continuous casting - rolling - controlled rolling - rapid cooling - medium-temperature tempering - warehousing, which can well meet the user's requirements of exempting from the quenching and tempering process and directly processing 12.9-grade wind power bolt steel.

[0047] Disclosed are 12.9-grade non-quenched and tempered steel for wind power bolts and a production method thereof. The steel comprises the following components by weight: C: 0.20%-0.25%, Si: 1.30%-1.50%, M: 1.90%-2.20%, Cr: 0.50%-0.70%, P≤0.015%, Mo: 0.18%-0.22%, V: 0.050%-0.070%, S≤0.008%, Al: 0.020%-0.050%, Nb: 0.020%-0.050%, Ni: 0.20%-0.30%, N: 0.0100%-0.0150%, and the remainder being Fe and unavoidable impurities.

[0048] The specific production process and technical parameters are as follows:

[0049] (1) Smelting: Molten iron and scrap steel are used as raw materials, with the weight of the molten iron entering the furnace being 130t±2 tons and the weight of the scrap steel being 20t±2 tons. The smelting is carried out in a top-bottom combined-blown converter, and the target composition of the final molten steel is controlled to be: C ≥ 0.05%, P ≤ 0.012%. The steel is tapped with slag blocking, and a composite deoxidizer is added during the tapping process to form slag and deoxidize.

[0050] (2) Refining: The LF furnace uses high basicity slag deoxidation and a one-step aluminum adjustment process. Aluminum wire is fed in the early stage to adjust the aluminum content of the molten steel to 0.070% to 0.090%. When the refining temperature rises to above 1580°C, nitrogen wire is fed to adjust the nitrogen content of the molten steel to 0.0130% to 0.0180%. Before leaving the station, calcium wire is fed to treat the molten steel with calcium, and then a covering agent is added to protect the molten steel. After leaving the station, the LF furnace enters the RH furnace. The RH furnace is evacuated to below 67 Pa and maintained for 15±2 minutes. After the vacuum is broken, the molten steel is hydrogenated and the hydrogen content is controlled to ≤1.5ppm. The RH furnace is turned on for circulation to increase nitrogen. The soft blowing time before leaving the station is 25±5 minutes.

[0051] (3) Continuous casting: Weak cooling is used for secondary cooling, with a water content of 0.25±0.1L / KG and a target value of tundish superheat of 20±5°C. Through a light reduction process, the crystallizer and the end electromagnetic stirring are combined to reduce the continuous casting segregation. The temperature of the straightening machine is controlled at 1030-1100°C to reduce the probability of straightening cracks. The billet is stacked and cooled for ≥36h.

[0052] (4) Rolling: adopt segmented heating mode, preheating section temperature is below 600℃, time ≥65min, high temperature section temperature is controlled at 1200±15℃, time ≥70min; rolling adopts large reduction process, cumulative reduction rate of first and second passes ≥50%; final rolling is controlled by KOCKS, final rolling temperature is 780-830℃.

[0053] (5) Rapid cooling after rolling: After rolling, strong water penetration is used for rapid cooling, and the upper cooling bed temperature is controlled at 580±30℃. The cooling bed is covered with an insulation cover for slow cooling.

[0054] (6) Annealing furnace: After exiting the insulation cover, enter the heat treatment furnace for medium-temperature tempering: the temperature is controlled at 400±20℃ and kept warm for 2h.

[0055] The following is further described with reference to the embodiments.

[0056] Each embodiment was produced according to the process flow of converter-refining-continuous casting-rolling. The chemical composition of the steel in each embodiment is shown in Table 1, with the remainder being Fe and unavoidable impurities. The metallographic structure results of the round steel produced in each embodiment and comparative example are shown in Table 2.

[0057] Example 1:

[0058] A 12.9 grade non-quenched and tempered steel for wind power bolts and a production method thereof, wherein the components thereof are as follows by weight: C: 0.21%, Si: 1.48%, M: 2.03%, Cr: 0.53%, P: 0.013%, Mo: 0.20%, V: 0.055%, S: 0.005%, Al T : 0.035%, Nb: 0.027%, Ni: 0.22%, N: 0.0145%, and the remainder is Fe and inevitable impurities.

[0059] The specific production process and technical parameters are as follows:

[0060] (1) Smelting: The weight of molten iron in the converter is 128.5 tons, and the weight of scrap steel is 21.5 tons; the end point of the molten steel is C: 0.06%, P: 0.011%; the steel is tapped with slag blocking, and a composite deoxidizer is added during the tapping process to form slag and deoxidize.

[0061] (2) Refining: The LF furnace is adjusted to Al: 0.073% in one step, and the basicity of the final slag of the LF furnace refining slag is 6.5; the nitrogen line is adjusted to N: 0.0168%; the RH furnace vacuum time is 14 minutes, and the hydrogen is set at 1.05ppm; circulating nitrogen is added; the soft blowing time before leaving the station is 23 minutes.

[0062] (3) The secondary cooling water volume of 280 cubic meters continuous casting is 0.24L / KG, the superheat of the tundish is 18℃, the temperature of the billet entering the straightening machine is 1040-1070℃, the electromagnetic stirring current of the crystallizer is 150A, the frequency is 2.5Hz, the electromagnetic stirring current of the end is 250A, the frequency is 8H; the soft pressure is 10mm; the billet is pile-cooled for 40h.

[0063] (4) Rolling: The preheating section temperature is below 515°C, the time is 69 minutes, the high temperature section temperature is 1195°C, the time is 75 minutes; the rolling adopts a large reduction process, and the cumulative reduction rate of the first and second passes is ≥50%; the KOCKS final rolling temperature is 785-798°C.

[0064] (5) Rapid cooling after rolling: After rolling, strong water penetration is used for rapid cooling. The upper cooling bed temperature is 558-596℃, and the cooling bed is covered with an insulation cover for slow cooling.

[0065] (6) Annealing furnace: medium temperature tempering at 388℃ for 2h.

[0066] The metallographic structure of the round steel produced according to the above method is as follows Figure 1 shown.

[0067] Example 2:

[0068] A 12.9-grade non-quenched and tempered steel for wind power bolts and a production method thereof, wherein the components thereof are as follows by weight: C: 0.23%, Si: 1.32%, M: 2.17%, Cr: 0.67%, P: 0.011%, Mo: 0.20%, V: 0.055%, S: 0.004%, Al T : 0.022%, Nb: 0.046%, Ni: 0.28%, N: 0.0118%, and the rest are Fe and inevitable impurities.

[0069] The specific production process and technical parameters are as follows:

[0070] (1) Smelting: The weight of molten iron in the converter is 130.2 tons, and the weight of scrap steel is 19.5 tons; the end point of the molten steel is C: 0.07%, P: 0.010%; the steel is tapped with slag blocking, and a composite deoxidizer is added during the tapping process to form slag and deoxidize.

[0071] (2) Refining: The LF furnace is adjusted to Al: 0.083% in one step, and the basicity of the final slag of the LF furnace refining slag is 7.2; the nitrogen line is adjusted to N: 0.0152%; the RH furnace vacuum time is 16 minutes, and the hydrogen is set at 1.01ppm; circulating nitrogen is added; the soft blowing time before leaving the station is 25 minutes.

[0072] (3) The secondary cooling water volume of 280 cubic meters continuous casting is 0.25L / KG, the superheat of the tundish is 22℃, the temperature of the billet entering the straightening machine is 1060-1080℃, the electromagnetic stirring current of the crystallizer is 200A, the frequency is 2.5Hz, the electromagnetic stirring current of the end is 300A, the frequency is 8H; the soft pressure is 12mm; the billet is pile-cooled for 36h.

[0073] (4) Rolling: The preheating section temperature is below 545°C, the time is 71 minutes, the high temperature section temperature is 1205°C, the time is 80 minutes; the rolling adopts a large reduction process, and the cumulative reduction rate of the first and second passes is ≥50%; the KOCKS final rolling temperature is 792-818°C.

[0074] (5) Rapid cooling after rolling: After rolling, strong water penetration is used for rapid cooling. The upper cooling bed temperature is 582-601℃, and the cooling bed is covered with an insulation cover for slow cooling.

[0075] (6) Annealing furnace: medium temperature tempering at 402℃ for 2h.

[0076] Example 3:

[0077] A 12.9 grade non-quenched and tempered steel for wind power bolts and a production method thereof, wherein the components thereof are as follows by weight: C: 0.25%, Si: 1.42%, M: 1.92%, Cr: 0.62%, P: 0.010%, Mo: 0.21%, V: 0.061%, S: 0.003%, Al T : 0.045%, Nb: 0.035%, Ni: 0.25%, N: 0.0133%, and the remainder is Fe and inevitable impurities.

[0078] The specific production process and technical parameters are as follows:

[0079] (1) Smelting: The weight of molten iron in the converter is 131.8 tons, and the weight of scrap steel is 18.5 tons; the end point of the molten steel is C: 0.08%, P: 0.009%; the steel is tapped with slag blocking, and a composite deoxidizer is added during the tapping process to form slag and deoxidize.

[0080] (2) Refining: The LF furnace is adjusted to Al: 0.088% at one time, and the basicity of the final slag of the LF furnace refining slag is 7.5; the nitrogen line is adjusted to N: 0.0177%; the RH furnace vacuum time is 17 minutes, and the hydrogen is set at 0.98ppm; circulating nitrogen is added; the soft blowing time before leaving the station is 29 minutes.

[0081] (3) The secondary cooling water volume of 280 cubic meters continuous casting is 0.26L / KG, the superheat of the tundish is 24℃, the temperature of the billet entering the straightening machine is 1069~1098℃, the electromagnetic stirring current of the crystallizer is 200A, the frequency is 2.5Hz, the electromagnetic stirring current of the end is 300A, the frequency is 8H; the soft pressure is 12mm; the billet is pile-cooled for 45h.

[0082] (4) Rolling: The preheating section temperature is below 585°C, the time is 80 minutes, the high temperature section temperature is 1213°C, the time is 92 minutes; the rolling adopts a large reduction process, and the cumulative reduction rate of the first and second passes is ≥50%; the KOCKS final rolling temperature is 802-826°C.

[0083] (5) Rapid cooling after rolling: After rolling, strong water penetration is used for rapid cooling. The upper cooling bed temperature is 595-607℃, and the cooling bed is covered with an insulation cover for slow cooling.

[0084] (6) Annealing furnace: medium temperature tempering at 417℃ for 2h.

[0085] Comparative Example 1: The production steps are consistent with those of Example 1, except that the chemical composition Si is 0.80%, which is not within the scope of the present invention. Other chemical elements are consistent with those of the present invention.

[0086] Comparative Example 2: The production steps are consistent with those of Example 2, except that the chemical composition of Mn is 1.65%, which is not within the scope of the present invention. Other chemical elements are consistent with those of the present invention.

[0087] Comparative Example 3: The chemical composition is consistent with that of Example 3. Only the final rolling temperature of 880-905°C, the upper cooling bed temperature of 810-830°C, and the tempering at 510-520°C for 2h controlled rolling and controlled cooling process are not within the scope of the present invention. The other steps are consistent with those of Example 3.

[0088] Table 1 Chemical composition of steel in each example (%)

[0089]

[0090] Table 2 Metallographic structure results of round steel samples in various embodiments and comparative examples

[0091]

[0092] It can be seen from the comparative experiments that the reduction of Si and Mn will lead to a decrease in the yield strength of the steel, while the change of rolling and cooling temperature will affect the microstructure formation of the steel and reduce other properties.

[0093] In summary, by adopting the technical solution of the present invention, non-quenched and tempered steel can directly reach the performance level of 12.9, R m ≥1200MPa, -40℃KV2≥30J, breaking through the strength-toughness bottleneck of existing non-quenched and tempered steel.

[0094] The above description is only a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are included in the scope of protection of the present invention.

Claims

1. A 12.9 grade non-quenched and tempered steel for wind power bolts, characterized in that: The composition by weight percentage is: C: 0.20%-0.25%, Si: 1.30%-1.50%, M: 1.90%-2.20%, Cr: 0.50%-0.70%, P≤0.015%, Mo: 0.18%-0.22%, V: 0.050%-0.070%, S≤0.008%, Al: 0.020%-0.050%, Nb: 0.020%-0.050%, Ni: 0.20%-0.30%, N: 0.0100%~0.0150%, and the rest is Fe and unavoidable impurities; the metallographic structure of the non-quenched and tempered steel for wind power bolts is tempered bainite structure, and the performance in the delivery state is -40℃ impact absorption energy KV2≥30J, R m ≥1200MPa, yield strength ratio R P0.2 / R m ≥0.9, elongation after fracture A≥10%, section shrinkage Z≥45%.

2. The method for producing a 12.9 grade non-quenched and tempered steel for wind power bolts according to claim 1, characterized in that: The key process steps include: (1) Smelting: Using molten iron and scrap steel as raw materials, smelting is carried out in a top-bottom double-blown converter, controlling the end point of the molten steel C ≥ 0.05% and P ≤ 0.012%, tapping with slag blocking, and adding a composite deoxidizer to slag and deoxidize during the tapping process; (2) Refining: The LF furnace adopts high basicity slag making and deoxidation, and a one-time aluminum adjustment process. In the early stage, aluminum wire is fed to adjust the aluminum content of the molten steel to 0.070%~0.090%. When the refining temperature rises to above 1580℃, nitrogen wire is fed to adjust the nitrogen content of the molten steel to 0.0130%~0.0180%. Before leaving the station, calcium wire is fed to treat the molten steel with calcium, and then a covering agent is added to protect the molten steel. After leaving the station, the LF furnace enters the RH furnace. The RH furnace is evacuated to below 67Pa and the vacuum is maintained for 15±2min. After that, the vacuum is broken to determine the hydrogen content of the molten steel and the hydrogen content is controlled to ≤1.5ppm. The RH furnace is turned on for circulation to increase nitrogen. The soft blowing time before leaving the station is 25±5min. (3) Continuous casting: Weak cooling is used for secondary cooling, with a water content of 0.25±0.1L / KG and a target value of 20±5°C for the tundish superheat. Through a soft reduction process, combined with electromagnetic stirring at the mold and end, continuous casting segregation is reduced. The temperature of the straightening machine is controlled at 1030-1100°C, and the billet is stacked and cooled for ≥36h. (4) Rolling: adopting segmented heating mode, rolling adopts large reduction process, the cumulative reduction rate of the first and second passes is ≥50%; final rolling is KOCKS controlled rolling, and the final rolling temperature is 780-830℃; (5) Rapid cooling after rolling: After rolling, strong water penetration for rapid cooling, and the cooling bed insulation cover for slow cooling; (6) Annealing furnace: After exiting the insulation cover, enter the heat treatment furnace for medium-temperature tempering, and control the temperature at 400±20℃ for 2h.

3. The method for producing a 12.9 grade non-quenched and tempered steel for wind power bolts according to claim 2, characterized in that: In the smelting step (1), the weight of the molten iron entering the furnace is 130t±2 tons, and the weight of the scrap steel is 20±2 tons.

4. The method for producing a 12.9 grade non-quenched and tempered steel for wind power bolts according to claim 2, characterized in that: In the (4) rolling step, the preheating section temperature is below 600°C and the time is ≥65 minutes, and the high temperature section temperature is 1200±15°C and the time is ≥70 minutes.

5. The method for producing a 12.9 grade non-quenched and tempered steel for wind power bolts according to claim 2, characterized in that: The (5) post-rolling rapid cooling step controls the upper cooling bed temperature to 580±30°C.

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