CN108179356A - A kind of high through hardening large scale bolt for wind power generation steel and its manufacturing method - Google Patents

Abstract

A steel for high-hardenable large-size wind power bolts and a manufacturing method thereof, belonging to the technical field of steel for high-strength wind power bolts, the chemical composition weight percent of the steel is: C 0.35-0.45%, Si≤0.20%, Mn 0.60-1.00% , P≤0.012%, S≤0.005%, Cr 1.00‑1.50%, Mo 0.15‑0.40%, B 0.0005‑0.003%, Ti 0.03‑0.08%, Als 0.02‑0.05%, N≤0.005%, Ca 0.0001‑0.0003 %, the rest is Fe and unavoidable impurities. It is mainly used to manufacture large-scale wind power bolts with grades 10.9-12.9 and sizes ranging from 42-75mm. High hardenability is obtained by controlling the content of acid-soluble aluminum in the steel, and the upper and lower line ranges of design elements are calculated by using the hardenability model to improve the stability of bolt section hardness and mechanical properties, and refine grains to improve the low-temperature impact toughness of large-size bolt steel .

Description

Steel for high-hardenable large-size wind power bolts and manufacturing method thereof

technical field

The invention belongs to the field of steel for high-strength wind power bolts, and in particular provides a steel with high hardenability and large size for wind power bolts and a manufacturing method thereof. It is mainly used to manufacture large-scale wind power bolts with grades 10.9-12.9 and sizes ranging from 42-75mm. High hardenability is obtained by controlling the content of acid-soluble aluminum in the steel, and the upper and lower line ranges of design elements are calculated by using the hardenability model to improve the stability of bolt section hardness and mechanical properties, and refine grains to improve the low-temperature impact toughness of large-size bolt steel .

Background technique

In recent years, my country's wind power industry, especially large-capacity megawatt-level large-scale wind turbines, has developed rapidly. Due to long-term field service, harsh environment and poor maintenance conditions, high-strength fasteners for wind power equipment require strong wind turbine stability. Under normal continuous working conditions, wind power bolts must have a service life of more than 20 years.

Due to the continuous increase of wind turbine capacity, the weight of wind towers continues to increase, so the application specifications of wind power bolts continue to increase. At present, the maximum diameter of offshore large-capacity wind turbine bolts is 72mm, and 42CrMo and B7 bolt steel are usually used to manufacture 10.9 wind power bolts. , but the maximum hardened diameter of 42CrMo bolt steel is 42mm. If 42CrMo steel is used to manufacture bolts above 42mm, the performance of the surface and the core will be very different (hardness fluctuations reach 5-8HRC, and the toughness of the core is lower than the index requirement - 40 ℃ ≥ 27J ), such a large performance fluctuation will cause local overload to cause bolt failure during use, resulting in major accidents. It shows that the use of traditional 42CrMo bolt steel to manufacture large-size wind power bolts will cause great safety hazards to wind turbines, and cannot meet the large-size 42mm bolts. The index requirements of the above bolts. Therefore, it is of great significance to develop a bolt steel that not only meets the high hardenability requirements of large-size wind power bolts, but also ensures the stable quality of bolt products. The purpose of the present invention is that the hardenability of bolt steel meets the requirement of full hardening of 42mm-75mm bolts, and the hardness difference between the core and surface of bolt products is less than 3HRC, so as to meet the safety and stability requirements of large-capacity fans for large-size bolts. For this reason, the present invention is mainly based on solving the following three problems: 1. Control hardness fluctuation; 2. Full hardening in the size of 42mm-75mm; 3. Improve the control of the low temperature toughness of the core.

To sum up, in view of the problems of large-size wind power bolt steel, it is necessary to provide a high-hardenability bolt steel and its manufacturing method that can meet the safety and stability of large-size wind power bolts in service.

Contents of the invention

The object of the present invention is to provide a high-hardenability large-size wind power bolt steel and its manufacturing method, the tensile strength is in the range of 1100-1400MPa, the performance is stable, the economy is high, and the wind power bolt steel has good hardenability .

The stability of mechanical properties of large-sized wind power bolts is mainly restricted by the hardenability of bolt steel. The main effect of alloying elements in steel on mechanical properties is to increase hardenability, so that bolts with larger cross-sections can also be hardened. Many alloying elements can make the tempering transformation slow, and the tempering resistance is good. Compared with the medium carbon steel, it needs a higher tempering temperature, and can get a better combination of strength and toughness. However, alloying elements also have adverse effects on quenching and tempering of high-strength bolts, among which temper brittleness is very important, which should be strictly avoided, otherwise the impact energy value will be greatly reduced. When high-strength bolts are tempered and quenched, more than 90% of the entire cross-section is required to obtain a martensitic structure, that is, the steel must be hardened. The quenching depth is not only related to the chemical composition of the steel, but also affected by the size of the sample, heating temperature, cooling medium, cooling method, etc. The critical diameter is commonly used in production to measure the hardenability of steel. 20MnTiB, 35VB and 35CrMo steels belong to alloy steels with low hardenability, and the critical diameter of oil quenching is generally not greater than Ф25mm, so this is only suitable for the manufacture of steel bolts below M24-M30. 30CrMnSi steel is recommended as an alternative steel for high-strength fasteners. It has good comprehensive properties, high strength and sufficient toughness in the quenched and tempered state, and the hardenability is not high. Oil quenching is critical. The diameter is Ф25mm; while the diameter of high-strength bolts for wind power is greater than Ф30mm, 42CrMo, B7 and 40CrNiMo steels are often used, and the critical diameter of oil quenching is Ф42-45mm. There are no mature materials for large cross-section bolts exceeding the critical diameter.

To sum up, at present, for wind power bolts with a diameter greater than 42mm, there is no material that can stably meet the hardenability requirements. The common method to improve hardenability in the industry is to add alloying elements such as Cr, Ni, Mo, etc., but the addition of a large amount of alloying elements will reduce Bolt toughness (wind power bolts require toughness -40℃AKv≥27J), so it is necessary to find other technical methods to solve the hardenability problem. Acid-soluble aluminum, a by-product of deoxidation in bolt steel smelting, is an important factor affecting the hardenability of steel materials that has not been paid attention to. The diameter of its content in the steel determines the level of hardenability of the material. Acid-soluble aluminum, and the content of acid-soluble aluminum in the finished material is guaranteed through manufacturing process control, so as to realize the stable hardenability of large-sized wind power bolt steel and meet the requirements of wind power bolts with a diameter of 42-75mm.

The chemical composition weight percent of the steel with high hardenability and large size for wind power bolts of the present invention is: C 0.35-0.45%, Si≤0.20%, Mn 0.60-1.00%, P≤0.012%, S≤0.005%, Cr 1.00-1.50 %, Mo 0.15-0.40%, B0.0005-0.003%, Ti 0.03-0.08%, Als 0.02-0.05%, N≤0.005%, Ca 0.0001-0.0003%, and the rest are Fe and unavoidable impurities.

Manufacturing method: converter + refining outside the furnace + vacuum degassing + continuous casting + bar rolling; the specific process parameters are:

After refining outside the furnace, blow argon for 10-15 minutes, cast in a hanging ladle, and control the superheat of the tundish at 15-35°C;

The continuous casting slab is rolled after being heated and kept at 1150-1250°C for 2 hours.

Rough rolling stage, 1050-1150°C; finish rolling stage 900-1050°C, cooling bed temperature 800-850°C, natural cooling after rolling.

The technical scheme principle that the present invention adopts is:

1. Calculation of hardenability of acid-soluble aluminum and alloy elements

Composition design based on hardenability bandwidth (≤3HRC): To solve the performance fluctuation of bolt steel after heat treatment,

The chemical composition needs to be narrowly controlled. Since the hardenability bandwidth corresponds to the fluctuation range of the hardness after quenching, based on the composition design of the hardenability bandwidth ≤ 3HRC, bolt steel with a hardness fluctuation of less than 3HRC after quenching can be obtained. Use the hardenability calculation formula:

h＝[6.9[Mn] ² +3.2[Si]+22.6[Cr] ² +23.1[Mo]+(13.0[Cr]+2.5[Mo]+9.7)[Ni]+7.9[B]×10 ³ + 2.5] [C]

_Jmax = 66.5-47.8exp(-4[C]) b = 0.22h-0.34

J _min ＝56.2[C]+11.0[Mn]+2.0[Si]+13.6[Cr]+28.0[Mo]+3.3[Ni]-2.6[B]×10 ³ -17.3

Calculate the upper and lower lines of the chemical composition of the main elements in the inventive steel satisfying the hardenability bandwidth at J25 ≤ 3HRC, as shown in the hardenability calculation curve. According to the calculation results, it is determined that the content of Mn, Mo, Cr, and B elements in the invented steel should be controlled within the following range to ensure that the hardenability bandwidth at J25 is not greater than 3HRC, where Mn: 0.60-1.00%, Cr: 1.00-1.50%, Mo : 0.15-0.40%, B: 0.0005-0.003%, Als: 0.02-0.05%. Similarly, using the above formula to calculate the hardenability bandwidth of ML42CrMo composition in GB/T 6478-×××× for comparison, it shows that the hardenability bandwidth of ML42CrMo bolt steel designed for narrow composition control without hardenability bandwidth calculation is 7HRC, Obviously, the narrow composition control of the invention steel can realize the hardenability bandwidth less than 3HRC, and the hardness fluctuation after heat treatment is small.

2. Design of nano-precipitated phase

Nano-precipitation behavior in steel under thermodynamic equilibrium conditions: Ti elements can form nano-scale precipitates during thermal processing and heat treatment. The precipitates can refine the original austenite grains on the one hand and produce precipitation strengthening effects on the other hand. Conducive to the stability of high temperature tempering. The effect of precipitation phase refinement and precipitation strengthening is determined by its size and precipitation amount. Therefore, mastering the precipitation behavior of Ti element is the key to alloy composition design. Through Thermo-calc thermodynamic software and laboratory phase analysis theory and experiment The combined method to determine the precipitation amount of Ti element under thermodynamic equilibrium conditions is shown in Figure 3. The theoretical calculation is consistent with the phase analysis experimental data, and all Ti elements are precipitated after thermal deformation and heat treatment. Figure 4 shows the distribution state of the percentage content of Ti element in precipitates of different sizes obtained by phase analysis. The results show that under the condition of thermodynamic equilibrium, Ti element precipitates in steel with different sizes of precipitates and precipitates in a certain proportion. The above theoretical calculation And the experimental results provide the basic data for the precipitation phase control.

Calculation of solid solution temperature of nano-precipitates based on precipitation behavior: The rolling process affects the precipitation behavior of precipitates in steel by applying strain. In order to obtain a large number of nano-sized precipitates, it is necessary to combine the content of Ti element in the invented steel to formulate heating temperature, rolling The specific technical scheme is as follows:

Heating temperature design: use Thermo-calc software to calculate the total solution temperature of different Ti contents (Figure 5), and then obtain the formula for calculating the Ti solution temperature:

Ti ° C (solution temperature) = 60 × exp (Ti% / 0.07) + 1076

Using this equation to calculate the full solution temperature of Ti in the range of 0.03-0.08% is 1150-1250°C, so heating the billet at the above temperature can ensure that all Ti elements are in solid solution and avoid the existence of large-sized Ti precipitates.

3. Organizational refinement control

Refine austenite grains to 8-12μm by using precipitated phase and heat treatment process

A large number of nano-scale precipitates formed by Ti element in the rolling and cooling process of the invention steel, followed by quenching heat treatment (recrystallization), nano-scale precipitates can inhibit the growth of recrystallized grains, refine the original austenite grains, The grain refinement effect of precipitated phases in steel can be quantitatively calculated by the following formula:

In the formula, Dc is the prior austenite grain size (μm), d is the precipitated phase size, and f is the precipitated volume fraction, and the basic parameters of precipitation in steel with different Ti contents obtained in technical scheme 2 are substituted into the formula to calculate the invention The grain size obtained by steel without deformation and natural cooling. It is calculated that the inventive steel with Ti content in the range of 0.03-0.08% can obtain 8-12 μm prior austenite grain structure after quenching heat treatment.

Based on the above technical content, the composition design is carried out, and the functions and contents of each element of the invented steel are based on the following:

①C: The main element to obtain high strength and hardenability, the C content must be above 0.35. The higher the carbon content, the higher the strength of the steel, and the lower the plasticity, so the upper limit of C content does not exceed 0.45%.

②Si: The impact of silicon on cold heading performance is second only to carbon. Si can improve the elastic limit of steel, but affects cold working performance. When Si≤0.20%, it has little effect on hardenability and does not affect the plastic elongation of steel And the reduction of area, so its content is controlled not to exceed 0.20%.

③Mn: Mn can improve the hardenability of steel, but it is unfavorable for the cold plastic deformation of steel. It will also increase the cold work hardening rate of the steel. Mn is easy to segregate during the solidification process of steel. During quenching and tempering, Mn is easy to segregate at the grain boundary, which promotes temper brittleness. Reducing the Mn content is beneficial to reduce segregation of the billet. In order to ensure stable performance, the calculation is based on the hardenability bandwidth control The Mn content is controlled at 0.60-1.00%.

④P: P improves the cold brittleness of steel and is a harmful residual element. Microscopic segregation is formed when the molten steel solidifies, which increases the delayed fracture sensitivity of the steel, so the P content is controlled below 0.012%.

⑤S: S improves the hot brittleness of steel and deteriorates hot workability; forms MnS inclusions (type A inclusions) in molten steel, which deteriorates cold workability and delayed fracture performance of steel, so its content is controlled below 0.005%.

⑥Cr: It can effectively improve the hardenability of steel, improve wear resistance, improve corrosion resistance, and help maintain strength at high temperatures, but too high a content will deteriorate the cold workability of steel; Cr is an element that reduces the tendency of decarburization, and bolts Steel has high requirements for surface decarburization after heat treatment, so in order to ensure the hardenability of steel at J25, the Cr content is controlled at 1.00-1.50% according to the hardenability bandwidth control calculation.

⑦Mo: It can control hardenability, reduce the sensitivity of steel to temper brittleness, prevent temper brittleness of steel after high temperature tempering, and have a great influence on improving the tensile strength under high temperature tempering conditions, but the content is too high It will damage the delayed fracture performance of the bolt, and the Mo content is controlled at 0.15-0.40% according to the hardenability bandwidth control calculation.

⑧Als: Acid-soluble aluminum can improve the hardenability of steel without damaging the toughness of steel. According to the requirement of complete hardenability at J25, the amount of acid-soluble aluminum in steel should be controlled at 0.02-0.05%.

⑨B: It is an element that is both economical and can significantly improve hardenability. According to the control of hardenability bandwidth, the B content is in the range of 0.0005-0.003%.

⑩N: Forming fine nitrides with Al and V in the steel can refine the grains, but excessive N will form large-sized inclusions with Ti at high temperatures. It is necessary to control the formation temperature of TiN below 1250°C, and calculate the N content according to the formula Should be less than 0.005%.

Ti: Ti can significantly refine the austenite grain size, inhibit the growth of high-temperature austenite grains, and form nano-scale TiC precipitates during high-temperature tempering. TiC is an effective hydrogen trap in steel and improves the delayed fracture performance of steel . According to the calculation of precipitation strengthening, the Ti content is at least 0.03%. Usually, the rolling heating temperature of steel does not exceed 1250°C. Considering the relationship between Ti content and full solution temperature, the upper limit of Ti content at 1250°C is 0.08%. Therefore, the Ti content in the invented steel should be between 0.03% and 0.08%.

⑿Ca: Al ₂ O ₃ inclusions are modified to form calcium aluminate to avoid nodules caused by Al ₂ O ₃ adhering to the nozzle during the casting process. The content is 0.0003-0.001%.

Manufacturing method:

The manufacturing process of the steel of the present invention is: converter + refining outside the furnace + vacuum degassing + continuous casting + bar rolling; the specific process parameters are: converter + refining outside the furnace + vacuum degassing + continuous casting + bar rolling; The specific process parameters are: blowing argon for 10-15 minutes after refining outside the furnace, hanging ladle casting, and controlling the superheat of the tundish at 15-35°C; Rolling stage, 1050-1150°C; finish rolling stage 900-1050°C, cooling bed temperature 800-850°C, natural cooling after rolling.

Description of drawings

Fig. 1 is a calculation chart of the design hardenability bandwidth of the narrow composition of the inventive steel.

Figure 2 is the calculation chart of hardenability bandwidth of traditional ML42CrMo steel.

Fig. 3 is a map of Ti precipitation in steels with different Ti contents under thermodynamic equilibrium conditions.

Fig. 4 is a diagram of Ti element content in precipitated phases of different sizes under the condition of thermodynamic equilibrium.

Fig. 5 is a calculation chart of total solid solution temperature with different Ti contents.

Fig. 6 is a diagram of the ultra-fine prior austenite grain structure obtained from the inventive steel.

Fig. 7 is a phase diagram of nanoscale precipitation in the inventive steel.

Detailed ways

The following will be described in conjunction with specific implementation examples, but the present invention is not limited to the following specific implementation examples. The chemical composition of the steel of the present invention is (weight%): C 0.35-0.45%, Si≤0.20%, Mn 0.60-1.00%, P≤0.012%, S≤0.005%, Cr 1.00-1.50%, Mo 0.15-0.40% , B 0.0005-0.003%, Ti 0.03-0.08%, Als0.02-0.05%, Ca 0.0003-0.001%, N≤0.005%, and the rest are Fe and unavoidable impurities. Adopt converter and laboratory induction furnace to smelt 3 heats of steel of the present invention altogether according to above-mentioned chemical composition requirement, and smelt 1 heat of 42CrMo commercial steel as contrast steel according to the requirement in GB/T 6478-2001, the chemical composition of embodiment and contrast steel is as table 1. The following are specific examples of the steel of the present invention.

Table 1. Chemical Composition of Examples and Comparative Steels, % by Weight, Balance Fe

Table 2 embodiment manufacturing process

Table 3 embodiment heat treatment process

Numbering Quenching temperature tempering temperature A 880 500 B 880 550

Table 4 specific embodiment and test result

It can be seen from Table 4 that at the strength level of 1100-1400MPa, the steel of the present invention has significantly improved hardness stability and toughness compared with the comparison steel 42CrMo, and meets the hardenability requirements of wind power bolts in the size range of 42-75mm. In addition, the steel of the present invention is calculated based on the hardenability of acid-soluble aluminum and alloy elements and controlled by a narrow hardenability bandwidth. After the bolt steel is heat-treated, the hardness difference between the core and the surface is less than 3HRC, while the hardness fluctuation of the comparison steel 42CrMo is ≥ 6HRC. The test results of the toughness of the inventive steel and the comparative steel show that the original austenite grains of the inventive steel are refined to 8-12 μm (Figure 7) by using the nano-precipitated phase of Ti (Figure 6). The low-temperature impact energy is greater than 27J, and there is a certain margin, which meets the requirements of wind power bolts for low-temperature impact energy (wind power bolts require low-temperature impact energy -40℃AKv≥27J), while the contrast steel has a diameter larger than The impact energy at -40°C low temperature of 42mm is lower than 27J.

In addition, the argon blowing process of the invention steel after refining outside the furnace ensures the homogenization of acid-soluble aluminum in the steel. The argon blowing process of Ca treatment promotes the floating of calcium aluminate inclusions and prevents Al ₂ O ₃ inclusions from adhering to the nozzle. Cause casting nodulation problem.

The above comparison results show that the invention steel not only has excellent hardenability, but also meets the requirements of material hardenability for the manufacture of bolts with a diameter greater than 42mm, and the difference in the surface hardness of the core after heat treatment is small, especially the toughness of the core meets the low temperature index requirements of wind power bolts .

Claims (2)

1. a kind of high-hardenability large scale bolt for wind power generation steel, which is characterized in that the specific chemical composition weight % of the steel is：C 0.35-0.45%, Si≤0.20%, Mn 0.60-1.00%, P≤0.012%, S≤0.005%, Cr 1.00-1.50%, Mo 0.15-0.40%, B 0.0005-0.003%, Ti 0.03-0.08%, Als 0.02-0.05%, N≤0.005%, Ca 0.0003-0.001%, surplus are Fe and inevitable impurity.

2. a kind of manufacturing method of high-hardenability large scale bolt for wind power generation steel described in claim 1, it is characterised in that：Converter + external refining+vacuum outgas+continuous casting+rolling bar；Specifically technological parameter is：

Argon 10-15 minutes after external refining, bull ladle casting,

The tundish degree of superheat is controlled at 15-35 DEG C；

Continuous casting billet is rolled by 1150-1250 DEG C of heating and thermal insulation after 2 hours, rough rolling step, 1050-1150 DEG C；Finish rolling rank 900-1050 DEG C of section, enters 800-850 DEG C of cold bed temperature, rolls rear natural cooling.