A production method of non-quenched and tempered steel for shafts

Abstract

A production method of non-quenched and tempered steel for shafts, with the process route being converter + LF furnace + RH furnace + continuous casting + hot rolling. The chemical composition of the steel is C = 0.37 - 0.44%, Si = 0.17 - 0.37%, Mn = 0.5 - 0.8%, P ≤ 0.030%, S ≤ 0.030%, Cr = 0.90 - 1.1%, Al = 0.005 - 0.025%, N = 0.003 - 0.008%, H ≤ 0.0002%, O ≤ 0.0020%, and the rest being Fe and inevitable impurities. The present invention adopts a medium-carbon low-alloy composition design, strictly controls the purity of the molten steel, and uses new technologies such as hot rolling with high temperature and large reduction, controlled rolling and controlled cooling, and on-line quenching and tempering hardening. The developed non-quenched and tempered steel for shafts has reached the strength and plasticity indexes of quenched and tempered material 40Cr. The produced steel is trial-produced into shaft parts such as motor shafts, and the strength performance requirements can be achieved without the whole-section quenching and tempering treatment of the parts, and it can be widely used in manufacturing motor shaft products.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical technology and relates to a production method of energy-saving, environment-friendly and high-toughness turning non-quenched and tempered steel. Background Art

[0002] At present, medium-carbon structural steel is generally used for motor shaft parts after quenching and tempering heat treatment and turning. For example, 40Cr is quenched and tempered. The quenching and tempering cost is high, and the production efficiency is low. Moreover, since the quenching and tempering process will increase energy consumption and pollute the environment, and there are also some waste losses, it does not meet the current energy-saving and environmental protection requirements. Therefore, in recent years, some non-quenched and tempered steels have been developed by various enterprises for shafts and rods. That is, on the basis of medium-carbon manganese steel, micro-alloying elements such as vanadium, titanium, and niobium are added. The fine carbides and nitrides precipitated by the micro-alloying elements vanadium, titanium, and niobium are used to strengthen the steel, so that the steel can reach the strength of quenched and tempered steel without quenching and tempering treatment.

[0003] Currently, the strength and other performance indicators of the non-quenched and tempered steels produced by each steel mill can reach the level of 40Cr quenched and tempered materials, but the economic benefits are poor. There is no obvious economic benefit for steel mills and downstream processing enterprises, and the effect of product use and promotion is not obvious. Therefore, it has practical research significance to develop a non-quenched and tempered steel for motor shafts with a diameter of 20-99 mm that has a suitable cost and can meet the use performance of 40Cr quenched and tempered materials. Summary of the Invention

[0004] The purpose of the present invention is to provide a production method of non-quenched and tempered steel for shafts, which produces non-quenched and tempered steel with a tensile strength ≥ 820 Mpa, a yield strength ≥ 540 Mpa, an elongation ≥ 22%, a reduction of area ≥ 60%, a grain size ≥ 7.0 grades, an impact energy Aku2 at room temperature ≥ 60 J, and a full-section hardness of 240-290 HBW for round steel with a diameter of 20 mm-99 mm.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A production method of non-quenched and tempered steel for shafts, the chemical composition of the steel by weight percentage is C = 0.37% - 0.44%, Si = 0.17% - 0.37%, Mn = 0.5% - 0.8%, P ≤ 0.030%, S ≤ 0.030%, Cr = 0.90% - 1.1%, Al = 0.005% - 0.025%, N = 0.003% - 0.008%, H ≤ 0.0002%, O ≤ 0.0020%, and the rest is Fe and inevitable impurities; the key process steps include:

[0007] 1) Smelting: The C content in the molten steel tapped from the converter is ≥0.08%. The LF+RH furnace refining process is adopted. Argon is blown throughout the refining process of the LF furnace for slag making and deoxidation. The holding time of white slag is ≥30 min, and the Als content is controlled at 0.015 - 0.025% during the process. Vacuum treatment is carried out in the RH furnace. The vacuum holding time is not less than 20 min under a vacuum degree of 0.5 tor. The H content in the molten steel at the end of tapping is ≤2 ppm, and the O content is ≤10 ppm.

[0008] 2) Continuous casting: The whole process of protective casting is adopted. The superheat temperature of the tundish for continuous casting is ≤25 °C for consecutive casting furnaces and ≤35 °C for the first casting furnace. The size of the billet is 280 mm×280 mm.

[0009] 3) Heating: The heating time of the billet is 280 - 480 min. The temperature of the preheating section is ≤600 °C, the temperature of the heating section is 1140 - 1200 °C, and the temperature of the soaking section is 1160 - 1220 °C.

[0010] 4) Rolling: The bar and wire two-high horizontal-vertical alternating 850 rolling mill is used for rolling with large reduction. The reduction of the first and second passes is controlled at 70 mm and 65 mm respectively. The starting rolling temperature is ≥1000 °C. The final rolling is carried out by a three-high rolling mill, and the final rolling temperature is 700 - 740 °C.

[0011] 5) Cooling: After rolling, the round steel is cooled by strong water through the water tank to ensure that the temperature on the cooling bed is 500 - 580 °C, and the round steel is placed on the cooling bed with every other tooth.

[0012] 6) Finishing: The round steel is cut by sawing. The finished round steel is inspected by ultrasonic flaw detection + magnetic particle surface flaw detection.

[0013] 7) Stress relief annealing: The finished round steel is subjected to stress relief annealing as a whole in a trolley furnace. The annealing temperature is 300 - 350 °C, and the holding time is 4 hours. Finally, a non-quenched and tempered steel with a tensile strength ≥820 Mpa, a yield strength ≥540 Mpa, an elongation ≥22%, a reduction of area ≥60%, a grain size ≥7.0 grades, an impact energy Aku2 at room temperature ≥60 J, and a full-section hardness of 240 - 290 HBW for round steel with a diameter of 20 - 99 mm is obtained.

[0014] Principle of the invention:

[0015] The present invention mainly ensures the strength, hardness and toughness of the product from the following aspects.

[0016] (1) Design of chemical composition: Non-quenched and tempered steel refers to adding trace amounts of strong carbide-forming elements to medium-carbon steel. After controlled rolling and controlled cooling and cooling to room temperature, precipitation strengthening of carbides (nitrides) is utilized to make it reach the strengthening level after quenching and tempering of medium-carbon steel, thus eliminating the quenching and tempering treatment process. It not only saves energy and protects the environment, shortens the production cycle, but also can avoid quenching deformation and cracking, improve product quality and reduce manufacturing costs.

[0017] Determination of C content: As the most economical and basic strengthening element in steel, C can significantly improve the strength of steel through solid solution strengthening and precipitation strengthening. However, too high C content will have an adverse impact on the center segregation, plasticity, toughness and welding performance of steel. The C content range of this invention is determined to be 0.37 - 0.44%, and the steel involved in this invention belongs to the medium carbon steel category;

[0018] Determination of Si content: Si here strengthens ferrite, can improve the matrix strength of the material, and makes significant contributions to the yield strength and tensile strength of the material. The Si content range of this invention is determined to be 0.17 - 0.37%;

[0019] Determination of Mn content: Here, Mn can improve the tensile strength of steel through solid solution strengthening, and at the same time improve the hardenability of steel. Moreover, as a deoxidizing element in the steelmaking process, Mn can also fix the form of sulfur in steel and form MnS and (Fe, Mn)S with less harm to the performance of steel, reducing or inhibiting the generation of FeS. However, Mn element belongs to the element prone to segregation, and too high content will cause serious center segregation of steel. The Mn content range of this invention is determined to be 0.5% - 0.8%;

[0020] Determination of Cr content: Cr is a carbide - forming element. Cr dissolved in austenite during heating can improve the hardenability. Part of the Cr in steel dissolves into ferrite, producing solid solution strengthening and improving the strength and hardness of ferrite. Cr can also reduce the overheating tendency and surface decarburization rate of steel. However, too high Cr content will combine with carbon in steel and easily form large carbide, which reduces the plasticity and toughness of steel. And too high Cr content makes the hardness of steel too large, which is not conducive to customer processing and use. Considering the above factors comprehensively, the Cr content range of this invention is determined to be 0.90 - 1.1%;

[0021] Determination of Al content: The Al element forms fine and dispersed aluminum nitride inclusions with N, which can refine the grains. However, when the Al content is large, brittle inclusions such as Al2O3 are easily formed during the melting process of molten steel, reducing the purity of molten steel. At the same time, Al is also added as a deoxidizing element in steel, reducing the dissolved oxygen in molten steel. The Al content range of this invention is determined to be 0.005 - 0.025%;

[0022] Determination of S element: The S element can combine with the Mn element to form hard and brittle MnS, improving the cutting performance of steel and promoting the formation of intragranular ferrite. However, too much S content will make the steel have hot brittleness, reduce the plasticity and toughness of steel, and increase the center segregation of steel. The S content range of this invention is determined to be ≤0.030%;

[0023] Determination of P content: P causes serious segregation during solidification in steel. P dissolves in ferrite, making the grains distorted and coarse, and increasing cold brittleness. The P content range of this invention is determined to be ≤0.030%;

[0024] Determination of N content: In the steel, both V and Al elements need to combine with N to form nitrides in order to play the role of improving strength and refining grains. Therefore, an appropriate amount of N element is required. According to the contents of V and Al and the stoichiometric ratio of the corresponding nitrides, the range of N content in the present invention is determined to be 0.003 - 0.008%.

[0025] (2) Reduce non-metallic inclusions in the steel. Due to the existence of non-metallic inclusions in the steel, the continuity of the metal matrix is damaged, stress concentration is likely to occur, and it becomes the origin of metal fatigue. Most cracks occur between oxides, point-like inclusions and the matrix. When the stress is large enough, cracks are generated and quickly expand to cause damage. The lower the plasticity of non-metallic inclusions and the sharper their shape, the greater the stress concentration. In order to improve the fatigue life of the product, control the vacuum degassing treatment and secondary refining outside the furnace well, reduce the content of non-metallic inclusions, and change the type and distribution state of inclusions.

[0026] (3) Optimize the rolling process to improve the grain size of the product. The surface layer of the round steel is a tempered structure, and the metallographic structure of the rest of the round steel is ferrite + pearlite, and the grain size is 7 - 9 grades; fine grains can improve the strength and toughness of the product, and correspondingly improve the fatigue resistance.

[0027] Advantages of the present invention: The present invention adopts a medium-carbon low-alloy composition design, LF + RH combined refining technology, strictly controls the purity of molten steel, and adopts new technologies such as hot rolling with heavy reduction at high temperature, controlled rolling and controlled cooling, and on-line quenching and tempering. The non-quenched and tempered steel for shafts developed has reached the strength and plasticity indexes of the quenched and tempered material 40Cr, with a tensile strength ≥ 820 Mpa, a yield strength ≥ 540 Mpa, an elongation ≥ 22%, a reduction of area ≥ 60%, a grain size ≥ 7.0 grades, a room temperature impact energy Aku2 ≥ 60 J, and a non-quenched and tempered steel with a full-section hardness of 240 - 290 HBW for round steel with a diameter of 20 mm - 99 mm. Its production cost is lower than that of quenched and tempered steel, with obvious economic benefits. The present invention breaks through the conventional production process, utilizes the existing equipment and process conditions of the steel mill, gives full play to the equipment advantages, is energy-saving and environmentally friendly, and the non-quenched and tempered steel produced with high toughness, uniform hardness and low cost can be widely used in manufacturing key parts such as motor shafts. Description of the Drawings

[0028] Figure 1 It is the metallographic picture of 1 / 4 diameter of the round bar in Example 1;

[0029] Figure 2 It is the metallographic picture of the surface layer 2 mm of the round bar in Example 1;

[0030] Figure 3 It is the metallographic picture of 1 / 4 diameter of the round bar in Example 2;

[0031] Figure 4Metallographic picture of the 2-mm surface layer of the round bar in Example 2;

[0032] Figure 5 Metallographic picture of the 1 / 4 diameter of the round bar in Example 3;

[0033] Figure 6 Metallographic picture of the 2-mm surface layer of the round bar in Example 3. Detailed implementation method

[0034] Example 1:

[0035] A production method of non-quenched and tempered steel for shafts. The chemical composition of the steel is in weight percentages as follows: C = 0.38%, Si = 0.25%, Mn = 0.7%, P = 0.010%, S = 0.005%, Cr = 1.0%, Al = 0.010%, N = 0.005%, H = 0.0002%, O = 0.0010%, and the rest is Fe and inevitable impurities. The key process steps include:

[0036] 1) Smelting: The C content of the molten steel tapped from the converter is 0.09%. The LF + RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process for slag making and deoxidation. The white slag holding time is 35 minutes, and the Als content is controlled at 0.020% during the process; RH vacuum treatment is carried out. Under a vacuum degree of below 0.5 tor, the vacuum holding time is 22 minutes. The H content of the molten steel leaving the station is 1.6 ppm, and the O content is 10 ppm;

[0037] 2) Continuous casting: The whole-process protected casting is adopted. The superheat of the tundish is 24°C for the continuous casting furnace and 30°C for the starting furnace. The size of the cast slab is 280 mm × 280 mm;

[0038] 3) Heating: The heating time of the cast slab is 300 minutes. The temperature of the preheating section is 580°C, the temperature of the heating section is 1160°C, and the temperature of the soaking section is 1180°C;

[0039] 4) Rolling: The final finished round steel specification is 72 mm; The bar and wire two-high horizontal-vertical alternating 850 rolling mill is used for rolling with large reduction. The reduction of the first and second passes is controlled at 70 mm and 65 mm respectively; The starting rolling temperature is 1020°C; The final rolling is carried out by a three-high rolling mill, and the final rolling temperature is 710°C;

[0040] 5) Cooling: The rolled round steel is cooled by passing through water in a water tank. The temperature on the cooling bed is 560°C, and the cooling bed is loaded with an interval tooth;

[0041] 6) Finishing: The round steel is cut by sawing. The finished round steel is inspected by ultrasonic flaw detection + magnetic particle surface flaw detection;

[0042] 7) Stress relief annealing: The whole finished round steel is stress relieved by a trolley furnace. The annealing temperature is 320°C, and the holding time is 4 hours.

[0043] The results of the performance inspection of the steel are shown in Tables 1 to 3. The metallographic diagram of the round steel can be seen in Figure 1 and Figure 2 .

[0044] Example 2:

[0045] A production method of non-quenched and tempered steel for shafts, the chemical composition of the steel is composed of the following weight percentages: C = 0.43%, Si = 0.25%, Mn = 0.75%, P = 0.010%, S = 0.005%, Cr = 1.05%, Al = 0.010%, N = 0.005%, H = 0.0002%, O = 0.0010%, and the rest is Fe and inevitable impurities. The key process steps include:

[0046] 1) Smelting: The C content of the molten steel tapped from the converter is 0.09%. The LF+RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process for slag making and deoxidation. The white slag holding time is 35 minutes, and the Als content is controlled at 0.020% during the process; RH vacuum treatment is carried out. Under a vacuum degree of less than 0.5 tor, the vacuum holding time is 25 minutes. The molten steel at the station has an H content of 1.6 ppm and an O content of 10 ppm;

[0047] 2) Continuous casting: The whole-process protected casting is adopted. The superheat of the tundish is 24 °C for the continuous casting furnace and 30 °C for the starting furnace. The size of the cast slab is 280 mm×280 mm;

[0048] 3) Heating: The heating time of the cast slab is 460 minutes, the temperature of the preheating section is 580 °C, the temperature of the heating section is 1180 °C, and the temperature of the soaking section is 1210 °C;

[0049] 4) Rolling: The final finished round steel specification is 65 mm; The 850 rolling mill with two-high horizontal and vertical alternating rolls for bars and wires is used for rolling with large reduction. The reduction of the first and second passes is controlled at 70 mm and 65 mm respectively; The starting rolling temperature is 1050 °C; The final rolling is carried out by a three-high rolling mill, and the final rolling temperature is 710 °C;

[0050] 5) Cooling: The rolled round steel is cooled by passing through water in a water tank. The temperature on the cooling bed is 540 °C, and the cooling bed is loaded in a staggered-tooth manner;

[0051] 6) Finishing: The round steel is cut by sawing. The finished round steel is inspected by ultrasonic flaw detection + magnetic particle surface flaw detection;

[0052] 7) Stress relief annealing: The whole finished round steel is stress relief annealed in a trolley furnace. The annealing temperature is 320 °C, and the holding time is 4 hours.

[0053] The results of the performance inspection of the steel are shown in Tables 1 to 3. The metallographic diagram of the round steel can be seen in Figure 3 and Figure 4 .

[0054] Example 3:

[0055] A production method of non - quenched and tempered steel for shafts. The chemical composition of the steel in weight percentage is C = 0.41%, Si = 0.25%, Mn = 0.55%, P = 0.010%, S = 0.005%, Cr = 0.92%, Al = 0.010%, N = 0.006%, H = 0.0002%, O = 0.0010%, and the rest is Fe and inevitable impurities. The key process steps include:

[0056] 1) Smelting: The C content of the molten steel tapped from the converter is 0.10%. The LF + RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process for slag making and deoxidation. The white slag holding time is 35 min, and the Als content is controlled at 0.020% during the process. For RH vacuum treatment, under a vacuum degree of less than 0.5 tor, the vacuum holding time is 28 min. The H content of the molten steel out of the station is 1.6 ppm, and the O content is 10 ppm.

[0057] 2) Continuous casting: The whole - process protected casting is adopted. The superheat degree of the tundish for continuous casting is 24 °C for consecutive casting furnaces and 30 °C for the first - casting furnace. The size of the billet is 280×280 mm.

[0058] 3) Heating: The heating time of the billet is 360 min, the temperature of the pre - heating section is 560 °C, the temperature of the heating section is 1165 °C, and the temperature of the soaking section is 1180 °C.

[0059] 4) Rolling: The final finished round steel specification is 43 mm. The 850 rolling mill with two - high horizontal and vertical alternating rolls for bars and wires is used for rolling with large reduction. The reduction of the first and second passes is controlled at 70 mm and 65 mm respectively. The starting rolling temperature is 1025 °C. The final rolling is carried out by a three - high rolling mill, and the final rolling temperature is 740 °C.

[0060] 5) Cooling: The rolled round steel is cooled by passing through water in a water tank. The temperature on the cooling bed is 580 °C, and the round steel is placed on the cooling bed with teeth separated.

[0061] 6) Finishing: The round steel is cut by sawing. The finished round steel is inspected by ultrasonic flaw detection + magnetic particle surface flaw detection.

[0062] 7) Stress - relieving annealing: The whole finished round steel is stress - relieved annealed in a trolley furnace. The annealing temperature is 320 °C, and the holding time is 4 hours.

[0063] The performance test results of the steel are shown in Table 1 - Table 3. The metallographic diagram of the round steel is shown in Figure 5 、 Figure 6 。

[0064] Table 1 Mechanical properties of the steel in the example

[0065] Serial number Tensile strength, MPa Yield strength, MPa Elongation, % Reduction of area, % Example 1 955 724 22.0 64 Example 2 980 720 22.4 66 Example 3 961 712 23.2 65

[0066] Table 2 Inspection of impact energy, hot - rolled hardness, grain size, and banded structure of the steel in the example

[0067] Serial number Impact energy, J Hot-rolled core hardness, HBW Hot-rolled surface hardness, HBW Grain size, grade Banded structure, grade Example 1 75 249 286 8 1.5 Example 2 80 245 290 8 2.0 Example 3 86 258 283 8.5 1.5

[0068] Table 3 Inspection of Non-Metallic Inclusions in Steel for Examples

[0069] Serial number A B C D Ds Example 1 1.5 0 0 1.0 0 Example 2 1.0 0 0 1.0 0 Example 3 1.0 0 0 1.0 0

[0070] As can be seen from the test results in Table 1 to Table 3, for the non-quenched and tempered steel for shafts produced by the method of the present invention, all inspection indicators are excellent, the gas content and non-metallic inclusions in the steel are low, and it can well meet the usage requirements of motor shaft products.

Claims

1. A production method of non - quenched and tempered steel for shafts, characterized in that: The chemical composition of the steel by weight percentage is C = 0.37% - 0.44%, Si = 0.17% - 0.37%, Mn = 0.5% - 0.8%, P ≤ 0.030%, S ≤ 0.030%, Cr = 0.90% - 1.1%, Al = 0.005% - 0.025%, N = 0.003% - 0.008%, H ≤ 0.0002%, O ≤ 0.0020%, and the rest is Fe and inevitable impurities; The key process steps include: 1) Smelting: The tapped steel from the converter has C ≥ 0.08%. The LF + RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process for slag making and deoxidation. The white slag holding time is ≥ 30 min, and the process control of Als is 0.015 - 0.025%. Vacuum treatment is carried out in the RH furnace. The vacuum holding time is not less than 20 min under a vacuum degree of 0.5 tor. The molten steel at the station has H ≤ 2 ppm and O ≤ 10 ppm; 2) Continuous casting: The whole - process protected casting is adopted. The superheat of the tundish for continuous casting is ≤ 25℃ for consecutive casting furnaces and ≤ 35℃ for the starting casting furnace. The size of the billet is 280 mm × 280 mm; 3) Heating: The heating time of the billet is 280 - 480 min. The temperature of the pre - heating section is ≤ 600℃, the temperature of the heating section is 1140 - 1200℃, and the temperature of the soaking section is 1160 - 1220℃; 4) Rolling: The bar and wire two - high horizontal - vertical alternating 850 rolling mill is used for rolling with large reduction. The reduction of the first and second passes is controlled at 70 mm and 65 mm respectively. The starting rolling temperature is ≥ 1000℃. The finishing rolling is carried out by a three - high rolling mill, and the finishing rolling temperature is 700 - 740℃; 5) Cooling: The rolled round steel is cooled by strong water penetration in a water tank to ensure that the temperature on the cooling bed is 500 - 580℃, and it is fed onto the cooling bed with alternate teeth; 6) Finishing: The round steel is cut by sawing. The finished round steel is inspected by ultrasonic flaw detection + magnetic particle surface flaw detection; 7) Stress - relieving annealing: The whole finished round steel is stress - relieved annealed in a trolley furnace. The annealing temperature is 300 - 350℃, and the holding time is 4 hours. Finally, a non - quenched and tempered steel with a tensile strength ≥ 820 Mpa, a yield strength ≥ 540 Mpa, an elongation ≥ 22%, a reduction of area ≥ 60%, a grain size ≥ 7.0 grades, a room - temperature impact energy Aku2 ≥ 60 J, and a full - section hardness of 240 - 290 HBW for round steel with a diameter of 20 - 99 mm is obtained.

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