Production method of non-quenched and tempered mechanical structural steel for direct cutting machining

Abstract

The present invention discloses a production method of a non-quenched and tempered mechanical structural steel for direct cutting machining. The non-quenched and tempered mechanical structural steel is formed by adding appropriate alloying elements on the basis of medium-carbon manganese steel. In the rolling process of the non-quenched and tempered mechanical structural steel: in the heating furnace heating process, the preheating section is controlled to be ≤780°C, the heating section temperature is: 1100±30°C, and the soaking section temperature is: 1180±30°C; the rolling temperature of the round steel entering the continuous rolling is controlled at 820°C±20°C; in the continuous rolling process, the cooling between the continuous rolling mills is carried out by using several groups of movable water-cooled through-tubes. Through special composition design and by adopting the controlled rolling and controlled cooling process, the present invention obtains excellent hot-rolled comprehensive mechanical properties through precipitation hardening and fine grain strengthening, replaces the subsequent quenching and tempering process, saves the quenching and tempering cost, shortens the delivery period, reduces environmental pollution, requires less equipment investment, and can realize the production of large-size non-quenched and tempered mechanical structural steel for cutting with a size of 100mm to 150mm.

Description

Technical Field

[0001] The present invention belongs to the field of steel manufacturing, and particularly relates to a production method of non-quenched and tempered mechanical structure steel for direct cutting processing. Background Art

[0002] Non-quenched and tempered steel is based on medium-carbon manganese steel and added with micro-alloying elements such as vanadium, titanium, and niobium, which are dissolved in austenite during the heating process. Since the solid solubility of vanadium, titanium, and niobium in austenite decreases with cooling, the micro-alloying elements vanadium, titanium, and niobium will precipitate in the form of fine carbides and nitrides in the prior-precipitated ferrite and pearlite. These precipitates maintain a coherent relationship with the matrix phase, strengthening the steel. The mechanical properties of this type of steel in the hot-rolled state reach or approach those of quenched and tempered steel, which not only shortens the production cycle but also saves energy. Compared with quenched and tempered steel, the cost can be saved by more than 1000 yuan per ton.

[0003] The mechanical properties of non-quenched and tempered steel depend on the matrix microstructure and the strengthening of the precipitated phase. Non-quenched and tempered steel is divided into non-quenched and tempered steel for hot forging and non-quenched and tempered steel for direct cutting. Among them, non-quenched and tempered steel for direct cutting is directly processed into parts from hot-rolled parts, such as piston rods, injection molding machine rods, etc., and has a good market prospect. In order to control the mechanical properties, steel mills generally make appropriate adjustments to the composition. However, it is difficult to achieve the mechanical properties of quenched and tempered steel only by relying on chemical composition. At present, domestic steel mills are restricted by manufacturing equipment and rolling processes and are difficult to produce high-strength and large-size non-quenched and tempered steel products for direct cutting on the production line. The diameter of the non-quenched and tempered steel in the mature market generally does not exceed Φ100mm, and there is a large gap between the market demand and supply of non-quenched and tempered steel with a diameter of more than Φ100mm. Summary of the Invention

[0004] The purpose of the present invention is to provide a production method of non-quenched and tempered mechanical structure steel for direct cutting processing.

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

[0006] A production method of non-quenched and tempered mechanical structure steel for direct cutting processing, the non-quenched and tempered mechanical structure steel is formed by adding appropriate alloy elements on the basis of medium-carbon manganese steel, the alloy elements include V, Nb, N, and the main components and their weight percentages of the non-quenched and tempered mechanical structure steel are as follows: C: 0.38 - 0.45%, Si: 0.15 - 0.6%, Mn: 0.8 - 1.5%, Cr ≤ 0.2%, V ≤ 0.15%, Nb ≤ 0.03%, N ≤ 0.03%, and the rest is Fe.

[0007] The design idea of chemical composition is as follows: on the basis of medium-carbon manganese steel, appropriate alloying elements are added, and the controlled rolling and controlled cooling process is adopted to obtain excellent comprehensive mechanical properties through precipitation hardening and fine grain strengthening. Carbon has the greatest influence on the strength and hardness of steel, but its content needs to be controlled, otherwise it will seriously affect the plasticity and toughness of steel. The carbon content is 0.38% - 0.45% (mass fraction, the same below). Adding appropriate amount of Mn to the steel is beneficial to both the strength and toughness of the non-quenched and tempered steel, and the content is controlled at 0.8% - 1.5%. Si is an important ferrite strengthening element, but excessive Si will seriously reduce the toughness of the steel, and the Si content is generally 0.15% - 0.6%. At the same time, appropriate amounts of V, Nb, and N are added. During the hot rolling process, V and Nb nitrides precipitate as the second phase, which plays a role in refining austenite grains, refining ferrite grains, and improving the pearlite lamellar morphology, ensuring the mechanical properties in the hot-rolled state.

[0008] The rolling process of the non-quenched and tempered mechanical structural steel is as follows: continuous casting → hot charging and hot delivery → heating in the heating furnace → BD rough rolling → air cooling and temperature reduction in the waiting device → continuous rolling (including several sets of water-cooling devices) → grouping at the grouping table → shearing the head and tail → slow cooling (stack cooling) → surface quality inspection → chamfering → weighing → packing → warehousing, where:

[0009] In the heating process of the heating furnace, the preheating section temperature is controlled ≤780°C, the heating section temperature is 1100 ± 30°C, and the soaking section temperature is 1180 ± 30°C. Preferably, the total heating time in the heating furnace heating process is controlled not less than 2.5 hours, and the soaking time is not less than 1 hour. The above heating system can, on the one hand, gradually dissolve carbides and nitrides such as V and Nb into austenite, promote the precipitation and strengthening of the second-phase particles in solid solution during cooling, thereby improving the strength and hardness of the steel; on the other hand, it can avoid the growth of austenite grains and the coarsening of the structure and the decrease of toughness caused by too high temperature. The reason for choosing such heating temperatures is that the AC3 temperature of this steel grade is 803 ± 10°C (obtained according to the steel composition), and the liquidus line is about 1480 ± 10°C. If the heating temperature exceeds 1280°C, overheating will occur, the grains will grow abnormally, and the toughness will decrease. However, if the heating temperature is too low, it cannot ensure the best plasticity and the minimum deformation resistance of the steel during the primary rolling process.

[0010] In the BD rough rolling process, the BD rolling mill uses a large reduction of 60 - 70 mm per single pass for rough rolling to play a role in welding the porosity of the continuous casting billet, breaking the original coarse austenite grains, refining the grains, and controlling the rolling temperature rise.

[0011] After air cooling and temperature reduction by the waiting temperature device, the rolling temperature of the round steel entering the continuous rolling is controlled at 820±20°C to achieve rolling in the austenite low-temperature recrystallization range, thereby obtaining fine austenite grains and reducing the cross-sectional temperature difference. According to the steel composition, the AC3 temperature of this steel grade is 803±10°C. Using 820±20°C, which is slightly higher than the AC3 temperature and within the austenite region, can ensure good processing plasticity. If the temperature is too high, the austenite grains will grow during further heating in the continuous rolling process, making it difficult to meet the mechanical properties of the non-normalized steel. If the temperature is too low, below the austenite region, the plasticity of the steel will decrease, which is not conducive to metal processing deformation.

[0012] In the continuous rolling process, the cooling between the continuous rolling mills is carried out by several groups of movable water-through cooling water tanks, so as to adjust the position of the water tanks, the water flow rate of the water tanks and the number of water tanks used according to different specifications of round steel, achieving the purpose of changing the ratio of ferrite pearlite in the steel, refining the ferrite and pearlite lamellar spacing, and making the steel have good strength and toughness. On the existing continuous rolling mills, the water-through cooling water tanks are basically only fixedly arranged before and after the finishing rolling, and are not arranged after the first rolling (medium rolling) process. However, for large-size steel materials that only require blooming + medium rolling, it takes a long time to be water-cooled after rolling. During this process, the grains have grown, resulting in the coarsening of ferrite and the increase of pearlite lamellar spacing, and the deterioration of the strength and toughness of the steel. Therefore, in the continuous rolling process, a specially designed water-through cooling water tank that can move back and forth along the rolling direction is used for water cooling, so as to adjust the position of the water tank, the water flow rate of the water tank and the number of water tanks used according to different specifications of round steel, achieve uniform cooling of large-size round steel with a diameter of more than 100mm after rolling, reduce the cross-sectional temperature difference of the round steel, and is crucial for ensuring the performance of non-normalized steel materials for direct cutting processing with a diameter of more than Φ100mm.

[0013] Preferably, in the continuous rolling process, the continuous rolling mill set includes 14 continuous rolling mills, and the cooling between the continuous rolling mill sets is completed by 4 groups of movable water-through cooling water tanks.

[0014] In the processes of grouping on the grouping table and shearing the head and tail, the temperature control on the upper grouping table: For this steel grade, the Ar1 temperature is 711±10°C, and the AC1 temperature is 741±10°C. To ensure that after water cooling, the round steel can generate self-tempering through the heat conduction from the core to the outside, and no phase transformation and grain growth occur, and a uniform mechanical property and metallographic structure of the cross-section are formed, it is necessary to control the temperature on the upper grouping table at 650±20°C and the temperature of the hot saw for shearing the head and tail at 605±20°C. At the same time, the sawing temperature cannot be too low, so as to avoid difficult sawing and too low slow cooling temperature. Since the Ar1 temperature of the steel grade is 711±10°C, the temperature of 650±20°C on the upper grouping table is slightly lower than the Ar1 temperature, which can ensure that no phase transformation occurs during self-tempering. If the temperature is too high, there may be phenomena of phase transformation and grain growth, and the mechanical properties are difficult to guarantee. If the temperature is too low, after grouping and cooling, the sawing temperature is too low, resulting in difficult hot sawing.

[0015] In the slow cooling process, after the round steel is cut to a fixed length and quickly collected through the slow cooling material collecting bench, it is hoisted into the slow cooling pit for slow cooling. The temperature of the round steel entering the slow cooling pit is controlled not to be lower than 480°C. The round steel is slowly cooled in the slow cooling pit for 24 - 48 hours and the finished product is obtained after coming out of the pit. By performing medium-temperature self-tempering at about 500°C, the purpose of releasing internal stress, reducing dislocations, and reducing aging can be achieved, so that users can directly perform cutting processing. About 500°C can ensure the recovery and recrystallization of deformed ferrite grains, eliminate the white spots in the steel, and avoid cracks caused by thermal stress and tissue stress during the subsequent cooling process. If the temperature is too low, the above effects cannot be achieved.

[0016] In the present invention, through a large amount of research and experiments, the inventor finally obtained the solution and effects of the present invention, among which:

[0017] A reasonable heating temperature can gradually dissolve carbonitrides such as V and Nb into austenite, which can promote the precipitation strengthening of the dissolved second-phase particles during cooling, thereby improving the strength and hardness of the steel. At the same time, the temperature should be avoided being too high to cause the growth of austenite grains and the coarsening of the structure, resulting in a decrease in toughness.

[0018] A reasonable rolling process combination: Adopt the process combination method of rough rolling and blooming with a BD rolling mill + air cooling and temperature reduction with a waiting temperature device + rolling with a continuous rolling mill. For the first rolling, a BD rolling mill that can achieve large reduction is used, which can play a role in welding the looseness of the cast billet, crushing the original coarse austenite grains, refining the grains, and controlling the rolling temperature rise. Before entering the continuous rolling, the rolling interval time between the first rolling and the intermediate rolling is adjusted through the waiting temperature device to control the rolling temperature of the round steel entering the continuous rolling mill, so as to realize rolling in the low-temperature recrystallization range of austenite, thereby obtaining fine austenite grains.

[0019] A reasonable water-through cooling process can improve the properties of steel through the diffusion phase transformation mechanism and grain refinement. The cooling device in the present invention is composed of several groups of movable water-through cooling water tanks, which can be adjusted according to the position of the water tanks, the water flow rate of the water tanks and the number of water tanks used for round steel of different specifications, so as to change the proportion of ferrite pearlite in the steel, refine the lamellar spacing of ferrite and pearlite, and make the steel have good strength and toughness.

[0020] By controlling the appropriate temperature on the upper marshalling table, the cooled round steel can produce a self-tempering effect through heat conduction from the inside to the outside, and form a uniform mechanical property and metallographic structure in the cross section.

[0021] By controlling the temperature of the round steel entering the slow cooling pit not to be lower than 480 °C, and then controlling the round steel to be slowly cooled in the slow cooling pit for 24-48 hours to obtain the finished product, the internal stress of the round steel can be released, the dislocations can be reduced and the aging can be reduced.

[0022] The beneficial effects of the present invention are as follows:

[0023] A production method of non-quenched and tempered mechanical structure steel for direct cutting processing according to the present invention, based on medium-carbon manganese steel through special composition design, adding appropriate alloying elements, and adopting controlled rolling and controlled cooling process, obtains excellent hot-rolled comprehensive mechanical properties through precipitation hardening and fine grain strengthening, replacing the subsequent quenching and tempering process. It can not only save the quenching and tempering cost by more than 1000 yuan / ton, but also shorten the delivery period of the user's quenching and tempering, reduce the environmental pollution and carbon emissions generated during the quenching and tempering process, and can be widely used in manufacturing industries such as automobiles, machine tools and construction machinery; in addition, compared with other production methods of non-quenched steel for cutting processing, the production method of the present invention has less additional equipment investment, can produce non-quenched and tempered mechanical structure steel for cutting with large sizes of 100 mm to 150 mm, and effectively solves the shortage problem of non-quenched and tempered mechanical structure steel for cutting with large sizes in the market. Brief Description of the Drawings

[0024] For easy explanation, the present invention will be described in detail by the following specific embodiments and drawings.

[0025] Figure 1 Schematic diagram of the rolling line structure used in the embodiment of the present invention;

[0026] Figure 2 Metallographic diagram of round steel with a diameter of 100 mm at 500 times after one rolling in the continuous rolling area without timely water cooling;

[0027] Figure 3 Metallographic diagram of round steel with a diameter of 100 mm at 500 times after one rolling in the continuous rolling area with timely water cooling by a water-through cooling water tank;

[0028] Among them, 1 is a BD rolling mill, 2 is a temperature-holding device, 3 is a continuous rolling area, 3.1 is an intermediate rolling unit, 3.2 is a finishing rolling unit, and 4 is a water-through cooling water tank. Specific implementation method

[0029] A production method of a non-quenched mechanical structural steel for direct cutting machining, the non-quenched mechanical structural steel is formed by adding appropriate alloying elements on the basis of medium-carbon manganese steel, the alloying elements include V, Nb, N, and the main components and their weight percentages of the non-quenched mechanical structural steel are as follows: C: 0.38 - 0.45%, Si: 0.15 - 0.6%, Mn: 0.8 - 1.5%, Cr ≤ 0.2%, V ≤ 0.15%, Nb ≤ 0.03%, N ≤ 0.03%, and the rest is Fe;

[0030] The rolling process of the non-quenched mechanical structural steel is: continuous casting → hot charging and hot delivery → heating furnace heating → BD rough rolling → temperature-holding device air cooling for temperature reduction → continuous rolling (including several sets of water-through cooling devices) → grouping table grouping → shearing head and tail → slow cooling → surface quality inspection → chamfering → weighing → packing → warehousing, and its BD rough rolling, temperature-holding device air cooling for temperature reduction, and continuous rolling processes use Figure 1 The production line shown, where 1 is a BD rolling mill, 2 is a temperature-holding device, 3 is a continuous rolling area, 3.1 is an intermediate rolling unit, 3.2 is a finishing rolling unit, and 4 is a water-through cooling water tank, among which:

[0031] In the heating furnace heating process, control the preheating section ≤ 780°C, the heating section temperature: 1100 ± 30°C, the soaking section temperature: 1180 ± 30°C, and the total heating time is not less than 2.5 hours, of which the soaking time is not less than 1 hour. The above heating system can, on the one hand, gradually dissolve carbides and nitrides such as V and Nb into austenite, promote the precipitation and strengthening of the second-phase particles in solid solution during cooling, thereby improving the strength and hardness of the steel, and on the other hand, can avoid the growth of austenite grains and the coarsening of the structure and the decrease of toughness caused by too high temperature.

[0032] In the BD rough rolling process, the BD rough rolling process is completed by using the BD rolling mill 1. The BD rolling mill uses a large reduction of 60 - 70 mm in a single pass for rough rolling to play the role of welding the looseness of the continuous casting billet, breaking the original coarse austenite grains, refining the grains, and controlling the rolling temperature rise.

[0033] After cooling down by the temperature-holding device air cooling, control the rolling temperature of the round steel entering the continuous rolling at 820°C ± 20°C (the AC3 temperature of this steel grade is 803°C ± 10°C) to achieve rolling in the austenite low-temperature recrystallization range, thereby obtaining fine austenite grains and reducing the cross-sectional temperature difference.

[0034] The continuous rolling process is completed within the continuous rolling area 3. The continuous rolling process includes the medium rolling process and the finishing rolling process. The medium rolling process is completed by the medium rolling mill set 3.1, and the finishing rolling process is completed by the finishing rolling mill set 3.2. In this embodiment, the continuous rolling mill set includes 14 continuous rolling mills. The cooling between the continuous rolling mill sets is carried out by 4 groups of movable water-through cooling water tanks 4 to enable adjustment of the water tank position, the water flow rate of the water tank, and the number of water tanks used according to different specifications of round steel, so as to achieve the purpose of changing the ratio of ferrite pearlite in the steel, refining the ferrite and pearlite lamellar spacing, and making the steel have good strength and toughness. The water-through cooling water tank itself can adjust the water flow rate of the water tank. On the existing continuous rolling mills, the water-through cooling water tanks are basically only fixedly arranged before and after the finishing rolling, and there is no one arranged after the first rolling (medium rolling) process. However, for large-sized steel bars that only require rough rolling and blooming + medium rolling, it takes a long time to be water-cooled after rolling. During this process, the grains have grown, resulting in the coarsening of ferrite and the increase of pearlite lamellar spacing, and the problem of poor strength and toughness of the steel. Therefore, in the continuous rolling process, a specially designed water-through cooling water tank that can move back and forth along the rolling direction is used for water cooling, enabling adjustment of the water tank position, the water flow rate of the water tank, and the number of water tanks used according to different specifications of round steel, achieving uniform cooling of large-sized round steel with a diameter of more than 100 mm after rolling, reducing the temperature difference across the cross-section of the round steel, which is crucial for ensuring the performance of non-adjustable steel for direct cutting with a diameter of more than Φ100 mm.

[0035] As Figure 2 shown is the metallographic diagram of a round steel with a diameter of 100 mm at 500 times magnification after the medium rolling process without timely water cooling. As Figure 3 shown is the metallographic diagram of a round steel with a diameter of 100 mm at 500 times magnification after the medium rolling process with timely water cooling by the water-through cooling water tank. It can be seen from the comparison that the grains of the round steel with timely water cooling by the water-through cooling water tank after the medium rolling in the continuous rolling process are more uniform and finer, achieving the purpose of making the steel have good strength and toughness.

[0036] In the grouping table grouping and cutting head and tail processes, the upper grouping temperature control: control the temperature of the upper grouping table at 650 ± 20 °C, and the temperature of the cutting head and tail hot saw at 605 ± 20 °C. The temperature of the upper grouping table at 650 ± 20 °C is slightly lower than the Ar1 temperature to ensure that self-tempering does not cause a phase change. If the temperature is too high, there may be phenomena of phase change and grain growth, and the mechanical properties are difficult to guarantee. If the temperature is too low, after grouping and cooling, the sawing temperature is too low, causing difficulties in hot sawing.

[0037] In the slow cooling process, after the round steel is cut to a fixed length and quickly collected through the slow cooling material collecting bench, it is hoisted into the slow cooling pit for slow cooling. The temperature of the round steel entering the slow cooling pit is controlled to be not lower than 480°C. The round steel is slow cooled in the slow cooling pit for 24 - 48 hours, and the finished product is obtained after coming out of the pit. By performing medium-temperature self-tempering at about 500°C, the purpose of releasing internal stress, reducing dislocations and reducing aging can be achieved, so that users can carry out direct cutting processing. About 500°C can ensure the recovery and recrystallization of deformed ferrite grains, eliminate the white spots in the steel and avoid cracks caused by thermal stress and tissue stress during the subsequent cooling process. If the temperature is too low, the above effects cannot be achieved.

[0038] As described above, it is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any change or replacement that is thought of without creative work should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope defined by the claims.

Claims

1. A production method of a non-quenched and tempered mechanical structural steel for direct cutting machining, characterized in that, The non-quenched and tempered mechanical structural steel is formed by adding appropriate alloying elements on the basis of medium-carbon manganese steel. The alloying elements include V, Nb, and N. The main components and their weight percentages of the non-quenched and tempered mechanical structural steel are as follows: C: 0.38 - 0.45%, Si: 0.15 - 0.6%, Mn: 0.8 - 1.5%, Cr ≤ 0.2%, V ≤ 0.15%, Nb ≤ 0.03%, N ≤ 0.03%, and the rest is Fe; The rolling process of the non-quenched and tempered mechanical structural steel is as follows: continuous casting → hot charging and hot delivery → heating in a heating furnace → BD rough rolling → air cooling and temperature reduction in a waiting temperature device → continuous rolling → grouping at a grouping table → cutting the head and tail → slow cooling → surface quality inspection → chamfering → weighing → packing → warehousing, where: In the heating process of the heating furnace, the preheating section is controlled to be ≤ 780 °C, the heating section temperature is: 1100 ± 30 °C, and the soaking section temperature is: 1180 ± 30 °C; After air cooling and temperature reduction in the waiting temperature device, the rolling temperature of the round steel entering the continuous rolling is controlled at 820 °C ± 20 °C; In the continuous rolling process, the cooling between the continuous rolling mills is carried out by using several groups of movable water-through cooling water tanks; In the grouping at the grouping table and cutting the head and tail processes, the upper grouping temperature control: the temperature of the upper grouping table is controlled at 650 ± 20 °C, and the temperature of the hot saw for cutting the head and tail is 605 ± 20 °C; In the BD rough rolling process, the BD rolling mill uses a large reduction of 60 - 70 mm in a single pass for rough rolling; In the slow cooling process, after the round steel is cut to a fixed length and quickly collected through the slow cooling material collecting bench, it is lifted into the slow cooling pit for slow cooling. The temperature of the round steel entering the slow cooling pit is controlled to be not lower than 480 °C, and the round steel is slowly cooled in the slow cooling pit for 24 - 48 h, and the finished product is obtained after coming out of the pit.

2. The production method of a non-quenched and tempered mechanical structural steel for direct cutting machining according to claim 1, characterized in that, In the continuous rolling process, the continuous rolling mill set includes 14 continuous rolling mills, and the cooling between the continuous rolling mills is completed by using 4 groups of movable water-through cooling water tanks.

Images