Production method of low-temperature-resistant free-cutting non-quenched and tempered steel

Abstract

A production method of low-temperature-resistant free-cutting non-quenched and tempered steel adopts a process route of a converter, an LF furnace, an RH furnace, continuous casting and hot rolling, and the steel comprises the following chemical components in percentage by weight: 0.35-0.39% of C, 0.60-0.75% of Si, 1.30-1.50% of Mn, less than or equal to 0.005% of P, 0.020-0.040% of S, 0.10-0.20% of Cr, less than or equal to 0.20% of Cu, 0.15-0.25% of Ni, 0.08-0.15% of V, less than or equal to 0.05% of Mo, 0.010-0.030% of Al, 0.010-0.040% of Ti, less than or equal to 0.03% of Sn, 0.010-0.025% of N, less than or equal to 0.0002% of H, less than or equal to The tensile strength of the obtained steel is larger than or equal to 780 Mpa, the yield strength is larger than or equal to 580 Mpa, the ductility is larger than or equal to 15%, the grain size is larger than or equal to 7.0 grades, the impact energy Aku2 at the temperature of-40 DEG C is larger than or equal to 27 J, the diameter of round steel ranges from 20 mm to 90 mm, the total section hardness ranges from 270 HBW to 300 HBW, and the steel can be widely used for manufacturing engineering machinery connecting pin shafts and the like used under the low-temperature weather condition.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a production method of energy-saving, environmentally friendly, low-temperature-resistant, easy-cutting non-quenched and tempered steel. Background Art

[0002] With the rapid development of the manufacturing industry, the performance requirements of steel in many fields such as automobiles, machinery, construction, and petrochemicals are increasing. As a high-strength and high-performance steel, non-quenched and tempered steel is increasingly used in these fields. Non-quenched and tempered steel is a high-strength steel with micro-alloying elements such as vanadium, titanium, and niobium added to medium-carbon manganese steel. These micro-alloying elements are dissolved in austenite during the heating process, and precipitate in the form of fine carbides and nitrides as the cooling process proceeds, thereby strengthening the steel. The application of microalloying technology has significantly improved the performance of non-quenched and tempered steel. Rolling and cooling processes, through advanced rolling and cooling processes, can produce steel that can be used without quenching and tempering treatment. The optimization of these processes not only improves the performance of steel, but also reduces production costs. In recent years, with the continuous development of materials science, the research and development of new non-quenched and tempered steels has also made significant progress. For example, the emergence of new steel grades such as bainitic non-quenched and tempered steel, niobium-based and vanadium-niobium composite non-quenched and tempered steels has provided more options for the application of non-quenched and tempered steels.

[0003] In automobile manufacturing, key parts such as connecting rods and crankshafts need to withstand large loads and impacts, and need to meet the requirements of high strength, high toughness and low temperature resistance. In the field of mechanical manufacturing, some precision parts need to have high cutting performance and low temperature resistance. In the field of new energy, such as wind power and nuclear power, the requirements for low temperature resistance and strength of steel are extremely high.

[0004] In summary, the application prospects of low-temperature resistant and easy-to-cut non-quenched and tempered steel will become increasingly broad. At the same time, with the rise of emerging fields such as new energy vehicles and wind power, higher requirements are placed on the low-temperature resistance and easy-to-cut properties of steel. The development of a low-temperature resistant and easy-to-cut non-quenched and tempered steel that can meet the market demand for these high-performance steels, and the material can be produced at low cost and improve its low-temperature performance, is a problem that everyone is concerned about and hopes to solve. However, the existing non-quenched and tempered steel is not good in terms of low-temperature resistance and easy cutting, and the impact energy at -40℃ is generally below 10J. Therefore, it is of great practical significance to develop a non-quenched and tempered steel with low cost, tensile strength ≥780Mpa, and -40℃ impact energy Aku2 ≥27J. Summary of the invention

[0005] The invention aims to provide a method for producing a non-quenched and tempered steel for direct turning which is resistant to low temperature and easy to cut. The method adopts a medium carbon steel microalloying design and combines steelmaking and rolling processes to control the organization and performance of hot-rolled round steel, and studies a non-quenched and tempered steel which is energy-saving, environmentally friendly, resistant to low temperature, easy to cut, has a tensile strength of ≥780Mpa, a yield strength of ≥580Mpa, an elongation of ≥15%, a grain size of ≥8.0, an impact energy of Aku2 of ≥27J at -40°C, a round steel diameter of 20-90mm, and a full-section hardness of 270-300HBW.

[0006] The purpose of the present invention is achieved through the following technical solutions: A production method of non-quenched and tempered steel for low-temperature resistant free-cutting direct turning, wherein the chemical composition of the steel is C=0.35%-0.39%, Si=0.60%-0.75%, Mn=1.30%-1.50%, P≤0.005%, S=0.020%-0.040%, Cr=0.10%-0.20%, Cu≤0.20%, Ni=0.15%-0.25%, V=0.08%-0.15%, Mo≤0.05%, Al=0.010%-0.030%, Ti=0.010%-0.040%, Sn≤0.03%, N=0.010%-0.025%, H≤0.0002%, O≤0.0020%, and the rest is Fe and essential impurities; the key process steps include: 1) Smelting: C≥0.08% and P≤0.003% of the steel tapped from the converter. The LF+RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process, slag is deoxidized, and the white slag retention time is ≥30min. The process is controlled by Als=0.015%~0.025%. The RH furnace is vacuum treated, and the retention time is not less than 20min at a vacuum degree below 0.5tor. Nitrogen is circulated throughout the RH furnace. After breaking the air, samples are taken for analysis of the nitrogen content. According to the test results, the nitriding core wire is fed with nitrogen to adjust the nitrogen content, and the nitrogen content is controlled to be 0.010%~0.025%. The H of the molten steel out of the station is ≤2ppm, and O is ≤10ppm. 2) Continuous casting: full protection casting is adopted, the overheat of the tundish continuous casting furnace is ≤20℃, the open casting furnace is ≤35℃, and the billet size is 280×280mm; 3) Heating: The heating time of the casting is 300-480 minutes, the temperature of the heating section is 1200-1260°C, and the temperature of the soaking section is 1220-1270°C; 4) Rolling: The bar and wire 850 two-roller horizontal and vertical alternating rolling mill adopts large reduction for rolling, and the first and second pass reductions are controlled at 70mm and 65mm respectively; the starting rolling temperature is ≥1100℃; the final rolling adopts a three-roller rolling mill, and the final rolling temperature is 680-720℃; 5) Cooling: After rolling, the round steel is cooled by strong water penetration in a water tank, the temperature of the upper cooling bed is controlled at 660-700℃, and a tooth-spaced upper cooling bed is used; 6) Finishing: The round steel is sawn, and the finished round steel is subjected to ultrasonic flaw detection + magnetic particle surface flaw detection to obtain a non-quenched and tempered steel with a round steel diameter of 20 to 90 mm, tensile strength ≥ 780 MPa, yield strength ≥ 580 MPa, elongation ≥ 15%, grain size ≥ 8.0, -40℃ impact energy Aku2 ≥ 27J, and full-section hardness of 270 to 300 HBW.

[0007] Principle of the invention: The present invention mainly ensures the strength, hardness and toughness of the product from the following aspects.

[0008] Chemical composition design. Non-quenched and tempered steel refers to adding a small amount of strong carbide-forming elements such as V, Nb, and Ti to medium carbon steel, cooling it to room temperature after controlled rolling and controlled cooling, and using the precipitation strengthening of carbon (nitride) to make it reach the strengthening level of medium carbon steel after quenching and tempering, thereby eliminating the quenching and tempering process. It is not only energy-saving and environmentally friendly, shortens the production cycle, but also avoids quenching deformation and cracking, improves product quality, and reduces manufacturing costs.

[0009] Determination of C content: C is the most economical and basic strengthening element in steel. It can significantly improve the strength of steel through solid solution strengthening and precipitation strengthening. However, too high C content will have an adverse effect on the central segregation, plasticity and toughness, and welding performance of steel. The C content of the present invention is determined to be in the range of 0.35% to 0.39%. The steel material involved in the present invention belongs to the category of medium carbon steel.

[0010] Determination of Si content: Si here strengthens ferrite, can improve the matrix strength of the material, and has a significant contribution to the yield strength and tensile strength of the material. The Si content range of the present invention is determined to be 0.60% to 0.75%.

[0011] 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 the steel and form MnS and (Fe, Mn)S, which have less harm to the performance of the steel, thereby reducing or inhibiting the formation of FeS. However, the Mn element is an element that is easy to segregate, and too high a content will cause serious central segregation of the steel. The range of the Mn content in the present invention is determined to be 1.30% to 1.50%.

[0012] Determination of Cr content: Cr is a carbide-forming element. Cr dissolved in austenite during heating can improve hardenability. Cr in steel partially dissolves in ferrite, producing solid solution strengthening, improving the strength and hardness of ferrite; Cr can also reduce the overheating tendency and surface decarburization rate of steel; however, if the Cr content is too high, it will combine with carbon in the steel to easily form large carbides. Such insoluble carbides reduce the plasticity and toughness of the steel; and if the Cr content is too high, the hardness of the steel is too high, which is not conducive to customer processing and use. Taking the above factors into consideration, the range of Cr content in the present invention is determined to be 0.10% to 0.20%.

[0013] Determination of Al content: Al element and N form dispersed and fine aluminum nitride inclusions to refine the grains, but a large Al content easily forms brittle inclusions such as Al2O3 during molten steel smelting, reducing the purity of the molten steel; at the same time, Al is also added as a deoxidizing element in steel to reduce the dissolved oxygen in the molten steel; the range of Al content in the present invention is determined to be 0.010% to 0.030%.

[0014] Determination of S element: S element can combine with Mn element to form hard and brittle MnS, thereby improving the cutting performance of steel and promoting the formation of intracrystalline ferrite. However, excessive S content will cause hot brittleness of steel, reduce the plasticity and toughness of steel, and increase the center segregation of steel. The range of S content in the present invention is determined to be 0.020% to 0.040%.

[0015] Determination of V element: V element has a high solubility in steel and is one of the most commonly used and most effective strengthening elements for microalloyed steel. The formation of V (C, N) affects the organization and performance of steel. It is mainly precipitated in the ferrite at the austenite grain boundary. During the rolling process, it can inhibit the recrystallization of austenite and prevent the growth of grains, thereby refining the ferrite grains and improving the strength and toughness of steel. Due to the precipitation of V (C, N), the formation of intracrystalline ferrite is promoted, and ferrite and pearlite are evenly distributed at the grain boundary and in the crystal, and the grains are significantly refined. The range of V content in the present invention is determined to be 0.08% to 0.15%.

[0016] Determination of P content: P in steel causes serious segregation during solidification. P dissolves in ferrite to cause grain distortion and coarsening, and increases cold brittleness. The range of P content in the present invention is determined to be ≤0.005%.

[0017] Determination of N content: V and Al elements in steel 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 content 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 0.010% to 0.025%.

[0018] Reduce non-metallic inclusions in steel. The presence of non-metallic inclusions in steel destroys the continuity of the metal matrix, easily generates stress concentration, and becomes the source of metal fatigue. Most cracks occur between oxides, point-like inclusions and the matrix. When the stress is large enough, cracks occur and rapidly expand and destroy. The lower the plasticity of non-metallic inclusions and the sharper the shape, the greater the stress concentration. In order to improve the fatigue life of the product, control the vacuum degassing treatment and refining outside the furnace, reduce the content of non-metallic inclusions, and change the type and distribution of inclusions.

[0019] Optimize the rolling process and improve the grain size of the product. The metallographic structure of round steel is ferrite + pearlite, and the grain size is 7 to 9; fine grains can improve the strength and toughness of the product, and correspondingly improve the fatigue resistance.

[0020] Beneficial effects of the present invention: The present invention uses special design of medium carbon microalloy components, LF+RH composite refining technology, strictly controls the purity of molten steel, and adopts high temperature and high pressure rolling, controlled rolling and controlled cooling new technologies to develop non-quenched and tempered steel that reaches strength and plasticity indicators, is resistant to low temperatures of -40°C, and has a production cost lower than that of quenched and tempered steel, with obvious economic benefits. The present invention breaks through conventional production processes, utilizes existing equipment and process conditions in steel mills, gives full play to equipment advantages, saves energy and protects the environment, and produces non-quenched and tempered steel with high toughness, uniform hardness, low temperature resistance and low cost, which can be widely used in key parts such as pins and rods of engineering machinery under low temperature conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a metallographic picture of a 1 / 4 diameter round rod in Example 1.

[0022] Figure 2 This is a metallographic picture of a 1 / 4 diameter round rod in Example 2.

[0023] Figure 3 This is a metallographic picture of a 1 / 4 diameter round rod in Example 3. DETAILED DESCRIPTION Example 1

[0024] The present embodiment is a production method of a low-temperature resistant free-cutting non-quenched and tempered steel for direct turning, characterized in that: the chemical composition weight percentage of the steel is C=0.36%, Si=0.65%, Mn=1.40%, P=0.004%, S=0.025%, Cr=0.15%, Cu=0.09%, Ni=0.20%, V=0.10%, Mo=0.01%, Al=0.020%, Ti=0.015%, Sn≤0.03%, N=0.012%, H=0.0001%, O=0.0015%, and the rest is Fe and essential impurities; the key process steps include: 1) Smelting: C=0.10% and P=0.002% of the steel tapped from the converter. The LF+RH furnace refining process was adopted. Argon was blown throughout the LF furnace refining process, slag was deoxidized, and the white slag holding time was 35 minutes. The process control Als=0.035%; the RH furnace was vacuum treated, with a vacuum degree below 0.5tor and a vacuum holding time of 25 minutes. Nitrogen was circulated throughout the RH furnace. After breaking the air, samples were taken for analysis of the nitrogen content. According to the test results, nitrogen was fed into the nitriding cored wire to adjust the nitrogen content to ensure that the nitrogen content was 0.015%. The H=1ppm and O=16ppm of the molten steel out of the station; 2) Continuous casting: full protection casting is adopted, the tundish superheat temperature of continuous casting furnace is 18℃, the open casting furnace is 32℃, and the billet size is 280×280mm; 3) Heating: The heating time of the casting is 360 minutes, the temperature of the heating section is 1220°C, and the temperature of the soaking section is 1230°C; 4) Rolling: The bar and wire 850 two-roller horizontal and vertical alternating rolling mill adopts large reduction for rolling, and the first and second pass reductions are controlled at 70mm and 65mm respectively; the starting rolling temperature is 1130℃; the final rolling adopts a three-roller rolling mill with a final rolling temperature of 685℃; 5) Cooling: After rolling, the round steel is cooled by strong water penetration in a water tank to ensure that the temperature of the upper cooling bed is 670℃, and the upper cooling bed is separated by teeth; 6) Finishing: The round steel is sawn, and the finished round steel is subjected to ultrasonic flaw detection + magnetic particle surface flaw detection.

[0025] The performance test results of the steel are shown in Tables 1 to 3. The metallographic diagram of the round steel is shown in the attached Figure 1 . Example 2

[0026] The present embodiment is a production method of a non-quenched and tempered steel for low-temperature resistant free-cutting direct turning, characterized in that: the chemical composition weight percentage of the steel is C=0.38%, Si=0.70%, Mn=1.42%, P=0.003%, S=0.030%, Cr=0.17%, Cu=0.08%, Ni=0.20%, V=0.12%, Mo=0.02%, Al=0.025%, Ti=0.020%, Sn≤0.03%, N=0.018%, H=0.0002%, O=0.0015%, and the rest is Fe and essential impurities; the key process steps include: 1) Smelting: C=0.12% and P=0.002% of the steel tapped from the converter. The LF+RH furnace refining process was adopted. Argon was blown throughout the LF furnace refining process, slag was deoxidized, and the white slag was kept for 35 minutes. The process control Als=0.035%; the RH furnace was vacuum treated, with a vacuum degree below 0.5tor and a vacuum holding time of 30 minutes. Nitrogen was circulated throughout the RH furnace. After breaking the air, samples were taken for analysis of the nitrogen content. According to the test results, nitrogen was fed into the nitriding core wire to adjust the nitrogen content to ensure that the nitrogen content was 0.019%. The H content of the molten steel out of the station was 2ppm and the O content was 15ppm. 2) Continuous casting: full protection casting is adopted, the tundish superheat temperature of continuous casting furnace is 17℃, the open casting furnace is 30℃, and the billet size is 280×280mm; 3) Heating: The heating time of the ingot is 480 minutes, the temperature of the heating section is 1255°C, and the temperature of the soaking section is 1270°C; 4) Rolling: The bar and wire 850 two-roller horizontal and vertical alternating rolling mill adopts large reduction for rolling, and the first and second pass reductions are controlled at 70mm and 65mm respectively; the starting rolling temperature is 1160℃; the final rolling adopts a three-roller rolling mill with a final rolling temperature of 720℃; 5) Cooling: After rolling, the round steel is cooled by strong water penetration in a water tank to ensure that the temperature of the upper cooling bed is 700℃, and the upper cooling bed is separated by teeth; 6) Finishing: The round steel is sawn, and the finished round steel is subjected to ultrasonic flaw detection + magnetic particle surface flaw detection.

[0027] The performance test results of the steel are shown in Tables 1 to 3. The metallographic diagram of the round steel is shown in the attached Figure 2 . Example 3

[0028] The present embodiment is a production method of a non-quenched and tempered steel for low-temperature resistant free-cutting direct turning, characterized in that: the chemical composition weight percentage of the steel is C=0.35-0.39%, Si=0.60-0.75%, Mn=1.30-1.50%, P≤0.005%, S=0.020-0.040%, Cr=0.10-0.20%, Cu≤0.20%, Ni=0.15-0.25%, V=0.08-0.15%, Mo≤0.05%, Al=0.010-0.030%, Ti=0.010-0.040%, Sn≤0.03%, N=0.010-0.025%, H≤0.0002%, O≤0.0020%, and the rest is Fe and essential impurities; the key process steps include: 1) Smelting: C≥0.08% and P≤0.003% of the steel tapped from the converter. The LF+RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process, slag is deoxidized, and the white slag retention time is ≥30min. The process control Als=0.015~0.025%; RH furnace vacuum treatment, the vacuum degree is below 0.5tor, and the vacuum retention time is not less than 20min. Nitrogen is circulated throughout the RH furnace. After breaking the air, samples are taken for analysis of the nitrogen content. According to the test results, the nitriding core wire is fed with nitrogen to ensure that the nitrogen content is 0.010~0.025%. The H of the molten steel out of the station is ≤2ppm, and O is ≤10ppm; 2) Continuous casting: full protection casting is adopted, the overheat of the tundish continuous casting furnace is ≤20℃, the open casting furnace is ≤35℃, and the billet size is 280×280mm; 3) Heating: The heating time of the casting is 300-480 minutes, the temperature of the heating section is 1200-1260°C, and the temperature of the soaking section is 1220-1270°C; 4) Rolling: The bar and wire 850 two-roller horizontal and vertical alternating rolling mill adopts large reduction for rolling, and the first and second pass reductions are controlled at 70mm and 65mm respectively; the starting rolling temperature is ≥1100℃; the final rolling adopts a three-roller rolling mill, and the final rolling temperature is 680-720℃; 5) Cooling: After rolling, the round steel is cooled by strong water penetration in a water tank to ensure that the temperature of the upper cooling bed is 660-700℃, and the upper cooling bed is separated by teeth; 6) Finishing: The round steel is sawn, and the finished round steel is subjected to ultrasonic flaw detection + magnetic particle surface flaw detection.

[0029] The performance test results of the steel are shown in Tables 1 to 3. The metallographic diagram of the round steel is shown in the attached Figure 3 .

[0030] It can be seen from the test results in Tables 1 to 3 that the non-quenched and tempered steel produced by the method of the present invention in the embodiment has excellent inspection indicators, low gas content and non-metallic inclusions in the steel, and can well meet the use requirements of low-temperature resistant engineering machinery pin products.

[0031] Table 1 Mechanical properties of example steel.

[0032] .

[0033] Table 2 Example steel impact energy, hot rolling hardness, grain size, banded structure inspection results.

[0034] .

[0035] Table 3 Inspection results of non-metallic inclusions in example steel.

[0036] .

Claims

1. A method for producing low temperature resistant free-cutting non-quenched and tempered steel, characterized in that: The chemical composition of steel is C=0.35%~0.39%, Si=0.60%~0.75%, Mn=1.30%~1.50%, P≤0.005%, S=0.020%~0.040%, Cr=0.10%~0.20%, Cu≤0.20%, Ni=0.15%~0.25%, V=0.08%~0.15%, Mo≤0.05%, Al=0.010%~0.030%, Ti=0.010%~0.040%, Sn≤0.03%, N=0.010%~0.025%, H≤0.0002%, O≤0.0020%, and the rest are Fe and essential impurities; The key process steps include: 1) Smelting: C≥0.08% and P≤0.003% of the steel tapped from the converter. The LF+RH furnace refining process is adopted. Argon is blown throughout the LF furnace refining process, slag is deoxidized, and the white slag retention time is ≥30min. The process is controlled by Als=0.015%~0.025%. The RH furnace is vacuum treated, and the retention time is not less than 20min at a vacuum degree below 0.5tor. Nitrogen is circulated throughout the RH furnace. After breaking the air, samples are taken for analysis of the nitrogen content. According to the test results, the nitriding core wire is fed with nitrogen to adjust the nitrogen content, and the nitrogen content is controlled to be 0.010%~0.025%. The H of the molten steel out of the station is ≤2ppm, and O is ≤10ppm. 2) Continuous casting: full protection casting is adopted, the overheat of the tundish continuous casting furnace is ≤20℃, the open casting furnace is ≤35℃, and the billet size is 280×280mm; 3) Heating: The heating time of the casting is 300-480 minutes, the temperature of the heating section is 1200-1260°C, and the temperature of the soaking section is 1220-1270°C; 4) Rolling: The bar and wire 850 two-roller horizontal and vertical alternating rolling mill adopts large reduction for rolling, and the first and second pass reductions are controlled at 70mm and 65mm respectively; the starting rolling temperature is ≥1100℃; the final rolling adopts a three-roller rolling mill, and the final rolling temperature is 680-720℃; 5) Cooling: After rolling, the round steel is cooled by strong water penetration in a water tank, and the temperature of the upper cooling bed is controlled at 660-700℃, and a tooth-spaced upper cooling bed is used; 6) Finishing: The round steel is sawn, and the finished round steel is subjected to ultrasonic flaw detection + magnetic particle surface flaw detection to obtain a non-quenched and tempered steel with a round steel diameter of 20 to 90 mm, tensile strength ≥ 780 MPa, yield strength ≥ 580 MPa, elongation ≥ 15%, grain size ≥ 8.0, -40℃ impact energy Aku2 ≥ 27J, and full-section hardness of 270 to 300 HBW.

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