Low temperature resistant tool steel having excellent dimensional stability and method for producing the same

Abstract

The present application relates to a kind of tool steel of excellent low temperature-resistant tool steel and its production method, and the chemical composition in steel is as follows by weight percentage: C 0.38%~0.70%, Si≤0.5%, Mn 0.4%~1.5%, Nb 0.01%~0.8%, Cr≤1.0%, Ni≤0.5%, 90≤C / N≤150, Ca 0.0005%~0.005%, wherein Si, Cr, Ni is not zero;Impurity element B≤0.0008%, P≤0.020%, S≤0.015%, the balance is Fe and inevitable impurities.The tool steel workpiece produced by the present application is above 50HRC in-20 ℃ hardness after heat treatment, the yield strength is above 500MPa, the impact energy is above 30J, the surface decarburization layer depth is not more than 10 μm, there is no intergranular oxidation, no microcrack, excellent wear resistance and impact toughness in low temperature environment, and the service life is obviously improved.

Description

Technical Field

[0001] This invention belongs to the field of tool steel production technology, and relates to a low-temperature resistant tool steel with excellent dimensions and its production method. Background Technology

[0002] With the development of mechanization in various tool manufacturing industries, agriculture, and animal husbandry, the demand for tools such as bimetal saws suitable for low-temperature operations, and for steels suitable for processing these tools, is increasing. Ordinary tool steels generally have high carbon content, are brittle, and have poor toughness and plasticity, making them prone to cracking in low-temperature environments, resulting in a short service life. In environments below -20°C, they suffer severe impact fracture failure. There is an urgent need to develop tool steels suitable for use in environments below -20°C. On the other hand, the surface quality of steel directly affects the manufacturing process and impact performance of processing tools. Steel with high surface quality can reduce grinding, increase manufacturing productivity, lower production costs, and improve impact resistance.

[0003] A novel 9SiCrAlBN alloy tool steel, application number 201210367813.5, has the following steel composition: C: 0.8–1.0; Si: 0.3–1.8; Mn: 0.3–1.8; Cr: 0.6–1.4; Na: 0–0.006; Mg: 0–0.003; Al: 0.8–1.8; B: 0–0.10; N: 0.01–0.10; nitrides 0.01–0.08. The addition of Na, Mg, Al, N, and nitrides (BN, AlN, TiN, etc.) significantly improves the wear resistance and strength of this alloy tool steel. However, it has poor toughness and plasticity, especially at low temperatures, making it unsuitable for use in low-temperature environments.

[0004] A method for preparing a low-temperature resistant steel pipe, application number 201310715230.1, has the following chemical composition by mass percentage: C 0.05-0.15, Si 0.1-0.3, Ni 2.2-2.8, Cr 1.5-2.0, Mn 0.8-1.2, Al 0.3-0.6, Mo 0.2-0.4, Nb 0.1-0.2, Ti 0.05-0.15, Nd 0.03-0.05, Er 0.02-0.04, N 0.004-0.008, V 0.002-0.005, S ≤0.01, p ≤0.02, with the balance being Fe. It exhibits excellent low-temperature impact toughness and can be used for extended periods in extremely low-temperature regions below -196℃. However, this steel has low carbon content, low strength and hardness, and poor wear resistance, making it unsuitable for use as tool steel.

[0005] A high-strength, low-temperature resistant steel and its heat treatment process are disclosed in application number 201510423279.9. The steel composition is as follows: nickel 2.0-7.05%; carbon 0.02-0.1%; silicon 0.02-0.1%; manganese 0.60-8.00%; chromium 0.3-0.5%; molybdenum 0.05-0.6%; copper 0.02-0.3%; aluminum 0.03-2.0%; phosphorus ≤0.010%; sulfur ≤0.002%; nitrogen 0.004-0.010%; oxygen 0.0005-0.002%; calcium 0.0005-0.005%. It has a good combination of low-temperature strength and toughness and can be used to manufacture structural steel for use in low-temperature environments. However, it has low carbon content, low strength and hardness, and poor wear resistance, making it unsuitable for use as tool steel.

[0006] A high-strength, low-temperature resistant steel and its production method, application number 201510720341.0, discloses the following composition: C 0.05-0.2%, Si 0.1-0.26%, Mn 0.60-8.00%, Cr 0.4-0.8%, Mo 0.12-0.28%, Cu≤0.03%, Al 0.02-0.06%, Ti 0.010-0.018%, P≤0.012%, S≤0.005%, N 0. The carbon content is 0.01%, O≤0.0012%, H≤0.00012%, Ca0.001-0.005%, Sb0.0001-0.0005%, Sn≤0.001%, As≤0.005%, B≤0.0010%, V≤0.08%, Nb≤0.03%. It exhibits significant low-temperature impact power, with impact energy values ​​of 260-350 J at -20℃ and 240-300 J at -40℃. However, its low carbon content results in low strength, hardness, and poor wear resistance, making it unsuitable for tool steel.

[0007] A D6A hot-rolled wide strip steel for bimetallic saw blade backing and its production method are disclosed in application number 201710014769.2. The chemical composition by mass percentage is: C 0.45–0.55, Si 0.05–0.30, Mn 0.6–1.0, P ≤0.025, S ≤0.010, Cr 0.9–1.20, V 0.08–0.15, Ni 0.50–0.70, Mo 0.90–1.20, Al 0.05–0.15, N 0.007–0.015, with the balance being Fe. The hot-rolled wide strip steel has a specification of (3.0–8.0) × (960–1500) mm. The process involves smelting in a 210t converter, refining in a ladle furnace and vacuum furnace, and casting in a slab continuous casting machine. The slabs are hot-charged and hot-delivered, with an initial furnace temperature exceeding 400℃ and a heating temperature exceeding or equal to 1200℃. The final hot rolling temperature is 860–920℃. The rolled strip undergoes layer cooling with a coiling temperature of 650–720℃. The hot-rolled steel is then slowly cooled in a slow-cooling pit or under a slow-cooling hood. The strip's microstructure is a mixture of ferrite, degraded pearlite, and granular bainite, with extremely fine and uniform grain size from the surface to the center, averaging below 5μm. This invention suffers from high alloy content, high cost, and lacks methods to control surface oxidation and decarburization, resulting in poor surface quality and low-temperature toughness and plasticity, making it unsuitable for low-temperature environments. The converter smelting process requires dual-furnace smelting, including LF refining and RH vacuum treatment, which is costly and inefficient.

[0008] A cold-rolled strip steel for bimetallic saw blade backing and its manufacturing method are disclosed in application number 201811343856.3. The hot-rolled steel coil undergoes softening annealing, pickling, spheroidizing annealing in a full-hydrogen bell-type furnace, cold rolling on a single-stand reversible cold rolling mill, polishing with abrasive cloth in a polishing unit, annealing in a full-hydrogen bell-type furnace, and leveling to obtain the finished product. This invention, based on ordinary steel production equipment, solves problems such as surface decarburization and surface grain boundary oxidation, difficulty in pickling annealed reduced sponge iron, and poor flatness control, achieving high-quality, low-cost production. The cold-rolled strip steel for bimetallic saw blade backing has an average ferrite grain size of 4–4.5 μm, a cementite size of less than 0.3 μm, and a spheroidization rate of over 95%; an average hardness of 256–280 HV; and a width-direction flatness not exceeding 0.5% of the width. However, this invention involves multiple annealing processes, resulting in high costs and poor low-temperature toughness and plasticity, making it unsuitable for use in low-temperature environments.

[0009] The steel grades and production methods mentioned above all have certain defects and are not suitable for processing tools for low-temperature environments. Summary of the Invention

[0010] To address the problems of poor toughness and plasticity at -20℃ and severe impact fracture failure in existing tool steels, this invention provides a low-temperature resistant tool steel with excellent dimensions and its production method. The tool steel produced using this invention exhibits a strip thickness variation of ≤0.6mm, crown ≤70μm, tensile strength difference within the same roll ≤50MPa, and hardness difference within the same roll ≤5HRB. After heat treatment at -20℃, it achieves a hardness above 50HRC, a yield strength above 500MPa, an impact energy above 30J, a surface decarburization layer depth not exceeding 10μm, no intergranular oxidation, and no microcracks. It demonstrates excellent wear resistance and impact toughness at low temperatures after heat treatment, resulting in a significantly improved service life.

[0011] To achieve the above objectives, the present invention employs the following technical solution:

[0012] A low-temperature resistant tool steel with excellent dimensional properties has the following chemical composition by weight percentage: C 0.38%–0.70%, Si ≤0.5%, Mn 0.4%–1.5%, Nb 0.01%–0.8%, Cr ≤1.0%, Ni ≤0.5%, N 0.003%–0.010%, Ca 0.0005%–0.005%, wherein Si, Cr, and Ni are not zero; impurity elements B ≤0.0008%, P ≤0.020%, S ≤0.015%, with the balance being Fe and unavoidable impurities. The ratio of C to N is 90 ≤ C / N ≤ 150.

[0013] The following details the mechanism of action of each alloy component in the structural steel of this invention, where the percentage symbol % represents a weight percentage:

[0014] Carbon (C) is the main solid solution strengthening element in steel. In this invention, sufficient C is needed to combine with nitrogen (N) to inhibit surface decarburization and intergranular oxidation. If the C content is below 0.38%, it is difficult to guarantee the strength of the steel plate. On the other hand, if the C content is above 0.70%, the strength is too high, which deteriorates the toughness and plasticity of the steel, affects the yield strength ratio, and results in poor formability. Therefore, the C content should be controlled between 0.38% and 0.70%.

[0015] Manganese (Mn) is relatively inexpensive and is an excellent deoxidizer and desulfurizer, essential for ensuring the strength and toughness of steel. Manganese and iron can form a solid solution indefinitely, increasing hardness and strength with relatively little impact on plasticity. Mn combines with sulfur (S) to form MnS, preventing the formation of FeS at grain boundaries, which can lead to hot cracking and affect the hot formability of tool steel. Mn is also a good deoxidizer and increases hardenability. Low Mn content in steel cannot meet the requirements for high strength and hardness, while excessive Mn content causes severe segregation, affecting weldability and formability, and increasing production costs. Therefore, considering both cost and performance requirements, the Mn content should be controlled between 0.4% and 1.5%.

[0016] Si is a common element in steel, used as a reducing agent and deoxidizer in the steelmaking process. Si is a ferrite-forming element, not a carbide-forming element. Si dissolved in ferrite improves hardenability and tempering resistance, increases strength and hardness, enhances wear resistance, significantly improves elastic limit, yield strength, and yield ratio, and increases fatigue strength, thus extending the service life of the steel. However, a Si content exceeding 0.5% will promote decarburization and oxidation on the surface of tool steel, forming a loose oxide layer and microcracks such as intergranular oxidation, severely affecting fatigue performance.

[0017] Cr is a medium-strong carbide-forming element. The carbides formed by adding Cr in this invention reduce the carbon concentration in pearlite, decrease the micro-segregation of carbon and other elements at the dendrite tips, thereby suppressing micro-segregation in the finished steel plate, increasing the pearlite spheroidization rate, and improving low-temperature toughness and plasticity. Cr also significantly improves strength, hardness, and wear resistance, enhances the steel's oxidation and corrosion resistance, raises the A3 and A1 temperatures, shifts the GS line to the upper left, slows down A decomposition, and improves hardenability; it also improves oxidation resistance, surface quality, tempering stability, and hardness, and enhances wear resistance. In this invention, Cr works synergistically with C and N to improve corrosion resistance and high-temperature oxidation resistance, and improve surface quality. Cr ≤ 1.0%.

[0018] In high-carbon conditions, nitrogen (Nb) reacts with carbon (C) to form NbC, resulting in high-melting-point, high-hardness, and highly dispersed carbides that significantly improve wear resistance; refine grain size, increase yield strength ratio, and enhance low-temperature impact toughness; reduce decarburization sensitivity, inhibit intergranular oxidation, and improve surface quality. This invention uses Nb to inhibit the formation of large lamellar pearlite, resulting in hard and fine pearlite lamellars, refining ferrite grains, and improving the plasticity and toughness of steel, especially low-temperature toughness, under the same strength conditions. Nb content: 0.01%–0.8%.

[0019] Ni can improve the strength of steel while maintaining good plasticity and toughness. Nickel has high resistance to corrosion from acids and alkalis, and provides rust prevention and heat resistance at high temperatures. Ni expands the γ-phase region, and instead of forming carbides, it enters austenite entirely, forming an infinite solid solution. Its maximum solubility in α-iron is about 10%, resulting in solid solution strengthening and improved hardenability. In this invention, Ni and Nb inhibit the formation of large lamellar pearlite, resulting in hard and fine pearlite. This refines the ferrite grains, improving the plasticity and toughness of steel, especially low-temperature toughness, under the same strength conditions. Ni ≤ 0.5%.

[0020] Nitrogen (N), with a ratio of 90 ≤ C / N ≤ 150, is generally considered a harmful element, and lower N levels in steel are better. This is because when steel contains supersaturated nitrogen, and is left for a prolonged period or subsequently heated or annealed at 200–300°C, the nitrogen and alloying elements precipitate as nitrides. These nitrides are generally considered non-metallic inclusions, affecting the steel's toughness and plasticity, and reducing the effectiveness of the alloying elements. This invention employs a nitrogen-containing composition design. During continuous casting, at least 0.003% N is introduced near the billet shell through the crystallizer, allowing it to dissolve extensively into the amorphous phase (A-phase) on the billet surface. This reduces carbon diffusion, inhibits decarburization and intergranular oxidation on the billet surface, thereby improving the surface quality of the steel plate. Furthermore, this invention utilizes the strong amorphous phase-forming effect of N to generate a large amount of γ-phase, reducing the tendency for grain coarsening, significantly hindering A-phase grain coarsening, refining the grains, and improving the steel's low-temperature toughness and weldability. This invention limits the ratio of C / N to 90 to 150 to ensure that N inhibits decarburization and intergranular oxidation on the surface of the billet and refines the grains, thereby improving the low-temperature toughness and plasticity of the steel.

[0021] P (≤0.025%) and S (≤0.015%) are unavoidable harmful impurities in steel, and their presence severely deteriorates the toughness of steel. Therefore, measures must be taken to minimize the P and S content in steel. This invention limits the maximum P content to 0.025% and the maximum S content to 0.015%. Sulfur exists in steel in the form of CaS and CaAlS, reducing the tendency to form MnS, which has poor hot brittleness. It eliminates the need for RH vacuum treatment, does not form MnS, which is highly detrimental to toughness and plasticity, and improves low-temperature toughness.

[0022] Boron (B), as an element that enhances hardenability, can significantly improve the hardenability of steel plates. However, boron is too reactive and difficult to control during smelting. In high-N steel, a high boron content leads to the formation of large amounts of BN compounds, which delays austenite recrystallization, increases the austenitizing temperature of the steel, and causes boron embrittlement and billet cracking. Therefore, the boron content in the high-N steel of this invention is controlled to be less than 0.0008%.

[0023] As a microalloying element, calcium (Ca) can refine grains, partially desulfurize, and improve the corrosion resistance, wear resistance, high-temperature and low-temperature performance of steel. It also enhances the impact toughness, fatigue strength, plasticity, and weldability of steel; and increases its cold heading ability, shock resistance, hardness, and contact creep strength. Tool steels have high carbon content, resulting in poor molten steel flow and difficulty in the floating of inclusions. This invention adds calcium to change the composition, quantity, and morphology of non-metallic inclusions, accelerates the flow of molten steel, promotes the full floating of inclusions, improves the purity of the steel, and ensures that the various non-metallic inclusions in the finished steel do not exceed grade 1.5. Furthermore, it improves the surface finish of the steel, eliminates anisotropy in the microstructure, improves low-temperature toughness, and extends service life. Ca content: 0.0005%–0.005%.

[0024] A method for producing a low-temperature resistant tool steel with excellent dimensional properties includes:

[0025] 1) Smelting process: The steel plate of this invention is rolled from a continuously cast billet produced by converter smelting and ANS electric furnace refining (without RH vacuum treatment). The billet thickness is 170-250mm. Before tapping from the converter, the molten steel has a carbon content of 0.3% ≤ C ≤ 0.5%. Refining is performed in an ANS electric furnace, with Ca added after the sulfur content is ≤ 0.015%. Continuous casting employs light pressure and electromagnetic stirring in the crystallizer, with an electromagnetic stirring current of 300-700A and a frequency of 2.2-2.9Hz. z, while adding N to adjust the N content of the molten steel, the dynamic flow rate of the cooling water in the crystallizer is 80-160 L / min, a special crystallizer protective slag is used for casting, the liquid level of the molten steel in the crystallizer is 750-850 mm; the superheat is 20-30℃; the continuous casting speed is 0.8-1.4 m / min, the secondary cooling water flow rate is 0.15-0.35 L / kg, the surface temperature deviation of the billet is not greater than 10℃, the equiaxed crystal ratio is above 50%, and the micro-segregation of the alloy liquid at the end of the columnar crystals is controlled;

[0026] 2) Billet heating process: The billet is hot-sent and hot-charged, and the temperature of the billet before entering the walking beam furnace is above 500℃; the furnace adopts a weak reducing atmosphere, and the total furnace time is 3 to 4 hours.

[0027] 3) Rolling process: High-pressure water descaling is carried out before roughing and finishing rolling, with a pressure of not less than 30MPa to ensure the surface quality of the steel plate; the reduction rate of each pass in roughing rolling is 15% to 35%, the roughing rolling temperature is 1000℃ to 1250℃, and the temperature difference between each pass of rolling the steel strip is ≤30℃; the reduction rate of each pass in finishing rolling is ≤30%, the finishing rolling temperature is 800℃ to 1100℃, and the temperature difference between each pass of rolling the steel strip is ≤20℃.

[0028] 4) Cooling process: After exiting the finishing mill, the steel plate enters the layer cooling process for uniform cooling at a rate of 10-50℃ / s. It is then cooled to 500-700℃ and coiled, and then slowly cooled to below 200℃ at a rate of 3-10℃ / h. It is then straightened and leveled with a force greater than 800MPa to obtain a mixed microstructure of fine lamellar pearlite, spheroidized pearlite, and blocky ferrite. The maximum diameter of ferrite is ≤20um, the interlamellar spacing of pearlite is ≤0.3um, and the proportion of spheroidized pearlite is not less than 20%.

[0029] 5) Heat treatment process: The steel strip is not annealed, but directly leveled, slit, and processed into various cutting tools and finished workpieces.

[0030] The finished workpiece is then heated to 780-880℃ for oil quenching and tempered at 300-400℃ to obtain a fine martensitic structure with lath bundles up to 20μm in length and MA islands up to 3μm in size.

[0031] This invention employs converter smelting and ANS electric furnace refining, eliminating the need for RH vacuum treatment. Before tapping from the converter, the molten steel has a carbon content of 0.3% ≤ C ≤ 0.5%, ensuring uniform carbon content and no macroscopic segregation in the billet after continuous casting. ANS electric furnace refining involves adding Ca after the sulfur content is ≤ 0.015%, altering the composition, quantity, and morphology of non-metallic inclusions, accelerating steel flow, promoting full floatation of inclusions, and improving steel purity. The finished steel contains no more than grade 1.5 non-metallic inclusions, improving surface finish, eliminating anisotropy, and enhancing low-temperature toughness. Continuous casting utilizes light pressure and electromagnetic stirring in the crystallizer, with an electromagnetic stirring current of 300A–700A and a frequency of 2.2–2.9Hz. Simultaneously, nitrogen is introduced to adjust the nitrogen content in the molten steel, allowing it to dissolve extensively into the amorphous matrix on the billet surface, reducing carbon diffusion, inhibiting decarburization and intergranular oxidation, thereby improving the surface quality of the steel plate and enhancing its low-temperature toughness. Because this invention employs a high N composition design, in order to avoid boron embrittlement caused by N and B, the B content of the converter steel is controlled to be ≤0.0008%.

[0032] The dynamic water flow rate in the crystallizer is 80–160 L / min, using a special crystallizer protective slag for casting. The crystallizer liquid level is 750–850 mm, and the superheat is 20–30 °C. Inclusions and segregation are controlled. Simultaneously, the continuous casting speed is 0.8–1.4 m / min, and the secondary cooling water flow rate is 0.15–0.35 L / kg. This ensures that the surface temperature deviation of the cast billet is no greater than 10 °C, the cooling rate of the billet is uniform, and the microstructure is homogeneous. This guarantees that the strength difference of the strip steel in the same coil after rolling is ≤50 MPa, and the hardness difference is ≤5 HRB. The equiaxed crystal ratio is above 50%, and micro-segregation of the alloy liquid at the ends of columnar crystals is controlled.

[0033] The billet is hot-charged and heated using a walking beam furnace. The preheating temperature of the billet before entering the heating section of the furnace must be above 500℃ to reduce the temperature difference on the billet surface. The furnace uses a weakly reducing atmosphere and is in the furnace for a total time of 3-4 hours to ensure the surface quality of the billet and prevent oxidation and decarburization.

[0034] High-pressure water descaling is performed before both roughing and finishing rolling, with a pressure of not less than 30 MPa to ensure the surface quality of the steel plate. In roughing, the reduction rate per pass is 15%–35%, and the temperature is 1000–1250℃, ensuring a rolling temperature difference of ≤30℃ between each pass along the entire length of the steel strip, resulting in uniform microstructure and properties. In finishing, the reduction rate per pass is ≤30%, and the rolling temperature is 800–1100℃, ensuring a rolling temperature difference of ≤20℃ between each pass along the entire length of the steel strip, resulting in uniform microstructure and properties. The thickness fluctuation of the entire coil is ≤0.6 mm, the crown is ≤70 μm, the tensile strength difference within the same coil is ≤50 MPa, and the hardness difference within the same coil is ≤5 HRB.

[0035] After exiting the finishing mill, the steel plate undergoes layer cooling for uniform cooling at a rate of 10–50°C / s. It is then coiled at 500–700°C and slowly cooled to below 200°C at a rate of 3–10°C / h, resulting in a mixed microstructure of fine lamellar pearlite, spheroidized pearlite, and blocky ferrite. The maximum diameter of the blocky ferrite is ≤20μm, the pearlite lamellar spacing is ≤0.3μm, and the spheroidization ratio is not less than 20%. The hot-rolled plate exhibits excellent formability; the strip requires no annealing before processing and can be directly leveled, slit, and used to manufacture various cutting tools and blades. The finished parts are heated to 780–880°C for oil quenching and then tempered at 300–400°C to obtain a fine martensitic microstructure. The longest lamellar bundle is 20μm, and the largest MA island is 3μm. At -20°C, the hardness is above 50HRC, the yield strength is above 500MPa, and the impact energy is above 30J. It exhibits excellent wear resistance and impact toughness at low temperatures, significantly improving service life.

[0036] Compared with the prior art, the beneficial effects of the present invention are:

[0037] Steel plates produced according to the above chemical composition and process are microalloyed with C and Nb. Simultaneously, process parameters such as billet heating, rough and finish rolling descaling pressure, rolling temperature, roll reduction, cooling rate, and coiling temperature are controlled to obtain a mixed microstructure of fine lamellar pearlite, spheroidized pearlite, and blocky ferrite. The maximum diameter of the blocky ferrite is ≤20μm, the pearlite lamellar spacing is ≤0.3μm, and the spheroidization ratio is not less than 20%. The strip coil thickness fluctuation is ≤0.6mm, the crown is ≤70μm, the tensile strength difference within the same coil is ≤50MPa, the hardness difference within the same coil is ≤5HRB, the surface is smooth, the surface oxidation and decarburization depth does not exceed 10μm, and there is no intergranular oxidation. After heat treatment, a fine martensitic microstructure is obtained, with lath bundles up to 20μm in length and MA islands up to 3μm in size. At -20℃, the hardness is above 50HRC, the yield strength is above 500MPa, the impact energy is above 30J, and the wear resistance and impact toughness at low temperatures are excellent, resulting in a significantly improved service life. Attached Figure Description

[0038] Figure 1 This is a metallographic diagram of the hot-rolled plate of the present invention.

[0039] Figure 2 This is a metallographic diagram of the workpiece after heat treatment of the tool steel of this invention. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0041] The chemical composition of the steel in the embodiments of the present invention is shown in Table 1, the production process of the embodiments is shown in Table 2, and the properties of the steel plate and heat-treated workpieces are shown in Table 3.

[0042] Table 1 Chemical composition and smelting process, %

[0043]

[0044] Table 2 Hot-rolled strip steel production process (I)

[0045]

[0046] Table 2 Hot-rolled strip steel production process (II)

[0047]

[0048] Table 3 Properties of Steel Plates and Heat-Treated Workpieces

[0049]

Claims

1. A low-temperature resistant tool steel with excellent dimensional properties, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C 0.38%~0.70%, Si≤0.5%, Mn 0.4%~1.5%, Nb 0.01%~0.8%, Cr≤1.0%, Ni≤0.5%, N: 0.003%~0.010%, Ca 0.0005%~0.005%, where Si, Cr, and Ni are not zero; impurity elements B≤0.0008%, P≤0.020%, S≤0.015%, with the balance being Fe and unavoidable impurities; Methods for producing low-temperature resistant tool steel with excellent dimensional properties include: 1) Smelting process: Converter smelting and ANS electric furnace refining are adopted; the molten steel before tapping from the converter is 0.3%≤C≤0.50%; after refining, the sulfur content is ≤0.015%, Ca is added; the casting superheat is 20~30℃; the continuous casting speed is 0.8~1.4m / min, the secondary cooling water flow is 0.15~0.35L / kg, and the surface temperature deviation of the billet is not greater than 10℃; 2) Billet heating process: The billet is hot-delivered and hot-charged, and the temperature of the billet before entering the heating furnace is above 500℃; the total time in the furnace is 3 to 4 hours. 3) Rolling process: The reduction rate per pass in roughing is 15% to 35%, the roughing temperature is 1000℃ to 1250℃, and the temperature difference between each pass of the steel strip is ≤30℃; the reduction rate per pass in finishing rolling is ≤30%, the finishing temperature is 800 to 1100℃, and the temperature difference between each pass of the steel strip is ≤20℃. 4) Cooling process: Layer cooling rate is 10-50℃ / s, cool to 500-700℃ and then roll up, and then slowly cool to below 200℃ at a cooling rate of 3-10℃ / h. The thickness fluctuation of the strip coil is ≤0.6mm, the crown is ≤70μm, the difference in tensile strength within the same coil is ≤50MPa, and the difference in hardness within the same coil is ≤5HRB.

2. The low-temperature resistant tool steel with excellent dimensional properties according to claim 1, characterized in that, 90≤C / N≤150.

3. A method for producing low-temperature resistant tool steel with excellent dimensional properties as described in claim 1 or 2, characterized in that, include: 1) Smelting process: Converter smelting and ANS electric arc furnace refining are adopted; the molten steel before tapping from the converter has a carbon content of 0.3% ≤ C ≤ 0.50%; Add Ca after refining sulfur content ≤0.015%; casting superheat 20~30℃; continuous casting speed 0.8~1.4m / min, secondary cooling water flow 0.15~0.35L / kg, billet surface temperature deviation not greater than 10℃; 2) Billet heating process: The billet is hot-delivered and hot-charged, and the temperature of the billet before entering the heating furnace is above 500℃; the total time in the furnace is 3 to 4 hours. 3) Rolling process: The reduction rate per pass in roughing is 15% to 35%, the roughing temperature is 1000℃ to 1250℃, and the temperature difference between each pass of the steel strip is ≤30℃; the reduction rate per pass in finishing rolling is ≤30%, the finishing temperature is 800 to 1100℃, and the temperature difference between each pass of the steel strip is ≤20℃. 4) Cooling process: Layer cooling rate is 10-50℃ / s, cool to 500-700℃ and then roll up, and then slowly cool to below 200℃ at a cooling rate of 3-10℃ / h.

4. The method for producing a low-temperature resistant tool steel with excellent dimensional properties according to claim 3, characterized in that, The billet thickness is 170-250mm; continuous casting uses electromagnetic stirring in the crystallizer with an electromagnetic stirring current of 300-700A and a frequency of 2.2-2.9Hz, while adding nitrogen to adjust the nitrogen content of the molten steel.

5. The method for producing a low-temperature resistant tool steel with excellent dimensional properties according to claim 3, characterized in that, High-pressure water descaling is performed before both roughing and finishing rolling, with a pressure of not less than 30 MPa.

6. The method for producing a low-temperature resistant tool steel with excellent dimensional properties according to claim 3, characterized in that, (Step 3 above) After finishing rolling, the strip thickness fluctuation is ≤0.6mm, the crown is ≤70μm, the tensile strength difference within the same roll is ≤50MPa, and the hardness difference within the same roll is ≤5HRB.

7. The method for producing a low-temperature resistant tool steel with excellent dimensional properties according to claim 3, characterized in that, Step 4 above yields a mixed microstructure of lamellar pearlite, spheroidized pearlite, and massive ferrite, with the maximum diameter of the massive ferrite ≤20μm, the interlamellar spacing of the lamellar pearlite ≤0.3μm, and the proportion of spheroidized pearlite not less than 20%.

8. A heat treatment process for low-temperature resistant tool steel workpieces with excellent external dimensions, characterized in that, The low-temperature resistant tool steel workpiece is processed using the low-temperature resistant tool steel with excellent dimensions as described in claim 1 or 2. The heat treatment process of the low-temperature resistant tool steel workpiece is oil quenching at 780-880℃ and tempering at 300-400℃.

9. The heat treatment process for a low-temperature resistant tool steel workpiece with excellent external dimensions according to claim 8, characterized in that, The low-temperature resistant tool steel workpiece after heat treatment has a martensitic microstructure, with lath bundles up to 20 μm in length and MA islands up to 3 μm in size.

10. The heat treatment process for a low-temperature resistant tool steel workpiece with excellent external dimensions according to claim 8, characterized in that, The low-temperature resistant tool steel workpiece has a hardness of over 50 HRC, a yield strength of over 500 MPa, and an impact energy of over 30 J at -20℃.

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