Pickling-free high-surface-quality steel for cutting tool and manufacturing method of pickling-free high-surface-quality steel

Abstract

The invention relates to pickling-free high-surface-quality steel for a cutting tool and a manufacturing method of the pickling-free high-surface-quality steel. The pickling-free high-surface-quality steel for the cutting tool comprises the following chemical components: 0.60%-1.10% of C, 0.10%-0.40% of Si, 0.7%-1.5% of Mn, 0.02%-1.5% of Cr, 0.01%-0.2% of Al, 0.0010%-0.010% of N, 0.0005%-0.02% of Ca, less than or equal to 0.0008% of B, less than or equal to 0.025% of P, less than or equal to 0.025% of S and the balance of Fe. 2 < = Al / N < = 20, 0.5 < = S / N < = 4; and the balance Fe and impurities. The steel plate does not need to be pickled, the surface of the steel plate is free of scale, the surface quality is high (reaching the FB level), the roughness is 3.0-5.0 microns, the surface hardness of the hot-rolled coiled plate after heat treatment is 45 HRC or above, the surface hardness difference is within + / -0.5 HRC, and the unevenness is smaller than or equal to 0.05 mm / m.

Description

Technical Field

[0001] The invention relates to the technical field of cutting tool steel production, and in particular to a pickling-free high-surface-quality cutting tool steel and a manufacturing method thereof. Background Art

[0002] The hardness of cutting tool steel after heat treatment is generally above 42HRC. With the rapid development of various fields, the use of cutting tool steel is becoming more and more extensive. The failure form of saw blades and other cutting tool steel parts during use is mainly fracture, which is mainly related to the poor flatness of the saw blade and the uneven heat treatment resulting in poor toughness and plasticity. Due to the high carbon content of cutting tool steel, surface oxidation and decarburization and the formation of iron oxide scale are the main factors affecting the flatness and hardness uniformity of heat treatment. During the heating process of heat treatment, the surface iron oxide scale of different thicknesses will cause stress to be generated due to uneven surface heating of the steel parts, resulting in poor plate shape and poor flatness; and the uneven temperature of the steel parts during quenching will also lead to uneven quenching hardness, thereby reducing wear resistance and toughness and plasticity. In addition, the surface iron oxide scale still exists after heat treatment, and shot blasting is required, resulting in low production efficiency.

[0003] Due to the limitations of the production process, the depth of the surface oxide scale of the hot-rolled coil of cutting tool steel is usually tens of microns. Even high-pressure water descaling cannot be completely removed. The flatness of the steel plate after heat treatment is about 0.10mm, and the minimum surface hardness difference is about 3HRC.

[0004] In order to reduce the failure of tool steel parts, improve production efficiency and reduce processing costs, many saw blade manufacturers use pickled plates instead of hot-rolled plates. However, pickling increases costs on the one hand and causes environmental pollution on the other (pickling production lines have been shut down in many areas in accordance with environmental protection requirements). Therefore, it is urgent to develop a high-surface quality blade steel plate that does not require pickling and has no iron oxide on the surface, while improving the flatness of the steel plate and the uniformity of hardness after heat treatment.

[0005] The Chinese invention patent with the authorization announcement number CN104745786B discloses "a method for producing thin-gauge tool steel using a CSP line without spheroidizing annealing", which involves thin tool steel plates with a thickness of 1-2.5mm, and controls the decarburization depth to not exceed 1% of the plate thickness. It has the characteristics of thin gauge and shallow decarburization layer, but cannot meet the application requirements of thick gauge tool steel plates. It adopts a production process of low-temperature rolling, rapid cooling to low-temperature coiling near the martensitic transformation temperature, and then tempering to obtain tempered troostite to reduce the hardness. On the one hand, it has extremely high requirements on the capacity of the coiler. On the other hand, martensitic phase transformation will occur during the coiling process, and martensitic brittleness is extremely easy to break. In addition, it needs to be tempered at 550-700℃ after coiling, and the production cost is relatively high.

[0006] The "Method for manufacturing a cutting tool steel plate with a three-layer composite blade" disclosed in the Chinese invention patent with authorization announcement number CN100482406C, and the "Method for manufacturing a cutting tool steel plate with a double composite blade" disclosed in the Chinese invention patent with authorization announcement number CN100462192C, both involve methods for manufacturing composite cutting tool steels, and heat treatment is required after composite, which results in complex processes and high production costs.

[0007] The Chinese invention patent with authorization announcement number CN103757546B discloses "High-alloy cutting tool steel for woodworking machinery rotary cutter blades and its hot processing technology". The steel has high content of alloys such as Cr, V, W, and Mo, and requires two quenching treatments. The process is complicated and the production cost is high.

[0008] The Chinese patent application with publication number CN105177430A discloses "an alloy tool steel and its production method", which relates to a medium carbon tool steel containing: C 0.5%, Si 0.2%, Mn 0.5%, Cr 5.0%, Mo 2.3%, V0.5%, S≤0.003%, P≤0.02%. The alloy tool steel contains a large amount of alloys such as Cr, Mo, and V. The smelting requires electroslag remelting and pouring ingots, with a low yield rate. It also requires multiple heat treatment processes such as softening annealing, spheroidizing annealing, quenching, and tempering. The production cost is high and it is not suitable for processing and manufacturing various tools with complex shapes.

[0009] The Chinese invention patent with the authorization announcement number CN103506380B discloses "a production method for reducing the thickness of the decarburized layer of high-carbon spring strip steel", which controls the depth of the decarburized layer of 2.5-3.55mm thick spring steel to no more than 0.02mm by controlling the heating temperature and furnace atmosphere, single-pass rolling reduction rate and cooling rate. However, the method is not suitable for thicker steel plates, and the temperature segmentation control precision of the heating furnace atmosphere and temperature is high, which makes the operation difficult.

[0010] The production methods of the above-mentioned steel plates do not mention the control of surface oxide scale, and the production processes have certain shortcomings and cannot meet the production requirements of high surface quality cutting tool steel. Summary of the invention

[0011] The present invention aims at the problems of poor uniformity of surface hardness, uneven hardness after heat treatment, poor wear resistance and easy fracture of existing tool steel. A pickling-free high-surface quality steel for cutting tools and a manufacturing method thereof are provided. The steel plate does not need pickling, has no iron oxide scale on the surface and has high surface quality (reaching FB grade), with a roughness of 3.0-5.0 μm, and the surface hardness of the hot-rolled coil after heat treatment is above 45 HRC, and the surface hardness difference is within ±0.5 HRC, and the unevenness is ≤0.05 mm / m.

[0012] In order to achieve the above object, the present invention adopts the following technical solutions:

[0013] A pickling-free high-surface-quality steel for cutting tools. The chemical composition of the cutting tool steel is, by weight percentage, C: 0.60%-1.10%, Si: 0.10%-0.40%, Mn: 0.7%-1.5%, Cr: 0.02%-1.5%, Al: 0.01%-0.2%, N: 0.0010%-0.010%, Ca: 0.0005-0.02%, B≤0.0008%, P≤0.025%, S≤0.025%; and meets 2≤Al / N≤20, 0.5≤S / N≤4; the remainder is Fe and unavoidable impurities.

[0014] The structure of the cutting tool steel is a mixed structure of uniform and fine spheroidized pearlite and lamellar pearlite, the diameter of the spheroidized pearlite is below 0.2 μm, and the spheroidization rate is above 10%.

[0015] The finished steel plate has no surface oxidation decarburization layer and iron oxide scale, the yield strength is 500-600MPa, the surface roughness is 3.0-5.0μm, and the surface quality is not lower than FB grade; the surface hardness after heat treatment is above 45HRC, the surface hardness difference is within ±0.5HRC, and the unevenness is ≤0.05mm / m.

[0016] A method for manufacturing pickling-free high-surface-quality steel for cutting tools, comprising the following process:

[0017] 1) Smelting and continuous casting:

[0018] The smelting adopts converter smelting + LF electric furnace refining process. Before the converter taps out steel, the C content in the molten steel is controlled to be 0.5%≤C≤0.9%; during LF electric furnace refining, the sulfur content in the steel is controlled to be ≤0.025%, and then alloys are added to adjust the composition of the molten steel. At the same time, nitrogen is introduced to adjust the N content in the steel to 0.0010%~0.010%;

[0019] Continuous casting adopts light pressure and crystallizer electromagnetic stirring, the electromagnetic stirring current intensity is 300-700A, the frequency is 2.2-2.9Hz, and the dynamic water volume of the crystallizer is 80-160L / min; the crystallizer protection slag is used for casting, the crystallizer liquid level is controlled at 750-850mm, and the superheat is controlled at 20-30℃; the continuous casting speed is controlled at 0.8-1.4m / min, and the secondary cooling water volume is 0.15-0.35L / kg; the surface temperature deviation of the ingot is not more than 10℃;

[0020] 2) Heat treatment of ingot:

[0021] The hot casting is transferred to the walking beam heating furnace for heating; the heating furnace adopts a weak oxidizing atmosphere, the preheating section temperature is 500-600℃, and the preheating section insulation time is more than 60min; the first heating section temperature is 900-1100℃, and the first heating section insulation time is more than 60min; the second heating section temperature is 1200-1350℃; the total time in the furnace is 3-4h;

[0022] 3) Rolling:

[0023] High-pressure water is used for descaling before rough rolling and finish rolling, and the pressure of high-pressure water is not less than 20MPa;

[0024] The first pass reduction rate of rough rolling is above 30%, the rough rolling temperature is 1100~1250℃, and the temperature difference of the whole length of the steel strip during rough rolling is ≤20℃;

[0025] The reduction rate of each of the first two passes of finishing rolling is 25% to 40%, the finishing rolling temperature is 800 to 900°C, edge heating devices are set on both sides of the steel plate, the compensation heating temperature is 35 to 45°C, the heating width is not less than 50mm, and the temperature difference of the full length of the steel strip during finishing rolling is ≤10°C;

[0026] 4) Cooling:

[0027] After the steel plate leaves the finishing mill, it enters layer cooling, with a cooling rate of 10-50℃ / s, and the cooling rate difference in the width direction at the same position is ≤1℃ / s; it is coiled when cooled to 500-700℃, and then cooled to below 300℃ at a cooling rate of 15-25℃ / h, and finally air-cooled to room temperature; a mixed structure of uniform and fine spheroidized pearlite and lamellar pearlite is obtained;

[0028] 5) Surface treatment:

[0029] The steel coil is straightened and leveled by a straightening machine with a straightening force greater than 600MPa, and then sandblasted;

[0030] 6) Heat treatment:

[0031] After the steel plate is processed into a workpiece blank, it is induction heated to 780-850°C, water quenched or oil quenched, and cooled to room temperature; then it is tempered at 250-450°C to obtain a fine martensite structure or troostite tempered structure.

[0032] During the smelting and continuous casting process, the continuous casting adopts a vertical bending arc continuous casting machine for casting, the arc radius of the vertical bending arc continuous casting machine is not less than 5m, and the thickness of the casting billet is 170-250mm.

[0033] During the surface treatment process, a 9-roll straightening machine is used for straightening and leveling; during sandblasting, the distance between the upper nozzle and the upper surface of the steel plate and the distance between the lower nozzle and the lower surface of the steel plate are 50-200 mm, the sandblasting pressure is 100-200 MPa, the belt threading speed is 5-20 m / min, the water temperature is 10-50° C., and the sand particle size is 0.5-2.0 mm.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1) A high-surface-quality steel for cutting tools is produced by combining chemical composition and production process, using Cr-Al-Mn-Ca alloying, and controlling the process parameters such as heating of the ingot, rough and fine rolling descaling pressure, rolling temperature, reduction rate, coiling temperature, cooling rate, surface sandblasting pressure and speed, etc., so that a uniform and fine mixed structure of spheroidized pearlite and lamellar pearlite can be obtained without pickling and annealing, the diameter of the spheroidized pearlite is less than 0.2 μm, the spheroidization rate is more than 10%, the hot-rolled steel plate has no surface oxidation decarburization layer and iron oxide scale, the surface roughness is 3.0-5.0 μm, and the surface quality reaches FB grade (the surface quality of the steel plate is divided into 4 grades, from high to low, namely FA, ​​FB, FC, and FD);

[0036] 2) Through controlled rolling and controlled cooling process, the yield strength of steel plate is 500-600MPa, the thickness fluctuation of strip steel is controlled within ±0.20mm, the convexity is ≤40μm, the tensile strength difference of the same roll is ≤20MPa, and the hardness difference of the same roll is ≤3HRB; the surface hardness of steel plate after heat treatment is above 45HRC, the surface hardness difference is within ±0.5HRC, and the unevenness is ≤0.05mm / m;

[0037] 3) The service life of the cutting tool made of the steel plate of the present invention is extended by more than 0.5 times under the same use conditions, and the service life is significantly improved. DETAILED DESCRIPTION

[0038] The invention discloses a pickling-free high-surface-quality cutting tool steel, wherein the chemical composition of the cutting tool steel is, by weight percentage, C: 0.60%-1.10%, Si: 0.10%-0.40%, Mn: 0.7%-1.5%, Cr: 0.02%-1.5%, Al: 0.01%-0.2%, N: 0.0010%-0.010%, Ca: 0.0005-0.02%, B≤0.0008%, P≤0.025%, S≤0.025%; and the steel satisfies 2≤Al / N≤20, 0.5≤S / N≤4; the remainder is Fe and unavoidable impurities.

[0039] The structure of the cutting tool steel is a mixed structure of uniform and fine spheroidized pearlite and lamellar pearlite, the diameter of the spheroidized pearlite is below 0.2 μm, and the spheroidization rate is above 10%.

[0040] The finished steel plate has no surface oxidation decarburization layer and iron oxide scale, the yield strength is 500-600MPa, the surface roughness is 3.0-5.0μm, and the surface quality is not lower than FB grade; the surface hardness after heat treatment is above 45HRC, the surface hardness difference is within ±0.5HRC, and the unevenness is ≤0.05mm / m.

[0041] The present invention provides a method for manufacturing a pickling-free high-surface-quality cutting tool steel, comprising the following process:

[0042] 1) Smelting and continuous casting:

[0043] The smelting adopts converter smelting + LF electric furnace refining process. Before the converter taps out steel, the C content in the molten steel is controlled to be 0.5%≤C≤0.9%; during LF electric furnace refining, the sulfur content in the steel is controlled to be ≤0.025%, and then alloys are added to adjust the composition of the molten steel. At the same time, nitrogen is introduced to adjust the N content in the steel to 0.0010%~0.010%;

[0044] Continuous casting adopts light pressure and crystallizer electromagnetic stirring, the electromagnetic stirring current intensity is 300-700A, the frequency is 2.2-2.9Hz, and the dynamic water volume of the crystallizer is 80-160L / min; the crystallizer protection slag is used for casting, the crystallizer liquid level is controlled at 750-850mm, and the superheat is controlled at 20-30℃; the continuous casting speed is controlled at 0.8-1.4m / min, and the secondary cooling water volume is 0.15-0.35L / kg; the surface temperature deviation of the ingot is not more than 10℃;

[0045] 2) Heat treatment of ingot:

[0046] The hot casting is transferred to the walking beam heating furnace for heating; the heating furnace adopts a weak oxidizing atmosphere, the preheating section temperature is 500-600℃, and the preheating section insulation time is more than 60min; the first heating section temperature is 900-1100℃, and the first heating section insulation time is more than 60min; the second heating section temperature is 1200-1350℃; the total time in the furnace is 3-4h;

[0047] 3) Rolling:

[0048] High-pressure water is used for descaling before rough rolling and finish rolling, and the pressure of high-pressure water is not less than 20MPa;

[0049] The first pass reduction rate of rough rolling is above 30%, the rough rolling temperature is 1100~1250℃, and the temperature difference of the whole length of the steel strip during rough rolling is ≤20℃;

[0050] The reduction rate of each of the first two passes of finishing rolling is 25% to 40%, the finishing rolling temperature is 800 to 900°C, edge heating devices are set on both sides of the steel plate, the compensation heating temperature is 35 to 45°C, the heating width is not less than 50mm, and the temperature difference of the full length of the steel strip during finishing rolling is ≤10°C;

[0051] 4) Cooling:

[0052] After the steel plate leaves the finishing mill, it enters layer cooling, with a cooling rate of 10-50℃ / s, and the cooling rate difference in the width direction at the same position is ≤1℃ / s; it is coiled when cooled to 500-700℃, and then cooled to below 300℃ at a cooling rate of 15-25℃ / h, and finally air-cooled to room temperature; a mixed structure of uniform and fine spheroidized pearlite and lamellar pearlite is obtained;

[0053] 5) Surface treatment:

[0054] The steel coil is straightened and leveled by a straightening machine with a straightening force greater than 600MPa, and then sandblasted;

[0055] 6) Heat treatment:

[0056] After the steel plate is processed into a workpiece blank, it is induction heated to 780-850°C, water quenched or oil quenched, and cooled to room temperature; then it is tempered at 250-450°C to obtain a fine martensite structure or troostite tempered structure.

[0057] During the smelting and continuous casting process, the continuous casting adopts a vertical bending arc continuous casting machine for casting, the arc radius of the vertical bending arc continuous casting machine is not less than 5m, and the thickness of the casting billet is 170-250mm.

[0058] During the surface treatment process, a 9-roll straightening machine is used for straightening and leveling; during sandblasting, the distance between the upper nozzle and the upper surface of the steel plate and the distance between the lower nozzle and the lower surface of the steel plate are 50-200 mm, the sandblasting pressure is 100-200 MPa, the belt threading speed is 5-20 m / min, the water temperature is 10-50° C., and the sand particle size is 0.5-2.0 mm.

[0059] The proportions (by weight percentage) of the alloy components and the mechanism of action of the pickling-free high-surface-quality cutting tool steel described in the present invention are as follows:

[0060] C: 0.60%~1.10%;

[0061] C is the main solid solution strengthening element in steel. In the present invention, sufficient C is required to form carbides with N and Al to inhibit surface oxidation and improve hardness uniformity. At the same time, the generated surface oxide scale is sparse, which is easy to remove thoroughly during sandblasting, and finally achieves the target surface roughness of 3.0 to 5.0 μm. If the C content is lower than 0.60%, it is difficult to ensure the hardness of the steel plate after heat treatment, and if the C content is higher than 1.10%, the surface is prone to oxidation and decarburization, which will deteriorate the toughness and plasticity of the workpiece after heat treatment, causing the workpiece to break easily.

[0062] Si: 0.1%~0.4%;

[0063] Si is one of the common elements in steel and is used as a reducing agent and deoxidizer in the steelmaking process. Si is a ferrite-forming element, not a carbide-forming element. Si solid solution in ferrite can improve hardenability and tempering resistance, improve strength and hardness, improve wear resistance, significantly improve elastic limit, yield strength and yield strength ratio, and improve fatigue strength, thereby extending the service life of steel. When the Si content exceeds 0.4%, the surface of the steel plate is prone to decarburization and oxidation, which seriously affects the hardness uniformity and fatigue performance. When the Si content in steel is 0.1% to 0.4%, loose iron oxide scale is formed on the surface of the steel, which is easy to remove, and then a steel coil with a surface roughness of 3.0 to 5.0 μm is obtained, and the flatness and surface hardness difference after heat treatment are further improved to avoid cracking of the workpiece during use.

[0064] Mn: 0.70%~1.5%;

[0065] Mn is relatively cheap, is a good deoxidizer and desulfurizer, and is an essential element to ensure the strength and toughness of steel. Manganese and iron can infinitely dissolve to form a solid solution, thereby improving the hardness and strength of steel, and the effect on plasticity is relatively small. Mn combines with S to form MnS, which can avoid the formation of FeS at the grain boundary and the resulting hot cracks that affect the hot formability of steel. At the same time, Mn is also a good deoxidizer and can increase hardenability. Low Mn content in steel cannot meet the requirements of high strength and hardness. Too high Mn content will lead to serious segregation, affect welding performance and formability, and increase production costs. The present invention comprehensively considers factors such as cost and performance requirements, and controls the Mn content to 0.70% to 1.5%.

[0066] Cr: 0.02%~1.5%;

[0067] Chromium is a medium-strong carbide-forming element. In the present invention, the composite carbide formed by Cr, carbon and Fe exists in the surface oxide scale, making the oxide scale loose and easy to remove. Chromium can also increase the temperature of A3 and A1, and the GS line moves to the upper left. It slows down the decomposition of A, improves hardenability, and improves the oxidation resistance and corrosion resistance of steel, improves tempering stability, and achieves high hardness and high wear resistance of steel.

[0068] Al: 0.01%~0.2%;

[0069] Al is used as a deoxidizer during steelmaking, which can refine grains, inhibit steel aging, improve steel toughness at low temperatures, and reduce the brittle transition temperature of steel. Al is a non-carbide forming element, which can improve the oxidation resistance of steel, inhibit surface oxidation and decarburization, and improve surface quality. The present invention controls 2≤Al / N≤20, and Al forms fine particles of ≤50nm with N, C, and S. The presence of Al in the surface oxide scale can make the oxide scale loose and easy to remove, and improve the flatness of the steel plate after heat treatment, ensuring that the unevenness is ≤0.05mm / m, and the surface hardness difference is controlled within ±0.5HRC.

[0070] N: 0.0010%~0.010%;

[0071] It is generally believed that N is a harmful element, and the lower the N content in steel, the better. The present invention adopts a composition design containing N. During refining, nitrogen gas of more than 0.001% is introduced, and 2≤Al / N≤20, 0.5≤S / N≤4 are controlled, so that N, Al, C, and S form fine particles of ≤50nm, which exist in the surface oxide scale, making the oxide scale loose and easy to remove, and obtaining a steel coil with a surface roughness of 3.0 to 5.0μm, improving the flatness and surface hardness difference after heat treatment, and avoiding cracking of the workpiece during use. When the N content exceeds 0.01%, the toughness and plasticity will be reduced.

[0072] B≤0.0008%, P≤0.025%, S≤0.025%;

[0073] P and S are both unavoidable harmful impurities in steel. Their presence will seriously deteriorate the toughness of steel, so measures should be taken to reduce the P and S contents in steel as much as possible. The present invention controls the P content to below 0.025% and the S content to below 0.025% to reduce the formation of MnS. At the same time, the B content in the steel is controlled to avoid boron embrittlement.

[0074] Ca: 0.0005%~0.02%;

[0075] As a microalloying element, Ca forms composite carbides with Cr and Al, which can inhibit microsegregation, reduce banded structure, promote carbide nucleation and spheroidization, make the surface easy to sandblast without brittle fracture, and improve the flatness and surface hardness difference after heat treatment.

[0076] The manufacturing method of the pickling-free high-surface-quality cutting tool steel described in the present invention has the following design principles for each process and parameter:

[0077] The present invention adopts the process of "converter smelting + LF electric furnace refining" without RH vacuum treatment; before the converter taps out steel, 0.5%≤C≤0.9% in the molten steel is controlled to ensure uniform carbon content in the ingot after continuous casting without macro segregation. LF electric furnace refining is adopted, and the sulfur content in the steel is controlled to be ≤0.025% during refining, and then other alloys are added to adjust the composition of the molten steel, and nitrogen is passed at the same time to adjust the N content in the molten steel, change the composition, quantity and form of non-metallic inclusions, speed up the flow of molten steel, promote the full floating of inclusions, improve the purity of steel, and ensure that various types of non-metallic inclusions in the finished steel plate do not exceed level 1.5. At the same time, the surface finish of the steel plate is improved and the anisotropy of the organization is eliminated.

[0078] The continuous casting adopts the process of "light pressure + crystallizer electromagnetic stirring", the electromagnetic stirring current intensity is 300-700A, and the frequency is 2.2-2.9Hz. The dynamic water volume of the crystallizer is 80-160L / min, and the crystallizer special protective slag is used for casting. The crystallizer liquid level is 750-850mm. The superheat of the first furnace of the casting is controlled at 30℃-40℃, and the superheat of other furnaces is controlled at 20℃-30℃ to control inclusions and segregation. In addition, the continuous casting pulling speed is controlled to 0.8-1.4m / min, and the secondary cooling water volume is 0.15-0.35L / kg to ensure that the surface temperature deviation of the ingot is no more than 10℃, the cooling rate of the ingot is uniform, the organization is uniform, and the strength difference of the same coil after rolling is ≤20MPa, and the hardness difference of the same coil is ≤3HRB. The equiaxed crystal rate is controlled to be above 50%, and the liquid micro-segregation of the alloy at the end of the columnar crystal is controlled.

[0079] The billet is cooled slowly without leaving the production line, and is hot transported and loaded, which can save energy and reduce consumption, and reduce the temperature difference on the billet surface. A weak oxidizing atmosphere is used in the heating furnace, with a preheating section temperature of 500℃~600℃ and a holding time of more than 60 minutes; a heating section temperature of 900℃~1100℃ and a holding time of more than 60 minutes; a second heating section temperature of 1200℃~1350℃; the total time in the furnace is controlled at 3~4 hours to ensure the surface quality of the billet and avoid oxidation and decarburization.

[0080] High-pressure water is used for descaling before rough rolling and finishing rolling, and the high-pressure water pressure is not less than 20MPa to ensure the surface quality of the steel plate. The first pass of rough rolling has a reduction rate of more than 30%, and the rough rolling temperature is 1100℃~1250℃, which provides nucleation power for the precipitation of various carbides and nitrides. The temperature difference of steel strip rolling is ≤20℃, so that the structure and performance are uniform. The reduction rate of each of the first two passes of finishing rolling is 25%~40%, and the finishing temperature is 800℃~900℃, which is conducive to obtaining a mixed structure of fine pearlite and ferrite after subsequent cooling, and the banded structure does not exceed level 3.0. Edge heating devices are set on both sides of the steel plate, the compensation heating temperature is 35-45℃, the heating width is not less than 50mm, and the rolling temperature difference of the steel strip is ≤10℃, which is conducive to uniform organizational properties and improved flatness; the strip thickness fluctuation is within ±0.20mm, the convexity is ≤40μm, the tensile strength difference of the same roll is ≤20MPa, and the hardness difference of the same roll is ≤3HRB.

[0081] After the steel plate leaves the finishing mill, it enters the layer cooling and is uniformly cooled at a cooling rate of 10 to 50°C / s. The cooling rate difference in the bandwidth direction at the same position is ≤1°C / s to ensure uniform performance of the same plate structure (strength difference ≤20MPa, hardness difference ≤3HRB). When it is cooled to 500 to 700°C, it is coiled and cooled to below 300°C at a cooling rate of 15 to 25°C / h, which is conducive to carbide nucleation and obtains a uniform and fine mixed structure of spheroidized pearlite, lamellar pearlite and ferrite. The diameter of the spheroidized pearlite is below 0.2μm, and the spheroidization rate is above 10%, which reduces the yield strength and facilitates surface treatment. When the cooling rate exceeds 25°C / h, it is easy to form lamellar pearlite, and when the cooling rate is lower than 15°C / h, the spheroidized pearlite is easy to grow, resulting in poor toughness and plasticity. On the other hand, the surface oxide scale formed at the above coiling temperature and cooling rate is relatively loose, which is conducive to subsequent surface sandblasting.

[0082] After coiling, the steel coil is air-cooled to room temperature, at which point the yield strength is 500-600 MPa. It is preferred to use a 9-roller straightening machine with a straightening force greater than 600 MPa for straightening and flattening to ensure the flatness of the plate shape. The distance between the upper and lower nozzles of the sandblasting equipment and the surfaces on both sides of the steel plate is controlled at 50-200 mm, the sandblasting pressure is 100-200 MPa, the belt threading speed is 5-20 m / min, the water temperature is 10-50 ° C, and the sand particle size is 0.5-2.0 mm. Ensure that the iron oxide scale on the surface of the steel plate is cleanly removed during sandblasting, and the surface quality reaches a higher FB level, with a surface roughness of 3.0-5.0 μm, no surface intergranular oxidation and decarburization, uniform hardness after heat treatment, and a surface hardness difference within ±0.5 HRC, improve flatness (unevenness ≤ 0.05 mm / m), and a surface hardness difference within ±0.5 HRC.

[0083] The part blank made of the steel plate prepared by the present invention is induction heated to 780-850°C, quenched (water quenched or oil quenched), cooled to room temperature, and then tempered at 250-450°C to obtain a fine martensite or troostite tempered structure. The surface hardness of the part blank after treatment is above 45HRC.

[0084] In order to more intuitively embody the present invention, the embodiments of the present invention are further described in conjunction with examples. The following examples are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any technical solution that can be obviously obtained by a person skilled in the art within the technical scope disclosed in the present invention, including simple changes or equivalent replacements, is within the protection scope of the present invention.

[0085] [Example]

[0086] The chemical composition of the steel in each embodiment is shown in Table 1, the smelting-cooling process parameters are shown in Table 2, the surface treatment, heat treatment process parameters and the structure of the steel are shown in Table 3, and the properties of the hot-rolled steel plate and after heat treatment are shown in Table 4.

[0087] Table 1 Chemical composition of steel (wt%)

[0088]

[0089]

[0090] Table 2 Smelting-cooling process parameters

[0091]

[0092] Table 3 Surface treatment, heat treatment process parameters and steel structure

[0093]

[0094]

[0095] Table 4 Properties of hot rolled steel plates and after heat treatment

[0096]

[0097] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A pickling-free high-surface quality cutting tool steel, characterized in that: The chemical composition of the cutting tool steel is C: 0.60% ~ 1.10%, Si: 0.10% ~ 0.40%, Mn: 0.7% ~ 1.5%, Cr: 0.02% ~ 1.5%, Al: 0.01% ~ 0.2%, N: 0.0010% ~ 0.010%, Ca: 0.0005 ~ 0.02%, B ≤ 0.0008%, P ≤ 0.025%, S ≤ 0.025%; and satisfies 2 ≤ Al / N ≤ 20, 0.5 ≤ S / N ≤ 4; the balance is Fe and unavoidable impurities.

2. The pickling-free high-surface-quality cutting tool steel according to claim 1, characterized in that: The structure of the cutting tool steel is a mixed structure of uniform and fine spheroidized pearlite and lamellar pearlite, the diameter of the spheroidized pearlite is below 0.2 μm, and the spheroidization rate is above 10%.

3. The pickling-free high-surface-quality cutting tool steel according to claim 1, characterized in that: The finished steel plate has no surface oxidation decarburization layer and iron oxide scale, the yield strength is 500-600MPa, the surface roughness is 3.0-5.0μm, and the surface quality is not lower than FB grade; the surface hardness after heat treatment is above 45HRC, the surface hardness difference is within ±0.5HRC, and the unevenness is ≤0.05mm / m.

4. A method for manufacturing pickling-free high-surface-quality cutting tool steel according to any one of claims 1 to 3, characterized in that: The process includes the following: 1) Smelting and continuous casting: The smelting adopts converter smelting + LF electric furnace refining process. Before the converter taps out steel, the C content in the molten steel is controlled to be 0.5%≤C≤0.9%; during LF electric furnace refining, the sulfur content in the steel is controlled to be ≤0.025%, and then alloys are added to adjust the composition of the molten steel. At the same time, nitrogen is introduced to adjust the N content in the steel to 0.0010%~0.010%; Continuous casting adopts light pressure and crystallizer electromagnetic stirring, the electromagnetic stirring current intensity is 300-700A, the frequency is 2.2-2.9Hz, and the dynamic water volume of the crystallizer is 80-160L / min; the crystallizer protection slag is used for casting, the crystallizer liquid level is controlled at 750-850mm, and the superheat is controlled at 20-30℃; the continuous casting speed is controlled at 0.8-1.4m / min, and the secondary cooling water volume is 0.15-0.35L / kg; the surface temperature deviation of the ingot is not more than 10℃; 2) Heat treatment of ingot: The hot casting is transferred to the walking beam heating furnace for heating; the heating furnace adopts a weak oxidizing atmosphere, the preheating section temperature is 500-600℃, and the preheating section insulation time is more than 60min; the first heating section temperature is 900-1100℃, and the first heating section insulation time is more than 60min; the second heating section temperature is 1200-1350℃; the total time in the furnace is 3-4h; 3) Rolling: High-pressure water is used for descaling before rough rolling and finish rolling, and the pressure of high-pressure water is not less than 20MPa; The first pass reduction rate of rough rolling is above 30%, the rough rolling temperature is 1100~1250℃, and the temperature difference of the whole length of the steel strip during rough rolling is ≤20℃; The reduction rate of each of the first two passes of finishing rolling is 25% to 40%, the finishing rolling temperature is 800 to 900°C, edge heating devices are set on both sides of the steel plate, the compensation heating temperature is 35 to 45°C, the heating width is not less than 50mm, and the temperature difference of the full length of the steel strip during finishing rolling is ≤10°C; 4) Cooling: After the steel plate leaves the finishing mill, it enters layer cooling, with a cooling rate of 10-50℃ / s, and the cooling rate difference in the width direction at the same position is ≤1℃ / s; it is coiled when cooled to 500-700℃, and then cooled to below 300℃ at a cooling rate of 15-25℃ / h, and finally air-cooled to room temperature; a mixed structure of uniform and fine spheroidized pearlite and lamellar pearlite is obtained; 5) Surface treatment: The steel coil is straightened and leveled by a straightening machine with a straightening force greater than 600MPa, and then sandblasted; 6) Heat treatment: After the steel plate is processed into a workpiece blank, it is induction heated to 780-850°C, water quenched or oil quenched, and cooled to room temperature; then it is tempered at 250-450°C to obtain a fine martensite structure or troostite tempered structure.

5. The method for manufacturing a pickling-free high-surface-quality cutting tool steel according to claim 4, characterized in that: During the smelting and continuous casting process, the continuous casting adopts a vertical bending arc continuous casting machine for casting, the arc radius of the vertical bending arc continuous casting machine is not less than 5m, and the thickness of the casting billet is 170-250mm.

6. The method for manufacturing a pickling-free high-surface-quality cutting tool steel according to claim 4, characterized in that: During the surface treatment process, a 9-roll straightening machine is used for straightening and leveling; during sandblasting, the distance between the upper nozzle and the upper surface of the steel plate and the distance between the lower nozzle and the lower surface of the steel plate are 50-200 mm, the sandblasting pressure is 100-200 MPa, the belt threading speed is 5-20 m / min, the water temperature is 10-50° C., and the sand particle size is 0.5-2.0 mm.

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