Manufacturing method of surface hardening agricultural machine tool

Abstract

The invention relates to the technical field of steel for agricultural machinery, in particular to a manufacturing method of a surface hardening agricultural machinery tool. Comprising the steps of smelting, continuous casting, casting blank treatment, rolling, cooling, surface treatment, part forming, heat treatment and local surface hardening treatment. The obtained hot-rolled plate is free of surface scale, the hardness after heat treatment is 53 HRC or above, the surface hardness difference is + / -0.5 HRC, the tensile strength is 1800 MPa or above, the ductility is larger than or equal to 8%, the ballistic work Akv at the temperature of 20 DEG C is larger than or equal to 20 J, tempering treatment, sand blasting and tumbling are not needed, the hardness of an agricultural machine tool obtained after surface local hardening treatment is 60 HRC or above, and the wear resistance is obviously improved.

Description

Technical Field

[0001] The present invention relates to the technical field of steel for agricultural machinery, and in particular to a method for manufacturing a surface hardened agricultural machinery tool. Background Art

[0002] Agricultural machinery tools are key components of agricultural machinery, which directly affect the quality and efficiency of agricultural machinery automation operations. When rotary tillers, deep plows and other soil-entering parts are subjected to hard sandy soil during operation, they wear quickly, are brittle and easy to break, and have low operating efficiency. my country's rotary tillers are mainly made of 60Si2Mn as raw materials, with a hot-rolled plate yield strength of more than 600MPa, a yield strength ratio of more than 65%, and a tensile strength of more than 900MPa. The oil quenching + tempering treatment is inefficient and not environmentally friendly. After heat treatment, the surface hardness is up to 50HRC, the tensile strength is 1300-1500MPa, and the impact energy AKv at 20℃ is less than 10J. On the other hand, in order to obtain good performance matching such as high toughness and fracture resistance and high hardness and wear resistance, the heat treatment process is used to make the organizational properties of different parts of the same rotary tiller different. The heat treatment process is complicated and the production efficiency is low.

[0003] Analysis shows that the low operating efficiency of the above-mentioned rotary tiller and other agricultural machinery parts is mainly related to the poor matching of strength and toughness. At present, there is no agricultural machinery tool steel with a strength of more than 1800MPa after heat treatment, impact toughness Akv ≥ 20J, matrix hardness of more than 53HRC, hardness difference within ±0.5HRC, and edge surface hardness of more than 60HRC. In order to improve the wear resistance of agricultural machinery parts, surface hardening treatment processes such as surfacing welding have appeared both at home and abroad in recent years. Before surface hardening treatment, the surface of the parts needs to be sandblasted and tumbled, which is a complicated process and has low production efficiency. On the other hand, the uneven surface hardness is mainly related to the presence of iron oxide on the surface of the parts before heat treatment.

[0004] Patent ZL201410250576.3 discloses a hot-rolled strip rake suitable for water quenching and a manufacturing method thereof. It is designed with a medium-carbon boron-containing component and Cr, Ti, V, and Nb are added at the same time. The cost is high, the activity of boron is particularly difficult to control, and the ratio of Ti, V, Nb to N needs to be controlled. The control accuracy is high, the smelting is difficult, the plate shape is not good, the flatness after heat treatment is poor, the coiling temperature is high, and the surface oxidation and decarburization are serious.

[0005] In summary, the mismatch of materials, manufacturing processes and component performance is the main reason that restricts the lagging development of my country's mid-to-high-end agricultural machinery tools. Hot-rolled plates have high yield strength, are difficult to process and form, have complex heat treatment processes that are not environmentally friendly, are prone to breakage due to mismatched strength and toughness, have low hardness and poor wear resistance, are frequently replaced and have low operating efficiency, making it difficult to meet the needs of large-scale, complex, high-speed development and continuous and efficient operation of fully automatic agricultural machinery. Summary of the invention

[0006] In view of the problems that the above-mentioned existing agricultural machinery tools have low surface hardness, poor uniformity, poor strength and toughness matching, insufficient wear resistance, poor toughness and easy breakage, low heat treatment efficiency and environmental pollution, and cannot meet the use requirements of high-end earth-entering agricultural machinery tools, the present invention provides a method for manufacturing surface hardened agricultural machinery tools.

[0007] In order to achieve the above object, the technical solution of the present invention is as follows:

[0008] The present invention provides a method for manufacturing a surface hardened agricultural tool, the method comprising the following steps:

[0009] (1) Smelting and continuous casting: The C content of the molten steel before tapping the converter is 0.25% to 0.4%. The molten steel enters the LF electric furnace for refining. After the sulfur content is ≤ 0.020%, alloying is performed. At the same time, N is introduced to adjust the N content of the molten steel to 0.001% to 0.010%, and continuous casting is performed to obtain a continuous casting billet.

[0010] (2) Ingot treatment: Ingots are heated at above 550°C and then put into a heating furnace for heating;

[0011] (3) Rolling: Descaling is performed before rough and finish rolling. The first pass of rough rolling has a reduction ratio of 20% to 25%, the start rolling temperature is 1200 to 1250°C, and the temperature difference of the steel strip over the entire length is ≤20°C. The first pass of finish rolling has a reduction ratio of 20% to 25%, the final rolling temperature is 950 to 1000°C, and the temperature difference of the steel strip over the entire length is ≤10°C.

[0012] (4) Cooling: After the steel plate leaves the finishing mill, it enters the layer cooling, cools to 580-650℃ and coils it, and then slowly cools it to below 300℃ after coiling;

[0013] (5) Surface treatment: The coiled steel coil is air-cooled to 50-100°C for flattening, and then sandblasted after being flattened;

[0014] (6) Heat treatment: the hot-rolled plate obtained in step (5) is processed into a part blank, induction heated to 880-920° C., hot stamped, water cooled to 180-230° C. and kept at this temperature for 5-10 min, and then air cooled to room temperature;

[0015] (7) Local surface hardening treatment: According to the design requirements of agricultural machinery tools, a flux-cored welding wire is used to locally form a hardened layer on the surface of the agricultural machinery tool blank. The flux-cored welding wire contains the following components in weight percentage: C 1.0% to 3.0%, Si 1.0% to 3.0%, Mn 0.8% to 2.0%, B 8.0% to 15.0%, Cr 20% to 30%, Nb 1.0% to 2.0%, Ni 0.8% to 1.6%, Mo 0.4% to 1.2%, and the balance is Fe and unavoidable impurities.

[0016] In the above technical scheme, in step (1), molten steel is poured by a vertical bending arc continuous casting machine, and the arc radius of the vertical bending continuous casting machine is not less than 5m; continuous casting adopts light pressure and electromagnetic stirring of the crystallizer, the electromagnetic stirring current is 400-600A, the frequency is 2.2-2.9Hz, the dynamic water volume of the crystallizer is 80-160L / min, the crystallizer protective slag is used for casting, the crystallizer liquid level is 750-850mm, the superheat is 20-30℃, the continuous casting pulling speed is 1.1-1.6m / min, and the secondary cooling water volume is 0.2-0.4L / kg; the thickness of the continuous casting billet is 170-250mm.

[0017] In the above technical scheme, in step (2), the heating furnace adopts a weak oxidizing atmosphere, the preheating section temperature is 500-600°C, the insulation time is more than 60 minutes, the first heating section temperature is 900-1100°C, the insulation time is more than 60 minutes, the second heating section temperature is 1200-1350°C, and the total time in the furnace is 3-4 hours.

[0018] In the above technical solution, in step (3), the rough rolling adopts 2 to 3 continuous rolling passes; the finishing rolling adopts 5 to 7 continuous rolling passes. In the finishing rolling process, edge heating devices are used on both sides of the steel plate, with a compensation temperature of 35 to 45°C and a width of 40 to 50 mm.

[0019] In the above technical solution, in step (4), the layer cooling rate is 20-30°C / s, and the cooling rate difference in the bandwidth direction at the same position is ≤1°C / s, and the slow cooling rate is 15-25°C / h.

[0020] In the above technical scheme, in step (5), a 9-roller straightening machine with a straightening force greater than 600 MPa is used for leveling; in the sandblasting process, the upper and lower nozzles are 50 to 200 mm away from the surface of the steel plate, the sandblasting pressure is 100 to 200 MPa, the belt threading speed is 5 to 20 m / min, the water temperature is 10 to 50°C, and the sand particle size is 0.5 to 2.0 mm.

[0021] In the above technical solution, the chemical composition of the hot-rolled plate obtained in step (5) is as follows by weight percentage: C 0.3%-0.50%, Si 0.01%-0.3%, Mn 0.5%-1.5%, B 0.0005%-0.005%, Cr0.1%-0.8%, Nb 0.01%-0.5%, Ti 0.01%-0.1%, N 0.0010%-0.010%, Ca0.0005%-0.02%, 1≤Nb / Ti≤10, P≤0.020% and S≤0.020%, and the balance is Fe and unavoidable impurities.

[0022] In the above technical solution, in step (7), a plasma surfacing method is used to prepare a hardened layer, and the thickness of the hardened layer is 2 to 4 mm.

[0023] The invention adopts converter smelting and LF electric furnace refining without RH vacuum treatment; the C content of molten steel before tapping the converter is 0.3%-0.5%, so as to ensure that the carbon content in the ingot after continuous casting of refined steel is uniform and has no macro segregation; LF electric furnace refining is adopted, and alloy is added after the refined S content is ≤0.020% for alloying, and nitrogen is passed at the same time to adjust the N content of molten steel, change the composition, quantity and form of non-metallic inclusions, accelerate the flow of molten steel, promote the full floating of inclusions, improve the purity of steel, various non-metallic inclusions in finished steel do not exceed level 1.5, improve the surface finish of steel, and eliminate the anisotropy of structure; the continuous casting adopts light pressure reduction and crystallizer electromagnetic stirring, and the electromagnetic stirring current is 400-600A, Frequency 2.2~2.9Hz; dynamic water volume of crystallizer 80~160L / min, use special crystallizer protective slag for pouring, crystallizer liquid level 750~850mm, superheat 20~30℃, control inclusion and segregation, continuous casting speed 1.1~1.6m / min, secondary cooling water volume 0.2~0.4L / kg, ensure that the surface temperature deviation of the ingot is no more than 10℃, the cooling rate of the ingot is uniform, the structure is uniform, the strength difference of the same roll of strip after rolling is ≤20MPa, the hardness difference of the same roll is ≤3HRB, the equiaxed crystal rate is more than 50%, and the liquid micro-segregation of the alloy at the end of the columnar crystal is controlled.

[0024] The ingot is hot-transferred and loaded into the step-beam heating furnace at a temperature above 550°C to save energy and reduce consumption; the heating furnace adopts a weak oxidizing atmosphere (excess air coefficient 1.02-1.1, air-fuel ratio 2.0-2.2), the preheating section temperature is 500-600°C, the holding time is more than 60 minutes, the first heating section temperature is 900-1100°C, the holding time is more than 60 minutes, the second heating section temperature is 1200-1350°C, and the total time in the furnace is 3-4 hours to ensure the surface quality of the ingot and avoid oxidation and decarburization.

[0025] Rough rolling adopts 2-3 continuous rolling, with the first-pass reduction rate of 20%-25%, and the starting rolling temperature of 1200-1250℃, which provides nucleation power for the precipitation of various carbides and nitrides. The temperature difference of the steel strip rolling over the entire length is ≤20℃, and the microstructure and performance are uniform. The high temperature and small reduction in rough rolling are conducive to obtaining a coarse pearlite and ferrite mixed structure after subsequent cooling, reducing the yield strength. Finishing rolling adopts 5-7 continuous rolling, with the first-pass reduction rate of 20%-25%, and the final rolling temperature of 950-1000℃, which reduces the strip shape and does not exceed 2.0 grades. In the finishing rolling process, edge heating devices are used on both sides of the steel plate, the compensation temperature is 35-45℃, the width is 40-50mm, the strip rolling temperature difference is ≤10℃, the microstructure and performance are uniform, the strip thickness fluctuation is within ±0.20mm, the convexity is ≤40μm, the tensile strength difference of the same coil is ≤20MPa, and the hardness difference of the same coil is ≤3HRB.

[0026] After the steel plate leaves the finishing mill, it enters the layer cooling, uniform cooling, cooling rate of 20-30℃ / s, cooling rate difference in the width direction of the same position ≤1℃ / s, forming ferrite volume accounting for 20-30%, ensuring uniform performance of the same plate structure, strength difference ≤20MPa, hardness difference ≤3HRB. Cooling to 580-650℃ for coiling, cooling to below 300℃ at a cooling rate of 15-25℃ / h, which is conducive to carbide nucleation, and obtaining a mixed structure of spheroidized pearlite, lamellar pearlite and ferrite, with a spheroidization rate of more than 30%. The cooling rate exceeds 25℃ / h, which is easy to form lamellar pearlite. The cooling rate is lower than 15℃ / h, and the spheroidized pearlite is easy to grow, with poor toughness and plasticity. On the other hand, the surface oxide scale formed by this coiling temperature and cooling rate is loose, which is conducive to the subsequent surface sandblasting.

[0027] The coiled steel coil is air-cooled to 50-100℃ and leveled with a 9-roller straightening machine with a straightening force greater than 600MPa to ensure the flatness of the plate shape; the upper and lower nozzles of the sandblasting equipment are 50-200mm away from the surface of the steel plate, the sandblasting pressure is 100-200MPa, the belt threading speed is 5-20m / min, the water temperature is 10-50℃, and the sand particle size is 0.5-2.0mm, ensuring that the iron oxide scale is cleanly removed during sandblasting, the surface quality FB is at a high level, the surface roughness is 3.0-5.0μm, which is conducive to surface hardening treatment, and there is no iron oxide scale and oxidized decarburization layer on the surface, which helps to improve the heat treatment hardness and hardness uniformity.

[0028] The blanks of agricultural machinery tools are induction heated to 880-920℃, hot stamped, water cooled to 180-230℃ for 5-10min, and then air cooled to room temperature to obtain fine lath martensite structure, which has excellent performance without tempering treatment. The tensile strength is above 1800MPa, the elongation is ≥8%, the impact energy Akv at 20℃ is ≥20J, the surface hardness is above 53HRC, and the surface hardness difference is within ±0.5HRC.

[0029] The agricultural machinery tool blanks after quenching heat treatment do not need tempering, sandblasting and tumbling. According to the design requirements, the plasma surfacing method is used to form a 2-4mm thick Fe-Cr-CB series flux-cored wire hardening layer on the local surface of the component. After air cooling, the surface hardness is above 60HRC.

[0030] The beneficial effects of the present invention are:

[0031] The hot-rolled plate obtained by the present invention has no surface iron oxide scale, a surface roughness of 3.0-5.0 μm, a ferrite volume proportion of 20-30%, a yield strength of 300-400 MPa, a tensile strength of 650-700 MPa, good formability, and the processed agricultural machinery tool is quenched and heat-treated with a coolant containing 60-90% water to obtain a uniform fine lath martensitic structure, the longest martensitic lath is no more than 25 μm, the tensile strength after heat treatment is above 1800 MPa, the elongation is ≥8%, the impact energy Akv at 20°C is ≥20J, the hardness is above 53HRC, the surface hardness difference is within ±0.5HRC, and the structure is martensite; no tempering treatment, sandblasting and rolling are required, the hardness is above 60HRC after local surface hardening treatment, the wear resistance is significantly improved, and the efficiency and yield rate are significantly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 The metallographic structures of the hot-rolled plate and the surface hardened layer of the agricultural machinery tool prepared in Example 1 of the present invention, a is the hot-rolled plate, and b is the surface hardened layer. DETAILED DESCRIPTION

[0033] The following examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0034] The chemical composition of the steel in Examples 1-15 and Comparative Example 1 is shown in Table 1; the smelting and continuous casting process parameters are shown in Table 2; the ingot processing process parameters are shown in Table 3; the rolling and cooling process parameters are shown in Table 4; the surface treatment and heat treatment process parameters are shown in Table 5; the microstructure and properties of the hot-rolled plate are shown in Table 6; the composition of the flux-cored welding wire in Examples 1-15 is shown in Table 7; and the finished product properties are shown in Table 8.

[0035] Table 1 Chemical composition of steel in Examples 1-15 and Comparative Example 1, %

[0036] serial number C Si Mn P S Nb Cr B Ti N Ca Nb / Ti Example 1 0.38 0.16 1.15 0.01 0.004 0.07 0.58 0.001 0.05 0.0048 0.0009 1.40 Example 2 0.37 0.24 1.08 0.014 0.003 0.32 0.25 0.002 0.09 0.0064 0.0008 3.56 Example 3 0.39 0.19 1.47 0.005 0.005 0.12 0.39 0.0025 0.02 0.0039 0.0035 6.00 Example 4 0.38 0.07 0.77 0.016 0.006 0.03 0.51 0.0043 0.015 0.0063 0.018 2.00 Example 5 0.46 0.14 0.86 0.006 0.004 0.05 0.32 0.0025 0.02 0.0058 0.0016 2.50 Example 6 0.45 0.09 0.58 0.021 0.008 0.02 0.29 0.0031 0.018 0.0078 0.0006 1.11 Example 7 0.39 0.05 0.98 0.013 0.019 0.15 0.15 0.0038 0.032 0.0091 0.0028 4.69 Example 8 0.42 0.19 1.18 0.02 0.01 0.06 0.36 0.0039 0.01 0.0056 0.0035 6.00 Example 9 0.44 0.25 0.85 0.019 0.017 0.07 0.55 0.0009 0.021 0.0047 0.0046 3.33 Example 10 0.33 0.22 1.21 0.01 0.008 0.04 0.75 0.0012 0.018 0.0046 0.019 2.22 Embodiment 11 0.45 0.09 0.99 0.018 0.013 0.25 0.37 0.0026 0.07 0.0051 0.0089 3.57 Example 12 0.38 0.26 1.3 0.012 0.005 0.32 0.38 0.0035 0.04 0.008 0.0011 8.00 Embodiment 13 0.34 0.18 1.18 0.016 0.003 0.46 0.28 0.0023 0.086 0.0058 0.0013 5.35 Embodiment 14 0.35 0.19 1.19 0.04 0.004 0.26 0.79 0.0034 0.05 0.0059 0.0086 5.20 Embodiment 15 0.31 0.04 1.15 0.011 0.014 0.18 0.66 0.0049 0.03 0.0072 0.0091 6.00 Comparative Example 1 0.65 1.70 1.21 0.011 0.009 0 0 0 0 0 0 -

[0037] Table 2 Smelting and continuous casting process parameters in Examples 1-15 and Comparative Example 1

[0038]

[0039] Table 3 Process parameters of casting process in Examples 1-15 and Comparative Example 1

[0040]

[0041]

[0042] Table 4 Rolling and cooling process parameters of Examples 1-15 and Comparative Example 1

[0043]

[0044]

[0045] Table 5 Surface treatment and heat treatment process parameters in Examples 1-15 and Comparative Example 1

[0046]

[0047] Table 6 Test results of the microstructure and properties of the hot-rolled plates in Examples 1-15 and Comparative Example 1

[0048]

[0049]

[0050] Table 7 Composition of flux-cored welding wire of Examples 1-15

[0051]

[0052]

[0053] Table 8 Test results of steel structure and properties after heat treatment in Examples 1-15 and Comparative Example 1

[0054]

[0055] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for manufacturing a surface hardened agricultural tool, characterized in that: The method comprises the following steps: (1) Smelting and continuous casting: The C content of the molten steel before tapping the converter is 0.25% to 0.4%. The molten steel enters the LF electric furnace for refining. After the sulfur content is ≤ 0.020%, alloying is performed. At the same time, N is introduced to adjust the N content of the molten steel to 0.001% to 0.010%, and continuous casting is performed to obtain a continuous casting billet. (2) Ingot treatment: Ingots are heated at above 550°C and then put into a heating furnace for heating; (3) Rolling: Descaling is performed before rough and finish rolling. The first pass of rough rolling has a reduction ratio of 20% to 25%, the start rolling temperature is 1200 to 1250°C, and the temperature difference of the steel strip over the entire length is ≤20°C. The first pass of finish rolling has a reduction ratio of 20% to 25%, the final rolling temperature is 950 to 1000°C, and the temperature difference of the steel strip over the entire length is ≤10°C. (4) Cooling: After the steel plate leaves the finishing mill, it enters the layer cooling, cools to 580-650℃ and coils it, and then slowly cools it to below 300℃ after coiling; (5) Surface treatment: The coiled steel coil is air-cooled to 50-100°C for flattening, and then sandblasted after being flattened; (6) Heat treatment: processing the hot-rolled plate obtained in step (5) into a blank for agricultural machinery tools, induction heating to 880-920° C., hot stamping forming, water cooling to 180-230° C. for 5-10 min, and air cooling to room temperature; (7) Local surface hardening treatment: According to the design requirements of agricultural machinery tools, a flux-cored welding wire is used to locally form a hardened layer on the surface of the agricultural machinery tool blank. The flux-cored welding wire contains the following components in weight percentage: C 1.0% to 3.0%, Si 1.0% to 3.0%, Mn 0.8% to 2.0%, B 8.0% to 15.0%, Cr 20% to 30%, Nb 1.0% to 2.0%, Ni 0.8% to 1.6%, Mo 0.4% to 1.2%, and the balance is Fe and unavoidable impurities.

2. The manufacturing method according to claim 1, characterized in that: In step (1), the molten steel is cast by a vertical bending arc continuous casting machine, and the arc radius of the vertical bending continuous casting machine is not less than 5m; Continuous casting adopts light pressure and crystallizer electromagnetic stirring, the electromagnetic stirring current is 400-600A, the frequency is 2.2-2.9Hz, the dynamic water volume of the crystallizer is 80-160L / min, the crystallizer protection slag is used for casting, the crystallizer liquid level is 750-850mm, the superheat is 20-30℃, the continuous casting speed is 1.1-1.6m / min, and the secondary cooling water volume is 0.2-0.4L / kg; The thickness of the continuous casting slab is 170-250mm.

3. The manufacturing method according to claim 1, characterized in that: In step (2), the heating furnace adopts a weak oxidizing atmosphere, the preheating section temperature is 500-600°C, the insulation time is more than 60 minutes, the first heating section temperature is 900-1100°C, the insulation time is more than 60 minutes, the second heating section temperature is 1200-1350°C, and the total time in the furnace is 3-4 hours.

4. The manufacturing method according to claim 1, characterized in that: In step (3), the rough rolling adopts 2 to 3 continuous rolling passes; the finishing rolling adopts 5 to 7 continuous rolling passes. During the finishing rolling process, edge heating devices are used on both sides of the steel plate, with a compensation temperature of 35 to 45° C. and a width of 40 to 50 mm.

5. The manufacturing method according to claim 1, characterized in that: In step (4), the layer cooling rate is 20-30°C / s, and the cooling rate difference in the bandwidth direction at the same position is ≤1°C / s, and the slow cooling rate is 15-25°C / h.

6. The manufacturing method according to claim 1, characterized in that: In step (5), a 9-roller straightening machine with a straightening force greater than 600 MPa is used for leveling; During the sandblasting process, the upper and lower nozzles are 50 to 200 mm away from the surface of the steel plate, the sandblasting pressure is 100 to 200 MPa, the belt threading speed is 5 to 20 m / min, the water temperature is 10 to 50°C, and the sand particle size is 0.5 to 2.0 mm.

7. The manufacturing method according to claim 1, characterized in that: The chemical composition of the hot-rolled plate obtained in step (5) is as follows by weight percentage: C 0.3%-0.50%, Si 0.01%-0.3%, Mn 0.5%-1.5%, B 0.0005%-0.005%, Cr 0.1%-0.8%, Nb 0.01%-0.5%, Ti 0.01%-0.1%, N 0.0010%-0.010%, Ca 0.0005%-0.02%, 1≤Nb / Ti≤10, P≤0.020% and S≤0.020%, and the balance is Fe and unavoidable impurities.

8. The manufacturing method according to claim 1, characterized in that: In step (7), a plasma surfacing method is used to prepare a hardened layer, and the thickness of the hardened layer is 2 to 4 mm.

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