CN113564470B - 1700MPa heat-resistant steel for agricultural machinery and its manufacturing method - Google Patents

Abstract

1700MPa heat-resistant steel for agricultural machinery and a manufacturing method thereof, wherein the steel comprises the following chemical components in percentage by weight: 0.70 to 1.1 percent of C, less than or equal to 0.1 percent of Si, 0.4 to 1.5 percent of Mn0.06 percent of Al, 0.0005 to 0.05 percent of Ca0.001 to 0.5 percent of Re0.001 to 0.5 percent of Sn0.001 to 0.5 percent of Mg0.0008 to 0.05 percent of Ca/S, more than or equal to 0.2 percent of Ca/Mg, more than or equal to 1 percent of Ca/Re, less than or equal to 0.020 percent of P and less than or equal to 0.015 percent of S; the balance being Fe and unavoidable impurities. The agricultural machine steel hot rolled plate produced by the invention has high surface quality, the hardness of the matrix after heat treatment is more than 55HRC, the tensile strength is more than 1700MPa, the hardness of the surface spray-welded spherical chromium-copper alloy granular powder is more than 65HRC, and the surface wear resistance, the heat resistance and the corrosion resistance are excellent.

Description

1700MPa heat-resistant steel for agricultural machinery and its manufacturing method

technical field

The invention relates to the field of steel material preparation, in particular to a 1700MPa heat-resistant steel for agricultural machinery and a manufacturing method thereof.

Background technique

The development and level of agricultural machinery and equipment are related to the major security of national food. The steel used for agricultural machinery parts is constantly updated. At present, there are two main types of steel for high-end buried agricultural machinery parts, one is 65Mn steel oil quenching, the finished product hardness is 42-45HRC, the strength is 1200MPa, the wear resistance is average, the environmental pollution is serious, and the high carbon hot-rolled plate has high strength and high hardness. , the yield strength ratio is generally more than 80%, the processing and forming is difficult, and the heat treatment is easy to crack, and the heat resistance and corrosion resistance are poor; the other is the medium carbon 30MnB5 agricultural machinery steel imported from abroad, and the hardness of the finished product is 46-48HRC after tempering , The strength is about 1300MPa, and the yield ratio is generally more than 70%. It is easy to crack after heat treatment, the plate shape is not good, and the heat resistance and corrosion resistance are not ideal. The above-mentioned parts processed with steel for agricultural machinery all have the problem of decreased wear resistance when the friction heats up during work in the soil. At present, there are no agricultural machinery parts with a hardness above 52HRC, a strength above 1500MPa, and excellent surface heat resistance, wear resistance and corrosion resistance. steel.

The patent document with the application number CN201510174384.3 "A method for producing thin gauge tool steel with CSP line without spheroidizing annealing" describes the production method of thin tool steel plates with a thickness of 1-2.5mm, which cannot meet the requirements for thick gauge tools. The use requirements of the steel plate; adopt the production process of low-temperature rolling, rapid cooling to low-temperature coiling near the martensitic transformation temperature, and then tempering to obtain tempered sorbite to reduce hardness. On the other hand, martensite phase transformation occurs during the coiling process, and the martensite is extremely brittle and easily broken. After coiling, a high temperature tempering treatment at 550-700 ° C is required, and the cost is relatively high. The process controls the depth of decarburization not to exceed 1% of the plate thickness.

CN201310076240.5, a production method of annealing-free medium-high carbon steel plate and CN201110411594.1, a production method of annealing-free hot-rolled S45C strip, adopting two-phase or ferrite zone rolling under low temperature and high pressure, high temperature coiling and stacking , get 60% ferrite and partial spheroidized pearlite, soften and reduce the hardness of the steel plate to 80-85HRB. The steel plate obtained by this process has a lot of ferrite and is large in size. It must be quenched and tempered before it can be used. Moreover, the hardness after heat treatment is low, which cannot meet the requirements of high-end tool steel.

The patent document with the application number 201510687941.1 discloses an alloy tool steel and its production method, involving a medium carbon tool steel, the steel contains: C 0.5%, Si 0.2%, Mn 0.5%, Cr 5.0%, Mo 2.3% , V 0.5%, S ≤ 0.003%, P ≤ 0.02%. This alloy tool steel is more alloyed with Cr, Mo, V and other alloys. The smelting needs to use electroslag remelting to cast ingots. The yield is low, and softening annealing is required. Multi-channel heat treatment processes such as spheroidizing annealing, quenching, and tempering are costly and are not suitable for processing and manufacturing various tools with complex shapes.

The steel plate described in the patent document "A Steel 55MnB for Household and Garden Tools and Its Preparation Method" with the application number CN104630618A is only 3-5 mm thick, which has a limited application range and cannot meet the needs of thinner or thicker specifications. This patent adopts boron-containing composition design, but does not emphasize the control of oxygen and nitrogen. Boron in molten steel is easy to combine with oxygen and nitrogen to form compounds, which not only weakens the effect of solid solution boron on improving hardenability, but also, The compound phases of B and N are easy to segregate into a network at the grain boundary, resulting in boron brittleness, resulting in defects such as high brittleness. Boron-containing steel has high thermal deformation resistance, and the hot-rolled plate shape is difficult to control. The hot-rolled plate shape is poor, and it is difficult to level the subsequent heat treatment process, which seriously affects the quality of the finished product. On the other hand, the yield strength of the steel plate described in this patent is 390MPa-470, the tensile strength is 750-820MPa, and the elongation is 17-26%. The strength is still high, and processing still has certain difficulties. Moreover, the described production process adopts high-temperature coiling at 750°C, the surface of the steel plate is seriously decarburized, the surface oxide scale is thick, and the amount of grinding by the user is large, which affects the production efficiency.

The patent document of ZL201610727654.3 "A kind of steel for gardening tools with excellent processing performance and its production method" obtains a steel plate structure of coarse flaky pearlite + spheroidized pearlite, with a decarburized layer of more than 0.01mm on the surface, and poor surface quality , Bending is easy to crack, the cost of adding Cr\Ni\Cu is high, the quenching hardness is 50HRC, the strength is less than 1500MPa, and the toughness after tempering is lower.

A hot-rolled strip steel rake suitable for water quenching and its manufacturing method ZL 201410250576.3, a hot-rolled steel strip for a rake and its production method, and a rake treatment method ZL 201410253770.7, a steel for a rake, a production method and a rake Sheet processing method 201410253772.6, a strip steel for rake sheet and its production method and rake sheet heat treatment method ZL201410250902.0, these four patent documents adopt the design of medium carbon and boron-containing components, and add Cr\Ti\V\Nb at the same time, the cost is high , Boron activity 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 is poor after heat treatment, the coiling temperature is high, and the surface is oxidized and decarburized serious.

None of the steel types and production methods mentioned in the above-mentioned prior art mentioned heat-resistant and corrosion-resistant technologies, and are not suitable for processing high-end heat-resistant and corrosion-resistant agricultural machinery accessories for use in soil. In order to adapt to the development of the times and meet the needs of upgrading of agricultural machinery parts, it is urgent to develop steel for agricultural machinery parts with a hardness of 55HRC, a strength of more than 1500MPa, and excellent heat resistance and corrosion resistance.

Contents of the invention

The object of the present invention is to provide a kind of 1700MPa heat-resistant agricultural machinery steel and its manufacture method, the agricultural machinery steel hot-rolled plate microstructure that the present invention produces is the thick lamellar pearlite with interlamellar interval 0.5 μ m or more, and the diameter of pearlite group is less than 10 μ m, The yield strength ratio is below 60%, there is no grain boundary oxide layer on the surface, the matrix hardness is above 55HRC under high temperature working conditions after heat treatment, and the tensile strength is above 1700MPa. The agricultural machinery accessories produced have a surface hardness above 65HRC and excellent wear resistance, heat resistance and corrosion resistance.

In order to achieve the above object, the present invention adopts the following technical solutions to realize:

1700MPa heat-resistant agricultural machinery steel, the chemical composition of the steel is: C 0.70%~1.1%, Si≤0.05%, Mn 0.4%~1.5%, Al≤0.06%, Ca 0.0005%~0.05%, Re 0.05% ~0.5%, Sn0.1%~1.0%, Mg0.0008%~0.05%, Ca/S≥0.2, Ca/Mg≥1, Ca/Re≥0.05, P≤0.020%, S≤0.015%; balance For Fe and unavoidable impurities.

The mechanism of action of each alloy composition of the structural steel of the present invention is set forth in detail below:

C is the main solid solution strengthening element in steel, and in the present invention, a sufficient amount of C needs to be combined with Sn, Re, etc. to improve the toughness. If the C content is lower than 0.70%, it is difficult to ensure the toughness of the steel plate. On the other hand, C If the content is higher than 1.1%, the strength of the hot-rolled sheet will be too high, the toughness and plasticity of the steel will be deteriorated, the yield ratio will be affected, and the formability will be poor. Therefore, the C content should be controlled at 0.70% to 1.1%.

Mn is relatively cheap, is a good deoxidizer and desulfurizer, and is a necessary element to ensure the strength and toughness of steel. Manganese and iron can dissolve indefinitely to form a solid solution, improve hardness and strength, and have relatively little influence on plasticity. The combination of Mn and S forms MnS, which avoids hot cracks caused by the formation of FeS at grain boundaries and affects the hot formability of tool steel. At the same time, Mn is also a good deoxidizer and increases hardenability. The Mn content in the steel is low, which cannot meet the requirements of high hardness. If the Mn content is too high, the segregation will be serious, which will affect the welding performance and formability, and increase the production cost. Therefore, considering factors such as cost and performance requirements, the Mn content should be controlled at 0.4 % ~ 1.5%.

Si is one of the common elements in steel. It is used as a reducing agent and deoxidizer in the steelmaking process. Si in solid solution can increase the yield strength and ductile-brittle transition temperature. Si0.17-0.37% in general agricultural machinery steel. The present invention does not add Si to control the residual Si in the steel as little as possible. The Si of the present invention can promote the decarburization and oxidation of the cutting tool steel surface to form a loose oxide layer, and there are microcrack defects such as grain boundary oxidation in the oxide layer, which seriously affects the surface hardness and fatigue performance. The invention has Si≤0.05%, low cost, avoids steel surface oxidation, improves surface quality, contributes to the compactness of the spray-welded alloy layer, and ensures wear resistance and corrosion resistance.

Al is used as a deoxidizing and nitrogen-fixing agent in steelmaking to refine grains, inhibit the aging of steel, improve the toughness of steel at low temperatures, and especially reduce the brittle transition temperature of steel; Al can also improve the oxidation resistance of steel, improve Corrosion resistance to hydrogen sulfide. If the Al content exceeds 0.06%, it is easy to form large particle oxide inclusions with oxygen in the steel, which will affect the fatigue performance.

As a microalloying element, Ca can refine grains, partially desulfurize, improve the corrosion resistance, wear resistance, high temperature and low temperature resistance of steel, and improve the impact toughness, fatigue strength, plasticity and welding performance of steel; Cold heading, shock resistance, hardness and contact durability of steel. Tool steel has high carbon, poor fluidity of molten steel, and inclusions are not easy to float up. The present invention adds calcium to change the composition, quantity and shape of non-metallic inclusions, speed up the flow of molten steel, promote the full floating of inclusions, and improve the purity of steel. The finished steel Various types of non-metallic inclusions do not exceed 1.5, and improve the steel surface finish, eliminate the anisotropy of the structure, improve the resistance to hydrogen-induced cracking and lamellar tearing, and prolong the service life of the tool. The denaturation effect of Ca inclusions has a great relationship with the sulfur content. In the present invention, Ca/S≥0.2 can ensure sufficient denaturation treatment of Ca inclusions. The addition of calcium in the present invention can increase the yield of rare earth to more than 60%, the yield of Mg to more than 40%, effectively improve the utilization rate of rare earth and magnesium, and give full play to their high temperature strength and hardness in steel. Oxidation resistance, corrosion resistance and heat resistance effect.

Re rare earth elements can improve the oxidation resistance and corrosion resistance of steel, make the steel structure compact, and improve the high temperature strength. The effect of RE oxidation resistance exceeds that of aluminum, titanium and other elements. The Re of the present invention can improve the fluidity of the steel, improve the surface smoothness of the steel plate, and improve the surface spray welding performance. Re can also change oxide and sulfide inclusions such as Al ₂ O ₃ and MnS into finely dispersed spherical inclusions, thereby eliminating the harmfulness of inclusions and improving fatigue performance. Only when Ca/Re≥0.05 in the present invention can there be enough Ca to increase the yield of rare earth to over 60%, and give full play to the effects of rare earth on oxidation resistance, corrosion resistance and heat resistance in steel. The rare earth in the invention makes the steel structure compact at high temperature, improves the high temperature strength, and improves the performance after heat treatment. Re, Mg and C in the steel form a series of carbon-magnesium rare earth compounds, which further promotes hot rolling to form a pearlite structure with a sheet spacing of 0.5 μm or more. Heat treatment carbon diffuses quickly, does not require high temperature, and saves energy and reduces consumption.

Mg is a very active metal element, and it has a strong affinity with oxygen, nitrogen, and sulfur. However, because Mg is too active, it is difficult to control during smelting, and the yield is low. The present invention adopts self-created refining and Re-Mg smelting technology, strictly and accurately controls the Mg content in steel, and utilizes the joint action of Ca and Mg to form composite inclusions such as CaO·MgO·Al ₂ O ₃ and CaO·MgO·MnS. The melting point is low, and it is easy to solidify and float up in molten steel, avoiding the problem of nozzle nodules in the continuous casting process, reducing the content of inclusions in molten steel, and controlling the level of inclusions in steel to not exceed 1.0. Only with Ca/Mg≥1 can there be enough Ca to increase the yield of Mg by more than 40%, effectively improve the utilization rate of magnesium, form CaO·MgO·Al ₂ O ₃ and CaO·MgO·MnS composite inclusions, and improve the high temperature strength after heat treatment and hardness. Another main function of Mg is to form a series of carbon-magnesium rare earth compounds with Re and C, so that a pearlite structure with a sheet spacing of more than 0.5 μm is formed in the hot-rolled sheet, and the heat-treated carbon diffuses quickly without high temperature, saving energy and reducing consumption.

Sn: the tin of the present invention can improve the wear resistance of steel and the fluidity of molten steel, the fluidity of molten steel is good, the inclusions can fully float up, and the purity of steel is good. The solid solution Sn of the present invention improves the toughness of handling. The present invention has high carbon content, and carbon is easy to segregate and gather at the solidification end of cast slab dendrites. When carbon segregates and gathers at the grain boundary on the surface of the steel plate, it will cause serious surface oxidation, surface grain boundary oxidation and microcracks, which will affect the surface spray welding. main reason for eligibility. On the other hand, the carbon content of the present invention is high, and the hot-rolled sheet is prone to liquid phase separation, and heat treatment such as annealing further precipitates graphite, which affects the performance such as bending forming and fatigue. The present invention adds a certain amount of tin to the steel, which can effectively hinder the carbon segregation and aggregation, cementite and graphitization precipitation of the tool steel, suppress microcracks on the surface of the steel plate, obtain an oxidized steel plate without surface grain boundaries, and improve the surface spray welding performance and strength , wear resistance and other comprehensive properties.

Solid solution Re, Mg and Sn, C in a specific production process to form spherical carbide tin compounds with a diameter of less than 20nm, such as _ReMgSnCM , _ReSnCM , _MgSnCM , etc., are evenly dispersed in the steel and become pearlite sheets. Nucleation of particles, so that the spacing of cementite sheets in the pearlite of the hot-rolled steel plate is more than 0.5 μm, and the carbon in the cementite diffuses easily and quickly, without high-temperature austenitization. The heating temperature during heat treatment is 670-750 ° C. Save energy. These spherical Re and Mg carbides with a diameter of less than 20nm are more heat-resistant than ordinary cementite, and they do not spread and decompose at 900°C. After heat treatment, they are still evenly distributed in the steel, which improves the hardness after heat treatment and further improves heat resistance. Performance and wear resistance, under the high temperature working conditions above 200 ℃, the tensile strength is above 1700MPa, and the hardness is above 55HRC.

Both P and S are unavoidable harmful impurities in steel, and their existence will seriously deteriorate the toughness of steel, so measures should be taken to reduce the content of P and S in steel as much as possible. According to the invention, the maximum P content is limited to 0.020% and the maximum S content is limited to 0.015%. Sulfur exists in the form of FeS and MnS in steel. In this invention, the higher the Mn, the higher the formation tendency of MnS. Although its higher melting point can avoid the generation of hot embrittlement, MnS can extend into strips along the processing direction during processing deformation. , The plasticity, toughness, and fatigue strength of steel are significantly reduced, so Ca and Mg are added to the steel for inclusion deformation treatment.

The hot-rolled steel plate of agricultural machinery steel has a coarse pearlite structure, the spacing between pearlite sheets is more than 0.5 μm, and the diameter of pearlite clusters is not greater than 10 μm; after heat treatment, the structure is a lath-shaped horse of spherical carbide tin oxide with a diameter of less than 20 nm. Body organization.

The steel plate for agricultural machinery has a yield ratio of less than 60%; after heat treatment, under the high temperature working condition of above 200°C, the hardness of the matrix is above 55HRC, and the tensile strength is above 1700MPa.

The non-metallic inclusions in the agricultural machinery steel plate do not exceed grade 1.0; the grain boundary oxide layer of the steel plate is 0mm; and the segregation does not exceed grade 1.5.

The manufacturing method of 1700MPa heat-resistant steel for agricultural machinery, the steel plate of the present invention is rolled by the continuous casting billet rolling of converter smelting, electric furnace refining, pouring, and method comprises:

1) Smelting process:

a) Deoxidize with Al deoxidizer, after refining the oxygen content ≤ 0.0020%, add Ca for at least 5 minutes, then add Mn, Re, Mg, and finally add Sn alloy;

b) The argon blowing time in the tundish is 5 to 8 minutes to ensure that the inclusions are fully floating and the pouring superheat is ≤25°C;

c) The continuous casting adopts electromagnetic stirring of the mold, the thickness of the billet is 170mm-250mm; the reduction is 2mm-10mm; the current intensity of the electromagnetic stirring of the continuous casting mold is 500A-1000A, and the electromagnetic stirring is 1 minute to 3 minutes;

d) Continuous casting casting speed 1.0m/min～1.4m/min;

2) Billet processing technology:

a) The cast slab is put into the slow cooling pit for more than 72 hours after being rolled off the assembly line, and the cast slab is heated by a walking heating furnace before rolling;

b) The heating furnace adopts a reducing atmosphere, the temperature of the preheating section is above 500°C, the temperature of the heating section is 1200°C~1350°C, the heating section time is 30 minutes to 50 minutes, and the total time in the furnace is 2 hours to 4 hours;

3) The rolling process includes three processes of rough rolling, finishing rolling and third rolling:

Before rough rolling, finishing rolling and the third rolling, multi-pass high-pressure water descaling is used, and the pressure of high-pressure water is not less than 30MPa to ensure the surface quality of the steel plate;

a) Rough rolling adopts the first pass ≥ 50% large reduction rate rolling to fully break the coarse grains of the slab;

b) Finish rolling is not less than 6-pass continuous rolling method, the total reduction rate is ≥80%, the first pass reduction rate is ≥30%, high temperature and fast rolling, rolling speed is ≥20m/s, and the rolling start temperature is 1100℃～ 1150℃, junction speed temperature 900℃～980℃;

c) After finishing rolling, cool down to 600℃～750℃ at a cooling rate ≥ 20℃/s, enter into two vertical rollers and four horizontal roller mills for continuous rolling for two passes, the upper and lower reduction ratios are 2% to 8%, and the side pressure reduction ratios 5% to 25%;

4) Cooling process:

a) The steel plate enters laminar flow cooling, quenching, and the cooling rate is ≥20°C/s;

b) In order to ensure the uniformity of the structure and properties of the whole roll and the whole roll without surface oxidation and grain boundary oxidation, the cooling method of the head, the middle and the end of the roll is adopted. Coiling at 550°C-600°C at the end of 0m-30m, and coiling at 520°C-580°C for the rest of the roll. The whole roll will obtain a thick flaky pearlite structure. 10μm;

5) Use the hot-rolled steel plate obtained in step 4) to process and manufacture the required shape of the agricultural machinery parts, heat it to 670°C-750°C, keep it warm for 10-30 minutes, oil quench to below 200°C, put it into a tempering furnace at 300-500°C and keep it warm for 24 ~36 hours, air-cooled.

The spray welding method of agricultural machinery accessories processed by using 1700MPa heat-resistant agricultural machinery steel, the method includes

1) After sandblasting for 10 to 30 minutes and rolling for 10 to 30 minutes for agricultural machinery parts, after surface treatment, the surface is heated to 800°C to 900°C, and the chrome-copper alloy powder layer containing 70% to 80% of chromium is thermally sprayed and welded, and air-cooled to room temperature;

2) The chromium-copper alloy powder spray-welded on the surface of agricultural machinery parts is a spherical granular powder with a diameter of 80 μm to 150 μm.

The thickness of the chrome-copper alloy granular powder layer spray-welded on the surface of the agricultural machinery parts is 0.2 mm to 0.5 mm, and forms a dense CrCuC transition layer of 0.1 mm to 0.2 mm with the carbon in the matrix under the steel surface. The hardness is above 65HRC, the wear rate is less than 20mg/km under the conditions of grinding pin rotation speed 300r/min and load 120N, and the corrosion rate in soil environment is less than 15g/m ² ·h.

The steel used for agricultural machinery in the present invention has high carbon content, difficult control of oxygen content, and poor fluidity of molten steel. In order to improve the fluidity of molten steel and deoxidize fully, deoxidize with Al deoxidizer first. After the oxygen content is ≤0.0020%, add Ca for at least 5 minutes and then add Re and Mg alloys. Ca, Mg, Re, and Sn in the composition of the present invention are all active elements, which are difficult to control during smelting, and the order of addition is very important. Al ₂ O ₃ inclusions will be generated in the steel after adding Al for deoxidation. If the furnace lining refractory material is poor, MgO·Al ₂ O ₃ inclusions will also be generated. These Al oxide inclusions have a high melting point and are not easy to solidify and float in the steel. On the one hand It will reduce the fluidity of molten steel and block the pouring nozzle. On the other hand, it will also increase the inclusions in the steel and affect the bending and fatigue properties of the steel. After adding Al for deoxidation and then adding Ca treatment, Ca will interrupt the original long strips of Al ₂ O ₃ and MgO Al ₂ O ₃ and MnS inclusions, and wrap them outside these intermittent inclusions to form diffusely distributed spherical CaO. MgO·Al ₂ O ₃ or CaO·Al ₂ O ₃ and CaO·MnS composite oxides refine and spheroidize the inclusions, and these small particle calcium-aluminate composite inclusions have a low melting point and are easy to solidify and float in molten steel. , to avoid the nozzle nodulation problem in the continuous casting process, reduce the content of inclusions in molten steel, and ensure that the level of inclusions in steel does not exceed 1.0. After Ca treatment for 5 minutes, the denatured inclusions fully floated up. After the molten steel was purified, Re and Mg alloys were added. At this time, the excess free Ca in the molten steel increased the yield of R and Mg, and Ca increased the yield of Re. Increased to more than 60%, the yield of Mg is as high as more than 40%. Give full play to their role in improving high temperature strength and hardness, oxidation resistance, corrosion resistance and heat resistance in steel.

The Ca, Re, and Sn of the present invention can denature oxide and sulfide inclusions, increase the fluidity of molten steel, and increase the floating speed of inclusions. The argon blowing time in the tundish is 5 to 8 minutes, which can promote the inclusions to fully float after denaturation treatment, ensuring that the inclusions in the steel All kinds of non-metallic inclusions do not exceed the purity of grade 1.0, and it saves nearly half of the time compared with ordinary tool steel, saves energy and reduces consumption, and improves productivity.

The carbon content of the invention is high, and the continuous casting is easy to produce component segregation, which affects the uniformity of the structure. Therefore, the continuous casting adopts the reduction technology and controls the degree of superheat to improve the macro segregation of the slab. The degree of superheat is less than or equal to 25°C, and the continuous casting reduction is 2 mm to 10 mm to reduce segregation and ensure that the cast slab does not have cracks. If the reduction is less than 2mm, the segregation is serious, exceeding grade 1.5, and if the reduction is greater than 10mm, the slab is prone to cracks.

The carbon content of the invention is high, and it is easy to form liquid micro-segregation at the end of the solidification of the columnar crystals of the slab, and the hot-rolled plate is easy to produce liquid phases, and the heat treatment further precipitates graphite, which affects the tensile strength and fatigue performance. In the present invention, a certain amount of tin is added to the steel, and at the same time, the crystallizer is used for electromagnetic stirring, the current intensity is 500A-1000A, the time is 1 minute-3 minutes, and the casting speed is 1.0m/min-1.4m/min to control the alloy at the end of the columnar crystal. Liquid micro-segregation effectively hinders carbon segregation aggregation and graphitization precipitation. The current intensity of electromagnetic stirring is lower than 500A, and the stirring time is less than 1 minute, which hinders carbon segregation and graphite precipitation. The current intensity of electromagnetic stirring is higher than 1000A, which wastes energy. If the stirring time is more than 3 minutes, the temperature of molten steel will drop, and the pouring water will be blocked. . Tin can not only effectively hinder carbon segregation and aggregation, cementite and graphitization precipitation, but also inhibit grain boundary oxidation on the surface of the steel plate, and improve surface spray welding performance, strength, wear resistance and other comprehensive properties.

The casting slab is put into the slow cooling pit for more than 72 hours to cool slowly for more than 72 hours, so as to avoid the cracking of the high carbon steel slab with relatively high alloy content under stress, and reduce the segregation of the slab.

The slab is heated by a walking heating furnace before rolling, and the temperature of the slab entering the preheating section of the heating furnace is above 500°C to prevent the temperature difference between the inside and outside of the slab in the heating section from being too large, resulting in internal stress and thermal stress cracking; the heating furnace adopts a reducing atmosphere , resistance to oxidation and decarburization of the surface of the slab. The heating temperature in the heating section is 1200°C to 1350°C, the heating section time is 30 minutes to 50 minutes, and the total time in the furnace is 2 hours to 4 hours to ensure uniform heating of the billet, uniform composition, and reduce segregation. Inhibit the grain boundary oxidation and decarburization on the surface of the steel plate, and improve the comprehensive properties such as surface spray welding performance, strength, and wear resistance.

2 descaling before rough rolling, 2 descaling before finishing rolling, 1 descaling before the third rolling, the descaling pressure is above 30MPa, the iron scale on the surface of the steel plate is cleaned, and there is no obvious oxidation and decarburization on the surface of the steel plate. Contributes to surface spray welding and improves strength and hardness under high temperature working environment. Rough rolling adopts ≥50% large reduction rate and 3 passes to fully break the coarse columnar grains of the slab, and the Re, Mg and C in the steel form a series of carbon magnesium rare earth compounds, which further promotes hot rolling to form a sheet spacing of 0.5μm The above pearlite organization.

The finish rolling adopts 6 high-temperature and fast rolling passes, the starting temperature is ≥1100°C, and the ending temperature is 900°C-980°C. The Sn dissolved in the austenite reacts with carbon to form a series of small spherical SnC _M carbon tin with a diameter of less than 10nm. compounds. The total reduction rate is ≥80%, the first pass reduction rate is ≥30%, and the austenite is refined to provide sufficient phase transition energy for the formation of carbon tin compounds; these SnC _M carbon tin compounds are used to form high temperature resistant Re, Mg Preparation of the carbon-tin compound. In order to reduce the heating temperature of the heat treatment, a process of high temperature and fast rolling with a rolling speed ≥ 20m/s is adopted to coarsen the distance between the cementite sheets in the pearlite to more than 0.5μm. When the rolling speed is ≥20m/s, these carbide-tin compounds have no time to grow up, and are evenly distributed in the steel, preparing for the formation of Re and Mg carbon-tin compounds. On the other hand, these fine tin carbides also become the nucleation particles of pearlitic cementite sheets. Because these carbides are easier to gather carbon, the cementite sheets basically use these carbides as particles, and because they They are all distributed, so the interlamellar spacing of cementite in the formed pearlite is greater than 0.5 μm, and heat treatment does not require high temperature. However, after the above-mentioned finish rolling process, the cementite sheet grows rapidly, resulting in a large diameter of pearlite clusters, poor toughness and plasticity of the steel plate, poor bending formability, and coarse martensite after heat treatment, which affects the performance after heat treatment.

After the steel plate comes out of the finishing mill, it is quenched at a cooling rate of ≥20°C/s to 600°C-750°C and then rolled into two vertical roll four-horizontal roll mills for two consecutive passes. The upper and lower reduction ratios are 2% to 8%. The reduction rate is 5% to 25%. The third low-temperature rolling mainly provides the phase transition energy and supercooling required for the reaction of SnC _M tin carbide with Re and Mg to form carbon-tin compounds such as ReMgSnC _M , ReSnC _M , and MgSnC _M2 . These uniform and fine carbon-tin compounds reduce the yield ratio to less than 60% in hot-rolled sheets and improve formability. These carbon-tin composites are more heat-resistant than ordinary cementite, and they do not diffuse and decompose at 900°C. After heat treatment, they are still evenly distributed in the Martensitic low lath, which improves the hardness after heat treatment, and further improves heat resistance and wear resistance. Performance, under the high temperature working condition above 200℃, the tensile strength is above 1700MPa, and the matrix hardness is above 55HRC. On the other hand, the third low-temperature rolling can break the large pearlite clusters formed by the above-mentioned finishing rolling in order to coarsen the pearlite sheets, so that the diameter of the pearlite clusters does not exceed 10 μm, and the toughness and plasticity are improved. The yield ratio is less than 60%, and the formability Excellent, providing microstructure guarantee for the formation of fine martensitic laths after subsequent heat treatment. 20°C/s is the critical undercooling degree required for the formation of carbon-tin complexes, so the cooling rate after exiting the finishing mill is ≥ 20°C/s, and at the same time, excessive pearlite clusters are suppressed. The rolling temperature is 600°C-750°C, the upper and lower reduction ratios are 2%-8%, and the side pressure reduction ratios are 5%-25%, so as to ensure sufficient production of carbon-tin compounds such as ReMgSnC _M , ReSnC _M , and MgSnC _M2 . Deformation performance ensures that the diameter of the pearlite group is broken down to less than 10 μm, the yield ratio of the steel plate is less than 60%, and the bending formability is excellent. Continuous two-pass rolling with four-horizontal rolling mill increases phase transformation energy. The rolling temperature is higher than 750°C, the reduction rate is lower than 2%, and the lateral pressure reduction rate is lower than 5%. The large pearlite clusters are not easily broken, and the phase transition energy of the carbon-tin composite is insufficient and the amount is small. The rolling temperature is lower than 600°C, the reduction rate is greater than 8%, and the lateral pressure reduction rate is greater than 25%. The rolling force is large, the load of the rolling mill is too large, the stored energy in the steel is too large, and the carbon-tin composite grows to more than 20nm. Can not play the role of high temperature resistance, and it is easy to generate fine flaky carbides in the ferrite matrix of the above-mentioned thick flaky cementite pearlite, the pearlite lamella becomes thinner, carbon is not easy to diffuse during heat treatment, and austenitization is difficult .

20°C/s is also the critical cooling rate for the invention to obtain full pearlite. If the cooling rate is greater than this rate, there will be no ferrite precipitation and surface oxidation and decarburization will be reduced. 500°C~600°C is the final cooling temperature for the production of full pearlite in this invention, adopting the cooling method of coil head, middle coil and tail coil, coiling head 0m~30m 500℃～550℃ coiling, coil tail 0m Coiling at 550°C to 600°C for ~30 meters, and coiling at 520°C to 580°C for other parts of the roll, which hinders the uniformity of the structure and properties of the entire steel plate, making the surface of the entire steel plate fine and dense, improving the surface quality of the entire roll, and the surface is smooth and free oxidation and decarburization. Coiling in this temperature range can effectively inhibit the surface grain boundary oxidation and decarburization, and at the same time help the carbon-tin complexes of Re and Mg to be evenly distributed and form thick lamellar pearlite with interlamellar spacing greater than 0.5 μm. The diameter of the cluster does not exceed 10 μm, which improves the uniformity of the tissue performance of the whole roll. If the coiling temperature is lower than 500°C, bainite or martensite brittle hard phases will appear, and the hardness of the steel plate is too high and easy to crack. If the coiling temperature is higher than 600 °C and the cooling rate is less than 20 °C/s, large pieces of pro-anatectoid ferrite will be precipitated, the strength is low, and the surface is re-oxidized, which affects the surface spray welding, and the forming is easy to crack.

The agricultural machinery accessories manufactured by processing the hot-rolled plate of the invention have a quenching temperature of 670° C. to 750° C., heat preservation for 10 to 30 minutes, and a tempering temperature of 300° C. to 500° C. and heat preservation for 24 to 36 hours. Since the structure is thick lamellar pearlite and the diameter of the pearlite group is small, the quenching temperature of the invention is lower than that of ordinary high-carbon steel, the quenching and tempering holding time is short, energy consumption is saved, and the requirements of energy conservation, emission reduction and environmental protection are met. When the quenching temperature is below 750°C, the cementite in the coarse pearlite sheet can be completely diffused and decomposed to form martensite when the intersheet spacing is above 0.5 μm. After low-temperature quenching, the carbides of Re and Mg do not diffuse and decompose, and are still uniformly dispersed in the steel, which improves the hardness after heat treatment, and further improves heat resistance and wear resistance. Under high temperature working conditions above 200°C, the tensile strength is 1700MPa Above, the matrix hardness is above 55HRC. On the other hand, low-temperature heat treatment helps to inhibit oxidation and decarburization of the steel surface.

Sand blasting for 10 to 30 minutes and rolling for 10 to 30 minutes. After surface treatment, the surface is heated to 800°C-900°C, and the chromium-copper alloy powder layer containing 70% to 80% chromium is thermally sprayed and welded, and the layer thickness is 0.2mm to 0.5mm , The powder is composed of spherical particles with a diameter of 80μm-150μm. These spherical particle powders are attached to the steel surface, which effectively improves the surface heat resistance and wear resistance. Under the high temperature working conditions above 200 ℃, the surface hardness is above 65HRC, and the wear rate is less than 20mg /km (grinding speed 300r/min, load 120N). Spherical particles constituting the powder have a diameter less than 80 µm or greater than 150 µm, which is not good in abrasion resistance. On the other hand, part of the powder sprayed on the surface penetrates into the substrate under the steel surface at high temperature, and forms a dense CrCuC layer with a thickness of 0.1mm to 0.2mm with carbon to improve the corrosion resistance of the steel. In the soil environment, the corrosion rate is lower than 15g/ m ² ·h.

In summary, compared with the prior art, the beneficial effects of the present invention are:

1) Utilize Sn, Re, Mg and carbon to form high-temperature-resistant spherical carbide-tin compounds with a diameter of less than 20nm to improve high-temperature toughness. After heat treatment, the structure is a lath-shaped martensite structure with fine carbon-tin compounds with a diameter of less than 20nm evenly distributed at 200°C Under the above high temperature working conditions, the hardness of the matrix is above 55HRC, the tensile strength is above 1700MPa, and the heat resistance and wear resistance are significantly improved;

2) Utilize the scattered and distributed carbon tin compounds generated by Sn, Re, Mg and carbon to coarsen the interlamellar spacing of pearlite to more than 0.5 μm, reduce the heat treatment temperature, and save heat treatment energy;

3) Three-time rolling technology is adopted to crush large pearlite clusters. The size of pearlite clusters in hot-rolled sheets does not exceed 10 μm, the yield ratio is below 60%, and the formability is excellent;

4) The present invention uses combined deoxidation of Ca and Mg to denature the inclusions in the steel, and all kinds of non-metallic inclusions do not exceed level 1.0;

5) Add Sn and use electromagnetic stirring to effectively prevent carbon segregation and graphitization precipitation;

6) Using the reduction technology, the segregation does not exceed 1.5 grades;

7) The composition design of low Si and Sn is adopted to suppress surface oxidation and prevent grain boundary oxidation microcracks;

8) The surface is spray-welded with spherical chrome-copper powder particles with a diameter of 80μm-150μm. The chrome-copper layer is 0.2-0.5mm thick, and forms a dense CrCuC layer of 0.1-0.2mm with the carbon in the base, which effectively improves the surface hardness, wear resistance, Heat resistance and corrosion resistance, under the condition of high temperature above 200 ℃, the surface hardness is above 65HRC, the wear rate is less than 20mg/km under the conditions of grinding pin rotation speed 300r/min, load 120N, and the corrosion rate in soil environment is less than 15g/m ² h.

detailed description

The present invention is described in more detail through examples, and these examples are only descriptions of the best implementation modes of the present invention, and do not limit the scope of the present invention in any way.

The chemical composition in the steel of the embodiment is shown in Table 1; the production process of hot-rolled strip steel is shown in Table 2 and Table 3; the spray welding process is shown in Table 4; the performance of the finished product is shown in Table 5.

Chemical composition in steel of table 1 embodiment, %

Table 2 Smelting Production Process

Table 3 billet treatment and hot rolling, cooling process

Table 4 heat treatment and spray welding process parameters

Table 5 steel plate and finished product properties

Claims (9)

1. 1700MPa heat-resistant steel for agricultural machinery, characterized in that the chemical composition in the steel by weight percentage is: C 0.70%-1.1%, Si≤0.04%, Mn 0.4%-1.5%, Al≤0.06%, Ca 0.0005%-0.05 %, RE 0.12%～0.5%, Sn 0.22%～1.0%, Mg0.0008%～0.05%, Ca/S≥0.2, Ca/Mg≥1, Ca/RE≥0.05, P≤0.020%, S≤0.015 %; the balance is Fe and unavoidable impurities. 2. 1700MPa heat-resistant steel for agricultural machinery according to claim 1, characterized in that, the hot-rolled steel plate structure of the steel for agricultural machinery is thick lamellar pearlite structure, the spacing between pearlite sheets is more than 0.5 μm, and the diameter of pearlite clusters is not greater than 0.5 μm. Greater than 10 μm; after heat treatment, the structure is a lath-shaped martensite structure with spherical carbide tin carbides with a diameter of less than 20 nm. 3. The 1700MPa heat-resistant steel for agricultural machinery according to claim 1, characterized in that the yield strength ratio of the hot-rolled steel plate for agricultural machinery steel is less than 60%; Above 55HRC, the tensile strength of the steel plate is above 1700MPa. 4. The 1700MPa heat-resistant steel for agricultural machinery according to claim 1, characterized in that the non-metallic inclusions in the steel plate for agricultural machinery do not exceed grade 1.0; the grain boundary oxide layer of the steel plate is 0mm; and the segregation does not exceed grade 1.5. 5. the manufacturing method of 1700MPa heat-resistant agricultural machinery steel as described in any one of claim 1-4, it is characterized in that, method comprises: 1) Smelting process: a) Deoxidize with Al deoxidizer, after refining the oxygen content ≤ 0.0020%, add Ca for at least 5 minutes, then add Mn, RE, Mg alloy, and finally add Sn; b) The continuous casting adopts the reduction process and the electromagnetic stirring of the mold, the thickness of the billet is 170mm-250mm, the reduction is 2mm-10mm; the current intensity of the electromagnetic stirring of the continuous casting mold is 500A-1000A, and the electromagnetic stirring is 1 minute to 3 minutes. Pulling speed 1.0m/min～1.4m/min; 2) Rolling process: Including rough rolling, finishing rolling and third rolling three processes: a) Rough rolling adopts ≥50% large reduction rate rolling; b) The continuous rolling method is adopted for finishing rolling, the total reduction rate is ≥80%, the first pass reduction rate is ≥30%, the rolling speed is ≥20m/s, the starting rolling temperature is 1100℃～1150℃, and the ending temperature is 900℃～980℃ ℃; c) After finishing rolling, cool down to 600℃～750℃ at a cooling rate ≥ 20℃/s, enter into two vertical rollers and four horizontal roller mills for continuous rolling for two passes, the upper and lower reduction ratios are 2% to 8%, and the side pressure reduction ratios 5% to 25%; 3) Cooling process: a) The steel plate enters laminar flow cooling, and the cooling rate is ≥20°C/s; b) Adopt the section cooling method of the head, middle and tail of the roll, coiling at the head of 0 m ~ 30 m at 500 ° C ~ 550 ° C, coiling at the end of 0 m ~ 30 m at 550 ° C ~ 600 ° C, and other parts of the coil Coiling at 520℃～580℃; 4) Use the hot-rolled steel plate obtained in step 3) to process and manufacture the required shape of agricultural machinery accessories, heat to 670°C-750°C, keep warm for 10-30 minutes, oil quench to below 200°C, and put it into a tempering furnace at 300°C-500°C for 24 hours. ~36 hours, air-cooled. 6. The manufacturing method of 1700MPa heat-resistant steel for agricultural machinery according to claim 5, characterized in that it also includes a slab treatment process: the slab is rolled off the assembly line and put into a slow cooling pit for slow cooling for more than 72 hours, and the slab is used before rolling Walking heating furnace heating; the heating furnace adopts a reducing atmosphere, the temperature of the preheating section is above 500°C, the temperature of the heating section is 1200°C~1350°C, the heating section time is 30 minutes to 50 minutes, and the total time in the furnace is 2~4 hours. 7. Utilize the spray-welding method of the agricultural machinery accessories processed by the 1700MPa heat-resistant agricultural machinery steel of claim 1, it is characterized in that, the method comprises: 1) Sand blasting for 10 to 30 minutes and rolling for 10 to 30 minutes for agricultural machinery parts. After surface treatment, heat the surface to 800°C to 900°C. Thermal spray welding of chrome-copper alloy powder layer containing 70% to 80% chromium, air cooling to room temperature; 2) The chromium-copper alloy powder spray-welded on the surface of agricultural machinery parts is a spherical granular powder with a diameter of 80-150 μm. 8. The spray-welding method of agricultural machinery accessories processed with 1700MPa heat-resistant steel for agricultural machinery according to claim 7, characterized in that the thickness of the chromium-copper alloy granular powder layer spray-welded on the surface of the agricultural machinery accessories is 0.2-0.5mm, CrCuC transition layer thickness 0.1 ~ 0.2mm. 9. The spray-welding method of agricultural machinery accessories processed with 1700MPa heat-resistant steel for agricultural machinery according to claim 7, characterized in that the surface hardness of the agricultural machinery accessories after spray welding is above 65HRC, the grinding speed is 300r/min, and the load is 120N The wear rate under the condition is less than 20mg/km, and the corrosion rate under the soil environment is less than 15gg/m ² ·h.