CN113462985B - Low-cost high-surface-hardness tool steel with excellent annealing-free bending performance - Google Patents
Low-cost high-surface-hardness tool steel with excellent annealing-free bending performance Download PDFInfo
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Abstract
The low-cost high-surface-hardness tool steel with excellent annealing-free bending performance comprises the following chemical components in percentage by weight: 0.4 to 0.6 percent of C, 0.1 to 0.27 percent of Si, 0.4 to 1.5 percent of Mn, less than or equal to 0.06 percent of Al, 0.0005 to 0.05 percent of Ca, 0.001 to 0.5 percent of Re, less than or equal to 0.05 percent of Mg, more than or equal to 0.5 percent of Ca/S, more than or equal to 1 percent of Ca/Mg, more than or equal to 0.05 percent of Ca/Re, less than or equal to 0.020 percent of P, less than or equal to 0.015 percent of S, and the balance of Fe and inevitable impurities. The invention provides high-strength tool steel with high surface quality and excellent bending performance and a processing method thereof. The tool steel produced by the invention has smooth surface, no oxidation decarburized layer, uniform and fine troostite structure containing 10-20% of granular phase and with the plate spacing of less than 0.15 mu m, tensile strength of more than 1000MPa, hardness of more than 37HRC, steel plate yield ratio of less than 70%, excellent bending performance, high production efficiency and low cost.
Description
Technical Field
The invention relates to tool steel and a production method thereof, in particular to low-cost high-surface-hardness tool steel with excellent annealing-free bending performance.
Background
With the development of times, the tool steel has wider and wider application, and not only the hardness after heat treatment is required, but also the processed shape is more and more complex. The conventional tool steel has high carbon content, hot rolling structure mainly comprises pearlite, and the conventional tool steel has high brittleness, is not suitable for bending and processing parts with complex shapes, and cracks when being bent at a very small angle below 90 degrees. In order to meet the bending processing requirements of some complex parts, cold-rolled annealing tool steel is usually adopted as a raw material, and the bending use requirements can be met only through multi-pass annealing and cold rolling, so that the cost is high. The components and the structure of the hot rolled plate, the level of internal inclusions, surface oxidation, decarburization and the like are main factors influencing the bending performance, and the hot rolled plate has deep decarburization layer, large grinding amount and low production efficiency. At present, the surface decarburized layer depth of the high carbon tool steel is generally controlled to about 2.0% of the plate thickness, and the full thickness specification is preferably controlled to a level not exceeding 1.0% of the plate thickness, and the control becomes difficult as the thickness becomes thicker. And the hardness of the common tool steel is improved by heat treatment, so the cost is high. In order to adapt to the development of times, meet the requirements of tool steel for processing parts with complex shapes, reduce the surface polishing amount, improve the production efficiency and reduce the cost, the tool steel with high surface hardness, which has excellent bending performance, low cost, low depth of a decarburized layer and no oxidation microcracks on the surface, is urgently needed to be developed.
The patent 'a method for producing thin tool steel by using a CSP line without spheroidizing annealing' describes a production method of a thin tool steel plate with the thickness of 1-2.5mm, which can not meet the use requirement of a thick tool steel plate; the production process of reducing the hardness of the tempered sorbite by adopting low-temperature rolling, quick cooling to the temperature near the martensite transformation temperature for low-temperature coiling and tempering is adopted, on one hand, the requirement on the capability of a coiling machine is extremely high, on the other hand, the martensite transformation occurs in the coiling process, the martensite brittleness is extremely high, the strip is easily broken, and the high-temperature tempering treatment at the temperature of 550-700 ℃ is carried out after coiling, so that the cost is high. The process controls the decarburizing depth to not more than 1% of the plate thickness.
A method for manufacturing an annealing-free medium-high carbon steel plate CN201310076240.5 and a method for producing an annealing-free hot-rolled S45C plate strip CN201110411594.1 adopt two-phase region or ferrite region low-temperature large-pressure rolling, high-temperature coiling and stacking to obtain 60% ferrite and partial spheroidized pearlite, and the hardness of the steel plate is reduced to 80-85HRB through softening. The steel plate obtained by the process has the advantages of large ferrite amount, large blocks, low hardness after heat treatment and incapability of meeting the use requirements of high-end tool steel, and can be used only after quenching and tempering.
The patent document CN200710010942.8 "manufacturing method of three-layer composite edge cutting tool steel plate" and the patent document CN200710010941.3 "manufacturing method of double composite edge cutting tool steel plate" are both manufacturing methods of composite cutting tool steel, and heat treatment is also required after compounding, so that the process is complicated and the cost is high.
The patent document CN201410016729.8 "high alloy cutting tool steel for wood working machinery rotary cutter blade and its hot working process" has high content of Cr, V, W, Mo and other alloys, and needs two times of quenching treatment, the process is complex, and the cost is high.
Three patent documents, namely 201910821762.0 steel for a high-strength fastening belt with excellent electroplating performance, 201910821761.6 steel for an electroplating fastening belt buckle and a manufacturing method V202010127798.1 steel for an electroplating plate hook with high fatigue life, mainly control the surface quality of a steel plate through controlled rolling and controlled cooling, ensure that the surface decarburization depth does not exceed 1.5 percent of the plate thickness, and improve the electroplating performance. The hardness of the hot rolled plate is below 100HRB, and the hardness of the hot rolled plate can reach the hardness above 37HRC required by use only by a plurality of heat treatment processes such as quenching, tempering and the like, so that the cost is high and the efficiency is low.
Patent document No. 201510687941.1 discloses an alloy tool steel and a production method thereof, relating to a medium carbon tool steel, which comprises: 0.5 percent of C, 0.2 percent of Si, 0.5 percent of Mn, 5.0 percent of Cr, 2.3 percent of Mo, 0.5 percent of V, less than or equal to 0.003 percent of S and less than or equal to 0.02 percent of P, more alloys such as Cr, Mo, V and the like are added into the alloy tool steel, electroslag remelting and casting ingot casting are needed for smelting, the yield is low, a plurality of heat treatment procedures such as softening annealing, spheroidizing annealing, quenching, tempering and the like are needed, the cost is high, and the alloy tool steel is not suitable for processing and manufacturing various tools with complex shapes.
Patent document ZL201210204255.0 "a production method for reducing the thickness of the decarburized layer of high-carbon spring steel strip" controls the depth of the decarburized layer of spring steel of 2.5-3.55mm to be not more than 0.02mm by controlling the heating temperature, the atmosphere in the furnace, the rolling reduction rate of single pass and the cooling rate. The method is not suitable for thicker steel plates, and the requirements on the atmosphere and temperature sectional control precision in the heating furnace are high, so that the operation is difficult.
The steel grade and the production method mentioned in the prior art do not mention the bending performance, have certain defects and are not suitable for the use requirement of tool steel with bending and high surface hardness.
Disclosure of Invention
The invention aims to provide a low-cost high-surface-hardness tool steel with excellent annealing-free bending performance, and provides a tool steel with high surface hardness and excellent bending performance and a processing method thereof, aiming at the technical problems of serious decarburization and oxidation, more inclusions, unsuitability for bending and high heat treatment cost of the surface of the existing tool steel. The tool steel produced by the method has the advantages of tensile strength of more than 1000MPa, yield ratio of less than 60%, no need of deep desulfurization and annealing, and excellent bending performance; the surface has no crystal boundary oxidation layer and decarburized layer, the surface hardness does not need to be treated by heat more than 53HRC, the wear resistance is excellent, the production efficiency is high, and the cost is low.
In order to achieve the purpose, the invention adopts the following technical scheme:
the low-cost high-surface hardness tool steel with excellent annealing-free bending performance comprises the following chemical components in percentage by weight: 0.4 to 0.6 percent of C, less than or equal to 0.27 percent of Si, 0.4 to 1.5 percent of Mn, less than or equal to 0.06 percent of Al, 0.0005 to 0.05 percent of Ca, 0.001 to 0.5 percent of Ba, 0.001 to 0.5 percent of Mg, more than or equal to 1 percent of S/Ca, more than or equal to 0.1 percent of Ca/Mg, more than or equal to 0.1 percent of Ca/Ba, less than or equal to 0.020 percent of P, 0.001 to 0.06 percent of S, and the balance of Fe and inevitable impurities.
The depths of a grain boundary oxidation layer and a decarburized layer on the surface of the steel plate are both 0mm, nonmetal inclusions are not more than 1.0 grade, the tensile strength is more than 1000 MPa-1200 MPa, the yield ratio is less than 60%, the bending forming performance is excellent, the surface hardness is more than 53HRC, and the steel plate replaces heat-treated steel.
The action mechanism of each alloy component of the structural steel of the present invention is explained in detail below, wherein the percentage symbol% represents the weight percentage:
c0.40% -0.6%: c is a main solid solution strengthening element in steel, and in the invention, sufficient C is needed to improve the hardness, if the C content is lower than 0.40%, the hardness of the steel plate is difficult to ensure, and if the C content is higher than 0.6%, the strength of the hot rolled plate is too high, the toughness and the plasticity of the steel are deteriorated, the yield ratio is influenced, and the formability is poor. Therefore, the C content is controlled to be 0.40-0.6%.
0.40-1.5% of Mn: mn is relatively cheap, is a good deoxidizer and desulfurizer, and is an essential element for ensuring the strength and toughness of steel. The manganese and the iron can be infinitely dissolved to form a solid solution, so that the hardness and the strength are improved, and the influence on the plasticity is relatively small. Mn and S are combined to form MnS, so that the influence of hot cracks caused by FeS formed at the grain boundary on the hot formability of the tool steel is avoided. Meanwhile, Mn is also a good deoxidizer. The steel has low Mn content, cannot meet the requirement of high strength, and has over high Mn content, serious segregation, influence on welding performance and formability, and increase production cost, so the Mn content should be controlled to be 0.4-1.5 percent by comprehensively considering factors such as cost, performance requirements and the like.
Si is less than or equal to 0.27%: si is one of common elements in steel, is used as a reducing agent and a deoxidizing agent in the steelmaking process, and the Si in a solid solution form can improve the yield strength and the ductile-brittle transition temperature. According to the invention, the high Si content can promote decarburization and oxidation of the surface of the cutting tool steel to form a loose oxide layer, and the oxide layer has microcrack defects such as grain boundary oxidation and the like, so that the surface hardness, the bending performance and the fatigue performance are seriously influenced. The invention Si: si is less than or equal to 0.27 percent, the cost is low, the oxidation and decarburization of the steel surface are avoided, the surface quality is improved, and the bending performance is improved.
Al is less than or equal to 0.06%: al is used as a deoxidizing and nitrogen-fixing agent in steelmaking, so that crystal grains are refined, the aging of steel is inhibited, the toughness of the steel at low temperature is improved, and the brittle transition temperature of the steel can be particularly reduced; al can also improve the oxidation resistance of the steel and improve the corrosion resistance to hydrogen sulfide. The Al content exceeds 0.06 percent, and the Al easily forms large-particle oxide inclusion with oxygen in steel, thereby influencing the fatigue performance.
Ca 0.0005% -0.05%: ca deoxidation is carried out, so that the corrosion resistance, the wear resistance, the high temperature resistance and the low temperature resistance of the steel are improved; the cold heading property, the shock resistance, the hardness and the contact endurance strength of the steel are improved. The invention adds calcium, changes the components, quantity and form of non-metallic inclusions, accelerates the flow of molten steel, promotes the inclusions to float sufficiently, improves the purity of steel, ensures that various non-metallic inclusions in finished steel do not exceed 1.0 grade, improves the surface smoothness of steel, eliminates the anisotropy of tissues, improves the hydrogen induced cracking resistance and the lamellar tearing resistance, and prolongs the service life of tools. In the invention, calcium is added for oxide modification treatment and deoxidation, and after the Ca is added for deoxidation, the yield of Ba is improved to more than 60 percent, the yield of Mg is improved to more than 40 percent, and the utilization rate of Ba and Mg is effectively improved. Ca. After the steel plate is treated by Ba and Mg, the distance between the cementite lamella of the steel plate is less than 0.15 mu m, the tensile strength is 1000MPa to 1200MPa, the yield ratio of the steel plate is less than 60 percent, and the steel plate does not crack when being subjected to cold bending at 0 to 180 degrees under the condition that the diameter of a bending core is not less than the thickness of the steel plate.
0.001-0.5% of Ba, more than or equal to 0.1% of Ca/Ba: barium element improves the oxidation resistance of the steel and improves the high-temperature strength. In the invention, Ba not only has the solid solution strengthening effect, but also utilizes Ba, Mg, C and S to generate Mg-BaCM、Mg-BaSMAnd barium carbide and barium sulfide. Mg-BaCMThe steel plate is a cementite nuclear particle, so that the steel plate is promoted to form a large number of fine cementite pieces, and finally a fine troostite structure with the piece spacing less than or equal to 0.15 mu m is formed, the tensile strength is more than 1000MPa, the yield ratio is less than 60%, and the steel plate does not crack when being subjected to cold bending at 0-180 ℃ under the condition that the bending core diameter is not less than the thickness of the steel plate; a large amount of Mg-BaS with the diameter less than or equal to 30nm is formed under the surface of the steel plateMThe particle phase is dispersed and distributed, and the volume of the particle phase accounts for 50 to 60 percent, so that the surface of the steel plate is hardThe degree is above 53HRC, and the oxidation and decarburization of the steel sheet surface are inhibited. Ba can improve the fluidity of steel and improve the surface smoothness of the steel plate. Ba also makes Al2O3The oxide inclusions with the same size are changed into fine and dispersed spherical inclusions, so that the harmfulness of the inclusions is eliminated, and the bending performance and the fatigue performance are improved. The Ca/Ba is more than or equal to 0.1, the yield of Ba can be ensured to be more than 60 percent, and the effects of refining the interlayer spacing and granulating troostite of Ba are ensured. The Ba of the invention can also effectively prevent the carbon segregation and aggregation in the tool steel and inhibit the graphite precipitation. BaS formed by Ba and S under the surface of a casting blank can inhibit the surface oxidation and decarburization of the steel plate, so that the depths of a grain boundary oxidation layer and a decarburized layer on the surface of the steel plate are both 0 mm.
0.001-0.5% of Mg, and the ratio of Ca/Mg is more than or equal to 0.1: magnesium is a very active metal element, which has strong affinity with oxygen, nitrogen, and sulfur. However, because magnesium is too active, it is not easy to control during smelting, and it is very easy to form inclusions with oxygen, nitrogen, etc. to affect the purity of steel. The invention adopts a self-created smelting technology of adding Al and Ca for deoxidation and then adding Ba and Mg, and utilizes the coaction of Ba, Ca and Mg to generate BaO, CaO, MgO and Al2O3The inclusion has low melting point, is easy to solidify, float upwards and remove in molten steel, avoids the problem of nozzle nodulation in the continuous casting process, reduces the inclusion content in the molten steel, and controls the inclusion level in steel not to exceed 1.0 level. Enough Ca and Mg can form BaO, CaO, MgO and Al when the Ca/Mg ratio is more than or equal to 0.12O3And the Mg yield can be ensured to be more than 40 percent only by compound inclusion. In another aspect, the invention utilizes Ba, Mg, and C and S to form Mg-BaCM、Mg-BaSMAnd barium carbide and barium sulfide. Mg-BaCMThe core particles are cementite-shaped particles, so that the steel plate is promoted to form a large number of fine cementite pieces, and finally a fine troostite structure with the piece spacing less than or equal to 0.15 mu m is formed, the strength is improved, and the yield ratio is reduced; a large amount of Mg-BaS with the diameter less than or equal to 30nm formed below the surface of the steel plateMThe particle phase is dispersed and distributed, the volume accounts for 50-60%, the surface hardness of the steel plate is over 53HRC, the surface oxidation and decarburization of the steel plate are inhibited, and the electroplating and bending performances are improved.
P is less than or equal to 0.020%, S is 0.002% -0.06%, S/Ca is more than or equal to 1: it is generally accepted that P and S are inevitable harmful impurities in steel, and the lower the content, the better. But the sulfur can not be completely eliminated in the actual smelting, and the lower the control content is, the higher the smelting desulfurization cost is. According to the invention, Ca, Ba and Mg are added into steel at the same time, and S/Ca is more than or equal to 1, Ca/Mg is more than or equal to 0.1 and Ca/Ba is more than or equal to 0.1 by controlling the reasonable dosage of the three components, so that sulfur with higher content below the surface of the casting blank after electromagnetic stirring and the three components generate fine and uniform Mg-BaSM、Ca-BaSMAnd the like. They are distributed under the steel surface in the form of fine grains with a diameter not exceeding 30nm, and enhance the surface hardness and inhibit the surface oxidation and decarburization. According to the invention, the surface structure morphology of the steel plate can be formed by controlling the S content to be 0.002-0.06%, the requirements of high surface hardness and excellent bending performance are met, and the production cost of desulfurization is saved.
The production method of the low-cost high-surface hardness tool steel with excellent annealing-free bending performance comprises the following steps:
1) the smelting process comprises the following steps:
the steel plate is formed by rolling continuous casting billets which are smelted in a converter, refined in an electric furnace and poured, and the thickness of the plate blank is 170-250 mm;
a) deoxidizing by using an Al deoxidizer, adding Ca alloy after refining the oxygen content of less than or equal to 0.0020%, and adding Ba and Mg alloy along the wall of the tundish after at least 5 minutes;
b) the argon blowing time of the tundish is 5-8 minutes, so that impurities are fully floated, and the degree of superheat of pouring is less than or equal to 25 ℃;
the continuous casting adopts a pressing process and electromagnetic stirring of a crystallizer;
c) electromagnetic stirring is carried out on a crystallizer for 1-3 minutes, the current intensity is more than 1000A, the continuous casting drawing speed is 1.0-1.4 m/min, the casting blank is cooled to 900-1000 ℃ and is reduced under the pressure, and the reduction is 10-30 mm;
2) the casting blank treatment process comprises the following steps:
a) the casting blank is subjected to off-line stacking and cover buckling waste heat slow cooling for more than 24 hours, and the casting blank is heated by a stepping heating furnace before rolling, wherein the temperature of the casting blank in the heating furnace is required to be more than 500 ℃;
b) the heating furnace adopts reducing atmosphere, the temperature of the preheating section of the heating furnace is more than 500 ℃, the temperature of the heating section is 1200-1350 ℃, and the total time in the furnace is 2-4 hours;
3) the rolling process comprises the following steps:
the method comprises three processes of rough rolling, finish rolling and third rolling: high-pressure water is adopted for descaling before rough rolling, finish rolling and third rolling, the high-pressure water pressure is not less than 30MPa, and the surface quality of the steel plate is ensured:
a) the rough rolling is carried out at a high reduction rate of more than or equal to 50% in the first pass, and the coarse grains of the casting blank are fully crushed;
b) the finish rolling adopts at least 6-pass continuous rolling mode, the total rolling reduction rate is more than or equal to 80 percent, the first pass rolling reduction rate is more than or equal to 30 percent, the high-temperature rapid rolling is carried out, the rolling speed is more than or equal to 25m/s, the initial rolling temperature is 1050-1150 ℃, and the finishing temperature is 900-980 ℃;
C) after rough rolling and finish rolling of the steel plate, carrying out laminar cooling at a cooling speed of 25 ℃/s-100 ℃/s, cooling to 500-680 ℃, and continuously rolling in a double-vertical-roller four-horizontal-roller rolling mill for two times, wherein the upper and lower reduction ratios are 10-30%, and the side pressure reduction ratio is 10-30%;
4) and (3) a cooling process:
and after the steel plate is rolled for the third time, quenching at the cooling speed of 25 ℃/s-100 ℃/s, immediately coiling when the steel plate is cooled to 470-550 ℃, and cooling in air.
Carrying out third rolling and laminar flow quenching, and carrying out air cooling after coiling to obtain a fine troostite structure with a sheet pitch of less than 0.15 mu m in the steel plate, wherein the tensile strength is 1000-1200 MPa, the yield ratio is less than 60%, and the steel plate does not crack when being subjected to cold bending at 0-180 ℃ under the condition that the diameter of a bending core is not less than the thickness of the steel plate; forming a granulated troostite layer containing 50-60% of fine granular phase of barium sulfide below the surface of the steel plate, wherein the depth of the granulated troostite layer is not less than 10% of the thickness of the steel plate, the maximum diameter of the granular phase is 30nm, the interval between carbon sheets is not more than 0.15 mu m, the surface hardness is over 53HRC, the wear rate is not more than 25mg/km (the grinding rotation speed is 300r/min, the load is 120N), and replacing heat-treated steel; or, annealing is not needed, the steel plate is coiled at 470-550 ℃ and then slowly cooled to below 200 ℃ by using waste heat, the cooling speed is less than 5 ℃/h, the room temperature tissue is more than or equal to 80 percent of spheroidized fine sorbite, the yield ratio is less than 50 percent, and the steel plate does not crack when the bending core diameter is less than the thickness of the steel plate and is subjected to cold bending at 0-180 ℃; after slow cooling, the particles in the grained troostite layer below the surface of the steel plate are not decomposed, and the hardness of the steel plate is still over 53 HRC.
High carbon content in tool steel, difficult control of oxygen content and molten steel flowThe fluidity is poor, in order to improve the fluidity of the molten steel, the deoxidation is sufficient, Al deoxidizer is firstly used for deoxidation, after the oxygen content is less than or equal to 0.0020 percent, Ca is added for at least 5 minutes, and then Ba and Mg alloy are added. In the components of the invention, Ca, Mg and Ba are active elements, which are difficult to control during smelting, and the adding sequence is crucial. Al is generated in the steel after Al addition and deoxidation2O3Impurities, if the refractory material of the lining is poor, MgO. Al will be formed2O3And the melting point of the Al oxide inclusions is high, the Al oxide inclusions are not easy to solidify and float in steel, on one hand, the fluidity of molten steel is reduced, a pouring nozzle is blocked, on the other hand, the inclusions in the steel are increased, and the bending and fatigue properties of the steel are influenced. Adding Al for deoxidation, and then adding Ca for treatment, wherein the Ca can break the original long-strip-shaped Al2O3And MgO. Al2O3The impurities are wrapped outside the intermittent impurities to generate spherical CaO, MgO and Al which are dispersed and distributed2O3Or CaO. Al2O3The composite oxides refine spheroidized inclusions, the small-particle calcium aluminate composite inclusions have low melting point and are easy to solidify, float and get rid of in molten steel, the problem of nozzle nodulation in the continuous casting process is avoided, the inclusion content in the molten steel is reduced, and the inclusion level in the steel is ensured not to exceed 1.0 level. The generation and floating of the overlapped inclusion takes a period of time, so that the Ba and Mg containing alloy is added after the molten steel is purified after the Ca treatment is carried out for at least 5 minutes, and the yield of Ba and Mg is respectively improved by more than 60 percent and 40 percent by the free Ca in the molten steel.
Ca. Ba and Mg can modify oxide and sulfide inclusions, improve the fluidity of molten steel, increase the floating speed of the inclusions, and ensure that various non-metal inclusions in the steel can not exceed the purity of 1.0 level by fully floating after the denaturation treatment of the inclusions after the argon blowing time of the tundish is 5-8 minutes.
The Ba and the Mg added along the wall of the tundish in the later smelting period can improve the fluidity of steel, and the Ba, the Mg and the S are uniformly diffused and distributed near the inner wall of the tundish under the action of electromagnetic stirring centrifugal force. The electromagnetic stirring coil is made of pure carbon material, the current intensity is more than 1000A, the electromagnetic stirring is carried out for 1 to 3 minutes, and the diffusion capacity of Ba, Mg and S is strongAnd simultaneously controlling the continuous casting drawing speed to be 1.0-1.4 m/min, forming a surface layer with high Ba, Mg and S contents below the surface of the casting blank after continuous casting and drawing, wherein the depth of the surface layer with high alloy content is not less than 15% of the thickness of the casting blank, and the Ba, Mg and S contents in the surface layer are 5-10 times of the core content of the casting blank. When the casting blank is cooled to 900-1000 ℃, the reduction is 10-30 mm, the high content of Ba, Mg, S and C under the upper and lower surfaces of the casting blank react under the action of external force deformation energy to generate BaS and BaC, which are Mg-BaS generated in the later rolling processM、Mg-BaCMAnd nucleation particles of barium sulfide and barium carbide. Barium sulfide can be pinned at the grain boundary of the surface layer, so that surface oxidation and decarburization are effectively inhibited, the electroplating qualification rate is improved, and the surface hardness is improved. The barium carbide is a nucleation mass point of cementite, promotes the formation of a fine troostite structure with a plate spacing of less than 0.15 mu m, and improves the strength and the formability. The current intensity is less than 1000A, the stirring time is less than 1 minute, the diffusion of Ba and Mg is insufficient, the alloy content under the surface of the casting blank is 5-10 times of the core alloy content, the depth of the casting blank is not less than 15% of the thickness of the casting blank, and the surface hardness is influenced after rolling. The stirring time is longer than 3 minutes, the alloy elements are seriously diffused, and the surface of the casting blank is easy to crack. The reduction less than 10mm does not provide sufficient transformation energy for BaS and BaC, and the casting blank with the reduction more than 30mm is cracked. The casting blank reduction temperature is too low or too high, which is not beneficial to the generation of BaS and BaC. The BaC can also effectively prevent the carbon segregation in the tool steel from gathering and inhibit the graphite from precipitating. BaS under the surface of the casting blank can effectively inhibit surface oxidation and decarburization.
After the casting blank is off-line, the casting blank is placed into a slow cooling pit for slow cooling for more than 24 hours, so that the cracking of the tool steel casting blank with relatively high alloy content under the action of stress is avoided, and the generation of BaS and BaC in the casting blank is promoted. BaS is used as a nucleation mass point of barium sulfide to harden the surface of the steel plate; and BaC is used as a barium carbide nucleation particle to further promote the generation of a large number of carbon carbide pieces, reduce the piece spacing, and improve the strength and the yield ratio.
The casting blank is heated by a stepping heating furnace before rolling, the temperature of the casting blank in the heating furnace is more than 500 ℃, and the temperature of the preheating section of the heating furnace is more than 500 ℃, so that the phenomenon that the temperature difference between the inside and the outside of the casting blank in the heating section is too large, and internal stress and thermal stress cracking are generated is prevented; the heating furnace adopts reducing atmosphere to resist the surface oxidation and decarburization of the casting blank, and the depth of a grain boundary oxidation layer and the depth of a decarburized layer on the surface of the steel plate are both 0 mm. The heating temperature of the heating section is 1200-1350 ℃, the total time of the heating section in the furnace is 2-4 hours, uniform heating and uniform components of a casting blank are guaranteed, segregation is reduced, and the bending performance is improved.
The method has the advantages that the multi-channel high-pressure water descaling with the pressure of more than 30MPa is continuously adopted before rough rolling, finish rolling and third rolling, the rolling speed is more than or equal to 25m/s, the method is favorable for removing the iron scales on the surface of the broken steel plate, the surface is ensured to have no crystal boundary oxidation layer and decarburized layer, the surface quality and the bending performance of the steel plate are improved, and the grinding amount is reduced; the rough rolling adopts the first rolling with a large reduction ratio of more than or equal to 50 percent, fully crushes the primary iron scale on the surface of the casting blank, simultaneously crushes the coarse columnar grains of the casting blank, promotes the formation of fine austenite, and prepares for the formation of fine troostite.
The finish rolling adopts at least 6 times of high-temperature rapid rolling, the total reduction rate is more than or equal to 80 percent, the first time reduction rate is more than or equal to 30 percent, the starting temperature is 1100-1150 ℃, the finishing temperature is 900-980 ℃, the rolling speed is more than or equal to 25m/s, fine austenite grains are formed, secondary oxidation and decarburization on the surface are inhibited, and the bending performance is improved.
BaS and BaC are easy to grow into sheets and influence the obdurability of steel, but because Mg is added into steel, under the condition of high temperature and high pressure rolling of rough rolling and finish rolling, BaC is taken as mass point in a steel plate to form a large amount of uniform fine Mg-BaCM、Ca-Mg-BaCMAnd the like contain magnesium barium carbides, which are spherical and inhibit the growth of BaC. During the subsequent cooling process, they are used as cementite particles to promote the formation of a large number of cementite pieces, so as to obtain a fine troostite structure with the piece spacing of less than 0.15 mu m and improve the strength and the formability.
Because BaS and Mg generate Mg-BaSMThe phase transformation energy required is more, and the transformation of BaS can not be realized after rough rolling and finish rolling, especially the BaS exists in a large amount under the surface of steel, and the BaS is easy to grow to influence the strength and the formability. To solve this problem, the steel sheet is rolled for the third time after it comes out of the finishing mill.
The steel plate is taken out of the finishing mill and then is quenched to 500-680 ℃ at the cooling speed of 25-100 ℃/s for the third rolling, the reduction rate is 10-30 percent, and the steel plate is rolledThe machine adopts a four-horizontal-roller mill, and continuously rolls for two times with lateral pressure, wherein the lateral pressure reduction rate is 10-30%. Barium sulfide under the surface of steel generates a large amount of Mg-BaS with Mg and Ca under the conditions of rough rolling, finish rolling, third rolling and quenchingM、Ca-Mg-BaSMAnd the like. The cooling speed before the third rolling is lower than 25 ℃/s, and the barium sulfide is easy to grow up; the plate shape is not good when the cooling speed is higher than 100 ℃/s. The third rolling temperature is higher than 680 ℃, the reduction rate of the vertical roll and the side roll is lower than 10%, and the transformation energy of the formed magnesium-containing barium sulfide is insufficient; the rolling temperature is lower than 500 ℃, the reduction rate is too high, the rolling force is large, the load of a rolling mill is too large, the storage energy in steel is large, formed particles grow up, and the bending performance is influenced.
The fine barium carbide particles are used as cementite nucleation particles, a large number of fine cementite pieces are formed at a cooling speed of 25-100 ℃/s, the piece spacing is thinned, and a troostite structure with the piece spacing of 0.15 mu m is formed. 25 ℃/s is the critical cooling rate of the fine troostite structure obtained by the method, and 470-550 ℃ is the forming temperature area of the fine troostite structure, so that the fine troostite structure with the space of less than 0.15 mu m can be obtained by adopting a cooling process of layer cooling and rapid cooling at the cooling rate of 25 ℃/s-100 ℃/s and coiling at the temperature of 470-550 ℃. Meanwhile, magnesium-containing barium sulfide with the diameter of below 30nm formed below the surface of the steel plate after the steel plate is coiled and rolled for three times at the temperature does not grow, the hardness and the surface quality of the steel plate are improved, the surface of the steel plate is clean and compact, grain boundary oxidation and surface decarburization are avoided, and surface microcracks are not formed during bending. The coiling temperature is lower than 470 ℃, the cooling speed is higher than 100 ℃/s, bainite or martensite brittle-hard phases can be generated, the hardness of the steel plate is too high, and the steel plate is easy to crack. The coiling temperature is higher than 550 ℃, the cooling speed is less than 25 ℃/s, proeutectoid ferrite or proeutectoid cementite and coarse lamellar pearlite can be precipitated, the strength is low, the surface is easy to be subjected to secondary oxidation delamination, and the bending is easy to crack.
Therefore, after coiling and air cooling, troostite structures with the interval between the carbide pieces being less than or equal to 0.15 mu m are formed in the steel plate, the tensile strength is more than 1000MPa, the yield ratio of the steel plate is less than 60%, and the steel plate does not crack when being subjected to cold bending at 0-180 ℃ under the condition that the bending core diameter is not less than the thickness of the steel plate; a grained troostite layer containing 50-60% of barium sulfide fine granular phase is formed below the surface of the steel plate, the depth of the grained troostite layer is not less than 10% of the thickness of the steel plate, the diameter of the granular phase is not more than 30nm, the distance between troostite carburant pieces is not more than 0.15 mu m, the surface decarburization and oxidation depth is 0, the surface hardness is over 53HRC, the wear rate is not more than 25mg/km (the rotating speed of a grinding pin is 300r/min, the load is 120N), and the steel plate can replace heat-treated steel.
And for the bending property requiring that the bending core diameter is smaller than the thickness of the steel plate, coiling the steel coil at 470-550 ℃, slowly cooling the steel coil to below 200 ℃ by utilizing waste heat, wherein the cooling speed is less than 5 ℃/h, obtaining the spheroidized fine sorbite with the room-temperature structure of more than or equal to 80 percent, the yield ratio is less than 50 percent, and the steel coil is not cracked by cold bending at 0-180 ℃ under the condition that the bending core diameter is smaller than the thickness of the steel plate. Furthermore, the subsurface barium sulfide is not decomposed, and the surface hardness is maintained at 53HRC or higher. Slow cooling speed is too fast, spheroidization rate is low, and bending performance of the steel plate is poor.
Compared with the prior art, the invention has the beneficial effects that:
1) the invention adopts the design of adding Ca, Ba and Mg into high-C and high-S components, and through continuous casting electromagnetic stirring, pressing, three-time rolling and quenching, a grained troostite layer containing 50-60% of barium sulfide fine granular phase is formed under the surface of a steel plate, the depth of the grained troostite layer is not less than 10% of the thickness of the steel plate, the spacing of carbon carbide pieces is not more than 0.15 mu m, the maximum diameter of the granular phase is 30nm, the surface oxidation and decarburization depth is 0mm, the surface hardness is improved to be more than 53HRC, the wear rate is not more than 25Mg/km (the rotating speed of a grinding pin is 300r/min, the load is 120N), and the heat treatment steel is replaced;
2) the interior of the steel plate is a fine troostite structure with the interval between the cementite pieces being less than or equal to 0.15 mu m, the tensile strength is 1000MPa to 1200MPa, the yield ratio of the steel plate is less than 60 percent, and the steel plate does not crack under the condition of 0-180 DEG cold bending when the bending core diameter is not less than the thickness of the steel plate;
3) after the slow cooling treatment by utilizing the waste heat, the structure of the spheroidized sorbite is more than 80 percent, the yield ratio is less than 50 percent, and the cold bending at 0-180 degrees does not crack under the condition that the diameter of a bending core is less than the thickness of a steel plate;
4) after the waste heat treatment, the lower hardening particle phase of the surface of the steel plate is not decomposed, and the surface hardness is still over 53 HRC;
5) the steel has excellent bending performance without annealing, high surface hardness without heat treatment, better toughness matching than heat treatment steel, no need of desulfurization and low cost, and can replace heat treatment steel;
6) al is used for deoxidation, Ca is used for treating the deformation of the inclusion, and various non-metal inclusions are not more than 1.0 grade;
7) the yield of Ba and Mg is respectively up to 60% and 40%.
Drawings
FIG. 1 is a structural morphology (10 μm, 1000 times) of a fine troostite having a surface layer containing a barite sulfide-containing particulate phase and a carbon sheet spacing of 0.15 μm or less in example 3 of the present invention.
FIG. 2 is a graph (10 μm, 1000 times) of the morphology of 90% spheroidized sorbite after the slow cooling treatment with residual heat in example 3 of the present invention.
FIG. 3 is a structural morphology (20 μm, 500 times) of a conventional hot-rolled tool steel of comparative example 2.
Detailed Description
The present invention is described in more detail by way of examples, which are merely illustrative of the best mode of carrying out the invention and are not intended to limit the scope of the invention in any way.
The chemical compositions of the steels of the examples are shown in Table 1; the production process of the hot-rolled strip steel is shown in tables 2 and 3; the hot rolled steel coils of each example were subjected to two cooling processes of air cooling and waste heat slow cooling, and the properties of the finished steel sheets are shown in table 4.
Table 1: chemical composition (c) of
| Number of | C | Si | Mn | P | Als | S | Mg | Ca | Ba | Ca/Mg | Ca/Ba | S/Ca |
| 1 | 0.45 | 0.12 | 0.9 | 0.015 | 0.02 | 0.05 | 0.5 | 0.05 | 0.5 | 0.10 | 0.10 | 1.00 |
| 2 | 0.49 | 0.1 | 1.42 | 0.004 | 0.035 | 0.009 | 0.02 | 0.002 | 0.012 | 0.10 | 0.17 | 4.50 |
| 3 | 0.48 | 0.16 | 0.91 | 0.008 | 0.012 | 0.02 | 0.08 | 0.01 | 0.09 | 0.13 | 0.11 | 2.00 |
| 4 | 0.5 | 0.05 | 0.65 | 0.01 | 0.032 | 0.008 | 0.05 | 0.005 | 0.032 | 0.10 | 0.16 | 1.60 |
| 5 | 0.55 | 0.14 | 0.8 | 0.012 | 0.025 | 0.04 | 0.04 | 0.04 | 0.4 | 1.00 | 0.10 | 1.00 |
| 6 | 0.58 | 0.05 | 0.75 | 0.014 | 0.056 | 0.004 | 0.003 | 0.0006 | 0.005 | 0.20 | 0.12 | 6.67 |
| 7 | 0.52 | 0.21 | 0.88 | 0.013 | 0.023 | 0.008 | 0.02 | 0.003 | 0.007 | 0.15 | 0.43 | 2.67 |
| 8 | 0.57 | 0.17 | 1.32 | 0.014 | 0.028 | 0.04 | 0.006 | 0.009 | 0.05 | 1.50 | 0.18 | 4.44 |
| 9 | 0.42 | 0.05 | 0.99 | 0.012 | 0.035 | 0.06 | 0.2 | 0.03 | 0.2 | 0.15 | 0.15 | 2.00 |
| 10 | 0.45 | 0.06 | 1.18 | 0.01 | 0.045 | 0.009 | 0.04 | 0.007 | 0.07 | 0.18 | 0.10 | 1.29 |
| 11 | 0.6 | 0.09 | 0.5 | 0.008 | 0.035 | 0.03 | 0.03 | 0.025 | 0.006 | 0.83 | 4.17 | 1.20 |
| 12 | 0.58 | 0.13 | 1.32 | 0.013 | 0.036 | 0.05 | 0.002 | 0.008 | 0.016 | 4.00 | 0.50 | 6.25 |
| 13 | 0.53 | 0.08 | 1.38 | 0.013 | 0.046 | 0.007 | 0.005 | 0.0008 | 0.007 | 0.16 | 0.11 | 8.75 |
| 14 | 0.5 | 0.25 | 0.92 | 0.014 | 0.059 | 0.05 | 0.3 | 0.05 | 0.3 | 0.17 | 0.17 | 1.00 |
| 15 | 0.46 | 0.03 | 0.59 | 0.012 | 0.036 | 0.05 | 0.4 | 0.04 | 0.25 | 0.10 | 0.16 | 1.25 |
| 16 | 0.49 | 0.08 | 0.68 | 0.01 | 0.027 | 0.01 | 0.06 | 0.006 | 0.03 | 0.10 | 0.20 | 1.67 |
| 17 | 0.56 | 0.15 | 0.8 | 0.015 | 0.042 | 0.03 | 0.01 | 0.01 | 0.009 | 1.00 | 1.11 | 3.00 |
| 18 | 0.45 | 0.07 | 1.32 | 0.004 | 0.036 | 0.01 | 0.004 | 0.0009 | 0.002 | 0.23 | 0.45 | 11.11 |
| Comparative example 1 | 0.53 | 0.28 | 0.6 | 0.012 | - | 0.001 | - | - | - | - | - | - |
| Comparative example 2 | 0.83 | 0.2 | 0.62 | 0.01 | - | 0.002 | - | - | - | - | - | - |
Table 2: smelting production process of hot-rolled strip steel
Table 3: rolling and cooling production process
Table 4: hot rolled steel coil performance, waste heat slow cooling process and slow cooling performance
Claims (6)
1. The low-cost high-surface hardness tool steel with excellent annealing-free bending performance is characterized in that the steel comprises the following chemical components in percentage by weight: 0.42 to 0.6 percent of C, less than or equal to 0.14 percent of Si, 0.4 to 1.5 percent of Mn, less than or equal to 0.06 percent of Al, 0.0005 to 0.05 percent of Ca, 0.05 to 0.5 percent of Ba, 0.02 to 0.5 percent of Mg, more than or equal to 1 percent of S/Ca, more than or equal to 0.1 percent of Ca/Mg, more than or equal to 0.1 percent of Ca/Ba, less than or equal to 0.020 percent of P, 0.04 to 0.06 percent of S, and the balance of Fe and inevitable impurities.
2. The low-cost high-surface hardness tool steel with excellent annealing-free bending performance according to claim 1, wherein uniform and fine troostite structures with a lamella spacing of 0.15 μm or less are obtained inside the coiled air-cooled steel plate, the tensile strength is 1000MPa to 1200MPa, the steel plate yield ratio is less than 60%, and the tool steel does not crack when subjected to cold bending at 0-180 ℃ under the condition that the bending core diameter is not less than the thickness of the steel plate.
3. The tool steel with low cost and high surface hardness and excellent non-annealing bending performance according to claim 1, is characterized in that a grained troostite layer containing 50-60% of barium sulfide fine granular phase is formed below the surface of the tool steel, the depth of the grained troostite layer is not less than 10% of the thickness of a steel plate, the maximum diameter of grains is 30nm, the interval between cementite pieces is not more than 0.15 μm, the surface hardness is not required to be subjected to heat treatment and is more than 53HRC, the wear rate is not more than 25mg/km, and the test conditions are that the grinding pin rotating speed is 300r/min and the load is 120N.
4. The method for producing a low-cost tool steel with high surface hardness and excellent annealing-free bending property according to claim 1, wherein the method comprises:
1) the smelting process comprises the following steps:
a) deoxidizing by using an Al deoxidizer, adding Ca for treatment after the refined oxygen content is less than or equal to 0.0020%, and adding Ba and Mg alloy after the Ca treatment is carried out for at least 5 minutes;
b) the argon blowing time of the tundish is 5-8 minutes, and the casting superheat degree is less than or equal to 25 ℃;
c) continuous casting adopts a reduction technology and crystallizer electromagnetic stirring, the crystallizer electromagnetic stirring is carried out for 1-3 minutes, the current intensity is more than 1000A, the continuous casting pulling speed is 1.0-1.4 m/min, a casting blank is cooled to 900-1000 ℃ and reduced, and the reduction is 10-30 mm;
2) a casting blank treatment process:
a) the casting blank is off-line, stacked, covered and slowly cooled for more than 24 hours, and the temperature of the casting blank in a heating furnace is more than 500 ℃;
b) the temperature of the preheating section of the heating furnace is more than 500 ℃, the temperature of the heating section is 1200-1350 ℃, and the total time of the heating furnace is 2-4 hours;
3) the rolling process comprises the following steps:
the method comprises three processes of rough rolling, finish rolling and third rolling:
a) the first pass reduction of rough rolling is more than or equal to 50 percent of large reduction rate rolling;
b) the finish rolling adopts a continuous rolling mode, the total rolling reduction rate is more than or equal to 80 percent, the primary rolling reduction rate is more than or equal to 30 percent, the rolling speed is more than or equal to 25m/s, the starting temperature is 1050-1150 ℃, and the finishing temperature is 900-980 ℃;
c) after the steel plate is finely rolled, the steel plate enters laminar cooling and quenching, the cooling speed is 32 ℃/s-100 ℃/s, the steel plate is cooled to 500-628 ℃, and the steel plate enters a double-vertical-roller four-horizontal-roller final rolling mill to be continuously rolled for two times, wherein the upper and lower reduction rates are 10-30%, and the side pressure reduction rate is 10-30%;
4) and (3) a cooling process:
and after the third rolling, the steel plate enters laminar cooling at the cooling speed of 25-100 ℃/s, and is cooled to 470-528 ℃ for coiling.
5. The method for producing a low-cost tool steel with high surface hardness and excellent non-annealing bending properties according to claim 4, wherein the high-pressure water is used for descaling before the rough rolling, the finish rolling and the final rolling, and the high-pressure water pressure is not less than 30 MPa.
6. The method for producing a low-cost tool steel with high surface hardness and excellent annealing-free bending property according to claim 4, wherein annealing is not required, the steel plate is coiled at 470-550 ℃, then slowly cooled to 200 ℃ or below by using waste heat, the cooling rate is less than 5 ℃/h, the room temperature structure is a spheroidized fine sorbite of not less than 80%, the yield ratio is not more than 50%, and the steel plate is cold-bent at 0-180 ℃ without cracking when the core bending diameter is less than the thickness of the steel plate.
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