CN118222929A - High-nitrogen hot working die steel and preparation method thereof - Google Patents
High-nitrogen hot working die steel and preparation method thereof Download PDFInfo
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Abstract
The invention belongs to the technical field of hot work die steel, and particularly relates to high-nitrogen hot work die steel and a preparation method thereof. The high-nitrogen hot working die steel provided by the invention comprises the following chemical components ：0.10～0.30％C,0.80～1.20％Si,0.40～0.50％Mn,4.90～5.10％Cr,1.30～1.70％Mo,0.20～0.50％V,0.10～0.40％N,0.50～1.00％Nb,0.01～0.05％Nd,0.016～0.019％Mg,0.01～0.10％Te in percentage by mass, and the balance of Fe and unavoidable impurity elements. The invention adopts the design ideas of 'carbon nitrogen substitution' and 'niobium vanadium regulation' and combines with Nd, mg and Te microalloying to prepare the high-nitrogen hot work die steel with high heat stability and high strength and toughness, thereby prolonging the service life of the die.
Description
Technical Field
The invention belongs to the technical field of hot work die steel, and particularly relates to high-nitrogen hot work die steel and a preparation method thereof.
Background
Hot work die steel is mainly used for manufacturing dies for press working materials in a high temperature state. With the rapid development of industrial technology, the performance requirements on hot work die steel are increasing. Particularly, under severe working environments such as high temperature, high pressure, high abrasion and the like, the performance indexes such as the thermal stability, the oxidation resistance, the toughness and the like of the die steel become key factors for determining the service life and the processing efficiency.
The die steel widely used at present meets the requirements of industrial production to a certain extent, but has great defects in the aspect of high-temperature stability. Under the high temperature condition, the die steel is easy to soften, oxidize, thermally expand and the like, so that the die is deformed, the precision is reduced, even the die is broken, and the application of the die steel in the field of high temperature processing is severely restricted. In addition, the balance of high strength and high toughness of die steel is also a technical challenge. The traditional die steel is high in strength, meanwhile, part of toughness is often sacrificed, brittle fracture is easy to occur in the using process, and the service life of the die is reduced. Therefore, developing a hot work die steel with high heat stability and high strength and toughness has important significance for improving the high temperature resistance of the die, prolonging the service life and improving the processing efficiency.
Disclosure of Invention
In view of the above, the invention provides a high-nitrogen hot-work die steel and a preparation method thereof, and the hot-work die steel provided by the invention has high thermal stability and high strength and toughness, and can effectively prolong the service life of the die steel.
In order to solve the technical problems, the invention provides high-nitrogen hot work die steel, which comprises the following chemical components in percentage by mass:
The balance of Fe and other unavoidable impurity elements.
Preferably, the composite material comprises the following chemical components in percentage by mass:
The balance of Fe and other unavoidable impurity elements.
Preferably, the method comprises the following steps:
mixing an iron raw material, a chromium raw material, a molybdenum raw material, a silicon raw material, a manganese raw material, a niobium raw material, a carbon raw material, a vanadium raw material, a tellurium raw material, a magnesium raw material and a neodymium raw material according to a proportion, smelting, and casting to obtain an ingot; gas phase nitriding is carried out in the smelting process;
And sequentially carrying out homogenization treatment, forging, isothermal spheroidizing annealing treatment, quenching and tempering treatment on the cast ingot to obtain the high-nitrogen hot work die steel.
Preferably, the smelting comprises the steps of:
Adding part of carbon raw materials into the first molten materials of the iron raw materials, the chromium raw materials, the molybdenum raw materials, the silicon raw materials, the manganese raw materials and the niobium raw materials for deoxidization to obtain deoxidized molten steel; the partial carbon raw material accounts for 40-55% of the total mass of the carbon raw material;
Adding residual carbon raw materials, vanadium raw materials, tellurium raw materials, magnesium raw materials and neodymium raw materials into the deoxidized molten steel after performing first gas phase nitriding to perform second melting; the nitriding pressure of the first gas phase is P 1 units of MPa, and P 1 is calculated according to formula 1;
Carrying out second gas-phase nitriding on the molten steel obtained by the second melting; the pressure of the second gas phase nitriding is P 2, the unit is MPa, and P 2 is calculated according to the formula 2;
P 2＝aP1/pθ +b is formula 2;
Wherein, p θ is the standard atmospheric pressure, which is 101.325KPa; t is smelting temperature unit K; [%M ] is the mass percentage of the constituent element M, a is 0.1-0.2, b is 0.1-0.5.
Preferably, the first melting cleaning is carried out under a protective atmosphere, wherein the protective atmosphere is argon, and the pressure of the protective atmosphere is 20-50 kPa;
the temperature of the first melting clear is 1550-1580 ℃;
The vacuum degree in the deoxidized furnace is 10-15 Pa;
the temperature of the second melting clear is 1530-1550 ℃;
the casting temperature is 1520-1540 ℃.
Preferably, the temperature of the homogenization treatment is 1150-1250 ℃, and the heat preservation time of the homogenization treatment is 8-12 h.
Preferably, the forging temperature is 1100-1200 ℃, the heat preservation time of the forging is 1-2 h, and the forging ratio of the forging is 4-6.
Preferably, the isothermal spheroidizing annealing treatment comprises a high-temperature spheroidizing annealing treatment and a low-temperature spheroidizing annealing treatment which are sequentially carried out;
The high-temperature spheroidizing annealing treatment temperature is 850-900 ℃, and the heat preservation time of the high-temperature spheroidizing annealing treatment is 2-4 hours;
The temperature of the low-temperature spheroidizing annealing treatment is 720-760 ℃, and the heat preservation time of the low-temperature spheroidizing annealing treatment is 3-6 h.
Preferably, the quenching heat preservation temperature is 1030-1080 ℃, and the heat preservation time is 0.5-2 h; the quenching cooling mode is oil cooling.
Preferably, the tempering treatment is secondary tempering, the temperature of each tempering treatment is independently 550-600 ℃, and the heat preservation time of each tempering treatment is independently 1.5-2.5 h.
The invention provides high-nitrogen hot working die steel which comprises the following chemical components ：0.10～0.30％C,0.80～1.20％Si,0.40～0.50％Mn,4.90～5.10％Cr,1.30～1.70％Mo,0.20～0.50％V,0.10～0.40％N,0.50～1.00％Nb,0.01～0.05％Nd,0.016～0.019％Mg,0.01～0.10％Te in percentage by mass and the balance of Fe and other unavoidable impurity elements. In the invention, the component design of reducing the carbon content and increasing the nitrogen content by using the nitrogen substituted carbon not only compensates the problems of insufficient hardness and strength and the like caused by reducing the carbon content, but also fully plays roles of fine crystal strengthening, precipitation strengthening and solid solution strengthening of nitrogen, is beneficial to improving the toughness and the thermal stability of the high-nitrogen hot working die steel, and further improves the service performance. Meanwhile, the high-nitrogen hot-working die steel provided by the invention obviously reduces the quantity of large-size liquid separation carbonitrides through the component design of 'niobium-vanadium regulation and control', and is beneficial to improving the structural uniformity and toughness, so that the thermal stability and toughness of the high-nitrogen hot-working die steel are improved. In addition, nd and Mg can react with impurities and inclusions in molten steel, and the contents of harmful elements O, P, S and the like are reduced, so that the molten steel is obviously purified, and the metallurgical quality of products is improved. Nd and Mg can refine grains and inhibit aggregation of harmful elements with low melting point among crystals, thereby effectively preventing crack expansion among the crystals and improving hot workability. Te microalloying can obviously refine the structure and inhibit carbide dissolution in the isothermal softening process, thereby further enhancing the thermal stability. In addition, te can also obviously enhance the ductility of steel and improve the processing property, thereby better meeting the severe requirements of the modern industry on high-precision die products.
Drawings
FIG. 1 is a schematic flow chart of homogenizing, forging, isothermal spheroidizing annealing, quenching and tempering an ingot;
FIG. 2 is a structure diagram of the high nitrogen hot work die steel prepared in example 13;
FIG. 3 is a structure diagram of hot work die steel prepared in comparative example 1;
FIG. 4 is a microstructure of example 13 high nitrogen hot work die steel after isothermal softening treatment;
FIG. 5 is a microstructure of the hot work die steel prepared in comparative example 1 after isothermal softening treatment.
Detailed Description
The invention provides high-nitrogen hot working die steel, which comprises the following chemical components in percentage by mass:
The balance of Fe and other unavoidable impurity elements.
The high-nitrogen hot working die steel provided by the invention comprises 0.10-0.30% of C, preferably 0.21-0.29% of C by mass percent. In the present invention, carbon plays an important role in improving the hardness and strength of steel as a main constituent element of a strengthening phase in hot work die steel. The high carbon content can increase the hardness and strength of the steel, but also can increase the number of large-size carbides at the austenitic grain boundaries, and obviously reduce the plasticity and toughness of the steel. Compared with carbon, nitrogen has the unique advantages of improving the properties of material hardness and the like without reducing the toughness, and the fine-grain strengthening effect of the nitrogen is mainly reflected in increasing the quantity of undissolved M (C, N) carbonitrides during quenching, so that the capability of preventing the migration of original austenite grain boundaries and coarsening of grains is enhanced. The precipitation strengthening and solid solution strengthening effects of nitrogen mainly change the type, size, quantity and distribution of the precipitated phases by adding nitrogen elements, and increase the solid solution nitrogen content in the matrix, so that the microstructure and mechanical properties of the high-nitrogen hot work die steel are regulated and controlled. The invention adopts the design mode of 'carbon nitride substitution' to achieve the purpose of improving the toughness and the thermal stability of the steel.
The high-nitrogen hot working die steel provided by the invention comprises 0.80-1.20% of Si, preferably 0.80-0.84% by mass percent.
The high-nitrogen hot working die steel provided by the invention comprises 0.40-0.50% of Mn, preferably 0.40-0.43% by mass percent.
The high-nitrogen hot working die steel provided by the invention comprises 4.90-5.10% Cr, preferably 5.06-5.10% Cr by mass percent.
The high-nitrogen hot working die steel provided by the invention comprises 1.30-1.70% of Mo, preferably 1.34-1.41% by mass percent.
The high-nitrogen hot working die steel provided by the invention comprises 0.20-0.50% of V, preferably 0.21-0.49% of V by mass percent.
The high-nitrogen hot working die steel provided by the invention comprises 0.10-0.40% of N, preferably 0.11-0.38% by mass percent. In the invention, vanadium is used as a main secondary hardening element in hot working die steel, and plays an important role in refining original austenite grains, improving hardness and thermal stability and the like. However, since vanadium is a strong segregation element, excessive addition will aggravate element segregation during solidification of hot-work die steel and increase the amount of liquid-out carbonitride, thereby decreasing toughness. In addition, vanadium is high in price and large in fluctuation, and the alloy cost of hot work die steel is greatly increased. Compared with vanadium, the niobium-enriched carbonitride with small size formed by niobium, carbon, nitrogen and other elements has stronger capability of pinning grain boundary, and is beneficial to reducing the quantity of large-size liquid-separated vanadium carbonitride. The invention further reduces segregation and the content of liquid separation carbonitride through 'niobium vanadium regulation and control', and greatly reduces the alloy cost.
The high-nitrogen hot working die steel provided by the invention comprises 0.50-1.00% of Nb, preferably 0.51-0.99% by mass percent.
The high-nitrogen hot working die steel provided by the invention comprises 0.01-0.05% of Nd, preferably 0.012-0.048% by mass percent.
The high-nitrogen hot working die steel provided by the invention comprises 0.016-0.019% of Mg, preferably 0.0161-0.0189% by mass percent. In the invention, nd and Mg have remarkable molten steel purifying effect in the smelting process. They can react with impurities and inclusions in the molten steel to form more stable, easier to separate compounds, thereby reducing the content of these harmful elements O, P and S, etc. in the steel. In this way, the cleanliness of the hot work die steel can be remarkably improved, and the die failure risk caused by impurities and inclusions is reduced. In addition, nd and Mg elements can refine grains, strengthen grain boundaries, inhibit segregation at the grain boundaries and aggregation with low-melting-point harmful elements, and block the propagation of intercrystalline cracks so as to improve hot workability; in addition, the addition of the trace Mg element effectively improves the size and distribution of carbide, and is beneficial to improving the toughness of the high-nitrogen hot work die steel.
The high-nitrogen hot working die steel provided by the invention comprises 0.01-0.10% Te, preferably 0.011-0.096% by mass percent. In the invention, te microalloying can obviously refine the structure, inhibit the dissolution of carbide in the softening process, and is beneficial to improving the thermal stability. In addition, te can enhance the ductility of steel, so that the processed steel surface is brighter, and the generation of cracks and crazes is reduced, thereby meeting the requirements of modern industry on high-precision die products.
The invention adopts the design ideas of 'carbon nitrogen substitution' and 'niobium vanadium regulation' and combines with Nd, mg and Te microalloying to prepare the high-nitrogen hot work die steel with high heat stability and high strength and toughness, thereby prolonging the service life of the die.
The high-nitrogen hot work die steel provided by the invention also comprises the balance of Fe and other unavoidable impurity elements in percentage by mass.
The invention obtains the high-nitrogen hot work die steel with high heat stability and high strength and toughness through reasonably optimizing main alloying elements such as C, N, V, nb and micro alloying elements such as Nd, mg, te and the like.
The invention also provides a preparation method of the high-nitrogen hot work die steel, which comprises the following steps:
Mixing an iron raw material, a chromium raw material, a molybdenum raw material, a silicon raw material, a manganese raw material, a niobium raw material, a carbon raw material, a vanadium raw material, a tellurium raw material, a magnesium raw material and a neodymium raw material according to element proportions, smelting, and casting to obtain an ingot; gas phase nitriding is carried out in the smelting process;
And sequentially carrying out homogenization treatment, forging, isothermal spheroidizing annealing treatment, quenching and tempering treatment on the cast ingot to obtain the high-nitrogen hot work die steel.
The invention mixes iron raw material, chromium raw material, molybdenum raw material, silicon raw material, manganese raw material, niobium raw material, carbon raw material, vanadium raw material, tellurium raw material, magnesium raw material and neodymium raw material according to element proportion, and then performs smelting and casting to obtain cast ingot. In the present invention, the smelting preferably includes the steps of:
Adding part of carbon raw materials into the first molten materials of the iron raw materials, the chromium raw materials, the molybdenum raw materials, the silicon raw materials, the manganese raw materials and the niobium raw materials for deoxidization to obtain deoxidized molten steel; the partial carbon raw material accounts for 40-55% of the total mass of the carbon raw material;
Adding residual carbon raw materials, vanadium raw materials, tellurium raw materials, magnesium raw materials and neodymium raw materials into the deoxidized molten steel after performing first gas phase nitriding; the nitriding pressure of the first gas phase is P 1 units of MPa, and P 1 is calculated according to formula 1;
carrying out second gas-phase nitriding on the molten steel obtained by the second melting; the pressure of the second gas-phase nitriding is P 2 units of MPa, and P 2 is calculated according to formula 2;
P 2＝aP1/pθ +b is formula 2;
Wherein, p θ is the standard atmospheric pressure, which is 101.325KPa; t is smelting temperature unit K; [%M ] is the mass percentage of the constituent element M, and a is 0.1-0.2, preferably 0.1-0.15; b is 0.1 to 0.5, preferably 0.1 to 0.4.
In the present invention, all the raw materials are conventionally commercially available products unless otherwise specified.
The method comprises the steps of adding partial carbon raw materials into the first molten materials of iron raw materials, chromium raw materials, molybdenum raw materials, silicon raw materials, manganese raw materials and niobium raw materials for deoxidization to obtain deoxidized molten steel. In the invention, the iron raw material is preferably industrial pure iron, and the purity of the industrial pure iron is preferably more than or equal to 99.9%; the chromium raw material is preferably metallic chromium, and the purity of the metallic chromium is preferably more than or equal to 99.5%; the molybdenum raw material is preferably metallic molybdenum, and the purity of the metallic molybdenum is preferably more than or equal to 99.9%; the silicon raw material is preferably metallic silicon, and the purity of the metallic silicon is preferably more than or equal to 99.5%; the manganese raw material is preferably electrolytic manganese, and the purity of the electrolytic manganese is preferably more than or equal to 99.9%; the niobium raw material is preferably metal niobium, and the purity of the metal niobium is preferably more than or equal to 99.5%; the carbon raw material is preferably graphite, and the purity of the graphite is preferably more than or equal to 99.5%.
In the present invention, the temperature of the first melt-down is preferably 1550 to 1580 ℃, more preferably 1550 to 1560 ℃, and in the present invention, the first melt-down is performed under a protective atmosphere, wherein the protective atmosphere is argon, and the pressure of the protective atmosphere is preferably 20 to 50kPa, more preferably 20 to 30kPa. The invention preferably firstly vacuumizes the smelting container to 2-4 Pa, then introduces argon, and preferably vacuumizes the smelting container to 2-3 Pa.
In the present invention, the partial carbon raw material preferably accounts for 40 to 55% of the total mass of the carbon raw material, more preferably 45 to 50%.
In the present invention, the vacuum degree in the deoxidized furnace is preferably 10 to 15Pa, more preferably 12 to 15Pa.
After deoxidized molten steel is obtained, the deoxidized molten steel is subjected to first gas phase nitriding, and then the residual carbon raw material, the vanadium raw material, the tellurium raw material, the magnesium raw material and the neodymium raw material are added for second melting; the pressure of the first gas phase nitriding is P 1 units of MPa, and P 1 is calculated according to formula 1. In the invention, the vanadium raw material is preferably metal vanadium, and the purity of the metal vanadium is preferably more than or equal to 99.7%; the tellurium raw material is preferably metal tellurium, and the purity of the metal tellurium is preferably more than or equal to 99.7%; the magnesium raw material is preferably nickel-magnesium alloy, and in the invention, the mass percentage of magnesium in the nickel-magnesium alloy is preferably 15-25%, more preferably 20%; the neodymium raw material is preferably neodymium metal, and the purity of the neodymium metal is preferably more than or equal to 99.9%.
In the present invention, the temperature of the second melt-down is preferably 1530 to 1550 ℃, more preferably 1530 to 1540 ℃.
After the second melting, carrying out second gas-phase nitriding on molten steel obtained by the second melting; the pressure of the second gas-phase nitriding is P 2 units of MPa, and P 2 is calculated according to formula 2;
In the present invention, the smelting is preferably performed in a pressurized induction furnace.
In the present invention, the casting temperature is preferably 1520 to 1540 ℃, more preferably 1520 to 1530 ℃. The invention has no special requirement on the casting mode, and can be realized by adopting a mode conventional in the field.
The invention adopts a gas phase nitriding method to increase nitrogen, thereby avoiding the problem that extra impurities enter molten steel due to the addition of nitriding alloy.
After the ingot is obtained, the ingot is sequentially subjected to homogenization treatment, forging, isothermal spheroidizing annealing treatment, quenching and tempering treatment to obtain the high-nitrogen hot work die steel.
In the present invention, the temperature of the homogenization treatment is preferably 1150 to 1250 ℃, more preferably 1200 to 1250 ℃; the incubation time for the homogenization treatment is preferably 8 to 12 hours, more preferably 10 to 12 hours. The method can reduce segregation through high-temperature homogenization treatment to obtain homogenized cast ingots;
In the present invention, the forging temperature is preferably 1100 to 1200 ℃, more preferably 1150 to 1200 ℃; the holding time of the forging is preferably 1 to 2 hours, and the forging ratio of the forging is preferably 4 to 6, more preferably 4 to 5. In the present invention, the final forging temperature of the forging is preferably greater than 900 ℃, more preferably 950 to 980 ℃. According to the invention, dendrites can be fully crushed through forging, and a forging piece is obtained.
In the present invention, the post-forging preferably further comprises: and air-cooling the forged product to 20-30 ℃ to obtain the forged piece.
In the present invention, the isothermal spheroidizing annealing treatment preferably includes a high temperature spheroidizing annealing treatment and a low temperature spheroidizing annealing treatment which are sequentially performed. In the present invention, the temperature of the high-temperature spheroidizing annealing treatment is preferably 850 to 900 ℃, more preferably 850 to 880 ℃; the heat preservation time of the high-temperature spheroidizing annealing treatment is preferably 2-4 hours. In the present invention, the heating rate to the temperature of the high-temperature spheroidizing annealing treatment is preferably 5 to 8 ℃/min, more preferably 5 to 6 ℃/min. The invention can austenitize the forging through high-temperature spheroidizing annealing treatment.
In the invention, the temperature of the low-temperature spheroidizing annealing treatment is preferably 720-760 ℃, more preferably 740-750 ℃; the holding time of the low-temperature spheroidizing annealing treatment is preferably 3 to 6 hours, more preferably 4 to 5 hours. The invention preferably cools to the temperature required for the low-temperature spheroidizing annealing treatment based on the temperature of the high-temperature spheroidizing annealing treatment. In the present invention, the cooling rate of the cooling is preferably 5 to 8 ℃ per minute, more preferably 5 to 6 ℃ per minute. In the invention, the low-temperature spheroidizing annealing treatment preferably further comprises the steps of cooling the product after the low-temperature spheroidizing annealing treatment to 400-500 ℃ along with a furnace, and then air-cooling to room temperature. In the invention, the cooling rate of the furnace cooling is preferably 50-80 ℃/h, more preferably 60-70 ℃/h. In the present invention, the temperature of the room temperature is preferably 20 to 35 ℃, more preferably 25 to 30 ℃.
The invention can improve the spheroidizing degree of carbide and reduce the banded segregation through isothermal spheroidizing annealing treatment.
In the invention, the heat preservation temperature of quenching is preferably 1030-1080 ℃, more preferably 1040-1060 ℃; the heat preservation time of the quenching is preferably 0.5-2 h, more preferably 0.5-1 h; the heating rate to the temperature required for quenching is preferably 5 to 7℃per minute, more preferably 5 to 6℃per minute.
In the invention, the quenching cooling mode is oil cooling, and the temperature after oil cooling is preferably 20-30 ℃.
In the invention, the tempering treatment is preferably secondary tempering, and the temperature of each tempering treatment is preferably and independently 550-600 ℃, more preferably and independently 560-580 ℃; the heat preservation time of each tempering treatment is preferably independently 1.5-2.5 h, more preferably independently 1.5-2 h.
Fig. 1 is a schematic flow chart of homogenizing, forging, isothermal spheroidizing annealing, quenching and tempering an ingot, specifically homogenizing, forging, isothermal spheroidizing annealing, quenching and secondary tempering the ingot in sequence.
The technical solutions provided by the present invention are described in detail below in conjunction with examples for further illustrating the present invention, but they should not be construed as limiting the scope of the present invention.
The smelting equipment adopted in the embodiment of the invention is a 25kg pressurized induction furnace, the ultimate vacuum degree is 0.1Pa, and the capacity of the furnace in the crucible is 19-20 kg.
In the embodiment of the invention, a direct-reading spectrometer (ARL 4460) and a LECO TC500 oxygen nitrogen analyzer are adopted to analyze chemical components in the product.
In the examples of the present invention, the microstructure was observed using a JSM-7001F scanning electron microscope.
The examples of the present invention were subjected to hardness tests according to ASTM E18-15.
The thermal stability test (isothermal softening treatment) system of the present invention was a treatment of heat preservation at 600℃for 48 hours, followed by a hardness test according to ASTM E18-15 to evaluate thermal stability.
In the embodiment of the invention, an impact toughness experiment is carried out according to an ASTMA370 standard brick, and the size of a sample is 10 multiplied by 55mm and is provided with a V-shaped notch.
In the embodiment of the invention, a dog bone-shaped sample with the processing diameter of 5mm and the specification length of 35mm is adopted for the stretching experimentAnd tested according to ISO-6892 standard.
Example 1
(1) Smelting: batching according to the set chemical components, and calculating the pressure required by gas phase nitriding in the smelting process according to the steps 1 and 2; the high-nitrogen hot working die steel comprises the following chemical components in percentage by mass: 0.29% C,0.84% Si,0.43% Mn,5.08% Cr,1.36% Mo,0.45% V,0.10% N,0.51% Nb,0.012% Nd,0.0162% Mg,0.011% Te, the balance being iron and unavoidable impurity elements;
Placing industrial pure iron with purity of 99.9%, metallic chromium with purity of 99.5%, metallic molybdenum with purity of 99.9%, metallic silicon with purity of 99.5%, electrolytic manganese with purity of 99.9% and metallic niobium with purity of 99.5% in a crucible, placing graphite with purity of 99.5%, metallic vanadium with purity of 99.7%, metallic neodymium with purity of 99.9%, nickel-magnesium alloy with magnesium mass percent of 20% and metallic tellurium with purity of 99.7% in a stock bin, vacuumizing and starting heating; pumping to vacuum degree of 3Pa, then introducing 30000Pa argon and heating to 1560 ℃; after the furnace burden in the crucible is completely melted, adding graphite into the crucible, and then carrying out carbon deoxidation under the condition that the vacuum degree is 15 Pa; filling nitrogen with the pressure of 0.1MPa by adopting a gas phase nitriding method, and then adding residual graphite, metal vanadium, metal tellurium, nickel-magnesium alloy and metal neodymium into a crucible to carry out desulfurization and deep deoxidation at 1530 ℃ in a melting way; filling nitrogen with pressure of 0.3MPa (a is 0.1 and b is 0.2 in formula 2) again by adopting a gas phase nitriding method, controlling the temperature of molten steel to 1530 ℃, and pouring to obtain an ingot; part of graphite accounts for 40% of the total mass of the graphite;
(2) Homogenizing: preserving the temperature of the ingot for 10 hours at 1200 ℃ to carry out homogenization treatment to obtain a homogenized ingot;
(3) Forging: the homogenized cast ingot is subjected to forging after being kept at 1150 ℃ for 2 hours, the forging mode is three-drawing three-pier, the forging ratio is 4, the final forging temperature is 950 ℃, and the forging is obtained after air cooling to 30 ℃;
(5) Isothermal spheroidizing annealing: heating the forge piece to 880 ℃ according to a heating rate of 5 ℃/min, preserving heat for 3 hours, cooling to 740 ℃ at a cooling rate of 5 ℃/min, preserving heat for 4 hours, and finally taking out the forge piece for air cooling along with furnace cooling (the cooling rate is 50 ℃/h) to 500 ℃ to obtain an annealed forge piece;
(6) Quenching and tempering: heating the annealed forging to 1080 ℃, preserving heat for 0.5h, and oil-cooling to normal temperature to finish quenching; and then carrying out secondary tempering at the temperature of 580 ℃ for 2 hours each time to obtain the high-nitrogen hot work die steel.
Examples 2 to 18
A high nitrogen hot work die steel was prepared as in example 1 except for the nitriding pressure, the contents of the components being matched to the different nitrogen contents. The elemental composition is specifically referred to in table 1.
In the step of example 2, after carbon deoxidation is finished, 0.6MPa nitrogen is filled in by adopting a gas phase nitriding method, and then residual graphite, metal vanadium, metal tellurium, nickel-magnesium alloy and metal neodymium are added into a crucible to carry out desulfurization and deep deoxidation at 1530 ℃ in a melting way; nitrogen gas of 1.2MPa (in formula 2, a is 0.15 and b is 0.3) was again charged by gas phase nitriding. The remaining steps were the same as in example 1.
In the step of example 3, after carbon deoxidation is finished, 0.7MPa nitrogen is filled in by adopting a gas phase nitriding method, and then residual graphite, metal vanadium, metal tellurium, nickel-magnesium alloy and metal neodymium are added into a crucible to carry out desulfurization and deep deoxidation at 1530 ℃ in a melting way; nitrogen gas of 1.5MPa (in formula 2, a is 0.2 and b is 0.1) was again charged by gas phase nitriding. The remaining steps were the same as in example 1.
In the step of example 4, after carbon deoxidation is finished, nitrogen with the pressure of 0.9MPa is filled in by adopting a gas phase nitriding method, and then residual graphite, metal vanadium, metal tellurium, nickel-magnesium alloy and metal neodymium are added into a crucible to carry out desulfurization and deep deoxidation at the temperature of 1530 ℃; nitrogen gas of 1.9MPa (in formula 2, a is 0.2 and b is 0.1) was again charged by gas phase nitriding. The remaining steps were the same as in example 1.
The gas phase nitriding pressure in the steps of examples 5 to 18 was the same as that of example 2.
Comparative example 1
Smelting: proportioning according to the set chemical components, placing industrial pure iron with the purity of 99.5%, metal chromium with the purity of 99.5%, metal molybdenum with the purity of 99.9%, metal silicon with the purity of 99.5% and electrolytic manganese with the purity of 99.9% into a crucible, placing graphite with the purity of 99.5% and metal vanadium with the purity of 99.7% into a storage bin, vacuumizing and heating; pumping to 3Pa, then introducing 30000Pa argon and heating; after the furnace burden is completely melted, adding part of graphite into a crucible for carbon deoxidation, and then pumping to the vacuum degree of 15Pa; then introducing 30000Pa argon, and adding the rest graphite into the crucible, wherein the purity of the metal vanadium is 99.9%; finally controlling the temperature of the molten steel to be 1530 ℃ and pouring to obtain an ingot; part of graphite accounts for 40% of the total mass of the graphite;
The ingot was subjected to homogenization treatment, forging, isothermal spheroidizing annealing treatment, quenching (quenching temperature: 1050 ℃ C.) and tempering (tempering temperature: 600 ℃ C.) in the same manner as in example 1 to obtain hot work die steel.
Comparative examples 2 to 3
A hot-work die steel was prepared in accordance with the method of comparative example 1, except for the content of the elemental components, with specific reference to table 1.
Table 1 chemical composition of steel grades of examples 1 to 18 and comparative examples 1 to 3
The hot work die steels prepared in example 13 and comparative example 1 were ground, polished and etched, and then the microstructure was observed by a scanning electron microscope to obtain a microstructure chart as shown in fig. 2 to 3, wherein fig. 2 is a microstructure chart of the high nitrogen hot work die steel prepared in example 13, and fig. 3 is a microstructure chart of the hot work die steel prepared in comparative example 1. After isothermal softening treatment (incubation at 600 ℃ for 48 h), significant reversion of the martensitic structure and carbides occurs, i.e. decomposition of the martensitic substructure and precipitation, dissolution and coarsening of the carbides. As can be seen from FIG. 2, the microstructure of the high nitrogen hot work die steel of example 13 is composed mainly of tempered martensite and spherical and fine-bar-like carbides. As can be seen from FIG. 3, the microstructure of the hot work die steel prepared in comparative example 1 is mainly composed of tempered martensite and spherical and bulk carbides. The high nitrogen hot work die steel of example 13 was subjected to isothermal softening treatment and then observed by a scanning electron microscope to obtain a heat stability microstructure, as shown in fig. 4, consisting essentially of recovered martensite, ferrite and coarsened carbide. The heat-stable microstructure obtained by scanning electron microscope observation after isothermal softening treatment of the hot-work die steel prepared in comparative example 1 is shown in fig. 5, and mainly consists of ferrite and coarsened carbide. The more remarkable reversion of example 13 compared with comparative example 1, the substantial disappearance of the martensitic substructure and the more remarkable coarsening of the carbide resulted, which further represents the excellent heat stability of the high nitrogen hot work die steel.
The mechanical properties of the hot work die steels prepared in examples 1 to 18 and comparative examples 1 to 3 were examined as follows, and the results are shown in Table 2. Hardness: five point hardness values were measured on each sample to be tested using an HRS-150 rockwell hardness tester; intensity: impact toughness was measured on a SUNS PTM2302-B tester to measure a charpy V-notch specimen of dimensions 10 x 55 mm; impact toughness: processing dog bone sample with diameter of 5mm and specification length of 35mmTensile testing was performed on Shimadzu AGS100 at a strain rate of 1 mm/min; thermal stability hardness: incubate at 600℃for 48h, followed by five point hardness values at each sample using an HRS-150 Rockwell hardness tester. As can be seen from Table 4, the high nitrogen hot work die steel provided by the invention has high thermal stability and high toughness. /(I)
Table 2 mechanical properties of hot work die steels prepared in examples 1 to 18 and comparative examples 1 to 3
As can be seen from Table 2, the high nitrogen hot work die steel provided by the invention has high thermal stability and high toughness. The properties of the optimal steel grade (example 13) are as follows: high thermal stability, heat preservation at 600 ℃ for 48 hours, and hardness of more than 39 HRC; high hardness and toughness, and tempering hardness and toughness of 50HRC and 26J; high strength and elongation, and the tensile strength and yield strength can reach 1705MPa and 1465MPa, and the elongation is 15%.
Although the foregoing embodiments have been described in some, but not all, embodiments of the invention, it should be understood that other embodiments may be devised in accordance with the present embodiments without departing from the spirit and scope of the invention.
Claims (10)
1. The high-nitrogen hot work die steel is characterized by comprising the following chemical components in percentage by mass:
The balance of Fe and other unavoidable impurity elements.
2. The high nitrogen hot work die steel as claimed in claim 1, comprising the following chemical components in percentage by mass:
The balance of Fe and other unavoidable impurity elements.
3. The method for preparing the high nitrogen hot work die steel according to claim 1 or 2, which is characterized by comprising the following steps:
mixing an iron raw material, a chromium raw material, a molybdenum raw material, a silicon raw material, a manganese raw material, a niobium raw material, a carbon raw material, a vanadium raw material, a tellurium raw material, a magnesium raw material and a neodymium raw material according to a proportion, smelting, and casting to obtain an ingot; gas phase nitriding is carried out in the smelting process;
And sequentially carrying out homogenization treatment, forging, isothermal spheroidizing annealing treatment, quenching and tempering treatment on the cast ingot to obtain the high-nitrogen hot work die steel.
4. A method of manufacture according to claim 3, wherein the smelting comprises the steps of:
Adding part of carbon raw materials into the first molten materials of the iron raw materials, the chromium raw materials, the molybdenum raw materials, the silicon raw materials, the manganese raw materials and the niobium raw materials for deoxidization to obtain deoxidized molten steel; the partial carbon raw material accounts for 40-55% of the total mass of the carbon raw material;
Adding residual carbon raw materials, vanadium raw materials, tellurium raw materials, magnesium raw materials and neodymium raw materials into the deoxidized molten steel after performing first gas phase nitriding to perform second melting; the nitriding pressure of the first gas phase is P 1 units of MPa, and P 1 is calculated according to formula 1;
Carrying out second gas-phase nitriding on the molten steel obtained by the second melting; the pressure of the second gas phase nitriding is P 2, the unit is MPa, and P 2 is calculated according to the formula 2;
P 2＝aP1/pθ +b is formula 2;
Wherein, p θ is the standard atmospheric pressure, which is 101.325KPa; t is smelting temperature unit K; [%M ] is the mass percentage of the constituent element M, a is 0.1-0.2, b is 0.1-0.5.
5. The method according to claim 4, wherein the first melting is carried out under a protective atmosphere of argon gas, and the pressure of the protective atmosphere is 20 to 50kPa;
the temperature of the first melting clear is 1550-1580 ℃;
The vacuum degree in the deoxidized furnace is 10-15 Pa;
the temperature of the second melting clear is 1530-1550 ℃;
the temperature of the casting and pouring is 1520-1540 ℃.
6. The method according to claim 3, wherein the temperature of the homogenization treatment is 1150-1250 ℃ and the incubation time of the homogenization treatment is 8-12 hours.
7. The method according to claim 3, wherein the forging temperature is 1100-1200 ℃, the forging holding time is 1-2 hours, and the forging ratio is 4-6.
8. The method according to claim 3, wherein the isothermal spheroidizing annealing treatment comprises a high-temperature spheroidizing annealing treatment and a low-temperature spheroidizing annealing treatment performed sequentially;
The high-temperature spheroidizing annealing treatment temperature is 850-900 ℃, and the heat preservation time of the high-temperature spheroidizing annealing treatment is 2-4 hours;
The temperature of the low-temperature spheroidizing annealing treatment is 720-760 ℃, and the heat preservation time of the low-temperature spheroidizing annealing treatment is 3-6 h.
9. The preparation method according to claim 3, wherein the quenching has a heat preservation temperature of 1030-1080 ℃ and a heat preservation time of 0.5-2 h; the quenching cooling mode is oil cooling.
10. The method according to claim 3, wherein the tempering treatment is a secondary tempering, the temperature of each tempering treatment is independently 550-600 ℃, and the heat preservation time of each tempering treatment is independently 1.5-2.5 h.
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