Application of low carbon martensite in mould

Generally, the steel for the mold is made of alloy tool steel, carbon tool steel and high-speed tool steel. The quenched structure is mainly a substructure of twin crystal, and has high-carbon needle-like and sheet-shaped martensite with microcracks. Due to the high carbon content and the large number of carbide forming elements, the network will inevitably have network, ribbon and chain carbides, which often cause brittle fracture of the mold during use. Because the low carbon martensite substructure is dislocation, it has high strength (hardness is 45HRC~50HRC, yield strength is 1000MPa~1300MPa), good plasticity (δ5≥10%, ψ≥40%) and toughness (AKV) ≥59J) and good cold workability, weldability and heat treatment distortion. Therefore, how to obtain more or even all low-carbon martensite in the quenched structure of the die steel is the key to improving the toughness of the die and prolonging the service life of the die. The current development direction is that the quenched structure of low carbon structural steel hopes to obtain all lath martensite to ensure high toughness; for high carbon steel, it tends to reduce carbon content or minimize quenching temperature to reduce the number of twin martensite. In order to ensure the toughness of the material; for medium carbon alloy steel, it tends to use high temperature quenching to obtain more or all low carbon martensite structure to improve fracture toughness.

Application of low carbon martensite in plastic mold

1 Low carbon martensite carburized plastic mold steel low carbon steel, low carbon alloy steel mold after carburizing, quenching and low temperature tempering after forming, so that the surface of the mold can obtain high carbon fine needle tempered martensite + particles Carbide + a small amount of retained austenite, the core tissue is mainly low carbon martensite. Therefore, the mold surface has high hardness (58HRC~62HRC) and high wear resistance, while the core has high toughness (30HRC~45HRC), which is used for manufacturing various requirements, good wear resistance, complex shape and load bearing. Higher plastic forming dies.

2 Ultra-low carbon cold extrusion molding plastic mold steel widely used in ultra-low carbon steel cold extrusion molding process to manufacture plastic molds. Due to the need for cold extrusion molding, in addition to low carbon content, after softening annealing, the hardness is lower (≤160HBS, ≤130HBS when compacting complex cavity), especially suitable for cold plastic deformation. This type of steel is carburized, quenched, and tempered after cold extrusion. Therefore, it has the advantages of high production efficiency, short manufacturing cycle, and high precision of the mold. Typical steel grades are American P series low carbon die steel, German X6CrMo4 steel (≤0.07%C, mass fraction, the same below) and domestic LJ-08Cr3NiMoV steel (≤0.08%C).

3 low carbon martensitic steel (≤0.25% C) strong and tough plastic mold steel low carbon martensitic steel including low carbon carbon steel and low carbon alloy steel after strong quenching treatment with low carbon martensite, toughness Higher low-carbon martensite structure, instead of medium carbon steel quenching and tempering treatment or low carbon steel carburizing and nitriding treatment, it is suitable for manufacturing various plastic molds, which can significantly shorten the mold manufacturing cycle, reduce manufacturing costs and improve service life. It is widely used in heat treatment of plastic molds.

4 low carbon maraging time hardening plastic mold steel (1) low carbon maraging hardening steel This steel has a higher yield ratio, good cutting and welding performance and simple heat treatment process. The typical steel grade is the ultra-low carbon 18Ni steel series (≤0.03% C). Due to the high price of 18Ni steel, it is rarely used in China. In recent years, 06Ni6CrMoVTiAl (referred to as 06 steel, ≤0.06% C) is a low-nickel, low-carbon maraging-age hardening steel. The steel is heated at 800 ° C ~ 880 ° C, water or oil cooling solution treatment, (500 ~ 540) ° C × (4 ~ 8) h aging treatment of low carbon martensite + precipitation strengthening phase Ni3Al, Ni3Ti, TiC and TiN have a hardness of 42 HRC to 45 HRC and a yield strength of 1100 MPa to 1400 MPa. 06 steel is usually used to make tape cassette molds with an average life of >1.1 million pieces, which is a promising steel grade.

(2) Domestic low-carbon martensite precipitation (precipitation) hardening steel Typical steel grades are 25CrNi3MoAl and 10Ni3MnCuAl (referred to as PMS), which are suitable for making precision mirrors with deformation rate requirements of <0.05%, mirror requirements or surface requirements lithography Plastic molds. After 25CrNi3MoAl steel is solution treated at 830 °C, the hardness can reach 50HRC, after aging at 540°C×4h, the hardness is 39HRC~42HRC; PMS steel is solution treated at 870°C×1h, and the metallurgical treatment after aging at 510°C×4h The microstructure is a low-carbon martensite matrix dispersed in a large number of fine intermetallic compounds, the hardness of which is 40HRC ~ 43HRC, tensile strength is 1000MPa ~ 1300MPa.

5 low carbon martensite corrosion resistant plastic mold steel (1) low carbon martensitic stainless steel main steel grade is 2Cr13 steel (0.16% ~ 0.25% C) and 1Cr17Ni2 steel (0.11% ~ 0.17% C), suitable for manufacturing Plastic mold under the action of corrosive medium, transparent plastic product mold.

(2) Low carbon martensite precipitation (precipitation) hardening type stainless steel is made of domestic 07Cr16Ni4Cu3Nb steel (≤0.07%C, referred to as CR). The steel is quenched at 1050 °C to obtain a single low carbon martensite structure. For 32HRC~35HRC, it can be directly processed by cutting; after aging treatment from 460°C to 480°C, the hardness is 42HRC~44HRC, good mechanical properties and corrosion resistance. For example, the polychlorotrifluoroethylene valve cover mold, the original 45 steel chrome-plated mold, the service life of 1000 ~ 4000 pieces, later changed to low-carbon martensitic PCR steel, life expectancy of up to 10,000 ~ 20,000 pieces.

Application of low carbon martensite in cold working die

1 Low carbon martensitic cold quenching of low carbon steel is strongly quenched. Some countries have included low carbon martensitic steel (≤0.25% C) in the cold work die steel standard, for example in ISO 4957.1999 Tool Steel has the following steel grades: 5CrMo4 steel (≤0.07%C), 7CrMoNi2 steel (≤0.10%C), 20Cr13 steel (0.16%~0.25%C); German tool steel grade: 21MnCr5 steel (0.16%~0.24) %C), X6CrMo4 steel (≤0.07%C), 15NiCr18 steel (0.10%~0.17%C).

There are also many examples of cold-working dies that are used in the hard quenching process of low-carbon martensitic steels.
(1) 20 steel sawtooth lock washer die This cold die is used for stamping 65Mn steel strip (179HBS ~ 217HBS). Originally used T10 steel (48HRC ~ 50HRC), service life <1500 times; later using Cr12MoV steel (48HRC ~ 50HRC), service life 3000 times; and using 20 steel (46HRC ~ 48HRC), the service life is up to 30,000 to 40,000 times, The treatment process is: (910±10) °C heating, 10% NACL brine cooling, direct use without tempering, and low cost.

(2) Q235 steel shearing machine blade bolt forging tire mold punch used GCr15, 9SiCr, T8 and 45 steel, due to the bite mold and sticking phenomenon of the workpiece, the forging blank is difficult to punch out from the mold, resulting in the punch Produce plastic deformation, bending and fracture, short life, 800-1200 bolts per 10 punches; and use Q235 steel as punch material, heated by 950 ° C × 5 min salt furnace, 10% NACL brine after cooling and quenching It can be directly used without tempering. The hardness is 36HRC~40HRC. Each 10 punches can punch more than 2000 bolts, and the failure mode is plastic deformation, which eliminates the fragmentation phenomenon in the use of the punch and ensures the personal safety of the operator.

(3) 20CrMnTi steel pressed aluminum sleeve cold extrusion die D16, D20 type pressed steel wire rope aluminum sleeve cold extrusion die original CrWMn steel after quenching and tempering, the hardness is 45HRC ~ 50HRC, although the hardness requirement is lower in the cold extrusion die However, due to the serious non-uniformity of carbides in CrWMn steel, it is difficult to avoid mesh and banded carbides, resulting in chipping, cracking and early failure. The service life is only more than 1000 pieces, and some are only a few hundred or even a few. Ten pieces. The 20CrMnTi steel is used to make D16 and D20 aluminum sleeve cold extrusion die. After quenching by heating at 950 °C, it is used without tempering. The hardness of the mold is 46HRC~48HRC, and more than 2000 pieces of pressed aluminum sleeve are still used.

(4) 20Cr steel cold work die After the steel is strengthened by carburizing and quenching, the surface of the die can be obtained by fine needle-like tempered martensite with hardness of 58HRC~62HRC; the core is low carbon martensite structure, hardness 35HRC~40HRC, the matrix has high toughness, which can meet the performance requirements of high hardness, high strength and high toughness of cold working die. For example, automotive hose zinc alloy joint octagonal die punch, hardness requirements 58HRC ~ 62HRC. The octagonal mold originally made of Cr12MoV steel has a very short service life and often breaks less than 2,000 pieces. After replacing the punch material with 20Cr steel and after carburizing, the depth of the layer is 1.0mm~1.2mm and the hardness is 60HRC~62HRC. The service life has been increased to 30,000 pieces.

2 Low-carbon martensite high-temperature quenching of medium carbon alloy steel cold-working die For medium-carbon alloy steel cold-working die, the austenite grain can be more uniform and refined by increasing the austenitizing temperature, and the carbon content tends to decrease. The Ms point is increased, thereby obtaining more low-carbon martensite structure, improving the toughness of the mold, and prolonging the service life of the cold-working mold.

(1) High temperature quenching of 5CrMnMo steel cold heading machine cold rolling die The quenching temperature of 5CrMnMo steel is increased to 900 ° C ~ 950 ° C, which can obtain almost a single low carbon martensite structure. For example, the Z41.24 multi-station cold heading machine nut cold heading die made of 5CrMnMo steel is heated by 900 ° C ~ 930 ° C, graded quenching at 180 ° C, 200 ° C × 2 times low temperature tempering, the service life is better than Cr12MoV The steel is 2.6 times higher, which can completely replace the T10A, 9CrSi and Cr12MoV steels to make large-section cold heading molds.

(2) 60Si2MnA steel cold heading nut 4 step die high temperature quenching 60Si2MnA steel after high temperature quenching, the number of lath martensite in the structure increases, and even all lath martensite structure can be obtained, which has higher fracture toughness , impact toughness and excellent wear resistance. 60Si2MnA steel refrigeration 镦 nut 4 sequence die, after quenching at 920 ° C ~ 950 ° C, the service life is about 2 times higher than the normal temperature quenching die.

(3) 5CrW2Si steel cold shearing die high temperature quenching 5CrW2Si steel cold shearing die, after 950 °C quenching and 250 °C tempering, get lath martensite plus a small amount of evenly refined carbide and distributed in the slats The fine residual austenite structure at the boundary of the body can significantly improve the strength and toughness of the cold shear die cutting edge, and the life is increased by 1.5 to 5 times.

3High carbon steel cold work die low carbon martensite low temperature quenching high carbon steel cold work die quenching temperature is lower, austenite solid solution carbon content is also lower, Ms temperature is higher, the number of low carbon martensite is more . Therefore, the mold has high wear resistance and toughness, and the probability of deformation and cracking is small.
(1) T10A steel cold die low temperature short time heating quenching T10A steel cold die (light punch), according to the original process (780 ° C × 20min salt bath heating quenching) to obtain the twin martensite structure, there are micro cracks Under the impact load, due to poor toughness, chipping often occurs. Now it is heated and quenched by 750 °C×14min salt bath. After tempering at 200°C, fine flake martensite and low carbon martensite above 50% (volume fraction) can be obtained, which reduces microcracks. High hardness, high toughness, and nearly double the service life.

(2) Low-temperature quenching of CrWMn steel small molds Generally, CrWMn steel is used to manufacture small punches for watches. After conventional heat treatment, the service life of the molds is 1000-10000 times, and the damage forms are mostly partial chipping or whole head fracture. . The low carbon martensitic toughening treatment process of CrWMn steel compact punch is: 700 ° C × 10 min preheating, .790 ° C × (3 ~ 4) min salt bath furnace heating, after 180 ° C paraffin medium temperature 15 min , air cooling, hardness is 59HRC ~ 61HRC; after 200 °C × 2h tempering, the hardness is 58HRC ~ 60HRC, the sharpening life of the mold can reach 10,000 to 20,000 times, the total life can reach 10 to 200,000 times. Moreover, it is less prone to chipping and cracking during use, and exhibits extremely high fracture resistance.

(3) Cold-pressed LD steel cold extrusion punch of 7Cr7Mo3V2Si (LD) steel cold extrusion punch, after heating and quenching according to the conventional process of 1100 ° C ~ 1150 ° C, the service life is only 3000 ~ 4000 times. After heating to 1050 ° C × 25 min salt bath for oil cooling and 200 ° C × 2.5 h tempering, the lath martensite + dispersed carbide structure was obtained, the hardness was 60HRC ~ 61HRC, and the service life was increased to 20,000 times.

Application of low carbon martensite in hot working die

The high temperature heating of the hot work die steel can dissolve the alloy carbide in the steel into the austenite, improve the thermal strength and thermal stability of the matrix, and at the same time increase the tempering temperature accordingly, so that the slats in the structure can be The share of martensite has increased substantially, and the fracture toughness and small energy multi-shot life have increased. If the heating temperature is properly selected, the impact toughness and plasticity can be maintained at a certain level, so that the service life of the hot working die can be greatly improved. Therefore, the quenching heating temperature of the hot working die currently has a tendency to develop toward a high temperature. Another development trend of hot work die steel is carbon reduction and low carbonization. At present, some countries have included low carbon martensitic steel (≤0.25% C) in the hot work die steel standard. For example, the United States H25 steel (≤0.25% C), Germany 21CrMo10 steel (0.16% ~ 0.23% C), France 20MoNi34.13 steel (0.18% ~ 0.23% C) and so on. For product parts with simple shape, small production batch and short delivery time, low-carbon alloy structural steel can be used, especially low-carbon low-alloy heat-resistant structure containing 20CrMo, 20CrMnMo and other Mo-containing low-temperature heat-resistant structure with good high temperature strength and good creep resistance. The steel is made into a hot work die and directly subjected to intense quenching of low carbon martensite.

(1) High temperature quenching of 3Cr2W8V steel hot work die 3Cr2W8V steel hot work die After high temperature quenching and high temperature tempering process, more lath martensite structure can be obtained, which has higher tempering stability and high temperature strength. , high temperature wear resistance and thermal fatigue, can significantly improve the life of the mold. Using MCr × W8V steel M12 × 50 cylindrical studs (material 40Cr) hot forging die, using 1180 ° C heating quenching, 640 ° C × 2 h × 2 tempering, can get more than 90% of the lath martensite, the average use The life span has been increased from 3,000 to 5,000 pieces to 30,000 to 40,000 pieces.

(2) 5CrNiMo steel hammer forging die high temperature quenching 5CrNiMo steel 480mm × 450mm × 295mm hammer forging die, the original heat treatment process is 860 ° C × 5h oil quenching and 500 ° C × 10h tempering, hardness of 42HRC. When forging 2,500 pieces of gear blanks, the forging die will be plastically deformed and can be used after repairing the mold. Switch to the new process, 900 ° C × 5h oil quenching, 500 ° C × 10h tempering, hardness of 43HRC, forging the same gear blank 8100 pieces can still continue to use. Tests at other domestic plants have shown that the quenching temperature is increased from 860 ° C to 960 ° C, and the service life of the forging die can be increased by 2.5 times.

(3) Hot work die high temperature double quenching treatment 4Cr5MoSiV1 steel high temperature double quenching process: 1160 ° C quenching + 720 ° C tempering +1050 ° C quenching + 350 ° C tempering. Double quenching reduces the size and volume fraction of undissolved carbides and increases the spacing of carbide particles. When the average pitch of the second phase particles is increased, the fracture toughness can be increased. High temperature quenching can significantly reduce the twin martensite and increase the dislocation martensite, so that the mold has high fracture toughness. Therefore, the high-temperature double quenching can increase the fracture toughness by 30% to 40% compared with the ordinary quenching (1020~1050) °C, which is beneficial to increase the fatigue resistance of the hot work die steel and the resistance of the thermal fatigue cracking, so that the mold It has the best thermal fatigue properties to increase the service life of hot work dies.

(4) 20Cr steel hot forging die low carbon martensite strong quenching cable cold casting anchor cup cable buckle body is the key force fixed end piece of cable-stayed bridge, the technical requirement must be die forging. Since the contract quantity is only 88 pieces, the 20kg steel with high toughness, impact resistance and certain heat strength is used to manufacture the cable forging hot forging die. The processing flow is: blanking-forging-forging waste heat normalizing (deformation normalizing)-roughing-quenching, tempering-finishing. The heat treatment process is: 920 ° C heating, 10% NACL brine quenching, tempering at 430 ° C. The process not only eliminates quenching soft spots, improves mold hardenability, but also improves mold yield strength and impact toughness and ensures mold service life. The hot forging die of the cable joint body treated by the above process has an average hardness of 40HRC, and the metallographic structure is a fine lath martensite, and the contract task is satisfactorily completed.

(5) Q345B steel rivet welding hot forging die low carbon martensite strong quenching XKJ44.5mm ~ 48mm large stadium diagonal cable wire casting joint body is die forging (quality 98.5KG), because the batch is only 40 pieces, the thickness is selected The 40mm Q345B steel plate is made into a 400mm×400mm×290mm hot forging die by a riveting process. The process flow is: blanking - drilling - riveting - welding - machining - quenching - tongs repair. It is rapidly heated (20s/mm) in a box furnace at 950 °C. The brine is quenched and quenched and cooled, and it is used directly without tempering. The average hardness of the mold is 46HRC. After 40 pieces are completed, it can still be used.

Conclusion

(1) Low-carbon martensite has a relatively high toughness and is an ideal quenching structure for die steel.
(2) Low-carbon martensitic steel (≤0.25% C) is subjected to intense quenching and surface strengthening treatment, which can replace various molds for mold steel.
(3) The medium carbon steel mold adopts high temperature quenching to obtain more or even all low carbon martensite structure, which significantly improves the toughness of the mold.
(4) The high carbon steel mold adopts low temperature quenching, which can obtain more low carbon martensite structure and significantly improve the service life of the mold.
(5) Low carbon martensiticization of mold steel quenching structure, carbon reduction and low carbonization of die steel, which is an important development trend of heat treatment of die steel.

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