High manganese steel refers to alloy steel with manganese content above 10%. 1882, Englishman hadfield obtained high manganese steel with austenite structure for the first time, and obtained a patent in 1883, so the standard Mn 13 high manganese steel is also called hadfield steel. High manganese steel can be divided into two categories according to its different uses:
1, wear-resistant steel. Manganese is 10% ~ 15%, and carbon is generally 0.90% ~ 1.50%, most of which are above 1.0%. Its chemical composition (%): c: 0.90 ~1.50; Mn: 10.0 ~ 15.0； si:0.30 ~ 1.0； s:≤0.05； P: ≤ 0. 10 This kind of high manganese steel is used the most and has no magnetism. It is especially suitable for the working conditions of impact abrasive wear and high-stress grinding abrasive wear, and is often used to manufacture impact-resistant and wear-resistant castings, such as ball mill linings, hammer crushers, jaw plates of jaw crushers, mortar walls and crushing walls of cone crushers, bucket teeth and walls of excavators, railway switches, tractor and tank track plates, etc. High manganese steel is also used for bulletproof steel plates and safety steel plates.
The as-cast structure of high manganese steel is usually composed of austenite, carbide and pearlite, and sometimes contains a small amount of phosphorus crystals. When the amount of carbide is large, it often appears on the grain boundary in the form of a network. Therefore, as-cast high manganese steel is brittle and needs solution treatment. The common heat treatment method is solution treatment, that is, the steel is heated to1050 ~1100℃, and the as-cast structure is eliminated by heat preservation to obtain single-phase austenite structure, and then quenched with water to keep this structure at room temperature. After heat treatment, the microstructure is transformed into single austenite or austenite containing a small amount of carbide, and the strength, plasticity and toughness of steel are greatly improved, so this heat treatment method is often called water toughening treatment. High manganese steel is a typical wear-resistant steel, and its as-cast structure is austenite plus carbide. After solution treatment, there will still be a small amount of undissolved carbides in high manganese steel, which can also be used when the quantity is small enough to meet the inspection standards.
China's national standard for high manganese steel castings (GB/T5680- 1998) is: ZGMN13-1:C1.00-1.45, Mn1. zgmn 13-2:C 0.09- 1.35，Mn 1 1.0- 14.00，Si 0.30- 1.00，S≤0.04，P≤0.07。 Zgmn13-3: c0.95-1.35, Mn1.0-14.00, Si 0.30-0.80, S≤0.035, p. zgmn 13-4:c 0.09- 1.30，Mn 1 1.0- 14.00，Si 0.30-0.80，Cr 1.50-2.50，S ≤ 0。 zgmn 13-5:c 0.75- 1.30 Mn 1 1.0- 14.00，Si 0.30- 1.00，Mo 0.90- 1.20。
High manganese steel has good manganese increasing effect. Improve the stability of austenite and prevent the precipitation of carbide, thereby improving the strength and plasticity of steel and improving the work hardening ability and wear resistance of steel. For example, the service life of ZGMn 18 railway turnout used in the north is 20%-25% longer than that of ZGMn 13. At present, many wear-resistant high manganese steel manufacturers in the market simply reheat with waste manganese steel in order to reduce costs. The manganese content of the products is not up to standard, let alone Cr and Mo content, but the sulfur and phosphorus content are too high. The "70 1" brand high manganese steel products of Guofan Industrial and Mining Company and Ye Xiang Machinery are based on Mn 13Cr2, Mn 13Mo and Mn65438+. Manganese content is higher than 13%. Ultra-high manganese steel products are all cast according to Mn 18Cr2, Mn 18Cr2Mo and Mn 18Cr2MoV standards. Mn content is higher than 18%, which effectively removes sulfur and phosphorus impurities. Chromium, molybdenum and vanadium can increase wear resistance and impact resistance. Therefore, the wear resistance and impact resistance of the products reach or even far exceed the national standards Mn 13-4 and Mn 13-5 (ultra-high manganese steel does not exceed the national standards, but the manganese content should be greater than 18%). Although the casting cost is greatly increased, it has great advantages in crushing high-hardness stones.
In the process of cold deformation under impact load, steel is strengthened due to the large increase of dislocation density, dislocation transmission, dislocation plugging and the interaction between dislocation and solute atoms. This is an important reason for work hardening. Another important reason is that the stacking fault energy of high manganese austenite is low, which is easy to occur during deformation, thus creating conditions for the formation of ε martensite and deformation twins. In the deformation hardening layer of high manganese steel with conventional composition, high density dislocation, dislocation plugging and entanglement can often be seen. The appearance of ε martensite and deformation twins makes it difficult for steel to deform, especially the latter. All the above factors have greatly strengthened the hardened layer of high manganese steel and greatly improved its hardness.
The most important feature of high manganese steel with austenitic structure is that under the conditions of strong impact and extrusion, the surface layer quickly undergoes plastic deformation. Due to deformation strengthening, the deformation layer has obvious work hardening phenomenon, and the surface hardness is greatly improved (HB (Brinell hardness) can reach 300-400, and the material surface can reach HB500-550 after being treated by anti-wear technology, and HB500-800 under high impact load. The depth of hardened layer can reach 10-20mm under different impact loads. Under the condition of strong impact abrasive wear, high manganese steel has excellent wear resistance, which makes it maintain the good toughness and plasticity of central austenite, and the hardened layer has good wear resistance, so it is often used to make wear-resistant parts. This is beyond the reach of other materials. Under the condition of low impact, because the work hardening effect is not obvious, high manganese steel can not give full play to the characteristics of the material. The wear resistance of high manganese steel is superior only when there are enough conditions to form work hardening, but it is poor in other cases. Due to work hardening, a small amount of high manganese steel is processed by forging, so it is necessary to avoid processing castings as much as possible. Holes and grooves in castings should be cast as much as possible. But the processing of high manganese steel is not completely impossible. Tool dressing can be carried out after one feed, and the inevitable machining should be enlarged in the casting process design so that the machining feed can avoid the work hardening layer.
The castability of high manganese steel is better. The melting point of steel is low (about 1400℃), the temperature range between liquid phase and solid phase of steel is small (about 50℃), the thermal conductivity of steel is low, the fluidity of molten steel is good, and it is easy to cast. The linear expansion coefficient of high manganese steel is 1.5 times that of pure iron and twice that of carbon steel, so the volume shrinkage and linear shrinkage are large during casting, and stress and cracks are easy to appear.
In order to improve the properties of high manganese steel, many studies have been carried out in alloying, microalloying, carbon and manganese content adjustment and precipitation strengthening treatment, and they have been applied in production practice. Compared with cast steel, the appearance of metastable austenitic manganese steel can greatly reduce the content of carbon and manganese in steel and improve the deformation strengthening speed of steel, which is suitable for high and medium-low impact loads and is a new development of high manganese steel.
High manganese steel is mainly used in severe working conditions such as impact, extrusion and material wear. The main failure forms are wear and tear, and some parts are broken and deformed. There are three kinds of wear: friction wear in which the surfaces of metal parts contact and move; Abrasive wear caused by the impact of other metal or nonmetal materials on the metal surface and erosion wear caused by the contact between flowing gas or liquid and metal. The wear resistance of wear-resistant steel depends on the material itself, and wear-resistant steel shows different wear resistance under different working conditions, and both the material itself and working conditions can determine its wear resistance. Austenitic manganese steel is the main casting wear-resistant steel and wear-resistant steel. Low alloy steel with proper heat treatment also has good effect under certain conditions, while graphite steel is used for lubrication and friction conditions.
2. Non-magnetic steel. The manganese content of this steel is more than 17%, and the carbon content is generally lower than 1.0%, which is often used to manufacture retaining rings in automobile industry. The density of this steel is 7.87-7.98g/cm3. Due to the high content of carbon and manganese, the thermal conductivity of steel is poor. The thermal conductivity is12.979 w/(m℃), which is about 1/3 of that of carbon steel. Because steel is austenitic and nonmagnetic, its magnetic permeability μ is 1.003- 1.03(H/m).
Second, the factors affecting the mechanical properties of high manganese steel.
1, the influence of carbide on properties. Reduce the impact toughness and tensile strength of high manganese steel.
2. Effect of nonmetallic inclusions on properties of high manganese steel. When molten steel solidifies, a large amount of manganese oxide precipitates in the form of nonmetallic inclusions on the periphery of steel, which reduces the impact toughness of steel and increases the tendency of hot cracking of castings.
3. Selection of chemical composition of high manganese steel and its influence on properties. (1) carbon content and manganese content. When the carbon content in steel is too low, it is not enough to produce effective work hardening effect; When the carbon content is too high, a large number of carbides, especially coarse carbides, will appear as cast. Therefore, in order to avoid carbide precipitation, the carbon content must be controlled not to be too high. In order to ensure the performance of high manganese steel, there must be enough manganese content. When the manganese content is too low, a single austenite structure cannot be formed; However, too high manganese content is unnecessary. Generally speaking, WMn is controlled at11.0%-14.0%, and WC is controlled at 0.9%- 1.3%. It should be pointed out that the manganese content and carbon content should be properly matched, that is, there should be an appropriate manganese-carbon ratio, which is generally controlled at Mn/C= 10. (2) Silicon content. The specification content of Wsi in high manganese steel is 0.3%-0.8%. Silicon will reduce the solubility of carbon in austenite, promote carbide precipitation and reduce the wear resistance and impact toughness of steel, so the silicon content should be controlled at the lower limit of the specification. (3) Phosphorus content. The specification content of high manganese steel is Wp≤0.7%. When smelting high manganese steel, the phosphorus content in ferromanganese is relatively high. Because phosphorus will reduce the impact toughness of steel and make castings easy to crack, the phosphorus content in steel should be reduced as much as possible. (4) Sulfur content. The specification of high manganese steel requires Ws≤0.05%. Because of its high manganese content, most of the sulfur and manganese in steel combine with each other to form manganese sulfide (MnS), so the sulfur content in steel is often low (generally less than 0.03%). Therefore, the harmful effect of sulfur in high manganese steel is higher than that of phosphorus.
Third, the casting process of high manganese steel.
Under the working conditions of high energy impact, high manganese steel and ultra-high manganese steel castings have a wide range of applications. Many foundries lack the necessary knowledge about producing this kind of steel castings. The specific operation is briefly described for the reference of producers. High manganese steel castings mostly adopt sand molding process scheme, and sand casting is widely popularized because of its mature technology and high production efficiency. There is also a special casting method, which is obviously different from sand casting in mold material, modeling method, filling form of molten metal and solidification conditions of metal in mold. Special casting includes investment casting, permanent mold casting, pressure casting, low pressure casting, back pressure casting, extrusion casting, centrifugal casting, lost foam casting, gypsum casting, ceramic precision casting, continuous casting, vacuum suction casting, fine grain casting, electromagnetic casting and so on.
1, chemical composition. High manganese steel is divided into five grades according to national standards, the main difference is carbon content, ranging from 0.75% to 1.45%. The greater the influence, the lower the carbon content. The manganese content is between11.0%-14.0%, and generally should not be lower than 13%. There is no national standard for ultra-high manganese steel, but the manganese content should be greater than 18%. Silicon content has a great influence on impact toughness, so the lower limit should be taken, and it is appropriate to not exceed 0.5%. Low phosphorus and sulfur is the most basic requirement. Because high manganese content naturally plays a role in desulfurization, reducing phosphorus is the most important thing, and try to reduce phosphorus below 0.07%. Chromium can improve the wear resistance, generally around 2.0%. Molybdenum can improve the hardness, generally around 1.0%.
2. charge. The material fed into the furnace is determined by the chemical composition. The main charges are high-quality carbon steel (or steel ingot), high-carbon ferromanganese, medium-carbon ferromanganese, high-carbon ferrochromium and high-manganese steel. Some people mistakenly think that as long as the chemical composition is suitable, they can use more recycled materials, which is the reason why the quality of products in some factories is poor. Not only high manganese steel and ultra-high manganese steel, but also all metal castings shall not use too much return charge, and the return charge shall not exceed 25%.
Step 3 melt. Pay attention to the feeding order. Carbon steel is always smelted first, whether in intermediate frequency furnace or electric arc furnace, and various precious alloy materials such as ferromanganese should be put into the furnace several times, and finally a small amount of precious elements should be added to reduce the burning loss. The block should be as small as possible, preferably 50-80 mm After melting, when the furnace temperature reaches 1580- 1600℃, aluminum wire, calcium silicon alloy or SiC can be used for deoxidation, dehydrogenation and denitrification. The deoxidizer must be pressed deep into the furnace. At this time, the metal liquid level is tightly covered with covering agent to isolate the outside air. Calm down for a period of time, so that oxides and inclusions have enough time to float. However, many enterprises just spread aluminum wires or even aluminium scrap on the metal surface without covering it, which is a waste. At the same time, the contents of manganese and carbon are adjusted in time with medium carbon ferromanganese. Before the molten steel is tapped, the ladle must be baked above 400℃. Modified with trace elements such as ferrovanadium, ferrotitanium and rare earth. It is a necessary means to refine primary crystallization, and its influence on product performance is very important.
4. Loading and molding materials. It is necessary to distinguish the properties of steel and lining. Manganese steel is alkaline, and the furnace lining is of course magnesium oxide. Tamping furnace lining shall be carried out in turn, and transposition operation shall be repeated. The lining material should not be too thick, about 80 cm at a time, and it should be baked at low temperature for a long time after tamping. During operation, the furnace charge should be placed next to the furnace mouth for preheating, then the furnace charge should be placed slowly next to the furnace mouth for preheating with a clamp, and then the furnace charge should be placed slowly along the furnace wall with a clamp. Modeling materials and coatings should also be consistent with the properties of molten metal, or use neutral materials (such as chromite and brown corundum). If you want to obtain a refined aggregate, it is correct to use chromite with large heat storage capacity, especially in EPC foundry, which will overcome the shortcomings of slow heat dissipation.
5. Casting process design. High manganese steel is characterized by large solidification shrinkage and poor heat dissipation. Accordingly, the shrinkage of castings in process design is 2.5%-2.7%, and the longer the casting growth time, the higher the upper limit should be. The concession between molding sand and sand core must be good. The gating system is on. A plurality of dispersed internal sprues are introduced from the thin wall of the casting, and they are flat and wide trumpet-shaped, and the cross-sectional area near the casting is larger than that associated with transverse sprues, so that molten metal can be injected into the mold quickly and smoothly, and the temperature difference in the whole mold can be prevented from being too large. The diameter of the riser is larger than that of the hot spot, close to the hot spot, and the height is 2.5-3.0 times of the diameter. It is necessary to use hot risers or even pouring risers to make enough high-temperature molten metal to make up for the vacancy of castings during solidification and shrinkage. Put the sprue and riser in a high place (there are 5-8 sand boxes. ) is also correct. When pouring, pour as soon as possible at low temperature. Once solidified, loosen the sand box in time. It is necessary to make good use of chilled iron, including inner chilled iron and outer chilled iron, which can not only refine the primary crystal, eliminate shrinkage cavity and porosity, but also improve the process yield. The inner cooling iron should be clean and easy to melt, and the dosage should be less. The functional relationship between the three-dimensional size of the external cooling iron and the three-dimensional size of the cooled object is 0.6-0.7 times. Too small won't do, too big will cause the casting to crack. Castings should be placed in the mold for a long time until the temperature is lower than 200℃ before unpacking.
6. Heat treatment. Heat treatment cracking is caused by excessive temperature rise in the low temperature stage. Therefore, the correct operation is below 350℃, the heating rate is 750℃, the casting is in a plastic state, and the temperature can be raised quickly. When the temperature reaches 1050℃, the holding time is determined according to the casting thickness, and then it is increased to above 1 100℃. Leave room for the furnace to cool down and enter the water as soon as possible. At high temperature, too slow temperature rise, too short heat preservation time and too long time interval (not more than: 0.5 minutes) from discharging to water inflow all affect the casting quality. Inlet water temperature should be
7. Cutting and welding. When high manganese steel is reheated, there is a brittle temperature range of carbide precipitation between 250-800°C, and there are reticular carbides and casting stress in as-cast high manganese steel, so the welding performance is very poor. For high manganese steel castings, the riser should be cut or welded to repair the defects after water toughening treatment, and the castings should be cooled quickly after welding. Due to the poor thermal conductivity of manganese steel, great attention should be paid to cutting risers. It is best to put the casting in water to expose the cutting part, leave some stubble when cutting, and grind it off after heat treatment. In order to eliminate or reduce the heat affected zone as much as possible, the operation methods of small current, weak arc, intermittent welding, small bead multi-layer welding or pouring while welding are adopted, and the low temperature and low heat are always maintained. Tap during welding to relieve stress. Important castings must be inspected. Covered electrode adopts high manganese steel covered electrode or austenitic stainless steel covered electrode (austenitic D256 or D266 manganese nickel covered electrode), with slender specifications, φ3.2mm×350mm, and the external coating is alkaline. If there is work hardening layer, it should be removed before welding.
8. Matters needing attention in production. What producers should consider is not only to reduce production costs, but more importantly, not to produce waste products, to maximize the production of high-quality products, and then to maximize market share. This seems slow and expensive, but it's actually fast and economical.