Induction Furnace Smelting of ZG120Mn13Cr2 High Manganese Steel Castings
Abstract: The effect of chemical composition on the properties of ZG120Mn13Gr2 high manganese steel was studied. The key technology of smelting ZG120Mn13Cr2 high manganese steel by induction furnace is proposed. The performance of ZG120Mn13Gr2 steel smelted with magnesia lining and silica sand lining induction furnace is compared. The results show that the performance of ZG120Mn13Gr2 steel smelted with magnesia lining induction furnace is better than that of ZG120M13Cr2 steel smelted with silica sand lining induction furnace. The problems of smelting high manganese steel with silicon sand furnace lining induction furnace are pointed out.
Among the wear-resistant cast steels, high-manganese steel is known for its strong impact resistance and good toughness. When subjected to a large impact load or extrusion stress, the surface austenite will cause work hardening due to the force, the hardness is increased to more than 500 HBW, and it has high wear resistance. This good combination of high surface hardness and high internal toughness makes high manganese steel a high-performance wear-resistant steel with internal and external softness, high impact load resistance, and high abrasion resistance without cracking. However, the high hardness of high-manganese steel is low. For the working conditions with insufficient impact hardening ability, its excellent wear resistance cannot be fully exerted. Working condition. This paper introduces the process control of induction furnace smelting production of ZG120Mn13Cr2 high manganese wear-resistant steel, as well as some experience and experience in stabilizing and improving the metallurgical quality of high manganese steel wear-resistant products and reducing scrap rate.
Control of Chemical Composition of ZG120Mn13Cr2 High Manganese Wear-resistant Steel
1.C content and Mn Content:
C has an important influence on the wear resistance and impact toughness of ZG120Mn13Cr2 high manganese wear-resistant steel. Within a certain range, with the increase of C content, the wear resistance of steel increases and the impact toughness decreases accordingly. When the C content is low and the Mn content is high, it is not enough to produce an effective work hardening effect, the toughness of the steel is high, and the hardness is low. When the contents of C and Mn are too low, part of the martensite structure will appear after heat treatment, making the high manganese steel workpiece brittle and prone to early fracture during service; when the C content is too high and the Mn content is insufficient, After heat treatment, the network carbides in the austenite grain boundary cannot be eliminated, and it is inevitable that coarse carbides will precipitate, which will reduce the overall performance of the high manganese steel and also cause the early fracture of the high manganese steel workpiece during use. In general, ZG120Mn13Cr2 wear-resistant castings that are used under low temperature or strong impact conditions should have a C content of less than 1.15% in order to ensure sufficient toughness; service under strong impact loads and require both high wear resistance and requirements For high impact toughness, the content should be controlled at 1.15% to 1.25%; for non-strong impact service conditions, in order to increase wear resistance, the C content can be increased to 1.35%. Under the service conditions of thick wall parts, complex structural parts and hard materials with strong impact or extrusion, the low C content should be selected. Thin-walled parts and soft materials, low stress wear conditions should choose a higher C content. Mn is the main alloying element in high-manganese steel and the main element of stable austenite. In order to ensure that high-manganese steel forms a single austenite structure, sufficient Mn content is needed. The content of Mn is in the range of 9% -14%. With the increase of Mn content, the impact toughness of steel increases rapidly, especially the improvement of low temperature impact toughness2. However, Mn is an element that strongly promotes the growth of austenite grains, which can make the austenite grains coarse and easily produce columnar crystals, and also increase the amount of condensation shrinkage of high manganese steel to form thermal cracks. The most appropriate reduction of Mn content can reduce the stability of austenite and promote the work hardening ability of high manganese steel. Excessively high Mn content is not conducive to the work hardening performance of high manganese steel. The content of Mn in high-manganese steel and the content of C are mainly determined by factors such as working conditions, casting wall thickness and structural complexity, and the two should have an appropriate combination. / C (manganese-carbon ratio) = 9-11, thin-walled, simple parts under low-stress service conditions, containing Mni generally less than 12%, to facilitate work hardening, improve wear resistance, but toughness decreases; under high stress conditions The thick-walled, complex-structured parts in service should have a suitably high Mn content, generally greater than 12%, to prevent carbide precipitation, improve toughness, and prevent the workpiece from breaking during use.
Because the yield strength of high-manganese steel is low, it can not fully exert its strength. Therefore, adding 2% chromium to ZG120Mn 13Cr2 ф improves the yield strength and initial hardness of the steel, overcomes the plastic rheology caused by the impact, and strengthens the workpiece The ability to resist deformation and improve wear resistance. At the same time, Cr can also reduce the stability of austenite, improve the work hardening properties of steel, and enhance the wear resistance. A certain amount of Cr content can also improve the hardenability and oxidation resistance of steel, and improve the high temperature wear resistance and wet corrosion resistance of workpieces. But the addition of Cr will reduce the impact toughness of high manganese steel, especially the low temperature impact toughness. During smelting, the Cr content is often controlled at about 1.8%.
The main function of silicon in high-manganese steel is deoxidation, followed by silicon can significantly improve the yield strength of high-manganese steel, and with the increase of Si content, the yield strength of steel increases, while the tensile strength does not change much. However, Si and Mn-like elements are overheat-sensitive. Increasing the Si content can reduce the solubility of carbon in austenite, promote the precipitation of carbides, and make the carbides coarse, thereby making the steel grain coarse, anti-wear performance and Impact toughness is reduced. If the Si content is greater than 0.6%, it will easily lead to coarse grains of high-manganese steel, and the impact toughness of the steel will be significantly reduced. When smelting ZG120Mn13Cr2 high manganese wear-resistant steel, the Si content is generally controlled at 0.40% ~ 0.55%. When the Si content is below 0.5%, with the increase of Si content, the metallurgical quality is improved due to deoxidation, and the toughness of high manganese steel is also improved.
4.S Content And P Content
S and P are harmful elements in steel, but the content of S in high-manganese steel is low. This is because high-manganese steel contains a large amount of Mn. During the refining process, S and Mn form a high-melting MnS inclusion that floats into the slag. S also reacts with lime to form Cas, which also enters the slag and is removed by scraping. When tapping, rare earth is added to further remove S, so the harmful effect of S in high-manganese steel is small. However, high manganese steel will contain higher P. Due to the large amount of ferromanganese (up to 0.4% P content) that must be added in the later stage of smelting, a large amount of P is also brought in while increasing manganese, so it is generally high Manganese steel has higher P content than other steel types. P can deteriorate the mechanical properties and casting properties of high manganese steel and reduce the toughness, plasticity and strength of the steel. The high P content will promote the generation of cold cracking and heat treatment cracks in high manganese steel, and during the service process of the workpiece, the brittle P eutectic zone will initiate cracks under repeated loads and cause the cracks to gradually expand under stress. Cause the workpiece to break. When the content of P is higher than 0.06%, the elongation of steel decreases, and it is easy to cause cracks. Reducing the P content of high manganese steel will effectively prevent the occurrence of cracks and early fracture of the workpiece, and increase the service life of the workpiece. High P content will also directly reduce the high-temperature plasticity of high-manganese steel, making it very sensitive to hot cracking, and the P segregation of thick-walled castings is particularly serious, which will exacerbate the harm to high-manganese steel. In daily smelting, the effect of P segregation is generally reduced by appropriately reducing the C content, because the high C content will exacerbate the P segregation, and the interaction between C and P makes the crack tendency of the workpiece more serious. However, reducing the C content will reduce the wear resistance of the steel, so the lower the P content, the better. The P content of important castings should be less than 0.04%. Low P and low S are the most basic requirements for high manganese steel.
Main Points of Process Control For Smelting ZG120Mn13Cr2 High Manganese Wear-resistant Steel Induction Furnace
P-reduction is one of the key technical points of smelting ZG120Mn13Cr2 high manganese wear-resistant steel. However, it is difficult to remove P by oxidation method when using induction furnace to smelt high-manganese steel. In actual practice, the following measures can be taken to control and remove P. (1) At least 40% of low-C and low-P clean scrap steel should be mixed in the batching, and high manganese steel cannot be used to return the material, and it is best to control the average P content of the raw materials used to be below 0.05%. Controlling the content of P in the smelting process does not exceed the standard and improving the performance of the casting can play a guarantee role. (2) Use bottom slag dephosphorization to remove P. When charging, the bottom of the induction furnace is filled with metallurgical lime, which accounts for about 2% of the weight of the charge. During the melting of the charge, a small lime block is formed on the molten pool to make alkaline slag. The P in the steel reacts with the slag to produce calcium phosphate and enter the slag. Note that the P removal operation requires an alkaline lining, and the furnace temperature should not be too high (<1500 ℃), after sufficient P removal, then add new slag material to make new slag. (3) High-quality, low-P high-carbon and medium-carbon ferromanganese should be used for pre-deoxidation and alloying. For important castings, an appropriate amount of electrolytic manganese should be used for batching to prevent the return of P in the later stage of the refining and make the P content higher.
2.Control of Mn / C
Mn / C (manganese-carbon ratio) is a key factor in the performance of high-manganese steel and a key control index in the smelting process of high-manganese steel. For a specific workpiece, the appropriate Mn / C determines the toughness, strength, wear resistance and service reliability of the casting. Practice shows that when Mn / C <10, carbides are distributed on the austenite matrix of ZG120Mn13Cr2 high manganese wear-resistant steel, the wear resistance is improved, the toughness is reduced, and fracture phenomenon is easy to occur during use; control Mn / C> 10, water After toughening, a single austenite can be obtained, and then there will be a good combination of toughness and wear resistance. It is necessary to clearly grasp and strictly control the original C content, Mn content of the raw materials of the ingredients, and the carbon and manganese addition of a large number of ferroalloys during alloying. Therefore, the rapid analysis of C, Mn, Si, P, Cr elements during smelting is very necessary and important, which is of great significance for the adjustment of the chemical composition of the molten steel in the later stage of smelting and the production of qualified molten steel.
3.Furnace Temperature Control During Smelting
At the end of the melting process, the furnace temperature should be controlled not to be too high (furnace temperature <1500 ℃) to facilitate the removal of P. If the melting temperature is too high (melting temperature> 1550 c), the molten steel will have poor P removal effect, easy to get air, and steel slag The slag is difficult to clean, and the erosion effect of the molten steel on the furnace village is increased, increasing the amount of foreign inclusions in the molten steel. Moreover, if the smelting temperature is too high, the grains of the steel are likely to be coarse, and the waiting time for pouring is prolonged. But as a high-carbon and high-alloy steel, the smelting temperature should not be too low, otherwise it will prolong the melting time of the ferroalloy, increase the amount of alloy burnout, and the chemical composition of the molten steel is not uniform, which is not conducive to the production of qualified molten steel. In actual refining, the furnace temperature is usually controlled at 1500 ~ 1550 C in the later stage of smelting, and the tapping temperature is controlled at about 1530 ℃.
4.Control of Inclusions In Steel
To minimize the amount of foreign inclusions in the molten steel, use a well-knotted furnace lining to improve the quality of the furnace lining construction and reduce MgO foreign inclusions. It is also necessary to use high-quality, clean raw materials, and strengthen deoxidation to reduce inclusions in the smelting process. Poor deoxidation of high-manganese steel will produce a large number of Mno inclusions, which have a high melting point and are not easily melted in the molten steel. Finally, they are concentrated on the grain boundary, which deteriorates the mechanical properties of the steel, reduces the toughness of the steel, and generates heat. The main factor of hot cracking of manganese steel. High manganese steel is easy to cold crack and hot crack, and it is a casting steel that is easy to crack. To prevent thermal cracking, deoxidation is the key. Pre-deoxidation + final deoxidation and final deoxidation use pure aluminum and use rare earth to strengthen deoxidation.At the same time, the molten steel is subjected to composite metamorphism treatment. The deoxidized products are easy to float up.The secondary deoxidized products produced when the molten steel solidifies are less than 5, thereby improving the high manganese steel. Metallurgical quality. After the final deoxidation of molten steel and aluminum, the deoxidation capacity will gradually decline, and the pouring should be completed within 10 minutes.
5.Pre-deoxidation And Alloying
When smelting ZG120Mn13Cr2 high manganese wear-resistant steel, ferrochromium should be added together with the high manganese steel return material at the end of melting, and a large amount of ferromanganese that needs to be added is divided into two after the furnace material is melted, the slag is dephosphorized, and the sample test is performed. Add three batches to the molten pool (if equipped with high-carbon fierce iron, high-carbon fierce iron should be added for pre-deoxidation) to prevent the molten pool from cooling too much, prolong the smelting time, and increase the amount of alloy loss. The amount of ferrosilicon added is very small. When the furnace temperature is close to 1 530 ℃ 7-10 min before tapping, the slag layer is pushed away and added to the molten pool for deoxidation. After the ferrosilicon is melted, take the molten steel as a cup sample and check the deoxidation situation.If the shrinkage is good, then change the slag, power off and clean the scum to get out of the steel. After enhanced deoxidation, slag is tapped to make steel.
6.Grain refinement and composite final deoxidation
The as-cast crystalline structure of high-manganese steel is often relatively coarse, which will directly affect the structure and performance after heat treatment. Adding rare earth modifier to the high manganese steel can obviously refine the as-cast structure, improve the shape, size and distribution of inclusions, and greatly reduce the harmful effects of inclusions. Rare earth and s, N, H, 0 and other elements in the molten steel form rare earth compounds into the slag, which can purify the molten steel, further desulfurize, deoxidize and fix nitrogen and solid oxygen, and reduce the blowhole defects and hydrogen embrittlement hazards of castings. The rare earth also forms a high melting point compound with MnO, Feo, etc. in the molten steel. The compound is already in a solid state before the molten steel solidifies, thus forming a dispersed distribution of crystals during the solidifying process of the molten steel
The core greatly improves the distribution of inclusions in the grains, refines the austenite grains, hinders the generation of hot cracks, inhibits the growth of coarse columnar crystals, and thus improves the impact resistance of high manganese steel serving under strong impact loads Performance and wear resistance. The greater the impact load, the harder the material and the more irreplaceable the rare earth. The addition of rare earth can not only improve the yield strength and impact toughness of high manganese steel, but also enhance the work hardening ability of high manganese steel, accelerate the speed of work hardening, and also improve the fluidity of molten steel and enhance the filling of molten steel. Type ability. Method: Insert 1.5 kg / t aluminum wire or aluminum block to the bottom of the molten steel in the furnace for final deoxidation before tapping. When tapping, 3 kgt of 1 “rare earth ferrosilicon alloy is punched into the ladle to strengthen the final deoxidation, at the same time, the composite modification is carried out and the crystal Grain refinement. Practice has shown that this treatment method has a significant effect on purifying molten steel, refining the structure, eliminating columnar crystals, improving metallurgical quality and improving the comprehensive mechanical properties of steel.
7 Pouring of Molten Steel
The casting temperature of high manganese steel should be reduced as much as possible on the premise of ensuring a good casting shape, but it should not be too low, otherwise it will cause defects such as unclear casting contours, insufficient casting and porosity. The casting temperature of high-manganese steel has a significant effect on the primary crystalline structure. The thickness of the as-cast structure is very sensitive to the casting temperature. The higher the casting temperature, the coarser the grains and the lower the impact toughness. When the pouring temperature exceeds 1460 ℃, coarse crystals and columnar crystals are easily generated, and the grain boundaries and inclusions of coarse grains are often the source of fracture cracks. Generally, the casting temperature should be controlled according to the wall thickness of the casting at 1 400-1 440 ℃, the greater the wall thickness, the lower the casting temperature. Thin-walled and complex-structured parts should be poured quickly to avoid insufficient pouring, and the riser should be refilled in time. It is very beneficial to reduce the cracks of the casting after the pouring is timely. In addition, in order to prevent high manganese steel castings from chemical sticky sand defects, corundum powder coating should be applied to the sand part in contact with the molten steel and dried.
ZG120Mn13Cr2 High Manganese Wear-resistant Steel Induction Furnace Smelting Production Examples
1 Using Alkaline Lining
1. Smelting Equipment and Production Conditions
A 500 kg intermediate frequency furnace is used to produce the tooth cap castings of the crusher of the coal preparation plant by the non-oxidizing method. The material is ZG120Mn13Cr2. The weight of the casting is 43 kg and the main wall thickness is 55 mm. Magnesia furnace lining is used, quicklime is used for slagging material, and temperature is measured by thermocouple. For ferroalloys, high-carbon ferrochrome, high-carbon ferrite, medium-carbon ferromanganese and 65 ferrosilicon, with a block size of 50-80 mm. Scrap steel plate scrap, I-beam, ZG25 steel return material, etc., no high manganese steel return material. The final deoxidizer and modifier are made of aluminum wire and 1 “rare earth ferrosilicon alloy (grain size 0.5-3mm), and poured with 60kg small ladle.
2. Smelting Production Process Control
Before charging, metallurgical lime, which accounts for about 2% of the weight of the charge, is placed in the bottom of the furnace. After the charge is melted, pay attention to the dephosphorization operation at the low temperature stage. If the steel slag is too thick, it can be adjusted with a small amount of fluorite. Throughout the smelting process, if there is molten steel exposed in the air, slag should be added in time to maintain coverage. Sampling analysis and adding high carbon ferrochromium and heating according to the c.s.Mn, Si.P.Cr element test results and ingredient calculation results. To add ferromanganese, first add high-carbon ferromanganese, and then add medium-carbon ferromanganese in two batches. When the ferromanganese melts quickly, push the slag layer and add ferrosilicon. After the ferrosilicon is melted and the temperature of the molten steel reaches 1530 ℃, take the molten steel as a cup sample, observe the shrinkage is good, change the slag, and then power off to remove the slag to produce steel. Before tapping, put 1.5 kg / t of aluminum wire into the small ladle according to the calculated amount, and when the molten steel is filled with 1/3 of the ladle, pour into 3 kg / prepared 1 “rare earth ferrosilicon alloy. Sprinkle with covering agent and let stand for 4-6 min. When measuring the temperature of molten steel about 1420 ℃, remove the scum in the ladle and pour quickly.
3. Chemical Composition of Molten Sample and Finished Sample and Quality of Finished Casting
Taking the 10.6-1 furnace as an example, the chemical composition of the 10.6-1 furnace molten steel and the finished product using the alkaline lining smelting are shown in Table 1. After the sand of this furnace is cleaned, the surface is neat, the outline is clear, there are no casting defects, and the appearance quality is better.
2 Using Acid Refractory Lining
1.Smelting Equipment and Production Conditions
At that time, due to the lack of magnesia furnace lining material, and eager to produce and deliver, tried to use the newly knotted silicon sand furnace lining with only 3 furnaces of ZG45 steel, using a 500 kg intermediate frequency furnace, and using non-oxidation smelting to produce coal crusher teeth Ring hammer casting, material ZG120Mn13Cr2, single casting weight 47 kg, wall thickness 70 mm. Adopt cast steel slag removing agent and thermocouple for temperature measurement. Steel plate scrap for scrap steel, 25 “round steel material head, old gear ring hammer (the same type of steel return material, 60%). Ferro-alloy high carbon ferrochrome, medium carbon ferromanganese and 65 ferrosilicon, block 50 -80 mm. The final deoxidizer and modifier are made of aluminum wire and 1 “rare earth ferrosilicon alloy (grain size 0.5 ~ 3 mm). Use a 60 kg small package to pour and pour.
2. Smelting Production Process Control
Because a large number of old gear ring hammers are used as the charge, the C, Si, P content is relatively high, and the acid furnace lining is used without removing P, so the content of C, Si, P is controlled to ensure the Mn content. First select low-C, P clean scrap steel and high-quality iron alloy, ferrochrome and old gear ring hammer to join at the end of melting. Sampling and analyzing the elements C, Si, Mn, Cr after smelting the furnace charge, and performing slag replacement operation. High-carbon ferrochromium is added according to the test results and ingredients calculation results, and medium-carbon ferromanganese is added in two batches. When the fierce iron will be melted, push the slag layer away and add a small amount of ferrosilicon (silicon will not be added when the amount of silicon reaches 0.4%, instead of 0.1% calcium silicon block). , Take the molten steel as the round cup sample, observe the shrinkage, when it is good, change the slag, and then power off to clean the slag to make steel. The addition of the final deoxidizer and modifier and the pouring of molten steel are the same as the smelting example of the alkaline intermediate frequency furnace described above. 3.2.3 The chemical composition of the molten sample and the finished product and the quality of the finished product are shown in Table 2 after the molten steel of No. 2 furnace melted with acidic lining and the chemical composition of the finished product are shown in Table 2. defect.
3. The chemical composition of the clean samples and finished products and the quality of the finished products are shown in Table 2. The molten steel of No. 2 furnace melted with acidic lining and the chemical composition of the finished products are shown in Table 2. defect.
4.Problems With ZG120Mn13Cr2 High Manganese Wear-resistant Steel By Acid Furnace
(1) Due to the use of acidic furnace lining and no dephosphorization, P tends to be too high, which is not conducive to improving the overall performance of castings. It is difficult to guarantee the service life of castings under strong impact conditions.
(2) The Mn burns seriously. Only the excessive amount of ferromanganese can make the content of Mn reach the standard, which consumes a lot of ferromanganese and electricity, increases the production cost, and passively increases the content of C, Si, P.
(3) Mn / C is difficult to control properly, which will seriously affect the impact toughness, wear resistance and reliability of high manganese steel castings.
(4) During the refining process, the molten steel erodes the furnace lining seriously, and the risk of the molten steel passing through the furnace is very large. Only 5 furnaces are smelted, and the furnace lining can no longer be used.
- ZG120Mn13Cr2 high manganese steel produced by induction furnace smelting should be knotted with high-quality magnesia furnace lining, which has high refractoriness, long furnace age, and can be dephosphorized. Liquid steel.
- If an acid lining that does not match the properties of the high-manganese steel is used, the furnace age is short, the molten steel contains high P and the Mn content is low, and Mn / C will be inappropriate. Moreover, in order to make the content of Mn qualified, the amount of ferromanganese added is increased, the smelting time is prolonged, the power consumption is increased, the production cost is increased, and the content of C.Si.P is passively increased, and the scrap rate is high.
- The production of ZG120Mn13Cr2 high manganese wear-resistant steel by smelting requires the addition of a large amount of ferromanganese and more ferrochromium. C and P tend to be too high. Carbon smelting, dephosphorization, deoxidation and inclusion removal should be strengthened during smelting. Otherwise, it is difficult to produce high-quality qualified molten steel to ensure the excellent characteristics of high toughness, high strength, high wear resistance and long service life of the castings in service under strong impact conditions.
- High-quality castings not only need to optimize the chemical composition of materials reasonably, but also improve the metallurgical quality under the instruction of chemical composition, smelt highly clean metal liquid to produce castings, and be equipped with appropriate heat treatment process.
- In daily smelting, the scope of chemical composition control can be narrowed (mass fraction,%): C is 1.05 -1.30, Si is 0.40 ~ 0.55, Mn is 11-13, P <0.05, S <0.03, Cr is 1.5 -2.0, • # Control Mn / C = 10 ~ 11.
- The grain refinement of ZG120Mn13Cr2 high manganese wear-resistant steel is very heavy for high at. 7B, the service life of ZG120Mn13Cr2 wear-resistant castings with the same type I and metamorphic treatment can be nearly doubled compared with the same type of castings without metamorphic treatment.
(7) The casting temperature of high manganese steel must not be too high, otherwise it is easy to produce coarse and columnar structure defects, reduce the toughness and wear resistance of high manganese steel, deteriorate its performance, increase the cracking tendency of castings and Early fracture momentum during service. The tapping temperature should be controlled at about 1530 ℃, the pouring temperature should be 1400 ~ 1440C, and fast pouring at low temperature.
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