A carburized medium manganese steel and an efficient carburizing method thereof

Abstract

The application provides a high-efficiency carburizing method for carburized medium manganese steel and belongs to the technical field of metal materials. The present research takes solid carburizing technology as the core and aims to explore the influence of different carburizing processes on the thickness and quality of the carburized layer. From the perspective of process design, by analyzing the medium manganese steel samples under different carburizing time and temperature conditions, it is found that under certain carburizing time and temperature conditions, ideal carburized layer thickness and carbon content can be obtained. The carburized layer thickness obtained by double-temperature carburizing is 615-952 mu m, and the hardness is as high as 276.7 HV. By controlling the final carburizing temperature and holding time, the control of different carburized layer thickness is realized. Through the present research, important clues for optimizing process parameters and controlling the thickness of the carburized layer are provided, and beneficial references for further optimization of medium manganese steel materials in terms of high strength, wear resistance and the like are provided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of medium manganese steel carburizing, and in particular to a carburized medium manganese steel and an efficient carburizing method thereof. BACKGROUND

[0002] In the field of medium manganese steel solid carburizing, solid carburizing is a key technology that plays a crucial role in improving the hardness, wear resistance, toughness, and corrosion resistance of steel. In the field of medium manganese steel, the development of solid carburizing technology has great potential in improving the overall performance of medium manganese steel and expanding its application fields. The application of solid carburizing technology in medium manganese steel can achieve precise control of material properties by precisely controlling process parameters such as carburizing time, temperature, and carburizing agent ratio, thereby improving hardness and wear resistance while reducing deformation and damage in high-temperature and corrosive environments. However, in the past, there have been problems such as unstable carburizing process, uneven carburizing layer, and low production efficiency. Traditional carburizing methods often require long carburizing times and high carburizing temperatures, and due to the complexity of the carburizing process, the thickness of the carburizing layer is not uniform, which affects the service life and performance stability of the parts. SUMMARY

[0003] Therefore, the present application aims to provide an efficient and stable solid carburizing method for medium manganese steel to improve carburizing efficiency, uniformity, and production efficiency. The present application aims to adjust carburizing temperature, carburizing time, and other key process parameters to enable medium manganese steel parts to quickly obtain a uniform and high-hardness carburizing layer. This will help improve the service life, wear resistance, and corrosion resistance of medium manganese steel parts to meet the increasing demand for material performance in the engineering field, and has important practical significance and broad development prospects.

[0004] To achieve the above-mentioned application purposes, the present application provides the following technical solutions: an efficient carburizing method for carburized medium manganese steel, comprising the following steps:

[0005] (1) Place a 3mm medium manganese steel hot-rolled plate in a sealed box containing a carburizing agent, heat the sealing to above A3 temperature, specifically 820-850℃, and keep it for 1-2h, and perform preliminary carburizing under a carbon atmosphere; the purpose is to allow the carburizing agent to better volatilize and decompose, and to form a good carbon atmosphere before final carburizing, also known as pre-gasification treatment;

[0006] (2) Heat the medium manganese steel after preliminary carburizing to the final carburizing temperature, keep it for a certain time, and cool it in the furnace to obtain carburized medium manganese steel. The cooling rate of furnace cooling is 0.1-1℃ / s, and by precisely controlling the cooling rate, undesirable phase transformation and organizational segregation phenomena can be effectively avoided, ensuring the uniformity and stability of the material.

[0007] Further, the medium manganese steel has the component mass percentage of C: 0.15-0.3%, Mn: 5.0-7.0%, Si: 0.4-0.6%, Al: 0.5-1.5%, and the balance of Fe, and the specific component design can provide good strength and wear resistance, and ensure that the weldability and processability of the material are met.

[0008] Further, the medium manganese steel hot-rolled plate is subjected to surface treatment before carburizing, mainly to remove surface oxides and impurities, and improve the carburizing effect.

[0009] Further, the carburizing agent is composed of graphite and carburizing aid calcium carbonate in a mass ratio of 9:1, and the sample is carburized with at least 20mm thick carburizing agent around the sample, and the carburizing aid can improve the carburizing speed and promote the diffusion of carbon atoms, so as to realize the uniformity and density of the carburized layer.

[0010] Further, the temperature rising rate in steps (1) and (2) is 7-9℃ / min, and the target temperature is reached in a continuous heating mode.

[0011] Further, the final carburizing temperature is 950-1050℃, and the final carburizing holding time is 10-14h.

[0012] The carburized medium manganese steel obtained by the method has a surface complete pearlite layer with a thickness of 0.5-0.9mm and a core lower bainite structure.

[0013] The method provided by the application has the beneficial effects that: the method provided by the application obtains a complete pearlite structure on the surface and a lower bainite structure in the core. The lower bainite has higher hardness and strength than ordinary pearlite, can significantly improve the wear resistance and impact resistance of the steel material, has excellent hardness and strength, can better absorb energy when impacted or vibrated, and avoids the failure of parts caused by brittle fracture. The hardness gradient from the surface to the core finally obtains the surface soft-phase pearlite and the core hard-phase lower bainite. The results of the embodiment of the application show that the carburized layer thickness (that is, the complete pearlite layer) of the medium manganese steel obtained by the heat treatment method provided by the application is 0.5-0.9mm, and the surface carbon content is 0.4-0.9wt%.

[0014] The cross-section hardness of the carburized medium manganese steel shows obvious layering phenomenon. In the pearlite region, the carbon content gradually decreases with the increase of the depth, and the corresponding hardness also gradually decreases. While in the bainite region near the core, the hardness is obviously higher than that of the carburized layer pearlite region, but it also gradually decreases with the decrease of the carbon content. This is because the existence of carbon atoms in the steel can increase the hardness of the steel. When the carbon concentration in the steel is high, carbon atoms will form a solid solution with iron atoms, forming a so-called solid solution hardening effect. However, with the decrease of the carbon concentration, the solid solution hardening effect gradually weakens. When the carbon concentration is too low, the carbon atoms in the solid solution cannot continue to effectively affect the dislocation density of the crystal interface, so as to continue to increase the hardness of the steel. BRIEF DESCRIPTION OF DRAWINGS

[0015] The application will be further described in detail below in combination with the drawings and specific embodiments

[0016] Figure 1 The microstructure diagram of the carburized medium manganese steel obtained in Example 1 of the application, (a), (d) are the carburized layer and core structure of 950℃-6h carburizing, (b), (e) are the carburized layer and core structure of 950℃-10h carburizing, (c), (f) are the carburized layer and core structure of 950℃ carburizing-14h;

[0017] Figure 2 The carbon content characterization curve of the carburized medium manganese steel obtained in Example 1 of the application;

[0018] Figure 3 The hardness characterization curve of the carburized medium manganese steel obtained in Example 1 of the application;

[0019] Figure 4 The microstructure diagram of the carburized medium manganese steel obtained in Example 2 of the application, (a) is 950℃-6h; (b) is 1000℃-6h; (c) is 1050℃-6h;

[0020] Figure 5 The carbon content characterization curve of the carburized medium manganese steel obtained in Example 2 of the application;

[0021] Figure 6 The hardness characterization curve of the carburized medium manganese steel obtained in Example 2 of the application. DETAILED DESCRIPTION

[0022] In the application, the medium manganese steel comprises the following components in mass percentage: C 0.15-0.3%, Mn 5.0-7.0%, Si 0.4-0.6%, Al 0.5-1.5%, and the balance of Fe.

[0023] In the present invention, the mass percentage of C is 0.15-0.3%, preferably 0.19-0.25%, more preferably 0.20%. In the present invention, C is an element for stabilizing austenite at room temperature, and is a necessary element for ensuring the retention of austenite; the role of carbon element (C) is to increase the surface hardness of the steel. By solid carburizing, carbon atoms diffuse into the surface layer of the steel, forming a region with higher carbon content, which improves the wear resistance and fatigue resistance of the steel. Appropriate carbon content helps to form carbides, which can further improve the hardness and strength of the material to ensure the desired mechanical properties.

[0024] In the present invention, the mass percentage of Mn is 5.0-7.0%, preferably 5.0-6.0%, more preferably 5.0%. Manganese, as an austenite stabilizing element, can enhance the toughness and strength of the carburized layer, while reducing the brittleness of the carburized layer; prevent grain boundary oxidation, at high temperatures, manganese can form oxides with residual oxygen, helping to prevent grain boundary oxidation, which is crucial for improving the uniformity and overall performance of the carburized layer; affect the formation of carbides, manganese can affect the morphology and distribution of carbides in the steel, which is beneficial to the formation of finer and more uniform carbides, thereby improving the mechanical properties of the material;

[0025] In the present invention, the mass percentage of Al is 0.5-1.5%, preferably 0.8-1.2%, more preferably 1.0%. In the present invention, aluminum is an effective grain refiner that can help form fine and uniform austenite grains, thereby improving the mechanical properties of the steel during quenching and tempering after carburizing; aluminum can affect the formation and stability of carbides (such as ferrite and austenite carbides), which plays an important role in adjusting the hardness and strength of the steel; Al as a ferrite stabilizing element, helps to increase the volume fraction of ferrite, improving the machinability of the steel plate.

[0026] In the present invention, the mass percentage of Si is 0.4-0.6%, preferably 0.4-0.5%, more preferably 0.5%.

[0027] In the present invention, Si as a ferrite stabilizing element, helps to increase the volume fraction of ferrite, improving the machinability of the steel plate; silicon can promote the formation of silicon carbides, thereby enhancing the hardness of the material; improve the oxidation resistance, silicon in the steel helps to form an oxide layer, enhancing the oxidation resistance of the steel, which is particularly important at high temperatures.

[0028] The present invention does not have special requirements for the source of the medium manganese steel, and the technology known to those skilled in the art can be used; in the embodiments of the present invention, commercially available medium manganese steel products can be used, or the technology known to those skilled in the art for preparing medium manganese steel can be used to prepare it.

[0029] The medium manganese steel hot-rolled plate is obtained by the following steps: ①firstly, the hot-rolled plate is heated to 1200℃ for 1.5h, then 5 passes of hot-rolling treatment are carried out, and finally the hot-rolled plate is cooled to 600℃ for 2h and then naturally cooled to room temperature; ②the hot-rolled plate is a 3mm thick steel plate, which is further prepared into a sample with a size of 10mm×5mm×3mm; ③the surface of the sample is polished by using No.800 sandpaper and then cleaned with alcohol.

[0030] The technical solutions of the present application are further described in detail below by examples, which are an explanation rather than a limitation of the present application.

[0031] Example 1

[0032] According to the following weight percentages: C: 0.2%; Mn: 5%; Si: 0.5%; Al: 1.5%; the rest is Fe and other unavoidable impurities, smelting in a 50Kg medium frequency induction furnace, after casting and cooling, the billet is forged into a 60mm thick steel billet. The steel billet is heated to 1200℃ for 1.5h in a box-type heating furnace, and then 5 passes of hot-rolling are carried out, and finally the hot-rolled plate is cooled to 600℃ for 2h and then air-cooled to room temperature. The thickness of the hot-rolled plate is 3mm.

[0033] The 3mm thick hot-rolled plate is subjected to carburizing heat treatment process: the test sample is further prepared into a sample with a size of 10mm×5mm×3mm. The surface of the sample is polished by using No.800 sandpaper and then cleaned with alcohol. The sample is placed in a carburizing box with a size of 100mm×100mm×60mm for treatment. The carburizing agent is graphite and carburizing agent calcium carbonate in a ratio of 9:1, and during operation, 20mm thick carburizing agent is first laid on the bottom of the sealed box, and then the sample is placed, keeping a distance of 20mm between the samples. The sealed box containing the carburizing agent and the sample is subjected to subsequent carburizing treatment.

[0034] Firstly, isothermal holding at 820℃ for 2h, and then isothermal holding at 950℃ for 6h, 10h and 14h respectively; then furnace cooling to room temperature, the obtained carburized medium manganese steel is subjected to carbon concentration gradient characterization and hardness characterization. Among them, the carburized layer without pearlite for 6h of final carburizing, the thickness of the carburized layer increases from 635μm to 952μm for 10h and 14h of final carburizing, and the highest hardness reaches 282±8HV. Since the thickness of the carburized layer reaches 952μm at 14h, the carburized layer accounts for 63.47% of the whole sample (0.952×2) / 3×100%, and the high proportion of the thickness of the carburized layer will increase the brittleness of the surface of the part, thereby affecting the toughness and ductility of the part. Therefore, it is not necessary to further explore the carburizing of the hot-rolled medium manganese steel at 950℃ of the final carburizing temperature in this embodiment.

[0035] The obtained carburized medium manganese steel is subjected to observation of the microstructure, and the microstructure is as follows: Figure 1As shown in the figure, the microstructure of the prepared carburized medium manganese steel includes the outermost complete pearlite and the lower bainite structure in the core.

[0036] Example 2

[0037] According to the following weight percentages: C: 0.2%; Mn: 5%; Si: 0.5%; Al: 1.5%; the rest is Fe and other inevitable impurities, smelting in a 50 Kg medium frequency induction furnace, casting and cooling to open die forging into a 60 mm thick billet. The billet is heated to 1200℃ in a box-type heating furnace for 1.5h, and after 5 passes of hot rolling, it is cooled to 600℃ for 2h, and then air-cooled to room temperature. The hot-rolled plate is a 3mm thick steel plate.

[0038] The hot-rolled plate is subjected to a carburizing heat treatment process: first isothermal holding at 820℃ for 2h, and then isothermal holding at 950℃, 1000℃, 1050℃ for 6h respectively; then furnace cooling to room temperature, and the obtained carburized medium manganese steel is subjected to carbon concentration gradient characterization and hardness characterization. The carbon concentration gradient characterization is divided into thermo-calc simulation and electron probe (EPMA) detection. After pre-carburizing at 820℃ for 2h and final carburizing at 950℃, 1000℃ for 6h, no obvious carburized layer is observed at 950℃, while a carburized layer with a thickness of 615μm and a hardness of 276.7HV is obtained at 1000℃. When the final carburizing temperature is 1050℃, the entire cross-section presents a uniform carburized layer structure, which does not conform to the structure of the present application, i.e. the surface complete pearlite structure and the core lower bainite structure. The metallographic structure of the sample cross-section after carburizing at different temperatures for 6h is shown in Figure 4 As shown in the figure, the microstructure of the prepared carburized medium manganese steel includes the outermost complete pearlite and the lower bainite structure in the core. Figure 4 As shown in the figure, the microstructure of the prepared carburized medium manganese steel includes the outermost complete pearlite and the lower bainite structure in the core. Similarly, the carburized layer thickness at 1000℃ for 6h is already relatively high, and there is no need to explore the carburizing at 1050℃ for 6h for complete penetration, and there is no need for research.

Claims

1. A high efficiency carburizing method of carburizing medium manganese steel, characterized by, The method comprises the following steps: (1) placing the medium manganese steel hot-rolled plate in a sealed box containing carburizing agent, heating the sealed box to a temperature above A3, and keeping the temperature for 1-2 hours to perform preliminary carburizing in a carbon atmosphere, the carburizing agent is composed of graphite and carburizing aid calcium carbonate in a mass ratio of 9:1, the sample is carburized with at least 20 mm thick carburizing agent around the sample, and the A3 temperature is 820 ℃; (2) heating the medium manganese steel after preliminary carburizing to a final carburizing temperature, keeping the temperature, and furnace cooling to obtain carburized medium manganese steel, the final carburizing temperature is 950-1050 ℃, and the final carburizing keeping time is 10-14 hours.

2. The method of claim 1, wherein, The medium manganese steel comprises the following components in mass percentage: C: 0.15-0.3%, Mn: 5.0-7.0%, Si: 0.4-0.6%, Al: 0.5-1.5%, and the balance of Fe.

3. The method of claim 1, wherein, The medium manganese steel hot-rolled plate is subjected to surface treatment before carburizing.

4. The method of claim 1, wherein, The heating rate in steps (1) and (2) is 7-9 ℃ / min.

5. A carburized medium manganese steel obtained by the method according to any one of claims 1-4, characterized in that, The carburized medium manganese steel has a structure of a complete pearlite layer on the surface and lower bainite structure in the center.

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