A method for manufacturing a cast steel of g26crmo4 without casting cracks

Abstract

The application relates to a manufacturing method of G26CrMo4 cast steel without casting cracks, which comprises the following steps: S1: smelting to obtain G26CrMo4 cast steel liquid, the composition of the G26CrMo4 cast steel liquid is as follows: C: 0.22-0.29%, Si: less than or equal to 0.6%, Mn: 0.5-0.8%, Cr: 0.8-1.2%, Mo: 0.15-0.3%, Al: 0.3-0.8%, P: less than or equal to 0.02%, S: less than or equal to 0.02%, and the rest is Fe and inevitable impurities; S2: casting, the G26CrMo4 cast steel liquid is cast into a casting mold; S3: mold stripping, the stripping temperature T is controlled to satisfy: T2<=T<=T1, wherein: T1=518-14.6[C]-12.52[Mn]-18.45[Cr]-10.22[Mo]+3.63[Si]+24.8[Al], unit: degree Celsius; T2=416+128.5[C]+197.2[S]-3.62[Si]-3.43[Mn]-2.58[Cr]-1.17[Mo]-1.81[Al], unit: degree Celsius; [C], [Mn], [Cr], [Mo], [Si], [Al] and [S] respectively represent the mass percentage content of C, Mn, Cr, Mo, Si, Al and S in the cast steel.

Description

Technical Field

[0001] This invention relates to cast steel, and in particular to a method for manufacturing G26CrMo4 cast steel without casting cracks. The G26CrMo4 cast steel prepared by this method is free of casting cracks and is widely applicable to components such as train couplers, valve bodies, cylinders, and bogies. Background Technology

[0002] Cast steel possesses strength, toughness, and ductility, making it widely used in various mechanical parts. G26CrMo4 is a European standard cast steel grade, and in recent years, more and more factories have begun producing cast steel parts made of this material. However, in actual production, it has been found that G26CrMo4 cast steel parts have many casting cracks, affecting their various mechanical properties. After the G26CrMo4 cast steel parts are cast, factories generally use welding to repair the cracks. However, welding increases process and labor costs; moreover, the welded hardened area (HAZ) often produces secondary cracks or affects toughness and other mechanical properties, resulting in substandard casting performance and a high scrap rate. Therefore, the production cost of high-quality G26CrMo4 cast steel parts remains high. Summary of the Invention

[0003] This invention provides a method for manufacturing G26CrMo4 cast steel without casting cracks. The resulting G26CrMo4 cast steel parts have good casting quality, and the cast steel parts are free of cracks after magnetic particle inspection. No repair operations such as welding are required, resulting in a high product yield. This method significantly reduces process, labor, and production costs, and is conducive to the low-cost mass production of high-quality G26CrMo4 cast steel.

[0004] The technical objective of this invention is achieved through the following means.

[0005] The purpose of this invention is to provide a method for manufacturing G26CrMo4 cast steel without casting cracks, comprising the following steps:

[0006] S1: Smelting to obtain G26CrMo4 cast steel liquid. The composition of G26CrMo4 cast steel liquid is: C: 0.22-0.29%, Si≤0.6%, Mn: 0.5-0.8%, Cr: 0.8-1.2%, Mo: 0.15-0.3%, Al: 0.3-0.8%, P≤0.02%, S≤0.02%, with the remainder being Fe and unavoidable impurities.

[0007] S2: Casting, pouring G26CrMo4 molten steel into the mold;

[0008] S3: Mold unpacking, controlling the unpacking temperature T to satisfy: T2≤T≤T1, where:

[0009] T1 = 518 - 14.6[C] - 12.52[Mn] - 18.45[Cr] - 10.22[Mo] + 3.63[Si] + 24.8[Al], in °C;

[0010] T2 = 416 + 128.5[C] + 197.2[S] - 3.62[Si] - 3.43[Mn] - 2.58[Cr] - 1.17[Mo] - 1.81[Al], in °C;

[0011] [C], [Mn], [Cr], [Mo], [Si], [Al], and [S] represent the mass percentages of C, Mn, Cr, Mo, Si, Al, and S in cast steel, respectively.

[0012] The composition design principle of this invention will be introduced below.

[0013] During production practice, the inventors discovered the problem of cracks in G26CrMo4 cast steel parts and conducted in-depth analysis. They found the main causes of these cracks to be as follows: First, the solidification temperature range of this cast steel is relatively wide. When the surface layer of the casting has solidified while only many dense dendritic crystals have formed inside, some molten steel remains between the dendrites. At this point, the steel's strength is very low, and cracks occur when the casting's shrinkage is hindered. Second, the timing of mold removal has a significant impact on crack development in cast steel parts. If the mold removal temperature is too high, the casting remains at a high temperature, resulting in poor overall strength. The cast steel loses the support of the mold, leading to crack initiation. If the mold removal temperature is too low, the casting's shrinkage is constrained by the sand core and mold, causing significant stress in the casting, which becomes the starting point for cracks.

[0014] Therefore, based on the above understanding, the inventors of this invention conducted extensive practical exploration. They ultimately discovered that adding a certain amount of Al to the elemental composition of G26CrMo4 cast steel can narrow the solidification temperature range of the cast steel and slow down cracking. Furthermore, precise control of the casting temperature is necessary, ultimately resulting in the production of G26CrMo4 cast steel without casting cracks.

[0015] The G26CrMo4 cast steel prepared by the manufacturing method of this invention has no cracks after magnetic particle testing, exhibiting excellent casting quality, greatly improving production efficiency and yield, and reducing production costs.

[0016] The box-opening temperature described in this invention refers to the temperature of the casting when the box is opened. As a non-limiting description, this temperature can be obtained by monitoring after the steel casting is poured by pre-installing thermocouples inside the mold cavity or on the side wall of the mold cavity.

[0017] Regarding the specific molding temperature, as mentioned earlier, it cannot be too high or too low. The inventors have discovered that the upper limit of the molding temperature is closely related to the ability of the cast steel to resist deformation and cracking at a specific temperature. If the temperature is too high, the cast steel part will not be able to resist cracking. The lower limit of the molding temperature is mainly affected by the solidification shrinkage of the cast steel. If the temperature is too low, the shrinkage of the cast steel will be forcibly constrained by the sand core and mold, leading to cracking. In other words, the molding temperature of this invention needs to be controlled within a temperature range where the cast steel part has sufficient strength to resist deformation and cracking, but has not yet begun to undergo large-scale shrinkage.

[0018] Through repeated experiments and research, the inventors discovered that both temperatures are related to the composition of the cast steel. Therefore, considering the specific elemental system and Al content of G26CrMo4 cast steel, the inventors, based on extensive experiments, finally determined the upper limit T1 and lower limit T2 of the mold-making temperature, and expressed them using the following formula:

[0019] T1 = 518 - 14.6[C] - 12.52[Mn] - 18.45[Cr] - 10.22[Mo] + 3.63[Si] + 24.8[Al], in °C;

[0020] T2 = 416 + 128.5[C] + 197.2[S] - 3.62[Si] - 3.43[Mn] - 2.58[Cr] - 1.17[Mo] - 1.81[Al], in °C;

[0021] [C], [Mn], [Cr], [Mo], [Si], [Al], and [S] represent the mass percentages of C, Mn, Cr, Mo, Si, Al, and S in cast steel, respectively.

[0022] In other words, during the casting process of the cast steel of the present invention, it is only necessary to test the composition of the cast steel liquid before casting, or to cast a test block separately and then quench it in water after casting and test the composition. This can quickly determine the required temperature range for casting, thereby better guiding production and ensuring that G26CrMo4 cast steel without casting cracks is obtained.

[0023] By controlling the Al content and the molding temperature, this invention ultimately yielded crack-free G26CrMo4 cast steel. Compared to conventionally produced G26CrMo4 cast steel, the G26CrMo4 cast steel of this invention, due to the absence of casting cracks, eliminates the need for welding repair, simplifying the production process, reducing production costs, and increasing the product yield, thus enabling large-scale, low-cost production of G26CrMo4 cast steel.

[0024] Next, based on the aforementioned principles, the function of each element in this invention will be introduced.

[0025] Carbon: Carbon is an element that ensures the strength and hardness of cast steel, but excessive carbon content affects the plasticity and toughness of the cast steel. Furthermore, the inventors discovered that carbon content has a significant impact on the shrinkage rate of cast steel during the cooling process. This invention controls the carbon content to 0.22-0.29%.

[0026] Silicon: Silicon is an element that ensures the strength and hardness of cast steel. Silicon helps improve casting fluidity, but excessive silicon content affects the plasticity and toughness of cast steel. Furthermore, silicon is prone to segregation and leads to the formation of non-metallic inclusions, deteriorating the performance of cast steel. This invention controls the silicon content to below 0.6%.

[0027] Manganese: Manganese is an element that improves the strength of cast steel. If the content is too low, the strength cannot be guaranteed, while if the content is too high, the toughness and plasticity deteriorate. Furthermore, manganese is an element that improves hardenability, and if the manganese content is too high, the tendency for crack initiation during the solidification process of cast steel increases. This invention controls the manganese content to be 0.5-0.8%.

[0028] Chromium: Chromium is an element that improves the strength and wear resistance of cast steel. If the content is too low, the strength and wear resistance cannot be guaranteed, while if the content is too high, the toughness and plasticity deteriorate. Chromium is also an element that improves hardenability, and if the chromium content is too high, the tendency for crack initiation during the solidification process of cast steel increases. This invention controls the manganese content to be 0.8-1.2%.

[0029] Molybdenum: Molybdenum is an element that improves strength; too low a content will compromise strength, while too high a content will deteriorate toughness and plasticity. Furthermore, molybdenum improves hardenability; excessive molybdenum content increases the tendency for crack initiation during the solidification process of cast steel. This invention controls the manganese content to be 0.15-0.3%.

[0030] Aluminum: Aluminum is an intentionally added element in this invention. The inventors discovered that adding 0.3-0.8% aluminum to the G26CrMo4 cast steel system can narrow the solidification temperature range of the cast steel and inhibit the occurrence of cracks during solidification. If the aluminum content is too low, the above-mentioned effects will not be obvious; if the aluminum content is too high, it will lead to segregation and the appearance of non-metallic inclusions, which will instead lead to casting cracks and deteriorate the performance of the cast steel. This invention controls the aluminum content to be 0.3-0.8%.

[0031] Phosphorus and sulfur: Both phosphorus and sulfur are impurity elements in cast steel. P tends to increase the brittleness of cast steel, while S tends to form inclusions, affecting the performance of cast steel. In particular, the S content has a significant impact on the shrinkage rate of cast steel during the cooling process. Therefore, this invention controls the P and S contents to be below 0.02%.

[0032] In addition to the elements mentioned above, as stated earlier, this invention also specifies the upper and lower limits of the temperature for unpacking cast steel parts.

[0033] T1 = 518 - 14.6[C] - 12.52[Mn] - 18.45[Cr] - 10.22[Mo] + 3.63[Si] + 24.8[Al], in °C;

[0034] T2 = 416 + 128.5[C] + 197.2[S] - 3.62[Si] - 3.43[Mn] - 2.58[Cr] - 1.17[Mo] - 1.81[Al], in °C;

[0035] [C], [Mn], [Cr], [Mo], [Si], [Al], and [S] represent the mass percentages of C, Mn, Cr, Mo, Si, Al, and S in cast steel, respectively.

[0036] In the production process of the cast steel of this invention, the mold-forming temperature T needs to be controlled to satisfy: T2≤T≤T1. If the mold-forming temperature is too high, the cast steel part will not be able to resist cracks, leading to the appearance of casting cracks. If the mold-forming temperature is too low, the shrinkage of the cast steel will be forcibly constrained by the sand core and mold, leading to the appearance of casting cracks. In other words, the mold-forming temperature of this invention needs to be controlled within a temperature range where the cast steel part has sufficient strength to resist deformation and cracks, but has not yet begun to undergo large-scale shrinkage.

[0037] In this way, by controlling the casting temperature within the corresponding range after casting, cast steel with the composition of this invention can be obtained without casting cracks. Therefore, the welding repair step for disordered crack repair simplifies the production process, reduces production costs, and improves the product yield, realizing large-scale, low-cost production of G26CrMo4 cast steel.

[0038] As a further improvement, the manufacturing method also includes a step of detecting the composition of the molten steel before casting, and determining T1 and T2 based on the composition of the molten steel. This allows the range of the casting temperature to be determined during casting, thereby enabling accurate control of the casting temperature of the steel casting.

[0039] As a further improvement, the casting mold in step S2 includes product casting mold and sample casting mold. That is, a batch of cast steel is used to cast both products and samples. After casting, the sample casting mold is rapidly cooled (e.g., directly water quenched) and the sample composition is analyzed and tested. The sample composition is the same as the product composition. Therefore, T1 and T2 can be determined based on the sample composition, that is, the range of the mold opening temperature is obtained. Thus, the mold opening temperature of the cast steel parts can be accurately controlled.

[0040] As a further improvement, the aluminum source is added last in the smelting process. Aluminum is easily burned off and oxidized. Therefore, it is added as close as possible to the end of the smelting process to ensure the aluminum yield and reduce the formation of non-metallic oxide inclusions in aluminum, thereby improving casting quality.

[0041] As a further improvement, the casting process employs protective casting. Since aluminum is an easily oxidized element, protective casting during the casting process can ensure the aluminum yield and prevent the formation of non-metallic oxide inclusions in aluminum, thereby improving casting quality.

[0042] As a further improvement, the protective casting is gas-protected casting, which includes selecting inexpensive inert gases such as nitrogen, argon, and carbon dioxide for protective casting.

[0043] As a further improvement, the manufacturing method also includes step S4: slow cooling after molding. Slow cooling of the cast steel part after molding can reduce the temperature stress of the cast steel part after molding, avoid stress concentration, and further improve the stability and reliability of the mechanical properties of the cast steel part during its later service life.

[0044] As a non-limiting description, the slow cooling is carried out in a heat treatment furnace, and the cooling rate of the slow cooling is preferably controlled at 2-10℃ / h. The slower the slow cooling rate, the more beneficial it is to avoid stress concentration. However, the slow cooling rate is not economical in terms of cost and is not conducive to improving production efficiency and reducing production costs. If the slow cooling rate is too fast, the stress release is insufficient and it is easy to affect the service life of the cast steel parts in the later stage.

[0045] By way of non-limiting description, the G26CrMo4 cast steel prepared by the manufacturing method of the present invention is used in components such as train couplers, valve bodies, cylinders, and bogies, but is not limited to these components.

[0046] The present invention has the following technical effects.

[0047] This invention provides a method for manufacturing G26CrMo4 cast steel that controls the process from two aspects: elemental configuration and process parameters. By adding a certain amount of Al to the G26CrMo4 cast steel elemental composition system, the solidification temperature range of the cast steel is narrowed, thus slowing down the occurrence of cracks. Simultaneously, this invention also precisely controls the molding temperature based on the specific composition, providing a corresponding molding temperature range. Production personnel can quickly obtain the most suitable molding temperature by considering the specific cast steel composition, thereby better guiding production and ensuring the production of G26CrMo4 cast steel free of casting cracks. Compared to G26CrMo4 cast steel produced by conventional methods, the G26CrMo4 cast steel prepared by the method of this invention is free of casting cracks, thus eliminating the need for welding repairs, simplifying the production process, reducing production costs, improving product yield, and ensuring the performance of the cast steel parts. Detailed Implementation

[0048] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, the following detailed explanation is provided in conjunction with specific experimental examples.

[0049] The molten steel is smelted according to the designed composition. Aluminum ingots are added after other components have passed inspection. The molten steel is inspected online before casting. Each batch of molten steel is then cast into several ingots under argon protection. These ingots are right-angled (70mm thick, 120mm wide, and 350mm on each side). This allows for inspection of both the casting quality of ordinary surfaces and the casting quality at right-angle bends (i.e., locations prone to stress concentration). Specific composition is shown in Table 1. After casting, the casting is unpacked according to the unpacking temperature T specified in Table 2. The unpacked steel is then cooled to room temperature in a heat treatment furnace at a rate of 8℃ / h. Magnetic particle testing is then performed to detect casting cracks and defects, which are recorded in Table 2. T1 and T2 are calculated using the formulas described in the instruction manual.

[0050] Table 1: Composition of each cast steel, %, balance is Fe.

[0051]

[0052] For the cast steels numbered 1 to 10, different cooling temperatures T were controlled during the cooling process. At the same time, magnetic particle testing was performed on the final castings to detect any cracks. The results are shown in Table 2.

[0053] Table 2: Packing temperature and crack detection results.

[0054]

[0055] The above-mentioned inventive examples and comparative examples will be further analyzed and explained below with reference to Tables 1-2.

[0056] In Table 1, test numbers 1-7 all meet the composition requirements of this invention. In Table 2, regarding the crushing temperature, the crushing temperatures for crushing numbers 1-2, 1-3, 2-2, 2-3, 3-2, 3-3, 4-2, 4-3, 5-2, 5-3, 6-2, 6-3, 7-2, and 7-3 are all within the T2-T1 range calculated for their corresponding compositions. Ultimately, the resulting cast steel parts showed no cracks after magnetic particle testing. The crushing temperatures T for crushing numbers 1-1, 1-4, 2-1, 2-4, 3-1, 3-4, 4-1, 4-4, 5-1, 5-4, 6-1, 6-4, 7-1, and 7-4 are all outside the T2-T1 range. Ultimately, the resulting cast steel parts all showed cracks after magnetic particle testing. Therefore, it can be confirmed that simply meeting the composition requirements of the invention without controlling the molding temperature is insufficient to produce the crack-free G26CrMo4 cast steel of this invention. Excessive molding temperature leads to insufficient crack resistance in the cast steel, resulting in casting cracks; conversely, insufficient molding temperature causes the shrinkage of the cast steel to be forcibly constrained by the sand core and mold, also leading to casting cracks. This demonstrates that controlling the molding temperature of the cast steel within a reasonable range is crucial for obtaining crack-free G26CrMo4 cast steel. However, crack-free G26CrMo4 cast steel cannot be obtained solely through controlling the molding temperature, as further explained below with reference to experiments 8-10.

[0057] Test numbers 8-10 in Table 1 do not meet the requirements of the invention.

[0058] Experiment No. 8 was based on Experiment No. 1 without the addition of Al, and had the same composition as conventional G26CrMo4 cast steel. As can be seen from the boxing serial numbers 8-1 and 8-2, even if the boxing temperature was within the T2~T1 range calculated from the composition, the final manufactured G26CrMo4 cast steel parts still had crack defects. This indicates that the addition of Al plays an important role in narrowing the solidification temperature range of cast steel and resisting casting cracks.

[0059] Experiment 9 increased the amount of Al added based on Experiment 2, exceeding the Al content requirement range of this invention. As can be seen from the batching serial numbers 9-1 and 9-2, even if the batching temperature was within the T2~T1 range calculated from the composition, the final manufactured G26CrMo4 cast steel parts still had crack defects. This indicates that an appropriate amount of Al is required to effectively narrow the solidification temperature range of the cast steel and resist casting cracks. Excessive Al content can easily cause segregation and increase non-metallic inclusions, becoming the starting point for crack initiation.

[0060] Experiment 10 reduced the amount of Al added based on Experiment 3, falling below the Al content requirement range of this invention. As can be seen from the batching serial numbers 10-1 and 10-2, even if the batching temperature was within the T2~T1 range calculated from the composition, the final manufactured G26CrMo4 cast steel parts still had crack defects. This indicates that an appropriate amount of Al is required to effectively narrow the solidification temperature range of cast steel and resist casting cracks. If the Al content is too low, the effect of narrowing the solidification temperature range of cast steel and resisting casting cracks is not significant enough.

[0061] In summary, it is clear that manufacturing G26CrMo4 cast steel parts requires not only the addition of an appropriate amount of Al but also precise control of the molding temperature to ultimately produce G26CrMo4 cast steel parts free of casting cracks. This invention, through compositional improvements and precise control of the molding temperature, ultimately achieves the production of G26CrMo4 cast steel parts free of casting cracks.

[0062] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

[0063] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for manufacturing G26CrMo4 cast steel without casting cracks, characterized in that, Includes the following steps: S1: Smelting to obtain G26CrMo4 cast steel liquid. The composition of G26CrMo4 cast steel liquid is: C: 0.22-0.29%, Si≤0.6%, Mn: 0.5-0.8%, Cr: 0.8-1.2%, Mo: 0.15-0.3%, Al: 0.3-0.8%, P≤0.02%, S≤0.02%, with the remainder being Fe and unavoidable impurities. S2: Casting, pouring G26CrMo4 molten steel into the mold; S3: Mold unpacking, controlling the unpacking temperature T to satisfy: T2≤T≤T1, where: T1 = 518 - 14.6[C] - 12.52[Mn] - 18.45[Cr] - 10.22[Mo] + 3.63[Si] + 24.8[Al], in °C; T2 = 416 + 128.5[C] + 197.2[S] - 3.62[Si] - 3.43[Mn] - 2.58[Cr] - 1.17[Mo] - 1.81[Al], in °C; [C], [Mn], [Cr], [Mo], [Si], [Al], and [S] represent the mass percentages of C, Mn, Cr, Mo, Si, Al, and S in cast steel, respectively.

2. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 1, characterized in that, It also includes the step of testing the composition of the molten steel before casting, and determining T1 and T2 based on the composition of the molten steel.

3. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 1, characterized in that, The casting molds used in step S2 include product molds and sample molds. After casting, the sample molds are rapidly cooled and the sample composition is tested. T1 and T2 are determined based on the sample composition.

4. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 1, characterized in that, The aluminum source is added last during the smelting process.

5. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 1, characterized in that, The casting process employs protective casting.

6. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 5, characterized in that, The protective casting is gas-protected casting.

7. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 1, characterized in that, The manufacturing method further includes step S4: slow cooling after packaging.

8. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 7, characterized in that, The slow cooling is carried out in a heat treatment furnace.

9. The method for manufacturing G26CrMo4 cast steel without casting cracks according to claim 7, characterized in that, The slow cooling rate is controlled at 2-10℃ / h.

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