A method for producing a thick-walled steel casting
By adjusting the chemical composition and heat treatment process, especially by increasing Nb and B elements and by high-temperature annealing, quenching, and tempering, the problems of uneven material and poor weldability of thick-walled lifting lug cast steel parts were solved, and high-performance cast steel parts production was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- KOCEL STEEL
- Filing Date
- 2023-06-27
- Publication Date
- 2026-06-09
Abstract
Description
Technical Field
[0001] This invention belongs to the field of casting technology and mainly relates to a method for producing thick-walled cast steel parts. Background Technology
[0002] This invention relates to cast steel handle-type lifting lugs for large hulls used in large offshore FPSO mooring systems. As the tonnage of deep-sea FPSOs increases, the dimensions of these cast steel handle-type lifting lugs also increase, especially the cylindrical portion of the lug that directly bears the anchor chain tension. The casting steel lug is made of ASTM A148 90-60, a low-carbon steel, with dimensions of 2400*2200*1000mm. The maximum wall thickness (cylinder lug) is 400mm, and the thinnest is 100mm (bottom plate), showing significant thickness variation. The standard requirements for this cast steel part are a tensile strength Rm ≥ 550MPa and a yield strength Rp at room temperature. 0.2 ≥415MPa, elongation A≥20%, reduction of area Z≥40%, and the average impact toughness Akv at -20℃ is required to be ≥50J.
[0003] Since the cast steel handle lug is a key component of the FPSO, it has high performance requirements and needs to be assembled and welded with the sinking cylinder, which also requires high weldability. Therefore, how to produce this cast steel component is an urgent issue to be resolved. Summary of the Invention
[0004] In view of the above description, it is necessary to propose a production method for thick-walled cast steel parts, mainly involving a production method for thick-walled lifting lug cast steel parts, to overcome the problem that existing technologies cannot produce qualified thick-walled lifting lug cast steel parts. The method specifically includes the following steps:
[0005] A method for producing thick-walled cast steel parts includes the following steps:
[0006] Chemical composition design of the cast steel parts: The chemical composition of the cast steel parts by mass percentage is as follows: 0.12%≤C≤0.14%, 0.5%≤Si≤0.6%, 1.2%≤Mn≤1.6%, P≤0.02%, S≤0.005%, 0.08%≤Cr≤0.13%, 0.1%≤Mo≤0.18%, 0.9%≤Ni≤1.1%, 0.01%≤Al≤0.02%, 0.02%≤Cu≤0.06%, 0≤V≤0.02%, Ti≤0.02%, 0.018%≤Nb≤0.03%, 0.002%≤B≤0.005%, with the remainder being Fe and residual elements.
[0007] Heat treatment process design: The cast steel parts are treated with a heat treatment method of high temperature annealing + quenching + high temperature tempering in sequence.
[0008] In one embodiment, in the chemical composition design step of the cast steel part, the chemical composition of the cast steel part is determined based on the control range of carbon equivalent Ceq combined with the calculation formula of hardenability DI, wherein the range of carbon equivalent Ceq is 0.46≤Ceq≤0.49; the calculation formula of hardenability is as follows:
[0009] DI(in) = 0.54C*(0.7Si+1)*(3.3333Mn+1)*(2.16Cr+1)*(3Mo+1)*(0.363Ni+1)*(0.365Cu+1)*(1.73V+1), where each element is a mass percentage.
[0010] In one embodiment, the carbon equivalent Ceq = C + 1 / 6Mn + 1 / 5*(Cr + Mo + V) + 1 / 15*(Ni + Cu) ≤ 0.50, where each element is a mass percentage.
[0011] In one embodiment, the formula for calculating hardenability satisfies C% < 0.4%.
[0012] In one embodiment, in the heat treatment process design step, the high-temperature annealing holding temperature is 940°C to 960°C, and the holding time is 14h to 15.3h.
[0013] In one embodiment, in the heat treatment process design steps, after the high-temperature annealing and heat preservation of the cast steel part is completed, the cast steel part is cooled with the furnace to below 300°C and then naturally air-cooled.
[0014] In one embodiment, the heat treatment process design step involves a quenching and holding temperature of 910°C to 930°C and a holding time of 8 hours to 13 hours.
[0015] In one embodiment, in the heat treatment process design steps, after the quenching and holding time of the cast steel part is completed, when the minimum surface temperature of the cast steel part is ≥850℃, the cast steel part is placed in water for rapid and uniform cooling. After the overall temperature of the cast steel part is cooled down to below 200℃, it can be taken out of the water and prepared for furnace tempering.
[0016] In one embodiment, in the heat treatment process design step, the high-temperature tempering temperature is 630°C to 660°C, and the holding time is 10h to 14h.
[0017] In one embodiment, in the heat treatment process design step, after the high-temperature tempering and holding time of the cast steel part is completed, the cast steel part is taken out of the furnace and placed in water for rapid cooling.
[0018] The method for producing thick-walled cast steel parts provided by this invention significantly improves the hardenability of the cast steel material by adjusting the composition of elements within a wide range as required by the customer and adding trace elements Nb and B as strengthening elements when designing and optimizing the internal control composition. Furthermore, it combines a high-temperature annealing + quenching + high-temperature tempering heat treatment process, designing reasonable parameter ranges and steps for each heat treatment process to ensure that the entire cross-section of the cast steel part obtains a uniform tempered sorbite structure. The method also ensures that the average strength, plasticity, and -20℃ impact toughness Akv of the surface, half-radius, and core of the cast steel part all meet standard requirements. The final performance compliance rate of the obtained cast steel parts is 100%. Detailed Implementation
[0019] To facilitate understanding of the present invention, a more comprehensive description is provided below, along with preferred embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.
[0020] This embodiment relates to a production method for thick-walled cast steel parts. Taking the production method of a thick-walled lifting lug cast steel part as an example, the mass percentage of the chemical composition of the cast steel part as required by the customer's technical specifications is as follows: C≤0.18%, Si≤0.60%, 0.90%≤Mn≤1.7%, P≤0.02%, S≤0.01%, Cr≤0.15%, Ni≤1.20%, Mo≤0.20%, Al≤0.05%, Cu≤0.25%, Nb≤0.03%, V≤0.08%, Ti≤0.02%, with the remainder being Fe and unavoidable impurities; the carbon equivalent Ceq≤0.50, and it is specified that Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15 (Ceq is Carbon equivalent, this formula is the internationally accepted formula of the International Institute of Welding, and each element is a mass percentage content, %). Existing production methods for thick-walled lifting lug cast steel parts with large diameters exhibit significant variations in cross-sectional properties, insufficient subsurface and core strength, and poor and unstable ductility and toughness, failing to meet standard technical requirements. Through comprehensive analysis of the composition and heat treatment process parameters of previous cast steel parts, and metallographic analysis of tensile and impact samples at different depths of the anatomical surface of the lifting lug cast steel parts, it was found that the main issues are limited hardenability depth in existing compositional internal control designs and insufficient content of strength-enhancing elements. Furthermore, the large diameter of the lifting lug cast steel parts results in relatively slow cooling of the subsurface and core during casting solidification, leading to coarse dendrites and severe compositional segregation. This results in coarse initial grains and uneven microstructure, leading to low and unstable performance. Therefore, this embodiment provides a production method for thick-walled cast steel parts that overcomes the aforementioned problems and obtains cast steel parts that meet standard technical requirements. The method mainly includes the following steps:
[0021] Step 01, Chemical composition design of the cast steel parts: The chemical composition of the cast steel parts by mass percentage is as follows: 0.12%≤C≤0.14%, 0.5%≤Si≤0.6%, 1.2%≤Mn≤1.6%, P≤0.02%, S≤0.005%, 0.08%≤Cr≤0.13%, 0.1%≤Mo≤0.18%, 0.9%≤Ni≤1.1%, 0.01%≤Al≤0.02%, 0.02%≤Cu≤0.06%, 0≤V≤0.02%, Ti≤0.02%, 0.018%≤Nb≤0.03%, 0.002%≤B≤0.005%, with the remainder being Fe and residual elements.
[0022] Specifically, the chemical composition of the cast steel part is determined based on the control range of carbon equivalent combined with the calculation formula of hardenability DI, and the range of carbon equivalent Ceq is 0.46≤Ceq≤0.49.
[0023] The carbon equivalent Ceq = C + 1 / 6Mn + 1 / 5*(Cr + Mo + V) + 1 / 15*(Ni + Cu) ≤ 0.50, where each element is a mass percentage.
[0024] The formula for calculating hardenability is as follows:
[0025] DI(in) = 0.54C*(0.7Si+1)*(3.3333Mn+1)*(2.16Cr+1)*(3Mo+1)*(0.363Ni+1)*(0.365Cu+1)*(1.73V+1), where each element is a mass percentage (%). The calculation formula satisfies C% < 0.4%.
[0026] It should be noted that during the design of the chemical composition of cast steel parts, the chemical composition of the cast steel parts is optimized according to the material characteristics and operating conditions of the cast steel parts. The trace elements Nb and B are newly added to achieve micro-alloying, which achieves the effects of fine grain strengthening and solid solution strengthening. This improves the high hardenability of thick-walled castings and the comprehensive matching function of strength and toughness at different depths of the cross section, laying the foundation for achieving the high mechanical properties required by customers.
[0027] From the above formulas for calculating Ceq and DI(in), it is clear that C, Mn, Cr, and Mo elements have a significant effect on increasing the carbon equivalent and improving hardenability of materials. Therefore, appropriately increasing the content of C and Si elements in this invention is beneficial for improving the strength of cast steel parts. However, excessively high content will reduce toughness. Although Si is not reflected in the Ceq formula, increasing Si will not lead to an increase in Ceq. However, in general, Si should be kept ≤0.5% in cast steel materials, and should not exceed 0.6%. Mn can also improve the strength of materials without significantly reducing plasticity and impact toughness. However, when the content reaches 1% or more, especially 1.7% or more, it has a significant impact on reducing plasticity and toughness. Mo can refine grains and improve the strength of steel, while also improving the tempering resistance of cast steel parts. Ni can improve both the strength of castings and enhance the low-temperature toughness of materials.
[0028] From the customer's perspective, a relatively low carbon equivalent (Ceq) is required to ensure good weldability when welding the lifting lug base plate and the cylinder structure. From the casting manufacturer's perspective, a relatively high Ceq is desired to improve material strength. The casting manufacturer also desires a high hardenability (DI(in)) to increase the hardening depth of the large lifting lug cast steel parts. However, considering the range of elements in the chemical composition provided by the customer, due to the relatively small amount of added alloying elements, the strengthening effect of alloying elements is weaker compared to high-alloy materials, and there will be no substantial change in performance improvement, including improvements in ductility and toughness. Based on previous experience in the microalloying strengthening production of low-carbon steel and low-alloy steel castings, this product also underwent microalloying design. This primarily involves adding Nb and B elements to the original internally controlled composition to improve the mechanical properties of the castings. Generally, when 0.001%-0.1% Nb is added to steel, it combines with carbon, nitrogen, and sulfur in the steel to form stable carbides and carbonitrides, resulting in fine-grain strengthening and dispersion strengthening, improving the strength and toughness of the cast steel, and benefiting weldability. According to relevant research and testing, when adding 0.1% of alloying elements, the maximum increase in yield strength is as follows: Nb increases by 118 MPa, V increases by 71.5 MPa, Mo increases by 40 MPa, Mn increases by 17.6 MPa, and Ti increases by zero. Furthermore, Nb can reduce the temper brittleness of the material, which is beneficial for improving impact toughness. Boron (B) significantly refines the grain size of alloy materials, purifies grain boundaries, and improves the density of castings (reducing shrinkage porosity). Its main function is to improve the hardenability of materials, thereby saving other rare and precious metals such as Cr, Ni, and Mo. Its addition content is generally specified in the range of 0.001%-0.005%, which can replace 1.6% of Ni, 0.3% of Cr, and 0.2% of Mo. However, while Mo can prevent or reduce temper brittleness, B has a slight tendency to promote it. Therefore, B cannot completely replace Mo. The effect of B weakens and eventually disappears as the carbon content in steel increases; it is generally added to materials with carbon content below a certain level. Finally, through extensive experimental comparisons, the optimal internal chemical composition (mass percentage, %) for the cast steel lifting lugs was determined as follows: C: 0.12-0.14, Si: 0.5-0.6, Mn: 1.2-1.6, P≤0.02, S≤0.005, Cr: 0.08-0.13, Ni: 0.9-1.10, Mo: 0.1-0.18, Al: 0.01-0.02, Cu: 0.02-0.06, V: 0-0.02, Ti≤0.02, Nb: 0.018-0.03, B: 0.002-0.005. Furthermore, the carbon equivalent (Ceq) was controlled between 0.46 and 0.49. Actual measurements of the hardness (DI) across a 400mm diameter section showed minimal difference, and the hardening depth was more than doubled compared to the previous method.
[0029] Step 02, Heat treatment process design: The cast steel parts are treated sequentially by high-temperature annealing + quenching + high-temperature tempering.
[0030] Specifically, the high-temperature annealing holding temperature is 940℃ to 960℃, and the holding time is 14h to 15.3h. After the high-temperature annealing holding of the cast steel parts is completed, the cast steel parts are cooled with the furnace to below 300℃ and then naturally air-cooled.
[0031] The quenching and holding temperature is 910℃ to 930℃, and the holding time is 8h to 13h. After the quenching and holding time of the cast steel parts is completed, when the minimum surface temperature of the cast steel parts is ≥850℃, the cast steel parts are placed in water for rapid and uniform cooling. After the overall temperature of the cast steel parts has cooled down to below 200℃, they can be removed from the water and prepared for tempering in the furnace.
[0032] The high-temperature tempering temperature is 630℃ to 660℃, and the holding time is 10h to 14h. After the holding time for the high-temperature tempering treatment of the cast steel parts is completed, the cast steel parts are taken out of the furnace and placed in water for rapid cooling.
[0033] It should be noted that, based on the determined internal composition of the cast steel parts, and because the lifting lug cast steel parts are relatively large and contain high-melting-point Nb and B elements, a high-temperature annealing process was added before quenching and tempering. Therefore, the complete heat treatment process for the lifting lug cast steel parts is high-temperature annealing + quenching + high-temperature tempering.
[0034] High-temperature annealing: Based on the dimensions and wall thickness of the cast steel part, it belongs to a large cast steel part. According to the specific composition of the material, the AC3 of this material is 832℃. According to the heat treatment process rules and the mechanism of high-temperature annealing, the high-temperature annealing temperature is generally relatively high, higher than the subsequent quenching temperature. The high-temperature annealing temperature is generally AC3+ (80~130℃), and lower than the austenite grain coarsening temperature of 960℃ for this type of material. Finally, it was determined that the casting would be annealed at 940~960℃ with the furnace. Too high an annealing temperature will lead to the risk of grain coarsening in the material structure, thereby reducing the strength and toughness of the material, and increasing the risk of cracking of the casting during quenching. If the annealing temperature is too low, or even lower than AC3, the matrix structure cannot obtain a high-temperature austenite structure, and the various precipitates in the as-cast structure cannot be dissolved back into the matrix, nor can other new structures such as ferrite and pearlite be obtained through cooling transformation. Therefore, the grains cannot be refined, and at most, it can only relieve stress. The annealing holding time is calculated based on the maximum wall thickness of the cast steel part:
[0035] t(annealing) = δmax * K / 60
[0036] In the formula:
[0037] —t represents the heat preservation time, in hours (h).
[0038] —δmax is the maximum wall thickness of the casting, in mm;
[0039] —K represents the thermal insulation coefficient, measured in min / mm. It is primarily determined by a combination of factors, including the casting material type and its thermal conductivity, the type of heat treatment furnace and heating method, the loading method of the casting, and the purpose of the heat treatment. Generally, the industry uses a coefficient between 0 and 2. In this embodiment, the cast steel is made of carbon steel with micro-alloying elements, and a single-piece, non-stacked loading method is used. Furthermore, a natural gas heat treatment furnace is employed. Based on the furnace reaching temperature time, the casting reaching temperature time, and the casting's heat penetration time, a thermal insulation coefficient of 2.0–2.3 is selected.
[0040] -60 is the time unit conversion factor, with units in min / h.
[0041] The high-temperature annealing holding time for the cast steel lifting lug is 14–15.3 hours, and can be set to 15 hours. A suitable annealing holding temperature combined with sufficient holding time can effectively homogenize the undesirable microstructure and compositional segregation in the thick walls and center of the cast steel lifting lug. After the holding time is completed, the casting can be cooled in the furnace to below 300°C and then naturally air-cooled.
[0042] Quenching: After high-temperature annealing, the cast steel parts can be reheated in a furnace for quenching. Based on the specific composition of the material and simulating its thermophysical parameters, the AC3 of the material is found to be 832℃. Therefore, the quenching temperature range is determined to be AC3 + (50~100℃), which is lower than the annealing temperature. The final quenching temperature is determined to be 910℃~930℃. This quenching temperature ensures complete austenitization of the cast steel parts without causing grain enlargement. Furthermore, the lower quenching temperature also prevents quenching cracks in the cast steel parts.
[0043] t(quenching) = δmax * K / 60
[0044] In the formula:
[0045] —t represents the heat preservation time, in hours (h).
[0046] —δmax is the maximum wall thickness of the casting, in mm;
[0047] —K is the heat preservation coefficient, measured in min / mm. It is primarily determined by the type of cast steel material and its thermal conductivity, the type of heat treatment furnace and heating method, the loading method of the cast steel, and the purpose of heat treatment. Generally, the industry uses a coefficient of 0 to 2. In this embodiment, the cast steel material is carbon steel with microalloying elements. It is loaded into the furnace as a single piece without stacking, and a natural gas heat treatment furnace is used. The heat preservation time is selected based on the furnace temperature reaching time, the cast steel temperature reaching time, and the through-heating time of the casting. Furthermore, the cast steel has undergone prior high-temperature annealing, resulting in a basically homogeneous microstructure. Therefore, the key to the quenching holding time is ensuring that the thick-walled core of the casting reaches the austenitizing process temperature; a slightly longer holding time is sufficient. Thus, the quenching holding time coefficient is relatively lower than that for high-temperature annealing. A heat preservation coefficient of 1.0 to 2.0 is selected overall.
[0048] -60 is the time unit conversion factor, with units in min / h.
[0049] Based on the above quenching and holding coefficients, the quenching and holding time for cast steel parts is generally controlled between 8 and 13 hours. After the holding time, the cast steel parts are removed from the furnace for cooling. Because the carbon content of the cast steel parts is low, the quenching cracking is low, so tap water with a faster cooling rate can be selected for quenching. From the time the cast steel parts are removed from the furnace to the time they are placed in the water, the hoisting and moving time of the parts should be carefully controlled (during this period, the temperature of the cast steel parts will drop), ensuring that the minimum surface temperature of the cast steel parts is ≥850℃ before quenching. After the cast steel parts are placed in the water, the water in the tank should be thoroughly and evenly stirred, and the cast steel parts can also move back and forth, left and right, up and down in the water tank to ensure that the cast steel parts, especially the thick and large lifting lug parts, can be cooled quickly and evenly. Faster cooling is beneficial to increasing the quenching depth of the cast steel parts. After the overall temperature of the cast steel parts has cooled down to below 200℃, they can be removed from the water and prepared for tempering in the furnace.
[0050] Tempering: The tempering temperature has a critical impact on performance. The tempering temperature affects the rate of transformation and decomposition of the quenched matrix structure, as well as the rate of precipitation and diffusion of supersaturated carbides within the quenched martensite. Generally, under a given quenching process, as the tempering temperature increases, strength decreases while ductility and toughness increase. High-temperature tempering is beneficial for cast steel parts to achieve suitable strength and good ductility and toughness. Therefore, based on the internal control composition designed in this embodiment and the quenching process used, high-temperature tempering was selected for implementation. Multiple experiments verified that tempering at 630℃~660℃ was adopted.
[0051] t(tempering) = δmax * K / 60
[0052] In the formula:
[0053] —t represents the heat preservation time, in hours (h).
[0054] —δmax is the maximum wall thickness of the casting, in mm;
[0055] —K is the heat retention coefficient, measured in min / mm. It is primarily determined by the type of cast steel material and its thermal conductivity, the type of heat treatment furnace and heating method, the loading method of the cast steel, and the purpose of heat treatment. Generally, the industry uses a range of 0 to 2. In this embodiment, the cast steel material is carbon steel with microalloying elements. It is loaded into the furnace as a single piece without stacking, and a natural gas heat treatment furnace is used. The furnace temperature is selected based on the furnace's heating time, the cast steel's heating time, and the through-heating time of the casting. After quenching, the main function of tempering is to eliminate the residual stress from quenching. Simultaneously, tempering at different temperatures causes the hard and brittle structure obtained from quenching to undergo decomposition and transformation. The tempering temperature and holding time affect the degree of this transformation, thus further influencing the final properties of the cast steel. Therefore, tempering holding temperature and holding time are two key parameters affecting the cast steel, especially when the tempering temperature is basically determined; the length of the tempering time affects the mechanical properties of the casting. Comprehensive tests determined that the tempering insulation coefficient of this cast steel part is 1.5 to 2.0;
[0056] -60 is the time unit conversion factor, with units in min / h.
[0057] Based on the above formula, the high-temperature tempering holding time for the lifting lugs is calculated to be 10-14 hours, and should be controlled accordingly. The tempering temperature is relatively lower than the quenching temperature, slowing down the heat penetration rate of the cast steel parts in the heat treatment furnace. Therefore, the core of the cast steel parts needs a slightly longer time to reach the required temperature. The overall holding time is longer than the quenching holding time. If the tempering holding time is insufficient, the high-strength, low-ductility martensite structure obtained after quenching will not undergo sufficient tempering decomposition and transformation, resulting in an uneven tempered sorbite structure. In particular, the subsurface and core structures of the cast steel parts may exhibit underdeveloped structures, leading to high strength but poor ductility and toughness. If the tempering holding time is too long, the high-strength, low-ductility martensite structure obtained after quenching will undergo complete tempering decomposition and transformation, resulting in more supersaturated carbides precipitating and even growing, leading to lower strength and better ductility and toughness in the cast steel parts. Therefore, it is necessary to combine the specific composition and quenching process, and select an appropriate tempering temperature and a suitable tempering time to ensure that the quenched structure on the surface and center of the lifting lug is fully homogenized, resulting in better overall mechanical properties of the entire lifting lug cross section and meeting the standard requirements.
[0058] In addition, to reduce the brittleness of cast steel parts after high-temperature tempering, the cast steel parts are directly removed from the furnace and immersed in water for rapid cooling after tempering and holding, instead of the traditional furnace-cooled tempering process. This can, to some extent, prevent or reduce the brittleness tendency of cast steel parts and improve their impact toughness.
[0059] Using the production method provided in the above embodiments, by adjusting the wide range of elemental contents required by the original customer, an optimized internal control composition was designed, and trace amounts of Nb and B strengthening elements were added. This significantly improved the hardenability of the cast steel material. Furthermore, through a heat treatment process of high-temperature annealing + quenching + high-temperature tempering, the holding temperature and holding time of each heat treatment step were designed and controlled. The cooling in the high-temperature tempering furnace was changed to water cooling, resulting in a uniform tempered sorbite structure across the entire cross-section of the cast steel. The strength, plasticity, and average Akv value of the -20℃ impact toughness of the surface, half-radius, and core of the cast steel all met the standard requirements. Particularly noteworthy was the achievement of the most difficult-to-meet standard requirements for the half-radius and core mechanical properties, with Rm ranging from 560 MPa to 600 MPa and yield strength Rp. 0.2 The range is 425 MPa to 460 MPa, the elongation A2 is 21% to 24%, the reduction of area Z is 42% to 50%, and the average impact toughness Akv at -20℃ is 60 to 80 J, ultimately achieving a 100% performance compliance rate.
[0060] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.
[0061] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.
Claims
1. A method for producing thick-walled cast steel parts, characterized in that, Includes the following steps: Chemical composition design of the cast steel parts: The chemical composition of the cast steel parts by mass percentage is as follows: 0.12%≤C≤0.14%, 0.5%≤Si≤0.6%, 1.2%≤Mn≤1.6%, P≤0.02%, S≤0.005%, 0.08%≤Cr≤0.13%, 0.1%≤Mo≤0.18%, 0.9%≤Ni≤1.1%, 0.01%≤Al≤0.02%, 0.02%≤Cu≤0.06%, 0≤V≤0.02%, Ti≤0.02%, 0.018%≤Nb≤0.03%, 0.002%≤B≤0.005%, with the remainder being Fe and residual elements. The carbon equivalent Ceq range is 0.46≤Ceq≤0.
49. Heat treatment process design: The cast steel parts are treated sequentially by high-temperature annealing + quenching + high-temperature tempering; in the heat treatment process design steps, the high-temperature annealing holding temperature is 940℃ to 960℃, and the holding time is 14h to 15.3h; the quenching holding temperature is 910℃ to 930℃, and the holding time is 8h to 13h.
2. The method for producing thick-walled cast steel parts according to claim 1, characterized in that, The carbon equivalent Ceq = C + 1 / 6Mn + 1 / 5*(Cr + Mo + V) + 1 / 15*(Ni + Cu), where each element is a mass percentage.
3. The method for producing thick-walled cast steel parts according to claim 1, characterized in that, In the heat treatment process design steps, after the high-temperature annealing and heat preservation of the cast steel parts is completed, the cast steel parts are cooled with the furnace to below 300°C and then naturally air-cooled.
4. The method for producing thick-walled cast steel parts according to claim 1, characterized in that, In the heat treatment process design steps, after the quenching and holding time of the cast steel parts is completed, when the minimum surface temperature of the cast steel parts is ≥850℃, the cast steel parts are placed in water for rapid and uniform cooling. After the overall temperature of the cast steel parts is cooled down to below 200℃, they can be taken out of the water and prepared for furnace tempering.
5. The method for producing thick-walled cast steel parts according to claim 1, characterized in that, In the heat treatment process design steps, the high-temperature tempering temperature is 630℃ to 660℃, and the holding time is 10h to 14h.
6. The method for producing thick-walled cast steel parts according to claim 5, characterized in that, In the heat treatment process design steps, after the high-temperature tempering and holding time of the cast steel parts is completed, the cast steel parts are taken out of the furnace and placed in water for rapid cooling.