A structural Cu-containing Mo-V microalloy cast steel and a method for producing the same

Abstract

The application provides a structure Cu-containing Mo-V micro-alloy cast steel and a preparation method thereof. The cast steel material is composed of the following components in percentage of mass fraction: C: 0.06-0.13%, Mn: 1.3-1.9%, Si: 0.2-0.4%, Ni: 0.5-1%, Mo: 0.18-0.42%, V: 0.05-0.09%, and Cu: 0.25-0.65%. The method comprises the following steps: step one, adding a blank into an electric arc furnace or an induction furnace; step two, performing conventional smelting on the blank to obtain a molten steel with a required mass target component; step three, adjusting the temperature of a semi-finished cast steel and tapping the molten steel to obtain the cast steel; step four, performing heat treatment on the cast steel; and step five, judging the qualification of the cast steel to obtain a high-quality finished cast steel. Through the structure Cu-containing Mo-V micro-alloy cast steel and the preparation method thereof, the yield strength grade of the non-quenching + tempering type micro-alloyed precipitation strengthening type cast steel can be further improved, the low-temperature toughness of a large-size structure cast piece under a conventional process can be improved, and the cost consumption of the large-size structure cast piece under the conventional process can be reduced.

Description

Technical Field

[0001] This invention relates to the field of cast steel technology, and more specifically, to a Cu-containing Mo-V microalloyed cast steel for structural use and its preparation method. Background Technology

[0002] Microalloyed steel technology, which enhances steel strength by forming dispersed carbides through elements such as V, Nb, and Ti, has been applied for over fifty years. Its significant technical and economic advantages have led to its widespread use in the steel industry. Microalloyed low-carbon high-strength steel (HSLA) accounts for approximately 15% of global steel production. Compared to high-strength steel obtained through quenching and tempering heat treatment to form martensitic tempered structures (tempered martensite, tempered troostite, tempered sorbite), the strengthening of microalloyed steel primarily relies on the precipitation of second phases rather than the addition of large amounts of hardenability-enhancing alloying elements. When the rolling process is appropriate, fine ferrite or bainite grains and high-density nano-second phases can be obtained, with the steel's yield strength exceeding 1200 MPa. Furthermore, its alloying element content is significantly lower than that of quenched and tempered steels of the same strength level, which is beneficial for improved weldability and cost reduction. Therefore, in many industries and operating conditions, HSLA steel, which achieves precipitation strengthening through controlled rolling and cooling, has replaced traditional quenched and tempered steel.

[0003] In shipbuilding, petrochemical, marine engineering, and construction industries, large-rigid structural components often require thick-walled cast steel, which demands high comprehensive performance in terms of strength, toughness, and weldability. To ensure high strength and hardenability, quenched and tempered cast steel requires increased C and alloying element content, but this negatively impacts toughness and weldability. Due to this contradiction, it is difficult to simultaneously achieve high strength, toughness, heat treatment properties, and weldability in thick-walled cast steel. Compared to controlled-rolling and controlled-cooling steel plates, cast steel has a lower dislocation density and lacks deformation-induced fine grain and second-phase particle nucleation processes. Achieving high-density nano-second-phase precipitation strengthening solely through composition design and heat treatment process adjustments is very challenging. Because of the limited means of performance control for microalloyed low-carbon cast steel, there is currently far less research on it compared to HSLA steel plates. Among the results published in existing domestic papers, the yield strength of non-quenched and tempered microalloyed cast steel parts for structural use generally does not exceed 450 MPa, and their toughness level is relatively low. Usually, only U-shaped Charpy low-temperature impact toughness or room-temperature V-shaped Charpy impact toughness is examined. Due to the low dislocation density in cast steel, feasible routes to improve the strength of non-quenched and tempered microalloyed cast steel include fine-grain strengthening and precipitation strengthening. However, even with various process measures, it is difficult for the grain size of sand-cast parts to reach grade 9 or above. Microalloyed cast steel mainly uses a single type of carbonitride or boride as the strengthening phase. These phases are relatively hard and brittle, and are semi-coherent or incoherent with the matrix. Since the precipitation strengthening effect is proportional to the half power of the volume fraction of the second phase and inversely proportional to the size of the second phase particles, and the size of these strengthening phases is easy to exceed the critical radius, the precipitation strengthening of microalloyed cast steel is usually a dislocation bypass mechanism, which is much less effective than the dislocation shear mechanism. Increasing the volume fraction requires adding more C and microalloying elements, which damages toughness and weldability. Currently, a significant proportion of microalloyed cast steel parts with a strength of 450MPa or higher undergo quenching followed by tempering to form tempered martensite or tempered troostite, achieving high strength rather than relying on precipitation strengthening. Cast steels with this structure are typically small in size and are mostly used in mechanical structures. The main purpose of adding microalloying elements is to refine the grains and reduce costs while ensuring the hardenability and toughness of the castings.

[0004] A summary of foreign papers and patents on non-quenched and tempered microalloyed cast steel reveals that Germany designed a C-Mn-Mo-Nb-V microalloyed cast steel with a C mass fraction of 0.1%, a Mo mass fraction of 0.4%, and a Mn mass fraction of 1.65%. After normalizing and tempering, it obtained a polygonal ferrite structure with a yield strength of 460 MPa, an elongation of 29%, and a reduction of area of ​​67%. US Patent 4634476 discloses a Cu-containing cast steel with the following elemental mass percentages: C: 0.07-0.12%; Si: 0.20-0.60%; Mn: 0.90-1.20%; Mo: 0.3-0.5%; Ni and Cr ≤ 0.35%; Cu ≤ 0.35%; V: 0.05-0.1%. This cast steel has a yield strength of 450-655 MPa, a tensile strength of 620-795 MPa, and a KV2 of 54 J to 95 J at -40℃. According to the patent text, this material is primarily used in the manufacture of US warships to replace HY80 and HY100 cast steels. US Patent US2012 / 018019112A1 discloses a low-alloy cast steel with a total mass percentage of C: 0.1-0.2%; Si: 0.1-0.5%; Mn: 0.40-1.20%; Cr: 0.2-0.7%; Ni: 2-3%; Mo: 0.1-0.5%, a yield strength of not less than 600 MPa, and KV2 ≥ 70 J at 0℃. However, it actually employs a quenching and tempering process. Other examples include Centrishore V, a Mn-Mo-Nb microalloyed cast steel from Pont-a-Mousson in France, and HRS, a microalloyed cast steel from Paris-Outreau, used for steel pipes. Both have yield strengths exceeding 600 MPa and primarily use Nb as a microalloying element, but require a centrifugal casting process. In summary, although microalloyed steels with yield strengths exceeding 600 MPa exist abroad, their heat treatment processes still employ quenching and tempering, or use special casting processes, which limits their application range.

[0005] A summary of domestic papers and patents on non-quenched + tempered microalloyed cast steel reveals the following: Lei Yong et al. from Central South University disclosed a low-temperature 0.07C-1.4Mn-0.35Ni-0.3Cr-0.3Mo-0.05V cast steel in CN103194687A. After normalizing and tempering, its yield strength is 475 MPa, and its Charpy impact absorption energy at -40℃ with a U-notch is 113 J. However, its primary strengthening mechanism is not second-phase dispersion strengthening, but rather bainite strengthening. Among currently published patents on normalized + tempered cast steel, those using microalloyed carbides for precipitation strengthening as the primary strengthening method include CN1916217A, CN100491573C, CN1570182A, CN1330788C, and CN1033845A. Among them, the cast steel involved in patents CN1916217A and CN100491573C is mainly used for diesel engine bases. Its microstructure is ferrite + dispersed carbides, and its yield strength is 480-500MPa. The cast steel involved in patents CN1570182A and CN1330788C has a carbon content of up to 0.5%. Its microstructure is bainite + martensite. This composition and microstructure limit the maximum size of the castings and make them non-weldable. It is mainly used for mechanical parts. The cast steel involved in patent CN1033845A has better performance under working conditions and is mainly used in structures. Its main elements are C, Mn, Nb, Ti, and Mo. However, its yield strength is low below 450MPa.

[0006] In summary, through literature and patent searches, although the cast steel composition described in US Patent US4634476 contains 0.07-0.12% carbon, 0.20-0.60% silicon, 0.90-1.20% manganese, 0.30-0.50% molybdenum, 0.05-0.10% vanadium, 0.009-0.0125% nitrogen, a maximum sulfur content of 0.010%, a chromium content not exceeding 0.30%, a maximum nickel content of 0.30%, a maximum titanium content of 0.01%, and a maximum copper content of 0.3%, the results show that the steel composition is suitable for cast steel. The maximum aluminum content is 0.07%, and the maximum phosphorus content is 0.015%. The main technical idea is to promote the precipitation of bainite and Cu aging phases during quenching and high-temperature tempering by adding no more than 0.5% Mo and a small amount of Cu. A small amount of Ni and Cr are also added to improve the hardenability of the cast steel. However, it is not very effective in improving the yield strength of non-quenched and tempered microalloy precipitation-strengthened cast steel, and the cost of cast steel prepared by this method is high. Therefore, it is of great significance to study how to reduce the casting cost while improving the low-temperature toughness of cast steel.

[0007] Patent CN1644748A mentions a high-sulfur anti-friction and wear-resistant cast steel and its production method. The high-sulfur anti-friction and wear-resistant cast steel suitable for use below 550℃ is generally used in the as-cast state, but can also undergo stress-relieving annealing or normalizing, or slow quenching (with a suitable composition formula); or tempering at 500–620℃ after casting. The heat treatment for high-sulfur anti-friction and wear-resistant cast steel suitable for use at 550–700℃ is: heating to 900–1120℃, holding, then air-cooling or oil-cooling quenching, followed by tempering at 600–720℃, or tempering at 700–780℃ after casting. Different heat treatment methods are used at different temperatures for the cast steel, but quenching is still required. The effect of improving the yield strength of cast steel without quenching is still relatively poor. Summary of the Invention

[0008] In view of this, the present invention aims to propose a Cu-containing Mo-V microalloyed cast steel for structural use and its preparation method, in order to solve the problems of poor yield strength improvement of non-quenched + tempered microalloyed precipitation-strengthened cast steel, low low-temperature toughness and high cost of large-size structural castings under conventional processes in the existing technology; thereby achieving a further improvement in the yield strength of non-quenched + tempered microalloyed precipitation-strengthened cast steel, improving the low-temperature toughness of large-size structural castings under conventional processes, and reducing the cost of large-size structural castings under conventional processes.

[0009] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0010] This invention relates to a Cu-containing Mo-V microalloyed cast steel for structural applications and its preparation method. Through a series of thermodynamic analyses, DFT theoretical analyses, and experiments, an alloy for a 450MPa grade cast steel was determined, which exhibits higher low-temperature toughness and carbon equivalent than general carbide microalloyed cast steels. The Cu-containing Mo-V microalloyed cast steel for structural applications is composed of the following components by mass fraction percentage: C: 0.06-0.13%, Mn: 1.3-1.9%, Si: 0.2-0.4%, Ni: 0.5-1%, Mo: 0.18-0.42%, V: 0.05-0.09%, Cu: 0.25-0.65%.

[0011] A method for preparing Cu-containing Mo-V microalloyed cast steel for structural applications, the method comprising the following steps:

[0012] Step 1: Add the billet to the electric arc furnace or induction furnace;

[0013] Step 2: Perform conventional smelting on the billet to obtain molten steel with the required quality target composition;

[0014] Step 3: Adjust the temperature of the molten steel and tap it out to obtain semi-finished cast steel;

[0015] Step 4: Heat treat the semi-finished cast steel to obtain the finished cast steel;

[0016] Step 5: Determine the quality of the cast steel to obtain high-quality finished cast steel.

[0017] Furthermore, in step two, conventional smelting refers to smelting using common cast steel smelting methods. When smelting cast steel, since the range of Mn, Si, and Ni in copper-containing steel is relatively wide, micro-alloying can be achieved by adding V and Nb, and any one or more elements from Cr and Mo can be added in combination.

[0018] Furthermore, in step three, the temperature of the molten steel is adjusted, and the molten steel is poured into the mold to obtain semi-finished cast steel.

[0019] Furthermore, the pouring temperature is between 1540℃ and 1580℃.

[0020] Furthermore, in step four, the heat treatment method includes the following steps:

[0021] Step S41: Homogenize the semi-finished cast steel;

[0022] Step S42: Perform normalizing treatment;

[0023] Step S43: After normalizing, perform tempering treatment;

[0024] Step S44: Post-tempering treatment: After tempering, perform rapid cooling.

[0025] Furthermore, in step S41, the semi-finished cast steel after casting is homogenized, and the homogenization temperature range is 980℃-1100℃.

[0026] Furthermore, in step S42, the normalizing temperature range is set to 860℃-940℃, and the normalizing holding time T2 is calculated as follows: T2=t+D×t1, where t is the preset time, t1 is the preset error time value, and D is the effective wall thickness of the cast steel.

[0027] Furthermore, in step S43, the tempering temperature range is 550℃-680℃, and the tempering time T3 ranges from 1h to 4h.

[0028] Furthermore, in step five, the qualification of cast steel is determined by calculation and analysis based on the crack sensitivity coefficient PCM formula for low alloy high strength steel and the carbon equivalent CE formula recommended by the International Institute of Welding. This is used to further determine the quality of cast steel and screen out high-quality finished cast steel.

[0029] Compared with the prior art, the Cu-containing Mo-V microalloyed cast steel for structural use and its preparation method described in this invention have the following beneficial effects:

[0030] By employing the aforementioned structure and heat treatment method, the soft Cu-containing phase can partially replace the hard carbide second phase, thereby enhancing the dislocation shearing effect and precipitation strengthening increment. This further improves the yield strength grade of non-quenched and tempered microalloyed precipitation-strengthened cast steel. Furthermore, by replacing the soft Cu-containing phase partially replaces the hard carbide second phase, the dislocation shearing effect and precipitation strengthening increment are enhanced, thus improving the low-temperature toughness of large-size structural castings under conventional processes. Additionally, the simple heat treatment process reduces the overall cost of large-size structural castings. Attached Figure Description

[0031] The accompanying drawings, which constitute a part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0032] Figure 1 This is a schematic diagram of the preparation method;

[0033] Figure 2 This is a schematic diagram of the heat treatment process.

[0034] Figure 3a The results of precipitate kinetic analysis (570℃) are shown in the schematic diagram of the average radius of FCC-Cu (dislocation nucleation).

[0035] Figure 3b The results of precipitate kinetic analysis (570℃) are shown in the schematic diagram of FCC-Cu volume fraction (dislocation nucleation).

[0036] Figure 3c The results of precipitate kinetic analysis (570℃) are shown in the schematic diagram of the critical radius (dislocation nucleation) of FCC-Cu.

[0037] Figure 3d The results of the kinetic analysis of the precipitates (570℃) are shown in the schematic diagram of the average radius (bulk nucleus) of (Mo,V)C.

[0038] Figure 3e The results of the kinetic analysis of the precipitates (570℃) are shown in the schematic diagram of the volume fraction (Mo,V)C (bulk nuclei).

[0039] Figure 3f The results of precipitate kinetic analysis (570℃) are shown in the schematic diagram of the average radius (bulk nucleation) of FCC-Cu.

[0040] Figure 3g The results of precipitate kinetic analysis (570℃) are shown in the schematic diagram of FCC-Cu volume fraction (bulk nucleation).

[0041] Figure 4a The results of the precipitate kinetic analysis are shown in the schematic diagram of the average radius of (Mo,V)C (volume nucleation at 640℃).

[0042] Figure 4b The results of the precipitate kinetic analysis are shown in the schematic diagram of (Mo,V)C volume fraction (bulk nucleation at 640℃).

[0043] Figure 4c The results of the precipitate kinetic analysis are shown in the schematic diagram of the average radius of (Mo,V)C (volume nucleation at 600℃).

[0044] Figure 4d This is a schematic diagram of the volume fraction of (Mo,V)C (volume nucleation at 600℃) as part of the kinetic analysis results of the precipitates. Detailed Implementation

[0045] The inventive concepts of this disclosure will be described below using terminology commonly used by those skilled in the art to communicate the essence of their work to others skilled in the art. However, these inventive concepts may be embodied in many different forms and should not be construed as limited to the embodiments described herein.

[0046] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0047] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0048] In existing technologies, structural cast steel is an indispensable material in shipbuilding, marine engineering, mining, power, and mechanical engineering, mainly used in complex shapes, weldable applications, and areas requiring high rigidity. However, quenched and tempered cast steel requires the addition of numerous alloying elements to improve hardenability and ensure strength and toughness, leading to increased costs and a higher tendency for weld cracking. Although microalloyed steels with yield strengths exceeding 600 MPa exist internationally, their heat treatment processes still employ quenching and tempering, or utilize specialized casting processes, limiting their application. Domestically, when used primarily in structural applications, the main elements are C, Mn, Nb, Ti, and Mo, resulting in lower yield strengths and higher costs in some cases. Furthermore, based on the empirical formula for the yield strength of Orowan-type precipitation steel and thermodynamic and first-principles calculations, it is known that with V, Ti, and Nb... The yield strength limit of cast steel strengthened by the precipitation of semi-coherent carbonitrides of certain microalloying elements such as Zr and Ta is about 400 MPa. Even with solid solution and fine grain strengthening, it can only be increased to about 450 MPa. Although the inventors added Mo to V-Nb alloy cast steel, the yield strength can be increased to over 500 MPa by simultaneously precipitating multiple carbides and making full use of C solid solution strengthening. However, there is still room for improvement in its low-temperature toughness. After heat treatment by different processes, the highest average Charpy V-type impact value at 0℃ and -40℃ is only 41 J and 35 J, respectively, which cannot meet the requirements of some working conditions with high requirements for low-temperature toughness.

[0049] To address the problems of poor yield strength improvement in non-quenched + tempered microalloyed precipitation-strengthened cast steel, low low-temperature toughness in conventional processes for large-sized structural castings, and high cost in existing technologies, this embodiment, based on the application of electronic structure theory in the study of second-phase strengthened hull steel (LW190102), proposes a Cu-containing Mo-V microalloyed cast steel for structural use and its preparation method. Through a series of thermodynamic analyses, DFT theoretical analyses, and experiments, a 450M... The Pa-grade cast steel alloy possesses higher low-temperature toughness and carbon equivalent than ordinary carbide microalloyed cast steel. The aforementioned Cu-containing Mo-V microalloyed cast steel material for structural use, by mass fraction percentage, comprises the following components: C: 0.06-0.13%, Mn: 1.3-1.9%, Si: 0.2-0.4%, Ni: 0.5-1%, Mo: 0.18-0.42%, V: 0.05-0.09%, Cu: 0.25-0.65%, primarily based on the following:

[0050] The composition of V, Mo, and C elements: Compared to existing technologies, V alone can form VC with C, and under equimolar conditions, the volume fraction of V forming the VC second phase is second only to Zr. Furthermore, VC has a smaller mismatch with the matrix and a lower tendency to grow; therefore, V is chosen as the main microalloying element. The simultaneous addition of Mo and V significantly increases the total fraction of the second phase in the steel, and the P63 / mmc structure (Mo,V)C becomes the main precipitated carbide. At the nanoscale, this phase exhibits a more pronounced hardening effect than FCC-type MC carbides, but may be more prone to growth and steel embrittlement. Mo also makes the microstructure closer to bainite, but if the Mo content is too high, P63 / mmc will occur. -m2 type MoC precipitation, this phase has a more obvious tendency to be hard and brittle; this invention adds Mo to the CV base to introduce P63 / mmc type (Mo,V)C to increase the total amount of carbides, and increases the C content while minimizing the total amount of P6-m2 type MoC and ensuring weldability; since there is also Cu aging phase in the material, the required strength can be achieved with a low total amount of carbides, which is beneficial to reducing the C equivalent of the material and ensuring good low-temperature toughness and weldability; therefore, the final determined values ​​are: V mass fraction 0.05%-0.1%, Mo mass fraction 0.25%-0.40%, and C mass fraction 0.06%-0.13%.

[0051] Ni and Mn element settings: Mn and Ni are added to improve toughness. Since Ni is expensive and the degree of damage to low-temperature toughness caused by carbides in this cast steel should not be too high, the mass fraction of Ni added is 0.5%-1%, and the mass fraction of Mn is slightly higher, at 1.3%-1.9%. Too high a Mn content will reduce the low-temperature toughness of the material.

[0052] Cu element setting: Adding a small amount of Cu can replace semi-coherent carbides to achieve precipitation strengthening. Cu precipitates generally have a lower tendency to grow in size and a lower shear modulus. Therefore, Cu is more conducive to ensuring high precipitation strengthening effect than carbides and can reduce damage to the toughness of materials. Based on the calculation and analysis results and the target strength range, the mass fraction of Cu that can be added is between 0.35% and 0.65%.

[0053] In summary, the research results indicate that the permissible content ranges of Ni, Mn, and Si in Cu-containing steel are relatively wide. However, while Si has some solid solution strengthening and interfacial strengthening effects between the Cu-aged phase and the matrix, it has little effect on improving the toughness of the matrix. Although Cr is beneficial for improving material strength, improper Cr content may lead to the precipitation of complex-structured carbides such as Cr7C3 and Cr3C2 in the matrix, which adversely affects the toughness of the material. Although V and Mo have some effect on inhibiting Cr carbide formation, the adjustable range of Cr content remains relatively low. After weighing the pros and cons, Cr and Si are controlled as residual elements, and their mass fraction should not exceed 0.4%. Furthermore, the mass fractions of impurity elements S, P, and residual Al can be controlled according to the general requirements for cast steel, not exceeding 0.01%, 0.015%, and 0.06%, respectively.

[0054] Specifically, in determining the types and ranges of the above elements, a combination of experimental, theoretical, and computational analysis was mainly used; the following material strengthening increment formula was used in the material design:

[0055] Considering solid solution strengthening Δσ sol Dislocation strengthening Δσ dis Fine grain strengthening Δσ reiine Precipitation strengthening Δσ y Total intensity increment σ:

[0056] σ=(Δ 2 σ dis +Δ 2 σ refine 0.5+Δσ sol +Δσ y (1)

[0057] Dislocation enhancement Δσ dis The following relationship typically exists between the dislocation density ρ and the dislocation density:

[0058] Δσ dis =2αGbρ0.5+75.46 (foundation strength, unit MPa) (2)

[0059] G is the shear modulus, calculated to be 69 GPa; b is the modulus of the Burgers vector for dislocations with a value of <111> / 2, which is 0.249 nm; α is the proportionality coefficient, which is 0.5 for α-Fe-based materials.

[0060] The second-phase enhancement is based on the Orowan particle precipitation enhancement Δσ derived by Gladman. y Incremental formula calculation:

[0061] Δσ y = (0.538Gbf) 1 / 2 / X)ln(X / 2b) (3)

[0062] It can be seen that the enhancement increment of the second phase is related to the volume fraction f and the particle diameter X, and the effect is best when the particle diameter X is 1-10nm.

[0063] Interface enhancement components:

[0064] Δτ y =2(γ) s / b) 3 / 2 / (bLT) 1 / 2 (4)

[0065] Coherent elastic distortion reinforcement:

[0066] Δτ y =4·1Gε 3 / 2 (rf / b) 1 / 2 (5)

[0067] Modulus strengthening:

[0068]

[0069] The sum of formulas (4)-(6) above, multiplied by the Taylor factor 2.73, represents the total increment in yield strength caused by the second phase. The matrix Burgers vector b, the shear modulus G of the matrix and the second phase, and the mismatch degree ε in the formulas are all substituted with their corresponding values. s This is the unstable stacking fault energy; the specific value is obtained from the measurement results.

[0070] The study found that simply precipitating FCC phases such as VC, TiC, and ZrC does not produce sufficient precipitation strengthening in cast steel to achieve a total yield strength of over 450 MPa. The main reason is that the volume fraction of carbides is too low and they are prone to growth. Although C-Mo-V can achieve a yield strength exceeding 550 MPa with reasonable formulation design, and the optimal strengthening ratio of C-Mo-V has been identified in the previous study, the large difference in modulus between the matrix and the second phase makes the interface prone to cracking, which means there is still room for further improvement in the low-temperature toughness of cast steel.

[0071] Preferably, during the smelting of cast steel, since the range of adjustable Mn, Si, and Ni in copper-containing steel is relatively wide, it is possible to perform microalloying by adding V and Nb, and to combine it with any one or more elements from Cr and Mo to improve the performance of the cast steel. Specifically, it is considered to use a copper-containing phase to replace part of (Mo,V)C. Previous calculations, analysis and experimental verification have also shown that the range of adjustable Mn, Si, and Ni in Cu-containing steel is relatively wide, and it is feasible to add V and Nb for microalloying and combine it with elements such as Cr and Mo.

[0072] Therefore, based on the optimal strengthening ratio of C-Mo-V, the C and V contents were reduced, and analysis was conducted. The strengthening increment of the second phase was calculated. For the copper-containing phase: the mismatch degree ε was taken as the average value of BCC-Cu and FCC-Cu, and the interfacial energy was taken as 0.41 J / m from the Prisma database. 2 The particle radius r, volume fraction f, and average particle spacing L are obtained from Prisma calculations. For (Mo,V)C, the dislocation bypass mechanism is the main factor, so the strengthening mechanism represented by formulas (4)-(6) is basically ineffective. Calculations and analyses were carried out on the cases where the second phase is dominated by bulk nucleation and dislocation nucleation. By combining experimental results, it was clarified that the copper-containing phase of bulk nucleation contributed more than half of the strengthening increment, and the relationship between tempering temperature and time and aging increment and the change of the second phase nucleation mechanism was explored: at 570℃, the copper-containing phase is mainly dominated by dislocation nucleation, with a low volume fraction, and the strength is mainly contributed by (Mo,V)C; at 640℃, although the strengthening effect of (Mo,V)C decreases, the yield strength is significantly improved due to the precipitation of a large amount of copper-containing phase and the good strengthening effect; if the temperature is further increased, the strength will decrease due to over-aging.

[0073] The precipitation strengthening increment was also predicted to be 145MPa-156MPa when aging for 1-4 hours at 640℃. Combined with the strengthening contribution of (Mo,V)C at this temperature, FCC-Cu contributed 70%-80% of the aging strengthening increment. When aging for 1-4 hours at 570℃, the precipitation strengthening components were 11MPa-17MPa and 32MPa-76MPa under the combined action of FCC-Cu (weighted) and (Mo,V)C, respectively. When aging for 1-4 hours at 680℃, the precipitation strengthening components were 17MPa-78MPa or 122MPa-148MPa under the combined action of FCC-Cu and (Mo,V)C, as shown in Tables 1-2 below.

[0074] Table 1. Partial results of the research process for the trial production of CV-Mo-Cu system cast steel.

[0075]

[0076]

[0077] Table 2. CV-Mo-Cu series trial cast steels and their solid solution strengthening components

[0078]

[0079] Therefore, it can be seen that by controlling the prepared cast steel material and its mass fraction percentage, it is possible to prepare a non-quenched + tempered microalloy precipitation-strengthened cast steel based on the replacement of quenched and tempered steel plates with precipitation-strengthened HSLA steel plates. This will further improve the yield strength grade of the non-quenched + tempered microalloy precipitation-strengthened cast steel, enhance the low-temperature toughness of large-size structural castings under conventional processes, and improve the dislocation shear effect and precipitation strengthening increment by replacing part of the hard carbide second phase with soft Cu phase. This will further improve the yield strength grade of the non-quenched + tempered microalloy precipitation-strengthened cast steel, enhance the dislocation shear effect and precipitation strengthening increment by replacing part of the hard carbide second phase with soft Cu phase, improve the low-temperature toughness of large-size structural castings under conventional processes, and reduce the overall cost of large-size structural castings through a simple heat treatment process.

[0080] A method for preparing Cu-containing Mo-V microalloyed cast steel for structural applications, the method comprising the following steps:

[0081] Step 1: Add the billet to the electric arc furnace or induction furnace;

[0082] Step 2: Perform conventional smelting on the billet to obtain molten steel with the required quality target composition;

[0083] Step 3: Adjust the temperature of the molten steel and tap it out to obtain semi-finished cast steel;

[0084] Step 4: Heat treat the semi-finished cast steel to obtain the finished cast steel;

[0085] Step 5: Determine the quality of the cast steel to obtain high-quality finished cast steel;

[0086] In step two, conventional smelting refers to smelting using common cast steel smelting methods. When smelting cast steel, since the range of Mn, Si, and Ni in copper-containing steel is relatively wide, micro-alloying can be achieved by adding V and Nb, and any one or more elements from Cr and Mo can be added in combination. In step three, the temperature of the molten steel is adjusted, and the molten steel is poured into a mold for casting steel, and the steel is tapped to obtain semi-finished cast steel. The pouring temperature is between 1540℃ and 1580℃. In this embodiment, a sand mold is used for casting steel to reduce costs and improve the plasticity of the cast steel.

[0087] The method described above can reduce the cost of conventional processes for large-size structural castings. Compared to existing technologies, where controlled rolling and cooling methods cannot fully refine grains and increase dislocation and nanoscale second-phase density, the overall performance of precipitation-strengthened cast steel is currently low, with yield strength rarely exceeding 450 MPa and low low-temperature toughness, thus limiting the application range of microalloyed precipitation-strengthened cast steel. Through the aforementioned preparation method and the setting of the cast steel material composition, combined with material calculation methods, a Cu-containing Mo-V microalloyed precipitation-strengthened high-strength cast steel for structural use and its heat treatment method can be invented, achieving a yield strength that... While maintaining a stable strength of over 450 MPa, the steel also retains a high level of low-temperature toughness. Furthermore, the cast steel in this application has no special requirements for smelting and casting. The target composition can be obtained by using a conventional electric arc furnace or induction furnace and smelting according to ordinary cast steel smelting methods. Finally, the steel is tapped after adjusting the temperature of the molten steel, and the pouring temperature is controlled at 1540℃-1580℃, which is comparable to the smelting and casting process of ordinary low-alloy cast steel. This further simplifies the complexity of cast steel preparation, improves the efficiency of cast steel preparation, enhances the quality of cast steel preparation, reduces the cost of cast steel, increases the possibility of mass production of cast steel, and improves the economic benefits of cast steel.

[0088] In step four, the heat treatment process mainly includes homogenization treatment + normalizing + tempering, and the method for performing the heat treatment includes the following steps:

[0089] Step S41: Homogenize the semi-finished cast steel;

[0090] Step S42: Perform normalizing treatment to dissolve alloying elements and refine grains during cooling;

[0091] Step S43: After normalizing, tempering is performed to precipitate the strengthening phase;

[0092] Step S44: Post-tempering treatment: After tempering, the material should be cooled as quickly as possible, i.e., the temperature should be lowered as quickly as possible.

[0093] In step S41, the semi-finished cast steel is homogenized to eliminate segregation. The homogenization temperature range is 980℃-1100℃, and the holding time T1 is set according to the wall thickness of the cast steel. In step S42, the purpose of normalizing is to dissolve alloying elements and refine grains during cooling. Based on the characteristics of the target composition, the normalizing temperature range is set to 860℃-940℃. The normalizing holding time T2 is calculated as: T2=t+D×t1, where t is the preset time, t1 is the preset error time value, and D is the effective wall thickness of the cast steel. In this embodiment, t=30min, and t1 ranges from 1min to 2min, but it is not limited to this in practice and can be set according to requirements. In step S43, the tempering temperature range is 550℃-680℃, and the tempering time T3 ranges from 1 to 4 hours. Specifically, tempering is mainly to induce the precipitation of strengthening phases. Considering the changes in the volume fraction and size of carbides and Cu-rich precipitates due to different holding temperatures and holding times, the tempering process is 550℃-680℃×1h-4h. In step four, steps S41-S43 can be repeated twice, or step S43 can be repeated twice, to improve the quality and performance of cast steel. That is, higher cast steel performance can be obtained by performing two homogenization treatments + normalizing + tempering, or by performing two-step tempering. In this embodiment, the specific values ​​of T1, T2, T3, t, t1, and D are all set according to the requirements.

[0094] The preparation method includes a Cu-containing Mo-V microalloyed precipitation-strengthened cast steel and its heat treatment method, which allows for a wider range of adjustable strength in the cast steel. By selecting appropriate composition and a homogenization treatment + normalizing + tempering heat treatment process, the cast steel can achieve a yield strength of over 450 MPa while ensuring high low-temperature toughness. Furthermore, the prepared cast steel breaks through the yield strength limit of existing microalloyed precipitation-strengthened cast steels. High strength can be achieved through a simple homogenization treatment + normalizing + tempering heat treatment process, meeting the requirements of shipbuilding, marine engineering, mining, power, and mechanical engineering, which have high requirements for cast steel strength and low-temperature toughness. It is particularly suitable for structures and working conditions where high requirements are placed on casting wall thickness and low-temperature toughness, where quenching + tempering (i.e., tempering) is inconvenient, and welding is required. Moreover, the low alloy element content helps reduce material and manufacturing costs, making it a technology worthy of promotion. Furthermore, the steel is smelted using an electric arc furnace or induction furnace and sand casting, and the finished product is obtained through a homogenization treatment + normalizing + tempering heat treatment process. Composite strengthening is achieved through the precipitation of multiple carbides and C solid solution strengthening. The homogenization treatment temperature is 980℃-1100℃; the normalizing temperature is 860℃-940℃; and the tempering process is 550℃-680℃×1h-4h. The performance is tested using φ160mm ingots, and its performance meets the following requirements: yield strength Rp0.2≥450MPa, tensile strength Rm≥610MPa, reduction of area A≥22%, elongation Z≥70%. It has high Charpy V-type impact values ​​at low temperatures of 0℃ and -40℃, and the average Charpy impact value can guarantee KV2≥80J at 0℃ and KV2≥41J at -40℃. Compared with ordinary microalloy precipitation-strengthened cast steel, the strength is significantly improved, and compared with microalloyed cast steel that relies solely on carbide strengthening, the low-temperature toughness is significantly improved.

[0095] In step five, the qualification of the cast steel is determined by calculation and analysis based on the crack sensitivity coefficient PCM formula for low-alloy high-strength steel and the carbon equivalent CE formula recommended by the International Institute of Welding. This is used to further determine the quality of the cast steel, screen out high-quality finished cast steel, and improve the quality inspection of the prepared cast steel. Specifically, based on the above target composition, the crack sensitivity coefficient P of low-alloy high-strength steel proposed by Ito et al. is used... CM Calculate the P of the cast steel using the formula and the carbon equivalent CE formula recommended by the International Institute of Welding. CM The CE ranges are 0.173%-0.275% and 0.380%-0.616%, respectively, calculated using the following formulas:

[0096]

[0097]

[0098] Among them, P cmIn the calculation formula, elements not present in the cast steel are substituted as zero. The above results show that the cast steel can be welded, but preheating is required to avoid cold cracking. Compared with HY80 and 15Ni3CrMnMo cast steels with similar strength grades, and US4634476 with similar composition, the results are shown in Table 3 below:

[0099] Table 3

[0100]

[0101] As can be seen from Table 3 above: cast steel CE and P cm Significantly lower than tempered HY80 and 15Ni3CrMnMo cast steel; compared with US4634476, P cm The values ​​are slightly lower for Pcm and slightly higher for CE, but there is a large overlap between the two results. Since US4634476 only specifies the upper limit for the content of Ni, Cr and Cu in cast steel, it has a certain impact on the calculation results of Pcm and CE. If the actual composition is used for calculation, the difference is not significant, and the weldability and hardenability of the two are basically at the same level.

[0102] Therefore, by casting steel through sand casting during the preparation process to obtain finished cast steel castings, and by screening the castings for qualification, the performance and quality of cast steel can be determined through specific theoretical data, thereby greatly improving the qualification rate of cast steel and distinguishing the quality grades of castings. This can effectively identify castings with high performance, further improve the economic benefits of castings, reduce the cost of cast steel, and enhance the stability and quality of cast steel.

[0103] Example 1: The steel was smelted in an induction furnace and sand-cast, and then heat-treated using the technical approach of this invention. Test bars with a diameter of φ140mm were cast according to the normal smelting process. The homogenization temperature was 1050℃, the normalizing temperature was 880℃, and the tempering process was 570℃ for 1 hour.

[0104] Example 2: The steel was smelted in an induction furnace and sand-cast, and then heat-treated using the technical approach of this invention. Test bars with a diameter of φ180mm were cast according to the normal smelting process. The homogenization temperature was 1030℃, the normalizing temperature was 900℃, and the tempering process was 620℃ for 2 hours.

[0105] Example 3: The steel was smelted in an induction furnace and sand-cast, and then heat-treated using the technical approach of this invention. Test bars with a diameter of φ140mm were cast according to the normal smelting process. The homogenization temperature was 1030℃, the normalizing temperature was 890℃, and the tempering process was 640℃ for 4 hours.

[0106] Example 4: The steel was smelted in an induction furnace and sand-cast, and then heat-treated using the technical approach of this invention. Test bars with a diameter of φ180mm were cast according to the normal smelting process. The homogenization temperature was 1030℃, the normalizing temperature was 880℃, and the tempering process was 660℃ for 2 hours.

[0107] Example 5: The steel was smelted in an induction furnace and sand-cast. Using the technical route of the present invention, the cast steel was heat-treated. The steel was cast into φ140mm test bars according to the normal smelting process. The homogenization temperature was 1030℃, the normalizing temperature was 900℃, and the tempering process was 680℃×1h.

[0108] Example 6: The steel was smelted in an induction furnace and sand-cast. Using the technical route of the present invention, the cast steel was heat-treated. The steel was cast into φ180mm test bars according to the normal smelting process. The homogenization temperature was 1030℃, the normalizing temperature was 880℃, and the tempering process was 570℃×4h.

[0109] The composition of the materials in the above embodiments is shown in Table 4, and the measured results of their mechanical properties are shown in Tables 5 and 6.

[0110] Table 4 Measured values ​​of chemical composition of materials

[0111] Example C Mn Si Ni Mo V Cu Example 1 0.091 1.62 0.222 0.78 0.337 0.075 0.35 Example 2 0.083 1.41 0.233 0.60 0.362 0.088 0.5 Example 3 0.1 1.52 0.243 0.90 0.286 0.092 0.45 Example 4 0.07 1.82 0.201 0.83 0.304 0.071 0.5 Example 5 0.12 1.30 0.223 0.85 0.286 0.070 0.35 Example 6 0.064 1.82 0.201 0.83 0.304 0.071 0.63

[0112] Table 5 Measured values ​​of mechanical properties

[0113] Example [R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A / ％ Z / ％ 0℃KV2 / J -40℃KV2 / J Example 1 454 621 23.5 70 126 89 Example 2 469 644 22 71 103 66 Example 3 511 657 24 75 86 48 Example 4 487 655 26.5 74 90 46 Example 5 502 662 24.5 73 95 49 Example 6 478 653 25.5 74 117 75

[0114] Comparing the following copper-free normalized + tempered composite microalloyed cast steels or steel plates (steel plates are easier to obtain higher precipitation strengthening increments and yield strength, but their yield strength ratio will also increase), it is shown that this patent has lower carbon content / carbon equivalent while obtaining higher yield strength and low temperature toughness, as shown in Tables 6 and 7 below.

[0115] Table 6 Measured values ​​of mechanical properties

[0116]

[0117]

[0118] Table 7 Measured values ​​of mechanical properties

[0119] Example <![CDATA[R p0.2 / MPa]]> <![CDATA[R m / MPa]]> A / ％ Z / ％ 0℃KV2 / J -40℃KV2 / J Comparative Example 1 562 747 18.5 66 28 / Comparative Example 2 403 536 22 60 17 / Comparative Example 3 562 747 18.5 66 36 / Comparative Example 4 417 547 26 59 / 16.5 Comparative Example 5 495 624 21 48 / 17 Comparative Example 6 425 500 35 74 / 57 Comparative Example 7 507 597 20 70 / 64

[0120] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A Cu-containing Mo-V microalloyed cast steel for structural use, characterized in that, The cast steel material, by mass fraction percentage, consists of the following components: C: 0.06-0.13%, Mn: 1.3-1.9%, Si: 0.2-0.4%, Ni: 0.5-1%, Mo: 0.18-0.42%, V: 0.05-0.09%, Cu: 0.25-0.65%; the balance being Fe and unavoidable impurities. The cast steel is prepared by the aforementioned method for preparing Cu-containing Mo-V microalloyed cast steel for structural applications; the method includes the following steps: Step 1: Add the billet to the electric arc furnace or induction furnace; Step 2: Perform conventional smelting on the billet to obtain molten steel with the required quality target composition; Step 3: Adjust the temperature of the molten steel and tap it out to obtain semi-finished cast steel; Step 4: Heat treat the semi-finished cast steel to obtain the finished cast steel; Step 5: Determine the quality of the cast steel to obtain high-quality finished cast steel; In step three, the temperature of the molten steel is adjusted and poured into a mold to obtain semi-finished cast steel; the pouring temperature is between 1540℃ and 1580℃. In step five, the qualification of cast steel is determined by calculation and analysis based on the crack sensitivity coefficient PCM formula for low alloy high strength steel and the carbon equivalent CE formula recommended by the International Institute of Welding. This is used to further determine the quality of cast steel and screen out high-quality finished cast steel. Specifically, based on the above target composition, and according to the formulas for the crack sensitivity coefficient (PCM) and carbon equivalent (CE) of low-alloy high-strength steel, the PCM and CE ranges of this cast steel are calculated to be 0.173%-0.275% and 0.380%-0.616%, respectively. The calculation formulas are as follows: ； In the calculation formula for Pcm, elements not present in the cast steel are substituted as zero. Step four involves the following steps for heat treatment: Step S41: Homogenize the semi-finished cast steel; Step S42: Perform normalizing treatment; Step S43: After normalizing, perform tempering treatment; Step S44: Post-tempering treatment: After tempering, rapid cooling is performed; Specifically, in step S41, the semi-finished cast steel after casting is homogenized, and the homogenization temperature range is 980℃-1100℃. In step S42, the normalizing temperature range is set to 860℃-940℃, and the normalizing holding time T2 is calculated as follows: T2=t+D×t1, where t is the preset time, t1 is the preset error time value, and D is the effective wall thickness of the cast steel. In step S43, the tempering temperature range is 550℃-680℃, and the tempering time T3 ranges from 1h to 4h.

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