Low-temperature cast steel for polar region ship and heat treatment process of low-temperature cast steel

Abstract

The invention provides low-temperature cast steel for polar ships and a heat treatment process of the low-temperature cast steel. The cast steel comprises the following chemical components in percentage by weight: 0.03-0.07% of C, 0.10-0.20% of Si, 1.3-1.5% of Mn, 3.0-4.0% of Ni, 0.80-1.00% of Al, 1.5-2.5% of Cu, not more than 0.05% of Cr and Mo, not more than 0.015% of S, not more than 0.020% of P and the balance of Fe and inevitable impurities. The cast steel has the advantages of ultra-large wall thickness, high strength, low-temperature high toughness, weldability, corrosion resistance and the like, and can meet the building requirements of polar region ship stem columns.

Description

Technical Field

[0001] The present invention relates to the technical field of alloy cast steel, in particular to a low-temperature cast steel for polar ships and a heat treatment process thereof, and more particularly to a heat treatment process thereof for ultra-thick, high-strength and tough weldable low-temperature cast steel for polar ships. Background Art

[0002] Polar regions hold broad development prospects, possessing significant scientific and economic value, and represent a crucial direction for my country's future development. The polar ocean environment experiences persistently low temperatures (down to -60°C) and is subject to the presence of ice and floating ice. In such low temperatures, alloy steel loses its toughness and significantly weakens its ability to resist crack initiation and propagation, significantly increasing the likelihood of widespread low-stress structural failure and posing a serious threat to structural safety and reliability. Furthermore, the additional ice loads borne by ships during surface icebreaking, icebreaking, and surface collisions with floating ice can also impact structural safety.

[0003] Cast steel is an indispensable material for the construction of polar vessel hull structures, primarily used in the critical bow structure. This structure, crucial for icebreaking, is subject to harsh operating conditions, complex shapes, and heavy weight (up to 30 tons per unit, with a maximum thickness of 600 mm). It is welded to the main hull. Therefore, to ensure safe and reliable navigation, its construction requires thick, high-strength, corrosion-resistant, and weldable low-temperature cast steel. According to the China Classification Society's "Materials and Welding Specifications," hull structural steel castings include carbon steel / carbon-manganese steel and alloy steel. However, their low-temperature impact test temperature is only 0°C, which is insufficient to meet the requirements of polar vessel construction. Therefore, to meet the development of polar equipment, it is necessary to develop high-strength and tough low-temperature cast steel for polar vessels. This requires a yield strength of at least 450 MPa and an impact energy of at least 80 J at -60°C for the construction of polar vessel bows.

[0004] In the prior art, invention patent CN102206790A proposes a low-temperature steel casting material and process, with a yield strength of no less than 690 MPa. However, the quenching + heat treatment process limits the maximum wall thickness, and the impact energy at -45°C is only ≥32J.

[0005] Invention patent CN107475487B proposes a method for producing low-carbon, low-alloy, high-strength, and low-temperature toughness steel castings. Its yield strength is 680MPa to 760MPa, but its average impact energy at -40°C is only 48J to 65J.

[0006] Invention patent CN102424935A proposes a low-temperature cast steel containing Cr and Ni. Its yield strength is 500-519 MPa, but its impact energy at -45°C is only 30-32 J, and its carbon content is not less than 0.25%, making it difficult to weld.

[0007] Invention patent CN102776449B proposes a thick-walled weldable cast steel with a yield strength of ≥370MPa and an impact energy of only ≥27J at -40°C.

[0008] Invention patent CN111172477B proposes a high-yield, low-temperature, high-toughness seat frame casting and its manufacturing method for deep-sea ships. The yield strength of the casting is required to be no less than 520MPa, and the impact absorption energy value at -20℃ is only no less than 65J, which is a certain distance from the polar requirements, and its wall thickness is only 120mm.

[0009] It can be seen that the cast steel in the existing technology is difficult to meet the construction requirements of extremely low-pressure ships in terms of maximum thickness, yield strength, low-temperature toughness and other properties.

[0010] In summary, existing cast steel products or patented technologies for ship hull structures cannot meet the construction requirements of polar ship bows. It is necessary to develop weldable cast steel with a wall thickness of up to 600 mm, a yield strength of not less than 450 MPa, and excellent low-temperature toughness at -60°C. Summary of the Invention

[0011] In view of this, the present invention aims to propose a low-temperature cast steel for polar ships and its heat treatment process, so as to solve the problem that cast steel in the existing technology is difficult to meet the requirements of polar ship bow construction, especially the requirement for low-temperature toughness at -60°C.

[0012] To achieve the above object, the technical solution of the present invention is achieved as follows:

[0013] A low-temperature cast steel for polar ships has the following chemical composition (by weight): C content of 0.03% to 0.07%, Si content of 0.10% to 0.20%, Mn content of 1.3% to 1.5%, Ni content of 3.0% to 4.0%, Al content of 0.80% to 1.00%, Cu content of 1.5% to 2.5%, Cr and Mo content of no more than 0.05%, S content of no more than 0.015%, P content of no more than 0.020%, and the balance being Fe and unavoidable impurities.

[0014] Furthermore, Ni / Cu>1.20, and Ni+Cu≥5.0%.

[0015] Furthermore, the yield strength of the cast steel is ≥450 MPa, and the impact energy at -60°C is ≥105 J.

[0016] Furthermore, the cast steel has a yield strength of 452 MPa to 497 MPa, a tensile strength Rm of 560 MPa to 617 MPa, an elongation after fracture A of 25.5% to 28.5%, and a cross-sectional reduction rate Z of 72% to 78%.

[0017] Furthermore, the average corrosion rate of the cast steel is ≤1.02 mm / year.

[0018] A heat treatment process for low-temperature cast steel for polar ships is provided, which is used for the low-temperature cast steel for polar ships. The heat treatment process comprises: a normalizing process, air cooling; a two-phase zone normalizing process, air cooling; and a tempering process, air cooling.

[0019] Furthermore, the holding temperature of the normalizing process is 890° C. to 910° C., and the holding time is 4 min / mm to 6 min / mm.

[0020] Furthermore, the holding temperature of the two-phase zone normalizing process is 700° C. to 740° C., and the holding time is 2 min / mm to 3 min / mm.

[0021] Furthermore, the holding temperature of the tempering process is 540° C. to 560° C., and the holding time is 8 min / mm to 10 min / mm.

[0022] Compared with the prior art, the low-temperature cast steel for polar ships and the heat treatment process thereof described in the present invention have the following advantages:

[0023] The low-temperature cast steel for polar ships and its heat treatment process described in the present invention enable the cast steel to have the advantages of ultra-thick wall, high strength, high toughness at low temperature, weldability, corrosion resistance, etc., and can meet the construction requirements of polar ship bows. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0025] Figure 1 This is a microstructure photograph of the cast steel (surface) obtained in Example 1 of the present invention;

[0026] Figure 2 This is a microstructure photograph of the cast steel (T / 4 position) obtained in Example 1 of the present invention;

[0027] Figure 3 This is a microstructure photograph of the cast steel (core) obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The inventive concepts of the present disclosure will be described below using terms commonly used by those skilled in the art to convey the essence of their work to other persons skilled in the art. However, these inventive concepts can be embodied in many different forms and should not be considered limited to the embodiments described herein.

[0029] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0030] The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with embodiments.

[0031] In order to solve the problem that cast steel in the existing technology is difficult to meet the construction requirements of polar ship bows, especially the requirement for low-temperature toughness at -60°C, this embodiment proposes a low-temperature cast steel for polar ships, the chemical composition (weight percentage) of which is as follows: C content of 0.03% to 0.07%, Si content of 0.10% to 0.20%, Mn content of 1.3% to 1.5%, Ni content of 3.0% to 4.0%, Al content of 0.80% to 1.00%, Cu content of 1.5% to 2.5%, Cr and Mo contents of not more than 0.05%, S content of not more than 0.015%, P content of not more than 0.020%, and the balance being Fe and unavoidable impurities.

[0032] Among them, Ni / Cu (the ratio of Ni content to Cu content) should be greater than 1.20, and Ni+Cu (the sum of Ni content and Cu content) should not be less than 5.0%.

[0033] Therefore, the cast steel of this application adopts a low-carbon design, and the C content is controlled at 0.03% to 0.07%, which reduces casting segregation and cementite content, and improves the uniformity and low-temperature toughness of the cast steel; the addition of 1.5% to 2.5% Cu plays a precipitation strengthening role on the one hand to ensure higher strength, and at the same time can improve casting fluidity and corrosion resistance. In order to avoid the adverse effects of the addition of Cu on the hot working performance (hot cracking) and further improve the low-temperature toughness of the cast steel, appropriate Ni must be added, Ni / Cu is greater than 1.20, and Ni+Cu is not less than 5.0%, which can effectively inhibit the influence of Cu on the hot working performance and reduce the probability of hot cracking of the cast steel during hot working. At the same time, the synergistic effect of Ni and Cu can significantly improve the low-temperature toughness of the cast steel, while taking into account the strength and corrosion resistance of the cast steel; by adding a certain amount of Al, in the subsequent heat treatment process, it combines with Ni to form a NiAl precipitation phase, which further improves the strength of the cast steel without reducing the low-temperature toughness of the cast steel.

[0034] Specifically, for ease of understanding, this application introduces the content of each element one by one.

[0035] Carbon (C) is the primary solid solution strengthening element, significantly increasing the strength of cast steel. However, it negatively impacts impact energy, particularly upper platform impact energy, and significantly reduces weldability. Therefore, the cast steel of the present invention utilizes a low-carbon design, with the carbon content controlled within a range of 0.03% to 0.07%.

[0036] Silicon (Si) is an essential element for deoxidation in steelmaking and also has a certain strengthening effect. However, excessive Si content can reduce the cleanliness, toughness, and weldability of the steel. Based on comprehensive considerations, the silicon content in this invention is limited to 0.10% to 0.20%.

[0037] Manganese (Mn) is an effective element for increasing strength and deoxidation. However, due to its high tendency to segregate, its content should not be too high. To ensure strength and avoid severe segregation, the present invention limits the manganese content to 1.3% to 1.5%.

[0038] Nickel (Ni) prevents thermal cracking during heating and improves the steel's low-temperature toughness. However, excessive Ni content can easily lead to compositional deviations and increase the steel's tendency to harden, resulting in the formation of bainite, which can dramatically reduce the steel's low-temperature toughness. The present invention limits the Ni content to 3.0% to 4.0%.

[0039] Aluminum (Al) combines with nickel to form NiAl precipitates, which can play a precipitation strengthening role, but too high a content will significantly reduce the weldability of the steel. In the present invention, the aluminum content range is limited to 0.80% to 1.00%.

[0040] Copper (Cu) has solid solution strengthening and precipitation dispersion strengthening effects, which can improve yield strength, tensile strength, and corrosion resistance. However, excessive copper content can easily cause surface cracking. In this invention, the copper content is limited to 1.5% to 2.5%.

[0041] Chromium (Cr) has a significant solid solution strengthening effect and can improve the corrosion resistance of steel. However, when the Ni content is high, adding too much Cr will promote the formation of martensite structure and reduce the plasticity and toughness of the steel. In the present invention, the chromium content range is limited to no more than 0.05%.

[0042] Molybdenum (Mo) is a substitutional solid solution alloying element that can significantly improve the hardenability of steel, delay ferrite transformation, and promote the formation of bainite. In the present invention, the molybdenum content is limited to no more than 0.05%.

[0043] Phosphorus (P) and sulfur (S) are common impurity elements in steel. Phosphorus tends to segregate at grain boundaries, reducing the toughness of cast steel; sulfur easily forms inclusions with other elements, reducing the strength-toughness balance. In this invention, the phosphorus content is limited to no more than 0.015%, and the sulfur content is limited to no more than 0.020%.

[0044] Furthermore, the low-temperature cast steel for polar ships is produced through conventional cast steel smelting processes. Since the chemical composition is defined, the improvements in this application regarding the cast steel manufacturing process are not in the smelting process, and this application only briefly describes the relevant smelting processes. Specifically, this application uses a conventional electric arc furnace or vacuum furnace for smelting according to conventional cast steel smelting methods. During the melting phase, short arc operation with low voltage and high current is used to improve thermal efficiency. After the melt is cleared, quicklime is added to create slag, and the alkalinity is adjusted appropriately. Oxygen blowing is used for decarburization, and slag is removed at the end of oxidation. Quicklime is added for secondary slag production to maintain white slag deoxidation. The steel temperature is adjusted to 1540°C to 1580°C before tapping. Sand casting is used, and bottom casting is used for pouring.

[0045] For the low-temperature cast steel for polar ships, as the main improvement point of this application in the cast steel manufacturing process, in order to achieve a good strength-toughness match for the cast steel, this application further proposes a heat treatment process for the low-temperature cast steel for polar ships, including: a normalizing process, a holding temperature of 890°C to 910°C, a holding time of 4 min / mm to 6 min / mm, and air cooling; a two-phase zone normalizing process, a holding temperature of 700°C to 740°C, a holding time of 2 min / mm to 3 min / mm, and air cooling; a tempering process, a holding temperature of 540°C to 560°C, a holding time of 8 min / mm to 10 min / mm, and air cooling.

[0046] The main purpose of the normalizing process is to refine the grain size. Cast steel must be fully austenitized and then held in a coolant. Excessively high temperatures will result in coarse grains, affecting final performance. Excessively low temperatures will make it difficult to eliminate the initial coarse grains, limiting the grain refinement effect. Air cooling after normalizing forms a ferrite + pearlite structure in the cast steel, resulting in low internal and surface stresses and a low cracking potential. Based on the steel's composition, calculations and heat treatment process tests were conducted, and the holding temperature during normalizing was set at 890°C to 910°C, with a holding time of 4 to 6 minutes per millimeter (mm), followed by air cooling.

[0047] The main purpose of the two-phase zone normalizing process is to redistribute the composition and refine the grains. The cast steel is held in the two-phase zone to form a mixed structure of ferrite and austenite. The alloy content in ferrite is low, while the alloy content in austenite is high. The uneven distribution of Ni and Al, in particular, is mainly to ensure the subsequent precipitation of NiAl phases. The holding time must ensure the full formation of ferrite and austenite. The cooling method has a significant impact on the final structure. The structure of air-cooled cast steel is ferrite + pearlite + bainite, with low internal and surface stresses and low cracking. Combined with the steel composition, calculation analysis and heat treatment process tests were conducted. The holding temperature of the two-phase zone normalizing process was set at 700℃~740℃, the holding time was 2min / mm~3min / mm, and the cooling was air-cooled.

[0048] The tempering process is mainly to precipitate NiAl phase and copper-rich phase to improve the strength of cast steel. If the tempering temperature is too high, the precipitated phase will grow excessively, weakening the strengthening effect. If the tempering temperature is too low, the precipitation is insufficient, which will also lead to insufficient strengthening effect and low low-temperature toughness. If the tempering time is too short, the NiAl phase and copper-rich phase in bainite and ferrite cannot be fully precipitated. If the tempering time is too long, the precipitated NiAl phase and copper-rich phase will grow, which will have an adverse effect on performance. If the cooling rate after tempering is too slow, the surface and core of the cast steel will have large differences in cooling rate. Temper brittleness is more likely to occur in areas with slower cooling rate, affecting the low-temperature toughness of the steel. Combined with the steel composition, calculation analysis and heat treatment process test were carried out. The holding temperature of the tempering process is 540℃~560℃, the holding time is 8min / mm~10min / mm, and air cooling is used.

[0049] In order to clearly illustrate the technical features of this solution, the present invention is further described below in conjunction with specific embodiments, but the present invention is not limited to the following embodiments.

[0050] Examples 1-3 employed the design composition and heat treatment process of the present invention. The chemical composition of the cast steel used in these examples is shown in Table 1 (Ni / Cu ratio greater than 1.20, and Ni + Cu not less than 5.0%), the maximum wall thickness of the cast steel and the heat treatment process are shown in Table 2, and the mechanical property test results are shown in Table 3. For ease of reference and comparison, Example 1 was conducted on cast steel with two different maximum wall thicknesses: a maximum wall thickness of 600 mm and a maximum wall thickness of 100 mm.

[0051] The results show that the cast steel of the present invention (Examples 1 to 3) has a good match between strength and toughness, with a yield strength of 452 MPa to 497 MPa, a tensile strength Rm of 560 MPa to 617 MPa, an elongation after fracture A of 25.5% to 28.5%, a cross-sectional shrinkage Z of 72% to 78%, and an average impact energy of not less than 105 J at -60°C; the cast steel has good uniformity, and the tensile properties and low-temperature impact properties of samples taken at different positions fluctuate little; this proves that the composition of the cast steel and the heat treatment process are reasonable and feasible.

[0052] Compared to the cast steel compositions of the present invention, Examples 4-6 incorporate certain amounts of chromium (Cr) and molybdenum (Mo) into the alloy composition, while the other alloying element contents are comparable to those of Example 2. The maximum wall thickness and heat treatment process for the cast steels are consistent with those of Example 2. It can be seen that the addition of chromium (Cr) and molybdenum (Mo) significantly increases the strength of the cast steels, with the yield strength increasing by approximately 100 MPa. However, the plasticity and toughness of the cast steels are significantly reduced, with the elongation after fracture decreasing to 13.5% and the impact energy at -60°C dropping to a minimum of 23 J. These results demonstrate that the patent's limit of ≤0.05% for the alloying elements chromium (Cr) and molybdenum (Mo) in the cast steel composition is reasonable.

[0053] Compared with the heat treatment process of the present invention, Examples 7-9 share the same composition and maximum wall thickness as Example 2, but differ in heat treatment parameters. Specifically, the two-phase normalizing temperature in Example 7 differs from that in Example 2, and the tempering temperatures in Examples 8 and 9 differ from those in Example 2. Compared with Example 2, Example 7 exhibits significantly higher strength and a -60°C impact energy reduction to below 100; Example 8 exhibits a significant decrease in yield strength; and Example 9 exhibits decreased strength and uneven -60°C impact energy. These results demonstrate that the patent's restrictions on cast steel heat treatment parameters are crucial for ensuring "yield strength no less than 450 MPa and -60°C impact energy no less than 105 J."

[0054] Table 1 Chemical composition of cast steel of Examples 1-6 (wt%)

[0055]

[0056] Table 2 Maximum wall thickness of cast steel and heat treatment process of Examples 1-9

[0057]

[0058] Table 3 Mechanical properties test results of cast steel of Examples 1-9

[0059]

[0060] Samples of the cast steels from Examples 1-3 were subjected to accelerated corrosion testing under laboratory conditions. For comparison, the corrosion rate of 420 MPa-grade EH47 steel plates was also measured using a 3.5% NaCl solution. The average corrosion rates of the samples after 30 days of corrosion testing are shown in Table 4. This demonstrates that the cast steels of the present invention exhibit excellent corrosion resistance, with an average annual corrosion rate of no more than 1.02 mm / year, lower than that of the EH47 steel plates.

[0061] Table 4 Average corrosion rate of cast steel and EH47 steel plate of Examples 1-3 after 30 days of full immersion corrosion

[0062]

[0063] In addition, the present application conducted metallographic observation on the cast steel with a maximum thickness of 600 mm in Example 1. Figure 1-3 The following are microstructure photos of the cast steel surface, T / 4, and core observed through an optical microscope (500 times), and the structures are all ferrite + bainite tempered structure + a small amount of pearlite.

[0064] From the above content, we can see that:

[0065] The low-temperature cast steel for polar ships of the present invention has the advantages of ultra-large wall thickness, high strength, high toughness at low temperature, and weldability. In particular, its yield strength is not less than 450 MPa, and the impact energy at -60°C is not less than 105 J, which can meet the construction requirements of polar ship bows.

[0066] The maximum casting wall thickness of the present invention can reach 600mm with good uniformity and excellent mechanical properties. It can be used in polar ship structures, marine engineering, mining, electric power and mechanical engineering, and other fields with high requirements on the strength and thickness of cast steel. It is especially suitable for structures and components with high requirements on the consistency of casting surface and internal performance and large wall thickness.

[0067] The low-temperature cast steel for polar ships of the present invention exhibits excellent corrosion resistance, with an average annual corrosion rate of no more than 1.02 mm / year, making it suitable for use in marine environments. Furthermore, the low-temperature cast steel for polar ships of the present invention exhibits excellent weldability, enabling tailored welding between castings, making it suitable for welding ship structures.

[0068] 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 in the scope of protection of the present invention.

Claims

1. A low-temperature cast steel for polar ships, characterized in that: The chemical composition (weight percentage) is: C content is 0.03% to 0.07%, Si content is 0.10% to 0.20%, Mn content is 1.3% to 1.5%, Ni content is 3.0% to 4.0%, Al content is 0.80% to 1.00%, Cu content is 1.5% to 2.5%, Cr and Mo contents are not higher than 0.05%, S content is not higher than 0.015%, P content is not higher than 0.020%, and the balance is Fe and unavoidable impurities.

2. The low-temperature cast steel for polar ships according to claim 1, characterized in that: Ni / Cu>1.20, and Ni+Cu≥5.0%.

3. The low-temperature cast steel for polar ships according to claim 1, characterized in that: The yield strength of the cast steel is ≥450 MPa, and the impact energy at -60°C is ≥105J.

4. The low-temperature cast steel for polar ships according to claim 3, characterized in that: The cast steel has a yield strength of 452 MPa to 497 MPa, a tensile strength Rm of 560 MPa to 617 MPa, an elongation after fracture A of 25.5% to 28.5%, and a cross-sectional shrinkage Z of 72% to 78%.

5. The low-temperature cast steel for polar ships according to claim 1, characterized in that: The average corrosion rate of the cast steel is ≤1.02 mm / year.

6. A heat treatment process for low-temperature cast steel for polar ships, characterized in that: The heat treatment process is used for the low-temperature cast steel for polar ships according to any one of claims 1 to 5; the heat treatment process comprises: Normalizing process, air cooling; Two-phase zone normalizing process, air cooling; Tempering process, air cooling.

7. The heat treatment process for low-temperature cast steel for polar ships according to claim 6, characterized in that: The holding temperature of the normalizing process is 890° C. to 910° C., and the holding time is 4 min / mm to 6 min / mm.

8. The heat treatment process for low-temperature cast steel for polar ships according to claim 6, characterized in that: The holding temperature of the two-phase zone normalizing process is 700° C. to 740° C., and the holding time is 2 min / mm to 3 min / mm.

9. The heat treatment process for low-temperature cast steel for polar ships according to claim 6, characterized in that: The holding temperature of the tempering process is 540° C. to 560° C., and the holding time is 8 min / mm to 10 min / mm.

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