450mpa grade ultra-thick section high-nickel low-temperature cast steel and heat treatment process thereof

Abstract

The application provides a 450MPa-grade super-thick-section high-nickel low-temperature cast steel and a heat treatment process thereof, and relates to the technical field of cast steel materials.The chemical components of the cast steel are as follows in terms of percentage by weight: C: 0.01%-0.05%; Si: ≤0.20%; Mn: 0.3%-0.7%; Ni: 9.5%-10.5%; Cu: 0.1%-0.3%; S: ≤0.010%; P: ≤0.015%, and the balance is iron Fe and impurities.The cast steel has good weldability, the maximum casting wall thickness can reach 1000mm, has good uniformity, the yield strength is not lower than 450MPa, and the impact absorption energy at-60 DEG C is not lower than 165J, and can be used in the fields of large ship hull structures, ocean engineering, mines, power and mechanical engineering and the like which have high requirements on the strength and thickness of cast steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cast steel materials, in particular to a 450MPa-grade ultra-thick-section high-nickel low-temperature cast steel and a heat treatment process thereof. BACKGROUND

[0002] Cast steel parts are indispensable materials for the construction of large ship hull structures, mainly used for the manufacture of key structures such as bow columns, stern columns, rudder supports, tail shaft supports, hanging rudder arms, tail shaft pipe hubs, rudder blades, etc. As key structures, cast steel parts have the characteristics of harsh working conditions, complex shape, large single weight (some single weight up to 50 tons, maximum thickness up to 1000 mm), etc., and are connected to the main hull by welding, so in order to ensure the safe and reliable navigation of the ship, the construction of the cast steel parts must use low-temperature cast steel materials with large thickness, high strength, corrosion resistance and good weldability. In the Material and Welding Specification of China Classification Society, the yield strength of cast steel parts for ship structure is only 240 MPa, and the low-temperature impact test temperature is only 0℃. With the increase in size of large ships in China, and the demand for global and full-sea navigation, especially in the polar region where the temperature is low (the lowest temperature can reach -60℃) and there are ice layers and floating ice, there is a demand for high-strength and low-temperature toughness cast steel. In view of this, in order to meet the needs of large ships for full-sea navigation, it is necessary to develop ultra-thick-section high-strength and toughness low-temperature cast steel to meet the design requirements of maximum thickness not less than 1000 mm, yield strength not less than 450 MPa, and impact at -60℃ not less than 100 J.

[0003] According to relevant patents and literature, the invention patent CN102206790A proposes a material and process for cast steel for low temperature, the yield strength of which is not less than 690MPa, but the process of quenching + heat treatment limits the maximum wall thickness (the wall thickness of the embodiment is only 100mm), and the-45℃ impact is only 32J. Similarly, the invention patent CN107475487B proposes a production method of low-carbon low-alloy high-strength high-low-temperature toughness cast steel, the yield strength of which is not less than 600MPa, but the-40℃ impact is only 27J; the invention patent CN102424935A proposes a Cr and Ni containing low-alloy low-temperature cast steel, the yield strength of which is not less than 500MPa, but the-45℃ impact is only 30-32J, and the carbon content is not less than 0.25%, which is difficult to weld; the invention patent CN102776449B proposes a large wall thickness weldable cast steel, the yield strength of which is not less than 390MPa, and the-40℃ impact is only 33J; the invention patent CN111172477B proposes a high-yield low-temperature high-toughness seat frame cast steel for deep-sea ships and a manufacturing method, the yield strength of which is not less than 520MPa, but the-20℃ impact is only 65J, and the wall thickness is only 120mm. It can be seen that the maximum thickness, yield strength, low-temperature toughness and other properties of the existing cast steel are difficult to meet the construction requirements of the ultra-low ship.

[0004] In summary, the existing cast steel products or patent technologies for ship hull structures cannot meet the requirements of large-scale ship navigation in all sea areas, and it is urgent to develop a weldable cast steel with a wall thickness of 1000mm, a yield strength of not less than 450MPa, and excellent low-temperature toughness at-60℃. SUMMARY

[0005] Therefore, the present application aims to provide a 450MPa grade ultra-thick section high-nickel low-temperature cast steel and a heat treatment process thereof, so as to solve the problem that the yield strength and low-temperature impact toughness of the existing ultra-thick cast steel for ship hull structures cannot reach a high performance.

[0006] To achieve the above-mentioned purpose, the technical scheme of the present application is as follows:

[0007] A 450MPa grade ultra-thick section high-nickel low-temperature cast steel, the chemical composition of which is as follows in terms of percentage by weight: C: 0.01%-0.05%; Si: ≤0.20%; Mn: 0.3%-0.7%; Ni: 9.5%-10.5%; Cu: 0.1%-0.3%; S: ≤0.010%; P: ≤0.015%, and the balance is iron Fe and impurities.

[0008] Further, the cast steel has a yield strength of 478-534 MPa, a tensile strength of 626-682 MPa, an elongation of 23.5-26.5%, a reduction of area of 72-76%, and an impact energy at -60℃ of not less than 165 J.

[0009] A heat treatment process of a 450 MPa grade ultra-thick section high-nickel low-temperature cast steel for preparing the cast steel described above, comprising the following steps:

[0010] S1, diffusion annealing treatment;

[0011] S2, full normalizing treatment;

[0012] S3, two-phase region normalizing treatment;

[0013] S4, tempering treatment.

[0014] Further, in step S1, the diffusion annealing holding temperature is 1000-1050℃, the holding time is t1=h×(8-12)min / mm, h is the wall thickness of the casting in mm, and furnace cooling is used.

[0015] Further, in step S2, the full normalizing holding temperature is 840-860℃, the holding time is t2=h×(3-5)min / mm, h is the wall thickness of the casting in mm, and one of air cooling, air blast cooling, and spray cooling is used.

[0016] Further, in step S3, the two-phase region normalizing holding temperature is 670-700℃, the holding time is t3=h×(3-5)min / mm, h is the wall thickness of the casting in mm, and one of air cooling, air blast cooling, and spray cooling is used.

[0017] Further, in step S4, the tempering process holding temperature is 550-590℃, the holding time is t4=h×(10-12)min / mm, h is the wall thickness of the casting in mm, and one of air cooling, air blast cooling, and spray cooling is used.

[0018] Further, in steps S2-S4, when the wall thickness of the casting is ≤300 mm, air cooling is used to cool to room temperature.

[0019] Further, in steps S2-S4, when 300 mm

[0020] Further, in steps S2-S4, when 1000 mm≥the wall thickness of the casting>600 mm, a spray cooler is used.

[0021] Compared with the prior art, the 450MPa-grade ultra-thick-section high-nickel low-temperature cast steel and the heat treatment process thereof have the following advantages:

[0022] (1) The cast steel adopts low-C design, and the content of C is controlled to be 0.01% to 0.05%, so that the casting segregation and cementite content are reduced, and the uniformity and low-temperature toughness of the cast steel are improved; Ni is controlled to be 9.5% to 10.5%, which plays a solid solution strengthening role to ensure higher strength, and can improve the low-temperature toughness and corrosion resistance; a certain amount of copper Cu is added, and the strength of the steel is ensured through precipitation strengthening in the subsequent heat treatment process.

[0023] (2) The ultra-large wall thickness high strength and toughness weldable low-temperature cast steel prepared by the method has high strength and toughness, wherein the yield strength is not less than 450MPa, and the impact energy at-60℃ is not less than 165J.

[0024] (3) The maximum casting wall thickness can reach 1000mm and the uniformity is good, and the cast steel can be used in fields such as large ship hull structures, ocean engineering, mining, power and mechanical engineering which have high requirements on the strength and thickness of cast steel, and is especially suitable for structures and parts with high requirements on strength and toughness and large wall thickness.

[0025] (4) The ultra-large wall thickness high strength and toughness weldable low-temperature cast steel has good weldability, can realize splicing welding between castings, and is suitable for structure manufacturing with high requirements on welding work.

[0026] (5) The microstructure of the cast steel is ferrite + bainite tempered structure + a small amount of residual austenite, as shown in FIG. 1. Figure 1 BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 FIG. 1 is a microstructure diagram of the core of Example 1 observed by an optical microscope (OM). DETAILED DESCRIPTION

[0028] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below with reference to the drawings.

[0029] The present application provides a 450MPa-grade ultra-thick-section high-nickel low-temperature cast steel, which is an ultra-large wall thickness high strength and toughness weldable low-temperature cast steel for polar ship, and the chemical components of the cast steel are as follows in terms of percentage by weight: C: 0.01% to 0.05%; Si: ≤0.20%; Mn: 0.3% to 0.7%; Ni: 9.5% to 10.5%; Cu: 0.1% to 0.3%; S: ≤0.010%; P: ≤0.015%, and the balance is iron Fe and impurities.

[0030] ​In the chemical composition of the cast steel of the present application, the main alloying elements are defined as follows:

[0031] C is the most important solid solution strengthening element, which can significantly improve the strength of the cast steel, but is not conducive to impact toughness, especially the upper platform impact energy, and also significantly reduces the welding performance. Therefore, the cast steel involved in the present application adopts a low-carbon design, with a carbon content of 0.01% to 0.05%, reducing casting segregation and cementite content, and improving the uniformity and low-temperature toughness of the cast steel.

[0032] Si is one of the necessary elements for steelmaking deoxidization and also has a certain strengthening effect. However, too high silicon content will reduce the cleanliness of the steel, and the toughness and weldability will be poor. Considering comprehensively, the silicon content range is limited to ≤0.20%.

[0033] Mn is an effective element for improving strength and deoxidization. However, manganese has a high segregation tendency, so its content should not be too high. In order to ensure the strength and avoid serious segregation, the manganese content range is limited to 0.3% to 0.7% in the present application.

[0034] Ni is one of the most important alloying elements in the present application, which can significantly improve the stability of austenite and is beneficial to finally obtain stable austenite. In addition, nickel can improve the hardenability of the steel and the corrosion resistance, but too high content of nickel is easy to produce composition deviation, and also increases the hardening tendency of the steel, thereby producing bainite structure, which leads to a sharp decrease in low-temperature toughness of the steel. The Ni content range is limited to 9.5% to 10.5% in the present application, which on the one hand plays a solid solution strengthening role to ensure higher strength, and on the other hand improves low-temperature toughness and corrosion resistance.

[0035] Cu has solid solution strengthening and precipitation dispersion strengthening effect, which can improve yield strength, tensile strength and corrosion resistance. However, too high Cu will easily cause surface cracking. In the present application, the Cu content range is limited to 0.10 to 0.3%, which can ensure the strength of the steel through subsequent heat treatment process precipitation strengthening.

[0036] P and S are common impurity elements in steel. Phosphorus easily segregates at grain boundaries, reducing the toughness of the cast steel; sulfur easily forms inclusions with other elements, reducing the strength and toughness matching. In the present application, the phosphorus content is limited to not higher than 0.015%, and the sulfur content is limited to not higher than 0.010%.

[0037] The steel smelting of the application adopts the method of electric arc furnace smelting + refining + vacuum smelting, the molten steel is coarsely refined by alkaline electric furnace, refined by a refining furnace and poured after vacuum degassing (vacuum degree ≤98Pa, vacuum time ≥20min). In order to ensure the smelting quality of the castings, the electric furnace charge, ferroalloy and smelting auxiliary materials are carefully selected and matched to ensure that the contents of harmful elements such as As, Sn and Sb are as low as possible; at the same time, the contents of P and S are strictly controlled during smelting to ensure that the chemical components of the castings meet the design requirements. Before pouring, the cavity is baked: the outlet temperature of the hot blast furnace is greater than 200℃, the baking time is not less than 24 hours; according to the volume calculation of the cavity, argon is filled into the cavity; after pouring, the riser is covered with a heat preservation agent, the height of the heat preservation agent added is ≥100mm; after the heat preservation is completed and the riser root is ≤200℃, the box is knocked and the sand is removed, the residual sand around the riser and the castings is removed, and the castings are lifted out and thoroughly cleaned.

[0038] In order to realize good matching of strength and toughness of the cast steel, the application provides a heat treatment process for 450MPa grade ultra-thick section high-nickel low-temperature cast steel, which is used for preparing the cast steel and comprises the following steps:

[0039] S1, diffusion annealing treatment;

[0040] S2, full normalizing treatment;

[0041] S3, two-phase region normalizing treatment;

[0042] S4, tempering treatment.

[0043] Specifically, in step S1, the diffusion annealing holding temperature is 1000-1050℃, the holding time is t1, the holding time is t1=h×(8-12)min / mm, h is the wall thickness of the castings, the unit is mm, and furnace cooling is adopted.

[0044] Specifically, in step S2, the full normalizing holding temperature is 840-860℃, the holding time is t2=h×(3-5)min / mm, h is the wall thickness of the castings, the unit is mm, and one of air cooling, air cooling and spray cooling is adopted.

[0045] Specifically, in step S3, the two-phase region normalizing holding temperature is 670-700℃, the holding time is t3=h×(3-5)min / mm, h is the wall thickness of the castings, the unit is mm, and one of air cooling, air cooling and spray cooling is adopted.

[0046] Specifically, in step S4, the tempering process holding temperature is 550-590℃, the holding time is t4=h×(10-12)min / mm, h is the wall thickness of the castings, the unit is mm, and one of air cooling, air cooling and spray cooling is adopted.

[0047] More specifically, for full normalizing, two-phase region normalizing, and tempering process cooling, when the wall thickness of the casting is not greater than 300 mm, air cooling is used to cool to room temperature; when the wall thickness of the casting is greater than 300 mm and not more than 600 mm, a blower is used for air blowing cooling; when the wall thickness of the casting is greater than 600 mm and not more than 1000 mm, a blower is used for spray cooling.

[0048] In the steel casting preparation process of the application, the main performance heat treatment process parameter limitation reasons are described as follows:

[0049] (1) The diffusion annealing process is mainly to homogenize the composition of the steel casting. In the casting cooling process, alloy segregation inevitably exists, which leads to uneven composition and also leads to uneven organization and performance. Through high-temperature diffusion annealing, the composition can be homogenized and distributed, and the uneven degree can be reduced. Since the segregation is mainly Ni and Mn elements, the atomic diffusion is slow at a lower temperature, and the atomic diffusion is fast at a higher temperature, which is easy to realize homogenization, but will lead to local overburning, thereby causing the casting to be scrapped. Combined with the alloy composition design and calculation analysis, the diffusion annealing holding temperature is 1000-1050℃, the holding time t1 is the casting wall thickness h x (8-12) min / mm, and the furnace cooling is used.

[0050] (2) The full normalizing process is mainly to refine the grain size. After the diffusion annealing of the steel casting, the grain is coarse. Full normalizing needs complete austenitization for holding. If the temperature is too high, the grain will grow again, which will affect the final performance. If the temperature is too low, it will be difficult to eliminate the coarse grain in the previous stage, and the grain refinement effect will be limited. Combined with the composition of the steel, calculation analysis and heat treatment process test, the normalizing process holding temperature is set to 840-860℃, and the holding time t2 is the casting wall thickness h x (3-5) min / mm.

[0051] (3) The two-phase region normalizing process is mainly to redistribute the composition and refine the grain. The steel casting is held in the two-phase region to form a mixed organization of ferrite and austenite. The alloy content in the ferrite is low, and the alloy content in the austenite is high. The non-uniform distribution is mainly to ensure the formation of a small amount of residual austenite in the subsequent process. The holding time needs to ensure the stable generation of ferrite and austenite. Combined with the composition of the steel, calculation analysis and heat treatment process test, the two-phase region normalizing process holding temperature is set to 670-700℃, and the holding time t3 is the casting wall thickness h x (3-5) min / mm.

[0052] (4) The tempering process is mainly for the formation of a small amount of bainite carbide and the precipitation of copper-rich phase in ferrite to improve the strength of the cast steel. If the tempering temperature is too high, the precipitated phase will grow excessively, and the strengthening effect will be weakened. If the tempering temperature is too low, the precipitation will be insufficient, which will also lead to insufficient strengthening effect and low low-temperature toughness. If the tempering time is too short, the bainite and copper-rich phase cannot be fully precipitated, and if the tempering time is too long, the precipitated carbide and copper-rich phase will grow, which will adversely affect the performance. Combined with the composition of the steel, the heat treatment process test is calculated and analyzed, the tempering process is kept at 550-590°C, and the holding time is t4 for the wall thickness h x (10-12) min / mm.

[0053] (5) During the complete normalizing, two-phase region normalizing and tempering process of the cast steel, the cooling speed has a great influence on the performance. A faster cooling speed can easily lead to higher local stress and even cracks. A slower cooling speed can also cause great differences in the microstructure and the size of the precipitated phase, which can also increase the degree of uneven performance of the cast steel. The cooling rate of the cast steel is greatly related to its wall thickness. The greater the wall thickness, the slower the cooling rate. Through comprehensive calculation and analysis and experimental verification, in order to ensure the performance and quality of the cast steel, during the complete normalizing, two-phase region normalizing and tempering process, when the wall thickness of the cast steel is not greater than 300 mm, air cooling is used to cool to room temperature; when the wall thickness of the cast steel is greater than 300 mm and not more than 600 mm, a blower is used for blowing cooling; when the wall thickness of the cast steel is greater than 600 mm and not more than 1000 mm, a blower is used for spray cooling.

[0054] In order to clearly illustrate the technical features of the present scheme, the present application will be further described below in combination with specific embodiments, and the present application is not limited to the following embodiments.

[0055] The chemical composition, smelting and heat treatment process designed in the application are used in Examples 1-4. The cast steel used in Examples 1-4 is smelted by electric arc furnace + refining + vacuum smelting method, the molten steel is refined in an alkaline electric furnace, and after refining in a refining furnace and vacuum degassing, the vacuum degree is ≤82 Pa, and the vacuum time is 26-32 min; the outlet temperature of the baking cavity is 227-296 ℃ before pouring, and after baking for 32 hours, argon is filled into the cavity; the riser heat preservative is added to a height of 128-156 mm; the temperature of the riser root is 36-158 ℃ when the box is knocked out and the sand is dropped. The chemical composition of the cast steel in Examples 1-4 is shown in Table 1, the contents of P and S in the steel are relatively low, and each chemical composition meets the design requirements. 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. The results show that the cast steel has good strength and toughness matching, the yield strength is 478-534 MPa, the tensile strength is 626-682 MPa, the elongation after fracture is 23.5%-26.5%, the reduction of area is 72%-76%, the impact energy at -60 ℃ is not less than 165 J; the cast steel has good uniformity, the tensile properties and impact properties of samples taken at different positions have small fluctuations; it is proved that the composition of the cast steel and the heat treatment process are reasonable and feasible.

[0056] The chemical composition of Comparative Examples 1, 2 and 3 is the same as that of the 800 mm size test block in Example 2, but the heat treatment process parameters are different, specifically, the cooling system of Comparative Example 1 is different, Comparative Example 2 is not tempered, and Comparative Example 3 is not normalized in the two-phase region. Compared with Example 2, the strength of Comparative Example 1 is significantly reduced, and the impact energy at -60 ℃ is reduced to less than 100 J; the strength of Comparative Examples 2 and 3 is significantly increased, the plasticity (elongation after fracture, reduction of area) is significantly reduced, the impact energy at -60 ℃ is reduced to less than 100 J and the fluctuation is increased. The results show that the limitation of the heat treatment process parameters of the application is an important guarantee for ensuring that the yield strength is not less than 450 MPa and the impact energy at -60 ℃ is not less than 165 J.

[0057] Table 1 Chemical composition of cast steel of the application (wt%)

[0058]

[0059] Table 2 Wall thickness of cast steel and heat treatment process of the application

[0060]

[0061] Table 3 Mechanical property test results of cast steel of the application

[0062]

[0063] Although the present application has been disclosed with reference to the above embodiments, the application is not limited to the above embodiments. It will be apparent to those skilled in the art that various modifications and changes can be made thereto without departing from the spirit and scope of the application. The scope of the application should be limited only by the appended claims.

Claims

1. A 450MPa grade ultra-thick section high-nickel low-temperature cast steel, characterized in that, Its chemical composition by weight percentage is as follows: C: 0.01%~0.05%; Si: ≤0.20%; Mn: 0.3%~0.7%; Ni: 9.5%~10.5%; Cu: 0.1%~0.3%; S≤0.010%; P: ≤0.015%, with the balance being iron (Fe) and impurities; the heat treatment process of the cast steel includes the following steps: S1. Diffusion annealing treatment; S2, Full normalizing treatment; S3, Two-phase zone normalizing treatment; S4. Tempering treatment; The microstructure of the cast steel is ferrite + bainite tempered structure + retained austenite; In step S1, the diffusion annealing holding temperature is 1000~1050℃, the holding time is t1 in min, the wall thickness is h in mm, t1=h×8~12min / mm, and furnace cooling is used; In step S2, the complete normalizing temperature is 840-860℃, the holding time is t2 (in minutes), the wall thickness is h (in mm), and t2 = h × 3-5 min / mm. One of the following cooling methods is used: air cooling, wind cooling, or spray cooling. In step S3, the normalizing temperature in the two-phase region is 670-700℃, the holding time is t3 (in minutes), the wall thickness is h (in mm), and t3 = h × 3-5 min / mm. One of the following cooling methods is used: air cooling, wind cooling, or spray cooling. In step S4, the holding temperature during the tempering process is 550~590℃, the holding time is t4 (in minutes), the wall thickness is h (in mm), and t4 = h × 10~12 min / mm. One of the following cooling methods is used: air cooling, wind cooling, or spray cooling.

2. The cast steel according to claim 1, characterized in that, The yield strength of cast steel is 478MPa~534MPa, the tensile strength is 626MPa~682MPa, the elongation after fracture is 23.5%~26.5%, the reduction of area is 72%~76%, and the impact energy at -60℃ is not less than 165J.

3. A heat treatment process for 450MPa grade ultra-thick cross-section high-nickel low-temperature cast steel, used for the cast steel as described in claim 1 or 2, characterized in that, In step S1, the diffusion annealing holding temperature is 1000~1050℃, the holding time is t1 in min, the wall thickness is h in mm, t1=h×8~12min / mm, and furnace cooling is used.

4. The heat treatment process according to claim 3, characterized in that, In steps S2 to S4, when the casting wall thickness is ≤300mm, air cooling is used to cool it to room temperature.

5. The heat treatment process according to claim 3, characterized in that, In steps S2 to S4, when 300mm < casting wall thickness ≤ 600mm, a blower is used for air cooling.

6. The heat treatment process according to claim 3, characterized in that, In steps S2 to S4, when 1000mm ≥ casting wall thickness > 600mm, a blower is used for spray cooling.

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