A composite heat-treated fine-grained pressure vessel steel plate and its manufacturing method

Abstract

The present invention discloses a composite heat-treated fine-grained pressure vessel steel plate and a manufacturing method thereof. The composition of the steel plate is as follows by weight percentage: C: 0.20 - 0.30%, Si: 4.0 - 5.0%, Mn: 0.70 - 0.80%, P ≤ 0.015%, S ≤ 0.005%, Cr: 1.0 - 2.0%, Ni: 0.10 - 0.20%, Mo: 3.0 - 4.0%, Nb: 0.01 - 0.02%, Ti: 0.02 - 0.03%, Cu: 0.80 - 0.90%, V: 0.02 - 0.03%, B: 0.001 - 0.002%, Als: 2.0 - 3.0%, N: 0.06 - 0.08%, and the balance is Fe and unavoidable impurities. The steel plate of the present invention not only has high room temperature strength and plasticity, but also has high hardness, high temperature strength, and low temperature toughness.

Description

Technical Field

[0001] The present invention relates to a composite heat-treated fine-grained pressure vessel steel plate and a manufacturing method thereof, belonging to the technical field of iron and steel. Background Art

[0002] Traditional medium carbon steel, due to its high hardness and strength, is suitable for the production of tool steel and die steel with low toughness requirements. However, due to its high brittleness, it cannot be used for the production of structural materials such as pressure vessels. With the continuous development of the industry, the development direction of equipment tends to be large-scale, high-performance, and long-term service, which puts higher requirements on the raw materials for equipment manufacturing. The strength and toughness of traditional low-carbon alloy container steel plates can no longer meet the manufacturing requirements of high-end pressure vessel equipment. As is well known, the properties of materials depend on the type of microstructure. Uniform and fine refinement of the microstructure will greatly improve the comprehensive mechanical properties of materials and achieve a high degree of matching of strength and toughness. At present, low-carbon steel has successfully achieved microstructure refinement, but there is little research on the microstructure refinement of medium and high-carbon steel.

[0003] CN201911106651.8 discloses a method for refining the solidification structure of steel continuous casting billets based on the target equiaxed crystal size and ratio. During the steelmaking process, according to the target equiaxed crystal ratio and size under corresponding working conditions, one or more specified amounts of modification elements are added at one or more nodes in the smelting, refining, and continuous casting processes, and the molten steel containing the modification elements is cast into billets to achieve the refinement of its solidification structure; CN201510900912.9 discloses a method for refining and purifying the solidification structure of thick-walled cast steel parts. Based on pre-deoxidation with ferrosilicon, ferromanganese, and calcium silicate barium alloy in the furnace, the final deoxidation and the process of refining the solidification structure are combined and carried out in the ladle, thereby achieving the refinement and purification of the solidification structure of thick-walled cast steel parts; CN201510112841.6 discloses a method for refining the steel microstructure and improving its mechanical properties by adding external nanoparticles. After mixing and uniformly dispersing the nanoparticles and alloy nanopowders, they are filled into slender steel pipes and compacted and sealed. At a fixed continuous casting drawing speed, the steel pipes are slowly inserted into the lower part of the molten steel flow in the mold at a certain speed. Relying on the continuous impact of the molten steel flowing from the tundish and the violently disturbed flow field inside the mold, the nanoparticles enter the molten steel and quickly disperse. However, the above-mentioned microstructure refinement methods have limited refinement of grain size, so the improvement of properties is also limited. CN201610841911.6 discloses a fine-grained modifier prepared by mixing powders obtained by atomizing ferrotitanium, ferrovanadium, and ferroniobium according to a set ratio and then mixing with rare earth ferrosilicon particles according to a set ratio. Although the refinement effect has been improved, its by-products such as AlCl3, HCl, and chlorine will cause serious damage to the human body, environment, and equipment. Summary of the Invention

[0004] The object of the present invention is to overcome the above problems and deficiencies, and to provide a fine-grained pressure vessel steel plate by composite heat treatment and a manufacturing method thereof.

[0005] The present invention realizes the refinement of the microstructure of the composite structure by using microalloying + high-temperature short-time homogenization treatment + two-stage rolling + composite heat treatment and other grain refinement technologies, so that the medium-carbon steel not only has high room-temperature strength, but also has good room-temperature plasticity. Through heat treatment, it can also obtain higher hardness and high-temperature strength.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows:

[0007] On the one hand, the present invention provides a fine-grained pressure vessel steel plate. The composition of the steel plate is as follows by weight percentage: C: 0.20 - 0.30%, Si: 4.0 - 5.0%, Mn: 0.70 - 0.80%, P ≤ 0.015%, S ≤ 0.005%, Cr: 1.0 - 2.0%, Ni: 0.10 - 0.20%, Mo: 3.0 - 4.0%, Nb: 0.01 - 0.02%, Ti: 0.02 - 0.03%, Cu: 0.80 - 0.90%, V: 0.02 - 0.03%, B: 0.001 - 0.002%, Als: 2.0 - 3.0%, N: 0.06 - 0.08%, and the balance is Fe and unavoidable impurities.

[0008] The action mechanisms of the alloying elements in the steel plate of the present invention are as follows:

[0009] (1) C: C is the main component element of the steel type. The strength of the steel mainly depends on the content of C element in the steel. Too high content of C element will lead to poor toughness, plasticity and welding performance of the steel; too low content of C element will lead to low strength of the steel and low performance after simulated stress relief treatment. In order to ensure good matching of low-temperature impact toughness, strength and welding performance of the steel plate during use, the C content in the steel of the present invention is preferably controlled within the range of 0.20 - 0.30%.

[0010] (2) Si: Adding Si to medium-carbon steel has an impact on the thermodynamics of the iron-carbon system and the kinetics of carbide formation and dissolution. As a ferrite stabilizing element, silicon will increase the Ac1 point of the steel. The addition of silicon causes the carbon content of the eutectoid composition to decrease, thereby increasing the number of proeutectoid carbides and increasing the volume fraction of carbides used to pin grain boundaries in the γ + θ two-phase region. In this way, the growth of ferrite and austenite grains is inhibited. Silicon is insoluble in carbides. When carbides precipitate, silicon is distributed around the carbides, locally forming a high-concentration region of silicon. Silicon is also an element that increases the activity of carbon. In the high-concentration region of silicon, the activity of carbon also increases accordingly, which reduces the diffusion flux of carbon to carbides, and thus can inhibit the coarsening of carbides. Therefore, the Si content of the present invention is controlled at 4.0% - 5.0%.

[0011] (3) Mn: In the steel, the Mn element can strengthen ferrite by solid solution strengthening. C-Mn strengthening is also the main way to increase the strength of low-carbon steel. However, if the Mn content is too high, while increasing production costs, the Mn element is likely to combine with the S element to form MnS, reducing the hydrogen-induced crack resistance of the material. At the same time, too high a Mn content will reduce the activity of carbon elements. Therefore, the Mn content in the steel is required to be controlled at 0.70 - 0.80%.

[0012] (4) P: Phosphorus is a harmful element in steel, increasing the cold brittleness of steel, deteriorating the welding performance, reducing plasticity, making the cold bending performance worse, and P is also particularly sensitive to irradiation embrittlement. Therefore, the lower the P content in the steel, the better. In this invention, it is required to be less than 0.015%.

[0013] (5) S: Sulfur is usually a harmful element. S usually easily forms brittle sulfides with alloying elements in steel, causing hot brittleness of steel, reducing the ductility and toughness of steel, and at the same time, S also has a tendency to accelerate irradiation embrittlement. Therefore, in this invention, the S content in the steel is required to be limited to less than 0.005%.

[0014] (6) Als: Alloying with aluminum can also increase the Ac1 point of steel, make the proeutectoid cementite refined and evenly distributed, and inhibit the formation of network carbide. After adding aluminum, the grain boundary carbide and Widmanstatten structure carbide both disappear, and a refined fully ferrite structure is obtained. Therefore, the Al content in the steel is required to be 2.0 - 3.0% in this invention.

[0015] (7) V: V belongs to microalloying elements. Microalloying of V in steel can form fine second-phase particles, playing the role of pinning grain boundaries and precipitation strengthening, and can effectively refine grains, greatly improving the comprehensive mechanical properties such as strength, toughness, ductility and thermal fatigue resistance of steel. Therefore, the range of adding V in the steel is 0.02 - 0.03%.

[0016] (8) Ni: Ni is a solid solution strengthening element in steel, which can increase the strength of steel. Ni reduces the dislocation movement resistance in the steel, makes the stress relax, and then changes the substructure of the matrix tissue, thereby improving the toughness of steel, especially the low-temperature toughness. However, too high a Ni content in medium-carbon steel will increase the phase transformation temperature. Therefore, the Ni content is controlled at 0.10 - 0.20%.

[0017] (9) Cr: Chromium is an element that stabilizes carbides. Adding chromium reduces the dissolution rate of carbides. Therefore, when adopting the hot deformation microstructure refinement process, even if the heating temperature is increased or the heating time is extended, the eutectoid transformation can still be avoided and a refined structure can be obtained. Chromium can also inhibit the graphitization of ultra-high carbon steel containing silicon and aluminum. Therefore, the Cr content in the steel is required to be controlled at 1.0 - 2.0% in this invention.

[0018] (10) Cu: The prominent role of Cu in steel is to improve the corrosion resistance of plain carbon low-alloy steel, increase the strength and yield ratio of the steel, and have no adverse effect on the welding performance. When the copper content exceeds 0.75%, after solution treatment and aging, age hardening can occur. At the same time, its role is similar to that of nickel, and it can play a certain role in saving nickel and reducing costs. However, when the content is relatively high, copper embrittlement occurs during hot deformation processing. Therefore, the present invention requires that the Cu content in the steel be controlled at 0.80 - 0.90%.

[0019] (11) Nb: As a strong carbide-forming element, Nb forms NbC phases with large dispersion and good high-temperature stability in steel, playing a precipitation strengthening role. Through multi-stage rolling, the grains can be effectively refined, and the reduction in toughness caused by precipitation strengthening can be improved, so that the steel plate obtains comprehensive properties of high strength and high toughness. In addition, in the steel with composite addition of Nb-Mo, Mo can also segregate at the NbC matrix interface, preventing the coarsening of NbC particles, thus greatly improving the high-temperature strength of the steel. Therefore, the Nb content is controlled at 0.01 - 0.02%.

[0020] (12) Mo: Mo mainly relies on solid solution strengthening and grain boundary strengthening to increase the strength of the steel; secondly, Mo increases the stability of supercooled austenite, shifts the austenite-to-ferrite transformation curve to the right, and a finer ferrite structure is obtained after phase transformation; in addition, when Ti and Mo are combined, a large amount of nano-sized Ti-Mo(CN) carbides precipitate in the steel, and the refined carbides pin dislocations, greatly improving the strength and toughness of the steel. Therefore, the present invention requires that the Mo content in the steel be controlled at 3.0% - 4.0%.

[0021] (13) Ti: Adding an appropriate amount of Ti can form a large number of finely dispersed TiN or Ti2O3 particles, which can serve as heterogeneous nucleation cores for acicular ferrite during solidification of the structure, thus refining the structure. Ti also has a deoxidation effect, ensuring that B is not oxidized and nitrided. And B can reduce the transformation temperature from austenite to ferrite, promoting the formation of acicular ferrite within the grains, playing a role in refining the grains. However, when w(Ti) ≥ 0.09%, the content of acicular ferrite will decrease, deteriorating the low-temperature toughness of the steel plate. Therefore, the present invention requires that the Ti content in the steel be controlled at 0.02 - 0.03%.

[0022] (14) B: B can reduce the transformation temperature from austenite to ferrite, promoting the formation of acicular ferrite within the grains, playing a role in refining the grains. Therefore, the present invention requires that the B content in the steel be controlled at 0.001 - 0.002%.

[0023] (15) N: N can combine with Ti to form a large number of finely dispersed TiN, which can serve as heterogeneous nucleation cores for acicular ferrite during solidification of the structure, thus refining the structure. Therefore, the present invention requires that the N content in the steel be controlled at 0.06 - 0.08%.

[0024] In the above technical solution, further, the microstructure of the steel plate is composed of cementite and ferrite, wherein the grain size of the cementite is 0.1 - 0.5 μm, the grain size of the ferrite is 0.5 - 1.0 μm, and large-angle grain boundaries are presented between the grains, and the misorientation between adjacent grains is between 30 - 40°. The existence of the large-angle grain boundaries will effectively hinder the propagation of cracks, thereby further improving the low-temperature toughness of the material.

[0025] In the above technical solution, further, the tensile strength of the steel plate is 600 - 740 MPa, the yield strength is 375 - 575 MPa, the elongation after fracture is 60 - 70%, the impact energy at -80 °C is 260 - 360 J, the surface Brinell hardness is 290 - 390 HBW, and the high-temperature tensile yield strength at 450 °C is 265 - 405 MPa.

[0026] In the above technical solution, further, the thickness of the steel plate is 15 - 95 mm.

[0027] On the other hand, the present invention provides a method for manufacturing the above fine-grained pressure vessel steel plate, and the method includes the following steps:

[0028] (1) Smelting: The thickness of the continuous casting billet for smelting is 150 - 350 mm;

[0029] (2) Homogenization treatment: After the continuous casting billet is smelted, it is directly hot-charged into a heating furnace for homogenization treatment; this step is to perform homogenization annealing in a single-phase austenite region to fully dissolve carbon elements;

[0030] (3) Two-stage rolling: After the homogenization treatment, the continuous casting billet is hot-transported to a rolling mill for rolling. First, the continuous casting billet is rough-rolled to open the billet, and after rough rolling, the intermediate billet is air-cooled with the roller table reciprocating and swinging. When the temperature drops to 600 - 650 °C, continuous multi-pass rapid finish rolling is carried out; the thermal deformation in this process helps to break the pro-eutectoid cementite precipitated from austenite and avoid the formation of network carbide;

[0031] (4) Composite heat treatment: The composite heat treatment is carried out in four stages: The first stage is homogenization heat treatment, with the temperature of 1210 - 1230 °C, pure heat preservation for 4 - 6 h, cooling in the furnace to 910 - 930 °C, forming a complete pearlite structure, without coarsened proeutectoid ferrite laths and undissolved ferrite particles; The second stage is normalizing heat treatment in the austenite + ferrite two-phase region. The steel plate is cooled in the furnace to 910 - 930 °C for heat preservation, and after continuing to heat preserve for 10 - 20 min, it is taken out of the furnace and air-cooled to room temperature to spheroidize the refined ferrite laths; The third stage is solution heat treatment, with the temperature of 1060 - 1080 °C, pure heat preservation for 50 - 70 min, water-cooled to room temperature, quenching in the austenite + pearlite two-phase region to obtain a fully refined martensite structure, improving the strength and hardness of the material; The fourth stage is normalizing heat treatment at the Ac1 point, with the temperature of 960 - 980 °C, pure heat preservation for 20 - 40 min, air-cooled to room temperature to recrystallize the martensite and obtain an ultra-refined ferrite + cementite duplex structure, improving the toughness and plasticity of the material. Before the heat treatment of the steel plate, the grain boundaries are mainly small-angle grain boundaries with an orientation difference less than 10°, which are composed of a series of edge dislocations separated by a certain distance. The grain boundary layer is relatively thin and the grain boundary energy is relatively small. Through multi-stage heat treatment, the grain boundary energy is greatly increased, and the small-angle grain boundaries are transformed into large-angle grain boundaries with an orientation difference between adjacent grains of 30 - 40°.

[0032] In the above technical solution, further, in step (2), the homogenization temperature treatment is: heat preservation at a temperature of 1140 - 1160 °C for 1 - 2 h.

[0033] In the above technical solution, further, in step (3), the rough rolling starting temperature is 1080 - 1110 °C, the finishing rolling temperature is 810 - 840 °C, ensuring a reduction rate of 15 - 25% for each pass, and the thickness of the intermediate billet is 2 - 3 times the thickness of the finished steel plate; The finishing rolling ensures a reduction rate of 5 - 15% for each pass to the thickness of the final finished steel plate.

[0034] The beneficial effects of the present invention are as follows:

[0035] 1. The present invention conducts high-temperature short-time homogenization treatment, carries out homogenization annealing in the single-phase austenite region to fully dissolve carbon elements; two-stage rolling helps to break the proeutectoid cementite precipitated from austenite and avoid the formation of network carbide; After composite heat treatment, an ultra-refined ferrite + cementite duplex structure is obtained, improving the comprehensive mechanical properties of the material, not only having high room temperature strength, but also having good room temperature plasticity, relatively high hardness and high temperature strength;

[0036] 2. The steel plate prepared by the present invention can obtain a perfect global microstructure, the grain size of cementite reaches 0.1-0.5 μm, the grain size of ferrite reaches 0.5-1.0 μm, and there are large-angle grain boundaries between grains. The orientation difference between adjacent grains is between 30 and 40°. The presence of large-angle grain boundaries will effectively hinder the expansion of cracks, thereby further improving the low-temperature toughness of the steel plate;

[0037] 3. The finished steel plate of the present invention has a tensile strength of 600-740 MPa, a yield strength of 375-575 MPa, an elongation after fracture of 60-70%, an impact energy of 260-360 J at -80°C, a surface Brinell hardness of 290-390 HBW, and a high-temperature tensile yield strength of 265-405 MPa at 450°C. Such good process performance and mechanical properties make it expected to be widely used in the field of pressure vessel equipment manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a metallographic structure photograph of the steel plate of Example 1. DETAILED DESCRIPTION

[0039] The following examples are used to specifically illustrate the present invention. These examples are only general descriptions of the present invention and do not limit the present invention.

[0040] The compositions of the steel plates in Examples 1-6 of the present invention are shown in Table 1.

[0041] Table 1 Composition of steel plates of Examples 1-6 (wt%)

[0042]

[0043] The method for manufacturing the fine-grained pressure vessel steel plate comprises the following steps:

[0044] (1) Smelting: Molten steel is smelted into continuous casting billets;

[0045] (2) Homogenization treatment: After the continuous casting billet is smelted, the continuous casting billet is directly hot-charged into a heating furnace for homogenization treatment; the main process parameters of smelting and homogenization treatment of Examples 1-6 are shown in Table 2;

[0046] Table 2 Main process parameters of smelting and homogenization treatment of Examples 1-6

[0047] Example Continuous casting slab thickness / mm Finished steel plate thickness / mm Insulation temperature / ℃ Insulation time / h 1 150 15 1145 1.2 2 200 24 1157 1.5 3 230 32 1148 1.6 4 250 54 1156 1.9 5 300 77 1152 1.1 6 350 95 1146 1.5

[0048] This step performs homogenization annealing in the single-phase austenite region to fully dissolve carbon elements. The carbon elements are fully dissolved by high-temperature short-time homogenization treatment.

[0049] (3) Two-stage rolling: After homogenization treatment, the continuous casting billet is hot-transported to a double-stand rolling mill for rolling. First, the continuous casting billet is rough-rolled to break the billet, and after rough rolling, the intermediate billet is cooled by reciprocating swing on the roller table. After cooling, continuous multi-pass finish rolling is carried out. The thermal deformation in this process helps to break the proeutectoid cementite precipitated from austenite and avoid the formation of network carbide;

[0050] Table 3 Main process parameters of two-stage rolling treatment for Examples 1-6

[0051]

[0052]

[0053] (4) Composite heat treatment: Composite heat treatment is carried out in four stages: The first stage is homogenization heat treatment to form a complete pearlite structure without coarsened proeutectoid ferrite laths and undissolved ferrite particles; The second stage is normalizing heat treatment in the austenite + ferrite two-phase region to spheroidize the refined ferrite laths; The third stage is solution heat treatment to obtain a fully refined martensite structure by quenching in the austenite + pearlite two-phase region, improving the strength and hardness of the material; The fourth stage is normalizing heat treatment at the Ac1 point to recrystallize the martensite and obtain an ultra-refined ferrite + cementite duplex structure, improving the toughness and plasticity of the material; Through multi-stage heat treatment, the grain boundary energy is greatly increased, and small-angle grain boundaries are transformed into large-angle grain boundaries with the misorientation between adjacent grains ranging from 30° to 40°.

[0054] The main process parameters of composite heat treatment for Examples 1-6 are shown in Table 4.

[0055] Table 4 Main process parameters of composite heat treatment for steel plates of Examples 1-6

[0056]

[0057] As Figure 1 shown, the microstructure of the steel plate in Example 1 is composed of cementite and ferrite. The performance of the above-mentioned fine-grained pressure vessel steel plate is tested, and the grain size and comprehensive mechanical properties of the steel plate are shown in Table 5.

[0058] Table 5 Grain size and comprehensive mechanical properties of steel plates of Examples 1-6

[0059]

[0060]

[0061] In order to describe the present invention, the present invention has been described appropriately and sufficiently through embodiments above. The above embodiments are only used to illustrate the present invention and are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can still make various changes and modifications without departing from the spirit and scope of the present invention. Any modifications, equivalent replacements, improvements, etc. should be included within the protection scope of the present invention. The patent protection scope of the present invention shall be defined by the claims.

Claims

1. A fine-grained pressure vessel steel plate, characterized in that, The composition of the steel plate is as follows by weight percentage: C: 0.20 - 0.30%, Si: 4.0 - 5.0%, Mn: 0.70 - 0.80%, P ≤ 0.015%, S ≤ 0.005%, Cr: 1.0 - 2.0%, Ni: 0.10 - 0.20%, Mo: 3.0 - 4.0%, Nb: 0.01 - 0.02%, Ti: 0.02 - 0.03%, Cu: 0.80 - 0.90%, V: 0.02 - 0.03%, B: 0.001 - 0.002%, Als: 2.0 - 3.0%, N: 0.06 - 0.08%, and the balance is Fe and unavoidable impurities; The manufacturing method of the steel plate includes the following steps: (1) Smelting: The thickness of the continuous casting billet for smelting is 150 - 350 mm; (2) Homogenization treatment: After the continuous casting billet is smelted, it is directly hot charged into the heating furnace for homogenization treatment; (3) Two-stage rolling: After homogenization treatment, the continuous casting billet is hot sent to the rolling mill for rolling. First, the continuous casting billet is rough rolled to open the billet. After rough rolling, the intermediate billet is air-cooled with the roller table reciprocating and swinging. When the temperature drops to 600 - 650 °C, continuous multi-pass finish rolling is carried out; (4) Composite heat treatment: The composite heat treatment is carried out in four stages: The first stage is homogenization heat treatment, with a temperature of 1210 - 1230 °C, a net heat preservation of 4 - 6 h, and furnace cooling to 910 - 930 °C; The second stage is normalizing heat treatment in the austenite + ferrite two-phase region. The steel plate is furnace cooled to 910 - 930 °C for heat preservation, and after continuing to heat preserve for 10 - 20 min, it is taken out of the furnace and air-cooled to room temperature; The third stage is solution heat treatment, with a temperature of 1060 - 1080 °C, a net heat preservation of 50 - 70 min, and water-cooled to room temperature; The fourth stage is normalizing heat treatment at the Ac1 point, with a temperature of 960 - 980 °C, a net heat preservation of 20 - 40 min, and air-cooled to room temperature.

2. The fine-grained pressure vessel steel plate according to claim 1, wherein The microstructure of the steel plate is composed of cementite and ferrite. Among them, the grain size of cementite is 0.1 - 0.5 μm, the grain size of ferrite is 0.5 - 1.0 μm, and large-angle grain boundaries are presented between the grains. The orientation difference between adjacent grains is between 30 - 40°.

3. The fine-grained pressure vessel steel plate according to claim 1, characterized in that, The tensile strength of the steel plate is 600 - 740 MPa, the yield strength is 375 - 575 MPa, the elongation after fracture is 60 - 70%, the impact energy at -80 °C is 260 - 360 J, the surface Brinell hardness is 290 - 390 HBW, and the high-temperature tensile yield strength at 450 °C is 265 - 405 MPa.

4. The fine-grained pressure vessel steel plate according to claim 1, wherein The thickness of the steel plate is 15 - 95 mm.

5. The fine-grained pressure vessel steel plate according to claim 1, wherein In the step (2), the homogenization temperature treatment is: heat preservation at a temperature of 1140 - 1160 °C for 1 - 2 h.

6. The fine-grained pressure vessel steel plate according to claim 5, characterized in that, In the step (3), the rough rolling starting temperature is 1080 - 1110 °C, the finishing rolling temperature is 810 - 840 °C, ensuring a reduction ratio of 15 - 25% for each pass. The thickness of the intermediate billet is 2 - 3 times the thickness of the finished steel plate; The finish rolling ensures a reduction ratio of 5 - 15% for each pass to the thickness of the final finished steel plate.

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