A divorced eutectoid transformation fine-grained pressure vessel steel plate and its manufacturing method

Abstract

The invention discloses a divorced eutectoid transformation fine-grained pressure vessel steel plate and a manufacturing method thereof, and belongs to the technical field of steel preparation. The chemical composition of the divorced eutectoid transformation fine-grained pressure vessel steel plate of the invention is as follows by weight percentage: C: 0.20-0.30%, Si: 0.15%-0.40%, Mn: 0.90%-1.50%, P≤0.015%, S≤0.005%, Cr: 1.0%-2.0%, Ni: 0.20%-0.30%, Mo: 1.0%-2.0%, Nb: 0.01-0.02%, Ti: 0.02-0.03%, Cu: 0.80-0.90%, V: 0.02-0.03%, B: 0.0 01~0.002%, Als: 0.025%~0.035%, N: 0.06%~0.08%, the balance is Fe and inevitable impurities, and the microstructure refinement technology such as chromium-copper alloying + high temperature short-time homogenization treatment + two-stage rolling + short-time normalizing is adopted to realize the refinement of the micro-composite structure of the medium carbon container steel plate, and obtain a perfect global microstructure. The cementite grain size reaches 1.0~2.5μm, and the ferrite grain size reaches 0.5~2.0μm. At the same time, it has good comprehensive mechanical properties, which makes it expected to be widely used in the field of pressure vessel equipment manufacturing.

Description

Technical Field

[0001] The invention belongs to the technical field of steel preparation, and in particular relates to a divorced eutectoid transformation fine-grained pressure vessel steel plate and a manufacturing method thereof. Background Art

[0002] The traditional view is that medium carbon steel can obtain high hardness and strength, and is suitable for the production of tool steel and die steel that do not require high toughness. However, due to its high brittleness, it cannot be used in 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. It is well known that the performance of materials depends on the type of organization. Uniform and refined organization will greatly improve the comprehensive mechanical properties of materials and achieve a high degree of matching of strength and toughness.

[0003] The invention patent with application number 201710037112.8 discloses the manufacturing process of large-size 42CrMo4 quenched and tempered steel for the outer main shaft of the wind power speed increaser. The patent adopts the die casting + 3500t fast forging machine forging process, which has high production cost, high energy consumption, slow production rhythm, and poor uniformity of product plate shape and performance. The invention patent with application number CN201710919045.2 discloses a normalizing method for achieving microstructure refinement of low-activated ferrite / martensitic steel. The normalizing process is heated to 900-930℃ at 50-60℃ / s, kept warm for 1-2s, and then air-cooled to room temperature 20-25℃ to refine the duplex stainless steel structure. The invention only provides heat treatment process, does not specify the rolling process, and does not clearly propose the degree and grade of microstructure refinement. Up to now, low carbon steel has successfully achieved microstructure refinement, but related research on microstructure refinement of medium and high carbon steel has not been reported. Summary of the invention

[0004] In view of this, the purpose of the present invention is to provide a divorced eutectoid transformation fine-grained pressure vessel steel plate and a manufacturing method thereof. The present invention adopts chromium-copper alloying + high temperature short-time homogenization treatment + two-stage rolling + short-time normalizing and other organizational refinement technologies to achieve the refinement of the microscopic composite structure of the medium carbon container steel plate, so that the medium carbon steel not only has high room temperature strength, but also has good low temperature toughness. Through heat treatment, it can also obtain a higher elongation after fracture. Pearlite transformation does not occur during the divorced eutectoid transformation process, and carbides are precipitated in a spherical shape near the undissolved carbides on the austenite grain boundary and inside the austenite grains. After the above treatment, a perfectly globalized microstructure can be obtained, the cementite grain size reaches 1.0-2.5μm, the ferrite grain size reaches 0.5-2.0μm, and it has good comprehensive mechanical properties. The good process performance and mechanical properties of medium carbon steel make it expected to be widely used in the field of pressure vessel equipment manufacturing.

[0005] The object of the present invention is achieved in the following manner:

[0006] One of the technical solutions of the present invention is to provide a divorced eutectoid transformation fine-grained pressure vessel steel plate. The chemical composition of the steel plate is as follows by weight percentage: C: 0.20 - 0.30%, Si: 0.15% - 0.40%, Mn: 0.90% - 1.50%, P ≤ 0.015%, S ≤ 0.005%, Cr: 1.0% - 2.0%, Ni: 0.20% - 0.30%, Mo: 1.0% - 2.0%, 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: 0.025% - 0.035%, N: 0.06% - 0.08%, and the balance is Fe and unavoidable impurities.

[0007] The mechanism of action of the above components is as follows:

[0008] C: C is the main component element of the steel grade. The strength of the steel mainly depends on the content of C element in the steel. Excessive 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%.

[0009] 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 pro-eutectoid 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, thus inhibiting the coarsening of carbides. Therefore, the Si content of the present invention is controlled within the range of 0.15% - 0.40%.

[0010] Mn: The Mn element can strengthen ferrite by solid solution strengthening in the steel grade. C-Mn strengthening is also the main way to increase the strength of low-carbon steel. However, when the Mn content is too high, while increasing production costs, the Mn element is easy to combine with the S element to form MnS, reducing the hydrogen-induced cracking resistance of the material. Therefore, the Mn content in the steel of the present invention is required to be controlled within the range of 0.90% - 1.50%.

[0011] P: Phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates the welding performance, reduces plasticity, makes the cold bending performance worse, and P is also particularly sensitive to irradiation embrittlement. Therefore, the present invention requires that the P content in steel be as low as possible, and the present invention requires it to be less than 0.015%.

[0012] S: Sulfur is usually a harmful element. S usually easily forms brittle sulfides with alloying elements in steel, causing hot brittleness in steel, reducing the ductility and toughness of steel, and at the same time, S also has a tendency to accelerate irradiation embrittlement. Therefore, the present invention requires that the S content in steel be limited to less than 0.005%.

[0013] Als: Alloying with aluminum can also increase the Ac1 point of steel, refine and evenly distribute the proeutectoid cementite, and inhibit the formation of network carbides. After adding aluminum, the grain boundary carbides and Widmanstatten structure carbides both disappear, and a refined fully pearlitic structure is obtained. Therefore, the present invention requires that the Al content in steel be 0.025% - 0.035%.

[0014] V: V belongs to microalloying elements. Microalloying of V in steel can form fine second-phase particles, which play a role in pinning grain boundaries and precipitation strengthening, can effectively refine grains, and greatly improve the comprehensive mechanical properties such as the strength, toughness, ductility, and thermal fatigue resistance of steel. Therefore, the present invention requires that the range of adding V in steel be 0.02 - 0.03%.

[0015] Ni: Ni is a solid solution strengthening element in steel, which can improve the strength of steel. Ni reduces the dislocation movement resistance in the steel grade, relaxes the stress, and then changes the substructure of the matrix tissue, thereby improving the toughness of steel, especially the low-temperature toughness. Therefore, the present invention requires that the Ni content be controlled at 0.20 - 0.30%.

[0016] 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 microstructure can be obtained. Chromium can also inhibit the graphitization of ultra-high carbon steel containing silicon and aluminum. Therefore, the present invention requires that the Cr content in steel be controlled at 1.0 - 2.0%.

[0017] Cu: The prominent role of Cu in steel is to improve the corrosion resistance of plain carbon low alloy steel, and it can also improve the strength and yield ratio of steel, without having an 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 role in saving nickel and reducing costs to a certain extent. However, when the content is relatively high, copper embrittlement occurs during hot deformation processing. Therefore, the present invention requires that the Cu content in steel be controlled at 0.80 - 0.90%.

[0018] Nb: As a strong carbide-forming element, Nb forms NbC phase with high dispersion and good high-temperature stability in the steel, which plays a role in precipitation strengthening. Through multi-stage rolling, the grain size can be effectively refined, and the reduction of toughness caused by precipitation strengthening can be improved, so that the steel plate can obtain 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 interface of the NbC matrix, preventing the coarsening of NbC particles, thus greatly improving the high-temperature strength of the steel. Therefore, the Nb content in the steel of this invention is required to be controlled within 0.01 - 0.02%.

[0019] Mo: Mo mainly relies on solid-solution strengthening and grain-boundary strengthening to improve the strength of the steel; secondly, Mo increases the stability of supercooled austenite, shifts the transformation curve of austenite to ferrite to the right, and a finer ferrite structure is obtained after phase transformation; in addition, Ti and Mo combine, and a large number 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 Mo content in the steel of this invention is required to be controlled at 1.0 - 2.0%.

[0020] Ti: Adding an appropriate amount of Ti can form a large number of fine TiN or Ti2O3 particles distributed dispersedly, which can serve as heterogeneous nucleation cores of acicular ferrite during solidification of the structure, thus refining the structure. Ti also has a deoxidizing effect to ensure that B is not oxidized and nitrided. And B can reduce the transformation temperature from austenite to ferrite phase, promoting the formation of acicular ferrite in the grains, playing a role in refining 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 Ti content in the steel of this invention is required to be controlled at 0.02 - 0.03%.

[0021] B: B can reduce the transformation temperature from austenite to ferrite phase, promoting the formation of acicular ferrite in the grains, playing a role in refining grains. Therefore, the B content in the steel of this invention is required to be controlled at 0.001 - 0.002%.

[0022] N: N can combine with Ti to form a large number of fine TiN distributed dispersedly, which can serve as heterogeneous nucleation cores of acicular ferrite during solidification of the structure, thus refining the structure. Therefore, the N content in the steel of this invention is required to be controlled at 0.06 - 0.08%.

[0023] Based on the above technical solutions, further, the microstructure of the steel plate is a completely spheroidized microstructure, the cementite grain size reaches 1.0 - 2.5 μm, and the ferrite grain size reaches 0.5 - 2.0 μm.

[0024] Based on the above technical solution, further, the tensile strength of the steel plate is 600-740 MPa, the yield strength is 360-500 MPa, the elongation after fracture is 40-50%, the impact energy at -40°C is 360-500 J, the surface Brinell hardness is 290-360 HBW, and the high-temperature tensile yield strength at 450°C is 265-405 MPa.

[0025] The second technical solution of the present invention is to provide a manufacturing method for the above-mentioned divorced eutectoid transformation fine-grained pressure vessel steel plate, which mainly includes high-temperature short-time homogenization treatment, two-stage rolling and short-time normalizing treatment, and includes the following steps:

[0026] (1) High-temperature short-time homogenization treatment: The continuous casting billet is kept at 1130-1170°C for 1-2 h for homogenization annealing in the single-phase austenite region to fully dissolve carbon elements;

[0027] (2) Two-stage rolling: The continuous casting billet after homogenization treatment is hot-transported to the rolling mill for rolling. The rough rolling starting temperature is 1080-1110°C, the finishing rolling temperature is 810-840°C, the reduction rate per pass is 15%-25%, and it is air-cooled to 600-650°C for finish rolling. The finishing rolling temperature is 510-620°C, and the reduction rate per pass is 5%-15%;

[0028] (3) Short-time normalizing treatment: The rolled steel plate is hot-transported to a continuous heat treatment furnace for austenitization in the range of 20-40°C above the Ac1 temperature, and the net holding time is 15-30 min, and then air-cooled to room temperature.

[0029] Based on the above technical solution, further, the thickness of the continuous casting billet in step (1) is 150-350 mm.

[0030] Based on the above technical solution, further, the homogenization temperature in step (1) is 1140-1160°C.

[0031] Based on the above technical solution, further, the thickness of the intermediate billet in step (2) is 2-3 times the thickness of the finished steel plate.

[0032] Based on the above technical solution, further, in step (2), the rough rolling starting temperature is 1093-1108°C, the finishing rolling temperature is 813-838°C, the finish rolling starting temperature is 640-650°C, and the finishing rolling temperature is 514-613°C.

[0033] Based on the above technical solution, further, the austenitization temperature in step (3) is 820-840°C.

[0034] The beneficial effects of the present invention compared with the prior art are as follows:

[0035] While the present invention uses chromium copper alloying to improve the strength and toughness matching of the steel plate, chromium, as an element that stabilizes carbides, reduces the dissolution rate of carbides. Therefore, when adopting the two-stage rolling 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 microstructure can be obtained; high-temperature short-time homogenization treatment is carried out for 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 carbides; during the short-time normalizing process, pearlite transformation does not occur, and carbides precipitate spheroidally near the undissolved carbides on the austenite grain boundaries and inside the austenite grains. After the above treatment, a fully spheroidized microstructure can be obtained, with the cementite grain size reaching 1.0 - 2.5 μm and the ferrite grain size reaching 0.5 - 2.0 μm. The final finished steel plate has a tensile strength of 600 - 740 MPa, a yield strength of 360 - 500 MPa, an elongation after fracture of 40 - 50%, an impact energy at -40°C of 360 - 500 J, a surface Brinell hardness of 290 - 360 HBW, and a high-temperature tensile yield strength at 450°C of 265 - 405 MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention, the drawings related to the embodiments will be briefly introduced below.

[0037] Figure 1 Metallographic diagram of the steel plate prepared for Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The present invention will be described in detail below in conjunction with the embodiments. However, the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only partial embodiments of the present invention. For those skilled in the art, without creative efforts, obtaining other similar embodiments will fall within the protection scope of the present invention.

[0039] Examples 1 - 6

[0040] This embodiment provides a manufacturing method for a divorced eutectoid transformation fine-grained pressure vessel steel plate. The chemical composition and weight percentage of the steel plate are shown in Table 1;

[0041] Table 1 Chemical composition and weight percentage (wt%) of the steel billets in Examples 1 - 6

[0042]

[0043] It includes the following steps:

[0044] (1) High-temperature short-time homogenization treatment: Continuously cast slabs with a thickness of 150 - 350 mm are directly hot-charged into a heating furnace and held at 1145 - 1157 °C for 1.1 - 1.9 h for homogenization annealing in the single-phase austenite region to fully dissolve carbon elements. The main process parameters of the high-temperature short-time homogenization treatment are shown in Table 2;

[0045] (2) Two-stage rolling: After homogenization treatment, the continuously cast slabs are hot-transported to a double-stand rolling mill for rolling. First, the continuously cast slabs are rough-rolled to break the billet, so that proeutectoid cementite forms in the form of fine particles at the austenite grain boundaries and in the high-density dislocation regions within the grains. The rough rolling starting temperature is 1093 - 1108 °C, and the final rolling temperature is 813 - 838 °C, ensuring a reduction ratio of 17% - 24% per pass. The thickness of the intermediate billet is 2 - 3 times the thickness of the finished steel plate. After rough rolling, the intermediate billet is cooled in the roller table by reciprocating swing to 640 - 650 °C for continuous multi-pass rapid finish rolling. The final rolling temperature is 514 - 613 °C, ensuring a reduction ratio of 5% - 15% per pass. The thermal deformation in this process helps to break the proeutectoid cementite precipitated from austenite and avoid the formation of network carbides. The main process parameters of the two-stage rolling are shown in Table 2;

[0046] Table 2 Main process parameters of high-temperature short-time homogenization treatment and two-stage rolling for Examples 1 - 6

[0047]

[0048] (3) Short-time normalizing: After the steel plate is rolled, it is hot-transported to a continuous heat treatment furnace for austenitization for a short time in the range of 20 - 40 °C above the Ac1 temperature (measured as 800 °C), and then air-cooled to room temperature after a net holding time of 17 - 30 min to complete the short-time normalizing heat treatment (this process is called divorced eutectoid transformation). During the divorced eutectoid transformation process, pearlite transformation does not occur, and carbides precipitate in the form of spheres near the undissolved carbides at the austenite grain boundaries and inside the austenite grains. After the above treatment, a fully spheroidized microstructure can be obtained, and the steel plate has good mechanical properties. The main process parameters of the short-time normalizing are shown in Table 3, and the grain size and comprehensive mechanical properties of the steel plates obtained in Examples 1 - 6 are shown in Table 4.

[0049] Table 3 Main process parameters of short-time normalizing of steel plates for Examples 1 - 6

[0050]

[0051] Table 4 Grain size and comprehensive mechanical properties of steel plates for Examples 1 - 6

[0052]

[0053] 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 also 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.

Claims

1. A divorced eutectoid transformation fine-grained pressure vessel steel plate, characterized in that, The chemical composition of the steel plate is as follows by weight percentage: C: 0.20 - 0.30%, Si: 0.15% - 0.40%, Mn: 0.90% - 1.50%, P ≤ 0.015%, S ≤ 0.005%, Cr: 1.0% - 2.0%, Ni: 0.20% - 0.30%, Mo: 1.0% - 2.0%, 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: 0.025% - 0.035%, N: 0.06% - 0.08%, and the balance is Fe and inevitable impurities; The microstructure of the steel plate is a fully spheroidized microstructure, the cementite grain size reaches 1.0 - 2.5 μm, and the ferrite grain size reaches 0.5 - 2.0 μm; The steel plate has a tensile strength of 600 - 740 MPa, a yield strength of 360 - 500 MPa, an elongation after fracture of 40 - 50%, an impact energy at -40°C of 360 - 500 J, a surface Brinell hardness of 290 - 360 HBW, and a high-temperature tensile yield strength at 450°C of 265 - 405 MPa; The manufacturing method of the divorced eutectoid transformation fine-grained pressure vessel steel plate includes high-temperature short-time homogenization treatment, two-stage rolling, and short-time normalizing treatment, and comprises the following steps: (1) High-temperature short-time homogenization treatment: The continuous casting billet is held at 1130 - 1170°C for 1 - 2 h for homogenization annealing in the single-phase austenite region to fully dissolve carbon elements; (2) Two-stage rolling: The continuous casting billet after homogenization treatment is hot-transported to the rolling mill for rolling. The rough rolling starting temperature is 1080 - 1110°C, the finishing rolling temperature is 810 - 840°C, the reduction rate per pass is 15% - 25%, and it is air-cooled to 600 - 650°C for finish rolling. The finishing rolling temperature is 510 - 620°C, and the reduction rate per pass is 5% - 15%; (3) Short-time normalizing treatment: The rolled steel plate is hot-transported to a continuous heat treatment furnace for austenitization in the range of 20 - 40°C above the Ac1 temperature, and the net holding time is 15 - 30 min, and then air-cooled to room temperature.

2. The divorced eutectoid transformation fine-grained pressure vessel steel plate according to claim 1, wherein, In step (1), the thickness of the continuous casting billet is 150 - 350 mm.

3. The divorced eutectoid transformation fine-grained pressure vessel steel plate according to claim 1, characterized in that, In step (1), the homogenization temperature is 1140 - 1160°C.

4. The divorced eutectoid transformation fine-grained pressure vessel steel plate according to claim 1, characterized in that, In step (2), the thickness of the intermediate billet is 2 - 3 times the thickness of the finished steel plate.

5. The divorced eutectoid transformation fine-grained pressure vessel steel plate according to claim 1, characterized in that, In step (2), the rough rolling starting temperature is 1093 - 1108°C, the finishing rolling temperature is 813 - 838°C, the finish rolling starting temperature is 640 - 650°C, and the finishing rolling temperature is 514 - 613°C.

6. The divorced eutectoid transformation fine-grained pressure vessel steel plate according to claim 1, characterized in that, In step (3), the austenitization temperature is 820 - 840°C.

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