A low alloy container steel plate for a normalizing type ultra-high temperature molten salt storage tank and a manufacturing method thereof

Abstract

The present application relates to a kind of low alloy container steel plate for normalizing type superhigh temperature molten salt storage tank and its manufacturing method, chemical composition and its mass percentage composition are as follows:C:0.14~0.18%, Si:0.30~0.50%, Mn:1.30~1.60%, P≤0.010%, S≤0.002%, Al≥0.020%, Cu≤0.10%, Cr≤0.10%, Ni≤0.10%, Mo:0.12~0.20%, Nb:0.02~0.04%, V:0.02~0.04%, Ti:0.01~0.03%, the balance is Fe and inevitable impurities.The steel plate is produced by the processes of converter smelting+secondary refining+vacuum treatment+slab continuous casting+controlled rolling and controlled cooling+normalizing heat treatment, and the thickness is 20-60mm, with good weldability, forming machinability, high temperature tensile property and low temperature impact toughness:room temperature tensile yield strength R p0.2 ≥350MPa, tensile strength R m ≥540-620MPa, elongation A 50 ≥25%; core-20 ℃ transverse impact single value ≥100J; 300-400 ℃ high temperature tensile yield strength R p0.2 ≥300MPa, tensile strength R m ≥505MPa, elongation A 50 ≥25%; surface hardness ≤180HB.

Description

Technical Field

[0001] This invention belongs to the field of special steel smelting technology, specifically relating to a low alloy container steel plate for normalized ultra-high temperature molten salt storage tanks and its manufacturing method. Background Technology

[0002] my country's green energy transition is accelerating, making energy storage an essential component of the new power system. New energy storage mainly includes electrical energy storage, thermal energy storage, and hydrogen energy storage, with thermal energy storage primarily referring to molten salt energy storage, applied in concentrated solar power (CSP). As of the end of 2022, my country's cumulative installed CSP capacity was only 0.588 GW. Therefore, with favorable policies, molten salt energy storage is poised for rapid development.

[0003] From the perspective of concentrated solar power (CSP) market applications, CSP plants generally use dual storage tanks for molten salt (cold / hot). The cold molten salt tank is typically made of normalized carbon steel plate with a yield strength of 345 MPa, and its operating temperature range is 290-400℃, classifying it as an ultra-high temperature storage tank (design temperature > 250℃). Therefore, to ensure safety during equipment operation and maintenance, the high-temperature tensile properties of the steel plate at 300-400℃ must not be lower than the requirements of ASME Section II, Part D, SA516 Gr70, i.e., high-temperature tensile yield strength at 300℃ / 325℃ / 350℃ / 375℃ / 400℃ ≥ 204 MPa / 199 MPa / 193 MPa / 187 MPa / 181 MPa, respectively, and tensile strength at 300℃ / 325℃ / 350℃ / 375℃ / 400℃ ≥ 485 MPa / 485 MPa / 485 MPa / 476 MPa, respectively; and the transverse impact absorption energy (KV8) at -20℃ in the core must be ≥ 54 J.

[0004] Patent (CN114645209A) discloses a steel plate for medium and high temperature pressure vessels and its preparation method. The heat treatment process uses normalizing. The Charpy impact absorption energy at 0℃ is ≥160J, the yield strength in the high-temperature tensile test at 200-400℃ is ≥225MPa, the tensile strength is ≥460MPa, and the elongation after fracture is ≥25.5%. However, its high-temperature tensile strength at 300-400℃ does not meet the requirements of ASME Section II, Part D, SA516Gr70, and it does not address the impact performance at -20℃.

[0005] Patent (CN108754340A) discloses a Q345R steel plate for manufacturing pressure vessel heads and its production method. The heat treatment process involves two normalizing and tempering processes, achieving a Charpy impact absorption energy of ≥100J at -20 to 0℃; a yield strength of ≥220MPa, a tensile strength of ≥450MPa, and an elongation after fracture of ≥24% in a high-temperature tensile test at 200 to 400℃. However, its high-temperature tensile strength at 300-400℃ does not meet the requirements of SA516Gr70 in ASME Section II, Part D, and the heat treatment process of this patented product is complex, with a long production cycle and high cost.

[0006] Patent (CN115354219A) discloses an SA516Gr70 steel plate with excellent high-temperature strength at 200-400℃ and its manufacturing method. The heat treatment process uses normalizing (water cooling) + tempering, with a core Charpy impact absorption energy of ≥100J at -30℃, a yield strength of ≥320MPa and a tensile strength of ≥500MPa at 200-400℃. Although its high-temperature tensile properties at 300-400℃ meet the requirements of SA516Gr70 in ASME Section II, Part D, the heat treatment process of this patented product is complex, with a long production cycle and high cost. In addition, water cooling is used after normalizing, while the tempering temperature is 530-570℃. Based on existing production experience, the steel plate has high surface hardness and poor processability. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a low alloy container steel plate for normalized ultra-high temperature molten salt storage tanks and its manufacturing method, which is in contrast to the above-mentioned prior art. The steel plate has a thickness of 20-60mm and has good weldability, formability, high temperature tensile properties and low temperature impact toughness.

[0008] The technical solution adopted by this invention to solve the above problems is as follows: a low-alloy steel plate for normalized ultra-high temperature molten salt storage tanks, wherein the chemical composition and mass percentage of the steel plate are: C: 0.14-0.18%, Si: 0.30-0.50%, Mn: 1.30-1.60%, P≤0.010%, S≤0.002%, Al≥0.020%, Cu≤0.10%, Cr≤0.10%, Ni≤0.10%, Mo: 0.12-0.20%, Nb: 0.02-0.04%, V: 0.02-0.04%, Ti: 0.01-0.03%, with the balance being Fe and unavoidable impurities. Furthermore, the carbon equivalent (CEV) of the steel plate of this invention is ≤0.45%, wherein the carbon equivalent is calculated using the formula CEV(%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

[0009] The low-alloy steel plate for normalized ultra-high temperature molten salt storage tanks of the present invention meets the following performance requirements: room temperature tensile yield strength Rp0.2 ≥350MPa, tensile strength R m ≥540-620MPa, elongation after fracture A 50 ≥25%; transverse impact strength at -20℃ ≥100J; high-temperature tensile yield strength R at 300-400℃ p0.2 ≥300MPa, tensile strength R m ≥505MPa, elongation after fracture A 50 ≥25%; Surface hardness ≤180HB.

[0010] Chemical composition is a major factor affecting the performance of steel plates. The main chemical elements in steel and their roles are as follows:

[0011] Carbon (C) is one of the strongest strengthening elements, increasing pearlite content and significantly improving the strength of steel, but it has adverse effects on plasticity, toughness, and weldability. In this invention, the C content is controlled at 0.14–0.18%.

[0012] Si is a strong solid solution strengthening element that can significantly improve the strength of steel, but a high Si content is detrimental to toughness and leads to an increase in the brittle transition temperature. In this invention, the Si content is controlled at 0.30–0.50%.

[0013] Mn is one of the main solid solution strengthening elements for ferrite. Upon cooling, it can lower the γ→α phase transformation temperature, refine ferrite grains, and simultaneously widen the range between the recrystallization stop temperature and the phase transformation start temperature, which is beneficial for controlled rolling processes. In this invention, the Mn content is controlled at 1.30–1.60%.

[0014] P and S tend to segregate in steel, causing low-temperature brittleness and temper brittleness, and are therefore considered harmful elements. In this invention, the P content is controlled to ≤0.010%, and the S content is controlled to ≤0.002%.

[0015] Nb, V, and Ti are strong carbonitride forming elements. Nb has the greatest effect on increasing the temperature of the non-recrystallization region of austenite and its grain refinement effect through controlled rolling is most significant. V has a strong precipitation strengthening effect in ferrite. Ti and N have the strongest bonding force, and the formed TiN particles significantly hinder the coarsening of austenite grains during reheating. Composite microalloying often produces a comprehensive effect, resulting in excellent strength and toughness. In this invention, the Nb content is controlled at 0.02–0.04%, the V content at 0.02–0.04%, and the Ti content at 0.01–0.03%.

[0016] Mo exhibits significant high-temperature solid solution strengthening effects, improving high-temperature performance. Simultaneously, it increases the solubility of Nb in austenite, which is beneficial for enhancing the precipitation strengthening effect of ferrite. In this invention, the Mo content is controlled at 0.12–0.20%.

[0017] This invention also provides a method for manufacturing low-alloy steel plates for normalized ultra-high temperature molten salt storage tanks, the steps of which are smelting and continuous casting, slab heating, rolling, and heat treatment, as detailed below:

[0018] (1) Smelting and continuous casting: LF refining time ≥30min; RH vacuum treatment, vacuum degree ≤0.67mbar, vacuum holding time ≥15min, soft blowing argon time ≥10min; continuous casting superheat 15~35℃, casting speed 0.45~0.57m / min, cumulative dynamic light reduction 10-15mm.

[0019] (2) Slab heating: The preheating section heating temperature is ≤900℃, the first heating section temperature is 980~1200℃, the second heating section temperature is 1120℃~1230℃, the soaking section temperature is 1130~1220℃, and the total heating time of the second heating section and the soaking section is ≥200min.

[0020] (3) Steel rolling: A two-stage rolling process of roughing and finishing is adopted. The initial rolling temperature of roughing is 1050-1150℃, and the final rolling temperature is ≥950℃. At least 3 of the last 4 passes have a reduction of ≥40mm. The initial rolling temperature of finishing is 820-880℃, and the final rolling temperature is 780-820℃. The cumulative reduction rate is ≥60%. After rolling, ACC water cooling is performed at a cooling rate of 2-5℃ / s. The final cooling and reddening temperature is 690-750℃.

[0021] (4) Heat treatment: normalizing process is adopted, with a normalizing holding temperature of 880-910℃ and a holding coefficient of 0.5-0.8min / mm. After exiting the furnace, air cooling is performed.

[0022] This invention employs a C-Mn basic composition system, supplemented with trace amounts of Nb, V, Ti, and Mo. Through pure steelmaking technology, high-homogeneity continuous casting technology, controlled rolling and cooling technology, and precise heat treatment technology, a 20-60mm thick normalized low-alloy steel plate for ultra-high temperature molten salt storage tanks has been developed. The steel plate has a carbon equivalent (CEV) ≤ 0.45% and a room temperature tensile yield strength R0. p0.2 ≥350MPa, tensile strength R m ≥540-620MPa, elongation after fracture A 50 ≥25%; transverse impact strength at -20℃ ≥100J; high-temperature tensile yield strength R at 300-400℃ p0.2 ≥300MPa, tensile strength R m ≥505MPa, elongation after fracture A 50 ≥25%; surface hardness ≤180HB; possesses excellent strength, toughness, high-temperature performance and low surface hardness.

[0023] Compared with the prior art, the advantages of the present invention are as follows:

[0024] (1) The chemical composition design adopts the C-Mn system as the main system; the addition of Nb, V and Ti elements for composite microalloying plays a comprehensive role in grain refinement and precipitation strengthening; the addition of Mo element has a significant effect on high-temperature solid solution strengthening, improves high-temperature performance, and increases the solubility of Nb in austenite, enhancing the precipitation strengthening effect; the CEV is controlled to be ≤0.45%, so that the steel plate has good weldability.

[0025] (2) High purity of steel is ensured by double refining with LF+RH; the quality of the core of the continuously cast billet is improved by strictly controlling the superheat, casting speed and dynamic light reduction parameters, and ensuring that the low magnification quality is not lower than the center segregation grade C1.0 / center porosity grade 0.5.

[0026] (3) The continuous casting billet adopts a low temperature and long time heating process to refine the original austenite grains and further improve the segregation of the billet core.

[0027] (4) The rolling process uses low speed and high reduction to fully compress the porous core; at the same time, the combination of controlled rolling and cooling technology with microalloying significantly refines the grains and improves the microstructure, thereby improving both the strength and toughness of the steel.

[0028] (5) A uniform and fine ferrite + pearlite structure is obtained through normalizing heat treatment, which gives the steel plate a good balance of strength, hardness, toughness and high temperature performance. Attached Figure Description

[0029] Figure 1 The typical microstructure of the 20mm thick steel plate in Example 1 is mainly composed of ferrite and pearlite.

[0030] Figure 2 The typical microstructure of the 40mm thick steel plate in Example 2 is mainly composed of ferrite and pearlite.

[0031] Figure 3 The typical microstructure of the 60mm thick steel plate in Example 3 is mainly composed of ferrite and pearlite. Detailed Implementation

[0032] The technical solution of the present invention will be described in more detail with reference to preferred embodiments. However, these embodiments are merely descriptions of preferred implementations of the present invention and should not be construed as limiting the scope of the invention.

[0033] Example 1

[0034] The low-alloy steel plate for the normalized ultra-high temperature molten salt storage tank in this embodiment has a thickness of 20 mm and is produced using a 370 mm continuous casting billet. Its chemical composition and mass percentage are as follows: C: 0.15%, Si: 0.38%, Mn: 1.47%, P: 0.007%, S: 0.001%, Al: 0.033%, Cu: 0.02%, Cr: 0.03%, Ni: 0.02%, Mo: 0.15%, Nb: 0.026%, V: 0.033%, Ti: 0.019%, with the balance being Fe and unavoidable impurities; carbon equivalent (CEV): 0.440%.

[0035] The manufacturing process of this steel plate is as follows:

[0036] (1) Smelting and continuous casting: LF refining time 41min; RH vacuum treatment, vacuum degree 0.65mbar, vacuum holding time 25min, soft blowing argon time 15min; continuous casting superheat 27℃, casting speed 0.50m / min, cumulative dynamic light reduction 12mm.

[0037] (2) Slab heating: The preheating section heating temperature is ≤900℃, the first heating section temperature is 980~1200℃, the second heating section temperature is 1120℃~1230℃, the soaking section temperature is 1130~1220℃, and the total heating time of the second heating section and the soaking section is 237min.

[0038] (3) Steel rolling: A two-stage rolling process of roughing and finishing is adopted. The roughing rolling start temperature is 1102℃, the finishing rolling temperature is 962℃, and the reduction of the last four passes is 44mm, 43mm, 43mm, and 41mm respectively; the finishing rolling start temperature is 877℃, the finishing rolling temperature is 801℃, and the cumulative reduction rate is 86%; after rolling, ACC water cooling is performed at a cooling rate of 3.7℃ / s, and the final cooling and reddening temperature is 727℃.

[0039] (4) Heat treatment: normalizing temperature 897℃, holding coefficient 0.77min / mm, air cooling after removal from the furnace.

[0040] The mechanical properties of the 20mm thick steel plate produced by the above manufacturing process are shown in Table 1.

[0041] Example 2

[0042] The low-alloy steel plate for the normalized ultra-high temperature molten salt storage tank in this embodiment has a thickness of 40 mm and is produced using a 370 mm continuous casting billet. Its chemical composition and mass percentage are as follows: C: 0.15%, Si: 0.37%, Mn: 1.48%, P: 0.008%, S: 0.001%, Al: 0.037%, Cu: 0.02%, Cr: 0.03%, Ni: 0.02%, Mo: 0.15%, Nb: 0.025%, V: 0.035%, Ti: 0.021%, with the balance being Fe and unavoidable impurities; carbon equivalent (CEV): 0.442%.

[0043] The manufacturing process of this steel plate is as follows:

[0044] (1) Smelting and continuous casting: LF refining time 45min; RH vacuum treatment, vacuum degree 0.55mbar, vacuum holding time 18min, soft blowing argon time 19min; continuous casting superheat 21℃, casting speed 0.53m / min, cumulative dynamic light reduction 13.5mm.

[0045] (2) Slab heating: The preheating section heating temperature is ≤900℃, the first heating section temperature is 980~1200℃, the second heating section temperature is 1120℃~1230℃, the soaking section temperature is 1130~1220℃, and the total heating time of the second heating section and the soaking section is 209min.

[0046] (3) Steel rolling: A two-stage rolling process of roughing and finishing is adopted. The roughing rolling start temperature is 1081℃, the finishing rolling temperature is 974℃, and the reduction of the last four passes is 42mm, 44mm, 41mm, and 38mm, respectively; the finishing rolling start temperature is 845℃, the finishing rolling temperature is 813℃, and the cumulative reduction rate is 73%; after rolling, ACC water cooling is performed at a cooling rate of 3.1℃ / s, and the final cooling and reddening temperature is 718℃.

[0047] (4) Heat treatment: normalizing temperature 894℃, holding coefficient 0.71min / mm, air cooling after removal from the furnace.

[0048] The mechanical properties of the 40mm thick steel plate produced by the above manufacturing process are shown in Table 1.

[0049] Example 3

[0050] The low-alloy steel plate for the normalized ultra-high temperature molten salt storage tank in this embodiment has a thickness of 60 mm and is produced using a 370 mm continuous casting billet. Its chemical composition and mass percentage are as follows: C: 0.16%, Si: 0.40%, Mn: 1.46%, P: 0.006%, S: 0.001%, Al: 0.036%, Cu: 0.01%, Cr: 0.02%, Ni: 0.01%, Mo: 0.16%, Nb: 0.027%, V: 0.033%, Ti: 0.019%, with the balance being Fe and unavoidable impurities; carbon equivalent (CEV): 0.447%.

[0051] The manufacturing process of this steel plate is as follows:

[0052] (1) Smelting and continuous casting: LF refining time 35min; RH vacuum treatment, vacuum degree 0.59mbar, vacuum holding time 22min, soft blowing argon time 23min; continuous casting superheat 19℃, casting speed 0.55m / min, cumulative dynamic light reduction 14.5mm.

[0053] (2) Slab heating: The preheating section heating temperature is ≤900℃, the first heating section temperature is 980~1200℃, the second heating section temperature is 1130℃~1230℃, the soaking section temperature is 1120~1220℃, and the total heating time of the second heating section and the soaking section is 231min.

[0054] (3) Steel rolling: A two-stage rolling process of roughing and finishing is adopted. The roughing rolling start temperature is 1087℃, the finishing rolling temperature is 983℃, and the reduction of the last four passes is 44mm, 37mm, 43mm, and 42mm, respectively; the finishing rolling start temperature is 831℃, the finishing rolling temperature is 798℃, and the cumulative reduction rate is 66%; after rolling, ACC water cooling is performed at a cooling rate of 2.6℃ / s, and the final cooling and reddening temperature is 724℃.

[0055] (4) Heat treatment: normalizing temperature 901℃, holding coefficient 0.58min / mm, air cooling after removal from the furnace.

[0056] The mechanical properties of the 60mm thick steel plate produced by the above manufacturing process are shown in Table 1.

[0057] Table 1 Mechanical properties of steel plates from various embodiments

[0058]

[0059] Although preferred embodiments of the present invention have been described in detail above, it should be clearly understood that various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for manufacturing a low-alloy steel plate for a normalized ultra-high temperature molten salt storage tank, characterized in that: The chemical composition and mass percentage of the steel plate are as follows: C: 0.14–0.18%, Si: 0.30–0.50%, Mn: 1.30–1.60%, P≤0.010%, S≤0.002%, Al≥0.020%, Cu: 0.01–0.02%, Cr: 0.02–0.03%, Ni: 0.01–0.02%, Mo: 0.12–0.20%, Nb: 0.02–0.04%, V: 0.02–0.04%, Ti: 0.01–0.03%, with the balance being Fe and unavoidable impurities. The thickness of the steel plate is 20–60 mm, and the microstructure is uniform and fine ferrite + pearlite. The room temperature tensile yield strength R of the steel plate is... p0.2 ≥350MPa, tensile strength R m ≥540-620MPa, elongation after fracture A 50 ≥25%; transverse impact strength at -20℃ ≥100J; high-temperature tensile yield strength R at 300-400℃ p0.2 ≥300MPa, tensile strength R m ≥505MPa, elongation after fracture A 50 ≥25%; surface hardness ≤180HB; the method mainly includes the following steps: (1) Smelting and continuous casting: LF refining time ≥30min; RH vacuum treatment, vacuum degree ≤0.67mbar, vacuum holding time ≥15min, soft blowing argon time ≥10min; continuous casting superheat 15~35℃, casting speed 0.45~0.57m / min, cumulative dynamic light reduction 10-15mm; (2) Slab heating: The preheating section heating temperature is ≤900℃, the first heating section temperature is 980~1200℃, the second heating section temperature is 1120℃~1230℃, the soaking section temperature is 1130~1220℃, and the total heating time of the second heating section and the soaking section is ≥200min; (3) Steel rolling: The two-stage rolling process of roughing and finishing is adopted. The roughing rolling temperature is 1050-1150℃ and the finishing rolling temperature is ≥950℃. At least 3 of the last 4 passes have a reduction of ≥40mm. The finishing rolling temperature is 820-880℃ and the finishing rolling temperature is 780-820℃. The cumulative reduction rate is ≥60%. After rolling, ACC water cooling is carried out at a cooling rate of 2-5℃ / s. The final cooling and reddening temperature is 690-750℃. (4) Heat treatment: normalizing process is adopted, with a normalizing holding temperature of 880-910℃ and a holding coefficient of 0.5-0.8min / mm. After exiting the furnace, air cooling is performed.

2. The method for manufacturing a low-alloy steel plate for a normalized ultra-high temperature molten salt storage tank according to claim 1, characterized in that: The carbon equivalent (CEV) of the steel plate is ≤0.45%, and the carbon equivalent is calculated using the formula CEV (%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

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