Normalizing type low-alloy container steel plate for ultrahigh-temperature molten salt storage tank and manufacturing method of normalizing type low-alloy container steel plate

Abstract

The invention relates to a normalizing type low-alloy container steel plate for an ultrahigh-temperature molten salt storage tank and a manufacturing method of the normalizing type low-alloy container steel plate. The normalizing type low-alloy container steel plate comprises the following chemical components in percentage by mass: 0.14-0.18% of C, 0.30-0.50% of Si, 1.30-1.60% of Mn, less than or equal to 0.010% of P, less than or equal to 0.002% of S, more than or equal to 0.020% of Al, less than or equal to 0.10% of Cu, less than or equal to 0.10% of Cr, less than or equal to 0.10% of Ni, 0.12-0.20% of Mo, 0.02-0.04% of Nb, 0.02-0.04% of V, 0.01-0.03% of Ti and the balance of Fe and inevitable impurities. The steel plate is produced through the procedures of converter smelting, external refining, vacuum treatment, slab continuous casting, controlled rolling and controlled cooling, normalizing heat treatment and the like, the thickness of the steel plate is 20-60 mm, and the steel plate has good weldability, forming machinability, high-temperature tensile property and low-temperature impact toughness, the room-temperature tensile yield strength Rp0.2 is larger than or equal to 350 MPa, the tensile strength Rm is larger than or equal to 540-620 MPa, and the percentage elongation after fracture A50 is larger than or equal to 25%; the-20 DEG C transverse impact unit value of the core part is not less than 100J; the tensile yield strength Rp0.2 at the high temperature of 300-400 DEG C is greater than or equal to 300 MPa, the tensile strength Rm is greater than or equal to 505 MPa, and the percentage elongation after fracture A50 is greater than or equal to 25%; and the surface hardness is less than or equal to 180 HB.

Description

Technical Field

[0001] The invention belongs to the technical field of special steel smelting, and specifically relates to a normalizing type low alloy container steel plate for ultra-high temperature molten salt storage tank and a manufacturing method thereof. Background Art

[0002] my country's energy green transformation is accelerating, and energy storage has become an essential part of the new power system. New energy storage mainly includes electrical energy storage, thermal energy storage and hydrogen energy storage, among which thermal energy storage mainly refers to molten salt energy storage, which is applied in the field of solar thermal power generation. As of the end of 2022, the cumulative installed capacity of solar thermal power generation in my country is only 0.588GW. Therefore, with the favorable policies, molten salt energy storage will surely usher in vigorous development.

[0003] From the perspective of the application of CSP market, CSP power stations generally use cold / hot molten salt dual storage tanks to store molten salt. The cold molten salt tank is usually made of normalized carbon steel plate with a yield strength of 345MPa. Its operating temperature range is 290-400℃, which belongs to ultra-high temperature storage tank (design temperature>250℃). Therefore, in order to ensure the safety of equipment operation and maintenance, the high temperature tensile performance of steel plate at 300-400℃ shall not be lower than the provisions of SA516Gr70 in ASME Volume II, Part D, that is, the high temperature tensile yield strength of 300℃ / 325℃ / 350℃ / 375℃ / 400℃ is ≥204MPa / 199MPa / 193MPa / 187MPa / 181MPa, respectively, and the tensile strength is ≥485MPa / 485MPa / 485MPa / 485MPa / 476MPa respectively; at the same time, the core -20℃ lateral impact absorption energy KV8 is ≥54J.

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

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

[0006] The patent (CN115354219A) discloses a SA516Gr70 steel plate with excellent high temperature strength at 200-400℃ and its manufacturing method. The heat treatment process adopts normalizing (water cooling) + tempering, the core -30℃ Charpy impact absorption energy ≥100J, the yield strength of 200-400℃ high temperature tensile strength ≥320MPa, and the tensile strength ≥500MPa. Although its 300-400℃ high temperature tensile performance meets the requirements of SA516Gr70 in ASME Volume II, Part D; however, the heat treatment process of the patented product is complicated, the production cycle is long, and the cost is high; at the same time, water cooling is adopted after normalizing, and the tempering temperature is 530-570℃. According to existing production experience, the surface hardness of the steel plate is high and the processing formability is poor. Summary of the invention

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

[0008] The technical solution adopted by the present invention to solve the above problems is: a normalizing type ultra-high temperature molten salt storage tank low alloy container steel plate, 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%, and the balance is Fe and unavoidable impurities. At the same time, the carbon equivalent CEV of the steel plate of the present invention is ≤0.45%, wherein the carbon equivalent calculation formula is CEV (%) = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15.

[0009] The properties of the normalized ultra-high temperature molten salt storage tank low alloy container steel plate of the present invention meet the following requirements: room temperature tensile yield strength Rp0.2 ≥350MPa, tensile strength R m ≥540-620MPa, elongation after fracture A 50 ≥25%; core -20℃ lateral impact single value ≥100J; 300-400℃ high temperature tensile yield strength R p0.2 ≥300MPa, tensile strength R m ≥505MPa, elongation after fracture A 50 ≥25%; surface hardness ≤180HB.

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

[0011] C is one of the strongest strengthening elements, which can increase the pearlite content and significantly improve the strength of steel, but has an adverse effect on plasticity, toughness and weldability. The C content of the present invention is controlled at 0.14-0.18%.

[0012] Si is a strong solid solution strengthening element and can significantly improve the strength of steel, but a higher Si content is not good for toughness and leads to an increase in the brittle transition temperature. The Si content of the present invention is controlled at 0.30-0.50%.

[0013] Mn is one of the main ferrite solid solution strengthening elements. It can reduce the γ→α phase transformation temperature during cooling, refine the ferrite grains, and at the same time increase the interval between the recrystallization stop temperature and the phase transformation start temperature, which is beneficial to the implementation of the controlled rolling process. The Mn content of the present invention is controlled at 1.30-1.60%.

[0014] P and S are easily segregated in steel, causing low-temperature brittleness and temper brittleness, and are harmful elements. In the present invention, the P content is controlled at ≤0.010%, and the S content is controlled at ≤0.002%.

[0015] Nb, V, and Ti are strong carbonitride-forming elements. Nb has the greatest effect on increasing the temperature of the unrecrystallized zone of austenite, and has the most significant effect on grain refinement through controlled rolling; V has a strong precipitation strengthening effect in ferrite; Ti and N have the strongest binding force, and the TiN particles formed have a significant inhibitory effect on the coarsening of austenite grains during reheating. Composite microalloying often produces a comprehensive effect and obtains excellent strength and toughness. The Nb content of the present invention is controlled at 0.02-0.04%, the V content is controlled at 0.02-0.04%, and the Ti content is controlled at 0.01-0.03%.

[0016] The Mo element has a significant high temperature solid solution strengthening effect, improves high temperature performance, and can increase the solubility of Nb in austenite, which is beneficial to enhance the precipitation strengthening effect of ferrite. The Mo content of the present invention is controlled at 0.12-0.20%.

[0017] The present invention also provides a method for manufacturing a normalizing type low alloy container steel plate for ultra-high temperature molten salt storage tank, which comprises smelting and continuous casting, slab heating, steel rolling and heat treatment in sequence, and the specific process is as follows:

[0018] ⑴ Smelting and continuous casting: LF refining time ≥30min; RH vacuum treatment, vacuum degree ≤0.67mbar, vacuum holding time ≥15min, soft argon blowing time ≥10min; continuous casting superheat 15~35℃, pulling speed 0.45~0.57m / min, cumulative dynamic soft reduction 10-15mm.

[0019] ⑵ Slab heating: preheating section heating temperature ≤ 900℃, first heating section temperature 980~1200℃, second heating section temperature 1120℃~1230℃, soaking section temperature 1130~1220℃, total heating time of second heating section and soaking section ≥ 200min.

[0020] ⑶ Steel rolling: adopts two-stage rolling process of rough rolling and finishing rolling. The starting temperature of rough rolling is 1050-1150℃, the final rolling temperature is ≥950℃, and the reduction of at least 3 of the last 4 passes is ≥40mm; the starting temperature of finishing rolling is 820-880℃, the final rolling temperature is 780-820℃, and the cumulative reduction rate is ≥60%; ACC water cooling is carried out after rolling, the cooling rate is 2-5℃ / s, and the final cooling temperature is 690-750℃.

[0021] ⑷ Heat treatment: normalizing process is adopted, normalizing holding temperature is 880-910℃, insulation coefficient is 0.5-0.8min / mm, and air cooling is carried out of the furnace.

[0022] The composition design of the present invention adopts the C-Mn basic system, supplemented by trace Nb, V, Ti, and Mo elements, and develops a 20-60mm thick normalized ultra-high temperature molten salt storage tank low alloy container steel plate through pure steel smelting technology, high homogeneity continuous casting technology, controlled rolling and controlled cooling technology, and precise control heat treatment technology. The carbon equivalent CEV of the steel plate is ≤0.45%; the room temperature tensile yield strength R p0.2 ≥350MPa, tensile strength R m ≥540-620MPa, elongation after fracture A 50 ≥25%; core -20℃ lateral impact single value ≥100J; 300-400℃ high temperature tensile yield strength R p0.2 ≥300MPa, tensile strength R m ≥505MPa, elongation after fracture A 50 ≥25%; surface hardness ≤180HB; has excellent strength, toughness, high temperature performance and low surface hardness.

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

[0024] ⑴ The chemical composition design adopts C-Mn system as the main system; adding Nb, V, Ti elements for composite microalloying plays a comprehensive role of fine grain strengthening and precipitation strengthening; adding Mo element has obvious high temperature solid solution strengthening effect, improves high temperature performance, and increases the solubility of Nb in austenite, enhancing the precipitation strengthening effect; controlling CEV≤0.45% to make the steel plate have good weldability.

[0025] ⑵ Ensure high purity of steel through LF+RH double refining; improve the quality of the core of continuous casting billet by strictly controlling superheat, pulling speed and dynamic light reduction parameters, and ensure that the low-multiple quality is not lower than center segregation C1.0 level / center porosity 0.5 level.

[0026] ⑶ The continuous casting billet adopts a low-temperature, long-time heating process to refine the original austenite grains while further improving the segregation in the center of the billet.

[0027] (4) The steel rolling adopts a low-speed, high-pressure process to fully compact the loose core; at the same time, through the combination of controlled rolling and controlled cooling technology with micro-alloying, the grains are significantly refined and the microstructure is improved, which not only improves the strength of the steel but also improves the toughness of the steel.

[0028] ⑸ Through normalizing heat treatment, a uniform and fine ferrite + pearlite structure is obtained, so that the steel plate has good steel plate strength, hardness, toughness matching and high temperature performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a typical microstructure of the 20 mm thick steel plate of Example 1, which is mainly ferrite and pearlite.

[0030] Figure 2 This is a typical microstructure of the 40 mm thick steel plate of Example 2, which is mainly ferrite and pearlite.

[0031] Figure 3 This is a typical microstructure of the 60 mm thick steel plate of Example 3, which is mainly ferrite and pearlite. DETAILED DESCRIPTION

[0032] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0033] Example 1

[0034] The thickness of the low-alloy container steel plate for the normalizing type ultra-high temperature molten salt storage tank in this embodiment is 20 mm. It is produced by 370 mm continuous casting billet. Its chemical composition and mass percentage are 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%, and the balance is Fe and unavoidable impurities; carbon equivalent CEV: 0.440%.

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

[0036] ⑴ Smelting and continuous casting: LF refining time 41min; RH vacuum treatment, vacuum degree 0.65mbar, vacuum holding time 25min, soft argon blowing time 15min; continuous casting superheat 27℃, pulling speed 0.50m / min, cumulative dynamic soft reduction 12mm.

[0037] ⑵ Slab heating: The heating temperature of the preheating section is ≤900℃, the temperature of the first heating section is 980~1200℃, the temperature of the second heating section is 1120℃~1230℃, the temperature of the soaking section is 1130~1220℃, and the total heating time of the second heating section and the soaking section is 237min.

[0038] ⑶ Steel rolling: A two-stage rolling process of rough rolling and finishing rolling is adopted. The starting rolling temperature of rough rolling is 1102℃, the final rolling temperature is 962℃, and the reduction of the last 4 passes is 44mm, 43mm, 43mm, and 41mm respectively; the starting rolling temperature of finishing rolling is 877℃, the final rolling temperature is 801℃, and the cumulative reduction rate is 86%; ACC water cooling is carried out after rolling, the cooling rate is 3.7℃ / s, and the final cooling temperature is 727℃.

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

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

[0041] Example 2

[0042] The thickness of the low-alloy container steel plate for the normalizing type ultra-high temperature molten salt storage tank in this embodiment is 40 mm. It is produced by 370 mm continuous casting billet. Its chemical composition and mass percentage are 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%, and the remainder is Fe and unavoidable impurities; carbon equivalent CEV: 0.442%.

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

[0044] ⑴ Smelting and continuous casting: LF refining time 45min; RH vacuum treatment, vacuum degree 0.55mbar, vacuum holding time 18min, soft argon blowing time 19min; continuous casting superheat 21℃, pulling speed 0.53m / min, cumulative dynamic soft reduction 13.5mm.

[0045] ⑵ Slab heating: The heating temperature of the preheating section is ≤900℃, the temperature of the first heating section is 980~1200℃, the temperature of the second heating section is 1120℃~1230℃, the temperature of the soaking section is 1130~1220℃, and the total heating time of the second heating section and the soaking section is 209min.

[0046] ⑶ Steel rolling: A two-stage rolling process of rough rolling and finishing rolling is adopted. The starting rolling temperature of rough rolling is 1081℃, the final rolling temperature is 974℃, and the reduction of the last 4 passes is 42mm, 44mm, 41mm, and 38mm respectively; the starting rolling temperature of finishing rolling is 845℃, the final rolling temperature is 813℃, and the cumulative reduction rate is 73%; ACC water cooling is carried out after rolling, the cooling rate is 3.1℃ / s, and the final cooling temperature is 718℃.

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

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

[0049] Example 3

[0050] The thickness of the low-alloy container steel plate for the normalizing type ultra-high temperature molten salt storage tank in this embodiment is 60 mm. It is produced by 370 mm continuous casting billet. Its chemical composition and mass percentage are 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%, and the balance is Fe and unavoidable impurities; carbon equivalent CEV: 0.447%.

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

[0052] ⑴ Smelting and continuous casting: LF refining time 35min; RH vacuum treatment, vacuum degree 0.59mbar, vacuum holding time 22min, soft argon blowing time 23min; continuous casting superheat 19℃, pulling speed 0.55m / min, cumulative dynamic soft reduction 14.5mm.

[0053] ⑵ Slab heating: The heating temperature of the preheating section is ≤900℃, the temperature of the first heating section is 980~1200℃, the temperature of the second heating section is 1130℃~1230℃, the temperature of the soaking section is 1120~1220℃, and the total heating time of the second heating section and the soaking section is 231min.

[0054] ⑶ Steel rolling: A two-stage rolling process of rough rolling and finishing rolling is adopted. The starting rolling temperature of rough rolling is 1087℃, the final rolling temperature is 983℃, and the reduction of the last 4 passes is 44mm, 37mm, 43mm, and 42mm respectively; the starting rolling temperature of finishing rolling is 831℃, the final rolling temperature is 798℃, and the cumulative reduction rate is 66%; ACC water cooling is carried out after rolling, the cooling rate is 2.6℃ / s, and the final cooling temperature is 724℃.

[0055] (4) Heat treatment: Normalizing holding temperature 901℃, insulation coefficient 0.58min / mm, air cooling after furnace exit.

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

[0057] Table 1 Mechanical properties of steel plates in various examples

[0058]

[0059] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A normalized low alloy container steel plate for ultra-high temperature molten salt storage tanks. Features: The chemical composition of the steel plate and its mass percentage 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%, and the balance is Fe and unavoidable impurities.

2. A normalized low alloy container steel plate for ultra-high temperature molten salt storage tank according to claim 1, Features: The carbon equivalent CEV of the steel plate is ≤0.45%, wherein the carbon equivalent calculation formula is CEV(%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

3. A normalized low alloy container steel plate for ultra-high temperature molten salt storage tank according to claim 1, Features: The steel plate has a thickness of 20-60 mm and a microstructure of uniform and fine ferrite+pearlite.

4. A normalized low alloy container steel plate for ultra-high temperature molten salt storage tank according to claim 1, Features: The room temperature tensile yield strength R of the steel plate p0.2 ≥350MPa, tensile strength R m ≥540-620MPa, elongation after fracture A 50 ≥25%; core -20℃ lateral impact single value ≥100J; 300-400℃ high temperature tensile yield strength R p0.2 ≥300MPa, tensile strength R m ≥505MPa, elongation after fracture A 50 ≥25%; surface hardness ≤180HB.

5. A method for manufacturing a normalized low alloy container steel plate for ultra-high temperature molten salt storage tank according to claim 1, Features: The method mainly comprises the following steps: ⑴ Smelting and continuous casting: LF refining time ≥30min; RH vacuum treatment, vacuum degree ≤0.67mbar, vacuum holding time ≥15min, soft argon blowing time ≥10min; continuous casting superheat 15~35℃, pulling speed 0.45~0.57m / min, cumulative dynamic soft pressure reduction 10-15mm; (2) Slab heating: preheating section heating temperature ≤ 900℃, first heating section temperature 980~1200℃, second heating section temperature 1120℃~1230℃, soaking section temperature 1130~1220℃, second heating section and soaking section total heating time ≥ 200min; ⑶ Steel rolling: adopts two-stage rolling process of rough rolling and finishing rolling, the starting rolling temperature of rough rolling is 1050-1150℃, the final rolling temperature is ≥950℃, and the reduction of at least 3 of the last 4 passes is ≥40mm; the starting rolling temperature of finishing rolling is 820-880℃, the final rolling temperature is 780-820℃, and the cumulative reduction rate is ≥60%; ACC water cooling is carried out after rolling, the cooling rate is 2-5℃ / s, and the final cooling temperature is 690-750℃; ⑷ Heat treatment: normalizing process is adopted, normalizing holding temperature is 880-910℃, insulation coefficient is 0.5-0.8min / mm, and air cooling is carried out of the furnace.

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