Steel for pressure vessels

Abstract

The present application relates to the technical field of metallurgical casting, in particular to a steel for pressure vessel, which has more than 50% of grains with size of less than 5 microns and less than 15% of grains with size of more than 20 microns after one post-weld heat treatment; more than 45% of grains with size of less than 5 microns and less than 20% of grains with size of more than 20 microns after two post-weld heat treatments; more than 40% of grains with size of less than 5 microns and less than 25% of grains with size of more than 20 microns after three post-weld heat treatments. The steel for pressure vessel can ensure greater strength and toughness after post-weld heat treatment by adjusting the composition and process, and can ensure good strength and toughness even after two or more post-weld heat treatments to meet the requirements.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metallurgical casting, in particular to a steel for pressure vessels. BACKGROUND

[0002] The steel for pressure vessels, such as SA516Gr70 medium and low temperature pressure vessel steel, is commonly used in the petroleum, chemical, power station, boiler and other industries. In order to improve the performance of the welding area and eliminate the harmful effects of welding residual stress, the steel for pressure vessels generally needs post-weld heat treatment process after welding, and sometimes even needs multiple post-weld heat treatments.

[0003] However, the steel for pressure vessels provided by the related art has a large decrease in strength and toughness after welding heat treatment, and even after secondary simulated post-weld heat treatment, it no longer meets the use requirements. SUMMARY

[0004] The purpose of the present application is to provide a steel for pressure vessels, which can ensure greater strength and toughness after welding heat treatment, and even after secondary and more than secondary post-weld heat treatment, it can also ensure good strength and toughness to meet the requirements.

[0005] The present application is implemented as follows:

[0006] The present application provides a steel for pressure vessels, the composition of the steel for pressure vessels comprises, in terms of mass percentage:

[0007] C: 0.17-0.20%, Si: 0.23-0.40%, Mn: 1.05-1.30%, P≤0.010%, S≤0.0015%, Ni: 0.15-0.20%, Mo: 0.04-0.09%, Cr: 0.10-0.20%, Nb: 0.01-0.02%, V: 0.01-0.02%, Ti: 0.01-0.02%, Alt: 0.02-0.04%, B≤0.0005%, Sn≤0.0005%, As≤0.0005%, the balance being Fe and unavoidable impurities; wherein,

[0008] After one post-weld heat treatment, the steel for pressure vessels has more than 50% of grains with a size of ≤5μm and less than 15% of grains with a size of ≥20μm; after two post-weld heat treatments, the steel for pressure vessels has more than 45% of grains with a size of ≤5μm and less than 20% of grains with a size of ≥20μm; after three post-weld heat treatments, the steel for pressure vessels has more than 40% of grains with a size of ≤5μm and less than 25% of grains with a size of ≥20μm.

[0009] In an optional embodiment, the temperature of the post-weld heat treatment is 630±10℃, and the treatment time is 2-3h.

[0010] In an alternative embodiment, the method for preparing the steel for pressure vessels comprises: smelting and rolling; wherein,

[0011] In the smelting step, during the tapping of the converter, lime 5-6 kg / t, fluorite 1-2 kg / t are added, and Alt≥0.02% is controlled; at the end of the LF refining, lime 4.5-5.5 kg / t is added, and the Alt of the molten steel at the outlet is controlled to be≥0.05%; when the VD refining bottom blowing flow rate reaches 55-65 m 3 / h, the holding time under a vacuum degree≤100 Pa is≥15 min.

[0012] In an alternative embodiment, in the smelting step, the reduction rate at the end of continuous casting is 1.5-2.5 mm / m.

[0013] In an alternative embodiment, in the smelting step, during continuous casting, the magnetic field intensity of the crystallizer electromagnetic stirring is controlled to be 80×10 -4 -120×10 -4 T.

[0014] In an alternative embodiment, the method for preparing the steel for pressure vessels comprises: smelting and rolling; wherein,

[0015] In the rolling step, the opening rolling temperature of rough rolling is controlled to be≥1050℃, the finish rolling temperature of rough rolling is controlled to be≥920℃, at least 2 passes of the reduction amount is≥18%, and the rolling speed of rough rolling is controlled to be 0.6-0.95 m / s.

[0016] In an alternative embodiment, in the rolling step, it comprises: a preheating section, a heating section and a soaking section; wherein, the temperature of the soaking section is 1200-1260℃, the time of the soaking section is≥H×0.5 min / mm, the total heating time=H×(0.85-1.05) min / mm, and H is the thickness of the steel plate.

[0017] In an alternative embodiment, in the rolling step, the finish rolling temperature and the reduction amount are controlled according to the thickness of the steel plate; wherein,

[0018] When the thickness of the steel plate is >6 mm and≤12 mm, the opening rolling temperature of finish rolling is controlled to be 980±10℃, the finish rolling temperature is controlled to be 860-880℃, and the cumulative reduction deformation amount is controlled to be≥50%;

[0019] When the thickness of the steel plate is >12 mm and≤70 mm, the opening rolling temperature of finish rolling is controlled to be 900±10℃, the finish rolling temperature of finish rolling is controlled to be≤840℃, and the cumulative reduction deformation amount is controlled to be≥50%;

[0020] When the thickness of the steel plate is > 70 mm and ≤ 100 mm, the rolling temperature of the controlled rolling is 910 ± 10 ℃, the finish rolling temperature of the controlled rolling is ≤ 840 ℃, and the accumulated reduction deformation amount is ≥ 40%.

[0021] In an optional embodiment, the method for preparing the steel for pressure vessel further comprises: a heat treatment, in the step of the heat treatment, the normalizing temperature is 850 ± 10 ℃, the holding time = H x (2-3) min / mm, H is the thickness of the steel plate, and the holding time is ≥ 30 min; wherein when the holding time calculated by the formula holding time = H x (2-3) min / mm is less than 30 min, the holding time is controlled to be 30 min.

[0022] In an optional embodiment, the step of the heat treatment further comprises:

[0023] When the thickness of the steel plate is > 6 mm and ≤ 12 mm, the steel plate is discharged and air-cooled to control the cooling rate to be 5-10 ℃ / s;

[0024] When the thickness of the steel plate is > 12 mm and ≤ 70 mm, the steel plate is discharged and air-cooled to control the cooling rate to be 5-10 ℃ / s;

[0025] When the thickness of the steel plate is > 70 mm and ≤ 100 mm, the steel plate is discharged and air-cooled and simultaneously water-mist-cooled to control the cooling rate to be 5-10 ℃ / s.

[0026] The present application comprises the following beneficial effects:

[0027] The steel for pressure vessel provided by the present application improves the low-temperature toughness of the steel material by controlling the components and adding Ni, Nb and Ti elements, improves the strength of the steel material by adding Cr, Mo, V, Nb and Ti elements, and reduces the harmful elements S, P, Sn, As and B in the steel, thereby ensuring the grain refinement of the steel plate and the strength of the grain and the grain boundary. Moreover, after one post-weld heat treatment, the proportion of grains with a size ≤ 5 μm is greater than 50%, and the proportion of grains with a size ≥ 20 μm is less than 15%; after two post-weld heat treatments, the proportion of grains with a size ≤ 5 μm is greater than 45%, and the proportion of grains with a size ≥ 20 μm is less than 20%; and after three post-weld heat treatments, the proportion of grains with a size ≤ 5 μm is greater than 40%, and the proportion of grains with a size ≥ 20 μm is less than 25%. In this way, the strength and toughness of the steel for pressure vessel after the post-weld heat treatment can not be greatly reduced, and the steel for pressure vessel can still maintain good strength and toughness after at least two post-weld heat treatments. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. It should be understood that the following drawings only show some of the embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those of ordinary skill in the art, other related drawings can also be obtained from these drawings without creative labor.

[0029] Figure 1 Microstructure of the steel plate after normalizing in Example 1 of the present application;

[0030] Figure 2 Microstructure after heat treatment after first welding in Example 1 of the present application;

[0031] Figure 3 Microstructure after heat treatment after second welding in Example 1 of the present application;

[0032] Figure 4 Microstructure after heat treatment after third welding in Example 1 of the present application. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be clearly and completely described as follows. If the specific conditions are not indicated in the embodiments, the conventional conditions or the conditions suggested by the manufacturers are adopted. If the manufacturers of the reagents or instruments are not indicated, they are all the conventional products which can be purchased in the market.

[0034] The present application provides a kind of steel for pressure vessel, it can refer to a kind of steel for low temperature pressure vessel.

[0035] The steel for pressure vessel includes, in terms of mass percentage, C: 0.17-0.20% (for example, 0.17%, 0.18%, 0.19%, 0.20%, etc.), Si: 0.23-0.40% (for example, 0.23%, 0.25%, 0.27%, 0.30%, 0.32%, 0.35%, 0.37%, 0.40%, etc.), Mn: 1.05-1.30% (for example, 1.05%, 1.10%, 1.20%, 1.30%, etc.), P≤0.010% (for example, 0.010%, 0.008%, 0.006%, etc.), S≤0.0015% (for example, 0.0015%, 0.0013%, 0.0011%, etc.), Ni: 0.15-0.20% (for example, 0.15%, 0.18%, 0.20%, etc.), Mo: 0.04-0.09% (for example, 0.04%, 0.05%, 0.09%, etc.), Cr: 0.10-0.20% (for example, 0.10%, 0.15%, 0.20%, etc.), Nb: 0.01-0.02% (for example, 0.01%, 0.015%, 0.02%, etc.), V: 0.01-0.02% (for example, 0.01%, 0.015%, 0.02%, etc.), Ti: 0.01-0.02% (for example, 0.01%, 0.015%, 0.02%, etc.), Alt: 0.02-0.04% (for example, 0.02%, 0.03%, 0.04%, etc.), B≤0.0005% (for example, 0.0005%, 0.0004%, 0.0003%, etc.), Sn≤0.0005% (for example, 0.0005%, 0.0004%, 0.0003%, etc.), As≤0.0005% (for example, 0.0005%, 0.0004%, 0.0003%, etc.), and the balance of Fe and inevitable impurities.

[0036] After one post-weld heat treatment, the proportion of grains with a size of ≤5 μm is greater than 50%, and the proportion of grains with a size of ≥20 μm is less than 15%; after two post-weld heat treatments, the proportion of grains with a size of ≤5 μm is greater than 45%, and the proportion of grains with a size of ≥20 μm is less than 20%; after three post-weld heat treatments, the proportion of grains with a size of ≤5 μm is greater than 40%, and the proportion of grains with a size of ≥20 μm is less than 25%.

[0037] The pressure vessel steel improves the low-temperature toughness of the steel material by controlling the composition, adding Ni, Nb and Ti elements, improves the strength of the steel material by adding Cr, Mo, V, Nb and Ti elements, and reduces the harmful elements S, P, Sn, As and B in the steel, on the one hand, ensures that the grain of the steel plate is refined, and on the other hand, ensures the strength of the grain and the grain boundary; and after one post-weld heat treatment, the proportion of grains with a size of less than or equal to 5 microns is greater than 50%, and the proportion of grains with a size of greater than or equal to 20 microns is less than 15%; after two post-weld heat treatments, the proportion of grains with a size of less than or equal to 5 microns is greater than 45%, and the proportion of grains with a size of greater than or equal to 20 microns is less than 20%; after three post-weld heat treatments, the proportion of grains with a size of less than or equal to 5 microns is greater than 40%, and the proportion of grains with a size of greater than or equal to 20 microns is less than 25%. In this way, the strength and toughness of the pressure vessel steel after post-weld heat treatment can be ensured not to be greatly reduced, and the pressure vessel steel can still maintain good strength and toughness after at least two post-weld heat treatments.

[0038] Specifically, Cr, Mo and V are all strong carbide forming elements, which can enhance the binding force between atoms in the solid solution, and Cr, Mo and V can form a composite reinforcement to effectively improve the strength of the steel plate; wherein Cr can increase the temper brittleness transition temperature of the steel and promote temper brittleness, so the use of Cr needs to be limited and the amount needs to be strictly controlled.

[0039] Ni can significantly improve the low-temperature impact toughness and reduce the brittleness temperature.

[0040] S and P are both harmful elements, which can increase the brittleness of the steel, reduce the impact toughness and welding performance, therefore, reducing the content of S and P is beneficial to ensure the toughness and strength of the steel after post-weld heat treatment.

[0041] Nb can significantly improve the austenite recrystallization temperature of the steel, expand the recrystallization range, facilitate high-temperature rolling, and has the effects of fine-grain strengthening and precipitation strengthening, so the addition of Nb can ensure the toughness and strength of the steel after post-weld heat treatment.

[0042] Ti can form carbonitride with C and N elements, has the effects of delaying austenite recrystallization and refining ferrite grains, can improve the strength and toughness of the steel plate at the same time, and through the addition of Ti, the toughness and strength of the steel after post-weld heat treatment can be ensured.

[0043] Al is mainly used for deoxidization and grain refinement; through grain refinement, the toughness and strength of the steel after post-weld heat treatment can also be ensured.

[0044] Sn, As and B are all harmful elements, which are easy to enrich in the intergranular, weaken the grain boundary, and reduce the content of Sn, As and B, which can ensure the toughness and strength of the steel after post-weld heat treatment.

[0045] It should be noted that the temperature of the post-weld heat treatment described above can be 630±10℃ (for example: 620℃, 630℃, 640℃, etc.), and the treatment time is 2-3h (for example: 2h, 2.5h, 3h, etc.).

[0046] It should also be noted that the pressure vessel steel of the present application has a transverse Charpy impact energy at -40℃ of ≥70J after three post-weld heat treatments. The microstructure is 65% ferrite + 35% pearlite.

[0047] The preparation method of the pressure vessel steel of the present application comprises smelting, rolling and heat treatment.

[0048] The smelting comprises KR desulfurization → converter → CAS → LF refining → VD → continuous casting; wherein, during the tapping of the converter, 5-6kg / t (for example: 5kg / t, 5.5kg / t, 6kg / t, etc.) of lime, 1-2kg / t (for example: 1kg / t, 1.5kg / t, 2kg / t, etc.) of fluorite, and control of Alt≥0.02% (for example: 0.02%, 0.025%, 0.03%, etc.) are added; at the end of LF refining, 4.5-5.5kg / t (for example: 4.5kg / t, 5kg / t, 5.5kg / t, etc.) of lime is added, and the Alt of the molten steel at the outlet is controlled to be≥0.05% (for example: 0.05%, 0.055%, 0.06%, etc.); when the VD refining single porous plug bottom blowing flow reaches 55-65m 3 / h (for example: 55m 3 / h, 60m 3 / h, 65m 3 / h, etc.), the limit vacuum is maintained for≥15min (for example: 15min, 17min, 20min, etc.), and the limit vacuum refers to a vacuum degree≤100Pa. In this way, harmful elements such as S and P in the steel can be removed, on the one hand to strengthen the grain boundary, and on the other hand to reduce element segregation, avoiding the increase of the strength and plasticity of the steel plate.

[0049] In order to ensure the performance requirements after multiple simulated welding heat treatments, the center segregation of the casting blank needs to be strictly controlled; wherein, the technology of large pressing at the end of continuous casting is adopted, with a pressing rate of 1.5-2.5mm / m (for example: 1.5mm / m, 1.8mm / m, 2.0mm / m, 2.2mm / m, 2.5mm / m, etc.); in this way, the problem of abnormal grain growth and the inability to obtain uniform grain distribution can be improved, thereby significantly improving the comprehensive performance of the steel plate; the technology of large pressing can also effectively press the shrinkage of the casting blank, reduce element segregation, avoid the segregation of elements such as C and Mn, and ensure that the steel has good strength and toughness after post-weld heat treatment.

[0050] Further, during continuous casting, the magnetic field strength of the mold electromagnetic stirring is controlled to be 80×10-4 -120×10 -4 T, for example: 80×10 -4 T, 90×10 -4 T, 100×10 -4 T, 110×10 -4 T, 120×10 -4 T, etc. Thus, the element segregation and dendritic segregation of the casting blank can be reduced, and under the magnetic field strength, the dendrites can be sufficiently broken, the number of crystal nuclei is increased, and the effect of refining the grains is achieved, so as to ensure that the steel has good strength and toughness after welding and heat treatment.

[0051] It should be noted that after a series of operations of the continuous casting step, the center segregation level of the casting blank is ≤0.5 level, the center porosity level is ≤0.5 level, and the levels of triangular zone cracks, intermediate cracks, and corner cracks are ≤0.5 level.

[0052] In the rolling step of the present application, a preheating section, a heating section, and a soaking section are included. The temperature of the soaking section is 1200-1260℃ (for example: 1200℃, 1220℃, 1240℃, 1260℃, etc.), the time of the soaking section is ≥H×0.5 min / mm, the total heating time is H×(0.85-1.05) min / mm (for example: H×0.85 min / mm, H×0.95 min / mm, H×1.05 min / mm, etc.), and H is the thickness of the steel plate.

[0053] It should be noted that the process of the preheating section and the heating section can refer to the process parameters of the related art for preparing SA516Gr70, which is a steel for medium and low temperature pressure vessels, and will not be described here.

[0054] In the rough rolling stage of the present application, the "high temperature low speed large reduction" rolling technology is used to penetrate the heart of the steel plate, reduce the center porosity and segregation of the steel plate, and improve the heart grain of the steel plate. The uniformity of the grains in all directions and thickness of the steel plate is ensured.

[0055] In the rough rolling, the rough rolling temperature is controlled to be≥1050℃ (for example, 1050℃, 1080℃, 1100℃, etc.), the finish rolling temperature is controlled to be≥920℃ (for example, 920℃, 930℃, 950℃, etc.), the reduction of at least two passes is≥18% (for example, 18%, 20%, 21%, etc.), and the rolling speed is 0.6-0.95m / s (for example, 0.6m / s, 0.7m / s, 0.8m / s, 0.9m / s, 0.95m / s, etc.). Controlling the reduction of at least two passes to be≥18% can ensure the degree of fragmentation of austenite and slow rolling, which is beneficial to the effective transmission of rolling force to the center of the steel plate, and the uniformity of the structure / grain of the steel plate, thereby ensuring that the steel has good strength and toughness after welding and heat treatment.

[0056] In the finish rolling, the temperature and reduction are controlled according to the thickness of the steel plate. Specifically, when the thickness of the steel plate is >6mm and≤12mm, the opening rolling temperature of the finish rolling is controlled to be 980±10℃ (for example, 970℃, 980℃, 990℃, etc.), the finish rolling temperature is 860-880℃ (for example, 860℃, 870℃, 880℃, etc.), the finish rolling temperature is≤880℃ (for example, 880℃, 870℃, 860℃, 850℃, etc.), and the cumulative reduction is≥50% (for example, 50%, 52%, 55%, etc.); when the thickness of the steel plate is 12mm< steel plate thickness≤70mm, the opening rolling temperature is 900±10℃ (for example, 910℃, 900℃, 890℃, etc.), the finish rolling temperature is≤840℃ (for example, 840℃, 830℃, 820℃, etc.), and the cumulative reduction is≥50% (for example, 50%, 52%, 55%, etc.); when the thickness of the steel plate is 70mm< steel plate thickness≤100mm, the opening rolling temperature is 910±10℃ (for example, 920℃, 918℃, 915℃, 913℃, 910℃, etc.), the finish rolling temperature is≤840℃ (for example, 840℃, 830℃, 820℃, etc.), and the cumulative reduction is≥40% (for example, 40%, 42%, 45%, 48%, 50%, etc.).

[0057] In the process of finish rolling, a certain cumulative deformation is obtained, which can increase the number of nucleation and the dislocation density, form more nucleation cores in the subsequent normalizing process, and obtain finer grains; thereby, after three times of simulated post-welding heat treatment, it can still ensure that the grain size of≤5μm accounts for more than 40%, and the grain size of≥20μm accounts for less than 25%, thereby ensuring that the steel still has good strength and toughness.

[0058] In the present application, the normalizing temperature in the heat treatment step is 850±10℃ (for example: 840℃, 850℃, 860℃, etc.), the holding time = H x (2-3) min / mm, H is the thickness of the steel plate, and the holding time ≥ 30 min; wherein when the holding time calculated by the formula holding time = H x (2-3) min / mm is less than 30 min, the holding time is controlled to be 30 min; when the holding time calculated by the formula holding time = H x (2-3) min / mm is greater than or equal to 30 min, the holding time is according to the calculated time.

[0059] Further, during cooling, different cooling methods are adopted according to the thickness of the steel plate; wherein when 6mm < thickness of the steel plate ≤ 12mm, air cooling after discharge is adopted to control the cooling rate to be 5-10℃ / s; when 12mm < thickness of the steel plate ≤ 70mm, air cooling after discharge is adopted to control the cooling rate to be 5-10℃ / s; when 70mm < thickness of the steel plate ≤ 100mm, air cooling + water mist cooling after discharge is adopted to control the cooling rate to be 5-10℃ / s.

[0060] During the normalizing process, the temperature is controlled to be 840-860℃, Nb, Ti, C, N diffuse to vacancies, grain boundaries, etc. with the elements, and after the cooling of the steel plate, they stay at the position, which can hinder the subsequent growth of the grains in the heat treatment process, that is, the strength and toughness of the steel after the post-weld heat treatment can be ensured not to decrease greatly. Controlling the cooling rate to be 5-10℃ / s (for example: 5℃ / s, 6℃ / s, 7℃ / s, 8℃ / s, 9℃ / s, 10℃ / s, etc.) is beneficial to the precipitation of fine and dispersed V(C / N), TiC, TiN, NbCN, etc. lattice dislocation, which is beneficial to the formation of more high-angle grain boundaries, which can hinder the growth of the grains in the simulated post-weld heat treatment, and then ensure the strength and toughness after the post-weld heat treatment.

[0061] The present application is further described in detail below in combination with the embodiments.

[0062] Embodiment 1

[0063] The composition of the steel for pressure vessel, in terms of mass percentage, includes: C: 0.17%, Si: 0.40%, Mn: 1.30%, P: 0.010%, S: 0.0015%, Ni: 0.15%, Mo: 0.04%, Cr: 0.10%, Nb: 0.02%, V: 0.01%, Ti: 0.02%, Alt: 0.04%, B: 0.0005%, Sn: 0.0005%, As: 0.0005%, and the balance is Fe and unavoidable impurities.

[0064] Smelting:

[0065] During the tapping process of the converter, lime 5.5kg / t, fluorite 1.5kg / t, and Al2S 0.02% are added; at the end of LF refining, lime 5kg / t is added, and Al2S 0.05% is added to the molten steel at the exit; the bottom blowing flow rate of the VD refining single-permeable brick is 60m 3 / h, and maintain the ultimate vacuum (vacuum degree 100Pa) for 15min.

[0066] The continuous casting end is subjected to large reduction, with a reduction rate of 2 mm / m; the magnetic field strength of the electromagnetic stirring of the crystallizer is controlled to be 80×10 - 4 T.

[0067] Rolling:

[0068] Preheating section, heating section and soaking section; among them, the temperature of the soaking section is 1230℃, the thickness of the steel plate is 10mm, the time of the soaking section is 5min, and the total heating time is 8.5min.

[0069] During rough rolling, the starting rolling temperature is 1050°C, the finishing rolling temperature is 920°C, the reduction in two passes is 18%, and the rolling speed is 0.6 m / s.

[0070] During finishing rolling, the starting rolling temperature is 980°C, the final rolling temperature is 880°C, and the cumulative reduction deformation is 50%.

[0071] Heat treatment:

[0072] Normalizing temperature is 850℃, time is 30min, cooling method is air cooling after leaving the furnace, cooling rate is 5℃ / s.

[0073] The steel plate of Example 1 was subjected to post-weld heat treatment, wherein, after one post-weld heat treatment (630°C), the proportion of grains with a size of ≤5 μm was 51%, and the proportion of grains with a size of ≥20 μm was 14%; after two post-weld heat treatments (630°C), the proportion of grains with a size of ≤5 μm was 48%, and the proportion of grains with a size of ≥20 μm was 18%; after three post-weld heat treatments (630°C), the proportion of grains with a size of ≤5 μm was 42%, and the proportion of grains with a size of ≥20 μm was 22%.

[0074] The microstructure diagram of the steel plate after normalizing, the microstructure diagram after the first welding heat treatment, the microstructure diagram after the second welding heat treatment and the microstructure diagram after the third welding heat treatment are shown in the table below. Figures 1-4 ; It can be seen from the figure that the proportion of grains with a size of ≤5μm and the proportion of grains with a size of ≥20μm in the steel prepared in Example 1 after heat treatment are effectively controlled, thereby ensuring that the steel after heat treatment after multiple welding still has good strength and toughness.

[0075] Example 2

[0076] The composition of the steel for pressure vessel includes, in terms of mass percentage, C: 0.20%, Si: 0.23%, Mn: 1.05%, P: 0.008%, S: 0.0013%, Ni: 0.20%, Mo: 0.09%, Cr: 0.20%, Nb: 0.01%, V: 0.02%, Ti: 0.01%, Alt: 0.02%, B: 0.0004%, Sn: 0.0004%, As: 0.0004%, and the balance of Fe and inevitable impurities.

[0077] Smelting:

[0078] During the converter tapping process, 6 kg / t of lime, 1 kg / t of fluorite and 0.03% of Als are added; 4.5 kg / t of lime is added at the end of LF refining, and the Als of the molten steel at the exit is 0.06%; the VD refining single porous plug bottom blowing flow rate is 55 m 3 / h, and the limit vacuum (vacuum degree 87 Pa) is maintained for 20 min.

[0079] The end of continuous casting is adopted with large reduction, and the reduction rate is 2.5 mm / m; the magnetic field strength of the electromagnetic stirring of the crystallizer is controlled to be 120 x 10 -4 T.

[0080] Rolling:

[0081] The preheating section, heating section and soaking section; wherein the temperature of the soaking section is 1200 DEG C, the thickness of the steel plate is 50 mm, the time of the soaking section is 30 min, and the total heating time is 52.5 min.

[0082] During rough rolling, the roughing temperature is 1150 DEG C, the finishing temperature is 950 DEG C, the reduction of 3 passes is 20%, and the rolling speed is 0.95 m / s.

[0083] During finish rolling, the roughing temperature is 910 DEG C, the finishing temperature is 840 DEG C, and the cumulative reduction deformation is 50%.

[0084] Heat treatment:

[0085] The normalizing temperature is 840 DEG C, and the time is 35 min. The cooling mode is air cooling after discharging, and the cooling rate is 10 DEG C / s.

[0086] The steel plate of Example 2 is subjected to post-weld heat treatment, wherein after one post-weld heat treatment (630℃), the proportion of grains with a size of ≤5μm is 52%, and the proportion of grains with a size of ≥20μm is 13%; after two post-weld heat treatments (630℃), the proportion of grains with a size of ≤5μm is 46%, and the proportion of grains with a size of ≥20μm is 15%; after three post-weld heat treatments (630℃), the proportion of grains with a size of ≤5μm is 45%, and the proportion of grains with a size of ≥20μm is 20%.

[0087] Example 3

[0088] The composition of the steel for pressure vessels, in terms of mass percentage, includes: C: 0.18%, Si: 0.30%, Mn: 1.15%, P: 0.009%, S: 0.0014%, Ni: 0.18%, Mo: 0.07%, Cr: 0.15%, Nb: 0.015%, V: 0.015%, Ti: 0.015%, Alt: 0.03%, B: 0.0003%, Sn: 0.0003%, As: 0.0003%, and the balance is Fe and inevitable impurities.

[0089] Smelting:

[0090] During the tapping process of the converter, 5kg / t of lime, 2kg / t of fluorite, and 0.03% of Als are added; at the end of LF refining, 5.5kg / t of lime is added, and the Als of the molten steel at the exit is 0.06%; the VD refining single porous plug bottom blowing flow rate is 65m 3 / h, and the limit vacuum (vacuum degree 65Pa) is maintained for 18min.

[0091] The continuous casting end large reduction is adopted, and the reduction rate is 2mm / m; the magnetic field intensity of the controlled crystallizer electromagnetic stirring is 100×10 -4 T.

[0092] Rolling:

[0093] Preheating section, heating section, and soaking section; wherein the temperature of the soaking section is 1260℃, the thickness of the steel plate is 80mm, the time of the soaking section is 42min, and the total heating time is 80min.

[0094] During rough rolling, the opening rolling temperature is 1100℃, and the final rolling temperature is 930℃, wherein the 4-pass reduction amount is 18%, and the rolling speed is 0.7m / s.

[0095] During finish rolling, the opening rolling temperature is 920℃, and the final rolling temperature is 840℃, and the cumulative reduction deformation amount is 40%.

[0096] Heat treatment:

[0097] The normalizing temperature is 860℃, and the time is 32min. The cooling mode is air cooling and water mist cooling, and the cooling rate is 8℃ / s.

[0098] The steel plate of Example 1 is subjected to post-weld heat treatment, wherein after one post-weld heat treatment (630℃), the proportion of grains with a size of ≤5μm is 55%, and the proportion of grains with a size of ≥20μm is 10%; after two post-weld heat treatments (630℃), the proportion of grains with a size of ≤5μm is 55%, and the proportion of grains with a size of ≥20μm is 18%; after three post-weld heat treatments (630℃), the proportion of grains with a size of ≤5μm is 43%, and the proportion of grains with a size of ≥20μm is 24%.

[0099] Comparative Example 1

[0100] Comparative Example 1 is similar to Example 1, except that the content of Cr is 0.5%, the content of Ni is 0.3%, the content of Nb is 0.03%, and the content of Ti is 0.03%; other processes refer to Example 1.

[0101] Comparative Example 2

[0102] Comparative Example 2 is similar to Example 1, except that the content of Mo is 0.1%, and the content of V is 0.03%; other processes refer to Example 1.

[0103] Comparative Example 3

[0104] Comparative Example 3 is similar to Example 1, except that the content of Cr is 0.5%, the content of Ni is 0.3%, the content of Nb is 0.03%, the content of Ti is 0.03%, the content of Mo is 0.1%, and the content of V is 0.03%; other processes refer to Example 1.

[0105] Comparative Example 4

[0106] Comparative Example 4 is similar to Example 1, except that the content of Sn and As in the molten iron during smelting is greater than 0.0005%, the content of S, P, and B during smelting is greater than 0.0015%, 0.010, and 0.0005% respectively, i.e. the content of Sn, As, S, P, and B is controlled outside the range limited by the application, and other processes refer to Example 1.

[0107] Comparative Example 5

[0108] Comparative Example 5 is similar to Example 1, except that lime is only added during the tapping process of the converter, the VD refining bottom blowing flow rate is 50m 3 / h, the ultimate vacuum is maintained for 10min, and other processes refer to Example 1.

[0109] Comparative Example 6

[0110] Comparative Example 6 is similar to Example 1 except that the press-down ratio of continuous casting is 1 mm / m, and other processes refer to Example 1.

[0111] Comparative Example 7

[0112] Comparative Example 7 is similar to Example 1 except that the electromagnetic stirring of the crystallizer is 70 x 10 -4 T, and other processes refer to Example 1.

[0113] Comparative Example 8

[0114] Comparative Example 8 is similar to Example 1 except that the electromagnetic stirring of the crystallizer is 130 x 10 -4 T, and other processes refer to Example 1.

[0115] Comparative Example 9

[0116] Comparative Example 9 is similar to Example 1 except that only one pass of the rough rolling has a reduction of more than 18%, and other processes refer to Example 1.

[0117] Comparative Example 10

[0118] Comparative Example 10 is similar to Example 3 except that the cumulative reduction deformation is 30% during the finish rolling, and other processes refer to Example 3.

[0119] Comparative Example 11

[0120] Comparative Example 11 is similar to Example 1 except that after the normalizing heat treatment, the cooling is air cooling with a cooling rate of 12 °C / s, and other processes refer to Example 1.

[0121] Comparative Example 12

[0122] Comparative Example 12 is similar to Example 3 except that after the normalizing heat treatment, the cooling is air cooling with a cooling rate of 3 °C / s, and other processes refer to Example 3.

[0123] Comparative Example 13

[0124] Comparative Example 13 is similar to Example 1 except that the lime is only added during the converter tapping, the VD refining bottom blowing flow rate is 50 m 3 / h, the limit vacuum is maintained for 10 min, the press-down ratio of continuous casting is 1 mm / m, the electromagnetic stirring of the crystallizer is 130 x 10 -4 T, only one pass of the rough rolling has a reduction of more than 18%, after the normalizing heat treatment, the cooling is air cooling with a cooling rate of 12 °C / s, and other processes refer to Example 1.

[0125] Comparative Example 14

[0126] Comparative Example 14 is similar to Example 1, except that the content of Cr is 0.5%, the content of Ni is 0.3%, the content of Nb is 0.03%, the content of Ti is 0.03%, lime is only added in the converter tapping process, the VD refining bottom blowing flow rate is 50 m 3 / h, the ultimate vacuum is maintained for 10 min, the press-down rate of continuous casting is 1 mm / m, the mold electromagnetic stirring is used, the magnetic field strength is 130 x 10 -4 T, during rough rolling, only one pass has a press-down amount greater than 18%, after normalizing heat treatment, the cooling is air cooling, the cooling rate is 12°C / s, and the other processes refer to Example 1.

[0127] The strength, toughness and other properties of the steel plates of each example and comparative example after one welding and heat treatment, two weldings and heat treatment, and three weldings and heat treatment are compared, and the results are shown in the following tables:

[0128]

[0129]

[0130]

[0131]

[0132]

[0133]

[0134]

[0135]

[0136]

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144]

[0145]

[0146]

[0147] In summary, the steel for pressure vessel of the present application can ensure greater strength and toughness after welding heat treatment by adjusting the composition and process, even after secondary and more than secondary welding heat treatment, it can ensure good strength and toughness to meet the requirements.

[0148] The preferred embodiments of the present application have been described above with the purpose not to limit the present application but to more fully and completely disclose the present application. Accordingly, various modifications and changes can be made to the present application by those skilled in the art which will reflect the principles of the present application and these should be considered as within the scope of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A steel for a pressure vessel, characterized in that: The composition of the pressure vessel steel, calculated by mass percentage, includes: C: 0.17-0.20%, Si: 0.23-0.40%, Mn: 1.05-1.30%, P≤0.010%, S≤0.0015%, Ni: 0.15-0.20%, Mo: 0.04-0.09%, Cr: 0.10-0.20%, Nb: 0.01-0.02%, V: 0.01-0.02%, Ti: 0.01-0.02%, Alt: 0.02-0.04%, B≤0.0005%, Sn≤0.0005%, As≤0.0005%, the balance being Fe and unavoidable impurities; wherein, After the first post-weld heat treatment, the proportion of grains with a size of 5 μm or less is greater than 50%, and the proportion of grains with a size of 20 μm or more is less than 15%; after the second post-weld heat treatment, the proportion of grains with a size of 5 μm or less is greater than 45%, and the proportion of grains with a size of 20 μm or more is less than 20%; after the third post-weld heat treatment, the proportion of grains with a size of 5 μm or less is greater than 40%, and the proportion of grains with a size of 20 μm or more is less than 25%; The preparation method of the pressure vessel steel includes: smelting, rolling and heat treatment; wherein, In the smelting process, lime 5-6 kg / t and fluorite 1-2 kg / t are added during the tapping process of the converter, and the Alt is controlled to be ≥ 0.02%; lime 4.5-5.5 kg / t is added at the end of LF refining, and the Alt of the molten steel at the exit is controlled to be ≥ 0.05%; when the bottom blowing flow rate of VD refining reaches 55-65 m 3 / h, the maintenance time is ≥15min under the condition of vacuum degree ≤100Pa; The reduction rate at the end of continuous casting is 1.5-2.5 mm / m; During continuous casting, the magnetic field strength of the electromagnetic stirring of the crystallizer is controlled to be 80×10 -4 -120×10 -4 T; In the rolling step, the starting rolling temperature of the rough rolling is controlled to be ≥1050°C, the finishing rolling temperature of the rough rolling is controlled to be ≥920°C, the reduction of at least two passes is ≥18%, and the rolling speed of the rough rolling is controlled to be 0.6-0.95m / s; and The finishing rolling temperature and reduction are controlled according to the thickness of the steel plate; When the thickness of the steel plate is greater than 6mm and less than or equal to 12mm, the start rolling temperature of the finishing rolling is controlled at 980±10℃, the final rolling temperature is 860~880℃, and the cumulative reduction deformation is controlled at ≥50%; When the thickness of the steel plate is greater than 12mm and less than or equal to 70mm, the start rolling temperature of the finishing rolling is controlled at 900±10℃, the final rolling temperature of the finishing rolling is controlled at ≤840℃, and the cumulative reduction deformation is controlled at ≥50%; When the thickness of the steel plate is greater than 70mm and less than or equal to 100mm, the start rolling temperature of the finishing rolling is controlled at 910±10℃, the final rolling temperature of the finishing rolling is controlled at ≤840℃, and the cumulative reduction deformation is controlled at ≥40%; In the heat treatment step, the normalizing temperature is 850±10°C, the holding time = H×(2-3) min / mm, H is the thickness of the steel plate, and the holding time is ≥30 min; wherein, when the holding time calculated by the formula holding time = H×(2-3) min / mm is less than 30 min, the holding time is controlled to be 30 min; the heat treatment also includes controlling the cooling rate to 5-10°C / s.

2. The pressure vessel steel according to claim 1, characterized in that: The temperature of the post-weld heat treatment is 630±10° C., and the treatment time is 2-3 hours.

3. The pressure vessel steel according to claim 1, wherein: The rolling process includes: a preheating section, a heating section, and a soaking section; wherein the temperature of the soaking section is 1200-1260° C., the soaking section time is ≥H×0.5 min / mm, and the total heating time = H×(0.85-1.05) min / mm, where H is the thickness of the steel plate.

4. The pressure vessel steel according to claim 1, wherein: The heat treatment step further comprises: When the thickness of the steel plate is greater than 6mm and less than or equal to 12mm, it should be air-cooled after leaving the furnace to control the cooling rate to 5-10℃ / s; When the thickness of the steel plate is greater than 12mm and less than or equal to 70mm, it is air-cooled after leaving the furnace to control the cooling rate to 5-10℃ / s; When the thickness of the steel plate is greater than 70mm and less than or equal to 100mm, it is cooled by air and water mist at the same time after leaving the furnace to control the cooling rate to 5-10℃ / s.

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