Production method of steel for low-temperature pressure vessels and steel for low-temperature pressure vessels

Abstract

The present invention discloses a production method of steel for low-temperature pressure vessels and the steel for low-temperature pressure vessels, belonging to the technical field of production of steel for low-temperature pressure vessels. The steps of the present invention include: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → controlled rolling → controlled cooling → coiling. Among them, the molten steel is smelted according to the following chemical components: C: 0.14 - 0.16%, Si: 0.20 - 0.40%, Mn: 1.40 - 1.60%, P ≤ 0.015%, S ≤ 0.008%, Nb: 0.020 - 0.030%, Als: 0.020 - 0.040%, and the rest is Fe and inevitable inclusions. Through the composition design of low carbon, low Nb and appropriate amounts of Si and Mn, and at the same time, through the good matching of each hot rolling process, the steel strip can be cooled at a suitable cooling rate to generate a uniform and fine target structure, ensuring strength and toughness and improving the performance uniformity.

Description

Technical Field

[0001] The present invention belongs to the technical field of steel production for low-temperature pressure vessels. More specifically, it relates to a production method for steel used in low-temperature pressure vessels and the steel for low-temperature pressure vessels. Background Art

[0002] With the rapid development of the petrochemical industry in China, the market for steel used in low-temperature pressure vessels has gradually increased. The existing steel plates for low-temperature containers mainly fall into two directions: one is mainly medium and heavy plates, adding two or more of Nb, V, Ti, and Ni on the basis of Si and Mn to meet the requirements of low-temperature toughness. However, the key points of composition, process design, and process control of medium and heavy plate products are not applicable to hot continuous rolling; the other is mainly thin plates, which are produced by hot continuous rolling + off-line heat treatment. During the heat treatment process, the as-rolled structure re-austenitizes, diffuses during sufficient holding time, and the structure formed after cooling is uniform and fine, and the strength and toughness are well improved. However, the process is cumbersome and the cost is relatively high.

[0003] After retrieval, a patent application with the Chinese patent application number 201210123638.5 discloses a steel plate for large-thickness low-temperature pressure vessels and its production method. The steel plate for large-thickness low-temperature pressure vessels in this patent application is composed of the following chemical components by weight percentage: C: 0.07 - 0.12%, Si: 0.20 - 0.40%, Mn: 1.40 - 1.60%, Ni: 0.60 - 0.80%, P ≤ 0.010%, S ≤ 0.003%, Al: 0.020 - 0.045%, Nb: 0.02 - 0.05%, and the balance is Fe and unavoidable impurities. The steel plate production method in this patent application includes: smelting → casting → heating → rolling → heat treatment → finished product.

[0004] Another example is a patent application with the Chinese patent application number 201611152263.X, which discloses a steel plate for pressure vessels with a yield strength of 345 MPa and its production method. In this patent application, the chemical composition and weight percentage of the steel plate are: C: 0.15 - 0.18%, Si: 0.15 - 0.35%, Mn: 1.40 - 1.50%, P ≤ 0.008%, S ≤ 0.002%, Al: 0.020 - 0.040%, Nb: 0.020 - 0.035%, O ≤ 0.003%, and the balance is Fe and unavoidable impurities. The production method in this patent application includes heating, rolling, and heat treatment processes.

[0005] In the above two patents, although steel for low-temperature pressure vessels with relatively high purity can be produced, off-line heat treatment processes are required in the production methods, resulting in relatively cumbersome production processes and high costs. Summary of the Invention

[0006] 1. Problems to be Solved

[0007] In view of at least some of the problems existing in the above prior art, the present invention provides a production method for steel used in low-temperature pressure vessels and the steel for low-temperature pressure vessels. By using the production method of the present invention, on the basis of the main economic strengthening elements Mn and Si, the alloy element Nb is added, which has good low-temperature toughness on the premise of ensuring strength, is conducive to safe service, and at the same time, without offline heat treatment, only through hot continuous rolling, the steel for low-temperature pressure vessels with uniform structure can be produced.

[0008] 2. Technical Solutions

[0009] In order to solve the above problems, the technical solutions adopted by the present invention are as follows:

[0010] The present invention provides a steel plate for low-temperature pressure vessels and its normalizing rolling method. Its chemical composition and weight percentage content are C: 0.14 - 0.16%, Si: 0.20 - 0.40%, Mn: 1.40 - 1.60%, P≤0.015%, S≤0.008%, Nb: 0.020 - 0.030%, Als: 0.020 - 0.040%, and the rest is Fe and inevitable inclusions.

[0011] The design principle of each alloy element and its mass percentage of the steel plate for low-temperature pressure vessels of the present invention is as follows:

[0012] C is the element with the highest cost performance for improving strength. However, if the C content is too high, in addition to being unfavorable for toughness and weldability, it is also easy to generate severe banded structure, resulting in anisotropy of the strip structure and properties, so that it is not easy to be effectively improved even after isothermal normalizing. Considering the above factors, C is controlled at 0.14 - 0.16%.

[0013] Si has the functions of deoxidation and solid solution strengthening in steel. And adding an appropriate amount of Si can expand the process window for ferrite formation and promote ferrite formation. However, if the Si content is too high, it is easy to form dense scale on the steel surface, which is not conducive to subsequent polishing treatment. Therefore, Si is controlled at 0.20 - 0.40%.

[0014] Mn is one of the main and economic elements for improving strength in steel. Mn improves strength and toughness by substitutional solid solution, grain refinement and increasing the amount of ferrite. However, if the Mn content is too high, it is easy to cause center segregation, which is not conducive to the uniform distribution of the strip structure and properties. Therefore, Mn is controlled at 1.40 - 1.60%.

[0015] P and S are inevitable harmful elements, which are very easy to form segregation in steel. In production, their contents should be reduced as much as possible to avoid adverse effects on the performance of steel plates or strips. Therefore, P and S are controlled at P≤0.015% and S≤0.008%.

[0016] Nb is an important alloying element in the low-temperature pressure vessel steel produced by normalizing rolling. On the one hand, the precipitation of Nb carbonitrides at high temperatures can effectively pin the austenite grain boundaries, prevent the growth of austenite grains, and at the same time effectively inhibit the recrystallization growth of deformed austenite grains, improving the strength and low-temperature toughness of the steel strip. On the other hand, as the only alloying element in this steel grade, except that the added Nb meets the requirements of strength and toughness (normalizing rolling also improves toughness to a certain extent), the Nb content should be reduced as much as possible. On the one hand, it is to reduce costs, and on the other hand, it is to reduce the dendritic segregation of carbon and Nb during casting, thereby improving the banded structure. Therefore, Nb is controlled at 0.020 - 0.030%.

[0017] Als is an important deoxidizing element in steel, but the Als content cannot be too high, as it is easy to form coarse AlN particles with N. Therefore, Als is controlled at 0.020 - 0.040%.

[0018] In addition to the above chemical composition control, the present invention also provides a control method for the steel plate for low-temperature pressure vessels and its normalizing rolling process, including the following steps: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → controlled rolling → controlled cooling → coiling.

[0019] 1) Smelting, refining, and continuous casting processes

[0020] The molten steel is smelted according to the above chemical composition. During the hot metal pretreatment, slag skimming is required before and after, and after hot metal desulfurization, S ≤ 0.0050% is required. During converter smelting, tapping deoxidation alloying is carried out, and slag blocking operation is strengthened at the same time. Through LF furnace refining treatment and calcium treatment, on the one hand, the key elements are adjusted to the target values; on the other hand, by dehydrogenating and deoxidizing, the morphology, quantity, and size of non-metallic inclusions in the molten steel are controlled, the formation of non-metallic inclusions is reduced, and the purity of the molten steel is improved. During the continuous casting process, the use of dynamic soft reduction to a certain extent can reduce the internal defects of the slab. After the slab cutting is completed and taken offline, the slab is inspected and cleaned.

[0021] 2) Controlled rolling and controlled cooling processes

[0022] The slab is heated in the heating furnace, and the temperature is controlled at 1230 - 1250 °C, and the holding time is 2 - 2.5 h. The purpose is to enable the slab to be fully austenitized during the solid-state phase transformation process, and each element diffuses evenly. Especially, carbon elements can diffuse well to other positions, so that the banded structure can be improved during the subsequent rolling process. Moreover, compared with the conventional products, the heating temperature is increased by 20 - 40 °C. In addition to ensuring that the final rolling temperature can be increased by 20 - 30 °C compared with normal rolling, and it can not significantly increase the rolling speed, ensuring safe and efficient production, creating conditions for subsequent normalizing cooling. After the slab is taken out of the furnace, the scale on the slab surface is removed by high-pressure water descaling, and then it enters the rolling stage.

[0023] In the rough rolling stage, multi-pass reciprocating rolling is carried out in the austenite recrystallization zone. The deformed austenite undergoes recovery and recrystallization multiple times. After rough rolling, the temperature is controlled at 1040 - 1080 °C, the cumulative reduction ratio of rough rolling is ≥ 82%, and the thickness of the intermediate billet after rough rolling is controlled at 39 - 41 mm. The entry temperature of finish rolling is controlled at 1020 - 1030 °C. By means of cumulative large deformation, the deformation bands and dislocation density in the deformed austenite are increased, thereby increasing the phase transformation nucleation points and refining the grain size. The cumulative reduction ratio of finish rolling is ≥ 85%, and the finishing temperature of finish rolling is controlled at 900 - 920 °C. The entry temperature and finishing temperature of finish rolling are increased by 10 - 25 °C and 30 - 35 °C respectively compared with the normal production process; on the one hand, by optimizing the heating temperature, and on the other hand, by slightly adjusting the rolling speed from 6 - 7 m / s to 7 - 8 m / s. After finish rolling, air cooling is carried out. The cooling water pipes on the laminar cooling line are closed, and at the same time, the cooling water on the roller table and the side spray water are closed. The cooling rate is controlled at 6 - 7 °C / s, which is much lower than the normal cooling rate of 20 - 25 °C / s in normal hot strip rolling. After laminar cooling without opening water, the coiling temperature is 770 - 790 °C, and the strip steel after cooling is coiled and parked in the low-temperature area offline. From the end of finish rolling, no water is opened in the laminar cooling to slow cooling in the low-temperature area to achieve true air cooling, in order to achieve the purpose of online normalizing.

[0024] 3. Beneficial effects

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] A production method of steel for low-temperature pressure vessels and the steel for low-temperature pressure vessels of the present invention add alloy element Nb on the basis of the main economic strengthening elements Mn and Si, ensuring good low-temperature toughness on the basis of strength, which is beneficial to safe service; at the same time, through various hot rolling processes, mainly including: good matching of heating temperature, finishing temperature, rolling speed, opening and closing status of cooling water on the laminar cooling line, coiling temperature and placement in the low-temperature area of the hot coil, so that the steel strip can be cooled at a suitable cooling rate to generate a uniform and fine target structure, ensuring strength and toughness and improving performance uniformity. There is no need for offline heat treatment, and only through hot strip rolling, steel for low-temperature pressure vessels with uniform structure can be produced, and the banded structure is grade 1.5, which is basically the same as the structure, performance and banded structure of the steel for low-temperature pressure vessels produced by the conventional process, and improves the anisotropy of the strip steel structure and performance, shortens the process flow, and greatly reduces the production cost at the same time. Brief description of the drawings

[0027] Figure 1 It is the metallographic structure diagram of the hot-rolled steel strip in Example 1.

[0028] Figure 2 It is the metallographic structure diagram of the hot-rolled steel strip in Comparative Example 1.

[0029] Figure 3Metallographic structure diagram of the hot-rolled steel strip of Comparative Example 1 after heat treatment. Detailed implementation manners

[0030] The present invention will be further described below in conjunction with specific embodiments.

[0031] The present invention provides a production method of steel for low-temperature pressure vessels and steel for low-temperature pressure vessels, the chemical composition and weight percentage content are C: 0.14 - 0.16%, Si: 0.20 - 0.40%, Mn: 1.40 - 1.60%, P ≤ 0.015%, S ≤ 0.008%, Nb: 0.020 - 0.030%, Als: 0.020 - 0.040%, and the rest is Fe and inevitable inclusions.

[0032] The present invention adopts a hot continuous rolling process to achieve normalizing rolling production, and the process flow includes: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → controlled rolling → controlled cooling → coiling.

[0033] The chemical compositions of the examples and comparative examples of the present invention are shown in Table 1. The component detection is carried out according to GB / T 4336 "Determination of Multi-Element Contents in Carbon Steels and Medium and Low Alloy Steels - Spark Discharge Atomic Emission Spectrometry (Conventional Method)".

[0034] Table 1 Chemical compositions of the examples and comparative examples of the present invention

[0035]

[0036] The main rolling process parameters of the examples and comparative examples of the present invention are shown in Table 2.

[0037] Table 2 Main process parameters of the rolling process

[0038]

[0039]

[0040] The mechanical properties of the examples and comparative examples of the present invention are shown in Table 3.

[0041] Table 3 Mechanical properties

[0042]

[0043] Combined with, such as 1 - Figure 3As can be seen from Table 3, by selecting raw materials according to the weight percentage of chemical components given in the present invention and then producing according to the manufacturing method given in the present invention, a low-temperature pressure vessel steel plate with uniform structure, high low-temperature toughness, low-grade banded structure and good transverse and longitudinal property differences can be obtained. Among them, the yield strength is ≥345 MPa, the tensile strength is 490-640 MPa, A≥30%, the impact energy at -80 °C is ≥25 J (specimen size: 2.5×10×55 mm); the microstructure is ferrite + pearlite, the grain size of the structure is 11.0-11.5, and the banded structure is Grade 1.5. It is basically consistent with the structure, properties and banded structure of the low-temperature pressure vessel steel produced by the conventional process, and improves the anisotropy of the strip structure and properties.

[0044] In Comparative Examples 1 to 3, since the process temperature and laminar flow cooling do not meet the conditions, the purpose of online normalizing cannot be achieved, and only normal ferrite and pearlite structures are obtained, the banded structure grade is high, and subsequent normalizing treatment is required.

[0045] The above schematically describes the present invention and its embodiments. The description is not restrictive. What is shown in the drawings is only one of the embodiments of the present invention, and the actual structure is not limited thereto. Therefore, if those of ordinary skill in the art are inspired by it and design similar structural forms and embodiments to this technical solution without creative work without departing from the purpose of the present invention, they shall fall within the protection scope of the present invention.

Claims

1. A production method of steel for low-temperature pressure vessels, characterized in that, It includes the following steps: hot metal pretreatment → converter smelting → LF refining → continuous casting → slab heating → controlled rolling → controlled cooling → coiling → stacking. Among them, for the steelmaking, by weight percentage, the molten steel is smelted according to the following chemical components: C: 0.14 - 0.16%, Si: 0.20 - 0.40%, Mn: 1.40 - 1.60%, P ≤ 0.015%, S ≤ 0.008%, Nb: 0.020 - 0.030%, Als: 0.020 - 0.040%, and the rest is Fe and inevitable inclusions; In the slab heating step, the slab heating temperature is controlled at 1230 - 1250 °C; In the controlled rolling step, the temperature after rough rolling is controlled at 1040 - 1080 °C; the entry temperature of finish rolling is controlled at 1020 - 1030 °C; the final rolling temperature of finish rolling is controlled at 900 - 920 °C; the rolling speed is 7 - 8 m / s; In the controlled cooling step, after finish rolling, air cooling is carried out, and the cooling rate is controlled at 6 - 7 °C / s; In the coiling step, the coiling temperature is 770 - 790 °C; In the stacking step, after the strip is coiled, it is taken offline and parked in the low - temperature area.

2. The production method of steel for low-temperature pressure vessels according to claim 1, characterized in that: In the slab heating step, the slab holding time is 2 - 2.5 h.

3. In the controlled rolling of the production method of steel for low-temperature pressure vessels according to claim 2, the cumulative reduction ratio in rough rolling is ≥ 82%, and the thickness of the intermediate billet after rough rolling is controlled at 39 - 41 mm.

4. The production method of steel for low-temperature pressure vessels according to claim 3, characterized in that: In the controlled rolling step, the cumulative reduction ratio of finish rolling ≥ 85%.

5. The production method of steel for low-temperature pressure vessels according to any one of claims 1 - 4, characterized in that: In the controlled cooling, the cooling water pipes on the laminar cooling line are closed, and at the same time, the cooling water on the roller table and the side spray water are closed.

6. The production method of steel for low-temperature pressure vessels according to claim 5, characterized in that: In the hot metal pretreatment step, it is required to skim the slag before and after, and after hot metal desulfurization, it is required that S ≤ 0.0050%.

7. The production method of steel for low-temperature pressure vessels according to claim 6, characterized in that: In the converter smelting step, deoxidation alloying is carried out during tapping, and at the same time, the slag - blocking operation is strengthened.

8. The production method of steel for low-temperature pressure vessels according to claim 7, characterized in that: In the LF refining step, the key elements are adjusted to the target values through calcium treatment; at the same time, by dehydrogenating and deoxidizing, the morphology, quantity and size of non - metallic inclusions in the molten steel are controlled.

9. A steel for low-temperature pressure vessels, characterized in that, Obtained by the production method described in claim 8, it includes the following components by weight percentage: C: 0.14 - 0.16%, Si: 0.20 - 0.40%, Mn: 1.40 - 1.60%, P ≤ 0.015%, S ≤ 0.008%, Nb: 0.020 - 0.030%, Als: 0.020 - 0.040%, and the rest is Fe and inevitable inclusions; among them, the yield strength ≥ 345 MPa, the tensile strength: 490 - 640 MPa, A ≥ 30%, the impact energy at - 80 °C ≥ 25 J, and the banded structure is grade 1.5.

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