Round steel for phi 300mm large-size petroleum valve body and production method of round steel

Abstract

The invention discloses round steel for a phi 300mm large-specification petroleum valve body and a production method of the round steel. The round steel comprises the following chemical components in percentage by weight: 0.17%-0.20% of C, 0.20%-0.30% of Si, 1.23%-1.35% of Mn, 0.015%-0.030% of Al, 0.005%-0.015% of V and the balance of iron and inevitable impurities. On the basis of solution strengthening of C and Mn, V, Al and other fine-grain alloy elements are designed and added, dispersed precipitation of carbon and nitride fine particles and solution strengthening of the V and Al elements are achieved through controlled rolling to refine grains, the strength and toughness of the round steel are greatly improved, it is guaranteed that the grains are fine, and the normalized V-type-50 DEG C low-temperature impact of the round steel is larger than 100 J; and the grain size in the hot rolling state is 6.5-7.0 grade, and the requirements of downstream customers can be completely met.

Description

Technical Field

[0001] This invention belongs to the field of steel technology, specifically relating to a φ300mm large-size round steel bar for oil valve bodies and its production method. Background Technology

[0002] The extraction and transportation environments in the petroleum, natural gas, and other chemical industries are typically characterized by high pressure, low temperature, and strong corrosiveness. As a critical pressure-bearing component, the performance of the oil valve body directly affects the safety, stability, and service life of the entire transportation system. Round steel, as the core raw material for oil valve bodies, must meet stringent industry standards and usage requirements in terms of its mechanical properties, low-temperature toughness, and microstructure uniformity.

[0003] As oil and gas extraction expands into extreme environments such as the deep sea and polar regions, downstream customers are placing higher demands on the specifications and performance of round steel used in oil valve bodies. Among these, large-diameter round steel (φ300mm and above) is increasingly used in large chemical plants and long-distance pipelines because it can adapt to large valve body structures, reduce splicing processes, and improve the overall strength of the valve body. Meanwhile, extreme low-temperature environments such as -50℃ and below require round steel to possess excellent low-temperature impact toughness to prevent valve body failure due to brittle fracture under low-temperature conditions. Furthermore, a refined grain structure not only improves the strength-toughness balance of the round steel but also enhances its corrosion resistance and machinability, ensuring dimensional accuracy and forming quality during valve body processing.

[0004] However, in actual production, due to unsuitable process conditions, the grain size of the φ300 large-diameter round steel was too low, rated at 5.0-5.5, failing to meet the customer's requirement of no less than 6.5. The impact of grain size on steel used in oil valve bodies is mainly reflected in mechanical properties; the smaller the grain size, the better the hardness, tensile strength, and toughness of the steel. For example, steel with a small grain size can better resist deformation and fracture under external forces, exhibiting higher strength and toughness. In terms of processing performance, steel with a small grain size has better processing performance and lower production costs. Fine grains make the steel easier to deform and form during processing, thereby reducing processing difficulty and cost. Summary of the Invention

[0005] To solve the above-mentioned technical problems, the present invention provides a φ300mm large-specification round steel for oil valve bodies and its production method. The round steel material has a V-shaped -50℃ low-temperature impact strength greater than 100J and a grain size of 6.5-7.0, which can meet the requirements of downstream customers to process it into valves for use in the petroleum, natural gas and other chemical fields.

[0006] The technical solution adopted in this invention is as follows:

[0007] A type of round steel bar for large-size oil valve bodies with a diameter of φ300mm, wherein the chemical composition and weight percentage of the round steel bar for large-size oil valve bodies are as follows: C 0.17%~0.20%, Si 0.20%~0.30%, Mn 1.23%~1.35%, Al 0.015%~0.030%, V 0.005%~0.015%, P≤0.015%, S≤0.003%, with the balance being iron and unavoidable impurities.

[0008] The metallographic structure of the hot-rolled round steel for the φ300mm large-size oil valve body is ferrite and pearlite, with a grain size of 6.5-7.0.

[0009] The hot-rolled round steel used for the φ300mm large-size oil valve body has a lower yield strength ≥340MPa, a tensile strength ≥510 MPa, and an elongation after fracture ≥35%.

[0010] The V-type low-temperature KV2 of the normalized round steel used for the φ300mm large-specification oil valve body is 120-150J at -50℃.

[0011] The preparation method of the φ300mm large-specification oil valve body round steel in normalized state is as follows: the φ300mm large-specification oil valve body round steel in hot-rolled state is normalized, and the normalizing conditions are: normalizing temperature is 910±10℃, and holding time is 60±5min.

[0012] The present invention also provides a method for producing the φ300mm large-size round steel for oil valve bodies, the method comprising the following steps: converter smelting – LF furnace refining – RH furnace refining – φ600mm round billet continuous casting – billet surface inspection – heating – rolling.

[0013] In the heating process, the furnace temperature in zone 1 is 650-900℃; the furnace temperature in zone 2 is 850-1150℃; the furnace temperature in zone 3 is 1200-1270℃; the temperature in the soaking zone is 1220-1270℃; the flue gas temperature is ≤300℃; and the heating time in the furnace is 7.0-8.0h to ensure that the added fine-grained alloying elements diffuse fully.

[0014] In the rolling process, the initial rolling temperature is 1090±10℃ and the final rolling temperature is 890±10℃.

[0015] In the rolling step, after rolling is completed, the material is air-cooled to room temperature.

[0016] In the rolling step, high-pressure water descaling is performed before rolling, with a water pressure of 20-22 MPa.

[0017] In the φ300mm large-size oil valve body round steel provided by this invention, the roles of each chemical element in the steel are as follows:

[0018] C: C plays a role in solid solution strengthening in steel, and is the main element that increases strength. It also increases the pearlite content in the microstructure. Excessive pearlite content can reduce the plasticity of the steel and easily lead to excessive CEV content. In this invention, the mass fraction of C is controlled between 0.17% and 0.20%.

[0019] Si: Si plays a solid solution strengthening role, which can improve the strength of steel, and it does not participate in the carbon equivalent (CEV) calculation. However, excessive content will affect the drawing performance. In this invention, the mass fraction of Si is controlled at 0.20% to 0.30%.

[0020] Mn: Mn plays a solid solution strengthening role, significantly improving the strength of steel, while also increasing its hardenability and pearlite content, and participating in the calculation of carbon equivalent (CEV). In this invention, the mass fraction of Mn is controlled between 1.23% and 1.35%.

[0021] V: V has a very strong pinning effect on grain boundaries and dislocations in steel, and can refine grains, significantly improving material strength, while also increasing plasticity and toughness. In this invention, the V element is controlled at 0.005% to 0.015%.

[0022] Al: Al can strongly shrink the austenite phase region in steel and refine the inherent grains of steel, increasing the temperature at which steel grains coarsen. It can also improve toughness and ductility, and improve the toughness of steel at low temperatures. In this invention, the Al content is controlled at 0.015% to 0.030%.

[0023] P and S: S reduces the plasticity of steel and is a harmful element. S exists in the form of FeS, which has a low melting point of 1190℃, while steel is typically heated to 1100-1200℃. This causes the FeS at the grain boundaries to melt, significantly weakening the bonding force between grains and leading to hot brittleness. P has a strong solid solution strengthening and cold work hardening effect in steel. Its greatest harm is severe segregation, increasing the brittleness of steel and significantly increasing its plasticity and toughness, making it prone to brittle fracture during cold working, a phenomenon known as "cold brittleness." P and S are harmful elements and should be strictly controlled. The P content should be controlled below 0.015%, and the S content below 0.003%.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] Based on C and Mn solid solution strengthening, this invention refines the grain size by designing and adding fine-grained alloying elements such as V and Al, and controlling the rolling process to achieve the dispersed precipitation of fine carbon and nitride particles and the solid solution of V and Al elements. This greatly improves the strength and toughness of round steel, ensures fine grain size, and guarantees that the normalized round steel material meets the requirements of a V-type -50℃ low-temperature impact strength greater than 100J and the hot-rolled round steel material has a grain size of 6.5~7.0 grade, which can fully meet the needs of downstream customers. Attached Figure Description

[0026] Figure 1 This is a grain size diagram of the round steel produced in Example 1. Detailed Implementation

[0027] The present invention provides a φ300mm large-specification round steel for oil valve bodies, the chemical composition and weight percentage of which are as follows: C 0.17%~0.20%, Si 0.20%~0.30%, Mn 1.23%~1.35%, Al 0.015%~0.030%, V 0.005%~0.015%, P≤0.015%, S≤0.003%, with the balance being iron and unavoidable impurities.

[0028] The production method of the φ300mm large-specification oil valve body round steel includes the following steps: converter smelting - LF furnace refining - RH furnace refining - φ600mm round billet continuous casting - billet surface inspection - heating - rolling.

[0029] In the heating process, the furnace temperature in zone 1 is 650-900℃; the furnace temperature in zone 2 is 850-1150℃; the furnace temperature in zone 3 is 1200-1270℃; the temperature in the soaking zone is 1220-1270℃; the flue gas temperature is ≤300℃; and the heating time in the furnace is 7.0-8.0h.

[0030] In the rolling process, the initial rolling temperature is 1090±10℃ and the final rolling temperature is 890±10℃.

[0031] In the rolling step, after rolling is completed, the material is air-cooled to room temperature.

[0032] In the rolling step, high-pressure water descaling is performed before rolling, with a water pressure of 20-22 MPa.

[0033] The present invention will now be described in detail with reference to the embodiments.

[0034] The chemical composition and weight percentage of the round steel used for the φ300mm large-size oil valve body in each embodiment and comparative example are shown in Table 1, with the balance being iron and unavoidable impurities.

[0035] Table 1

[0036]

[0037] The production process parameters of the φ300mm large-size oil valve body round steel in each embodiment and comparative example are shown in Table 2.

[0038] Table 2

[0039]

[0040] After rolling, the properties and grain size of the φ300mm large-size oil valve body round steel in each embodiment and comparative example after normalizing are shown in Table 3.

[0041] Table 3

[0042]

[0043] The φ300mm large-size oil valve body round steel obtained in each embodiment and comparative example was subjected to V-type -50℃ low temperature impact KV2 after being held at 910±10℃ for 60±5min. The results are shown in Table 4.

[0044] Table 4

[0045]

[0046] As can be seen from the above, the round steel in the various embodiments obtained according to the present invention meets the requirements of V-type -50℃ low temperature impact greater than 100J and the grain size of the hot-rolled state reaches 6.5~7.0 grade, which can fully meet the needs of downstream customers.

[0047] In Comparative Example 1, due to insufficient content of key grain-refining elements (Al, V), one of the core designs of this invention is to refine grains through the synergistic effect of Al and V elements: Al can shrink the austenite phase region, increase the grain coarsening temperature, and avoid grain growth during heating / rolling; V can pin grain boundaries and dislocations, promote the dispersed precipitation of carbides and nitrides, further refine the grains and improve strength and toughness. The Al content (0.012%) is lower than the 0.015% to 0.030% range specified in this invention, the V content (0.003%) is lower than the 0.005% to 0.015% range specified in this invention, and the S content (0.010%) exceeds the upper limit of ≤0.003% specified in this invention. Furthermore, the final rolling temperature (915℃) is higher than the 890±10℃ range specified in this invention. As a result, the grain size of the hot-rolled round steel is only grade 5.5 (not meeting the 6.5-7.0 grade requirement), and the normalized V-type -50℃ low-temperature impact energy is only 83J (lower than the 100J requirement). At the same time, the lower yield strength (330MPa), tensile strength (490MPa), and elongation after fracture (33%) are all lower than the levels of the example, which cannot meet the needs of downstream customers.

[0048] In Comparative Example 2, because the Al content (0.010%) is lower than the 0.015% to 0.030% range specified in this invention, the V content (0.004%) is lower than the 0.005% to 0.015% range specified in this invention, and the S content (0.016%) far exceeds the ≤0.003% upper limit specified in this invention, and the final rolling temperature (922℃) is higher than the 890±10℃ range specified in this invention, the grain size of the hot-rolled round steel is only grade 5.0 (not meeting the 6.5-7.0 grade requirement), the normalized V-type -50℃ low-temperature impact energy is only 86J (lower than the 100J requirement), and the lower yield strength (315MPa), tensile strength (475MPa), and elongation after fracture (32%) are all significantly lower than the levels of the example, which cannot meet the needs of downstream customers.

[0049] In Comparative Example 3, because the Al content (0.008%) is lower than the 0.015% to 0.030% range specified in this invention, the V content (0.003%) is lower than the 0.005% to 0.015% range specified in this invention, and the S content (0.012%) exceeds the ≤0.003% upper limit specified in this invention, and the final rolling temperature (927℃) is higher than the 890±10℃ range specified in this invention, the grain size of the hot-rolled round steel is only grade 5.5 (not meeting the 6.5-7.0 grade requirement), the normalized V-type -50℃ low-temperature impact energy is only 81J (lower than the 100J requirement), and the lower yield strength (325MPa), tensile strength (485MPa), and elongation after fracture (30%) are all lower than the levels of the example, which cannot meet the needs of downstream customers.

[0050] The above detailed description of a φ300mm large-size oil valve body round steel and its production method, with reference to the embodiments, is illustrative rather than limiting. Several embodiments can be listed according to the defined scope. Therefore, changes and modifications without departing from the overall concept of the present invention should be within the protection scope of the present invention.

Claims

1. A type of round steel bar for a large-size oil valve body with a diameter of φ300mm, characterized in that, The chemical composition and weight percentage of the round steel used for the φ300mm large-size oil valve body are as follows: C 0.17%~0.20%, Si 0.20%~0.30%, Mn 1.23%~1.35%, Al 0.015%~0.030%, V 0.005%~0.015%, P≤0.015%, S≤0.003%, with the balance being iron and unavoidable impurities.

2. The φ300mm large-size oil valve body round steel according to claim 1, characterized in that, The metallographic structure of the hot-rolled round steel for the φ300mm large-size oil valve body is ferrite and pearlite, with a grain size of 6.5-7.

0.

3. The φ300mm large-size oil valve body round steel according to claim 1, characterized in that, The KV2 of the normalized V-type -50℃ low temperature steel used for the φ300mm large-specification oil valve body is 120-150J.

4. The φ300mm large-size oil valve body round steel according to claim 3, characterized in that, The preparation method of the φ300mm large-specification oil valve body round steel in normalized state is as follows: the φ300mm large-specification oil valve body round steel in hot-rolled state is normalized, and the normalizing conditions are: normalizing temperature is 910±10℃, and holding time is 60±5min.

5. The method for producing φ300mm large-size oil valve body round steel as described in any one of claims 1-4, characterized in that, The production method includes the following steps: converter smelting - LF furnace refining - RH furnace refining - continuous casting of φ600mm round billet - billet surface inspection - heating - rolling.

6. The production method according to claim 5, characterized in that, In the heating process, the furnace temperature in zone 1 is 650-900℃; the furnace temperature in zone 2 is 850-1150℃; the furnace temperature in zone 3 is 1200-1270℃; the temperature in the soaking zone is 1220-1270℃; the flue gas temperature is ≤300℃; and the heating time in the furnace is 7.0-8.0h.

7. The production method according to claim 5, characterized in that, In the rolling process, the initial rolling temperature is 1090±10℃ and the final rolling temperature is 890±10℃.

8. The production method according to claim 5, characterized in that, In the rolling step, after rolling is completed, the material is air-cooled to room temperature.

9. The production method according to claim 5, characterized in that, In the rolling step, high-pressure water descaling is performed before rolling, with a water pressure of 20-22 MPa.

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