Production method of 450MPa-grade hot-rolled plate coil for thin-specification low-Mn longitudinal welded pipe

Abstract

The invention discloses a production method of a 450MPa-grade hot-rolled plate coil for a thin-specification low-Mn longitudinal welded pipe. The production method comprises the following steps: limiting chemical components and contents of the hot-rolled plate coil in percentage by mass; the invention relates to a production process flow. KR molten iron desulphurization pretreatment, converter top and bottom blowing, LF furnace refining, RH vacuum treatment, slab continuous casting, reheating furnace treatment, rough rolling high-pressure water descaling, fixed-width press treatment, E1R1 roughing mill treatment, E2R2 roughing mill treatment, flying shear treatment, finish rolling high-pressure water descaling, F1-F7 finishing mill treatment, encryption type laminar cooling, coiling, tray transportation system treatment, sampling and inspection. According to the method, based on the advantages of 2250 mm hot rolling unit equipment, a high-grade pipeline steel product which is 5-10 mm thick and used for manufacturing the longitudinal welded pipe can be produced, meanwhile, it is guaranteed that the produced pipeline steel has high obdurability, and the production cost is low.

Description

Technical Field

[0001] The invention belongs to the technical field of low-alloy high-strength steel products, and in particular relates to a method for producing a 450MPa-grade hot-rolled coil for a thin-gauge low-Mn straight seam welded pipe. Background Art

[0002] This patent involves a method for producing a 450MPa hot-rolled coil for thin-gauge low-Mn straight-seam welded pipe, which is suitable for the manufacture of oil and gas straight-seam (high-frequency / submerged arc) welded pipes with a yield strength of 450MPa and a wall thickness of 5-10mm. Due to the influence of downstream pipe making process and Bauschinger effect, thin-gauge coils are generally required to have not only high strength, low yield strength ratio, but also good welding performance. However, excessive Mn content can easily aggravate the center segregation of the plate thickness, deteriorate the weld performance, and reduce the welding quality. Therefore, more and more downstream users have begun to limit the Mn content of the parent material.

[0003] 1. The chemical composition of Document 1, "A Low-Temperature L450M Pipeline Steel and Its Manufacturing Method", is Mn: 1.61%-1.70%, and the thickness of the ingot is ≤200mm. However, the Mn content of this patent is 1.20%-1.35%, and the thickness of the ingot is 230mm. The strength loss after the reduction of Mn content is compensated by a large compression ratio, thereby reducing the alloy cost.

[0004] 2. The chemical composition disclosed in Document 2, "X65 pipeline steel with excellent low temperature toughness and its manufacturing method", adopts the C+Mn-Nb-V-Ti-Cr alloy design system, and V is added to the alloy composition. However, this patent does not add V, and the Mn content is 1.20%-1.35%, which is significantly lower than Document 2.

[0005] 3. The process flow disclosed in Document 3 "A Method for Producing X65M Pipeline Steel" is a steel plate production process, with a slab thickness of 250mm-300mm, an intermediate slab thickness of 80mm-120mm, no coiling process and stacking and natural tempering, and a finished product thickness of 26.97mm-31.75mm. However, the slab thickness of this patent is 230mm and the intermediate slab thickness is 38-68mm, matching 6 or 8 rough rolling and 7-stand continuous finishing rolling process, followed by rapid laminar cooling to 550℃-600℃ coiling, which is mainly suitable for the manufacture of thin-gauge (thickness 5-10mm) straight seam welded pipes. Summary of the invention

[0006] The purpose of the present invention is to provide a method for producing 450MPa grade hot-rolled coils for thin-gauge low-Mn straight seam welded pipes. Based on the advantages of 2250mm hot rolling mill equipment, the method can produce high-grade pipeline steel products with a thickness of 5-10mm for the manufacture of straight seam welded pipes, while ensuring that the produced pipeline steel has high strength and toughness and low production cost.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The invention discloses a method for producing a 450MPa hot-rolled coil for a thin-gauge low-Mn straight seam welded pipe. The chemical composition of the hot-rolled coil is as follows by mass percentage: C≤0.08%, Si≤0.20%, Mn: 1.20%-1.35%, Nb+Ti≤0.065%, Cr≤0.30%, Alt: 0.020%-0.050%, P≤0.018%, S≤0.008%, and the balance is Fe and unavoidable impurities. The production process is as follows: KR molten iron desulfurization pretreatment - converter top and bottom blowing - LF furnace refining - RH vacuum treatment - slab continuous casting - reheating furnace - rough rolling high-pressure water descaling - fixed width press - E1R1 rough rolling mill - E2R2 rough rolling mill - flying shear - finishing high-pressure water descaling - F1-F7 finishing mill - encrypted laminar cooling - coiling - pallet transportation system - sampling and inspection; wherein:

[0009] Smelting and continuous casting process: After the molten iron undergoes KR molten iron desulfurization pretreatment, converter top and bottom blowing, LF furnace refining, RH vacuum treatment and slab continuous casting process, the formed continuous casting billet meets P≤0.015%, S≤0.008%; the vacuum degree of RH vacuum treatment is required to be ≤2.6mbar, and the vacuum maintenance time is ≥10min; during the RH vacuum treatment, the circulating pure degassing time is guaranteed to be ≥8min; and after the RH vacuum treatment, the calcium wire is fed for calcium treatment, and the soft blowing time is guaranteed to be greater than 10min after the wire feeding;

[0010] Slab heating and rolling process: The continuous casting slab is 230mm thick. The continuous casting slab is heated to the target temperature of 1180±20℃ by a walking beam heating furnace and then discharged from the furnace. It is then subjected to two-stage controlled rolling by the rough rolling and finishing rolling units. The rolling passes are 3+3 or 3+5, a total of 6-8 passes. The intermediate slab thickness is 38-45mm. The finishing rolling start temperature is ≤1030℃, and the finishing rolling final temperature is 830-870℃. Subsequently, a distributed laminar cooling method is adopted, i.e., uniform cooling at a speed of 10-20℃ / s is adopted, and coiling is performed at 550-600℃.

[0011] Furthermore, the coil sampling method is as follows: after the coil is coiled, it is cooled to room temperature and then sampled at a distance of ≥1 meter from the tail of the coil. The sampling angle is perpendicular to the rolling direction to test various mechanical properties.

[0012] Furthermore, the transverse mechanical properties of the hot-rolled coil meet the following requirements: yield strength R t0.5 ≥490MPa, tensile strength R m ≥545MPa, elongation A 50 ≥30%, -10℃ Half-size Charpy impact energy A kv ≥100J, Vickers hardness HV 10 ≤200.

[0013] Furthermore, it is suitable for the manufacture of straight seam high frequency / submerged arc welded pipes with a wall thickness range of 5-10mm.

[0014] Furthermore, the chemical composition of the hot-rolled coil is, by mass percentage, C: 0.07%, Si: 0.15%, Mn: 1.28%, Nb+T i: 0.053%, Cr: 0.23%, Alt: 0.045%, P: 0.013%, S: 0.001%, and the remainder is Fe and unavoidable impurities.

[0015] Furthermore, the chemical composition of the hot-rolled coil is, by mass percentage, C: 0.06%, Si: 0.14%, Mn: 1.29%, Nb+T i: 0.052%, Cr: 0.24%, Alt: 0.031%, P: 0.012%, S: 0.002%, and the remainder is Fe and unavoidable impurities.

[0016] Furthermore, the chemical composition of the hot-rolled coil is, by mass percentage, C: 0.06%, Si: 0.16%, Mn: 1.31%, Nb+Ti: 0.054%, Cr: 0.23%, Alt: 0.033%, P: 0.014%, S: 0.001%, and the remainder is Fe and unavoidable impurities.

[0017] The main elements were chosen for the following reasons:

[0018] C: As the carbon content in steel increases, the yield strength and tensile strength increase, but the plastic toughness decreases. For pipeline steel, if the carbon content is too high, the toughness of the steel will drop sharply and the weldability will deteriorate. Therefore, the low-carbon component design concept is the premise of high-toughness pipeline product design. Therefore, the present invention controls the carbon content to ≤0.08%.

[0019] Mn: Manganese can be infinitely substituted with iron for solid solution, and is the most economical solid solution strengthening element. However, considering the user's individual demand that the Mn content should not be higher than 1.35%, the manganese content of the present invention is 1.20-1.35%.

[0020] Nb / Ti: Niobium and titanium have the function of improving the strength and toughness of steel, and have become the most typical and widely used microalloying elements. Niobium has a significant fine grain strengthening effect, and titanium has a strong affinity with carbon and nitrogen, forming insoluble second phase particles to hinder grain boundary migration and dislocation movement, and has a strong effect of preventing grain growth, which has a significant effect on improving the fracture toughness of the heat-affected zone during steel welding. Therefore, the present invention controls the total content of niobium and titanium to ≤0.065%.

[0021] Cr: Chromium can improve hardenability and make the steel have better comprehensive mechanical properties after quenching. Compared with niobium and vanadium alloys, ferrochrome alloys are cheaper, so when the addition of manganese is limited, the chromium content can be appropriately increased to control its cost-effectiveness.

[0022] Compared with the prior art, the beneficial technical effects of the present invention are:

[0023] (1) The design is simple and reasonable, mainly based on low-carbon medium manganese steel. Adhering to the principles of low cost, high efficiency and customer first, the Cr element is used to replace the Mn element to make up for the loss of steel strength caused by reducing the Mn element content, and the low-manganese chemical composition product design and industrial production and supply are realized, thereby meeting the diversified product needs of users.

[0024] (2) Based on the advantages of the 2250 hot rolling unit equipment, a large deformation multi-stand continuous rolling process parameter is used to obtain a mixed structure of ferrite + pearlite. The structure has fine grains and good low-temperature toughness, thereby achieving a product with a thickness of 5-10mm and excellent strength and toughness. The main mechanical performance indicators meet the standard requirements and the surplus is moderate. The products involved are suitable for the manufacture of straight seam welded pipes.

[0025] Performance meets: yield strength R t0.5 ≥490MPa, tensile strength R m ≥545MPa, elongation A 50 ≥30%, -10℃ Half-size Charpy impact energy A kv ≥100J, Vickers hardness HV 10 ≤200. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention will be further described below in conjunction with the accompanying drawings.

[0027] Figure 1 This is a metallographic structure diagram of a hot-rolled coil obtained in an embodiment of the present invention; the metallographic structure is polygonal ferrite + pearlite, and the grain size is 12 levels. DETAILED DESCRIPTION

[0028] The following examples are used to specifically illustrate the present invention. These examples are only general descriptions of the present invention and do not limit the present invention.

[0029] Example

[0030] The chemical compositions of Examples 1 to 3 are shown in Table 1 below, the specific process systems used in each Example are shown in Table 2 below, and the mechanical properties of the hot-rolled coils obtained in each Example are shown in Table 3 below.

[0031] Table 1: Chemical composition of example steel (wt, %)

[0032] Example C Si Mn P S Alt Cr Nb Ti 1 0.07 0.150 1.28 0.013 0.001 0.045 0.23 0.035 0.018 2 0.06 0.140 1.29 0.012 0.002 0.031 0.24 0.032 0.020 3 0.06 0.160 1.31 0.014 0.001 0.033 0.23 0.030 0.024

[0033] Table 2: Process system of the embodiment

[0034]

[0035] Table 3: Mechanical properties of steel in the examples

[0036]

[0037]

[0038] It can be seen from the examples that the chemical composition and mechanical properties of the steel in the examples of the present invention meet the requirements of API 5L standard with sufficient margin, and the yield strength R t0.5 ≥450MPa (even up to ≥500MPa), tensile strength R m ≥535MPa MPa (even up to ≥560MPa), elongation A 50 ≥30%; -10℃ and transverse half-size Charpy impact energy average Ak v8 ≥80J, -10℃ and transverse drop weight shear section rate ≥90%, Vickers hardness HV 10 ≤200. Other technical indicators are in full compliance with the technical requirements of X65M hot-rolled coil.

[0039] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for producing 450MPa grade hot rolled coil for thin gauge low Mn straight seam welded pipe, characterized in that: The chemical composition of the hot rolled coil is calculated by mass percentage as follows: C≤0.08%, Si≤0.20%, Mn:1.20%-1.35%, Nb+Ti≤0.065%, Cr≤0.30%, Alt:0.020%-0.050%, P≤0.018%, S≤0.008%, and the balance is Fe and inevitable impurities; the production process is as follows: KR hot metal desulfurization pretreatment - converter top and bottom blowing - LF furnace refining - RH vacuum treatment - slab continuous casting - reheating furnace - rough rolling high pressure water descaling - fixed width press - E1R1 rough rolling mill - E2R2 rough rolling mill - flying shear - finishing high pressure water descaling - F1-F7 finishing mill - encrypted laminar cooling - coiling - pallet transportation system - sampling and inspection; wherein: Smelting and continuous casting process: After the molten iron undergoes KR molten iron desulfurization pretreatment, converter top and bottom blowing, LF furnace refining, RH vacuum treatment and slab continuous casting process, the formed continuous casting billet meets P≤0.015%, S≤0.008%; the vacuum degree of RH vacuum treatment is required to be ≤2.6mbar, and the vacuum maintenance time is ≥10min; during the RH vacuum treatment, the circulating pure degassing time is guaranteed to be ≥8min; and after the RH vacuum treatment, the calcium wire is fed for calcium treatment, and the soft blowing time is guaranteed to be greater than 10min after the wire feeding; Slab heating and rolling process: The continuous casting slab is 230mm thick. The continuous casting slab is heated to the target temperature of 1180±20℃ by a walking beam heating furnace and then discharged from the furnace. It is then subjected to two-stage controlled rolling by the rough rolling and finishing rolling units. The rolling passes are 3+3 or 3+5, a total of 6-8 passes. The intermediate slab thickness is 38-45mm. The finishing rolling start temperature is ≤1030℃, and the finishing rolling final temperature is 830-870℃. Subsequently, a distributed laminar cooling method is adopted, i.e., uniform cooling at a speed of 10-20℃ / s is adopted, and coiling is performed at 550-600℃.

2. The method for producing 450MPa grade hot rolled coil for thin gauge low Mn straight seam welded pipe according to claim 1, characterized in that: Coil sampling method: After the steel coil is coiled, it is cooled to room temperature and then sampled at a distance of ≥1 meter from the tail of the steel coil. The sampling angle is perpendicular to the rolling direction to test various mechanical properties.

3. The method for producing 450MPa grade hot rolled coil for thin gauge low Mn straight seam welded pipe according to claim 1, characterized in that: The transverse mechanical properties of the hot-rolled coil meet the following requirements: yield strength R t0.5 ≥490MPa, tensile strength R m ≥545MPa, elongation A 50 ≥30%, -10℃ Half-size Charpy impact energy A kv ≥100J, Vickers hardness HV 10 ≤200.

4. The method for producing 450MPa grade hot rolled coil for thin gauge low Mn straight seam welded pipe according to claim 1, characterized in that: Suitable for straight seam high frequency / submerged arc welded pipe manufacturing, wall thickness range 5-10mm.

5. The method for producing 450MPa grade hot rolled coil for thin gauge low Mn straight seam welded pipe according to claim 1, characterized in that: The chemical composition of the hot-rolled coil is calculated by mass percentage as follows: C: 0.07%, Si: 0.15%, Mn: 1.28%, Nb+Ti: 0.053%, Cr: 0.23%, Alt: 0.045%, P: 0.013%, S: 0.001%, and the remainder is Fe and unavoidable impurities.

6. The method for producing 450MPa grade hot rolled coil for thin gauge low Mn straight seam welded pipe according to claim 1, characterized in that: The chemical composition of the hot-rolled coil is calculated by mass percentage as follows: C: 0.06%, Si: 0.14%, Mn: 1.29%, Nb+Ti: 0.052%, Cr: 0.24%, Alt: 0.031%, P: 0.012%, S: 0.002%, and the remainder is Fe and unavoidable impurities.

7. The method for producing 450MPa grade hot rolled coil for thin gauge low Mn straight seam welded pipe according to claim 1, characterized in that: The chemical composition of the hot-rolled coil is calculated by mass percentage as follows: C: 0.06%, Si: 0.16%, Mn: 1.31%, Nb+Ti: 0.054%, Cr: 0.23%, Alt: 0.033%, P: 0.014%, S: 0.001%, and the remainder is Fe and unavoidable impurities.

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