Economical L415 pipeline steel hot-rolled coiled plate and preparation method thereof

Abstract

The invention discloses an economical L415 pipeline steel hot-rolled coiled plate and a preparation method thereof, and belongs to the technical field of steel smelting. The invention provides an L415 pipeline steel hot-rolled coil plate and a preparation method thereof. The L415 pipeline steel hot-rolled coil plate comprises the following chemical components: 0.05 to 0.09 percent of C, 0.15 to 0.25 percent of Si, 1.25 to 1.40 percent of Mn, 0.025 to 0.040 percent of Nb, 0.015 to 0.025 percent of Ti, less than or equal to 0.018 percent of P, less than or equal to 0.008 percent of S, 0.015 to 0.045 percent of Als and the balance of Fe and inevitable impurities. The preparation method comprises the steps of molten iron pretreatment, converter smelting, external refining, continuous casting, slab reheating, high-pressure water descaling, rough rolling, finish rolling, laminar cooling and coiling. According to the method, the addition amount of alloy elements is remarkably reduced, and the L415-grade pipeline steel with good obdurability matching can be obtained by adopting a conventional smelting-continuous casting-hot continuous rolling process.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and in particular relates to an economical L415 pipeline steel hot-rolled coil and a preparation method thereof. Background Art

[0002] Natural gas is a clean energy source with the advantages of high unit calorific value, less exhaust pollution and high safety in use. It has now become the main civilian fuel. Long-distance high-pressure transmission pipelines usually use X70 and above steel grade pipeline steel, while various branch pipelines, industrial and urban pipelines use lower steel grade pipeline steel. L245-L415 grade pipeline steel is the most important material for branch and urban pipelines. Based on the consideration of cost and performance balance, economy is the main competitiveness indicator of L245-L415 pipeline steel. As the highest steel grade L415 pipeline steel, low-cost production can significantly improve the market competitiveness of the product.

[0003] CN105063482B discloses an X60 pipeline steel and a production method thereof, wherein the chemical composition thereof is 0.05-0.10% C, 0.10-0.30% Si, 1.20-1.40% Mn, P≤0.020%, S≤0.010%, 0.020-0.034% Nb, 0.05-0.08% V, 0.008-0.019% Ti, and the rest is F. e and unavoidable impurities; its hot rolling method includes heating the slab to 1185-1220℃, finishing rolling entrance temperature 960-1000℃, final rolling temperature 830-870℃, and finally cooling to 590-620℃ at a cooling rate of 10-40℃ / s for coiling, to obtain pipeline steel with yield strength>480MPa, tensile strength>560MPa, yield strength ratio 0.84-0.87, and -20℃ impact energy>160J. However, the invention adopts Nb-V composite microalloying system, and the alloy cost is high.

[0004] CN118127419A discloses a hot-rolled coil of L415 pipeline steel for ground gathering and transportation and a production method thereof, wherein the chemical composition is 0.061-0.070% C, 0.15-0.25% Si, 0.39-0.48% Mn, P≤0.012%, 0.0028-0.003% S, 0.030-0.045% Als, 0.024-0.030% Nb, 0.032-0.038% Ti, 0.25-0.31% Cr, N≤0.004%, and the rest Fe and unavoidable impurities; the obtained pipeline steel hot-rolled coil has a yield strength of 426-477MPa, a tensile strength of 557-570MPa, an elongation after fracture ≥37%, a Charpy impact energy of -40℃ ≥319J, a corrosion rate of 0.0240-0.0276mm / a, a corrosion potential ≥-0.458V, and a corrosion current density ≤1.08×10-5 A.cm -2 The L415 pipeline steel described in the invention has good corrosion resistance, but the addition amount of Cr and Ti in the alloy is relatively high, and the strength surplus is relatively low, so it cannot be produced at low cost.

[0005] CN118127399A discloses a hot-rolled coil of X52M pipeline steel with a thickness of 15-20 mm and a manufacturing method thereof, wherein the chemical composition is 0.040-0.085% C, 0.10-0.30% Si, 1.25-1.40% Mn, 0.020-0.035% Nb, 0.30-0.40% Cr, 0.015-0.040% Al, Ti≤0.018%, Ca≤0.0035%, P≤0.018%, S≤0.006%, N≤0.006%, H≤0. 0003%, the balance is Fe and inevitable trace impurities, and KR method pre-desulfurization-LD top and bottom blowing-LF desulfurization-RH deep degassing-hot rolling-coiling is adopted, wherein "3+3" passes of austenite recrystallization rolling and 7 passes of austenite non-recrystallization rolling (start rolling temperature ≤950℃, final rolling temperature ≤850℃, cumulative reduction ≥68%) are adopted, and after finish rolling, the product is cooled to 530-590℃ at a cooling rate of 25-35℃ / s and coiled to obtain X52M pipeline steel hot-rolled coil with a thickness of 15-20mm with excellent tensile properties and low-temperature toughness. The thick-gauge X52M pipeline steel described in the invention has a high Cr addition amount to ensure hardenability; the smelting process requires RH degassing, and the finishing rolling start temperature is relatively low, ≤950℃, and the intermediate billet needs to be cooled by means such as swing rolling, which increases the process time and cost; the cooling rate after rolling is required to be 25-35℃ / s, which places high demands on the cooling system; in addition, the invention does not describe the key control parameters in detail, and only gives a relatively brief reason. Summary of the invention

[0006] The purpose of the present invention is to make up for the lack of a method for producing economical L415 pipeline steel using conventional smelting-continuous casting-hot rolling process in the prior art, to develop a L415 pipeline steel (thickness 6.5-11.9 mm) with lower alloy and process costs and detailed supporting production process parameters, to cancel the addition of alloys Cr and V, to cancel the RH degassing process, and to have a production process method with lower requirements for hot rolling production line equipment.

[0007] To achieve the above object, the present invention first provides a method for preparing an economical L415 pipeline steel hot-rolled coil, wherein the chemical composition of the L415 pipeline steel hot-rolled coil includes, by weight percentage: C: 0.05-0.09%, Si: 0.15-0.25%, Mn: 1.25-1.40%, Nb: 0.025-0.040%, Ti: 0.015-0.025%, P≤0.018%, S≤0.008%, Als: 0.015-0.045%, and the rest is Fe and unavoidable impurities;

[0008] The preparation method comprises the following steps: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting to obtain slabs, slab reheating → high-pressure water descaling → rough rolling → finishing rolling → laminar cooling → coiling to obtain L415 pipeline steel hot-rolled coils;

[0009] Among them, converter smelting: blowing argon throughout the process, pulling carbon once to the target value [C] ≤ 0.04%, avoiding re-blowing; phosphorus content [P] of steel ≤ 0.012%, temperature ≥ 1660℃; adding aluminum iron for deoxidation, and adding low-carbon ferrosilicon and low-nitrogen metal manganese to complete Si alloying and Mn alloying, and finally feeding aluminum wire for oxygen determination;

[0010] Refining outside the furnace: after entering the station, slag desulfurization operation is carried out, the time is ≥15min, and the exit target [S] is ≤0.004%; after slag desulfurization, Nb alloy and Ti alloy are added to complete Nb alloying and Ti alloying; the exit temperature is 1570-1595℃;

[0011] Continuous casting: full protection pouring, using peritectic steel protection slag; tundish superheat 15-40℃; secondary cooling water adopts weak cooling mode, special light reduction model containing Nb peritectic steel is used, and casting speed is 0.9-1.3m / s;

[0012] Slab reheating: Continuous casting slab is hot or cold charged into the furnace. When the furnace temperature is ≤200℃, the furnace time is ≥180min. Taking the furnace temperature of 200℃ as the benchmark, the furnace time is reduced by 5min for every 100℃ increase in the furnace temperature (rounded down), and the total furnace time is ≥150min; the target steel tapping temperature is 1180-1200℃;

[0013] Rough rolling: Rough rolling is carried out by 5 reciprocating rolling, with a single-pass reduction of ≥18%, and descaling between odd-numbered passes. The slab running speed is 2.9-3.0m / s to obtain the intermediate slab; according to the finished product rolling thickness, the intermediate slab thickness is 4-5 times the finished product thickness, and the minimum thickness is not less than 46mm;

[0014] Finishing rolling: The intermediate billet enters the finishing rolling, the inlet temperature is not higher than 990℃, after 7 continuous rolling passes, the deformation of the last 3 passes is not less than 17%, 12%, and 9% respectively, the cooling water between F1 and F4 stands is used, and the final rolling temperature is 830-850℃;

[0015] Laminar cooling and coiling: After finishing rolling, the strip enters the laminar cooling section and adopts a double-stage cooling mode. The strip running speed is 3.5-4.5m / s, the average cooling rate is 15.8-20.5℃ / s, and it is cooled to 500-550℃ for coiling.

[0016] Among them, in the preparation method of the above-mentioned economical L415 pipeline steel hot-rolled coil, the molten iron is pretreated: desulfurization is carried out, the target sulfur content is ≤0.001%, and the desulfurization slag is scraped off after the treatment.

[0017] Among them, in the above-mentioned method for preparing the economical L415 pipeline steel hot-rolled coil, the thickness of the slab obtained by continuous casting is 200-230 mm.

[0018] Among them, in the preparation method of the above-mentioned economical L415 pipeline steel hot-rolled coil, high-pressure water descaling: after the slab is taken out of the furnace, 17-20MPa high-pressure water is used to descale the entire length of the slab, and the descaling operation speed is 1.5-1.6m / s.

[0019] Among them, in the above-mentioned method for preparing the economical L415 pipeline steel hot-rolled coil, the thickness of the L415 pipeline steel hot-rolled coil is 6.5-11.9 mm.

[0020] The present invention also provides an L415 pipeline steel hot-rolled coil, which is prepared by the above-mentioned method for preparing the economical L415 pipeline steel hot-rolled coil.

[0021] Among them, the yield strength of the above-mentioned L415 pipeline steel hot-rolled coil is ≥420MPa, the tensile strength is 510-550MPa, the yield strength ratio is not higher than 0.90, the elongation is above 40%, the impact energy of the full-size specimen at -20℃ is above 160J, the single value of the shear area of ​​the -5℃ drop hammer test is not less than 70%, and the average value is not less than 85%.

[0022] Beneficial effects of the present invention:

[0023] The invention optimizes steel composition, cancels the addition of Cr and V, significantly reduces the addition amount of alloy elements, and reduces the alloy cost; adopts a conventional smelting-continuous casting-hot rolling process, cancels the RH process, reduces the process cost and reduces the process time, removes S and P and alloys through converter smelting and furnace refining (LF) mode, so as to obtain L415 grade pipeline steel with good strength and toughness matching, the yield strength of which is 420-500MPa, the tensile strength of which is 520-560MPa, the yield strength ratio is not higher than 0.91, the elongation is more than 22%, the impact energy of a full-size specimen at -20°C is more than 160J, the single value of the shear area of ​​a -5°C drop hammer test is not less than 70%, the average value is not less than 85%, the thickness specification is 6.5-11.9mm, and the steel has good economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a surface microstructure diagram of the L415 pipeline steel hot-rolled coil obtained in Example 1 of the present invention.

[0025] Figure 2 This is the core microstructure diagram of the L415 pipeline steel hot-rolled coil obtained in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] Specifically, a method for preparing an economical L415 pipeline steel hot-rolled coil, wherein the chemical composition of the L415 pipeline steel hot-rolled coil includes, by weight percentage: C: 0.05-0.09%, Si: 0.15-0.25%, Mn: 1.25-1.40%, Nb: 0.025-0.040%, Ti: 0.015-0.025%, P≤0.018%, S≤0.008%, Als: 0.015-0.045%, and the rest is Fe and unavoidable impurities;

[0027] The preparation method comprises the following steps: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting to obtain slabs, slab reheating → high-pressure water descaling → rough rolling → finishing rolling → laminar cooling → coiling to obtain L415 pipeline steel hot-rolled coils.

[0028] The chemical composition restrictions of the hot-rolled coil of the present invention are described below:

[0029] Carbon (C): The main function of carbon is to produce solid solution strengthening and form strong carbides with microalloying elements such as niobium and titanium to prevent abnormal grain growth during slab heating and rolling. Too high a carbon content will reduce the toughness and plasticity of the steel and affect the welding performance at a relatively low coiling temperature. The carbon content in pipeline steel is usually below 0.10%. The present invention controls the C content to 0.05-0.09%.

[0030] Silicon (Si): Silicon acts as a reducing agent and deoxidizer during the smelting process of pipeline steel, and also provides a certain solid solution strengthening effect. However, too high a silicon content will reduce the plasticity, toughness and welding performance of the steel. Therefore, the designed addition amount of Si is 0.15-0.25%.

[0031] Manganese (Mn): The main function of manganese is solid solution strengthening, which is used to compensate for the decrease in strength caused by the decrease in carbon content. At the same time, manganese can reduce the austenite phase transition temperature, stabilize the austenite phase, promote low-temperature phase transition, refine the phase transition structure, and reduce the tough-brittle transition temperature. A high manganese content can easily cause the ferrite-pearlite phase separation, resulting in a decrease in yield strength. The present invention controls the Mn content to 1.25%-1.40%.

[0032] Niobium (Nb) and titanium (Ti): Niobium and titanium are the main microalloying elements. The carbonitrides precipitate at different stages of slab heating and hot rolling, which play a role in grain refinement. Niobium can refine austenite grains and appropriately increase the austenite recrystallization temperature. Titanium can precipitate carbonitrides at higher temperatures to pin the grain boundaries and optimize the weld structure and performance. Considering the addition effect and alloy cost, the present invention controls the Nb content to 0.025-0.040% and the Ti content to 0.015-0.025%.

[0033] Acid-soluble aluminum (Als): The main deoxidizer in the smelting stage. Too low a content cannot ensure that oxygen in the molten steel is effectively removed, while too high a content can easily lead to problems such as non-metallic inclusions. Therefore, the acid-soluble aluminum content is controlled at 0.015-0.045%.

[0034] The smelting method of the economical L415 pipeline steel hot-rolled coil provided by the present invention comprises the following smelting process: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting to obtain slabs. The key steps are as follows:

[0035] (1) Pretreatment desulfurization: target sulfur content ≤ 0.001%, and all desulfurization slag shall be scraped off after treatment;

[0036] (2) Converter smelting: argon blowing throughout the process, carbon drawing in one time to the target value [C] ≤ 0.04%, avoid re-blowing; phosphorus content [P] of steel tapping ≤ 0.012%, temperature ≥ 1660℃; add aluminum iron for deoxidation, and add low-carbon ferrosilicon and low-nitrogen metal manganese to complete Si alloying and Mn alloying, and finally feed aluminum wire for oxygen determination;

[0037] (3) Refining outside the furnace: after entering the station, slag desulfurization operation is carried out, the time is ≥15min, and the exit target [S] is ≤0.004%; after the slag desulfurization is completed, Nb alloy and Ti alloy are added to complete Nb alloying and Ti alloying; the exit temperature is 1570-1595℃;

[0038] The present invention eliminates the RH process, and completes alloying and removes S and P through converter smelting and external furnace refining (LF).

[0039] (4) Continuous casting: The whole process is protected by pouring, and peritectic steel protective slag is used; the superheat of the tundish is 15-40℃; the secondary cooling water adopts weak cooling mode, and a special light reduction mold containing Nb peritectic steel is used, and the casting speed is 0.9-1.3m / s.

[0040] In the continuous casting of the present invention, the protective pouring is to prevent nitrogen addition, oxygen addition and resulfurization during the pouring process; the secondary cooling system and the light reduction system ensure that the Nb micro-alloying component L415 pipeline steel slab has cracks and central segregation defects, and improves the equiaxed crystal ratio; too high or too low superheat of the tundish will cause coarse grains and alloy segregation, and too high or too low pulling speed may cause steel leakage or central segregation.

[0041] The rolling method of the above-mentioned economical L415 pipeline steel hot-rolled coil provided by the present invention comprises a slab rolling process flow including: slab reheating → high-pressure water descaling → rough rolling → finishing rolling → laminar cooling → coiling, which is specifically as follows:

[0042] (5) Slab reheating: Continuous casting slabs are allowed to be hot-charged or cold-charged. When the furnace temperature is ≤200℃, the furnace time is ≥180min. Based on the furnace temperature of 200℃, the furnace time is reduced by 5min for every 100℃ increase in the furnace temperature, and the total furnace time is ≥150min. The target steel tapping temperature is 1180-1200℃.

[0043] Slab reheating: The main function is to make the slab completely austenitized, dissolve the alloy elements and evenly distribute them. The hot charging method can make full use of the slab waste heat and save energy; control the approximate quantitative relationship between the slab time in the furnace and the temperature entering the furnace during the heating process, and shorten the time in the furnace according to the slab charging temperature to avoid abnormal grain growth, save energy and shorten the process time.

[0044] Conventional hot rolling production lines are in a continuous cooling process from the time the slab comes off the line to the time it is sent into the heating furnace. If no temperature compensation device is added, the temperature entering the furnace is lower than the temperature exiting the continuous casting furnace. The function of the heating furnace is to compensate for the temperature and to make the temperature uniform. Therefore, the present invention does not need to specify an upper limit on the temperature entering the furnace, and the temperature entering the furnace is generally lower than the temperature exiting the continuous casting furnace.

[0045] Due to continuous production, in addition to reducing the rolling load of the steel billet and the performance of the finished product, the slab reheating in the furnace can coordinate the overall production rhythm. For example, when the hot rolling equipment fails, the slab can stay in the heating furnace and wait, and the time in the furnace will be extended, so no upper limit requirement is set. For some steel types that are sensitive to the heating process, an upper limit time is set. If the upper limit time is exceeded, the steel is directly returned from the furnace to terminate the rolling process. The L415 pipeline steel of the present invention is low-temperature heating and has low temperature sensitivity, so no upper limit is set.

[0046] (6) High-pressure water descaling: After the slab comes out of the furnace, 17-20MPa high-pressure water is used to descale the entire length of the slab, and the descaling operation speed is 1.5-1.6m / s;

[0047] High-pressure water descaling: The purpose is to remove the primary iron oxide scale produced by long-term heating of the slab to avoid it being pressed into the slab surface during rough rolling. Using a suitable operating speed can ensure the descaling effect and reduce the temperature drop of the slab.

[0048] (7) Rough rolling: Rough rolling is performed by 5 reciprocating passes, with a single pass reduction of ≥18%, and descaling between odd passes. The slab running speed is 2.9-3.0 m / s;

[0049] Rough rolling: The main function is to refine the austenite grains by controlling the dynamic recrystallization of austenite. The deformation of a single rough rolling pass must be greater than the critical deformation, otherwise it will easily lead to insufficient deformation at the thickness center. Therefore, the deformation of a single rough rolling pass is controlled at 18% in order to fully recrystallize and refine the austenite grains.

[0050] (8) According to the finished product rolling thickness, the intermediate billet thickness is 4-5 times the finished product thickness, and the minimum thickness is not less than 46 mm. The intermediate billet thickness is specified as the finishing rolling reduction ratio applicable to the L415 pipeline steel of the present invention.

[0051] In the present invention, the thickness of the finished product of L415 pipeline steel hot-rolled coil is 6.5-11.9 mm. Combined with the above-mentioned intermediate billet thickness control requirements, when the thickness of the finished product is 6.5-9.2 mm, the minimum intermediate billet thickness of 46 mm is adopted, and when the thickness of the finished product is greater than 9.2 mm, the intermediate billet thickness is determined according to 4-5 times the thickness of the finished product.

[0052] (9) Finishing rolling: The intermediate billet enters the finishing rolling, and the inlet temperature is not higher than 990°C. After 7 continuous rolling passes, the deformation of the last 3 passes is not less than 17%, 12%, and 9% respectively. Cooling water between F1 and F4 stands is used, and the final rolling temperature is 830-850°C;

[0053] Finishing rolling: The main function is to deform the austenite grains through non-recrystallization rolling, providing nucleation energy and nucleation particles for the ferrite phase transformation. In the present invention, the finishing rolling compression ratio is 4-5, which ensures that the non-recrystallized austenite grains are fully deformed while avoiding the occurrence of mixed crystals. The characteristic of the present invention is that the cooling water between the racks is used to control the deformation temperature of the pass. The F1-F3 racks allow a small amount of austenite recrystallization to further refine the grains. The F4-F7 racks are mainly rolled in the non-austenite recrystallization zone. The purpose of increasing the deformation amount of the last 3 racks is to fully deform the recrystallized grains. The purpose of limiting the lower limit of the final rolling temperature (≥830°C) is to prevent the occurrence of proeutectoid ferrite phase transformation.

[0054] (10) Laminar cooling and coiling: After finishing rolling, the strip enters the laminar cooling section and adopts a double-stage cooling mode. The strip running speed is 3.5-4.5 m / s, the average cooling rate is 15.8-20.5 °C / s, and it is cooled to 500-550 °C for coiling. The specific opening method of the "double-stage cooling mode" is to open the second group of the first 7 groups of headers, and open the first 4 groups of the last 7 groups of headers.

[0055] Laminar cooling: The laminar cooling water system forms a cooling rate and phase change temperature range suitable for ferrite phase transformation. The running speed and temperature of the steel strip are process parameters suitable for phase transformation. The double-stage cooling mode can ensure that the coil temperature control is more accurate under the premise of controlled phase transformation, reducing the fluctuation of organizational properties.

[0056] The present invention is further described in detail below by way of examples, but the protection scope of the present invention is not limited to the scope of the examples.

[0057] Embodiments 1 to 4

[0058] The smelting process of the L415 pipeline steel hot-rolled coil of Examples 1 to 4 includes: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting to obtain a slab, and the slab rolling process includes: slab reheating → high-pressure water descaling → rough rolling → finishing rolling → laminar cooling → coiling. The finished product composition of the L415 pipeline steel hot-rolled coil is shown in Table 1, which is specifically as follows:

[0059] (1) Pretreatment desulfurization: target sulfur content ≤ 0.001% (see Table 2 for details), and the desulfurization slag is scraped off after treatment;

[0060] (2) Converter smelting: argon blowing throughout the process, carbon drawing to the target value [C] ≤ 0.04% (see Table 2 for details), avoid re-blowing; phosphorus content [P] of steel ≤ 0.012%, temperature ≥ 1660℃; add aluminum iron for deoxidation, and add low-carbon ferrosilicon and low-nitrogen metal manganese at the same time, and finally feed aluminum wire for oxygen determination;

[0061] (3) Refining outside the furnace: after entering the station, slag desulfurization operation is carried out, the time is ≥15min (see Table 2 for details), and the exit target [S] is ≤0.004%; after slag desulfurization, Nb alloy and Ti alloy are added; the exit temperature is 1570-1595℃;

[0062] (4) Continuous casting: full protection pouring, using peritectic steel protection slag; the tundish superheat is 15-40℃ (see Table 2 for details); the secondary cooling water adopts weak cooling mode, a special light reduction mold for Nb-containing peritectic steel is used, and the casting speed is 0.9-1.3m / s (see Table 2 for details);

[0063] (5) Slab reheating: Continuous casting slabs can be charged hot or cold. The furnace entry temperature, furnace time and target tapping temperature are shown in Table 3.

[0064] (6) High-pressure water descaling: After the slab comes out of the furnace, 20MPa high-pressure water is used to descale the entire length of the slab, and the descaling speed is 1.5-1.6m / s;

[0065] (7) Rough rolling: Rough rolling is performed by 5 reciprocating passes, with a single pass reduction of ≥18%, and descaling between odd passes. The slab running speed is 2.9-3.0 m / s;

[0066] (8) According to the rolling thickness of the finished product, the intermediate billet thickness is 4-5 times the finished product thickness, and the minimum thickness is not less than 46mm (see Table 3 for details).

[0067] (9) Finishing rolling: The intermediate billet enters the finishing rolling, and the inlet temperature is not higher than 990°C (see Table 3 for details). After 7 consecutive rolling passes, the deformation of the last 3 passes is not less than 17%, 12%, and 9% respectively. Cooling water between F1 and F4 stands is used, and the final rolling temperature is 830-850°C (see Table 3 and Table 4 for details);

[0068] (10) Laminar cooling and coiling: After finishing rolling, the strip enters the laminar cooling section and adopts a double-stage cooling mode. The strip running speed is 3.5-4.5 m / s, the average cooling rate is 15.8-20.5 °C / s, and it is cooled to 500-550 °C (see Tables 3 and 4 for details) and coiled. The specific opening method of the "double-stage cooling mode" is to open the second group of the first 7 groups of headers, and open the first 4 groups of the last 7 groups of headers. L415 pipeline steel hot-rolled coil is obtained.

[0069] Table 1 is the composition of the finished product of the embodiment of the present invention, Table 2 is the key process parameters of the steelmaking process of the embodiment of the present invention, Table 3 is the key process parameters of the hot rolling process of the embodiment of the present invention, Table 4 is the key process parameters of the finishing rolling and cooling of the embodiment of the present invention, and Table 5 is the measured performance of the finished product of the embodiment of the present invention.

[0070] Table 1 Example composition / wt%

[0071] C Si Mn P S Nb Ti Als Example 1 0.06 0.20 1.31 0.010 0.004 0.029 0.018 0.035 Example 2 0.06 0.20 1.29 0.010 0.005 0.025 0.016 0.035 Example 3 0.09 0.19 1.25 0.008 0.001 0.030 0.018 0.035 Example 4 0.05 0.21 1.37 0.009 0.004 0.033 0.017 0.035

[0072] Table 2 Key process parameters of steelmaking process

[0073]

[0074] Table 3 Key process parameters of hot rolling process

[0075]

[0076]

[0077] Table 4 Finishing rolling and cooling rate

[0078]

[0079] Table 5 Measured performance of plate rolling in the embodiment

[0080]

[0081] Note: In Table 5, the Charpy impact specimen is a 3 / 4 size specimen, and the impact energy of 120J meets the impact energy of 160J for the full size specimen.

[0082] Figure 1 This is a surface microstructure diagram of the L415 pipeline steel hot-rolled coil obtained in Example 1 of the present invention. Figure 2 This is a microstructure diagram of the core of the L415 pipeline steel hot-rolled coil obtained in Example 1 of the present invention. It can be seen that the microstructure type of the L415 pipeline steel hot-rolled coil of the present invention from the surface to the core is ferrite + a small amount of pearlite, and the grain size is uniform in the thickness direction.

Claims

1. A method for preparing economical L415 pipeline steel hot-rolled coil, characterized in that: The chemical composition of the L415 pipeline steel hot-rolled coil includes, by weight percentage: C: 0.05-0.09%, Si: 0.15-0.25%, Mn: 1.25-1.40%, Nb: 0.025-0.040%, Ti: 0.015-0.025%, P≤0.018%, S≤0.008%, Als: 0.015-0.045%, and the rest is Fe and unavoidable impurities; The preparation method comprises the following steps: molten iron pretreatment → converter smelting → refining outside the furnace → continuous casting to obtain slabs, slab reheating → high-pressure water descaling → rough rolling → finishing rolling → laminar cooling → coiling to obtain L415 pipeline steel hot-rolled coils; Among them, converter smelting: blowing argon throughout the process, pulling carbon once to the target value [C] ≤ 0.04%, avoiding re-blowing; phosphorus content [P] of steel ≤ 0.012%, temperature ≥ 1660℃; adding aluminum iron for deoxidation, and adding low-carbon ferrosilicon and low-nitrogen metal manganese to complete Si alloying and Mn alloying, and finally feeding aluminum wire for oxygen determination; Refining outside the furnace: after entering the station, slag desulfurization operation is carried out, the time is ≥15min, and the exit target [S] is ≤0.004%; after slag desulfurization, Nb alloy and Ti alloy are added to complete Nb alloying and Ti alloying; the exit temperature is 1570-1595℃; Continuous casting: full protection pouring, using peritectic steel protection slag; tundish superheat 15-40℃; secondary cooling water adopts weak cooling mode, special light reduction model containing Nb peritectic steel is used, and casting speed is 0.9-1.3m / s; Slab reheating: Continuous casting slab is hot or cold charged into the furnace. When the furnace temperature is ≤200℃, the furnace time is ≥180min. Based on the furnace temperature of 200℃, the furnace time is reduced by 5min for every 100℃ increase in the furnace temperature, and the total furnace time is ≥150min. The target steel tapping temperature is 1180-1200℃. Rough rolling: Rough rolling is carried out by 5 reciprocating rolling, with a single-pass reduction of ≥18%, and descaling between odd-numbered passes. The slab running speed is 2.9-3.0m / s to obtain the intermediate slab; according to the finished product rolling thickness, the intermediate slab thickness is 4-5 times the finished product thickness, and the minimum thickness is not less than 46mm; Finishing rolling: The intermediate billet enters the finishing rolling, the inlet temperature is not higher than 990℃, after 7 continuous rolling passes, the deformation of the last 3 passes is not less than 17%, 12%, and 9% respectively, the cooling water between F1 and F4 stands is used, and the final rolling temperature is 830-850℃; Laminar cooling and coiling: After finishing rolling, the strip enters the laminar cooling section and adopts a double-stage cooling mode. The strip running speed is 3.5-4.5m / s, the average cooling rate is 15.8-20.5℃ / s, and it is cooled to 500-550℃ for coiling.

2. The method for preparing the economical L415 pipeline steel hot-rolled coil according to claim 1, characterized in that: Hot metal pretreatment: desulfurization is carried out, the target sulfur content is ≤0.001%, and the desulfurization slag is scraped off after treatment.

3. The method for preparing the economical L415 pipeline steel hot-rolled coil according to claim 1, characterized in that: The thickness of the slab obtained by continuous casting is 200-230mm.

4. The method for preparing the economical L415 pipeline steel hot-rolled coil according to claim 1, characterized in that: High-pressure water descaling: After the slab comes out of the furnace, 17-20MPa high-pressure water is used to descale the entire length of the slab, and the descaling operation speed is 1.5-1.6m / s.

5. The method for preparing the economical L415 pipeline steel hot-rolled coil according to any one of claims 1 to 4, characterized in that: The thickness of the L415 pipeline steel hot-rolled coil is 6.5-11.9 mm.

6. The L415 pipeline steel hot-rolled coil is prepared by the method for preparing the economical L415 pipeline steel hot-rolled coil according to any one of claims 1 to 5.

7. The L415 pipeline steel hot rolled coil according to claim 6, characterized in that: Its yield strength is ≥420MPa, tensile strength is 510-550MPa, yield strength ratio is not higher than 0.90, elongation is more than 40%, impact energy of full-size specimen at -20℃ is more than 160J, and the single value of shear area in -5℃ drop hammer test is not less than 70% and the average value is not less than 85%.

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