High-purity pipeline steel and smelting method thereof

Abstract

The invention discloses high-purity pipeline steel and a smelting method thereof. The pipeline steel comprises the following components in percentage by weight: 0.03 to 0.15 percent of C, 0.09 to 0.26 percent of Si, 1.16 to 1.69 percent of Mn, 0.009 to 0.066 percent of Nb, 0.009 to 0.021 percent of Ti, 0.021 to 0.049 percent of Al and the balance of Fe. The smelting method comprises the following steps: converter smelting: the final slag alkalinity is 3.8-4.3, MgO accounts for 8.0-10.0%, total iron accounts for 15-17%, and the tapping temperature is 1650-1670 DEG C; during tapping, alloying and slagging are carried out, and the aluminum adding amount MAl is equal to 0.0010 MO + 0.90; and RH vacuum refining is conducted, specifically, vacuum degassing, alloying, net circulation and vacuum breaking hydrogen determination are conducted in sequence, and the tapping temperature is (1563-1583) DEG C + k * (tRH-10). Therefore, the purity can be improved, the inclusion content and size are reduced, and subsequent residual stress control is facilitated.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel material preparation, and relates to a high-purity pipeline steel and a smelting method thereof. BACKGROUND

[0002] Pipeline steel plays an irreplaceable role in the energy transportation industry such as petroleum and natural gas. In the production process of pipeline steel plates, due to the comprehensive action of rolling, cooling, heat treatment and other processes, residual stress will be generated in the steel plate.

[0003] The existence of residual stress has many adverse effects on the performance of pipeline steel and the safety of pipelines.

[0004] Therefore, how to control and reduce residual stress and how to make the residual stress of pipeline steel plates uniform become difficult problems to be solved in the field of pipeline steel manufacturing.

[0005] It has been found through research that large-size inclusions in pipeline steel billets will cause discontinuous microstructure and stress concentration, which is not conducive to the control of residual stress when the pipeline steel billets are subsequently prepared into steel plate products. SUMMARY

[0006] The present application aims to provide a high-purity pipeline steel and a smelting method thereof.

[0007] To achieve the above-mentioned application purpose, an embodiment of the present application provides a smelting method of pipeline steel. The chemical composition of the pipeline steel includes, in terms of mass percentage: C 0.03~0.15%, Si 0.09~0.26%, Mn 1.16~1.69%, Cr 0~0.29%, Ni 0~0.29%, Mo 0~0.18%, Cu 0~0.29%, Nb 0.009~0.066%, Ti 0.009~0.021%, Al 0.021~0.049%, P≤0.0150%, S≤0.0040%, O≤0.0025%, N≤0.0048%, H≤0.0002%, and the rest is iron and unavoidable impurities. The smelting method includes a molten iron KR desulfurization process, a converter smelting process, an LF refining process, an RH vacuum refining process and a casting billet process in sequence. In the converter smelting process: the basicity of the final slag is controlled to be 3.8~4.3, the mass percentage of MgO is 8.0~10.0%, and the mass percentage of total iron is 15~17%, and the temperature of the molten steel when tapping is 1650~1670℃; alloying and slagging are carried out in the order of lime, silicon-manganese alloy or medium-carbon manganese-iron alloy, aluminum ingot and lime when tapping, and the mass of Al added by the aluminum ingot is M Al M Al =0.0010M O+ 0.90, wherein M O is the mass fraction of O in the molten steel at tapping, in ppm, M Al is kg / t; In the RH vacuum refining process: vacuum degassing, alloying, net circulation and breaking vacuum and hydrogen fixation are sequentially performed, and then the molten steel is tapped, the tapping temperature T = T RH + T RHXZ , T RH is valued at 1563-1583℃, T RHXZ = k x (t RH - 10), k is valued at 0.3-0.5, and t RH is the time from tapping to pouring, in min.

[0008] Preferably, the chemical composition of the pipeline steel further satisfies any one, two, three or all of the following conditions in mass percentage: CEV = C + Mn / 6 + (Cr + Mo) / 5 + (Cu + Ni) / 15 is 0.223-0.571; Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15 is 0.091-0.294; wherein the element symbols in the formula represent the mass percentage of the corresponding element.

[0009] Preferably, in the hot metal KR desulfurization process: the outgoing hot metal temperature is ≥1300℃, and S≤0.0025%.

[0010] Preferably, in the converter smelting process: the ladle bottom argon blowing is started before tapping, the argon blowing pressure is 0.5-0.6 MPa, and the bright circle diameter of the molten steel is 240-340 mm; when the tapping reaches 3 / 4, the argon blowing pressure is reduced to 0.42-0.52 MPa, and the bright circle diameter of the molten steel is 120-220 mm.

[0011] Preferably, in the LF refining process: after the ladle is in place, the bottom argon blowing is connected first and initial alloying is performed, and then main alloying and slagging are performed under strong stirring; power refining is then performed, during which alloy is added; finally, white slag is formed, and the molten steel is tapped.

[0012] Preferably, in the LF refining process: The bottom argon blowing is started after the bottom argon blowing is connected, and the argon blowing pressure is 0.4-0.6 MPa; When main alloying and slagging are performed, the flow rate of the bottom argon blowing is 700-860 NL / min, the argon blowing pressure is 0.50-0.60 MPa, and the strong stirring time is ≤6 min; In the supplementing alloying process, the argon bottom blowing flow rate is 650-800 NL / min, the argon blowing pressure is 0.45-0.55 MPa, and the strong stirring time is ≤4 min; The argon bottom blowing flow rate in the remaining time is 300-400 NL / min, the argon blowing pressure is 0.40-0.50 MPa, and the bright circle diameter of the molten steel is <150 mm.

[0013] Preferably, in the LF refining process: when white slag is formed, the CaO / SiO2 in the slag is controlled to be 5.0±0.2, and the CaO / Al2O3 is controlled to be 1.8±0.5, and the white slag is maintained for 15 min or more.

[0014] Preferably, in the LF refining process: the tapping temperature is 1598-1618 ℃.

[0015] Preferably, in the LF refining process: the tapping temperature corresponding to the start of pouring is 1608-1618 ℃, and the tapping temperature corresponding to continuous pouring is 1598-1608 ℃. In the RH vacuum refining process: the T RH corresponding to the start of pouring is 1573-1583 ℃, and the T RH corresponding to continuous pouring is 1563-1573 ℃.

[0016] Preferably, in the RH vacuum refining process: the vacuum degree during vacuum degassing is ≤2.0 mbar, the degassing time is 20-25 min, titanium ferroalloy is added during alloying, the treatment time during net circulation is 8-10 min, and the H mass percentage during breaking vacuum and hydrogen fixation is ≤1.5 ppm.

[0017] Preferably, in the casting blank process: a continuous casting machine is used to prepare a continuous casting blank, and full protection casting is used, and the superheat is 26-30 ℃.

[0018] To achieve the above-mentioned purposes, an embodiment of the present application provides a pipeline steel. The chemical composition of the pipeline steel includes, in mass percent: C 0.03-0.15%, Si 0.09-0.26%, Mn 1.16-1.69%, Cr 0-0.29%, Ni 0-0.29%, Mo 0-0.18%, Cu 0-0.29%, Nb 0.009-0.066%, Ti 0.009-0.021%, Al 0.021-0.049%, P ≤0.0150%, S ≤0.0040%, O ≤0.0025%, N ≤0.0048%, H ≤0.0002%, and the rest is iron and inevitable impurities. The pipeline steel is a billet with a thickness of 200 mm or more, and the number of inclusions with a size of ≥15 μm on the cross section of the pipeline steel is ≤5 pieces / cm 2 , and the number of inclusions with a size of ≥10 μm and <15 μm is ≤15 pieces / cm.2 number of inclusions with size ≥ 5 μm and < 10 μm ≤ 150 pieces / cm 2 .

[0019] Preferably, the pipeline steel has a center segregation ≤ 0.5 level, a cross-section manganese segregation ratio ≤ 1.10, a center porosity ≤ 0.5 level, and A, B, C, D, Ds type inclusion ratings all ≤ 1 level and a total ≤ 2.5 level.

[0020] Compared with the prior art, the embodiment of the present application has the beneficial effect that, on the basis of the designed chemical composition, by controlling the final slag in the converter smelting, and controlling the Al addition amount based on the O content, combined with the tapping temperature precision control in the RH vacuum refining, the purity of the molten steel can be optimized, that is, the purity of the molten steel is greatly improved, thereby improving the internal quality of the pipeline steel, reducing the inclusion content and size, and thus facilitating the control of residual stress during subsequent processing of the pipeline steel plate. DETAILED DESCRIPTION

[0021] To make the objectives, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely below in combination with the specific embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments.

[0022] The embodiment of the present application provides a smelting method of a pipeline steel.

[0023] The chemical composition of the pipeline steel includes, in terms of mass percentage, C 0.03~0.15%, Si 0.09~0.26%, Mn 1.16~1.69%, Cr 0~0.29%, Ni 0~0.29%, Mo 0~0.18%, Cu 0~0.29%, Nb 0.009~0.066%, Ti 0.009~0.021%, Al 0.021~0.049%, P ≤ 0.0150%, S ≤ 0.0040%, O ≤ 0.0025%, N ≤ 0.0048%, H ≤ 0.0002%, and the rest is iron and inevitable impurities.

[0024] The main effects of each element and the selection of the amount thereof are specifically analyzed and described below.

[0025] C: Carbon is the most economical strengthening element in steel, which has a solid solution strengthening effect, and forms carbides with niobium, titanium, chromium, molybdenum, etc., which has a precipitation strengthening effect; the increase of the carbon content has a significant effect on improving the strength and hardness of the steel plate, but too high carbon will cause poor low-temperature toughness and ductility, and reduce the low-temperature drop hammer performance of the steel plate; the formation of carbides and martensite will induce an increase of residual stress. In the present application, the carbon content is controlled to be 0.03~0.15%.

[0026] Preferably, the carbon content can be controlled to be 0.03-0.07%, 0.04-0.08%, 0.05-0.09%, 0.07-0.11% or 0.11-0.15%.

[0027] Si: Silicon has a solid solution strengthening effect in steel, but can increase the grain boundary segregation of phosphorus, sulfur and other elements, reduce low-temperature toughness and plasticity, and too much silicon can easily produce Fe2SiO4 on the surface of continuous casting billets, which is not conducive to the control of the surface quality of the steel plate. In an embodiment, the silicon content is controlled to be 0.09-0.26%.

[0028] Preferably, the silicon content can be further controlled to be 0.14-0.26%, 0.12-0.24%, 0.10-0.22% or 0.09-0.21%.

[0029] Mn: Manganese has a solid solution strengthening effect in steel, which increases strength and hardness. Too much manganese can cause center segregation of the billet, which is not conducive to toughness. Furthermore, increasing the manganese content can also cause the banded structure level to increase, and the higher the banded structure level, the more uneven the structure distribution. In addition, manganese is prone to form manganese sulfide inclusions. Center segregation, banded structure, inclusions and the like can all cause an increase in residual stress. In an embodiment, the manganese content is controlled to be 1.16-1.69%.

[0030] Preferably, the manganese content can be further controlled to be 1.16-1.24%, 1.31-1.39%, 1.46-1.54%, 1.51-1.59%, 1.56-1.64% or 1.61-1.69%.

[0031] Cr: Chromium has a solid solution strengthening effect in steel; and chromium can form stable carbides and increase the stability of martensite, so that a larger residual stress is generated during rapid cooling. The higher the chromium content, the more complex the residual stress distribution after cooling. In the present application, chromium can be selectively added or not added. Specifically, the chromium content is controlled to be 0-0.29%.

[0032] Preferably, in an embodiment, chromium is not contained, that is, the chromium content is 0 or close to 0, for example, no chromium alloy is actively added in the form of alloying during the production process, so that the chromium content in the steel is 0, or only chromium introduced as an impurity in the raw material.

[0033] Preferably, in another embodiment, chromium is added, and the chromium content can be controlled to be 0.06-0.24%, 0.09-0.27% or 0.11-0.29%.

[0034] Ni: Nickel in steel plays a role of solid solution strengthening, which improves the strength of the steel without significantly increasing the hardness of the steel. Nickel can expand the austenite phase region, thereby reducing residual stress; and nickel can also improve the toughness of the steel and reduce stress concentration. In the present application, nickel can be selectively added or not added. Specifically, the content of nickel is controlled to be 0-0.29%.

[0035] Preferably, in one embodiment, nickel is not contained, i.e. the content of nickel is 0 or close to 0, for example, nickel alloy is not actively added in the form of alloying during the production process, so that the content of nickel in the steel is 0, or only nickel introduced as an impurity in the raw material.

[0036] Preferably, in another embodiment, nickel is added, and the content of nickel can be controlled to be 0.02-0.18%, 0.06-0.24% or 0.11-0.29%.

[0037] Mo: Molybdenum can significantly improve the hardenability of the steel, and improve the strength, toughness, and also can play a role in refining grains and improving corrosion resistance. Molybdenum can improve the tempering stability of the steel and reduce the increase of residual stress. In the present application, molybdenum can be selectively added or not added. Specifically, the content of molybdenum is controlled to be 0-0.18%.

[0038] Preferably, in one embodiment, molybdenum is not contained, i.e. the content of molybdenum is 0 or close to 0, for example, molybdenum alloy is not actively added in the form of alloying during the production process, so that the content of molybdenum in the steel is 0, or only molybdenum introduced as an impurity in the raw material.

[0039] Preferably, in another embodiment, molybdenum is added, and the content of molybdenum can be controlled to be 0.02-0.14% or 0.06-0.18%.

[0040] Cu: Copper can promote the precipitation of niobium and make up for the loss of strength caused by the decrease of carbon content, and adding a certain amount of nickel at the same time can effectively inhibit surface cracks. In one embodiment, copper can be selectively added or not added in the present application. Specifically, the content of copper is controlled to be 0-0.29%.

[0041] Preferably, in one embodiment, copper is not contained, i.e. the content of copper is 0 or close to 0, for example, copper alloy is not actively added in the form of alloying during the production process, so that the content of copper in the steel is 0, or only copper introduced as an impurity in the raw material.

[0042] Preferably, in another embodiment, copper is added, and the content of copper can be controlled to be 0.11-0.29%.

[0043] Nb: Niobium is an important grain refining element in steel. During hot rolling, niobium strongly suppresses austenite recrystallization and its precipitation in austenite, pins austenite grain boundaries, and refines recrystallized grains. During cooling, the dissolved niobium can continue to precipitate in the form of niobium carbonitride, significantly refining the structure of the material after phase transition, further improving the strength and toughness of the steel. Niobium can reduce residual stress through grain refinement and precipitation strengthening. In the present application, the niobium content is controlled to be 0.009-0.066%.

[0044] Preferably, the niobium content can be further controlled to be 0.009-0.021%, 0.014-0.026%, 0.019-0.031%, 0.024-0.036%, 0.034-0.046%, 0.044-0.056% or 0.054-0.066%.

[0045] Ti: Titanium is a nitrogen-fixing element in steel, which can form a dispersed distribution of carbonitride, inhibit austenite grain coarsening during billet heating and hot rolling, and refine grains, thereby reducing residual stress. However, when the titanium content is high, coarse carbonitride precipitates can form in the center of the billet, affecting the low-temperature toughness of the steel plate. And large particle TiN can easily cause stress concentration. In the present application, the titanium content is controlled to be 0.009-0.021%.

[0046] Al: Aluminum is a deoxidizing element in steel. Excessive aluminum can increase Al2O3 inclusions in steel, affecting the low-temperature toughness of the steel, and coarse inclusions can cause discontinuous structure and stress concentration. In the present application, the aluminum content is controlled to be 0.021-0.049%.

[0047] P, S, N, O, H: All are impurity elements in steel, which can cause deterioration of the properties of the steel plate, including but not limited to increasing residual stress. In the present application, P is controlled to be ≤0.0150%, S is controlled to be ≤0.0040%, O is controlled to be ≤0.0025%, N is controlled to be ≤0.0048%, and H is controlled to be ≤0.0002%.

[0048] Preferably, the P content is controlled to be 0.0120% or less, 0.0100% or less, and can also be controlled to be 0.0050% or more.

[0049] Preferably, the S content is controlled to be 0.0030% or less, 0.0020% or less, and can also be controlled to be 0.0005% or more.

[0050] Preferably, the N content is controlled to be 0.0022% or more.

[0051] Preferably, the O content is controlled to be 0.0011% or more.

[0052] Preferably, the H content is controlled at 0.00018% or less, and can also be controlled at 0.00005% or more.

[0053] Further, the chemical composition of the pipeline steel plate satisfies any one or both of the following ① and ② in terms of mass percentage: ① CEV is 0.223-0.571, and ② Pcm is 0.091-0.294.

[0054] CEV and Pcm can be calculated by the following two formulas respectively: CEV = C + Mn / 6 + (Cr + Mo) / 5 + (Cu + Ni) / 15; Pcm = C + Si / 30 + (Mn + Cu + Cr) / 20 + Ni / 60 + Mo / 15.

[0055] Herein, in the formulas of CEV and Pcm, each element symbol represents the mass percentage of the corresponding element. For example, if the content of C element in the pipeline steel plate is 0.03%, then "C" in the formula represents the mass percentage of 0.03.

[0056] The smelting method comprises a sequential hot metal KR desulfurization process, a converter smelting process, an LF refining process, an RH vacuum refining process and a casting blank process, wherein: In the converter smelting process: the basicity of the final slag is controlled at 3.8-4.3, the mass percentage of MgO is 8.0-10.0%, and the mass percentage of total iron is 15-17%, and the temperature of the molten steel at tapping is 1650-1670℃; alloying and slagging are performed in the order of lime, silicon-manganese alloy or medium-carbon manganese-iron alloy, aluminum ingot and lime at tapping, and the mass of Al added by the aluminum ingot is M Al satisfies M Al =0.0010M O +0.90, wherein M O is the mass percentage of O in the molten steel at tapping, in ppm, and M Al is in kg / t; In the RH vacuum refining process: vacuum degassing, alloying, net circulation and hydrogen breaking are sequentially performed, and then the molten steel is tapped, and the tapping temperature T = T RH + T RHXZ , T RH is valued at 1563-1583℃, and T RHXZ =k×(t RH -10), k is valued at 0.3-0.5, and t RH is the time from tapping to pouring, in min.

[0057] Thus, based on the designed chemical composition, by controlling the final slag in the converter smelting, and controlling the Al addition amount based on the O content, combined with the tapping temperature control in the RH vacuum refining, the purity of the molten steel can be optimized, that is, the purity of the molten steel is greatly improved, thereby improving the internal quality of the pipeline steel, reducing the inclusion content, and further greatly reducing the residual stress of the final pipeline steel.

[0058] In the converter smelting process, the composition of the molten steel can be detected at the time of tapping to obtain the O mass fraction M O in the molten steel, and then the Al mass M Al added by the aluminum ingot is determined based on the value. For example, the detected O mass fraction is 80 ppm, and M Al in the formula M O = 0.0010M O + 0.09 is substituted with 80.

[0059] Here, the "Al mass M Al added by the aluminum ingot" refers to the mass of Al elements added by the aluminum ingot. M Al is kg / t, which refers to the Al mass per ton of molten steel.

[0060] Further, "t RH is the time from tapping to pouring" refers to the time from the tapping moment to the pouring start moment. The tapping moment refers to the moment when the RH vacuum refining process ends (i.e., the ladle car leaves the converter position), and the pouring start moment refers to the moment when the molten steel reaches a predetermined liquid level in the tundish of the continuous casting machine and the water nozzle is opened (so that the molten steel starts to be injected into the crystallizer).

[0061] The t RH determined by the tapping moment and the pouring start moment can be determined in advance based on the PLC control program in production, so that t RH is determined according to the production control, so that T RHXZ can be confirmed before tapping, and the tapping temperature T is accurately regulated.

[0062] Further, in the hot metal KR desulfurization process: the incoming hot metal temperature is ≥ 1300℃, and S≤0.0025%. Thus, the purity of the hot metal can be ensured, and the difficulty of subsequent steelmaking is reduced.

[0063] In order to reduce the production difficulty of the KR desulfurization process, the incoming hot metal temperature and composition of the hot metal KR desulfurization process can also be controlled, for example, the incoming hot metal temperature is ≥ 1395℃, P≤0.15% in the incoming hot metal, S≤0.045%, 0.3%≤Si≤0.6%, Mn≤0.40%.

[0064] Furthermore, in the hot metal KR desulphurization process, the slag grade can also be controlled at A level or above, so as to minimize the desulphurization slag into the converter and cause the sulfur element to rise.

[0065] Next, in the converter smelting process: open the ladle bottom argon blowing before tapping, the argon blowing pressure is 0.5~0.6MPa, the bright circle diameter of the liquid steel is 240~340mm; when the tapping reaches 3 / 4, the argon blowing pressure is reduced to 0.42~0.52MPa, and the bright circle diameter of the liquid steel is 120~220mm. This can promote the slag in the liquid steel to float up, improve the purity of the liquid steel, and also ensure that the terminal P content of the converter is low.

[0066] In addition, the converter smelting process can also use the slag retaining technology, for example, the amount of retained slag is 50~65% of the total slag of the previous furnace.

[0067] Furthermore, in the converter smelting process, the slag basicity is controlled at 2.0~2.2, and the total iron mass fraction in the molten slag is 14~17%.

[0068] Further, in the LF refining process: after the ladle is in place, first connect the bottom argon blowing and perform initial alloying, then perform main alloying and slag making under strong stirring; then perform power refining, during which the alloy is supplemented; finally, white slag is made, and the steel is tapped.

[0069] Specifically, in the LF refining process: after the ladle is in place, connect the bottom argon blowing and open the bottom argon blowing, the argon blowing pressure is 0.4~0.6MPa; during main alloying and slag making, large bottom blowing technology is used, specifically, the bottom argon blowing flow is 700~860NL / min, the argon blowing pressure is 0.50~0.60MPa, and the strong stirring time is ≤6min; during the alloy supplementing, medium bottom blowing technology is used, specifically, the bottom argon blowing flow is 650~800NL / min, the argon blowing pressure is 0.45~0.55MPa, and the strong stirring time is ≤4min; during the rest of the time, small bottom blowing technology is used, specifically, the bottom argon blowing flow is 300~400NL / min, the argon blowing pressure is 0.40~0.50MPa, and the bright circle diameter of the liquid steel is <150mm.

[0070] In this way, by optimizing the large, medium and small bottom blowing flow, the molten pool stirring is strengthened, the molten slag is promoted to melt quickly, and the composition and temperature of the liquid steel are uniform. At the same time, the small flow soft blowing operation is optimized, the inclusions are promoted to float up and be removed, the inclusion content in the liquid steel is further reduced, the inclusion morphology is improved, and the generation of large size inclusions is reduced.

[0071] Among them, the bottom argon blowing flow during main alloying and slag making is the largest, the bottom argon blowing flow during alloy supplementing is the second, and the bottom argon blowing flow during the rest of the time is relatively small.

[0072] Further, in the main alloying and slagging, a slag surface deoxidizer can be used for deoxidation, but is not limited thereto.

[0073] In the electric refining, lime, fluorite and other slag materials can be used to adjust the slag; in addition, calcium carbide can be added to diffuse and deoxidize the molten steel.

[0074] In addition, in the LF refining process: in the white slag making, the CaO / SiO2 in the slag is controlled to be 5.0±0.2, and the CaO / Al2O3 is controlled to be 1.8±0.5, and the white slag is kept for more than 15 minutes.

[0075] Preferably, in the LF refining process: the tapping temperature is 1598-1618℃.

[0076] More specifically, in the LF refining process: the tapping temperature corresponding to the start of pouring is 1608-1618℃, and the tapping temperature corresponding to the continuous pouring is 1598-1608℃.

[0077] Next, preferably, in the RH vacuum refining process: the T RH corresponding to the start of pouring is 1573-1583℃, and the T RH corresponding to the continuous pouring is 1563-1573℃. In this way, it is beneficial to obtain a more pure molten steel, reduce inclusions, and thus facilitate the residual stress control of the pipeline steel.

[0078] Further, in the RH vacuum refining process: the vacuum degree during vacuum degassing is ≤2.0mbar, the degassing time is 20-25min, titanium ferroalloy is added during alloying, the treatment time during net circulation is 8-10min, and the H mass percentage of the broken vacuum hydrogen is ≤1.5ppm. In this way, through deep degassing, the gas content and inclusion content in the molten steel can be further reduced, and the purity of the molten steel can be improved.

[0079] In addition, the RH vacuum refining process can prohibit the oxygen blowing operation throughout the process.

[0080] After the RH vacuum refining process, the ladle is placed on the rotary table for 13-18min before pouring, which can further promote the floating of inclusions.

[0081] Next, in the casting slab process: a continuous casting machine is used to prepare a continuous casting slab, and full protection casting is used, and the superheat is 26-30℃.

[0082] In this way, through full protection casting and low superheat casting, the internal quality of the continuous casting slab is improved, the center segregation, center porosity and inclusion content are reduced, and the quality of the continuous casting slab is ensured to meet the requirements of subsequent low residual stress steel plate production.

[0083] Specifically, the specific operation of full protection casting can include: using a large ladle long nozzle, a tundish covering agent, an immersion nozzle and argon sealing.

[0084] The large nozzle is connected to high-purity argon gas, and the flow rate is controlled at 180-220 L / min. The lower end of the nozzle is inserted into the molten steel in the tundish below 220-280 mm, effectively isolating air and stabilizing the steel flow.

[0085] At the same time, an independent argon gas channel is provided in the sliding nozzle mechanism area. The argon gas flow rate for the upper nozzle is 4-8 L / min, and the argon gas flow rate for the mechanism body is 4-12 L / min. Argon gas is also introduced into the stopper and submerged nozzle, and the flow rate is maintained at 4-8 L / min to prevent air from being sucked in and inclusions from being generated.

[0086] All argon sealing interfaces maintain a back pressure of not less than 0.05 Bar to ensure reliable sealing effect, thereby strictly controlling the nitrogen increase during continuous casting to be ≤0.0002% (i.e. 2 ppm).

[0087] In addition, the fluctuation amplitude of the molten steel level in the mold is controlled within ±2.0 mm to ensure uniform growth of the shell and reduce surface defects. The molten steel temperature in the tundish is accurately controlled in the range of 1536-1550°C. Through strict control of raw materials and baking system, the hydrogen content in the steel is not more than 2.0 ppm.

[0088] The tundish can use a double-layer covering structure: the bottom layer is a low-carbon alkaline covering agent for adsorbing floating inclusions and inhibiting molten steel oxidation; the upper layer is carbonized rice husk, which provides excellent thermal insulation performance, reduces temperature drop and maintains thermal stability.

[0089] Next, during casting, the fluctuation amplitude of the molten steel level in the mold is controlled to be not more than ±5 mm; an electromagnetic stirring device is configured in the secondary cooling zone, and the working frequency is set to 5.0-5.5 Hz, and the current intensity is controlled at 550-600 A, which effectively promotes the formation of equiaxed crystals and breaks the dendritic network, and inhibits composition segregation.

[0090] At the solidification end of the continuous casting, dynamic soft reduction technology is implemented, and the reduction position and amount are dynamically adjusted according to the real-time solidification state of the casting blank, and the total reduction amount is controlled between 5%-7% of the thickness of the casting blank, to compensate for solidification shrinkage, compact the center porosity and significantly reduce center segregation.

[0091] The fan-shaped segment taper of the continuous casting machine is designed to be 1.1%-1.3%. The mold cooling system uses high-precision water control parameters: the cooling water flow rate is maintained at 450-550 L / min, the water inlet temperature is controlled at 26-38°C, and the water outlet temperature difference is maintained at 5-8°C, to ensure stable heat conduction and uniform growth of the shell, providing reliable protection for the production of high-quality continuous casting blanks.

[0092] Next, in an embodiment of the present application, the pipeline steel is prepared by the smelting method, and has a thickness of 200-320 mm, for example, a continuous casting billet with a thickness of 220 mm or 320 mm.

[0093] The number of inclusions with a size of ≥15 μm on the cross section of the pipeline steel is ≤5 pieces / cm 2 The number of inclusions with a size of ≥10 μm and <15 μm is ≤15 pieces / cm 2 The number of inclusions with a size of ≥5 μm and <10 μm is ≤150 pieces / cm 2 That is, the number of inclusions is small, and the size is small, stress concentration is avoided, thereby facilitating the reduction of residual stress, and the subsequent processing needs of low residual stress can be met.

[0094] In addition, the center segregation of the pipeline steel is ≤0.5 level, the manganese segregation ratio of the cross section is ≤1.10, and the center porosity is ≤0.5 level. That is, the pipeline steel has excellent center quality, further avoiding stress concentration, thereby facilitating the reduction of residual stress.

[0095] Further, the A, B, C, D, and Ds type inclusions of the pipeline steel are all ≤1 level and the sum is ≤2.5 level, that is, the sum of the grades of the A, B, C, D, and Ds type inclusions is ≤2.5 level.

[0096] In the present application, the type and rating of non-metallic inclusions of the pipeline steel are detected according to GB / T 10561-2023 “Determination of Non-metallic Inclusions in Steel - Microscopic Examination Method of Standard Rating Diagram”; the size distribution and number per unit area of inclusions are quantified based on the same metallographic sample using an automatic image analysis system according to the size classification principle in the appendix of the standard. The center segregation level and the center porosity level are rated according to GB / T 1979-2001 “Structural Steel Macrostructure Defect Rating Diagram”; the manganese segregation ratio is obtained by sampling at the center line and 1 / 4 radius position of the cross section of the billet, preparing chemical analysis samples according to GB / T 20066-2006 “Sampling and Sample Preparation Method for Determination of Chemical Composition of Steel and Iron”, and calculating the manganese content after determining the manganese content according to GB / T 223.63-2022 “Determination of Manganese Content of Steel and Iron - Sodium (Potassium) Periodic Acid Spectrophotometric Method”.

[0097] Further, the pipeline steel prepared by the smelting method is a continuous casting billet (that is, a pipeline steel billet), which can be further processed to prepare a steel plate product (that is, a pipeline steel plate), and the steel plate product can have low residual stress based on the continuous casting billet.

[0098] In an embodiment, the continuous casting billet can be further processed to prepare a steel plate product with low residual stress by existing technologies, which will not be described again.

[0099] The residual stress of the steel plate product is ≤130 MPa.

[0100] More preferably, the residual stress of the steel plate product is not more than 120 MPa, or not more than 110 MPa, or not more than 90 MPa, or not more than 70 MPa, or not more than 60 MPa, or even 50 MPa.

[0101] For example, the residual stress of the steel plate is ≤130 MPa, and the residual stress difference of any two of the head, the middle, the tail, the upper portion and the lower portion is ≤50 MPa.

[0102] The residual stress difference of any two of the middle, the tail, the upper portion and the lower portion can be not more than 40 MPa, or not more than 35 MPa, or not more than 30 MPa, or even not more than 25 MPa.

[0103] The above introduces the technical spirit of the application and the basic situation of each embodiment. The following provides several embodiments to show the beneficial effects of the application. Of course, these embodiments are only a part of the numerous changed embodiments contained in the application, not all.

[0104] First, the pipeline steel billet is prepared by the smelting method of the application, specifically including: 1) hot metal KR desulfurization The outgoing hot metal temperature is ≥1300℃, and S≤0.0025%; 2) converter smelting The basicity of the final slag is controlled to be 3.8~4.3, the mass proportion of MgO is 8.0~10.0%, and the mass proportion of total iron is 15~17%, and the steel liquid temperature at tapping is 1650~1670℃; alloying and slagging are carried out in the order of lime, silicon-manganese alloy or medium-carbon manganese-iron alloy, aluminum ingot and lime at tapping, and the mass of Al added by the aluminum ingot is M Al Satisfies M Al =0.0010M O +0.90; 3) LF refining After the ladle is in place, the bottom blowing argon is connected first, and then the bottom blowing argon is started, and the argon blowing pressure is 0.4~0.6 MPa; Initial alloying is carried out, and then main alloying and slagging are carried out under strong stirring, the flow rate of the bottom blowing argon is 700~860 NL / min, the argon blowing pressure is 0.50~0.60 MPa, and the strong stirring time is ≤6 min; Then, power refining is carried out, and alloy is added during the period, the flow rate of the bottom blowing argon is 650~800 NL / min, the argon blowing pressure is 0.45~0.55 MPa, and the strong stirring time is ≤4 min; Finally, white slag is formed, CaO / SiO2=5.0±0.2, CaO / Al2O3=1.8±0.5 in the slag is controlled, and the white slag is kept for more than 15 min; then the molten steel is tapped, and the tapping temperature is 1598-1618℃; 4) RH vacuum refining Vacuum degassing, alloying, net circulation and hydrogen breaking are sequentially performed, and then the molten steel is tapped, and the tapping temperature T=T RH +T RHXZ , T RH is valued at 1563-1583℃, and T RHXZ =k×(t RH -10), k is valued at 0.3-0.5; 5) Casting blank A continuous casting machine is used to prepare a continuous casting blank, and full protection casting is used, and the superheat is 26-30℃.

[0105] Referring to Table 1, the chemical composition of the pipeline steel blank of several embodiments is shown. In the table, "-" indicates that the corresponding element is not added, and the content of the corresponding element is 0 or exists as an impurity element and is maintained at 0.02% or less.

[0106] [Table 1]

[0107] Each pipeline steel blank is sampled and detected, and the measurement results are shown in Table 2, wherein ">15", "[10, 15)", and "[5, 10)" respectively represent "the number of inclusions with a size ≥15 μm on the cross section", "the number of inclusions with a size ≥10 μm and <15 μm on the cross section", and "the number of inclusions with a size ≥5 μm and <10 μm on the cross section".

[0108] [Table 2]

[0109] It can be seen that the pipeline steel blanks of furnace numbers 1# to 7# of the present application are all prepared by the smelting method of the present application, all have the advantages of few and small inclusions, and further reduce center segregation and center porosity, avoid the discontinuity of the microstructure caused by center segregation, center porosity and large particle inclusions, and further avoid stress concentration and increase residual stress, so that the pipeline steel blank is beneficial to control the residual stress in the subsequent preparation of the steel plate product, to obtain a steel plate product with residual stress.

[0110] [Table 3]

[0111] For example, based on the pipeline steel billets with furnace numbers 1# to 7#, the difficulty of controlling residual stress is greatly reduced by adopting the conventional technology known in the art (for example, the conventional technology of continuous casting billet heating-two-stage hot rolling-cooling) for rolling, and the obtained steel plate products are sampled and tested for mechanical properties according to GB / T 2975-2018 “Steel and Steel Products—Position and Preparation of Specimens for Mechanical Testing” and GB / T 228.1-2021 “Metallic Materials—Tensile Testing—Part 1: Method of Test at Room Temperature”, and the residual stress of the steel plate products with yield strength of 245 MPa, 290 MPa, 360 MPa, 415 MPa, 450 MPa, 485 MPa, and 555 MPa obtained as shown in Table 3 can be as low as 50 MPa, 60 MPa, 70 MPa, 90 MPa, 100 MPa, 120 MPa, and 130 MPa, respectively, and the residual stress difference of any two of the head, middle, tail, upper, and lower parts can be as low as 25 MPa, 25 MPa, 30 MPa, 35 MPa, 35 MPa, 40 MPa, and 50 MPa, respectively.

Claims

1. A method of smelting a pipeline steel, characterized in that, The pipeline steel has the following chemical components in percentage by mass: C 0.03-0.15%, Si 0.09-0.26%, Mn 1.16-1.69%, Cr 0-0.29%, Ni 0-0.29%, Mo 0-0.18%, Cu 0-0.29%, Nb 0.009-0.066%, Ti 0.009-0.021%, Al 0.021-0.049%, P≤0.0150%, S≤0.0040%, O≤0.0025%, N≤0.0048%, H≤0.0002%, and the rest is iron and inevitable impurities. The smelting method comprises the following sequential processes: hot metal KR desulfurization process, converter smelting process, LF refining process, RH vacuum refining process and casting blank process. In the converter smelting process: the basicity of final slag is controlled to be 3.8~4.3, the mass percentage of MgO is 8.0~10.0%, the mass percentage of total iron is 15~17%, the temperature of molten steel when tapping is 1650~1670℃; alloying and slagging are carried out in the order of lime, silicon-manganese alloy or medium-carbon manganese-iron alloy, aluminum ingot and lime when tapping, the mass of Al added by the aluminum ingot is M Al =0.0010M Al +0.90, wherein M O is the mass percentage of O in the molten steel when tapping, the unit is ppm, and the unit of M O is kg / t; Al ​ In the RH vacuum refining process: sequentially performing vacuum degassing, alloying, clean circulation and breaking vacuum hydrogen fixation, and then tapping, the tapping temperature T = T RH + T RHXZ , T RH is valued at 1563~1583℃, T RHXZ = k x (t RH - 10), k is valued at 0.3~0.5, t RH is the time from tapping to pouring, in min.

2. The method of smelting a pipeline steel according to claim 1, characterized in that, The pipeline steel has the following chemical components in percentage by mass: C 0.03-0.15%, Si 0.09-0.26%, Mn 1.16-1.69%, Cr 0-0.29%, Ni 0-0.29%, Mo 0-0.18%, Cu 0-0.29%, Nb 0.009-0.066%, Ti 0.009-0.021%, Al 0.021-0.049%, P≤0.0150%, S≤0.0040%, O≤0.0025%, N≤0.0048%, H≤0.0002%, and the rest is iron and inevitable impurities. The pipeline steel has the following chemical components in percentage by mass: C 0.03-0.15%, Si 0.09-0.26%, Mn 1.16-1.69%, Cr 0-0.29%, Ni 0-0.29%, Mo 0-0.18%, Cu 0-0.29%, Nb 0.009-0.066%, Ti 0.009-0.021%, Al 0.021-0.049%, P≤0.0150%, S≤0.0040%, O≤0.0025%, N≤0.0048%, H≤0.0002%, and the rest is iron and inevitable impurities. The pipeline steel has the following chemical components in percentage by mass: C 0.03-0.15%, Si 0.09-0.26%, Mn 1.16-1.69%, Cr 0-0.29%, Ni 0-0.29%, Mo 0-0.18%, Cu 0-0.29%, Nb 0.009-0.066%, Ti 0.009-0.021%, Al 0.021-0.049%, P≤0.0150%, S≤0.0040%, O≤0.0025%, N≤0.0048%, H≤0.0002%, and the rest is iron and inevitable impurities. In the hot metal KR desulfurization process, the temperature of the outgoing hot metal is ≥1300℃, and S≤0.0025%.

3. The method of smelting a pipeline steel according to claim 1, characterized in that, In the converter smelting process, the bottom argon blowing of the ladle is started before tapping, the argon blowing pressure is 0.5-0.6MPa, and the bright circle diameter of the molten steel is 240-340mm; when the tapping reaches 3 / 4, the argon blowing pressure is reduced to 0.42-0.52MPa, and the bright circle diameter of the molten steel is 120-220mm.

4. The method of smelting a pipeline steel according to claim 1, characterized in that, In the LF refining process, after the ladle is in place, the bottom argon blowing is started and initial alloying is performed, then main alloying and slagging are performed under strong stirring, then electric refining is performed, during which alloy is added, and finally white slag is formed and the steel is tapped.

5. The method of smelting a pipeline steel according to claim 1, characterized in that, In the LF refining process:

6. The method of smelting a pipeline steel according to claim 5, characterized in that, After the bottom argon blowing is started, the bottom argon blowing is started, and the argon blowing pressure is 0.4-0.6MPa; During the main alloying and slagging, the flow rate of the bottom argon blowing is 700-860NL / min, the argon blowing pressure is 0.50-0.60MPa, and the strong stirring time is ≤6min; During the addition of alloy, the flow rate of the bottom argon blowing is 650-800NL / min, the argon blowing pressure is 0.45-0.55MPa, and the strong stirring time is ≤4min; The flow rate of the bottom argon blowing during the rest of the time is 300-400NL / min, the argon blowing pressure is 0.40-0.50MPa, and the bright circle diameter of the molten steel is <150mm. In the LF refining process, during the formation of white slag, the CaO / SiO2 in the slag is controlled to be 5.0±0.2, and the CaO / Al2O3 is controlled to be 1.8±0.5, and the white slag is maintained for 15min or more.

7. The method of smelting a pipeline steel according to claim 5, characterized in that, In the LF refining process, the tapping temperature is 1598-1618℃.

8. The method of smelting a pipeline steel according to claim 1, characterized in that, In the LF refining process, the tapping temperature corresponding to the start of pouring is 1608-1618℃, and the tapping temperature corresponding to continuous pouring is 1598-1608℃.

9. The method of smelting a pipeline steel according to claim 1, characterized in that, ​ T in the RH vacuum refining process: T corresponding to the start of tapping RH T in the RH vacuum refining process: T corresponding to the start of tapping RH T in the RH vacuum refining process: T corresponding to the start of tapping 10. The method of smelting a pipeline steel according to claim 1, characterized in that, In the RH vacuum refining process: the vacuum degree during vacuum degassing is ≤2.0 mbar, the degassing time is 20-25 min, titanium-iron alloy is added during alloying, the treatment time during net circulation is 8-10 min, and the H mass percentage of broken vacuum hydrogen is ≤1.5 ppm.

11. The method of smelting a pipeline steel according to claim 1, characterized in that, In the casting blank process: a continuous casting blank is prepared by using a continuous casting machine, full protection casting is adopted, and the superheat is 26-30 ℃.

12. A pipeline steel, characterized in that, The pipeline steel is prepared by using the smelting method in any one of claims 1 to 11; The pipeline steel is a billet with a thickness of 200 mm or more, and the number of inclusions with a size of 15 μm or more on the cross section is 5 or less per cm 2 The number of inclusions with a size of 10 μm or more and less than 15 μm is 15 or less per cm 2 The number of inclusions with a size of 5 μm or more and less than 10 μm is 150 or less per cm 2 .

13. A pipeline steel according to claim 12, characterized in that, The pipeline steel has a central segregation of ≤0.5 level, a manganese segregation ratio of the cross section of ≤1.10, a central porosity of ≤0.5 level, and A, B, C, D, and Ds type inclusion ratings of all ≤1 level and a total of ≤2.5 level.

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