Converter phosphorus-controlled smelting method for low-temperature steel and production method for high-quality low-temperature steel

Abstract

A converter phosphorus-controlled smelting method for low-temperature steel and a production method for high-quality low-temperature steel. The smelting method comprises the following steps: firstly, adding molten iron and scrap steel to a converter for blowing, wherein the alkalinity of the slag is 2.0-2.5, the total iron thereof is 25-35%, the lance position during the first minute is 1.8-2 m, with oxygen being blown at 28,000-30,000 Nm3 / h, then the lance position is controlled to rise and the oxygen blowing is controlled to to reduce twice at the same time, and blocking slag and tapping the steel after the blowing is completed, so as to obtain semi-steel molten steel; adding the semi-steel molten steel, a nickel plate and a second batch of scrap steel to the converter for deep blowing, which is divided into an early stage and a later stage of 3-4 minutes each. During the later stage of blowing, the alkalinity of furnace slag is 5.5-9.5, the temperature of the end point molten steel is 1,580-1,620ºC, P is less than or equal to 0.0035%, and the content of C is 0.02-0.05%.

Description

Converter phosphorus controlled smelting method for low-temperature steel and production method of high-quality low-temperature steel

[0001] This application claims priority to a Chinese patent application filed on March 29, 2024, with application number 202410377187.0, entitled “Converter phosphorus controlled smelting method for low-temperature steel and method for producing high-quality low-temperature steel,” the entire contents of which are incorporated herein by reference. Technical Field

[0002] The present invention relates to a converter phosphorus-controlled smelting method for low-temperature steel and a method for producing high-quality low-temperature steel, belonging to the technical field of steel material production and manufacturing. Background Art

[0003] The large-scale transportation and storage of liquefied natural gas places extremely stringent requirements on container materials. Among them, low-temperature steels of different grades and uses, represented by 9Ni, 5Ni, 3.5Ni, etc., can be used as natural gas storage and transportation container materials.

[0004] Nickel-based low-temperature steel refers to a series of nickel-containing special steels for welded structures that can serve at low temperatures of -70℃ to -196℃. It is mainly used in energy industries such as petroleum and chemical industries, and is used to manufacture various production and storage containers for liquefied petroleum gas, liquid ethylene, liquid oxygen, liquid nitrogen, liquefied natural gas, etc.

[0005] Due to its unique application, low-temperature steel requires extremely high cleanliness. Controlling the phosphorus content, for example, has been addressed in patent applications such as CN112680557A, CN101328529A, and CN112662839A. However, these technologies suffer from limitations such as suboptimal dephosphorization, operational difficulties, significant equipment loss, and long refining cycles. Therefore, efficient and stable control of phosphorus content is a crucial component of low-temperature steel production.

[0006] In addition, when producing low-temperature steel, in addition to the P element, it is also necessary to take into account the control of elements such as S, O, and N to obtain high-purity low-temperature steel. Summary of the Invention

[0007] The object of the present invention is to provide a converter phosphorus-controlled smelting method for low-temperature steel and a method for producing high-quality low-temperature steel.

[0008] To achieve the above-mentioned purpose, an embodiment of the present invention provides a converter phosphorus-controlled smelting method for low-temperature steel. The converter phosphorus-controlled smelting method comprises:

[0009] Add desulfurized molten iron and the first batch of scrap steel into the converter for blowing to produce slag with a basicity of 2.0-2.5 and a T.Fe content of 25-35%. The oxygen lance position of the first minute of blowing is 1.8-2.0m, and the top blowing oxygen volume is 28000-30000Nm 3 / h, from the second minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen amount is controlled to decrease twice simultaneously, argon is blown from the bottom during the entire blowing process and the molten steel temperature is controlled at 1380-1420°C. After the end of blowing, the bottom blowing of argon is continued for 1-3 minutes, and then the slag is blocked and the steel is tapped. The final molten steel temperature is 1360-1400°C, the C content is 3.3-3.8%, and the P content is ≤0.018%, thereby obtaining semi-steel molten steel;

[0010] After the converter is cleaned of slag and excess steel, the semi-steel molten steel, nickel plates and a second batch of scrap steel are added to the converter for blowing. The second batch of scrap steel accounts for 50-70% of the total weight of the first and second batches of scrap steel. The entire blowing process is divided into an early blowing process lasting 3-4 minutes and a late blowing process. During the early blowing process, lime and light-burned dolomite are added at the beginning of the blowing process. After blowing for 1 minute, pellets are added in 3-5 batches to maintain the T.Fe content in the slag at ≥13% throughout the early blowing process. After the early blowing process is completed, argon is continuously blown from the bottom for 1-2 minutes, and then the slag is partially drained and the late blowing process begins. The late blowing process produces slag with a basicity of 5.5-9.5. The terminal molten steel temperature of the late blowing process is 1580-1620° C., P≤0.0035%, and C content of 0.02-0.05%. The slag is drained and the steel is tapped to obtain the final steel smelted in the converter.

[0011] Compared with the prior art, the beneficial effect of one embodiment of the present invention is that: first, a portion of scrap steel (i.e., the first batch of scrap steel) and molten iron are preliminarily blown in a converter together to produce a slag with low to medium alkalinity (e.g., 2.0-2.5) and high oxidizability (e.g., T.Fe content of 25-35%), and the preliminarily blowing is divided into three stages: an initial low lance position + short-time (e.g., 1 minute) blowing with high oxygen content, a middle high lance position + low oxygen content stage, and a final higher lance position + lower oxygen content stage, and bottom blowing is performed throughout the process while maintaining the molten steel temperature at 1380-1420°C, thereby accelerating the oxidation heat release of Si elements, promoting efficient slag dephosphorization in the early stage of smelting, and realizing rapid melting of the slag, and then gradually achieving efficient dephosphorization by increasing the lance position twice and reducing the oxygen content twice, and the melting of the dephosphorized slag is very good, avoiding rephosphorization in the later stage; next , and then the remaining scrap steel (i.e., the second batch of scrap steel), nickel plates and semi-steel molten steel are poured back into the cleaned converter for deep blowing. In this stage, a higher degree of dephosphorization is achieved, and decarburization is also combined. The deep blowing is divided into two stages: early blowing and late blowing. In the early blowing, pellets are added in batches at 1 minute to maintain the T.Fe content in the slag at ≥13% during the entire early blowing period, thereby controlling the slag drying, greatly improving the dephosphorization and decarburization effects, and avoiding rephosphorization. In the late blowing stage, high-basicity slag is produced to promote slag dephosphorization and reduce late rephosphorization, thereby stabilizing the dephosphorization effect. In this way, through three small stages of preliminary blowing and two large stages of deep blowing, efficient and stable dephosphorization is achieved, and converter smelting of ultra-low phosphorus molten steel is achieved. Moreover, the production process can be completed by a single converter, with high efficiency, little equipment damage and low cost.

[0012] Preferably, the above “controlling the oxygen lance position to rise and the top-blowing oxygen volume to decrease twice from the 2nd minute to the end of blowing” means: the oxygen lance position from the 2nd to 3rd minute is 2.2-2.5m, the top-blowing oxygen volume is 26000-28000Nm 3 / h, after the 4th minute, the oxygen lance is positioned at 2.5-2.8m, and the top-blowing oxygen volume is 23,000-26,000 Nm 3 / h.

[0013] Preferably, the “bottom blowing argon throughout the blowing process and controlling the molten steel temperature at 1380-1420° C.” includes:

[0014] Before the second simultaneous control of the oxygen lance position rising and the top blowing oxygen volume decreasing, the bottom blowing argon volume is 1200~1500Nm 3 / h, then bottom blowing argon gas volume 600~800Nm 3 / h. Thus, the initial blowing is divided into three stages, and in each stage, the oxygen lance position and top-blowing oxygen volume are controlled while the bottom-blowing argon volume is also controlled. This results in the following three stages of initial blowing: an initial short-term blowing with a low lance position, high oxygen volume, and high argon volume (for example, the first minute); a middle stage with a high lance position, low oxygen volume, and high argon volume; and a final stage with an even higher lance position, lower oxygen volume, and low argon volume. This combination of lance position, oxygen volume, and argon volume enables rapid and precise adjustment of the molten steel's temperature, slagging rate, and composition, further improving the dephosphorization effect.

[0015] Preferably, in the "bottom blowing of argon continues for 1 to 3 minutes after the blowing is completed", the amount of bottom blowing argon is 800 to 1000 Nm 3 / h.

[0016] Preferably, in the step of "adding desulfurized molten iron and the first batch of scrap steel into the converter for blowing", a first slag-forming agent is used for slag-forming, and the first slag-forming agent includes: lime, pellets, and light-burned dolomite.

[0017] Preferably, in the early blowing process, the oxygen lance position in the first minute is 1.8-2.0 m, and the top blowing oxygen volume is 33,000-35,000 Nm 3 / h, then the oxygen lance is at 2.2-2.5m, and the top-blowing oxygen volume is 25000-28000Nm 3 / h; In the early stage of blowing, argon is blown throughout the whole process, and the amount of argon blown at the bottom of the first minute is 800~1000Nm 3 / h, then bottom blowing argon gas volume 600~800Nm 3 / h. Thus, in the early stages of blowing, a short-term blowing process with a low lance position and high oxygen volume is first performed, followed by a switch to a high lance position and low oxygen volume blowing process. This can further facilitate slag control, allowing for decarburization and dephosphorization while preventing rephosphorization. Furthermore, by combining the control of lance position and oxygen volume with the control of argon volume, dephosphorization slag removal can be more efficiently promoted.

[0018] Preferably, in the early blowing, a second slagging agent is used for slagging, and the second slagging agent includes: lime, pellets, and light-burned dolomite.

[0019] Preferably, lime, pellets and light-burned dolomite are added to form slag during the later blowing process.

[0020] Preferably, in the later stage of blowing, the oxygen lance is initially positioned at 1.8 to 2.0 m and the top blowing oxygen volume is 36,000 to 38,000 Nm 3 / h. From 4.5 minutes to 0.5 minutes before the end of blowing, the oxygen lance position and top-blowing oxygen volume are adjusted twice simultaneously. The first adjustment lowers the oxygen lance position and increases the top-blowing oxygen volume. The second adjustment raises the oxygen lance position and decreases the top-blowing oxygen volume. This can significantly reduce rephosphorization on top of efficient decarburization and dephosphorization, and stabilize the final dephosphorization effect.

[0021] Preferably, in the later stage of blowing, 4 minutes before the end of blowing, the oxygen lance position is lowered to 1.6-1.8m and the top blowing oxygen volume is increased to 38000-40000Nm 3 / h. One minute before the end of blowing, the oxygen lance position is adjusted to 2.2-2.5m for the second time, and the top blowing oxygen volume is reduced to 28,000-30,000 Nm 3 / h.

[0022] Preferably, in the later blowing process, the amount of argon blown at the bottom at the beginning is 1000-1200 Nm 3 / h, while lowering the oxygen lance position and increasing the top-blowing oxygen volume for the first time, increase the bottom-blowing argon volume to 1300-1500Nm 3 / h and maintain it for 1 to 2 minutes after the end of the post-blowing, then end the bottom argon blowing and pour the slag and tap the steel.

[0023] To achieve the above-mentioned purpose, an embodiment of the present invention provides a method for producing high-quality low-temperature steel. The production method comprises:

[0024] The molten iron is treated in the KR desulfurization equipment with an outlet temperature of 1350-1400°C and a sulfur content of ≤0.0010%;

[0025] The desulfurized molten iron is smelted into molten steel using the converter phosphorus control smelting method for low-temperature steel;

[0026] The molten steel obtained from converter smelting is transported to LF furnace for refining, including sequential heating, alloying, and slag formation, with the outlet temperature reaching 1610-1630℃. The molten steel from LF furnace is then transported to RH vacuum refining furnace for vacuum treatment and steelmaking.

[0027] The molten steel is hoisted from the RH vacuum refining furnace to the continuous casting platform for standing, and then poured to obtain continuous casting billets.

[0028] Thus, the production method of one embodiment of the present invention adopts the converter phosphorus control smelting method on the basis of the KR desulfurization-converter smelting-refining-continuous casting process route, and realizes efficient and stable dephosphorization through three small stages of preliminary blowing and two large stages of deep blowing, and realizes converter smelting of ultra-low phosphorus molten steel. Moreover, the production process can be completed by a single converter, with high efficiency, little equipment damage and low cost. Moreover, refining and continuous casting are carried out based on the low-phosphorus molten steel produced by the converter, which is conducive to the preparation of low-phosphorus continuous casting billets, so as to realize the preparation of high-purity continuous casting billets.

[0029] Preferably, in the tapping process of "slag pouring and steel tapping to obtain converter smelting final steel", deoxidation alloying and slag making are carried out successively, and the ladle bottom blowing flow rate during the tapping process is 400-600 NL / min, and the ladle bottom blowing flow rate is increased to 800-1000 NL / min after the tapping is completed;

[0030] When 10-20% of the steel is tapped, metallic aluminum, low-titanium and low-aluminum ferrosilicon and metallic manganese are added in sequence to deoxidize and alloy the molten steel. When 60-70% of the steel is tapped, all of them are added. Then, lime and calcium aluminate synthetic slag are added for slagging. When 80-90% of the steel is tapped, all of them are added.

[0031] Preferably, the calcium aluminate synthetic slag has a particle size of 10 to 50 mm and a chemical composition, by weight, of 50-60% CaO, 35-45% Al2O3, 2-4% MgO, ≤3% SiO2, and other unavoidable impurities. The 12CaO·7Al2O3 phase accounts for at least 80% of the composite phase of the calcium aluminate synthetic slag. Thus, while achieving efficient dephosphorization, the calcium aluminate synthetic slag, primarily composed of the low-melting-point 12CaO·7Al2O3 phase, can also achieve rapid and superior desulfurization, significantly reducing the sulfur content in molten steel.

[0032] Preferably, during the slag-making stage of refining in the LF furnace, the slag is deoxidized with calcium carbide and aluminum granules to adjust the slag composition to contain, by weight percentage, 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, less than 1.5% T.Fe+MnO, and other inevitable impurities;

[0033] When vacuum treatment is carried out in the RH vacuum refining furnace, the steel is first treated at a vacuum degree below 1.5 mbar for 15 to 20 minutes, then at a vacuum degree above 5 mbar for 10 to 20 minutes, and finally the steel is tapped.

[0034] Preferably, during the slag-making stage of refining in the LF furnace, calcium carbide and a low-carbon steel slag surface deoxidizer are used to deoxidize the slag to adjust the slag composition to 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, 2-5% T.Fe+MnO, and other inevitable impurity components by weight. Thus, in this embodiment, calcium carbide and a low-carbon steel slag surface deoxidizer are used to adjust the slag composition, especially to increase the T.Fe+MnO content from the traditional low percentage to 2-5%. The inventors have found that this can greatly reduce the nitrogen absorption of molten steel.

[0035] When the RH vacuum refining furnace is used for vacuum treatment, the vacuum degree is reduced to below 1.5 mbar within 4 minutes after the molten steel arrives at the station, and then metallic aluminum is added to the molten steel, and 2 to 4 kg / t of low-carbon steel slag surface deoxidizer is added to the ladle slag surface, and then the vacuum treatment is continued for 15 to 20 minutes; then the treatment is continued for 10 to 20 minutes at a vacuum degree above 5 mbar, and finally the steel is tapped; in this way, on the one hand, a deep vacuum is quickly drawn, and on the other hand, metallic aluminum is added to the molten steel under vacuum and a low-carbon steel slag surface deoxidizer is added to the slag surface. The RH vacuum refining process can also be used to deoxidize the molten steel at the same time. It can make use of the tiny bubbles formed by the C-O reaction of the molten steel, the argon bubbles blown in by the large flow rate lifting gas under deep vacuum, and the deep vacuum molten steel interface reaction to comprehensively degas and reduce the O and N content of the molten steel. Then, metal aluminum and low-carbon steel slag surface deoxidizer are added under vacuum conditions to avoid the oxidation and alloying of metal aluminum and the inhalation of air during slag making. In addition, under the condition of the slag composition design in the LF refining process, the treatment of the RH vacuum refining process can also avoid the high oxidizing problem caused by the slag composition design in the LF refining process.

[0036] Preferably, in the slag-making stage of refining in the LF furnace, 0.3-0.5 kg / t of calcium carbide and 1.0-2.0 kg / t of second calcium aluminate synthetic slag are added to the molten steel to adjust the slag composition to contain, in mass percentage, 50-55% CaO, 30-35% Al2O3, 1-3% CaF2, less than 3% SiO2, 4-6% MgO, less than 1% T.Fe+MnO and other inevitable impurity components.

[0037] Preferably, the components of the second calcium aluminate synthetic slag include, by mass percentage, 40-45% CaO, 10-15% Al2O3, 5-10% CaF2, less than 3% SiO2, 2-5% MgO, 5-10% CaC2, and 15-20% elemental aluminum, wherein the mass percentage of the phase 12CaO·7Al2O3 exceeds 30%, and the rest is a single phase or a composite phase of CaO, CaF2, SiO2, and MgO.

[0038] Preferably, when vacuum treatment is carried out in the RH vacuum refining furnace, the vacuum is firstly carried out at a vacuum degree of 200 mbar or more and a pressure of 80 to 100 Nm. 3 / h of the elevated gas flow for 3 to 5 minutes, and then at a vacuum degree below 1.5 mbar, at a pressure of 150 to 200 Nm 3 / h of lifting gas flow for 10 to 15 minutes, and then at a vacuum degree of more than 50mbar, at a rate of 150 to 200Nm 3 / h of lifting gas flow for more than 5 minutes, and finally break the air to tap the steel. In this way, first use high pressure (for example, more than 200mbar), low lifting gas flow (for example, the lifting gas flow is maintained at 80-100Nm 3 / h), and then use deep vacuum (specifically, for example, below 1.5mbar), high lifting gas flow (specifically, for example, the lifting gas flow is 150-200Nm 3 / h), and finally continue to treat with a higher pressure (for example, above 50mbar). In this way, the molten steel is first treated with a low circulation volume to promote the floating of inclusions while reducing the erosion of refractory materials. The molten steel is then treated with a deep vacuum and high circulation volume to quickly and more effectively remove inclusions (i.e., deoxidation). Finally, the circulation volume is reduced to reduce the inclusions introduced by the erosion of the refractory materials while taking away a large amount of inclusions. In summary, the total oxygen content is controlled, the purity of the molten steel is improved, and the low-temperature performance of the final low-temperature steel is guaranteed.

[0039] Preferably, after the vacuum is reduced to 500 mbar, calcium wire is fed into the ladle from an area near the downcomer at a rate of 1.0 to 1.5 m / t at a speed of 4 to 6 m / s. This addition of calcium wire to the molten steel further deoxidizes the steel and improves the composition of inclusions, thereby further enhancing the purity of the molten steel.

[0040] Preferably, after the vacuum level in the RH vacuum refining furnace drops below 1.5 mbar, a continuous treatment period of 10 to 15 minutes is followed by the addition of 0.5 to 1.0 m / t of calcium wire into the ladle from an area near the riser tube at a feed rate of 1 to 1.5 m / s. This method of feeding calcium wire near the riser tube, combined with a low feed rate and thin iron sheet, allows the calcium to enter the middle and upper portions of the molten steel and quickly come into contact with the oxygen element in the molten steel to form inclusions. This majority of the calcium element is rapidly drawn from the riser tube into the vacuum chamber of the RH vacuum refining furnace along with the molten steel, thereby achieving rapid deoxidation. The remaining small amount of calcium element can dissolve in the molten steel in the ladle and participate in the deoxidation cycle. The inventors have surprisingly discovered that this deoxidation method can further reduce the TO content by 2 to 5 ppm.

[0041] Preferably, the downcomer and riser of the RH vacuum refining furnace are symmetrically distributed in a mirror plane, and the ladle at the RH vacuum refining furnace has a fitting surface passing through the center of the riser and parallel to the mirror plane, and the "area close to the riser" is between the fitting surface and the mirror plane.

[0042] Preferably, the chemical composition of the continuous casting billet includes, by weight percentage: TO≤10ppm, P≤0.0045%, S≤0.0015%, N≤0.0025%, H≤1.5ppm.

[0043] Preferably, more than 90% of the oxide inclusions in the continuous casting billet have an Al2O3 content of ≥80%, more than 96% of the oxide inclusions have a size of ≤5 μm, and the largest oxide inclusion does not exceed 25 μm.

[0044] Preferably, the “molten steel is hoisted from the RH vacuum refining furnace to the continuous casting platform for standing, and then cast to obtain the continuous casting billet” includes:

[0045] The molten steel is cast using a slab continuous casting machine to obtain a billet with a thickness of 220 mm or 320 mm and a width of 1500 to 2300 mm; the superheat of the molten steel in the tundish is 30 to 50° C., a low-melting-point alkaline mold slag with a melting point of 1100 to 1200° C. is used in the crystallizer, the casting speed v is 1.05 to 1.35 m / min, and the taper of the crystallizer is 1.05 to 1.2%. In this way, based on the control of the casting speed v and the crystallizer taper, combined with the high superheat and low-melting-point alkaline mold slag, the melting rate of the mold slag has an excellent protective and lubricating effect on the shell of the billet, the thermal conductivity is guaranteed, and the cooling uniformity of the billet is improved;

[0046] After the billet leaves the crystallizer, it is cooled in the secondary cooling zone, which is divided into 10 sections according to the water volume. The water volume in the first section is less than 1 / 10 of the water volume in the crystallizer, the water volume in sections 2 to 4 is greater than that in the first section and decreases in sequence, the water volume in section 5 is less than that in section 1, the water volume in the inner arc of section 6 is less than that in the outer arc, and the water volume in the edges of the inner and outer arcs of sections 7 to 10 is less than that in the middle of the inner and outer arcs. In this way, the cooling intensity of the molten steel in the crystallizer is large (the water volume is large), and the cooling intensity is greatly reduced in section 1 after leaving the crystallizer (the water volume drops by more than 9 / 10), so that The shell of the billet is properly warmed up, and the water volume in the second to fourth sections is increased again, thereby preventing the shell from thinning during the warming process and causing steel leakage; at the same time, the water volume in the second to fourth sections is reduced in sequence, that is, the water volume in the second section > the water volume in the third section > the water volume in the fourth section. In this way, while ensuring the avoidance of steel leakage, it can also prevent fatigue overcooling that causes internal cracks in the billet during straightening, cutting and stacking; furthermore, starting from the sixth section, the water volume of the inner and outer arcs, and the water volume of the edges and the middle are precisely adjusted to ensure the uniformity of the billet cooling and prevent cracks caused by overcooling of the edges / corners;

[0047] The billet is pressed in sections 8 to 10, with reductions of 25%, 25%, and 50% allocated to each of the three sections. The reductions for billets with thicknesses of 220 mm and 320 mm are 3 to 5 mm and 6 to 8 mm, respectively. By regulating the billet's reduction and the distribution of the reduction, the internal quality and segregation of the billet can be optimized.

[0048] After the billets leave all the sectors, they are cut and stacked for slow cooling to obtain continuous casting billets.

[0049] Preferably, adjusting the taper of the crystallizer, the water volume of the crystallizer, and the water volume of sections 1 to 5 according to the change of the casting speed v includes:

[0050] When the pulling speed v is below 1.20m / min, the taper of the crystallizer is 1.1-1.2%; otherwise, the taper of the crystallizer is 1.05-1.15%. This can reduce the uneven cooling caused by the shrinkage of the billet during the cooling process.

[0051] When the casting speed v is below 1.20m / min, the water volume on the wide side of the crystallizer is 3600~3900NL / min, and the water volume on the narrow side is 390~420NL / min; otherwise, the water volume on the wide side of the crystallizer is 3900~4100NL / min, and the water volume on the narrow side is 420~450NL / min. In this way, on the one hand, the crystallizer adopts a large cooling intensity, and at the same time, the water volume of the crystallizer is adjusted according to the casting speed v, reducing the uneven cooling caused by the shrinkage of the billet during the cooling process.

[0052] The inner and outer arcs of sections 1 to 5 have a standard water volume corresponding to a pulling speed of v=1.05m / min and an enhanced water volume corresponding to a pulling speed of v>1.05m / min, wherein the enhanced water volume is the sum of the standard water volume and the incremental water volume ΔT=T×floor((v-1.05m / min) / 0.05m / min), and T is 5~10NL / min; wherein, the standard water volumes of the inner and outer arcs of sections 1 to 5 corresponding to a pulling speed of v=1.05m / min are 250~290NL / min, 575±25NL / min, 525±25NL / min, 475±25NL / min, and 175±25NL / min, respectively; in this way, controlling the water volumes of the inner and outer arcs of sections 1 to 5 according to the standard water volume and the incremental water volume can properly warm up the shell of the billet, and while ensuring that steel leakage is avoided, further avoid fatigue overcooling that causes internal cracks in the billet during straightening, cutting and stacking.

[0053] Preferably, the central carbon segregation of the continuous casting billet is below level 1.5, and the surface cracks are ≤1.5 mm.

[0054] Preferably, the production method further comprises:

[0055] The continuous casting billet is fed into a heating furnace for heating. The maximum temperature of the preheating section is 750-850°C, the maximum temperature of the heating section is 1100-1200°C, the maximum temperature of the soaking section is 1150-1200°C, the heating rate of the preheating section is 20-30°C / min, and the heating rate of the heating section is 30-50°C / min.

[0056] Hot rolling the heated continuous casting billet into a hot-rolled plate with a thickness of 5 to 60 mm, with a starting rolling temperature of 1030 to 1130° C. and a finishing rolling temperature of 800 to 850° C.;

[0057] The hot-rolled plate is naturally air-cooled to below 200°C, then subjected to a secondary quenching and tempering heat treatment, and then naturally cooled to room temperature to obtain a finished plate. The temperature of the first quenching is 800-900°C, and the temperature of the second quenching is 700-800°C. If h ≤ 20 mm, the tempering temperature is 600-620°C; if h ≥ 40 mm, the tempering temperature is 560-580°C; if 20 mm < h < 40 mm, the tempering temperature is > 580°C and < 600°C. The production method can be used to produce low-temperature steel plates with excellent mechanical properties, excellent low-temperature performance, and excellent surface quality.

[0058] Preferably, the first quenching time in the furnace is (2.0-2.2) min / mm×h+(5-10) min, the second quenching time in the furnace is (2.1-2.3) min / mm×h+(5-10) min, and the tempering time in the furnace is (2.5-2.8) min / mm×h+(5-10) min, where h is the thickness of the hot-rolled plate.

[0059] Preferably, the Z-direction cross-sectional shrinkage of the finished plate product is ≥70%, and the single values ​​of low-temperature impact energy at -80°C and -196°C are both ≥215J. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] FIG1 is a schematic flow diagram of a converter phosphorus-controlled smelting method for low-temperature steel according to one embodiment of the present invention;

[0061] FIG2 is a schematic flow diagram of a method for producing low-temperature steel according to an embodiment of the present invention;

[0062] FIG3 is a schematic diagram of a partial structure of an RH vacuum refining furnace in one embodiment of the present invention;

[0063] FIG4 is a cross-sectional view along line AA in FIG3 . DETAILED DESCRIPTION

[0064] One embodiment of the present invention provides a converter-controlled phosphorus smelting method for low-temperature steel, which can achieve efficient and stable control of phosphorus content in a single converter. Specifically, as shown in FIG1 , the converter-controlled phosphorus smelting method includes:

[0065] Add desulfurized molten iron and the first batch of scrap steel into the converter for blowing to produce slag with a basicity of 2.0-2.5 and a T.Fe content of 25-35%. The oxygen lance position of the first minute of blowing is 1.8-2.0m, and the top blowing oxygen volume is 28000-30000Nm 3 / h, from the second minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen amount is controlled to decrease twice simultaneously, argon is blown from the bottom during the entire blowing process and the molten steel temperature is controlled at 1380-1420°C. After the end of blowing, the bottom blowing of argon is continued for 1-3 minutes, and then the slag is blocked and the steel is tapped. The final molten steel temperature is 1360-1400°C, the C content is 3.3-3.8%, and the P content is ≤0.018%, thereby obtaining semi-steel molten steel;

[0066] After the converter is cleaned of slag and excess steel, the semi-steel molten steel, nickel plates and a second batch of scrap steel are added to the converter for blowing. The second batch of scrap steel accounts for 50-70% of the total weight of the first and second batches of scrap steel. The entire blowing process is divided into an early blowing process lasting 3-4 minutes and a late blowing process. During the early blowing process, lime and light-burned dolomite are added at the beginning of the blowing process. After blowing for 1 minute, pellets are added in 3-5 batches to maintain the T.Fe content in the slag at ≥13% throughout the early blowing process. After the early blowing process is completed, argon is continuously blown from the bottom for 1-2 minutes, and then the slag is partially drained and the late blowing process begins. The late blowing process produces slag with a basicity of 5.5-9.5. The terminal molten steel temperature of the late blowing process is 1580-1620° C., P≤0.0035%, and C content of 0.02-0.05%. The slag is drained and the steel is tapped to obtain the final steel smelted in the converter.

[0067] Thus, the converter phosphorus control smelting technology of one embodiment of the present invention is to first carry out preliminary blowing of a portion of scrap steel (i.e., the first batch of scrap steel) and molten iron in a converter to produce a slag with low-medium alkalinity (e.g., 2.0-2.5) and high oxidizability (e.g., T.Fe content 25-35%). The preliminary blowing is divided into three stages: the initial low gun position + short time (e.g., 1 minute) blowing with high oxygen content, the middle high gun position + low oxygen content stage, and the final higher gun position + lower oxygen content stage. The bottom blowing is carried out throughout the process and the temperature of the molten steel is maintained at 1360-1420°C. On the one hand, the oxidation heat release of Si element is accelerated, and the efficient slag dephosphorization in the early stage of smelting is promoted, and the slag is quickly melted. Then, the gun position is increased twice and the oxygen content is reduced twice, so that efficient dephosphorization is gradually achieved, and the melting of the slag after dephosphorization is very good, avoiding phosphorus reversion in the later stage. Next, the remaining scrap steel ( That is, the second batch of scrap steel), nickel plates and semi-steel molten steel are poured back into the cleaned converter for deep blowing. In this stage, a higher degree of dephosphorization is achieved, and decarburization is also combined. The deep blowing is divided into two stages: early blowing and late blowing. Lime and light-burned dolomite are added at the beginning of blowing. After blowing for 1 minute, pellets are added in batches to maintain the T.Fe content in the slag of the furnace at least 13% during the entire early blowing period. This achieves control of slag drying, greatly improves the dephosphorization and decarburization effects, and avoids rephosphorization. High-basicity slag is produced in the late blowing stage to promote slag dephosphorization and reduce late rephosphorization, thereby stabilizing the dephosphorization effect. In this way, through three small stages of preliminary blowing and two large stages of deep blowing, efficient and stable dephosphorization is achieved, and converter smelting of ultra-low phosphorus molten steel is realized. The production process can be completed by a single converter, with high efficiency, little equipment damage and low cost.

[0068] In this application, the basicity and total iron (i.e., T.Fe) content of the slag can be obtained in the following manner: during the smelting process, the slag is placed in a small iron sampling bucket, and the slag is crushed after cooling, and then measured by XRF fluorescence method to obtain the mass percentage of various components in the slag (including CaO, SiO2, T.Fe, Al2O3, MgO, etc.), that is, the content of each component; and the basicity is the ratio of the mass percentage of CaO / SiO2.

[0069] Prior to converter smelting, the total weight of the desulfurized molten iron, scrap steel, and nickel plates is roughly determined based on the target chemical composition of the low-temperature steel to be produced. For example, the weight of the desulfurized molten iron is 175-185 tons, and the total weight of the scrap steel and nickel plates is 20-25 tons. The weight of the scrap steel M1 can be, for example, 5-20 tons. In this embodiment, the total scrap steel weight M1 is divided into two batches: one batch, approximately 30-50% of the weight M1, known as the first batch of scrap steel, and the other batch, approximately 50-70% of the weight M1, known as the second batch of scrap steel. The first batch of scrap steel is initially blown together with the desulfurized molten iron. After tapping to obtain semi-steel molten steel, the second batch of scrap steel is blown together with the semi-steel molten steel. This eliminates the need to feed all the scrap steel into converter smelting at the beginning of converter smelting, as is conventional practice. This facilitates control of molten steel temperature and slag during converter smelting, thereby achieving precise phosphorus control.

[0070] Optionally, the chemical composition of the nickel plate includes, by weight percentage, Ni≥99%, P≤0.025%, S≤0.03%, and the remainder is Fe and other inevitable impurities.

[0071] The chemical composition of the scrap steel includes, by weight percentage, Si≤0.6%, Mn≤1.8%, Al≤0.08%, P≤0.02%, S≤0.01%, and the rest is Fe and other inevitable impurities.

[0072] Optionally, the size of a single piece of scrap steel does not exceed 500 mm×500 mm×200 mm. Of course, the size of a single piece of scrap steel in the present application is not limited thereto.

[0073] Furthermore, the above “from the 2nd minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen volume is controlled to decrease twice” means: from the 2nd to the 3rd minute, the oxygen lance position is 2.2 to 2.5m, and the top blowing oxygen volume is 26000 to 28000 Nm 3 / h, that is, at the second minute, the oxygen lance position is raised and the top-blowing oxygen volume is reduced for the first time; after the fourth minute, the oxygen lance position is 2.5-2.8m and the top-blowing oxygen volume is 23000-26000Nm 3 / h, that is to say, at the 4th minute, the oxygen lance position is controlled to rise and the amount of top-blown oxygen is controlled to decrease for the second time.

[0074] Preferably, the "bottom blowing argon during the whole blowing process and controlling the molten steel temperature at 1380-1420°C" includes: before the second simultaneous control of the oxygen lance position rising and the top blowing oxygen amount decreasing, the bottom blowing argon amount is 1200-1500Nm 3 / h, then the bottom blowing argon volume is 1200~1500Nm 3 / h. In this way, the initial blowing is divided into three stages, and in each stage, the oxygen lance position and the amount of top blowing oxygen are controlled while the amount of bottom blowing argon is also controlled. This results in the following three stages of initial blowing: an initial short-term blowing with low lance position, high oxygen amount, and high argon amount (for example, the first minute); a middle stage with high lance position, low oxygen amount, and high argon amount; and a final stage with even higher lance position, even lower oxygen amount, and low argon amount. This combination of lance position, oxygen amount, and argon amount enables rapid and precise adjustment of the molten steel's temperature, slagging rate, and composition, further improving the dephosphorization effect.

[0075] In the above “bottom blowing argon continues for 1 to 3 minutes after blowing is completed”, when the top blowing of oxygen is completed at the end of blowing, the oxygen lance can be lifted first. Of course, the bottom blowing of argon is still continued during this period, and the bottom blowing argon volume is 800 to 1000 Nm 3 / h, that is, starting from the end of top blowing oxygen, at 800~1000Nm 3 Continue to blow argon at the bottom at a flow rate of / h for about 1 to 3 minutes to achieve sufficient stirring of the molten steel, and then use slag blocking (such as slide plate slag blocking method) to tap the steel.

[0076] In the process of "adding desulfurized molten iron and the first batch of scrap steel into the converter for blowing", a first slag-forming agent is used for slag-forming. There are multiple options for the first slag-forming agent. For example, in an optional embodiment, the first slag-forming agent may include: lime, pellets, and light-burned dolomite.

[0077] Furthermore, after the "slag blocking and steel tapping to obtain semi-steel molten steel", the slag and excess steel in the converter can be cleaned first, and at the same time, the ladle containing the semi-steel molten steel can be transported from the bottom track of the converter to the bottom of the furnace platform of the modified converter, the platform steel plate is opened in advance, and then the ladle is lifted so that the semi-steel molten steel can be re-added into the cleaned converter to facilitate the subsequent deep smelting.

[0078] As mentioned above, the subsequent blowing process of the semi-steel molten steel is divided into an early blowing process lasting 3 to 4 minutes and a late blowing process. Preferably, in the early blowing process, the oxygen lance position of the first minute is 1.8 to 2.0 meters, and the top blowing oxygen volume is 33,000 to 35,000 Nm 3 / h, then the oxygen lance is at 2.2-2.5m, and the top-blowing oxygen volume is 25000-28000Nm 3 / h. In this way, in the early stage of blowing, short-term blowing with low lance position and high oxygen volume is carried out first, and then it is switched to blowing with high lance position and low oxygen volume. This can further promote the regulation of slag, decarbonize and dephosphorize while avoiding rephosphorization.

[0079] At the same time, during the initial blowing process, argon was blown from the bottom throughout the entire process, and the amount of argon blown from the bottom in the first minute was 800 to 1000 Nm 3 / h, then bottom blowing argon gas volume 600~800Nm 3 / h. In this way, combined with the control of the gun position and the amount of oxygen, by controlling the amount of argon, the removal of dephosphorization slag can be promoted more efficiently.

[0080] In addition, in the above statement “after the initial blowing is completed, the bottom blowing of argon is continued for 1 to 2 minutes, and then the slag is partially dumped”, the top blowing of oxygen is completed at the end of the initial blowing, and the oxygen lance can be raised first. Of course, the bottom blowing of argon is still continued during this period, and the bottom blowing argon volume is 600 to 800 Nm 3 / h, that is, from the second minute of the initial blowing to the end of the initial blowing (i.e., stop blowing oxygen) for 1 to 2 minutes, the bottom blowing argon volume is maintained at 600 to 800 Nm 3 / h; and then partially dumping the slag (for example, dumping 70-80% by weight of the steel slag).

[0081] Preferably, a second slag-forming agent is used for slag formation during the preliminary blowing. The first slag-forming agent has multiple options. For example, in an optional embodiment, the second slag-forming agent includes lime, pellets, or light-burned dolomite. In this application, for ease of understanding, the slag-forming agent used in the preliminary blowing is referred to as the second slag-forming agent, and the slag-forming agent used in the initial blowing of the converter is referred to as the first slag-forming agent. Of course, the chemical compositions of the first and second slag-forming agents may be the same or different.

[0082] Next, optionally, lime, pellets, and light-burned dolomite are added to the post-blowing process for slagging. Of course, the slagging agents used for slagging are not limited to these components.

[0083] In a preferred embodiment, in the later blowing process, the oxygen lance is initially positioned at 1.8 to 2.0 m and the top blowing oxygen volume is 36,000 to 38,000 Nm 3 / h, from 4.5min to 0.5min before the end of blowing, the oxygen lance position and top-blowing oxygen volume were adjusted twice simultaneously. The first adjustment was to lower the oxygen lance position and increase the top-blowing oxygen volume, and the second adjustment was to raise the oxygen lance position and lower the top-blowing oxygen volume. In other words, in this process, before the first adjustment was to lower the oxygen lance position and increase the top-blowing oxygen volume, the oxygen lance position was maintained at 1.8-2.0m and the top-blowing oxygen volume was maintained at 36000-38000Nm 3 / h, then through the first adjustment, the oxygen lance position is lowered and the top-blowing oxygen volume is increased. Then through the second adjustment, the oxygen lance position is raised and the top-blowing oxygen volume is reduced. In this way, on the basis of efficient decarburization and dephosphorization, phosphorus reversion can be greatly reduced and the final dephosphorization effect can be stabilized.

[0084] For example, in the later stage of blowing, 4 minutes before the end of blowing, the oxygen lance position is lowered to 1.6-1.8m and the top blowing oxygen volume is increased to 38000-40000Nm 3 / h. One minute before the end of blowing, the oxygen lance position is adjusted to 2.2-2.5m for the second time, and the top blowing oxygen volume is reduced to 28,000-30,000 Nm 3 / h. This is equivalent to maintaining the oxygen lance position at 1.6-1.8m and the top-blowing oxygen volume at 38,000-40,000 Nm 3 / h state for about 3 minutes, the oxygen lance position of the last minute is 2.2-2.5m, and the top blowing oxygen volume is 28000-30000Nm 3 Of course, the timing of the first and second adjustments does not necessarily have to occur 4 minutes and 1 minute before the end of blowing, but can be changed between 4.5 minutes and 0.5 minutes before the end of blowing.

[0085] In this application, the term "4.5 minutes to 0.5 minutes before the end of blowing" is used, for example, if the duration T of the late blowing is 6 minutes. Then, the duration T of the late blowing is 1.5 minutes to 5.5 minutes, which is the "4.5 minutes to 0.5 minutes before the end of blowing," and the duration T of the late blowing is 4 minutes before the end of blowing. It will be appreciated that the duration of each blowing period (e.g., the early blowing period and the late blowing period) can be determined according to methods known in the art (e.g., obtained based on a smelting system model or set by the manufacturer).

[0086] Preferably, in the later blowing process, the amount of argon blown at the bottom at the beginning is 1000-1200 Nm 3 / h, while lowering the oxygen lance position and increasing the top-blowing oxygen volume for the first time, increase the bottom-blowing argon volume to 1300-1500Nm3 / h and maintained until 1 to 2 minutes after the end of the post-blowing (top blowing of oxygen is ended at this time), and then the bottom blowing of argon is ended and the slag is poured out.

[0087] Furthermore, an embodiment of the present invention also provides a method for producing high-quality low-temperature steel, which can be used to produce high-purity low-temperature steel products, such as low-temperature steel billets or low-temperature steel plates.

[0088] Referring to Figure 2, the production method includes:

[0089] KR desulfurization process: The molten iron is treated in the KR desulfurization equipment, with the outlet temperature of 1320-1380℃ and the sulfur content of the outlet molten iron ≤0.0010%;

[0090] Converter smelting process: The desulfurized molten iron is subjected to molten steel smelting using the converter phosphorus control smelting method according to one embodiment of the present invention; that is, the converter smelting process is the same as the converter phosphorus control smelting method described above, and will not be repeated here;

[0091] Refining process: The molten steel obtained from converter smelting is transported to LF furnace for refining, including sequential heating, alloying, and slag formation, with the outlet temperature reaching 1610-1630℃. The molten steel from LF furnace is then transported to RH vacuum refining furnace for vacuum treatment and steelmaking.

[0092] Continuous casting process: The molten steel is hoisted from the RH vacuum refining furnace to the continuous casting platform for static placement, and then poured to obtain continuous casting billets.

[0093] Thus, the production method of one embodiment of the present invention adopts the converter phosphorus control smelting method based on the process route of KR desulfurization-converter smelting-refining-continuous casting, and performs refining and continuous casting based on the low-phosphorus molten steel produced by the converter, which is conducive to the preparation of low-phosphorus continuous casting billets, so as to achieve the preparation of high-purity continuous casting billets.

[0094] Specifically, in the KR desulfurization process, molten iron is treated in the KR desulfurization equipment. For example, molten iron from a blast furnace is fed into the KR desulfurization equipment for desulfurization. After desulfurization, the outlet temperature is 1320-1380°C, and the S content of the outlet molten iron is ≤0.0010%.

[0095] It is uniformly explained here that the "content" in this application generally refers to the weight percentage. For example, the "S content of the outgoing molten iron" refers to the weight percentage of S in the outgoing molten iron.

[0096] In one embodiment, the molten iron has a temperature of 1380-1460° C. upon arrival at the KR desulfurization equipment, a S content of ≤0.040%, a C content of 4.2-4.6%, a Si content of 0.30-0.65%, a P content of ≤0.10%, and the remainder being Fe and unavoidable impurities. Of course, the temperature and chemical composition of the molten iron upon arrival at the KR desulfurization equipment in this application are not limited thereto.

[0097] Furthermore, in the KR desulfurization process, after desulfurization is completed, slag is removed so that the surface area of ​​the molten iron does not exceed 15cm 2 The desulfurization slag agglomerates and the exposed area of ​​molten iron in the KR desulfurization equipment accounts for more than 97%.

[0098] The desulfurized molten iron obtained in the KR desulfurization process is put into a converter and smelted according to the converter phosphorus control smelting method to obtain converter smelting final steel and then hoisted to the LF furnace for refining.

[0099] Among them, in the converter smelting process, preferably, in the tapping process of "pouring slag and tapping steel to obtain the final steel of converter smelting" mentioned above, deoxidation alloying and slag making are carried out successively, and the ladle bottom blowing flow rate during the tapping process is 400~600NL / min, and the ladle bottom blowing flow rate at the end of tapping is increased to 800~1000NL / min.

[0100] Specifically, when 10-20% of the steel is tapped, metallic aluminum, low-titanium and low-aluminum ferrosilicon, and metallic manganese are added in sequence to deoxidize and alloy the molten steel, and all of the addition is completed when 60-70% of the steel is tapped. The chemical composition of the metallic aluminum, in terms of weight percentage, includes: Al ≥ 99%, P ≤ 0.010%, S ≤ 0.015%, and other unavoidable impurities. The chemical composition of the low-titanium and low-aluminum ferrosilicon, in terms of weight percentage, includes: Si 75-79%, P ≤ 0.015%, S ≤ 0.006%, and the remainder is Fe and unavoidable impurities. The chemical composition of the metallic manganese, in terms of weight percentage, includes: Mn ≥ 99%, P ≤ 0.008%, S ≤ 0.005%, and other unavoidable impurities.

[0101] After the addition of metallic aluminum, low-titanium, low-aluminum ferrosilicon, and metallic manganese, lime and calcium aluminate synthetic slag are added to the molten steel for slagging. This addition is complete when 80-90% of the steel has been tapped. The calcium aluminate synthetic slag has a particle size of 10-50 mm and a chemical composition, by weight, of 50-60% CaO, 35-45% Al2O3, 2-4% MgO, ≤3% SiO2, and other unavoidable impurities. The 12CaO·7Al2O3 phase accounts for more than 80% of the composite phase in the calcium aluminate synthetic slag. This allows for efficient dephosphorization, while also achieving rapid and superior desulfurization, significantly reducing the sulfur content in the molten steel.

[0102] Next, in the refining process, the molten steel obtained from the converter is refined, including the first LF refining and the subsequent RH vacuum refining. Regarding LF refining and RH vacuum refining, this application provides three implementation methods, which are introduced below.

[0103] [First embodiment]

[0104] In this embodiment, LF refining involves refining molten steel in an LF furnace, sequentially undergoing a heating phase, an alloying phase, and a slag-forming phase. Specifically, upon arrival at the LF furnace, the molten steel undergoes temperature measurement and sampling at the inlet station, followed by heating to adjust the temperature (i.e., the heating phase). Next, alloying is performed by adding alloying materials and carbon powder based on the chemical composition measured during inlet sampling (i.e., the alloying phase). Finally, slag-forming materials are added to the molten steel for slag-forming (i.e., the slag-forming phase).

[0105] The tapping temperature of LF refining is 1610~1630℃.

[0106] In the slag-making stage, calcium carbide and aluminum particles are used to deoxidize the slag to adjust the slag composition to contain 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, less than 1.5% T.Fe+MnO and other inevitable impurity components in weight percentage.

[0107] Furthermore, in LF refining, the bottom blowing argon of the ladle is started after the molten steel enters the station. The bottom blowing argon flow rates in the power-on heating stage, alloying stage, and slag-making stage are 400-500NL / min, 300-400NL / min, and 500-600NL / min, respectively. The bottom blowing argon flow rates in the remaining stages are 150-250NL / min, respectively.

[0108] In LF refining, when the chemical composition of the slag and the temperature of the molten steel meet the target, the molten steel is transported to the RH vacuum refining furnace for RH vacuum refining.

[0109] In RH vacuum refining, that is, when vacuum treatment is carried out in an RH vacuum refining furnace, the steel is first treated at a vacuum degree below 1.5 mbar for 15 to 20 minutes, then at a vacuum degree above 5 mbar for 10 to 20 minutes, and finally the steel is tapped.

[0110] For example, referring to FIG3 , a first-stage steam pump E1 , a second-stage steam pump E2 , a third-stage steam pump E3 , a fourth-stage steam pump E4 , and two-stage water circulation pumps W1 & W2 are sequentially arranged in the vacuum exhaust pipeline of the vacuum chamber 10 of the RH vacuum refining furnace.

[0111] Within 3 minutes after the molten steel arrives at the station, open the two-stage water circulation pumps W1 & W2, the fourth-stage steam pump E4, the third-stage steam pump E3, the second-stage steam pump E2, and the first-stage steam pump E1 in sequence, and reduce the vacuum degree to below 1.5mbar within 4 minutes after the molten steel arrives at the station; and the lifting gas flow rate within 3 minutes after the molten steel arrives at the station is 100-120Nm 3 / h, the lifting gas flow rate after 3 minutes is 230~250Nm 3 / h; After the treatment is continued for 15 to 20 minutes at a vacuum degree below 1.5 mbar, the two-stage vacuum pump of the vacuum chamber is turned off. After the vacuum chamber rises to above 5 mbar, the treatment is continued for 10 to 15 minutes, during which the gas flow rate is increased to 180 to 200 Nm 3 / h; then break through the air and produce steel.

[0112] [Second embodiment]

[0113] In this embodiment, LF refining involves refining molten steel in an LF furnace, sequentially undergoing a heating phase, an alloying phase, and a slag-forming phase. Specifically, upon arrival at the LF furnace, the molten steel undergoes temperature measurement and sampling at the inlet station, followed by heating to adjust the temperature (i.e., the heating phase). Next, alloying is performed by adding alloying materials and carbon powder based on the chemical composition measured during inlet sampling (i.e., the alloying phase). Finally, slag-forming materials are added to the molten steel for slag-forming (i.e., the slag-forming phase).

[0114] The tapping temperature of LF refining is 1610~1630℃.

[0115] During the slag-making stage, 0.15-0.35 kg / t of calcium carbide and a low-carbon steel slag surface deoxidizer are used to deoxidize the slag, adjusting the slag composition to 50-55% by weight of CaO, 30-35% of Al2O3, 3-6% of SiO2, 4-7% of MgO, 2-5% of T.Fe+MnO, and other unavoidable impurities. The T.Fe+MnO content is further preferably 3-5%. Thus, compared to the first embodiment described above, this embodiment uses calcium carbide and a low-carbon steel slag surface deoxidizer to adjust the slag composition, particularly increasing the T.Fe+MnO content from the traditional low percentage to 2-5%. The inventors have found that this can significantly reduce nitrogen absorption by molten steel.

[0116] Furthermore, during LF refining, bottom argon blowing is initiated in the ladle after the molten steel enters the station. The bottom blowing argon flow rates during the power-up, temperature-raising, alloying, and slag-forming stages are 400-500 NL / min, 300-400 NL / min, and 500-600 NL / min, respectively. The bottom blowing argon flow rates during the remaining stages are 150-250 NL / min. This, by focusing on small and medium-sized bottom blowing to avoid violent churning of the molten steel, combined with the weak deoxidation technology used in the previous converter smelting process and the slag composition design in the LF refining process, can further reduce nitrogen absorption.

[0117] In LF refining, when the chemical composition of the slag and the temperature of the molten steel meet the target, the molten steel is transported to the RH vacuum refining furnace for RH vacuum refining.

[0118] In RH vacuum refining, that is, when vacuum treatment is carried out in the RH vacuum refining furnace, the vacuum degree is first reduced to below 1.5mbar, metallic aluminum is added to the molten steel, and 2-4kg / t of low-carbon steel slag surface deoxidizer is added to the ladle slag surface, and then the vacuum treatment is continued for 15-20 minutes; then the treatment is continued at a vacuum degree of more than 5mbar for 10-20 minutes, and finally the air is broken to tap the steel.

[0119] Specifically, for example, referring to FIG3 , the vacuum exhaust pipeline of the vacuum chamber 10 of the RH vacuum refining furnace includes a first-stage steam pump E1 , a second-stage steam pump E2 , a third-stage steam pump E3 , a fourth-stage steam pump E4 , and two-stage water circulation pumps W1 & W2 .

[0120] Within 3 minutes after the molten steel arrives at the station, open the two-stage water circulation pumps W1 & W2, the fourth-stage steam pump E4, the third-stage steam pump E3, the second-stage steam pump E2, and the first-stage steam pump E1 in sequence, and reduce the vacuum degree to below 1.5mbar within 4 minutes after the molten steel arrives at the station; and the lifting gas flow rate within 4 minutes after the molten steel arrives at the station is 100-120Nm 3 / h, the lifting gas flow rate after 4 minutes is 230~250Nm3 / h; after the vacuum degree drops to 1.5mbar, add metallic aluminum to the molten steel, add 2-4kg / t of low-carbon steel slag surface deoxidizer to the ladle slag surface, and then continue vacuum treatment for 15-20min; then turn off the two-stage vacuum pump of the vacuum chamber, and after the vacuum chamber rises to above 5mbar, reduce the lifting gas flow rate to 180-200Nm 3 / h; then continue the process for 10 to 15 minutes, then break the air and tap the steel.

[0121] In this way, on the one hand, by quickly drawing a deep vacuum, and on the other hand, by adding metallic aluminum to the molten steel and adding a low-carbon steel slag surface deoxidizer to the slag surface under a vacuum state to simultaneously deoxidize, it is possible to utilize the tiny bubbles formed by the C~O reaction in the molten steel, the argon bubbles blown in by the large-flow lifting gas under deep vacuum, and the deep vacuum molten steel interface reaction to comprehensively and significantly degas, thereby reducing the O and N content in the molten steel. Then, metallic aluminum and the low-carbon steel slag surface deoxidizer are added under vacuum conditions to avoid the oxidation and alloying of metallic aluminum and the inhalation of air during slag formation. In addition, under the condition of the slag composition design in the LF refining process, the treatment of the RH vacuum refining process can also avoid the problem of high oxidizability caused by the slag composition design in the LF refining process.

[0122] In this embodiment, the specific components of the low carbon steel slag surface deoxidizer used in LF refining and RH vacuum refining are CaO 25-35%, Al2O3 10-20%, CaF2 5-10%, metallic aluminum 45-55%, and other inevitable components by weight percentage.

[0123] [Third embodiment]

[0124] In this embodiment, LF refining involves refining molten steel in an LF furnace, sequentially undergoing a heating phase, an alloying phase, and a slag-forming phase. Specifically, upon arrival at the LF furnace, the molten steel undergoes temperature measurement and sampling at the inlet station, followed by heating to adjust the temperature (i.e., the heating phase). Next, alloying is performed by adding alloying materials and carbon powder based on the chemical composition measured during inlet sampling (i.e., the alloying phase). Finally, slag-forming materials are added to the molten steel for slag-forming (i.e., the slag-forming phase).

[0125] The tapping temperature of LF refining is 1610~1630℃.

[0126] Preferably, in the slag-making stage, 0.3-0.5 kg / t of calcium carbide and 1.0-2.0 kg / t of calcium aluminate synthetic slag are added to the molten steel to adjust the slag composition to contain, in mass percentage, 50-55% CaO, 30-35% Al2O3, 1-3% CaF2, less than 3% SiO2, 4-6% MgO, less than 1% T.Fe+MnO and other inevitable impurity components.

[0127] The calcium aluminate synthetic slag used in this process is different from the calcium aluminate synthetic slag used in the converter tapping process. For clarity, the calcium aluminate synthetic slag used in this process is referred to as the second calcium aluminate synthetic slag in this application. Specifically, the second calcium aluminate synthetic slag comprises, by mass percentage, 40-45% CaO, 10-15% Al2O3, 5-10% CaF2, up to 3% SiO2, 2-5% MgO, 5-10% CaC2, and 15-20% elemental aluminum. The mass percentage of the 12CaO·7Al2O3 phase exceeds 30%, with the remainder being a single phase or a composite phase of CaO, CaF2, SiO2, and MgO.

[0128] In this way, the LF refining process uses a large amount of slag and strong deoxidation synthetic slag for rapid deoxidation and slagging, which can reduce the total oxygen content of molten steel, reduce adsorbed inclusions, and improve the purity of molten steel.

[0129] Furthermore, during the LF refining process, argon is blown from the bottom throughout the entire process. The flow rate of the bottom blowing argon during the power-on and heating period is 400-500NL / min, the flow rate of the bottom blowing argon during the alloying period is 300-400NL / min, the flow rate of the bottom blowing argon during the slag forming period is 500-600NL / min, and the flow rate of the bottom blowing argon during the rest of the time is 150-250NL / min.

[0130] In LF refining, when the chemical composition of the slag and the temperature of the molten steel meet the target, the molten steel is transported to the RH vacuum refining furnace for RH vacuum refining.

[0131] In RH vacuum refining, that is, when vacuum treatment is carried out in an RH vacuum refining furnace, the steel is first treated at a vacuum degree above 200 mbar for 3 to 5 minutes, then continuously treated at a vacuum degree below 1.5 mbar for 10 to 15 minutes, and then continuously treated at a vacuum degree above 50 mbar for more than 5 minutes, and finally the air is broken to tap the steel.

[0132] Specifically, as shown in FIG3 , a first-stage steam pump E1 , a second-stage steam pump E2 , a third-stage steam pump E3 , a fourth-stage steam pump E4 , and two-stage water circulation pumps W1 & W2 are sequentially arranged in the vacuum exhaust pipeline of the vacuum chamber 10 of the RH vacuum refining furnace.

[0133] After the molten steel is transported to the RH vacuum refining furnace, open the two-stage water circulation pump W1 & W2 within 1 minute to maintain the vacuum degree above 200mbar and the lifting gas flow rate at 80-100Nm 3 / h, process for 3 to 5 minutes; then, turn on the fourth-stage steam pump E4, the third-stage steam pump E3, the second-stage steam pump E2, and the first-stage steam pump E1 in sequence, and increase the lifting gas flow rate to 150 to 200 Nm 3 / h, after the vacuum degree drops below 1.5mbar, continue the treatment for 10 to 15 minutes; then turn off the third-stage steam pump E3, the second-stage steam pump E2, and the first-stage steam pump E1, adjust the vacuum degree to above 50mbar, and then treat for more than 5 minutes. During this period, the gas flow rate remains unchanged, that is, maintained at 150 to 200Nm 3 / h, finally breaking through the air and producing steel.

[0134] Thus, firstly adopt high pressure (specifically, above 200 mbar) and low lifting gas flow (specifically, the lifting gas flow is maintained at 80-100 Nm 3 / h), and then use deep vacuum (specifically, for example, below 1.5mbar), high lifting gas flow (specifically, for example, the lifting gas flow is 150-200Nm 3 / h), and finally continue to treat with a higher pressure (for example, above 50mbar). In this way, the molten steel is first treated with a low circulation volume to promote the floating of inclusions while reducing the erosion of refractory materials. The molten steel is then treated with a deep vacuum and high circulation volume to quickly and more effectively remove inclusions (i.e., deoxidation). Finally, the circulation volume is reduced to reduce the inclusions introduced by the erosion of the refractory materials while taking away a large amount of inclusions. In summary, the total oxygen content is controlled, the purity of the molten steel is improved, and the low-temperature performance of the final low-temperature steel is guaranteed.

[0135] Referring to Figure 3 , the lifting gas can be introduced through a gas lift pipe 11, which is connected to a riser pipe 32 below the vacuum chamber 10. More specifically, two pipelines are provided below the vacuum chamber 10 for circulating molten steel. One of the pipelines, designated as the riser pipe 32, connects to the gas lift pipe 11 and serves as a passage for the molten steel to flow upward from the ladle into the vacuum chamber. The other pipeline, designated as the downcomer 31, serves as a passage for the molten steel to return from the vacuum chamber to the ladle.

[0136] Furthermore, in one embodiment, referring to FIG4 , after the vacuum level is reduced to 500 mbar, calcium wire is fed from area S1 of the downcomer 31 near the bottom of the vacuum chamber 10 at a rate of 1.0 to 1.5 m / t at a speed of 4 to 6 m / s. In this manner, by feeding calcium wire into the molten steel, calcium is used to further deoxidize the molten steel and improve the composition of inclusions, thereby further enhancing the purity of the molten steel.

[0137] As a more preferred alternative embodiment, referring to Figure 4 , after the vacuum level drops below 1.5 mbar, calcium wire is fed into the molten steel at a rate of 0.5 to 1.0 m / t at a speed of 1 to 1.5 m / s at a location S2 located closer to the riser 32 than the downcomer 31 below the vacuum chamber 10, for a period of 10 to 15 minutes. It will be appreciated that the minimum distance between any point in area S2 and the riser 32 is less than the minimum distance between any point in area S2 and the downcomer 31. In this way, the implementation method of feeding the calcium wire in the area S2 near the riser 32, combined with a low feeding speed + thin iron sheet, allows Ca to be fed into the middle and upper part of the molten steel and quickly come into contact with the O element in the molten steel to form inclusions. This majority of the Ca element is quickly drawn into the interior of the vacuum chamber 10 along with the molten steel from the riser 32, thereby achieving rapid deoxidation, while the remaining small portion of the Ca element can be dissolved in the molten steel in the ladle 20 and participate in the molten steel deoxidation cycle. The inventors have unexpectedly found through research that the deoxidation effect of this method exceeds the method of feeding the calcium wire in the area S1 near the downcomer 31. For example, the TO content can be further reduced by 2 to 5 ppm compared to the method of feeding the calcium wire in the area S1.

[0138] The inner part of the calcium wire is passivated metal calcium powder with a Ca content of more than 95%, and the outer layer is an iron sheet with an outer diameter of 8 to 10 mm and a thickness of 1 to 2 mm.

[0139] The chemical composition of the iron sheet of the calcium wire includes, by mass percentage, Al: 0.005-0.035%, Si: 0.2-0.4%, Mn: 0.3-0.5%, P≤0.001%, S≤0.003%, and the rest is Fe and unavoidable impurities.

[0140] Here, the dividing plane M between regions S1 and S2 is determined by the relationship between the minimum distances between each location within region S1 and the ascending tube 32 and the minimum distance between each location and the descending tube 31. For example, the minimum distance between any location in region S1 and the ascending tube 32 is greater than the minimum distance between any location in region S2 and the ascending tube 32; the minimum distance between any location in region S2 and the descending tube 31 is less than the minimum distance between any location on dividing plane M and the ascending tube 32. Alternatively, dividing plane M represents the plane of symmetry (or mirror image) between the ascending tube 32 and the descending tube 31.

[0141] Furthermore, referring to FIG4 , a fitting surface T is defined in the ladle 20. The fitting surface T passes through the center O of the riser 32 and is parallel to the mirror plane M. The region S2 is preferably a region S21 between the fitting surface T and the mirror plane M, such as the portion marked with oblique lines in FIG4 . In other words, the calcium wire is fed between the fitting surface T and the mirror plane M. This further enhances the deoxidation effect.

[0142] Next, in the continuous casting process, the molten steel is hoisted from the RH vacuum refining furnace to the continuous casting platform for standing, and then poured to obtain a continuously cast billet.

[0143] Specifically, the molten steel is hoisted from the RH vacuum refining furnace to the continuous casting platform and allowed to stand for 10 to 20 minutes before pouring. Of course, the standing time is not limited to this.

[0144] In practice, a slab continuous caster can be used for casting, producing billets with a thickness of 220 mm or 320 mm and a width of 1500 to 2300 mm. Specifically, for example, multiple ladles containing molten steel can be sequentially poured into a tundish, which is then poured into the mold of the slab continuous caster and then pulled out of the mold to produce billets. The resulting billets are 220 mm thick and 1500 to 2300 mm wide, or 320 mm thick and 1500 to 2300 mm wide.

[0145] In a preferred embodiment, the superheat of the molten steel in the tundish is 30-50°C, the crystallizer uses a low-melting-point alkaline protective slag with a melting point of 1100-1200°C, the pulling speed v is 1.05-1.35 m / min, and the taper of the crystallizer is 1.05-1.2%. In this way, on the basis of controlling the pulling speed v and the taper of the crystallizer, combined with high superheat and low-melting-point alkaline protective slag, the melting rate of the protective slag has excellent protection and lubrication effects on the shell of the billet, the thermal conductivity effect is guaranteed, and the cooling uniformity of the billet is improved.

[0146] The combined mass percentage of Na2O and KO in the low-melting-point alkaline mold flux is 8-10%, with Na2O accounting for 50-80% of the total of Na2O and KO. This high content of alkaline oxides (e.g., Na2O and KO) in the low-melting-point alkaline mold flux protects the billet's crust, provides lubrication, and ensures thermal conductivity, further enhancing cooling uniformity.

[0147] In one specific embodiment, the basicity (i.e., CaO / SiO2) of the low-melting-point basic mold slag is 1.2-1.6. In addition to CaO, SiO2, Na2O, and K2O, its chemical composition, by mass percentage, includes: Al2O3: 2-5%, MgO: 1-3%, Fe2O3: 0.6-1.5%, F: 8-10%, C: 4-6%, and H2O ≤ 0.2%. In addition to the aforementioned components, the remaining components of the low-melting-point basic mold slag are unavoidable impurities.

[0148] The low-melting-point alkaline mold slag has a particle size of 1 to 3 mm, accounting for more than 96% by mass, while particles less than 1 mm and larger than 3 mm account for less than 4% by mass. This particle size ensures the melting rate of the low-melting-point alkaline mold slag in the crystallizer, thereby improving its protective effect.

[0149] In one embodiment, the consumption of low-melting-point alkaline protective slag in the crystallizer is 0.4-0.55 kg / t, that is, the low-melting-point alkaline protective slag is added to the crystallizer according to the ratio of adding 0.4-0.55 kg of the low-melting-point alkaline protective slag per ton of molten steel.

[0150] After exiting the crystallizer, the billet is cooled in a secondary cooling zone, which is divided into 10 sections according to different water volumes.

[0151] Among them, the water volume in the first section is less than 1 / 10 of the water volume in the crystallizer, the water volume in sections 2 to 4 is greater than that in section 1 and decreases in sequence, the water volume in section 5 is less than that in section 1, the water volume in the inner arc of section 6 is less than that in the outer arc, and the water volume in the edges of the inner and outer arcs of sections 7 to 10 is less than that in the middle of the inner and outer arcs; the billet is pressed down in sections 8 to 10 and the three sections are respectively allocated 25%, 25% and 50% of the pressing amount, and the pressing amount of billets with thicknesses of 220mm and 320mm are 3 to 5mm and 6 to 8mm respectively; after the billet leaves all the fan-shaped segments, it is cut and stacked for slow cooling to obtain a continuously cast billet.

[0152] In this way, on the one hand, the cooling intensity of the molten steel in the crystallizer is high (the amount of water is large). After leaving the crystallizer, the cooling intensity is greatly reduced in the first section (the amount of water drops by more than 9 / 10), so that the shell of the billet is properly warmed up, and the amount of water in the second to fourth sections is increased again, thereby preventing the shell from thinning during warming and causing steel leakage; at the same time, the amount of water in sections 2 to 4 decreases successively, that is, the amount of water in section 2 > the amount of water in section 3 > the amount of water in section 4. In this way, while ensuring the avoidance of steel leakage, fatigue overcooling can also be avoided, which may cause internal cracks in the billet during straightening, cutting and stacking; furthermore, starting from section 6, the amount of water in the inner and outer arcs, and the amount of water in the edges and middle are precisely adjusted to ensure the uniformity of billet cooling and prevent cracks due to overcooling of the edges / corners.

[0153] In a preferred embodiment, the mold taper can be adjusted based on the casting speed v. For example, when the casting speed v is below 1.20 m / min, the mold taper is 1.1-1.2%; otherwise, the mold taper is 1.05-1.15%. This can reduce uneven cooling caused by billet shrinkage during cooling.

[0154] Furthermore, the water volume in the crystallizer and the water volume in sections 1 to 5 can be adjusted according to the change of the casting speed v.

[0155] For example, when the casting speed v is within the range of 1.05-1.20 m / min, the water volume on the wide side of the mold is 3600-3900 NL / min, and the water volume on the narrow side is 390-420 NL / min. When the casting speed v is greater than 1.20 m / min and less than or equal to 1.35 m / min, the water volume on the wide side of the mold is 3900-4100 NL / min, and the water volume on the narrow side is 420-450 NL / min. This, on the one hand, allows the mold to adopt a high cooling intensity, and on the other hand, adjusts the water volume in the mold according to the casting speed v, reducing the uneven cooling caused by the shrinkage of the billet during the cooling process.

[0156] For another example, the inner and outer arcs of sections 1 to 5 have standard water volumes corresponding to a casting speed v = 1.05 m / min. That is, when the casting speed v is 1.05 m / min, the inner and outer arcs of sections 1 to 5 each have their own standard water volumes. The standard water volumes corresponding to the casting speed v = 1.05 m / min for the inner and outer arcs of sections 1 to 5 are 250-290 NL / min, 575±25 NL / min, 525±25 NL / min, 475±25 NL / min, and 175±25 NL / min, respectively.

[0157] The inner and outer arcs of sections 1 to 5 also have enhanced water volumes corresponding to a pulling speed of v>1.05m / min, that is, when the pulling speed v>1.05m / min, the inner and outer arcs of sections 1 to 5 respectively have their own enhanced water volumes, and the enhanced water volumes are the sum of the standard water volume and the incremental water volume ΔT=T×floor((v-1.05m / min) / 0.05m / min), and T is between 5 and 10NL / min.

[0158] Wherein, floor represents a flooring function, that is, (v-1.05m / min) / 0.05m / min is rounded down. For example, when the pulling speed v takes a value of 1.14m / min in the range of 1.05-1.35m / min mentioned above, v=1.14m / min is substituted into floor((v-1.05m / min) / 0.05m / min) to obtain floor(1.8)=1; for another example, when the pulling speed v takes a value of 1.31m / min in the range of 1.05-1.35m / min mentioned above, v=1.31m / min is substituted into floor((v-1.05m / min) / 0.05m / min) to obtain floor(5.2)=5.

[0159] In this way, by controlling the inner and outer arc water volume of sections 1 to 5 according to the standard water volume and the incremental water volume, the shell of the billet can be properly warmed up, and while ensuring that steel leakage is avoided, fatigue overcooling can be further avoided, which may cause internal cracks in the billet during straightening, cutting and stacking.

[0160] In a specific embodiment, the water volume of the inner arc of the 6th section is 100-120NL / min, and the water volume of the outer arc is 150-170NL / min; the water volume of the middle inner arc of the 7th and 8th sections is 50-60NL / min and 70-80NL / min respectively, the water volume of the inner arc edge is 40-50NL / min and 60-70NL / min respectively, the water volume of the middle outer arc is 90-100NL / min and 150-160NL / min respectively, and the water volume of the outer arc edge is 80-90NL / min and 140-150NL / min respectively; the water volume of the middle inner and outer arcs of the 9th and 10th sections is both 50-60NL / min, and the water volume of the inner and outer arc edges is both 40-50NL / min. In this way, for sections 6 to 10, the amount of water in the inner and outer arcs, and the amount of water in the edges and middle are precisely adjusted to ensure uniform cooling of the billet and prevent cracks caused by overcooling of the edges / corners.

[0161] Furthermore, the water volume on the narrow side of the first section is 40 to 60 NL / min.

[0162] Furthermore, in one embodiment, the tundish utilizes a high-basicity coating agent, specifically, a high-basicity coating agent is added to the molten steel within the tundish. The high-basicity coating agent has a melting point of 1300-1450°C and a basicity (i.e., CaO / SiO2) ratio of ≥15. This high-basicity, low-melting-point coating agent maintains a molten bottom layer in contact with the molten steel, thereby adsorbing alumina and silicate inclusions in the molten steel and improving its cleanliness.

[0163] In one specific embodiment, in addition to CaO and SiO2, the chemical composition of the high-basicity covering agent includes, by mass percentage, the following: Al2O3: 33-36%, MgO: 4-6%, Fe2O3: 1-2%, C ≤ 1%, and H2O ≤ 0.2%. In addition to the aforementioned components, the remaining components of the high-basicity covering agent are unavoidable impurities.

[0164] In one embodiment, during the pouring process, 400-450 kg of the high-basicity covering agent is added to the tundish before the molten steel from the first ladle is poured into the crystallizer. Thereafter, starting with the pouring of the molten steel from the second ladle into the tundish, 5-10 kg of the high-basicity covering agent and 5-10 kg of lime powder particles are added to the tundish for each ladle (i.e., each ladle after the second ladle). This ensures that the high-basicity covering agent protects and purifies the molten steel.

[0165] The lime powder particles with a particle size of 1 to 3 mm account for more than 90% by mass, and CaO accounts for more than 95% by mass.

[0166] In one embodiment, during the pouring process, the total weight of the molten steel in the tundish is 50±2 tons when pouring into the mold, 55-60 tons during the stable pouring period, and 50±2 tons during the ladle change. This method of controlling the molten steel in the tundish achieves a stable flow field, prevents molten steel from agitating and causing slag entanglement, and promotes the adsorption of alumina and silicate inclusions in the molten steel by the high-basicity coating agent, further improving the cleanliness of the molten steel.

[0167] During the pouring process, full-process protective pouring is carried out, the argon blowing flow rate of the long nozzle is 150-250NL / min, the argon blowing flow rate of the stopper rod and submerged nozzle is 3-5NL / min, and the tundish is blown with argon for more than 5 minutes before pouring.

[0168] All fan-shaped segments are divided into arc zones, straightening zones and horizontal zones in sequence. In one embodiment, the surface temperature of the blank in the straightening zone is ≥950°C, the temperature when leaving all fan-shaped segments is ≥750°C, and the surface temperature after cutting is ≥650°C.

[0169] In a preferred embodiment, the slab continuous casting machine includes a secondary cooling electromagnetic stirring device. During the casting process, the secondary cooling electromagnetic stirring device operates at a power of 6-8 Hz and a current of 200-400 A. This electromagnetic stirring further controls the internal quality and segregation of the continuously cast slab, thereby improving the quality of the resulting continuously cast slab.

[0170] In one embodiment, when the billets are stacked and slowly cooled, they are stacked and slowly cooled at a location surrounded by enclosures. After slowly cooling to below 200° C., the billets are destackered to prepare continuous casting billets.

[0171] Furthermore, the chemical composition of the continuous casting billet includes, by weight percentage, C: 0.03-0.10%, Si: 0.15-0.35%, Mn: 0.5-1.6%, Ni: 0.4-10.0%, Al: 0.015-0.055%, Cu≤0.015%, Mo≤0.50%, Cr≤0.70%, Nb≤0.035%, and the remainder is iron and unavoidable impurities.

[0172] In low temperature steel, the role of each element in this chemical composition is as follows.

[0173] Carbon is a steel-strengthening element and austenite-stabilizing element. Reversing carbon-enriched austenite significantly lowers the Ms point and improves its stability. However, excessive carbon content increases the ductile-brittle transition temperature, negatively impacting the low-temperature toughness of the HAZ. Therefore, while maintaining strength, the carbon content should be as low as possible.

[0174] Si: A deoxidizing element in the steelmaking process, Si is crucial for reducing the harmful oxygen content in nickel-based steels. It also improves strength. In addition to inhibiting Mn segregation when present in steel in a certain ratio with Mn, Si also inhibits P segregation at grain boundaries. Excessive Si content can negatively impact weldability, but reducing it can improve the low-temperature toughness of the base metal and the weld heat-affected zone (HAZ).

[0175] Mn: An austenite-stabilizing element, its enrichment in austenite helps reverse austenite stability. Mn is also a matrix-strengthening element, increasing strength through solid solution strengthening and precipitation strengthening (forming fine MnS particles). Mn also has a strong influence on hardenability, significantly improving the material's hardenability. Too low a Mn content results in suboptimal strength, while too high a Mn content can easily form large MnS inclusions, degrading toughness. The optimal content is 0.6 wt%. The presence of Mn and Si in a certain ratio in steel also helps inhibit Si segregation.

[0176] Al: used as a deoxidizing and nitrogen fixing agent in steelmaking, it refines grains, inhibits the aging of low-carbon steel, improves the toughness of steel at low temperatures, especially reduces the brittle transition temperature of steel, and improves the oxidation resistance of steel.

[0177] Nickel: Nickel can increase the strength of steel while maintaining good plasticity and toughness. Nickel has high corrosion resistance to acids and alkalis, and is rust-proof and heat-resistant at high temperatures.

[0178] Nb: It increases the strength of steel. Nb forms high-hardness carbides with carbon, enhancing the strength and hardness of steel, and improving its wear resistance and tensile strength. Nb stabilizes the formation of carbides, limiting corrosion and hydrogen-induced cracking at grain boundaries, and improving the steel's corrosion resistance and weathering resistance.

[0179] Cu: Copper improves strength and toughness, especially atmospheric corrosion resistance. Its disadvantage is that it can easily become brittle during hot working. A copper content exceeding 0.5% significantly reduces plasticity. A copper content below 0.50% has no effect on weldability.

[0180] Molybdenum (Mo): Molybdenum refines steel's grain size, improves hardenability and thermal strength, and maintains sufficient strength and creep resistance at high temperatures (deformation caused by prolonged stress at high temperatures is called creep). Adding molybdenum to structural steel improves mechanical properties. It also suppresses the brittleness of alloy steels caused by heat. In tool steel, it improves reddening properties.

[0181] Cr: In structural and tool steels, chromium significantly increases strength, hardness, and wear resistance, but also reduces plasticity and toughness. Chromium also improves steel's oxidation and corrosion resistance, making it an important alloying element in stainless and heat-resistant steels.

[0182] Cu is an impurity element rather than an alloying element. Mo, Cr, and Nb may be impurity elements rather than alloying elements in some specific embodiments, but may be alloying elements in other embodiments. For example, in one embodiment in which Mo, Cr, and Nb are impurity elements, the chemical composition of the continuous casting billet is, by mass percentage, as follows: C: 0.03-0.10%, Si: 0.15-0.35%, Mn: 0.5-0.9%, Ni: 0.4-10.0%, Al: 0.015-0.055%, Cu≤0.015%, Mo≤0.010%, Cr≤0.015%, Nb≤0.006%, TO≤10ppm, P≤0.005%, S≤0.002%, N≤0.002%, H≤1.5ppm, with the remainder being iron and unavoidable impurities.

[0183] As previously mentioned, the chemical composition of the continuous casting ingot, in addition to C, Si, Mn, Ni, Al, Cu, Mo, Cr, and Nb, is composed of iron and unavoidable impurities. These impurities include, for example, TO, P, S, N, and H. In one embodiment of the present invention, based on the production method, the chemical composition of the continuous casting ingot, measured in weight percentage, includes: TO ≤ 10 ppm, P ≤ 0.0045%, S ≤ 0.0015%, N ≤ 0.0025%, and H ≤ 1.5 ppm. The impurity element content is low, and the continuous casting ingot is a high-purity, low-temperature steel continuous casting ingot. Even, TO of the continuous casting billet is ≤7ppm, ≤6ppm, ≤5ppm, ≤4ppm or ≤3ppm; P of the continuous casting billet is ≤0.004%, ≤0.0035%, ≤0.003%, ≤0.0025%, ≤0.002% or ≤0.0015%; S of the continuous casting billet is ≤0.0014%, ≤0.0013%, ≤0.0010%, ≤0.0009%, ≤0.0008% or ≤0.0005%; N of the continuous casting billet is ≤0.0025%, ≤0.0022%, ≤0.0020%, ≤0.0018%, ≤0.0016% or ≤0.0015%; H of the continuous casting billet is ≤1.3ppm, ≤1.2ppm, ≤1.0ppm, ≤0.8ppm or ≤0.6ppm.

[0184] In low temperature steel, the effects of these impurity elements are as follows.

[0185] P and S: They tend to segregate at grain boundaries, forming Fe3P with iron. This weakens the bonding between Fe atoms and surrounding Fe atoms, reduces the grain boundary's resistance to crack growth, and worsens low-temperature toughness. Therefore, both S and P are detrimental to low-temperature toughness. S easily forms MnS precipitates with the metallic element Mn, reducing low-temperature toughness.

[0186] O / N: O and N easily form high-melting-point precipitates, Al2O3 and AlN, with Al. These precipitates are large, reaching several microns in diameter. This can easily cause stress concentration near the precipitates, becoming a crack source and severely impacting the matrix's low-temperature toughness. Therefore, the content of these elements should be minimized. Like hydrogen, oxygen adversely affects the mechanical properties of steel. Not only the oxygen concentration, but also the quantity, type, and distribution of oxygen-containing inclusions play a significant role.

[0187] H: When the hydrogen content in steel is greater than 2ppm, hydrogen plays an important role in the so-called "scaling" phenomenon. This scaling phenomenon is generally more obvious when internal cracks and fractures occur during the cooling process after rolling and forging, and is more often found in large sections or high-carbon steels.

[0188] In addition, more than 90% of the oxide inclusions in the obtained continuous casting billet have an Al2O3 content of ≥85%, more than 96% of the oxide inclusions have a size of ≤5μm, and the largest oxide inclusion does not exceed 25μm.

[0189] The composition and size analysis methods for oxide inclusions are conventional techniques in this field. For example, a scanning electron microscope (SEM) is used for automatic inclusion statistical analysis. Five samples (20 mm long, 20 mm wide, and 15 mm thick) are uniformly collected from the inner and outer arcs at the 1 / 4 position along the width of the continuous casting ingot. Each sample is scanned with a SEM, and the oxide inclusions within a 10 mm x 10 mm area are counted. During the scanning process, the electron microscope's energy spectrum analysis provides the content of each element. Converted to aluminum oxide, the Al2O3 content in each oxide inclusion can be obtained. The SEM can also automatically calculate the size of each oxide inclusion.

[0190] The central carbon segregation of the obtained continuous casting billet is below level 1.5, or even below level 1 or below level 0.5; the surface cracks of the continuous casting billet are ≤1.5mm, or even ≤1.3mm, ≤1.2mm, ≤1.0mm, ≤0.9mm or ≤0.8mm; the width deviation of the obtained continuous casting billet is within 5mm, and the thickness deviation is within 2mm.

[0191] Further, referring back to FIG2 , in one embodiment of the present invention, the production method further comprises:

[0192] Heating process: The continuous casting billet is fed into the heating furnace for heating. The maximum temperature of the preheating section is 750-850℃, the maximum temperature of the heating section is 1100-1200℃, the maximum temperature of the soaking section is 1150-1200℃, the heating rate of the preheating section is 20-30℃ / min, and the heating rate of the heating section is 30-50℃ / min.

[0193] Hot rolling process: After leaving the heating furnace, the continuous casting slab is hot rolled into hot-rolled plates with a thickness of 5 to 60 mm. The starting rolling temperature is 1030 to 1130°C, the finishing rolling temperature is 800 to 850°C, and the rolling amount per pass is 10 to 15%;

[0194] The hot-rolled plate is naturally air-cooled to below 200°C, and then subjected to secondary quenching and tempering heat treatment, and then naturally cooled to room temperature to obtain a finished plate; wherein, the temperature of the first quenching is 800-900°C, and the temperature of the second quenching is 700-800°C; if h≤20mm, the tempering temperature is 600-620°C; if h≥40mm, the tempering temperature is 560-580°C; if 20mm<h<40mm, the tempering temperature is >580°C and <600°C.

[0195] In this way, the production method can be used to prepare low-temperature steel plates with excellent mechanical properties, excellent low-temperature properties, and excellent surface quality.

[0196] Specifically, the Z-direction cross-sectional shrinkage of the resulting plate product is ≥70%, and the low-temperature impact energy values ​​at -80°C, -95°C, -125°C, and -196°C are all ≥185J. Furthermore, the Z-direction cross-sectional shrinkage is ≥75%, ≥80%, ≥85%, or even ≥90%, and the low-temperature impact energy values ​​at -80°C, -95°C, -125°C, and -196°C can reach 215J or more, 250J or more, or even 320J or more. In this application, the room-temperature tensile properties test of the resulting plate product can be conducted on a hydraulic universal testing machine in accordance with GB / T 228.1-2010 "Tensile testing of metallic materials - Part 1: Room-temperature test methods," and the low-temperature impact test of the resulting plate product can be conducted on an impact testing machine in accordance with GB / T 229-2020 "Charpy pendulum impact test method for metallic materials" to obtain the room-temperature tensile properties and low-temperature impact properties of the product.

[0197] Specifically, during the heating process, after the continuous casting billet is cooled to room temperature and before entering the heating furnace, the surface can be ground, and then a high-temperature anti-oxidation coating with a thickness of 0.8 to 1.5 mm is sprayed on the ground surface before the continuous casting billet is sent into the heating furnace.

[0198] During the heating process, the heating rate is controlled to be low to avoid internal cracks and other problems caused by uneven heat inside and outside the billet. In addition, the soaking section has a holding time of 65 to 85 minutes. The soaking section adopts high temperature and long-term holding method to ensure that the billet is heated evenly, thereby avoiding surface cracks in subsequent hot rolling.

[0199] Furthermore, for continuous casting billets with a thickness of 220 mm, the total time the continuous casting billets spend in the furnace during the heating process is preferably controlled within 150 to 180 minutes; and for continuous casting billets with a thickness of 320 mm, the total time the continuous casting billets spend in the furnace during the heating process is preferably controlled within 200 to 230 minutes.

[0200] Next, the heat treatment process includes an initial secondary quenching stage and a subsequent tempering heat treatment stage. In the secondary quenching stage, the temperature for the first quenching is 800-900°C, and the temperature for the second quenching is 700-800°C. Based on this, the temperatures for the first and second quenching can be fine-tuned based on the Ni content in the steel. For example, when the Ni content is 3-4%, the first quenching temperature is 820±20°C, and the second quenching temperature is 720±20°C. When the Ni content is 8-10%, the first quenching temperature is 880±20°C, and the second quenching temperature is 780±20°C.

[0201] More preferably, the first quenching time in the furnace is (2.0-2.2) min / mm×h+(5-10) min, the second quenching time in the furnace is (2.1-2.3) min / mm×h+(5-10) min, and the tempering time in the furnace is (2.5-2.8) min / mm×h+(5-10) min, where h is the thickness of the hot-rolled plate.

[0202] The above is a detailed introduction to the production method of one embodiment of the present invention. The following introduces several examples of using the production method of the present invention to prepare finished plate products. In these examples, in addition to the parameters mentioned below, some other important parameters are as described above and are not repeated here.

[0203] The first set of embodiments

[0204] Generally speaking, this set of examples prepares continuous casting billets according to the process of KR desulfurization - converter smelting - LF refining - RH vacuum refining - continuous casting, and the LF refining - RH vacuum refining process is implemented using the first embodiment described above. The specific process is as follows.

[0205] The blast furnace molten iron is put into the KR desulfurization equipment for desulfurization treatment. After the desulfurization is completed, the slag is skimmed and the S content of the molten iron leaving the station is ≤0.0010%.

[0206] The desulfurized molten iron is fed into a converter for converter smelting. The specific process is as described in the converter phosphorus control smelting method and / or the production method described in the above embodiment. Some important processes include:

[0207] Desulfurized molten iron and the first batch of scrap steel were added to a single converter for preliminary blowing. The basicity and T.Fe content of the slag produced are shown in Table 1. The oxygen lance position of the first minute of blowing was 1.8-2.0m, and the top blowing oxygen volume was 28000-30000Nm 3 / h, from the second minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen amount is controlled to decrease twice simultaneously, argon is blown from the bottom during the entire blowing process and the molten steel temperature is controlled at 1380-1420°C. After the end of blowing, the bottom blowing of argon is continued for 1-3 minutes, and then the slag is blocked and the steel is tapped. The final molten steel temperature is 1360-1400°C, the C content is 3.3-3.8%, and the P content is ≤0.018%, thereby obtaining semi-steel molten steel;

[0208] After the converter is cleaned of slag and excess steel, the semi-steel molten steel, nickel plate and second batch of scrap steel are added to the converter for blowing. The entire blowing process is divided into an early blowing process lasting 3 to 4 minutes and a late blowing process. During the early blowing process, lime and light-burned dolomite are added at the beginning of blowing. After blowing for 1 minute, pellets are added in 3 to 5 batches to maintain the minimum T.Fe content in the slag as shown in Table 1. After the early blowing process is completed, argon is continuously blown from the bottom for 1 to 2 minutes, and then the slag is partially drained and the late blowing process begins. The basicity of the slag produced by the late blowing process is shown in Table 1. The terminal molten steel temperature, P content and C content of the late blowing process are shown in Table 1, and the slag is drained and the steel is tapped.

[0209] [Table 1]

[0210] The molten steel from the converter was transported to the LF furnace for refining, which included a power-on heating stage, an alloying stage, and a slag-making stage in sequence. The tapping temperatures were shown in Table 2. During the slag-making stage, calcium carbide and aluminum particles were used to adjust the slag composition. The specific slag composition is shown in Table 2.

[0211] [Table 2]

[0212] Then, the molten steel obtained from the LF furnace refining is transported to the RH vacuum refining furnace for RH vacuum refining. It is first treated at a vacuum degree below 1.5 mbar for 15 to 20 minutes, then at a vacuum degree above 5 mbar for 10 to 20 minutes, and finally the steel is tapped. The specific operation process is as described in the first embodiment above.

[0213] Molten steel was hoisted from the RH vacuum refining furnace onto the continuous casting platform and allowed to rest before pouring began, producing continuous cast ingots with a thickness of 220 mm or 320 mm. The chemical composition of the resulting continuous cast ingots is shown in Table 3. In addition to the elements listed in the table, the balance is iron and unavoidable impurities.

[0214] [Table 3]

[0215] [Table 3 continued]

[0216] In addition, testing shows that in the continuous casting ingot, approximately M% of the oxide inclusions have an Al2O3 content ≥80%, and approximately N% of the oxide inclusions have a size ≤5μm, where M and N are respectively shown in Table 4. Furthermore, the maximum size, central carbon segregation level, maximum surface crack, width deviation, and thickness deviation of the oxide inclusions are also shown in Table 4.

[0217] [Table 4]

[0218] Next, after cooling to room temperature, the surfaces of each continuous-cast slab in this set of examples were ground. A high-temperature, anti-oxidation coating with a thickness of 0.8 to 1.5 mm was then sprayed onto the ground surfaces. The slabs were then heated in a heating furnace at a rate of 20 to 30°C / min in the preheating stage and 30 to 50°C / min in the heating stage. After leaving the heating furnace, the slabs were hot-rolled into sheets. The preheating, heating, and soaking temperatures in the heating furnace, the thickness of the sheets obtained after hot rolling, and the start and finish rolling temperatures are shown in Table 5.

[0219] [Table 5]

[0220] After the hot-rolled plates were naturally air-cooled to below 200°C, they were subjected to secondary quenching and tempering heat treatments. After cooling to room temperature, the finished plates were obtained. The temperature and furnace time for the first quenching, the temperature and furnace time for the second quenching, and the tempering temperature and furnace time are shown in Table 6.

[0221] [Table 6]

[0222] The finished plate products were sampled and their performance tested. Specifically, in accordance with GB / T 228.1-2010 "Tensile tests on metallic materials - Part 1: Room temperature test methods", the finished plate products were subjected to room temperature tensile performance tests on a hydraulic universal material testing machine. In addition, in accordance with GB / T 229-2020 "Charpy pendulum impact test method for metallic materials", the finished plate products were subjected to low-temperature impact tests on an impact testing machine. The results are shown in Table 7 below.

[0223] [Table 7]

[0224] As can be seen from Table 7, the finished plate products exhibit excellent room temperature and low-temperature mechanical properties, with low-temperature impact energy values ​​of ≥185 J at -80°C, -95°C, -125°C, and -196°C. Furthermore, the Z-direction shrinkage of the finished plate products of each embodiment is ≥70%.

[0225] Second set of embodiments

[0226] Generally speaking, this set of examples prepares continuous casting billets according to the process of KR desulfurization - converter smelting - LF refining - RH vacuum refining - continuous casting, and the LF refining - RH vacuum refining process is implemented using the second embodiment described above. The specific process is as follows.

[0227] The blast furnace molten iron is put into the KR desulfurization equipment for desulfurization treatment. After the desulfurization is completed, the slag is skimmed and the S content of the molten iron leaving the station is ≤0.0010%.

[0228] The desulfurized molten iron is fed into a converter for converter smelting. The specific process is as described in the converter phosphorus control smelting method and / or the production method described in the above embodiment. Some important processes include:

[0229] Desulfurized molten iron and the first batch of scrap steel were added to a single converter for preliminary blowing. The basicity and T.Fe content of the slag produced are shown in Table 8. The oxygen lance position of the first minute of blowing was 1.8-2.0m, and the top blowing oxygen volume was 28000-30000Nm 3 / h, from the second minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen amount is controlled to decrease twice simultaneously, argon is blown from the bottom during the entire blowing process and the molten steel temperature is controlled at 1380-1420°C. After the end of blowing, the bottom blowing of argon is continued for 1-3 minutes, and then the slag is blocked and the steel is tapped. The final molten steel temperature is 1360-1400°C, the C content is 3.3-3.8%, and the P content is ≤0.018%, thereby obtaining semi-steel molten steel;

[0230] After the converter is cleaned of slag and excess steel, the semi-steel molten steel, nickel plate and second batch of scrap steel are added to the converter for blowing. The entire blowing process is divided into an early blowing process lasting 3 to 4 minutes and a late blowing process. During the early blowing process, lime and light-burned dolomite are added at the beginning of blowing. After blowing for 1 minute, pellets are added in 3 to 5 batches to maintain the minimum T.Fe content in the slag as shown in Table 8. After the early blowing process is completed, argon is continuously blown from the bottom for 1 to 2 minutes, and then the slag is partially drained and the late blowing process begins. The basicity of the slag produced by the late blowing process is shown in Table 1. The terminal molten steel temperature, P content and C content of the late blowing process are shown in Table 8, and the slag is drained and the steel is tapped.

[0231] [Table 8]

[0232] The molten steel from the converter was transported to the LF furnace for refining, which included a power-on heating stage, an alloying stage, and a slag-making stage in sequence. The tapping temperatures were shown in Table 9. During the slag-making stage, calcium carbide and aluminum particles were used to adjust the slag composition. The specific slag composition is shown in Table 9.

[0233] [Table 9]

[0234] Then, the molten steel obtained from the LF furnace refining is transported to the RH vacuum refining furnace for RH vacuum refining. First, the vacuum degree is reduced to below 1.5 mbar, metallic aluminum is added to the molten steel, and 2 to 4 kg / t of low-carbon steel slag surface deoxidizer is added to the ladle slag surface. Then, the vacuum treatment is continued for 15 to 20 minutes; then, the treatment is continued at a vacuum degree of more than 5 mbar for 10 to 20 minutes, and finally the air is broken to tap the steel. The specific operation process is as described in the second embodiment above.

[0235] Molten steel was hoisted from the RH vacuum refining furnace onto the continuous casting platform and allowed to rest before pouring began, producing continuous cast ingots with a thickness of 220 mm or 320 mm. The chemical composition of the resulting continuous cast ingots is shown in Table 10. In addition to the elements listed in the table, the balance consists of iron and unavoidable impurities.

[0236] [Table 10]

[0237] [Table 10 continued]

[0238] In addition, testing shows that in the continuous casting ingot, approximately M% of the oxide inclusions have an Al2O3 content ≥80%, and approximately N% of the oxide inclusions have a size ≤5μm, where M and N are respectively shown in Table 11. Furthermore, the maximum size, central carbon segregation level, maximum surface crack, width deviation, and thickness deviation of the oxide inclusions are also shown in Table 11.

[0239] [Table 11]

[0240] Next, after cooling to room temperature, the surfaces of each continuous-cast slab in this set of examples were ground. A high-temperature, anti-oxidation coating with a thickness of 0.8 to 1.5 mm was then sprayed onto the ground surfaces. The slabs were then heated in a heating furnace at a rate of 20 to 30°C / min in the preheating stage and 30 to 50°C / min in the heating stage. After leaving the heating furnace, the slabs were hot-rolled into sheets. The preheating, heating, and soaking temperatures in the heating furnace, the thickness of the sheets obtained after hot rolling, and the start and finish rolling temperatures are shown in Table 12.

[0241] [Table 12]

[0242] After the hot-rolled plates were naturally air-cooled to below 200°C, they were subjected to secondary quenching and tempering heat treatments. After cooling to room temperature, the finished plates were obtained. The temperature and furnace time for the first quenching, the temperature and furnace time for the second quenching, and the tempering temperature and furnace time are shown in Table 13.

[0243] [Table 13]

[0244] The finished plate products were sampled and tested for performance. Specifically, the room temperature tensile performance test was performed on the hydraulic universal material testing machine in accordance with GB / T 228.1-2010 "Tensile tests on metallic materials - Part 1: Room temperature test methods". The finished plate products were also subjected to a low-temperature impact test on an impact testing machine in accordance with GB / T 229-2020 "Charpy pendulum impact test method for metallic materials". The results are shown in Table 14 below.

[0245] [Table 14]

[0246] As can be seen from Table 14, the finished plate products exhibit excellent room temperature and low-temperature mechanical properties, with low-temperature impact energy values ​​of ≥185 J at -80°C, -95°C, -125°C, and -196°C. Furthermore, the Z-direction shrinkage of the finished plate products of each example is ≥70%.

[0247] The third set of embodiments

[0248] Generally speaking, this set of examples prepares continuous casting billets according to the process of KR desulfurization - converter smelting - LF refining - RH vacuum refining - continuous casting, and the LF refining - RH vacuum refining process is implemented using the third embodiment described above. The specific process is as follows.

[0249] The blast furnace molten iron is put into the KR desulfurization equipment for desulfurization treatment. After the desulfurization is completed, the slag is skimmed and the S content of the molten iron leaving the station is ≤0.0010%.

[0250] The desulfurized molten iron is fed into a converter for converter smelting. The specific process is as described in the converter phosphorus control smelting method and / or the production method described in the above embodiment. Some important processes include:

[0251] Desulfurized molten iron and the first batch of scrap steel were added to a single converter for preliminary blowing. The basicity and T.Fe content of the slag produced are shown in Table 15. The oxygen lance position of the first minute of blowing was 1.8-2.0m, and the top blowing oxygen volume was 28000-30000Nm 3 / h, from the second minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen amount is controlled to decrease twice simultaneously, argon is blown from the bottom during the entire blowing process and the molten steel temperature is controlled at 1380-1420°C. After the end of blowing, the bottom blowing of argon is continued for 1-3 minutes, and then the slag is blocked and the steel is tapped. The final molten steel temperature is 1360-1400°C, the C content is 3.3-3.8%, and the P content is ≤0.018%, thereby obtaining semi-steel molten steel;

[0252] After the converter is cleaned of slag and excess steel, the semi-steel molten steel, nickel plate and second batch of scrap steel are added to the converter for blowing. The entire blowing process is divided into an early blowing process lasting 3 to 4 minutes and a late blowing process. During the early blowing process, lime and light-burned dolomite are added at the beginning of blowing. After blowing for 1 minute, pellets are added in 3 to 5 batches to maintain the minimum T.Fe content in the slag as shown in Table 15. After the early blowing process is completed, argon is continuously blown from the bottom for 1 to 2 minutes, and then the slag is partially drained and the late blowing process begins. The basicity of the slag produced by the late blowing process is shown in Table 1. The terminal molten steel temperature, P content and C content of the late blowing process are shown in Table 15, and the slag is drained and the steel is tapped.

[0253] [Table 15]

[0254] The molten steel from the converter is transported to the LF furnace for refining, which includes a power-on heating stage, an alloying stage, and a slag-making stage in sequence. The tapping temperature is shown in Table 16. In the slag-making stage, calcium carbide and aluminum particles are used to adjust the slag composition. The specific slag composition is shown in Table 16.

[0255] [Table 16]

[0256] The molten steel obtained from the LF refining process is then transferred to the RH vacuum refining furnace for RH vacuum refining. The refining process is initially conducted at a vacuum level of 200 mbar or higher for 3-5 minutes, followed by a vacuum level of 1.5 mbar or lower for 10-15 minutes, and finally at a vacuum level of 50 mbar or higher for 5 minutes or more. Finally, the steel is tapped. The detailed operation is as described in the third embodiment. During this time, at a vacuum level of 1.5 mbar or lower, calcium wire is fed into the molten steel from region S21 near the riser 32 at a rate of 1-1.5 m / s.

[0257] Molten steel was hoisted from the RH vacuum refining furnace onto the continuous casting platform and allowed to rest before pouring began, producing continuous cast ingots with a thickness of 220 mm or 320 mm. The chemical composition of the resulting continuous cast ingots is shown in Table 17. In addition to the elements listed in the table, the balance consists of iron and unavoidable impurities.

[0258] [Table 17]

[0259] [Table 17 continued]

[0260] In addition, testing has shown that in the continuous casting ingot, approximately M% of the oxide inclusions have an Al2O3 content ≥80%, and approximately N% of the oxide inclusions have a size ≤5μm, where M and N are shown in Table 21 respectively. Furthermore, the maximum size of the oxide inclusions, the central carbon segregation level, the maximum surface crack, the width deviation, and the thickness deviation are also shown in Table 18, respectively.

[0261] [Table 18]

[0262] Next, after cooling to room temperature, each continuous-cast slab in this set of examples was surface-ground. A high-temperature, anti-oxidation coating with a thickness of 0.8 to 1.5 mm was then sprayed onto the ground surface. The slabs were then heated in a heating furnace at a preheating rate of 20 to 30°C / min and a heating rate of 30 to 50°C / min. After leaving the furnace, the slabs were hot-rolled into sheets. The preheating, heating, and soaking temperatures in the heating furnace, the thickness of the hot-rolled sheets, and the start and finish rolling temperatures are shown in Table 19.

[0263] [Table 19]

[0264] After the hot-rolled plates were naturally air-cooled to below 200°C, they were subjected to secondary quenching and tempering heat treatments. After cooling to room temperature, the finished plates were obtained. The temperature and furnace time for the first quenching, the temperature and furnace time for the second quenching, and the tempering temperature and furnace time are shown in Table 20.

[0265] [Table 20]

[0266] The finished plate products were sampled and their performance tested. Specifically, the room temperature tensile performance test was performed on the hydraulic universal material testing machine in accordance with GB / T 228.1-2010 "Tensile tests on metallic materials - Part 1: Room temperature test methods". The finished plate products were subjected to a low temperature impact test on an impact testing machine in accordance with GB / T 229-2020 "Charpy pendulum impact test method for metallic materials". The results are shown in Table 21 below.

[0267] [Table 21]

[0268] As can be seen from Table 21, the finished plate products exhibit excellent room temperature and low-temperature mechanical properties, with low-temperature impact energy values ​​of ≥185 J at -80°C, -95°C, -125°C, and -196°C. Furthermore, the Z-direction shrinkage of the finished plate products of each embodiment is ≥70%.

[0269] In summary, one embodiment of the present invention adopts the converter phosphorus-controlled smelting method, and realizes efficient and stable dephosphorization through three small stages of preliminary blowing and two large stages of deep blowing, thereby realizing converter smelting of ultra-low-phosphorus molten steel, and the production process can be completed by a single converter, with high efficiency, little equipment damage and low cost; and, refining and continuous casting are carried out based on the low-phosphorus molten steel produced by the converter, which is conducive to the preparation of low-phosphorus continuous casting billets, meeting the preparation requirements of high-purity steel.

Claims

1. A converter phosphorus-controlled smelting method for low-temperature steel, characterized in that: The converter phosphorus control smelting method comprises: Add desulfurized molten iron and the first batch of scrap steel into the converter for blowing to produce slag with a basicity of 2.0-2.5 and a T.Fe content of 25-35%. The oxygen lance position of the first minute of blowing is 1.8-2.0m, and the top blowing oxygen volume is 28000-30000Nm 3 / h, from the second minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen amount is controlled to decrease twice simultaneously, argon is blown from the bottom during the entire blowing process and the molten steel temperature is controlled at 1380-1420°C. After the end of blowing, the bottom blowing of argon is continued for 1-3 minutes, and then the slag is blocked and the steel is tapped. The final molten steel temperature is 1360-1400°C, the C content is 3.3-3.8%, and the P content is ≤0.018%, thereby obtaining semi-steel molten steel; After the converter is cleaned of slag and excess steel, the semi-steel molten steel, nickel plates and a second batch of scrap steel are added to the converter for blowing. The second batch of scrap steel accounts for 50-70% of the total weight of the first batch of scrap steel and the second batch of scrap steel. The entire blowing process is divided into an early blowing process lasting 3-4 minutes and a late blowing process. During the early blowing process, lime and light-burned dolomite are first added at the beginning of the blowing process. After blowing for 1 minute, pellets are added in 3-5 batches to maintain the T.Fe content in the slag at ≥13% throughout the early blowing process. After the early blowing process is completed, argon is continuously blown from the bottom for 1-2 minutes, and then the slag is partially drained and the late blowing process begins. The late blowing process produces slag with a basicity of 5.5-9.

5. The terminal molten steel temperature of the late blowing process is 1580-1620° C., P≤0.0035%, and C content of 0.02-0.05%. The slag is drained and the steel is tapped to obtain the final steel smelted in the converter.

2. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 1, characterized in that: The above “from the 2nd minute to the end of blowing, the oxygen lance position is controlled to rise and the top blowing oxygen volume is controlled to decrease twice” means: from the 2nd to the 3rd minute, the oxygen lance position is 2.2 to 2.5m, and the top blowing oxygen volume is 26000 to 28000 Nm 3 / h, after the 4th minute, the oxygen lance is positioned at 2.5-2.8m, and the top-blowing oxygen volume is 23,000-26,000 Nm 3 / h.

3. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 1, characterized in that: The "bottom blowing of argon throughout the blowing process and controlling the molten steel temperature at 1380-1420°C" includes: Before the second simultaneous control of the oxygen lance position rising and the top blowing oxygen volume decreasing, the bottom blowing argon volume is 1200~1500Nm 3 / h, then bottom blowing argon gas volume 600~800Nm 3 / h.

4. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 3, characterized in that: In the above “bottom blowing argon continues for 1 to 3 minutes after blowing is completed”, the bottom blowing argon volume is 800 to 1000 Nm 3 / h.

5. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 1, characterized in that: In the step of "adding desulfurized molten iron and the first batch of scrap steel into a converter for blowing", a first slag-forming agent is used for slag formation, and the first slag-forming agent includes lime, pellets, and light-burned dolomite.

6. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 1, characterized in that: In the early blowing process, the oxygen lance position of the first minute is 1.8-2.0m, and the top blowing oxygen volume is 33000-35000Nm 3 / h, then the oxygen lance is at 2.2-2.5m, and the top-blowing oxygen volume is 25000-28000Nm 3 / h; During the initial blowing process, argon was blown from the bottom of the furnace, and the amount of argon blown from the bottom of the furnace in the first minute was 800-1000 Nm 3 / h, then bottom blowing argon gas volume 600~800Nm 3 / h.

7. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 1, characterized in that: During the early blowing, a second slagging agent is used for slagging, and the second slagging agent includes lime, pellets, and light-burned dolomite.

8. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 1, characterized in that: During the later blowing process, lime, pellets and light-burned dolomite are added to form slag.

9. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 1, characterized in that: In the later stage of blowing, the oxygen lance is initially positioned at 1.8 to 2.0 m and the top blowing oxygen volume is 36,000 to 38,000 Nm 3 / h, from 4.5min to 0.5min before the end of blowing, the oxygen lance position and the top-blowing oxygen volume were adjusted twice simultaneously. The first adjustment was to lower the oxygen lance position and increase the top-blowing oxygen volume, and the second adjustment was to raise the oxygen lance position and reduce the top-blowing oxygen volume.

10. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 9, characterized in that: 4 minutes before the end of blowing, the oxygen lance position was lowered to 1.6-1.8m and the top blowing oxygen volume was increased to 38,000-40,000 Nm 3 / h. One minute before the end of blowing, the oxygen lance position is adjusted to 2.2-2.5m for the second time, and the top blowing oxygen volume is reduced to 28,000-30,000 Nm 3 / h.

11. The converter phosphorus-controlled smelting method for low-temperature steel according to claim 10, characterized in that: In the latter stage of blowing, the amount of argon gas blown at the bottom at the beginning is 1000~1200Nm 3 / h, while lowering the oxygen lance position and increasing the top-blowing oxygen volume for the first time, increase the bottom-blowing argon volume to 1300-1500Nm 3 / h and maintain it for 1 to 2 minutes after the end of the post-blowing, then end the bottom argon blowing and pour the slag and tap the steel.

12. A method for producing low-temperature steel, characterized in that: The production method comprises, The molten iron is treated in the KR desulfurization equipment with an outlet temperature of 1350-1400°C and a sulfur content of ≤0.0010%; The desulfurized molten iron is smelted into molten steel using the converter phosphorus control smelting method for low-temperature steel according to claim 1; The molten steel obtained from converter smelting is transported to LF furnace for refining, including sequential heating, alloying, and slag formation, with the outlet temperature reaching 1610-1630℃. The molten steel from LF furnace is then transported to RH vacuum refining furnace for vacuum treatment and steelmaking. The molten steel is hoisted from the RH vacuum refining furnace to the continuous casting platform for standing, and then poured to obtain continuous casting billets.

13. The method for producing low-temperature steel according to claim 12, characterized in that: In the tapping process of "slag pouring and steel tapping to obtain the final steel smelted in the converter", deoxidation and alloying and slag making are carried out successively, and the ladle bottom blowing flow rate during the tapping process is 400-600 NL / min, and the ladle bottom blowing flow rate is increased to 800-1000 NL / min after the tapping is completed; When 10-20% of the steel is tapped, metallic aluminum, low-titanium and low-aluminum ferrosilicon and metallic manganese are added in sequence to deoxidize and alloy the molten steel. When 60-70% of the steel is tapped, all of them are added. Then, lime and calcium aluminate synthetic slag are added for slagging. When 80-90% of the steel is tapped, all of them are added.

14. The method for producing low-temperature steel according to claim 13, characterized in that: The calcium aluminate synthetic slag has a particle size of 10 to 50 mm and a chemical composition, by weight percentage, of CaO 50 to 60%, Al2O3 35 to 45%, MgO 2 to 4%, SiO2 ≤ 3%, and other inevitable impurity components, wherein the phase 12CaO·7Al2O3 accounts for more than 80% of the composite phase of the calcium aluminate synthetic slag.

15. The method for producing low-temperature steel according to claim 12, characterized in that: In the slag-making stage of refining in the LF furnace, the slag is deoxidized with calcium carbide and aluminum particles to adjust the slag composition to 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, less than 1.5% T.Fe+MnO and other inevitable impurities in weight percentage; When vacuum treatment is carried out in the RH vacuum refining furnace, the steel is first treated at a vacuum degree below 1.5 mbar for 15 to 20 minutes, then at a vacuum degree above 5 mbar for 10 to 20 minutes, and finally the steel is tapped.

16. The method for producing low-temperature steel according to claim 12, characterized in that: In the slag-making stage of refining in the LF furnace, the slag is deoxidized using calcium carbide and low-carbon steel slag surface deoxidizer to adjust the slag composition to 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, 2-5% T.Fe+MnO and other inevitable impurity components in weight percentage; When vacuum treatment is carried out in the RH vacuum refining furnace, the vacuum degree is reduced to below 1.5 mbar within 4 minutes after the molten steel arrives at the station, and then metallic aluminum is added to the molten steel, and 2 to 4 kg / t of low-carbon steel slag surface deoxidizer is added to the ladle slag surface. The vacuum treatment is then continued for 15 to 20 minutes; then the treatment is continued at a vacuum degree of more than 5 mbar for 10 to 20 minutes, and finally the air is broken to tap the steel.

17. The method for producing low-temperature steel according to claim 12, characterized in that: During the slag-making stage of refining in the LF furnace, 0.3-0.5 kg / t of calcium carbide and 1.0-2.0 kg / t of second calcium aluminate synthetic slag are added to the molten steel to adjust the slag composition to contain, in mass percentage, 50-55% CaO, 30-35% Al2O3, 1-3% CaF2, less than 3% SiO2, 4-6% MgO, less than 1% T.Fe+MnO and other inevitable impurities.

18. The method for producing low-temperature steel according to claim 17, characterized in that: The components of the second calcium aluminate synthetic slag include, by mass percentage, 40-45% CaO, 10-15% Al2O3, 5-10% CaF2, less than 3% SiO2, 2-5% MgO, 5-10% CaC2, and 15-20% elemental aluminum, wherein the mass percentage of the phase 12CaO·7Al2O3 exceeds 30%, and the rest is a single phase or a composite phase of CaO, CaF2, SiO2, and MgO.

19. The method for producing low-temperature steel according to claim 17, characterized in that: When vacuum treatment is performed in the RH vacuum refining furnace, firstly, the vacuum degree is above 200mbar and the pressure is 80~100Nm 3 / h of the elevated gas flow for 3 to 5 minutes, and then at a vacuum degree below 1.5 mbar, at a pressure of 150 to 200 Nm 3 / h of lifting gas flow for 10 to 15 minutes, and then at a vacuum degree of more than 50mbar, at a rate of 150 to 200Nm 3 / h of lifting gas flow is continuously processed for more than 5 minutes, and finally the air is broken and the steel is discharged.

20. The method for producing low-temperature steel according to claim 19, characterized in that: After the vacuum degree drops to 500 mbar, 1.0 to 1.5 m / t of calcium wire is fed into the ladle from the area near the downcomer at a feeding speed of 4 to 6 m / s.

21. The method for producing low-temperature steel according to claim 19, characterized in that: After the vacuum degree of the RH vacuum refining furnace drops below 1.5 mbar, 0.5 to 1.0 m / t of calcium wire is fed into the ladle from the area near the riser at a feeding speed of 1 to 1.5 m / s during the continuous treatment for 10 to 15 minutes.

22. The method for producing low-temperature steel according to claim 21, characterized in that: The downcomer and riser of the RH vacuum refining furnace are symmetrically distributed along a mirror plane. The ladle at the RH vacuum refining furnace has a fitting surface passing through the center of the riser and parallel to the mirror plane. The "area close to the riser" is located between the fitting surface and the mirror plane.

23. The method for producing low-temperature steel according to claim 12, characterized in that: The process of "lifting the molten steel from the RH vacuum refining furnace to the continuous casting platform for standing, and then pouring to obtain the continuous casting billet" includes: The molten steel is cast using a slab continuous casting machine to obtain a billet with a thickness of 220 mm or 320 mm and a width of 1500-2300 mm; the superheat of the molten steel in the tundish is 30-50° C., a low-melting-point alkaline mold slag with a melting point of 1100-1200° C. is used in the crystallizer, the casting speed v is 1.05-1.35 m / min, and the taper of the crystallizer is 1.05-1.2%; After the billet exits the crystallizer, it is cooled in the secondary cooling zone, which is divided into 10 sections according to different water volumes; the water volume in the first section is less than 1 / 10 of the water volume in the crystallizer, the water volumes in sections 2 to 4 are greater than the water volume in section 1 and decrease in sequence, the water volume in section 5 is less than the water volume in section 1, the water volume in the inner arc of section 6 is less than the water volume in the outer arc, and the water volume at the edges of the inner and outer arcs in sections 7 to 10 is less than the water volume in the middle of the inner and outer arcs; The blank is pressed in sections 8 to 10, with the three sections each allocated 25%, 25%, and 50% of the reduction. The reductions for blanks with thicknesses of 220 mm and 320 mm are 3 to 5 mm and 6 to 8 mm, respectively. After the billets leave all the sectors, they are cut and stacked for slow cooling to obtain continuous casting billets.

24. The method for producing low-temperature steel according to claim 23, characterized in that: Adjust the mold taper, the water volume in the mold, and the water volume in sections 1 to 5 according to the change of the casting speed v, including: When the pulling speed v is below 1.20m / min, the taper of the crystallizer is 1.1-1.2%; otherwise, the taper of the crystallizer is 1.05-1.15%; When the casting speed v is below 1.20m / min, the water volume on the wide side of the crystallizer is 3600~3900NL / min, and the water volume on the narrow side is 390~420NL / min; otherwise, the water volume on the wide side of the crystallizer is 3900~4100NL / min, and the water volume on the narrow side is 420~450NL / min; The inner and outer arcs of sections 1 to 5 have a standard water volume corresponding to a pulling speed of v=1.05m / min and an enhanced water volume corresponding to a pulling speed of v>1.05m / min. The enhanced water volume is the sum of the standard water volume and the incremental water volume ΔT=T×floor((v-1.05m / min) / 0.05m / min), and T is 5~10NL / min; among them, the standard water volume of the inner and outer arcs of sections 1 to 5 corresponding to a pulling speed of v=1.05m / min are 250~290NL / min, 575±25NL / min, 525±25NL / min, 475±25NL / min, and 175±25NL / min, respectively.

25. The method for producing low-temperature steel according to claim 12, characterized in that: The production method further comprises: The continuous casting billet is fed into a heating furnace for heating. The maximum temperature of the preheating section is 750-850°C, the maximum temperature of the heating section is 1100-1200°C, the maximum temperature of the soaking section is 1150-1200°C, the heating rate of the preheating section is 20-30°C / min, and the heating rate of the heating section is 30-50°C / min. Hot rolling the heated continuous casting billet into a hot rolled plate with a thickness of 5 to 60 mm, with a starting rolling temperature of 1030 to 1130° C. and a finishing rolling temperature of 800 to 850° C.; The hot-rolled plate is naturally air-cooled to below 200°C, and then subjected to secondary quenching and tempering heat treatment, and then naturally cooled to room temperature to obtain a finished plate; wherein, the temperature of the first quenching is 800-900°C, and the temperature of the second quenching is 700-800°C; if h≤20mm, the tempering temperature is 600-620°C; if h≥40mm, the tempering temperature is 560-580°C; if 20mm<h<40mm, the tempering temperature is >580°C and <600°C.

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