High-purity low-temperature steel and production method therefor

Abstract

High-purity low-temperature steel and a production method therefor. The method comprises KR desulfurization, converter steelmaking, LF-RH refining, continuous casting, hot rolling, and heat treatment; the converter steelmaking period relates to three-stage blowing in a single converter; in the first stage, the lance position is 1.8-2 m in the first minute, the oxygen supply intensity is high, the bottom blowing intensity is high, then the lance position is 2.8-3 m, the oxygen supply intensity is low, the bottom blowing intensity is high, slag having an alkalinity of 1.8-2.5 and a total iron content of 20%-30% is formed, the slag is poured, and the end-point temperature is 1370-1420°C; in the second stage, pellets are added to form slag having an alkalinity of 4.5-8.5 and a total iron content of 18%-26%, the lance position is reduced by 10%-20%, the oxygen supply intensity is high, the bottom blowing intensity is low, the slag is poured, and the end-point temperature is 1550-1580°C; and in the third stage, the lance position is 2.8-3.0 m, the oxygen supply intensity is high, the bottom blowing intensity is high, slag having an alkalinity of 6-12 and a total iron content of 20%-30% is formed, the end-point temperature is 1600-1630°C, and slag cutoff tapping is carried out.

Description

High-purity low-temperature steel and production method thereof

[0001] This application claims priority to a Chinese patent application filed on March 22, 2024, with application number 202410330140.9, entitled “High-purity low-temperature steel and its production method,” the entire contents of which are incorporated herein by reference. Technical Field

[0002] The invention relates to high-purity low-temperature steel and a production method thereof, and belongs to the technical field of steel material production and manufacturing. Background Art

[0003] With rapid economic growth, market demand for high-quality clean energy, such as natural gas, is increasing. The large-scale transportation and storage of liquefied natural gas (LNG) places extremely stringent demands on container materials. Low-temperature steels of varying grades and uses, such as 9Ni, 5Ni, and 3.5Ni, are seeing increasing market demand as natural gas storage and transportation container materials, offering broad development prospects.

[0004] Nickel-based low-temperature steel refers to a series of nickel-containing special steels for welded structures that serve at low temperatures of -70°C to -196°C. It is one of the wide and thick plate products with the highest technical requirements and the greatest industry impact. 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 the particularity of its use, low-temperature steel has extremely high requirements for product cleanliness, and P, S, O, N, etc. are all required to be controlled at extremely low levels. There are many technical disclosures about P control, such as patent applications CN112680557A, CN101328529A, CN112662839A, etc., but these technologies have some defects. For example, CN112680557A has the problem of serious rephosphorization and damage to the ladle car cables, CN101328529A has the problem of long refining cycle, low production efficiency and high cost, CN112662839A has the problem of serious corrosion of refractory materials, poor purity of molten steel, and difficulty in operation. There are also some other dephosphorization technologies, or there are defects such as large equipment loss, or there are defects such as unsatisfactory dephosphorization effect, etc. Summary of the Invention

[0006] The object of the present invention is to provide a low-temperature steel and a production method thereof.

[0007] To achieve the above-mentioned purpose, an embodiment of the present invention provides a method for producing low-temperature steel. The method comprises:

[0008] Step 1: Treat molten iron in KR desulfurization equipment, with an outlet temperature of 1350-1400°C and a sulfur content of ≤0.0010%;

[0009] Step 2: The desulfurized molten iron, scrap steel and nickel plate are blown in a single converter in three stages; in the first stage of blowing: the oxygen lance position in the first minute is 1.8-2.0m, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.10Nm 3 / t / min, the oxygen lance position starting from the 2nd minute is 2.8~3.0m, and the top blowing oxygen intensity is 2.7~3.0Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, in this stage, slag with a basicity of 1.8-2.5 and a T.Fe content of 20-30% is produced. After the blowing is completed, argon is blown from the bottom for 1-2 minutes, and then a portion of the slag is poured. The final molten steel temperature is 1370-1420℃; the second stage of blowing: 8-16 kg / t of pellets are added to produce slag with a basicity of 4.5-8.5 and a T.Fe content of 18-26%. Compared with the first stage of blowing, the oxygen lance position is lowered by 10-20% after the second minute, and the top blowing oxygen intensity is 3.0-3.5 Nm 3 / t / min, bottom blowing argon intensity 0.05~0.08Nm 3 / t / min, part of the slag is poured before the end of blowing, and the final molten steel temperature is 1550-1580℃; the third stage of blowing: the oxygen lance is at a position of 2.8-3.0m, and the top blowing oxygen intensity is 3.2-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, in this stage, a slag with a basicity of 6.0-12.0 and a T.Fe content of 20-30% is produced. After blowing, a portion of the slag is removed. The final molten steel temperature is 1600-1630°C, and the slag is retained for tapping to obtain a molten steel with a C content of 0.02-0.05% and a P content of ≤0.0035%.

[0010] Step 3: The blown molten steel is refined in the LF furnace, which includes heating, alloying, and slag formation in sequence, with the outlet temperature reaching 1610-1630°C. The molten steel from the LF furnace is then transported to the RH refining furnace for vacuum treatment and tapping.

[0011] Step 4: The molten steel is hoisted from the RH refining furnace to the continuous casting platform for standing, and then poured to obtain continuous casting billets.

[0012] As a preferred embodiment, in step 2, in the first stage of blowing: 10-16 kg / t of lime and 6-15 kg / t of pellets are used for slagging, which are added in at least two batches, with the first batch adding 70-80% of the total weight of the lime and pellets respectively;

[0013] Second stage blowing: slag is made using 14-22 kg / t of lime, 8-16 kg / t of pellets, 7-10 kg / t of light-burned dolomite, and 2-4 kg / t of magnesium balls. These are added in at least two batches, with the first batch containing 70-80% of the total weight of the lime, pellets, light-burned dolomite, and magnesium balls.

[0014] The third stage of blowing: 8-12 kg / t of lime, 3-5 kg / t of pellets, and 1-2.5 kg / t of light-burned dolomite are used for slag making.

[0015] As a preferred embodiment, in step 2, deoxidation and alloying and slag making are carried out successively during the tapping process, 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;

[0016] 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.

[0017] As a preferred embodiment, in step 2, the chemical composition of the calcium aluminate synthetic slag includes, by weight percentage, the following: CaO 50-60%, Al2O3 35-45%, MgO 2-4%, 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.

[0018] As a preferred embodiment, in step 3, during the slagging stage, the slag is deoxidized using calcium carbide and aluminum particles 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 impurity components;

[0019] When vacuum treatment is carried out in the RH refining furnace, the treatment is first continued for 15 to 20 minutes at a vacuum degree below 1.5 mbar, then continued for 10 to 20 minutes at a vacuum degree above 5 mbar, and finally the air is broken to tap the steel.

[0020] As a preferred embodiment, in step 3, during the slagging stage, the slag is deoxidized using calcium carbide and a low-carbon steel slag surface deoxidizer to adjust the slag composition to contain, by weight percentage, 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, 2-5% T.Fe+MnO, and other inevitable impurity components;

[0021] When performing vacuum treatment in the RH 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. The vacuum treatment is then continued for 15-20 minutes; then the treatment is continued at a vacuum degree above 5mbar for 10-20 minutes, and finally the air is broken to tap the steel.

[0022] As a preferred embodiment, in step 3, during the slagging stage, 0.3-0.5 kg / t of calcium carbide and 1.0-2.0 kg / t of a second calcium aluminate synthetic slag are added to the molten steel to adjust the slag composition to contain, by 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;

[0023] When vacuum treatment is carried out in the RH refining furnace, the steel is first treated at a vacuum degree of more than 200 mbar for 3 to 5 minutes, then continuously treated at a vacuum degree of less than 1.5 mbar for 10 to 15 minutes, and then continuously treated at a vacuum degree of more than 50 mbar for more than 5 minutes, and finally the steel is tapped.

[0024] As a preferred embodiment, in step 3, after the vacuum degree of the RH refining furnace is reduced to 500 mbar, 1.0-1.5 m / t of calcium wire is fed from the area near the downcomer at a feeding speed of 4-6 m / s.

[0025] As a preferred embodiment, in step 3, after the vacuum degree of the RH refining furnace drops below 1.5 mbar, calcium wire is fed from an area near the riser at a rate of 0.5 to 1.0 m / t at a speed of 1 to 1.5 m / s during a continuous treatment period of 10 to 15 minutes.

[0026] As a preferred embodiment, in step three, 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, and the bottom blowing argon flow rates in the remaining stages are 150-250NL / min, respectively.

[0027] As a preferred embodiment, in step 4, a slab continuous casting machine is used for casting 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., the crystallizer uses a low-melting-point alkaline mold slag with a melting point of 1100 to 1200° C., the casting speed v is 1.05 to 1.35 m / min, and the taper of the crystallizer is 1.05 to 1.2%;

[0028] After exiting the crystallizer, the billet is cooled in a secondary cooling zone, which is divided into 10 sections according to the amount of water. The water amount in the first section is less than 1 / 10 of the water amount in the crystallizer, the water amount in sections 2 to 4 is greater than that in section 1 and decreases in sequence, the water amount in section 5 is less than that in section 1, the water amount in the inner arc of section 6 is less than that in the outer arc, and the water amount 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 allocated 25%, 25% and 50% of the pressing amount respectively. The pressing amount of billets with a thickness of 220 mm and 320 mm is 3 to 5 mm and 6 to 8 mm respectively. After the billet leaves all the fan-shaped sections, it is cut and stacked for slow cooling to obtain a continuously cast billet.

[0029] As a preferred embodiment, in step 4, when the pulling speed v is below 1.20 m / min, the taper of the crystallizer is 1.1-1.2%, the water volume on the wide side of the crystallizer is 3600-3900 NL / min, and the water volume on the narrow side is 390-420 NL / min; otherwise, the taper of the crystallizer is 1.05-1.15%, the water volume on the wide side of the crystallizer is 3900-4100 NL / min, and the water volume on the narrow side is 420-450 NL / min;

[0030] 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 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; 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.

[0031] As a preferred embodiment, in step four, the chemical composition of the obtained 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%, TO≤10ppm, P≤0.005%, S≤0.0020%, N≤0.0020%, and H≤1.5ppm.

[0032] As a preferred embodiment, in step 4, more than 90% of the oxide inclusions in the obtained continuous casting billet have an Al2O3 content of ≥80%, more than 98% of the oxide inclusions have a size of ≤5 μm, and the largest oxide inclusion does not exceed 20 μm.

[0033] As a preferred embodiment, the method further comprises:

[0034] Step 5: 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.

[0035] Step 6: After leaving the heating furnace, the continuous casting slab is hot rolled into a plate with a thickness of 5 to 60 mm, with a starting rolling temperature of 1030 to 1130° C., a finishing rolling temperature of 800 to 850° C., and a rolling amount of 10 to 15% per pass;

[0036] Step 7: The hot-rolled plate is subjected to secondary quenching and tempering heat treatment; the temperature of the first quenching is 800-900°C, and the temperature of the second quenching is 700-800°C; for plates with a thickness of less than 20 mm, the tempering temperature is 600-620°C; for plates with a thickness of more than 40 mm, the tempering temperature is 560-580°C; for plates of other thicknesses, the tempering temperature is greater than 580°C and less than 600°C;

[0037] Step 8: After cooling to room temperature, the finished board is obtained.

[0038] As a preferred embodiment, in step seven, 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 plate.

[0039] To achieve the above-mentioned object, one embodiment of the present invention further provides a low-temperature steel. The chemical composition of the low-temperature steel comprises, 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%, TO≤10ppm, P≤0.005%, S≤0.0020%, N≤0.0020%, H≤1.5ppm;

[0040] More than 90% of the oxide inclusions in the low-temperature steel have an Al2O3 content of ≥80%, more than 98% of the oxide inclusions have a size of ≤5 μm, and the largest oxide inclusion does not exceed 20 μm.

[0041] As a preferred embodiment, the low-temperature steel is a plate with a thickness of 5 to 60 mm, a Z-direction cross-sectional shrinkage ratio of ≥ 70%, and a low-temperature impact energy value of -196°C of ≥ 180J.

[0042] Compared with the prior art, the beneficial effects of one embodiment of the present invention are as follows: through this three-stage blowing technology, efficient dephosphorization can be achieved and rephosphorization can be avoided, and the production process causes little damage to the equipment. In addition, a single converter can be used to complete efficient smelting of molten steel, avoiding the problem of poor dephosphorization effect in traditional converter smelting, and also avoiding the problems of equipment occupation, low efficiency and high cost in the double-converter smelting process. The refined molten steel achieves high-purity smelting and meets the smelting requirements of high-purity molten steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] FIG1 is a schematic diagram of a method for producing low-temperature steel according to one embodiment of the present invention;

[0044] FIG2 is a schematic diagram of a partial structure of an RH refining furnace in one embodiment of the present invention;

[0045] FIG3 is a schematic diagram of a partial structure of an RH refining furnace in another embodiment of the present invention;

[0046] FIG4 is a cross-sectional view taken along line AA in FIG3 ;

[0047] FIG5 is another cross-sectional view along the AA section line in FIG3 . DETAILED DESCRIPTION

[0048] One embodiment of the present invention provides a method for producing low-temperature steel, which can be used to produce high-purity steel. Specifically, as shown in FIG1 , the method includes:

[0049] Step 1: Treat molten iron in KR desulfurization equipment, with an outlet temperature of 1350-1400°C and a sulfur content of ≤0.0010%;

[0050] Step 2: The desulfurized molten iron, scrap steel and nickel plate are blown in a single converter in three stages; in the first stage of blowing: the oxygen lance position in the first minute is 1.8-2.0m, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.10Nm 3 / t / min, the oxygen lance position starting from the 2nd minute is 2.8~3.0m, and the top blowing oxygen intensity is 2.7~3.0Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, in this stage, slag with a basicity of 1.8-2.5 and a T.Fe content of 20-30% is produced. After the blowing is completed, argon is blown from the bottom for 1-2 minutes, and then a portion of the slag is poured. The final molten steel temperature is 1370-1420℃; the second stage of blowing: 8-16 kg / t of pellets are added to produce slag with a basicity of 4.5-8.5 and a T.Fe content of 18-26%. Compared with the first stage of blowing, the oxygen lance position is lowered by 10-20% after the second minute, and the top blowing oxygen intensity is 3.0-3.5 Nm 3 / t / min, bottom blowing argon intensity 0.05~0.08Nm 3 / t / min, part of the slag is poured before the end of blowing, and the final molten steel temperature is 1550-1580℃; the third stage of blowing: the oxygen lance is at a position of 2.8-3.0m, and the top blowing oxygen intensity is 3.2-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, in this stage, a slag with a basicity of 6.0-12.0 and a T.Fe content of 20-30% is produced. After blowing, a portion of the slag is removed. The final molten steel temperature is 1600-1630°C, and the slag is retained for tapping to obtain a molten steel with a C content of 0.02-0.05% and a P content of ≤0.0035%.

[0051] Step 3: The blown molten steel is refined in the LF furnace, which includes heating, alloying, and slag formation in sequence, with the outlet temperature reaching 1610-1630°C. The molten steel from the LF furnace is then transported to the RH refining furnace for vacuum treatment and tapping.

[0052] Step 4: The molten steel is hoisted from the RH refining furnace to the continuous casting platform for standing, and then poured to obtain continuous casting billets.

[0053] Thus, the method for producing low-temperature steel according to one embodiment of the present invention employs a process route of KR desulfurization - converter smelting - LF refining - RH vacuum refining - continuous casting, wherein a single converter is used for three-stage blowing during converter smelting. The specific techniques of the three-stage blowing generally include:

[0054] In the first stage of blowing, a medium-alkalinity and highly oxidizing slag is produced. A low lance position, high oxygen supply intensity, and strong bottom blowing are used for a short blowing time at the start of blowing. This promotes efficient slag dephosphorization in the early stages of smelting, accelerates the oxidation heat release of Si in the molten iron, promotes early furnace temperature rise, and rapidly melts the slag. Smelting is then switched to a high lance position, low oxygen supply intensity, and strong bottom blowing. At the end of blowing, bottom blowing and stirring are used to promote slag-metal reaction dephosphorization. A portion of the slag is then discarded (e.g., 80-90% of the pre-slag by weight). This stage, serving as the early stages of the entire converter smelting, achieves efficient dephosphorization. The dephosphorized slag melts well after dephosphorization, significantly improving the dephosphorization effect.

[0055] In the second stage of blowing, a large amount of pellets are added to inhibit slag drying, reduce rephosphorization, and produce a high-basicity and high-oxidizing slag. Compared with the first stage of blowing, the lance position is lowered, the oxygen supply intensity is increased, and the bottom blowing intensity is reduced. This slows down the decarburization reaction, avoids excessive temperature increase and reduced slag oxidation, and stabilizes the slag oxidation. Before the end of blowing (for example, from 2 to 3 minutes before the end of blowing to the end of blowing), a portion of the slag (for example, 50 to 65% by weight) is discarded. This reduces rephosphorization and improves the dephosphorization effect.

[0056] In the third stage of blowing, slag is re-made, and ultra-high basicity + high oxidizing slag is made, and the high lance position and strong bottom blowing of the first stage of blowing are restored. Compared with the second stage of blowing, the oxygen supply intensity is further increased, which promotes slag dephosphorization and reduces later phosphorus regeneration, and stabilizes the dephosphorization effect. After the blowing is completed, a part of the slag is poured out (for example, 40-60% of the slag is poured out by weight), that is, the slag is retained for tapping.

[0057] Therefore, through this three-stage blowing technology, efficient dephosphorization can be achieved and rephosphorization can be avoided. The production process causes little damage to the equipment, and a single converter can be used to complete the efficient smelting of molten steel, avoiding the problem of poor dephosphorization effect in traditional converter smelting, and also avoiding the problems of equipment occupation, low efficiency and high cost in the double-converter smelting process. The refined molten steel achieves high-purity smelting and meets the smelting needs of high-purity molten steel.

[0058] Furthermore, the chemical composition of the obtained continuous casting billet includes, by weight percentage, TO≤10ppm, P≤0.005%, S≤0.0020%, N≤0.0020%, and H≤1.5ppm, and the content of impurity elements is low.

[0059] In addition, the resulting continuous casting ingot has the following chemical composition by weight: 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%, and Nb ≤ 0.035%, representing a high-purity low-temperature steel continuous casting ingot. Of course, based on the composition design of various low-temperature steel grades, elements such as Cu, Mo, Cr, and Nb can be added as alloying elements during the steelmaking process in some embodiments, or can be controlled as impurity elements rather than intentionally added as alloying elements in other embodiments.

[0060] In low temperature steel, the role of each element in this chemical composition is as follows.

[0061] 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.

[0062] 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).

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] In addition, more than 90% of the oxide inclusions in the obtained continuous casting billet have an Al2O3 content of ≥80%, more than 98% of the oxide inclusions have a size of ≤5μm, and the largest oxide inclusion does not exceed 20μm.

[0074] 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.

[0075] In addition, the central carbon segregation of the obtained continuous casting billet is below level 1.5, the surface crack is ≤1.5mm; the width deviation is within 5mm, and the thickness deviation is within 2mm.

[0076] Specifically, in step 1, the molten iron is treated in a KR desulfurization device, for example, the blast furnace molten iron is put into the KR desulfurization device for desulfurization treatment. After the desulfurization treatment, the outlet temperature is 1350-1400° C., and the S content of the outlet molten iron is ≤0.0010%.

[0077] 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.

[0078] 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.

[0079] Furthermore, in step 1, 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%.

[0080] In step 2, the desulfurized molten iron obtained in step 1 is combined with scrap steel and nickel plates for converter smelting. The total weight of the scrap steel and nickel plates is within 12.5% ​​of the total weight of the molten iron, scrap steel, and nickel plates. For example, the desulfurized molten iron weighs 175-185 tons, and the scrap steel and nickel plates weigh 20-25 tons.

[0081] The chemical composition of nickel plate, by weight, includes Ni ≥ 99%, P ≤ 0.025%, S ≤ 0.03%, with the remainder being Fe and other unavoidable impurities. The chemical composition of scrap steel, by weight, includes Si ≤ 0.6%, Mn ≤ 1.8%, Al ≤ 0.08%, P ≤ 0.02%, S ≤ 0.01%, with the remainder being Fe and other unavoidable impurities.

[0082] In an optional embodiment, 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.

[0083] In step 2, three-stage blowing is carried out in a single converter, which can achieve efficient dephosphorization while avoiding the problems of low efficiency and high cost.

[0084] During the first stage of blowing, a slag having a basicity of 1.8-2.5 and a T.Fe content of 20-30% is produced. In a further preferred embodiment, during the first stage of blowing, 10-16 kg / t of lime and 6-15 kg / t of pellets are used to produce the slag, which are added in at least two batches, with the first batch comprising 70-80% of the total weight of the lime and pellets, respectively. It will be appreciated that the remaining lime and pellets are added evenly in batches.

[0085] During the second stage of blowing, a slag with a basicity of 4.5-8.5 and a T.Fe content of 18-26% is produced. In a further preferred embodiment, during the second stage of blowing, 14-22 kg / t of lime, 8-16 kg / t of pellets, 7-10 kg / t of light-burned dolomite, and 2-4 kg / t of magnesium balls are used for slag production. These are added in at least two batches, with the first batch accounting for 70-80% of the total weight of the lime, pellets, light-burned dolomite, and magnesium balls, respectively. It will be appreciated that the remaining slag-making materials (including lime, pellets, light-burned dolomite, and magnesium balls) are added evenly in batches. That is, in this preferred embodiment, in addition to pellets, lime, light-burned dolomite, and magnesium balls are also used as slag-making materials.

[0086] In the third stage of blowing, slag with a basicity of 6.0-12.0 and a T.Fe content of 20-30% is produced. In a further preferred embodiment, in the third stage of blowing, 8-12 kg / t of lime, 3-5 kg / t of pellets, and 1-2.5 kg / t of light-burned dolomite are used for slag production.

[0087] 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.

[0088] Furthermore, in step 2, deoxidation alloying and slag making are carried out successively during the tapping process, 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.

[0089] 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.

[0090] 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.

[0091] Next, in step three, the molten steel obtained from the converter is refined, including LF refining first and RH vacuum refining later. Regarding LF refining and RH vacuum refining, this application provides three implementation methods, which are introduced below respectively.

[0092] [First embodiment]

[0093] 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).

[0094] The tapping temperature of LF refining is 1610~1630℃.

[0095] 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.

[0096] 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.

[0097] 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 refining furnace for RH vacuum refining.

[0098] In RH vacuum refining, that is, when vacuum treatment is carried out in the RH 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.

[0099] For example, referring to FIG2 , 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 refining furnace.

[0100] 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.

[0101] [Second embodiment]

[0102] 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).

[0103] The tapping temperature of LF refining is 1610~1630℃.

[0104] 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.

[0105] 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.

[0106] 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 refining furnace for RH vacuum refining.

[0107] In RH vacuum refining, that is, when vacuum treatment is carried out in the RH 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. 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.

[0108] Specifically, for example, referring to FIG2 , 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 refining furnace.

[0109] 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 230~250Nm 3 / 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.

[0110] 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.

[0111] 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.

[0112] [Third 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] 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.

[0116] 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.

[0117] In this way, based on the large slag volume and strong deoxidation synthetic slag in the converter smelting process, the LF refining process also uses a large slag volume and strong deoxidation synthetic slag for rapid deoxidation and slagging, which can further reduce the total oxygen content of the molten steel, reduce adsorbed inclusions, and improve the purity of the molten steel.

[0118] 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.

[0119] 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 refining furnace for RH vacuum refining.

[0120] In RH vacuum refining, that is, when vacuum treatment is carried out in the RH refining furnace, the steel is first treated at a vacuum degree of more than 200 mbar for 3 to 5 minutes, then continuously treated at a vacuum degree of less than 1.5 mbar for 10 to 15 minutes, and then continuously treated at a vacuum degree of more than 50 mbar for more than 5 minutes, and finally the steel is tapped.

[0121] 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 refining furnace.

[0122] After the molten steel is transported to the RH refining furnace, open the two-stage water circulation pump W1 & W2 within 1 minute, maintain the vacuum degree above 200mbar, and maintain 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.

[0123] 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., deoxidize). 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.

[0124] 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.

[0125] 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.

[0126] As a more preferred alternative embodiment, referring to Figure 5 , 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 duration 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] Furthermore, referring to FIG5 , 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 FIG5 . In other words, the calcium wire is fed between the fitting surface T and the mirror plane M. This further enhances the deoxidation effect.

[0131] Next, in step four, the molten steel is hoisted from the RH refining furnace to the continuous casting platform for standing, and then poured to obtain a continuous casting billet.

[0132] Specifically, the molten steel is hoisted from the RH 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.

[0133] 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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] After exiting the crystallizer, the billet is cooled in a secondary cooling zone, which is divided into 10 sections according to different water volumes.

[0140] 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.

[0141] In this way, on the one hand, the cooling intensity of the molten steel in the crystallizer is high (the water volume is large). After leaving the crystallizer, the cooling intensity is greatly reduced in the first section (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 warming and causing steel leakage; at the same time, the water volume in the second to fourth sections decreases successively, 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, fatigue overcooling can also be avoided, which may cause 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 due to overcooling of the edges / corners; on the other hand, by pressing the billet down and regulating the distribution of the pressing amount, the internal quality and segregation problems of the billet can be optimized.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] 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.

[0146] 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.

[0147] 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.

[0148] 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.

[0149] 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.

[0150] Furthermore, the water volume on the narrow side of the first section is 40 to 60 NL / min.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] 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.

[0158] 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.

[0159] In one embodiment, when the blanks are stacked and slowly cooled, they are stacked and slowly cooled in a location surrounded by enclosures, and are destackered after slowly cooling to a temperature below 200°C.

[0160] As of step 4, the production method produces a high-purity low-temperature steel continuous casting billet; and further, in one embodiment, the method further includes:

[0161] Step 5: 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.

[0162] Step 6: After leaving the heating furnace, the continuous casting slab is hot rolled into a plate with a thickness of 5 to 60 mm, with a starting rolling temperature of 1030 to 1130° C., a finishing rolling temperature of 800 to 850° C., and a rolling amount of 10 to 15% per pass;

[0163] Step 7: The hot-rolled plate is subjected to secondary quenching and tempering heat treatment; the temperature of the first quenching is 800-900°C, and the temperature of the second quenching is 700-800°C; for plates with a thickness of less than 20 mm, the tempering temperature is 600-620°C; for plates with a thickness of more than 40 mm, the tempering temperature is 560-580°C; for plates of other thicknesses, the tempering temperature is greater than 580°C and less than 600°C;

[0164] Step 8: After cooling to room temperature, the finished board is obtained.

[0165] In this way, the production method uses the continuous casting billets obtained in the previous steps one to four to further produce finished plate products through a heating process, a hot rolling process, a tempering heat treatment process and a cooling process. While having the same alloy elements as the prior art, a plate with better low-temperature performance and room-temperature performance can be obtained.

[0166] In a specific embodiment, the Z-direction cross-sectional shrinkage of the obtained plate is ≥70%, and the low-temperature impact energy values ​​at -80°C, -95°C, -125°C and -196°C are all ≥180J. Even the low-temperature impact energy values ​​at these temperatures are ≥210J, or above 245J, or even above 320J.

[0167] Specifically, in step five, 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, and then the continuous casting billet is sent into the heating furnace.

[0168] 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.

[0169] 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.

[0170] Next, step seven includes the initial secondary quenching stage and the 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.

[0171] 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 plate.

[0172] The above is a detailed introduction to the production method of one embodiment of the present invention. The following introduces several embodiments of using the production method of the present invention to prepare finished plate products. In these embodiments, in addition to the parameters mentioned below, some other important parameters are as described above and are not repeated here.

[0173] Examples 1 to 6

[0174] In general, Examples 1 to 6 prepare 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.

[0175] The blast furnace molten iron is put into the KR desulfurization equipment for desulfurization treatment. After the desulfurization is completed, the slag is skimmed. The outlet molten iron temperature is shown in Table 1. The S content of the outlet molten iron is ≤0.0010%.

[0176] According to the weight relationship in Table 1, the obtained blast furnace iron, scrap steel and nickel plate are put into a single-seat converter for three-stage blowing.

[0177] In the first stage of blowing, the oxygen lance position in the first minute is 1.8-2.0m, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.10Nm 3 / t / min, the oxygen lance position starting from the 2nd minute is 2.8~3.0m, and the top blowing oxygen intensity is 2.7~3.0Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, lime and pellets are used for slag making, the slag basicity and T.Fe content are shown in Table 1, argon is blown continuously at the bottom for 2 minutes after blowing, and the amount of slag discarded is shown in the slag dumping rate in Table 1, and the terminal molten steel temperature is shown in Table 1.

[0178] [Table 1]

[0179] In the second stage of blowing, the oxygen lance position is lowered by 10-20% compared with the oxygen lance position after the second minute of the first stage of blowing, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.05~0.08Nm 3 / t / min, lime, pellets, light-burned dolomite and magnesium balls are used for slag making. The slag basicity, T.Fe content, approximate slag discharge rate before the end of blowing and terminal molten steel temperature are shown in Table 2.

[0180] In the third stage of blowing, the oxygen lance is located at 2.8 to 3.0 m and the top blowing oxygen intensity is 3.2 to 3.5 Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, lime, pellets, and light-burned dolomite are used for slag making. The slag basicity, T.Fe content, approximate slag discharge rate after blowing, and terminal molten steel temperature are shown in Table 2. The C content of the terminal molten steel is 0.02-0.05%, and the P content is ≤0.0035%.

[0181] [Table 2]

[0182] The steel is tapped with slag. 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. All of the additions are completed when 60-70% of the steel is tapped. Afterwards, lime and calcium aluminate synthetic slag are added to the molten steel for slagging. All of the additions are completed when 80-90% of the steel is tapped.

[0183] 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 temperatures are shown in Table 3. In the slag-making stage, calcium carbide and aluminum particles are used to adjust the slag composition, as shown in Table 3.

[0184] [Table 3]

[0185] Then, the molten steel obtained from LF furnace refining is transported to the RH refining furnace for RH vacuum refining, firstly treated at a vacuum degree below 1.5mbar for 15 to 20 minutes, then treated at a vacuum degree above 5mbar for 10 to 20 minutes, and finally the steel is tapped.

[0186] Molten steel was hoisted from the RH refining furnace onto the continuous casting platform and allowed to rest before pouring began, producing continuous casting ingots with a thickness of 220 mm or 320 mm and a width of 1500 to 2300 mm. The chemical compositions of the resulting continuous casting ingots are shown in Table 4.

[0187] [Table 4]

[0188] [Table 4 continued]

[0189] In addition, M% of the oxide inclusions in the continuous casting slab have an Al2O3 content ≥ 80%, and N% of the oxide inclusions have a size ≤ 5μm, where M and N are shown in Table 5 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 5 respectively.

[0190] [Table 5]

[0191] Next, Examples 1 to 6 were prepared into finished board products using the following process:

[0192] After the continuous casting billet was cooled to room temperature, the surface was ground, and then a high-temperature anti-oxidation coating with a thickness of 0.8 to 1.5 mm was sprayed on the ground surface. The continuous casting billet was then sent to a heating furnace for heating. The maximum temperatures of the preheating section, heating section, and soaking section are shown in Table 6. The heating rate in the preheating section was 20 to 30°C / min, and the heating rate in the heating section was 30 to 50°C / min.

[0193] After leaving the heating furnace, the continuous casting slab is hot rolled into a plate. The thickness, starting rolling temperature, and finishing rolling temperature of the hot rolled plate are shown in Table 6. The rolling amount of each pass is 10-15%.

[0194] [Table 6]

[0195] The hot-rolled plate was subjected to a secondary quenching and tempering heat treatment and cooled to room temperature to obtain a finished plate. The temperature and holding time of the first quenching, the temperature and holding time of the second quenching, and the tempering temperature and holding time are shown in Table 7.

[0196] [Table 7]

[0197] The finished plate products obtained in each embodiment were sampled and tested. The Z-direction cross-sectional shrinkage of the obtained plate was ≥70%, and the mechanical strength and low-temperature impact energy values ​​were shown in Table 8.

[0198] [Table 8]

[0199] Examples 7 to 12

[0200] In general, Examples 7 to 12 prepare 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.

[0201] The blast furnace molten iron was put into the KR desulfurization equipment for desulfurization treatment. After the desulfurization was completed, the slag was skimmed. The outlet molten iron temperature was shown in Table 9. The S content of the outlet molten iron was ≤0.0010%.

[0202] According to the weight relationship in Table 9, the obtained blast furnace molten iron, scrap steel and nickel plate were put into a single-seat converter for three-stage blowing.

[0203] In the first stage of blowing, the oxygen lance position in the first minute is 1.8-2.0m, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.10Nm 3 / t / min, the oxygen lance position starting from the 2nd minute is 2.8~3.0m, and the top blowing oxygen intensity is 2.7~3.0Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, lime and pellets are used for slag making, the slag basicity and T.Fe content are shown in Table 9, argon is blown continuously at the bottom for 2 minutes after blowing, and the approximate slag pouring rate and terminal molten steel temperature are shown in Table 9.

[0204] [Table 9]

[0205] In the second stage of blowing, the oxygen lance position is lowered by 10-20% compared with the oxygen lance position after the second minute of the first stage of blowing, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.05~0.08Nm 3 / t / min, lime, pellets, light-burned dolomite and magnesium balls are used for slag making. The slag basicity, T.Fe content, approximate slag discharge rate before the end of blowing and terminal molten steel temperature are shown in Table 10.

[0206] In the third stage of blowing, the oxygen lance is located at 2.8 to 3.0 m and the top blowing oxygen intensity is 3.2 to 3.5 Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, lime, pellets and light-burned dolomite are used for slag making. The slag basicity, T.Fe content, approximate slag discharge rate after blowing and terminal molten steel temperature are shown in Table 10. The C content of the terminal molten steel is 0.02-0.05% and the P content is ≤0.0035%.

[0207] [Table 10]

[0208] The steel is tapped with slag. 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. All of the additions are completed when 60-70% of the steel is tapped. Afterwards, lime and calcium aluminate synthetic slag are added to the molten steel for slagging. All of the additions are completed when 80-90% of the steel is tapped.

[0209] 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 temperatures are shown in Table 11. During the slag-making stage, calcium carbide and the low-carbon steel slag surface deoxidizer are used to adjust the slag composition, as shown in Table 11.

[0210] [Table 11]

[0211] Then, the molten steel obtained from LF furnace refining is transported to the RH refining furnace for RH vacuum refining. First, the vacuum degree is 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. Then, the vacuum treatment is continued for 15-20 minutes; then, the treatment is continued at a vacuum degree above 5mbar for 10-20 minutes, and finally the steel is tapped.

[0212] Molten steel was hoisted from the RH refining furnace onto the continuous casting platform and allowed to rest before pouring began, resulting in billets with a thickness of 220 mm or 320 mm and a width of 1500 to 2300 mm. The chemical composition of the resulting continuous casting billets, expressed in weight percentage, is shown in Table 12.

[0213] [Table 12]

[0214] [Table 12 continued]

[0215] In addition, M% of the oxide inclusions in the continuous casting ingot have an Al2O3 content ≥80%, and N% of the oxide inclusions have a size ≤5μm, where M and N are shown in Table 13 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 13 respectively.

[0216] [Table 13]

[0217] Next, Examples 7 to 12 were prepared into finished board products using the following process:

[0218] After the continuous casting billet was cooled to room temperature, the surface was ground, and then a high-temperature anti-oxidation coating with a thickness of 0.8 to 1.5 mm was sprayed on the ground surface. The continuous casting billet was then sent to a heating furnace for heating. The maximum temperatures of the preheating section, heating section, and soaking section are shown in Table 14. The heating rate in the preheating section was 20 to 30°C / min, and the heating rate in the heating section was 30 to 50°C / min.

[0219] After leaving the heating furnace, the continuous casting slab is hot rolled into a plate. The thickness, starting rolling temperature, and finishing rolling temperature of the hot rolled plate are shown in Table 14. The rolling amount of each pass is 10-15%.

[0220] [Table 14]

[0221] The hot-rolled plates were subjected to a secondary quenching and tempering heat treatment and cooled to room temperature to obtain finished plates. The temperature and holding time for the first quenching, the temperature and holding time for the second quenching, and the tempering temperature and holding time are shown in Table 15.

[0222] [Table 15]

[0223] The finished plate products obtained in each embodiment were sampled and tested, and the Z-direction cross-sectional shrinkage of the obtained plate was ≥70%, and the mechanical strength and low-temperature impact energy values ​​were shown in Table 16.

[0224] [Table 16]

[0225] Examples 13 to 18

[0226] In general, Examples 13 to 18 prepared 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 was implemented using the third embodiment described above.

[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. The outlet molten iron temperature is shown in Table 17. The S content of the outlet molten iron is ≤0.0010%.

[0228] According to the weight relationship in Table 17, blast furnace iron, scrap steel and nickel plates are put into a single-seat converter for three-stage blowing.

[0229] In the first stage of blowing, the oxygen lance position in the first minute is 1.8-2.0m, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.10Nm 3 / t / min, the oxygen lance position starting from the 2nd minute is 2.8~3.0m, and the top blowing oxygen intensity is 2.7~3.0Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, lime and pellets are used for slag making, the slag basicity and T.Fe content are shown in Table 17, argon is blown continuously at the bottom for 2 minutes after blowing, and the approximate slag pouring rate and terminal molten steel temperature are shown in Table 17.

[0230] [Table 17]

[0231] In the second stage of blowing, the oxygen lance position is lowered by 10-20% compared with the oxygen lance position after the second minute of the first stage of blowing, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.05~0.08Nm 3 / t / min, lime, pellets, light-burned dolomite and magnesium balls are used for slag making. The slag basicity, T.Fe content, approximate slag discharge rate before the end of blowing and terminal molten steel temperature are shown in Table 18.

[0232] In the third stage of blowing, the oxygen lance is located at 2.8 to 3.0 m and the top blowing oxygen intensity is 3.2 to 3.5 Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, lime, pellets and light-burned dolomite are used for slag making. The slag basicity, T.Fe content, approximate slag discharge rate after blowing and terminal molten steel temperature are shown in Table 18. The C content of the terminal molten steel is 0.02-0.05%, and the P content is ≤0.0035%.

[0233] [Table 18]

[0234] The steel is tapped with slag. 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. All of the additions are completed when 60-70% of the steel is tapped. Afterwards, lime and calcium aluminate synthetic slag are added to the molten steel for slagging. All of the additions are completed when 80-90% of the steel is tapped.

[0235] Molten steel from the converter is transported to the LF furnace for refining, which includes a sequential heating stage, an alloying stage, and a slag-making stage. The tapping temperatures are shown in Table 19. During the slag-making stage, calcium carbide and a second calcium aluminate synthetic slag are used to adjust the slag composition, as shown in Table 19.

[0236] [Table 19]

[0237] The molten steel obtained from the LF refining process is then transferred to the RH refining furnace for RH vacuum refining. The refining is initially conducted at a vacuum level above 200 mbar for 3-5 minutes, then below 1.5 mbar for 10-15 minutes, and finally above 50 mbar for at least 5 minutes before being tapped. During this period, calcium wire is fed into the molten steel from area S21 near the riser 32 at a rate of 1-1.5 m / s at a vacuum level below 1.5 mbar.

[0238] Molten steel was hoisted from the RH refining furnace onto the continuous casting platform and allowed to rest before pouring began, resulting in billets with a thickness of 220 mm or 320 mm and a width of 1500 to 2300 mm. The chemical composition of the resulting continuous casting billets, expressed in weight percentage, is shown in Table 20.

[0239] [Table 20]

[0240] [Table 20 continued]

[0241] In addition, M% of the oxide inclusions in the continuous casting ingot have an Al2O3 content ≥80%, and 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 21 respectively.

[0242] [Table 21]

[0243] Next, Examples 13 to 18 were prepared into finished plate products using the following process:

[0244] After the continuous casting billet was cooled to room temperature, the surface was ground, and then a high-temperature anti-oxidation coating with a thickness of 0.8 to 1.5 mm was sprayed on the ground surface. The continuous casting billet was then sent to a heating furnace for heating. The maximum temperatures of the preheating section, heating section, and soaking section are shown in Table 22. The heating rate in the preheating section was 20 to 30°C / min, and the heating rate in the heating section was 30 to 50°C / min.

[0245] After leaving the heating furnace, the continuous casting slab is hot rolled into plates. The thickness, starting rolling temperature and finishing rolling temperature of the hot rolled plates are shown in Table 22.

[0246] [Table 22]

[0247] The hot-rolled plates were subjected to a secondary quenching and tempering heat treatment and cooled to room temperature to obtain finished plates. The temperature and holding time for the first quenching, the temperature and holding time for the second quenching, and the tempering temperature and holding time are shown in Table 23.

[0248] [Table 23]

[0249] The finished plate products obtained in each embodiment were sampled and tested, and the Z-direction cross-sectional shrinkage rate of the obtained plate was ≥70%, and the mechanical strength and low-temperature impact energy values ​​were shown in Table 24 respectively.

[0250] [Table 24]

[0251] In summary, one embodiment of the present invention, through the three-stage blowing technology, can achieve efficient dephosphorization and avoid rephosphorization. At the same time, the production process causes little damage to the equipment, and a single converter can be used to complete the efficient smelting of molten steel, avoiding the problem of poor dephosphorization effect in traditional converter smelting, and also avoiding the problems of equipment occupation, low efficiency and high cost in the double-converter smelting process. The refined molten steel achieves high-purity smelting, meeting the smelting requirements of high-purity molten steel.

Claims

1. A method for producing low-temperature steel, characterized in that: The method comprises, Step 1: Treat molten iron in KR desulfurization equipment, with an outlet temperature of 1350-1400°C and a sulfur content of ≤0.0010%; Step 2: The desulfurized molten iron, scrap steel and nickel plate are blown in a single converter in three stages; in the first stage of blowing: the oxygen lance position in the first minute is 1.8-2.0m, and the top blowing oxygen intensity is 3.0-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.10Nm 3 / t / min, the oxygen lance position starting from the 2nd minute is 2.8~3.0m, and the top blowing oxygen intensity is 2.7~3.0Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, in this stage, slag with a basicity of 1.8-2.5 and a T.Fe content of 20-30% is produced. After the blowing is completed, argon is blown from the bottom for 1-2 minutes, and then a portion of the slag is poured. The final molten steel temperature is 1370-1420℃; the second stage of blowing: 8-16 kg / t of pellets are added to produce slag with a basicity of 4.5-8.5 and a T.Fe content of 18-26%. Compared with the first stage of blowing, the oxygen lance position is lowered by 10-20% after the second minute, and the top blowing oxygen intensity is 3.0-3.5 Nm 3 / t / min, bottom blowing argon intensity 0.05~0.08Nm 3 / t / min, part of the slag is poured before the end of blowing, and the final molten steel temperature is 1550-1580℃; the third stage of blowing: the oxygen lance is at a position of 2.8-3.0m, and the top blowing oxygen intensity is 3.2-3.5Nm 3 / t / min, bottom blowing argon intensity 0.08~0.15Nm 3 / t / min, in this stage, a slag with a basicity of 6.0-12.0 and a T.Fe content of 20-30% is produced. After blowing, a portion of the slag is removed. The final molten steel temperature is 1600-1630°C, and the slag is retained for tapping to obtain a molten steel with a C content of 0.02-0.05% and a P content of ≤0.0035%. Step 3: The blown molten steel is refined in the LF furnace, which includes heating, alloying, and slag formation in sequence, with the outlet temperature reaching 1610-1630°C. The molten steel from the LF furnace is then transported to the RH refining furnace for vacuum treatment and tapping. Step 4: The molten steel is hoisted from the RH refining furnace to the continuous casting platform for standing, and then poured to obtain continuous casting billets.

2. The method for producing low-temperature steel according to claim 1, characterized in that: In step 2, the first stage of blowing: lime 10-16 kg / t and pellets 6-15 kg / t are used for slagging, which are added in at least two batches, with the first batch adding 70-80% of the total weight of lime and pellets respectively; Second stage blowing: slag is made using 14-22 kg / t of lime, 8-16 kg / t of pellets, 7-10 kg / t of light-burned dolomite, and 2-4 kg / t of magnesium balls. These are added in at least two batches, with the first batch containing 70-80% of the total weight of the lime, pellets, light-burned dolomite, and magnesium balls. The third stage of blowing: 8-12 kg / t of lime, 3-5 kg / t of pellets, and 1-2.5 kg / t of light-burned dolomite are used for slag making.

3. The method for producing low-temperature steel according to claim 1, characterized in that: In step 2, deoxidation, alloying and slag making are carried out successively during the tapping process, 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.

4. The method for producing low-temperature steel according to claim 3, characterized in that: In step 2, the chemical composition of the calcium aluminate synthetic slag includes, by weight percentage, CaO 50-60%, Al2O3 35-45%, MgO 2-4%, 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.

5. The method for producing low-temperature steel according to claim 1, characterized in that: In step 3, during the slagging stage, the slag is deoxidized using calcium carbide and aluminum granules to adjust the slag composition to contain, by weight, 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, less than 1.5% T.Fe+MnO, and other inevitable impurities; When vacuum treatment is carried out in the RH refining furnace, the treatment is first continued for 15 to 20 minutes at a vacuum degree below 1.5 mbar, then continued for 10 to 20 minutes at a vacuum degree above 5 mbar, and finally the air is broken to tap the steel.

6. The method for producing low-temperature steel according to claim 1, characterized in that: In step 3, during the slagging stage, the slag is deoxidized using calcium carbide and a low-carbon steel slag surface deoxidizer to adjust the slag composition to contain, by weight percentage, 50-55% CaO, 30-35% Al2O3, 3-6% SiO2, 4-7% MgO, 2-5% T.Fe+MnO, and other inevitable impurities; When performing vacuum treatment in the RH 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. The vacuum treatment is then continued for 15-20 minutes; then the treatment is continued at a vacuum degree above 5mbar for 10-20 minutes, and finally the air is broken to tap the steel.

7. The method for producing low-temperature steel according to claim 1, characterized in that: In step 3, during the slagging stage, 0.3-0.5 kg / t of calcium carbide and 1.0-2.0 kg / t of a second calcium aluminate synthetic slag are added to the molten steel to adjust the slag composition to 50-55% by mass of CaO, 30-35% by mass of Al2O3, 1-3% by mass of CaF2, less than 3% by mass of SiO2, 4-6% by mass of MgO, less than 1% by mass of T.Fe+MnO, and other inevitable impurities; When vacuum treatment is carried out in the RH refining furnace, the steel is first treated at a vacuum degree of more than 200 mbar for 3 to 5 minutes, then continuously treated at a vacuum degree of less than 1.5 mbar for 10 to 15 minutes, and then continuously treated at a vacuum degree of more than 50 mbar for more than 5 minutes, and finally the steel is tapped.

8. The method for producing low-temperature steel according to claim 7, characterized in that: In step 3, after the vacuum degree of the RH refining furnace is reduced to 500 mbar, 1.0-1.5 m / t of calcium wire is fed from the area near the downcomer at a feeding speed of 4-6 m / s.

9. The method for producing low-temperature steel according to claim 7, characterized in that: In step 3, after the vacuum degree of the RH refining furnace drops below 1.5 mbar, the calcium wire is fed from the area near the riser at a rate of 0.5 to 1.0 m / t at a speed of 1 to 1.5 m / s for 10 to 15 minutes.

10. The method for producing low-temperature steel according to claim 1, characterized in that: In step three, 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, and the bottom blowing argon flow rates in the remaining stages are 150-250NL / min, respectively.

11. The method for producing low-temperature steel according to claim 1, characterized in that: In step 4, a slab continuous casting machine is used for casting 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, the crystallizer uses a low-melting-point alkaline mold slag with a melting point of 1100 to 1200°C, the casting speed v is 1.05 to 1.35 m / min, and the taper of the crystallizer is 1.05 to 1.2%; After exiting the crystallizer, the billet is cooled in a secondary cooling zone, which is divided into 10 sections according to the amount of water. The water amount in the first section is less than 1 / 10 of the water amount in the crystallizer, the water amount in sections 2 to 4 is greater than that in section 1 and decreases in sequence, the water amount in section 5 is less than that in section 1, the water amount in the inner arc of section 6 is less than that in the outer arc, and the water amount 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 allocated 25%, 25% and 50% of the pressing amount respectively. The pressing amount of billets with a thickness of 220 mm and 320 mm is 3 to 5 mm and 6 to 8 mm respectively. After the billet leaves all the fan-shaped sections, it is cut and stacked for slow cooling to obtain a continuously cast billet.

12. The method for producing low-temperature steel according to claim 11, characterized in that: In step 4, when the casting speed v is below 1.20 m / min, the taper of the crystallizer is 1.1-1.2%, the water volume on the wide side of the crystallizer is 3600-3900 NL / min, and the water volume on the narrow side is 390-420 NL / min; otherwise, the taper of the crystallizer is 1.05-1.15%, the water volume on the wide side of the crystallizer is 3900-4100 NL / min, and the water volume on the narrow side is 420-450 NL / 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 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; 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.

13. The method for producing low-temperature steel according to claim 1, characterized in that: In step 4, the chemical composition of the obtained 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%, TO≤10ppm, P≤0.005%, S≤0.0020%, N≤0.0020%, and H≤1.5ppm.

14. The method for producing low-temperature steel according to claim 1, characterized in that: In step 4, more than 90% of the oxide inclusions in the obtained continuous casting billet have an Al2O3 content of ≥80%, more than 98% of the oxide inclusions have a size of ≤5 μm, and the largest oxide inclusion does not exceed 20 μm.

15. The method for producing low-temperature steel according to claim 1, characterized in that: The method further comprises, Step 5: 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. Step 6: After leaving the heating furnace, the continuous casting slab is hot rolled into a plate with a thickness of 5 to 60 mm, with a starting rolling temperature of 1030 to 1130° C., a finishing rolling temperature of 800 to 850° C., and a rolling amount of 10 to 15% per pass; Step 7: The hot-rolled plate is subjected to secondary quenching and tempering heat treatment; the temperature of the first quenching is 800-900°C, and the temperature of the second quenching is 700-800°C; for plates with a thickness of less than 20 mm, the tempering temperature is 600-620°C; for plates with a thickness of more than 40 mm, the tempering temperature is 560-580°C; for plates of other thicknesses, the tempering temperature is greater than 580°C and less than 600°C; Step 8: After cooling to room temperature, the finished board is obtained.

16. The method for producing low-temperature steel according to claim 15, characterized in that: In step seven, 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 plate.

17. A low temperature steel, characterized in that: The chemical composition of the low-temperature steel 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%, TO≤10ppm, P≤0.005%, S≤0.0020%, N≤0.0020%, H≤1.5ppm; More than 90% of the oxide inclusions in the low-temperature steel have an Al2O3 content of ≥80%, more than 98% of the oxide inclusions have a size of ≤5 μm, and the largest oxide inclusion does not exceed 20 μm.

18. The low-temperature steel according to claim 17, characterized in that The low-temperature steel is a plate with a thickness of 5 to 60 mm, a Z-direction cross-sectional shrinkage rate of ≥70%, and a low-temperature impact energy value of -196°C of ≥180J.

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