CN118109661A - Method for producing low-temperature steel with controlled total oxygen content - Google Patents

Abstract

The present invention discloses a method for producing low-temperature steel with controlled total oxygen content. The method comprises: KR desulfurization, with an outlet temperature of 1300-1350°C and S≤0.0010% by mass percentage; converter smelting, with a tapping temperature of 1580-1620°C, and the tapping molten steel having a C content of 0.02-0.05%, an O content of 0.025-0.040%, a P content of ≤0.0055%, and an S content of ≤0.0025% by mass percentage; LF refining, with a tapping temperature of 1610-1630°C; vacuum treatment after the molten steel is transported to the RH vacuum furnace, and after the vacuum degree drops below 1.5 mbar, a calcium wire of 0.5-1.0 m/t is fed at a position close to the riser and the wire feeding speed is 1-1.5 m/s; casting is performed to obtain a continuous casting billet. In this way, most of the calcium is quickly drawn into the vacuum chamber, and a small amount of calcium is dissolved in the molten steel in the ladle, thereby achieving rapid and efficient deoxidation.

Description

Production method of low temperature steel with controlled total oxygen content

Technical Field

The invention belongs to the technical field of steel material preparation, and relates to a method for producing low-temperature steel with controlled total oxygen content.

Background technique

As energy demand continues to increase, the demand for low-temperature steel in shipbuilding, bridges, natural gas pipelines, offshore platforms and other fields is also increasing. Especially with the rapid economic growth, the market demand for high-quality clean energy such as natural gas is increasing. The large-scale transportation and storage tanks of liquefied natural gas have extremely stringent requirements on materials. The market demand for low-temperature container steel of different grades and uses represented by 9Ni, 5Ni, 3.5Ni, etc. is expanding, and has broad development prospects.

Nickel-based low-temperature steel refers to a series of special steels containing Ni for welding structures that can be used 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 industry, and is used to manufacture various production and storage containers for liquefied petroleum gas, liquid ethylene, liquid oxygen, liquid nitrogen, liquefied natural gas, etc.

Due to the special use of low-temperature steel, the cleanliness of the product is extremely high, and P, S, O, N, H, etc. are required to be controlled at extremely low levels. In particular, the total oxygen content directly affects the size, quantity and type of inclusions in the steel, and has a profound impact on the performance of low-temperature steel.

Summary of the invention

The object of the present invention is to provide a method for producing low-temperature steel with controlled total oxygen content.

To achieve the above-mentioned purpose, an embodiment of the present invention provides a method for producing low-temperature steel with controlled total oxygen content. The method comprises:

KR desulfurization process: the outlet temperature of molten iron is 1300-1350℃, and S is ≤0.0010% by mass percentage; converter smelting process: the tapping temperature of molten steel is 1580-1620℃, and the tapping molten steel has a C content of 0.02-0.05%, an O content of 0.025-0.040%, a P content of ≤0.0055%, and a S content of ≤0.0025% by mass percentage;

LF refining process: molten steel temperature 1610～1630℃;

RH vacuum refining process: after the molten steel is transported to the RH vacuum furnace, it is vacuum treated. During the process, after the vacuum degree drops below 1.5 mbar, a calcium wire of 0.5-1.0 m/t is fed into the position closer to the ascending pipe than the descending pipe below the vacuum chamber, and the feeding speed is 1-1.5 m/s; the interior 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 8-10 mm and a thickness of 1-2 mm; finally, the vacuum is broken and the steel is tapped;

Continuous casting process: The molten steel from the RH vacuum refining process is poured to obtain a continuous casting billet.

Preferably, in the RH vacuum refining process, an RH vacuum furnace is used in which a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump and a two-stage water circulation pump are sequentially arranged in the vacuum exhaust pipeline of the vacuum chamber; after the molten steel is transported to the RH vacuum furnace, the two-stage water circulation pump, the fourth-stage steam pump, the third-stage steam pump, the second-stage steam pump and the first-stage steam pump are sequentially opened to perform vacuum treatment, and after the vacuum degree drops below 1.5 mbar, the treatment is continued for 10 to 15 minutes, during which calcium wire is fed.

Preferably, in the RH vacuum refining process, the two-stage water circulation pump is turned on within 1 minute after the molten steel is transported to the RH vacuum furnace, the vacuum degree is maintained at above 200 mbar, and the lifting gas flow rate is maintained at 80-100 Nm ³ /h, and the process is carried out for 3-5 minutes; then, the fourth-stage steam pump, the third-stage steam pump, the second-stage steam pump, and the first-stage steam pump are turned on in sequence, and the lifting gas flow rate is increased. After the vacuum degree drops below 1.5 mbar, the process is continued for 10-15 minutes; then the third-stage steam pump, the second-stage steam pump, and the first-stage steam pump are turned off, the vacuum degree is adjusted to above 50 mbar, and then the process is carried out for more than 5 minutes, and finally the air is broken and the steel is tapped.

Preferably, 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.

Preferably, the downcomer and the riser below the vacuum chamber are symmetrically distributed in a mirror plane, the ladle below the vacuum chamber has a fitting surface passing through the center of the riser and parallel to the mirror plane, and the position of the feeding calcium wire is between the fitting surface and the mirror plane.

Preferably, in the converter smelting process: 15-18m ³ /t of oxygen is blown into the molten steel, and after the oxygen blowing is completed, the first slag retention and slag removal is carried out; after the first slag retention and slag removal is completed, oxygen blowing is carried out again until the C content in the molten steel reaches below 0.05%, and then the oxygen blowing is terminated, and then the second slag retention and slag removal is carried out; after the second slag retention and slag removal is completed, 2-4kg/t of ferrosilicon particles and 1-3kg/t of ferromagnesium particles are sequentially added into the converter, and bottom argon blowing is started at a flow rate of 0.05-0.10Nm ³ /min and continued for 1.5-3min before tapping the steel.

Preferably, in the converter smelting process, deoxidation and alloying are performed during the steel tapping process. After the alloying is completed, 5-8 kg/t of lime and 15-20 kg/t of calcium aluminate synthetic slag are added for slagging, and all of the addition is completed before the steel tapping is completed; the components of the calcium aluminate synthetic slag include, by mass percentage, 40-45% of CaO, 10-15% of Al ₂ O ₃ , 5-10% of CaF ₂ , less than 3% of SiO ₂ , 2-5% of MgO, 5-10% of CaC ₂ , and 15-20% of elemental aluminum, wherein the mass percentage of the phase 12CaO·7Al ₂ O ₃ exceeds 30%, and the rest are CaO, CaF ₂ , SiO ₂ , single phase or composite phase of MgO; and, the flow rate of argon gas blown at the bottom of the ladle during the steel tapping process is 300-500NL/min, and after the steel tapping is completed, the flow rate of argon gas blown at the bottom of the ladle is reduced to 200-300NL/min, and then the treatment is continued for 2-5min, and then transported to the LF refining furnace for refining.

Preferably, in the LF refining process, after slag making, 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, by mass percentage, 50-55% of CaO, 30-35% of Al ₂ O ₃ , 1-3% of CaF ₂ , within 3% of SiO ₂ , 4-6% of MgO, within 1% of T.Fe+MnO and other inevitable impurity components.

Preferably, in step 3), argon is blown from the bottom throughout the entire process, with a flow rate of 400-500NL/min during power-on and heating, 300-400NL/min during alloying, 500-600NL/min during slag making, and 150-250NL/min at the rest of the time.

Preferably, the chemical composition of the continuous casting billet is T.O≤8ppm in mass percentage.

Compared with the prior art, the beneficial effects of the present invention are as follows: during the RH vacuum refining process, calcium wire is fed at a position near the riser, combined with a low wire feeding speed + thin iron sheet, so that Ca can be fed into the middle and upper part of the molten steel and quickly contact the O element in the molten steel to form inclusions. Most of the Ca elements are quickly pumped into the vacuum chamber from the riser along with the molten steel, thereby achieving rapid deoxidation, while the remaining small amount of Ca elements can be dissolved in the molten steel in the ladle and participate in the molten steel deoxidation cycle. The inventors have found that the deoxidation effect of this method is unexpectedly far superior to the method of feeding calcium wire at other positions, for example, the total oxygen content is 2 to 5 ppm lower than that at other positions, thereby improving the purity of the molten steel, thereby ensuring the low-temperature performance of the final low-temperature steel, and can be used to prepare low-temperature steel plate products with excellent low-temperature toughness, which can meet the market performance requirements for low-temperature steel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a partial structural schematic diagram of an RH vacuum furnace in one embodiment of the present invention;

Fig. 2 is a cross-sectional view along the A-A section line in Fig. 1.

Detailed ways

The object of the present invention is to provide a method for producing steel, which is suitable for preparing low-temperature steel and can greatly reduce the total oxygen content, i.e., the T.O. content, in the steel. The method comprises a KR desulfurization process, a converter smelting process, a LF refining process, a RH vacuum refining process, and a continuous casting process. The specific implementation contents of each process are as follows.

<KR desulfurization process>

The blast furnace molten iron is subjected to KR desulfurization treatment. The outlet temperature of the molten iron after desulfurization is 1300-1350°C, and S is ≤ 0.0010% by mass percentage.

<Converter Smelting Process>

The desulfurized molten iron in the KR desulfurization process is used to smelt molten steel in a converter, the tapping temperature of the molten steel is 1580-1620°C, and the tapping molten steel has a C content of 0.02-0.05%, an O content of 0.025-0.040%, a P content of ≤0.0055%, and an S content of ≤0.0025% by mass.

<LF refining process>

The molten steel from the converter smelting process is transported to the LF refining furnace for refining, during which power is turned on to increase temperature, alloy and slag are made in sequence, and the steel tapping temperature is 1610-1630°C.

<RH vacuum refining process>

The molten steel from the LF refining furnace is lifted and transported to the RH vacuum furnace for vacuum refining. For example, as shown in FIG1 , the molten steel from the LF refining furnace is lifted and transported to the RH vacuum furnace by a ladle 20 and subjected to vacuum treatment.

It is understandable that in RH vacuum refining, it is inevitably necessary to use lifting gas to realize the circulation of molten steel between the ladle 20 and the vacuum chamber 10. Two pipelines for the circulation of molten steel are arranged under the vacuum chamber 10. One of them is connected to the lifting gas pipe 11 and serves as a channel for the molten steel to enter the vacuum chamber from the ladle upward, and is named as the riser 32. The other one serves as a channel for the molten steel to return to the ladle from the vacuum chamber downward, and is named as the downcomer 31.

In the present invention, during the vacuum treatment, referring to FIG. 2 , after the vacuum degree drops below 1.5 mbar, the calcium wire is fed at a speed of 1 to 1.5 m/s at a position S2 closer to the ascending tube 32 than the descending tube 31 below the vacuum chamber 10. Here, it can be understood that the minimum distance between any position point in the position S2 and the ascending tube 32 is less than the minimum distance between any position point and the descending tube 31.

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; finally, the wire is broken and steel is produced.

In this way, the implementation method of feeding the calcium wire at the position S2 near the riser 32, combined with the low wire feeding speed + thin iron sheet, enables Ca to be fed into the middle and upper part of the molten steel and quickly contact the O element in the molten steel to form inclusions. This majority of the Ca elements are quickly drawn into the vacuum chamber 10 along with the molten steel from the riser 32, thereby achieving rapid deoxidation, and the remaining small portion of the Ca elements can be dissolved in the molten steel in the ladle 20 to participate in the molten steel deoxidation cycle. The inventors have found that the deoxidation effect of this method is unexpectedly far superior to the method of feeding the calcium wire at other positions, for example, the total oxygen content is 2 to 5 ppm lower than that at other positions.

<Continuous Casting Process>

The molten steel from the RH vacuum refining process is hoisted to a continuous casting platform for casting to obtain a continuous casting billet having a very low total oxygen content, for example, below 8 ppm.

In summary, the present invention implements the calcium wire feeding method at position S2 near the riser 32, combined with low wire feeding speed + thin iron sheet, so that Ca can be fed into the middle and upper part of the molten steel and quickly contact with the O element in the molten steel to form inclusions. Most of the Ca elements are quickly pumped into the vacuum chamber 10 from the riser 32 along with the molten steel, thereby achieving rapid deoxidation, and the remaining small amount of Ca elements can be dissolved in the molten steel in the ladle 20, participate in the molten steel deoxidation cycle, achieve the control of the total oxygen content, and obtain a continuous casting billet with a low total oxygen content, thereby improving the purity, thereby ensuring the low-temperature performance of the final low-temperature steel, and can be used to prepare low-temperature steel plate products with excellent low-temperature toughness, which can meet the market performance requirements for low-temperature steel.

Optionally, 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.

Further, the downcomer 31 and the riser 32 are symmetrically distributed along a mirror plane M. The minimum distance between any position point on the mirror plane M and the riser 32 is equal to the minimum distance between any position point and the downcomer 31. Here, a fitting surface T is defined in the ladle 20, and the fitting surface T passes through the center O of the riser 32 and is parallel to the mirror plane M. The position S2 is preferably located between the fitting surface T and the mirror plane M, such as the portion marked by the oblique line in FIG. 2 , that is, the calcium wire is fed between the fitting surface T and the mirror plane M. In this way, the deoxidation effect can be further improved.

Furthermore, in the converter smelting process, the desulfurized molten iron of the KR desulfurization process, as well as the nickel plate and scrap steel, are put into the converter together for molten steel smelting. Among them, the weight of the desulfurized molten iron accounts for more than 80% of the total mass of the desulfurized molten iron, nickel plate and scrap steel, and correspondingly, the total weight of the nickel plate and scrap steel accounts for less than 20% of the total mass of the desulfurized molten iron, nickel plate and scrap steel. The actual amount of nickel plate put in can be determined within the above weight ratio according to the target chemical composition of the low-temperature steel to be produced.

The chemical composition of the nickel plate used may include Ni≥99%, P≤0.015%, S≤0.003% by mass percentage, and the balance Fe and other inevitable impurities. The chemical composition of the scrap steel may include P≤0.012%, S≤0.005% by mass percentage, and of course the scrap steel may also include Fe and elements such as Al, Si, and Mn.

Furthermore, in the converter smelting process, ^15-18m3 /t of oxygen is first blown into the molten steel, and the first slag removal is performed after the oxygen blowing is completed; after the first slag removal is completed, oxygen blowing is performed again until the C content in the molten steel reaches below 0.05%, and then the oxygen blowing is terminated, and then the second slag removal is performed; after the second slag removal is completed, 2-4kg/t of ferrosilicon particles and 1-3kg/t of ferromagnesium particles are added to the converter in sequence, and bottom argon blowing is started at a flow rate of ^0.05-0.10Nm3 /min and continued for 1.5-3min before steel is tapped. In this way, through two slag removals, the phosphorus content of the steel can be ensured to be low, the cleanliness of the steel can be further improved, and the low-temperature performance of the low-temperature steel can be improved.

The slag retention and slag pouring mentioned above refers to retaining a portion of slag in the molten steel during slag pouring. Specifically, during the first slag retention and slag pouring, the amount of slag retention is about 30-50%, and the amount of slag pouring is 50-70% accordingly; during the second slag retention and slag pouring, the amount of slag retention is about 20-40%, and the amount of slag pouring is 60-80%. Here, the measurement method of the slag pouring amount can be specifically: based on the ratio of the weight of the poured slag to the total weight of the slag in the molten steel before slag pouring; correspondingly, the slag retention amount is the difference between 100% and the ratio.

Furthermore, the chemical composition of the ferrosilicon particles includes, by mass percentage: Si: 45-55%, P≤0.015%, S≤0.008%, and the rest is Fe and other inevitable impurities; and more than 95% of the ferrosilicon particles have a particle size of 30-50 mm.

The chemical composition of the magnesium-iron particles includes, by mass percentage: Mg: 25-35%, P≤0.025%, S≤0.01%, and the rest is Fe and other inevitable impurities; and among them, more than 98% of the ferrosilicon particles have a particle size of 30-50 mm.

Furthermore, slag blocking can be used in the converter tapping process, such as using a slide plate to block slag, and the slag amount is controlled to be ≤2kg/t. The slag amount can be measured by inserting a thin iron wire into the ladle, measuring the slag thickness, and calculating the slag amount according to the slag density.

In addition, deoxidation and alloying are performed during the steel tapping process of the converter. For example, when 10-20% of the steel tapping is completed, deoxidation and alloying are performed according to the metal aluminum, ferrosilicon, metal manganese, and nickel plates, and when the steel tapping reaches 60-70%, all the alloys are added. Of course, the specific elements and alloy components used in deoxidation and alloying are not limited to this, and can be determined according to the target chemical composition of the low-temperature steel to be produced. In this way, on the basis of the previous slag pouring, the oxidizability of the molten steel and the slag can be appropriately reduced through deoxidation, so as to achieve low-carbon and low-oxygen steel tapping.

After alloying, 5-8 kg/t of lime and 15-20 kg/t of calcium aluminate synthetic slag are added for slagging, and all of them are added before steel tapping is completed. The components of the calcium aluminate synthetic slag include 40-45% of CaO, 10-15% of _Al2O3 , 5-10% of _CaF2 , less than ₃ % of _SiO2 , 2-5% of MgO, 5-10% of _CaC2 , and 15-20% of elemental aluminum in terms of mass percentage, and the mass percentage of the physical phase _12CaO · _7Al2O3 exceeds 30%, and the rest is a single phase or a composite phase of CaO, _CaF2 , _SiO2 , and MgO. In this way, the purity of molten steel can be further improved by rapidly deoxidizing and slagging with a large amount of slag and strong deoxidation synthetic slag.

Furthermore, during the steel tapping process, the flow rate of argon gas blown from the bottom of the ladle is 300-500NL/min. After the steel tapping is completed, the flow rate of argon gas blown from the bottom of the ladle is reduced to 200-300NL/min, and then the treatment is continued for 2-5 minutes before being transported to the LF refining furnace for refining.

Further preferably, in the RH vacuum refining process, a first-stage steam pump E1, a second-stage steam pump E2, a third-stage steam pump E3, a fourth-stage steam pump E4, and two-stage water circulation pumps W1&W2 are sequentially arranged in the vacuum exhaust pipeline of the vacuum chamber 10 of the RH vacuum furnace used.

After the molten steel is transported to the RH vacuum furnace, the two-stage water circulation pump 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 are sequentially turned on for vacuum treatment. After the vacuum degree drops below 1.5 mbar, the treatment is continued for 10 to 15 minutes, during which the calcium wire feeding treatment is performed. In this way, while feeding the calcium wire, the treatment is performed in a deep vacuum (specifically, below 1.5 mbar), which can further improve the deoxidation effect.

In a preferred specific operation, after the molten steel is transported to the RH vacuum furnace, the two-stage water circulation pumps W1 & W2 are turned on within 1 minute to maintain the vacuum degree above 200 mbar, and the lifting gas flow rate is maintained at 80-100 Nm ³ /h, and the treatment is carried out for 3-5 minutes; then, 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 are turned on in sequence, and the lifting gas flow rate is increased to 150-200 Nm ³ /h, and after the vacuum degree drops below 1.5 mbar, the treatment is continued for 10-15 minutes; then, the third-stage steam pump E3, the second-stage steam pump E2, and the first-stage steam pump E1 are turned off, and the vacuum degree is adjusted to above 50 mbar, and then the treatment is carried out for more than 5 minutes, during which the lifting gas flow rate remains unchanged, that is, maintained at 150-200 Nm ³ /h. In this way, high pressure + low lifting gas flow rate is used first, then deep vacuum + high lifting gas flow rate is used, and finally medium pressure is used to continue the treatment, so that the low circulation volume is controlled to treat the molten steel first, which promotes the floating of inclusions while reducing the erosion of refractory materials, and then deep vacuum and high circulation volume are used to treat the molten steel, so as to quickly and more strongly remove inclusions (that is, deoxidation), and finally the circulation volume is reduced to reduce the inclusions introduced by the erosion of refractory materials while taking away a large number of inclusions.

Further optionally, in the LF refining process, after slag making, 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, by mass percentage, 50-55% of CaO, 30-35% of Al ₂ O ₃ , 1-3% of CaF ₂ , within 3% of SiO ₂ , 4-6% of MgO, within 1% of T.Fe+MnO and other inevitable impurity components.

The components of the calcium aluminate synthetic slag include, by mass percentage, 40-45% CaO, 10-15% Al ₂ O ₃ , 5-10% CaF ₂ , less than 3% SiO ₂ , 2-5% MgO, 5-10% CaC ₂ , and 15-20% elemental aluminum, wherein the mass percentage of the phase 12CaO·7Al ₂ O ₃ exceeds 30%, and the rest are single phases or composite phases of CaO, CaF ₂ , SiO ₂ , and MgO.

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.

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 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 making period is 500-600NL/min, and the flow rate of the bottom blowing argon during the rest of the time is 150-250NL/min.

In addition, the continuous casting process can be implemented by any technology known in the art. Optionally, when the molten steel arrives at the continuous casting station, it is first allowed to stand for more than 5 minutes before pouring. During the pouring, the pulling speed is 0.8-1.1 m/min, and protective pouring is adopted throughout the process. The argon blowing flow rate of the long nozzle is 150-250 L/min, and the argon blowing flow rate of the stopper rod and the immersed nozzle is 3-5 L/min. Argon blowing starts in the tundish 5 minutes before pouring, and stops after the first round of tundish covering agent addition is completed.

In summary, the beneficial effects of the present invention are at least that: during the RH vacuum refining process, the calcium wire is fed at position S2 near the riser 32, combined with a low wire feeding speed + thin iron sheet, so that Ca can be fed into the middle and upper part of the molten steel and quickly contact the O element in the molten steel to form inclusions. This majority of the Ca elements are quickly drawn into the vacuum chamber 10 from the riser 32 along with the molten steel, thereby achieving rapid deoxidation, and the remaining small portion of the Ca elements can be dissolved in the molten steel in the ladle 20 to participate in the molten steel deoxidation cycle. The inventors have found that the deoxidation effect of this method is unexpectedly far superior to the method of feeding the calcium wire at other positions, for example, the total oxygen content is 2 to 5 ppm lower than that at other positions, thereby improving the purity of the molten steel, thereby ensuring the low-temperature performance of the final low-temperature steel, and can be used to prepare low-temperature steel plate products with excellent low-temperature toughness, which can meet the market performance requirements for low-temperature steel.

Several embodiments are provided below. In these embodiments, continuous casting billets are prepared by the method provided by the present invention with the following target chemical composition. The target chemical composition of these continuous casting billets includes, by mass percentage: C: 0.03-0.10%, Si: 0.15-0.35%, Mn: 0.5-1.6%, Ni: 0.4-10.0%, Al: 0.015-0.055%, Cu≤0.015%, Mo≤0.50%, Cr≤0.70%, Nb≤0.035%, and the rest is iron and unavoidable impurities. Of course, within the above range, MO, Cr and Nb can be used as impurity elements (e.g. from scrap steel) instead of alloying elements, or as alloying elements in the steelmaking process according to actual product requirements.

After the method of the present invention is implemented, the chemical composition and thickness of the continuous casting slabs obtained in these examples are shown in Table 1 below.

[Table 1]

[Table 1 continued]

It can be seen that in addition to the elements in the target chemical composition mentioned above, the impurity elements in these embodiments are very low, including P≤0.005%, S≤0.0015%, N≤0.0025%, H≤1.5ppm, T.O≤8ppm, and even T.O≤6ppm.

Sampling is performed on the continuous casting billets obtained in various embodiments, for example, cross-section sampling is performed, and 5 metallographic samples of 20 mm×20 mm are taken from the inner arc to the outer arc; each sample is scanned by a scanning electron microscope, and inclusions are counted, and the statistical area of each sample is 100 mm ² . It is found that all these samples meet the following requirements: the number of oxide inclusions larger than 5 μm is ≤0.5/mm ² , the maximum size of inclusions does not exceed 30 μm, the Al ₂ O ₃ content in the oxide inclusions is 40-80%, the CaO content is 15-45%, the sum of the contents of SiO ₂ , MnO and MgO is ≤15%, and the rest are other inevitable components.

The continuous casting billets of each embodiment are processed by conventional heating-hot rolling-primary quenching-secondary quenching-tempering to obtain low-temperature steel finished plates. The mechanical properties and low-temperature properties of the low-temperature steel finished plates of each embodiment are tested, and the results are shown in Table 2.

[Table 2]

Claims (10)

1. A method of producing a low temperature steel for controlling total oxygen content, the method comprising:

KR desulfurization process: the outlet temperature of molten iron is 1300-1350 ℃, and S is less than or equal to 0.0010% in percentage by mass;

Converter smelting process: the temperature of the tapping molten steel is 1580-1620 ℃, and the tapping molten steel comprises 0.02-0.05% of C, 0.025-0.040% of O, less than or equal to 0.0055% of P and less than or equal to 0.0025% of S in percentage by mass;

LF refining procedure: the temperature of the tapping molten steel is 1610-1630 ℃;

RH vacuum refining procedure: vacuum treatment is carried out after molten steel is conveyed to an RH vacuum furnace, and after the vacuum degree is reduced to below 1.5mbar, calcium wires are fed at a position which is closer to a lifting pipe than a descending pipe below a vacuum chamber, wherein the feeding speed is 1-1.5 m/s; the inside of the calcium wire is passivated metal calcium powder with the Ca content of more than 95 percent, and the outer layer is iron sheet with the outer diameter of 8-10 mm and the thickness of 1-2 mm; finally, breaking the blank and tapping;

Continuous casting process: and pouring the tapping molten steel in the RH vacuum refining process to obtain the continuous casting blank.

2. The method for producing a low-temperature steel with controlled total oxygen content according to claim 1, wherein in the RH vacuum refining process, an RH vacuum furnace is adopted, in which a first-stage steam pump, a second-stage steam pump, a third-stage steam pump, a fourth-stage steam pump and a two-stage water circulation pump are sequentially arranged in a vacuum exhaust pipeline of a vacuum chamber; after molten steel is conveyed to an RH vacuum furnace, a two-stage water circulation pump, a fourth-stage steam pump, a third-stage steam pump, a second-stage steam pump and a first-stage steam pump are sequentially started to carry out vacuum treatment, and after the vacuum degree is reduced to below 1.5mbar, the treatment is continued for 10-15 min, and a calcium line is fed in the period.

3. The method for producing a low-temperature steel with controlled total oxygen content according to claim 2, wherein in the RH vacuum refining process, a two-stage water circulation pump is turned on within 1min after molten steel is transported to an RH vacuum furnace, the vacuum degree is maintained at 200mbar or more, the flow rate of lifting gas is maintained at 80-100 Nm ³/h, and the treatment is performed for 3-5 min; then, sequentially opening a fourth-stage steam pump, a third-stage steam pump, a second-stage steam pump and a first-stage steam pump, improving the flow of lifting gas, and continuously treating for 10-15 min after the vacuum degree is reduced to below 1.5 mbar; and then the third-stage steam pump, the second-stage steam pump and the first-stage steam pump are turned off, the vacuum degree is adjusted to be more than 50mbar, the treatment is carried out for more than 5 minutes, and finally, the air breaking and the tapping are carried out.

4. The method for producing a low-temperature steel with controlled total oxygen content according to claim 1, wherein the chemical composition of the iron scale of the calcium line comprises, in mass percent: 0.005-0.035% of Al, 0.2-0.4% of Si, 0.3-0.5% of Mn, less than or equal to 0.001% of P, less than or equal to 0.003% of S, and the balance of Fe and unavoidable impurities.

5. The method of producing a cryogenic steel with controlled total oxygen content according to claim 1, characterized in that the down-comer and the riser below the vacuum chamber are symmetrically distributed with a mirror plane, the ladle below the vacuum chamber has a fitting plane passing through the centre of the riser and parallel to the mirror plane, said location of the feed calcium line being between said fitting plane and said mirror plane.

6. The method for producing a low-temperature steel for controlling total oxygen content according to claim 1, wherein in the converter smelting process: blowing oxygen of 15-18 m ³/t into molten steel, and carrying out first slag-retaining and deslagging after oxygen blowing is completed; after the first slag-reserving slag-pouring is finished, oxygen blowing is carried out again until the C content in the molten steel is less than 0.05%, and then the second slag-reserving slag-pouring is carried out; after the second slag-remaining and deslagging is finished, adding 2-4 kg/t of ferrosilicon particles and 1-3 kg/t of ferromagnesium particles into the converter in sequence, starting bottom blowing argon according to the flow of 0.05-0.10Nm ³/min for 1.5-3 min, and tapping.

7. The method for producing a low-temperature steel with controlled total oxygen content according to claim 1, wherein in the converter smelting process, deoxidation and alloying are performed in the tapping process, after the alloying is finished, 5-8 kg/t lime and 15-20 kg/t calcium aluminate synthetic slag are added for slag formation, and all the materials are added before the tapping is finished; the calcium aluminate synthetic slag comprises, by mass, 40-45% of CaO, 10-15% of Al ₂O₃, 5-10% of CaF ₂, 3% of SiO ₂, 2-5% of MgO, 5-10% of CaC ₂ and 15-20% of elemental aluminum, wherein the mass percentage of phase 12 CaO.7Al ₂O₃ exceeds 30%, and the balance is single phase or composite phase of CaO, caF ₂、SiO₂ and MgO; and the flow of the bottom blowing argon of the steel ladle is 300-500 NL/min in the tapping process, the flow of the bottom blowing argon of the steel ladle is reduced to 200-300 NL/min after the tapping is finished, and then the steel ladle is continuously treated for 2-5 min and then conveyed to an LF refining furnace for refining.

8. The method for producing a low-temperature steel with controlled total oxygen content according to claim 1, wherein 0.3-0.5 kg/t of calcium carbide, 1.0-2.0 kg/t of calcium aluminate synthetic slag are added to molten steel after slagging in the LF refining process to adjust slag components to 50-55% CaO, 30-35% Al ₂O₃, 1-3% CaF ₂, 3% SiO ₂, 4-6% MgO, 1% t.fe+mno, and other unavoidable impurity components in mass percent.

9. The method for producing a low temperature steel with controlled total oxygen content according to claim 1, wherein in step 3), argon is bottom-blown in the whole process, the flow rate of the bottom-blown argon is 400-500 NL/min during the power-on heating, the flow rate of the bottom-blown argon is 300-400 NL/min during alloying, the flow rate of the bottom-blown argon is 500-600 NL/min during slag formation, and the flow rate of the bottom-blown argon is 150-250 NL/min for the rest of time.

10. The method for producing a low-temperature steel with controlled total oxygen content according to claim 1, wherein the chemical composition of the continuous casting billet is less than or equal to 8ppm in mass percent.