CN111719080A

CN111719080A - Inclusion control method for prestressed steel strand

Info

Publication number: CN111719080A
Application number: CN201910218750.9A
Authority: CN
Inventors: 刘志璞
Original assignee: Bengang Steel Plates Co Ltd
Current assignee: Bengang Steel Plates Co Ltd
Priority date: 2019-03-21
Filing date: 2019-03-21
Publication date: 2020-09-29

Abstract

The invention relates to the field of high-carbon steel, in particular to an inclusion control method for a prestressed steel strand. The steelmaking control adopts a high carbon-drawing process, the end point [ C ] of the converter is required to be more than or equal to 0.40 wt%, and the converter is drawn in place once, so that nitrogen increase caused by low carbon and large amount of carbon increase due to reblowing is prevented; the refining process of the LF refining furnace adopts micro-positive pressure operation, and the pressure is 0.01-0.03 MPa relative to the atmospheric pressure; the alkalinity of the slag is controlled within the range of 1.0-1.2, and all chemical components are controlled according to target values. The invention develops a high-carbon low-phosphorus control technology of the converter blowing end point, the converter blowing end point is effectively controlled, and C, P, T can meet the requirements of quality and process. The smelting adopts refining processes of Al-free deoxidation, low-alkalinity refining slag and the like to carry out plasticizing treatment on inclusions in the steel, and the technical problem of brittle inclusions in the steel is effectively solved.

Description

Inclusion control method for prestressed steel strand

Technical Field

The invention relates to the field of high-carbon steel, in particular to an inclusion control method for a prestressed steel strand.

Background

The SWRH82B high-carbon wire rod is a main raw material for producing steel wires and steel strands for a high-strength low-relaxation prestressed concrete structure, and the end products of the steel wires and the steel strands, namely PC steel wires and steel strands, are widely applied to key projects such as high-rise buildings, bridges, petrochemical engineering, railways and the like, so that the SWRH82B high-carbon wire rod is required to have stable chemical components, pure steel, excellent mechanical properties, uniform metallographic structures and higher sorbite degrees.

The SWRH82B high-carbon wire rod is a main raw material for producing steel wires and steel strands for a high-strength low-relaxation prestressed concrete structure, is mainly used for producing plain steel wires, indented steel wires, female thread steel wires and steel strands of various specifications and grades, and is widely applied to various fields of railway sleepers, expressways, bridges, urban overpasses, large cement water pipes, high-rise earthquake-resistant buildings, mine supports and the like.

The wire rod is required to have better extensibility and drawing performance because the wire rod needs to be further drawn to an end product through a next process. Therefore, the wire rod has stable chemical composition requirement and small fluctuation, the microstructure requires high sorbite content with good plasticity in the wire rod, and does not allow the appearance of non-deformable reticular cementite and martensite, the steel quality of the wire rod is required to be pure, and the type and the size of inclusions are also strictly limited. In order to facilitate the acid washing and further drawing of the downstream process, the wire rod is also required to have small tensile strength fluctuation, high wire rod section shrinkage, smooth surface and no oil stain.

The SWRH82B high-carbon wire rod is a main raw material for producing steel wires and steel strands for a high-strength low-relaxation prestressed concrete structure, is mainly used for producing plain steel wires, indented steel wires, female threaded steel wires and steel strands of various specifications and grades, is widely applied to various fields of railway sleepers, expressways, bridges, urban overpasses, large cement water pipes, high-rise earthquake-resistant buildings, mine supports and the like, and has large application amount.

Disclosure of Invention

The invention aims to provide an inclusion control method of a prestressed steel strand, and develops a high-carbon low-phosphorus control technology of a converter blowing end point, so that the converter blowing end point is effectively controlled, and C, P, T can meet the quality and process requirements. The smelting adopts refining processes of Al-free deoxidation, low-alkalinity refining slag and the like to carry out plasticizing treatment on inclusions in the steel, and the technical problem of brittle inclusions in the steel is effectively solved.

The technical scheme of the invention is as follows:

a steel-making control adopts a high carbon-drawing process, the end point [ C ] of a converter is required to be more than or equal to 0.40 wt%, and the converter is drawn in place once, so that nitrogen increase caused by low-carbon and large-amount carbon increase due to blowing supplement is prevented; the refining process of the LF refining furnace adopts micro-positive pressure operation, and the pressure is 0.01-0.03 MPa relative to the atmospheric pressure; the alkalinity of the slag is controlled within the range of 1.0-1.2, and all chemical components are controlled according to target values.

According to the inclusion control method of the prestressed steel strand, the prestressed steel adopts SWRH82B, and the chemical components are as follows according to the mass percentage:

when the rolling specification is 8.0-11 mm, 0.79-0.83% of C, 0.20-0.30% of Si, 0.72-0.80% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S, 0.18-0.25% of Cr and the balance of Fe;

when the rolling specification is greater than 11.0 mm-13 mm, 0.79-0.83% of C, 0.20-0.30% of Si, 0.72-0.82% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S, 0.24-0.30% of Cr and the balance of Fe;

when the rolling specification is greater than 13.0mm to 15mm, 0.79 to 0.83 percent of C, 0.20 to 0.30 percent of Si, 0.72 to 0.82 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.015 percent of S, 0.30 to 0.33 percent of Cr, 0.035 to 0.045 percent of V, and the balance of Fe.

The inclusion control method of the prestressed steel strand comprises the following process control steps of a steelmaking process:

(1) when the rest 50-90 tons of the continuous casting ladle are obtained, the molten steel reaches an LF furnace for refining, and the constant continuous casting pulling speed is ensured to be 1-3 m/min;

(2) the method comprises the following steps of carrying out pre-desulfurization and slagging-off treatment on molten steel from steel making in a desulfurization station, wherein desulfurizing agents are magnesium powder and lime powder, and the blowing volume ratio of the magnesium powder to the lime powder is 1: 3.5-4.5, controlling the S component in the treated molten iron to be less than or equal to 0.015 wt%, and slagging off to achieve the mirror surface effect.

The inclusion control method of the prestressed steel strand comprises the following steps of converter process control:

firstly, converter smelting adopts a high-carbon low-phosphorus control technology, the slag alkalinity is controlled to be 3.5-4.0, the end point C is controlled to be 0.50-0.70 wt%, and the end point P is controlled to be 0.005-0.015 wt%;

the tapping time of the tapping hole is 5-7 min;

and thirdly, introducing argon into the steel ladle for 2-4 min before tapping, and blowing argon for not less than 4min in the tapping process.

The inclusion control method of the prestressed steel strand adopts an LF furnace refining process for control:

adding 650-750 kg of lime into the furnace, and correspondingly increasing the quartz sand consumption over 750kg, wherein the proportion is as follows: adding 50kg of quartz sand into 100kg of lime, taking slag samples from two furnaces before the next pouring, and analyzing, wherein the alkalinity R is controlled to be 0.80-1.2;

secondly, the power-on time is required to be not less than 8min after the quartz sand is added;

thirdly, the addition amount of the rice husk carbonized in the station is not less than 25 bags, the rice husk carbonized in the station is uniformly covered, the constant pulling speed is ensured to be 1-3 m/min, and the continuous casting furnace superheat degree is 20-30 ℃.

The inclusion control method of the prestressed steel strand adopts low-alkalinity refining slag for refining, and the inclusion Al in the steel₂O₃15～25wt％，CaO 30～50wt％，SiO₂40～60wt％。

The invention has the advantages and beneficial effects that:

the process route of the invention is feasible for producing the prestressed steel stranded wires SWRH82B by adopting a blast furnace mixer → molten iron pretreatment (S removal and slag removal) → 120 tons of top-bottom combined blown converter (sublance) → ladle bottom argon blowing → LF furnace refining → 150mm × 150mm square billet continuous casting.

Through a series of research works on the production process flow of the prestressed steel strand SWRH82B, inclusions in steel meet the technical conditions of products.

According to the invention, the production process is improved, the prestressed steel strand SWRH82B is produced smoothly, and the batch production is realized.

The SWRH82B produced according to the prior art is excellent in performance, meets the requirements of downstream users and terminal users, and reaches the domestic advanced level.

Through the development project of the prestressed steel strand, the quality of the steel for the steel strand is improved, and the technical economic benefit and the social benefit are remarkable.

Drawings

FIG. 1 shows the S element control condition of the molten iron SWRH 82B.

FIG. 2 shows the control situation of the end point C of the converter smelting SWRH 82B.

FIG. 3 shows the control of the terminal point P of SWRH82B in the converter smelting process.

Detailed Description

In the specific implementation process, the production process route of the prestressed steel strand is as follows:

iron mixing furnace → molten iron pretreatment (S removal and slag removal) → 120-ton top-bottom combined blown converter → ladle bottom argon blowing → 120-ton LF furnace refining → 150mm × 150mm square billet continuous casting (crystallizer electromagnetic stirring and solidification end electromagnetic stirring) → billet slow cooling (winter) → rolling.

In the production process, each operation area strictly implements each measure, and the process is controlled in place. The raw material operation area ensures that low molten iron with S, P content is charged into the furnace and the charging amount is stable; the key point of steel-making control is a high-carbon-drawing process, the end point [ C ] of the converter is required to be more than or equal to 0.40 wt%, and the converter is drawn in place once, so that nitrogen increase caused by low-carbon and large-amount carbon increase due to blowing supplement is prevented, and all actual control reaches the process requirement; the refining process of the LF refining furnace adopts micro-positive pressure operation to produce low-alkalinity slag, and all chemical components are controlled according to target values. In order to match with the pouring process of continuous casting with low pulling speed and low superheat degree, the adding amount of the carbonized rice hulls is increased, and a good ladle heat-preservation effect is obtained. The outlet temperature of the next ladle is timely adjusted according to the outlet temperature of the previous ladle and the temperature change condition in the last casting stage, so that the guarantee is provided for scientifically and reasonably reducing the superheat degree; the constant drawing speed of the continuous casting operation area is 2m/min, and practice proves that the casting speed is changed, so that the central defects and microcracks of the casting blank caused by the growth of liquid cavities, the growth of columnar crystals, bridging and the like in the crystallization process of molten steel are reduced. A specially-assigned person is arranged in the shipping operation area to be responsible for the heat preservation effect of the slow cooling field, and the requirement of slow cooling for more than 3 days is met; meanwhile, the technical quality department, the production department and the scheduling room actively pursue and organize production, and the smooth development and quality requirements of the variety steel are also ensured.

(I) metallurgical quality requirements

1. Chemical composition of steel

The chemical composition of the SWRH82B hot rolled wire rod is controlled as shown in table 1.

TABLE 1 SWRH82B chemical composition requirements (wt%)

TABLE 2 SWRH82B coil bar partial Heat chemical composition (wt%)

Smelting number	C	Si	Mn	P	S	Cr	V	Fe
										1	0.82	0.23	0.77	0.015	0.012	0.18		Surplus
2	0.81	0.24	0.75	0.014	0.01	0.19		Surplus
									3	0.8	0.22	0.77	0.014	0.013	0.20		Surplus
4	0.8	0.23	0.75	0.015	0.008	0.21		Surplus
									5	0.82	0.24	0.77	0.015	0.013	0.22		Surplus
6	0.8	0.22	0.77	0.015	0.014	0.23		Surplus
									7	0.82	0.25	0.76	0.015	0.011	0.24		Surplus
8	0.82	0.24	0.77	0.014	0.012	0.25		Surplus
									9	0.81	0.25	0.76	0.013	0.011	0.24		Surplus
10	0.8	0.22	0.76	0.015	0.012	0.25		Surplus
									11	0.84	0.24	0.77	0.018	0.014	0.26		Surplus
12	0.81	0.23	0.76	0.015	0.012	0.27		Surplus
									13	0.8	0.23	0.8	0.014	0.011	0.28		Surplus
14	0.81	0.27	0.76	0.013	0.012	0.29		Surplus
									15	0.82	0.25	0.78	0.013	0.01	0.30		Surplus
16	0.82	0.22	0.77	0.014	0.008	0.28		Surplus
									17	0.81	0.24	0.78	0.014	0.009	0.30	0.035	Surplus
18	0.8	0.23	0.75	0.014	0.011	0.31	0.036	Surplus
									19	0.81	0.24	0.76	0.013	0.015	0.32	0.037	Surplus
20	0.8	0.26	0.75	0.012	0.014	0.33	0.038	Surplus
									21	0.8	0.29	0.73	0.014	0.011	0.30	0.039	Surplus
22	0.79	0.25	0.77	0.015	0.011	0.31	0.040	Surplus
									23	0.81	0.25	0.78	0.015	0.012	0.32	0.041	Surplus
24	0.81	0.23	0.74	0.013	0.009	0.33	0.042	Surplus

2. Control of non-metallic inclusions in steel

The existence of non-metallic inclusions in the steel destroys the continuity of a steel matrix, increases the nonuniformity of a steel structure and seriously affects the performance of the steel. Aiming at the requirements that the A, B, C, D inclusions are less than or equal to 1.5 grade and the target is less than or equal to 1.0 grade, the following process control mode is adopted in the casting blank smelting process:

the method has the advantages of improving the content of the converter end point [ C ] (high carbon-pulling), reducing the oxygen content of molten steel, reducing the addition amount of a recarburizing agent, and recarburizing without using composite carbon iron. The control standard of the steel converter end point [ C ] for the steel strand is more than 0.40 wt%.

And the whole process of the ladle is subjected to soft argon blowing, so that impurities are promoted to float.

⑶ strengthening LF furnace refining diffusion deoxidation, and using ferrosilicon powder to deoxidize slag in batches, increasing adsorption capacity of refining slag to inclusion, reducing inclusion in steel, the refining process adopts micro-positive pressure operation to prevent air from entering molten steel to cause secondary oxidation of molten steel, and slag alkalinity (CaO and SiO)₂Content of) is controlled within a range of 1.0 to 1.2, so that Al in the steel is contained₂O₃The inclusion achieves the plasticizing effect, and the technical problem of brittle inclusion is effectively solved.

And optimizing the number of continuous casting furnaces of the continuous casting tundish, keeping the casting speed constant, and ensuring the liquid level height (more than 600mm) of the tundish and the fluctuation of the liquid level of the crystallizer within +/-5 mm by adopting an automatic crystallizer liquid level control system to prevent the tundish from rolling slag.

Non-metallic inclusions in the wire rod reduce the plasticity, toughness and drawability of the steel. The requirements for each type of inclusion are shown in table 3.

TABLE 3 nonmetallic inclusion requirement (grade) in SWRH82B coil bar steel

TABLE 4 partial heat metallographic examination of SWRH82B wire rod

3. Low power test of casting blank

The control target of each defect of the casting blank is below 1.0 level. The specific requirements are shown in table 5.

TABLE 5 casting blank Low power requirement (grade) of SWRH82B

Process control of steel-smelting procedure

1. Scheduling system production organization and preparation

Firstly, controlling the production rhythm, and refining molten steel in an LF furnace when 50-90 tons of residual molten steel in a continuous casting ladle are obtained;

secondly, ensuring the constant continuous casting speed to be 2.0 m/min;

2. raw material process control

Firstly, the molten iron of steel making is pre-desulfurized and slag-off treated in a desulfurizing station. The desulfurizer is magnesium powder and lime powder, and the blowing volume ratio of the magnesium powder to the lime powder is 1: 3.8. the control target of the S component in the treated molten iron is less than or equal to 0.015 wt%, and the slag skimming achieves the mirror surface effect;

secondly, refined heavy scrap steel (such as cutting blank head, blank tail, non-fixed length, cutting scrap steel and the like) is adopted, the scrap steel is clean and dry, sundries such as dregs and the like cannot be contained, scrap steel with high nickel and copper content (such as SPA-H and the like) and scrap steel of Ti-containing steel welding wires cannot be used.

As shown in fig. 1, the S content was controlled after the entire hot metal pretreatment in the production of SWRH 82B. The desulfurization hit rate reached 96.43% in the analyzed statistical data. The partial molten iron composition control is shown in table 6.

TABLE 6 composition and temperature after pretreatment of part of molten iron

3. Converter process control

And directly charging the pre-desulfurized molten iron into a converter for smelting steel, wherein the total charging amount is controlled to be 165-170 t. The specific control requirements are as follows:

firstly, converter smelting adopts a 'high-carbon low-phosphorus' control technology. Smelting is carried out with high-carbon and low-phosphorus end point control to reduce the oxidability of molten steel and reduce the consumption of carburant and deoxidizer; on one hand, the production cost can be reduced, on the other hand, nitrogen and other inclusions are reduced to enter molten steel, and the purity of the steel is ensured to the maximum extent.

In order to improve the adverse effect of high-tension carbon on molten steel dephosphorization, the dephosphorization process in the early stage of smelting is strengthened in the smelting process. Namely, a high lance position is adopted in the early stage of blowing, the content of slag (FeO) is increased, lime melting is accelerated, the slag alkalinity is increased, and the like, so that early-stage dephosphorization is promoted. Such a process ensures the desired phosphorus content of the molten steel. Controlling the alkalinity of the slag to ensure that R is more than or equal to 3.2 (the target is 3.5-4.0); the end point [ C ] is not less than 0.45 wt%, and the steel is once pulled in place to prevent the large amount of carburetion caused by carbon reduction due to the after-blowing, and if the end point [ C ] is less than 0.35 wt%, the steel is changed into other steel types.

As shown in the figure 2 and the figure 3, the statistics of the converter smelting SWRH82B data show that the control of the end point C of SWRH82B is more than 0.45 wt%, wherein the end point C of 96.4% of the heat is controlled in the range of 0.50 wt% to 0.70 wt%, the highest C is 0.53 wt%, and the lowest C is 0.72 wt%. The current end point P is 0.005 wt% -0.015 wt%, and the average is 0.011 wt%.

Cleaning and drying alloy and auxiliary materials (sampling silicon manganese and low-aluminum silicon iron); tapping time of the tapping hole is more than 5min (the optimal time is 5-7 min), and the residual steel outside the tapping hole is processed cleanly in time to prevent tapping from scattering;

thirdly, the air permeability and cleanliness of the steel ladle must be checked before tapping, strong argon is introduced into the steel ladle for about 3min before tapping, argon is blown for not less than 4min in the tapping process, argon is blown for not less than 8min in an argon station, the flow of the argon is controlled, and the molten steel is prevented from being violently stirred and absorbing nitrogen;

fourthly, in the later stage of blowing (after 10 minutes), no ore is added for slag mixing and cooling;

fifthly, the temperature for starting LF treatment is more than or equal to 1490 ℃, and the target is more than or equal to 1500 ℃.

4. Refining process control

The SWRH82B steel is refined by an LF furnace, steel quality is purified, and components are adjusted. The refining process comprises the following aspects:

firstly, the opening degree of an air door is required to be adjusted, and micro-positive pressure operation (0.01-0.03 MPa higher than atmospheric pressure) is ensured. The furnace door is opened except for temperature measurement and sampling in the whole refining process, and the furnace door is closed in the rest of time to ensure the reducing atmosphere in the furnace;

650-750 kg of lime is added into each furnace, the quartz sand consumption is correspondingly increased when the lime consumption exceeds 750kg, 50kg of quartz sand is added into 100kg of lime, slag samples are taken from the two furnaces before pouring, analysis is carried out, and R is controlled to be 0.80-1.2;

thirdly, after the quartz sand is added, the electrifying time is required to be not less than 8min, and the target is as follows: the temperature before the quartz sand reaches the requirement, the low-grade high-current temperature rise after the quartz sand is avoided, and the electrode recarburization during the electrifying period after the quartz sand is added is considered;

fourthly, the addition amount of the rice husk carbonized in the off-station is not less than 25 bags, the rice husk carbonized in the off-station is uniformly covered, the constant pulling speed is ensured to be 2.0m/min, the superheat degree of a continuous casting furnace is not more than 35 ℃, and the target temperature is 20-30 ℃.

5. Process control for continuous casting process

The SWRH82B square billet is cast by a six-flow arc square billet continuous casting machine, and the specification of the casting billet is 150mm multiplied by 12000 mm. The key points and main measures of the continuous casting process comprise the following aspects:

firstly, selecting a heat-insulating plate with a flat ladle bottom, a middle ladle and a water feeding port which are outwards extended as far as possible (ensuring that the extended length is more than 90mm), wherein the heat-insulating plate has cracks or has obvious quality problems and is forbidden to be used, cleaning residual refractory materials, sundries and the like in the middle ladle and an impact groove area after the construction is finished, and sealing the periphery of a ladle cover of the middle ladle by adopting the refractory materials;

secondly, the sealing of the external casing is ensured, the use of the protective casing is forbidden under the condition without a sealing ring, when the protective casing is installed, the casting machine is lifted to align the bowl part of the protective casing, the bowl part of the casing and the water gap are ensured to be completely matched with a vertical contact surface, and the sealing ring is found to be damaged and needs to be replaced. Strictly detecting the sealing condition of the sleeve in each furnace, and finding out reasons and timely processing the reasons if an air suction phenomenon exists;

checking the protective pouring system before production to confirm that the device for introducing argon into the tundish is intact and the argon is unblocked, starting pouring and purging the tundish with argon by a ladle changing furnace, wherein the purging time is not less than 2min, and introducing argon in the pouring process;

and fourthly, pouring the tundish at a high liquid level, wherein the liquid level height is required to be more than or equal to 700 mm. The furnace is required to measure the thickness of the tundish slag in the middle and later stages of the casting time, and slag is required to be discharged if the thickness of the slag is more than or equal to 80 mm;

fifthly, an aluminum carbon nozzle is used for pouring to ensure that the nozzle is aligned with the crystallizer, and the insertion depth of the nozzle is proper (the standard insertion depth requires 80 mm-100 mm);

sixthly, measuring the temperature of the middle and later period of the continuous casting furnace, particularly when the superheat degree of the tundish is lower than 20 ℃.

6. Management of casting blanks

After the SWRH82B casting blank is poured, slow cooling is needed to be carried out in a finished casting blank storehouse. The purpose of slow cooling of the casting blank is as follows:

promoting the release of hydrogen elements in a casting blank and improving the mechanical property index of a wire rod;

eliminating internal stress of the casting blank and reducing crack defects caused by uneven cooling inside and outside the casting blank;

the surface scab of the finished product wire rod caused by the crack defect of the casting blank is reduced, and the yield of the product is improved;

the casting blank slow cooling field is required to be relatively closed, and no large air convection is generated. The billet blanks are compactly stacked, and the upper and lower chopped billet blanks can be isolated from the outside by other billet blanks, so that the billet blanks are slowly cooled.

(III) measures and research on inclusion control

1. Control of endogenous inclusions

The endogenous inclusions are distributed more uniformly and with smaller particles, and proper operation and rational process measures can reduce their quantity and change their composition, size and distribution, but are generally unavoidable.

Research on a converter high-carbon low-phosphorus control technology: one of the methods for reducing non-metallic inclusions in steel is to reduce the total oxygen content of the steel. The quantity and size of the endogenous inclusions in the steel, namely the deoxidation products of the steel, are greatly determined by the oxygen content at the smelting end point of the converter. The converter blowing adopts a high C and low P smelting technology with variable lance position and variable oxygen flow, and can greatly reduce the oxygen and nitrogen contents and other impurity elements at the smelting end point of the converter. And because the end-point oxygen content is reduced, an aluminum-containing deoxidizer is not adopted for deoxidation, so that the deoxidation products of aluminum are reduced, and the cleanliness of steel is improved.

The converter blowing dephosphorization reaction molecular formula is as follows:

2[P]+5(FeO)+4(CaO)＝(4CaO·P₂O₅)+5[Fe]

or

2[P]+5(FeO)+3(CaO)＝(3CaO·P₂O₅)+5[Fe]

Analyzing main influence factors and dephosphorization limit of a dephosphorization process under the condition of steelmaking of the combined blown converter by utilizing a Healy formula (1) of dephosphorization and an STB combined blown converter dephosphorization experience formula (2):

lg(％P)/[％P]＝22 350/T-16.0+2.5·lg(％T.Fe)+0.08(％CaO)------(1)

lg(％P)/[％P]＝12 210/T-9.332+0.745·lg(％T.Fe)+2.358lg(％CaO)------(2)

according to the formulas (1) and (2), the distribution coefficient of phosphorus between slag and steel under different working condition parameter combination conditions can be calculated.

Since the Hewler equation is obtained under the condition that the system reaches chemical equilibrium, the distribution coefficient of phosphorus between slag and steel calculated under various conditions is very high. The STB empirical formula comprehensively considers thermodynamic and kinetic factors of dephosphorization in the actual blowing process of the combined blown converter, and the obtained distribution coefficient of the phosphorus is lower than that calculated by the Heiler formula and can be used as a reference basis for the dephosphorization process of the combined blown converter. In order to ensure the reliability of operation, the STB empirical formula is adopted to calculate the corresponding slag amount of steel-slag balance when the phosphorus content of molten steel is removed to the same extent (0.11 wt% → 0.010 wt%) under two different working conditions. Table 7 shows a comparison of the above calculations.

TABLE 7 slag amount required for slag dephosphorization reaction equilibrium under operating conditions

As can be seen from Table 7, the most important factor affecting the dephosphorization of the combined blown converter is the temperature. The low temperature is beneficial to dephosphorization, and even under the conditions of lower slag alkalinity and lower T.Fe content in the slag, the small amount of the slag can ensure better dephosphorization effect. The distribution ratio of phosphorus between slag and steel is 700 at 1350 ℃; and under the high temperature condition (1660 ℃), even if the T.Fe and slag alkalinity in the slag are improved (beneficial to dephosphorization), the corresponding phosphorus distribution ratio is only 97 (less than 1/7 of the distribution coefficient under the low temperature condition). Therefore, the corresponding theoretical slag amount difference under the two working conditions is also large.

Therefore, the optimal time for dephosphorization in the blowing process of the combined blown converter is in the early stage of blowing, and the effect of early dephosphorization mainly depends on the dynamic conditions of a molten pool: (1) the molten pool has strong enough stirring kinetic energy to promote the balance between steel and slag; (2) ensuring a certain dephosphorization time.

In order to ensure that the dynamic conditions of the molten pool are achieved, the influence of the addition amount of the scrap on the end point temperature, the control of the slag alkalinity, the control of FeO in the slag, the control of a lance position, the control of the process temperature, the control of oxygen supply intensity and the like are researched, and the control technology of the high carbon and low phosphorus of the converter is realized.

The control of the converter smelting endpoint C, P, T is shown in Table 8.

TABLE 8 converter blowing end-point control

Item	Highest point of the design	Lowest level of	Average
				End point C/wt%	0.72	0.53	0.62
End point P/wt%	0.016	0.005	0.011
				End point temperature/. degree.C	1635	1615	1628

Selection and study of refining slag systems: by optimizing the refining slag system, the alkalinity of the refining slag is reduced, and the SiO content in the slag is improved₂The ratio (alkalinity R of refining slag is 0.8-1.2) is used for changing the components of the inclusions so as to improve the SiO content in the inclusions₂In the mass fraction of (3), correspondingly lower Al₂O₃In an amount such that the components of the inclusions areCaO-A1₂O₃-SiO₂The location points in the ternary inclusion map are shifted up to the left, approaching the plastic region. Refining with low-alkalinity refining slag to obtain Al inclusions in steel₂O₃15～25wt％，CaO 30～50wt％，SiO₂40-60 wt%, and the inclusion is in the low melting point region.

The slag system is also adopted for producing the cord steel, and considering that the corrosion of low-alkalinity slag on a steel ladle is serious, and a steel strand is thinner than the cord steel, and has relatively loose requirements on inclusions, so that the alkalinity of the slag is controlled to be 1.0-1.2, and the typical 3-furnace LF slag components are shown in a table 9.

TABLE 9 typical three-furnace refining slag composition

2. Control of foreign inclusions

Generally speaking, the particles of foreign inclusions are larger and are distributed more randomly in the steel, and the particles of endogenous inclusions are smaller and are distributed more uniformly. The large-particle inclusions with diameters of 50um or more are generally a concentrated source of drawing stress and one of the causes of brittle fracture in the drawing process.

Macroscopic observations showed that the fractures of this batch of broken filaments were all cup-cone shaped. The fracture origin of the fracture is mainly analyzed by a scanning electron microscope, and from the composition of inclusions, the fracture origin contains Na, K and Mg mold powder. Therefore, the reason of wire breakage can be judged to be caused by the fact that the crystallizer is involved in the covering slag to form large-particle inclusion, and the source of the large-particle inclusion is mainly caused by falling of the refractory material of the tundish and the covering slag.

Measures for preventing the crystallizer casting powder from rolling up: at a high drawing speed, slag entrapment is easily caused to the casting blank, and the internal quality of molten steel cannot be guaranteed because the residence time of the molten steel in a tundish is short; under the low pulling speed state, the residence time of molten steel in the tundish is relatively long, the floating effect of impurities is good, but when the pulling speed is too low, the molten steel amount and the flow rate flowing out from the submerged nozzle are reduced, the heat supply to the meniscus of the crystallizer is insufficient, and the molten protective slag is not well melted and is easily captured by slag shells. In order to prevent the crystallizer from slag entrapment in the steel drawing process, the drawing speed is set to be constant at 2.3m/min, and the fluctuation of the liquid level of the crystallizer is not more than 5 mm.

The results of the examples show that:

the converter has the advantages that the SWRH82B belongs to high-carbon steel, a high-carbon and low-phosphorus control technology of a converter blowing end point is developed, the converter blowing end point is effectively controlled, and C, P, T can meet quality and process requirements.

The refining process of Al-free deoxidation, low-alkalinity refining slag and the like is adopted for smelting, the impurities in the steel are subjected to plastic treatment, and the technical problem of brittle impurities in the steel is effectively solved.

According to the invention, the number and size of foreign impurities are strictly controlled by optimizing the number of the continuous casting furnaces of the casting machine.

Claims

1. A method for controlling inclusions of a prestressed steel strand is characterized in that a high-carbon-drawing process is adopted for steel making control, the end point [ C ] of a converter is required to be more than or equal to 0.40 wt%, and the converter is drawn in place once, so that nitrogen increase caused by low-carbon and large-amount carbon increase due to blowing supplement is prevented; the refining process of the LF refining furnace adopts micro-positive pressure operation, and the pressure is 0.01-0.03 MPa relative to the atmospheric pressure; the alkalinity of the slag is controlled within the range of 1.0-1.2, and all chemical components are controlled according to target values.

2. The inclusion control method for the prestressed steel strand as claimed in claim 1, wherein the prestressed steel is SWRH82B, and the chemical composition of the prestressed steel is as follows by mass percent:

3. The inclusion control method for the prestressed steel strand as recited in claim 1, wherein the steel making process comprises:

4. The inclusion control method for the prestressed steel strand as recited in claim 1, wherein the converter process control comprises:

the tapping time of the tapping hole is 5-7 min;

5. The inclusion control method for the prestressed steel strand as recited in claim 1, wherein the inclusion control is performed by an LF furnace refining process:

6. The inclusion control method for prestressed steel strand as claimed in claim 5, wherein refining with low-basicity refining slag is used to control the inclusion Al in the steel₂O₃15～25wt％，CaO 30～50wt％，SiO₂40～60wt％。