SU1731826A1 - Method of microalloying of steel with nitrogen - Google Patents

Abstract

The invention relates to ferrous metallurgy and is intended for use in the smelting of steels containing nitro-forming elements. The aim of the invention is to improve the quality of steel by reducing the contamination by large single and line inclusions of oxides, increasing the level of mechanical properties of finished rolled products. The metal smelted in the steelmaking unit, deoxidized and blown into the ladle with nitrogen, is subjected to additional treatment with gaseous nitrogen during casting into molds. To do this, the metal stream is flushed with nitrogen with an intensity of 0.008-0.020 nm / t, supplied by a continuous, coaxial metal stream with an internal diameter of 2 to 3.5 times the diameter of the metal stream. In addition, the flow rate of nitrogen for purging in the ladle and the flow rate of nitrogen for blowing the jet are set in a ratio of 0.1 to 5-0.75. 1 hp ff, 1 tab. THX

Description

The invention relates to ferrous metallurgy and can be used in the smelting of steels containing nitroforming elements, for example, aluminum, vanadium, niobium, boron, titanium, etc.

The aim of the invention is to improve the quality of steel by reducing the pollution by large single and line inclusions of oxides and nitrides, by increasing the mechanical properties of finished rolled products.

The essence of the method of micro-alloying of steel with nitrogen is that the metal smelted in the steel-making unit is deacidified, blown into the ladle with nitrogen and during the process of casting into the mold metal stream is additionally blown with gaseous nitrogen with an intensity of 0.005-0.020 nm / t min supplied by a continuous coaxial jet of metal with a stream with an internal diameter equal to 2.0-3.5 diameter of the metal jet. In addition, the ratio of nitrogen flow for blowing in the ladle to the flow rate of nitrogen for blowing the jet is 0.15-0.75.

When the steel is purged with nitrogen in the ladle, some of the gas is dissolved in the liquid metal,

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I interact primarily with active nitride-forming elements (titanium, chromium, etc.), which are present in almost all steel grades. The resulting large inclusions of nitrides, which can reduce the quality of the finished metal, are partially removed from the melt during its mixing and soaking in the ladle. In addition, these nitride-forming elements, which actively bind nitrogen when blowing into the ladle, prevent the formation of finely dispersed vanadium, aluminum, niobium and other microalloying nitrides, which have a favorable effect on the properties of finished rolled products.

During the casting process, the metal jet is additionally blown with nitrogen gas supplied by a continuous coaxial metal jet with a stream of a certain intensity and internal diameter related to the jet diameter at the level of the slice of the pouring nozzle.

As a result, the inner part of the gas flow is attracted to the metal stream and envelops irregularities due to the turbulent nature of the flow of the ravplav, a part of nitrogen is drawn by the metal stream into the mold. Favorable conditions are created for the absorption of nitrogen by the metal, providing a high specific surface area of its contact with the gas, both in the jet and when bubbling nitrogen from the volume of the mold emerges. At this stage, the kinetic impediments to the flow of this interaction, associated at the previous stage of processing with blocking of the surface by active oxygen atoms, largely disappear. This contributes to the formation of finely dispersed nitrides of elements that increase the level of properties of finished rolled products. The externally continuous surface of the gas flow blocks the flow of atmospheric air to the metal stream and prevents the secondary oxidation of elements having a significant affinity for oxygen. In addition, the external part of the nitrogen stream, striking the metal surface in the mold, spreads along it and prevents the metal surface from contacting with the oxygen of the air. At the same time, additional refining of the melt in the mold from large oxide and nitride inclusions, carried out by nitrogen bubbles emerging from the volume of the metal, occurs. Thus, the effect of nitrogen on the metal during purging in the ladle and in the casting process allows to remove large nonmetallic inclusions of oxides and nitrides from the melt, to form on

the final stage of processing is fine nitride inclusions, which, remaining in the metal, favorably affect the level of its mechanical properties.

The proposed parameters of the method were tested experimentally. To do this, under the conditions of the open-hearth shop in 430-ton open-hearth furnaces, steel was melted according to the known and proposed methods.

0 mark М76В. The chemical composition of rail steel, wt.%: C 0,72-0,76; Mp 0.8-0.9; Si 0.2-0.3; P 0.035; S 0.025; V 0.03-0.07. With the release of smelting, ferroalloys were introduced into the ladle under the stream of metal, kg / t, steel:

5 CMN17 5.14; FS45 1.40; SACd 3.27; FVd (V 45%) 0.86. The composition of the V-ligature SCWd, wt.%: V 1-3; Ca 15-20; Si 45-55; Ti 0.5. After melting, the metal was blown into the ladle with nitrogen supplied through a submerged lined tuyere with a gas flow rate of 20–125 m / h for 6–8 min. According to a known method, simultaneously with blowing steel with nitrogen, aluminum was injected in the amount of 0.5 kg / t of steel into the zone of bubbling bubbles. Then the metal was poured through a glass with a diameter of 70 mm into ingots with a mass of 8.23 tons. According to the proposed method, a jet of metal was blown with nitrogen using special nozzles with an annular nozzle of various diameters. The ratio of the diameters of the annular nozzle and the diameter of the metal jet was 1.8-3.7. The flow rate of nitrogen supplied to the jet is varied from 0.003 to 0.025 Nm3 / t min.

5 The test results are summarized in table.

The results of the experiments showed that the flow rate of nitrogen for blowing the jet is less than 0.005 Nm3 / t min (option 9) is insufficient for nitriding

0 metal and does not ensure the achievement of the required level of mechanical properties of finished rolled products. In addition, with insufficient nitrogen consumption, the process of secondary oxidation of the metal jet is not suppressed,

5 which leads to an increase in the oxidation of steel and the appearance of lines of oxide nonmetallic insertions.

An increase in nitrogen flow rate of more than 0.020 Nm3 / t min (variant 10) leads to an increase in processing costs and an increase in metal rejection due to surface defects, which is associated with an increase in metal heaps on the walls of the mold.

Reducing the diameter of the nitrogen stream used for blowing the jet below 2.0 the diameter of the metal jet (option 11) does not effectively protect the liquid metal from secondary oxidation due to the discontinuity of the gas flow, which leads to an increase in oxidation, the appearance of lines of oxide nonmetallic inclusions , an increase in the loss of micro-alloying elements and, as a result, a decrease in the level of mechanical properties of the finished steel.

An increase in the ratio of the diameter of the gas flow to the diameter of the metal stream in excess of 3.5 leads to the fact that the flow of nitrogen does not sufficiently envelop the surface of the metal, as a result of which the required degree of nitriding is not achieved. This prevents the formation of a sufficient amount of nitrides that harden the metal and obtain the required high level of mechanical properties of the steel.

The ratio of the specific intensity of metal treatment with nitrogen in the steel-casting ladle and during steel casting also has a significant impact on the quality and properties of the finished metal. Thus, a decrease in the ratio of these costs below 0.15 does not allow complete binding of harmful nitride-forming elements to nitrides when purging with nitrogen in the ladle (version 13). This leads to the fact that elements such as titanium, chromium and others form large nitrides when interacting with nitrogen, forming a jet during casting, which are not removed from the metal and form stitches of nitrides in finished steel, resulting in deterioration of the mechanical properties of the steel.

Exceeding this ratio above 0.75 (option 14) causes the metal surface in the ladle to be exposed during processing, further oxidizes it, increases the waste of deoxidizing elements and microalloying and, accordingly, deteriorates the mechanical properties of the finished steel. In addition, when gas consumption increases at the melt processing in the ladle, nitrogen will also be spent on interaction with vanadium, aluminum, niobium and other microalloying elements, reducing the effectiveness of their beneficial effect on the properties of finished rolled products.

According to the results of the experimental heats given in the table, it can be seen that the metal treated according to variants 2-8 possesses the best characteristics - the temporary tensile strength of the metal increases by 325-455 MPa or 1.25-1.45

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times, the impact viscosity is 8.0-14.0 J / cm (1.30-1.52 times), there are no large single and stitched nitrides in the metal, which reduces the cost of processing compared to the known method. Example 1. Under the conditions of the oxygen converter shop in a 160-ton converter, 09G2 steel with a chemical composition is melted, wt%: C 0.12; Si 0.57; Mp 1.51;

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phosphorus, sulfur, chromium less than 0.030, 0.035, 0.16, respectively. The metal is deoxidized with aluminum in an amount of 0.9 kg / ton. In the process of release, the metal is treated with nitrogen supplied through a porous tube with a flow rate of 60 Nm3 / h. The purge is completed 5 minutes after filling the bucket. Subsequently, the steel is poured from above into molds with a capacity of 8.2 tons through a beaker with a diameter of 60 mm. During casting, a stream of metal is flushed with nitrogen supplied through a nozzle with an annular slotted nozzle. The internal diameter of the annular gas flow thus obtained is 196

5 mm. The ingots are rolled onto beam no. 55. Metal samples were taken from rolled products to determine the mechanical properties, impact toughness at -40 ° C, as well as samples to determine the content of gases and metallographic analysis of non-metallic inclusions.

After this treatment, the metal is well averaged by chemical composition and temperature. Unacceptable macrostructure defects 5 rounds in bars are absent. Residual chromium is largely bound to nitrides, with large inclusions. An increased amount of fine aluminum nitrides is observed in the metal.

0 enhancing the level of mechanical properties. The tensile strength is 570 MPa, i.e. increased by 30 rel.%, impact viscosity at –40 ° С increased by 10 J / cm2 and amounted to 39 J / cm2. The cost of steel processing decreased mainly due to saving of acidizers.

EXAMPLE 2. Steel grade M76B is smelted in a 430-ton open-hearth furnace. In

0, the release time of the smelting, ferroalloys, kg / t, steel were introduced into the ladle: SMP 5.14; FS45 1.40; SLE d 3.27; FVd (Y45%) 0.86; Composition of V-l and SCWd gaiters, wt.%: V 1-3; Ca 15-20; Si 45-55; Ti 0.5. After melting, the metal is pro duced through a submerged lance with nitrogen at a flow rate of 60 nm / h. Blowing turn 8 minutes after the release of melting. During casting through a beaker with a diameter of 70 mm into ingots weighing 8.23 tons, a stream of metal is flushed with nitrogen at a flow rate of 150 Nm3 / h through a nozzle with an annular nozzle having an internal diameter of 196 mm. Casting time 45 min. The chemical composition of the finished steel, wt.%: C 0,72; Mp 0.80; Si 0.27; P 0.028; S

5 0.022; V 0.06; N 0.008. Ingots are rolled onto rails of type P65. The metal treated in this way is well averaged by chemical composition and temperature, so there are no defects in the macrostructure in the ingots. Residual chrome and titanium in zn0

The reading measure is associated with nitrogen, and large inclusions are removed from the metal while vigorously stirring it with nitrogen trapped in a steel volume in a mold. The amount of fine vanadium nitrides increases in the metal. The temporary tensile strength of steel is 1,670 MPa (increased by 1.28 times), the impact strength (20 ° C) increased by 12 J / cm2 and amounted to 39 J / cm. The cost of steel processing has decreased due to savings of deoxidizers and vanadium.

When applying the proposed method as compared with the known, the temporary tensile strength of the metal is increased by 1.20-1.35 times; metal toughness increases by 8-14 J / cm, there are no large single and line inclusions of nitrides in the metal; reduced steel processing costs.

Claims (2)

1. Method of micro-alloying of steel with nitrogen, including smelting of metal in a steel-smelting unit, deoxidation, blowing the metal in the ladle with nitrogen and then pouring it into molds, characterized in that, in order to improve the quality of steel by reducing the contamination with large single and line inclusions of oxides and nitrides, increasing the mechanical properties of the finished steel, the metal stream during the casting process is additionally blown nitrogen with an intensity of 0.005-0.020 Nm3 / t min supplied continuous coaxial jet of metal with a flow with an internal diameter of 2.0-3.5 diameters of the jet of metal. 2. A method according to claim 1, characterized in that the ratio of the nitrogen flow rate for blowing in the bucket to the nitrogen flow rate for blowing the jet is 1: (0.15-0.75).