RU2380431C1 - Nitriding method of steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: method includes vacuumizing of steel in ladle with simultaneous blowing off by inert gas through bottom porous tuyeres under high-basic slag. Blowing off in ladle at vacuumising is implemented through bottom porous tuyeres by nitrogen with intensity 3-25 m³/h and consumption 0.02-0.35 m³/t of steel. Oxidation of slag not more than 1.2% FeO and ratio CaO/ SiO₂ 2.1-2.8, herewith height of slag in ladle is provided equal to 80-150 mm.

EFFECT: usage of invention provides reduction of expenditures at nitriding of steel, reduction of oxygen concentration in steel and pollution level by nonmetallic inclusions, increasing of impact elasticity at negative temperatures.

Description

The invention relates to ferrous metallurgy, in particular to methods for the production of steel using vacuum.

Nitrogen-containing steels are known that have increased toughness at low temperatures due to nitride and carbonitride hardening [1]. The necessary properties of such steels are achieved by alloying steel with nitrided ferroalloys. However, the production of nitrided ferrovanadium alloys is associated with high costs, they are expensive and, when introduced into steel, the degree of nitrogen assimilation is low.

There is also known a method of producing rail steel, including smelting steel in a furnace, its pouring into a ladle, deoxidation and subsequent purging of steel in a ladle with gaseous nitrogen through a lance, in which gaseous nitrogen is fed through a slotted bottom refractory lance having a gap thickness of up to 0.1 mm, within 15-30 minutes with a flow rate of 40-65 nm ³ / h at a pressure of (6-8) · 10 ⁵ Pa and a total nitrogen flow rate of 0.10-0.30 nm ³ / t of liquid steel [2].

The technical disadvantages of this method are:

- unstable assimilation of nitrogen during purging and obtaining different values of impact strength with other equal chemical composition;

- an increased level of contamination of rail steel with non-metallic oxide inclusions due to the use of nitrogen of insufficient degree of purity in oxygen concentration;

- reduction of the mechanical properties of rails due to the high concentration of oxide non-metallic inclusions in the rail steel;

- low productivity with nitriding.

A known method of smelting and evacuating steel selected as a prototype is known, including smelting metal, processing with refining slag, vacuum, in which the metal in the ladle is first treated with the main reducing slag by pouring metal from the furnace into the ladle onto solid slag-forming materials while simultaneously blowing the melt with argon with an intensity of 0 , 01-0.07 m ³ / t · min and after 30-90 s with the main oxidizing slag and argon with a purge intensity of 0.2-0.8 of the initial purge for 30-180 s, after which 20-40% is removed m ssy slag sits down converter and perform vacuuming metal [3].

Technical disadvantages of this method of smelting and evacuation of steel are:

- the impossibility of the operation of nitriding due to the use of argon as the gas used;

- the impossibility of combining the operation of reducing the oxygen content and oxide non-metallic inclusions with the operation to saturate the steel with nitrogen due to the use of argon.

The desired technical results of the invention are: reducing costs during nitriding of steel, reducing the concentration of oxygen in steel and the level of contamination with non-metallic inclusions; increase in toughness at low temperatures.

To this end, a method of steel nitriding is proposed, which includes evacuating steel in a ladle while simultaneously blowing with inert gas through porous bottom tuyeres under highly basic slag, characterized in that purging in the ladle during evacuation is carried out through bottom porous tuyeres with nitrogen with an intensity of 3-25 m ³ / h and with a flow rate of 0.02-0.35 m ³ / t of steel under the slag with an oxidation of not more than 1.2% FeO and a CaO / SiO ₂ ratio of 2.1-2.8, the slag height in the ladle being equal to 80-150 mm.

The claimed limits are selected experimentally.

The purging of steel with nitrogen through porous lances is carried out with a flow rate of 0.02-0.35 m ³ / t and an intensity of 3-25 m ³ / h, selected on the basis of the following premises. With a decrease in flow rate of less than 0.02 nm ³ / t of smelted steel and intensity of less than 3 m ³ / h, the rate of steel saturation with nitrogen decreases. With an increase in flow rate of more than 0.35 nm ³ / t and the intensity of steel purging with nitrogen more than 25 nm ³ / h per ton of steel being melted, intensive foaming of steel and slag in the ladle occurs, which complicates the processing due to splashes of steel and slag from the ladle into the vacuum chamber , which leads to emergency operation.

When slag oxidation increases by more than 1.2% FeO during steel processing, emergency situations occur due to sharp boiling of steel in the ladle and emissions of the vacuum tank into the tank, as a result of which the further evacuation process stops.

The ratio of CaO / SiO ₂ equal to 2.1-2.8 was chosen on the basis that, when the ratio is lower than 2.1, the required level of contamination of the steel with oxide exogenous inclusions is not ensured due to erosion of the steel lining of the ladle by ladle slag, in addition, fluidity slag does not allow to effectively saturate steel with nitrogen due to the rapid removal of nitrogen bubbles through the slag. With an increase in this ratio of more than 2.8, the slags are “thick” and refractory, which complicates the process of adding nitrogen-forming ferroalloys (vanadium, titanium, aluminum, etc.) to steel, since the passage of ferroalloys through the slag layer is difficult due to the absence of steel necessary nitride-forming elements, seated with ferroalloys, the process of steel nitriding slows down, and therefore, the duration of the vacuum treatment increases.

When the slag height in the ladle is less than 80 mm, the low slag height does not protect the metal from the absorption of gases from the atmosphere. With a slag height of more than 150 mm, the processing time on the vacuum increases.

The inventive method of nitriding steel was implemented in the production of rail steel grade NE76F with processing on a chamber type vacuum cleaner VD. Smelted steel in a steel-pouring ladle after heating on a ladle-furnace unit was fed to a vacuum vessel for processing. At the same time, the content in the ladle slag was not more than 1.2% FeO and the CaO / SiO ₂ ratio equal to 2.1-2.8. The slag height in the ladle was 80-150 mm. At a slag height of more than 150 mm, slag was downloaded by a slag loading machine through the side of a steel pouring ladle. Before creating a vacuum, the bucket with metal was installed in the chamber and nitrogen purging was started through porous bottom tuyeres with an intensity of 3-25 m ³ / h and a flow rate of 0.02-0.35 m ³ / t of steel. Then the vacuum cap was approaching and within 5 minutes a pressure of less than 0.3 torr was created. The exposure time under vacuum was 15-60 minutes. After the processing operation, the vacuum was removed, the lid was opened and the ladle was fed to continuous casting. Steel was cast on 4-strand continuous casting machines with a mold section of 300 × 330 mm. Then, continuously cast billets were heated in a walking beam furnace and rolled to P65 rails.

When using the proposed method, the consumption of nitrided vanadium-containing ferroalloys is reduced by 0.5-0.7 kg / t of smelted steel.

The oxygen content is reduced to 12-20 ppm, the nitrogen content is 100-250 ppm. The line length of oxide inclusions is reduced from 1.5 mm to 0.5 mm. Impact strength of heat-strengthened rails increased from 25 J / cm ² to 45 J / cm ² .

Information sources

1. Filippenkov A.A. Vanadium-containing steels for castings. - Yekaterinburg: Ural Branch of the Russian Academy of Sciences. - 2001. -345 p.

2. RF patent No. 2161205, class C21C 7/00, 7/072.

3. A.s No. 968078, class C21C 7/10.

Claims (1)

A method of steel nitriding, including evacuation of steel in a ladle with simultaneous inert gas purging through bottom porous tuyeres under highly basic slag, characterized in that purging in the bucket during evacuation is carried out through bottom porous tuyeres with nitrogen with an intensity of 3-25 m ³ / h and flow rate 0, 02-0.35 m ³ / t of steel, under a slag with an oxidation of not more than 1.2% FeO and a CaO / SiO ₂ ratio of 2.1-2.8, and the slag height in the ladle is 80-150 mm.