GB2576026A

GB2576026A - Manufacturing steel

Info

Publication number: GB2576026A
Application number: GB1812580.7A
Authority: GB
Inventors: Kirk Anthony; Stuart Millman Maurice
Original assignee: Materials Proc Institute
Current assignee: Materials Proc Institute
Priority date: 2018-08-02
Filing date: 2018-08-02
Publication date: 2020-02-05
Also published as: GB201812580D0

Abstract

A method of manufacturing steel includes introducing a charge material into a basic oxygen converter (501). Oxygen gas is blown at e.g. supersonic speed into the charge to oxidise and remove impurities such as carbon from the charge (502). The step of blowing removes the carbon from the charge material to form carbon monoxide gas (506). During the critical early stages of a slopping event / blow, large amounts of gas rising up through the slag with high apparent viscosity resulting from excessively high levels of carbon monoxide gas generation are reduced by introducing nitrogen gas (504) while blowing oxygen, e.g. into the flow stream. The oxygen flow rate may be comparable to the subsequent combined flow rate and the nitrogen flow may be discontinued before the end of the oxidation stage. The charge material may be combined scrap and blast furnace iron comprising silicon. The nitrogen bleed profile supply may increase with the silicon content (fig 6). A lime-based material may be added to neutralize acidic compounds in the slag.

Description

Manufacturing Steel

CROSS REFERENCE TO RELATED APPLICATIONS

This application represents the first application for a patent directed towards the invention and the subject matter.

BACKGROUND OF THE INVENTION

The present invention relates to a method of manufacturing steel.

The basic oxygen steelmaking (BOS) process has been utilised commercially for around sixty years. The process involves blowing oxygen gas at supersonic speed into blast furnace iron in a BOS converter to oxidise and remove carbon, silicon and other impurities from the blastfurnace iron thereby producing liquid low-carbon steel.

A problem with the current process is that during the initial stages of BOS converter operation, oxidation of iron and silicon leads to the production of large volumes of silica and iron oxide (and also manganese oxide). This oxidation also forms a type of slag known as Fayalite which is highly acidic in composition due to the combination of silica and iron oxide. Lime-based additions (basic) are charged to the BOS converter at this time to neutralise the Fayalite and to produce a working slag that is able to take up phosphorus. Very little carbon oxidation takes place at this stage of the process.

When the silicon content in the blast furnace iron drops below around zero point one five and zero point two percent (0.15% - 0.20%) and the temperature rises above around one thousand four hundred degrees Celsius (1400°C), oxidation of silicon switches to oxidation of carbon in the blast furnace iron and a rapidly increasing amount of carbon monoxide gas is formed as the blow progresses.

The slag is very viscous in these early stages of production. Metal droplets that result from the impact of the oxygen gas jets onto the liquid metal surface are thrown-up into the slag to form a slag/metal/gas emulsion. Because the slag is viscous, the carbon monoxide gas bubbles cannot easily pass through it and as a consequence, the slag/metal/gas/emulsion can be lifted by the large volumes of generated carbon monoxide gas and become foamy.

When the slag/metal/gas emulsion is lifted excessively and over the top of the converter mouth then this is known as a slop. Slopping events occur when the slag/metal/gas emulsion is lifted and escapes from the converter. In extreme circumstances, slopping events can be very rapid and explosive in nature and as such these tend to be unpredictable, uncontrollable and consequently dangerous. Additionally, explosive slopping events can lead to considerable downtime as the volumes of material released are difficult and time consuming to clear up. In addition, apart from causing a safety hazard and environmental damage, slopping events increase operating costs and required levels of maintenance while reducing productivity and liquid steel yield.

Once a slopping event has started, current systems seek to regain control of the converter operation by altering the position of the oxygen lance or oxygen gas flow rate from the oxygen lance used to deliver the oxygen gas jets that impact on the liquid metal surface at supersonic speed. In general, this is only done once the slopping event has already started as it is difficult to anticipate as and when such an event is likely to occur. Consequently, this approach only provides a limited amount of control and does not prevent the slopping event itself. Furthermore, variation in the oxygen flow rate or a change in the oxygen lance height destabilises the physical and chemical balance BOS converter operation meaning there is a period of time before the changes are effective and stable operation is restored.

In addition to this, systems based on audio detection, or oxygen lance or converter vessel vibrations can be utilised to attempt to detect when potential slopping events will occur. These systems are typically unreliable and often do not produce sufficient warning to prevent slopping events from taking place.

There remains therefore a need to prevent slopping events from occurring in basic oxygen steelmaking.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a method of manufacturing steel, comprising the steps of: introducing a charge material into a basic oxygen converter; and blowing oxygen into said charge material to oxidise and remove impurities from said charge material, said impurities comprising carbon; wherein continuation of said step of blowing oxygen comprises removal of said carbon from said charge material to form carbon monoxide; and further comprising the step of: delaying the high rate of formation of carbon monoxide by introducing nitrogen into said converter at a controlled rate during said step of blowing oxygen.

In an embodiment, nitrogen is introduced into the oxygen gas input stream during the early stages of the oxygen blow whilst maintaining constant the total input gas flow-rate. By introducing nitrogen gas into the oxygen gas input stream in this way, the volume of displaced carbon monoxide gas that would be generated without this intervention, is halved. As a consequence, slopping events are avoided.

The introduction of nitrogen gas into the oxygen gas input stream during the early stages of the BOS converter process results in a delay in production of the high volumes of carbon monoxide gas which lead to the slopping events described herein. Controlled nitrogen gas introduction also leads to increased control of the oxidation reactions occurring in the converter.

In an embodiment, nitrogen gas is introduced directly into the oxygen gas input flow stream. The total gas flow rate (of oxygen gas and nitrogen gas) into the basic oxygen converter is also configured to remain consistent with the original oxygen flow. This ensures that the physical characteristics of the gas jet interaction with the liquid metal in the BOS converter remains undisturbed. In this way, the total gas volume bubbling up through the slag can be reduced, the volume of nitrogen introduced being half that of the displaced carbon monoxide gas. As a consequence, slopping events can be avoided.

Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings. The detailed embodiments show the best mode known to the inventor and provide support for the invention as claimed. However, they are only exemplary and should not be used to interpret or limit the scope of the claims. Their purpose is to provide a teaching to those skilled in the art. Components and processes distinguished by ordinal phrases such as “first” and “second” do not necessarily define an order or ranking of any sort.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figure 1 shows a basic oxygen steelmaking (BOS) converter for use in manufacturing steel;

Figure 2 shows a schematic illustration of chemical reactions in a BOS converter during the production of steel;

Figure 3 shows a plot of frequency of slopping events during the BOS process;

Figure 4 shows a schematic illustration of the stages of oxidation during the BOS process;

Figure 5 shows steps in a method of manufacturing steel;

Figure 6 shows example nitrogen bleed profiles for use in accordance with the present invention;

Figure 7 shows the difference between the rate of decarburisation against blow time during the BOS process in accordance with the present invention and conventional methods;

Figure 8 shows the difference in temperature profiles during the BOS process in accordance with the present invention and conventional methods; and

Figure 9 shows a vessel suitable for introduction of nitrogen into the blown oxygen.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 1

The basic oxygen steelmaking (BOS) process utilises a converter which is represented schematically in Figure 1 as converter 101.

Converter 101 comprises a vessel 102 having a mouth 103 into which materials can be added into the vessel 102. Vessel 102 typically comprises a refractory lining 104 which protects the converter vessel shell from the molten material therein. Lining 104 is typically comprised of a brick-based material having a graphite substrate. The brick material is resistant to the heat of the molten metal and slag when steelmaking is in progress.

Vessel 102 further includes a tap hole 105 for removal of processed steel to enable the steel to be removed from the inside of vessel 102 for further production.

Converter 101 further comprises a lance 106 which, in the embodiment is a water-cooled lance. In conventional methods, this lance 106 is utilised to blow oxygen gas at supersonic speed into the contents added into the vessel 102

In the production of steel, a charge material is introduced into basic oxygen converter 101 through mouth 103. This charge material resides in the bottom of vessel 102 and oxygen gas is blown at supersonic speed into the charge material by means of lance 106 so as to remove impurities from the charge material thereby leading to the formation of liquid low-carbon steel.

The charge material typically comprises a blast furnace iron in a molten form comprising iron and further components including carbon, silicon and manganese. Other components such as phosphorous and sulphur are also usually present. The molten blast furnace iron typically includes carbon in the region of four to four and a half percent (4 - 4.5%).

In addition to the blast furnace iron, recycled scrap steel is also added to vessel 102 and is used in predetermined quantities in order to control the temperature rise of the blast furnace iron from a temperature in the region of one thousand three hundred degrees Celsius (1300°C) to around one thousand seven hundred degrees Celsius (1700°C).

During production, oxygen gas is blown at supersonic speed into the charge material by means of water-cooled vertical lance 106 at a relatively high flow rate. In an embodiment, the oxygen gas is blown into the charge material typically at a flow rate of one thousand cubic metres per minute (1000 m³/min). The actual oxygen gas flow-rate will depend on the BOS converter geometric constraints. This introduction is typically conducted at supersonic speed, that is above the speed of sound, and in an example embodiment, this occurs in the region of Mach 2.2.

Blowing oxygen gas at supersonic speed into the charge material results in the formation of steel from hot metal 107. As blown oxygen reacts with the charge material, impurities in the blast furnace iron are oxidised to their oxides. In particular, silicon oxidises into silica, iron oxidises into various iron oxides (FexOy) and manganese oxidises into manganese oxides. These oxides are highly acidic in terms of their composition.

As part of the process, slag 108 is also formed from the iron oxides and silica. As noted previously, the slag has high acidity and is also highly viscous. A lime-based material is usually added to the slag to help neutralise any acidic compounds in the slag, however, this does not prevent slopping events from occurring.

In accordance with the present invention, the method herein delays the formation of excessive amounts of gas, bubbling up through the highly viscous early-formed slag during the initial stages of BOS Converter operation by introducing controlled rates of nitrogen gas into the oxygen gas stream passing through the lance while maintaining a constant total gas flow rate through the lance. In the embodiment, the volume of nitrogen introduced is half that of the displaced carbon monoxide gas that would be generated without this intervention. In this way, slopping events are avoided.

Figure 2

A schematic diagram indicating the chemical reactions occurring in the process described in Figure 1 is shown in Figure 2.

In accordance with the invention, oxidising gas 201 is blown at supersonic speed into the charge material 202 in BOS converter 101 resulting in various output compounds from charge material 202. These include iron oxides (FexOy) 203 formed from the iron in the charge material when oxidised, and silica 204 formed from the combination of silicon and the oxygen in blown gas 201. Carbon monoxide 205 is also produced along with other oxides 206 typically including elements such as manganese and phosphorous which are often found in typical blast furnace iron materials.

In accordance with the invention, blown oxidising gas 201 comprises a percentage of nitrogen introduced at specific periods so as to delay the formation of excessive amounts of carbon monoxide gas 205, bubbling up through the highly viscous early-formed slag during the initial stages of BOS Converter operation. When added to the converter 101 at specific periods in the process, the input nitrogen is not absorbed within the hot metal and is simply output as nitrogen molecules 207. The nitrogen gas however delays the formation of excessive amounts of carbon monoxide gas, bubbling up through the highly viscous early-formed slag during the initial stages of BOS Converter operation. In this way, slopping events are avoided and this will be described further in particular with respect to Figures 4 to 7.

Figure 3

A plot of data illustrating the frequency of slopping events in conventional BOS processes is graphically illustrated in Figure 3.

Figure 3 shows a plot of the frequency of slopping events against the time period during which oxygen is blown for a typical conventional method of basic oxygen steelmaking. The data in Figure 3 covers slopping events where slopping lasts over thirty seconds (30s) duration and is therefore highly deleterious to BOS converter operation. These slopping events are significantly reduced by the claimed invention.

Referring to the plot of Figure 3, a peak 301 indicates that a high frequency of slopping events occurs between five and eight minutes (5 to 8 mins) of a typical blow cycle. At peak 302, a further increased frequency of slopping events occurs during the period between twelve and fourteen minutes (12 to 14 mins) of a typical blow cycle. A typical oxygen blow cycle lasts in the region of sixteen to eighteen minutes (16 to 18 mins) and it is noted that between the start of the blow and around four minutes into the cycle, shown by 303, slopping events do not frequently occur. The invention therefore seeks to take advantage of this initial period thereby reducing peaks 301 and 302 and consequently avoiding high frequency slopping events.

Figure 4

A schematic breakdown of stages during the process of blowing oxygen over the charge material is shown in Figure 4. The process can be considered as taking place over four separate stages of which stage 401 is an initiation stage, stage 402 is a transition stage and stages 403 and 404 are primary and secondary oxidation stages. The schematic shows example time periods in minutes in which each of the stages may begin in an embodiment. It is appreciated that these time periods may vary and are used in this figure as an example of a typical blow process lasting in the region of sixteen to eighteen minutes (16-18 mins). The lengths of time of each of the stages may also vary depending on the composition of the materials included in the converter and the chemical and physical interactions between the hot metal and the slag.

Initiation stage 401 typically lasts in the region of three to four minutes (3-4 mins) and results in the formation of iron oxides and silica along with other oxides such as manganese oxide. During this stage, the iron oxides and silica combine to form a highly acidic (siliceous) slag known as Fayalite. A limebased material comprising calcium oxide or similar is added to the converter in order to neutralise acidic compounds in the slag. The lime-based material also develops the slag to produce a working slag which can absorb other materials such as phosphorous in the hot metal. During the initiation stage there is very little oxidisation of the carbon in the furnace iron.

Initiation stage 401 is commenced by the activation of blowing oxygen and after around three to five minutes (3-5 mins) of this process transition stage 402 typically occurs.

At the transition stage, the silicon content in the hot metal is typically reduced to around zero point two percent (0.2%). Iron oxide formation continues and it is at this stage that the oxidation of the silicon switches to oxidation of carbon in the blast furnace iron. In the embodiment, during this transition stage nitrogen gas is introduced into the oxygen blow whilst maintaining the total input gas flow at a constant rate and this consequently results in a reduction of the production of carbon monoxide gas and also a reduction in the total amount of gas bubbles travelling up through the slag.

After around five to six minutes (5-6 mins) of oxygen being blown, the primary oxidation stage is reached. The introduction of nitrogen gas is continued through the primary oxidation stage and discontinued prior to the end of the primary oxidation stage 403.

In conventional BOS converters it is during this primary oxidation stage in which the peak 301 in Figure 3 occurs and slopping events are most frequent. By continuing to introduce nitrogen into the oxygen blow during this period, excess amounts of gas going into the slag are avoided and this consequently prevents slopping events from occurring.

In order to avoid nitrogen pick-up or absorption into the hot metal, the nitrogen needs to be introduced into the converter at a relatively low temperature and a relatively high carbon level in the hot metal in comparison to conventional methods. Thus, during the initial three stages of the oxygen blow and while the carbon level in the hot metal is closer to the four percent (4%) provided from the blast furnace, the nitrogen does not interfere with the liquid hot metal and is not absorbed.

Prior to the end of primary oxidation stage 403 however the introduction of nitrogen is discontinued so that the process continues with full oxygen flow for the latter part of the process throughout the secondary oxidation stage 404. Secondary oxidation stage 404 typically commences around eight minutes (8 mins) from the start of the oxygen blow and the commencement of the initiation stage.

This process not only prevents the peak of slopping event frequency during the primary oxidation stage illustrated by peak 301 in Figure 3 but because it avoids over-oxidation of the slag at the start of the secondary oxidation stage, it also prevents peak 302 from arising during the secondary oxidation stage.

It is noted that even once the introduction of nitrogen gas has been discontinued, oxygen gas continues to be blown at supersonic speed and now at maximum rate into the charge material to enable the further removal of impurities from the charge material. Thus, by introducing nitrogen during the transition and primary oxidation stages the generation of excess amounts of gas passing upwards through the slag of carbon monoxide is delayed and slopping events are prevented from arising during the process.

Figure 5

A method of manufacturing steel is shown in Figure 5 illustrating the steps of the process. At step 501 a charge material is added to the basic oxygen converter 101 and at step 502 oxygen gas is blown at supersonic speed into the charge material to remove impurities from the charge material. This leads to the oxidation of silicon and iron in particular at step 503 and nitrogen gas is subsequently introduced at step 504 prior to the oxidation stage as described previously in respect of Figure 4.

The step of introducing nitrogen involves introducing the nitrogen gas directly into the oxygen gas flow stream through the lance described in Figure 1. The total input gas flow rate through the lance is not altered and this presents a step away from conventional methods for preventing slopping. In conventional methods, the flow rate of the input oxygen gas is typically lowered in order to reduce the production of carbon monoxide gas. In contrast, in the present invention, the introduction of nitrogen gas avoids the need to substantially alter the total input gas flow rate. The introduction of one unit volume of nitrogen is half that of the displaced carbon monoxide gas and therefore this results in the reduction in the production of total gas volume rising up through the slag without the necessity of reducing the total input gas flow rate.

After a predetermined period of time the introduction of nitrogen gas is discontinued at step 505 however, the oxygen gas is continued to be blown into the charge material to remove impurities. This continues to result in the oxidation of carbon to form carbon monoxide.

The rate of formation of carbon monoxide gas is suppressed during the early part of the blow while during the latter part of the blow it is enhanced. Also, over-oxidation of the slag during later stages of the blow is avoided and therefore, slopping events during the secondary oxidation stage 404, are averted. In conventional methods, slopping events occur due to chemical and physical interactions between the molten metal and the slag formed in the initial stages of the process. In the present invention, the introduction of nitrogen gas into the oxygen gas stream while maintaining the total gas flow lowers the rate of formation of carbon monoxide gas and reduces the total gas volume while the converter process is allowed sufficient time to stabilise enabling avoidance of the slopping event.

Figure 6

Example nitrogen bleed profiles are shown in Figure 6 for alternative calculated percentages of silicon content in the hot metal.

Profile 601 illustrates a silicon content in the hot metal of over one percent (1%) silicon. During the initiation period there is no nitrogen bled into the converter using this profile however, once the transition period has been reached, an increasing amount of nitrogen is introduced and retained for a specified period before the bleed is reduced. The reduction coincides with the end of the primary oxidation stage and the nitrogen bleed is therefore discontinued for the secondary oxidation stage until the end of the blow.

Profiles 602, 603, 604 and 605 correspond to alternative examples whereby the silicon contact in the hot metal is between zero point eight and one percent (0.8 and 1 %), zero point six and zero point eight percent (0.6 and 0.8%), zero point four and zero point six percent (0.4 and 0.6%) and zero point two and zero point four percent (0.2 and 0.4%) respectively. Thus, it can be seen that the nitrogen bleed profile comprises an increased percentage of nitrogen with increased silicon content in the charge material. In particular, prior to initiation of the process, the percentage of silicon in the hot metal can be determined such that a suitable nitrogen bleed profile can be selected and included in the process by means of a conventional controller and computer system.

It should be noted that the variation in hot metal silicon level is one example of how nitrogen bleed profiles might be derived. It is appreciated that, in alternative embodiments, other example variants might be used either singly or in combination to derive suitable nitrogen bleed profiles.

Figure 7

An illustration of the rate of decarburisation of the off-gas from the basic oxygen converter against the blow time for a conventional process and the invention is shown with respect to Figure 7.

Plot 701 illustrates a conventional curve for a blow of oxygen without the introduction of nitrogen. This illustrates that the decarburisation begins at a point in the blow time soon after initiation of the blow before stabilising for a period of time and then decreasing towards the end of the blow.

In accordance with the invention, plot 702 illustrates that, comparatively, the rate of decarburisation is offset such that rate of decarburisation is relatively suppressed during the early stages of the blow whilst being relatively enhanced during the later stages of the blow.

Figure 8

An illustration of the temperature profile of the process described herein and a conventional process as a plot of temperature against blow time is shown in Figure 8.

Plot 801 illustrates a conventional curve for a blow of oxygen without the introduction of nitrogen. The process is initiated at a given temperature which rises as the process continues and reactions occur.

In contrast, plot 802 illustrates a curve in accordance with the present invention. The introduction of nitrogen gas into the blow ensures that during the earlier stages of the blow (typically the initiation and transition stages described previously) the temperature of the blow is relatively supressed. As the blow continues, however, and the nitrogen introduction is discontinued, the temperature is permitted to rise and is relatively enhanced.

As a result of the application of the present invention, the total blow time may or may not be extended relative to conventional operation.

Figure 9

A vessel 102 suitable for introduction of nitrogen gas into the oxygen blow is shown with respect to Figure 9.

In conventional BOS vessels, it is important that the refractory lining wear rates are kept as low as possible within suitable financial, commercial and engineering constraints. To achieve this, conventional vessels use oxygen lance 106 to introduce nitrogen gas at a high flow rate to blow remaining slag left in the converter after tapping a heat onto the inside surface of the converter to coat with slag to protect refractory wall 901 from physical and chemical attack on the next heat. Nitrogen gas in this case is used because of its relatively low cost and convenience.

This means however that lance 106 in conventional systems can be easily adapted to provide a nitrogen gas bleed into the oxygen flow by the introduction of minor engineering adaptations such as the inclusion of additional valves and control systems. Thus, the system can be retrofitted without the need for significant cost implications.

The improved method described herein provides a series of advantages in terms of the production of steel by the basic oxygen steel making process. This introduction of nitrogen gas into the oxygen gas stream while maintaining 5 a constant total gas flow rate reduces the operating cost and increases the productivity by avoiding downtime due to slopping events. By reducing slopping events a higher liquid steel yield can be obtained and there is further improved safety and improved environmental impact by reducing clean-up requirements and the copious amounts of fume that accompany slopping 10 events. The process therefore provides a preferred method to be utilised.

Claims

The invention claimed is:

1. A method of manufacturing steel, comprising the steps of: introducing a charge material into a basic oxygen converter; and blowing oxygen into said charge material to oxidise and remove impurities from said charge material, said impurities comprising carbon; wherein continuation of said step of blowing oxygen comprises removal of said carbon from said charge material to form carbon monoxide; and further comprising the step of:

delaying the high rate of formation of carbon monoxide by introducing nitrogen into said converter at a controlled rate during said step of blowing oxygen.

2. A method of manufacturing steel according to claim 1, wherein said charge material comprises blast furnace iron and scrap material, said blast furnace iron comprising iron, carbon and silicon.

3. A method of manufacturing steel according to claim 1 or claim 2, wherein said impurities further comprise silicon and said step of blowing further comprises removal of silicon from said charge material to form silica.

4. A method of manufacturing steel according to any one of claims 1 to 3, wherein said oxygen of said step of blowing oxygen is introduced at supersonic speed.

5. A method of manufacturing steel according to any preceding claim, wherein said step of blowing oxygen into said charge material typically occurs at a flow rate of 1000 cubic metres per minute.

6.

A method of manufacturing steel according to any preceding claim, wherein said step of blowing oxygen produces an oxygen flow stream and said nitrogen is introduced directly into said oxygen flow stream.

7. A method of manufacturing steel according to claim 6, wherein the magnitude of the flow rate of the blown oxygen flow stream is substantially similar to the flow rate of the blown oxygen with nitrogen flow stream.

8. A method of manufacturing steel according to any preceding claim, further comprising the step of:

discontinuing said introduction of nitrogen after a predetermined period of time; and continuing to blow oxygen into said charge material to remove further impurities from said charge material.

9. A method of manufacturing steel according to any preceding claim wherein said method comprises a process having an initiation stage immediately followed by a transition stage and then an oxidation stage; and said introduction of nitrogen occurs during said transition stage.

10. A method of manufacturing steel according to claim 9, wherein said introduction of nitrogen continues through at least part of said oxidation stage.

11. A method of manufacturing steel according to claim 9 or claim 10, wherein said introduction of nitrogen is discontinued prior to the end of said oxidation stage.

12. A method of manufacturing steel according to any one of claims 9 to 11, wherein the initiation of said step of blowing oxygen commences said initiation stage and said transition stage occurs between 3 and 5 minutes of the commencement of said initiation stage.

13. A method of manufacturing steel according to any one of claims 9 to 12, wherein said oxidation stage comprises a primary oxidation stage and a secondary oxidation stage, and further wherein said primary oxidation stage occurs between 6 and 8 minutes ofthe commencement of said initiation stage.

14. A method of manufacturing steel according to any preceding claim, wherein said step of introducing nitrogen is conducted by means of a vertical water-cooled lance.

15. A method of manufacturing steel according to any preceding claim, wherein said introduction of nitrogen is determined by a nitrogen bleed profile.

16. A method of manufacturing steel according to claim 15, wherein said nitrogen bleed profile comprises an increased percentage of nitrogen with increased silicon content in said charge material.

17. A method of manufacturing steel according to any preceding claim, wherein said method results in the formation of slag and further comprising the step of:

adding a lime-based material to neutralise acidic compounds in said slag.