Title of Invention

METHOD OF PRODUCING LOW PHOSPHORUS STEEL IN LD VESSELS ON OPTIMIZED MAINTENANCE OF END BLOW CARBON RANGE

Abstract

The present invention relates to a method of consistently and reliably producing a low phosphorus steel (P < 0.010%) in LD vessel by optimized maintenance of end blow carbon range (0.005-0.030%) comprising the step of charging high phosphorus containing hot liquid metal of in wt% C- 4.3-4.4, Si- 0.4-1,P- 0.100-0.300 including other normal constituents (like Ti and S) and rest iron in a LD vessel of MgO lining converting the hot liquid metal to steel by a single blowing of high purity oxygen jet along with few additives like lime, iron ore at a temperature range of 1670-1710°C and maintaining end blow carbon (final carbon in the steel after blowing) in the range of 0.03 to 0.05% to obtain final steel with low phosphorus as low as <0.010% in a single shot.

Full Text

FIELD OF THE INVENTION The present invention relates to a method of producing low phosphorus steel in LD vessels on optimized maintenance of end blow carbon range by single blow/single slagging operation. BACKGROUND OF THE INVENTION The prime purpose of LD steelmaking is to convert iron into steel. Iron contains carbon (C ~ 4-4.5%), silicon (Si ~0.4-1.0%), phosphorus (P ~ 0.100 - 0.300%) etc, which needs to be removed or lowered during steelmaking. Production of low phosphorus steel (P single LD blow has always been a challenge (demand of low phosphorus steels is growing of late) in steel making industry. In the conventional practice of phosphorus removal during steel making usually, hot metal containing high hot metal phosphorus (P> 0.200%) is either A. prior de-phosphorized before LD blow, or B. two stage blow is employed in LD vessel e.g. double slag practice or LD- ORP (LD-ORP has been developed by Japanese steel companies) The methods have disadvantages in terms of lower yield and drop in hot metal temperature (in case of A) and higher cycle time and logistics issues (in case of B). The present invention has proposed a new approach in process development for low phosphorus steel through single blow/single slagging by maintaining end blow carbon in a particular range. DESCRIPTION OF THE INVENTION De-phosphorization (i.e. phosphorus lowering or removal) in LD steelmaking depends on many factors e.g. slag chemistry (basicity and Fe-oxides), steel temperature, hot metal phosphorus, blow profile etc. there is an adequate mention of all these parameters in the text books and in various technical articles published on this subject. OBJECTS OF THE INVENTION One object of the invention is to de-phosphorize hot iron during LD steelmaking through employment of another variable called steel carbon (i.e. carbon obtained after LD blow is complete, also called end blow carbon), the effect of this variable has not yet been referred so far in any prior state of art. Another object of the invention is to produce steel by LD process to obtain phosphorus level in the final steel as low as 0.009% on a consistent basis, for a range of steel carbon (end blow carbon). A further objective of the invention is to produce low phosphorus steel from higher phosphorus containing hot metal by a single blow steel making process without going for its pre-dephosphorisation or double blow LD process. Iron ore and coal reserves in India generally contain higher phosphorus, which result higher phosphorus in hot metal produced by blast furnaces. In absence of any pretreatment or any other methods, to get lower steel phosphorus ( provided a gate way to produce low phosphorus steel in a single blow LD furnace using this strong parameter (end blow carbon) than the other known variables used so far for this purpose. According to the invention there is provided a method of producing a low phosphorus steel in LD vessels on optimized maintenance of end blow carbon range comprising the steps of charging high phosphorus containing hot metal of in wt% C - 4.4, Si - 0.4-1, P - 0.100-0.300, Al 0.021-0.06, other normal constituents and rest iron into a LD vessel of MgO lining, converting the hot metal to steel by single blowing/single slagging step at a temperature range of 1670° - 1690°C, controlling the steel forming operation on maintaining end blow carbon (final carbon in the steel after blowing) in the range of 0.03 to 0.05% to obtain final steel with low phosphorus as low as The proposed invention has carried out an in-depth statistical analysis using datamining methodology, which shows clear-cut trend and correlations based on which the underlying mechanism of dephosphorization behavior in the LD vessel has been proposed. This addresses mysteries and uncertainties associated with de-phosphorization to a large extent. Dephosphorization behaves in an orderly and predictable manner provided role of each of the blowing parameters (during vessel blowing) is clearly understood. Reactions in LD vessel are primarily oxidizing and dephosphorization is a function of the state of oxidation. The state of oxidation is better represented in terms of end blow carbon (traditionally slag total Fe is the measure of oxidation state). The role of other blowing parameters e.g. end blow temperature, basicity, blow profile, hot metal silicon or slag volume etc. are important, but not as important as that of end blow carbon. The proposed invention has been developed through studies of the conditions and variables regarding Dephosphorisation, TBM (Bottom blowing), End Blow conditions, Slag-metal interface, Combined Blowing etc. Half of the heat downgrading at steel melting shop is phosphorus related i.e. the whole heat (~138 tons) is downgraded to lower grades (in extreme case, rendered defective and results in SNO - Steel Not Ordered). Implications of such downgrading are quite significant in terms of productivity, cost, delivery compliance etc. The reason for phosphorus downgrading is primarily two fold: 1. End blow phosphorus is more than required (most of the steel specification in flat products are below 0.015%). 2. Phosphorus reversion (i.e. even if end blow phosphorus is below 0.015%, but during (Lacjde Finish) LF treatment, slag phosphorus reverts back to steel leading to steel phosphorus more than required) The obvious preventive action to reduce such downgrading is to develop the capability to produce lower phosphorus at the end blow ( dephosphorization potential of LD vessel. Market requirements of future steel grades also points towards lower levels of steel phosphorus. Lower levels of phosphorus in steel further enhance product attributes (in terms of physical properties) leading to customer delight. Present average of end blow phosphorus at steel melting shops is 0.013% (range of variation between 0.005-0.04%), Table-1. It has been reported that Steel plants have achieved even lower average steel phosphorus to the tune of 0.008% or lower by 1. lowering hot metal phosphorus (selecting low phosphorus iron ore, coal etc), and/or 2. prior hot metal pre-treatment, and/or 3. modifying the blowing practice of LD vessel (double blowing/refining of hot metal in some of the Japanese steel plants is quite common). Hot metals at Indian Steel Plant have high phosphorus (avg: 0.180%) and high silicon (avg: 0.8%). High phosphorus and silicon is the result of high phosphorus in the raw materials (e.g. coal, iron ore etc.) and operating practices at the blast furnaces. Sources of phosphorus at Indian conditions generates as 43% from iron ores, 43% from coal and 14% from other sources. Steelmakers always believed high hot metal silicon as bad owing to its adverse effects during LD refining e.g. slopping, yield loss, lime consumption, slag carry- over etc. High hot metal silicon gives high slag volume during LD refining and this helps flushing out phosphorus through slag. High slag volume reduces the phosphorus activity in the slag which favours the dephosphorization reaction. High slag volume, of course, poses other problems e.g. slag disposal, recycling or reuse etc. In absence of any pre-treatment process, treating high hot metal phosphorus in a single blow with low hot metal silicon (i.e. phosphorus on consistent basis is a challenging task. Therefore, high hot metal silicon (to the extent of 0.7-0.9%) is to be considered an advantage and to this effect, blowing parameters/profile during LD refining should be designed appropriately to take advantage of high hot metal silicon. The invention will be better understood from the following description with reference to the accompanying drawings in which Figure 1 represents interpretation of intelligent Miner output / graph. Figure 2 represents effect of end blow carbon and temperature on end blow phosphorus Figure 3 represents effect of end blow carbon on end blow phosphorus. Figure 4 represents effect of end blow carbon on average slag Mgo. Figure 5 represents effect of end blow carbon on steel N and OLP. Figure 6 represents in graphs correlating end blow carbon with high and low end phosphorus achieving. An extensive statistical analysis (bivariate analysis using datamining methodology by Intelligent Miner) has been carried out to understand correlation amongst various parameters affecting vessel dephosphorization. Datamining methodology is a tool to analyze industrial data statistically. It produces an output in the form of two overlapping histogram, by which one know the relationship between the dependent and independent variable (y and x respectively). It segregates the ranges of x variable to optimize y variable. Over 2500 heats have been taken for analysis. Heats are segregated into two categories e.g. low hot metal phosphorus (avg: 0.185%) and high hot metal phosphorus (avg: 0.240%). Other parameters considered for bi-variate analysis are as follows: Hot Metal : silicon and phosphorus BOF Charge : amount of hot metal and scrap Blowing : amount of oxygen and iron ore, re-blowing End Blow Conditions : temperature, steel phosphorus and carbon, slag analysis The result generated by the Intelligent Miner shows two overlapping distributions in the form of a histogram (Fig. 1). Fig. 1 shows the effect of end blow temperature (X-axis, °C) on steel phosphorus. The faint dots show overall distribution in which Y-axis is represented as proportion of heats i.e. distribution of heats for different ranges of end blow temperature. The solid white portion line shows other distribution in which Y axis is represented as proportion of heats with end blow phosphorus temperature increases beyond 1680°C, the proportion of heats with end blow phosphorus temperature for achieving end blow phosphorus End blow phosphorus depends on many parameters. However, extensive datamining study shows strong effect of two parameters on end blow phosphorus. They are: 1. End blow carbon : threshold value - 0.03% 2. End blow temperature : threshold value - 1680°C (others parameters e.g. slag basicity, slag T, Fe, slag volume etc. are by and large operating in an optimum range and therefore doesn't affect much.) Figure 6 shows mining graph to correlate end blow carbon (similar output has also been generated to assess the effect of other parameters). Fig. 6 has two parts - one for low hot metal phosphorus and other for high hot metal phosphorus. For each part, there are two graphs e.g. one for achieving high end blow phosphorus (>0.015%, Graph-1 & 3) and other for achieving low end blow phosphorus ( carbon is amply clear from each output graph. Figure 2 shows higher end blow carbon (>0.03%) and lower end blow temperature ( higher end blow carbon and lower end blow temperature) produces even low end blow phosphorus ( blow carbon (i.e. lower or same level of end blow phosphorus with adverse end blow temperature). Figure 3 shows strong correlation of end blow carbon on end blow phosphorus (irrespective of all other parameters/conditions). Dephosphorization performance (as represented by lower end blow/turn down steel phosphorus) improves as end blow steel carbon increases and it peaks when end blow carbon is in the range of 0.04-0.05%. In this figure R2 represents a correlation co-efficient which varies between 0-1. Higher value of R2 indicates better correlation between dependent and independent variables i.e. x and y variables. It has been observed from the test result that End blow carbon determines the state of blown heat e.g. overblown, under- blown etc. End blow carbon varies between 0.015 - 0.050% with present blowing practice (avg. end blow carbon being 0.023%). Typical trend of end blow carbon has been summarized in Table-2 (result of 4665 heat data). Approximately 40% of heats shows end blow carbon below 0.02% can be termed as overblown heat, which adversely affects end blow phosphorus (see Fig. 3). Fig. 3 shows decreasing avg. end blow phosphorus with increasing end blow carbon till end blow carbon reaches 0.05%. The underlying mechanism/explanation is assessed from the above test datas. High hot metal silicon leads to higher lime consumption to maintain same level of slag basicity. This produces enough slag to take care of hot metal phosphorus. Hot metal phosphorus removed as end blow carbon lowers down to 0.03-0.05% (i.e. phosphorus in the hot metal leaves steel and joins slag during the blow-ideal condition for forward dephosphorisation reaction owing to high slag volume and lower temperature of steel-slag interface). As blow proceeds further end blow carbon reduces below 0.03%, phosphorus reverses from the slag and joins back to the steel. With lower end blow carbon ( blowing oxygen is required to reduce carbon (end blow carbon and blowing oxygen follows an exponential relationship). In the fag end of the blow, the lance height is also raised. Both of these effects (i.e. higher oxygen and raised lance) increases slag-metal interface temperature, which facilitate phosphorus reversal. Phosphorus reversal further aggravates if TBM (bottom blowing) is not so effective because rise in the temperature in the interface region does not get homogenized quickly enough. In order to get higher end blow carbon, the aimed carbon in the level-II blowing model has been changed from 0.03% to 0.04%. Level II model is a supervisory model which ascertain through computer calculation the effect of heat generating reactions (e.g. Si, C, P, Oxidation etc) and heat desorbing reactions (e.g. cold additions like lime, scrap, iron ore etc. including heat losses) to achieve desired temperature of steel at the blow end. As a result, proportion of heats with end blow carbon >0.03% has been raised from 8% to 13%. Raising the proportion further requires additional fine tuning of the blowing module i.e. controlling steel temperature in a close range of ± 10°C between achieved and aimed steel temperature. OTHER EFFECTS OF END BLOW CARBON: Lower end blow carbon results in relatively high oxidation state of metal and slag. The effect of end blow carbon on phosphorus is already explained in the foregoing descriptions. The effect of end blow carbon on other steelmaking parameters has been assessed through indirect measurements. Indirect measurements include few interrelated effects commensurate with the higher oxidation state of metal and slag e.g. refractory wear, steel nitrogen, Al (aluminum) consumption etc. The result of indirect measurements shown up good correlation with end blow carbon. Slag MgO: Lower end blow carbon ( any addition of MgO source in the vessel, higher slag MgO indirectly represents wear of vessel lining (lining is MgO based). Overoxidized slag is rather corrosive and result in lining wear. Higher slag MgO can adversely affect campaign life of the vessel. Fig. 4 shows correlation between slag MgO and end blow carbon. Slag Mgo is higher with lower end blow carbon for each range of hot metal silicon. Al Consumption and Steel Al: Al consumption is directly correlated with oxidation state of metal and slag. Lower end blow carbon (C therefore, likely to have higher Al consumption (amount of steel oxygen will depend upon actual end blow carbon and effectiveness of bottom blowing). Table-3 shows the effect of end blow carbon on Al consumption. Al consumption for lower end blow carbon (C Table-3 also shows variability i.e. range of actual steel Al w.r.t end blow carbon. Variability of steel Al is on the higher side with lower end blow carbon i.e. C points towards relatively high oxidation state of metal and slag with lower end blow carbon. In the fag end of the blow (when blow is 85% complete), vessel goes under suction as steel carbon reduces below 0.1-0.2% resulting in lower amount of CO evolution (partial pressure of CO gas becomes very low), steel becomes susceptible to N pick up from atmosphere air. Therefore, in over blown heats, threes is a greater chance of N pick up. This is also reflected in actual result of steel N analysis at OLP (end blow N analysis is not done) - see Fig 5. The distribution of steel N shifts towards higher value with lower end blow carbon i.e. C Influence of end blow carbon on vessel dephosphorization is a new finding. End blow carbon primarily depends on accurate prediction of end point determination of the blow. At present, blow conditions are such that 80-85% of heats are having end blow carbon which are nothing but overblown heats and adversely affect end blow phosphorus). The benefits of keeping end blow carbon between 0.035-0.05% are plenty. The most important benefit is to develop the capability to produce lower steel phosphorus i.e. hot metal phosphorus!). Other benefits include higher vessel campaign life and favourable effects on steel N and Al. The invention as narrated hereinabove and illustrated should not be read and construed in a restrictive manner, as various modifications, alterations and adaptations regarding conditions, variables for a range of compositions of steel are possible with in the scope and ambit of the invention as defined in the encompassed appended claims. WE CLAIM: 1. A method of consistently and reliably producing a low phosphorus steel (P carbon range (0.003-0.05%) comprising the step of: charging high phosphorus containing hot liquid metal of in wt% C-4.3- 4.4, Si-0.4-1, P-0.100-0.300 including other normal constituents (like Ti and S) and rest iron in a LD vessel of MgO lining converting the hot liquid metal to steel by a single blowing of high purity oxygen jet along with few additives like lime, iron ore at a temperature range of 1670- 1690°C and maintaining end blow carbon (final carbon in the steel after blowing) in the range of 0.03 to 0.05% to obtain final steel with low phosphorus as low as 2. A method of producing low phosphorus steel as claimed in claim 1 wherein the hot metal charged in the LD vessel is not pretreated for dephosphorization. 3. A method of producing low phosphorus steel as claimed in claim 1 wherein the end blow carbon level is determined through bivariate analysis using datamining methodology by Intelligent Miner from its correlation with various parameters affecting vessel dephosphorization such as silicon and phosphorus in hot metal, amount of hot metal and scrap in BOF charge, amount of oxygen and iron ore during blowing, end blow temperature and carbon, slag composition during end blow, on the basis of the categories of heats as low hot metal phosphorus (average 0.185%) and high hot metal phosphorus (average 0.240%). 4. A method of producing low phosphorus steel as claimed in the preceding claims wherein dephosphorization performance represented by lower end blow/turn down steel phosphorous, improves when end blow steel carbon increases and it peaks when end blow carbon is in the range of 0.03-0.05%. 5. A method of producing low phosphorus steel in LD vessels on optimized maintenance of end blow carbon range as herein described and illustrated. ABSTRACT METHOD OF PRODUCING LO WPHOSPHORUS STEEL IN LD VESSELS ON OPTIMIZED MAINTENANCE OF END BLOW CARBON RANGE The present invention relates to a method of consistently and reliably producing a low phosphorus steel (P carbon range (0.005-0.030%) comprising the step of charging high phosphorus containing hot liquid metal of in wt% C- 4.3-4.4, Si- 0.4-1,P- 0.100-0.300 including other normal constituents (like Ti and S) and rest iron in a LD vessel of MgO lining converting the hot liquid metal to steel by a single blowing of high purity oxygen jet along with few additives like lime, iron ore at a temperature range of 1670-1710°C and maintaining end blow carbon (final carbon in the steel after blowing) in the range of 0.03 to 0.05% to obtain final steel with low phosphorus as low as shot.

Documents:

00453-kol-2007-abstract.pdf

00453-kol-2007-claims.pdf

00453-kol-2007-correspondence others 1.1.pdf

00453-kol-2007-correspondence others 1.2.pdf

00453-kol-2007-corrospond others.pdf

00453-kol-2007-description complete.pdf

00453-kol-2007-drawings.pdf

00453-kol-2007-form 1-1.1.pdf

00453-kol-2007-form 1.pdf

00453-kol-2007-form 18.pdf

00453-kol-2007-form 2.pdf

00453-kol-2007-form 3.pdf

00453-kol-2007-gpa.pdf

453-KOL-2007-(30-11-2012)-CORRESPONDENCE.pdf

453-kol-2007-abstract 1.1.pdf

453-KOL-2007-ABSTRACT.pdf

453-KOL-2007-AMANDED CLAIMS.pdf

453-KOL-2007-AMNDED PAGES OF SPECIFICATION.pdf

453-KOL-2007-CANCELLED PAGES-1.1.pdf

453-KOL-2007-CANCELLED PAGES.pdf

453-KOL-2007-CLAIMS 1.1.pdf

453-KOL-2007-CORRESPONDENCE-1.1.pdf

453-KOL-2007-CORRESPONDENCE.pdf

453-KOL-2007-DESCRIPTION (COMPLETE) 1.1.pdf

453-KOL-2007-DESCRIPTION (COMPLETE).pdf

453-KOL-2007-DRAWINGS.pdf

453-KOL-2007-EXAMINATION REPORT.pdf

453-KOL-2007-FORM 1 1.2.pdf

453-KOL-2007-FORM 1.pdf

453-KOL-2007-FORM 18.pdf

453-KOL-2007-FORM 2 1.2.pdf

453-KOL-2007-FORM 2.pdf

453-KOL-2007-FORM 5.pdf

453-KOL-2007-GPA.pdf

453-KOL-2007-GRANTED-ABSTRACT.pdf

453-KOL-2007-GRANTED-CLAIMS.pdf

453-KOL-2007-GRANTED-DESCRIPTION (COMPLETE).pdf

453-KOL-2007-GRANTED-DRAWINGS.pdf

453-KOL-2007-GRANTED-FORM 1.pdf

453-KOL-2007-GRANTED-FORM 2.pdf

453-KOL-2007-GRANTED-FORM 3.pdf

453-KOL-2007-GRANTED-FORM 5.pdf

453-KOL-2007-GRANTED-SPECIFICATION-COMPLETE.pdf

453-KOL-2007-OTHERS.pdf

453-KOL-2007-PA.pdf

453-KOL-2007-REPLY TO EXAMINATION REPORT-1.1.pdf

453-KOL-2007-REPLY TO EXAMINATION REPORT.pdf

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