|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.|
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
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
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%),
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
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
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
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-
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.
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
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
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.
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
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
5. A method of producing low phosphorus steel in LD vessels on
optimized maintenance of end blow carbon range as herein described
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.
|Indian Patent Application Number||453/KOL/2007|
|PG Journal Number||51/2013|
|Date of Filing||22-Mar-2007|
|Name of Patentee||TATA STEEL LIMITED.|
|Applicant Address||RESEARCH AND DEVELOPMENT AND SCIENIFIC SERVICES DIVISION, JAMSHEDPUR|
|PCT International Classification Number||C21C7/00; C21D8/12|
|PCT International Application Number||N/A|
|PCT International Filing date|