JP2003181602A - Method for manufacturing ultralow carbon steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a sound ultralow carbon steel, which is free from defects derived from inclusions and can meet a very high quality requirement, in a highly efficient and stable manner. <P>SOLUTION: In this method for manufacturing an ultralow carbon steel which includes the steps of subjecting a molten steel to RH vacuum degassing and then continuously casting the molten steel, molten steel treatment conditions and/or casting conditions are regulated so as to satisfy the following equation (1) [wherein NSL is a total oxygen concentration of slab, (ppm); VL is the length of vertical part of a continuous casting machine from a meniscus, (m); NFM is the concentration of (FeO)+(MnO) in ladle slag after the completion of RH, (%); W is the capacity of tundish, (ton); TP is a throughput, (ton/min); h is the depth of tundish bath, (m); and VP and α are each a constant]. <P>COPYRIGHT: (C)2003,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION [0001] The present invention relates to a converter and RH vacuum desorption.
Production of ultra-low carbon steel using gas equipment and continuous casting equipment
Relates to the method of manufacturing. [0002] 2. Description of the Related Art Continuous casting with vacuum degassing
Takes out the molten steel refined in the converter into a ladle,
After decarburization with an empty degasser, adjust components such as deoxidation
Next, the molten steel is taken from the ladle to the tundish (hereinafter “T
/ D ”), and the continuous casting machine from Tundish
The mold is continuously poured into a slab. Ultra-low carbon steel mainly used for automobiles
Then, the quality requirement level is improving year by year. others
Therefore, inclusion reduction measures are used in the steelmaking process as described above.
It is an important issue. [0004] As a measure for reducing inclusions, refining in a converter.
Slag reforming process and vacuum degassing
Promoting separation in the room, promoting levitation in the tundish,
There are various methods such as promotion of floating in the mold of the continuous casting machine.
It is being considered. For example, as a countermeasure for reducing inclusions after refining,
JP-A-10-298629, JP-A-11-158537,
Japanese Laid-Open Patent Publication No. 2000-119732 and Japanese Laid-Open Patent Publication No. 06-256837
As shown, various lower oxide concentrations in ladle slag
This reduces the amount of reoxidation of molten steel.
Many methods have been published. Japanese Patent Laid-Open Nos. 08-141708 and 09-3875
In No. 3 publication, inclusions in the molten steel remaining at the refining stage are slurried.
In order to avoid bringing them into the product stage.
Increase the residence time of the molten steel in the
A means to improve the flotation separation efficiency in the dish
It has been proposed. Japanese Patent Application Laid-Open No. 11-33687 discloses a continuous casting installation.
In the preparation, a vertical part is provided below the meniscus of the molten steel,
It is brought into the slab through an electromagnetic stirring device.
A method for reducing the amount of inclusions has been proposed. [0008] However, the above-mentioned problem is not solved.
Various methods reduce slab inclusions and improve cleanliness.
Although there is an effect to increase, it is accompanied by an increase in manufacturing cost
Not only tundish and continuous casting equipment
Such as those that require a large amount of investment. For example, lower oxide concentration in ladle slag
When performing slag reforming as a method of reducing,
High cost due to use of deoxidizer containing metal Al
Ascend. It also extends the residence time in the tundish.
Methods such as tundish shape change and throughput
(The hot water supply speed from the tundish to the mold)
However, it is possible to change the shape of the tundish.
A large investment is required, and if throughput is reduced
Due to temperature loss due to lower production efficiency or longer casting time
Cost increase occurs. To provide a vertical part in a continuous casting machine
Requires huge capital investment for equipment modification,
As long as you are operating with existing equipment,
Absent. A continuous casting machine already has a vertical section and a large tank.
In continuous casting equipment that uses dishes
Has a high ability to remove inclusions, so inclusions in the molten steel stage
Even if the quantity reduction measures are mild, the quality requirement level
Can be satisfied. However, as mentioned above,
The amount of inclusions remaining in the slab continuously cast from ultra-low carbon steel
The specifications and capabilities of the equipment used, and various operating conditions
Affects. In addition, these inclusion floating removal means
It also has various effects on the manufacturing cost and productivity of the lab. This
Therefore, slabs with minimal inclusions are stably
It was not easy to get by strike. The object of the present invention is to eliminate defects caused by inclusions.
Healthy, very low coal that can meet strict quality requirements
To provide a method for producing raw steel efficiently and stably
There is to be. [0012] [Means for Solving the Problems] Continuously Low Carbon Steel Slab
There are various amounts of inclusions remaining in the slab during casting.
Operating conditions will affect. In addition, there is a means for removing the floating of inclusions.
Various methods are considered, but each method
Effects on production, manufacturing costs, or productivity
It is. Moreover, these are the specifications and capabilities of the equipment used.
It depends on the situation. Therefore, the present inventor has cast from the steel refining stage.
The various operating factors up to
If you can clearly understand the impact on the real thing,
By always selecting the optimum operating conditions, the desired product
High quality ultra-low carbon steel to meet the specifications and capabilities of the equipment used
In response, the idea that it can be obtained stably. In particular, the refining of steel under various operating conditions.
Operation costs from the time of continuous casting to continuous casting, especially the rise of inclusions
If the cost required for removal can be grasped, this and the above inclusions
Combined with reduction methods, the desired quality is achieved.
To manufacture products that are efficient and stable at low cost
I knew I could do it. The present invention is based on these ideas.
As a result of research, it was completed based on the obtained knowledge.
The summary is that the molten steel refined in the converter is put into a ladle,
This is decarburized by RH vacuum degassing equipment and continues
After deoxidizing, cast using continuous casting equipment.
Satisfying the following formula (1)
Adjust the treatment and / or casting conditions of the molten steel
It is the manufacturing method of the ultra-low carbon steel characterized by adjusting. [0016] [Expression 2] However, N_FM: (FeO) + in ladle slag after completion of RH treatment
(MnO) concentration (mass%), V_L: Vertical length of continuous casting machine from meniscus (m), W: Tundish capacity (tons), h: tundish bath depth (m), T_P: Throughput (ton / min), V_P: Inclusion ascent rate (m / min) = 0.4 α: Inclusion removal rate constant (m^-1) = 1.0 [0018] BEST MODE FOR CARRYING OUT THE INVENTION Next, the slab as the basis of the present invention will be described.
Of the formula (1) that expresses the relationship between inclusion concentration and various operating conditions
Explain the contents. A. In the production of ultra-low carbon steel,
The refined molten steel is taken out into a ladle, and this is RH vacuum degassing equipment
After decarburization in the furnace and adjusting components such as deoxidation, remove the molten steel.
Move from pan to tundish and continuously pour into mold
And cast into a slab. In the steel making process, vacuum degassing treatment
(Hereinafter referred to as RH treatment) Lower level in slag in ladle after
The oxide concentration is determined by the tandem when the molten steel is transferred to tundish.
This greatly affects the total oxygen concentration of the molten steel in the dish. FIG. 1 shows the above result according to the research result of the present inventor.
The lower oxide concentration and the total oxygen concentration of the molten steel (in FIG. 1, “T
-[O] "). FIG.
As you can see, there is a strong positive correlation between the two.
The Lower oxide concentration in the ladle slag shown in FIG.
The relationship between the temperature and the total oxygen concentration of molten steel in tundish
It can be expressed by equation (2). N_TD= 3N_FM  +10 (2) However, N_TD: Total oxygen concentration in molten steel in tundish (single
The order is ppm), N_FM: (FeO) + in slag in the ladle after RH treatment
 (MnO) concentration (unit: mass%). Thus, the amount of inclusions contained in the steel is steel.
It has a good correlation with the total oxygen concentration. So the whole of steel
Oxygen concentration should be used as an indicator of steel cleanliness
Therefore, in the present invention, both in the molten steel and in the slab
The total oxygen concentration was used as an index representing the amount of inclusions. B. Molten steel is continuously activated from tundish.
Poured into the lud. When pouring hot water from inside the tundish
The amount of inclusions brought into the mold
Stipulated by operating conditions such as the shape and throughput. That is, the inclusions float in the tundish.
To increase the amount of inclusions in the mold.
However, the inclusion removal efficiency is tundish.
The longer the residence time of the molten steel, the better. Also tan
Inclusions rise more easily as the depth of the dish bath is shallower. FIG. 2 shows the result of the inventor's research.
Influence of operating conditions on the levitation of inclusions in a dish.
It is a graph which shows. N is the total oxygen concentration of molten steel in the mold.
_MD(Unit: ppm), the total oxygen concentration of molten steel in tundish
N_TDIf the unit is ppm, the inclusions in the tundish
As you surface, N_MD/ N_TDDecrease. This meaning
N of the vertical axis in FIG._MD/ N_TDFor molten steel in tundish
This represents the floating degree of inclusions. The horizontal axis in FIG. 2 represents the ease of inclusion floating.
The inclusion levitation index. This means that the inclusion flotation speed is V
_P (Unit: m / min), tundish
The amount of molten steel in the shell is W (unit is tons), and the throughput is T_P
(Unit: ton / min), tundish
When the depth of molten steel in the h is h (unit is m), _P
・ W / T_P-It is represented by h). In the inclusion levitation index, W / T_PIs a tande
It is the residence time of inclusions in the dish. Shown in Figure 2
As the residence time is longer and the tundish depth is shallower,
The decrease in the total oxygen concentration of the molten steel in the mold is large,
It can be seen that ascent is promoted. FIG. 2 shows that the total oxygen concentration in the molten steel in the mold.
Can be represented by the following formula (3). [0030] [Equation 3]Where V_PIs the inclusion ascending speed (unit: m / m
min), and its value may be considered constant at 0.4 m / min. c. Continuous casting to prevent inclusions from remaining in the slab
A vertical part effective for floating separation of inclusions is installed in the machine mold.
It is effective to cut. FIG. 3 shows the mode according to the results of the inventor's research.
Influence of the vertical part on the floating separation of inclusions in the mold
It is a graph which shows. The vertical axis in FIG. 3 indicates the molten steel in the mold.
Total oxygen concentration (N_MD) Total oxygen concentration in the slab (N
_SL, Unit is ppm) ratio (N_SL/ N_MD) And N_SL/ N
_MDThe smaller the is, the higher the inclusion floating in the mold.
It means that the past is being promoted. As shown in FIG. 3, continuous from the meniscus
The vertical part of the casting machine (almost mold + vertical part of the continuous casting machine
In the mold as the length increases.
Floating separation of existing objects is promoted. This relationship is shown in FIG.
It can be represented by the following formula (4). [0034] N_SL= N_MD× α (−0.031 × V_L+0.91) (4) However, V_LIs the vertical length of the continuous casting machine from Meniscus
(Unit is m), α is inclusion removal rate constant (unit is
m^-1) And its value is 1.0m^-1It may be considered constant. D. Solving the above equations (2) to (4) simultaneously
The following formula (5) can be obtained. [0036] [Expression 4] According to the equation (5), all the slabs are
Predict oxygen concentration and estimate slab inclusions
it can. Note that the length of the vertical part of the continuous casting machine is already known to those skilled in the art.
Obviously, inside the cast molten steel
This is the vertical length of the region where the object rises. E. As the inclusions in the slab increase,
The defective rate of the products that are produced increases. Product defect rate here
The above slab is hot-rolled steel sheet, cold-rolled steel sheet, or these
Processed into final products such as various plated steel sheets
If there is a failure due to material failure,
The defect rate is indexed. Product
If the good rate is high, the cost loss due to the worsening of the yield will increase.
The FIG. 4 shows the result of the investigation by the present inventor.
The total oxygen concentration (N_SL) Shows the effect on product defect rate
It is a graph. In Fig. 4, the product defect rate is indexed and manufactured.
It was shown as a product defect index. As shown in Figure 4, the slab
As the total oxygen concentration increases, the product defect index increases,
The increase is especially remarkable when it exceeds 27ppm. The present inventor further describes various operating conditions.
Inclusion reduction at the manufacturing stage from steel refining to continuous casting
Various expenses (steel-making loss costs) required for measures and the results
Cost loss due to devaluation at the product inspection stage
Slab total oxygen concentration and total
Cost loss (steel making loss cost + devaluation cost)
The relationship was investigated. FIG. 5 shows a scan obtained from the above research results.
Effect of total oxygen concentration of lab on total cost loss
It is a graph which shows. In FIG. 5, the total axis on the vertical axis
The Scost index is the steelmaking loss cost and the cutting loss cost.
Total rosco based on general operating conditions
This is an index of strikes. As shown in FIG. 5, the total oxygen concentration of the slab is
Loss cost is minimized at 27ppm. Total loss
The cost index shows that the total oxygen concentration in the slab is less than 27ppm
As the cost of steelmaking increases,
The cut-off cost increases as it exceeds 27 ppm.
To become bigger. The product defect index is kept low, and there are few inclusions.
To obtain a stable and healthy product, the total oxygen concentration of the slab
It is better to manufacture under the condition that is 27ppm or less.
The value calculated on the left side of Equation (1) is 27 or less, preferably
Operating conditions and special characteristics in the steelmaking process
To adjust the manufacturing conditions and / or casting conditions of molten steel
Is preferred. The method for adjusting the operating conditions is practically continuous.
Casting equipment layout and tundish shape are determined
Because it is almost the case, it is sloop during casting
The lower oxide concentration in the slag and slag is within the range that satisfies equation (1)
It is desirable to set and operate for each continuous casting facility. [0045] [Example] The molten steel refined in the converter is put into a ladle, which is
Decarburized with RH vacuum degasser, followed by deoxidation
Three types of continuous casting equipment is used for molten steel
Produce ultra-low carbon steel slabs and final coil shape
The rate of product disqualification that occurred during product inspection and shipment inspection
investigated. Combined with the refining of the steel in each case
Investigate the operating costs up to continuous casting and
Based on the operating cost under conventional operating conditions
The cost index was calculated. Table 1 shows the equipment specifications of the continuous casting machine, and Table 2
And the inclusion concentration in the slab calculated by equation (1)
And the cost index for each. [0047] [Table 1][0048] [Table 2] Case 1 is usually provided by continuous casting equipment A.
This is the case of manufacturing under the conditions. In this case, the left side of equation (1)
The calculated value of
The number was high and product collection was not stable. Case 1
Continuous casting machine with operating cost from refining to continuous casting
It was set as the standard of the operation cost in the case of A. Case 2 is more efficient than Case 1
-Putting speed is reduced and casting speed (slab drawing speed)
As the case 3, the slag modifier is
Reduced lower oxide concentration in slag by using a large amount
The case was evaluated. In either case, calculate the left side of equation (1)
The value is below the target of 27, the product defect index is low,
The removal was stable and good. In both cases, the operating cost is
Higher compared to case 1, but lower oxidation in slag
In case 3 with reduced material concentration, as described above
Cost of slag modifier is higher than case 2
The index increased. This is a problem for production efficiency.
If not, reduce the throughput as in Case 2,
It is more effective to extend the residence time in the tundish
It is. Case 4 performs slag reforming more powerfully.
As a result, the lower oxide concentration in the slag is reduced to more than Case 3.
The casting speed is reduced in the same manner as in Case 2.
The In this case, the calculated value on the left side of Equation (1) is even smaller.
However, the improvement effect of the product defect index is close to saturation.
It did not improve. On the other hand, the increase in operating costs is significant
In addition, the cost disadvantage has increased. This is a must
It means that it is not necessary to improve the cleanliness more than necessary.
The Case 5 is refined under the same conditions as Case 1.
Cast the molten steel that has been vacuum degassed in continuous casting equipment B.
It is an example when it is made. Continuous casting equipment B is tundish
The volume is small and the residence time in the tundish is short.
Therefore, it is a continuous casting facility that is disadvantageous for the removal of inclusions. this
Therefore, the calculated value on the left side of equation (1) is for continuous casting equipment A
The product defect index was high. Case 6
Or as shown in Case 7, reduce the throughput.
If the casting speed is reduced, or slag reforming
A large amount of material is used to lower the lower oxide concentration in the slag.
The target slab is obtained and the final product
The product defect index was small and good. The operating cost is
Case 6 is more advantageous than S7
It was the same as in the case of machine A. Continuous casting shown in case 8 and case 9
Since the facility C is a continuous casting facility having no vertical portion,
Inclusion reduction measures were required more than continuous casting equipment B. [0054] The present invention satisfies the formula (1) defined by the present invention.
By adjusting the operating conditions to the desired quality requirement level
Produce stable and efficient ultra-low carbon steel
be able to.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the lower oxide concentration in slag after RH treatment and the total oxygen concentration of molten steel. FIG. 2 is a graph showing the influence of operating conditions on the floating of inclusions in a tundish. FIG. 3 is a graph showing the influence of the mold vertical part on the floating separation of inclusions in the mold. FIG. 4 is a graph showing the effect of the total oxygen concentration (N _SL ) of a slab on the product defect rate. FIG. 5 is a graph showing the influence of the total oxygen concentration of the slab on the total cost loss.

──────────────────────────────────────────────────── ─── Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme code (reference) C21C 7/06 C21C 7/06 7/068 7/068 7/10 7/10 A

Claims (1)

[Claim 1] After removing molten steel refined in a converter into a ladle, this is decarburized by an RH vacuum degasser, and subsequently deoxidized, In a method for producing an ultra-low carbon steel cast using a continuous casting facility, the processing conditions and / or casting conditions of the molten steel are adjusted so as to satisfy the following formula (1). Production method. [Expression 1] However, N _FM: RH treatment after the end of the ladle in the slag of (FeO) + (
MnO) concentration (% by mass), V _L : vertical length of continuous casting machine from meniscus (m), W: tundish capacity (ton), h: tundish bath depth (m), T _P : throughput ( Ton / min), V _P : Inclusion ascent rate (m / min) = 0.4, α: Inclusion removal rate constant (m ^-1 ) = 1.0