JP3460595B2 - Melting method for extremely low sulfur steel - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an extremely low-sulfur steel which is economically and highly efficiently produced from extremely low-sulfur steel whose needs are increasing due to strong demands for improvement of toughness depending on its use.
In particular, it relates to a method for producing low phosphorus / ultra low sulfur steel.

[0002]

2. Description of the Related Art Recently, the demand for extra-low-sulfur steel, represented by high-strength sour-resistant line pipes, has increased, and the requirements have become stricter.

Conventionally, as a method for melting ultra-low sulfur steel, for example, JP-A-6-207212 and "Iron and Steel '83 -S1" have been used.
The method published in "83" is proposed. For example,
In Japanese Patent Laid-Open No. 6-207212, hot metal is desulfurized and dephosphorized to make S: 0.003% or less and P: 0.030% or less, and then decarburization in a converter, and then to a ladle. During and / or after tapping, the slag modifier is added to adjust the slag component to satisfy (T.Fe) + (MnO) ≤ 7%, and then, in an RH vacuum treatment device, Along with deoxidizing, the flux containing CaO as the main component
It discloses a method of performing desulfurization treatment of adding 4 kg / t or more in terms of CaO to molten steel in an RH vacuum tank and adjusting S: 0.0020% or less. On the other hand, "Iron and Steel '83 -S183" is [S] ≤ 10ppm by blowing fluorite mixed quicklime powder into the ladle.
States that it is achievable.

[0004]

However, in the conventional ultra-low-sulfur steel melting process, the secondary refining load is large, the problem of matching with continuous casting, high cost of molten steel desulfurization (thermal cost, flux cost, fire resistance) It was an issue that has not yet been fully examined.

An object of the present invention is to provide a method for economically and efficiently producing extremely low-sulfur steel while reducing the secondary refining load. More specifically, an object of the present invention is to reduce the load of secondary refining, thereby suppressing resource loss and energy saving while suppressing melting damage of refractory materials such as ladle and RH vacuum processing equipment. An object of the present invention is to provide a method for melting ultra low sulfur steel.

[0006]

As described in the section of the prior art, each of the methods proposed in both of the above reports is
Desulfurization to [S] ≤ 0.002% is possible. However, in these conventional techniques, there is no idea of utilizing tapping desulfurization.
There is no suggestion about the technical idea that if the desulfurization of tapping steel can be efficiently performed, the function of desulfurization can be added to the process of tapping steel to reduce the role of secondary refining.

Therefore, in the prior art, the load of secondary refining after tapping is large, and it is essential to raise the tapping temperature extremely or to compensate the heat after tapping (power input, oxidative heating of Al, etc.). Therefore, improvement in efficiency has been demanded, and improvements have been demanded from the viewpoint of extending the life of refractory materials.

In the present invention, as an improvement thereof, specifically, by forming a slag having a high desulfurization ability in the tapped steel, it becomes possible to melt the economical and highly efficient ultra-low sulfur steel. .

Here, the present inventors have conducted the following studies in order to achieve such an object. That is, first, in order to quickly form a slag with a high desulfurization capacity at the time of tapping, the added flux conditions were examined by a phase diagram, etc., and the effect was confirmed by a test. It turned out that it is effective to add 2 to 10 kg / t of flux.

[Al] level in molten steel is 3CaO + 2 [Al] +3 [S] = 3
Considering the reaction of (CaS) + Al ₂ O ₃ , desulfurization is promoted as the amount of [Al] is higher. Therefore, considering the [Al] standard and the amount of heat rise after tapping due to the heat generated by the oxidation of Al. The [Al] level was set so that [Al] ≥ 0.060% in the molten steel in the ladle after tapping.

The present invention is as follows. (1) Perform preliminary desulfurization of hot metal to achieve [S] ≤ 0.003% and preliminary dephosphorization to achieve [P] ≤ 0.040%, and to perform converter blowing of the obtained desulfurized and dephosphorized hot metal. What to do: When pre-melted with a composition having a melting point of 1500 ° C or less, a desulfurizing flux with a particle size of 30 mm or less and a mixed product of 3 mm or less is used at the time of tapping of the converter in the amount of 2 to 10 kg / ton of molten steel and Al. Was added to the molten steel and [Al] ≧ 0.060% in the molten steel in the ladle to accelerate the desulfurization of the tapping steel, and then the secondary refining RH treatment or ladle treatment was used to perform final desulfurization. ] Economic and highly efficient smelting method for ultra-low sulfur steel, including ≤ 0.002%.

(2) In the method for smelting ultra-low sulfur steel according to the above (1), the preliminary desulfurization of hot metal is performed by a KR method, and preferably the desulfurization / dephosphorization hot metal is blown in a converter from the bottom. The composition ratio of CaO and Al ₂ O ₃ in the slag after the final desulfurization is performed by a two-stage countercurrent refining method using a blow-type composite converter, and further, the final desulfurization is performed by a powder top blowing method by RH treatment. 1.0 ≦ CaO /
[P] ≤ 0.012%, characterized by Al ₂ O ₃ ≤ 1.8
And the method for melting low phosphorus and ultra low sulfur steel with [S] ≤ 0.001%.

(3) In the method for melting ultra-low-sulfur steel according to (1) above, the preliminary desulfurization of the hot metal is performed by the KR method, and the desulfurization / dephosphorization hot metal is blown in a converter from the bottom. The two-stage countercurrent refining method using the composite converter of No. 1 and the above-mentioned final desulfurization is performed by powder injection into the ladle molten steel or bubbling method, and CaO and Al ₂ O in the slag after the final desulfurization.
_A method for melting low-phosphorus / extremely low-sulfur steel with [P] ≤ 0.012% and [S] ≤ 0.001%, characterized in that the composition ratio of ₃ is 1.0 ≤ CaO / Al ₂ O ₃ ≤ 1.8.

(4) For the target desulfurization rate, the following (demolition steel + finishing) desulfurization flux basic unit and (demolition steel + finishing)
The method for melting low-phosphorus / extremely low-sulfur steel according to claim 2 or 3, wherein an amount of desulfurization flux defined by a relational expression with a desulfurization rate is added.

20- {5 [70-desulfurization rate (%)]} ^0.5 ≤ flux basic unit (kg / t) ≤ 20- {5 [90-desulfurization rate (%)]} ^0.5 where (steel + finish ) Desulfurization rate is defined by the following formula: {(Before tapping [S] -After finishing desulfurization [S]) / Before tapping [S]} × 10
0 (%)

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the processing conditions in the present invention as described above and the operation thereof will be described respectively.

First, the reason why [S] ≦ 0.003% is set in the hot metal pretreatment is that the [S] at tapping should be 0.004% or less even if the [S] rise in the converter is 0.001%. . Usually, the molten steel desulfurization rate is 50 to 90% when targeting extremely low sulfur steel,
Since [S] ≤ 0.002% is aimed,
0.004% or less is required. Further, when comparing the hot metal desulfurization cost and the molten steel desulfurization cost, generally, the molten steel desulfurization costs several times to 10 times. This is because thermodynamically, the higher [C] is, the easier it is to desulfurize, and the hot metal treatment has a lower treatment temperature of about 300 ° C than the molten steel treatment, which is advantageous in preventing damage to refractories.

Therefore, in the present invention, the hot metal pretreatment is used.
It is desirable to keep [S] as low as possible, that is, [S] ≤ 0.002%. In order to suppress [S] pickup in the converter as much as possible, the influence (contamination) of metal attached to converter and slag pay attention to. To do this, for example, avoid smelting extra low sulfur steels immediately after high [S] steels in the casting scheme. [S] ≤ 0.003% can be achieved by the KR method, the injection method, etc. using an economical quicklime desulfurizing agent.

[P] ≤ 0.040% in the hot metal pretreatment means melting of extremely low sulfur / low phosphorus steel, and recovery P from slag / inexpensive alloy iron accompanying extremely low sulfurization (product [ Mn] = 1.00%
This is the result of taking into account the normal P) of about 0.004% at the level. In order to save Mn alloys, the dephosphorization rate in low phosphorus steel blowing, which is conscious of high output steel [Mn], is usually 40% to 80%.

In order to economically raise the steel output [Mn] as much as possible, the product [P] ≦ 0.
To smelt 012% low-phosphorus steel, the steel from the converter [P] ≤ 0.008%
[0.008% ← 0.012% (product) −0.004% (reconstitution P)] is required, and [P] ≦ 0.040% [0.040% ← 0.008% /
(1-0.8)] is the required condition.

In order to improve the Mn yield, it is preferable that the Mn distribution ratio (= (MnO) / [Mn]) shown in FIG. 1 is low, and the basicity is 3 or more. When the basicity exceeds 5, the dephosphorization effect becomes saturated
(From the viewpoint of slag formation, the higher the basicity, the more difficult it becomes). In consideration of economic efficiency, desulfurization / dephosphorization hot metal and basicity of 3 to 5 at the time of blowing in a converter are preferable. The lower the content of slag (T.Fe), the higher the Mn yield, but in consideration of dephosphorization (T.Fe), most heat is 12 to 20%.

Regarding the desulfurization of tapping steel, the point is to form a slag having a desulfurization ability at an early stage. The conditions for adding desulfurization flux were examined with a phase diagram and the effect at that time was confirmed by various tests.

As a result, a desulfurization flux having a composition with a melting point of 1500 ° C. or less and a particle size of 30 mm or less is used for 2 tons of molten steel per ton.
It has been found that it is important to use 10 kg (hereinafter referred to as Kg / t). At this time, the desulfurization flux may be a mixture of one kind or two or more kinds of fluxes having different compositions.

With a flux composition having a melting point of more than 1500 ° C., the desired result could not be obtained due to insufficient slag formation during tapping. That is, the desulfurization rate was less than 20%. As a specific composition, for example, in the CaO-Al ₂ O ₃ system phase diagram of FIG. ₂ , the molar ratio of CaO: Al ₂ O ₃ , which has the lowest melting point, is 12: 7 (about 4 by weight ratio).
8:52) with a melting point of 1413 ° C and a weight ratio of
CaO: CaF ₂ : SiO ₂ = 60%: 30%: 10% melting point 1350 ° C flux and the like.

It was also found that a sufficient effect cannot be obtained unless the particle size of the desulfurization flux is 30 mm or less for the melt-processed flux (hereinafter referred to as a premelt product) and 3 mm or less for the mixed product. It is desirable to use a premelted product with a flux having a diameter of 5 mm or less from the desulfurization surface. 5 kg pre-melted product
In the test example in which only the amount of / t was added, the diameter was approximately 30 mm and 10 m
When desulfurization was carried out using fluxes with the same composition of m and 3 mm in size, the desulfurization rate of the steel output was 38%, 43%,
It was 49%. In the case of a mixed product having a diameter of about 3 mm, the desulfurization rate of steel output was 28%.

Once the slag capable of desulfurization is formed at an early stage of 2 kg / t or more, after that, the [S] value before tapping, steel type and demand
It is also possible to use desulfurization flux exceeding 1500 ° C, for example quicklime, corresponding to each condition of [S]. That is,
A slag with desulfurization ability is formed, and Al added at the same time is Al
_{This is because, when 2} O ₃ is formed and contained in the slag, the added quick lime acts to lower the melting point as shown in FIG. Considering the cost of desulfurization flux only, quick lime is generally cheaper, and it is higher in the order of the mixed product and the pre-melted product in which a melting point depressant is added so as to easily form slag.

Therefore, for example, when using a total flux of 10 kg / t during tapping, first add 3 kg / t of a premelted product flux having a melting point of 1500 ° C. or less, and within 2 minutes after that, CaO-Ca.
F ₂ (eg, 70%: 30% by weight) type flux may be added in a case-by-case manner such as adding 7 kg / t or adding 10 kg / t of a premelted product flux having a melting point of 1500 ° C or less.

The addition amount of the flux having a melting point of 1500 ° C. or less is 2
The reason for limiting to ~ 10 kg / t is as shown in Fig. 3 by the relationship between the basic unit of flux having a melting point of 1500 ° C or less and the desulfurization rate of tapped steel.
This is because the effect of desulfurization on tapping steel cannot be obtained at less than 2 kg / t (the desulfurization rate of tapping steel is less than 20%), and the desulfurization effect on tapping steel becomes saturated at 10 kg / t.

By the way, the desulfurization reaction is 3CaO + 2 [Al] +3 [S] =
It is shown by the reaction formula of 3 (CaS) + Al ₂ O ₃ , and the conditions for molten steel are [A
l] is high, and from the viewpoint of slag composition, the lower the sum of lower oxides, that is, (FeO) + (MnO), is (for desulfurization in a ladle, for example, 3% or less, more preferably 1% or less) It is well known to progress.

[0030] Therefore, the output steel was Al completely deoxidized steel, and the proper [Al] level was examined and tested. As a result, (FeO) + (MnO)
It was found that [Al] ≧ 0.060% to satisfy ≦ 3%. The higher [Al] is, the more convenient it is for desulfurization. However, considering the [Al] standard and the subsequent temperature increase due to Al oxidation, [Al] ≧ 0.06
It was set at 0%. It is of course possible to use a slag modifier at the end of tapping and / or immediately after tapping to reduce (FeO) + (MnO), if necessary.

The finish desulfurization is carried out by RH treatment or ladle treatment depending on the [S] value in the pot after tapping and the target [S] value. With the above method, after desulfurization of tapping steel with a probability of 80% or more, [S] ≤ 0.
Since 002% can be obtained, the desulfurization load in the secondary refining will be greatly reduced, and steel with [S] ≤ 0.002% can be mass-produced more economically and stably.

As the finishing desulfurization method, various means such as adding a lump flux in the RH vacuum treatment tank and / or spraying a desulfurizing agent powder, a powder injection into a ladle molten steel or a bubbling method can be applied. .

In one embodiment of the present invention, KR → P removal
Low phosphorus / extremely low sulfur steel is melted through the steps of furnace → de-C furnace → steel desulfurization → RH (preferably RH-PB desulfurization). The current hot metal desulfurization process uses the KR method, which has a lower running cost than the powder injection method and is advantageous for extremely low sulfurization [S]
Desulfurize to ≤0.003%.

The above-mentioned de-P furnace → de-C furnace process is, for example, the method disclosed in Japanese Patent Publication No. 3-77246, which is proposed by the applicant of the present invention, and is the SRP (Simple Refining Pr
It may be a two-stage countercurrent refining method using a composite blowing converter called "ocess", which is also an excellent method for melting low phosphorus steel and is preferably adopted. Desulfurization during tapping may be performed in the same manner as described above.

Finishing desulfurization after tapping desulfurization is a method disclosed in, for example, Japanese Patent Publication No. 61-59376, which is proposed by the present applicant, and is called RH-PB (RH-Powder Blowing). A powder top blowing method under reduced pressure may be used. Secondary refining is RH
Only the processing may be performed.

After finishing desulfurization by blowing powder on the RH treatment, the weight composition ratio of slag main components (CaO, Al ₂ O ₃ ) is set to 1.0 ≦ CaO / Al ₂ O ₃ ≦ 1.8. The reason for the numerical limitation is as shown in Fig. 2 where the composition is in a relatively low melting point region (the actual slag is not a binary system but contains SiO ₂ and MgO, and a small amount of FeO, MnO and S, but the composition ratio CaO / Al ₂ O ₃ is important) and desulfurization capacity (Under conditions where slag formation can be secured, the larger CaO / Al ₂ O ₃ is, the better)
Was decided in consideration.

That is, if CaO / Al ₂ O ₃ is less than 1.0, the desulfurization ability is significantly reduced (a weight composition ratio of 1.0 is good in slagification property), and if CaO / Al ₂ O ₃ exceeds 1.8, desulfurization is performed. This is because the performance is high and good, but it becomes difficult to slag and the stability is impaired.

Thus, according to the present invention, the economy of ultra-low sulfur / low phosphorus steel required for high-grade, high-strength sour resistant pipes, specifically [S] ≤ 0.002% and [P] ≤ 0.012% steel And stable mass production becomes possible.

According to yet another embodiment of the present invention, the final desulfurization is performed by powder injection or bubbling into molten steel ladle. This is because for steel grades that require degassing, degassing and desulfurization can be performed only by RH treatment using the method described above, but if final desulfurization is required without degassing, powder injection into ladle molten steel is required. Alternatively, it is possible to melt by just bubbling. In addition, the function is shared with the RH vacuum treatment device by using existing equipment (the bubbling device has a simple structure and has been widely used for more than 20 years), local conditions (difficulty in procurement of pressure reducing steam, etc.), etc. May be.

FIG. 4 shows the relationship between the desulfurization flux basic unit and the desulfurization rate (steel + finish). Here, the desulfurization rate of (steel + finish) is defined by the following formula: {(before steel [S] -after finish desulfurization
[S]) / Before tapping [S]} × 100 (%) The present inventors have found that the desulfurization flux 2 having a melting point of 1500 ° C. or less
A large amount of data was arranged under the conditions of 5 kg / t and before tapping [S] = 0.0024% to 0.0044%, and the relationship between (decasting + finishing) desulfurization flux basic unit and (decasting + finishing) desulfurization rate was investigated. As a result, the following relational expression was obtained with good accuracy although it was an approximate expression.

70-0.2 [20-Desulfurization flux basic unit (kg /
t)] ² ≤ desulfurization rate (%) ≤ 90−0.2 [20− desulfurization flux basic unit
(kg / t)] ² That is, 20- {5 [70-desulfurization rate (%)]} ^0.5 ≤ desulfurization flux basic unit (kg / t) ≤ 20- {5 [90-desulfurization rate (%)]} ^0.5 Therefore, when the target desulfurization rate is determined, the desulfurization flux basic unit defined by the above formula is used based on the desulfurization rate, and the ultra-low-sulfur and low-phosphorus steel is melted by any of the methods described so far. be able to. According to this aspect, it is possible to use a desulfurization flux amount that is necessary and sufficient for the desired desulfurization rate.

[0042]

[Example] In this example, KR → de-P furnace → de-C furnace → tap steel desulfurization →
Low-phosphorus / ultra-low-sulfur steel was melted from each process of ladle desulfurization (-PH degassing) or RH (RH-PB desulfurization).

Example 1 In Example 1, a low melting point flux of 4 kg / t (particle size: 5 mm or less, composition: CaO: Al ₂ O ₃ = 50:
42, melting point: 1472 ° C), 2.3kg / t Al and 4kg / t quicklime added in the latter half to desulfurize the steel, and greatly reduce the load on the RH vacuum treatment equipment. Quicklime was added at 4 kg / t, and then CaO (80%)-CaF ₂ (20%) powder (100 mesh under) 4 kg / t was sprayed under the condition of a vacuum degree of 2 torr (RH-PB).
. After such finish desulfurization, [S] = 0.0005% was obtained. Since the melting loss of the lower tank of the RH vacuum processing device can be reduced and the processing time can also be shortened, the disadvantage of the conventional method that the RH processing is rate limiting can be solved, and as a result, in continuous casting,
12 times continuous casting was completed.

On the other hand, in Conventional Example 1, 6 kg / t of quick lime was put into the tapped steel, and after heating to 80 ° C. in an RH vacuum treatment device, 6 kg / t of massive quick lime was put, and then CaO (80%)- CaF ₂ (20%)
6 kg / t of powder (100 mesh under) was sprayed under the condition of vacuum degree of 2 torr (RH-PB). After such finishing desulfurization, [S] = 0.
0005% was obtained. However, RH treatment was a bottleneck, and continuous casting could only be performed three times continuously. The results are summarized in Table 1.

Example 2 Example 2 and Conventional Example 2 are examples of ladle desulfurization. In this example, Example 1 was repeated except that ladle desulfurization was performed instead of RH treatment. It was The secondary refining maximum operating temperature of the conventional example is higher than that of the present invention by 35 ° C, and problems such as melting damage of the lance and ladle lid ([N] pickup becomes remarkable when the lid adhesion is poor) occur. 2 continuous casting
Only continuous casting was possible. The results are summarized in Table 1.

[0046]

[Table 1]

[0047]

According to the present invention, the load of secondary refining can be significantly reduced by effectively utilizing the tapping desulfurization.
It has become possible to produce ultra-low sulfur steel or ultra-low sulfur / low phosphorus steel economically and with high efficiency. The effects of the present invention can be summarized as follows.

(I) Desulfurization of tapping steel facilitated finish desulfurization, and the temperature rise in secondary refining could be reduced by about 30 ° C. (ii) Therefore, in the case of heating by Al oxidation, not only the Al and oxygen were saved but also the wear of the refractory could be reduced by almost half. The suppression of refractory wear is due to the reduction of the secondary refining maximum temperature and the processing time.

(Iii) Since it is possible to melt ultra-low-sulfur steel in a secondary refining treatment time of about 2/3 of the conventional time, matching with continuous casting can be improved and the index can be increased to more than double. That is, according to the present invention, the A tank (6
Even if the tank was changed from the heat treatment) to the B tank (6 heat treatments), continuous casting was possible and a total of 12 continuous castings were possible.

(Iv) Since extremely low sulfurization is achieved by tapping desulfurization and finish desulfurization, desulfurization can be performed accurately and stably with respect to the target [S] level. (v) Demand for high-strength sour-resistant line pipes, etc. is expanding, realizing efficient mass production of [S] ≤ 0.001% and [P] ≤ 0.0012% low phosphorus / ultra-low sulfur steel, and In terms of protection, it is also a manufacturing method that saves energy and resources.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between basicity and Mn distribution ratio.

FIG. 2 is a phase diagram of an Al ₂ O ₃ —CaO system.

FIG. 3 is a graph showing a relationship between a desulfurization flux basic unit having a melting point of 1500 ° C. or less and a desulfurization rate of tapped steel.

FIG. 4 is a graph showing a relationship between a basic unit of desulfurization flux in (steel + finish) and a desulfurization rate in (steel + finish).

─────────────────────────────────────────────────── --- Continuation of the front page (56) Reference JP-A-8-73923 (JP, A) JP-A-9-217110 (JP, A) JP-A-6-207212 (JP, A) JP-A-10- 219335 (JP, A) JP 8-246030 (JP, A) JP 5-171247 (JP, A) JP 11-209817 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21C 7/064 C21C 7/10

Claims (4)

(57) [Claims] 1. [S] ≤ 0.003% by subjecting hot metal to pre-desulfurization
In addition, perform preliminary dephosphorization to make [P] ≤ 0.040%, blow the obtained desulfurized and dephosphorized hot metal into a converter, and have a composition with a melting point of 1500 ° C or less when tapping the converter. Having a pre
By adding 2 to 10 kg of desulfurization flux with a particle size of 30 mm or less for molten products and 3 mm or less for mixed products to molten steel per ton of molten steel, and making [Al] ≧ 0.060% in molten steel in the ladle. Economical and highly efficient smelting method of ultra-low-sulfur steel, including accelerating desulfurization of tapping steel, and then performing final desulfurization by RH treatment or ladle treatment of secondary refining to make [S] ≤ 0.002% . 2. The method for producing ultra-low-sulfur steel according to claim 1, wherein the preliminary desulfurization of hot metal is performed by a KR method, and the final desulfurization is performed by an RH treatment by a powder top-blowing method. The composition ratio of CaO and Al ₂ O ₃ in the slag after 1.0 ≦
CaO / Al ₂ O ₃ ≤1.8, [P] ≤0.01
2% and [S] ≤ 0.001%, low-phosphorus / ultra-low-sulfur steel melting method. 3. The method for producing ultra-low-sulfur steel according to claim 1, wherein the preliminary desulfurization of the hot metal is performed by the KR method, and further the final desulfurization is performed by powder injection or bubbling into the ladle molten steel. Perform and finish desulfurization of slag
The composition ratio of CaO and Al ₂ O ₃ is 1.0 ≦ CaO / Al ₂ O ₃ ≦ 1.8, [P] ≦ 0.012% and [S] ≦ 0.001
% Low-phosphorus / ultra-low-sulfur steel melting method. 4. A desulfurization flux of an amount defined by a relational expression between the following (demolition steel + finishing) desulfurization flux basic unit and (demolition steel + finishing) desulfurization rate is added to a target desulfurization rate, The method for producing a low-phosphorus / extremely low-sulfur steel according to claim 2 or 3. 20- {5 [70-desulfurization rate (%)]} ^0.5 ≤ flux basic unit (kg
/ t) ≤ 20- {5 [90-desulfurization rate (%)]} ^0.5 where (dealing steel + finishing) Desulfurization rate is defined by the following formula: {(before tapping [S] -after finishing desulfurization [S ]) / Before tapping [S]} × 10
0 (%)