JP6642174B2 - Continuous casting method of high carbon molten steel - Google Patents

Description

  The present invention relates to a continuous casting method for high carbon molten steel having a carbon concentration of 0.5% by mass or more.

  High-carbon steels generally have high strength, low ductility and low toughness, so they are often used for parts that do not require complicated processing and are not used in harsh environments. Therefore, except for some steel materials such as bearing steel, there were few strict requirements regarding cleanliness. On the other hand, automobiles have become lighter due to recent environmental problems, and steel sheets having higher strength have been used. Under these needs, even in the case of high-carbon thin steel sheets where the required levels of workability, fatigue properties and toughness were not strict until now, in order to improve these properties, the required level of cleanliness It is becoming more stringent.

Factors that affect workability, fatigue characteristics, and toughness include not only the influence of the metal structure but also inclusions that serve as starting points of defects. As the inclusions, it is known that extensible inclusions such as MnS and coarse inclusions adversely affect the quality. The most effective way to prevent MnS formation is to lower the S concentration, and desulfurization techniques therefor are widely known. In addition, as a measure against the solute S concentration portion in the center segregation portion, addition of Ca for controlling the form of the formed sulfide is a generally known technique. As described above, desulfurization technology and Ca addition technology are generally used as a technology for improving cleanliness, but the generation of low-melting CaO-Al ₂ O ₃ -based low-melting oxide generated at the time of Ca addition is further increased. From the viewpoint of prevention, there is also disclosed a technique in which a rare earth element and Ca are added to control from a CaO-Al ₂ O ₃ -based low melting point oxide to a rare earth element and Ca oxysulfide.

  Patent Literature 1 is a technology for manufacturing a high-strength structural steel plate (C = 0.08 to 0.22% by mass), and has a problem of adding Ca or a rare earth element (Ca excess) in order to prevent MnS generation. As a technique for solving the problem of nozzle erosion prevention by addition and nozzle clogging prevention by the addition of rare earth elements, the problem is solved by adding a rare earth element and Ca to form a composite oxysulfide of the rare earth element and Ca. is there.

Patent Document 2 relates to a steel material (C = 0.03 to 0.10% by mass) excellent in hydrogen-induced cracking resistance among line pipes and thick steel plates, which prevents MnS generation and reduces CaO—Al ₂ O _3. An object of the present invention is to solve the problem by controlling the composition of oxide-based inclusions by adding a rare earth element and Ca for the purpose of avoiding deterioration of resistance to hydrogen-induced cracking due to inclusions at the melting point.

Patent Document 3 relates to an electric resistance welded steel pipe (C = 0.03 to 0.15% by mass) excellent in weld quality suitable for oil country tubular goods and line pipes and a method for producing the same, and has SSC resistance and low-temperature toughness. In order to improve the hardness, CaO-Al ₂ O ₃ -based oxides generated at the time of adding Ca are converted into hard inclusions (complex oxysulfides containing rare earth elements, Ca, and Al) by adding rare earth elements and Ca. The problem is solved by controlling.

  Patent Document 4 relates to a high-strength steel sheet (C = 0.03 to 0.25% by mass) excellent in stretch flangeability and bending workability, and by adding a rare earth element and Ca, a fine and hard interposition is obtained. The formation of MnS and the formation of coarse alumina-based oxides are controlled by controlling the composition (complex oxysulfide containing rare earth elements, Ca and Al).

  The target steel types in the above-mentioned conventional technology are line pipes, high-strength thin steel sheets for automobiles, and high-strength thick steel sheets. Mainly steel type. There is no case where these prior arts attempt to apply to a high carbon steel type having a carbon concentration of 0.5% by mass or more.

JP-A-2011-68949 JP 2012-34662 A Japanese Patent No. 5765597 JP 2014-109056 A

Iron and Steel Making 3rd Edition Iron and Steel Handbook II 690-693

  When a method of adding a rare earth element and Ca as described in Patent Documents 1 to 4 is applied to continuous casting of high carbon steel having a C concentration of 0.50% or more, large slabs are produced in the produced slab. It was found that CaS-based inclusions remained to lower the cleanliness of the steel, and that clogging of the continuous casting submerged nozzles increased, resulting in deterioration of continuous casting operability.

Furthermore, the CaO-Al ₂ O ₃ based low-melting inclusions level included in the steel, it is required to further reduce than achieved level of the conventional low carbon steel.

  The present invention suppresses the generation of MnS by adding a rare earth element and Ca during continuous casting of a high carbon steel having a C concentration of 0.50% or more, and reduces both large-size CaS-based inclusions and low-melting-point inclusions. It is another object of the present invention to provide a continuous casting method of high carbon molten steel, which can prevent occurrence of clogging of a submerged nozzle.

That is, the gist of the present invention is as follows.
(1) In continuously casting a high-carbon molten steel having a C concentration of 0.5% by mass or more from a converter and deoxidized using aluminum, after desulfurizing using a plurality of desulfurizing agents,
A continuous process of high carbon molten steel, characterized in that metal Ca is added after the rare earth element is added and when the S concentration is 0.0030% by mass or less and desulfurized, and the molten steel subjected to the desulfurization treatment is continuously cast. Casting method.
(2) The continuous casting method according to the above (1), wherein T.I. A continuous casting method for high carbon molten steel, characterized by continuously casting molten steel containing less than 10 ppm of O.
(3) The high-carbon molten steel according to the continuous casting method according to the above (1) or (2), wherein the steel is continuously cast within a range satisfying the following formulas (1) to (3). Continuous casting method.
0.0005 ≦ [REM] ≦ 0.0015 (1)
0.0010 ≦ [Ca] ≦ 0.0020 (2)
[S]-[Ca] × 0.64 ≦ 0.0008 (3)
However, [S], [Ca], and [REM] mean the concentration of each element in steel (REM is the total concentration of rare earth elements) (% by mass).

  In the present invention, when a rare-earth element and Ca are added to a high-carbon steel having a C concentration of 0.50% or more and continuous casting is performed, metal Ca is added after the rare-earth element is added, and the S concentration is 0.0030% by mass or less. The addition of metal Ca suppresses the precipitation of CaS in molten steel and suppresses the formation of low-melting oxides in spite of being a high carbon steel, thereby improving the cleanliness of the steel and reducing the clogging of the immersion nozzle. Can be prevented.

  C in the molten steel is an element that reduces the reactivity of O in the molten steel and increases the reactivity of S. Therefore, in the low-carbon region and the high-carbon region, the generation ratio of oxides and sulfides in molten steel when a rare earth element or Ca is added is different.

Conventionally, when controlling the inclusion morphology by adding a rare earth element and Ca, a low carbon steel grade having a carbon concentration of 0.25 mass% or less has been targeted. In these low-carbon steels, the reactivity of O in molten steel is high and the reactivity of S is low, so Ca added to molten steel exists in the form of CaO in molten steel. S and Al reacted to produce CaS and Al ₂ O ₃ . That is, in the molten steel stage, the generation of CaS was suppressed.

  On the other hand, in a high carbon region having a carbon concentration of 0.50% by mass or more, CaS is preferentially generated in molten steel. Although the generation of CaS itself is effective in preventing the generation of MnS, if excessive CaS precipitates in the molten steel stage, coarse clusters are generated due to aggregation of CaS and the like, and coarse CaS clusters are formed in the slab. It has been found that by being taken in, the cleanliness is deteriorated, and that CaS clusters precipitate on the immersion nozzle and cause clogging of the immersion nozzle.

Hereinafter, in a high-carbon steel having a carbon concentration of 0.50 mass% or more, even in continuous casting in which a rare earth element and Ca are added to molten steel, CaS precipitation in the molten steel stage is suppressed, and at the same time, low melting point CaO-Al ₂ O is used. _The present invention, which can suppress generation of _three- system inclusions, will be described in detail.

[C concentration in molten steel is 0.5% by mass or more]
C in the molten steel has an effect of reducing the reactivity of “O” and increasing the reactivity of “S”, so that the C concentration in the molten steel is higher than the low carbon region by 0.5% by mass or more. CaS is very easily generated in the carbon region. Therefore, when the method disclosed in the prior art is applied, CaS is excessively generated, and the application of the present invention is required. Therefore, it is specified that the C concentration in the molten steel is 0.5% by mass or more. are doing.

[Addition of metal Ca after rare earth element addition]
After the molten steel discharged from the converter is deoxidized using aluminum, rare earth elements and Ca are added. In the present invention, the first point is the order of adding the rare earth element and Ca. The rare earth element is a general term for a total of 17 elements of scandium (Sc), yttrium (Y), and lanthanoids (15 elements from lanthanum La to lutetium Lu in the periodic table). If Ca is added first after Al deoxidation, T.P. O has a large variation. O low-melting oxides (CaO-Al ₂ O ₃ based oxide) is likely generated when high. This cannot ensure the required workability of steel. Therefore, it is preferable to add a rare earth element to molten steel before adding metal Ca to reduce the amount of dissolved oxygen in the molten steel.
The deoxidation using aluminum described above may be a commonly used deoxidation form. For example, the amount of dissolved oxygen in the molten steel after deoxidation is 10 ppm or less, and the content of metallic aluminum is about 0.01 to 0.1% by mass.

By reducing the amount of dissolved oxygen in the molten steel to 10 ppm or less by deoxidation using aluminum, the oxide after the addition of the rare earth element becomes almost a rare earth element oxide, and the low melting point inclusion (CaO-Al ₂ O ₃ -based oxide) is suppressed. Further, under the condition that the amount of dissolved oxygen in the molten steel after deoxidation using aluminum is 5 ppm or less, at least the T.V. O amount is less than 10 ppm, the generation of the If this condition low melting inclusions (CaO-Al ₂ O ₃ based oxide) can be effectively suppressed. Here, it is conceivable to add, for example, Zr (for example, 10 ppm or more as a steel component) as an element that can be expected to have a deoxidizing effect instead of aluminum, but its oxide has a large specific gravity and cannot be expected to have a floating removal effect. Due to the strong tendency to suspend, Since it becomes difficult to lower the O value, it is desirable to avoid adding an element having a large specific gravity of the oxide.

In the present invention, by adding a rare earth element before Ca, T.D. For the first time, it has been clarified that it is possible to absorb variations in O, suppress generation of CaO-Al ₂ O ₃ -based low-melting-point oxides, and realize high cleanliness. Further, an effect is exhibited by adding a small amount of Ca.

  That is, by adding metal Ca after adding a rare earth element, a C-based inclusion is made into CaO-REM oxide or CaS in a high carbon steel having a C concentration of 0.50% or more in the steel, thereby having a low melting point. Since the generation of Ca-based oxides can be suppressed, it is possible to manufacture steel having an excellent level of cleanliness, in which the residual level of low-melting-point oxides has been improved as compared with the related art.

[The S concentration before adding Ca is set to 0.0030% by mass or less]
If the S concentration when Ca is added is high (exceeding 0.0030% by mass), in the present invention in a high carbon steel, the amount of CaS generated in the molten steel stage increases, and the CaS agglomeration and coarsening in the molten steel are reduced. It is easy to invite and worsens the cleanliness, and the generated CaS precipitates at the immersion nozzle and causes clogging of the immersion nozzle.

  The present invention suppresses the amount of CaS generated in molten steel by setting the S concentration before adding Ca to 0.0030% by mass or less, despite being a high carbon steel with C ≧ 0.50%. It is possible to prevent deterioration of the cleanliness of steel due to coarse CaS and prevent clogging of the immersion nozzle.

  In order to reduce the S concentration in the molten steel before the addition of Ca to 0.0030% by mass or less, the S concentration is reduced in advance by hot metal pretreatment, and the S concentration in blowing in a converter or tapping from a converter is reduced. It is preferable to prevent the S component from being mixed in from the auxiliary raw material. Further, by adding metal Al or CaO for the purpose of slag reforming (reducing the degree of oxidation of slag) and deoxidizing molten steel during tapping from the converter, the S concentration before Ca addition can be reduced. As well as reducing the total amount of oxygen in the molten steel, a stable effect can be expected with a small amount of addition. When the S concentration in the molten steel does not become 0.0030% by mass or less before the addition of Ca after the adjustment of the alloy components, the sulfur concentration is reduced to 0.0030% by mass using a desulfurizing agent containing CaO as a main component. The following is assumed.

[Other features]
The present invention is also characterized in that molten steel is desulfurized using a plurality of desulfurizing agents and then continuously cast. The “plurality of desulfurizing agents” include those containing alloy components (rare earth elements and Ca) that generate oxides and sulfides typified by CaO. As described above, in high-carbon steel, when Ca is added to molten steel, CaS is generated in the molten steel stage, and a part of the CaS floats and separates on the surface of the molten steel.

  The present invention can exhibit the effects of the present invention by continuously casting the molten steel subjected to the desulfurization treatment as described above.

[Preferred requirements of the present invention]
It is preferable to define the steel components of the total rare earth element concentration and the Ca concentration during continuous casting as in the following equations (1) and (2). In the following formulas (1) to (3), [S], [Ca], and [REM] mean the concentration of each element in steel (REM is the total concentration of rare earth elements) (% by mass).
0.0005 ≦ [REM] ≦ 0.0015 (1)
0.0010 ≦ [Ca] ≦ 0.0020 (2)

When the concentration of the rare earth element is less than 0.0005% by mass, the melting point of the low-melting CaO-Al ₂ O ₃ -based oxide may not be sufficiently increased, and the effect of suppressing inclusion stretching during hot rolling may be insufficient. May be insufficient. On the other hand, if it exceeds 0.0015% by mass, it may remain as a high-melting REM oxide, which does not affect cleanliness, but may lead to adhesion or clogging of the immersion nozzle during continuous casting.

  If the Ca concentration is less than 0.0010% by mass, the effect of preventing MnS due to the generation of CaS may be insufficient depending on the application. On the other hand, when the content exceeds 0.0020% by mass, the amount of CaS generated slightly increases, and the cleanliness may be insufficient depending on the application.

The relationship between the Ca concentration and the S concentration during continuous casting may be defined as in the following equation (3).
[S]-[Ca] × 0.64 ≦ 0.0008 (3)
Investigation of the ratio of CaS and CaO generated in the molten steel after the addition of Ca in a high carbon steel having a C concentration of 0.5% by mass or more revealed that 80% or more was present as CaS. In order to prevent the generation of MnS, which is an extensible inclusion, it is necessary to increase the amount of S fixed by Ca and reduce the concentration of free S that becomes MnS generated after continuous casting. As a standard of the free S concentration for not generating MnS, a value obtained by subtracting the S fixed amount by Ca (80% of Ca fixes S as CaS) is 0.0008% or less (that is, 8 ppm as a free S concentration). If it is, MnS is not generated, and extremely high cleanliness can be realized. Since the S / Ca atomic weight ratio is 0.8 and the Ca ratio that becomes CaS is 0.8 (80%) as described above, the two are multiplied to obtain [Ca] in the above formula (3). The coefficient is 0.64.

  When the lower limit of Ca is 0.0010% by mass, the S concentration satisfying the above formula (3) for not generating MnS is 0.00144% by mass. The S concentration that satisfies the above formula (3) for not being generated is 0.00208% by mass. For this reason, when the S concentration during continuous casting is set to 0.0020% by mass or less, extremely high cleanliness can be realized. Further, when the content is 0.0015% by mass or less, further high cleanliness can be obtained.

[Molten steel treatment]
The addition of REM and Ca is performed on the molten steel in the ladle at the molten steel stage. As the molten steel processing means therefor, any of a simple ladle refining apparatus as described below or a molten steel vacuum degassing apparatus such as RH can be used.

  The simple ladle refining device can be selected from the methods described in Non-Patent Document 1 (in particular, FIGS. 13 and 65 on page 690). In the CAS method, ladle slag is separated by a closed tank made of refractory, and an alloy is added to the molten steel surface in the closed tank while an inert gas is sent from the bottom of the ladle toward the closed tank. . The SAB method (Sealed Argon Bubbling) and the CAB method (Capped Argon Bubbling) (both are blown with an inert gas) may be used. The alloy component may be blown into the molten steel from the tip of the injection lance immersed in the ladle molten steel. It is good to use the inert gas bottom blowing together.

[Ingredients of steel]
Regardless of the high carbon steel having a C concentration of 0.50% or more, the present invention is applied to continuous casting of any component steel, rare earth elements and Ca are added to suppress MnS generation, and In this case, both CaS-based inclusions and low-melting-point inclusions can be reduced, and clogging of the immersion nozzle can be prevented. The preferred REM content, Ca content, and S content in the molten steel are also as described above.

  After the desulfurization and dephosphorization of the hot metal in the hot metal pretreatment, the hot metal is decarburized and blown in a converter, and when the molten steel is tapped from the converter to the ladle, C and Al are added to achieve a predetermined C concentration and aluminum. Deacidification was performed. In addition, at the time of tapping, slag modification is sufficiently performed by adding CaO. By these processes, T.T. O Lowers the degree of oxidation (T.Fe + MnO <5% by mass) in the slag, which is effective for stabilizing the low level and accelerating desulfurization. As a secondary refining device, a CAS method, which is a simple ladle refining device, was used. After adjusting the alloy components by the CAS method, it was confirmed that the S concentration was 0.0030% by mass or less, and an alloy containing at least one rare earth element was added, and then Ca was added. After the adjustment of the molten steel component by the secondary refining, a slab was produced with a ladle-tundish-continuous caster. The cast slab was hot rolled, pickled, and annealed, then cold rolled and annealed to obtain a product plate.

  In the treatment by the CAS method, while performing Ar bubbling from the bottom of the ladle while adding an alloy including Al and adjusting the components, it was confirmed that the S concentration was 0.0030% by mass or less. An alloy containing one or more rare earth elements was added. When the S concentration exceeded 0.0030% by mass, it was confirmed that the Ca concentration was 0.0030% by mass or less after CaO was added to the inside of the CAS cylinder and sufficiently stirred. Thereafter, metallic Ca was added as a Fe-Si alloy having a Ca content of 30% by mass. The molten steel after adding each alloy was sampled, and S and T.P. A component analysis such as O was performed. In addition, the level which changed the addition order of the rare earth element and Ca was also implemented as a comparison. Table 2 shows the S concentration before the addition of Ca, the S concentration after the addition of the alloy, REM (total rare earth element concentration), and the Ca concentration.

  Molten steel was received from a ladle having completed secondary refining into a boat-shaped tundish, and continuous casting was performed using a one-strand curved-type slab continuous casting apparatus. The immersion nozzle for injecting molten steel from the tundish into the mold is mainly composed of alumina graphite, the size of the mold is 1100 mm, the thickness: 250 mm, the throughput is 2 ton / min, and the casting time is about 180 minutes per charge. There is (the amount of molten steel in the ladle is 300 ton).

  As a quality index relating to the present invention, cleanliness was selected. In addition, an index of clogging of the immersion nozzle during continuous casting was used for stable operability. As a quality characteristic, it is an index of cleanliness, but comprehensive evaluation including superiority and inferiority when operability is also taken into account.

[Index regarding cleanliness]
The results of measuring the cleanliness by a microscope on the L section (plane parallel to the rolling direction) of the product sheet rolled to a sheet thickness of 2.4 mm were indexed and evaluated according to the following criteria. ◎, ○, △ are acceptable.
◎: no inclusions with a major axis of 20 μm or more (measurement field 1 cm ² )
：: no inclusion with a major axis of 50 μm or more (measuring visual field 1 cm ² )
Δ: The number density of inclusions having a major axis of 50 μm or more is 1 (pieces / cm ² )
×: The number density of inclusions having a major axis of 50 μm or more is 2 (pieces / cm ² ) or more

[Index related to operability]
The following evaluation criteria were used to determine whether the casting speed was reduced due to clogging of the immersion nozzle during continuous casting. △ and △ both passed without the need to interrupt casting.
○: No decrease in casting speed △: Decreased casting speed

  Table 1 shows the composition of the molten steel sampled by the tundish, and Table 2 shows the production conditions and the evaluation results. In Tables 1 and 2, items outside the scope of the present invention are underlined. The molten steel composition of the tundish is substantially the same as the composition of the product plate. In the present invention, the components of the molten steel and the components of the product plate are collectively referred to as the components of the steel.

  Inventive Examples 1 to 12 of Table 2 are inventive examples, and Comparative Examples 1 and 2 are comparative examples. Inventive Example 1 is the base of the present invention (C = 0.7% by mass), and Inventive Examples 11 and 12 have a C concentration of 0.5% by mass (lower limit). With respect to Inventive Examples 1, 3, and 11, all the production conditions satisfied the preferred conditions of the present invention, and the cleanliness was “◎” and the operability was “○”, and good results were obtained. did it.

  In Example 2 of the present invention, the Ca concentration deviated from the lower limit of the preferable range, and the left side of the formula (3) deviated from the upper limit. In the present invention example 3, since the Ca concentration was outside the upper limit of the preferable range, the index of the cleanliness was “○”, and the operation level was “△” because the molten metal level was changed due to clogging of the immersion nozzle and the speed was reduced. . It is considered that the CaS amount was slightly large.

  Inventive Example 4 had an S concentration of 0.0016% by mass, which exceeded the most preferable range of 0.0015% by mass, but the cleanliness was good because the left side of Formula (3) was good.

  In Inventive Example 5, the REM concentration deviated from the lower limit of the preferable range, and the left side of the equation (3) deviated from the upper limit. This is probably due to the generation of a small amount of MnS. In Example 6 of the present invention, since the REM concentration was outside the upper limit of the suitable range, the clogging tendency of the immersion nozzle was observed and the speed was reduced, so that the operation index was “Δ”. In addition, due to clogging of the immersion nozzle, the cleanliness was uneven, and the index of the cleanliness was “「 ”.

  In Inventive Example 7, the S concentration exceeded the upper limit of the preferred range of 0.0020% by mass, and the left side of the formula (3) deviated from the upper limit. This is probably due to the generation of a small amount of MnS.

  In Inventive Example 8, the Ca concentration deviated from the upper limit of the preferred range, the CaS amount was large, the index of cleanliness was “○”, the molten metal surface fluctuation caused by clogging of the immersion nozzle caused by CaS was large, and the operation index was “ Δ ”. In Inventive Example 9, the Ca concentration deviated from the lower limit of the preferable range, and the left side of the formula (3) deviated from the upper limit. Therefore, the index of cleanliness was “○” due to MnS generation.

  In Example 10 of the present invention, the REM concentration was out of the upper limit of the suitable range, the clogging tendency of the immersion nozzle was observed, and the speed was reduced. In addition, due to clogging of the immersion nozzle, the cleanliness was uneven, and the index of the cleanliness was “「 ”.

In Example 12 of the present invention, the Ca concentration deviated from the lower limit of the preferred range, and the left side of equation (3) deviated from the upper limit. Since the O value was also slightly higher, the index of cleanliness was “△” due to the generation of MnS and CaO—Al ₂ O ₃ -based oxides.

  In Comparative Example 1, since REM was added after Ca was added, the cleanliness deteriorated due to the formation of a low-melting oxide, and the evaluation was "x".

  In Comparative Example 2, since the S concentration before the addition of REM was 0.0032% by mass, the cleanliness was deteriorated due to excessive generation of CaS, and the evaluation was “x”. In addition, since the casting speed was reduced, the operation index was “△”.

Claims (3)

In continuous casting of high-carbon molten steel deoxidized with aluminum having a C concentration of 0.5% by mass or more, which has been discharged from a converter, using a plurality of desulfurizing agents,
A continuous process of high carbon molten steel, characterized in that metal Ca is added after the rare earth element is added and when the S concentration is 0.0030% by mass or less and desulfurized, and the molten steel subjected to the desulfurization treatment is continuously cast. Casting method.   2. The continuous casting method according to claim 1, wherein T.I. A continuous casting method for high carbon molten steel, characterized by continuously casting molten steel containing less than 10 ppm of O. 3. The continuous casting method according to claim 1, wherein the steel component is continuously cast within a range satisfying the following formulas (1) to (3).
0.0005 ≦ [REM] ≦ 0.0015 (1)
0.0010 ≦ [Ca] ≦ 0.0020 (2)
[S]-[Ca] × 0.64 ≦ 0.0008 (3)
However, [S], [Ca], and [REM] mean the concentration of each element in steel (REM is the total concentration of rare earth elements) (% by mass).