JP3500894B2 - Continuous casting of steel - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuous casting method for steel, and more particularly to a method for continuously cracking steel such as medium carbon steel and ferritic and martensitic stainless steels. In high-speed continuous casting (casting speed: 1.5m / min or more), vertical cracks on the slab surface, which were inevitable, were reduced, improving the quality of the slab and reducing costs by reducing the surface treatment of the slab. It is intended to eliminate the unstable operations such as breakout and stabilize the continuous casting operation. [0002] C content is 0.08 to 0.16 wt% (hereinafter simply referred to as%
In the continuous casting for so-called medium carbon steel, vertical cracks are likely to occur on the surface of the slab during casting, and various measures have been taken to prevent such cracks. [0003] There have been many studies on the mechanism of vertical cracking, which is a problem in continuous casting of medium carbon steel, and longitudinal cracks are likely to occur in medium carbon steel.
Because the C content is in the peritectic transformation region of 0.08 to 0.16%, a large difference occurs in the growth rate between the slow and fast growing portions of the solidified shell due to the addition of transformation stress during the solidification process of the steel, that is, It has been clarified that the cause is the increased non-uniformity of the solidified shell. [0004] The non-uniformity of the growth of the solidified shell has a correlation with the amount of heat removed in the initial stage in the mold. It is already known that the air gap can be mitigated by eliminating the air gap and suppressing the variation in the thickness of the mold flux (hereinafter simply referred to as flux) film between the mold and the solidified shell. [0005] As a prior art relating to these, for example, Japanese Patent Application Laid-Open No. 50-59229 (a method for producing a wide continuous cast slab having a small number of surface flaws) employs both an oil casting method and a flux casting method to obtain molten steel in a mold. There has been proposed and disclosed a technique for preventing surface vertical cracks caused by non-uniform cooling. Japanese Patent Application Laid-Open No. 61-92756 (continuous casting method for preventing slab surface cracking and a mold) discloses a method for forming a continuous casting mold by forming a plurality of longitudinal grooves having an appropriate shape on the upper surface in the mold. A method of performing continuous casting while uniformly cooling the solidified shell and avoiding surface cracks by supplying a medium carbon steel melt and slowly cooling only at the top of the mold using Has been proposed and disclosed. Further, as a prior art relating to the flux, the viscosity and the crystallization temperature of the flux are set low in order to prevent the non-uniform cooling of the solidified shell by uniformly flowing the flux between the mold and the solidified shell. Japanese Patent Application Laid-Open No. 63-235054 (Method of preventing vertical cracks during high-speed casting of continuous cast slabs) proposes a technique for applying the method only to the center of the mold. Raising and increasing the solid phase thickness in the flux layer between the mold and the slab, and reducing the radiant heat flux, a technique to prevent cracking is described in `` CAMP-ISIJ, 5
(1992), p.283 ”, the flux was maintained at a basicity of about 1.2, ZrO <sub>2</sub> was added by about 3%, and the freezing point was raised to a higher temperature to reduce radiant heat transfer and reduce the mold temperature. The technology to reduce the cooling by increasing the contact thermal resistance between the solder and the flux solidification layer and prevent cracking is `` CAMP-ISIJ,
6 (1993), p. 283 ". However, in the above, Japanese Patent Laid-Open No.
The technology disclosed in the 9229 publication, in addition to the flux casting equipment that is currently commonly used, it is necessary to install a new oil casting equipment, in addition to increasing the number of operating personnel, There was a drawback in that costs such as equipment costs and work costs were unavoidable. Further, the technique disclosed in Japanese Patent Application Laid-Open No. 61-92756 requires the use of a special mold for casting medium-carbon steel. In addition to being disadvantageous in terms of surface, the mold surface temperature is estimated to be 400 ° C. or higher, so that the mold itself and the plating are significantly deteriorated, and this method is difficult to achieve in terms of the life of the mold. Further, the technique disclosed in Japanese Patent Application Laid-Open No. 63-235054 discloses that two types of flux are applied from the center of the long side surface of the slab.
Since the flux is mixed properly in the area of about 200mm, unstable heat removal occurs due to the mixing of the flux in this area, and although the effect of suppressing some vertical cracking is recognized, the degree of improvement is extremely slight. Was something. The technology using a high freezing point flux disclosed in “CAMP-ISIJ, 5 (1992), p. 283” and “CAMP-ISIJ, 6 (1993), p. about
Due to the high temperature of 1200 ° C, the thickness of the liquid phase portion of the flux film interposed between the mold and the slab cannot be sufficiently secured, resulting in poor lubrication and breakout easily occurring. From the center to the bottom of the direction, flux inflow tends to be poor and local temperature changes are likely to occur.Therefore, if a breakout alarm system that measures mold temperature is installed, high frequency In some cases, a false alarm was generated, disrupting operations and reducing productivity. [0012] In addition, due to the high solidification temperature of the flux, a cohesive layer of flux called a slag rim develops thickly and non-uniformly just above the so-called meniscus where the upper surface of the molten steel and the mold come into contact. However, this method still has problems such as interfering with the flow between the mold and the slab, causing the solidified shell to be restrained in order to cause non-uniform inflow, and possibly causing breakout. SUMMARY OF THE INVENTION The present invention is intended to advantageously solve the above-mentioned problems, and is mainly composed of a medium carbon steel, a ferritic stainless steel and a martensitic stainless steel having high crack susceptibility. In the continuous casting of steel, etc., we propose a continuous casting method of steel that can suppress the uneven growth of the solidified shell, prevent the occurrence of surface defects such as cracks, and also avoid the occurrence of breakouts and false alarms of breakouts. The purpose is to do. SUMMARY OF THE INVENTION The present inventors have found that casting is a particular problem in continuous casting of medium carbon steel in the peritectic transformation region and ferritic and martensitic stainless steels having low solidification shell strength. As a result of repeated experiments and studies on preventing surface cracking of the piece and improving the lubricity between the mold and the slab, the flux layer interposed between the solidified shell and the mold, the so-called flux film, adhered to the mold surface New knowledge that it is possible to control the heat transfer inside the mold while maintaining sufficient lubrication and to prevent vertical cracks in the slab by controlling the interfacial heat transfer resistance between the solidified flux layer and the mold To achieve the present invention. That is, the gist of the present invention is as follows. [0015] In continuous casting of steel, the solidification temperature is 1080
C. or higher, 1300 ° C. or lower, and a basicity of 1.05 or more and less than 1.5, and a mold flux containing TiO ₂ and MgO in a range of 2 wt% or more and less than 5 wt%, respectively, at least in a vertical direction around the meniscus. The surface roughness of the inner wall of the mold over a range of 20 mm is R _max A continuous casting method for steel, characterized in that the casting is performed using a continuous casting mold having a size of 10 μm or more and 50 μm or less. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The operation and effect of the present invention will be described below together with experimental and examination results. First, the situation in which a solidified shell is being formed in the vicinity of the so-called meniscus in the upper part of the continuous casting mold will be described below. The heat removal of the molten steel and the solidified shell in the mold is all performed through the flux between the molten steel and the solidified shell and the mold. The solid phase layer (solidified layer of flux)
Is formed, and in a portion in contact with the solidified shell, a liquid phase is formed because the flux exceeds its solidification temperature. The heat transfer performed through such a flux film is determined by the heat transfer resistance between the mold and the flux solidified layer, that is, the state of contact between the two. Degree of unevenness of solidified layer surface
The greater the (roughness), the greater the heat transfer resistance and the slower the slab cools. Here, FIG. 1 is an explanatory view of the heat transfer mechanism between the mold and the flux. In FIG. 1, the upper half of the drawing shows a crystal phase in which crystals are precipitated up to the surface of the flux solidification layer in contact with the mold, and the surface has irregularities. In this case, the heat transfer resistance increases and the slab is slowly cooled. on the other hand,
The lower half of the drawing shows the case where the surface of the flux solidified layer in contact with the mold is a smooth glass phase. In this case, the heat transfer resistance is small and the slab is rapidly cooled. Therefore, for example, by using a flux having a high freezing point temperature as shown in the prior art, a crystal phase is generated directly from the liquid phase of the flux on the mold side, and a 10 μm
A uniform slow cooling is achieved by obtaining a thermal resistance with the mold surface corresponding to the degree of unevenness of the solidified layer surface. However, with this technology,
Lubrication becomes unstable because the liquid phase thickness of the flux cannot be sufficiently secured, and furthermore, due to the high solidification temperature of the flux, the fixation of the flux called a slag rim just above the so-called meniscus where the upper surface of the molten steel and the mold contact. The layer grows thickly and unevenly, which interferes with the flow of the molten flux between the mold and the slab during the vibration of the mold and causes uneven flow, thereby increasing the risk of breakout. Therefore, the present inventors have studied the casting method of the present invention based on the following eight points which summarize the new findings of the results of experiments and studies. 1) The liquid phase flux contacts the mold surface,
When the glass phase is instantaneously solidified and the glass phase is stable, the interface heat transfer resistance between the mold and the glassy flux is extremely close to zero. Then, at this time, the roughness of the contact surface of the glassy flux with the mold is several μm or less. 2) The liquid phase flux contacts the mold surface,
When crystal nuclei are generated and grown and solidified directly on the contact surface, the interface heat transfer resistance is relatively small, about 0.1 to 0.3 m ² / kw, and the roughness of the contact surface with the mold at this time is 10 μm or less. . 3) The liquid phase flux contacts the mold surface,
When it instantaneously solidifies into a glassy state, and then crystals are sufficiently precipitated in the glass phase and the precipitated phase reaches the mold surface,
The interface heat transfer resistance is about 0.2 to 0.5 m ² / kw, and the roughness of the contact surface with the mold at this time is 15 μm or less. 4) The slag rim thickness becomes thinner as the roughness of the contact surface of the flux solidified layer with the mold becomes larger. Therefore, even if the solidification temperature of the flux is relatively high, if this roughness can be increased, the formation of slag rim is suppressed, and the occurrence of breakout due to poor lubrication can be prevented. Here, Figure 2 shows the graph of roughness R _max and Suragurimu thickness of the contact surface between the mold flux solidified layer. As is apparent from this figure, Suragurimu thickness gradually decreases roughness R _max increases. 5) The roughness of the contact surface of the flux solidified layer with the mold and the apparent thermal conductivity of the flux film become lower as the roughness becomes larger. Here, Figure 3 shows the graph of apparent thermal conductivity of roughness R _max and flux film contact surface with the mold flux solidified layer.
As apparent from this figure, the apparent thermal conductivity decreases as the roughness increases. The apparent thermal conductivity is determined by the parallel plate method (Kawatetsu Technical Report, Vol. 28 (1996),
The values obtained in (59-65) are taken into account when considering the flux layer thickness: 0.3 mm. 6) As the minimum crystal precipitation temperature in the glass phase is lower, the crystal phase of the flux solidified layer in contact with the mold surface becomes more stable, and the interface heat transfer resistance with the mold becomes relatively uniform (0.3 m ² / about kw). 7) When fine irregularities are present on the mold surface (inner wall surface), the interface of the flux in contact with the mold also has an irregular shape, and the flux in contact with the concave portion of the mold locally becomes high in temperature, and this portion is removed. As a starting point, a crystal phase is precipitated from the glassy flux solid phase, and a uniform heat resistance layer is formed between the mold and the flux film. 8) By adding an appropriate amount of TiO ₂ and MgO, the roughness of the contact surface with the mold can be made about 50 μm. Therefore, the mechanism for uniform cooling and stable lubrication during continuous casting in the present invention is to reduce the solidification temperature.
In the range of 1080 to 1300 ° C, the basicity is set to 1.05 or more and less than 1.5, and TiO ₂ and MgO are respectively 2% or more and 5%
By using a flux containing less than 10%, the number of crystal nuclei generated is increased, and fine irregularities (roughness R _max 10 to 50 μm), the flux solidified layer in contact with the mold is rapidly and uniformly crystallized, and the heat transfer resistance between the mold and the flux film is also made uniform. Here, FIG. 4 is an explanatory view of a heat transfer mechanism between the mold and the flux when minute irregularities are provided on the inner wall surface of the mold. In this figure, the upper half of the figure has irregularities on the inner surface of the mold. In the case where the flux layer in contact with this becomes a crystal phase and the surface has irregularities, the heat transfer resistance is increased and the slab is slowly cooled, and the lower half also has the inner wall surface of the mold,
The case where the flux layer (glass phase) in contact with this is also smooth, shows a state where the heat transfer resistance is small and the slab is strongly cooled. In addition, the volume shrinkage during the precipitation reaction of the crystal phase from the glass phase is large, and the roughness of the mold contact surface of the flux during crystallization becomes 50 μm or more. Heat transfer resistance increases, and the formation of slag rims is suppressed, so that the flow of flux between the mold and the slab does not become uneven,
High lubrication conditions can be provided. Although the solidification temperature of the flux used in the present invention is relatively high in the range of 1080 to 1300 ° C., the formation of the slag rim is suppressed for the above-mentioned reason, and the breakage caused by the increase in the slag rim thickness is caused. Out can be eliminated. Next, the reasons for limitation of the present invention will be described. The flux used in the present invention contains TiO ₂ and MgO in a range of 2% or more and less than 5%, respectively.
If the content of either O ₂ or MgO is less than 2%,
The roughness of the contact surface with the mold during crystallization of the flux solidified layer
When it is 30 μm or less, the effect of slow cooling is not clear. On the other hand, when the content of either TiO ₂ or MgO is 5% or more, the stability of the glass phase of the flux solidified layer increases, so that the solid phase film partially remains in the glass phase, and Induces uniform cooling. The basicity of the flux is not less than 1.05 and 1.
When the basicity is less than 1.05, the temperature at which the crystal phase is precipitated from the glass phase exceeds 750 ° C.
No crystal precipitation reaction occurs on the contact surface of the flux film cooled by the mold with the mold, and a stable and uniform heat resistance layer is not formed. Conversely, when the basicity is 1.5 or more, the flux directly forms a crystal phase when it comes into contact with the mold surface, and from the above-mentioned findings, the contact with the mold becomes relatively dense, and as a result, it becomes relatively uniform. Although the cooling effect is high, the apparent thermal conductivity of the flux film is increased, so that the slag rim is remarkably developed, the flow of the liquid phase flux is hindered, and a breakout is caused. Further, the solidification temperature of the flux is set to 1080 ° C.
As described above, the temperature is set to 1300 ° C. or less. However, if the solidification temperature is lower than 1080 ° C., it becomes impossible to maintain the contact surface roughness of the flux film with the mold constant. In particular, when the surface roughness of the flux film is 50 μm, the temperature of the surface of the flux film may be higher than the softening temperature of the flux. At this time, the contact with the mold becomes partially dense, resulting in uneven cooling. May cause. Conversely, when the solidification temperature of the flux exceeds 1300 ° C., a liquid phase flux film having a sufficient thickness is not formed, lubrication between the mold and the slab is impaired, and the frequency of breakouts is significantly increased. The flux was used as a base material of a normal mold flux, SiO ₂ : 20 to 50 wt%, CaO: 20
5050 wt%, Al ₂ O ₃ : 1 to 10 wt%, Na ₂ O: 1 to 15 wt% and F: 1 to 10 wt%, or B
₂ O ₃ : It is preferable to contain each in the range of 8 wt% or less. Further, in order to adjust the viscosity and the solidification temperature, Li ₂ O: 5 wt% or less and / or B ₂
O: may be contained in a range of 10 wt% or less. On the other hand, the continuous casting mold used in the present invention has fine irregularities on the inner wall surface of the mold near the meniscus, and has a surface roughness R _max. Is in the range of 10 μm or more and 50 μm or less. This roughness R _max Is less than 10 μm, the promotion of crystallization of the flux film in contact with the mold is not sufficient,
If it exceeds 50 μm, the temperature of the portion of the flux film in contact with the mold may be higher than the softening temperature of the flux, and at this time, the contact between the mold and the flux film becomes partially dense, causing the uneven cooling. May occur. Also, the surface roughness R _max Is 10 μm or more and 50 μm or less, the range of at least 20 mm (40 mm in total) in the vertical direction around the meniscus. Normally, in continuous casting of steel, the mold is vibrated (oscillated) in the vertical direction, so that the meniscus position moves up and down relative to the mold. In the present invention, the surface roughness R _{max of the} inner wall surface of the mold is Is a center position of the relatively fluctuating meniscus as a reference for defining a region where is not less than 10 μm and not more than 50 μm. Also, the surface roughness R _max : In the area where 10 to 50 μm is provided, the preferred area upward from the center position is up to about 40 mm, and even if applied above, there is no influence on the slab surface properties. On the other hand, the effect of the present invention is more stable in the downward preferred region as the distance from the center of the meniscus becomes longer, but when the distance exceeds 150 mm, the effect is no longer saturated. It is better to do. EXAMPLES Various fluxes shown in Table 1 were used, and
Medium-carbon steel was cast under the following casting conditions using continuous casting molds with different inner dimensions (roughness and range) as shown in Table 1 near the meniscus with inner dimensions of 1000 mm long side and 200 mm short side. Were continuously cast, and the breakout, the state of vertical cracks on the surface of the slab, and the like were investigated. [Table 1] Casting conditions: Mold vibration conditions: Stroke: 8 mm Negative speed ratio (Ns ratio): 20% (rate at which mold lowering speed due to vibration of mold exceeds drawing speed of slab) f = Vc / 2 × 8・ (1 + Ns / 100) × 1000 where f: Mold frequency (cpm) Vc: Casting speed (m / min) ○ Casting speed Vc: 1.5 to 2.20 m / min ○ ΔT: 34 to 50 ° C (injection into mold) The difference between the molten steel temperature and the liquidus temperature of the steel) ○ The composition range of the main components of the steel: C: 0.08 to 0.16%, Si: 0.05
0.40.40% and Mn: 0.201.21.20% The results of these investigations are collectively shown in Table 1. Note that the vertical crack index in Table 1 is the value of “crack length / total slab length” when the length of a vertical crack occurrence portion is expressed in units of 1 m.
It is indexed based on o.1. The breakout index is a value obtained by reducing the number of charges in which a breakout or breakage mark of the solidified shell (breakout mark) has occurred on the slab surface by the total length of the slab based on the sample No. 1. . As is evident from Table 1, in the case of a continuous casting mold conforming to the present invention, using a flux conforming to the present invention, the conforming example did not have any vertical cracks on the surface of the slab, and the breakout occurred. The number of occurrences has been drastically reduced to 1/10 or less of that of the comparative example which is used as a reference in an index. According to the present invention, there is provided a continuous casting mold in which the solidification temperature and basicity are specified, and the roughness of the inner wall surface of the mold near the meniscus is specified using a mold flux containing an appropriate amount of TiO ₂ and MgO. According to the present invention, according to the present invention, a medium-carbon steel or a ferrite based material having high crack susceptibility,
Even in continuous casting of martensitic stainless steel or the like, good slabs without vertical cracks can be cast stably without occurrence of breakout.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of a heat transfer mechanism between a mold and a flux. FIG. 2 Roughness R of the contact surface of the flux solidified layer with the mold
It is a graph of the relationship between _max and slag rim thickness. FIG. 3 Roughness R of the contact surface between the flux solidification layer and the mold
₅ is a graph showing the relationship between _max and the apparent thermal conductivity of a flux film. FIG. 4 is an explanatory diagram of a heat transfer mechanism between a mold and a flux when fine irregularities are provided on the inner wall surface of the mold.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) B22D 11/07 B22D 11/00

Claims (1)

(57) [Claims 1] In continuous casting of steel, the solidification temperature is 1080
C. or higher, 1300 ° C. or lower, and a basicity of 1.05 or more and less than 1.5, and a mold flux containing TiO ₂ and MgO in a range of 2 wt% or more and less than 5 wt%, respectively, at least in a vertical direction around the meniscus. The surface roughness of the inner wall of the mold over a range of 20 mm is R _max A continuous casting method for steel, characterized in that the casting is performed using a continuous casting mold having a size of 10 μm or more and 50 μm or less.