JP2013128940A - CONTINUOUS CASTING METHOD OF Si-CONTAINING STEEL - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a continuous casting method that can effectively prevent surface cracking of a continuously cast slab in Si-containing steel.SOLUTION: The continuous casting method of the Si-containing steel containing Si at 0.5 mass% or more includes setting the specific water amount SW of secondary cooling water when the surface temperature of the cast slab is in a temperature range from 1,177°C to Tobtained by the following expression (3), to at most a value of SWobtained by the following expressions (1)-(3) to carry out casting: (1) SW=(T/1177)×SW; (2) SW=1.6×V; and (3) T=1177-(940×Cu+9,450×Sn+10,928×Sb)/3; wherein SWis an upper limit value (l/kg) of the secondary cooling specific water amount of Si-containing steel; SWis the secondary cooling specific water amount (l/kg) of ordinary steel; Tis a lower limit temperature (°C) at which internal oxidation scale with a depth of 150 μm or more is formed; Vis a casting speed (m/min), and symbols of elements are the contents (mass%) of the elements.

Description

  The present invention relates to a method for continuously casting Si-containing steel, and more particularly, to a continuous casting method for preventing surface cracks occurring in a continuous cast slab of steel containing 0.5 mass% or more of Si.

  In recent years, high-tensile steel with increased strength has been used in a wide range of fields for the purpose of reducing the weight and improving safety of various steel products including automobiles. There are various methods for increasing the strength of steel, but a method of solid solution strengthening by adding a strengthening element such as Si or Mn is generally used.

  Conventionally, the content of Si added to steel is generally about 0.3 mass% or less, except in the case of electromagnetic steel sheets. This is because, in general hot-rolled steel sheets and general cold-rolled steel sheets, when the Si content is increased, surface defects due to scale, plating unevenness in zinc plating, adhesion defects, and the like are likely to occur. Therefore, the steel sheet mainly has a Si content of 0.1 mass% or less. However, in recent years, many steels added with 0.5 mass% or more of Si have been manufactured in response to the increasing demand for higher strength.

  On the other hand, in recent years, recovery and recycling of iron resources have been actively promoted from the viewpoint of resource saving and energy saving. Accordingly, from the viewpoint of preventing surface cracks in slabs in continuous casting and surface cracks in the hot rolling process, iron scrap containing a large amount of Cu, Sn, etc., which has been restricted in use. Reuse is increasingly desired.

  In general, surface cracks in continuous cast slabs correspond to the embrittlement temperature range where the hot ductility of steel decreases, and the slabs from the casting mold are bent and unbent (corrected) along the oscillation mark. It is known to occur. Therefore, as a countermeasure for surface cracking of the slab, casting is performed while avoiding bending and correction of the slab in the above embrittlement temperature range.

In addition to the impurity elements such as Cu and Sn as described above, the following are known as causes for causing the surface crack of the slab, and measures are taken according to the respective causes.
1) The range referred to as a subperitectic crystal having a C content of 0.08 to 0.12 mass% is likely to generate austenite having a coarse particle diameter due to non-uniform solidification, and surface cracks are likely to occur. Design excluded ingredients.
2) Since steel containing Nb, V, Al, N, etc. at a high concentration, grain boundaries become brittle due to precipitation of carbonitrides such as Nb (C, N), V (C, N), AlN, etc. (A) Secondary cooling to bend and straighten the slab while avoiding the embrittlement temperature range, (b) Add N elements such as Ti and Ca to fix N, for example, Ti is stoichiometric An amount of 3.3 or more is added to Ti / N.

  In addition to the above-described measures, surface cracks in the slab have been greatly reduced due to recent advances in secondary cooling technology for controlling the slab surface temperature. However, recently, there is a tendency for surface cracks, which are considered to have different generation mechanisms, to increase from the previously known cracks described above. As one of them, there is a surface crack in a continuous cast slab (slab) of steel containing 0.5 mass% or more of Si, which is typified by a high-tensile steel plate. Although Si may reduce the hot ductility at 1200 ° C. or higher, Si does not have a significant adverse effect on the ductility in the temperature range of 700 to 1000 ° C. where slab bending and straightening are performed. Therefore, it is considered that the generation mechanism of the slab surface crack in the Si-containing steel is different from the conventionally known slab surface crack.

Patent Document 1 discloses the influence of scale generation on the surface crack of a slab. However, this technique has a problem with the formation of subscale formed by the oxidation state of Ni oxide and Fe ₂ SiO ₄ scale in Ni-containing steel, and subscale is not generated in steel not containing Ni. The situation differs from the slab surface crack in the present invention.

Japanese Patent No. 3456498

  This invention is made | formed in view of the said problem which a prior art has, The objective is to propose the continuous casting method which can prevent effectively the surface crack of the continuous cast slab in Si containing steel. There is.

The inventors have conducted intensive studies on the cause and countermeasures for the occurrence of surface cracks in a continuous cast slab of Si-containing steel. As a result, in the continuous cast slab containing Si by 0.5 mass% or more, the oxide scale formed on the slab surface is Fe ₂ SiO from a flat form mainly composed of FeO when less than 0.5 mass%. ₄ (firelight) is mainly used to change the shape of the projections and depressions, and when Si, Cu, Sn and Sb, which are low melting point metals, are contained in addition to Si, the above-mentioned Fe ₂ SiO ₄ is mainly used. Oxide scale formation is promoted to enter the inside of the slab along the grain boundary of the solidified structure, and the unevenness becomes more severe, and the oxide scale that has entered the inside of the slab along the grain boundary is secondary cooled. In order to cause surface cracking of the slab due to thermal stress at the time, and therefore to prevent surface cracking of the slab, the thermal stress generated on the surface of the slab during secondary cooling is reduced or further The inventors have found that the oxide scale formed on the surface of the slab may be removed by some means, and have developed the present invention.

That is, according to the present invention, in the continuous casting method of Si-containing steel containing 0.5 mass% or more of Si, the secondary cooling is performed in the temperature range of T _sc where the surface temperature of the slab is calculated from 1177 ° C. to the following formula (3). Specific water amount SW of water is the following (1) to (3);
SW _max = (T _sc / 1177) ^0.5 × SW ₀ (1)
SW ₀ = 1.6 × V _c ^0.2 (2)
T _sc = 1177− (940 × Cu + 9450 × Sn + 10928 × Sb)} / 3
(3)
(Where SW _max : upper limit value of secondary cooling specific water amount of Si-containing steel (l / kg), SW ₀ : secondary cooling specific water amount of ordinary steel (l / kg), T _sc : depth of 150 μm or more Lower limit temperature (° C.) at which a grain boundary oxide scale is formed, V _c : casting speed (m / min), element symbol: content of the element (mass%), Cu: exceeding 0.1 mass% is 0 0.1 mass%, Sn: 0.1 mass% exceeding 0.1 mass%, Sb: 0.05 mass% exceeding 0.05 mass%)
This is a continuous casting method for Si-containing steel, characterized in that casting is performed at a value equal to or lower than the value of SW _max obtained in step (1).

In the continuous casting method of the Si-containing steel of the present invention, the surface temperature of the slab is 1177 ° C. to the collision temperature of the secondary cooling water in the temperature range of T _sc obtained by the above equation (3) is 20 kPa or more. It is characterized by removing oxide scale.

  Further, in the continuous casting method of the Si-containing steel of the present invention, when continuous casting is performed with a vertical bending type continuous casting machine, the collision pressure of secondary cooling water in the vertical portion is 20 kPa or more, and continuous casting with a curved type continuous casting machine. In this case, the impact pressure of the secondary cooling water in the correction part from the curved part is set to 20 kPa or more, and the oxide scale on the slab surface is removed.

  Further, the continuous casting method of the Si-containing steel of the present invention uses mechanical means instead of controlling the collision pressure of the secondary cooling water or in addition to controlling the collision pressure of the secondary cooling water. And removing oxidized scale on the surface of the slab.

  Moreover, the said Si containing steel in the continuous casting method of Si containing steel of this invention is Cu: 0.01-0.1mass%, Sn: 0.01-0.1mass%, and Sb: 0.001-0.05mass. % Of at least 1% or more.

According to the present invention, it is possible to prevent the surface crack of the slab caused by the highly uneven oxide scale mainly composed of Fe ₂ SiO ₄ formed on the surface of the continuous cast slab of high Si content steel. Not only can casting be achieved and productivity can be improved, but also the slab can be sent directly to the next process without lowering the yield due to surface maintenance of the slab and the loss of thermal energy. Furthermore, since the technology of the present invention is also effective for high-tensile steel, which is expected to have higher demands for development in the future, the effect exerted industrially is great.

It is a graph which shows the relationship between Si content and the unevenness | corrugation amount of an oxide scale / base metal interface. It is a schematic diagram of the experimental apparatus which evaluates a heat transfer rate and scale peelability. It is a graph which shows the influence which Si content has on the relationship between the amount of cooling water and a heat transfer coefficient. It is a graph which shows the relationship between the collision pressure of a cooling water, and a scale residual amount.

As described above, the crack generated on the surface of the continuous cast slab (slab) of Si-containing steel is different from the conventionally known surface crack generation mechanism. Therefore, the inventors first conducted the following investigation focusing on the influence of the Si content on the oxide scale formed on the slab surface.
C: 0.10 mass%, Mn: 1.8 mass%, P: 0.015 mass%, S: 0.007 mass%, Al: 0.025 mass% and N: 0.004 mass%, and Si content Steel changed to 0 mass%, 0.05 mass%, 0.3 mass%, 0.5 mass%, 1.0 mass% and 2.0 mass% was melted in a 20 kg atmospheric melting furnace, poured into a mold, and steel ingot It was. In addition, content of Cu, Sn, and Sb of the said steel was made constant at Cu: 0.02 mass%, Sn: 0.02 mass%, and Sb: 0.001 mass%.

  Next, a test piece including an oxide scale formed on the surface was collected from the steel ingot, and the cross section of the steel ingot surface layer was observed with EPMA. As a result, the property of the scale formed on the surface with Si: 0.5 mass% as a boundary changes greatly. That is, when Si is less than 0.5 mass%, the steel ingot surface is covered with a flat oxide scale. On the other hand, at 0.5 mass% or more, as schematically shown in FIG. 1, the oxide scale was changed to a rugged structure in which the scale was wedged inside the steel ingot.

In addition, as a result of qualitative analysis of the oxide scale formed on the surface of the steel ingot with EPMA, Fe ₂ SiO ₄ (firelight) was observed in a part of the scale mainly composed of FeO when Si was less than 0.5 mass%. On the other hand, when Si was 0.5 mass% or more, it was found that Fe ₂ SiO ₄ entered the wedge shape from the interface between the oxide scale and the ground iron, resulting in a form with severe irregularities.

  Next, the inventors have the same C: 0.10 mass%, Mn: 1.8 mass%, P: 0.015 mass%, S: 0.007 mass%, Al: 0.025 mass%, and N: 0.00. Steel containing 004 mass% and further added with various components such as Cu, Sn, Sb, Nb, Ti, and Ca are melted in the same manner as in the above-described experiment, cast into a 20 kg steel ingot, In the same manner as in the experiment described above, the oxide scale formed on the surface of the steel ingot was investigated, and the influence of the additive element on the properties of the oxide scale was investigated.

  As a result, when the Si content is merely 0.5 mass% or more, an oxide scale developed along the grain boundary is partially observed, but the degree is not so large, but Cu, Sn, Sb, etc. When the low melting point metal is added, the oxide scale penetrates deeply into the slab along the grain boundary of the solidified structure and the unevenness is more severe (hereinafter, this oxide scale is also referred to as “grain boundary oxide scale”). In the steel ingot in which this grain boundary oxide scale is formed from the interface between the scale and the base iron to the inside (depth) of 150 μm or more, even after no mechanical strain is applied, It became clear that fine cracks progressed along the grain boundary from the surface of the steel ingot to the inside due to the thermal stress at the time of steel ingot cooling.

  Regarding the generation of firelight, there have been some research reports in the past, such as a study by Fukakawa Tomoki et al. ("Effect of S in steel on descaling property of high-pressure water of Si-added hot-rolled steel sheet", According to Iron and Steel, 81 (1995), No. 5, p. 559), a scale mainly composed of firelite is generated with an increase in Si content, and a study by Masano Kanno (“Cu, According to “Effect of water vapor on surface red hot brittleness of Sn-containing steel”, Iron and Steel, 89 (2003), No. 6, p.659), it is stated that the water vapor atmosphere greatly affects the scale formation. . However, none of the above documents describes the effects of Cu, Sn, and Sb on the oxide scale in Si-containing steel.

Therefore, the inventors further investigated the reason why Fe ₂ SiO ₄ enters the inside of the slab along the grain boundary when the low melting point metals Cu, Sn, and Sb are contained, and the unevenness becomes severe. As a result, on the phase diagram, Fe ₂ SiO ₄ exists in the liquid phase up to the eutectic point of 1177 ° C. and exists as a solid at temperatures lower than that, but in the oxide scale formed on the surface of the slab. When the liquid phase contained in the material is only Fe ₂ SiO ₄ , the oxide scale is formed flat on the surface of the slab. That is, when the slab surface temperature is 1177 ° C. or higher where only Fe ₂ SiO ₄ exists as a liquid phase, the liquid phase grain boundary oxide scale hardly grows.

However, when the surface temperature of the slab becomes 1177 ° C. or less, when the steel contains Cu, Sn, Sb, which are low melting point metals, these elements are liquid at the interface between the oxide scale and the ground iron. It is clear that this low melting point liquid promotes the grain boundary oxidation of the solidified structure and enters the inside of the slab along the grain boundary to form a grain boundary oxidation scale with severe irregularities. became. Here, Cu, melting point _{T mp} of Sn and Sb each _{element, T mp (Cu): 1083} ℃, T mp (Sn): 232 ℃ and _T mp (Sb): a 630.6 ° C..

Therefore, the inventors investigated the relationship between the content of the low melting point metals Cu, Sn, and Sb and the effect of each element promoting the formation of grain boundary oxide scale. As a result, the inventors were almost proportional to the content. However, when Cu exceeds 0.1 mass%, Sn exceeds 0.1 mass%, and Sb exceeds 0.05 mass%, it was confirmed that the effect is almost saturated.
That is, here, the Cu content (mass%) in the steel is [Cu], the Sn content (mass%) is [Sn], the Sb content (mass%) is [Sb], and the above effect is saturated. When the contents to be expressed are [Cu] _cri , [Sn] _cri and [Sb] _cri , the effect of forming the grain boundary oxidation scale of each element of Cu, Sn and Sb is Cu: 0.1 mass% or less , Sn: 0.1 mass% or less, Sb: 0.05 mass% or less, ([Cu] / [Cu] _cri ) = ([Cu] /0.1), ([Sn] / [Sn), respectively ] _Cri ) = ([Sn] /0.1) and ([Sb] / [Sb] _cri ) = ([Sb] /0.05). However, the above effects are saturated when Cu exceeds 0.1 mass%, Sn exceeds 0.1 mass%, and Sb exceeds 0.05 mass%. In this case, [Cu], [Sn] and [Sb] Are calculated as 0.1 mass%, 0.1 mass%, and 0.05 mass%, respectively.

Further, as a result of investigating the temperature range in which the formation of grain boundary oxide scale is promoted by the low melting point metals Cu, Sn and Sb, the inventors are closely related to the melting point of each element. The temperature range in which the formation of the grain boundary oxide scale is continued until the melting point is reached, that is, the temperature range in which each element promotes the formation of the grain boundary oxide scale is Cu (1177 ° C. to T _mp (Cu)) = 1177 to It was found that 1083 ° C., Sn (1177 ° C. to T _mp (Sn)) = 1177 to 232 ° C. and Sb (1177 ° C. to T _mp (Sb)) = 1177 to 630.6 ° C.

Therefore, the inventors further describe the lower limit temperature T _sc at which a grain boundary oxide scale having a depth of 150 μm or more that causes surface cracking is formed when Cu, Sn, and Sb are contained in combination. , Sn and Sb elements content [Cu], [Sn] and [Sb], Fe ₂ SiO ₄ melting point T _mp (Fe ₂ SiO ₄ ), and Cu, Sn and Sb element melting points T _mp ( As a result of examining the relational expression between Cu), T _mp (Sb) and T _mp (Sn) by trial and error, it was found that the lower limit temperature T _sc can be expressed by the following expression (3).
Serial _{T sc = T mp (Fe 2} SiO 4) - {(T mp (Fe 2 SiO 4) -T mp (Cu)) × ([Cu] / [Cu] cri) + (T mp (Fe 2 SiO 4 _{) -T mp (Sn)) ×} ([Sn] / [Sn] cri) + (T mp (Fe 2 SiO 4) -T mp (Sb)) × ([Sb] / [Sb] cri)} / 3
= 1177− (940 × [Cu] + 9450 × [Sn] + 10928 × [Sb]) / 3
... (3)
Incidentally, the lower limit temperature T _sc when Cu: 0.1 mass%, Sn: 0.1 mass%, and Sb: 0.05 mass% is about 650 ° C.

As described above, the generation mechanism of the grain boundary oxide scale formed along the grain boundary from the slab surface to the inside has been clarified. Therefore, the inventors further examined a method for preventing the surface crack of the slab caused by the grain boundary oxide scale.
As described above, it is considered that the surface crack of the continuous cast slab of Si-containing steel occurs due to thermal stress during cooling of the slab when the grain boundary oxide scale is greater than a certain depth (150 μm or greater). However, it is virtually impossible to prevent the formation of an oxide scale mainly composed of Fe ₂ SiO ₄ (Phlite) on the surface of the continuous cast slab. Therefore, as a method of preventing the surface crack of the slab, the thermal stress generated on the surface of the slab is relaxed as much as possible in the temperature range where the grain boundary oxide scale is formed, and the oxidation formed on the surface of the slab It is considered effective to remove the scale by some means and prevent it from growing to a grain boundary oxide scale.

Therefore, first, a method for relaxing the thermal stress generated on the surface of the slab was examined.
As described above, in the temperature range of the eutectic temperature of Fe ₂ SiO ₄ scale: 1177 ° C. to T _sc, an oxide scale with severe irregularities is formed on the surface of the slab. Since this oxide scale has high adhesion, the heat transfer rate is higher than that when an oxide scale with low adhesion is formed. It is strongly cooled by the next cooling.

  On the other hand, the grain boundary oxide scale with severe irregularities is not uniformly formed over the entire slab, and therefore, the thickness of the oxide scale varies greatly depending on the location. As a result, coupled with the strong cooling of the secondary cooling, the cooling unevenness is increased, and it is considered that the surface cracking is promoted by the thermal stress caused by the local temperature difference on the surface of the slab.

Therefore, for the high Si content steel containing 1.0 mass% of Si and the normal steel containing 0.25 mass% of Si, using the experimental apparatus shown in FIG. 2, imitating secondary cooling of the slab surface, The relationship between the amount of cooling water in the water spray and the average value of the heat transfer coefficient in the temperature range of 1200 to 600 ° C. at which the grain boundary oxide scale continues to be formed was investigated, and the result is shown in FIG. From FIG. 3, the high Si content steel containing 1.0 mass% of Si has a larger heat transfer coefficient even with the same amount of water than the ordinary steel with Si content of 0.25 mass%. It was confirmed that the amount of water with which the heat transfer coefficient can be obtained is about 80% of the amount of water in ordinary steel, that is, the amount of cooling water can be reduced by about 20% in order to achieve the same cooling conditions as in ordinary steel.
In addition, the reason why the heat transfer coefficient of the high Si content steel is increased is that, in addition to the high adhesion of the oxide scale, the minimum heat flux point (MHF) that changes from the film boiling region to the transition boiling region in the boiling curve. However, in the case of strong cooling, it is considered that shifting to a higher temperature side also has an effect.

Therefore, the inventors investigated the amount of cooling water that can obtain the same heat transfer coefficient as that of ordinary steel in the same manner as described above for steels in which the addition amount of Cu, Sn, and Sb was changed in addition to Si. As a result, in the Si-containing steel, the cooling water amount SW at which the same heat transfer coefficient as that of the ordinary steel is changed according to the contents of Cu, Sn and Sb, and when the cooling water amount SW ₀ of the ordinary steel is used as a reference, The following formula (1) obtained by multiplying SW ₀ by (T _sc / 1177) ^0.5 ;
SW (l / kg) = (T _sc / 1177) ^0.5 × SW ₀ (l / kg) (1)
And found that it can be obtained approximately.
Here, the cooling water amount SW ₀ of ordinary steel is generally expressed by the following equation (2):
SW ₀ (l / kg) = 1.6 × V _c ^0.2 (2)
(Where V _c : casting speed (m / min)
Given in.

And as a result of repeating experiments based on the above results, in the continuous casting of Si-containing steel, the surface temperature of the slab is in a temperature range of 1177 ° C. to T _sc so as to be equal to or less than the same cooling condition as ordinary steel. The cooling water amount at the time of secondary cooling in is reduced and cast so as to be equal to or less than the cooling water amount SW given by the above equation (1), that is, the SW obtained by the above equation (1) is converted into the secondary cooling specific water amount. It has been found that by casting as the upper limit SW _max , the thermal stress generated on the surface of the slab of Si-containing steel can be relaxed and cracking of the surface of the slab can be prevented.

  From the viewpoint of reducing the stress on the surface of the slab, from a continuous casting machine, a portion of the continuous slab that is distorted by bending or unbending (correcting), for example, in a vertical bending type continuous casting machine, a vertical portion to a curved portion In the section where the transition is made to the bending type continuous casting machine, it is desirable that the amount of secondary cooling water in the section where the transition is made from the bending portion to the correction portion is particularly strictly controlled.

Next, the inventors examined a method for preventing surface cracking of the slab by removing the oxide scale formed on the surface of the continuous cast slab and preventing it from growing to a grain boundary oxide scale.
Using the apparatus shown in FIG. 2, a steel sample containing Si: 1.0 mass%, Cu: 0.05 mass%, Sn: 0.1 mass%, and Sb: 0.001 mass% was heated at 1250 ° C. with a propane gas burner. After forming an oxide scale on the surface of the sample by simulating secondary cooling of the slab, the scale remains when the collision pressure of the cooling water sprayed from the spray nozzle is changed in the range of 0 to 40 kPa The amount (thickness) was investigated, and the result is shown in FIG.

FIG. 4 shows that the scale thickness can be reduced (removed) to 50 μm or less by setting the collision pressure of the cooling water sprayed from the nozzle to 20 kPa or more. Therefore, in the present invention, it is preferable that the collision pressure of the secondary cooling water is 20 kPa or more when the surface temperature of the slab where the grain boundary oxide scale is formed is 1177 ° C. to T _sc .

The temperature range of 1177 ° C. to T _sc varies depending on the contents of Cu, Sn, and Sb. Therefore, as a rough guideline, as the portion where the collision pressure of the secondary cooling water is 20 kPa or more, for example, in the case of continuous casting with a vertical bending type continuous caster, the interval from the vertical portion to the curved portion In the case of continuous casting with a curved continuous casting machine, it is preferable to use a section from the curved portion to the correction portion.

  Further, as a means for removing the oxide scale, in addition to the method of increasing the collision pressure of the secondary cooling water as described above, for example, a method of removing with a roll having protrusions, a method of peeling off the scale with a brush roll, etc. The following mechanical means may be used.

  In the above description, steel containing 0.5 mass% or more of Si and further containing any one or more of Cu, Sn, and Sb has been described as a steel type for the continuous casting method of the present invention. However, it is also effective for a continuous casting method of steel such as for electrical steel sheets containing Si in a large amount of 2 mass% or more.

C: 0.10 mass%, Si: 1.0 mass%, Mn: 1.8 mass%, P: 0.015 mass%, S: 0.007 mass%, Al: 0.025 mass%, and N: 0.0040 mass% Further, a steel whose contents of Cu, Sn and Sb are changed as shown in Table 1 is melted and cast by a curved continuous casting machine at a casting speed of 1.0 m / min. 1500 mm × thickness: 215 mm slab (No. 1-28). At this time, the secondary cooling of the slab in the continuous casting was performed by a water spray method, and the specific amount of cooling water and the collision pressure were changed as shown in Table 1. As a reference example, cast pieces (Nos. 29 to 34) were produced in the same manner as described above except that Si: 0.25 mass% was used.
The cast slab thus obtained was then subjected to a penetrant test (PT) after cutting the surface of the slab by 2 mm with a scarfing equipment, and examined for the presence of surface cracks. A case where a slight crack was confirmed was evaluated as Δ, and a case where a clear crack was confirmed was evaluated as ×.

Table 1 shows the results of the investigation on the presence or absence of surface cracks. From Table 1, in the slab containing only 0.25 mass% of Si (reference examples of No. 29 to 34), the occurrence of surface cracks was recognized even when the specific water amount SW of the secondary cooling was higher than the SW _{max of the} present invention. It is not done.
On the other hand, in the slab containing 1.0 mass% of Si, the slab of the example of the present invention in which the specific water amount SW of the secondary cooling is reduced to the SW _max or less of the present invention in consideration of the surface crack caused by grain boundary oxidation ( In Nos. 12 to 24), the occurrence of surface cracks is not observed. In particular, after setting the specific water amount SW of the secondary cooling to an appropriate range, the cooling water collision pressure was 20 kPa. In Nos. 23 and 24, the slab surface properties were very good (indicated by ◎).
In contrast, without considering the surface cracks due to intergranular oxidation, as with ordinary steel low _Si, the comparative example in which the secondary cooling at a ratio water SW exceeding _{SW max} slab (No.1~11) In the temperature range (1177 ° C. to T _sc ) of the present invention, the secondary cooling specific water amount SW was not reduced below SW _max . In 25 and 26, the occurrence of cracks was observed on the surface of any slab. No. 27 and 28, it is understood that when the secondary cooling is not performed in consideration of the surface crack caused by the grain boundary oxidation, the slab surface crack cannot be prevented only by increasing the collision pressure of the secondary cooling water. .

  In addition, a sample was taken from the slab where the occurrence of surface cracks was observed, and when the cross-sectional structure of the crack occurrence site was observed with EPMA, the cracks developed from the grain boundary oxidized scale into the slab, It was estimated that the part where the grain boundary oxidation was remarkable developed into cracks due to the thermal stress during cooling. From these results, it was confirmed that in the slab production by continuous casting of high-Si steel, it is necessary to perform secondary cooling in consideration of surface cracks caused by oxide scale, which has not been considered in the past.

  The technique of the present invention relates to a technique for preventing surface cracking of a continuous cast slab, but is not limited to this. For example, the scale control in a continuous casting or hot rolling process, or via scale control. It can also be applied to cooling rate control.

Claims (5)

In the continuous casting method of Si-containing steel containing 0.5 mass% or more of Si,
The specific water amount SW of the secondary cooling water in the temperature range of T _sc obtained by the surface temperature of the slab from 1177 ° C. to the following equation (3) is equal to or less than the value of SW _max obtained by the following equations (1) to (3). A continuous casting method of Si-containing steel, characterized in that
SW _max = (T _sc / 1177) ^0.5 × SW ₀ (1)
SW ₀ = 1.6 × V _c ^0.2 (2)
T _sc = 1177− (940 × Cu + 9450 × Sn + 10928 × Sb)} / 3
(3)
(Where SW _max : upper limit value of secondary cooling specific water amount of Si-containing steel (l / kg), SW ₀ : secondary cooling specific water amount of ordinary steel (l / kg), T _sc : depth of 150 μm or more Lower limit temperature (° C.) at which a grain boundary oxide scale is formed, V _c : casting speed (m / min), element symbol: content of the element (mass%), Cu: exceeding 0.1 mass% is 0 0.1 mass%, Sn: 0.1 mass% exceeding 0.1 mass%, Sb: 0.05 mass% exceeding 0.05 mass%) The surface temperature of the slab is from 1177 ° C. to T _sc obtained in the above equation (3), and the impact pressure of the secondary cooling water is set to 20 kPa or more to remove the oxide scale on the surface of the slab. The continuous casting method of Si-containing steel according to 1. When continuously casting with a vertical bending type continuous casting machine, the impact pressure of the secondary cooling water in the vertical part is 20 kPa or more. When continuously casting with a curved type continuous casting machine, secondary cooling is performed from the curved part to the correction part. The continuous casting method for Si-containing steel according to claim 1, wherein the oxide scale on the surface of the slab is removed with a water collision pressure of 20 kPa or more. In place of controlling the collision pressure of the secondary cooling water, or in addition to controlling the collision pressure of the secondary cooling water, the mechanical scale is used to remove the oxide scale on the surface of the slab. The continuous casting method of Si-containing steel according to claim 2 or 3. The Si-containing steel contains at least one of Cu: 0.01 to 0.1 mass%, Sn: 0.01 to 0.1 mass%, and Sb: 0.001 to 0.05 mass%. The continuous casting method for Si-containing steel according to any one of claims 1 to 4.

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