JP2007275987A - Light rolling method of cast steel slab in continuous casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light rolling method of a cast steel slab which can securely restrain center segregation. <P>SOLUTION: In the invented method, molten steel which contains 0.70-0.90 wt.% C, 0.15-0.25 wt.% Si and 0.45-0.55 wt.% Mn and of which the degree of superheat is 10-45°C is cast at the rate of 0.50-0.65 m/min while being cooled with spray water with a specific water ratio of 0.25-1.0 l/kg, so as to manufacture a cast steel slab of which the thickness T is 380±30 mm and the ratio of the width B to the thickness T, namely B/T is 1-2. In this case, the decreasing gradient Y (mm/m) of the face-to-face distance between a pair of reduction rolls 4 in the casting direction is set as follows with respect to a distance X (m) in the casting direction from the meniscus of the molten steel:0≤Y≤5.0 for a range of 10.0≤X≤22.3, 2.0≤Y≤3.0 for 22.3≤X≤25.9, 0.6≤Y≤2.6 for 25.9≤X≤27.5, and 0.5≤Y≤1.5 for 27.5≤X≤32.3. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for lightly reducing a slab cast by continuous casting for the purpose of improving center segregation.

  Conventionally, when producing semi-finished products (slabs) such as slabs and blooms by continuous casting, carbon (C), silicon (Si), manganese (Mn) contained in the molten steel at the center of the slab So-called center segregation, in which components such as) segregate.

  It is known that this center segregation occurs by the following mechanism. That is, when dendrites (dendritic crystals) grow from the surface side of the slab toward the center at the end of solidification of the molten steel, unsolidified portions remain between the dendrites at the center of the slab. When the remaining unsolidified portion is solidified and contracted, the concentrated molten steel containing components such as C, Si, and Mn described above at a high content rate flows into the center of the slab. The components will segregate.

  Such center segregation causes a decrease in steel toughness, hydrogen-induced cracking, and the like, and is preferably suppressed as much as possible. Therefore, at the end of solidification of molten steel, the slab is squeezed from the outside to compensate for the solidification shrinkage of the remaining unsolidified part (hereinafter referred to as light reduction), thereby preventing the flow of concentrated molten steel as much as possible. In recent years, methods for suppressing segregation have been generally adopted.

  By the way, if the amount of reduction at the time of light reduction is too small, the reduction is not transmitted to the center part of the slab, and the center segregation is not sufficiently improved. On the other hand, if the amount of reduction is too large, the concentrated molten steel between dendrites will be squeezed out to the center part of the slab, and the center segregation will be worsened. Therefore, in order to effectively suppress the center segregation, it is necessary to perform light reduction with an appropriate reduction amount according to the solidified state of the slab. Therefore, in the conventional light reduction method, reduction conditions such as a reduction section and a reduction amount are determined according to the solid phase ratio inside the slab (for example, see Patent Documents 1 and 2).

JP 2001-259810 A JP-A-8-132204

  In the light reduction method disclosed in Patent Documents 1 and 2, it is essential to accurately grasp the solid phase ratio inside the slab. Here, since it is almost impossible to detect the solid phase rate in the slab in an actual continuous casting process, the solid phase rate is generally predicted by calculation.

  In order to accurately perform the solidification heat transfer calculation in this continuous casting process, at least the physical property data (for example, solidification latent heat / thermal conductivity / specific heat) of the steel grade and the external heat removal conditions (inside the mold) It is necessary to accurately grasp calculation conditions such as heat removal at the heat / heat transfer coefficient by spray or mist cooling in the secondary cooling zone / heat transfer coefficient by roll cooling. Among these calculation conditions, the ones that greatly affect the calculation results are as follows: (1) (physical property data) solidification latent heat and (2) (external heat removal conditions) heat transfer coefficient / roll in the secondary cooling zone And a heat transfer coefficient by cooling.

The former (1) solidification latent heat generally has a value of about 55 to 65 cal / g, but it is very difficult to accurately determine the solidification latent heat of steel containing many elements.
In addition, the heat transfer coefficient in the secondary cooling zone of the latter (2) is generally estimated based on the measurement results of temperature changes when steel is cooled at a predetermined spray flow rate in experiments. Yes.

  However, it has been reported that the heat transfer coefficient of spray / mist cooling in the secondary cooling zone is expressed as a complicated function in which various parameters are linked (Mitsuka et al .: Iron and Steel, 69 (1983), 262. / Mitsuka: Iron and steel, 91 (2005), see 1). The parameters are, for example, spray flow rate / water droplet size and momentum / air amount and pressure / slab surface temperature.

  And, even if these parameters are appropriately determined, the present condition is that the heat transfer coefficient varies greatly depending on the measurement conditions. In addition, in the above experiment, (a) the difference in cooling capacity between the upper and lower surfaces of the slab, the change caused by the movement of the slab, (b) the effect of clogging of the immersion nozzle, (c) the accumulation between the guide rolls Naturally, it is impossible to estimate various effects that can occur in actual machines, such as the effects of water, (d) the effects of cooling from low-temperature rolls, and (e) the effects of slab oxidation (scale adhesion thickness).

As described above, as long as the calculation conditions for solidification heat transfer calculation include many uncertain factors, it is extremely difficult to accurately predict the solid fraction in the slab according to each steel type / casting condition. Have difficulty.
As a reference, an example of a calculation result of solidification heat transfer calculation is shown in FIG. This FIG. 12 is calculated based on the prediction formula described in the above-mentioned Mitsuka et al. Literature, assuming that the solidification latent heat is 55 cal / g or 65 cal / g. In FIG. 12, a solid line indicates a case where the latent heat is 65 cal / g, and a broken line indicates a case where the calorie is 55 cal / g. As can be seen from this figure, it can be seen that due to the difference in solidification latent heat that is not actually obtained accurately, a large deviation of, for example, several m order occurs in the relationship between the solid phase ratio and the meniscus distance. Further, the above-mentioned prediction formula of Mitsuka et al. Does not necessarily meet all casting conditions, and depending on the prediction formula used, it can be easily predicted that a deviation will occur in the relationship between the solid phase ratio and the meniscus distance. .

  For the above reasons, as shown in Patent Documents 1 and 2 described above, even if the rolling conditions (the rolling section and the rolling amount) are determined based on the solid phase ratio, the center segregation is actually sufficiently suppressed. There are many things that cannot be done.

  An object of the present invention is to provide a light reduction method for a slab that can reliably suppress center segregation by determining a reduction condition based on actual casting conditions, not a solid phase ratio obtained by calculation. It is to be.

Means for Solving the Problems and Effects of the Invention

The light reduction method of the slab in the continuous casting of the first invention is that the carbon content is 0.70 to 0.90 wt%, the silicon content is 0.15 to 0.25 wt%, and the manganese content is 0.45. Steel for steel cord in the range of 0.55 wt%, 0.50 to 0.65 m / min while cooling a molten steel with a superheat degree of 10 to 45 ° C. with a specific water amount of 0.25 to 1.0 l / kg. When casting a slab having a thickness T of 380 ± 30 mm and a ratio B / T of width B to thickness T of 1-2, the slab is lightened by a plurality of roll pairs. A method of rolling down,
When the casting direction distance from the meniscus of the molten steel is X [m], and the decreasing gradient of the distance between the surfaces of the roll pair with respect to the casting direction is Y [mm / m],
In the range of 10.0 ≦ X ≦ 22.3, 0 ≦ Y ≦ 5.0
In the range of 22.3 ≦ X ≦ 25.9, 2.0 ≦ Y ≦ 3.0
In the range of 25.9 ≦ X ≦ 27.5, 0.6 ≦ Y ≦ 2.6
In the range of 27.5 ≦ X ≦ 32.3, 0.5 ≦ Y ≦ 1.5
It is characterized by being.

  According to the first invention, after the casting conditions such as the steel composition, the size of the slab, and the casting speed are determined, the gradient Y (the casting direction) of the distance between the surfaces of the roll pair that performs light reduction is further reduced. The amount of decrease in the distance between the roll surfaces per unit distance (1 m) (mm), also referred to as a rolling gradient), is specifically determined based on the casting direction distance X from the meniscus. In other words, instead of the solid phase ratio inside the slab, which is difficult to predict in practice, the reduction condition is determined based on the actual casting conditions. It is possible to suppress the center segregation of the slab more effectively.

  The light reduction method of the slab in the continuous casting of the second invention is characterized in that, in the first invention, 2.0 ≦ Y ≦ 3.5 in the range of 10.0 ≦ X ≦ 22.3. It is what.

  In particular, when producing a slab having a relatively large thickness, if the amount of reduction in the initial reduction stage is small, the concentrated molten steel with a high content of carbon or the like flows toward the center of the slab. Thus, a segregation pattern (called V segregation) having a V-shaped cross section tends to occur around the center of the slab. However, according to the second invention, since the reduction is performed with a certain amount of reduction in the initial reduction stage, it is possible to prevent the occurrence of V segregation.

  Next, an embodiment of the present invention will be described. First, a continuous casting machine (hereinafter referred to as a continuous casting machine) of this embodiment will be described with reference to FIG. As shown in FIG. 1, the continuous casting machine 100 is arranged in parallel on a mold 1 for solidifying molten steel into a predetermined shape, a tundish 2 for pouring molten steel into the mold 1, and a downstream side of the mold 1. A plurality of rolls 3 are provided.

  The plurality of rolls 3 are respectively disposed on both sides of this path along a predetermined casting path that extends vertically below the mold 1 and then bends in an arc shape and finally extends in the horizontal direction. A pair of rolls 8 is composed of two rolls 3 arranged so as to sandwich the path, and the two rolls 3 of each pair of rolls 8 are arranged with a predetermined plane therebetween. In addition, a cooling water injection device (not shown) is provided between the rolls 3 to cool the slab by spraying water onto the slab.

  The molten steel poured into the mold 1 forms a shell (solidified portion) from a portion in contact with the mold 1 and becomes a slab having an unsolidified portion inside. The slab is cooled by water sprayed from between the rolls 3 and is sent to the downstream in the casting direction by a plurality of pairs of rolls 8 so that the shell grows, and finally the casting is solidified completely to the inside. It becomes a piece.

  Here, the casting conditions for continuous casting by the continuous casting machine 100 of the present embodiment are as follows. First, the content of the elements contained in the target steel is as follows: carbon (C) content is 0.70 to 0.90 wt%, silicon (Si) content is 0.15 to 0.25 wt%, manganese The (Mn) content is in the range of 0.45 to 0.55 wt%. The content of elements other than C, Si, and Mn is not particularly limited as long as it is within a generally used range. For example, the phosphorus (P) content is 0.03 wt% or less, and the sulfur (S) content is 0.01 wt% or less.

  And the continuous casting machine 100 of this embodiment is the amount of specific water (amount of water given with respect to 1 kg of steel) 0.25-1. While cooling at 0 l / kg, casting is performed at a casting speed of 0.50 to 0.65 m / min to produce a slab.

  Furthermore, as shown in FIG. 2, the slab obtained by continuous casting by the continuous casting machine 100 has a substantially rectangular cross section, and the ratio B / T is 1 when the width is B and the thickness is T. It is a slab (bloom) which becomes -2. Furthermore, the thickness T of this slab is 300 ± 30 mm.

  By the way, as described above, at the end of solidification of the molten steel, elements contained in the molten steel such as carbon, silicon, manganese, phosphorus, and sulfur are formed in the center of the slab along with the solidification shrinkage of the unsolidified portion inside the slab. Concentrated molten steel containing at a high content rate flows. Therefore, as shown in FIG. 3, the center segregation 20 tends to occur at the center of the slab. In addition, FIG. 3 has shown the cross section when the width direction center of a slab is cut | disconnected by the vertical surface.

  Therefore, as shown in FIG. 1, in the continuous casting machine 100, a plurality of reduction roll pairs 4 follow the casting path at a position downstream of the roll 3 in the region where the casting path is horizontal. It is provided so as to sandwich the slab to be sent. The two rolling rolls 5 constituting each rolling roll pair 4 are respectively arranged on the upper and lower sides of the path with a predetermined inter-surface distance.

  In order to prevent the center segregation from occurring in the slab, the plurality of reduction roll pairs 4 reduce the slab at the end of solidification of the molten steel to compensate for the solidification shrinkage of the unsolidified portion inside the slab ( (Under light pressure). The plurality of rolling roll pairs 4 are arranged so that the space between the rolling roll pairs 4 becomes narrower toward the downstream side in the casting direction, and as the slab is fed to the downstream side in the casting direction between these rolling roll pairs 4, The slab is squeezed from both sides in the thickness direction by the upper and lower two rolling rolls 5 of the rolling roll pair 4.

  Here, as shown in FIG. 4, when considering two pairs of rolling roll pairs 4, the distance between the surfaces of the rolling roll pair 4 positioned on the upstream side in the casting direction of the two rolling roll pairs 4 is determined. G1 [mm], G2 [mm] is the distance between the surfaces of the reduction roll pair 4 located downstream in the casting direction, L [m] is the separation distance in the casting direction of the two reduction roll pairs 4, and 2 pairs of reduction rolls Y = (G1-G2) where Y [mm / m] is a decrease gradient of the distance between the roll surfaces between the pair 4 (the amount of decrease in the inter-surface distance per unit distance with respect to the casting direction, hereinafter also referred to as the rolling gradient). It is represented by / L.

And in this embodiment, when the casting direction distance from the meniscus which is the molten steel surface of the casting_mold | template 1 is set to X [m],
In the range of 10.0 ≦ X ≦ 22.3, 0 ≦ Y ≦ 5.0
In the range of 22.3 ≦ X ≦ 25.9, 2.0 ≦ Y ≦ 3.0
In the range of 25.9 ≦ X ≦ 27.5, 0.6 ≦ Y ≦ 2.6
In the range of 27.5 ≦ X ≦ 32.3, 0.5 ≦ Y ≦ 1.5
It has become.

  In this way, after the casting conditions such as the slab size and casting speed are determined, the reduction gradient (rolling gradient) Y of the distance between the rolling roll pairs 4 that performs light rolling is the casting direction from the meniscus. It is determined according to the distance X. In other words, since the reduction condition is determined based on the actual casting conditions instead of the solid phase ratio of the slab, which is difficult to predict in practice, the final solidification part of the slab can be ensured under appropriate reduction conditions. It is possible to reduce the center segregation generated at the center of the slab.

  When the amount of reduction in the initial reduction stage is relatively small, the concentrated molten steel flows toward the center of the slab, and as shown in FIG. A segregation pattern having a V-shaped cross section, that is, a so-called V segregation 21, which is inclined so as to spread toward the downstream side in the direction, is likely to occur. Therefore, in order to suppress the occurrence of V segregation by performing a relatively large amount of reduction in the initial reduction stage, 2.0 ≦ Y ≦ 3.5 is set in the range of 10.0 ≦ X ≦ 22.3. preferable.

  The light reduction method of the present embodiment described above was verified by more specific examples and comparative examples.

[Casting conditions]
First, as examples of the present invention, the thickness of the slab, the contents of elements (carbon, silicon, and manganese) contained in the steel, the casting speed, the degree of superheat of the molten steel, and the specific water amount are as follows. Each slab was cast as determined within the range.
Slab thickness (mold thickness): 380 ± 30 mm
Carbon content: 0.70-0.90wt%
Silicon content: 0.15-0.25 wt%
Manganese content: 0.45-0.55 wt%
Casting speed: 0.50 to 0.65 m / min
Molten steel heating degree: 10-45 ° C
Specific water amount: 0.25 to 1.0 l / kg

  In addition, it supplements a little regarding the measurement of the above-mentioned molten steel superheat degree (DELTA) T. In continuous casting, the molten steel from the ladle is received in a tundish, a container lined with a refractory, and the molten steel is poured into the mold from the bottom of the tundish. During casting, molten steel is always stored in the tundish. And the temperature measurement of molten steel is performed by immersing a thermometer in from the molten steel surface of this tundish. Specifically, a thermocouple type thermometer is used as the thermometer, and the temperature detector provided at the tip of the thermometer is output from the thermometer in a state where it is immersed 50 mm or more from the surface of the molten steel in the tundish. The value was taken as the molten steel temperature at that time. And the molten steel superheat degree is calculated | required as a value which subtracted the solidification start temperature decided only from the component of the molten steel from the molten steel temperature measured by the method mentioned above.

  On the other hand, the slab of the comparative example with respect to an Example was cast by setting so that at least any one casting condition may remove | deviate from the above-mentioned range. In all of the examples and comparative examples, continuous casting was performed so that the width of the slab was 600 mm.

[Concrete configuration of rolling roll pair]
Next, the specific structure in the Example and comparative example of the reduction roll pair 4 is demonstrated. FIG. 5 is a schematic view showing an arrangement configuration of the rolling roll pair 4, and FIG. 6 is an enlarged view of a portion surrounded by a one-dot chain line in FIG. 5. As shown in FIGS. 5 and 6, the four rolling rolls 5 arranged in the casting direction are provided on one roll stand 6, 7 on both sides of the casting path through which the slab is fed. Here, the roll stand 7 located below the casting path is fixed. On the other hand, the roll stand 6 positioned on the upper side of the casting path is configured to be adjustable in its inclination and vertical position. That is, by adjusting the inclination and the vertical position of the upper roll stand 6, it is possible to freely set the inter-surface distance of the reduction roll pair 4.

  Further, the diameter D of the rolling roll 5 is 300 mm, and the four rolling rolls 5 provided on one roll stand 6 and 7 are arranged at equal intervals, and the interval Lc (distance between roll centers) is 320 mm. Further, the first roll stands 6 and 7 from the left in FIG. 5 are installed so that the casting direction distance X1 from the meniscus of the reduction roll 5 located on the most downstream side in the casting direction is 22.3 m. Similarly, the casting direction position X2 of the most downstream roll 5 of the third roll stand 6, 7 from the left is 25.9 m, and the casting direction position X3 of the most downstream roll 5 of the fourth roll stand 6, 7 from the left is 27. The casting direction position X4 of the most downstream roll 5 of the roll stands 6 and 7 from the left, 7th from the left (first from the right in FIG. 5) is 32.3 m.

  Then, by appropriately changing the inclination and vertical position of the upper roll stand 6 based on the casting direction distance from the meniscus, the space between the uppermost downstream roll pair 4 and the lowermost downstream roll pair 4 is respectively changed. Adjust and set the decreasing gradient of the distance between roll surfaces to an appropriate value.

  For example, when setting the decreasing gradient of the inter-surface distance when the distance X is between 25.9 m and 27.5 m to a desired value, the most upstream roll pair 4A and the most downstream roll provided on the roll stands 6A and 7A The distance between the 4D surfaces is adjusted. First, as shown in FIG. 6, the inter-plane distance Gx of the most downstream roll pair 4X of the roll stands 6 and 7 located on the upstream side of the roll stands 6A and 7A is measured. And when this inter-surface distance Gx is, for example, 376 mm, the most upstream roll is used to set the decreasing gradient of the inter-surface distance when the distance X is between 25.9 m and 27.5 m to 1.1 mm / m. The distance between the surfaces of the pair 4A and the most downstream roll pair 4D may be set as follows.

  First, the inter-surface distance Gd of the most downstream roll pair 4D is Gd = Gx−1.1 [mm / m] × 1.6 [m] = 374.24 [mm]. On the other hand, since the distance Lc between the rolling roll pairs 4A, 4B, 4C, and 4D is 320 mm, the inter-surface distance Ga of the most upstream roll pair 4A is Ga = 1.1 [mm / m] × (0.32 [ m] × 3) + Gd = 375.296 [mm].

In the same manner, the distance between the faces of the reduction roll pairs 4 provided on the other roll stands 6 and 7 was set so as to have a desired decreasing gradient based on the casting direction distance from the meniscus. As an example of the present invention, when the casting direction distance from the meniscus, which is the molten steel surface of the mold, is X [m], the decreasing gradient of the distance between the rolling roll pairs (the rolling gradient) Y [mm / m ]
In the range of 10.0 ≦ X ≦ 22.3, 0 ≦ Y ≦ 5.0
In the range of 22.3 ≦ X ≦ 25.9, 2.0 ≦ Y ≦ 3.0
In the range of 25.9 ≦ X ≦ 27.5, 0.6 ≦ Y ≦ 2.6
In the range of 27.5 ≦ X ≦ 32.3, 0.5 ≦ Y ≦ 1.5
A light reduction was performed within the range. On the other hand, as a comparative example for this example, light pressure reduction was also performed under the condition that the value of Y was out of the aforementioned range in at least any one section.

[Center segregation evaluation]
Next, a method for evaluating the degree of center segregation occurring in the cast slab will be described.
A typical steel type in which central segregation is a problem is a steel cord material used as a reinforcing material for tires. This steel cord material is manufactured by the following processes, for example. First, a cast slab having a width of 600 mm and a thickness of 380 mm is heated in a heating furnace for about 3 hours, and then formed into a 155 mm square billet. And by rolling this billet, a wire rod having a diameter of 5.5 mm is obtained.

  Here, when center segregation occurs in the slab, the content of elements such as carbon, silicon, and manganese in the center of the slab is high. Therefore, in the wire obtained by rolling such a slab, the shaft core portion becomes hard and is easily broken at the time of wire drawing.

  For example, when carbon is taken as an example of an element contained in molten steel, the degree of center segregation of carbon is determined by the carbon content C0 of the sample taken from the molten steel in the tundish and the carbon in the center of the cast slab. It can be evaluated by a ratio with the maximum value Cmax of content, Cmax / C0. When Cmax / C0 is 1, there is no center segregation. Conversely, the greater the Cmax / C0, the greater the degree of carbon center segregation.

  As a result of investigations by the inventors of the present application, a relationship as shown in FIG. 7 was found between Cmax / C0 and the number of breaks during wire drawing of a wire produced from the slab. As can be seen from FIG. 7, when the value of Cmax / C0 is 1.1 or less, the wire material hardly breaks, but when Cmax / C0 is 1.2 or more, the wire breakage occurs. Become. That is, Cmax / C0 ≦ 1.1 is required to prevent disconnection due to carbon center segregation.

  Similarly, the degree of center segregation of silicon can be evaluated by the ratio Simax / Si0 between the silicon content Si0 of the sample taken out from the molten steel and the maximum value Simax of the silicon content at the center of the slab. The degree of central segregation of manganese can be evaluated by the ratio of the manganese content Mn0 of the sample taken from the molten steel to the maximum value Mnmax of the manganese content at the center of the slab, Mnmax / Mn0. In addition, similar relationships were found between Simax / Si0 and the number of disconnections, and between Mnmax / Mn0 and the number of disconnections, as in carbon. These relationships are shown in FIGS. 8 and 9, respectively. 8 and 9, it is necessary to satisfy Simax / Si0 ≦ 1.2 and Mnmax / Mn0 ≦ 1.1 in order to prevent disconnection.

  Cmax, Simax, and Mnmax were measured as follows. First, as shown in FIG. 10, a rectangular sample having a predetermined length (for example, 250 mm) in the casting direction in a vertical cross section passing through the center in the width direction of a cast piece (for example, width 600 mm × thickness 380 mm). Cut out the piece Sa. Further, as shown in FIG. 11, the contents of carbon, silicon, and manganese in the chips collected by hollowing out the central portion in the thickness direction of the sample piece Sa with a 5.0 mmφ drill are measured. Such chips are collected at a total of 25 positions separated from each other at a pitch of 10 mm in the casting direction, and the most of the carbon, silicon, and manganese contents measured from these 25 positions. The large values are Cmax, Simax, and Mnmax, respectively. The center segregation of the slab is evaluated depending on whether all the conditions of Cmax / C0 ≦ 1.1, Simax / Si0 ≦ 1.2, and Mnmax / Mn0 ≦ 1.1 are satisfied.

[V segregation]
Further, whether or not V segregation occurs around the center of the slab is confirmed as follows. First, similarly to the above-described measurement of center segregation, a sample piece Sa (see FIGS. 10 and 11) is cut out from a cast slab. Then, after polishing the surface of the sample piece Sa, the sample piece Sa is corroded with hydrochloric acid for about 10 minutes, and then whether or not a V-shaped segregation pattern (V segregation 21 shown in FIG. 3) appears on the surface is visually checked. Confirm with.

[Evaluation results]
Then, in the examples and comparative examples, the mold thickness (ie, slab thickness), the content of component elements, the casting speed, the molten steel superheat degree (ΔT), the specific water amount, the distance between the surfaces of the rolling roll pair with respect to the distance from the meniscus. Tables 1 to 5 show various conditions of the decrease gradient (rolling gradient) and the segregation situation in the slab obtained by casting. In the column of the rolling gradient in the table, the range of the distance X is displayed as a simple value obtained by rounding off the boundary value (for example, an interval of 25.9 ≦ X ≦ 27.5 is expressed as 26-28. it's shown).

  From Tables 1 to 5, the casting conditions and the rolling gradient Y are all within the above-described range (indicated by a circle in the table). In the slabs of Examples, Cmax / C0 ≦ 1.1, Simax / Si0 It is understood that the three conditions of ≦ 1.2 and Mnmax / Mn0 ≦ 1.1 are satisfied (indicated by ◯ in the table), and the central segregation of carbon, silicon and manganese is suppressed. On the other hand, in the slab of the comparative example in which at least one of the casting conditions and the rolling gradient Y is not within the above-described range (indicated by x in the table), Cmax / C0 ≦ 1.1, Simax / Si0 It can be seen that at least one of the three conditions of ≦ 1.2 and Mnmax / Mn0 ≦ 1.1 is not satisfied (indicated by x in the table), and the center segregation is not sufficiently improved.

  Furthermore, among the examples in which the suppression of the center segregation is confirmed, when the distance X is in the range of 10.0 ≦ X ≦ 22.3, the rolling gradient Y is 2.0 ≦ Y ≦ 3.5 ( In the examples (Nos. 20, 29, 36, 43, and 49) that are ◎ in the table, V segregation was not observed (indicated by ◎ in the column of V segregation in the table).

It is a schematic block diagram of the continuous casting machine which concerns on embodiment of this invention. It is a figure which shows the cross-sectional size of slab. It is a vertical sectional view of a slab schematically showing a state in which center segregation and V segregation occur in the slab. It is explanatory drawing explaining the relationship between the distance between roll surfaces, and a rolling-down gradient. It is the schematic which shows the arrangement structure of a reduction roll pair. FIG. 6 is an enlarged view of a portion surrounded by an alternate long and short dash line in FIG. 5. It is a graph which shows the relationship between the center segregation (Cmax / C0) of carbon, and the frequency | count of disconnection of a wire. It is a graph which shows the relationship between the center segregation (Simax / Si0) of silicon, and the frequency | count of disconnection of a wire. It is a graph which shows the relationship between the center segregation of manganese (Mnmax / Mn0), and the frequency | count of wire breakage of a wire. It is explanatory drawing regarding extraction of the sample piece for the center segregation evaluation from slab. It is a figure which shows the surface of a sample piece when extract | collecting chips with a drill. It is a graph which shows the calculation result of solidification heat transfer calculation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Mold 4 Reducing roll pair 5 Reducing roll 6,7 Roll stand 20 Center segregation 21 V segregation 100 Continuous casting machine

Claims (2)

Steel cord steel having a carbon content of 0.70 to 0.90 wt%, a silicon content of 0.15 to 0.25 wt%, and a manganese content of 0.45 to 0.55 wt%. Is continuously cast at a casting speed of 0.50 to 0.65 m / min while cooling with a specific water amount of 0.25 to 1.0 l / kg, and a thickness T of 380 ± 30 mm, When producing a slab having a ratio B / T of width B and thickness T of 1-2, the slab is lightly reduced by a plurality of pairs of rolls,
When the casting direction distance from the meniscus of the molten steel is X [m], and the decreasing gradient of the distance between the surfaces of the roll pair with respect to the casting direction is Y [mm / m],
In the range of 10.0 ≦ X ≦ 22.3, 0 ≦ Y ≦ 5.0
In the range of 22.3 ≦ X ≦ 25.9, 2.0 ≦ Y ≦ 3.0
In the range of 25.9 ≦ X ≦ 27.5, 0.6 ≦ Y ≦ 2.6
In the range of 27.5 ≦ X ≦ 32.3, 0.5 ≦ Y ≦ 1.5
A method of lightly rolling a slab in continuous casting, characterized in that:   The method of light reduction of a slab in continuous casting according to claim 1, wherein 2.0≤Y≤3.5 in the range of 10.0≤X≤22.3.

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