JP2005211920A - Method for continuously casting pipe material suitable for production of seamless steel pipe with skew rolling method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To lighten the load at boring time of a seamless steel pipe with skew rolling, by that linear segregation except the central part in a bloom with a continuous casting is lightened to form only the center segregation and easily perform the boring of the central part, and micro-porosity at the peripheral part is reduced and the development of scab and bulging flaw caused by oxidation is reduced. <P>SOLUTION: In a multi-steps type light rolling-reduction facility composed of multi-steps of drawing-out strand in the continuous casting, the linear segregation in the cross sectional face in the thickness direction of the bloom except the central part, is restrained by applying the rolling-reduction of the bloom at 1.0-1.5 mm/step in each step of the rolling-reduction rolls disposed at the downstream of the mold in the range of 0.2-0.6 center solid-phase ratio of the bloom. Further, in each step of the rolling-reduction rolls at the rolling-reduction end stage zone of the drawing-out strand, the rolling-reduction action in the central part at the solidified end stage, is eliminated by opening the rolls, so as to develop the intense segregation only to the central part. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method of manufacturing a pipe material applied to the manufacture of a seamless steel pipe by a tilt rolling method, for example, the manufacture by a Mannesmann drilling method, and in particular, the tilt rolling manufactured by lightly reducing an unsolidified portion in a continuous cast strand. The present invention relates to a method for producing a continuous casting bloom of a pipe material suitable for the production of seamless steel pipes by the method.

  As shown in FIG. 2, when a pipe 9 made of continuous casting bloom is punched by a Manningman drilling method with a piercing mill to produce a steel pipe 11, the inner surface of defects caused by the continuous casting bloom of the pipe 9 as a rolling material. On the inner peripheral surface 12 of the punched steel pipe 11, for example, a spatula 13 or a blister 14 may be generated. The reason why these inner surface flaws are generated is considered to be due to the fact that the microporosity due to the segregation in the bloom which is the tube material to be perforated, that is, the central segregation, is oxidized when heated in the rotary furnace for heating. That is, when end face oxidation occurs in the rotary furnace along micro cracks existing in the pipe material, and this end face oxidized portion is eccentric from the central axis of the pipe material, it is confined together with the gas in the thickness at the time of drilling and becomes blistered. Thus, it is considered that the working surface of the tube material bloom by the Mannesmann drilling method is subjected to a considerable shear stress, thereby leading to the occurrence of the above-described defects. Therefore, in order to prevent the occurrence of these defects, it was necessary to concentrate the shear stress on the center of the bloom.

  On the other hand, in continuous casting of steel, the slab is rolled down with a roll at the end of solidification to improve center segregation of the slab, and lightly reduced at the end of solidification to suppress the flow of concentrated molten steel based on solidification shrinkage, etc. ( For example, see Patent Literature 1, Patent Literature 2, and Patent Literature 3.). However, even in these cases, the effect of improving segregation and center porosity is insufficient or the effect of improvement varies. Furthermore, to improve these, the molten steel heating temperature in the tundish is adjusted to 50 ° C. or less, and the molten steel in the strand is electromagnetically stirred to make equiaxed crystals, thereby finely solidifying the solidified structure in the center. The amount of shrinkage and fluidity in the solid-liquid coexisting phase are made uniform, the flow of concentrated molten steel in the center and the accumulation in the center are reduced, and the central solid phase ratio fs called the end of solidification is about 0.6 A method of improving center segregation and center porosity by lightly reducing the range of 0.8 and the range before and after this range (see, for example, Patent Document 4) has been proposed.

Japanese Patent Publication No.59-16862 Japanese Patent Publication No.59-39225 Japanese Patent Publication No.62-34460 JP-A-6-126405

  The problem to be solved by the present invention is to improve a part of the light reduction technology in the multi-stage reduction equipment for continuous casting strands, which is performed to reduce the center segregation of bloom due to continuous casting, and to reduce the reduction before reaching the end of solidification. Completion reduces line segregation other than the center of the slab, and concentrates segregation at the center, which facilitates drilling of the center part, reduces microporosity in the peripheral part, and dents due to oxidation An object of the present invention is to provide a method for reducing the load at the time of drilling in the production of seamless steel pipes by inclined rolling by the Mannesmann method or the like by reducing the occurrence of state defects.

  The means of the present invention for solving the above problems will be described below. In the drawn strand in continuous casting of steel, segregation control is performed by light reduction, and the reduction method concentrates the segregation only on the central part of the bloom, which is a continuous cast piece. For this reason, in the first aspect of the present invention, in the multi-stage light reduction equipment comprising a plurality of continuous drawn drawn strands, each of the reduction rolls in which the center solid phase ratio fs of the bloom is in the range of 0.2 to 0.6. By applying a bloom reduction of 1.0 to 1.5 mm / step in the step, linear segregation in the cross section in the thickness direction of the bloom excluding the central portion is suppressed, and the end-of-roll zone located downstream of the drawn strand For the production of seamless steel pipes by the inclined rolling method, the rolls are opened at each stage to eliminate the rolling action at the center of the end of solidification and cause strong segregation only at the center. This is a suitable continuous casting method for pipe materials. In the present invention, the center portion means a range from the center line to a region outside 5% of the diameter in the cylindrical tube material.

  In the above means, the reason why the central solid phase ratio fs of the bloom is limited to the reduction by the reduction roll in the range of 0.2 to 0.6 is that even if the central solid ratio fs is less than 0.2, the reduction is effective. However, as shown in FIG. 1 (c), the state is close to no reduction, and when the central solid fraction fs exceeds 0.6, the degree of segregation is improved, as shown in FIG. 1 (b). It is because it becomes the same as the normal light pressure under the prior art.

  In the invention of claim 2, the Bloom reduction in which each stage of the reduction roll in which the central solid phase ratio fs of the bloom is in the range of 0.2 to 0.6 is 1.0 to 1.5 mm / stage is the center of the Bloom The solid phase ratio fs is reduced within a range of 1.0 to 10.0 mm within a range of 0.2 to 0.6, and is suitable for the production of seamless steel pipes by the inclined rolling method of the means of claim 1 This is a continuous casting method for pipe materials.

  In the above means, the reason why the reduction is limited to the range of 1.0 to 10.0 mm is that the reduction is less than 1.0 mm, and there is no effect even if the reduction is more than 10 mm. This is because.

  In the invention of claim 3, the ratio W / L of the length L to the width W of the cross section perpendicular to the drawing direction of the bloom is 1.0 to 0.5. Or it is the continuous casting method of the pipe material suitable for manufacture of the seamless steel pipe by the inclination rolling method of 2 means.

  In the above means, the reason why the ratio W / L of the length L to the width W of the Bloom cross section is limited to 1.0 to 0.5 is that the ratio of the cross section is square or close to it is prismatic in the lump process. It is because it is easy to process from bloom to a cylindrical pipe.

  According to a fourth aspect of the present invention, the strong segregation generated only in the center part of the bloom has a carbon segregation degree of 1.2 or more by the inclined rolling method according to any one of the first to third aspects. This is a continuous casting method for pipe materials suitable for the production of seamless steel pipes.

  In the above means, the reason for limiting the carbon segregation degree to 1.2 or more is that when the carbon is segregated in the central part, the workability of the central part is better and the central part is perforated to produce a seamless steel pipe. It is because it is suitable for.

  As described above, according to the present invention, in a drawn strand in continuous casting, segregation control by light pressure is performed to cause strong segregation such that the carbon segregation degree is 1.2 or more only at the center portion of the slab. As a result, a lot of porosity is generated in the center segregation, the load at the time of drilling Mannesmann can be reduced, and excellent effects such as effective prevention of flaws on the inner surface of the steel pipe after drilling can be achieved.

  The best mode for carrying out the present invention will be described with reference to the following examples using tables and drawings.

  First, for SUJ2, which is a bearing steel specified by JIS, the molten steel is continuously cast in a bloom multi-stage light reduction facility of a vertical continuous casting apparatus, and a drawing strand of 16.65 to 22.35 m downstream from the meniscus in the mold. In the rolling roll arranged in the slab, the slab is reduced by 1.0 to 1.5 mm per stage using only the rolling roll corresponding to the zone having the center solid phase ratio fs of 0.2 to 0.6. The central part of the slab is reduced in this zone by releasing the roll interval so that the reduction roll does not perform the reduction in the zone at the end of the lower stage of the lower stage. The bloom, which is a cast slab in which segregation occurs only in the central part, without exerting a rolling effect on the steel.

  The bloom obtained in this way is subjected to ingot rolling by a conventional method, and then rolled into a round billet. The obtained round billet is heated to an extraction temperature of 1120 ° C. in a rotary heating furnace, drilled by a piercing mill by the Mannesmann method to form a raw pipe, and subsequently pipe-rolled by an elongator, and reheated to 960 ° C. in a reheating furnace. Adjust the dimensions with a sinking mill, and use a rotary sizer to produce roundness to make a product tube.

  Table 1 shows the relationship among the reduction roll position (m), reduction amount (mm / stage), casting speed Vc (m / min), and central solid fraction fs in the above-described method in the vertical continuous casting apparatus.

As shown in Table 1, the present invention continuously casts with a plurality of rolling rolls having a meniscus distance of 16.65 to 24.25 m and a central solid fraction fs of 0.2 to 0.6. The pieces are reduced by 1.0 to 1.5 mm for each step, and when the central solid fraction fs exceeds 0.6, the reduction is stopped, and the bloom is obtained simply by drawing.
That is, the rolling rolls themselves are arranged in eight stages, but when the casting speed is 0.40 m / min, the rolling speed is reduced by 1.0 mm with only the rolling rolls having a central solid phase ratio fs of 0.51, and the casting speed is 0. In the case of .55 m / min, a total of 8.5 mm is crushed by a seven-stage tumbling roll having a central solid phase ratio fs in the range of 0.23 to 0.47.
In addition, about all the stage reduction, reduction is performed in all these 8 rolls.

  In the drawn strand that is the current general operation, the bloom obtained by non-compression, the bloom obtained by full-stage reduction that is a test method, and the bloom obtained by the partial reduction method of the present invention are compared with each other. Table 2 shows the inner surface flaw occurrence rate at the mass ratio of the steel pipe obtained from these and the test results of the macro corrosion judgment of the pipe material. In the test of Table 2, the target steel type is SUJ2 steel, the rolling conditions are heating furnace extraction temperature 1120 ° C., the pre-plug draft rate is 6.0 to 12.5 (lead standard is 15 to 35 mm, gorge standard is 79 to 84mm), and the inspection condition is the visual inspection of the inner surface of the steel pipe to detect the bulges and folds on the inner surface of the steel pipe to obtain the weight ratio of the steel pipe, and the pipe material according to JIS G0553 “Method for testing the macro structure of steel” A macro corrosion test was conducted. The carbon segregation degree is obtained by collecting samples from the cross section of the pipe material from the center and other parts, and analyzing the carbon amount by the combustion-infrared absorption method in JIS G1211 “Iron and steel-carbon determination method”. Was determined as the degree of segregation.

  Furthermore, the solidification state and macro structure of the slab 1 in a drawing strand are shown in FIG. (A) of FIG. 1 is an example of the present invention, and among the eight stages of rolling rolls arranged on the drawing strands of 16.65 to 22.35 m downstream from the meniscus in the mold, the upper six stages of rolling rolls are pressed. It is a slab obtained by a partial reduction method in which the lower part 5 is reduced and the latter two-stage reduction roll is used as the non-lower part 6 and the solidification end stage is opened. Thus, a strong central segregation 4 having a segregation degree of carbon of 1.2 or more remains. The liquid phase is denoted by 3. FIG. 1 (b) shows a slab 1 obtained as a material for reducing central segregation due to multi-stage reduction, with all eight stages of reduction rolls used as the indented part 5. The segregation 7 has a degree of segregation to a degree that cannot be confirmed by the macrostructure test method. (C) of FIG. 1 has the center segregation 4 and the thick linear segregation 7 in the periphery with the slab 1 obtained under the non-pressing which is the present general operation.

  As shown in FIG. 1 (a), according to the present invention, there is no linear segregation 7 in the periphery, and a strong central segregation 4 is left only at the center of the solid phase 2. Therefore, the central portion is easy to perforate, there is no microporosity in the peripheral portion, and all the central segregation 4 is pressed against the inner peripheral surface 12 of the steel pipe 11 along the surface of the plug 10 as shown in FIG. Since the steel pipe 11 according to the present invention disappears, the inner peripheral surface 12 can be effectively prevented from being bent or swollen.

Sectional drawing in the drawing direction L of the slab showing the macrostructure of the slab in the drawn strand and sectional view in the thickness direction T of the slab. (A) is according to the present invention, (b) is the whole by multistage pressing. (C) shows the result of non-reduction of the current general operation. (A) is a schematic cross-sectional view of the pipe material at the time of drilling the Assel, (b) is a view showing the inner surface of the steel pipe, (c) is a cross-sectional view taken along line AA of (b).

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Cast slab 2 Solid phase 3 Liquid phase 4 Center segregation 5 Indentation part 6 Non-indentation part 7 Linear segregation 8 Negative segregation 9 Tubing material 10 Plug 11 Steel pipe 12 Inner peripheral surface 13 Heel 14 swell

Claims (4)

  In a drawn strand having multi-stage light reduction equipment in continuous casting of steel, each stage of the reduction roll in which the central solid phase ratio fs of the bloom is in the range of 0.2 to 0.6 is 1.0 to 1.5 mm / stage. By suppressing the linear segregation in the cross section perpendicular to the pulling direction of the bloom excluding the central part by applying the Bloom reduction, and further opening the reduction roll in the end-solidification zone following the above-mentioned reduction roll A continuous casting method of a pipe material suitable for the production of seamless steel pipes by a tilt rolling method, characterized by causing strong segregation only at the center of bloom at the end of solidification.   In Bloom reduction where each stage of the reduction roll in the range of 0.2 to 0.6 in the center solid fraction fs of the bloom is 1.0 to 1.5 mm / stage, the center solid fraction fs of the bloom is 0. The continuous casting method of a pipe material suitable for manufacturing a seamless steel pipe by the inclined rolling method according to claim 1, wherein the range of 2 to 0.6 is reduced within a range of 1.0 to 10.0 mm.   3. The inclined rolling according to claim 1, wherein the section in a direction perpendicular to the drawing direction of the bloom has a ratio W / L of the length L to the width W of 1.0 to 0.5. This is a continuous casting method for pipe materials suitable for the production of seamless steel pipes.   The strong segregation generated only in the center of the bloom has a carbon segregation degree of 1.2 or more, and is suitable for the production of seamless steel pipes by the inclined rolling method according to any one of claims 1 to 3. Continuous casting method for pipes.

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