JP2001059137A - High carbon slab for seamless steel pipe and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of internal defects without causing the reduction of the productivity even in the case a seamless steel pipe is produced with a high carbon slab as the stock. SOLUTION: This high carbon slab for a seamless steel pipe contains, by weight, 0.65 to 1.50% C and <=0.050% P and satisfies the conditions represented by the following inequality: [C]+([P]×100)-FT/10<=1.6, where [C]: the C content (%), [P]: the P content (%), and FT: the time (Hr) for soaking the slab at >=1100 deg.C before the production of the seamless steel pipe. As to the method for producing a high carbon slab for a seamless steel pipe, at the time of rolling a slab of high carbon contg. 0.65 to 1.50% C and <=0.050% P and obtd. by a continuous casting method or an ingot making method and producing a slab for a seamless steel pipe, the conditions shown in the above formula are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel slab for use in the production of a seamless carbon steel pipe (seamless steel pipe) of high carbon steel containing 0.65 to 1.50% by weight of C, and more particularly to a method of producing the same. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon steel slab in which the structure of a steel slab as a raw material is adjusted in advance before the pipe is manufactured so that no inner surface flaw is generated when the steel pipe is manufactured, and a method for manufacturing the same.

[0002]

2. Description of the Related Art As a mechanical steel pipe or the like that requires wear resistance, a so-called high carbon steel seamless steel pipe containing C at about 1.00% by weight is widely used. Normally, a seamless steel pipe is manufactured from a round steel piece by manufacturing a hollow shell by mannesmann drilling, press drilling, etc.
The outer diameter is reduced with a reducing mill such as a stretch reducer,
Manufactured by finishing into steel pipes.

[0003] In the case of manufacturing the above-mentioned seamless steel pipe of high carbon steel, a round steel slab obtained by rolling a slab manufactured by continuous casting or ingot ingot casting method is used as a raw material for pipe making. At this time, the steel slab used as the material is generally manufactured by continuous casting or ingot ingot ingot, has a rectangular cross-section cast (bloom), and is heated to a uniform temperature. It is manufactured by a method of hot rolling into a round shape by a blooming mill or the like, or a method of directly casting into a round slab by continuous casting.

[0004] In the hot pipe making of a seamless steel pipe, since the high-carbon steel has a low melting temperature, the central portion and the vicinity of the steel slab are melted by the heat generated during processing or overheating.
Internal defects may occur in the steel pipe after pipe production. For example, when an inner surface defect such as a middle flaw (hereinafter referred to as an inner surface flaw) occurs in a steel pipe, not only does the yield of the product decrease, but also the production of a piercing rolling mill, a rolling mill, and a rolling mill. The entire tube mill may have to be shut down, in which case production efficiency will be severely impaired.

Conventionally, as a measure to prevent the occurrence of inner surface flaws in a hot steel pipe made of a seamless steel pipe, the heating temperature of the raw material is reduced in order to reduce the degree of processing at the time of pipe manufacturing and to avoid melting due to processing heat. For example, measures such as lowering the pressure are adopted. However, these measures are all based on the premise that the productivity of the hot pipe is reduced, and cannot be said to be appropriate preventive measures.

[0006] Further, in Japanese Patent Application Laid-Open No. 3-99708, when a seamless steel pipe is manufactured by using a hard-to-work material such as high alloy steel, cracks are generated on the inner surface due to high deformation resistance. A method for manufacturing a seamless steel pipe that avoids the above has been proposed. More specifically, the inner surface of the material is cooled with cooling water while the material is being drilled, so that an appropriate rolling temperature is controlled. However, in the proposed manufacturing method, since the cooling means is provided to the perforated plug in terms of equipment, an excessive capital investment is required, and in actual operation, the pipe production conditions are always fluctuated. There is a problem that proper temperature control cannot be achieved. For this reason, the proposed method of manufacturing a seamless steel pipe cannot be an effective manufacturing method when hot-forming high carbon steel slabs.

[0007]

As described above, the conventional measures for preventing inner surface flaws used in the production of seamless steel pipes of difficult-to-machine materials such as high carbon steel are to reduce the degree of work and lower the heating temperature. Since it is a means, it is essentially a hindrance to efficient production. Further, even the method of cooling the inner surface of the material during drilling and controlling the temperature to an appropriate rolling temperature has a serious problem in equipment and operation and cannot be an effective preventive measure.

[0008] The present invention has been made in view of the above problems, and when producing a seamless steel pipe using a high carbon steel slab as a material, the generation of internal flaws without reducing the productivity. It is an object of the present invention to provide a high carbon steel slab for a seamless steel pipe and a method for producing the same, which can be prevented.

[0009]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventor has conducted various studies on the prevention of melting of high carbon steel slab during hot pipe making and found that the content of the specified component was reduced. In addition to stipulating, it was clarified that it is effective to adjust the metallurgical structure of the steel slab before forming the seamless steel pipe.

When examining additional components effective for preventing the melting of the inside of the steel slab during hot pipe making, C is the most influential element. However, the C content is often determined uniquely with respect to the performance required of the product, and it is often difficult to appropriately change the content in the present invention. On the other hand, the P element has a small effect on the product performance, but the P content has a large effect on the melting temperature of the steel.

Therefore, as a result of study, the content was specified by specifying the elements C and P, and the soaking time in the slab or slab stage before the production of the seamless steel pipe was adjusted to a predetermined condition. It has been found that a steel slab in which internal surface flaws are less likely to occur even in the case of a high-carbon steel hot-worked tube can be obtained.

The present invention has been completed on the basis of the above findings, and has the following (1) high carbon steel slab for seamless steel pipes and (2) method for producing high carbon steel slabs for seamless steel pipes The main point is.

(1) A high-carbon steel slab for a seamless steel pipe characterized by containing, by weight%, 0.65 to 1.50% of C and 0.050% or less of P and satisfying the following condition (a): It is.

[C] + ([P] × 100) −FT / 10 ≦ 1.6 (a) where [C]: C content (%), [P]: P content (%) FT : Before casting a seamless steel pipe, add slab or slab to 110
Time to soak above 0 ° C (Hr) (2)% by weight obtained by continuous casting or ingot making
In producing a steel slab for a seamless steel pipe by rolling a high carbon slab containing 0.65 to 1.50% and P: 0.050% or less, it is necessary to satisfy the condition represented by the above equation (a). This is a method for producing a high carbon steel slab for a seamless steel pipe.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION The high carbon steel slab for a seamless steel pipe according to the present invention and the method for producing the same will be described in detail by dividing them into chemical compositions and production conditions as described above.

1. Chemical composition C: 0.65 to 1.50% C is added to enhance hardenability, improve strength, and secure abrasion resistance. However, if the content is less than 0.65%, it is not possible to secure a predetermined strength to be secured by using high carbon steel. On the other hand, C
Excessive addition deteriorates the brittleness, so the upper limit is
1.50%.

P: 0.050% or less P is inevitably present in steel as an impurity element, and the lower the content, the better. If the content exceeds 0.050%, the toughness of the high-strength material is deteriorated. Therefore, the upper limit is set to 0.050%. In particular, P has the property of lowering the melting point of steel, and is likely to lead to the generation of internal flaws. Therefore, the content of P is desirably 0.012% or less.

In general, P tends to cause segregation in steel, and the tendency is remarkable in high carbon steel. For this reason, the action of lowering the melting point of steel is also easily affected by segregation, and even if a small amount of P is added, due to the uneven distribution of P, processing heat or the like may be generated to cause internal melting. Therefore, it is effective in terms of quality that P is suppressed as much as an unavoidable impurity. However, excessive reduction of P requires a difficult process in the melting process, and raises the melting cost. Therefore, there is a certain limitation in reducing the P content.

The high-carbon steel slab of the present invention is defined as described above by limiting to the C and P components, but the other components are usually within the range of components permitted by the carbon steel. For example, the following chemical composition is exemplified.

That is, Si: 0.010 to 1.00%, Mn: 0.20
~ 2.0%, S: 0.050% or less, Cr: 1.80% or less, Mo: 1.00
%, Cu: 1.00% or less, Ni: 1.00% or less, Ti: 0.050
% Or less, Al: 0.050% or less, N: 0.015% or less, B: 0.00
50% or less, Nb: 0.050% or less, V: 0.20% or less and C
a: 0.0050% or less.

[0021] Further, one or two or more of the components of Co: 0.20% or less, Mg: 0.02% or less, Zr: 0.02% or less, Sb: 0.02% or less, Sn: 0.02% or less, and Pb: 0.02% or less.
More than one species can be included.

2. Manufacturing Conditions The high-carbon steel slab of the present invention is designed to reduce the influence of P segregation due to the soaking heat treatment, assuming the C content and P content specified above, before casting a seamless steel pipe, Or a billet
It is necessary that the soaking time at 1100 ° C. or more satisfies the condition shown by the following equation (a).

[C] + ([P] × 100) −FT / 10 ≦ 1.6 (a) where [C]: C content (%), [P]: P content (%) FT : Before casting a seamless steel pipe, add slab or slab to 110
Time for soaking at 0 ° C. or higher (Hr) As described above, C and P are effective as additional components for lowering the melting point of a billet, but the C content is uniquely determined by product performance. In many cases. On the other hand, P is an unavoidable impurity and tends to cause segregation in the steel. Therefore, the addition of P should be reduced as much as possible.

Therefore, in the present invention, the P content is reduced as much as technically possible, and at the same time, the P segregation in the steel is reduced, and the localized high concentration of P is not localized. . In the present invention, by combining the reduction of the P content and the diffusion of the P segregation, it is possible to uniformly reduce the P contained in the steel and to prevent a decrease in the melting point. Become.

The above technical idea of the present invention is quantified by the above equation (a). In order to determine the diffusion effect of P segregation, the soaking time (FTHr) at the slab or slab stage is determined. Has been introduced. By manufacturing a steel slab to be used for the production of a seamless steel pipe while satisfying the condition represented by the equation (a), it is possible to surely suppress internal surface flaws generated by internal melting.

FIG. 1, which will be described later, is a diagram in which the relationship between the above equation (a) and the incidence rate of the inner surface flaw of the steel pipe is arranged based on the results of the embodiment. Have a big impact,
It can be seen that as long as the relationship of the expression (a) is satisfied, it is effective in preventing the occurrence of internal flaws.

In the above equation (a), the heating temperature is set to 1100 ° C. or higher in order to secure a temperature sufficient for diffusing the segregated P component. In general, when held at high temperatures,
The diffusion effect is expected to increase, but when heated above 1280 ° C, the slab melts and subsequent slab rolling,
It is desirable to heat in the range of 1100 to 1280 ° C., since adverse effects may occur during the production of the seamless steel pipe.

The heating targeted for the soaking time, that is, FT (Hr), includes not only the heating of the cast slab, but also the heating of the steel slab before the production of the seamless steel pipe. This is because each of them acts on the diffusion of the segregated P component. Therefore, it does not include the reheating time after elongation rolling of a mandrel mill or the like. During the soaking time, it is not always necessary to keep the temperature constant, and if it is 1100 ° C. or more, the temperature may fluctuate in the temperature.

The high carbon steel slab produced while satisfying the condition shown in the above equation (a) is sent to a pipe making process to be used as a material for a seamless steel pipe. Even when the pipe production is performed by a usual method, it is possible to obtain a product in which internal melting is suppressed and the generation of internal surface flaws is extremely small. Hereinafter, the effects of the present invention will be specifically described with reference to examples.

[0030]

EXAMPLES In the examples, samples of 13 steel types shown in Table 1 were melted, steel pieces were manufactured using these as materials for seamless steel pipes, and the occurrence of inner surface flaws in seamless steel pipes manufactured therefrom was examined. investigated. At this time, the production process of the slab, the slab, and the steel pipe was changed from A to E described below, and the soaking time (F
THr) was measured, and the occurrence rates under each manufacturing condition were compared.

[0031]

[Table 1]

(Process A) 410 m made by continuous casting
A slab having a thickness of 530 mm and a width of 530 mm was subjected to lump heating at 1220 ° C. for 6 hours, followed by finish rolling to a 191 mmφ round steel slab. The soaking time at 1100 ° C. or more in the bulk heating was 5 hr.

The obtained round steel slab was sent to a seamless steel pipe forming process, and a steel pipe having an outer diameter of 60 mmφ and a wall thickness of 8 mm was manufactured by the Mannesmann method. At this time, before drilling, the steel slab was 1200 ° C x 3
After maintaining the heating of Hr, the soaking time of 1100 ° C. or more was set to 1 Hr, and then perforation was performed.

After the above-mentioned perforation step, the diameter was reduced by a sizing mill, elongation-rolled by a mandrel mill, re-heated, and squeezed by a stretch laser to finish to a predetermined outer diameter and thickness.

(Process B) 410 m made by continuous casting
A slab having a thickness of 530 mm and a width of 530 mm was subjected to bulk heating at 1220 ° C. × 11 hours and then finish rolled into a 191 mmφ round steel slab. The soaking time at 1100 ° C or more in the lump heating was 10 Hr.

The obtained round steel slab is sent to a seamless steel pipe forming process, and the outer diameter is 60 mm in the same Mannesmann process as in process A.
A steel pipe of φ × wall thickness 8 mm was manufactured.

(Process C) 410 m made by continuous casting
A slab having a thickness of 530 mm and a width of 530 mm was subjected to bulk heating at 1220 ° C. × 18 hours and then rolled into a 225 mmφ round steel slab. The soaking time at 1100 ° C or more in the bulk heating was 17Hr.

The obtained round steel slab is sent to a seamless steel pipe forming process, and the outer diameter is 160 mmφ and the wall thickness is 1 by the Mannesmann method.
A 0 mm steel pipe was manufactured. At this time, the piercing step, the elongation rolling step, and the drawing rolling step after reheating were the same as those in the process A.

(Process D) A slab having a thickness of 650 mm and a width of 750 mm produced by the slab method was subjected to slab heating at 1220 ° C. × 12 hours and then finish rolled into a 191 mmφ round steel slab. The soaking time at 1100 ° C or more in the mass heating was 11Hr.

The obtained round steel slab was sent to a seamless steel pipe forming process, and a steel pipe having an outer diameter of 60 mmφ and a wall thickness of 8 mm was manufactured by the Mannesmann method. At this time, the piercing step, the elongation rolling step, and the drawing rolling step after reheating were the same as those in the process A.

(Process E) A 191 mmφ round cast piece was directly cast by a continuous casting method. The cast round slab was sent to a seamless steel pipe forming process without soaking heat treatment, and a steel pipe having an outer diameter of 60 mmφ and a wall thickness of 8 mm was manufactured by the Mannesmann method. At this time, the piercing step, the elongation rolling step, and the drawing rolling step after reheating were the same as those in the process A.

[0042] Samples of the 13 steel types shown in Table 1 above were used as test materials, and after finishing steel pipes of predetermined dimensions in combination with the above-mentioned processes A to E, the production conditions were arranged and the occurrence rate of internal flaws was reduced. The inspection was carried out by the ratio of the number of scratches generated to the total number of inspections. Table 2 shows the results.

[0043]

[Table 2]

FIG. 1 is a graph showing the relationship between the above equation (a) and the incidence rate of inner surface flaws of a steel pipe in the embodiment. As is clear from FIG. 1 and Table 2, the expression (a) has a large effect on the occurrence of inner surface flaws, and effectively prevents the inner surface flaws occurring in the seamless steel pipe as long as the relationship of the expression (a) is satisfied. be able to.

[0045]

According to the high carbon steel slab for a seamless steel pipe of the present invention and the method for producing the same, the content is specified by specifying the elements C and P, and the slab before the seamless steel pipe is produced. Alternatively, by ensuring the soaking time in the slab stage under predetermined conditions, it is possible to obtain a slab capable of suppressing the occurrence of inner surface flaws even in a high-carbon steel hot tube. As a result, even when a seamless steel pipe is manufactured using the difficult-to-work material as a raw material, the product yield can be improved without a decrease in productivity.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the above-mentioned equation (a) and the incidence rate of inner surface flaws of a steel pipe in Examples.

Claims (2)

[Claims] (1) C: 0.65 to 1.50% and P: 0.
A high-carbon steel slab for seamless steel pipes, containing 050% or less and satisfying the following condition (a). [C] + ([P] × 100) −FT / 10 ≦ 1.6 (a) [C]: C content (%), [P]: P content (%) FT: Seamless Before casting steel pipe, slab or slab
Time to soak above 0 ° C (Hr) 2. A steel for seamless steel pipe obtained by rolling a high-carbon slab containing C: 0.65 to 1.50% and P: 0.050% or less by weight obtained by a continuous casting method or an ingot-making method. A method for producing a high-carbon steel slab for a seamless steel pipe, which satisfies the condition represented by the following formula (a) when producing a slab. [C] + ([P] × 100) −FT / 10 ≦ 1.6 (a) [C]: C content (%), [P]: P content (%) FT: Seamless Before casting steel pipe, slab or slab
Time to soak above 0 ° C (Hr)