JPH06172854A - Production of seamless steel tube having fine grain structure - Google Patents

Abstract

PURPOSE:To obtain a steel tube as rolled having a fine grain structure of over an austenitic grain size No 8 by cooling after a hollow steel tube stock containing the prescribed quantity of Nb is worked under the prescribed condition with an inclined rolling mill and forming after successively worked with a final inclined rolling mill. CONSTITUTION:A hollow steel tube stock, which holds high temp. heat produced by piercing and rough rolling a billet containing 0.01-0.1 Nb heated to high temp., is rolled with two or more inclined rolling mills arranged in row. By the inclined rolling mill of an elongater, the hollow steel stock is subjected to cooling to 1000-1100 deg.C before the inclined rolling mill in front of the final rolling mill and working at 20-70% reduction rate of area with the next inclined rolling mill located immediately after thereof, it is further made to a fine grain over No.6 to lower the recrystallization temp. The steel tube is cooled to 900-1000 deg.C, successively, subjected to working at 20-70% reduction rate of area with the final rolling mill and then to form rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a seamless steel pipe having an austenite grain size number of 8 or more as it is rolled.

[0002]

2. Description of the Related Art To make austenite grains finer,
It has the effect of improving the material such as high strength, toughness and sour resistance.
This is an important subject in the manufacturing method of seamless steel pipes used for machine structural steel pipes. Conventionally, in order to obtain a fine-grained structure of No. 8 or more, it was manufactured by cooling after rolling and then heating again in the austenite region. However, from the viewpoint of process saving and energy saving, it is desired to obtain fine-grained steel equivalent to reheating in the as-rolled state. For example, as shown in JP-A-3-64415, Ti and Nb are appropriately added. Although attempts have been made to reduce the grain size, only as-coarse-grain steel with a grain size number of about 6 was obtained in the as-rolled state (such as direct quenching). That is, it was difficult to obtain fine-grained steel as it was rolled.

[0003]

SUMMARY OF THE INVENTION In view of the above-mentioned conventional problems, the present invention aims to provide a manufacturing method for obtaining a seamless steel pipe having a fine grain structure of No. 8 or more as rolled. And

[0004]

In order to achieve the above object, the present inventors have studied in detail the relationship between rolling conditions and grain size in various rolling mills in the production of seamless steel pipes. As a result, the inclusion of Nb suppresses the growth of the crystal grains of the steel running between rolling mills by retaining high temperature heat, and the inclined rolling mills (elongator rolling mills, etc.) in the production of seamless steel pipes.
It was found that when a reduction in cross-sectional area reduction of 20% or more is applied, a seamless steel pipe having a fine grain structure of No. 8 or more can be obtained as rolled.

The present invention is constructed by combining the above findings, and its gist is to manufacture a steel slab heated to a high temperature and containing 0.01 to 0.1% of Nb by piercing and rough rolling. The hollow raw steel pipes that retain the high temperature heat are cooled to a temperature of 1000 to 1100 ° C. in front of the inclined rolling mill before the final inclined rolling mill in the inclined rolling process in which two or more units are continuously installed. The reduction rate of cross-sectional area is 2 with the inclined rolling mill located immediately after.
After processing 0 to 70%, it is cooled to 900 to 1000 ° C., and then the cross-sectional area reduction rate is 20 to 70% in the final tilt rolling mill.
Is a method for producing a seamless steel pipe having a fine-grained structure which is shaped after being processed.

The present invention will be described in detail below. A steel slab containing 0.01 to 0.1% of Nb heated to a high temperature is subjected to piercing / rough rolling in a seamless steel pipe manufacturing process to manufacture a hollow raw steel pipe having high temperature heat. Nb in steel
The component is added as an effective component for suppressing the growth of crystal grains that occurs between rolling mills for producing seamless steel pipes,
If the content is less than 0.01%, the effect of suppressing grain growth at a high temperature of 1000 ° C. or higher is insufficient, and the content is 0.1%.
Even if added in excess, the effect is in the saturation region.

The hollow raw steel pipe produced in this manner is
Rolling is performed continuously by two or more inclined rolling mills. The inclined rolling mill is a rolling mill (elongator rolling mill) in which the axes of two or more rolls are tilted with respect to the material axis, and other rolling mills used for seamless steel pipes (mandrel rolling mill, plug rolling mill). Machines) and steel plate rolling machines have very large shear strain components. FIG. 1 shows the same conditions as in the case where a hollow raw steel pipe containing 0.03% of Nb was heated to a high temperature and then rolled from that temperature to each temperature by using an inclined rolling mill. Is a comparison of austenite grain size numbers in the case of manufacturing with a normal plate rolling machine. Figure 1
As is clear from the above, the inclined rolling mill can obtain a fine grain structure of about No. 9, and has a fine grain structure of about No. 2 as compared with the plate rolling mill.

[0008] As described above, forming a hollow raw steel pipe into a product shape by means of an inclined rolling mill is a constituent requirement necessary for achieving the object of the present invention. In addition, if two or more inclined rolling mills (base) are installed, the recrystallization temperature of the subsequent rolling mill can be expanded to the low temperature side by performing the grain refinement in the preceding rolling mill, and low temperature rolling can be performed. A steel pipe exhibiting a grain structure is easily manufactured. For that purpose, the high temperature hollow raw steel pipes to be rolled are placed in front of the inclined mill before the final inclined mill.
It is necessary to cool to 0 to 1100 ° C., and then perform processing at that temperature with an inclined rolling mill located immediately after that at a cross-sectional area reduction rate of 20 to 70%. These rolling conditions are prerequisites for obtaining a fine-grained microstructured steel pipe having a grain size of 8 or more, that is, conditions for producing a hollow raw steel pipe of 6 or more. That is, at a low temperature of less than 1000 ° C. before the final rolling mill and at a temperature of more than 1100 ° C., coarse grains are formed, and in the subsequent rolling, a non-recrystallized structure is exhibited and each characteristic value required for the steel pipe. Because it becomes difficult to obtain. Also, 1
In order to recrystallize by cooling to 000 to 1100 ° C. and rolling from that temperature, it is necessary to process at a cross-sectional area reduction rate of 20% or more. On the other hand, if the work is excessively processed to exceed 70%, the heat generated by the work will increase and the rolling temperature will rise, making it difficult to manufacture a steel pipe having the required characteristic values.

The hollow raw steel pipe manufactured through such a rolling process is further cooled to 900 to 1000 ° C., and subsequently subjected to final tilt rolling with a cross-sectional area reduction rate of 20 to 70% at that temperature to obtain fine grains. A Nb-containing steel pipe having a chemical structure is produced. As shown in FIG. 2 showing the relationship between the rolling temperature and the austenite (γ) grain size, the rolling in this case results in a non-recrystallized structure at a temperature lower than 900 ° C., and conversely at a temperature higher than 1000 ° C. Tend to do. That is, the Nb-containing steel pipe is a solid solution of Nb at a rolling temperature of 900 to 1000 ° C.
In the rolling process, it precipitates and suppresses grain boundary migration to form a fine-grained structure, but a steel pipe containing no Nb causes grain growth immediately after rolling and has a coarse-grained structure. Further, the reduction rate of the cross-sectional area during rolling also needs to be controlled at 20 to 70% for the reason described above.

The steel pipe thus obtained is usually formed by a rolling mill having a small rolling ratio to adjust its shape and size, and then cooled. When quenching and accelerated cooling are performed, tempering may be performed as necessary to adjust the strength and reduce residual strain. The steel pipe manufactured by the method of the present invention as described above has a fine grain structure.

[0011]

EXAMPLES Using the steel pieces having the chemical compositions of the present invention example and the comparative example shown in Table 1, a cooling device was installed immediately before each of the two elongator rolling mills continuously arranged in the seamless steel pipe factory. Installed and confirmed the effect. The manufacturing process starts with 12 pieces of steel.
The hollow raw steel tube obtained by heating at 50 ° C. and punching was cooled to a predetermined temperature by a cooling device installed for testing, and then rolled by a first elongator rolling machine. Since the hollow raw steel pipe after rolling has a high temperature of 1000 ° C. or higher due to heat generated by processing,
After cooling to a predetermined temperature with a cooling device installed for testing, the product was rolled with a second elongator rolling mill and then cooled through a plug mill, reeler mill and sizer mill, and the austenite grain size of the manufactured steel pipe was investigated. Table 2 shows the production conditions and the austenite grain size. For those in which the austenite grains could not be observed due to the air cooling, the grain size was estimated from the ferrite grain size by using the conversion ratio of the austenite grains and the ferrite grains, which was separately obtained.

From the results shown in Table 2, the steel pipe manufactured by the method of the present invention has a fine grain structure of No. 9 or more, whereas the comparative method deviating from the present invention does not obtain the fine grain structure.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

As described above, according to the present invention, by specifying the rolling conditions, it is possible to form a fine grain structure having a grain size of 8 or more in the as-rolled state, and to obtain a seamless steel pipe having excellent characteristics. Obtainable.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of rolling processing on austenite grain size number when Nb-containing steel is subjected to tilt rolling and normal plate rolling.

FIG. 2 is a diagram showing a relationship between rolling temperature and austenite grain size of Nb-containing steel and Nb-free steel.

Claims (1)

[Claims] 1. Nb heated to a high temperature is 0.01 to.
Two or more hollow steel pipes with high temperature heat produced by piercing and roughly rolling a steel slab containing 0.1% are installed in succession. The inclination before the final inclination rolling mill in the inclination rolling process. It is cooled to a temperature of 1000 to 1100 ° C. in front of the rolling mill, and 20 to 70% of the cross-sectional area reduction rate is processed by an inclined rolling mill located immediately after this, followed by cooling to 900 to 1000 ° C., followed by a final inclination A method for producing a seamless steel pipe having a fine-grained structure, which comprises performing processing by a rolling mill at a cross-sectional area reduction rate of 20 to 70% and then shaping.