JPH0718328A - Manufacture of seamless steel tube having grain refining organization - Google Patents

Abstract

PURPOSE:To provide a manufacturing method of a seamless steel tube having grain fining organization. CONSTITUTION:After a square steel billet is heated to a high temperature of 900 to 1050 deg.C, the billet is rolled with a press rolling mill into a round steel billet. A hollow tube stock which is pierce rolled with a successive front stage inclined rolling/mill is forcively cooled from inside surface or both of inside and outside surfaces. A uniforming treatment from Ar3 point to 1000 deg.C, and inclined rolling of 20 to 70% are applied and continuously hot rolling is performed to corrected the shape. Thus, a seamless steel tube having grain refining organization is manufactured.

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 a fine grain structure.

[0002]

2. Description of the Related Art In order to obtain a seamless steel pipe having a fine grain structure with a hot-rolled seamless steel pipe, for example, JP-A-52-77813 is used.
As described in Japanese Patent Laid-Open Publication No. H11-096, the hollow shell that has been hot-rolled and rough-rolled is forcibly cooled once to a temperature of Ar ₁ point or less and then heated again to the austenitizing temperature, and then immediately quenched after finishing rolling is completed. Then, there is a method of tempering after that, or a method of quenching and tempering after completion of usual finish rolling.

[0003]

However, in any of the above-mentioned methods, there is a problem in that the manufacturing process is complicated in addition to the problem of thermal efficiency caused by performing heat treatment a plurality of times. On the other hand, in the direct quenching process after the hot seamless rolling so far, the austenite grain size is A
STM No. There was a problem that a low alloy seamless steel pipe with a fine grain structure having characteristics such as SSC (stress corrosion cracking) resistance and low temperature toughness could not be obtained because the grain size was 3 to 6 and the variation was large.

[0004]

Means for Solving the Problems The inventors of the present invention conducted a number of experiments for the purpose of producing a seamless steel pipe having a fine-grained structure, and as a result, examined the results of fine-graining by controlling hot rolling conditions. It was discovered that a seamless steel pipe with a structure is manufactured.

The present invention is constructed on the basis of this finding. After heating a steel slab to a temperature of 900 to 1050 ° C.,
A round steel piece of a predetermined size is rolled by a press roll rolling machine and then pierced and rolled by an inclined rolling machine in the previous stage. Forced cooling is performed from the inner surface of the raw pipe in front of the inclined rolling mill, or gentle cooling is performed from the outer surface of the raw pipe in addition to the forced cooling from the inner face of the raw pipe, and the entire raw pipe is Ar _{3 to} 100 points.
After performing a homogenizing treatment at a temperature between 0 ° C., a 20 to 70% inclined rolling is performed at a wall thickness reduction rate, and a shape-correcting hot continuous rolling is then performed. Is a method for manufacturing seamless steel pipes.

[0006]

The reason for limiting the seamless rolling conditions of the present invention as described above will be described below. Molten in a melting furnace such as a converter or an electric furnace, or after further vacuum degassing treatment,
A steel piece having an arbitrary shape such as a square shape or a round shape is manufactured by the continuous casting method or the ingot-agglomeration method. The steel piece thus obtained is immediately or once cooled and then heated to a high temperature for hot rolling. The heating temperature is an important means depending on how the rolling temperature in the final stage inclined rolling mill can be expanded to a low temperature region. That is, the lower the heating temperature is, the finer grain γ structure is used for rolling, and as a result, the γ grain size after the final stage tilt rolling can be made finer.

FIG. 1 shows C: 0.21, Si: 0.21, M.
n: 1.02, P: 0.019, S: 0.004, A
1: 0.025, Nb: 0.015, Ti: 0.02
8, B: 0.0010, N: 0.0040 (wt%) shows the relationship between the heating temperature and the γ grain size. Table 1 shows the γ grain size and the recrystallizable temperature. Can be rolled

[Table 1] Indicates temperature. When the heating temperature of the steel slab is set to 1050 ° C. or less, the γ grain size of about 7 can be achieved, but at a higher temperature than that, rapid coarsening occurs. Further, by making the γ grain size finer from No. 4 to No. 7 at present, the rollable temperature in the inclined rolling mill can be lowered by 50 ° C. However, in order to perform hot piercing and rolling immediately after rolling the square billet into the round billet, the heating temperature must be sufficiently high, but if the temperature is 900 ° C. or higher, the hot rolling of the square billet into the round billet is performed. The lower limit temperature is 90 because there is no hindrance in processing and hot drilling.
The temperature was 0 ° C., and the upper limit temperature was 1050 ° C. for the above reason.

The steel slab heated to a temperature of 900 to 1050 ° C. is rolled into a round steel slab of a predetermined size corresponding to the steel pipe of the size required by the press roll rolling machine, and then the inclined rolling machine of the preceding stage. Perform piercing and rolling. The steel billet and the hollow shell after rolling are heated to a high temperature of 1000 ° C or higher due to the heat generated by rolling, and if the final stage of the inclined rolling mill is used in this state, the γ grain size after rolling becomes coarse and has large variations. . Therefore, in order to obtain the required γ grain size before the final stage of the inclined rolling mill, it is necessary to reduce the temperature of the raw pipe and at the same time make the entire raw pipe a uniform temperature. FIG. 2 shows the γ grain size in the thickness direction of the raw pipe and the rolling temperature after the final stage tilt rolling mill was performed. The temperature range within the tube is 100 ° C or less, preferably 50
If the temperature is controlled to within ℃, there will be no difference in γ particle size in the direction of wall thickness of the raw tube, and a sized γ state will be obtained. The particle size of the sized γ varies depending on the temperature level, and becomes finer at lower temperatures. In view of the use in severity, especially SSC resistance and cryogenic today oil well development environment, gamma grain size number 8 or more is required to tilt rolling mill before the temperature of the final stage Ar ₃ point to 1000 ° C. From this point You need to be in between. Further, in the forced cooling, it is impossible to make the temperature uniform in the raw pipe unless the inner surface side, which has a higher temperature than the outer surface side, is forcibly cooled. The cooling may be water alone, mist, or compressed air.

After the temperature in the tube is made uniform, stretch rolling is carried out by a final-stage tilt rolling machine. In tilt rolling, recrystallization mostly occurs dynamically, so that the grain size does not depend on the processing amount. However, the critical strain for recrystallization needs to be exceeded.
The lower limit of the rolling reduction was set to 20% in consideration of the fact that recrystallization statically occurs even after the rolling is completed. On the other hand, if the reduction ratio is too large, rolling becomes difficult and the formability and surface quality of the pipe deteriorate, so the upper limit was made 70%. After the completion of rolling, the element pipe refined by the final stage of tilt rolling is subjected to shape-correction continuous rolling. Although the rolling temperature is not particularly limited, it is desirable that it is not lower than the austenite range temperature from the viewpoint of formability. There is no variation in γ grain size obtained under the above production conditions, and it is effective for producing a seamless steel pipe having a fine-grained structure.

[0010]

EXAMPLES Next, examples of the present invention will be described. Table 2
Steel composition produced by melting in a converter and continuous casting: 0.
2C-0.2Si-1.0Mn-0.2Mo-0.01
5Ti-0.015Nb-0.0010B-0.004
The γ grain size of a steel pipe obtained by hot-seam rolling a steel slab of 0 N (wt%) and the balance substantially Fe is shown. At this time, the heating temperature was set to 1000 ° C., and the temperature range of the raw material pipe before the final stage inclined rolling mill was set to 1000 to 1100 ° C.

[0011]

[Table 2]

It can be seen that the steel pipe produced according to the present invention has a fine γ grain size and a controlled grain size as compared with the comparative material.

[0013]

The steel pipe produced by the method of the present invention as described above is fine-grained and thus has excellent sour resistance and is used in cold regions in the far north.

[Brief description of drawings]

FIG. 1 shows the relationship between heating temperature and γ particle size.

FIG. 2 shows the γ grain size in the thickness direction of the raw pipe after the final stage of the inclined rolling mill.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Yoshikazu Kikuchi Inventor, 1-1 Hibahata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corp. Yawata Works

Claims (2)

[Claims] 1. A steel slab is heated to a temperature of 900 to 1050 ° C., then rolled into a round steel slab of a predetermined size by a press roll rolling machine, and subsequently pierced and rolled by an inclined rolling machine in the preceding stage. The hollow shell that has reached a temperature of 1000 ° C. or higher due to the heat generated by processing during rolling is forcibly cooled from the inner surface of the shell before the final stage of the inclined rolling mill, and the entire shell is set to Ar _{3 to} 1000 points.
After performing homogenization treatment at a temperature between ℃, 20 ~ 70% of the thickness reduction cross-sectional inclination rolling is carried out, and then shape-correcting hot continuous rolling is carried out. Seamless steel pipe manufacturing method. 2. A steel slab is heated to a temperature of 900 to 1050 ° C., rolled into a round steel slab by a press roll rolling machine, and subsequently pierced and rolled by an inclined mill in the preceding stage. The hollow shell that has reached a temperature of 1000 ° C. or higher is subjected to forced cooling from the inner surface of the shell in front of the final stage of the inclined rolling mill and further to slow cooling from the outer surface of the shell, so that the entire shell is Ar ₃ to 1 points.
After performing a homogenizing treatment at a temperature between 000 ° C., an inclined rolling of 20 to 70% is performed at a wall thickness reduction rate, and then a shape-correcting hot continuous rolling is performed, which is a fine-grained structure. Seamless steel pipe manufacturing method.