JP3059993B2 - Manufacturing method of low alloy seamless steel pipe with fine grain structure - Google Patents

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 low alloy seamless steel pipe having a fine grain structure.

[0002]

2. Description of the Related Art To obtain a low alloy seamless steel pipe having a grain refined structure from a hot-rolled seamless steel pipe, for example, Japanese Patent Application Laid-Open No. 52-77 / 1982
As described in Japanese Patent Publication No. 813, the hollow shell which has been hot-rolled and roughly rolled is forcibly once lowered the temperature of the steel to _one point or less of Ar, then heated again to the austenitizing temperature, and quenched and tempered immediately after finishing the subsequent finish rolling. Or quenching and tempering after normal finish rolling.

[0003]

However, any of the above-mentioned methods has a drawback that the manufacturing process is complicated in addition to the problem of thermal efficiency. On the other hand, in the conventional direct quenching treatment after hot seamless rolling, the austenite grain size is ASTM No. There was a problem that a low alloy seamless steel pipe having a grain refined structure could not be obtained because of coarse grains of 1 to 6 and large variations.

[0004]

Means for Solving the Problems The present inventors have developed a low alloy seamless steel pipe having a grain refined structure and a seamless steel pipe excellent in sulfide stress cracking resistance (hereinafter referred to as SSC resistance) or high toughness. As a result of conducting many experiments for the purpose of manufacturing a seamless steel pipe, it was found that a low-alloy seamless steel pipe having a refined grain structure can be manufactured by controlling the steel composition and hot rolling conditions.

The present invention has been made on the basis of this finding, and the gist of the invention is that C: 0.03-0.
35%, Si: 0.01 to 0.5%, Mn: 0.15
~ 2.5%, P: 0.020% or less, S: 0.
010% or less, Al: 0.005 to 0.1%, T
i: 0.005 to 0.1%, Nb: 0.005 to 0.
1%, N: 0.01% or less, Cr: 0.1-1.5%, Mo: 0.05-
0.5%, Ni: 0.1 to 2.0%, V: 0.
01-0.1%, B: 0.0003-0.0030%
Or one or more of the above, or if necessary, a rare earth element: 0.001 to 0.05%, Ca: 0.00
1-0.02%, Co: 0.05-0.5%, C
u: a steel slab containing 0.1 to 0.5% of one or more kinds and the balance substantially consisting of Fe is 1000 to 1250
° C and then subjected to hot piercing and rolling. The hollow shell, which was heated to 1000 ° C or higher due to the heat generated by the rolling, was forcibly cooled from the inner surface of the shell in front of the final inclined rolling mill, and the whole shell was Ar ₃ After performing a homogenization treatment at a temperature between the point and 1100 ° C., performing an inclined rolling at a reduction rate of 20 to 70% in thickness cross section, and further performing a shape correcting hot continuous rolling, and then performing an Ar ₃ point to 900 ° C
The hollow shell was lowered to a temperature higher than the temperature Ar ₃
After reheating to the point + 50 ° C. to 1000 ° C., the finishing temperature is Ar
Either hot finish rolling at ₃ points + 50 ° C. or more is performed, or when the raw tube is maintained at a sufficiently high temperature of Ar ₃ points + 50 ° C. or more, the finishing temperature is Ar ₃ points + 5 without reheating.
After hot finish rolling at 0 ° C. or higher, and then quenching treatment for quenching from a temperature of Ar ₃ points or higher, then Ac
_This is a method for manufacturing a low-alloy seamless steel pipe having a grain refined structure that performs a tempering process of heating and cooling to a temperature of _one point or less.

[0006]

The production method of the present invention will be described below in detail.
First, the reason why the present invention is limited to the above steel components will be described. C and Mn are important for increasing the quenching effect, increasing the strength, stably obtaining a high-tensile steel having a yield point of 40 to 80 kgf / mm ² , and reducing the grain size. If the amount is too small, the effect is not obtained, and if the amount is too large, quenching cracks are induced and the hardness is increased, and the SSC resistance is lowered. Therefore, the content is set to 0.03 to 0.35% and 0.15 to 2.5%, respectively. S
i is an effective component for increasing the strength with the deoxidizing agent remaining. When the amount is too small, the effect is not obtained. When the amount is too large, the number of inclusions is increased and the SSC resistance is reduced.
0.5%. P is a harmful component that easily causes cracks during processing due to grain boundary segregation, and its content is set to 0.020% or less because it causes deterioration of low-temperature toughness. S
Forms a layered structure by forming MnS-based inclusions and stretching by continuous hot rolling to improve the fracture propagation performance of steel. If the amount is too small, the effect is not obtained, and if the amount is too large, inclusions increase and the properties of the steel become brittle, so that the content is set to 0.01%. Al is Si
In the same manner as described above, the deoxidizing agent remains and combines with N contained as an impurity component in steel to suppress the growth of crystal grains and improve the fracture propagation performance of steel. If the amount is too small, the effect is not obtained. If the amount is too large, inclusions increase to embrittle the properties of the steel.
005 to 0.1%.

[0007] Both Ti and Nb are the most important elements among the components of the present invention as crystal grain size controlling elements during seamless rolling. Ti combines with N contained as an impurity component in steel to control the crystal grains during hot rolling and suppress the growth of crystal grains after hot rolling, improve the fracture propagation performance of the steel, deoxidize, Due to the action of denitrification, quenching properties of B described below are exhibited to increase the strength. If the amount is too small, the effect is not obtained. If the amount is too large, TiC is precipitated and the steel is embrittled.
It was set to 5 to 0.1%. On the other hand, Nb is an important element for suppressing the growth of crystal grains during inclined rolling and for suppressing the abnormal coarsening of γ grains when the pipe, whose temperature has been lowered after continuous rolling, is heated to an Ar ₃ point to 1000 ° C. higher than the temperature. It is. If the amount is too small, the effect is not obtained, and if the amount is too large, the effect is saturated and the cost is extremely high. N
Has a content of 0.01% or less as a harmful component that lowers the hardenability of B described later. In the case of further increasing the strength of the steel having the above composition, components such as Cr are selectively added as necessary.

[0008] Cr, Mo, Ni, V are added to increase the hardenability of steel and to increase the strength. If the amount is too small, the effect is not obtained. If the amount is too large, the effect saturates and is very expensive.
1.5%, 0.05-0.5%, 0.1-2.0%,
It was set to 0.01 to 0.1%. B remarkably improves the hardenability and increases the strength. If the amount is too small, the effect is not obtained, and if the amount is too large, the effect does not change, and the toughness and the hot workability are deteriorated.

Further, the present invention has been developed in view of the use environment of seamless steel pipes in recent years.
In order to improve the C property, a component such as a rare earth element is selectively added as needed. The rare earth element, Ca, is an effective component that makes the form of inclusions spherical and harmless. If the amount is too small, the effect is not obtained. If the amount is too large, the number of inclusions increases and the SSC resistance is lowered.
5%, 0.001 to 0.02%. Co and Cu are
It has the effect of suppressing hydrogen intrusion into steel and works effectively on SSC resistance. If the amount is too small, the effect is not obtained. If the amount is too large, the effect is saturated.
0.5%.

Next, the reason why the hot seamless rolling conditions are limited as described above will be described. The steel having the above composition is melted in a melting furnace such as a converter or an electric furnace or further subjected to a vacuum degassing process, and a steel slab is manufactured by a continuous casting method or an ingot lump method. The slab is heated immediately after or once cooled and then subjected to hot piercing and rolling. The heating temperature must be sufficiently high to facilitate hot piercing and rolling, and is preferably as low as possible from the viewpoint of obtaining a fine-grained austenite structure. 1 within the range of the component of the present invention.
At a temperature of 000 ° C. or higher, there is no problem in hot drilling, and from 1250 ° C. or higher, coarse particles are formed from the viewpoint of grain refinement.

The hollow shell subjected to hot piercing and rolling has a high temperature of 1000 ° C. or more due to a heat generation phenomenon due to processing, and the temperature of the shell is high and uneven on the inner surface. As shown in FIG.
The crystal grain size is mainly governed by the rolling temperature. Therefore, in order to obtain uniform and fine-grained austenite in the final-stage inclined rolling mill, it is necessary to lower the tube temperature immediately before rolling and at the same time to make the tube temperature uniform. In order to obtain a fine grain structure within the component range of the present invention, the temperature before the final stage of the tilt rolling mill is Ar ₃ point to 11 points.
It is necessary to raise the temperature to 00 ° C.
It is necessary to forcibly cool the inner surface side where the temperature is higher than the outer surface. The cooling may be water alone, mist, or compressed air.

[0012] In the inclined rolling mill, since recrystallization occurs mostly dynamically, the crystal grain size does not depend on the processing amount. But,
The critical strain for recrystallization must be exceeded. The lower limit of the rolling reduction was set to 20% in consideration of the fact that recrystallization occurs statically even after the end of rolling. On the other hand, if the rolling reduction is too large, rolling becomes difficult and the formability and surface quality of the pipe deteriorate, so the upper limit was set to 70%.

[0013] After the end of the rolling, the blank refined by the final-stage inclined rolling is subjected to continuous rolling for shape correction.
hollow shell was lowered to a temperature of r ₃ point to 900 ° C. is reheated to or the plain tube is Ar ₃ point + 50 ℃ above temperature high enough to secure a high Ar ₃ point + 50 ℃ to 1000 ° C. than the temperature If so, hot final rolling is performed without reheating. Reheating temperature must be at a high temperature to ensure the quenching temperature from the final finishing after austenite has been a too high and the oxide scale is causative for Ar ₃ point of many resulting scratches occur + 50 ℃ ~1000 ℃.

Immediately after continuous rolling for shape correction or after reheating, hot final finishing rolling is performed at a temperature of ₃ points of Ar + 50 ° C. or more. Rolling temperature was Ar ₃ point + 50 ℃ or it becomes difficult becomes too low secure shape. A quenching process is performed to rapidly cool from a completely austenite state after the final finish rolling. The quenching start temperature was set to _three points or more of Ar in order to secure a sufficient quenched structure and required strength, sour resistance and toughness. The cooling rate during quenching is not particularly limited, but is set to a rate faster than air cooling. After quenching
Perform tempering. The tempering temperature is set to ensure the stabilization of strength, sour resistance and toughness.
₁ point or less. The heating method is not particularly limited. There is no variation in the γ grain size obtained under the above manufacturing conditions, and this is effective for manufacturing a low alloy seamless steel pipe having a refined grain structure.

[0015]

Next, an embodiment of the present invention will be described. Table 1
Shows the γ-grain size, sour resistance, and toughness of a steel pipe produced by smelting in a converter and performing continuous casting and then directly quenching and tempering steel slabs by hot seamless rolling. The SSC resistance is NACE
Σth (Thresh) by the load method according to TM01-77
old Stress). It can be seen that the steel pipe manufactured according to the present invention has a fine γ grain size and can obtain sour resistance and high toughness.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

The steel pipe manufactured by the method of the present invention as described above is excellent in sour resistance because of its fine grain, and is suitable for use in cold regions in the far north.

[Brief description of the drawings]

FIG. 1 shows the influence of the γ-grain size and rolling temperature after a conventional steel plate rolling method and inclined rolling.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yoshikazu Kikuchi 1-1, Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Nippon Steel Corporation Yawata Works (56) References JP-A-1-278902 (JP, A) JP-A-3-64415 (JP, A) JP-A-7-18327 (JP, A) JP-A-6-88128 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) ) C21D 8 / 10,9 / 08 B21B 17/00-25/06

Claims (4)

(57) [Claims] C .: 0.03 to 0.35%, Si: 0.01 to 0.1% by weight.
5%, Mn: 0.15 to 2.5%, P: 0.020% or less, S: 0.010% or less, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, and the balance is substantially 1000%.
After heating to １２1250 ° C., hot piercing and rolling is performed, and the hollow shell, which has been heated to 1000 ° C. or more due to the heat generated by the rolling, is forcibly cooled from the inner surface of the hollow shell in front of the final stage of the inclined rolling mill, and the entire hollow shell is cooled. After performing a homogenization process at a temperature between Ar ₃ points and 1100 ° C., performing an inclined rolling at a reduction rate of 20 to 70% in a thickness section reduction rate, further performing a shape correcting hot continuous rolling, and then performing an Ar ₃ point After the hollow shell dropped to a temperature of about 900 ° C. is reheated to an Ar ₃ point higher than that temperature + 50 ° C. to 1000 ° C., hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C. or higher, or mother tube is subjected to hot finish rolling finishing temperature is above Ar ₃ point + 50 ℃ without if it is ensured Ar ₃ point + 50 ℃ more sufficiently high temperature reheat, then quenched from Ar ₃ point or more temperature after performing hardening processing of, followed by less than ₁ point Ac warm A method for producing a low-alloy seamless steel pipe having a fine-grained structure, comprising performing a tempering treatment of heating and cooling each time. 2. C: 0.03 to 0.35%, and Si: 0.01 to 0.
5%, Mn: 0.15 to 2.5%, P: 0.020% or less, S: 0.010% or less, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, Cr: 0.1-1.5%, Mo: 0.05-0.
5%, Ni: 0.1-2.0%, V: 0.01-0.
1%, B: A steel slab containing one or more of 0.0003 to 0.0030%, and the balance substantially consisting of Fe
After heating to 1250 ° C., hot piercing and rolling is performed, and the hollow shell, which has been heated to 1000 ° C. or more due to the heat generated by the rolling, is forcibly cooled from the inner surface of the hollow shell in front of the final inclined rolling mill. After performing a homogenization process at a temperature between Ar ₃ points and 1100 ° C., performing an inclined rolling at a reduction rate of 20 to 70% in a thickness section reduction rate, further performing a shape correcting hot continuous rolling, and then performing an Ar ₃ point After the hollow shell dropped to a temperature of about 900 ° C. is reheated to an Ar ₃ point higher than the temperature + 50 ° C. to 1000 ° C., hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C. or higher is performed, or mother tube is subjected to hot finish rolling finishing temperature is above Ar ₃ point + 50 ℃ without if it is ensured Ar ₃ point + 50 ℃ more sufficiently high temperature reheat, then quenched from Ar ₃ point or more temperature after performing hardening processing of, followed by less than ₁ point Ac warm A method for producing a low-alloy seamless steel pipe having a fine-grained structure, comprising performing a tempering treatment of heating and cooling each time. 3. As% by weight, C: 0.03 to 0.35%, Si: 0.01 to 0.
5%, Mn: 0.15 to 2.5%, P: 0.020% or less, S: 0.010% or less, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, Rare earth element: 0.001 to 0.05%, Ca: 0.00
1 to 0.02%, Co: 0.05 to 0.5%, Cu: 0.1 to 0.5
% Of a steel slab containing one or two or more kinds and the balance substantially consisting of Fe was heated to 1000 to 1250 ° C., and then subjected to hot piercing and rolling. The hollow shell is forcibly cooled from the inner surface of the shell in front of the final stage of the inclined rolling mill, and the entire shell is homogenized to a temperature between Ar ₃ point and 1100 ° C., and then, the wall thickness reduction rate is 20%. ~
After performing 70% inclined rolling and further performing shape-correcting hot continuous rolling, the hollow shell dropped to a temperature of Ar ₃ point to 900 ° C. is cooled to an Ar ₃ point higher than the temperature and 50 ° C. to 1000 ° C.
After reheating ° C., the finishing or the temperature is subjected to hot finish rolling or Ar ₃ point + 50 ° C., or the plain tube is Ar ₃ point + 50
If a sufficiently high temperature of at least 3 ° C. is secured, hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C. or more without reheating, and then a quenching process of rapidly cooling from a temperature of Ar ₃ or more is performed. And a tempering treatment of heating and cooling to a temperature of not more than _one point of Ac, followed by performing a tempering treatment of the low alloy seamless steel pipe having a grain refined structure. 4. C: 0.03 to 0.35%, and Si: 0.01 to 0.
5%, Mn: 0.15 to 2.5%, P: 0.020% or less, S: 0.010% or less, Al: 0.005 to
0.1%, Ti: 0.005 to 0.1%, Nb: 0.005 to
0.1%, N: 0.01% or less, Cr: 0.1-1.5%, Mo: 0.05-0.
5%, Ni: 0.1-2.0%, V: 0.01-0.
1%, B: one or more of 0.0003 to 0.0030%, rare earth element: 0.001 to 0.05%, Ca: 0.00
1 to 0.02%, Co: 0.05 to 0.5%, Cu: 0.1 to 0.5
% Of a steel slab containing one or two or more kinds and the balance substantially consisting of Fe was heated to 1000 to 1250 ° C., and then subjected to hot piercing and rolling. The hollow shell is forcibly cooled from the inner surface of the shell in front of the final stage of the inclined rolling mill, and the entire shell is subjected to a homogenization treatment at a temperature between Ar ₃ point and 1100 ° C., and then the reduction rate of the wall thickness is 20%. ~
After performing 70% inclined rolling and further performing shape-correcting hot continuous rolling, the hollow shell dropped to a temperature of Ar ₃ point to 900 ° C. is cooled to an Ar ₃ point higher than the temperature and 50 ° C. to 1000 ° C.
After reheating ° C., the finishing or the temperature is subjected to hot finish rolling or Ar ₃ point + 50 ° C., or the plain tube is Ar ₃ point + 50
If a sufficiently high temperature of at least 3 ° C. is secured, hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C. or more without reheating, and then a quenching process of rapidly cooling from a temperature of Ar ₃ or more is performed. And a tempering treatment of heating and cooling to a temperature of not more than _one point of Ac, followed by performing a tempering treatment of the low alloy seamless steel pipe having a grain refined structure.