JPH06172855A - Low Yield Ratio High Toughness Seamless Steel Pipe Manufacturing Method - Google Patents

Abstract

(57) [Summary] [Objective] A high toughness SSC steel pipe with a low yield ratio is obtained by air cooling treatment from a non-recrystallized state of 40% or more in a seamless process. [Configuration] The hollow shell of the heated steel slab obtained by piercing and rolling to 1100 ° C. or higher, cooled to Ar ₃ point C. to 1100 ° C. and Ar ₃ point to 900 ° C. before the inclined rolling of the front and last stage The cross-sectional area reduction rate of each inclined rolling mill is 20 ~
70% processing is applied to make the unrecrystallized structure rate 40% or more,
Then, it is cooled at a cooling rate of 10 ° C./s or less. [Effect] Since the inclined rolling mill has a large shear strain, the number of dislocations introduced is much larger than that of other ordinary rolling mills. A fine ferrite structure can be obtained by cooling in an austenite state with a high dislocation density, and a low yield ratio and high toughness SSC resistance.
A steel pipe is obtained.

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 low yield ratio and high toughness.

[0002]

2. Description of the Related Art In recent years, due to depletion of energy resources,
The development of gas and oil wells in the far north has become active. Therefore, it has a low yield ratio and high toughness (guaranteed at -60 ° C) and high strength (X52 or more) to withstand the use of high-pressure operation in cold regions for seamless steel pipes used as transportation equipment for products. Qualities are required. To give a low yield ratio to high strength materials, for example, “Iron and Steel, '87 -S131
5 ”, it is reported that the amount of C is increased and the tempering temperature after quenching is decreased. However, an increase in the amount of C significantly reduces the weldability, which is the basic use performance of line pipe steel, and as a result, preheating is required before on-site welding, and workability during line pipe laying is significantly reduced. . On the other hand, since the low temperature toughness of the line pipe steel manufactured by the low temperature tempering process becomes unstable, its use in cold regions is restricted.

[0003]

SUMMARY OF THE INVENTION In view of the above situation, it is an object of the present invention to provide a method for producing a low yield ratio and high toughness seamless steel pipe by controlling the steel composition and hot rolling conditions. To do.

[0004]

The present invention has the following structures in order to achieve the above object. Ie% by weight
As C: 0.03 to 0.20%, Si: 0.0
1-2.5%, Mn: 0.15-2.5%,
P: 0.020% or less, S: 0.010% or less,
Al: 0.005-0.1%, Ti:
0.005-0.1%, Nb: 0.005-
0.1% N: 0.01% or less, and if necessary, Cr: 0.1 to 1.5%, Mo: 0.0
5 to 0.5%, Ni: 0.1 to 2.0%,
V: 0.01 to 0.1%, B: 0.0003 to
0.0033%, rare earth elements: 0.001-0.05
%, Ca: 0.001 to 0.02%, Co: 0.05
˜0.5%, Cu: 0.1 to 0.5%, a steel slab containing 1 or 2 or more and the balance substantially consisting of Fe is heated to 1100 ° C. or higher, and then hot piercing rolling is performed. The obtained hollow shell was cooled to a point of Ar _{3 to} 1100 ° C., and immediately after that, it was subjected to a thickness cross-section reduction rate of 20 to 70 by a preceding stage inclined rolling mill.
% Forming process, and further cooling the hollow rough tube heated by heat generated during processing by the preceding stage inclined rolling mill to Ar ₃ point to 900 ° C. in front of the final stage inclined rolling machine, and the final inclined rolling immediately after that. Machined at a reduction rate of wall thickness of 20 to 70% with a machine to leave an unrecrystallized structure of 40% or more of the total thickness of the cross section, and then after shape-correcting hot continuous rolling, Ar ₁ point to 900
The hollow rough tube that has been lowered to a temperature of ℃
This is a method for producing a low yield ratio and high toughness seamless steel pipe, which comprises heating to 950 ° C., hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C. or higher, and cooling treatment at a cooling rate of 10 ° C./s or less.

[0005]

The operation of the present invention will be described in detail below.
First, the reason for limiting the above steel components in the present invention will be described. C and Mn are important in order to secure the strength and to reduce the grain size. If it is too small, the effect is not obtained, and if it is too large, the weldability is deteriorated. Therefore, the respective amounts were made 0.03 to 0.20% and 0.15 to 2.5%.

[0006] Si is an effective component for increasing the strength because it remains a deoxidizer. If it is too small, it will have no effect,
If it is too large, inclusions increase and the properties of the steel deteriorate, so the content was made 0.01 to 2.5%.

P and S are particularly important elements for improving the toughness in the present invention. P is a harmful component that easily causes cracks during processing due to segregation of grain boundaries, and S is Mn.
S-based inclusions were formed and stretched by hot continuous rolling, and their contents were 0.010% or less and 0.010% or less as components harmful to low temperature toughness.

Al, like Si, has a deoxidizing agent remaining, and is combined with N contained as an impurity component in steel to suppress the growth of crystal grains and improve the SSC resistance and the low temperature toughness. If it is too small, the effect is not obtained, and if it is too large, inclusions increase and the properties of the steel become brittle.
It was set to 1%.

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, which is contained as an impurity component in the steel, to control the crystal grains during hot piercing and to perform hot piercing and rolling on the hollow shell before the final stage of the inclined mill.
_{It is} cooled to ₃ points to 900 ° C., and immediately after that, it is possible to suppress coarsening of the crystal grain size after the forming process of 20 to 70% in the thickness reduction section by the final inclination rolling mill to improve the low temperature toughness, and to deoxidize,
From the effect of denitrification, the hardenability of B described below is exerted to enhance the strength. If it is too small, the effect is not obtained, and if it is too large, TiC precipitates and the steel is embrittled, so the content was made 0.005 to 0.1%. On the other hand, Nb is crystal grain growth suppression during tilt rolling, and abnormal coarseness of γ grains in the case where the material pipe that has fallen to a temperature of 900 ° C. to Ar ₁ point after continuous rolling is heated to 900 to 950 ° C. higher than the temperature. It is an important element that suppresses formation. If it is too small, it will not have the effect, and if it is too large, the effect will be saturated.
Moreover, since it is very expensive, it was set to 0.005 to 0.1%.

N is a harmful component that reduces the effect of B, and its content is set to 0.01% or less.

When the steel having the above-mentioned composition is used to further increase the strength of the steel, Cr, Mo, Ni, V and other components are selectively added as required. Cr, Mo, Ni and V are added to enhance the strength. If the amount is too small, there is no effect, and if the amount is too large, the effect saturates and it is very expensive.
.About.0.5%, 0.1 to 2.0%, and 0.01 to 0.1%. B suppresses the precipitation of ferrite and increases the strength.
If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will not change and the toughness and hot workability will be deteriorated.
It was set to 0.003%.

Further, in view of the use environment of the seamless steel pipe in recent years, the present invention further provides an SSC resistant steel having the above-mentioned composition.
In order to improve the 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 inclusions spherical and harmless. If it is too small, there is no effect, and if it is too large, inclusions increase and SSC resistance decreases, so 0.001 to 0.0, respectively.
5% and 0.001-0.02%. Co and Cu are
It has an effect of suppressing hydrogen invasion into the steel and works effectively for SSC resistance. If the amount is too small, the effect will not be obtained, and if the amount is too large, the effect will be saturated, so 0.05 to 0.5% and 0.1 to 0.1%, respectively.
It was set to 0.5%.

Next, the reason why the hot seamless rolling conditions are limited as described above will be explained. Steel having the above-described composition is melted in a melting furnace such as a converter or an electric furnace or further subjected to vacuum degassing treatment, and a steel slab is manufactured by a continuous casting method or an ingot agglomeration method. The steel slab is immediately or once cooled and then heated to a high temperature for hot piercing and rolling. The heating temperature must be sufficiently high to facilitate hot piercing and rolling. Within the composition range of the present invention, no problem occurs in hot drilling at a temperature of 1100 ° C. or higher,
The temperature was 1100 ° C. or higher.

The hollow shell subjected to piercing rolling is cooled to a temperature of Ar ₃ point to 1100 ° C. in front of the inclined rolling machine in the preceding stage, and immediately subjected to rough rolling for rough processing. The inclined rolling mill (elongator mill, etc.) has a very large shear strain component, unlike other rolling mills (mandrel mill, plug mill, etc.) used for rolling seamless steel pipes, and steel plate rolling mills.
Therefore, the substantial strain amount is much larger than the strain amount predicted from the cross-sectional area reduction rate. For this reason, in the inclined rolling mill, even if processing with a small cross-sectional area reduction rate, processing heat generation is large and the final stage inclined rolling temperature is set to Ar ₃ point to 900 ° C.
Need to be cooled. The hollow rough tube cooled to a temperature of Ar ₃ point to 900 ° C. is subjected to tilt rolling in which the outer diameter and wall thickness close to the final shape of the steel tube are roughly processed by a final stage tilt rolling machine. Since the inclination rolling mill has a very large shear strain component, the substantial strain amount is significantly increased, and even if the work with a small cross-sectional area reduction rate is performed, the austenite structure undergoes a large deformation, and during subsequent ferrite transformation. A fine ferrite structure is generated.

The photograph of FIG. 1 shows the same cross-sectional area reduction rate (40
%) Of the elongator mill (A) and the plate rolling machine (B). From this structure photograph, it can be seen that the deformation of the austenite structure is larger in the elongator mill than in the plate rolling mill. That is, in the elongator rolling at a low temperature, many dislocations are introduced into the unrecrystallized austenite structure and the generation of a fine ferrite structure is promoted. In this case, when the rolling temperature is 900 ° C. or higher, the unrecrystallized austenite structure ratio is reduced, the formation of a fine ferrite structure is suppressed, and the target fine-grained ferritic steel cannot be obtained, so the upper limit of the rolling temperature is set to 900 ° C. did. On the other hand, the increased rolling load and the rolling temperature decreases significantly decreases the moldability of the steel, and an outer diameter, since the wall thickness is difficult to obtain Ar ₃ point or more as a target.

The temperature of the hollow rough tube may be controlled by either standing cooling or forced cooling. If the rolling reduction is small, a fine ferrite structure is not formed, so the lower limit is 20.
%. 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 final stage of tilt rolling, the hollow rough tube is further continuously rolled to correct the shape, and Ar ₁ point to 900 ° C.
The temperature of the rough tube lowered to the temperature of
Reheat to 950 ° C. When the reheating temperature is high, recrystallization proceeds and the dislocation density is lowered so that a fine ferrite structure is not formed. Therefore, the upper limit is set to 950 ° C. On the other hand, if the temperature is too low, the rolling temperature in the final finishing rolling after reheating becomes low and it becomes difficult to secure the shape, so the lower limit was made 900 ° C.

After reheating, hot final finish rolling is performed at a temperature of Ar ₃ + 50 ° C. or higher. The rolling temperature was set to Ar ₃ + 50 ° C. or higher, which makes it difficult to secure the shape when the rolling temperature is too low. After hot final finish rolling, cooling is performed from the completely γ state. The cooling start temperature was set to Ar ₃ point or higher in order to secure a uniform structure and required characteristics. The structure after cooling is preferably a ferrite structure because a quenched structure or an intermediate stage structure appears and the yield ratio is increased. Therefore, the cooling rate during cooling is 10 ° C.
/ S or less. The steel obtained under the above manufacturing conditions is effective for manufacturing an SSC-resistant seamless steel pipe having a low yield ratio and excellent toughness.

[0019]

EXAMPLES Next, examples of the present invention will be described. Table 1
Shows the yield ratio, toughness, and SSC resistance of a steel pipe that is directly quenched and tempered by performing hot seamless rolling of a steel slab produced by melting in a converter and continuous casting. SSC resistance is N
Σt by constant load method according to ACE TM01-77
h (Threshold Stress) was obtained and evaluated. It can be seen that the steel pipe manufactured according to the present invention has high toughness at a low yield ratio and improved SSC resistance.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

The steel pipe produced by the method of the present invention as described above has excellent low temperature toughness and SSC resistance because it has a low yield ratio and finer grains, and is used in cold regions in the far north and sulfide stress corrosion environments. To be done.

[Brief description of drawings]

FIG. 1 (A) is a 100 × magnified photograph showing a non-recrystallized metal structure of a material after the final stage of tilt rolling and (B) after plate rolling.

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[Procedure amendment]

[Submission date] September 3, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

[0019]

EXAMPLES Next, examples of the present invention will be described. Table 1
Shows the yield ratio, toughness, and SSC resistance of a steel pipe which is produced by melting in a converter and through continuous casting, and hot-seam-rolled to allow the steel pipe to cool . SSC resistance is NACE ™
01-77 by σth (Thre
The evaluation was made by obtaining the "holdStress". It can be seen that the steel pipe manufactured according to the present invention has high toughness at a low yield ratio and improved SSC resistance.

Claims (4)

[Claims] 1. As weight%, C: 0.03 to 0.20%, Si: 0.01 to 2.5%, 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 Then, the steel slab consisting essentially of Fe
After heating above ℃, hot-hole piercing and rolling the hollow shell to Ar
_{It is} cooled to _{3 to} 1100 ° C, and immediately after that, it is subjected to forming processing of 20 to 70% with a reduction rate of wall thickness section in the former stage inclined rolling mill,
Further, the hollow rough tube whose temperature was raised by the heat generated by processing by the former-stage inclined rolling mill was cooled to Ar ₃ point to 900 ° C. in front of the final-stage inclined rolling mill, and immediately after that, the wall thickness cross-section reduction rate was 2%.
Hollow roughened to a temperature of Ar ₁ point to 900 ° C. after performing 0 to 70% forming processing to leave 40% or more of the entire thickness cross section of the unrecrystallized structure and then performing shape straightening hot continuous rolling. After the tube is heated to 900 to 950 ° C. higher than the temperature, hot finishing rolling is performed at a finishing temperature of Ar ₃ points + 50 ° C. or more, and a cooling treatment at a cooling rate of 10 ° C./s or less is performed. Yield ratio High toughness Seamless steel pipe manufacturing method. 2. As weight%, C: 0.03 to 0.20%, Si: 0.01 to 2.5%, 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 Then, Cr: 0.1 to 1.5%, Mo: 0.05 to 0.5%, Ni: 0.1 to 2.0%, V: 0.01 to 0.1%, B: A steel slab containing 0.0003 to 0.0033% of 1 type or 2 types or more and the balance of which is substantially made of Fe is heated to 1100 ° C. or higher, and then a hot blanked tube is hot-rolled and rolled into an Ar ₃ point. 〜1100 ° C, and immediately after that, it is 20-70 at the reduction rate of wall thickness section in the inclined rolling mill in the previous stage.
% Forming process, and further, the hollow rough tube heated by the heat generated by the processing in the inclined rolling mill in the previous stage is Ar in front of the inclined rolling mill in the final stage.
_{It is} cooled to ₃ points to 900 ° C., and immediately after that, it is subjected to forming processing of 20 to 70% at the reduction rate of the thickness section by a final inclination rolling mill to leave an unrecrystallized structure of 40% or more of the entire thickness section, and thereafter, After performing the shape-correcting hot continuous rolling, the hollow coarse tube, which has dropped to a temperature of Ar ₁ point to 900 ° C., has a temperature of 900 to 95 higher than the temperature.
Manufacture of a seamless steel pipe with a low yield ratio and high toughness, which is characterized by performing hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C or higher after heating to 0 ° C and performing cooling treatment at a cooling rate of 10 ° C / s or less. Law. 3. As a weight%, C: 0.03 to 0.20%, Si: 0.01 to 2.5%, 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 Further, rare earth element: 0.001 to 0.05%, Ca: 0.001 to 0.02%, Co: 0.05 to 0.5%, Cu: 0.1 to 0.5%, 1 After heating a steel slab containing at least one kind or two or more kinds, and the balance being substantially Fe, to 1100 ° C. or higher, the hot piercing-rolled hollow shell is cooled to Ar ₃ point to 1100 ° C. The reduction rate of wall thickness is 20 ~ 70
% Forming process, and the hollow rough tube heated by the heat generation in the former stage of the inclined mill is heated in front of the final stage of the inclined mill.
_{It is} cooled to ₃ points to 900 ° C., and immediately after that, it is subjected to forming processing of 20 to 70% at the reduction rate of the thickness section by a final inclination rolling mill to leave an unrecrystallized structure of 40% or more of the entire thickness section, and thereafter, After performing the shape-correcting hot continuous rolling, the hollow coarse tube, which has dropped to a temperature of Ar ₁ point to 900 ° C., has a temperature of 900 to 95 higher than the temperature.
Manufacture of a seamless steel pipe with a low yield ratio and high toughness, which is characterized by performing hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C or higher after heating to 0 ° C and performing cooling treatment at a cooling rate of 10 ° C / s or less. Law. 4. C: 0.03 to 0.20%, Si: 0.01 to 2.5%, Mn: 0.15 to 2.5%, P: 0.020% or less, as weight%. 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 Then, Cr: 0.1 to 1.5%, Mo: 0.05 to 0.5%, Ni: 0.1 to 2.0%, V: 0.01 to 0.1%, B: 0.0003 to 0.0033% of one kind or two or more kinds and rare earth elements: 0.001 to 0.05%, Ca: 0.001 to 0.02%, Co: 0.05 to 0.5%, Cu: A hollow shell prepared by heating a steel slab containing 0.1 to 0.5% of 1 type or 2 types or more and the balance of which is substantially Fe to 1100 ° C. or higher, and then hot piercing-rolling the hollow shell. Ar ₃ points to 1100 ° C., and immediately after that, the thickness of the cross-section reduction rate is 20 to 70 in the preceding stage inclined rolling mill.
% Forming process, and further, the hollow rough tube heated by the heat generated by the processing in the inclined rolling mill in the previous stage is Ar in front of the inclined rolling mill in the final stage.
_{It is} cooled to ₃ points to 900 ° C., and immediately after that, it is subjected to forming processing of 20 to 70% at the reduction rate of the thickness section by a final inclination rolling mill to leave an unrecrystallized structure of 40% or more of the entire thickness section, and thereafter, After performing the shape-correcting hot continuous rolling, the hollow coarse tube, which has dropped to a temperature of Ar ₁ point to 900 ° C., has a temperature of 900 to 95 higher than the temperature.
Manufacture of a seamless steel pipe with a low yield ratio and high toughness, which is characterized by performing hot finishing rolling at a finishing temperature of Ar ₃ points + 50 ° C or higher after heating to 0 ° C and performing cooling treatment at a cooling rate of 10 ° C / s or less. Law.